Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets

Vapor-phase hydrothermal transformation of HTiOF3 intermediates into {001} faceted anatase single-crystalline nanosheets

For the first time, a facile, one-pot hydrofluoric acid vapor-phase hydrothermal (HF-VPH) method is demonstrated to directly grow single-crystalline anatase TiO(2) nanosheets with 98.2% of exposed {001} faceted surfaces on the Ti substrate via a distinctive two-stage formation mechanism. The first stage produces a new intermediate crystal (orthorhombic HTiOF(3) ) that is transformed into anatase TiO(2) nanosheets during the second stage. The findings reveal that the HF-VPH reaction environment is unique and differs remarkably from that of liquid-phase hydrothermal processes. The uniqueness of the HF-VPH conditions can be readily used to effectively control the nanostructure growth.

Mesoporous NiO crystals with dominantly exposed {110} reactive facets for ultrafast lithium storage

Faceted crystals with exposed highly reactive planes have attracted intensive investigations for applications such as hydrogen production, enhanced catalytic activity, and electrochemical energy storage and conversion. Herein, we report the synthesis of mesoporous NiO crystals with dominantly exposed {110} reactive facets by the thermal conversion of hexagonal Ni(OH)₂ nanoplatelets. When applied as anode materials in lithium-ion batteries, mesoporous NiO crystals exhibit a high reversible lithium storage capacity of 700 mAh g⁻¹ at 1 C rate in 100 cycles and an excellent cyclability. In particular, the dominantly exposed {110} reactive facets and mesoporous nanostructure of NiO crystals lead to ultrafast lithium storage, which mimics the high power delivery of supercapacitors.

Aerosol-assisted molten salt synthesis of NaInS(2) nanoplates for use as a new photoanode material

NaInS(2) , a H(2) -evolving photocatalyst, is synthesized as single-crystalline hexagonal plates by coupling a molten salt synthesis with ultrasonic spray pyrolysis (USP) for the first time. USP NaInS(2) films are used as a new photoanode material and have an initial photocurrent of ≈37 μA/cm(2) upon illumination and activities 25 times greater than films made from a standard non-aerosol NaInS(2) sample.

Synthesis of carbon nanotube-anatase TiO₂ sub-micrometer-sized sphere composite photocatalyst for synergistic degradation of gaseous styrene

The carbon nanotube (CNT)-sub-micrometer-sized anatase TiO₂ sphere composite photocatalysts were synthesized by a facile one-step hydrothermal method using titanium tetrafluoride as titanium source and CNTs as structure regulator. Various technologies including X-ray diffraction, UV-visible absorption spectra, N₂ adsorption-desorption, scanning electron microscopy, and transmission electron microscopy were employed to characterize the structure properties of the prepared composite photocatalysts. The results indicated that the composite photocatalysts consisted of CNTs wrapping around the sub-micrometer-sized anatase TiO₂ spheres with controllable crystal facets and that the aggregated particles with average diameter ranged from 200 to 600 nm. The fabricated composite photocatalysts were used to degrade gaseous styrene in this work. As expected, a synergistic effect that remarkably enhancing the photocatalytic degradation efficiency of gaseous styrene by the prepared composite photocatalysts was observed in comparison with that the degradation efficiency using pure anatase TiO₂ and the adsorption of CNTs. Similar results were also confirmed in the decolorization of liquid methyl orange. Further investigation demonstrated that the synergistic effect in the photocatalytic activity was related to the structure of the sub-micrometer-sized anatase TiO₂ spheres and the significant roles of CNTs in the composite photocatalysts. By controlling the content of CNTs, the content of TiO₂ or the temperature during the hydrothermal synthesis process, anatase TiO₂ spheres with controllable crystallite size and dominant crystal facets such as {001}, {101}, or polycrystalline could be obtained, which was beneficial for the increase in the synergistic effect and further enhancement of the photocatalytic efficiencies.

The synthesis of nitrogen/sulfur co-doped TiO2 nanocrystals with a high specific surface area and a high percentage of {001} facets and their enhanced visible-light photocatalytic performance

Nitrogen/sulfur co-doped anatase TiO2 nanocrystals with a high specific surface area and a high percentage of {001} facets were synthesized by a solvent-thermal process followed by the calcination with thiourea at an optimum heat treatment temperature. Under current experimental conditions, the optimum heat treatment temperature was found at 300°C, which successfully introduced nitrogen and sulfur dopants into the anatase lattice to replace a small portion of oxygen atoms while preserving the geometry of these anatase TiO2 nanocrystals to maintain a high percentage of {001} facets. These nitrogen/sulfur co-doped anatase TiO2 nanocrystals demonstrated a largely enhanced light absorption in the whole visible-light range and exhibited much higher photocatalytic performance than both P25 TiO2 nanoparticles and anatase TiO2 nanocrystals with a high percentage of {001} facets under visible-light illumination.

Nanosize storage properties in spinel Li4Ti5O12 explained by anisotropic surface lithium insertion

Nanosizing is a frequently applied strategy in recent years to improve storage properties of Li-ion electrodes and facilitate novel storage mechanisms. Due to particle size reduction, surface effects increasingly dominate, which can drastically change the storage properties. Using density functional theory calculations we investigate the impact of the surface environment on the Li-ion insertion properties in defective spinel Li(4+x)Ti(5)O(12), a highly promising negative electrode material. The calculations reveal that the storage properties strongly depend on the surface orientation. The lowest energy (1 1 0) surface is predicted to be energetically favorable for Li-ion insertion into the vacant 16c sites. The (1 1 1) surface allows capacities that significantly exceed the bulk capacity Li(7)Ti(5)O(12) at voltages greater than 0 V by occupation of 8a sites in addition to the fully occupied 16c sites. One of the key findings is that the surface environment extends nanometers into the storage material, leading to a distribution of voltages responsible for the curved voltage profile commonly observed in nanosized insertion electrode materials. Both the calculated surface-specific voltage profiles and the calculated particle size dependent voltage profiles are in good agreement with the experimental voltage profiles reported in literature. These results give a unique insight into the impact of nanostructuring and further possibilities of tailoring the Li-ion voltage profiles and capacities in lithium insertion materials.

Full solution-processed synthesis of all metal oxide-based tree-like heterostructures on fluorine-doped tin oxide for water splitting

Well-ordered tree-like functional heterostructures, composed of the environmentally friendly oxides ZnO, TiO(2) , and CuO, on a fluorine-doped tin oxide substrate are realized by a practical, cost-effective, solution-processable strategy. The heterostructures are demonstrated to be an efficient light-harvesting medium in a photo-electrochemical cell to split water for hydrogen-gas generation, and the developed strategy provides a highly promising, cheap, green way to fabricate multifunctional hierarchically branched structures for many potential applications.

Synthesis of amphiphilic brookite nanoparticles with high photocatalytic performance for wide range of application

Brookite TiO(2) nanoparticles with amphiphilic properties were successfully synthesized with a water-soluble titanium glycolate complex precursor using an oleate-modified hydrothermal growth process. The ~20-40 nm TiO(2) nanoparticles were highly dispersible in both water and cyclohexane. The activity of the brookite nanoparticles for the degradation of acetaldehyde under UV irradiation was higher than that of Degussa P-25 TiO(2) powder. A spin-coated brookite film prepared on a polyimide substrate exhibited photoinduced hydrophilicity. Thus, these synthesized brookite nanoparticles could be applied as a photocatalytic coating solution.

Two-dimensional nanoarchitectures for lithium storage

Two dimensional nanoarchitectures are of great interest in lithium storage for energy-storage devices, in particular lithium-ion batteries, due to its shortened paths for fast lithium ion diffusion and large exposed surface offering more lithium-insertion channels. Their competitive lithium-storage features provide huge potentials to develop next-generation high-performance lithium-ion batteries. This review is devoted to the recent progress in the fabrication of innovative 2D structures with various synthetic strategies and their applications for lithium storage in lithium-ion batteries. These 2D architectures are categorized into six styles, i.e., nanoporous nanosheets, ultrathin nanosheets, flower-like structures assembled by nanosheets, sandwich-like nanosheets, corrugated nanosheets, and nanosheets with specific facets. Based on the lithium-storage manner, we further summerized their electrochemical performance for lithium storage with four classified themes including surface Li storage, zero or low-strain Li storage, volume-variation Li storage and synergic-effect Li storage. Finally, the outlook and perspective on 2D lithium-storage materials is concisely provided.

Direct Synthesis of Anatase Films with ~100% (001) Facets and [001] Preferred Orientation

Anatase films exhibiting ~100% (001) reactive facets at the surface were grown hydrothermally on gold substrate from a homogeneous solution of TiF(4) and NaF. In addition to NaF, it was found that TiO(2) films with very similar properties could be prepared with the fluoride salts LiF, CsF, HF, NH(4)F, and N(CH(2)CH(3))(4)F. The polycrystalline anatase films are continuous, approximately 1 μm thick, and evenly coat the substrate. The surface grain size is ~400 nm. Grazing angle XRD measurements show that the films exhibit a high degree of preferred orientation with the c-axis normal to the substrate surface. SEM images reveal that the grains span the thickness of the films. Annealing the films at 500 °C removes fluorine and causes crystallites within the grains to restructure as shown by SEM, XRD, and Raman spectroscopy. Supported anatase films grown from this one-pot method may serve as oxidative photocatalysts and electrodes for photoelectrochemical applications such as solar cells and hydrogen evolution.

Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries

The search for new electrode materials for lithium-ion batteries (LIBs) has been an important way to satisfy the ever-growing demands for better performance with higher energy/power densities, improved safety and longer cycle life. Nanostructured metal oxides exhibit good electrochemical properties, and they are regarded as promising anode materials for high-performance LIBs. In this feature article, we will focus on three different categories of metal oxides with distinct lithium storage mechanisms: tin dioxide (SnO(2)), which utilizes alloying/dealloying processes to reversibly store/release lithium ions during charge/discharge; titanium dioxide (TiO(2)), where lithium ions are inserted/deinserted into/out of the TiO(2) crystal framework; and transition metal oxides including iron oxide and cobalt oxide, which react with lithium ions via an unusual conversion reaction. For all three systems, we will emphasize that creating nanomaterials with unique structures could effectively improve the lithium storage properties of these metal oxides. We will also highlight that the lithium storage capability can be further enhanced through designing advanced nanocomposite materials containing metal oxides and other carbonaceous supports. By providing such a rather systematic survey, we aim to stress the importance of proper nanostructuring and advanced compositing that would result in improved physicochemical properties of metal oxides, thus making them promising negative electrodes for next-generation LIBs.

Noble-metal-free carbon nanotube-Cd0.1Zn0.9S composites for high visible-light photocatalytic H2-production performance

Visible light photocatalytic H(2) production from water splitting using solar light is of great importance from the viewpoint of solar energy conversion and storage. In this study, a novel visible-light-driven photocatalyst multiwalled carbon nanotube modified Cd(0.1)Zn(0.9)S solid solution (CNT/Cd(0.1)Zn(0.9)S) was prepared by a simple hydrothermal method. The prepared samples exhibited enhanced photocatalytic H(2)-production activity under visible light. CNT content had a great influence on photocatalytic activity and an optimum amount of CNT was determined to be ca. 0.25 wt%, at which the CNT/Cd(0.1)Zn(0.9)S displayed the highest photocatalytic activity under visible light, giving an H(2)-production rate of 78.2 μmol h(-1) with an apparent quantum efficiency (QE) of 7.9% at 420 nm, even without any noble metal cocatalysts, exceeding that of pure Cd(0.1)Zn(0.9)S by more than 3.3 times. The enhanced photocatalytic activity was due to CNT as an excellent electron acceptor and transporter, thus reducing the recombination of charge carriers and enhancing the photocatalytic activity. Furthermore, the prepared sample was photostable and no photocorrosion was observed after photocatalytic recycling. Our findings demonstrated that CNT/Cd(0.1)Zn(0.9)S composites were a promising candidate for the development of high-performance photocatalysts in photocatalytic H(2) production. This work not only shows a possibility for the utilization of low cost CNT as a substitute for noble metals (such as Pt) in the photocatalytic H(2)-production but also for the first time shows a significant enhancement in the H(2)-production activity by using metal-free carbon materials as effective co-catalysts.

Nonaqueous synthesis of TiO2 nanocrystals using TiF4 to engineer morphology, oxygen vacancy concentration, and photocatalytic activity

Control over faceting in nanocrystals (NCs) is pivotal for many applications, but most notably when investigating catalytic reactions which occur on the surfaces of nanostructures. Anatase titanium dioxide (TiO(2)) is one of the most studied photocatalysts, but the shape dependence of its activity has not yet been satisfactorily investigated and many questions still remain unanswered. We report the nonaqueous surfactant-assisted synthesis of highly uniform anatase TiO(2) NCs with tailorable morphology in the 10-100 nm size regime, prepared through a seeded growth technique. Introduction of titanium(IV) fluoride (TiF(4)) preferentially exposes the {001} facet of anatase through in situ release of hydrofluoric acid (HF), allowing for the formation of uniform anatase NCs based on the truncated tetragonal bipyramidal geometry. A method is described to engineer the percentage of {001} and {101} facets through the choice of cosurfactant and titanium precursor. X-ray diffraction studies are performed in conjunction with simulation to determine an average NC dimension which correlates with results obtained using electron microscopy. In addition to altering the particle shape, the introduction of TiF(4) into the synthesis results in TiO(2) NCs that are blue in color and display a broad visible/NIR absorbance which peaks in the infrared (λ(max) ≈ 3400 nm). The blue color results from oxygen vacancies formed in the presence of fluorine, as indicated by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) studies. The surfactants on the surface of the NCs are removed through a simple ligand exchange procedure, allowing the shape dependence of photocatalytic hydrogen evolution to be studied using monodisperse TiO(2) NCs. Preliminary experiments on the photoreforming of methanol, employed as a model sacrificial agent, on platinized samples resulted in high volumes of evolved hydrogen (up to 2.1 mmol h(-1) g(-1)) under simulated solar illumination. Remarkably, the data suggest that, under our experimental conditions, the {101} facets of anatase are more active than the {001}.

Morphology evolution of TiO2 facets and vital influences on photocatalytic activity

Modulation of anatase toward highly active facets has been attracting much attention, but the mechanism and photoactivity are still ambiguous. Here we demonstrate the inherent mechanisms for facets nucleation and morphology evolution, and clarify some vital influences of facets and surface nature on the photoactivity. Simply tuning the Ti/F ratio in the synthetic mixture leads to single anatase crystal exposed with different facets like {001}, {010}, or {110}. And complex sphere structure exposed with {001} facets can be formed by secondary nucleation and growth. Prolonging the hydrothermal treatment time causes selective etching on {001} facets, whereas defluorination via thermal calcination produces many pores on the surface. The photodegradation of positively and negatively charged, and zwitterionic dyes indicates that the type of reactant, adsorption mode and surface area play significant roles in photocatalysis. This work makes a step toward understanding the formation of facet-mediated structure and designing highly active materials for environmental remediation, hydrogen production, and dye-sensitized solar cells.

Template-free fabrication of TiO2 hollow spheres and their photocatalytic properties

Submicrometer-sized anatase TiO(2) hollow spheres were fabricated through a template-free solvothermal route using TiCl(4) as a raw material and a mixture of alcohols-acetone as solvent. Control of the hollow spheres' size was achieved by adjusting the ratio of alcohols to acetone. Products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM, X-ray photoelectron spectra (XPS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and thermogravimetric (TG) analysis. It was found that the formation process of the TiO(2) hollow spheres might include the hydrolysis of Ti(IV) with the water formed from the solvothermal etherification reaction, the aggregation of the anatase TiO(2) nanoparticles, and the Ostwald ripening. Furthermore, the as-prepared TiO(2) hollow nanostructures exhibited good photocatalytic activity for the degradation of phenol.

Anatase TiO2 nanocrystals with exposed {001} facets on graphene sheets via molecular grafting for enhanced photocatalytic activity

Owing to their extensive practical applications and fundamental importance, the controllable synthesis of well-faceted anatase TiO(2) crystal with high percentage of reactive facets has attracted increasing attention. Here, nano-sized anatase TiO(2) sheets mainly dominated by {001} facets had been prepared on graphene sheets by using a facile solvothermal synthetic route. The percentage of {001} facets in TiO(2) nanosheets was calculated to be ca. 64%. The morphologies, structural properties, growth procedures and photocatalytic activities of the resultant TiO(2)/graphene nanocomposites were investigated. In comparison with commercial P25 and pure TiO(2) nanosheets, the composite exhibited significant improvement in photocatalytic degradation of the azo dye Rhodamine B under visible light irradiation. The enhancement of photocatalytic activity and stability was attributed to the effective charge anti-recombination of graphene and the high catalytic activity of {001} facets.