Effect of method of synthesis and different Effect of method of synthesis and different metal doping on the photocatalytic activity of metal doping on the photocatalytic activity of

titaniatitania

Department of ChemistryDepartment of Chemistry

Anna University Anna University

ChennaiChennai

P. VijayanP. Vijayan

In the year 1972, Fujishima and Honda discovered the photocatalytic In the year 1972, Fujishima and Honda discovered the photocatalytic splitting of water on titania semiconductor electrode. Later on Sato and splitting of water on titania semiconductor electrode. Later on Sato and White decomposed water on platinum loaded TiOWhite decomposed water on platinum loaded TiO2 2 powder. powder.

After that researchers concentrated towards the usage of semiconductor After that researchers concentrated towards the usage of semiconductor photocatalysis for environmental protection.photocatalysis for environmental protection.

Photocatalysis has been exploited for various environmental process Photocatalysis has been exploited for various environmental process such as deodorization, water purification, air purification, sterilization such as deodorization, water purification, air purification, sterilization and soil proof. and soil proof.

Many researchers have put their effort on degradation and Many researchers have put their effort on degradation and mineralization of variety of toxic substances by photocatalytic oxidation. mineralization of variety of toxic substances by photocatalytic oxidation. Carbon dioxide has been reduced efficiently with water by Carbon dioxide has been reduced efficiently with water by photocatalysis. photocatalysis.

Different types of semiconductors were used as photocatalyst. Among Different types of semiconductors were used as photocatalyst. Among these titania is the widely used photocatalyst due to its strong oxidizing these titania is the widely used photocatalyst due to its strong oxidizing power, stable at different pH and favorable band gap energy.power, stable at different pH and favorable band gap energy.

PhotocatalysisPhotocatalysis

33

PhotocatalystsPhotocatalysts and their band gap energyand their band gap energy

SemiconductorValence

bandConductance

bandBand gap

(eV)Band gap

wavelength (nm)

SnO2 +4.1 +0.3 3.8 318

ZnO +3.0 -0.2 3.2 390

ZnS +1.4 -2.3 3.7 336

CdS +2.1 -0.4 2.5 497

CdSe +1.6 -0.1 1.7 730

TiO2 +3.1 -0.1 3.0 380

GaP +1.3 -1.0 2.3 540

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Even though titania is the widely used semiconductor, it has some disadvantage like Even though titania is the widely used semiconductor, it has some disadvantage like low surface area, fast recombination and wavelength maximum lies in UV region.low surface area, fast recombination and wavelength maximum lies in UV region.

Commercial titania P-25 has average particle size of 40-50nm. Since larger particles Commercial titania P-25 has average particle size of 40-50nm. Since larger particles increases the specific surface area of the catalyst, reduction of particle size become increases the specific surface area of the catalyst, reduction of particle size become essential.essential.

Recombination are of two types (i) surface recombination (ii) volume Recombination are of two types (i) surface recombination (ii) volume recombination.recombination.

Surface recombination can be avoided by doping some foreign material like metal, Surface recombination can be avoided by doping some foreign material like metal, non-metals and coupling titania with another semiconductor. Volume recombination non-metals and coupling titania with another semiconductor. Volume recombination can be reduced by preparing titania in nanoscale. Wavelength of maximum can also can be reduced by preparing titania in nanoscale. Wavelength of maximum can also increased to higher wavelength by doping metals or non-metals.increased to higher wavelength by doping metals or non-metals.

It is well known that wavelength and surface area are mainly depend on particle size. It is well known that wavelength and surface area are mainly depend on particle size. Altering the size of the particle alters the degree of confinement of the electrons, and affects the electronic structure of the solid, in particular ‘band edges’, which are tunable with particle size.

From the above fact that the synthesis of titania with controlled nanometer From the above fact that the synthesis of titania with controlled nanometer dimension and doped with altervalent ion is essential.dimension and doped with altervalent ion is essential.

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Size Dependence of Light AbsorptionSize Dependence of Light Absorption

300 400 500 600 7000

1

2

3

4 (a)

a1 (12nm) a2 (17nm) a3 (23nm)

Abs

orba

nce

Wavelength

300 400 500 600 700

a3

a2a1

Ismat shah etal studied the effect of particle size for the degradation of 2-chlorophenol using titania synthesized by metallo organic chemical vapour deposition method with different particle size.

Particle size decrease from 29 to 17 nm showed red shift and further decrease i.e less than 17nm leads to blue shift.

Titania with 17nm have shown remarkable catalytic activity towards the degradation of 2-chlorophenol than the catalysts having 23nm and 12nm.

Comparison of band gaps B17nm < B12nm < B23nm

Appl.Catal.B:Environ, Vol. 68, 2006, Pages. 1-11

66

Methods usedMethods used

Sol-gel Sol-gel

Liquid phase depositionLiquid phase deposition

Pulse laser depositionPulse laser deposition

Chemical vapour depositionChemical vapour deposition

Magnetic sputteringMagnetic sputtering

Direct hydrothermal methodDirect hydrothermal method

77

Sol-gelSol-gel

Generation of a dispersion of colloidal particles suspended in Brownian Generation of a dispersion of colloidal particles suspended in Brownian motion within a fluid matrix.motion within a fluid matrix.

Colloids are suspension of particles of linear dimensions between 1nm Colloids are suspension of particles of linear dimensions between 1nm and 1 and 1 m. The colloidal suspensions can subsequently convert to viscous m. The colloidal suspensions can subsequently convert to viscous gels and then to solid materialsgels and then to solid materials

Sol-gel preparation leads to the greatest possible homogeneous Sol-gel preparation leads to the greatest possible homogeneous distribution of the dopant ion in the host matrix.distribution of the dopant ion in the host matrix.

Products have high purity and homogeneity, ease of processing and Products have high purity and homogeneity, ease of processing and composition controlcomposition control

Sol-gel synthesis involving following stepsSol-gel synthesis involving following steps AgeingAgeing GelationGelation DryingDrying Densification Densification

88

Hydrothermal treatmentHydrothermal treatment

The hydrothermal process including aqueous solvents as reaction medium is The hydrothermal process including aqueous solvents as reaction medium is eco-friendly since it is carried out in a closed system and the contents can be eco-friendly since it is carried out in a closed system and the contents can be recovered and reused after cooling down to room temperature.recovered and reused after cooling down to room temperature.

The equipment and processing required are simpler and reaction is low energy The equipment and processing required are simpler and reaction is low energy consumption, consumption,

By controlling hydrothermal temperature and duration of the treatment, various By controlling hydrothermal temperature and duration of the treatment, various crystalline products with different composition, structure and morphology could crystalline products with different composition, structure and morphology could be obtained. be obtained.

Fine particle size can be obtained with more uniform distribution and high Fine particle size can be obtained with more uniform distribution and high dispersion either in polar and nonpolar solventsdispersion either in polar and nonpolar solvents

In this way the energy band structure becomes discrete and titania nanoparticles In this way the energy band structure becomes discrete and titania nanoparticles exhibit improved optical and photocatalytic properties.exhibit improved optical and photocatalytic properties.

99

Sol-gel +hydrothermalSol-gel +hydrothermal

Materials can be obtained with high purityMaterials can be obtained with high purity

Particle size can be reduced and controlledParticle size can be reduced and controlled

Highly crystalline material can be obtainedHighly crystalline material can be obtained

Material with higher light harvesting characterMaterial with higher light harvesting character

Phase formation can be decided before the heat treatment i.e, calcinationPhase formation can be decided before the heat treatment i.e, calcination

Eco-friendlyEco-friendly

1010

2,4,6-Trichlorophenol2,4,6-Trichlorophenol

Chlorophenols are deadly toxic compounds, present in wastewater which Chlorophenols are deadly toxic compounds, present in wastewater which mainly arise from chemical intermediates or by-products in petrochemical, mainly arise from chemical intermediates or by-products in petrochemical, paper making, plastic, pesticidal and water disinfectionpaper making, plastic, pesticidal and water disinfection

PCDD/PCDF are the most toxic chlorinated substances which are 1000 times PCDD/PCDF are the most toxic chlorinated substances which are 1000 times poisonous than arsenic. Due to handling problem of these chemicals in the poisonous than arsenic. Due to handling problem of these chemicals in the laboratory, the model pollutant such as chlorophenol and chlorobenzene laboratory, the model pollutant such as chlorophenol and chlorobenzene which are having structural similarities with them are takenwhich are having structural similarities with them are taken

19 different chlorophenols have been listed by the US-EPA as priority 19 different chlorophenols have been listed by the US-EPA as priority pollutants pollutants

Among these 2,4,6-trichlorophenol is one of the most vulnerable water Among these 2,4,6-trichlorophenol is one of the most vulnerable water pollutants, which causes serious damage to the vital organs of human beings . pollutants, which causes serious damage to the vital organs of human beings . Thus, the removal of the chlorophenols from the waste water is highly Thus, the removal of the chlorophenols from the waste water is highly imperative.imperative.

In this work, we have used a modified sol-gel followed by hydrothermal In this work, we have used a modified sol-gel followed by hydrothermal treatment to synthesize metals (Fe, Cr, Co and Zr ) doped titania and used treatment to synthesize metals (Fe, Cr, Co and Zr ) doped titania and used them for the oxidative degradation of 2,4,6-TCP.them for the oxidative degradation of 2,4,6-TCP.

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ExperimentalExperimental

Pure titania was synthesized by sol-gel, hydrothermal and combination of Pure titania was synthesized by sol-gel, hydrothermal and combination of these two methods these two methods

Parameters affecting synthesis such as Water : Ethanol ratio, hydrothermal Parameters affecting synthesis such as Water : Ethanol ratio, hydrothermal temperature and duration were optimized for effective synthesis.temperature and duration were optimized for effective synthesis.

Doped titania were synthesized by sol-gel + hydrothermal method with Doped titania were synthesized by sol-gel + hydrothermal method with various weight percentage of loading (0.3, 0.5, 0.7 and 1wt%).various weight percentage of loading (0.3, 0.5, 0.7 and 1wt%).

ReactorReactor

Slurry photoreactorSlurry photoreactor

8x8w lamp 8x8w lamp

Emits 365nm wave lengthEmits 365nm wave length

1212

Procedure adopted------Procedure adopted------Sol-gelSol-gel

1313

Procedure adopted-Procedure adopted-------hydrothermalhydrothermal

1414

Procedure adopted----Procedure adopted----sol-gel + hydrothermalsol-gel + hydrothermal

1515

Effect of synthesis parametersEffect of synthesis parameters

1616

XRD pattern of (a) sol-gel (b) hydrothermal and (c ) XRD pattern of (a) sol-gel (b) hydrothermal and (c ) sol-gel + hydrothermal synthesized titaniasol-gel + hydrothermal synthesized titania

In sol-gel, hydrothermal and In sol-gel, hydrothermal and combination of sol-gel and combination of sol-gel and hydrothermal methods, only hydrothermal methods, only anatase phase is formed.anatase phase is formed.

Particle size and Crystallinity of Particle size and Crystallinity of titania with different method of titania with different method of synthesis follows the ordersynthesis follows the order sol-sol-gel< sol-gel + hydrothermal < gel< sol-gel + hydrothermal < hydrothermal.hydrothermal.

Most of the particle are in same Most of the particle are in same size in both the hydrothermal size in both the hydrothermal and sol-gel + hydrothermal and sol-gel + hydrothermal methodsmethods

In sol-gel restriction of particle In sol-gel restriction of particle size within a range is not size within a range is not possible. Particles are in wide possible. Particles are in wide range of sizes.range of sizes.

20 30 40 50 60 70 80

2 Theta (degree)

Inte

nsit

y (a

.u)

sol-gel

Hydrothermal

sol-gel + hydrothermal

1717

Histogram of (a) sol-gel (b) hydrothermal and (c) Histogram of (a) sol-gel (b) hydrothermal and (c) sol-gel + hydrothermalsol-gel + hydrothermal

(a) (b)

(c)

1818

UV-Vis DRS spectra of (a) sol-gel (b) hydrothermal and UV-Vis DRS spectra of (a) sol-gel (b) hydrothermal and (c )sol-gel + hydrothermal synthesized titania(c )sol-gel + hydrothermal synthesized titania

0

0.4

0.8

1.2

1.6

2

200 300 400 500 600 700 800

Wavelength (nm)

Abs

orba

nce

sol-gel + hydrothermal

sol-gel

hydrothermal

Titania synthesized by sol-gel (11nm), Titania synthesized by sol-gel (11nm), hydrothermal (24 nm) and hydrothermal (24 nm) and combination of these two methods combination of these two methods (17nm) show different electrical and (17nm) show different electrical and optical propertiesoptical properties

Titania synthesized by combination of Titania synthesized by combination of sol-gel followed by hydrothermal sol-gel followed by hydrothermal show red shift while titania show red shift while titania synthesized by sol-gel and synthesized by sol-gel and hydrothermal separately show blue hydrothermal separately show blue shiftshift

From the above observation it can be From the above observation it can be concluded that the particle size greatly concluded that the particle size greatly influenced on the electrical and optical influenced on the electrical and optical properties of the catalystsproperties of the catalysts

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XRD pattern of metals (Fe, Cr, Co and Zr) doped XRD pattern of metals (Fe, Cr, Co and Zr) doped titaniatitania

Metals doping does not alter the Metals doping does not alter the phase formationphase formation

Metal doping reduces the crystallite Metal doping reduces the crystallite growthgrowth

There is no peak observed for the There is no peak observed for the formation of metal oxideformation of metal oxide

Particle size was calculated using Particle size was calculated using scherrer equation. Among the scherrer equation. Among the metals doped, cobalt doped titania metals doped, cobalt doped titania having the smaller size.having the smaller size.

20 30 40 50 60 70 80

2 Theta (degree)

Inte

nsity

(a.u

)

Co/TiO2 Cr/TiO2

Fe/TiO2 Zr/TiO2

2020

UV-Visible DRS of metals doped titaniaUV-Visible DRS of metals doped titania

Cr, Fe and Co doping shifted the Cr, Fe and Co doping shifted the wavelength maximum towards wavelength maximum towards visible region. Whereas visible region. Whereas zirconium showed blue shift zirconium showed blue shift when compared with bare when compared with bare titania.titania.

The band gap energies of Co, Cr, The band gap energies of Co, Cr, Fe and Zr are Fe and Zr are 2.85, 2.92, 2.95 2.85, 2.92, 2.95 and 3.18 eVand 3.18 eV respectively. respectively.

Among the different metal Among the different metal doping, cobalt doped titania doping, cobalt doped titania shows an extra absorption at shows an extra absorption at 500-700nm.500-700nm.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

200 300 400 500 600 700 800

Wavelength (nm)

Abs

orba

nce

Cr/TiO2 Co/TiO2

Fe/TiO2 Zr/TiO2

2121

EPR spectral analysis of metals doped titaniaEPR spectral analysis of metals doped titania

(A) (B)

(C)

Spectra shows the EPR signal of (A) Fe/TiO2 (B) Co/TiO2 and (C) Cr/TiO2 catalyst respectively

All the spectra show only one signal corresponding to (A) Fe3+, (B) Co2+ and (C) Cr3+ in anatase matrix respectively.

2222

Transmission Electron MicroscopyTransmission Electron Microscopy

2323

Extent of degradationExtent of degradation

Among the different metal ion doped, cobalt doped titania which is having smaller particle size and increased wavelength absorption showed higher activity

Iron and chromium doped titania are almost same in catalytic activity. Among these two, iron doped titania shows better activity.

Zirconium doped titania showed poor catalytic activity when compared with others. Wavelength maximum shifts to blue line of the spectrum as zirconium concentration is increased may be the reason for this.

0

20

40

60

80

100

0 30 60 90 120 150 180

Time in minutes

% o

f Con

cent

ratio

n

Co/TiO2 Fe/TiO2

Zr/TiO2 Cr/TiO2

2424

Extent of mineralizationExtent of mineralization

Mineralization profile follows the same order with degradation profile

Rate of Mineralization follows the order: Cobalt > Iron > Chromium > Zirconium

0

0.2

0.4

0.6

0.8

1

1.2

0 30 60 90 120 150 180

Time in minutes

C/C

o

Co/TiO2 Fe/TiO2

Cr/TiO2 Zr/TiO2

2525

ConclusionConclusion Sol-gel method can be used for the preparation of nanosized titania with Sol-gel method can be used for the preparation of nanosized titania with

high purity but size restriction is not possible anywhere in the preparation high purity but size restriction is not possible anywhere in the preparation procedure. Sol-gel method does not produce phase purityprocedure. Sol-gel method does not produce phase purity

Titania synthesized by hydrothermal method have more uniform Titania synthesized by hydrothermal method have more uniform distribution in particle size. High purity and reduced particle size is not distribution in particle size. High purity and reduced particle size is not possible in hydrothermal methods.possible in hydrothermal methods.

Titania synthesized by combination of these two methods have the Titania synthesized by combination of these two methods have the advantages of sol-gel and hydrothermal methods and overcame the advantages of sol-gel and hydrothermal methods and overcame the disadvantages.disadvantages.

Catalysts synthesized by combination methods have lower band gap Catalysts synthesized by combination methods have lower band gap

energy and controlled particle size ( 15-20 nm) than sol-gel and energy and controlled particle size ( 15-20 nm) than sol-gel and hydrothermal methods.hydrothermal methods.

These observations conclude that titania synthesized by the combination These observations conclude that titania synthesized by the combination method is the opt one for photocatalytic activity.method is the opt one for photocatalytic activity.

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Iron, cobalt and chromium doping shifts the wavelength absorption Iron, cobalt and chromium doping shifts the wavelength absorption maximum in to visible region whereas zirconium doping shifts towards maximum in to visible region whereas zirconium doping shifts towards UV region.UV region.

From the XRD patterns of metal doped titania, metal doping doesn’t affect From the XRD patterns of metal doped titania, metal doping doesn’t affect the phase formation of core species i.e titania. But restricted the crystallite the phase formation of core species i.e titania. But restricted the crystallite growth and thereby reduced the particle size. growth and thereby reduced the particle size.

From the EPR spectra, all the dopant ion present in anatase matrix and not From the EPR spectra, all the dopant ion present in anatase matrix and not in the surface is confirmed. in the surface is confirmed.

From the TEM histogram, the uniform distribution of particle size From the TEM histogram, the uniform distribution of particle size acquired by combination methods is confirmed. From the TEM picture, acquired by combination methods is confirmed. From the TEM picture, particle size of all the doped catalyst is ranging between 15-20nm is particle size of all the doped catalyst is ranging between 15-20nm is observed.observed.

Cobalt doped titania have smaller particle size and higher wavelength of Cobalt doped titania have smaller particle size and higher wavelength of absorption than the other doped titania.absorption than the other doped titania.

Above synthesized catalysts was tested for the photocatalytic oxidative Above synthesized catalysts was tested for the photocatalytic oxidative degradation of 2, 4, 6- Trichlorophenol in aqueous suspension.degradation of 2, 4, 6- Trichlorophenol in aqueous suspension.

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From the degradation and mineralization profiles, cobalt shows higher From the degradation and mineralization profiles, cobalt shows higher catalytic activity than other doped catalysts.catalytic activity than other doped catalysts.

Iron and chromium doped titania shows almost similar activity profile, but Iron and chromium doped titania shows almost similar activity profile, but iron doping shows higher catalytic activity.iron doping shows higher catalytic activity.

Zirconium doped titania which showed blue shift as the metal content Zirconium doped titania which showed blue shift as the metal content increased shows poor catalytic activity.increased shows poor catalytic activity.

On comparison with other dopant ion, cobalt shows higher catalytic activity On comparison with other dopant ion, cobalt shows higher catalytic activity even at higher concentration (1 wt%).even at higher concentration (1 wt%).

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