Aixian Shan

Monodispersed, ultrathin NiPt hollow nanospheres with tunable diameter and composition via a green chemical synthesis

Monodispersed NiPt hollow nanospheres with citric acid as surfactant were synthesized by a one-pot wet chemical method in water at room temperature. The diameter is adjustable from 13 to 100 nm with the shell thickness of 2–3 nm, roughly the size of nanocrystallites. The composition is also tunable with the ratio of Ni : Pt from 1 : 9 to 9 : 1. A phase formation mechanism is proposed for the controllability of the diameter from the sacrificial templates of intermediate Ni–B amorphous nanocompounds. Molecular dynamic calculations show that 2 nm is the lower-bound shell thickness for a stable hollow spherical structure to avoid distortion or collapse. Electrocatalytic properties for the oxidation of ethylene glycol are measured for 35 nm Ni50Pt50 hollow nanospheres. Its performance is apparently better than that using commercial Pt/C catalyst. This makes it a potential application as a fuel cell catalyst.

Phase formations and magnetic properties of single crystal nickel ferrite (NiFe2O4) with different morphologies

Nickel ferrite (NiFe2O4) nanomaterials with different morphologies, including nano-spheres (10–25 nm in diameter), nano-rods (50–60 nm in diameter and ~1 μm in length), and nano-octahedrons (side length ~150 nm), have been synthesized by a single mild hydrothermal method at 160 °C without any surfactant. The crystal structures have been investigated by TEM, HRTEM, and studied by simulations using the program Materials Studio. The variations in the morphology, as well as the preferential crystal growth directions, depend only on the pH value of the reaction solution. A phase formation mechanism is thus proposed. Magnetization measurements at T = 300 K indicate that the NiFe2O4 nano-spheres of 10–25 nm diameter are superparamagnetic with non-saturating magnetization at H = 7 kOe. The saturation magnetization, MS, of the nanorods is 40 emu g−1, less than the bulk value, MS = 46.7 to 55 emu g−1. The coercivity is HC = 40 Oe, reduced from the bulk value of 100 Oe. On the other hand, the nano-octahedrons have a saturation magnetization of MS = 50 emu g−1, the same as the bulk value. However the coercivity, HC = 50 Oe is also much reduced from the bulk value.

Magnetic anisotropy and anomalous transitions in TbMnO3 thin films

TbMnO3 thin films with different crystallographic orientations were deposited epitaxially on LaAlO3 [100] and SrTiO3 [100] single crystal substrates by using pulsed laser deposition. Magnetization measurements were performed along the [110], [−110], and [001] directions of the films. Obvious magnetic anisotropy and low temperature ferromagnetism were found. Transitions at T ∼ 10, 32, 120, and 125 K along different directions are observed. The susceptibility anomalies, remarkably anisotropic, are discussed on the frame of the domain wall and strain-induced distortion. Particularly, the lattice mismatch and the anisotropic thermal expansion between the films and substrates are likely responsible for the distortion behaviors.

Phase formation, magnetic and optical properties of epitaxially grown icosahedral Au@Ni nanoparticles with ultrathin shells

The synergistic effect between the metallic elements in the core–shell nanostructures has attracted increasing interest. In the system of Au@Ni nanostructure, it is challenging to epitaxially grow Ni atoms on Au nanocrystals due to the large lattice mismatch. In this paper, Au@Ni core–shell nanostructures have been synthesized by a facile one-pot wet chemical method. The Ni shell is epitaxially grown on the (111) planes of the icosahedral Au cores. The diameter of the icosahedral Au core is about 10–20 nm and the thickness of the Ni shell is of only several nanometers, providing an ideal structure for the study of synergistic effect. The Curie temperature of the Ni shells is estimated to be lower than 400 K by the field-cooling/zero field-cooling M(T) measurements. It is suppressed considerably from that of the bulk phase, mainly attributed to the finite size effect. The optical properties of the Au@Ni core–shell nanostructures are studied by absorption spectroscopy. The spectral blue-shift tendency is consistent with the results described by the plasmon hybridization theory.

Co2P nanostructures by thermal decomposition: phase formation and magnetic properties

We report a controlled synthesis of Co2P nanostructures. The products showing Curie–Weiss paramagnetic behavior have an orthorhombic structure with novel morphology. Nanoneedles with a tip of less than 10 nm homogeneously grew out of the center. The effects of temperature and the morphological evolutions of the intermediates are investigated.

Magnetism in undoped ZnS nanotetrapods

The magnetism of undoped ZnS nanotetrapods, synthesized by a solvothermal method, has been investigated by magnetization measurements and first principle numerical calculations. The background magnetic impurity concentrations of Fe, Co and Ni were determined at ppm level by inductively coupled plasma mass spectrometry (ICP-MS). Hysteresis loops of weak ferromagnetism were observed, attributable to the magnetic impurities. However, the total magnetic moments analyzed from the paramagnetism are far beyond the explanations from the presence of these magnetic impurities, by about two orders of magnitude larger. It implies a different origin of the magnetic moments. Electron microscopy analysis reveals that there are defects in the sample. Numerical simulations indicate that the excessive magnetic moments might arise from the local band structure of polarized electrons associated with the defects of cation deficiency. This study elaborates on the understanding of magnetic properties in the non-magnetic II-VI semiconductor nanomaterials.

Phase formations, magnetic and catalytic properties of Co3O4 hexagonal micro-boxes with one-dimensional nanotubes

One-dimensional Co3O4 nanotubes were grown in alignment on the surface of micron-sized Co3O4 hexagonal-box via a mild template-free wet chemical method. The nanotubes are ∼250 nm in diameter with several micrometers in length. The growth mechanism and phase formation for the vertically grown nanotubes on the surface of the boxes have also been investigated. The intermediates of α-Co(OH)2 and β-Co(OH)2 towards the final products of Co3O4 are also characterized. The magnetization, M(T), measurements for the micro-boxes with nanotubes reveals that the Neel temperature is suppressed from the bulk value of 40 K down to 22 K, possibly due to finite size effect. The Curie–Weiss analysis on the paramagnetic behavior above the Neel temperature gives the average effective moment of 4.4 μB per formula unit, slightly higher than 4.14 μB of its bulk counterparts. The electrocatalytic experiments for the oxidation of methanol reveal that the final product exhibits better catalytic performance compared with the intermediate product due to its higher specific surface area.

Morphology-Controlled Synthesis of Hematite Nanocrystals and Their Optical, Magnetic and Electrochemical Performance

A series of α-Fe2O3 nanocrystals (NCs) with fascinating morphologies, such as hollow nanoolives, nanotubes, nanospindles, and nanoplates, were prepared through a simple template-free hydrothermal synthesis process. The results showed that the morphologies could be easily controlled by SO42− and H2PO4−. Physical property analysis showed that the α-Fe2O3 NCs exhibited shape- and size-dependent ferromagnetic and optical behaviors. The absorption band peak of the α-Fe2O3 NCs could be tuned from 320 to 610 nm. Furthermore, when applied as electrode material for supercapacitor, the hollow olive-structure exhibited the highest capacitance (285.9 F·g−1) and an excellent long-term cycling stability (93% after 3000 cycles), indicating that it could serve as a candidate electrode material for a supercapacitor.

Giant enhancement and anomalous temperature dependence of magnetism in monodispersed NiPt2 nanoparticles

A simple yet general one-step solvothermal method is applied to synthesize sub-7 nm monodispersed single-crystal NiPt2 nanoparticles (NPs) with the morphology of truncated octahedrons in the alloying state of disordered atomic arrangements. The effective magnetic moments of these NPs exhibit an anomalous temperature dependency, increasing from approximately 0.9 μB/atom at 15 K to 1.9 μB/atom at 300 K. This is an increase by a factor of more than three compared with bulk Ni. On the basis of experiments involving X-ray absorption near-edge spectroscopy of the L3 edge for Pt and density functional theory calculations, the observed novel magnetism enhancement and its anomalous temperature dependence are attributed to the electron transfer arising from the thermal-activation effects.