Photoinduced Reactions of Surface-Bound Species on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FTIR Study

Photoinduced conversion of surface-bound species on titania nanotubes that were first oxidized and then reduced (Ti–NT–O₂–H₂) and on platinized titania nanotubes subjected to oxidation and reduction (Pt–Ti–NT–O₂–H₂) has been investigated by means of in situ FTIR spectroscopy. Bidentate and monodentate carbonates as well as bicarbonates and carboxylates are formed subsequent to exposure of both Ti–NT–O₂–H₂ and Pt–Ti–NT–O₂–H₂ to CO₂. Formic acid was only observed on Pt–Ti–NT–O₂–H₂. UV illumination of the nanotubes led to an increase in the number of surface-bound species as a result of the further reaction with gas-phase CO₂ with a greater increase in surface species on Ti–NT–O₂–H₂ than on Pt–Ti–NT–O₂–H₂. The underlying basis of the photoinduced increase in adsorbed species is discussed for both types of nanotubes. Photoinduced reactions of surface species also take place and are remarkably different on the two types of nanotubes. UV illumination of Ti–NT–O₂–H₂ converts bidentate carbonates and bicarbonates to monodentate carbonates and carboxylates. There are less, and different, photoinduced reactions of surface species on Pt–Ti–NT–O₂–H₂: bicarbonates and monodentate carbonates convert to bidentate carbonates on the platinized titania nanotubes, and there is no obvious reaction involving carboxylates and formic acid upon irradiation of the platinized nanotubes. These differences in reactive behavior are discussed in the context of platinum acting as an efficient trap for photoelectrons which mitigates against reduction of Ti⁴⁺ to Ti³⁺, stabilizes holes, and alters the surface photochemistry taking place on the two different types of nanotubes. Photoinduced holes play an important role in photochemistry via oxidation of “structural water” and concomitant production of undercoordinated titania sites