KEY POINTSTohoku and Hokkaido researchers make nickel oxide photocathodes from 10-20 nanometer nanocrystals using supercritical hydrothermal synthesis and push-coatingPush-coated NiO films show about three times higher photocurrent conversion efficiency than Langmuir-Schaefer films under 330-nanometer ultraviolet lightMethod forms 100-500 nanometer semiconductor layers with lower solvent use and material loss for artificial photosynthesis applications
Researchers used supercritical hydrothermal synthesis and push-coating to form 100-500 nanometer NiO films for artificial photosynthesis electrodes. Photo by Veli Batuhan Aytaç on Unsplash
Veli Batuhan Aytaç
Researchers at Tohoku University and Hokkaido University have developed a way to produce nickel oxide semiconductor electrodes for reduction reactions by laying down nanocrystals in thin films, according to a joint release issued on June 29.
The method could provide a lower-environmental-burden route to making electrode materials for applications such as artificial photosynthesis, where semiconductor electrodes use light energy to drive chemical reactions including hydrogen generation and carbon dioxide reduction.
The research team, led by Associate Professor Tomoya Oshikiri and Professor Masaru Nakagawa at Tohoku University’s Institute of Multidisciplinary Research for Advanced Materials, worked with Professor Takaaki Tomai of Tohoku University and Professor Yasutaka Matsuo and Associate Professor Xu Shi of Hokkaido University’s Research Institute for Electronic Science.
The researchers synthesized highly crystalline p-type nickel oxide nanocrystals with particle sizes of about 10 to 20 nanometers using supercritical hydrothermal synthesis. In the experiment, water was brought into a supercritical state and the reaction was carried out at 400 degrees Celsius and about 38 megapascals, yielding nanocrystals of the targeted size when oleic acid was present.
The team then formed nickel oxide layers about 100 to 500 nanometers thick using a push-coating method with a flat stamp. In that process, nanocrystals dispersed in n-octane were placed on a transparent conductive glass substrate and spread with a flat polydimethylsiloxane stamp, forming a film as solvent was removed from the narrow space between the stamp and substrate.
The resulting nickel oxide layer functioned as a semiconductor electrode for reduction reactions, or photocathode, and showed current responses under illumination that changed regularly with film thickness. The release also said incident photon-to-current conversion efficiency normalized by coverage was inversely proportional to film thickness, indicating the push-coating method can be used to design photocathodes that operate uniformly across the electrode surface.
The researchers also found that the push-coated films had smaller surface irregularities than films made by the conventional Langmuir-Schaefer method. Under irradiation with 330-nanometer ultraviolet light, the push-coated films showed about three times higher incident photon-to-current conversion efficiency than films produced by the Langmuir-Schaefer technique. The release attributed the improvement to fewer large gaps between particles and tighter junctions among secondary particles, which made it easier for holes to move through the film.
Nickel oxide is a chemically stable p-type semiconductor with a wide band gap and electronic states favorable for reduction reactions. Photocathodes are one of the core components in artificial photosynthesis systems, which aim to convert solar energy into chemical fuels.
The paper was published in the Royal Society of Chemistry journal Nanoscale Advances on June 18. The research was supported by grants from the Japan Society for the Promotion of Science and other Japanese government-backed research programs, according to the release.
AloJapan.com