Pd Nanodendrites Research Articles

Tunable synthesis of highly branched Pd nanodendrites for enhanced electrocatalysis

Hierarchical porous noble-metal based structures are of great interest for energy electrocatalysis on account of their unique shape which not only provides high accessible surface area and consequently numerous catalytic active sites but also facilitates mass transport for rapid kinetics.We report a wet chemical approach to synthesize highly branched Pd nanodendrites with tunable sizes ranging from 25 to 125 nm via sequential oriented attachment growth and epitaxial growth. With the merits of mesoporous structure, atomic steps and corners, and grain boundaries, the 25 nm Pd nanodendrites deliver peak current density of 1625.5 mA mg−1Pd toward formic acid oxidation, the highest value among all reported branched Pd nanostructures, and exhibit half-wave potential of 0.882 V (vs. RHE) for the oxygen reduction reaction, 25 mV more positive than the Commercial Pt/C. The synthetic strategy demonstrated in this work can be further extended to create advanced nanocrystals for broad applications, such as in energy and biosensing systems and surface-enhanced Raman scattering.

Controllable synthesis of oxygenated carbon supported palladium nanodendrites for highly efficient nitroaromatics reduction

Palladium nanodendrites supported on oxygenated carbon layer (Pd/ O-carbon) have been prepared herein through modulating the mass ratio of Pd (II) precursor and carbon dots derived from sucrose without additional reducing agent. Characterizations of FT-IR, XPS spectra and TEM graphs reveal that oxygen-bearing groups on carbon dots play important role in reducing Pd(II) into Pd(0), controlling nanoparticle growth and stabilizing Pd nanodendrites. The as-prepared Pd/ O-carbon presents size-dependent activity on catalytic p-nitrophenol (4-NP) reduction using NaBH4 as hydrogen donor. The reaction rate constant of 4-NP on Pd/ O-carbon with Pd particle size of 4–7 nm almost triples that on catalyst with Pd nanoparticles of 6–10 nm. Pd / O-carbon can be reused for 10 times without obvious decreased activity, showing a long-term chemical stability in 4-NP reduction reaction. In addition, Pd / O-carbon achieves ∼100% selectivity on nitrobenzene conversion and > 95% selectivity for the hydrogenation of substituted nitrobenzene compounds with methyl-, chloro- and amino- groups to corresponding aniline through tandem reaction with NH3BH3 as hydrogen donor in methanol/ water solvent (volume ratio of 1:3).

Bifunctional Pd@RhPd Core-Shell Nanodendrites for Methanol Electrolysis.

Methanol electrolysis is a promising strategy to achieve energy-saving and efficient electrochemical hydrogen (H2) production. In this system, the advanced electrocatalysts with high catalytic performance for both the methanol oxidation reaction (MOR) and hydrogen evolution reaction (HER) are highly desirable. Inspired by the complementary catalytic properties of rhodium (Rh) and palladium (Pd) for MOR and HER, herein, several Pd core-RhPd alloy shell nanodendrites (Pd@RhPd NDs) are synthesized through the galvanic replacement reaction between Pd nanodendrites (Pd NDs) and rhodium trichloride. For MOR, Pd@RhPd NDs exhibit Rh content-determined catalytic activity, in which Pd@Rh0.07Pd NDs have an optimal combination of oxidation potential and oxidation current due to the synergistic catalytic process of Pd/Rh double active sites. For HER, the introduction of Rh greatly improves the catalytic activity of Pd@RhPd NDs compared to that of Pd NDs, suggesting that Rh is the main activity site for HER. Unlike MOR, however, the HER activity of Pd@RhPd NDs is not sensitive to the Rh content. Using Pd@Rh0.07Pd NDs as robust bifunctional electrocatalysts, the as-constructed two-electrode methanol electrolysis cell shows a much lower voltage (0.813 V) than that of water electrolysis (1.672 V) to achieve electrochemical H2 production at 10 mA cm-2, demonstrating the application prospect of methanol electrolysis for H2 production.

Bifunctional Palladium Hydride Nanodendrite Electrocatalysts for Hydrogen Evolution Integrated with Formate Oxidation.

The rational design of advanced electrocatalysts and energy-saving electrolysis strategies is highly desirable for achieving high-efficiency electrochemical H2 generation yet challenging. In this work, we report highly branched Pd hydride nanodendrites (PdH-NDs) formed by a very facial solvothermal method and a succedent chemical H intercalation method in N,N-dimethylformamide. The electrocatalytic performance of PdH-NDs is experimentally and theoretically correlated with the morphology and composition, which has demonstrated substantially enhanced electrochemical activity and stability for formate oxidation reaction and hydrogen evolution reaction in alkaline electrolyte compared with Pd nanodendrites. Density functional theory calculations suggest a downshift of the Pd d-band center of PdH-NDs due to the dominant Pd-H ligand effects that weaken the binding energies of the intermediate catalytic species and toxic carbon monoxide. The asymmetric formate electrolyzer based on bifunctional PdH-ND electrocatalysts is first constructed, which only requires a low voltage of 0.54 V at 10 mA cm-2 for continuous H2 generation. This study reveals significant insights about the morphology/composition-performance relationship for palladium hydrides with bifunctional electroactivity.

Synthesis of Palladium Nanodendrites Using a Mixture of Cationic and Anionic Surfactants.

Surfactants are used widely to control the synthesis of shaped noble-metal nanoparticles. In this work, a mixture of hexadecyltrimethylammonium bromide (CTAB), a cationic surfactant; sodium oleate (NaOL), an anionic surfactant; palladium chloride; and a reducing agent were used in the seed-mediated synthesis of palladium nanoparticles. By controlling the surfactant mixture ratio, we initially discovered that palladium nanodendrites with narrow size distribution were formed instead of the traditional nanocubes, synthesized with only CTAB. In order to investigate the optimal ratio to produce Pd nanodendrites with a high yield and narrow size distribution, samples synthesized with multiple molar ratios of the two surfactants were prepared and studied by transmission electron microscopy, dynamic light scattering, conductance, and ultraviolet–visible spectroscopy. We propose that the addition of NaOL alters the arrangement of surfactants on the Pd seed surface, leading to a new pattern of growth and aggregation. By studying the nanodendrite growth over time, we identified the reduction period of Pd2+ ions and the formation period of the nanodendrites. Our further experiments, including the replacement of CTAB with hexadecyltrimethylammonium chloride (CTAC) and the replacement of NaOL with sodium stearate, showed that CTA+ ions in CTAB and OL– ions in NaOL play the main roles in the formation of nanodendrites. The formation of palladium nanodendrites was robust and achieved with a range of temperatures, pH and mixing speeds.

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, knor = 59.66 mmol−1 s−1) for PNP and 0.3181 min−1 (knor = 17.6 mmol−1 s−1) for TNT with the corresponding turnover frequencies (TOFs) as 16.06 and 40.80 h−1, respectively.

Benzophenone assisted UV-activated synthesis of unique Pd-nanodendrite embedded reduced graphene oxide nanocomposite: a catalyst for C-C coupling reaction and fuel cell.

In this work we report the use of benzophenone (BP) for the synthesis of a palladium (Pd) embedded on reduced graphene oxide (rGO) nanocomposite (Pd/rGO) using a simple aqueous solution and UV irradiation. The simple and facile evolution of thermodynamically unstable branched Pd(0) nanodendrites was achieved by BP photoactivation, circumventing the growth of more stable nanomorphologies. The synthesis of Pd(0)-embedded rGO nanosheets (PRGO-nd) was made possible by the simultaneous reduction of both the GO scaffold and PdCl2 by introducing BP into the photoactivation reaction. The nanocomposites obtained in the absence of BP were common triangular and twinned Pd(0) structures which were also implanted on the rGO scaffold (PRGO-nt). The disparity in morphologies presumably occurs due to the difference in the kinetics of the reduction of Pd2+ to Pd0 in the presence and absence of the BP photoinitiator. It was observed that the PRGO-nd was composed of dense arrays of multiple Pd branches around nucleation site which exhibited (111) facet, whereas PRGO-nt showed a mixture of (100) and (111) facets. On comparing the catalytic efficiencies of the as-synthesized nanocatalysts, we observed a superiority in efficiency of the thermodynamically unstable PRGO-nd nanocomposite. This is due to the evolved active facets of the dendritic Pd(0) morphology with its higher surface area, as testified by Brunauer–Emmett–Teller (BET) analysis. Since both PRGO-nd and PRGO-nt contain particles of similar size, the dents and grooves in the structure are the cause of the increase in the effective surface area in the case of nanodendrites. The unique dendritic morphology of the PRGO-nd nanostructures makes them a promising material for superior catalysis, due to their high surface area, and the high density of surface atoms at their edges, corners, and stepped regions. We investigated the efficiency of the as-prepared PRGO-nd catalyst in the Suzuki–Miyaura coupling reaction and showed its proficiency in a 2 h reaction at 60 °C using 2 mol% catalyst containing 0.06 mol% active Pd. Moreover, the electrochemical efficiency for the catalytic hydrogen evolution reaction (HER) was demonstrated, in which PRGO-nd provided a decreased overpotential of 68 mV for a current density of 10 mA cm−2, a small Tafel slope of 57 mV dec−1 and commendable stability during chronoamperometric testing for 5 h.

Poly-l-lysine mediated synthesis of palladium nanochain networks and nanodendrites as highly efficient electrocatalysts for formic acid oxidation and hydrogen evolution

The morphology- and size-tunable synthesis of nanocatalysts has attracted substantial research interest especially in catalysis. In this work, we synthesized free-standing Pd nanochain networks (Pd NCNs) and Pd nanodendrites (Pd NDs) through a direct poly-l-lysine (PLL)-mediated one-pot aqueous method. The presence of PLL and its concentrations were critical in this regard, showing PLL as the structure-directing and capping agents during the nucleation and crystal growth procedures. The synthesized architectures exhibited improved catalytic activity and enhanced durability towards formic acid oxidation and hydrogen evolution reactions relative to commercial Pd black catalyst. Moreover, the electrochemical active surface area and the electrocatalytic performance of Pd NCNs were dramatically enhanced in comparison to Pd NDs mainly owing to the unique network-like structure of Pd NCNs.

Controlled Synthesis of Hierarchical Pd Nanodendrites and Their Electrocatalytic Properties.

Well-defined hierarchical Pd nanodendrites with (111) surface were successfully synthesized with Pd(acac)2 as a precursor, polyvinylpyrrolidone (PVP) as a stabilizer, CO as a reducing agent in the presence of appropriate amounts of NaAc and oleylamide (OAm) at 100 °C for 3 h under atmospheric pressure. OAm played an important role in controlling the growth of the hierarchical dendritic nanostructures of the final products. The formation of the hierarchical Pd dendrites was ascribed to the OAm-confined growth from the tips along 〈111〉 axis directions. This kind of hierarchical Pd nanodendrites exhibited an enhanced electrocatalytic activity for the electro-oxidation of formic acid compared with Pd black.

Photocatalytic H2 Generation Efficiencies of TiO2 Nanotube-based Heterostructures Grafted with ZnO Nanorods, Ag Nanoparticles, or Pd Nanodendrites

TiO2 nanotube-based heterostructures grafted with ZnO nanorods, Ag nanoparticles, or Pd nanodendrites were synthesized for photocatalytic H2O/CH3OH splitting and H2 generation. Compared with P25 TiO2 nanoparticles and bare TiO2 nanotubes, these heterostructures, in particular, the one grafted with Pd nanodendrites, were found to present a markedly enhanced photocatalytic H2 generation efficiency (net H2 generation rate ∼143 μmol/h). Rather than the surface area of the photocatalysts, the lifetime (separation) of photogenerated carriers and, in particular, the surface plasmon resonance-stimulated carrier excitation dominated the number of total effective carriers. A power relationship with an exponent of 0.2 between the H2 generation rate and the number of total photogenerated carriers was determined, which suggests that the number of effective carriers or the efficiency in H2O/CH3OH splitting might decay in an exponential way.

Facile synthesis of palladium nanodendrites supported on graphene nanoplatelets: an efficient catalyst for low overpotentials in lithium–oxygen batteries

Morphology-controlled Pd nanodendrites were synthesized on CPC-functionalized graphene and they exhibited excellent catalytic activity towards oxygen reduction and evolution reactions.

Direct synthesis of graphene nanosheets support Pd nanodendrites for electrocatalytic formic acid oxidation

We report a solvothermal method preparation of dendritic Pd nanoparticles (DPNs) and spherical Pd nanoparticles (SPNs) supported on reduced graphene oxide (RGO). Drastically different morphologies of Pd NPs with nanodendritic structures or spherical structures were observed on graphene by controlling the reduction degree of graphene oxide (GO) under mild conditions. In addition to being a commonplace substrate, GO plays a more important role that relies on its surface groups, which serves as a shape-directing agent to direct the dendritic growth. As a result, the obtained DPNs/RGO catalyst exhibits a significantly enhanced electro-catalytic behavior for the oxidation of formic acid compared to the SPNs/RGO catalyst.

Monitoring Ligand‐Mediated Growth and Aggregation of Metal Nanoparticles and Nanodendrites by In Situ Synchrotron Scattering Techniques

AbstractIn situ synchrotron wide‐angle X‐ray scattering (WAXS) and ultra‐small angle X‐ray scattering (USAXS) were used to study the growth of Pd nanodendrites and quasi‐spherical Pd nanoparticles. The metal–ligand environments of both the precursors and growing nanoparticles contribute to nanostructure formation. Specifically, the use of Pd(hfac)2 (hfac=hexafluoroacetylacetonate) as a precursor facilitated nanostructure formation more facilely compared to Pd(acac)2 (acac=acetylacetonate), as quantified by in situ WAXS. This enhanced rate is attributed to the better leaving group properties of the hfac− ligands, and this result demonstrates that fundamental principles from coordination chemistry can guide the selection of precursors in nanomaterial syntheses. This in situ approach also revealed concomitant nucleation, coalescence, and growth processes during the synthesis of monodisperse Pd nanoparticles. Significantly, this finding identifies self‐limited growth mediated by the local ligand environment of the particles as the central means of size distribution control and contradicts the typically cited mechanism (LaMer‐like growth) for such chemical systems. These observations highlight the importance of applying in situ approaches to study nanostructure formation as intertwined processes can be revealed.

Facile and rapid synthesis of Pd nanodendrites for electrocatalysis and surface-enhanced Raman scattering applications.

Numerous properties from metal nanostructures can be tuned by controlling both their size and shape. In particular, the latter is extremely important because the type of crystalline surface affects the surface electronic density. This paper describes a simple approach to the synthesis of highly-structured, anisotropic palladium nanostructured dendrites. They were obtained using an eco-friendly biomolecule 5-hydroxytryptophan, which acts as both a reducing and stabilizing agent. The growth mechanism is proposed for the evolution of dendrites morphology. It was found that the concentration of 5-hydroxytryptophan played a vital role on the morphology of the nanostructured Pd dendrites. This nanomaterial shows enhanced electrocatalytic performance towards the oxidation of formic acid, and it exhibits surface-enhanced Raman scattering properties towards the prostate specific antigen. These properties may be explored in fuel cells and biosensors, respectively.

Rapid room-temperature synthesis of Pd nanodendrites on reduced graphene oxide for catalytic oxidation of ethylene glycol and glycerol

In this paper, a simple, fast, and green method is developed for preparation of uniform Pd nanodendrites anchored on reduced graphene oxide (Pd/RGO) at room temperature, with the assistance of octylphenoxypolye thoxyethanol (NP-40) as a soft template, while no seed, organic solvent, or special apparatus involved. The as-prepared nanocomposites show the improved CO tolerance, enhanced catalytic activity, and better stability for ethylene glycol (EG) and glycerol (Gly) electrooxidation in alkaline media, compared with commercial Pd black and Pd/C catalysts. The synthetic strategy can be extended to fabricate other electrocatalysts in direct alcohol fuel cells.

Facile synthesis of Pt–Pd nanodendrites and their superior electrocatalytic activity

Porous Pt–Pd nanodendrites were fabricated by a co-chemical reduction method using both PVP and urea as the co-stabilizing and co-structure-directing agents. The Pt–Pd nanodendrites displayed high catalytic activity and stability toward the electrooxidation of methanol and ethylene glycol in alkaline media.

Growing metal trees on tubular semiconductor land: TiO2/(Zn,Sn)Pd heterostructures with high SERS and photocatalytic activity

By using sacrificial ZnO nanorods, a self non-uniform electric field was established to yield the aqueous growth of 〈111〉 oriented, single-crystalline (Zn,Sn)-doped Pd nanodendrites on TiO2 nanotubes for synthesizing three-dimensional hydrophobic heterostructures. Structural analyses revealed that primary dendrite arms grew along a 〈111〉 direction group; symmetric branches were built on the basal plane of the arms but sprouted in another [111] direction, similar to a twin structure. Sequential growth processes different from a diffusion-limited aggregation model were suggested, including the dissolution of ZnO nanorods for forming a self non-uniform electric field, the protruding of primary arms towards a Zn2+ concentrated zone, and then the development of branches through a cation depletion zone. A strong surface enhanced Raman scattering and high photocatalytic activity, owing to the strong surface plasmon resonance on the large-surface symmetric structures and the inhibited recombination of photogenerated electron–hole pairs, suggested the high potential of the heterostructures for applications in self-cleaning photocatalysts and nano-optoelectronic devices.

Graphene-induced Pd nanodendrites: A high performance hybrid nanoelectrocatalyst

A facile and green approach has been developed for the in situ synthesis of hybrid nanomaterials based on dendrite-shaped Pd nanostructures supported on graphene (RG). The as-synthesized hybrid nanomaterials (RG-PdnDs) have been thoroughly characterized by high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy and electrochemical techniques. The mechanism of formation of such dendrite-shaped Pd nanostructures on the graphene support has been elucidated using TEM measurements. The RG induces the formation of, and plays a decisive role in shaping, the dendrite morphology of Pd nanostructures on its surface. Cyclic voltammetry and chronoamperometry techniques have been employed to evaluate the electrochemical performance of RG-PdnDs towards oxidation of methanol. The electrochemical (EC) activities of RG-PdnDs are compared with graphene-supported spherical-shaped Pd nanostructures, Pd nanodendrites alone and a commercial available Pd/C counterpart. The combined effect of the graphene support and the dendrite morphology of RG-PdnDs triggers the high electrocatalytic activity and results in robust tolerance to CO poisoning. Open image in new window

One-Step Synthesis of Au–Pd Alloy Nanodendrites and Their Catalytic Activity

Au–Pd bimetallic nanocrystals with well-defined dendritic morphology, uniform size, and homogeneous alloy structure were synthesized in an aqueous solution by using ascorbic acid as the reductant and triblock copolymer P123 as the stabilizing agent. In this synthesis, ascorbic acid played a key role in directing the nanodendritic morphology while the presence of Pd was mandatory for the formation of well-defined nanodendrites. Without Pd, only faceted nanoparticles of gold were formed under the same reaction conditions. Other reaction variables such as the addition sequence and the dropping rate of the metal precursors, the type and concentration of the stabilizing agents, and the reaction time and temperature were all found to affect the size uniformity and morphology perfectness to some extent. The Au/Pd atomic ratio could be tuned in a wide range without deteriorating the nanodendritic morphology. The as-prepared Au–Pd nanodendrites exhibited excellent catalytic activities toward electroxidation of methanol and reduction of 2-nitrophenol, and the catalytic performances could be effectively tuned by the Au/Pd ratio.

Dendritic palladium decorated with gold by potential pulse electrodeposition: Enhanced electrocatalytic activity for H2O2 electroreduction and electrooxidation

Dendritic Au@Pd nanocomposite is uniformly electrodeposited on carbon fiber cloth via a two-step potential pulse technique. The electrode (Au@Pd/CFC) is characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffractometer. It shows a unique open three dimensional structure allowing the full utilization of electrocatalyst. Au is uniformly distributed on the surface of Pd nano-dendrites. H2O2 electrooxidation in KOH solution and electroreduction in H2SO4 solution on the dendritic Au@Pd/CFC electrode are studied by linear sweep voltammetry, chronoamperometry and electrochemical impedance. The electrode exhibits a remarkably higher catalytic activity for both H2O2 electrooxidation and electroreduction than the Pd/CFC electrode. The effects of H2O2, KOH and H2SO4 concentration on the catalytic performance were investigated and the suitable ratios of [H2SO4]/[H2O2] and [KOH]/[H2O2] are determined to be 1 and 4. The cell using it both as the anode and cathode exhibits a much higher performance than those reported in the literatures, that an open circuit voltage is as high as 0.9V and the peak power density can reach 20.7mWcm−2 at 20°C.