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The electronic structures and optical properties of pure, Cr, S single- and Cr/S co-doped rutile TiO2 were calculated by the first-principle plane wave pseudopotential method based on density functional theory. The calculated results indicate that the three different doping ways can lead to lattice distortion in the rutile TiO2 and introduce local electronic states in the forbidden band of TiO2. The local energy levels in the forbidden band of TiO2 are mainly contributed by Cr-3d and S-3p orbital. Compared with pure TiO2, the absorption edges (i.e. the edge of the main peak) of the doped TiO2 have different blue shifts; however, the light response ranges of the doped systems are extended, especially in the case of Cr single- and Cr/S co-doped TiO2. The extension of the visible light response range of the doped TiO2 may enhance its visible light photocatalytic performance. In addition, the co-doped TiO2 has a stronger oxidation ability, which may increase the catalytic efficiency of TiO2.

A photocatalyst which consists of zero-dimension (0D) ZnFe2O4 (ZFO) and two-dimension (2D) Fe-doped g-C3N4 (Fe-CN) has been prepared successfully through a facile solvothermal method. The Fe doping into g-C3N4 lattice has been identified by both the infrared (IR) and X-ray photoelectron spectra (XPS), respectively. Introducing Fe into CN extends the spectral response to 700 nm while ZFO serves as a near infrared (NIR) photosensitizer of the Fe-CN component. The formation of Fe-CN heterostructure not only contributes to visible light absorption but also promotes the separation of photo-generated electron-hole pairs. Furthermore, the density functional theory (DFT) calculation indicated that the N2 molecules prefer to be adsorbed onto the Fe sites with a reduced potential barrier for N2 fixation. More importantly, the Fe atoms doped into the CN lattice acted as the reaction sites for accelerating photocatalytic N2 fixation reaction, which was also demonstrated by DFT. These factors lead to a high photocatalytic NH4+ generation rate of 25.3 mg g−1 h−1 under a full-spectrum irradiation, which is much higher than the value (4.4 mg g−1 h−1) of pure CN catalysts. This research might provide a novel approach for rationally designing full-spectrum-response N2 fixation photocatalyst.

Previous studies have shown that hydrogen treatment leads to the formation of blue to black TiO_2, which exhibits photocatalytic activity different from that of white pristine TiO_2. However, the underlying mechanism remains poorly understood. Herein, density functional theory is combined with comprehensive analytical approaches such as X-ray absorption near edge structure spectroscopy and transient absorption spectroscopy to gain fundamental understanding of the correlation among the oxygen vacancy, electronic band structure, charge separation, charge carrier lifetime, reactive oxygen species (ROS) generation, and photocatalytic activity. The present work reveals that hydrogen treatment results in chemical reduction of TiO_2, inducing surface and subsurface oxygen vacancies, which create shallow and deep sub-band gap Ti(III) states below the conduction band. This leads to a blue color but limited enhancement of visible light photocatalytic activity up to 440 nm at the cost of reduced ultraviolet photocatalytic activity. The extended light absorption spectral range for reduced TiO_2 is ascribed to both the defect-to-conduction band transitions and the valence band-to-defect transitions. The photogenerated charge carriers from the defect states to the conduction band have lifetimes too short to drive photocatalysis. The Ti(III) deep and shallow trap states below the conduction band are also found to reduce the lifetime of photogenerated charge carriers under ultraviolet light irradiation. The ROS generated by the reduced TiO_2 are less than those generated by pristine TiO_2. Consequently, the reduced TiO_2 exhibits ultraviolet-responsive photocatalytic activity worse than that of pristine TiO_2. This report shows that increasing the light absorption spectral range of a semiconductor by doping or introduction of defects does not necessarily guarantee an increase in photocatalytic activity.

In this work, graphene oxide quantum dots (GOQDs) are introduced as an electron-trapped layer in Pd/Zr[Formula: see text]Hf[Formula: see text]O2 (ZHO)/SiO2/Si memory device. This structure possessed longer than 104 s retention capability, a low operation voltage around [Formula: see text][Formula: see text]V and 2.61[Formula: see text]V storage windows. GOQDs contained carbon–carbon and carbon–oxygen single/double bonds based on the analysis of C-1[Formula: see text] and O-1[Formula: see text] X-ray photoelectron spectra. It is proposed that the GOQDs’ wide bandgap with many oxygen-containing functional groups favors charge capture to a greater extent. This new type of charge-trapping memory can be a promising candidate for wide application to storing information with non-volatility in the big data era.

The effect of isovalent lanthanum (La) doping on the monoclinic Y2WO6 photoluminescence was studied. Introducing the non-activated La3+ into Y2WO6 brings new excitation bands from violet to visible regions and strong near-infrared emission, while the bands position and intensity depend on the doping concentration. It is interesting to find that doping La3+ into Y2WO6 promotes the oxygen vacancy formation according to the first-principle calculation, Raman spectrum and synchrotron radiation analysis. Through the Rietveld refinement and X-ray photoelectron spectroscopy results, La3+ is found to mainly occupy the Y2 (2f) site in low-concentration doped samples. With increasing doping concentration, the La3+ occupation number at the Y3 (4g) site increases faster than those at the Y1 (2e) and Y2 (2f) sites. When La3+ occupies different Y sites, the localized energy states caused by the oxygen vacancy pair change their position in the forbidden band, inducing the variation of the excitation and emission bands. This research proposes a feasible method to tune the oxygen vacancy emission, eliminating the challenge of precisely controlling the calcination atmosphere.