光催化分解水制氫

1、光催化分解水制氫 Nanjing University of Aeronautics and AstronauticsInstitute of Nanoscience21 九月 2022IV-VIPbS 0.4117PbSe？PbTe0.3130II-VICdS2.425.4CdSe1.7010.0CdTe1.5610.2ZnTe2.410.4ZnSe2.829.2ZnS3.688.9ZnO3.359.0WO3TiO2CuO2eV=1240/ 光波波長對應(yīng)的能量200nm 6.2eV 400nm 3.1eV600nm 2.067eV 800nm 1.55eVDoping atomsRu,Eu,

2、21 九月 2022氫的主要來源電解水制氫（商業(yè)化電解水的效率85%）熱化學(xué)法分解水制氫石油產(chǎn)品催化重整制氫生物質(zhì)原料催化重整制氫生物制氫硫化氫裂解制氫光催化分解水制氫Nanjing University of Aeronautics and AstronauticsInstitute of Nanoscience納米粒子光催化分解水的要求強吸收太陽光（主要可見光）化學(xué)性質(zhì)穩(wěn)定合適的能帶適合水的氧化還原在半導(dǎo)體中電荷能有效轉(zhuǎn)移氧化還原反應(yīng)時具有低的超電勢低成本，高效率Nanjing University of Aeronautics and AstronauticsInstitute of N

3、anoscienceNanjing University of Aeronautics and AstronauticsInstitute of Nanoscience半導(dǎo)體光催化分解水熱力學(xué)原理示意圖+3.0+2.0+1.00.0-1.0Band gapH+H2H2OO2H+/H2O2/H2Oh+ h+ h+ h+ h+e- e- e- e- e- Water reductionWater oxidationhvValence bandConduction band H2O H2 + 1/2O2G0 = 238 kJ/mol(E = -Go/nF = -1.23 eV)V/NHE最佳能隙范圍

4、半導(dǎo)體納米粒子的能隙大于熱力學(xué)分解電壓（1.23eV）+熱動力學(xué)損失（0.4eV）+超電勢（0.30.4）,約1.9eV，對應(yīng)的波長約為650nm；在400nm（3.1eV）以下太陽光強度急劇下降；半導(dǎo)體納米粒子的最佳能隙范圍（1.93.1eV）（400-650nm）Nanjing University of Aeronautics and AstronauticsInstitute of NanoscienceIntensity of sunlight versus wavelength for AM1.5 conditions.Nanjing University of Aeronauti

5、cs and AstronauticsInstitute of NanoscienceEnergy band positions for various semiconductors at pH 14, the reduction and oxidation potentials of water vary with -59 mV per pH unit. Nanjing University of Aeronautics and AstronauticsInstitute of Nanoscience納米材料Si, GaAs, GaP, CdS,ZnO(unstable）AMWO6(A=Rb

6、,Cs;M=Nb,Ta)SrTiO3, BaTi4O9K4Nb6O17, K2La2Ti3O10,MTaO3, ZrO2, Ta2O5, TiO2(3.2eV), SnO2(3.6eV), Fe2O3(2.1-2.2eV), CdS, CdSe, WO3, Cu2O, Nanjing University of Aeronautics and AstronauticsInstitute of Nanoscience主要的優(yōu)化方法摻雜（調(diào)控能帶）(C,N,過渡金屬或稀土摻雜等)包覆（降低超電勢，增加穩(wěn)定性，提高電子空穴分離效率，提供析氫活性中心）（貴金屬等）染料分子或者稀土配合物敏化。Nanji

7、ng University of Aeronautics and AstronauticsInstitute of Nanoscience加大電子和空穴的遷移率。金屬氧化物的導(dǎo)帶和價帶分別和金屬的3d軌道、O的2p軌道相關(guān)。金屬的3d軌道重疊越多，電子的遷移率越高。O 2p軌道的重疊程度影響空穴的遷移率。盡量減少半導(dǎo)體納米粒子的缺陷，減少電子/空穴對的再結(jié)合位點。Nanjing University of Aeronautics and AstronauticsInstitute of NanoscienceTiO2粒子中光生電子、空穴的衰減過程示意圖+AA-體相復(fù)合表面復(fù)合hvhvEg Ti

8、O2 粒子DABCD+-+-+-導(dǎo)帶價帶-+D+-+TiO2納米粒子催化性能改進方法制備更細的納米粒子，提高比表面積，減少空穴遷移到表面的距離，減少電子空穴對再結(jié)合的機會；摻雜過渡金屬陽離子（Fe， Cr）；摻雜C， N, S, P, F, ClNanjing University of Aeronautics and AstronauticsInstitute of NanoscienceEnergy diagram of a PEC cell for the photo-electrolysis of water. The cell is based on an n-type semico

9、nducting photo-anode.Nanjing University of Aeronautics and AstronauticsInstitute of NanoscienceNanjing University of Aeronautics and AstronauticsInstitute of NanoscienceTiO2中光生電子、空穴的不同衰減過程的特征弛豫時間 電子、空穴的產(chǎn)生: TiO2 + hv hvb+ + ecb- fs載流子被捕獲過程： hvb+ + TiIVOH TiIVOH + 10ns ecb- + TiIVOH TiIIIOH 輕度捕獲 100ps

10、ms (動力學(xué)平衡) ecb- + TiIV TiIII 深度捕獲 10 ns (不可逆）電子、空穴的復(fù)合: ecb- + h + hv or ps ecb- + TiIVOH + TiIVOH 100nss hvb+ + TiIIIOH TiIVOH 10ns表面電荷轉(zhuǎn)移： etr- + Ox TiIVOH + Ox - 很慢 ms 主要過程 特征時間尺度 TiIVOH + + Red TiIVOH + Red + 100nsNano-sized TiO2 photocatalyst : opportunity & challenge reporter：youshun Luansupervi

11、ser：Prof. hengyong XuDalian Institute of Chemical PhysicsChinese Academy of SciencesSeminar II4 / 2006Aim: Net solar-to-hydrogen conversion efficiency of 10%Main content Introduction Advantage & shortage of TiO2 Modification methods Conclusion & outlook其其煤石油天然氣其他中國石油煤天然氣其他世界Situation of energy resou

12、rce & environment Solar energy is an abundant, economic, clean reversible resource Photocatalysis (UV-vis) is a promising field for our energy supply(H2OH2) and control of pollution (VOC oxidation)Mechanism of photocatalysis+BB+-AA-hCBVBh+e-hv+Volume recombination+surface recombinationWhy TiO2?1 n-t

13、ype TiO2 electrode 2 platinum black counter electrode3 ionically conducting separator 4 gas buret5 load resistance 6 voltmeterFujishima A.Honda K.,Nature，1972，37(1):238-245. Good photoactivity (band gap=3.2ev) oxidation of most VOC & water Photo & chemical stability, non-toxicity Low cost, ease of a

14、vailability Photocatalysis goes to TiO2 era!Challenge of TiO2!Because TiO2 has a high band gap ( 3.2 eV), it is excited only by UV light ( 2.43 eVFactors which have been considered in Photocatalysis of Water SplittingBand gap and band positionBand BendingCocatalysts; Junction structureSurface areaDe

15、fects in crystal structure and compositionSolution pH; External additiveStability against photocorrosionPHOTOCATALYSIS - CHALLENGES AND POTENTIALSProf. B. ViswanathanDepartment of chemistryIndian Institute of Technology -Madras Photocatalysis Conventional redox reaction Oxidizing agent should have m

16、ore positive potential Photocatalysis - simultaneous oxidation and reduction The redox couple capable of promoting both the reactions can act as photocatalyst Metals, Semiconductors and Insulatorsreaction assisted by photonscatalyst70MetalsNo band gapOnly reduction or oxidationDepends on the band po

17、sition InsulatorsHigh band gapHigh energy requirementMetalsVBCBVBCBVBCBH+/H2H2O/O2InsulatorsSCEWHY SEMICONDUCTOR ?71For conventional redox reactions, one is interested in either reduction or oxidation of a substrate. For example consider that one were interested in the oxidation of Fe2+ ions to Fe 3

18、+ ions then the oxidizing agent that can carry out this oxidation is chosen from the relative potentials of the oxidizing agent with respect to the redox potential of Fe2+/Fe3+ redox couple. The oxidizing agent chosen should have more positive potential with respect to Fe3+/Fe2+ couple so as to affe

19、ct the oxidation, while the oxidizing agent undergoes reduction spontaneously. This situation throws open a number of possible oxidizing agents from which one of them can be easily chosen. Concepts Why semiconductors are chosen as photo-catalysts?72Water splitting - carry out both the redox reaction

20、s simultaneously - reduction of hydrogen ions (2H+ + 2e- H2) as well as (2OH- + 2h+ H2O + 1/2O2 ) oxygen evolution from the hydroxyl ions. The system that can promote both these reactions simultaneously is essential. Since in the case of metals the top of the valence band (measure of the oxidizing p

21、ower) and bottom of the conduction band (measure of the reducing power) are almost identical they cannot be expected to promote a pair redox reactions separated by a potential of nearly 1.23 V.where the top of the valence band and bottom of the conduction band are separated at least by 1.23V in addi

22、tion to the condition that the potential corresponding to the bottom of the conduction band has to be more negative with respect to be more negative with respect to while the potential of the top of the valence band has to be more positive to the oxidation potential of the reaction 2OH- + 2h+ H2O +

23、O2. 73 This situation is obtainable with semiconductors as well as in insulators. Insulators are not appropriate due to the high value of the band gap which demands high energy photons to create the appropriate excitons for promoting both the reactions. The available photon sources for this energy g

24、ap are expensive and again require energy intensive methods. Hence insulators cannot be employed for the purpose of water splitting reaction. Therefore, it is clear that semiconductors are alone suitable materials for the promotion of water splitting reaction.74Criterion one has to use for the selec

25、tion of the semiconductor materials and also how one can fine tune the material thus chosen for the water splitting reaction.Essentially for photo-catalytic splitting of water, the band edges (the top of valence band and bottom of the conduction band or the oxidizing power and reducing power respect

26、ively) have to be sifted in opposite directions so that the reduction reaction and the oxidation reactions are facile. 75Ionic solids as the ionicity of the M-O bond increases, the top of the valence band (mainly contributed by the p- orbitals of oxide ions) becomes less and less positive (since the

27、 binding energy of the p orbitals will be decreased due to negative charge on the oxide ions) and the bottom of the conduction band will be stabilized to higher binding energy values due to the positive charge on the metal ions which is not favourable for the hydrogen reduction reaction. More ionic

28、the M-O bond of the semiconductor is, the less suitable the material is for the photo-catalytic splitting of water. The bond polarity can be estimated from the expressionPercentage ionic character (%) = 76The percentage ionic character of the M-O bond for some of the semiconductors SemiconductorM-OP

29、ercentage ionic characterTiO2SrTiO3Fe2O3ZnOWO3CdSCdSeLaRhO3LaRuO3PbOZnTeZnAsZnSeZnSGaPCuSeBaTiO3MoS2FeTiO3KTaO3MnTiO3SnO2Bi2O3Ti-OTi-O-SrFe-OZn-OW-OCd-SCd-SLa-O-RhLa-O-RuPb-OZn-TeZn-AsZn-SeZn-SGa-PCu-SeBa-O-TiMo-SFe-O-TiK-O-TiMn-O-TiSn-OBi-O59.568.547.355.557.517.616.553.053.526.5 5.06.818.419.53.51

30、0.070.84.353.572.759.042.239.677The oxide semiconductors though - suitable for the photo-catalytic water splitting reaction in terms of the band gap value which is greater than the water decomposition potential of 1.23 V. Most of these semiconductors have bond character more than 50-60 % and hence m

31、odulating them will only lead to increased ionic character and hence the photo-catalytic efficiency of the system may not be increased as per the postulates developed Therefore from the model developed in this presentation the following postulates have been evolved.78The photo-catalytic semiconducto

32、rs are often used with addition of metals or with other hole trapping agents so that the life time of the excitons created can be increased. This situation is to increase the life time of the excited electron and holes at suitable traps so that the recombination is effectively reduced. In this mode,

33、 the positions of the energy bands of the semiconductor and that of the metal overlap appropriately and hence the alteration can be either way and also in this sense only the electrons are trapped at the metal sites and only reduction reaction is enhanced. 79Hence we need stoichiometrically both oxi

34、dation and reduction for the water splitting and this reaction will not be achieved by one of the trapping agents namely that is used for electrons or holes. Even if one were to use the trapping agents for both holes and electrons, the relative positions of the edge of the valence band and bottom of

35、 the conducting band may not be adjusted in such a way to promote both the reactions simultaneously 80Normally the semiconductors used in photo-catalytic processes are substituted in the cationic positions so as to alter the band gap value. Even though it may be suitable for using the available sola

36、r radiation in the low energy region, it is not possible to use semiconductors whose band gap is less than 1.23 V and any thing higher than this may be favourable if both the valence band is depressed and the conduction band is destabilized with respect to the unsubstituted system. Since this situat

37、ion is not obtainable in many of the available semiconductors by substitution at the cationic positions, this method has not also been successful. 81In addition the dissolution potential of the substituted systems may be more favourbale than the water oxidation reaction and hence this will be the pr

38、eferred path way. These substituted systems or even the bare semiconductors which favour the dissolution reaction will undergo only preferential photo-corrosion and hence cannot be exploited for photo-catalytic pathway. In this case ZnO is a typical example.82Very low value of the ionic character al

39、so is not suitable since these semiconductors do not have the necessary band gap value of 1.23 V. - the search for utilizing lower end of the visible region is not possible for direct water splitting reaction. If one were to use visible region of the spectrum, then only one of the photo-redox reacti

40、ons in water splitting may be preferentially promoted and probably this accounts for the frequent observation that non-stiochiometric amounts of oxygen and hydrogen were evolved in the photo-assisted splitting of water. 83Therefore it is deduced that the systems which has ionic bond character of abo

41、ut 20-30% with suitable positions of the valence and conduction band edges may be appropriate for the water splitting reaction. This rationalization has given one a handle to select the appropriate systems for examining as photo-catalysts for water splitting reaction. 84There are some other aspects

42、of photo-catalysts on which some remarks may be appropriate. Though they have been derived from the solid state point of view like flat band potential , band bending, Fermi level pinning, these parameters also can be understood in terms of the bond character and the redox chemical aspects by which t

43、he water splitting reaction is dealt. 85PROCESSES ON THE PHOTO-EXCITED SEMICONDUCTOR SURFACE AND BULKA. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1 86Photodecomposition of waterPhotocatalytic formation of fuelPhotocatalysis in pollution abatementTYPICAL PHOTOCATALYTIC PROCES

44、S87HYDROGEN PRODUCTIONThere are various methods and technologies that have been developed and a few of them have already been practiced. These technologies can be broadly classified as:Thermo-chemical routes for hydrogen production Electrolytic generation of hydrogenPhotolytic means of hydrogen form

45、ation Biochemical pathways for hydrogen evolution and Chemical (steam ) reformation of naphtha88Photo electrolysis of Water-Holy Grail of ElectrochemistryHistorically, the discovery of photo-electrolysis of water directly into oxygen at a TiO2 electrode and hydrogen at a Pt electrode by the illumina

46、tion of light greater than the band gap of TiO2 3.1 eV is attributed to Fujishima and Honda though photo catalysis by ZnO and TiO2 has been reported much earlier by Markham in 1955 89 2H + + 2e- H2 0.00V O2 + 4e- + 4H+ 2H2O 1.229V The band edges of the electrode must overlap with the acceptor and do

47、nor states Minimum band gap 1.23 eV Charge transfer from the surface of the semiconductor must be fast - prevent photo corrosion Shift of the band edges resulting in loss of photon energyCHALLENGES IN PHOTODECOMPOSITION OF WATER90PHOTO-ELECTROCHEMICAL CELL FOR THE PHOTO CLEAVAGE OF WATER91NHE0.001.2

48、3OR Type Oxidation & ReductionR Type ReductionO Type OxidationX type - None H+/H2H2O/O2TYPES OF SEMICONDUCTORS BASED ON WATER ELECTOLYSIS CHOICE OF MATERIALS eV Semiconductor materials that satisfy the band gap requirement (1.4 eV) - susceptible for photo corrosion. Stable materials with a wider ban

49、d gap absorb light only in the UV region. 92Conditions for photo electrolysis of water For the direct photo electrochemical decomposition of water to occur, several key criteria have to be met with. These can be stated at the first level as follows:The band edges of the electrode must overlap with t

50、he acceptor and donor states of water decomposition reaction, thus necessitating that the electrodes should at least have a band gap of 1.23 V, the reversible thermodynamic decomposition potential of water. This situation necessarily means that appropriate semiconductors alone are acceptable as elec

51、trode materials for water decomposition. The charge transfer from the surface of the semiconductor must be fast enough to prevent photo corrosion and shift of the band edges resulting in loss of photon energy.93What modifications?various conceptual principles have been incorporated into typical TiO2

52、 system so as to make this system responsive to longer wavelength radiations. These efforts can be classified as follows:Dye sensitizationSurface modification of the semiconductor to improve the stability Multi layer systems (coupled semiconductors)Doping of wide band gap semiconductors like TiO2 by

53、 nitrogen, carbon and Sulphur New semiconductors with metal 3d valence band instead of Oxide 2p contribution Sensitization by doping.All these attempts can be understood in terms of some kind sensitization and hence the route of charge transfer has been extended and hence the efficiency could not be

54、 increased considerably. In spite of these options being elucidated, success appears to be eluding the researchers. 94Conditions to be satisfied?The band edges of the electrode must overlap with the acceptor and donor states of water decomposition reaction, thus necessitating that the electrodes sho

55、uld at least have a band gap of 1.23 V, the reversible thermodynamic decomposition potential of water. This situation necessarily means that appropriate semiconductors alone are acceptable as electrode materials for water The charge transfer from the surface of the semiconductor must be fast enough

56、to prevent photo corrosion and shift of the band edges resulting in loss of photon energy.95 without deterioration of the stability should increase charge transfer processes at the interface should improvements in the efficiencyENGINEERING THE SEMICONDUCTOR ELECTRONIC STRUCTURES96Positions of bands

57、of semiconductors relative to the standard potentials of several redox couples97 Identifying and designing new semiconductor materials with considerable conversion efficiency and stability Constructing multilayer systems or using sensitizing dyes - increase absorption of solar radiation Formulating

58、multi-junction systems or coupled systems - optimize and utilize the possible regions of solar radiation Developing nanosize systems - efficiently dissociate waterTHE AVAILABLE OPPORTUNITIES98 high surface area morphology presence of surface states wide band gap position of the VB & CB edgeCdS appro

59、priate choice for the hydrogen productioneVADVANTAGES OF SEMICONDUCTOR NANOPARTICLES 99The opportunitiesThe opportunities that are obviously available as such now include the following:Identifying and designing new semiconductor materials with considerable conversion efficiency and stabilityConstruc

60、ting multilayer systems or using sensitizing dyes so as to increase absorption of solar radiation.Formulating multi-junction systems or coupled systems so as to optimize and utilize the possible regions of solar radiation.Developing catalytic systems which can efficiently dissociate water.100Opportu