Tsutomu Shinagawa

Oriented Transformation from Layered Zinc Hydroxides to Nanoporous ZnO: A Comparative Study of Different Anion Types

Thermal decomposition of layered zinc hydroxides (LZHs) is a simple and convenient way to achieve porous ZnO nanostructures. The type of anion contained in an LZH determines the fundamental characteristics of the LZH and thus affects the formation process of the resulting porous ZnO. Here we report a comparative study on the crystal orientation relationship between LZH precursors and the corresponding porous ZnO products by using well-faceted and highly oriented LZH crystals with three different anions, i.e., NO3-, SO42-, and Cl-. Highly oriented LZH crystals were prepared on layer-by-layer coated indium tin oxide substrates by electrodeposition in aqueous solution and were transformed into porous ZnO by calcination in air. The synthesized materials were characterized by X-ray diffraction, scanning electron microscopy with electron backscatter diffraction, Fourier transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. The layered structure of the highly oriented LZHs was parallel to the substrate surface and all transformed to nanoporous ZnO with a ⟨0001⟩ preferred orientation. The ⟨0001⟩ orientation degree and in-plane orientation of the nanoporous ZnO differed significantly depending on the type of anion but not the decomposition temperature, revealing that the initial formation process of ZnO from the LZHs is crucial. Finally, a possible transformation mechanism explaining the difference in the resulting ZnO orientation by anions (NO3-, SO42-, and Cl-) is discussed on the basis of their layered structure and thermal decomposition processes.

Effects on external quantum efficiency of electrochemically constructed n-ZnO/p-Cu2O photovoltaic device by annealing

Cuprous oxide (Cu2O), a terrestrial abundant, low cost, nontoxic, intrinsically p-type oxide semiconductor with bandgap energy of about 2eV, has recently received increasing attention as a light absorbing layer in solar cells. However, the performances of electrochemically constructed Cu2O solar devices are poor compared to the theoretical power conversion efficiency. This research was conducted focusing on the EQE performance, which is closely related to the short circuit current of a solar device. ZnO/Cu2O-PV-devices were constructed electrochemically with 3-electrode cell on Ga:ZnO/SLG substrates; ZnO layers were deposited from an aqueous solution of 8 mmolL-1 zinc nitrate hexahydrate at 63°C, 0.01 Coulomb cm−2, and -0.8V, while Cu2O layers were deposited from aqueous solution containing 0.4 molL-1 copper (II) acetate monohydrate (pH12.5), at 40°C, 1.5 Coulomb cm−2, and -0.4V. Devices were then annealed under different temperatures of 150°C, 200°C, 250°C, and 300°C for 60 minutes with a Rapid Thermal A...

Preparation of Hollow Titanium Dioxide Shell Thin Films from Aqueous Solution of Ti-Lactate Complex for Dye-Sensitized Solar Cells

As photovoltaic devices possessing potential for low processing costs and flexible architectures, dye-sensitized solar cells (DSSCs) using nanocrystalline TiO2 (nc-TiO2) electrodes have been extensively studied.(Bisquert et al., 2004; O’Regan & Gratzel, 1991) Congruently with increasingly urgent dissemination of solar cells against crisis of a depletion of fossil fuel, DSSCs are as promising alternative to conventional silicon-type solar cells. The main trend of investigations of DSSCs originates from the epoch-making works by Gratzel and co-workers in the early 1990s. (O’Regan & Gratzel, 1991) A typical construction of the cells are composed of dye-molecules (usually Ru complexes) coated nc-TiO2 electrodes on transparent-conductive (TC) backcontact (usually fluorine-doped tin oxide (FTO)) glass substrate and counter Pt electrodes sandwitching triiodine/iodine [I3–/I–] redox liquid electrolyte layer maintaining electrical connection with the counter Pt electrode. The voids of the network of TiO2 nanoparticles connection form nanopores which are efficiently filled with electrolyte solution. An operation mechanism of DSSC begins with harvesting incident light by dye-molecules via photoexcitation of electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The photoexcited electrons are transferred to the conduction band of the nc-TiO2 and diffuse in TiO2 matrix to TC layer followed by ejection to outer electric load. The oxidized dye is reduced by the electrolyte (I–) and the positive charge is transported to Pt counter electrode. As well as close fitting of photo-absorption spectra of dyes to the spectrum of sunlight mainly in visible light region (nearly panchromatic dyes) (Nazeeruddin et al., 2001) the strong dye-TiO2 coupling leading to rapid electron transfer from excited dye to TiO2 (Tachibana et al., 1996) realizes practically promising solar-to-electrical conversion efficiency: more than 10 %. The charge separation of DSSCs occurs at the interface TiO2 nanoparticles / dye molecules / [I–/I3–] electrolyte. Therefore the combination of Rucomplex and TiO2 is currently almost ideal choice in DSSC. Some problems of the TiO2 nanoparticles electrode, however, remain room to investigate. Connection points of TiO2 nanoparticles decrease an effective area of interface, and play a role on electron scattering sites, leading to restrict the conversion efficiency.(Enright & Fizmaurice, 1996; Peng et al., 2003) Though denser films seemingly improve the electron migration, they result in decrease of surface area for dye adsorption. Additionally TiO2 nanoparticles electrodes are