ZnO Multipods Research Articles

Facile synthesis of highly uniform ZnO multipods as the supports of Au and Ag nanoparticles

Through the direct reaction of the aqueous solutions of zinc salts and KOH, highly uniform ZnO multipods consisted of rod-shaped branches are synthesized in an extremely high yield. The synthesis is conducted at low temperature, does not require any organic capping agents to affect the crystal habit, and is independent on the species of zinc sources. Moreover, by changing the reaction temperature from 25 to 70 °C, the tips of the multipods can be adjusted from taper to regular hexagon. The mechanism responsible for the formation of ZnO multipods is the self-regulation growth guided by the inherently asymmetric structure of hexagonal ZnO along the c-axis. After the surface modification with amino groups, ZnO multipods show good affinity to Ag and Au nanoparticles, which results in the spontaneous migration of Au and Ag nanoparticles to the surface of ZnO to form well-defined Au/ZnO and Ag/ZnO heterostructures. The fabricated hybrids are found to be effective surface-enhanced Raman scattering (SERS) substrates because of their distinct geometrical shape, large surface area, and the ability to create a strong local electromagnetic field by the metal–semiconductor heterojunction.

Effect of ZnO seed layer on the catalytic growth of vertically aligned ZnO nanorod arrays

We have grown vertically aligned ZnO nanorods and multipods by a seeded layer assisted vapor–liquid–solid (VLS) growth process using a muffle furnace. The effect of seed layer, substrate temperature and substrate material has been studied systematically for the growth of high quality aligned nanorods. The structural analysis on the aligned nanorods shows c-axis oriented aligned growth by homoepitaxy. High crystallinity and highly aligned ZnO nanorods are obtained for growth temperature of 850–900 °C. Depending on the thickness of the ZnO seed layer and local temperature on the substrate, some region of a substrate show ZnO tetrapod, hexapods and multipods, in addition to the vertically aligned nanorods. Raman scattering studies on the aligned nanorods show distinct mode at ∼438 cm −1, confirming the hexagonal wurtzite phase of the nanorods. Room temperature photoluminescence studies show strong near band edge emission at ∼378 nm for aligned nanorods, while the non-aligned nanorods show only defect-emission band at ∼500 nm. ZnO nanorods grown without the seed layer were found to be non-aligned and are of much inferior quality. Possible growth mechanism for the seeded layer grown aligned nanorods is discussed.

From single ZnO multipods to heterostructured ZnO/ZnS, ZnO/ZnSe, ZnO/Bi2S3 and ZnO/Cu2S multipods: controlled synthesis and tunable optical and photoelectrochemical properties

This manuscript describes the selective modification of ZnO multipods with a series of wide bandgap and narrow bandgap semiconductors (ZnS, ZnSe, Bi2S3, and Cu2S). ZnO multipods are firstly synthesized via the aqueous phase reaction of Zn2+ ions with KOH at a temperature lower than 70 °C, which is simple, facile and free of organic capping agents. Through the reaction of the chalcogen source (Na2S2O3 or Na2SeSO3) with the multipods supported by zinc or copper sheet, heterostructured ZnO/ZnS, ZnO/ZnSe, ZnO/Bi2S3 and ZnO/Cu2S multipods are formed. The particles of ZnS and ZnSe are selectively deposited on the tips of the multipods but Cu2S and Bi2S3 are randomly distributed on the entire surfaces of the multipods. The difference among the deposition sites of ZnS, ZnSe, Cu2S, and Bi2S3 is related to the different structural affinity of these semiconductors to ZnO multipods. The heterostructures have integrated the optical absorption properties of ZnO multipods and the deposited semiconductors, and as a result, the optical absorption range of ZnO multipods is expanded (335∼1000 nm). The photoelectrochemical properties of the heterostructures are also different from that of bare ZnO multipods. The photocurrent densities yielded by bare ZnO multipods, heterostructured ZnO/ZnS, ZnO/ZnSe, and ZnO/Bi2S3 multipods are 0.08 mA cm−2, 0.10 mA cm−2, 0.30 mA cm−2, and 0.03 mA cm−2, respectively. As for ZnO/Cu2S multipods, the photocurrent is unexpectedly smaller than the dark current.

Synthesis, characterization and optical properties of multipod ZnO whiskers

Multipod ZnO whiskers were synthesized successfully by two steps: pulsed laser deposition (PLD) and thermal evaporation process. First, a thin layer of Zn films were deposited on Si(1 1 1) substrates by PLD. Then the whiskers grew on Zn-coated Si(1 1 1) substrate by the simple thermal evaporation oxidation of the metallic zinc powder at 900 °C in the air without any catalysts or additives. The pre-deposited Zn films by PLD on the substrate can promote the growth of ZnO multipod whiskers effectively. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the whiskers are highly crystalline with the wurtzite hexagonal structure. Room temperature photoluminescence (PL) spectrum of the whiskers shows a UV emission peak at ∼393 nm and a broad green emission peak at ∼517 nm, which was assigned to the near band-edge emission and the deep-level emission, respectively.

Formation of tetrapod and multipod ZnO whiskers

ZnO tetrapod and multipod nanostructures were synthesized by vapor-phase transport method using the mixtures of ZnO (or ZnO:In 2O 3) and graphite powders as source materials. By controlling the growth condition, the tetrapod whiskers with homogeneous pillar, awl and inverse-awl shaped legs, and the multipods with the similar leg shapes were obtained. Combining X-ray diffraction pattern, scanning electron microscopy, and high-resolution transmission electron microscopy image of the products in initial and final growth stages, the growth process was investigated systematically. The influences of the source, growth temperature, and vapor pressure on the final morphology were discussed by considering the surface energy of nucleus and the kinetic energy of the incoming atoms.

Surface-State-Mediated Electron Transfer at Nanostructured ZnO Multipod/Electrolyte Interfaces

Redox kinetics of cyanoferrate(III) species adsorbed at an n-type ZnO multipod/electrolyte interface is explored using electrochemical techniques like cyclic voltammetry and impedance spectroscopy. The electrochemical impedance results are analyzed using a fluctuating energy level model, assuming isoenergetic tunneling of majority carriers through the Helmholtz layer. A shift in the slope of Mott−Schottky plots (Csc-2 versus E) together with evidence from cyclic voltammetry shows that the electron-transfer process is mediated by surface states formed because of the adsorption of ferricyanide ions (as evident from the results of Fourier transform infrared spectroscopy). More significantly, the pH of zero charge (point of zero zeta potential, pzzp) of ZnO multipods is found to be 4.5 (from capacitance vs pH plots) compared to that of bulk ZnO (pH 9.5), which could be explained on the basis of a lowering in the work function of the nanostructured semiconductor and its consequent susceptibility to the formation of surface states. This is in excellent agreement with our earlier observation of ultralow threshold field emission with this material in the light of the linear dependence of pzzp with work function of the electrode material. The flat-band potential of the nanostructures is found to be 200 mV more negative than that reported for bulk n-type ZnO electrodes, indicating a higher doping density in the former. A three-dimensional mapping of charge distribution in the surface states is attempted by correlating the capacitance response of the system subjected to a sinusoidal potential modulation to the semiconductor electrode with that resulting from a systematic variation of the redox potential of the dissolved acceptor (achieved by varying the pH of the electrolyte) which further reveals the polyenergetic nature of the surface states.

Multipod ZnO 3D microstructures

ZnO 3D microstructures with nano-multipod have been self-assembly synthesized on a post-annealed Ag-coated silicon substrate by sintering the mixture of ZnO and graphite powders in Ar flow. SEM images show that the microstructure has a spherical core and multiple radial nanorods grown around its outside with diameter of about 1–2 μm and 50–200 nm, respectively. XRD investigation shows the ZnO multipods are of a wurtzite structure. The room temperature photoluminescence spectrum of the microstructures reveals a strong broad green emission band at 530 nm. It is found that the post-deposition annealing treatment of the Ag thin film is a key factor to control the formation of ZnO 3D microstructures. The possible growth process based on VLS mechanism is proposed.

Multipod ZnO Nanoforms: Low Temperature Synthesis and Characterization

ZnO nanotetrapods were synthesized by a simple thermal evaporation of Zn powder at a relatively low temperature approximately 600 degrees C. The tetrapods have four legs with hexagonal cross-section. Interpenetrating growth was observed in some of these nanotetrapods. Multipod ZnO nanoforms were produced at higher temperature. The optical characterizations such as optical absorbance, photoluminescence and Raman spectroscopy reveal excellent crystal qualities of these nanoforms. The field emission studies indicated that these nanoforms could be utilized in field emission based devices.

ZnO Multipods, Submicron Wires, and Spherical Structures and Their Unique Field Emission Behavior

A simple method of vapor deposition for the shape selective synthesis of ZnO structures, namely, multipods, submicron wires, and spheres, has been successfully demonstrated. A plausible growth mechanism based on the studies of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) is proposed. Our studies suggest that the growth of a multipod structure is governed by the screw dislocation propagation while the vapor-liquid-solid (VLS) mechanism is responsible for the formation of submicron wires and spheres. Moreover, the flow rate of the carrier gas plays a crucial role in governing the morphology. Further, these structures exhibit an enhanced field emission behavior. The nonlinearity in the Fowler-Nordheim (F-N) plot, a characteristic feature of electron emission from semiconductors, is explained by considering the contributions from both the conduction and the valence bands of ZnO.

A general combustion approach to multipod ZnO and its characterization

By a general approach of combustion oxidation at high temperature, multipod ZnO was synthesized without any catalysts or additives. The morphology and optical properties of the multipod ZnO were studied in detail. The growth mechanism was discussed preliminarily. An ultraviolet (UV) emission peak at 374 nm and a broad green emission peak centered at 502 nm are observed in photoluminescence spectrum of the multipod ZnO. The multipod structure exhibits significant enhancement of UV emission intensity and green light emission intensity compared with the tetrapod structure, which are attributed to less structural defects and increased surface area respectively. Furthermore, compared with nano-particle and micro-particle ZnO, UV emission peak of multipod ZnO appears a slight blue shift. Due to slim tips of the legs, quantum size effect cause a slight blue shift of UV emission peak. We believe that these optical properties of the multipod structure have extensive applications in nanoscale optical devices.

Catalyst-free synthesis, growth mechanism and optical properties of multipod ZnO with nanonail-like legs

Abstract Multipod ZnO with nanonail-like legs was synthesized successfully by a facile combustion oxidation method at 880 °C without any catalysts. The morphologies of three samples obtained from three growth stages, namely sprout, growing and multipod with nanonail-like legs, were investigated. The possible growth mechanism was discussed in detail. Compared with the other two structures, the multipod with nanonail-like legs exhibited a significant enhancement in the green emission region. Its unique structure together with its good optical properties suggest that it is a promising candidate for the fabrication of optical nanodevices.

Synthesis and optical properties of ZnO multipod nanorods

Multipod ZnO nanorods modified by poly ( N-vinyl-2-pyrrolidone) (PVP) were synthesized by a simple solution method. Transmission electron microscopy, ultraviolet–visible spectrum and room-temperature photoluminescence spectrum were used to characterize the as-prepared products. The results indicate that the multipod ZnO nanorods are composed of six pods with the wurtzite structure. Each arm of the multipod ZnO has the average diameter of 45 nm.The as-prepared product shows wide ultraviolet emission peak around 385 nm excited by Xe lamp at the room temperature.

Ultralow threshold field emission from a single multipod structure of ZnO

The field emission of individual ZnO multipods and a single arm of a multipod structure grown by a vapor deposition were carried out. A current of 1 nA with an ultralow onset voltage of 40 V was observed repeatedly for the single multipod as well as for the arm. The nonlinearity observed in the Fowler–Nordheim plots have been interpreted on the basis of the theory of electron emission from semiconductors and a scheme explaining the field emission behavior in both the high- and low-field regions owing to the very high geometrical factor has been picturized.

Synthesis and characterization of multipod, flower-like, and shuttle-like ZnO frameworks in ionic liquids

A fast and environment-friendly microwave-heating route to synthesize multipod, flower-like and shuttle-like ZnO nano-/microstructures in room-temperature ionic liquids has been reported. The products were characterized by X-ray diffraction (XRD), scanning electron microcopy (SEM) and transmission electron microscopy (TEM). These ZnO frameworks are composed of needle-like, cone-like and shuttle-like rods growing homocentrically, respectively.

Synthesis and properties of ZnO multipod structures

We report fabrication and properties of multipod ZnO structures, synthesized from a mixture of ZnO, GeO2, and graphite. We studied the morphology, composition, and optical properties of these structures for different ZnO:GeO2 ratios. The formation mechanism of the multipods is discussed. The multipod structures exhibit significant enhancement of the green emission which can be attributed to the increased surface area. Enhancement of the green emission is of interest for typical applications of ZnO phosphors, such as field emissive display technology.

Evaporation growth of multipod ZnO whiskers assisted by a Cu 2+ etching technique

Large and uniform multipod ZnO whiskers were successfully synthesized by a Cu 2+ etching-assisted evaporation growth method. Products were characterized with X-ray diffraction, scanning electron microscopy, atomic emission spectrum and energy dispersive X-ray analysis. Results showed that the products had pure hexagonal phase with radical shapes. The functions of etching technique for the formation of final products were discussed.