Aspect Ratio Of ZnO Nanorods Research Articles

Effect of aspect ratio on the nonlinear optical properties of ZnO nanorods

In this work, we synthesized ZnO nanorods with two different aspect ratios using the solution method. Morphological and crystalline structures were investigated using TEM and XRD analysis. Also investigated the effect of aspect ratio on the optical properties of ZnO nanorods. With an increasing aspect ratio of the nanorods, the absorption peak is red-shifted and near-band edge emission is suppressed. The nonlinear absorption is enhanced with the increasing aspect ratio of ZnO nanorods.

Fabrication of color-toned micro/nanopattern surface by submerged photosynthesis method

Nanosurface patterning can be advantageous for novel opto-electrical devices application. In this study, we utilized the electron beam lithography (EBL) technique to fabricate ZnO nanorods(NRs)/ZnO/Si surface nanopatterning via galvanic submerged photo-synthesis of crystallites (G-SPSC) method, which combines galvanic reactions between two semiconducting surfaces and a photo-induced crystallization in water. By controlling the UV illumination time, high aspect ratio of ZnO NRs were precisely grown on 500 nm size of circular, linear, and lattice patterns. Using bromothymol blue (BTB) gel, a “photo-induced” galvanic reaction was visualized in G-SPSC. Room-temperature photoluminescence (PL) and cathodoluminescence (CL) results dictates oxygen vacancy (VO), ZnO/Si interface luminescence, as well as localized surface plasmon resonance (LSPR) phenomena on the ZnO NRs/ZnO/Si surface. Distinct luminescence behavior was illustrated by color-toned surface patterning (CTSP), opening a possibility for facile fabrication of miniaturized LED application in nanoscale surface area.

Fabrication and Characterization of High-Quality UV Photodetectors Based ZnO Nanorods Using Traditional and Modified Chemical Bath Deposition Methods

Ultraviolet (UV) photodetectors (PDs) based on high-quality well-aligned ZnO nanorods (NRs) were fabricated using both modified and conventional chemical bath deposition (CBD) methods. The modified chemical bath deposition (M-CBD) method was made by adding air bubbles to the growth solution during the CBD process. The viability and effectiveness of M-CBD were examined by developing UV PDs based on ZnO NRs. The ZnO nano-seed layer was coated on a glass substrate utilizing radiofrequency (RF) sputtering. The impact of the different growth-times on morphology, growth rate, crystal structure, and optical and chemical properties were investigated systematically using different characterization techniques, such as field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) analysis, UV–VIS double beam spectrometer, and energy dispersive X-ray analysis (EDX), respectively. The Al/ZnO UV PDs based on ZnO nanorods were fabricated with optimum growth conditions through the two methods of preparation. This study showed that the synthesized ZnO NRs using the M-CBD method for different growth times possess better properties than the conventional method under similar deposition conditions. Despite having the highest aspect ratio and growth rate of ZnO NRs, which were found at 4 h growth duration for both methods, the aspect ratio of ZnO NRs using the M-CBD technique was comparatively higher than the conventional CBD method. Besides, the UV PDs fabricated by the M-CBD method at 5 V bias voltage showed high sensitivity, short response time, quick recovery time, high gain, low dark current, and high photocurrent compared with the UV PD device fabricated by the conventional CBD method.

Evolution of ZnO flowerets from dealloying of Cu-Zn alloy powder

The synthesis of flower-like morphology of ZnO nanorods labelled as 'ZnO flowerets' via a novel approach comprising of dealloying of mechanically alloyed powder particles of Cu-Zn of mean size 2.9 ± 0.6 μm is reported in the present work. The mechanical alloying of the powder blend was performed in a planetary ball mill. Subsequently, the dealloying was performed in a basic solution of NaOH. The optimization of type of basic solution, pH of the solution, dose of alloy powder and time of dealloying was performed to find the best possible process parameters for dealloying. The ZnO flowerets were characterized by X-ray diffraction (XRD) and field emission electron microscope (FE-SEM) attached with energy dispersive spectrophotometer (EDS). The aspect ratio of ZnO nanorods and the EDS atomic ratio of Zn/Cu in the ZnO flowerets were measured to be ∼ 11 (length = ∼520 nm and width = 48 nm) and ∼ 1.2, respectively. The results of inductively coupled plasma mass spectroscopy (ICP-MS) showed the concentration of Cu and Zn ions to be 25 and 1200 ppm, respectively, in NaOH solution after experimentation for 24 h. It also indicated the predominant dissolution of Zn relative to Cu from the alloyed powder.

Size Dependency of a ZnO Nanorod-Based Piezoelectric Nanogenerator Evaluated by Conductive Atomic Force Microscopy

ZnO nanorods are one of the most studied materials because it can be facilely grown on a wide range of substrates at low temperature. ZnO exhibits piezoelectricity as well as semiconducting properties, and hence is applicable to piezoelectric nanogenerators and sensors. In the present work, the effect of ZnO nanorods’ size on piezoelectric performance was systematically studied using conductive atomic force microscopy (C-AFM). We measured the total C-AFM signal for an observing area and evaluated the piezoelectric performance of the ZnO nanorods based on this total C-AFM signal. First, five samples of ZnO nanorod with distinct aspect ratios were hydrothermally grown on silicon substrates. Afterwards, two types of AFM tips with different spring constants were used to conduct C-AFM as a function of aspect ratio. When the AFM tip with a 42 N⁄m spring constant was used for the C-AFM measurement, the total C-AFM signal continuously increased with increasing aspect ratio. The total C-AFM signal increased with increasing normal force, but fluctuated with increasing scan rate. The results of the C-AFM experimental measurements were compared with the open-circuit voltage and short-circuit current of ZnO-nanorod based piezoelectric nanogenerator. We show that using C-AFM is a facile and effective method for investigating the optimized aspect ratio of ZnO nanorods for piezoelectric power generation. Key words: nanorod, zinc oxide, size effect, piezoelectric, conductive AFM, nanogenerator

Effects of the aspect ratio of ZnO nanorods on the performance of piezoelectric nanogenerators

Abstract This paper presents an investigation on the performance of ZnO nanorod (ZNR)-based piezoelectric nanogenerators (PENGs). ZNRs on the ZnO seed layer coated double-sided flexible polyethylene terephthalate (PET) at different molar concentrations (0.01, 0.05 and 0.1 M) were synthesized by controlling the aspect ratio (length/diameter) of ZNRs, that are closely related to the piezoelectric output potential voltage using a simple hydrothermal method. ZNR PENGs were fabricated with an opposite electrode of gold-coated PET (Au/PET), which was placed on both the top and bottom of the ZNR-coated double-sided PET. X-ray diffraction and field emission scanning electron microscopy images revealed that as the molar concentration increased, the orientation of the as-grown ZNRs became non-uniformly distributed along the c-axis and also along with the decreased aspect ratio. At a low molar concentration (0.01 M), the ZNR PENGs exhibited a relatively high output potential voltage (∼4.48 V) under an external pressing mass (500 g). The ZNR samples grown at 0.05 and 0.1 M exhibited a lower piezoelectric voltage 2.48 and 1.84 V, respectively. These results confirmed that ZNR PENGs with a small diameter, long length (i.e. high aspect ratio) and good alignment tend to be bent more easily for the efficient generation of the piezoelectric potential.

Morphology Controlled Synthesis of ZnO Nanorods for Glucose Enzymatic Biosensor

The use of zinc oxide nanorods is a promising option to increase the specific area for efficient glucose oxidase immobilization which further enhance the signal performance of glucose detection. In this work ZnO nanorods were prepared via hydrothermal process on sol-gel ZnO seed layer. The effects of different temperature of annealed seed layer on the morphology and properties of grow ZnO nanorods were investigated. The ZnO seed layers were annealed at various temperatures ranging from 300 to 600 °C for 2 h. The study demonstrated that the process condition of ZnO seed layer had a strong influence on the morphology and crystallinity of ZnO nanorods grow. X-ray diffraction analysis and scanning electronic microscopy were employed to characterize the crystal structure and morphology of the prepared ZnO seed layer and grow ZnO nanorods. The performance of different aspect ratio of ZnO nanorods on glucose detection were measured by using electrochemical analysis. With optimized ZnO nanorods, the glucose biosensor exhibited an enhanced in signal performance with a high sensitivity of 11.36 μA mM-1 cm-2 in the range of 0.05-1 mM. Such high performance was due to more immobilization on the well-aligned ZnO nanorods array and direct electron conduction between the nanorods and the electrodes

Dimensional effect of ZnO nanorods on gas-sensing performance

High aspect ratio ZnO nanorods with controlled diameters and lengths have been successfully prepared. The seed-mediated growth method was used to prepare the samples. An extensive investigation on the performance of the ethanol gas sensor based on ZnO nanorods has been reported. The effects of dimensions on gas-sensing performance and operating temperatures have evidently been discussed. This paper shows that the response to gas has a strong dependence on ZnO nanorod dimensions.

Control of oxygen vacancies in ZnO nanorods by annealing and their influence on ZnO/PEDOT:PSS diode behaviour

ZnO/PEDOT:PSS diodes were produced using high aspect-ratio ZnO nanorods with diode behaviour dependent on annealing conditions.

Solution Process Synthesis of High Aspect Ratio ZnO Nanorods on Electrode Surface for Sensitive Electrochemical Detection of Uric Acid

This study demonstrates a highly stable, selective and sensitive uric acid (UA) biosensor based on high aspect ratio zinc oxide nanorods (ZNRs) vertical grown on electrode surface via a simple one-step low temperature solution route. Uricase enzyme was immobilized on the ZNRs followed by Nafion covering to fabricate UA sensing electrodes (Nafion/Uricase-ZNRs/Ag). The fabricated electrodes showed enhanced performance with attractive analytical response, such as a high sensitivity of 239.67 μA cm−2 mM−1 in wide-linear range (0.01–4.56 mM), rapid response time (~3 s), low detection limit (5 nM), and low value of apparent Michaelis-Menten constant (Kmapp, 0.025 mM). In addition, selectivity, reproducibility and long-term storage stability of biosensor was also demonstrated. These results can be attributed to the high aspect ratio of vertically grown ZNRs which provides high surface area leading to enhanced enzyme immobilization, high electrocatalytic activity, and direct electron transfer during electrochemical detection of UA. We expect that this biosensor platform will be advantageous to fabricate ultrasensitive, robust, low-cost sensing device for numerous analyte detection.

Microwave-assisted Facile and Ultrafast Growth of ZnO Nanostructures and Proposition of Alternative Microwave-assisted Methods to Address Growth Stoppage.

The time constraint in the growth of ZnO nanostructures when using a hydrothermal method is of paramount importance in contemporary research, where a long fabrication time rots the very essence of the research on ZnO nanostructures. In this study, we present the facile and ultrafast growth of ZnO nanostructures in a domestic microwave oven within a pressurized environment in just a few minutes. This method is preferred for the conventional solution-based method because of the ultrafast supersaturation of zinc salts and the fabrication of high-quality nanostructures. The study of the effect of seed layer density, growth time, and the solution’s molar concentration on the morphology, alignment, density, and aspect ratio of ZnO nanorods (ZNRs) is explored. It is found in a microwave-assisted direct growth method that ~5 mins is the optimum time beyond which homogeneous nucleation supersedes heterogeneous nucleation, which results in the growth stoppage of ZNRs. To deal with this issue, we propound different methods such as microwave-assisted solution-replacement, preheating, and PEI-based growth methods, where growth stoppage is addressed and ZNRs with a high aspect ratio can be grown. Furthermore, high-quality ZnO nanoflowers and ZnO nanowalls are fabricated via ammonium hydroxide treatment in a very short time.

An Investigation of Growth Time of Homoepitaxial Synthesis of ZnO Nanorods via Hydrothermal Method

In this research, ZnO nanorods were prepared via hydrothermal method on the seed layer. The seed layer was prepared through the sol-gel high-speed spin coating process. The effect of concentration of precursors and growth time on the size and alignment ordering of these hexagonally structured nanorods was studied by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction. Results showed that under the conditions of hydrothermal growth and in the presence of seed layer, the concentration of precursors influences diameter, length, and density of nanorods. Concentration also improves the alignment ordering of the nanorods, so that compared with lower concentrations, despite the increase of rod diameter by increase of concentration, the alignment ordering of the rods significantly improves. Also, the growth time has significant effect on the length, diameter, and aspect ratio of ZnO nanorods.

Study of NBE emission enhancement with an absence of DL emission from ZnO nanorods through controlled growth on ultra-thin Ag films

ZnO nanorods (NRs) exhibiting enhanced ultra-violet near band edge (UV-NBE) emission without a broad visible deep level (DL) emission has been investigated on catalytically grown ZnO/Ag hybrid nanostructure. The hybrid structure is fabricated in two steps, (1) Thermal evaporation of ultra-thin catalytic layer of Ag with mass thickness ∼1nm on glass substrate followed by annealing process from 50 to 250°C and (2) vertical growth of ZnO NRs by hydrothermal reaction process on all Ag films. The surface properties of Ag layer such as particle size, inter-particle distance, particles number density, surface roughness and surface coverage area were altered through annealing process. Annealing at 100°C modifies Ag from quasi-amorphous to nanocrystalline leading to high density growth and high aspect ratio of ZnO NRs where as a random and less density growth was realized at 250°C due to increase of both particle size and inter-particles distance in Ag layer. X-ray diffraction reveals a predominant growth of (002) plane at 100°C confirming the formation of wurtzite phase of ZnO NRs with highest texture coefficient of 2.35. Raman spectra verify the chemical structure of ZnO with very good crystallinity. Absorption spectra demonstrates the overlapping of surface plasmon resonance (SPR) and exciton bands up to 200°C while the excitonic absorption band is resolved at above 200°C because of the red shift in SPR due to change in surface properties of Ag layer. At 250°C, a broad optical absorption spectra from 300 to 800nm attributed to the dominant properties of SPR and exciton. Besides acting as a catalyst, Ag interlayer enhances the NBE emission at above 200°C through electrons transfer from Ag to ZnO which is quite possible because of the direct contact between them, explained by giving energy band diagram. The morphology is such that there is an increase in passage for light interaction due to less density and random growth of ZnO NRs leading to increase light scattering over absorption of Ag interlayer.

Patterned growth of zinc oxide nanorods using poly(vinyl alcohol)-N-methyl-4(4′-formylstyryl)pyridinium methosulfate acetal and zinc acetate mixture as a seed layer

This paper reports a new method for fabricating two-dimensional ZnO nanorod patterns. A water soluble mixture of poly(vinyl alcohol)-N-methyl-4(4′-formylstyryl)pyridinium methosulfate acetal (PVA-Sbq) and zinc acetate (ZnA) was used as a negative photoresist to produce the desired patterns using conventional photolithography. Hydrothermally-grown ZnO nanorods were grown selectively on the calcined PVA-Sbq/ZnA patterns. The appropriate concentration of PVA-Sbq and ZnA that can produce the desirable seed layer pattern was determined experimentally. Furthermore, the effects of the calcination time on the morphology and vertical alignment of ZnO nanorods were investigated. The vertically-aligned ZnO nanorods were generated by sufficient calcination of the patterned seed layer. On the other hand, the aspect ratio of ZnO nanorods decreased slightly with increasing calcination time. This new approach provides a simple and cost-effective method for fabricating ZnO nanorod patterns which can be beneficial in various solid-state devices and optoelectronic applications.

Optimization and tuning of the aspect ratio of hydrothermally grown ZnO nanorods by varying the hydrothermal temperature and their electron transport properties

In this investigation, a facile method to synthesize 1-D ZnO nanorod having different aspect ratio by varying the hydrothermal temperature was demonstrated. The hydrothermal reaction temperatures were optimized to obtain high aspect ratio ZnO nanorods with well-defined 1-D structures. The aspect ratio depended electron transport properties, charge recombination, chemical capacitance, life-time and charge diffusion length of 1-D ZnO nanostructures were investigated. It was observed a clear correlation between the aspect ratio and the electron transport properties of ZnO nanorods. The highest aspect ratio of 7.6 was obtained for the ZnO nanorods synthesized at 100–120 °C and was found to have the highest electron transport properties. It was demonstrated the formation of highly crystalline, high-aspect ratio 1-D ZnO nanorods with enhanced electron transport properties at low hydrothermal temperatures which will be beneficial for device applications.

Effect of hydroxide anion generating agents on growth and properties of ZnO nanorod arrays

In chemical growth of 1-D ZnO nanorods the selection of hydroxide anion generating agent plays a crucial role in determining the growth rate and morphological features. Due to misconception regarding the role of hexamethylenetetramine (HMTA) in the growth of ZnO nanorods, it is heavily used as hydroxide anion generating agent for ZnO nanorod growth. Though it needs repetitive renovation of growth solution and longer time deposition for the growth of high aspect ratio ZnO nanorods. Here we have carried out a comparative study on the effect of hydroxide anion generating agents (HMTA and ammonia) on growth, morphological, structural, chemical, optical, and photo-electrochemical properties of ZnO nanorods, in view to assist in the selection of the these agents in ZnO nanorod growth.

Aspect-Ratio Dependent Electron Transport and Recombination in Dye-Sensitized Solar Cells fabricated with one-dimensional ZnO nanostructures

Though one-dimensional (1-D) ZnO nanrods are promising transport electron transport material in the photoanode of Dye Sensitized Solar Cells (DSSC), 1-D ZnO nanorod based DSSCs exhibit poor energy conversion efficiencies. In this study, DSSCs were fabricated with 1-D ZnO nanorods having different aspect ratio and the dependence of solar cell performance on aspect ratio of ZnO nanorods was investigated. Photoanodes fabricated with different 1-D ZnO nanorods having aspect ratios of 4.4, 5.4, 5.8, 6.8 and 7.6 showed increasing solar cell performance with the increase of aspect ratio where 2.1 and 4.7% light conversion efficiencies were observed respectively for the lowest and highest aspect ratio of 1-D ZnO nanostructures. We study the electrical and operational differences between DSSC made with ZnO nanostructures with different aspect ratio. Electrochemical impedance spectroscopy (EIS) is used to quantify the aspect ratio depended electron transport properties, charge recombination, life-time and charge diffusion lengths of excited electrons in 1D ZnO nanorods and electron transport properties are correlated to the observed cell performance. In addition, effect of aspect ratio of ZnO nanorods on dye loading amount and light scattering properties were also investigated.

Growth of Variable Aspect Ratio ZnO Nanorods by Solochemical Processing

In this work, variable aspect ratio (length divided by diameter) zinc oxide nanorods were synthesized through a simple solochemical method by reacting a Zn2+ precursor with sodium hydroxide at low reaction temperatures. The analysis of the X-ray diffraction data indicated that the samples had hexagonal wurtzite structure and nanometric size crystallites. The transmission electron microscopy (TEM) images of the products prepared at 60 and 80 °C exhibited rod-like architecture, showing that the reaction temperature did not affect the ZnO morphology. The average aspect ratio of the ZnO nanorods decreased from 3.4 to 2.4 when the reaction temperature was raised from 60 to 80 °C. The samples presented a blue shift in the excitonic absorption compared to ZnO bulk that increased alongside with reaction temperature. In addition, this research investigated the results obtained by varying the concentration of zinc chloride solution. At the same temperature, it could be verified that when the zinc concentration was increased, the diameter of the ZnO nanorods also slightly increased, and much shorter nanorods were achieved, especially in the reactions performed at 50 and 70 °C. Finally, the growth mechanism of the ZnO nanostructures was proposed based on the results obtained by changing the zinc precursor concentration and reaction temperature.

Effect of the Diameter on the Band Gap of ZnO Nanorods

ZnO nanomaterials have different properties from the bulk. One interesting property is band gap. It is found that the band gap is dependent on the dimensions of the nanostructures. Many methods have been used by researchers [1-7] to produce ZnO nanostructures but amongst the soft chemistry methods, chelating agents are normally used [1,6,7]. In this work zinc oxide nanostructures have been synthesized using the sol-gel synthesis method. The effect of a synthesis with and without the use of gelling agent is investigated. One sample is done using a sol-gel synthesis route using tartaric acid and the other sample is synthesized without using a gelling agent at all. Results showed that the samples are pure, however the sample prepared without using the gelling agent are made up of rods with long aspect ratios while the one obtained using the gelling agent are made up of rods with lower aspect ratios. It is found that the band gap of the high aspect ratio ZnO is smaller than the one with low aspect ratio. This is attributable to the lattice expansion of the lower aspect ratio ZnO nanorods as obtained from quantitative X-Ray diffraction. Therefore, a characteristic of ZnO nanomaterials is dependent on their dimensions.

Large-scale manufacture of ZnO nanorods by flame spray pyrolysis

Large quantities of ZnO nanorods (>3 kg/h throughput) were produced in the gas-phase by flame spray pyrolysis (FSP) of a zinc nitrate–ethanol precursor solution without employing any catalysts or dopants. The nanorods with diameters of 20–30 nm and aspect ratios as high as seven were collected as a dry powder. Several rods self-aligned by forming junctions at the basal planes, while some even assembled into tetrapods. The aspect ratio of the nanorods could be controlled by the concentration of the Zn ions in the starting precursor solution, its delivery rate, and the oxygen flow into the reactor. To the best of our knowledge, this is the first time that synthesis of high aspect ratio ZnO nanorods by FSP is reported. Previous lab-scale experiments always yielded rather spherical albeit slightly elongated nanoparticles unless dopants were added. Such a product powder was obtained here when the ethanol in the precursor solution was replaced by methanol at otherwise constant process conditions. This is attributed to different temperature–time histories of the particles in the flame based on which a mechanism for ZnO nanorod formation in spray flames is proposed.