Structural and electronic properties of anodes in photoelectrolytical cells

Solar Energy Materials 11 (1984) 209-221 North-Holland, Amsterdam

209

S T R U C T U R A L A N D ELECTRONIC P R O P E R T I E S OF A N O D E S IN PHOTOELECTROLYTICAL

CELLS

N.A. M A N C I N I , A. P E N N I S I a n d F. S I M O N E Istituto Dipartimentale di Fisica dell' Universiti~ Catania, Gruppo Nazionale di Struttura della Materia del CNR Unit~ di Catania, Progetto Finalizzato Energetica del CNR, Centro Universitario di Microscopia Elettronia Catania, Corso Italia, 57, 1 95129 Catania, Italy

The physical properties of TiO2 rutile n-type semiconductor as an anode in photoelectrolytical cells have been widely investigated from many authors in order to explain the exhibited photoresponces. In this paper the attention is focused in particular to characterize by backscattering, PIXE (proton induced X-ray emission), and X-ray diffraction analysis titanium surfaces oxidized at different temperatures in oxygen flux. Using a Schottky barrier model for the surface of the semiconductor in contact with the electrolyte, the meaningful physical quantities like minority carrier diffusion length, barrier width and the quantum efficiency of cells result strongly dependent on oxidation temperature of the TiO2 electrode. The behaviour of these quantities is interpreted in terms of structural properties of the anode and analyzed with the above mentioned methods.

1. Introduction P h o t o e l e c t r o c h e m i c a l cells have recently been s t u d i e d with the a i m of realizing an e c o n o m i c a l a n d alternative source of energy b y p r o d u c i n g h y d r o g e n as fuel. A l t h o u g h the theoretical b a c k g r o u n d of the p h o t o e l e c t r o l y t i c effect is quite well known, recent efforts have been m a d e in the direction of u n d e r s t a n d i n g the charge transfer m e c h a n i s m between an i l l u m i n a t e d s e m i c o n d u c t i n g surface a n d redox c o u p l e s in an electrolytic solution where the same s e m i c o n d u c t o r is immersed. F o r this p u r p o s e it is necessary to c o m p a r e the p h o t o r e s p o n c e s e x h i b i t e d b y s e m i c o n d u c tors with the p r e p a r a t i o n m e t h o d of the samples, as suggested b y T o m k i e v i c z a n d F a y [1]. In this p a p e r we r e p o r t e x p e r i m e n t a l results o b t a i n e d b y observing the p h o t o c u r r e n t g e n e r a t e d in p h o t o e l e c t r o c h e m i c a l cells having t i t a n i u m d i o x i d e (n-type) as a s e m i c o n d u c t i n g a n o d e as electrode. A t t e n t i o n is n o t p a i d to the p r o d u c t i o n of h y d r o g e n which, as well known, when a T i O 2 a n o d e is used, requires a p h o t o a s s i s t e d electrolysis b y the a p p l i c a t i o n of an external bias to the cell. Peculiar a t t e n t i o n has b e e n p a i d to correlate the p h o t o r e s p o n c e exhibited b y the cell with crystalline a n d chemical p r o p e r t i e s of T i O 2 s a m p l e s which were o b t a i n e d b y t h e r m a l o x i d a t i o n at different temperatures. O b s e r v e d results are a n a l y z e d using a S c h o t t k y b a r r i e r m o d e l of the s e m i c o n d u c t o r - e l e c t r o l y t e interface. T h e m e c h a n i s m of the p h o t o e l e c t r o l y s i s in now well k n o w n a n d can be s c h e m a t i z e d as in fig. 1 [2-7]. If r a d i a t i o n of o p p o r t u n e frequency ( h p > E C) is a b s o r b e d b y the s e m i c o n d u c t o r , an e l e c t r o n - h o l e p a i r is generated. This p a i r is then s e p a r a t e d b y the electric field 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

210

N.A. Mancini et al. / ,4 nodes #1 photoelectro(vtical ceUs

present in the depletion layer so that a redox process is possible when redox levels are present in the electrolyte, also if rate limiting effects like recombination processes have to be taken into account. There is in practice, in reverse bias, a transfer of electrons to the cathode and of holes from the semiconductor surface to the electrolyte. In fig. 1 the band bending is also shown, caused by the space charge region at the semiconductor-electrolyte interface due to the difference in the electrochemical potential of the two regions. This diagram also shows the thypical energy levels for an n-type semiconductor and an electrolytical solution containing the redox species of water, where the flat band potential Vm is slightly more negative than the electron affinity of the oxidized species H +. When, as in the case of TiO 2, the flat band potential is more positive than the hydrogen electrode potential, there will be no reaction and electrolysis cannot proceed without an external bias. A large amount of theoretical work is in progress to interpret the J ( V ) characteristics exhibited by photoelectrolyical cells [8-3]. These studies essentially concern the extension of the simple model proposed by G~rtner [14] in order to include recombination effects which are present both in the space charge region (SCR) and in the surface through surfacial states [15]. In the case of reverse bias, when the semiconductor surface is depleted, it can be assumed that the potential drop across the Helmoltz double layer is constant and that the voltage variations due to applied bias or incident light, drop entirely across the space charge region. For an n-type semiconductor under illumination the steady state balance requires that the minority carrier flux flowing out of the depletion region must equate the net pair generation within the depletion layer. The current flowing out of the depletion layer is given by the difference between the hole current flowing from the semiconductor to the electrolyte (Jp) minus the flux of holes that, generated in the neutral region, reach by diffusion the depletion region (jpD). The net generation rate within the depletion layer is the generation due to photoabsorption (Js~R) minus the surface recombination ( j R ) and the space charge region recombination (JR-R). The balance equation can be written as follows: jp _jpD =Js~R - J #

- JS~'R-

(1) E .eV~b

to

counter elect ro(:le

E6 ,/.//

/.//~

metal

h-~

~ 3 eV O,/H,O

type-n el ectrolyte semiconductor

metallic counter electrode

Fig. 1. Energy level diagram of an electrolytical cell.

N.A. Mancini et al. / Anodes in photoelectrolytical cells

211

In the above equation a term of dark current was neglected. This current is due to majority carrier transfer from the conduction band to acceptor ions in the solution under reverse bias conditions [16]. This term is in any case negligible under high reverse bias. Following the procedure proposed by Ahlgren [17] eq. (1) can be rewritten as:

otLpGI exp( - aW) -~p (p-w-p°)+ e(1 +aLe)

Jp _ Lp

_

GI [exp ( - a W ) - l] -vs( P*w- P o ) - R( P* ]'/Z exp [ e ( V - Vie) ] e ~o] KT

(2) The second and the third term of the left-hand side of this equation represent j D as deducible from the solution of the steady state continuity equation for the minority carriers, Lp and rp being the minority carrier diffusion length and their lifetime, respectively Pw* and P0 are the concentrations of holes at the inside edge of the depletion region assumed to be of width W, respectively in non-equilibrium and equilibrium conditions, P0 is determined by the thermal generation of electron-holes pairs, e is the electron charge. a = a ( ~ ) is the absorption coefficient of the radiation, I is the total radiation intensity, G is a factor defined in such a way that the ratio GI/e gives the same total number of pairs created and also the same number of pairs created within the same distance from the surface. In other terms G can be obtained by normalizing the charge transfer flux to the incident light, so G is strongly dependent on the reflectivity of the semiconductor surface. The first term of the right-hand side of eq. (2) is the drift current density due to carriers generated inside the depletion layer (JGR). The term vs ( p * - P 0 ) defines the surfacial recombination, vs being the velocity of such recombination [12, 15-19]. Finally the quantity

R/ Pw/Po , , ,1/2 ) e x p [ e ( V - Vtc)/KT ] is the space charge region recombination current density: R being a factor which depends on the voltage and on the barrier profile [16,20,21], Vt¢ is the threshold voltage for the dark current intensity, K the Boltzmann constant and T the absolute temperature. The barrier width, in the depletion approximation, as a function of applied potential can be expressed by [22] W= tb(V-

Vfb)1/2,

(3)

where Vfb is the semiconductor potential for which there is no bending of energy bands, so that the potential energy of the carriers is a constant from the bulk to the surface (flat band conditions). In this situation there is no electric field at the semiconductor surface which can separate the charge carriers of opposite sign. Under these conditions strong processes as recombination are present, so that the probability for carriers to reach the surface is almost negligible. This potential is not

212

N.A. Mancini et al. / Anodes' in photoelectrolrtical cells

always deducible from the J(V) characteristics because the onset of the current with the voltage can be controlled by recombination effects [23]. The quantity

L b = (2Ceo/eUo)1/2

(4)

is the depletion layer width for a potential of 1 V across it, Nt) is the density of the donor centres, c is the semiconductor relative dielectric constant, co is the permittivity of the free space. Eq. (2) solved for the ratio (P*/P0) gives:

P* G =I I+GIq)s(V)-JP q ) ~J~( V ) 2J, eR- v~/'J~R2 ~ Sp- ]+ 4p[1 +~ .

L

,

(5)

L

where exp(-aW) 1 + aLp and

(6)

(Lp )

J~ epo ,rp =

- -

-

v~

(7)

is the saturation value of the dark current density. The last term on the right-hand side of eq. (5), due to the SCR recombination, can be neglected if the minority carrier diffusion length Lp is much larger than the width of the depletion layer so that each hole within this region can reach the surface. In these conditions only the surfacial recombination has to be taken into account and eq. (1) becomes jp _jpD = J ~ R _ j R .

(8)

Ahlgren [17] derived the J(V) equations in the hypothesis that the semiconductor follows a Tafel law. The net photocurrent, when the surface is depleted, is given by:

J(V)-

Jdark( V ) =

GI@(V),

(9a)

where (9b) is the quantum yield and KT v,/2

= V,c - - ape

Jp,,

(10)

exp --ff

and

Jdark(V)=Js{1--exp[

e(VKT-Vtc)]}/(1 + e x p [

eap(V-KTVtc)1}"

(11)

ap is the transfer coefficient for electrons crossing the semiconductor-electrolyte

N.A. Mancini et al. / Anodes in photoelectrolvtical cells

213

interface from the electrolyte to the semiconductor valence band. Jpo and Jdark are the hole exchange and the dark current density, respectively.

2. Experimental arrangement and samples preparation The necessity to detect very low current densities due to the low intensity of the monochromatic radiation and the high time constants of the processes involved required an automatic control system to guarantee a good reproducibility for each set of measurements. The controlled parameters were the incident light wavelength and the time constant of a single measurement, while the acquired data were the external circuit photocurrent and potentials of the electrodes. It was necessary to use an automatic shutter to detect the net photocurrent Jx,v(light)-Jx.v(dark ). For these reasons we used an Automatic Data Acquisition System H.P. 3050B (A.D.A.S.). Steady conditions for each measurement were obtained by an appropriate choice of the time constant in data acquisition and of voltage and wavelength steps. For this reason several hours were necessary for the detection of each characteristic. The experimental apparatus is shown schematically in fig. 2. The light source was a point wire tungsten halogen lamp (150 W) quartz enveloped; the light was focused on the entrance slit of a McPherson 218 grating monochromator (regulated for a bandwidth of 50 ,A) or on a narrow band interference filter with a bandwidth of 10 ,h,. The power of the radiation leaving the filter or the monochromator was detected by an Eppley thermopile in 2600-6000 ,A wavelength range. The knowledge of this spectrum was necessary in order to normalize the experimentally obtained characteristics. The largest intensity obtained in this range was of the order 20 ~tW/cm 2. The photoelectrolytical cell which was furnished with an UV grade quartz

Vl i--~'

I

~

~

I C'r--"~ ~ ' ~ I

L_ J

Source Grating Shutter lamp monocrom,

I

I

~

Automahc D~,~ I Acquisition System

Cell with u.~z grade quartz window

Fig. 2. Schematic experimental arrangement.

214

N.A. Mancini et al. / Anodes in photoelectrolvtical eel&

window, consisted of the three standard elements, i.e. the TiO 2 semiconducting electrode, the black platinum counterelectrode and the N.H.E. reference electrode. These three electrodes were immersed in N a O H 1 Normal solution. The semiconducting electrode dimensions were 5 × 1 × 10 m m 3. The potential of the electrodes with respect to the reference electrode were directly measured using a high impedance ( > 1014 ~) voltmeter which was driven by the A.D.A.S. To obtain open circuit measurements an electronic switch S driven by the A.D.A.S. was inserted in the external circuit. The bias voltage V was applied to the cell by a D / A power supply programmer H.P. 59501A. This instrument allows minimum voltage steps of 2 mV. The voltage drop on 500 f~ calibrated resistance was measured in order to detect the photocurrent. This voltage drop was amplified by a dc nanovolt amplifier Keithley 140. Titanium dioxide of the rutile structure was obtained by thermal oxidation of titanium foils (purity 99.5%, furnished by Leico Inc.) in a dry oxygen flux. Each sample was thermally treated for three hours and a set of samples was obtained for each different oxidation temperature (500, 600, 700, 800 and 900 o C). The electrical contact on metallic bulk titanium was made by a Ti wire ( ~ = 0.5 mm) electrically solded to the foil before the oxidation process. A Siemens Type F diffractometer was used to carry out the analysis by X-ray diffraction, while P.I.X.E. (proton induced X-ray emission) and backscattering analysis were carried out by a Van de G r a a f accelerator, using 2 MeV protons and 2 MeV He + ions as impinging beams, respectively.

3. Experimental results Samples obtained as previously described have been analyzed by the X-ray diffraction technique. The radiation incident on the samples was the copper K , line (~ = 1.5405 A), the resolution in angle was 0.05 °. In fig. 3 the relative reflected intensities are shown as a function of the deviation angle 20 (0 being the Bragg angle), for samples oxidized at 600, 700, 800 and 900 o C. The peaks centered at 28 = 27.46, 38.46 and 40.15 correspond, respectively, to the reflections from (100) planes of TiO 2 rutile, from (111) planes of aluminum and from (011) planes of titanium [24]. All these three peaks correspond to the most intense reflections from the three elements. From the same figure it is deducible that raising the oxidation temperature Tox, the percentage of crystalline rutile also increases in the sample, while the presence of crystalline Ti is lower and disappears at Tox = 900 o C. The titanium foils employed showed the presence of a large percentage of aluminum, which exists in crystalline form up to Tox = 800°C. The apparent thickness of crystalline grains has been determined by the measurement of the half peak width of the reflection lines. The following expression has been used [25] X/t(cos 0) = 8(20),

(12)

where t indicates the grain thickness and 8(20) is the half peak width. The results obtained are listed in table 1. The presence of aluminum and other impurities, even

N.A. Mancini et aL / Anodes in photoelectrolytical cells

215

Table 1 Tox ( o C) 500 600 700 800 900

Grains apparent thickness (A) 180 240 350 470

P.I.X.E.

Backscattering

NAI/NTi

N~-X/NTi

1.44 × 10- i 10 -1 9.1 X 10 - 2 6 X 10 - 2 4.7 >(10 - 2

0.98 0.33 0.30 0.30

if non in crystalline form, has been veryfied and calculated by the emission of X-rays induced by a b o m b a r d m e n t with 2 MeV protons [26]. The resulting spectra showed the presence of AI and other impurities as Fe, Mn, Ni, Si, Ca, these were present in a very low percentage. A semiquantitative analysis was carried out on the spectra. In table 1 the quantity NAI/NTi is also reported, NAI and NTi being the n u m b e r of A1 and Ti atoms respectively. This quantity is corrected for a factor that accounts for autoabsorption p h e n o m e n a in the samples. These data also show a lowering in A1 presence when Tox is increased. Furthermore the backscattering spectra [27,28] supplied useful knowledge on the formation of oxidized layers. In fact, the Ti concentration has been calculated in the various samples against the concentration measured in non-oxidized samples. This quantity is in fact reduced by the formation of oxide layers. In table 1 the determined ratios are also reported. In fig. 4 the typical backscattering spectra obtained are shown. As is evident from

1 -Ti o x i d i z e d at 600'£

2.

.

.

.

700"C

.

3. . . . . 4 . . . . .

80O °C

900'C

1"

-~3

~ '

2746

3

II

J2 J I

38.46

-J~3

II

/.015

20

Fig. 3. X-ray relative intensities (arb. units) versus deviation angle 20 exhibited from titanium foils oxidized at various temperatures.

216

N.A. Manc'ini et al. / A nodes in photoelectroA, tical celL~

table 1, yields at the characteristic energy of backscattered He + ions from surface Ti atoms decrease when Tox is higher. Following the procedure proposed in ref. [17], the values of ap, V and Jpo have been obtained by plotting the quantity In[@( V)/(@~ - @( V))] vs. V according to eq. (9) and obtaining J~ and V,~ values directly from the J ( V ) characteristics. The experimental values of @(V) and its saturation value @~ were obtained using eq. (9) by calculating the slope:

G@(V)- dJ(V)dl v

(13)

of the straight lines belonging to the family obtained by plotting the current as a function of the radiation intensity with the voltage as the parameter. In fig. 5 the quantity: In [@(V)/(@~ - @(V))] vs. V is shown for samples oxidized at To~ = 500, 600, 700 and 800°C. Dots represent the experimental points, solid lines have been obtained fitting the data by a linear regression. The figs. 6a and b show typical experimental net photocurrent (dotted lines) flowing in the external circuit versus the

~\~Oo ~o <~ v o~° >-

o ~OoO~oOoOoO Oo~%o o %% o

%-,%%o°

o z

. . . . . . . ooo

T,

°°°%°°. °°°%°°°°'.°°°°°%OoOoO'bO°o°°°o°.o° °~

%:

k..

d3

05

I0

" Energy

i

• 15

(MeV)

Fig. 4. Backscattering spectra of metallic titanium (O) and titanium oxidized at To~ = 6 0 0 ° C (O) obtained with 2 MeV He + particles. The arrows indicate the energies of particles backscattered from surface atoms of titanium and oxygen.

b I

•

500°C

•

600°C

. 700°£ o 800oc

~ J o

. o / ,, ~ ,~Y ,~/.

2~-

dle~

?q Volt

Fig. 5. Plot of In[ ~ (

V ) / ( ~ s - ~( V))] vs. V for samples oxidized at various temperatures (see text).

N.A. Mancini et al. / Anodes in photoelectrolvtical cells

217

s e m i c o n d u c t i n g electrode p o t e n t i a l for samples oxidized at the s a m e t e m p e r a t u r e as above. Characteristics in fig. 6a were d e t e c t e d using m o n o c h r o m a t i c r a d i a t i o n with A = 3400 ,~ (AA = 10 ,~) as light source. The c o r r e s p o n d i n g a b s o r p t i o n coefficient a is 2 x 1 0 5 c m - t . In fig. 6b r a d i a t i o n with X = 3 8 0 0 ,~ ( a = 8 . 5 x 1 0 3 c m 1) a n d = 4000 ,~ (ct = 6.6 x 102 cm -1) were used respectively for samples oxidized at Tox = 700 a n d Ton = 800 o C. The a b s o r p t i o n s p e c t r u m for T i O 2 p r e s e n t e d in refs. [7] a n d [29] was used. In the s a m e figs. 6a a n d b the best fits of the e x p e r i m e n t a l d a t a are r e p o r t e d (solid lines). Eq. (9) in c o n j u n c t i o n with eq. (6) and (3) was used with G, a L h, c~Lp a n d Vrb as a d j u s t a b l e p a r a m e t e r s . D u e to the large n u m b e r of the involved p a r a m e t e r s , a c o m p u t e r p r o g r a m for function m i n i m i z a t i o n to d e t e r m i n e the best p a r a m e t e r values was used [30]. In table 2 the o b t a i n e d values of the p a r a m e t e r s are s u m m a r i z e d for different samples. Electrical resistance exhibited by these s a m p l e s is listed in the last c o l u m n of table 2. T h e electrical resistance of the samples has been m e a s u r e d observing the difference of p o t e n t i a l between an electrical c o n t a c t m a d e up of the Ti wire c o n n e c t e d

•

5000C

600°C ;k=3AO0~, 0.=210 5cm'~

8

•

t~A 0.5

1Volt

0 0.5

• -700°C

>800°C

X=38ooAX=~oooA IJ.A

025

O'=8"5'103r4 Ct= 6"6"102crr ]=13.95#W l=21.321J.W

, ~ ~

0-I ---

I

0

l

I Vott

Fig. 6. The experimental net photocurrent versus external applied bias under monochromatic lighting.

Solid lines show the best fit of eq. (9) to the experimental data.

218

N.A. Mancini et al. / Anodes in photoelectrolytical cells"

? m~

xx

~ 0 0 0

~ X X X X

0 ~ 0 ~

X X X X

?

mi

?

I

X X X X

N.A. Mancini et al. / Anodes in photoelectrolvtical cells

219

to the bulk titanium and a thick metallic layer deposited by evaporation on the oxidized surface. On this metallic layer a wire was attached by a low melting temperature solder. The value of the resistance measured in this way is considered to be due to the semiconducting oxide layer.

4. Discussion and conclusions As indicated in fig. 5, since the experimental data lie on a straight line, this accords with the assumption that recombination effects in the depletion region are negligible. If they were taken into account, eq. (9) (see also eq. (5)) should include a non-linear term. This hypothesis is also supported by a comparison between the observed values of the minority carriers diffusion length Lp and the barrier width Lb: as is clear from table 2, in general Lp > L b. The slope of the straight lines depends essentially on the value of the electron transfer coefficient at the interface ap. This parameter is rather constant for the different samples. As said before, the value of the flat band potential is obtained from the numerical fit on the experimental data. It was therefore not directly detected from the J(V) characteristics as the onset potential of the net photocurrent. This procedure was followed in order to avoid errors in the evaluation of Vfb due to surface recombination effects [23]. As visible in table 2 and in fig. 6a the highest quantum efficiency was exhibited by samples oxidized at Tox = 600 ° C. This efficiency is in part correlated to the high value of the exchange current Jpo shown by these samples. This fact means that the best conditions for charge exchange with the electrolyte were realized in these cases. The differences in actual responces between these samples and those prepared at Tox = 5 0 0 ° C can be explained by the fact that the TiO 2 rutile phase formation temperature, at atmospheric pressure, is about 6 0 0 ° C as we observed by x-ray diffractometry data and in agreement with the literature [31]. When Tox is raised above 600 °C, the percentage of crystalline TiO 2 is higher while the electrical conductivity is lower, as is evident from the resistance values reported in table 2. This behaviour can be explained by a decreasing presence of oxygen vacancies because there is a progressive saturation of titanium free bonds. It is well known [32], in fact, that the TiO 2 electrical conductivity can be produced by creating oxygen vacancies that leave Ti atoms in the 3 + state, so that they can act as donors. So, in spite of the fact Lp is higher, reduction in quantum efficiency can be explained by the increased electrical resistivity. On the other hand the formation of a more ordered crystalline structure can account for the resulting value of Lp. Measurements performed on 900 ° C oxidized samples showed no photoelectrochemical induced currents: in this case the very high resistivity is an excessively limiting factor [331. The barrier width L b also slightly increases with Tox. This parameter can be related with the effective density of donor centres N D by means of eq. (4). The reduction in the N D value can be attributed to a lower presence of Ti atoms in 3 + state and of AI acceptor impurities (see table 1) when the samples are prepared at Tox = 700, 800 and 900 ° C.

220

N.A. Mancini et al. / Anodes in photoelectrolytical cells

It is rather difficult to a t t r i b u t e the lower efficiencies to the insufficient thickness of T i O 2. The thickness of the semiconductor, although never m e a s u r e d in a direct way, was larger than b o t h the s t o p p i n g range of He ÷ ions and of p r o t o n s (of o r d e r of 5 ~m) in this material. It must therefore be c o n c l u d e d that light a b s o r p t i o n was quite satisfactory in the s e m i c o n d u c t i n g layer. The r e p o r t e d results can be s u m m a r i z e d as follows: i) the most limiting factor in the b e h a v i o u r of the studied p h o t o e l e c t r o c h e m i c a l cells is represented b y the a n o d e electrical conductivity. Large values of the resistivity of s e m i c o n d u c t i n g materials can cancel the effect of an increased m i n o r i t y carriers diffusion length, also if g o o d light a b s o r p t i o n c o n d i t i o n s are satisfied. ii) The f o r m a t i o n of a very resistive s e m i c o n d u c t i n g layer is u n a v o i d a b l e with the m e t h o d of thermal o x i d a t i o n of t i t a n i u m in an oxygen atmosphere. If a reduction process for p r o d u c i n g T i O 2 c o n d u c t i v i t y is utilized, the best o p e r a t i o n temperature seems to be a b o u t 600 ° C, in o r d e r to o b t a i n g o o d efficiences of the devices. These results are obviously valid when samples are i l l u m i n a t e d with m o n o c h r o m a t i c r a d i a t i o n for which the s e m i c o n d u c t o r exhibits a well d e t e r m i n e d a b s o r p t i o n coefficient. W h e n solar r a d i a t i o n is used, it is necessary to take into account the spectral b e h a v i o u r of the a b s o r p t i o n coefficient a n d it must be c o n s i d e r e d that only p h o t o n s with an energy larger than the b a n d g a p are useful to the p h o t o c o n v e r s i o n process. To o b t a i n this, a t t e m p t s are carried out to reduce the effective width of the b a n d g a p b y d o p i n g the s e m i c o n d u c t i n g electrode [7,34,35].

Acknowledgements The authors are grateful to prof. E. Rimini, Dr. M.G. G r i m a l d i a n d Dr. L. Torrisi for their kind assistance in p e r f o r m i n g b a c k s c a t t e r i n g a n d P I X E m e a s u r e m e n t s a n d to prof. A. Lo G i u d i c e for X - r a y diffraction analysis a n d for useful discussions. T h a n k s are also due to Mr. G. Bruno for technical assistance.

References [1] M. Tomkievicz and H. Fay, Appl. Phys. 18 (1979) 1. [2] A. Many, Y. Goldstein and N.D. Grover, Semiconductor Surfaces (North-Holland, Amsterdam, 1965). [3] D.E. Scaife, Solar Energy 25 (1980) 41. [4] H. Gerischer, in: Advances in Electrochemistry and Electrochem. Eng., vol. 1, ed. P. Delahay (1981). [5] H.P. Maruska and A.K. Ghosh, Solar Energy 20 (1978) 443. [6] R. Memming, Electrochim. Acta 25 (1980) 77. [7] A.K. Ghosh and H.P. Maruska, J. Electrochem. Soc. 124 (1977) 1516. [8] W.J. Albery and Ph.N. Bartlett, J. Electrochem. Soc. 129 (1981) 1492. [9] J.O. Bockris and K. Uosaki, J. Electrochem. Soc. 125 (1978) 233. [10] S. Bruckenstein and S. Miller, J. Electrochem. Soc. 129 (1982) 2029. [11] J.N. Chazalviel, J. Electrochem. Soc. 129 (1982) 963. [12] J.F. McCann and D. Hanemann, J. Electrochem. Soc. 129 (1982) 1134.

N.A. Mancini et al. / Anodes in photoelectrolytical cells

[13] [141 [151 [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

221

K. Rajeshwar, J. Electrochem. Soc. 129 (1982) 1003. W.W. G~irtner, Phys. Rev. 116 (1959) 84. R.H. Wilson, J. Appl. Phys. 48 (1977) 4292. J. Reichmann, Appl. Phys. Lett. 36 (1980) 574. W.L. Ahlgren, J. Electrochem. Soc. 128 (1981) 2123. R.N. Hall, Phys. Rev. 87 (1952) 387. W. Shockley and W.T. Read, Phys. Rev. 87 (1952) 835. C.H. Henry, R.A. Logan and F.R. Merritt, J. Appl. Phys. 48 (1978) 3530. C.T. Sah, R.N. Noyce and W. Shockley, Proc. IRE 45 (1957) 1228. M.A. Butler, J. Appl. Phys. 48 (1977) 1914. R.H. Wilson, J. Electrochem. Soc. 127 (1980) 228. Annual Book ASTM of Standard. (ASTM, Philadelphia, 1977) p. 526, 572, 669. N.F.M. Henry, H. Lipson and W.A. Wooster, The interpretation of X-ray diffraction Photographs (McMillan, London, St. Martin's press, New York, 1961). [26] F. Folkmann, in: Ion Beam Surface Layer Analysis, vol. 2 (Plenum, New York, London, 1976). [27] W.K. Chu, J.W. Mayer, M.A. Nicolet, T.M. Buck, G. Amsel and F. Eisen, Thin Solid Films 17 (1973) 1. [28] E. Rimini, Proc. Course on Physics of Thin Films of the Intern. College on Applied Phys. eds. N.A. Mancini and I.F. Quercia, Catania (1974). [29] D.M. Eagles, J. Phys. Chem. Solids 25 (1964) 1243. [301 F. James and M. Roos, Comput. Phys. Commun. 10 (1975) 343. [311 J.C. Jameson and B. Olinger, The American Mineralogist 54 (1969) 1477. [32] L.A. Harris and R. Schumacher, J. Electro, chem. Soc. 128 (1980) 1186. [33] S.M. Sze, Physics of Semiconductor Devices (J. Wiley, New York, 1969). [34] M.A. Butler, Abramovich, F. Deker and J.F. Juliao, J. Electrochem. Soc. 128 (1981) 200. [35] Y. Matsumoto, J. Kurimoto, T. Shimitzu and F. Sato, J. Electrochem. Soc. 128 (1981) 1040.