Single-crystalline Anatase TiO2 Research Articles

Axis-Oriented, Anatase TiO2 Single Crystals with Dominant {001} and {100} Facets

Owing to wide-ranging industrial applications and fundamental importance, tailored synthesis of well-faceted single crystals of anatase TiO2 with a high percentage of photoactive facets has attracted much research interest. In this work, single crystalline anatase TiO2 has been prepared using hydrothermal conditions with precursors such as TiF4 and TiCl3. Fluorine species (herein HF) are introduced as a stabilizing agent to reduce surface energies of anatase TiO2, especially {001} facets, which leads to a large percentage of the reactive {001} facets exposed in final assemblies or films of titania with the largest exposure area. Anatase TiO2 single crystals with various degrees of truncation are prepared as a result of differences in relative concentration ratios of HF to TiF4. The “rice-like” anatase single crystals with exposed {100} facets were prepared in the system using TiCl3 precursor. Carboxymethyl cellulose (CMC), used as a dispersing agent, helps to control the reaction rate and suppress the agg...

Hydrothermal Stability of {001} Faceted Anatase TiO2

Due to its great importance in fundamental research and practical applications, tailored synthesis of anatase TiO2 dominated with highly energetic {001} facets has been extensively studied during the past few years. However, clean (001) surface of anatase TiO2 has been evidenced to be unstable and usually tends to reconstruct under ultrahigh-vacuum conditions. Thus, the stability of surface structure under other ambient conditions might be one of the most critical issues for anatase TiO2 with exposed high-reactive {001} facets. In this study, the hydrothermal stability of {001} faceted anatase TiO2 was systematically investigated by using single-crystalline anatase TiO2 nanosheets with 80% {001} facets as a model starting material. Under hydrothermal conditions (200 °C in deionized water), anatase TiO2 nanosheets can grow into larger single crystals with a truncated bipyramidal shape through an oriented attachment process along the [001] crystallographic direction, driven by the minimization of surface energy. Furthermore, the coarsening behavior and growth mechanism were discussed with the assistance of standard and high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry, as well as theoretical calculations. A modified kinetic model was also developed to elucidate the asymptotic growth of anatase TiO2 nanosheets via oriented attachment mechanism. In addition, pH value and the solvents adopted during the treatments were revealed to have significant influence on the oriented attachment-based crystal growth due to suppression of hydrolysis of Ti–F groups on the surface. For example, the anatase TiO2 nanosheets remained their original morphology unchanged when ethanol, propanol, butanol, or 1.5 M HCl aqueous solution was used as reaction medium.

Plasma Surface Treatments of TiO2 Photoelectrodes for Use in Dye-Sensitized Solar Cells

Single crystalline anatase TiO2 nanoparticles were synthesized and sintered to obtain mesoporous TiO2 photoelectrodes. Surface treatment was performed on the resulting TiO2 photoelectrodes using oxygen, nitrogen, or nitrogen + methane (1:1) plasma. X-ray absorption near-edge structure analysis and x-ray photoelectron spectroscopy were used to show the differences among the surface treatments. Dye-sensitized solar cells fabricated from as-sintered and plasma-treated TiO2 photoelectrodes exhibit different cell efficiencies. Compared to the as-sintered sample, the nitrogen + methane plasma treatment enhances the cell efficiency by 24.1%, followed by the oxygen plasma treatment (19.7%) and the nitrogen plasma treatment (5.0%). The differences in the cell efficiency are attributed to the differences in the photoelectrode structures (oxygen vacancy and C–C sp2 to sp3 bonding ratio) and the resulting properties (incident photon to conversion efficiency and dye absorption).

Synthesis of Single Crystalline Anatase TiO2 (001) Tetragonal Nanosheet‐Array Films on Fluorine‐Doped Tin Oxide Substrate

Single crystalline anatase TiO2 tetragonal nanosheet‐array film on a transparent conductive fluorine‐doped tin oxide (FTO) substrate was successfully prepared by hydrothermal method. The products were characterized with XRD, SEM, and transmission electron microscopy measurements. The formation mechanism of the tetragonal (001) TiO2 nanosheets is discussed and the effect of growth parameters, such as acidity, growth time, growth temperature, initial reactant concentration, and additives, are investigated. This work demonstrates a clean and effective route to the morphology‐controlled preparation of large area single‐crystalline TiO2 nanostructured array films on FTO glass.

Synthesis of anatase TiO2 rods with dominant reactive {010} facets for the photoreduction of CO2 to CH4 and use in dye-sensitized solar cells

Single crystalline anatase TiO(2) rods with dominant reactive {010} facets are directly synthesized by hydrothermally treating Cs(0.68)Ti(1.83)O(4)/H(0.68)Ti(1.83)O(4) particles. The nanosized rods show a comparable conversion efficiency in dye-sensitized solar cells (DSSCs), and a superior photocatalytic conversion of CO(2) into methane to the benchmark P25 TiO(2) nanocrystals.

Single-crystalline and reactive facets exposed anatase TiO2 nanofibers with enhanced photocatalytic properties

Single-crystalline anatase TiO2 nanofibers with highly reactive {001} facets were synthesized from layered potassium titanate K2Ti2O5via topotactic transformation in ion-exchange and dehydration. The cuboid fibers showed one-dimensional (1-D) orientation in the [010] direction and {001} as well as {100} facets enclosing along the longitudinal dimension. The structural evolution was deduced from XRD, SEM and TEM characterizations, which revealed that the highly reactive {001} facets were derived from the interlayer splitting and exfoliation of the layered precursors in a surfactant-free way. Photoluminescence (PL) measurements witnessed the efficient separation and transfer of photoinduced charge carriers in the single-crystalline and reactive facets enclosed TiO2 nanofibers. Sequently, highly efficient photocatalytic property of the nanofibers was demonstrated by phenol degradation and H2 evolution. Phenol degradation rate of nanofibers is 2.7 times of the irregular-shaped nanoparticle counterparts with the same crystal phase and similar specific surface area. In photocatalytic evolution of H2, nanofibers presented much higher and more stable activity than both nanoparticle counterparts and P25 benchmark. Charge carrier highways provided by single-crystalline 1-D structures and efficient surface reactivity offered by exposed facets are the two key factors for the enhanced properties.

Hydrothermal Fabrication of Quasi‐One‐Dimensional Single‐Crystalline Anatase TiO2 Nanostructures on FTO Glass and Their Applications in Dye‐Sensitized Solar Cells

One-dimensional and quasi-one-dimensional semiconductor nanostructures are desirable for dye-sensitized solar cells (DSSCs), since they can provide direct pathways for the rapid collection of photogenerated electrons, which could improve the photovoltaic performance of the device. Quasi-1D single-crystalline anatase TiO(2) nanostructures have been successfully prepared on transparent, conductive fluorine-doped tin oxide (FTO) glass with a growth direction of [101] through a facile hydrothermal approach. The influences of the initial titanium n-butoxide (TBT) concentration, hydrothermal reaction temperature, and time on the length of quasi-1D anatase TiO(2) nanostructures and on the photovoltaic performance of DSSCs have been investigated in detail. A power conversion efficiency of 5.81% has been obtained based on the prepared TiO(2) nanostructure photoelectrode 6.7 μm thick and commercial N719 dye, with a short-circuit current density of 13.3 mA cm(-2) , an open-circuit voltage of 810 mV, and a fill factor of 0.54.

Shape-Enhanced Photocatalytic Activity of Single-Crystalline Anatase TiO2 (101) Nanobelts

Particle size is generally considered to be the primary factor in the design of nanocrystal photocatalysts, because the reduction of particle size increases the number of active sites. However, the benefit from the size reduction can be canceled by a higher electron-hole recombination rate due to the confined space in sphere-shaped nanoparticles. Here we report a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. Single-crystalline anatase TiO(2) nanobelts with two dominant surfaces of (101) facet exhibit enhanced photocatalytic activity over the nanosphere counterparts with an identical crystal phase and similar specific surface area. The ab initio density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O(2), facilitating the generation of superoxide radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals; (ii) fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O(2) on the (101) facet. Our results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals, which provides a new concept for rational design and development of high-performance photocatalysts.

Room-Temperature Synthesis of Single-Crystalline Anatase TiO2 Nanowires

Single-crystalline anatase TiO2 nanowires have been synthesized using a novel, room-temperature method involving the use of seed particles and a radio frequency magnetron sputter deposition technique. Also, the growth time required was less than 60 min. Transmission electron microscopy and X-ray absorption near-edge structure analyses show that the obtained nanowires are single-crystalline anatase TiO2. Cathodoluminescence spectroscopy spectrum shows that the nanowires’ band-to-band emission exhibits a blue shift from to 3.20 (387) to 3.46 eV (358 nm). Conventional vapor−liquid−solid and vapor−solid−solid growth modes are not applicable to explain the growth of the obtained single-crystalline anatase TiO2 nanowires at room-temperature. A folded-growth mode is therefore suggested.

Tetragonal faceted-nanorods of anatase TiO2 single crystals with a large percentage of active {100} facets

Tetragonal faceted-nanorods of single-crystalline anatase TiO(2) with a large percentage of higher-energy {100} facets have been synthesized by hydrothermal transformation of alkali titanate nanotubes in basic solution.

Synthesis of highly-active single-crystalline TiO 2 nanorods and its application in environmental photocatalysis

Highly-active anatase TiO 2 nanorods have been successfully synthesized via a simple two-step method, hydrothermal treatment of anatase/rutile titanium dioxide nanoparticle powder in a composite-hydroxide eutectic system of 1:1 M KOH/NaOH, followed by acid post-treatment. The morphology and crystalline structure of the obtained nanorods were characterized using XRD, TEM, SEM/EDX and BET surface area analyzer. The obtained TiO 2 nanorods have a good crystallinity and a size distribution (about 4–16 nm); with the dimensions of 200–300 nm length and of 30–50 nm diameter. Compared with its precursor anatase/rutile TiO 2 nanoparticles and the titanate nanotubes, the pure anatase TiO 2 nanorods have a large specific surface area with a mesoporous structure. The photocatalytic performance of the prepared nanorods was tested in the degradation of the commercial Cibacrown Red (FN-R) textile dye, under UV irradiation. Single-crystalline anatase TiO 2 nanorods are more efficient for the dye removal.

Single-Crystalline Anatase TiO2 Dous Assembled Micro-Sphere and Their Photocatalytic Activity

Using anodized TiO2 nanotubes as the starting materials, unique structural single-crystalline anatase TiO2 dous assembled microspheres are fabricated by a hydrothermal method. The single-crystalline anatase TiO2 dou is formed through the transformation of a TiO2 nanotube, which is formed through a complex process including the expansion of the nanotube, dissolution−recrystallization, and recombination. The opening and hollow structural dou exposes one {001} face and eight {101} faces, which displays excellent photocatalytic activity. And it is also envisaged to be of high promise for many other potential applications. The approach might be exploited for the synthesis of single-crystalline nanomaterials with special morphologies.

Nonaqueous sol–gel synthesis and growth mechanism of single crystalline TiO 2 nanorods with high photocatalytic activity

In this paper, we report on a nonaqueous synthesis of single crystalline anatase TiO 2 nanorods by reaction between TiCl 4 and benzyl alcohol at a low temperature of 80 °C. The resulting samples were characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, nitrogen adsorption, X-ray photoelectron spectrometry and UV–vis diffuse reflectance spectroscopy. We proposed that the TiO 2 nanorods were formed through an oriented attachment mechanism. More importantly, these single crystalline anatase TiO 2 nanorods exhibited significantly higher photocatalytic activities than commercial photocatalyst P25. This study provides an environmentally friendly and economic approach to produce highly active TiO 2 photocatalyst.

Water Dissociation on Single Crystalline Anatase TiO2(001) Studied by Photoelectron Spectroscopy

The adsorption of water on the anatase TiO2(001)-(4 × 1) surface is studied using synchrotron radiation-excited core level photoelectron spectroscopy. The coverage-dependent adsorption of water at low temperature is monitored and compared to the sequence obtained after heating of a water multilayer. Two adsorption phases of submonolayer coverage can be defined: Phase 1 consists only of dissociated water, observed as OH-groups. This phase is found at low coverage at low temperature (190 K) and is the only state of adsorbed water above ∼230 K. The saturation coverage of phase 1 is consistent with dissociation on the 4-fold-coordinated Ti ridge atoms of the (4 × 1) surface reconstruction. Phase 2 is found at higher coverage, reached at lower temperature. It consists of a mixture of dissociated and molecular water with a ratio of 1:1 at 170 K. The molecular water is found to bond to the hydroxyl groups. The hydroxyl coverage of phase 2 is approximately 2 times that of phase 1. The results suggest that the OH and H2O species of phase 2 are confined to the ridges of the surface.

Enhanced Optical Absorption Induced by Dense Nanocavities inside Titania Nanorods

Dense polyhedral nanocavities inside single-crystalline anatase TiO2 nanorods are successfully synthesized by simply heating H2Ti3O7 nanorods. The size of the nanocavities is typically about 10 nm. The surfaces of the nanocavity polyhedron are determined to be the crystallographic low-index planes of the TiO2 crystal (see figure).

Single-crystalline TiO2 nanorods: Highly active and easily recycled photocatalysts

Large-scale and single-crystalline anatase TiO2 nanorods were synthesized via hydrothermal reaction of H-titanate fibers. It was found that the pH plays an important role in the shape and crystal structures of the products by hydrothermal post-treatment of H-titanate fibers in acidic solutions. At pH 2–7, the pure anatase phase was formed and the samples reserved the rod-like morphology with a diameter of 20–200nm, in contrast to the formation of particles with a mixture of rutile and brookite phases at pH 0. Photocatalytic experiments indicated that the obtained single-crystalline anatase TiO2 nanorods are highly active for photodegradation of organic pollutants. Furthermore, we demonstrated that these nanorod catalysts can be easily recycled without decrease of the photocatalytic activity.

Dye-Sensitized Solar Cells Based on Anatase TiO2 Nanoparticle/Nanowire Composites

Dye-sensitized solar cells were fabricated based on the composites of anatase TiO2 nanoparticles and single crystalline anatase TiO2 nanowires. Nanoparticle/nanowire composites can possess the advantages of both building blocks, i.e., the high surface area of nanoparticle aggregates and the rapid electron transport rate and the light scattering effect of single-crystalline nanowires. Three different composites were prepared with 5 wt %, 20 wt %, and 77 wt % nanowires, respectively. The performances of composite solar cells were compared with pure nanoparticle cells at a series of film thickness. With low nanowire concentrations (5 wt % and 20 wt %), the composite films maintain similar specific surface area as the pure nanoparticle films, while the composite cells show higher short-circuit current density and open-circuit voltage. An enhancement of power efficiency from 6.7% for pure nanoparticle cells to 8.6% for the composite cell with 20 wt % nanowires has been achieved under 1 Sun AM1.5 illumination (100 mW/cm2). For the composite film with 77 wt % nanowires, the nanowires became the major phase. Their less compact packing resulted in significant decrease of the specific surface area, and thus the current density. However, with the increase of film thickness, the current density showed a continuous increase in the whole thickness range up to 17 microm, indicating the improved electron diffusion length due to the formed nanowire network. The nanowires also helped to preserve crack-free thick films. These results show that employing nanoparticle/nanowire composites represents a promising approach for further improving the efficiencies of sensitized solar cells.

Hydrothermal Synthesis of Single-Crystalline Anatase TiO2 Nanorods with Nanotubes as the Precursor

Preparation of anatase TiO2 nanorods from solutions in the absence of surfactants or templates has rarely been reported. The present work has found that hydrothermal treatment of titanate nanotube suspensions under an acidic environment resulted in the formation of single-crystalline anatase nanorods with a specific crystal-elongation direction. The nanotube suspensions were prepared by treatment of TiO2 in NaOH, followed by mixing with HNO3 to different pH values. The crystal size of the anatase nanoparticles obtained from the hydrothermal treatment increased with the pH of the suspensions, and nanorods with an aspect ratio up to 6 and a long axis along the anatase [001] were obtained at a pH slightly less than 7. A mechanism for the tube-to-rod transformation has been proposed on the basis of the crystalline structures of the tubes and rods. The local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites have been suggested to be the key steps involved in the nanorod formation.

Highly Efficient Dye-sensitized Solar Cells Based on Single Crystalline TiO2 Nanorod Film

Abstract Single crystalline anatase TiO2 nanorods were successfully synthesized by surfactant-assisted hydrothermal method. A high light-to-electricity conversion yield of 7.06% was achieved by applying the TiO2 nanorods as thin film of dye-sensitized solar cells.

Strain-induced charge separation in the photocatalytic single crystalline anatase TiO 2 film

A single crystalline anatase TiO 2 film was grown on a LaAlO 3 substrate. Photocatalytic reduction of Ag ions resulted in Ag precipitation on the surface of the film. The density and the morphology of the Ag precipitation varied at the twin boundaries of the substrate. The anatase film should suffer from the strain caused by this twinning and the strain distribution should be different in each twin domains. Hence, this twin-domain dependent Ag precipitation is a possible clue for the electron–hole charge separation centers induced by the stress concentration in the anatase film which act as active sites for photocatalytic reaction.