Investigation of the photoelectronic properties of nanocrystalline carbon- and nitrogen-doped titanium dioxide

This paper reports the results of the study of photoelectronic properties of nanocrystalline titanium dioxide doped with carbon and nitrogen. Photo-generated paramagnetic radicals were detected and identified in nanocrystalline titanium dioxide by the electron paramagnetic-resonance method. The concentrations of the radicals versus the carbon and oxygen content were determined in the dark and under light exposure. The presence of C and CO ₂ ^? -radicals in carbon-doped titanium dioxide and mainly N and NO radicals in nitrogen-doped titanium dioxide is shown. The concentration of the radicals increases under light exposure, which indicates the recharging processes of electronic states in the bandgap, due to the presence of a dopant, accompanied by generation of paramagnetic particles. The electron paramagnetic resonance data correlate with the kinetics of formaldehyde decomposition on the titanium dioxide samples under study.     

Photoactive lead ions in ferroelectric Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> and paramagnetic resonance

The recharging of lead matrix ions upon exposure to light in pure, doped, and nonstoichiometric lead germanate crystals has been studied using electron paramagnetic resonance. It has been shown that the maximum concentration of metastable Pb³⁺ ions is achieved in crystals doped with chlorine, fluorine, titanium, and in samples with excess lead oxide. The annealing activation energy and the parameters of the superhyperfine interaction of Pb³⁺ paramagnetic centers have been determined.     

Radiolabelling of TiO<Subscript>2</Subscript> nanoparticles for radiotracer studies

Industrially manufactured titanium dioxide nanoparticles have been successfully radiolabelled with ⁴⁸V by irradiation with a cyclotron-generated proton beam. Centrifugation tests showed that the ⁴⁸V radiolabels were stably bound within the nanoparticle structure in an aqueous medium, while X-ray diffraction indicated that no major structural modifications to the nanoparticles resulted from the proton irradiation. In vitro tests of the uptake of cold and radiolabelled nanoparticles using the human cell line Calu-3 showed no significant difference in the uptake between both batches of nanoparticles. The uptake was quantified by Inductively Coupled Plasma Mass Spectrometry and high resolution γ-ray spectrometry for cold and radiolabelled nanoparticles, respectively. These preliminary results indicate that alterations to the nanoparticles’ properties introduced by proton bombardment can be controlled to a sufficient extent that their further use as radiotracers for biological investigations can be envisaged and elaborated.     

Red photoluminescence mechanism in SnO<Subscript>2</Subscript> nanostructures

Tin dioxide nanoribbons were fabricated for clarifying the origin of the red photoluminescence band. It is found that the red band abruptly decreases its intensity after annealing the nanoribbons in O₂. The time-resolved photoluminescence decay curve shows that the red band has a luminescence lifetime of ns. The electron spin resonance spectrum discloses that the red band is related to a kind of combined oxygen-vacancy (V _o⁺ and V _o⁺⁺) centers. Spectral analysis and theoretical calculation confirm that the red band arises from a transition between the combined oxygen-vacancy defect states in the band gap.     

Anomalous magnetism of the nanocrystalline oxide TiO<Subscript>2</Subscript> surface

The magnetic properties of an oxygen-deficient nanocrystalline undoped titanium dioxide synthesized by the gas-phase, electric-explosion, and chemical method have been studied. The defect state was controlled using reduction treatments in vacuum or in a hydrogen atmosphere. It is shown that the defect state of the surface of nanocrystalline oxides (for example, the existence of vacancies in the anion sublattice and other defects) has a dominant influence on the formation of the magnetic properties of the samples under study. The main contributions to the magnetism of TiO₂ nanoparticles after the reduction treatments are the paramagnetic contribution of the matrix, the paramagnetic Curie–Weiss contribution, and the contribution of the spontaneous magnetic moment provided by the existence of regions with different spin ordering. A heterogeneous magnetic state is found to exist in the TiO₂ nanopowders; for example, at low temperatures, shifted hysteresis loops are observed as a result of a possible set of magnetic states with different spin orders. It is shown that a soft compaction or grinding of nanopowders in an agate mortar lead to substantial increase in the magnetization, sometimes, by a factor of more than two, regardless of the nanopowder synthesis method and the initial phase state of TiO₂ (anatase or rutile structures). This experimental fact proves the key role of the surface defects and the magnetic moment carriers with different spin configurations localized mainly on the nanoparticle surface. The compaction changes the magnetization only in the case when the initial magnetic state has a nonlinear “quasi-superparamagnetic” character of the magnetization curve. As a result of predominant exchange interaction between the nanoparticles with a frustrated character of spin ordering on the nanoparticles surface, the ferromagnetic contribution increases as nanoparticles contact.     

Luminescence properties of transparent ceramics Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Yb

We present the results of studying the luminescence properties of transparent ceramics Y₃Al₅O₁₂:Yb obtained by the vacuum sintering and nanocrystalline technology. In the course of research, we measured the luminescence and luminescence excitation spectra, as well as the temperature and kinetic behavior of luminescence. Our results are analyzed in comparison with the characteristics of corresponding single crystals. We revealed that processes of generation and relaxation of electronic excitations that occur in ceramics, in particular, in the charge transfer state, are similar to processes occurring in crystals. The behavior of two charge-transfer luminescence bands at 340 and 490 nm is studied. In the range 300–600 nm, we revealed a broad emission band of radiation of other type, which is also observed in spectra of undoped ceramics. This broad band is attributed to F⁺ centers. Emission and excitation spectra of charge transfer luminescence at a maximum of the temperature dependence of 100 K are measured for the first time. We found that, upon excitation in the charge transfer band, luminescence in ceramics is more intense than in single crystals with similar concentrations of Yb and has a higher quenching temperature.     

Titanium-modified Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> with a synergistic effect of surface modifying,bulk doping,and size reducing

In this work, a one-step solid-phase sintering process via TiO₂ and Li₂CO₃ under an argon atmosphere, with ultra-fine titanium powder as the modifying agent, was used to prepare a nano-sized Li₄Ti₅O₁₂/Ti composite (denoted as LTO–Ti) at 800 °C. The introduction of ultra-fine metal titanium powder played an important role. First, X-ray photoelectron spectroscopy demonstrates that Ti⁴⁺ was partially changed into Ti³⁺, through the reduction of the ultra-fine metal titanium powder. Second, X-ray diffraction revealed that the ultra-fine metal titanium powder did not react with the bulk structure of Li₄Ti₅O₁₂, while some pure titanium peaks could be seen. Additionally, the size of LTO–Ti particles could be significantly reduced from micro-scale to nano-scale. The structure and morphology of LTO–Ti were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Electrochemical tests showed a charge/discharge current of 0.5, 1, 5, and 10 C; the discharge capacity of the LTO–Ti electrode was 170, 161, 140, and 111 mAh g^?1. It is believed that the designed LTO–Ti composite makes full use of both components, thus offering a large contact area between the electrolyte and electrode, high electrical conductivity, and lithium-ion diffusion coefficient during electrochemical processes. Furthermore, ultra-fine titanium powder, as the modifying agent, is amenable to large-scale production.     

The Formation of Dimeric Centers of Copper,Iron and Gadolinium in Modified Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript>

It is discovered that the electron paramagnetic resonance (EPR) spectrum of the doubly charged copper centers occurs in single crystals of Pb₅Ge₃O₁₁ doped with gadolinium or iron after annealing in an atmosphere containing chlorine and bromine. Similar annealing of the crystals doped with copper in a chlorine and fluorine atmosphere leads to redistribution of the intensities of the EPR spectra of two types of Cu²⁺ centers. The influence of annealing on the ongoing intensity of spectra of the dimeric triclinic centers Fe³⁺–A, Gd³⁺–A (where A represent Cl^?, Br^?, O^2?, F^?) was the subject of this research. Consideration is given to the mechanisms for changing the charge state and association of copper center with defects.     

X-ray emission study of the electronic structure of nanocrystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Valence states of metal ions and the phase composition of nanocrystalline Al₂O₃ (of the original oxide and the oxide irradiated by high-energy Fe⁺ ions) are studied by using x-ray emission Al L_{2, 3} and O Kα spectra. It is established that the shape of the Al L_{2, 3} spectra strongly changes as one goes from the original (bulk) Al₂O₃ to nanocrystalline oxide, while the O Kα spectra remain practically unchanged. Moreover, irradiation by high-energy Fe⁺ ions results in slight additional changes in the x-ray spectral characteristics of the aluminum oxides under study. The obtained experimental data are compared with the results of theoretical calculations of the electronic structure of α and γ phases of Al₂O₃ performed using the LDA formalism. Using the results of x-ray spectral studies, electronic structure calculations, and x-ray diffraction analysis, it is shown that the revealed spectral differences between the nanocrystalline state of aluminum oxide and the bulk material can be interpreted as a phase transition from the α phase to the γ phase of Al₂O₃ with an addition of bayerite.     

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

⁷¹Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga₂O₃ samples prepared by sol-gel and ball-milling techniques. ²⁹Si and ²⁷Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. ⁷¹Ga NMR parameters are reported for the α- and β-Ga₂O₃ phases, and more tentatively for the δ-Ga₂O₃ phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga₂O₃ using Gaussian distributions in χ_Q, the extent of disorder in the Ga₂O₃ crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the ⁷¹Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga₂O₃, as well as the smallest nanocrystal sizes. Authors' address: Mark E. Smith, Department of Physics, University of Warwick, Coventry CV4 7AL, UK     

Light intensity and temperature dependence on performance of a photoelectrochemical cells of ITO/TiO<Subscript>2</Subscript>/PVC-LiClO<Subscript>4</Subscript>/graphite

The photovoltaic characteristics of a photoelectrochemical cell of ITO/TiO₂/PVC-LiClO₄/graphite are reported. This paper is concerned with the influence of light intensity and temperature on short-circuit current density, J_sc and open-circuit voltage, V_oc of the device. The photoelectrochemical cell material was a screen-printed layer of titanium dioxide onto an ITO-covered glass substrate, which was used as a working electrode of the cell. The solid electrolyte was polivinylchloride-lithium perchlorate. The graphite film serves as a counter electrode of the cell. The current density–voltage characteristics of the device under an illumination of 20, 40, 60, 80 and 100 mW cm⁻² light from a tungsten halogen lamp were recorded at 40 °C as well as under an illumination of 100 mW cm⁻² at 30, 35, 40, 45 and 50 °C, respectively. It was found that the short-circuit current density, J_sc of the device increases with both light intensity and temperature. The J_sc obtained at 100 mW cm⁻² was 1.0 μAcm⁻² and that at 50 °C was 0.7 μAcm⁻².     

Properties of ferromagnetic resonance in Fe<Subscript>73.5</Subscript>CuNb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> nanocrystalline alloys

The structural properties and parameters of ferromagnetic resonance have been studied for Fe_73.5CuNb₃Si_13.5B₉ nanocrystalline alloys produced from the initial amorphous state via annealing under different conditions. The dependence of the linewidth of the ferromagnetic resonance on the grain size ΔH ∼ D ⁶ has been found. The result is discussed within the framework of the random magnetic anisotropy model.     

Transient optical absorption and luminescence of lithium triborate LiB<Subscript>3</Subscript>O<Subscript>5</Subscript>

The transient optical absorption and luminescence of LiB₃O₅ (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li⁰ electronic centers, which represent neutral lithium atoms. At 290 K, the Li⁰ centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B²⁺ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.     

Radiative transitions and spectral-hole burning in MgO: Cr<Superscript>3+</Superscript> nanocrystals

Luminescence spectra of Cr³⁺ ions in nanocrystalline MgO obtained by sol-gel technology were studied. The radiative lifetime of excited ²E states of cubic and tetragonal Cr³⁺ centers was found to be considerably longer than that of bulk single crystals. This effect is caused by a modified effective refractive index of the inhomogeneous medium. Burning of long-lived spectral holes in the ⁴A₂-²E transition profile of cubic centers was observed, which sets nanocrystalline samples strongly apart from bulk crystals, where no hole burning was found.     

Temperature dependence of the lowest frequency <Emphasis Type="BoldItalic">E</Emphasis><Subscript><Emphasis Type="Bold">g</Emphasis></Subscript> Raman mode in laser-synthesized anatase TiO<Subscript>2</Subscript> nanopowder

Nanosized titanium dioxide (TiO₂) powder was prepared by a laser-induced pyrolysis. Specific surface area of the as-grown powder measured by BET method was 109 m²/g. The grain size (14.5 nm) estimated from these data coincides well with the crystallite size (12.3 nm) determined by XRD measurements. The average grain size (∼35 nm) obtained from the subsequent SEM measurements refers to considerable agglomeration of nanoparticles. Raman spectroscopy has been used to investigate the structural properties of TiO₂ nanopowder and its anatase structure is confirmed. The blueshift and broadening of the lowest frequency E_g Raman mode at temperature range ∼25–550 K have been analyzed using a phonon-confinement model. Dominant influence of the strong anharmonic effect at higher temperatures was demonstrated. PACS 81.07.Wx; 78.30.-j; 63.22.+m     

Transient hole-polaron optical absorption in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li₆Gd(BO₃)₃ (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li⁰ centers. At 290 K, the Li⁰ centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.     

Near-infrared broadband luminescence in Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript> binary glass system

Near-infrared broadband luminescence from 1100 to 1600 nm was observed in Bi₂O₃-GeO₂ binary glasses. The strongest emission can be observed with 30 mol % Bi₂O₃ when pumped at 808 nm. The lifetimes of all samples are longer than 200 μs. The glass network was studied by Raman spectra and Bi⁺ ions are proposed as the infrared luminescence centers in this glass system. Thermal treatment in air results in partly oxidation of Bi⁺ to Bi²⁺.     

Two-dimensional quantum ferromagnet,magnetic chains,and oxygen ordering in La<Subscript>2</Subscript>NiO<Subscript>4.125</Subscript> single crystals

A method using the atom-vacancy ordering phenomenon for the visualization of structural vacancies in crystals was suggested and implemented. The ordered nonstoichiometric titanium monoxide Ti₅O₅ was taken as the object of investigation, because the structural vacancies in this compound can be observed due to the formation of continuous vacancy channels in certain crystallographic directions. The structural vacancies in the specially oriented sample of ordered titanium monoxide were directly observed by high-resolution transmission electron microscopy with a magnification of 4×10⁶.     

EPR of trivalent iron centers in SrF<Subscript>2</Subscript>: Fe crystals

Paramagnetic centers of three types are found in SrF₂: Fe(0.2 at. %) crystals. Two types are observed in the untreated crystals, and the third type appears only in the crystals irradiated by x-rays. The EPR spectra of one type of centers in a nonirradiated crystal and of the centers that appear after irradiation are described by the orthorhombic Hamiltonians with an effective spin S _eff = 5/2. In both cases, the centers are observed at 4.2 and 77 K. The principal axes of the spin Hamiltonians for them are along the 〈001〉, 〈1 \(\overline 1 \) 0〉, and 〈110〉 axes. However, the fine-structure parameters of their EPR spectra differ significantly. An analysis of the superhyperfine structure (SHFS) of the EPR spectra shows that the radiation center forms through substitution of a Fe²⁺ ion for a Sr²⁺ cation. Under x-ray irradiation, the Fe²⁺ ion transforms into the Fe³⁺(⁶ A _1g ) state and is displaced to an off-center position along the C ₂ axis of its coordination cube. The absence of a SHFS in the EPR spectra of the orthorhombic centers in a nonirradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model is proposed for this structure. The EPR spectra of centers of the third type were observed only at 4.2 K, and the structure of these centers was not studied.     

Formation and structure of nanocrystals in bulk Zr<Subscript>50</Subscript>Ti<Subscript>16</Subscript>Cu<Subscript>15</Subscript>Ni<Subscript>19</Subscript> metallic glass

The possible formation of a nanocrystalline structure in controlled crystallization of a bulk Zr₅₀Ti₁₆Cu₁₅Ni₁₉ amorphous alloy has been studied using differential scanning calorimetry, transmission and high-resolution electron microscopy, and x-ray diffraction. It was established that crystallization of the alloy at temperatures above the glass formation point occurs in two stages and brings about the formation of a nanocrystalline structure consisting of three phases. Local spectral x-ray analysis identified the composition and structure of the phases formed.