Rutile Single Crystals Research Articles

Electron transfer dynamics of single quantum dots on the (110) surface of a rutile TiO2 single crystal

The interfacial electron transfer (IET) dynamics of single CdSe core/multilayer shell (CdS2MLZnCdS1MLZnS1ML) quantum dots (QDs) on the (110) surface of a rutile TiO2 single crystal and TiO2 nanoparticles have been compared. The fluorescence decay rates of single QDs on TiO2 are faster than those on glass, an insulating substrate, due to IET from the QDs to TiO2. Whereas the average IET rates are similar for QDs on the single crystal and nanoparticles, the distribution of IET rates is much broader in the latter, indicating a broad distribution of QD adsorption sites on the TiO2 nanoparticles.

Structural and Magnetic Properties of Ni-Implanted Rutile Single Crystals

The structural and magnetic properties of Ni-implanted rutile single crystals are discussed. Ni nanocrystals (NCs) are formed in TiO2 after ion implantation. Their crystalline sizes increase with increasing post-annealing temperature. Metallic Ni nanocrystals inside the TiO2 matrix are stable up to an annealing temperature of 1073K. The Ni NCs formed inside TiO2 are the major contribution to the measured ferromagnetism.

Ultraviolet-Induced Grafting of Alkenes to TiO2 Surfaces: Controlling Multilayer Formation

Photochemical grafting of alkenes has emerged as a versatile way to form functionalized surfaces and enables multistep surface chemistry on a range of substrates. However, the self-terminating nature of the reaction remains poorly understood. Here, we use X-ray photoelectron spectroscopy (XPS) to explore multilayer formation during photochemical grafting of functional alkenes on single-crystal rutile (110) TiO2 surfaces. We demonstrate that simple alkenes such as 1-hexene stop at 1 monolayer, but alkenes with an additional alcohol group easily form multilayers. Atomic force microscopy (AFM) and XPS studies show that the multilayers are highly conformal and that the alcohol groups remain chemically accessible even within multilayer films. Small amounts of water enhance multilayer formation by facilitating formation of •OH or other radical species that initiate oligomerization processes, while oxygen has little effect. We show that the multilayer formation can be reduced or eliminated by using small amounts of a radical scavenger or by modifying the molecular structure of the alkene. Overall these studies show that multilayer formation is controlled by the ability to form alkyl radicals under ultraviolet (UV) illumination.

Photodeposition of Ag or Pt onto TiO2 Nanoparticles Decorated on Step Edges of HOPG

Ordered linear arrays of titanium dioxide nanoparticles were fabricated on highly oriented pyrolytic graphite utilizing a step edge decoration method. Ag- or Pt-based nanoparticles were then photodeposited onto the titanium dioxide nanoparticles (∼18 nm) to simultaneously verify photocatalytic activity and to demonstrate a viable route to load the titanium dioxide nanoparticles with metals. Scanning electron microscopy and atomic force microscopy determined the morphology, size, and distribution of the particles. X-ray photoelectron spectroscopy confirmed the identity of the titanium dioxide nanoparticles, and transmission electron microscopy showed that some of the particles were rutile single crystals. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy determined the chemical composition of the metal-based nanoparticles selectively loaded on the linear arrays of titanium dioxide nanoparticles.

Synthesis and photocatalytic activity of sea urchin-shaped rutile TiO 2 nanocrystals

Sea urchin-shaped rutile nanostructures (SUR NSs) with abundant {110} surfaces are synthesized at 60 °C under atmospheric pressure. The SUR NSs were ~ 420 nm in diameter and contain a number of needle-like rutile single crystals grown parallel to the (110) face from a single nucleus in a radial fashion. The photocatalytic activity of the SUR NSs is significantly higher than that of commercial rutile nanoparticles with the same specific surface area. This demonstrates that {110} facets are effective in enhancing photoactivity.

Rutile TiO2 microspheres with exposed nano-acicular single crystals for dye-sensitized solar cells

Uniquely structured rutile TiO2 microspheres with exposed nano-acicular single crystals have been successfully synthesized via a facile hydrothermal method. After calcination at 450 °C for 2 h, the rutile TiO2 microspheres with a high surface area of 132 m2/g have been utilized as a light harvesting enhancement material for dye-sensitized solar cells (DSSCs). The resultant DSSCs exhibit an overall light conversion efficiency of 8.41% for TiO2 photoanodes made of rutile TiO2 microspheres and anatase TiO2 nanoparticles (mass ratio of 1:1), significantly higher than that of pure anatase TiO2 nanoparticle photoanodes of similar thickness (6.74%). Such a significant improvement in performance can be attributed to the enhanced light harvesting capability and synergetic electron transfer effect. This is because the photoanodes made of rutile TiO2 microsphere possess high refractive index which improves the light utilisation efficiency, suitable microsphere core sizes (450–800 nm) to effectively scatter visible light, high surface area for dye loading, and synergetic electron transfer effects between nanoparticulate anatase and nano-acicular rutile single crystals phases giving high electron collection efficiency. Open image in new window

Wettability conversion and surface friction force variation of polycrystalline rutile ceramics under UV illumination

Photoinduced surface friction variations of the polished surface of polycrystalline rutile ceramics under UV illumination in different atmospheres was evaluated using FFM. When UV was irradiated onto the rutile surface, the surface friction force first increased and then decreased. Once the water contact angle of the rutile surface reached the lower limit, the surface friction force increased gradually. Increased friction force on the surface in the early stage of UV illumination was attributable to decomposition of the surface organic contaminant and the resultant increase of surface energy from the fitting of calculated contact angles to practical values. However, the friction force decrease and gradual increase after a certain period of UV illumination are attributed to the lubrication effect and either a capillary effect or the increased adsorbed water layer by humidity control. From the force curve measurement on the highly hydrophilic rutile single crystal (1 0 0), the repulsion force was detected from 2 to 3 nm separation distance.

Effects of the diameter of rutile (TiO 2) single crystals grown using tilting-mirror-type infrared heating image furnace on solid–liquid interface and etch pit density

The effects of the diameter of a rutile (TiO 2) crystal on interface shape during TiO 2 crystal growth by the floating zone (FZ) method using a tilting-mirror-type image furnace and on the etch pit density (EPD) of rutile single crystals were examined. To investigate the diameter effect on the interface shape, the grown crystal diameter was varied from 6 to 11 mm at two different tilting angles ( θ) of 0° and 20°, respectively. The interface shape was characterized by the convexity ( h/r) of interface, where h and r represent the height of the interface and the radius of the grown crystals. The h/r of the crystal–melt interface decreased from 0.15 to 0.0 with an increase in the crystal diameter from 6.5 to 11.2 mm at θ=20°, whereas it increased from 0.45 to 0.58 with an increase in the crystal diameter from 6.0 to 11.0 mm at θ=0°. This result suggests that the molten zone becomes stable at θ=20° for the growth of crystals with a larger diameter. To investigate the crystal diameter effect on the EPD, rutile single crystals with diameters of 10, 16 and 19 mm were grown at θ=20°. The EPDs at the center were 1.0−1.4×10 4 cm −2, which were much smaller than those at the periphery, i.e., 8.0−13.0×10 4 cm −2, for all the grown crystals with different diameters.

Synthesis of a Nitrogen-Doped TiO2 (Rutile) Single Crystal and Its p-Type Behaviors

We have synthesized nitrogen-doped (N-doped) TiO2 (rutile) single crystals by annealing in a 1.0 MPa N2 atmosphere after reduction treatment. Transmittance, electrical resistivity, and sample weight decreased with the reduction treatment, but recovered after the subsequent N-doping process. The N 1s X-ray photoelectron spectroscopy (XPS) signal disappeared after 8 min of Ar sputtering. Diodelike behaviors were observed in the voltage–current (V–I) characteristics of the TiO2–metal contact for the as-received TiO2 and N-doped TiO2. They demonstrated that the as-received TiO2 is an n-type semiconductor and the N-doped TiO2 is a p-type semiconductor. Photoinduced voltage was measured under irradiation with a Xe lamp. The sign of the photoinduced voltage in the N-doped TiO2 was found to be opposite to that of the as-received or reduced TiO2, which also indicates that the N-doped TiO2 is a p-type semiconductor. We discuss the electronic states of dopant nitrogen atoms.

Transforming insulating rutile single crystal into a fully ordered nanometer-thicktransparent semiconductor

Rutile single crystals treated with ion-beam preferential etching (IBPE) areinvestigated with electrical transport and transmission electron microscopy.The initially insulating single crystals show the formation of an oxygenvacancy-rich, highly ordered, thin conducting layer, below a crystalline rutileTiO2 surface layer. Carrierconcentrations of 1019 cm − 3 and very high mobilities of the order of300 cm2 V − 1 s − 1 are observed in the nanolayers. The observations indicate that rutile single crystals can beeffectively transformed into controlled conducting material using IBPE for creating a newbreakthrough in transparent conducting media.

Differences in adsorption behavior of N3 dye on flat and nanoporous TiO2 surfaces

The adsorption of N3 dye on two types of TiO2 surfaces (rutile single crystals and anatase nanocrystalline films) was studied. We used highly sensitive absorption spectroscopy to acquire absorption spectra of sub-monolayer amounts of dye adsorbed on rutile TiO2 single crystals. We found that absorption spectra were not sensitive to dye amount in single crystals, whereas the peak shifted toward longer wavelength region with increasing dye amount in nanocrystalline films. We also found that much less N3 dye was adsorbed on the nanoporous surface than on the flat (single-crystal) surface. Possible reason for these experimental findings will be discussed.

Reduced Etch Pit Density of Rutile (TiO2) Single Crystals by Growth Using a Tilting-Mirror-Type Infrared Heating Image Furnace

Rutile single crystals were grown by the floating zone (FZ) method using a tilting-mirror-type image furnace. The tilting angle (θ) of the mirrors during the growth was changed from 0 to 10 and 20° to examine the effects of θ on crystal quality. The etch pit density (EPD) at the center of the crystals was low compared with that at the periphery. The EPDs at the periphery and center of the crystals grown at θ = 0° were (11−16) × 104 and 5 × 104 cm−2, respectively. Both values significantly decreased with an increase in θ. In the crystal grown at θ = 20°, the EPDs at the periphery and center were 7.9 × 104 and 1.5 × 104 cm−2, respectively. In particular, at the center of the crystals, the EPD for θ = 20° was 30% of that for θ = 0°. This result suggests that the quality of rutile crystals can be improved by tilting the focusing mirrors used in crystal growth.

Effect of ultraviolet and x-ray radiation on the work function of TiO2 surfaces

The work functions of nanocrystalline anatase (TiO2) thin films and a rutile single crystal were measured using photoemission spectroscopy (PES). The nanocrystalline titanium dioxide films were deposited in-vacuum using electrospray thin film deposition. A comparison between ultraviolet photoemission spectroscopy (UPS) and low intensity x-ray photoemission spectroscopy (LIXPS) work function measurements on these samples revealed a strong, immediate, and permanent work function reduction (>0.5 eV) caused by the UPS measurements. Furthermore, it was found that regular XPS measurements also reduce the work function after exposure times ranging from seconds to minutes. These effects are similar in magnitude to artifacts seen previously on indium tin oxide (ITO) substrates characterized with XPS and UPS, and are likely related to the formation of a surface dipole through the photochemical hydroxylation of oxygen vacancies present on the TiO2 surface.

Surface residual stress dependence on photoinduced highly hydrophilic conversion and back-reaction in the dark of rutile single crystals

The wettability changes of TiO(2) surface, photo-induced hydrophilic conversion and back-reaction in the dark, were evaluated using rutile (100) and (001) single crystals with diamond polishing (DP) and chemical mechanical polishing (CMP) treatments. Dynamic hardness measurements indicated that the DP surface had a residual compressive stress; however, the CMP surface did not. The rate of hydrophilic conversion was greatly suppressed (approximately one fourteenth) on the DP surface compared to the CMP surface, showing that the photoinduced hydrophilicity was greatly suppressed on the surface with compressive stress although the number of photogenerated carriers at the DP surface was estimated to decrease to only ca. half that at the CMP surface. In addition, when a heat treatment relaxed the compressive stress, the hydrophilicity was greatly increased. The back-reaction in the dark, i.e., the degradation of the photo-produced hydrophilic state, was ca. two times faster on the DP surface with compressive stress. The results strongly suggest that the pressure effect caused by the compressive stress could be the main reason for the degradation of the hydrophilicity and acceleration of the back reaction.

Electron spin resonance investigation of the substitution of Fe3+ for Ti4+ ions in rutile TiO2 single crystal

A Fe doped rutile TiO2 single crystal is grown in an O2 atmosphere by the floating zone technique. Electron spin resonance (ESR) spectra clearly demonstrate that Fe3+ ions are substituted for the Ti4+ ions in the rutile TiO2 matrix. Magnetization measurements reveal that the Fe:TiO2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K. The Fe3+ ions possess weak magnetic anisotropy with an easy axis along the c axis. The annealed Fe:TiO2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.

High-temperature ferromagnetism in Co-implanted TiO2 rutile

We report on structural, magnetic and electronic properties of Co-implanted TiO2(1 0 0) rutile single crystals for different implantation doses. Strong ferromagnetism at room temperature and above is observed in TiO2 rutile plates after cobalt ion implantation, with magnetic parameters depending on the cobalt implantation dose. While the structural data indicate the presence of metallic cobalt clusters, the multiplet structure of the Co L3 edge in the XAS spectra provides evidence that a sizeable portion of the dopants occupy substitutional Co2+ sites. The detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted TiO2 plates. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the rutile phase, and the second one is a superparamagnetic phase which originates from the formation of metallic cobalt clusters in the implanted region. Using x-ray resonant magnetic scattering, the element specific magnetizations of cobalt, oxygen and titanium in Co-implanted TiO2 single crystals are investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. Anomalous Hall effect measurement indicates n-type carriers in Co-implanted TiO2 rutile. The interaction mechanism, which leads to ferromagnetic ordering of substituted cobalt ions in the host matrix, is also discussed.

Ferromagnetism induced in rutile single crystals by argon and nitrogen implantation

The magnetic properties of Ar- and N-implanted single crystallineTiO2 rutile were studied and correlated with the induced disorder in order toclarify the influence of defects in the magnetic behaviour. Nominal fluences of1 × 1017 and2 × 1017 cm−2 and an implantation energy of 100 keV were used. The as-implanted single crystalsexhibited ferromagnetic behaviour up to 380 K. Annealing at 1073 K induced recovery of thelattice structure and the decrease of the ferromagnetic moment in the case of Ar-implantedsamples, but the magnetic moment did not change significantly for the samples implantedwith nitrogen. No impurities, other than the implanted species were detected andconsequently the ferromagnetic behaviour is attributed to defects created duringimplantation, which in the case of nitrogen remained in the lattice even after partialrecovery of the structure.

Bi- and Uniaxially Oriented Growth and Plasmon Resonance Properties of Anisotropic Ag Nanoparticles on Single Crystalline TiO2 Surfaces

Anisotropic Ag nanoparticles were grown epitaxially on UV-irradiated rutile TiO2(100) and (110) single crystals in an aqueous AgNO3 solution by photoelectrochemical reduction of Ag+. Under different conditions (0.1−10 mW cm−2 UV irradiation for 1 min−21 h in the presence or absence of ethanol or acetaldehyde or both) and on different substrates, biaxially and uniaxially orientated crystalline nanorods, uniaxially oriented triangular nanoplates (i.e., so-called nanoprisms), or square pyramids were grown preferentially, without need for a masking layer or alignment step. This represents a new route for the fabrication of polarized, visible−near-infrared sensitizing thin layers on TiO2.

Effect of Oxygen Content on Dielectric Properties of Float Zone Grown Titanium Dioxide (TiO2) Crystal

Dielectric studies have been carried out to see the effect of oxygen content on two different sets of rutile (TiO2) single crystals grown by float zone technique. One set is having pale yellow colour which is the correct characteristic colour of rutile crystals whereas another have dark blue colour which contains oxygen deficiency. The variation of dielectric constant (ϵ′), the dielectric loss (tan δ) and the electric conductivity (σ) at different temperature and frequency of the applied electric field was studied for both these crystals. There is significant change in the values of dielectric constant for blue sample as compared to yellow sample. The dielectric constant in most of the region for oxygen deficient crystal (blue colour) is large as compared to true stiochoimetric ratio (pale yellow) of the rutile crystal. Based on conductivity behaviour, it was found that the correlated barrier hopping (CBH) model is the most predominant hopping mechanism in the case of the yellow crystal, whereas in the case of the blue crystal, hopping may be due to overlap of large polarons.

Analysis of Dielectric Response of TiO2 in Terahertz Frequency Region by General Harmonic Oscillator Model

The terahertz range dielectric response of polycrystalline TiO2 (rutile) ceramics and rutile single crystals were investigated by analyzing infrared-active phonons by both Fourier-transform far-infrared (FT-FIR) reflectivity measurement and terahertz time-domain spectroscopy (THz-TDS). The dielectric functions at room temperature measured by THz-TDS are well expressed using the general harmonic oscillator model. We found that the imaginary part of the dielectric permittivity of the TiO2 ceramic is threefold higher than that of the rutile single crystal at 300 K. The large dielectric loss is attributed to the additional damping of the A2u mode or/and an increase in the phonon damping. The low-temperature dielectric loss of the rutile single crystals is exceedingly low. However, TiO2 ceramics shows little temperature dependence. These results indicate that extrinsic scattering significantly affects the dielectric loss of the TiO2 ceramics in the THz region.