Mineralizer-Assisted Shape-Control of Rare Earth Oxide Nanoplates

Rare earth oxides are important emerging materials because of their unique properties. Nanosized, two-dimentional (2D) materials have received increasing attention due to their high surface-to-volume ratios and ultrathin layered structures. Here, we synthesize 2D rare earth (RE) oxide nanoplates in the presence of a mineralizer. The use of a mineralizer not only facilitates the synthesis of RE oxide nanoplates (i.e., increases the yield and allows mild reaction parameters), but also allows for shape-control. To emphasize the importance of RE oxide nanoplates in materials science and engineering, we demonstrate that (1) ceria nanoplates can be used in a ceria/Cu inverse catalyst to enable an enhanced CO oxidation activity, and (2) Y₂O₃ nanoplates doped with Eu³⁺ enable photon energy down-conversion

Synthesis, Shape Control, and Methanol Electro-oxidation Properties of Pt–Zn Alloy and Pt₃Zn Intermetallic Nanocrystals

We report the first synthesis of highly monodisperse Pt₃Zn nanocrystals (NCs). Shape-controlled synthesis generates cubic and spherical Pt–Zn NCs. Reaction temperature is the key to incorporate Zn into Pt, even in the absence of a strong reducing agent. The Pt–Zn NCs are active toward methanol oxidation, with the spherical NCs exhibiting higher activity than the cubic NCs. The Pt–Zn alloy phase can be transformed into the Pt₃Zn intermetallic phase, upon annealing. The intermetallic Pt₃Zn shows better performance than the alloy phase Pt–Zn. Besides the activity toward methanol oxidation, Pt–Zn NCs show excellent poisoning tolerance. With activities comparable to the commercial Pt catalyst, enhanced poisoning tolerance and lower cost, Pt–Zn and Pt₃Zn NCs are a promising new family of catalysts for direct methanol fuel cells

Ag–Sn Bimetallic Catalyst with a Core–Shell Structure for CO₂ Reduction

Converting greenhouse gas carbon dioxide (CO₂) to value-added chemicals is an appealing approach to tackle CO₂ emission challenges. The chemical transformation of CO₂ requires suitable catalysts that can lower the activation energy barrier, thus minimizing the energy penalty associated with the CO₂ reduction reaction. First-row transition metals are potential candidates as catalysts for electrochemical CO₂ reduction; however, their high oxygen affinity makes them easy to be oxidized, which could, in turn, strongly affect the catalytic properties of metal-based catalysts. In this work, we propose a strategy to synthesize Ag–Sn electrocatalysts with a core–shell nanostructure that contains a bimetallic core responsible for high electronic conductivity and an ultrathin partially oxidized shell for catalytic CO₂ conversion. This concept was demonstrated by a series of Ag–Sn bimetallic electrocatalysts. At an optimal SnO_<i>x</i> shell thickness of ∼1.7 nm, the catalyst exhibited a high formate Faradaic efficiency of ∼80% and a formate partial current density of ∼16 mA cm^–2 at −0.8 V vs RHE, a remarkable performance in comparison to state-of-the-art formate-selective CO₂ reduction catalysts. Density-functional theory calculations showed that oxygen vacancies on the SnO (101) surface are stable at highly negative potentials and crucial for CO₂ activation. In addition, the adsorption energy of CO₂^– at these oxygen-vacant sites can be used as the descriptor for catalytic performance because of its linear correlation to OCHO* and COOH*, two critical intermediates for the HCOOH and CO formation pathways, respectively. The volcano-like relationship between catalytic activity toward formate as a function of the bulk Sn concentration arises from the competing effects of favorable stabilization of OCHO* by lattice expansion and the electron conductivity loss due to the increased thickness of the SnO_<i>x</i> layer

Multimetallic Core/Interlayer/Shell Nanostructures as Advanced Electrocatalysts

The fine balance between activity and durability is crucial for the development of high performance electrocatalysts. The importance of atomic structure and compositional gradients is a guiding principle in exploiting the knowledge from well-defined materials in the design of novel class of core–shell electrocatalysts comprising Ni core, Au interlayer, and PtNi shell (Ni@Au@PtNi). This multimetallic system is found to have the optimal balance of activity and durability due to the synergy between the stabilizing effect of subsurface Au and modified electronic structure of surface Pt through interaction with subsurface Ni atoms. The electrocatalysts with Ni@Au@PtNi core-interlayer-shell structure exhibit high intrinsic and mass activities as well as superior durability for the oxygen reduction reaction with less than 10% activity loss after 10 000 potential cycles between 0.6 and 1.1 V vs the reversible hydrogen electrode

Design of Pt–Pd Binary Superlattices Exploiting Shape Effects and Synergistic Effects for Oxygen Reduction Reactions

Large-area icosahedral-AB₁₃-type Pt–Pd binary superlattices (BNSLs) are fabricated through self-assembly of 6 nm Pd nanocrystals (NCs) and 13 nm Pt octahedra at a liquid–air interface. The Pt–Pd BNSLs enable a high activity toward electrocatalysis of oxygen reduction reaction (ORR) by successfully exploiting the shape effects of Pt NCs and synergistic effects of Pt–Pd into a single crystalline nanostructure. The Pt–Pd BNSLs are promising catalysts for the oxygen electrode of fuel cells

Highly Active Pt₃Pb and Core–Shell Pt₃Pb–Pt Electrocatalysts for Formic Acid Oxidation

Formic acid is a promising chemical fuel for fuel cell applications. However, due to the dominance of the indirect reaction pathway and strong poisoning effects, the development of direct formic acid fuel cells has been impeded by the low activity of existing electrocatalysts at desirable operating voltage. We report the first synthesis of Pt₃Pb nanocrystals through solution phase synthesis and show they are highly efficient formic acid oxidation electrocatalysts. The activity can be further improved by manipulating the Pt₃Pb–Pt core–shell structure. Combined experimental and theoretical studies suggest that the high activity from Pt₃Pb and the Pt–Pb core–shell nanocrystals results from the elimination of CO poisoning and decreased barriers for the dehydrogenation steps. Therefore, the Pt₃Pb and Pt–Pb core–shell nanocrystals can improve the performance of direct formic acid fuel cells at desired operating voltage to enable their practical application

Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives

We report an improved synthesis of colloidal gold nanorods (NRs) by using aromatic additives that reduce the concentration of hexadecyltrimethylammonium bromide surfactant to ∼0.05 M as opposed to 0.1 M in well-established protocols. The method optimizes the synthesis for each of the 11 additives studied, allowing a rich array of monodisperse gold NRs with longitudinal surface plasmon resonance tunable from 627 to 1246 nm to be generated. The gold NRs form large-area ordered assemblies upon slow evaporation of NR solution, exhibiting liquid crystalline ordering and several distinct local packing motifs that are dependent upon the NR’s aspect ratio. Tailored synthesis of gold NRs with simultaneous improvements in monodispersity and dimensional tunability through rational introduction of additives will not only help to better understand the mechanism of seed-mediated growth of gold NRs but also advance the research on plasmonic metamaterials incorporating anisotropic metal nanostructures

Atomic Structure of Pt₃Ni Nanoframe Electrocatalysts by <i>in Situ</i> X‑ray Absorption Spectroscopy

Understanding the atomic structure of a catalyst is crucial to exposing the source of its performance characteristics. It is highly unlikely that a catalyst remains the same under reaction conditions when compared to as-synthesized. Hence, the ideal experiment to study the catalyst structure should be performed <i>in situ</i>. Here, we use X-ray absorption spectroscopy (XAS) as an <i>in situ</i> technique to study Pt₃Ni nanoframe particles which have been proven to be an excellent electrocatalyst for the oxygen reduction reaction (ORR). The surface characteristics of the nanoframes were probed through electrochemical hydrogen underpotential deposition and carbon monoxide electrooxidation, which showed that nanoframe surfaces with different structure exhibit varying levels of binding strength to adsorbate molecules. It is well-known that Pt-skin formation on Pt–Ni catalysts will enhance ORR activity by weakening the binding energy between the surface and adsorbates. <i>Ex situ</i> and <i>in situ</i> XAS results reveal that nanoframes which bind adsorbates more strongly have a rougher Pt surface caused by insufficient segregation of Pt to the surface and consequent Ni dissolution. In contrast, nanoframes which exhibit extremely high ORR activity simultaneously demonstrate more significant segregation of Pt over Ni-rich subsurface layers, allowing better formation of the critical Pt-skin. This work demonstrates that the high ORR activity of the Pt₃Ni hollow nanoframes depends on successful formation of the Pt-skin surface structure

Heterogeneous Catalysts Need Not Be so “Heterogeneous”: Monodisperse Pt Nanocrystals by Combining Shape-Controlled Synthesis and Purification by Colloidal Recrystallization

Well-defined surfaces of Pt have been extensively studied for various catalytic processes. However, industrial catalysts are mostly composed of fine particles (e.g., nanocrystals), due to the desire for a high surface to volume ratio. Therefore, it is very important to explore and understand the catalytic processes both at nanoscale and on extended surfaces. In this report, a general synthetic method is described to prepare Pt nanocrystals with various morphologies. The synthesized Pt nanocrystals are further purified by exploiting the “self-cleaning” effect which results from the “colloidal recrystallization” of Pt supercrystals. The resulting high-purity nanocrystals enable the direct comparison of the reactivity of the {111} and {100} facets for important catalytic reactions. With these high-purity Pt nanocrystals, we have made several observations: Pt octahedra show higher poisoning tolerance in the electrooxidation of formic acid than Pt cubes; the oxidation of CO on Pt nanocrystals is structure insensitive when the partial pressure ratio <i>p</i>_O2/<i>p</i>_CO is close to or less than 0.5, while it is structure sensitive in the O₂-rich environment; Pt octahedra have a lower activation energy than Pt cubes when catalyzing the electron transfer reaction between hexacyanoferrate (III) and thiosulfate ions. Through electrocatalysis, gas-phase-catalysis of CO oxidation, and a liquid-phase-catalysis of electron transfer reaction, we demonstrate that high quality Pt nanocrystals which have {111} and {100} facets selectively expose are ideal model materials to study catalysis at nanoscale

Engineering Catalytic Contacts and Thermal Stability: Gold/Iron Oxide Binary Nanocrystal Superlattices for CO Oxidation

Well-defined surface, such as surface of a single crystal, is being used to provide precise interpretation of catalytic processes, while the nanoparticulate model catalyst more closely represents the real catalysts that are used in industrial processes. Nanocrystal superlattice, which combines the chemical and physical properties of different materials in a single crystalline structure, is an ideal model catalyst, that bridge between conventional models and real catalysts. We identify the active sites for carbon monoxide (CO) oxidation on Au-FeO_<i>x</i> catalysts by using Au-FeO_<i>x</i> binary superlattices correlating the activity to the number density of catalytic contacts between Au and FeO_<i>x</i>. Moreover, using nanocrystal superlattices, we propose a general strategy of keeping active metals spatially confined to enhance the stability of metal catalysts. With a great range of nanocrystal superlattice structures and compositions, we establish that nanocrystal superlattices are useful model materials through which to explore, understand, and improve catalytic processes bridging the gap between traditional single crystal and supported catalyst studies