M. A. Melkozerova

Effect of Doping with 3d Elements (Co, Ni, Cu) on the Intrinsic Defect Structure and Photocatalytic Properties of Nanostructured ZnO with Tubular Morphology of Aggregates

The precursor-derived nanostructured solid solutions Zn0.95M0.05O (M = Co, Ni, Cu) with tubular aggregates have been investigated using optical absorption spectroscopy and electron paramagnetic resonance. The dependences of the concentration of intrinsic defects Vo+ and the effective band gap on the dopant type have been determined. It has been shown using the oxidation reaction of hydroquinone dissolved in water as an example that an increase in the photocatalytic activity in the series ZnO → Zn0.95Ni0.05O → Zn0.95Co0.05O Zn0.95Cu0.05O in the ultraviolet and visible spectral regions correlates with a decrease in the band gap and with an increase in the concentration of oxygen vacancies VO+.

Synthesis, crystal structure, and luminescence properties of CaY2Ge3O10:Ln3+, Ln = Eu, Tb

The crystal structure and luminescence properties of CaY2Ge3O10:Ln3+ (Ln = Eu, Tb) germanates synthesized via a conventional solid-state reaction and an ethylenediaminetetraacetic acid complexing process are studied. The CaY2 − xLnxGe3O10 (Ln = Eu, Tb; x = 0–1.0, 2.0; Δx = 0.1) solid solutions have a monoclinic structure (space group P21/c, Z = 4), in which dopant ions occupy three nonequivalent noncentrosymmetric sites with different Ca2+/Ln3+ ratios. The effect of the synthesis methods, dopant concentrations, and excitation wavelengths on the luminescence properties of the compounds obtained is determined.

Nature of defects in nanocrystalline zinc oxide with particles of tubular morphology

Nanostructured ZnO powders with particles of tubular form were synthesized by a precursor method. A narrow symmetrical line with a g factor of 2.002 was detected in the ESR spectra of the materials. On the basis of analysis of the dependence of the line intensities on the heat treatment conditions it was concluded that the signal belongs to singly charged oxygen vacancies VO+.

Synthesis, microstructure, and native defects of photoactive Zn 1 − x Cu x O solid solutions (0 ≤ x ≤ 0.1) with tubular aggregates

258 Design of photocatalysts based on nanosized zinc oxide for oxidation of toxic and colored organic com pounds has been a perspective direction of research in recent years [1–3]. The efficiency of such materials is determined by their composition, size, and aggregate morphology and microstructure, as well as by the existence of native structural defects. In particular, doping of ZnO with 3d metals or formation of particles of extended shape causes a considerable enhancement of the photocatalytic activity of zinc oxide and often leads to the shift of the catalytic process from the char acteristic UV to the visible spectrum [4, 7]. To fabri cate photocatalysts with enhanced functional charac teristics being active under visible light, we synthesized the Zn1 – xMxO solid solutions (M = Co, Mn, Fe, Ni) with tubular morphology of aggregates [5, 8].

Synthesis, structure and spectroscopic characteristics of Ti(O,C)2/carbon nanostructured globules with visible light photocatalytic activity

A morphology-controlled facile synthesis of titanium-glycolate precursors with subsequent annealing in He and air atmospheres has been exploited for the production of nanostructured composite globules, whiskers and plates of C-modified titanium dioxide. Characterisation tests proved the as-obtained globule composites to exclusively exhibit high-specific surface area (up to 150–170 m2 g−1), thus being useful for photocatalytic applications in the visible-light region. The combination of the electron paramagnetic resonance, X-ray photoelectron spectroscopy, absorption spectroscopy and transmission electron microscopy revealed the presence of three kinds of carbon in the globules: a small bandgap (with measured width of 0.8 eV) amorphous carbon surrounding the anatase nanocrystallites, C-containing radicals including carbonates on the surface of TiO2 and interstitial carbon in the oxygen position of the TiO2 lattice. It was found that the maximum visible-light photocatalytic activity of the globules is determined by the optimal surface concentration of amorphous carbon of about 0.002 wt.% m−2. Under these conditions, the highest synergic photosensitising effect on TiO2 nanocrystallites of all three kinds of carbon is expected.

Peculiarities of EPR in polycrystalline solid solutions Zn 0.95 Fe 0.05 O with different particles morphology: The role of intrinsic defects in formation of magnetic properties

Single-phase polycrystalline samples of Zn0.95Fe0.05O having the wurtzite structure and different morphology of particles were obtained using precursor technologies. It was found by electron spin resonance that iron enters the ZnO crystal lattice in the Fe3+ charge state. Magnetic studies revealed that the obtained samples show weak ferromagnetic properties at 300 K. A correlation between the number of oxygen vacancies and the degree of ferromagnetic ordering was found.

Synthesis, optical properties, and defective structure of carbon-doped titanium dioxide

211 Titanium dioxide is a wide gap semiconductor and is widely used in different fields of modern technics. Materials based on highly dispersed titanium dioxide are currently believed to be among the best candidates for design of visible light active photocatalysts for decomposition of organic chemicals and decontami nation of water and air [1, 2]. As known, the photocat alytic activity of such materials significantly depends on their defective structure: the presence of dopants and intrinsic defects in the semiconductor lattice determines its energy spectrum and has a noticeable effect on the concentration and mobility of free charge carriers (electrons and holes) involved in redox reac tions [3–5]. In addition, point defects localized at the surface can act as electron and hole traps and promote the generation of active species, e.g., and OH•, on the catalyst surface [6, 7].

Structural, Optical, and Photocatalytic Properties of Quasi-One-Dimensional Nanocrystalline ZnO, ZnOC:nC Composites, and C-doped ZnO

Various thermolysis rotes of zinc glicolate complexes are considered for the synthesis of quasi-one-dimensional nanostructured aggregates ZnO and Zn–O–C used as photocatalysts. Structural features of quasi-one-dimensional aggregates Zn–O–C and ZnO are investigated in detail. Transmission electron microscopy, Raman spectroscopy, and electron paramagnetic resonance spectroscopy methods demonstrate that the aggregates Zn–O–C have either composite structure (ZnO crystallites in amorphous carbon matrix) or a C-doped ZnO single-phase structure depending on heat treatment conditions, and that all the aggregates exhibit as a rule a tubular morphology, a nanocrystalline structure with a high specific surface area, and a high concentration of singly charged oxygen vacancies. The mechanism of the nanocrystalline structure formation is discussed and the effect of thermolysis condition on the formation of the textured structure of aggregates is investigated. The results of examination of the photocatalytic and optical absorption properties of the synthesized aggregates are presented. The photocatalytic activity for the hydroquinone oxidation reaction under ultraviolet and visible light increases in the series: the reference ZnO powder, quasi-one-dimensional ZnO, quasi-one-dimensional aggregates C-doped ZnO, and this tendency correlates with the reduction of the optical gap width. As a result of our studies, we have arrived at an important conclusion that thermal treatment of ZnO:nC composites allows a C-doped ZnO with high catalytic activity. This increasing photoactivity of C-doped ZnO aggregates is attributed to the optimal specific surface area and electron-energy spectrum restructuring to be produced owing to the presence of singly charged oxygen vacancies and carbon dissolved in the ZnO lattice.

Novel IR Phosphor Based on Sr3La2(Ge3O9)2 : Nd3+,Ho3+ Germanate

Cyclogermanate Sr3La2(Ge3O9)2, isostructural to silicate Sr3Er2(Si3O9)2, activated by neodymium and holmium is obtained for the first time by the precursor method. Ion Nd3+ in the structure of Sr3La2(Ge3O9)2 is a sensitizer of the infrared luminescence of Ho3+. Excitation by radiation with a wavelength of 808 nm leads to a series of emission lines in the luminescence spectra of Sr3La2-xNdx(Ge3O9)2 : Ho3+ in the short-wave and middle-IR ranges (1.0–3.4 μm). The highest intensity of lines at 2.1 and 2.7 μm, associated with the 5I7 → 5I8 and 5I6 → 5I7 transitions in the Ho3+ ion, is found for compositions containing traces of holmium. Based on the analysis of the concentration dependences of the luminescence intensity, an optimal composition of the phosphor is determined, which ensures the maximum efficiency of conversion of laser radiation energy. The data obtained are interpreted in the assumption of cross-relaxation energy transfer from Nd3+ to Ho3+.

Structure, magnetic and optical properties of Sr3RE2(Ge3O9)2 cyclogermanates (RE = La–Gd)

A new series of Sr3RE2(Ge3O9)2 (RE = La, Pr, Nd, Sm, Eu, Gd) cyclogermanates has been prepared using an ethylenediaminetetraacetic acid assisted route. The powder XRD study reveals that the compounds crystallize in the monoclinic space group C2/c, Z = 4. A decrease in the crystal radius of RE3+ ions in Sr3RE2(Ge3O9)2 (RE = La–Gd) causes a morphotropic phase transition followed by a step-like change of the lattice parameters. The transition is accompanied by internal reorganization, including the redistribution of rare earth and strontium ions among three symmetry independent Sr/RE sites, a conformation change of [Ge3O9]6− rings and a variation of Sr/RE(2) atom coordination from seven-fold to nine-fold. Density functional theory calculations reveal that Sr3RE2(Ge3O9)2 compounds are indirect bandgap materials with a band gap around 3.0 eV. The magnetic property measurements indicate that Sr3RE2(Ge3O9)2 (RE = Pr, Sm, Gd) are Curie–Weiss paramagnets up to 2 K. For Sr3Eu2(Ge3O9)2 an anomaly of magnetic susceptibility is found at about 60 K. A photoluminescence study of the Sr3Sm2(Ge3O9)2 and Sr3Eu2(Ge3O9)2 samples reveals an emission in the visible spectral range, corresponding to the characteristic 4f–4f transitions in rare earth ions.