The grand materials chemistry challenge for photoelectrochemical hydrogen production is development of new semiconductor anodes for water oxidation side of the water splitting reaction. Currently, the most widely studied candidates are metal oxides (e.g. Fe2O3, TiO2, BiVO4) and III-V compound semiconductors (e.g. GaP, InGaAs). Despite the significant progress made in recent years towards improvement of charge transport of the former and the increasing the operational stability of the latter, the search for alternative materials options is still on. Metal nitrides constitute an interesting class of compounds for this application because of stability similar to metal oxides and charge transport properties similar to compounds semiconductors. In particular tin (IV) nitride, Sn3N4, is a semiconductor composed of Earth-abundant elements, making it a potential candidate for very larger scale hydrogen production applications. Here we report on synthesis and semiconducting properties of Sn3N4 for the hydrogen production, as studied by thin-film experiments and first-principles theory. Polycrystalline thin films of Sn3N4 with spinel crystal structure were synthesized by reactive sputtering of Sn target in atomic nitrogen atmosphere as reported earlier for Cu3N [1]. The optical absorption onset was determined at 1.6 eV in good agreement with 1.5 eV calculated from GW theory. The electron concentration was found to be 1018 cm-3 with mobility of ~1 cm2/Vs. Most importantly, calculations and experiments agree that Sn3N4 band gap straddled the water oxidation and reduction potentials. The measured photocurrent in the polycrystalline Sn3N4 thin films was limited by the 50-100 nm minority carrier (holes) diffusion length, on the order of the grain sizes, calling for further improvement in thin film growth process to achieve larger grains. Another contribution to this short minority carrier diffusion length is the calculated large hole effective mass (12.9me), which contrasts with very small electron effective mass (0.18me). Theoretical calculations suggest that alloying Sn3N4 with other group-IV nitrides can improve hole effective masses by both changing the electronic structure (band engineering) and crystal structure (polymorphism). [1] Mater. Horiz., 1, 424 (2014)

Tin nitride spinel semiconductor for photoelectrochemical water oxidation

249th National Meeting (2015)

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