Exploring the undiscovered: Synthesis and analysis of a new orthorhombic Sn3O4 polymorph

A new optimized hydrothermal synthesis approach.

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Due to their versatility, tin oxides (SnxOy) are used in numerous current technologies. Tin oxides are endowed with electroconductivity, photocatalysis, and numerous functional features thanks to its multivalent oxidation states, Sn2+ and Sn4+. A low bandgap for visible-light absorption is necessary for the photocatalysis application of tin oxides to utilize various solar energy.

While there are many theoretical and computational predictions of new stable SnxOy, there 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 novel tin oxide in response to this call to action. In recent work, they introduced a new, improved hydrothermal synthesis technique that produced a SnO4 polymorph with a never-before-reported orthorhombic crystal structure.

The discovery could help improve the efficiency of many environmentally significant photocatalytic reactions like water splitting and CO2 reduction.

The project leader, Prof. Miyauchi, explained, “The aim of our study was two-fold. First was synthesizing a new tin oxide polymorph, and the second was applying it for a visible-light sensitive photocatalyst.”

To prepare Sn3O4, the team built up 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 level of filling of the precursor solution in the first series one set. The Teflon liners were filled with ambient air, pure oxygen, and pure nitrogen in the second series while maintaining the filling level at 20%.

The created products were then subjected to Rietveld analysis, X-ray spectroscopy, and first-principles calculations. The study results revealed that the novel form of Sn3O4 has the chemical formula Sn(II)2Sn(IV)O4. Based on empirical and computational assessments, its X-ray diffraction pattern—which has never been reported—is categorized as belonging to an orthorhombic crystal phase.

The orthorhombic polymorph was only produced when the degree of filling was high or when the gas admitted was inert and had less oxygen, according to comparative tests for tuning of gas composition and degree of fill. Hence, the scientists proposed that focusing on the oxygen supply might be the secret to achieving more precise hydrothermal synthesis.

Compared to a conventional monoclinic Sn3O4, the novel orthorhombic Sn3O4 polymorph has a smaller bandgap. Meanwhile, it has a higher efficiency in absorbing visible light. Furthermore, the conduction band of the orthorhombic polymorph is high enough to drive CO2 reduction reaction.

Scientists noted“This study finds that the often-neglected parameters in hydrothermal synthesis drastically affect the crystal structure. This finding is informative for the discovery of numerous new oxide materials.”

Journal Reference:

  1. Yang-Shin Liu, Akira Yamaguchi et al. Synthesis and Characterization of the Orthorhombic Sn3O4 Polymorph. Angewandte Chemie International Edition. DOI: 10.1002/anie.202300640
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