Influence of solution composition on sprayed ZnO nanorods properties and formation process: Thermoanalytical study of the precursors

ZnO is an important inorganic material for numerous applications. Different physical and chemical methods have been applied to deposit ZnO. Spray pyrolysis method being simple, rapid and low-cost is amongst the many options and has been chosen for this study. Fabrication of ZnO nanorods crystals by chemical spray pyrolysis was performed using an acidic solution of ZnCl₂1 or a basic ammonia-containing solution of ZnCl₂2. All layers were studied using X-ray diffractometry and Scanning Electron Microscopy. The formation of ZnO nanorods from 2 appeared at 450?°C, whereas spraying acidic solution 1 yielded ZnO nanorods like morphology at 550?°C.Thermal decomposition of precursors for ZnO layers prepared by de-watering of acidic aqueous solution of ZnCl₂ with pH =?3 (1) and basic solution of ZnCl₂ and NH₄OH with pH =?10 (2) was monitored by simultaneous thermogravimetric and differential thermal analysis (TG/DTA) in air coupled online with evolved gas analyses by Fourier transformed infrared spectroscopy (FTIR).The precursor (1) is ZnCl₂ *nH₂O; the precursor (2) is a mixture of (NH₄)₃(ZnCl₄)Cl, Zn(NH₃)₂Cl₂, NH₄Cl(NH₃)₃ and Zn(OH)₂ phases. The thermal decomposition of (1) and (2) in the temperature range of 30–700?°C consists of two steps with total mass losses of 86.2% and 93.8%, respectively. The main evolved gases from (1) are H₂O and HCl, whereas the main evolved gases from (2) are H₂O, NO_x and NH₃. Degradation of (1) and (2) is completed by 670 and 620?°C, respectively. The final decomposition product of (1) and (2) at 700?°C is ZnO.This study shows that the use of basic solutions enables to decrease the temperature of ZnO formation and the deposition temperature of ZnO nanorods layers.     

Nanoseed-assisted rapid formation of ultrathin ZnO nanorods for efficient room temperature NO2 detection

The influence of ZnO nanoseeds on the formation of ZnO nanorods from ε-Zn(OH)₂ in NaOH solution at 80 °C was investigated, using ZnO nanoparticles with a diameter of 4–10 nm as the seeds. The experimental results indicated that the presence of ZnO nanoseeds promoted the rapid heterogeneous formation of ultrathin ZnO nanorods. Compared with the ZnO submicron rods with a diameter of 0.5–1.0 µm, the ultrathin ZnO nanorods with a diameter of 10–15 nm were found to be more sensitive for detecting NO₂ at room temperature owing to their higher variation of channel conduction to the diameter.     

Facile Synthesis of ZnO Nanorods by Microwave Irradiation of Zinc–Hydrazine Hydrate Complex

ZnO nanorods have been successfully synthesized by a simple microwave-assisted solution phase approach. Hydrazine hydrate has been used as a mineralizer instead of sodium hydroxide. XRD and FESEM have been used to characterize the product. The FESEM images show that the diameter of the nanorods fall in the range of about 25–75 nm and length in the range of 500–1,500 nm with an aspect ratio of about 20–50. UV–VIS and photoluminescence spectra of the nanorods in solution have been taken to study their optical properties. A mechanism for microwave synthesis of the ZnO nanorods using hydrazine hydrate precursor has also been proposed.     

Nucleation and growth of ZnO nanorods on the ZnO-coated seed surface by solution chemical method

ZnO nanorods on ZnO-coated seed surfaces were fabricated by solution chemical method using supersaturated (ZnNO₃)₂/NaOH at 70 °C. The seed surfaces were coated on glass substrates by sol–gel processing, and their texture was dominated by heating temperatures, cooling styles and layer thickness per dipping. The effects of the seed surface on the morphology of the resultant nanorods were primarily discussed. The orientation and morphology of both the seed surface and successive nanorods were analyzed by using XRD and SEM. It is proved that when the seed size increases from 15 to 50 nm with temperature increasing, the average diameter of the resultant nanorods increase from 25 to 50 nm, with a length of 800 nm after growing for 1.5 h. The seed surface prepared by heating at 300–400 °C, fast cooling or drawing at lower speed has better orientation and few surface defects, which leads to higher density of nuclei on the seed surface and thus to the optimal preferred crystal growth of ZnO rods standing perpendicular onto substrates.     

Hydrothermal growth of ZnO nanostructures from nano-ZnO seeded in P(MMA-co-BA) matrix

Nano-ZnO synthesized by hydrothermal reaction were embedded in poly(methyl methacrylate-co-butyl acrylate) matrix (P(MMA-co-BA)) to produce the nano-ZnO/P(MMA-co-BA) nanocomposites via in-situ polymerization at 85 °C. The nano-ZnO/P(MMA-co-BA) nanocomposites were hydrothermal treated in the mixture solution of Zn(NO₃)₂·6H₂O and NH₄OH at 90 °C under various pH (i.e.7, 8, 9 and 10) and treatment time (i.e. 4, 6, 8, 10, 12 and 24 hrs). The nano-ZnO could act as seeding particles for hydrothermal growth of ZnO nanostructures on the surfaces of nanocomposites. The higher pH of basic solutions used in the hydrothermal treatment, the higher amount of Zn(OH)₄²⁻nuclei would be created, leading to a modification of the ZnO morphology from nano-nuclei to nanorods, nanorods bushes (flower-like nanostructure) and nanofibers with nanospine. The increase of hydrothermal treatment time resulted in the increases of amount and length of multidirectional grown ZnO nanorods. Data of the contact angle measurement exhibited the increase of hydrophobicity of the nano-ZnO/P(MMA-co-BA) nanocomposites after hydrothermal growth of ZnO nanostructures. The nanocomposites treated at pH = 10 for 24 hrs shows the highest hydrophobicity with the contact angle of 121˚. In addition, the thermal stability of the nano-ZnO/P(MMA-co-BA) could be improved by the formation of hydrothermal grown ZnO nanostructure on the nanocomposite surface.     

The Influence of the Temperature on the Optical Absorption Properties and Morphologies of WO<Subscript>3</Subscript> Nanorods Synthesized by the Hydrothermal Method

Crystalline WO₃ nanorods of less than 100 nm in diameter have been successfully synthesized at 240 °C for 48 h at pH 1.5 by the hydrothermal method with sodium tungstate as a tungsten source and potassium sulphate as a subsidiary salt. The morphologies and structures of WO₃ rods were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and selected-area electron diffraction (SAED). SEM analysis confirms that the slenderness ratio of the WO₃ rods is enlarged with the increase of the reaction temperature at pH 1.5. SAED analysis shows that the synthesized WO₃ nanorods are crystalline. Ultraviolet–visible (UV-VIS) absorption spectroscopy shows that absorbent power of UV light for WO₃ nanorods enhances with an increase of the slenderness ratio.     

ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Enhanced CO2 photocatalytic conversion into CH3OH over visible‐light‐driven Pt nanoparticle-decorated mesoporous ZnO–ZnS S-scheme heterostructures

In the present contribution, the design and fabrication of Pt nanoparticle-decorated mesoporous ZnO–ZnS heterostructures were described and used effectively for photocatalytic CO₂ conversion to yield CH₃OH. TEM images of the mesoporous Pt/ZnS–ZnO heterostructure demonstrated spherical ZnO NPs ~20 nm, and Pt NPs ~3 nm were well dispersed on the porous ZnS–ZnO heterostructure. The formation of CH₃OH over the Pt/ZnS–ZnO heterostructure was 78, 39 and 20 times larger than that bare ZnS, ZnO NPs and ZnS–ZnO, respectively. The optimal Pt/ZnO–ZnS heterostructure exhibited a high CH₃OH formation rate of 81.1 μmolg^-1h^-1, which is about 44, 22 and 20 times larger than that of bare ZnS (1.86 μmolg^-1h^-1), ZnO (3.72 μmolg^-1h^-1), and ZnO–ZnS (4.15 μmolg^-1h^-1), respectively. The significantly enhanced reduction of CO₂ was imputed to the synergistic effects of the ZnO–ZnS heterostructure and the incorporation of Pt NPs. The synthesized photocatalyst provides a new transfer route through which carriers can migrate to the outer surface as well as pore channels of the mesoporous ZnO–ZnS, therefore significantly minimizing the transfer distance for carriers, inhibiting photoinduced electron-hole recombination, and diminishing the mobility resistance, as determined using photoluminescence, photocurrent response, and electrochemical impedance spectra measurements.     

Highly sensitive C2H2 gas sensor based on Ag modified ZnO nanorods

The sliver (Ag) modified zinc oxide (ZnO) nanorods were successfully obtained with a simplified and environmentally friendly solvothermal method. Materials characterization indicated that the metallic Ag was located on the outside of ZnO nanorods after annealing. In comparison with ZnO nanorods, Ag modified ZnO (Ag–ZnO) nanorods exhibited a considerably enhanced response to C₂H₂. The response of the 3 at% Ag–ZnO based sensor operating at 175 °C is 539 (R_a/R_g), which is the highest value among all the sensors in detecting 100 ppm C₂H₂. The Ag–ZnO based sensors exhibited fast response speed, lower operation temperature and higher selectivity.     

Synthesis and characterization of 3D ZnO superstructures via a template-free hydrothermal method

The ZnO microrods and flower-like 3D superstructures have been synthesized via a template-free hydrothermal method. The prepared ZnO samples are characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the flower-like 3D ZnO superstructures can be obtained simply by tuning the volume ratio of Zn(OH)₂⁴⁻ precursor solution to water, or the volume ratio of ethylene glycol to water. When the volume ratio of Zn(OH)₄²⁻ precursor solution to water is 1:7, the flower-like 3D ZnO superstructures are obtained, in which the diameter of an individual pencil-like rod is about 1.0 μm, and the tip size is several nanometers. While when the volume ratio of Zn(OH)₄²⁻ precursor solution to ethylene glycol (EG) is 1:1, the flower-like 3D ZnO superstructure consists of hexagonal rods with about 500 nm in diameter. A plausible formation mechanism of flower-like 3D ZnO superstructures is discussed in detail.     

Gradient doping of Cu(I) and Cu(II) in ZnO nanorod photoanode by electrochemical deposition for enhanced photocurrent generation

This study describes the synthesis and characterization of Cu-doped ZnO nanorods (NRs) by an electrochemical method in the presence of two different Cu precursor (Cu⁺² and Cu⁺) in order to improve photocurrent generation. Analyses of the resulting materials by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis and electrochemical photocurrent (ECP) spectroscopy confirm the formation of well-aligned ZnO Würtzite nanostructures in the form of hexagonal rods. For both doping source with a concentration of up to 0.5%, the following changes were observed: a distortion of the ZnO morphology, an increase in transmittance to 96% for ZnO doped with Cu⁺², and a reduction of the energy gap from 3.36 eV to 3.06 and 3.02 eV for ZnO doped with Cu⁺² and Cu⁺, respectively. From photoelectrochemical tests, the photo-current density was improved up to 0.05 mA cm^-2 in the presence of Cu doping, which is twelve times superior to that of undoped ZnO nanorods, which means that the incorporation of Cu+2 or Cu + significantly improves the separation efficiency of photogenerated electron-hole pairs. These results can be considered promising for optoelectronic and photocatalysis applications.     

ZnO Nanorods as Antireflective Coatings for Industrial‐Scale Single‐Crystalline Silicon Solar Cells

In this work, both planar and textured, industrial scale (156 mm × 156 mm) single‐crystalline silicon (Si) solar cells have been fabricated using zinc oxide (ZnO) nanorods as antireflection coating (ARC). ZnO nanorods were grown in a few minutes via hydrothermal method within a commercially available microwave oven. Relative improvement in excess of 65% in the reflectivity was observed for both planar and textured Si surfaces. Through ZnO nanorods, effective lifetime (τ_eff) measurements were presented to investigate the surface passivation property of such an ARC layer. ZnO nanorods increased the τ_eff from 9 to 71 μs at a carrier injection level of 10¹⁵ cm^?3. Increased carrier lifetime revealed the passivation effect of the ZnO nanorods in addition to their ARC property. 33% and 16% enhancement in the photovoltaic conversion efficiency was obtained in planar and textured single‐crystalline solar cells, respectively. Our results reveal the potential of ZnO nanorods as ARC that can be deposited through simple solution‐based methods and the method investigated herein can be simply adapted to industrial scale fabrication.     

Synthesis and Characterization of ZnO Nanorods and Nanodisks from Zinc Chloride Aqueous Solution

ZnO nanorods and nanodisks were synthesized by solution process using zinc chloride as starting material. The morphology of ZnO crystal changed greatly depending on the concentrations of Zn²⁺ ion and ethylene glycohol (EG) additive in the solution. The effect of thermal treatment on the morphology was investigated. Photocatalytic activities of plate-like Zn₅(OH)₈Cl₂ · H₂O and rod-like ZnO were characterized. About 18% of 1 ppm NO could be continuously removed by ZnO particles under UV light irradiation.     

One-step solution synthesis of urchin-like ZnO superstructures from ZnO rods

The urchin-like ZnO superstructures have been directly prepared by the assistance of poly (acrylic acid) (PAA, M_w 5000) under a one-step solution-based process. X-ray diffraction (XRD) patterns indicate that the crystal structure of the special ZnO urchins is hexagonal. The results of Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) tests show that the urchins are composed of rods and the average aspect ratio of them is about 10 with a length of about 1.5 μm. Selected area electron diffraction (SAED) pattern reveals that the rods are single crystal in nature, which preferentially grow up along the 〈0001〉 direction. Furthermore, the sizes and aspect ratios of the rods can be easily controlled by regulating the concentration of ZnSO₄ solution. It is believed that the process of crystallization, including nucleation and crystal growth, happens along PAA chains resulting in the production of rods and assembly of them into superstructures.     

Sorption of palladium on carbon adsorbents from nitric acid solutions

Sorption recovery of palladium from nitric acid solutions on carbon adsorbents BAU, LKAU-7, ABG and UC has been investigated using model solutions with concentrations 8 × 10⁻⁴–8 × 10⁻³ mol/l for palladium and 1, 2 and 5 mol/l for nitric acid. The recovery degrees of Pd(II) depend on the concentration of palladium in contacting solutions as well as on the type of sorbent used. On average, they reach 60%–100% with the maximum in 1 M HNO₃ The palladium desorption by 10% thiocarbamide solution in 1M H₂SO₄ proceeds completely for the sorbent LKAU-7. The use of thiocarbamide solutions in 0.1 M NaOH increases the desorption of palladium from the sorbents BAU and UC up to 80%–85%     

Ultrasound-assisted synthesis of metal–organic framework nanorods of Zn-HKUST-1 and their templating effects for facile fabrication of zinc oxide nanorods via solid-state transformation

In this paper, the Zn-based analogue of Cu₃(btc)₂ (HKUST-1), Zn₃(btc)₂ nanorods (Zn-HKUST-1; btc = 1,3,5-benzenetricarboxylate) have been synthesized through direct precipitation in solution phase under ultrasound condition without the assistance of any surfactant, catalyst, or template. Zn-HKUST-1 nanorods werecharacterized by Fourier transform infrared studies, elemental analysis, powder X-ray diffraction and field emission scanning electron microscopy. Concentration of initial reagent effects on size and morphology of the Zn-HKUST-1 has been studied and shows that low concentrations of initial reagents decreased widths, thicknesses, and lengths of nanorods. Subsequently, ligand-free ZnO nanorods and nanoparticles are selectively prepared via a thermal decomposition method using the corresponding nanorods and single crystals as a precursor, respectively. This solid-state transformation route is promising for the preparation of crystalline nanomaterials with desired morphologies because of its simplicity and the low cost of the starting reagents.     

Mechanochemical synthesis of zinc oxide nanocrystalline

Pure zinc oxide (ZnO) nanocrystalline was successfully prepared by the mechanochemical method followed by a heat-treatment using ZnCl₂ and Na₂CO₃ as reagents, while NaCl was used as a diluent. The samples were characterized by Transmission Electron Microscopy (TEM) and X-ray Diffraction analysis (XRD). The effects of the milling time and the heat-treatment temperature on ZnO nanocrystalline size were investigated in this work. Experimental results show that increasing the milling time can effectively reduce the nanocrystalline size of ZnO. The average crystal size is about 21 nm for the ZnO nanocrystallites by the calcination of the after-milled powder at 600 °C in air and removal of NaCl through washing. The crystal size increases with increasing heat-treatment temperature. The mechanism of nanocrystallite growth is discussed.     

Preparation of ZnO/Eu mixed films by electrochemical precipitation

The electrochemical preparation of europium doped zinc oxide and europium oxide/hydroxide as thin films is investigated. First, a thermodynamic study of the Eu-Cl-H₂O system has been carried out at 25 and 70 °C in order to predict the electrochemical behaviour of Eu(III) dissolved in aqueous solution containing chloride ions. A comparison of the Eu-Cl-H₂O and Zn-Cl-H₂O systems indicates the possible coprecipitation of ZnO and Eu(OH)₃ from deposition solutions containing well-adjusted Eu(III)/Zn(II) concentrations ratio. The thermodynamic predictions have been confirmed experimentally by the electrochemical co-deposition of ZnO/Eu thin films on conducting electrode substrates at −1.4 V versus MSE. The presence of europium in the film is detected for Eu(III)/Zn(II) concentration ratio at (0.6 mM/5 mM) which is lower than the predicted value. Increasing Eu(III) concentration leads to the rapid appearance of two phases: dispersed zinc oxide nanorods and, at the bottom of the rods, a covering layer containing Eu(OH)₃ and zinc. The density of ZnO rods decreases and the rod size increases with increasing Eu(III) concentration in the bath. Above 1 mM EuCl₃, a dramatic fall in the current density is observed with the formation of a less conducting ZnO/Eu mixed deposit.     

Sonochemical assisted preparation of ZnS–ZnO/MCM-41 based on blast furnace slag and electric arc furnace dust for Cr (VI) photoreduction

Iron/steel making industry is a weed that produces large quantities of slag and dust. The objective of the present study was to develop a procedure for obtaining and characterizing photocatalysts derived from this waste for chromium remediation. The MCM-41 was synthesized via sodium silicate (Na₂SiO₃) derived from Blast Furnace Slag (BFS), and ZnO and ZnS were synthesized based on zinc extracted from Electric Arc Furnace Dust (EAFD). Subsequently, ZnO/ZnS were sono-chemically loaded on the MCM-41 and were tested for the Cr (IV) photoreduction. The resultant ZnO, ZnS, MCM-41, and composites were characterized by X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), N2adsorption–desorption isotherms, Fourier-transform infrared (FT-IR) spectrometry, Dynamic Light scattering, and Transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). A regular hexagonal structure of typical mesoporous MCM-41 had been proven by small-angle XRD, HRTEM, and N2 adsorption–desorption. The photoreduction activity of ZnS–ZnO/MCM-41 nanocomposite has obvious efficiency compared to ZnO and ZnO/MCM-41, achieving a 94% photoreduction of Cr (VI) in 180 min under UV irradiation. The slight activity loss after 4 cycles (84.7%) reveals the good photoreduction properties of catalysts. Based on these results, ZnS–ZnO/MCM-41 composite material seems to be high efficiency, green, stable, environment, and economical alternative to be used as a photocatalyst for the reduction of Cr (VI).     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.