TiO₂ nanomaterials are the research hotspots among recent nanotechnologies, and those are demonstrated having the crucial application prospects in the solar light photocatalytic splitting water into hydrogen, the photocatalytic reduction of CO₂, as well as dye-sensitized solar cells. This article mainly reviewed recent research trends of TiO₂-based nanomaterials, the existing problems, and the advances in the clean energy utilization. Especially, the hot research issues such as non-metallic element doping, high energy facets exposed titania, and compact film for dye-sensitized solar cells, were commented and prospected.

Photocatalysts could utilize solar energy to remedy environmental pollutions thus attract world wide attention. Some bismuth-containing complex oxides could be activated by visible light and mineralize organic pollutants. In this paper we reviewed recent progresses on the development of Bi₂WO₆, BiVO₄ and Bi₂MoO₆ photocatalysts. By controlling the particle size, morphology, crystallinity and other microstructures via different methods, the photocatalytic activities in the degradation of organic dyes, colorless model pollutants such as phenol and acetaldehyde, and disinfection of these visible light induced photocatalysts were greatly enhanced. Through further development, bismuth- containing complex oxides are hopeful to be applied in the field of environmental remediation.

Cu-BiVO₄ photocatalysts were prepared by hydrothermal method using Bi(NO₃)₃·5H₂O, NH₄VO₃ and Cu(NO₃)₂·3H₂O as raw materials. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscope (XPS) and UV-Vis diffusion reflectance spectra (UV-Vis). The results show that highly crystalline monoclinic scheelite structure of Cu-BiVO₄ is obtained by increasing the pH value, and the crystal structures of BiVO₄ are no change with the increasing of Cu dopant. XPS results reveal that the doped Cu species exists in the form of CuO and Cu₂O. The Cu-BiVO₄ catalyst has a significant red-shift in the absorption band in the visible region, and the absorption intensity increases greatly for the composite catalyst compared with pure BiVO₄. The Cu-BiVO₄ photocatalysts enhance the photocatalytic activities for degradation of methylene blue (MB) under visible light irradiation. When pH value is 5.0 and Cu dopant is 1.0wt%, the photocatalytic activities reach the maximum. After visible light irradiation for 60 min, the removal rate of MB by Cu-BiVO₄(1.0wt% Cu) at 10 mg/L initial concentration increases to 97.8% while the removal rate of MB by pure BiVO₄ is only 54.7%. The mechanism of enhanced photocatalytic activity is also discussed.

Visible-light photocatalysts BiVO₄ with different crystal structures and morphologies were prepared via a mild additive-free hydrothermal method. The as-prepared samples were well-determined by X-ray diffraction (XRD), scanning electron microscope (SEM), UV-Vis diffuse reflectance spectroscope (DRS) and low-temperature N₂ adsorption. The results indicated that both hydrothermal temperature and pH value of reaction solution had significant influence on the formation of crystal structures. Hydrothermal temperature of 180℃ and acidic condition facilitated the formation of monoclinic BiVO₄. The pH value of reaction solution also affected the morphologies of samples remarkably, which resulted in three different types: sheet-, cube- and rod-like shapes. All the samples were applied to the photodegradation of methylene blue (MB) under visible light and sheet-like BiVO₄ obtained in pH=2 reaction solution exhibited the best photocatalytic activity (4 h, 92.0%). In addition, influence factors of this photocatalytic system were thoroughly investigated, which included photocatalyst dosage, initial concentration and the existence of electron acceptors. The optimal photocatalytic condition was as follow: the dosage of catalyst was 1.0 g/L, the initial concentration of methylene blue was 10 mg/L, and KBrO₃ was used as electron acceptor.

Fe-Ni/TiO₂ was used as catalyst to synthesize carbon nanotube(CNT)/Fe-Ni/TiO₂ composite photocatalyst by fluidized bed chemical vapor deposition (FBCVD) method. Carbon nanotube grew in situ on TiO₂ surface. The CNT/Fe-Ni/TiO₂ composite photocatalyst were characterized by X-ray diffraction (XRD), UV-Vis absorption spectroscope and field emission scanning electron microscope (FESEM). The photocatalytic activities of prepared samples were investigated by the photodegradation of methylene blue (MB). The results showed that Ni was characterized as catalytic center for CNT growth in FBCVD process. In CNT/Fe-Ni/TiO₂ composite photocatalyst, Fe (III) cation was characterized as an electron capture center and efficiently promoted the separation of photogenerated holes and electrons. The electron transfer from TiO₂ was facilitated by Ni particles and CNTs, which could reduce the electron - hole recombination. The synergistic effects of Fe-Ni and CNT significantly enhanced the photocatalytic activity of the catalysts. The degradation ratio of MB of CNT/Fe-Ni/TiO₂ doped with 0.25mol% Fe and 4.75mol% Ni was increased about 70% than that of pure TiO₂.

TiO₂ nanobelts deposited with cubic p-type semiconductor Cu_1.8S particles were prepared using Cu₂O-deposited TiO₂ nanobelts and thiourea as precursors under hydrothermal condition by Cu₂O sacrificial template process. The measurement results show that the reaction temperature, reaction time and thiourea concentration have strong influence on the purity and morphology of Cu_1.8S particles. A mixture phases of Cu_1.8S and Cu₂O on TiO₂ nanobelts were obtained at 120℃ for 25 h within 0.5 mol/L of thiourea concentration. While reaction temperature was raised to 160℃, the Cu_1.8S particles with high purity exhibited obvious aggregation, whereas the Cu_1.8S particles with high purity and well dispersion were prepared under 0.25 mol/L of thiourea concentration. The photocatalytic activities in degradation of Rhodamine B solution demonstrate that the Cu_1.8S or Cu₂O-loading catalysts show lower photodegradation activities as compared with that of pure TiO₂ nanobelts.

Highly crystalline and monodisperse anatase TiO₂ nanorods were synthesized successfully via an improved solvothermal method. The shape evolution of TiO₂ nanorod was investigated by adjusting the reaction parameters, such as reaction duration and temperature. The phase structures, morphologies, and sizes of as-prepared TiO₂ nanoparticles were investigated in detail by XRD, TEM, and HRTEM. The photocatalytic properties of the product were measured by decomposition of methylene blue under full spectrum light irradiation. When Ostwald Ripening is dominant, the TiO₂ nanorods grow along the <001> crystallographic direction. When Ostwald Ripening is depressed at lower temperature, Oriented Attachment occurs. And primary nanoparticles join together by sharing a common (001) facet. The driving force of shape evolution and crystal growth of TiO₂ nanocrystals is reducing the high surface free energy. Compared with P25, the as-prepared TiO₂ nanorods exhibit a superior photocatalytic activity, which is attributed to the high crystallinity.

Vertically oriented single-crystalline TiO₂ nanorod arrays sensitized with CuInS₂ quantum dots (QDs) were synthesized directly on fluorine-doped tin oxide (FTO) substrates by two-step solvothermal method. The as-prepared nanostructure assembly was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results indicated that CuInS₂ QDs were uniformly covered on the surface of the single-crystalline TiO₂ nanorods. Ultraviolet visible (UV-Vis) absorption spectra showed that the absorbance of the TiO₂/CuInS₂ nanocomposite film was extended to the visible region. Under illumination of simulated AM 1.5 G (100 mW/cm²) at one sun intensity, the QD sensitized solar cells (QD-SSCs) assembled with FTO/TiO₂/CuInS₂ photoelectrodes exhibited an open-circuit voltage of 0.29 V and a short-circuit current density of 0.15 mA/cm².

TiO₂ nanotube arrays were made on the titanium foil by the method of anodic oxidation, and then crystallized by different heat-treatment processes. The morphology and structure of the TiO₂ nanotube arrays were characterized by SEM and XRD. The results showed that the TiO₂ nanotube arrays prepared by anodic oxidation and annealed at the temperature range of 450-750℃ were nanocrystalline structure, and the average measurement of the crystal particle grew with the increasing of the annealing temperature. The average crystal size of TiO₂ nanotube arrays annealed in the nitrogen atmosphere is smaller than that annealed in the air atmosphere at the same temperature. Annealing in the nitrogen atmosphere can widen the limit of the transformation temperature from Anatase to Rutile. Annealed above 650℃, the TiO₂ nanotube arrays can be doped into a small amount of nitrogen. The results of light open-circuit potential tests and the steady-state polarization curves tests show that the TiO₂ namotube arrays electrode has the best photoelectric response properties after being annealed in the nitrogen atmosphere at 650℃ for 2 h. The arrays have mixed crystal structure of Anatase and Rutile.

PbS quantum dots(QDs)/ZnO nanosheets composite films were synthesized via a two-step method. ZnO nanosheet films were firstly grown on fluorine-doped SnO₂ conducting glass(FTO) by electrodeposition techniques, then PbS QDs were decorated on ZnO nanosheets to form composite films by sequential chemical bath deposition. The morphology and crystalline phase of obtained films were detailedly characterized by scanning electron microscope (SEM), X-ray diffraction (XRD). The UV-Vis absorption, photoelectrochemical and surface photovoltage properties of ZnO nanosheets and PbS QDs/ZnO nanosheets composite films were investigated respectively. The results show that the absorption and surface photovoltage response region of PbS QDs/ZnO nanosheets composite films are more abroad than that of pristine ZnO nanosheets, extended from UV to visible light region. The photoelectrochemical property of PbS QDs/ZnO nanosheets composite films is dramatically enhanced compared to non-decorated ZnO nanosheets films, from 0.1 to 0.7 mA/cm² of short circuit current density (J_sc), from 0.04% to 0.57% of convert efficiency. Compared with ZnO nanosheets, the surface photovoltage response intensity is enhanced in PbS QDs/ZnO nanosheets, indicating heterojunction formed between PbS and ZnO. Photo-generated charge can effectively separate through this heterojunction, that is the reason for enhancement in photoelectric properties of PbS QDs/ZnO nanosheets composite films.

Nb-doped (2.5wt%) TiO₂ thin films was deposited on glass substrate by DC magnetron sputtering from a ceramic target and the films thickness was controlled in the range from 300 nm to 350 nm. The structure, surface morphology and optical properties of the films deposited at different substrate temperatures were investigated by X-ray diffraction, scanning electron microscope, and optical transmission spectroscope. The films deposited at 150℃, 250℃ and 350℃ were amorphous, anatase and rutile, respectively. The grain size of the typical anatase film deposited at 250℃ reached the maximum of 32 nm. The roughness of the films decreased and their density increased with the rising of substrate temperature. The average optical transmittance of films were around 70% when the substrate temperatures were below 250℃. As the substrate temperature were risen to 350℃, the films’ transmittance dropped to 59%. It indicated that the transmittance of visible light was hindered by the rutile phase in the Nd-doped TiO₂ films. The optical band gap of the films were in the range from 3.68 eV to 3.78 eV, and the optical band gap of the typical anatase film deposited at 250℃ reached the highest value of 3.78 eV.

Hierarchical porous ceria with nanocrystalline was successfully synthesized using maple leaf as a biotemplate. Unique biomorphic microstructures were characterized by Field Emission Scanning Electron Microscope (FESEM), transmission electron microscope (TEM) and nitrogen absorption-desorption technique. The obtained ceria material shows the repetitious biomimetic structure consisting of the stoma porous about several μm and nanopores with 2-4 nm apertures. The small crystal grain (6-8 nm) and the high specific surface area (64.4 m²/g) of porous CeO₂ are detected by wide-angle X-ray diffraction (XRD), high resolution TEM (HRTEM) and the BET method. The catalytic property is evaluated by the oxygen storage/release capacity (OSC). The test confirms that hierarchical porous material possesses more surface active oxygen than nonporous ceria does. While the concentration of acid fuchsine is 40 mg/L, the porous sample has a higher decoloring rate in a shorter time than others. The decoloring rate can reach 100% after decolored for 300 min. The investigation infers that the hierarchical porous ceria exhibits better catalytic activity and higher adsorptive capacity in dye wastewater purification due to smaller crystallite size, higher surface area and enhanced OSC.

A new kind of electrochromic device composed of a CeO₂ modified TiO₂ nanotube array counter electrode and a working electrode of P3MeT film was reported. The counter electrode was fabricated by electrodepositing CeO₂ nanoparticles onto the walls of anodized TiO₂ nanotubes. The CeO₂ modified TiO₂ nanotube array counter electrode keeps a high transparency under working voltage. The charge capacity of CeO₂ modified TiO₂ nanotube array has an increase of 30% than that of un-modified TiO₂ nanotube array electrode. Compared with the electrochromic device with a TiO₂ nanotube array film electrode, the CeO₂ modified TiO₂ nanotube counter electrode based EC device has a fast decoloration time of 0.8 s. The fast response attributes to the high effective surface area and the fast charge transportation along the nanotube walls.

Cu₂ZnSnS₄ (CZTS) semiconductor is a promising materials for thin film solar cells. Cu₂ZnSnS₄(CZTS) nanoparticles were prepared via a hot-injection processing under high-purity N₂ atmosphere, using Cu(acac)₂, Zn(OAc)₂, SnCl₂•2H₂O, sulfur powder as the precursors, oleylamine (OLA) as the solvent and the capping molecules. The CZTS thin films were deposited on glass substrates by drop-casting from the Sol of CZTS nanocrystals in toluene. The influence of reaction temperature on the phase structure and morphology of nanoparticles was studied. The samples were investigated by powder X-ray diffraction (XRD), Raman spectroscope, transmission electron microscope (TEM), scanning electron microscope (SEM) and UV-Vis-NIR spectroscope. The results indicated that the CZTS nanoparticles with 10 nm in size, good dispersion and an optical band gap of 1.5 eV was synthesized under the optimum reaction temperature of 260℃.

Carbon counter electrode was fabricated using high surface area mesoporous carbon (MC) as catalytic material at low temperature. The integral structure and the photovoltaic performances of dye-sensitized solar cells (DSC) consisting of the obtained carbon counter electrode were emphatically optimized. The results show that both contact interface among MC particles and that between carbon catalytic film and substrate are improved by adding Triton X100 into MC slurry, leading to an increase of 7.1% in the efficiency of DSC. With the increasing thickness of TiO₂ film, the efficiency of DSC sharply increases at first and tends to stabilization later on. The change in the efficiency is the result of competition between the adsorption amount of dye and the transmission path of electron in the TiO₂ film. The resistance of electrolyte is reduced by mixing tributyl phosphate into electrolyte, resulting in an increase of 23.1% in the efficiency of DSC. The optimum efficiency of DSC employing carbon counter electrode reaches 4.82%.

Graphene-Ag nanoparticles composites were prepared by one step in situ synthesis method, using nontoxic green glucose as reducer. Graphite oxide and ammoniacal silver ions were reduced at the same time without stabilizing agent under mild reaction conditions of aqueous solution. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscope, scanning electronic microscope (SEM), and transmission electronic electron microscope (TEM) were used to characterize the resulting composites. The results of analysis indicate that graphite oxide and ammoniacal silver ions are reduced by glucose simultaneously. Ag nanoparticles (AgNPs) uniformly distribute in the graphene sheets, and most of AgNPs show twin boundary. The quantity of silver nitrate influences the size and range of sizes of the AgNPs. The range of sizes of AgNPs on the graphene sheets centralizes at 25 nm under a suitable concentration of silver ions. The intensities of the Raman signals of graphene in the composites increase 7 fold by the loaded AgNPs.

The Ni/Pt core-shell nanoparticles with different shell thicknesses were prepared by composite coating of colloid particles and then characterized by HRTEM, EDS, XPS and XRD. Electrochemical properties for the oxygen electro-reduction reaction and methanol electro-oxidation were measured by potentiodynamic and cyclic voltammetry. The experimental results show that Ni/Pt core-shell nanoparticles are spherical and the mean particle size of Ni₁-Pt_0.067 is about 7 nm with shell thickness of 1 nm. Compared with Pt/C, Ni/Pt core-shell nanoparticels possess higher activity. Ni₁-Pt_0.067/C has the highest catalytic activity in these prepared catalysts with different thickness, and its peak current density can reach 143.06 mA/mg in 0.5 mol/L H₂SO₄ solution, 1.4 times of that of Pt/C for O₂ electro-reduction reaction. Peak current density of Ni₁-Pt_0.067/C for methanol electro-oxidation reaches 538.3 mA/mg in 0.5 mol/L H₂SO₄+1.0 mol/L CH₃OH solution, which is 5.2 times of that of Pt/C. Based on 1 mg Pt, the specific mass activity of Ni₁-Pt_0.067/C is 30-fold higher than that of Pt/C nanoparticles.

3D meshes of TiO₂ were fabricated by direct writing and subsequently sintered at 1150℃ for 2 h in an air atmosphere for photocatalytic degradation (PCD) of methylene blue(MB) solution. The crystal structure and morphology of the samples were determined by X-ray diffraction(XRD) and scanning electron microscope(SEM). PCD properties of the sintered meshes were obtained in an ultraviolet-visible light spectrophotometer, and were found to be better than that of cylindrical samples with the same weight. Three kinds of mesh structures were designed, sintered and measured. The results show that reasonable design could cause significant enhancement in the PCD activity by more than 60%. This enhancement could be attributed to irradiation absorption increment, which was caused by structure design of photocatalyst.

The present research demonstrates the significant influence of fluoride content on the morphology, phase structure and chemical composition of ZrO₂ nanotube arrays which were prepared via a direct electrochemical anodization of zirconium foil in glycerol-based electrolytes composed of 5vol% H₂O and different contents of NH₄F from 0.1 mol/L to 1.1 mol/L. The as-prepared nanotube arrays with external diameter of 90-130 nm and length of 4.8- 9.1 μm were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS). The results show that the highly ordered and vertically aligned tetragonal ZrO₂ nanotube arrays can be obtained in the electrolyte containing 0.7 mol/L NH₄F, and they become amorphous at lower NH₄F concentration and collapse at higher NH₄F concentration. Moreover, the F/O atomic ratio near the top surface of nanotubes is also dependent on the F^- concentration in the electrolyte.