Nanostructured Ti_1‑xS_xO_2‑yN_y Heterojunctions for Efficient Visible-Light-Induced Photocatalysis

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 Ti⁴⁺–thiourea complex upon reaction between peroxo–titania complex and thiourea. Decomposition of the Ti⁴⁺–thiourea complex and formation of visible-light-active S,N-codoped TiO₂ heterojunctions are confirmed using X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and UV/vis spectroscopic studies. Existence of sulfur as sulfate ions (S⁶⁺) 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-TiO₂ 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.