Hydrothermal synthesis of dumbbell-shaped ZnO microstructures

Dumbbell-shaped ZnO microstructures have been successfully synthesized by a facile hydrothermal method using only Zn(NO₃)₂·6H₂O and NH₃·H₂O as raw materials at 150 °C for 10 h. The results from X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) show that the prepared ZnO samples exhibit dumbbell-shaped morphology and hexagonal wurtzite structure. The length of ZnO dumbbells is about 5–20 μm, the diameters of the two ends and the middle part are about 1–5 μm and 0.5–3 μm, respectively. The dumbbell-shaped ZnO microstructures may be formed by self-assembly of ZnO nanorods with 1–5 μm in length and 100–200 nm in diameter. The photoluminescence (PL) spectrum of dumbbell-shaped ZnO microstructures at room temperature shows three emission peaks at about 362, 384 and 485 nm.     

Synthesis of Ag3PO4–ZnO nanorod composites with high visible-light photocatalytic activity

Ag₃PO₄ nanoparticles with 50–100 nm in size distributed on the surface of ZnO nanorods with ca. 20 nm in diameter and 1–2 μm in length have been synthesized by a facile method. The Ag₃PO₄–ZnO nanorod composites had much higher photocatalytic activity toward degradation of Rhodamine B (RhB) under visible light irradiation than pure ZnO nanorods, and had better recyclability and stability than pure Ag₃PO₄ nanoparticles. The Ag₃PO₄–ZnO nanorod composite with the molar ratio of Ag₃PO₄:ZnO = 1:40 exhibited the highest photodegradation efficiency of RhB (93%), which was 1.5 times of pure ZnO nanorods.     

Core–shell BaMoO4@SiO2 nanospheres: Preparation,characterization, and optical properties

Core–shell BaMoO₄@SiO₂ nanospheres were prepared in reverse microemulsions and exhibited enhanced photoluminescence (PL) intensity as compared to that of the uncoated BaMoO₄. Characterization was performed using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDX), and X-ray powder diffraction (XRD). It was found that the silica shell could increase the PL intensity, but the shell is not the thicker the better. The PL emission can be decomposed into three individual Gaussian components: two UV emissions at 308 nm and 369 nm and a visible emission at 448 nm. Such short emission wavelengths can be attributed to quantum size effect of the small BaMoO₄ cores (～16 nm).     

Ethanol sensing properties of tungsten oxide nanorods prepared by microwave hydrothermal method

Tungsten oxide nanorods have been prepared by a simple microwave hydrothermal (MH) method via Na₂SO₄ as structure-directing agent at 180 °C for 20 min. The structure and morphology of the products are characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The obtained nanorods are about 20–50 nm in diameter and several micrometers in length. The ethanol sensing property of as-prepared tungsten oxide nanorods is studied at ethanol concentration of 10–1000 ppm and working temperature of 370–500 °C. It was found that the sensitivity depended on the working temperatures and also ethanol concentration. The results show that the tungsten oxide nanorods can be used to fabricate high performance ethanol sensors.     

Intense photoluminescence emission at room temperature in calcium copper titanate powders

A study was undertaken about the structural and photoluminescent properties at room temperature of CaCu₃Ti₄O₁₂ (CCTO) powders synthesized by a soft chemical method and heat treated between 300 and 800 °C. The decomposition of precursor powder was followed by thermogravimetric analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and photoluminescence (PL) measurements. XRD analyses revealed that the powders annealed at 800 °C are becoming ordered and crystallize in the cubic structure. The most intense PL emission was obtained for the sample calcined at 700 °C, which is not highly disordered (300–500 °C) and neither completely ordered (800 °C). From the spectrum it is clearly visible that the lowest wavelength peak is placed around 480 nm and the highest wavelength peak at about 590 nm. The UV/vis absorption spectroscopy measurements showed the presence of intermediate energy levels in the band gap of structurally disordered powders.     

Nanoseed-assisted rapid formation of ultrathin ZnO nanorods for efficient room temperature NO2 detection

The influence of ZnO nanoseeds on the formation of ZnO nanorods from ε-Zn(OH)₂ in NaOH solution at 80 °C was investigated, using ZnO nanoparticles with a diameter of 4–10 nm as the seeds. The experimental results indicated that the presence of ZnO nanoseeds promoted the rapid heterogeneous formation of ultrathin ZnO nanorods. Compared with the ZnO submicron rods with a diameter of 0.5–1.0 µm, the ultrathin ZnO nanorods with a diameter of 10–15 nm were found to be more sensitive for detecting NO₂ at room temperature owing to their higher variation of channel conduction to the diameter.     

Luminescent and structural properties of manganese-doped zinc aluminate spinel nanocrystals

Manganese-doped zinc aluminate spinel (ZnAl₂O₄:Mn; Mn=0–6.0 mol%) phosphor nanoparticles were prepared by the sol–gel process. The effects of thermal annealing and dopant concentration on the structure, microstructure and luminescence of the powder phosphors were investigated. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) results confirmed that a single-phase spinel started to crystallize at around 600 °C for the investigated powders. On heating at 600–1200 °C, the powders had the average crystallite sizes of around 12–33 nm. The crystallite size and lattice constant increased as the doping level of Mn increased. FT-IR spectra exhibited only absorption bands of the AlO₆ octahedral groups, which suggested that the powder phosphors mainly crystallized in a normal spinel structure. Scanning electron microscopy (SEM) investigations showed the primary particle sizes were around 20–25 nm for the powders annealed at 1000 °C, and less than ca. 50 nm for those annealed at 1200 °C. Photoluminescence (PL) spectra under UV or visible light excitation exhibited a strong green emission band centered at 510 nm, corresponding to the typical ⁴T₁(⁴G)—⁶A₁(⁶S) transition of tetrahedral Mn²⁺ ions. The most intense PL emission was obtained by exciting at 458 nm. The PL intensity was significantly enhanced by the improved crystallinity and diminished OH^? groups. Optimum brightness occurred at a doping of 3.0 mol% Mn.     

Enhanced NO2 sensing properties of Zn2SnO4-core/ZnO-shell nanorod sensors

Zn₂SnO₄-core/ZnO-shell nanorods were synthesized using a two-step process: synthesis of Zn₂SnO₄ nanorods the thermal evaporation of a mixture of ZnO, SnO₂, and graphite powders, followed by atomic layer deposition (ALD) of ZnO. The nanorods were 50–250 nm in diameter and a few to a few tens of micrometers in length. The cores and shells of the nanorods were face-centered cubic-structured single crystal Zn₂SnO₄ and wurtzite-structured single crystal ZnO, respectively. The multiple networked Zn₂SnO₄-core/ZnO-shell nanorod sensors showed a response of 173–498% to NO₂ concentrations of 1–5 ppm at 300 °C. These response values are 2–5 times higher than those of the Zn₂SnO₄ nanorod sensor over the same NO₂ concentration range. The NO₂ sensing mechanism of the Zn₂SnO₄core/ZnO-shell nanorods is discussed.     

Direct microwave synthesis of graphitic C3N4 with improved visible-light photocatalytic activity

The graphitic carbon nitride (g-C₃N₄) was rapidly synthesized via direct high-energy microwave heating approach. During the preparation process, only low-cost melamine and artificial graphite powders were used, without any metal catalysts or inert protective gas. The microstructure was investigated by using X-ray diffraction (XRD), Flourier transformed infrared (FT-IR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM). The spectra of XRD and HRTEM indicated that the obtained g-C₃N₄ had a high crystallinity. The optical spectra covering Photoluminescence (PL) and Ultraviolet-visible (UV–vis) were also measured at room temperature. PL peak and UV–vis absorption edge of the g-C₃N₄ were shown at 455 nm and 469 nm, respectively, indicating visible-light photocatalytic property. Finally, the photocatalytic activity of g-C₃N₄ was investigated and evaluated as photocatalyst for the photo-degradation of Rhodamine B (RhB) and Methyl Orange (MO) in aqueous solution under visible-light (λ>420 nm) irradiation, respectively. Results indicated that the g-C₃N₄ sample displayed an excellent performance of removing of RhB and MO due to the improved crystallinity and large surface area of 126 m²/g. After the visible-light photocatalytic reaction for 40 min, the decolorization ratios of RhB and MO reached up to 100% and 94.2%, respectively.     

Synthesis of fluorene- and anthracene-based π-conjugated polymers and dependence of emission range and luminous efficiency on molecular weight

In the present work poly[9,9-dioctylfluorene-co-2-pentyl-9,10-bis(4-vinylphenyl)anthracene], a fluorene- and anthracene-based copolymer, is synthesized through a Heck coupling reaction. In order to synthesize polymers with high-molecular weight, DMF (P1), DMF/p-Xylene = 1/1 (P2), p-Xylene (P3), and 1,4-Dioxane (P4) are used as solvents, which are an important factor in the synthesis process. The number of average molecular weights (M_n) of the synthesized polymers P1–P4 do not differ significantly, standing at 22,309, 12,369, 29,192, and 39,464, respectively, while their weight average molecular weights (M_w) show considerable differences (i.e. 50,055; 24,042; 125,406; and 231,053). Polymers P1–P4 demonstrate little difference in the results of a thermal analysis, electrochemical analysis, UV–vis analysis, and photoluminescence (PL) spectrum measurement. With regard to electroluminescence (EL) spectrum measurement, however, P1 and P2 show main luminous peaks at 508 nm, while P3 and P4's luminous peaks are seen at 516 nm. Moreover, luminous shoulder peaks were red-shifted with increase of molecular weight of polymers from 460 to 544 nm. In this process, the luminous area is red-shifted from greenish-blue to yellowish-green. The I–V–L measurement results show that the maximum brightness of P1, P2, and P3 ranges from 164 to 303 cd/m² and their luminous efficiency is low at 0.031–0.054 cd/A. Meanwhile, the turn-on voltage of P4, having greater molecular weight, is 9.5 V, and its maximum brightness and corresponding luminous efficiency are 736 cd/m² and 0.08 cd/A, respectively, implying that the luminous efficiency of devices improves as the molecular weight becomes greater.     

An electron spin resonance and photoluminescence investigation of the effect of annealing temperature on Gd-doped La2Zr2O7 nano-ceramics

Lanthanum zirconate pyrochlore (La₂Zr₂O₇=LZO) is considered as a potential ceramic host matrix for nuclear waste immobilization. In this context, incorporation of Gd ions, a surrogate for trivalent minor actinides, in the LZO host was investigated. Undoped and Gd doped LZO samples were synthesized using a low temperature gel-combustion route and characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS), electron spin resonance (ESR) and photoluminescence (PL) spectroscopic techniques. It was observed that even at 500 °C, a single phase pyrochlore could be stabilized without any impurity phase. The average particle size of the synthesized product was estimated to be 85–90 nm. At lower temperatures, it was observed that, the Gd ions were present mostly on the surface of the LZO host which diffused into the lattice and replaced the La³⁺ ions after annealing at 700 °C for 5 h. Due to difference in the ionic radii, the environment around the Gd ion was highly asymmetric. This was further confirmed by PL investigations wherein upon excitation with UV radiation, the nano ceramics exhibited strong and sharp transition at 310 nm suggesting the existence of Gd³⁺ions in a D_2d geometry. Upon annealing, agglomeration of the particles was taking place reducing the surface defects and increasing the PL intensity as well as life time values.     

Template free synthesis of highly ordered mullite nanowhiskers with exceptional photoluminescence

An efficient method to produce highly ordered mullite nanowhiskers using B₂O₃-doped molten salt synthesis was reported in this work. The morphology and optical properties of the obtained nanowhiskers were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) spectroscopy. The results show that highly ordered mullite nanowhiskers with uniform morphology in large scale are obtained at 1000 °C for 3 h in air. The structure of mullite is Al-rich and single crystalline. The diameter is in the range of 90–110 nm and length up to 20–30 μm. The reaction mechanism of the highly ordered mullite nanowhiskers is attributed to local concentration gradient. Due to the Al-rich structure, mullite nanowhiskers demonstrate strong photoluminescence (PL) emission at 399 nm, 452 nm and 468 nm, implying mullite nanowhiskers can be applied as a promising candidate in optical and electronic fields.     

Scintillation,TSL and RPL properties of MgF2 transparent ceramic and single crystal

We have prepared MgF₂ transparent ceramics by spark plasma sintering (SPS) and investigated the scintillation and dosimeter properties compared with those of a single crystal. Under X-ray irradiation, a transparent ceramic showed scintillation with a broad feature over 300–600 nm with decay times of 310 and 1940 μs while the single crystal did not show such emission. It was found that MgF₂ show radio-photoluminescence (RPL), which is a generation of new luminescence center by ionizing radiation. The RPL was only observed in the transparent ceramic samples, which appeared as two emission bands peaking around 415 and 700 nm under 340±40 nm excitation. During the PL reading, the emission band at 415 nm effectively increased over 50 s. This result suggested that the RPL in MgF₂ shows a build-up phenomenon. From PL emission/excitation characteristics and decay time, it was suggested that the RPL peak around 415 nm was due to M(C_2h) center.     

Nucleation and growth of ZnO nanorods on the ZnO-coated seed surface by solution chemical method

ZnO nanorods on ZnO-coated seed surfaces were fabricated by solution chemical method using supersaturated (ZnNO₃)₂/NaOH at 70 °C. The seed surfaces were coated on glass substrates by sol–gel processing, and their texture was dominated by heating temperatures, cooling styles and layer thickness per dipping. The effects of the seed surface on the morphology of the resultant nanorods were primarily discussed. The orientation and morphology of both the seed surface and successive nanorods were analyzed by using XRD and SEM. It is proved that when the seed size increases from 15 to 50 nm with temperature increasing, the average diameter of the resultant nanorods increase from 25 to 50 nm, with a length of 800 nm after growing for 1.5 h. The seed surface prepared by heating at 300–400 °C, fast cooling or drawing at lower speed has better orientation and few surface defects, which leads to higher density of nuclei on the seed surface and thus to the optimal preferred crystal growth of ZnO rods standing perpendicular onto substrates.     

Growth mechanism and photocatalytic activity of chrysanthemum-like anatase TiO2 nanostructures

Chrysanthemum-like hierarchical anatase TiO₂ nanostructures self-assembled by nanorods have been successfully fabricated by a simple solvothermal route without using template materials or structure-directing additives. The products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectrometer system (Raman), UV–vis absorption spectroscopy (UV–vis) and N₂ adsorption–desorption measurement. The results indicate that synthesized chrysanthemum-like hierarchical anatase TiO₂ nanostructures have a spherical shape with an average diameter of 1.5 μm and they are composed of nanorods with a width of about 30 nm and a length of about 300 nm. The pore distribution of the sample exhibits two kinds of pores. Such mesoporous structure of the sample might be extremely useful in photocatalysis because they possess efficient transport pathways to the interior and supplies higher specific area for more pollutant molecules to be absorbed. In addition, the synthesized TiO₂ nanostructures show enhanced photocatalytic activity compared with commercial P25 for the degradation of RhB under UV light irradiation, which can be attributed to their special hierarchical structure and high light-harvesting capacity.     

Template-free synthesis of neodymium hydroxide nanorods by microwave-assisted hydrothermal process,and of neodymium oxide nanorods by thermal decomposition

Based on the 150 °C and 1 h microwave-assisted hydrothermal reaction of Nd(NO₃)₃ dissolved in deionized water with pH 10 adjusted by concentrated NH₄OH solution, the phase and morphology of the product, characterized by XRD, SEM and TEM analyses, were specified as hexagonal Nd(OH)₃ nanorods 50 nm in diameter and 700 nm long, growing along the [0 0 1] direction. TGA analysis showed the evaporation of adsorbed water and dehydration of Nd(OH)₃ to synthesize the final pure Nd₂O₃ product. With 500 °C and 2 h calcination of the Nd(OH)₃ self-template precursor, Nd₂O₃ nanorods were synthesized, retaining both the morphology and growth direction of the precursor.     

Photoluminescence properties of Tb3+ doped Al2O3 microfibers via a hydrothermal route followed by heat treatment

Uniform Al₂O₃:Tb³⁺ microfibers were synthesized via a hydrothermal route and thermal decomposition of a precursor of Tb³⁺ doped ammonium aluminum hydroxide carbonate, and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD results indicate that various crystallographic phase Al₂O₃:Tb³⁺ microfibers are obtained by postannealing at different temperatures. SEM results show that the length and diameter of these Tb³⁺ doped α-Al₂O₃ microfibers are about 6–8 μm and 300 nm, respectively. The PL spectra indicate that the ⁵D₄ → ⁷F₅ (545 nm) electric dipole transition is the most intensive when excited at 240 nm. It is shown that the 2.0 mol% of doping concentration of Tb³⁺ ions in α-Al₂O₃:Tb³⁺ is optimum. According to Dexter's theory, the critical distance between Tb³⁺ ions for energy transfer was determined to be 12.7 Å. It is found that the decay curves follow the single-exponential decay.     

The O2-dependent growth of ZnO nanowires and their photoluminescence properties

ZnO nanowires were massively synthesized on a Ni(NO₃)₂-coated silicon substrate under oxygen-containing argon atmosphere by a simple chemical vapor deposition method. The average diameter of the ZnO nanowires was about 50 nm and the average length was about 20 μm. The morphologies of the ZnO nanowires strongly depended on oxygen content in the growth atmosphere. At low oxygen concentration (about 5–10 ppm), ZnO nanocones and nanoneedles were obtained, while at high oxygen concentration (about ∼250 ppm), ZnO nanoparticles deposited on the substrate. The room temperature photoluminescence (PL) spectrum of the ZnO nanowires revealed that a strong UV band at 384 nm dominated the whole spectrum. These results indicate that the ZnO nanowires grown under oxygen-containing atmosphere possess better crystalline quality and UV luminescence properties than those grown in reducing hydrogen atmosphere. Based on the analysis of oxygen effect on the ZnO nanostructures, a vapor–liquid–solid mechanism assisted by the redox growth mode was proposed to understand the growth of the ZnO nanowires.     

Synthesis and photoluminescence of F and N co-doped reduced graphene oxide

F and N co-doped reduced graphene oxide (F,N-RGO) was synthesized by annealing the mixture of N-doped reduced graphene oxide (N-RGO) and XeF₂ at 180 °C. The photoluminescence (PL) properties of F,N-RGO were studied. The results show that F,N-RGO exhibits two strong ultraviolet (UV) PL centered at 365 and 310 nm, and the PL at 310 nm can be appropriately tuned by tailoring the N content further.     

Effects of Cr content and morphology on the luminescence properties of the Cr-doped alpha-Al2O3 powders

Al₂O₃:Cr³⁺ samples were synthesized via hydrothermal and microwave solvothermal methods and thermal decomposition of Cr³⁺ doped precursors. The sample characterizations were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD results indicated that Cr³⁺ doped samples were pure α-Al₂O₃ phase after being calcined at 1573 K. SEM results showed that the length and diameter of these Cr³⁺ doped alumina microfibers by hydrothermal route were about 2–5 μm and 100–300 nm, respectively; the obtained α-Al₂O₃ based powders via the microwave solvothermal method were microspheres with an average diameter about 1–2 μm. PL spectra showed that the Al₂O₃:Cr³⁺ samples presented a broad R band at 696 nm. It is shown that the 0.3 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ is optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 24 Å. It is found that the curve followed the single-exponential decay. Furthermore, the luminescence properties of the samples are also dependent on the morphology.