J. Phys. Chem. 1981, 85,1679-1682
figuration space, which would favor forming only the stable isomer. It has been suggested15 that this is not the case in proton transfer from H3+to CO, but that both HCO+ and the metastable HOC+ can be formed. There appears to be no evidence (either theoretical or experimental) which bears on this auestion. This is unfortunate since the astrophysical qukstion which prompted this study (15)Herbst, E.;Norbeck, J. M.; Certain, P. R.; Klemperer, w. Astrophys. J. 1976,207,110.
1679
depends on whether proton transfer reactions produce all energetically accessible metastable isomers or only the most stable product. In the former case, HOCN is expected to be produced in interstellar space, but not in the latter. Further study of branching ratios in such reactions is reauired. Acknowledgment. I thank Dr. Charles A. Wight, Professor F*Wm. Chickering, and an anonymous referee for helpful comments.
Behavior of Some n-Type Unmodified and Chemically Modified Metal Oxide Electrodes in Nonaqueous Solvents Kablr-ud-Din, Robert C. Owen, and Marye Anne Fox* Department of Chemistty, The Universiv of Texas at Austin, Austin, Texas 78712 (Receivd: October 15, 1980; In Final Form: December 3 1, 1980)
Flat-band positions of several n-type semiconductor electrodes (single-crystal and polycrystalline TiOz,polycrystalline SnOz, and chemically modified SnOz) have been measured by determining capacitance-voltage dependence or the onset of photopotential in five nonaqueous solvents. The effects which give rise to deviations from ideality in Schottky-Mott plots apparently cause significant differences in flat-band potentials measured by these two techniques. For metal oxide semiconductor electrodes, interface energetics are not dramatically altered by derivatization.
acetonitrile, of p-GaAs'O and n-Ti0;' in liquid ammonia, The importance of characterizing the band positions in semiconductor electrodes and of understanding the role and of n-GelZin Nfl-dimethylformamide have been made. of surface states or intermediate levels in the band-gap The band positions of derivatized n-GaAs,13n-Si,14n-Ge,16 region has been recognized for some time. Such data are and n-SnOZl6have also been determined. In no case, critical in devising photosensitive electrodes for solar enhowever, has a comprehensive study of flat-band position ergy applications since these positions establish external been conducted in a series of solvents. bias requirements and the maximum attainable photoWe have surveyed band positions of some naked and voltage in liquid junction photoelectrochemical cells.' derivatized metal oxide electrodes in several nonaqueous solvents to determine whether band positions determined Since the photoelectrochemical production of hydrogen from water has been such a widely accepted goal, most in water can be related in a direct way to those observed such studies have been concerned with properties of semin nonaqueous media and whether deviations from ideal iconductors in contact with aqueous electrolytes and redox behavior (i.e., that predicted from semiconductor band systems. Here, the effect of pH of the medium on the theory) were more or less severe in the absence of protic conduction band Dosition of metal oxide semiconductors surface eauilibria. has been clearly described, the pH-induced shifts having Section been attributed to protonation of surface oxide groups.2 Electrodes. The single-crystal TiOa electrodes used in Less information is available regarding band positions this Study were cut from a boule containing 0.05% A1 of semiconductors in nonaqueous solvents. Because such (National Lead CO.). The exposed (001) surface Was media offer a wider stability range for electrochemical Polished (0.3-pm A1203 final Polish), oxidized 60 min at 900 studies, easier purification (so that misleading adsorption "C in Oz, reduced 30 min at 500 "C in Hz, and cleaned for impurity effects can be minimized), and an environment 30 S in ConCentrated at 55 "c. The PolYcrYstalline better suited for electrochemical studies of organic redox TiOz dectrodes were Prepared by chemical vapor depossystems, a number of studies of flat-band positions for various semiconductors in a sinale nonaaueous solvent are available. For example, studyes of n-TiOz,334I I - Z ~ O , ~ ~ ~ (7)Kohl, P.A.;Bard, A. J. J. Electrochem. SOC.1979,126,598. (8)Kohl, P.A.;Bard, A. J. J. Electrochem. SOC.1979,126,59. n-CdS,3i5*6 n- and p-InP,' n- and p-GaAs,8 n-and p-Sig in (9)Laser, D.; Bard, A. J. J . Phys. Chem. 1976,80,459.
(10)Malpas, R.E.;Itaya, K.; Bard, A. J. J. Am. Chem. SOC.1979,101,
3.535 -I--.
(1)For a review, see: Nozik, A. J. Annu. Reu. Phys. Chem. 1978,29, 189. (2)Watanabe, T.;Fujishima, A.; Honda, K. Chem. Lett. 1974,897. (3)Nakatani, K.;Tsubomura, H. Bull Chem. SOC.Jpn. 1977,50,783. (4)Frank, S. N.; Bard, A. J. J . Am. Chem. SOC.1975,97,7427. (5)Kohl, P. A.;Bard, A. J. J . Am. Chem. SOC.1977,99,7531. (6)Landsberg, R.;Janietz, P.; Dehmlow, R. 2.Phys. Chem. (Leipzig) 1976,257,657. 0022-3654/81/2085-1679$01.25/0
(11)Fox, M.A,; Kabir-ud-Din J. Phys. Chem. 1979,83,1800. (12)Krotova, M. D.; Pleskov, Yu. V. Solid State Phys. 1963,3,2119. (13)Bolts, J. M.;Wrighton, M. S.J. Am. Chem. SOC.1979,101,6179. (1.4)Bolts, J. M.; Bocarsly, A. B.; Palazzotto, M. C.; Walton, E. G.; Lewis, N. S.;Wrighton, M. S.J. Am. Chem. SOC.1979,101, 1378. (15)Bolts, J. M.; Wrighton, M. S.J . Am. Chem. SOC.1978,100,5257. (16)Fox, M.A.; Nobs, F. J.; Voynick, T. A. J . Am. Chem. SOC.1980. 102,4029.
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 12, 198 1
ition on titanium meta1,l’ and the polycrystalline n-Sn02 (NESA) glass electrodes were obtained from PPG Industries. The polycrystalline electrodes were washed successively with acid, base, and distilled water before being dried overnight a t 150 “C under vacuum. The modified tin oxide electrodes were prepared and pretreated as described previously.16 Copper wire leads were attached to the semiconductor either with alligator clips or with silver conductive paint (E-Kote 3030, Acme Chemicals and Insulation, New Haven, CT). In the latter case, the wire was threaded through a glass tube for support, and the electrode and the wire were mounted with epoxy resin (Devcon Corp., Denver, CO) so that all of the silver paint surfaces were covered with epoxy. This reinforced electrode was then covered on the back and the sides with silicone rubber. Before each experiment, the naked surfaces were exposed to an HF etching solutionlaand treated by a previously described procedure.8 Electrochemical Cell. A three-compartment cell was used with the reference and auxiliary compartments being separated from the main body of the cell by medium-porosity frits. The working compartment had an optically flat Pyrex window, and the semiconductor being examined was mounted in a Teflon holder space -1 mm from the inside of the optical flat. The quasi-reference electrode was a silver wire, and the auxiliary electrode was a coiled Pt wire. Materials. Acetonitrile (MeCN) (Matheson Coleman and Bell (MCB) Spectroquality) was purified and dried by the methods described by Frank and Bard.3 N,N-Dimethylformamide (DMF) (Mallinckrodt Spectral Grade) was stirred over CaO for 24 h, distilled under vacuum from fresh CaO, and stored under argon over 9A sieves. Dimethyl sulfoxide (Me2SO), (MCB Spectroquality) was stirred over KOH for 24 h, distilled under vacuum from fresh KOH, and stored under argon on 9A sieves. Tetrahydrofuran (THF) (MCB Reagent Grade) was heated under reflux overnight with sodium benzophenone ketyl and distilled into a receiving flask containing lithium aluminum hydride. After 1 h of stirring at room temperature, the solvent was redistilled directly into the electrolysis cell. Ammonia (NH,) was distilled into a trap containing sodium metal before redistillation into the electrolysis cell was allowed to occur. Water was triple distilled in glass from KMnO,. Polarographic-grade tetrabutylammonium perchlorate (TBAP), lithium perchlorate, or potassium iodide was dried for a period of >3 days a t 150 “C. A cyclic voltammogram at a Pt disk electrode was obtained a t the beginning and the end of each experiment to ensure the purity of all chemicals used and to calibrate the reference electrode potential. Procedure. A PAR 173 potentiostat and a PAR 175 Universal programmer (Princeton Applied Research Corp.) were used to obtain cyclic voltammograms. Positive feedback was used to compensate for solution resistance and the internal resistance of the semiconductors. The i-V curves were recorded on a Houston Instruments x-y recorder. Polychromatic light from a 250-W high-pressure mercury arc, quartz focused (Applied Photophysics Ltd., London), was used as a light source. Capacitance measurements were made by calculation from the charging currents measured in the cyclic voltammograms at various scan rates (20-1000 mV/s). For wavelength-dependent studies, the light from a 1000-Whigh-pressure mercury arc (Hanovia) was allowed to pass through a Bausch and
Kabir-ud-Din et ai.
Laumb refraction monochromator with bandwidth of nm.
-
10
Results and Discussion Flat-band potentials ( Vfi) can be determined for semiconductors by several methods. Among these are the following: (1) characterization of the change in spacecharge capacitance produced by the production of a depletion layer at the surface of a semiconductor; l9 (2) determination of the potentials for the onset of photoanodic current in irradiated n-type semiconductors; (3) differential stress measurements with attached piezoelectric detectors; 2o (4) observation of voltammetric response in the dark of redox couples spanning a potential range above and below Vfi; and (5) theoretical predictions based upon the electronegativities of the constituent atoms and the zero { p ~ t e n t i a l . ~ lIn - ~this ~ study, we have employed the first two methods to experimentally determine Vfi at single-crystal Ti02 (I),polycrystalline Ti02 (2), polycrystalline SnOz (3), and derivatized electrodes 4 and 516 in 395
’I
4
c‘
I
5
nonaqueous solvents of varying dielectric constant containing 0.1 M inert electrolyte. The first method involves measuring differential capacitance as a function of applied voltage and is based on the Schottky-Mott equation
1/C,: = 2(A&
-
kT/e)/(ttoeND)
(1)
where C,, is the space-charge capacitance, t the dielectric constant of the semiconductor, to the permittivity of free space, NDthe dopant density, and A the potential difference (in eV) between the flatbanrpotential and the potential at which the measurement is made. Thus an ideal plot of 1/cZ vs. V should be linear with an x intercept corresponding to V,. Previous studies have shown that sometimes significant frequency dependence and/or deviation from linearity is encountered. This has been attributed to the effect of surface states, recombination effects, inhomogeneous doping, insufficient etching, or to nonnegligible contributions of the Helmholtz layer to the interfacial capacitance. Cases are known in which frequency dispersion of capacitance measurements leads to either convergent or nonconvergent x intercepts in Schottky-Mott plots.24 In the second method, the determination of open-circuit photopotential, these same effects may yield onset photopotentials less negative than the actual Vfi value. The results that we have obtained are summarized in Tables I and 11. Typical Schottky-Mott plots from which (19) Gerischer, H. In “Physical Chemistry: An Advanced Treatise”; Eyring, H., Henderson, D., Jost, W., Eds.; Academic Press: New York, 1970: Vol. IXA. (20) Handley, L. J.; Bard, A. J. J. Electrochem. SOC.1980, 127, 338. (21) Butler, M. A.; Ginley, D. S. In “Semiconductor Liquid-Junction Solar Cells”; Heller, A., Ed.; Electrochemical Society: Princeton, NJ, 1977; p 290. (22) Gomes, W.P.; Cardon, W. F. In “Semiconductor Liquid-Junction Solar Cells”; Heller, A., Ed.; Electrochemical Society: Princeton, NJ, 1977; p 120. (23) McCaldin, J. 0.;McGill, T. C. In “Thin Film Semiconductors and Interfacial Reactions”; Poate, J., Ed.; Wiley-Interscience: New York, 1977. ~. . ..
(17) Hardee, K.L.;Bard, A. J. J. Electrochem. SOC.1975, 122, 739. (18) Fujishima, A,; Honda, K. Bull. Chern. SOC.Jpn. 1971, 44, 1148.
(24) Dutoit, E. C.; Cordon, F.; Gomes, W. P. Ber. Bunsenges. Phys. Chem. 1976,80,475.
The Journal of Physical Chemistry, Vol. 85, No. 12, 1981 1681
Behavior of Some n-Type Metal Oxide Electrodes
TABLE I:
Flat-Band Potentials for Several Semiconductors in Nonaqueous Solvents Determined by Methods A and Ba Vfh, eV (k0.2 eV vs. SCE) dielectric constant
solvent THF 3 "
MeCN DMF Me ,SO H,O (PH 7)
polycrystalline TiO, ( 2 )
single-crystal TiO, ( 1 )
A
B
7 27
-1.00 (0.05)b -1.20 (0.10)
36 37 49
-1.30 -0.95 -1.10 -0.65
-0.90 -1.15 -0.80 -0.80 - 0.80
78
(0.30) (0.50) (0.40) (0.10)
B
A
-0.70
(0.35) (0.35)
-0.80 -0.85 -0.75 -0.80
(0.20)
-0.80
(0.10)
-0.50
-0.95 (O.lO)b
-1.00 (0.10) -1.50 -0.90 - 0.90 -0.45
polycrystalline SnO, ( 3 )
A
B
-0.80 (0.15)b -1.05 (0.10)
-0.50 -0.95
-0.70 (0.20)
-0.40 - 0.40
-0.75 (0.30) -0.80 (0.30) -0.40 (0.25)
-0.50
-0.40
Method A: x intercept of least-squares best fit of Schottky-Mott plot (C,,-* vs. V). Method B: onset of photopotential (irradiation source : unfiltered output of a 200-W high-pressure mercury point source). Observed photooxidation : adIn method A, the value presented is an average for data ventitious or added (10 - 3 M) water or M hydroquinone. taken at six frequencies in three independent trials. The values in parentheses represent the spread of frequency dependence (range of x intercepts for scan rates of 20-1000 mV/s). a
TABLE 11: Effect of Surface Modification on Flat-Band Potentials for Modified Tin Oxide Electrodes in Acetonitrile
Vfb, eV (i 0.2eV vs. SCE) method: capacitancea onset of photopotentialb
3
4
5
-0.70 (0.20) - 0.40
-0.65 (0.15) -0.45
-0.65 (0.25) - 0.50
a Capacitance : Schottky-Mott x intercept (average value of least-squares best fit). The values in parentheses represent Irradiation source: unfiltered output of a 200-W high-pressure mercury point source. frequency dispersion (see Table I). I
-IO
e
12
Pot.nt,ol
01
lev
"I
06
SCE 1
Flgure 1. Schottky-Mott plots of capacitance vs. electrode potential 200 Hz; (0) 1000 for single-crystal n-TiO, in ammonia, 1 M KI: (0) Hz.
these data are derived are shown in Figures 1 and 2 (electrodes 1 and 3 in liquid ammonia). As can be seen, such plots are reasonably linear. Although some frequency dependence is observed, the x intercepts are reasonably convergent in THF and ammonia. In nonaqueous solvents of higher dielectric constant, a greater spread is observed. This range of x intercepts in experiments employing scan rates from 20 to 1000 mV/s are listed in parentheses behind the average Schottky-Mott flat-band potentials listed in Table I. Note that comparable frequency dispersion is observed in single-crystal and polycrystalline electrodes. We have also found that repeated HF etching cannot remove this frequency dependence. The plots shown in Figure 3 and represent the dependence of photoanodic current on applied bias. For systems with little frequency dispersion in Schottky-Mott plots, the onset of photocurrent corresponds well with the convergency of the x intercepts. For those electrodes, however, where large ranges of x intercepts are encountered, poor correspondence between flat-band potentials obtained by the two methods are generally observed. This result is consistent with previous suppositions that Schottky-Mott
0
-05
Potential
ieV
vs
03
SCE)
Figure 2. Schottky-Mott plot of capacitance vs. electrode potential for polycrystalllne (NESA) n-SnO, in ammonla, 1 M KI: (0)200Hz; (0) 1000 Hz. Potential
0I
I
( eV
VI
-0.5
SCE)
I
-1.0
Figure 3. Onset of photooxidation. (a) Oxidative current vs. potential for single-crystal n-TiO, in DMF, M hydroquinone, 1 M TBAP: (---) dark; (-) illuminated. (b) Oxidative current vs. potential for polycrystalline (NESA) n-SnO, in DMF, lo3 M hydroquinone, 1 M TBAP (- - -) dark; (-) illuminated.
J. Phys. Chem. 1981, 85. 1682-1685
1682
plots are meaningful only for systems exhibiting convergent intercepkl (In almost every case, the flat-band potential obtained by the onset of photopotential was less negative than that determined by capacitance measurements.) The fact that better correspondence of the two methods is observed in the two solvents of lowest dielectric constant is probably coincidental for close correspondence of these values is also observed in water, the medium of highest dielectric constant. No monotonic variation of linearity in Schottky-Mott plots with solvent dielectric was observed. The data in Table I1 compare band positions in acetonitrile for n-SnOzpolycrystalline electrodes, both in naked and derivatized form. In electrode 4, the attached molecule exhibits redox chemistry only at potentials more negative than V,, but, in electrode 5 , the attached molecule should be electroactive in the band-gap region. It is fairly remarkable than that the V, of these three electrodes should apparently lie so close in energy. Our observation that derivatization causes only minor shifts in band-gap position is consistent with the minor shifts upon derivatization observed by others.13-16 Our observed shifts are toward more negative potentials, however, whereas others have observed shifts to less negative potentials. The slopes of the Schottky-Mott plots from which the data in Table I1 were derived can also be used to determine donor density. If a value of 24 is used in eq 1for e (Sn02),25 we calculate donor densities on the order of 1019, the specific value varying with pretreatment. We note that, if care is not taken to ensure near monolayer coverage by the derivative,16 i.e., if extensive polymerization occurs (25)Bogoriditaki, N.;Mityureva, I.; Fridberg, I. Sou. Phys.-Solid State 1963,4,1753.
upon derivitization, significant nonlinearity and frequency dispersion is observed in the Schottky-Mott plots and that a large difference between an average n intercept and the onset of photopotential can be observed. Perhaps the most striking conclusion derivable from our data is the rough similarity of band position in solvents of widely differing physical properties and in naked and derivatized electrodes (3 vs. 4 or 5) irrespective of whether the attached molecule undergoes redox reactions at potentials inside (5) or outside (4)the band gap. Thus, the common assumption that interface energetics are not dramatically altered by derivatization seems justifiable. Caution should be exercised in refraining from overgeneralizing these conclusions though, since larger deviations from ideality and more profound solvent dependence has been observed in less stable, smaller band-gap materials. In these latter materials, Fermi level pinningz6 and/or inversion effect^'^^^^^^ may complicate observed behavior. Furthermore, quantitative interpretatation of photoelectrochemical results will be strongly affected by even the small band shifts reported here. Acknowledgment. This work was generously supported by the U.S.Department of Energy, Office of Basic Energy Science. M.A.F. acknowledges support as an Alfred P. Sloan Research Fellow, 1980-82. We thank Dr. Ronald Wilson of General Electric Research for his gift of the single-crystal TiOz electrodes used in this study. (26)Bard, A. J.; Bocarsly, A. B.; Fan, F. R. F.; Walton, E. G.; Wrighton, M. S. J. Am. Chem. SOC.1980,102, 3671. (27)Kautek, W.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1980,84, 645. (28)Turner, J. A; Manassen, J.; Nozik, A. J. Appl. Phys. Lett. 1980, 37,489.
Photophysical Properties of Saturated Amines in Slightly Polar Media. Saturated Ethers Arthur M. Halpern Department of Chemistry, Northeastern University, Boston, Massachusetts 02 1 15 (Received: October 22, 1980; In Final Form: March 3, 198 1)
The fluorescence properties of a prototype saturated amine, NJV-diethylmethylamine (DEMA),were examined in diethyl ether and in tetrahydrofuran (THF). In going from a nonpolar medium (n-hexane) to more polar media (diethyl ether and THF), the fluorescence of DEMA undergoes a substantial red shift (292-308-340nm), along with a diminution in quantum efficiency and an increase in lifetime. The shift in the fluorescence spectrum occurs continuously as the amount of THF in THF/n-hexane mixed solvents is increased. These observations are discussed within the framework of universal and specific interactions. Consideration is also taken of the change in geometry of DEMA in the ground and excited states, and some properties of a “rigid” amine, l-azabicyclo[2.2.2]octane,are compared and discussed. The question of specific complexation of the amine excited state with the solvent molecules is raised (amine/ether exciplex), and it is concluded on the basis of the properties of aminoether model compounds that a 1:l exciplex cannot account for the observed photophysical properties. It seems possible that the original view of Muto, Nakato, and Tsubomura that the ether solvent media stabilize the Rydberg or Rydberg-like excited state of the amines may account satisfactorily for the observations. Introduction The fluorescence quantum efficiencies of saturated tertiary amines in the vapor phase’P2 and in nonpolar solvents such as saturated hydrocarbons have been shown
to be quite large.3i4 For example r # ~for ~ triethylamine in n-hexane is 0.69, and this value is typical of many other simple monoamines. Because of the potential exploitation of saturated tertiary amines fluorescence as a probe or an
(1)A . M . Halpern, Chem. Phys. Lett., 6,296 (1970). (2)A. M. Halpern and T. Gartman, J. Am. Chem. SOC.,96, 1393 (1974).
(3) A. M. Halpern and A. L. Lyons, Jr., J.Am. Chem. SOC.,98,3242 (1976). (4)A. M. Halpern and D. K. Wong, Chem. Phys. Lett., 37,416(1976).
0022-3654/81/2085-1682$01.25/00 1981 American Chemical Society
