Zhaowu Wang

Implementing Metal‐to‐Ligand Charge Transfer in Organic Semiconductor for Improved Visible‐Near‐Infrared Photocatalysis

The coordination of organic semiconductors with metal cations can induce metal-to-ligand charge transfer, which broadens light absorption to cover the visible-near-infrared (vis-NIR) spectrum. As a proof-of-concept demonstration, the g-C3 N4 -based complex exhibits dramatically enhanced photocatalytic H2 production with excellent durability under vis-NIR irradiation.

Graphitic carbon nitride supported single-atom catalysts for efficient oxygen evolution reaction

Based on DFT calculations, we propose a TM@CN hybrid structure, in which the single-atom transition metal (TM = Pt, Pd, Co, Ni, Cu) is supported by graphitic carbon nitride (g-CN), as a promising high-performance OER catalyst. Our work reveals the importance of local TM coordination in catalysts for the OER, which would lead to a new class of low-cost, durable and efficient OER catalysts.

Highly anisotropic Dirac fermions in square graphynes

We predict a family of 2D carbon (C) allotropes, square graphynes (S-graphynes) that exhibit highly anisotropic Dirac fermions, using first-principle calculations within density functional theory. They have a square unit-cell containing two sizes of square C rings. The equal-energy contour of their 3D band structure shows a crescent shape, and the Dirac crescent has varying Fermi velocities from 0.6 × 10(5) to 7.2 × 10(5) m/s along different k directions. Near the Fermi level, the Dirac crescent can be nicely expressed by an extended 2D Dirac model Hamiltonian. Furthermore, tight-binding band fitting reveals that the Dirac crescent originates from the next-nearest-neighbor interactions between C atoms. S-graphynes may be used to build new 2D electronic devices taking advantages of their highly directional charge transport.

Sequential insulator-metal-insulator phase transitions of VO2 triggered by hydrogen doping

We report modulation of a reversible phase transition in VO2 films by hydrogen doping. A metallic phase and a new insulating phase are successively observed at room temperature as the doping concentration increases. It is suggested that the polarized charges from doped hydrogens play an important role. These charges gradually occupy V3d-O2p hybridized orbitals and consequently modulate the filling of the VO2 crystal conduction band-edge states, which eventually evolve into new valence band-edge states. This demonstrates the exceptional sensitivity of VO2 electronic properties to electron concentration and orbital occupancy, providing key information for the phase transition mechanism.

Protonation-Induced Room-Temperature Phosphorescence in Fluorescent Polyurethane

Room-temperature phosphorescence (RTP) from purely organic systems is of practical importance in biological imaging, oxygen sensing and displaying technologies. The key step to obtaining RTP from organic molecules is efficient intersystem crossing (ISC), which is usually low compared to inorganic materials. Here we show that protonation of a dye molecule, a thioflavin derivative, in strongly polar polyurethane can be used to effectively harness RTP. Prior to protonation, the predominant transition is π-π* for the polymer, which has nearly undetectable RTP due to the large singlet-triplet energy splitting (0.87 eV); when Brønsted acids are gradually added to the system, increasingly strong RTP is observed due to the presence of a new intramolecular charge-transfer state (ICT). The ICT state serves to lower the singlet-triplet energy gap (0.46 eV). The smaller gap results in more efficient ISC and thus strong RTP under deoxygenated conditions. The thioflavin-polyurethane system can be tuned via proton concentration and counterions and opens new doors for RTP-based polymeric sensors and stimuli-responsive materials.

Non-catalytic hydrogenation of VO 2 in acid solution

Hydrogenation is an effective way to tune material property1-5. Traditional techniques for doping hydrogen atoms into solid materials are very costly due to the need for noble metal catalysis and high-temperature/pressure annealing treatment or even high energy proton implantation in vacuum condition5-8. Acid solution contains plenty of freely-wandering protons, but it is difficult to act as a proton source for doping, since the protons always cause corrosions by destroying solid lattices before residing into them. Here we achieve a facile way to hydrogenate monoclinic vanadium dioxide (VO2) with protons in acid solution by attaching suitable metal to it. Considering the Schottky contact at the metal/VO2 interface, electrons flow from metal to VO2 due to workfunction difference and simultaneously attract free protons in acid solution to penetrate, forming the hydrogens dopants inside VO2 lattice. This metal-acid treatment constitutes an electron-proton co-doping strategy, which not only protects the VO2 lattice from corrosion, but also causes pronounced insulator-to-metal transitions. In addition, the metal-acid induced hydrogen doping behavior shows a ripple effect, and it can spread contagiously up to wafer-size area (>2 inch) even triggered by a tiny metal particle attachment (~1.0mm). This will stimulate a new way of simple and cost-effective atomic doping technique for some other oxide materials.Hydrogenation is an effective way to tune the property of metal oxides. It can conventionally be performed by doping hydrogen into solid materials with noble-metal catalysis, high-temperature/pressure annealing treatment, or high-energy proton implantation in vacuum condition. Acid solution naturally provides a rich proton source, but it should cause corrosion rather than hydrogenation to metal oxides. Here we report a facile approach to hydrogenate monoclinic vanadium dioxide (VO2) in acid solution at ambient condition by placing a small piece of low workfunction metal (Al, Cu, Ag, Zn, or Fe) on VO2 surface. It is found that the attachment of a tiny metal particle (~1.0 mm) can lead to the complete hydrogenation of an entire wafer-size VO2 (>2 inch). Moreover, with the right choice of the metal a two-step insulator–metal–insulator phase modulation can even be achieved. An electron–proton co-doping mechanism has been proposed and verified by the first-principles calculations.Hydrogenation is an effective way to tune the property of metal oxides. Here, the authors report a simple approach to hydrogenate VO2 in acid solution under ambient conditions by placing a small piece of low workfunction metal on VO2 surface.

Insight into Electronic and Structural Reorganizations for Defect-Induced VO2 Metal–Insulator Transition

An oxygen vacancy defect in monoclinic VO2 has been shown to modulate the metal-insulator transition (MIT) at room temperature. However, as the electronic and structural reorganizations occur simultaneously, the origin of MIT is still unclear. Here we performed first-principles calculations to examine electronic variations separately from structural reorganizations during MIT. It was found that the oxygen defect induces electronic reorganization by creating polarized 3d orbitial electrons, while structure reorganization makes the conduction band edge states available for occupation. The conduction band states thus hold polarized charges that delocalize over space, bestowing metallic property on the originally insulated VO2. A linear relationship for the number of polarized electrons and the defect concentration is revealed, which would lead to cost-effective control of VO2 MIT behavior by defect engineering.