J. Phys. Chem. 1992, 96, 8692-8694
8692
A chemisorption of nitrogen on ammonia synthesis catalysts such as Fe, W, and C O . ~The reaction equilibrium in ammonia synthesis is maintained by virtue Of the Of the to adsorb nitrogen dissociatively (type A adsorption) and to decompose ammonia into its constituent elements. It is generally known that the catalysts which d a m p o s e ammonia into constituent elements at a particular temperature can easily adsorb nitrogen dissociatively at the same temperature. Since ZSM-5 zeolite does not decompose ammonia into its constituent elements even at higher temperatures, the complete dissociative adsorption of nitrogen on ZSM-5 is ruled out.
References and Notes (1) See,for example: Anderson, J. R.; Foger, K.; Mole, T.; Rajadhyaksha, R, A.; Sanders, J. V, J , Coral. 1979, 58, 114. V&ine, J. C,; Auroux, A,; &lis, V.;bjaifve, P.;Naccache, C.; Wierzchowski, P.;Deroune, E. G.;Nagy, J . B.;Gilson, J . P.; van Hooff, J. H. C.; van den Berg, J. P.; Wolthuizen, J. J . Cural. 1979,59,249. Nayak, V. S.;Choudhary, V. R. J . Catal. 1983,81, 26. Choudhary, V. R.; Nayak, V. S. Appl. Catal., 1982,4, 31. (2) Nayak, V . S.; Riekert, L. Acta Phys. Chem. 1985, 31, 157. (3) Nayak, V . S.;Riekert, L. Appl. Catal. 1986, 23, 403. (4) Kokotailo, G.T.;Lawton, S. W.; Olson,D. H.; Meier, W. M. Nature 19’78, 272. ( 5 ) Bond, G . C. In Catalysis by Metals; Academic Press: London, 1962.
An in Situ Characterization of the p-InP-In/Electrolyte Contact Formed by Cathodic Corrosion of the Semiconductor J. Schefold Universitat Stuttgart, Institut fur Physikalische Elektronik, Pfaffenwaldring 47, D- 7000 Stuttgart 80, FRG (Received: July 2, 1992; In Final Form: August 19, 1992)
Hydrogen evolution at illuminated pInP/electrolyte contacts is accompanied by a corrosion reaction leading to indium formation at the semiconductor surface. After this process the electrode response is analyzed in 1 M H$04 using current-voltage curves and Mott-Schottky as well as impedance spectroscopic measurements. Mott-Schottky data indicate a shift of the flat-band potential due to In formation whereas barrier heights at the semiconductor surface remain almost constant (t#,B = 1 eV). Impedance data under photocurrent flow reveal a nearly ideal Schottky contact after separation of the electrochemical charge transfer at the In surface.
Introduction The p-InP/electrolyte contact has been the subject of many studies in semiconductor photoelectrochemistry, mainly due to an efficient solar hydrogen generation at pInP-Pt (e.g., refs 1-3). It is known that H2evolution at the bare electrode is accompanied by a corrosion process in acid solution:’” InP 3H+ 3eIn PH3. Previous works7+’ showed that the electrode response can be described by the model of a Schottky contact in series with the electrochemical charge-transfer reaction, both with and without Pt coating. Indium formation at the surface was expected to lead to a high-quality Schottky barrier (Le., without an intermediate oxide layer) similar to barriers obtained by ultrahigh-acuum deposition techniques. Experimental evidence for such a contact is given in the following, including impedance data with a remarkably low frequency dispersion.
+
+
+
-
Experimental Section The p-InP single crystals ((loo), Zn-doped, doping concentration =l X lOI7 cm-), lEvB- EFI 120 meV) were purchased from MCP, G.B. Measurements were taken in a single-compartment quartz cell under potentiostatic control (PAR 273) and N2or H2 purging. Either a tungsten halogen lamp or a red light emitting diode served as light sources. Impedance data were taken using a Solartron 1255 frequency response analyzer. Further experimental details have been given el~ewhere.~-~ Results and Discussion Figure 1 shows the current-voltage curve for a hydrogen evolving p I n P electrode under illumination. The shift of the i / U curve toward negative potentials, the anodic current peak after cathodic prepolarization, and an observed decrease of the saturation photocurrent iphfor higher illumination levels (liphi> 1-2 mA/cm2) are consistent with indium formation at the semiconductor surface. After cathodic prepolarization of the illuminated electrode, the i / U curves’ intersection with the potential axis (point
A in Figure 1) varied from -0.2 to -0.4 V/SCE. This variation depended on the amount of In formed at the surface. As discussed in ref 7, the poor electrocatalytic properties of this surface for the hydrogen evolution reaction (exchange current density io= IO-’ A/cm2 for metallic Inlo) give rise to a large overpotential 1 under photocurrent flow and thereby to a low solar-to-chemical energy conversion efficiency. The i / U curve of an electrochemically Pt-coated (not corroding surface) electrode is added for comparison (1= 0’). After indium formation, the flat-band potential U, derived from Mott-Schottky (MS) measurements is shifted about 200 mV to more negative potentials (Figure 2). The MS measurement range is limited to U I4 3 V/SCE since at more positive potentials the In layer would immediately be oxidized (cf. Figure 1). After correction for the electrolyte series resistance, capacitance data used for the U, evaluation are characterized by an average phase angle of -(89.5-89.7)’ in the range from 10 Hz to 100 kHz, corresponding to an extremely small capacitance change of -l%/decade. The frequency dependence of the extrapolated flat-band potential is only +7 mV per frequency decade. Data indicate an excellent bulk quality of the semiconductor, a negligible influence of surface roughness on the capacitance measurements, and a uniform distribution of the barrier heights 4Bacross the surface. Under open-circuit conditions, the potential of the In layer is defined by a mixed potential due to reduction of H+ and oxidation of In (standard potentials V(In+/In) = -0.38 V and V(In3+/In) = -0.59 V/SCEI1). The potential is, therefore, more negative than V(H+/H,) which is the relevant redox potential of a not corroding H 2 evolving electrode. (Measured dark open-circuit potentials varied from -0.2 to +0.05 V/SCE.) Since both redox and flat-band potential shift toward negative values, the barrier height +B = q(U, - Urcdox) + ~EVB - EFI does not change significantly = 1-1.2 eV). This is in agreement with generally large barrier heights at solid-state p-InP Schottky contacts (dB
0022-3654/92/2096-8692!$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 8693
Letters
TABLE I: Stationary Parameters and Fitting Results for the Impedance Data of Figure 4"
Figure 4a 4b 4c
stationary parameters i,, (pA/cm2) i (pA/cm2) U (mV/SCE) -880 -160 -350 -400 -528 -432 -152 -848 -480
R, (S2.cm2) 34 f 0.8% 56 f 0.5% 205 0.3%
*
fitting elements R,,(S2cm2) C, (pF/cm2) 200 f 1% 0.138 f 1.6% 83 f 0.6% 0.137 f 1% 41 f 3% 0.133 f 0.4%
Current density variation during the measurements is S*2%. The slightly different values of iph= (i and reflection at the growing indium layer.
H
A'
Ai /
n
CHH(pF/cm2)
+ -1.0 (30 mV/s). Dark reverse currents are lidll < 20 nA/cm2. The sample was prepolarized at -1.3 V/SCE for some minutes before the scan start. A' denotes the open-circuit potential in the dark and A under illumination (after cathodic current flow). The dotted curve refers to the Pt-coated contact (12 mC/cm2 equivalent to =12 monolayers of Pt). Results are identical for N2 and H2 purging.
U ,,
(p-lnP)
1.15
1.14 1.20
+ irs) are due to changes of light absorption
I/
$.-l
Figure 1. Current-voltage curves for illuminated p-InP in 1 M H2S04
38 f 1% 31 f 1% 33 f 6%
diode ideality factor n (eq 2)
--
'
electrolyte
I=///
F i e 3. Approximate band diagram of the pInP-In/electrolyte contact at pH = 0 (dark). U,(p-InP) denotes the flat-band potential value before In formation.
current density (
