Growth Of ZnO Nanorods Research Articles

A Review on Synthesis and Optoelectronic Applications of Nanostructured ZnO

Nanostructured ZnO has gained a lot of interest as a suitable material for various applications, especially sensing, energy conversion, and storage. ZnO nanostructures can be synthesized in several ways. It is one of the materials that can be prepared in a variety of morphologies including hierarchical nanostructures. This review article presents a review of current research activities on the growth of ZnO Nanorods. The article covers various water-based routes of synthesis and is further characterized by the type of substrate used for the growth. The growth factors involved in the hydrothermal and chemical bath deposition methods are discussed. These factors include the variety of precursors, time, temperature, and the seeding method employed. At the end, applications such as gas sensing and improvement in Opto-electric properties are discussed.

Highly efficient and morphology dependent antibacterial activities of photocatalytic CuxO/ZnO nanocomposites

This study reports on morphology dependent antibacterial activities of CuxO/ZnO nanocomposites towards Escherichia coli. The CuxO/ZnO nanocomposites were formed by aqueous photoreduction of CuxO nanostructures on solution grown ZnO nanorods on Si substrates. By controlling the photoreduction time and temperature, various CuxO nanostructures including nanocubes, nanoclusters, nanospikes, and nanowebs were created as verified by scanning electron microscopy. X-ray diffraction analysis reveals the good crystallinity of the ZnO nanorods. X-ray photoelectron spectroscopy confirms the successful deposition of CuO and Cu2O nanostructures on the nanorods. Antibacterial tests were performed in dark and under low-intensity blue-light irradiation. The nanoweb sample has an antibacterial efficiency of 95.8% for 10 min in dark and the nanospike sample has an efficiency of 99.5% for 10 min under light. For the nanospike sample, remarkable bacterial survival ratios of 3 × 10–4 and 10–5 in dark and under light both for a 60 min reaction time, respectively, have been achieved. The excellent antibacterial performance is attributed to combined effects of mechanical piercing of ZnO nanorods, release of Zn2+, Cu2+, and Cu+ ions, and effective generation of reactive oxygen species due to charge separation.

Modulating the magnetic-optical properties of Zn1−xCoxO nanocrystals with x-content

In this study, magnetic-optical properties of Zn1−xCoxO NCs were modulated with x-content. The optical, structural, morphological, and magnetic properties were investigated by optical absorption (OA), fluorescence (FL), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and electronic paramagnetic resonance (EPR). The XRD and Raman results confirm the growth of Zn1−xCoxO NCs, with x greater than 1.0 having occurred for the formation of the nanocomposite (Co-doped ZnO/ZnCo2O4). The formation of the nanocomposite allows new and exciting optical and magnetic properties. SEM images show inhibition in ZnO nanorod growth with Co-content and formation of ZnCo2O4 NCs. OA and FL spectra have changed with x-content in Zn1−xCoxO NCs. These changes are strongly related to the symmetric environment of the Co2+ ions within ZnO NCs and the formation of ZnCo2O4 NCs. A linear dependence of FL intensity was observed with the x-content in the red-green region. The symmetric environment and the nanocomposite formation also alter the magnetic properties confirmed by EPR. Therefore, Zn1−xCoxO NCs can be a material platform suitable for photonic devices, magnetic-optical devices, or spintronics.

Au‐Decorated ZnO Nanorod Powder and Its Application in Photodegradation of Organic Pollutants in the Visible Region

A simple, green, and cost‐effective approach to synthesizing powdered Au‐decorated ZnO (ZnO–Au) nanorods having high photocatalytic activity by growing ZnO nanorods and decorating the grown ZnO nanorods with Au nanoparticles via hydrothermal reactions is reported. The morphology and structure of the synthesized samples are characterized by field‐emission scanning electron microscopy, transmission electron microscopy, selected‐area electron diffraction, X‐ray diffraction and Raman backscattering measurements. The optical properties are examined through optical absorbance and photoluminescence measurements. The ZnO nanorods have a hexagonal wurtzite structure, whereas the Au nanoparticles are uniformly distributed on the surfaces of the ZnO nanorods and present a face‐centered cubic structure. Compared with bare ZnO nanorods, the synthesized ZnO–Au nanorods exhibit a significant increase in ultraviolet and visible light absorption and noticeable quenching of PL, revealing efficient generation of electron–hole pairs by light excitation in a wide spectral region and effective suppression of recombination of photogenerated electrons and holes. In addition to the enhanced photocatalytic activity in the ultraviolet region, the ZnO–Au nanorods are expected to have a high visible photocatalytic activity and good stability as visible photocatalysts, which are demonstrated by the photodegradation of organic Rhodamine B and methylene blue dyes under visible light illumination.

Improved Ultraviolet Photoresponse Properties of ZnO Nanorods Grown by Using Hydrothermal Method Applied Rotating Precursor Solution

We grew ZnO nanorods (NRs) using a hydrothermal method while rotating the precursor solution, and report the effect of rotation on surface modification, and the optical and ultraviolet (UV) photoresponse properties of the resulting ZnO nanorods. The ZnO NRs grown in the rotating precursor solution at 100 rpm had the longest length and diameter, which decreased with increasing rotation rate above 100 rpm. Also, the intensity of the diffraction peaks from ZnO (002) and (100) was strongest and weakest for the ZnO NRs grown in a rotating solution at 150 rpm, respectively, indicating improvement in the c-axis orientation of the ZnO NRs. In the PL spectra, near-band-edge (NBE) and deep-level (DL) emissions were observed from all of the ZnO NRs. The intensity of the NBE emission gradually increased with increasing rotation rate, due to the increase in surface area. Also, the intensity of the DL emission gradually increased with increasing rotation rate because of the increasing number of interstitial oxygens. Analysis of the UV photoresponse found the photocurrent of the ZnO NRs grown in the rotating precursor solution was higher than that of ZnO NRs grown in a non-rotated precursor solution. In particular, ZnO NRs grown in a rotating precursor solution at 150 rpm exhibited the highest value of photosensitivity, with high reproducibility.

Well-aligned ZnO nanorod array covered with ruthenium layers for alternative counter electrodes in dye-sensitized solar cells

As an alternative material for conventional counter electrodes (CEs), which are usually made from Pt/fluorine doped tin oxide (FTO), well aligned ZnO nanorod arrays have great potential, their extremely high surface area can significantly increase the number of interfacial reaction sites in contact with the electrolyte. However, in order to use ZnO nanorod arrays as CEs, inferior charge collecting efficiency is a crucial obstacle that must be overcome. Here, patterned ZnO nanorod arrays are synthesized using a hydrothermal method and an Ru metal layer is deposited using a radio frequency sputtering process. We found the optimal conditions for ZnO nanorod growth that enable us to form vertically aligned ZnO nanorod arrays on only the ZnO seed layer. The Ru metal layer, coated onto the ZnO nanorod array serves both the role of the catalyst and the charge collecting electrode, its deposition process is controlled in such a way to maximize electrode performance. The results found in this study reveal the combination of Ru metal and the ZnO nanorod array has great potential as a lower cost alternative to Pt and FTO.

Photoelectrochemical performance of MWCNT–Ag–ZnO ternary hybrid: a study of Ag loading and MWCNT garnishing

Herein, by using chemical methods such as successive ionic layer adsorption and reaction (SILAR) and spin coating we have demonstrated a novel strategy for the synthesis of ternary hybrid to study photoelectrochemical (PEC) performance. To the best of our knowledge, for the first time we have represented a case study of achieving optimum SILAR cycles for Ag nanoparticles decoration on ZnO nanorods and a discussion was made on a role of multi-walled carbon nanotube (MWCNT) as a top layer over Ag–ZnO nanostructures for better PEC performance. Firstly, Ag nanoparticles loading over SILAR grown ZnO nanorods was varied for different SILAR cycles to optimize better photocurrent. This Ag–ZnO hybrid showed higher photocurrent density of 0.45 mA/cm2 at 1 V bias (vs SCE) and photoconversion efficiency (PCE) of 0.21% (0.45 V vs SCE). Thereafter, MWCNTs were garnished by using spin coating as a top layer on Ag–ZnO hybrid leading to the formation of ternary hybrid of MWCNT–Ag–ZnO for further enhancement of PEC activity. We believe that top layer of MWCNT plays a vital role of electron and hole transfer and bridges Ag decorated ZnO nanorods together leading to well-connected conducting pathways for efficient charge collection and transport. The appropriate band bending of MWCNT–Ag–ZnO hybrid leads to the formation of active interface helping out in charge separation leading to excellent photocurrent density of 0.56 mA/cm2 at 1 V bias (vs SCE) and photoconversion efficiency of 0.26% (0.45 V vs SCE). Enhanced light harvesting, higher donor density, appropriate band bending, lowest charge transfer radius of C–Ag–ZnO hybrid signifies that efficient charge transfer and restriction to charge recombination leading to the enhanced PEC performance.

Self-assembly nuclei with a preferred orientation at the extended hydrophobic surface toward textured growth of ZnO nanorods in aqueous chemical bath deposition

Textured growth of ZnO nanorods with no restriction of the substrate material is beneficial to their applications. The approaches to grow ZnO nanorods with texture are based on preparing suitable surface structure on the growth substrate, e.g. using a crystalline substrate with a specific surface structures or pre-depositing seed layers by high-temperature annealing of precursors. In the aqueous nutrient solution of the chemical bath deposition (CBD) process for ZnO growth, the concentration of Zn2+ ions at the extended hydrophobic surface is sufficiently high for forming self-assembly nuclei with a preferred orientation, resulting in the subsequent textured growth of ZnO nanorods. In this research, the hydrophobic surface is prepared by modifying Si surface with a self-assembly octadecyltrimethoxysilane (OTMS) monolayer. The formation mechanism of the nuclei on this hydrophobic surface for the textured growth of ZnO nanorods is investigated. It is shown that the nuclei form at the beginning of the CBD process and later transform into the Wurtzite structure to seed ZnO growth. An alternative approach to prepare seed layers is therefore involved in the aqueous CBD process, which is applicable to a range of hydrophobic substrates for textured growth of ZnO nanorods.

Synthesis of high quality hydrothermally grown ZnO nanorods for photoelectrochemical cell electrode

One-dimensional ZnO nanorods (ZnO NRs) have the edge over other nanostructures due to their unique properties. Current work fabricated ZnO NRs on pre-seeded ITO glass substrate through hydrothermal synthesis using customized hydrothermal set-up. The sources of Zn2+ and OH− ions were supplied continuously from zinc nitrate hexahydrate and hexamethylenetetramine (HMTA), respectively whereby the concentration of both precursors and the temperature of growth solution varied from 0.01 M to 0.05 M and from 80 °C to 120 °C, respectively. The impact of concentration and temperature variation on hydrothermal reaction and corresponding PEC cell performance is discussed. Field emission scanning electron microscopy (FESEM) results revealed that the morphology of prepared ZnO NRs arrays is concentration- and temperature-dependent. X-ray diffraction (XRD) reflected strong orientation along (002) direction for the hexagonal wurtzite ZnO NRs, and the film crystallinity greatly improved when the temperature increased. At optimal concentration and hydrothermal temperature, the ZnO NRs photoanode exhibited a bandgap value of 3.22 eV and achieved an impressive photocurrent density of 0.483 mA/cm2 that was attributed to the ordered rods alignment and improved optical properties. In addition, the electrochemical impedance spectroscopy (EIS) demonstrated optimized one-dimensional (1D) NRs sample showed better charge separation and transfer rate, which was two times larger than ZnO nanoparticles. The present work exhibited better photoelectrochemical performance than the other reported literature showing the high quality of ZnO NRs produced.

Growth of ZnO nanorods array on PCB for enhanced UV photosensors

Ultraviolet (UV) photosensitive device is fabricated using ZnO nanorods array on commercially available printed circuit board (PCB). Facile open aqueous solution deposition (OASD) method is used to deposit ZnO nanorods array in a trenched region between closely spaced Cu-electrodes. X-ray diffraction (XRD) patterns confirm the wurtzite phase of ZnO nanorods arrays and scanning electron microscopy (SEM) characterizes the multidimensional growth of ZnO nanorods across the trenched part of PCB. The current-voltage (I-V) characteristics of the device in dark is analogous to the back-to-back diode behaviour. However, the device exhibits an excellent photoresponsivity wherein the photocurrent value increases by two orders of magnitude at 2 V bias under the illumination ultraviolet (UV wavelength  ~ 254 nm) light source. The enhancement in photocurrent is mainly due to formation of multiple contacts between neighbouring grain-boundaries of the nanorods arrays extending to the Cu-electrodes. The prototype optoelectronic device displays almost four-times increment in the UV photocurrent at 5 V external bias potential.

Optimizing zinc oxide nanorods based DSSC employing different growth conditions and SnO coating

Effect of seed layer thickness, growth time and tin oxide coating was studied for optimized performance in ZnO nanorods (NRs) based Dye-sensitized solar cells (DSSCs). Three different seed layer thickness of 150 nm, 250 nm and 350 nm were used for hydrothermal growth of ZnO NRs varying the growth time from 12 to 20 h. Among all devices, cell fabricated employing ZnO NRs grown for 20 h using 150 nm thick seed layer yielded the highest power conversion efficiency (PCE) of 0.25%, with a short circuit current density (Jsc) of 1.549 mA/cm2, open circuit voltage (Voc) of 0.5 V and a fill factor of 32.68. A significant increase in short circuit current density was observed by employing tin oxide coating on zinc oxide nanorods based DSSCs compared to simple ZnO based DSSCs. This increase of Jsc is attributed to better electron transfer, increased dye loading, more charge carriers generation and lower recombination of charge carriers. The DSSC fabricated with SnO coated ZnO NRs also showed an increase in open circuit voltage and fill factor because of the higher conduction band edge of SnO as compared to ZnO. A maximum efficiency of 0.81% was achieved when seed layer thickness was kept at 150 nm, hydrothermal growth of ZnO NRs was continued for 20 h, and 2 h of SnO coating was performed followed by 350 °C annealing. An increase of 224%, 61.39% and 38% was observed for PCE, short circuit current density (Jsc) and open circuit voltage (Voc) respectively, compared to the device fabricated without SnO coating of ZnO NRs with the same seed layer thickness and growth time. Scanning electron microscopy was performed to visualize and understand function of tin oxide coating on zinc oxide nanorods used in dye-sensitized solar cell.

High aspect ZnO nanorod growth over electrodeposited tubes for photocatalytic degradation of EtBr dye.

In this work, vertically grown rod type ZnO nanostructures have been synthesized on metallic nickel tube films fabricated through the cost-effective process of electroforming. The use of tubular metal substrates for the growth of ZnO nanorods has been found to be advantageous for the photocatalytic degradation of EtBr dye because of their high surface area-to-volume ratio. The nickel tubes with ZnO nanorods were characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The developed system was utilized for the photocatalytic degradation of EtBr dye and its efficacy was revealed through the comprehensive mineralization of the dye within 150 min. The mechanism of the degradation process has been revealed through total organic carbon (TOC), chemical oxygen demand (COD) and high-performance liquid chromatography (HPLC) studies. A substantially lower amount of the photocatalyst has been used because of the homogeneously distributed growth of nanorods on the substrate. Density functional theory-based analysis has also been performed to study the photocatalytic degradation and adsorption properties of EtBr on ZnO nanorods. Using first principles DFT theory, geometry optimizations and vibrational analysis are performed which show a negative charge transfer from the substrate to the photocatalyst. For the first time this article reports the use of DFT analysis for investigating the adsorption of EtBr on ZnO nanorods, and the experimental growth of nanorods over electroformed Ni tubes.

Improved photoresponse properties of hydrothermally grown ZnO nanorods by controlling the Ga doping location

In this study, two types of ZnO nanorods (NRs) were hydrothermally grown on Si substrates by controlling the location of the Ga dopant. The growth behavior of ZnO NRs grown on the Ga-doped ZnO (GZO) seed layer (NR1s) was affected by the particle size controlled by the Ga dopant, while that of ZnO NRs grown in a Ga-doped aqueous solution (NR2s) was affected by complexes caused by the Ga dopant in the aqueous solution. The photoresponsivity of the ZnO NR1s overall increased with increasing concentration of Ga dopant due to the large surface area related to the density of ZnO NR1s. However, in case of the ZnO NR2s, the photoresponsivity of the undoped ZnO (UZO) NR2s was larger than that of the GZO NR2s due to the larger surface area for oxygen molecule adsorption in the UZO NR2s. Finally, under a 3-mM Ga doping concentration, the photoresponsivity of the ZnO NR1s was approximately 11.5 times higher than that of the ZnO NR2s, indicating that the ZnO NR1s are promising candidates for use in ultraviolet (UV) photodetector applications. These results indicate that when the ZnO NRs are grown using the hydrothermal method, doping at the seed layer is more suitable for high-performance UV photodetectors.

Selective growth of ZnO nanorods by thickness contrast in In-doped ZnO quantum dots seed layer

Selective growth of ZnO nanorods (NRs) have been demonstrated using thickness contrast in In-doped ZnO (IZO) quantum dot (QD) seed layer. The use of IZO QD as a seed layer has enabled the direct growth of ZnO NRs on soft substrates such as polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS). Depending on the annealing temperature, the seed layers show different grain sizes: as the annealing temperature increases, the seed grain size also increases accordingly. Interestingly, the hydrothermal growth of ZnO NRs has been found to depend on the seed grain size: the larger grain seed sample shows earlier start of growth compared to smaller seed grain counterpart. The same growth behavior has been found in the growth of ZnO NRs on seed layers having different thickness, due again to the difference in seed grain size. To advantageously exploit the observed growth behavior, the IZO QDs seed layers have been patterned by soft lithographic technique, which led to the formation of alternating thin/thick region periodically. On this patterned seed surface, the thin regions showed earlier start of NRs growth compared to thick regions, enabling the spatially selective growth of ZnO NRs. When applied for acetone gas sensors, the selectively grown sample showed better performance than the non-selectively grown counterpart. The low resistance in air, due to increased amount of chemisorbed oxygen, has been found to be responsible for the inferior sensor performance with non-selectively grown sample.

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.

High Performance Zinc Oxide Nanorod-Doped Ion Imprinted Polypyrrole for the Selective Electrosensing of Mercury II Ions

A biomimetic, ion-imprinted polymer (IIP) was prepared by electropolymerization of pyrrole at the surface of gold electrodes decorated with vertically grown ZnO nanorods. The vertical growth of the nanorods was achieved via an ultrathin aryl monolayer grafted by reduction of diazonium salt precursor. Pyrrole was polymerized in the presence of L-cysteine as chelating agent and Hg2+ (template). Hg2+-imprinted polypyrrole (PPy) was also prepared on a bare gold electrode in order to compare the two methods of sensor design (Au-ZnO-IIP vs. Au-IIP). Non-imprinted PPy was prepared in the same conditions but in the absence of any Hg2+ template. The strategy combining diazonium salt modification and ZnO nanorod decoration of gold electrodes permitted us to increase considerably the specific surface area and thus improve the sensor performance. The limit of detection (LOD) of the designed sensor was ~1 pM, the lowest value ever reported in the literature for gold electrode sensors. The dissociation constants between PPy and Hg2+ were estimated at [Kd1 = (7.89 ± 3.63) mM and Kd2 = (38.10 ± 9.22) pM]. The sensitivity of the designed sensor was found to be 0.692 ± 0.034 μA.pM-1. The Au-ZnO-IIP was found to be highly selective towards Hg2+ compared to cadmium, lead and copper ions. This sensor design strategy could open up new horizons in monitoring toxic heavy metal ions in water and therefore contribute to enhancing environmental quality.

Binder-Free Electrode Based on ZnO Nanorods Directly Grown on Aluminum Substrate for High Performance Supercapacitors.

Herein, for the first time, the growth of ZnO nanorods directly on aluminum (Al) substrate via a low temperature (80 °C) wet chemical method, and used as binder-free electrode for supercapacitors were reported. XRD pattern and HRTEM images showed that high crystalline nanorods grown on Al substrate with c-axis orientation. Morphological studies revealed that the nanorods possessed well defined hexagon phase with length and diameter of ~2 µm and 100–180 nm, respectively. Raman spectrum of ZnO nanorods showed that the characteristic E2H mode corresponds to the vibration associated with the oxygen atoms of ZnO. The optical properties of ZnO nanorods studied using Room-temperature PL spectra revealed a near-band-edge (NBE) peak at ~388 nm emission and deep level (DLE) at ~507 nm. Electrochemical measurements showed that ZnO nanorods on Al substrate exhibited remarkably enhanced performance as electrode for supercapacitors with a value of specific capacitance of 394 F g−1 measured with scan rate of 20 mV s−1. This unique nanorods structures also exhibited excellent stability of >98% capacitance retention for 1000 cycles that were measured at 1A g−1. The presented easy and cost-effective method might open up the possibility for the mass production of binder-free electrodes for efficient electrochemical energy storage devices.

Photocatalytic performance improvement by utilizing GO_MWCNTs hybrid solution on sand/ZnO/TiO2-based photocatalysts to degrade methylene blue dye.

In this work, sand/zinc oxide (ZnO)/titanium dioxide (TiO2)-based photocatalysts were hybridized with graphene oxide (GO) and GO_multi-walled carbon nanotubes (MWCNTs) hybrid solution. The novel hybrid was then used in photocatalysis to degrade dye contamination. The nanocomposite photocatalyst was initially fabricated by growing ZnO nanorods (NRs) via sol-gel immersion followed by synthesizing TiO2 NRs for different times (5 and 20 h) using a hydrothermal method on sand as a substrate. Prior to the hybridization, the initial GO was synthesized using electrochemical exfoliation and further mixed with 1 wt% MWCNTs to form GO_MWCNTs hybrid solution. The synthesized GO and GO_MWCNTs hybrid solution were then incorporated onto sand/ZnO/TiO2 nanocomposite-based photocatalysts through immersion. Various sand/ZnO/TiO2-based photocatalysts were then tested for methylene blue (MB) dye degradation within 3 days. On the basis of UV-Vis measurement, the highest MB degradation was achieved by using sand/ZnO NRs/TiO2 NRs (5 h)/GO_MWCNTs (92.60%). The high surface area and high electrical conductivity of GO_MWCNTs prolonged the lifetime of electron/hole separation and thus enhanced the photocatalytic performance.

Nickel Doping in ZnO Nanorod Synthesis: Effects of Nickel Concentration on Physical Properties of the Nanorod

ZnO nanorods have successfully been grown on the surface of FTO. The growth of ZnO nanorods was carried out using the seed-mediated growth method, whereas the Ni doping was carried at different concentrations. The effect of nickel doping on ZnO nanorods was observed using field emission scanning electron microscope (FESEM), UV-Vis spectroscopy, and X rays diffraction (XRD). ZnO nanorods having hexagonal shape were shown by FESEM images, and overlap of ZnOnanorods forming a nanoflowers with wider rod size. UV-Vis spectra showed that strong absorption occurred at a wavelength of 390-300 nm, the highest intensity of absorption was resulted by 15mM sample. The energy gap of the sample decreased as increasing its concentration. XRD pattern of the samples showed the diffraction peaks occurred at angles of 2θ = 31.72°, 34.45°, 36.20°, 47.50°, and 56.57° that represented the crystal plane orientation of (100), (002), (101), (102), and (110).

Role of oxygen vacancies on the green photoluminescence of microwave-assisted grown ZnO nanorods

Mechanism of commonly seen defect mediated photoluminescence (PL) from ZnO nanostructures is a topic of much discussion mainly due to its broad nature originating from several types of defects. Identification of such surface related defects requires suitable specimen having high surface to volume ratio characterized by complimentary techniques. Here, ZnO nanorods of different aspect ratio within the quantum confinement regime have been prepared using microwave-assisted synthesis method by varying time in the range of 1–4 min. The band gap as well as intensity of visible PL emission is found to decrease with the increase in size of nanorods obtained by allowing longer preparation time. Excitation wavelength dependent PL shows a strong green emission at ∼540 nm for above the band gap excitation (ABE) as well as below the band gap excitation (BBE) near to the band edge. Whereas a blue emission at ∼485 nm is seen for excitations far below the band gap. A systematic shift in emission from green to blue is observed for decreasing values of BBE, whereas almost no shift was seen for green emission with the variation of ABE. It is proposed that the green PL can be seen when ABE generated electrons in the conduction band transit rediatively to singly ionized oxygen vacancy (Vo+) and finally to reach the valence band whereas the blue emission may appear by direct excitation of electrons to a sub-band within the band gap by BBE. The presence of Vo+ in ZnO nanorods is confirmed by x-ray photoelectron spectroscopic studies and correlated with the green PL that shows increased intensity for higher concentration of Vo+ in smaller nanorods.