Here, the hydrogen evolution reaction (HER) activities at the edge and basal-plane sites of monolayer molybdenum disulfide (MoS ) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected-area lithography. Reaction windows are opened by e-beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)-covered monolayer MoS triangles. The HER properties of MoS edge sites are obtained by subtraction of the activity of the basal-plane sites from results containing both basal-plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10 s , found for 1T'-, 2H-MoS edges, and 1T'-, 2H-MoS basal planes, respectively. Edge sites of both 2H- and 1T'-MoS are proved to have comparable activities to platinum (≈1-10 s ). When fitted into the HER volcano plot, the MoS active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition-metal dichalcogenides (TMDs) and metal catalysts.
The mechanism of the size-dependent outer/inner phase transformation in ultrafine TiO 2 particles with narrowed size distribution was intensively investigated using UV Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Particle size is found to be the critical parameter determining the onset transition temperature and nucleation performance. The transformation temperature was decreased with the decrease of initial particle size. Rutile nucleates at interfaces of the contacting anatase grains (<60 nm). The free surface, interface, and bulk are all likely to work as rutile nucleation sites for large particles(g60 nm). Thermal stability of nanoscale titania is significantly enhanced by the presence of La 2 O 3 even with surface coverage less than 5%, which blocks the outer/inner phase transformation of nano anatase. A model was devised to describe the phase transformation as a function of particle size and to predict the phase stability.
We have developed a cluster model of a TiO(2) nanoparticle in the dye-sensitized solar cell and used first-principles quantum chemistry, coupled with a continuum solvation model, to compute structures and energetics of key electronic and structural intermediates and transition states. Our results suggest the existence of shallow surface trapping states induced by small cations and continuum solvent effect as well as the possibility of the existence of a surface band which is 0.3-0.5 eV below the conduction band edge. The results are in uniformly good agreement with experiment and establish the plausibility of an ambipolar model of electron diffusion in which small cations, such as Li(+), diffuse alongside the current carrying electrons in the device, stabilizing shallowing trapping states, facilitating diffusion from one of these states to another, in a fashion that is essential to the functioning of the cell.
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