The chemical reactivity of different surfaces of titanium dioxide (TiO2) has been the subject of extensive studies in recent decades. The anatase TiO2(001) and its (1 × 4) reconstructed surfaces were theoretically considered to be the most reactive and have been heavily pursued by synthetic chemists. However, the lack of direct experimental verification or determination of the active sites on these surfaces has caused controversy and debate. Here we report a systematic study on an anatase TiO2(001)-(1 × 4) surface by means of microscopic and spectroscopic techniques in combination with first-principles calculations. Two types of intrinsic point defects are identified, among which only the Ti3+ defect site on the reduced surface demonstrates considerable chemical activity. The perfect surface itself can be fully oxidized, but shows no obvious activity. Our findings suggest that the reactivity of the anatase TiO2(001) surface should depend on its reduction status, similar to that of rutile TiO2 surfaces.

The water splitting reaction based on the promising TiO(2) photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti(5c)) sites of the reduced TiO(2) (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO(2). This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OH(br), and the other either occurs as OH(t) at Ti(5c) sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OH(t) or oxygen adatoms exactly at the original Ti(5c) sites, without the trace of OH(br). Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.

Effects of the inertia-induced radial confinement on the dynamic increase factor (DIF) of a mortar specimen are investigated in split Hopkinson pressure bar (SHPB) tests. It is shown that axial strain acceleration is unavoidable in SHPB tests on brittle samples at high strain-rates although it can be reduced by the application of a wave shaper. By introducing proper measures of the strain-rate and axial strain acceleration, their correlations are established. In order to demonstrate the influence of inertia-induced confinement on the dynamic compressive strength of concrete-like materials, tubular mortar specimens are used to reduce the inertia-induced radial confinement in SHPB tests. It is shown that the DIF measured by SHPB tests on tubular specimens is lower than the DIF measured by SHPB tests on solid specimens. This paper offers experimental support for a previous publication , Int. J. of Solids and Struct., 40,[343][344][345][346][347][348][349][350][351][352][353][354][355][356][357][358][359][360], which claimed that inertia-induced radial confinement makes a large contribution to the dynamic compressive strength enhancement of concrete-like materials when the strain-rate is greater than a critical transition strain-rate between 10 1 and 10 2 s -1 . It is concluded that DIF formulae for concrete-like materials measured by split Hopkinson pressure bar tests need to be corrected if they are going to be used as the unconfined uniaxial compressive strength in the design and numerical modelling of structures made from concrete-like materials to resist impact and blast loads.