Effect of the mixing rate on the morphology and photocatalytic activity of ZnO powders prepared by a precipitation method

The urchin-like shape of ZnO powders was prepared by mixing of Zn²⁺ and NaOH solutions at various mixing rates. In this work, ε-Zn(OH)₂ was the first precipitant that was subsequently transformed to ZnO in the alkaline medium during heating. The size of the urchin-like shape of the ZnO powder decreased with a decrease of the mixing rate. The large urchin-like shape also had a large diameter of its hexagonal facet (0 0 0 1) and showed the highest photocatalytic degradative activity on methylene blue.     

Thermal decomposition synthesis of 3D urchin-like α-Fe2O3 superstructures

Urchin-like α-Fe₂O₃ superstructures have been deposited on Si substrate using thermal decomposition FeCl₃ solution at 200–600 °C in the oven. The morphologies and structures of the synthesized urchin-like superstructures have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that urchin-like α-Fe₂O₃ superstructures were a polycrystal with the rhombohedral structure and typical diameters of 16–20 nm and lengths up to 1.0 μm. The as-prepared α-Fe₂O₃ superstructures have a high Brunauer–Emmett–Teller (BET) surface area of about 60.24 m²/g. The photoluminescence spectrum of the urchin-like α-Fe₂O₃ superstructures consists of one weak emission peak at 548 nm (2.26 eV). A possible new mechanism for the formation of the urchin-like superstructures was also preliminarily discussed.     

Anionic surfactant-assisted hydrothermal synthesis of high-aspect-ratio ZnO nanowires and their photoluminescence property

ZnO nanowires with high-aspect-ratio of up to ca. 600 were synthesized in a quaternary reverse microemulsion containing sodium dodecyl sulfate (SDS) / water / heptane / n-hexane via a hydrothermal method. SDS, as an anionic surfactant, plays an important role in the formation of morphologies. Subsequently, we studied lots of key influencing factors including the molar ratio (w) value of NaOH to Zn(OAc)₂, the reaction temperature, and the instance without the quaternary reverse microemulsion. The selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) reveal the single-crystal nature of the ZnO nanowires. The morphologies and crystalline structure of the as-obtained products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and powder X-ray diffraction (XRD), respectively. Through this route, we can obtain a mass of products and the method is both convenient and reproducible. Finally, we measured the photoluminescence (PL) spectra and found that the ZnO nanowires exhibited green-orange emission at 525 nm and short ultraviolet emission at 380 nm and the ZnO nanomaterials with different aspect ratio (length to diameter) (L / D) showed PL intensity disciplinary change. Aiming at this phenomenon, we propose a reasonable mechanism to explain the PL spectra of the ZnO nanomaterials in detail.     

Fabrication of n-Zn1−xGaxO/p-(ZnO)1−x(GaP)x thin films and homojunction

Ga doped ZnO (GZO) and GaP codoped ZnO (GPZO) thin films of different concentrations (1–4 mol%) have been grown on sapphire substrates by RF sputtering for the fabrication of ZnO homojunction. The grown films have been characterized by X-ray diffraction (XRD), photoluminescence (PL), Hall measurement, energy dispersive spectroscopy (EDS), time-of-flight secondary ion mass spectrometer (ToF-SIMS), UV–Vis–NIR spectroscopy and atomic force microscopy (AFM). Unlike in conventional codoping, here we directly doped (codoped) GaP into ZnO to realize p-ZnO. The Hall measurements indicate that 2 and 4% GPZO films exhibit p-conductivity due to the sufficient amount of phosphorous incorporation while all the monodoped GZO films showed n-conductivity as expected. Among the p-ZnO films, 2% GPZO film shows low resistivity (2.17 Ωcm) and high hole concentration (1.8 × 10¹⁸ cm^?3) by optimum incorporation of phosphorous due to best codoping. Similarly, among the n-type films, 2% GZO shows low resistivity (1.32 Ωcm) and high electron concentration (2.02 × 10¹⁹ cm^?3) by optimum amount of Ga incorporation. The blue shift and red shift in NBE emission observed from PL acknowledged the formation of n- and p-conduction in monodoped and codoped films, respectively. The neutral acceptor bound exciton recombination (A⁰X) observed by low temperature PL for 2% GPZO confirms the p-conductivity. Further, the high concentration of P atoms than Ga observed from ToF-SIMS (2% GPZO) also supports the p-conductivity of the films. The fabricated p–n junction with best codoped p-(ZnO)_0.98(GaP)_0.02 and best monodoped n-Zn_0.98Ga_0.02O films showed typical rectification behavior of a diode. The diode parameters have also been estimated for the fabricated homojunction.     

Enhanced photoluminescence and electrical properties of 0-3 type ZnO/Bi3.6Eu0.4Ti3O12 nanocomposite thin films

0-3 type ZnO/Bi_3.6Eu_0.4Ti₃O₁₂ (BEuT) nanocomposite films with ZnO nanopowders in BEuT host were prepared by chemical solution deposition. The effects of ZnO content on the structure, photoluminescence, and electrical properties of the films were investigated. The ZnO/BEuT molar ratio strongly affected the grain size and growth orientation of BEuT, dielectric and ferroelectric properties, as well as emission intensity. The nanocomposite films showed strong red emission peaks due to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺ ions. Good electrical properties with high dielectric constant of 480 (at 1 kHz) and large remanent polarization (2P_r) of 32 μC/cm² were obtained for the nanocomposite films having a ZnO/BEuT molar ratio of 1:2. The mechanisms for enhanced photoluminescence and electric properties were discussed. The results suggest that the nanocomposite thin films are promising candidate materials for multifunctional optoelectronic devices.     

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.     

Synthesis,growth mechanism,photoluminescence and field emission properties of metal–semiconductor Zn–ZnO core–shell microcactuses

Metal–semiconductor Zn–ZnO core–shell microcactuses have been synthesized on Si substrate by simple thermal evaporation and condensation route using NH₃ as carrier gas at 600 °C under ambient pressure. Microcactuses with average size of 65–75 μm are composed of hollow microspheres with high density single crystalline ZnO rods. The structure, composition and morphology of the product were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscope (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A vapor–liquid–solid (VLS) based growth mechanism was proposed for the formation of Zn–ZnO core–shell microcactuses. Room temperature photoluminescence (PL) investigations revealed a strong and broad blue emission band at 441 nm associated with a weak ultraviolet (UV) peak at 374 nm. This blue emission (BE) is different from usually reported green/yellow-green emission from Zn–ZnO or ZnO structures. The field emission (FE) measurements exhibited moderate values of turn-on and threshold fields compared with reported large field emissions for other materials. These studies indicate the promise of Zn–ZnO core–shell microcactuses for the applications in UV-blue light display and field emission microelectronic devices.     

Microwave-assisted hydrothermal synthesis of zinc oxide particles starting from chloride precursor

Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn²⁺ ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)₆·H₂O), and the conversion from Zn₄CO₃(OH)₆·H₂O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 °C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved.     

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.     

Synthesis of sea urchin-like cuprous oxide with hollow glass microspheres as cores and its preliminary application as a photocatalyst

Cuprous oxide (Cu₂O) microcrystals with sea urchin-like morphologies were successfully prepared on the surface of hollow glass microspheres (HGMs) using sodium sulfite (Na₂SO₃) as the reducing agent and sodium acetate–acetic acid (NaAc–HAc) as buffer solution in copper sulfate (CuSO₄) solution. The products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal-thermogravimetry (DTA-TG), and visible spectrophotometer. Based on the SEM images of the as-obtained samples, it was found that the HGMs played a crucial role in the formation of sea urchin-like Cu₂O. Meanwhile, the stirring time was also important for coating process. The as-prepared sea urchin-like microcrystals are cubic phase Cu₂O. The as-prepared products can be oxidized at 240 °C. The preliminary study on the photocatalytic behavior of the sea urchin-like Cu₂O showed that the photodegradation efficiency of 40 mg/L methyl orange (MO) reached 95.15% within 30 min.     

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.     

Synthesis and photoluminescence of Y,Eu co-doped ZnO red phosphor

A red emitting ZnO·Y₂O₃:Eu phosphor has been prepared using pyrolysis technique at temperatures ≤1000 °C. When NH₄Cl was used as an ingredient, its luminescence efficiency was quite high indicating that Cl^? ions act as charge compensators since the introduction of Y³⁺ and Eu³⁺ cations in ZnO lattice demands the introduction of equal amount of excess anions. However, Na⁺ or Li⁺ quenches the luminescence efficiency of ZnO·Y₂O₃:Eu. Due to ZnO host absorption, the excitation peaks of ZnO·Y₂O₃:Eu phosphor near 260 nm and 394 nm are suppressed while the one at 468 nm is intense. This red emitting phosphor may find applications when monochromatic excitation such as lasers are involved. XRD data of (Zn_0.93Y_0.07)O_z:Eu³⁺,Cl^? shows the presence of the ZnO phase as well as the Y₂O₃ phase. It shows that Y₂O₃ forms a sublattice within ZnO host. This is supported by the PL data of (Zn_0.93Y_0.07)O_z:Eu³⁺,Cl^? which showed no significant change in the PL efficiency with increase in ZnO molar concentration.     

Influence of SOP modes on Raman spectra of ZnO(Fe) nanoparticles

Nanocrystaline samples of ZnO(Fe) were synthesized by traditional wet chemical method followed by calcinations. Samples were characterized by X-ray diffraction to determine composition of the samples (ZnO, ZnFe₂O₄ and Fe₂O₃) and the mean crystalline size (from 8 to 51 nm). In this paper we report the experimental spectra of Raman scattering (from 200 to 1600 cm⁻¹) with surface optical phonons (SOP) in range of 500–550 cm⁻¹. The phonon of registered phase’s exhibit effects connected to phase concentration, while the SOP phonon mode exhibit significant confinement effect.     

Synthesis and Characterization of Urchin-like Mischcrystal TiO2 and Its Photocatalysis

The urchin-like mischcrystal TiO₂ using acid attapulgite as an introducer was synthesized after a subsequent low-temperature hydrolyzation and crystallization followed by removal of acid attapulgite. The samples were characterized by transmission electron microscope, X-ray diffraction, Fourier transform infrared spectra and X-ray photoelectron spectroscopy. Acid attapulgite plays a critical role in the morphology and crystal structure of TiO₂. The results suggest that the perfect urchin-like mischcrystal TiO₂ is fabricated when the mass ratio of TiO₂ and acid attapulgite is 0.7:1. The single urchin-like TiO₂ is comprised of a nanosphere and plentiful nanoneedles. The nanoneedles grow radially on the surface of the nanosphere. The urchin-like TiO₂ is around 100 nm, and the nanoneedles have a diameter ranging from 2 to 5 nm. It has been confirmed that the chemical groups of acid attapulgite have a significant influence on the growth of TiO₂. In addition, the urchin-like mischcrystal TiO₂ exhibits excellent activity to assist photodegradation of Rhodamine B aqueous solution under ultraviolet light, and the degradation rate is about 94.15% for 80 min. The photocatalytic kinetics can be well described by the pseudo-first rate equation.     

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.     

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.     

Synthesis and photoluminescence spectroscopy of BaGeF6:Mn4+ red phosphor

We synthesized Mn⁴⁺-activated BaGeF₆ red phosphor by the chemical reaction method from HF, H₂SiF₆, BaF₂, KMnO₄, and GeO₂ powder. The structural and optical properties of BaGeF₆:Mn⁴⁺ were investigated using X-ray diffraction analysis, secondary electron microscopy observation, electron spin resonance measurement, photoluminescence (PL), PL excitation (PLE) and Raman scattering spectroscopies, and luminescence decay time measurement. Temperature dependence of the PL intensity was measured from T = 20 to 500 K and analyzed by taking into consideration the Bose–Einstein phonon occupation number. The PLE spectra measured at T = 20 and 300 K and luminescence decay time at T = 20–460 K were also analyzed based on the Franck–Condon and conventional thermal quenching models, respectively. Comprehensive discussion was given on the Mn⁴⁺-related PL properties and Raman scattering behaviors in a family of the barium hexafluorometallate phosphors.     

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.     

Urchin-like m-LaVO4 and m-LaVO4/Ag microspheres: Synthesis and characterization

The semiconductor nanostructures decorated with noble metals have attracted increasing attention due to their interesting physical and chemical properties. In this work, urchin-like monoclinic (m-) LaVO₄ microspheres were prepared by a hydrothermal method and used as a template to fabricate Ag nanoparticle-decorated m-LaVO₄ composites. The morphology and structure were characterized by transmission electron microscope, high-resolution transmission electron microscope, scanning electron microscope, and energy-dispersive X-ray. It was found that Ag nanoparticles with narrow size distribution were uniformly loaded on urchin-like m-LaVO₄ microspheres, and the resulted composite microspheres showed distinct surface plasmon absorption band compared to pure m-LaVO₄ microspheres. Photocatalytic activities of as-prepared samples were examined by studying the degradation of methyl orange solutions under visible-light irradiation (> 400 nm). Results clearly showed that urchin-like m-LaVO₄/Ag microspheres possess much higher photocatalytic activity than pure m-LaVO₄ microspheres and P25.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.