Tiandou Hu

Vacancy-Induced Ferromagnetism of MoS2 Nanosheets

Outstanding magnetic properties are highly desired for two-dimensional ultrathin semiconductor nanosheets. Here, we propose a phase incorporation strategy to induce robust room-temperature ferromagnetism in a nonmagnetic MoS2 semiconductor. A two-step hydrothermal method was used to intentionally introduce sulfur vacancies in a 2H-MoS2 ultrathin nanosheet host, which prompts the transformation of the surrounding 2H-MoS2 local lattice into a trigonal (1T-MoS2) phase. 25% 1T-MoS2 phase incorporation in 2H-MoS2 nanosheets can enhance the electron carrier concentration by an order, introduce a Mo(4+) 4d energy state within the bandgap, and create a robust intrinsic ferromagnetic response of 0.25 μB/Mo by the exchange interactions between sulfur vacancy and the Mo(4+) 4d bandgap state at room temperature. This design opens up new possibility for effective manipulation of exchange interactions in two-dimensional nanostructures.

New observations on the optical properties of PPV/TiO2 nanocomposites

Abstract The optical properties of poly(phenylenevinylene) (PPV)/TiO 2 nanocomposites, prepared from mixtures of PPV precursor and titanium butoxide ethanol solution in a sol–gel process, are investigated by UV–VIS, FT-IR, PL spectroscopy and TEM. The TEM images showed that the TiO 2 nano-aggregates took the form of a sphere and finally ellipsoid with an alignment, as the content of TiO 2 increased. FT-IR spectra indicated that the titanium butoxide hydrolyzed to form Ti organic compound, which can result in the alignment structure of TiO 2 nanoparticles; at the same time, PPV in TiO 2 matrix had the trans -PPV configuration. PL spectra revealed that the light emission of the PPV/TiO 2 nanocomposites was from PPV, and the relative intensity of the vibrant components changed with the formation of the TiO 2 alignment structure. These phenomena suggest that the TiO 2 matrix can form confined environments to control PPV molecular aggregate states and further effect the optical property of PPV.

Hexane-Driven Icosahedral to Cuboctahedral Structure Transformation of Gold Nanoclusters

Whether and how nanoclusters possessing a rich diversity of possible geometric configurations can transform from one structural type to another are critical issues in cluster science. Here we demonstrate an icosahedral-to-cuboctahedral structural transformation of Au nanoclusters driven by changing the chemical environment. For icosahedral Au(13) clusters protected by a mixture of dodecanethiol and triphenylphosphine ligands, solvent exchange of ethanol by hexane leads to quick selective desorption of the thiolate layers from the cluster surface. The surviving Au cores then undergo a much slower energy-minimization process via structural rearrangement, stabilized in the cuboctahedral structure and protected by triphenylphosphine in the hexane environment. In response to the dramatically changed atomic structure, the character of the electronic structure of the Au clusters is converted from semiconducting to metallic. This work addresses the structure-property correlation and its strong dependence on the chemical environment for metal nanoclusters.

Oxygen vacancy effect on room-temperature ferromagnetism of rutile Co:TiO2 thin films

The x-ray absorption near edge structure spectroscopy and first-principles calculations were combined to study the local and electronic structures of rutile Co:TiO2 thin film with room-temperature ferromagnetism. It was revealed that the CoTi2+ substituting the Ti site forms CoTi2+-VO complex with the O vacancy generated in the annealing process. The O vacancy induces a spin-split donor impurity band with a t2g character within the gap region. We proposed that the strong exchange interaction between the localized carriers captured by O vacancy and the substitutional Co ions leads to the ferromagnetism of the Co:TiO2 thin film.

XANES study on the valence transitions in cerium oxide nanoparticles

The aim of this work is the determination of Ce environment and valence state in Cerium oxide nanoparticles prepared by the microemulsion method. X-ray absorption near-edge structure measurements at Ce L3 edge were performed on the nanoparticles as a function of annealing temperature, ranging from 298K to 873K under air condition. The experimental results support the conclusion that Ce ion, in the investigated systems, is in trivalence state when the annealing temperature is below 473K. As the temperature increases up to 623K, the XANES spectrum shows the coexistence of Ce3+ and Ce4+ states. When the temperature is higher than 623K, the spectra become identical to that of CeO2 with a distinct double-peak structure, corresponding to the Ce4+ state.

Study on the optical properties of PPV/TiO2 nanocomposites

The optical properties of poly(phenylenevinylene) (PPV)/TiO2 nanocomposites, prepared via the sol-gel process, were investigated by UV-VIS, FT-IR, FL spectroscopy and TEM. TEM images indicated that the surfaces of TiO2 nanoparticles became rough, and finally formed the connected network as the TiO2 content increased. FT-ER spectra showed the titanium butoxide hydrolyzed to form Ti organic compound, which can result in the connected network of TiO2 nanoparticles. The conjugation of PPV polymer chains can be interrupted by the TiO2 network structure, as evidenced in UV-VIS spectra. The investigation of the PL spectra revealed that the emitted light of the PPV/ TiO2 nanocomposites blue shifted without fine structure,and the PL intensity enhanced, when the TiO2 network formed. These phenomena suggested that the optical properties of the PPV/TiO2 nanocomposites were dependent on the interfacial structure between PPV acid TiO2 nanoparticles

Unidirectional Thermal Diffusion in Bimetallic Cu@Au Nanoparticles

Understanding the atomic diffusions at the nanoscale is important for controlling the synthesis and utilization of nanomaterials. Here, using in situ X-ray absorption spectroscopy coupled with theoretical calculations, we demonstrate a so far unexplored unidirectional diffusion from the Au shell to the Cu core in thermally alloying Cu@Au core@shell architecture of ca. 7.1 nm. The initial diffusion step at 423 K is found to be characterized by the formation of a diffusion layer composed of a Au-dilute substitutional CuAu-like intermetallic compound with short Cu-Au bond length (2.61 Å). The diffusion further happens by the migration of the Au atoms with large disorder into the interior Cu matrix at higher temperatures (453 and 553 K). These results suggest that the structural preference of a CuAu-like compound, along with the nanosized effect, plays a critical role in determining the atomic diffusion dynamics.

XAFS at Eu–L3 edge and UV–VUV excited luminescence of europium doped strontium borophosphate prepared in air

The local structure and the valences of europium in SrBPO5:Eu prepared in air were checked by means of XAFS at Eu-L-3 edge. From the EXAFS results, it was discovered that the doped europium atoms were nine-coordinated by oxygen atoms and the distances of bond Eu-O were 2.42 Angstrom in the host. From the XANES data, it was found that the divalent and trivalent europium coexisted in the matrix. The emission spectra excited by VUV or UV exhibited a prominent broad band due to the 4f(6)5d-4f(7) transition of Eu2+ ions, which indicated that the trivalent europium ions were reduced in air in the matrix at high temperature by the defects [V-Sr] formed by aliovalent substitution between Sr2+ and Eu3+ ions. The VUV excitation spectra in 100-200 nm range showed that the matrix had absorption bands with the maxima at about 130 and 150 nm, respectively.

An EXAFS study on the local structure around iron in atmospheric aerosols collected in the Qingdao area

The existing forms of Fe are of great interest since they have a profound effect on the biological availability of Fe. In this work, aerosol samples collected in different seasons and at different locations in the Qingdao region were examined by means of extended X-ray absorption fine structure (EXAFS) K-edge analysis of Fe, X-ray diffraction (XRD) and Fe content analysis. The results showed that an iron ion in aerosol particles is surrounded on average by 5.8 (coordinated) O ions. For the six samples examined, the coordination number of the first Fe-O coordination subshell is always 3 with a coordination distance (with O) in the range of 1.952~1.966±0.002 Å, while the coordination number of the second subshell varies from 2.2 to 3.0 with a coordination distance of 2.108±0.002 Å. The coordination is approximately consistent with that of α-Fe2O3, suggesting that iron in aerosol samples is mainly present in the form of α-Fe2O3. The fact that the coordination number in the second subshell is smaller than that of α-Fe2O3 might be an indication that there is a small amount of FeO mixed with α-Fe2O3 in aerosol particles. Existence of FeO is confirmed by a later XRD experiment.

Adsorption kinetic process of thiol ligands on gold nanocrystals

Understanding the kinetic mechanism during ligand adsorption on gold nanocrystals is important for designing and fine-tuning their properties and implications. Here, we report a kinetic study on the adsorption process of dodecanethiol ligands on Au nanocrystals of 3.3 nm by an in situ time-resolved X-ray absorption fine structure technique. A two-step process of dodecanethiol adsorption on Au NC surfaces is proposed based on the obtained ligand coverage, which shows a quick increase from 0 to 0.40 within the first 20 min, followed by a much slower increase to the limiting value of 0.94. In-depth analysis suggests that the first stage involves the quick adsorption of dodecanethiol to the corner and edge sites of Au NCs surfaces, leading to remarkable surface Au-Au bond length relaxation (from 2.79 to 2.81 Å) and pronounced gold-to-ligand charge transfer. The second step that corresponds to the much slower adsorption process to the surface facets could be described by the Langmuir kinetics equation with an adsorption rate constant of 0.0132 min(-1) and an initial coverage of 0.41, in good agreement with the initially preferable adsorption of thiols to the most favorable sites.