Semiconductor-based Photocatalytic Hydrogen GenerationXiaobo Chen,*,†Shaohua Shen,†,‡Liejin Guo,‡and Samuel S.Mao†Lawrence Berkeley National Laboratory,Berkeley,California94720,United States,and State Key Laboratory of Multiphase Flow in PowerEngineering,Xi’an Jiaotong University,Xi’an,Shaanxi710049,ChinaReceived May29,2010Contents1.Introduction65032.Basic Principles of Photocatalytic HydrogenGeneration65052.1.Fundamental Mechanism of PhotocatalyticHydrogen Generation65052.2.Main Processes of Photocatalytic HydrogenGeneration65052.3.Evaluation of Photocatalytic Water Splitting65072.3.1.Photocatalytic Activity65072.3.2.Photocatalytic Stability65073.UV-Active Photocatalysts for Water Splitting65073.1.d0Metal Oxide Photocatalyts65073.1.1.Ti-,Zr-Based Oxides65073.1.2.Nb-,Ta-Based Oxides65143.1.3.W-,Mo-Based Oxides65173.1.4.Other d0Metal Oxides65183.2.d10Metal Oxide Photocatalyts65183.3.f0Metal Oxide Photocatalysts65183.4.Nonoxide Photocatalysts65184.Approaches to Modifying the Electronic BandStructure for Visible-Light Harvesting65194.1.Metal and Nonmetal Doping65194.1.1.Metal Ion Doping65194.1.2.Nonmetal-Ion Doping65294.1.3.Metal/Nonmetal-Ion Codoping65314.2.Controlling Band Structure through SolidSolutions65324.2.1.(Oxy)sulfide Solid Solutions65324.2.2.Oxide Solid Solutions65334.2.3.Oxynitride Solid Solutions65344.3.Dye Sensitization to Harvest Visible Light65354.3.1.Sensitization Using Ruthenium ComplexDyes65354.3.2.Sensitization Using Other Transition-MetalComplex Dyes65364.3.3.Sensitization Using Metal-Free Dyes65364.4.Developing Novel Single-PhaseVisible-Light-Responsive Photocatalysts65374.4.1.d-block Metal Oxides65384.4.2.p-block Metal Oxides65394.4.3.f-block Metal Oxides65404.4.4.Miscellaneous Photocatalysts65415.Approaches for Efficient Photogenerated ChargeSeparation65415.1.Cocatalyst Loading65415.1.1.Noble Metal Cocatalysts65425.1.2.Transition-Metal Oxide Cocatalysts65425.1.3.Nonmetal-Oxide Cocatalysts65435.2.Semiconductor Combinations65445.3.Modification of Crystal Structure andMorphology65475.3.1.Modification of Crystal Structure65475.3.2.Modification of Size and Morphology65486.Photocatalytic Hydrogen Generation Systems65526.1.Hydrogen Generation Systems ContainingSacrificial Reagents65526.1.1.Inorganic Sacrificial Reagent Systems6552anic Sacrificial Reagent System65556.2.Overall Water-Splitting Systems65566.2.1.Pure Water-Splitting System65566.2.2.Biomimetic Z-Scheme Water-SplittingSystem65567.Summary and Prospects65578.Acknowledgments65589.References6558 1.IntroductionBecause of its high energy capacity and environmental friendliness,hydrogen has been identified as a potential energy carrier in many low greenhouse gas(GHG)energy scenarios.1,2In a proposed hydrogen energy system,3hydrogen-containing compounds such as fossil fuels,biomass,or even water are potential sources of hydrogen.4-11When hydrogen is derived from hydrocarbons such as fossil fuels or biomass, CO2capture and sequestration are requirements in a low GHG scenario.12-14On the other hand,hydrogen produced from water does not present the challenge of unwanted emissions at the point of conversion,but it does require that energy be supplied from an external resource.15If this energy can be obtained from a renewable energy source such as solar energy,hydrogen can then be considered a green energy alternative capable of powering everything from laptops to submarines.Figure1shows a diagram of photocatalytic hydrogen generation in the hydrogen energy system.Such an approach to energy production is one that exhibits due concern for environmental issues and that is becoming increasingly relevant in our world.16,17However,the technol-ogy to produce hydrogen in a cost-effective,low-GHG manner has not yet been developed.Since the discovery of hydrogen evolution through the photoelectrochemical splitting of water on n-type TiO2*Corresponding author:Xiaobo Chen.E-mail:chenxiaobolbl@(Xiaobo Chen);shshen_xjtu@(Shaohua Shen);lj-guo@(Liejin Guo);SSMao@(Samuel S.Mao).†Lawrence Berkeley National Laboratory.‡Xi’an Jiaotong University.Chem.Rev.2010,110,6503–6570650310.1021/cr1001645 2010American Chemical SocietyPublished on Web11/10/2010electrodes,18the technology of semiconductor-based photo-catalytic water splitting to produce hydrogen using solar energy has been considered as one of the most important approaches to solving the world energy crisis.19Hence,the development of the necessary semiconductor photocatalysts has undergone considerable research.Over the past40years, many photocatalysts reportedly exhibited high photocatalytic activities for splitting water into a stoichiometric mixture of H2and O2(2:1by molar ratio)in the ultraviolet(UV)light region.These include La doped NaTaO3,20Sr2M2O7(M) Nb,Ta),21La2Ti2O7,22K2La2Ti3O10,23and -Ge3N4,24among others.Of particular note is the NiO/NaTaO3:La photocata-lyst,which shows the highest activity with a quantum yield amounting to56%at270nm.20However,these oxide photocatalysts are only active under UV irradiation.With respect to the solar spectrum,only a small fraction(ca.4%) of the incoming solar energy lies in the ultraviolet region, whereas the visible light in the solar spectrum is far more abundant(ca.46%).It is essential,therefore,as an alternative to UV-active photocatalysts to develop visible-light-driven photocatalysts that are stable and highly efficient for the practical,large-scale production of hydrogen using solar energy.Over the recent years,continuing breakthroughs have been made in the development of novel visible-light-driven photocatalysts,leading to the enhancement of photocatalytic activity for water splitting and inspiring great enthusiasm.A large number of semiconductor materials have been developed as photocatalysts for water splitting to hydrogen under visible-light irradiation.A significant process has been achieved on semiconductor-based photocatalytic hydrogen generation through waterDr.Xiaobo Chen is at Lawrence Berkeley National Laboratory.He obtained his Ph.D.Degree in Chemistry from Case Western Reserve University. His research interests include materials and devices development, renewable energy science and technology,environmental pollution,and health.Dr.Shaohua Shen is an assistant professor at Xi’an Jiaotong University, China.He obtained his Ph.D.Degree in Thermal Engineering from Xi’an Jiaotong University in2010.During2008-2009,he worked as a guest researcher at Lawrence Berkeley National Laboratory,U.S.A.His research interests include photocatalysis,photoelectrochemistry,and the related materials and devices development.Dr.Liejin Guo is a professor and the director of the State Key Laboratory of Multiphase Flow in Power Engineering in Xi’an Jiaotong University, China.He obtained his Ph.D.Degree in Engineering Thermophysics from Xi’an Jiaotong University in1989.His research interest includes multiphase flow,heat transfer,and renewable energy technologies.Dr.Samuel S.Mao is a career staff scientist at Lawrence Berkeley National Laboratory and an adjunct faculty at The University of California at Berkeley.He obtained his Ph.D.degree in Engineering from The University of California at Berkeley in2000.He is leading a multidisciplinary research team developing solar-active materials and devices and investigating fundamental energy conversion processes.Figure1.Schematic diagram of photocatalytic hydrogen genera-tion in the hydrogen energy system.6504Chemical Reviews,2010,Vol.110,No.11Chen etal.splitting over the past several decades,25-31and many excellent reviews have been published.32-62In this review, we aim to put together the research effort having been made so far,with a view of providing a good reference and inspiring new ideas for tackling this important challenge. Starting with a brief introduction to semiconductor-based photocatalysts for hydrogen generation from water splitting, we overview the development of high-efficiency,visible-light-driven photocatalysts.A number of synthetic and modification techniques for adjusting the band structure to harvest visible light and improve the charge separation in photocatalysis are discussed.Photocatalytic systems for water splitting are also reviewed and classified into two main kinds: sacrificial reagent-containing water-splitting systems and overall water-splitting systems.2.Basic Principles of Photocatalytic Hydrogen Generation2.1.Fundamental Mechanism of Photocatalytic Hydrogen GenerationIn Fujishima and Honda’s pioneering work,the electro-chemical cell they constructed for the decomposition of water into hydrogen and oxygen is shown in Figure2.18When the surface of the TiO2electrode was irradiated by UV light,as a result of a water oxidation reaction,oxygen evolution occurred at the TiO2electrode.Concomitant reduction led to hydrogen evolution at the platinum black electrode.This concept,which emerged from the use of photoelectrochemi-cal cells with semiconductor electrodes,was later applied by Bard to the design of a photocatalytic system using semiconductor particles or powders as photocatalysts.63-65 A photocatalyst absorbs UV and/or visible(Vis)light irradiation from sunlight or an illuminated light source.The electrons in the valence band of the photocatalyst are excited to the conduction band,while the holes are left in the valence band.This,therefore,creates the negative-electron(e-)and positive-hole(h+)pairs.This stage is referred to the semiconductor’s“photo-excited”state,and the energy dif-ference between the valence band and the conduction band is known as the“band gap”.This must correspond to the wavelength of the light for it to be effectively absorbed by the photocatalyst.After photoexcitation,the excited electrons and holes separate and migrate to the surface of photocatalyst. Here,in the photocatalytic water-splitting reaction,they act as reducing agent and oxidizing agent to produce H2and O2,respectively.A schematic representation of the principle of the photocatalytic system for water is depicted in Figure 3.Water splitting into H2and O2is an uphill reaction.It needs the standard Gibbs free energy change∆G0of237kJ/mol or1.23eV,as shown in eq1.Therefore,the band gap energy(E g)of the photocatalyst should be>1.23eV(<1000nm)to achieve water splitting. However,to use visible light,it should be<3.0eV(>400 nm).To facilitate both the reduction and oxidation of H2O by photoexcited electrons and holes,the match of the band gap and the potentials of the conduction and valence bands are important.Both the reduction and oxidation potentials of water should lie within the band gap of the photocatalyst. The bottom level of the conduction band has to be more negative than the reduction potential of H+/H2(0V vs normal hydrogen electrode(NHE)),whereas the top level of the valence band has to be more positive than the oxidation potential of O2/H2O(1.23V).Parts A and B of Figure4 show the conduction band edge and valence band edge of some oxide-and sulfide-based semiconductor materials.66We can see that there are many semiconductor systems whose electronic structures match well with the redox potential of water into hydrogen and oxygen molecules.The band structure requirement is a thermodynamic requirement for water splitting.Other factors,such as overpotentials,charge separation,mobility,and lifetime of photogenerated electrons and holes,affect the photocatalytic generation of hydrogen from water splitting as well.For example,the band edges of the semiconductor photocatalyst usually vary with the change of pH,as shown in Figure4C.66The phase stability of the semiconductor photocatalyst changes in different pH environments as well.2.2.Main Processes of Photocatalytic Hydrogen GenerationThe processes in the photocatalytic generation of hydrogen are illustrated in Figure5.They include light absorption of the semiconductor photocatalyst,generation of excited charges(electrons and holes),recombination of the ex-cited charges,separation of excited charges,migration of the charges,trap of excited charges,and transfer of excitedFigure2.Schematic representation of a photoelectrochemical cell (PEC).Reprinted with permission from ref18.Copyright1972 Nature Publishing Group.Figure3.Fundamental principle of semiconductor-based photo-catalytic water splitting for hydrogen generation.H2O f12O2+H2;∆G)+237kJ/mol(1)Photocatalytic Hydrogen Generation Chemical Reviews,2010,Vol.110,No.116505charges to water or other molecules.All of these proces-ses affect the final generation of hydrogen from the semi-conductor photocatalyst system.The total amount of hydrogen generated is mainly determined by the amount of excited electrons in the water/photocatalyst interface in reducing water.Apparently,any other processes that consume excited electrons should be avoided in order to maximize the efficiency of the hydrogen generation of the photocatalyst system.Any process that generates excited electrons should be considered to act in a possible way to improve the efficiency.Thus,if we look atthe charge-generation process,the semiconductor photocata-lyst should first have a low band gap to absorb as much light as possible,and reflection or scattering of light by the photocatalyst should be minimized.Second,using the absorbed photons,the semiconductor photocatalyst should have a high efficieny in generating excited charges,instead of generating phonons or heat.After excited charges are created,charge recombination and separation/migration processes are two important com-petitive processes inside the semiconductor photocatalyst that largely affect the efficiency of the photocatalytic reaction for water splitting.67Charge recombination reduces the excited charges by emitting light or generating phonons.It includes both surface and bulk recombination and is classified as a deactivation process,and it is ineffective for water splitting.The separation of excited electrons and holes sometimes may need to overcome an energy barrier,which is the sometimes binding energy of the excited electron -hole pairs,excitons.Charge separation and migration,on the other hand,is an activation process.This is as a result of the charges being on the surface of the photocatalyst ready for the desired chemical reaction.It is beneficial for hydrogen generation through water splitting.Efficient charge separation and fast charge transport,avoiding any bulk/surface charge recombination avoided,are fundamentally important for photocatalytic hydrogen generation through water splitting.Any approach beneficial to the charge separation and transport should be taken into account such as design of internal-built electric field and use of high photoconductive semiconductor materials.The reaction of photogenerated H 2and O 2to form H 2O on the photocatalyst surface is normally called “surface back-reaction (SBR)”.It will inevitably have a negative effect on any enhancement of the photocatalytic activity,because it reduces the amount of H 2emitted from the photocatalyst.There are two main approaches to suppress SBR:one involves the addition of sacrificial reagents into the photo-catalytic reaction environment and the second creates a separation of the photoactive sites on the surface of the photocatalysts.In general,the electron donor and acceptor sacrificial reagents that are added work as an external driving force for the surface chemical reaction and depress the H 2O formation from H 2and O 2.The separation of the photoactive sites necessary for hydrogen and oxygen evolution,andFigure4.Calculated energy positions of conduction band edges and valence band edges at pH 0for selected metal oxide (A)and metal sulfide (B)semiconductors.The bottom of open squares represent conduction band edges,and the top of solid squares represent valence band edges.The solid lines indicate water stability limits.(C)pH dependence of the conduction band edge and valence band edge in an aqueous electrolyte solution.Reprinted with permission from ref 66.Copyright 2000The Mineralogical Society of America.Figure 5.Processes in photocatalytic water splitting.Reprinted with permission from ref 67.Copyright 1995American Chemical Society.6506Chemical Reviews,2010,Vol.110,No.11Chen et al.which is always accompanied by the surface separation ofthe photogenerated electrons and holes,has been shown tobe greatly affected by the surface properties of the photo-catalysts.As well as the surface reaction sites themselves,the surface states and morphology also play an importantrole.Taking into consideration the basic mechanism andprocesses of photocatalytic water splitting,there are two keysto developing a suitable high-efficiency semiconductor forthe visible-light-driven photocatalytic splitting of water intoH2and/or O2:(1)A photocatalyst should have a sufficientlynarrow band gap(1.23eV<E g<3.0eV)to both harvest visible light and possess the correct band structure.(2)Photoinduced charges in the photocatalyst should be sepa-rated efficiently in order to avoid bulk/surface electron/holerecombination.In addition,they must migrate to the pho-tocatalyst surface for hydrogen and/or oxygen evolution at the respective photocatalytic active sites.58In summary,it is generally accepted that the correct band structure for efficient visible-light harvesting and effective separation between the photoexcited electrons and the holes is essential to improve the photocatalytic properties of the semiconduc-tor.In addition,with the development of efficient visible-light-driven photocatalysts,it is important that economical, highly efficient photocatalytic systems for light-to-hydrogen energy conversion,in which aqueous solutions containing sacrificial reagents can be used to depress the backward reaction of hydrogen and oxygen to water on the surface of photocatalysts,can be constructed.2.3.Evaluation of Photocatalytic Water Splitting There are two apparent indicators that should be paid attention in evaluating the hydrogen generation through photocatalytic water splitting.One is photocatalytic activity, and the other one is photocatalytic stability.2.3.1.Photocatalytic ActivityThe efficiency of photocatalytic hydrogen generation from water splitting can be measured directly on the amount of hydrogen gas evoluted or indirectly on the electrons trans-ferred from semiconductor to water within a certain time period under light irradiation.Different photocatalytic setup configurations,such as inner irradiation type and top irradia-tion type,and light sources,such as Xe lamp and Hg lamp, are commonly used by different research groups and scientists,which may give different rates of gas evolution when exactly the same photocatalyst is used.This makes it difficult to make direct comparison across the results from different research groups and photocatalytic hydrogen gen-eration systems.Nevertherless,it seems helpful to get approximate correlations between various results if we normalize the rates of gas evolution to the amount of photocatalyst employed within a unit of time.Here,we use the rate of gas(O2and H2)evolution with units such as µmol·h-1andµmol·h-1·g-1catalyst to make the mensurable comparison between different photocatalysts under similar experimental conditions.The(apparent)quantum yield,as an extension from the overall quantum yield in a homogeneous photochemical system,becomes important and acceptable to evaluate the photocatalytic activity for water splitting.The overall quantum yield and apparent quantum yield are defined by eqs2and3,respectively.68The apparent quantum yield is estimated to be smaller than the total quantum yield because the number of absorbed photons is usually smaller than that of incident light.In addition to the quantum yield,the solar energy conversion efficiency that is usually used for evaluation of solar cells is also sometimes reported in the literature.It is defined as 2.3.2.Photocatalytic StabilityAs a good photocatalyst,it should have a good stability for H2and/or O2production,besides a high photocatalytic activity or quantum yield.To test the photocatalytic stability, a long-time experiment or a repeated experiment is always necessary.Photocorrosion is considered to be the main reason causing the poor stability of photocatalysts,especially the metal sulfide photocatalysts.CdS has frequently been reported to be unstable for photocatalytic H2evolution.S2-in CdS rather than water is self-oxidized by photoinduced holes in the valence band of CdS.The photocorrosion reaction occurs as in eq5,553.UV-Active Photocatalysts for Water SplittingA wide range of semiconducting materials have been developed as photocatalysts for use under UV irradiation. These are shown in Table1.On the basis of their electronic configuration properties,these UV-active photocatalysts can be typically classified into four groups:(1)d0metal(Ti4+, Zr4+,Nb5+,Ta5+,W6+,and Mo6+)oxide photocatalysts,(2) d10metal(In3+,Ga3+,Ge4+,Sn4+,and Sb5+)oxide photo-catalysts,(3)f0metal(Ce4+)oxide photocatalysts,and(4)a small group of nonoxide photocatalysts.3.1.d0Metal Oxide Photocatalyts3.1.1.Ti-,Zr-Based OxidesTiO2is thefirst reported photocatalyst for water splitting under UV irradiation.69TiO2can produce hydrogen and/or oxygen from water vapor,pure water,and aqueous solutions containing electron donor.70-77It was found that,under UV irradiation,colloidal TiO2combined with ultrafine Pt and RuO2particles generated H2with a high quantum yield of 30(10%and O2in stoichiometric proportions from water.70 The reaction solution had a pH of1.5,which was adjusted Overall quantum yield(%))Number of reacted electronsNumber of absorbed photons×100%(2)(Apparent)Quantum yield(QY,%))Number of reacted electronsNumber of incident photons×100%)2×Number of evolved H2moleculesNumber of incident photons×100%(for H2evolution))4×Number of evolved O2moleculesNumber of incident photons×100%(for O2evolution)(3)Solar energy conversion(%))Output energy of hydrogen evolvedEnergy of incident solar light×100(4)CdS+2h+f Cd2++S(5)Photocatalytic Hydrogen Generation Chemical Reviews,2010,Vol.110,No.116507T a b l e 1.U V -L i g h t -A c t i v e P h o t o c a t a l y s t s f o r W a t e r S p l i t t i n g t o H y d r o g e n a n d /o r O x y g e n a c t i v i t y (µm o l ·h -1·g -1)p h o t o c a t a l y s t s y n t h e t i c m e t h o dm a s s (g )l i g h t s o u r c e i n c i d e n t l i g h t a q u e o u s r e a c t i o n s o l u t i o n c o -c a t a l ./H 2c o -c a t a l ./O 2Q Y (%)r e f e r e n c eT i O 2(a n a t a s e )M C B T i O 20.3500-W H g b250-400n m w a t e r v a p o r R h /149729(340n m )74T i O 2(a n a t a s e )M C B T i O 21450-W H g q u a r t z fil t e r N a O H N i O x /32N i O x /1475T i O 2(a n a t a s e ,78%)P 25T i O 20.3400-W H g q u a r t z fil t e r N a 2C O 3P t /1893P t /95771T i O 2(a n a t a s e ,78%)P 25T i O 20.3250-W H g P y r e x fil t e r p u r e w a t e r P t /353P t /1771.4(300-400n m )76T i O 2h y d r o l y s i s ,c a l c i n a t i o n 0.2300-W X e cP y r e x fil t e r C H 3O H P t /∼330072r u t i l e /a n a t a s e T i O 2i m p r e g n a t i o n ,c a l c i n a t i o n 0.2300-W X e P y r e x fil t e r C H 3O H P t /∼670073c o l l o i d T i O 2h y d r o l y s i s r e a c t i o n 0.025450-W H g P y r e x fil t e r H C l P t -R u O 2/400030(310n m )70m e s o p o r o u s T i O 2s o l g e l m e t h o d 0.2300-W H g P y r e x fil t e r C H 3O H P t /6925889,934,941T i O 2n a n o w i r e s e l e c t r o s p i n n i n g a n d s o l -g e l 1450-W H g P y r e x fil t e r C H 3O H 54964,975T i O 2n a n o t u b e s h y d r o t h e r m a l m e t h o d 1450-W H g P y r e x fil t e r C H 3O H 285966T i O 2n a n o s h e e t s h y d r o t h e r m a l m e t h o d 1450-W H g P y r e x fil t e rC H 3O H 117.61003N i -i n t e r c a l a t e d N a 2T i 2O 5n a n o t u b e s h y d r o t h e r m a l m e t h o d 0.1450-W H g C H 3O H P t /∼85099T i O 2:G a s o l v o t h e r m a l m e t h o d 1.5U V -l a m p s P y r e x fil t e r P u r e w a t e r 20.8679T i O 2:N i h y d r o t h e r m a l m e t h o d 0.3300-W H g P y r e x fil t e r C H 3O H P t /∼566.780T i O 2:S c h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼750082T i O 2:Y h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼780082T i O 2:L a h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼768082T i O 2:C e h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼480082T i O 2:P r h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼660082T i O 2:N d h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼930082T i O 2:S m h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼1020082T i O 2:G d h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼1080082T i O 2:E u h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼1320082T i O 2:T b h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼480082T i O 2:D y h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼840082T i O 2:H o h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼1020082T i O 2:E r h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼840082T i O 2:T m h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼870082T i O 2:Y b h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼840082T i O 2:L u h y d r o l y s i s ,c a l c i n a t i o n 0.1300-W H g P y r e x fil t e r C H 3O H P t /∼720082T i O 2:S n /E u p o l y o l m e t h o d 0.2288-WF dq u a r t z fil t e r C H 3O H P d /∼9240.481T i O 2/C u x O i m p r e g n a t i o n m e t h o d 1400-W H g q u a r t z fil t e r C H 3O H 1850085-87T i O 2/S n O 2p o l y o l m e t h o d 0.05288-W F q u a r t z fil t e r C H 3O H P d /∼8383A g x O /T i O 2i m p r e g n a t i o n m e t h o d 0.05s o l a r l i g h t q u a r t z fil t e rC H 3O H 6700088-90S r T i O 3/T i O 2s o l i d -s t a t e r e a c t i o n 0.1150-W H g H C O O H 56091B /T i o x i d e s o l -g e l m e t h o d 0.3400-W H g q u a r t z fil t e r p u r e w a t e r P t /73P t /36.792,93m e s o -T i O 2/Z r O 2e v a p o r a t i o n i n d u c e d s e lf -a s s e m b l y p r o c e s s 0.06500-W Hg P y r e x fil t e r C H 3O H P t /∼2484C a T i O 3S a k a i C h e m i c a l I n d u s t r y 0.5500-W H g s i l i c a g l a s s N a O H P t /76P t /18120S r T i O 3A l f a -V e n t r o n 0.5400-W H g P y r e x fil t e r N a O H N i O x /∼70N i O x /∼32109-114S r T i O 3:L a s o l i d -s t a t e r e a c t i o n 0.1400-W H g q u a r t z fil t e r N a 2C O 3C o O x /∼2800C o O x /∼1300116S r T i O 3:T a s o l i d -s t a t e r e a c t i o n 0.3450-W H g P y r e x fil t e r p u r e w a t e r R h x C r 2-x O 3/∼14160R h x C r 2-x O 3/∼7000117S r T i O 3:N a s o l i d -s t a t e r e a c t i o n 0.3450-W H g P y r e x fil t e r p u r e w a t e r R h x C r 2-x O 3/∼22220R h x C r 2-x O 3/∼11110117S r 3T i 2O 7p o l y m e r i z e d c o m p l e x m e t h o d 1400-W H g q u a r t z fil t e r p u r e w a t e r N i O x /144N i O x /72118S r 4T i 3O 10p o l y m e r i z e d c o m p l e x m e t h o d 1400-W H g q u a r t z fil t e r p u r e w a t e r N i O x /1704.5(360n m )119K 2L a 2T i 3O 10p o l y m e r i z e d c o m p l e x m e t h o d 1450-W H g q u a r t z fil t e r K O H N i O x /2186N i O x /113123,135,139R b 2L a 2T i 3O 10s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r R b O H N i O x /869N i O x /4305(330n m )135R b 1.5L a 2T i 2.5N b 0.5O 10s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r R b O H N i O x /725N i O x /358135R b L a 2T i 2N b O 10s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r R b O H N i O x /79N i O x /30135C s 2L a 2T i 3O 10s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /700N i O x /340135C s 1.5L a 2T i 2.5N b 0.5O 10s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /540N i O x /265135C s L a 2T i 2N b O 10s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /115N i O x /50135L a T i O 3s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /137125L a 2T i O 5s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /442125L a 2T i 3O 9s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /386125L a 4T i 3O 12p o l y m e r i z e d c o m p l e x m e t h o d 0.5400-W H g q u a r t z fil t e r p u r e w a t e r N i O /714N i O /358133L a 2T i 2O 7p o l y m e r i z e d c o m p l e x m e t h o d 1450-W H g q u a r t z fil t e r p u r e w a t e r N i O x /960N i O x /4782722,122-127,131,134L a 2T i 2O 7:S r s o l i d -s t a t e r e a c t i o n 1450-W H g q u a r t z fil t e rp u r e w a t e r N i O x /15101256508Chemical Reviews,2010,Vol.110,No.11Chen et al.。
