Tuning the optical and photoelectrochemical properties of surface-modified TiO2

Abstract

Surface-modification of TiO₂ is found to be a powerful tool for manipulating the fundamental optical and photoelectrochemical properties of TiO₂. High surface area nanocrystalline TiO₂ was modified by urea pyrolysis products at different temperatures between 300 °C and 500 °C. Modification occurs through incorporation of nitrogen species containing carbon into the surface structure of titania. The N1s XPS binding energies are 399–400 eV and decrease with increasing modification temperature whereby the Ti2p_3/2 peak is also shifted to lower binding energies by about 0.5 eV. With increasing modification temperature the optical bandgap of surface-modified TiO₂ continuously decreases down to ∼2.1 eV and the quasi-Fermi level of electrons at pH 7 is gradually shifted from –0.6 V to –0.3 V vs.NHE. The surface-modified materials show enhanced sub-bandgap absorption (Urbach tail) and exhibit photocurrents in the visible down to 750 nm. The maximum incident photon-to-current efficiency (IPCE) was observed for the materials modified at 350 °C and 400 °C (IPCE ∼ 14% at 400 nm, and IPCE ∼ 1% at 550 nm, respectively). The efficiency of photocurrent generation is limited by surface recombination, which leads to a significant decrease in IPCE values and significantly changes the shape of the IPCE spectra in dependence on the optical bandgap.