1808
J. Phys. Chem. 1984, 88, 1808-1815
Photoelectrocatalysis and Electrocatalysis on p-Silicon M. Szklarczykt and J. O'M. Bockris* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: August 8, 1983)
The effect of metal additions on the photoelectrochemical evolution of h drogen on p-Si has been studied by using Pt, Ni, Au, Co, Pb, and Cd. Six ex situ methods, and ellipsometry, have been use Jtd study the type of surface formed by the etching and the metal additives. The mode of study is that of measuring the photocurrent-potential relations; the dependence of photocurrent on light intensity and wavelength at constant potential and the dependence of photocurrent density on time were studied. Surface preparation, particularly etching, was found to increase the photocurrent at constant potential; the effect of the metals depended upon the exchange current density of the ddrk evolution of hydrogen on the metals themselves. Pt-rich areas consisted of spheres about 250 A in diameter, while the areas themselves were ca. 4000 %I across. The photocurrent depends linearly upon the light intensity for the limiting current region but becomes relatively independent of intensity in the exponential region. Stability was maintained for more than 500 h. Quantum efficiency exceeded unity for some electrodes. The etching forms Si0 on the surface. In a model in which the increase due to the metal is explained in terms of effects at the semiconductor-metal interface, the increase of current due to the metal would be expected to increase with decrease of work function. However, the present results show the converse trend. They are consistent with an electrocatalytic view of the effect of the metal. The lack of dependence of the photocurrent in the exponential region upon light intensity is interpreted in terms of a two-statemodel for surface sites. Quantum efficiencies above unity were explained in terms of Compton scattering.
Introduction Recently, there has been much work published on the effects of submonolayer metal deposits on the photoactivity of semiconductor cathodes.'-12 A gain in photoelectrochemical rate was ~bserved.',~,~~~,~~'~,~~ In these publications, there are two views as to the mechanism of the effects. Either the semiconductor properties, e.g., flat band potential, barrier heights, surface states,'^^ etc. are affected by the metal or the metal causes a change in the rate-determining step to which is then a c ~ e l e r a t e d . ~ J ' J ~ We have tried to carry out experiments which throw light on these mechanisms. Experimental Section Electrodes. The electrodes were of Si, p-type, boron doped, (100) oriented. Two suppliCs of Si were used, one from Metron, Inc., and one from Monsanto Co. The resistivities were 0.62 ?! cm (1.3 X 10l6 ~ m - and ~ ) 0.86 R cm. The p-Si disks were cleaned ultrasonically in ethanol. On the backside of the Si electrode, an Ohmic contact was made by a gallium-indium alloy to a copper plate. The Cu wire was soldered to the Cu plate. The electrode was placed at the bottom of a cylindrical Teflon holder 6 cm long and 1.5 cm in diameter (Figure la). The area exposed td light was about 0.25 cm2. The electrode ensemble was pressed down upon a Teflon O-ring mounted inside the electrode holder. Pressure was applied by means of a threaded Teflon cover. The arrangement was found to be leakproof. N o epoxy cement was used. The surface of the electrode was prepared by immersion in aqua regia-48% H F solution (supplied by MCB) for 1-5 min and then reimmersed in a 48% HF-water solution. The surface was then multiply washed with triply distilled water. In general, a fresh Si electrode was used for each set of experiments with each metal deposit. However, from time to time the metal deposits were removed by means of an appropriate solvent. The semiconductor electrode, after reetching and receiving a new metal deposit, was used again. The same results were found for metalized electrodes usipg fresh and reprepared silicon wafers. The metalization of the p-Si surface was carried out under galvanostatic pulsed conditions during illumination of the Si surface. The formal charge passed corresponded to a few monolayers of meta1.l3 The metals studied were as follows: Pb, Cd, Co, Au, Ni, Pt. The baths prepared consisted of Pb(N0)3 + H N 0 3 , CdO + Cd + NaCN, C0S04 + H3BO3, h 2 0 3 + HC1, NiC& + H3B03, and 'On leave from Department of Chemistry, Warsaw University, Zwirki i Wigury 101, 02-089 Warsaw, Poland.
0022-3654/84/2088-1808$01.50/0
H2PtC16,re~pectively.'~,'~ The materials were supplied by Fisher, MCB, and Alfa Ventron. After metalization, the electrode was rinsed with triply distilled water and placed in the experimental photocell. Reference Electrodes. Two different reference electrodes were used. The first was a saturated calomel electrode (Fisher). The second was a saturated mercurous sulphate electrode made in the laboratory. The potential of the first electrode should be 0.242 V and the second, 0.690 V at 25 OC. The difference was measured and found to be 444 & 3 mV. All potentials quoted in this paper were recalculated to the normal hydrbgen scale. Solutions. In the present work the minimum current density at which measurements had sensibly to be made was in the order of 20 FA cm-*. Under these conditions, electrode kinetic measurements are very sensitive to impurities in the solution, and consequently, great care in respect to the impurity content of the solution must be taken in carrying out the measurements. Corresponding to this, all cells were carefully washed in concentrated sulfuric acid followed by the copious passage of tap water, distilled water, and triply distilled water. The washing with distilled and low-conductance water was carried out two to three times. (1) Y. Nakato, S. Tonomura, and H. Tsubomura, Ber. Bunsenges. Phys. Chem., 80, 1002 (1976). (2) J. O'M. Bockris and J. McCann, unpublished results, 1975; cf. J. McCann, Thesis, Flinders University, Australia, 1978. (3) K. Ohashi, J. McCann, J. O'M. Bockris, Energy Res. Abstr. 1, 259 (1977). (4) W. Kautek, J. Gobrecht, and H. Gerischer, Ber. Bunsenges. Phys. Chem., 84, 1034 (1980). ( 5 ) J. M. Lehn, J. P. Sauvage, and R. Ziessel, Nouu. J . Chim., 4, 623 (1980). ( 6 ) A. Heller and R. G. Vadimsky, Phys. Reu. Lett., 46, 1153 (1981). (7) A. Heller, R. G. Vadimsky, W. D. Johnston, Jr., K. E. Strege, H. J. Leamy, and B. Miller, "Proceedings of the 15th IEEE Photovoltaic Specialists Conference", Institute of Electrical and Electronics Engineers, New York, 1981, p 1422. (8) R. N. Dominey, N. S. Lewis, J. A. Bruce, D. C . Bookbinder, and M. S. Wrighton, J . Am. Chem. SOC.,104, 467 (1982). (9) C. Levy-Clement, A. Heller, W. A. Bonner, and B. A. Parkinson, J . Electrochem. SOC.,129, 1701 (1982). (10) A. Heller, E. Aharon-Sholom, W. A. Bonner, and B. Miller, J . Am. Chem. SOC.,104, 6942 (1982). (11) M. Szklarczyk and J. O'M. Bockris, Appl. Phys. Lett., 42, 1035 (1983). (12) J. O'M. Bockris and M. Szklarczyk, Appl. Phys. Commun., in press. (13) Optical examination (see below) showed that the coverage of the surface with metal was in the submonolayer region. (14) A. G. Gray, "Modern Electroplating", New York, Wiley, 1953. (15) "Modern Electroplating", The Electrochemical Society, Inc., New York, 1942.
0 1984 American Chemical Society
Photoelectrocatalysis and Electrocatalysis on p-Si a
b A
5
2’
U
Figure 1. (a) Design of the photoelectrode Teflon holder ensemble: 1, p-Si disk; 2, Teflon O-ring; 3, Ga-In alloy layer; 4,copper plate; 5, tin solder; 6, inner cylindrical part of Teflon holder: 7 , outer cylindrical part of Teflon holder; 8, threaded Teflon cover; 9, an electrical wire. (b) Photoelectrochemicalcell: 1, quartz window; 2, photoelectrode holder; 3, counterelectrode compartment; 4,reference electrode; 5, Luggin capillary; 6, bubbler outlet of argon; 7 , 8, inlet and outlet of solution, when experiments with continuous flow of electrolyte are carried out; 9, inlet of Ar (or solution from preelectrolyzer) (a) under and (b) above solution
level. The water used originated from a water still and was further purified by three distillations. The first was made in air, the second in acidic KMnO, solution and under nitrogen, and the third under a nitrogen atmosphere. All solutions were prepared originally from chemicals of very high purity. Reagent grade concentrated HzSO4, HCl, and HNO, (MCB) were used. Preelectrolysis was carried out for about 12 h on platinized platinum gauze electrodes which had a geometric area of about 100 cmz in an argon atmosphere. The overall cell potential during preelectrolysis was maintained potentiostatically at 1.1 V. At the end of the preelectrolytic period, the solution was pressured by purified argon into the cell in which the photoelectrochemical measurements were carried out. Argon (Airco Inc.) was purified over copper and platinum columns at 280 and 780 OC, respectively. Oxygen. Depolarization by competing oxygen reactions is of importance in many studies of hydrogen evolution, and for this reason argon was at first bubbled through the solution and kept as a blanket over it. In solutions which were preelectrolyzed, argon gas was bubbled through the solution in a separate vessel and then passage of Ar was continued through the solution in the cell. In experiments in which the semiconductor electrodes contained no Pt, no effects of O2on the photocurrents were observable. In experiments on metalized electrodes, currents in the region of 50 MAcm-2 were sensitive to the removal of oxygen by argon. Electrode Resistance. The resistance of the electrode was measured by making Ohmic contacts to the Si wafers using Ga-In alloys in contact with Cu sheets. The measured value was 2.2 f 0.4 Q . A Luggin capillary was used, its end being 1-5 mm from the electrode surface. Cells. The cell is shown in Figure lb. A quartz window is utilized to facilitate the passage of light from the irradiating source. The counterelectrode was separated from the main cell by means of a fritted-glass disk, medium porosity. The cell was protected from the ingress of air by means of water traps through which exited argon used to blanket the solution. The electrode-containing holder was detachable (Figure la). An alternative cell was also used, the main difference being that the photoelectrode was placed at the bottom of the cell, the electrode being horizontal to avoid the aggregation of hydrogen bubbles. Light was introduced into the cell by means of a flat mirror through a quartz window at the cell top. Light Source. The usual light source was a 1000-W Xe lamp, Model 6142 (Oriel), operated at 20-A current to produce a 50 mW cm-2 light beam. The light was passed through an IR glass filter and water filter (continuously cooled) for filtering out the
The Journal of Physical Chemistry, Vol. 88. No. 9, 1984 1809 long-wavelength part of the radiation. Neutral density filters (transmittances 50%, 39.8%, lo%, and 1%; Oriel) were used for obtaining varying light intensities. The i-V dependences were carried out with a high-intensity monochromator (Bausch & Lomb). Solar light was simulated with a solar simulator, AMI, Model 6730/6/41 (Oriel). Long-term experiments were carried out with a tungsten-halogen lamp. The light flux incident at the electrode surface was measured with a radiometer, Model PSP (EPLAB) . Electronics. The current-voltage behavior was monitored with a combination of EG & G PAR 175 universal programmer; EG & G PAR 363 potentiostat-galvanostat, and an X-Y recorder, Model 70048 (Hewlett Packard). The current-time behavior was studied with a Model 549 ECO potentiostat, the current being monitored with a digital multimeter, Model 3466A (HewlettPackard). Optical Examinations.’6 Several kinds of optical examinations were made. In the first, electron microscope photographs were taken of the Si surface, before immersion in the solution and after etching in the normal way, and of the metalized Si surface. X-ray examinations were made of these situations described above. A JEOLCO JSM-35U scanning electron microscope (SEM) with dual, automated, wavelength dispersive X-ray spectrometers (Tracor Northern), X- and Y-stage automation, and digital beam control integrated with an LSI-l1/2364k minicomputer was applied. A 20- or 15-keV electron beam with a beam current of microamps was applied. The pressure was maintained at lo4 torr. The shape of deposits and geometrical coverage were determined with particle recognition and characterization (PRC) software (Tracor Northern). This procedure utilizes secondary electron imaging and X-ray techniques, simultaneously. The backscattering electron image mode was applied to allow the interpreter to correlate signals due to the chemical composition of the surface and its topography. A JEOL lOOX transmission electron microscope (TEM) was applied for the determination of the shape of the platinum islets carried out in Shell Research (Westhollow). The SIMS study was carried out in the Advanced R & D Inc. Laboratory, St. Paul, MN. A UTI 100 C spectrometer was used. An He3+ 1.5-keV, 600-nA beam was applied. A 1.5-mm2 area of electrode was studied. A Hewlett-Packard 5950A ESCA XPS spectrometer with a monochromatized A1 Ka X-ray source was applied in order to determine the surface composition. The angle tilt was 0’. The torr. Initially, pressure of the analyzer chamber was 1 X a survey scan was recorded to identify the elements present at the surface. Subsequently, pertinent levels were recorded over a narrow energy range (20-eV window width). The instrument is provided with a dedicated computer, and this facilitated signal averaging and spectral acquisition by repetitive scans over the energy region of interest. Standard spectra recorded were Si 2p (120 scans), 0 1s (60 scans), and Pt 4f (180 scans). The software package used for the spectrum analysis was obtained from Surface Science Laboratories (ESCA Data System Revision E). All peaks were corrected initially according to the “sigmoid background” procedure and then deconvoluted. An automatic thin film ellipsometer, Type 43603-200E (Rudolph Research, Inc.), was utilized in characterization of the silicon oxide layer thickness. The ellipsometric experiments were carried out in all electrodes subjected to optical examination before study.
Results 1. Effect of Preparation of the Surface. The very large effect of chemical etching with aqua regia is seen in Figure 2. In the exponential region the increases of current caused by etching is (16) A detailed report and discussion on optical examination of p-Si electrodes will be published elsewhere. (17) B. E. Conway and J. O’M. Bockris, J . Chern. Phys., 26, 532 (1957). (1 8) R. Parsons, ”Handbook of Electrochemical Constants”, Butterworths, London, 1959. (19) S. Trasatti, J . Electroanul. Chern., 39, 163 (1972).
I810
Srklarczyk and Bozkris
The Jolrrnul of Physical Chemistry, Val. 88, No. 9, I984
-
-04
-06
-08
-10
-12
EIVNHE
rograph of p-SI-Pt photocathode (b) TEM Pt spheres In a rich area on a p-SL-Pt electrode
Figure 2. Potentiadynamic $-E relations showing the influence of preparation on the photoelectrochemical response ofp-Si (50 rnW Xe light, 0.5 M H,SO,). -32
2
4
6
8
io
O/mC x cm-* Figure 3. Dependence of a photocurrent on
deposition. more than an order of magnitude. The effect of continuing swee a t around 200 mV s-! between about -0.4 and -0.7 V vs. N H E for 15 min is shown. After this time, no further significant change in the photocurrent was observed. The result of the sweeps is to increase the velocity by an additional factor of 5 compared with the situation before successive sweeping. Upon introduction of Pt islets onto surfaces prepared thus, the increase in velocity is about 7 times compared with the situation of the etched surface after sweeping a t -0.1 V on the NHE. 2. Efiect of the Platinum Deposits on Figure 3 is shown the photocurrent-po function of the charge passed during plati the surface. A maximum catalytic effect three monolayers (the so-called “formal m mated on the assumption of a uniform coverage of p-Si by the metal). 3. Opticul Study of the Si-Metal Deposited Surfuce. The micrographs of p-Si-Pt surfaces are shown on Figure 4a,b. Figure 4a, taken in SEM, shows bright platinum-rich areas. Figure 4b, taken in TEM, shows that these areas are constructed of small (250-A average diameter) platinum spheres (see Discussion). The coverage by Pt aggregates is 0.14.2 of the geometric p s i surface. The average dimensions of the Pt-rich areas obtained hy the particle recognition and characterization software applied to S E M and X-ray data and are presented in Figure 5 . The area X-ray analysis did not detect platinum, but spot analysis, focused on white regions (Figure 4a), shows the presence of platinum, (Figure 6a,b)
-
-
ra? area
analyais oi p-Si-Pi electrode (small low- and high-energy peaks for M and L shells. respectively)
The Journal ojPhysiw1 Chemistry, Yo/. 88. No. 9, 1984 1811
Photoelectiocatalyais and E~ectiocata~ysis on p-sl
re&s
-401
X-ray spot analysis of p-Si-Pb electrode.
'TABLI'. I:
-____~ -
Results of ESCA-SPS Studies'
veak xidtb, eV
9,of pcak
LlcinLnt
bindim cncig,, CV
si 2p
99 84
2 16
I0206 I03 46 532.15 533.30
2 26 1 96 2.13 2.14
8 04 I 1 98 13 97 73.92 26.08
0 is
silicon base (b) X-ray area analysis of p-S$-Au electrode
JIC~
compd SI SI0 StO, Si0 SiO,
" Carbon -8 cm-' because of their large spin-orbit coupling constants. Many other diatomics involving first-row transition metals were believed prepared but were not observed, and their likely lowest electronic states are discussed, These are Fez, Co,, Niz, CrFe, MnFe, FeCo, FeNi, FeCu, FeAg, MnCo, CoNi, CoCu, CoAg, CrNi, MnNi, NiCu, NiAg, and NiZn.
Introduction In extending our ESR studies of transition-metal molecules, we have attempted to observe most of the first-row homo- and heteronuclear diatomics. This has been successful for ScZ,' Mn2,2 and C ~ C U but , ~ ,there ~ have also been a large number of negative results. This is not unexpected because some of these molecules may be diamagnetic or have orbitally degenerate ground states5 or large zero-field splittings. However, these negative results are usually informative, particularly if the diatomic molecule is known to be bonded in the gas phase from mass spectrometry research.6 Therefore, we will speculate here also about the lowest states of the numerous diatomics that we believe were surely prepared but Present address: Department of Chemistry, Indiana University, Bloomington, IN 47405.
0022-3654/84/2088-1815$01.50/0
were undetected in our ESR spectra. The inclusion of heavier transition metals to form a homologous series, such as in the earlier study of CrCu, CrAg, and CrAu4 (referred to as I below), can lead to interesting variations in electronic properties of the diatomics. This is demonstrated here (1) L. B. Knight, Jr., R. J. Van Zee, and W. Weltner, Jr., Chem. Phys. Lett.. 94. 296 (1983). (2) C:A. Baumahn, R. J. Van Zee, S.V. Bhat, and W. Weltner, Jr., J . Chem. Phys., 78, 190 (1983). (3) R. J. Van Zee and W. Weltner, Jr., J . Chem. Phys., 74,4330 (1981); erratum, ibid., 75, 2484 (1981). (4) C. A. Baumann, R. J. Van Zee, and W. Weltner, Jr., J . Chem. Phys., 79, 5272 (1983); designated as I throughout this paper. ( 5 ) R. J. Van Zee, C. M. Brown, K. Z. Zeringue, and W. Weltner, Jr., Acc. Chem. Res., 13, 237 (1980). (6) K. A. Gingerich, Symp. Faraday SOC.,No. 14, 109 (1980).
0 1984 American Chemical Society
