Hexagonal Microrods Research Articles

Hexagonal microrods architectured MoO3 thin film for supercapacitor application

Hexagonal microrods containing MoO3 thin films are synthesized by simple, low cost and scalable chemical bath deposition approach with acidification of ammonium molybdate ([NH4]6Mo7O24·4H2O). The structural and morphological analyses are carried out through X-ray diffraction and field emission scanning electron microscopy (FE-SEM) respectively. Hexagonal microrods of width ~5 µm and length of ~20–50 µm are manifested in FE-SEM analysis. Supercapacitive tests of electrode are performed with cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) techniques in 1 M Na2SO4 electrolyte. The highest specific capacitance of 194 Fg−1 is obtained from CV study. The energy and power densities of 7.33 Wh kg−1 and 1200 W kg−1, respectively are obtained from GCD study. An electrochemical impedance spectroscopic study is also carried out.

Hexagonal crown-capped NaYF4:Ce3+/Gd3+/Dy3+ microrods: Formation mechanism, energy transfer and luminescence properties

In this work, hexagonal crown-capped β-NaYF4:Ce3+/Gd3+/Dy3+ microrods with remarkably uniform morphology and size have been successfully synthesized for the first time via a facile hydrothermal route. The phase and morphology evolution of β-NaYF4 crystals are carefully tracked during hydrothermal growth by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron diffraction (TEM), respectively. The possible growth mechanism of hexagonal crown-capped NaYF4 microrods has been proposed. Furthermore, the luminescence properties of β-NaYF4:Ce3+/Gd3+/Dy3+ are systemically investigated by the photoluminescence excitation spectra, emission spectra and decay curves. Impressively, upon the allowed Ce3+ 4f–5d electronic transition excitation, the efficient energy transfer from sensitizer (Ce3+) to activator (Dy3+) occurs via energy migration process Ce3+ → (Gd3+)n → Dy3+, in which Gd3+ ions act as bridge centers to induce intense yellow luminescence from Dy3+ activators.

Intense and fast UV emitting ZnO microrods fabricated by low temperature aqueous chemical growth method

Intense and fast ultraviolet (UV) emitting zinc oxide (ZnO) microrods are successfully fabricated by low temperature aqueous chemical growth (ACG) method. Uniform and hexagonal microrods of varying dimensions are synthesized using different zinc acetate dihydrate (ZnAc) and hexamethelynetetramine (HMTA) molar concentration ratios. Although exhibiting broad orange emissions, the microrods have intense UV emissions with lifetimes as fast as 30–40ps. Analyses show that there is no definite correlation between the microrod dimensions and the optical emissions. Nevertheless, with the ease on the microcrystal fabrication, these ZnO microrods with intense and fast UV emissions have potential use for future scintillator applications.

Lasing from an individual ZnO hexagonal microrod on the Au surface coated by a nanometer-scaled SiO2 layer

We demonstrate that an individual ZnO hexagonal microrod on the surface of Au substrates coated by a nanometer-scaled SiO2 layer can become a source for manufacture miniature ZnO plasmon lasers by surface plasmon polariton coupling with whispering-gallery modes (WGMs), and the SiO2 layer can tune the modes and intensity of ZnO emission. We achieve an abnormally low-threshold plasmonic WGM lasing from a ZnO hexagonal rod, and the threshold is 0.36 KW/cm2. These investigations show that this coupling mode holds a potential of ZnO hexagonal micro- and nanorods for data storage, bio-sensing, optical communications, as well as all-optic integrated circuits.

Dissolution–recrystallization and regrowth mechanism of hydrothermally derived six faceted prismatic hexagonal ZnO microrods

Zinc oxide (ZnO) microstructures have been grown using simple hydrothermal method. X-ray data confirms well crystalline nature of obtained ZnO powder with hexagonal (wurtzite-type) crystal structure. Scanning electron microscopic observations reveals that the microstructure consists of well defined sharp prismatic hexagonal microrods with angular walls. The microrods grown with controllable diameter in the range of 1.2–1.5μm with length in the range of 8–10μm and grow along the [0001] direction. The aspect ratio of ZnO microrods increases with increase in reaction time. It is due to the dissolution–recrystallization mechanism and regrowth process. Morphological observations confirm that the reaction time plays a vital role to determine dimensionality of the synthesized ZnO microrods. The photoluminescence (PL) spectrum of the as-grown ZnO nanostructure reveals a near-band-edge (NBE) emission peak (399nm) and a blue emission peak (460nm). The increase in reaction time reveals enhancement in photoluminescence intensity.

Molten salt synthesis, growth mechanism of β-NaYF4 and tunable luminescence properties of β-NaYF4:Tb3+ microrods

Pure β-NaYF4 hexagonal microrods have been prepared via a facile and mass production molten salt method without using any surfactant. The phase and morphology evolution process as well as the formation mechanism were presented systematically on the basis of XRD analysis, SEM and TEM observations of the products at the different reaction time periods. Moreover, the systematical investigation on the luminescence properties of β-NaYF4:xTb3+ (x=0.5, 1, 2.5, 5, 10, 20, 40mol%) microrods shows the characteristic emissions of Tb3+ (5DJ→7FJ′, J=3, 4; J′=3, 4, 5, 6). Under the excitation of single wavelength light of 368nm, the luminescence colors of the corresponding products can be tuned feasibly from greenish blue to green then to green-yellow by changing the doping concentration of Tb3+.

Super low threshold plasmonic WGM lasing from an individual ZnO hexagonal microrod on an Au substrate for plasmon lasers.

We demonstrate an individual ZnO hexagonal microrod on the surface of an Au substrate which can become new sources for manufacturing miniature ZnO plasmon lasers by surface plasmon polariton coupling to whispering-gallery modes (WGMs). We also demonstrate that the rough surface of Au substrates can acquire a more satisfied enhancement of ZnO emission if the surface geometry of Au substrates is appropriate. Furthermore, we achieve high Q factor and super low threshold plasmonic WGM lasing from an individual ZnO hexagonal microrod on the surface of the Au substrate, in which Q factor can reach 5790 and threshold is 0.45 KW/cm2 which is the lowest value reported to date for ZnO nanostructures lasing, at least 10 times smaller than that of ZnO at the nanometer. Electron transfer mechanisms are proposed to understand the physical origin of quenching and enhancement of ZnO emission on the surface of Au substrates. These investigations show that this novel coupling mode holds a great potential of ZnO hexagonal micro- and nanorods for data storage, bio-sensing, optical communications as well as all-optic integrated circuits.

Growth of a seven pointed star shaped of vertical and uniform ZnO nanostructures on optical fiber via catalyst-free VLS mechanisms

In this paper, the growths of ZnO nano- and microstructures by VLS mechanism have been studied. A piece of silica fiber (SMF-28 optical fiber) was used as a substrate. By controlling the experimental conditions, nano- and microstructures of ZnO with different shapes and sizes were grown around the optical fiber, which can be used as an optical fiber sensor. A star-shaped ZnO microstructure was obtained in this research. Pyramidal structures were grown on each side of vertical hexagonal microrods which formed the seven-pointed star-shaped microstructures. This growth process was carried out in a special quartz container in a tube furnace at temperatures around 400–550 °C without any additional catalyst. XRD, SEM and PL spectroscopy were used for the characterization of the produced nanostructures. The influence of temperature, reactant and carrier gases flow rate on the morphology of ZnO nanostructure was investigated.

Electron-hole plasma induced band gap renormalization in ZnO microlaser cavities.

We report electron-hole plasma (EHP) lasing in hexagonal ZnO microrods and thin nanobelts. Under the excitation of 325 nm line femtosecond pulsed laser, ultraviolet whispering-gallery mode (WGM) lasing was observed from hexagonal ZnO microrods. When EHP was formed at high excitation energy density, the center wavelength of the WGM lasing band presented a redshift from 387.5 nm to 397.5 nm, and the full width of half maximum (FWHM) of the WGM lasing band increased from 2.5 nm to 7 nm. Each lasing mode showed obvious blueshift and broadening. Such lasing characteristics were attributed to the band gap renormalization (BGR) due to the high carrier concentration at the EHP condition. In addition, EHP Fabry-Perot (F-P) mode lasing from thin ZnO nanobelt was also observed and discussed. According to the phenomenological BGR calculation with including the carrier density dependent screening effect, the values of the band gap of ZnO at different excitation energy densities were obtained, which agree well with the experimental results.

Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence enhancement of ZnO microrods.

More than 170-fold ultraviolet emission enhancement was obtained from the hexagonal ZnO microrods synthesized by a simple vapor-phase transport process and decorated by Al nanoparticles (NPs). Based on the stable and transient photoluminescence (PL) spectra of the ZnO microrods sputtered with Al NPs for different times, the underlying mechanism was deduced and can be attributed to the metal surface plasmon resonance (SPR) coupling with ZnO. Interestingly, with increasing of the sputtering time, the ratio of the band gap emission to the defect-related emission was increased from 0.1 to 42.7. Our results demonstrated that ZnO microrods decorated with Al NPs shed light on developing stable and high-efficiency excitonic optoelectronic devices such as light-emitting diodes and lasers.

Improving the Optical Properties of Self-Catalyzed GaN Microrods toward Whispering Gallery Mode Lasing

GaN microrods were grown self-catalyzed by a fast and simple metal–organic vapor phase epitaxy method without any processing before or after deposition. The prismatic microrods with a regular hexagonal cross-section, sharp edges, straight, and smooth sidewall facets act as a microresonator, as seen by the appearance of whispering gallery modes in the yellow defect band range. To improve their optical properties, a reduced Ga precursor flow is required during growth. However, their hexagonal microrod morphology is not maintained under these growth conditions. The approach to start growth with a high Ga precursor flow and applying a ramp to a reduced precursor flow yield in significant enhancement of the near band edge emission in the upper part of the microrods. Whispering gallery modes in superposition with the near band edge emission can now be detected by cathodoluminescence measurements. These improvements lead to stimulated emission of a single whispering gallery mode up to ∼2 MW/cm2 and multimode las...

Excitation wavelength dependent photoluminescence intensity of six faceted prismatic ZnO microrods

This article presents the investigation on the large-scale synthesis of ZnO microrods with a simple low temperature hydrothermal method without using surfactants, organic solvents, or catalytic reagents. The synthesized ZnO powder is characterized with different techniques. The X-ray diffraction study reveals the excellent crystal quality of the ZnO product possessing the hexagonal (wurtzite-type) crystal structure. The scanning electron microscope observation confirms the formation of six faceted prismatic hexagonal ZnO microrods with the aspect ratio of 10. It also reveals that the ZnO microrods grow along the (0001) direction and finally emerge with a sharp tip because of the existence of polar faces. The UV–vis spectrum shows a sharp absorption peak centered at 370nm, which is in a good agreement with the equivalent bulk band gap value. The strong UV absorption peak implies the excellent crystal quality of the synthesized ZnO microrods. Room temperature photoluminescence spectroscopic study of the ZnO microrods with different excitation wavelengths reveals a strong band edge emission peak centered at 398nm and a defect related visible blue emission peak at 460nm. The decrease in photoluminescence intensity with negligible red shift in peak position is observed with increasing excitation wavelength.

Preparation of novel layer-stack hexagonal CdO micro-rods by a pre-oxidation and subsequent evaporation process

Novel layer-stack hexagonal cadmium oxide (CdO) micro-rods were prepared by pre-oxidizing Cd granules and subsequent thermal oxidation under normal atmospheric pressure. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed to characterize the phase structure and microstructure. The pre-oxidation process, vapor pressure and substrate nature were the key factors for the formation of CdO micro-rods. The diameter of micro-rod and surface rough increased with increasing of thermal evaporation temperature, the length of micro-rod increased with the increasing of evaporation time. The formation of hexagonal layer-stack structure was explained by a vapor–solid mechanism.

Furfural-induced hydrothermal synthesis of ZnO@C gemel hexagonal microrods with enhanced photocatalytic activity and stability.

Here we report the synthesis of ZnO@C coaxial gemel hexagonal microrods with a thin hydrothermal carbon (HTC) layer on their surface by a facile one-step hydrothermal method with furfural as the carbon precursor. The furfural has a unique dual role, which not only induces the nucleation of ZnO in the initial stage of hydrothermal process, but also forms a thin HTC layer deposited on the ZnO surface. The thickness of the surface HTC layer increases with the hydrothermal time until 16 h under the conditions adopted in the present study. It has been found that the HTC layer has resulted in a significant improvement in the photocatalytic activities and photostabilities of the ZnO@C microrods for the UV-irradiated photodegradation of methylene blue solution. The mechanism involved in the process is proposed and discussed in terms of the photodegradation scheme and the properties of the ZnO@C microrods.

Hierarchical Growth of ZnO Particles by a Hydrothermal Route

The crystallization of ZnO microrods by hydrothermal treatment of a suspension formed from reaction of zinc acetate and sodium hydroxide has been examined using scanning and transmission electron microscopy. Polycrystalline hexagonal ZnO microrods first appeared after 0.5 h reaction time at 120°C. These early stage rods were composed of stacks of hexagonal layers, each ~50 nm in thickness containing closely aligned assemblies of nanocrystallites <20 nm in size. Further growth of the microrods involved columns of nanoparticles extending from the basal layers of the preformed hexagonal stacks. Re-crystallization produced single-crystal microrods, many of which existed as twin particles.

微米级NaYF4∶Eu3+的水热合成及其荧光性能研究

分别以EDTA-2Na和柠檬酸钠为辅助剂,采用温和的水热法合成了六方相Na YF4∶Eu3+荧光材料。利用XRD、SEM、荧光分光光度计和FT-IR对样品的结构、形貌和荧光进行了表征。在反应温度为140℃、反应时间为24 h的水热条件下,得到的样品均为纯六方相的Na YF4。以EDTA-2Na为络合剂合成的Na YF4∶Eu3+为20μm的微米级球形体,而以柠檬酸三钠为辅助剂合成的Na YF4∶Eu3+为长度约为1.8μm的六棱柱微晶棒。样品的尺寸分布均匀、分散性好。辅助剂在微晶体的形貌控制过程中起了很大作用。在395 nm光的激发下,Na YF4∶Eu3+发射出蓝白色光,微米级球形体的荧光强度明显强于微晶棒。

Metal–organic frameworks MIL-88A hexagonal microrods as a new photocatalyst for efficient decolorization of methylene blue dye

Metal-organic frameworks (MOFs) MIL-88A hexagonal microrods as a new photocatalyst show an active performance for methylene blue (MB) dye decolorization using visible light. MB decolorization over the MIL-88A photocatalyst follows first-order kinetics. The addition of a H2O2 electron acceptor can markedly enhance the photocatalytic MB decoloration performance of MIL-88A. Moreover, MIL-88A showed a very stable activity for MB decoloration after four consecutive usages. Owing to the advantages of the visible light response, low cost and abundance in nature, this active MIL-88A MOF photocatalyst would have great potential for environmental purification.

Robust Superhydrophobic Zinc Oxide Film

Abstract A robust superhydrophobic surface of hexagonal ZnO microrods with a sole microscale structure was achieved via a controlled shape-preserving hydrothermal reaction. The obtained surface showed superhydrophobic properties with good mechanical properties by means of a tribological tester, which may provide an insight into designing robust water-repellent surfaces on various substrates and offer opportunities for developing various promising applications for biomimetic superhydrophobic surfaces.

Facile synthesis of β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) microcrystals with down- and up-conversion luminescence

β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) hexagonal microrods have been successfully synthesized through a facile molten salt method without any surfactant. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra were used to characterize the samples. It is found that at a preferred reaction temperature of 400 °C, the structure of β-NaYF4 can gradually transform from microtubes to microrods as reaction time extends from 0.5 to 4 h. Furthermore, as the molar ratio of NaF:RE3+ (RE represents the total amount of Y3+ and the doped rare earth elements such as Eu3+, Tb3+, Yb3+/Er3+, or Yb3+/Tm3+) increased, the phase of sample transforms from YF3 into NaYF4. Under the excitation of 395 nm ultraviolet light, β-NaYF4:5 %Eu3+ shows the emission lines of Eu3+ corresponding to 5D0-3 → 7FJ (J = 1–4) transitions from 400 to 700 nm, resulting in red down-conversion (DC) light emission. When doped with 5 % Tb3+ ions, the strong DC fluorescence corresponding to 5D4 → 7FJ (J = 6, 5, 4, 3) transitions with 5D4 → 7FJ (green emission at 544 nm) being the most prominent group that has been observed. Moreover, upon 980 nm laser diode excitation, the Yb3+/Er3+- and Yb3+,Tm3+- co-doped β-NaYF4 samples exhibit bright yellow and blue upconversion (UC) luminescence, respectively, by two- or three-photon UC process. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.

Hexagonal microrods of anatase tetragonal TiO2: self-directed growth and superior photocatalytic performance

Novel TiO2 mesocrystalline microrods with unique hexagonal structure were prepared via a facial thermal method. Their photocatalytic performance under UV irradiation was much higher than that of the Degussa P25 benchmark, one of the best commercial polycrystalline TiO2 photocatalyst. The hexagonal microrods were formed through self-directed self-assembly within intermediate scaffolds.