Jing Hui He

Tuning the electronic and structural properties of WO3 nanocrystals by varying transition metal tungstate precursors

Oxygen vacancy is one type of the most important defects affecting the photocatalytic performance of WO3. In this paper, WO3 nanoplates with a high density of oxygen vacancies were synthesized from MWO4 (M = Zn, Cd, Co, Ni) precursors using a sacrificial template method. The structures and morphologies of WO3 nanoplates were investigated by field emission scanning electron microscopy (FE-SEM), high resolution Transmission Electron Microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, Photoluminescence (PL), Diffuse Reflectance UV-Vis (DRS UV-Vis) and Time-correlated single-photon counting (TCSPC). The metal tungstates were found to not only act as the precursors but also as structure-directing agents during the growth of WO3 nanoplates. XRD data revealed that two phases of WO3·xH2O (x = 1 or 2) were obtained after acid treatment of MWO4. WO3 nanoplates derived from NiWO4 were found to have the highest ratio of WO3·2H2O, highest concentration of oxygen vacancies, narrowest band gap, longest electron–hole recombination time, and in turn the highest rate of photodegradation of azo dye methylene blue. These results show that the structural, electronic and photocatalytic properties of synthesized WO3 nanoplates can be tuned by varying the transition metal tungstate precursors.

Enhancement of the photocatalytic efficiency of WO3 nanoparticles via hydrogen plasma treatment

Surface defect engineering is able to effectively enhance the photocatalytic performance of WO3 nanoparticles. In this paper, radio frequency hydrogen plasma was employed to create surface defects on WO3 nanoparticles. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analysis confirmed that hydrogen plasma modification increases the density of oxygen vacancies on the surface of WO3. The broadening of characteristic WO3 peaks in Raman spectra indicates the increase of oxygen vacancies by increasing voltage in hydrogen plasma treatment. The sample treated with hydrogen plasma at 20 volts shows enhancement in photocurrent density by an order of magnitude, attributable to the band-gap narrowing and subsequent increase of quantum yield in the visible range. Consistent results were also obtained from photocatalytic O2 evolution from water oxidation.

Orbital resolution of molecules covalently attached to a clean semiconductor surface

Understanding the chemical and electronic nature of molecules attached to semiconductors is of great importance in the study of molecule-based electronic devices. Resolving individual molecular orbitals using scanning tunnelling microscopy is a straightforward approach but remains challenging on the semiconductor surfaces because of their highly reactive dangling bonds. Here we show that hybridized molecular orbitals of pyridazine molecules covalently attached to Ge(100) surfaces can be resolved by scanning tunnelling microscopy. Pyridazine binds to Ge(100) through single/double dative bond(s) and presents two types of features with three and four lobes. These features resemble the lowest unoccupied molecular orbitals of free pyridazine, which are hybridized by the surface states in the adsorbed state. The adsorbing sites, binding mechanisms, orientations and electronic properties of the adsorbed molecules are convincingly determined. Our results indicate that orbital resolution of molecules covalently attached to semiconductors is accessible despite of their high reactivity.

Resolving molecular orbitals self-decoupled from semiconductor surfaces

Resolving orbitals using scanning tunneling microscopy (STM) provides an in-depth understanding of the chemical and electronic nature of molecule/substrate junctions. Most orbital resolving work was performed for molecules physisorbed on metal surfaces by inserting interfacial layers to decouple the interaction between the molecules and substrates. It remains challenging to image the orbitals of molecules directly chemisorbed on native surfaces because the linking chemical bonds likely induce severe coupling. Here we demonstrate that the π orbitals of the phenyl rings of nitrosobenzene chemisorbed on Ge(100) are electronically decoupled from the semiconductor surface and can be resolved by STM. Four types of dumbbell-like molecular features are imaged, corresponding to the intradimer and interdimer [2 + 2] nitrosoadducts. In these products, nitrosobenzene binds to Ge(100) through its NO group, which spatially separates and electronically decouples the phenyl ring (C6H5–) from the substrate. Theoretical calculations and STM simulation reveal that the dumbbell-like features resemble the occupied π orbitals of benzene. Our results show that electronic decoupling and orbital resolving can be achieved for molecules binding to highly reactive surfaces via the sacrifice of a double bond as the anchoring/spacing group.