Surface engineering of Rh-modified Pd nanocrystals by colloidal underpotential deposition for electrocatalytic methanol oxidation

The development of methods to control the surface structures of metallic nanocatalysts is of vital importance for their application as heterogeneous catalysts in chemical conversions of energy and environmental and chemical engineering. The underpotential deposition (UPD) phenomenon has received considerable interest as a tool for the controllable synthesis of metal nanocrystals and engineering their catalytic performances. Herein, the discovery of UPD of Rh on Pd nanocrystals is reported. More importantly, the UPD of Rh is explored as a strategy to direct the synthesis of Rh-modified Pd nanocrystals with controllable shapes and surface structures. The mechanism of the UPD of Rh on Pd is elucidated in terms of electronegativity difference considerations. Compared with pristine Pd octahedral nanocrystals and commercial carbon-supported Pd catalysts, the Rh-modified Pd octahedral nanocrystals exhibit remarkable electrocatalytic performances during the methanol oxidation reaction in alkaline media. Our discovery heralds a new paradigm for UPD-mediated growth of metal nanocrystals and may provide a mechanistic understanding for the guided design of other colloidal UPD systems in the synthesis and surface engineering of metal nanocrystals.

Synthesis and characterization of size controlled alloy nanoparticles

Bimetallic and multimetallic alloy nanoparticles are emerging as a class of critical nanomaterials in electronic, optical and magnetic fields due to their unique physic-chemical properties. In particular, precise control of the nanoparticle size can endow them with broad versatility and high selectivity. This chapter reviews some tremendous achievements in the development of size controlled bimetallic and multimetallic alloy nanoparticles, with special emphasis on general preparation methods, characterization methodologies and instrumentation techniques. Some key factors and future perspectives on the development of size-controlled bimetallic and multimetallic alloy nanoparticles are also discussed.

Seed-mediated synthesis of Au@PtCu nanostars with rich twin defects as efficient and stable electrocatalysts for methanol oxidation reaction

Pt-based nanocrystals with a twinned structure are highly desirable to achieve high performances in both catalytic activity and durability for methanol oxidation reaction (MOR). However, it still remains great challenge for producing such twinned nanocrystals due to the high internal strain energy of Pt. Here we present a seed-mediated approach to generate Au@PtCu nanostars with a five-fold twinned structure using Au decahedra as seeds. The composition of Pt/Cu in the nanocrystals was tuned by varying the molar ratio of Pt to Cu salt precursors with the amount of Au seeds being the same. Through composition optimization, Au@Pt1.2Cu nanostars achieved the highest specific (1.06 mA cm-2) and mass (0.18 mA mgPt -1) activities for MOR, which were about 5.9 and 1.6 times higher than those of commercial Pt/C, respectively. After accelerated stability test (ADT) for 1500 cycles, such nanostars remained ∼95% of specific activity compared to a loss of ∼28% for commercial Pt/C, indicting their superior durability for MOR. We believed that this enhancement may arise from the unique twinned structure and possible synergetic effect between Pt and Cu components.

Facile Surfactant-Free Synthesis of Composition-Tunable Bimetallic PtCu Alloy Nanosponges for Direct Methanol Fuel Cell Applications

Sponge-like metal nanomaterials have been paid great attention due to their unique structure for wide applications in hydrogen storage, filtration, sensors, heterogeneous catalysis, and fuel cells. Here, we first use a facile, bottom-up method to successfully prepare composition-tunable PtCu alloy nanosponges constructed with sub-4.5 nm particle building blocks. Due to the porous structure, structure defects, and synergetic effect of Pt and Cu, the PtCu alloy nanosponges exhibit good electrocatalytic performances towards methanol oxidation. Compared with pure Pt nanosponges, the specific/mass activity on PtCu2 alloy nanosponges is 5.84/2.93 times that on pure Pt nanosponges. Furthermore, the stability and reactivation ability of PtCu alloy nanosponges are also superior to pure Pt nanosponges.

Monodispersed sub-5.0 nm PtCu nanoalloys as enhanced bifunctional electrocatalysts for oxygen reduction reaction and ethanol oxidation reaction

The development of effective electrocatalysts with enhanced activity and stability for both the anode and the cathode reaction in fuel cells still remains a challenge. Here, we report a one-pot route to prepare monodispersed, uniform sub-5.0 nm PtCu alloy polyhedra with a narrow size distribution. These PtCu alloy polyhedra exhibit enhanced electrocatalytic activity for both cathode and anode reactions as compared to the commercial Pt/C catalyst under alkaline conditions. The specific activity and mass activity on Pt₆₈Cu₃₂ nanoalloys are 15 and 2.8 times that on Pt/C catalyst toward oxygen reduction reaction (ORR), respectively. And the peak current density and mass activity on Pt₆₈Cu₃₂ nanoalloys are 11.8 and 2.12 times that on Pt/C catalyst toward ethanol oxidation reaction (EOR), respectively. Furthermore, the as-synthesized Pt₆₈Cu₃₂ nanoalloys have much higher stability than commercial Pt/C black for both ORR and EOR. These experimental results show an effective approach to the development of monodispersed, sub-5.0 nm PtCu nanoalloys as bifunctional electrocatalysts for both the cathode and the anode reaction in fuel cells.

Structural evolution of concave trimetallic nanocubes with tunable ultra-thin shells for oxygen reduction reaction

For the large-scale commercial application of Pt-based catalysts, minimizing the usage of Pt while retaining the high catalytic activity is crucial. The fabrication of Pd@Pt core-shell structures, good control of surface atom arrangement, and the formation of an alloy structure with 3d transition metals have been approved as efficient approaches to address this issue; however, it remains a challenge to meet all these three requirements. This paper describes a facile green route for preparing concave Pd@PtNi catalysts with controllable structural evolution. The concavity and thickness of the shells could be controlled by tuning specific experimental parameters. We quantitatively analyzed the deposition rate and diffusion rate of adatoms and found that the concave surfaces of Pd@PtNi nanocrystals were due to the distinct atom deposition and diffusion rate. The as-prepared concave Pd@PtNi nanocubes exhibited greatly enhanced catalytic activities (e.g., oxygen reduction reaction) and durability than commercial Pt/C catalysts. DFT calculations further suggest that, the cooperated Ni reduced the adsorption strength of OH, resulting in the enhancement of the electrochemical properties. These results provided an attractive strategy for designing catalysts with controllable morphology and composition by depositing active metals as ultrathin shells.

Shape-controlled synthesis of Au–Pd bimetallic nanocrystals for catalytic applications

This review describes recent progress in the design and synthesis of shape-controlled Au–Pd bimetallic NCs and their emerging catalytic applications.

Crystalline Control of {111} Bounded Pt3Cu Nanocrystals: Multiply-Twinned Pt3Cu Icosahedra with Enhanced Electrocatalytic Properties

Despite that different facets have distinct catalytic behavior, the important role of twin defects on enhancing the catalytic performance of metallic nanocrystals is largely unrevealed. The key challenge in demonstrating the importance of twin defects for catalysis is the extreme difficulties in creating nanostructures with the same exposed facets but tunable twin defects that are suitable for catalytic investigations. Herein, we show an efficient synthetic strategy to selectively synthesize {111}-terminated Pt3Cu nanocrystals with controllable crystalline features. Two distinct {111}-bounded shapes, namely, multiply-twinned Pt3Cu icosahedra and single-crystalline Pt3Cu octahedra, are successfully prepared by simply changing the types of Cu precursors with the other growth parameters unchanged. Electrocatalytic studies show that the {111}-terminated Pt3Cu nanocrystals exhibit the very interesting crystalline nature-dependent electrocatalytic activities toward both the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) with multiply-twinned Pt3Cu icosahedra demonstrating enhanced electrocatalytic activities compared to the single-crystalline Pt3Cu octahedra due to their additional yet important effect of twin defect. As a result, under the multiple tuning conditions (alloy, shape, and twin effects), the multiply-twinned Pt3Cu icosahedra exhibit much enhanced electrocatalytic activities in both ORR and MOR with respect to the Pt black. The present work highlights the importance of twin defects in enhancing electrocatalytic activities of metallic nanocrystals.

Highly active and durable platinum-lead bimetallic alloy nanoflowers for formic acid electrooxidation

The Pt84Pb16 (atomic ratio) bimetallic alloy nanoflowers (Pt84Pb16 BANFs) are synthesized by a simple one-pot hydrothermal reduction method that effectively enhance the dehydrogenation pathway of the formic acid oxidation reaction (FAOR) due to the ensemble effect and the electronic effect. As a result, the mass activity of Pt84Pb16 BANFs for the FAOR is 16.7 times higher than that of commercial Pt black at 0.3 V potential.

Composition-tunable synthesis of Pt–Cu octahedral alloy nanocrystals from PtCu to PtCu3via underpotential-deposition-like process and their electro-catalytic properties

Octahedral Pt–Cu alloy nanocrystals with tunable composition from PtCu to PtCu₃ was successfully synthesized via UPD-like process.

CTAB-reduced synthesis of urchin-like Pt–Cu alloy nanostructures and catalysis study towards the methanol oxidation reaction

Urchin-like PtCu alloy nanostructures were fabricated using a CTAB-reduced approach, and they exhibited enhanced catalytic performance towards MOR.

Component, Microstructure and Simulation Calculation Study of Bimetallic Pt-Cu Alloys Towards Catalyzing Methanol Oxidation Reaction

Three Pt - Cu alloys ( Pt 0.3 Cu 0.7, Pt 0.5 Cu 0.5, and Pt 0.7 Cu 0.3) with different Cu contents were synthesized by adjusting the Pt / Cu precursor ratio, and their electrocatalytic activities for methanol oxidation reaction (MOR) were systematically studied. Component and microstructure study revealed that the relationship between the lattice parameters and Cu content followed the Vegard's law. Electrochemistry measurement showed that the MOR catalytic activity for Pt - Cu alloys displayed a dependency on the Cu content, and it decreased following: Pt 0.3 Cu 0.7 > Pt 0.5 Cu 0.5 > Pt 0.7 Cu 0.3. Among the three Pt - Cu alloys, the Pt 0.3 Cu 0.7 alloy exhibited the highest oxidation current density and best CO tolerance activity. Density functional theory simulation calculation, taking into account the shrinking of Pt - Cu alloy's lattice after Cu incorporation, confirmed that the adsorption energy of CO also displayed a dependency on the Cu content in Pt - Cu alloys, and it increased following: Pt 0.3 Cu 0.7 < Pt 0.5 Cu 0.5 < Pt 0.7 Cu 0.3 < Pt , which could rationally explain the best CO tolerance ability for the Pt 0.3 Cu 0.7.

Learning from nature: introducing an epiphyte-host relationship in the synthesis of alloy nanoparticles by co-reduction methods

This communication reports an epiphytic co-reduction method which can overcome the common tendency of sequential deposition in the synthesis of alloy nanoparticles. In this method the reduction of one of the metals (the epiphyte-metal) is only turned-on and rendered more facile by the in situ generated fresh surfaces of the other metal (the host-metal).

Dendritic Au/Pt and Au/PtCu nanowires with enhanced electrocatalytic activity for methanol electrooxidation

The high-yield synthesis of dendritic Au/Pt and Au/PtCu nanowires is achieved through an effective heterogeneous, epitaxial growth strategy conducted in the water-phase to grow dendritic Pt and PtCu nanoshells on Au nanowires. The synthesized products exhibit excellent electrocatalytic activity towards methanol electrooxidation.

Wet chemical synthesis of intermetallic Pt3Zn nanocrystals via weak reduction reaction together with UPD process and their excellent electrocatalytic performances

Platinum based alloy nanocrystals are promising catalysts for a variety of important practical process. However, it remains a great challenge to synthesize platinum-based intermetallic compound nanocrystals with well-defined surface structures. In this communication, taking the synthesis of concave cubic intermetallic Pt3Zn nanocrystals with {hk0} facets as an example, we proposed a new synthesis strategy for intermetallic compounds by reduction of noble metal precursors via a slow reduction process and reduction of transition metal ions via an underpotential deposition (UPD) process in wet chemical synthesis. The as-prepared intermetallic Pt3Zn nanocrystals exhibited superior CO poisoning tolerance and high electro-catalytic activity in both methanol and formic acid oxidation reactions in comparison with solid solution Pt3Zn nanocrystals and Pt/C.

Autocatalysis and selective oxidative etching induced synthesis of platinum-copper bimetallic alloy nanodendrites electrocatalysts

The controllable synthesis of noble metal alloy nanostructures with highly branched morphology has attracted much attention because of their specific physical and chemical properties. This article reports the synthesis of platinum-copper bimetallic alloy nanodendrites (Pt-Cu BANDs) by a facile, one-pot, templateless, and seedless hydrothermal method in the presence of poly(allylamine hydrochloride) (PAH) and formaldehyde (HCHO). The morphology, composition, and structure of Pt-Cu BANDs are fully characterized by various physical techniques, demonstrating Pt-Cu BANDs are highly alloying, porous, and self-supported nanostructures. The formation/growth mechanism of Pt-Cu BANDs is explored and discussed based on the experimental observations. The autocatalytic growth and interdiffusion are responsible for the formation of Pt-Cu alloy whereas selective oxidative etching results in dendritic morphology of Pt-Cu alloy nanostructures. In addition, the electrocatalytic activity and stability of Pt-Cu BANDs for the methanol oxidation reaction (MOR) are investigated by various electrochemical techniques. The synthesized Pt-Cu BANDs show higher electrocatalytic activity and stability than commercially available Pt black.

Octahedral noble-metal nanoparticles and their electrocatalytic properties

Octahedrally shaped noble-metal nanocrystals are fascinating for their unique properties, such as electrocatalytic, catalytic, plasmonic, and optical behavior, owing to their exclusively exposed {111} facets; Oh symmetric structure; and close-packed surface atoms in low-index surface categories, which are normally stable in a reaction. A series of protocols in the preparation of noble-metal nano-octahedra through a wet-chemical synthetic strategy have been developed in recent years. Herein, advances in synthetic approaches and mechanistic studies of noble-metal nano-octahedra are systematically discussed and key factors, including reduction kinetics, selective capping, and epitaxial growth, are outlined. Their unique performance as advanced electrocatalysts towards fuel-cell reactions is highlighted as well.