Cu(In,Ga)(S,Se)2 thin film solar cell with 10.7% conversion efficiency obtained by selenization of the Na-doped spray-pyrolyzed sulfide precursor film.

Selenium-rich Cu(In,Ga)(S,Se)2 (CIGSSe) thin films on an Mo-coated soda-lime glass substrate were fabricated by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)2, In(NO3)3, Ga(NO3)3, and thiourea followed by selenization at 560 °C for 10 min. We studied the effects of intentional sodium addition on the structural and morphological properties of the fabricated CIGSSe films by dissolving NaNO3 in the aqueous precursor solution. The addition of sodium was found to affect the morphology of the final CIGSSe film: the film had denser morphology than that of the CIGSSe film obtained without addition of NaNO3. Photoelectrochemical measurements also revealed that the acceptor density of the nondoped CIGSSe film was relatively high (N(a) = 7.2 × 10(17) cm(-3)) and the addition of sodium led to a more favorable value for solar cell application (N(a) = 1.8 × 10(17) cm(-3)). As a result, a solar cell based on the sodium-modified CIGSSe film exhibited maximum conversion efficiency of 8.8%, which was significantly higher than that of the cell based on nondoped CIGSSe (4.4%). In addition, by applying MgF2 antireflection coating to the device, the maximum efficiency was further improved to 10.7%.

Fabrication of pores in a silicon carbide wafer by electrochemical etching with a glassy-carbon needle electrode.

An electrochemical method for making pores in a silicon carbide (SiC) wafer, in which a glassy-carbon (GC) needle electrode was used for processing, is described. By bringing the GC electrode into contact with SiC at its tip end in 20 mol dm(-3) HF solution and applying an anodic potential of or higher than 4 V vs Ag/AgCl to it, SiC was etched at the SiC/GC contact area, leading to pore formation in SiC. The diameter of the pore was almost the same as the diameter of the tip of the GC electrode (about 130 µm). By addition of sulfuric acid to the HF solution, the rate of pore formation was increased. As a result, the depth of pores formed after processing for 5 h at 10 V vs Ag/AgCl was increased from 15.3 µm to about 33 µm by addition of sulfuric acid at a concentration of 3.0 mol dm(-3).

Pore formation in a p-type silicon wafer using a platinum needle electrode with application of square-wave potential pulses in HF solution.

By bringing an anodically biased needle electrode into contact with n-type Si at its tip in a solution containing hydrofluoric acid, Si is etched at the interface with the needle electrode and a pore is formed. However, in the case of p-type Si, although pores can be formed, Si is likely to be corroded and covered with a microporous Si layer. This is due to injection of holes from the needle electrode into the bulk of p-type Si, which shifts its potential to a level more positive than the potential needed for corrosion and formation of a microporous Si layer. However, by applying square-wave potential pulses to a Pt needle electrode, these undesirable changes are prevented because holes injected into the bulk of Si during the period of anodic potential are annihilated with electrons injected into Si during the period of cathodic potential. Even under such conditions, holes supplied to the place near the Si/metal interface are used for etching p-type Si, leading to formation of a pore at the place where the Pt needle electrode was in contact.

Formation of through-holes in Si wafers by using anodically polarized needle electrodes in HF solution.

Electrochemical pore formation in Si using an anodized needle electrode was studied. In the electrochemical process, a Pt, Ir or Pd needle with a diameter of 50-200 µm was brought into contact at its tip with a Si wafer, which was not connected to an external circuit, in HF solution. By applying an anodic potential to the needle electrode against a Pt counter electrode, a pore with a diameter slightly larger than the diameter of the needle electrode was formed in both p-type and n-type Si, of which current efficiency was higher for n-type Si. Through-holes were electrochemically formed in p-type and n-type Si wafers at speeds higher than 30 µm min(-1) using a sharpened Ir needle electrode. A model was proposed to explain the results, in which the pore formation was attributed to successive dissolution of Si atoms near the 3-phase (Si/metal/HF solution) boundary by positive holes injected from the needle electrode to the surface of Si.

Fabrication of CuInS2 films from electrodeposited Cu/In bilayers: effects of preheat treatment on their structural, photoelectrochemical and solar cell properties.

Polycrystalline CuInS(2) films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films in a Cu-rich composition at 520 °C in H(2)S (5% in Ar). Structural analyses revealed that the adherence of the thus-formed CuInS(2) film to the Mo substrate was strongly dependent on heating profiles of the Cu/In bilayer film: a CuInS(2) film with poor adherence having many crevices was formed when the Cu/In bilayer film was heated monotonously from room temperature to 520 °C in Ar within 25 min followed by sulfurization, whereas CuInS(2) films with good adherence were obtained when the Cu/In films were pretreated at 110 °C in Ar for 10-60 min just before increasing the temperature up to 520 °C for sulfurization. It was also clarified that the CuInS(2) film obtained without 110 °C pretreatment had pinholes inside the film, whereas the CuInS(2) films formed after 110 °C pretreatment showed no notable pinholes. Photoelectrochemical responses of these CuInS(2) films in an electrolyte solution containing Eu(III) indicated that the CuInS(2) films obtained after 110 °C pretreatment had higher external quantum efficiency (EQE) values than those of films obtained without 110 °C pretreatment, mainly due to better adherence of 110 °C pretreated CuInS(2) films to the Mo substrate than the CuInS(2) film obtained without 110 °C pretreatment. The performance of solar cells with an Al:ZnO/Zn(S,O)/CdS/CuInS(2)/Mo structure also depended on the structural characteristics of the CuInS(2) films, i.e., preliminary conversion efficiencies of ca. 5% were obtained for devices based on the CuInS(2) films obtained after 110 °C pretreatment, whereas the device prepared by the CuInS(2) film without 110 °C pretreatment showed the conversion efficiency less than 1.5%.

Photoreduction of water by using modified CuInS2 electrodes.

Polycrystalline CuInS(2) films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films. Structural analyses revealed that the CuInS(2) film formed compact agglomerates of crystallites with grain sizes of ca. 0.5-1.5 µm. Photoelectrochemical characterization revealed that the film was p-type with a flat band potential of 0.3-0.4 V (vs Ag/AgCl at pH 4), which is suitable for water reduction but cannot be for water oxidation. Upon loading Pt deposits, the film worked as a hydrogen (H(2)) liberation electrode under cathodic polarization. Moreover, by introduction of n-type thin layers such as CdS and ZnS on the CuInS(2) surface before the Pt loading, appreciable improvements of H(2) liberation efficiency were achieved: for the CdS modified sample, spectral response data showed incident photon to current efficiency as high as 20 % at wavelengths ranging from ca. 500 to 750 nm. Appreciable H(2) evolution on this sample under potentials of power-producing regions was also confirmed.

Catalytic activity and regeneration property of a Pd nanoparticle encapsulated in a hollow porous carbon sphere for aerobic alcohol oxidation.

A core-shell composite consisting of a palladium (Pd) nanoparticle and a hollow carbon shell (Pd@hmC) was employed as a catalyst for aerobic oxidation of various alcohols. The core-shell structure was synthesized by consecutive coatings of Pd nanoparticles with siliceous and carbon layers followed by removal of the intermediate siliceous layer. Structural characterizations using TEM and N(2) adsorption-desorption measurements revealed that Pd@hmC thus-obtained was composed of a Pd nanoparticle core of 3-6 nm in diameter and a hollow carbon shell with well-developed mesopore (ca. 2.5 nm in diameter) and micropore (ca. 0.4-0.8 nm in diameter) systems. When compared to some Pd-supported carbons, Pd@hmC showed a high level of catalytic activity for oxidation of benzyl alcohol into benzaldehyde using atmospheric pressure of O(2) as an oxidant. The Pd@hmC composite also exhibited a high level of catalytic activity for aerobic oxidations of other primary benzylic and allylic alcohols into corresponding aldehydes. The presence of a well-developed pore system in the lateral carbon shell enabled efficient diffusion of both substrates and products to reach the central Pd nanoparticles, leading to such high catalytic activities. This core-shell structure also provided high thermal stability of Pd nanoparticles toward coalescence and/or aggregation due to the physical isolation of each Pd nanoparticle from neighboring particles by the carbon shell: this specific property of Pd@hmC resulted in possible regeneration of catalytic activity for these aerobic oxidations by a high-temperature heat treatment of the sample recovered after catalytic reactions.

Multicomponent sulfides as narrow gap hydrogen evolution photocatalysts.

A series of mixed crystals composed of Cu(2)ZnSnS(4), Ag(2)ZnSnS(4) and ZnS was prepared by co-precipitation of the corresponding metal ions in aqueous sodium sulfide followed by annealing in a sulfur atmosphere. Ideal solid solutions of Cu(2)ZnSnS(4) and Ag(2)ZnSnS(4) with a kesterite structure ((Cu(x)Ag(1-x))(2)ZnSnS(4) (0 ≤x≤ 1)) were successfully obtained by this procedure, as confirmed by their X-ray diffraction (XRD) patterns and energy-diffuse X-ray (EDX) analyses. On the other hand, the solubility of ZnS in these kesterite compounds was found to be limited: the upper limit of the ratio of ZnS to (Cu(x)Ag(1-x))(2)ZnSnS(4) was less than 0.1, regardless of the Cu-Ag ratio in (Cu(x)Ag(1-x))(2)ZnSnS(4). Based on the results for dependence of their photoabsorption properties on atomic compositions, a plausible band structure is discussed. Evaluation of the photocatalytic activity for H(2) evolution of these mixed crystals from an aqueous solution containing S(2-) and SO(3)(2-) ions upon loading Ru catalysts under simulated solar radiation (AM 1.5) revealed that active compounds for this reaction should contain both dissolved ZnS and Ag components. The dissolved ZnS in (Cu(x)Ag(1-x))(2)ZnSnS(4) gave upward shifts of their conduction band edges. Moreover, the presence of Ag in the solid solution provided n-type conductivity, leading to efficient migration of photogenerated electrons to the surface to induce water reduction into H(2).

Determination of oxygen sources for oxidation of benzene on TiO2 photocatalysts in aqueous solutions containing molecular oxygen.

Photocatalytic oxidation of benzene to CO(2) was studied in aqueous solutions using different kinds of TiO(2) powders, and isotopic oxygen tracers (H(2)(18)O and (18)O(2)) were used to investigate the oxidation process. Phenol was produced as a main intermediate in solution. When anatase powders, which showed high activity for oxidation of benzene, were used, 70-90% of oxygen introduced into phenol was from water. On the other hand, when rutile powders were used, only 20-40% of the oxygen was from water. The rest was from molecular oxygen in both cases. The rate of phenol production by using molecular oxygen was nearly the same between anatase and rutile powders. Hence, the high activity of anatase powders for oxidation of benzene to CO(2) is attributed to their high activity for oxidation of benzene to phenol, which is considered to be the rate-determining step, using water as the oxygen source. The processes using water and molecular oxygen as the oxygen sources are ascribed, respectively, to oxygen transfer and hole transfer processes in the initial step of benzene oxidation.

An efficient and reusable carbon-supported platinum catalyst for aerobic oxidation of alcohols in water.

Platinum nanoparticles embedded in a hollow porous carbon shell prepared by a photocatalytic reaction acted as a reusable catalyst for the aerobic oxidation of alcohols under atmospheric pressure of oxygen in water.

High sintering resistance of platinum nanoparticles embedded in a microporous hollow carbon shell fabricated through a photocatalytic reaction.

Platinum (Pt) nanoparticles encapsulated in microporous carbon with a hollow structure (nPt@hC) were fabricated on the basis of a titanium(IV) oxide (TiO2) photocatalytic reaction. From the tomogram of a sample studied by using a transmission electron microscope (TEM), the Pt nanoparticles were found to be embedded in the carbon shell and were physically separated from each other by the carbon matrix. Owing to this unique structure, the Pt particles showed high resistance to sintering when subjected to thermal treatment at temperatures up to 800 degrees C. As a result, hydrogenation reactions using various heat-treated nPt@hCs as catalysts indicated that loss of catalytic activity was minimized. Thus, the present system will be a promising system for optimizing catalyst nanostructures utilized in processes requiring rigorous conditions.

Size-selective photocatalytic reactions by titanium(IV) oxide coated with a hollow silica shell in aqueous solutions.

A novel core-shell composite photocatalyst, commercially available titanium(IV) oxide (TiO(2)) particles directly incorporated into a hollow amorphous silica shell, was fabricated by successive coating of TiO(2) with a carbon layer and a silica layer followed by heat treatment to remove the carbon layer. The composite induced efficient photocatalytic reactions when relatively small substrates were used, such as methanol dehydration and decomposition of acetic acid, without any reduction in the intrinsic activity of original TiO(2), but did not exhibit efficient photocatalytic activity for decomposition of large substrates, methylene blue and polyvinyl alcohol. The unique size-selective properties of the composites are due to their structural characteristics, i.e., the presence of a pore system and a void space in the silica shell and between the shell and medial TiO(2) particles, respectively. The loading of alkylsilyl groups on the surface of the composite led to highly photostable floatability: the floated sample also induced efficient photocatalytic reaction for decomposition of acetic acid while retaining floatation at the gas/water interface.

Encapsulation of titanium(IV) oxide particles in hollow silica for size-selective photocatalytic reactions.

A core-shell composite of TiO2 particles encapsulated in a hollow silica was fabricated, and the core-shell composite showed size-selective photocatalytic activity for decomposition of organics without reducing the intrinsic activity of the naked TiO2 core.

Synthesis of a coumarin compound from phenanthrene by a TiO2-photocatalyzed reaction.

Phenanthrene was converted into a coumarin compound by a TiO2-photocatalyzed reaction in an acetonitrile solution containing 8 wt% water and molecular oxygen in 45% yield.

Porous polystyrene microspheres having dimpled surface structures prepared within micellar assemblies of amphiphilic silica particles in water.

Production of porous polystyrene microspheres having dimpled surface structures was demonstrated using amphiphilic and hydrophobic silica particles as structure-directing agents.

Asymmetrically modified silica particles: a simple particulate surfactant for stabilization of oil droplets in water.

Spherical silica particles that are able to assemble at a phase boundary of a dual-phase mixture of water and an immiscible organic solvent were prepared by a partial modification of their surface hydroxyl groups with an alkylsilylation agent. Scanning electron microscopic observation of these particles in which their remaining surface hydroxyl groups had been selectively modified with colloidal gold particles revealed that each particle has an asymmetric surface structure: one side of the surface is hydrophilic and the other is hydrophobic. We found that these particles could form a micellar structure in water in the presence of an organic solution of a toluene/polystyrene mixture. The micellar structure was evidenced by formation of golf-ball-like polystyrene particles with dimples imprinting morphologies of the hydrophobic part of modified silica particles.