742
J. Phys. Chem. 1803, 87,742-744
The n-GaAs/Electrolyte Interface: Evldence for Specificity in Lattice Ion-Electrolyte Interactions, Dependence of Interfacial Potential Drops on Crystal Plane Orientation to the Electrolyte, and Implications for Solar Energy Conversion K. RaJeshwar"and 1.Mraz Department of Electrlcal Engineering, Colorado State Unlversiv, Fort Collins, Colorado 80523 (Received: August 30, 1982)
Mott-Schottky measurements on n-GaAs electrodes in contact with aqueous electrolytes ranging in pH from -2 to -12 reveal flat-band potentials (VFB) which are dependent on the orientation of the crystallographic planes exposed to the electrolyte. While the VFBvalues shift to more negative potentials at the rate of -55 mV/pH for the (111A) and (111B) planes (the latter case being consistent with the findings of previous authors), the (100)orientation reveals a dV,/d(pH) value of only -15 mV. These data are discussed in terms of a similar orientation-dependent behavior previously observed by us for the n-GaAs/room temperature molten salt electrolyte interface. Implications of these data in tailoring the interface to optimum performance for solar energy conversion are finally discussed.
Introduction The importance of the semiconductor/electrolyte interface in controlling the behavior of photoelectrochemical (PEC) systems is well recognized.' The extent of potential drop and the nature of charge distribution at the interface play a crucial role in charge-transfer dynamics and thereby the ultimate performance of the corresponding devicea2v3 Processes which contribute to these interface effects are mainly those which cause some modifications of the semiconductor surface upon contact with the electrolyte. These include specific ion adsorption and surface compound formation. The precise nature of the aforementioned modifications is poorly understood at present. In the course of our continuing research on the role of interfacial phenomena in PEC sy~tems,2-~ we have observed a variation of the flat-band potential (Vm)6 with electrolyte composition for n-GaAs electrodes which is intriguing in that the extent of this variation is dependent on the orientation of the semiconductor crystallographic planes with respect to the electrolyte. We are prompted to report our preliminary findings primarily because such an orientation-dependent variation in V,, appears to have received only scant attention in previous studies on semiconductor/electrolyte interfaces in general.' Furthermore, as we shall attempt to show later, this specificity in the nature of interfacial effects may have important consequences for the use of PEC systems in solar energy conversion schemes. Experimental Section Wafers of single-crystal n-GaAs (Te doped) were obtained from commercial sources. Donor densities (ND)for these materials ranged from 10l6to 1017~ m - Three ~. orientations, namely, (111 A), (111 B), and (100) were employed for the measurements to be described below. The wafers were degreased with xylene followed by a chemomechanical polish with 1% Br,/MeOH. As is well-known, only the (111B) and (100) surfaces yield a shiny metallic finish by this treatment. Electrode fabrication and ohmic contacting followed procedures described previ~usly.~ Before measurements, each electrode was etched in 10% HC1 for 15 min to remove native oxide layers. This was followed by a 15-s etch in 3:l:l H2S04/H202/H20 followed by a deionized water rinse and a final etch in 6 M HCl. After a final rinse in deionized water and EtOH, the
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0022-365418312087-0742$0 1.50/0
electrode was dried in vacuo and stored under N2. Mott-Schottky measurements'' were performed by an automated admittance spectroscopy technique described el~ewhere.~JlThis technique enables estimates of V,, which are free from the complicating effects of frequency dispersion. A conventional three-electrode cell geometry was employed in all cases. A Pt foil was employed as the counterelectrode. The geometric area of this electrode was always at least ten times larger than that of the working electrode (-0.1-0.5 cm2). A saturated calomel electrode (SCE) was used as the reference. Electrolyte preparation in all cases involved the use of Analar grade chemicals and deionized (18MQ)water. The following buffers were used: 0.09 M KCl + 0.01 M HC1 (PH ~ 2 )1;M KCl + 0.05 M CH&OOH 0.05 M CH3COONa (pH ~ 4 ) 1; M KC1 + 0.1 M NaB40, (pH ~ 9 and ) 1 M KC1 + 0.01 M KOH (pH ~ 1 2 ) . All measurements were performd in the dark and under a positive pressure of ultrapure Ar.
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Results Figure 1 illustrates the dependence of Vm on pH for the n-GaAs/aqueous electrolyte interface. In Figure la, the data are shown for the (111) orientation for the cases where the Ga-terminated (111A) planes and the As-terminated (111B) surface are exposed to the electrolyte. For (1) See, for example, 'Photoelectrochemical Cells: Fundamental Processes and Measurement Techniques", W. Wallace, A. Nozik, S. Deb, and R. Wilson, Ed., Electrochemical Society Softbound Proceedings Series, Princeton, NJ, 1982. (2)P.Singh, R. Singh, R. Gale, K. Rajeshwar, and J. DuBow, J . Appl. ~ h y s .51,6286 , (1980). (3)K.Rajeshwar, J . Electrochem. Soc., 129,1003 (1982). (4)R. Gale, P.Smith, P. Singh, K. Rajeshwar, and J. DuBow, ACS Symp. Ser., No. 146, 343 (1981). (5) P.Singh and K. Rajeshwar, J.Electrochem. SOC.,128,1724(1981). (6)This parameter is a useful indicator of interfacial phenomena because it includes the contribution of potential drops across the semiconductor/electrolyte interface. (7) To our knowledge, the only published studies broadly relevant to the question at hand are those of Brattain and Boddf and Harvey et al.,9 who report distinct differences in the nature of iodide interaction with the (111)Ge surface relative to the (100)and (110)cases. (8)W.H. Brattain and P. J. Boddy, Surf. Sci., 4, 18 (1966). (9)W.W.Harvey, W. J. LaFleur, and H. C. Gatos, J . Electrochem. SOC..109.155 (1962). ,( l O ) ~ ( a ) NT F. Mott, h o c . R. S O ~London, . Ser. A , 171,27 (1939):(b) W. Schottky, 2. Phys., 113, 367 (1939). (11)P. Smith, T. Mraz, J. DuBow, and K. Rajeshwar in ref 1, p 444. ~~~
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0 1983 American Chemical Society
The Journal of Physical Chemistty, Vol. 87, No. 5, 1983 743
2.3kT E = constant - -pH 4
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3
5
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I
7
9
i
II
PH
Flgure 1. Dependence of the flat-band potentlal, V m on pH for n-GaAs (1 11) planes (a) and (100) surface (b) exposed to the electrolyte. I n the case of (I 11 B), data (e)are compared with literature values taken for the (111 from ref 10 (A).The donor densities were 10'' ~ (111 B) case. A) and (100) cases and 10'' ~ m for- the
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the latter case, our measurements are compared with those reported by Laflere et These two sets of values agree within limits of experimental error (denoted by error bars in the figure) yielding a slope of - 4 5 mV/pH. The (111 A) planes yield essentially the same behavior although the Vm value at each pH lies well positive of the corresponding value for the (111B) sample. This difference cannot be rationalized on the basis of the different ND values in the two cases since the shift ( - 2 0 0 mV) is too large to be accounted for by the relatively small difference in ND (-lOle cm-3 for (111A) vs. -1017 cm-3 for (111 B)). Figure l b shows the dependence of VFB on pH for the (100) plane exposed to the electrolyte. The magnitude of the slope (-15 mV/pH) is significantly smaller than the cases shown in Figure la.
Discussion For oxide and certain elemental semiconductors (e.g., Ge), a shift in VFB with pH has been explained in terms of equilibria involving "surface hydroxides", cf. eq l.13 In S-OH + H2O s S-0- + H30' (1)
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eq 1, S denotes a semiconductor lattice atom. Thermodynamic arguments based on dissociation of surface hydroxyl groups lead to a dependence of electrode potential, E, on pH given by eq 2. (12) W. H. Laflere, F. Cardon, and W. P. Gomes, Surf. Sci., 41, 541 (1974). (13) H. Gerischer in "Physical Chemistry-An Advanced Treatise", Vol. IXA, H. Eyring, Ed., Academic Press, New York, 1970, Chapter 5, p 463. See also references therein.
At T = 25 "C, the slope of Vm (or E ) vs. pH plots should therefore correspond to -58 mV according to this model. Agreement with this model has been observed for nZnO/aqueous electr01yte.l~n-Ti02/aqueous electrolyte,12 and Gelaqueous ele~trolyte'~ interfaces where a change in VFB of -59 mV/pH has been observed in accordance with eq 2. Our studies on the latter interface reveal a somewhat smaller slope (-55 mV/pH),16 comparable to that observed here and by previous a u t h o d 2 for the nGaAs (111) case (cf. Figure la). Hoffman-Perez and Gerischer17 also report a variation in VFBof only -50 to -56 mV/pH for the Ge/electrolyte interface. It is not clear whether these differences can be attributed to experimental errors in the determination of Vm Precedence for dVFB/d(pH) values signifhmtly lower than the theoretically predicted value is found for the n-MoS2/aqueous electrolyte and n-CdSe/aqueous electrolyte case^.^^^^^ Changes in VFB of 30 mV18 and 15 mV19 per pH unit, respectively, have been recorded for these systems. In the former case, the observed dVFB/d(pH) value has been rationalized on the basis that two elementary charges are involved in the dissociation step. Notwithstanding the mechanism(s)which are operative in these cases, the point to emphasize here is that dVFB/d(pH) values much lower than -60 mV are indeed possible and this is precisely the behavior we observe for the n-GaAs (100) case (cf. Figure lb). The simplest explanation for the observed trends for the (111)and (100) surfaces is the differing nature of the potential distribution across the inner Helmholtz layer in the two cases. The dipole contribution to this potential distribution is expected to be altered when both Ga and As atoms are present at the surface relative to the case ((111) planes) when only As or Ga is present. The manner in which surface OH groups interact with each other (thereby affecting their ionization) should also be dependent on surface orientation. While more speculation is of little value at this stage, that there exists some specificity in the interaction of OH- groups with Ga and As surface atoms is in itself noteworthy in our opinion. One final point regarding the sensitivity of VFB values to crystal orientation needs to be noted. In our previous work on the n-GaAs/room temperature molten salt electrolyte interface, we have observed a similar trend for the (100) and (111B) orientation^.^^^ The model electrolyte system that we employed for these studies was the AlCl3-n-butylpyridinium chloride (BPC) mixture, where a pC1 value can be defined at -log,, acl analogous to the pOH parameter for aqueous electrolytes. A dVFB/d(pC1) value of -130 mV was observed for the (111B) case,2 whereas the (100) plane showed a corresponding variation of only -65 mV.4 (These values correspond to ca. 2( 2 . 3 k T / q ) and 2 . 3 k T / q , respectively, for T = 40 "C.) The precise nature of the interaction of n-GaAs surface with C1- ions, however, may not be entirely comparable with the present situation involving hydroxyl groups.20 (14) M. Tomkiewicz, J. Electrochem. Soc., 126, 1505 (1979). (15) P. J. Boddy and W. H. Brattain, J . Electrochem. SOC.,110, 570 (1963). (16) R. J. Gale and K. Rajeshwar, unpublished data. (17) M. Hofmann-Perez and H. Gerischer, 2.Electrokem., 65, 771 (1961). (18) S. M. Ahmed, Electrochim. Acta, 27, 707 (1982). (19) K. W. Frese, Jr., J. Appl. Phys., 53, 1571 (1982). (20) Note that the Esin-Markov effect has been invoked to account for dVFB/d(pCl) values greater than 2.3KT/q, in the former case (see ref 2).
744
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983 0 ) Equilibrium
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Electrolyte lnterf oce
Semiconductw
Electrolyte Semiconductor Interface
AV
C
D AVc # AVO
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h ”
”h
Figwe 2. Schematic of the interfacial energetics at equilibrium (a) and nonequiiibrium (b) Conditions for model systems. See text for description. EF is the Femvl level at equilibrium and €Ffl is the quasi-Fermi level for maJoritycarriers (electrons in this case). E,, is the redox potential. It is assumed that negllglble counterelectrode polarization obtains under operating conditions. An n-type semiconductor is assumed in all the cases for specificity.
Rajeshwar and Mraz
face interaction with the electrolyte species. In Scheme A, negatively charged groups accumulate at the surface, thereby inducing a large “band bending” at the surface. In the extreme case, an inversion layer may be set up at the semiconductor s u r f a ~ e . ~Charge ~ . ~ ~ accumulation and, correspondingly, the degree of band bending is less for the situation shown in Scheme B. Scheme A as illustrated corresponds to a situation at high pH values in the electrolyte whereas Scheme B would apply to an acidic electrolyte. Figure 2b shows the situation under nonequilibrium conditions for the two cases where the interfacial potential drops (AV) are different. It is clear that the interface which affords a greater degree of negative charge accumulation (Scheme D)also facilitates a higher photovoltage Vph. Consistent with this expectation and with the data presented in Figure 1, we have observed higher photovoltages for PEC systems based on n - G d s electrodes with the (111B) plane exposed to the electrolyte relative to the (100) orientation.22 The AlC1,-BPC mixture was used as the electrolyte in this study, although essentially similar considerations should apply to the aqueous electrolyte case. The key result emerging from this study is that it should be possible in principle to “tune” a given semiconductor/electrolyte interface to optimum performance by varying the orientation of crystal planes exposed to the electrolyte. Similar orientation effects, albeit of entirely differently origin, are well-known for layer chalcogenide^.^^ The data presented here suggest that such effects may be common to other (nonlayered) semiconductors also.25 Further experimental verification in terms of implications for solar energy conversion is, however, undoubtedly necessary.
Implications for Solar Energy Conversion. As mentioned in a preceding paragraph, the nature of charge distribution and the potential drop at the semiconducAcknowledgment. It is a pleasure to acknowledge distorlelectrolyte interface have a profound influence on PEC cussions with Professor K. A. Jones during the course of performance. The case where redox couples interact with this work. the semiconductor surface (e.g., by specific ion adsorption) Registry No. Gallium arsenide, 1303-00-0. was discussed in previous papers from this l a b o r a t ~ r y . ~ ~ ~ The following discussion21is more general in that species (22)P.Singh, R.Singh, K. Rajeshwar, and J. DuBow, J. Electrochem. from the background electrolyte (e.g., OH- groups) are also SOC..128. 1145 (1981). included. Figure 2a illustrates the equilibrium (dark) (23) K.Rajeshwar; P. Singh, and J. DuBow, J . Solar Energy Eng. situation for two cases involving different degrees of sur(Trans. ASME), 104, 133 (1982). ~~
(2j) An n-type semiconductoris assumed for specificity in the following discussion.
(24) See for, e.&, 24, 705 (1979).
S. M. Ahmed and H. Gerischer, Electrochim. Acta,
(25) Experimenta in progress on the n-InP/AIC&-BPC interface reveal orientation-dependenteffects similar to the n-GaAs case.
