Spectra analysis and O_2 evolution for TiO_2 photocatalyst compounded with indirect transition semiconductors

The photo absorbing, photo transmitting and photoluminescence performances of WiO2 photocatalysts compounded with V2O5 or WO3 were investigated by UV-Vis spectra, transmitting spectra, and PL spectra, respectively. The energy band structures of TiO2 photocatalysts were analyzed. The photocatalytic activities of the TiO2 photocatalysts were investigated by splitting of water for 02 evolution. The results indicate that the band gaps of WO3 and V205 are about 2.8 and 2.14 eV, respectively, and the band gap of rutile TiO2 is about 3.08 eV. Speeds of water splitting for 2%WO3-TiO2 and 8%V2O5-TiO2 photocatalysts are 420 and 110 μmol/（L.h）, respectively, under UV light irradiation. V2O5 and WO3 compounded with suitable concentration can improve the photocatalytic activity of TiO2 with Fe3＋ as electron acceptor.

Strain-induced direct-indirect bandgap transition and phonon modulation in monolayer WS2

In situ strain photoluminescence （PL） and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition （CVD）-grown monolayer tungsten disulphide （WS2） under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition （K-K） and indirect bandgap transition （F-K, ≥2.5%）, exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E＇ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide （TMD） materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

UV-Vis and Photoluminescent Spectra of TiO_2 Films

TiO 2 films have been deposited on glass substrates using DC reactive magnetron sputtering at different oxygen partial pressures from 0.10Pa to 0.65Pa.The transmittance (UV vis) and photoluminescence (PL) spectra of the films were recorded.The results of the UV vis spectra show that the deposition rate of the films decreased at oxygen partial pressure P(O 2)≥0.15Pa,the band gap increased from 3.48eV to 3.68eV for direct transition and from 3.27eV to 3.34eV for indirect transition with increasing the oxygen partial pressure.The PL spectra show convincingly that the transtion for films was indirect,and there were some oxygen defect energy levels at the band gap of the films.With increasing the O 2 partial pressure,the defect energy levels decreased.For the films sputtered at 0.35 and 0.65Pa there were two defect energy levels at 2.63eV and 2.41eV,corresponding to 0.72eV and 0.94eV below the conduction band for a band gap of 3.35eV,respectively.For the films sputtered at 0.10Pa and 0.15Pa,there was an energy band formed between 3.12eV and 2.06eV,corresponding to 0.23eV and 1.29eV below the conduction band.