•Three different TiO2 with surface oxygen vacancy and/or bulk SETOV were fabricated.•Coexistence of two defects exhibited an enhanced activity for CO2 photo-reduction.•Increasing the ratio of SO/SETOV facilitates to separate the photoexcited e-h pairs.Oxygen vacancies play an important role in many photocatalytic reaction, and have attracted enormous attention from the scientists and engineers. The surface or bulk oxygen vacancies have a different function in the photo-reaction process. Herein, three different TiO2 nanoparticles possessing surface oxygen vacancies (SO) and/or bulk single-electron-trapped oxygen vacancy (SETOV) were fabricated by dehydration or reduction of different titania precursors. The three kinds of TiO2 nanoparticles were characterized systematically by XRD, TEM, Raman, XPS, ESR, TG, UV–vis DRS, and PL techniques. The photocatalytic reduction results of CO2 indicated that both the bulk SETOVs and surface oxygen vacancies contributed to the enhancement of the light absorption, while the surface vacancies facilitated to the separation of the photo-generated charge carriers, and on the contrast, the bulk SETOVs acted as the recombination center. The co-existence of the surface and bulk oxygen vacancies exhibited a synergistic effect to improve the photoreduction efficiency of CO2 to CH4. Through adjusting the ratio of the surface and bulk oxygen vacancies and analyzing the positron lifetime and relative intensity by positron annihilation, the photoreduction efficiency of CO2 improved with the increase of the ratio of surface oxygen vacancies to bulk SETOVs.Oxygen vacancies with different types and concentrations play important roles in many photocatalytic reaction, and attracts much attention recently. Herein, we fabricated three different TiO2 nanomaterials with surface oxygen vacancies and/ or bulk single-electron-trapped oxygen vacancy (SETOVs) successfully, and found that they exhibited different function in photocatalytic reduction of CO2. Through adjusting the ratio of the surface oxygen vacancies and bulk SETOVs, and analyzing the positron lifetime and relative intensity by positron annihilation, we found that the photo-reduction efficiency of CO2 improved with the increase of the ratio of surface oxygen vacancies.Download high-res image (172KB)Download full-size image

•The g-C3N4-TiO2 composites were obtained by simple solid state sintering.•The composites were direct contact Z-scheme without an electron mediator.•Annealing temperature had vast effect on the photocatalytic activity of composites.•Thinner porous g-C3N4 nanosheets improved the separation of photoexcited e-h pairs.Z-scheme g-C3N4-TiO2 heterojunctions containing g-C3N4 nanosheets with different thickness were prepared by sintering the mixture of g-C3N4 and nanotube titanic acid (denoted as NTA) at different temperatures in air. As-prepared Z-scheme g-C3N4-TiO2 heterojunctions were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible light diffuse reflectance spectrometry, electron spin resonance, and photoluminescence spectrometry. Findings indicate that the annealing temperature has crucial effects on the visible-light photocatalytic activity (λ ≥ 420 nm) of the as-prepared Z-scheme g-C3N4-TiO2 heterojunctions. The Ti3+ and porous g-C3N4 nanosheets formed upon the calcination at 600 °C as well as the low concentration of bulk single-electron trapped oxygen vacancy are favorable to the transport of the photoexcited charge carriers. This, in association with the Z-scheme system, contributes to improving the photocatalytic activity of g-C3N4-TiO2 photocatalysts. As a result, g-C3N4-TiO2 photocatalyst prepared at 600 °C exhibits good photocatalytic activity towards the degradation of propylene and hydrogen generation by water-splitting under visible light irradiation.A series of g-C3N4-TiO2 direct contact Z-scheme heterojunction composites were prepared via simple calcination of the mixture of NTA and bulk g-C3N4 at different temperatures in air and the vast difference of photocatalytic activity of composites was mainly due to the influence of calcination temperature to the structure of g-C3N4. The g-C3N4 in the composites was stable plane structure below 600 °C while it would partially decompose to form the thinner porous nanosheets at 600 °C and the latter provided plentiful exposed surface, numerous active sites, a short transport length for increasing the separation efficiency of interface photoexcited charge carrier and the more irradiation of the incident light. In addition, comparing with the sample of 400 °C, the bulk Vo concentration in the g-C3N4-TiO2-600 composites decreased sharply and formed Ti3+ in the process of roasting which greatly promoted the separation of photogenerated charge carrier and so powerfully enhanced its photocatalytic oxidation propylene activity under visible light.Download high-res image (140KB)Download full-size image

BaTiO3/TiO2 heterostructure nanotube arrays were fabricated by in situ hydrothermal method using TiO2 nanotubes as both template and reactant. The BaTiO3/TiO2 heterostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. Compared with pure TiO2 nanotube arrays, the BaTiO3/TiO2 heterostructures exhibited enhanced photocurrent under UV light irradiation. The electrochemical impedance spectra (EIS) showed that the impedance arc radius of BaTiO3/TiO2 heterostructures was much smaller, indicating an improved charge carrier separation ability was achieved. In addition, the BaTiO3/TiO2 heterostructure nanotube arrays displayed a higher photocatalytic activity for methylene blue degradation.

•BaTiO3/TiO2 heterostructure nanotube arrays were fabricated.•Ag nanoparticles were loaded by two different reduction methods.•Ag nanoparticles obtained by the chemical reduction were small and uniform.•Ag/BT-C exhibited higher separation efficiency of the photo-generated carriers.•Ag/BT-C exhibited higher photocatalytic activity for MB degradation.BaTiO3/TiO2 (BT) heterostructure nanotube arrays were fabricated by in situ hydrothermal method using TiO2 nanotubes as both template and reactant. Compared with pure TiO2 nanotube arrays, the BT heterostructures exhibited enhanced photocurrent under UV light irradiation. For further improving the photoelectrochemical performance, Ag nanoparticles were loaded on the surface of BT heterostructure by two different photo-reduction (Ag/BT-P) and chemical reduction (Ag/BT-C) methods. The results showed that the Ag nanoparticles on Ag/BT-C were uniform and dispersed homogeneously, but the Ag nanoparticles on Ag/BT-P were very large, which resulted in the tailored and integrated nanotube structure destroyed. The electrochemical impedance spectra (EIS) indicated that the impedance arc radius of Ag/BT-C was much smaller than Ag/BT-P and the pure BT nanotube arrays, indicating that the enhanced charge carrier separation was achieved on Ag/BT-C. In addition, the Ag/BT-C nanotube arrays exhibited a higher photocatalytic activity for methylene blue (MB) degradation.

Zr/N co-doped TiO2 nanostructures were successfully synthesized using nanotubular titanic acid (NTA) as precursors by a facile wet chemical route and subsequent calcination. These Zr/N-doped TiO2 nanostructures made by NTA precursors show significantly enhanced visible light absorption and much higher photocatalytic performance than the Zr/N-doped P25 TiO2 nanoparticles. Impacts of Zr/N co-doping on the morphologies, optical properties, and photocatalytic activities of the NTA precursor-based TiO2 were thoroughly investigated. The origin of the enhanced visible light photocatalytic activity is discussed in detail.

In this paper, we report on the co-doping nitrogen and sulfur has been achieved in the TiO2 nanotube array films by treatment with thiourea and calcination under vacuum at 500 °C for 3 h. The samples were characterized by scanning electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible diffuse reflectance spectroscopy. XPS spectra revealed that N might coexist in the forms of NTiO and NOTi, S was incorporated into the lattice of TiO2 through substituting oxygen atoms in the N + S co-doped TiO2 nanotube array films. XRD patterns indicated that improved crystallinity was obtained for N + S co-doped TiO2 nanotube arrays as compared to that of undoped TiO2 nanotube arrays. In photoelectrochemical measurements, the photocurrent of N + S co-doped TiO2 nanotube array films was greatly enhanced compared to that of undoped samples under visible light irradiation. And the photocatalytic activities of the samples were evaluated on the removal of methylene blue under visible light irradiation. The N + S co-doped TiO2 nanotube array films showed a better photocatalytic activity than the undoped sample due to the N, S doping.

Au core Ag shell composite structure nanoparticles were prepared using a sol method. The Au core Ag shell composite nanoparticles were loaded on TiO2 nanoparticles as support using a modified powder–sol method, enabling the generation of Au @ Ag/TiO2 photocatalysts for photocatalytic decomposition and elimination of ozone. The sols were characterized by means of ultraviolet–visible light (UV–Vis) reflection spectrometry, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The activity of the Au @ Ag/TiO2 photocatalysts for photocatalytic decomposition and elimination of ozone was evaluated and the effect of Cl− anions on the photocatalytic activity of the catalysts was highlighted. Results showed that Au @ Ag/TiO2 prepared via the modified powder–sol route in the presence of an appropriate amount of NaCl solid as demulsifier had better activity in the photocatalytic decomposition and elimination of ozone. At the same time, Au @ Ag/TiO2 catalysts had better ability to resist poisonous Cl− anions than conventional Au/TiO2 catalyst. The reasons could be, first, that NaCl was capable of reducing the concentration of free Ag+ by adsorption on the surface of Ag particles forming AgCl and enhancing the formation of Au core Ag shell particles, leading to a better resistance to Cl− anions of the catalysts, and, second, AgCl took part in the photocatalytic decomposition of ozone together with Au @ Ag/TiO2 catalysts and had a synergistic effect on the latter, resulting in better photocatalytic activity of Au @ Ag/TiO2 catalysts.

Although numerous papers are available about the origin of visible light photocatalytic activity of N-doped TiO2, it still remains conflicting how nitrogen-doping affects the visible light photocatalytic activity of TiO2. Thus N-doped TiO2 was prepared by heat treatment of commercial P25-TiO2 in flowing NH3, aiming at revealing the origin of visible light sensitization of N-doped TiO2. The resulting N-doped TiO2 was characterized by means of electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), diffusion reflectance spectrometry (DRS), and X-ray diffraction (XRD). Results indicate that N-doped TiO2 shows triplet g value ESR signals (g = 1.987, 2.004 and 2.024), which is attributed to single-electron-trapped oxygen vacancy (denoted as SETOV) in a certain chemical environment. Its visible light photocatalytic activity is proportional to the intensity of the triplet g value signals, which implies that the visible light photocatalytic activity of N-doped TiO2 is closely correlated to the formation of SETOV during heat treatment in flowing NH3. Besides, N-doped TiO2 catalyst calcinated at 600 °C possesses the highest photocatalytic activity, but that calcinated at 700 °C has drastically decreased photocatalytic activity and shows no XPS signal of nitrogen. Moreover, N-doped TiO2 shows visible light absorption in a wavelength range of 400–520 nm, which is attributed to the formation of SETOV and phase transformation from anatase to rutile. It is suggested that the visible light photocatalytic activity of N-doped TiO2 is co-determined by the formation of SETOV in TiO2 matrix and existence of doped-N on the surface. In other words, in the absence of either SETOV in TiO2 matrix or doped-nitrogen on the surface, N-doped TiO2 will not show visible light photocatalytic activity; and the higher the SETOV concentration is, the better the visible light photocatalytic activity will be.Graphical abstractESR results of P25-TiO2 and N-doped TiO2. The results were obtained at room temperature in air.Download full-size imageResearch highlights▶ Triplet g value ESR signal ascribed to single-electron-trapped oxygen vacancy was observed for N-doped TiO2. ▶ The visible light photocatalytic activity of N-doped TiO2 is proportional to the intensity of triplet ESR signal. ▶ Doped-N functions to prevent photoinduced electrons and holes from recombination.