Towards the design of contrived photosynthesizers

Solar Energy Materials 1 (1979) 141-156 (~)North-Holland Publishing Company

TOWARDS THE DESIGN OF CONTRIVED PHOTOSYNTHESIZERS V. G U R U S W A M Y and J. O'M. BOCKRIS School of Physical Sciences, Flinders University, Adelaide, Australia and

Chemistry Department, Texas A&M University, College Station, Texas 77843,USA Received 21 September 1978

Photosynthesis may be contrived to give H 2 and 02. Bio,systems are complex to work with; semiconductors in contact with electrolytic solutions offer more easily comprehended situations and may give enhanced efficiency of conversion of solar light (when compared with bio-conversion). What is needed are criteria for optimization (i) in respect to solid state factors connected with photoelectrochemical energy conversion; (ii) in respect to the competing side reactions which destroy the electrode materials ; and (iii) with respect to the enhancement of conductivity which is often needed. Some quasi-quantitative numbers are derived.

1. Introduction

1.1. Rationale for contrived photosynthesis The present energy situation, which is dominated by the social and economic consequences of the exhaustion of the liquid and gaseous fossil fuels, is connected to the present almost total dependence upon the photosynthetic products, oil and coal. It is natural, therefore, to look to some kind of repetition of the former synthetic processes to give us a future fuel supply, and, as it is known that the solar energy arriving on the earth is much greater than the energy at present used, it is worth asking whether an acceptable amount of land would give rise to the necessary energy supply, at the average photosynthetic conversion efficiency of around 19/o. The amount of land is, of course, some 10 times larger than would be needed with the 10-20~o conversion efficiency which is assumed to be available from solarthermal and photovoltaic approaches. Correspondingly, the cost of a natural photosynthetic plant could be less than that of a photovoltaic one because of the absence of the need for an artificial collection system. A disadvantage arises, however, from any such scheme in respect to the fact that the product is carbonaceous. It does not provide a fuel for transportation without payment for a secondary plant for conversion of e.g., wood to, e.g., liquid natural gas. 141

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1.2. Biological techniques Another possibility arises, and that is to concentrate upon the fact that there are some enzymes (e.g., hydrogenase) which give rise directly to hydrogen from water. Such work has already been carried out by Bennerman [2], and by Nicholas, Neill, Bockris and McCann [3]. Were it possible to use hydrogen instead of methane, there would be an advantage in efficiency, because the burning of hydrogen in internal combustion engines occurs at about 1.2-1.4 times higher efficiency than that of methane [4] and the hydrogen from the photosynthetic product would be able to be mixed with the hydrogen fuel which must be the main product from atomic breeders and also solar-thermal and OTEC plants. The difficulties of utilising (e.g.) blue-green algae-which contain hydrogenase-are significant. For one thing, the system does not function for long without losing its activity [3]. Perhaps it is a matter of feeding the enzyme with an appropriate amount of carbon dioxide in the presence of light, and there would be a possibility of the periodic renewal of the system. However, the work of Nicholas et al. [3] showed that other difficulties were at work : lyolysis occurred, breaking down the plant and exposing the enzyme. Biological conversion systems are very complicated and adjustment and tampering are difficult to carry out. Thus, it may be more reasonable to contrive a photosynthetic generator. The reason for this paper is to describe the material problems in achieving artificial photosynthesis. The biological cell is an electrochemical device, and Delducca and Fucsoe [5] gave rise to a qualitative description of the electrochemical version of the natural processes in 1964. Bockris [6] introduced electrode kinetic and solid state concepts to considerations in a paper called "Is interfacial electron transfer a basic biological step?". Bockris and Tunulli recently diagrammated and illustrated the solid state and electrochemical viewpoint in the electrochemical mechanism of the photochemical activity of biolipid membranes [7] (see fig. 1). The Mitchell hypothesis of ® T

4,

IPS-I*+~~H30" ~..~.-~... h U.n.

H30*-.F~O2t i

Pnotocathode Photoanode Fig. 1. The separate electrodes reactions which are suggested as making up the electrode reactions in thylakoid membranes. In reality, the m e m b r a n e surfaces will tend to be the two sides of the membrane. Each electrode reaction site corresponds to the sites of the photochemical receptors in the membrane.

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the action of electrochemical cells [8] is already a kind of latent electrochemical mechanism which is not different in principle from that proposed by Delducca and others. In fact, the concept that self-activating electrochemical mechanisms are in fact micro-fuel cells was really implicit in the work of Evans and Hoar in 1936 [9] and from the more well known work of Wagner and Traud 1938 [10]. Such concepts were formulated into quantitative relations by Bockris in 1954. Developing bio-synthetically along the Bockris and Tunulli lines would perhaps be helpful, but the same type of situation would probably arise as with the work of Nicholas et al.-the considerations become too complicated and cell breakdown occurs too readily. The advantage of cheapness of material may compensate for this. In the present paper we consider what kind of progress can be made if we change the material from the biochemical to inorganic.

1.3. Inorganic techniques 1.3.1. Why semiconductor and not metal electrodes The photochemical activity of the electrode solution interface is much less for metals than semiconductors. The mechanism is easy to see: the penetration of the photon in metals as compared to semiconductors is the same depth, but the coefficient of absorption of metals is less, and, in particular, electrons which are generated by means of the photon absorption easily deactivate on their way to the interface, falling below the Fermi level, whereas in semiconductors there is an energy gap, and transfer across this gap, as a result of deactivating collisions suffered during the diffusional process, is less easy. Hence, whereas the efficiency of photoelectrochemical conversion at the metal-solution interface [11] is ~ 10-2~, the efficiency of solar conversion at the semiconductor-solution interface is around 1~ [12]. 1.3.2. Criteria for improving efficiency What is needed is clearly two semiconductors placed together in a cell, each irradiated by solar light. Such a system was achieved by Ohashi, McCann and Bockris [13] in the first entirely light-driven electrochemical convertor to give hydrogen and electricity at the same time (a hydrogen photosynthesiser, therefore), and the question is less the principle of the operation, and more what the materials should be. It is necessary that for the process to be viable the efficiency of conversion should be upgraded to at least 5~o. Although suitable cathode materials have been found [13], suitable and stable anode materials are necessary for upgrading of the efficiency. Semiconductor anodes which have energy gaps which are too high will not be able to absorb solar light; the majority of solar photons have energies in the region of 2 eV. However, the energy gap and absorptive properties of the semiconductor will not be simple, particularly if we return, as has increasingly been done in recent times, to studies of oxygen catalysis on tungsten bronze-like substances [14, 15]. With these substances the band theory is not applicable [16]: bonds replace bands in the consideration of the electron transitions. Hence, the advantage obtained with low band-

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gap materials may be lost by the lower probabilities of transition of electrons compared with the situation with simple band-gap materials. The second most important consideration will be the flat band potential, because this determines the conductivity band, and for doped semiconductors, the Fermi level. The basic principle of electron transfer at the interface is the transfer of an electron from within the solid phase to levels outside in the solution, and these are fixed (e.g., they are levels near to the ground state in the lowest electron state of H30 + ; similar levels in the case of water in alkaline solution; and for the evolution of oxygen, the levels will be those of O H - , and water in acid solutions). It will not be possible to deviate too much (0.5 eV?) from the ground state levels of these materials, and this defines the regions of acceptable conductivity bands (for the cathodic reaction to form hydrogen) and valency band for the donor reactions of electrons to holes in the semiconductor, which will be associated with the evolution of oxygen. A neglected aspect is comprised by'the surface states. With a sufficiently high concentration of surface states, a semiconductor-solution interface becomes metal-like. Previous theorists who have treated this area have concerned themselves with the electrode semiconductor as if it were a semiconductor in contact with a gas, and the potential drop has been regarded as though it were in the semiconductor, which is incorrect. Furthermore, the surface state concentration is likely to play a large role in a semiconductor-solution interface because there is anion adsorption which would provoke surface states not present at the gas phase. The third most important aspect of a contrived photosynthetic convertor is that of conductivity. The net current available in the cell is the difference of potential of the two electrodes decreased by the I R drop in these electrodes and that in the solutions, where I is the current through the cell and R is the sum of the resistances in the solutions and the electrodes. If this is too high, little net potential will result. It is not simple, particularly with the transition metal oxides, to find systems which are dopable, and principles of how one dopes such materials must be evaluated. The last consideration is the one that pertains most directly to this journal's origin, properties leading to instability. Most of the anodic semiconductor materials of suitable band gap appear to be unstable, perhaps due to corrosion and perhaps due to competing anodic reactions involving the substrate. It is necessary to find an approach whereby the likelihood of such reactions can be judged.

2. The basic theory of the photoelectrochemical energy and substance generator There has been confusion in the literature concerned with the theory of selfactivating photoelectrochemical cells because a number of the solid state physicists who deal with them have treated the interfaces as np junctions. Electrochemical cells cannot be treated in this way because they do not consist ofnpjunctions. One junction is indeed an np junction, but that is not the junction which is irradiated with light, but rather the junction between the two substances, for example, gallium phosphide and titanium oxide, which make up the cell. If, as would be usual, a metallic contact is interposed between these two substances, this makes no difference to the theory because the potential difference which arises at the semiconductor (1) with the metal

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is eliminated by the converse potential difference at the metal-semiconductor (2) contact. The other two contacts are semiconductor-ionic solution contacts, and it is at these that the theory must be evaluated. Some of this theory has been evaluated by Bockris and Uosaki [17]. The model is easy to describe. The photon enters the semiconductor, and is partly absorbed in the valency band. The Fermi statistics are then used to calculate the number of whole electron pairs to which this photon gives rise. The energy gap and the Fermi energy is needed in this calculation. Once one knows the number of electrons which have come into the conductivity band, and therefore the number of holes which have come into the valency band, the situation is that the fate of these electrons and holes can then be examined in terms of diffusion theory, taking into account their electronelectron and hole-hole collisions, or their collisions with impurities, in terms of their lifetime. It is necessary to take into account some complex considerations in respect to scattering, and also in respect to the direction of motion of electrons inside the semiconductor [18]. Eventually, the flux of electrons arriving at the surface of the semiconductor is calculated as a function energy of the electrons get to the surface of the electrode (considering momentarily a cathode) they have reached the bottom of the conduction band. At this point, the emission of electrons into the solution is calculated. At present, the Gamow method has been used to achieve this calculation, although Bockris and Khan [20] have devised a superior treatment which, however, involves much more computation. It is lastly necessary to take into account the receptor density in the solution and the distribution of receptor states, and when this is done it is possible to obtain an expression for the dependence of the current upon potential, and to show that this is different from the Tafel situation in thermal electrode kinetics. The situation as to what is the rate determining step in all this cannot be answered in general. There is certainly a situation at sufficiently high current densities, when the current enters the saturation region, when transport inside the electrode is rate determining. There is a region at lower current densities, at least in some photo cathodes, where another rate determining step, perhaps the electron transfer across the interface, is rate determining [19]. As the overpotential builds up greatly at high current densities, thus diminishing the net available potential of the cell, the nontransport controlled region will tend to be that in which practical cells may be developed. The equations and theory of the Bockris and Uosaki method give relations between the flat band potential, the energy gap, and the so-called critical potential, the potential corresponding to that at which electrons are transferred to the solution by radiationless transfer, according to the Gurney criterion [20].

3. Photo-corrosion of anodes

When we come to consider the situation in the photoelectrochemical case, it is clearly different from normal anodic corrosion in respect to the fact that two kinds

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of dissolution are possible in competition with photo-induced oxygen evolution. On the one hand, thermal semiconductor reaction can be in competition with those which occur without light activation or acceleration. Although subject fundamentally to the same mechanism consideration as the photo ones, there is an important difference in that these anodic reactions will go increasingly with p-type semiconductors rather than with n-type in contradiction to the situation with photoactivated electrodes. Gerischer made the first attempt to give a scientific basis to the light activated corrosion of photo anodes on the basis of Nernstian thermodynamic treatment [21]. The corrosion of the anode was attributed to holes attaching the electrode material MX such that M ÷ went into solution. If AG for this mechanism is known, then ED the dissolution potential can be expressed as, (1)

ED = - A G / n F .

The condition for stability was the scavenging of the holes by O H - ions such that Eo < Eredox. But this zeroeth approximation does not take into effect the overpotential of the competing reactions where the condition for stability is Eo2/H20+r/o2 > ED +r/D.

(2)

Up to now Gerischer, Memming and subsequent workers in this field have seemed to assume that Eo2/H20 and EH+/H2 occurred in the same potential region as in .SELF OaP cathode , SrTiO 3 anode

GENERATII4G

PHOTO

CELL Platimzed

CdTe

c a t h o d e , SrTIO 3 anode

a 0.4

O.

EO2/H20 therrna~ pHlZ,

0.4 Current [ m A . c m -2 )

LtJ

-F

[ j

-0.4

/EH+/H 2 photo

EO2/H20 thermal pill/.

01.4

0.8 z~Ld

Current

-0.4.~.~*/H2

I 0.8

( rnA cm -2)

photo

2 -0.8 ~ therrna[

~, ~-~'~'~EH*/H 2 thermal pill/,

E02 !H2 0 photo

-1.2

Eo~ /H20 photo -1.2

Fig. 2. (a) Results for the current potential curves in a self-activating photoelectrochemical generator (SrTiOs GAP). The position on the normal hydrogen scale at which 02 and H2 are evolved is to be noted. (b) The same, but for SrTiO 3 and platinized CdTe.

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thermal cells [21, 22]. However, examination of the experimental data of Ohashi, McCann and Bockris [13] for the SrTiO3 and CdTe couple and SrTiOa and GaP couple indicate that oxygen evolves at much more negative potentials than in thermal cells and hydrogen at more positive potentials (see fig. 2). Under thermal conditions in alkali, oxygen begins to evolve at around +0.42 V on the N hydrogen scale while hydrogen is evolved at -0.81 V on the same scale and the difference is 1.23 eV, the energy necessary for the decomposition of water. However, the photo catalytic decomposition of water 02 is somewhere in the region of - 1.0 V and H2 somewhere in the region of - 0 . 5 V. The advent of a photon at energies ~>Eg creates a hole and if the photons are created at energy levels lower than the top of the valancy band it would be possible for oxygen to be evolved at more negative potentials than with thermal reactions. Therefore, the potential at which oxygen will evolve will move downwards in a more negative direction away from that at which oxygen would evolve without light in the normal way. As holes will be used up by the anodic mechanism, the tendency would be for the reaction of more holes at these levels. Correspondingly, photons with energies higher than Eg would cause electrons with energies higher than the top of the conduction band and hence supply electrons at higher energies for hydrogen evolution such that hydrogen is evolved at more positive potentials. This is helpful as it makes the energetics of the p and n semiconductor more hopeful and will be discussed further on. Oxygen evolution thus moves into a situation which is less corroding and hydrogen, which one does not need to fear in respect to corrosion so much, into a position where corrosion is more dangerous.

3.1. Stability with respect to oxygen The fact that all stable electrodes found up to now are oxides, namely TiO2, SrTiO3, Fe203, YFeO 3 and SnO2, is indicative that due to their high oxidation states and high lattice energies, they would be more stable to oxygen evolution than other materials. However, materials such as gallium phosphide and cadmium sulphide have been found to form n type anodes basically unstable to oxygen. Competing thermal electrochemical reactions will not be considered as they would presumably occur at different potentials.

3.2. Competing photo reactions If the oxygen evolution could shift by - 1.2 eV it is reasonable to assume that Eo would also shift. In the event of thermodynamic instability, the relative positions of E o can be shifted with respect to Eo2/H2o by manipulation. ED is a function of the solvation energy of the cation M +. Working with aprotic solvents it has been possible to change the solvation energy of an ion by _ (1-2) eV [23]. By suitable choice of solvent, ED can be shifted. Eo2/H2Oshifts to more positive potentials by 0.59 eV per pH in aqueous media. Aprotic solvent water mixtures have been found to have pH's in the range of 14-30 [24] and it would be reasonable to assume that Eo2m2o could also be shifted. Hence, the possibility of thermodynamically stabilising an electrode by using aprotic/water mixtures is an area to be explored.

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Although Memming [22] introduced kinetic criteria (in isoenergetic charge transfer, the position of the valency band edge for an anodic mechanism must overlap with the solvent levels and vice versa for cathodic mechanism), the levels of comparison appear open to question. He uses Eo2/H2O rather than the ground state of OH ion from which electrons tunnel directly into the electrode. In view of the shift of Eo2/H2o, as described above, this comparison may be invalid. Hence, in the determination of the optimum flatband potentials, direct comparisons can be made with an (OH)ion and the valency band and under these conditions minimal corrosion takes place. Similar treatment has previously been made by Bockris and Uosaki [17].

4. Characteristics of suitable anodes

The main problem in devising suitable photoelectrochemical generators is in the anode, for cathodes are already relatively satisfactory in the fact that the gallium phosphide and cadmium telluride have energy gaps which are between 1 and 2 eV and appear to work over significant times without deterioration. On the other hand the principal difficulty with the work which has been done hitherto is that the energy gap of the anodes has been too large (typically 3 eV) so that little solar energy is absorbed, and the consequent overall efficiency of the electrode-generatton of oxygen is less than 1~o*. In the following we attempt to deduce guidelines by which workers might select anode materials. It is clear that what we shall deduce are not more than guidelines, i.e. exact calculations are not possible. Further, the choice of an electrode would always be a fairly complex compromise between various conflicting trends. We shall discuss the effect of three main variables: the energy gap, the fiat band potential, and the electron affinity. 4.1. Relations with the energy 9ap

The overall energetic equation for the working of a photoelectro-chemical cell may be seen in the concept that the maximum cell potential available at zero current will be the difference in energy of the light which has irradiated the electrode diminished by the energy gap which the photons have overcome. One obtains, therefore : hvocat h -k-hYoa n --(EG,cath "~-EG,an )--E+IR.

(3)

This equation is derived from the implications of Eo2/u:o and E n ÷/n2 shifting due to photo effects: Iis the total current across the electrode, R is the total ohmic resistance of the cell including that of the electrodes and the electrolyte. Ecenj

~- Erev,photo

"~-E?~ •

These overpotential terms can be expanded to give * However, efficienc~es over small wavebands are very high, e.g., 90~o.

(4)

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E = Erev photo '

RT

-

-

i¢

~.¢~In - -

i0,tcath)

R T , i,n In .

~anF

i0(an)

149

(5)

The relationship between I and i is simply that: i = I/A, where A is the area of the electrode. Such equations enable relationships to be derived between the current density, the radiation frequency, and the energy gap etc. The equations hold for a given light intensity. As we are nearly always interested in the solar spectrum, eq.(3) can be used to obtain some kind of idea of the needed energy gaps. Thus, at maximum, it should have an energy gap of less than 2 eV. On the other hand, the energy gap should not be too low because of increasing deactivation. A lower limit of about 0.7 eV should be adhered to. The sum of the energy gaps, which the above relationships suggest, should be greater than this because of the need to compensate the over-potential by giving the cell more energy than the reversible value. Thus, the cell will not produce significant current unless the energy gap is overcome substantially. Thus, Ec, + EG> E ....thermal. The latter value of course is 1.23, but in practice, to obtain a current density of, say 1 mA, it is necessary to have a potential of more than about 1.6 V in the thermal case so that it would seem that another basic limit would be, approximately : EG,cath+EG,an i>1.7 V.

(6)

Summarising these conditions for energy gaps, we find that the energy gap of cathode and anode.should add up to at least 1.7 V; that none should be greater than 2 eV and none less than 0.7 eV. The greater the energy gap the more negative will be the potential of hydrogen evolved, and the more positive the potential at which the first evolution will be seen.

4.2. The flat band potential for anodes The basic concept for the flat band potential for anodes is that the top of the valency band should be roughly equal to the level of the bottom of the electron level distribution in the solution of the O H - . Thus: Evb ~EoH-

(7)

in solution where these potentials are with reference to the potential electron in a vacuum. Now it is obvious that

-Eo =Eo

(8)

or

Erop=EC+Eo.-

(9)

where all these potentials are on the electron and vacuum scale. The standard enthalpy change for electron transfer from the electron level of the hydroxyl ion to the top of the valency band is calculated from the following thermodynamic cycle.

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q-S.C.(hole) +OHa~

An , q-S.C.-- O H - - H 2 0

tl-S.C.(hole) + O H - + H 2 0

tI-S.C- - O H + HE0

r / - S . C . ( h o l e ) + O H + e + H 2 0 , E,+E, q-S.C.+OH+HEO, where A', R', E.A., Ea, L0, Eg represent the heat of adsorption of an OH group on the semiconductor, O H - H 2 0 repulsive force, the electron affinity of the OH group, the electron affinity of the semiconductor, the hydration energy of the hydroxyl ion and the energy gap of the semiconductor, respectively. Therefore, AH = -L~ + E.A.- E~ -EG+ A'+ R',

(10)

R ' = - 0 . 1 3 eV, L ~ = - 3 . 7 8 eV, E.A.=l.82 eV, Ea=3.5-5.5 eV, E~=0.7-3 eV, A ' = -0.13 eV. R', L', E.A. and A' from K. Uosaki [29]• The absolute value of Eon- can be calculated from the following expression: Eon = - AH - AG + Efblabs~

(11 )

Efb~abs) fixes the conduction band. Eon- is calculated from experimental values for Ec, Efb and calculated values of Ea [25]• SrTiO3.

EG =3.2 eV, E, =3.71 eV, Efb(NHE)= --0.2 eV, EfbINHE)=Eabs - 4 . 3 ;

EoH-=2.3 eV. FezO3. TiO2.

Ec, =2.2 eV, Ea =4.71 eV, EfbfNHE~ =0.7 eV; Eon- =2.73 eV. Ec;=3.0 eV, Ea =4.33 eV, EfbiNHEt=0.05 eV; Eo8- =2.59 eV.

The variation is due to the experimental error in Efb and the difference between E~ (calculated) and Ea (actual) and other values in determining AH. However, taking Eon- =(2.5+0.2) eV and utilising eqs. (10) and (11): -Ea+Eeo= -7.17+2Eg. Now if a value of +0.5 eV is used for band bending then a suitable flat band potential is one which coincides with the O H - level. Efblabs~= Eg + 2.5 + 0.5, Etl,INHEI = Eg + 3 -- 4.3 = Eg -- 1.3, Eg = 1.0, EfbINHEI= --0.3 ; Eg = 1.3 EIbINHE).=0, Eg= 1.5 EfbINHE~= --0.2 eV, Eg=2.0 Ejb~HE~= +0.7 eV. Hence, the optimum flat band potential is very much a function of Eg. It is sccn that Eg, Efb and Ea are all interelated. Hence, when a material of suitable band gap is found all other properties must also fall into place•

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4.3. Electron affinity A tendency towards a high negative electron affinity should occur. However, empirical correlations tend to suggest that in the increase of a negative electron affinity, the flat band potential is sufficiently negative [25]. In terms of actual values, a suitable range seems to be more negative than 5 eV, but if the value goes to more than 5.6, the change in the flat band potential is too great. This gives a rather narrow delineation of the electron affinity.

4.4. Summary of anode characteristics The full characteristics are suggested above. Suitable anodes should have an energy gap between 0.7 and 2 eV ; an electron affinity between about - 5 and - 5.6, and a fiat band potential of - 0 . 3 to - 0 . 8 V on the NHE. These characteristics do not take into account competing anodic reactions.

4.5. Choosin9 a semieonductor material Up to now most of the attention has been confined to binary compounds but due to the complexity of the properties required namely, band gap, lattice energies restability, dopability. Naturally interest has veered to more complex ternary and quaternary semiconductor oxides whose composition may be manipulated to give optimum properties. Some of the ternary structures which have been deduced to have semiconductor properties are : (a) tetrahedral diamond type (b) octahedral (1) spinels (2) perovskites (3) garnets (c) pyrochlor and (d) fl alumina types, namely magneto plumbates. Type (a) may or may not have transition elements and have been typified by Groryunova [26] as not infinite and of the following structure types : AInlvc'vl"2 "-'3,

*AIDIV(-~VIz. "-'3at2x., ~A llBlVCV, *AIBIIICVl,

*A1BVC'"~4 Vl3 •

Those marked with asteriks would allow the inclusion of oxygen to form an oxide and which would be of interest to the present discussion. Octahedral structures mostly involve a transition element. A large number of perovskites have been studies for suitable photochemical properties. Examples are: YFeO3, SrTiO3 and BaTiO3. They have been found stable. Other candidates are V Vl the spinels which have the structure type A l I B2C4 but no photochemical studies have been done on them with one exception [28] up to now. Recent work by Kuhn et al. [27], indicates that magneto plumbates e.g., PbFe12019 and pyrochlor struc-

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tures of the type rt 2AIilDVF' !u2~,_.7 V would also be suitable candidates, e.g., Hg2Ta20 7. It is possible that other structure types may also have suitable semiconductor properties. If the structure types are known it is possible to make an estimate of the band gap empirically. The criteria is that one constituent binary oxide must have a low band gap, then the new material would have a band gap +0.3 eV to this in general, although a few exceptions have been found. This appears reasonable; one could expect transitions from the smallest bands to dominate (table 1). The band gaps of most binary oxides have been determined; hence, it would be possible to determine the compositions of a large range of small band gap ternary compounds. The electron affinity can be calculated from the electronegativities of the constituent atoms by the following expression [25]. 2 1/3 1 EA =[XAZB] --~-E~

(12)

where XAand Xa are the individual electronegativities of A and B in AB 2. From this the flat band potential can be calculated [23].

5. Attaining sufficient conductivity Although a large number of complex crystal structures have been studied [27] for suitable semiconductor anode properties, little attention appears to have been given to the dopability of the material. Thus, in Cr203, the band gap is 1.6 eV and the fiat band potential - 0 . 5 eV on the NHS, with a good chance of stability. However, it has not been doped n-type, and so the material cannot be used as a photoanode. It would appear that most workers in this field have depended hitherto on opportune doping such as reduction with hydrogen as in TiO2 and Fe203 [28, 29], unknown impurities and even reduction during pressing [27]. Doping of binary and ternary compounds involving no transition elements, present no problem. Doping n-type is effected by replacing a few atoms in the crystal with one atom which has a valency state one higher than that of the atom replaced. Table 1 Correlations between band gaps of constituent oxide in ternary oxides

Ternary oxide Hg2Nb207 BaTiO 3 SrTiO 3 FeTiO3 YFeO3 PbFel:Ol9 Hg2TaO7

Narrow E~ of constituent binary oxide [28][33] HgO=2.1 TiO2 = 3.0 TiO2 = 3 FeO = ? Fe203 = 2.32 Fe203=2.34 HgO=2.1

Wide Eg of constituent binary oxide [28] Nb2Os=3A BaO = 3.2 SrO = 5,7 TiO 2 = 3.0 Y203 = 5.6 PbO=2.9 TazO5 =4.0

E 8 of ternary oxide [271128] 1.8 3.18 3.12 3.2 2.6 2.32 1.8

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The confining condition is Eo
Eo =Im*/e,

(13)

where I is the first ionization energy, reduced by the dielectric e of the crystal according to the Bohr model and m* is the effective mass of the electron. The values obtained could be two or three times larger than actual values due to limitations of the model. However, the technique outlined does not work when transition elements are involved in a semiconductor, due to the build up of the d levels. Much of the attention of new n-type semiconductors is confined to oxides and the following discussion is limited to such compounds. The fact that in transition metal oxides the optical transitions are not between bands but bonds has considerable implications for the intensity of absorption. The transition probability between bands must be less than that for split energy levels of the bands [19]. The excitation processes depend on level to level of the split d orbitals [27] and not band to band transitions. Hence, the method by which the conductivity of the oxide can be increased will not depend on the normal mechanism of doping. In these oxides, an impurity, e.g., Ti +++ +, in Fe20 3 introduced into the lattice or employed as a reducing agent, e.g., H 2 or CO to put in or remove an electron from the d orbitals, causing an excess of electrons (n-type doping) in these levels or holes (p-type doping), respectively. The objective is in contradiction to the band gap semiconductor: controlled valency states bring about conduction by hopping. A condition is that the energy involved from state to state such as Fe + + to Fe + + + should be kT. The energy difference between the Fe* * and the Fe + + + ion causes a tendency to equalise energy levels, giving rise to transitions throughout the crystal structure by hopping. However, if the energy change from state to state is large, the conductivity is low.

5.1. Selecting dopants References to experimentation in this area can be found in several textbooks [30]. For any type of doping band or transition split level semiconductors, a solubility of more than 0.01~o is necessary. If the impurity to be introduced is compatible in the crystal, it is probable that these solubilities can be achieved. Some first approximation can be obtained for dilute solutions from classical thermodynamics [31].

5.2. Example of cadmium selenide : CdSe for n-type doping (non transition elements) Condition 1. An impurity with a higher valency state than the one it replaces must be chosen. It is decided to replace Cd + + in the crystal ; candidates would be group III elements and those with a normal valency state of three such as actinides and lanthanides. Hence, possible candidates are Sc, Y, La, Ac, B, Al, Ga, In, Tl, Gd, Dy, Ho, Er, Lu. Condition 2. The ionic radius must be similar to the ion it is replacing. Hence, one looks for an ionic radius near to 0.97 ,~, the value for Cd + +. Those which fit into this category are I n + + + = 0 . 8 4 ~, N d + + + = 0 . 9 9 5 ~, G d + + + = 0.938 ~, Dy ÷ ÷ ÷ =0.908 A, Er =0.881 ,~, Lu=0.85 ,~ [32].

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Using eq. (1), taking 7 as the dielectric constant and ½ for the reduction in mass due to the attraction of the atoms in the crystal, together with values of the ionisation energies [32], In, Nd, Gd, Dy, Er would satisfy the above condition. Values for Lu were not available but probably would also satisfy the above condition. At this point, a judgement can be made on the cost and the ease with which the impurity can be introduced into the crystal. From these criteria, indium is chosen as a suitable candidate.

5.3. Example of transition metal oxides The principles differ here to the extent that one starts (assuming the oxide has a low electronic conductivity) with a situation in which the two nearest valency states in the cation in the oxide (e.g., Cu ÷ and Cu ÷ +) are much further apart than kT. One then seeks to introduce an impurity oxide into the CuO, the cations of which would ionize easily in donative interaction with the cations in the CuO. In this way, a hopping mechanism of conductivity may be introduced.

6. Summary (1) It may be better to attempt to achieve a light-based water-splitting by means of semiconductor-solution junctions than by the use of biological materials. (2) The decomposition of water to form hydrogen and oxygen, with by-product electricity has been achieved but only at 0.6~ efficiency. (3) The four important parameters which have to be varied are: (a) the energy gaps ; (b) the flat band potential ; (c) electrode conductivity; and (d) competing electrode reactions. (4) A fundamental error has been made in previous theoretical considerations of the present systems: they have been treated as though the semiconductor solution interface was an npjunction. Semiconductors in contact with electrolyte solutions usually contain a significant degree of surface states so that the location of the potential-difference is not inside the semiconductor (as assumed in the previous work) but partly across the Helmholtz double-layer. The interface must be treated like a semiconductor-solution interface, as first shown by M. Green. (5) A second fundamental error has been made in previous considerations of this field : it has been assumed that the rate-determining step is transport of chargecarriers in the semiconductor. Although all interfacial reactions will be controlled by this factor at sufficiently high rates (when the interfacial reaction is being driven by an outside source), the cause of the current-potential relation at low overpotential exhibits a region where d2//dq2 > 0, which is indicative of another rate-determining step, probably interfacial charge transfer. (6) The potential at which oxygen is evolved in a spontaneously-acting photohydrogen generator is more negative than that at which hydrogen is evolved and is far displaced from the region in which 02 is evolved in the absence of light. This eases some of the (thermal) corrosional possibilities.

V. Guruswamy and J. O' M. Bockris / Contrived photosynthesizers

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(7) Chemical attack with evolved oxygen on anodes does not constitute a problem. (8) It is proposed that competing photoreactions (which may destroy the anode) are shifted away from a potential at which they may interfere with 02 evolution by the use of aprotic solvents. (9) In considering competing corrosion reactions, the key criterion is the fiat band potential of the semiconductor. With the energy gap this determines the potential range in which the semiconductor may undergo competitive (instability-causing) reactions. (10) Detailed theoretical considerations suggest that the criteria for suitable anodes in photogenerators is: an energy gap of 0.7 to 2 eV, an electron affinity between - 5 and - 5 . 6 eV, and a fiat band potential of - 0 . 3 to - 0 . 8 on the hydrogen scale of potentials. (11) Among interesting candidates for photo-anodes are transition metal oxides. They do not behave like band gap semiconductors. Activation of individual bonds is the important factor. (12) The principles of doping, well known for band gap semiconductors, must be modified for transition metal oxides; the introduction of impurities giving energy states within k T of the prevailing valency state is the key to provoking sufficient electronic conductivity. There may be no appropriate impurities in a significant number of cases.

Acknowledgement Thanks are due to Dr. O. Greiss for discussion of doping and inorganic structures.

References [1] T. B. Reed and R. M. Lerner, Hydrogen Energy, ed. T. N. Vezroghu (Plenum Press, New York, 1975) part B, p. 1266. [2] J. R. Benermann, J. A. Berenson, N. D. Kaplan and M. D. Kamen, Proc. Nat. Acad. Sci. 70 (1973) 2317. [3] G. Neil, D. J. D. Nicholas, J. O'M. Bockris and J. F. McCann, Heliotech. Develop. 1 (1976) 481. [4] R. R. Adt, H. Greenwall and M. R. Swain, Hydrogen Energy, ed. T. N. Vezroghu (Plenum Press, New York, 1975) part B, p. 727. [5] Delducca and Fucsoe, J. Sci. Technol. 1 (1965) 42. [6] J. O'M. Bockris, Nature 224 (1969) 775. [7] J. O'M. Bockris and M. S. Tunulli, in course of publication. [8] P. Mitchell, Nature 191 (1961) 144. [9] T. P. Hoar and U. R. Evans, Proc. Roy. Soc., London A137 (1932) 363. [10] C. Wagner and W. Traud, Z. Electrochem. (1938). [11] J. O'M. Bockris, S. U. M. Khan and D. D. Mathews, J. Res. Inst. Catalysis Hokkaido Univ. 21 (1974) 7 [12] K. Ohashi, J. McCann and J. O'M. Bockris, Nature 226 (1974) 610. [13] K. Ohashi, J. McCann and J. O'M. Bockris, Energy Res. 1 (1977) 259. [14] A. C. C. Tseung and S. Jasem, Electrochim. Acta 22 (1977) 31. [15] A. Damjanovic, D. Sepa and J. O'M. Bockris, J. Res. Inst. Catalysis Hokkaido Univ. 16 (1968) 1.

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