Tuning the optical and photoelectrochemical properties of surface-modified TiO2.

Surface-modification of TiO(2) is found to be a powerful tool for manipulating the fundamental optical and photoelectrochemical properties of TiO(2). High surface area nanocrystalline TiO(2) was modified by urea pyrolysis products at different temperatures between 300 degrees C and 500 degrees 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 TiO(2) continuously decreases down to approximately 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 degrees C and 400 degrees C (IPCE approximately 14% at 400 nm, and IPCE approximately 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.