Because of their versatility, tin oxides (SnXOj) are used in many current technologies. Tin oxides are endowed with electrical conductivity, photocatalysis and numerous functional characteristics due to its polyvalent oxidation states, Sn2+ and sn4+. A low visible light absorption bandgap is necessary for the application of photocatalysis of tin oxides to utilize various solar energies.
While there are many theoretical and computational predictions of new stable SnXOjthere remains a need for experimental studies that can turn the predictions into reality.
Scientists from the Tokyo Institute of Technology, National Defense Academy and Mitsubishi Materials Corporation have created a new tin oxide in response to this call to action. In recent work, they introduced a new improved hydrothermal synthesis technique that produced an Sn3O4 polymorph with an orthorhombic crystal structure never reported before.
The discovery could help improve the efficiency of many environmentally damaging photocatalytic reactions, such as water splitting and CO2 reduction.
The project leader, Prof. Miyauchi, explained: “The purpose of our research was twofold. The first was to synthesize a new tin oxide polymorph, and the second was to apply it to a visible photocatalyst.”
To prepare Sn3O4, the team built many thermal hydrothermal reactors using the same raw material. By filling 20, 40, 60 and 80% of a 100 ml Teflon liner, they changed the fill level of the precursor solution in the first series one set. The Teflon liners in the second run were filled with ambient air, pure oxygen and pure nitrogen while maintaining the fill level at 20%.
The manufactured products were then subjected to Rietveld analysis, X-ray spectroscopy and first principles calculations. The study results showed that the new form of Sn3O4 has the chemical formula Sn(II)2Sn(IV)O4. Based on empirical and computational assessments, the X-ray diffraction pattern – which has never been reported – is categorized as belonging to an orthorhombic crystal phase.
The orthorhombic polymorph was produced only if the filling degree was high or if the admitted gas was inert and contained less oxygen, according to comparative tests for matching gas composition and filling degree. So the scientists suggested that focusing on the oxygen supply could be the secret to achieving more accurate hydrothermal synthesis.
Compared to a conventional monoclinic Sn3O4the new orthorhombic Sn3O4 polymorph has a smaller bandgap. Meanwhile, it has higher efficiency in absorbing visible light. In addition, the conduction band of the orthorhombic polymorph is high enough to drive the CO2 reduction reaction.
Scientists noted, “This study shows that the often neglected parameters in hydrothermal synthesis drastically influence the crystal structure. This finding is informative for the discovery of numerous new oxide materials.”
- Yang-Shin Liu, Akira Yamaguchi et al. Synthesis and characterization of the orthorhombic Sn3O4 Polymorph. Angewandte Chemie International Edition. DOI: 10.1002/anie.202300640