Selection Criteria for Metal Precursors and Solvents for Targeted Synthesis of Metallic Nanostructures Via Kinetic Control in the Polyol Process

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.

In situ Investigations of the Formation Mechanism of Metastable γ-BiPd Nanoparticles in Polyol Reductions

Synthesizing intermetallic phases containing noble metals often poses a challenge as the melting points of noble metals often exceed the boiling point of bismuth (1560 °C). Reactions in the solid state generally circumvent this issue but are extremely time consuming. A convenient method to overcome these obstacles is the co-reduction of metal salts in polyols, which can be performed within hours at moderate temperatures and even allows access to metastable phases. However, little attention has been paid to the formation mechanisms of intermetallic particles in polyol reductions. Identifying crucial reaction parameters and finding patterns are key factors to enable targeted syntheses and product design. Here, we chose metastable γ-BiPd as an example to investigate the formation mechanism from mixtures of metal salts in ethylene glycol and to determine critical factors for phase formation. The reaction was also monitored by in situ X-ray diffraction using synchrotron radiation. Products, intermediates and solutions were characterized by (in situ) X-ray diffraction, electron microscopy, and UV-Vis spectroscopy. In the first step of the reaction, elemental palladium precipitates. Increasing temperature induces the reduction of bismuth cations and the subsequent rapid incorporation of bismuth into the palladium cores, yielding the γ-BiPd phase.

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.

New insights into pertinent Fe-complexes for the synthesis of iron via the instant polyol process

Chemically synthesized iron is in demand for biomedical applications due to its large saturation magnetization compared to iron oxides. The polyol process, suitable for obtaining Co and Ni particles and their alloys, is laborious in synthesizing Fe. The reaction yields iron oxides, and the reaction pathway remains unexplored. This study shows that a vicinal polyol, such as 1,2-propanediol, is suitable for obtaining Fe rather than 1,3-propanediol owing to the formation of a reducible Fe intermediate complex. X-ray absorption spectroscopy analysis reveals the ferric octahedral geometry and tetrahedral geometry in the ferrous state of the reaction intermediates in 1,2-propanediol and 1,3-propanediol, respectively. The final product obtained using a vicinal polyol is Fe with a γ-Fe2O3 shell, while the terminal polyol is favourable for Fe3O4. The distinct Fe-Fe and Fe-O bond lengths suggest the presence of a carboxylate group and a terminal alkoxide ligand in the intermediate of 1,2-propanediol. A large Fe-Fe bond distance suggests diiron complexes with bidentate carboxylate bridges. Prominent high-spin and low-spin states indicate the possibility of transition, which favors the reduction of iron ions in the reaction using 1,2-propanediol.

Deciphering the role of water and a zinc-doping process in a polyol-based approach for obtaining Zn/Co/Al-based spinels: toward "green" mesoporous inorganic pigments

Two new families of zinc/cobalt/aluminum-based pigments, with a unique composition, were obtained through the polyol method. The hydrolysis process of a mixture of Co(CH3COO)2, Zn(acac)2 and Al(acac)3 (acac- = acetylacetonate ion) in 1,4-butanediol afforded dark blue gels (wPZnxCo1-xAl), in the presence of a supplementary amount of water, and light green powders (PZnxCo1-xAl), respectively, for the water-free procedure (x = 0, 0.2, 0.4). The calcination of the precursors yielded dark green (wZnxCo1-xAl) and blue (ZnxCo1-xAl) products. XRD measurements and Rietveld refinement indicate the co-existence of three spinel phases, in different proportions: ZnxCo1-xAl2O4, Co3O4 and the defect spinel, γ-Al2.67O4. The Raman scattering and XPS spectra are in agreement with the compositions of the samples. The morphology of wZnxCo1-xAl consists of large and irregular spherical particle aggregates (ca. 5-100 mm). Smaller agglomerates (ca. 1-5 mm) with a unique silkworm cocoon-like hierarchical morphology composed of cobalt aluminate cores covered with flake-like alumina shells are formed for ZnxCo1-xAl. TEM and HR-TEM analyses revealed the formation of crystalline, polyhedral particles of 7-43 nm sizes for wZnxCo1-xAl, while for ZnxCo1-xAl, a duplex-type morphology, with small (7-13 nm) and larger (30-40 nm) particles, was found. BET assessment showed that both series of oxides are mesoporous materials, with different pore structures, with the water-free samples exhibiting the largest surface areas due, most likely, to the high percent of aluminum oxide. A chemical mechanism is proposed to highlight the role of the water amount and the nature of the starting compounds in the hydrolysis reaction products and, further, in the morpho-structural features and composition of the resulting spinel oxides. The CIE L*a*b* and C* colorimetric parameters indicate that the pigments are bright, with a moderate degree of luminosity, presenting an outstanding high blueness.

Metal-Micelle Interaction Leading to Spontaneous Formation of Ligand-Free Palladium(0) Nanoparticles: Highly Efficient Catalysis Enabling Biaryl Ketone Formation from Carboxylic Acid Derivatives

A novel strategy has been developed to spontaneously form ligand-free Pd(0) nanoparticles (NPs) from water- and air-sensitive Pd₂dba₃ in water. These NPs are thoroughly characterized by IR, NMR, and mass spectrometry, revealing that the metal-micelle binding plays a critical role in their stability and activity. High-resolution transmission electron microscopy supported the ultrasmall nature of NPs, whereas X-ray photoelectron spectroscopy analysis confirmed the zero-oxidation state of Pd. The shielding effect of micelles and enhanced stability of NPs enabled fast cross-couplings of water-sensitive triazine adducts of carboxylic acid to form nonsymmetrical biaryl ketones. These naturally formed NPs are more efficient than new synthetic NPs formed under a hydrogen atmosphere and traditional NPs formed using the air-sensitive Grignard reagent as a reductant. The activity of naturally formed NPs is compared with that of synthetic NPs over 34 substrates, revealing that naturally formed NPs are much more efficient than synthetic NPs.

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.

Pt-Based Multimetal Electrocatalysts and Potential Applications: Recent Advancements in the Synthesis of Nanoparticles by Modified Polyol Methods

In our review, we have presented a summary of the research accomplishments of nanostructured multimetal-based electrocatalysts synthesized by modified polyol methods, especially the special case of Pt-based nanoparticles associated with increasing potential applications for batteries, capacitors, and fuel cells. To address the problems raised in serious environmental pollution, disease, health, and energy shortages, we discuss and present an improved polyol process used to synthesize nanoparticles from Pt metal to Pt-based bimetal, and Pt-based multimetal catalysts in the various forms of alloy and shell core nanostructures by practical experience, experimental skills, and the evidences from the designed polyol processes. In their prospects, there are the micro/nanostructured variants of hybrid Pt/nanomaterials, typically such as Pt/ABO3-type perovskite, Pt/AB2O4-type ferrite, Pt/CoFe2O4, Pt/oxide, or Pt/ceramic by modified polyol processes for the development of electrocatalysis and energy technology. In the future, we suggest that both the polyol and the sol-gel processes of diversity and originality, and with the use of various kinds of water, alcohols, polyols, other solvents, reducing agents, long-term capping and stabilizing agents, and structure- and property-controlling agents, are very effectively used in the controlled synthesis of micro/nanoparticles and micro/nanomaterials. It is understood that at the levels of controlling and modifying molecules, ions, atoms, and nano/microscales, the polyol or sol-gel processes, and their technologies are effectively combined in bottom-up and top-down approaches, as are the simplest synthetic methods of physics, chemistry, and biology from the most common aqueous solutions as well as possible experimental conditions.

In situ investigation of the formation mechanism of α-Bi2Rh nanoparticles in polyol reductions

The study of Bi2Rh formation in a polyol process revealed a two-step mechanism. BiRh is formed by co-reduction of bismuth and rhodium cations and converted into Bi2Rh by Bi diffusion. Various starting materials and reaction parameters are examined.

Formation and selected catalytic properties of ruthenium, rhodium, osmium and iridium nanoparticles

The synthesis and applications in catalysis of nanoparticles formed from ruthenium, rhodium, osmium and iridium have been reviewed.

Formation of Bi2Ir nanoparticles in a microwave-assisted polyol process revealing the suboxide Bi4Ir2O

Intermetallic phases are usually obtained by crystallization from the melt. However, phases containing elements with widely different melting and boiling points, as well as nanoparticles, which provide a high specific surface area, are hardly accessible via such a high-temperature process. The polyol process is one option to circumvent these obstacles by using a solution-based approach at moderate temperatures. In this study, the formation of Bi₂Ir nanoparticles in a microwave-assisted polyol process was investigated. Solutions were analyzed using UV-Vis spectroscopy and the reaction was tracked with synchrotron-based in situ powder X-ray diffraction (PXRD). The products were characterized by PXRD and high-resolution transmission electron microscopy. Starting from Bi(NO₃)₃ and Ir(OAc)₃, the new suboxide Bi₄Ir₂O forms as an intermediate phase at about 160 °C. Its structure was determined by a combination of PXRD and quantum-chemical calculations. Bi₄Ir₂O decomposes in vacuum at about 250 °C and is reduced to Bi₂Ir by hydrogen at 150 °C. At about 240 °C, the polyol process leads to the immediate reduction of the two metal-containing precursors and crystallization of Bi₂Ir nanoparticles.

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.