Nguyen Dang Phu

Crystallization process and magnetic properties of amorphous iron oxide nanoparticles

This paper studied the crystallization process, phase transition and magnetic properties of amorphous iron oxide nanoparticles prepared by the microwave heating technique. Thermal analysis and magnetodynamics studies revealed many interesting aspects of the amorphous iron oxide nanoparticles. The as-prepared sample was amorphous. Crystallization of the maghemite ?-Fe2O3 (with an activation energy of 0.71?eV) and the hematite ?-Fe2O3 (with an activation energy of 0.97?eV) phase occurred at around 300??C and 350??C, respectively. A transition from the maghemite to the hematite occurred at 500??C with an activation energy of 1.32?eV. A study of the temperature dependence of magnetization supported the crystallization and the phase transformation. Raman shift at 660?cm?1 and absorption band in the infrared spectra at 690?cm?1 showed the presence of disorder in the hematite phase on the nanoscale which is supposed to be the origin of the ferromagnetic behaviour of that antiferromagnetic phase.

Magnetic properties of FePt nanoparticles prepared by sonoelectrodeposition

Sonoelectrodeposition is a useful technique to make metallic nanoparticles, using ultrasound during electrodeposition to remove nanoparticles as they grow on the cathode surface. This paper reports some structural and magnetic properties of FePt nanoparticles prepared by this method. The as-prepared Fe45Pt55 nanoparticles were ferromagnetic at room temperature. Upon annealing at 700°C for 1 h under H2 atmosphere, the saturation magnetization and the coercivity of the nanoparticles were improved significantly. The annealed nanoparticles showed a high coercivity of 13.5 kOe at 2 K and of 9 kOe at room temperature. Sonoelectrodeposition is a promising technique to make large quantity of FePt nanoparticles.

Arsenic removal from water by magnetic Fe1− x Co x Fe2O4 and Fe1− y Ni y Fe2O4 nanoparticles

This article studies the effects of Co and Ni replacement in Fe1− x Co x Fe2O4 and Fe1− y Ni y Fe2O4 (x, y = 0, 0.05, 0.1, 0.2, 0.5) nanoparticles, pH, weight of nanoparticles/mL of water, and time of stirring on the arsenic removal ability. The results showed that a small amount (0.25 g L−1) of Fe3O4 nanoparticles after stirring time of 3 min can reduce the arsenic concentration from 0.1 to 0.01 mg L−1. The removal was also affected by the pH of the water. Absorption of arsenic by nanoparticles was effective when pH was smaller than seven and reduced with the increase of pH. At pH of 13, there was a strong release of arsenic ions from arsenic-absorbed nanoparticles back to water. The time of stirring was studied from 1 min to 2 h and the optimal time was about few minutes. Co and Nis presence was reported to keep saturation magnetisation stable under working conditions. For Co replacement, absorption does not change significantly when x ≤ 0.1 and slightly reduces when x > 0.1. The presence of Ni improved the absorption in most cases.

Co–Pt nanoparticles encapsulated in carbon cages prepared by sonoelectrodeposition

Co-Pt nanoparticles encapsulated in carbon cages have been prepared by sonoelectrodeposition followed by annealing in a CO atmosphere. Sonoelectrodeposition is a useful technique to make metallic nanoparticles, using ultrasound during electrodeposition to remove nanoparticles as they grow on the cathode surface. We used an electrolyte containing chloroplatinic acid and cobalt chloride and found that the atomic ratio of Co:Pt in the as-formed materials varied from 0.2 to 0.8 as the deposition current density was changed from 15 to 35 mA cm(-2). However, the as-deposited materials were inhomogeneous, comprising a mixture of Pt-rich and Co-rich nanoparticles. X-ray diffraction indicated that subsequent heat treatment (700 °C for 1 h) under CO gas created an ordered CoPt alloy phase that exhibited hard magnetic properties. Transmission electron microscopy showed many of the resulting nanoparticles to be encapsulated in carbon cages, which we ascribe to Co-catalyzed decomposition of CO during annealing. The thickness of the carbon cages was about ten layers, which may have helped reduce sintering during annealing. The size of the resultant nanoparticles was about 100 nm diameter, larger than the typical 5-10 nm diameter of as-deposited nanoparticles.

Control of crystal phase of BiVO4 nanoparticles synthesized by microwave assisted method

Different crystalline phases BiVO4 nanoparticles (tetragonal-zircon, monoclinic-scheelite, and tetragonal-zircon/monoclinic-scheelite heterophase) have been prepared by fast microwave assisted method and annealing treatment. For the heterophase, the ratio of tetragonal-zircon and monoclinic-scheelite phases can be well controlled by controlling the annealing temperature. Furthermore, a tight interface junction has been formed between tetragonal-zircon BiVO4 and monoclinic-scheelite BiVO4 in a nanosize level. This tight interface junction can modify the electronic structure of BiVO4 nanoparticles, which would be very helpful for achieving high photocatalytic activity.

Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Gd-doping via microwave assisted method

Gd-doped Bi2WO6 nanoparticles were successfully prepared through microwave assisted synthesis. The incorporation of Gd3+ ions into Bi2WO6 crystallite was analyzed by X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The optical properties of the nanoparticles were characterized by UV–Vis absorption and photoluminescence spectroscopy. The photocatalytic activity of the nanoparticleswas investigated by degradation of Rhodamine B under visible-light irradiation. Our results showed that Gd-doping plays an important role for enhancing the visible light photocatalytic activity, and the enhanced photocatalytic activity of Gd-doped Bi2WO6 nanoparticles would be mainly correlated with the effective decrease of recombination rate of photogenerated electron–hole pairs.

Influence of Mn Doping on the Structure, Optical,and Magnetic Properties of

In this paper, the influences of the Mn2+ doping on the structure, optical, and magnetic properties of PbTi1-x MnxO3 (with x = 0.00, 0.03, 0.06, 0.08, 0.10, and 0.12), prepared by the sol-gel method have been investigated by X-ray diffraction (XRD), Raman scattering, optical absorption, and vibrating sample magnetometer measurements. The detailed analysis of XRD patterns showed that the tetragonal lattice distorted ratio (c/a) decreased undesirably with the increasing of Mn content. However, c/a ratio increased again after calcining at higher temperature. The shift to the lower wavenumber and the broadening of some Raman peaks, such as E(2TO) and A1(2TO), indirectly indicated the incorporation of Mn2+ ions into PbTiO3 (PTO) lattice. Absorption spectra exhibited a remarkable decrease of effective optical bandgap (Eg) from 2.99 eV for pure PTO to 1.5 eV for Mn-doped PTO. Therefore, Mn-doped PTO material may be considered as a promising material for photovoltaic and photocatalytic application. In addition, the diamagnetic nature of pure PTO disappeared and the ferromagnetism gradually increased as the Mn content increases. The saturation magnetization Ms varied from 0.014 emu/g for pure PTO to 0.064 emu/g for the PbTi0.88 Mn0.12O3 (Mn12) sample. The nature of this ferromagnetic order in samples could be assigned to the incorporation of ion Mn2+ in PTO lattice.

{\rm PbTiO}_{3}

The Bi2WO6/BiVO4 nanocomposites were successfully synthesized using fast microwave assisted method. The photocatalytic activities of Bi2WO6/BiVO4 nanocomposites were evaluated by the degradation of Rhodamine B under visible irradiation. The optimal BiVO4 content of 30% was observed for improving the photocatalytic activity of Bi2WO6/BiVO4 nanocomposites, and the mechanism of photocatalytic activity enhancement was investigated. Our results suggest that for the Bi2WO6/BiVO4 nanocomposites, particle surface area plays the most important role for improving photocatalytic activity, and electron–hole recombination rate plays more important role than amount of light absorbed by nanocomposites for improving photocatalytic activity.Graphical abstract

Material

We investigate the crystallization process of NiFe2O4 nanoparticles prepared by microwave heating technique, which were then characterized using X-ray diffraction, high-resolution transmission electron microscopy, the differential scanning calorimetry, Raman scattering, Fourier transformed infrared spectra (FTIR), and magnetic measurements. The results showed that crystallization of the amorphous NiFe2O4 nanoparticles occurred at around 300 °C. The best crystallization quality would be obtained by annealing the sample at 500 °C. After crystallization, the sample shows ferromagnetic behavior with a saturation magnetization of about 30 emu/g. The FTIR and Raman measurements also provided further information about the crystallization process and the phase transformation of the NiFe2O4 nanoparticles.

Study of photocatalytic activities of Bi2WO6/BiVO4 nanocomposites

Amorphous CoFe2O4 nanoparticles have been successfully synthesized by the chemical microwave-assisted method. The crystallization process and the hard magnetic properties of prepared nanoparticles were systematically investigated in terms of thermal analysis and in situ measurement of magnetization dynamics. The as-prepared sample was amorphous, which was confirmed by various techniques, such as high resolution transmission electron microscopy, with electron diffraction and X-ray diffraction. The thermal analysis using differential scanning calorimetry showed that crystallization from CoFe2O4 amorphous state to CoFe2O4 crystalline started at 290°C. Crystallization activation energy was determined using the Kissinger model with a value of 1.09 eV. A study of the temperature dependence of magnetization supported the crystallization process of cobalt ferrite nanoparticles. Magnetic measurement indicated that the as-prepared amorphous particles were superparamagnetic. Compared with the bulk material, the observed value of saturation magnetization of the annealed sample was significantly low (27 emu/g) due to its nanocrystalline nature.