Discovering the Unexplored: Synthesis and Analysis of a New Orthorhombic Sn3O4Polymorph
Tuning the reaction conditions such as degree of filling and gas composition can have amajor impact on the products obtained by hydrothermal synthesis obtained. This wasclearly represented in the new Tokyo Tech study where they synthesized an unreported
orthorhombic polymorph of Sn3O4 instead of conventional monoclinic phase by optimizing
the conditions inside the hydrothermal reactor. The orthorhombic Sn3O4 has a narrower
bandgap than the conventional one, thus making it useful as a visible-light active
photocatalyst.

Oxides of tin (SnxOy) are found in many of modern technologies due to their versatilenature. The multivalent oxidation states of tin—Sn2+ and Sn4+—impart tin oxides withelectroconductivity, photocatalysis, and various functional properties. For the photocatalysis
application of tin oxides, a narrow bandgap for visible-light absorption is indispensable toutilize a wide range of solar energy. Hence, the discovery of new SnxOy could help improvethe efficiency of many environmentally significant photocatalytic reactions like water
splitting and CO2 reduction. While there are many theoretical and computational
predictions of new stable SnxOy, there still remains a need for experimental studies that canturn the predictions into reality.
Taking this as a call to action, researchers from Tokyo Institute of Technology, National
Defense Academy, and Mitsubishi Materials Corporation have designed a new tin oxide. In
their recent breakthrough published in Angewandte Chemie International Edition, Mr. Y. Liuet al. presented a new optimized hydrothermal synthesis approach that led to the synthesisof a Sn3O4 polymorph with a previously unreported orthorhombic crystal structure. The
research was performed in the Mitsubishi Materials Sustainability Innovation Collaborative
Research Cluster with the support of the Tokyo Institute of Technology Open Innovation
Platform.
The project leader, Prof. Miyauchi explains the driving force behind the study, “The aim ofour study was two-fold. First was the synthesis of a new tin oxide polymorph and the second
was applying it for a visible-light sensitive photocatalyst.”
The team set up multiple thermal hydrothermal reactors with the same starting material for
preparing Sn3O4. In the first series one set, they altered the degree of filling of the precursorsolution by filling 20, 40, 60, and 80% of a 100 ml Teflon liner. For the second series, theykept the degree of filling constant at 20% and the Teflon liners were filled with ambient air,pure oxygen, and pure nitrogen respectively.
The team then carried out Rietveld analysis, X-ray spectroscopy, and first-principles
calculations on the products formed. The analysis showed the new Sn3O4 polymorph has thechemical formula of Sn(II)2Sn(IV)O4. Its X-ray diffraction pattern has never been reported andis assigned to an orthorhombic crystal phase based on empirical and computational
analyses. The comparative studies for tuning of gas composition and degree of filling
showed that the orthorhombic polymorph was only formed when the degree of filling washigh or when the gas introduced was inert and has less oxygen. The team hence suggested
that paying attention to the oxygen source could be the key to more precise hydrothermal
synthesis.
The novel orthorhombic Sn3O4 polymorph reported in this study has a smaller bandgap thana conventional monoclinic Sn3O4, indicating a higher efficiency of absorbing visible light.
Furthermore, the conduction band of the orthorhombic polymorph is enough high to drive
CO2 reduction reaction.
Hydrothermal method is a widely used method of materials synthesis. This study finds thatthe often-neglected parameters in hydrothermal synthesis drastically affect the crystalstructure. This finding is informative for the discovery of numerous new oxide materials.

Reference
Yang-Shin Liu, 1 Akira Yamaguchi, 1 Yue Yang, 1 An Niza El Aisnada,1 Sho Uchida,1 HidekiAbe,2 Shigenori Ueda,3,4,5 Kenji Yamaguchi,6 Toyokazu Tanabe,7 and Masahiro
Miyauchi*1Title:
Synthesis and Characterization of the Orthorhombic Sn3O4 Polymorph
Journal:Angewandte Chemie International Edition
DOI: 10.1002/anie.202300640
DOI:
Affiliations: 1 Department of Materials Science and Engineering, School of Materials and Chemical
Technology, Tokyo Institute of Technology, Japan
2Center for Green Research on Energy and Environmental Materials, National Institutefor Materials Science, Japan.
3
Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, Japan.
4
Research Center for Advanced Measurement and Characterization, National Institutefor Materials Science, Japan.
5
Research Center for Functional Materials, National Institute for Materials Science,Japan.
6
Innovation Center, Mitsubishi Materials Corporation, Japan.
7
Department of Materials Science and Engineering, National Defense Academy, Japan.
Authors:*Corresponding author’s email: mmiyauchi@ceram.titech.ac.jp
Further information
Professor Masahiro Miyauchi
Department of Materials Science and Engineering
School of Materials and Chemical Technology
Tokyo Institute of Technology, Japanmmiyauchi@ceram.titech.ac.jp