Mono-doped TiO2 Research Articles

Gd–La codoped TiO2 nanoparticles as solar photocatalysts

Gd–La codoped TiO2 nanoparticles with diameter of 10nm were successfully synthesized via a sol–gel method. The photocatalytic activity of the Gd–La codoped TiO2 nanoparticles evaluated by photodegrading methyl orange was significantly enhanced compared to that of undoped or Gd or La mono-doped TiO2. Ti4+ may substitute for La3+ and Gd3+ in the lattices of rare earth oxides to create abundant oxygen vacancies and surface defects for electron trapping and dye adsorption, accelerating the separation of photogenerated electron–hole pairs and methyl orange photodegradation. It is believed that the formation of an excitation energy level below the conduction band of TiO2 from the binding of electrons and oxygen vacancies decreases the excitation energy of Gd–La codoped TiO2, resulting in versatile solar photocatalysts. The results suggest that Gd–La codoped TiO2 nanoparticles are promising for future solar photocatalysts.

Copper and Nitrogen co-doped TiO2 photocatalyst with enhanced optical absorption and catalytic activity

Cu–N-codoped TiO2 photocatalyst is synthesized by sol–gel method to obtain enhanced optical absorption in the visible region. Optimum concentrations of Cu and N were obtained by maximizing the photocatalytic activity for the monodoped (Cu or N) TiO2. These optimized concentrations were used for synthesizing (Cu, N)–codoped TiO2. The photocatalysts were characterized using XRD, micro-Raman, SEM, XPS, BET surface area analyzer, UV–vis diffuse reflectance spectroscopy and photoluminescence. XPS study suggests the incorporation of Cu2+ into TiO2 lattice, which assists N to substitutionally replace oxygen in codoped TiO2, while maintaining the anatase phase even after doping. N-doping creates minor variation in the energy band gap of TiO2 reducing it upto 3.0eV, while Cu doping was able to narrow the band gap to 2.2eV mainly due to the localized levels of Cu-3d states and shifting of Ti-3d states to lower energy (due to oxygen vacancies) in the band-gap as deduced from XPS data and confirmed by the DFT calculation. In (Cu, N)–codoped TiO2, visible light absorption is higher than the other TiO2 samples, a feature that is mainly attributed to the formation of an isolated intermediate band (IB) occurring due to the strong hybridization between Cu 3d and N 2p orbitals. This IB contributes to visible light absorption by two step optical transition with the first transition from valence band (VB) to IB and the second from IB to conduction band (CB). The dopant species may also act to reduce the charge carrier recombination by acting as the trapping sites for photogenerated charges. (Cu, N)–codoped TiO2 was able to degrade Methylene Blue dye and p-Nitrophenol solution under light irradiation with significantly better rate in comparison to monodoped and undoped TiO2. High photocatalytic activity is attributed to the presence of IB in the energy band gap of TiO2, which creates the synergic effect by higher visible light absorption and lower recombination of photogenerated charges.

Synergistic effects of nonmetal co-doping with sulfur in anatase TiO2: a DFT + U study.

Using DFT + U calculations, the crystal structure and electronic properties of nonmetal co-doping with sulfur in anatase TiO2 are systematically investigated. The initial purpose of this work is to improve the photocatalytic performance of S mono-doped TiO2, in which S occupies the lattice Ti site and acts as a recombination center. Among eight nonmetal impurities that occupy the interstitial site of a TiO6 octahedron, the synergistic effects of B, C, and O with S could achieve this purpose: suppressing the recombination of photogenerated electron-hole pairs by inducing a local inner built-in electric field and eliminating the deep impurity energy bands of S mono-doped TiO2. Furthermore, the photon absorption could be extended to the visible-light region, owing to the overlap of impurity energy bands with the top of the valence band or the bottom of the conduction band. Thus, Ti1-xO2SxBy, Ti1-xO2SxCy and Ti1-xO2SxOy could be considered as promising efficient photocatalysts. Furthermore, the underlying mechanism and tendency of these synergistic effects have been discussed, according to the relationship between the photocatalytic performance and the crystal or electronic structure.

Density functional study of X monodoped and codoped (X = C, N, S, F) anatase TiO2

Using density-functional theory (DFT) calculations within the generalized gradient corrected approximation, the models that nonmetallic impurities X (X=C, N, S, F) substituted for O or Ti sites in anatase TiO2 were investigated. By calculating the formation energy of X-monodoped TiO2 with X substituted for O, we suggested that X dopants existed as C4−, N3−, S2− and F− ions, respectively. Meanwhile, the X dopants existed as C4+, N3+, and S6+ for X-monodoped (X=C, N, S) TiO2 with X substituted for Ti. The conclusion of the valence states of nonmetallic impurities X substituted for O or Ti sites in TiO2 is also supported by the results of optimized cell parameters and the local structures. Furthermore, an effective nonmetallic passivated codoping approach to modify the band edges of TiO2 is proposed. Based on the first-principle calculations, we suggested that nonmetallic passivated groups such as (S2−+C4+) and (C4−+S6+) could reduce the band gap largely and make less perturbation in conduction band minima (CBM), thus lead to an ideal visible-light absorption region without affecting the reducing power. This work provides some new results and is anticipated to give some inspiration and guidance for design visible-light-driven TiO2 photocatalyst with high efficiency.

First principles calculations of electronic and optical properties of Mo and C co-doped anatase TiO2

Using the first principles calculations, the electronic and optical properties of C, Mo and Mo-C-doped anatase TiO2 are studied. For the Mo mono-doped TiO2, the band gap reduces little, and the largest perturbation occurs at the CBM of TiO2. C mono-doping suppresses the effective band gap, but the partially occupied subbands in the gap probably also serve as the recombination centers for electrons and holes. Therefore, the Mo-C co-doping is investigated for the charge compensation consideration. We discuss six doped configurations and find that the total energy of the system is increased with increasing distance of C and Mo. It is found that co-doped configurations with C nearest to Mo possess the lowest total energy. Then, we focus on discussing three possible Mo-C adjacent co-doped configurations. The subbands mainly induced by C-2p states in the band gap become fully occupied because the Mo atom contributes sufficient electrons to C anion for compensation. At the same time, the effective band gap is narrowed about 0.9 eV and the perturbation at the CBM occurred in Mo mono-doped TiO2 disappears, which means the band edges of doped system still straddle the redox potentials of water. Furthermore, the optical properties of the compensated Mo-C adjacent co-doped TiO2 and pure TiO2 are calculated. The optical absorption edges of the Mo-C co-doped TiO2 shift towards the visible light region.

Solar photocatalysts from Gd–La codoped TiO2 nanoparticles

Gd–La codoped TiO2 nanoparticles with diameter of 10 nm were successfully synthesized via a sol–gel method. The photocatalytic activity of the Gd–La codoped TiO2 nanoparticles evaluated by photodegrading methyl orange has been significantly enhanced compared to that of undoped or Gd or La monodoped TiO2. Ti4+ may substitute for La3+ and Gd3+ in the lattices of rare earth oxides to create abundant oxygen vacancies and surface defects for electron trapping and dye adsorption, accelerating the separation of photogenerated electron–hole pairs and methyl orange photodegradation. The formation of an excitation energy level below the conduction band of TiO2 from the binding of electrons and oxygen vacancies decreases the excitation energy of Gd–La codoped TiO2, resulting in versatile solar photocatalysts. The results suggest that Gd–La codoped TiO2 nanoparticles are promising for future solar photocatalysts.

Electronic and optical properties of C and Nb co-doped anatase TiO2

The electronic and optical properties of C, C–Nb and C–2Nb doped anatase TiO2 are studied using the first principles calculations. For the C-doped TiO2, three subbands in the band gap are generated due to the C-2p states, which suppress the effective band gap for electron transfer, while the gap states are partially occupied and probably serve as the recombination centers for electrons and holes. Therefore, the C–2Nb co-doping is investigated for the charge compensation consideration. Two possible co-doping configurations are comprehensively studied and compared. The three subbands in the band gap of the C mono-doped TiO2 become fully occupied because the two Nb atoms contribute sufficient electrons for compensation. Furthermore, the optical properties of the co-doped TiO2 and pure TiO2 are calculated. It is found that the optical absorption of the C–2Nb co-doped TiO2 extends its coverage over the visible light region.

Efficient photocatalytic degradation of organic water pollutants using V–N-codoped TiO2 thin films

TiO2, N-doped TiO2, V-doped TiO2, and V–N-codoped TiO2 thin films have been prepared using RF-magnetron sputtering and their photocatalytic activities have been investigated. The codoping strategy was adopted to decrease both the band gap of TiO2 and the recombination rate of the photo-generated electron–hole pairs. Low concentration doping with single element (V or N) preserves the anatase dominated phase in TiO2 film while codoping with V and N produces a mixed phase of nearly equal amount of rutile and anatase as inferred from the XRD and Raman spectroscopy studies. XPS studies reveal that, for N-doped TiO2 elemental N resides at interstitial lattice positions but codoping with V permits N to reside in both substitutional and interstitial sites in TiO2 lattice. UV–vis studies indicate that the band gap of TiO2 (3.2eV) reduced to 3.0eV, 2.8eV and 2.5eV, by N-doping, V-doping and V–N codoping, respectively. The photocatalytic activity of pure, N-doped, V-doped, and V–N codoped TiO2 thin films were tested by examining the degradation of methylene blue, chlorophenol and nitrophenol as a function of time. It was observed that the codoped TiO2 gave the highest photocatalytic activity in comparison to the mono-doped and undoped TiO2 because of high visible light absorption and possible reduction in the recombination of photo-generated charges. Density of states calculated using density functional theory (DFT) showed that the narrowing of band-gap for the codoped TiO2 is obtained by the formation of isolated energy levels of V 3d and N 2p states below the conduction band and above the valence band of pure TiO2, respectively. While for the mono-doped TiO2 the narrowing of the band gap is only contributed by impurity levels formed near any one of the band edges. It is concluded that for the codoped TiO2, high visible light absorption is caused by the formation of impurity energy states near both the band edges which also act as the trapping sites for both the photo-generated charges to reduce the recombination process.

Electronic and optical performances of Si and Fe-codoped TiO2 nanoparticles: A photocatalyst for the degradation of methylene blue

The effect of substitutional Si, Fe, and Si and Fe (co)doping on the electronic structure and optical properties of anatase and rutile TiO2 have been investigated by the density functional theory. The calculated results indicate that Si doping TiO2 will induce an obvious band gap narrowing by mixing O 2p with Si 3p states, and a series of impurity energy levels (Fe 3d) appear in edge of the VB and the CB through Fe doping. The synergistic effects of Si and Fe codoping can extend optical absorption edge, which leads to higher visible-light photocatalytic activities than pure and monodoped anatase TiO2. Si and Fe-codoped TiO2 nanoparticles was prepared by a sol–gel-solvothermal method, and the photocatalytic activity of TiO2 nanoparticles was examined by measuring the rate of methylene blue decomposition. The experimental results show that Si and Fe-codoped TiO2 sample exhibits better optical absorption performance and higher photocatalytic activities for the degradation of methylene blue than pure and monodoped TiO2, which verifies the reliability of our calculated results.

Codoping titanium dioxide nanowires with tungsten and carbon for enhanced photoelectrochemical performance

Recent density-functional theory calculations suggest that codoping TiO2 with donor-acceptor pairs is more effective than monodoping for improving photoelectrochemical water-splitting performance because codoping can reduce charge recombination, improve material quality, enhance light absorption and increase solubility limits of dopants. Here we report a novel ex-situ method to codope TiO2 with tungsten and carbon (W, C) by sequentially annealing W-precursor-coated TiO2 nanowires in flame and carbon monoxide gas. The unique advantages of flame annealing are that the high temperature (>1,000 °C) and fast heating rate of flame enable rapid diffusion of W into TiO2 without damaging the nanowire morphology and crystallinity. This is the first experimental demonstration that codoped TiO2:(W, C) nanowires outperform monodoped TiO2:W and TiO2:C and double the saturation photocurrent of undoped TiO2 for photoelectrochemical water splitting. Such significant performance enhancement originates from a greatly improved electrical conductivity and activity for oxygen-evolution reaction due to the synergistic effects of codoping.

A comparative study on characterization and photocatalytic activities of gadolinium–boron codoped and mono-doped TiO2 nanoparticles

Gadolinium–boron codoped and mono-doped TiO2 nanoparticles were prepared using a sol–gel method, and tested for photocatalytic activity by the UV light after a further calcination process. For comparison, a pure TiO2 sample was also prepared and tested under the same conditions. The prepared catalysts were characterized by X-ray diffraction, scanning electron microscope, and UV–Vis spectra. The photocatalytic activity of the samples was evaluated through the photo-degradation of three different dyes under UV light. The experiments demonstrated that the gadolinium–boron codoped TiO2 (Gd–B–TiO2) sample calcined at 500 °C possessed the best photocatalytic activity, and the photodegradation rate of the Reactive Brilliant Red K2G aqueous solution could reach to 95.7% under UV irradiation for 80 min. The results showed that Gd–B–TiO2 has smaller crystallite size and higher photocatalytic activity than that of mono-doped TiO2 samples and undoped TiO2.

Enhanced visible-light photocatalytic activity of anatase TiO2 through N and S codoping

The density of states and optical properties of N, S, and (N, S)-doped anatase TiO2 are calculated based on the density functional theory. The results indicate that the doping S atoms occupy the Ti-atom sites in anatase TiO2 lattice. The mixing of O 2p, N 2p, S 3p, and Ti 3d states in the forbidden gap of (N, S)-codoped TiO2 can result in the higher visible-light photocatalytic activities than those monodoped TiO2. For comparison, (N, S)-codoped TiO2 was also synthesized by one-step hydrothermal method, and the optical absorption spectra obtained by experiments verified the reliability of our calculation.

Synergy Effect of Ce/N Co-Doping on Anatase TiO<sub>2</sub> Photocatalysts from First Principles Calculation

Recent experiments have indicated that TiO2 co-doped with cerium (Ce) and nitrogen (N) may show enhanced photocatalytic activity in the visible region with respect to TiO2 doped only with Ce. Prompted by these findings, we have investigated Ce-N co-doped TiO2 through a theoretical study. Density functional theory (DFT) calculations have been carried out with the generalized gradient approximation (GGA) to describe the electronic structure and photocatalytic activity of Ce-, N- and N/Ce-doped anatase TiO2; substitutional locations of Ce and N in the TiO2 lattice were considered: an O (Ti) atom is replaced by an N (Ce) atom. It was found that the Ce-N co-doped case narrows the band gap more significantly than the corresponding mono-doped anatase TiO2. These findings give the theoretical understanding for recent experimental results.

Enhanced photocatalytic activity of (Mo, C)-codoped anatase TiO2 nanoparticles for degradation of methyl orange under simulated solar irradiation

C-doped, Mo-doped, and (Mo, C)-codoped TiO2 photocatalysts were prepared by a sol-gel process. The photocatalytic activity was evaluated by the photocatalytic degradation of methyl orange (MO) under simulated solar irradiation. Results indicated that both monodoped and codoped TiO2 exhibited better visible light absorption behavior and narrower energy gap than pure TiO2, and codoped TiO2 showed a slightly higher adsorption property in the dark because of higher Brunauer–Emmett–Teller-specific surface area. The photocatalytic activity of monodoped TiO2 was also enhanced, and the (0.04% Mo, C)-codoped sample had the best photocatalytic activity for degrading MO among all of the samples. The reason can be ascribed to the synergistic effect due to Mo and C doping. Furthermore, the transfer pathways of photoinduced carriers and photocatalytic reaction mechanism of (Mo, C)-codoped TiO2 was first investigated.