Takuya Masuda

Boron Nitride Nanosheet on Gold as an Electrocatalyst for Oxygen Reduction Reaction: Theoretical Suggestion and Experimental Proof

Boron nitride (BN), which is an insulator with a wide band gap, supported on Au is theoretically suggested and experimentally proved to act as an electrocatalyst for oxygen reduction reaction (ORR). Density-functional theory calculations show that the band gap of a free h-BN monolayer is 4.6 eV but a slight protrusion of the unoccupied BN states toward the Fermi level is observed if BN is supported on Au(111) due to the BN-Au interaction. A theoretically predicted metastable configuration of O2 on h-BN/Au(111), which can serve as precursors for ORR, and free energy diagrams for ORR on h-BN/Au(111) via two- and four-electron pathways show that ORR to H2O2 is possible at this electrode. It is experimentally proved that overpotential for ORR at the gold electrode is significantly reduced by depositing BN nanosheets. No such effect is observed at the glassy carbon electrode, demonstrating the importance of BN-substrate interaction for h-BN to act as the ORR electrocatalyst. A possible role of the edge of the BN islands for ORR is also discussed.

Hysteresis-free and highly stable perovskite solar cells produced via a chlorine-mediated interdiffusion method

Although lead-halide perovskite-based solar cells hold the promise of a breakthrough in the production of next-generation photovoltaic devices, anomalous hysteresis in current–voltage curves and inadequate stability remain as major challenges. Here, we demonstrate the production of low-temperature solution-processed perovskite solar cells (ITO/PEDOT:PSS/perovskite/PC61BM/Ca/Ag) with hysteresis-free current–voltage characteristics, excellent photostability, and high reproducibility via the inclusion of methyl ammonium chloride (MACl) using the interdiffusion method. The best-performing devices exhibited a power conversion efficiency of over 12%. Our devices showed promising stability by maintaining more than 90% of their initial performance over long periods of time at ambient conditions with encapsulation using common techniques, as well as no obvious degradation after 2 h of continuous light exposure. We statistically compared fabrication processes using the interdiffusion method with or without MACl by creating a histogram of over 120 devices for each method. The results clearly indicated that including MACl gave better reproducibility and a higher average efficiency of 9.5%, as well as improved device stabilities.

In situ x-ray photoelectron spectroscopy for electrochemical reactions in ordinary solvents

In situ electrochemical X-ray photoelectron spectroscopy (XPS) apparatus, which allows XPS at solid/liquid interfaces under potential control, was constructed utilizing a microcell with an ultra-thin Si membrane, which separates vacuum and a solution. Hard X-rays from a synchrotron source penetrate into the Si membrane surface exposed to the solution. Electrons emitted at the Si/solution interface can pass through the membrane and be analyzed by an analyzer placed in vacuum. Its operation was demonstrated for potential-induced Si oxide growth in water. Effect of potential and time on the thickness of Si and Si oxide layers was quantitatively determined at sub-nanometer resolution.

In situ observation of carrier transfer in the Mn-oxide/Nb:SrTiO3 photoelectrode by X-ray absorption spectroscopy

The Mn-oxide/Nb:SrTiO3 photoelectrode for oxygen evolution reaction was investigated by in situ Mn K-edge XAFS spectroscopy under UV irradiation. The oxidization of the Mn oxide was observed via photoexcited carrier transfer, which results in the positive potential shift of the Mn oxide cocatalyst toward oxygen evolution reaction.

Molecular Catalysts Confined on and Within Molecular Layers Formed on a Si(111) Surface with Direct Si–C Bonds

Two examples of confined molecular catalysts are presented. PtCl(4) (2-) complexes are attached to a thiol-terminated monolayer by ligand exchange of Cl(-) with a thiolate group and incorporated in a multilayer of viologen moieties by ion exchange. All Cl(-) ligands are replaced by OH(-) or H(2) O before HER takes place. Ex situ and in situ XAFS measurements confirm that the Pt complexes accelerate HER without being converted into Pt particles.

Potential-Dependent Adsorption/Desorption Behavior of Perfluorosulfonated Ionomer on a Gold Electrode Surface Studied by Cyclic Voltammetry, Electrochemical Quartz Microbalance, and Electrochemical Atomic Force Microscopy

Potential-dependent adsorption/desorption behavior of perfluorosulfonated ionomer (PFSI) on a gold electrode was investigated by cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), and electrochemical atomic force microscopy (EC-AFM) in a Nafion (i.e., PFSI) dispersed aqueous solution without any other electrolyte. It was found that PFSI serves as an electrolyte and that electrochemical measurements can be performed in this solution without any significant IR drop. PFSI molecules were adsorbed on the Au surface in the lying-down configuration in the potential range between 0 and 0.45 V, the amount of adsorbed PFSI increased when the potential was made more positive than 0.75 V, and the adsorbed PFSI fully desorbed from the surface at potentials more positive than 1.4 V where gold oxide was formed. Once the gold oxide had been reduced, PFSI readsorbed on the surface, albeit slowly. PFSI desorbed from the surface as the potential was made more negative than 0 V. These processes took place reversibly.

Enhancement in efficiency and optoelectronic quality of perovskite thin films annealed in MACl vapor

We analyzed and compared quantitatively the optoelectronic characteristics of perovskite PV devices with and without annealing the perovskite layer in a methyl ammonium chloride vapor atmosphere (MACl treatment). We found that the MACl treatment resulted in the mitigation of defect states, reduced defect density, improvement in the carrier profile, and passivation of recombination activities, which we infer as natural consequences of significantly improved film quality with better crystallinity and grain morphology of the perovskite layer. MACl-treated devices are more efficient with the best efficiency of ∼15.1% with small standard deviation (std.) (0.50%) and improved stability compared to devices without MACl treatment having the best efficiency of 12.4% with std. of 0.66%.

Functionalization of silicon surfaces with catalytically active Pd complexes and application to the aerobic oxidation of benzylic alcohols

A single-crystal silicon surface was modified with a bisoxazoline-Pd molecular layer and utilized as a highly efficient (catalyst turnover number up to 780,000, 110 degrees C, 72 h) and recyclable catalyst in the aerobic oxidation of benzylic alcohols.

Charge transport at the interface of n-GaAs (100) with an aqueous HCl solution: Electrochemical impedance spectroscopy study

Charge transport processes at the interface of n-GaAs (100) with an aqueous HCl solution are studied by electrochemical impedance spectroscopy. It is found that when open-circuit potential and anodic potentials are applied to the semiconductor the impedance spectra contain two capacitive semicircles corresponding to the capacitances of the space charge layer and surface states. In the case of open-circuit potential, semiconductor band bending at the interface with the solution is about 0.7 eV and the density of occupied surface states in the dark and under daylight conditions is 1.6 and 2.8 × 1012 cm2 eV−1, respectively. When cathode potentials are applied to GaAs, hydrogen evolution begins at the semiconductor/electrolyte interface and an additional inductive loop appears in the impedance spectra. At the same time, the density of occupied surface states increases considerably both due to a straightening of the semiconductor bands and to the appearance of As-H bonds. Thus, charge transport through the n-GaAs (100)/aqueous HCl solution interface is always mediated by semiconductor surface states.

Origin of the enhancement of electrocatalytic activity and durability of PtRu alloy prepared from a hetero bi-nuclear Pt–Ru complex for methanol oxidation reactions

PtRu alloy, prepared by pyrolysis of hetero bi-nuclear Pt–Ru complexes, showed a higher electrocatalytic activity and durability in the methanol oxidation reaction (MOR) than PtRu alloy prepared by pyrolysis of a mixture of K2PtCl4 and RuCl3. The origin of this activity enhancement was investigated by XPS and Ru K edge XAFS measurements. Although both alloys were composed of metallic Pt and RuO2, the RuO2 in the PtRu alloy prepared from the Pt–Ru complex had greater oxygen deficiency than that in the PtRu alloy prepared from the mixture of K2PtCl4 and RuCl3. XAFS measurements showed that the Ru–O distance of the former was longer than that of the latter, suggesting the presence of the weakly coordinated oxygen species, i.e., H2O, in the PtRu alloy prepared from the Pt–Ru complex. The H2O coordinated to Ru can become an active oxidant, i.e., surface Ru–OH, under electrochemical conditions and enhance MOR activity and durability.