Vertical Nanorod Arrays Research Articles

Enhanced synergistic effect of NiCo sulfide nanorod arrays with high mass loading on carbon cloth for hybrid supercapacitors: The role of compositional regulation

Achieving an enhanced synergistic effect and controlling the morphology of active materials to facilitate electrolyte access are crucial for the design of high-mass-loading electrode. In this work, NiCo sulfide nanorod arrays were grown on a carbon cloth (CC) substrate via a two-step hydrothermal deposition strategy. The compositional effect was employed to modify the growth behavior, hierarchical morphology, and synergistic effect of the as-obtained electrodes. The contents of Ni-S and Co-S species and the resistance nature of the electrodes can be optimized by adjusting the Co/Ni feed ratio. The optimized Ni1.5Co1.5S4/CC electrode with a high mass loading (5.54 mg cm−2) comprises vertical nanorod arrays with open channels for electrolyte access, and exhibits a high capacity of 711 C g−1 at 2 A g−1 (3.95 C cm−2 at 10 mA cm−2) with good cyclic stability. The as-assembled Ni1.5Co1.5S4/CC//activated carbon cloth (ACC) hybrid supercapacitor delivers a high areal capacity of 1.96 C cm−2 at 5 mA cm−2 with excellent rate performance and achieves a high energy density of 0.49 mWh cm−2 at a power density of 4.5 mW cm−2. These achievements represent a promising concept for the practical design and regulation of NiCo sulfide electrodes at high mass loading levels.

Schottky Contacts on Polarity-Controlled Vertical ZnO Nanorods.

Polarity-controlled growth of ZnO by chemical bath deposition provides a method for controlling the crystal orientation of vertical nanorod arrays. The ability to define the morphology and structure of the nanorods is essential to maximizing the performance of optical and electrical devices such as piezoelectric nanogenerators; however, well-defined Schottky contacts to the polar facets of the structures have yet to be explored. In this work, we demonstrate a process to fabricate metal-semiconductor-metal device structures from vertical arrays with Au contacts on the uppermost polar facets of the nanorods and show that the O-polar nanorods (∼0.44 eV) have a greater effective barrier height than the Zn-polar nanorods (∼0.37 eV). Oxygen plasma treatment is shown by cathodoluminescence spectroscopy to affect midgap defects associated with radiative emissions, which improves the Schottky contacts from weakly rectifying to strongly rectifying. Interestingly, the plasma treatment is shown to have a much greater effect in reducing the number of carriers in O-polar nanorods through quenching of the donor-type substitutional hydrogen on oxygen sites (HO) when compared to the zinc-vacancy-related hydrogen defect complexes (VZn-nH) in Zn-polar nanorods that evolve to lower-coordinated complexes. The effect on HO in the O-polar nanorods coincides with a large reduction in the visible-range defects, producing a lower conductivity and creating the larger effective barrier heights. This combination can allow radiative losses and charge leakage to be controlled, enhancing devices such as dynamic photodetectors, strain sensors, and light-emitting diodes while showing that the O-polar nanorods can outperform Zn-polar nanorods in such applications.

Spontaneous growth of III-nitride 1D and 0D nanostructures on to vertical nanorod arrays

In this work, we report on the growth of AlGaN nanostructures by the droplet epitaxy process. Initially, well-oriented vertical AlN nanorod clusters were grown on to c-plane sapphire substrates by Plasma Assisted Molecular Beam Epitaxy (PA-MBE). On top of these AlN nanorods an Al0.76Ga0.24N layer was deposited, followed by 40 pairs of Al0.76Ga0.24N/AlN Multiple Quantum Wells (MQWs). Spontaneously formed nanodots and nanowires were observed by Field Emission Scanning Electron Microscopy (FESEM) on the top planes of these vertical nanorod arrays. For nearly stoichiometric conditions, 20 nm diameter AlGaN nanodots were formed selectively at the step edges generated during step-flow growth. For growth under excess group III conditions, lateral nanowires were formed perpendicular to the edges of the vertical nanorods, with widths varying from 20 nm near the center to 70 nm at the edge for a length of ∼150 nm. An enhancement of at least 15 times was obtained in the Cathodoluminescence (CL) emission peak intensity (290 nm) from the top of the nanorod structures. We believe these spontaneously formed well-ordered nanodot and horizontal nanowire structures are generated by the sequential formation and Nitridation of metal nanodroplets on the growth surface. Their properties can be controlled by optimization of the deposition parameters by varying the surface diffusivity of Ga and Al adatoms.

Separating light absorption layer from channel in ZnO vertical nanorod arrays based photodetectors for high-performance image sensors

Photoconductor arrays with both high responsivity and large ON/OFF ratios are of great importance for the application of image sensors. Herein, a ZnO vertical nanorod array based photoconductor with a light absorption layer separated from the device channel has been designed, in which the photo-generated carriers along the axial ZnO nanorods drive to the external electrodes through nanorod-nanorod junctions in the dense layer at the bottom. This design allows us to enhance the photocurrent with unchanged dark current by increasing the ratio between the ZnO nanorod length and the thickness of the dense layer to achieve both high responsivity and large ON/OFF ratios. As a result, the as-fabricated devices possess a high responsivity of 1.3 × 105 A/W, a high ON/OFF ratio of 790, a high detectivity of 1.3 × 1013 Jones, and a low detectable light intensity of 1 μW/cm2. More importantly, the developed approach enables the integration of ZnO vertical nanorod array based photodetectors as image sensors with uniform device-to-device performance.

Extraordinarily high-rate capability of polyaniline nanorod arrays on graphene nanomesh

We report the high-rate capacitive performance of polyaniline (PANI) coated on graphene nanomesh (GNM). The PANI on GNM sheets shows vertical nanorod arrays that offer large electrochemically active surface area and numerous ion-accessible gaps. Benefiting from the highly pseudocapacitive PANI and ion-penetrable GNM, GNM-PANI with 70 wt% PANI exhibits a specific capacitance of 452 F g−1 at 1.0 A g−1, which remains at 404 F g−1 even at high discharge rate of 50 A g−1 in 1.0 M H2SO4. More importantly, a symmetric capacitor built with this electrode exhibits a relaxation time constant of 2.61 s, a value comparable to 2–5 s with activated carbon as electrode, and much shorter than 4–6 s of capacitors with chemically converted graphene as electrodes. The performance of GNM-PANI electrode reported herein is far superior to that of most other PANI-based electrodes, demonstrating that GNM is a promising scaffold that can help to realize the full potential of PANI due to its abundant in-plane nanopores.

An Ultrahigh-Resolution Digital Image Sensor with Pixel Size of 50 nm by Vertical Nanorod Arrays.

The pixel size limit of existing digital image sensors is successfully overcome by using vertically aligned semiconducting nanorods as the 3D photosensing pixels. On this basis, an unprecedentedly high-resolution digital image sensor with a pixel size of 50 nm and a resolution of 90 nm is fabricated. The ultrahigh-resolution digital image sensor can heavily impact the field of visual information.

Control growth of single crystalline ZnO nanorod arrays and nanoflowers with enhanced photocatalytic activity

Single crystalline vertical nanorod arrays and nanoflowers of ZnO have been grown in situ on cheap zinc foils under hydrothermal conditions, by means of hexamethylenetetramine and ethanolamine, respectively. Their morphologies and crystal structures are characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The nanorods and flowers of ZnO grew along the $$ \{ 10\bar{1}1\} $$ and $$ \{ 0001\} $$ planes, respectively. Both types of ZnO display high photocatalytic ability toward the degradation of methylene orange under UV irradiation. The ZnO nanorods show better performance than that of the ZnO nanoflowers, and the $$ \{ 10\bar{1}1\} $$ facets of the ZnO nanorods have higher photoactivity than that of the $$ \{ 000\bar{1}\} $$ or $$ \{ 10\bar{1}0\} $$ crystal planes. This is because the weaker coordinated O atoms on the surface are more likely to be saturated by H atoms in aqueous solution, thereby releasing more free OH radicals. A facile method was developed for selective control synthesis of ZnO nanoflowers and nanorod arrays on Zinc foil, with the assistance of ethanolamine and the hexamethylenetetramine, respectively. The illustration shows the time evolution of the two ZnO structures.

Large-scale floated single-crystalline TiO2 flower-like films: synthesis details and applications

Large-scale floated single-crystal flower-like films were synthesized without any surfactants in a one-step hydrothermal reaction. Enhanced light-harvesting performance was obtained compared to vertical nanorod arrays, and significant improvements of photoelectric conversion efficiency have been achieved in the dye-sensitized solar cells using the films as photoanodes or scattering layers.

A fabrication method for vertical oxide nano-rod array by internal oxidation of alloys at high-temperatures

A simple metallurgical process for fabricating vertical oxide nano-rod array structures with internal oxidation of alloys is described. Oxidation at high-temperatures and under low oxygen partial pressures can develop internally rod-like oxide precipitates in some dilute alloys such as Ni(Al) solid solutions. The vertical oxide nano-rod arrays can be developed on the metal substrates or components by removing the metallic matrix of the internal oxidation zone. Al2O3 or NiAl2O4 spinel nano-rod arrays were prepared by using M(Al) solid solutions. Pack cementation process to develop Ni(Al) solid solution zone on a metal surface was used for the fabrication of vertical nano-rod arrays on Ni substrates with desired shape. Near-net shape Ni substrates with vertical oxide nano-rod array structures on their surfaces can be prepared by using pack cementation and internal oxidation. Fabrication of vertical nano-rod arrays with Co(Al) and Fe(Al) solid solutions were also described briefly.

Enhanced Photocatalytic Activity by Aligned WO3/TiO2 Two-Layer Nanorod Arrays

The photocatalytic and structural properties of TiO2/WO3 two-layer nanostructures made by electron beam deposition were studied and compared with single-layer TiO2 nanostructures. Three different two-layer structures, thin film, tilted nanorods, and vertical nanorods, were fabricated by a normal deposition, oblique angle deposition, and glancing angle deposition. The as-deposited and annealed samples at Ta = 300 °C and Ta = 400 °C, respectively, were studied by SEM, XRD, Raman spectroscopy, and UV−vis spectrophotometry. The photocatalytic properties of those samples were investigated and found to be improved significantly by increasing the surface and interfacial area, annealing at a proper temperature, the enhanced charge separation due to the WO3−TiO2 coupling, and the arrangement of the two layers. The optimal structure was found to be the two-layer vertical TiO2/WO3 nanorods after annealing at Ta = 300 °C, which had a photodegradation rate about 10 times higher than a single-layer TiO2 vertical nanorod array.

Ce-induced single-crystalline hierarchical zinc oxide nanobrushes

High-density hierarchical brush-like nanostructures of Ce-doped ZnO are first synthesized by a two-step approach (sol–gel and chemical vapor deposition). This novel structure, called nanobrush, is a new member in the family of hexagonal crystal ZnO nanostructures, which is composed of two parts: a micron-sized prism-like base and nano-sized vertical nanorod arrays. The nanobrush is enclosed by the (0001) and { 01 1 ¯ 0 } facets and grows towards the [0001] direction. Room-temperature photoluminescence (RTPL) spectra show a weak UV emission but a strong broad green emission.

Template-Synthesized Cobalt Porphyrin/Polypyrrole Nanocomposite and Its Electrocatalysis for Oxygen Reduction in Neutral Medium

For the first time, a linear J-aggregate of cobalt porphyrin and its templated, nanostructured cobalt porphyrin/polypyrrole composite are formed in neutral aqueous solutions. With the assistance of ultrasonication under different preparation procedures, the nanocomposite can be electrochemically synthesized with uniform 2-D and 3-D nanostructures. The 3-D nanocomposite is synthesized by a three-step method and has an interesting regular nanoarray-structure: a vertical nanorod array with uniform diameters (∼50 nm). The resultant nanoarray catalyzes the oxygen reduction mainly through a four-electron pathway to form H2O in a neutral medium, exhibiting excellent electrocatalytic activity. The mechanisms of formation of the nanocomposite and its good electrocatalysis for oxygen reduction are suggested. The new nanocomposite could have broad potential applications in energy storage systems and electrochemical sensors, particularly in a neutral medium for the biofuel cells and biosensors.

Shape‐selective synthesis of II–VI semiconductor nanowires

AbstractPolar II–VI semiconductors can nucleate in complex shapes ranging from nanowires to nanoribbons, nanosaws and multipods. Here we demonstrate the deterministic and fully reproducible shape‐selective growth of several morphologies of CdSe and ZnTe nanocrystals by a steady‐state vapour transport process. A simple pressure‐based precursor‐flow shutter excludes any effects of temperature ramping, ensuring reproducible shape selectivity for each set of deposition parameters. Once thermal gradients are eliminated, we show that the transition from one nanocrystal shape to another is controlled just by the interplay of precursor impinging on the substrate (ruled by the powder temperature T P) and sample surface kinetics (ruled by the sample temperature T S). Furthermore, a regime is found where seeded, epitaxial growth of CdSe nanorods becomes dominant over the conventional catalyst‐assisted nucleation. This allows the fabrication of vertical nanorod arrays free of any metal contamination. Seeded growth of branched and tetrapod‐like nanocrystals is also possible by further optimisation of the growth parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)