Use of surface photovoltage spectroscopy to probe energy levels and charge carrier dynamics in transition metal (Ni, Cu, Fe, Mn, Rh) doped SrTiO3 photocatalysts for H2 evolution from water

Abstract

Doping with transition metal ions is widely employed to adjust the optical and photocatalytic properties of wide band semiconductors, however, quantitative information about the energetics and charge transfer dynamics of the impurity states is often difficult to obtain. Here we use surface photovoltage spectroscopy (SPS), optical spectroscopy, and irradiation experiments to study the effect of several dopants on the ability of SrTiO₃ nanocrystals to generate a photovoltage and to catalyse H₂ evolution from aqueous methanol. Phase pure SrTiO₃:TM nanocrystals with TM = Ni, Cu, Fe, Mn, Rh were synthesized by hydrothermal reaction of TiO₂, Sr(OH)₂, KOH, and transition metal chlorides and nitrates in water. SPS data was obtained on thin films of these nanocrystals on fluorine doped tin oxide substrates under vacuum atmosphere. All samples are n-type, which can be gauged from the negative photovoltage caused by the transfer of electrons into the FTO substrate. All dopants produce sub-bandgap states in the SrTiO₃ lattice, whose energetic positions can be determined from the photovoltage onset energy in SPS and from optical absorption spectra. The reversibility and size of the photovoltage provide information about the photohole dynamics and their ability to oxidize sacrificial electron donors at the nanocrystal surface. Overall, this work provides an explanation for the inability of Ni, Cu, Fe, Mn dopants to enhance visible light photocatalytic activity in SrTiO₃, and it establishes SPS as a useful tool to map the energetics and photochemistry of impurity states in metal oxide nanocrystals.