Int. J. Hydrogen Energy, Vol. 11, No. 6, pp. 367-371, 1986. Printed in Great Britain.

0360-3199/86 $3.00 + 0.00 Pergamon Journals Ltd.

© 1986 International Association for Hydrogen Energy.

CHARACTERIZATION OF PHOTORESPONSES IN SEMICONDUCTOR- ELECTROLYTE SYSTEMS A N D DETERMINATION OF FLAT-BAND

POTENTIALS

A, RoY, S. S, BHATrACHARYYA and S. ADITYA

Department of Chemical Technology, University College of Science and Technology, 92 Acharya Prafulla Chandra Road, Calcutta 700009, India

(Received for publication 7 November 1985)

Abstraet--A new technique to determine the flat-band potential (V~) of semiconductors (SC), as well as to distinguish the direct photoresponse from indirect processes, has been developed. The anomalous cathodic photocurrent, as observed by Spriinken et al. (Faraday Disc. Chem. Soc. 70, 55--60) with hydrogenated n-TiO 2 is not a direct photoresponse; rather the enhancement of the cathodic current under illumination is due to electrochemical reduction of the species produced in the anodic half under illumination. Chopped light is used for illuminating the SC and the oscillating photocurrent is measured in a storage oscilloscope at varying potentials. More reliable values of Vfb can be obtained by illuminating the SC with high frequency chopped light during the potential scan, with a rate much slower than the chopping frequency, and recording the current after amplification in an oscilloscope in AC mode, so as to block the slowly changing electrochemical dark current and to increase the photocurrent to dark current ratio (e.g. chopping frequency 100 Hz, scan rate 65 mV s-l). Cathodic photoresponse is observed with TiO2 prepared by thermal oxidation of Ti without further thermal hydrogenation or doping. Vfb of this is different from that of the former one. Their energy level diagrams and the nature of surface states are discussed.

INTRODUCTION

Fundamental processes in SC-electrolyte systems have attracted much attention because of their potential usefulness in the field of energy research. One of the important features is the determination of Veo, as it approximately locates the band positions of the semi- conductor. Of the various methods, two have been adopted most frequently to defermine this. In the measurement of differential capacitance as a function of electrode potential according to Mott-Schottky's equation [2-4], which is applicable to a double layer in equilibrium in a SC/solution system, a plot of CSI: vs the potential gives a straight line. The intercept with the potential axis gives the value of Vro. The measurement of capacitance is often affected by surface disorder, non- uniform doping concentration and varying adsorption and desorption at the electrode surface, which gives non-linear Mott-Schottky plots [5-9] or plots that are functions of the perturbation frequency [10] used in measuring the capacitance. Moreover, most SCs have the Vro-well in the cathodic region. When they are polarized cathodically they show appreciable reductive current, which disturbs the equilibrium in the double layer. As such, capacitance values do not obey the relation. Another method, first suggested by Butler [11] is based on the principle that the photocurrent starts to flow onward from the fiat-band position given by the relation V - Vro c¢ j2. The intercept of the plot of j2 (where J is the current density) against potential is a measure of Vro. A number of authors [12, 13] deter- mined Vfb from the photocurrent vs voltage curve,

367

where the photocurrent o~set potential is the fiat-band value. Spriinken et al. [1] reported some complications using the method suggested by Butler. They found that with n-TiO2 and n-SrTiO3 an additional cathodic photoprocess (which is very unusual with n-type semi- conductors) appeared when operating in the reverse bias mode and in the presence of reducible species. This cathodic photoprocess, according to the authors, may give rise to an apparent onset potential Von for the appearance of anodic photocurrent and thus deter- mination of Vfb from photocurrent onset potential may lead to erroneous results. In the present paper we report a modification of the onset potential method to over- come the complications discussed above. Using this technique it has also been shown that the cathodic .response with n-TiO2 in reverse bias mode is not a direct photoresponse; rather the additional cathodic current in the presence of light is due to electrochemical reduction of the species, which are produced under illumination in the anodic region.

EXPERIMENTAL

Pure Ti foil was first cleaned with acetone and then heated in a muffle furnace at 750°C in air for 1 h to produce polycrystalline TiO2. Some was further acti- vated by heating in a H2 atmosphere at 600°C for a further i h. Ohmic contacts were developed in the rear face using Cu wire attached with conducting Ag paste (Dupont conductor composition 4922). The rear face and the side of the foil was then encased with perspex/

368

teflon and epoxy resin and mounted on glass tubing through which the Cu wire was inserted, resulting in an exposed area of 1 x 1.5 cm 2. The electrodes were then used without etching or any other surface treatment.

The electrochemical experiments were carried out in a closed single compartment cell using a three-electrode system; the counter-electrode being Pt foil and the reference a saturated calomel electrode.

Cyclic voltametric experiments were performed with equipment developed in the laboratory using IC oper- ational amplifiers and were recorded on a storage oscilloscope in X - Y mode (Tektronics, Model DM 64). The light source used was a Hanovia medium pressure Hg vapour lamp, without any filter. However, on pass- ing through the glass wall of the cell, the low u.v. emissions are cut out. Reagent grade chemicals were used without further purification. All solutions were prepared from doubly distilled water and were deox- ygenated for at least 30min with N 2 before each experiment.

Photoresponse measurement

The photoresponse of the SC-electrolyte system was measured with a steady and a chopped light source. Steady light was obtained by operating the Hanovia lamp with a DC source, and oscillating light (100 Hz) by operating it with an AC source (50Hz). Lower frequency chopped light (2 Hz) was obtained by mech- anical chopping. The signals from the voltage and cur- rent follower of the cyclic voltameter were recorded directly (without using a lock-in amplifier) on the oscillo- scope in X - Y mode.

A. ROY, S. S. BHATTACHARYYA AND S. ADITYA

TH Ti02 in 1..5 M NoOH. - - - - i n dork; - - u n d e r illuminotion.

onodic

. . . . . . . . . . . . 0

"%. \ co,,o ,c

I I I I 1 I I - o . e - o . a - i . o - ~ . z - ~ . 4

POTENTIAL V$ SCE IN VOLTS

Fig. 1. Cyclic voltamogram of TH TiO2 in 1.5 M NaOH in the dark (dashed line) and under illumination with steady l i g h t

(solid line).

the presence of light occurs only when the potential of the electrode is allowed to sweep in the anodic range and then return to the cathodic half. Even in the dark similar enhancement of cathodic current is obtained if, during the scan, the potential is held for a few moments in the anodic range; the magnitude of current increases on increasing the holding time. Thus the additional

RESULTS AND DISCUSSION

Figure 1 shows the cyclic voltamogram of H2 treated n-TiO2 (TH TiO2) in 1.5 M NaOH in the dark as well as in the presence of steady light. The potential was swept in the range 0.6 to -1 .3 V vs SCE. The curves well resemble those obtained in [1]. In addition to the usual anodic photoresponse, an enhancement of cathodic current was observed in the region of -0 .8 to - 1 . 2 V when the potential was swept from positive values to cathodic directions in the presence of light. This increase in the cathodic current has been referred to by Spr/inken et al. as cathodic photocurrent.

Figure 2 shows the cyclic voltamogram of TH TiO2 in the same solution in the dark and in the presence of high frequency (100 Hz) chopped light. It shows that the anodic photoresponse reflects the chopping behav- iour of the light while the light induced cathodic current does not, its response is steady as with non-modulated light. Lower frequency chopped light (assuming that the cathodic photoresponse may have a slower response time) gave the same result. With increased light inten- sity, the anodic photoresponse increased as evidenced by the increase in wave amplitude, while the cathodic part shows only an increase in peak height. It was also observed that the enhancement of cathodic current in

TH TiO 2 in 1.5M NoOH; . . . . . in dark i under chopped I;ght,

h-~ ~ with higher intensity

onodic

II I I ~ " k " ~ x ' x ~ cothodic

l I L l I I I i

- 0 ' 6 - -0 "8 - I ' 0 -1-2 - -1 '4 P O T E N T I A L V• $Cg IN VOLTS

Fig. 2. Cyclic voltamogram of T H T i O : in 1.5 M in dark (- . . . . . . ) and under illumination with oscillatifig light

(100Hz): higher intensity ( ), lower intensity ( . . . . ).

PHOTORESPONSES AND FLAT-BAND POTENTIALS OF SC-ELECTROLYTE SYSTEMS

cathodic current in the presence of light is not a direct photoresponse of the electrode. It is probably due to reduction of some species generated in the anodic half in presence of light or on holding the potential in the anodic region. First the problem of identifying Vfb will be discussed. The nature of the species involved in this cathodic process will be discussed later on.

On the basis that the potential at which the anodic photocurrent starts is Vro (Fig. 1), a and a', the inter- sections of the forward and reverse curves in the vol- tamograms in light and dark, give Vro: cf. Fig. 3(a) in [1]. Different speculative explanations have been offered to explain the difference. The curves in Fig. 2 indicate that the intersections of the curves in the voltamograms in light and dark do not indicate that there is no anode photocurrent but rather that the cathodic and anodic currents are equal.

The anodic photocurrent extends well within the cathodic current region (Fig. 2) and hence it is apparent that the DC photocurrent-potential curve within this overlapping region will show an average of the anodic photocurrent and the light induced cathodic current. Hence intersection of the DC photocurrent-potential curve with the dark current-potential curve (when sweeping from positive to negative potential) is not the point of exact zero photocurrent and this is the reason the two intersection points in the forward and reverse sweeps do not lie on the same potential (Fig. 2). Similar observations have been reported [1]. Thus the DC pho- tocurrent-potential curve gives two apparently different values of Vro. These difficulties, however, do not arise in the case of the AC photocurrent-potential curve and the value of the potential, where the oscillating photocurrent ceases, is the same irrespective of the scan

Photocurrent of (a) NH TI02 (b) TH TiO;~, in [ [ 1'5 M NoOH under chopped l ight, onodic I

o a)hhhhhhhhhhhhhhhhhhh , o | !)piJiJI)~l)~!)l~l)P~!)i)~uv~v . . . . ~'~'utJUppv ['

; co,.o i¢ I !/llllllllllll/l/lll/n hh 4 0 . 2 0 - O ' 2 - 0 . 4 - 0 - 6 - O . 8 - I . 0 - I , 2

pOTENTIAL VS SCE IN VOLTS

Fig. 3. Photocurrent-potential curve of TiO2 in 0.1 M NaOH in AC mode, under illumination with oscillating light (2 Hz);

(a) NH TiO2, (b) TH TiO2.

369

direction (Fig. 2, a and a'). This is the true flat-band value.

Another problem associated with measurement of the photocurrent onset potential with most SCs is that the fiat-band values lie well within the cathodic region, where an appreciable dark cathodic current is present. In the presence of this high dark current, it is often very difficult to measure low photoresponse in the fiat-band region. Tomkiewicz [14] reported the flat-band poten- tials of TiO2 to be c a 200 mV more negative than the values obtained from DC photocurrent vs potential measurements. To overcome this, the sweep rate of the potential was kept low and the current measured in the oscilloscope in AC mode, so as to block the slowly changing dark current. The high frequency oscillating photocurrent generated on illumination with 100Hz chopped light, which bypasses the blocking condenser, can be measured more accurately (Fig. 3).

In Fig. 4, (a) shows the cyclic voltamogram of TiO2 produced by thermal oxidation only (without thermal H: treatment) in 0.1 M NaOH solution and (b) shows the same for TiO2 in 0.1M NaOH after cathodic reduction at -2 .0 V vs SCE. The cathodic peak C and the high anodic current after 0 .5V which appeared on electrochemical reduction are identical with that obtained with thermal H: treated TiO2. The only dif- ference is that with the electrochemically reduced one, the peaks disappear slowly after a few cycles while with the latter, they are permanent. A distinct difference in photoresponse properties also exists between the thermally hydrogenated (TH) and non-hydrogenated (NH) samples. In aqueous electrolyte at pH 12.3, the fiat-band potential of the NH TiO: as determined by the AC photoresponse vs voltage method, is -0.75 V, while with TH TiO: the value is - 1.05 V vs SCE. More- over, the NH one showed cathodic photoresponse in the range -0.75 V to - 1 . 1 V vs SCE (Fig. 5) while with the TH one no such cathodic photocurrent was observed. On the basis of the above results the energy

C.V. of (e) NHTIO 2 , (b ) NHTiO 2 after electro chem. reduc t ion a t - 2 . 0 V vs. S C E , in 0.1 M N a 0 H .

• \ onocJlC \ X ~ . . / P P x

cethodic ~ ~ ' * x C x'~. \ ~

" \ \ \ \

t I I L 1 I I +1"4 +0"8 0 -0"8 -1"4

POTENTIAL VS SCE iN VOLTS

Fig. 4. Cyclic voltamogram of (a) NHTiO 2 ( ); (b) NH TiO: ( . . . . ) after electrochemical reduction at -2.0 V vs

SCE; in 0.1 M NaOH.

370 A. ROY, S. S. BHATTACHARYYA AND S. ADITYA

anodic

o ~T~.h~ ~ ( a ) cathodic o

lllnn, "-<

}

NH rio m 04 M .ooH _ \ \ ( ~ under chopped light ~ \ in reverse mode(forward scan not shown). ~ 1

I , ] J f J J t ] i J r i I -0"5 -0"7 -a .g -H

POTENTIAL VS SCE IN VOLTS

Fig. 5. Current-potential curve of NH T i O 2 in 0.1 M NaOH in the dark (solid line) and in presence of chopped light (only anodic to cathodic sweep is shown in the presence of light).

level diagrams of the two types of TiO2 may be repre- sented as shown in Fig. 6.

With the NH type, on polarizing the electrode to positive values of flat-band Fermi level (EF(ro)), there is an electron depletion region resulting in an anodic photoresponse while the polarization of the electrode to values negative relative to Er(ro) and positive relative to CB produces a hole depletion region with the photo- generated electrons moving to the surface and holes being transferred through the external circuit. On pola- rizing to further negative values, i.e. where the Fermi level goes above the CB position, the SC behaves as a metal as evidenced by high dark current. With the TH type, the flat band in 0.1 M NaOH is located at -1 .05 V vs SCE, Absence of cathodic photoresponse and pres- ence of high dark current beyond that value indicates that keeping the CB and VB position the same, the flat- band Fermi level shifts to a more negative value (i.e. closer to the CB) and the SC becomes more extrinsic. This suggests that thermal reduction in H2 developed some electron donor level in between the VB and CB thus shifting the flat-band Fermi level to more negative values. The presence of a donor level between the CB and VB is further evidenced by the cathodic peak c and higher anodic current above 0.5 V vs SCE with TH TiO2 (Fig. 4). Some authors [1, 15] reported earlier that the peak c is probably due to reduction of 02 generated in the anodic half. But is has been observed that (1) the peak height is independent of the presence of 02 in solution and (2) the peak is present even with TiO2

immersed in alcohol (with LiCI as supporting electro- lyte) in the absence of O2 and the peak height increases on potential scanning during illumination (where no 02 evolution occurs at all). Thermal reduction in a H2 atmosphere may produce the lower valence cations namely Ti 3÷ and at the surface these reduced ions may act as traps for holes [16]. The existence of surface Ti 3÷ ions on reduced TiO2 has already been shown by XPS and UPS studies [17, 18]. The high anodic current above 0.5 V vs SCE with TH TiO2 may be due to injection of electrons from Ti 3+ to the CB of the SC. During the reverse scan reduction of these surface states to Ti 3+ again gives the cathodic peak c.

Photocurrent efficiency is also higher with TH TiO2 than NH TiO2. The higher photoresponse o fTH TiO2 is probably due to efficient trapping of the photogenerated holes by the surface traps which subsequently react slowly with water to generate 02 (Fig. 6. III), whereas direct reaction of H ÷ with an electrolyte molecule will be thermodynamically a much slower process, thereby increasing hole-electron recombination. The formation of higher surface state oxidized species in the presence of light is the reason for enhancement of cathodic cur- rent (Peak c) in the reverse cycle, while no such effect is observed with NH TiO2. After cathodic polarization at - 2 . 0 V vs SCE for a few seconds, the NH TiO2 on the first scan shows higher anodic photocurrent and the flat-band value also shifts to a somewhat negative potential; the cathodic photoresponse almost ceases. On further cycles the anodic photocurrent slowly decreases and the Vro returns to the original value; the cathodic photoresponse arises again.

With TH TiO2 the flat-band potential shifts with pH

~ ~( f ~ . . . . . ~ - I l O~ E F ( f ~ ) . . . . . . . . . . . . . O ' ~ S

VB ~-+1.9 E VS SCE vB -4-1-9

( I ) (n)

i

jSS e

CB e

EF

VB--

(till

Fig. 6. Energy band diagram of (I) TH TiO2, (II) NH TiO) in 0.1 M NaOH, (III) TH TiO2 under anodic bias showing surface

state.

PHOTORESPONSES AND FLAT-BAND POTENTIALS OF SC-ELECTROLYTE SYSTEMS 371

in an aqueous medium in accordance with the Nernst equation, while with NH TiO2, the variation of fiat- band potential with pH is much lower as evidenced by the values given in Table 1. The fiat-band value of TH TiO2 varies to some extent from sample to sample in the range of -0 .95 to -1 .10 V vs SCE at pH 12.3 probably depending on the hole trap density.

Table 1. Flat-band potential at dif- ferent pH values (vs SCE)

pH TH TiO2 NH TiO2

12.30 -1.05 -0.75 2.50 -0.55 -0.35 0.15 -0.40 -0.30

Determination of fiat-band Fermi level in different redox couples is in progress. Results so far obtained with THTiO2 corroborate well with Bard's [19,20] observation of Fermi level pinning. At the same pH, the fiat-band Fermi level' shifts with the redox potential of the electrolyte indicating that the change in potential difference between EF/ro) and redox potential takes place almost entirely at the Helmholtz layer rather than at the space charge layer. With NH TiO2 the fiat-band Fermi level shifts with electrolyte redox potential to a much lesser extent. Therefore, while doping increases the photoresponse efficiency of a SC, its Fermi level gets pinned by the surface states; i.e. the photopotential becomes independent of electrolyte redox potential. Thus tuning of photopotential by varying the redox couple is not possible with highly doped SCs, while it can be done with more intrinsic SCs.

Acknowledgements--Financial assistance from the Council of Scientific and Industrial Research, New Delhi is gratefully acknowledged.

REFERENCES

1. H. R. Sprtinken, R. Schumacher and R. N. Schindler, Disc. Faraday Chem. Soc. 70, 55-60 (1980).

2. P. J. Boddy, J. electroanal. Chem. 10, 19%244 (1965). 3. J. F. Dewald, J. Phys. Chem. Solids 14, 155-161 (1960). 4. H. Gerischer, Semiconductor electrochemistry. In H.

Eyring, D. Henderson and W. Jost (Eds), Physical Chem- istry, Vol. IXA, pp. 462-542. Academic Press, New York (1970).

5. P. J. Boddy, J. electrochem. Soc. 115, 19%203 (1968). 6. F. Moilers, H. J. Tolle and R. Memming, J. electrochem.

Soc. 121, 1160-1167 (1974). 7. V. A. Myamlin and Yu. V. Pleskov, Electrochemistry of

Semiconductors, pp. 306--310. Plenum, New York (1967). 8. R. DeGryse, W. P. Gomes, F. Cardon and J. Vennik, J.

electrochem. Soc. 122,711-712 (1975). 9. L. A. Harris and R. Wilson, A. Rev. Mater, Sci. 8, 99-134

(1978). 10. E. C. Dutoit, R. L. Van Meirhaeghe, F. Cardon and W.

P. Gomes, Bet. Bunsenges Phys. Chem. 79, 1206-1213 (1975).

11. M. A. Butler, J. appl. Phys. 48, 1914-1920 (1977). 12. H. H. Kung, H. S. Jarrett, A. W. Sleight and A. Ferretti,

J. appl. Phys. 48, 2463-2469 (1977). 13. J. M. Bolts and M. S. Wrighton, J. Phys. Chem. 80, 2641-

2645 (1976). 14. M. Tomkiewicz, J. electrochem. Soc. 126, 1505-1510

(1979). 15. R. H. Wilson, J. electrochem. Soc. 127,228-234 (1980). 16. 16. C. Gutierrez and P. Salvador, J. electroanal. Chem.

138, 457-463 (1982). 17. W. J. Lo, Y. W. Chung and G. A. Somorjai, Surf. Sci. 71,

19%219 (1978). 18. V. E. Henrich, G. Dresselhaus and H. J. Zeiger, Phys.

Rev. Lett. 36, 1335-1339 (1976). 19. A. J. Bard, A. B. Bocarsly, Fu-Ren F. Fan, E. G. Walton

and M. S. Wrighton, J. Am. Chem. Soc. 102, 3671-3677 (1980).

20. A. J. Bard, Fu-Ren F. Fan, A. S. Gioda, G. Naga- subramanian and H. S. White, Discuss. Faraday Chem. Soc. 70, 1%31 (1980).