J. Phys. Chem. 1063, 87, 4446-4453
-
4446
which qualitatively accounted for the rapidity of the S1 Mg+.-H2-. step and the much slower (5 X lo9 s-l) decay of this E T product to X and the ground state (see Scheme I). An implicit assumption of this model was that the rate of decay of the S1 state to So was similar to that of other l(a,r*)decays in related porphyrins (i.e., k [ ,< lo9 s-l). This implies an E T quantum yield >99 % . Our measured value of 0.9 f 0.1 is in good agreement with this expectation. Electrochemical measurements have previously shown that -90% of the S1state's free energy was stored in the Mg+-Hp photoproduct.' Thus, three important criteria for successfully replicating the primary E T step of PSI1 (and other photosystems as well) have now been satisfied for ET from S1in the Mg-H2 model: (1)a >lo" s-l rate,
(2) high energy-storage efficiency, and (3) near unity quantum yield. Recent work has also shown that by changing the solvent from CH2C12to NJV-dimethylformamide one can extend the lifetime of the Mg+-Hp product from 200 ps to 1.3 1x3.~This makes it likely that Mg+-Hp will also be able to reduce an associated secondary electron acceptor with a high quantum yield.
Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the U.S.Department of Energy, Washington, DC, under contract no. DEAC02-76CH00016 at Brookhaven National Laboratory and by the Research Corp. and the National Science Foundation at Michigan State University. Registry No. [Fe(~hen)~]~+, 14708-99-7;Mg-H2, 83221-92-5.
Improvement of the Photoelectrochemical Oxidation of Halides by Piatinization of Metal Dichaicogenlde Photoanodes Richard A. Shnon, Antonlo J. R~CCO, D. Jed HarrhKM, and Mark S. Wrlghton' Depertment of Cbmisby, MassachwettsInstitute of Technobw, C a m b w , Massachusetts 02139 (Received:February 15, 1983)
Eledrochemical deposition of Pt onto the surface of n - M a or n-WS2electrodes results in significant improvement in the efficiency of the photoelectrochemical oxidation of C1- or Br- in aqueous or nonaqueous media. The optimum amount of Pt is in the 10-8-10-7 mol/cm2 range where electron microscopy and Auger and X-ray photoelectron spectroscopy show that the Pt incompletely covers the MS2surfaces. Phenomenologically, the deposited F't behaves as a catalyat for the two-electron oxidation of X- and does not affect the interfacialenergetics of the n-MS2/liquidelectrolyte. Improvement in efficiency for X- oxidation results from enhanced photovoltages and fill factors. Improvement in the oxidation of C1- in 15 M LiCl is most notable; for one n-MoS2electrode the platinization improved the energy conversion efficiency from 0.7% to 9.8% for 632.8 nm (36 mW/cm2) and for one n-WSzelectrode the efficiency was improved from 7.4% to 13.4% for 632.8 nm (15 mW/cm2). The improved efficiency can be maintained at -10 mA/cm2 for periods of -1 h, but the Pt catalyst is slowly oxidatively dissolved and the output eventually declines to that associated with naked n-MS2photoanodes.
Introduction Metal &chalcogenide,MY2 (M = Mo, W, Y = S, Se, Te), semiconductors have been shown to be durable, relatively efficient, small band gap photoanodes for the conversion of visible light to electrical energy or chemical energy in the form of oxidation products, including such strong oxidants as C12 and Br2.1-5 Recently, n-WS2 has been re~
~~~~
(1) (a) Tributsch, H.; Bennett, J. C. J. Electroaml. Chem. 1977,81, 97. (b) Tributsch, H. Z . Naturjorsch. A 1977,32A, 972; J. Electrochem. SOC.1978,125,1086,1981,128,1261; Ber. Bumenges. Phys. Chem. 1977, 81,361; 1978,82,169; Sol. Energy Mater. 1979,1,257; Discuss. Faraday SOC.1980, 70, 189. (c) Gobrecht, J.; Tributach, H.; Gerischer, H. J . Electrochem. SOC.1978,125,2085; Ber. Bumenges. Phys. Chem. 1978, 82, 1331. (d) Ahmed, S. M.; Gerischer, H. Electrochim. Acta 1979,24, 705. (e) Kautek, W.; Gerischer, H.; Tributsch, H. Ber. Bumenges. Phys. Chem. 1979,83,1000;J. Electrochem. Soc. 1980,127,2471. (fJTributsch, H.; Gerischer, H.; Clemen, C.; Bucher, E. Ibid. 1979,83,655. (g) Kautek, W.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1980,84,645. (h) Kautek, W.; Gobrecht, J.; Gerischer, H. Ibid. 1980,84, 1034. (i) Jaeger, C. D.; Gerischer, H.; Kautek, W. Ibid. 1982,86,20. (j) Clemen, C.; Saldana, X. I.; Muny, P.; Bucher, E. Phys. Status Solidi A 1978, 49, 437. (2) (a) Lewerenz, H. J.; Heller, A.; DiSalvo, F. J. J. Am. Chem. Soc. 1980,102,1877. (b) Menezes, S.; DiSalvo, F. J.; Miller, B. J. Electrochem. SOC.1980,127,1751. (c) Menezes, S.; Schneemeyer, L. S.;Lewerenz, H. J. Appl. Phys. Lett. 1981, 38, 949. (d) Lewerenz, H. J.; Ferris, S. D.; Doherty, C. J.; Leamy, H. J. J. Electrochem. SOC.1982, 129, 418. QO22-3654/83/2087-4446$01SO10
ported to yield 6.9% efficient visible light-assisted (632.8 nm) oxidation of C1- to C12in aqueous 15 M LiCl and up to 12% efficiency for Br- oxidation in aqueous 12 M LiBr.6 Despite the fact that MoS2, MoSe2, and WS2 have been shown to be durable photoanodes for the oxidation of C1in 15 M LiC1,4I6 the efficiency has been disappointing compared to the efficiency for the oxidation of either I(3) (a) Fan, F.-R. F.; White, H. S.; Wheeler, B.; Bard, A. J. J. Electrochem. SOC.1980,127, 518. (b) Fan, F.-R. F.; White, H. S. Wheeler, B. L.; Bard, A. J. J.Am. Chem. SOC.1980,102,5142. ( c ) Abruna, H. D.; Bard, A. J. J. Electrochem. Soc. 1982,129,673. (d) White, H. S.;Abruna, H. D.; Bard, A. J. Ibid. 1982,129, 265. (e) White, H. S.; Fan, F.-R. F.; Bard, A. J. Ibid. 1981,128,1045. (0 Nagasubramanian, G.; Bard, A. J. Ibid. 1981, 128, 1055. (4) (a) Schneemeyer, L. F.; Wrighton, M. S. J.Am. Chem. SOC.1979, 101, 6496; 1980, 102, 6964. (b) Schneemeyer, L. F.; Wrighton, M. S.; Stacy, A.; Sienko, M. J. App. Phys. Lett. 1980,36, 701. (c) Kubiak, C. P.; Schneemeyer, L. F.; Wrighton, M. S. J.Am. Chem. SOC.1980,102, 6898. (d) Calabrese, G. S.; Wrighton, M. S. Ibid. 1981,103,6273. (5) (a) Kline, G.; Kam, K.; Canfield, D.; Parkinson, B. A. Sol. Energy Mater. 1981,3,301. (b) Canfield, D.; Parkinson, B. A. J.Am. Chem. SOC. 1981,103,1279. (c) Furtak, T. E.; Canfield, D.; Parkinson, B. A. J. Appl. Phys. 1980,51,8018. (d) Parkinson, B. A.; Furtak, T. E.; Canfield, D.; Kam, K.; Kline, G. Discuss. Faraday SOC.1980, 70,233. (6) Baglio, J. A.; Calabrese, G. S.; Kamieniecki, E.; Kershaw, R.; Kubiak, C. P.; Ricco, A. J.; Wold, A.; Wrighton, M. S.; Zoski, G. D. J. Electrochem. SOC.1982,129,1461.
0 1983 American Chemical Society
Photoelectrochemical Oxidatlon of Halldes
or Br-.l* We suspected that the inefficiency for Cl- oxidation might be due to poor heterogeneous kinetics. The heterogeneous rate for halogen generation at a given electrode can be expected to depend on the electrode material and its surface proper tie^.^ The modification of semiconductor surfaces to enhance stability or electron-transfer rates is an area of active research.8 The deposition of noble metal films or catalyst systems onto p-type semiconductors such as Si,*ll InP,12J3 GaAs,14 GaP," and the layered compounds WSe21hand WSJ5 greatly improves the photoelectrochemical efficiency for H2 evolution from these electrodes. Recently, attention has turned to electrocatalysis of inner-sphere reactions at photoanodes, and n-type Si,'6 CdS,17and GaP18have been the subjects of investigation. The deposition of Ru02, a known C12-evolution cataly~t,'~ on n-&/iridium silicide interfaces has produced a durable photoanode with efficiencies of about 5%'" for the generation of C12. So far the improvement in the rate of O2or C12evolution has been brought about on a photoelectrode where the output photovoltages are not dominated by the semiconductor/ liquid electrolyte interface; rather the photovoltages are controlled by an overlayer of conducting p~lymer,'~ silicide or oxide.16 We have examined the effect of Pt electrodeposition onto n-MS2 (M = Mo, W) on the energy conversion efficiency for Cl- and Br- oxidation. We have chosen to study Pt, since Pt has been well studied with respect to mechanism and is known to be a reversible electrode for the C12/C1In >1.0 M C1- media the Pt has less than 0.1 monolayer of platinum oxide when C12is ev01ved.l~ The inherent durability of the MS2 materials in concentrated aqueous LiBr or LiCl and nonaqueous solutions allows ready evaluation of the effects from surface platinization by examining photocurrent-voltage properties of the electrodes. The main finding is that significant improvements in efficiency can be brought about by the deposition of islands of Pt (10-8-10-7mol/cm2) onto the (7)Bockris, J. O'M.; Reddy, K. N. "Modern Electrochemistry"; Plenum Press: New York, 1970. (8)Wrighton, M. S.ACS Symp. Ser. 1982,No. 192,99. (9)(a) Bruce, J. A.; Murehashi, T.; Wrighton, M. S. J. Phys. Chem. 1982,86,1552. (b) Dominey, R. N.; Lewis, N. S.; Bookbinder, D. C.; 1982,104,467. (c) Bookbinder, D.C.; Bruce, J. A. J. Am. Chem. SOC. Bruce, J. A.; Lewis, N. S.;Dominey, R. N. h o c . Natl. Acad. Sci., U.S.A. 1980,77,6280.Bruce, J. A.;Wrighton, M. 5.Zsr. J. Chem. 1982,22,184. (10)Abruna, H. D.; Bard, A. J. J. Am. Chem. SOC. 1981,103,6898. (11)Nakato, Y.; Tonomura, S.;Tsubomura, H. Ber. Bumenges. Phys. Chem. 1976,80,1289. (12)(a) Heller, A.;Miller, B.; Lewerenz, H. J.; Bachman, K. J. J.Am. 1980,102,6555.(b) Heller, A,; Vadimsky, R. G. Phys. Rev. Chem. SOC. Lett. 1981,46,1153. (13)(a) Dominey, R. N.; Stalder, C.; Wrighton, M. S., to be submitted for publication. (b) Dominey, R. N., Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1982. (14)(a) Fan, F.-R. F.; Reichman, B.; Bard, A. J. J. Am. Chem. SOC. 1980,102,1488. (b) Menezes, S.;Heller, A.; Miller, B. J. Electrochem. Soc. 1980,127,1268. (15)Baglio, J. A.; Calabrese, G. S.; Harrison, D. J.; Kamieniecki, E.; Ricco, A. J.; Wrighton, M. S.; Zoski, G. D. J. Am. Chem. SOC. 1983,105, 2246. (16)(a) Thompson, L.; Dubow, J.; Rajeshwar, K. J. Electrochem. SOC. 1982,129,1934.(b) Fan, F.-R. F.; Hope, G. A.; Bard, A. J. Zbid. 129,1647. (c) Fan, F.-R. F.; Keil, G.; Bard, A. J., submitted for publication. (17)(a) Frank, A. J.; Honda, K. J. Phys. Chem. 1982,86,1933. (b) Gissler, W.;McEvoy, A. J.; Gratzel, M. J.Electrochem. SOC. 1982,129, 1733. (18)Nakato, Y.; Abe, K.; Tsubomura, H. Ber. Bumenges. Phys. Chem. 1976,80, 1002. (19)For a recent review, see: Novak, D. M.; Tilak, B. V.; Conway, B. E. In "Modern Aspects of Electrochemistry, No. 14";Bockris, J. O'M., Conway, B. E., White, R. E., Eds.; Plenum Press: New York, 1982. (20)Bard, A. J., Ed 'Encyclopedia of Electrochemistry of the Elements"; Marcel Dekker: New York, 1973;Vol. 6,pp 213-7. (21)Payer, S.;Noumahn, K. Chem.: Exp. Didakt. 1976,2,253. (22)Tilak, B. V. J.Electrochem. SOC. 1979,126,1343.
1
0
104
,
108
tl2
,
I
0
io4
!
to8
,
,
112
POTENTIAL, V vs S C E
Flgure 1. Steady-state photocwrent-voltage curves for naked (-- -) and platinized (-) kM0S2(bn)and n-WS, (rlght) In 15 M LiCl (top) and 12 M LiBr (bottom). Redox potentials poised with Cl,(g) or Br2(l). Illuminationis in mW/cm2 at 632.8 nm. Maximum power point (e or m) efficiencies are lndlcated in % .
surface under conditions where the output photovoltage is controlled by the n-MS2/liquid electrolyte contact, not by the n-MS2/Pt interface.
Experimental Section Materials, Chemicals, and Electrochemical Procedure. Sources of single-crystal MS2 and the procedure for fabricating electrodes are described elsewhere.4s6J5 Chemicals, solvents, and electrolytes were commercially available reagent grade, purified as necessary! Procedures for cyclic voltammetry and the electrochemicalinstrumentation have been reported.6 Steady-state photocurrent voltage curves were recorded at low scan rates (610 mV/s) in well-stirred solutions at 25 "C under conditions of light intensity limited current using a single-compartment, three-electrode cell with Pt counterelectrode and saturated calomel reference electrode (SCE). Provided hysteresis (difference in forward and reserve scans) was minimal, higher scan rates were sometimes used in 15 M LiC1/C12 solutions to avoid the current fluctuations associated with C12 bubble formation on the photoanode surface. The most satisfactory results were obtained in LiC1/C12 solutions by using a small circulating pump to direct a stream of electrolyte onto the photoanode face, dislodging any bubbles of C12and providing efficient stirring. A He-Ne laser providing up to -40 mW/cm2 at 632.8 nm was used as the illumination source. Solutions of LiCl were purged with C12 for >15 min before beginning experiments and a slow C12bubbling rate was maintained during current-voltage measurements to ensure a constant solution electrochemical potential. A few drops of Br2were added to LiBr solutions to poise the solution potential. Durability measurements were made using the same apparatus as for current-voltage curves. Electroplating of Pt. The n-MS2 electrode was initially potentiostated at -0.2 V vs. SCE in the dark in a wellstirred Ar-or N2-purged solution of 1M NaC104,buffered to pH 7 with potassium phosphate, containing 1 mM K2PtC14. In most cases, very little current increase occurred when the cell was switched on; the electrode potential was moved negative to - 4 . 4 V until a current density of -30 bA/cm2 flowed. As plating progressed, it
4440
Simon et al.
The Journal of phvsical Chemistry, Vol. 87, No. 22, 1983
TABLE I: Representative O u t p u t Parameters for Naked and Platinized n-MS,-Based Photoelectrochemical Cellsa
lo-@x electrode WS, no. 51
MoS, no. 130
MoS, no. 9
WS, no. 51
MoS, no. 130
WS, no. 10
Eredox, v vs. SCE 1.02 1.06 1.04 1.02 1.06 1.04 1.04 1.04 1.03 1.04 1.04 1.03 1.02
0.62 0.63 0.62 0.62 0.63 0.62 0.63 0.65 0.65 0.63 0.65 0.65 1.02 1.02
a
( a m t of light Pt), intensity, m o l / c m Z mW/cm2
i n p u t pwr, PW
Q e at
Ev(oc)/Ev at '?max,mV
'?mu,%
FF
Part A. Aqueous 15 M LiCl, Saturated C1, 0 36 640 0.67 3 0.63 18 0.57 0 9.6 170 0.66 3 0.64 18 0.57 0 36 64 0 0.33 3 0.59 18 0.50 0 9.6 170 0.25 3 0.59 18 0.51 0 28 1400 0.41 0.43
6501260 6901380 6901500 6 3 0 13 2 0 6801440 6501500 440/100 5901340 6201450 3601100 5401380 5401370 500/140 7001430
7.4 10.5 13.4 9.0 11.9 13.5 0.7 8.3 9.8 0.5 8.3 7.8 1.8 6.6
0.33 0.46 0.65 0.43 0.53 0.70 0.09 0.46 0.61 0.13 0.50 0.54 0.16 0.43
Part B. Aqueous 12 M LiBr, 0 36 64 0 3 18 0 9.6 170 3 18 0 36 640 3 18 0 9.6 170 3 18
7101320 7301510 7301500 6601360 6901510 7001520 5001170 6201450 6101430 4301150 5501390 5701390
10.0 15.8 15.8 11.9 15.6 16.2 3.1 12.2 10.3 2.6 9.4 8.9
0.39 0.63 0.60 0.49 0.68 0.67 0.19 0.61 0.58 0.17 0.54 0.56
6201320 6401480 6001320 6001480
8.7 12.1 9.9 14.8
0.43 0.60 0.46 0.68
Eredox
-
5 mM Br, 0.69 0.66 0.69 0.71 0.64 0.66 0.63 0.62 0.56 0.68 0.61 0.54
Part C. In CH,CN, 1M [Et,N]Cl, C1, 0 48 380 0.64 1 0.62 0 14 110 0.71 1 0.71
I n p u t optical power a t 632.8 n m ; data uncorrected for reflection or solution absorption of light.
was necessary to gradually move the potential more positive to maintain this current density, though the potential was moved no more positive than -0.1 V vs. SCE. Plating was terminated when the desired charge, measured by a PAR Model 179 digital coulometer, had passed. Plating of Pt onto p-WS2was effected by illuminating (632.8 nm, -40 mW/cm2) the p-WS2electrode held between 0.0 and +0.2 V vs. SCE in the plating solution. Surface Analysis. Scanning electron micrographs were obtained on an AMR Model lOOOA equipped with a Tracor Northem Model TN2000 energy-dispersiveX-ray analyzer, using a 20-kV electron beam. Gold coating of samples was generally unnecessary. Auger spectra and elemental maps were obtained on a Physical Electronics Model 590 scanning Auger spectrometer utilizing a 5-kV electron beam at a current of 150-200 nA. X-ray photoelectron spectra were obtained on a computer-controlled Physical Electronics Model 548 spectrometer equipped with Mg anode. Electrodes were mounted as samples, after removal of the Pyrex tube, by fastening the Cu wire to the sample holder, ensuring electrical grounding. Elemental signals were compared to the literature23for identification. Results and Discussion Photoelectrochemical Oxidation of Halides in Aqueous and Nonaqueous Media. We have found that modification (23) (a) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. G. 'Handbook of Auger Electron Spectroscopy", 2nd ed.; Perkin-ElmerCorp.: Eden Prairie, MN, 1972. (b) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G . E. "Handbook of X-Ray Photoelectron S p e c t r o ~ ~ p yPerkin-Elmer "; Corp.: Eden Prairie,
MN, 1979.
of the n-MS2 (M = Mo, W) surface by electrodeposition of Pt enhances, by as much as 2-fold for "good" electrodes and over 10-fold for poor ones, the photoelectrochemical conversion of optical energy in CH&N or H20 electrolyte solutions of X,/X- (X = C1, Br). Figure 1 shows steadystate photocurrent-voltage curves for naked and platinized n-MS2-basedaqueous cells, poised to Erdox= +1.04 V vs. SCE in 15 M LiCl and Eredox = +0.64 in 12 M LiBr. In such curves key parameters include open-circuit photovoltage, Ev(oc), the quantum yield for electron flow at Erdor,@e, and the fill factor, FF.'j The maximum energy conversion efficiency, q-, can be deduced from the photocurrent-voltage curves.6 Output parameters for a number of electrodes in aqueous C12/LiC1and Br2/LiBr and CH3CN/X2/X- solutions are given in Table I. (Additionaloutput parameter data are provided as supplementary material. See paragraph at end of text regarding supplementary material.) In both aqueous and nonaqueous media the full cell chemistry can be represented by eq 1and 2 for the pho2X-
-
X2 + 2e-
X2 + 2e-
-
2X-
(n-MS2 photoanode)
(1)
(counterelectrode)
(2)
toanodic and counterelectrode processes, respectively. In such cases, the output from the cell is electricity and no net chemical change occurs. In aqueous 15 M LiC1/C12 solutions the relatively efficient (-7%) n-WS2 photoanodes can be nearly doubled in output efficiency (- 13%) when platinized to 1.8 X lov7 mol of Pt/cm2 (Figure 1, Table I). Interestingly, the same
Photoelectrochemical Oxidation of Halldes
platinized surface proved to be 13% efficient in CH3CN/ 1 M [Et4N]C1/C12solution. The n-MoS2 also improves substantially in efficiency with Pt deposition. In some cases poor electrodes (no. 130,0.7%) could be dramatically enhanced (to 10%1, while more efficient naked surfaces showed an approximately threefold increase in q- (Table I). The output characteristics differ from electrode to electrode and not all electrodes show large improvements when platinized. We suspect that in some cases this variation may be due to uneven deposition of Pt on the surface (vide infra). While the naked MS2semiconductors are more efficient for Br- oxidation in aqueous 12 M LiBr than for C1- oxidation in 15 M LiC1, platinization is still seen to improve the performance; n-WS2 increases from 10% to -15% conversion efficiency, and n-MoS2shows about a threefold increase (Figure 1, Table I). Likewise, the efficiency for C1- oxidation in CH3CN/1 M [Et4N]C1/C12is relatively good compared to aqueous 15 M LiCl for the naked electrodes, but significant improvement in efficiency can be realized by platinization. As can be seen in Figure 1 and Table I, the improved output characteristics are largely a result of increased fill fador, and to a lesser extent a larger open-circuit voltage, Ev(oc). The large fill factors reported (0.5-0.7) appear to be the best found to date for the oxidation of C1- at photoanodes.16 Improvement in qmax is a function of the amount of Pt deposited, with fill factor and Ev(oc) increasing as the quantity of Pt is increased. However, there is a competing effect since the buildup of Pt results in "shading" of the photoactive surface, causing a reduction in quantum yield, @'e, and hence the photocurrent. Figure 1 clearly shows that the plateau region of the photocurrent-voltage curves is lower for the platinized electrodes where qmar has been optimized. The optimum coverage varies with the electrode but lies in a range between 3 X and 4 X lo-' mol/cm2. The data in Figure 1and Table I unambiguously show that deposition of Pt onto MY2electrodes can dramatically improve the performance for the photoelectrochemical oxidation of X- in aqueous or nonaqueous media. The remainder of this paper establishes aspects of the mechanism of the improvement in efficiency, the nature of the n-MS2/Pt/liquid electrolyte interface, and the durability of the photoanodes. Interfacial Energetics. The deposition of a metal onto a semiconductor can ordinarily result in the formation of an ohmic contact or a Schottky barrier. While an n-type semiconductor would generally be expected to give a Schottky barrier when contacted by a metal like Pt that has a large work the interface chemistry upon deposition can sometimes result in a low resistance contact. Indeed, some n-type semiconducting photoanodes have been ohmically contacted by deposition of Pt onto the surface.26 For n-MS2 surfaces though, electrochemical deposition onto the surface does not give an ohmic contact to the electrolyte solution. The formation of an ordinary Schottky barrier by depositing Pt onto the surface of the n-MS2electrodes would still result in an electrode that gives a good photoeffect. However, if the n-MS2/Pt interface is a Schottky barrier and the n-MS2surface is completely coated by the Pt, then the value of Ev(oc) would be independent of the electro-
The Journal of Physical Chemistry, Vol. 87, No. 22, 1983 4449
Scheme I. Interfacial Energetics at the Flat-Band Condition for n-MS, Electrodes in CH,CN/O.l M [n-Bu,N]ClO, from Ref 4 and 6 n-K2
N
(24) Sze, S. M. 'Physics of Semiconductor Devices", 2nd ed.; WileyInterscience: New York, 1981. (25) (a) Wrighton, M. S.;Wolc~an~ki, P. T.;Ellis,A. B.J. Solid State Chem. 1977,22,17. 6)Wagner, F. T.;Somorjai, G. A. J. Am. Chem. SOC. 1980,102,6494. (c) Hope,G. A.; Bard, A. J., private communication and submitted for publication.
n-MoS2
EtB=-O.3
0
T
Ef---
ECB = r 0 . 2
--'
IyB
= +1.4
chemical potential of the solution Edox because the photovoltage would be controlled by the energetics associated with the n-MY2/Pt interface and not by the n-MS2/liquid electrolyte interface as for the naked electrode. The variation of Ev(oc) with Eredox has been reported for nMoS2 and n-WS2 in CH3CN/0.1 M [n-Bu4N]C104by examining the cyclic voltammetry for a variety of fast, oneelectron, outer-sphere redox reagenta4s6 We have carried out a limited set of such measurements for platinized n-MS2 electrodes that are improved by the platinization procedure for the oxidation of C1- in 15 M LiC1. The absence of a uniform n-WS2/Pt Schottky barrier in contact with the liquid electrolyte is established by examination of the cyclic voltammetry for 1mM TCNQ in CH3CN at the platinized n-WS2surfaces! In the dark, platinized n-WS2(no. 51 of Figure 1)has a single, reversible wave with the anodic peak, EW,at -0.27 V vs. SCE, as for a Pt electrode. When illuminated, two waves are found with anodic peaks at -0.27 and +0.10 V vs. SCE. Taking the photovoltage as E" = IEpa(MS2)- Epa(Pt)1,6 this represents Ev = 170 mV for the more positive wave, within 10 mV of the Ev = 160 mV found for n-WS2(no. 51) before platinization. Similar results were obtained for other n-WS2/Pt electrodes in contact with TCNQ. For the nMoS2/Pt interface the absence of a uniform Schottky barrier is established in a similar fashion but using TMPD in CH3CN which has two reversible waves straddling the n-MoS2 conduction band edge.4a The absence of a photoeffect for the first wave of the TCNQ at n-WS2/Pt or TMPD at n-MoS2/Pt indicates that a uniform Schottky barrier, for which Ev should be independent of Eredo=,is not formed upon deposition of Pt onto the n-MS2surfaces. It is apparent that the interface energetics remain dominated by the interaction of n-MS2 with the liquid electrolyte solution. This suggests that the n-MS, is incompletely coated with the deposited Pt; microscopy and X P S analyses (vide infra) substantiate this hypothesis. When C1- oxidation is examined by cyclic voltammetry in CH3CNat the n-WS2/Pt and n-MoS2/Pt interfaces, the anodic peaks are shifted cathodically by 480 and 320 mV compared to the naked surfaces, respectively (Figure 2).
The Journal of Physical Chemistry, Vol. 81, No. 22, 1983 Pt in :H3CN,0.1M [n-Bu,N] i t 100mV/sec :l-ot
C104
I
I
I
1
:I-at n-MoS2*35 in :H3CN,G.I M[n-Bu4N] CIO,, i t 100 mV/sec
I
I
I
1
2mM[Et4N] C I
--- NAKED
T
IOpA
iG W
1
I
:l-at n-WS2*23
I
I
1
I
I
I
in
:H3CN,0 I k4[n-Bu4N] i t 100 mV/sec
CIO,
,
2mM[Et4N] C1
T
0.I p A
i
I
1
0
I
I +0.4
1
I
[email protected] I
+1.2
POTENTIAL,V vs SCE
FlgUfe 2. Comparison of the cyclic voltammetry of 2 mM [Et,N]CI at Pt (top), naked (- -) and platinized (-) n-MoS, (center), and n-WS, (bottom) in CH CN/O.l M [n-Bu,N]CIO, at 100 mV/s. Illumination is -40 mW/cmd at 632.8 nm.
-
The relative positions of anodic peaks for n-WS2/Pt and n-MoS2/Pt are consistent with a more negative flat-band potential for n-WSz than n-MoS2,as previously reported.48 In the case of n-MoS2/Pt the onset potential of photoanodic current coincides with the flat-band potential, E F B , for n-MoS2, -+0.3 V vs. SCE. Scheme I shows the interface energetics for n-MS2 electrodes used in this study. The photovoltages, Ev, on the platinized surfaces, Ev = 690 mV for n-WS2/Pt and Ev = 560 mV for n-MoS2/Pt, are in agreement with the value predicted by cyclic voltammetry of fast, one-electron, outer-sphere species in the same potential regime.lg*kSThis is in contrast to the lower photovoltages obtained at the naked n-MSz surfaces and suggests that the two-electron formation of C12 is more kinetically facile on the platinized semicondudor surfaces. This result is consistent with the improvements in efficiency upon platiniiation. The main conclusions from the cyclic voltammetry of n-MS2/Pt in CHsCN are that the kinetics for the oxidation of Cl- are improved and that the interface energetics remain dominated by the contact of n-MSz with the electrolyte.
Simon et ai.
It has been suggested that both n-WS2 and n-MoSz become Fermi-level pinned by surface states at sufficiently positive Eredo=, resulting in a constant photovoltage for redox couples at more positive values.@ The Ev measured by cyclic voltammetry for ClZ/Cl- and the open-circuit photovoltages Ev(oc) observed for n-WS2/Pt in CH3CN or H20 indicate that platinization does not affect the surface states responsible for pinning of the Fermi level at about 700 mV positive of EFB. The case for n-MoSz is less clear. The Ev(oc) and Ev measured for C12/C1-in H20 and CH3CN, respectively, agree with the value predicted by examination of fast one-electron c ~ u p l e s . ~ However, g*~~ the onset for Br- photooxidation in H20, -+0.05 V vs. SCE, lies negative of Em (Em = +0.3 V vs. SCE, Scheme I).4a Onset of Br- oxidation negative of E F B could be explained by adsorption of Br,/Br- onto the n-MS2resulting in a negative shift of the flat-band potential in analogy to the result from adsorption of 13-/1- onto n-MY2.'+ Onset of Br- oxidation current negative of E F B for n-MoSz has been observed in this laboratory before," but such behavior may, in many cases, be unobservable as a result of poor kinetics for Br- oxidation offsetting the effect of a negative shift in E F B . Platinization of the surface also enhances the dark C12 reduction current. This is clearly evidenced by the current-voltage curves in Figure l. Additionally, in the cyclic voltammetry of C1- in CH3CN the C12 reduction wave is almost nonexistent at the naked surface, in contrast to the well-defined C12reduction wave at the platinized surfaces (Figure 2). This dark C12 reduction current is a majority carrier process and is unaffected by changes in recombination rate, suggesting that the increased current results from improvement of the charge-transfer rate for the back-reaction. This is to be expected since Pt also has good kinetics for the reduction of C12 to C1-.21 A noteworthy feature of the dark current-voltage curves is that the oxidation of C1- does not occur in the dark at Edm, and even for potentials significantly positive (-400 mV) of Eredo= the dark current for oxidation of C1- is low. This is consistent with the conclusion above that the deposited Pt is not in ohmic contact with the n-MS2. Microscopic Surface Structure. Further insight into the nature of the platinized surfaces comes from scanning electron microscopy ($EM) of n-MS2/Pt surfaces. The Pt is deposited predominantly in an island structure (Figure 3). Both X-ray fluorescence and scanning Auger techniques reveal that the islands are Pt and that the areas between deposits are relatively free of Pt. X-ray photoelectron spectroscopy at low lateral resolution reveals signals for both Pt and substrate, consistent with an island structure rather than a thin continuous Pt film. The island growth is almost invariably more dense along steps on the surface than on smooth regions at lower Pt coverages, but the surface is usually covered relatively uniformly at the optimum Pt coverages. Steps and other faults have been and identified as regions of increased dark conductivity1d,26 greater negative charge density and this may account for the greater initial density of Pt nucleation sites on the steps. Island formation of metals deposited on semiconductor surfaces is a fairly common phenomenon for a number of deposition technique^.^^-^^ The discontinuous nature of the film is consistent with the interfacial energetics deduced from current-voltage curves. A relatively large (26) Ahmed, S. M. Electrochim. Acta 1982,27, 707. (27) Harris, L. A.; Hugo, J. A. J. Electrochem. SOC.1981,128, 1203. (28) Gerischer, H.; Lubke, M. 2.Phys. Chem. (Frankfurt am Main) 175, 98, 317. (29) Goldatein, B.; Szostak, D. J. J. Vac. Sci. Technol. 1980,17, 718.
n-MoS,
130/Pt,
1 . 8 ~ 1 0 ' ' mol/cm2
'p
J
n-MoS, +M 1 0 / P t ,
p-WS,+3/Pto
3x10"
3x10'8
mol/cm2
1
1olJ
mol/cm2
Flguro 3. Scanning electron micrographs of platinked W S 2 and n-WS2 surfaces. Mght '@islands'@ are Pt and dark areas are the naked MS2 surface. Upper left W S 2 no. 130, 1.8 X io-' md/cm2Pt. Upper right: n-MoS2no. M10,3 X 10" mol/cm2 Pt, showing "decoration" of steps on the surface by the elecb.odeposlted Pt. Lower left n-WS, no. 52, 1.5 X 10'' mo1/cm2 Pt, showtng that some Islands are comprised of a number of smalbr particles. LOWW right pWS, no. 3 , 3 X lo4 mol/cm2 ~ t .
fraction of the n-MS2 surface remains free of Pt, accounting for the Edox dependence of Ev(oc). The increased deposition of Pt at defect sites which are believed to act as recombination centers1&-*is of interest, but the effect of the interaction on these surface states is unknown. Noble metal ions have been reported to improve the efficiency of n-GaAs photoelectrochemical cells by interacting with, and passivating, surface recombination centers,30but there is evidence that the noble metal ions serve only as a catalyst system for interfacial electron transfer a t Plating of larger quantities of Pt is associated with increased density of Pt islands per unit area; however, the long hole diffusion length parallel to the surface (-200 pm)6means that photogenerated carriers are within range of a Pt site even at low coverages. The improved efficiencies found at higher Pt coverages may be due to an increase in the Pt surface area. The current density for (30)(a) Relson, R J.; Williams, J. S.; Leamy, H. J.; Mier, B.; Heller, A. Appl. Phys. Lett. 1980,36,76. (b) Heller, A; Lewerenz, H. J.; Miller, B. Ber. Bunsenges. Phys. Chem. 1980, 84, 592. (c) Parkinaon, B. A.; Heller, A.; Miller, B. Appl. Phys. Lett. 1978,33,521; J. Electrochem. Soc. 1979,126,954. (31) Ginley, D. Chem. Eng. News 1982,60 (no. 40),24.
C12 evolution a t Pt anodes is known to saturate at high overpotentials (Figure 4).32 Thus, the better fill factors found at higher Pt coverages may be a result of the greater density of islands, creating a larger area of Pt. p- WS,,Examining the Dark Reaction. For pWS, (Em = +1.0 V vs. SCE)16the Cl2/Cl- couple lies positive of the valence band edge, allowing oxidation of C1- to occur in the dark via the majority carrier (h+) process. The rest potential for pWS2 in the dark or when illuminated is the same as for Pt in 15 M LiCl saturated with ClP. Although the onset of anodic current at p-WS, occurs at approximately the same potential as at Pt, it is found that a significant overvoltage is required to drive the oxidation a t a reasonable rate (Figure 4). For p-WS2 oxidation is a majority carrier process not requiring photoexcitation. Thus, recombination is not significant and this finding unequivocally shows a kinetic limitation to the oxidation of c1-. We have recently reported the improvement of kinetics of H2evolution on pWS2 by Pt or Pd electroplated onto the surface.15 The interfacial energetics remain controlled by the solution redox potential at the optimum coverage (32) Conway, B. E.; Novak, D. M.J. Chem. Soc., Faraday l h n s . 1
1979, 75,2454.
Simon et at.
The Journal of Physical Chemistry, Vol. 87, No. 22, 1983
4452
TABLE 11: Output Parameters as a Function of Time for a Typical p-WS,/Pt-Based c1, Evolution Cell light intensity," @,at p-WS, (history) mW/cmz Eredox ~ v ( o c )mV ,~ qmax, % naked 1.4 X
lo-'
mol of Pt/cmZ
initial after 6.72 h at +0.55 V after 25.3 h at +0.55 V after 31.9 h at + 0.55 V final (49 h )
a
4 1 (730)
0.63
680 (300)
41 (730)
0.50 0.50 0.52 0.55 0.58
750 710 690 690 670
(310)
7.8
0.36
12.0 8.6 7.5 7.6 7.3
0.63 0.48
0.41 0.39 0.37
Values in parentheses are input power (in microwatts). b Values in parentheses are Ev at qmsx (in millivolts). I
I
I
300
400
I
+2.0
N -
E
ea
E
Y
7
(540) (410) (360) (320)
fill factor
0
H Y
W
0
-I
-1.0
-2.0 -
L IO0 200
OVERVOLTAGE E-Eredox (mV)
m e 4.
Comparison of Tafel pkts for Pt (0)and naked and plathized pWS2 (A)in 15 M Uci poised at +1.02 V vs. SCE by purging w b C12. Electrodes were not illuminated.
of metal, indicating that the role of the noble metal is electrocatalytic in nature. The ClZ evolution rate was investigated on similarly prepared p- WSz/Pt surfaces (Figure 4)and is enhanced by 1.5-2 orders of magnitude at low overpotentials. Deposition of about 1.6 X lo-'' mol of Pt/cm2 resulted in current-potential characteristics very similar to a Pt anode, without forming an ohmic contact to electrolyte solutions for photoelectrochemical Hz evolution. While the improved electron-transfer rates are dependent on coverage, even the lowest coverages examined (-5 X lo4 mol of Pt/cm2) resulted in enhancement of current density by a factor of 10 a t low overpotential. Results for p-WSz/Pt are clear evidence that Pt on MSz electrodes provides a facile kinetic pathway for the twoelectron oxidation of Cl-. The poor kinetics found for ClZ evolution at the p-WSz surface support the conclusion that a similar effect at the platinized n-WSz and n-MoSz photoanodes accounts, at least in part, for the improvement in the ClZ/Cl- photocell performance. The improvement in efficiency for Br- oxidation in aqueous solution and C1oxidation in CH&N is also a result of improved kinetics. Stability of the Modified Surface. The reproducible nature of the current-voltage curves presented indicates that the platinized surface is moderately stable, and the Pt is not rapidly corroded away or mechanically separated from the surface. Examination of freshly platinized surfaces and those which have been used briefly (- 10 min to 1h) for Clz evolution by SEM shows that no substantial changes occur in the Pt island structure. However, the output parameters do decline over a period of hours, eventually approaching those of the naked surface, as
shown for a platinized n-WSz electrode (Table 11). After 49 h of operation in aqueous 15 M LiC1/ClZat +0.55 V vs. SCE under 41.3 mW/cm2 of 632.8-nm illumination the electrode was examined by microscopy and Auger and X-ray fluorescence spectroscopies. The Pt island structure was not observed and no Pt was detected, while the substrate appeared undamaged. A similar stability run was performed with n-MoSz/Pt at a current density of -40 mA/cm2 with tungsten lamp illumination with the same result; after 20 h of operation the current-voltage characteristics were similar to the initial naked surface and no Pt was detected on the surface. These observations show that the Pt undergoes oxidative dissolution. This is consistent with reports of the slow oxidative dissolution of Pt metal anodes during Clz evolution in far less concentrated C1- solutions.1g~20 The n-MS,/Pt electrodes do not show rapid loss of Pt when immersed in the 15 M LiC1/C12 solution without illumination. The loss of Pt is thus significantly accelerated during the Clzevolution, as expected. An interesting fact from the data in Table I1 is that the open-circuit photovoltage and the fill factor decline significantly before there is an increase in the quantum yield for electron flow. This result suggests that early degradation of the Pt reduces the catalytic activity without removing the gross amounts of Pt that shade the electrode.
Conclusion A limiting factor in achieving high efficiency for the photoelectrochemical oxidation of C1- or Br- at naked n-MSz surfaces is the slow rate of photogenerated hole equilibration with Xz/X-. This problem can be alleviated by the deposition of small amounts of Pt onto the MS2 surface. The mechanism for the improvement of photoelectrochemical Xz generation is associated with electrocatalysis by Pt, giving an increase in the rate of the reaction of the photogenerated holes with X-. The variation in the extent of the improvement in efficiency for halogen generation as Pt coverage is increased can be ascribed to several factors. The rate of increase of efficiency with increase in quantity of Pt is maximum at low coverages, consistent with lowering of the kinetic overpotential by deposition of a catalyst. In some cases, however, much of the initially deposited Pt may grow along steps and other surface defects, explaining why additional Pt enhances the catalytic effect. Efficiency also increases simply as a result of the increase in size of the Pt islands lowering the effective current density that each island must carry at a given light intensity. Finally, the shadowing effect that the Pt islands have on the surface eventually results in lowering of the efficiency with increasing Pt coverage. Stability is the most important criterion in the selection of n-type semiconductor/catalyst systems for generating strong oxidants. The strong oxidizing conditions needed for Clz generation lead to the tendency for the surface catalyst to be converted into a higher valent species (e.g., oxide or M"+(aq)) yielding dissolution or deactivation of
J. Phys. Chem. 1983, 87, 4453-4461
the catalyst. In this case the oxidative dissolution of Pt(0) is the limiting factor in the stability and long-term use of n-MS2/Pt photoanodes for C12generation. Active research is being pursued in our labs toward development of durable catalyst coatings for n-type Semiconductors for C12and O2 generation. The ideal catalyst coating will have to show stability toward oxidation and/or attack by C1-, OH-, C12, and O2 and possibly enable the use of n-type semiconductors in less concentrated electrolyte solutions. Acknowledgment. We thank the United States Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, and GTE Laboratories for support of this work. A.J.R. acknowledges support as an
4453
M.I.T. NPW Fellow, 1982-1983, and D.J.H. acknowledges support for graduate study from NSERC of Canada. Use of the Central Facilities of the Center for Materials Science and Engineering is gratefully acknowledged. R&t4 NO.MoS2,1317-33-5; WS2,1213809-9;Pt, 7440-06-4; C1-, 16887-00-6;Br-, 24959-67-9.
Supplementary Material Available: Three tables giving output parameters for a variety of platinized n-WS2 and n-MoS2 photoanode-based cells in aqueous C12/LiC1 and Br,/LiBr, and in CH3CN/Cl2/C1-and CH3CN/Br2/Br- (4 pages). Ordering information is given on any current masthead page.
Quadrupole Echo Study of Internal Motions in Polycrystalline Media Leslie J. Schwartz,t Eva MeirovHch,tt:John A. Ripmeester,§ and Jack H. Freed'+ 8aker Laboratory of Chemlsby, Cornell Unlversliy, Ithaca, New Ywk 14853; Isotope Department, Welzmann Institute of Science, Rehovot 76 000 Israel: and Natbnal Research Councll of Canada, Divlsbn of Chemlsby, Ottawa, Ontarlo, Canada K1A OR9 (Recelved: February 23, 1983; I n Flnal Form: June 7, 1983)
Quadrupole echo spectra of polycrystalline trimethylsulphoxonium-dg iodide (TMSI-dg) and hexamethylbenzene-dls (HMB-d'a) in the slow- and fast-motionaltemperature regions are analyzed via a dynamic line-shape formalism which incorporates the stochastic Liouville operator. Both molecules are found to exhibit two approximately independent motional modes: methyl-group rotation, and discrete jumps about a single axis. For TMSI-dg, this latter mode consists of 3-fold jumps about an axis tilted 67.5 f LOo with respect to the S-C axis, with an activation energy EA = 11.1f 0.9 kcal/mol and an inverse frequency factor ro 6 X s, while HMB-dIsreorients about its hexad axis with EA = 5.3 f 0.2 kcal/mol and ro 2 X s. Simulations based on the Fourier transforms of calculated free induction decays (instead of calculated echoes) are inadequate for much of the temperature ranges studied.
I. Introduction The development of solid-state NMR techniques' has made it possible to obtain well-resolved NMR line shapes from polycrystalline materials. Contrary to liquids, where rapid molecular reorientation can effectively average out any effects that slow, anisotropic internal motions have on the spectra, in solids, internal motions (involving molecular segments rather than entire molecules) can be the only ones present. Changes in NMR line shapes will result when the rates of the internal dynamic processes are on the NMR time scale. For many years, dynamic processes in solids were followed by dielectric relaxation, broad-line NMR, and spin-lattice relaxation measurements2 In many instances, the exact dynamic model presented in such NMR studies depended on second moment calculations based on crystal structure data, assumed structural features, and likely models for the motion. More recently, with the advent of multiple-pulse, cross-polarization,and dipolar decoupling techniques,' the ability to obtain well-resolved NMR spectra of spin 1/2 nuclei has led to the development of complete dynamic line-shape formulation^^-^ which can yield very specific information on both motional models and rates. For spin 1 nuclei (such as 2H) with large quadrupole splittings, it is impossible to obtain the NMR line shape t Cornell
University.
* Weizmann Institute of Science. 1 NRC
of Canada.
directly by Fourier transformation of the free induction decay (fid), as it is impossible to detect an undistorted fid within a few microseconds of the rf pulse. In this instance, the quadrupole echo sequence6may be used to refocus the magnetization and the line shape can be obtained by Fourier transformation of the half-echo. In the presence of slow motions, 2H spectra obtained from quadrupole echoes may differ significantly from those expected from the fid' because of irreversible dephasing during the echo pulse separation time. However, line shapes obtained from quadrupole echoes in the slowmotional region can still give quite specific information on motional models and rates, provided that unavoidable experimental distortions are properly accounted for. For (1) (a) A. Pines, M. G. Gibby, and J. S. Waugh, J. Chem. Phys., 69, 569 (1973); (b) U. Haeberlen, Adu. Magn. Reson., S1 (1976); (c) M. Mehring in "NMR Basic Principles and Progress", Vol. 11, P. Diehl, E. Fluck, and R. Kosfeld, Eds., Springer-Verlag, New York, 1976. (2) P. S.Allen, Phys. Chem. (MTP), 4, 43 (1972). (3) D. E. Wemmer, Ph.D. Thesis, University of California, Berkeley, CA, 1978; see also ref IC,p 39. (4) H. W. Spiess in 'NMR Basic Principles and Progress", Vol. 15, P. Diehl, E. Fluck, and R. Kosfeld, Eds., Springer-Verlag, New York, 1978, and references cited within. (5) R. F. Campbell, E. Meirovitch, and J. H. Freed, J. Phys. Chem., 83,525 (1979). (6) (a) I. Solomon, Phys. Rev., 110, 61 (1958); (b) J. H. Davis, K. R. Jeffrey, M. Bloom, M. 1. Valic, and T. P. Higgs, Chem. Phys. Lett., 42, 390 (1976). (7) (a) H. W. Spiess and H. Sillescu, J . Magn. Reson., 42,381 (1981); (b)D. E. Woeasner, B. S. Snowden, Jr., and G. H. Meyer, J. Chem. Phys., 51, 2968 (1969).
0022-3654/83/2087-4453~01.50/0 0 1983 American Chemical Society
