Narrow Results

Based on density functional theory (DFT) and local density approximation (LDA), the band structure and the density of states of all models of pure and V-doped anatase TiO₂ systems were calculated using plane-wave ultra-soft pseudo potential method after geometry optimization. The calculation results show that, with the increase of the doping concentration of V in anatase TiO₂, the free electron concentration and electron effective mass increase; the electronic mobility and electronic conductivity decrease, which is consistent with the change trend of the experimental results.

A half-metal diluted magnetic semiconductor (DMS) can be formed in heavy V-doped TiO₂. Contradictory experimental results in the literature have reported about the absorption spectra blueshift and redshift results in heavy V-doped TiO₂. This study aims to reveal the mechanism of half-metal DMS in heavy V-doped TiO₂ and solve the problem of absorption spectra blueshift and redshift in the doping system. In this study, models of the unit cells of pure anatase TiO₂ and two V heavy-doped supercells of Ti${}_{0.96875}$V${}_{0.03125}$O₂ and Ti${}_{0.9375}$V${}_{0.0625}$O₂ were constructed based on density functional theory, which uses the first-principles plane-wave ultrasoft pseudopotential method. All models were obtained through geometry optimization. Local density approximation $(\mathrm{\text{LDA}})+U$ was used to calculate the band structure, density of states (DOS), orbital charge and absorption spectrum of the doping system. The calculated results under the condition of electron spin showed that in the heavy doping concentration range, the volume of supercells increases, the total energy and formation energy decrease and the stability of the supercells increases as V doping concentration increases. Furthermore, the interaction of $p$–$p$ states is weaker than that of $p$–$d$ states, which results in the valence band maximum shifting toward the low-energy region, and also the optical bandgap becomes narrower as well as the redshift and intensity of the absorption spectrum become more notable. Noticeably, the hybrid coupling effect of Ti-3$d$ and V-3$d$ states becomes stronger, and the magnetic moment increases. The Fermi levels of spin-up band structure within the conduction band, which form the $n$-type degenerate semiconductors, and the Fermi levels of spin-down band structure within the bandgap indicate that the doping system has semiconductor features. Therefore, V-doped anatase TiO₂ is an extremely promising DMS because of its high electron polarizability of nearly 100%. The calculation results are consistent with the experimental data; these results explain the problems reasonably and adequately. Therefore, the research findings can help solve the contradiction of the redshift and blueshift in the preparation of photocatalysts and half-metal diluted magnetic semiconductors of V heavy-doped anatase TiO₂.

Theoretical calculations on the effects of different Sr-doped ratios and oxygen vacancy concentrations on the magnetic mechanism of anatase TiO₂ are rarely reported. For this problem, generalized gradient approximation (GGA) plane wave ultra-soft pseudopotential $+$U based on the spin density functional theory framework was adopted in this work. The effect of different Sr doping ratios and O vacancy concentrations on the magnetic properties of anatase TiO₂ was studied by first principles. Results show that Sr replaces Ti and O vacancies at different Sr:V${}_{\mathrm{\text{O}}}$ ratios (1:0, 1:1, 1:2, 2:1 and 2:2), the doping system is magnetic when the Sr:V${}_{\mathrm{\text{O}}}$ ratio is 1:2. The systems with Sr${}_{\mathrm{\text{i}}}$:V${}_{\mathrm{\text{O}}}$ ratios of 1:0 and 2:1 are nonmagnetic, whereas those with 1:1, 1:2, and 2:2 Sr${}_{\mathrm{\text{i}}}$:V${}_{\mathrm{\text{O}}}$ ratios are magnetic. Regardless of whether or not Sr replaces Ti and O vacancies or interstitial Sr and O vacancies in different ratios of anatase TiO₂, the effect of magnetic switching can be achieved by adjusting the concentration. In this study, the largest magnetic moment of Ti${}_{16}$Sr${}_{\mathrm{\text{i2}}}$O${}_{30}$ system is obtained at Sr${}_{\mathrm{\text{i}}}$:V${}_{\mathrm{\text{O}}}$ ratio of 2:2. The Curie temperature of the doping system is above room-temperature, and the doping system produces 100% electron spin polarizability and is half-metallized. These features are valuable to the design and preparation of novel dilute magnetic semiconductors with spin-electron injection sources. The main magnetic sources of the Ti${}_{16}$Sr${}_{\mathrm{\text{i2}}}$O${}_{30}$ system are the holes generated by doped Sr and O vacancy complexes, which cause the spin polarization double exchange of Ti-3d electron orbitals and O-2p electron orbits near O vacancies. This result is consistent with the average field approximation and dual exchange mechanism theories.

The electronic and optical properties of S- and/or Ce-(co)doped anatase titanium dioxide (TiO₂) are investigated using density functional theory plus U (DFT$+$U) calculations. The optimized total energy suggests that TiO₂ codoping by Ce and S favours the configuration of one substitutional Ce atom occupied on a Ti site with one substitutional S atom either on its nearest neighboring O or Ti site. The calculated results show that all doping configurations exhibit remarkable red-shift and excellent photocatalytic properties compared with pure TiO₂. These reinforced features can mainly be ascribed to the appearance of S $3p$ states in the top of valence band (VB) and Ce $4f$ states in the bottom of conduction band (CB) as well as the contribution from the increasing octahedral dipole moments. The synergetic effects of cationic Ce and anionic S can extend optical absorption edge, which results in higher absorption coefficient in the visible light region than that of the anionic S monodoping and cationic Ce monodoping case; in the same time, decreasing the codoping concentration leads to reduced optical absorption. Additionally, Ce and S as cations incorporating into TiO₂ lattices can induce stronger redox potential with a lower defect formation energy under O-rich condition compared with other doping systems.

Nanoparticles of Titanium dioxide $({\mathrm{\text{TiO}}}_2)$ with its unique optical and electronic characteristics is an important material for photochemical catalysis. The efficiency of catalytic activity of ${\mathrm{\text{TiO}}}_2$ anatase nanostructures is greatly influenced by the photo-generated bound excitons. It is found that the interaction of bound excitons generated in ${\mathrm{\text{TiO}}}_2$ enhances the cubic nonlinearity of the system due to strong oscillation of photo-generated bound excitons. The trapped electron hole pair concentration is directly proportional to the photocatalytic efficiency of the ${\mathrm{\text{TiO}}}_2$ anatase nanostructures. In our report, we show how these photo-generated bound excitons play a significant role in origin of third-order optical nonlinearities. In particular, we have measured large phase shift and seen two photon absorption process through closed and open aperture Z-scan, respectively, using femtosecond pulses at 532nm.

Synthesis of uniform nanocrystalline anatase TiO₂ by sol–gel method was investigated using different thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Thermal treatment of the precursor at 350°C for 2 h in air results in the formation of nanocrystalline anatase TiO₂ with the average crystal size of 21.5 nm. Quantitative analysis through energy dispersive X-ray spectroscopy (EDS) indicated that the atomic ratio of Ti:O ≈ 1:1 is close to the stoichiometric ratio of TiO₂, leading to the evident fact that the prepared nanoparticles are indeed the TiO₂ material.

We report a facile easy method to deposit anatase titania (TiO${}_2)$ on cellulose paper. The anatase TiO₂/cellulose paper (ATCP) was characterized by scanning electron microscopy, transmission electron microscope, energy dispersive X-ray spectrometer, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. This hybrid paper with the anatase TiO₂ content of around 13.86wt.% can serve as an eco-friendly flexible photocatalyst, which can rapidly degrade blue indigo carmine dye into a colorless solution within 30min under UV radiation. Moreover, compared to commercially available TiO₂ P25 and anatase TiO₂ powder, a faster decomposition rate of indigo carmine dye was acquired when using ATCP. These results suggest that this hybrid paper might be useful in the treatment of organic dye wastewater.

Recently, semiconductor nanomaterials with exposed high percentage of reactive facets have attracted great attention due to their excellent photocatalytic performances. In this report, anatase titanium dioxide (TiO₂) nanobelts with dominant {100} facets were synthesized by a hydrothermal method. The percentage of desired {100} facets were as high as up to 74.8%, and the photoactivity of anatase TiO₂ {100} and {001} facets were evaluated and compared by methylene blue and rhodamine B photodegradation. The {100} facets show higher photocatalytic activity than {001} facets, which could be ascribed to the superior electronic band structure and higher surface active sites density of {100} facets than that of {001} facets.