J . Phys. Chem. 1984, 88, 1310-1317
1310
et al.’ is also shown. It seems that a linear relationship holds for the plot of In kd vs. AE(Sz-Sl). The slope of the plot is three times larger than that for azulene and its derivative^.'^ The value for C estimated from this result by assuming huM= 3000 cm-I for the C-H vibration was too large for the electronic matrix element. This may be due to the small Sz-S1 energy gap in the porphyrins investigated here. Anyway, the rate of S2-S1internal conversion of metal-TPP complexes depends mainly on the Sz-S, energy gap as expected from the energy gap law. Further studies on this subject are necessary to clarify the relation between the radiationless transition rate and the energy gap.
2 28
C
6.5
7.0
7.5
A E I 10~cm-l Figure 3. The plot of In kd vs. AE(S,-SI).
plexes than in TPP complexes. Then, the Soret fluorescence of metal-OEP complexes cannot be observed in contrast to the case of metal-TPP complexes. The results in Table I can be the test for eq 5 . Figure 3 shows the plot of In kd vs. AE(S2-SI). In Figure 3 the result of Bajema
Acknowledgment. We thank Mr. M. Arakawa for the synthesis of CdTPP. Registry No. AICITPP, 7 1102-37-9; ZnTPP, 14074-80-7; CdTPP, 14977-07-2.
(14) Murata, S.; Iwanaga, C.; Toda, T.; Kokubun, H. Ber. Bunsenges. Phys. Chem. 1972, 76, 1176.
Systematic Studies of the Semiconductor/Liquid Junction: n-Gallium Arsenide Phosphide Anodes in Aqueous Se2-/Se;Solutions Chris M. Gronet and Nathan S. Lewis* Department of Chemistry, Stanford University, Stanford, California 94305 (Received: June 9, 1983)
Epitaxial layers of n-GaAsl,P, (0 < x < 1) have been studied as photoelectrodes and as Au Schottky junctions. We observe increases in open-circuit voltage, V,, with increases in P content for n-GaAsl-,P, (x I 0.4) and decreases in V,, for x 2 0.6. Under 88 mW/cm2 of ELH-type (3350 K color temperature with a dichroic rear reflector) tungsten-halogen irradiation, we observe that ~ - G ~ A S , , anodes , ~ P ~exhibit ~ ~ a V, of 0.95-0.99 V, short-circuit currents of 15-17 mA/cm2, and energy conversion efficiencies of 13.0 f 1.O%. Irradiation at 632.8-nm yields monochromatic conversion efficiencies of greater than 30%, and solar irradiation (85-100 mW/cmz) yields efficiencies of 11.0 f 1.0% in 1.0 M KOH/l.O M Sez-/O.Ol M Se2solutions. The n-GaAso72P0.28 anodes exhibit stable photocurrent for passage of over 3000 C/cmZ at AM1 photocurrent densities. Ru(HZ0)2+ions are effective in improving photocurrent-voltage characteristics for n-GaAsl,P, (0 Ix < 1) anodes but have no effect for n-Gap, indicating chemical interactions of the Ru ion with As or As oxide sites at the semiconductor/liquidjunction. Direct comparison of V, for n-GaAs,,P,/Au junctions with n-GaAsl-xPx/Se2-barriers indicates that liquid junctions have higher V, values than some Schottky barriers and that pinning of the Fermi level by intrinsic surface states may not play a dominant role in determining interface parameters for these junctions.
There have been very few controlled studies of the properties of a semiconductor/liquid junction with continuous, incremental changes in semiconductor composition. Therefore, models that would enable prediction of junction parameters are difficult to formulate from the data available. This is due, in part, to the substantial differences in experimental conditions, as well as to variations in electronic properties and surface chemistry of the materials that have been compared. Of particular interest is the response of junction properties to substitution of isovalent ions in the semiconductor lattice. Related studies of variation in interface properties with changes in lattice composition have been reported for semiconductor/metal contacts.’-3 Although the properties of liquid junction systems are often compared to those of Schottky contacts, rarely are the two types of experiments performed under controlled conditions on identical samples. We report studies which indicate that deliberate variation in bulk (1) Sze, S. M. “Physics of Semiconductor Devices”; Wiley: New York, 1981. (2) McCaldin, J. 0;McGill, T.C.; Mead, C. A. J . Vac.Sci. Technol. 1976, 13, 802. (3) Rideout, V. L. Solid-Stare Electron. 1974, 17, 1107.
0022-3654/84/2088-1310$01.50/0
elemental composition of the electrode can be correlated with the properties of the semiconductor/liquid interface. In addition, we find substantial differences in interfacial behavior of metal and liquid junctions. A general rule advanced for 111-V photoanodes4J suggests that the electronegativity of the lattice anion controls the position of the semiconductor valence band This notion has been adopted from a similar proposal in the solid-state Schottky barrier literature which states that the barrier height for holes in Au Schottky barriers will be determined by the nature of the semiconductor lattice anion.2 The reported values of the open-circuit voltage, V,, for n-GaAs9 and n-Gap5 photoanodes in aqueous (4) Ellis, A. B.; Bolts, J. M.; Wrighton, M. S. J . Electrochem. Soc. 1977, 124, 1603. ( 5 ) Ellis, A. B.; Bolts, J. M.; Kaiser, S. W.; Wrighton, M. S. J . Am. Chem. SOC.1977, 99, 2848. (6) Gerischer, H. J . Electroanal. Chem. Inferfacial Electrochem. 1975, 58, ‘263. (7) Wrighton, M. S. Acc. Chem. Res. 1979, 12, 303. (8) Fonash, S. J. “Solar Cell Device Physics”; Academic Press: New York, 1981. (9) Parkinson, B. A,; Heller, A,; Miller, B. J. Electrochem. SOC.1979, 126, 954.
0 1984 American Chemical Society
n-GaAsl-,P, Anodes in Se2-/SeZ2-Solution KOH/SeZ-solutiong are very similar; thus, incrementally varying the semiconductor comlhsition by increasing the electronegativity of the lattice aniorlis expected to produce little change in V,. Our studies represent an.effort in liquid junction systems to verify these predictions for a family of similar single-crystal electrode materials. The materials chosen for study are solid solutions of GaAs and Gap, GaAsl-,P,. Vapor-phase epitaxial growth techniques can conveniently yield any desired composition of this alloy series. The electronic and materials properties of these solid solutions (band gap vs. elemental composition, luminescence properties, mobilities, and dopants, etc.) are reported in detail in the solid-state literature.1°‘12 A study of the n-GaAsl,P, system is also of significance because one of the parent compounds, n-GaAs, is the material used in one of the highest efficiency semiconductor/liquid junction solar cells reported to date.l3,l4 A contributing factor to the high efficiencies of the n-GaAs/KOH/Se2-/Se2- system is the ability to passivate surface recombination sites by chemisorption of metal ions from solution.13 In contrast, preliminary investigations of the related 111-V material, n-Gap, suggest that low quantum efficiencies (0.2-0.3) and modest energy conversion parameters are obtained in KbH/Se2-/Se22- solutions, even with monochromatic irradiation of energy well in excess of semiconductoi band gap.5 Thus,-of particular interest in our studies are the variations in output properties and changes in the chemistry of surface recombination sites for the series of n-GaAs,-,P,/ KOH/SeZ- liquid junctions. An understanding of the fundamental mechanisms for the high efficiency of the n-GaAs system and low efficiency of the n-GaP system should allow design of more ideal semiconductor/liquid interfaces with other anode materials. We observe substantial, reproducible changes in V, for Schottky barriers and liquid junctions with the n-GaAsl,P, series. In the region of composition where the materials have a direct band gap, we find remarkable increases in V , with increases in x . These observations modify the approach to efficient semiconductor/liquid junction interfaces. For solar irradiation, an increase in x is expected to result in decreased short-circuit current, ,Z because of the diminished number of photons with energy above E,. If the values of V, do not change, as might be predicted by the Z with increasing x could not produce “anion rule”, a decrease in , cells that are comparable in solar efficiency to that of GaAs. If the value of V , increases with phosphorus content, improved efficiencies are possible despite concurrent increases in E,. The implications of our observations are illustrated in the nGaAsl_,P,/KOH/SeZ-/Se22- liquid junction cells. Although efficient photoelectrodes are rare, with only five anodes to date exceeding 10% solar-to-electrical conversion efficiency,13-19the n-GaAsl-~,/KOH/Se2-/Se2”series provides a family of efficient and stable semiconductor/liquid junction cells. For example, n-Ga Aso72Po,28 anodes are found to yield solar-to-electrical energy conversion efficiencies of over lo%, and efficiencies for conversion of monochromatic visible light, 632.8 nm, of over 30%. Both the solar conversion efficiency and monochromatic visible light efficiency compare favorably with the best reported values for any laboratory photochemical system to date. The results of our studies on the properties of the GaAsl-,P, system, and the implications for control of fundamental parameters of semiconductor/liquid interfaces, are described below.
(10) Lorenzy, M. A,; Blakeslee, A. E. “Gallium Arsenide and Related Compounds”; Institute of Physics: London, 1973;Conf. Ser. No.17,p 106. (1 1) Onton, A. Adu. Solid State Phys. 1973, 13, 59. (12) Williams, C. K.; Glisson, T. H.; Hauser, J. R.; Littlejohn, M. A. J . Electron. Mater. 1978, 7 , 639. (13) Heller, A,; Parkinson, B. A,; Miller, B. Appl. Phys. Lett. 1978, 33, 512. (14)Noufi, R.; Tench, D. J . Electrochem. SOC.1980, 127, 188. (15) Heller, A,; Parkinson, B. A.; Miller, B. Con$ Rec. IEEE Photovoltaic Spec. Conf. 1978, 12, 1253. ( 16) Kline, G.; Kam, K.; Canfield, D.; Parkinson, B. A. Sol. Energy Muter. 1981, 4, 301. (17) Gronet, C. M.; Lewis, N. S. Nature (London) 1983, 300, 733. (18) Gronet, C. M.; Lewis, N. S. Cogan, G.; Gibbons, J. Proc. Nutl. Acud. Sci.’ U:S.A.1983, 80, 1152. (19) Gronet, C. M.; Lewis, N. S. Appl. Phys. Lett. 1983, 43, 1 1 5.
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1311 Experimental Section Photoelectrode substrates were polished wafers of n+-GaAs (Nd = 5 X lOI7carriers/cm3) or n+-GaP oriented to expose the (100) face. Ohmic contacts were made by evaporating In onto samples at 5 X 10“ torr and annealing them in 95% N, and 3.2% H2 for 15 min at 450 “C. The electrode was attached to a coil of Cu wire with Ag paint, and the Cu wire was passed through a glass tube to provide insulation from the solution. The exposed Cu and Ag were then sealed with epoxy cement to expose only the electrode face to the solution. Sample areas ranged from 0.05 to 0.2 cm2, as measured photomicrographically within A5%. A Pyrex cell with an optically flat bottom was used. Light was directed vertically through the bottom of the cell with a large-area mirror. The distance between the electrode surface and optical flat was no greater than 1.0 mm. All solutions were stirred magnetically. The 1.O M KOH/ 1.O M Se2- solution was prepared by addition of water to Al2Se3(Alfa Ventron) to generate H2Se in situ. The H,Se was bubbled through 3.0 M KOH until the appropriate stoichiometry had been reached. Formation of K2Se2was regulated by controlled addition of oxygen to the solution. The solutions were stable for extended periods under a N2 atmosphere. RuCl, (Alfa Ventron) was used as received. Etching solutions were mixed from reagent grade acids and H202,cooled to 25 OC, and used within 3 h. All n-GaAsl-,P, surfaces were etched in 1:l H F / H 2 0 2 for 10 s and rinsed with H 2 0 before use. Potential control was accomplished with a PAR Model 173 potentiostat equipped with a Model 175 voltage programmer. Traces were recorded on a Houston Instruments Model 2000 X-Y recorder, and current-time measurements were recorded with a Linear, Inc., Model 555 strip chart recorder. Schottky barrier impedance measurements were recorded on a computer-controlled system at 1 MHz at the Stanford Center for Materials Research. Light sources were either a 5.0-mW polarized He/Ne laser (632.8 nm) from Aerotech, Inc., an ELH-type tungsten-halogen bulb (3350 K color temperature, dichroic rear reflector) with a ground-glass diffuser, direct winter sunlight between 11 a.m. and 1 p.m. at Palo Alto, CA, or an ENH-type bulb (3250 K color temperature, dichroic rear reflector) with a ground-glass diffuser (for stability runs). Laser intensities were varied by rotation of a Hoya polarizer in the path of the polarized laser beam. The laser beam was expanded with a 5X beam diffuser and collimator from Aerotech, Inc. Laser intensities were measured with a Newport Research Corp. Model 8 15 photometer which was periodically verified for accuracy within 5% relative to a Scientech, Inc., calorimeter. Solar intensities were measured with a Solarex Si secondary standard 4-cmZ solar cell. Direct insolation was generally between 85 and 95 mW/cm2, and the photoelectrochemical cell had a Pyrex optical flat normal to the sun and in the same plane as the reference cell. For laboratory solar simulation measurements, the ELH bulb was always maintained at line voltage and the distance of the source was adjusted to achieve the desired light intensity (as measured by the short-circuit current of the Solarex cell) in the plane of the photoelectrochemical cell. Short-circuit photocurrents determined by this technique were in acceptable agreement with measurements in actual direct sunlight for samples with E, less than 1.8 eV (see Discussion). Spectral response curves were obtained with a computer-controlled system at SERA Solar Corp., Santa Clara, CA. The incident light beam was chopped at 13 Hz and provided an average intensity of 1 mW/cm2 over a range of 400-1100 nm. The reference cell was a Si detector from United Detector Technologies, Culver City, CA. Data acquisition and plotting were performed with a Hewlett-Packard Model 9826 computer, yielding quantum efficiencies accurate to &lo%. Schottky barrier formation was accomplished by evaporation of 2.0-mm-diameter Au dots in an ambient vacuum of less than 1.0 X torr. The films used transmitted approximately 40%-60% of the incident light for the 1100-400-nm wavelength region. Metal thickness was monitored by use of an oscillator-type acoustic impedance thickness monitor from R. D. Mathis, Inc. (Model TM-100). Satisfactory electrical contact to the Au films
1312 The Journal of Physical Chemistry, Vol. 88, No. 7 , 198 4
Gronet and Lewis
ax
::
"7
d
x'
0.3
"
i
u
1.40.30
0.50
1.00
x 5aAS1.x~x
Figure 1. Representation of structure of n-GaAs,,P,
photoanodes. Graded P region avoids lattice mismatch at the epilayer/substrate interface. Graph on right indicates band gap as a function of elemental composition of the epilayer. Dark circles represent direct gap-type absorptions, while empty circles indicate indirect electronic transitions. TABLE I
Eg , eV
E g eV
Figure 2. Plots of photocurrent-voltage properties of Au Schottky barriers with n-GaAsl,P,. Irradiation is 100 mW/cmz from an EL€€-type
tungsten-halogen source. Chart I 11
Nd 3
sam-
ple
microprobe compn
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A
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GaAs,~,,P,~,,
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n+-GaAs 1.61 undoped, OMVPE n+-GaAs 1.77 PL = 703 nm: undoped n+-GaAs 1.77 Te doped n+-GaAs 1.88 Te doped n+-GaP 2.08 indirect gap n+-GaP 2.13 indirect gap n+-Gap 2.22 indirect gap 2.25 indirect gap
was obtained with standard micromanipulators. Materials. The general structure of the materials employed here consists of a thin epilayer (usually 5-20 pm) of the desired composition of lightly doped n-GaAsl-,P,. This is followed by a thicker region of graded phosphorus composition, and finally a heavily doped, n+-GaAs or n+-GaP substrate. The situation is depicted schematically in Figure 1. The graded region prevents lattice mismatch and barrier formation at the substrate/epilayer interface. The epilayers have been grown by chloride vapor-phase epitaxy techniques. In addition, we have studied the properties of some samples grown by organometallic vapor-phase epitaxy. The relevant materials' characterization data for all samples are summarized in Table I. The elemental composition of the epilayers has been verified by both photoluminescence datal' and electron microprobe analysis. The dopant densities were determined by differential capacitance measurements on Au Schottky barriers at 1-MHz frequency and are generally between 1 X 1015 and 1 X 10'' carriers/cm3. The electronic properties of the n-GaAsl,P, series are well documented.10-'2 Increases in P content lead to increases in band gap, E,, with a minimum for n-GaAs (direct) of 1.44 eV and a maximum for n-GaP (indirect) of 2.25 eV. A direct-to-indirect transition in the band gap occurs at an elemental composition of x = 0.44. The E , for the series is found to obey Vegard's law, with the bowing parameters of 1.091 and 0.210 for this alloy series.I2 The composition and E, of our representative set of solid solutions are included in Table I. Results Schottky Barriers with n-GaAs,,P,. We have studied the properties of n-GaAs,,P,/Au Schottky barriers using semiconductor epilayers that are identical with those used in the liquid junction photoelectrochemical experiments. The Schottky barrier data serve to verify the quality of the epilayer/substrate assembly in the n-GaAsl,P, series, as well as to provide a basis for direct comparisons of metal interface parameters with the properties of liquid junction systems. We have concentrated upon measurement of V,, and , Z under 100 mW/cm2 of tungsten-halogen irradiation, and of the short-circuit spectral response vs. incident photon energy
(A'IA)
t/ .1
for the n-GaAs,_,P, Schottky series. 1 . Open-circuit Voltage and Barrier Height Measurements. Previous reports of the barrier height for Au Schottky barriers in the n-GaAsl-,P, system, as measured by differential capacitance and internal photoemission techniques, indicate linear increases in &, the barrier height, with increases in E, for the direct gap materials20(see Chart I). In addition, the +b for n-GaP is reported to be consistent with this trend, having values of between 1.3 and 1.5 V.' We find that increases in V, parallel the increases in &, for materials with compositions of x I 0.30 (Figure 2 ) . Remarkably, we observe that a 0.3-V increase in E, (from x = 0.0 to x = 0.28) leads to a 0.3-V increase in V,. At higher levels of P, we observe significant declines in V,. V, is the physically relevant quantity in a Schottky barrier solar cell, and the increase in V, with increasing x (x I0.3) suggests that control of E, is a simple, direct method to manipulate Voc. Furthermore, the correlation of both V , and &, with changes in E , indicates substantial variation in the position of ECBrelative to the work function of Au. The relationship between V, in Schottky systems and the properties of n-GaAsl,P,/liquid junction interfaces are discussed in detail below. 2. Short-circuit Current Measurements. We have also recorded the short-circuit photocurrent response of these Schottky diodes vs. the energy of incident light. A typical example is depicted in Figure 3. The n-GaAs,,P,/Au diodes generally exhibit onsets of photocurrent in accord with the epilayer band gap energy, and the spectral response profile is in agreement with a direct band gap transition for epilayer compositions with x I 0.44and an indirect transition for compositions with x > 0.44. The values of the short-circuit current, Zsc, for the nGaAsl-,P,/Au Schottky barrier series under 100 mW/cm2 of ELH-type solar simulated irradiation are summarized in the right-hand side of Figure 2. Consistent with the measured spectral response of the n-GaAs,-,P, series and the intensity profile of our tungsten-halogen lamp:' increases in Eg lead to decreased values (20) Stern, R. J. Appl. Phys. Comrnun. 1981, 1, 43. (21) Seaman, C.H.; Anspaugh, B. E.; Downing, R. G.;Esrey, R. S. Con5 Rec. IEEE Photovoltaic Spec. Conf. 1980, 14, 494.
n-GaAs,,P,
" 0.80 t
0
1
6
8
°
.... GaP/Au
0
-
The Journal of Physical Chemistry, Vol. 88, No. 7 , 1984 1313
Anodes in Sez-/Se22- Solution
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'
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4
,.
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600
700
800
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.
Figure 4. Plots of photocurrent-voltage properties of liquid junctions with n-GaAs,-,P,. The solution is 1.0 M KOH/1.0 M K2Se/0.01M K2Sez, and irradiation source is 88 mW/cm2 from an ELH-type tungstenhalogen lamp. n-GaAs is etched to a shiny finish with 1:l H,S0,/H20z, n-GaP is treated with 12 M HC1, and n-GaAs,,P, anodes (0 < x < 1) are etched with 1:1 HF/H2OZimmediately before use.
Figure 3. Spectral response data for n-GaAs,,$0.28 and n-GaP Schottky barriers with Au overalyers. The cell is maintained at short circuit during
to be significantly higher for all anodes investigated. Clearly, for the solid solutions with x < 0.4, increases in V , for both Au the measurement. Onset of photocurrent for n-GaAso,72Po.z8 is in excontacts and Se2- liquid junctions can be correlated directly with cellent agreement with the epilayer E,, indicating no appreciable phoincreases in the bulk E,. toactivity due to the substrate. n-GaP shows a less steep rise in current, A small contribution to the decline in V , for large values of as expected for an indirect electronic transition. x is due to the wavelength dispersion of the W-halogen lamp and of Zsc, However, the spectral response curves, as in Figure 3, the light absorption by the colored (e = 1000, X = 450 nmZ4 indicate that the quantum efficiency of the n-GaAsl-,P,/Au polyselenide solutions. However, we have investigated the desystems is fairly constant for incident light of energy above E,. pendence of V, on the photocurrent density for n-GaP in detailz5 The high values of these quantum yields, after consideration of and find that, even at photocurrent densities of over 20 mA/cm2, the reflectivity losses in the metal overlayer, indicate that efficient the V, for n-GaP is only 0.88 V in 1 M KOH/Sez- solutions. The charge separation and carrier collection can be obtained with our decline in V, for materials of E, greater than 2.0 eV as depicted materials and attest to the high quality of the family of epilayers in Figure 4 is thus an accurate representation of the trend under study. We thus expect that the quantum efficiencies in throughout the n-GaAsl_,P,/KOH/SeZ-/Sezz- series. liquid junction systems should also be excellent, and poor quantum 2. Short-circuit Current Data. Consistent with results for the efficiencies in liquid systems would thus reflect surface defects n-GaAs,_,P,/Au Schottky barriers, increasing x (E,) results in and not bulk recombination sites. the expected decrease in .,Z The data for all samples are reported 3. Liquid Junction Data in 1 .O M K O H / l .O M K2Se. The in the right-hand side of Figure 4 and Table 11. Photoaction primary focus of our work was to investigate the photoelectrospectra indicate that the quantum efficiency for electron flow is chemical parameters of the n-GaAs,-,P, anode series in aqueous very high for all samples with x I0.4, and the value of I,, is thus Se2- solutions. Both n-GaAs and n-GaP have been previously a simple convolution of the spectral distribution of the polyshown to be stable at AM1 solar current densities in solutions chromatic source with the absorption spectrum of the epilayer material. Notably, we do not observe the drastic decrease in containing greater than 0.5 M Se2-.5J33z2The n-GaAsl-xP, system is thus expected to be compatible with the KOH/Se" electrolyte short-circuit quantum efficiency with increasing x which might and will allow reproducible measurements on semiconductor/liquid be expected on the basis of comparisons of earlier studies of interfaces where photocorrosion and photopassivation processes n-GaAs and n-GaP in KOH/SeZ-.5~z6 are minimized. The n-GaAs/Sez- system also offers the opporThe dependence of quantum efficiency on photon wavelength tunity for study of an efficient semiconductor/liquid junction is explicitly displayed in photoaction spectra of the nsystem, as this cell has been reported to yield 12% solar-to-elecGaAs,-,P,/liquid junctions under short-circuit conditions. The trical energy conversion efficiencies under AM 1 condition^.'^ In spectral response of photocurrent vs. incident photon energy of addition, n-GaAso6P0,4has recently been studied in aqueous 7 M two members of the series, and n-GaAs, are KOH/Tez- solutions and is reported to yield 16% conversion represented in Figure 5. The spectral response for n-GaAs efficiency for monochromatic 458-nm light into e l e c t r i ~ i t y . ~ ~ (Si-doped, Nd = 8 X lOI5 carriers/cm3, (100) oriented material) Curiously, despite its larger E,, n-GaP is reported to yield lower is in excellent agreement with previously published data for this operating voltages at maximum power than n-GaAsSSWe hoped system.26 The response profile for the ~ - G ~ A S ~28 ,system , ~ P ~is that an investigation of the entire series of compositions would very similar, except that the onset in photocurrent is shifted to yield some insight into the chemical reasons for this behavior, as higher photon energies due to the higher E, of the epilayer mawell as elucidate the reasons for the high efficiency of the nterial. The absence of current below 1.7 eV verifies that the GaAs/Se2- system. We have thus investigated the trends in V,, n+-GaAs substrate does not contribute to photocurrent production. Zsc, and spectral response characteristics for the series of nThe decline in quantum yield at short wavelengths for both n-GaAs GaAs1-,P,/KOH/Se2-/Se2z- interfaces. and n-GaAso,72Po,2x is due to light absorption by the 0.1 M KzSe2 1. Open-circuit Voltage Measurements. We find very conin the 0.5-mm path length of the electrochemical cell. The insistent photoelectrode behavior throughout the series of compotegrated quantum efficiency based upon photons above E, is sitions of n-GaAsl,P, anodes. The values of V , as a function similar for n-GaAso,7zPo,2x and n-GaAs, with n-GaAs having a of x (E,), obtained under 88 mW/cm2 of tungsten-halogen irslightly superior response in 0.1 M K2Se2 due to the smaller radiation from our solar simulator, are indicated in the left-hand fraction of light with energy greater than E , which is absorbed side of Figure 4. As was found for n-GaAsl-,P, Schottky barriers, by the solution. we observe that for x I0.4, manipulation of V , is possible simply 3. Efficiency Measurementsfor the n-GaAsl-,P,/KOH/Se2by variation of the semiconductor composition. The general trend System. The energy conversion properties for light into electricity in behavior resembles the n-GaAs,,P,/Au Schottky barrier data quite closely, except that the V , for liquid junction systems seems (22) Chang, K. C.; Heller, A.; Schwartz, B.; Menenzes, S.; Miller, B. Science (Washington, D.C.) 1977, 196, 1097. (23) Hobson, W. S.; Ellis, A. B. Appl. Phys. Lett. 1982, 41, 891.
(24) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J . Am. Chem. SOC.1917, 99, 2839. ( 2 5 ) Romer, B.; Gronet, C. M.; Lewis, N. S., manuscript in preparation. (26) Heller, A,; Chang, K . C.; Miller, B. J . Am. Chem. SOC.1978, 100, 684.
1314
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Gronet and Lewis
TABLE 11: Efficiency Parameters for n-GaAs,_,P, . .. .. Anodes in 1.0 M KOH/1.0 M Se2-/0.01 M Se,'- Solutions sample light source intens, mW/cm2 vow v Isc, mA/cm2 0.68-0.72 21-23 ELH 88 21-23 90 0.65-0.70 Sun ELH 88 0.82-0.87 19-21 ELH 0.95-0.99 15-17 88 0.90-0.95 13-14 Sun 90 ELH 0.97-1.03 9.5-10.0 88 ELH 0.95-1.00 3.7-4.3 88 ELH 0.90-0.93 2.2-2.7 88 ELH 0.84-0.8 8 1.8-2.0 88 ELH 88 0.78-0.81 0.30-0.36 1.0
I
1
'
1
eff, o/c 12.5 i. 1.0 12.5 i 1.0 12.7 i 1.0 13.0+ 1.0 11.0 i. 1.0 8.5 i 1.0 3.0 f 0.5 1.5 i 0.5 HOTON ENERGY(eV)
Spectral response characteristics for n-GaAs and nGaAso72P028 in 1.O M KOH/0.8 M K2Se/0.1M K2Se2.Average light intensity is 1 mW/cm2,and the photoanode is maintained at short circuit vs. a Pt-foil counterelectrode. Optical path length through solution is approximately 1.0 mm. Spectra are normalized to unity for both samples. Extensive light absorption for hv G O O nm by the solution under these conditions will affect solar response of ~ - G ~ A S ~ ,more , ~ P severely ~,~, than n-GaAs. Maximum internal quantum efficiencies for both systems are very similar, with shiny (1:l H202/H2S04 etched) n-GaAs and shiny (1 :1 HF/H202)n-GaAso72P0,28 anodes exhibiting maximum absolute external quantum efficiencies of 0.8. Figure 5.
for the n-GaAs1,P, system in KOH/Se" solutions are reported in Table 11. We have performed experiments both in actual sunlight and under 88 mW/cm2 of irradiation from our ELH-type simulator. The wavelength dispersion and use of ELH lamps as solar simulators have been discussed p r e v i o ~ s l y . ' ~ -The ~ ~ -sim~~ ulated spectrum possesses somewhat greater intensity than sunlight above 650 nm and is correspondingly weaker below 650 ~1111.~~ As reported in Table I1 and as determined previously, the agreement in efficiency values between the ELH source and actual sunlight is within f l 5 % . We do expect substantially higher discrepancies between the ELH source and actual sunlight for materials with E, > 1.8 eV or in cell configurations with materials of E, < 1.8 eV if significant light is absorbed by the polyselenide species. In these situations, the spectral mismatch of the ELH simulator with actual sunlight can lead to serious errors in efficiency measurements. As indicated in Table 11, relevant ELH measurements have been verified for consistency by measurements on identical samples under both actual sunlight and ELH irradiation. In most (27) Godfrey, R. B.; Green, M. A. Appl. Phys. Lett. 1979, 34, 790.
cases, we prefer the simulator source for routine determinations due to the convenience and reproducibility of this method. We find extremely high energy conversion efficiencies for photoanodes with x i 0.3. A typical current-voltage curve is represented in Figure 6 for an epitaxially grown layer of nG ~ A s ~ ,(not ~ ~deliberately P ~ , ~ ~ doped) in a solution of 1.0 M KOH/1.0 M Se2-/0.01 M Se22-. These solutions contain less oxidized species than some studies of n-GaAs in Se2-/Se$- l 3 to avoid excessive light absorption in our system. The epilayers are also treated with RuCl, in order to passivate recombination sites, as for n-GaAs.13 Under these conditions (88 mW/cm2 of ELH irradiation), we observe a V, of 0.95-0.99 V, an I,, of 15.0-17.0 mA/cm2, and energy conversion efficiencies at the maximum power point (0.76480 V) of over 12%. We have performed these experiments for a number of samples in several different electrolyte solutions and routinely find values of 13.0 f 1.O for n - G a A ~ ~ , ~ ~ p , , ~ , samples. Very similar efficiencies, 1 1.O f 1.O%, are obtained in 85-100 mW/cmz of actual sunlight (Table 11, Figure 6). These efficiencies are based upon power incident onto the cell as measured by a calibrated Solarex Si secondary standard cell and are not corrected for light absorption by the electrolyte or reflection losses in the electrochemical cell. Use of a two-electrode configuration with a 2.0-cm2 Pt-foil counterelectrode 2 cm from the working electrode yields conversion efficiencies within 7% of those depectied for the three-electrode measurement in Figure 6. Under identical conditions, we repeatedly observe 12.5 f 1.0% solar efficiency for the Nd = 8 X 10l5,Si-doped, matte-etched13n-GaAs photoelectrodes. The 11% solar efficiency for the nG ~ A S ~ , ~ ~ P ~ , ~ ~system / K O is H one / S ~of ~the - highest values reported for any semiconductor/liquid junction to date. In order to provide comparison to previous data, we have also studied the photocurrent-voltage characteristics of the n-
n-GaAs,,P,
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1315
Anodes in Sez-/Se$- Solution I
I
i
'
l
'
l
'
that exceeds the 30% value observed with n-GaAso,7zPo,28 at 632.8 nm.
Discussion
POTENTIAL ( V
VS.
SCE)
Figure 7. Photocurrent-voltage characteristics (50 mV/s) for an epitaxial layer of n-GaAso,,zPo,,,on n+-GaAs. The solution is 1.0 M KOH/1.0 M K2Se/0.01M KzSe2,and incident light is at 632.8 nm from a He/Ne laser. The light beam is expanded and collimated to provide uniform intensity irradiation over approximately 80% of the exposed
electrode area. Quantitative efficiency data for this system are reported in Table 111. TABLE 111: Photocurrent-Voltage Parameters for n-GaAs,.,,P,.,, in 1.0 M KOH/1.0 M Se2-/0.01 M Se,'- Solutions
input: b %c % mW/m2 @sc VOC P ' IPmax 24.0-25.0 1.33 0.78-0.82 0.83-0.85 0.77-0.79 0.85-0.87 0.77-0.79 24.5-25.5 2.56 0.78-0.82 4.96 0.80-0.83 0.86-0.88 0.76-0.78 28.0-29.0 7.28 0.81-0.83 0.88-0.90 0.77-0.79 29.0-30.0 9.68 0.81-0.83 0.88-0.90 0.77-0.79 29.2-30.3 16.24 0.81-0.83 0.89-0.91 0.79-0.81 29.5-30.5 a Input is at 632.8 nm from a beam-expanded He/Ne laser. Actual illuminated area is 0.037 cm2. Voltage at maximum power point relative to equilibrium solution potential, -0.94 V vs. SCE. Maximum energy conversion efficiency for monochromatic light to electricity. A typical set of data is represented in Figure 7. Data are uncorrected for cell reflection losses, solution absorption of light, or resistance losses. Efficiencies have an estimated relative error of 5%-7%. GaAs,,Px series with monochromatic laser irradiation. A series of current-voltage curves for n-GaAso,72Po,28 in 1.O M K2Se/0.01 M K2Se2is depicted in Figure I . The incident photon wavelength is 632.8 nm from a He/Ne laser, and the light intensity is varied nfrom 1.0 to 16 mW/cmz. With the Nd = 3.3 X sample used in this study, we observe values of V, ranging from 0.8 to 0.9 V and short-circuit quantum efficiencies in excess of 0.8 at all light intensities investigated. These parameters are consistent with the V, and spectral response data presented above for this composition of n-GaAsl-,P,. The excellent fill factors, combined with the high values of V , and I,,, lead to efficiencies for conversion of monochromatic 632.8-nm light to electricity of over 30.0%! The quantitative efficiency data, as tabulated in Table 111, indicate that these conversion efficiencies can be maintained over a wide range of light intensities and that the fill factor does not change substantially at any of the light intensities investigated. The ability to manipulate V, by deliberate variation of substrate composition is a key factor in the attainment of these high-energy conversion parameters. We are not aware of any reported monochromatic efficiency measurement for visible light conversion to fuel or electricity in wet photochemical sys-
1. Trends in V, and &, According to the "anion rule" as stated for liquid junctions, the efficiency of the n-GaAsl-,P, system under polychromatic irradiation would be expected to be lower than that of n-GaAs, due to a constant V, and large decreases in Zsc. Reference to the data in Tables I1 and I11 and Figures 2 and 4 indicates clearly that a study of the full series of n-GaAs,-,P, anodes does not substantiate this finding. For both Au Schottky barrier and SeZ-electrolyte contacts, we find large changes in V, with variation of the lattice anion from As to P. We are thus led to conclude that the value of V, for n-GaAsl,P, contacts is more sensitive to the semiconductor composition than to the electronegativity of the particular lattice anion. In the region of composition x > 0.4, the V, under ELH irradiation levels off and then starts to decline. This behavior might not have been predicted based only upon V, measurements of the individual n-GaAs and n-GaP members of the series. The properties of the n-GaP/Au Schottky barrier have been described in detail in a separate p ~ b l i c a t i o n .We ~ ~ note here that the values of V, for n-GaP/Au Schottky barriers and for n-GaAs,,P,/Au systems with x > 0.6 are significantly lower than the values of & for these interfaces.l This can be ascribed to gap states, which decrease the available photovoltage at n-GaP/Au interfaces. Changes in surface stoichiometry upon deposition of metal overlayers or formation of Ga/metal alloy junctions can lead to pinning of the surface Fermi level in Schottky systems.3340 For liquid junctions, Butler and Ginley have suggested that electronegativity calculations may predict the flat band potential of the semiconductor, provided that these measurements are referenced to the point of zero {potential (pzzp) at the semiconductor/liquid i n t e r f a ~ e . ~The ~ . ~different ~ surface chemistry of n-GaAs and n-GaP in chalcogenide solutions, where strong adsorption of redox species has often been p o ~ t u l a t e d > ~may - ~ ~tend to shift the pzzp with different surface compositions and thus dominate the observed trend of V, with E , in the n-GaAsl-,P, series. In acetonitrile solvent with outer-sphere redox reagents, the decline of V, for n-GaP relative to ~ - G ~ A S ~ , ,is, not P ~ observed," ,~~ and nonaqueous systems may provide a more generally useful model for changes in interface parameters than aqueous chalcogenide solutions. Our data certainly do not indicate that the V, will be independent of the lattice anion, and even in the aqueous solutions, we observe increases in V, with increasing P content for epilayers with x I 0.4.
(28) Wrighton, M. S. Chem. Eng. News. 1979, 57, 29. (29) Heller, A. Acc. Chem. Res. 1981, 14, 154. (30) Wrighton, M. S. Acc. Chem. Res. 1979, 12, 303. (31) Nozik, A. J. Annu. Reu. Phys. Chem. 1978, 29, 189. (32) Bard, A. J. Science (Washington, D.C.) 1980, 207, 139. (33) Spicer, W. E.; Chye, P. W.; Skeath, P. R.; Su, C. Y.; Lindau, I. J . Vac. Scr. Technol. 1979, 16, 1422. (34) Williams, R. H. J . Vac. Sci. Technol. 1981, 18, 929. (35) Spicer, W. E.; Lindau, I.; Skeath, P. R.; Su. C. Y.; Chye, P. W. Phys. Reu. Lett. 1980, 44, 420. (36) Spicer, W. E.; Lindou, I.; Skeath, P. R.; Su, C. Y. J . Vac. Sci. Technol. 1980, 17, 1019. (37) Bard, A. J.; Bocarsly, A. B.; Fan, F.-R. F.; Walton, E. G.; Wrighton, M. S.J . Am. Chem. SOC.1980, 102, 3671. (38) Bard, A. J.; Fan, F.-R. F.; Giodu, A. S.; Nagasubramanian, G.; White, H. S. Discuss. Faraday SOC.1980, 70, 19. (39) Tanaka, S.; Bruce, J. A.; Wrighton, M. S. J . Phys. Chem. 1981,85,
3778.
(40) Mead, C. W.; Spitzer, W. R. Phys. Rev. [Sect.]A 1964, 134, 713. (41) Butler, M. A,; Ginley, D. S. Chem. Phys. Lett. 1977, 47, 319. (42) Butler, M. A.; Ginley, D. S. J . Electrochem. SOC.1978, 125, 228. (43) Ellis, A. 9.;Kaiser, S.W.; Wrighton, M. S. J . Am. Chem. SOC.1976, 98, 6855. (44) Miller, 9.; Heller, A. Narure (London) 1976, 262, 680. (45) Bard, A. J.; Wrighton, M. S. J . Electrochem. SOC.1977, 124, 1706.
1316 The Journal of Physical Chemistry, Vola88, No. 7, 1984
It is tempting to conclude that the position of EcBrelative to Ercdox is varying as x increases, especially in view of the excellent correlations in all systems studied for x C 0.4. However, it is also possible that the potential drop measured by the V, and @b determinations is predominantly due to pinning of the Fermi level by defect levels or surface states in the s e m i c o n d ~ c t o r . ~If~ the -~ energy of these levels were to become more positive with increases in x, then it would be possible to observe an increase in V, and $Jb without any movement of ECB. Further experimentation, preferably with direct spectroscopic observation of these surface states, is needed in order to definitively assess the situation. The general guideline that emerges from our studies is that, for the n-GaAs,-,P, system for both Schottky barriers and Se2liquid junctions, it is possible to manipulate V, by deliberate variation of the semiconductor composition. In the region of direct electronic gap composition, x C 0.4, this trend is apparrent for all n-GaAs,-,P, barriers studied to data. We note that a similar trend of increasing V, with increasing E, in n-GaAsl,P, liquid junctions has been reported for the ferrocene (Fc)/Fc+/CH3CN cell as well.I7 In this system, the increase in V, extends through the indirect-direct transition to compositions of x = 1.0. Clearly there will be situations where the surface chemistry of the interface will control the magnitude of V,, as for the n-GaP/Se2- and n-GaP/Au systems. However, it is reasonable to expect that under optimum conditions it should be possible to observe larger values of V, with larger E, materials. The existence of this situation in CH3CN solvent likely represents the fact that n-GaP/CH,CN junctions approach ideal behavior more closely than do nGaP/KOH/SeZ- or n-GaP/Au interfaces. Preliminary experiments in our laboratory indicate that the Al,-,Ga& anode system in nonaqueous solvents behaves in a similar fashion to the nG~AS,-~P,/CH~CN series. The observation of similar trends upon variation of the cation, Al, as opposed to variation of the anion, P, would strongly suggest that the increases of V, with increases in E, observed in the n-GaAsl,P, system may be quite general. 2. Quantum Efficiency Trends. The high quantum efficiencies of our samples reflect efficient charge separation and collection mechanisms at the junctions investigated. Although the spectral response measurements described above have been obtained at relatively low light intensities, the data correlate with the quantum efficiencies of the KOH/SeZ- junctions under actual solar conditions. AM1.5 sunlight (83 mW/cmz) provides 17 X 10l6 photons/(cm2 s) above 1.44 eV and 11 X 10l6photons/(cm2 s) above 1.77 eV.46 In actual sunlight (90 mW/cm2), we observe 22 mA/cm2 for n-GaAs and 13 mA/cm2 for n-GaAso 72P028 in 1.0 M Sez-/O.Ol M SeZ” solutions. Quantum efficiencies are thus in excess of 0.7 as based upon electrons collected per photons available above the semiconductor E,. Differences between low light intensity measurements and short-circuit currents under solar fluxes can occur if substantial surface defects are present.26 The excellent agreement of the measured Zscvalues under solar conditions with the calculated values based upon the spectral response characteristics reflects the high quality of both the epilayer material and the solid/liquid junctions in the n-GaAsI,P,/SeZsystem. 3. Efficiency Considerations. Although terrestrial solar conversion applications favor use of the 1.44-eV E, n-GaAs, use of n-GaAso 72Po z8 may have some advantages. The maximum efficiencies of the two systems theoretically differ by about 10% (36%-32%),47 and we find that in KOH/Se2- solutions the two cells have very similar efficiencies. Use of high concentrations of the strongly colored Sez2- species in long optical path lengths would tend to favor n-GaAs cells because of the lower Egof GaAs. However, the relatively large operating voltage available in the n-GaAso7zPo2,system (0.8 V) would be an advantage in the unassisted photoelectrochemical production of fuels. The main p i n t here is that deliberate variation in bulk electrode composition (46) Brandhont, H. W., Jr. ERDA-NASA Technical Report 1022-77116,
“TerrestrialPhotovoltaic Measurement Procedures”; June 1977,NASA Lewis Research Center, Cleveland, OH. (47) Bolton, J. R. Science (Washington, D.C.)1978, 202, 705.
Gronet and Lewis b,
I
n-GaAs0.85P0,15
II I -1.8
-1.4
-1.0 POTENTIAL,
-1.8
-1.4
-1.0
\I vs SCE
Figure 8. Photocurrent-voltage characteristics of epitaxial layers of n-GaAsl,Px in 1.0 M KOH/0.9 M K2Se/0.05M K2Se2.Dashed curves indicate properties after etching in 1:l HF/H202, while solid curves represent properties after a subsequent 30-s dip in 0.01 M RuC13in 0.1 M HN03 and HzO rinse. Light in a-c is 88 mW/cmz of ELH irradiation; light in d is at 514 nm provided from an Ar ion laser.
can produce systematic changes in semiconductor/liquid junction behavior and can yield interfaces that produce high output voltages with little sacrifice in solar energy conversion efficiency. Although the monochromatic conversion efficiencies for all of the n-GaAsl,Px materials would be quite high at the appropriate wavelengths, the values of such monochromatic efficiencies are not a useful indicator of the solar conversion properties. For example, in the KOH/Se2- liquid junction system, a 16% monochromatic efficiency measurement a t 457 nm for nGaAso 6P0 4 (E, = 1.97 eV) would correspond to a substantially lower solar conversion efficiency than a 10% monochromatic efficiency for n-GaAs (Eg= 1.44 eV) . Under polychromatic ELH or solar irradiation, increases in x above 0.3 lead to substantial decreases in Zsc, which cannot be compensated by the increase in V,. This produces declines in solar conversion efficiency for members of the series with x 2 0.4. We note here that n-GaP resembles n-Fe203 in both the value of its band gap and the indirect nature of its transition. Although n-Fe203electrodes have been suggested recently as potential candidates for photochemical water splitting,48the greatly decreased values of I,, at these large values of Eg certainly favor the use of smaller E,, direct gap materials such as n-GaA%,7zPo,z8 for efficient electricity production. The smaller photocurrent densities under solar irradiation are expected to lead to improved stability for the n-GaAsl,P, system relative to n-GaAs. Stabilization of photoanodes is well established to be strongly dependent on the photocurrent density, with lower photocurrent densities generally providing lower rates of electrode At present, we have sustained photocurrent at 15 mA/cm2 through a 5-pm epilayer of n-GaAso72P0z8 in 1.O M KOH/0.8 M Se2-/0.1 M Sez2-solution for passage of over 3000 C/cm2 of charge. A 6-electron decomposition process of this epilayer would require less than 10 C/cmZto consume the entire material. During this period, we observed no deterioration in photocurrent-voltage properties or evidence for visible surface damage to the epilayer material. Because the rate of photocorrosion of the anode is so low, extended stability studies would be needed to establish the differences, if any, between n-GaAs and (48) Leygraf, C.; Hendewerk, M.; Somorjai, G . h o c . Natl. Acad. Sci. U.S.A. 1982, 79, 5739. (49) Gomes, W.; Van Overmeire, F.; Vanmaekelbergh, D.; Vanden Kerchove, F.; Cardon, F. ACS Symp. Ser. 1981, No. 146, 119.
J. Phys. Chem. 1984,88, 1317-1320 other members of the n-GaAsl-,P, series. We certainly expect that the long-term stability of the n-GaAsl-,P, series will be comparable to the low etch rate (- 1 pm/year) of the n-GaAs system.22 The investigation of the properties of the n-GaAsl-,P, series can also be useful in understanding the reported surface chemistry of n-GaAs. Treatment of n-GaAs with RuC13 has been reported to be effective in passivating surface states in KOH/SeZ- solut i o n ~ . ’ ~ This * ’ ~ is also reported to yield decreased recombination velocity and increased luminescence lifetimes at the n-GaAs/ vacuum interface.50 We observe substantial improvement in photocurrent-voltage properties of the n-GaAsl,P, series for x < 1.0 after exposure to RuC13. An example of the difference in photocurrent-voltage properties is depicted in Figure 8 for several compositions of n-GaAsl,P,. As for n-GaAs, the main effect of RuC13 is to improve the fill factor in KOH/Se2- solutions. The values of V, and I , are unaffected by this procedure. We observe similar effects for every member of the series n-GaAsl,P, except for n-GaP itself. We can thus associate the effect of RuC13 treatment to changes in interfacial kinetics arising from coordination with arsenic or arsenic oxide sites on the electrode surface. This is consistent with theoretical treatments of the electron distribution at a GaAs surface, which place increased electron density on the As sites.51 Studies of the bonding of RuC1, to As in the dilute matrix provided by n-GaAso,05Po,95 are being performed in order to establish a more detailed picture of the interactions in this system. Finally, we note that the values of V , for the semiconductor/liquid junctions exceed that for Au Schottky barriers for every member of the n-GaAs,,P, series. It has been suggested that the Fermi level of n-GaAs is so severely pinned by surface states ~
~~
~
Nelson, R. J.; Willarns, J. S.; Leamy, H. J.; Miller, B.; Parkinson, B. A,; Heller, A. Appl. Phys. Lett. 1980, 36, 76. (50)
(51) Goddard, W. A., III; Barton, J. J.; Redondo, A.; McGill, T. C. J . Vuc. Technol. Sci. 1978, 15, 1274.
1317
that formation of any barrier, whether a metal overlayer or a liquid junction, would not alter the amount of band bending in the resulting d e ~ i c e . ~ ~InJ *direct comparisons of Schottky barriers and liquid junctions formed with identical semiconductor material, we consistently find that for n-Si in MeOH,18 n-GaAsI9 and n-GaAsl_,P, in acetonitrile,” and now for n-GaAsl,P, in aqueous KOH/Se2-, the values of liquid junction V, exceed those demonstrated for simple Schottky systems under identical illumination conditions. In some cases, such as with n-Gap, this difference can be as large as 0.5 V.” There is increasing evidence that the interfacial chemistry of the semiconductor/contact j ~ n c t i o n ’ ” ’ ~ ~ ~ ~ ~ ~ ~ is more responsible for the values of &, and V, than is the intrinsic distribution of surface states in the semiconductor itself. Simply replacing the metal overlayer (even a so-called noble metal) in a Schottky barrier by a liquid junction may result in a sufficiently large chemical change at the interface to invalidate predictions of V, for liquid systems based upon measured values of Schottky contacts. Design of systems that take advantage of these differences between metal and liquid interfaces is the key to formulating efficient semiconductor/liquid junction devices. Acknowledgment. We gratefully acknowledge C.L.R. Lewis of Varian Associates, Palo Alto, CA, and R. Farraro and L. Stinson of Hewlett-Packard, Inc., Palo Alto, for growth and characterization of some of the n-GaAsl,Px samples used in this study. We also thank G. Cogan, J. Gibbons, L. Christel, and G. Moddel of SERA Solar Corp., Santa Clara, CA, for many helpful discussions and for the use of metal evaporation and spectral response facilities. This work was supported by the Stanford Center for Materials Research, funded by the National Science Foundation, and by the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. Ruthenium(3+), 22541-88-4; gold, 7440-57-5. (52) Fan, F-R F.; Hope, G. A.; Bard, A. J. J. Electrochem. SOC.1982, 129, 1647.
Geometry of the *CH20R Radical in X-Irradiated Crystals of Methyl 0-D-Galactopyranoside: An ESR/ENDOR Study William A. Bernhard,* Tex L. Homing, and Kermit R. Mercer Department of Radiation Biology and Biophysics, The University of Rochester School of Medicine and Dentistry, Rochester, New York 14642 (Received: June 21, 1983)
-
A CHzOR radical is trapped in single crystals of methyl P-D-galactopyranosideX-irradiated at 12 K. ENDOR measurements at -6 K were used to determine the two a-hydrogen and one y-hydrogen hyperfine coupling tensors. The a-hydrogen tensors, determined to a high degree of accuracy, were used to calculate the geometry about the free-radical center. The .CH20R radical is slightly bentwith 8, = 2.3.h l.Oo, where 6 is the angle between the nodal plane of the lone electron orbital and the three a-bonds (0-C, H-C, H’-C). The three angles H-C-H’, 0-C-H, and 0-C-H’ are 125.7 f 0.7’, 116.9 f 0.8’, and 117.2 f 0.8O, respectively. With a two-center dipole approximation the “H-C bond lengths are found to be ca. 1.00 8,that is, about 0.10 8, shorter than the H-C bonds in the nonradical. The unpaired electron densities are p c 1 0.83 and po 50.17.
Introduction A question of long-standing interest is to what degree, if any, oxygen substituents induce nonplanarity (bending) about the free-radical center of oxyalky radicals. Simple alkyl radicals such as .CH3 and .CH20H have played a pivotal role in answering this question. Measurements of isotopic 13Chyperfine splitting a(13C) in a series of small substituted alkyl radicals led Fessenden and co-~orkers’-~ to conclude that the -CH3radical oscillates about
an equilibrium conformation that is essentially planar’pz but that the C H 2 0 H and .CH20- radicals are bent slightly in their equilibrium conformation^.^ The angle of bending, 8, between each of the three a-bonds and the plane normal to the lone electron orbital (LEO) was estimated from a(13C) measurements to be -4O. They also noted that changes in hyperfine splitting (hfs) of the a hydrogen a(*H) might be the result of bending, as 0 increases a(*H) becomes less negative, and then turns positive.1,2
(1) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 43,2704 (1965).
(2) R. W. Fessenden, J. Phys. Chem., 71, 74 (1967) (3) G. P. Laroff and R. W. Fessenden, J. Chem. Phys., 57, 5614 (1972).
0022-3654/84/2088-1317$01.50/0
0 1984 American Chemical Society
