Xiangfeng Guan

Facile synthesis of composite g-C3N4/WO3: a nontoxic photocatalyst with excellent catalytic activity under visible light

A novel nontoxic g-C3N4/WO3 composite photocatalyst with an increased surface area but a reduced defect concentration was synthesized, which shows an excellent catalytic performance under visible light irradiation.

Preparation of cereal-like Y V O4:Ln3 + (Ln = Sm, Eu, Tb, Dy) for high quantum efficiency photoluminescence

In this work, preparation of cereal-like architectures Y V O(4) and Y V O(4):Ln(3 + ) (Ln = Eu, Sm, Dy, Tb) was initiated using a hydrothermal method. During the formation reaction, Na(3)C(6)H(5)O(7).2H(2)O was used to effectively adjust the concentration of Y(3 + ) species necessary for cereal-like architectures. Phase structure, surface chemistry, morphology, and photoluminescence were characterized by x-ray powder diffraction, Fourier transformed infrared spectra, scanning electron microscopy, transmission electron microscopy, and photoluminescence spectra. All samples crystallize in a tetragonal zircon structure, stably showing a homogeneous cereal-like morphology. This special morphology was constructed by self-assembly of tiny primary particles with a dimension of 31-32 nm. With increasing atomic number of Ln(3 + ), the lattice dimension of the cereal architectures became monotonously enlarged. This cereal-like architecture is proved unique in significantly improving the quantum efficiencies: the internal quantum efficiencies of (5)D(0) for Ln = Eu and (4)F(9/2) for Ln = Dy were 14.6% and 11.4%, respectively, which are all superior over those of the counterparts of nanoparticles reported in the literature. The average lifetime of the (5)D(0) level for Ln = Eu was calculated to be 98 micros, which is longer than that of 50 micros of the (4)F(9/2) level for Ln = Dy. The strong photoluminescence might be the consequence of the effective energy transfer due to the greatly reduced defect centers from this special self-assembly structure.

Porous SnIn4S8 microspheres in a new polymorph that promotes dyes degradation under visible light irradiation

Porous SnIn(4)S(8) microspheres were initially synthesized through a facile solvothermal approach and were investigated as visible-light driven photocatalysts for dyes degradation in polluted water. The photocatalysts were characterized by XRD, SEM, TEM, N(2) adsorption-desorption, and UV-vis diffuse reflectance techniques. Results demonstrated that the as-synthesized SnIn(4)S(8) was of a new tetragonal polymorph, showing a band-gap of 2.5 eV, a specific surface area of 197 m(2) g(-1), and an accessible porous structure as well. The photocatalytic activity of the porous SnIn(4)S(8) was evaluated by decomposition of several typical organic dyes including methyl orange, rhodamine B, and methylene blue in aqueous solution under visible light irradiation. It is demonstrated that porous SnIn(4)S(8) was highly photoactive and stable for dyes degradation, showing photocatalytic activity much higher than binary constituent sulfides like In(2)S(3), SnS(2), or even ternary chalcogenide ZnIn(2)S(4) photocatalyst. The excellent photocatalytic performance of porous SnIn(4)S(8) is the consequence of its high surface area, well-defined porous texture, and large amount of hydroxyl radicals.

Self-adjusted oxygen-partial-pressure approach to the improved electrochemical performance of electrode Li[Li0.14Mn0.47Ni0.25Co0.14]O2 for lithium-ion batteries

We initiated a self-adjusted oxygen-partial-pressure approach to prepare high-performance Li2MnO3–LiMO2 cathode material. Four different lithium resources, lithium acetate, lithium hydrate, lithium carbonate, and lithium nitrate were used to create the local oxygen partial pressure over the samples. Since the melting points or decomposition temperatures for these lithium resources decrease in a sequence, Li2CO3 ≈ LiOH > LiNO3 > CH3COOLi, the oxygen partial pressure of the four crucibles that contain these lithium salts increases in a sequence, S4 ≈ S3 < S2 < S1 ≈ air in muffle furnace (S1: CH3COOLi·2H2O, S2: LiNO3, S3: LiOH·H2O, and S4: Li2CO3). Regardless of the lithium resources, the decomposed gases reduced the local oxygen partial pressures, leading to an incomplete oxidation of Mn ions in the final product Li[Li0.14Mn0.47Ni0.25Co0.14]O2. That is, some of the Mn3+ ions existed in the final product Li[Li0.14Mn0.47Ni0.25Co0.14]O2, and the amount of Mn3+ ions was closely related to the oxygen partial pressure. The lower oxygen partial pressure gave rise to a larger amount of Mn3+ in the final products, as confirmed by X-ray photoelectron spectroscopy. Electrochemical tests showed that the products prepared using lithium carbonate exhibited the best electrochemical performance: the initial discharge capacity was 279.4 mA h g−1 at a current density of 20 mA g−1, which remained as high as 187.2 mA h g−1 even at a much higher current density of 500 mA g−1. Such excellent electrochemical performance could be ascribed to the presence of Mn3+ that decreased the surface layer resistance and charge transfer resistance, and that further increased the conductivity and Li+ ion diffusion coefficient.

Novel synthesis of Li1.2Mn0.4Co0.4O2 with an excellent electrochemical performance from −10.4 to 45.4 °C

Lithium-ion batteries continue to dominate the market and transportation applications of portable electronics, while these applications are still very difficult at low or elevated temperatures. In this work, the cathode material Li1.2Mn0.4Co0.4O2 was initially synthesized via an oxalate-precursor method. During sample preparation, lithium ions were co-precipitated with transition metal ions to form a uniform distribution of reactants at the molecular level. As a consequence, the current preparation method gave rise to a uniform cation distribution inside the target materials with no need of the additional process of mixing with lithium salt, which is however always required when using conventional co-precipitation methods. Due to the uniform cation distribution inside the material, the Li1.2Mn0.4Co0.4O2 cathode thus prepared was found to exhibit an excellent electrochemical performance. At room temperature, the initial discharge capacity and capacity retention ratio after 20 cycles were 284 mA h g−1 and 82.75%, respectively, which are superior to 246 mA h g−1 and 79.27%, the best results ever reported for the counterparts. Further, the low and elevated-temperature electrochemical performance for this cathode was also explored. It was found that the maximal discharge capacity measured at a current density of 20 mA g−1 between 2.0 and 4.6 V was maintained as high as 296 and 200 mA h g−1, at 45.4 and −10.4 °C, respectively. The change in the state of health (SOH) in the temperature range −10.4 to 45.4 °C was investigated by EIS. It was demonstrated that the %SOH operation window for Li1.2Mn0.4Co0.4O2/Li cells was somewhat broad, which indicated a potential application at low/elevated temperatures. The synthetic route described in this work is new, and may help to prepare more advanced cathode materials essential for a broad class of applications.

Low-concentration donor-doped LiCoO2 as a high performance cathode material for Li-ion batteries to operate between −10.4 and 45.4 °C

The majority of electrode materials suffer from severe capacity fading on cycling at elevated temperatures or poor conductivity and diffusion of Li+ at low temperatures, which have made it very difficult for lithium-ion batteries to operate at low temperatures and/or elevated temperatures without loss of electrochemical performance. In this work, we report on a new strategy for tackling this issue through low-concentration donor-doping of higher valence Mn ions in LiCoO2, a typical commercial cathode for many lithium-ion batteries. Firstly, low-concentration Mn-doped LiCoO2 was successfully synthesized using a molten-salt method, in which solvent NaOH provides an alkaline environment that makes the reactant mixture uniform in reaction process and ensures the valence state of Mn ion at +4. Secondly, the chemical compositions for all samples were systematically tuned, while retaining the single-phase nature. The electrochemically inert Mn4+ was found to significantly enhance the structure stability, conductivity, and diffusion rate of LiCoO2. As a consequence, the cathode material with a composition of LiCo0.95Mn0.05O2 exhibited an excellent electrochemical performance in a temperature range from −10.4 to 45.4 °C. The finding reported in this work will be conducive to the applications of lithium-ion batteries under different temperature conditions.

Synthesis of FeTiO3 nanosheets with {0001} facets exposed: enhanced electrochemical performance and catalytic activity

Ionic-liquid-assisted solution chemistry was initiated to prepare FeTiO3 nanosheets with {0001} polar facets exposed predominantly, which yielded improved electrochemical performance and excellent catalytic activity towards thermal decomposition of ammonium perchlorate.

pH-driven hydrothermal synthesis and formation mechanism of all BiPO4 polymorphs

Understanding of formation mechanism of inorganic solids by solution chemistry is always disturbed by the interference effect of the impurities from starting materials. Herein, we exploited a new two-step route to the selective synthesis of BiPO4 of different polymorphs with an aim to eliminate the impurity interference effect. The first step is the room-temperature solution synthesis of a hexagonal phase (HP), and the second step involves sufficient washing of HP and a subsequent hydrothermal treatment of HP under given conditions. The formation mechanism of BiPO4 nanocrystals of different polymorphs was studied by monitoring the reaction parameters like pH, reaction temperature, time, and impurity ions as well as by sample characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. It is found that the pH of the solution is the determinant parameter for the selective synthesis of BiPO4 polymorphs. Namely, at 240 °C and under strong acidic conditions (pH 750 °C. Based on these observations, two kinds of phase transition mechanisms were discussed.

Preparation and morphology-sensitive luminescence properties of Eu3+-doped YVO4: a defect chemistry viewpoint of study

YVO4:Eu3+ of four different morphologies was successfully synthesized by simply varying the pH under hydrothermal conditions. Systematic characterization indicates that by increasing the pH from 3 to 14, the morphology was changed from uniform hollow microspheres, potato-cakes, microhamburgers, to lichee-like architectures in sequence, which is followed by a colour variation from wine, red-purple, pink-purple, to red. Among all the morphologies, the lichee-like shape showed a superior luminescence performance as represented by a high quantum efficiency of 55.5% and long lifetime of 1.15 ms. Such a morphology-sensitive luminescence property was first interpreted in terms of the defect chemistry. The findings reported in this work are fundamentally important, which may give a deep insight into the origin of morphology-dependent properties necessary for exploring new luminescent materials.

MgAl2O4 nanoparticles: A new low-density additive for accelerating thermal decomposition of ammonium perchlorate

Developing low-density additives with high activities is significantly important for thermal decomposition of ammonium perchlorate and the relative technical applications. In this paper, nanoparticles of magnesium aluminate (MgAl2O4) with a low density of 3.02–3.17 g cm−3 were synthesized by a self-generated template path, in which citric acid was introduced to ensure homogeneous distribution of metal cations at atomic level and serve as the carbon source of the self-generated carbon template. Carbon template was generated by pyrolysis and carbonization of citrate after calcinations at 800 °C in N2, and then removed with annealing in air at high temperatures, resulting in porous MgAl2O4 with a specific surface area as high as 291 m2 g−1. Depending on the annealing temperatures, the primary sizes of MgAl2O4 nanoparticles that built up the porous structures were adjusted from 6.2 to 24.7 nm. The structural characteristics of MgAl2O4 nanoparticles were systematically studied by X-ray diffraction, transmission electron microscopy, Braun-Emmet-Teller analysis, and the catalytic role were evaluated by thermal decomposition of ammonium perchlorate. All samples were indicated to be X-ray-pure MgAl2O4, among which MgAl2O4 nanoparticles with a largest surface area of 291 m2 g−1 and pore volume of 0.24 cm3 g−1 was proved to have an optimum catalytic activity: exothermic peaks of AP thermal decomposition shifted towards the lower temperatures by 78.3 °C for high-temperature decomposition process. The relative kinetic process was also investigated.