J. Phys. Chem. 1983, 87, 4919-4929
4919
Photoelectrochemlcal Hydrogen Evolution and Water-Photolyzing Semiconductor Suspensions: Properties of Platinum Group Metal Catalyst-Semiconductor Contacts in Air and in Hydrogen D. E. Aspnes
and A. Heller"
Bell Laboratories, Murray Hlll, New Jersey 07974 (Received: July 7, 1983)
We have measured the properties of electrical contacts between catalytically active metals of different work functions (Pt, Rh, and Ru) and semiconductors of interest in photoelectrochemical cells and photochemical water-splitting suspensions (n-Ti02,n-SrTi03, n-CdS, and p-InP). All air-exposed contacts form Schottky junctions, with barrier heights ranging from 0.1 eV for n-TiOz/Ru to 1.84 eV for n-CdS/Pt. Exposure to a dry hydrogen atmosphere reversibly converts the three n-TiOzcontacts to true Ohmic behavior, the three n-SrTi03 contacts to near-Ohmic behavior, and reduces the barrier heights for the three n-CdS contacts. The barrier heights for the three p-InP contacts increase to the same final value of 0.86 V upon hydrogenation, indicating that the Fermi level on the hydrogenated (100) surface is unpinned and is determined by the hydrogen work function at the interface and by the electron affinity of the semiconductor. The significant reduction in barrier heights upon hydrogenation for n-type semiconductors correlates with and explains the observations of photolytic hydrogen evolution in suspensions of catalyzed n-TiOz, n-SrTi03, and n-CdS by Gratzel, Lehn, Darwent, and their co-workers. Our results show that once hydrogen evolution is initiated at a particular microcontact, the barrier to electron transfer is lowered, leading to further increases in hydrogen evolution and further reductions of barrier height at that microcontact. This positive-feedback mechanism ensures that photoelectron transfer occurs only at a fraction of the microcontacts, thereby providing a natural explanation for the contact asymmetry necessary for hydrogen generating microcells. The model implies that photochemical hydrogen evolution in catalyzed semiconductor particle suspensions must initially be an autocatalytic process with unpredictable incubation and delay times. We find that Ohmic or nearly Ohmic behavior can be irreversibly induced for contacts on n-TiOz and n-SrTi03 also by driving the as-deposited contacts several volts into depletion. This tendency to trap and retain charge provides some insight into the controversy as to the "actual" Schottky barrier heights in these materials. Contacts to n-CdS and p-InP showed no trapping anomalies except for n-CdS/Pt, where large ideality factors, large apparent donor concentrations, differences between barrier heights determined from capacitance-voltage and from current-voltage data, and a bias-, illumination-, and hydrogen-dependent hysteresis were observed. All indicate a significant disruption of the CdS surface region due to a solid-state reaction between CdS and Pt during deposition. Our data show that Pt islands on n-CdS particles cannot act as hydrogen evolving microcathodes in water splitting suspensions. Although the apparent barrier heights for Pt on CdS suggest that photodecomposition of HzOshould be possible when the Pt islands act as oxygen-evolving microanodes, hydrogenation does not appear to provide enough asymmetry to make this process feasible. The ambient gas induced barrier height changes observed in this work are attributed to a change of the surface-dipole component of the metal work function.
Introduction The light-to-hydrogen conversion efficiency of semiconducting photoelectrodes and of semiconductor particle suspensions depends on the relative rates of the desired photoelectrochemical reaction and the undesired carrier recombination and leakage processes. Near the reversible potential the rate of hydrogen evolution at a clean or oxidized semiconductor surface is usually negligible. But hydrogen evolution can be greatly accelerated, and the Gibbs free energy efficiency of light-tohydrogen conversion correspondingly increased, by incorporating a small amount of a metal catalyst in the surface of the semiconductor. The literature on catalyzed semiconductor photoanodes and photocathodes is very extensive, but ref 1-11 represent (1)F. Mollers, H.J. Tolle, and R. Memming, J.Electrochem. Sac., 121, 1160 (1974). (2)N.Yoshihiro, S.Tonomura, and H. Tsubomura, Ber. Bunsenges. Phys. Chem., 80, 1289 (1976). (3)A. V. Bulatov and M. L. Khidekel, Izu. Akad. Nauk SSSR, Ser. Khim.,1902 (1976). (4)M. S.Wrighton, P. T. Wolczanski, and A. B.Ellis, J. Solid State Chem., 22, 7 (1977). (5)W. Kautek, J. Gobrecht, and H. Gerischer, Ber. Bunsenges. Phys. Chem., 84, 1034 (1980). 0022-3654/83/2087-49 19$01.50/0
a sampling of the hundreds of papers on the subject. Quantitative examples include the incorporation of Pt islands (at an average 7 f 3 A thickness of the metal) on the surfaces of p-InP photocathodes, which increases the quantum or current efficiency at +0.5 V vs. SHE by a factor of 104,12and the incorporation of hydrogenated Rh islands on the same semiconductor, which yields a solarto-hydrogen conversion Gibbs free energy efficiency of 13.3%, the highest reported so far for any photochemical, photobiological, or photoelectrochemical systern.l3 The deposition of a metal on the surface of an n-type semiconductor also results in the formation of a metal(6) s. Sat0 and J. M. White, J. Am. Chem. Soc., 102, 7206 (1980). (7)V. Guruswamy, P.Keillor, G. L. Campbell, and J. O'M. Bockris, Solar Energy Mater., 4, 11 (1980). (8)F. T. Wagner and G. A. Somorjai, J. Am. Chem. SOC.,102,5494 (1980). (9)A. Heller and R. G. Vadimsky, Phys. Reu. Lett., 46, 1153 (1981). (10)F.R. Fan, G. A. Hope, and A. J. Bard. J . Electrochem. SOC.,129, 1647 (1982). (11)Y. Taniguchi, H. Yoneyama, and H. Tamura, Chem. Lett., 269 (1983). (12)A. Heller, E. Aharon-Shalom, W. A. Banner, and B. Miller, J. Am. Chem. Soc., 104,6942 (1982). (13)E. Aharon-Shalom and A. Heller, J. Electrochem. SOC.,129,2865 (1982).
0 1983 American Chemical Society
4920
The Journal of Physical Chemistty, Vol. 87,
InP
z W
0
a
-1
Aspnes and Helier
No. 24, 1983
SrTi03
-RU
--
-Rh ANOC
-Pt
2 h't
HpO
-
CATHODE 2e-+ 2 H+ x,
2H++1/2 0
HZ
W W
n-TiOe
Figure 2. Energy diagram for a semiconductor with symmetric junctions. Flgure 1. Work functions of metals and band edges of semiconductors used In this work, shown relative to the potential of the standard hydrogen electrode (from ref 14-16).
semiconductor junction, or Schottky barrier, which acts to prevent electrons from reaching the catalytically active metal where proton reduction should be taking place. Figure 1shows the work function^'^ and electrons affinitied5 for the metal-semiconductor combinations discussed here, plotted with respect to the work function of the H+/H2 couple.16 It is eminently evident that in well-behaved contacts relatively high barriers, of the order of 1 V, should be formed. If the catalyst metals are to be cathodes or microcathodes, the first law of thermodynamics requires that these barriers be surmounted, no matter whether the electron proceeds directly to the metal from the bulk of the semiconductor or via an uncoated part of the surface. For this reason, it appeared puzzling that hydrogen should be photochemically evolved at platinum group catalysts on n-type semiconductor crystals or particles such as n-Ti02,3,6,8J7-19 n-SrTi03,4p20or n-CdS.21-26 Even more puzzling is the reported fact that the very same metal catalyst/ semiconductor combination accelerates the thermodynamically uphill process of the simultaneous evolution of both hydrogen and o ~ y g e n . At ~ *f~i s t glance this appears not merely to be a puzzle but a physical impossibility, because no conversion of light into chemical or electrical energy can occur in a photovoltaic cell having a pair of identical junctions. As illustrated in Figure 2, any energy gained by holes as a result of the barrier in the (14) Work functions for polycrystalline metals were compiled by H. B. Michaelson in "Handbookof Chemistry and Physics", 62nd ed, R. C. Weast, Ed., Chemical Rubber, Cleveland, 1981, p E-79. (15) The values used by R. Memming, Electrochim. Acta, 25, 77 (1980), have been updated and reconciled with those of other authors. (16) R. Gomer and G. Tryson, J . Chem. Phys., 66, 4413 (1977). (17) T. Kawai and T. Sakata, J. Chem. Soc., Chem. Commun., 1047 (1979); Chem. Phys. Lett., 72, 87 (1980). (18) E. Borgarello, J. Kiwi, E. Pelizetti, M. Visca, and M. Graetzel, Nature (London),289,158 (1981); J . Am. Chem. SOC.,103,6324 (1981). (19) J. Kiwi, E. Borgarello, E. Pelizetti, M. Visca, and M. Graetzel, Angew. Chem., 92,663 (1980). (20) J. M. Lehn, J. P. Sauvage, and R. Ziessel, Nouv. J . Chim., 4,623 (1980); 5, 291 (1981), Isr. J. Chem., 22, 168 (1982). (21) J. R. Darwent and G. Porter, J. Chem. Soc., Chem. Commun. 145 (1981). (22) J. R. Darwent, J. Chem. SOC.,Faraday Trans. 2,77,1703 (1981). (23) E. Borgarello, K. Kalyanasundaram, M. Graetzel, and E. Pelizetti, Helu. Chim. Acta, 65, 243 (1982). (24) K. Kalyanasundaram,E. Borgarello,and M. Graetzel, Helv. Chim. Acta, 64, 362 (1981). (25) K. Kalyanasundaram, E. Borgarello, D. Duonghong, and M. Graetzel, Angew. Chem., 93, 1012 (1981). (26) D. J. Meissner, R. Memmina, and B. Kastening, Chem. Phys. Lett., 96, 34 (1983).
anodic oxygen evolution process is necessarily lost in a symmetric cell by electrons in the cathodic hydrogen evolution process. In this paper, we show that these difficulties can be resolved, thanks to indications found in earlier work by Tsubomura and colleagues who studied hydrogen effects on a wide range of semiconductol-metal contacts including n-Ti02/Pd and n-CdS/Pd,np28by Steele and MacIver, who studied n-CdS/Pd hydrogen sens0rs,2~by Lund~trOm,3@~~ F o n a ~ h P, ~~ ~t e a t and , ~ ~co-workers, who studied siliconbased hydrogen sensors, by Yoneyama and co-w0rkers,3~?~~ who studied n-ZnO/Pd hydrogen sensors, by Heller et al., who studied p-InP/platinum group metal hydrogenevolving photocathodes,12and by the authors who studied n-GaAs/platinum group metal contacts.37 The common theme of ref 27-36 is the reductions of barrier heights of n-semiconductor/metal contacts by exposing them to dilute hydrogen ambients. Here, we investigate catalystcoated semiconductor crystals under the atmosphericpressure hydrogen ambients more nearly characteristic of the actual situations in hydrogen-evolving photocathodes. We find that hydrogen exposure greatly reduces the barrier heights of n-type semiconductorjunctions. In many cases, including those of Pt on n-Ti02or on n-SrTi03, these very substantial barriers collapse upon hydrogenation to true Ohmic or nearly Ohmic contacts. In most, but not in all cases, the interconversions of large barrier Schottky junctions to Ohmic contacts is completely reversible and can be achieved by exposing the hydrogenated contacts to air. This observation provides a natural mechanism that leads to the asymmetry necessary for photochemical evolution of oxygen and hydrogen in aqueous suspensions of nominally identical catalyst-coated semiconductor parti(27) N. Yamamoto, S. Tonomura, T . Matsuoka, and H. Tsubomura, Surf. Sci., 92, 400 (1980). (28) N. Yamamoto, S. Tonomura, T. Matsuoka, and H. Tsubomura, J. Appl. Phys., 52, 6230 (1981). (29) M. C. Staele and B. A. MacIver, Appl. Phys. Lett., 28,687 (1976). (30) I. Lundstrom,M. S. Shivaraman,C. Svensson, and L. Lundquist, Appl. Phys. Lett., 26, 55 (1975). (31) I. Lundstrom, M. S. Shivaraman, and C. Svensson,J.Appl. Phys., 46, 3876 (1975);Surf. Sci., 64, 497 (1977). (32) M. S. Shivaraman, I. Lundstrom, C. Svensson, and H. Hammarsten, Electron. Lett., 12, 483 (1976). (33) P. F. Ruths, S. Ashok, S. J. Fonash, and J. M. Ruths, IEEE Trans. Electron Deuices, ED-28, 1003 (1981). (34) T. L. Poteat and B. Lalevic, IEEE Trans. Electron Devices, ED29, 123 (1982). (35) W. B. Li, H. Yoneyama, and H. Tamura, Chem. Abstr., 94,95157 (1981). (36) H. Yoneyama, W. B. Li, and H. Tamura, Chem. Abstr., 93,214257 (1980). (37) D. E. Aspnes and A. Heller, J. Vac. Sci. Technol., B1,602 (1983).
Water-Photolyzing Semiconductor Suspensions
cles, even for a single metal-semiconductor combination, because both Ohmic contacts and Schottky junctions become possible by merely changing the atmosphere. High barriers, necessary for the oxidation of water with oxygen evolution on n-type particles, are further enhanced when the junctions are exposed to oxygen and Ohmic contacts essential for the reduction of water with hydrogen evolution on n-type particles, are obtained when the junctions are exposed to hydrogen. The positive-feedback nature of this process ensures continuing photoelectrolysis of water in suspensions of catalyzed n-type semiconductor particles such as n-TiOz, n-SrTi03, and n-CdS, because once started, oxygen is expected to be generated at some catalyst islands while hydrogen evolves at others. This mechanism therefore explains the physically "impossible" observation that a seemingly single metal-semiconductor combination will convert light to chemical energy via an uphill reaction. Our data account for the observation that photoelectrolysis on semiconductor particles has a finite incubation time. Little or no gas evolution can take place until the separate hydrogen- and oxygen-enveloped catalyst islands are established. The data also explain why photoelectrolysis experiments with suspended particles are not always reproducible. The time that it takes to establish the different atmospheres around the islands is not predictable, and moreover, the initial barrier heights tend to depend on charge trapping phenomena that vary from spot to spot. We suggest, however, that the incubation time may be reduced significantly and some of the experimental uncertainties eliminated by introducing traces of hydrogen into the photoelectrolyzing suspensions. Our transport data also allow some conclusions to be drawn about the nature of the Schottky barriers formed with these metal-semiconductor combinations. The asdeposited contacts on Ti02and SrTi03 show a range of barrier heights. The variations can be understood through the observation that these junctions can be made irreversibly Ohmic or nearly Ohmic by charge injection, that is, by driving the contact a few volts in the reverse-bias (depletion) direction. The permanent nature of this change-in contrast to the reversible nature of that induced by hydrogenation-indicates that these oxide semiconductors are very susceptible to trapping effects and-by implication-that the hydrogen remains outside the semiconductor under the room-temperature conditions of our experiments. The fact that very large apparent capacitance values were observed for TiOz and SrTi03 at MHz frequencies is consistent with previous measurements3* showing that the capacitance increases with increasing frequency. This behavior is exactly opposite to that expected for the deep traps usually invoked to explain the observed frequency dispersion in capacitance. We attribute the large highfrequency capacitance and the abnormal dispersion to polarization effects within the semiconductor, probably related to the polaron transport mechanism in these incipient ferroelectrics. The results suggest that accurate estimates of donor concentrations and barrier heights cannot be obtained from capacitance measurements. Contacts to CdS and InP follow more clmsical behavior, with apparently good I-V and C-V characteristics obtained for both. However, the data for n-CdS/Pt contacts show anomalies such as poor agreement between barrier heights deduced from I-V and C-V data, excessively large donor concentrations, and large bias-, illumination-, and hydrogen-dependent trapping behavior, all of which suggest that (38) S. N. Frank and A. J. Bard, J. Am. Chem. SOC.,97,7427 (1975).
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983 4921
the CdS surface is substantially disrupted by reaction with the deposited Pt. No such anomalies were observed for Rh and Ru contacts, indicating that the reaction was specific to Pt. Nevertheless, only the n-CdS/Pt contacts showed apparent barrier heights sufficiently high to allow photodecomposition of water. But these very same high barriers persist even upon hydrogenation and should inhibit the electron transfer process to platinum. Thus, while we shall explain the photoelectrolysis of water with catalyzed suspensions of n-TiOz and n-SrTiO,, we are left puzzled by the report on water photoelectrolysis with catalyzed, n-CdS particles where hydrogen is evolved on Pt island^.^^^^^ We note, however, that since classical behavior is not observed for n-CdS/Pt, the actual barrier heights may be quite different from their apparent Valu e ~ . ~ ~ ~ ~ Contacts to p-InP show no such anomalies. Good agreement is found between barrier heights deduced from I-V and from C-V measurements. These contacts show a remarkably constant barrier height of VB= -0.86 f 0.05 V after hydrogenation, independent of the contact metal, suggesting that the barrier height in this case is determined almost entirely by the hydrogen. In contrast to GaAs, where strong pinning prevents a change of the barrier by more than 0.2 V upon hydr~genation,~' we found an increase of the barrier height by 0.5 V for p-InP/Rh, indicating negligible pinning for this material. We observe that the ideality factor of p-InP contacts increases upon hydrogenation and attribute these increases to variations in the barrier height over the surface as proposed by Freeouf et al.42 In no case do we observe barrier heights of -1.0 or -1.2 V, corresponding to the locations of the surface defects in InP.43v44
Experimental Section Crystals. Single crystalline n-Ti02 (rutile) slices polished and hydrogen reduced for 30 min at 750 "C, and n-SrTiO, slices polished and vacuum-reduced at 1320 "C for 90 min were generously provided by David S. Ginley of Sandia Laboratories. n-CdS (A plates) were obtained from Cleveland Crystals. The CdS crystals were first mechanically polished with Linde B (500-A corundum particles) and then chemically polished with 1 M HC1. The (100) p-InP crystals were cut, mechanically polished with Linde B, chemically polished with methanol-1 vol % bromine, and rinsed with a 1:l solution of NH40H:H20just prior to loading in the evaporator. Ohmic Back Contacts. Ohmic contacts to n-Ti02, nSrTiO,, and n-CdS were made by rubbing a solid indium amalgam into the back surfaces until the contact resistance between separate 3 mm2 areas dropped to 2 s2 or less. Ohmic contacts to p-InP were made by alloying evaporated films of Zn (200 A) and Au (2000 A) at 450 "C for 30 s. These contacts were checked by making duplicates on the back of the same crystal, or by sandblasting a channel through individual contacts and measuring the resistance between the divided halves. Catalyst Films. Arrays of 1.25-mm-diameter dots of -400-A-thick films of Pt, Rh, or Ru were electron-beam evaporated onto the crystals. Scanning electron microscopy at 72 000 X magnification showed these films to be (39) S. J. Fonash, J. Appl. Phys., 46, 1286 (1975). (40) S.J. Fonash, t o be published. (41) G. P. Schwartz and G. J. Gualtieri, unpublished. (42) J. L. Freeouf, T. N. Jackson, S.E. Lam, and J. M. Woodall, Appl. Phys. Lett., 40, 634 (1982). (43) W. E. Spicer, P. W. Chye, P. R. Skeath, C. Y. Su, and I. Lindau, J. Vuc. Sci. Technol., 16, 1422 (1979). (44) W. E. Spicer, S. Eglash, I. Lindau, C. Y. Su, and P. R. Skeath, Thin Solid Films, 89, 447 (1982).
4922
Aspnes and Heller
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
I s
4ot-
I
0.1
0
f A I R
Y
0.2 0.4 BIAS, VOLTS
0
0.6
-2
0.8
Figure 3. Current-voltage characteristics on a linear scale for a n-TiO,/Pt contact in air and In atmospheric-pressure hydrogen.
+05
0.2
0.4 BIAS, VOLTS
0.6
0.8
Flgure 5. As Figure 3. for a n-TiO,/Rh contact.
r
N
E 12Or
’ O 0I BO
I I
BIAS, VOLTS -3.5
L
Flgure 6. As Flgure 3, for a n-TIO,/Ru contact.
0
0.1
0.2
0.3 0.4
0.5
0.6
0.7
08
BIAS, VOLTS
Flgure 4. As Figure 3, on a logarithmic scale.
uniform and free of pinholes. External Contacts. For transport measurements, the samples were placed on the ground plate of a probe. The catalyst layers were contacted with the tip of a phosphor bronze spring. Hydrogenation Atmosphere. A hydrogen envelope was created by flooding the sample with commercial tank H2 a t 1 atm pressure. The oxygen (air) content in the proximity of the catalyst was well below the ignitable H 2 / 0 2 ratio, as evidenced by the fact that the authors, to their delight, experienced no explosions or fires in spite of the ample presence of Hz-02 recombination catalysts. Electrical Measurements. Current-voltage (I-V) data were obtained in the 1-10-Hz frequency range by measuring the voltage across the sample and the current through a calibrated series resistor. Capacitance-voltage (C-V) data were obtained by observing the change of sample voltage obtained by injecting a known amount of charge into the interface. The injection was accomplished by applying a 1-MHz square wave to a 100-pF capacitor connected to the junction. Photovoltage (PV) measurements were made by focusing the output of a high-intensity microscope lamp onto the contact and measuring the resultant voltage with a high-impedance voltmeter. The same light source was used to test for illumination-dependent trapping effects. The -400-A metal films were optically thick. Thus the photovoltage measurements show only trends. The limiting barrier heights were not approached at the light levels reaching the semiconductors.
Results and Discussion A . Ti02. Current-voltage data for a Pt-Ti02 contact are shown on a linear scale in Figure 3 and on a logarithmic scale in Figure 4 before and after hydrogenation. Similar results are shown in Figures 5 and 6 for the Rh and Ru contacts. The effective barrier heights in air decrease in the order Rh 2 Pt >> Ru. While a small barrier for Ru and a large barrier for Pt are expected from the positions
of the band edges of TiOz relative to the metals (Figure 11, the barrier for Rh is much greater than expected. The striking feature of these data is the complete change of contact character from rectifying to Ohmic upon exposure to hydrogen. This change is essentially instantaneous, with the front-to-back sample resistance becoming equal to that between the split halves of the back contact. This indicates that the Ohmic quality of the hydrogenated contacts is as good as that of the back contacts themselves. The change is reversible, with the original I-V characteristic being restored upon exposure to air. The ideality factor of 4.2observed for the as-deposited Pt contact is far larger than possible for recombination process within the barrier, and indicates that the applied voltage is probably being dissipated by resistive effects at the semiconductor-metal interface or by the injection and storage of charge in traps in the space-charge region. Evidence for charge storage is seen in a 100-mV shift of the I-V curves to lower voltage after strong forward bias and in a 50-mV shift to higher voltage after strong reverse bias, with a time constant of approximately 20 s for each. The temporary lowering of the barrier under strong forward bias indicates that holes from the metal are being injected into the space charge region near the interface. Small current-induced shifts were also observed for the Rh contacts. For the Ru contacts, these shifts were more dramatic in that a large current in the forward direction made the contact nearly Ohmic, and the effect was permanent. The Ru contacts so modified were made completely Ohmic by exposure to hydrogen. Ideality factors are not shown explicitly for the Rh and Ru contacts, but are also large as with Pt. Barriers heights are normally deduced from I-V data by means of the Richardson equation.45 This equation is based on thermionic emission and on the assumption of simple parabolic energy bands. The large ideality factor shows that the thermionic emission model is not applicable, while the extremely large conduction band effective mass,46mc* zi lome, implies that the conduction bands (45) S. M. Sze, “Physics of Semiconductor Devices”, 2nd ed, Wiley, New York, 1981,Chapter 5.
80-
'061
AIR
v)
0
60-
$ 40i,
8 20-
NE
d
-0.6
0
3 0
1 1 ,I
.
-0.1 0 0.1
,
j
1
I
,
0.3 0.6 BIAS, VOLTS
/
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983 4923
Water-Photolyzing Semiconductor Suspensions
P 5 - 2.6
= - 1.6
,
1
-
0
0.9
0.7
Figure 7. Current-voltage characteristics on a linear scale for a n-SrTiO,/Pt contact in air and in atmospheric-pressure hydrogen.
'3
r
-3.56
/
Y
02
I
I
0.4
I
0.6
!
0.8 1.0 BIAS, VOLTS
I
I
12
1.4
I
1.6
Figure 10. As Figure 9, on a logarithmic scale,
Y
108-
v)
6-
c* 3 0
I
'O
0.1
012
Oi3
014
Oi5
0:s
0.7
BIAS, VOLTS
Oi8
'
Figure 8. As Figure 7, on a logarithmic scale.
42-
-0.4
0
0.4 BIAS, VOLTS
0.8
1.2
Figure 11. As Figure 7, for a n-SrTiO,/Ru contact.
2 300
3
2501
I
+0.6 -
-0.5 BIAS, VOLTS Figure 9. As Figure 7, for a n-SrTiO,/Rh contact.
cannot be parabolic over a significant fraction of an electronvolt. We conclude that any attempt t~ deduce barrier heights from the I-V data using the Richardson equation is not legitimate. Barrier estimates based on the approximate values of the I-V curves are shown in Table I. These are meant only to represent trends. Similarly, no meaningful Mott-Schottky plots were obtained for these n-TiOz samples. The high-frequency capacitances were extremely large, of the order of pFd/cm2, and essentially independent of voltage. While the large capacitance could be a consequence of a large impurity concentration, its independence of bias voltage is anomalous. Anomalous capacitance behavior in n-TiOz has also been observed by previous workers: Frank and Bard38 reported that C2decreased by 30% as the frequency was increased (!) form 200 to 1000 Hz. (We did not attempt low-frequency C-V measurements, because these are strongly affected by slow trapping levels and hence give an unrealistic estimate for the barrier height in this material). A detailed analysis of the space charge region and (46) G. A. Acket and J. Volger, Physica, 32, 1680 (1966).
-3.6
0.1
0.3
0.6 0.7 BIAS, VOLTS
0.9
1.1
Figure 12. As Figure 11, on a logarithmic scale
transport properties in n-TiOz would clearly be a formidable undertaking. Indeed, the heroic analysis of the n-Ti02/electrolyte interface indicates the complexity of the problem.4g50 (47) Deleted in proof. (48) M. Tomkiewicz, J . Electrochem. SOC.,126, 2220 (1979). (49) M. Tomkiewicz, J . Electrochem. Soc., 129, 1240 (1982). (50) S. Withana and M. Tomkiewicz, Phys. Reu. Lett., 50,443 (1983).
4924
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Aspnes and Heller
TABLE I: Summary of the Transport Data for the 1 2 Metal-Semiconductor Combinations Studied in This Work" vB,
semiconductor metal N D , NA,l o L 6cm-3 n-TiO, n-SrTiO, n-CdS p-lnP
"
Pt Rh Ru Pt Rh Ru Pt Rh Ru Pt Rh Ru
r)
4.2 (ohmic)
I-v
v
c-v
PV, mV 0.0 (0.0)
20.5 ( 0 ) 2 0 . 6 (0)
17 (0.0)
20.1 ( o j
0.9 (0.9) 0.14 (0.22) 0.32 (0.28) 10 (11) 10 (10) 1 2 (11)
7.2 ( 1 . 3 ) 2.9 (1.0) 3.3 (2.9) 1.69 ( - 2 ) 1.50 (1.44) 2.34 (0.97) 1.21 (1.19) 1.06 (1.24) 1.18 (1.44)
2 0 . 6 (20.1)
0.0 (0.0) 0.0 (0.0)
20.7 ( 2 0 . 2 ) -1.1 (-1.0) 0.78 ( 0 . 5 3 ) -0.9 (0.49) -0.61 (-0.81) -0.45 (-0.84) -0.52 (-0.83)
1.0 (1.0) 1.0 (1.0) 200 (90) 150 (10) 150 (10) 0 (20) 7 (360) 100 (350)
21.0i2o.i j
1.84 (1.63) 1.00 (0.75) 1.28 (0.75) -0.72 (-0.91)
-0.31 (-0.82) -0.57 (-0.85) Values of the parameters for the contacts in atmospheric-pressure hydrogen are shown in parentheses.
Photovoltage measurements also failed to yield definitive results. The only contacts that showed photovoltages above the millivolt level were those formed by Rh, the metal which also gave the largest apparent barrier height from the I-V data shown in Figure 3. The results on barriers and hydrogen effects of Pt, Rh, and Ru contacts are, in general, consistent with those of Yamamoto et al.27,28on n-TiO,/Pd contacts. The latter have barriers of -0.75 eV in air and to become Ohmic at >10 000 ppm hydrogen. B. SrTiO,. Results for Pt, Rh, and Ru contacts to n-SrTiO, are shown in Figures 7-12. Contacts to SrTiO, follow the same general pattern as contacts to TiOz, namely, large ideality factors in the as-deposited condition and large reductions in barrier height upon hydrogenation. In addition, all contacts showed small temporary bias changes if driven strongly in the forward direction. The Pt and Rh contacts could be changed permanently to low-resistance, nearly Ohmic, behavior by a strong negative bias, making it necessary to avoid reverse bias potentials in excess of 4 V. Again, photovoltages were negligibly small (51mV). The high-frequency capacitance was also very large and showed little dependence on bias. The SrTiO, contacts differed significantly from those on Ti02: although barrier heights were greatly reduced by exposure to H,, the contacts remained nonohmic. As seen in Figures 8 and 10, the ideality factors for Pt and Rh were also greatly reduced, approaching within 30% their ideal values. The shift of the linear I-V characteristic upon hydrogenation exceeded 1 V for Rh. The I-V behavior for Ru was more complicated, showing definite hysteresis effects with the contact tending to be more Ohmic on returning from forward bias (Figure 11). The data are summarized in Table I. Implications of the Results to Water Photolysis with Catalyzed n-TiO, and n-SrTiO, Suspensions. From the results it is evident that catalyzed evolution of hydrogen on illuminated n-TiO, and n-SrTiO, cannot take place unless part of the catalyst is hydrogen saturated, because substantial barriers prevent electrons from reaching the catalysts. Selective hydrogenation of some of the islands has two effects: it eliminates or greatly lowers the barriers that prevent electron access and increases the asymmetry of the barriers of the hydrogen- and the oxygen-evolving catalyst contacts. This asymmetry represents the upper limit to the reduction in the 1.23-V bias voltage required for electrolysis of water. Our first conclusion is therefore that water photolysis, in its initial phase, must be autocatalytic. The rate of photolysis is increased as hydrogen evolution on some catalyst islands and oxygen evolution on other create the necessary >1.23-eV asymmetry in barrier heights. From
the autocatalytic nature of the reaction it follows that if one plots the integrated volume of gases evolved vs. elapsed time, the linear region should be preceded by a nonlinear region representing an "incubation period" or "delay time". Indeed an overlooked detail of a figure showing the time dependence of photoelectrolysis of H2S on RuO, and Pt catalyzed n-CdS particles reveals a 30-min delay time.,, Our second conclusion is that by introducing small amounts of hydrogen (and oxygen) into the water-photolyzing cells the delay times should be reduced. The only process by which hydrogen evolution could start at a low rate, evolution on the uncatalyzed semiconductor itself, must depend on the surface chemistry of the particles. Even if this chemistry is meticulously controlled, the necessary presence of adsorbable impurities, such as ions from the glassware, will cause some irreproducibility in the delay times. Indeed, unless hydrogen is purposefully introduced into the cells it is conceivable that the reaction may never start. Our third conclusion is that the n-SrTi03/Rh contact has the greatest asymmetry, i.e., the largest difference in barrier heights in air and in hydrogen, of the contacts studied. This conclusion supports the results of Lehn, Sauvage, and Ziessel,20who found that Rh-activated nSrTi0, suspensions were superior in the photolysis of water. Our results also suggest that the RuO, incorporated by Borgarello et al.1*319in the surface of anatase-phase n-Ti02 to catalyze oxygen evolution may also have a second, quite different, role. If present in its amorphous (non-rutile) form, which can readily be reduced, it may form Ru islands. Because the barrier of the Ru junctions is much lower than that of the Pt junctions, the evolution of the first trace of hydrogen that is required to lower the barrier a t the Pt islands may take place on Ru. Thus, Ru may act as primer, or co-catalyst, of the Pt. C. n-CdS. In contrast to TiO, and SrTiO,, contacts on CdS showed good Mott-Schottky behavior and relatively large photovoltages in both as-deposited and hydrogenated conditions. The relevant transport data are shown in Figures 13-18. The ideality factors 17 are listed in Table I, except for the hydrogenated Pt contact. In the nCdS/Pt contact, hysteresis and trapping were so extensive that only an estimate of 17 could be obtained. Barrier heights were deduced from the I- V data using the Richardson equation J = A * * F exp(q(V - VB)kT) N
where A** = (m*/m,)A, m* = 0.20mCis the conduction band mass for CdS,5l and A = 120 A/cm-2 K-2 is the Richardson constant for free electron^.^^ Whenever the ideality factor was large, we used our previous argument37
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Water-Photolyzing Semiconductor Suspensions
4925
+0.5r
BIAS, VOLTS
Flgure 13. Current-voltage characteristics on a logarithmic scale for a n-CdS/Pt contact in air. Data for the junction in atmosphericpressure H, are strongly bias dependent and only a rough estimate of the ideality factor could be obtained.
-4.5 1 0
1
0.1
/I
0.2
1
0.3 BIAS, VOLTS
I
I
I
0.5
0.4
0.6
Flgure 16. As Figure 14, for the n-CdS/Rh contact of Figure 15.
N
5 a
-0.5 -
7-
Y
P -F
-1.5 -
-2.5 BIAS, VOLTS
-3.5 -
Flgure 14. Mott-Schottky plot for the n-CdS/Pt contact of Figure 13.
0.0
300 T
0.1
0.2
0.3 0.4 BIAS, VOLTS
0.5
0.6
0.7
Flgure 17. As Figure 13, for a n-CdS/Ru contact.
Q
\
?OI A E B = -0.2SV L
-3.0
-2.5
-2.0
-1.5
1.oov 0.75V -1.0 -0.5 0 BIAS, VOLTS
+0.5
+1.0
+1.5
+2.0
0.75V
Figure 15. As Figure 13, for a n-CdS/Rh contact.
about the measured current being an upper limit to J to obtain an estimate of VB. Values so obtained are also shown in Table I. Barriers heights and impurity concentrations were obtained from the C-V data in the usual way by determining the slope and intercept of the linear portions of the data plotted as C2vs. V. To obtain the impurity concentration, we used the value 8.g51for the static dielectric constant (51) J. I. Pankove, 'Optical Processes in Semiconductors",PrenticeHall, Englewood Cliffs, 1971, Appendix 2.
\I I
-3.0
-2.5
-2.0
-1.5
I
1.28V 1
I
-1.0 -0.5 0 BIAS, VOLTS
+0.5
+1.0
+1.5
+2.0
Flgure 18. As Figure 14, for the n-CdS/Ru contact of Figure 17.
of CdS. These results are also given in Table I. By every criterion, the n-CdS-Pt contacts showed anomalous behavior. The apparent impurity concentration was much higher for the Pt-contacted material than that for Rh and Ru, even though it was made from the same face of the same crystal by the same technique. The
4928
The Journal of Physical Chemistry, Vol. 87,
No. 24, 1983
difference between barrier heights determined by I-V and C-V measurements was -0.6V, about twice that for the other contacts that yielded values. In addition, the Pt contacts showed strong bias- and illumination-dependent trapping effects, in contrast to those of Rh and Ru for which no time-dependent effects were observed. The time dependences in the initial (as-deposited, air-exposed) condition could only be explained by assuming that two types of traps were present. The first type caused a shift of the barrier by about 0.1 V to higher energy when the junction w8s driven strongly in the forward direction. This shift relaxed under open-circuit conditions with a time constant of about 200 s in the dark, and essentially instantaneously under strong illumination. The same trap could be populated by subjecting the sample to strong illumination while under negative bias. The trap of the second type had a time constant of 10 s and slowed the recovery of the barrier height after strong illumination. Upon hydrogenation, a third trap was found to come into play. This trap caused the I-V characteristic of the illuminated contact to overshoot its quiescent dark value by -50 mV. When the illumination was discontinued, the I-V characteristic relaxed with a time constant of minutes to its original condition. Other measurements under the conditions described in the preceding paragraph show that the first two types of traps are already present before hydrogenation, and are not affected by the presence of hydrogen. As a result of the time-dependent behavior and particularly that due to the third trap type activated by hydrogen, we experienced some difficulty in obtaining I-V and C-V data for n-CdS/Pt contacts. Nevertheless, the values shown in Table I were all taken under near-equilibrium conditions in the dark and all except for the ideality factor should be reliable. The value of 2 is approximate, but adequately represents the fact that 7 is unquestionably larger than 1. The Mott-Schottky plot indicates a threshold reduction of 0.21 V upon hydrogenation, which is larger than, but consistent with, the measured reduction of photovoltage by 0.10 V. The Rh and Ru contacts showed no trapping anomalies under any condition. The respective data are summarized in Table I. Hydrogenation slightly improved the ideality factor for Rh and markedly improved it for Ru, with the Ru contact showing a larger change in barrier height upon hydrogenation. Differences between thresholds determined from I-V and C-V measurements indicate that these contacts are not ideal, but are complicated by the presence of interfacial layers of a composition differing from those of the pure semiconductor and the pure metal. We note that the reported hydrogen effects on nCdS/Pd b a r r i e r ~ are~ not ~ ~ in ~ agreement ~ with each other and differ from our results on Pt, Rh, and Ru. According to Steele and MacIver, who like ourselves used e-beam evaporated metal contacts and calculated barrier heights from the forward characteristic^,^^ the barrier of the nCdS/Pd contact in air is 0.53 eV and zero in hydrogen above 5000 ppm. According to Yamamoto et al. the barrier height of the n-CdS/Pd contact in air is 0.8 eV, dropping on to 0.7 eV at 10000 ppm of H2. The cause of these differences may be associated with the sensitivity of CdS to surface damage caused by polishing abrasives. We found, for example, that considerable time was required to remove mechanical damage by chemomechanical polishing with l M HC1. The reason for the much higher barriers that we measured is most probably due to our success in effectively removing the damaged regions by chemical polishing.
-
AsDnes and Heller
. E
BIAS, VOLTS
Figure 19. Current-voltage characteristics on a linear scale for a p-InP/R contact in air and in atmospheric-pressure hydrogen.
Implications to Photolysis with Catalyzed n-CdS Particles. Even though hydrogenation of catalyst contacts with n-CdS causes a decrease in the barrier heights, very substantial barriers remain. The barrier of the n-CdS/Pt contact after hydrogenation remains particularly large, well in excess of 1 eV (Table I). With such a residual barrier, it is unlikely, if not impossible, that the 21.23-eV barrier asymmetry needed to water photolysis could be achieved between hydrogen-evolving Pt contacts and oxygen-evolving RuOz contacts in Pt/n-CdS/Ru02 micro cell^.^^^^^ To achieve the asymmetry needed, it would be necessary to prepare the semiconductor particles in such a way that each particle have two quite different surface regions, one undamaged, to form high barrier contacts with a catalysts like Ru02, and one heavily damaged to form low barrier contacts to Pt. While recent work by H o d e shows ~ ~ ~ that sputtering of metals onto n-CdS can sufficiently damage the surface to form Ohmic contacts, (even though high barrier contacts are formed on the undamaged semiconductor surface by other metal deposition techniques), it appears that the reproducible preparation of n-CdS particles with the two types of surface region^^^-^^ must be quite difficult. Futhermore, if the n-CdS particles are damaged, electron-hole recombinations centers will reduce the quantum efficiency. Thus high light-to-chemical conversion efficiencies cannot be reached with high quality and certainly not with damaged n-CdS/Pt hydrogen evolving micro cathode^.^^-^^ While water photolysis should prove difficult on catalyzed n-CdS, other photolytic hydrogen evolving reactions, proceeding at a lesser electrochemical bias, should be and are indeed found to be feasible. Subject to variations in incubation times, these reactions should be quite reproducible, as found to be the case by Darwent and Porter for hydrogen evolution accompanied by oxidation of EDTA or cysteine,21tn by Gratzel et al. for photolysis of H2S,23and by Meissner et al. for hydrogen evolution accompanied by oxidation of sulfide and EDTA solutions a t monograin n-CdS membranes.26 We note that our results suggest that for hydrogen evolution on n-CdS, Rh and Ru should be preferred over Pt, as these metals form, upon hydrogenation, lower barriers to electron transport to the catalyst. That this may well be the case is suggested by the result of Meissner et a1.,26who found no hydrogen evolution a t illuminated n-CdS monograin membranes platinized on one side and exposed on the other to an oxidizable EDTA solution. They did, however, observe H2 evolution when the Pt was replaced by Ru02 and EDTA by an oxidizable sulfide, and measured a ;=0.5-eV barrier height for the sputtered Ru02/n-CdS contact. Note that the work function of Ru02 is 4.8 eV, close to that of Ru itself.53 (52) G. Hodes, unpublished results. (53) M. Tomkiewicz, Y. S. Huang, and F. H. Pollak, J. Electrochem. Soc., 130, 1514 (1983).
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Water-Photolyzing Semiconductor Suspensions
+0'5
4927
+O'T
/
0I
/
I
-0.5
"I -2.5
-3.5
0.2
0
0.4
0.6
-
0.8
I
I
I
I
02
0.4 BIAS, VOLTS
0.6
0.8
BIAS, VOLTS
Figure 20. As Figure 19, on a logarithmic scale.
Figure 23. As Figure 22, on a logarithmic scale.
'T
3l1 --
N
t
HYDRo -
2 t
Y
N
Y c
't
10 MHz
10 MHz I
-1
0
+1 BIAS, VOLTS
I
I
+2
+3
Figure 21. Mott-Schottky plot for the p-InP/Pt contact of Figure 19.
I
I
-1
0
"
I
I
1
I
+1 BIAS, VOLTS
+2
+3
Figure 24. Mott-Schottky plot for the p-InP/Rh contact of Figure 22.
a
2 200 I-'
!
3
800
2 2
400
600 BOO O -0.6
-0.4
-0.2
0
0.2
0.4
BIAS, VOLTS
2 -0.6
-0.4
-02 0 BIAS, VOLTS
0.2
0.4
Figure 22. As Figure 19, for a p-InP/Rh contact.
D. p-InP. Current-voltage and Mott-Schottky data for Pt, Rh, and Ru contacts to p-InP are shown in Figures 19-27. These junctions show ideality factors very close to unity for air-exposed junctions. The ideality factors increase slightly upon exposure to hydrogen. Barrier heights were determined from the I-V data using the Richardson equation with the hole conductivity mh* = 0.40me.Good Mott-Schottky plots were obtained in all cases. Barrier heights and carrier concentrations determined by analysis of C2vs. V plots with5' ,€, = 12.1 are listed in Table I. Values of N Aare independent of contact material and state of hydrogenation, in contrast to the results for n-CdS. No bias- or illumination-dependent trapping effects were seen.
Figure 25. As Figure 19, for a p-InP/Ru contact.
In the p-InP junctions there is also good agreement between barrier heights determined from I-V or C-V measurements. All barrier heights show a remarkable similarity after hydrogenation,'2 which indicates that the metal work function is no longer relevant and that the interface potential is dominated, also in this case, by interfacial hydrogen. Upon hydrogenation the barrier height change is small for Pt and relatively large for Ru, indicating that the metal work functions are not as measured under vacuum but that another constituent, possibly oxygen, interacts chemically with the metals at the semiconductor interfa~e.~'The small increase in ideality factor upon hydrogenation for Rh and Ru is opposite to the effect seen in n-type semiconductors including n-GdAs. This may be due to an increase in the effective number of recombination centers activated as the barrier is driven deeper into
4828
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
Aspnes and Heller
+0.5 r
The present results, obtained with 200-400-A thick continuous, pinhole free catalyst films in dry experiments support the results on the photoelectrochemical cells. As in the photoelectrochemical cells all contacts, though greatly different prior to hydrogenation behave similarly after hydrogenation. As seen in Figures 21,24, and 27 and in Table I hydrogenation invariably increases the barrier height to 0.86 f 0.05 eV. In the photoelectrochemical cells, the ideality factor of the hydrogen-evolving p-InP photocathode with Ru islands (determined by measuring the shift in threshold potential for hydrogen evolution as a function of the incident monochromatic photon flux and determined to be 59 mV for each tenfold increase) was found to be close to unity.12 The dry hydrogenated p-InP/Rh contacts made with thicker and continuous Rh films have, after hydrogenation, BIAS, VOLTS an ideality factor of 1.24 (Figures 22 and Table I). This Figure 28. As Figure 25, on a logarithrnlc scale. shows that recombination and leakage at the p-InP/hydrogen-saturated acid interface is lower than at the hydrogenated catalyst/p-InP interface and that there is an advantage to densely spaced catalyst islands over continuous catalyst films in hydrogen-evolving p-InP photocathodes. The key conclusion of our studies of dry and wet pInP/catalyst contacts both with catalyst islands covering -4% of the surface and with continuous, pinhole free catalyst films is that the two behave similarly. The reason for this is, in retrospect, simple. An electron that is photogenerated in a semiconductor must reach the catalyst in order to reduce the solution species. In order to do so it must pass the barrier that exists at the semiconductor-catalyst junction, irrespective of the path it takes. -0.m Thus, in a catalyzed photoelectrochemical cell, as well as 10 MHr in suspended catalyzed microcells, the relevant barrier I I I I height cannot differ from that of the dry contact of the -1 0 +1 +2 +3 semicondudor and the catalyst. One can, therefore, apply BIAS, VOLTS the very substantial body of information that has been Figure 27. Mott-Schottky plot for the p-InP/Ru contact of Figure 25. gathered in the past four decades by the solid-state physics community on semiconductor contacts with metals that depletion, or by a chemical nonuniformity introduced into chemists and electrochemists know as catalysts and electhe semiconductor-catalyst interface upon hydrogenation. trocatalysts. Using this body of knowledge one should find We note that the barrier heights do not correspond to the values -1.0 and -1.2 V of the unified defect m0de1,4~*~~ it possible to rationally build photoelectrochemical cells and microcells with the highly asymmetrical barrier pairs indicating that pinning by defects is not a factor in these that are the key to efficient photoelectrolysis. junctions. Nature of the Ambient Gas-Induced Change. There are Implications to Hydrogen-Evolving p-InP Photocaat least three processes by which the ambient gas can thodes. In an earlier paper12we discussed hydrogen evochange the barrier heights: change in the surface dipole lution on p-InP (100 face) photocathodes without catalysts component of the metal work function; change of the and with island-forming thin films of Pt, Rh, and Ru. The Fermi level of the metal upon hydrogen alloying, reversed catalyst islands were -400 in diameter, were separated by the complete stripping of hydrogen from the alloy by from each other by -1500 A, and covered -4% of the oxygen; and doping of the semiconductor space charge surface.54 We found that in these photocathodes the bias region by electrically active hydrogen and/or oxygen. required for the photolysis of water was reduced upon There is ample evidence in the literature to show that equilibration with hydrogen. As a result we were able to protic solutions can dope n-Ti02 electrode^.^^-^^ achieve a solar-to-hydrogen conversion Gibbs free energy The data on p-InP contacts demonstrate that no elecefficiency of trically active gas is penetrating significantly into the space We found that the threshold potentials for hydrogen charge region of this material. The Mott-Schottky plots evolution shift upon hydrogen equilibration to common have identical slopes and therefore the samples have values, irrespective of the presence or nature of the catalyst identical carrier concentrations. While surface doping very and concluded that the contacts formed with any of the near the interface (-5 A) cannot be ruled out, such doping hydrogenated catalysts as well as with the hydrogen-saturated acid solutions themselves behave as if they were (55)P.F.Chester and D. H. Broadhurst,Nature (London),199,1056 proper p-InP/hydrogen contacts. Upon intensely illu(1963). minating the photocathode, we measured a 0.8-V reduction (56)D. S.Ginley and M. L. Knotek, J. Electrochem. SOC.,126,2163 (1 ~ 979) -_- _ , _ in the bias required for electrolysis and deduced that the (57)L.A. Harris, M. E. Gerstner, and R. H. Wilson, J. Electrochem. effective barrier heights of the hydrogenated photocaSoc., 126,850 (1979). thodes must be of 10.8 eV. (58)L.A. Harris and R. Schumacher,J. Electrochem. SOC.,127,1186
'T
~
~~
~
~
~~
(54) A. Heller and R. G. Vadimsky, unpublished results.
~~
~
(1980). (59)M.F.Weber, L. C. Schumacher, and M. J. Dignam, J. Electrochem. SOC.,129,2022 (1982).
J. Phys. Chem. l903, 87,4929-4932
is equivalent to changing the interfacial dipole. Change in the Fermi level of the metal is inconsistent with our parallel measurements of the optical properties of these metal films with changing ambients: Significant changes in the metal Fermi level would have caused large changes in the Drude tail of the dielectric functions of the metals. Such changes were not observed. Contributions from hydrogen doping of semiconductors other than InP cannot be ruled out, as shown for example by the reversible hydrogen-induced trapping state that we have observed in CdS. However, the general mechanism, that bears on all contacts, is the change in the surface dipole component of the metal work function upon changing the ambient gas.
4929
Acknowledgment. Many of the questions resolved in this paper were raised in enjoyable discussions between Jean-Marie Lehn and Adam Heller while the latter was a Guest Professor a t the College de France. The authors are indebted to David S. Ginley of Sandia Laboratories for the n-Ti02 and the n-SrTi03 crystals. They thank David B. Colavito for extensive assistance in the experiments and A. J. Nozik and M. Tomkiewicz for useful discussions of the band structure, dielectric functions, and electron masses of TiOz and SrTi03. Registry No. Hydrogen, 1333-74-0;water, 7732-18-5; platinum, 7440-06-4; rhodium, 7440-16-6; ruthenium, 7440-18-8; titanium dioxide, 13463-67-7; strontium titanate, 12060-59-2; cadmium sulfide, 1306-23-6; indium phosphide, 22398-80-7.
Stereochemical Consequences of Halogen-for-Halogen Substitutions in the Gas Phase Kar-Chun To, A. P. Wolf;
and E. P. Rack'
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, and Department of Chemistry, University of Nebraska, Lincoin, Nebraska 68588-0304 (Received: November 22, 1982)
The stereochemistry of translationally energetic fluorine-for-halogenand chlorine-for-halogensubstitution was studied in gaseous 2(S)-and 2(R)-halopropionyl halides. While net inversion of configuration was observed for halogen using 34mClas the displacing agent, predominant retention of configuration was found when 18F was used as the displacing agent on the chiral centers of the 2-halopropionyl halides. The extent of inversion or retention in these energetic substitution reactions appears to be sensitive to the mass of the incoming atom, to steric hindrance to back-side attack, and to the nature of the halogen leaving group.
Resolution of this point can provide insight into the dynamics of hot-atom reactions. All recoil tritium substitution reactions'-' regardless of substrate molecules studies, or phase, occur with predominant retention of configuration. On the other hand, recoil fluorine,8c h l ~ r i n e ? ~bromine,11J2 J~ and iodine12substitu-
tion reactions in the gas phase with substrate molecules containing two asymmetric centers occur mainly with retention of configuration but can in certain cases show appreciable yields of the inverted product. In the condensed phase'+l8 the results are not that clear-cut, ranging from 75 7' retention of configuration for 38C1-for-chlorine substitution in neat dl-dichl~robutanel~ to around 50% for several other systems. An advance in the mechanistic aspects of these substitution reactions was made by Stocklin et al.4J3in their studies of recoil chlorine-for-chlorine substitution in liquid solutions of 2,3-dichlorobutane (DCB) and gas, highpressure, and condensed-phase 1,2-dichloro-1,2-difluoroethane (DCDFE). They suggested that halogen-forhalogen substitution may involve two reaction channels:
(1)M. Henchman and R. Wolfgang, J. Am. Chem. SOC., 81, 2991 (1961). (2)Y.N.Tang, C. T. Ting, and F. S. Rowland, J. Am. Chem. Soc., 86, 2525 (1964). (3)G. F.Palino and F. S. Rowland, J. Phys. Chem., 75, 1299 (1971). (4)H.-J. Machulla and G. Stocklin, J. Phys. Chem., 78,658 (1974). (5)J. G.Kay, R. P. Malsan, and F. S. Rowland, J . Am. Chem. SOC., 81,5050 (1959). (6)H. Keller and F. S. Rowland, J . Phys. Chem., 62, 1373 (1958). (7)Y.N.Tang, C. T. Ting, and F. S. Rowland, J. Phys. Chem., 74,675 (1976). (8)G.F. Palino and F. S. Rowland, Radiochim. Acta, 15,57 (1971). (9)F.S.Rowland, C. M. Wai, C. T. Ting, and G. Miller and 'Chemical Effects of Nuclear Tranformations", Vol. 1, International Atomic Energy Agency, Vienna, 1965,p 333. (10)C. M. Wai and F. S. Rowland, J. Phys. Chem., 71, 2752 (1967).
(11) C. M. Wai, C. T. Ting, and F. S. Rowland, J. Am. Chem. SOC., 86, 2525 (1964). (12)S. M. Daniel, H. J. Ache, and G. Stocklin, J . Phys. Chem., 78, 1043 (1974). (13)L.Vasaros, H.-J. Machulla, and G. Stocklin, J . Phys. Chem., 76, 501 (1972). (14)(a) A. P. Wolf, E. P. Rack, and R. R. Pettijohn, "Abstracts", 7th International Hot Atom Chemistry Symposium, Julich, West Germany, 1973;(b) A. P. Wolf, P. Schueler, R. R. Pettijohn, K.-C. To, and E. P. Rack, J . Phys. Chem., 83, 1237 (1979). (15)J. Wu and H. J. Ache, J. Am. Chem. SOC.,99, 6021 (1977). (16)T. R. Acciani, Y. Y. Su, H. J. Ache, and E. P. Rack, J . Phys. Chem., 82, 975 (1978). (17)Y.Y.Su and H. J. Ache, J . Phys. Chem., 80,659 (1976). (18)J. Wu, T. E. Boothe, and H. J. Ache, J . Chem. Phys., 68,5285 (1978).
Introduction One of the mechanistically important questions in recoil atom chemistry concerns retention vs. inversion of configuration in homolytic substitution (SHH2) of recoil atoms a t sp3-hybridized carbon in the gas phase. Xi*
+ RXj
X, for X i ___+
RXi* + X j
0022-3654/83/2087-4929$0 1.50/0
0 1983 American Chemical Society
