Preparation and characterization of ceria nanospheres by microwave-hydrothermal method

Crystalline ceria (CeO₂) nanoparticles have been successfully synthesized by a simple and fast microwave-hydrothermal method at 130 °C for 20 min. As-synthesized CeO₂ powders were calcinated at 500 °C for 1, 2 and 4 h. The products were characterized by thermogravimetric analysis (TG-DTA), X-ray powder diffraction (XRD), field-emission scanning electron microscopy/STEM mode (FEG/STEM), Fourier Transformed-IR and RAMAN spectroscopies. It is shown that synthesized ceria powders have a spherical shape with particle size below 10 nm, a narrow distribution and exhibit weak agglomeration. The FTIR spectrum of the ceria exhibits strong broad band below 700 cm^− 1 which is due to the δ (Ce-O-C) mode. Raman spectrum is characterized by the presence of a very strong band to 464.5 cm^− 1. The microwave-hydrothermal method enabled cerium compounds to be synthesized at low temperature and shorter time.     

Novel method to control the size of well-crystalline ceria particles by hydrothermal method

Well-crystalline ceria particles were synthesized by heating peptized ceria sol as precursor under hydrothermal conditions. The morphology and the crystallites size of hydrothermal ceria particles were controlled by varying the dielectric property of solvent used in preparation of the ceria precursor. The synthesized particles exhibit cubic fluorite structure with size ranged from 20 to 400 nm without the formation of hard aggregates. In this work, the relationships between dielectric property of the solvent and particle size were investigated in terms of the supersaturation of solute. In addition, the influences of precipitation participating anions (OH⁻) and acidic hydrothermal medium on crystallites size of ceria particles were studied.     

Synthesis of boron carbide powder from polyvinyl borate precursor

Polyvinyl borate (PVBO) was prepared by the condensation of poly(vinyl alcohol) (PVA) and boric acid, and used as a precursor for boron carbide. Boron carbide powder was synthesized by the pyrolysis of the PVBO precursor in air at 600 °C for 2 h, followed by heat treatment in Ar flow at 1300 °C for 5 h, which is a relatively low temperature compared with conventional carbothermal methods. Pyrolysis of the PVBO precursor resulted in submicron-size particles of B₂O₃ dispersed in a carbon matrix. In addition, the pyrolysis temperature governed the carbon content in the pyrolyzed product of the PVBO precursor, which led to the synthesis of crystalline boron carbide powder with little free carbon.     

Effect of process parameters on size,shape, and distribution of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

Antimony trioxide (Sb₂O₃) nanoparticles with particle sizes ranging from 2 to 12 nm, spherical in shape, and well-distributed were successfully synthesized by chemical reduction method. The nanoparticles were synthesized in the presence of hydrazine as a reduction agent in ethylene glycol through the reaction between antimony trichloride and sodium hydroxide. Effects of reaction temperature, reaction time, precursor concentration and boiling temperature on the particle size, shape, and distribution of the Sb₂O₃ nanoparticles were investigated. Morphology of the nanoparticles was examined by transmission electron microscope (TEM). It was revealed that the particle size increased when reaction temperature, reaction time and concentration of precursor were increased. Moreover, the mixture needed to be boiled prior to the addition of hydrazine as a reduction agent, in order to obtain an optimum reduction. X-ray diffraction (XRD) was employed to study the crystallinity and phase of the nanoparticles. The nanoparticles were determined as cubic phase of Sb₂O₃ (ICDD file no. 00-043-1071) by XRD. Interrelation between UV–vis absorption spectra of the nanoparticles and their particle size were obtained.     

Energy-efficient green synthesis of Nd:Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowder by microwave gel combustion

A novel method for the synthesis of nanocrystalline neodymium-doped yttria (Nd:Y₂O₃) by gel combustion in microwave without long period of calcination at high temperature is described. The method leads to rapid formation of phase pure product with saving of time and energy. Gels were prepared by selecting citrate to nitrate (C/N) ratios of 0.5:1 & 1:1 followed by combustion in microwave for drying of gel leading to precursor formation. Thermogravimetric-differential thermal analysis (TG-DTA) of precursor with (C/N) ratios of 1:1 exhibited a total loss in weight of about 31.7% up to 1300 °C and did not show M–O bonds characteristic of yttria by FTIR indicating requirement of calcination at high temperature. Crystallization to phase pure yttria took place only on thermal treatment at 1000 °C for 4 h in oxygen atmosphere confirmed by XRD with particle size 40 nm. While the microwave combusted precursor with C/N ratio of 0.5:1 exhibited a weight loss of only 4.7% up to 1300 °C and showed M–O bonds characteristic of yttria in FTIR spectrum without calcinations at higher temperature. This microwave combustion precursor with C/N ratio of 0.5:1 was found to be crystalline phase pure yttria by X-ray diffraction (XRD) with primary particle size 28 nm by Scherrer’s equation and 30–50 nm of uniform morphology by transmission electron microscopy (TEM). In the present work the ratio of citric acid to nitrates is playing a crucial role in terms of saving time and energy involved in calcinations of microwave combusted precursor for the composition with higher citrate content.     

Investigation of reactive cerium-based oxides for H2 production by thermochemical two-step water-splitting

This study focuses on the use of cerium-based mixed oxides for hydrogen production by solar-driven thermochemical two-step water-splitting. Mixed cerium oxides are proposed in order to decrease the reduction temperature of ceria and to avoid material sublimation occurring above 2,000 °C during the high-temperature solar step. Ceria-based nanopowders were synthesized by soft chemistry methods including the modified Pechini method. The influence of the synthesis method, the type of cationic element mixed with cerium, and the content of this added element was investigated by comparing the reduction temperatures of the derived materials. The synthesized powders were characterized by X-ray diffraction, thermogravimetric analysis, SEM, and Raman spectroscopy. Results showed that the synthesized pure cerium oxide is more reactive toward reduction than a commercial powder. Among the different elements added to ceria that were screened, the addition of zirconium significantly improved the reduction of ceria at temperatures below 1,500 °C. Increasing zirconium content further favored cerium reduction yield up to 70%. Water-splitting tests were performed to demonstrate the reactivity of the developed materials for H₂ production. The amount of H₂ evolved was enhanced with a temperature increase, the maximum H₂ production from Ce_0.75Zr_0.25O_2−δ was 0.24 mmol/g at 1,045 °C, and the powder reactivity upon cycling was demonstrated via thermogravimetry through two successive reduction–hydrolysis reactions.     

Poly(furfuryl alcohol)-assisted pyrolysis synthesis of ceramic nanoparticles for solid oxide fuel cells

A pyrolysis synthesis method was developed to prepare ceramic nanoparticles for the fabrication of solid oxide fuel cells. Furfuryl alcohol was used as a polymerizable solvent to dissolve metal nitrates and then polymerized into poly(furfuryl alcohol) (PFA). During the pyrolysis at 600 °C, a mixture of nitrates/PFA was converted into ceramic nanoparticles/carbon networks nanocomposite, and the carbon networks act as a barrier to prevent the aggregation of newly formed nanoparticles during particle crystallization. Dispersible nanoparticles with particle sizes ranging from 40 nm to 200 nm were obtained after burning off carbon networks in air. As an example, Ce_0.8Sm_0.2O_1.9 nanoparticles were synthesized to prepare solid oxide fuel cells, and the fuel cells achieved maximum power densities of 444.5, 625.5 and 684 mW cm^?2 at 500 °C, 550 °C and 600 °C, respectively. Our study shows that the pyrolysis synthesis method described here is promising for the effective synthesis of high quality ceramic nanoparticles.     

A novel method to prepare Cr2O3 nanoparticles

By the reaction system of CrO₃ and HCHO in aqueous solution, Cr₂O₃ nanoparticles were first prepared via hydrothermal synthesis. The process can be easily scaled up. The reaction time was only 1 h and the reaction temperature was 170 °C. The products were loosely agglomerated Cr₂O₃ particles of 50-70 nm in average particle size calculated from the Scherrer's formula, whose microstructure and that of the precursor were investigated by SEM. And IR, TG and BET were other characterization methods to study the process. The findings showed that the higher calcination temperature and the higher total concentration were factors to result in the larger average particle size.     

Synthesis of deer horn-like and spherical ZnO nanoparticles by microwave decomposition of bis(2-pyridinethiol <Emphasis Type="Italic">N</Emphasis>-oxide)zinc using factorial and Taguchi experimental designs

Deer horn-like and spherical nanoparticles of ZnO have been prepared via microwave heating (MWH) of Bis (2-pyridinethiol N-oxide) Zinc (II) [BPTZ] complex. The product was characterized by XRD, SEM, LLS, BET, FTIR and chemical analysis. The 2³ factorial and the Taguchi L ₄ designs were used for factors effect estimation and determination of optimum conditions for spherical ZnO nanoparticle synthesis. The three main factors considered were power of microwave, temperature of pyrolysis and time of thermal decomposition. The time of pyrolysis had the most influence on the average particle size and the size distribution of product. The average particle size for the spherical ZnO at optimum conditions was found to be 16 nm and the particle range was 16 ± 13 nm.     

Ultrasonic spray pyrolysis of surface modified TiO2 nanoparticles with dopamine

Spherical, submicronic TiO₂ powder particles were prepared in the low temperature process of ultrasonic spray pyrolysis (150 °C) by using as a precursor aqueous colloidal solutions consisting of surface modified 45 Å TiO₂ nanoparticles with dopamine. Detailed structural and morphological characterization of colored submicronic TiO₂ spheres was performed by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser particle size analysis and FTIR techniques. Also, optical characterization of both dopamine-modified TiO₂ precursor nanoparticles and submicronic TiO₂ powder particles was performed using absorption and diffuse reflectance spectroscopy, respectively. A significant decrease of the effective band gap (1.9 eV) in dopamine-modified TiO₂ nanoparticles compared to the band gap of bulk material (3.2 eV) was preserved after formation of submicronic TiO₂ powder particles in the process of ultrasonic spray pyrolysis under mild experimental conditions. Due to the nanostructured nature, surface-modified assemblage of TiO₂ nanoparticles preserved unique ability to absorb light through charge transfer complex by photoexcitation of the ligand-to-TiO₂ band, conventionally associated with extremely small TiO₂ nanoparticles (d < 20 nm) whose surface Ti atoms, owing to the large curvature, have penta-coordinate geometry.     

Nanocrystalline ceria powders through citrate-nitrate combustion

Nanocrystalline ceria powders have been synthesized by combustion technique using citric acid as a fuel and nitrate as an oxidizer. The auto-ignition of the gels containing cerium nitrate and citric acid resulted in ceria powders. A theory based on adiabatic flame temperature for different citric acid-to-cerium nitrate molar ratios has been proposed to explain the nature of combustion reaction and its correlation with the powder characteristics. Specific surface area and primary particle size of the ceria powder obtained through fuel-deficient precursor was found to be approximately = 127 m2/g and 2.5-10 nm, respectively. The combustion synthesized ceria powder when cold pressed and sintered in air at 1250 degrees C for 1 hour resulted in approximately = 96% of its theoretical density with sub-micron grains.     

纳米二氧化铈的低温水热一步法合成

在低温水热条件下,通过一步法合成了尺寸可控的二氧化铈纳米粒子.采用XRD和SEM对合成的样品进行表征.XRD结果表明,在三乙烯四胺(C6H18 N4)存在的条件下,通过水热法在80℃加热48h即可直接合成出二氧化铈纳米粒子,而无需将产物在高温下处理.SEM图显示,在80℃反应48h得到的二氧化铈为球形纳米颗粒,其平均粒径约为13nm.实验发现通过改变反应温度可以很容易地控制二氧化铈纳米粒子的粒径,其随着反应温度的升高而增大.由于整个反应过程可以在低温下完成,因而此合成方法具备实现工业化生产的潜力.     

Preparation and characterization of nanosized Zn-Co spinel oxide by solid state reaction method

ZnCo₂(C₂O₄)₃·6H₂O nanocrystal was prepared by a one-step solid state reaction method at room temperature under ambient conditions using oxalic acid as a chelating agent. Adding oxalic acid to the mixture of zinc acetate dihydrate and cobalt acetate tetrahydrate, grinding and subsequently calcining the precursor at different temperatures, the pink precursor was attained. Thermogravimetric analysis and differential scanning calorimetry have been used to study the thermal behaviors of the precursor, which reveal that the precursor is a compound. Powder X-ray diffraction data show that the sample obtained by heating the precursor between 350 °C and 550 °C for 3 h is a single-phase cubic material having the spinel-type structure, which can be confirmed by infrared spectra. Transmission electron microscopy images show that the grain size of the precursor and the products are about 50 nm and 20 nm, respectively.     

Preparation of monodisperse ZnO/SiO2 composite particles using spray drying with an anionic surfactant template and their photoluminescence characteristics

The photoluminescence (PL) characteristics of ZnO/SiO₂ composite particles were investigated. ZnO/SiO₂ composite particles were synthesized utilizing the consecutive sol–gel spray drying method by incorporating sodium lauryl sulfate (SLS) as a particle morphology control agent. The effect of SLS concentration and ZnO ratio on precursors was studied further on the composite particle morphology and PL performance. Elevating the SLS concentration exhibited a reduction in the particle diameter and an increase in particle uniformity. The particle diameter without SLS was 6.18 µm and reduced to 2.6 µm with the addition of SLS at 3 critical micelle concentrations (CMC). The decrease in ZnO concentration also reduced the particle diameter of the ZnO/SiO₂ composite to 1.74 µm at a ZnO concentration of 25% mol. In addition, the increase in the excitation wavelength from 230 nm to 320 nm indicates a shift in the peak emission intensity at higher wavelengths from 467 nm to 645 nm. The excitation wavelength-dependent photoluminescence phenomenon was exhibited by incorporating silica into the ZnO precursor pre- and post-drying to deliver composite particles. The addition of silica to the composite particles can augment the PL emission intensity without causing a shift in the PL emission peaks when excited at the same wavelength. The 25% mol ZnO composite particles with the addition of SLS 3 CMC had the highest PL emission intensity. The amount of silica nanoparticles sufficient to trap the ZnO nanoparticles in the droplet is an important factor besides the size and uniformity of the particles, which causes the high intensity of PL emission.     

Effect of processing methods on physicochemical properties of titania nanoparticles produced from natural rutile sand

Titania (TiO₂) nanoparticles were produced from natural rutile sand using different approaches such as sol–gel, sonication and spray pyrolysis. The inexpensive titanium sulphate precursor was extracted from rutile sand by employing simple chemical method and used for the production of TiO₂ nanoparticles. Particle size, crystalline structure, surface area, morphology and band gap of the produced nanoparticles are discussed and compared with the different production methods such as sol–gel, sonication and spray pyrolysis. Mean size distribution (d₅₀) of obtained particles is 76 ± 3, 68 ± 3 and 38 ± 3 nm, respectively, for sol–gel, sonication and spray pyrolysis techniques. The band gap (3.168 < 3.215 < 3.240 eV) and surface area (36 < 60 < 103 m² g^?1) of particles are increased with decreasing particle size (76 > 68 > 38 nm), when the process methodology is changed from sol–gel to sonication and sonication to the spray pyrolysis. Among the three methods, spray pyrolysis yields high-surface particles with active semiconductor bandgap energy. The effects of concentration of the precursor, pressure and working temperature are less significant for large-scale production of TiO₂ nanoparticles from natural minerals.     

Magnetic and dielectric properties of HoMnO3 nanoparticles synthesized by the polymerized complex method

In this paper, we report on the magnetic and dielectric properties of HoMnO₃ nanoparticles with different size synthesized by a polymerized complex method have been investigated. The HoMnO₃ nanoparticles crystallized in hexagonal perovskite-type structure. The zero-field-cooled magnetic susceptibility curve of HoMnO₃ nanoparticles with averaged size of 30 nm shows that complicated magnetic transitions occurred in a temperature range from 2 to 100 K, which was confirmed by magnetic hysteresis loops. With increasing the particle size, the antiferromagnetic (AFM) transition temperature increases from 56 to 77 K, due to the reduced surface-to-volume ratio. Moreover, with a decrease in particle size, the Mn-spin reorientation temperature (T_SR) is enhanced from 44 to 48 K.     

Preparation of nanometer CuFe2O4 by auto-combustion and its catalytic activity on the thermal decomposition of ammonium perchlorate

Nanometer copper ferrite was synthesized by auto-combustion method using cupric nitrate, ferric nitrate and malic acid as raw materials. The precursor and as-burnt sample were characterized by X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) . Average particle size of sample is 26nm. The catalytic performance of nanometer CuFe₂O₄ on the thermal decomposition of ammonium perchlorate (AP) was investigated by DTA. The results show that the nanometer CuFe₂O₄ has high a catalytic activity, and the thermal decomposition temperature of AP shift 105 °C downward with the effect of nanometer copper ferrite. When the content of CuFe₂O₄ comes to 5%, the catalytic performance is the best.     

P-type ZnO thin film deposited by spray pyrolysis technique: The effect of solution concentration

The aim of this research is to study the role of concentration variations on precursor solution of nitrogen doped ZnO (ZnO:N) thin films which has been prepared by spray pyrolysis technique. SEM micrographs show that ZnO:N films in 0.1 ML concentration have a mono-disperse surface with nano-spheres of 50 nm in diameter. In higher molarities the nano-spheres agglomerate leading to particle formation. For 0.4 ML concentrations this change is observed, where plume like particles are seen over the surface of ZnO:N thin film. This change corresponds also to changes observed in the XRD spectra, where crystal orientation of ZnO:N thin films changes from (002) to (100). All of the ZnO:N thin films have kept their sharp ultra violet absorption edge, but the transparency in visible spectra region decreases as the molarities in precursor solution increase. Photoluminescence spectra at room temperature revealed emissions at 2.33 eV, 2.54 eV and 3.16 eV that can be attributed to the presence of nitrogen in ZnO structure. We also observe that all samples analyzed show a p-type Hall effect behavior, and that as the molarities in the precursor solution increase, the electrical resistivity of the films decreases, due to an enhancement of free carriers, while the mobility decreases. These data prove the capability of spray pyrolysis as a viable technique in preparing p-type TCO materials and so, fully transparent CMOS-like devices.     

Preparation of SiC nanopowder using low-temperature combustion synthesized precursor

SiC nanopowder was synthesized by carbothermal reduction of a low-temperature combustion synthesized (LCS) precursor derived from silicic acid, polyacrylamide (PAM), nitric acid, urea, and glucose mixed solution. The results showed that the LCS precursor is a kind of porous blocky particles. The precursors were subsequently calcined under argon at 1100–1500 °C for 2 h. The transformation of SiO₂ to SiC occurred at 1200 °C, and complete transformation of SiO₂ to SiC was achieved at 1500 °C. The SiC powder synthesized at 1500 °C is mostly composed of near-spherical particles with the diameter of 50–100 nm. Moreover, the SiC powder also contains very rare amount of whiskers with a diameter of 80 nm and a length of up to several micrometers. It is proposed that the present holes in the precursor particles during calcination are responsible for the formation of whiskers. Furthermore, the formation of mainly SiC near-spherical nanoparticles is ascribed to coarse surface of precursor particles during calcination, intimate contact among SiO₂ and C particles, uniformly formed free space during reduction reaction, and separation effect of unreacted carbon.