One-step electrodeposition of Ag-decorated ZnO nanowires

A new route for synthesizing Ag-decorated ZnO nanowires (NWs) on conductive glass substrates using a one-step electrodeposition technique is described here. The structural, optical, and photoelectrochemical properties of Ag-decorated ZnO nanowires were studied in detail using techniques such X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-visible spectroscopy, photoluminescence, and photoelectrochemical measurements. Both pure and Ag-decorated ZnO nanowires were found to crystallize in the wurtzite structure, irrespective of their Ag contents. Increasing the Ag content from pure ZnO NWs to 3% Ag ZnO NWs decreases the photoluminescence intensity, shifts the optical band gap to the red, and increases the photocurrent up to threefold. This behavior was attributed to the surface plasmon resonance effect induced by the Ag nanoparticles, which inhibits charge recombination and improves charge transport on the ZnO surface.     

Enhanced electron field emission from ZnO nanoparticles-embedded DLC films prepared by electrochemical deposition

ZnO nanoparticles-embedded diamond-like amorphous (DLC) carbon films have been prepared by electrochemical deposition. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) results confirm that the embedded ZnO nanoparticles are in the wurtzite structure with diameters of around 4 nm. Based on Raman measurements and atomic force microscope (AFM) results, it has been found that ZnO nanoparticles embedding could enhance both graphitization and surface roughness of DLC matrix. Also, the field electron emission (FEE) properties of the ZnO nanoparticles-embedded DLC film were improved by both lowering the turn-on field and increasing the current density. The enhancement of the FEE properties of the ZnO-embedded DLC film has been analyzed in the context of microstructure and chemical composition.     

Photoelectrochemical investigation of ultrathin film iron oxide solar cells prepared by atomic layer deposition

Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)(6)](3-/4-) electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ～20 nm. Mott-Schottky analysis of differential capacitance measurements indicated a depletion region of ～17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.     

Tailoring nanoporous materials by atomic layer deposition

Atomic layer deposition (ALD) is a cyclic process which relies on sequential self-terminating reactions between gas phase precursor molecules and a solid surface. The self-limiting nature of the chemical reactions ensures precise film thickness control and excellent step coverage, even on 3D structures with large aspect ratios. At present, ALD is mainly used in the microelectronics industry, e.g. for growing gate oxides. The excellent conformality that can be achieved with ALD also renders it a promising candidate for coating porous structures, e.g. for functionalization of large surface area substrates for catalysis, fuel cells, batteries, supercapacitors, filtration devices, sensors, membranes etc. This tutorial review focuses on the application of ALD for catalyst design. Examples are discussed where ALD of TiO(2) is used for tailoring the interior surface of nanoporous films with pore sizes of 4-6 nm, resulting in photocatalytic activity. In still narrower pores, the ability to deposit chemical elements can be exploited to generate catalytic sites. In zeolites, ALD of aluminium species enables the generation of acid catalytic activity.     

Barrier properties of polymer/alumina nanocomposite membranes fabricated by atomic layer deposition

Novel polymer/ceramic nanocomposite membranes were fabricated, characterized and tested for their barrier performance. Atomic layer deposition (ALD) was used to deposit alumina films on primary, micron-sized (16 and 60 μm) high-density polyethylene (HDPE) particles at a rate of 0.5 nm/cycle at 77 °C. Well-dispersed polymer/ceramic nanocomposites were obtained by extruding alumina coated HDPE particles. The dispersion of alumina flakes can be controlled by varying the number of ALD coating cycles and substrate polymer particle size. The diffusion coefficient of fabricated nanocomposite membranes can be reduced to half with the inclusion of 7.29 vol.% alumina flakes. However, a corresponding increase in permeability was also observed due to the voids formed at or near the interface of the polymer and alumina flakes during the extrusion process, as evidenced by electron microscopy. The low surface wettability of the alumina outerlayers was believed to be one of the main reasons of void formation. Particle surface wettability was improved using 3-aminopropyltriethoxysilane (APS) to coat the particle ALD surface modified polymer particles prior to extrusion. The diffusion coefficient and permeability of the membrane using surfactant-modified particles decreased by 20%, relative to the non-modified case.     

Raman scattering and efficient UV photoluminescence from well-aligned ZnO nanowires epitaxially grown on GaN buffer layer

Optical phonon confinement and efficient UV emission of ZnO nanowires were investigated in use of resonant Raman scattering (RRS) and photoluminescence (PL). The high-quality ZnO nanowires with diameters of 80-100 nm and lengths of several micrometers were epitaxially grown through a simple low-pressure vapor-phase deposition method at temperature 550 degrees C on the precoated GaN(0001) buffer layer. The increasing intensity ratio of n-order longitudinal optical (LO) phonon (A(1)(nLO)/E(1)(nLO)) with increasing scattering order in RRS reveals the phonon quantum confinement as shrinking the diameter of ZnO nanowires. The exciton-related recombination near the band-edge transition dominate the UV emissions at room temperature as well as at low temperature that exhibits almost no other nonstoichiometric defects in the ZnO nanowires.     

原子层沉积技术制备单原子催化剂

贵金属单原子催化剂因具有独特的催化性能和高的利用率而迅速引人关注.原子层沉积(ALD)逐渐成为大批量合成稳定单原子的有力工具.本文总结了采用ALD合成单原子的最新进展,以及未来的研究方向和趋势.     

Growth of iron cobalt oxides by atomic layer deposition

Thin films of iron cobalt oxides with spinel-type structure are made by the atomic layer deposition (ALD) technique using Fe(thd)3 (Hthd = 2,2,6,6-tetramethylheptane-3,5-dione), Co(thd)2, and ozone as precursors. Pulse parameters for ALD-type growth are established and such growth can be achieved at deposition temperatures between 185 and 310 degrees C. Films have been deposited on amorphous soda-lime glass and single-crystalline substrates of Si(100), MgO(100), and alpha-Al2O3(001) which all provide crystalline films, but with various orientations and crystallite sizes. Application of an external magnetic field during the film growth does not influence film growth characteristics (growth rate, crystallinity, topography etc.). Magnetization data are reported for phase-pure films of spinel-type structure with composition Fe2CoO4.     

Enhanced ultraviolet emission from ZnS-coated ZnO nanowires fabricated by self-assembling method

A simple chemical route for ZnS-coated ZnO nanowires with preferential (002) orientation is reported. Sodium sulfide and zinc nitrate were employed to supply S and Zn atoms at 60 degrees C to form ZnS-coated ZnO nanowires structures. Electron diffraction measurement shows that the ZnO/ZnS core-shell nanostructure is single crystalline. Interesting features are found in the photoluminescence (PL) spectra of ZnS-coated ZnO nanostructures. After coating, the UV emission of nanorods is dramatically enhanced at the expense of the green emission. The core/shell structure with higher band gap shell material and reduced surface states should be responsible for this PL enhancement.     

Synthesis and properties of ZnO nanofibers prepared by electrospinning

ZnO nanofibers were prepared from zinc acetate/polyvinyl alcohol (PVA) by electrospun method. The morphological features, crystallinity, mechanical and optical properties of the ZnO nanofibers were studied. The results show the specific surface area of the ZnO nanofibers was influenced by the electrospun conditions. The specific surface area reached 389.7 m²g⁻¹ as the average diameter was 232 nm. The XRD date reveals the nanofibers consist of a single phase of well-crystallized ZnO with hexagonal structure. The elastic modulus of a single ZnO nanofiber was also characterized by nano-scale three-point bending test.     

Enhanced super-hydrophobic and switching behavior of ZnO nanostructured surfaces prepared by simple solution--immersion successive ionic layer adsorption and reaction process

A simple and cost-effective successive ionic layer adsorption and reaction (SILAR) method was adopted to fabricate hydrophobic ZnO nanostructured surfaces on transparent indium-tin oxide (ITO), glass and polyethylene terephthalate (PET) substrates. ZnO films deposited on different substrates show hierarchical structures like spindle, flower and spherical shape with diameters ranging from 30 to 300 nm. The photo-induced switching behaviors of ZnO film surfaces between hydrophobic and hydrophilic states were examined by water contact angle and X-ray photoelectron spectroscopy (XPS) analysis. ZnO nanostructured films had contact angles of ~140° and 160°±2 on glass and PET substrates, respectively, exhibiting hydrophobic behavior without any surface modification or treatment. Upon exposure to ultraviolet (UV) illumination, the films showed hydrophilic behavior (contact angle: 15°±2), which upon low thermal stimuli revert back to its original hydrophobic nature. Such reversible and repeatable switching behaviors were observed upon cyclical exposure to ultraviolet radiation. These biomimetic ZnO surfaces exhibit good anti-reflective properties with lower reflectance of 9% for PET substrates. Thus, the present work is significant in terms of its potential application in switching devices, solar coatings and self-cleaning smart windows.     

Photocatalytic properties of ZnO micromaterials prepared by hydrothermal method

ZnO microrods were prepared by a simple and inexpensive hydrothermal method. The structure, morphology and optical properties of the micromaterials were characterized by XRD, FE-SEM and UV–Vis spectroscopy. The FE-SEM micrographs indicated that the sizes of the microrods were 0.2–1 μm in width and 2–5 μm in length. With methyl orange as a model degradation target, photocatalytic performance of the microrods under ultraviolet radiation was evaluated. The results indicate that the photocatalytic activity of ZnO microrods was satisfactory even under low power ultraviolet irradiation.     

Preliminary studies in the electrodeposition of PbSe/PbTe superlattice thin films via electrochemical atomic layer deposition (ALD)

This paper concerns the electrochemical growth of compound semiconductor thin film superlattice structures using electrochemical atomic layer deposition (ALD). Electrochemical ALD is the electrochemical analogue of atomic layer epitaxy (ALE) and ALD, methods based on nanofilm formation an atomic layer at a time, using surface-limited reactions. Underpotential deposition (UPD) is a type of electrochemical surfaced-limited reaction used in the present studies for the formation of PbSe/PbTe superlattices via electrochemical ALD. PbSe/PbTe thin-film superlattices with modulation wavelengths (periods) of 4.2 and 7.0 nm are reported here. These films were characterized using electron probe microanalysis, X- ray diffraction, atomic force microscopy (AFM), and infrared reflection absorption measurements. The 4.2 nm period superlattice was grown after deposition of 10 PbSe cycles, as a prelayer, resulting in an overall composition of PbSe0.52Te0.48. The 7.0 nm period superlattice was grown after deposition of 100 PbTe cycle prelayer, resulting for an overall composition of PbSe0.44Te0.56. The primary Bragg diffraction peak position, 2theta, for the 4.2 superlattice was consistent with the average (111) angles for PbSe and PbTe. First-order satellite peaks, as well as a second, were observed, indicating a high-quality superlattice film. For the 7.0 nm superlattice, Bragg peaks for both the (200) and (111) planes of the PbSe/PbTe superlattice were observed, with satellite peaks shifted 1 degrees closer to the (111), consistent with the larger period of the superlattice. AFM suggested conformal superlattice growth on the Au on glass substrate. Band gaps for the 4.2 and 7.0 nm period superlattices were measured as 0.48 and 0.38 eV, respectively.     

Defect formation and magnetic properties of carbon-doped ZnO nanowires by the first principles

Theoretical calculation based on density functional theory (DFT) and local density approximation (LDA) with Hubbard parameters has been carried out in studying defect formation energy, transition energy and ferromagnetism of carbon-doped ZnO nanowires (NW). The formation and ionization characteristics of the defects [C_O (B), C_O (S), C_Zn (B), V_O (B), V_Zn (B), I_O (oct) and I_Zn (oct)] in ZnO NW are analyzed in the text. Ferromagnetic (FM) and antiferromagnetic (AFM) coupling between C atoms are also investigated by 9 different configurations. The FM and AFM stability are explained by the interaction of C energy level. In addition, the vacancies [V_O (B) and V_Zn (B)] and interstitials [I_O (oct) and I_Zn (oct)] affecting the FM coupling are also investigated. It is found that magnetic moment of C 2p can be mediated by these defects.     

Morphological control of ZnO nanostructures by electrodeposition

We report here an electrodeposition route for the preparation of oriented and well-defined ZnO nanostructures by kinetically controlling the growth rates of various facets of the deposit by appropriate capping agents. We demonstrated that adsorption of Cl(-) takes places preferentially onto the (0001) planes to hinder the crystal growth along the c-axis, and results in the formation of platelet-like crystals. It is also shown that the morphology evolved from hexagonal tapers to hexagonal rods and rhombohedral rods by changing the compositions of the capping agents. Furthermore, strong UV emissions at 380 approximately 390 nm and negligible green bands at around 500 nm were observed, indicating that these ZnO electrodeposits are highly crystallized and of excellent optical quality.     

Structural and optical properties of ZnMgO nanostructures formed by Mg in-diffused ZnO nanowires

ZnMgO nanostructures with wurtzite phase were prepared by thermal diffusion of Mg into the ZnO nanowires. As ZnO light-emitting devices have been operated by using ZnMgO layers as energy barrier layers to confine the carriers, it is essential to realize the characterization of ZnMgO particularly. In this work, the Mg content in Zn_1−xMg_xO alloy determined by X-ray diffraction (XRD) and photoluminescence (PL) shows a good coincidence. The variation of lattice constant and the blueshift of near-band-edge emission indicate that Zn²⁺ ions are successfully substituted by Mg²⁺ ions in the ZnO lattice. In Raman-scattering studies, the change of E₂(high) phonon line shape in ZnO:Mg nanostructures reveals the microscopic substitutional disorder. In addition to the host phonons of ZnO, two additional bands around 383 and 510 cm⁻¹ are presumably attributed to the Mg-related vibrational modes.