Catalyst-free synthesis and luminescence of aligned ZnO nanorods

Quasi-aligned undoped ZnO nanorods with diameter in the range 100–300 nm and length of several micrometers have been grown catalyst-free on Si(1 0 0) wafer in a one-step process by direct heating of Zn powders. All nanowires are single crystals and are aligned vertically to the substrate surface with c-axis preferred orientation. XRD, HRTEM and Raman studies revealed that the ZnO nanorods have wurtzite phase, are highly crystalline and well aligned with the lattice parameters a=0.32 nm and c=0.52 nm. The PL spectra measured at different temperatures are dominated by excitonic emission at 380 nm and less intense below band gap emission band centered at 520 nm.     

Optical investigation of the red band emission of CdS nanoparticles

Cadmium sulfide (CdS) nanoparticles, with different Cd/S molar ratios, were chemically produced and characterized in this study. High quality CdS nanoparticles were grown in the hexagonal phase, as indicated by X-ray diffraction data and Raman spectroscopy. Photoluminescence was used to investigate the surface effects caused by submitting the nanoparticle-based samples at different laser treatments and vacuum pressure.     

40% Efficiency enhancement in solar cells using ZnO nanorods as shell prepared via novel hydrothermal synthesis

Herein, rod-like ZnO nanostructures were synthesized via a novel hydrothermal route using Zn(OAc)₂, ethylenediamine and hydrazine as a new set of starting reagents. The as-synthesized products were characterized by techniques including XRD, EDS, SEM, XPS, Pl and FTIR. The prepared ZnO nanostructures were utilized as shell on TiO₂ film in DSSCs. Effect of precursor type, morphology and thickness of ZnO shell (number of electrophoresis cycle) on solar cells efficiency were well studied. Our results showed that ethylenediamine has crucial effect on morphology of synthesized ZnO nanostructures and using ZnO nanostructures leads to an increase in DSSCs efficiency compared to bare TiO₂ from 4.66 to 7.13% (~40% improvement). Moreover, highest amount of solar cell efficiency (7.13%) was obtained by using ZnO nanorods with two cycle of electrophoresis for deposition.     

Sodium dodecyl sulfate-assisted synthesis of CoWO4 nanorods

CoWO₄ nanorods were synthesized at 453 K for 12 h by a hydrothermal technology from Na₂WO₄ · 2H₂O and CoCl₂ · 6H₂O in the presence of sodium dodecyl sulfate (SDS). The as-synthesized CoWO₄ nanorods were characterized by various techniques of X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray detector. Luminescent properties of the samples were measured at room temperature. The results showed that CoWO₄ products are nanorods with diameters of about 20 nm, and lengths ranging between 100 and 200 nm. CoWO₄ nanorods display a very strong PL peak at 453 nm with the excitation wavelength 300 nm. The possible formation mechanism of CoWO₄ nanorods was suggested.     

Formation of aligned ZnO nanorods on self-grown ZnO template and its enhanced field emission characteristics

We report a novel method for producing aligned ZnO nanorods (ANR) on self-grown ZnO template in a single step process involving growth of ZnO by vapor transport, followed by quenching of growing ZnO flux in liquid nitrogen. In the present study Zn powder turns into ZnO sheet under oxygen flow at ∼900 °C and bottom surface of the sheet acts as template for the growth of ANR. It is revealed from XRD and EDAX analysis that the bottom of the sheet is Zn rich region and acts as self catalyst for the growth of ANR. The grown nanorods have length up to several tens of micrometers with diameters ranging from ∼100 to 150 nm. Microstructural analysis of ANR indicates the fractal like configuration. The field emission properties have been investigated for ANR with fractal geometry using the ANR on self-grown ZnO template as a cathode directly. The turn-on electric field required to draw current density of ∼1.0 μA/cm² has been found to be ∼0.98 V/μm. The field enhancement factor based on Fowler-Nordheim (F-N) plot was found to be ∼7815 for ANR. The fractal geometry of ANR has been shown to be advantageous for achieving improved field emission features. The present investigations of synthesis involving formation of ANR over self-grown ZnO template, together with fractal configuration of the as-synthesized ANR, are first of their type.     

Detection of hydrogen with SnO2-coated ZnO nanorods

SnO₂-coated ZnO nanorods on c-plane sapphire substrates were synthesized by pulsed laser deposition. The thickness of the polycrystalline SnO₂ was ∼10 nm, as determined by high-resolution transmission electron microscopy, while the diameter of the ZnO nanorods was ∼30 nm. The sensitivity of the SnO₂/ZnO structures to hydrogen was tested by depositing Ti/Au Ohmic contacts on a random array of the nanorods and measuring the current at fixed voltage. There was no response to 500 ppm H₂ in N₂ at room temperature, but we obtained a sensitivity of ∼70% at 400 °C. The SnO₂/ZnO structures exhibit drift in their recovery characteristics and for sequential detection of hydrogen, as generally reported for SnO₂ thin film sensors.     

Spontaneous organisation of ZnS nanoparticles into monocrystalline nanorods with highly enhanced dopant-related emission

A natural self-assembly process of semiconductor nanoparticles leading to the formation of doped, monocrystalline nanorods with highly enhanced dopant-related luminescence properties is reported. ∼4 nm sized, polycrystalline ZnS nanoparticles of zinc-blende (cubic) structure, doped with Cu⁺-Al³⁺ or Mn²⁺ have been aggregated in the aqueous solution and grown into nanorods of length ∼400 nm and aspect ratio ∼12. Transmission electron microscopic (TEM) images indicate crystal growth mechanisms involving both Ostwald-ripening and particle-to-particle oriented-attachment. Sulphur-sulphur catenation is proposed for the covalent-linkage between the attached particles. The nanorods exhibit self-assembly mediated quenching of the lattice defect-related emission accompanied by multifold enhancement in the dopant-related emission. This study demonstrates that the collective behavior of an ensemble of bare nanoparticles, under natural conditions, can lead to the formation of functionalized (doped) nanorods with enhanced luminescence properties.     

Hydrothermal synthesis, characterization and Raman studies of Eu activated Gd2O3 nanorods

Eu³⁺ (8 mol%) activated gadolinium oxide nanorods have been prepared by hydrothermal method without and with surfactant, cityl trimethyl ammonium bromide (CTAB). Powder X-ray diffraction (PXRD) studies reveal that the as-formed product is in hexagonal Gd(OH)₃:Eu phase and subsequent heat treatment at 350 and 600 °C transforms the sample to monoclinic GdOOH:Eu and cubic Gd₂O₃:Eu phases, respectively. The structural data and refinement parameters for cubic Gd₂O₃:Eu nanorods were calculated by the Rietveld refinement. SEM and TEM micrographs show that as-obtained Gd(OH)₃:Eu consists of uniform nanorods in high yield with uniform diameters of about 15 nm and lengths of about 50-150 nm. The temperature dependent morphological evolution of Gd₂O₃:Eu without and with CTAB surfactant was studied. FTIR studies reveal that CTAB surfactant plays an important role in converting cubic Gd₂O₃:Eu to hexagonal Gd(OH)₃:Eu. The strong and intense Raman peak at 489 cm⁻¹ has been assigned to A_g mode, which is attributed to the hexagonal phase of Gd₂O₃. The peak at ∼360 cm⁻¹ has been assigned to the combination of F_g and E_g modes, which is mainly attributed to the cubic Gd₂O₃ phase. The shift in frequency and broadening of the Raman modes have been attributed to the decrease in crystallite dimension to the nanometer scale as a result of phonon confinement.     

Thioglycolic acid (TGA) assisted hydrothermal synthesis of SnS nanorods and nanosheets

Nanorods and nanosheets of tin sulfide (SnS) were synthesized by a novel thioglycolic acid (TGA) assisted hydrothermal process. The as prepared nanostructures were characterized by X-ray diffraction (XRD) study, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD study reveals the formation of well-crystallized orthorhombic structure of SnS. Diameter of the SnS nanorods varied within 30-100 nm. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns identify the single crystalline nature for the SnS nanocrystals. The mechanism for the TGA assisted growth for the nanosheets and nanorods have been discussed.     

Hydrothermal synthesis and the enhanced blue upconversion luminescence of NaYF4:Nd3+,Tm3+,Yb3+

Nd³⁺, Tm³⁺ and Yb³⁺ co-doped NaYF₄ upconversion (UC) material was synthesized by the hydrothermal method. The structure of the sample was characterized by the X-ray diffraction, and its UC luminescence properties were investigated in detail. Under the 980 nm semiconductor laser excitation, its UC spectra exhibited distinct emission peaks at 451 nm, 475 nm and 646 nm respectively. On the basis of the comparison of UC spectra between NaYF₄:Nd³⁺,Tm³⁺,Yb³⁺ and NaYF₄:Tm³⁺,Yb³⁺, it was indicated that the existence of Nd³⁺ ion enhanced the blue emission intensity. The law of luminescence intensity versus pump power proved that the blue emission at 475 nm, and the red emission at 646 nm were the two-photon processes, while the blue emission at 451 nm was a three-photon process.     

Synthesis, characterization and upconversion emission properties of the nanocrystals of Yb/Er-codoped YF3-YOF-Y2O3 system

Nanocrystalline Yb³⁺, Er³⁺-codoped fluoride (YF₃), oxyfluoride (YOF), and oxide (Y₂O₃) phosphors have been synthesized by a facile pyrolysis of a yttrium trifluoroacetate precursor. YF₃, YOF and Y₂O₃ nanoparticles were demonstrated to be good host materials for lanthanides. Varied hosts led to different optical properties. Red, green, and blue up-conversion (UC) was observed upon excitation in the NIR spectral range in all synthesized compounds. The UC mechanisms were also analyzed.     

Growth and electron field emission of ZnO nanorods on diamond films

Zinc oxide (ZnO) nanorods grown on chemical vapor deposited diamond films by thermal vapor transport method have been investigated. In the initial growth status, the semi-spherical ZnO nuclei were preferably deposited near the growth steps on the terraces and the boundaries of diamond grains. With increasing the growth time, the [0 0 0 1] orientated ZnO nanorods appeared and further covered the whole diamond film. It is found that the size of diamond grains would determine the diameter of ZnO nanorods. The electron field emission properties of the ZnO nanorods/diamond system have been significantly improved with respect to pure diamond film. The feature of the ZnO nanorods grown on diamond films played an important role in further enhancing the electron field emission performances.     

Exciplex emission in the blend of two blue luminescent materials

Organic light-emitting diodes (OLEDs) based on the blend of two blue luminescent materials N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-diphenyl-4,4′-diamine (NPB) and 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) were fabricated. The electroluminescence (EL) spectra of this device showed a new emission that is different from their intrinsic exciton emission. Compared with the photoluminescence (PL) spectra of single layer NPB and PBD, respectively, there was an apparent red shift in that of their blend. Thus the exciplex formation in the blend can be concluded due to the similar emission in both PL and EL spectra. The exciplex formation process and the effect of applied voltage were analyzed by Gaussian fitting.     

Survey and research on up-conversion emission character and energy transition of Yb/Er/Tm co-doped phosphate glass and glass ceramic

By conventional high-temperature melting method, Yb³⁺/Er³⁺/Tm³⁺ co-doped phosphate glass was synthesized. After annealing the precursor glass, the phosphate glass ceramic (GC) was obtained. By measuring the X-ray diffraction (XRD) spectrum, it is proved that the LiYbP₄O₁₂ and Li₆P₆O₁₈ nano-crystals have existed in the phosphate GC. The up-conversion (UC) emission intensity of the GC is obvious stronger compared to that of the glass. The reason is that the shorter distance between rare earth ions in the glass ceramic increases the energy transitions from the sensitized ions (Yb³⁺) to the luminous ions (Er³⁺ and Tm³⁺). By studying the dependence of UC emissions on the pump power, the 523 and 546 nm green emissions of Er³⁺ ions in the glass are two-photon processes. But in the glass ceramic, they are two/three-photon processes. The phenomenon implies that a three-photon process has participated in the population of the two green emissions. Using Dexter theory, we discuss the energy transitions of Er³⁺ and Tm³⁺. The results indicate the energy transition of Tm³⁺ to Er³⁺ is very strong in the GC, which changes the population mechanism of UC emissions of Er³⁺.     

Chemical synthesis of zinc oxide nanorods for enhanced hydrogen gas sensing

Zinc oxide(ZnO) nanorods are prepared using equimolar solution of zinc nitrate((Zn(NO3)2) and hexamethylenetetramine(C6H12N4) by the hydrothermal technique at 80 C for 12 h. Epitaxial growth is explored by X-ray diffraction(XRD) patterns, revealing that the ZnO nanorods have a hexagonal(wurtzite) structure. Absorption spectra of ZnO are measured by UV–visible spectrometer. The surface morphology is investigated by field emission scanning electron microscopy(FESEM). The synthesized ZnO nanorods are used for detecting the 150 C hydrogen gas with a concentration over 1000 ppm. The obtained results show a reversible response. The influence of operating temperature on hydrogen gas detecting characteristic of ZnO nanorods is also investigated.     

Low temperature synthesis of hexagonal ZnO nanorods and their hydrogen sensing properties

The growth of hexagonal ZnO nanorods was demonstrated by low temperature chemical synthesis approach. X-ray diffraction (XRD) analysis revealed a wurtzite hexagonal structure of the ZnO nanorods. The optical properties were measured by UV-vis spectrophotometer at room temperature. X-ray photoelectron spectroscopy (XPS) confirmed high purity of the ZnO nanorods. The hydrogen sensor made of the ZnO nanorods showed reversible response. The hydrogen gas tests were carried out in presence of ambient air and the influence of operation temperature on the hydrogen gas sensing property of ZnO nanorods was also investigated.     

Room temperature synthesis of needle-shaped ZnO nanorods via sonochemical method

Single crystalline needle-shaped zinc oxide nanorods were synthesized via sonochemical methods using zinc acetate dihydrate and sodium hydroxide at room temperature. Morphological investigation revealed that the nanoneedles are of hexagonal surfaces along the length. The typical diameter and length vary from 120 to 160 nm and 3 to 5 μm, respectively. Sonication time appears to be a critical parameter for the shape determination. Detailed structural characterization confirmed that the nanorods are single crystalline with wurtzite hexagonal phase. A standard peak of zinc oxide was observed at 520 cm⁻¹ from the Fourier transform infrared spectroscopy. The ultra-violet visible and room temperature photoluminescence (PL) spectroscopic results demonstrate that the synthesized material has good optical properties.     

Floating synthesis with enhanced catalytic performance via acoustic levitation processing

Acoustic levitation supplies a containerless state to eliminate natural convection and heterogeneous crystal nucleation and thus provides a highly uniform and ultra clean condition in the confined levitating area. Herein, we attempt to make full use of these advantages to fabricate well dispersed metal nanoparticles. The gold nanoparticles, synthesized in an acoustically levitated droplet, exhibited a smaller size and improved catalytic performance in 4-nitrophenol reduction were synthesized in an acoustically levitated droplet. The sound field was simulated to understand the impact of acoustic levitation on gold nanoparticle growth with the aid of crystal growth theory. Chemical reducing reactions in the acoustic levitated space trend to occur in a better dispersed state because the sound field supplies continuous vibration energy. The bubble movement and the cavitation effect accelerate the nucleation, decrease the size, and the internal flow inside levitated droplet probably inhibit the particle fusion in the growth stage. These factors lead to a reduction in particle size compared with the normal wet chemical synthetic condition. The resultant higher surface area and more numerous active catalytic sites contribute to the improvement of the catalytic performance.     

Optimization red emission of SrMoO4: Eu3+ via hydro-thermal co-precipitation synthesis using orthogonal experiment

In the present study, the SrMoO₄:Eu³⁺ phosphors has been synthesized through hydro-thermal co-precipitation method, and single factor and orthogonal experiment method was adopted to find optimal synthesis condition. It is interesting to note that hydro-thermal temperature is a prominent effect on the luminescent intensity of SrMoO₄:Eu³⁺ red phosphor, followed by co-precipitation temperature, calcining time, and the doping amount of Eu³⁺. The optimal synthesis conditions were obtained: hydro-thermal temperature is 145 °C, co-precipitation temperature is 35 °C, the calcining time is 2.5 h, and the doping amount of activator Eu³⁺ is 25%. Subsequently, the crystalline particle size, phase composition and morphology of the synthesized phosphors were evaluated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results show that these phosphors possess a scheelite-type tetragonal structure, and the particle size is about 0.2 μm. Spectroscopic investigations of the synthesized phosphors are carried out with the help of photo-luminescence excitation and emission analysis. The studies reveal that SrMoO₄: Eu³⁺ phosphor efficiently convert radiation of 394 nm–592 and 616 nm for red light, and the luminescence intensity of SrMoO₄:Eu³⁺ phosphors is improved. SrMoO₄:Eu³⁺ phosphors may be a potential application for enhancing the efficiency of white LEDs.     

Infrared-to-visible upconversion emission of Er3+/Yb3+-codoped SrMoO4 phosphors as wide-range temperature sensor

Er³⁺/Yb³⁺-codoped SrMoO₄ phosphors were prepared by a high-temperature solid-state reaction method. At room temperature, all the as-prepared samples exhibited strong upconversion properties and the emission intensity increased dramatically with the increase of Yb³⁺ ion concentration, reaching its maximum value when the concentration was 5 mol%. The dependence of emission intensity on the pump power suggested that the upconversion emission was a two-photon process. Furthermore, the optical temperature sensing properties based on green upconversion emissions of the SrMoO₄:0.01Er³⁺/0.05Yb³⁺ phosphor were studied. It is found that the SrMoO₄:0.01Er³⁺/0.05Yb³⁺ phosphor can be operated over a very wide temperature range of 93–773 K with a maximum sensitivity of ∼0.0128 K⁻¹, indicating that low- and high-temperature thermometry can be simultaneously realized in this phosphor.