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.     

Galvanic deposition of ZnO using mixed electrolyte and their photoluminescence properties

The effects of Zn(OAc)₂ concentrations and chemical nature of supporting electrolytes on the galvanic deposition of ZnO have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) microanalysis. The results show that the taper-like ZnO crystals are apt to be produced at lower Zn(OAc)₂ concentrations, while the rod-like ZnO crystals tend to be grown at higher Zn(OAc)₂ concentrations. The photoluminescence of as-prepared ZnO nanorods shows that there exist a strong UV emission band, a broad blue band at 468 nm, and a very weak green band at 550 nm. The blue-shift of UV emission is attributed to the Cl doping of ZnO in chloride electrolyte.     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

Defect induced room temperature ferromagnetism in well-aligned ZnO nanorods grown on Si (100) substrate

In this work, we report the fabrication of high quality single-crystalline ZnO nanorod arrays which were grown on the silicon (Si) substrate using a microwave assisted solution method. The as grown nanorods were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photo-luminescence (PL) and magnetization measurements. The XRD results indicated that the ZnO nanorods are well oriented with the c-axis perpendicular to the substrate and have single phase nature with the wurtzite structure. FE-SEM results showed that the length and diameter of the well aligned rods is about ~ 1 μm and ~ 100 nm respectively, having aspect ratio of 20-30. Room-temperature PL spectrum of the as-grown ZnO nanorods reveals a near-band-edge (NBE) emission peak and defect induced green light emission. The green light emission band at ~ 583 nm might be attributed to surface oxygen vacancies or defects. Magnetization measurements show that the ZnO nanorods exhibit room temperature ferromagnetism which may result due to the presence of defects in the ZnO nanorods.     

Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles

Electrochemical deposition of ZnO nanorods having a diameter of 80-150 nm and length ~ 2 μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100 s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375 nm and a broad defect-related emission band centered at ~ 596 nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50 s. The enhancement factor reached a maximum value for the Au sputtering time of 50 s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity.     

Hybrid nanostructures of Au nanocrystals and ZnO nanorods: Layer-by-layer assembly and tunable blue-shift band gap emission

A layer-by-layer assembly technique was developed to synthesize the hybrid nanostructures of Au nanocrystals with diameter of about 5 nm and ZnO nanorods via the electrostatic interaction. In comparison with ZnO nanorods, the Au-ZnO hybrid nanostructures exhibited the broadened and red-shifted surface plasmon band, enhanced band gap emission, and suppressed defect emission due to the strong interfacial coupling between Au and ZnO. Moreover, the band gap emission of the Au-ZnO hybrid nanostructures is controllably blue-shifted with decreasing distance between the Au nanocrystals and ZnO nanorods tuned by the amount of the polyelectrolyte layers due to the exciton and plasmon interactions.     

Optical and structural characteristics of ZnO thin films grown by rf magnetron sputtering

Nanostructured ZnO thin films on Pyrex glass substrates were deposited by rf magnetron sputtering at different substrate temperatures. Structural features and surface morphology were studied by X-ray diffraction and atomic force microscopy analyses. Films were found to be transparent in the visible range above 400 nm, having transparency above 90%. Sharp ultraviolet absorption edges around 370 nm were used to extract the optical band gap for samples of different particle sizes. Optical band gap energy for the films varied from 3.24 to 3.32 eV and the electronic transition was of the direct in nature. A correlation of the band gap of nanocrystalline ZnO films with particle size and strain was discussed. Photoluminescence emission in UV range, which is due to near band edge emission is more intense in comparison with the green band emission (due to defect state) was observed in all samples, indicating a good optical quality of the deposited films.     

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.     

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.     

Synthesis and photoluminescence properties of aligned Zn2GeO4 coated ZnO nanorods and Ge doped ZnO nanocombs

Aligned Zn₂GeO₄ coated ZnO nanorods and Ge doped ZnO nanocombs were synthesized on a silicon substrate by a simple thermal evaporation method. The structure and morphology of the as-synthesized nanostructure were characterized using scanning electron microscopy and transmission electron microscopy. The growth of aligned Zn₂GeO₄ coated ZnO nanorods and Ge doped ZnO nanocombs follows a vapor-solid (VS) process. Photoluminescence properties were also investigated at room temperature. The photoluminescence spectrum reveals the nanostructures have a sharp ultraviolet luminescence peak centered at 382 nm and a broad green luminescence peak centered at about 494 nm.     

Rapid preparation, characterization, and photoluminescence of ZnO films by a novel chemical method

A novel and simple chemical route was developed for the deposition of ZnO film from aqueous solution, integrating the merits of successive ionic layer adsorption and reaction and chemical bath deposition. ZnO thin films on glass and Si(1 0 0) substrates were deposited with the precursor of zinc-ammonia complex. As-deposited ZnO film exhibits good crystallinity with the hexagonal wurtzite crystalline structure and the preferential orientation along (0 0 2) plane. With a dense and continuous appearance, the film is composed of ZnO particles in even size of 200-300 nm. Under the excitation of 340 nm, strong and sharp near band gap emission (∼391 nm) dominates the photoluminescence spectra with several weak emission peaks related to the deep level (∼450-500 nm). In addition, the mechanism for the deposition process of ZnO from aqueous solution was preliminarily discussed.     

Structural properties and growth mechanism of flower-like ZnO structures obtained by simple solution method

Flower-shaped zinc oxide (ZnO) structures have been synthesized in the reaction of aqueous solution of zinc nitrate and NaOH at 90 °C. To examine the morphology of ZnO nanostructures, time-dependent experiments were carried out. Detailed structural observation showed that the flower-like structures consist of triangular-shaped leaves, having sharpened tips with wider bases. Photoluminescence spectrum measured at room temperature show a sharp UV emission at 381 nm and a strong and broad green emission at 480-750 nm attributed to structural defects. A possible growth mechanism for the formation of flower-shaped ZnO structures is discussed in detail.     

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.     

Characterization and humidity sensing of ZnO/42° YX LiTaO3 Love wave devices with ZnO nanorods

Love mode surface acoustic wave devices based on ZnO/42° YX LiTaO₃ were characterized with the thickness of the sputtered ZnO guiding layer varied from 250 nm to 1.18 μm. Phase velocity, temperature coefficient of resonant frequency, sensitivity, electromechanical coupling coefficient and humidity sensing of the Love mode SAW devices were studied as a function of the ZnO layer thickness. With increasing ZnO thickness over the range of thickness values we have examined, the sensitivity of 42° YX LiTaO₃ to liquid loading increased and the values of electromechanical coupling coefficient decreased. The device with a thickness of 250 nm showed the best humidity response. ZnO nanorods were grown on this device and its humidity sensing performance has been further improved due to their large surface-to-volume ratio of the ZnO nanorods.     

Cathodoluminescence of CaWO4 and SrWO4 thin films prepared by spray pyrolysis

Thin films of CaWO₄ and SrWO₄ were prepared on glass substrates by spray pyrolysis. The effects of preparation conditions and monovalent, bivalent and trivalent cation doping on cathodoluminescence (CL) properties of the films were studied. Polycrystalline CaWO₄ and SrWO₄ films formed a scheelite structure after being annealed above 300°C. They exhibited analogous cathodoluminescence consisting of a blue emission band at 447 nm and a blue-green emission band at 487 nm. The blue and blue-green emission intensities increased with substrate and annealing temperature. Annealing atmosphere and doping with Ag⁺, Pb²⁺ and La³⁺ did not influence the characteristics of the blue and blue-green emissions, whereas Eu³⁺ did. The results indicated both the blue and blue-green emissions originated from the WO₄²⁻ molecular complex. The luminance and efficiency for CaWO₄ film were 150 cd/m² and 0.7 lm/W at 5 kV and 57 μA/cm².     

Synthesis, characterization and optical properties of sheet-like ZnO

Sheet-like ZnO with regular hexagon shape and uniform diameter has been successfully synthesized through a two-step method without any metal catalyst. First, the sheet-like ZnO precursor was synthesized in a weak alkaline carbamide environment with stirring in a constant temperature water-bath by the homogeneous precipitation method, then sheet-like ZnO was obtained by calcining at 600 °C for 2 h. The structures and optical properties of sheet-like ZnO have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and UV-vis-NIR spectrophotometer. The results reveal that the product is highly crystalline with hexagonal wurtzite phase and has appearance of hexagon at (0 0 0 1) plane. The HRTEM images confirm that the individual sheet-like ZnO is single crystal. The PL spectrum exhibits a narrow ultraviolet emission at 397 nm and a broad visible emission centering at 502 nm. The band gap of sheet-like ZnO is about 3.15 eV.     

Effect of ZnO seed layer on the catalytic growth of vertically aligned ZnO nanorod arrays

We have grown vertically aligned ZnO nanorods and multipods by a seeded layer assisted vapor–liquid–solid (VLS) growth process using a muffle furnace. The effect of seed layer, substrate temperature and substrate material has been studied systematically for the growth of high quality aligned nanorods. The structural analysis on the aligned nanorods shows c-axis oriented aligned growth by homoepitaxy. High crystallinity and highly aligned ZnO nanorods are obtained for growth temperature of 850–900 °C. Depending on the thickness of the ZnO seed layer and local temperature on the substrate, some region of a substrate show ZnO tetrapod, hexapods and multipods, in addition to the vertically aligned nanorods. Raman scattering studies on the aligned nanorods show distinct mode at ∼438 cm⁻¹, confirming the hexagonal wurtzite phase of the nanorods. Room temperature photoluminescence studies show strong near band edge emission at ∼378 nm for aligned nanorods, while the non-aligned nanorods show only defect-emission band at ∼500 nm. ZnO nanorods grown without the seed layer were found to be non-aligned and are of much inferior quality. Possible growth mechanism for the seeded layer grown aligned nanorods is discussed.     

Photoluminescence characteristics of ZnO clusters confined in the micropores of zeolite L

Sub-nanometric ZnO clusters were prepared in the micropores of zeolite L by the incipient wetness impregnation method. The X-ray patterns (XRD), transmission electron microscope (TEM), N₂ adsorption-desorption isotherms, UV-vis absorption spectra (UV-vis) and photoluminescence spectra (PL) were used to characterize the composite materials. The results indicate that a small amount of sub-nanometric ZnO clusters can be introduced into the channel of zeolite L, however, when the amount of ZnO loading exceeds 20 wt%, macrocrystalline ZnO appears on the external surface of zeolite L. Different from bulk ZnO materials, these sub-nanometric ZnO clusters exhibit their absorption onset below 255 nm and a blue luminescence band in the range of 404-422 nm. The temperature-dependent luminescence demonstrates that the amount of the ZnO loading significantly affects the exciton-phonon interaction between the ZnO clusters and the zeolite host. The ZnO clusters exhibit a picosecond scale emission lifetime at room temperature.     

Synthesis of novel zinc oxide microphone-like microstructures

Novel microphone-like ZnO microstructures were grown at a very high density via a simple thermal evaporation process using commercially available ZnO powder in ambient air at ∼ 1050 ± 20 °C in 1 h. The unique as-grown microstructures were characterized in detail in terms of their structural and optical properties. The structural properties of the synthesized products confirmed that they were wurtzite hexagonal phase for the as-grown products. Raman-scattering spectra exhibited a strong and dominated Raman-active E₂ (high) mode at 441 cm^− 1, confirming the wurtzite hexagonal phase for the as-grown microphone-like ZnO morphologies. The cathodoluminescence (CL) spectrum shows a suppressed near band edge emission at ∼ 380 nm and strong green emission at ∼ 500 nm.