Rutile Single Crystals Research Articles

Constructed Single-Crystal Rutile TiO2 Cluster and Plasmon Synergistic Effect for Dye-Sensitized Solar Cells

We demonstrate a method for incorporating plasmon metallic nanoparticles in hierarchical rutile TiO2 clusters (RTC) assembled from single-crystal nanospindles. The RTC could efficiently improve the diffusion of the photoelectrons, which can be ascribed to the improvement of the connectivity by bridging the neighbouring microflowers through the single-crystal nanospindles. But not all the nanospindles are tightly interconnected, hence organic colloid has been prepared for post-treatment of the device based on RTC by the generation of TiO2 nanoparticles. When added into Au nanoparticles, localized electric fields can be produced, because Au can excite dye molecules more intensively than incident far-field light. The surface plasmon synergistic effect had been investigated by Uv-vis absorption spectrum of Au@ organic colloid and the relative change of the IPCE. As a result, the cell based on RTC exhibits an overall conversion efficiency of 7.68%, indicating a 17% promotion compared with that derived from commercial P25 (6.58%) which could be ascribed to faster electron transfer of single-crystal nanospindles. With the Au nanoparticles incorporation in RTC, the device achieves a conversion efficiency of 9.15%, resulting in a 11% increase compared to the RTC device post-treated by organic colloid without Au nanoparticles (8.24%), which is attributed to the surface plasmon synergistic of Au nanoparticles.

Carrier lifetime measurements on various crystal faces of rutile TiO2 single crystals

Carrier lifetimes in rutile TiO2 single crystals were measured by the microwave photoconductivity decay method. The measurement results showed that carrier lifetimes in rutile TiO2 single crystals were dominated by the surfaces recombination but not recombination centers in the bulk. The surface recombination at the (110) face is the slowest among the crystal faces, and thus photocatalytic activity would be relatively high for a crystal with the (110) face.

Investigations of the spin Hamiltonian parameters and local structures for Fe3+, Cr3+ and Mn4+ in rutile TiO2 single crystal

The spin Hamiltonian parameters (g factors, hyperfine structure constants and zero-field splittings (ZFSs)) and local structures for the rhombic substitutional Fe3+, Cr3+ and Mn4+ in rutile (TiO2) single crystal are theoretically investigated from the high order perturbation calculations based on the cluster approach including both the crystal-field (CF) and charge-transfer (CT) contributions to the g factors and hyperfine structure constants. The impurity centers are found to undergo the local axial distortions ΔZ (≈0.22, 0.14 and –0.18Ǻ) and the planar bond angle variations Δφ (≈4.3, 5.9 and 0.2°) for Fe3+, Cr3+ and Mn4+, respectively. The signs for ZFSs D and E are analyzed in the light of those for ΔZ and rhombic distortion angle δφ (=φ′–π/4) related to an ideal octahedron. The magnitudes of ΔZ and Δφ are conveniently illustrated by the axial and perpendicular ZFS relative variations ΔF and ΔG for the deviations of D and E based on the local distortion parameters from those (DH and EH) based on the host structural data of Ti4+ site. The validity of the above local structures is discussed in view of size and charge mismatch of the various impurity centers. The CT contributions to g-shift are opposite in sign and about 13–56% in magnitude compared with the CF ones, indicating the increasing importance (Cr3+<Fe3+<Mn4+). The CT contributions to hyperfine structure constants are the same in sign and about 19–22% in magnitude with respect to the corresponding CF contributions.

Resistive Switching of a Quasi‐Homogeneous Distribution of Filaments Generated at Heat‐Treated TiO2 (110)‐Surfaces

Resistive switching of thermally treated rutile single crystals with (110) orientation is studied. A heat treatment procedure is developed that involves reduction and oxidation steps and allows to induce low resistance states in switchable regions at the surface by low‐voltage electrical stimulation with the conducting tip of an atomic force microscope. This way, it is possible to electrically imprint quasi‐homogeneous switchable regions over several square micrometers. These regions are identified to consist of nanofilaments crossing the surface with a density of around 1012 cm−2, much higher in density than observed for single crystals so far. Experimental evidence is given that these nanofilaments are not related to inherent structural imperfections such as dislocations, but may originate from the linear agglomeration of oxygen vacancies as predicted by theory. Ab initio calculations and electrical simulations are performed to analyze the filamentary structures and their network in the effort to explain the observed filamentary switching of heat‐treated single‐crystalline TiO2.

Optical Properties of Rutile Single Crystals Grown Under Pressure by Tilting-Mirror-Type Floating Zone Method

Rutile single crystals with high quality and high transparency were grown successfully by tilting-mirror-type floating zone (TMFZ) technique. The grown crystals were characterized by polarized optical microscopy, X-ray diffractometer, UV-visible spectrophotometer and etch pits density measurement. The effect of applied pressure during growth on the optical properties and defect density of the rutile crystals were investigated. It was found that the rutile crystals grown under the high oxygen pressure are bright and transparent due to the lower concentration of defects, but rutile crystals grown at ambient or lower oxygen pressure exhibit deep blue color due to presence of defects such as dislocations and grain boundaries. The crystals grown under high oxygen pressure contains no low-angle grain boundaries, indicating that it can be used for high efficiency optical devices.

UV-black rutile TiO2: An antireflective photocatalytic nanostructure

This work presents an experimental study on the specific quantitative contributions of antireflective and effective surface areas on the photocatalytic and antibacterial properties of rutile TiO2 nanospikes. They are studied when continuously distributed over the whole surface and when integrated into well-defined microstructures. The nanospikes were produced following MeV ion beam irradiation of bulk rutile TiO2 single crystals and subsequent chemical etching. The ion beam irradiation generated embedded isolated crystalline nanoparticles inside an etchable amorphous TiO2 layer, and nanospikes fixed to the not etchable TiO2 bulk substrate. The produced nanospikes are shown to resist towards aggressive chemical environments and act as an efficient UV antireflective surface. The photocatalytic activity experiments were performed under the ISO 10678:2010 protocol. The photonic and quantum efficiency are reported for the studied samples. The combined micro- and nanostructured surface triples the photonic efficiency compared to the initial flat surface. Results also revealed that the antireflective effect, due to the nanostructuring, is the dominating factor compared to the increase of surface area, for the observed photocatalytic response. The obtained results may be taken as a general strategy to design and precisely evaluate photoactive nanostructures.

Controllable synthesis of rutile titania with novel curved surfaces

We report a facile organic-free hydrothermal strategy to fabricate rutile TiO2 single crystals with a new type of curved surfaces which consist of high-energy-facet-bound nanohills rather than quasi-continuous miller-index microfacets.

The pH dependence of OH radical formation in photo-electrochemical water oxidation with rutile TiO2 single crystals.

It has been believed that photocatalytic oxidation in water proceeds with the reaction of OH radicals generated on the photocatalysts. To explore the actual contribution of OH radicals to photocatalytic oxidation, OH radicals were detected by fluorescence probe method in photoelectrolysis with rutile TiO2 of (100) and (110) facets. The effect of hydrogen peroxide on the OH radical formation at pH 6.7 was investigated to confirm the relevant intermediate which was suggested in our previous report for water oxidation. In alkaline solutions at pH 9.6 and 12.5, the current efficiencies of OH radical formation were 0.01-0.05%, which are far smaller than those at pH 6.7 (0.2-0.6%) due to the deprotonation of the reaction intermediate as confirmed by FT-IR measurements. These experimental results support a plausible reaction mechanism that the surface Ti-O-O-Ti structure is an intermediate of the water oxidation process, by which mechanism the O2 production becomes favorable in alkaline solution.

Ion implantation induced blistering of rutile single crystals

The rutile single crystals were implanted by 200keV He+ ions with a series fluence and annealed at different temperatures to investigate the blistering behavior. The Rutherford backscattering spectrometry, optical microscope and X-ray diffraction were employed to characterize the implantation induced lattice damage and blistering. It was found that the blistering on rutile surface region can be realized by He+ ion implantation with appropriate fluence and the following thermal annealing.

One-step synthesis of TiO2 nanorod arrays on Ti foil for supercapacitor application

Titanium dioxide (TiO2) nanorod arrays grown directly on Ti metal foil were prepared by a facile one-step hydrothermal method, in which the Ti foil serves as both substrate and precursor. The nanorods are tetragonal rutile single crystal with growth orientation along the [001] direction. The electrochemical properties of the TiO2 nanorod arrays were systematically investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy using a three-electrode system. As a result, the TiO2 nanorod arrays exhibit good areal specific capacitance and excellent cyclic stability by retaining more than 98% of the initial specific capacitance after 1000 cycles. In addition, a good flexibility of the Ti foil with TiO2 nanorod arrays was demonstrated by the stable electrochemical performance under different bending angles, which indicates that TiO2 nanorod arrays grown on Ti foil could be a promising electrode material for flexible supercapacitor application.

Optical Properties of Al- and Zr-Doped Rutile Single Crystals Grown by Tilting-Mirror-Type Floating Zone Method and Study of Structure-Property Relationships by First Principle Calculations

High quality and transparent single crystals of undoped rutile TiO2, Al-doped rutile (Al : TiO2), and Zr-doped rutile (Zr : TiO2) have been grown successfully by tilting-mirror-type floating zone (TMFZ) using travelling solvent floating (TSFZ) technique. The effect of doping on the electronic and optical properties of rutile has been studied experimentally as well as by simulation calculations. The effect of doping on the quality of crystals was also investigated by observing optical micrograph and measuring etch pits density that reveals the presence of defects. Undoped rutile crystals were dark blue and comprised many low-angle grain boundaries. Al+3 and Zr+4 ions pin down the migration of dislocations during the cooling and create oxygen vacancies. Doping of the impurities would improve the electronic and optical properties of rutile. The elastic properties might be changed for doping of the impurities in the rutile crystals.

Insights from arsenate adsorption on rutile (110): grazing-incidence X-ray absorption fine structure spectroscopy and DFT+U study.

Insights into the bonding of As(V) at the metal oxide/aqueous interface can further our understanding of its fate and transport in the environment. The motivation of this work is to explore the interfacial configuration of As(V) on single crystal rutile (110) using grazing-incidence X-ray absorption fine structure spectroscopy (GI-XAFS) and planewave density functional calculations with on-site repulsion (DFT+U). In contrast to the commonly considered corner-sharing bidentate binuclear structure, tetrahedral As(V) binds as an edge/corner-sharing tridentate binuclear complex on rutile (110), as evidenced by observation of three As-Ti distances at 2.83, 3.36, and 4.05 Å. In agreement with the GI-XAFS analysis, our DFT+U calculations for this configuration resulted in the lowest adsorption energy among five possible alternatives. In addition, the electron density difference further demonstrated the transfer of charge between surface Ti atoms and O atoms in AsO4. This charge transfer consequently induced the formation of a chemical bond, which is also confirmed by the partial density of states analysis. Our results may shed new light on coupling the GI-XAFS and DFT approaches to explore molecular-scale adsorption mechanisms on single crystal surfaces.

Defect types and room temperature ferromagnetism in N-doped rutile TiO2 single crystals

The magnetic properties and defect types of virgin and N-doped TiO2 single crystals are probed by superconducting quantum interference device (SQUID), X-ray photoelectron spectroscopy (XPS), and positron annihilation analysis (PAS). Upon N doping, a twofold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancy, rather to unpaired 3d electrons in Ti3+, arising from titanium vacancies and the replacement of O with N atoms in the rutile structure. The production of titanium vacancies can enhance the room temperature ferromagnetism (RTFM), and substitution of O with N is the onset of ferromagnetism by inducing relatively strong ferromagnetic ordering.

Selective binding of oligonucleotide on TiO2 surfaces modified by swift heavy ion beam lithography

We have used swift heavy-ion beam based lithography to create patterned bio-functional surfaces on rutile TiO2 single crystals. The applied lithography method generates a permanent and well defined periodic structure of micrometre sized square holes having nanostructured TiO2 surfaces, presenting different physical and chemical properties compared to the surrounding rutile single crystal surface. On the patterned substrates selective binding of oligonucleotides molecules is possible at the surfaces of the holes. This immobilisation process is only being controlled by UV light exposure. The patterned transparent substrates are compatible with fluorescence detection techniques, are mechanically robust, have a high tolerance to extreme chemical and temperature environments, and apparently do not degrade after ten cycles of use. These qualities make the patterned TiO2 substrates useful for potential biosensor applications.

Charge and magnetic states of rutile TiO2 doped with Cr ions

We observe that the electronic and magnetic properties of Cr-doped rutile single crystals are highly dependent on growth conditions. The ferromagnetic component of magnetic susceptibility is observed to be enhanced for samples grown under oxygen-rich conditions. To understand the charge state of Cr dopants and their role in response to an external magnetic field, we carry out density functional theory calculations for Cr-doped rutile . Using the results of formation energy calculations in the presence of oxygen vacancies and Cr atom substitution at the Ti sites, we demonstrate that the state is a source of Curie–Weiss-type magnetic response, whereas the defect states contribute to the ferromagnetic component. We also provide the electronic structures of various defect configurations and attempt to explain the optical and electronic properties of the Cr-doped system.

Why is anatase a better photocatalyst than rutile? - Model studies on epitaxial TiO2 films

The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high –quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5 nm thick, while rutile films reach their maximum activity for ~2.5 nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.

Defect annealing in Mn/Fe-implanted TiO2 (rutile)

A study of the annealing processes and charge state of dilute Fe in rutile TiO2 single crystals was performed in the temperature range 143–662 K, utilizing online 57Fe emission Mössbauer spectroscopy following low concentrations (<10−3 at%) implantation of 57Mn (T1/2 = 1.5 min). Both Fe3+ and Fe2+ were detected throughout the temperature range. Three annealing stages were distinguished: (i) a broad annealing stage below room temperature leading to an increased Fe3+ fraction; (ii) a sharp annealing stage at ∼330 K characterized by conversion of Fe3+ to Fe2+ and changes in the hyperfine parameters of Fe2+, attributed to the annealing of Ti vacancies in the vicinity of the probe atoms; and (iii) an annealing stage in the temperature range from 550 to 600 K, where all Fe ions are transformed to Fe3+, attributed to the annealing of the nearby O vacancies. The dissociation energy of MnTi–VO pairs was estimated to be 1.60(15) eV. Fe2+ is found in an environment where it can probe the lattice structure through the nuclear quadrupole interaction evidencing the extreme radiation hardness of rutile TiO2. Fe3+ is found in a paramagnetic state with slow spin–lattice relaxation which follows a ∼Tn temperature dependence with 4.1 < n < 6.3 at T > 350 K.

Manipulating solar absorption and electron transport properties of rutile TiO2 photocatalysts via highly n-type F-doping

In this work, we report a facile and nontoxic one-pot hydrothermal method for synthesizing F-doped rutile single crystalline TiO2 with tuneable solar absorption. The optical band gap of the catalyst can be easily manipulated from 3.05 to 2.58 eV via altering the initial F : Ti molar ratio of reaction precursors. The photoanodes made of rutile TiO2 single crystals with appropriate F-doping concentration show excellent photoelectrocatalytic activity towards water oxidation under ultraviolet and visible light illumination. The best photoelectrocatalytic performance under UV irradiation can be obtained by F-doped TiO2 with an initial F : Ti molar ratio of 0.1, which is almost 15 times higher than that of un-doped TiO2. Further, the F-doped TiO2 photoanodes also exhibit superior photoelectrocatalytic activity under visible irradiation, and the best performance can be achieved by F-doped TiO2 photoanode with an initial F : Ti molar ratio of 0.05. The superior photoelectrocatalytic activity could be attributed to the highly n-type dopant introduced by fluorine, which significantly tunes the electrical conductivities and band structures of the resulting TiO2 photoanodes, and thus the photoelectrocatalytic activities under both UV and visible irradiation. Different techniques have been employed to characterize the electrical conductivity, charge carrier density and band structures of the F-doped rutile TiO2 films, such as photoelectrochemical method, electrical impedance spectroscopy (EIS) measurements, Mott–Schottky plots and XPS valence band spectra.

Highly mobile and reactive state of hydrogen in metal oxide semiconductors at room temperature

Hydrogen in metal oxides usually strongly associates with a neighboring oxygen ion through an O-H bond and thus displays a high stability. Here we report a novel state of hydrogen with unusually high mobility and reactivity in metal oxides at room temperature. We show that freshly doped hydrogen in Nb2O5 and WO3 polycrystals via electrochemical hydrogenation can reduce Cu2+ ions into Cu0 if the polycrystals are immersed in a CuSO4 solution, while this would not happen if the hydrogenated polycrystals have been placed in air for several hours before the immersion. Time-dependent studies of electrochemically hydrogenated rutile single crystals reveal two distinct states of hydrogen: one as protons covalently bonded to oxygen ions, while the other one is highly unstable with a lifetime of just a few hours. Observation of this mobile and reactive state of hydrogen will provide new insight into numerous moderate and low temperature interactions between metal oxides and hydrogen.

Mesoporous TiO2 Single Crystals: Facile Shape-, Size-, and Phase-Controlled Growth and Efficient Photocatalytic Performance

In this work, we have succeeded in preparing rutile and anatase TiO2 mesoporous single crystals with diverse morphologies in a controllable fashion by a simple silica-templated hydrothermal method. A simple in-template crystal growth process was put forward, which involved heterogeneous crystal nucleation and oriented growth within the template, a sheer spectator, and an excluded volume, i.e., crystal growth by faithful negative replication of the silica template. A series of mesoporous single-crystal structures, including rutile mesoporous TiO2 nanorods with tunable sizes and anatase mesoporous TiO2 nanosheets with dominant {001} facets, have been synthesized to demonstrate the versatility of the strategy. The morphology, size, and phase of the TiO2 mesoporous single crystals can be tuned easily by varying the external conditions such as the hydrohalic acid condition, seed density, and temperature rather than by the silica template, which merely serves for faithful negative replication but without interfering in the crystallization process. To demonstrate the application value of such TiO2 mesoporous single crystals, photocatalytic activity was tested. The resultant TiO2 mesoporous single crystals exhibited remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single-crystal nature, and the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. It was found that {110} facets of rutile mesoporous single crystals can be considered essentially as reductive sites with a key role in the photoreduction, while {001} facets of anatase mesoporous single crystals provided oxidation sites in the oxidative process. Such shape- and size-controlled rutile and anatase mesoporous TiO2 single crystals hold great promise for building energy conversion devices, and the simple solution-based hydrothermal method is extendable to the synthesis of other mesoporous single crystals beyond TiO2.