Despite the excellent photo-generated charge separation and transport properties of the 1D rutile TiO2 nanowire (NW) array, insufficient light harvesting due to a low surface area is a key factor that limits the photovoltaic performance of TiO2 NW-based solar cells. Herein, we apply the Sb2S3 semiconductor as a light absorber and sprayed p-CuI as a hole conductor for TiO2 NW solar cells. The Sb2S3-sensitized device displays significantly improved light absorption compared to its corresponding dye-sensitized device, with a peak incident-photon-to-current conversion efficiency (IPCE) of 64%. Moreover, CuI film deposited by the spray technique enables improved pore filling and better electrical contact between the Sb2S3 absorber and CuI, as well as CuI crystals themselves, and facilitates hole transfer from Sb2S3 to CuI crystals and hole transportation in the CuI layer. As a result, the TiO2 NW/Sb2S3/CuI-spray/Au device exhibits an overall power conversion efficiency of 1.18% under AM 1.5G simulated solar irradiation, which is about 2.88 times and 2.11 times higher than TiO2 NW/N719/CuI-spray/Au and TiO2 NW/Sb2S3/CuI-drop coating/Au devices, respectively. This study thus demonstrates the superiority of the Sb2S3 sensitizer for TiO2 NW solar cells and the spray technique for the preparation of the p-CuI hole conductor.

The structural design of TiO2 photoanodes has proved to be a useful approach in improving the energy conversion efficiency of dye-sensitized solar cells (DSSCs). Herein, a bilayer TiO2 photoanode consisting of rutile nanowires (NWs) infiltrated with anatase nanoparticles (NPs) as a bottom layer and spherical voids (450 nm) in a NP film as the top layer is prepared via a simple one-time spray technique on a 1D TiO2 NW array. The bilayer structure exhibits an excellent dye-loading property and efficient light scattering ability, as confirmed by dye-desorption, diffuse transmittance and reflectance spectra, and incident-photon-to-current conversion efficiency (IPCE) results. As a result, the DSSC based on the bilayer photoanode ((1.6 + 2.6) μm) exhibits remarkably higher photocurrent output and overall energy conversion efficiency (3.2 times and 3.5 times higher, respectively) than a 1D TiO2 NW device (1.6 μm) under AM 1.5G simulated solar irradiation. The enhanced device performance can be ascribed to the synergistic effect of a large surface area for sufficient dye-loading, efficient light scattering for sufficient light harvesting, fast charge transport for efficient charge collection and a highly porous structure of the top spherical voids layer for fast diffusion of the I−/I3− electrolyte in the bilayer TiO2 photoanode. This study provides a facile route towards improving the photovoltaic performance of 1D TiO2 NW array solar cells based on constructing bilayer structures via a simple one-time spray technique.

Visible light-active TiO2(B)/anatase heterojunction nanotubes (NTs) are prepared via a process combining a two-step alkaline hydrothermal technique, ion exchange and heat treatment in a vacuum. The TiO2 NTs inherit the skeleton structural features of NT precursors and thereby lead to the formation of NTs with good crystallinity and large surface area (298 m2 g−1 at 350 °C and 279 m2 g−1 at 400 °C). UV-vis diffuse reflectance and PL measurements confirm the distinct visible light absorption due to bandgap defect states introduced by vacuum treatment. Heat treatment in a vacuum at 350 °C and 400 °C induces the formation of TiO2(B)/anatase heterostructure in the TiO2 NTs as confirmed by HRTEM results. As a result, visible light photocatalytic tests demonstrate that TiO2 NTs vacuum treated at 350 °C and 400 °C exhibit significantly better photocatalytic activity for acetaldehyde degradation and CO2 generation in comparison with commercial Degussa P25 nanoparticles. The high photocatalytic activity can be ascribed to the synergistic effect of high surface area, good crystallinity and efficient charge separation promoted by the TiO2(B)/anatase heterojunction. This study provides an effective approach in improving the visible light photocatalytic activity of TiO2 nanostructures based on the introduction of native bandgap defects and heterostructure via vacuum heat treatment.

Doping TiO2 photocatalysts with foreign ions has been deemed an effective method to enhance visible light absorption and thus increase their photocatalytic performance. Herein, we report that Nb-doped TiO2 porous microspheres prepared by ultrasonic spray pyrolysis of peroxide precursor solution are yellow, and the yellow coloration becomes increasingly conspicuous with increasing Nb dopant concentration. Comprehensive spectral analyses show that both surface peroxo species and bulk Ti3+ are introduced into TiO2 microsphere samples together by charge compensation with Nb5+ dopant and are responsible for the coloration of TiO2. The Nb-doped microspheres show higher photocatalytic rates than undoped TiO2 for the degradation of gaseous acetaldehyde under visible irradiation but slower rates under ultraviolet light. Moreover, the photocatalytic mineralization rates of acetaldehyde to CO2 are lowered with Nb doping under both visible and UV irradiation. Correlation between the results of surface photovoltage spectroscopic (SPS) characterizations and photocatalytic tests suggests that surface peroxo states are relevant to the visible-light-stimulated charge separation and photocatalytic reactions, albeit holes trapped in these states have lower reactivity than in valence band. On the other hand, the enhanced photoluminescence in the near-infrared region, reduced SPS response in the UV region, and photochromic phenomena during photocatalytic process indicated that Ti3+ defects serve as charge carrier recombination centers and display adverse effect to photocatalytic activity of Nb-doped TiO2, especially under UV irradiation.

Ag/TiO2 nanocomposite films present a stable optical memory based on localized surface plasmon resonance but still suffer from the problem of the low efficiency of holographic storage. Here, we report that the response time and diffraction efficiency of the high-density holographic storage of Ag/TiO2 nanocomposite films at 403.4 nm can be improved significantly and further modulated by introducing the auxiliary 532 nm irradiation with s or p linear polarization state. Absorbance at ∼600 nm, contrast of holographic fringes, and brightness of reconstruction image were all enhanced under the bicolor excitation. The observations were explained by Ag+ ions migration, Ag nanoparticle dissolution, and their redeposition, with the help of concentration and electronic-field gradient forces. Taking these factors in account, a phenomenological model describing the growth of two competitive-phase gratings is proposed. The localized surface plasmon resonance with the composite wave provides new possibilities for Ag/TiO2 nanocomposite films in application of long-life and high-density optical memory.

In this work, we report a novel approach to fabricate hierarchical TiO2 microspheres (HTMS) assembled by ultrathin nanoribbons where an anatase/TiO2(B) heterojunction and high energy facet coexist. The as-adopted approach involves (1) nonaqueous solvothermal treatment of a mixture of tetrabutyl titanate and acetic acid and (2) topotactical transformation into HTMS via thermal annealing. By this approach, the TiO2(B) phase usually synthesized from an alkaline treatment route could be initially formed. Subsequently, phase transition from TiO2(B) to anatase TiO2 occurs upon thermal treatment. It is demonstrated that such phase transition is accompanied by crystallographic orientation along the c-axis of anatase and TiO2(B) crystals, resulting in not only a coherent interface between two phases but also oriented attachment of anatase mesocrystals along the [001] direction, and finally high-energy (001) facet exposure. Interestingly, this work provides an alternative fluorine-free route for the synthesis of TiO2 crystals with high-energy (001) facet exposure. The structural analysis reveals that lattice-match induced topotactic transformation from TiO2(B) to anatase is the sole reason for the (001) facet exposure of anatase TiO2. The photocatalytic test for acetaldehyde decomposition shows that HTMS with anatase/TiO2(B) heterojunction and high-energy (001) facet exhibits superior photocatalytic efficiency compared with the relevant commercial product P25, which can be ascribed to the synergistic effect of large surface area, anatase/TiO2(B) heterojunction as well as high-energy facet exposure.

Construction of a micro-/nanostructured TiO2 film has been considered to be an important way to enhance the electrochemical properties and electrochromic performance of the material. Herein, we investigated the electrochromic properties of a nanocomposite TiO2 film prepared by decorating rutile a TiO2 nanowire (TiO2 NW) array with anatase TiO2 nanoparticles (TiO2 NPs) via a facile two-step synthesis method. Owing to its large active surface area and high transparency, the TiO2 NW–NP composite film supported a greater number of sites for Li+ ion intercalation and extraction. As a result, the TiO2 NW–NP film displayed higher optical contrast, coloration efficiency and transient current density compared with NW and NP films, demonstrating that enhanced electrochromic properties could be achieved using the NW–NP composite structure.

Waveband-dependent photochemical reduction properties of graphene oxide (GO) were investigated. The ratio of oxygenated carbon to graphitic carbon of the GO film was reduced to 39% and 28% by 400–800 nm and 350–800 nm irradiation, respectively from the initial value of 47%, while this ratio changed negligibly by 520–800 nm irradiation. This waveband-dependent photoreduction is closely related to the wide size distribution of sp2 carbon clusters with different band gaps within the GO sheet, which play the role of the photochemical reacting centers. In addition, the photochemical activity of GO was also demonstrated by photoreduction of Ag+ in water. Our research suggests the possibilities of tuning the microstructure of GO by careful selection of irradiation conditions and of preparing noble metal nanoparticles with a neat surface on GO sheets.

TiO2 nanoporous films loaded with Ag nanoparticles exhibited distinctive photochromism and photoanisotropy under the visible linearly polarized irradiation. Based on such properties, a pure polarization holographic grating was recorded in the photochromic film using two orthogonal circularly polarized green beams (532 nm) and reconstructed with a red beam (632.8 nm). The diffraction efficiency of the holographic grating and the brightness of the reconstruction image were strongly dependent on the polarization state of the probe beam. The hologram can be erased simply by the irradiation of single green beam. This recording–erasing process can be repeated with little loss, which may be benefited from the reciprocating mobility of Ag+ ions, reversible deformation and re-growth of Ag nanoparticles under the alternate irradiation of linearly and circularly polarized light. The novel nanocomposite system with photoanisotropy makes a new range of applications in the field of high-density optical memory media.

The structural and interfacial design of TiO2 photoanodes plays an important role in improving the solar energy conversion performance of dye-sensitized solar cells (DSSCs). Herein, we report that a rutile nanowire (NW) array infiltrated with anatase nanoparticles (NPs) can combine the advantages of the one dimensional electron transportation and light scattering of a NW array and the large dye-loading capacity of NPs, due to the presence of a rutile–anatase heterojunction. The dye-sensitized NW–NP composite film (1.4 μm thick) with a roughness factor of ∼114.7 displays a significantly improved light harvesting ability than a NW array (roughness factor ∼28.2), as manifested by diffuse transmittance and reflection spectra, and even higher light harvesting than a NP film (1.5 μm thick) with a roughness of ∼414.7. Moreover, the dye-sensitized NW–NP composite film shows slower charge recombination kinetics than both the NW array and NP film, as measured by open-circuit photovoltage decay and transient absorption spectroscopy. As a result, the dye-sensitized TiO2 NW–NP composite photoanode exhibits 2.2 times and 1.5 times higher overall efficiency than NW array and NP film photoanodes, respectively, under AM 1.5G simulated solar irradiation, demonstrating the synergistic effect of rutile NW and anatase NP for photoelectrochemical solar energy conversion.

Two series of K1−xNaxSrPO4:0.005Eu2+ and K0.4Na0.6Sr0.995−yPO4:0.005Eu2+, yTb3+ phosphors are synthesized via a high-temperature solid-state reaction. Their emission color can be tuned from deep blue to blue–green by modulating the crystal field strength and energy transfer. Partial substitution of K+ with Na+ results in a contraction and distortion of the unit cell of the K1−xNaxSr0.995PO4:0.005Eu2+ host, tuning the emission from 426 to 498 nm. The red-shifted emission is attributed to an increased crystal field splitting for Eu2+ in a lowered symmetry crystal field. The tunable emission is further demonstrated in the cathodoluminescence spectra, which indicates that the luminescence distribution of the K1−xNaxSr0.995PO4:0.005Eu2+ phosphor is very homogenous. Additionally, utilizing the principle of energy transfer, the emission color can be further tuned by co-doping with Tb3+. The chromaticity coordinates for the co-doped phosphor, K0.4Na0.6Sr0.995−yPO4:0.005Eu2+, yTb3+, can be adjusted from (0.202, 0.406) for y = 0 to (0.232, 0.420) for y = 0.09. The energy transfer processes from the sensitizer (Eu2+) to the activator (Tb3+) are studied and demonstrated to have a resonance-type dipole–dipole interaction mechanism, with the critical distance of the energy transfer calculated to be 12.46 Å using a concentration quenching method.

A novel green synthesis strategy was developed to synthesize a film photocatalyst containing anatase TiO2 nanorods wholly dominated with {100} and {101} facets by employing a simple hydrothermal reaction in the presence of NaCl solution and hydrogen titanate nanosheet array film. The formation mechanism of the anatase TiO2 nanorods was deduced from X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations, which revealed that the highly reactive {100} facets were derived from the enhanced structural stability of hydrogen titanate induced by Na+ and selective adsorption of Cl− on the {100} facets of anatase TiO2 crystals. Photocatalytic performance has indicated that the TiO2 nanorods with {100} facet exposure exhibit much higher activities than that of rhombic nanoparticles with {101} facet exposure for the degradation of organic contaminants, which may be primarily ascribed to the high surface energy of {100} facets.

A titanate nanosheet (TN) film is employed as a seed layer for the hydrothermal growth of a single-crystalline rutile TiO2 nanowire array on SnO2:F (FTO) conductive glass for dye-sensitized solar cell (DSSC) applications. TiO2 nanowires grown on a TN film appear to be thinner, more uniform, and well separated from each other, compared with those grown directly on FTO conductive glass. Besides FTO conductive glass, TiO2 nanowires can also be grown on silicon wafers and glass slides when a TN film is involved. Scanning electron microscope observations showed that the TN film underwent surface coarsening and thinning during the nucleation and growth of the TiO2 nanowires, suggesting that the film may act as a sacrificial seed layer. When applied in a DSSC, the nanowire photoanode with a TN film involved is significantly superior to that without the film in terms of all cell parameters, and gave an overall solar energy conversion efficiency of over 3% under AM 1.5G solar irradiation, about 3.4 times greater than that without the TN film. The combination of increased dye loading amount and reduced charge recombination at the FTO glass/electrolyte interface due to the involvement of the TN film should contribute to the significant improvement in cell performance.

Ultrasonic spray pyrolysis method was used to prepare Nb-doped TiO2 porous microspheres with an average diameter of 500 nm for solar photocatalytic applications. The effect of Nb-doping on morphology, structure, surface area, as well as spectral absorption properties of TiO2 microspheres was investigated with SEM, TEM, XRD, Raman spectra, BET, and UV–Vis absorption spectra. The Nb-doping decreased the grain size of TiO2 porous microsphere, and influenced its surface area and pore size distribution dependent on the doping concentration, but changed negligibly the morphology and size of TiO2 microspheres. Moreover, the Nb-doping was observed to extend the spectral absorption of TiO2 into visible spectrum, and the absorption onset was red-shifted for about 88 nm at a doping level of 5% compared to pristine TiO2 microspheres. Under solar or visible irradiation, Nb-doped TiO2 microspheres showed higher photocatalytic activity for methylene blue degradation compared with TiO2 microspheres, which could be ascribed to the extended light absorption range and the suppression of electron-hole pair recombination.Highlights► Ultrasonic spray pyrolysis method was used to prepare Nb-doped TiO2 mesoporous microspheres. ► The absorption edge shifted to longer wavelengths with the increasing of Nb content. ► Nb-doped TiO2 showed higher photocatalytic activity than TiO2 under visible and solar light.

Cotton textile was coated with ZnO@SiO2 nanorods in order to obtain superhydrophobic and ultraviolet (UV)-blocking properties. The coating process was conducted in mild conditions, which involved the low-temperature preparation of ZnO seeds, hydrothermal growth of ZnO nanorods, bioinspired layer-by-layer deposition of a SiO2 shell on the surface of ZnO nanorods, and hydrophobic modification of ZnO@SiO2 nanorods with octadecyltrimethoxysilane. Despite the highly curved morphology of cotton fibers, the ZnO@SiO2 nanorods coated the textile densely and uniformly. The treated cotton textile was found to have a large UV protection factor (UPF = 101.51) together with UV-durable superhydrophobicity, as determined by contact-angle measurement under long-term UV irradiation. The good UV-blocking property can be ascribed to the high UV absorbance and scattering properties of ZnO nanorods, and the UV-durable superhydrophobicity is a result of suppression of the photoactivity of ZnO nanorods by a SiO2 shell.Keywords: cotton textile; superhydrophobicity; surface modification; ultraviolet blocking; ZnO@SiO2 core−shell nanorod

A water–dichloromethane interface-assisted hydrothermal method was employed to grow rutile TiO2 nanowires (NWs) on electrospun anatase TiO2 nanofibers (NFs), using highly reactive TiCl4 as precursor. The water–dichloromethane interface inhibited the formation of rutile NWs in water phase, but promoted the selective radial growth of densely packed rutile NWs on anatase NFs to form a branched heterojunction. The density and length of rutile NWs could be readily controlled by varying reaction parameters. A formation mechanism for the branched heterojunction was proposed which involved (1) the entrapment of rutile precursor nanoparticles at water–dichloromethane interface, (2) the growth of rutile NWs on anatase NFs via Ostwald ripening through the scavengering of interface-entrapped rutile nanoparticles. The heterojunction formed at anatase NF and rutile NW enhanced the charge separation of both under ultraviolet excitation, as evidenced by photoluminescence and surface photovoltage spectra. The branched TiO2 heterostructures showed higher photocatalytic activity in degradation of rodamine B dye solution than anatase NFs, and the mixture of anatase NFs, and P25 powders, which was discussed in terms of the synergistic effect of enhanced charge separation by anatase–rutile heterojunction, high activity of rutile NWs, and increased specific area of branched heterostructures.Graphical abstractA water–oil interface assisted fabrication approach was employed to fabricate rutile TiO2 nanowires-anatase TiO2 nanofibers heterostructures. The as-fabricated branched heterostructures display higher photocatalytic activities than pure TiO2 nanofibers as well as P25 powders.Highlights► We employ water–oil interface assisted hydrothermal technique to TiO2 based heterostructure fabrication. ► The as-fabricated heterostructure displayed interesting branched morphology. ► The branched heterostructure showed higher photocatalytic activity than pristine TiO2 nanofibers and P25 powders.

Si4+ was introduced to the lattice of LiEuMo2O8 by solid-state reaction to prepare a new kind of red-emitting LiEuMo2−xSixO8 (0