UNIVERSITATEA DIN CRAIOVA

FACULTATEA ŞTIINŢE

SPECIALIZAREA FIZICĂ

METTALIC OXIDE SEMICONDUCTOR:

SYNTHESIS, PROPERTIES &

APPLICATIONS

Îndrumător ştiinţific: Absolvent:

Prof. univ. dr. DINESCU Maria Ing. BĂNICI Ana-Maria

CRAIOVA

2019

Thanks !

At the end of my phD thesis elaboration process, I want to address my sincere gratitude

to Prof. Dr. Maria Dinescu, my thesis scientific coordinator, for her scientific support given

throughout my doctoral school.

I owe special gratitude and thanks to Dr. Claudiu Fleaca, Dr. Sandu Ion and Dr. Marius

Dumitru Grivei, for their high quality scientific guidance and professionalism throughout the

elaboration of my thesis.

I am deeply grateful to Dr. Ruxandra Birjega for her support, guidance,useful discussions

and suggestions which have been of real use during my thesis.

I would especially like to thank Dr. Cristian Mihailescu, Dr. Natalia Mihailescu and

Dr. Catalin Luculescu without whom it wouldn`t have been possible to attain morpho-structural

and elemental characterizations for my scientific results.

Sincere thanks to all of those who have contributed in any way,especially by fellow

research colleagues from “Laser induces photochemistry”: Dr. Ion Morjan, Dr. Monica

Scarisoreanu, Dr. Lavinia Gavrila, Dr. Dumitrache Florian.

Introduction 5

1.1. Oxides: definition, clasification, chemical and physical properties 7

1.2. Metal Oxides Semiconductors: definition, classifications, physical and chemical properties 7

2 Nanometric metal oxide semiconductor (MOS) 8

2.1 Nanomaterials 8

2. 1. 1. Nanomaterials: definition, properties, application 8

2.1.2 General methods for obtaining nanomaterials: 8

2.2. Oxide nanomaterials. Gas sensors based on nanomaterials 9

2.2.3. Factors influencing the gas-sensing mechanism: 10

2.2.4. Band theory: 10

2.2.5. Sensor response disruption : 10

2.2.6. Sensor parameters 11

2.2.7. Type of gas sensors 11

2.2.8 Gas-sensing mechanism for hydrogen 12

3. Synthesis method of nanostructured mettalic oxides 12

3.1. Solid state combustion synthesis method 12

3.1.1 Nanopowder synthesis 13

3.1.2. Principal parameters of combustion process 14

3.2.Synthesis of nanomaterials by laser pyrolysis 14

3.2.1. Principle of the method and basic phenomena of laser pyrolysis 14

3.2.2 Influence of laser beam characteristics on pyrolysis processes 14

3.2.3. Set-up experimental de sinteza a nanoparticulelor prin piroliza laser 15

3.3. Synthesis of thin films using laser techniques 16

3.3.1. Pulsed laser deposition 16

3.3.2 (Matrix assisted pulsed laser evaporation) 17

4. Characterisation techniques of metalic oxides semiconductors 18

4.1 X-ray Diffraction 18

4.2 (AFM) Atomic force microscopy 19

4.3 Scanning electronic Microscopy 19

5. SnO2 si TiO2/SnO2 nanostructures/nanoparticles synthesis by laser techniques : obtaing,

characterisation and application 19

5.1. TiO2/SnO2 nanoparticles synthesis 19

5.2 The thin films sythesis using the MAPLE technique,starting from the TiO2 / SnO2 nanoparticles 22

5.3. Composite nanoparticles synthesis 26

5.4 Thin films synthesis with the help of MAPLE technique from SnO/SnO2 nanoparticles optained by

laser pyrolisis 29

6. Synthesis by combustion of solid solution of mixed oxide nanoparticles: ternary and quaternary

compounds; synthesis, characterization and applications 33

6.1. Ternary mixed oxides obtained by mediated glycine combustion 33

5.2. Quaternary mixed oxides Ti – Sn – Fe – Co 36

6.3. Mixed cuarternary oxides Ti-Sn-Fe-Ni 39

6.4. SnO2 NPs synthesis using solid state combustion : synthesis, characterisation and applications 45

7. Conclusion 47

Introduction

Nanotechnology is a well-known field of research since the last century, more precisely since Nobel laureate

Richard P. Feeyman held his famous lecture called “There’s Plenty of Room at the Bottom (1959). Nanotechnology

produced materials of various types at nanoscale level. Nanoparticles (NPs) are wide class of materials that include

particulate substances, which have one dimension less than 100 nm at least.

For a long time, polluting gases comming from industry and internal combustion engines have been

responsible for many human health problems and for global climate change. Regarding that, several groups of

researchers have focused on the research and development of new sensors materials for the detection and monitoring

of polluting gases, such as CO and C3H8. In particular, sensors that are made of semiconductor metal oxides have been

a valuable choice because they have a good chemical stability, low price and easy integration into electronic circuits.

However, it is still necessary to improve sensor parameters, such as selectivity, sensitivity and operating temperature.

In 1962, Naoyoshi Taguchi (Taguchi, 1962) designed the first SnO2 – based semiconductor gas sensor. Thus,

over 50 million Taguchi sensors have been widespread in domestic gas alarms only in Japan in the period from 1968

to 990 (FIGARO, 1990). Nevertheless, the main drawback of this type of sensor is their lack of selectivity; for

example, cross sensitivity to ethanol, carbon monoxide and methane prevents accurate hydrogen detection [4].

The research in the field of gas sensors based on semiconductor metallic oxides is guided by the innovations

in materials science, by the need to miniaturize the devices that contain the sensors in order to include them in domestic

/ industrial detection systems with the lowest operating costs and a deeper understanding of the mechanism / process

that takes place at the material / gas interface.

Most scientific publications in this field show poor selectivity for the gases to be measured, working

temperatures too high and response times too long, these parameters requiring continuous research and development.

Understanding the fundamental physico-chemical phenomena that determine certain parameters of the

sensors requires an overview between the structure, properties and the final architecture (design) of the sensor device,

corroborated with a good knowledge of the environment in which the sensors will be implemented.

On this thesis, the main focus was on the synthesis of materials with a controlled structure and morphology,

which could provide the increase of the working parameters of the sensor, the major improvement being the reduction

of the time response of the sensor and the obtaining of a selectivity at low gas concentrations; progress was also made

regarding the working temperature (150 ° C for hydrogen detection, on a sensor based on SnO2 nanoparticles, obtained

by laser pyrolysis). High working temperatures are usually required for rapid adsorption and desorption of gases; also,

dopants play a very important role in improving the sensitivity and selectivity of a gas sensor.

Another requirement on the research and development is the sensor stability, from the point of view of the

electronic components and the methods of encapsulation of the device, because the surface should be sensitive to the

composition of the surrounding gas atmosphere and special encapsulation is needed in order to exclude disturbing

environmental effects, but still enable gas flow or diffusion inside.

Magnetic nanoparticles like spinel / invers spinel have draw the attention of scientists in the last decade

because their methods of obtaining (synthesis methods) are simple, cheap and with a very high degree of

reproducibility, having a rigorous control of the process parameters, on which the morphological and structural

properties will depend : mean crystallite size, ratio of crystalline phases, dopant concentration, degree of crystallinity.

CoFe2O4 nanoparticles have received special attention because they are used on a broad applications in

several technological fields including ferrofluids, high-density recording media, drug delivery or hyperthermia

applications because of their unique properties ( magnetic, electronics, and optics properties) like: remanence,

moderate coercivity,high blocking temperature, strong anisotropy, high saturation magnetisation and coercivity

along with good mechanical hardness and chemical stability.The in vivo and in vitro studies shown that this NPs

could be used like biomaterials for magnetic hyperthermia application. Magnetic hyperthermia is dominated by energy

loss through three main heating mechanisms: loss of hysteresis, Neel relaxation and Brown relaxation - these

mechanisms being crucial influenced by the average crystallite size; heat dissipation of superparamagnetic

nanoparticles is rarely accompanied by hysteresis loss while ferromagnetic nanoparticles exhibit significant hysteresis

loss, which should improve heating efficiency.

The use of iron oxide nanoparticles in the treatment of cancer by magnetic hyperthermia was first

demonstrated by Gilchrist et.al in 1957. In 2004, first clinical system for treatement using hyperthermia was developed

at Medical University Charite from Berlin, and some years later, Magforce obtained the European approval to treat

pacients with brain tumors using this method.

This doctoral thesis is structured in 7 chapters as follows:

On the first chapter I made a brief presentation of the theoretical considerations regarding the materials that

I will synthesize and characterize in this thesis, respectively definitions, classifications and physical / chemical

properties of semiconductor metal oxides.

Chapter II presents the semiconductor metallic oxides at the nanometric scale and is made as well an

introduction on the field of nanomaterials, more specifically of the oxide nanomaterials used for the manufacture of

toxic gas sensors. We have presented in detail the operating mechanism of the conductometric sensors based on

semiconductor nanomaterials, the factors that influence this mechanism, the parameters that we need to improve,

respectively the elements that can disturb the response of the gas sensor during its operation.

Chapter III presents theoretical aspects regarding four types of methods for obtaining / synthesis of

semiconductor oxide nanomaterials; the first method is a chemical route, more precisely the chemical combustion of

solid state solutions (Solid State Combustion - SSC), the second is a laser technique, called laser pyrolysis :- with this

method, oxide nanoparticles of the TiO2 have been obtained , and nanocomposites SnO2 or TiO2 / SnO2, SnO2 / SnO.

The last two laser techniques aim to obtain the thin films needed for the achievement of the gas sensors, the pulsed

laser deposition (PLD) and the Matrix Assisted Pulse Laser Evaporation (MAPLE), both techniques based on the use

of solid state laser or excimer lasers on the ultraviolet regim ( wavelength), λ = 266nm - the fourth harmonic of the

Nd: YAG laser and λ = 248nm, the fundamental wavelength of the KrF laser.

In Chapter IV I made a quick review of the characterization techniques used for the study of nanoparticles

and thin films obtained by the laser techniques mentioned above; the main meths was X-ray Diffraction ( XRD)-

necessary for the identification of the crystalline phases present on the sample, for the determination of the residual

stress, lattice straind and the degree of crystallinity, the mean crystallite size and the lattice parameters. As a

complementary tool I used 2 types of electronic Microscopy : Scanning Electronic Microscopy (SEM) and High

resolution Transmision Electronic Microscopy ( HR-TEM). The topography and the morphology of the thin films

surface was studied with the help of Atomic Force Microscopy (AFM), which provided us information regarding the

thickness of the thin layer and its roughness, very important parameters for sensing tests, being attented the

achievement of thin films with a high roughness and an orderly arrangement of the nanoparticles, in order to have a

specific surface area higher as possible. The data obtained at AFM were confirmed by Optical Profilometry ( the

roughness and the thinkness of the thin film).

Chapter V is structured in two parts: part I - in which we presented the results obtained after the process of

laser pyrolysis, respectively the nanoparticles of TiO2, SnO2 and the nanocomposites TiO2 / SnO2, SnO2 / SnO. During

the experiments I tried to vary the precursor concentrations, the geometry of the experimental set-up and the laser

parameters in order to obtain a very strict control of the properties of the nanoparticles: mean crystallite size, crystalline

phase ratio. As obtained nanoparticles were studied morphological and structural with the techniques already

presented above. The second part of this chapter is focused on the thin film synthesis on silicon substrate and

interdigitated contacts, using MAPLE; the samples (thin films) obtained on this way were tested at various gases and

different concentrations. The obtained results from gas testing procedure present a major improvement on the time

response of the sensor at very low hydrogen concentrations ( up to 1ppm CO on synthetic air) and a low working

temperature ( T=150°C).

Chapter VI is dedicated to the method of chemical combustion synthesis (Solid State Combustion - SSC),

the method by which we obtained SnO2 oxide nanoparticles, with the goal of comparing those obtained by laser

assisted techniques (laser pyrolysis) and ternaries mixed oxides based on Ti-Sn-Fe (having TiO2, SnO2 and α-Fe2O3

main crystalline phases), respectively quaternary mixed oxides Ti-Sn-Fe-Co / Ni, which were synthesized starting

from the ternary compounds mentioned above by adding metallic compounds ( Cobalt and Nickel ) in different

concentrations and atomic ratios between elements, in order to obtain spinel and inverse spinel structures, with some

carefully controlled parameters for testing them in applications on the field of toxic gas sensors, photovoltaic cells,

photocatalysis, hyperthermia treatment , high frequency storage media, etc.

Chapter VII is dedicated to the original results obtained during the elaboration of the present doctoral thesis,

focusing on the experimental results regarding the applications in the field of toxic gas sensors, respectively the

synthesis of spinel / Inverse spinel composite nanostructures.

The results were partially published in ISI journals and presented at national and international conferences

on the field of materials science, X-ray diffraction and high-power lasers.

1.1. Oxizii: definiţie, clasificări, proprietăţi fizice şi chimice

1.1. Oxides: definition, classifications, physical and chemical properties

Oxides are the compounds of oxygen with the other elements. In the molecule of an oxide, both valences of

oxygen are satisfied by the other element which may be metal or nonmetal [9]. Depending on their property of reacting

with acids or bases, oxides can be classified into two groups: a) salts-producing oxides and b) non-salts-producing

oxides.

a) Salt-producing oxides: are those that form salts as a result of chemical reactions with acids (basic oxides) or with

bases (acid oxides).

b) Oxides that do not produce salts: - they are named like that because they do not react with acids or bases and

therefore don’t give rise to salts.

The oxides producing salts, according to their composition and chemical properties, are classified in: basic

oxides and acid oxides.[9] The basic oxides are mostly oxygen compounds with metals, also forming at normal temperature, being

generally solid, crystallized or amorphous substances. In most cases the basic oxides are colored; their color varies

not only from one oxide to another but even to the same oxide, either with temperature or with the state in which they

are (crystals, powders, or impurities).

Acid oxides are generally oxygen compounds with non-metals. This category includes a large number of

non-metallic oxides (metalloids), for example: CO2, SO2, SO3, P2O5, etc .; these oxides react with the bases

(hydroxides) giving rise to salts;

Amphoteric oxides have on the same time the properties of a basic and acidic as we have shown above. They

behave as basic oxides in the face of strong acids and as acidic oxides in the face of strong bases; one example of

amphoteric oxide is Al2O3, another being ZnO.

Inorganic peroxides are combinations of elements such as H, Na, K, Ba, etc. with oxygen, in the molecules

to which two oxygen atoms are linked together by a valence, and with the other valence each of the oxygen atoms is

linked to the atom of another element. The -O-O- group is called peroxo or peroxide, being characteristic of

peroxides:[9]. Generally, oxides can be obtained by several methods:

a) By directly combining oxygen with metals or non-metals (oxidation)

b) By thermal decomposition of oxygenated salts

c) By dehydrating the acids that contain oxygen

1.2. Oxizi metalici semiconductori: definiţie, clasificări, proprietăţi fizice şi chimice 1.2. Metal Oxides Semiconductors: definition, classifications, physical and chemical properties

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as

metallic copper, and an insulator, such as glass. Its resistance falls as its temperature rises; metals are the opposite. Its

conducting properties may be altered in useful ways by introducing impurities ("doping") into the crystal structure.

Where two differently-doped regions exist in the same crystal, a semiconductor junction is created [10]. The behavior

of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors

and all modern electronics. Semiconductors are the foundation of modern electronics; semiconductor devices are:

transistor, solar cells, several types of diodes, including luminescent diode and integrated circuits. Photovoltaic cells

are semiconductor devices that convert light energy into electrical energy. In a conducting metal, the current is

represented by the flow of electrons. In a semiconductor, the current is represented either by the flow of electrons or

by the flow of "holes" from the electronic structure of the material. The conductance of the current in a semiconductor

occurs through the movement of the free electrons with negative charge (-) and the "holes" with positive charge (+),

these being known as charge conductors. By adding impurity atoms into a semiconductor material (a process called

doping), the number of charge conductors in a semiconductor can increase substantially [11]. When in a semiconductor

the conduction take place due to the holes, it is called a p-type semiconductor, and in the case of a semiconductor the

electrical conduction takes place mainly due to free electrons, it is called a n-type semiconductor. Only one

semicondctor can have much more regions n-type and p-type; those can be coupled in junctions p-n, n-p-n or p-n-p,

which establish the electrical behavior of the diode or transistor type.

There are two types of semiconductor materials - elements and compounds. The unique arrangement of the

silicon and germanium atoms makes these two elements the most used in the preparation of semiconductor materials.

New discoveries in the field of semiconductors have made it possible to increase the complexity and speed of

microprocessors and memory devices.

Semiconductor devices can have many useful properties, such as passing the current in one direction more

easily than in the other, having variable resistances, sensitivity to light or heat. Because the electrical properties of a

semiconductor material change under the action of impurities, electric fields, or light, devices made of semiconductor

materials can be used to amplify, transform, or energy saving.

Semiconductor properties:

a)variable conductivity

b)heterojonction

c)excited electrons

d)light emission

e)heat energy conversion

2 Nanometric metal oxide semiconductor (MOS)

2.1 Nanomaterials

2. 1. 1. Nanomaterials: definition, properties, application

One of the most complete definitions of nanomaterials belongs to R. W. Siegel who defines them as:

[12] : , Materials that have atoms placed in nanometer-sized clusters that form granules or parts of the bulk

body ("bulk") or,

. Any material having at least a size between 1 nm and 100 nm.

The most convenient classification of nanomaterials can be achieved according to their dimensionality. Thus,

we can have materials of the following type:

0 D, for example: quantum dots, particles, precipitates, colloids, catalysts, etc.

1 D, for example: nanotubes, fibers, yarns, etc.

2 D, for example: surface coatings, thin layers, junctions, interfaces, etc.

The dependence of the physical and chemical properties of the materials on low range dimensions becomes

evident when:

The physical properties of the materials no longer follow the laws of classical physics but are rather

described by Quantum Mechanics.

The specific interface effects are dominant.

The physical properties are generated by a limited number of constituents of the material (no statistical

mediation can be made as in the case of macroscopic bodies).

Let us further detail some of the types of dependence of the physical properties of their nanometric materials

of reduced dimensionality:

a) Nano-effects determined by the quantum behavior of the materials

b) Nano-effects determined by specific interface effects

c) Nano-effects determined by a limited number of constituents

2.1.2 General methods for obtaining nanomaterials: The obtaining of materials with nanometric dimensions is the object of what is known as "Nanotechnologies".

We present below some of the most synthetic definitions of this field:

Nanotechnology include those fields of technology in which their dimensions and tolerances in the range

0.1 - 100 nm play a crucial role [13]

The object of Nanotechnology is: the production, analysis and applications of functional structures whose

dimensions do not exceed 100 nm. Nanotechnology deals with the intermediate area between individual atoms or

molecules and their expanded assemblies. In this area new physical phenomena appear that cannot be detected at

macroscopic objects.

Nanostructured materials can be synthesized using techniques and technologies that can be classified into the

following categories:

Techniques that start from precursors in atomic or molecular form that provide a basis on which larger

blocks can be built. For example, common techniques used in this class are: gas condensation, chemical precipitation,

sol-gel, aerosol reactions, pyrolysis, laser pyrolysis, laser ablation, CVD.

Techniques that are based on size reduction by mechanical fragmentation of micron particles to nanometric

range using ball milling.

Techniques based on phase transformations of materials.

2.2. Oxide nanomaterials. Gas sensors based on nanomaterials 2.2.1. Nanostructured metallic oxides semiconductor

Semiconductor metal oxides are widely studied and used in industry with different roles, especially in

detecting harmful, explosive or easily flammable gases, etc. In the last decade, the chemo-resistive sensors based on

semiconductor metal oxides have attracted a great deal of attention, being used in air / environmental quality

monitoring, monitoring and testing of gas emissions or food safety testing, due to low production cost and the property

of selectively detecting certain gases and the simplicity of use[14–17].

It has been known since 1962 that the absorption or desorption of a gas on the surface of a metal oxide

changes the conductivity of the material, this phenomenon being first demonstrated using thin layers of zinc oxide. The surface sensitivity of a gas can be very high, reaching up to several parts per billion (ppb). It is desirable that the

semiconductor metallic oxide type sensors have a larger surface area, so that they absorb as much of the target

analyte as possible on the surface, giving a stronger and more easily measurable response (especially at low

concentrations)[19].

The working principle of sensors of this type is based on changing the balance of surface reactions, associated

with the target gas. Semiconductor oxide gas sensors offer a number of advantages, including low cost of production,

simplicity of final device structure, robustness in practical applications, and adaptability to a wide variety of oxidizing

and reducing gases.

Recent studies have shown that MOS sensors of "p" type have received less attention compared to those of

the "n" type; Hübner et al. [55] suggested that the response of a p-type oxide semiconductor gas sensor to a given gas

was equal to the square root of that of an n-type oxide semiconductor gas sensor to the same gas when the

morphological configurations of both sensor materials were identical [11,12]. However, the "p" type has its own

advantages, such as low humidity dependence [23] and high catalytic properties [24]. Given that a single type of MOS

cannot meet all the requirements for monitoring difficult environmental conditions, the combination of types should

be considered when will choose the sensor materials [19].

2.2.2. Operating mechanism:

The gas sensing mechanism based on metalic oxide semiconductors is mainly described by 2 models: the

first model is ion-absorption, and the second is the model of oxygen valences. The first model considers that it is the

effect of the charge separation or the modification of the surface electric potential, which results from the adsorption

of the gas, the ionization and the redox reaction. The second model focuses on the reaction between the valence of

oxygen and the gas molecules on the one hand, and on the variation of the excess of the valence of oxygen located in

the immediate surrounding of the surface, respectively on the mechanism of reduction-reoxidation.

Fig. 2.5. [19] Gas-sensing mechanism and conduction model based on metattalic oxide semiconductor type ’n’ & „p”

2.2.3. Factors influencing the gas-sensing mechanism: • The particle size

• Pore size

• Structural defects

2.2.4. Band theory:

Band theory states that within a lattice there exists a valence band and a conduction band. The separation

between these two bands is a function of energy, particularly the Fermi level, defined as the highest available electron

energy levels at a temperature.

Fig. 2.6. [28] Schematic band diagrams of an insulator, semi-conductor and conductor

Insulators have a large gap between the valence and conduction band (typically taken to be 10 eV or more),

as such a lot of energy is required to promote the electron in to the conduction band and so electronic conduction does

not occur. The Fermi level is the highest occupied state at T = 0 [29]. Semiconductors have a sufficiently large energy

gap (in the region of 0.5–5.0 eV) so that at energies below the Fermi level, conduction is not observed. Above the

Fermi level, electrons can begin to occupy the conduction band, resulting in an increase in conductivity. Conductors

have the Fermi level lying within the conduction band.

2.2.5. Sensor response disruption :

The presence of other gases in the sensor headspace is important. The measurement of conductivity should

ideally be only for the target analyte. If a contribution to the charge carrier concentration (whether this be an increase

or decrease) comes from another gas, the sensor reading will be inaccurate, providing a false measurement. Some of

the important disrupting gases are listed below[28]: .

Ozone

Water

Volatile Organic Compounds

Factors Influencing Sensor Design

Many factors must be addressed when designing a new metal oxide semiconductor gas sensor; such as the

material’s sensitivity and specificity to the gas in question or if the sensitivity of the material is appropriate for the

application. The same gas sensor may not be appropriate in two different environments: a carbon dioxide sensor for

the inside of a car exhaust should be designed for high concentrations, whereas one car cabin air quality should be far

more sensitive to carbon dioxide at lower concentrations. The sensor would not be accurate enough to register

concentration changes of 10 ppm if its sensitive range is 1,000–10,000 ppm.[28]

2.2.6. Sensor parameters The most important parameters of a gas sensor are: sensitivity, response, recovery time, gas selectivity, limit

of detection, stable working temperature; while the role of humidity and long-term stability have been poorly studied.

The S sensitivity of a resistive sensor can be defined as S = R0/Rg for a semiconductor ‚,n” type and S =

Rg/R0 for a semiconductor ,,p”-type, while some define it as the relative change in resistance S = ΔR/Rg or S = ΔR/

Rg, where Ro si Rg are the measured resitance before and after the exposure at hydrogen or another reducing gas.

Trasp (time of response) is definde as the necesary time required to reach 90% of the signal for a constant

response.

Trec (recovery time) is defined as the necesary time required to remake 90% from de baseline of the original

resitance ( the gas sensor resistance before gas exposure).

Selectivity is defined as a ratio of the response of the target gas to another gas, and is used to evaluate the

performance of a MOS sensor on different gases. Most sensors encounter the problem of poor sensitivity or low

selectivity.

LOD- limit of detection of gas sensor; according to its definition, the LOD must be the lowest concentration

that can be detected, but 3 times higher than the background noise. Therefore, LOD should be estimated by

extrapolation as a function dependent on concentration S at 3σ / R0, where σ is the standard deviation of R0. However,

most studies reported LOD through direct measurement of concentration during experiments, while other studies did

not mention LOD at all.

2.2.7. Type of gas sensors

a) CO gas sensor

b) CO2 gas sensor

c) NOx gas sensor

d) NO3 gas sensor

Currently there are several types of commercial hydrogen sensors available: electrochemical based on

semiconductors, thermoelectric, metallic, optical, and acoustic. Of all these, semiconductor metal oxide sensors have

high sensitivity, fast response, long-term stability and potential for integration into high-performance hydrogen

detection systems. Nanostructured semiconductors and thin films have been used like gas sensing materials to increase

the sensing performance, due to the specific surface area and new electron-transport properties. For example,

mesoporous films or nanotube networks obtained by anodic oxidation exhibit improvements in sensor properties over

conventional films, due to the specifically enlarged specific surface. As for size reduction, 1D nanostructures exhibit

increased properties, such as higher sensitivity, higher detection limit (LOD), lower working temperature, and

improved response time. Figure 2.9. shows a typical structure of a semiconductor metallic oxide resistive hydrogen

sensor consisting of a metal oxide layer on an insulating substrate and 2 electrodes, as well as a heater under the

sensitive layer.

Fig. 2.9. Gas senzor diagram [31]

During operation, the sensitive layer will be heated to a certain temperature to increase sensing performance.

This temperature depending on the materials used is usually of the order of several hundred degrees Celsius. The

resistance (R) of the sensitive layer will change due to exposure to hydrogen gas. The variation of the resistance will

depend on the concentration of H2 and has a relation approximately linear with the concentration of H2 up to a certain

domain. The resistive type mechanism of the semiconductor metallic oxide has been intensively investigated. The

general-accepted mechanism is based on the variation of the space charge area due to the reaction between H2 and the

chemosorbed oxygen on the active surface.

2.2.8 Gas-sensing mechanism for hydrogen

First, oxygen is absorbed by the surface of the WHO when it is heated in air. Oxygen absorption forms ionic

species, such as O2-, O-, O2- that accumulate electrons in the conduction band. The kinetic reactions are explained by

the following formulas

O2(gaz) ↔ O2 (adsorbit) (3)

O2 (adsorbit) + e- → O2- (4)

O2- + e- → 2 O- (5)

O- + e- → O2- (6)

The transfer of electrons from the conduction band to the chemosorbed oxygen leads to a decrease in the

concentration of electrons and an increase in the resistance of the oxide film. The hydrogen sensing behavior of a

MOS film comes from the desorption of O- and O2- ions at a working temperature between 250 - 350 ° C or above 300

° C.

H2 + O- (ads.) → H2O + e- (7)

H2 + O-2 (ads.) → H2O + 2e- (8)

When the film is exposed to hydrogen, it reacts with chemosorbed oxygen species, thus releasing electrons

back into the conduction band.

3. Synthesis method of nanostructured mettalic oxides

3.1. Solid state combustion synthesis method Solution combustion is an effective method for the synthesis of nanometer-sized materials and has been used

to produce a variety of complex oxide powders (over 1000 powders) for various applications, including catalysts, fuel

cells and biotechnologies. It is well-known that techniques based on combustion or synthesis by self-propagation at

high temperatures are effective energy saving methods for the synthesis of a larger quantity of powder.

Nanoscale oxidic and ceramic powders can be prepared using this technique using the combination of metal nitrates and a fuel in a usually aqueous solution. Glycine, citric acid and urea are particularly suitable because they can act as a metal ion complexing agent in solution and also serve as a fuel for the synthesis of nanocrystalline metal oxides. The properties of the resulting powders (crystalline structure, amorphous structure, crystallite size, purity, specific surface area, particle agglomeration) depend to a large extent on the parameters of the chosen process.

Combustion synthesis [46] has emerged as an important technique for the synthesis and processing of advanced

ceramics (structural and functional), catalysts, composites, intermetallic alloys and nanomaterials. In combustion

synthesis, the extothermicity of the redox chemical reaction (oxidation-reduction or electron transfer) is used to

produce a very fine material. Depending on the nature of the reactants - elements or compounds (solids, liquids or

gases) and exothermicity (temperature, non / adiabatic process), combustion synthesis is described as: synthesis by

high temperature self-propagation (SHS), combustion synthesis at low temperature (LCS), synthesis by solution

combustion (SCS), gel combustion, sol-gel combustion, emulsion combustion. The combustion synthesis process is

characterized by high temperatures, rapid heating rates, and very short reaction times. The SHS method is

characterized by the fact that once the exothermic reaction mixture is ignited by an external heat source, a very fast

reaction wave (0.1-10 cm / s), and a very high temperature (1000 ° C-3000). ° C) propagates throughout the

heterogeneous composition in a controlled manner, so as to lead to the formation of the solid material without

involving any additional energy.The very high temperature required to initiate the process (> 1500 ° C) can be obtained

with the help of laser radiation, an electric arc or a chemical furnace [46]. In another case combustion synthesis, for

example VCS (volume combustion synthesis), the entire sample is uniformly heated until the reaction is self-initiated

throughout the entire volume of the sample

These characteristics make combustion synthesis an attractive method for the technological production of useful

materials at low costs compared to the conventional chemical process. Other advantages of combustion synthesis

would be:

• use of simple equipment;

• formation of high purity products;

• stability of the metastable phases;

• virtual formation of any size and shape of the final product.

3.1.1 Nanopowder synthesis According to A. Kopp Alves et. of [45], there are a variety of ways in which ceramic powders can be

synthesized: solution, solid-solid, solid-gas process. The processes in the solution have been used more as a result of

the specific characteristics. The purpose of all these processes is to produce high purity powders with small particle

size, small aggregation / agglomeration and low production costs.

Synthesis by combustion of solutions is a method based on the principle that once a reaction is thermally

initiated, an exothermic reaction occurs that self-sustains over a certain period of time resulting in a powder as the

final product. The exothermic reaction starts at the combustion temperature and generates a certain amount of heat

manifested as maximum temperature or combustion temperature. This type of synthesis has the advantage of rapidly

producing fine and homogeneous powders. Since it is a self-sustaining and exothermic process with a high rate of heat

release, it can be explosive and must be carried out with additional measures to protect the work. The process of

synthesis through combustion is a quick and easy process with the advantage of saving time and money. This process

is used directly in the production of high purity and homogeneous ceramic oxide powders. This method is versatile

for the synthesis of a wide range of nanoparticles, including nanometric alumina powder. It is interesting that a redox

metal complex consisting of nitrate-glycine-acetate or mixtures of metal-nitrogen combustion systems of aluminum-

urea, did not require the presence of the flame to obtain the oxide nanoparticles.

The foundations for the combustion synthesis technique consist of the thermodynamic concepts used in

explosives and propulsion fuels and their interpretations are extrapolated by many researchers to the combustion of

ceramic oxides. The success of this process is closely linked to the appropriate fuel mixture or complexing agent (citric

acid, urea or glycine) in water and an exothermic redox reagent between fuel and oxidant (eg nitrogen) [45].

The reaction mechanisms in the combustion process are much more complex, there are many parameters that

influence the reaction such as: type of fuel, fuel-oxidant rate, use of excess oxidant, combustion temperature, amount

of water contained by the mixture of precursors. In general, a combustion synthesis has no violent reactions, does not

produce toxic gases, acting as a complexing agent for metal cations

The characteristics of the powder, the size of the crystallite, the specific surface, the nature of the

agglomeration (strong or weak) are governed in principle by enthalpy and flame temperatures generated during

combustion which in turn depend on the nature of the fuel and the type of fuel-oxidant ratio used in reagents.

The rapid formation of a large volume of gas during combustion dissipates heat from the process and limits

the increase of temperature, reducing the possibility of premature sintering between the primary particles. The

generation of gas also helps the contact between the particles resulting in a more powdery product. The combustion

technique seems to be controlled by the mass of the mixture and the volume of the container.

3.1.2. Principal parameters of combustion process The main parameters studied extensively in the specialized literature are:

a) the type of flame; b) temperature; c) the gases generated; d) the air-fuel-oxidant ratio; e) the chemical composition

of the reactive precursors

a) a) The type of flame

b) b) Characteristic temperatures

c) During the combustion reaction from the combustion process there are four temperatures that can affect the

reaction process and the properties of the final product:

d) • Initial temperature (T0) is the average temperature of the reagent solution before starting the combustion

reaction;

e) • The combustion temperature (Tig) represents the point where the combustion reaction is activated

dynamically without any additional amount of external heat;

f) • The adiabatic flame temperature (Tad) is the maximum combustion temperature achieved in adiabatic

conditions;

g) The maximum flame temperature (Tmax) is the maximum temperature at which the configuration reached,

but in non-adiabatic conditions.

h) c) Gas generation

i) d) Atmosphere

j) e) The fuel-oxidant ratio

k) f) The chemical composition of the precursors

l) g) Fuel

m) h) Oxidants

3.2.Synthesis of nanomaterials by laser pyrolysis

3.2.1. Principle of the method and basic phenomena of laser pyrolysis Initiated by Haggerty in 1981 for the preparation of silicon-based ultrafine powders [48], the laser synthesis

method of nanostructured powders is based on the absorption of laser radiation by a reactant or a photosensitizer when

the wavelength of the laser beam is identical or close to one of the absorption bands of the reagent or the

photosensitizer.

Fig. 3.1. Molecular mechanism of excitation of precursors

This process takes place in the fundamental electronic state (of electronic energy 0). Through this process

the molecules of the fundamental vibration state are raised on a higher level placed at a distance equal to the frequency

of excitatory IR radiation (ie 944 cm-1 which corresponds to a wavelength of 10.6 μm - if it is the 10P20 line of CO2

laser)

3.2.2 Influence of laser beam characteristics on pyrolysis processes

high intensity of the beam

duration of radiation

spatial coherence

acordabilitatea

monochromaticity and temporal coherence they allow extremely high selectivity, even at low frequencies of absorption of the chemical species with which they work.

the process allows the control of the reaction temperature by the variation of the laser power, the concentration of

the reactants, the stationary time in the beam, the geometry of the laser beam, etc.

easy adjustment of gas flow rate, important in controlling reaction stoichiometry and other process parameters

the process is carried out in a continuous working regime

the process can be raised from the laboratory scale to the pilot and even industrial scale, allowing the obtaining of

large quantities of nanopowders by modifying the geometry of the nozzle - from the circular section to the

rectangular section with the laser flow and of course by increasing the laser flow and the reactant flows.

the method has a high degree of reproducibility

the reaction of producing the pyrolysis flame laser in orthogonal configuration, a very stable one, provided that the

gas and laser parameters are kept constant.

the absence of radiation of other wavelengths due to the monochromaticity of the laser radiation (extremely

important for example in the case of the fulerenes that are destroyed by the UV radiation).

In the case of using CO2 lasers, the overall efficiency of the process is comparable and often higher than that of

other conventional sources of heating such as various types of plasma, flames or heated tubes.

the method is very versatile and a wide variety of nanometric powders can be obtained: metals or non-metals as

well as compounds and composite materials at the nanometric scale.

In conclusion, the main features of laser pyrolysis would be

the reaction area is well defined

there is no physical contact of the reaction zone with the walls of the reaction chamber

there is no chemical contamination

allow to reach hight reaction temperatures

high heating / cooling speeds 105 – 106 °C/sec

These features of laser pyrolysis give the nanometric powder the following advantages:

reduced dimensionality (nanometric) and large specific surface of the particles

high degree of dimensional monodispersion of the particles;

morphology and controlled structure of particles

high purity of the obtained powders

Disadvantages can be stated:

- the obligation of the presence of a sensitizer which, under certain conditions, decomposes and impurities

the powder produced;

- in many cases the nanometric powders are agglomerated in aggregates that can no longer develop them

through ordinary scrapers, aggregates that can reach micrometric dimensions

3.2.3. Set-up experimental de sinteza a nanoparticulelor prin piroliza laser The general mechanism of powder formation involves several stages

decomposition of reactive species as a result of collisions with the formation of highly reactive species

(free radicals, atoms, molecular fragments);

formation of the first nuclei ("seeds") (several nm) by condensation of these species - the nucleation

stage;

growth of nuclei by collisions with other reactive species including other nuclei with the formation of

final nanoparticles;

aggregation of nanoparticles formed in various forms (chains, networks, spheroidal agglomerates

The interaction between the gas flow and the laser beam can be done at an angle between 0 and 180 degrees,

the most widely used being verified in practice - the perpendicular configuration

An advantage of this set-up is the area of absorption laser larger, but there is a big disadvantage, in that

powder formed and driven by the gas flow can be deposited on the windows constituting the centers of absorption of

the laser beam which heats local. This phenomenon can be partially avoided by blowing inert gas inside the windows

through a series of circularly arranged holes

The most used geometry, namely the orthogonal one, in which the intersection of the flows is made on a

narrow field far from the two windows, ensures a superior preservation of the cleanliness of the windows, which in

this case are blown with an inert gas. Thus, the reaction chamber has a cross geometry, the longer, cylindrical

horizontal arm, being crossed by the laser beam and bordered by the two windows and the perpendicular arm on it

which can be either horizontal or vertical being crossed by the gas flow. The reaction chamber can also have the third

arm perpendicular to the two, it can contain the viewing window and the connection to the pressure gauge that

measures the pressure in the room. Also, to better capture and prevent the flow of gas through the chamber after the

exit from the reaction zone, a cylindrical device with a bell or cone termination can be used, which continues to the

other end with a chamber. in which the filter collector of nanopowders is found.

Fig. 3. 2 Schematic presentation of the orthogonal configuration of laser pyrolysis

In order to maintain a laminar flow of the reactive gases and to avoid the contact of the reactive flow that

carries the powder resulting from the reaction with the walls and especially with the windows, a second flow of an

inert gas (argon, helium or even nitrogen) surround the reactive flow.This inert gas also has a cooling role, and it can

also be injected into the internal flow where it can play the role of diluent or carrier gas for vapors or extrafine droplets

if precursors in the vapor state are used, respectively. in liquid state.

3.3. Synthesis of thin films using laser techniques

3.3.1. Pulsed laser deposition In 1965, lasers were first used in thin film experiments, when H.M. Smith and A.F.Turner ablated the material

from a target in a low temperature reaction chamber using a ruby laser; the films did not have uniform thickness and

did not adhere to the substrate. In the following decades, laser ablation was used more for the analysis / characterization

of materials and subsequently for obtaining thin films. Development of high-power lasers and sources offering pulses

nano-, pico- and femtoseconds, led to the emergence of various laser techniques synthesis of thin films, in the literature

there are currently more than 20,000 publications, reviews and proceeding sites by PLD, RF-PLD and MAPLE

3.3.1.1. Tehnica laser-asistata si fenomenele implicate The principle of laser ablation is illustrated in Figure (1). The basic idea of the techniqueis to exploit high-

power laser pulses, e.g., from an excimer, a Nd:YAG or another similarlaser, in order to evaporate a small amount of

matter from a solid target.The focused laser pulses are absorbed at the target surface in a small volume. The absorbed

energy density is sufficient to break any chemical bonds of the molecules withinthat volume

Fig. 3.4. Laser ablation technique set-up: reaction chamber for the radiofrequency laser plasma assisted deposition (a) Radiofrequency

source in detail

What essentially happens is that high-pressure gas is produced in the surface layer. As a result of the pressure

gradient, a supersonic jet of particles is ejected normal to the target surface. In practice, the process is far more

complicated than the idealizedmodel discussed above. The particle cloud absorbs a large amount of energy from the

laserbeam producing an expansion of hot plasma (plume) through the deposition chamber. The ablated species

condense on the substrate placed opposite to the target forming a thin film after some hundreds or thousands of laser

pulses. Laser-pulse energy density,fluence[J/cm2], on the target surface is one of the most important ablation

parameters. When the fluence is sufficiently high, rapid evaporation of material occurs in a thin surface layer,which

is necessary for stoichiometric transfer of material from a multicomponent target.PLD can take place both in vacuum

and in the presence of some dilute background gaswhichisusedtoinfluence the composition of the film [50]

Under proper process parameters (e.g., the background gas pressure, the substratetemperature, and the

laserfluence), the film grows epitaxially and the stoichiometry of the film is a replica of that of the target. Process

parameters have also an effect on the growth rate of thefilm but when the fluence and the target-substrate distance

have been optimized, the rate remains nearly constant (approximately 1 Å/pulse for oxides).

A tipical deposition setup is presented in Figure (2), and consists of a target holder (a carousel, if it is a multi-

target system), a substrate holder (sometimes rigurously heated,as in some experiments a high temperature is needed

when depositing thinfilms) and anRF-plasma gun for the RF-PLD experiments, where needed; usualy, oxygen and

nitrogenare the most common used gases (sometimes mixed together)

3.3.1.2 Laser target interaction The interaction between the laser pulse and the target depends strongly on the laser pulse intensity. At PLD,

the intensity is of the order of 102 W / cm2, corresponding to a pulse duration of several nanoseconds (Nd:YAG laser).

Therefore, it is sufficient time for the pulse to absorb, to warm the target surface and finally to remove the material.

There are several mechanisms by which the energy is transferred onto the target and some of them will be discussed.

In this context the term "sputtering" is used to describe different phenomena. In the "collision sputtering", the

momentum of the incident beam is transferred to the target resulting in an expulsion of the particles on the surface.

The mechanism is very important if the incident beam contains large particles such as ions

3.3.1.3. The plume formation The material that is ablated from the target is hot and, therefore, part of the atoms in thevapour are ionized.

In addition, the particle cloud absorbs energy from the laser beam andbecomes more ionize. Finally, a fully ionized

plasma is formed in the vicinity (∼50μm)of the target. The plasma expands away from the target, with a strongly

forward-directed supersonic velocity distribution. The visible part of the particle jet is referred to as an ablation plume.

Moreover, clusters of the various target compounds are observed near the target surface. The visible light of the plasma

cloud is due to the fluorescence and the recombination processes in the plasma. Although atomic transitions have

typical lifetime of a few nanoseconds ; collisions can re-excite atoms such that the emission lines are observed several

microseconds after the initial laser pulse.

3.3.1.4. Thin film growth The growth processes of the thin films, ie the deposition of the target material ablated on a substrate can be

explained by the following sequence: the arriving particles must be absorbed by the surface of the substrate and then

they can diffuse at a certain distance before reacting with each other on the surface and starting the nucleation. The

way in which the particles nucleate will determine the structure and morphology of the grown film. In some cases, for

example high temperature of the substrate, the diffusion interactions inside the film and the substrate, will

subsequently change the composition and properties of the film.

3.3.2 (Matrix assisted pulsed laser evaporation) MAPLE is a thin film deposition technique derived from pulsed laser deposition (PLD) in order to make it

possible to deposit sensitive materials (polymers, organic, bio -) [52]. On the other hand, the MAPLE technique

assumes that the target is an ice cream solution, formed from a suspension of the material in the matrix-host. The term

"laser evaporation" contained in the acronym of the method implies a milder expulsion of the material than the laser

ablation. Studies through simulations and experiments [53,54] have shown that the initially proposed evaporation

model in which the laser energy is mainly absorbed by the matrix, evaporates and transfers the thermal energy of the

molecules, which in turn is easily desorbed, is not suitable for the process description. The complex physics involved

in the ablation of a composite targetConsequently, the two initial conditions required for obtaining a MAPLE deposit

(the material having very high solubility in the matrix, and the radiation being absorbed mainly by the matrix and only

a negligible part by the depositing material) can be eliminated. Already several authors have demonstrated

experimentally that these conditions are not strictly necessary to obtain a thin film by MAPLE technique, this

technique being used also for depositing nanoparticle suspensions [56,57] and transparent molecules in the solvent.

With these results, the MAPLE deposition technique should be characterized only by: a) the presence of a

matrix in the target and b) the laser beam does not affect / damage the molecules in the matrix. The publications of

the last years that contain deposits of polymers, biomaterials and organic molecules for high number of applications

explain very well both the evaporation mechanism of the "host" material from the matrix and the importance /

influence of the deposition parameters [60,61] (the matrix / material ratio) deposited, boiling temperature and vapor

pressure of the matrix, temperature of material destruction, laser pulse energy and repetitive rate, depth of laser

radiation penetration, substrate temperature during deposition) on the characteristics of the obtained MAPLE films.

Another important parameter is the laser wavelength (or photon energy) used to evaporate the target. The

wavelength must be chosen in such a way as to avoid damage to the molecules (IR corresponds to low energy photons)

but to be absorbed by the matrix (solvent) - most common solvents are transparent to IR radiation and absorbed into

UV. In most cases, KrF (248nm, UV, 5eV) or ArF (193nm, UV, 6.42eV) lasers or Nd: YAG - fourth harmonic (266nm,

4.66eV) lasers are used. as the energy source for target ablation / evaporation. However, the use of the Nd: YAG laser

has the advantage of selecting the wavelength (UV or IR) and also the lower energy / photon. The use of VIS or IR

wavelengths has the advantage of not destroying sensitive molecules but also the disadvantage that many solvents are

transparent to the radiation from VIS-IR.

A classic MAPLE deposition system [62] is shown in Figure 3.8. The target consists of a diluted solution of

material in a volatile solvent, frozen at liquid nitrogen temperature. The laser pulse that hits the target removes both

solvent and material molecules from the surface, eventually reaching the substrate several inches away from the target.

The solvent molecules are extracted from the reaction chamber by the vacuum system. The presence of the solvent

(matrix) eliminates or minimizes the photodynamic destruction of delicate molecules.

Fig. 3. [50] MAPLE technique schematic presentation: vacuum chamber (a) and details regarding the target, the evaporation and the deposition process (b)

4. Characterisation techniques of metalic oxides semiconductors

4.1 X-ray Diffraction X-ray diffraction technique is the main source of information on the crystallographic structure of materials,

despite the information provided by electron or neutron diffraction. Usually the interatomic distances in the crystalline

solids are of the order of an angstrom. The X-rays have characteristic energy values in the range (1 - 120 keV). W.L.

Bragg represented this considering a crystal as being made of parallel atomic planes separated at an interplanar

distance "d". The conditions for maximum intensity of scattered radiation are the following:

X-rays must be reflected by ions in a plane

• X-rays reflected from successive planes must interfere

BRAGG law:

2dhklsinθ=nλ

A diffractometer is composed of a radiation detector, which must detect the angle and intensity of the

diffracted beam. Using the Bragg-Bretano or θ-2θ configuration, the detector is rotated at a double angular speed

relative to the sample. A recorder automatically traces the intensity of the diffracted beam when the detector moves

in a circle on the goniometer, synchronized with the sample within a range of 2θ. In this way both values, both the

diffracted angle and the intensity can be recorded at the same time. The X-ray diffraction pattern is therefore a

representation of the intensity of the diffracted beam against the diffraction angle 2θ, the intensity of the maxima and

angular positions 2θ providing information about the crystallographic structure of the material; the angular position

2θ is directly correlated with certain distances between the crystalline planes dhkl, and therefore with certain planes of

lattices (hkl). Bragg's law defines the conditions for maximum diffraction in an infinite crystal with a perfect 3D

structural order. In practice, the maximum diffraction has a finite width induced by an instrumental enlargement (due

to imperfect X-ray optical elements, wavelength dispersion, sample transparency, detector resolution or sample

surface) or deviations from an ideal crystalline structure, such as they would be finite crystallite dimensions, lattice

strain or lattice defects.We analyze whether from this width it is possible to extract information about the

microstructure of the material, more precisely the average crystallite size, using Scherrer's formula:

t = 𝐾𝜆

𝐵𝑐𝑜𝑠𝜃ℎ𝑘𝑙 (1)

where t = average crystallite size, K = constant whose values depend on the form of crystallites, usually with

values between 0.9 and 1; B = value of the maximum intensity (usually in radians 2θhkl), centered on the Bragg angle

θhkl corresponding to the reflection (hkl). It should be mentioned to not confuse the crystallite size with the particle

size, which can be associated with the presence of polycrystalline aggregates.

4.2 (AFM) Atomic force microscopy Atomic force microscopy is that type of high resolution scanning microscopy in which the topographic image

of the sample surface can be obtained from the interaction between a cantilever and the sample surface. The AFM was

invented in 1986 by Gerd Binning et. of IBM Zurich, based on STM - Scanning Tunneling Microscopy, already

present in 1981. While the first depends on conductive samples, the AFM allows the use of non-conductive samples.

A classic AFM consists of a cantilever (measuring head) with a small tip at the end, a laser, a diode and a scanner.

4.3 Scanning electronic Microscopy The first scanning electron microscope (SEM, Scanning Electron Microscope) was built in 1935 by Max

Knoll, shortly after the first TEM was completed [65]. In the first studies the resolution was limited to ~ 100 μm due

to the fact that the condenser lenses were not used to reduce the lateral size of the electron beam on the sample.

An SEM consists of an electron gun and a series of electromagnetic lenses, similar to TEM systems. The

electron gun is of the same type as the one used in TEM, but the acceleration voltage for obtaining the electron beam

is approximately one order smaller (1–40 kV). In contrast to TEM, in SEM the electron beam is focused so that it has

an extremely small diameter at the surface of the sample, of the order of nanometers (1–10 nm). The electromagnetic

lenses of SEM have the role of shaping the beam; they do not contribute to the direct formation of the image as it

happens at TEM.

5. SnO2 si TiO2/SnO2 nanostructures/nanoparticles synthesis by laser techniques : obtaing, characterisation and application

5.1. TiO2/SnO2 nanoparticles synthesis

The basic principle of laser pyrolysis as a method of obtaining pure, Fe, C, N, or S doped TiO2 nanoparticles

has been described in detail in previous chapters and studies. The precursors used for obtaining of TiO2 nanoparticles

with tin content were gases or vapors originated from bubbling carrier gas through volatile liquids as follows: titanium

tetrachloride (as precursor for Ti), tin tetrachloride (as precursor for Sn) and air (as oxidizing agent)

The carrier gas used to bringing of vapor liquid precursor in the reaction zone was air or an inert gas, argon

− used simultaneously for its role in the confinement of reactive gas flow and as reaction chamber windows protectant

(ZnSe windows which are transparent to the CO2 laser IR radiation and prone to laser-induced damage if some powder

attaches to them). These precursors enter in the reaction chamber (through an injector endowed with three concentric

nozzles − to prevent chemical interaction) orthogonally encountering the focused CW CO2, laser radiation

(wavelength 10.6 μm)

Usually, when reactive gases meet the laser beam, it is required that at least one of the precursors should

absorb the laser radiation in order for the reaction to occur. Because none of the precursors listed above present the

required IR absorption band, it was necessary to introduce a sensitizer − ethylene, which absorbs infrared laser

radiation and then transfer the energy by collisions to the molecules of precursors. The process of pyrolysis occurs in

the volume delimited by the intersection of the reactants with laser radiation, seen as a flame from which the hot

particles emerge as a smoke, being quickly cooled and thus remaining at nanoscale dimensions.

Fig 5.1 Laser pyrolysis experimental set-up used for obtaining composite TiO2/SnO2 NP

This method allows the synthesis of nanoparticles with desired morphological and structural properties

having strict control over process parameters such as: laser power density, flow / flow of precursors and auxiliary

gases (sensitizer and oxidant), pressure in the reaction chamber, reaction time, particle stationary in the pyrolysis

flameThe last parameter is a derived one and can be approximated using total reactive flow, system pressure and also

temperature and volume of the flame, for a related a laser pyrolysis process as described by F.Tenegal, I.Voicu et al

[71]. The values of the synthesis parameters used to obtain TiO2 / SnO2 powders can be found in the table1. The

samples were synthesized by varying one of the two parameters (while the rest of the parameters were kept constant),

namely the geometry of introducing the precursor into the reaction zone and the concentration of the tin precursor

introduced into the reaction. In the case of the first geometry, the gases were introduced into the reaction chamber as

follows: the precursor of Sn together with the sensitizer (ethylene) through the central nozzle (together with a

complementary flow of Ar for the constant flow rate), the precursor of Ti together with the oxidizing agent through

the median ring nozzle, and the confining argon was introduced through the intermediate nozzle and the outer

concentric nozzle. This geometry was used to obtain 3 samples (SnTiO2-0 reference, SnTiO2-2.7, SnTiO2-4.8),

varying the flow rate of tin precursor, SnCl4 (0, 10 and 20 sccm) [70]

Tabel I.1 EDX measurements and experimental parametres for TiO2/SnO2

In the case of the second geometry, the gases enter the reaction chamber as follows: the precursors of Sn and

Ti are introduced through the inner nozzle together with the sensitizer, the oxidant through the intermediate nozzle,

the confining Ar being introduced through the outer nozzle. Using this geometry, 3 powders (SnTiO2-1.1, SnTiO2-

1.2, SnTiO2-1.8) were obtained, varying the flow rate of SnCl4 (5, 7 and 12 sccm). It should be noted that the notation

of the samples was made according to the elemental concentration of Sn in powder: SnTiO2-at%, at%-atomic

concentration of Sn resulting from EDX analysis.

After synthesis all the samples were calcined to eliminate the possibility of their contamination with Cl

(which came from both precursors, SnCl4 and TiCl4), and to decrease the C content (which came from the partial

decomposition of the ethylene sensitizer), at a temperature which prevents phase transformations from anatase to

rutile. And annealing them at 450 °C for 3 h, in a furnace with circulation of air.

Samples resulting from laser pyrolysis process were further characterized for morphological, structural and

compositional through different techniques: scanning electron microscopy, SEM, AFM Atomica- force microscopy

and X-ray Diffraction (XRD).

The X-ray Diffraction patterns were collected on a Panalytical X’Pert MPD theta–theta system in continuous

scan mode: 0.02° step size/20 s- time/step, interval on 2θ (20°–70°) degrees, 40 kV and 30 mA. In the diffracted beam

a Ni filter a curved graphite monochromator and a divergence slit were used (λ = 0.15418 nm). Estimations about the

crystallographic characteristics were performed using the HighScore Plus software and ICDD data base (International

Center of Diffraction Data). After comparing the as recorded diffractograms on the samples obtained from laser

pyrolisis and compared with the Data Base ICDD we found three major crystalline phases : TiO2-anatase ( JCPDS

00-021-1271) , TiO2-rutile(JCPDS 00-021-1276) and SnO2 –casiteritte( JCPDS 00-041-1445) The ratio between

these phases ranges function of the Sn content and on the synthesis conditions.

The superposed diffractograms (presented in Fig. 2a) allow a direct comparison between the pure sample

SnTiO2-0 (containing only one phase, the TiO2 anatase) with the highest Sn content sample- SnTiO2-4.8, where all

three phases are present. n calculating the average crystallite size, Dhkl, the Debye-Scherrer formula was used :

Dhkl =𝐾𝜆

𝐵ℎ𝑘𝑙 𝑐𝑜𝑠𝜃ℎ𝑘𝑙 (1)

is a form factor, most often it is approximately 0.9 ; λ is the wavelenght of incident X-ray beam ; Bhkl is the FWHM

of the of reflection (hkl), θhkl the Bragg angle of reflection (hkl). FWHM need an instrumental correction of 0.08°.

Fig. 5.2 X-ray defractogram samples obtained using a) separated precursors and b) pre-mixed precorsors; the insets represent the

deconvolution of peaks from the 2θ=24-30 deg range for the SnTiO2-4.8 sample

In the case of TiO2 phases, the average crystallite size were calculated using the Bhkl (FWHM) value

corresponding to the most intense reflections of each TiO2 phase: the diffraction peak corresponding to the crystalline

plane (101) for anatas at 2θ ≈ 25.4 ° respectively the maximum corresponding to the plane ( 110) for rutile at 2θ ≈27.5

The mean crystallite size of the SnO2 phase was mediated on 3 peaks of SnO2 (110),(110) and (220). Table 5.2

presents the values of the ratio between the two major TiO2 phases and the average crystallite dimensions of the same

phase. The ratio between the anatase and rutile phases was calculated using the same two most intense maxima of the

two TiO2 phases according to the empirical formula of Spurr and Myers.

IA={1/[1+1,26(IR/IA)]}x100 (%) (2)

where, as mentioned above, IR represents the intensity of reflection (110) of the TiO2-rutile phase and IA the intensity

of reflection (101) of the TiO2-anatas phase

Tabel 5.2. Crystallographic parameters estimated by XRD for TiO2/SnO2 samples [70]

Heat treatment did not influence the morpho-structural characteriscs of the powders obtained by laser

pyrolysis. This behavior is demonstrated by the total absence of the rutile phase in the SnTiO2-0 reference sample

Fig. 5.3 Images of the SnTiO2-0 reference sample (without tin) and the highest degree tin content sample SnTiO2-4.8

From the TEM images above, we can observe a slightly wavy aspect of the nanoparticle surface, which can

be an advantage due to the increase of the specific surface area. The mean size of the nanoparticles is 20 nm, in

agreement with the results obtained by X-ray diffraction measurements [70] if we consider the contribution to the

average crystallite size of the TiO2-anatas major phase.

5.2 The thin films sythesis using the MAPLE technique,starting from the TiO2 / SnO2 nanoparticles TiO2 / SnO2 composite nanoparticles (SnTiO2 -2.7 and SnTiO2-4.8) were dispersed in deionized water at a

concentration of 5g / l in the ultrasound bath for 1 hour. The solution was gradually immersed in liquid nitrogen,

resulting in a solid target, frozen to about 77 K. This target was immediately mounted inside the reaction chamber, on

a rotary holder (to ensure uniform erosion of the target), continuously cooled with liquid nitrogen, guaranteeing a

temperature of about 145 K and a slow rotation. In the reaction chamber a high vacuum was made, with a value of

approx. 5x10-4 mbar, using a combined system of preliminary / rotary pump and turbomolecular vacuum pump. The

target was irradiated using a Surelite / Continuum Nd: YAG laser system, using a fourth harmonic, wavelength 266

nm, with a pulse duration of 5-7 ns and a repetition rate of 10 Hz.

Fig. 5.4 MAPLE experimental set-up

The Si substrates (100) and the interdigitated electrical contacts were mounted parallel to the target at a

distance of 50 mm and rotated slowly. Several deposits were made, which varied the energy per pulse (6-9 mJ / pulse)

and the number of laser pulses. The laser spot area was kept constant, 0.5mm2.

Table 5.3. MAPLE experimental parameters

Sample target Substrate Puls

number

Laser

energie/puls

[mJ]

525 SnTiO2-2.7

5% wt in water

Si (001) 180000 6

582 SnTiO2-4.7

0.25% wt in water

Si (001) 180000 9

Although the deposition rate per pulse is very low, so that a very large number of pulses is used (a deposition

time of approximately 10-12 hours), this allows us a precise control of the thickness of the layer, because its thickness

is proportional to number of pulses. The layer thickness control allows the sensor to be adapted to the desired

applications, by controlling the basic resistance (when the sensor does not come in contact with the gas to be

monitored; the electrical resistance varies with the layer thickness). Decreasing or increasing the resistance can use

cheap electronic circuits, reducing operating cost by adapting sensors to the sensor electronics and eliminating the

need to design complex control systems

The resulting films were morphologically and structurally characterized by X-ray diffraction using a X'Pert

Pro MPD Panalytical Diffractometer, in Brag-Bretano configuration, with an incident beam emitted by a Cu tube (λ

= 0.15418nm). . The diffractograms were recorded at 45kV and 40mA, with a step size of 2θ = 0.02 ° and a scan time

that varied between 5 and 20 seconds per step, depending on the thickness of the sample to be analyzed.

Fig. 3.5 Superposed difractograms: green – target used power and blue – 582 thin filn

Fig. 5.6. SEM images of MAPLE deposed thin film composites

Surface topography obtained from (MAPLE), was studied using atomic force microscopy (AFM) and

scanning electron microscopy (SEM). With the aid of cross-section SEM measurement of the film could highlight the

homogenity of the active layer from the surface of the deposited film

The AFM measured an approximate roughness of 126 nm. A roughness as determined is advantageous for

gas detection due to the large specific surface. In this case we have a very large number of active centers necessary

for the gas sensing process, as it was presented in the theoretical chapter for gas detection mechanisms.

Fig. 5.8. Topography of AFM (AFM) a TiO2/SnO2 [582]

Fig. 5.9. Optical prophilometry (TiO2/SnO2 thin films)

The thin films synthesised using MAPLE were studies with optical profilometry as well, like complementary

technique for AFM and SEM, with the aim of a accurate determination of the roughness and thinkness of nanoparticles

thin film, deposited on Si(100) substrate. By this technique it has obtained a film thickness ranging from 150-160 nm

and a surface roughness ranging between 60-100 nm, depending on the energy per pulse and the number of pulses

used.

Also, from AFM and SEM images we can see that the active layers are formed by compacting the

nanoparticles used in the target preparation; they are deposited without changing their structure, preserving the initial

characteristics and property of presenting the active centers. Obtaining on this way the active layers is due to the

working principle of the MAPLE technique. Other techniques either damage the particles during deposition such as

PLD, which evaporate or merge the nanoparticles, producing a layer with good adhesion but losing the advantage of

preserving the initial characteristics of the nanoparticles, or techniques such as dip-coating, spin-coating, drop-cast,

which retain the initial characteristics of the materials but produce layers with very poor adhesion to the substrate,

resulting in a very low and unstable life time sensor. The MAPLE technique presents the benefits of the PLD technique

from which it derives, in the sense of producing a layer with good adhesion but also has the advantage of the

aforementioned non-laser techniques, preserving the initial characteristics of the nanoparticles. This is very important

because it allows us to select nanoparticles with the desired properties, properties that can be characterized before

deposition, for a rigorous control of the manufacturing process of the sensors, the latter reflecting these properties.

Within INFLPR- Magurele group led by dr. Gabriel Socol, it was made a special modular experimental

system for measuring electrical gas sensor, which allows safe testing procedures. This system allows to obtain a

controlled atmosphere in which the pressure, temperature and humidity can be set with the flow of test gases and their

concentration.

This testing system is characterized by :

• Performs sensitivity and selectivity tests;

• The system is automatically controlled by the PC using specialized software;

• Up to 4 different gases can be used in mixtures with controlled concentrations

Fig. 5.6 [http://llasem.inflpr.ro/setup.html ] Experimental set-up – gas sensors measurements; on the left we have the schematised mass-

flow controlers, LABVIEW software interface, multimeter and testing chamber and on the right the test chamer image itself with

potential accessories

For the measurements made on the sensors presented in this paper, the following were used:

Three different gases: CO, CH4 and synthetic air used as buffer / buffer;

A metering system consisting of three reducers adapted for the used gases and three mass flow

controllers;

Automatic control system of the PC monitored detection process for generating the gas flow

(starting / stopping the monitored gas flow - ON / OFF gas);

Reaction enclosure made of stainless steel, which allows vacuuming and incorporates a

thermostatic heating device and various passages for electrical circuits;

Sensor clamping system

The active layers were deposited by MAPLE on a sital (ceramic) support with Au electrodes and were

analyzed as sensors for CO and CH4 detection. Figure 5.7 shows the sensor response at 10 ppm CO concentration in

synthetic air (a), 100 ppm CO - synthetic air and c) 1000 ppm CH4. During the measurements, a current of 100 μA

was applied, a number of 10 cycles with a duration of 1200 sec / cycle (600 sec gas ON- 600 sec gas OFF). The

working temperature of the sensor was 300 ° C.

Fig. 5.7 The obtained sensor response from composite TiO2/SnO2 nanoparticles in CO atmosphere a) at 10 ppm and b) and 100 ppm

It is found that the sensor response is very fast at relatively low concentrations, and can be used in cases

where the rapid response is much more important than the sensitivity to the gas concentration.

a b

Fig. 5.8. 525 sensor response at 1000 ppm CH4

From the above images a good response to the detection of CO can be observed (Figure 5.8. A-10 ppm, b -

100 ppm), relative to the response for the CH4 detection (Fig. 10 c - 1000 ppm). We consider that sensor 525 could

be used in a sensor array for selective gas detection

5.3. Composite nanoparticles synthesis The experiments performed to obtain the SnO2 / SnO composite nanoparticles aimed to obtain powders with

different tin concentrations that involve a different SnO2 / SnO ratio. During the pyrolysis process it was used as

ethylene sensitizer, because the SnMe4 precursor does not absorb at the wavelength of the CO2 laser radiation To

avoid the contamination of nanoparticles with F or S, the usual SF6 sensitizer was removed, and an Ar mixture with

O2 was used instead, and as ethylene or ethylene carrier gas mixed with Argon. The working pressure in the reaction

chamber was 450 mbar. Figure 5.9 shows schematically the experimental set-up consisting of a reaction chamber with

two intersecting perpendicular tubes. Through the horizontal tube endowed with transparent windows at the

wavelength of the laser used, it passes the laser beam that is focused inside the enclosure by means of a lens with a

large focal length; through the vertical tube the reactive gas mixture is injected through a system of concentric nozzles

through the lower part of the system. The gases injected in this way into the system meet the laser beam right at the

focal point where the actual reaction takes place and oxide nanoparticles are generated, which are collected by means

of a filter in the collection chamber and the purified gases are extracted by means of a vacuum pump. The gas pressure

in the reaction chamber is kept constant, by adjusting the incoming gas flows, by means of inlet valves. Also, the

pressure difference on the filter is controlled through an outlet valve. Thus, the output flow is kept constant while the

particulate filter is charged, which lowers its permeability to the gas stream. The windows of the reaction chamber are

transparent to the laser radiation with CO2 in IR but they are liable to damage if powder is deposited on their surface.

It is necessary to use an argon flow which confines the reaction gases; this argon flow is introduced through the outer

concentric nozzle.

Fig. 5.9. Experimental set-up for obtaining SnO2/SnO nanoparticles by laser pyrolisis

Tabelul 4. Process parameters by laser pyrolisis

Proba Debitul interior al duzelor Debitul

duzei

circulare

Ar

Debitul

ferestrei

Ar

Φiduză

interioară

Φduză

circulară

inelara

densitatea

puterii

laserului

inainte si dupa

utilizare

presiune

[sccm] [sccm] [sccm] [mm] [mm] [W/cm2] [mbar]

SNO-24 8.89 15 22.5 4000 2x75 2.9 14.1 708/680 425

SNO-8 15 10 10 5000 2x75 0.9 8.8 920/892 450

In FIG. 5.12 shows the superposition of the RX diffractograms of two samples, samples from the extremes

of the range of varied parameters in which the ratio of SnMe4 and O2 flow rates was varied between 0.5 and 1.5 by

modifying the oxygen flow in mixture with argon. The laser power was kept constant throughout the experiments at

a value of 55 W. No notable results were obtained at this laser power because the temperature of the pyrolysis flame

(the area where the precursor Sn (Me) 4 decomposes) is directly proportional with the laser power density, the

pyrolysis reaction being a thermal threshold reaction.. The samples resulting from the experiments were analyzed with

the help of X-ray diffraction to establish the crystallographic structure and with the help of energy dispersion X-ray

spectroscopy (EDS) to establish the composition at elementary level. Two phases of tin oxide were identified: SnO -

romarchite, tetragonal system with elemental cell parameters a = 3.7986Å, c = 4.8408Å and Volcel= 69.85Å3

(according to JCPDS 00-055-0837) and 2-casiterite ( cassiterite) which also crystallizes in the tetragonal system and

has the following lattice constants a=4.738Å si c=3.1871Å , Volcel =71.55 Å3 (conf. JCPDS 00-041-1445).

In the SnO8 sample the presence of the metallic Sn phase - (JCPDS 00-004-0673) was also identified. The

average crystallite dimensions of the 2 major phases - romarchite and casiterite fluctuate depending on the variation

of the experimental parameters but are within a range of 12-20 nm, according to table 5. The crystallite size

calculations were performed using the Debye-Scherrer formula implemented in the HighScore Plus software using

FWHM of each diffraction peak corresponding to each crystalline phase. The ratio of crystalline phases was

determined with the Highscore software developed by Panalytical.

Table 5.5. Average mean crystalite size of the selected samples, calculated with the help of the Debye-Scherrer Formula as well as the

crystaline phazes ration found in samples

Proba DSnO(nm) DSnO2(nm) DSn(nm) %SnO %SnO2 %Sn

metalic

SnO24 16 14 - 27 73 -

SnO24T 8 14 - 13 87 -

SnO8 16 14 41 77 14 9

SnO8T 12 11 - 65 35 -

Fig. 5.12.X-rays superposed difractograms obtained for SnO8 and SnO24 samples

The samples shown in Figure 5.12 are obtained using two totally different ratios of Sn (Me) 4 to O2. For the

SnO 8 sample the flow ratio was R = D Sn (Me) 4 / DO2 = 1.5 and for the SnO24 sample it was 0.5. This difference

resulted in a very different phase composition between the two samples, which is very important to study because in

obtaining active layers for gas sensors it is important to have a dominant phase of SnO2 (casiterite); it seems we can

improve this by changing the ratio of precursor / oxygen flow rates.

Fig. 5.13. Imagini de Microscopie electronică prin transmisie de înaltă rezoluţie HRTEM Images

The samples were annealed in the oven for 3 hours in the oxygen atmosphere at 350 ° C in order to eliminate

the carbon content from the decomposition of methyl radicals upon the breakdown of the Sn (Me)4 molecule and of

the sensitizer used - ethylene.

The heat treatment performed can notice a decrease in crystalline phase romarchit SnO favoring the

increasing of SnO2 phase cassiterite. Also, in the case of the SnO8 sample which also had a percentage of 9% mettalic

tin , its disappearance can be observed at the same time with the increasing of SnO2 phase.

20 25 30 35 40 45 50 55 60 65 70

2Theta (°)

0

1000

2000

3000

4000

Inte

ns

ity

(c

ou

nts

)

Sn

O (

00

1)

Sn

O2

(1

10

)

Sn

O (

10

1)

Sn

(2

00

)

Sn

(1

01

)S

nO

(1

10

)

Sn

O2

(1

01

)

Sn

O (

00

2)

Sn

O2

(1

01

)

Sn

(2

20

)

Sn

(2

11

)

Sn

O (

20

0)

Sn

O (

11

2)

Sn

O2

(2

11

)

Sn

O2

(2

20

)

Sn

O

(21

1)

Sn

O2

|(3

01

)

Sn

O (

10

3)

Sn

O2

(1

12

)

Sn

O (

20

2)

Sn

O (

22

0)

Sn

O2

(2

02

)

Fig. 5.14. X-rays superposed difractograms of heat treatment nanometric SnO8 _SnO8 powders

The heat treatment used for the removal of residual carbon ratio also leads to the change of phase ratio, further

oxidizing of tin monoxide, thereby improving the ratio of SnO2 / SnO, resulting in a stabilization of the nanoparticles

prior to the laser deposition process in the form of active layers. After laser deposition, the thin films were also thermal

treated (annealing) in order to final stabilize the sensing measurements for the desired gases, respectively CH4 and

CO.

5.4 Thin films synthesis with the help of MAPLE technique from SnO/SnO2 nanoparticles optained by laser pyrolisis

Table I.6 Experimental data used in the deposition of active layers with the help of the MAPLE technique

Sample number Material target Substrate Energie/puls [mJ] Nr.of laser pulses

281 SNO24 0.5% Si(100) 6 80.000

282 SNO24 0.25% Si(100) 6 80.000 283 SNO8 T 0.25% Si(100) 6 80.000 284 SNO24 T 0.25% Si(100) 6 80.000 527 SNO24T(0.25%)+

MWCNT (0.25%)+

CMCNa

Si(100) 9 250.000

591 SNO24 0.25% Si(100)

Contacte electrice

( Au pe sital)

9 400.000

The table above shows a selection of several sets of samples made from nanometric powders obtained using

laser pyrolysis. In order to obtain the active layers necessary for the gas sensors, two powders with different ratios of

crystallographic phases, respectively SnO / SnO2, were chosen. In addition to the raw (non-heat treated) samples, the

annealed powders were used for target, after a heat treatments at 350 ° C in the synthetic air atmosphere, for three

hours. The heat treatment was carried out in order to reduce the residual carbon content of the sample - carbon from

the ethylene sensitization as well as to promote the increase of the casiterite phase-SnO2. The treated samples were

referred to as SNO8T, respectively SNO24T.

The suspensions were maintained in the ultrasound bath for 60 minutes and before the preparation of the

target at liquid Nitrogen temperature, introduced at the sonotrode for a better dispersion – 90 seconds. Initially a

concentration of 0.25% w powder in demineralized water was used, subsequently increasing the concentration to 0.5%

w. The concentrations were varied in order to optimize the laser deposition rate and the overall appearance of the

active layer (homogenity and roughness). In order to improve sensor quality, the nanoparticle solution obtained by

dispersing SNO24 sample in demineralized water was mixed with multi-walled carbon nanotubes (MWCNT) at

different concentrations, with or without the help of surfactant (CMC) Na [carboxy-methyl-cellulose] with a

stabilizing role (preventing MWCNT aggregation) of the aqueous dispersion.

The target thus obtained was mounted in the reaction chamber at a distance of 40 mm from the substrates in

parallel with them, on a substrate with a slow rotation, so as to prevent erosion of the target in the same place (creating

a crater to shorten the target lifetime) and ensure a homogeneous deposition of the plasma cloud containing

nanoparticles on Si substrates (100). The deposition was made on substrates with gold electrical contacts deposited

on silicon, respectively on the electrical contacts interdigitated by Sital with gold electrodes.

Fig. 5.16 SEM: a) interdigitated electrods and b) Gold/Silicon electrods, on which the the active layers depositions were obtained for the

gas sensors

The experimental set-up used was the one presented previously used for depositing TiO2/SnO2 composite

thin films. It consisted of a system consisting of a reaction chamber, a suite of vacuum pumps - preliminary and

turbomolecular to achieve the advanced vacuum of approx. 4x10-5bar and the fourth harmonic of an Nd: YAG laser,

at wavelength λ = 266nm, τ = 5-7ns and a repetition rate of 10Hz.. It was focused using a lens with a focal length of

F = 30 cm positioned outside the reaction chamber. The energy per pulse was measured with the help of an energy

meter before the focus lens and in the reaction chamber, before the target ablation began. The energy per pulse was

varied between 6 and 9 mJ in order to increase the fluence and improve the deposition rate. The spot size was 0.5 mm2

leading to laser fluence between 1.2 - 1.8 J/cm2, necessary to remove the SnO2 nanoparticles solution, dispersed in

aqueous solution.

After MAPLE depositions, several sets of samples were obtained, depending on the variation of the laser

parameters or the powder concentration in the solution. The thin films with a higher thickness (approximately 100-

150 nm, depending on the number of laser pulses) were selected, for their morphological and structural

characterization using X-ray diffraction.

The results in Table 5.7 were obtained on a deposition where we used a pulse energy of 9 mJ, a concentration

of 0.25% w of SnO24 and a deposition time of 12 hours (about 430,000 laser pulses). This deposition was performed

with a very large number of pulses to facilitate the X-ray diffraction characterization. The optimal solution finally was

found: a relatively small concentration - 0.25% (concentration at which the suspension is stable enough in order to

prevent aggregation phoenomena durring freezing the MAPLE target), average energy per pulse - 6 mJ (increasing

the laser energy a large amount of target material is ablated, forming crater in it and micro drops on the Si substrate

or on the electrical contacts) and a number of pulses between 80,000 and 216,000, depending on the type of electrical

contacts used. Increasing the surface active area will be much more easy to record a cleaner signal, without noise, at

small concentrations of gas, the order of tens of ppm, on a thin film with a thickness of approximately 100 nm.

Table 5.7. Mean crystalite size measurements for bulk powder used as MAPLE target and the resulted thin film

Sample Raw powder (target) SnO2 thin film

Crystalite phase

ratio SnO2 (73%) SnO(27%) SnO2 (100%)

Mean size crystalite

14nm 16nm 15nm

The SnO2 / SnO composite nanopowders (referred to as SNO24) were deposited in 0.25% mass

concentration, both on Au electrical contacts deposited on Si and on electrodes formed from interdigitated electrical

contacts on Sital support. The first gas sensor tests were performed using the Au electric contacts, with a distance

between 100 µm contacts.

The diagram of the sensor test device for hydrogen detection can be seen in Figure 5.21. The experimental

set-up consists of an electric oven, two gas controllers (one of hydrogen and one of synthetic air), a rod-bracket on

which the plate was mounted with the sensor to be tested, a pressure controller inside the oven, a valve exhaust gas

for testing synthetic air, vacuum pump, two thermometers - which monitor the working temperature of the sensor and

the outside temperature and a multimeter that measures the variation of the electrical resistance R (Ω) when the test

gas is introduced.

Fig. 5.21. Testing device for H2 detecting sensors

Tests were performed at several hydrogen concentrations, in a range of 1000 to 10 ppm H2 in synthetic air.

It should be noted that the sensor was heat treated before the gas measurements in order to stabilize the

semiconductor material (SnO2 / SnO) from the crystallographic point of view, respectively the possible phase

transitions at the working temperature of the sensor (350 ° C). The samples were heat treated for three hours

in the synthetic air atmosphere.

Fig. 5.22 The sensor response obtained from SnO2/SnO composite nanoparticles at a 1000 ppm H2 concentration in synthetic air

Sensing measurements were recorded at a temperature of 250 ° C and a concentration of 1000 ppm H2 in

synthetic air for 3 hours, the sensor being sensitive up to a temperature of 160 ° C.

Fig. 5.24. Nanoparticle obtained sensors experimental set-up

The above images show the principle diagram of the sensors obtained using interdigitated electrical contacts;

the sensor consists of a ceramic support on which metal electrodes are deposited and on their surface were deposited

TIMP,NORMAT

SnO2 nanoparticles by MAPLE technique. The sensor thus obtained is located on a heating element shown in Figure

5.25 A; In our case, the heating element is an improvised one but can be reused for different tests. The sensors were

analyzed inside a special enclosure that allows the realization of a mixture of gas - synthetic air, generating accurate

reports even below 1 ppm. The electrical resistivity was analyzed using a Sourcemetter system. In Fig. 5.25 B and C

are presented sensor responses in the case of methane - synthetic air mixing for values of 100 ppm (B) and 1 ppm (C),

in this case the sensor architecture with interdigitated electrodes allowed to obtain a very good signal even in the case

concentration of 1 ppm and with a response speed of several tens of seconds. This sensor was tested under real working

conditions (normal atmosphere and ethyl alcohol vapors), in this case the concentration could not be controlled but

the sensor responded almost instantaneously to contact with these vapors (these tests are being carried out and not can

be presented in full in this paper).

Fig. 5.25. 591 sensor response to two Methane concentrations b) 100 ppm and c) 1 ppm CH4

In order to improve the properties of the sensors used in the detection of combustible gases, we proceeded to

mix SNO24 composite nanopowders with multi-walled carbon nanotubes (MWCNT), the concentration being 0.25%

by weight MWCNT compared to SNO24, subsequently stabilized with CMCNa. The final solution was dispersed in

ultrapure distilled water at a concentration of 0.25g / liter. For the active MAPLE layers we used the same Nd: YAG

laser, λ = 266nm, pulse duration = 5-7 ns, τ = 10 Hz and E = 9 mJ. Spot area≈ 0.5 mm2, number of laser pulses =

250,000. From the microscopy images in figure 5.27, a relatively homogeneous structure of the thin film can be

observed, with a maximum roughness of 50nm.

Fig. 5.27. AFM images of the thin film obtained from the SNO24 nanoparticles mixed with MWCNT

The thin layers obtained and characterized above are being tested from the sensing point of view, at different

gas concentrations and under conditions similar to the other tests already carried out, in order to compare the results

with the gas sensors obtained from the heat-treated and undoped SNO24 powder.( without MWCNT).

6. Synthesis by combustion of solid solution of mixed oxide nanoparticles: ternary and quaternary compounds; synthesis, characterization and applications

6.1. Ternary mixed oxides obtained by mediated glycine combustion Precursors

• Titan : tetraizopropoxidul de titan (TTIP),

• Staniu: 1,1,3,3-tetrabutil-1,3-diacetoxidistanoxan (TBDADS),

• Fier: acetilacetonatul feric (Fe(acac)3), (FEACAC)

Fig. 6.1 Experimental used in obtaining nanoparticles ternary and quaternary mixed ferites by chemical combustion

. Recipe I:=For the atomic ratio Ti: Sn: Fe = 1: 1: 1; Recipe II - For the atomic ratio Ti: Sn: Fe = 1: 1: 0.5. It is observed

that recipe II is the same as recipe I with half the amount of FEACAC added in solution A

Fig. 6.2. Reference nanometric powders obtained by solid state combustion with different atomic ratios of the compounds

The samples were rigorously characterized morpho-structurally with the help of X-ray diffraction and with

the help of High Resolution Transmission Microscopy (HRTM). The samples were scanned using a Panalytical X

'pert Pro MPD Diffractometer, scanning on continous mode in Bragg-Bretano configuration, using a CuKalpha X-ray

radiation source with the wavelenght λ=0.15418nm , range (15°-75°)deg 2θ, with a step size of 0.02° and a scanning

time of 20 sec/step at 45kV and 40mA. On the incident beam was used a divergence slit 1/2°, and on the diffracted

beam i had used a nichel filter and a curved graphitic monochromator. Using the ICCD data base and the HighScore

Software from Panalytical, the crystalline phase were identified, as follows:

In the powder Ti: Sn: Fe = 1: 1: 0.5 the following crystalline phases SnO2-casiterite, Fe2O3-hematite (JCPDS

04-002-5211) and TiO2-rutile (JCPDS 01-076-9000) were identified

- In the Ti: Sn: Fe = 1: 1: 1 powder, SnO2-casiterite, Fe2O3 - hematite and TiFe2O5 (JCPDS 04-010-9790)

were identified in about 17% - the mineral name being pseudo-brookite. The TiFe2O5 phase does not have

single reflections so in order to highlight its diffraction peak, carefully deconvolution of the recorded

diffractograms must be performed. For this reason it is expected that the semi-quantitative evaluation will

be affected by larger errors.

The presence of SnO2-cassiterite phase by the side of anatase and rutile can be due at one incipiente chemical

reaction at high temeparture between the rutile and hematite nanoparticles from the mixture.

Fig. 6.5 XRD detail in 2theta (32-38 degree) in order to highlight certain defraction peaks by deconvolution from the Ti:Sn:Fe =

1:1:1 atomic ratio nanopowder

Fig. 6.6 SEM and HRTM Ti:Sn:Fe (1:1:1) atomic ratio nanopowders

The transmission electron microscopy images presented above attest to the presence of aggregates of

nanoparticles of nanometric dimensions, in the range of 20-30 nm.

In this study we have synthesized as a preliminary two nanocomposite powders containing only titanium, tin and

iron as metallic elements in the following atomic proportions: for the one with higher iron content Ti: Sn: Fe = 1: 1: 1

and for the one with reduced iron content, Ti: Sn: Fe = 1: 1: 0.5, which were considered reference samples, and referred

to as REF.High reference, respectively REF.Low These samples were then compared from point the view of

crystallographic phases and other physical properties with other samples containing Ti, Sn and Fe in the proportions

of the reference samples and other metals in group VIIIb, in the case of Cobalt and Nickel, because It is known that

they can form as iron oxides with spinel / inverse spinel structures, ie : CoFe2O4 and NiFe2O4, which contain Fe3 + and

Co2 + or Ni2 +. These ferrites have much more pronounced magnetic properties than the most thermally stable form of

iron oxide (α-Fe2O3- hematite).

31 32 33 34 35 36 37 38

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Inte

nsi

ty (

a.u

)

Position 2q (deg) Cu Ka

Ti:Sn:Fe=( 1-1-1)

SnO2 (101)

TiFe2O5 (203)

Fe2O3 (104) Fe2O3 (110)

Fig. 6.7 Hysteresis of REF.High Ti-Sn-Fe

According to the hysteresis curve at room temperature in the figure above, the REF.High sample (Ti: Sn: Fe

= 1: 1: 1) shows a saturation magnetization of Ms≈ 0.6 emu / g. This low value can be explained by the existence in

this nanocomposite of iron-based phases with reduced magnetization: hematite and pseudobrookite. Thus, in art. [73]

the authors determined for submicron hematite crystals a saturation magnetization of about 0.25 emu / g, and for the

millimeter single-crystal of the same phase 0.29 emu / g, values quite close to that found by us. Furthermore, other

authors obtained pseudobbrookite nanocrystals, heated to 900 ° C with a saturation magnetization of about 0.9 emu /

g [74]. From the same figure, a trend to increase the saturation magnetization with decreasing temperature can be

observed, so that at 10K, Ms becomes approx. 1.5 emu / g at a field of 10 kOe. This trend was also observed for Fe @

C, Fe @ C-TiO2 and Fe @ C-SiO2 nanocomposites, synthesized by laser / sol-gel pyrolysis [75]. The origin of the

Ms increase trend with decreasing temperature can be related to the thinning of the paramagnetic layer from the particle

surface and implicitly to the increase of its average magnetic diameter, a phenomenon that has been observed by

Caizer et.al [76]. The coercivity of this sample at room temperature has a value below 0.5 kOe, indicating a trend

towards superparamagnetism, in correlation with the very small average size of hematite nanocrystallites (according

to XRD and HRTM). by chemical combustion of the solution was investigated with the help of Infrared Fourier

Transform (FTIR) spectroscopy. The spectra were recorded using a Shimadzu Corp equipment, 8400S instrument,

operating in the spectral range (350 - 7800) cm-1, with a spectral resolution of 0.4cm-1 and a signal / noise ratio of 1:

20,000; records represent a cumulative of 45 scans per sample. The spectra were recorded in the transmitting mode,

using the ATR module IRTracer-100 (attenuated total reflectance).

Fig. 6.8 ( FTIR) of the sample REF.High, with atomic ratio Fe:Sn:Ti = (1:1:1)

The IR spectrum above shows the ferrite sample selected as reference, with an atomic ratio of Fe: Sn: Ti =

1: 1: 1. From the crystallographic point of view, the majority phase is SnO2-casiterite. The sample also contains TiO2-

rutile and pseudo-brookite. Due to this mixture of phases, exact identification of spectral lines corresponding to the

vibrations of molecular bonds is very difficult. Peaks can be observed in the range (400 - 550) cm-1; according to the

literature, the absorption lines corresponding to the SnO2 nanoparticles appear in the spectral range (540-660) cm-1,

indicating the presence of stretching vibrational modes corresponding to the O-Sn-O or Sn-O chemical bonds [77].

The vibration of the Sn-O bond is generally observed around 670 cm-1 and 560 cm-1 in the bulk material; In

the REF.High sample, the symmetrical and asymmetrical stretch vibrations of the bond move to 520 cm-1 as an effect

of the nanometric dimensions of the SnO2 phase in this sample. [78].

In Figure 6.8 we can also observe a maximum absorption of 443 cm-1 wavwlenght, corresponding to the

scissoring bonds of Fe-O. The very wide absorption band from the region 3000 cm-1 can be attributed to the stretching

vibrations of the hydroxyl groups and / or the water molecules absorbed at the surface of the powder. It is notable the

appearance of the two absorption bands at 517cm-1 and 443 cm-1, due to the vibration of the same Fe-O bond in the

rhombohedral structure of hematite, as reported by Lassoued et.al [79], which recorded peak at 527 cm- 1, respectively

434 cm-1, in the case of α-Fe2O3 nanoparticles synthesized by chemical precipitation from FeCl3 precursor hydrate,

and calcined at 700 ° C. The shift of the absorption maxima of the vibrations of the Fe-O bond at a higher wavelength

is due to the morphology and the small size of the nanoparticles and the fuel - in our case glycine, the mixture of

selected precursors and the working temperature during the synthesis by chemical combustion. .

Considering that in the synthesis of these nanoparticles, the iron precursor used was the iron acetyl acetate

[Fe (acac) 3], it is possible that the absorption maxima in the spectral range (1500-2000) cm-1 correspond to the

vibration of the C = O bond, with a clear maximum at 1570 cm-1. Because the other absorption bands corresponding

to the chemical bonds of the iron precursor are not clearly reflected in the spectrum, it is possible that, during its

conversion to acetate during the temperature synthesis process, certain groups or acetate ions may have been absorbed

on the surface of the nanoparticles, as reported by Jinguang Cai et.al [80].

5.2. Quaternary mixed oxides Ti – Sn – Fe – Co Precursors:

• Titan : tetraizopropoxidul de titan (TTIP),

• Staniu: 1,1,3,3-tetrabutil-1,3-diacetoxidistanoxan (TBDADS),

• Fier: acetilacetonatul feric (Fe(acac)3), (FEACAC)

• cobalt: acetatul de cobalt (II) tetrahidratat (COAC

a1) High cobalt ferrite content recipe (atomic ratio Sn: Ti: Fe: Co = 1: 1: 1: 0.5)

We chose a mixture of precursors in which the atomic ratio Sn: Ti: Fe: Co is 1: 1: 1: 0.5 to form cobalt

CoFe2O4.

Fig.6.9 TEM and HRTM images of highest Cobalt content ferrite, named High FeCo

These images suggest a well-defined crystalline structure, and particle agglomerations (aggregates) of the

order of tens of nanometers.

Fig. 6.11 XPS (X-ray photoelectron spectroscopy) High FeCo sample

XPS measurements were performed using ESCALAB Xi + equipment, equipped with a hemispherical multi-

channel electron analyzer. The radiation source is Al Kα (hν = 1486.2 eV) and the reference energy of C 1s (284.4

eV). To record a quality XPS spectrum, the powders were easily pressed and vacuumed to a pressure of <2 × 10−8

Torr to remove surface adsorbed water at room temperature. The chemical composition of the surface and the

oxidation states were estimated from the XPS spectra by calculating the integral of each peak separately, after

extracting the background signal using the "S-shaped" Shirley-type approximation.

Fig. 6.13 Magnetic hysteresis of highest Cobalt content High FeCo

The sample with a high content of cobalt ferrite (High FeCO) has a saturation magnetization of approx. 9

emu / g, higher than the reference sample (Ti: Sn: Fe = 1: 1: 1). This can be explained by the increased presence of

the CoFe2O4 phase (32% - percentage determined from X-ray Diffraction), which has in bulk a saturation

magnetization of 80 emu / g, according to [81] and 56.7 emu / g respectively for nanoparticles. obtained by chemical

combustion of the solution, 55.8 emu / g for nanoparticles obtained by co-precipitation and 47.2 emu / g for the

samples obtained by precipitation [82].. From the inset of the image in which the hysteresis curve of the sample with

a high content of cobalt ferrite (32% CoFe2O4) is shown, the same behavior can be observed at low temperatures

(10K), as in the case of the reference sample, respectively the increase of the magnetization value. saturation with

decreasing temperature. The relatively low value of the coercivity at room temperature (approx. 650 Oe) indicates a

behavior close to the superparamagnetic one, specific to the very small nanoparticles, confirmed by TEM / HRTEM

images and X-ray diffractograms.The clear trend of the coercivity increase (from 650 Oe to approx. 8000 Oe) with

the decrease of the temperature can be seen in figure 36 by comparing the hysteresis curves at room temperature (300k

with red) and at 10K (with black). This trend has been observed and discussed extensively in the article Fleaca and

others [83], for the case of the Fe @ C and Fe-Fe2O3 @ Carbosiloxane nanoparticles synthesized by laser pyrolysis.

Fig. 6.14 FTIR of HighFeCo sample

In Figure 6.14 we can observe the FTIR transmission spectrum of the High FeCo sample, made in order to

support the CoFe2O4 structure, of spinel type, next to hematite and casitterite. The absorption band from 447 cm-1

corresponds to the stretching vibration of the Fe-O bond in the hematite structure, the band observed in the reference

sample Fe: Sn: Ti = 1: 1: 1 discussed previously in this thesis. The bands in the range 550-600 cm-1, observed in most

of the materials having ferrous composition, are due to the stretching vibrations of the group (Fe3+ - O2-), metal-oxygen

bond [84]. The absorption peak on the range (2000-3000) cm-1 belong to the stretching vibration of the O-H, C-H

bond, respectively to the shear vibration C-H, specific to the existence of hydroxyl groups or of the water absorbed

on the surface of the nanoparticle. The existence of these peak in the near-infrared domain is attributed to the

elementary spinel cell vibration in the tetrahedral positions (A), and the bands on the range (400-600) cm-1 appear due

to the vibrations induced by the metal-oxygen bonds, which occupy the octahedral positions (B). ).

a2) Recipe with low content of Co ferrite (atomic ratio Sn: Ti: Fe: Co = 1: 1: 0.5: 0.25)

First the precursor solutions are formed:

2.84 g TTIP (10 mmol) în 19 mL izopropanol

2.61 g TBDADS (5 mmol) în 50 mL izopropanol

1,765 g FEACAC (5 mmol) în 50 mL izopropanol pe baia de ultrasunete + 100 mL metanol

0,623 g COAC (2.5 mmol) în 30 mL izopropanol + 30 mL etanol absolut + la final 40 mL etanol

It is the same recipe used in the preparation of the High FeCo sample with half the amount of FEACAC and

COAC added.

Tabelul 6.1 Crystallin phase ratio obtained for Cobalt ferrites

Sample Fe2O3 SnO2 CoFe2O4

LOW FeCo 29.7% 38.6% 31.7%

HIGH FeCo 41% 25% 34%

Fig. 6.15 XRD difraction ratio, obtained by scanning resulted powders by the two above mentioned recepies a)high Cobalt ferrite

content, b) low Cobalt ferrite content and c) REF.High (Ti:Sn:Fe =1:1:1) reference sample

The inset from the picture presented above has the role of detailing and highlighting by means of

deconvolution, the presence of the diffraction peak: TiFe2O5, SnO2 and TiO2 - anatas phase. In the above table (table

6.1) a semi-qualitative analysis of the powders can be observed, based on reports of crystalline phases.

The above FTIR spectrum (Fig. 6.16) confirms the spinel-like structure of the cobalt ferrite, with peak slightly

less attenuated due to the low concentration of the cobalt ions, while maintaining the absorption band from 447 cm-1

corresponding to the vibration. for the extension of the Fe-O bond in the hematite structure, the band observed also in

the REF.High sample, discussed previously in this thesis. The bands in the range 550-600 cm-1, observed in most of

the materials having ferrous composition, are due to the stretching vibrations of the group (Fe3+-O2-), metal-oxygen

bond.

Fig. 6.16. FTIR of Low FeCo

6.3. Mixed cuarternary oxides Ti-Sn-Fe-Ni There are two factors that cause major differences in physical / chemical properties at the nanometric scale:

surface effects (determine the scaling of surface phenomena based on surface atomic fractions) and quantum effects

(discontinuous behavior due to the quantum confinement effect in materials with delocalized electrons) [85]. These

factors directly affect the chemical reactivity of materials and their physical properties such as mechanical, optical,

electrical or magnetic properties. Magnetic nanoparticles have unique properties with technological applications in

the field of high density recordings, ferrofluids, dye imaging or high frequency devices. Nanoparticles obtained from

magnetic ceramic materials are used as contrast substances in nuclear magnetic resonance imaging (NMR), replacing

radio-transients obtained from radioactive substances, and for drug-controlled release into well-defined areas of the

human body. From the point of view of these applications, the most important properties are magnetic saturation,

coercivity and magnetization, properties that change dramatically when the particles are at the nanometric scale

[88,89].

Spinel structure

In order to obtain quaternary mixed oxides which also contain a crystalline NiFe2O4 spinel type, we used

certain precursors, arriving at two different recipes depending on the atomic ratios of the precursors.

Precursors:

• Titan :tetraizopropoxidul de titan (TTIP),

• Staniu: 1,1,3,3-tetrabutil-1,3-diacetoxidistanoxan (TBDADS),

• Fier:acetilacetonatul feric (Fe(acac)3), (FEACAC)

• nichel este acetatul de nichel (II) tetrahidratat (NIAC)

a1) a1) Recipes with a high content of nickel ferrite

We will choose a mixture of precursors in which the atomic ratio Sn: Ti: Fe: Ni is 1: 1: 1: 0.5, in order to obtain

NiFe2O4

-2.84 g TTIP in 30 ml izopropanol

-2.61 g TBDADS in 10 ml izopropanol

-3.53 g FEACAC in 100 ml izopropanol pe baia de ultrasunete

-1,244 g NIAC in 75 ml izopropanol + 75 ml ethanol absolut izopropanol pe baia de ultrasunete

Fig. 6.18 The above image represents the semi-cantitative analysis of the crystaline phases obtained following X-rays difraction

The semi-quantitative analysis was performed using HighScore Plus software and the ICDD database; It was

first processed signal, using a pseudo-Voight2 function (a combination of Gaussian and Lorentz functions) for the

fitting (approximation) of each diffraction peak corresponding to a crystalline plane. After fitting, the matching of

crystalline phases from the database was made taking into account the interplanar distances. Using the most intense

diffraction peak and the Debye-Scherrer formula in which we introduced the FWHM corresponding to each peak, we

obtained the following values: DmedFe2O3=36nm , DmedNiFe2O4=45nm, DmedTiO2(r)=28nm si DmedSnO2=25nm.

Using XRD for structural characterization, the presence of the following crystalline phases was identified:

SnO2- casiterite (JCPDS 00-041-1445), Fe2O3 - hematite (JCPDS 00-033-0664), NiFe2O4 - trevorit (JCPDS 00-044-)

1485) and TiO2-rutile (JCPDS 00-021-1276). The identification of these crystalline phases was performed using the

HighScore Plus software attached to the XRD database from ICDD. In calculating the mean crystallite size, the most

intense diffraction peak corresponding to each phase was used, peak that did not require deconvolution due to their

singularity. Where the deconvolution was necessary to extract the value of FWHM (full width at the half maximum ),

the calculation was performed on several peaks, the values being mediated at the end.

The values of the mean crystallite size were obtained using the Debye-Scherrer formula, in which the shape factor

was considered 0.9 particles having a roughly spherical shape, according to the images of Transmission Electron

Microscopy (TEM) presented below.

Fig. 6.20 TEM (left) and HRTM (right) on the high content Ni, High FeNi sample

The lower saturation magnetization value of the sample with a high content of nickel ferrite (HIGH FeNI-

1.8 emu / g) compared to that of the sample with a high content of cobalt ferrite (HIGH FeCo-9 emu / g) may have

more explanation: on the one hand, the percentage of magnetic phase is higher in the High FeCo sample (32%

CoFe2O4) compared to the HiGH FeNi sample (11% NiFe2O4) and on the other hand, due to the bulk saturation

magnetization of CoFe2O4 (80 emu / g) considerably larger than 50 emu / g for bulk NiFe2O4.

Also, the increase of the saturation magnetization with the decrease of the temperature can be observed in

Figure 6.21, behavior similar to the reference sample discussed above. Also in this sample can be observed a behavior

close to the superparamagnetic, which suggests the presence of crystallites of very small size

In both samples (High FeNi and High FeCo) there is also a contribution of the hematite (Fe2O3) nanoparticle

to their magnetization, this contribution being more important in the case of the nickel ferrite sample, despite the fact

that the percentage of this phase is lower. This is due to the weaker magnetization of NiFe2O4 compared to the

magnetization of CoFe2O4.

Fig. 6.21 Magnetic hysterezis of HIGH FeNi

Fig. 6.22 FTIR of HIGH FeNI ( NiFe2O4)

Fig.6.23 Survey XPS ( X-ray photoelectron spectroscopy) of High FeNi sample

The chemical surface composition and oxidation states of the elements in the High FeNi sample were studied

using Photoelectron X-ray Spectroscopy (XPS). As expected, XPS-survey spectra indicate the presence of species

with Fe2p, Ni2p, Sn3d, Ti2p, O1s and C1s states. Following a high resolution scan, in the range (705-735) eV,

corresponding peak intensities of Fe 2p3/2 (711eV) and Fe 2p1/2 (725eV) and Sn 3p3/2 ( 715eV), maxima also

identified by Pengxi Li et. of [90] in the study of NiFe2O4 nanoparticles, obtained by hydrothermal route, used

for electro-catalytic and oxygen reduction properties.

a2) Recipe with low content of nickel ferrite

I choosed an mixture of precursors where the atomic ratio Sn:Ti:Fe:Ni to be 1:1:0.5:0.25

2.84 g TTIP in 19 ml izopropanol

2.61 g TBDADS in 50 ml izopropanol

1,765 g FEACAC in 50 ml izopropanol pe baia de ultrasunete + 100 ml metanol

0,622 g NIAC in 75 ml izopropanol + 75 ml ethanol absolut

The synthesis is similar to the powders obtained above.

Following the morpho-structural characterization made by X-ray diffraction, electron microscopy and EDS,

it can be observed that when we used an atomic ratio of Sn: Ti: Fe: Ni = (1: 1: 0.5: 0.25) NiFe2O4 crystalline phase is

no longer present but the percentage increase of iron oxide - hematite was favored.

Tabelul 6.2 XPS determined ternary/quaternary mixed oxides elementary composition

Sample O (%) Sn (%) Fe (%) Ni (%) Ti (%) Co (%)

High FeCO 61.11 8.34 14.07 - 9.67 6.8

Low FeCO 60.72 10.06 11.1 - 10.75 7.37

High FeNi 71.9 10.21 0.18 4.66 13.05 -

Low FeNi 61.3 9.75 10.84 5.57 12.53 -

Tabelul 6.3 EDS determined ternary/quaternary mixed oxides elementary composition

Sample Fe

[at.%]

Co

[at.%]

Ni

[at.%]

Ti

[at.%]

Sn

[at.%]

O

[at.%]

REF.High 13.8 - - 12.5 10.9 62.8

High FeCo 10.6 6.5 - 10.5 8.6 63.8

High FeNi 12 - 5.9 11.9 8.8 61.4

The ternary / quaternary compounds High FeCo / FeNi were analyzed using EDS to approximate the

elemental composition and the results are presented in the table above, confirming the atomic ratios of Fe: Sn: Ti: Ni

/ Co. These reports were established by the recipe used in the synthesis process, demonstrating the reliability of the

method. From Table 6.2 we observe a higher concentration of nickel or cobalt in the samples where we used a lower

concentration of Ni / Co precursor, compared to the samples where we used a higher concentration of Ni / Co

precursor. This observation of inversion concentrations counter of nickel or cobalt powders obtained from the

precursors can be explained considering the characteristics of type XPS analysis; this is an analysis by surface

excellence (a few nanaometers), and thus, the concentrations measured by this technique are actually superficial

concentrations.. It is quite possible that in the High FeCo / High FeNi type samples, the total Co / Ni concentration

will be higher than in the Low FeCo / Low FeNi type samples, and the latter will only be superficially enriched in

these elements, due to the fact that at lower concentrations of precursors they started to diffuse only partially.

Tabelul 6.4. Nickel based mixed oxides composition obtained by the semi-cantitative analysis of the XRD based on the identified

crystaline phases

PROBA Fe2O3 SnO2 TiO2 NiFe2O4

HIGH FeNi 17.8% 62.4% 8.9% 10.9%

LOW FeNi 38% 20% 42% -

In the case of the Low FeNi sample, the crystalline NiFe2O4 phase could not be detected due to the lower

concentration of Nickel used as precursor. In contrast, the X-ray diffractogram revealed a systematic shift of the peak

intensities of the α-Fe2O3 phase, identified in this sample, which can be explained by the interstitial presence of Ni2+

ions in the α-Fe2O3 network.Also, the presence of these divalent ions that replace Fe3+ ions requires in this case and

the presence of oxygen vacancies, in order to maintain the neutrality of the crystalline network. The absence of the

NiFe2O4 phase in the LOW FeNi sample contrasts with the presence of the CoFe2O4 phase in the LOW FeCo sample.

This could be due to the different invers spinel structure of the Nickel ferrite from the spinel structure of the Cobalt

ferrite. Moreover, even the peaks of intensity of the crystalline phase SnO2 show a displacement, which in turn can be

attributed to a substitution of Ni2+ ions (Ni2+ ionic radius = 0.72Å [91] versus Sn4+ ionic radius = 0.71Å [92 ]) in the

tetragonal structure of rutile accompanied of course, by the presence of some Oxygen vacations.

Fig. 6.26 X-ray Diffraction of powders HIGH FeNi/LOW FeNi

Fig. 6. 27 XPS ( X-ray photoelectron spectroscopy) on LOW FeNi sample

Fig. 6.28. FTIR spectra of LOW FeNi

Fig. 6.29. Superposed FTIR spectra of the ternary/cuaternary as obtained powders

Figures 6.28-29 present the infrared spectra of various nanopowders of metallic oxides in which can be seen

the presence of two absorbtion ranges, corresponding to the stretching or shear vibrations of the ions in the crystalline

structure. In the case of spinel structures, the general formula is MFe2O4 where M = Co, Ni, as mentioned above, the

unit cell contains 32 oxygen atoms in the cubic system with centered faces, and the cations of Fe respectively of

transition metal (M2+) can occupies 8 tetrahedral (Td) and 16 octahedral (Oh) positions. As we have shown, nickel

ferrite is a inverse spinel structure, in which half of Fe3+ ions occupy tetrahedral positions, and the other half occupy

octahedral positions, while Ni2+ ions can occupy only octahedral positions. Becouse of this, the spectral range (550-

560)cm-1 corespound in our case to the Fe-O bond, with already identified absorbtion peaks at 619. 622 and 623m-1.

The spectral range (380-450)cm-1 correspond to octhaedrical vibrations Mocthaedric-O , Ni –O, Fe-O , Sn-O, Ti-O type

with spectral bands at 378, 386, 401, 405 si 441 cm-1 The range (1900-2700)cm-1 can be due to some functional

groups resulted from fuel decomposition- glycine in our case, durring high pressure chemical combustion synthesis

process

6.4. SnO2 NPs synthesis using solid state combustion : synthesis, characterisation and applications In order to obtain SnO2 nanopowders by a chemical method, cheaper and relatively easier than laser

pyrolysis, several recipes have been tested for obtaining these nanoparticles from tin organo-metal precursor

(Tetrabutyl 1,3-diacetoxydistannoxane). and glycine (fuel).The synthesis by chemical combustion of the three

obtained samples, corresponding to the three recipes, was followed by a morpho-structural characterization using X-

ray diffraction. Only the presence of the castrated SnO2 phase was identified, with JCPDS 00-041-1445 from the

ICDD database. After phase identification, the Debye-Scherrer formula was applied to estimate an average crystallite

size. The average obtained dimensions were between 25-40nm.

In order to obtain thin films used as active layers for gas sensors, the powder with the smallest crystallite size

was selected. This was dispersed in a 5g/l aqueous solution, ultrasounded at the ultrasound bath for 1 hour and

sonicated with the help of the sonotrode for better dispersion, 30 seconds before the target required for the MAPLE

experiments was achieved.

Fig. 6.30. XRD of SnO2 NPs obtained obtained through three recipes different

The thin films were obtained using the MAPLE technique, using the fourth harmonic of the Nd: YAG laser,

respectively 266nm, a repetition rate of 10Hz and the pulse duration of 5-7ns. There were many experiments we varied

parameters such as pulse energy, energy fluence on the target, the target concentration. These experiments aimed to

optimize the deposition rate and the morphology of the deposited films so that they could be used as active layer for

the sensors. Due to the relatively low concentration of the active material in the target, the laser depostion rate is quite

small, a successful deposition requiring up to 10 hours of irradiation.

25 30 35 40 45 50 552Theta (°)

400

1600

3600

6400

10000

14400

Inte

nsity

(co

unts

)

(110)

(101)

(111)

(210)

(211)

(220)

(200)

Fig. 6.31 XDR characterization of the film obtained by MAPLE: Superposed of the MAPLE deposed film difractogram with the

chemically combustion obtained SnO2 (target) powder difractogram

The nanostructured thin films were deposited using the MAPLE laser technique on three types of substrates:

a) and (111) - the choice of this substrate was made in order to perform the morphological and structural

characterization of the surface, respectively X-ray diffraction (figure above) ), Atomic Force Microscopy, b) Au

electrods deposited on silicon, with simple geometry, c) interdigitated electrodes deposited on Sital.. In order to

highlight the selectivity of the sensor (a very important parameter that I have aimed to improve in the present thesis),

several sets of sensor measurements were made on the same nanostructured layers obtained from SnO2 nanoparticles,

on two different gases; CO, respectively CH4. It should be noted that the thin films were deposited on Au electrods on

silicon substrate, with simple geometry. The first gas tested was carbon monoxide (CO) at three different

concentrations: 1000ppm, 200ppm and 10ppm. At 1000 ppm CO in synthetic air, a very good response time is

observed, almost instantly.

Fig. 6.32. 525 Gas sensor response at CO detection obtained from SnO2 nanoparticles synthetized by Glicine assisted combustion from

an organic precursor

Using electrical contacts interdigitate is beneficial to obtain a high operating speed sensor (response time for

recovery between cycles); In the case of simple electrical contacts, the decrease of the distance between the electrodes

also leads to the decrease of the response time of the sensor. The configuration of the electrical contacts must always

be chosen according to the operating parameters of the sensor priority in implementation: sometimes an extremely

short response time is more important than the detection limit of tens of parts per million (ppm).

Figure 6.32 shows the response of the sensors obtained for the detection of CO where an increase of the

variation of the resistance with the increase of the concentration of CO in the synthetic air can be observed. Finally,

the sensor response is interpreted by an electronic circuit and this is effective if the electronics can use the generated

signal. Since modern electronics can detect very small variations in electrical resistivity, we can say that our sensor

can be used efficiently up to concentrations of 1 ppm. Noise appears at weak signals, but these can also be eliminated

with the help of signal filters.

The first desired application was to equip a flying drone with these sensors to discover the hazardous areas

due to the gas leaks around the industrial installations (gas lines, distribution points). In this case, it was not important

to have a very low detection limit but we were interested in having a very fast response time so that the drone could

immediately detect the leakage of gas during the flight over the emitted area.

Fig. 6.33. 526 sensor response at CH4 detection obtained by SnO2 nanoparticles synthetized by Glicine mediated combustion from an

organic precursor at a)200 ppm, b) 500 ppm CH4 in synthetic air

In Figure 6.33. the CH4 response is presented (we expect it to be a better response but the sensor may have

been damaged during assembly - disassembly from the test room). At 200 ppm methane we have a good enough signal

to use the sensor in methane monitoring applications (this becomes dangerous when it exceeds a few percent. [93].

Conclusion Laser pyrolysis technique is an efficient and versatile method for the synthesis of TiO2 / SnO2 composite

nanoparticles. The technique allows obtaining nanoparticles with morphological and structural properties

required / desired in applications, due to the good control of the process parameters: laser power density,

flow rate of precursors / auxiliary gases, pressure in the reaction chamber, flame temperature.

The variation of the geometry for introducing the precursor into the reaction zone and the concentration of

the tin precursor were the process parameters that proved to be the most important for the synthesis of high

performance nanoparticles.

The post-synthesis process of thermal treatment (calcination) at 450 ° C for 3 hours in order to eliminate /

decrease the carbon content, did not lead to crystallographic phase transformations.

The variation of the amount of tin introduced to the synthesis leads to the synthesis of nanopowders with

different percentages / ratios of crystalline phases; TiO2 rutile, TiO2 anatas si SnO2 casitterit.

The results from XRD are in good agreement with those comming from TEM.

The porosity of the thin films, determined at AFM and optical profilometry, of 126 nm, is suitable for the

sensory characteristics of the films.

The sensor response is very fast at relatively low concentrations, it is recommended to use where the response

time is more important than its sensitivity or the lower limit of detection and quantification.

From the measurements made, the sensor 525 can be used in a sensor array for selective gas detection

The use of ethylene as a sensitizer in place of SF6, avoided the contamination of samples with S or F

Accurate control of the gas flow is very important because the active layers for gas sensors require a

dominant SnO2 phase, with an mean crystalline size in the nanometric range.

The heat treatement influenced the ration between SnO and SnO2

The optimal parameters for MAPLE depositions were established being : concentration of nanoparticles,

laser energy per pulse, and laser pulse number

The roughness of thin films varied between 8 and 20nm, function number of laser pulses and solutions

concentration

The minimum temperature measured were the sensor is still sensitive was 160°C

Significant differences in sensor parameter values were recorded in exchange of the electrical contacts

architecture

Chemical combustion method from solid solution assisted by glycine (fuel) generated powders with

average crystallite size in the nanometric field

Magnetic measurements have shown a trend to increase saturation magnetization with decreasing

temperature.

Measurements made by Fourier Transform Spectroscopy (FTIR) confirmed the presence of vibrational

stretching modes that correspond to the chemical bonds O-Sn-O or Sn-O, respectively Fe-O

XRD morpho-structural characterization highlighted the presence of αFe2O3, SnO2 and CoFe2O4

phases in different reports, depending on the precursor reports used.

The saturation magnetization of the sample with a high content of CoFe2O4 was higher than the

reference sample, the relatively low value of the coercivity at room temperature indicates a behavior

close to the superparamagnetic one, specifically for the very small nanoparticles.

The low saturation magnetization value of HighFeNi sample compared to HighFeCo is due to the

higher percentage of magnetic phase (32% CoFe2O4 versus 11% NiFe2O4) and due to the significantly

higher saturation magnetization of CoFe2O4 (80 emu / g). than NiFe2O4 (50 emu / g). FTIR spectroscopy confirms the presence of the crystalline phases mentioned above by the absorption

peaks values corresponding to the NiFe2O4 inverse spinel structure in the range (550-650) cm-1.

Thin films have the same morphostructural structure as the material used for the target

The sensors were tested in an experimental set-up dedicated to these measurements on two different

gases: CO and CH4; the sensors tested at CO recorded a very good response time both at 1000 ppm

and at 100 ppm CO in synthetic air

BIBLIOGRAPHY :

[1] I. Khan, K. Saeed, I. Khan, Nanoparticles : Properties , applications and toxicities, Arab. J. Chem. (2017).

doi:10.1016/j.arabjc.2017.05.011. [2] Z.O. Nanoparticles, J.P. Morán-lázaro, F. López-urías, E. Muñoz-sandoval, Synthesis, Characterization, and Sensor Applications of

Spinel ZnCo 2 O 4 Nanoparticles, (2016). doi:10.3390/s16122162.

[3] N. Taguchi, Gas detecting device, 1972. [4] Ȧ S.I.H., Ȧ A.N.N., Ȧ Q.G.A., Enhanced Hydrogen Sensing Parameters of MWCNT – SnO 2 Thin Film, 4 (2014) 3954–3960.

[5] J. Mizsei, Forty years of adventure with semiconductor gas sensors, Procedia Eng. 168 (2016) 221–226.

doi:10.1016/j.proeng.2016.11.167. [6] P.T. Phong, N.X. Phuc, P.H. Nam, N. V Chien, D.D. Dung, P.H. Linh, Physica B : Condensed Matter Size-controlled heating ability of

CoFe 2 O 4 nanoparticles for hyperthermia applications, Phys. B Phys. Condens. Matter. 531 (2018) 30–34.

doi:10.1016/j.physb.2017.12.010. [7] S. Ammar, A. Helfen, N. Jouini, F. Fie, Ë. Villain, P. Molinie, M. Danot, Â.P. Diderot, P. Jussieu, F. E-mail, Â.P. Marie, P. Jussieu, I.J.

Rouxel, C. De Houssinie, Magnetic properties of ultra ® ne cobalt ferrite particles synthesized by hydrolysis in a polyol medium {,

(2001) 186–192. doi:10.1039/b003193n. [8] A. Cristina, S. Samia, Structural effects on the magnetic hypertermia properties of iron oxide nanoparticles, Prog. Nat. Sci. Mater. Int.

26 (2016) 440–448. doi:10.1016/j.pnsc.2016.09.004.

[9] I. Risavi, I. Ionescu, Chimie și probleme de chimie, Editura Tehnică București, 1971.

[10] D.A. Neamen, Semiconductor physics and devices, 3rd ed., McGraw Hill, 2003. doi:10.1016/S1369-7021(06)71498-5.

[11] W. Shockley, Electrons and holes in semiconductors, with applications to transistor electronics, R. E. Krieger Pub. Co., 1976.

[12] R.W. Siegel, Synthesis and properties of nanophase materials, Mater. Sci. Eng. A. 168 (1993) 189–197. doi:10.1016/0921-5093(93)90726-U.

[13] M. Decker, M. Decker, Überlegungen zur Ersetzbarkeit des Menschen Perspektiven der Robotik . Überlegungen zur Ersetzbarkeit des

Menschen, (2001). [14] S.. Hahn, N. Bârsan, U. Weimar, S.. Ejakov, J.. Visser, R.. Soltis, CO sensing with SnO2 thick film sensors: role of oxygen and water

vapour, Thin Solid Films. 436 (2003) 17–24. doi:10.1016/S0040-6090(03)00520-0.

[15] X. He, J. Li, X. Gao, L. Wang, NO2 sensing characteristics of WO3 thin film microgas sensor, Sensors Actuators B Chem. 93 (2003) 463–467. doi:10.1016/S0925-4005(03)00205-3.

[16] G. Korotcenkov, V. Brinzari, V. Golovanov, Y. Blinov, Kinetics of gas response to reducing gases of SnO2 films, deposited by spray

pyrolysis, Sensors Actuators B Chem. 98 (2004) 41–45. doi:10.1016/j.snb.2003.08.022. [17] S. Bai, D. Li, D. Han, R. Luo, A. Chen, C.L. Chung, Preparation, characterization of WO3–SnO2 nanocomposites and their sensing

properties for NO2, Sensors Actuators B Chem. 150 (2010) 749–755. doi:10.1016/j.snb.2010.08.007.

[18] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A New Detector for Gaseous Components Using Semiconductive Thin Films., Anal. Chem. 34 (1962) 1502–1503. doi:10.1021/ac60191a001.

[19] J. Zhang, D. Zeng, Q. Zhu, J. Wu, Q. Huang, C. Xie, Effect of Nickel Vacancies on the Room-Temperature NO 2 Sensing Properties of

Mesoporous NiO Nanosheets, J. Phys. Chem. C. 120 (2016) 3936–3945. doi:10.1021/acs.jpcc.5b12162. [20] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview, Sensors Actuators B

Chem. 192 (2014) 607–627. doi:10.1016/j.snb.2013.11.005.

[21] N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, J. Electroceramics. 25 (2010) 11–19. doi:10.1007/s10832-009-9583-x.

[22] M. Hübner, C.E. Simion, A. Tomescu-Stănoiu, S. Pokhrel, N. Bârsan, U. Weimar, Influence of humidity on CO sensing with p-type CuO thick film gas sensors, Sensors Actuators B Chem. 153 (2011) 347–353. doi:10.1016/j.snb.2010.10.046.

[23] H.-R. Kim, A. Haensch, I.-D. Kim, N. Barsan, U. Weimar, J.-H. Lee, The Role of NiO Doping in Reducing the Impact of Humidity on

the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies, Adv. Funct. Mater. 21 (2011) 4456–4463. doi:10.1002/adfm.201101154.

[24] S.S. Kaye, J.R. Long, Hydrogen Storage in the Dehydrated Prussian Blue Analogues M 3 [Co(CN) 6 ] 2 (M = Mn, Fe, Co, Ni, Cu, Zn),

J. Am. Chem. Soc. 127 (2005) 6506–6507. doi:10.1021/ja051168t. [25] A. Gurlo, R. Riedel, In Situ and Operando Spectroscopy for Assessing Mechanisms of Gas Sensing, Angew. Chemie Int. Ed. 46 (2007)

3826–3848. doi:10.1002/anie.200602597.

[26] N. Yamazoe, K. Shimanoe, Receptor Function and Response of Semiconductor Gas Sensor, J. Sensors. 2009 (2009) 1–21. doi:10.1155/2009/875704.

[27] R.M. Martin, Electronic Structure: Basic Theory and Practical Methods, 1st ed., Cambridge University Press, 2004.

doi:10.1080/00107514.2010.509989. [28] G.F. Fine, L.M. Cavanagh, A. Afonja, R. Binions, Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring, Sensors.

10 (2010) 5469–5502. doi:10.3390/s100605469.

[29] D. Shriver, M. Weller, T. Overton, J. Rourke, F. Armstrong, Inorganic Chemistry, 6th ed., Oxford University Press, 2014.

[30] N. Tamaekong, C. Liewhiran, A. Wisitsoraat, S. Phanichphant, Acetylene sensor based on Pt/ZnO thick films as prepared by flame

spray pyrolysis, Sensors Actuators B Chem. 152 (2011) 155–161. doi:10.1016/j.snb.2010.11.058.

[31] H. Gu, Z. Wang, Y. Hu, Hydrogen Gas Sensors Based on Semiconductor Oxide Nanostructures, 2012. doi:10.3390/s120505517. [32] J.S. Wright, W. Lim, D.P. Norton, S.J. Pearton, F. Ren, J.L. Johnson, A. Ural, Nitride and oxide semiconductor nanostructured

hydrogen gas sensors, Semicond. Sci. Technol. 25 (2010) 024002. doi:10.1088/0268-1242/25/2/024002.

[33] K. Potje-Kamloth, Semiconductor Junction Gas Sensors, Chem. Rev. 108 (2008) 367–399. doi:10.1021/cr0681086. [34] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: How to?, Sensors Actuators B Chem. 121 (2007) 18–35.

doi:10.1016/j.snb.2006.09.047.

[35] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal Oxide Gas Sensors: Sensitivity and Influencing Factors, Sensors. 10 (2010) 2088–2106. doi:10.3390/s100302088.

[36] Y. Shimizu, N. Kuwano, T. Hyodo, M. Egashira, High H2 sensing performance of anodically oxidized TiO2 film contacted with Pd,

Sensors Actuators B Chem. 83 (2002) 195–201. doi:10.1016/S0925-4005(01)01040-1. [37] Y. Shimizu, T. Hyodo, M. Egashira, H2 sensing performance of anodically oxidized TiO2 thin films equipped with Pd electrode,

Sensors Actuators B Chem. 121 (2007) 219–230. doi:10.1016/j.snb.2006.09.039.

[38] E. Şennik, Z. Çolak, N. Kılınç, Z.Z. Öztürk, Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor, Int. J. Hydrogen Energy. 35 (2010) 4420–4427. doi:10.1016/j.ijhydene.2010.01.100.

[39] C. Lu, Z. Chen, High-temperature resistive hydrogen sensor based on thin nanoporous rutile TiO2 film on anodic aluminum oxide,

Sensors Actuators B Chem. 140 (2009) 109–115. doi:10.1016/j.snb.2009.04.004. [40] J. Lee, D.H. Kim, S.-H. Hong, J.Y. Jho, A hydrogen gas sensor employing vertically aligned TiO2 nanotube arrays prepared by

template-assisted method, Sensors Actuators B Chem. 160 (2011) 1494–1498. doi:10.1016/j.snb.2011.08.001.

[41] S. Phanichphant, C. Liewhiran, K. Wetchakun, A. Wisitsoraat, A. Tuantranont, Flame-Made Nb-Doped TiO2 Ethanol and Acetone Sensors, Sensors. 11 (2011) 472–484. doi:10.3390/s110100472.

[42] L.F. Dong, Z.L. Cui, Z.K. Zhang, Gas sensing properties of nano-ZnO prepared by arc plasma method, Nanostructured Mater. 8 (1997)

815–823. doi:10.1016/S0965-9773(98)00005-1. [43] A.S. Mukasyan, P. Epstein, P. Dinka, Solution combustion synthesis of nanomaterials, Proc. Combust. Inst. 31 II (2007) 1789–1795.

doi:10.1016/j.proci.2006.07.052.

[44] A. Mukasyan, C. Lau, A. Varma, Influence of Gravity on Combustion Synthesis of Advanced Materials, AIAA J. 43 (2005) 225–245. doi:10.2514/1.8972.

[45] A. Kopp Alves, C.P. Bergmann, F.A. Berutti, Novel Synthesis and Characterization of Nanostructured Materials, (2013).

doi:10.1007/978-3-642-41275-2. [46] K. Patil, S.T. Aruna, S. Ekambaram, Combustion synthesis, Curr. Opin. Solid State Mater. Sci. 2 (1997) 158–165. doi:10.1016/S1359-

0286(97)80060-5.

[47] K.C. Patil, S.T. Aruna, T. Mimani, Combustion synthesis: An update, Curr. Opin. Solid State Mater. Sci. 6 (2002) 507–512.

doi:10.1016/S1359-0286(02)00123-7.

[48] J.I. Steinfeld, J.C. Jang, J.S. Haggerty, H.W. Galibarith, W.C. Danen, W.R. Cannon, J.R. Ackerhalt, Laser-Induced Chemical

Processes, Plennum Press, New York and London, 1981. [49] H.M. Smith, A.F. Turner, Vacuum Deposited Thin Films Using a Ruby Laser, Appl. Opt. 4 (1965) 147. doi:10.1364/AO.4.000147.

[50] E. Morintale, C. Constantinescu, M. Dinescu, Thin films development by pulsed laser-assisted deposition, Ann. Univ. Craiova, Phys.

20 (2010) 43–56. [51] G.K. Chrisey, D.B. and Hubler, Pulsed laser deposition of thin films, 1st ed., Wiley-Interscience, 1994.

[52] R. Eason, Pulsed Laser Deposition of Thin Films, Wiley, 2006. doi:10.1002/9780470052129.ch11. [53] E. Leveugle, L. V. Zhigilei, Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted

pulsed laser evaporation, J. Appl. Phys. 102 (2007) 074914. doi:10.1063/1.2783898.

[54] A. Sellinger, E. Leveugle, J.M. Fitz-Gerald, L. V. Zhigilei, Generation of surface features in films deposited by matrix-assisted pulsed laser evaporation: the effects of the stress confinement and droplet landing velocity, Appl. Phys. A. 92 (2008) 821–829.

doi:10.1007/s00339-008-4582-3.

[55] F. Bloisi, M. Barra, A. Cassinese, L.R.M. Vicari, Matrix-Assisted Pulsed Laser Thin Film Deposition by Using Nd:YAG Laser, J. Nanomater. 2012 (2012) 1–9. doi:10.1155/2012/395436.

[56] A.P. Caricato, A. Luches, R. Rella, Nanoparticle Thin Films for Gas Sensors Prepared by Matrix Assisted Pulsed Laser Evaporation,

Sensors. 9 (2009) 2682–2696. doi:10.3390/s90402682. [57] C.N. Hunter, M.H. Check, J.E. Bultman, A.A. Voevodin, Development of matrix-assisted pulsed laser evaporation (MAPLE) for

deposition of disperse films of carbon nanoparticles and gold/nanoparticle composite films, Surf. Coatings Technol. 203 (2008) 300–

306. doi:10.1016/j.surfcoat.2008.09.003. [58] T. Smausz, G. Megyeri, R. Kékesi, C. Vass, E. György, F. Sima, I.N. Mihailescu, B. Hopp, Comparative study on Pulsed Laser

Deposition and Matrix Assisted Pulsed Laser Evaporation of urease thin films, Thin Solid Films. 517 (2009) 4299–4302.

doi:10.1016/j.tsf.2008.11.141. [59] V. Califano, F. Bloisi, L.R.M. Vicari, D.M. Yunos, X. Chatzistavrou, A.R. Boccaccini, Matrix Assisted Pulsed Laser Evaporation

(MAPLE) of Poly(D,L lactide) (PDLLA) on Three Dimensional Bioglass® Structures, Adv. Eng. Mater. 11 (2009) 685–689.

doi:10.1002/adem.200900092. [60] D.M. Bubb, A.O. Sezer, J. Gripenburg, B. Collins, E. Brookes, Assessing the effect of the matrix in resonant infrared MAPLE, Appl.

Surf. Sci. 253 (2007) 6465–6470. doi:10.1016/j.apsusc.2007.01.109.

[61] A. Luches, A.P. Caricato, Matrix assisted pulsed laser evaporation: the surface cluster problem, Appl. Phys. B. 105 (2011) 503–508. doi:10.1007/s00340-011-4519-y.

[62] F. Bloisi, A. Cassinese, R. Papa, L. Vicari, V. Califano, Matrix-Assisted Pulsed Laser Evaporation of polythiophene films, Thin Solid

Films. 516 (2008) 1594–1598. doi:10.1016/j.tsf.2007.03.159. [63] G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Tunneling through a controllable vacuum gap, Appl. Phys. Lett. 40 (1982) 178–180.

doi:10.1063/1.92999.

[64] G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Surface studies by scanning tunneling microscopy, Phys. Rev. Lett. 49 (1982) 57–61. doi:10.1103/PhysRevLett.49.57.

[65] C.W. Oatley, The early history of the scanning electron microscope, J. Appl. Phys. 53 (1982). doi:10.1063/1.331666.

[66] A. Moldovan, Materials Characterization at the Micro- and Nanoscale by Atomic Force Microscopy and Scanning Polarization Force Microscopy, University Politehnica of Bucharest, n.d.

[67] M. Scarisoreanu, I. Morjan, R. Alexandrescu, C.T. Fleaca, A. Badoi, E. Dutu, A.M. Niculescu, C. Luculescu, E. Vasile, J. Wang, S.

Bouhadoun, N. Herlin-Boime, Enhancing the visible light absorption of titania nanoparticles by S and C doping in a single-step process, Appl. Surf. Sci. 302 (2014) 11–18. doi:10.1016/j.apsusc.2014.01.135.

[68] R. Alexandrescu, M. Scarisoreanu, I. Morjan, R. Birjega, C. Fleaca, C. Luculescu, I. Soare, O. Cretu, C.C. Negrila, N. Lazarescu, V.

Ciupina, Preparation and characterization of nitrogen-doped TiO2nanoparticles by the laser pyrolysis of N2O-containing gas mixtures, Appl. Surf. Sci. 255 (2009) 5373–5377. doi:10.1016/j.apsusc.2008.08.046.

[69] R. Alexandrescu, I. Morjan, F. Dumitrache, R. Birjega, C. Fleaca, C.R. Luculescu, E. Popovici, I. Soare, I. Sandu, E. Dutu, G. Prodana,

Development of Fe-doped SnO2- based nanocomposites prepared by single-step laser pyrolysis, J. Optoelectron. Adv. Mater. 12 (2010) 599–604.

[70] M. Scarisoreanu, C. Fleaca, I. Morjan, A.M. Niculescu, C. Luculescu, E. Dutu, A. Ilie, I. Morjan, L.G. Florescu, E. Vasile, C.I. Fort,

High photoactive TiO2/SnO2nanocomposites prepared by laser pyrolysis, Appl. Surf. Sci. 418 (2017) 491–498.

doi:10.1016/j.apsusc.2016.12.122.

[71] F. Ténégal, I. Voicu, X. Armand, N. Herlin-Boime, C. Reynaud, Residence time effect on fullerene yield in butadiene-based laser pyrolysis flame, Chem. Phys. Lett. 379 (2003) 40–46. doi:10.1016/j.cplett.2003.08.010.

[72] R.A. Spurr, H. Myers, Quantitative Analysis of Anatase-Rutile Mixtures with an X-Ray Diffractometer, Anal. Chem. 29 (1957) 760–

762. doi:10.1021/ac60125a006. [73] O. Ozdemir, D.J. Dunlop, Hysteresis and coercivity of hematite, J. Geophys. Res. Solid Earth. (2014) 2582–2594.

doi:10.1002/2013JB010739.Received.

[74] M. Enhessari, M.K. Razi, L. Etemad, A. Parviz, M. Sakhaei, Structural , optical and magnetic properties of the Fe 2 TiO 5 nanopowders, 8080 (2014). doi:10.1080/17458080.2011.649432.

[75] C. Teodor, F. Dumitrache, I. Morjan, R. Alexandrescu, C. Luculescu, A. Niculescu, E. Vasile, V. Kuncser, Applied Surface Science

Novel Fe @ C – TiO 2 and Fe @ C – SiO 2 water-dispersible magnetic nanocomposites, 278 (2013) 284–288. [76] C. Caizer, I. Hrianca, The temperature dependence of saturation magnetization of γ -Fe 2 O 3 / SiO 2 magnetic nanocomposite, 122

(2003) 115–122. doi:10.1002/andp.200310008.

[77] F.I. Hamzah, K. Khalid, Better adsorption capacity of SnO 2 nanoparticles with different graphene addition Better adsorption capacity of SnO 2 nanoparticles with different graphene addition, (2016). doi:10.1088/1742-6596/776/1/012039.

[78] A. Ayeshamariam, S. Ramalingam, M. Bououdina, M. Jayachandran, Preparation and Characterizations of SnO2 Nanopowder and

Spectroscopic (FT-IR, FT-Raman, UV-Visible and NMR) analysis using HF and DFT calculations, Spectrochim. ACTA PART A Mol. Biomol. Spectrosc. (2013). doi:10.1016/j.saa.2013.09.030.

[79] A. Lassoued, M. Saber, B. Dkhil, S. Ammar, Physica E : Low-dimensional Systems and Nanostructures Synthesis , structural ,

morphological , optical and magnetic characterization of iron oxide ( α -Fe 2 O 3 ) nanoparticles by precipitation method : Effect of

varying the nature of precursor, Phys. E Low-Dimensional Syst. Nanostructures. 97 (2018) 328–334. doi:10.1016/j.physe.2017.12.004.

[80] J. Cai, S. Chen, J. Hu, Organic additive-free synthesis of mesocrystalline hematite nanoplates via two-dimensional oriented attachment,

(2014). doi:10.1039/c3ce41716f. [81] J.-G. Lee, J.Y. Park, Y.-J. Oh, C.S. Kim, Magnetic properties of CoFe2O4 thin films prepared by a sol-gel method, J. Appl. Phys. 84

(1998) 2801–2804. doi:10.1063/1.368393.

[82] M. Houshiar, F. Zebhi, Z.J. Razi, A. Alidoust, Z. Askari, Synthesis of cobalt ferrite (CoFe2O4) nanoparticles using combustion, coprecipitation, and precipitation methods: A comparison study of size, structural, and magnetic properties, J. Magn. Magn. Mater. 371

(2014). doi:10.1016/j.jmmm.2014.06.059. [83] C.T. Fleaca, I. Morjan, R. Alexandrescu, F. Dumitrache, I. Soare, L. Gavrila-Florescu, F. Le Normand, A. Derory, Magnetic properties

of core-shell catalyst nanoparticles for carbon nanotube growth, Appl. Surf. Sci. 255 (2009) 5386–5390.

doi:10.1016/j.apsusc.2008.10.078. [84] R. Zhang, L. Sun, Z. Wang, W. Hao, E. Cao, Y. Zhang, Dielectric and magnetic properties of CoFe2O4prepared by sol-gel auto-

combustion method, Mater. Res. Bull. 98 (2018) 133–138. doi:10.1016/j.materresbull.2017.08.006.

[85] E. Roduner, Size matters: Why nanomaterials are different, Chem. Soc. Rev. 35 (2006) 583–592. doi:10.1039/b502142c. [86] D.F. J.L. Dormann, Magnetic Properties of fine particles, 1992.

[87] M. Kishimoto, Y. Sakurai, T. Ajima, Magneto‐optical properties of Ba‐ferrite particulate media, J. Appl. Phys. 76 (1994) 7506–7509. doi:10.1063/1.357981.

[88] I.M.L. Billas, A. Chatelain, W.A. De Heer, Magnetism tram the Atom to the Bulk in Iran, Cobalt, and Nickel Clusters, 265 (1994)

1682–1684. doi:10.1126/science.265.5179.1682. [89] S. Son, M. Taheri, E. Carpenter, V.G. Harris, M.E. McHenry, Synthesis of ferrite and nickel ferrite nanoparticles using radio-frequency

thermal plasma torch, J. Appl. Phys. 91 (2002) 7589–7591. doi:10.1063/1.1452705.

[90] Spinel Nickel Ferrite Nanoparticles Strongly Cross-linked with Multiwalled Carbon Nanotube as a Bi-efficient Electrocatalyst for the Oxygen Reduction and Oxygen Evolution, RSC Advances, RSC Adv. (2013). doi:10.1039/C5RA14713A.

[91] R. Kayestha, K. Hajela, ESR studies on the effect of ionic radii on displacement of Mn 2+ bound to a soluble fl-galactoside binding

hepatic lectin, 368 (1995) 285–288. [92] E.T. Selvi, S.M. Sundar, Effect of replacing Sn 4 + ions by Zn 2 + ions on structural , optical and magnetic properties of SnO 2

nanoparticles, Appl. Phys. A. 123 (2017) 1–11. doi:10.1007/s00339-017-0995-1.

[93] F. Deganello, A. Kumar, Progress in Crystal Growth and Characterization of Materials Solution combustion synthesis , energy and environment : Best parameters for better materials, Prog. Cryst. Growth Charact. Mater. 64 (2018) 23–61.

doi:10.1016/j.pcrysgrow.2018.03.001.