Low temperature solution synthesis and characterization of ZnO nano-flowers

Synthesis of flower-shaped ZnO nanostructures composed of hexagonal ZnO nanorods was achieved by the solution process using zinc acetate dihydrate and sodium hydroxide at very low temperature of 90 °C in 30 min. The individual nanorods are of hexagonal shape with sharp tip, and base diameter of about 300-350 nm. Detailed structural characterizations demonstrate that the synthesized products are single crystalline with the wurtzite hexagonal phase, grown along the [0 0 0 1] direction. The IR spectrum shows the standard peak of zinc oxide at 523 cm⁻¹. Raman scattering exhibits a sharp and strong E₂ mode at 437 cm⁻¹ which further confirms the good crystallinity and wurtzite hexagonal phase of the grown nanostructures. The photoelectron spectroscopic measurement shows the presence of Zn, O, C, zinc acetate and Na. The binding energy ca. 1021.2 eV (Zn 2p_3/2) and 1044.3 eV (Zn 2p_1/2), are found very close to the standard bulk ZnO binding energy values. The O 1s peak is found centered at 531.4 eV with a shoulder at 529.8 eV. Room-temperature photoluminescence (PL) demonstrate a strong and dominated peak at 381 nm with a suppressed and broad green emission at 515 nm, suggests that the flower-shaped ZnO nanostructures have good optical properties with very less structural defects.     

Preparation of ZnO nanorods through wet chemical method

The different morphologies of nanorods have been obtained via a simple wet chemical method in the present of polyethylene glycol (PEG, M_w = 4000) by using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and ammonium hydroxide (NH₃·H₂O) as the starting materials. Samples were characterized by XRD, EDS, TEM, SEM, ED and PL. XRD results prove the formation of ZnO with wurtzite structure. The ED and HRTEM reveal that single ZnO nanorod is single crystal and preferentially grows up along the [001] direction. The PL spectra showed that the ZnO nanorods have blue emission at 466 nm and green-yellow emission at 542 nm. The influence of reaction temperature, pH in system and evaporation of ammonia on the morphology has been investigated. A possible growth mechanism of ZnO with various morphologies is discussed.     

Vertically aligned Mn-doped zinc oxide nanorods by hybrid wet chemical route

Mn-doped zinc oxide (Mn:ZnO) nanorods were synthesized by incorporating manganese in aligned ZnO nanorods. For this, Mn was evaporated onto ZnO nanorods and the composite structure was subjected to rapid thermal annealing. The nanorods were preferentially oriented in (0 0 2) direction as indicated by the XRD measurement. Optical band gap was seen to decrease with increasing amount of Mn incorporation. XPS studies indicated that incorporated Mn was in Mn²⁺ and Mn⁴⁺ states. Mn²⁺ atomic concentration was found to be larger than Mn⁴⁺ concentration in all the samples. The Raman spectra of the Mn:ZnO nanorods indicated the presence of the characteristic peak at ∼438 cm⁻¹ for high frequency branch of E₂ mode of ZnO. The PL peak at ∼376 nm (∼3.29 eV) was ascribed to the band edge luminescence while the peak at ∼394 nm (∼3.15 eV) was assigned to the donor bound exciton (D^oX) and free exciton transition related to Mn²⁺ states.     

Solvothermal synthesis of nanorods of ZnO, N-doped ZnO and CdO

ZnO nanorods with diameters in the 80-800 nm range are readily synthesized by the reaction of zinc acetate, ethanol and ethylenediamine under solvothermal conditions. The best products are obtained at 330 °C with a slow heating rate. Addition of the surfactant Triton^®-X 100 gave nanorods of uniform (300 nm) diameter. By adding a small amount of liquid NH₃ to the reaction mixture, N-doped ZnO nanorods, with distinct spectroscopic features are obtained. CdO nanorods of 80 nm diameter have been prepared under solvothermal conditions using a mixture of cadmium cupferronate, ethylenediamine and ethanol at 330 °C. Similarly, Zn_1−xCd_xO nanorods of a 70 nm diameter are obtained under solvothermal conditions starting with a mixture of zinc acetate, cadmium cupferronate, ethanol and ethylenediamine.     

Ni-doped vertically aligned zinc oxide nanorods prepared by hybrid wet chemical route

Ni-doped zinc oxide (Ni:ZnO) nanorods were synthesized by incorporating nickel in vertically aligned ZnO nanorods. Ni was evaporated onto ZnO nanorods and the composite structure was subjected to rapid thermal annealing for dispersing Ni in ZnO nanorods. The optical band gap decreased with increasing amount of Ni incorporation. The origin of the photoluminescence peak at ∼ 400 nm was related to the defect levels introduced due to substitution of Ni²⁺ in the Zn²⁺ site with annealing. The Raman spectra indicated the presence of the characteristic peak at ∼ 436 cm^− 1 which was identified as high frequency branch of E₂ mode of ZnO. The Fourier Transformed Infrared spectra indicated the existence of the distinct characteristic absorption peak at 481 cm^− 1 for ZnO stretching modes. Current-voltage characteristics indicated that the current changed linearly with voltage for both the doped and undoped samples.     

Graphene oxide modified ZnO nanorods hybrid with high reusable photocatalytic activity under UV-LED irradiation

In this work, graphene oxide/zinc oxide (GO/ZnO) hybrid was prepared through a facile hydrothermal process. Transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectra and N₂ adsorption and desorption isotherms were used to investigate the morphology, crystal structure, optical properties and specific surface area of GO/ZnO hybrid. It was shown that the well-dispersed ZnO nanorods were deposited on GO homogeneously. Photocatalytic properties of GO/ZnO nanorods hybrid were evaluated under 375 nm light-emitting diode light irradiation for photodegradation of methylene blue (MB). The synergic effect between GO and ZnO was found to lead to an improved photo-generated carrier separation. An optimal GO content has been determined to be 3 wt%, and corresponding the apparent pseudo-first-order rate constant kapp$k_{\text{app}}$ is 0.0248 min⁻¹, 4.3 times and 2.5 times more than that of pure ZnO nanorods and commercial P25 photocatalyst, respectively. Moreover, the cyclic photocatalytic test indicated that GO/ZnO hybrid can be reused for degradation of MB, suggesting the possible application of GO/ZnO hybrid as excellent candidate for water treatment.     

Fabrication and growth mechanism of hexagonal zinc oxide nanorods via solution process

This article presents, the fabrication of perfectly hexagonal zinc oxide nanorods performed via solution process using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (HMT) at various concentrations of i.e. 1 × 10⁻³ to 10 × 10⁻² M in 50 mL distilled water and refluxed at 100 °C for 1 h. We used HMT because it acts as a template for the nucleation and growth of zinc oxide nanorods, and it also works as a surfactant for the zinc oxide structures. The X-ray diffraction patterns clearly reveal that the grown product is pure zinc oxide. The diameters and lengths of the synthesized nanorods lie in the range of 200–800 nm and 2–4 μm, respectively as observed from the field emission scanning electron microscopy (FESEM). The morphological observation was also confirmed by the transmission electron microscopy (TEM) and clearly consistent with the FESEM observations. The chemical composition was analyzed by the FTIR spectroscopy, and it shows the ZnO band at 405 cm⁻¹. On the basis of these observations, the growth mechanism of ZnO nanostructures was also proposed.     

Ethylene glycol assisted hydrothermal synthesis of flower like ZnO architectures

We describe a simple route to flower like ZnO architectures, based on the decomposition of zinc acetate precursor in water-ethylene glycol solution at 140-160 °C for 1d through hydrothermal method. The PXRD pattern reveals that the ZnO crystals are of hexagonal wurtzite structure. Ethylene glycol plays a key role on the morphology control of ZnO crystals. The SEM images of ZnO products prepared at 140 °C and 160 °C mainly exhibit flower like architecture composed of many rods. Whereas, the product prepared at 180 °C shows bunches accompanying a few number of free rods. TEM results reveal that the rods resemble swords with decrease in size from one end to another. From Raman spectrum, the peaks at 437 cm^− 1, 382 cm^− 1 and 411 cm^− 1 correspond to E₂ (high), A₁ (TO) and E₁ (TO) of ZnO crystals respectively. The photoluminescence spectrum exhibits strong UV emission at ~ 397 nm, which comes from recombination of exciton. The possible mechanism for the formation of flower like ZnO architecture is proposed.     

Morphology investigation of hydrothermally prepared ZnO micro-crystals influenced by Cu ions

Micrometer-sized ZnO with various shapes have been synthesized by a hydrothermal route with Zn(Ac)₂/NH₃·H₂O in the presence of copper ions. Powder X-ray diffraction and scanning electron microscopy were utilized to characterize the composition and structure of the samples, respectively. As-synthesized, the ultrasonic pretreatment, pH value, reaction time and hydrothermal temperature played important roles in crystal structure tailoring. Novel nut-like ZnO crystal with improved crystal quality was obtained with initial Zn²⁺/Cu²⁺ ratio of 10, at reaction temperature of 100 °C, with initial pH of 7.0 and in a reaction time of 4 h, with ultrasonic pretreatment. Time-dependent experiments show that transfer of the Cu ions plays a crucial role as it causes the difference in crystal growth velocity.     

Enhanced light to electricity conversion efficiency of CdS–ZnO composite nanorod based electrochemical solar cell

Cadmium sulfide–zinc oxide composite nanorods having at least 100 nm diameters were synthesized by a two-step chemical deposition technique. Polycrystalline nanorods of ZnO were grown on indium tin oxide coated quartz substrate by aqueous chemical growth technique. Cadmium sulfide was deposited on the surface of the ZnO nanorod thin film by chemical bath deposition. The X-ray diffraction results revealed the co-existence of polycrystalline CdS and ZnO, both having hexagonal structures. Neither any phase mixing nor any surface diffusion induced alloying was observed. Micro-Raman study detected a pair of optical phonons at 301 cm⁻¹ and 438 cm⁻¹ corresponding to hexagonal CdS and ZnO, respectively. An enhanced light to electricity conversion efficiency of 2.52% was recorded from CdS–ZnO photoanode based electrochemical solar cell under 0.5 sun illumination condition (50 mW cm⁻²). We observed a significant enhancement of short circuit current of the electrochemical solar cells due to addition of ionic salt solution to the electrolyte.     

Synthesis of Sn-doped ZnO nanorods and their photocatalytic properties

Sn-doped ZnO nanorods were fabricated by a hydrothermal route, and characterized by X-ray diffraction, field emission scanning electron microscope, UV-vis spectroscopy, Raman spectra, solid-state nuclear magnetic resonance (NMR) spectra, and room temperature photoluminescence spectroscopy. Solid-state NMR result confirms that Sn⁴⁺ was successfully incorporated into the crystal lattice of ZnO. Room temperature photoluminescence showed that all the as-synthesized products exhibited a weak UV emission (380 nm) and a strong visible emission (540 nm), but the intensities of the latter emission increased with increase in Sn concentration. The improvement of visible emission at 540 nm in the Sn-doped ZnO samples was suggested to be a result of the lattice defects increased by doping of Sn in zinc oxide. In addition, the photocatalytic studies indicated that Sn-doped ZnO nanorods are a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes.     

Seeded growth of ZnO nanorods from NaOH solutions

Perpendicularly aligned arrays of corrugated ZnO nanorods were grown onto gold patterned LiTaO₃ substrates, coated with a sputtered ZnO seed layer. During the growth process, these substrates were held submerged in an aqueous solution comprising a 1:40 mol ratio mix of zinc nitrate hexahydrate to sodium hydroxide. The substrates were placed in a custom apparatus residing in an autoclavable storage bottle. Scanning electron micrographs, which were taken at different deposition intervals, suggest that the growth mechanism of ZnO nanorods initiates with the etching of the ZnO sputtered seed layer into hexagonal bases (> 500 nm across), from where multiple protrusions (40 nm-100 nm in width) grow atop these hexagonal bases. Such nanoprotrusions later coalesce into larger nanorods. Uniformly distributed high density corrugated nanorods, with proximal spacing between adjacent nanorods of approximately 20 nm-50 nm, were observed over the entire surface.     

Facile one-pot synthesis and luminescence of hexagonal TbPO4·nH2O hollow spheres

Monodisperse hexagonal TbPO₄·nH₂O hollow spheres were successfully obtained by utilizing Tb(OH)CO₃ colloidal spheres as the precursor and NH₄H₂PO₄ as the phosphorus source through the hydrothermal process. The obtained hollow spheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), respectively. They have the average diameter of 200 nm. There are a number of tiny nanorods with the length of about 60 nm on the surface of the spheres. The obtained TbPO₄ hollow spheres exhibit green color emission from ⁵D₄ − ⁷F_J (J = 6, 5, 4, 3) transitions of the Tb³⁺ ions, which are expected to be applied in display applications and biological applications.     

Investigation of ZnO nanorods synthesized by a solvothermal method, using Al-doped ZnO seed films

ZnO nanorods were successfully grown on common glass substrates using a simple solvothermal method via the precursors of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and Hexamethylenetetramine (C₆H₁₂N₄) with equal molar concentration at 0.01 mol/l, 0.025 mol/l, 0.05 mol/l, and 0.075 mol/l. The ZnO nanorods were characterized by X-ray diffraction (XRD), Scanning electron microscopy, UV-Vis absorption spectrophotometer and photoluminescence (PL) spectrometer. XRD results indicated that all the ZnO nanorods were preferentially grown along [0 0 0 1] direction (c-axis). With an increase of Zn(CH₃COO)₂·2H₂O and C₆H₁₂N₄ concentration, the diffraction intensity of ZnO nanorod along c-axis also increased. Scanning electron microscopy images showed that the well-faceted hexagonal ZnO nanorods were grown vertically from the common glass substrates. In addition, with the increase of Zn(CH₃COO)₂·2H₂O and C₆H₁₂N₄ concentration, the exciton band of ZnO nanorods determined by UV-Vis absorption spectra gradually became narrow and the intensity of exciton band also remarkably augmented. Photoluminescence spectra showed that with the increase of Zn(CH₃COO)₂·2H₂O and C₆H₁₂N₄ concentration, the position of emission peak of ZnO nanorod blue-shifts towards shorter wavelength in UV region and the luminescence intensity remarkably enhances in visible emission range (470-630 nm).     

Low-temperature synthesis of ZnO nanorods using organic-inorganic composite as a seed layer

Well-aligned zinc oxide (ZnO) nanorods were synthesized using a low-temperature hydrothermal method employing a zinc/sodium dodecyl sulfate (Zn/SDS) composite as a seed layer. The results of X-ray diffraction measurements indicate that the Zn/SDS composite has a lamellar structure with an interlayer distance of 3.12 nm, which is shorter than that of the lamellar structure of SDS (3.82 nm) due to ion exchange between Zn and Na. The results of X-ray absorption fine structure analyses suggest that ZnO crystals start to grow after an induction period of 20-30 min. The length of nanorods and the aspect ratio of ZnO nanorods could be controlled by altering the molarity of ammonium and zinc nitrate in the growth solutions.     

Structure and photocatalytic activity of Ni-doped ZnO nanorods

The one-dimensional (1D) Zn_1−xNi_xO (x = 0, 0.02, 0.05, 0.10) nanorods have been synthesized by a simple hydrothermal method. New bands show at ∼130 cm⁻¹ in the Raman spectra of Ni-doped ZnO nanorods and their relative intensity depends on the doping concentration of nickel. The optical band gap of the ZnO nanorods have been tuned by Ni-doping, which is revealed by absorption spectra. The photocatalytic activity of Zn_1−xNi_xO was studied by comparing the degradation rate of rhodamine B (RB) under UV-light irradiation. It was found that Zn_0.95Ni_0.05O exhibited the highest photocatalytic degradation efficiency among the samples.     

Effects of growth variables on the properties of single crystalline ZnO thin film grown by inductively coupled plasma metal organic chemical vapor deposition

Single crystalline undoped and Ga-doped n-type zinc oxide (ZnO) films were grown on sapphire (Al₂O₃) substrates by inductively coupled plasma (ICP) metal organic chemical vapor deposition. Effects of growth variables on the structural, optical, and electrical properties of ZnO films have been studied in detail. Single crystal films with flat and smooth surfaces were reproducibly obtained, with application of sample bias and O₂ ICP. The best film properties were obtained at the growth condition of 650 °C, 400 W ICP power, − 94 V bias voltage, O/Zn (VI/II) ratio of 75. Single crystalline Ga doped n-ZnO films were also obtained, with free carrier concentration of about 1.5 × 10¹⁹/cm³ at 1 at.% Ga concentration.     

Synthesis of size-tunable ZnO nanorod arrays from NH3·H2O/ZnNO3 solutions

Well-aligned crystalline ZnO nanorod arrays were fabricated via an aqueous solution route with zinc nitrate and ammonia as precursors. Dip-coating was firstly utilized to form a ZnO film on ITO substrate as a seed layer for subsequent growth of ZnO nanorods. The effects of NH₃·H₂O/ZnNO₃ molar ratio, ZnNO₃ concentration, growth temperature and time on nanorod morphology were respectively investigated. It was found that the size of nanorod is mainly determined by the molar ratio and concentration. XRD demonstrates that ZnO nanorods are wurtzite crystal structures preferentially orienting in the direction of the c-axis. SEM confirms that ZnO nanorods grew up perpendicular to the substrate. The diameter and length were tunable in a broad range from 80 nm to 500 nm and 250 nm up to 8 μm, respectively. The aspect ratio changed from 3 to 17 mainly dependent on composition of the aqueous solution.     

Synthesis of nanometal oxides and nanometals using hot-wire and thermal CVD

We report the synthesis of nano-oxides of molybdenum, tungsten, and zinc. Molybdenum oxide (MoO₃) and tungsten oxide (WO_x) were produced by hot-wire CVD with molybdenum and tungsten filaments, respectively while zinc oxide (ZnO) was produced by thermal CVD. When high purity molybdenum wire was oxidized at ambient system atmosphere, nanorods and nanostraws of MoO₃ with length ranging from ∼ 20-80 nm and diameters ranging from ∼ 5-15 nm were produced. Also, the oxidation of the tungsten filament led to the deposition of tungsten oxide nanorods (10-25 nm diameter and 75-90 nm long) and nanospheres with diameters of ∼ 60 nm. Each oxide was reduced to its metallic form by annealing in a hydrogen environment to produce metallic nanoparticles. Nanorods and nanoribbons of ZnO with diameters ranging from 20-65 nm and lengths up to 2 μm were also produced.     

The role of ammonia hydroxide in the formation of ZnO hexagonal nanodisks using sol–gel technique and their photocatalytic study

ZnO nanomaterials with large surface area are desired particularly for the gas sensor, biosensor and photocatalyst applications. In this study, ZnO hexagonal nanodisks with thickness to diagonal aspect ratio (～1/80) were successfully synthesised via sol–gel approach. By using aluminium sulphate as a complexing agent and carefully controlling the amount of ammonia hydroxide, zinc oxide hexagonal nanodisks were produced. The ZnO nanodisks had perfect hexagonal shape with about 4 μm in diagonal and 50 nm in thickness. The growth of the nanodisks was favoured along the six symmetric directions of ±[1ī00], ±[01ī0] and ±[10ī0]. The growth mechanism of ZnO hexagonal nanodisks is proposed as follows. The formation of ZnO hexagonal nanodisks was mediated by the adsorption of aluminate ions, Al(OH)₄^?, on the polar surface of ZnO. The Al(OH)₄^? ions were produced as a result of reaction between Al₂(SO₄)₃ and NH₄OH. The Al(OH)₄^? ions were bonded to the positively charged Zn²⁺-terminated (0001) polar surface of ZnO. This suppressed the preferential growth of ZnO along [0001] direction but allowed the lateral growth of ZnO in <01ī0>. Eventually, ZnO hexagonal nanodisks with ±(0001) top/bottom surfaces and {1ī00} side surfaces were formed. The size of the ZnO hexagonal nanodisks could be adjusted via the synthesis duration and the amount of ammonia hydroxide. The photocatalytic study indicates that ZnO hexagonal nanodisks were a good photocatalyst for the degradation of Rhodamine B under ultraviolet light irradiation with a rate constant of 0.036 min^?1.