Chengxiang Wang

Metal Oxide Gas Sensors: Sensitivity and Influencing Factors

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Large scale synthesis and gas-sensing properties of anatase TiO2 three-dimensional hierarchical nanostructures.

Three-dimensional (3D) crystalline anatase titanium dioxide (TiO(2)) hierarchical nanostructures were synthesized through a facile and controlled hydrothermal and after-annealing process. The formation mechanism for the anatase TiO(2) 3D hierarchical nanostructures was investigated in detail. The 3D hierarchical nanostructures morphologies are formed by self-organization of several tens of radially distributed thin petals with a thickness of several nanometers with a larger surface area. The surface area of TiO(2) hierarchical nanostructures determined by the Brunauer-Emmett-Teller (BET) adsorption isotherms was measured to be 64.8 m(2) g(-1). Gas sensing properties based on the hierarchical nanostructures were investigated. A systematic study on sensitivity as a function of temperatures and gas concentrations was carried out. It reveals an improved ethanol gas sensing response property with a sensitivity of about 6.4 at 350 degrees C upon exposure to 100 ppm ethanol vapor for the TiO(2) hierarchical nanostructures. A gas sensing mechanism based on the adsorption-desorption of oxygen on the surface of TiO(2) is discussed and analyzed. This novel gas sensor can be multifunctional and promising for practical applications. Furthermore, the hierarchical nanostructures with high surface area can find variety of potential applications such as solar cells, biosensors, catalysts, etc.

Sol-gel growth of hexagonal faceted ZnO prism quantum dots with polar surfaces for enhanced photocatalytic activity.

The hexagonal faceted ZnO quantum dots (QDs) about 3-4 nm have been prepared via a sol-gel route by using oleic acid (OA) as the capping agent. It is revealed by electron diffraction patterns and high resolution transmission electron microscopy lattice images that the profile surfaces of the highly crystalline ZnO QDs are mainly composed of {100} planes, with the Zn-terminated (001) faces and the opposite (001) faces presented as polar planes. Compared with spherical ZnO QDs, the hexagonal faceted ZnO QDs show enhanced photocatalytic activity for photocatalytic decomposition of methylene blue. A mechanism for the enhanced photocatalytic activity of the hexagonal faceted ZnO QDs for degradation of methylene blue is proposed. In addition to the large specific surface areas due to small size and high crystalline, the enhanced photocatalytic activity can mainly be ascribed to the special hexagonal morphology. The Zn-terminated (001) and O-terminated (001) polar faces are facile to adsorb oxygen molecules and OH(-) ions, resulting in a greater production rate of H(2)O(2) and OH(*) radicals, hence promoting the photocatalysis reaction. The synthesized hexagonal-shaped ZnO QDs with high photocatalytic efficiency will find widespread potential applications in environmental and biological fields.

Platinum-Nanoparticle-Modified TiO2 Nanowires with Enhanced Photocatalytic Property

Highly crystalline Pt nanoparticles with an average diameter of 5 nm were homogeneously modified on the surfaces of TiO(2) nanowires (Pt-TiO(2) NWs) by a simple hydrothermal and chemical reduction route. Photodegradation of methylene blue (MB) in the presence of Pt-TiO(2) NWs indicates that the photocatalytic activity of TiO(2) NWs can be greatly enhanced by Pt nanoparticle modification. The physical chemistry process and photocatalytic mechanism for Pt-TiO(2) NWs hybrids degrading MB were investigated and analyzed. The Pt attached on TiO(2) nanowires induces formation of a Schottky barrier between TiO(2) and Pt naonoparticles, leading to a fast transport of photogenerated electrons to Pt particles. Furthermore, Pt incoporation on TiO(2) surface can accelerate the transfer of electrons to dissolved oxygen molecules. Besides enhancing the electron-hole separation and charge transfer to dissolved oxygen, Pt may also serve as an effective catalyst in the oxidation of MB. However, a high Pt loading value does not mean a high photocatalytic activity. Higher content loaded Pt nanoparticles can absorb more incident photons which do not contribute to the photocatalytic efficiency. The highest photocatalytic activity for the Pt-TiO(2) nanohybrids on MB can be obtained at 1 at % Pt loading.

Metal–Organic Frameworks Derived Porous Core/Shell Structured ZnO/ZnCo2O4/C Hybrids as Anodes for High-Performance Lithium-Ion Battery

Metal-organic frameworks (MOFs) derived porous core/shell ZnO/ZnCo2O4/C hybrids with ZnO as a core and ZnCo2O4 as a shell are for the first time fabricated by using core/shell ZnCo-MOF precursors as reactant templates. The unique MOFs-derived core/shell structured ZnO/ZnCo2O4/C hybrids are assembled from nanoparticles of ZnO and ZnCo2O4, with homogeneous carbon layers coated on the surface of the ZnCo2O4 shell. When acting as anode materials for lithium-ion batteries (LIBs), the MOFs-derived porous ZnO/ZnCo2O4/C anodes exhibit outstanding cycling stability, high Coulombic efficiency, and remarkable rate capability. The excellent electrochemical performance of the ZnO/ZnCo2O4/C LIB anodes can be attributed to the synergistic effect of the porous structure of the MOFs-derived core/shell ZnO/ZnCo2O4/C and homogeneous carbon layer coating on the surface of the ZnCo2O4 shells. The hierarchically porous core/shell structure offers abundant active sites, enhances the electrode/electrolyte contact area, provides abundant channels for electrolyte penetration, and also alleviates the structure decomposition induced by Li(+) insertion/extraction. The carbon layers effectively improve the conductivity of the hybrids and thus enhance the electron transfer rate, efficiently prevent ZnCo2O4 from aggregation and disintegration, and partially buffer the stress induced by the volume change during cycles. This strategy may shed light on designing new MOF-based hybrid electrodes for energy storage and conversion devices.

Three-dimensional Mn-doped Zn2GeO4 nanosheet array hierarchical nanostructures anchored on porous Ni foam as binder-free and carbon-free lithium-ion battery anodes with enhanced electrochemical performance

We report on three dimensional Mn-doped Zn2GeO4 hierarchical nanosheet arrays anchored on porous Ni foam as binder-free lithium-ion battery (LIB) anodes with enhanced electrochemical performance. Homogeneous Mn doping effectively induces a great microstructure evolution from nanowire arrays of pure Zn2GeO4 to nanosheet arrays of Mn-doped Zn2GeO4 samples. LIB anodes based on 7% Mn-Zn2GeO4 nanosheet array hierarchical nanostructures anchored on Ni foam display significantly improved electrochemical Li storage performance, showing a superior reversible capacity of 1301 mA h g−1 at a current density of 100 mA g−1 after 100 cycles, almost two times higher than that of 660 mA h g−1 of pure Zn2GeO4 samples. An extraordinarily excellent rate capability with a capacity of 500 mA h g−1 at a current density of 2 A g−1 can be obtained for LIB anodes based on Mn-doped Zn2GeO4 hierarchical nanostructures. The great enhancement of the electrochemical lithium storage performance can be attributed to three-dimensional interconnected conductive channels composed of Ni foam, which not only serves as the current collector but also buffers the volume change of the active material upon cycling. Additionally, Mn doping can greatly improve charge transport kinetics at the interface between the electrode and the electrolyte.

Novel Au inlaid Zn2SnO4/SnO2 hollow rounded cubes for dye-sensitized solar cells with enhanced photoelectric conversion performance

We developed a facile strategy for the fabrication of uniform Au inlaid Zn2SnO4/SnO2 hollow rounded cubes with an adjustable Au loading content using ZnSn(OH)6 as the precursor, chloroauric acid as the Au source and ascorbic acid as the reducing agent. The Au inlaid Zn2SnO4/SnO2 hollow rounded cubes show enhanced light absorption ability and reduced recombination rate of photogenerated electron–hole pairs compared with pure Zn2SnO4/SnO2. The hollow rounded cube structured Au–Zn2SnO4/SnO2 sample displays a high surface area and high dye adsorption ability. As photoanodes for DSSCs, the Au–Zn2SnO4/SnO2 hollow rounded cubes demonstrate a greatly enhanced Jsc and an improved power conversion efficiency of up to 2.04%, almost 73% higher compared to the photovoltaic conversion efficiency of pure Zn2SnO4/SnO2 based DSSCs. The greatly improved power conversion efficiency of DSSCs based on the Au inlaid Zn2SnO4/SnO2 photoanode can be attributed to the following three factors. Firstly, the localized surface plasmon resonance of Au nanoparticles plays a crucial role in the enhancement of visible light absorption. Secondly, the potential barrier on the Zn2SnO4/SnO2 surface caused by Au nanoparticles can suppress electron–hole recombination, diminish the loss of electrons during the transfer process and improve the photocurrent density. Thirdly, the enhanced photovoltaic performance can also be attributed to the unique structural characteristics of hollow rounded cubes.

Hierarchical Cu0.27Co2.73O4/MnO2 nanorod arrays grown on 3D nickel foam as promising electrode materials for electrochemical capacitors

Cu0.27Co2.73O4/MnO2 hybrid hierarchical nanostructure arrays directly on pressed nickel foam, with thin MnO2 nanoflakes homogeneously wrapped on Cu0.27Co2.73O4 nanorod arrays, are successfully synthesized by a simple hydrothermal and post heat-treatment method. The microstructures, phase structure, crystalline state, chemical components and chemical bonding state of the hybrids are systematically characterized using XRD, XPS, SEM and TEM. The electrochemical performance of the Cu0.27Co2.73O4/MnO2 hybrid electrode for electrochemical capacitors (EC) is investigated using cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) techniques. A superior areal capacitance of ∼4 F cm−2 is obtained for the Cu0.27Co2.73O4/MnO2 hybrid electrode even at a high current density of ∼18.6 mA cm−2. The 3D Cu0.27Co2.73O4/MnO2 hierarchically porous nanorod array structures endow the electrodes with a large specific surface area and improve the pathway for ion diffusion, thus leading to high areal capacitance and excellent rate capability, and indicate their promising application as binder-free electrodes for high performance EC and potentiality triggering the exploration of improved areal capacitance for miniaturized devices.

Heteroatomic interface engineering in MOF-derived carbon heterostructures with built-in electric-field effects for high performance Al-ion batteries

Confronted with challenges in promoting fast AlxCly− anion diffusion and intercalation for aluminum ion batteries (AIBs), it is of vital importance to rationally design gradient hetero-interfaces with an ideal built-in interfacial electric potential to enhance charge diffusion and transfer kinetics. Herein, we demonstrate an effective strategy to realize accurate tuning gradient heteroatom N and P doping in MOF-derived porous carbon in C@N-C@N,P-C graded heterostructures. Importantly, gradient N and P doping could modify the electronic structure of MOF-derived carbon as certified by DFT calculations, and lead to charge redistribution to induce graded energy levels and a built-in electric field in the C@N-C@N,P-C graded heteroatomic interface, thus boosting interfacial charge transfer and accelerating reaction kinetics. Furthermore, the large surface area and high porosity of C@N-C@N,P-C graded heterostructures could efficiently absorb electrolyte and enhance anion transport kinetics. As expected, the designed gradiently N,P-doped C@N-C@N,P-C heterostructure with a built-in interfacial electric field could facilitate electron and AlCl4− anion transfer spontaneously between N,P-C, N-C and C gradient components, exhibiting a superior capacity of 98 mA h g−1 at a high current density of 5 A g−1 after 2500 cycles. This strategy reveals new insights about the gradient energy band for designing high-performance electrochemical energy storage devices.