The recent renaissance of black phosphorus (BP) as a two-dimensional (2D)layered

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We report our investigation on the intrinsic chemical activity of the anatase TiO 2 (001)-(1 × 4) reconstructed surface, using epitaxially grown anatase TiO 2 (001) thin films and using methanol molecules as a probe, characterized by combining scanning tunneling microscopy and temperature-programmed desorption. Our results provide direct evidence that the perfect (1 × 4) lattice sites of the surface are intrinsically quite inert for the reaction of methanol. We obtain that the activation energy for desorption of molecular methanol is about 0.55−0.64 eV, which is in good agreement with our first-principle calculations based on the structural model with 5-fold coordinated Ti atoms at the ridges of (1 × 4) reconstruction. We find that two types of defect sites, that is, reduced Ti pairs and partially oxidized Ti pairs, are responsible for the chemical activity of the surface, evidenced by the desorption of water due to the dehydrogenation of methanol at the defect sites. The methoxy left at the reduced Ti-pair sites further produced CH 3 radical, and the methoxy near the partially oxidized Ti-pair sites produced formaldehyde and methanol through disproportionation reaction. The determination of these intrinsic properties can be important to understanding the conflicting results from this surface in the literature and thus to reveal the actual reaction mechanisms.

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We investigate the dynamic processes of formaldehyde (HCHO) molecules on 5-fold-coordinated titanium (Ti 5c ) sites of rutile TiO 2 (110) surface using scanning tunneling microscopy (STM) together with density functional theory simulations. Our results show that the adsorbed HCHO molecules at Ti 5c sites are present as two types of protrusions, either centered at Ti 5c rows or centered at bridging oxygen (O b ) rows in the STM images, corresponding to the monodentate adsorption configuration through a O−Ti 5c bond and to the bidentate adsorption configuration through both O−Ti 5c and C−O b bonds, respectively, which can be well supported by the simulated images. It is also observed that the monodentate adsorption tends to spontaneously switch to bidentate adsorption. Our results confirm the existence of the energetically more favored bidentate adsorption for HCHO at Ti 5c sites. We obtain that the energy barriers are approximately 0.28 and 0.75 eV for the adsorbed HCHO molecules switching from monodentate adsorption to bidentate adsorption and reversely switching from bidentate adsorption to monodentate adsorption, respectively, from measurements of their dynamic processes. Our findings can well elucidate the missing signature of the energetically more favored bidentate configuration in some previous experiments and provide insightful understanding of formaldehyde on TiO 2 (110) surface.