7028
J. Phys. Chem. 1991, 95, 7028-7032
measured V, values for the alkali-metal-etched samples were 15-20 mV lower than the bulk-diffusion/recombinationlimited values for HF-etched samples. This discrepancy between experimental and theoretical values was presumably due to increased interfacial recombination induced by the vigorous etching procedure, because similar behavior was observed for a set of alkali-metal-etched n-Si samples that had not been exposed to any metal deposition steps. The quantitative agreement between the V, of metalized photoelectrodes in contact with CH3OH-Me2Fc+/Owith values measured for n-Silmetal Schottky barriers operated in air strongly indicates that the current transport properties of metal island modified junctions are controlled by thermionic emission of majority carriers. This is consistent with previous work which has shown that metal overlayers on n-Si pin the surface Fermi level at 0.7-0.9 eV below the conduction band and that thermionic emission controls the current transport properties of Schottky barrier type metalized Si devices. The metalized Si systems thus appear to exhibit predictable and normal I-Vproperties that fit within the established framework of conventional majority carrier thermionic emission theory, and the stable n-Si/CH30H junctions conform to the predictions of conventional bulk-diffusion/recombination theory Over a wide range of experimental conditions. In summary, no metalized Si sample prepared or investigated in our laboratory has shown the high voltages characteristic of the n-Si/CH30H-Me2Fc+/0 contact. Metalization sufficient to prevent corrosion in aqueous electrolytes yielded Schottky barrier behavior and resulted in low V, values that were consistent with pinning of the surface Fermi level. Metalization followed by
alkali-metal etching to form hybrid metal/oxide/semiconductor structures removed all metal from the semiconductor surface and yielded electrodes that behaved similarly to HF-etched Si samples. The n-Si/CH30H-Me,Fc+/0 interface showed a small contribution from majority carrier-based processes at the highest accessible Me2Fc+concentrations, but at lower concentrations, minority carrier dominated bulk-diffusion/recombinationprocesses were observed to dominate the junction behavior under all accessible conditions of cell temperature and acceptor concentration. At present, the Shockley diode treatment appears to be quite adequate to describe the behavior of this class of photovoltaic devices, and the 670-mV V, value obtained from 0.015 QDcm resistivity n-Si samples (J h = 20.0 mA/cm2, T = 300 K) remains the highest reported piotovoltage measured under controlled conditions in a regenerative-type Si/liquid photoelectrochemical cell. The small deviations from bulk-diffusion/recombination behavior at high acceptor concentrations could signify the onset of majority carrier processes, and the magnitude of these effects will be quantified in subsequent reports describing the photoelectrochemical prop erties of n-Si/CH30H junctions in the presence of redox couples that produce lower V ,values than those exhibited by the Me,Fc+/O system. Acknowledgment. We acknowledge the National Science Foundation, Grant CHE-8814694, for support of this work. A.K. acknowledges the Department of Education for a research fellowship, and the authors are grateful to Prof. H. Tsubomura for helpful discussions and for generously providing a metalized Si sample for use in our work.
Determlnatlon of the Isoelectric Polnts of Several Metals by an Adhesion Methodt N.Kallay,***2. Torbit,* M. Golie> and E.Matijevit Department of Chemistry, Clarkson University, Potsdam, New York 13699 (Received: March 4, 1991; In Final Form: April 23, 1991)
An adhesion method for the determination of the isoelectric point (iep) of metals in aqueous media is described. The procedure consists of measuring the rate of the uptake of charged latex particles on metal beads as a function of the pH. The iep of different metals are well correlated with those of corresponding metal (hydr)oxides but not with their points of zero charge or redox potentials. This result shows that the surfaces of different metals are coated with (hydrous) oxide layers, which are responsible for their surface charge.
Introduction Recently, it has been demonstrated1I2that the zero charge condition of metals can be determined by monitoring the deposition rate of well-defined colloids. The principle of this "adhesion method" is rather simple: it has been e~tablished~*~ that fine particles adhere rapidly to a solid surface, due to attractive London-van der Waals for-, as long as they are not electrically repelled. The latter takes place when the particles bear the charge of the same sign as the substrate; at low ionic strengths (
