Conghui Wang

Synthesis and photoluminescence of single-crystalline In2O3 nanowires

Single-crystalline In2O3 nanowires have been successfully synthesized from indium grains by a vapor–solid method at 1030 °C in 90% Ar and 10% O2 atmosphere. These nanowires are uniform with diameters of 40–120 nm and lengths of about 15–25 µm, and crystallize in a cubic structure. The growth of these nanowires is controlled by a spiral growth mechanism. Photoluminescence (PL) measurements show a green–blue PL band in the wavelength range of 400–700 nm with a peak at 470 nm (2.64 eV), caused by oxygen vacancies in the In2O3 nanowires.

Strong Quantum Confinement in Ordered PbSe Nanowire Arrays

Ordered PbSe crystalline nanowire arrays have been successfully fabricated in the nanochannels of porous anodic alumina membrane by direct current electrodeposition. X-ray diffraction and selected area electron diffraction investigations demonstrate that the PbSe nanowires have a uniform cubic structure. Electromicroscopy results show that the nanowires are quite ordered with diameters of about 50 nm and lengths up to 5 micrometers. X-ray energy dispersion analysis indicate that Pb:Se is very close to 1:1. The optical absorption spectrum of these PbSe nanowires show that there exist two peaks at 280 and 434 nm, respectively, attribute to excitonic absorption peaks.

Facile synthesis of porous nitrogen-doped holey graphene as an efficient metal-free catalyst for the oxygen reduction reaction

Nitrogen-doped graphene is a promising candidate for the replacement of noble metal-based electrocatalysts for oxygen reduction reactions (ORRs). The addition of pores and holes into nitrogen-doped graphene enhances the ORR activity by introducing abundant exposed edges, accelerating mass transfer, and impeding aggregation of the graphene sheets. Herein, we present a straightforward but effective strategy for generating porous holey nitrogen-doped graphene (PHNG) via the pyrolysis of urea and magnesium acetate tetrahydrate. Due to the combined effects of the in situ generated gases and MgO nanoparticles, the synthesized PHNGs featured not only numerous out-of-plane pores among the crumpled graphene sheets, but also interpenetrated nanoscale (5–15 nm) holes in the assembled graphene. Moreover, the nitrogen doping configurations of PHNG were optimized by post-thermal treatments at different temperatures. It was found that the overall content of pyridinic and quaternary nitrogen positively correlates with the ORR activity; in particular, pyridinic nitrogen generates the most desirable characteristics for the ORR. This work reveals new routes for the synthesis of PHNG-based materials and elucidates the contributions of various nitrogen species to ORRs.

Ultrathin N-rich boron nitride nanosheets supported iron catalyst for Fischer–Tropsch synthesis

The boron nitride nanosheets (BNNSs) have attracted great interest in the field of energy storage and heterogeneous catalysis. In this paper, BNNSs supported iron (Fe/BNNSs) catalysts were prepared by one-pot solid state reaction and used in Fischer–Tropsch synthesis (FTS) for the first time. The microscopic structure, morphology and metal–support interaction of the Fe/BNNSs catalysts were investigated by TEM, FT-IR, 1H MAS NMR and H2-TPR. The average thickness of the N-rich BNNSs support was 4–8 nm, and the mean size of the iron nanoparticles was 25–40 nm. The CO conversion, CH4 and C5+ selectivity of typical Fe/BNNSs catalyst with 33 wt% Fe-loading were 47%, 13.7% and 48% at 270 °C, respectively. No obvious deactivation was observed even after 270 h running. The conversion, selectivity and the iron time yield (FTY) of Fe/BNNSs catalysts were highly related to the loading, dispersion of iron nanoparticles. The lower loading and better dispersion of the iron nanoparticles in Fe/BNNSs catalyst resulted in the better FTY and C5+ selectivity. The N-rich defects of BNNSs and porous structure of BNNSs anchored active phases to prevent them from growing larger. Therefore, the BNNSs support plays an important role in retarding the catalyst from deactivation.

Thermochromic VO2 films from ammonium citrato-oxovanadate(IV) with excellent optical and phase transition properties

Thermochromic VO2 film is a potential material for energy-saving windows in future buildings. Considering the difficulty in controlling the valence state and polymorphism during the formation of VO2 (M) film, we propose a facile and safe solution method of fabricating monoclinic (M) VO2 film directly from the newly synthesized ammonium citrato-oxovanadate(IV) compound [(NH4)4[V2O2(C6H4O7)2]·2H2O, denoted as CA-V(IV)], as a vanadium(IV) precursor to stabilize vanadium in the 4+ valence state without utilizing V2O5 as an intermediate or complex post-treatment in vacuum. This new ambient-stable compound CA-V(IV) contains centrosymmetric dinuclear complex anions [{VO(C6H4O7)}2]4−, in which the carboxylate groups are coordinated to the V4+ ions in a monodentate fashion and each vanadium atom exhibits distorted octahedral geometry. With a perfect monoclinic phase, the VO2 films possessed excellent thermochromic and visible transmissive properties. Accompanying a change in film thickness from 119 nm to 41 nm, the integral visible transmittance Tvis at 25 °C of VO2 films ranged from 38.4% to 70.0%. The maximum visible transmittance (Tmax) reached 77.2% for VO2 film with a thickness of 41 nm, which shows that the phase transition of VO2 did not greatly affect the visible transmittance of the film. For the VO2 film with a thickness of 41 nm, of which the visible transmittance modulation (ΔTvis = 2.1%) was the largest among the three samples, the solar energy modulation (ΔTsol) reached 11.3%. Moreover, the transition temperature of our VO2 films was far below that of bulk VO2 (68 °C), and the best transition temperature was as low as 50.8 °C.

Nonspherical hollow α-Fe2O3 structures synthesized by stepwise effect of fluoride and phosphate anions

Despite the significant progress in making hollow structures, it is still a challenge to synthesize some specialized hollow structures. In the present work, we obtained a new hollow hematite structure, tube-in-dodecahedron, by using the stepwise influences of fluoride and phosphate anions. Based on condition-dependent experiments, we proposed a “nucleation–aggregation–recrystallition and etching” mechanism, which also directed us to synthesize a series of hematite hollow structures, including hollow dodecahedron and hollow ellipsoid. The concentration of phosphate was found to play a decisive role in the control of these hollow structures. 0.08 mM is the critical point for keeping the top facets of dodecahedral hematite particles while 0.2 mM is the upper limit for keeping the lateral facets. The magnetic properties of these synthesized hollow hematite structures were found to be closely associated with the structures. The synthesized tube-in-dodecahedral hematite particles exhibited excellent photocatalytic reactivity toward organic dyes.

A hydrophobic and abrasion-resistant MgF2 coating with an ultralow refractive index for double-layer broadband antireflective coatings

An ultralow-index top layer is a prerequisite to preparing high-performance broadband AR coatings. When coupled with hydrophobicity and the abrasion-resistance required in practical applications, the challenge becomes even greater. In this work, a MgF2 AR coating was studied because of the low refractive index of MgF2 and its easily realized strong adhesion through low-temperature heat treatment. In order to obtain an ultralow refractive index and endow the coating with hydrophobicity, we designed several experimental routes and finally adopted MTES/TEOS co-precursors to direct MgF2 particles to form a honeycomb-like network structure without a template. A final refractive index of 1.15 and good hydrophobicity, with a water contact angle of 122°, were obtained using the MgF2–SiO2(CH3) coating. These superior properties were attributed to the incorporation of methyl groups, which not only endowed the coating with hydrophobicity, but also changed the original linear assembly of MgF2 particles to a circular assembly. Using this hydrophobic ultralow-index coating as a top layer, a high-performance double-layer AR coating was fabricated with a high average transmittance of 99.43% in the wavelength range of 400–1000 nm, good abrasion-resistance, and damp heat resistance after a low-temperature heat treatment of 250 °C. This MgF2 double-layer AR coating may be used in display devices or lenses.

Iron cation-induced biphase symbiosis of h-WO3/o-WO3·0.33H2O and their crystal phase transition

Herein, tungsten oxide hexagonal prisms with a biphase of h-WO3 and o-WO3·0.33H2O were prepared by a facile hydrothermal method using Fe3+ cations. The combination of instrumental characterization and software simulation proved that two phases coexisted in one nanoparticle with same morphology. The ratio of two phases could be changed by adjusting the concentration of Fe3+ cations. On the basis of controlled experiments, a mechanism was proposed to illustrate the formation of this biphase WO3 structure, and it was also proved that the self-growth of Fe species was unfavorable for the coexistence of two phases.