Guangshe Li

Magnetic crossover of NiO nanocrystals at room temperature

The nature of room-temperature ferromagnetism in diluted magnetic semiconductors, ZnO:M (M is Ni, Co, Fe, etc.), remains in debate, most likely because previous theoretical and experimental studies have excluded the magnetic contribution from secondary phases of transition metal oxide nanocrystals including NiO and CoO. In this work, the authors initiated a study on NiO nanocrystals that demonstrated room-temperature ferromagnetic behavior with relatively large coercive forces, in apparent contradiction to the previous conjectures in the literature. With size reduction, NiO nanocrystals showed an abnormal magnetic crossover, which is closely related to weakened superexchange interactions in their multisublattices.

Size-induced symmetric enhancement and its relevance to photoluminescence of scheelite CaWO4 nanocrystals

This work explores size-induced lattice modification and its relevance to photoluminescence properties of scheelite nanostructures. CaWO4 nanocrystals, a prototype scheelite compound, exhibited a lattice expansion and an increased symmetry of structural units with physical dimension reduction, which is in contradiction to the trend previously reported in bulk CaWO4 at high pressures or high temperatures. Lattice variations in CaWO4 nanocrystals are probably due to the “negative pressures” that originated from strong defect dipole interactions on surfaces. The increased structural symmetry along with surface citric modifications produced a significant enhancement in photoluminescence of CaWO4 nanocrystals, indicating a quantitatively structural control over the electronic properties.

Nature of the abnormal band gap narrowing in highly crystalline Zn1−xCoxO nanorods

Highly crystalline Zn1−xCoxO nanorods were prepared using a hydrothermal method. With increasing Co2+ dopant concentration, the lattice volume enlarged considerably, which is associated with the enhanced repulsive interactions of defect dipole moments on the wall surfaces. This lattice modification produced a significant decrease in band gap energies with its magnitude that followed the relationship, ΔEg=ΔE0∙(e−xB−1), where x and B are Co2+ dopant concentration and a constant, respectively. The abnormal band gap energies were indicated to originate from the sp-d exchange interactions that are proportional to the square of lattice volume.

Solid solubility and transport properties of Ce 1− x Nd x O 2−δ nanocrystalline solid solutions by a sol-gel route

Ce 1− x Nd x O 2−δ ( x = 0.05 to 0.55) nanocrystalline solid solutions were prepared by a sol-gel route. X-ray diffraction analysis showed that Ce 1− x Nd x O 2−δ crystallized in a cubic fluorite structure. The lattice parameter for the solid solutions increased linearly with the dopant content. The solid solubility of Nd 3+ in ceria lattice was determined to be about x = 0.40 in terms of the nearly constant lattice parameters at a dopant level larger than x = 0.40. First-order Raman spectra for Ce 1− x Nd x O 2−δ at a lower dopant content exhibited one band associated with the F 2 g mode. At higher dopant contents, F 2 g mode became broadened and asymmetric, and a new broad band appeared at the higher frequency side of the F 2 g mode. This new band was assigned to the oxygen vacancies. The electron paramagnetic resonance spectrum for x = 0.05 showed the presence of O 2 − adsorbed on sample surface at g = 2.02 and 2.00 and of Ce 3+ with a lower symmetry at g = 1.97 and 1.94. Further increasing dopant content led to the disappearance of the signals for O 2 − . Impedance spectra showed the bulk and grain boundary conduction in the solid solutions. The bulk conduction exhibited a conductivity maximum and an activation energy minimum with increasing dopant content. Ce 0.80 Nd 0.20 O 2−δ was determined to give promising conduction properties such as a relatively high conductivity of σ 700 °C = 2.44 × 10 −2 S cm −1 and moderate activation energy of E a = 0.802 eV. The variations of conductivity and activation energy were explained in terms of relative content of oxygen vacancies V o and defect associations {Ce Ce ′V o }/{Nd Ce ′V o }.

Synthesis and thermal decomposition of nitrate-free boehmite nanocrystals by supercritical hydrothermal conditions

Abstract Single-phase boehmite nanocrystals that are free of nitrate contamination could be prepared at supercritical hydrothermal conditions at 400 °C and 35 MPa for 30 min. XRD analysis showed that the single phase boehmite was formed in the absence of alkali species. Two well-defined and strong absorption bands observed at 3300 and 3092 cm −1 in the present infrared spectra showed that the boehmite by supercritical hydrothermal conditions was highly crystallized. Furthermore, IR spectra confirmed that the boehmite samples were free of NO 3 − -related impurities in contrast to boehmite produced by hydrothermal conditions as reported by Music et al. [Mater. Lett., 40 (1999) 269] and Mishra et al. [Mater. Lett., 42 (2000) 38]. At a temperature of 725 °C, the present boehmite powders were decomposed into 10 nm γ-Al 2 O 3 . Further heating of the samples to 1250 °C led to the formation of single-phase α-Al 2 O 3 .

Structure, Luminescence, and Transport Properties of EuVO4

Metastable scheelite EuVO_4 was stabilized by a high temperature and pressure method, which was transformed into a stable zircon phase by annealing treatment in air. Scheelite EuVO_4 gave strong emissions with a dominant peak at 617 nm associated with the ^(5)D_0-^(7)F_2 transition of Eu^(3+). ^(151)Eu Mossbauer spectra indicated that the isomer shift for the metastable scheelite phase was ca. 0.5 mm/s lower than that for the zircon phase, which was ascribed to a reduced covalency in the Eu-O bond originated via a charge transfer from oxygen to Eu3+ in scheelite lattice by producing an enhanced shielding of 4f electrons on the s orbital as well as a decrease in s electron density around Eu^(3+) nucleus. Impedance spectra for the zircon phase clearly demonstrated an ionic hopping in the bulk with a conductivity of ca. 1.0×10^(–3) S cm^(–1) at 500°C. EuVO_4 is proved to be both a potential phosphor and a potential ionic conductor.

δ-MnO2–Mn3O4 Nanocomposite for Photochemical Water Oxidation: Active Structure Stabilized in the Interface

Pure phase manganese oxides have been widely studied as water oxidation catalysts, but further improvement of their activities is much challenging. Herein, we report an effective method to improve the water oxidation activity by fabricating a nanocomposite of Mn3O4 and δ-MnO2 with an active interface. The nanocomposite was achieved by a partial reduction approach which induced an in situ growth of Mn3O4 nanoparticles from the surface of δ-MnO2 nanosheets. The optimum composition was determined to be 38% Mn3O4 and 62% δ-MnO2 as confirmed by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The δ-MnO2-Mn3O4 nanocomposite is a highly active water oxidation catalyst with a turnover frequency (TOF) of 0.93 s-1, which is much higher than the individual components of δ-MnO2 and Mn3O4. We consider that the enhanced water oxidation activity could be explained by the active interface between two components. At the phase interface, weak Mn-O bonds are introduced by lattice disorder in the transition of hausmannite phase to birnessite phase, which provides active sites for water oxidation catalysis. Our study illustrates a new view to improve water oxidation activity of manganese oxides.

Valence variations in titanium-based perovskite oxides by high-pressure and high-temperature method

Typical titanium-based perovskite oxides Eu1‐xBaxTiO3 (x 4 0.6‐0.8), Eu1‐xKxTiO3 (x 4 0.2,0.32), and La0.7(Na,K)0.3TiO3 were synthesized by high pressure and temperature using RE2O3 (RE 4 La,Eu), TiO2, alkaline, or alkaline earth carbonates as the starting materials. X-ray diffraction data analysis showed that there was a structural transformation in Eu1‐xBaxTiO3 by varying Ba content [i.e., from cubic (x 4 0.6,0.7) to tetragonal (x 4 0.8)], and that samples Eu1‐xKxTiO3 and

Activation of oxide-ion conduction in KNbO3 by addition of Mg2+

Niobate perovskite oxides of KNb1−xMgxO3−δ were synthesized directly at high temperature and pressure. X-ray diffraction confirmed the formation of single-phase orthorhombic structures. High-temperature Raman spectroscopy in combination with thermal analysis showed that phase transitions occurred from orthorhombic to tetragonal, to pseudo cubic, and finally to cubic, in sequence. Addition of Mg2+ at the Nb5+ sites of perovskite KNbO3 lattice led to a dramatic enhancement of oxide-ion conduction. This enhancement in oxide-ion conduction was through the suppression of the contribution from p-type holes, which shows that these high-temperature phases can be promising candidates as conductive materials.

Unprecedented catalytic performance in amine syntheses via Pd/g-C3N4 catalyst-assisted transfer hydrogenation

The preparation of amine compounds is very important for both the chemical industry and renewable feedstock processing. Nevertheless, difficulties remain in finding a catalytic system that is sufficiently active and environmentally benign for producing amine compounds. In this work, we report that g-C3N4 nanosheets as support materials can significantly boost the efficiency of Pd nanoparticles for the reduction of nitro compounds to primary amines. Using formic acid as a hydrogen donor and water as a solvent, the optimized 5 wt% Pd/g-C3N4 catalyst exhibited an unprecedented performance in the conversion of nitrobenzene into aniline (achieving almost full conversion with an extremely high turnover frequency of 4770 h−1 at room temperature), yielding the best activity ever reported for heterogeneously catalyzing nitro compound reduction. Pd/g-C3N4 catalyst was also active for the one-pot reductive amination of carbonyl compounds with nitro compounds to obtain the corresponding secondary amines with excellent selectivity (>90%). We proposed that the protic N–H+ and hydridic Pd–H− on Pd/g-C3N4 are the active species for the transfer hydrogenation reaction of nitro compounds. Furthermore, Pd/g-C3N4 catalyst was highly stable with a wide scope in the syntheses of various amine compounds. This work will open up a new approach for the transfer hydrogenations of nitro compounds to produce primary or secondary amines in green chemistry.