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MCC025 MCC025 NanoscienceNanoscience

Introduction to Environmental Nanotechnology (3)Introduction to Environmental Nanotechnology (3)

(http://fy.chalmers.se/(http://fy.chalmers.se/~fogelstr/GordonYang2006.html)~fogelstr/GordonYang2006.html)

Professor Gordon C. C. YangProfessor Gordon C. C. Yang

Institute of Environmental Engineering Institute of Environmental Engineering National Sun National Sun YatYat--SenSen UniversityUniversity

Kaohsiung 80424, TaiwanKaohsiung 80424, Taiwanee--mailmail:: gordon@mail.nsysu.edu.twgordon@mail.nsysu.edu.tw

September 2006September 2006

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PhotocatalystsPhotocatalysts for Applications in Environmental Treatment and for Applications in Environmental Treatment and SensingSensing

(September 13, 2006)(September 13, 2006)

In this lecture the principles of In this lecture the principles of photocatalysisphotocatalysis will be introduced will be introduced first. Then various commonly used first. Then various commonly used photocatalystsphotocatalysts and their and their applications in environmental treatment and sensing will be applications in environmental treatment and sensing will be followed.followed.

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Photocatalysis: IntroductionPhotocatalysis: Introduction

Lotus Effect -- For microrough surfaces having contact angles greater than 130o, the adhesion of water and particles on these surfaces is extremely reduced. Water contacts such surfaces will be immediately contracted to droplets. The particles of contaminants adhere to the water droplets would be removed from the rough surface when the droplets roll of.

Upon exposure to the sun's ultraviolet radiation, photocatalysts (e.g., TiO2 and ZnO) are capable of decomposing organic materials and creating the super-hydrophilic properties on the surface of materials. As a result, it would yield the effects of anti-fouling, anti-fogging, sterilizing, deodorizing, air purifying and water purifying functionality.

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((Source: Wikipedia))

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Lotus EffectLotus Effect

((Benedix et al., 2000))

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Photocatalysis: IntroductionPhotocatalysis: Introduction

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Principles of Heterogeneous PhotocatalysisPrinciples of Heterogeneous Photocatalysis((Benedix et al., 2000))

A heterogeneous photocatalytic system consists of semiconductor particles (photocatalyst) which are in close contact with a liquid or gaseous reaction medium. Exposing the catalyst to light excited states would be generated yielding subsequent processes like redoxreactions and molecular transformations.Due to their electronic structure, which is characterized by a filled valence band (VB) and an empty conduction band (CB), semiconductors (metal oxides or sulfides as ZnO, CdS, TiO2, Fe2O3, and ZnS) can act as sensitizers for light-induced redox processes. The energy difference between the lowest energy level of the CB and the highest energy level of the VB is the so-called band gap energy Eg. It corresponds to the minimum energy of light required to make the material electrically conductive.

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Photochemical Excitation of a PhotocatalystPhotochemical Excitation of a Photocatalyst((Benedix et al., 2000))

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Band Gap Eneregies of Typical SemiconductorsBand Gap Eneregies of Typical Semiconductors((Benedix et al., 2000))

1010

Principles of Heterogeneous PhotocatalysisPrinciples of Heterogeneous Photocatalysis((Benedix et al., 2000))

When a photon with an energy of hν exceeds the energy of the band gap an electron (e-) is promoted from the valence band to the conduction band leaving a hole (h+) behind. In electrically conducting materials, i.e. metals, the produced charge-carriers are immediately recombined. In semiconductors a portion of this photoexcited electron-hole pairs diffuse to the surface of the catalytic particle (electron-hole pairs are trapped at the surface) and take part in the chemical reaction with the adsorbed donor (D) or acceptor (A) molecules. The holes can oxidize donor molecules (1) whereas the conduction band electrons can reduce appropriate electron acceptor molecules (2).

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Principles of Heterogeneous PhotocatalysisPrinciples of Heterogeneous Photocatalysis((Benedix et al., 2000))

A characteristic feature of semiconducting metal oxides is the strong oxidation power of their holes h+. They can react in an one-electron oxidation step with water (3) to produce the highly reactive hydroxyl radical (•OH). Both the holes and the hydroxyl radicals are very powerful oxidants, which can be used to oxidize most organic contaminants. In general, air oxygen acts as electron acceptor (4) by forming the super-oxide ion O2•−. Super-oxide ions are also highly reactive particles, which are able to oxidize organic materials.

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TiO2 exists in three crystalline forms: rutile, anatase, and brookite. Compared with rutile and brookite, anatase shows the highest photoactivity.

TiOTiO22 As A CatalystAs A Catalyst

(Chen, 2006)(Chen, 2006)

TiOTiO22

(Chou, 2006)(Chou, 2006)

TiOTiO2 2 Nanofiber FilmNanofiber Film/550/550 ℃

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Environmental Applications of TiOEnvironmental Applications of TiO22 (1)(1)((Benedix et al., 2000))

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In the field of wastewater detoxification, many systems are considered as such that the fine TiO2

photocatalyst powder was dispersed in liquid suspension. However, these systems are not easy to handle mainly due to the difficulties of removing TiO2 from the suspension after the degradation process under irradiation with UV light. A recent development of the simultaneous electrocoagulationand electrofiltration (EC/EF) process has been proven to be effective in solving this problem (Yang and Chuang, 2005).

Environmental Applications of TiOEnvironmental Applications of TiO22 (2)(2)

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Treatment of Spent Acetic Acid by TiOTreatment of Spent Acetic Acid by TiO22/UV/UV((Yang and Tsai, 2004))

400

600

800

1000

1200

1400

1600

1800

2000

0 50 100 150 200 250 300Time (min)

CO

D (m

g/L)

pH=3.21

pH=5.03

pH=9.98

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Degradation of Degradation of MethyleneMethylene Blue by TiOBlue by TiO22/UV/UV((Chen, 2006))

Photoactivity

time(min)

0 10 20 30 40 50 60 70

% m

ethy

lene

blu

e

0

20

40

60

80

100TiO2

Au/TiO2

Ag/TiO2

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Degradation of Degradation of MethyleneMethylene Blue by TiOBlue by TiO22

((Chen, 2006))

TiO2 Au/TiO2 Ag/TiO2

% D

egra

datio

n

0

20

40

60

80

100

visible lightUV light

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Simultaneous Simultaneous Electrocoagulation/ElectrofilitrationElectrocoagulation/ElectrofilitrationProcess vs. Process vs. NanoparticleNanoparticle--Containing WastewatersContaining Wastewaters

((Yang Group))

Nanosized TiO2-containing wastewater

Fab C’s oxide-CMP wastewater

EC/EF permeate

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In the last decade, for large-scale treatment reactors were designed where the titanium dioxide is fixed on a glass, ceramics or metal surface. Various types of reactors have been developed such as thin-film-fixed-bed reactors, concentration-type reactors, compound parabolic collector-type reactors, and so on. In these types of reactors industrial wastewater is passing a TiO2 coated material (glass, polystyrene, methacrylate).

LargeLarge--Scale Photocatalytic ReactorsScale Photocatalytic Reactors

2020

ThinThin--FilmFilm--FixedFixed--Bed Reactor for Wastewater TreatmentBed Reactor for Wastewater Treatment((Institute for Solar Energy Research Hamelin/Institute for Solar Energy Research Hamelin/EmmerthalEmmerthal (ISFH), University of Hannover, Germany(ISFH), University of Hannover, Germany))

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Demonstration Facility for Photocatalytic Treatment Demonstration Facility for Photocatalytic Treatment of Wastewater of Wastewater

((PlataformaPlataforma Solar de Solar de AlmerAlmerííaa (PSA), Spain(PSA), Spain))

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Solar Compound Parabolic Collectors for Solar Compound Parabolic Collectors for Photocatalytic Treatment of Wastewater Photocatalytic Treatment of Wastewater

(Source: Spainish Ministry of Science and Technology)(Source: Spainish Ministry of Science and Technology)

Overview of CADOX TechnologyOverview of CADOX Technology

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American Plans for an OnAmerican Plans for an On--Site Solar Purification SystemSite Solar Purification System(Source: ISFH, (Source: ISFH, UniversitUniversitäätt HannoverHannover))

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Photocatalytic oxidation (using TiO2) has been applied for removing and decomposing pollutants in indoor air. The used reactors trap and chemically oxidize organic compounds, converting them primarily to CO2 and water. These reactors operate at room temperature and with negligible pressure. Therefore, they may be readily integrated into new and existing heating, ventilation, and air conditioning systems.

Environmental Applications of TiOEnvironmental Applications of TiO22 (3)(3)((Benedix et al., 2000))

(Source: Bkc , Inc., Tokyo, 1999)(Source: Bkc , Inc., Tokyo, 1999)

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While TiO2 is a useful photocatalyst, it is not effective to sense the presence of organic molecules in its immediate vicinity.ZnO is another important large band gap semiconductor that has wide applications in electronic, display and catalytic devices. Its band gap and band energies are very similar to those of TiO2 (Eg = 3.2 eV; ECB= -0.5 V versus NHE (normal

hydrogen electrode)). While the photocatalytic activity of colloidal suspension has been explored in early work, its effectiveness in environmental remediation is yet to be explored fully.

NanoNano--ZnO for Environmental ApplicationsZnO for Environmental Applications((Kamat et al., 2002))

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Many researches, but not all, indicate that nano-TiO2outperforms nano-ZnO in photocatalytic treatment of organic pollutants. In most instances, however, nano-ZnO can be used either as a catalyst and/or a sensor for environmental applications.Sensors for Monitoring Pollutants and Chemicals of Concern: Acetone (Si et al., 2006; Xu et al., 2006), Ammonia (Xu et al., 2006), Benzene (Xu et al., 2006), CO (Gong et al., 2006; Xu et al., 2006), Cyclohexane (Si et al., 2006), Ethanol (Rout et al., 2006; Si et al., 2006; Xu et al., 2006), HCHO (Xu et al., 2006), H2S (Xu et al., 2006), Humidity (Zhang et al., 2005), LPG (Xuet al., 2006), Hydrogen (Rout et al., 2006), 90#Gasoline (Si et al., 2006), NO2 (Baratto et al., 2004; Shishiyanu et al., 2005), Ozone (Suchea et al., 2005), Touluene (Xu et al., 2006), and Xylene (Xu et al., 2006).

NanoNano--ZnO for Environmental Sensing ZnO for Environmental Sensing (1)(1)

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NanoNano--ZnO for Environmental Sensing ZnO for Environmental Sensing (2)(2)

((Sarala Devi et al., 2006))

ZnO Nanorods

((Xu et al., 2006))

ZnO ZnO NanoparticlesNanoparticles/500 /500 ℃

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NanoNano--ZnO for Environmental Sensing ZnO for Environmental Sensing (3)(3)

((Rout et al., 2006))

ZnO Nanotubes

((Rout et al., 2006))

ZnO ZnO NanowiresNanowires

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ZnO has an added advantage that it emits quite strongly in the visible light region. The visible emission of ZnO, which usually arises from anionic vacancies, is very sensitive to holescavengers. The emission is quantitatively quenched by hole scavengers such as iodide ions.A desirable feature for the detoxification of air and water is to develop a catalyst system that can simultaneously sense and destroy toxic chemicals. Such a catalyst system is especially useful to trigger the photocatalytic operation on demand, i.e., photocatalysis becomes operational only when the system senses the presence of a aromatic compound in the immediate surrounding. ZnO promises to be an ideal candidate for such specialized environmental applications (Scheme 1).

ZnO As a Catalyst and Sensor ZnO As a Catalyst and Sensor (1) (1) ((Kamat et al., 2002))

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ZnO As a Catalyst and Sensor ZnO As a Catalyst and Sensor (2) (2) ((Kamat et al., 2002))

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ZnO As a Catalyst and Sensor ZnO As a Catalyst and Sensor (3) (3) ((Kamat et al., 2002))

Excitation (excitation wavelength 325 nm ) and emission (emission wavelength 520 nm) spectra of ZnO film

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A “sense and shoot” approach has been developed for the photocatalytic degradation of organic contaminants from aqueous solutions. The presence of aromatic compounds such as chlorinated phenols is readily detected from the quenching of visible emission of ZnO semiconductor film. The emission quenching is quantitative and can be analyzed on the basis of adsorption equilibrium between ZnO and organic molecules. High apparent association constant values (Ka = (1-5) x 104 M-1) make possible the detection of these organic molecules at very low concentration levels. For example, 1 ppm of 4-chlorocatechol in water results in a 15% decrease of ZnO emission. Interestingly, the same ZnO film is capable of degrading aromatic compounds present in water under UV irradiation.

ZnO As a Catalyst and SensorZnO As a Catalyst and Sensor----SummarySummary (4) (4) ((Kamat et al., 2002))

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So, what is the role ofSo, what is the role ofelectrons in eelectrons in e----hh+ + pairs?pairs?

Remember, if there isRemember, if there isphotooxidation, there shouldphotooxidation, there should

exist photoreduction.exist photoreduction.Campus of National Sun Yat-Sen University, Taiwan

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If sunscreen containsIf sunscreen containsnanonano--TiOTiO22, is it safe to use it?, is it safe to use it?

Think before use it!Think before use it!

Campus of National Sun Yat-Sen University, Taiwan