Dianqing Li

Low-temperature hydrothermal synthesis of WO3 nanorods and their sensing properties for NO2

Tungsten trioxide (WO3) nanorods with an aspect ratio of ∼50 have been successfully synthesized by hydrothermal reaction at a low temperature of 100 °C. The crystal structure, morphology evolution and thermal stability of the products are characterized in detail by XRD, FESEM, FTIR, and TG/DTA techniques. The diameter evolution and distribution of WO3 nanorods strongly depend on hydrothermal temperature and time. Hydrothermal conditions of 100 °C and 24 h ensure the formation of well-defined WO3 nanorods. The transition of the crystal structure from monoclinic WO3 to hexagonal WO3 occurs after calcination at 400 °C. The appropriate calcination conditions of the WO3 nanorods are defined to be 600 °C and 4 h for gas-sensing applications. Response measurements reveal that the WO3 sensor operating at 200 °C exhibits high sensitivity to ppm-level NO2 and small cross-sensing to CO and CH4, which makes this kind of sensor a competitive candidate for NO2-sensing applications. Moreover, impedance measurements indicate that a conductivity mechanism of the sensor is mainly dependent on the grain boundaries of WO3 nanorods. A possible adsorption and reaction model is proposed to illustrate the gas-sensing mechanism.

Quantum-sized ZnO nanoparticles: Synthesis, characterization and sensing properties for NO2

Quantum-sized ZnO nanoparticles have been synthesized at room temperature by a mild sol–gel process using tetraethylorthosilicate (TEOS) as the capping agent to control the particle growth of ZnO. The crystal structure, particle size and optical properties have been investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectra and Raman spectra, respectively. The results show that the ZnO nanoparticles exhibit hexagonal wurtzite structure and the average crystallite size is 5.7 nm which is a little less than TEM results. It has been testified by room-temperature PL spectra that the TEOS capped the surface of ZnO nanoparticles and obviously reduced grain size, as an emission at 520 nm almost disappeared and a new peak with an anomalous blue shift as great as 9 nm, appeared for the TEOS capped ZnO. The sensing tests indicate that the ZnO based sensors not only show a high response to NO2 but also exhibit high selectivity over CO and CH4 at a low operating temperature of 290 °C. The response increases with NO2 concentration and decreases with calcination temperature, and is in agreement with Raman and XRD results.

Synthesis mechanism and gas-sensing application of nanosheet-assembled tungsten oxide microspheres

Nanosheet-assembled tungsten oxide microspheres have been synthesized using rapid sonochemistry followed by thermal treatment. Transient observation of controllable synthesis reveals that the morphological evolution of the product is highly dependent on the ultrasonication time. An assembly mechanism based on oriented attachment and reconstruction is proposed for the sonochemical formation of the nanosheet-assembled microspheres. The obtained samples possess intrinsic non-stoichiometry and a hierarchically porous nano/microstructure, which is beneficial for their utilization in sensing materials and for fast diffusion of gas molecules. The maximum response of the tungsten oxide hierarchical microspheres is 3 times higher than that of commercial nanoparticles for NO2 gas. The gas adsorption–desorption kinetics during the sensing process were mathematically simulated by a derivative method. The first-principles calculation reveals that the NO2 molecule is most likely adsorbed at the terminal O1c site of tungsten oxide, leading to the introduction of new surface states, which are responsible for the intrinsic NO2-sensing properties.

SYNTHESIS, FLAME-RETARDANT AND SMOKE-SUPPRESSANT PROPERTIES OF A BORATE-INTERCALATED LAYERED DOUBLE HYDROXIDE

Reaction of a Mg-Al carbonate layered double hydroxide (LDH) with boric acid leads to a borate-pillared LDH with the stoichiometry [Mg0.65Al0.35(OH)2][B3O5]0.35.0.65H2O and an interlayer spacing of 1.07 nm. Infrared and 11B magic angle spinning nuclear magnetic resonance data are consistent with the presence of polymeric triborate anions of the type [B3O4(OH)2]nn- in the interlayer galleries so that the material can be formulated as [Mg0.65Al0.35(OH)2][B3O4(OH)2]0.35.0.30H2O. The flame-retardant properties of the borate-pillared material and the carbonate precursor in composites with ethylene vinyl acetate copolymer were compared. Introduction of the borate anion leads to a significant enhancement in smoke suppression during combustion without compromising the flammability of the material. This is related to the synergistic effect between the host layers of the LDH and the borate anions uniformly distributed in the interlayer region.

Mechanism enhancing gas sensing and first-principle calculations of Al-doped ZnO nanostructures

Al-doped flower-like ZnO nanostructures have been synthesized by a facile hydrothermal method at 95 °C for 7 h. The structure and morphology of the product were characterized by XRD, FTIR and SEM analysis. The sensing tests reveal that the response is significantly enhanced by Al doping, and the 0.3 wt% Al-doped sample exhibits the highest response of 464 to 10 ppm CO at an operating temperature of 155 °C. A change of the structural defects in Al-doped ZnO is responsible for the enhancement of the sensing properties, which has been confirmed by the room temperature photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS). The response time is reduced disproportionately with the increase in CO concentration by modeling the transient responses of the sensor using the Langmuir–Hinshelwood reaction mechanism. The band structures and density of states for pure ZnO and Al-doped supercells have been calculated using first principles based on density functional theory (DFT). The calculated results show that the band gap is narrowed and the conductance is increased by Al doping, which coincides with the experimental results of gas sensing.

Size-controlled hydrothermal synthesis and high electrocatalytic performance of CoS2 nanocatalysts as non-precious metal cathode materials for fuel cells

Non-precious metal chalcogenides are considered as a potential alternative to Pt-based cathode catalysts in polymer electrolyte membrane fuel cells because of their promising electrocatalytic performance and low cost. However, size-controlled synthesis of this class of materials still remains a big challenge. In this paper, we directly prepared CoS2 nanocatalysts by a hydrothermal route without any post treatment, developed a facile way to tune the particle size by adjusting the initial Co2+ concentration in the reaction system in the presence of a surfactant, and investigated the corresponding electrocatalytic performance for the oxygen reduction reaction (ORR) in alkaline medium in detail. The results show that the ORR activity mainly depends on the CoS2 mass loading on the electrode disk surface and the average particle size of the CoS2 nanocatalysts. The CoS2 catalyst with an average particle size of 30.7 nm exhibits excellent electrocatalytic performance with an OCP (open circuit potential) of 0.94 V vs. RHE, a half-wave potential (E1/2) of ca. 0.71 V vs. RHE, and complete methanol tolerance for the ORR in 0.1 M KOH. This OCP value is the largest among non-precious metal chalcogenides to date, much close to that of 0.99 V vs. RHE for commercial Pt/C catalyst (E-TEK). In addition, the CoS2 nanocatalyst has comparable durability to the Pt/C catalyst in 0.1 M KOH. The CoS2 nanocatalyst is a promising candidate for alkaline membraneless fuel cell systems.

Electrocatalytic Cobalt Nanoparticles Interacting with Nitrogen-Doped Carbon Nanotube in Situ Generated from a Metal–Organic Framework for the Oxygen Reduction Reaction

A metal organic framework (MOF), synthesized from cobalt salt, melamine (mela), and 1,4-dicarboxybezene (BDC), was used as precursor to prepare Co/CoNx/N-CNT/C electrocatalyst via heat treatment at different temperature (700-900 °C) under nitrogen atmosphere. Crystallites size and microstrain in the 800 °C heat-treated sample (MOFs-800) were the lowest, whereas the stacking fault value was the highest among the rest of the homemade samples, as attested to by the Williamson-Hall analysis, hence assessing that the structural or/and surface modification of Co nanoparticles (NPs), found in MOFs-800, was different from that in other samples. CNTs in MOFs-800, interacting with Co NPs, were formed on the surface of the support, keeping the hexagonal shape of the initial MOF. Among the three homemade samples, the MOF-800 sample, with the best electrocatalytic performance toward oxygen reduction reaction (ORR) in 0.1 M KOH solution, showed the highest density of CNTs skin on the support, the lowest ID/IG ratio, and the largest N atomic content in form of pyridinic-N, CoNx, pyrrolic-N, graphitic-N, and oxidized-N species. Based on the binding energy shift toward lower energies, a strong interaction between the active site and the support was identified for MOFs-800 sample. The number of electron transfer was 3.8 on MOFs-800, close to the value of 4.0 determined on the Pt/C benchmark, thus implying a fast and efficient multielectron reduction of molecular oxygen on CoNx active sites. In addition, the chronoamperometric response within 24 000 s showed a more stable current density at 0.69 V/RHE on MOFs-800 as compared with that of Pt/C.

Surface decoration of WO3 architectures with Fe2O3 nanoparticles for visible-light-driven photocatalysis

Zero-dimensional Fe2O3 nanoparticles were successfully decorated on three-dimensional WO3 architectures to constitute a photocatalyst of Fe2O3@WO3 heterojunction. The obtained samples were characterized in detail by X-ray diffraction, scanning electron microscopy, elemental mapping, X-ray photoelectron spectroscopy and UV-Vis absorption spectra. The results indicate that rhombohedral α-Fe2O3 nanoparticles are homogeneously decorated on the surface of monoclinic WO3 architectures, and the constituted n+–n heterojunction results in “redshift” of the optical absorption. The photocatalyst of 1%Fe2O3@WO3 annealed at 400 °C exhibits the highest photocatalytic activity for degradation of Rhodamine B under visible light irradiation. The degradation obeys first-order reaction kinetics with an apparent rate constant of 0.057 min−1. It is suggested that the potential-energy difference between Fe2O3 and WO3 accelerates the separation of photogenerated electron–hole pairs, dominating the enhanced photocatalytic activity. The results presented herein provide new insight for development of a novel visible-light-driven photocatalyst and its potential application in harmful pollutant degradation.

Gas sensing properties of Cd-doped ZnO nanofibers synthesized by the electrospinning method

We used electrospinning followed by thermal treatment to successfully synthesize Cd-doped ZnO nanofibers using different doping concentrations. The research interest is aimed at achieving a significant enhancement of the sensing performance toward CO, which was achieved due to the doping changing the state of the native defect of the ZnO, and which has been confirmed by PL and XPS spectra. The competitive influence of the specific surface and the crystallinity of the ZnO on sensing response is discussed in detail. The sensing mechanism of Cd-doped ZnO sensors for the detection of CO is also discussed. In addition, the band structures and densities of the states for undoped and Cd-doped ZnO were also calculated using first-principles based on a local density approximation LDA + U scheme. The calculated results show that the band gap is significantly narrowed by doping, which concurs with the results determined from UV-Vis spectra.

SYNTHESIS OF Cu-CONTAINING LAYERED DOUBLE HYDROXIDES WITH A NARROW CRYSTALLITE-SIZE DISTRIBUTION

Hydrotalcite-like layered double hydroxides (LDHs) containing different ratios of Ni2+, Cu2+, Mg2+ and Al3+ in the layers have been prepared by a new method, the key features of which are a very rapid mixing and nucleation process in a colloid mill followed by a separate ageing process. The compositions and structural parameters of the materials synthesized using the two routes are very similar, although the degree of crystallinity is slightly higher for the LDHs produced using the new method. The major advantage of the new method is that it produces smaller crystallites, having a very narrow range of distribution of crystallite size. In the conventional coprecipitation process at constant pH, the mixing process takes a considerable time during which nuclei formed at the beginning of the process have a much longer time to undergo crystal growth than those formed at the end of the process. The consequence is that a wide dispersion of crystallite sizes is obtained. In the colloid mill process, however, the mixing and nucleation is complete in a very short time and is followed by a separate ageing process.