Formation of Pt decorated Ni-Pt nanocubes through low temperature atomic diffusion--time-resolved elemental analysis of nanoparticle formation

Synthesis of low-cost multi-element Pt-based alloy nanoparticles as catalysts for the oxygen reduction reaction

Due to their high catalytic activity, stability, and economic benefits, Pt-based multi-element alloyed nanoparticles (NPs) are considered promising electrodes for oxygen reduction reactions. However, a synthesis method capable of controlling the reduction reaction of elements with different redox potentials to synthesize multimetallic alloy NPs is yet to be developed. In this study, monodisperse NiPtPd alloy NPs with varying compositions were synthesized using 1-heptanol as a reducing solvent. The selection of low-reducing noble metal precursors and complexing agents is done strategically to adjust the reduction time of metal ions. The spectroscopic results confirmed that olelylamine (OAm) preferentially coordinates with Pt ions, while trioctylphosphine (TOP) preferentially coordinates with Pd ions. Consequently, control of the elemental distribution within the particle is successfully achieved by adjusting the OAm/Pt and TOP/Pd molar ratios. Subsequently, Ni78Pt11Pd11 alloy NPs were designed, and their catalytic properties as electrodes in the oxygen reduction reaction (ORR) were examined. Despite a low noble metal content of 22%, the catalytic performance and stability were superior to and comparable to those of commercial Pt NPs, respectively.

Kinetically Controlled Direct Synthesis of B2- and A1-Structured Cu-Pd Nanoparticles

Atomic arrangement in Cu-Pd alloy nanoparticles (NPs) has been reported to influence the catalytic activity, but they have yet to be studied in detail. Unlike previous studies, where the B2 structure Cu-Pd NPs are obtained by heat treating the A1 structure, this study reports the one-pot direct syntheses of A1- and B2-structured Cu-Pd NPs using an alcohol reduction method. The alcohol reduction technique facilitates the kinetic control of the reduction reaction by selecting the appropriate alcohol type and complexing agent to delay the reduction of easily reducible metallic elements to realize control over the reduction kinetics for coreduction. Different formation mechanisms for A1- and B2-structured CuPd NPs were confirmed by in situ ultraviolet-visible (UV-vis) measurements and morphological and structural analyses of samples withdrawn during the reaction. Finally, the direct formation of single-phase B2-structured Cu-Pd NPs with an average diameter of 18.6 ± 7.6 nm was realized using tri-n-octyl phosphine as a complexing agent. The noticeable crystal structural dependence of the electrocatalytic CO2 reduction reaction properties of A1- and B2-structured CuPd NPs was demonstrated.

Synthesis of Electromagnetic Wave-Absorbing Co-Ni Alloys and Co-Ni Core-Shell Structured Nanoparticles

Co-Ni alloy nanoparticles, a potential candidate for microwave absorption material, were successfully synthesized by tuning the reduction timing of Co and Ni ions by introducing oleylamine as a complexing agent and 1-heptanol as a reducing solvent. The formation mechanism elucidated using time-resolved sampling and in situ X-ray absorption spectroscopy (XAS) and ultraviolet-visible (UV-vis) spectrophotometry measurements suggested that the delay in the reduction of Co ions via complexation with oleylamine facilitated the co-reduction of Co with Ni ions and led to the formation of Co-Ni alloys. The successful synthesis of Co-Ni alloys experimentally confirmed the differences in magnetic properties between alloy and core-shell structured Co₅₀Ni₅₀ particles. Further, the syntheses of Co-Ni alloys with different compositions were also possible using the above technique. In addition, the microwave absorption properties were measured using the free-space method utilizing a vector network analyzer of Co₅₀Ni₅₀─polyethylene composite with different sheet thicknesses. A reflection loss (RL) value of -25.7 dB at 13.6 GHz for the alloy structure was more significant than the core-shell counterpart. The above values are high compared to results reported in the past. The validity of the measurements was confirmed by utilizing the parameter retrieval method to extract permittivity and permeability from the scattering parameter (S) and recalculation of the RL as a function of frequency.

Strategy to Design-Synthesize Bimetallic Nanostructures Using the Alcohol Reduction Method

Bimetallic nanomaterials have attracted much attention from various fields such as catalysis, optics, magnetism, and so forth. The functionality of such particles is influenced very much by the intermetallic interactions than their individual contribution. However, compared with the synthesis of monometallic nanoparticles, the reaction parameters that need to be controlled for tuning the size, shape, composition, and crystal structure of bimetallic nanoparticles becomes challenging. This study focuses on synthesizing of bimetallic nanostructures using the alcohol reduction method, where the control over the reducing power is conceivable by varying the combination of the alcohol type, complexing agent, and metal salts. Consequently, various Cu-Co nanostructures such as Cu-Co core-shell (size ranged between 40 and 15 nm) and hollow alloy nanoparticles and nanotubes were successfully synthesized by incorporating diffusion and etching phenomena during the reduction reaction. Moreover, time-resolved sampling revealed that the formation of a Cu-Co alloy hollow nanostructure has been realized by the diffusion of the Cu core into the Co shell by controlling the reduction time gap between Cu and Co and the crystal structure besides the reduction sequences. It should be noted that the synthesis of a high-temperature (∼1300 °C) Cu-Co alloy phase was carried out at 170 °C. Among the Cu-Co alloy nanostructures, Cu-Co hollow alloy nanoparticles exhibited enhanced catalytic activity compared to metallic Cu and other Cu-Co nanostructures from the degradation reaction of methylene blue. The enhanced catalytic performance was considered to be mainly due to the alloy structure.

Theoretical and Experimental Evaluation of the Reduction Potential of Straight-Chain Alcohols for the Designed Synthesis of Bimetallic Nanostructures

Recently, the development of bimetallic nanoparticles with functional properties has been attempted extensively but with limited control over their morphological and structural properties. The reason was the inability to control the kinetics of the reduction reaction in most liquid-phase syntheses. However, the alcohol reduction technique has demonstrated the possibility of controlling the reduction reaction and facilitating the incorporation of other phenomena such as diffusion, etching, and galvanic replacement during nanostructure synthesis. In this study, the reduction potential of straight-chain alcohols has been investigated using molecular orbital calculations and experimentally verified by reducing transition metals. The alcohols with a longer chain exhibited higher reduction potential, and 1-octanol was found to be the strongest among alcohols considered. Furthermore, the experimental evaluation carried out via the synthesis of metallic Cu, Ni, and Co particles was consistent with the theoretical predictions. The reaction mechanism of metallic particle formation was also studied in detail in the Ni-1-octanol system, and the metal ions were confirmed to be reduced via the formation of nickel alkoxide. The results of this investigation were successfully implemented to synthesize Cu-Ni bimetallic nanostructures (core-shell, wire, and tube) via the incorporation of diffusion and etching besides the reduction reaction. These results suggest that the designed synthesis of a wide range of bimetallic nanostructures with more refined control has become possible.

Pt distribution-controlled Ni–Pt nanocrystals via an alcohol reduction technique for the oxygen reduction reaction

The catalytic performance and durability of Ni–Pt alloy nanoparticles synthesized using an alcohol reduction technique were enhanced by controlling the metallic Pt distribution.

Efficient polyalcohol oxidation electrocatalysts enabled by PtM (M = Fe, Co, and Ni) nanocubes surrounded by (200) crystal facets

Due to the high-density (200) crystal planes and abundant active sites, cubic platinum nanomaterials have become outstanding electrocatalysts in promoting fuel cell reactions. However, because of the fact that the facet-controlled synthesis is difficult, it is still a grand challenge to synthesize a sequence of Pt-based nanocubes via a universal method. Herein, we report a general and simple eco-friendly solvothermal method to prepare (200)-enclosed PtM nanocubes. Different from the other nanomaterials, nanocubes are conducive to mass transfer. Moreover, the synergistic and electronic effects between M and Pt are profitable to improve the utilization of precious metals. We used (200)-encapsulated nanocrystals to evaluate their electrocatalytic performance towards glycerol and ethylene glycol oxidation reactions in an alkaline medium. In particular, Pt4Co nanocubes showed superior mass activities in glycerol and ethylene glycol oxidation reactions, which are 6.2- and 5.0-fold higher than those obtained for commercial Pt/C catalysts, respectively. Meanwhile, Pt4M catalysts manifested excellent stability in the endurance test, which is attributed to the alloying effect promoting the electrooxidation of intermediates. Our study provides an ideal method for the construction of Pt-based bimetallic nanocubes, which can be used for anode reactions of polyol fuel cells and beyond.

Site-Specific Wetting of Iron Nanocubes by Gold Atoms in Gas-Phase Synthesis

A key challenge in nanotechnology is the rational design of multicomponent materials that beat the properties of their elemental counterparts. At the same time, when considering the material composition of such hybrid nanostructures and the fabrication process to obtain them, one should favor the use of nontoxic, abundant elements in view of the limited availability of critical metals and sustainability. Cluster beam deposition offers a solvent- and, therefore, effluent-free physical synthesis method to achieve nanomaterials with tailored characteristics. However, the simultaneous control of size, shape, and elemental distribution within a single nanoparticle in a small-size regime (sub-10 nm) is still a major challenge, equally limiting physical and chemical approaches. Here, a single-step nanoparticle fabrication method based on magnetron-sputtering inert-gas condensation is reported, which relies on selective wetting of specific surface sites on precondensed iron nanocubes by gold atoms. Using a newly developed Fe-Au interatomic potential, the growth mechanism is decomposed into a multistage model implemented in a molecular dynamics simulation framework. The importance of growth kinetics is emphasized through differences between structures obtained either experimentally or computationally, and thermodynamically favorable configurations determined via global optimization techniques. These results provide a roadmap for engineering complex nanoalloys toward targeted applications.

Preparation of Pt-skin PtRhNi Nanoframes Decorated with Small SnO2 Nanoparticles as an Efficient Catalyst for Ethanol Oxidation Reaction

Pt-based nanoframes are one of the most promising catalysts for ethanol oxidation reaction in direct ethanol fuel cells. It is important to understand the mechanisms responsible for creating these hollow nanoframe-based catalysts. Herein, for the first time, Pt-skin PtRhNi rhombic dodecahedral nanoframes were decorated with small SnO₂ nanoparticles and were used as an efficient catalyst for the ethanol oxidation reaction. Moreover, by combining the ex situ scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy observations at various stages of synthesis, along with density functional theory calculations, it was possible to track the synthesis route of solid rhombic dodecahedral PtRhNi nanoparticles, which are the precursors of PtRhNi nanoframes. After the chemical etching of the Ni core from solid PtRhNi nanoparticles, the obtained nanoframes were decorated with SnO₂ nanoparticles. The resulting SnO₂@PtRhNi heteroaggregates were deposited on high-surface-area carbon and electrochemically tested, showing a 6-fold higher mass activity and 10-fold higher specific activity toward ethanol oxidation reaction than commercially available Pt catalysts.

Design of monoalcohol - Copolymer system for high quality silver nanowires

Research to improve the dimensional properties of silver nanowires (Ag NWs) for transparent conductive film (TCF) applications are being carried out intensively. However, the protocol for the designed synthesis of high-quality Ag NWs is yet to be developed due to the inadequacy of knowledge on the role of parameters. Here, we attempt to elucidate the role played by the parameters and propose a monoalcohol-copolymer based system for the designed synthesis of Ag NWs superior in quality to the one synthesized using conventional ethylene glycol (EG)-polyvinylpyrrolidone (PVP) system. The key findings of the study are as follows: (1) the solubility of Ag source and the partially formed AgCl particles in monoalcohols was found to play an important role not only in the reduction to metallic Ag but also on the uniaxial growth, (2) the adsorption of capping agents on Ag NWs was carried through O and N atoms in the base molecule and their binding energies indicated that the strength is the key parameter to obtain Ag NWs with high aspect ratio, (3) the properties of nanowire could be enhanced through copolymerization of VP and base molecules that have O- and N-based ligands, and (4) the influence of copolymerization on the physical and chemical properties of the surface active agent has been theoretically and experimentally verified. Consequently, we succeeded in the development of a new technique to synthesize high yield of Ag NWs with improved aspect ratio than EG-PVP system by using benzyl alcohol as reducing solvent and N-vinylpyrrolidone/N,N-diethylaminoethyl metacrylate copolymer as a capping agent. The optical transmittance and electrical resistivity of TCFs prepared using the Ag NWs with an average diameter of 43 nm, and an average length of 13 μm were 98.6% and R: 49.1 Ω/□, respectively.

Monocrystalline platinum-nickel branched nanocages with enhanced catalytic performance towards the hydrogen evolution reaction

Single crystalline noble metal nanocages are the most promising candidates for heterogeneous catalysis due to their large specific surface area, well-defined structure and enhanced structural stability. Herein, based on the observation of an unexpected phenomenon that the alloying of Pt and transition metals by co-reduction is more preferential than the formation of pure Pt NCs, we propose a feasible one-pot strategy to synthesize a uniformly epitaxial core-shell Pt-Ni structure with a Ni-rich alloy as the core and a Pt-rich alloy as the shell. The as-prepared Pt-Ni core-shell structures are subsequently etched into monocrystalline Pt-Ni branched nanocages with the wall thickness being 2.8 nm. This unique structure exhibits excellent catalytic performance and stability for the hydrogen evolution reaction (HER) in alkaline solution which is of great significance for the energy-intensive water-alkali and chlor-alkali industry.

Synergy of facet control and surface metalloid modification on hierarchical Pt–Ni nanoroses toward high electrocatalytic activity

We demonstrated a highly active nitrogen species-decorated Pt–Ni–N electrocatalyst with tunable architectures, facets, and catalytic performance.