Anatase TiO2 Single Crystals Research Articles

Controllable Nanocarving of Anatase TiO2 Single Crystals with Reactive {001} Facets

Owing to its wide applications in photocatalysis, photo/ electrochromics, solar cells, sensors, and smart surface coatings, anatase titanium dioxide (TiO2) has attracted intensive research interests during the past decades. Most available and stable anatase TiO2 single crystals have truncated octahedral-bipyramid shapes, in which the majority of the surface area is normally surrounded by thermodynamically stable {101} facets, rather than the more reactive {001} facets. For a long period, tailored synthesis of {001}-faceted anatase TiO2 single crystals has been a great challenge. Recently, we made a breakthrough in the hydrothermal synthesis of anatase TiO2 single crystals with a large percentage (35–47%) of highly reactive {001} facets by using hydrofluoric acid (HF) as a specific morphology-controlling agent. Later, with the assistant of isopropanol (iPrOH)—a synergistic capping agent together with HF, anatase TiO2 single crystals with a higher ratio of {001} facets were prepared through a water–isopropanol solvothermal synthetic route. During these processes, HF was reported to act as a capping agent by reversing the relative stability of {001} and {101} facets and therefore promoting the growth of {001}faceted surfaces of anatase TiO2. By using similar strategies, a series of studies by research groups worldwide have demonstrated the syntheses and applications of anatase TiO2 with exposed reactive {001} facets. Unfortunately, few researchers have explored the etching effect of HF, which was traditionally considered a strong etching agent for metal oxides. Thus studying the chemical stability of {001}-faceted anatase TiO2 in the presence of HF is quite important and can help to understand the real function of HF during these important chemical processes. Herein, the dual role of HF—as capping agent and etching agent—is revealed through the controllable nanocarving of {001}-faceted anatase TiO2 single crystals. This process was realized through thermal treatment of anatase TiO2 single crystals with 35% of {001} facets by using HF-containing solutions or just by one-step solvothermal reactions of aqueous titanium tetrafluoride (TiF4) solutions in mixed solvents containing HF. In addition, a feasible mechanism is proposed to clarify the etching process involving HF and the growth process of the anatase TiO2 products with diverse morphologies. Figure 1a presents a typical SEM image of anatase TiO2 single crystals with 35% of reactive {001} facets, which was synthesized according to the method described in our earlier

Anatase TiO2 Crystals with Exposed High‐Index Facets

Put your best face forward: The performance of TiO2 anatase crystals in energy and environmental applications is normally correlated with the TiO2 crystal facets exposed, and increasing the percentage of highly reactive surfaces is extremely important. A new gas-phase oxidation process using TiCl4 as precursor now yields anatase TiO2 single crystals with primarily high-index {105} facets, which can cleave water photocatalytically. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

On the Unusual Properties of Anatase TiO2 Exposed by Highly Reactive Facets

As an important metal oxide, anatase titanium dioxide has been widely investigated because of its many promising properties. The properties of anatase TiO2 crystals are largely determined by exposed external surfaces. Since the breakthrough in synthesizing anatase TiO2 single crystals with a large percentage of highly reactive {001} facets in 2008, many unusual properties and applications of these {001} facets dominant in anatase TiO2 have been explored theoretically and experimentally, showing the industrial importance of this semiconductor material. This Perspective focuses on the theoretical simulations and application explorations of the unusual properties of anatase TiO2 bound by highly reactive facets. Research opportunities as well as the challenges for future research in this emerging frontier are also highlighted.

Graphite-like carbon deposited anatase TiO2 single crystals as efficient visible-light photocatalysts

Graphite-like carbon deposited single-crystal anatase TiO2 with exposed {001} facets was fabricated through a two-step solvothermal process by using glucose as carbon source. The physicochemical properties of the as-prepared samples were investigated by X-ray diffraction, Brunauer-Emmett-Teller, transmission electron microscopy, Raman, UV–vis diffuse reflectance spectra, electrochemical impedance spectroscopy and surface photovoltage spectroscopy. These results demonstrated that graphite-like carbon layers were deposited on the surface of TiO2 single-crystal nanosheets with exposed highly reactive {001} facets via the dehydration of glucose during the process of hydrothermal treatment. The loading of the graphite-like carbon layers could effectively extend the light absorption edge of the single-crystal anatase TiO2 nanosheets to visible light region and accelerate the separation of photo-generated electrons and holes, contributing an excellent visible-light driven photocatalytic performance to the graphite-like carbon deposited single-crystal anatase TiO2 nanosheets for the degradation of methyl orange.

Double‐Layered Photoanodes from Variable‐Size Anatase TiO2 Nanospindles: A Candidate for High‐Efficiency Dye‐Sensitized Solar Cells

Little and large: A double-layered photoanode constructed from small and large single-crystal anatase TiO2 nanospindles achieves 8.3 % energy-conversion efficiency in a dye-sensitized solar cell (see picture). One layer of the double-layer structure serves mainly to harbor numerous dye molecules, and the other primarily enhances light harvesting by multiple scattering.

Anatase TiO2 single crystals with exposed {001} and {110} facets: facile synthesis and enhanced photocatalysis

Anatase TiO(2) single crystals with exposed {001} and {110} facets have been successfully synthesized using a modified hydrothermal technique in the presence of hydrogen peroxide and hydrofluoric acid solution; these single crystals exhibited enhanced photocatalytic activities for degradation of Methylene Blue dye under ultraviolet light irradiation.

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.

Nanosized anatase TiO2 single crystals for enhanced photocatalytic activity

Nanosized anatase TiO(2) single crystals with 18% {001} facets have a raised conduction band minimum by 0.1 eV, and exhibit photocatalytic activity both for generating *OH radicals and for splitting water into hydrogen that is markedly superior--by factors of 5.6 and 8.2, respectively--to reference ca. 3 microm anatase TiO(2) with 72% {001} facets.

Anatase TiO2 single crystals with a large percentage of reactive facets

Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied. Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. A typical example is titanium dioxide (TiO2), which has promising energy and environmental applications. Most available anatase TiO(2) crystals are dominated by the thermodynamically stable {101} facets (more than 94 per cent, according to the Wulff construction), rather than the much more reactive {001} facets. Here we demonstrate that for fluorine-terminated surfaces this relative stability is reversed: {001} is energetically preferable to {101}. We explored this effect systematically for a range of non-metallic adsorbate atoms by first-principle quantum chemical calculations. On the basis of theoretical predictions, we have synthesized uniform anatase TiO(2) single crystals with a high percentage (47 per cent) of {001} facets using hydrofluoric acid as a morphology controlling agent. Moreover, the fluorated surface of anatase single crystals can easily be cleaned using heat treatment to render a fluorine-free surface without altering the crystal structure and morphology.

Absorption spectra of anatase TiO2 single crystals heat-treated under oxygen atmosphere

Anatase TiO2 single crystals large enough to measure the optical properties have been grown by chemical vapor transport reaction. As-grown crystals having blue color were heat-treated under oxygen atmosphere. The change in crystal color from blue through yellow to colorless depending on oxygen annealing was detected by optical absorption spectra. The blue color is due to free carrier absorption. The yellow color is originated from an oxygen vacancy which is a color center. Models are discussed for these change during the oxygen annealing.

Surface Investigations on Single Crystal Anatase TiO2

AbstractUtilizing a chemical transport reaction, we succeeded in growing large and clean anatase TiO2 single crystals whose surfaces could be characterized by standard physical and electro-chemical techniques. The examination of the structure of the clean (101) and (001) faces by low energy electron diffraction (LEED) and secondary electron imaging (SEI) showed that these surfaces are bulk terminated and thermodynamically stable. Impedance spectroscopy in aqueous solution revealed a slight difference in the flatband potential between the (101) and the (001) faces. This shift is also manifested in a different photocurrent onset potential and can be rationalized by a different water adsorption on the two surface structures. Voltammetry in aprotic solutions showed a different lithium insertion behavior for the two surfaces. This is explained by a different structural transparency of the anatase lattice in the two directions. Both findings favor the (001) over the (101) surface. These orientational dependencies may have some important technological relevance for the mesoscopic TiO2 films used in solar cells and lithium batteries.