Evidence for Site-Specific Interaction of Redox Species at the FeS2/Electrolyte Interface W. Seeliger, G. L. Troughton, N. Alonso-Vante, and H. Tributsch Hahn-Meitner-lnstitut, Abt. So/are Energetik, 14109 Berlin, Germany ABSTRACT Adsorption of organic ligands [e.g., (1H-benzotriazol-l-yl)-methylisocyanide; btic, Fig. 1] and deposition of platinum on pyrite (FeS2) surfaces leads to the selective blocking of specific surface sites for electron transfer. In electrochemical experiments a drastic decrease of currents was observed with an Fe2+/3+ electrolyte, but only slightly diminished currents for an I- electrolyte. From this it is inferred that the two different redox species interact with chemically different sites on the surface of pyrite. Introduction In the course of our efforts to improve the semiconductor proper- ties of pyrite (FeS2) by chemical surface treatment, we observed some unusual behavior of the pyrite/electrolyte interface. We believe that high dark currents and low photovoltages, as well as Fermi level pinning of the material, are caused by a high number of surface states] This work in our group has been carried out aiming at the chemical passivation of these unfavorable surface states. Our approach is to either coordinatively binding organic ligands to unsaturated Fe-centers on the pyrite surface, or to de- posit metals (chemically or electrochemically) onto the pyrite.2 Experimental Pyrite electrodes, cut in the [100] direction from natural crystals, contacted on their back side with silver epoxy to a holder and E O electrically insulated with 3M Scotchcast, were surface treated by <: first polishing, second electrochemical etching (polarization to -0.6 V NHE in 0.5M H2SO 4 for 2 min) and third by immersing the electrode for 20 min in a saturated (H-benzotriazol-l-yl)-methyliso- cyanide; btic solution in THF, or by dipping it for 60 s in a 0.01M H2(PtCl6) in 0.1M HCI N2-purged aqueous solution. Subsequently "0 the electrode was rinsed with the appropriate solvent. The photoac- tivity of the treated sample was tested in a standard three-electrode ,.P electrochemical cell. Illumination intensity was ca. 400 mW cm -2, O using a 475 nm cutoff filter. As electrolytes we used 2M KI and 0.1 M FeSO4/Fe2(SO4)a in 0.5M H2SO4 for the experiments with btic and 0.5M KI and 0.5M FeSO4 in the case of Pt-adsorption. The pres- ence of Pt on the pyrite surface was confirmed by Rutherford backscattering. Results When we applied btic to a pyrite surface we found a drastic effect on the measured currents for an Fe2+/S+-electrolyte: the dark cur- rent as well as the illumination current of the btic-treated electrode dropped considerably compared to the untreated, electrochemi- cally etched surface (Fig. 2a). The photo to dark current ratio, ~' however, improved from roughly 5:1 to approximately 15:1 at I= 0.65 V vs. NHE. In order to increase the efficiency of the charge- transfer, we repeated the experiment in an iodide electrolyte. As- .E. tonishingly, we did not find the expected high suppression of the dark current, but only a reduction by ca. 10% (see Fig. 2b). The .~ same is true for the illumination current. Analogous results were obtained with a pyrite electrode, which "o was, again after electrochemical etching, briefly dipped into an "~ H2(PtCI~)- solution. The Pt-treated surface showed a distinctly di- ~. minished electrochemical activity toward the Fe2+-electrolyte (re- t~ duction of dark and illumination current by ca. 50% each, slightly increased Ip~/leark ratio at a potential of 0.85 V vs. NHE; Fig. 3a), while in the case of the iodide solution the currents were completely unaffected (Fig. 3b). I CH2NC Discussion Our currently favored model explaining the reasons for this be- havior is a surface site selectivity of the interacting redox species for chemically and/or energetically different sites on the pyrite sur- face. We think it possible that the Fe2+-ions from the solution mainly interact with the pyrite-sulfur for charge-transfer, while the iodide preferentially adsorbs to the iron centers of the pyrite surface. We then could explain our experimental results by postulating the ad- sorption/reaction of the btic with the sulfur (formation of thio- cyanate?), which would lead to a blocking of the main charge-trans- 8 6 4 2 0 -2 i ' i , i 0.3 r , , i ] l l , . 0.5 .... ! .... I 0.7 0.9 Pot. vs NHE [V] 80 ' ' ' b. 60 ........... 4O 2O 0 -20 , oJ 0.3 0.7 1.1 1.5 Pot. vs NHE [V] ( 1H-Benzotriazole- l-yl)-methylisocyanide Fig. 1. Fig. 2. The diagram shows the effect of btic-treatment on the current/potential behavior of a pyrite photoelectrode: the drawn-out curves represent the currents before the treatment, the dotted ones the behavior after btic-treatment. Figure 2(a) was recorded in 0,1M Fe2+~+-sulfate (pH 0.3), Fig. 2(b) in 2M KI electrolyte (pH 0.3). Chopped illumination (0.2 Hz) was applied. L166 J. Electrochem. Soc., Vol. 142, No. 9, September 1995 9 The Electrochemical Society, Inc. Downloaded 17 May 2009 to 90.16.255.165. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp