Aligned ZnO Research Articles

A novel approach to the green synthesis of zinc oxide nanorods using Thymus kotschyanus plant extract: effect of ammonium hydroxide and precursor concentration

This research introduces a pioneering green method for synthesizing zinc oxide nanorods (ZnO NRs) on a glass substrate using Thymus kotschyanus plant extract. The study delves into the intricate effects of ammonium hydroxide and precursor concentrations on the morphology, size, alignment, and crystalline structure of ZnO NRs. Through systematic experimentation, it was found that specific concentrations of these substances play vital roles in the formation and properties of the nanorods. Notably, a low concentration of the precursor coupled with a high concentration of ammonium hydroxide led to well-aligned hexagonal ZnO NRs with a remarkable aspect ratio. Variations in these concentrations were also found to influence the length, diameter, and alignment of the nanorods. The findings were corroborated using a diverse array of analytical techniques, including transmission and scanning electron microscopy, x-ray diffraction, UV–vis spectroscopy, and energy-dispersive x-ray analysis. The UV–vis spectra provided further insights into the optical properties and band gap energy of the ZnO NPs, while EDX analysis confirmed the elemental composition. This work represents a significant advancement in eco-friendly nanomaterial synthesis, providing detailed insights into the controlled fabrication of aligned ZnO NRs. Its innovative approach and extensive investigation into influencing factors make it a valuable contribution to the field of nanoscience.

Room temperature excitonic emission in highly aligned ZnO nanostructures prepared by glancing angle Xe+ ion irradiation

Highly aligned ZnO nanostructures have been prepared using glancing angle ion irradiation technique. ZnO thin films were deposited on Si substrates using pulsed laser deposition method. These films were irradiated with 100 keV Xe+ ions of different fluencies ranging from 1 × 1016 to 1 × 1017 ions/cm2 at a glancing angle of 70° with respect to the sample normal. The scanning electron and atomic force microscopy studies reveal a randomly oriented flower petal like structure in the pristine ZnO film. However after ion irradiation, these petals are aligned in the beam direction. Further, with the increase of the ion fluence the petals are transformed to the nanostripe, nanowire like structures and at highest fluence wrinkled structure are formed. Interestingly, on closer examination, these wrinkles consist of nanopillar like structures aligned in the beam direction. The X-ray diffraction analysis reveals that the formed ZnO nanostructures are of hexagonal wurtzite structure with preferred orientation along the c-axis. Further, the ion beam modified nanostructures exhibit interesting photoluminescence emissions in the UV regime. The exciton emission of these ion beam modified ZnO nanostructures has been discussed with the aid of X-ray absorption near edge spectroscopy at O K-edge.

ZnO Nanorods Grown on Flexible Polyurethane Foam Surfaces for Photocatalytic Azo Dye Treatment

A flexible photocatalyst hybrid structure made up of aligned ZnO nanorods and three-dimensional networks of polyurethane (PU) foam struts was fabricated. The surfaces of the PU struts were first functionalized by the two-step chemical solution treatment method. The functional groups of the PU foam and the nanohybrid structure were characterized using Fourier transmission infrared spectroscopy (FTIR). Their crystal structures and morphologies were examined by using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The adequacy of the chemical surface modification process was examined by analyzing the differences in the −N ═ C ═ O and O–H regions of the FTIR spectrum. The hybrid photocatalyst samples were also tested in terms of their structural integrities under ultrasonic irradiation. It was observed that the ZnO layers remained intact and adherent on the surfaces even after 1 h of sonication. The optical properties of the samples were also characterized by ultraviolet–visible (UV–vis) and photoluminescence (PL) spectroscopies. Aqueous Acid Red 88 (AR88) azo dye solutions were prepared and used to assess both the color and the total organic carbon (TOC) removal efficiencies of the samples utilizing the laboratory-scale photoreactor. The maximum color removal of 97% was reached in 180 min under UVA light irradiation as a consequence of simultaneous adsorption and photocatalysis mechanisms.

Effects of Reaction Parameters on the Geometry and Crystallinity of Hydrothermally Synthesized ZnO Nanorods for Bio-Fouling Applications

In this present work, we have successfully synthesized vertical aligned ZnO nanorod arrays (ZnO NRs) on conductive stainless-steel substrate. These ZnO NRs were obtained by using a low temperature, low cost, and eco-friendly hydrothermal approach. The effect of the sol-gel deposited ZnO seed layer on the vertical alignment ZnO NRs has been thoroughly studied. Characterizations of the seed layer and the as-obtained nanorods were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HR-TEM), and water contact angles (WCA) measurement. SEM analysis shows that these nanorods were grown with a high uniformity and density over the entire substrate. TEM and XRD studies confirmed the hexagonal wurtzite structure of ZnO nanorods. Finally, field immersion tests using the optimized samples were conducted in a coastal sea environment to study biofouling settlement. To the best of our knowledge, there are few articles that investigate the influence of the seed layer deposited on stainless steel on the formation of vertically aligned nanorods for antifouling applications. The novelty of this work is manifested by optimizing the ZnO seed layer to obtain either thick or fine ZnO nanorods (NRs). Moreover, another novelty of this study is the use of the hydrothermal deposition of ZnO NRs on stainless steel for biofouling to replace paints in marine applications.

Growth of highly aligned ZnO nanorod arrays on zinc plates: Morphological and structural characterization

In this work, uniform and highly aligned ZnO nanorod arrays were simply synthesized on zinc plates by hydrothermal method using zinc–ammonia complex solution. Effects of ZnO seed layer formation, Zn(NO3)2 concentration and NH3/Zn(NO3)2 mole ratio (R) in the reaction medium on the morphology and crystal structure of nanorods were investigated in detail. ZnO nanorods grown on zinc surfaces were highly crystalline and had a hexagonal wurtzite structure, as revealed by XRD analysis. In addition, the dominant crystal growth direction was in c-axis, indicating the verticality of the nanorods. SEM images showed that one-dimensional (1D) ZnO nanorods with high verticality and number density were synthesized on seeded plates whereas randomly oriented arrays were grown on non-seeded surfaces. Increase in the Zn2+ concentration changed the top ends of the rods from tapering to hexagonal ends; and also led to an increase in the average size and verticality of the nanorods. The increase in R value (or i.e., amount of NH3) caused the rods to misalign on the surface and decrease in size. The size of well-aligned nanorods can be adjusted from 50 to 260 nm in diameter and 0.11 to 6 m in length by changing the reaction parameters, implying their large potential to be used in photocatalytic and also in electronic applications.

Homogeneous and aligned ZnO nanorods@Jute fibers for enhanced mechanical/microwave absorbing performance and thermal stability

With the needs of lightweight and sustainable development, the structure-function integration of natural fibers through multiscale surface modification has attracted much attention. Jute fibers (JFs) stand out from other natural alternatives because of their unique hierarchical structure, lower production cost and better mechanical properties. In this work, a facile and effective strategy for preparing ZnO nanorods (NRs) @JFs was proposed by an in-situ method. The introduction of ZnO NRs not only reinforces the fiber, but also endows additional functionalities. The components and morphologies of ZnO@JFs were characterized. The structurally optimized ZnO@JFs exhibit a significant increase in surface energy, tensile strength, elongation at break, thermal stability with 150.0%, 16.8%, 8.5%, 216.9%, respectively, as well as remarkable enhancement in microwave absorption with 40.0%. These homogeneous and aligned ZnO NRs@JFs that integrate reinforced structure and additional functions have great application potential in the fields of automotive industry, building construction and aerospace.

Study on preparation process and enhanced piezoelectric performance of pine-needle-like ZnO@PVDF composite nanofibers

The effect of hydrothermal process on the morphology and piezoelectric performance of zinc oxide/polyvinylidene fluoride (ZnO@PVDF) composite nanofibers is rarely studied. In this work, highly aligned ZnO nanorods were epitaxially grown on the electrospun PVDF nanofibers to form ZnO@PVDF composite nanofibers with a pine-needle-like bionic structure. Nanofibers before and after hydrothermal reaction were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and piezoelectric analysis. Results demonstrated that hydrothermal reaction conditions, including mass ratio of ZnCl2 to HMTA, ammonium hydroxide volume, hydrothermal reaction time and temperature should be carefully controlled and cooperated with each other to produce ZnO nanorods with ideal microstructure and piezoelectric performance. FTIR and XRD results confirmed the transformation of β phase in PVDF nanofibers from α phase in PVDF powder due to the elongation under strong electric field during electrospinning. Results of SEM and XRD certified the successful growth of ZnO nanorods with hexagonal wurtzite structure, which plays an important role in facilitating the percentage of β phase and the resulted piezoelectric capacity of ZnO@PVDF composite nanofibers. All of these characterizations suggest that the proposed pine-needle-like ZnO@PVDF composite nanofibers demonstrate promising potential in the wide application of electromechanical energy conversion.

Energy Alignment of Quantum-Confined ZnO Particles with Copper Oxides for Heterojunctions with Improved Photocatalytic Performance.

The ability to control electronic states by utilizing quantum confinement of one of the material components in heterojunctions is a promising approach to perform energy-level matching. In this work, we report the possibility to achieve optimum energy alignment in heterojunctions made from size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu2O, Cu4O3, and CuO. Quantum confinement effects on the ZnO nanoparticles in the diameter range 2.6-7.4 nm showed that the direct optical band gap decreased from 3.99 to 3.41 eV, with a dominating shift occurring in the conduction band (CB) edge, and thus the possibility to obtain close to 0.6 eV CB edge shift by controlling the size of ZnO. The effect was utilized to align the electronic bands in the ZnO Q-dot/copper oxide heterojunctions to allow for charge transfer between the materials and to test the ability to improve the photocatalytic performance for the system, evaluated by the transformation of a dye molecule in water. The catalyst materials were investigated by X-ray diffraction, scanning electron microscopy, ultraviolet-visible (UV-vis), photoluminescence, and Raman spectroscopy. The most promising material combination was found to be the Cu2O copper oxide in combination with an energy aligned ZnO Q-dot system with approximately 7 nm diameter, showing strong synergy effects in good agreement with the energy-level analysis, outperforming the added effect of its individual components, ZnO-Q-dots and Cu2O, by about 140%. The results show that utilization of a heterojunction with controllable energy alignment can provide a drastically improved photocatalytic performance. Apart from increased photocatalytic activity, specific surface states of ZnO are quenched when the heterojunction is created. It is anticipated that the same approach can be utilized in several material combinations with the added benefit of a system with controllable overpotential and thus added specificity for the targeted reduction reaction.

Cu-doped p-type ZnO nanostructures as unique acetone sensor at room temperature (~25 °C)

We demonstrate prominent specificity on acetone gas sensing at room temperature (~25 °C) enabled solely by rendering ZnO nanostructures p-type via Cu doping of 0.8–3.7 at%, synthesized by chemical vapor deposition at 575 °C. Structural and chemical bonding analysis confirmed the successful substitution of Cu+1 for Zn+2 lattice sites leading to the conversion of n-type ZnO to p-type. Besides, Cu doping caused significant change in the morphology of the resulting ZnO nanostructures from highly aligned ZnO nanowires to randomly-aligned nanobelts, nanoribbons, and needle-like nanowires with length of 15–20 μm. The electrical measurement based on field-effect transistors comprising of individual nanowires revealed the transition of n- to p-type at a doping concentration of 1.1 at. %. The same devices were then exposed to acetone to monitor the gas sensing properties through chemiresistive responses as gas sensors. Both the un-doped and the Cu-doped n-type ZnO nanowires did not respond toward acetone exposure at room temperature. Exclusively, only the p-type Cu-doped ZnO nanostructures (>1.1 at% Cu) exhibited striking responses toward the exposure of acetone at even 1 ppm with supreme specificity over various gas species (acetone, ammonia, oxygen and alcohols) at room temperature. These results promise the unique implementation of the devices into wearable gadgets for early diagnosis of diabetes.

Hierarchical aligned ZnO nanorods on surface of PVDF/Fe2O3 nanofibers by electrospinning in a magnetic field

The electrospinning was applied to fabricate aligned nanofibers in a magnetic field. The Fe2O3 nanoparticles were added to PVDF/Zn(CHCOOH)2 solution, and heat treatment of the nanofiber mats was made to produce PVDF/Fe2O3 nanofibers containing ZnO nanoparticles. Hierarchical composites were obtained via a facile hydro-thermal growth process, where radially oriented ZnO nanorods were found. The morphology of the as-synthesized samples was investigated by using the scanning electron micrograph.

Growth of ZnO nanorods on cotton fabrics via microwave hydrothermal method: effect of size and shape of nanorods on superhydrophobic and UV-blocking properties

In this present research, a novel microwave hydrothermal method was applied to cotton fabrics to develop a superhydrophobic surface by rapid synthesis of aligned ZnO nanorods on the surface of the cotton fabric. A two-step approach was used to grow the ZnO nanorods. Firstly, the cotton fabric was coated by a seed layer of ZnO nanocrystals, synthesized using the microwave-assisted sol gel method. Secondly, the ZnO nanorods were grown rapidly on the seeded cotton fabrics using the microwave hydrothermal method. Moreover, a layer of non-fluorinated silane was applied on the as-grown nanorods to fabricate a superhydrophobic surface. Non-fluorinated silane was selected as it is less harmful to the skin than fluorinated silanes. The effect of the zinc nitrate hexahydrate (Zn(NO3)2·6H2O) concentration, reaction time and microwave power on the growth of ZnO nanorods was investigated in detail using scanning electron microscopy. The surface topography and roughness of the nanorods grown on the fabrics were studied using atomic force microscopy. EDS analysis and X-ray diffraction techniques were used to study the structural properties of the ZnO nanorods. The ultraviolet protection properties were investigated by a UV–Vis-NIR spectrophotometer. The ZnO nanorods grown on the cotton fabrics exhibited excellent UV blocking properties. A maximum UPF value of 114 was observed for the ZnO nanorods grown on the cotton fabric with 100 mM of zinc nitrate hexahydrate (Zn(NO3)2·6H2O). The superhydrophobic properties were examined based on contact angle and roll-off angle measurements, where a maximum water contact angle of 170.2° and a minimum roll-off angle of 1° were found for 25 mM of zinc nitrate hexahydrate. The ZnO-OTMS coated fabrics were evaluated for superhydrophobic durability against mechanical abrasion, laundering, chemical and UV action. Moreover, the ZnO-OTMS coated fabrics showed excellent potential for separation of floating and underwater oil layers or an oil–water mixture.

Optimising ZnO seed layer to improve the growth of the dense, aligned ZnO nanorods as an electron transport layer in perovskite solar cell applications

ABSTRACT Vertically aligned ZnO nanorods were grown using the chemical deposition method. The presence of various ZnO seed layers in the growth process was examined. Perovskite cells, with these films, were also studied. XRD pattern confirmed the deposition of nanorods with hexagonal phasing in the (001) direction for all seeds, though with different intensities. Consistent with the FESEM images, peak intensity was directly related to the improvement in the alignment and density of the nanorods. Film transparency was also directly associated with the alignment of the ZnO structure. The optical bandgap of the nanorods was close to 3.2 eV for different seed layers. Open circuit voltage and fill factor of the cells were both found to increase from 0.55 V to 0.66 V and 0.44 to 0.49, respectively, through improving the density and alignment of the nanorods. The highest efficiency for the fabricated cells was 4.56%.

Nanometer-Thick Films of Aligned ZnO Nanowires Sensitized with Au Nanoparticles for Few-ppb-Level Acetylene Detection

Wireless devices for environmental and health monitoring require the development of stable and highly sensitive thin metal oxide gas-sensing films. This paper reports one-step Langmuir–Blodgett assembly of ultrathin (20 nm) nanostructured films of aligned ZnO nanowires with 10 nm-wide v-grooves on the surface, followed by sensitization with Au nanoparticles (Au NPs) by sputtering and postannealing. The ridges of the nanopatterns are revealed as diffusion barriers capable of preventing Au NPs from sintering via particle migration and growth (PMC). The resulting ZnO Langmuir–Blodgett film decorated with high dispersion of small Au NPs functions as a highly responsive conductance switch, which demonstrates unprecedented sensitivity (37 ppm–1) and detection limit (3 ppb) toward acetylene (C2H2), a key marker gas for air pollution caused by anthropogenic emission. The fabrication method for the stabilized nanometric metal on the Langmuir–Blodgett film reported in this work not only provides a general strategy to improve the sensitivity of the thin sensing film but also can be applied to many metal nanodot-based applications, such as solar cells, plasmon-resonance waveguides, and biosensors.

(Invited) Nanopatterning Functional Metal Oxide Nanostructures By Vapor-Phase Infiltration in Polymer Templates

Infiltration synthesis is a process technique derived from atomic layer deposition (ALD), where polymeric templates, such as those lithographically patterned and self-assembled, are infiltrated by vapor-phase inorganic precursors to form novel organic-inorganic hybrids with enhanced materials properties. These hybrids can be further transformed by selectively removing the organic matrix to finally form metal oxide nanostructures with morphology and positional registry as dictated and designed by the starting polymer templates. In this talk, I will highlight a few examples of our recent efforts to generate functional inorganic nanostructures by using infiltration synthesis including, patterning arbitrary metal oxide nanostructures with sub-40 nm linewidth and over 15 aspect ratio, structural characteristics extremely difficult to achieve by conventional microfabrication techniques; fully CMOS compatible, wafer-scale synthesis and integration of in-plane aligned ZnO nanowire array phototransistors; and three-dimensional (3D) ZnO nanomesh structures derived from self-assembled block copolymer (BCP) templates with layer-number-dependent percolative electrical conductance.

Photo electrochemical ability of dense and aligned ZnO nanowire arrays fabricated through electrochemical anodization

In the present study, dense and aligned zinc oxide nanowire arrays annealed at 350 °C, with an average diameter of 99 ± 6 nm measured from histogram by Lorentz curve fitting, are fabricated through electrochemical anodization in a potassium bicarbonate aqueous solution at room temperature. During anodization, a transition in the surface morphology from nanoflowers to aligned nanowires with respect to growing time was observed. Photocurrent density of as fabricated ZnO nanowires were studied using three-electrode electrochemical cell using aqueous 1.0 M of Na2SO4 electrolyte and was found to be 30 μA/cm2 which indicates its competence to be used in UV–visible driven photoelectrode material applications.

Improvement in photoluminescence behavior of well-aligned ZnO nanorods by optimization of thermodynamic parameters

In this paper, we introduce a novel strategy for improvement in preparing aligned ZnO nanorods through the hydrothermal method. By optimization of thermodynamic parameters of the hydrothermal method, the photoluminescence (PL) properties of ZnO transparent conducting oxide (TCO) films modified interestingly. Solution volume is one of the most important parameters of nanorods growth through this method. High amounts of growth solutions can preferably change the crystal structure and PL behavior of prepared ZnO nanorods. The results confirmed that nanorods ordered more parallelly as the volume of the solution increased. Such that, the sample prepared with the highest solution volume showed an intense UV emission with a sharp PL peak. The high peak intensity of the PL spectra attributes to the high crystallinity of ZnO nanorods.

CdS quantum dots/Au nanoparticles/ZnO nanowire array for self-powered photoelectrochemical detection of Escherichia coli O157:H7

In this paper, the hydrothermally grown ZnO nanowire array (NWs) was modified by Au nanoparticles (NPs) and CdS quantum dots (QDs) to construct a high-performance photoelectrochemical (PEC) electrode. The aligned ZnO NWs, which decorated Au NPs and CdS QDs have the effective light absorption range from UV to visible region. This hybrid structure provided a self-powered PEC electrode with a favorable energy-band configuration for fast charge separation and transportation. Meanwhile, the Au NPs and CdS QDs also made increase of the surface area to improve the immobilization of the analytes. After assembling aptamer as recognition element, this composite nanoarray was further developed as a self-powered PEC biosensor by synergizing above multiple enhancement factors. The PEC aptasensor exhibited a rapid response in a wide linear range of 10–107 CFU/mL with the detection limit as low as 1.125 CFU/mL to Escherichia coli O157:H7 (E. coli O157:H7). This approach would offer an alternative PEC transduction for fast environment monitoring and clinical diagnosis related to pathogenic bacteria.

Conduction mechanisms in ZnO nanowires based Schottky diode grown under an electric field

We present a cathodoluminescence (CL) and electrical study of aligned ZnO nanowires based Schottky diodes synthesized by applying an AC electric field between two Au microcontacts. Our results reveal that the applied electric field aligns the ZnO nanowires between the electrodes and inhibits the formation of ZnO oxygen vacancies (VO). Local CL measurements of ZnO nanowires grown at different zones of the device show that the applied electric field inhibited the formation of oxygen vacancies (VO). Furthermore, CL spectra display an energy shift of the ZnO band edge emission, generated by changes in the relative intensity of two CL bands centered at 3.23 and 3.27 eV that correspond to the donor-acceptor pair and free electrons-acceptor transitions, respectively. We propose the formation of zinc vacancies (VZn) in nanowires that act as acceptor centers in the generation of these two electronic transitions. I-V curves acquired at room temperature reveal the photoresponse of the ZnO nanowires based Schottky diode exposed under UV (365 nm) illumination, exhibiting photocurrent intensities several times higher than that observed under dark conditions for applied bias lower than 1 V. The electrical conduction mechanisms in aligned ZnO nanowires of the device were tunneling and thermionic-emission for applied bias lower than 400 and 700 mV under dark and UV (365 nm) illumination conditions, respectively. For higher bias values, the device showed a conduction mechanism type field-emission.

Solution processed aligned ZnO nanowires as anti-reflection and electron transport layer in organic dye-sensitized solar cells

Aligned one-dimensional nanostructure-based solar cells have lower-performance than traditional dye-sensitized solar cells (DSSC); due to optical and electrical losses. In this letter, we minimized optical/electrical-loss by using low-temperature solution-processed ZnO nanowires (NW) as anti-reflection(AR)/electron-transport(ET) layer in newly designed indoline dye based-DSSCs. AR-layered organic-DSSC exhibit better durability performance and enhanced photocurrent density (Jsc), >26% compared to the bare devices; due to the enhanced light-coupling. Additionally, this letter debates the techniques to increase performance of the DSSCs in terms of current and voltage in detail. Such AR/ET layered architecture will be of broad academic and industrial interest, for example in photodetectors and so on.

Effect of ZnO Seed Layer on the Growth of ZnO Nanorods on Silicon Substrate

Abstract Highly vertically aligned ZnO nanorods (NRs) are preferred for the fabrication of devices such as solar cells, integrated circuits and light emitting diodes. However, ZnO NRs are randomly grown on silicon (Si) (100) substrate attributed to a large lattice mismatch (∼40 %) between Si and ZnO. The alignment of ZnO NRs could be improved by pre-deposition of a highly (002) oriented ZnO seed layer on the of surface Si substrate. In this work, ZnO seed layer was pre-deposited onto the Si substract by spin coating and followed by annealing. Parameters such as rotation speed, number cycles of spin coating and annealing temperatures were systematically studied. The results showed that 1000 rpm spin coating rate with the 11 spin cycles produced the most uniform coverage of ZnO NRs on Si with annealing temperature of 350°C. Micrographs of the samples show that the ZnO NRs are well-aligned with average diameters of ∼ 23 nm. In addition, the XRD analysis indicated highly aligned ZnO NRs were grown in [0001], i.e. perpendicular to (002) of the ZnO seed layer.