Chunyan Xi

Hierarchical NiO hollow microspheres assembled from nanosheet-stacked nanoparticles and their application in a gas sensor

A facile and robust route for the mass preparation of hollow NiO microspheres assembled from nanosheet-stacked nanoparticles is developed. The Ni(HCO3)2 precursor with a hollow spherical structure was firstly prepared by a hydrothermal reaction without any surfactants or organic additives. The reaction generated gas bubble may act as a template for the formation of Ni(HCO3)2 hollow microspheres. These are converted into hierarchical NiO hollow microspheres assembled from nanoparticles (with diameter of ∼25 nm) upon calcination, which are further assembled by the stacking of ultrathin nanosheets. The hierarchical NiO hollow structures are attractive for catalyst, sensor and environmental applications, benefiting from their large surfaces. The sensing properties of the hierarchical NiO hollow microspheres were evaluated. They show high sensitivity, short response and recovery times, and good response and recovery characteristics to n-butanol. As compared with NiO nanoparticles with similar dimensions (∼20–35 nm), the nanoparticle assembled NiO hollow microspheres exhibit enhanced gas sensing properties. The effective integration of several nanostructures in one microunit would provide a novel way to design new materials for nanodevices.

Nanosheet-Based Hierarchical Ni2(CO3)(OH)2 Microspheres with Weak Crystallinity for High-Performance Supercapacitor

Three-dimensionally hierarchical oxide/hydroxide materials have recently attracted increasing interest by virtue of their exciting potential in electrochemical energy conversion and storage. Herein, hierarchical Ni2(CO3)(OH)2 microspheres assembled from ultrathin nanosheets were successfully synthesized by a one-pot/one-step hydrothermal route. In this method, common nickel salts and urea were selected as raw materials. The influence of urea concentration on the final product was studied. The hierarchical Ni2(CO3)(OH)2 microspheres show weak crystallinity and contain crystalline water. It was found that they exhibit excellent rate capacity when used as supercapacitor electrode. Under current density of 0.5 and 10 A/g, the optimized Ni2(CO3)(OH)2 electrode with loading density of 5.3 mg/cm(2) exhibited specific capacitances of 1178 and 613 F/g with excellent cycling stability. The excellent electrochemical property is possibly attributed to the intrinsic nature of Ni2(CO3)(OH)2, the ultrathin thickness of nanosheet units, and the sufficient space available to interact with the electrolyte. This facile synthesis strategy and the good electrochemical properties indicate that hydroxycarbonates are promising materials for supercapacitor application. This study suggests a large library of materials for potential application in energy storage systems.

CN foam loaded with few-layer graphene nanosheets for high-performance supercapacitor electrodes

Recently, three-dimensional porous carbon-based foams have attracted increasing interest owing to their exciting potential applications in various fields. Herein, hierarchical porous monoliths, CN foam loaded with free few-layer graphene nanosheets, have been prepared. In this method, graphene oxide sheets were first loaded on the usual melamine foam that was selected as the raw material for the CN framework. With a subsequent annealing process in N2 atmosphere, the melamine foam was converted into a CN foam; simultaneously, the graphene oxide sheets were converted into reduced graphene oxide, to form a reduced graphene oxide–CN composite. The loading density of graphene on the CN framework can be tuned by the dosage of graphene oxide. The obtained composite monoliths exhibit excellent cycling performance and good rate capacity when used as pseudocapacitive electrode materials. At current densities of 0.5 and 10 A g−1, the optimized electrode exhibits areal specific capacitance of 1067 and 463 mF cm−2, respectively, with excellent cycling stability. The excellent property can be attributed to the unique macropore structures that endow sufficient space available to interact with the electrolytes, and the pseudocapacitive contribution originated from the nitrogen and oxygen composition. This facile synthesis strategy and the good electrochemical properties suggest that the synthesized CN–graphene composites are promising materials for supercapacitor application.

Synthesis of Cu3P nanocubes and their excellent electrocatalytic efficiency for the hydrogen evolution reaction in acidic solution

Cu3P nanocubes are synthesized through a facile two-step strategy, which consists of a simple solution based method followed by a low-temperature phosphidation process. The Cu3P nanocubes have an average size of about 198 nm, and show a three-dimensional (3D) cubic architecture with hollow interiors and thin cubic shells. The material as an electrocatalyst for the hydrogen evolution reaction (HER) is investigated in acidic solution. It is found that the Cu3P nanocubes exhibit a low overpotential (145 mV), a small Tafel slope (70.2 mV per decade), and a large exchange current density (0.016 mA cm−2). Moreover, the Cu3P nanocubes show great electrochemical stability in acidic solution since no obvious decay in current density is observed after 1000 cycles. The excellent electrocatalytic performance can be associated with the electronic structures of Cu and P, as well as the hollow interior structure of Cu3P nanocubes, which supplies more active sites for HER. The approach used here provides an effective route for synthesizing metal phosphides with various microstructures and functions.

Facile synthesis and gas-sensing performance of Sr- or Fe-doped In2O3 hollow sub-microspheres

Sr- or Fe-doped In2O3 hollow sub-microspheres were successfully fabricated without the assistance of any additives or templates. The obtained hollow In2O3 sub-microspheres show relatively uniform size and band gap of ∼3.1 eV. Benefited from the hollow microstructure and doping effect, the doped In2O3 sub-microspheres show excellent gas sensing performance towards a series of organic solvents. For 100 ppm of formaldehyde, the Sr- and Fe-doped In2O3 sensors demonstrate sensing responses of 9.4 and 5.5, respectively. These values are much higher than previously reported In2O3-based gas sensors. Interestingly, Fe-doped In2O3 shows relatively higher sensing response to propanol than that of Sr-doped In2O3, although towards formaldehyde, ethanol, acetone and heptane, Sr-doped In2O3 shows relatively higher sensing responses. This investigation therefore indicates that doped In2O3 hollow microstructures could be an effective platform for sensing hazardous indoor formaldehyde gas, and that sensing selectivity could be improved through the doping effect.

Platelet-like nickel hydroxide: Synthesis and the transferring to nickel oxide as a gas sensor

Free-standing Ni(OH)2 platelet-like nanostructures with average width of 124 nm and thickness of 19 nm were successfully prepared through a simple one-pot polymer assisted process. The preparation involves the assistance of poly (sodium-4-styrene sulfonate) (PSS) and is easy to perform. The influence of pH value on the structure of the Ni(OH)2 product was investigated and it was found that pH value plays an important role for the formation of free-standing Ni(OH)2 nanoplatelets. A possible formation mechanism is proposed. In addition, from the obtained Ni(OH)2 nanoplatelets, a NiO sensor was fabricated and tested, which exhibits a 5 ppm sensing sensitivity to ethanol and propanol.

Polymer guided synthesis of Ni(OH)2 with hierarchical structure and their application as the precursor for sensing materials

Hierarchical Ni(OH)2 microstructures with sizes of several micrometers have been synthesized through a polymer mediated self-assembly route. The hierarchical Ni(OH)2 microstructures are built of sheet-like building blocks, which are further composed of “side to side” stacking of tiny sheet-like units. The influence of various experimental parameters on the microstructure was investigated. It is believed that the polymer, poly(sodium-4-styrene sulfonate), selectively bonded on the surface of Ni(OH)2 clusters mediates the formation of hierarchical Ni(OH)2 microstructures. In addition, through the chemical conversion of Ni(OH)2 to NiO, an oxide sensor was fabricated, which exhibits excellent gas sensing performances to ethanol and propanol, suggesting the potential applications. It is also anticipated that these Ni(OH)2 microstructures will have successful applications in Ni-based batteries, electrodes, and so on.