lec-16 prospective usage of photoelectrochemistry for environmental control.pdf

7/28/2019 Lec-16 PROSPECTIVE USAGE OF PHOTOELECTROCHEMISTRY FOR ENVIRONMENTAL CONTROL.pdf

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PROSPECTIVE USAGE OF PHOTOELECTROCHEMISTRY FOR ENVIRONMENTAL CONTROL

S.K. Haram and K.S.V. Santhanam

Chemical Physics Group, Tata Instituteof Fundamental Research, ColabaBombay 400 005, India

1. INTRODUCTION:

Solar energy has been playing a vital role in maintaining acleaner environment ever since this universe was formed. The photosynthesiswhich is occurring in green plants has been converting COz into usefulproducts; however, the deforestation during the last two decades hascaused an alarm as the process of C02 removal has been reduced. A globalwarming due to an increasing concentration of CO and the green houseeffect are of considerable concern in todays environment. The idealcontrol of the concentration of C02 in the atmosphere would be to convertit at the point of its entry into the atmosphere. If we examine todaysinput to this toxic waste (C02 is less toxic than CO; toxicity level ofCO = 5000 ppm, toxicity level of CO= 50 ppm) it arises from industrialwastes and automobile exhausts. The decontamination of this can be broughtabout by mimicing the photosynthetic process in a modified form i.e.,

2tilising the photons of the sun in an appropriate manner to convert COinto useful or less toxic waste. Among other toxic wastes entering into ouratmosphere are H S, N 0, O, HCN, hydrocarbons etc..

2

2

2 2

Photoelectrochemistry can play a significant role in creating acleaner environment by converting the above chemicals into other valueadded products. Although this concept has been in discussion in thelast two decades, the practicality of it is slowly building up in recenttimes as a result of a large number of fundamental studies at thesemiconductor/electrolyte interface. Several oxidative and reductiveprocesses have been brought out in the last several years (1-10) at thesemiconductor electrodes. An excellent review ( 1 ) on photoelectrochemicaloxidation processes summarises the developments in this area. The presentreview discusses the photoelectrochemical reductive processes that have


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been carried out at the semiconductor electrodes which have a bearing onthe environmental control by the polluting gases.

----

2. PHOTOELECTROCHEMICAL REDUCTION OF C02

a) At bare semiconductors:The semiconductors which are suitable for photoelectrochemical

reduction of C02 are shown in Figure 1.

-'TwIv,ui>> 1dS

Eo= -0.096VAlAs CO*/CH30H

Lo= -0.16VC02/HCH0EO= -0.1 1vco,/co

Figure 1. Band diagram of selected semiconductors for C02 reduction

A wide selection of semiconductors is capable of effecting this reduction.Out of these several semiconductors, atleast three of them have beenexamined in detail. p-GaAs single crystal electrode has been used by Zafriret a1 (11) . The photocurrent onset potential for CO was observed at +O.O5Vin 3.1 M CaC12 containing 0.87 M HC1 and 0.07 M V( I I ) chloride. TheV(II)/V(III) couple was used to bring about efficient electrontransfer reaction at the semiconductor electrode. A faradaic yield of 1.2%for HCOOH has been obtained at a potential of -0.7 at a current densityof 8 . 5 mWcm . The optical to chemical energy conversion efficiency is

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2


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estimated at 0.2%. The other products have the following faradaic yields;HCHO = 0.3% nd CH OH = 0.13%. he following step wise conversion of C02 isproposed at the semiconductor electrode.3

co2 + e -> COO-' ( 1 )COO-' + H +- HCOO' (2)-HCOO'+ e -> HCOO (3)

This conversion produces H2 as the bye product through direct reduction ofH+. The p-CdTe and p-InP electrodes have also been investigated (12);while the reaction scheme is essentially as indicated above, the netformation of formic acid is demonstrated to be a function of the pH of themedium. Hence using the solutions of supporting electrolytes such ascarbonates, sulphates, phosphates and perchlorates of alkali salts ortetra alkyl ammonium salts, the reaction ( 4 ) has been effected.

co-'+ H ~ O e ->2The possibility of adsorbed COconversion at p-CdTe has beenindicated 7o,.C02+ e

HCOO-+ OH- ( 4 )

playing a role in the photoelectrochemicalreported (13). The following steps are

( 5 )-co ' + co2-- 5 o=c-0-co-0- ( 6 )2

O=C-0-CO-O-+- o=c-o-co-6 ( 7 )o=c-0-co-0- > coz- + co (8)This mechanism has been invoked to account for the different productsobtained at different electrodes such as Pb and p-CdTe. Table 1 summarisesthe conversion efficiencies of CO at the semiconductor electrodes. It isinteresting to note that at p-GaP in Na2C03 electrolyte, a very highconversion of CO to HCOOH is reached, while at p-CdTe in the sameelectrolyte a very low conversion is obtained. A l so the conversion of

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C02 to CO is efficient at p-CdTe in non-aqueous electrolyte. Theconversion of C02 to CH OH is very high at p-GaAs at a neutral pH. While norationalisation is possible on the basis of the band diagram, it appearssome specific adsorption of the electrolyte and C02 might be playing a rolein the conversion.

3

Table 1: Photoelectrochemical reduction of C02 at semiconductorelectrodes.

Incident Medium Bias Faradaipower Applied yieldemiconductor Effi Refc ency/oV 1 2 3 4mW/cm

D-GaAs 980 0.32 M -0.50 0.032 0.0180.007 - 0.045 11VClz in4 M HC1 -0.65(8.4 ours)

90 0.07M 0.012 0.003 0.0013 - 0. 0 11VC12 in4 M HC1

p-GaAs

p-GaAs

p-CdTe

- - -. 5 M -1.05 0.07 0.003 - 1laKC1

- -Na2S04 -1.15 - - 0.1 - llbto 0.8H2

- - - -0.1 M -1.6 - 15TBAPin DMF0.1MNH4C104

- - - - --CdTe 3.2 0.1 M 13TBAF inCH3CN

p-CdTe 0.8 0.1 M -1.20 0.48 - - 0.066 - 12


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Table 1 (contd)Na2C03

p-CdTe 0.8

p-CdTe -

p-GaP -

p-GaP -

p-InP -

p-si -

0. 1 M -1.20 0.20 - - 0.65 -TEAP0.1 M -1.20 0.076 - - 0.78 -TEABr0.1 M -1.20 0.088 - - 0.51 -TEACl

12

- 1 d.1 M -1.35 - - 0.78TBAPH20/DMF0.5 M -0.75 0.78 0.024 0.005 - - l l aNa2C03R N'C10; -1.00 0.12 - - 0.03 - llc4H2

0.1 M -1.20 0.36 - - 0.26 - 12TEAP0. 1 M -1.20 0.28 - - 0.31 -TEABr0.1 M -1.20 0.31 - - 0.28 -TEAC10.1 M -1.20 0.17 - 0.08 - -Na2C030. 1 M -1.20 0.04 - 0.16 - -Na2S040.1 M -1.20 0.023 - 0.10 - -Na3P040.1 M -0.75 - - 0.63 - - 17LiC104CH CN/H203

The numbers i n the table header represents:1.HCOOH 2.HCHO 3. CH30H 4. CO


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The photoelectrochemical reduction of C02 at semiconductorthe H2 yieldelectrodes also produces H2 in appreciable amounts (14);

depends on the nature of the semiconductor surface.

The mediation of photoelectrochemical reduction of CO, byammonium ions has been reported by Bockris et a1 (15). The first step inthis mediation is the reduction of NH; and follows sequences (9) through

II0.S n i A cm- '

I I I 1-0 5 -1.0 -1.5 -2 -2.5wv vs SCE

Figure 2. Current-voltage curves in DMF containing 5% H200.1 M TBAP (in Ar atmosphere).

containingNH4C104in C02A. 0.1 M NH C104 (in Ar atmosphere). B. 0.1 M

atm~sphere,~C.Taken from reference (15).


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The current efficiency for CO formation in the presence of NH C10 dependson the water content. Figure 2 shows the current-potential curves at p-CdTeelectrode in DMF in the presence of NH4C104. It is interesting to notethat the onset of photocurrent is substantially shifted to a negativepotential in the presence of 0 . 1 M TBAP. The ammonium ion mediated reductionis obvious in the current-voltage curves.

4 4

b) Semiconductors with deposited Catalysts:

Several semiconductors coated with catalysts have beeninvestigated (14-18) or increasing the efficiency. At a metal coated p-GaP(14) the faradaic efficiency for C02 reduction is about 50% in propylenecarbonate containing tetra alkyl ammonium salt; in comparison the aqueousmedium produced a faradaic yield of a few percent. The products innon-aqueous solvents are (COOH)2, HCOOH, CO and H2, The earliest attempt tocatalyse the C02 reduction at p-Si was carried out by usingtetra-aza-macrocyclic complexes of Co(I1) or Ni(II1 (17,191. t thismodified semiconductor, the catalysed electrochemical reduction of CO

2O occurs at -0.55 V vs. SHE; the photoelectrolytic reduction produces Hbesides CO in the ratio of CO/H of 2:l. The modified electrodes can bringabout the reduction of C02 to CO in 100% faradaic yield. However, poisoningof the semiconductor surface by either carbon or CO has been postulatedfor the decreasing efficiency with time.

2 to

2

Table 2. The photoelectrochemical reduction of CO at a metal coated2semiconductors*Serni- Incident Medium Bias Faradai Efficiency Refconductor power

2%oltage yield (%I

vs.Ag/ 1 2 3 4c m AgC--CaP 1.7 0.1 - 1 . W 2.6 - 2.6 65 14

(uncoated) TEAPPC

p-GaP/Au 1.7 0.1 M -1.2v 8 6 94 trace 108TEAPPC14


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Table 2 (contd)

p-GaP/In 1.7

p-GaP/Zn 1.7

p-GaP/Pb 1.7

p-GaP/ 2+ -Ni(cyclam1

p-GaP/ 2+ -Ni(cyclam1p-GaAs/ 2+ -Ni(cyclam1

p-Si/Tetra -azo macrocyclic comp-lexes of Ni

0.1 M -1.2 7 7 107 trace 121 14TEAPPC0.1 M -1.5 1 8 100 trace 109 14TEAPPC0.1 -1.2 38 11 41 trace 90 14TEAPPC0.1 M -0.2 - 83 16 - - 16NaC O4

0.1 M -0.75 3 3 85 8 - 16NaClo40.1 -0.95 - - 47 30 - 16KC104H20.1 M -0.75 - - 63 32 - 17CH3CN/H20LiC104

The numbers in the table header represents* Faradaic efficiency

1.(COOH)2 2.HCOOH 3.CO 4.H2

Bockris et a1 (20a) proposed a model for photoelectrocatalysisat metallised semiconductors.

3. PHOTOCATALYTIC REDUCTION OF C02

The use of photocatalytic semiconducting powders foreffectively converting C02 into one organic such as HCOOH or HCHO or CH30Hor CH3CH0 or C H OH has been carried out by a large number of investigators(21-25).Ulman et a1 (21,221 ave used SrTi03 or Ti02 powders for effecting2 5


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the above conversion. At Sic the reduction of C02 proceeds to yield CH3CH0and C H OH (23); by metallising SIC the photocatalytic reduction follows adifferent route as shown in Table 3.2 5

Table 3. Photocatalytic reduction of GO at semiconductors2Semiconductor Metal HCOOH HCHO CH30H CH CHO EtOH Effici Refermol% p~ p~ p~ p i l p~ ency ence

%

SiC/Pb

SiC/Pd

SiC/Pd

SiC/Fe

SiC/Fe

SiC/Pt

s c/cusic/cu

sic

0.44

0.011

0.020

0.80

1.03

0.30

0.25

0.93

1.2 1 . 1

1.6 1.3

2 .0 1.6

1.6 0.95

2.3 0.86

2.1 1.2

1.5 0.90

1.3 0.74

1.9 0.40

0.20

1.6

1.3

1 . 1

1.5

1 . 1

0.78

0.65

0.50

0.018 4.4

0.014 9.8

0.14 10.0

6 . 8

0.05 11.0

0.03 8.0

0.016 5.8

- 4.8

- 4.2

24

24

24

24

24

24

24

24

24

Heat of combustion of productsx 100Eff ciency = Incident 1ight energy

The formation of methanol and ethanol is considered throughphotoexcitation of Sic powder and HCHO formed in the reaction. For example,the ethanol formation is sequenced as

wHCHO + HCHO-CHO-CHO + 2 H+ + 2 e (13)


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CHO-CHO + 2 H++ 2e -> CH CHO + 1/202 (14 )3CH~CHO + 2 H++ 2 e- ~ H ~ O H (15)

where the electron is contributed by the photoexcited Sic powder; H+ issupplied by H20 via. the oxidation by Sic.

4. PHOTOELECTROCHEMI AL DECOMPOSITION OF H2S

The photoelectrochemical conversion of H S to H was reported byKalyanasundaram and Gratzel (26a1, Crzyll et a1 (26b) and Chun et a1 (83).Kalyanasundaram and Gratzel carried out the conversion of H S to H2efficiently with CdS loaded with 0.5% Ru02 at a quantum efficiency of 35% .Chun et a1 conducted the photoelectrolysis of aqueous sulphide at n-Sicoated with polypyrrole films and then loaded it with Ru02. The followingsequence of reactions is postulated for the decomposition of S

2 2

2

2-

n-Si +hv -> h++ e (16)

2 H++ 2e - ~ (17)The photocorrosion of the semiconductor has been reduced by the polymercoating. The simultaneous generation of H2 is an advantage of this process.This method gives a power conversion efficiency of 0.6% and a quantumefficiency of nearly 1%.

5. PHOTOELECTROCHEMICAL CONVERSION OF SO2

The photoassisted reduction of SO at p-WS2, p-Si and p-InP hasbeen reported by Calabrese et a1 (27). The light assisted reductionproduces S20z- via. the reaction

2

SO2 + SO2 + 2 e - s 2 0 y (19)


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The current efficiency for this conversion is reported to be >90% at allthe electrodes; the naked semiconductors such as p-InP show sluggishkinetics and hence requires metallisation. Figure 3 shows current-voltagecurves for SO2 reduction at these semiconductor electrodes. At p-InPmodified by Pt, a power saving efficiency of

I I l l um i na t ed

T

P t') - l m M S 0 20 [o -Euq N] C 104 /

CH,CN100m V/s

I l lumina ted/ I n pplat in izellurninalQ -1nP

(naked)

1 ! I 1 1 1-I 2 -08 - 0 4

Potent ia l , V v s A g / A g t,

FIGURE 3. Current-voltage curves for 1 mH SO2 in 0.1 bl TBAP in CH3CNat Pt, P-si p-InPzand P-usz. Irradiation wavelength:632.8 nm(40 mW/cm 1 .Taken from reference (27).

2about 11% has been reported using 514.5 nrn (100 m W c n ). The solarefficiency for the conversion of SO2 to S20:- is expected to besignificant.


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6. PHOTOELECTROCHEMICAL REDUCTION OF O2

The photoelectrochemical reduction of O2 to H20z is anindirect process.The early investigation into it was carried out byCalabrese and Wrighton (28). The photoelectrochemical process of reductionof 2-t-butyl-9,lO anthraquinone in CH3CN was carried out in the presence ofCH3COOH using 632.8 nm radiation.electron transfer) process occurs resulting in the formation of thecorresponding hydroquinone. A 2% photoelectrochemical energy conversionefficiency was obtained for this process. The hydroquinone reacts with O2to yield >0.18 H202 by the following scheme

An ECE (electron transfer-chemical-

Keita and Nadjo (29) used a single redox couple, sodium9,lO-anthraquinone 2,6-disulphonate in the regenerative photocellsconstructed with p-WSe2 and p-Si photocathodes where the followingreaction occurs

( 2 0 )O2 - H202+ I2HI +The synthesis of H202 by the following route has also been proposed by theabove authorsCat a1 yst

AQ -> AQHZH2

(21)

AQH, + 02-> AQ + H22 (22)

where AQ represents anthraquinone. This scheme is similar to the one


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proposed earlier; however, there is a possibility of a side reaction ofH202 with AQH2 reducing the overall yield. The formation of H 0 at theilluminated Ti02 film electrode by the sol-gel method has also beenproposed (30 .

2 2

7. PHOTOELECTROCHEMICAL GENERATION OF H2

Perhaps one of the exciting interests in this field whenFujishima and Honda reported (31) photoelectrolysis of H 0 at n- Ti02 wasthe generation of H2 at the cathode. As this electrolysis uses a widebandgap semiconductor (E =3.2 V), the practical utilisation of it has beenlimited. The prospects of using small band gap semiconductors has beenexplored in the last few years by a large number of investigators (32-35).The previous attempts by others have already been reviewed (36-43) n theliterature. The photoelectrochemical hydrogen evolution using metallisedsemiconductors have been investigated (44,55-60) s the metal catalystsincreased the Gibbs free energy efficiency upto about 13%. Aspnesand Heller (44) roposed that the work function of the metal catalyst withreference to the semiconductor brings a true ohmic or nearly ohmic contactwith a substantial collapse of the barrier between the metal and thesemiconductor upon hydrogenation. Figure 4 depicts the work function ofmetals and band edges of semiconductors and the processes occurring at thesemiconductor. It is observed (44) hat there is a finite delay time whenno gas evolution takes place until the separate hydrogen and oxygenenveloped the catalyst islands are established. However, the oxidesemiconductors produce trapping effects which are not reproducible. Thehydrogen evolution at small band gap semiconductors such as n-CdS would beaccelerated by Rh or Ru than Pt. This conclusion is well supported by thehydrogen evolution on n-CdS deposited with Ru02 (44) nd absence of it whendeposited with Pt (for this discussion the work function of Ru02 is 4.8 Vwhich is close to Ru metal). The formation of ohmic contact due to thesurface damage during sputtering of metals onto the semiconductors has alsobeen reported (46); this has been demonstrated for n-CdS where surfaceregions are sufficiently damaged. The hydrogen evolution at a Pt modifiedp-InP electrode has been investigated in detail (32). This modificationproduced an efficient and stable solar to chemical energy conversionefficiency of 9.2 % by etching the electrodes in Conc. HC1. The formation

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g


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z -63I-VWJW

a

-1

-8

c

(1

xzI-VW-I

ANODE2h++n20-n '+ vz o

Cd s-1;;?

A I

-I 3IB

CATHODE2C-+ZH++#-HZ

I n - T i O p

FIGURE 4. A. Semiconductor band edges and work function of metalsB. The band bendings of a semiconductor in assymnetric junctionsTaken from reference ( 4 4 1 .

of minute Pt islands on InP is recommended where the InP is in directcontact with the solution. The deposition of Pd on the semiconductor i n theplace of Pt produces a very poor response. The efficient cell performanceis attributed to the interface states produced by hydrogen; the adsorptionof hydrogen on the metal surface and consequently decreasing the metalwork function and the introduction of high barrier have been considered tobe ineffective.


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Several doped oxide materials have been considered for efficientproduction of hydrogen in the photoelectrolysis experiments (34,351.Somarji's group (35) produced hydrogen from water electrolysis byilluminating Mg doped (p-type) and Si doped (n-type) iron oxides. Thephotoelectrochemical properties of p-CaFe 0 and p-SrFe10022 with band gapsof 1.8 eV and 1.9 eV respectively have been considered for hydrogengeneration (34). Figure 5 shows the band diagram for thesesemiconductors. he deposition of Pt on the surface of both electrodesincreases the hydrogen evolution reaction. The conversion efficiency fromlight to H2 and O2 was estimated to be about The long termstability of CaFe204 is higher than Sr7Fe10022. Several other oxides havingFe in their lattice have been considered as photoanodes with band gapenergies of about 2.1 eV on the basis of the stability considerations(36-43).The kinetics of photoelectrochemical oxidation of water has beenconsidered at n-Ti02 semiconductor electrode (46-48). he possibility of

2 4

I Current ( log i )

FIGURE 5. A description of the electron transfer process atand band structure description for CaFe 0 and Sr2Fe100z24s and d orbitals is shown. e 1,and awith trigonal distribution of the lattice are also indicated inthe figure. Taken from reference (34).

CaFe 0e 5. band formed by crystal field spligting of irong . AI hopping bands8 1g


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photointercalation in semiconductors has been considered (49,501 and it hasbeen shown that the intercalation in p-WSe2 causes a high overpotential forhydrogen evolution reaction. The quantity of hydrogen corresponding to

20 monolayers coverage has been obtained by heating p-WSeTungsten-silicon and other modified electrodes have been examined for thestability in the continuous generation of H2 in photoelectrolysis (51).p-Si surface stabilised by hydrogen has been shown to produce H2 for morethan 48 h at a c.d. 240 mA/cm with no degradation. The characteristics ofNi modified p-Si have also been examined in KOH solutions (52). The metalcatalysts incorporated into p-Si have been investigated for the hydrogenevolution (53) in acidic solutions. It has been shown that Pt/p-Si contactis nearly ohmic; good rectifying character develops by etching thiselectrode in alkali. A similar result has been obtained with a Au layercovering p-Si. In effect this indicates the barrier height is increased atthe junction. The structure of the metal layer plays a role in the photoactivity. If the semiconductor is coated with minute islands of the metalit causes H2 evolution at positive potentials than the bare Pt or barep-Si. A discontinuous metal covering produces a good photovoltaic activity(53). Photosensitised reduction of water at Ru02 has also been examined(54) and the activation energy for hydrogen evolution has beensubstantially changed by the addition of anthracene carboxylates. Thephotoevolution of H2 at metal catalysts has been examined by a number ofinvestigators (55-60); conclusive evidence exists that the metal islandsare necessary for a good activity. A short circuited photoelectrochemicalcell has been used with sintered CdS pellets with Pt deposits for theoxidation of HCOOH and simultaneous evolution of H2 (61);3 po l of H2 in180 minutes was generated with an input energy of 1.2 kJ m s . Thesimultaneous generation of H2 and 0 using Ti02 and Ru(bpy):+ has been2considered as a function of grain size and amount of Ti02 (62). Theaddition of thiourea and 2-aminopyridine has been shown to suppress thephotocorrosion of Ti02 (65). A physical model for water splitting to H2has been described using semiconductors (63). The need for developing newmaterials for producing H has also been emphasised (64,661.Popkirov andPleskov (67) proposed a new system for hydrogen production by waterelectrolysis and Siege1 and Schott (68) have suggested the optimisation ofphotovoltaic cells for hydr0gen.A derivatized p-InP has been used in thephotoelectrochemical production of H2 (69).

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-2 -1

2


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The generat ion of hydrogen using small bandgapsemiconductors while stabilising from photo-corrosion has been quitechallenging and has been innovatively solved by the concept of vectorialcharge transfer in vertical and circular configurations (70,711. n thismethod a higher driving force for the decomposition of water has beenobtained by the vectorial addition of voltages. The ultimate decompositionof water to hydrogen and oxygen occurs at the two Pt electrodes. A longterm stability of the semiconductor is reached by using a polysulphide orpolyselenide medium (72). polymer coated semiconductor electrode in placeof the naked electrode in the vectorial charge transfer assembly has alsobeen used successfully (73-75). The production of H2 using p-typetransition metal phosphides and p-type molybdinium sulphides have also beenreported (76,77). The function of Co and Pt on p-InP in the evolution ofH has been explored by Goodman and Wessels (78,791 nd Kobayashi et a1(80). Pt intercalated to K4Nb6017 has also been examined for waterdecomposition (81).2

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