The interest in heterogeneous photocatalysis is intense and increasing, as shown by the number of publications on this theme which regularly appear in this journal, and the fact that over 2000 papers have been published on this topic since 1981. This article is an overview of the field of semiconductor photocatalysis : a brief examination of its roots, achievements and possible future. The semiconductor titanium dioxide (TiO 2 ) features predominantly in past and present work on semiconductor photocatalysis; as a result, in the most of the examples selected in this overview to illustrate various points the semiconductor is TiO 2 .

An overview of semiconductor photocatalysis

Controlling surface structure at the atomic scale is paramount to developing effective catalysts. For example, the edge sites of MoS(2) are highly catalytically active and are thus preferred at the catalyst surface over MoS(2) basal planes, which are inert. However, thermodynamics favours the presence of the basal plane, limiting the number of active sites at the surface. Herein, we engineer the surface structure of MoS(2) to preferentially expose edge sites to effect improved catalysis by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS(2) bicontinuous network with nanoscaled pores. The high surface curvature of this catalyst mesostructure exposes a large fraction of edge sites, which, along with its high surface area, leads to excellent activity for electrocatalytic hydrogen evolution. This work elucidates how morphological control of materials at the nanoscale can significantly impact the surface structure at the atomic scale, enabling new opportunities for enhancing surface properties for catalysis and other important technological applications.

Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis

Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX2) might offer properties superior to those of graphene. So far, only very few MX2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA...

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Stable, Single-Layer MX2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure

Molybdenum disulphide nanostructures are of interest for a wide variety of nanotechnological applications ranging from the potential use of inorganic nanotubes in nanoelectronics to the active use of nanoparticles in heterogeneous catalysis. Here, we use atom-resolved scanning tunnelling microscopy to systematically map and classify the atomic-scale structure of triangular MoS2 nanocrystals as a function of size. Instead of a smooth variation as expected from the bulk structure of MoS2, we observe a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulphur excess present at the cluster edges. By analysing of the atomic-scale structure of clusters, we identify the origin of the structural transitions occurring at unique cluster sizes. The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS2 nanomaterials with superior performance.

Size-dependent structure of MoS2 nanocrystals.

Based on first-principles plane-wave calculations, we studied the functionalization of the two-dimensional single-layer MoS2 structure through adatom adsorption and vacancy defect creation. Minimum-energy adsorption sites were determined for 16 different adatoms, each of which gives rise to diverse properties. Bare, single-layer MoS2, which is normally a nonmagnetic, direct-band-gap semiconductor, attains a net magnetic moment upon adsorption of specific transition-metal atoms, as well as silicon and germanium atoms. The localized donor and acceptor states in the band gap expand the utilization of MoS2 in nanoelectronics and spintronics. Specific adatoms, such as C and O, attain significant excess charge upon adsorption onto single-layer MoS2, which might be useful for tribological applications. Each MoS2 triple vacancy created in a single layer of MoS2 gives rise to a net magnetic moment, whereas other vacancy defects related to Mo and S atoms do not influence the nonmagnetic ground state. The present re...

Kam Keung Kam

Papers

Further studies of the photoelectrochemical properties of the group VI transition metal dichalcogenides