Single-crystalline Anatase Research Articles

Efficient Light Harvesting and Charge Collection of Dye-Sensitized Solar Cells with (001) Faceted Single Crystalline Anatase Nanoparticles

With a comparable specific surface area, 17% enhanced dye-loading capacity was observed for the first time in the (001) faceted TiO2 single crystals compared to that of benchmark P25 nanoparticles, thus generating a significant enrichment in both short-circuit photocurrent density and power conversion efficiency of dye-sensitized solar cells (DSCs). Such a remarkably increased dye-loading capacity was primarily ascribed to the higher density of 5-fold-coordinnated Ti atoms on the (001) surfaces. Furthermore, kinetic studies revealed that such single crystals confer a higher electron lifetime and charge collection efficiency compared with conventional P25 electrode, which might originate from the specific surface configuration in these high energetic facet dominant single crystals. Our study provides straightforward evidence for the superior reactivity of (001) facets and implies that such single TiO2 crystals with high-energetic facets would be a promising electrode material for DSCs.

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.

Higher water splitting hydrogen generation rate for single crystalline anatase phase of TiO2nanotube arrays

This paper presents a detailed investigation on the effect of heat-treatment process on the highly ordered titanium dioxide (TiO 2 ) nanotube arrays in connection with the photoelectrochemical (PEC) response and hydrogen evolution rate. TiO 2 nanotube arrays have been systematically heat-treated to control the transformation of as-anodized TiO 2 amorphous structure to crystalline anatase and rutile phases. In this study, single crystalline TiO 2 anatase phase exhibited a higher PEC response and hydrogen evolution rate at 400 °C heat treatment. The photocurrent density increase was mainly attributed to the effective transport of photo-induced electrons within the single crystal anatase phase. However, polycrystalline anatase and rutile phases showed the fluctuation in lower photocurrent density upon heat treatment above 500 °C. The mobility of photo-induced electrons was obviously hindered due to the recombination losses in defect sites between the anatase and rutile phase.

Hydrogen-Plasma-Assisted Growth of Anatase TiO2 Nanoneedles on Ti Plates

Pure anatase TiO2 nanoneedles are prepared on substrates via a dry process that involves hydrogen plasma treatments followed by the oxidation of Ti plates at a low temperature of 500 °C. Scanning electron microscopy and high-resolution transmission electron microscopy images show that the nanoneedles have diameters and lengths in ranges of 20–50 nm and 100–200 nm, respectively, and are single-crystalline anatase structures that grow along the [116] direction. The TiO2 nanoneedles nucleate on microgrooves that result from the oxidation of microcracks induced by the hydrogen plasma. The diameters, lengths, and area densities of the TiO2 nanoneedles can be varied by adjusting the power of the hydrogen plasma. A stress-assisted vapor–solid mechanism for the formation of the nanoneedles is proposed. The room-temperature photoluminescence spectrum of the TiO2 nanoneedles exhibits a relatively strong ultraviolet emission centered at 375 nm and broad visible emission at around 640 nm. This study proposes a route...

Synthesis of single-crystalline anatase nanorods and nanoflakes on transparent conducting substrates

Single-crystalline anatase nanorods and nanoflakes were grown on transparent conducting fluorine-doped tin oxide (FTO) substrates through hydrolysis of titanium tetrachloride (TiCl(4)) followed by heating to 450 °C.

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.

Synthesis and characterization of flexible one-dimensional TiO2 nanocrystalline films

The flexible oriented one-dimensional (1-D) TiO2 nanocrystalline films with large length-diameter ratios were synthesized in 1 M NaOH solution by hydrothermal treatment of the thin titanium layer which was deposited on flexible stainless steel substrates by direct current magnetron sputtering. The films were characterized, respectively by field emission scanning electron microscopy (FESEM), X-Ray diffraction (XRD), and transmission electron microscopy (TEM). Results showed that the TiO2 films were composed of oriented single-crystalline anatase nanowires, and the morphology of the nanowire film was determined by hydrothermal conditions. The photoelectric property studies revealed that the photoelectric property of TiO2 films was improved with the increase of the hydrothermal temperature. This approach provides an alternative method to synthesize 1-D TiO2 nanocrystalline films on non-Ti substrates.

Effect of photoelectrode morphology of single-crystalline anatase nanorods on the performance of dye-sensitized solar cells

TiO2 terpineol-based pastes with nanorods (NRs) of over 25μm thickness have been prepared for the photoactive electrodes of the dye-sensitized solar cells (DSSCs). The NRs, with a length of approximately 80nm and an aspect ratio of about 3, are made by a two-step hydrothermal process. They have the single crystalline anatase structure and can be dispersed well in water and ethanol. With a high thermal stability and larger surface area (47.2m2g−1) than commercial TiO2 particles (P25, 39.1m2g−1), the well-dispersed anatase NR films with aggregate-free morphology are transparent. For the photocurrent–voltage measurements, the NR cell exhibits high short-circuit photocurrent (JSC) under 1 Sun AM 1.5 simulated sunlight due to the higher surface area and transmittance. The open-circuit voltage (VOC) of NR films is not obviously reduced with incremental thickness, which results from the one-dimensional single crystalline structure of NR due to less surface defects. As compared with the P25 cell, DSSCs made with NRs have a higher fill factor (FF) because of the uniform void spaces. An enhancement of conversion efficiency from 4.88% for P25 to 5.67% for NR is achieved. The P25 particles are incorporated in NR films as light-scattering centers, while the R1P1 containing 50wt.% of P25 has a high VOC and FF as compared with P25, but the JSC is still lower than that of the NR.

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.

Hydrothermal growth of multi-facet anatase spheres

Titania with various nanostructures can be synthesized by several F −-mediated procedures. In this paper, we report the synthesis of a novel multi-facet microsphere consisting of etched single-crystalline anatase by simply immersing metallic Ti plates in an HF aqueous solution under hydrothermal conditions. The etched multi-facet sphere was found to grow through the nucleation and growth of truncated bipyramids on a previously precipitated one to assemble a microsphere, and its subsequent etching by HF to expose the thermodynamic stable {1 0 1} facets. The photocatalytic activity of such etched multi-facet sphere thin films was evaluated utilizing rhodamine B and sulfonic salicylic acid in water as target molecules and compared with commercial Degussa P25 titania nanoparticles.

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.

High-Efficiency Solid-State Dye-Sensitized Solar Cells: Fast Charge Extraction through Self-Assembled 3D Fibrous Network of Crystalline TiO2 Nanowires

Herein, we present a novel morphology for solid-state dye-sensitized solar cells based on the simple and straightforward self-assembly of nanorods into a 3D fibrous network of fused single-crystalline anatase nanowires. This architecture offers a high roughness factor, significant light scattering, and up to several orders of magnitude faster electron transport to reach a near-record-breaking conversion efficiency of 4.9%.

Synthesis of TiO2 nanotube arrays and its application in mini-3D dye-sensitized solar cells

Here we report a mini-three-dimensional dye-sensitized solar cell (3D DSSC) based on TiO2 nanotube arrays (TNAs). TNAs were directly grown on spiral-shaped titanium wire via a facile potentiostatic anodization. Furthermore, the TNAs film was characterized by scanning electron microscopy, x-ray diffraction and transmission electron microscopy, indicating that the annealed TNAs were composed of single-crystalline anatase particles. Unlike conventional flat DSSC, this mini-3D DSSC could easily hold liquid electrolyte due to the capillary force which facilitated sealing the cell. This mini-3D DSSC showed an energy conversion efficiency of 4.1% under the AM 1.5 condition, which was much higher compared with that (3.2%) of the backside illuminated TNAs based DSSC of the same projected area.

Preparation of Spherical Titania Nanoparticles by CO2 Laser Evaporation and Process‐Integrated Particle Coating

The CO2 laser vaporization (LAVA) method was used to prepare titania nanopowders. Because this versatile method does not require special precursors, a coarse anatase raw powder was applied as starting material. Powder samples produced under varied process parameters were characterized by transmission electron microscopy (TEM), X‐ray diffraction measurements, and Brunauer–Emmett–Teller surface area measurements. The laser‐generated powders consist of spherical, single crystalline and pure anatase nanoparticles, merely softly agglomerated by weak van der Waals forces. Using TEM analysis, the influence of the process parameters on the resulting particle size distribution was investigated. The results are discussed with respect to the particle formation by gas phase condensation. The potential of a process integrated, i.e. in situ, coating procedure for the surface modification of the anatase nanoparticles is demonstrated. As an exemplary representative of organic layer materials stearic acid was chosen. The organic coating was characterized by TEM and Raman spectrometry. Because of the unavoidable soft agglomeration the coating covers entire agglomerates rather than individual primary particles. Thus, the influence of the LAVA process parameters on the agglomerate sizes was systematically studied using a scanning mobility particle sizer.

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.