Abstract
Highly visible-light-active S,N-codoped anatase-rutile heterojunctions are reported for the first time. The formation of heterojunctions at a relatively low temperature and visible-light activity are achieved through thiourea modification of the peroxo-titania complex. FT-IR spectroscopic studies indicated the formation of a Ti(4+)-thiourea complex upon reaction between peroxo-titania complex and thiourea. Decomposition of the Ti(4+)-thiourea complex and formation of visible-light-active S,N-codoped TiO(2) heterojunctions are confirmed using X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and UV/vis spectroscopic studies. Existence of sulfur as sulfate ions (S(6+)) and nitrogen as lattice (N-Ti-N) and interstitial (Ti-N-O) species in heterojunctions are identified using X-ray photoelectron spectroscopy (XPS) and FT-IR spectroscopic techniques. UV-vis and valence band XPS studies of these S,N-codoped heterojunctions proved the fact that the formation of isolated S 3p, N 2p, and Π* N-O states between the valence and conduction bands are responsible for the visible-light absorption. Titanium dioxide obtained from the peroxo-titania complex exists as pure anatase up to a calcination temperature as high as 900 °C. Whereas, thiourea-modified samples are converted to S,N-codoped anatase-rutile heterojunctions at a temperature as low as 500 °C. The most active S,N-codoped heterojunction 0.2 TU-TiO(2) calcined at 600 °C exhibits a 2-fold and 8-fold increase in visible-light photocatalytic activities in contrast to the control sample and the commercial photocatalyst Degussa P-25, respectively. It is proposed that the efficient electron-hole separation due to anatase to rutile electron transfer is responsible for the superior visible-light-induced photocatalytic activities of S,N-codoped heterojunctions.