Wet chemical synthesis of ZnO nanocrystals: dependence of growth and morphology on the solvent composition

Zinc oxide (ZnO) nanocrystals were prepared using a wet chemical route starting from zinc acetate dihydrate dissolved in pure ethanol, pure water, and mixtures of ethanol and water. X-ray diffraction (XRD) studies along with thermogravimetric analyses results show that ZnO begins to crystallize at a temperature lower than 100 °C in a starting solution having 1:4 ethanol–water volume ratio. For other starting solutions, ZnO forms above 150 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies confirm the formation of nanoparticles of size ~15–20 nm and XRD analysis shows that the particles crystallize in the wurtzite structure. SEM and TEM studies show that ZnO particles grown in pure ethanol, pure water and in ethanol–water mixtures (other than the 1:4 mixture) have similar morphology, with the nanocrystals forming randomly grouped clusters. In the case of 1:4 solvent, however, the morphology is different, ZnO in this case growing in the form of chain like structures which appear like rods. Room temperature photoluminescence spectra exhibit a strong emission band in the red region probably caused by transitions between deep levels involving zinc interstitials.     

Effect of Magnetized Ethanol on the Shape Evolution of Zinc Oxide from Nanoparticles to Microrods: Experimental and Molecular Dynamic Simulation Study

In the current research, ethanol was exposed to an external magnetic field, called magnetized ethanol, and then, used as a solvent in the solvothermal method to synthesize various ZnO structures. Moreover, the morphologies of the synthesized structures are compared with those obtained using ordinary ethanol. The attained results evidently demonstrated the formation of ZnO nanoparticles and microrods by using ordinary and magnetized ethanol, respectively. Moreover, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were utilized for characterizing the synthesized ZnO structures. The XRD results demonstrated that the synthesized products are in Zincite hexagonal phase. Besides, molecular dynamics simulation suggested that the molecular mobility is diminished upon using the magnetic field. It was found that the interactions among ZnO particles were enhanced by the slight increase in the magnetic field while the number of interactions between ZnO and solvent was reduced revealing the magnetic-field-induced particle growth from the molecular level insight.     

Rapid microwave-assisted synthesis of hierarchical ZnO hollow spheres and their application in Cr(VI) removal

A rapid one-pot synthesis of hierarchical ZnO hollow spheres consisting of nanoparticles was realized by a facile microwave-assisted solvothermal method using ethanol as the solvent. According to the time-dependent observation of the formation process, a tentative mechanism based on ethyl acetate bubble-templating self-assembly of ZnO nanoparticles was proposed for the formation of the ZnO hollow spheres. Compared with the conventional heating, the microwave irradiation resulted in a significantly shortened reaction time (within 30?min) and considerably improved quality of the ZnO hollow spheres, such as narrower size distribution and more regular morphology, owing to the high heating rate and thus the accelerated reaction rate. It was shown that the microwave-assisted synthesis of ZnO nanostructures with tunable morphologies can be realized by judicious selection of appropriate solvents. The obtained ZnO hollow spheres exhibited an excellent adsorption capacity towards Cr(VI) ions in water because of their high surface area for adsorption and a good ability to preserve the accessible surface.     

Solvent-dependent tuning of blue–green emission of chemically synthesized ZnO nanomaterials with high colour purity and electroluminescence efficiency

A series of nanomaterials of ZnO have been synthesized via chemical co-precipitation method with fixed proportions of precursors and varied solvents. X-ray diffraction confirms wurtzite crystalline structure with nanometric crystallites (< 23 nm). Texture coefficient of crystallographic orientations show remarkable change for switching the solvents from water to alcohol. Morphological study reveals nanomaterials resembling prolate, sphere and oblate shaped structures for the solvents water, methanol and ethanol, respectively, with increasing particle size. All the nanomaterials show a similar absorption band in the UV region; though, more absorption covering a wider region in visible range is observed for nanomaterials prepared in alcoholic solutions. Red shifting in band gap of nanomaterials is correlated with band-tail effect. Variation in Urbach energy indicates that the nature of solvent plays a vital role in creating defects in ZnO, justifying enhanced absorptions in visible region. Photoluminescence (PL) spectra show various emission bands consisting of blue, green and yellow emissions corresponding to different intrinsic defects in nanomaterials. PL displays a tuning trend for blue–green emission by changing solvent from water to alcohol. However, overall enhanced PL intensity and particularly intense blue emission have been achieved by replacing water with alcohol. Tunability in emission colours and high colour purity is observed in the CIE chromaticity analysis. Theoretically, estimated electroluminescence of ZnO prepared in alcohol shows superiority compared to ZnO prepared in water. The mechanism of solvent-mediated defect creation and emission in ZnO will be beneficial for future QD LED applications.

     

Liquid-Phase Preparation and Characterization of Zinc Oxide Nanoparticles

Zinc oxide (ZnO) nanoparticles have been prepared by a wet chemical method, from zinc acetate and LiOH ethanol-based solutions. The resulted nanoparticles were dispersed in a solvent. The effect of solvent (ethanol or butanol) and surfactant (polyethylene glycol-PEG 200) on the average size and size distribution of the nanoparticles was investigated by light scattering measurements. Smaller size was observed for ZnO nanoparticles dispersed in butanol and PEG 200. The statistical parameters of the Gaussian size distribution curves were calculated. ZnO nanoparticles have been prepared to be used as seeds on a substrate for the growth of ZnO nanowires. The morphology, surface roughness, crystalline structure, and orientation of the nanoparticles deposed on silicon substrate were characterized by atomic force microscopy and x-ray diffraction, respectively.     

Ordered ZnO nanorods synthesized by combustion chemical vapor deposition

Aligned single-crystal ZnO rod arrays with diameters ranging from 50 nm to 300 nm and length up to 10 microm have been synthesized on silicon substrate under open-air conditions using a combustion chemical vapor deposition (CVD) method. The ZnO rods grew in the direction of (0001) with six [1120] peripheral surface planes. A single ZnO rod solid-state gas sensor fabricated on an interdigitated electrode pattern demonstrated prompt response to ethanol vapor at 400 degrees C.     

Water-processed self-assembles of monolayers as interface modifier for ZnO/P3HT hybrid solar cells

Benzoic acid based molecules were used for surface modification of ZnO nanorods as self-assembled monolayers (SAMs) in order to improve the interface interaction between ZnO and poly(3-hexylthiophene) in hybrid solar cells. The dipole moment of the molecules and the solvent used for the surface modification were investigated in relation to the performance of the hybrid devices. A linear relationship between the dipole moment of the interface modifiers and the open-circuit voltage (V_oc) of the devices were found. The V_oc are enhanced by the use of the molecules with their dipole moment pointing away from the ZnO surface and vice versa. The enhancement in V_oc is attributed to the shift of the vacuum level of ZnO when the surface is modified with the SAMs. When water was used as the solvent for surface modification process, aggregate was found in the solution state. This implies an ordered orientation of SAMs attached onto the ZnO surface, resulting in the improvement of the V_oc. An enhancement in the incident photon to current efficiency spectra was also obtained in the devices prepared from aqueous solution. The SAMs prepared from aqueous solution have better charge collecting properties in comparison with that prepared from ethanol solution.     

Re-dispersible Li+ and Eu3+ co-doped nanocrystalline ZnO: luminescence and EPR studies

Nano-crystals of ZnO, Eu3+ doped ZnO, and Li+, Eu3+ co-doped ZnO have been prepared by urea hydrolysis in ethylene glycol medium at 150 degrees C. Ethylene glycol acts as capping agent for nanoparticles. Three colors 437 (blue), 540 (green) and 615 nm (red) from 2 at.% Li+ and 5 at.% Eu3+ co-doped ZnO have been observed from luminescence studies compared to that from 5 at.%. Eu3+ doped ZnO, which shows emission at 437 and 615 nm. It is established that green light is originated from the oxygen vacancy brought by Li+ incorporation into ZnO. Particles are redispersible in organic solvent such as ethanol, and are able to incorporate into polymer-based material such as SiO2 matrix.     

多壁碳纳米管负载镁掺杂氧化锌纳米颗粒的研究

利用醋酸锌和醋酸镁为原料,无水乙醇为溶剂,草酸为沉淀剂,通过共沉淀法和后续热处理实现多壁碳纳米管表面负载镁掺杂氧化锌纳米颗粒,并使用X射线衍射仪、扫描电子显微镜、透射电子显微镜和紫外吸收光度计研究复合材料的形貌和光学性能。电子显微镜结果表明,在450℃热处理下,多壁碳纳米管表面能够均匀地负载镁掺杂氧化锌纳米颗粒,其颗粒粒径大约为10～20nm。紫外吸收光谱结果说明,镁掺杂显著地提高多壁碳纳米管/氧化锌复合材料的紫外吸收性能,而且其紫外吸收峰向短波方向蓝移达22nm。     

La-doped ZnO nanoparticles: Simple solution-combusting preparation and applications in the wastewater treatment

La-doped ZnO nanoparticles have been successfully synthesized by a simple solution combustion method via employing a mixture of ethanol and ethyleneglycol (v/v = 60/40) as the solvent. Zinc acetate and oxygen gas in the atmosphere were used as zinc and oxygen sources, and La(NO₃)₃ as the doping reagent. The as-obtained product was characterized by means of powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy. Experiments showed that La-doped ZnO nanoparticles exhibited the higher capacities for the removal of Pb²⁺ and Cu²⁺ ions in water resource than undoped ZnO nanoparticles.     

Controllable fabrication of various ZnO micro/nanostructures from a wire-like Zn-EG-AC precursor via a facile solution-based route

In this paper, ZnO particles with various morphologies were prepared though a facile solution-based route. The complexes Zn-EG-AC (EG: ethylene glycol; AC: CH₃COO⁻ groups) obtained by reaction of anhydrous zinc acetate and EG were used as precursors. It is found that the precursor could transform into ZnO in water with no need of assistant of additional alkali as it is sensitive to water. At the same time, it is well dispersed in reaction medium (water and ethanol). Experimental results showed that ZnO particles with various morphologies, such as the hexagonal rings, the hexagonal plates, the tubes, the prisms, and some interesting hierarchical structures, could be obtained by controlling hydrolysis of precursor in water and water/ethanol medium through finely tuning the experimental parameters. The success of shape-controllable fabrication was related intimately with the Zn-EG-AC precursor used in our synthesis.     

Photovoltaic properties of a ZnO nanorod array affected by ethanol and liquid-crystalline porphyrin

A vertically aligned array of ZnO nanorods, fabricated on conductive ITO substrate in aqueous solution, was characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and UV-visible transmission spectroscopy. Surface photovoltage (SPV) techniques based on a lock-in amplifier and a Kelvin probe were both employed to study the photogenerated charges in the system. The effects of ethanol solvent and a liquid-crystalline porphyrin, [5-(para-dodecyloxy)phenyl-10,15,20-tri-phenyl] porphyrin (DPTPP), on the photovoltage enhancement in the ZnO nanorod array were studied via SPV comparison between different irradiation directions on the system. We demonstrate that the ethanol adsorption could induce the space charge region to expand towards the ZnO/ITO interface. In the absence of ethanol, the ZnO nanorod array with the DPTPP adsorption showed enhanced SPV with reduced attenuation rate of photogenerated charge carriers. We found that the separation of photogenerated charges could be further improved by coating the surface with DPTPP and ethanol together. Furthermore, the SPV spectra patterns of the composite system with opposite incident-light directions reveal that the DPTPP molecules adsorbed just at the surface of ZnO nanorods adopt a more monomeric alignment in contrast to the aggregative state in the DPTPP bulk.     

A simple one-step solvothermal synthesis of hierarchically structured ZnO hollow spheres for enhanced selective ethanol sensing properties

A simple one-step solvothermal method, using ethanolamine as solvent without any additives except zinc source, has been employed to synthesize hierarchically structured ZnO hollow spheres consisting of numerous orderly and radical nanorods with diameter of several tens nanometers and length of 2–3 μm. The ethanolamine and the solvothermal process play the critical role in the synthesis of the ZnO hollow spheres by the primary formation of ZnO crystal nucleus and subsequent transformation into nanorods, which self-assemble into hollow spheres. The morphology and structure of the spheres have been characterized by transmission electron microscopy, field emission scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and Brunauer–Emmett–Teller N₂ adsorption–desorption analyses. The results also indicate that the sensor based on the prepared ZnO hollow spheres exhibit good ethanol sensing performance, which can be attributed to its structural defects and high surface-to-volume ratio that significantly facilitate the absorption of oxygen species and diffusion of target gas. Besides, the sensor shows high selectivity to ethanol because ZnO as a basic oxide is favored for dehydrogenation of ethanol.     

Effect of solvents on the structural, optical and morphological properties of Zn0.96Cu0.04O nanoparticles

Zn_0.96Cu_0.04O nanoparticles were synthesized by co-precipitation method using different solvents like ethanol, water and mixer of ethanol and water in 50:50 ratios. Crystalline phases and optical studies of the nanoparticles were studied by X-ray diffraction (XRD) and UV–visible photo-spectrometer. The XRD showed that the prepared nanoparticles have different microstructure without changing a hexagonal wurtzite structure. The calculated average crystalline size was high for ethanol (27.3 nm) due to the presence of more defects and low for water (26 nm) due to the reduction of defects and vacancies. The energy dispersive X-ray analyses confirmed the presence of Cu in ZnO system and the weight percentage is nearly equal to their nominal stoichiometry within the experimental error. The presence of lower Zn and Cu percentage in the sample prepared using ethanol than other solvents was due low reaction rate which was confirmed by XRD spectra. Water solvent has relatively stronger transmittance in the visible region which leads to the industrial applications especially in opto-electronic devices. The average crystalline size is slowly decreased from 27.3 nm (ethanol) to 26 nm (water) whereas energy gap is steadily increased from 3.56 eV (ethanol) to 3.655 eV (water) when water concentration is increased from 0 to 100 % in ethanol. Existence of functional groups and bonding were analyzed by FTIR spectra. The observed blue shift of UV emission from ethanol (349 nm) to water (340 nm) solution and the high I_UV/I_G ratio in water solution in photoluminescence spectra was due to the decrease of crystalline size and defects/secondary phases. The intensity of blue–green band emission was gradually decreased due to the reduction of defects and vacancies when water concentration is increased from 0 to 100 % in ethanol solution, which was consistent with the XRD observation.     

The effects of solvents on the highly oriented ZnO films prepared using sol–gel method

The effects of solvents in ZnO precursor solutions on highly oriented ZnO films is studied in this work. Three different solvents, including ethanol, isopropanol and 2-methoxyethanol, are used for the preparation of ZnO film on fused quartz wafer substrates in this study. The structural properties of ZnO films are investigated by X-ray diffraction and scanning electron microscopy analysis. It is found that ZnO film formed using 2-methoxyethanol precursor solution shows a strong preferred orientation and has the smoothest surface among the films. The properties of ZnO sol solutions are also studied using the thermal gravimetric-differential thermal analysis and ultraviolet–visible spectroscopy. The absorbance spectrum of the sol using 2-methoxyethanol as solvent shows an apparent ZnO absorbance edge while the others do not, which means ZnO nano-particles are formed in the sol. The mechanism of the formation of highly oriented ZnO films using different solvents is also discussed in this paper.     

Hierarchical structures of ZnO spherical particles synthesized solvothermally

We review the solvothermal synthesis, using a mixture of ethylene glycol (EG) and water as the solvent, of zinc oxide (ZnO) particles having spherical and flower-like shapes and hierarchical nanostructures. The preparation conditions of the ZnO particles and the microscopic characterization of the morphology are summarized. We found the following three effects of the ratio of EG to water on the formation of hierarchical structures: (i) EG restricts the growth of ZnO microcrystals, (ii) EG promotes the self-assembly of small crystallites into spheroidal particles and (iii) the high water content of EG results in hollow spheres.     

Hierarchical structures of ZnO spherical particles synthesized solvothermally

Abstract

We review the solvothermal synthesis, using a mixture of ethylene glycol (EG) and water as the solvent, of zinc oxide (ZnO) particles having spherical and flower-like shapes and hierarchical nanostructures. The preparation conditions of the ZnO particles and the microscopic characterization of the morphology are summarized. We found the following three effects of the ratio of EG to water on the formation of hierarchical structures: (i) EG restricts the growth of ZnO microcrystals, (ii) EG promotes the self-assembly of small crystallites into spheroidal particles and (iii) the high water content of EG results in hollow spheres.     

Control of epitaxial growth orientation in ZnO nanorods on c-plane sapphire substrates

This paper presents a study on low temperature hydrothermal growth of ZnO nanorods (NRs) on pre-seeded (0001) sapphire substrates. Prior to hydrothermal growth of ZnO NRs, epitaxial ZnO seeds were grown by metal-organic chemical vapour deposition under various process conditions. Findings show that the majority of ZnO NRs inclined at a specific angle of about 38° to the direction perpendicular to the substrate surface and exhibited a preferential in-plane alignment, besides other NRs growing vertically from the sapphire surface. X-ray diffraction φ-scan measurements reveal that the ZnO nanorods displayed two distinct epitaxial relationships with sapphire which were (0001)_ZnO//(0001)_sapphire and (0001)_ZnO//(101?4)_sapphire, respectively. Reduced lattice mismatch between ZnO and sapphire is responsible for the inclined ZnO NRs growth. The growth direction of ZnO NRs is remarkably dependent on the growth conditions of ZnO seeds and sapphire substrate pre-treatment. The epitaxial orientations of ZnO seeds grown on the sapphire substrate dominate the subsequent ZnO NRs growth and can be controlled through adjusting growth conditions.     

Microstructural evolution of ZnO by wet-etching using acidic solutions

The characteristics of the wet-etching of ZnO thin films were investigated using hydrochloric and phosphoric acid solutions as etchants. The etch rate of ZnO films, using a highly diluted hydrochloric acid solutions at a concentration of 0.25% in deionized water, was determined to be about 120 nm/min, and linearly increased with increasing the acid concentration. The surface of ZnO(0001) etched by an HCl solution, observed by scanning electron microscopy, showed a rough morphology with a high density of hexagonal pyramids or cones with sidewall angles of about approximately 45 degrees. Moreover, the sidewall angles of the masked area were similar to those of the pyramids on the surface. In comparison, the surface of ZnO(0001) etched by a phosphoric acid had a smooth surface morphology. The origin of this difference is from the very initial stage of etching, indicating that the etch-mechanism is different for each solution. Furthermore, when H3PO4 was added to the HCl aqueous solution, the morphology of the etched surface was greatly enhanced and the sidewall angle was also increased to about 65 degrees.     

A novel route to ZnO nanorods with small diameters of 20 nm

The ZnO nanorods with small diameters of 20 nm were prepared successfully by an easy “in situ consumed template” route. In the synthesis, Zn(Ac)₂ were used as Zn source and dodecanethiol (DT) was used as coordinated agents in ethanol solvent. The samples were characterized detailed by XRD, TEM and IR techniques. The results indicated that the ZnO rods were uniform in diameters with good crystallinity. Time-dependent experiments indicated that the ZnO rods are grown within the Zn–DT complex (a complex composed of Zn and DT) that was formed at the beginning of the reaction. With prolonging the reaction time, the Zn–DT “template” was gradually in situ consumed and transformed into ZnO, and finally, the ZnO nanorods with diameters of 20 nm were obtained. The method here provides the new route for ZnO nanorods with small diameters.