Tsang-Lang Lin

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 (3u2005nm in rod length and 2u2005nm 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 ~30u2005nm. We also demonstrated that Pt nanoparticles can be synthesized by PVP reduction at 323u2005K 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.

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.

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.

Capacitive performance enhancements of RuO2 nanocrystals through manipulation of preferential orientation growth originated from the synergy of Pluronic F127 trapping and annealing

The capacitive performances of RuO2 prepared by oxidation precipitation of Ru precursors (RuCl3·xH2O) surrounded with tri-block co-polymer, Pluronic F127, in aqueous media can be enhanced through manipulating its preferential orientation growth of nanocrystals. From the heterogeneous surface chemistry viewpoints with the support of structure characterizations, such enhancement originates from the preferential orientation growth of the {101} facet due to the adsorption of the highly polarisable, non-ionic ligands of Pluronic F127 on the high surface energy facets on RuO2 nanocrystallites. In this case, the F127-trapped sample with annealing at 300 °C enhances the specific capacitance 1.6-fold in comparison to its counterpart without F127. With the mechanistic insight into the heterogeneous surface crystal growth pathways, our results materialize the development of RuO2 with tuneable capacitive performances. Furthermore, due to the different propagation models of RuO2 with and without F127 trapping, a schematic diagram is proposed to interpret such a unique crystal growth evolution phenomenon.

Size distribution and coarsening kinetics of δ' precipitates in Al-Li alloys considering temperature and concentration dependence

Abstract While the aging temperature is increased to low undercooling, 180xa0°C for Al–9.7xa0at.% Li alloy, the transitions of both the coarsening mechanism and the size distribution of δ′ precipitates in Al–Li alloy are found and studied using small-angle X-ray scattering (SAXS) technique. A modified precipitation mechanism considering this transition is explored with respect to both the determined precipitate size distribution and the growth rate constant. Additionally, a more adequate method is established for estimating the growth rate constant as a function of aging temperature and Li concentration, and those results are compared with corresponding measurement results.

Core-dependent growth of platinum shell nanocrystals and their electrochemical characteristics for fuel cells

We demonstrated that the reactivity, electrochemically active area (ECSA), and long-term structural stability of Pt-based core–shell nanoparticles (NPs) under the methanol oxidation reaction (MOR) can be controlled by the crystal structure and the chemical composition of the core. This approach is inspired by the structural relaxation of shell Pt atoms on two types of crystalline cores (e.g., Ru and Co) due to the differences in surface free energy between facets. Due to the isotropic facet energy of the Co cores, the initial formation of the Pt shell is under kinetic control when the shell thickness is less than two atomic layers (∼3 A). After the surface layer is formed, the subsequent growth of the Pt shell atoms is under surface diffusion control. For Ru core crystallites with significantly different facet energies, the NPs are disk-like shaped, indicating that the shell atoms preferentially deposit onto the radial edges in the absence of nucleation barriers. In this case, the ECSA of the Rucore–Ptshell NPs is 2.23-fold higher than that of the Cocore–Ptshell NPs, which is promising in the electrocatalytic oxidation of methanol and H2O2 for the applications of fuel cells and sensors, respectively.

Heterojunction confinement on the atomic structure evolution of near monolayer core–shell nanocatalysts in redox reactions of a direct methanol fuel cell

This research demonstrated that the methanol oxidation reaction triggers the protonation corrosion on the multi-walled carbon nanotube supported bimetallic nanocatalysts (NCs) in different geometrical configurations. Such corrosion pathways are confined by the geometrical configuration of the heterogeneous interfaces at the bimetallic oxide nanoparticles. In bimetallic alloyed NCs, the platinum (Pt) and ruthenium (Ru) domains were exposed simultaneously to the redox environment, therefore, the geometrical confinement on the atomic restructure is weak. This leads to substantial dissolution followed by the regrowth of hydrophilic components in redox conditions. For core–shell NCs, the shell crystal would prevent the core from direct contact with the redox environments. The heteroatomic junction forms ordered Pt atomic stacking at the ruthenium oxide crystallite. Such conformation confinement enhanced the Pt to Ru charge donation, thus the CO tolerance factor was also enhanced by more than two orders of magnitude when compared to the module durability of the direct methanol fuel cell compared to that of PtRu alloy.

The structure modification and activity improvement of Pd–Co/C electrocatalysts by the addition of Au for the oxygen reduction reaction

In this study, Pd75Co25−xAux/C ternary catalysts with varying x content are synthesized by the deposition–precipitation approach with hydrogen reduction at 390 K for the oxygen reduction reaction (ORR). The roles of Au in the modification of structures, surface species and electrochemical properties of PdCo/C catalysts are investigated. X-ray diffraction results reveal that although the low reduction temperature does not benefit the Co alloying with Pd, Pd–Au alloys are preferentially formed. Moreover, it confirms that the incorporation of Au into a Pd–Co system contributes to the generation of inhomogeneous alloy structure. Fine structural details determined by X-ray absorption spectroscopy indicate that Au addition improves the heteroatomic intermixing extent of alloy nanocatalysts, especially for Pd75Co10Au15/C (Au15) catalysts. Surface characterization by temperature programmed reduction suggests that a Pd-rich surface gradually changes to Pd, Au and alloy mixed surfaces when the Au content is larger than 15 at%. Regarding the electrochemical results, Au15 displays the superior ORR performance among all samples due to the improved heteroatomic intermixing extent, large electrochemical surface area and multiple coexisting surface species. Furthermore, it also displays a better stability than Pd/C and Pd75Co25/C catalysts after accelerated durability tests.

δ' precipitation in Al-9.7at%Li alloy using small-angle X-ray scattering

Abstract This study presents a novel method for small-angle X-ray scattering (SAXS) to reconstruct the particle size distribution using the indirect transform method plus hard-sphere model. Effectiveness of the proposed method is demonstrated by a simulation case study, in which our results correlate well with simulation results. The method proposed herein, which is an improvement over the conventional one, is then applied to analyze a series of experimental SAXS intensities from δ ′ precipitation in Al–9.7at%Li alloy. More accurate parameters involving structural and thermodynamic information are obtained. Moreover, SAXS analysis is performed to investigate the kinetic model of δ ′ particle coarsening. Comparing the experimentally obtained asymptotic size distributions of δ ′ particles with those predicted by the theoretical models and other experimental results demonstrates that MLSW theory is more appropriate than the others. The experimental results indicate the MLSW model interprets more accurately than the other SAXS studies. Dynamic scaling is used to examine the time-dependent SAXS profiles during this phase separation.