No abstract

The low-temperature sinterability of TiO 2 -CuO systems was examined through a two-step sintering process. The first-step heat treatment was carried out at 900 °C for either 5 min or 1 h. The subsequent second-step sintering by reheating after cooling proceeded at 800 °C for various times. Micron-sized (2 µm) powders as received were used as the starting materials. In the present system, the densification behaviors through grain-boundary diffusion were similar to those in nanocrystalline ceramics at low temperatures. The difference in temperature sensitivity for densification or grain growth was thus exploited to enhance densification and hinder grain growth. As a result, enhanced densification without notable grain growth was achieved after second-step sintering, as opposed to conventional sintering at 800 °C. This implies that the grain-boundary diffusion contributing to densification was more active than the grain-boundary migration at 800 °C. In this regard, the current results show a trait of activated sintering as well.

We investigated the effects of stoichiometry on photovoltages and photocurrents in CuBi2O4 thin-film photocathodes grown by pulsed laser deposition under different oxygen partial pressures to manipulate their stoichiometry. While the X-ray diffraction patterns show crystalline phases in the CuBi2O4 thin films, it is found that the Cu/Bi ratio of the CuBi2O4 thin films varied from ~0.3 to ~0.5 which are analyzed by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. The slightly off-stoichiometric CuBi2O4 thin-film photocathode with a Cu/Bi ratio of ~0.44 shows the highest photocurrent density in the CuBi2O4 thin films. More interestingly, the off-stoichiometric CuBi2O4 thin-film photocathode with a Cu/Bi ratio of ~0.44 exhibited a stable open-circuit voltage difference of ~0.2 V RHE without severe degradation over time. On the other hand, the photovoltage of the stoichiometric CuBi2O4 thin-film photocathode with a Cu/Bi ratio of ~0.5 gradually decreased as a function of time. Our results suggest that stoichiometry manipulation can be one of the promising strategies to achieve long-term stable Cu-based oxide photocathodes with the maintenance of a stable photovoltage.