New Insights into the Band‐Gap Narrowing of (N,P)‐Codoped TiO2 from Hybrid Density Functional Theory Calculations

The electronic properties of anatase‐TiO₂ codoped by N and P at different concentrations have been investigated via generalized Kohn–Sham theory with the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional for exchange‐correlation in the context of density functional theory. At high doping concentrations, we find that the high photocatalytic activity of (N, P)‐codoped anatase TiO₂ vis‐à‐vis the N‐monodoped case can be rationalized by a double‐hole‐mediated coupling mechanism [Yin et al., Phys. Rev. Lett. 2011, 106, 066801] via the formation of an effective N? P bond. On the other hand, Ti³⁺ and Ti⁴⁺ ions’ spin double‐exchange results in more substantial gap narrowing for larger separations between N and P atoms. At low doping concentrations, double‐hole‐coupling is dominant, regardless of the N? P distance.     

First‐principles calculation of electronic structure of V‐doped anatase TiO2

The energetic and electronic structures of V‐doped anatase TiO₂ have been investigated systematically by the GGA+U approach, including replacement of Ti by V in the absence and presence of oxygen vacancies and the presence of an interstitial site. It was found that V should exist as a V⁴⁺ ion in the replacement of Ti in the anatase lattice, the electron transitions of which to the conduction band from V 3d states are responsible for the experimentally observed visible light absorption. The influence of V dopant concentration on the electronic and magnetic properties is also discussed, such as the influence of the U value in systems containing oxygen vacancies and spin flip phenomena for interstitial V‐doping.     

Enhanced photocatalytic degradation of rhodamine B under visible light irradiation on mesoporous anatase TiO2 microspheres by codoping with W and N

Mesoporous anatase TiO₂ microspheres were prepared via solvothermal method. Ammonium tungstate was used as the W source, and ammonia gas flowing in an ammonothermal reactor as the N source for codoping. TiO₂:(W,N) mesoporous microspheres, which were prepared from solvothermal treatment at 160 °C for 16 h and thermal ammonolysis at 500 °C for 2 h after calcination, have high specific surface area of 106 m² g⁻¹. XPS results indicate the presence of N_O, N_i and W⁶⁺ in the codoped mesoporous TiO₂ microspheres. Monodoping with N shifts the absorption band edge of anatase TiO₂ from ultraviolet region to visible region. Although codoping with W makes the visible light absorbance decrease a little, the photocatalytic degradation of a cationic dye rhodamine B (RhB) on mesoporous TiO₂:(W,N) microspheres is increased to 1.7 times of that on mesoporous TiO₂:N microspheres. This may due to decreasing recombination centers by W-doping charge compensation.     

Ammonia‐Annealed TiO2 as a Negative Electrode Material in Li‐Ion Batteries: N Doping or Oxygen Deficiency?

Improving the chemical diffusion of Li ions in anatase TiO₂ is essential to enhance its rate capability as a negative electrode for Li‐ion batteries. Ammonia annealing has been used to improve the rate capability of Li₄Ti₅O₁₂. Similarly, ammonia annealing improves the Li‐ion storage performance of anatase TiO₂ in terms of the stability upon cycling and the C‐rate capability. In order to distinguish whether N doping or oxygen deficiencies, both introduced upon ammonia annealing, are more relevant for the observed improvement, a systematic electrochemical study was performed. The results suggest that the creation of oxygen vacancies upon ammonia annealing is the main reason for the improvement of the stability and C‐rate capability.     

Synthesis of W‐doped TiO2 by low‐temperature hydrolysis: Effects of annealing temperature and doping content on the surface microstructure and photocatalytic activity

A series of tungsten‐doped Titania photocatalysts were synthesized using a low‐temperature method. The effects of dopant concentration and annealing temperature on the phase transitions, crystallinity, electronic, optical, and photocatalytic properties of the resulting material were studied. The X‐ray patterns revealed that the doping delays the transition of anatase to rutile to a high temperature. A new phase W_yTi_1‐yO₂ appeared for 5.00 wt% W‐TiO₂ annealed at 900 °C. Raman and diffuse reflectance UV–Vis spectroscopy showed that band gap values decreased slightly up to 700 °C. X‐ray photoelectron spectroscopy showed that surface species viz. Ti³⁺, Ti⁴⁺, O^2?, oxygen‐vacancies, and adsorbed OH groups vary depending on the preparation conditions. The photocatalytic activity was evaluated via the degradation of methylene blue using LED white light. The degradation rate was affected by the percentage of dopants. The best photocatalytic activity was achieved with the sample labeled 5.00 wt% W‐TiO₂ annealed at 700 °C.     

Impact of Oxygen Vacancy on Band Structure Engineering of n-p Codoped Anatase TiO2

Doping with various impurities is an effective approach to improve the photoelectrochemical properties of TiO₂. Here, we explore the effect of oxygen vacancy on geometric and electronic properties of compensated (i.e. V-N and Cr-C) and non-compensated (i.e. V-C and Cr-N) codoped anatase TiO₂ by performing extensive density functional theory calculations. Theoretical results show that oxygen vacancy prefers to the neighboring site of metal dopant (i.e. V or Cr atom). After introduction of oxygen vacancy, the unoccupied impurity bands located within band gap of these codoped TiO₂ will be filled with electrons, and the position of conduction band offset does not change obviously, which result in the reduction of photoinduced carrier recombination and the good performance for hydrogen production via water splitting. Moreover, we find that oxygen vacancy is easily introduced in V-N codoped TiO₂ under O-poor condition. These theoretical insights are helpful for designing codoped TiO₂ with high photoelectrochemical performance.     

Carbon‐Coated and Interfacial‐Functionalized Mixed‐Phase MoxTi1−xO2‐δ Nanotubes as Highly Active and Durable PtRu Catalyst Support for Methanol Electrooxidation

A synchronous carbon‐coating and interfacial‐functionalizing approach is proposed for the fabrication of Mo‐doped Mo_xTi_1?xO_2‐δ nanotubes (C@IF‐MTNTs) under mild hydrothermal reaction with subsequent annealing as advanced catalyst supports for PtRu nanoparticles (NPs) towards methanol electrooxidation. The carbonation of glucose and Mo‐doping takes place simultaneously at the interface of pristine anatase TiO₂ nanotubes (TNTs), generating a unique concentric multilayered one‐dimensional (1D) structure with crystalline an anatase/rutile mixed‐phase TiO₂ core and Mo‐functionalized interface and subsequently a carbon shell. The obtained PtRu/C@IF‐MTNTs catalyst exhibits an over 2 times higher mass activity with comparable durability than that of the unmodified PtRu/C@TNTs catalyst and over 1.7 times higher mass activity with over 20 % higher stability than that of PtRu/C catalyst. Such superior catalytic performance towards methanol electrooxidation is ascribed to the Mo‐functionalized interface, concentric multilayered 1D architecture, and anatase/rutile mixed‐phase core, which facilitates the charge transport through 1D structural support and electronic interaction between C@IF‐MTNTs and ultrafine PtRu NPs. This work reveals the critical application of a 1D interfacial functionalized architecture for advanced energy storage and conversion.     

Effective band gap reduction of titanium oxide semiconductors by codoping from first‐principles calculations

Doping is an efficient approach to narrow the band gap of TiO₂ and enhance its photocatalytic activity. Here, we perform generalized gradient approximation (GGA)+U calculations to narrow the band gap of TiO₂ by codoping of X (F, N) with transition metals (TM = Fe, Co) to extend the absorption edge to longer visible‐light wavelengths. Our results show that all the codoped systems can narrow the band gap significantly, in particular, (F+Fe)‐codoped system could serve as remarkably better photocatalysts with both narrowing of the band gap and relatively smaller formation energies than those of (F+Co) and (N+TM)‐codoped systems. Our results provide useful guidance for codoped TiO₂ efficient for photocatalytic activity. © 2013 Wiley Periodicals, Inc.     

Characterization and activity of visible-light-driven TiO2 photocatalyst codoped with nitrogen and cerium

Nitrogen and cerium codoped TiO₂ photocatalysts were prepared by a modified sol-gel process with doping precursors of cerium nitrate and urea, and characterized by X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray photoelectron spectra (XPS) and ultraviolet-visible light diffuse reflectance spectra (UV-vis DRS). Results indicate that anatase TiO₂ is the dominant crystalline type in as-prepared samples, and CeO₂ crystallites appear as the doping ratio of Ce/Ti reaches to 3.0 at%. The TiO₂ starts to transform from amorphous phase to anatase at 987.1 K during calcination, according to the TG-DSC curves. The XPS show that three major metal ions of Ce³⁺, Ce⁴⁺, Ti⁴⁺ and one minor metal ion of Ti³⁺ coexist on the surface. The codoped TiO₂ exhibits significant absorption within the range of 400-500 nm compared to the non-doped and only nitrogen-doped TiO₂. The enhanced photocatalytic activity of the codoped TiO₂ is demonstrated through degradation of methyl orange under visible light irradiation.     

Probing the Electronic Structure and Photoactivation Process of Nitrogen‐Doped TiO2 Using DRS,PL, and EPR

The electronic structure and photoactivation process in N‐doped TiO₂ is investigated. Diffuse reflectance spectroscopy (DRS), photoluminescence (PL), and electron paramagnetic resonance (EPR) are employed to monitor the change of optical absorption ability and the formation of N species and defects in the heat‐ and photoinduced N‐doped TiO₂ catalyst. Under thermal treatment below 573 K in vacuum, no nitrogen dopant is removed from the doped samples but oxygen vacancies and Ti³⁺ states are formed to enhance the optical absorption in the visible‐light region, especially at wavelengths above 500 nm with increasing temperature. In the photoactivation processes of N‐doped TiO₂, the DRS absorption and PL emission in the visible spectral region of 450–700 nm increase with prolonged irradiation time. The EPR results reveal that paramagnetic nitrogen species (N_s^.), oxygen vacancies with one electron (V_o^.), and Ti³⁺ ions are produced with light irradiation and the intensity of N_s^. species is dependent on the excitation light wavelength and power. The combined characterization results confirm that the energy level of doped N species is localized above the valence band of TiO₂ corresponding to the main absorption band at 410 nm of N‐doped TiO₂, but oxygen vacancies and Ti³⁺ states as defects contribute to the visible‐light absorption above 500 nm in the overall absorption of the doped samples. Thus, a detailed picture of the electronic structure of N‐doped TiO₂ is proposed and discussed. On the other hand, the transfer of charge carriers between nitrogen species and defects is reversible on the catalyst surface. The presence of oxygen‐vacancy‐related defects leads to quenching of paramagnetic N_s^. species but they stabilize the active nitrogen species N_s^?.     

Structural and Electrochemical Study of Vanadium‐Doped TiO2 Ramsdellite with Superior Lithium Storage Properties for Lithium‐Ion Batteries

Titanium‐oxide‐based materials are considered attractive and safe alternatives to carbonaceous anodes in Li‐ion batteries. In particular, the ramsdellite form TiO₂(R) is known for its superior lithium‐storage ability as the bulk material when compared with other titanates. In this work, we prepared V‐doped lithium titanate ramsdellites with the formula Li_0.5Ti_1?xV_xO₂ (0≤x≤0.5) by a conventional solid‐state reaction. The lithium‐free Ti_1?xV_xO₂ compounds, in which the ramsdellite framework remains virtually unaltered, are easily obtained by a simple aqueous oxidation/ion‐extraction process. Neutron powder diffraction is used to locate the Li channel site in Li_0.5Ti_1?xV_xO₂ compounds and to follow the lithium extraction by difference‐Fourier maps. Previously delithiated Ti_1?xV_xO₂ ramsdellites are able to insert up to 0.8 Li⁺ per transition‐metal atom. The initial gravimetric capacities of 270 mAh g^?1 with good cycle stability under constant current discharge conditions are among the highest reported for bulk TiO₂‐related intercalation compounds for the threshold of one e^? per formula unit.     

Isomerism in Titanium‐Oxo Clusters: Molecular Anatase Model with Atomic Structure and Improved Photocatalytic Activity

Phase isomerism is a common and important phenomenon in inorganic materials, but the molecular isomerism of their nanocluster models is still rare. In this work, we report the first pair of isomeric titanium‐oxo clusters. Through combining pentagonal {Ti(Ti₅)} building units in corner‐sharing or edge‐sharing forms, the two obtained Ti₂₀‐oxo clusters take vertical and horizontal core configurations, respectively, which are both functionalized by the same organic ligands. More interestingly, the vertical type Ti₂₀‐oxo core contains an identical {Ti₈O₁₄} moiety as anatase, making it the first molecular model of anatase TiO₂. As a result, the vertical type cluster exhibits better photocatalytic H₂ evolution activity, higher photocurrent response and faster charge transfer to external acceptors than its horizontal isomer. Thus, this work provides a cluster isomer method to understand the structure–property relationship of import titanium oxide materials.     

掺杂二氧化锡的二氧化钛的光学性质及其光催化特性

二氧化钛多相催化是一种极具前途的环境污染深度净化技术。 本文以钛酸四丁酯和四氯化锡为原料,无水乙醇为溶剂,采用溶胶-凝胶法制备了掺杂二氧化锡的二氧化钛薄膜和复合氧化物粉体。通过测量薄膜的吸收光谱推算光学能隙,结果发现掺杂样品的光学能隙比纯二氧化钛样品有所变小。随着热处理温度的提高,掺杂和纯二氧化钛样品的光学能隙都略微降低。X-射线衍射分析表明,复合氧化物粉体的热处理温度对样品的晶体结构和光催化性能有重要影响。以掺杂二氧化锡5 % 摩尔比的样品与纯二氧化钛对照,500 ℃以下热处理样品以锐钛矿结构为主,600 ℃热处理样品为锐钛矿与金红石相共存,并显示了较好的光催化性能。透射电子显微镜观察显示,同样600 ℃热处理,掺杂样品要比纯二氧化钛具有更小的颗粒尺寸。在700 ℃热处理的样品中,掺杂样品只存在金红石相而纯二氧化钛样品中仍存有锐钛矿相。用阿伦尼乌斯经验关系式推测的晶粒生长的活化能,纯二氧化钛47.486 kJ.mol, 掺杂5 % 摩尔比的复合氧化物样品33.103 kJ.mol。以亚甲基蓝为降解物质,考察了掺杂量和热处理温度对样品的光催化性能。     

In Situ Preparation of a Ti3+ Self‐Doped TiO2 Film with Enhanced Activity as Photoanode by N2H4 Reduction

A new synthetic method to fabricate Ti³⁺‐modified, highly stable TiO₂ photoanodes for H₂O oxidation is reported. With Ti foil as both the conducting substrate and the Ti³⁺/Ti⁴⁺ source, one‐dimensional blue Ti³⁺/TiO₂ crystals were grown by a one‐step hydrothermal reaction. The concentration of Ti³⁺ was further tuned by N₂H₄ reduction, leading to a greater photoelectrocatalytic activity, as evidenced by a high photocurrent density of 0.64 mA cm^?2 at 1.0 V vs RHE under simulated AM 1.5 G illumination. Electron paramagnetic resonance and Mott–Schottky plots reveal that higher charge‐carrier density owing to N₂H₄ reduction contributes to the observed improvement. The generality of this synthesis method was demonstrated by its effectiveness in improving the performance of other types of photoanodes. By integrating the advantages of the 1D TiO₂ architecture with those of Ti³⁺ self‐doping, this work provides a versatile tool toward the fabrication of efficient TiO₂ photoanodes.     

The Origin of Higher Open‐Circuit Voltage in Zn‐Doped TiO2 Nanoparticle‐Based Dye‐Sensitized Solar Cells

Zn‐doped anatase TiO₂ nanoparticles are synthesized by a one‐step hydrothermal method. Detailed electrochemical measurements are undertaken to investigate the origin of the effect of Zn doping on the performance of dye‐sensitized solar cells (DSSCs). It is found that incorporation of Zn²⁺ into an anatase lattice elevates the edge of the conduction band (CB) of the photoanodes and the Fermi level is shifted toward the CB edge, which contributes to the improvement in open‐circuit voltage (V_OC). Charge‐density plots across the cell voltage further confirm the increase in the CB edge in DSSCs directly. Photocurrent and transient photovoltage measurements are employed to study transport and recombination dynamics. The electron recombination is accelerated at higher voltages close to the CB edge, thus leading to a negative effect on the V_OC.     

A Cation and Anion Dual Doping Strategy for the Elevation of Titanium Redox Potential for High‐Power Sodium‐Ion Batteries

Titanium‐based polyanions have been intensively investigated for sodium‐ion batteries owing to their superior structural stability and thermal safety. However, their low working potential hindered further applications. Now, a cation and anion dual doping strategy is used to boost the redox potential of Ti‐based cathodes of Na₃Ti_0.5V_0.5(PO₃)₃N as a new cathode material for sodium ion batteries. Both the Ti³⁺/Ti⁴⁺ and V³⁺/V⁴⁺ redox couples are reversibly accessed, leading to two distinctive voltage platforms at ca. 3.3 V and ca. 3.8 V, respectively. The remarkably improved cycling stability (86.3 %, 3000 cycles) can be ascribed to the near‐zero volume strain in this unusual cubic symmetry, which has been demonstrated by in situ synchrotron‐based X‐ray diffraction. First‐principles calculations reveal its well‐interconnected 3D Na diffusion pathways with low energy barriers, and the two‐sodium‐extracted intermediate NaTi_0.5V_0.5(PO₃)₃N is also a stable phase according to formation energy calculations.     

Water oxidation on N‐Doped TiO2 nanotube arrays

Photocatalytic splitting water into hydrogen and oxygen by utilizing solar energy is regarded as an effective strategy to solve oil crisis. By utilizing density functional calculations, we herein present the systemic studies with respect to water splitting mechanism on N‐doped TiO₂ nanotube arrays (NTAs), and focus on activation energy, thermodynamic properties, and effects of N‐doping on reaction process. Our results reveal that the impurity 2p states of doped nitrogen effectively change electronic structure of TiO₂ NTAs, which act as an electron acceptor and facilitate weakly bound electrons of valence band to be easily excited to acceptor level, as well as enhance the first H₂O adsorption and dissociation on the inside wall of N‐doped TiO₂ NTAs. Therefore, it is found that the rate‐determining step of water splitting is the formation reaction of HOO* on N‐doped TiO₂ NTAs rather than the formation of HO* from the first H₂O. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Synthesis and Modification of Zn‐doped TiO2 Nanoparticles for the Photocatalytic Degradation of Tetracycline

The synthesis of Zn‐doped TiO₂ nanoparticles by solgel method was investigated in this study, as well as its modification by H₂O₂. The catalyst was characterized by transmission electron microscopy, X‐ray diffraction, Brunauer–Emmett–Teller, UV–visible reflectance spectra and X‐ray photoelectron spectroscopy (XPS). The results indicated that doping Zn into TiO₂ nanoparticles could inhibit the transformation from anatase phase to rutile phase. Zn existed as the second valence oxidation state in the Zn‐doped TiO₂. Zn‐doped TiO₂ that was synthesized by 5% Zn doping at 450°C exhibited the best photocatalytic activity. Then, the H₂O₂ modification further enhanced the photocatalytic activity. Zn doping and H₂O₂ modifying narrowed the band gap and efficiently increased the optical absorption in visible region. The optimal degradation rate of tetracycline by Zn‐doped TiO₂ and H₂O₂ modified Zn‐doped TiO₂ was 85.27% and 88.14%. Peroxide groups were detected in XPS analysis of H₂O₂ modified Zn‐doped TiO₂, favoring the adsorption of visible light. Furthermore, Zn‐doped TiO₂ modified by H₂O₂ had relatively good reusability, exhibiting a potential practical application for tetracycline's photocatalytic degradation.     

Enhancing Photoactivity of TiO2(B)/Anatase Core–Shell Nanofibers by Selectively Doping Cerium Ions into the TiO2(B) Core

Cerium ions (Ce³⁺⁾ can be selectively doped into the TiO₂(B) core of TiO₂(B)/anatase core–shell nanofibers by means of a simple one‐pot hydrothermal treatment of a starting material of hydrogen trititanate (H₂Ti₃O₇) nanofibers. These Ce³⁺ ions (≈0.202 nm) are located on the (110) lattice planes of the TiO₂(B) core in tunnels (width≈0.297 nm). The introduction of Ce³⁺ ions reduces the defects of the TiO₂(B) core by inhibiting the faster growth of (110) lattice planes. More importantly, the redox potential of the Ce³⁺/Ce⁴⁺ couple (E°(Ce³⁺/Ce⁴⁺)=1.715 V versus the normal hydrogen electrode) is more negative than the valence band of TiO₂(B). Therefore, once the Ce³⁺‐doped nanofibers are irradiated by UV light, the doped Ce³⁺ ions—in close vicinity to the interface between the TiO₂(B) core and anatase nanoshell—can efficiently trap the photogenerated holes. This facilitates the migration of holes from the anatase shell and leaves more photogenerated electrons in the anatase nanoshell, which results in a highly efficient separation of photogenerated charges in the anatase nanoshell. Hence, this enhanced charge‐separation mechanism accelerates dye degradation and alcohol oxidation processes. The one‐pot treatment doping strategy is also used to selectively dope other metal ions with variable oxidation states such as Co^2+/3+ and Cu^+/2+ ions. The doping substantially improves the photocatalytic activity of the mixed‐phase nanofibers. In contrast, the doping of ions with an invariable oxidation state, such as Zn²⁺, Ca²⁺, or Mg²⁺, does not enhance the photoactivity of the mixed‐phase nanofibers as the ions could not trap the photogenerated holes.     

The Influence of Defects on Mo‐Doped TiO2 by First‐Principles Studies

TiO₂ doped with transition metals shows improved photocatalytic efficiency. Herein the electronic and optical properties of Mo‐doped TiO₂ with defects are investigated by DFT calculations. For both rutile and anatase phases of TiO₂, the bandgap decreases continuously with increasing Mo doping level. The 4d electrons of Mo introduce localized states into the forbidden band of TiO₂, and this shifts the absorption edge into the visible‐light region and enhances the photocatalytic activity. Since defects are universally distributed in TiO₂ or doped TiO₂, the effect of oxygen deficiency due to oxygen vacancies or interstitial Mo atoms is systemically studied. Oxygen vacancies associated with the Mo dopant atoms or interstitial Mo will reduce the spin polarization and magnetic moment of Mo‐doped TiO₂. Moreover, oxygen deficiency has a negative impact on the improved photocatalytic activity of Mo‐doped TiO₂. The current results indicate that substitutional Mo, interstitial Mo, and oxygen vacancy have different impacts on the electronic/optical properties of TiO₂ and are suited to different applications.