Shape-Controlled Synthesis of Pt Nanopeanuts

Quercetin-Derived Platinum Nanomaterials Influence Particle Stability, Catalytic, and Antimicrobial Performance

Quercetin possesses high biological properties but low bioavailability, poor solubility, and rapid body clearance. Its structural modification is imperative for enhanced applications. Herein, we demonstrate the catalytic and antimicrobial characteristics of shape-dependent (cuboidal and peanuts) platinum nanoparticles. Modified quercetin, 4'-QP, was employed as the reducing and stabilizing agent for the aqueous synthesis of PtNPs without extraneous reagents. Monodispersed platinum nanocubes (C-PtNPs) and nanopeanuts (P-PtNPs) were produced by reacting 4'-QP and Pt ions in the ratios of 3:1 and 1:1, respectively. TEM characterization confirmed the formation of Pt nanocubes and Pt nanopeanuts, with their corresponding sizes of 39.1 ± 0.20 and 45.1 ± 0.24 nm. The shape-dependency of PtNPs on the nosocomial-causing bacteria, Citrobacter freundii ATCC 8090 (C. freundii) was determined by the Agar well-diffusion assay. Under the same particle size and dose treatments, C-PtNPs and P-PtNPs exhibited 16.28 ± 0.10 and 4.50 ± 0.15 mm zones of inhibition with minimum inhibitory concentrations of 25 and 45 μg/mL, respectively. SEM analysis of C-PtNPs treated C. freundii showed a damaged cell membrane and confirmed contact-killing as the antibacterial mechanism. The catalytic conversion of 4-nitrophenol (4-NP) to 4-amino phenol (4-AP) was tested using a shape-dependent PtNPs catalyst in the presence of sodium borohydride. The conversion rates (k) of C-PtNPs and P-PtNPs in wastewater samples from New Jersey were 0.0108 and 0.00607 s-1, respectively.

Chemical Identity of Poly(N-vinylpyrrolidone) End Groups Impact Shape Evolution During the Synthesis of Ag Nanostructures

Ag nanocubes (AgNCs) are predominantly synthesized by the polyol method, where the solvent (ethylene glycol) is considered the reducing agent and poly(N-vinylpyrrolidone) (PVP) the shape-directing agent. An experimental phase diagram for the formation of Ag nanocubes as a function of PVP monomer concentration (C_m) and molecular weight (M_w) demonstrated end groups of PVP impact the final Ag product. Measured rates of the initial Ag⁺ reduction at different PVP C_m and M_w confirmed the reducing effect originates from end-groups. PVP with well-defined aldehyde and hydroxyl end groups lead to the formation of Ag nanocubes and nanowires respectively, indicating the faster reducing agent formed kinetically preferred nanowires. We demonstrate PVP end-groups induce initial reduction of Ag⁺ to form seeds followed by autocatalytic reduction of Ag⁺ by ethylene glycol (and not solvent oxidation products) to form Ag nanostructures. The current study enabled a quantitative description of the role of PVP in nanoparticle shape-control and demonstrates a unique opportunity to design nanostructures by combining nanoparticle synthesis with polymer design to introduce specific physicochemical properties.

Investigation of Polyol Process for the Synthesis of Highly Pure BiFeO3 Ovoid-Like Shape Nanostructured Powders

Exclusive and unprecedented interest was accorded in this paper to the synthesis of BiFeO₃ nanopowders by the polyol process. The synthesis protocol was explored and adjusted to control the purity and the grain size of the final product. The optimum parameters were carefully established and an average crystallite size of about 40 nm was obtained. XRD and Mössbauer measurements proved the high purity of the synthesized nanostructurated powders and confirmed the persistence of the rhombohedral R3c symmetry. The first studies on the magnetic properties show a noticeable widening of the hysteresis loop despite the remaining cycloidal magnetic structure, promoting the enhancement of the ferromagnetic order and consequently the magnetoelectric coupling compared to micrometric size powders.

Effects of Capping Agents on the Oxygen Reduction Reaction Activity and Shape Stability of Pt Nanocubes

We investigated the formation of Pt nanocubes (NCs) and their electrocatalytic oxygen reduction reaction (ORR) properties and structural stability using two different capping agents, namely, polyvinylpyrrolidone (PVP) and oleylamine (OAm). The mono-dispersity of the obtained Pt NCs and their interactions with PVP and OAm were analyzed by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The TEM data show a high mono-dispersity (82 %) and a large mean particle size (9-10 nm) for the Pt NCs obtained by the oleylamine-assisted method compared to those prepared via the PVP-assisted procedure (68 %, 6-7 nm). FTIR, XPS, and TGA data show that PVP and OAm still remain at the Pt surface, despite washing. Interestingly, the OAm-capped Pt NCs show significantly higher electrochemically active surface area (ECSA) and ORR activity than the PVP-capped ones. An accelerated stress protocol, however, reveals that the OAm-capped NCs possess a poor structural stability during electrochemical cycling. The loss of a defined surface arrangement in the NCs is connected with a transformation into a near-spherical particle shape. In contrast, the PVP-capped NCs mainly retain their particle shape due to their strong capping behavior. In addition, we have developed a degradation model for NCs as a function of electrochemical parameters such as upper potential and cycle number. Altogether, we provide fundamental insights into the electronic interactions between capping agent and Pt NCs and the role of the adsorption strength of the capping agent in improving the electrochemical ORR performance as well as the structural stability of shape-controlled nanoparticles.

Electrocatalysis of Oxygen Reduction Reaction on Shape-Controlled Pt and Pd Nanoparticles-Importance of Surface Cleanliness and Reconstruction

Shape-controlled precious metal nanoparticles have attracted significant research interest in the recent past due to their fundamental and scientific importance. Because of their crystallographic-orientation-dependent properties, these metal nanoparticles have tremendous implications in electrocatalysis. This review aims to discuss the strategies for synthesis of shape-controlled platinum (Pt) and palladium (Pd) nanoparticles and procedures for the surfactant removal, without compromising their surface structural integrity. In particular, the electrocatalysis of oxygen reduction reaction (ORR) on shape-controlled nanoparticles (Pt and Pd) is discussed and the results are analyzed in the context of that reported with single crystal electrodes. Accepted theories on the stability of precious metal nanoparticle surfaces under electrochemical conditions are revisited. Dissolution, reconstruction, and comprehensive views on the factors that contribute to the loss of electrochemically active surface area (ESA) of nanoparticles leading to an inevitable decrease in ORR activity are presented. The contribution of adsorbed electrolyte anions, in-situ generated adsorbates and contaminants toward the ESA reduction are also discussed. Methods for the revival of activity of surfaces contaminated with adsorbed impurities without perturbing the surface structure and its implications to electrocatalysis are reviewed.

Revisiting the Polyol Synthesis of Silver Nanostructures: Role of Chloride in Nanocube Formation

Chloride (Cl^-) is often used together with polyvinylpyrrolidone (PVP) in the polyol synthesis of Ag nanocubes. In the literature, shape control is attributed predominantly to the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets, whereas the role of Cl^- has not been well studied. Several hypotheses have been proposed regarding the role of Cl^-; however, there is still no consensus regarding the exact influence of Cl^- in the shape-controlled synthesis of Ag nanocubes. To examine the influence of Cl^-, we undertook a joint theoretical-experimental study. Experimentally, we examined the influence of Cl^- concentration on the shape of Ag nanoparticles (NPs) at constant H⁺ concentration. In the presence of H⁺, in situ formed HNO₃ etches the initially formed Ag seeds and slows down the overall reduction of Ag⁺, which promotes the formation of monodisperse Ag NPs. Ex situ experiments probed the evolution of Cl^- during the growth of Ag nanocubes, which involves the initial formation of AgCl nanocubes, and their subsequent dissolution to release Cl^-, which adsorbs onto the surfaces of single crystal seeds to impact shape evolution through apparent thermodynamic control. The formation of cubes is independent of the source of AgCl, indicating temporal control of the Cl^- chemical potential in solution leads to high-yield synthesis of Ag nanocubes. Increasing the concentration of Cl^- alone leads to a progression in shape from truncated octahedra, to cuboctahedra, truncated cubes, and ultimately cubes, directly demonstrating the importance of Cl^- in Ag NP shape control. We used ab initio thermodynamics calculations based on density functional theory to probe the role of Cl^- in directing shape control. With increasing Cl chemical potential (surface coverage), calculated surface energies γ of Ag facets transition from γ₁₁₁ < γ₁₀₀ to γ₁₀₀ < γ₁₁₁ and predict Wulff shapes terminated with an increasing (100) contribution, consistent with experimental observations. The combination of theory and experiment is beneficial for advancing the understanding of nanocrystal formation.