Uniform doping is usually recognized as an efficient method for enhancing photocatalysis of TiO2, while non-uniform doping is generally supposed to be inefficient because of the inhomogeneous dispersion of dopants. However, in this study, we present the first example of non-uniform doping (with Au)-better enhanced photocatalysis of TiO2 nanotubes, as compared to uniform doping, in terms of the photocatalytic organic dye degradation. The extent to which the non-uniform doping (achieved by liquid phase deposition (LPD)) can enhance photocatalysis is evaluated, along with a comparison with uniform doping. There exists an additional positive effect in the non-uniform doping system, that is, as generated interfaces between pure phase TiO2 and Au-doped TiO2, in contrast to the uniform doping system leading to a positive “platinum island” effect. Such double beneficial effects contribute to the highest performance in the photocatalytic organic dye degradation for the Au-non-uniformly doped TiO2 nanotubes as compared to other samples involved in this study. Both the “platinum island” and interfacial separation effects are helpful to isolate the photo-generated electrons and holes, resulting in enhanced photocatalytic activities.

•Titanium dioxide nanotube was synthesized by hydrothermal, modification and etching process.•Titanium dioxide nanotube has good morphology at high calcination temperature.•Titanium dioxide nanotube exhibits improved photocatalytic activity.The hydrothermally synthesized titanate nanotubes were first modified by silane coupling agent as a pillar support, and then calcined at 450 °C. Finally, the surface coated silica was etched by concentrated NaOH to obtain pure titanium dioxide nanotube. The nanotubes were characterized by X-ray diffraction, high-resolution scanning and transmission electron microscopy, N2 adsorption-desorption isotherm measurements and photoluminescence spectra. The results revealed that titanium dioxide nanotubes have good tubular morphology, high anatase crystallinity and large surface area. Due to the better 1D nano-tubular morphology and higher separation efficiency of the photogenerated electron-hole pairs, titanium dioxide nanotube exhibits improved photocatalytic activity for the degradation of methyl orange.

•A novel route of silane-modification then H2 calcination is developed.•Carbon and silica dots are highly dispersed on the TNT.•The CS@TNT retains its tubular morphology at as high as 500 °C.•A high activity (0.096 mmol/h) is achieved towards photocatalytic water splitting.•Effect of carbon/silica modification is investigated in detail.A novel carbon/silica co-decorated TiO2 nanotubes (TNT) photocatalyst (CS@TNT) was prepared for visible light driven water splitting by a facile two-steps process of amino-propyl-triethoxy silane modification and calcination. Structure characterizations (XRD, STEM, XPS, UV–vis spectra and PL spectra) revealed that the silica and carbon dots were homogeneously dispersed on the surfaces of TNT. The introduced silica resists crystallization of TNT, and keeps the tubular morphology intact and high surface area; the co-existence of carbon dots increases the visible light absorbance capacity and promotes the electron-hole separation efficiency. The CS@TNT preserves intact tubular morphology during calcination (500 °C), and a high activity of 0.096 mmol/h is achieved, which was 1.7 times higher than that of actual catalyst made of gold nanoparticles loading on TNT.Using amino-propyl-triethoxy silane (KH570) as silane couplers, a novel carbon/silica dots co-decorated TiO2 nanotube (CS@TNT) is prepared through silane modification and thermal calcination approach. The CS@TNT retains its tubular morphology at as high as 500 °C, and presents a high activity (0.096 mmol/h) towards photocatalytic water splitting.Download high-res image (93KB)Download full-size image

•Fe2O3@MnO2 spindles were designed for use in Fischer–Tropsch synthesis.•Different from the conventional Fe–Mn catalysts, no Fe–Mn spinel oxide was formed.•Mn promoter was used to weaken the C–O band and thus favoring chain growth.•The prepared novel catalysts displayed excellent selectivity to C5+ hydrocarbons.Manganese served as promoter for the Fe-based catalysts has been considered to have great potential in Fischer–Tropsch (FT) synthesis. However, many previous researchers mainly focus on the enhancement of lower olefins, primarily because the previous reported Fe–Mn spinel oxide hindered its wide applications. In this article, Fe2O3@MnO2 spindles were fabricated by coverage of hematite with MnO2 via a hydrothermal reaction and applied in FT synthesis reaction. Different from the conventional Fe–Mn catalysts, the synthesized core–shell catalysts exhibited enhanced catalytic performance, especially in C5+ hydrocarbons selectivity. It demonstrated that Mn promoter could accelerate the dissociation of CO and thus enhanced the concentration of active intermediates for chain growth. Compared with the pure Fe2O3 (Mn-free) catalyst, the selectivity toward C5+ hydrocarbons over Fe2O3@MnO2 (Mn-9) catalyst was increased from 44.6 to 66.6 wt%. Meanwhile, the undesired CH4 was decreased from 16.8 to 8.9 wt%.Fe2O3@MnO2 spindles were designed as catalysts for use in Fischer–Tropsch synthesis. Different from the conventional Fe–Mn catalysts, the synthesized catalysts displayed excellent selectivity to C5+ hydrocarbons and poor to CH4 as a side product.

•A novel Fe-based confinement catalyst was prepared by vacuum-assisted impregnation.•Catalyst with confinement effect shows an improved yield of oil phase product.•Fe-in-TNT catalyst has a higher C5+ and C12+ hydrocarbons selectivity than Fe-out-TNT.•Confinement effect of encapsulated Fe2O3 can enhance the reabsorption of α-olefin in FTS.The catalysts with high activity and selectivity, especially the latter, play a key role in Fischer–Tropsch synthesis technology. Herein, a novel Fe-based confinement catalyst, Fe2O3 encapsulated in TiO2 nanotubes, was prepared by a method of vacuum-assisted impregnation. The catalyst presented an excellent yield of oil phase hydrocarbons and C5+ selectivity due to the confinement effect. This interesting phenomenon was discussed from the view of the restrained molecular movement and enhanced readsorption of short-chain α-olefins in the confined space of TiO2 nanotube channels. The results over this novel confinement catalyst revealed a promising research prospect on adjusting the product distribution to long-chain hydrocarbons for Fe-based Fischer–Tropsch catalyst.

Hierarchical porous-structured Fe3O4 microspheres for use in Fischer–Tropsch (FT) synthesis were fabricated via a solvothermal reaction mediated by ethylene glycol. Each microsphere was observed to consist of primary nanoparticles that served as active oxides for the FT synthesis reaction. Without additional porous support, the pore structure was well constructed through the self-assembly of the Fe3O4 nanoparticles. Compared to the previously reported Fe catalyst, the current synthesized porous structure appeared to have improved the interparticle connectivity and promote the dispersion of active metal, and hence enhance the selectivity for C5+ hydrocarbons. In particular, 1PFe with an average pore size of 3.71 nm and Fe dispersion of 13.9% exhibited the best selectivity for C5+ (59.0 wt%) combined with the lowest CH4 product fraction (11.3 wt%), as well as a high CO conversion of 93.2%.

Fe3O4 nanoparticles with pore size of 12.4 nm were synthesized and employed as catalyst for Fischer–Tropsch (FT) synthesis. The as-prepared Fe3O4 catalyst achieved a CO conversion of 98.3% while yielding higher than 50 wt% gasoline range (C5–C11) hydrocarbons after FT reaction for 48 h. Furthermore, highly activated Ag-doped composites were designed through a one-pot solvothermal method, and then porous core/shell materials were obtained. Interestingly, active metal oxide (Fe3O4) nanoparticles were interspersed on the surface of the Ag promoter. Importantly, pores could enhance the dispersion of metal particles and facilitate heat and mass transfer. The addition of Ag promoter decreased the selectivity to CH4 and enhanced the yield of C2–C4 olefins. In particular, 0.8Ag/Fe3O4 displayed high CO conversion (96.4%) and optimum selectivity to C2–C4 olefins (28.3 wt%) while yielding a low selectivity to CH4 (12.1 wt%), as well as a good selectivity to C5–C11. More importantly, 0.8Ag/Fe3O4 showed the highest catalytic activity (>1.6 × 10−4 molco gFe−1 s−1) and the best total hydrocarbon yield (5.25 × 10−3 gHC gFe−1 s−1).

Ag-doped Fe-based microspheres were synthesized via a one-pot solvothermal method, and displayed excellent mechanical stability and catalytic activity. Especially, the catalyst at Ag/Fe = 1 showed the lowest selectivity to CH4 (12.6 wt%) and the highest to C2–4 olefins (29.0 wt%), as well as a good selectivity to gasoline-range (C5–11) fraction products (45.7 wt%). In addition, the catalytic activity and product selectivity of Ag/Fe measured at 280 °C was superior to that at 270 °C and 260 °C.

A novel Au@TNT catalyst, with gold nano-particles (NPs) entrapped in TiO2 nanotube (TNT), was prepared by a vacuum assisted-impregnation route. For the photocatalytic water splitting under visible light, the Au@TNT catalyst presented 2 times higher activity than the Au/TNT with most part of gold NPs on the outer surface of TNT. The enhanced activity can be ascribed to the confinement of TNT which can effectively decreases the particle size and modulates the electronic state of gold catalyst.

Titanate/N-doped anatase composite hierarchical microspheres have been obtained by calcination treatment of NH4-titanate hierarchical microspheres at proper temperatures. The phase structure is dependent on the calcination temperature and varies from titanate to anatase with increasing calcination temperature, with the heterophase in-between. The composite structure is verified by the results of XRD, visible and UV Raman spectra. The titanate/N-doped anatase composite sample calcined at 300 °C for 1 h exhibits enhanced activity in the degradation of Rhodamine B under visible light irradiation compared to the other samples. The enhanced activity can be ascribed to the synergistic effect of titanate and N-doped anatase.

Nanosheet-assembled hierarchical flower-like titanium phosphate (TiP) is synthesized via hydrothermal treatment of H-titanate nanotubes (Ti-NT) at the optimized conditions of 0.1 M of H3PO4 and hydrothermal temperature of 130 °C. A possible formation mechanism for the TiP flowers, involving the disintegration of Ti-NT and the growth and assembling of TiP nanosheets, is proposed. The main compositions of the uncalcined TiP flowers are titanium hydrogen phosphate hydrates (Ti(HPO4)2·xH2O), which can be transformed to titanium phosphate (TiP2O7) after high temperature calcination. The photocatalytic activity of the prepared TiP flowers is increased with the increased calcination temperature, which may be attributed to the better photocatalytic activity of TiP2O7 than Ti(HPO4)2·xH2O and the increased crystallization of TiP2O7.

A Pt@TNT catalyst with Pt nanoparticles entrapped in titanate nanotubes (TNT) was prepared by hydrophobic modification of the exterior surface of the TNT and impregnation with hexachloroplatinic acid (H2PtCl6) aqueous solution. The catalyst's enhanced activity towards the hydrogenation of phenol (as high as ∼3200 gphenol h−1 gPt−1 of qTOF) can be ascribed to the confinement effect.

CdS nanorod arrays were grown on fluorine-doped tin oxide glass substrates via a hydrothermal process and subsequently coated with a TiO2 nanolayer via a vacuum dip-coating process to fabricate a one-dimensional array structured photocatalyst. The TiO2 nanolayer improved the photocatalytic efficiency of CdS nanorod arrays for the degradation of methylene blue due to the effective separation of the electron–hole pairs, and the photocorrosion of CdS nanorod arrays was successfully inhibited.

ZnO/CdS/ZnS and ZnO/ZnS/CdS nanorod array photoelectrodes were constructed and fabricated by an alternative chemical bath deposition of CdS and ZnS layers onto the surface of the ZnO nanorod array. The photoelectrochemical performances of composite photoelectrodes were investigated, and the action mechanism of ZnS buffer layers was also considered. The results show that the ZnO/ZnS/CdS nanorod array photoelectrode possesses enhanced hydrogen production efficiency. The enhancement may be attributed to the ZnS buffer layers, which are advantageous in the separation and transportation of photogenerated electron–hole pairs in ZnO/ZnS/CdS nanorod array photoelectrodes.

Nb-doped TiO2 nanoparticles were prepared by hydrothermal treatment of titanate nanotubes in niobium oxalate aqueous solution. The effect of Nb doping and rutile content on the photoelectrochemical performance based on TiO2 powder electrodes was investigated. The results show that Nb-doped TiO2 with a small amount of rutile exhibits the enhanced photoelectric conversion efficiency for dye-sensitized solar cell. The highest photoelectric conversion efficiency of 8.53% is obtained for 1% Nb–TiO2 containing a small amount of rutile. When a small amount of rutile contained in 2% Nb–TiO2, a higher photoelectric conversion efficiency of 8.77% is achieved.

TiO2 nanotubes were prepared by hydrothermal treatment of TiO2 powder in NaOH aqueous solution and then calcined at various temperatures. The post-calcination treated TiO2 nanotubes were decorated with CdS by wetness impregnation and subsequently sulfurization to fabricate CdS/TiO2 composites. The photocatalytic performance of CdS/TiO2 composites toward hydrogen production from water splitting was investigated. The results show that the calcination temperature of TiO2 nanotubes has a significant effect on the photocatalytic performance of CdS/TiO2. With the increase of calcination temperature from 300 to 500 °C, the crystallinity of TiO2 nanotubes is increased resulting in the enhanced photocatalytic performance of CdS/TiO2. When the calcination temperature is higher than 500 °C, TiO2 nanotubes gradually transform into nanorods and finally completely collapse, which leads to the decrease of photocatalytic performance of CdS/TiO2. The CdS/TiO2 composite with TiO2 nanotubes calcined at 500 °C exhibits the highest hydrogen evolution rate, which could be attributed to its 1 D nanotubular structure and good crystallinity.

The composite photocatalyst with layered nanoarray structure, CdS-sensitized ZnO nanorod arrays (ZnONRA/CdS) grown on ordered TiO2 nanotube arrays (TiO2NTA), was constructed. The performance of composite photocatalyst toward to hydrogen production from water splitting was investigated. The ZnONRA/CdS composite photocatalyst with the substrate layer of ordered TiO2NTA has the enhanced rate of hydrogen production and the improved photostability. It may be attributed to the one-dimensional structure of TiO2NTA at the bottom of ZnONRA/CdS composite photocatalyst, which provides a direct transfer pathway of photoinjected electrons along the photoanode to enhance charge-collection efficiency and consequently reduce electron–hole recombination.

TiO2 nanotube was synthesized by a hydrothermal process using TiO2 nanoparticle as starting material, and then it was loaded with Ag through a silver mirror reaction. Further, the Ag loading TiO2 nanotube was decorated by TiO2 via TiCl4 hydrolysis to obtain the TiO2 nanotube with the wall sandwiched by Ag. The TiO2 nanotubes were characterized by transmission electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction and UV–vis spectrophotometer. The electrochemical impedance spectroscopy and transient photocurrent were investigated in a three-electrode system with the TiO2 nanotube films served as photoanodes. The photo-catalytic activity was evaluated by photo-degradation of methyl orange under UV light irradiation. The results show that the sandwiched Ag could inhibit the crystal phase transformation of TiO2 nanotube from anatase to rutile. And the sandwiched nanotube structure exhibits enhanced photo-catalytic performance due to the efficient separation of the photo-generated carriers.Highlights► TiO2 nanotube with the wall sandwiched by Ag was prepared. ► The sandwiched nanotube structure exhibits enhanced photo-catalytic performance. ► Improved photoelectrochemical performance is due to effective charge separation.

In2O3-sensitized ZnO nanowire (NW) array film with improving visible light activity was synthesized on F-doped SnO2 (FTO) glass substrate by a facile two-step process. The films, ZnO NW arrays and In2O3-sensitized ZnO NW arrays, were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectroscopy. For the films severed as the photoanodes in a three-electrode system, the photo-electrochemical behaviors were investigated and the photocatalytic activity was evaluated by the oxidation of glucose under visible light irradiation. The results show that the ZnO NW arrays are highly ZnO (0 0 2) orientated with a diameter of 100 nm and a length of 3 μm, and In2O3 nanoparticles grow and form a shell film on the wall of the ZnO NW array in the In2O3-sensitized ZnO NW array film. This composite structure shows a red shift from ultraviolet (λ = 385 nm) to the visible-light region (λ = 450 nm) in the maximum optical response. The incorporation of In2O3 into the ZnO NW array film remarkably promotes the photogenerated carrier separation and increases the utilization of photogenerated carriers in photocatalytic reactions, resulting in the higher photocatalytic activity under visible light.

WO3/TiO2 nanotube array electrode was fabricated by incorporating WO3 with TiO2 nanotube array via a wet impregnation method using ammonium tungstate as the precursor. TiO2 and WO3/TiO2 nanotube arrays were characterized by field emission scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. In order to characterize the photoelectrochemical properties of WO3/TiO2 electrode, electrochemical impedance spectroscopy, and steady-state photocurrent (iss) measurement at a controlled potential were performed in the supporting electrolyte containing different concentrations of glucose. The photoelectrochemical characterization results reveal that WO3/TiO2 nanotube array electrode possesses a much higher separation efficiency of the photogenerated electron–hole pairs and could generate more photoholes on the electrode surface compared with the pure TiO2 nanotube array electrode. The iss for glucose oxidation at WO3/TiO2 nanotube array electrode is much higher than that at the pure TiO2 nanotube array electrode.

Anatase-dominated anatase/rutile mixed-phase TiO2 nanotubes were prepared by decorating TiO2 on the hydrothermally synthesized TiO2-derived nanotubes via liquid phase deposition and subsequent annealing. The morphology and structure of as-prepared TiO2 nanotubes were characterized by transmission electron microscopy, X-ray diffraction, Raman spectra, and UV–Vis diffuse reflectance spectra. The separation efficiency of photogenerated carriers was investigated by photoluminescence spectra. The photocatalytic activity was evaluated by methyl orange decomposition under UV illumination. The results indicated that the mixed-phase TiO2 nanotubes consisting of anatase and rutile (82/18 w/w) were successfully fabricated. Due to the synergistic effect of mixed anatase and rutile phases, the anatase-dominated anatase/rutile mixed-phase TiO2 nanotubes exhibited superior photocatalytic activity. The apparent rate constant k of the photocatalytic reaction for the TiO2 nanotubes with and without decoration was 0.0528 and 0.0126, respectively.

The thin films of TiO2 doped by Ni non-uniformly were prepared by a modified dip-hoisting process of sol–gel method. The transmittance, absorbency, photocurrent and open-circuit potential of the thin films were studied by UV–vis spectrophotometer and electrochemical workstation. The activity of thin films was characterized by photocatalytic degradation of aqueous methyl orange under UV irradiation. The results showed that the photocatalytic activity of TiO2 thin films can be remarkably enhanced by Ni non-uniformly doping compared with the pure ones, while that of the uniformly doped can only be improved a little. The transmittance of the TiO2 thin films doped by Ni uniformly and non-uniformly doping showed “blue shift” and “red shift”, respectively. The absorbency of the non-uniformly doped extended to “red shift”. Under UV irradiation, the non-uniformly doped TiO2 thin films show higher open-circuit potential and photocurrent compared with that of the pure ones, which demonstrated that the photogenerated electron–hole pairs were effectively separated. And the electrode bias influenced the photocurrent of Ni non-uniformly doped TiO2 thin films greatly, which verified the structure of p–n junction. The photocatalytic activity mechanism of the non-uniformly doped TiO2 was discussed by p–n junction theory.

•A Ru-in/TNT catalyst with Ru NPs entrapped in inner pores of TNT is prepared.•The entrapped Ru NPs exhibited excellent resist-metal sintering and metal leaching ability.•The Ru-in/TNT presented significantly enhanced initial CO conversion activity towards FTS.•The enhanced catalytic activity and resist-deactivation capacity can be ascribed to the confinement effect of TNT.Ru nanoparticles (NPs) have been successfully deposited in the inner pores of TiO2 nanotubes (TNTs), denoted as Ru-in/TNT, by vacuum assisted impregnation method for Fischer-Tropsch synthesis (FTS). The composition, morphology and electronic properties have been characterized by the XRD, TEM, N2-sorption, H2-TPR/TPD and XPS, investigating the confinement effect of TNT on the entrapped Ru NPs’ FTS performance. The Ru-in/TNT exhibits high CO conversion activity and superior deactivation-resist ability over the Ru-out/TNT (with most of Ru NPs deposited on the outer surface of TNT) and Ru/P25 (using commercial P25 powder as support) catalysts. Besides above superiority, the Ru-in/TNT facilitates the formation of long chain (C19+) products as well. The enhanced activity and selectivity can be ascribed to the confinement effect of TNT: (i) strengthen charge transfer and modulate the electronic properties of Ru species resulting in more active center site; (ii) constrain the metal sintering and metal leaching to resist deactivation during FTS; (iii) facilitate the re-adsorption of α-olefin leading to long-chain product.Ru nanoparticles (NPs) have been successfully deposited in the inner pores of TiO2 nanotubes (TNTs), denoted as Ru-in/TNT, by vacuum assisted impregnation method. Applied in Fischer-Tropsch synthesis (FTS), the Ru-in/TNT exhibited high CO conversion activity and superior deactivation-resist ability over the Ru-out/TNT and Ru/P25 catalysts. The enhanced activity and selectivity can be ascribed to the confinement effect of TNT.Download high-res image (87KB)Download full-size image

The TiO2-supported zeolite with core/shell heterostructure was fabricated by coating aluminosilicate zeolite (ASZ) on the TiO2 inoculating seed via in situ hydrothermal synthesis. The catalysts were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), nitrogen physisorption (BET), and Fourier transform infrared spectroscopy (FT-IR). The surface acidity of the catalysts was measured by pyridine-TPD method. The catalytic performance of the catalysts for ethanol dehydration to ethylene was also investigated. The results show that the TiO2-supported zeolite composite catalyst with core/shell heterostructure exhibits prominent conversion efficiency for ethanol dehydration to ethylene.

Peroxo-titanium decorated H-titanate-nanotube-based hierarchical microspheres (PTHM) with a large surface area (368 m2 g−1) and mesoporous structure were prepared by an alkaline hydrothermal method in the presence of H2O2 followed by acid wash, and they exhibited improved activity in degradation of Rhodamine B under visible light irradiation.

CdS nanorod arrays were grown on fluorine-doped tin oxide glass substrates via a hydrothermal process and subsequently coated with a TiO2 nanolayer via a vacuum dip-coating process to fabricate a one-dimensional array structured photocatalyst. The TiO2 nanolayer improved the photocatalytic efficiency of CdS nanorod arrays for the degradation of methylene blue due to the effective separation of the electron–hole pairs, and the photocorrosion of CdS nanorod arrays was successfully inhibited.

A Pt@TNT catalyst with Pt nanoparticles entrapped in titanate nanotubes (TNT) was prepared by hydrophobic modification of the exterior surface of the TNT and impregnation with hexachloroplatinic acid (H2PtCl6) aqueous solution. The catalyst's enhanced activity towards the hydrogenation of phenol (as high as ∼3200 gphenol h−1 gPt−1 of qTOF) can be ascribed to the confinement effect.