A series of TiO2-N/SnO2X heterostructure photocatalysts were synthesized by a hydrolysis-deposition method. The structure, existing states of N and SnO2 heterostructure at the interface of TiO2-N/SnO2X were studied by EADX, XRD, Raman, FT-IR, XPS, and HRTEM. The band structure is investigated by both theoretical calculation and experiment characterization. It was found that the introduction of NOx surface species and SnO2 nanoparticles would enhance the absorption in visible region, increase reactive oxidative species and separate photogenerated electrons and holes efficiently. Therefore, the photocatalytic activity is improved significantly for TiO2-N/SnO2X, compared with TiO2-N and TiO2 under visible and UV light irradiation. This work may offer a new strategy to fabricate new photocatalyst with high photocatalytic performance.The TiO2-N/SnO2 heterostructured photocatalysts exhibit a significant visible-light response and an efficient separation of charges carriers due to the introduction of NOx and SnO2.

We introduce the idea of Pd catalysis used in cross-coupling reactions into photocatalysis. The −O–Pd–Cl surface species modified on nanoscale TiO2 can remarkably enhance the photocatalytic activity under visible-light irradiation in the degradation of 4-bromophenol. It is revealed that the catalytically active Pd0 is in situ generated by the reduction of photogenerated electrons from TiO2. The −O–Pd0–Cl species can react with 4-bromophenol to form hydroxyphenyl radicals via an oxidative addition reaction. The photocatalytic mechanism assisted by the oxidative addition reaction on −O–Pd–Cl species is also demonstrated. We were hence able to rationally tailor the coordination environment of Pd on the TiO2 surface to obtain high photocatalytic activity and selectivity.

A novel Pd-modified TiO2 photocatalyst composited with carbon nanoparticles (TiO2–Pd/C) was synthesized via a sol–gel method. Characterized by XRD, Raman, BET, HRTEM, EDAX, XPS, absorption spectra and PL techniques, the introduced Pd existed as unique O–Pd–O species, and C existed as the substance carbon nanoparticles on the surface of TiO2, behaving as a composited photocatalyst on a nanoscale. Based on the experimental results and theoretical calculations, it was found that the introduction of O–Pd–O surface species and the carbon nanoparticles extend the absorption into the visible light region and facilitate the separation of photogenerated charge carriers, resulting in a much better photocatalytic activity of the photo-reduction of CO2 into CH4, than would be achieved with TiO2–Pd and TiO2/C samples.

A series of CeO2-modified C3N4 (C3N4/CeO2X%) heterostructure photocatalysts was synthesized via a hydrothermal method. Their band structure was studied via theoretical calculation and experimental characterization. The behavior of charge carriers was investigated via absorption spectra, PL, and time-resolved PL decay curves. It was found that the modification of CeO2 on C3N4 matched the energy band, enhanced absorption in the visible region, separated photogenerated electrons and holes. Owing to the modification of CeO2 on C3N4, the photocatalytic performance was significantly improved as compared to that of C3N4. This study may offer a new strategy to fabricate and design a band structure of a new photocatalyst with high photocatalytic performance.

A series of Fe-doped TiO2 samples with different concentrations of Fe3+ ions annealed at different temperatures were prepared using a sol–gel method. For Fe-doped TiO2, the existing states of Fe3+, the transformation of these existing states and the phase transition are investigated. It is revealed that the existing states of Fe3+ and the transformation of these existing states (substitutional Fe3+, α-Fe2O3 and Fe2TiO5) are closely related to the annealing temperature and the concentration of Fe3+. The anatase–rutile (A–R) transition is promoted by the substitutionally doped Fe in the TiO2 lattice. In addition, the Fe-doped TiO2 with a high calcination temperature (800 °C) and high doping concentration (x > 20) shows obvious ultrasonic piezoelectric catalytic activity for the reduction of CO2 to CH4. This work may be useful in understanding the doping mechanisms of Fe-doped TiO2, and the fabrication of new catalytic materials.

A series of the metal ions (Pd, Cu, and Mn) modified TiO2 photocatalysts are synthesized via simple sol–gel method. Characterized by X-ray diffraction, Raman, UV–vis absorption spectra, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, time-resolved photoluminescence (PL) decay curves, and PL, it was revealed these introduced metal ions existed as O–Me–O species (Me: Pd, Cu, and Mn) on the surface of TiO2. The corresponding theory calculation is used to investigate the electronic density of states and band structure of the metal ions (Pd, Cu, and Mn) modified TiO2. The modified TiO2 photocatalysts exhibit an improved photocatalytic performance on reduction of CO2 and H2O into methane (CH4), attributed to the contribution of surface species by enhancing the visible absorption efficiently, separating charge carriers, and matching of the redox potential on the photoreduction of CO2 into CH4. This article could provide a wider understanding about the adjustment and matching of the energy level for the synthesis and design of functional materials with excellent photocatalytic performance.

The novel Pd- and Mn-comodified TiO2 photocatalyst (TiO2–Pd–Mn) was prepared via a simple sol–gel method. The introduced Pd and Mn existed as the −O–Pd–O– and −O–Mn–O– species on the surface of the photocatalyst. The band structure and density of states are studied by theoretical calculations, which is demonstrated by the experimental results. The modification with Pd and Mn ions results in the strong visible response and efficient separation of photogenerated carriers. Thus, the TiO2–Pd–Mn exhibit improved photocatalytic activity compared with pure TiO2, TiO2–Pd, and TiO2–Mn for photoreduction of CO2 and H2O into CH4. It is an effective method on developing the highly active TiO2-based materials by modification with double elements on the surface.

A series of TiO2/vanadate (Sr10V6O25, Ni3V2O8, Zn2V2O7) heterostructured photocatalysts were prepared by a simple sol–gel method. The theoretical calculations imply the possible energy band match between TiO2 and vanadates. Characterized by XRD, Raman, TEM, EDX, XPS, absorption spectra, PL and time-resolved PL decay curves, it is revealed that the vanadates, which exist on the surface of TiO2, could suppress the recombination of charge carriers, prolong the life-time of photogenerated electrons and provide surface reactive hole sites, improving the photocatalytic activity for photo-reduction of CO2 into CH4.

A novel copper and carbon co-modified TiO2 photocatalyst (TiO2–Cu/C) was prepared using a simple sol–gel method. The properties of the photocatalysts were investigated by XRD, Raman, BET, HRTEM, EDAX, XPS, absorption spectroscopy and PL techniques. It was revealed that copper was present as –O–Cu–O– species and carbon existed as elementary substance carbon nanoparticles on the surface of TiO2. Based on the experimental results, the band structure and electronic density of states of TiO2, TiO2–Cu and elementary substance carbon are predicted by theoretical calculations. The energy levels of the –O–Cu–O– species and the carbon nanoparticles were located above the valence band, which extends the absorption into the visible light region, facilitate the separation of photogenerated charge carriers and provide hole reactive sites on the surface. Thus, the photocatalytic activity of TiO2–Cu/C is much higher than that of pure TiO2, TiO2/C and TiO2–Cu for photoreduction of CO2 and H2O into CH4 under UV-light irradiation.

•The introduced Zn exist as O–Zn–Cl surface species.•The O–Zn–Cl species result in an enhanced photocatalytic activity on reduction of CO2 into CH4.•The band structure and photocatalytic mechanism of the samples are discussed.TiO2 modified with unique surface O–Zn–Cl species is prepared by a simple sol-gel method. The Zn modified TiO2 samples exhibit improved photocatalytic activity on photo-reduction of CO2 into CH4, compared with pure TiO2. The surface structure and photocatalytic properties are investigated by Raman, XRD, XPS, UV–vis absorption spectra, PL and time-resolved PL decay curves techniques. It is revealed that the existence of O–Zn–Cl species can extend the absorption into visible region, inhibit the recombination of charge carriers and prolong the lifetime of photogenerated electrons. Therefore the O–Zn–Cl modified TiO2 samples represent an improved photocatalytic performance on photo-reduction of CO2 into CH4.

A series of Zn doped TiO2 is prepared via a sol–gel method by changing the concentration of Zn ions and calcination temperature. To investigate the existing states of introduced Zn, the transformation of existing states and the phase transition for the Zn doped TiO2 samples, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques are applied. The existing states of Zn2+ ions and the transformation of existing states (O–Zn–Cl and ZnTiO3) in Zn doped TiO2 are closely related to the calcination temperature and concentration of introduced Zn ions, as well as the phase transition temperature from anatase to rutile. This work can be helpful for a better realization about the doping mechanism of Zn2+ ions in Zn doped TiO2.

A new kind of nanostructured photocatalyst, PbBiO2Cl is synthesized by a simple hydrothermal method. The proposed formation mechanism of PbBiO2Cl is carried out by analyzing the XRD patterns and SEM images of the products prepared under different conditions. The PbBiO2Cl nanostructure behaves as a truncated bipyramid, exposed with {002} and {103} facets. Moreover, theoretical calculation and absorption spectrum indicate the PbBiO2Cl shows strong absorption in the visible region with a band gap of 2.53 eV. The obtained PbBiO2Cl nanostructures exhibit significantly enhanced photocatalytic activity on degradation of methyl orange (MO) and 4-chlorophonel (4-CP). This work may offer a paradigm on designing and synthesizing visible photocatalyst exposed with reactive facets, which can be applied in many fields.

Nitrogen and zirconium co-doped TiO2 (TiO2–N–x%Zr) photocatalysts were synthesized via a sol–gel method. The existing states of the dopants (N and Zr) and their corresponding band structures were investigated via XRD, Raman, BET, XPS, TEM, FT-IR, UV–vis DRS, and PL techniques. It was found that N existed only as a surface species (NOx) and Zr4+ was doped in a substitutional mode; the doping of Zr4+ ions and modification of N extended the absorption into the visible region and inhibited the recombination of electrons and holes. Moreover, the excess Zr4+ ions existed as the ZrTiO4 phase when the content of Zr was sufficiently high, which could also contribute to the separation of the charge carriers. Therefore, the TiO2–N–x%Zr samples show enhanced visible-light photocatalytic activity compared with single-doped TiO2. These results offer a paradigm for the design and fabrication of optoelectronic functional materials such as solar cells and photocatalysts.Keywords: 4-chlorophenol degradation; substitutional Zr4+ ions; surface NOx species; TiO2; visible-light photocatalysis; ZrTiO4;

Density functional theory (DFT) calculation is carried out to access the band structure and density of states (DOS) based on the models of TiO2 nanoparticle, nanotube, and nanosheet, predicting the order of the photocatalytic activity for three different nanostructures. Sol–gel method and hydrothermal method are used to achieve desired morphologies: nanoparticles, nanotubes, and nanosheets (fragmentized nanotubes). The photocatalytic activity ranks in the order of nanosheets > nanotubes > nanoparticles, which is consistent with theoretical prediction. It was revealed that the enlargement of band gap is caused by the quantum confinement effect; the prolonged lifetime of photogenerated electrons and increased specific surface areas are dependent on the morphology of the nanostructure. All these factors contribute to the improvement of the photocatalytic activity for nanostructures. Our results can guide the design and selection of low-dimensional nanomaterials with desired morphology and improved photoelectric functional properties, which can be used in many fields, such as solar cells, photocatalysis, and photosynthesis.

A new type of heterostructured photocatalysts (N-TiO2/InBO3) were synthesized by coupling nitrogen-modified TiO2 (N-TiO2) with indium borate (InBO3) via a one-step sol–gel method. It was revealed that N-TiO2/InBO3 exhibited an improved photocatalytic performance compared with TiO2, N-TiO2, and InBO3 under both UV and visible light irradiation because of the formation of a heterostructure at the interface as well as the introduction of surface NOx species and InBO3. These results may provide a paradigm to fabricate and design the optoelectronic functional materials with high efficiency and performance.

We theoretically forecasted the narrowness of band gap for the nitrogen doped ZrO2, implying a possible energy level matching at the interface between TiO2 and ZrO2. Inspired by the theoretical simulation, the corresponding experiment was carried out and the band gap of ZrO2 is narrowed by doping nitrogen in substitutional mode. The TiO2–N/ZrO2–xNx composite photocatalyst exhibited an excellent photocatalytic performance under visible-light irradiation. The enhancement is caused by the introduction of doping energy level as well as the electrons’ transition at interface, which would enhance the visible absorption of composite photocatalyst and separate the photogenerated charge carriers efficiently. These results suggest that the selection of materials and dopants and the matching of energy levels at interface are of great importance to design and fabricate photocatalysts with high efficiency and performance.

•A new kind of visible light activated photocatalyst was prepared by combining ZnO with B-doped TiO2 via the sol–gel method.•B dopants energy level narrowed the band gap.•Combined ZnO suppressed the recombination of photogenerated carriers.A new type of composite photocatalysts (ZnO/TiO2–B) with Zinc oxide nanoparticles dispersed on boron doped titanium dioxide was prepared via a sol–gel method. The as-prepared powders were characterized by HRTEM, XRD, XPS, UV–vis DRS, and PL techniques. The results reveal that B3+ ions are doped into the TiO2 lattice in interstitial mode, while ZnO nanoparticles are dispersed on the surface of TiO2. The absorption of photocatalysts was extended into visible light region and the photogenerated electrons and holes were separated efficiently. Hence, ZnO/TiO2–B composite photocatalyst exhibits much better photocatalytic activity than those of pure TiO2 and TiO2–B on photodegradation of 4-chlorophenol under visible light irradiation.

TiO2 nanoparticles doped with different concentrations of Zr4+ ions were prepared by the sol–gel method and annealed at different temperatures. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM) techniques were used to investigate the existing states and doping mechanism of dopants as well as the phase transition of the Zr4+-doped TiO2 samples. It was revealed that the doping behavior of introduced Zr4+ ions was closely related to the doping concentration. The Zr4+ ions would replace the lattice Ti4+ ions directly in substitutional mode at a certain annealing temperature. Moreover, if the concentration of doped Zr4+ ions is high enough, excess Zr4+ ions would form ZrTiO4 on the surface of TiO2. In addition, the phase transition temperature from anatase to rutile increases significantly after doping Zr4+ ions, due to their larger electropositivity and radius than those of Ti4+ ions. Our results may afford a better understanding on the doping mechanism and aid in the preparation of Zr-doped TiO2 with high photoelectric performance.