Tsan-Yao Chen

Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies.

Removal of arsenic (As) from water supplies is needed to reduce As exposure through drinking water and food consumption in many regions of the world. Magnetite nanoparticles (MNPs) are promising and novel adsorbents for As removal because of their great adsorption capacity for As and easy separation. This study aimed to investigate the adsorption mechanism of arsenate, As(V), and arsenite, As(III), on MNPs by macroscopic adsorption experiments in combination with thermodynamic calculation and microspectroscopic characterization using synchrotron-radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS spectra suggested predominant formation of bidentate binuclear corner-sharing complexes ((2)C) for As(V), and tridentate hexanuclear corner-sharing ((3)C) complexes for As(III) on MNP surfaces. The macroscopic and microscopic data conclusively identified the formation of inner-sphere complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the oxidation of MNPs, the oxidation of As(III) and MNPs was expected, but the observed As(V) reduction was surprising because of the role played by the reactive Fe(II).

Adsorption mechanism of selenate and selenite on the binary oxide systems.

Removal of selenium oxyanions by the binary oxide systems, Al- or Fe-oxides mixed with X-ray noncrystalline SiO(2), was previously not well understood. This study evaluates the adsorption capacity and kinetics of selenium oxyanions by different metal hydroxides onto SiO(2), and uses X-ray absorption spectroscopy (XAS) to assess the interaction between selenium oxyanions and the sorbents at pH 5.0. The binary oxide systems of Al(III)- or Fe(III)-oxides mixed with SiO(2) were prepared, and were characterized for their surface area, point of zero charge (PZC), pH envelopes, X-ray diffraction analysis (XRD), and then macro-scale adsorption isotherm and kinetics of selenite and selenate, micro-scale adsorption XAS. The adsorption capacity of selenite and selenate on Al(III)/SiO(2) is greater than on Fe(III)/SiO(2). Adsorption isothermal and kinetic data of selenium can be well fitted to the Langmuir isotherm and pseudo-second-order kinetic models. Based on simple geometrical constraints, selenite on both the binary oxide systems forms bidentate inner-sphere surface complexes, and selenate on Fe(III)/SiO(2) forms stronger complexes than on Al(III)/SiO(2).

Effects of Pt Shell Thicknesses on the Atomic Structure of Ru–Pt Core–Shell Nanoparticles for Methanol Electrooxidation Applications

In this research, core-shell electrocatalysts comprising a Ru core covered with precisely controlled 1.5-3.6 atomic layers (ALs)-thick Pt atoms are synthesized. The sample with 1.5 ALs shows a 3.2-fold improvement in CO-tolerance and 2.4-fold current enhancement at the conventional battery operation potential (I(300), at 300 mV vs Ag/AgCl) during methanol oxidation as compared with conventional all-Pt nanoparticles. The origin of the enhanced performance and the atomic structure of the core-shell nanoparticles are elucidated to be mainly dominated by the lattice strain (possibly some slight effect of heteroatomic interactions) then by the combination of ligand effects and bifunctional mechanisms when the shell crystal is thicker than 2.7 ALs.

Fractal aggregates of the Pt nanoparticles synthesized by the polyol process and poly(N-vinyl-2-pyrrolidone) reduction

Small-angle X-ray scattering was used to characterize the size and aggregation behavior of the Pt nanoparticles synthesized by the polyol process and the unusual poly(N-vinyl-2-pyrrolidone) (PVP) reduction. With formaldehyde (HCHO) as the reduction agent, the Pt nanoparticles synthesized in aqueous solutions with a high PVP/PtCl4 weight ratio were characterized by short rods with a 70% polydispersity in rod length. The size and size distribution of the rod-like Pt nanoparticles (3 nm in rod length and 2 nm in rod diameter) are consistent with the corresponding transmission electron microscopy image. With a comparable PVP/PtCl4 weight ratio in the aqueous solution containing HCHO, the high number density of reduced Pt nanoparticles led to a fractal-like aggregation with a fractal dimension of 2.1 and a correlation length of ~30 nm. We also demonstrated that Pt nanoparticles can be synthesized by PVP reduction at 323 K without HCHO. The particle size and the clustering behavior of the Pt nanoparticles reduced by PVP are closely related to the PVP concentration in the solution. Both the Pt nanoparticles synthesized in the commonly used polyol process and the unusual PVP reduction form fractal-like clusters via the PVP–metal nanoparticle association when the number density of the Pt nanoparticles in the solutions is high.

Tetragonal and hexagonal polymorphs of BaTi1−xFexO3−δ multiferroics using x-ray and Raman analyses

The effect of Fe doping on the crystalline phase transformation and on the local environment around Fe dopant ions is investigated for BaTi1−xFexO3−δ (0.0 ≤ x ≤ 0.5) polycrystalline samples, using x-ray diffraction, x-ray absorption spectroscopy, and Raman scattering spectroscopy. Our experimental results show that the tetragonal-to-hexagonal transformation is gradually taken place when increasing the Fe content in the range 0.02 ≤ x ≤ 0.12. Although both hexagonal and tetragonal polymorphs coexist in this doping range, Fe ions preferably substitute for Ti sites in the hexagonal lattice and exist in both Fe3+ and Fe4+ forms. Our work is of paramount importance to provide a direct evidence to the preferable substitution of transition metal ions for Ti ions.

Selenium Speciation in Coal Ash Spilled at the Tennessee Valley Authority Kingston Site

Selenium (Se) in coal ash spills poses a threat to adjacent ecosystems because of its potential to mobilize and bioaccumulate in aquatic organisms. Given that the mobility and bioavailability of Se is controlled by its valence states, we aimed to define Se speciation in coal ash solids and examine the relationships between Se speciation and the magnitude of its mobilization from coal ash. We used coal ash samples from the Tennessee Valley Authority (TVA)-Kingston fossil plant and the site of a coal ash spill that occurred in 2008 in Tennessee. Results of X-ray absorption spectroscopic analyses showed that Se in coal ash samples was a mixture of elemental Se(0) and Se oxyanions. The amount of leachable Se increased with an increase of pH from 3 to 13. At the natural pH of coal ash samples (from pH 7.6 to 9.5), the leachable Se was comprised of Se oxyanions, mainly selenite. This was observed by both direct quantification of Se oxyanions in the leachate and the corresponding loss of Se oxyanions in the solid phase. At pH 12, however, the Se release appeared to derive from both desorption of Se oxyanions and oxidative dissolution of elemental Se(0). Our results indicate that Se oxyanions are the most labile species; however, the magnitude of Se mobilization will increase if the waste material is subjected to alkaline conditions.

Self-assembled tetraoctylammonium bromide as an electron-injection layer for cathode-independent high-efficiency polymer light-emitting diodes

Tetraoctylammonium bromide (TOAB), a general kind of quaternary ammonium salt, was spin-coated onto the surface of a green-emissive poly(9,9-dialkylfluorene) derivative (G-PF) to fabricate cathode-independent polymer light-emitting diodes (PLEDs). The electroluminescence efficiencies were 15.4, 11.4, and 9.1 cd A−1 for TOAB with Al, Ag, and Au as the cathode, respectively, which are better than that of the device with Ca/Al as the cathode (6.1 cd A−1). The molecular nanomorphologies of TOAB deposited on G-PF were investigated using synchrotron X-ray diffraction. Results show that TOAB molecules nucleated on the hydrophobic G-PF surface and self-assembled into a highly ordered lamellar structure during the spin-coating process. This unique structure produces suitable molecular dipoles between N+ and Br−, significantly improving the electron-injection ability from stable metals to G-PF. The direction of the molecular dipole between N+ and Br− can be reversed by using a hydrophilic ZnO for producing an efficient electron injection layer in an inverted device. The self-assembled molecules of TOAB create an anisotropic dipole on hydrophilic or hydrophobic surfaces, making them a potentially efficient electron-injection layer.

Accumulation of heavy metals and trace elements in fluvial sediments received effluents from traditional and semiconductor industries

Metal accumulation in sediments threatens adjacent ecosystems due to the potential of metal mobilization and the subsequent uptake into food webs. Here, contents of heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) and trace elements (Ga, In, Mo, and Se) were determined for river waters and bed sediments that received sewage discharged from traditional and semiconductor industries. We used principal component analysis (PCA) to determine the metal distribution in relation to environmental factors such as pH, EC, and organic matter (OM) contents in the river basin. While water PCA categorized discharged metals into three groups that implied potential origins of contamination, sediment PCA only indicated a correlation between metal accumulation and OM contents. Such discrepancy in metal distribution between river water and bed sediment highlighted the significance of physical-chemical properties of sediment, especially OM, in metal retention. Moreover, we used Se XANES as an example to test the species transformation during metal transportation from effluent outlets to bed sediments and found a portion of Se inventory shifted from less soluble elemental Se to the high soluble and toxic selenite and selenate. The consideration of environmental factors is required to develop pollution managements and assess environmental risks for bed sediments.

Thermal-induced growth of RuO2 nanorods from a binary Ru–Ti oxide composite and alteration in supercapacitive characteristics

The growth of crystalline RuO2 nanorods has been thermally induced from a binary Ru–Ti oxide prepared by oxidative precipitation in an aqueous solution consisting of RuCl3·xH2O, TiCl3 and H2O2. X-ray diffraction results show the formation of RuO2 nanocrystals as the annealing temperature is above/equal to 250 °C. The examination of surface morphology by transmission electron microscopic analysis confirms that the growth of RuO2 nanorods takes place in the high annealing temperature region. X-ray absorption spectroscopy (XAS) demonstrates that a minor amount of Ti atoms have been incorporated into crystalline RuO2, which partially occupy Ru sites in the octahedral RuO6 structure. The electrochemical results show that the binary Ru–Ti oxide annealed at 200 °C exhibits a much higher specific capacitance (ca. 2.5-fold improvement) and better capacitance retention in comparison with RuO2. Introducing Ti into RuO2 not only facilitates RuO2 utilization, but also enhances capacitance retention for the high-rate charge–discharge process. A schematic model is proposed to describe the growth mechanism of RuO2 nanorods from the binary Ru–Ti oxide and the function of Ti atoms within such composites for promoting the capacitive characteristics.

Improving interfacial electron transfer and light harvesting in dye-sensitized solar cells by using Ag nanowire/TiO2 nanoparticle composite films

Metallic Ag nanowires coated with a TiO2 nanolayer (AgNWs@TiO2) were employed as electron conductors in the mesoporous photoanodes (developed by TiO2 nanoparticles, P25) of dye-sensitized solar cells (DSSCs). Our results demonstrate that the AgNWs@TiO2 (in an optimized content of 3 wt% in the photoanode) improved the energy conversion efficiency (η) of the DSSC from 4.68 to 5.31% compared to the DSSC without AgNWs@TiO2 (pure P25). Such an improvement could be mainly attributed to the reduced TiO2/dye/electrolyte interfacial charge transfer impedance. To our surprise, the surface plasmon resonance (SPR) and light scattering effects of the nanowires were found to substantially boost the incident photon-to-electron conversion efficiency (IPCE) of such DSSCs. Regardless of the assembly techniques, our results demonstrate the possibilities of developing the photoanode with reduced interfacial impedance and superior light harvesting capability compared to the existing photoanodes in similar configurations by appropriately adjusting the AgNWs@TiO2 content in the photoanode with P25 based composites.