Tin Nitride as an Earth Abundant Photoanode for Water Splitting

Photoelectrochemical (PEC) water splitting--the conversion of water to hydrogen and oxygen using light--is an attractive route to the chemical storage of solar energy. We demonstrate that spinel tin nitride (Sn$_{3}$N$_{4}$) has conduction and valence bands that straddle the redox potentials of water and we study it as a photoannode material. Sn$_{3}$N$_{4}$ thin films have been grown on glass at ambient temperature by reactive sputtering of tin in a nitrogen atmosphere. The resulting materials were n-type semiconductors. Carrier concentration, carrier mobility, work function, and optical properties were measured. Results indicate that tin nitride has a band gap of $\sim$ 1.7 eV aligned around water's redox potentials. GW-corrected DFT-surface calculations that take into account water surface dipole interactions are consistent with experiment. Early PEC devices were made from Sn$_{3}$N$_{4}$ on fluorinated tin oxide with cobalt oxide catalysts and show a small but promising photoresponse ($\sim$ 0.1 mA/cm$^{2}$ at 1.23 V vs. RHE) under AM 1.5 illumination in 0.1 M potassium phosphate (pH= 7.25). Further work will focus on increasing the photocurrent in tin nitride devices by increasing film quality and identifying the proper catalyst.