八重 真治

Metal filled Si nanopores, that is, metal nanorods in an Si matrix, are produced by an electroless process that consists of three steps: (1) electroless displacement deposition of metal nanoparticles from a metal salt solution containing HF; (2) Si nanopore formation by metal-particle-enhanced HF etching; and (3) metal filling in nanopores by autocatalytic electroless deposition. Ag nanoparticles produce Si nanopores whose sizes are a few tens of nm in diameter and ca. 50 nm deep. Au nanoparticles produce finer and straighter nanopores on Si than the Ag case. These nanopores are filled with a Co or a Co-Ni alloy by autocatalytic deposition using dimethylamine-borane as a reducing agent. Phosphinate can be used as a reducing agent for the Au-deposited-and-pore-formed Si. The important feature of this process is that the metal nanoparticles, that is, the initiation points of the autocatalytic metal deposition, are present on the bottoms of the Si nanopores. (C) 2008 Elsevier B.V. All rights reserved.

Metal-enhanced HF etching of Si is an electroless method to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide. Pd exhibits high activity for enhancing the HF etching of Si without an oxidizing agent, even under dissolved oxygen free and dark conditions. Electrolessly deposited Pd particles on n-Si enhance the HF etching of Si, but produce no porous layer. Patterned Pd films localize the etching under and the boundary of the Pd deposited areas, and thus Pd can produce a micro-etch pattern on Si with simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to Pd-enhanced anodic oxidization of Si with water. © The Electrochemical Society.

Pd-particle-modified n-Si can be etched at a high rate in a hydrofluoric acid solution without a particular oxidizing agent under dissolved oxygen free and dark conditions. In this study, Pd thin film patterned n-Si is used. The etching is localized at the boundary between the 29-nm-thick-Pd film and the non-Pd-deposited area of the n-Si surface at the initial stage. Then Pd particles form on the non-Pd-deposited area toward which the etched area extends. Thin (4.3 nm) Pd films localize the etching under the films.

© 2009 by Nova Science Publishers, Inc. All rights reserved. We have been investigating electrochemical deposition of metal nanoparticles on silicon. The metal nanoparticles play a tremendously important role in photoelectrochemical solar cells for photovoltaic and photochemical conversion of solar energy, and in micro- to nano-processing of silicon with metal-enhanced hydrofluoric acid (HF) etching. The kind, particle density and size of nanoparticles influence the stability and conversion efficiency of solar cells, and the etching behavior of silicon. Electrochemical deposition from an aqueous solution of metal ions is one of the simplest methods to produce metal nanoparticles on silicon. This article describes the results of our study on (1) double potential step electrodeposition and (2) electroless displacement deposition of metal nanoparticles on silicon. (1) Double potential step electrodeposition: Fine platinum (Pt) particles were electrodeposited on n-type silicon (n-Si) electrodes from an aqueous solution of hydrogen hexachloroplatinate(IV) by the single potential step (SPS) and double potential step (DPS) methods. The particle density of Pt on n-Si was 108 cm-2 for the SPS method, whereas it increased from 109 to 1010 cm-2 by a shift of the pulse potential at the initial step of the DPS method. The size of the Pt particles enlarged from smaller than 30 nm with the charge density passing across the electrode surface. (2) Electroless displacement deposition: Various kinds of metal nanoparticles were deposited on n-Si by the immersion of Si substrates in a simple metal-salt solution containing HF. The particle density of metals varies widely from 106 to 1011 cm-2, depending on the kind of metal. Deposited metals can be classified into two types of nucleation behavior. One consists of the platinum group elements, including Pt, rhodium, and palladium, which display lower particle densities than elements of the other group and depend on the type of pretreatment of the n-Si, and thus the surface conditions of Si substrates. The second group consists of the copper group elements, including copper, silver, and gold, which display higher particle density than the first group and are independent of pretreatment. The size of deposited particles decreases from hundreds of nm to ten nm as the particle density increases. The displacement deposition of the Pt and silver particles onto n-Si are in progressive and instantaneous nucleation modes, respectively.

Porous silicon (Si) has a wide range of pore sizes between nanometers and tens of micrometers. Microporous Si that consists of nanometer-sized pores has attracted considerable attention due to its visible light emissions and large surface areas. Macroporous Si that consists of micrometer-sized pores is expected to reduce silicon reflectivity to improve the conversion efficiency of solar cells. Porous Si is usually prepared by such methods as electrochemical (p-type) or photoelectrochemical (n-type) anodic etching and chemical (stain) etching. Recently, we reported that porous Si is formed on Si wafers modified with fine metal particles by simply immersing the wafers in a hydrofluoric acid (HF) solution without a bias or a particular oxidizing agent. This chapter describes the structure of porous silicon produced by metal particle enhanced HF etching and its application for the antireflection of multicrystalline Si solar cells. The metal particles are deposited onto Si wafers by electroless displacement deposition from a metal salt solution that includes HF. The metal particle deposited Si wafers are immersed in a simple aqueous HF solution. For n-type Si, a porous layer is only formed on the metal particle deposited area of the wafers. Etching rate increases with oxygen concentration or photoillumination intensity. Both macro and microporous layers are formed on n-Si wafers modified with platinum (Pt), gold, or silver particles. In the case of palladium particles, the n-Si wafers are polish-etched at a rate ten times higher than other metal particles. For Pt-particle-deposited p-type Si, porous layers are formed by HF immersion. Under continuous photoillumination, porous Si is formed mainly on the nonphotoirradiation side surface of p-Si, as opposed to the photoirradiation side of n-Si. Such etching behavior can be described by the mechanism based on the formation of local cells on the surface and the energy band profile of metal particle deposited Si. For Pt-particle deposited multicrystalline n-Si wafers, uniform porous layers are formed by HF immersion. The porous layers reduce the visible light reflectance of the wafers from 30% to 6%. The photocurrent density of porous multicrystalline Si solar cells has a 25% higher value than the non-porous Si cells.© 2009 by Nova Science Publishers, Inc. All rights reserved.

We investigate the nucleation behavior in the electroless displacement deposition of metal particles (Pt, Rh, Pd, Cu, Ag, and Au) onto n-Si wafers from a metal-salt solution containing HE The particle density of metals varies widely from 106 (pt) to loll (Au) cm(-2), depending on the kind of metal. Deposited metals can be classified into two types of nucleation behavior. One consists of the platinum group elements, including Pt, Rh, and Pd, which display lower particle densities than elements of the other group and depend on the type of pretreatment of the n-Si wafer, and thus the surface conditions of Si. The second group consists of the copper group elements, including Cu, Ag, and An, which display higher particle density than the first group and are independent of pretreatment. The size of deposited particles decreases from hundreds nm to tens nm as the particle density increases. Moreover, the displacement deposition of the Pt and Ag particles onto n-Si are in progressive and instantaneous nucleation modes, respectively. (C) 2007 Elsevier Ltd. All rights reserved.

Hydrogen production using water splitting by photoelectrochemical solar cells equipped with a TiO2 photoelectrode has been attracting much attention. However, TiO2 encounters serious difficulty in achieving hydrogen evolution. One solution to this difficulty is using a hydrogen-producing semiconductor, such as silicon, and an oxidation reaction other than oxygen evolution, such as oxidation of iodide ions into iodine (triiodide ion). In this study, microcrystalline silicon (μc-Si:H) thin films are used as photoelectrodes in the photodecomposition of HI for low-cost and efficient production of solar hydrogen. An n-μc-3C-SiC:H and an i-μc-Si:H layer are deposited on glassy carbon substrates using the hot-wire cat-CVD method. The μc-Si:H electrodes are modified with platinum nanoparticles through electroless displacement deposition. The platinum nanoparticles improve the electrode's stability and catalytic activity. The electrodes produce hydrogen gas and iodine via photoelectrochemical decomposition of HI with no external bias under simulated solar illumination. We also attempt solar water splitting using a multi-photon system equipped with the μc-Si:H thin film and TiO2 photoelectrodes in series.

Electrodeposited. Co-P multilayer films were formed by means of controlling stirring conditions and current density under DC electrolysis. The multilayer films comprise amorphous and nanocrystalline layers having a periodicity of several tens of nanometers. The measurement results of the magnetic property of the films are consistent with the above result. (c) 2007 Elsevier B.V. All rights reserved.

The objective of the present study was to confirm the presence of the hydroxide in electrolessly deposited cobalt alloy films. The existence of Mn hydroxide in Co-Mn-P films was confirmed by glow discharge optical emission spectroscopy (GDOES). The results of the electrical resistivity and magnetic coercivity measurements of the films are also consistent with the above results.

Microcrystalline silicon (mu c-Si:H) thin films, which are prospective low-cost semiconductor materials, are used as photoelectrodes for the direct conversion of solar energy to chemical energy. An n-type microcrystalline cubic silicon carbide layer and an intrinsic mu c-Si:H layer are deposited on glassy carbon substrates using the hot-wire cat-CVD method. The mu c-Si:H electrodes are modified with platinum nanoparticles through electroless displacement deposition. The electrodes produce hydrogen gas and iodine via photoelectrochemical decomposition of hydrogen iodide with no external bias under solar illumination. Surface modification with platinum nanoparticles and surface termination with iodine improve the conversion efficiency. (c) 2006 Elsevier B.V. All rights reserved.

With a measurement technique called television holographic interferometry, we investigated the internal stress generated at the initial deposition stage in electrolessly deposited Cu films on silicon substrates. In a region of film thickness less than 100 nm, two inflection points, at which the direction of stress was reversed, were observed on the film thickness-stress curve. From cross-sectional transmission electron microscopy observation of these films, it was found that the films grew in the Volmer-Weber mode and that the above inflection points related to specific stages observed, such as islands growth, islands coalescence, the formation of continuous film, and grain growth. (c) 2007 The Electrochemical Society.

Antireflection of silicon (Si) surface is one key technology for the manufacture of efficient solar cells. Metal particle enhanced HF etching is applied to produce uniform antireflecting porous layer on multicrystalline Si wafers that cannot be uniformly texturized by anisotropic etching with an alkaline solution. Fine platinum (Pt) particles are deposited on multicrystalline n-Si wafers by electroless displacement reaction in a hexachloroplatinic acid solution containing HF. Both macroporous and luminescent microporous layers are uniformly formed by immersing the Pt-particle-deposited multicrystalline Si wafers in a HF solution. The reflectance of the wafers is reduced from 30% to 6% by the formation of porous layer. The photocurrent density of photoelectrochemical solar cells using porous multicrystalline n-Si has a 25% higher value than non-porous Si cells. (c) 2005 Elsevier Ltd. All rights reserved.

Metal-particle deposited Si can be etched by simply immersion in HF aqueous solution without a bias or a particular oxidizing agent. Both macroporous and luminescent microporous Si are formed by this metal particle enhanced etching. The present work applies this method to p-type Si and investigates structural changes in porous Si by controlling photoillumination intensity and dissolved O2 concentration during etching. Fine Pt particles are deposited on p-Si wafers by simple immersion in the HF solution containing H 2PtCl6. Both macroporous and luminescent microporous layers are formed on the Pt particle deposited p-Si wafer by immersion in HF solution under intermittent photoirradiation. Continuous photoirradiation and O2 gas bubbling increase the etching rate of Si. Ar gas bubbling decreases the rate under photoirradiation and stops the etching in the dark. The similarities and differences in the etching behavior between p- and n-type Si are discussed.

The objective of this study was to clarify the microstructure of crystalline-amorphous transition layers formed in electrolessly deposited Co-P films. Structurally graded Co-P films, including the transition layers, were prepared by controlling the solution pH in electroless plating solutions. TEM analysis indicated that the layers featured a modulated structure comprised of two phases, namely a crystalline phase with low P content and an amorphous phase with high P content.

The objective of this study is to clarify the effect of the composition of electroless pure Ni solutions on the microstructure and electrical conductivity of deposited films. The carbon, sulfur and boron contents included as impurities in the samples have been measured, showing changes in the carbon content ranging from 0. 01 to 0. 19% by mass. The crystal structure of the deposited films was investigated by means of cross-sectional TEM observation, and investigation of the obtained electron diffraction patterns reveals that the crystals of the deposited films are generally oriented randomly. The grain size is uniform throughout the investigated film (1 μm), though the diameter varies from 10 nm to 120 nm with changes in the solution composition: relatively coarse grains are observed in films deposited from a solution with a high hydrazine concentration. The addition of saccharin sodium generally induces grain growth. The results of the measurements suggest an adequate correlation between the carbon content and the grain size, and it has become evident that the electrical conductivity of the films decreases linearly with an increase of the carbon content. © 2005, The Japan Institute of Electronics Packaging. All rights reserved.

We have developed an autocatalytic plating solution for pure nickel deposits using hydrazine as a reducing agent. The solution is characterized by its extended lifetime, high deposition rate and high deposit brightness. In this study, we have found that a simple solution consisting of nickel acetate and hydrazine can deposit black nickel films with stability. The deposition rate was 0.79 nm/sec. The addition of a small amount of formaldehyde to the plating solution gave bright nickel films. The purity of the deposited nickel was higher than 99.7%. Its electrical conductivity increased with purity.

The electroless plating method has a particular advantage in that it accepts non-conductive substrates as its substrate materials. The catalyzation pretreatment process is essential for initiating electroless deposition. The most common two-step pretreatment process prior to electroless nickel plating was studied here, which includes the sensitizing step and the activating step. The objective of this study is to investigate the morphological changes of the adsorbate produced on the non-conductive substrates during the period before and after the deposition is initiated. The results obtained are as follows :<br> By using an ICP for the nickel precipitate weight gain measurement when the glass substrate which conducted the catalyzation process was immersed in an electroless Ni-P plating bath, the results show that deposition is already initiated with a low deposition rate even before the deposition can be confirmed with a visual inspection.<br> The observations of the catalyzed surfaces using SEM detect the aggregates of adparticles with sizes grown to reach several microns. The incidence of the grown aggregates has a tendency to be higher as the solubility of Sn in the activating solution is increases. Comparing the results of the morphological observation before and after the initiation of the deposition, it should be concluded that the aggregates produced during the activation step play the starting point of the massive nodular grains that exist on the surface of deposits.<br>

Electroless Plated Ni-P/TiO₂ composite films were deposited from an acidic solution or an alkaline solution containing photocatalytic TiO₂ particles (P-25). The dispersion of particles in the films was investigated by cross-sectional observation with a transmission electron microscope. The particle content in the films increased with an increase of its amount in the solutions. The deposition rate of composite films was independent of the amount of TiO₂ particles in the solutions(0˜100g dm⁻³). Photocurrent density of the films in 0.1M NaOH aqueous solution rose with particle content in the films.<br>

This paper describes the formation process and chemical state of adsorbates produced during the two-step catalysation pre-treatment prior to electroless deposition. Tin species adsorbed during sensitisation are divided into two types, namely one that adsorbs weakly and one that adsorbs strongly. The former exists in equilibrium with Sn2+ in the sensitising solution. It easily desorbed from the substrate during the following 110 immersion, and the loss is recovered during an ensuing sensitising step. The latter is not desorbed during the HCl immersion step. A dry process after sensitisation, which should promote oxidation, induces the transition from Sn2+ to Sn4+. In the activating step, two reactions proceed independently of each other; that is, an oxidation-reduction reaction between Sn2+ and Pd2+ takes place, which produces Pd-0 and Sn4+. Simultaneously, some Sn2+ is desorbed from the substrate. XPS analysis confirms that tin oxide (or hydroxide) and metallic palladium are the final products in the two-step catalysation pre-treatment.

SnCl₂ aqueous solution including HCl is used as a sensitizer of two-step catalyzation (sensitization-activation) pretreatment for electroless metal plating onto non-conducting substrates. The sensitizer is inactivated by aging. In this study, the relationships among the Sn²⁺ concentration of sensitizer, amount of adsorbates formed on the substrates, induction period of electroless Ni-P deposition and aging time have been investigated quantitatively. As the aging proceeded, the Sn²⁺ concentration decreased, while the amount of adsorbates, including Sn species formed by sensitization, increased. With the decrease of Sn²⁺ concentration, the amount of Pd adsorbates formed during activation decreased, and the induction period was extended. An independent investigation using mixture solutions of SnCl₂ and SnCl₄ gave similar results for aging. These results show that the aging of sensitizer induces the decrease of divalent Sn species and the rise of tetravalent Sn species, which is inactive for the formation of Pd adsorbates. Thus, the aging decreases the Pd adsorbates and extends the induction period.<br>

Electroless plating is recently used for electronic industry. But, during the deposition of thin films large internal stresses are frequently built into the films. The internal stress in films has been extensively investigated but its origin has not been clarified yet. In order to determine the origin of stress, measurements of stress are required in the initial stage of film growth. In this study, TV holography, which can capture holographic image at TV frame rates, is used to measure sensitively the change in the internal stress in films by recording the change in deflection of the free end of a cantilever beam. The dependence of internal stress upon film thickness for a Cu plating bath on a Be-Cu substrate was discussed. As the results, large stress variation is generated right after the plating, which occurred simultaneously with the adhesion of the plating particle on the substrate by SEM observation. The stress variation is large different in commercial bath and basic bath which removed small additives. This study shows that present method is useful as "in-situ measurement" of internal stress of electroless plating.

Porous Si was formed on n-Si wafers, modified with fine Pt particles, by simply immersing the wafers in a HF solution without a bias or an oxidizing agent. The Pt particles were deposited onto n-Si wafers by electrodeposition or electroless displacement deposition. SEM images show that many pores, ranging between 0.1 and 0.8 μm in diameter and covered with a luminescent nanoporous layer, were formed only on the Pt-modified area of the n-Si surface by immersion in 7.3 M HF solution for 24 h. The weight loss of Pt-electrodeposited n-Si wafer was 0.46 mg cm -2 , corresponding to ca. 2 μm in thickness. The weight loss and the structure of porous Si changed with the etching conditions, such as concentration of dissolved oxygen in the HF solution, distribution density of metal particles, and different kinds of metal particles. A photoelectrochemical solar cell equipped with a Pt-particle-modified porous n-Si electrode gave 13.3 mW cm -2 of maximum output power, which corresponds to a 13% conversion efficiency and is higher than that for the Pt-particle-modified flat n-Si electrode. ©2003 Elsevier Science B.V. All rights reserved.

For electroless Co-P based alloy films, corrosion resistance increases with increased W content due to passivity, but decreases with increasing Zn content. The coercivity of the films increases with the Zn content, and is independent of the W content. The measurement of film magnetisation using a magnetic balance is effective for the determination of quantitative corrosion loss of ferromagnetic films, such as Co alloys. The electroless Co-W-Zn-P film, about 3 mum in thickness, containing 1.2 atom % of W and 1.6 atom % of Zn gave high coercivity of 69 kA m(-1) and high corrosion resistance, three times that of Co-Zn-P films. XPS analysis indicated that an oxide of Co and WO3 were formed on, and Zn was eluted from, the surface of the Co-W-Zn-P film by the corrosion. The passive WO3 layer improves the corrosion resistance of films.

Electroless deposition of metal onto nonconductors, such as ceramics, glass and plastics, requires a catalyzation pretreatment of substrates. Adsorbates formed on glass or mica substrates by two-step catalyzation (sensitization-activation) have been investigated by quantitative analysis of Sn and Pd on substrates, XPS and AFM. Adsorbates, formed on the substrate by the sensitization, included divalent Sn and O, and no Cl was included. There were two kinds of Sn adsorbates, weak adsorbed ones and strong adsorbed ones. The former was in an equilibrium state to Sn²⁺ in the sensitizer, therefore, it was desorbed from the substrate by immersion into HCl solution, and was recovered to the same amount by the sensitization after the immersion into HCl solution. Elemental Pd which included no Cl was produced on the substrate by the activation after the sensitization. The amounts of Sn and Pd adsorbates were increased by the repetition of sensitization and activation. The activity of the catalyzed substrate for the electroless deposition was lost by the sensitization. AFM inspection showed that fine particles (10–15nm in size, 0.5–1.0nm in height, 4–5×10³ particles μm^-2 in density) were formed on the substrate by the sensitization, and the height and density of particles were enlarged by the activation. The process of two-step catalyzation was explained by using a model based on these results.

Fine platinum (Pt) particles were deposited electrochemically on n-type silicon (n-Si) electrodes from an aqueous hexachloroplatinic acid(IV) solution by the single potential step (SPS) and double potential step (DPS) methods. The distribution density of the Pt particles on n-Si was 10(8) cm(-2) for the SPS method, whereas it increased from 10(9) to loll cm(-2) by a shift of the pulse potential at the initial step of the DPS method from -1.0 to -4.0 V versus SCE and remained nearly constant at more negative potentials. The size of the Pt particles enlarged with the charge density passing across the electrode surface at a potential of -0.70 V versus SCE, which was applied throughout for the SPS method and at the second step for the DPS method. Photoelectrochemical (PEC) solar cells equipped with Pt-electrodeposited n-Si electrodes generated open-circuit photovoltages (V-OC) of 0.51-0.61 V, much higher than those for n-Si electrodes coated with continuous Pt layers (ca. 0.2-0.3 V). Solar cell characteristics changed with the pulsed potential and charge density passing across the electrode surface which changed the size and distribution density of the Pt particles. The characteristics were explained well by our previous theory on metal-dot coated n-Si electrodes. (C) 2001 Elsevier Science Ltd. All rights reserved.

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.