Zinc Oxide Nanopillars Research Articles

Wettability and Bactericidal Properties of Bioinspired ZnO Nanopillar Surfaces.

Nanomaterials of zinc oxide (ZnO) exhibit antibacterial activities under ambient illumination that result in cell membrane permeability and disorganization, representing an important opportunity for health-related applications. However, the development of antibiofouling surfaces incorporating ZnO nanomaterials has remained limited. In this work, we fabricate superhydrophobic surfaces based on ZnO nanopillars. Water droplets on these superhydrophobic surfaces exhibit small contact angle hysteresis (within 2-3°) and a minimal tilting angle of 1°. Further, falling droplets bounce off when impacting the superhydrophobic ZnO surfaces with a range of Weber numbers (8-46), demonstrating that the surface facilitates a robust Cassie-Baxter wetting state. In addition, the antibiofouling efficacy of the surfaces has been established against model pathogenic Gram-positive bacteria Staphylococcus aureus (S. aureus) and Gram-negative bacteria Escherichia coli (E. coli). No viable colonies of E. coli were recoverable on the superhydrophobic surfaces of ZnO nanopillars incubated with cultured bacterial solutions for 18 h. Further, our tests demonstrate a substantial reduction in the quantity of S. aureus that attached to the superhydrophobic ZnO nanopillars. Thus, the superhydrophobic ZnO surfaces offer a viable design of antibiofouling materials that do not require additional UV illumination or antimicrobial agents.

Hydrothermal synthesis of ZnO Crystals: Diverse morphologies and characterization of the photocatalytic properties

Zinc oxide (ZnO) microstructures including the ZnO nanorods (R-ZnO), ZnO nanopillars (C-ZnO), and multistage microspheres composed of ZnO nanosheets (M-ZnO) have been synthesized using a two-step hydrothermal approach by adjusting the concentrations of the growth solution. Scanning Electron Microscopy (SEM) was employed to observe the surface morphology of the fabricated samples. X-ray diffraction (XRD) was utilized for the structure characterization, which the results reveal a hexagonal wurtzite ZnO structure in all three samples with different microstructures. X-ray Photoelectron Spectroscopy (XPS) was used to investigate their defects, indicating that the oxygen vacancy ratios were 34.3 %, 21.8 %, and 51.5 % respectively for R-ZnO, C-ZnO, and M-ZnO. The band gaps of R-ZnO, C-ZnO, and M-ZnO, determined through Ultraviolet–Visible Absorption Spectroscopy (UV–Vis), were found to be 3.35 eV, 3.6 eV, and 3.19 eV, respectively. Finally, the photocatalytic performances of the prepared ZnO samples were investigated under simulated sunlight irradiation by observing the degradation of methyl orange, which the M-ZnO samples obtained a superior photocatalytic efficiency of 79 %.

Electrochemical Removal of Bacteria from Zinc Oxide Nanopillars Synthesized on Stainless Steel

Electrochemical Removal of Bacteria from Zinc Oxide Nanopillars Synthesized on Stainless Steel

Microfluidic Study of Bacterial Attachment on and Detachment from Zinc Oxide Nanopillars.

Nanopillars can influence how bacterial cells attach to a surface. Herein, we investigated whether self-assembled zinc oxide (ZnO) nanopillars synthesized on glass substrates via the conventional hydrothermal route possess anti-biofouling properties either by reducing the amount of initially attached cells or promoting the detachment of cells from the surface or both. To avoid complications associated with manual intervention methods of assessing bacterial attachment on nanopillar surfaces, we implemented a microfluidic approach. In our study, we synthesized two nanopillar topographies: a low surface density of ZnO nanopillars and a high surface density of ZnO nanopillars. Next, we mounted microfluidic channels to each of these substrates. This microfluidic approach allowed us to gently flow Pseudomonas aeruginosa, Staphylococcus aureus, or Bacillus subtilis cells onto the nanopillars for initial attachment before systematically increasing the flowrate to attempt to detach remaining attached cells without introducing air-liquid interface artefacts during the assay. Generally, initial bacterial attachment was similar across all substrates. However, cells consistently detached more readily from high-surface-density nanopillars compared to low-surface-density nanopillars. Electron microscopy revealed that cells that attached to high-surface-density nanopillars rested atop the nanopillars, fully exposed to microfluidic shear, whereas many cells became trapped in the void space between neighboring low-surface-density nanopillars, shielding these cells from detachment. Our findings indicate that self-assembled ZnO nanopillars can provide anti-biofouling properties under submerged flow but only if synthesized at high surface density.

A tunable color filter using a hybrid metasurface composed of ZnO nanopillars and Ag nanoholes.

We propose the design of symmetrical and asymmetrical tunable color filters (TCFs) by using hybrid metasurface nanostructures in the visible wavelength range. They are composed of circular zinc oxide (ZnO) nanopillars and silver (Ag) nanoholes on a silica substrate. These TCFs exhibit ultrahigh transmission intensity over 90%, different tuning ranges, and polarization-dependent/independent characteristics. By changing the distance between the ZnO nanopillars and silica substrate, the resonant wavelength of TCFs could be tuned remarkably. Moreover, we also demonstrate the stability of TCFs under different disturbances and angles of incident light. Furthermore, the resonant wavelengths are red-shifted by increasing the ambient refraction index. TCFs exhibit great tunability and ultrahigh transmission intensity up to 100%. This design opens up an avenue to widespread optoelectronic applications, such as ultrahigh resolution color displays, high-efficiency biosensors, pressure sensors, and selective color filters.

Ultrafast microwave-assisted synthesis of various zinc oxide nanostructures

The conventional hydrothermal process for the synthsis of zinc oxide (ZnO) nanostructures is a slow process and provides less control in terms of shape, size and nucleation time. Whereas, synthesis through microwave heating takes only a few minutes to produce high quality, ultra-pure zinc oxide nanostructures. In this study, we present a protocol to fabricate various ZnO nanostructures (vertically-aligned nanorods, vertically-aligned nanowalls, nanoflowers and nanopillars) using a domestic microwave oven. Based upon the process study, variation in diameter and length of vertically aligned ZnO nanorods with growth time is reported. Uniformly distributed ZnO nanowalls along with ZnO nanoflowers are fabricated in less than 5 minutes. In addition to this, ZnO nanopillars are fabricated for the first-time using evaporation and degradation phenomena in the microwave oven. Furthermore, the ZnO nanorods are found to exhibit a superhydrophobic behaviour, whereas the ZnO nanowalls, nanoflowers and nanopillars are observed to have a hydrophobic behaviour. The developed ZnO nanostructures may find their applications in the areas of optics, electronics, biomedical, solar cell, sensors and transistors etc.

Zinc oxide nanopillars as an electrocatalyst for direct redox sensing of cadmium

Abstract In this paper, we report an ultrasensitive and selective technique for direct electrochemical sensing of cadmium (Cd) by zinc oxide nanopillars (ZONPs) synthesized using a facile chemical method. The synthesized ZONPs have been characterized in terms of their topological, morphological, elemental, structural, and optical properties using various microscopic and spectroscopic techniques. A gold electrode was modified with the as-synthesized ZnO nanostructures and utilized for direct redox sensing of Cd using cyclic voltammetry (CV) and chronoamperometric techniques. This fabricated sensor demonstrated excellent sensitivity and selectivity for direct redox sensing of Cd in real and laboratory samples. Using chronoamperometry, the developed sensor demonstrated ultra-sensitivity (10 μA cm−2 ppb−1) with a detection limit of 4 ppb (p-value 0.99) in a linear range of 5–50 ppb. The enhanced reproducibility of the sensor in the presence of common interfering ions offers the potential for use in diagnostic applications involving food adulteration and in clinical healthcare.

Broadband Omnidirectional Light Trapping in Gold-Decorated ZnO Nanopillar Arrays

The photoluminescence (PL) and reflectivity characteristics of zinc oxide nanopillars (ZnO-NPs) grown on indium-tin-oxide (ITO)-coated glasses were investigated. The room temperature PL showed bright white-light emission for the undoped ZnO-NPs grown at 600 °C, suggesting the close relation between the optical characteristic and the growth conditions being carried out for obtaining the present ZnO-NPs. The reflectivity of the as-grown ZnO-NPs array was about ∼29% with the wavelength of the incident light ranging from 200 to 1800 nm. Nevertheless, the reflectance reduced significantly to less than 9.9% when a layer of gold (Au) was deposited on ZnO-NPs by sputtering for 5 min, corresponding to more than 65% reduction in Au-coated ZnO-NPs (Au/ZnO-NPs). Moreover, the angle-resolved reflectance measurements on the present Au/ZnO-NPs array show an omnidirectional light-trapping characteristic. These remarkable characteristics, broadband and omnidirectional light-trapping of Au/ZnO-NPs, are attributed to the extended effective optical path of the incident light due to subwavelength scattering resulting from the presence of Au nanoparticles.

Superhydrophobic surfaces based on ZnO-constructed hierarchical architectures

A hydrothermal process of zinc oxide (ZnO), which was suitable for both planar surfaces and non-planar surfaces, was proposed to obtain nanopillars in this work. The effects of process parameters including the growth time and solution concentration on the morphological properties of ZnO nanorods were investigated by experiments. After process optimization, the ZnO arrays were well patterned in large area with vertically aligned orientation. This developed process of ZnO was then introduced to combine with other techniques to obtain various hierarchical structures on different substrates. Furthermore, the superhydrophobicity and self-cleaning effect were respectively verified on the plexiglass tube with ZnO nanopillars and the tin–bronze surface with hierarchical architectures.

Light Extraction Efficiency Enhancement of Colloidal Quantum Dot Light‐Emitting Diodes Using Large‐Scale Nanopillar Arrays

A colloidal quantum dot light‐emitting diode (QLED) is reported with substantially enhanced light extraction efficiency by applying a layer of large‐scale, low‐cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, efficient method of non‐wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current efficiency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum efficiency (EQE) of 9.34% reaches the highest EQE value for green‐emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light‐extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the first account for QLEDs offering detailed insight into the light extraction efficiency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high‐performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.

Application of ZnO nanopillars and nanoflowers to field-emission luminescent tubes

Zinc oxide (ZnO) nanopillars on a ZnO seed layer and ZnO nanoflowers were synthesized by electrochemical deposition on linear wires. The morphologies and crystal orientation of the ZnO nanostructures were investigated by a scanning electron microscopy and an X-ray diffraction pattern, respectively. Detailed study on the field-emission properties of ZnO nanostructures indicates that nanopillars with a high aspect ratio show good performance with a low turn-on field of 0.16 V/μm and a high field enhancement factor of 2.86 × 104. A luminescent tube with ZnO nanopillars on a linear wire cathode and a transparent anode could reach a luminance of about 1.5 × 104 cd/m2 under an applied voltage of 4 kV.

Study on the bulk junction type organic solar cells with double zinc oxide layer

The optical and photovoltaic properties of a photovoltaic cell with a structure of indium–tin oxide (ITO)/double ZnO/poly(3-hexylthiophene) (PAT6):PCBM/Ag have been investigated. The double layer ZnO was a composite of a sputtered ZnO layer and oriented zinc oxide nanopillars layer which was fabricated by a new method at low temperature (343 K). It is concluded that the double layer ZnO plays an important role in collecting photogenerated electrons and acts as a conducting path to the electrode. Insertion of the double layer ZnO in the photovoltaic cells produced enhanced performance with the power conversion efficiency of 1.42% under AM1.5 illumination.

ZnO Nano-Cauliflower Array Dye-Sensitized Solar Cells

Zinc oxide (ZnO) nano-cauliflower arrays with high-aspect-ratio hexagonal ZnO nanopillars have been prepared by using a polystyrene (PS) sphere template and an electrodeposition technique in zinc nitrate aqueous solutions. The ZnO electrodeposition is a two-step process, which is composed of electrodepositon (i) in a higher Zn(NO3)2 concentration and (ii) in a lower concentration. The structural and optical characteristics were investigated by using a field-emission scanning electron microscope and spectrofluorometer. The ZnO nano-cauliflower array is so high-quality that it emits ultraviolet light with a phton energy of 3.2 eV at room temperature due to the recombination of bound exitons. Dye-sensitized solar cells using the ZnO nano-cauliflower array as a transparent electrode have been fabricated and an energy conversion efficiency of 0.15% was obtained.

Fabrication of oriented ZnO nanopillar self-assemblies and their application for photovoltaicdevices

Organic photovoltaic devices based on a blend of poly(3-hexylthiophene) and a fullerenehave been studied by inserting oriented zinc oxide nanopillars which were fabricated by anew method at low temperature (343 K). The dependence of the photovoltaicperformance on the zinc oxide morphology was investigated, and it is concluded thatthe oriented zinc oxide nanopillar array plays an important role in collectingphotogenerated electrons and acts as a conducting path to the electrode. Insertion ofthe oriented zinc oxide nanopillars in the photovoltaic cells produced enhancedperformance with a power conversion efficiency of 1.22% under AM1.5 illumination.

Growth of zinc oxide nanopillars on an iridium/yttria-stabilized zirconia/silicon substrate

Zinc oxide nanopillars were grown by a self-catalyzed growth process on an epitaxial Ir/yttria-stabilized zirconia/Si(111) multilayer structure in an optically heated tube furnace. The pillars obtained stand upright parallel to each other with their c-axis perpendicular to the sample surface. Problems due to alloying of Zn with Si are completely avoided, and no irregularities of the pillars in the initial growth phase are found. Cathodoluminescence measurements show narrow linewidths below 700μeV due to the excellent crystal quality. Termination of ZnO pillars with a flat metallic iridium layer is an attractive issue towards an optical cavity for laser action.

Cathodoluminescence study of single zinc oxide nanopillars with high spatial and spectral resolution

Single zinc oxide (ZnO) nanopillars grown by the vapor-liquid-solid process on a-plane sapphire are investigated by spatially resolved cathodoluminescence on the nanometer scale in a scanning electron microscope. Spectra were recorded by exciting luminescence along single pillars and across the underlying wetting layer. Luminescence from the excited pillar spots exhibited sharp bound exciton lines and intrinsic free-exciton recombination, whereas the wetting layer showed only very weak luminescence. Along the pillars, virtually no shift of the lines was observed. This is interpreted as evidence for strain-free growth of the pillars. In the bottom region of the pillars aluminum incorporation from the substrate material was found. There are indications for an incorporation of gallium along the pillars due to unintentional doping introduced by the source material. The spot-resolved results are compared to integral photoluminescence measurements with large-area excitation carried out on the same sample region.

Controlled catalytic growth and characterization of zinc oxide nanopillars on a-plane sapphire

Using the vapor-liquid-solid (VLS) technique, we have grown well-aligned nanopillars on [112¯0]-sapphire (a-plane) substrates at atmospheric pressure in a horizontal tube furnace employing gold catalyst seeds of different sizes and densities. It was the aim of the present work to find experimental conditions (source and template temperatures, temperature gradients, carrier gas flow, gold cluster size and density) under which controlled catalytic growth of nanopillars takes place. The VLS process is expected to result in a correlation of the zinc oxide (ZnO) pillar diameters with the gold catalyst cluster size. This is indeed found to hold true except for very small gold clusters. A minimum value of the pillar diameter of about 20 nm on a-plane sapphire is obtained in our experiments, which apparently represents a general limit. Structure characterization relies on high-resolution x-ray diffraction, atomic force microscopy, and high-resolution scanning electron microscopy. Electronic characterization is done by Raman and standard luminescence measurements with large area (≈1 mm2) excitation. In particular, we have also studied individual nanopillars by spatially highly resolved cathodoluminescence spectroscopy. We find that luminescence averaged over large areas can be dominated by features which are not typical for well-shaped pillars but come from regions of irregular ZnO growth.

Formation of Periodically Ordered Zinc Oxide Nanopillars in Aqueous Solution: An Approach to Photonic Crystals at Visible Wavelengths

ZnO nanopillar arrays with high aspect ratios, such as the one shown in the figure, are fabricated from aqueous solutions at low temperatures using a polymer mold. The shape and dimensions of the nanopillars can be tuned by appropriately patterning the polymer mold. A hexagonal ZnO nanopillar array shows photonic bandgaps at visible wavelengths, as predicted by photonic band structure calculations.

Electrochemical Growth of Nanopillar Zinc Oxide Films by Applying a Low Concentration of Zinc Nitrate Precursor

Zinc oxide nanopillars with a single crystalline structure on the indium tin oxide substrate were prepared by using an electrodepositing process in lower concentrations of aqueous Zn(NO 3 ) 2 and 0.1 M KCl solutions. Surface structure and crystal orientation of the prepared ZnO films were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, respectively. By varying the electrolyte composition, the morphology and crystal orientation of ZnO films may be changed dramatically. Well-oriented ZnO nanopillars can be obtained at a relatively low concentration of zinc ion in the electrolyte, and at a lower potential. A bandgap energy change of grown ZnO with different Zn(NO 3 ) 2 concentrations was observed. The effects of electrodepositing potential, deaerating, and annealing are also discussed.