Gang Xu

Size-controllable one-dimensinal SnO2 nanocrystals: synthesis, growth mechanism, and gas sensing property

Single crystalline one-dimensional (1-D) SnO(2) nanocrystals with controllable sizes, including the diameter and the aspect ratio, were synthesized by modulating the precursor concentration, reaction time and temperature via a solution method. By regulating the growth in a kinetic regime, a higher temperature range (220-240 degrees C) was beneficial to the growth of SnO(2) nanowires, while reactions below 220 degrees C only resulted in nanorods or even nanoparticles. The aggregates of SnO(2) nanocrystals in the forms of hollow spheres and dendrites were observed as the intermediates for the nanowires. Based on the TEM and SEM observations, the growth mechanism is discussed from the viewpoints of the nature of the reverse micelles and the crystal habit of rutile SnO(2). CO gas sensing measurements were also carried out for SnO(2) nanocrystals with different assembly styles. The results indicate that the sensitivity had close correlation to the specific surface area of the nanocrystals.

(ZrO2)0.85(REO1.5)0.15 (RE=Sc, Y) solid solutions prepared via three Pechini-type gel routes: 1—gel formation and calcination behaviors

Abstract (ZrO 2 ) 0.85 ( RE O 1.5 ) 0.15 ( RE =Sc, Y) nanoparticles with pure cubic fluorite structure have been synthesized by three Pechini-type gel routes, viz . poly(vinyl alcohol) containing-process (route I), poly(ethylene glycol) and formic acid-containing process (route II), and in situ polymerizable complex method (route III). The coordination modes between metal ions and polymers in the gels are shown to be highly correlative with the synthesis route used. The gels prepared by route III have the strongest coordination throughout their network and therefore the best chemical homogeneity. The altered variety of polymer and cross-linking within the gels adopted by these three routes has made the as-synthesized samples show appreciable differences in thermal behavior, powder reactivity and nanoparticle properties.

Nanocrystalline rare earth stabilized zirconia: solvothermal synthesis via heterogeneous nucleation-growth mechanism, and electrical properties

Homogeneous nanocrystalline (ZrO2)0.92(RE2O3)0.08 (RE = Y, Sc) with large specific surface areas and cubic structure were solvothermally synthesized from the urea coprecipitated (Zr,RE)-hydroxide gel or the gel-calcined oxide in water (or absolute ethanol) under mild conditions. Both the hydroxide and the oxide powders showed very high solvothermal reactivity. Mainly by X-ray diffraction and transmission electron microscopy, the solvothermal mechanism was explored. The heterogeneous nucleation-growth, i.e. an in situ transformation of hydrous (Zr,RE)-oxide with an ordered cubic structure via dissolution and recrystallization is supposed to predominately account for the nanocrystalline crystallization. The as-obtained nanocrystallite exhibited a good sinterability. Over the temperature range of 360–600 °C, nanocrystalline (ZrO2)0.92(Y2O3)0.08 displayed enhanced specific grain boundary conductivity due to the size-dependent grain boundary impurity segregation.

Doping and grain size effects in nanocrystalline ZrO2-SC2O3 system with complex phase transitions: XRD and Raman studies

Weakly-agglomerated nanocrystalline (ZrO2)1−x(Sc2O3)x (x = 0.02–0.16) powders with high surface area (106–186 m2 g−1) and uniform particle size (5.7–9.5 nm) were synthesized by a two-step hydrothermal process in the presence of urea, and their complex phase evolution upon calcinations over 400–1400 °C were studied by a combined utilization of X-ray diffraction, in situ high temperature X-ray diffraction, near infrared Fourier transform Raman and ultraviolet Raman spectroscopies. It was discovered that the growth of the grains comprised two stages, and the apparent activation energies, Ea1 (<800 °C) and Ea2 (>800 °C), are in the range of 5.5–12 kJ mol−1 and 80–140 kJ mol−1, respectively. The crystallite size, microstrain, surface area and growth activation energy of the samples showed a strong dependence on the Sc3+ doping concentration. The nanocrystalline (ZrO2)1−x(Sc2O3)x samples underwent complex phase transitions upon calcination from 800 to 1000 °C. Tetragonal and cubic phases have been preserved when the samples calcined below 800 °C as a result of crystallite size and doping effects. At elevated calcination temperature of 1000 °C, the compositions at x = 0.04, 0.10 and 0.14 seemed to be in the metastable phase boundaries of m + t, t + β (Sc2Zr7O17) and β + γ (Sc2Zr5O13), respectively. As calcined at 1400 °C, the grain size was in the micrometre regime, and t → m, c → t″, c → β and c → γ phase transitions occurred on the cooling stage for x ≤ 0.04, x = 0.08, 0.10 ≤ x ≤ 0.12 and x ≥ 0.14, respectively, resulting in the existence of m, t, t″, β and γ phases in turn at room temperature. Moreover, it was found that the tetragonal phase has been enriched at the surface region for the as-calcined (ZrO2)0.94(Sc2O3)0.06 and (ZrO2)0.92(Sc2O3)0.08 samples.

Unusual calcination temperature dependent tetragonal–monoclinic transitions in rare earth-doped zirconia nanocrystals

The dependence of tetragonal-monoclinic (t→m) transitions in the hydrothermally prepared (ZrO2)0.98(RE2O3)0.02 (RE=Sc, Y) nanocrystals upon the calcination between 400–1400°C were explored by means of X-ray diffraction (XRD), high temperature X-ray diffraction (HTXRD), near infrared Fourier transform (FT) Raman and ultraviolet (UV) Raman spectroscopies. XRD and FT Raman scattering analysis indicated that the monoclinic fraction varied discontinuously with increasing the temperature from 400 to 1400°C, which is unusual and is mainly ascribed to the microstrain effect. HTXRD analyses (from room temperature to 1400°C) revealed that the as-prepared samples underwent tetragonal→monoclinic (t→m) transformation at 500°C for (ZrO2)0.98(Sc2O3)0.02 and at 200°C for (ZrO2)0.98(Y2O3)0.02 during the cooling process. UV Raman spectra suggested that the t→m transformation initially occurred at the surface region of the calcined samples. Hence, the monoclinic phase was enriched on the surface of the zirconia grains. Finally, the size effect of the stabilizers (Y3+ and Sc3+) on the critical crystallite size and the metastability of the tetragonal phase were also discussed.

Extraction of scandium from ion-adsorptive rare earth deposit by naphthenic acid

Abstract An extraction method for producing high purity Sc(III) from ion-adsorptive rare earth deposit (IARED) is presented. Naphthenic acid diluted with iso -octanol and sulfated kerosene is selected as the extractant, chloride acid as the scrubbing and stripping solution. By two extraction processes in naphthenic acid system, scandium is concentrated from 0.02–0.04% to 15–20% (wt.), and further purified to 99.99–99.999% grade. The stoichiometry of the extraction of Sc(III) with naphthenic acid was also determined.

Co-doping effects of Y3+ and Sc3+ on the crystal structures, nanoparticle properties and electrical behavior of ZrO2 solid solutions prepared by a mild urea-based hydrothermal method

Weakly agglomerated nanocrystalline (ZrO2)0.92(Y2O3)0.08 − x(Sc2O3)x (x = 0–0.08) powders (8.4–9.9 nm in size) in a cubic structure with high surface area (145–153 m2 g−1) were synthesized by a two-step hydrothermal process in the presence of urea: a stock solution of metal nitrates and urea was heated at 80 °C for 24 h and then at 180 °C for 48 h. The co-doping effect of Y3+ and Sc3+ ions on the crystal structures, nanoparticle properties (crystallite size, surface area and microstrain), and sintering and electrical behaviors of the (ZrO2)0.92(Y2O3)0.08 − x(Sc2O3)x solid solutions were systematically investigated by means of X-ray diffraction and transmission electron microscopy combined with energy dispersive X-ray analysis, scanning electron microscopy, BET specific area analysis and ac impedance spectroscopy. The lattice parameter and average crystallite size of the as-prepared nanocrystallites tend to decrease with increasing scandia content. In the Arrhenius plots over the measurement temperature range of 400–800 °C, a curvature towards higher activation energy with decreasing temperature appears, which is remarkable for the compacted sample doped with 8 mol% Sc2O3 (yttria-free) after sintering at 1400 °C, although not for those doped with 8 mol% Y2O3 (scandia-free) and with 1–7 mol% Sc2O3. When scandia is substituted by yttria, the lattice conductivity decreases monotonically due to the difference in the sizes of Y3+ and Sc3+.

(ZrO2)0.85(REO1.5)0.15 (RE=Sc, Y) solid solutions prepared via three Pechini-type gel routes: 2—sintering and electrical properties

Dense (ZrO 2 ) 0.85 (REO 1.5 ) 0.15 (RE = Sc, Y) specimens have been obtained from the nanoparticulate powders prepared by three Pechini-type gel routes, viz. poly(vinyl alcohol) (PVA) containing-process (route I), poly(ethylene glycol) and formic acid-containing process (route II), and in situ polymerizable complex method (route III). During sintering over 1000-1400°C, (ZrO 2 ) 0.85 (ScO 1.5 ) 0.15 samples prepared by routes II and III, and (ZrO 2 ) 0.85 (YO 1.5 ) 0.15 samples prepared by the three routes retain pure cubic fluorite structure, however, a monoclinic phase segregation takes place for the Sc-doped zirconia prepared by route I. Various gel routes are shown to display significant impact on sinterability of the green body, crystallinity and particle size uniformity of the sintered body, and the ionic conductivity of the dense specimens. The specimens prepared from route III exhibit superior sinterability and ionic conductance to those prepared from routes I and II.