Kei Murakoshi

Importance of binding states between photosensitizing molecules and the TiO2 surface for efficiency in a dye-sensitized solar cell

The construction of dye-sensitized TiO2 solar cells is studied using a ruthenium complex, cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicar☐ylic acid)-ruthenium (II), as a photosensitizing molecule (dye) on TiO2 nanocrystallite films to determine the interfacial binding effect on photoenergy conversion efficiency. Photoconversion efficiency is improved by electrochemical treatment of TiO2 films, or reflux treatment for effective anchoring of the photosensitizing molecules on TiO2 nanocrystallite films. Characteristics of interfacial binding between dye molecules and the TiO2 surface is determined and compared using IR and UV-visible spectra. The ester-like linkage and the chelating car☐ylato linkage are formed between car☐ylic acid groups of ligands in dye molecules and TiO2 nanocrystallites. The ester-linkage is observed as major on the film which shows relatively high photoenergy conversion efficiency. The improvement of cell properties such as open circuit cell voltage (Voc) can be explained by the decrease in charge on bipyridine ligands of the dye molecule owing to the formation of the covalent-like linkage. The importance of binding states between photosensitizing molecules and TiO2 is discussed with a view to achieving high efficiency in dye-sensitized solar cell.

Fabrication of solid-state dye-sensitized TiO2 solar cells combined with polypyrrole

Abstract A solid-state solar cell was fabricated by photoelectrochemical polymerization of pyrrole on porous nanocrystalline TiO 2 electrode sensitized by the Gratzel dye, cis -di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)-ruthenium (II) dihydrate, [RuL 2 (NCS) 2 ]), or a newly synthesized cis -Ru(dcb) 2 (pmp) 2 (pmp=3-(pyrrole-1-ylmethyl)-pyridine). Polypyrrole successfully worked as a hole-transport layer with improvement of the cell characteristics when the TiO 2 cell with cis -Ru(dcb) 2 (pmp) 2 was compared with the similarly fabricated cells using [RuL 2 (NCS) 2 ]. The improvement by using Ru(dcb) 2 (pmp) 2 can be explained as due to direct molecular wiring of the polymer-chain to the excited metal center of the complex.

Strategies for enhancing photoluminescence of Nd3+ in liquid media

Abstract The excited state (4F3 2) of Nd3+ readily undergoes radiationless energy transfer via vibrational excitation of surrounding medium and dipole-dipole energy transfer via cross relaxation and energy migration, which makes it difficult to observe luminescence of Nd3+ in a liquid matrix. In order to suppress such de-excitation of the 4F3 2 state, two kinds of s-diketonato ligands which have no CH and OH bonds having high vibrational frequency were successfully designed for observing luminescence of Nd3+ in a liquid matrix. Luminescence of Nd3+ was observed for the first time by using deuterated tris-(hexafluoroacetylacetonato) neodymium(III), Nd(HFA-D)3, in deuterated organic solvents. The luminescence intensity, lifetime and quantum yield of the complex were measured in methanol-d4, acetone-d6, THF-d8, DMF-d7 and DMSO-d6. Deuterated tris(bis-(perfluorooactanoyl)methanol)neodymium(III), Nd(POM-D)3, gave enhanced luminescence in DMSO-d6 by minimizing energy migration during diffusional collisions in the liquid matrix. The intensity was independent of concentration in the micromolar range from 0.01 to 0.07M. The bulky perfluoroalkyl groups in the ligands effectively prevented de-excitation via vibrational excitation and cross relaxation in liquid media.

Near-infrared plasmon-assisted water oxidation

We report the stoichiometric evolution of oxygen via water oxidation by irradiating a plasmon-enhanced photocurrent generation system with near-infrared light (λ: 1000 nm), in which gold nanostructures were arrayed on the surface of TiO2 electrode. It is considered that multiple electron holes generated by plasmon-induced charge excitation led to the effective recovery of water oxidation after the electron transfer from gold to TiO2. The proposed system containing a gold nanostructured TiO2 electrode may be a promising artificial photosynthetic system using near-infrared light.

Mesoporous electrodes having tight agglomeration of single-phase anatase TiO2 nanocrystallites: Application to dye-sensitized solar cells

Abstract Single-phase anatase nanocrystalline HyCOM-TiO2 (Hydrothermal Crystallization in Organic Media) to label this method was synthesized by high-temperature hydrolysis of titanium tetrabutoxide in toluene. The resulting HyCOM-TiO2 nanocrystallites were found to be covered by n-butoxide, yielding mesoporous, transparent anatase films with a narrow pore size distribution and good electron transport characteristic when sintered at 350–550°C on optically transparent conducting glass. Dye-sensitized solar cells made of the Ru-dye-adsorbed mesoporous HyCOM films as photoanodes achieved better photo-energy conversion efficiency as compared to those prepared using commercially available Degussa P25 films.

Enhancement of luminescence of Nd3+ complexes with deuterated hexafluoroacetylacetonato ligands in organic solvent

Abstract Tris-(hexafluoroacetylacetonato)neodymium(III), [Nd(HFA-D)3], was prepared by chelation of Nd3+ ion with deuterated hexafluoroacetylacetone in CD3OD. Luminescence of the Nd3+ complex was observed for the first time in organic solvents and the quantum yield was estimated to be of the order of 10−2 in deuterated acetone solution. The absorption spectrum of [Nd(HFA-D)3] dissolved in acetone was comparable with that of Nd3+ ion in Y3Al5O15 matrix (Nd:YAG). Splitting of the 4 F 3 2 level was determined to be 82.3 cm−1 in this system. These spectral characteristics suggest that the physical nature of Nd3+ coordination environments should be uniform and well defined by coordination of HFA in solution.

Plasmon-Assisted Water Splitting Using Two Sides of the Same SrTiO3Single-Crystal Substrate: Conversion of Visible Light to Chemical Energy

A plasmon-induced water splitting system that operates under irradiation by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate (SrTiO3) single-crystal substrate. The water splitting system contains two solution chambers to separate hydrogen (H2) and oxygen (O2). To promote water splitting, a chemical bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a reduction co-catalyst, was twice the quantity of O2 evolved from an Au-nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen-evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon-induced charge separation at the Au/SrTiO3 interface promotes water oxidation and the subsequent reduction of a proton on the backside of the SrTiO3 substrate. The chemical bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.

Conductance of a single molecule anchored by an isocyanide substituent to gold electrodes

The effect of anchoring group on the electrical conductance of a single molecule bridging two Au electrodes was studied using disubstituted [isocyanide (CN–), thiol (S–), or cyanide (NC–)] benzene. The conductance of a single Au/1,4-diisocyanobenzene/Au junction anchored by isocyanide via a C atom (junction with the Au–CN bond) was 3×10−3G0(=2e2∕h). The value was comparable to 4×10−3G0 of a single Au/1,4-benzenedithiol/Au junction with the Au–S bond. The Au/1,4-dicyanobenzene/Au molecular junction with the Au–NC bond did not show well-defined conductance values. The metal-molecule bond strength was estimated by the distance over which the molecular junction was stretched before breakdown. The stretched length of the molecular junction with the Au–CN bond was comparable to that of the Au junction, indicating that the Au–CN bond was stronger than the Au–Au bond.

Preparation of size-controlled hexagonal CdS nanocrystallites and the characteristics of their surface structures

Hexagonal CdS nanocrystallites have been easily prepared by reacting Cd2+ and S2- (H2S or Na2S) in N,N-dimethylformamide (DMF) without adding any surface stabilizers. The mean diameter of the nanocrystallites can be rigorously controlled over the range 1.8–4.2 nm with keeping the hexagonal crystalline structure by varying the sulfur source (H2S and Na2S) and by regulating the preparation temperature. Use of Na2S as a sulfur source produces smaller CdS nanocrystallites than use of H2S. The size control and stability of CdS nanocrystallites are discussed in terms of the characteristics of the surface structures based on insitu Cd K-edge EXAFS analysis.

Conductance of single 1,4-disubstituted benzene molecules anchored to Pt electrodes

The authors have studied the conductance of a 1,4-disubstituted isocyanide(–NC) or thiol(–SH) benzene molecule anchored to two Pt electrodes. A single molecular junction showing a well-defined conductance value (∼3×10−2G0, G0=2e2∕h) was fabricated with the Pt electrodes. The conductance of the molecular junction was one order higher than the previously documented value using Au electrodes. These observations could be explained by differences in the local density of states of the contact metal atom at the Fermi level and the extent of the hybridization and energy difference between the molecular and metal orbitals. Further insight into the binding strengths of the metal-anchoring group bond was obtained by statistically analyzing the stretching length of the molecular junction.