Pu-Xian Gao

Dual-mode mechanical resonance of individual ZnO nanobelts

The mechanical resonance of a single ZnO nanobelt, induced by an alternative electric field, was studied by in situ transmission electron microscopy. Due to the rectangular cross section of the nanobelt, two fundamental resonance modes have been observed corresponding to two orthogonal transverse vibration directions, showing the versatile applications of nanobelts as nanocantilevers and nanoresonators. The bending modulus of the ZnO nanobelts was measured to be ∼52 GPa and the damping time constant of the resonance in a vacuum of 5×10−8 Torr was ∼1.2 ms and quality factor Q=500.

Nanopropeller arrays of zinc oxide

Polar surface dominated ZnO nanopropeller arrays were synthesized by a two-step high temperature solid-vapor deposition process. The axis of the nanopropellers is a straight nanowire along the c axis and enclosed by {2110} surfaces, which grew first; the sixfold symmetric nanoblades are later formed along the crystallographic equivalent a axes (〈2110〉) perpendicular to the nanowire; and the array is formed by epitaxial growth of nanoblades on the nanowire. The top surface of the nanoblade is the Zn terminated +c plane, showing surface steps and possible secondary growth of nanowires due to higher self-catalytic activity, while the back surface is the oxygen-terminated −c plane, which is smooth and inert.

Manganese-doped ZnO nanobelts for spintronics

Zinc oxide (ZnO) nanobelts synthesized by thermal evaporation have been ion implanted with 30 keV Mn+ ions. Both transmission electron microscopy and photoluminescence investigations show highly defective material directly after the implantation process. Upon annealing to 800 °C, the implanted Mn remains in the ZnO nanobelts and the matrix recovers both in structure and luminescence. The produced high-quality ZnO:Mn nanobelts are potentially useful for spintronics.

Rectangular Porous ZnO–ZnS Nanocables and ZnS Nanotubes

ZnS, an important wide-bandgap semiconductor, is a photo-luminescence material. [1] Due to its wide bandgap of 3.8 eV, doping of ZnS with elements such as Tb and Eu, [2] can produce a wide range of exciting optical properties. Quantum dots of core±shell structured CdSe/ZnS have been found to exhibit a lasing effect and can be used for fluoro-immunoas-says, biological imaging, and biosensors. [3,4] Quantum confinement induced lasing has been observed in CdSe/ZnS. Semiconducting ZnO is one of the most important functional oxides for smart devices and optoelectronics. Recently, ultra-long nanobelt structures of ZnO, SnO 2 , In 2 O 3 , CdO, Ga 2 O 3 , and PbO 2 have been synthesized by simply evaporating the desired commercial metal oxide powders at high temperatures. [6,7] The as-synthesized oxide nanobelts are pure, structurally uniform, single crystalline, and most of them are free from dislocations. They have a rectangular cross section with typical widths of 100±300 nm, width-to-thickness ratios of 5±10 nm, and lengths of up to a few millimeters. The nano-belts are an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and for building functional devices along individual nanobelts. Nanosize sensors and field-effect transistors based on individual nanobelts have been fabricated. Colloid based methods are likely the most popular techniques for synthesis of semiconductor quantum dots. In this paper, using as-synthesized ZnO nanobelts as a template , nanostructured ZnS nanocables and nanotubes have been synthesized by chemical reaction. The structure of the ZnS has been analyzed and the corresponding photolumines-cence properties have been measured. A small blue shift is observed for the ZnO±ZnS cable structures, suggesting a small quantum-confinement effect. The template-assisted method is demonstrated to be a unique technique for producing ZnS nanostructures with controlled morphology. We first present the structural change of the ZnO nanobelt pre-and post-reaction with H 2 S. Figure 1a shows a scanning electron microscope (SEM) image of the as-synthesized ZnO nanobelts, which are pure and structurally uniform. A transmission electron microscope (TEM) image of the nanobelts is given in Figure 2a, clearly showing its uniformity in shape. The ZnO nanobelts have two fast growth directions, [0001] and [1010]. [6] For the [0001] ZnO nanobelts, the top surfaces are ± (21 10), and the side surfaces are ± (0110). The contrast observed on the nanobelts is due to bending induced strain, which is the so-called bending contour in electron diffrac-tion. Based …

Nanoarchitectures of semiconducting and piezoelectric zinc oxide

Semiconducting and piezoelectric zinc oxide has two important structure characteristics: the multiple and switchable growth directions: ⟨011¯0⟩, ⟨21¯1¯0⟩, and ⟨0001⟩; and the {0001} polar surfaces. The fast growth directions create nanobelts of different crystallographic facets, and the polar surfaces result in bending of the nanobelt for minimizing the spontaneous polarization energy. A combination of these distinct growth characteristics results in a group of unique nanostructures, including several types of nanorings, nanobows, platelet circular structures, Y-shape split ribbons, and crossed ribbons. We present here the as-grown nanoarchitectures naturally created by combining some of the fundamental structure configurations of ZnO, which could be unique for many applications in nanotechnology.

Monolithically integrated spinel M(x)Co(3-x)O(4) (M=Co, Ni, Zn) nanoarray catalysts: scalable synthesis and cation manipulation for tunable low-temperature CH(4) and CO oxidation.

A series of large scale Mx Co3-x O4 (M=Co, Ni, Zn) nanoarray catalysts have been cost-effectively integrated onto large commercial cordierite monolithic substrates to greatly enhance the catalyst utilization efficiency. The monolithically integrated spinel nanoarrays exhibit tunable catalytic performance (as revealed by spectroscopy characterization and parallel first-principles calculations) toward low-temperature CO and CH4 oxidation by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behavior and surface defect population. This provides a feasible approach for scalable fabrication and rational manipulation of metal oxide nanoarray catalysts applicable at low temperatures for various catalytic reactions.

A review of NOx storage/reduction catalysts: mechanism, materials and degradation studies

The catalytic removal of nitrogen oxides (NOx) from lean-burn exhaust emissions is one of the major challenges in environmental catalysis. Among the NOx emission control technologies, NOx storage/reduction (NSR) is currently regarded as one of the most practical technologies for lean-burn gasoline and diesel vehicles. This review gives a comprehensive overview of NSR technology, including the NSR reaction mechanisms, degradation mechanisms and NSR catalyst developments. The NSR reaction and degradation mechanisms will be addressed based on a typical NSR catalyst such as Pt/BaO/Al2O3, along with the concurrent new NSR catalyst developments for enhancing the NSR performance and alleviating their sulfur poisoning and thermal degradation.

Synthesis, characterization, and photocatalytic properties of ZnO/(La,Sr)CoO3 composite nanorod arrays

ZnO/(La, Sr) CoO3 (ZnO/LSCO) core-shell composite nanorod arrays have been successfully synthesized by a sequential combination process of a hydrothermal synthesis followed by a pulsed laser deposition (PLD) process (or a colloidal deposition process). Compared to the colloidal deposition process, PLD produces a more uniform and efficient deposition of continuous and mesoporous LSCO thin films onto ZnO nanorod arrays. During the PLD process, the deposited film uniformity was found to be dependent on the nanorod diameter, array density, and thus specific surface area of the nanorod arrays, in addition to the PLD deposition parameters. Field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to investigate the surface morphologies and orientations of the composite nanorod arrays. With densely packed ZnO nanorod arrays as a unique support structure, the mesoporous LSCO thin film coated on top exhibited better photocatalytic properties than ZnO nanorod arrays and LSCO thin films deposited on flat Si substrates. With optimization of the structure, dimensionality, packing density, as well as the composition and interface structure, these unique composite nanoarchitectures could be a promising class of photocatalyst candidates for organic molecule degradation.

Low temperature synthesis and characterization of MgO/ZnO composite nanowire arrays

Large scale dendritic MgO/ZnO composite nanowire arrays have been successfully synthesized on Si substrates using a two-step sequential hydrothermal synthesis at low temperature for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and x-ray photoelectron spectroscopy (XPS) were systematically carried out to confirm and elaborate the potentially localized Mg surface alloying process into the ZnO nanowire arrays. Both room temperature and low temperature (40 K) photoluminescence results revealed an enhanced and blue-shifted near-band-edge (NBE) ultraviolet (UV) emission for the MgO/ZnO nanowires compared to those of the pure ZnO nanowire arrays, indicating the success of Mg alloying into ZnO nanowires. This enhancement might be due to the 155 degrees C hydrothermal process and the amorphous MgO layer in the MgO/ZnO nanowires. The specific template of densely packed ZnO nanowire arrays was suggested to be instrumental in enabling this type of MgO/ZnO composite nanowire growth.

Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction

Efficient bifunctional catalysts for electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable due to their wide applications in fuel cells and rechargeable metal air batteries. However, the development of nonprecious metal catalysts with comparable activities to noble metals is still challenging. Here we report a one-step wet-chemical synthesis of Ni-/Mn-promoted mesoporous cobalt oxides through an inverse micelle process. Various characterization techniques including powder X-ray diffraction (PXRD), N2 sorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) confirm the successful incorporation of Ni and Mn leading to the formation of Co-Ni(Mn)-O solid solutions with retained mesoporosity. Among these catalysts, cobalt oxide with 5% Ni doping demonstrates promising activities for both ORR and OER, with an overpotential of 399 mV for ORR (at -3 mA/cm(2)) and 381 mV (at 10 mA/cm(2)) for OER. Furthermore, it shows better durability than precious metals featuring little activity decay throughout 24 h continuous operation. Analyses of cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), Raman, and O2-temperature-programmed desorption (O2-TPD) reveal that redox activity of Co(3+) to Co(4+) is crucial for OER performance, while the population of surface oxygen vacancies and surface area determine ORR activities. The comprehensive investigation of the intrinsic active sites for ORR and OER by correlating different physicochemical properties to the electrochemical activities is believed to provide important insight toward the rational design of high-performance electrocatalysts for ORR and OER reactions.