Pt/C催化剂中纳米Pt颗粒团聚效应的小角X射线散射

 应用小角X射线散射(SAXS)技术,对乙二醇合成法、浸渍还原法和微波加热法制备的Pt/C催化剂粉体内纳米Pt颗粒的团聚效应进行了研究,得到了不同方法制备的Pt颗粒及其团聚体的特征尺寸、体积分布、表面积变化、团聚程度等信息,并利用透射电镜（TEM）对3种样品进行了测试。实验结果表明:微波加热法制备的催化剂中,Pt颗粒较好地分散于C载体上,且Pt颗粒具有尺度小、分布范围窄、总表面积大和团聚体较少等特征;常规浸渍和乙二醇还原两种方法制备的催化剂中Pt颗粒大小分布相似,但乙二醇还原法制备的催化剂总表面积和团聚体尺度更大,数量也更多。     

Structural transformation study of TiO2 nanoparticles annealing at different temperatures and the photodegradation process of eosin-Y

Hydrothermal method was used to prepare TiO₂ nanoparticles with annealing temperature at 500 °C–700 °C. The mixture of anatase-rutile phase was investigated by powerful tool of X-ray diffraction (XRD). The structural parameters of anatase and rutile mixture phaseTiO₂ nanoparticles were calculated from the Rietveld refinement. The transformation rate of rutile was increased linearly with an annealing temperature of 500 °C–700 °C. The spherical morphology of the anatase and rutile mixed phase were obtained by scanning electron microscope and transmission electron microscope. The spherical particle of the anatase and rutile TiO₂ shows with great aggregation with different size and within the range of few tens nm. The EDAX study revealed the presence of titanium and oxygen. The best photocatalytic activity was identified as the 87.04% of anatase and 12.96% of rutile mixer phase of TiO₂. Various factors could be involved for a better photocatalytic activity.     

Effects of temperature gradient induced nanoparticle motion on conduction and convection of fluid

The continuous synthesis of nickel nanoparticles (NiNPs) in a static microchannel T-mixer by the reduction of NiCl₂·6H₂O in the presence of ethylene glycol without a stabilizing/capping agent was investigated. The nanoparticles were formed in accordance with the modified polyol process with hydrazine used as a reducing agent and NaOH as a catalyst for nanoparticle formation. The reaction mechanism for NiNP formation was investigated in batch with the help of Fourier transform infrared spectroscopy and X-ray diffraction (XRD) techniques. Parameters were found for reducing reaction times from 60 to 1?min. The effects of temperature (60?C120?°C) and NaOH concentration (0.1 and 0.5?M) on batch-processed particle characteristics were also studied using XRD, transmission electron microscope and electron microprobe analysis. Average particle size was reduced from 9.2?±?2.9 to 5.4?±?0.9?nm at higher temperature and NaOH concentration. Adaptation of this chemistry to a static microchannel T-mixer for continuous synthesis resulted in smooth, spherical particles. Increases in the reaction temperature from 120 to 130?°C resulted in a narrow size distribution of 5.3?±?1?nm and also resulted in magnetic properties of 5.1?emu/g (saturation magnetization), 1.1?emu/g (remanent magnetization), and 62?Oe (coercivity).     

Synthesis and Optical Properties of Eu3+ Ion Doped Nanocrystalline Hydroxyapatites

The Ca₁₀(PO₄)₆(OH)₂ hydroxyapatite (HA) nanopowders doped with Eu³⁺ ions were prepared using a wet synthesis method. Their structure and morphology were investigated. The XRD analysis has proven a single-phase of HA nanocrystallites. The average sizes of HA nanocrystallites calcinated at 400°C and 700°C were determined to be about 20 nm and 30 nm, respectively. The emission and excitation spectra as well as the fluorescence decay rates of Eu³⁺ ion doped HA nanocrystallites were measured. Particular attention was given to the spectroscopic properties of Eu³⁺ ions as a luminescent probe of nanocrystalline HA structure as a result of varying annealing temperature and dopant concentration. The Judd-Ofelt analysis of f-f transitions of Eu³⁺:HA nanocrystallites was performed. The effect of calcination temperatures on grain sizes and luminescence properties is noted and discussed.     

A correlation between structural and optical properties of cobalt oxide nanoparticles for various annealing conditions

Two stable phases of cobalt oxide nanoparticles of controlled sizes have been synthesized using environmentally friendly inorganic precursor. Structural characterization using X-ray diffraction (XRD) shows a single-phase spinal Co₃O₄ structure up to annealing temperature of 800 °C and a mixed phase of Co₃O₄ and CoO particles for T>900 °C. Single-phase CoO nanoparticles are also obtained by annealing the particles at a temperature >900 °C and cooling in inert atmosphere. Average macro- and micro-strain were estimated using XRD data. Macrostrain was found to be the minimum for particles annealed at 600 °C, whereas microstrain was found to decrease with increasing annealing temperature up to 900 °C. A correlation between the density of localized states (DOS) in the band gap and strain is expected because the origin of both strain and DOS are defects and bond length distortions. Sub-gap absorption measurement and model calculations have been used for the determination of DOS. For cobalt oxide nanoparticle samples we find a correlation between estimated strain and density of states in the band gap.     

Effect of thermal annealing on r.f. sputtering-deposited nanocrystalline GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

GaNAs thin films were deposited on Corning glass substrates by radio frequency (r.f.) sputtering in molecular nitrogen ambient. The stoichiometry in the GaNAs alloy was controlled by changing the nitrogen incorporation in the film during the growth process, through the variation of the r.f. power in the range 30–80 watts which produced films with N concentrations in the range: x = 0.85–0.90. The structural and optical properties of the GaNAs thin films were studied by X-ray diffraction (XRD), photoacoustic (PA) and photoluminescence (PL) spectroscopies. XRD measurements show a broad diffraction band with a peak close to the (002) diffraction line of the GaN hexagonal phase, and a slight shoulder at the position corresponding to the (111) GaAs cubic phase. The PA absorption spectra showed a remarkable shift to higher energies of the absorption edge as the r.f. power decreases corresponding to the films with higher N concentrations. Thermal annealing of the GaNAs films at temperatures of 450 °C produced a GaAs nanocrystalline phase with grain sizes in the range 10–13 nm, as confirmed by the XRD measurements that showed a well-defined peak in the (111) GaAs direction, and also by the PA spectra which showed an absorption band at energies around 1.45 eV due to the quantum confinement effects. PL spectra of thermal-annealed GaNAs films showed a very intense emission at 1.5 eV which we have associated to transitions between the first electron excited level and acceptor states in the GaAs nanocrystallites.     

A structural study of biocompatible magnetic nanofluid with synchrotron radiation-based X-ray scattering techniques

In this paper, water colloidal solutions of nanoparticles of magnetite (magnetic nanofluids, (MNFs)) are investigated by synchrotron X-ray diffraction (XRD) and small-angle scattering (SAXS). To prevent aggregation, nanoparticles are coated with polyacrylic acid (PAA) in a single solution and citric (CA) in the other solutions. In both cases, the maxima of the particle size distribution from SAXS (9?C10 nm) correspond to the sizes of the magnetite crystallites that were estimated from the broadening of the diffraction lines. In addition, the SAXS data indicate the presence of a significant proportion of aggregates (up to 60 nm in diameter) in both colloidal solutions, although fundamental differences in the structures of aggregates between the MNFs stabilized by PAA and CA were not observed. In this study determination of the structural characteristics of MNFs were carried out in order to obtain stable dispersive non-aggregating nanoparticles of magnetite for use as contrast agents in magnetic resonance tomography, drug carriers, and other biomedical applications.     

Charge states of the activator and size effects in BaF2: Eu nanophosphors

ABSTRACT

According to the spectra of stationary X-ray excited luminescence (XEL) of BaF₂: Eu nanophosphors at 80 and 294 K, it was revealed that the thermal annealing of fine-grained nanoparticles (d?=?35?nm) in the range of 400–1000°C, which is accompanied by an increase of their sizes in the range of 58–120?nm, does not result in effective changes of the charge state of Eu^{3 +} → Eu^{2 +} activator, in contrast to CaF₂: Eu nanoparticles. The maximum light output of X-ray excited luminescence of BaF₂: Eu nanophosphors in the 590?nm emission band of Eu³⁺ ion was observed at an annealing temperature of 600°C with the average size of nanoparticles 67?nm. The subsequent growth of annealing temperatures, especially in the range of 800–1000°C, causes decrease in the light output of X-ray excited luminescence due to the increase of defect concentration in the lattice as a result of sharp increase of nanoparticle sizes and their agglomeration. In BaF₂: Eu nanoparticles of 58?nm size, according to the thermostimulated luminescence (TSL) spectrum, transformation of Eu³⁺ → Eu²⁺ under the influence of long-time X-ray irradiation was revealed for the peak of 151?K. Thus, X-ray excited luminescence spectra of BaF₂: Eu nanophosphors are formed predominantly due to the emission of Eu³⁺ ions, while emission of Eu²⁺ ions is observed in the TSL spectra.     

Preferred orientation of ZnO nanoparticles formed by post-thermal annealing zinc implanted silica

High quality zinc oxide nanoparticles with (002) preferred orientation were prepared by post-thermal annealing zinc implanted silica at 700 °C using two methods. One method was annealing zinc implanted silica at 700 °C for 2 h in oxygen ambient; the other method was sequentially annealing zinc implanted silica at 700 °C in nitrogen and oxygen ambient for 1 h, respectively. X-ray diffraction (XRD), absorption and microphotoluminescence (micro-PL) results indicated that the latter method could create high quality ZnO nanoparticles with (002) preferred orientation and narrow size-distribution. X-ray photoelectron spectra (XPS) showed the formation of ZnO nanoparticles on a silica surface, where the ZnO nanoparticle content increased with increasing oxidation time in an oxygen environment. The processes of the transformation from Zn to ZnO are discussed.     

Hierarchical nanoparticle morphology for platinum supported on SrTiO3 (0 0 1): A combined microscopy and X-ray scattering study

The morphology of metal nanoparticles supported on oxide substrates plays an important role in heterogeneous catalysis and in the nucleation of thin films. For platinum evaporated onto SrTiO₃ (0 0 1) and vacuum annealed we find an unexpected growth formation of Pt nanoparticles that aggregate into clusters without coalescence. This hierarchical nanoparticle morphology with an enhanced surface-to-volume ratio for Pt is analyzed by grazing incidence small-angle X-ray scattering (GISAXS), X-ray fluorescence (XRF), atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM). The nanoparticle constituents of the clusters measure 2-4 nm in size and are nearly contiguously spaced where the average edge-to-edge spacing is less than 1 nm. These particles make up the clusters, which are 10-50 nm in diameter and are spaced on the order of 100 nm apart.     

Preparation of ferromagnetic γ - Fe2O3 nanocrystallites by oxidative co-decomposition of PEG 6000 and ferrocene

Highly crystalline and ferromagnetic γ-Fe₂O₃ nanocrystallites were prepared by controlled oxidative co-decomposition of PEG 6000 and ferrocene at a temperature of 450 ^°C under air atmosphere. The morphology, crystalline structure and preliminary magnetic properties of the as-synthesized nanocrystallites have been characterized by using transmission electron microscope (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The highly crystalline γ-Fe₂O₃ nanocrystallites are in quasi-cubic shape with an average size of 30 nm and exhibit room-temperature ferromagnetism. The capping effect of PEG 6000 has also been investigated by thermogravimetry analysis (TGA) and Fourier transform infrared (FTIR) regarding controlling the size of the nanocrystallites and preventing the volatilization of ferrocene and thus raising the yield of the products. This simple method has a high yield of over 80% as well as low cost.     

Reaction mechanism studies for platinum nanoparticle growth by atomic layer deposition

Mass spectrometry is used to study the reaction mechanism of platinum (Pt) atomic layer deposition (ALD) on large quantities of high surface area silica gel particles in a fluidized bed reactor. (Methylcyclopentadienyl)trimethylplatinum [(MeCp)PtMe₃] and oxygen are used as precursors. Studies are conducted at a substrate temperature of 320 °C. The self-limiting behavior of ALD appears to be disrupted with overexposure of Pt precursor. The amount of the deposited Pt and the size of the Pt nanoparticles increase with an increasing overdose time of Pt precursor. This can be explained by the thermal decomposition of Pt precursor at the reaction temperature of 320 °C and the in situ sintering of Pt nanoparticles forming larger particles. This finding is significant and its understanding is essential for better control of Pt deposition to achieve desirable morphological and structural properties for different application requirements.     

Temperature‐driven directional coalescence of silver nanoparticles

Silver nanoparticles were synthesized with a chemical reduction method in the presence of polyvinylpyrrolidone as stabilizing agent. The thermal stability behavior of the silver nanoparticles was studied in the temperature range from 25 to 700°C. Thermal gravimetric analysis was used to measure the weight loss of the silver nanoparticles. Scanning electron microscopy and high‐resolution transmission electron microscopy were used to observe the morphology and the change in shape of the silver nanoparticles. In situ temperature‐dependent small‐angle X‐ray scattering was used to detect the increase in particle size with temperature. In situ temperature‐dependent X‐ray diffraction was used to characterize the increase in nanocrystal size and the thermal expansion coefficient. The results demonstrate that sequential slow and fast Ostward ripening are the main methods of nanoparticle growth at lower temperatures (<500°C), whereas successive random and directional coalescences are the main methods of nanoparticle growth at higher temperatures (>500°C). A four‐stage model can be used to describe the whole sintering process. The thermal expansion coefficient (2.8 × 10^?5 K^?1) of silver nanoparticles is about 30% larger than that of bulk silver. To our knowledge, the temperature‐driven directional coalescence of silver nanocrystals is reported for the first time. Two possible mechanisms of directional coalescence have been proposed. This study is of importance not only in terms of its fundamental academic interest but also in terms of the thermal stability of silver nanoparticles.     

An optical and structural investigation into CdTe nanocrystals embedded into the tellurium lithium borophosphate glass matrix

Cadmium telluride nanocrystals that form in the TeO2-Li2O-B2O3-P2O5 glass matrix have been synthesized and studied.They are investigated by X-ray diffraction(XRD),optical transmission and infrared spectroscopy.It has been shown that the long annealing time effect on present samples leads to the growth of CdTe nanoparticles and an increase of tellurium oxide on the surface of nanocrystallites.On the other hand,the infrared spectroscopy shows that the phosphate and borate networks of the glass matrices are mo...     

Synthesis of ZnFe2O4 nanocrystallites by mechanochemical reaction

Mechanochemical reaction of ZnO and α-Fe₂O₃ in a planetary mill formed an amorphous precursor, which was subsequently heated to successfully produce zinc ferrite (ZnFe₂O₄) nanocrystallites. The amorphous precursor and nanocrystallites were characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Calcination of the precursor powder at 600 °C led to the formation of ZnFe₂O₄ nanocrystallites of about 22 nm in crystal size, and most of particle was about 10-50 nm in diameter. Effect of calcination temperature on the crystal size of the nanoparticles was investigated. The mechanism of nanocrystallite growth was primarily investigated. The activation energy of ZnFe₂O₄ nanocrystallite formation during thermal treatment was calculated to be 18.5 kJ/mol.     

UV-Visible Photoluminescence of TiO2 Nanoparticles Prepared by Hydrothermal Method

TiO₂ colloidal nanoparticles and nanocrystals are prepared by hydrolysis of titanium isopropoxide employing a surfactant-free synthetic hydrothermal method. The synthesized samples are characterized by X-ray diffraction (XRD), HRTEM and FTIR. The XRD study confirms that the size of the colloidal nanoparticle is around 4?nm which the HRTEM analysis indicates the sizes of the colloidal nanoparticles are in the range of 2.5?nm. The fluorescence property of the TiO₂ colloidal nanoparticles studied by the emission spectrum confirms the presence of defect levels caused by the oxygen vacancies. We have observed new emission bands at 387?nm,421?nm, 485?nm, 530?nm and 574?nm wavelengths, first one (387?nm) being emission due to annihilation of excitons while remaining four could be arising from surface states. The emission spectrum of annealed nanocrystallites is also having these four band emissions. It is observed that the surface state emission basically consists of two categories of emission.     

Size controlled synthesis of semiconductor nanocrystals in a continuous-flow mode microcapillary reactor

A microcapillary reactor with 320 μm inner diameter was utilized for CdSe nanoparticle synthesis. The influence of the reaction temperature and flow rate of precursors on the size and size distribution of prepared CdSe nanoparticles was systematically studied. The as-prepared nanoparticles exhibit sharp excitonic absorption and photoluminescence peak (FWHM 30 nm) with a quantum-yield around 10–40%. The microcapillary reactor was also used for CdSe/ZnS core-shell nanoparticle synthesis in continuous-flow mode. The quantum yield of the core-shell nanoparticles was found to be considerably influenced by the reactor temperature and have a close correlation with the thickness of ZnS shell under growth. An optimized quantum yield up to 70% was obtained for the CdSe/ZnS core-shell nanoparticles.     

用SAXS和XRD方法研究TiO2纳米颗粒微观结构

用溶胶-凝胶方法制备了TiO₂纳米样品,并对该样品在300℃到800℃温度区域进行了退火处理.应用同步辐射X射线粉末衍射(XRD)方法研究了经不同热处理温度的TiO₂纳米颗粒的结构相变.应用同步辐射小角X射线散射(SAXS)方法研究了TiO₂纳米颗粒的表面分形与界面特性.得到纳米颗粒粒度与退火温度的变化规律,讨论了表面界面特征与相变的关系. 关键词： X射线小角散射 X射线衍射 2纳米颗粒')" href="#">TiO₂纳米颗粒     

Fabrication of metal-nanoparticle/carbon-fiber composites having a microtube-array morphology

In the current work, we succeeded in incorporation of Pt ions into sisal fiber (SF) – a biological matrix with a characteristic morphology of microtube array, and in subsequent in situ synthesis of Pt nanoparticles of ca. 3.6 nm. Carbonization of the SF with Pt nanopaticles at 400°C produced Pt-nanoparticle/carbon-fiber composite, preserving the initial microtube-array morphology of SF. It is interesting that the walls of neighboring microtubes and the middle lamella between these microtubes were fused by carbonization, and a homogeneous wall was formed. Although the size of Pt nanoparticles was enhanced to ca. 5.3 nm after carbonization, the solid matrices (from cellulose to carbon) acted as effective barriers against the growth of Pt nanoparticles. The Pt-nanoparticle/carbon-fiber composite combines several important aspects, including the morphologies of fiber and microtube-array, carbon matrix, and Pt nanoparticles. Thus it might be a novel type of catalyst and have potential applications in many fields.     

Synthesis of nanostructured lean-NO x catalysts by direct laser deposition of monometallic Pt-, Rh- and bimetallic PtRh-nanoparticles on SiO2 support

Monometallic Pt and Rh and bimetallic PtRh catalysts with a highly dispersed noble metal weight loading of ca. 1 wt% were produced via the direct deposition of nanoparticles on different SiO₂ supports by means of pulsed ultra-violet (248 nm) excimer laser ablation of Pt, Rh bulk metal and PtRh alloy targets. Backscattered electron microscopy (BSE), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) were employed to characterize the deposited nanoparticles, which were found to exhibit narrow size distribution centred around 2.5 nm. The catalytic activities for lean NO _x reduction of the monometallic and bimetallic catalyst samples were investigated in a flow reactor setup in the temperature range 100–400°C using a test gas mixture representative of oxygen rich diesel engine exhaust gas. For comparison a Rh/SiO₂ reference catalyst prepared by a conventional impregnation method was also tested. Further experiments were performed in which PtRh nanoparticles were deposited on a Rh/SiO₂ reference catalyst sample to study the possibility for controlled modification of its activity. The catalytic activity measurements revealed that among the samples solely prepared by laser deposition the PtRh–SiO₂ nanoparticle catalyst showed the highest activity for NO _x reduction at low temperatures 100–300°C. In addition, it could be demonstrated that the initially low NO _x reduction activity and the N₂ selectivity of the Rh/SiO₂ reference catalyst sample for temperatures below 250°C can be enhanced by post laser deposition of PtRh nanoparticles.