J. Phys. Chem. 1991, 95, 1373-1380
1
0
Peak Intensity vs. Pt deposition
1
2 3 Pt [ioLo atoms/cm*]
4
1
6
Figure 10. HREELS peak intensities as a function of Pt coverages, clearly showing dipole scattering for A120, modes,and impact scattering for adsorbate modes.
coverage is greater than about 2.2 X I O l 5 atoms/cm2. From the TEM analysis it can be roughly estimated that the average cluster size at this deposition is about 1.4 nm. The most plausible explanation for this is that there are not sufficient sites with three-fold symmetry available to the ethylene until the clusters approach this size. Although there was little change in the CO spectra after the sample was heated to 700 K, one change was that the HREELS elastic peak became larger after the annealing treatment. As stated in our previous papers on this subject, the elastic
1373
peak falls between 1 and 2 orders of magnitude after the clusters are deposited. This has been attributed to the roughening of the surface, reducing the specular reflectivity of the surface. After this surface was annealed, an increase in the elastic peak intensity could be interpreted as due to sintering of the Pt clusters, leaving more of the smoother underlying substrate exposed. A plot of the HREELS peak intensities versus the Pt coverage for the 165 K C2H4spectra (Figure 10) supports our previous conclusion that the scattering from these adsorbate covered clusters is dominated by the impact mechanism. This is shown by the nearly constant intensities of the hydrocarbon peaks, although the elastic peak drops by a factor of three. The alumina phonon mode, on the other hand, drops in intensity as the surface reflectivity decreases, as is the case for dipole-dominated scattering. Conclusions TEM micrographs have been presented that show Pt cluster distributions on an alumina substrate. On the basis of these and HREELS experiments in which the vibrational spectra of adsorbed CO and C2H4 was analyzed, it can be concluded that this catalyst system is a stable model of a Pt/A1203 catalyst. Evidence for ethylidyne formation on these clusters has been shown, for the case of Pt distributions with an average cluster size larger than about 1.4 nm. It is expected that this general purpose fabrication technique can be extended to study other transition-metal catalysts and other size distributions. Acknowledgment. This work was supported by the Office of Naval Research. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We also thank Brad Johnson for his assistance in operating the TEM. Registry No. Pt, 7440-06-4; CO, 630-08-0 C2H4,74-85-1.
Trends in the Open-Circuit Voltage of Semiconductor/Liquid Interfaces: Studies of n-AI, Gal, As/CH,CN-Ferracene+'O and n-AI, Gal, As/KOH-Se-'*-( aq) Junctions Louis G. Casagrande, Bruce J. Tufts, and Nathan S. Lewis* Division of Chemistry and Chemical Engineering, California Institute of Technology,t Pasadena, California 91 I25 (Received: June 4, 1990; In Final Form: August 10, 1990)
Trends in open-circuit voltage (V& short-circuit current density (Js), and energy conversion efficiency have been determined for the n-type AI,Ga,-,As series of photoelectrodes (x = 0.0,0.09,0.16, 0.24, 0.31) in contact with CH3CN-ferrocene+/0 and KOH-Se-/2-(aq) electrolytes. V, increased linearly with increases in bandgap energy (E ) of the n-AI,Ga,,As alloy clectrodes, with AV,/AE, = 0.45 f 0.04 V eV-l in CH$N and 0.41 f 0.09 V eV-' in KOH-de-12-(aq) at a light intensity sufficient to provide J , = 1.O mA cm-2. J , values under solar-simulatedillumination decreased monotonically with increasing bandgap energy. The relatively low value of AV,/AE implies decreases in optimal energy conveision efficiency as the mole fraction of AI in the AI,Ga,,As alloy is increased. +his is in contrast to the behavior of the n-GaAs,PI, alloy series in the same electrolytes. The lower value of AV,/AE, for n-AI,Gal,As also indicates that predictions of the Ycommonanion rule" in solid-state barriers do not apply to this family of Ill-V semiconductor/liquid junctions.
Introduction A major goal of our research program has been to develop a predictive framework for the electrical properties of the semiconductor/liquid interface. One of the basic tenets regarding these junctions is that, for a variety of n-type semiconductor surfaces in contact with a given redox couple-electrolyte-solvent system, the value of the equilibrium barrier height should change in a predictable fashion with changes in the electron affinity of the semicond~ctor.'-~To date, there have been few studies relating *Address correswndence to this author. 'Contribution no. 8149.
such variations to changes in interfacial properties of semiconductor/liquid junctions. We have been involved in a long-term project to address this issue and describe in this paper studies that attempt to correlate the key stationary-state observable, the (1 ) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (2) (a) Gerischer, H. J . Electroam/. Chem. 1983, 150, 553. (b) Gerischer. H. In Physical Chemistry, An Advanced Treatise; Eyring, H. Y., Henderson, D., Jcst, W., Eds.;Academic Press: New York, 1970; Vol. 9A, pp 463-542.
(3) Bard. A. J.; Bocarsly, A. B.; Fan, F. R. F., Walton, E.G.; Wrighton, M. S. J. Am. Chem. SOC.1980,102, 3671. (4) (a) Butler, M. A.; Ginley, D. S. Chem. fhys. Lett. l977,47,319. (b) Butler, M. A.: Ginley, D. S. J. Electrochem. SOC.1978,125, 228.
0022-3654/91/2095-1373$02.50/00 1991 American Chemical Society
1374 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 open-circuit voltage ( Vw), to changes in the composition and electron affinity of the bulk semiconductor phase. We have chosen the 111-V series of solid solutions for study because their synthesis and solid-state properties are well-documerited.$ GaAs, AIAs, and InP are well studied,6 and the conduction and valence band discontinuities for the 111-V ternary series relative to GaAs, AIAs, or InP are available from experimental and/or theoretical Use of ternary alloys in the (AI, Ga, In)/(P, As) system allows variation in either the lattice anion or lattice cation and affords the potential for stable photoanode operation in a variety of electrolyte-redox couplesolvent combinations. The ability to study the properties of the entire electrode series should allow us to address the effects of electrode kinetics and changes in surface chemistry with variation in the composition of the electrodes. There are a limited number of studies that have addressed similar issues at the semiconductor/liquid junction. Butler and Ginley attempted to correlate the behavior of oxygen-evolving photoanodes with the bulk properties of the metal oxide series4 Although no experimental electron affinity or work function data have been obtained for most of the samples studied, these authors were able to correlate the Sanderson electronegativity function to the potential at which a photocurrent onset for water oxidation was observed. Ellis and co-workers have studied luminescence and photocurrent-voltage properties for the single-crystal nCdS$el-, photoanode series in an aqueous KOH-S-12- electrolyte and have found only small changes in V, despite a 0.7-eV change in bandgap from CdS (EK= 2.4 eV) to CdSe (EK= 1.7 eV).lo Studies of n-CdSe,Te -, surfaces in contact with KOH-S-/2-(aq) or KOH-Fe(CN),'-J4-(aq) electrolytes have noted improved performance for n-CdSeo,65Teo,35 anodes relative to that of nCdSe." Additionally, Noufi et al. have investigated the behavior of polycrystalline n-CdS,Se,_, anodes in the KOH-S-12-(aq) electrolyte and found modest increases in V, for 0 C x C 0.9, followed by a sudden drop in V, for 0.9 C x C 1.0.l2 These variations have been proposed to arise either from large changes in the electron affinity of CdS when small amounts of Se are introduced into the bulk or possibly from changes in the potential drop across the Helmholtz layer.]* This study concentrates on the photoelectrochemical behavior of the n-AI,Ga,,As (0 5 x 50.31) series in contact with H20KOH-Se-/2- and CH,CN-LiC104-ferrocene (Fc)+/Oelectrolytes. The high-quality, epitaxial single-crystal n-A1,Gal-,As layers, grown by organometallic vapor-phase epitaxy,I3 provide an opportunity to examine changes in interfacial behavior without complications from heterogeneous sample composition or recombination effects at grain boundaries. If the increase in bandgap ( 5 ) (a) Lorenzy, M. A.; Blakeslee, A. E. Gallium Arsenide and Related Compounds; Institute of Physics: London, 1973; Conf. Ser. No. 17, p 106 (b) Blakemore, J . S.J . Appl. Phys. 1982, 53, R123. (6) (a) Adachi, S. J . Appl. Phys. 1985, 58, R1. (b) Ludowise, hA J. J . Appl. Phys. 1985, 58, R31. (c) Williams, C. K.; Glisson, T. H.; Fr .user, J. R.; Littlejohn, M. A. J . Electron. Mater. 1978, 7, 639. (d) Ontcn, A. Adv. Solid State Phys. 1973, 13, 59. (7) Harrison, W. Solid State Theory; Dover: New York, 1979. (8) (a) Wang, W. 1.; Stern, F. J . Voc. Sci. Technol. 1985, 8 3 , 1280. (b) Arnold, D.; Ketterson, A.; Henderson, T.; Klem, J.; Morkoc, H. Appl. Phys. Lett. 1984, 45, 1237. (c) Porcd, W.; Potz, W.; Ferry, D. K. J . Vac. Sei. Technol. 1985.83. 1290. (d) Drummond, T. J.; Fritz, 1. J. Appl. Phys. Lett. 1985.47, 284. (e) Dingle, R.; Wiegmann, W.; Henry, C. H. Phys. Reu. Lett. 1974, 33, 827. (9) (a) Gourley, P. L.; Biefield, R. M. Appl. Phys. Lett. 1984,45,749. (b) Samuelson, L.; Pistol, M.-E.; Nilsson, S. Phys. Rev. 1986, 833, 8776. (IO) (a) Streckert, H . H.; Tong, J.-R.; Carpenter, M. K.; Ellis, A. B. J . Electrochem. Soc. 1982, 129, 772. (b) Carpenter, M. K.; Streckert, H. H.; Ellis, A . B. J . Solid State Chem. 1982, 45, 51. (c) Streckert, H. H.; Ellis, A. B. J . P h p . Chem. 1982,86, 4921. (d) Carpenter, M. K.; Ellis, A. B. J . Electroanal. Chem. 1985, 184, 289. ( I I ) (a) Licht, S.;Tenne. R.; Dagan, G.; Hodes, G.; Manassen, J.; Cahen, D. Appl. Phys. Lett. 1985. 46,608. (b) Frese, K. W., Jr. Appl. Phys. Lett. 1982, 40, 275. (12) Noufi, R. N.; Kohl, P. A.; Bard, A. J. J . Electrochem. Soc. 1978, 125, 375. ( 1 3) (a) Lewis, C. R.; Ford, C. W.; Werthen, J. G. Appl. Phys. Lett. 1984, 45, 895. (b) Werthen, J. G.; Virshup, G. F.; Ford, C. W.; Lewis, C. R.; Hamaker, H . C. Appl. Phys. Lett. 1986, 48, 74.
Casagrande et al. energy at higher AI c o n t e d a results in a substantial increase in open-circuit voltage, it may be possible to obtain improved energy conversion efficiencies by utilizing members of the n-AI,Ga,-,As series in photoelectrochemical cells that are presently constructed with n-GaAs photoanodes.leI6 Alternatively, if the n-A1,Ga1,As series behaves as the n-CdS,Se,-, series,'"I2 then photovoltaic efficiencies will not be improved by increasing the bandgap of the photoanode in this alloy series. A comparison of the AI,Gal,As results with previous work on the behavior of n-GaAs,,P, ternary alloys in contact with both aqueous and nonaqueous medial7 will provide information on the effects of variation of the lattice cation vs the lattice anion for this family of alloys. These studies should bear on the general rule that the energy of the valence-band edge is controlled by the electronegativity of the lattice anion.'* Prior work on the behavior of n-AI,Ga,-,As photoelectrodes has focused on their photocorrosion properties in aqueous s o l ~ t i o n ,so ' ~ this work represents the first study to our knowledge of the photoelectrochemical properties of the n-AI,Ga,,As series in a regenerative electrochemical cell arrangement. Several recent studies of the photoelectrode behavior of AI,Gal-,As superlattices have been reported,m and information on the properties of the bulk alloy series should provide a useful background for interpretation of the superlattice electrode behavior. Experimental Section All samples were (100) oriented and were grown by organometalk vapor-phase epitaxy on n+-GaAs substrates (Nd> 5 X IO'*6m-j of Se) oriented 2O off of (100). The photoactive epilayers were 1&15-pm-thick, Nd = (1-2) X IOl7 cm-' Se-doped samples of GaAs or 5-pm-thick, Nd = 1 X I O i 7 cm-3 Se-doped samples of n-AI,Ga,_,As (x = 0.09, 0.16, 0.24, and 0.31). The compositions of the alloys were confirmed by electron microprobe measurements. The bandgap energy for each sample was verified by photoluminescence and spectral response measurements (vide infra). Ohmic contacts were provided either by electron beam deposition of a 1000-A layer of In or by thermal evaporation of a 1000-A layer of a 2% Ge-98% Au alloy. Both contacts were followed by annealing in forming gas at 475 OC for 5 min. Samples were prepared into electrodes as described previ~usly,'~ with typical exposed electrode areas of 0.05-0.15 cm2. All areas were accurately determined from enlargements of photographs of the electrode. The electrode surfaces were etched immediately prior to insertion into the photoelectrochemical cell. Exposure to air was kept as short %s possible and generally was limited to 5-10 s. Samples of AI composition of 24 and 31% required a 2-s treatment with I . ' H F (49% in H 2 0 ) : H 2 0 2(30% in H 2 0 ) before further etc' tng, presumably to remove surface impurities. The etching procedure used for all electrodes consisted of three cycles, each of which involved the following sequence:21 ( I ) immersion for 10-1 5 s in 1 .O M KOH; (2) rinsing with H 2 0 ; (3) drying under a stream of N2(g);(4) etching for 10-1 5 s in 0.05% Br2-CH30H; (14) (a) Chang, K.C.; Heller, A.; Schwartz, B.; Menezes, S.; Miller, B. Science 1977, 196, 1097. (b) Tufts, B. J.; Abraham, I. L.; Casagrande, L. G.; Lewis, N . S.J . Phys. Chem. 1989, 93, 3260. (c) Noufi, R.; Tench, D. J . Electrochem. Soc. 1980, 127, 188. (15) Gronet, C. M.; Lewis, N . S.Appl. Phys. Lett. 1983, 43, 115. (16) (a) Langmuir, M. G.; Hoenig, P.; Rauh, R. D. J . Electrochem. Soc. 1981. 128, 2357. (b) Langmuir, M. G.; Parker, M. A.; Rauh, R. D. J . Electrochem. Soc. 1982, 129, 1706. (17) (a) Gronet, C. M.; Lewis, N . S. J . Phys. Chem. 1984,88, 1310. (b) Gronet, C. M.; Lewis, N . S. Nature (London) 1982, 300, 733. (18) McCaldin, J. 0.;McGill, T. C.; Mead, C. A. J . Vac. Sei. Technol. 1976, 13, 802. (19) (a) Haroutiounian, E.; Sandino, J . P.; Clechet, P.; Lamouche, D.; Martin, J.-R. J . Electrochem. Soc. 1984, 131, 27. (b) Furtado, M. T.; Loural, M. S. S.; Sachs, A. C. J . Appl. Phys. 1987, 62, 4926. (c) Peng, R.; Chen, Z.; Shao, Y. J . Cryst. Growth 1984, 69, 469. (20) (a) Petersen, M. W.; Turner, J . A.; Parsons, C. A.; Nozik, A. J.; Van Hoof, C.; Borghs, G.; Hondre, R.; Markoc, H. Appl. Phys. Lett. 1988, 53, 2666. (b) Vojak, B. A.; Laidig, W. D.; Holonyak, N., Jr.; Camras, N . D.; Coleman, J . J.; Dapkus, P. D. J . Appl. Phys. 1981, 52, 621. (21) (a) Stocker, H. J.; Aspnes, D. E. Appl. Phys. Lett. 1983.42.85. (b) Aspnes, D. E. J . Vac. Sri. Technol. 1985, A3, 1018. (c) Aspnes, D. E.; Studna, A . A . Appl. Phys. Lett. 1985, 46, 1071. (d) Aspnes, D. E.; Studna, A . A. Appl. Phys. Lett. 1981, 39, 316.
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1375
Semiconductor/Liquid Interfaces
TABLE I: n-AI.Cal_.As/KOH(aa)-Se-/*Photoelectrochemical Cell Characteristics
xo
E,,b eV
0 0.09
1.42
0.16
I .63 1.72 I .82
0.24 0.3 1
1.51
v,: v (r = 88 mW m f 2 )
J,? mA%-2
(r = 88 mW
0.737 0.826 0.836 0.884 0.888
18.0 17.6 12.7 6.8 5.6
V,,e
7'
10.1 11.3 8.5 4.5 3.6
v
(Jph= 1.0 mA cm-2) 0.653 0.750 0.754 0.822 0.827 AV,/AE, = 0.41 f 0.09
Mole fraction of AI. Bandgap energy. 'The variation in V, between different electrodes was 1 1 0 mV, but the precision of each measurement was 11 mV. "Error associated with J , is fO.l mA cm-2. 'Percent cell efficiency calculated as the ratio of the cell output at maximum power over the incident illumination intensity, r (mW
(5) rinsing with CH30H; (6) rinsing with H20; (7) drying under between the two Si photocurrent signals, combined with the aba stream of N2(g). After three full cycles were completed, the solute spectral irradiance data for the NBS lamp, then yielded electrode was subjected to a final partial cycle consisting only of the spectral irradiance data for the ELH bulbs. steps 1-3, and the sample was then immersed into the electrolyte Current-voltage curves were collected by scanning at 50 mV of interest. No attempt was made to minimize further the res-I with the scans initiated at the cathodic limits. To obtain flectivity losses through any surface texturing procedures; reproducible behavior in log (Jph)vs V, plots (Jphis the lighttherefore, all electrodes were optically reflective, mirror-finished limited photocurrent density), each electrode was maintained surfaces. initially at low photocurrent densities and at potentials negative All chemicals were electronic grade or better and were purified of open circuit. The electrode was then scanned through several as follows: CH$N (Baker, Aldrich) and C2HSCN(Aldrich) were complete I-Vcycles in which a cathodic current in excess of the distilled under N2(g) from P20s and were stored over activated concentration-limited cathodic current was obtained at potentials Linde 3-A sieves in a N2(g) ambient. Ferrocene (Fc, Aldrich) negative of open circuit. This condition required use of a low initial was sublimed in vacuo (110 pmHg). [Fc+][PF(] was prepared concentration of oxidized species in the cell. The required cathodic by chemical oxidation of recrystallized Fc. The Fc+ cation was current was usually easily obtained within 200 mV negative of the open circuit potential. While I-Vscans were being collected precipitated with NH4PF6,and the salt was then dried in vacuo. the photocurrent density was then gradually increased until the LiClO, (Alfa) was dried by fusing the powder in vacuo. All chemicals for the electrochemical solutions were stored under full 1 sun light intensity was reached. After this point, the current N2(g) in a Vacuum Atmospheres Inc. drybox. Deionized H 2 0 could be lowered and raised with a reversible, linear response in was passed through a Barnstead 18-MQ deionizer. Chemicals log (J,,) vs V,. All open-circuit voltages were verified to approach for etching were used as received. RuCI3.3H20 was used as 0.00 V in the absence of illumination, indicating that parallel dark received from Alfa Inc. corrosion processes were not contributing to anomalously high V, All nonaqueous solutions were prepared in the drybox and were values in any of the junctions discussed in this work. kept free of H 2 0 and 02.Aqueous Se-/2- solutions were prepared V, vs temperature measurements were performed with as previously d e ~ c r i b e d 'and ~ ~ were . ~ ~ ~maintained under a N,(g) equipment described previou~ly.~~ The n-AI,Ga,..,As anode was ambient using Schlenk techniques. All solutions were stirred etched, inserted into the electrochemical cell (0.20 M magnetically. The light source was an ELH-type tungstenLiCIO4-(O.O20-O.0SO) M Fc-1 .O mM Fc+-C2HSCN), and I-V halogen bulb (color T = 3350 K with a dichroic rear r e f l e c t ~ r ) . ' ~ ~ ~curves ~ were collected as described above. To reduce the freezing The light source was directed up through the bottom of the point and to minimize precipitation of the solutes at low temphotoelectrochemical cell by using a mirror, and the solution path peratures, C2HSCNwas substituted for CH3CN, and the eleclength between the downward-facing electrode and the Pyrex trolyte and redox concentrations in the cell were lowered relative optical flat cell bottom was 1-2 mm. For efficiency measurements, to those used for efficiency measurements. After stable nthe lamp intensity was measured by using a homemade Si p-n AI,Gal,As I-V properties were obtained, the short-circuit current solar cell that was frequently calibrated relative to a 4-cm2 Solarex was adjusted to the desired value and the cell temperature was p-n solar cell. The Solarex cell was a secondary standard that then decreased. Typical cooling rates were 10-20 K mi&, and had been calibrated independently at both 1 sun AM1.O and 1 V, and T were simultaneously monitored at 1 Hz through 12-bit sun AM0 illumination. This'Solarex cell was additionally validated A/D converters. Analyses of the slopes and intercepts of V, vs for use as a calibration cell by comparison to the power measured T plots were performed using conventional linear least-squares in direct sunlight with an Eppley Co. thermopile. Efficiency routines. Typical correlation coefficients for an acceptable data measurements of GaAs using a Si calibration cell and an ELH set were >0.995 for the ~ 3 0 pairs 0 of V,-T data points collected source have been shown previously to be accurate to within 5%.22 in a given experiment. However, this procedure can lead to substantial errors for higher Results bandgap materials, and such discrepancies are discussed quanCurrent- Voltage Behavior of n-AI,Ga,-,As in Contact with titatively in the results of this study (vide infra). H20-KOH-Se-12-. Figure 1 shows the I-V behavior of the nThe electrochemical instrumentation was the same as described AI,Ga,-,As electrode series, in contact with aqueous 1.0 M p r e v i ~ u s l y . ' ~ The ~ ' ~ instrumentation ~*~~ used for spectral response M Se2--0.01 M fjez2- at 88 mW cm-2 of ELH-type measurements has been detailed previously in the l i t e r a t ~ r e ' ~ ~ , ~KOH-1.0 ~ tungsten-halogen illumination. Each sample in the series was and was used with only minor modifications for this work. Spectral exposed to 0.010 M RuCI, in 0.10 M aqueous H N 0 3 before irradiance data for two ELH bulbs used in this work were measured by Dr. B. Anspaugh of the Jet Propulsion Laboratory, immersion in the KOH-Se-I2-(aq) electrolyte solution.24 All of the AI,Ga,-,As electrodes showed reproducible increases in the Pasadena, CA. In this procedure, a Si p-n diode was illuminated fill factor (ff) and V , after this step, and the magnitude of the through a monochromator/filter combination by an ELH light increase for a given member of the alloy series was comparable source. The photodiode current response was measured over the to that displayed by t~-GaAs.:!~ The values of V,, the short circuit wavelength range 300-1090 nm, and the Si diode was then illuphotocurrent density (Js), arid the optical-to-electrical conversion minated through the identical optical arrangement with a caliefficiency ( q ) at an ELH-typ light intensity equivalent to 88 mW brated National Bureau of Standards source lamp. The ratio ~~~~~~
(22) (a) Seaman, C. H.; Anspaugh, B. E.; Downing, R. G.; Esrey, R. S. Con/. Rec. IEEE Pfiotovoltaic Spec. Conf. 1980. 14, 494. (23) Rosenbluth, M. L.; Lewis, N . S. J . Am. Cfiem.Soc. 1986, 108,4689.
~
(24) (a) Parkinson, B. A,; Heller, A.; Miller, B. J . Electrochem. Soc. 1979, 126, 954. (b) Parkinson, 8.A.; Heller, A.; Miller, B. Appl. Pfiys. Lett. 1978, 33, 521.
Casagrande et al.
1376 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
TABLE 11: n-AI,Gal-,As/CH~CN-Ferrocene+/O Photoelectrochemical Cell Characteristics 9 0
E,: eV I .42
0.09 0.16 0.24
1.51 I .63 1.72 I .82
0.2 I
v,: v (r = 88 mW cm-2)
:,v v
Jlc$ mA cm-*
(r = 88 mW 20.2 14.0 12.3 11.4 8.3
0.822 0.846 0.863 0.922 0.97
:,v v
(JPh= 5.0 mA
qc
(JPh= 1.0 mA
0.759 0.807 0.837 0.894 0.922
6.6 6.7 6.6 6.5 4.9
AV,/AE,
= 0.41 f 0.03
0.702 0.762 0.783 0.834 0.890 0.45 f 0.04
a Mole fraction of AI. Bandgap energy. 'The variation in V, between different electrodes was f I O mV, but the precision of each measurement was il mV. dError associated with J , IS h0.l mA rPercent cell efficiency, calculated as the ratio of the cell output at maximum power over thc incident illumination intensity, r (mW
2
15
0
,
.
,
.
,
I '
I
J
-
N
'5
-2 -g
.-mi
10,
Q
3
5-
0-
-0.80
-0.40
-0.80
0.00
vappl,vvaPt Figure 1. Current-voltage properties of n-AI,Ga,,As electrodes in contact with 1 .O M KOH(aq)-1 .O M K,Se-O.Ol M KzSez electrolyte. The curves were obtained at 50 mV/s in a three-electrode potentiostatic configuration versus a Pt wire poised at the equilibrium potential of the Se-I2-(aq) (-0.94 V vs SCE). Illumination was provided by a tungstenhalogen bulb at 88 m W cm-* intensity. The composition of the nAI,Gal-,As samples was as follows: solid line, x = 0.09; dashed line, x = 0.16; dash-dot, x = 0.24; dash-dot-dot, x = 0.31.
on the calibration cell are presented in Table 1. To facilitate correlations of V, with changes in bandgap energy of the alloy electrodes, V, values also were collected at light intensities that provided a constant photocurrent density throughout the electrode a linear increase in V, was series. For Jph= 1.0 mA observed with increasing AI,Gal,As bandgap energy, with a slope of 0.41 f 0.09 eV-' (Table I). n-AI,Gal,As electrodes with compositions x 2 0.4 were not investigated in this study because of the change in band structure from direct gap to indirect gap at higher AI compositions.6 Current-Voltage Behavior of n-AI,Gal-,As in Contact with CH3CN-LiC104-Fc+/Q.Figure 2 shows the I-V behavior for the n-AI,Ga,-,As series in 0.09 M Fc-1 mM FcPF6-0.70 M LiCI04-CH3CNat constant photocurrent density ( J p h = 5.0 mA V, values observed at light intensities providing J h = 5.0 and at 1 .O mA and values of V,, J,, and q at 88 m b of ELH irradiation are reported in Table 11. Under 88 mW mW of ELH illumination, a monotonic increase in V, and a monotonic decrease in J , were observed as the bandgap energy in the AI,GaI-& alloy series increased. The absolute values of V, measured in contact with CH3CN-Fc+/0 were larger than those in contact with KOH-Se-/2-(aq) at the same photocurrent density, which is consistent with the trend previously reported for n-GaAs and n-GaAs,PI-, samples in these two electr~lytes.'~J~J~ Despite the =50-mV change in absolute magnitude of V, between the two electrolytes, we observed similar dependences of V , on AEg at constant photocurrent density, with a slope of AV,/AE, = 0.45 f 0.04 V eV-' for the ~I-AI,G~~-,AS/CH~CN-FC+~~ junctions at Jph = 1 .o mA and 0.41 f 0.03 at J p h = 5.0mA cnr2 (Table
-0.40 Vappl, vvs. Pt
0.00
Figure 2. Current-voltage properties of n-AIxGa1-,As electrodes in contact with CH3CN-0.70 M LiC104-0.09 M Fc-1.0 mM Fc+ electroiyte. The curves were obtained at 50 mV/s in a three-electrode potentiostatic configuration versus a Pt wire poised at the equilibrium potential of the Fc+I0 redox couple (+0.18 V vs SCE). Illumination was provided by a tungsten-halogen bulb and was adjusted to obtain a constant light-limited photocurrent density of 5.0 mA cm-* for each sample. The composition of the n-AI,Gal,As samples was as follows: solid line, x = 0.09; dashed line, x = 0.16; dash-dot, x = 0.24; dash-dot-dot, x = 0.31.
larger in contact with the CH3CN-0.7 M LiC104-Fc+/oelectrolyte than in contact with KOH-Se-/2-(aq). This discrepancy was predominantly due to higher solution optical absorbance values in the particular Se-I2- composition and optical cell used for this experiment. Additionally, some of the variation in J , was due to differences in the minority carrier diffusion lengths for the n-AI,Ga,-,As samples used in the KOH-Se-l2-(aq) electrolyte vs those used in the CH3CN-Fc+loelectrolyte. These factors were identified by the quantum yield vs excitatidn wavelength properties of the alloy series in both electrolytes (vide infra). Optimization of the diffusion length and optical absorption properties of the electrolyte was not of primary importance in this study; howeber, previous work has shown that improvements in these parameters are readily attainable and directly translate into improved energy conversion efficiencies through increases in the value of J,.i5324325 Previous mechanistic studies of n-GaAs photoanodes in both KOH-Se-/2-(aq) and CH3CN-Fc+/0 electrolytes have shown that the V, obtained at a given photocurrent density is relatively insensitive to the value of the hole diffusion length,'4bso the values of AV,/AE, obtained at constant photocurrent density (Tables I and 11) were taken to be good representations of the trends in photovoltage for the n-AI,Ga,-,As alloy series in contact with these two electrolyte solutions. Current- Voltage Behavior of n-GaAs,PI-, in Contact with KOH-Se-I2-(aq) and CH3CN-Fc+IQ Electrolytes. Table 111 reports v, values obtained at light intensities sufficient to provide a constant photocurrent density (J,h = 5.0 mA for the
11).
At constant light intensity, the short-circuit photocurrent densities for the n-AI,Ga,-,As electrodes were generally slightly
( 2 5 ) Gibbons. J.
F.; Cogan, G. W.;Gronet, C. M.; Lewis, N. S.Appl.
Phys. Leu. 1984, 45, 1095.
The Journal of Physical Chemistry, Vol. 95, No. 3, I991 1377
Semiconductor/Liquid Interfaces TABLE 111: n-GaAs,P,_, Photoelectrochemical Cell Characteristics i
E8? eV
v,,. v (r = 88 mW
v,: v
J,,d mA
(r = 88 mW
(Jph= 5.0 mA
CH,CN-Ferrocene+/O
0 0.15 0.28 0.38
I .42 1.61 1.77 1.88
0.822 0.938 1.076 1.136
0 0.15 0.28 0.38
I .42 1.61 1.77 1.88
0.737 0.8 2-0.87 0.95-0.99 0.97-1.03
20.2 17.2 12.2 6.7
0.759 0.907 1.057 1.136 AV,/AE, = 0.80 f 0.03
18.0 19-21 15-1 7 9.5-10
0.72 0.88 0.98 1.01 AV,JAE, = 0.64 f 0.07
KOH (aq)-Sd2-
Bandgap energy. CThevariation in V , between different electrodes was *IO mV, but the precision of each measurement a Mole fraction of P. was *I mV. dError associated with measurement of J,, is f O . l mA
600
Wavelength, nm Figure 3. Spectral response vs wavelength of n-AIxGaI-,As electrodes in contact with 1.O M KOH(aq)-l .O M K2Se-0.01 M K2Se2electrolyte. Electrodes were etched to a mirror finish before use. The data are absolute external quantum yields and were not corrected for optical absorption or reflection losses at any of the interfaces in the electrochemical ccll. The decline in quantum yield at long wavelengths was due to shorter minority carrier collection lengths in these samples. The composition of the n-AI,Ga,,As samples was as follows: solid line, x = 0.09; dashed line, x = 0.16; dash-dot, x = 0.24; dash-dot-dot, x = 0.31.
n-GaAs,P,-, alloy series in both CH3CN-Fc+/0 and KOHSe-l2-(aq) electrolytes. Previous studies from our laboratory have described the energy conversion efficiency trends under constant illumination intensity;17 however, data at constant photocurrent density provide a better comparison of trends in AV,/AEg. For light intensities that provided Jph= 5.0 mA cm-2, V , values for the n-GaAs,P,-, anodes were somewhat larger in contact with CH3CN than in contact with KOH-Se-/”(aq), as was observed for the n-AI,Ga,-,As alloy series. The values of AV,/AE, were 0.80 f 0.03 for the ~ - G ~ A ~ , P , - , / C H , C N - F C interfaces +/~ and 0.64 f 0.07 for the n-GaAs,P,-,/KOH-Se-/2-(aq) interfaces. These data are consistent with the trends in V, obtained previously under constant-illumination intensity conditions.I7 The V, for n-GaAs in contact with CH3CN-Fc+lo is somewhat larger than previously due to the improved etching procedure2’ used in the present worksz6 Spectral Response Behavior. The short-circuit spectral response curves obtained for the n-AI,Gal-,As series in contact with KOH-Se-l2-(aq) solutions are displayed in Figure 3, and the spectral response curves for a different set of n-AI,Ga,-,As samples used in contact with CH3CN-LiCIO4-Fc+/O are shown in Figure 4. Both sets of semiconductor/liquid junctions exhibited excellent (26) (a) Tufts,B. J.; Casagrande, L. G.; Grunthaner, F. J.; Lewis, N. S. Appl. Phys. Lett. 1990, 57, 2262. (b) Lewis, N. S.;Roscnbluth, M. L.; Casagrande, L. G.; Tufts, B. J. NATO AS1 Ser. C 1986, 174, 343.
800
1000
Wavelenath. nm Figure 4. Same conditions as in Figure 3, except that the electrolyte is CH3CN-0.70 M LiCI04-0.09 M Fc-0.l mM Fc+. A different set of samples was used for this study than was used in Figure 3. The composition of the n-AI,Ga,,As samples was as follows: solid line, x = 0.09; dashed line, x = 0.16; dash-dot, x = 0.24; dash-dot-dot, x = 0.31.
quantum yields for photons of energy higher than the bandgap energy of the AI,Ga,-,As epilayer. The measured upper limits on the external quantum yield were consistent with internal quantum yields of 1.0, Le., only reflectivity losses limited the maximum external quantum yields to values less than unity. The lack of substantial photocurrent response at energies less than the epilayer bandgap energy verified that the substrate GaAs did not contribute to the photocurrents of the n-AI,Ga,-,As alloy samples. The decreases in response centered at 500 (Figure 3) and at 625 and 450 nm (Figure 4) were due to absorption by SeZ2-,Fc’, and Fc, respectively. The decreases in quantum yield at photon energies just above E in the n-AI,Ga,-,As samples in contact with the KOH-Se-/”(aqI electrolyte (Figure 3) resulted from moderate minority carrier diffusion lengths in some of the n-AI,Ga,-,As samples used in this work,’4b*27 but these decreased quantum yields did not in themselves reflect any intrinsic interfacial loss mechanisms at the semiconductor/liquid junctions. V, us Temperature Data for n-AI,Gal-,As/C2HSCNLiC104-Fc+/o Junctions. The temperature dependence of the open-circuit voltage has been shown to provide useful information concerning the activation barrier for carrier transport a t semiconductor/liquid junction^.'^**^*^^^ The dependence of V on temperature for the n-A1,Gal~xAs/C2HsCN-LiC104-Fc+~ interfaces at 1 sun illumination is shown in Figure 5 . CIHSCN was used as the solvent because its freezing point is lower than that of CH,CN; also, lower concentrations of the solutes were used to minimize precipitation at low temperatures. These changes
-
(27) (a) Gartner, W.W.Phys. Reo. 1959, 116,84. (b) Heller, A.; Chang, K. C.; Miller, B. J . Am. Chem. SOC.1978, 100,684.
Casagrande et al.
1378 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
J AlxCai.xAs
0.31
TABLE V: n-AIEa,_,As/CHEN-FerroceRe+/O Kinetic Parameteno EgSb eV 1.51
A C
1.19 1.26 1.34 1.24 1.26 f 0.06' 1.19 1.33 1.26 i 0.10' 1.50 1.53 1.30 1.44 f 0.13' 1.32 1.32 1.55 1.32 1.38 f 0.12'
I .63 I .72 -4l
0.901
1.82 0.80'
'
200
'
'
220
.
'
240
260
280
300
Cell Temperature, K
Figure 5. Dependence of open-circuit voltage on temperature for samples
of n-AI,Ga,,As
in contact with the C2H5CN-LiC104-Fct/oelectrolyte. The temperature was lowered at 0. The increase in V, at the semiconductor/liquid interfaces under ELH illumination for the n-AlaosGaosIAs/KOH-Se-/2-(aq) therefore must be sufficiently large to offset this decreased J,,. interface relative to that for the n-GaAs/K0H-Se-I2-(aq) system For n-GaAs,P,_, alloys, the relatively large value of A V , / A E , (Table I) will not be observed in optimized cells under solar favorably counteracts declines in J,, for alloy Eg values as high irradiance conditions. When normalized in this fashion, it is clear as I .8 eV.17 The lower value of AV,/AE, for the n-AI,Ga,-,As from a comparison of the AV,/AE, data and the data for V , at series tends to favor use of the lower AI content of the alloy series 88 mW cm-2 of ELH-type illumination that declines in V , (and for optimal energy conversion efficiencies under standard terrestrial in energy conversion efficiency) will be more rapid for the nsolar conditions. A1,Gal,As series than for the n-GaAs,PI, alloy series in contact The efficiency values reported in Tables I and 11, obtained under with either KOH-Se-l2-(aq) or CH3CN-Fc+/0 electrolytes. ELH-type solar simulated illumination, yield accurate repreActivation Barriers and Transport Properties. The data in sentations of the (unoptimized) solar efficiency values only for Figure 5 and Tables I, 11, and 1V indicate that the activation certain alloy/cell combinations used in this study. The efficiency barriers obtained from V, vs T plots and the absolute values of values in these measurements are generally lower than those V, under AMI.5 conditions are much higher for the nreported for some of the cells studied earlier (e&, n-GaAs/ AI,Ga,-,As/liquid junctions than for n-AI,Gal-,As/Mo and CH3CN-Fc+/O),ISbecause minimization of uncompensated ohmic n-AI,Ga,,As/Au Schottky barrier^?^^^^,^^ This is in accord with resistance losses and of optical absorption lossesZSwas not the previous comparisons of n-GaAs/liquid and n-GaAs/metal central focus of the present study. Reliable projections of these junctions, which have demonstrated that the liquid contacts are efficiencies to optimized values are not .straightforward, because not subject to Fermi level pinning restrictions on the photovoltage the experimental value of J,, measured in any specific cell is that are commonly f0ur.d for n-GaAs Schottky barrier interfacsensitive to the alloy bandgap, the minority carrier collection es.17328.33The transport data obtained in this work suggest that properties of the specific material under study, solution absorption surface or near-surface nonradiative recombination dominates the losses, and the output properties of the light source. Additionally, V, values in the n-A1,Gal_,As/CH3CN-Fc+/o series, because the the measured resistance and concentration overpotential losses V, values are clearly too small to be limited by bulk recombiin a cell depend on the current and thus on the J,, value, with nation/injection p r o c e ~ s e and s ~ are ~ ~too ~ ~large ~ ~ to ~ be consistent improved J , values leading generally to a compensating increase with a thermionic emission process of a magnitude indicated by in overpotential losses. This compensation accounted for the the activation barrier of the V, vs T data. This behavior is similarity in measured efficiencies for ~ - G ~ A S ~ , , ~ P ~ , ~ ~ / C Hconsistent ~ C N - with the observation that the AV, does not scale Fc+/Ojunctions under ELH and solar ill~mination,'~~ even though precisely with the change in conduction band energy for the the actual Jsc values differed somewhat, as expected, under the n-AI,Gal-,As alloy series, whereas such behavior has been obtwo different light sources. served for the n-AI,Ga,,As/Mo Schottky barrier height^.'^ Given these caveats, it is possible to obtain reasonable estimates Recent work has yielded a value of 0.65AEg for the conductionof the values of J,, that could be obtained in optimized nband discontinuity at AI,Ga,..,As/GaAs interfaces," which is close AI,Ga,-,As-containing cells under solar irradiance. This can be to (but somewhat higher) than the value of (O.41-0.45)AEg obperformed by calculating the total number of photons in a standard tained for AV, with the n-AI,Ga,-,As/liquid interfaces studied AMI .5 spectrum29that have energies higher than the bandgap in this work. A more quantitative explanation of the differences energy of the semiconductor alloy and then assuming a total ~~
(31) (a) Sanderson, R. T. J . Chem. Educ. 1952,29,539. (b) Sanderson, R. T.J . Chem. Phys. 1955, 23,2467. (c) Nethercot, A . H., Jr. Phys. Reu. Left. 1974, 33. 1088. (d) Frese, K.W., J r . J . Vac. Sci. Technol. 1979. 16, 1042.
(32) (a! Spicer, W. E.;Chye, P.W.;Skeath, P. R.;Su,C. Y.; Lindau, 1. J . Vac. Sa. Technol. 1979, 16, 1422. (b) Spicer, W. E.;Lindau, 1.; Sheath, P.;Su, C. Y. J . Vac. Sci. Technol. 1980. 17, 1019. (c) McCaldin, J. 0.; McGill, T. C.: Mead, C. A. J . Vac. Technol. 1976, 13,802. (33) Lewis, N. S . J . Elecrrochem. Soc. 1984, 131, 2496.
1380
J. Phys. Chem. 1991, 95, 1380-1383
in A V,/AE for n-AI,Ga,-,As/Mo Schottky barriers, nAI,Ga,-,As/fiquid junctions, and n-GaAs,P,-,/liquid junctions would presumably require information on the distribution and cross section of these interface recombination sites, which is not readily obtainable from the charge-transport data available at the present time. In general, the trends in the AI,Ga,,As series are that increased V, values and better junction rectification properties are obtained as the bandgap energy of the alloy is increased. This stands in contrast to the situation observed for the n-CdS>e,-, series, where V, remains relatively constant throughout the series, and the solar energy conversion efficiency therefore must decline rapidly as the value of the bandgap energy is increased. Although the increases in V, for the n-A1,Ga,-,As/CH3CN-LiCI04-Fc+/o and nAI,GaI-,As/KOH-Se-/2-(aq) junctions are not sufficiently large to obtain improved cell performance at higher Eg values, the ability
to vary V, using alloy electrodes indicates that such an approach might afford improved energy conversion performance in certain photoelectrochemical cell systems. It is clear that the factors that govern changes in V , with changes in E are numerous, and further study is required to ascertain genera! trends regarding the factors controlling V, in an arbitrary semiconductor alloy/liquid junction system. Acknowledgment. We thank the Department of Energy, Office of Basic Energy Sciences, for support of this work. We are indebted to Dr. C . R. Lewis of Varian Associates for generously providing the n-AI,Gal-,As alloy samples utilized in this study. We also thank Dr. B. Anspaugh of the Jet Propulsion Laboratory for performing some of the spectral irradiance measurements. N.S.L. acknowledges support as a Dreyfus Teacher-Scholar (1 985-1 990).
Thermal and Hydrothermal Stability of Molecular Sieve VPI-5 by in Situ X-ray Powder Diffraction Michael J. Annen, David Young, Mark E. Davis,*'+ Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
0. Burl Cavin, and Camden R. Hubbard High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received: June I I , 1990)
The framework stability of the molecular sieve VPI-5 is studied at various conditions of temperature, pressure, and humidity. Dehydration of VPI-5, by either rapid heating or pressure reduction, leads to a contraction in "a" and an expansion in "c" (hexagonal cell). These changes are reversible upon rehydration. The transformation of VPI-5 into AIP04-8 depends on the rate at which water is removed from the pores. Slow heating results in the irreversible formation of AIP04-8 from VPI-5 at about 100 "C. However, VPI-5 rapidly heated to at least 550 OC can withstand exposure to steam at these elevated temperatures with little loss of structural integrity
Introduction The VPI-5 family of molecular sieves has recently been reported in the literature'**and is the first to contain extralarge poresa3q4 Each pore consists of a unidimensional channel circumscribed by rings consisting of 18 tetrahedral atoms and possesses a free diameter of approximately 12 A between oxygen atoms. The thermal and hydrothermal stability of a molecular sieve with such a large pore is of interest. This paper is the first to address this issue by studying the thermal and hydrothermal stability of VPI-5 by in situ X-ray powder diffraction. Similarities exist in the X-ray powder diffraction patterns of VPI-5 and HI .5 H 1 is an aluminophosphate (A1P04) hydrate which transforms into AIP0,-tridymite upon heating to 110 "C5 Like HI, VPI-5 has been reported to be an AIPO, hydrate2" We will show that VPI-5 does not transform into AIPO4-tridymite. It is clear, however, that water does interact with the VPI-5 framework. One-third of the aluminum atoms contact two water molecules to adopt octahedral coordination.' Water is also physisorbed within the void spaces of VPI-5. Octahedral aluminum in VPI-5 is inconsistent with the X-ray structure refinement by Rudolf and Crowde$ since they claim to locate all water away from the aluminum. Octahedrally coordinated aluminum is not unique to VPI-5 since A1P04-5 has recently been shown to *To whom correspondence should be addressed. 'Current address: Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91 125.
chemisorb (formation of octahedral aluminum) and physisorb waters9 Our purpose is to begin to define the thermal and hydrothermal stability of VPI-5. In situ X-ray powder diffraction was used to monitor the VPI-5 framework from ambient temperature to 800 O C in moist and dry environments. These studies are the first to show the stability of a molecular sieve with a pore size larger than I O A. Experimental Section VP1-5 was synthesized by using procedures outlined previously.l0 However, the composition of the reaction mixture was modified ( I ) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988. 331, 698. (2) Davis, M. E.;Montes, C.; Hathaway, P. E.; Arhancet, J. P.; Hasha, D. L.; Garces, J. M. J. Am. Chem. SOC.1989, 1 1 1 , 3919. (3) Crowder, C. E.; Garces, J. M.; Davis, M. E. Ado. X-ray Anal. 1989, 32, 503. (4) Richardson, Jr., J. W.; Smith, J. V.;Pluth, J. J. J. Phys. Chem. 1989, 93, 8212. ( 5 ) d'Yvoire, F. Bull. SOC.Chim. 1961, 1762. ( 6 ) Grobet, P. J.; Martens, J. A.; Balakrishnan, 1.; Mertens. M.; Jacobs, P. A. Appl. Catal. 1989, 56, L21. (7) Wu, Y.;Chnielka, B. F.; Pines, A,; Davis, M. E.; Grobet, P. J.; Jacobs, P. A. Nature 1990, 346, 550. (8) Rudolf, P. R.; Crowder, C. E. Zeolites 1990, 10, 163. (9) Meinhold, R. H.;Tapp, N. J . J. Chem. SOC.,Chem. Commun. 1990, 219. (10) Davis, M. E.; Montes, C.; Hathaway, P. E.; Garces, J. M. Srud. Surf. Sci. Caral. 1989, 49A, 199.
0022-3654191 12095-1 380%02.50/0 -~ , 0 1991 American Chemical Society ,
I
