Large-scale synthesis of palladium concave nanocubes with high-index facets for sustainable enhanced catalytic performance

Temperature Dependent Growth Kinetics of Pd Nanocrystals: Insights from Liquid Cell Transmission Electron Microscopy

Quantifying the role of experimental parameters on the growth of metal nanocrystals is crucial when designing synthesis protocols that yield specific structures. Here, the effect of temperature on the growth kinetics of radiolytically-formed branched palladium (Pd) nanocrystals is investigated by tracking their evolution using liquid cell transmission electron microscopy (TEM) and applying a temperature-dependent radiolysis model. At early times, kinetics consistent with growth limited is measured by the surface reaction rate, and it is found that the growth rate increases with temperature. After a transition time, kinetics consistent with growth limited by Pd atom supply is measured, which depends on the diffusion rate of Pd ions and atoms and the formation rate of Pd atoms by reduction of Pd ions by hydrated electrons. Growth in this regime is not strongly temperature-dependent, which is attributed to a balance between changes in the reducing agent concentration and the Pd ion diffusion rate. The observations suggest that branched rough surfaces, generally attributed to diffusion-limited growth, can form under surface reaction-limited kinetics. It is further shown that the combination of liquid cell TEM and radiolysis calculations can help identify the processes that determine crystal growth, with prospects for strategies for control during the synthesis of complex nanocrystals.

Selenium-Doped Seeded Growth of Truncated Octahedral Gold Nanocrystals with Surface Concavities for Surface-Enhanced Raman Spectroscopy

Concave nanocrystals stand out as a testament to the importance of the nanoscale morphology in dictating the functional properties of materials. In this report, we introduce a facile synthesis method for producing gold (Au) nanocrystals with a truncated octahedral morphology that features surface concavities (Au CNTOs). The incorporation of selenium (Se) doping into the truncated octahedral Au seeds was essential for their enlargement and the formation of concave structures. By simply adjusting the quantity of seeds, we could control the size of the nanocrystals while maintaining their distinctive morphology and surface concavity. The formation mechanism suggests that Se doping likely passivates the side faces, thereby slowing growth and promoting atomic deposition at the edges and corners. The resulting Se-doped Au CNTOs exhibited strong localized surface plasmon resonance (LSPR) absorptions in the visible spectrum and the SERS performance of their assemblies was demonstrated through crystal violet detection, reaching enhancement factors around 105. This study presents an innovative approach to synthesizing concave Au nanocrystals through the incorporation of selenium during a seeded growth process, offering insights into the strategic design of plasmonic nanostructures.

Integrated Optimization of Crystal Facets and Nanoscale Spatial Confinement toward the Boosted Catalytic Performance of Pd Nanocrystals

Tuning the surface chemical property and the local environment of nanocrystals is crucial for realizing a high catalytic performance in various reactions. Herein, we aim to elucidate the structure sensitivity of Pd facets on the surface catalytic hydrogenation reaction and to identify what role the nanoconfinement effect plays in the catalytic properties of Pd nanocrystal catalysts. By controlling the coating structures of mesoporous silica (mSiO2) on Pd nanocrystals with different exposed facets that include {100}, {111}, and {hk0}, we present a series of Pd@mSiO2 nanoreactors in core-shell and yolk-shell structures and the discovery of a partial-coated structure, which can provide different types of nanoconfinement, and we propose a seed size-dominated growth mechanism. We demonstrate that a superior activity was exhibited in Pd nanocrystals enclosed by the {hk0} facet as compared to the Pd{100} and Pd{111} facets, and substantially enhanced efficiency and stability were achieved in Pd@mSiO2 particles with yolk-shell structures, indicating a crucial superiority of optimizing the configuration of crystal facets and nanoconfinement. Our study provides an efficient strategy to rationally design and optimize nanocatalysts for promoting catalytic performance.

Morphological Evolution Trajectory of Multifaceted Palladium Nanoparticles

Precise control over the morphology and facets of Pd nanomaterials has great importance in catalytic and sensing applications. In this study, we synthesized Pd nanoparticles with multiple types of low-Miller-index-faceted morphologies by systematically defining the synthesis conditions of the seed-mediated colloidal preparation method. We discovered the morphological evolution of Pd nanoparticles by following the trajectory of the surface Miller indices, which were determined by the cooperative effects of cetyltrimethylammonium bromide and ascorbic acid. By precise control of the morphological trajectory, Pd nanoparticles with a new cuborhombicube morphology, composed of 36 facets and concave edges, were discovered. This study provides important insight into the design of the surface Miller indices and morphologies of functional nanomaterials.

Uncovering the encapsulation effect of reduced graphene oxide sheets on the hydrogen storage properties of palladium nanocubes

Decades of research on solute-induced phase transformation of metal hydride systems have shown the possibility to enhance hydrogen storage properties through novel material design such as nanoconfinement engineering. Nevertheless, the fundamentals of mechanical stress effect on confined Pd nanoparticles remain yet to be elucidated due to the difficulty in linking with hydrogen sorption thermodynamics. Here, a thermodynamic tuning of Pd nanocubes associated with hydrogen sorption as a result of encapsulation by reduced graphene oxide (rGO) layers is demonstrated. Pd nanocubes are constrained by rGO to such a degree that the chemical potential and the pressure hysteresis of the system during hydrogen sorption drastically change while showing a size dependence. A thorough thermodynamic analysis elucidates the role of constraints on hydrogen uptake and release; despite the nanoscale regime, the thermodynamic parameters (enthalpy and entropy) during phase transition considerably increase, a phenomenon not seen before in unconstrained Pd nanoparticle systems.

Single-Crystalline Body Centered FeCo Nano-Octopods: From One-Pot Chemical Growth to a Complex 3D Magnetic Configuration

Single crystalline magnetic FeCo nanostars were prepared using an organometallic approach under mild conditions. The fine-tuning of the experimental conditions allowed the direct synthesis of these nano-octopods with body-centered cubic (bcc) structure through a one-pot reaction, contrarily to the seed-mediated growth classically used. The FeCo nanostars consist of 8 tetrahedrons exposing {311} facets, as revealed by high resolution transmission electron microscopy (HRTEM) imaging and electron tomography (ET), and exhibit a high magnetization comparable with the bulk one (M_s = 235 A·m²·kg^-1). Complex 3D spin configurations resulting from the competition between dipolar and exchange interactions are revealed by electron holography. This spin structures are stabilized by the high aspect ratio tetrahedral branches of the nanostars, as confirmed by micromagnetic simulations. This illustrates how magnetic properties can be significantly tuned by nanoscale shape control.

A review of the role and mechanism of surfactants in the morphology control of metal nanoparticles

Although great progress has been made in the synthesis of metal nanoparticles, good repeatability and accurate predictability are still difficult to achieve. This difficulty can be attributed to the synthetic method based primarily on observation and subjective experience, and the role of many surfactants remains unclear. It should be noted that surfactants play an important role in the synthetic process. Understanding their function and mechanism in the synthetic process is a prerequisite for the rational design of nanocatalysts with ideal morphology and performance. In this review article, the function of surfactants is introduced first, and then the mechanism of action of surfactants in controlling the morphology of nanoparticles is discussed according to the types of surfactants, and the promoting and sealing effects of surfactants on the crystal surface is revealed. The relationship between surfactants and the morphology structure of nanoparticles is studied. The removal methods of surfactants are discussed, and the existing problems in the current development strategy are summarized. Finally, the application of surfactants in controlling the morphology of metal nanocrystals is prospected. It is hoped that the review can open up new avenues for the synthesis of nanocrystals.

Synthesis, characterization and photothermal analysis of nanostructured hydrides of Pd and PdCeO2

Hyperthermia was shown to be an important co-adjuvant therapy to conventional cancer treatments. Nanoparticles can be used in the hyperthermia therapy to improve the localized absorption of energy imposed by external sources, in order to kill tumor cells solely by the effect of heat and with minimum thermal damage to surrounding healthy cells. Nanoparticles can also serve as carriers of drugs that specifically act on the tumor when heated, including hydrogen that can be desorbed to locally promote an antioxidant effect and reduce the viability of cancer cells. In this context, palladium hydride nanoparticles emerge as promising materials for the hyperthermia therapy. In this study, palladium nanocubes (PdNC) and PdCeO₂ nanoparticles were synthesized. Nanofluids produced with these nanomaterials were hydrogenated and then tested to examine their photothermal effects. Nanofluids made of PdH_x nanoparticles presented significant temperature increases of more than 30 °C under 3 min of diode-laser irradiation. On the other hand, nanofluids with PdCeO₂H nanoparticles presented temperature increases around 11 °C under the same experimental conditions. The behavior observed with the PdCeO₂H nanofluids can be attributed to the effect of H⁺ in reducing Ce⁺⁴ to Ce⁺³.

Reshaping of Truncated Pd Nanocubes: Energetic and Kinetic Analysis Integrating Transmission Electron Microscopy with Atomistic-Level and Coarse-Grained Modeling

Stability against reshaping of metallic fcc nanocrystals synthesized with tailored far-from-equilibrium shapes is key to maintaining optimal properties for applications such as catalysis. Yet Arrhenius analysis of experimental reshaping kinetics, and appropriate theory and simulation, is lacking. Thus, we use TEM to monitor the reshaping of Pd nanocubes of ∼25 nm side length between 410 °C (over ∼4.5 h) and 440 °C (over ∼0.25 h), extracting a high effective energy barrier of E_eff ≈ 4.6 eV. We also provide an analytic determination of the energy variation along the optimal pathway for reshaping that involves transfer of atoms across the nanocube surface from edges or corners to form new layers on side {100} facets. The effective barrier from this analysis is shown to increase strongly with the degree of truncation of edges and corners in the synthesized nanocube. Theory matches experiment for the appropriate degree of truncation. In addition, we perform simulations of a stochastic atomistic-level model incorporating a realistic description of diffusive hopping for undercoordinated surface atoms, thereby providing a visualization of the initial reshaping process.

Core-Shell Palladium/MOF Platforms as Diffusion-Controlled Nanoreactors in Living Cells and Tissue Models

Translating the potential of transition metal catalysis to biological and living environments promises to have a profound impact in chemical biology and biomedicine. A major challenge in the field is the creation of metal-based catalysts that remain active over time. Here, we demonstrate that embedding a reactive metallic core within a microporous metal-organic framework-based cloak preserves the catalytic site from passivation and deactivation, while allowing a suitable diffusion of the reactants. Specifically, we report the fabrication of nanoreactors composed of a palladium nanocube core and a nanometric imidazolate framework, which behave as robust, long-lasting nanoreactors capable of removing propargylic groups from phenol-derived pro-fluorophores in biological milieu and inside living cells. These heterogeneous catalysts can be reused within the same cells, promoting the chemical transformation of recurrent batches of reactants. We also report the assembly of tissue-like 3D spheroids containing the nanoreactors and demonstrate that they can perform the reactions in a repeated manner.

Facile preparation of porous palladium nanocubes via a one-pot process induced by 1-hexadecyl-3-methyl imidazolium bromide for methanol electro-oxidation

Pd porous nanocubes were synthesized by a one-pot method assisted by HMIB and exhibited higher activity than solid nanocubes and Pd/C.

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.

RNA-mediated, green synthesis of palladium nanodendrites for catalytic reduction of nitroarenes

Palladium (Pd)-catalyzed reactions mostly show structure sensitivity: i.e., the selectivity and activity of the reactions are highly dependent on the arrangement of Pd atoms. In this regard, branched Pd nanoparticles show enhanced catalytic performance owing to the presence of low coordinated Pd atoms. In this paper, a novel solution-phase synthesis of flower-like Pd nanodendrites using ribonucleic acid (RNA) as a capping agent and ascorbic acid as a reducing agent was described. On the other hand, the co-use of polyvinylpyrrolidone (PVP) and potassium bromide (KBr) instead of RNA at the same synthesis conditions led to cuboid nanoparticles, while the sole use of ascorbic acid resulted in faceted nanoparticles. The formation of nanodendritic morphology was attributed to the RNA-assisted growth through particle attachment. This scenario was supported by TEM analysis that demonstrated the aggregation of small particles to form larger nanoparticles at the onset of the reaction. The shape and size of the nanoparticles could be readily tuned by the RNA content used. XPS confirmed the formation of metallic Pd nanoparticles. The presence of crystalline planes of {1 1 1}, {2 0 0}, {2 2 0}, {3 1 1} and {2 2 2} was demonstrated by XRD and SAED analyses. The Pd nanodendrites were used for the reduction of p-nitrophenol (PNP) and 2,4,6-trinitrotoluene (TNT), and reduction rate constants (k) were calculated as 1.078 min^-1 (normalized rate constant, k_nor = 59.66 mmol^-1 s^-1) for PNP and 0.3181 min^-1 (k_nor = 17.6 mmol^-1 s^-1) for TNT with the corresponding turnover frequencies (TOFs) as 16.06 and 40.80 h^-1, respectively.

Facile Synthesis of Pd Nanocubes with Assistant of Iodide and Investigation of Their Electrocatalytic Performances Towards Formic Acid Oxidation

This article presents a facile, one-pot method using the aqueous phase for the synthesis of high-quality Pd nanocubes. In this study, Pd chloride was used as the precursor, sodium iodide as capping agent, and poly(vinylpyrrolidone) as surfactant and reducing agent. The effects of different halogens on the morphology of Pd nanocrystals were investigated. The results showed that, in this synthesis system, the selection and proper amount of sodium iodide was essential to the preparation of high-quality Pd nanocubes. When iodide was replaced by other halogens (such as bromide and chloride), Pd nanocrystals with cubic morphology could not be obtained. In addition, we have found that NaBH₄ can be used to efficiently remove inorganic covers, such as iodide, from the surface of Pd nanoparticles as synthesized. The Pd nanoparticles obtained were employed as electro-catalysts for formic acid oxidation, and they exhibited excellent catalytic activity and good stability towards this reaction.

Bio-synthesis of palladium nanocubes and their electrocatalytic properties

The bio-synthesis of palladium nanocubes (PdNCs) was realised using pine needle extract as the reducing agent and cetyl trimethyl ammonium bromide as the capping agent. As an eco-friendly and readily available biomass, pine needle extract avoided the use of highly polluting chemical reducing agents. The growth process of PdNCs was analysed using ultraviolet-vis and Fourier transform infrared spectroscopy. Flavonoids, esters, terpenoids and polyhydric alcohols, which contain reductive groups, were mainly responsible for the transition of Pd2+ ions to PdNCs. The morphology and structure of PdNCs were characterised using transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction and X-ray diffraction. It was indicated that the as-prepared PdNCs displayed a relatively high purity and good crystallinity with a face-centred cubic structure and exhibited sizes ranging from 6.11 to 29.51 nm with an average particle size of 11.18 nm. In the methanol electro-oxidation reaction, the PdNCs enclosed by {100} facets exhibited superior electro-catalytic activity to commercial Pd/C, which was rarely reported in other bio-synthesis processes for Pd catalysts. Meanwhile, the PdNCs showed excellent anti-poisoning ability and long-term stability. This study reveals the possibility of preparing shape-controlled PdNCs with a specific structure and excellent electro-catalytic activity.

Shape control in concave metal nanoparticles by etching

The shape control of nanoparticles constitutes one of the main challenges in today's nanotechnology. The synthetic procedures are based on trial-and-error methods and are difficult to rationalize as many ingredients are typically used. For instance, concave nanoparticles exhibiting high-index facets can be obtained from Pt with different HCl treatments. These structures present exceptional capacities when are employed as catalysts in electrochemical processes, as they maximize the activity per mass unit of the expensive material. Here we show how atomistic simulations based on density functional theory that take into account the environment can predict the morphology for the nanostructures and how it is even possible to address the appearance of concave structures. To describe the control by etching, we have reformulated the Wulff construction through the use of a geometric model that leads to concave polyhedra, which have a larger surface-to-volume ratio compared to that for nanocubes. Such an increase makes these sorts of nanoparticles excellent candidates to improve electrocatalytic performance.

Concave gold bipyramids bound with multiple high-index facets: improved Raman and catalytic activities

Concave nanocrystals usually exhibit a large electromagnetic-field enhancement and superior catalytic performance due to their sharp corners, negative curvature and high-index facets. Conventional gold bipyramids (AuBPs) possess intriguing plasmonic properties which are attractive for various applications while the surface curvature of the reported bipyramids has not been fine-tuned to concave or convex structures to date. Additionally, the longitudinal surface plasmon resonance (LSPR) wavelengths of conventional AuBPs are mostly located in the range of 650-1350 nm and the sizes of these nanoparticles are usually not beyond 350 nm, which are not facilitated to some nano-focusing and nanophotonic applications. Herein, we reported a facile and robust approach for fabricating concave AuBPs (CAuBPs) with multiple high-index facets which are distinct from the conventional AuBPs and nanojavelin structures. The length of the as-prepared CAuBPs can even extend up to 800 nm. The CAuBP nanoparticles exhibit a strikingly pronounced broader plasmonic tuning range (even exceeding 1800 nm) and provide much higher electromagnetic-field enhancements in comparison to the conventional AuBPs, which broaden the promising applications of CAuBPs for many single-particle analyses. More importantly, the surface-enhanced Raman scattering (SERS) signals of CAuBPs on the single-particle or aqueous solution both displayed an enhanced intensity compared to conventional AuBPs. The CAuBP nanoparticles also exhibited improved catalytic activity due to the incredible abundance of uncoordinated atoms as active sites.

Metal Bromide Controlled Interfacial Aromatization Reaction for Shape-Selective Synthesis of Palladium Nanostructures with Efficient Catalytic Performances

Herein, the effect of diverse metal bromides for the shape evolution of palladium nanostructures (Pd NS) has been demonstrated. Aromaticity-driven reduction of bromopalladate(II) is optimized to reproducibly obtain different Pd NS at the water/organic layer interface. In this soft interfacial strategy, a redox potential driven reaction has been performed, forming the thermodynamically more stable (>10(4) -fold) PdBr4 (2-) precursor from PdCl4 (2-) by adding extra metal bromides. In the process, the reductant, Hantzsch dihydropyridine ester (DHPE), is aromatized. Interestingly, alkali metal bromides devoid of coordination propensity exclusively evolve Pd nanowires (Pd NWs), whereas in the case of transition metal bromides the metal ions engage the 'N' donor of DHPE at the interface, making the redox reaction sluggish. Hence, controlled Pd nanoparticles growth is observed, which evolves Pd broccolis (Pd NBRs) and Pd nanorods (Pd NRs) at the interface in the presence of NiBr2 and CuBr2 , respectively, in the aqueous solution. Thus, the effect of diverse metal bromides in the reaction mixture for tailor-made growth of the various Pd NS is reported. Among the as-synthesized materials, the Pd NWs stand to be superior catalysts and their efficiency is almost 6 and 2.5 times higher than commercial 20 % Pd/C in the electrooxidation of ethanol and Cr(VI) reduction reaction by formic acid, respectively.

Formation of palladium concave nanocrystals via auto-catalytic tip overgrowth by interplay of reduction kinetics, concentration gradient and surface diffusion

A clear understanding of the growth mechanism involved in the shape-controlled synthesis of noble-metal nanocrystals with concave surfaces can provide useful information for the rational design of novel anisotropic nanostructures with controllable properties. In this paper, we conducted a systematic study of the detailed growth mechanism of the Pd arrow-headed tripods and revealed how the formation of the concave Pd nanocrystals was collectively controlled by the reduction kinetics, concentration gradient of Pd precursors, and surface diffusion of atoms. The formation of the arrow-headed tripods can be attributed to an auto-catalytic tip overgrowth process, where the Pd triangular nanoplate seeds formed under a suitably slow reduction rate can auto-catalyze the dehydrogenation of benzyl alcohol to generate hydrogen atoms [H]. The presence of [H] further dramatically accelerates the reduction of Pd(acac)2, which introduces a concentration gradient of Pd precursors in our non-stirring synthesis system and facilitates the kinetically-controlled tip overgrowth under a concentration gradient to form tripods with troughs on the arms. The final shapes of the concave nanocrystals depend on the relative rate of atom deposition and surface diffusion of atoms, which can be tuned by manipulating the reaction conditions such as the reaction temperature and the stirring conditions. This study presents a new possibility for the rational synthesis of various Pd nanostructures by manipulating the auto-catalytic process and tuning the relative rate of atom deposition and surface diffusion of atoms, which provides useful information for understanding the growth mechanism and the design of other anisotropic noble-metal nanostructures.

Nanostructured platinum-free electrocatalysts in alkaline direct alcohol fuel cells: catalyst design, principles and applications

A review of the fundamental principles that allow for the intelligent design and synthesis of non-precious metal nanostructured electrocatalysts for ADAFCs.

Flower-like Palladium Nanoclusters Decorated Graphene Electrodes for Ultrasensitive and Flexible Hydrogen Gas Sensing

Flower-like palladium nanoclusters (FPNCs) are electrodeposited onto graphene electrode that are prepared by chemical vapor deposition (CVD). The CVD graphene layer is transferred onto a poly(ethylene naphthalate) (PEN) film to provide a mechanical stability and flexibility. The surface of the CVD graphene is functionalized with diaminonaphthalene (DAN) to form flower shapes. Palladium nanoparticles act as templates to mediate the formation of FPNCs, which increase in size with reaction time. The population of FPNCs can be controlled by adjusting the DAN concentration as functionalization solution. These FPNCs_CG electrodes are sensitive to hydrogen gas at room temperature. The sensitivity and response time as a function of the FPNCs population are investigated, resulted in improved performance with increasing population. Furthermore, the minimum detectable level (MDL) of hydrogen is 0.1 ppm, which is at least 2 orders of magnitude lower than that of chemical sensors based on other Pd-based hybrid materials.