Hydrothermal synthesis of single-crystal bismuth ferrite nanoflakes assisted by potassium nitrate

Well-crystallized single-crystal BiFeO₃ (BFO) nanoflakes have been successfully synthesized for the first time by the hydrothermal method assisted by KNO₃. The as-prepared samples were characterized by X-ray powder diffraction (XRD), transmission electron microscope (TEM), high-resolution TEM (HRTEM), and physical property measurement system (PPMS). It was found that KNO₃ played a key role in the formation of BFO nanoflakes. The as-prepared BFO nanoflakes were single-crystal in structure and showed super paramagnetic behavior at room temperature. It is expected that this process may be extended to the synthesis of other kinds of nanostructure materials, which will be beneficial to their detailed experimental investigation of the size- and morphology-dependent properties.     

Synthesis of bismuth oxide nanoparticles by the polyacrylamide gel route

A polyacrylamide gel route has been adopted for the preparation of bismuth oxide nanoparticles. Thermal behaviour of the polyacrylamide gel has been studied by means of thermogravimetry (TGA). The formation of monoclinic Bi₂O₃ nanocrystallites is confirmed by X-ray diffraction (XRD). Transmission electron microscopy (TEM) investigation revealed the average particle size of Bi₂O₃ nanoparticles to range from 30 nm to 50 nm.     

Low-temperature synthesis of bismuth titanate by modified citrate amorphous method

Bismuth titanate is a lead-free piezoelectric ceramic with outstanding properties that strictly depend on the composition and microstructure. However, bismuth-based materials are difficult to synthesize due to bismuth volatilisation that causes secondary phases and stoichiometry deviations. In this work, we propose a low-temperature chemical route, i.e. a modified amorphous citrate method, that allows a reduction of thermal treatment temperature, when compared with solid-state or other chemical routes, to obtain single-phase bismuth titanate samples. Single-phase powders with particle size under 300 nm are produced by calcination at 700 °C, and prepared into homogeneous dense pellets (density above 95%), with only isolated pores. The pellets show two distinctive features in the electrical behaviours directly associated with their mica-like microstructure: planar oriented boundaries are responsible for oxygen conduction, while the bulk is dominated by electronic conductivity. The samples show a high dielectric constant, around 200 at room temperature, while maintaining a low loss factor. The pellets also achieved a maximum polarisation of 5.85 μC/cm² and an inverse piezoelectric coefficient of 7.4 pm/V. The dielectric and piezoelectric properties obtained are comparable or superior to the state-of-the-art.     

Electrochemical reaction of lithium with orthorhombic bismuth tungstate thin films fabricated by radio-frequency sputtering

Bi₂WO₆ thin films with fast deposition rate have been fabricated by radio-frequency (R.F.) sputtering deposition, and are used as positive electrodes in rechargeable thin film lithium batteries. An initial discharge capacity of 113 μAh/cm²-μm is obtainable for Bi₂WO₆ film electrode with good capacity reversibility. A multiple-center reactive mechanism associated with both Bi³⁺/Bi⁰ and W⁶⁺/W^x+(x < 6) is investigated by ex situ X-ray diffraction patterns, transmission electron microscopy and selected area electron diffraction techniques, apart from the direct comparison of Bi₂WO₆ electrochemical performance with those of Bi₂O₃ and WO₃ thin films. A possible explanation about smooth capacity loss of Bi₂WO₆ after long-term cycling is suggested from the incomplete reaction of Bi component. The advantages of Bi₂WO₆ thin films over the singer-center Bi₂O₃ or WO₃ thin films are shown in both the aspects of volumetric capacity and cycling life.     

Piezo-ferroelectric response of bismuth ferrite based thin films and their related photo/piezocatalytic performance

Bismuth ferrite based thin films were grown by RF magnetron sputtering under different experimental conditions. The effects of substrate temperature, Ar:O₂ mass flow ratio and gas mixture pressure on the films’ microstructure, phase evolution, optic, ferroelectric and magnetic properties were systematically investigated. The structural analysis results revealed an amorphous phase for the films deposited at a substrate temperature below 500 °C, while for the thin films deposited at 700 °C, a ε-Fe₂O₃ secondary phase was detected. The diffraction lines of the samples deposited at 600 °C were associated with Bi₂Fe₄O₉ and Bi₂₅FeO₄₀ phases. The increase in the mixture gas pressure up to 1 Pa showed an improved crystallinity of the deposited films, while, at higher working gas pressures, the films were found to be amorphous. The use of low O₂ to Ar mass flow ratio during the deposition led to a phase transformation process. EDX and RBS measurements exposed a uniform distribution of the main elements, revealing some stoichiometry changes induced by the pressure variation. The optical band gap values were influenced by the substrate temperature and pressure of the Ar:O₂ gas. The magnetic properties were correlated with the structural features, the highest magnetic response being observed for the sample deposited at 600 °C, 1 Pa and 3:1 Ar:O₂ gas pressure. According to the PFM results, the film deposited at 700 °C, Ar:O₂ ratio 3:1 and total gas pressure 1 Pa clearly outperformed the others due to their excellent ferroelectric properties and outstanding piezo-response. The sample deposited at 700 °C showed both visible light-driven degradation and piezodegradation activities. The piezocatalytic and photocatalytic activities were ascribed to the high piezoresponse and to a more efficient separation of electrons and holes induced by a built-in electric field that is caused by the larger remnant polarization of Bi₂Fe₄O₉ and Bi₂Fe₄O₉/ε-Fe₂O₃ hetero-junction.     

Controllable synthesis and growth mechanism of lead free bismuth sodium titanate nanowires

Highly uniform lead-free piezoelectric bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) nanowires were successfully synthesized via a hydrothermal method. Synthesized at a stirring speed range of 300–1000 rpm, the compositions and orientations of BNT nanowires were well controlled. The effects of stirring speeds on the formation of BNT nanoparticles and nanowires in the hydrothermal processes were systematically investigated. The BNT nanowires with a high aspect ratio were proven to be single crystals with [110] growth direction from high-resolution TEM analysis. The mechanism of growth of BNT nanowires out of nanoparticles in the hydrothermal processes was proposed.     

Production and dispersion stability of nanoparticles in nanofluids

This paper presents an experimental study on the homogeneous dispersion of nanoparticles in nanofluids. In this study, various physical treatment techniques based on two-step method, including stirrer, ultrasonic bath, ultrasonic disruptor, and high-pressure homogenizer were systematically tested to verify their versatility for preparing stable nanofluids. Initially carbon black and silver nanoparticles dispersed in base fluids with the presence of surfactant were found to be highly agglomerated with the hydrodynamic diameter of 330 nm to 585 nm, respectively. After both CB and Ag nanofluids were treated by various two-step methods, stirrer, ultrasonic bath, and ultrasonic disrupter was found to do a poor performance in deagglomeration process for the initial particle clusters. However, the high-pressure homogenizer produced the average diameter of the CB and Ag particles of 45 nm and 35 nm, respectively, indicating that among various physical treatment techniques employed in this study, the high-pressure homogenizer was the most effective method to break down the agglomerated nanoparticles suspended in base fluids. In order to prepare another nanofluid with much smaller primary nanoparticles, we also employed a modified magnetron sputtering system, in which the sputtered nanoparticles were designed to directly mix with the running surfactant-added silicon oil thin film formed on a rolling drum (i.e. one-step method). We observed that Ag nanoparticles produced by the modified magnetron sputtering system were homogeneously dispersed and long-term stable in the silicon oil-based fluid, and the average diameter of Ag nanoparticles was found to be ~ 3 nm, indicating that the modified magnetron sputtering system is also an effective one-step method to prepare stable nanofluids.     

One-step synthesis of platinum nanoparticles loaded in alginate bubbles

Composite particles with multifunctions have been extensively utilized for various applications. Bubble particles can be applied for ultrasound-mediated imaging, drug delivery, absorbers, cell culture, etc. This study proposes a one-step strategy to obtain Pt nanoparticles loaded in alginate bubbles. A needle-based droplet formation was used to generate uniform alginate particles about 2 mm in diameter. The hydrolysis reaction of NaBH₄ was utilized to produce gaseous hydrogen and then trapped within alginate particles to form bubbles. The Pt⁴⁺ mixed with alginate solution was dropped into the reservoir to react with reducing NaBH₄ and hardening CaCl₂ to form Pt nanoparticles-alginate composite bubbles. Results indicate that the size of bubbles decreases with the CaCl₂ concentration (1% ~ 20%), and size of bubbles increases with the NaBH₄ concentration (1 ~ 20 mM). The advantages for the present approach include low cost, easy operation, and effective production of Pt nanoparticles-alginate composite bubbles.     

Thermal expansion of bismuth magnesium tantalate and niobate pyrochlores

The thermal behavior of pyrochlores composing of Bi_1·5Mg_0.75M_1.5O_6.75 (M = Nb, Ta) (sp. gr. Fd-3m:2) was studied using the high-temperature X-ray powder diffraction in a wide temperature range of 30–1200 °C. At a temperature above 1080 °C both compounds thermally dissociate forming one of the reaction products MgMO₆ (M = Nb, Ta) whose reflexes are traced on X-ray diffraction patterns after cooling the sample down to room temperature. In the case of Bi_1·5Mg_0·75Nb_1·5O_6.75 orthoniobate α-BiNbO₄ forms at about 800–1020 °C as admixture. The thermal expansion analysis of Bi_1·5Mg_0.75M_1.5O_6.75 (M = Nb, Ta) showed that the compounds studied belong to slightly or moderately expanding materials. Thermal expansion for both compounds is isotropic. With a rise in temperature the unit cell parameter a and the thermal expansion coefficient (TEC) are increased uniformly and slightly for M = Ta (Nb): from 10.52822 (10.55325) Å (30 °C) up to 10.59181 (10.62801) Å (1050 °C) and from 3.8 (30 °C) up to 7.4 (8.9) × 10⁻⁶ °С⁻¹ at 800 °C respectively. The average TEC values in the range of 30–800 °C range are 5.6 (6.4) × 10⁻⁶ °C⁻¹ for M = Ta (Nb) respectively. Bi_1.5Mg_0.75Nb_1.5O_6.75 is characterized with the highest thermal expansion that may be related to the longer bond-length of Nb(Bi)–O in the polyhedra.     

Influence of carbon chain length on the synthesis and yield of fatty amine-coated iron-platinum nanoparticles

Iron oxide nanoparticles are among the most widely used and characterized magnetic nanoparticles. However, metal alloys such as superparamagnetic iron-platinum particles (SIPPs), which have better magnetic properties, are receiving increased attention. Scalable techniques to routinely synthesize SIPPs in bulk need further study. Here, we focus on the role played by the fatty amine ligand in the formation of the bimetallic FePt nanocrystal. More specifically, we compare the effect of varying lengths of fatty amine ligands on the shape, structure, uniformity, composition, and magnetic properties of the SIPPs. We synthesized SIPPs by employing a ‘green’ thermal decomposition reaction using fatty amine ligands containing 12 to 18 carbons in length. Greater fatty amine chain length increased the polydispersity, particle concentration, iron concentration, and the stability of the SIPPs. Additionally, longer reflux times increased the diameter of the particles, but decreased the iron concentration, suggesting that shorter reaction times are preferable. Fourier transform infrared spectroscopy of the SIPPs indicates that the ligands are successfully bound to the FePt cores through the amine group. Superconducting quantum interference device magnetometry measurements suggest that all of the SIPPs were superparamagnetic at room temperature and that SIPPs synthesized using tetradecylamine had the highest saturation magnetization. Our findings indicate that the octadecylamine ligand, which is currently used for the routine synthesis of SIPPs, may not be optimal. Overall, we found that using tetradecylamine and a 30-min reflux reaction resulted in optimal particles with the highest degree of monodispersity, iron content, stability, and saturation magnetization.

PACS

81.07.-b; 75.75.Fk; 61.46.Df     

Green sonochemical synthesis of cupric and cuprous oxides nanoparticles and their optical properties

Nanoparticles of cupric oxide (CuO) and cuprous oxide (Cu₂O) with various morphologies were synthesized by a green sonochemical process without any surfactants and templates. The Cu₂O nanoparticles with the truncated cubic, cubic octahedral and octahedral morphologies were prepared via the deoxidation of the CuO nanoparticles. The Cu₂O and CuO samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet visible absorption spectroscopy (UV–vis). The experimental results indicate that the molar ratio of sodium hydroxide to copper sulfate affects the morphology and size of the CuO and Cu₂O nanoparticles produced by the sonication. The band gap energy of CuO nanoparticles was 1.45–1.75 eV, the morphology had a great effect on the optical properties of CuO. The Cu₂O nanoparticles had broad emission peaks at the visible region, and the band gap energy was estimated to be 1.95–2.09 eV. The growth mechanisms of the CuO and Cu₂O nanoparticles are discussed.     

Low temperature polymer assisted hydrothermal synthesis of bismuth ferrite nanoparticles

BFO nanoparticles were successfully synthesized by a polymer assisted hydrothermal method at a temperature as low as 160 °C. The as-prepared powders, characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM), exhibited a pure BFO phase about 10 nm size and uniform sphere-like shape. It was found that the added polymer played a key role in decreasing the growing speed of BFO nuclei and resulted in the formation of BFO nanoparticles.     

A sonochemical method for synthesis of Fe3O4 nanoparticles and thermal stable PVA-based magnetic nanocomposite

Fe₃O₄ nanoparticles were synthesized via a simple surfactant-free sonochemical reaction. Room temperature synthesis without using inert atmosphere is the novelty of this work. The effect of different parameters on the morphology of the products was investigated. The magnetic properties of the samples were also investigated using an alternating gradient force magnetometer. Fe₃O₄ nanoparticles exhibit a ferromagnetic behavior with a saturation magnetization of 66 emu/g and a coercivity of 39 Oe at room temperature. For preparation magnetic nanocomposite, Fe₃O₄ nanoparticles were added to the polyvinyl alcohol (PVA). Nanoparticles can enhance the thermal stability and flame retardant property of the PVA matrix.     

Piezoelectricity and flexoelectricity of sodium bismuth titanate-based ceramics

Although sodium bismuth titanate-based lead-free piezoelectric ceramics are a good substitute for lead oxide-based piezoelectric ceramics, their piezoelectric properties are still inferior to those of lead oxide-based ceramics. In this paper, the variation of piezoelectricity of sodium bismuth titanate-barium titanate (NBBT) ceramics after high temperature reduction was researched. A kind of ceramic with piezoelectric and flexoelectric properties was obtained by reducing one surface of ceramics at a high temperature. The relationship between the reduction structure and piezoelectric properties was studied by analyzing the crystal structure and microstructure of the original samples and the reduced samples. It was found that the piezoelectricity of NBBT was improved after reduction, in which the piezoelectric constant d₃₃ reached 133 pC/N, while it was 120 pC/N before reduction. The maximum displacement of the bipolar strain curve increased 50% after reduction. The change of properties is attributed to the combination of the flexoelectric effect generated by the reduction and the original piezoelectric effect.     

Dual plasma synthesis and characterization of a stable copper-ethylene glycol nanofluid

This article addresses the issue of metallic nanoparticle suspension (nanofluid) stabilization without the use of traditional surface-stabilizing agents such as surfactants. A dual plasma process for the single-step synthesis of a copper-ethylene glycol nanofluid is presented. The process uses a combination of low-pressure pulsed cathodic arc erosion to generate metal nanoparticles and in-flight radio-frequency glow discharge plasma functionalization for surface stabilization. The surface-stabilized nanoparticles are collected in situ by a falling film of ethylene glycol, the vapours of which are used for plasma functionalization. Little to no agglomeration is observed in the suspension, although this process is shown to occur when the produced suspensions are heated to temperatures well beyond the range of effectiveness of typical surfactants. This thermally induced agglomeration is demonstrated to be fully reversible. The stability of the suspension is confirmed by UV-Vis absorption spectrometry and electron microscopy. The particles synthesized are also shown to be re-dispersible in aqueous media. Though no heat transfer enhancement was observed due to the low nanoparticle loading, this study demonstrates for the first time that high temperature-stable nanofluids containing metal nanoparticles can be synthesized in a single-step, without manipulation outside the controlled synthesis environment and without the post-addition of surfactants.     

Facile preparation of Bi nanoparticles by novel cathodic dispersion of bulk bismuth electrodes

A novel electrochemical approach has been developed to prepare clean bismuth nanoparticles (NPs) with a bulk Bi electrode in a 0.5 mol dm⁻³ NaOH solution under highly cathodic polarization of −8 V versus a saturated mercurous sulfate electrode, requiring no any precursor ions and organic protective agents. The bulk Bi electrode can be facilely dispersed into Bi NPs at the condition of intensive hydrogen evolution. This cathodic dispersion of the bulk Bi electrode involves the formation and decomposition of unstable bismuth hydrides and the aggregation of atomic bismuth from the decomposition. Moreover, Bi₂O₃ NPs have also been achieved by heating the precursor Bi NPs. Field-emission scanning electron microscopy, transmission electron microscope and X-ray diffraction were used to characterize these NPs. The as-prepared Bi NPs mainly existed in rhombohedral phase.     

Regulation of bismuth valence in nano-porous silica glass for near infrared ultra-wideband optical amplification

The ultra-wideband near-infrared (NIR) emission of Bi-doped glass and fiber makes it play an increasingly important role in laser technology and optical communication system. However, challenge remains to grasp the original of NIR emission and manipulate the valence state of bismuth for efficient Bi-doped fiber lasers and amplifiers. In this article, we present a facile method to control the valence of bismuth by using one kind of porous host, the nano-porous silica glass (NPSG). Bi-doped silicate glass with novel NIR emission was obtained in NPSG based on glass phase separation technology (GPST). The valence state and NIR luminescence of bismuth can be controlled by sintering of NPSGs impregnated with ${\text{Al}}^{3+}$ and ${\text{P}}^{5+}$ ions in inert atmosphere. Three kinds of bismuth active centers (BACs) with their own fluorescence characteristics are formed in NPSG. $\text{Al}$ can disperse bismuth active centers (BACs) to enhance O band NIR emission; $\text{P}$ can promote the reduction of bismuth in glass to produce up-conversion and S+C+L+U band NIR emission. This work pushes us to take a big step towards understanding the truth of NIR emission of bismuth. The GPST may serve as key technology for efficient Bi-doped fiber lasers and amplifiers.     

Low-temperature synthesis of redispersible iron oxide nanoparticles under atmospheric pressure and ultradense reagent concentration

Highly dispersed α-Fe₂O₃ nanoparticles ca. 3 to 8 nm in diameter were prepared at atmospheric pressure, low temperature, and at an ultradense reagent concentration by titrating an aqueous ammonia solution into a dense iron oleate/toluene mixture. A transparent suspension was obtained by redispersing the prepared particles in nonpolar solvents since they were redispersible to primary particles without aggregate formations. The prepared particles were characterized by TEM, XRD, and FT-IR, and their dispersion stability in organic solvents was determined by dynamic light scattering (DLS) and viscosity measurements. In order to analyze the formation process of the highly dispersed α-Fe₂O₃ nanoparticles, time-course measurements of DLS and viscosity during the nanoparticle synthesis in toluene were carried out. A significant increase in the suspension viscosity and the formation of an aggregated structure were observed as soon as the titration of the aqueous ammonia solution. The suspension viscosity and aggregated particle size gradually reduced with continuous vigorous stirring; finally, α-Fe₂O₃ nanoparticles that were completely redispersible in nonpolar solvents were obtained after ca. 24 h. The particle size could be controlled by the synthesis temperature, and such redispersible α-Fe₂O₃ nanoparticles were obtained even when the reagent concentration was increased to 2.8 mol/L.     

Electrodeposition of bismuth from nitric acid electrolyte

The electrodeposition of Bi from acidic nitrate solution was examined. Bismuth deposition was determined to be quasi-reversible on Au, with a current efficiency of 100%, based on integration of deposition and stripping voltammetric waves. No interference from nitrate reduction was found on Bi and Au, whereas nitrate reduction occurred on W and Cu electrodes. Analysis of current-time transients clearly shows nucleation on Au to be instantaneous. Field emission SEM revealed nodular deposits with moderate surface roughness. Nodule size varied from 1 to 5 μm depending on deposition potential and deposit thickness. X-ray diffraction (XRD) patterns for all deposits were indexed to rhombohedral bismuth. The deposits showed fairly strong (0 1 2) crystallographic texture at high deposition overpotentials.