Photoelectrochemical Study of the Band Structure of Zn2SnO4 Prepared by the Hydrothermal Method

It is fundamentally interesting to study the photoelectrochemical properties of complex oxides for applications in photovoltaics and photocatalysis. In this paper, we study the band gap (E-g) and energetics of the conduction band (CB) and valence band (VB) for films of zinc stannate (Zn2SnO4) nanoparticles (ca. 25 nm) of the inverse-spinel structure prepared by the hydrothermal method. UV-vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemistry, and photoelectrochemistry were used to study the films. The fundamental E-g for Zn2SnO4 is proposed to be 3.6-3.7 eV with a direct-forbidden transition. The position of the CB was approximated from the flat band potential, E-fb, measured by the photocurrent onset potential. In aqueous and nonaqueous solutions the E-fb of n-Zn2SnO4 was found to be more positive than TiO2 anatase in the electrochemical scale. In aqueous solutions E-fb of Zn2SnO4 was found to follow a 59 mV/pH slope with E-fb extrapolated at pH 0 of 0.08 V vs NHE. In acetonitrile solutions that simulate the electrolyte for dye-sensitized solar cells (DSCs) the E-fb of Zn2SnO4 was found to be strongly dependent on electrolyte composition and more positive than TiO2 vs the l(-)/l(3)(-) couple. The reverse trend observed for the open-circuit voltage in certain DSC electrolytes is explained in terms of the higher rates of electron-triiodide recombination of TiO2 despite the lower position of the Zn2SnO4 CB in the vacuum scale.