八重 真治

A widely used approach to obtain smooth oxide-free and (partially) H-terminated silicon (Si) surfaces is to immerse Si wafers into CP4A (a mixture of H2O, HNO3, CN3COOH and HF in a volume ratio of 22:5:3:3) and/or HF solutions of varying concentrations. It is usually assumed that such treatments result in a dramatic reduction of the surface density of states and that, therefore, no surface band bending can occur. In our experiments we investigated the electronic surface structure of a number of CP4A/HF treated n- and p-Si wafers with varying doping densities by x-ray photoelectron spectroscopy (XPS). XPS allows a straightforward detection of surface stoichiometry as well as one of band bending and surface photovoltages (SPV) on semiconductor materials because the positions of the core level peaks directly depend on the position of the Fermi level within the band gap at the surface. Our experiments show that on all surfaces investigated Fermi level pinning still exists after the samples were immersed in the CP4A/HF solutions and that the pinning states are located close to the conduction band. Most of the samples also showed SPV when measured under illumination. The measurements also show that up to 36.6% of the surfaces are covered by F atoms depending on the treatment and the doping density. From the amount of blind bending we estimated the density of surface states present on the various samples. (C) 1999 American Vacuum Society. [S0734-2101(99)02301-5].

A number of nanometer-sized holes, 20-80 nm wide and 0.6-0.7 nm deep, were produced at NH4F-etched, atomically nearly-flat n-Si(111) surfaces while platinum was electrochemically deposited at -0.40 V vs SCE in 5.0 x 10(-3) M H2PtCl6 + 0.5 M Na2SO4 of pH 2.9. The nano-holes were produced only during the Pt deposition at potentials more negative than -0.35 V. Experiments have shown that hole injection (or extraction of valence-band electrons) by H2PtCl6 plays no important role in the nano-hole formation. When the n-Si(111) surfaces were kept at -0.4 V in 0.18 M H2SO4 which has the same pH as the 5.0 x 10(-3) M H2PtCl6 + 0.5 M Na2SO4, much smaller nano-holes than the above case were produced, suggesting that electrochemical reductive dissolution of Si occurs in acidic solutions. It is proposed as a possible mechanism that deposited Pt atoms, while migrating along the n-Si surface, act as a catalyst for the reductive dissolution of Si, leading to acceleration of nano-hole formation. (C) 1999 Elsevier Science Ltd. All rights reserved.

Photoelectrochemical activity of nanocrystalline TiO2 film electrodes prepared by use of various TiO2 particles has been studied comparatively. It is confirmed that the photocurrent (or photoactivity) for anatase-type TiO2 is in general higher than that for the rutile-type. The photocurrent quantum yield (eta(pc)) for anatase-type TiO2 is nearly equal to that for rutile-type in long wavelengths but increases more and more with decreasing wavelength in contrast to the eta(pc) for rutile-type, and thus the eta(pc) for anatase-type becomes much higher than the eta(pc) for rutile-type in short wavelengths around 300 nm, indicating that the high photoactivity for anatase-type TiO2 is due to this high eta(pc) in short wavelengths. There are some discussions on the reasons for the high eta(pc) for anatase-type TiO2 in short wavelengths, including the possibility of contribution of hot electrons or holes in the TiO2 film.

Photoelectrochemical reduction of carbon dioxide (CO 2 ) on p-type silicon (p-Si) electrodes modified with small metal (Cu, Ag, or Au) particles has been studied. The electrodes in CO 2 -saturated aqueous electrolyte under illumination produce methane, ethylene, carbon monoxide, etc., similar to the metal (Cu, Ag, or Au) electrodes, but at ca. 0.5 V more positive potentials than the corresponding metal electrodes, contrary to continuous-metal-coated p-Si electrodes. The results clearly show that the metal-particle-coated p-Si electrodes not only have high catalytic activity for electrode reactions but also generate high photovoltages and thus work as an ideal type semiconductor electrode. It is discussed that the CO 2 photoreduction proceeds with an upward shift of the surface band energies of p-Si in order to get energy level matching between the semiconductor and solution reactants, though hydrogen photoevolution occurs without such an upward shift. It is also discussed that the control of surface structure on a nanometer-sized level, as well as on an atomic scale, is important for getting higher efficiencies.

A p-type silicon (p-Si) electrode modified with copper particles (particulate-Cu/p-Si) was applied to photoelectrochemical (PEC) reduction of carbon dioxide (CO2) in acetonitrile electrolyte solutions with and without 3.0 M H2O. The particulate-Cu/p-Si electrode generated high photovoltages of 0.50 to 0.75 V, and produced methane, ethylene, etc., under addition of 3.0 M H2O, similar to a Cu metal electrode, indicating that the particulate-Cu/p-Si electrode acted as an efficient electrode for the PEC reduction of CO2 in non-aqueous solutions.

A p-type silicon (p-Si) electrode modified with small metal (Cu, Au and Ag) particles works as an ideal-type electrode for photoelectrochemical reduction of carbon dioxide, producing carbon monoxide, methane, ethylene, etc., with a large photovoltage of 0.5 V. It is discussed that two types of surface-structure control on atomic and nanometer-sized levels are important for getting a high efficiency. (C) 1997 Elsevier Science B.V.

The flat-band potential (U-FB) of n-Si electrodes in concentrated hydrogen halide (HX) solutions, as determined from Morr-Schottky plots and the onset potential of photocurrent, shifts largely toward the negative with the decreasing electronegativity of halogen atoms. XPS studies have shown the presence of halogen atoms on the Si electrodes which were beforehand immersed in the HX solutions. The shift in U-FB can be explained as due to changes in surface potential by formation of Si-halogen surface termination bonds.

The mechanism of synchronous oscillations found for reduction currents on two Pt electrodes in 0.7 M H2O2-0.3 M H2SO4 has been investigated. Synchronization occurs even when the two Pt working electrodes are far apart from each other if the distance between the working and reference electrodes is large enough. The synchronization also occurs even when one of the working electrodes is separated from the other electrodes with an electrically conductive carbon box. These results clearly indicate that the synchronization is not induced by mass transport but by electrical interaction. It is concluded that the current oscillation in one of the two oscillation systems causes an oscillation in the true electrode potential for the neighboring oscillation system through the oscillation of the ohmic drop in the solution, which leads to the synchronization.

A new electrochemical oscillation (called oscillation B) appears, in addition to a reported one (called oscillation A), for reduction reactions on platinum electrodes in acid solutions containing hydrogen peroxide in case where the H2O2 concentration is made high. Also, oscillation A becomes more pronounced and more stable as the H2O2 concentration increases except the case of very high concentrations (greater than or equal to 1.1 M). Oscillation A appears in a narrow potential region just before hydrogen evolution, whereas oscillation B appears in a potential region of hydrogen evolution. Both oscillation A and B are strongly affected by the concentrations df H+ as well as H2O2, suggesting that they arise from the competition between the H2O2 and H+ reduction. It is concluded that the high and low current states for oscillation A correspond to the H2O2 reduction and almost no H2O2 reduction (nor hydrogen evolution), respectively, whereas the high and low current states for oscillation B correspond to the H2O2 reduction and hydrogen evolution, respectively. Mechanisms for sudden transitions between these states are discussed qualitatively.

A photoluminescence (PL) band peaked at 840 nm is observed for an n-TiO 2 (rutile) electrode in aqueous electrolyte solutions when illuminated under a slight anodic bias and was previously explained as arising from an intermediate of photooxidation reaction of water on the n-TiO 2 electrode. The chemical structure and properties of the PL-emitting species have been investigated in the present work by measurements of photocurrent, photoluminescence, electroluminescence, and transient luminescence as well as inspection of the electrode surface by transmission electron microscopy. It is concluded that the PL band should be assigned to an electronic transition from the conduction band to the vacant level of HO • radicals present in a bulk defect (atomic gaps) near the n-TiO 2 (rutile) surface and that the radicals act as an intermediate of the photooxidation reaction of water. The results give confirmative evidence to our previously proposed new mechanism that the surface Ti-OH group cannot be oxidized by photogenerated holes, and thus the reaction in acidic solutions is initiated by the oxidation of Ti-OH or OH - in the bulk defect near the surface.

The open-circuit photovoltages (V OC s) of photoelectrochemical (PEC) solar cells, equipped with n-Si electrodes modified with ultrafine platinum (Pt) particles, have been investigated as functions of the Pt-particle density and the post-heat-treatment temperature. Langmuir-Blodgett layers of Pt-colloid particles stabilized by polyvinylpyrrolidone (PVP) are used to control the Pt-particle density on the n-Si electrodes. The ideality factor (n) and the dark saturation current density (j 0 ) are determined from plots of logarithm of the short-circuit photocurrent density versus V OC . The n is unity for all the electrodes, independent of both the Pt density and the heat-treatment temperature, indicating that an ideal n-Si/solution junction is formed. The j 0 decreases and, hence, the V OC increases as the Pt density decreases and also as the post-heat-treatment temperature is decreased. The decrease of j 0 is caused by the decrease in the majority carrier dark current density (j 0n ). In cases where the Pt-modified n-Si electrodes are heat-treated at a low temperature of 150°C or not heat-treated, V OC is in the range of 0.61 to 0.63 V, independent of the Pt density, indicating that minority carrier controlled solar cells are obtained.

A new electrochemical oscillation is found for reduction reaction of silicon tetrachloride on a partially immersed single crystal n-Si electrode in a lithium chloride-potassium chloride eutectic melt electrolyte. The reduction of SiCl4, which is almost insoluble in the electrolyte, occurs mainly near the upper edge of an electrolyte meniscus on the electrode, and it is discussed that the oscillation is caused by a change in the height of the meniscus due to a change in the chemical structure (and hence the interfacial tension) of the electrode surface with progress of the silicon deposition reaction.

Synchronous oscillations are found for reduction currents on two independently potential-controlled Pt electrodes in 0.7 M H2O2-0.3 M H2SO4. It is shown that electrical interaction between the two oscillating electrochemical systems through the electrolyte solution is responsible for the synchronization.

The mechanism of generation of high open-circuit photovoltages (V(oc)s) of 0.62-0.63 V for n-Si (similar to 1 Omega cm) electrodes modified with colloidal Pt particles (4 nm in diameter) is investigated by measurements of minority-carrier (hole) diffusion length (L(p)) and temperature dependence of V-oc Langmuir-Blodgett (LB) layers of colloidal Pt particles are used to control the Pt density on n-Si. The L(p) value is determined to be 200 mu m, irrespective of whether n-Si is modified with Pt or not. The temperature dependences of V(oc)s at 203-298 K have been explained well by our previously proposed model. It is shown that heat treatments of the Pt-modified n-Si electrodes increase the area and the width of the direct Pt-Si contacts and thus decrease V-oc, but minority-carrier controlled (ideal) solar cells are obtained if the electrodes are prepared under appropriate conditions. Copyright (C) 1996 Elsevier Science Ltd

Various platinum (Pt) colloid solutions were prepared and used for the modification of n-type silicon (n-Si) electrodes. All the photoelectrochemical (PEC) solar cells, equipped with the Pt-colloid modified n-Si electrodes thus prepared, gave the open-circuit photo voltages (V oc ) of 0.550 to 0.630 V, much higher than those (0.25 — 0.30 V) for n-Si electrodes coated with continuous thin Pt layers, clearly showing the beneficial effect of the discontinuous metal coating. The PEC cells, equipped with the textured-surface n-Si electrodes modified with the Pt colloid particles prepared by Bredig’s method, yielded an energy conversion efficiency of 14.9% in a three-electrode configuration. The relation between the V oc and the size and distribution of the Pt particles is discussed. © 1994, The Electrochemical Society, Inc. All rights reserved.

A Langmuir layer of ultrafine platinum particles (2–6 nm in diam) has been developed on a water surface by dropping a Pt colloid solution, prepared by refluxing an ethanol-water (1:1) solution of hexachloroplatinic(IV) acid in the presence of poly(N-vinyl-2-pyrrolidone) as a stabilizer. The layer is transferred onto a single-crystal n-type silicon (n-Si) wafer by the horizontal lifting method. The Pt particles are rather homogeneously scattered on n-Si, and the particle density can be controlled on a nanometer scale by changing the area of the Langmuir layer at the time of transfer. The open-circuit photovoltage (V oc ) for photoelectrochemical (PEC) solar cells with such n-Si electrodes is inversely related to Pt-particle density, and reaches 0.635 V, much higher than that for n-Si coated with a continuous Pt layer (ca. 0.30 V) or that for the conventional p-n junction Si solid solar cell of a similar simple cell structure (ca. 0.59 V). This result is in harmony with our previously proposed theory, the above increase in V oc being explained by the decrease in the majority carrier dark saturation current density. © 1994, The Electrochemical Society, Inc. All rights reserved.