Enhanced Photocatalytic Hydrogen Evolution of Highly Transparent Ti_1–<i>x</i>Sn_<i>x</i>O₂ Aqueous Nanocolloids: Facile Preparation of a Monomeric Dispersion of Particle Size-/Band Structure-Tunable Nanoparticles

A transparent aqueous dispersion of mixed titanium/tin oxide (TSO: Ti1–xSnxO2) nanoparticles (NPs) was successfully prepared by a systematic variation in composition (x = 0, 0.12, 0.41, 0.60, 0.80, 1.0) through the propagative hydrolysis/condensation of mixed metal halides. The transparency of the aqueous dispersion of TSO was maintained in methanol/water (9/1, v/v, with 0.1 M HCl) for at least three years. TSO NPs exhibited an apparent single phase of solid solutions with the core rutile microcrystal covered by an amorphous surface layer except TiO2 (x = 0) in anatase as the core microcrystal covered with an amorphous layer. Measurements of band gaps and flatband potentials revealed that both the conduction and valence bands of TSO (x = 0.12–0.80) suffered a quantum size effect by their small particle sizes (3.8 ∼ 6.5 nm ϕ) and were situated at similar positions to those of TiO2, suggesting that even a small amount of Sn4+ provides the larger lattice of a rutile crystal where the smaller Ti4+ ion can be easily accommodated to exhibit a TiO2 (rutile)-like band structure. The amorphous surface structure of TSO NPs with an apparently random mixing of Ti4+/Sn4+ was verified by detailed studies on both the much larger surface area than the spherical model for the particle and the number of surface-OH groups. Hydrogen evolution as a typical photocatalytic reaction for TSO NPs in aqueous methanol by the irradiation with UV light (254 nm) showed an interesting volcano-type enhancement in reactivity against the Sn4+ content, which was successfully understood by the two contradicting effects of (1) decreasing reactivity with increasing -O–Sn–O- content against (2) enhancement by the decrease of the electron trap density within one particle. An efficient hole transfer from -O-Sn-O- into -O-Ti-O- sites should be a key process in the photocatalytic behavior of TSO. The inclusion of Sn4+ has the advantage of both providing a rutile lattice to maintain the conduction band position and a small number of electron traps to exhibit the enhanced photocatalytic hydrogen evolution.