Hexagonal ZnO Nanostructures Research Articles

Hole scavenger effect on hexagonal ZnO nanostructures decorated with SnO2 nanoparticles for generating high current densities via photoelectrochemical activity

This study investigated the effects of the heterostructure and interface kinetics on the photocurrents generated by ZnO-coupled SnO2 nanostructures. Electrochemical impedance spectroscopy was utilized to understand the nanostructure heterostructure and interface charge kinetics in a 0.1 M NaOH under a ON/OFF state; the lowest charge kinetic resistance of 370.84 Ω was attained for ZnO sample decorated with SnO2 particles in the light state, which is ten times lower than that of ZnO structures, with a current density of 0.94 mA cm−2. In addition, the effect of the applied potential on the charge production of all the electrodes was examined at an applied voltage of 0.55 V vs. reference electrode, which showed the maximum generation of induced-current density for ZnO nanostructures decorated with SnO2 particles compared with other pristine electrodes. Furthermore, the hole scavenger effect on the interface kinetics and photocurrent generation for all the photoelectrodes was investigated under similar conditions, such as without a scavenger and in the same electrolyte. All electrodes in the presence of the scavenger exhibited better resistance, charge transfer kinetics, and photocurrent densities than the electrodes without a scavenger. The maximum current density of 8.96 mA cm−2 was attained for ZnO nanostructures decorated with SnO2 nanoparticles. This was higher compared with other pristine electrodes with scavengers, owing to better charge migration and reduced recombination due to the hole scavenger.

Investigation of efficient photoconduction and enhanced luminescence characteristics of Ce-doped ZnO nanophosphors for UV sensors.

This study involves the single-step, mass-scale productive synthesis, photoconduction, and luminescence characteristics of pure and cerium rare-earth-ion-doped ZnO (CZO) nanophosphors with different Ce concentrations (Ce: 0, 2, 4, 6, and 8wt.%) synthesized using the solid-state reaction method. The synthesized nanophosphors were characterized for their structural, morphological, optical, and photoconductivity (PC) properties using X-ray diffraction (XRD), field-effect scanning electron microscopy (FE-SEM), energy dispersive spectroscopy, Fourier-transform infrared (FT-IR), photoluminescence (PL), and PC measurements. The sharp diffraction peaks of XRD results exhibit the formation of crystalline hexagonal wurtzite ZnO nanostructures. The decrease in diffraction peak intensities of CZO with an increase in Ce concentrations signifies the deterioration of the ZnO crystal. FE-SEM images exhibit the good crystalline quality of nanophosphors composed of spherical- and elongated-shaped nanoparticles that are distributed consistently on the surface. The energy dispersive X-ray pattern of the 4wt.% Ce-doped ZnO (CZO4 ) sample confirms the doping of Ce in ZnO. The presence of chemical bonds and functional groups corresponds to transmittance peaks established using FT-IR spectroscopy. Deconvoluted PL spectra show two major emission peaks, one in the UV region, which is near-band-edge, and the other in the visible region ranging from ~456 to 561 nm. In PC studies, current-voltage (I-V) and current-time (I-T) characteristics, that is, rise/decayin current under dark as well as UV light conditions, are also investigated. Efficient photoconduction is observed in CZO samples. The obtained results indicate the suitability to luminescent and photosensor applications.

Synthesis of crossed twin disks of ZnO and its growth mechanism

Crossed twin disks of zinc oxide were fabricated by the vapor-phase transport method using a mixture of zinc oxide, indium oxide, and graphite powders as the source materials. The obtained nanostructure consisted of two crossed hexagonal disks, where the size of the disks was about 2.5 μm in diagonal and 300 nm in thickness. XRD result indicated that the as-obtained samples were well-crystallized wurtzite hexagonal ZnO nanostructures. Photoluminescence (PL) properties of the ZnO microdisks was characterized by fluorescent spectroscope with 250 nm picosecond LED. In the proposed growth mechanism, indium played the role of the doping element in the formation of special nucleation habits, which led to the crossed growth of the nanodisks. It is anticipated that this type of ZnO nanodisks would have application potentials in various types of nanodevices, such as light emitters and sensors, information storage devices, transducers, etc.

Manganese (Mn2+) doped hexagonal prismatic zinc oxide (ZnO) nanostructures for chemiresistive NO2 sensor

Low-operating temperature gas sensor is of great significance in a range of environmental and health safety. Here, we present the operation and performance of hexagonal prismatic ZnO nanostructures (NSs) for detection of toxic nitrogen dioxide (NO2). Mn nanoparticles (NPs) were incorporated into ZnO at various concentrations, and X-ray diffraction (XRD) patterns delineated a wurtzite hexagonal phase. From Tauc's plot, the band gap (Eg) for the NSs found to be ∼3.15–3.17 eV. The photoluminescence (PL) spectra showed a strong near-band edge emission (NBE) concordant with the calculated Eg. The NO2 response measurements showed a significant perceptual behavior of NO2. The NO2 responses increased with increasing Mn dopant concentrations. The maximum response of 152.3 % and 126.9 % was acquired for 2 % and 4 % Mn-doped ZnO at 75 °C. The response and recovery times for the sensor towards ten ppm NO2 are 107 s and 158 s. Better repeatability was achieved with good stability. These results bring promise to low operating temperature, low cost NO2 sensor based on ZnO, which could possibly used as real time monitoring.

Morphological and optical characterizations of different ZnO nanostructures grown by mist-CVD

In this study, the effects of growth temperatures, post-growth treatment, and substrate cleaning recipes on structural, morphological, luminescence, and growth kinetics of ZnO nanostructures grown by mist Chemical Vapor Deposition method (mist-CVD) on SiO2/Si substrates are investigated. The grown ZnO nanostructures have polycrystalline wurtzite (wz) hexagonal crystal structure according to X-ray Diffraction (XRD) measurements. However, the lower growth temperature is found to lead to the formation of ZnO nanostructures oriented in the [0002] direction. From Atomic Force Microscope (AFM) measurements, it is seen that the Root Mean Square (RMS) values decrease with decreasing growth temperature. In addition, it is shown that the surface covering is tightly dependent on decreasing growth temperature of the ZnO nanostructures. The hexagonal and triangular pyramid-like ZnO nanostructures are grown on the surface according to the Scanning Electron Microscope (SEM) measurements. The wurtzite type of crystallization is also demonstrated by confocal Raman measurements. The characteristic vibration modes related to Zn and O motion in the lattice are found to be ∼435 cm−1 and ∼100 cm−1, respectively. The luminescence properties are studied by low-temperature (10 K) Photoluminescence (PL) measurements. The spectra are dominated by the transition related to the donor-bound exciton (DBE) with a peak position typically observed at ∼369 nm (3.362 eV) in hexagonal ZnO structures. Defect-related transitions are observed to be strongly correlated with growth temperature and post-growth treatment.

Ultrasound-assisted green biosynthesis of ZnO nanoparticles and their photocatalytic application

AbstractEmploying plant extracts to obtain nanomaterials is an ecofriendly and highly appreciated synthetic approach. In this work a simple, green chemistry method, based on sol–gel, was used for ZnO nanoparticles synthesis by using two Sudanese medicinal plant extracts:Adanosia digitata(ZnO-A) andBalanites aegyptiaca(ZnO-B) under ultrasonic energy. The X-ray diffraction (XRD) revealed the formation of wurtzite hexagonal ZnO nanostructures, while the scanning electron microscopy (SEM) analysis displayed their diverse morphologies. The X-ray photoelectron spectroscopy (XPS) data showed the impact of extract via the variation in of the O1s and Zn2p3/2and Zn2p1/2orbitals binding energy of Zn–O. The UV-visible investigation indicated a variation of bandgap energy (Eg), where the ZnO nanoparticles displayed the lowestEg. The synthesized nanomaterials have exhibited high photocatalytic efficiency towards the methylene blue (MB) dye. The findings revealed the possibility of obtaining nanoparticles with tailored properties by using plants extracts.

Rapid synthesis of ZnO nanowires and nanoplates with highly enhanced photocatalytic performance

ZnO nanowires and nanoplates with extremely enhanced photocatalytic performance were prepared via one-step facile thermal decomposition of zinc acetate dihydrate and well characterized by FESEM, TEM, STEM-HAADF, EDX, XRD, PL, XPS, UV–Vis and Raman spectroscopy. FESEM and TEM studies revealed ZnO nanowires, nanoplates and few nanorods in the sample prepared at 300 °C, while increase in the thermal decomposition temperature to 500 °C led to the formation of ZnO nanoparticles and nanorods. XRD results revealed hexagonal wurtzite ZnO nanostructures with crystallite size variation from 47 to 34 nm with increase in thermal decomposition temperature, while PL studies showed decrease in band gap from 3.31 to 3.26 eV along with significant quenching of defect emissions as the thermal decomposition temperature was increased from 300 to 500 °C. ZnO nanowires and nanoplates prepared at 300 °C exhibited extremely enhanced photocatalytic performance and led to almost complete decomposition of methylene blue (MB) and methyl orange (MO) in only 6 and 32 min, respectively. The extremely strong photocatalytic performance of ZnO nanowires and nanoplates makes them exciting candidates for solar-driven water purification. The possible mechanisms of growth of ZnO nanowires and nanoplates and their enhanced photocatalytic performance were proposed.

Hydrothermal synthesis of ZnO nanotubes for CO gas sensing

Carbon monoxide (CO) gas sensing response of hydrothermally synthesized hexagonal ZnO nanostructures was examined with respect to its morphologies. Synthesized ZnO nanostructures were characterized by X-Ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM), Energy Dispersive X- Ray Analysis (EDAX) and UV Visible spectroscopic techniques. The CO gas sensing response of ZnO nanostructures were investigated as change in resistance at different gas concentrations of 2, 4, and 6 ppm at room temperature (25 °C). ZnO nanostructures have shown high response to CO concentrations as low as 6 ppm. Response 1.7 s, 2 s and 3 s and recovery time was 2 s, 3 s, 4 s, respectively, for 2, 4 and 6 ppm. Response of the CO gas for ZnO nanostructures at room temperature (25 °C) was 75%.

Structural and optical properties of hexagonal ZnO nanostructures grown by ultrasonic spray CVD

In this paper, the growth, structural and optical properties of the hexagonal ZnO nanostructures grown by Ultrasonic Spray Chemical Vapor Deposition (USCVD) method at atmospheric pressure have been investigated. The characterizations of the grown thin films have been made with X-ray diffraction (XRD), Atomic Force Microscopy (AFM) and UV–vis Spectrophotometry measurements. Different samples have been grown on the soda-lime glass substrate with different power values of the ultrasonic transducer. Although the hexagonal ZnO nanostructures have been expected to show the polycrystalline structures, they have a strong the x-ray diffraction peaks in (0002) direction. The optical band gap, the grain size and the surface roughness of the hexagonal ZnO nanostructures have been determined. It is shown that, with the increasing power usage of the transducer, the optical transmittance of hexagonal ZnO nanostructures has decreased in the visible area. The optimum power usage of transducer used in USCVD method has been determined for the further studies.

Enhanced photocatalytic performance of ZnO nanostructures produced via a quick microwave assisted route for the degradation of rhodamine in aqueous solution

Water pollution resulting from toxic organic pollutants has become a global issue and identification of a suitable low cost and non-toxic material possessing catalytic behavior is indispensable in order to circumvent this enigma. For solving this problem, we herein report a quick, cost effective, surfactant-free microwave assisted route for the preparation of hexagonal wurtzite ZnO nanostructures and its photocatalytic performances. Structural, morphological and compositional aspects of the nanostructures were determined by various characterization techniques. The photocatalytic efficacies of the samples monitored for the UV induced degradation of aqueous rhodamine B (RhB) solution indicated that all the nanostructures have high photocatalytic capability with superior performance (C/Co) than many existing reports. The highest photocataytic efficiency was obtained for the sample prepared at 0.5min with 92% efficiency after three cycles experiment and rate constant of 0.042. The possible mechanism for the growth of ZnO nanostructures and photo degradation is also reported.

Ultrasonic-assisted synthesis and photocatalytic performance of ZnO nanoplates and microflowers

Nanoplates, multi-layered nanorod petal flowers and microflowers of nanoneedles with pyramid tips of hexagonal ZnO nanostructures were ultrasonically synthesized in the solutions containing zinc nitrate hexahydrate, ammonium hydroxide and cetyltrimethyl ammonium bromide (CTAB) with pH adjusting to 7–10. The as-synthesized wurtzite hexagonal ZnO was detected by X-ray diffraction (XRD). Fourier transform infrared (FTIR) spectroscopy revealed the standard peak of zinc oxide at 507–518cm−1, and Raman spectroscopy at 437cm−1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the products with the morphologies of nanoplates, multi-layered nanorod petal flowers and microflowers of nanoneedles with pyramid tips at the pH of 7, 8-9, and 10, respectively. In addition, the photocatalytic degradation of methylene blue (MB) under UV radiation by the hexagonal ZnO nanoplates is the highest at 97.54%.

Low-temperature hydrothermal synthesis of ZnO nanorods: Effects of zinc salt concentration, various solvents and alkaline mineralizers

ZnO has been produced using various methods in the solid, gaseous, and liquid states, and the hydrothermal synthesis at low temperatures has been shown to be an environmentally-friendly one. The current work utilizes a low reaction temperature (60°C) for the simple hydrothermal synthesis of ZnO nanorod morphologies. Furthermore, the effects of zinc salt concentration, solvent type and alkaline mineralizer type on ZnO nanorods synthesis at a low reaction temperature by hydrothermal processing was studied. Obtained samples were analyzed using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Increasing the concentration of the starting zinc salt from 0.02 to 0.2M changed ZnO nucleation system from the homogeneous to the heterogeneous state. The XRD results confirmed the production hexagonal ZnO nanostructures of with a crystallite size of 40.4nm. Varying the experimental parameters (mineralizer and solvent) yielded ZnO nanorods with diameters ranging from 90–250nm and lengths of 1–2μm.

Structural and Luminescence Features of Lithium-Doped p-Type Film-Like ZnO Nanorods.

We report the structural and optical characteristics of p-type lithium (Li)-doped ZnO film-like nano-structures prepared by utilizing a simple hydro-thermal method in an aqueous solution at a low temperature (< 90 degrees C). The diameters and densities of the Li-doped ZnO nanostructures were controlled by adjusting the molar concentration. A relatively high molar concentration resulted in hexagonal and flat surface-shaped ZnO nanostructures. In addition, a post-annealing process in the range of 400 to 600 degrees C effectively leads to the incorporation of lithium dopant as an acceptor, resulting in optical p-type behavior. The p-type features of synthesized Li-doped ZnO nanostructures were analyzed using a photoluminescence measurement using a He-Cd laser as an excitation source at 10 K. Closer investigation of the fine donor- and acceptor-bound exciton emission peaks from the low temperature PL spectra revealed the occurrence of several peaks related to free excitons (FX), excitons bound to acceptor (A(0)X), free electron to the acceptor transition peak (FA), and its LO phonon replicas.

Effect of cetylpyridinium chloride on surface passivation and photocatalytic activity of ZnO nanostructures

We report a facile soft-chemical approach for the preparation of water dispersible ZnO nanostructures by using cationic surfactant, cetylpyridinium chloride (CPC) as a shape directing agent. XRD and TEM analysis reveal the formation of highly crystalline single-phase hexagonal wurtzite ZnO nanostructures. It has been observed that the addition of surfactant strongly affects size and shape of ZnO particles. The presence of CPC predominantly favors the formation of pyramidal-shaped nanoparticles (10–20nm), whereas roughly spherical nanoparticles (∼10nm) were formed in absence of CPC. These ZnO nanostructures show good aqueous colloidal stability, which is important for their photocatalytic applications. Optical studies confirmed that the concentration of CPC controls both bandgap and intensity of defect-related emission of ZnO nanostructures. Further, we have explored the photocatalytic activity of these ZnO nanostructures using methyl orange and methylene blue as model dyes. It has been observed that the concentration of defect present on the surface of ZnO nanostructures strongly influences the photocatalytic degradation of organic dyes. The nearly complete mineralization of treated organic dye by ZnO photocatalyst was confirmed from the significant reduction in chemical oxygen demand (COD) values. Further, the photodegradation efficiency of organic dye is also found to be dependent on other parameters, such as nature and concentration of dye and amount of photocatalyst. More ever, these nanostructures could be promising recyclable photocatalyst for degradation of organic contaminants under UV light.

A novel method for synthesis of well-aligned hexagonal cone-shaped ZnO nanostructures in field emission applications

Well-aligned and high-density ZnO nanocone arrays were prepared on Si substrates using an electric field-assisted chemical bath deposition (FECBD) method. The photoluminescence (PL) spectra of ZnO nanostructures indicate that near-band-edge (NBE) emission shifted slightly toward lower wavelengths with the increase in procedure temperature, and the intensity increased with improved ZnO crystallization. The cone-shaped ZnO nanorods had the lowest turn-on electric field, largest field-enhancement factor, and good stability, which were attributed to the small emitter radius at the tip, uniform density distribution, and high aspect ratio. These results indicate that the cone-shaped ZnO nanostructures fabricated by FECBD are promising for application as field emission (FE) electron sources.

Visible light photocatalysis of Methylene blue by graphene-based ZnO and Ag/AgCl nanocomposites

The present investigation deals with the photocatalytic activity of two different graphene oxide-(GO) based nanocomposites i.e. Ag/AgCl/GO and ZnO/GO for the degradation of Methylene blue (MB). The composites were prepared by low-temperature hydrothermal method and were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD), and Raman spectroscopy. SEM analysis showed that GO was stacked between the hexagonal ZnO nanostructures and in the case of Ag/AgCl, it was grafted onto the distorted spheres of Ag/AgCl. This distorted structure may be due to the overloading of the silver precursor close to the saturation during the synthesis. XRD pattern confirms the formation of both the nanocomposites. Both the nanocomposites showed good performance in degrading the dye under sunlight. ZnO/GO being a semiconductor photocatalyst showed 95% removal of dye in 100 min, whereas plasmonic catalyst Ag/AgCl/GO was found to degrade 92% of MB within 50 min of reaction time due to its unique surface plasmon resonance property. Raman spectroscopy confirms the enhanced photocatalytic activity of both the nanocomposites, which is due to the electron-accepting property of the GO and hinders the recombination of electron–hole pair.

Seed/catalyst-free vertical growth of high-density electrodeposited zinc oxide nanostructures on a single-layer graphene

We report the seed/catalyst-free vertical growth of high-density electrodeposited ZnO nanostructures on a single-layer graphene. The absence of hexamethylenetetramine (HMTA) and heat has resulted in the formation of nanoflake-like ZnO structure. The results show that HMTA and heat are needed to promote the formation of hexagonal ZnO nanostructures. The applied current density plays important role in inducing the growth of ZnO on graphene as well as in controlling the shape, size, and density of ZnO nanostructures. High density of vertically aligned ZnO nanorods comparable to other methods was obtained. The quality of the ZnO nanostructures also depended strongly on the applied current density. The growth mechanism was proposed. According to the growth timing chart, the growth seems to involve two stages which are the formation of ZnO nucleation and the enhancement of the vertical growth of nanorods. ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics such as photovoltaic devices, sensing devices, optical devices, and photodetectors.

H2 sensing properties of two-dimensional zinc oxide nanostructures

In this work we have grown particular zinc oxide two-dimensional nanostructures which are essentially a series of hexagonal very thin sheets. The hexagonal wurtzite crystal structure gives them their peculiar shape, whose dimensions are few microns wide, with a thickness in the order of 25 nm. Such kind of nanostructure, grown by thermal oxidation of evaporated metallic zinc on a silica substrate, has been used to fabricate conductometric gas sensors, investigated then for hydrogen gas detection. The "depletion layer sensing mechanism" is clarified, explaining how the geometrical factors of one- and two-dimensional nanostructures affect their sensing parameters. The comparison with one-dimensional ZnO nanowires based structures shows that two-dimensional nanostructures are ideal for gas sensing, due to their tiny thickness, which is comparable to the depletion-layer thickness, and their large cross-section, which increases the base current, thus lowering the limit of detection. The response to H₂ has been found good even to sub-ppm concentrations, with response and recovery times shorter than 18s in the whole range of H₂ concentrations investigated (500 ppb-10 ppm). The limit of detection has been found around 200 ppb for H₂ gas even at relatively low working temperature (175 °C).

Synthesis and characterisation of flower shaped Zinc Oxide nanostructures and its antimicrobial activity

Flower shaped Zinc Oxide nanostructures was synthesized using a simple method without using any structure directing agents. Elemental analysis, crystalline nature, shape and size were examined using Powder X-ray Diffraction (XRD), scanning electron microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDAX). XRD revealed the formation of hexagonal ZnO nanostructures. SEM and TEM analyses revealed the formation of crystalline ZnO flowers in which a bunch of ZnO nanorods assembled together to form a leaf like structure followed by flower shaped ZnO nanostructures. Thus synthesised ZnO nanostructures showed good antimicrobial activity towards gram-positive bacteria Staphylococcus aureus as well as gram-negative bacteria Escherichia coli with a MIC/MBC of 25mg/L.

Vertical Growth and Resonator Properties of Hexagonally Shaped Zinc Oxide Nanonails

We synthesized vertically aligned nail-shaped ZnO nanocrystal arrays on silicon substrates via a combination of a carbothermal reduction method and textured ZnO seeding layers that were precoated on silicon substrates by thermally decomposing zinc acetate, and studied their optical properties using cathodoluminescence (CL) and photoluminescence techniques. The ZnO nanonails show a sharp band-gap edge UV emission and a defect-related broad green emission. Monochromatic CL images of an individual ZnO nanonail show variations in spatial distributions of respective CL bands that had different origins. We attribute the spatial variation of CL images to an uneven distribution of luminescent defects and/or a structure-related light out-coupling from hexagonal ZnO nanostructures. The most distinct CL feature from the hexagonal head of an individual ZnO nanonail was the occurrence of a series of distinct resonant peaks within the visible wavelength range. It appeared that the head of a nanonail played the role of a hexagonal cavity so that polarization-dependent whispering gallery modes were stimulated by electron beam excitation.