1. PHOTOCATALYSIS - CHALLENGES AND POTENTIALS
Prof. B. Viswanathan
Department of chemistry
Indian Institute of Technology -Madras

2. Conventional redox reaction
Photocatalysis
Conventional redox reaction
Oxidizing agent should have more positive potential
Photocatalysis - simultaneous oxidation and reduction
The redox couple capable of promoting both the reactions can act as photocatalyst
Metals, Semiconductors and Insulators
catalyst
reaction assisted by photons 2

3. WHY SEMICONDUCTOR ? Metals No band gap Only reduction or oxidation
Depends on the band position
Insulators
High band gap
High energy requirement
Metals VB CB
H+/H2
H2O/O2
Insulators SC E 3

4. Concepts –Why semiconductors are chosen as photo-catalysts?
For 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+ 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 affect 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. 4

5. Water splitting - carry out both the redox reactions simultaneously - reduction of hydrogen ions (2H e- → 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 power) 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 addition 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 h+ → H2O + ½ O2. 5

6. 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 gap 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. 6

7. Criterion one has to use for the selection 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 respectively) have to be sifted in opposite directions so that the reduction reaction and the oxidation reactions are facile. 7

8. Percentage ionic character (%) =
Ionic 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 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 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 expression
Percentage ionic character (%) = 8

9. The percentage ionic character of the M-O bond for some of the semiconductors
TiO2
SrTiO3
Fe2­O3
ZnO
WO3
CdS
CdSe
LaRhO3
LaRuO3
PbO
ZnTe
ZnAs
ZnSe
ZnS
GaP
CuSe
BaTiO3
MoS2
FeTiO3­
KTaO3
MnTiO3
SnO2
Bi2O3
Ti-O
Ti-O-Sr
Fe-O
Zn-O
W-O
Cd-S
La-O-Rh
La-O-Ru
Pb-O
Zn-Te
Zn-As
Zn-Se
Zn-S
Ga-P
Cu-Se
Ba-O-Ti
Mo-S
Fe-O-Ti
K-O-Ti
Mn-O-Ti
Sn-O
Bi-O
59.5
68.5
47.3
55.5
57.5
17.6
16.5
53.0
53.5
26.5
5.0
6.8
18.4
19.5
3.5
10.0
70.8
4.3
72.7
59.0
42.2
39.6 9

10. The 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 % and hence modulating 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. 10

11. The photo-catalytic semiconductors 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, 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. 11

12. Hence we need stoichiometrically both oxidation 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 the conducting band may not be adjusted in such a way to promote both the reactions simultaneously 12

13. Normally 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 solar 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 situation is not obtainable in many of the available semiconductors by substitution at the cationic positions, this method has not also been successful. 13

14. In addition the dissolution potential of the substituted systems may be more favourbale than the water oxidation reaction and hence this will be the preferred 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. 14

15. Very low value of the ionic character also 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 reactions 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. 15

16. Therefore it is deduced that the systems which has ionic bond character of about 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. 16

17. There are some other aspects 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 the water splitting reaction is dealt. 17

18. PROCESSES ON THE PHOTO-EXCITED SEMICONDUCTOR SURFACE AND BULK
A. Millis and S. L. Hunte J. Photochem. Photobiol. A: Chem 180 (1997) 1 18

19. TYPICAL PHOTOCATALYTIC PROCESS
Photodecomposition of water
Photocatalytic formation of fuel
Photocatalysis in pollution abatement 19

20. HYDROGEN PRODUCTION
There 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 hydrogen
Photolytic means of hydrogen formation
Biochemical pathways for hydrogen evolution and
Chemical (steam ) reformation of naphtha 20

21. Photo electrolysis of Water-Holy Grail of Electrochemistry
Historically, the discovery of photo-electrolysis of water directly into oxygen at a TiO2 electrode and hydrogen at a Pt electrode by the illumination 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 21

22. CHALLENGES IN PHOTODECOMPOSITION OF WATER
2H + + 2e H V
O2 + 4e- + 4H H2O V
CHALLENGES IN PHOTODECOMPOSITION OF WATER
The band edges of the electrode must overlap with the acceptor and donor 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 energy 22

23. PHOTO-ELECTROCHEMICAL CELL FOR THE PHOTO CLEAVAGE OF WATER23

24. TYPES OF SEMICONDUCTORS BASED ON WATER
ELECTOLYSIS – CHOICE OF MATERIALS
NHE
0.00
1.23
OR Type – Oxidation & Reduction
R Type – Reduction
O Type – Oxidation
X type None
H+/H2 eV
H2O/O2
Semiconductor materials that satisfy the band gap requirement (~1.4 eV) - susceptible for photo corrosion.
Stable materials with a wider band gap absorb light only in the UV region. 24

25. Conditions 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 the 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 electrode 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. 25

26. What modifications?
various conceptual principles have been incorporated into typical TiO2 system so as to make this system responsive to longer wavelength radiations. These efforts can be classified as follows:
Dye sensitization
Surface modification of the semiconductor to improve the stability
Multi layer systems (coupled semiconductors)
Doping of wide band gap semiconductors like TiO2 by 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 increased considerably. In spite of these options being elucidated, success appears to be eluding the researchers. 26

27. Conditions 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 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 electrode materials for water
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. 27

28. ENGINEERING THE SEMICONDUCTOR ELECTRONIC STRUCTURES
without deterioration of the stability
should increase charge transfer processes at the interface
should improvements in the efficiency 28

29. Positions of bands of semiconductors relative to the standard potentials of several redox couples29

30. THE AVAILABLE OPPORTUNITIES
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 multi-junction systems or coupled systems - optimize and utilize the possible regions of solar radiation
Developing nanosize systems - efficiently dissociate water 30

31. ADVANTAGES OF SEMICONDUCTOR NANOPARTICLES
high surface area
morphology
presence of surface states
wide band gap
position of the VB & CB edge eV
CdS – appropriate choice for the hydrogen production 31

32. The opportunities
The opportunities that are obviously available as such now include the following:
Identifying and designing new semiconductor materials with considerable conversion efficiency and stability
Constructing 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. 32

33. Opportunities evolved
Deposition techniques have been considerably perfected and hence can be exploited in various other applications like in thin film technology especially for various devices and sensory applications.
The knowledge of the defect chemistry has been considerably improved and developed.
Optical collectors, mirrors and all optical analysis capability have increased which can be exploited in many other future optical devices.
The understanding of the electronic structure of materials has been advanced and this has helped to our background in materials chemistry.
Many electrodes have been developed, which can be a useful for all other kinds of electrochemical devices. 33

34. Limited success – Why?
The main reasons for this limited success in all these directions are due to:
The electronic structure of the semiconductor controls the reaction and engineering these electronic structures without deterioration of the stability of the resulting system appears to be a difficult proposition.
The most obvious thermodynamic barriers to the reaction and the thermodynamic balances that can be achieved in these processes give little scope for remarkable improvements in the efficiency of the systems as they have been conceived and operated. Totally new formulations which can still satisfy the existing thermodynamic barriers have to be devised.
The charge transfer processes at the interface, even though a well studied subject in electrochemistry has to be understood more explicitly, in terms of interfacial energetics as well as kinetics. Till such an explicit knowledge is available, designing systems will have to be based on trial and error rather than based on sound logical scientific reasoning. 34

35. Nanocrystalline (mainly oxides like TiO2, ZnO, SnO and Nb2O5 or chalcogenides like CdSe) mesoscopic semiconductor materials with high internal surface area If a dye were to be adsorbed as a monolayer, enough can be retained on a given area of the electrode so as to absorb the entire incident light.
Since the particle sizes involved are small, there is no significant local electric field and hence the photo-response is mainly contributed by the charge transfer with the redox couple.
Two factors essentially contribute to the photo-voltage observed, namely, the contact between the nano crystalline oxide and the back contact of these materials as well as the Fermi level shift of the semiconductor as a result of electron injection from the semiconductor. 35

36. Another aspect of thee nano crystalline state is the alteration of the band gap to larger values as compared to the bulk material which may facilitate both the oxidation/reduction reactions that cannot normally proceed on bulk semiconductors.
The response of a single crystal anatase can be compared with that of the meso-porous TiO2 film sensitized by ruthenium complex (cis RuL2 (SCN)2, where L is 2-2’bipyridyl-4-4’dicarboxlate).
The incident photon to current conversion efficiency (IPCE) is only 0.13% at 530 nm ( the absorption maximum for the sensitizer) for the single crystal electrode while in the nano crystalline state the value is 88% showing nearly times higher value. 36

37. This increase is due to better light harvesting capacity of the dye sensitized nano crystalline material but also due to mesoscpic film texture favouring photo-generation and collection of charge carriers .
It is clear therefore that the nano crystalline state in combination with suitable sensitization is one another alternative which is worth investigating. 37

38. The second option is to promote water splitting in the visible range using Tandem ells. In this a thin film of a nanocrystalline WO3 or Fe2O3 may serve as top electrode absorbing blue part of the solar spectrum. The positive holes generated oxidize water to oxygen
4h+ + 2H2O --- O2 + 4 H+
The electrons in the conduction band are fed to the second photo system consisting of the dye sensitized nano crystalline TiO2 and since this is placed below the top layer it absorbs the green or red part of the solar spectrum that is transmitted through the top electrode. The photo voltage generated in the second photo system favours hydrogen generation by the reaction
4H e-­­­­­ --- 2H2
The overall reaction is the splitting of water utilizing visible light. The situation is similar to what is obtained in photosynthesis 38

39. Dye sensitized solid hetero-junctions and extremely thin absorber solar cells have also been designed with light absorber and charge transport material being selected independently so as to optimize solar energy harvesting and high photovoltaic output. However, the conversion efficiencies of these configurations have not been remarkably high.
Soft junctions, especially organic solar cells, based on interpenetrating polymer networks, polymer/fullerene blends, halogen doped organic crystals and a variety of conducting polymers have been examined. Though the conversion efficiency of incident photons is high, the performance of the cell declined rapidly. Long term stability will be a stumbling block for large scale application of polymer solar cells. 39

40. New Opportunities
New semi-conducting materials with conversion efficiencies and stability have been identified. These are not only simple oxides, sulphides but also multi-component oxides based on perovskites and spinels.
Multilayer configurations have been proposed for absorption of different wavelength regions. In these systems the control of the thickness of each layer has been mainly focused on. 40

41. New Opportunities
Sensitization by dyes and other anchored molecular species has been suggested as an alternative to extend the wavelength region of absorption.
The coupled systems, thus giving rise to multi-junctions is another approach which is being pursued in recent times with some success
Activation of semiconductors by suitable catalysts for water decomposition has always fascinated scientists and this has resulted in various metal or metal oxide (catalysts) loaded semi conductors being used as photo-anodes 41

42. New opportunities (Contd)
Recently a combinatorial electrochemical synthesis and characterization route has been considered for developing tungsten based mixed metal oxides and this has thrown open yet another opportunity to quickly screen and evaluate the performances of a variety of systems and to evolve suitable composition-function relationships which can be used to predict appropriate compositions for the desired manifestations of the functions.
It has been shown that each of these concepts, though has its own merits and innovations, has not yielded the desired levels of efficiency. The main reason for this failure appears to be that it is still not yet possible to modulate the electronic structure of the semiconductor in the required directions as well as control the electron transfer process in the desired direction. 42

43. PREPARATION OF CdS NANOPARTICLES
1 g of Zeolite (HY, H, HZSM-5)
1 M Cd(NO3)2 , stirred for
24 h, washed with water
Cd / Zeolite
1 M Na2S solution, stirred for
12 h, washed with water
CdS / Zeolite
48 % HF, washed with water
CdS Nanoparticles 43

44. XRD PATTERN OF CdS
M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press) 44

45. d SPACING AND CRYSTALLITE SIZE
Debye Scherrer Equation
= diffraction angle T = Crystallite size
 = wave length  = FWHM
d-spacing (Å)
Catalyst
(0 0 2)
(1 0 1)
(1 1 2)
Crystallite
Size(nm)
CdS (bulk)
1.52
1.79
2.97
21.7
(HF treated)
2.93
CdS-Y
1.53
2.96
8.8
CdS-
1.78
8.6
CdS-Z
7.2 45

46. UV –VISIBLE SPECTRA OF CdS SAMPLES
Band Gap (eV)
CdS – Z
CdS – Y
CdS - 
Bulk CdS
2.38
2.27
2.21
2.13
M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press) 46

47. PHOTOCATALYTIC PRODUCTION OF HYDROGEN
35ml of 0.24 M Na2S and
0.35 M Na2SO3 in Quartz cell
0.1 g CdS
400 W Hg lamp
N2 gas purged before the reaction and constant stirring
Hydrogen gas was collected over
water in the gas burette 47

48. AMOUNT OF HYDROGEN EVOLVED BY CdS PHOTOCATALYST48

49. TEM IMAGE OF CdS NANOPARTICLES
Catalyst
Particle Size
(nm)
Surface
area
(m2/g)
Rate of hydrogen
production
(  moles /h)
CdS - Y
8.8 36
102
CdS - Z 6 46 68
CdS -  11 26 67
CdS - Bulk 23 14 45
CdS-Z
100 nm
CdS-Z
CdS- 
100 nm 49

50. SCANNING ELECTRON MICROGRAPHS
CdS-Z
CdS-Y
CdS-
CdS- bulk 50

51. T.Sakata, et al Chem. Phys.Lett. 88 (1982) 50
PHOTOCATALYSIS ON Pt/TiO2 INTERFACE
Vacuum level
Electrons are transferred to metal surface
Reduction of H+ ions takes place at the metal surface
The holes move into the other side of semiconductor
The oxidation takes place at the semiconductor surface
Aq. Sol Pt
TiO2
Aq. Sol
pH = 7
C.B
H+/H2
pH=0 EF
V.B
T.Sakata, et al Chem. Phys.Lett. 88 (1982) 50 51

52. MECHANISM OF RECOMBINATION REDUCTION BY METAL DOPING
Conduction Band
e- e- e- e- e- e- e- e- e- e- e- e-
Valence Band
h+ h+ h+ h+ h+ h+ h+ h+ h+ h+
Electron/hole pair
recombination
generation
e-(M) <-- M+e- Eg
Metallic promoter attracts electrons from TiO2 conduction
band and slows recombination reaction 52

53. PHOTOCATALYTIC HYDROGEN EVOLUTION OVER METAL LOADED CdS NANOPARTICLES
Activity of the catalyst is directly proportional to work function of the metal and M-H bond strength. 53

54. HYDROGEN PRODUCTION ACTIVITY OF METAL LOADED CdS PREPARED FROM H-ZSM-5
Redox potential
(E0)
Metal- hydrogen bond energy
(K cal mol-1)
Work function
(eV)
Hydrogen evolution rate*
(µmol h-1 0.1g-1) Pt Pd Rh Ru
1.188
0.951
0.758
0.455
62.8
64.5
65.1
66.6
5.65
5.12
4.98
4.71
600
144
114 54
*1 wt% metal loaded on CdS-Z sample. The reaction data is presented
after 6 h under reaction condition.
M. Sathish, B. Viswanathan, R. P. Viswanath Int. J. Hydrogen Energy (In press) 54

55. EFFECT OF METALS ON HYDROGEN EVOLUTION RATEPt Pd
1000
Pt, Pd & Rh show higher activity
High reduction potential.
Hydrogen over voltage is less for Pt, Pd & Rh Rh Au Cu
100 Ag Ni 10 Fe Ru
3 % 55

56. EFFECT OF SUPPORT ON THE CdS PHOTOCATLYTIC ACTIVITY
2, 5,10 and 20 wt % CdS on support - by dry impregnation method
Alumina & Magnesia supports enhance photocatalytic activity
MgO support has higher photocatalytic activity - favourable band position 56

57. I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1
Pb2+/ ZnS
Absorption at 530nm (calcinations at K)
Formation of extra energy levels between the band gap by Pb s orbital
Low activity at 873K is due to PbS formation on the surface (Zinc blende to wurtzite) Eg
(a) 573 K, (b) 623 K, (c) 673 K, (d) 773 K, and (e) 873K
Band structure of ZnS doped with Pb.
I. Tsuji, et al J. Photochem. Photobiol. A. Chem 622 (2003) 1 57

58. PREPARATION OF MESOPOROUS CdS NANOPARTICLE
BY ULTRASONIC MEDIATED PRECIPITATION
250 ml of 1 mM Cd(NO3)2
Rate of addition ml / h
Ultrasonic waves
 = 20 kHz
250 ml of 5 mM
Na2S solution
The resulting precipitate was washed with distilled water until the filtrate was free from S2- ions 58

59. N2 ADSORPTION - DESORPTION ISOTHERM
The specific surface area and pore volume are 94 m2/g and cm3/g respectively
The adsorption - desorption isotherm – Type IV (mesoporous nature)
Mesopores are in the range of 30 to 80 Å size
The maximum pore volume is contributed by Å size pores 59

60. M. Sathish and R. P. Viswanath Mater. Res. Bull (Communicated)
X- RAY DIFFRACTION PATTERN
XRD pattern of as-prepared CdS -U shows the presence of cubic phase
The observed “d” values are 1.75, 2.04 and 3.32 Å corresponding
to the (3 1 1) (2 2 0) and (1 1 1) planes respectively - cubic
The peak broadening shows the
formation of nanoparticles
The particle size is calculated
using Debye Scherrer Equation
The average particle size of as- prepared CdS is 3.5 nm
M. Sathish and R. P. Viswanath Mater. Res. Bull (Communicated) 60

61. ELECTRON MICROGRAPHS
The growth of fine spongy particles of CdS-U is observed on the surface of the CdS-U
The CdS-bulk surface is found with large outgrowth of CdS particles
The fine mesoporous CdS particles are in the nanosize range
The dispersed and agglomerated forms are clearly observed for the as-prepared CdS-U
TEM
SEM
CdS-U
100 nm
CdS-U
CdS - Bulk 61

62. 1 wt % Metal loaded CdS – U is 2-3 times more active than the CdS-Z
PHOTOCATALYTIC HYDROGEN PRODUCTION
Na2S and Na2SO3 mixture used as sacrificial agent
Amount of hydrogen (µM/0.1 g)
Metal
CdS-U
CdS-Z
CdS
bulk - Rh Pd Pt 73
320
726
1415 68
114
144
600 45
102
109
275
1 wt % Metal loaded CdS – U is 2-3 times more active than the CdS-Z 62

63. LIMITED SUCCESS – WHY?
Difficulties on controlling the semiconductor electronic structure without deterioration of the stability
Little scope on the thermodynamic barriers and the thermodynamic balances for remarkable improvements in the efficiency
Incomplete understanding in the interfacial energetic as well as in the kinetics 63

64. THE OTHER OPPORTUNITIES EVOLVED
Deposition techniques -thin film technology, for various devices and sensory applications.
Knowledge of the defect chemistry has been considerably improved and developed.
Optical collectors, mirrors and all optical analysis capability have increased
Understanding of the electronic structure of materials
Many electrodes have been developed- useful for all other kinds of electrochemical devices. 64

65. Thank you all for
your kind attention