Shiming Zhou

Mono-Mercury Doping of Au25 and the HOMO/LUMO Energies Evaluation Employing Differential Pulse Voltammetry

Controlling the bimetal nanoparticle with atomic monodispersity is still challenging. Herein, a monodisperse bimetal nanoparticle is synthesized in 25% yield (on gold atom basis) by an unusual replacement method. The formula of the nanoparticle is determined to be Au24Hg1(PET)18 (PET: phenylethanethiolate) by high-resolution ESI-MS spectrometry in conjunction with multiple analyses including X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). X-ray single-crystal diffraction reveals that the structure of Au24Hg1(PET)18 remains the structural framework of Au25(PET)18 with one of the outer-shell gold atoms replaced by one Hg atom, which is further supported by theoretical calculations and experimental results as well. Importantly, differential pulse voltammetry (DPV) is first employed to estimate the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) energies of Au24Hg1(PET)18 based on previous calculations.

Size-dependent magnetic properties and Raman spectra of La2NiMnO6 nanoparticles

The structure and magnetic properties of La2NiMnO6 nanoparticles with sizes ranging from about 16 to 66 nm synthesized by a sol-gel method have been investigated. The magnetic measurements reveal that the rates of disordered phases increase as particle size decreases, which can be deduced from the changes of Curie temperature and the occurrence of glasslike behaviors, and the similar saturation magnetization was obtained at 4 K. While the particle size becomes smaller, two broad peaks of Raman spectrum at around 535 and 670 cm−1, which are assigned to antisymmetric stretching mode and symmetric stretching mode, move to higher and lower frequencies than the corresponding modes of the bulk compound, respectively. The two peaks also become broader, while the intensity becomes relatively weaker with the decrease in nanoparticle size. All results support that the trend of Mn4++Ni2+→Mn3++Ni3+ bring locally cooperative Jahn–Teller deformation and charge disproportion, and demonstrate that the nanometer effect is...

Size-dependent exchange bias in half-doped manganite nanoparticles

Magnetic properties of the nanosized half-doped manganite of Sm0.5Ca0.5MnO3 with different particle sizes prepared by a sol-gel method are investigated. Exchange-bias phenomena are observed in the field-cooled magnetic hysteresis loops for these nanoparticles. The values of the exchange field, coercivity, remanence asymmetry, and remanent magnetization are found to depend strongly on the particle size. Particularly, as the particle size decreases, the exchange field shows a nonmonotonic variation with a maximum at ∼120 nm. These magnetic behaviors have been explained in terms of size effects on the charge ordered and antiferromagnetic manganite nanoparticles.

Evidence of short-range magnetic ordering above TC in the double perovskite La2NiMnO6

The magnetic properties of the double perovskite La2NiMnO6 polycrystalline sample have been investigated by magnetometry and electron spin resonance. The magnetization curve shows a paramagnetic-ferromagnetic transition at TC∼280K. However, the electron spin resonance spectra reveal that a pure paramagnetic regime only exists above T*∼390K in which the behaviors of the spectra can be well described by the small polaron hopping model. Between TC and T*, a short-range magnetic ordering appears which maybe derives from the correlation of those magnetic polarons. The possible relationship between those polarons and the observed multifunctions in La2NiMnO6 is briefly discussed.

Engineering electrocatalytic activity in nanosized perovskite cobaltite through surface spin-state transition.

The activity of electrocatalysts exhibits a strongly dependence on their electronic structures. Specifically, for perovskite oxides, Shao-Horn and co-workers have reported a correlation between the oxygen evolution reaction activity and the eg orbital occupation of transition-metal ions, which provides guidelines for the design of highly active catalysts. Here we demonstrate a facile method to engineer the eg filling of perovskite cobaltite LaCoO3 for improving the oxygen evolution reaction activity. By reducing the particle size to ∼80 nm, the eg filling of cobalt ions is successfully increased from unity to near the optimal configuration of 1.2 expected by Shao-Horns principle. Consequently, the activity is significantly enhanced, comparable to those of recently reported cobalt oxides with eg∼1.2 configurations. This enhancement is ascribed to the emergence of spin-state transition from low-spin to high-spin states for cobalt ions at the surface of the nanoparticles, leading to more active sites with increased reactivity.

Near room-temperature magnetoresistance effect in double perovskite La2NiMnO6

Near room-temperature magnetoresistance (MR) effect in ferromagnetic semiconductor La2NiMnO6 has been reported. The magnetic field dependent MR curve exhibits two distinct regions: the low-field MR which shows a rapid increase in an applied field within 200 Oe and the high-field MR which shows a relatively slow increase with the field increasing (more than 200 Oe). Based on the analyses of magnetic and electronic properties, we suggest that the low-field MR comes from the increased electron tunneling probability accompanied by the alignment of ferromagnetic domains under external field and the high-field MR is due to the suppression of scattering from the spin defects arising from the Ni/Mn antisite disorders.

A low-dimensional molecular spin system with two steps of magnetic transitions and liquid crystal property.

A low-dimensional compound [C(6)-Apy][Ni(mnt)(2)] (1, where mnt(2-) = maleonitriledithiolate, C(6)-Apy(+) = 4-amino-1-hexylpyridinium) has been designed and synthesized, which has layer arrangement of anions and cations and shows two steps of magnetic transitions. The low temperature magnetic transition has an uncommon hysteresis loop, while the crystal structure investigations disclosed no structural transition with the magnetic transition. The high temperature magnetic transition exhibits two remarkable features: (1) it synchronously occurs with a crystalline-to-mesophase transition in the first heating process and (2) the structural changes that accompany the solid-mesophase transition are irreversible. A diamagnetic and isostructural compound, [C(6)-Apy][Cu(mnt)(2)], is further characterized by structure, DSC and POM techniques, which revealed also the existence of an irreversible crystalline-to-mesophase transition in the same temperature interval of [C(6)-Apy][Ni(mnt)(2)]. Therefore, the high-temperature magnetic transition in 1 is driven by release of the structural strains, but not magnetoelastic interactions. The mesophase exhibits the characteristic of smectic A phase, and the alkyl chain melting in the cation layers probably lead to the formation of mesophase. It is noticeable that the finding of a mesophase occurring in a hexyl hydrocarbon chain molecular system is in contrast to a suggested rule that at least a dodecyl chain is required. Our results will shed a light on the design and preparation of a new low-dimensional molecular system combining magnetic transition and liquid crystal properties.

Nature of short-range ferromagnetic ordered state above TC in double perovskite La2NiMnO6

Magnetic measurements of La2NiMnO6 reveal that its inverse susceptibility exhibits an upward derivation from Curie–Weiss behavior above TC, indicating the presence of a short-range ferromagnetic ordered state. It is suggested that the upward derivation is due to antiferromagnetic interactions arising from anitisite defects, which is supported by the magnetic studies on the samples where the antiferromagnetic interactions are modulated by partially doping La with Sr or by decreasing the sample size. Basing on these results, we propose that the short-range ferromagnetic ordering is induced by antisite defects against long-range ferromagnetic ordering of the Ni/Mn sublattice.

Griffiths phase, spin-phonon coupling, and exchange bias effect in double perovskite Pr2CoMnO6

The ceramic Pr2CoMnO6 of double perovskite structure is prepared by a solid-state reaction and the magnetic properties, phonon behaviors are studied in detail. Two ferromagnetic transitions at TC1 ∼ 172 K and TC2 ∼ 140 K are observed in the temperature-dependent magnetization curves, respectively. Furthermore, a detail analysis on the magnetic susceptibility reveals that a short-range ferromagnetic clustered state exists above TC1, which can be well described as the Griffiths phase with a well-defined Griffiths temperature TG ∼ 210 K. The presence of the B-site antisite defects is considered to contribute to the observed Griffiths singularity. Temperature-dependent Raman scattering experiment reveals an obvious softening of the phonon mode involving stretching vibrations of the (Co/Mn)O6 octahedra in FM temperature regions, indicating a close correlation between magnetism and lattice in Pr2CoMnO6. On the other hand, it is found that the phonon softening extends up to TG, which further confirms the preform...

Size-dependent structure and magnetic properties of DyMnO3 nanoparticles

The structure and magnetic properties of orthorhombic DyMnO3 nanoparticles with different particle sizes are investigated in this paper. With decreasing particle size, all the lattice parameters a, b, and c gradually decrease, whereas the orthorhombic distortion increases. Magnetic measurements reveal that the antiferromagnetic interaction of Mn ions is weakened due to the decrease in Mn-O-Mn bond angle. Above a critical field H*, DyMnO3 undergoes a field-induced metamagnetic transition at 4 K, which is related to the spin reversal of Dy moments. The critical field H* increases monotonically with size reduction, indicating an enhancement of the antiferromagnetic interaction of Dy ions due to the decreased distance between rare earth ions. The magnetization at 4 K and 5 T, i.e., M(4 K, 5 T) shows a non-monotonic variation with particle size d, i.e., M(4 K, 5 T) initially increases with size reduction but decreases again for d < 68 nm. A modified core-shell model, in which the ferromagnetic ordering (Dy mag...