Ultrathin sheets of oxide materials are of interest as highly efficient catalysts that drive a variety of chemical reactions on exposure to light. The photocatalytic efficiency of these materials under visible light can be significantly enhanced by introducing nitrogen into the atomic structure, although doing so has generally involved complex processes or harsh conditions. Researchers from Kumamoto University in Japan1 have now developed a light-assisted process that allows efficient nitrogen doping of oxide nanosheets under mild conditions.

Fig. 1: Models of Nb–O nanosheets and their band structures before and after UV-assisted photo-N doping.

The researchers, lead by Yasumichi Masumoto, prepared nanosheets of various niobium-based oxides by a process involving protonation (proton addition) of nano-layered starting materials, separation of the materials into individual nanosheets using tetrabutylammonium hydroxide (TBAH), loading with platinum, and finally restacking of the nanosheets under irradiation with ultraviolet light.

Ultraviolet irradiation in the final restacking step causes the TBAH (added to separate the nanosheets) to decompose, forming nitrogen dopants. The presence of platinum promotes this light-assisted nitrogen-doping process, which could only be achieved using the combination of TBAH and niobium.

Nitrogen doping narrows the band gap — the energy required to make an electron jump between energy bands — of the oxide nanosheets, making the material catalytically active under excitation with light at wavelengths longer than usual. This increases the efficiency of the material for absorbing solar light. The researchers demonstrated this improved photocatalytic performance by using the nitrogen-doped niobium-oxide catalyst to produce hydrogen gas from water when irradiated with light at visible wavelengths. Nanosheets that were not successfully nitrogen-doped, on the other hand, did not show any activity for this reaction.

The present findings demonstrate a novel approach for the doping of oxide nanosheets, and show promise for a number of catalytic processes for the splitting of molecules. Even beyond such processes, this method might have further implications. “We expect that in the near future unique nanosheets with other interesting functions are produced, which are doped by various elements,” says Yasumichi Masumoto.