Flame synthesis of zinc oxide nanocrystals

Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counter-flow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of ～99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 μm. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of ～0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells. The synthesized nanostructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED). The growth mechanism of the ZnO nanostructures is discussed.     

Synthesis and characterization of ZnO with hexagonal dumbbell-like bipods microstructures

High crystalline ZnO with hexagonal dumbbell-like bipods morphology was successfully synthesized via N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB)-assisted hydrothermal microemulsion route. X-ray diffraction, scanning electron microscopy, photoluminescence spectrum and Ultraviolet and Visible absorption spectroscopy were used to characterize the structure, morphologies and properties of the as-prepared samples. The results demonstrated that the hydrothermal reaction time had obvious influence on the shape and size of the ZnO products. The Ultraviolet and Visible absorption spectra indicated that the as-prepared ZnO had a strong and broad absorption band from ultraviolet to visible region. The photoluminescence spectra of the synthesized ZnO exhibited a very strong UV emission at ～380 nm, indicative of their high crystal quality. Meanwhile, the possible growth mechanism for the formation of hexagonal dumbbell-like ZnO microstructures was proposed.     

Fabrication and characterization of tetrapod-like ZnO nanostructures prepared by catalyst-free thermal evaporation

Tetrapod-like ZnO nanostructures were fabricated on ZnO-coated sapphire (001) substrates by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation process. First, the ZnO films were pre-deposited on sapphire (001) substrates by PLD. Then the ZnO nanostructures grew on ZnO-coated sapphire (001) substrate by the simple thermal evaporation of the metallic zinc powder at 900 °C in the air without any catalysts. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO nanostructures. The as-synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrum (FTIR). The results show that the tetrapod-like ZnO nanostructures are highly crystalline with the wurtzite hexagonal structure. Photoluminescence (PL) spectrum of as-synthesized nanostructures exhibits a UV emission peak at ～ 389 nm and a broad green emission peak at ～ 513 nm. In addition, the growth mechanism of ZnO nanostructures is also briefly discussed.     

Effects of deposition temperature and chemical composition on the ZnO crystal growth on the surface of Pd catalyst through electroless chemical reaction

Zinc oxide (ZnO) was site-selectively grown on the palladium (Pd) catalyst through the electroless deposition process under mild conditions, and the effects of deposition temperature and chemical composition on the ZnO crystal growth were investigated. ZnO crystals were synthesized on the UV-patterned Pd catalysts in the aqueous solutions of various dimethylamine borane (DMAB)/Zn(NO₃)₂ ratio at 30–70 °C. The site-selective deposition was confirmed by X-ray photoelectron spectroscopy (XPS) data and elemental maps of Pd, Zn and oxygen in energy-filtering transmission electron microscopy (EFTEM), and the crystal morphology was observed by scanning electron microscopy (SEM). A strong near band emission at around 390 nm and a weak green emission at around 470 nm were observed in the photoluminescence (PL) spectrum. The ZnO crystals were grown in the following three steps: (1) ZnO fibrils were generated on the Pd catalysts and became sphere-like particles, (2) hexagonal wurtzite crystals initiated to grow from the sphere-like particles, and (3) the crystals grew in two directions—longitudinal and lateral growths giving rod-type or needle-type hexagonal crystals. It was found that longitudinal growth rate increased with increasing deposition temperature or DMAB/Zn(NO₃)₂ ratio.     

Size and defect related broadening of photoluminescence spectra in ZnO:Si nanocomposite films

Nanocomposite films of zinc oxide and silicon were grown by thermal evaporation technique using varying ratios of ZnO:Si in the starting material. Structural analyses reveal the role of ZnO and amorphous silicon interface in contributing to the relatively less common blue photoluminescence emissions (at ～410 and 470 nm). These blue peaks are observed along with the emissions resulting from band edge transition (370 nm) and those related to defects (520 nm) of ZnO. Careful analysis shows that along with the grain size of ZnO, a suitable compositional ratio of ZnO to silicon is critical for the coexistence of all the four peaks. Proper selection of conditions can give comparable photoluminescence peak intensities leading to broad-band emission.     

Hydrothermally grown ZnO nanoflowers for environmental remediation and clean energy applications

Well-crystalline flower-shaped ZnO nanostructures were synthesized by simple hydrothermal process at low-temperature of 145 °C and utilized as a photocatalyst and photo-anode material for photocatalytic degradation and dye-sensitized solar cell applications, respectively. The detailed morphological and the structural characterizations revealed that the synthesized products were flower-shaped, grown in very high-density, and possessed well-crystalline wurtzite hexagonal phase. The chemical composition confirmed the pure phase and good optical properties of as-synthesized ZnO flowers. The as-synthesized ZnO flowers were used as an efficient photocatalyst for the photocatalytic degradation of Rhodamine B which exhibit ～84% degradation within 140 min. Moreover, the as-synthesized ZnO flowers were utilized as photo-anode material for the fabrication of dye-sensitized solar cells (DSSCs) which exhibited overall light-to-electricity conversion efficiency of ～1.38%, open-circuit current (V_OC) of 0.621 V, short-circuit current (J_SC) of ～3.52 mA/cm² and fill factor (FF) of 0.64.     

Synthesis of nanocrystalline GaN by the sol–gel method

Single-phase wurtzite GaN nanocrystals with an average diameter of 11 ± 3 nm were synthesized by the sol–gel technique from readily available Ga(NO₃)₃. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) confirmed they had a hexagonal structure and a narrow size distribution of the nanocrystals. X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurement showed that the GaN powder was of single-phase wurtzite structure with a considerable fraction of structural defects such as twin and stacking faults. The IR spectrum showed that only the Ga–N stretch is present at 600 cm⁻¹. The EDX pattern of as-prepared product showed their ratio approximate to 1:1. Room temperature photoluminescence (PL) measurement exhibited the band-edge emission of GaN at about 390 nm and defect emission peak at 610 nm.     

Liquid phase synthesis of ZnO microrods highly oriented on the hexagonal ZnO sheets

We have successfully synthesized zinc oxide microrods perpendicularly oriented on hexagonal ZnO sheets by a simple heat treatment approach using LDH (layered double hydroxide) precursor in an aqueous solution. The synthesized ZnO microrods have an average diameter of 500 nm and length of 2–3 μm, and form highly-oriented array. In this work, the effect of heating temperature and time on morphology and orientation of ZnO microrods was studied experimentally and the formation mechanism was discussed in detail. Transformation from hexagonal precursor to ZnO microrods can be attributed to dissolution and re-precipitation of the precursor, which should be caused by its thermal unstability under heating temperature.     

Synthesis and optical properties of MgO-doped ZnO microtubes using microwave heating

The Zn_1−xMg_xO (x = 0%, 2% and 5%) microtubes have been successfully synthesized via a microwave heating method. The as synthesized microtubes were carefully investigated. Field emission scanning electron microscope (FE-SEM) showed that all the microtubes exhibit an exact hexagonal hollow structure with smooth surfaces and straight characteristics throughout their whole lengths. UV–Vis measurement indicates that the absorption peak for ZnO microtube was shifted from 378.88 nm (3.27 eV) to 369.91 nm (3.35 eV) for Zn_0.95Mg_0.05O microtube. Room temperature photoluminescence (PL) spectra showed that the intensity of UV emission peak decreased with increase of MgO concentration and the visible emission band showed a blue shift from 538.06812 nm for ZnO microtube to 529.54114 nm for Zn_0.95Mg_0.05O microtube. Energy-dispersive spectrometer (EDS) analysis revealed the presence of Zn and O as the only elementary components with the absence of MgO as a doping material.     

Facile synthesis of ZnO nanostructures and investigation of structural and optical properties

ZnO nanostructures were synthesized by chemical bath deposition method, using zinc nitrate [Zn(NO₃)₂] and hexa-methylene-tetra-amine [(HMT),C(H₂)₆N₄] as precursors. Controlled size and shape evolution of ZnO nanostructures were achieved by changing the HMT concentration from 0.025 M to 0.1 M, whereas Zn(NO₃)₂ concentration kept constant. X-ray diffraction (XRD) and Raman study confirmed the formation of single crystalline, hexagonal wurtzite ZnO structure. Sharp peaks in Raman spectra, corresponding to E₂(low) and E₂(high) referred to wurtzite structure with higher order of crystallinity. Transmission electron microscopy (TEM) revealed that the shape and size of the nanostructures reduced, with increasing concentration of HMT. Further, effect of structure's size was observed in the band gap (shift). Photoluminescence study showed two peaks at ~ 380 nm and ~ 540 nm corresponding to the band to band transition and defect transitions. Modifications of properties are explained in detail on the basis of shape and size change of the structures and possible mechanism is discussed.     

Synthesis,strong room-temperature PL and photocatalytic activity of ZnO/ZnWO4 rod-like nanoparticles

Zinc oxide (ZnO)/zinc tungstate (ZnWO₄) rod-like nanoparticles with diameters in the range of 6–11 nm and length of about 30 nm were synthesized by a low temperature soft solution method at 95 °C in the presence of non-ionic copolymer surfactant. It was found that their crystallinity was enhanced with the increase of heating time from 1 h up to 120 h. The photoluminescence (PL) measurements showed very strong, narrow UV band peaked at 3.30 eV and a broad visible band peaking at 2.71 eV with a shoulder at about 2.53 eV, for λ_exc < 300 nm. Quite large variations in the intensities of the two PL bands were observed for different excitation wavelengths. The intensity of the main visible band decreases with decreasing excitation energy and disappears when samples are excited λ = 320 nm (E_exc = 3.875 eV). We found that observed optical properties originate from ZnO phase. UV band gap PL had high intensity for all applied excitations, probably induced by ZnWO₄ phase presence on the surface. In addition, two values were found for direct band-gap energy of ZnO/ZnWO₄ rod-like nanoparticles 3.62 and 3.21 eV, determined from reflectance spectrum. The photocatalytic behaviour of ZnO is strongly dependent on the formation of ZnWO₄ phase, of the obtained rod-like nanoparticles.     

Cu doped ZnO nanoparticle sheets

Cu doped ZnO nanoparticle sheets were synthesized via a proposed solution route with mixed Zn(NO₃)₂ and Cu(NO₃)₂ precursors at a low temperature of 95 °C. Scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectrometry results demonstrate that the nanostructues synthesized by solutions with higher Cu(NO₃)₂ concentration are nanoparticle sheets comprised of uniform Cu doped ZnO nanoparticles with diameters around 20 nm. Room-temperature photoluminescence spectra of the nanoparticle sheets show tunable near band emissions centered at 390–405 nm and strong yellow emissions at 585–600 nm. Absorbance spectra show gradual redshift in the UV range with the increase of Cu concentrations in the ZnO nanomaterials. The study provides a simple and efficient route to prepare Cu doped ZnO nanomaterials at low temperature. The as-synthesized products with both violet and yellow emissions are promising for white light-emitting diode applications.     

Mesoporous structure and evolution mechanism of hydroxycarbonate apatite microspheres

Monodisperse mesoporous hydroxycarbonate apatite microspheres (MHAMs) were fabricated by soaking calcium carbonate microspheres (CCMs) in a cetyltrimethylammonium bromide (CTAB)/Na₂HPO₄/cyclohexane/n-butanol emulsion system. After soaking CCMs in the emulsion system at 20 °C, hydroxycarbonate apatite nanocrystals nucleate heterogeneously on the surfaces of CCMs via a dissolution–precipitation reaction. The as-formed nanocrystals aggregate to form mesopores with the pore size of ～ 3.9 nm and ～ 9.0 nm. The smaller mesopores are derived from the direct aggregation of the nanocrystals, which do not change obviously with the reaction time. In contrast, the larger mesopores are formed by using CTAB micelles as templates, and their pore size decreases from ～ 9.0 nm to ～ 7.4 nm with increasing the reaction time from 6 h to 1 day. After reaction for 3 or 5 days, the larger mesopores disappear because of unstability of the CTAB micelles. If reaction temperature is kept at 50 °C, the conversion rate of CCMs to MHAMs is greater than that at 20 °C, and the corresponding mesoporous structure is unimodal with the pore size of ～ 3.9 nm. Simulated body fluid immersion tests reveal that MHAMs have a great in vitro bioactivity, which is attributed to the mesoporous structure and B-type CO₃^2? substitution of MHAMs.     

Facile ultrasound-assisted synthesis of ZnO nanorods in an ionic liquid

ZnO nanocrystals have been synthesized by ultrasound-assisted synthesis from Zn(CH₃COO)₂2H₂O and NaOH in the neat room-temperature ionic-liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C₄mim][Tf₂N]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the formed ZnO nanocrystals are of rod like shape with lengths from 50 to 100 nm and diameters of about 20 nm. X-ray diffraction (XRD) confirms the crystallinity as well as the sample purity. The band gap of the as-prepared ZnO nanorods was estimated to be 3.31 eV from UV–Vis absorption measurements. The photoluminescence spectrum shows the characteristic greenish emission of ZnO at room temperature (λ_max = 563 nm). The ZnO bonding levels have been determined by X-ray photoelectron spectroscopy (XPS). Nitrogen adsorption–desorption measurements show typical samples to have a specific surface area of 49.93 m²/g.     

Optical properties of petal-like aggregated nanocrystalline zinc oxide synthesized by laser ablation

The results of the investigations carried out on the third-order nonlinearity in zinc oxide (ZnO) nanocrystals (NCs) by Z-scan technique are included in this paper. ZnO NCs show negative nonlinearity and good nonlinear absorption behavior at 532 nm. The third-order optical susceptibility χ(3) increases with enlargement of NCs due to the size dependent enhancement of exciton oscillator strength.The synthesis of ZnO NCs was performed by laser ablation from a high-purity metallic target of Zn in distilled water medium. For the ablation process, a high frequency pulsed Nd:YAG laser was employed operating at 532 nm with 100 ns pulse duration. UV–vis absorption spectroscopy illustrated the enhancement of the size of ZnO NCs upon increasing the laser pulse energy applied in ablation process. Accordingly the corresponding optical band gap (E_g) decrease by increasing the size of NCs. X-ray diffraction (XRD) associated with transmission electron microscopy (TEM) was utilized to characterize the crystalline phase and also for determining the ZnO NCs morphology.     

Low temperature dielectric studies of zinc oxide (ZnO) nanoparticles prepared by precipitation method

Using zinc nitrate as a precursor and NaOH starch as a stabilizing agent, hexagonal zinc oxide (ZnO) nanoparticles has been synthesized by precipitation method. The transmission electron microscopy (TEM) images show particles of nearly uniform spherical size of around 40 nm. The infrared spectroscopy (FT-IR) measurement reveals the peak at 500 cm^?1, corresponding to the Zn–O bond. Dielectric studies of ZnO nanoparticles show frequency dependence dielectric anomaly at low temperature (85–300 K). Results reveal that the capacitance and loss tangent decrease with the frequency while these parameters improve with the increasing of temperature. The increase of a.c. conductivity with the temperature indicates that the mobility of charge carriers is responsible for hopping and electronic polarization in ZnO nanoparticles.     

Photoluminescence study of ZnO nanostructures grown on silicon by MOCVD

ZnO nanostructures with a size ranging from 20 to 100 nm were successfully deposited on (1 0 0)-Si substrates at different temperatures (500–800 °C) using MOCVD. It could be confirmed that the size of ZnO nanostructures decreased with increasing growth temperature. From photoluminescence (PL) studies it was found, that intensive band-edge PL of ZnO nanostructures consists of emission lines with maxima at 368.6 nm, 370.1 nm, 373.7 nm, 383.9 nm, 391.7 nm, 400.7 nm and 412 nm. These lines can be dedicated to free excitons and impurity donor-bound excitons, where hydrogen acts as donor impurity with an activation energy of about 65 meV. A UV shift of the band-edge PL line with increasing growth temperature of ZnO nanostructures was observed as a result of the quantum confinement effect. The results suggest that an increase of growth temperature leads to increased band-edge PL intensity. Moreover, the ratio of band-edge PL intensity to green- (red-) band intensity also increases, indicating better crystalline quality of ZnO nanostructures with increasing growth temperature.     

Study of optical properties of GeO2 nanocrystals as synthesized by hydrothermal technique

The GeO₂ nanocrystals (α-quartz type structure) with β-phase are synthesized at relatively lower temperature by hydrothermal route using autoclave. All samples are characterized by XRD, FESEM, EDS, TEM, photoluminescence (PL) and UV–vis absorption spectroscopy techniques. Synthesized nanocrystals have uniform shape and uniform size distribution for a particular synthesis condition, which is about 30–300 nm depending on synthesis conditions. The XRD results indicate that grown GeO₂ crystals only shows peak related to α-quartz structure with lattice parameters a = 4.985 Å and c = 5.648 Å. UV–vis absorption spectroscopy measurements reveal the bandgap energies corresponding to the GeO₂ α-quartz structure. Synthesized nanocrystals are capable to emit strong blue light around 425–435 nm under excitation of 300 nm and 325 nm and consequently the as synthesized material can be used in integrated optical devices.     

Quickest ever single-step mechanosynthesis of Cd0.5Zn0.5S quantum dots: Nanostructure and optical characterizations

Multiphase Cd_0.5Zn_0.5S quantum dots (QDs) are prepared for the first time in a record minimum time of 30 min by mechanical alloying the stoichiometric mixture of elemental Cd, Zn and S powders at room temperature under Ar. Initially, the hexagonal phase is formed as major one and then gradually transforms to the cubic phase which is formed coherently on the hexagonal lattice with (0 0 2)_hexagonal||(1 1 1)_cubic mechanism. In the course of milling up to 15 h, the molar ratio of cubic to hexagonal phase becomes ～0.7:0.3. Both high resolution transmission electron microscopy (HRTEM) and X-ray microstructure analyses reveal that these QDs are isotropic in nature, their size reduces to ～5 nm after 15 h of milling with a very narrow size distribution and they contain a significant amount of stacking and twin faults. These QDs show very distinct quantum confinement effect and their optical band gaps are higher than that of the bulk CdZnS.     

Photoluminescence and thermoluminescence studies of Tb3+ doped ZnO nanorods

Here in, the synthesis of the terbium doped zinc oxide (ZnO:Tb³⁺) nanorods via room temperature chemical co-precipitation was explored and their structural, photoluminescence (PL) and thermoluminescence (TL) studies were investigated in detail. The present samples were found to have pure hexagonal wurtzite crystal structure. The as obtained samples were broadly composed of nanoflakes while the highly crystalline nanorods have been formed due to low temperature annealing of the as synthesized samples. The diameters of the nanoflakes are found to be in the range 50–60 nm whereas the nanorods have diameter 60–90 nm and length 700–900 nm. FTIR study shows ZnO stretching band at 475 cm^?1 showing improved crystal quality with annealing. The bands at 1545 and 1431 cm^?1 are attributed to asymmetric and symmetric CO stretching vibration modes. The diffuse reflectance spectra show band edge emission near 390 nm and a blue shift of the absorption edge with higher concentration of Tb doping. The PL spectra of the Tb³⁺-doped sample exhibited bright bluish green and green emissions at 490 nm (⁵D₄ → ⁷F₆) and 544 nm (⁵D₄ → ⁷F₅) respectively which is much more intense then the blue (450 nm), bluish green (472 nm) and broad green emission (532 nm) for the undoped sample. An efficient energy transfer process from ZnO host to Tb³⁺ is observed in PL emission and excitation spectra of Tb³⁺-doped ZnO ions. The doped sample exhibits a strong TL glow peak at 255 °C compared to the prominent glow peak at 190 °C for the undoped sample. The higher temperature peaks are found to obey first order kinetics whereas the lower temperature peaks obey 2nd order kinetics. The glow peak at 255 °C for the Tb³⁺ doped sample has an activation energy 0.98 eV and frequency factor 2.77 × 10⁸ s^?1.