Legna Figueroa-Cosme

Pt-Based Icosahedral Nanocages: Using a Combination of {111} Facets, Twin Defects, and Ultrathin Walls to Greatly Enhance Their Activity toward Oxygen Reduction

Engineering the surface structure of noble-metal nanocrystals offers an effective route to the development of catalysts or electrocatalysts with greatly enhanced activity. Here, we report the synthesis of Pt-based icosahedral nanocages whose surface is enclosed by both {111} facets and twin boundaries while the wall thickness can be made as thin as six atomic layers. The nanocages are derived from Pd@Pt4.5L icosahedra by selectively etching away the Pd in the core. During etching, the multiply twinned structure can be fully retained whereas the Pt atoms in the wall reconstruct to eliminate the corrugated structure built in the original Pt shell. The Pt-based icosahedral nanocages show a specific activity of 3.50 mA cm(-2) toward the oxygen reduction reaction, much greater than those of the Pt-based octahedral nanocages (1.98 mA cm(-2)) and a state-of-the-art commercial Pt/C catalyst (0.35 mA cm(-2)). After 5000 cycles of accelerated durability test, the mass activity of the Pt-based icosahedral nanocages drops from 1.28 to 0.76 A mg(-1)Pt, which is still about four times greater than that of the original Pt/C catalyst (0.19 A mg(-1)Pt).

Facile synthesis of iridium nanocrystals with well-controlled facets using seed-mediated growth.

Iridium nanoparticles have only been reported with roughly spherical shapes and sizes of 1-5 nm, making it impossible to investigate their facet-dependent catalytic properties. Here we report for the first time a simple method based on seed-mediated growth for the facile synthesis of Ir nanocrystals with well-controlled facets. The essence of this approach is to coat an ultrathin conformal shell of Ir on a Pd seed with a well-defined shape at a relatively high temperature to ensure fast surface diffusion. In this way, the facets on the initial Pd seed are faithfully replicated in the resultant Pd@Ir core-shell nanocrystal. With 6 nm Pd cubes and octahedra encased by {100} and {111} facets, respectively, as the seeds, we have successfully generated Pd@Ir cubes and octahedra covered by Ir{100} and Ir{111} facets. The Pd@Ir cubes showed higher H2 selectivity (31.8% vs 8.9%) toward the decomposition of hydrazine compared with Pd@Ir octahedra with roughly the same size.

Synthesis and Characterization of Ru Cubic Nanocages with a Face-Centered Cubic Structure by Templating with Pd Nanocubes

Nanocages have received considerable attention in recent years for catalytic applications owing to their high utilization efficiency of atoms and well-defined facets. Here we report, for the first time, the synthesis of Ru cubic nanocages with ultrathin walls, in which the atoms are crystallized in a face-centered cubic (fcc) rather than hexagonal close-packed (hcp) structure. The key to the success of this synthesis is to ensure layer-by-layer deposition of Ru atoms on the surface of Pd cubic seeds by controlling the reaction temperature and the injection rate of a Ru(III) precursor. By selectively etching away the Pd from the Pd@Ru core-shell nanocubes, we obtain Ru nanocages with an average wall thickness of 1.1 nm or about six atomic layers. Most importantly, the Ru nanocages adopt an fcc crystal structure rather than the hcp structure observed in bulk Ru. The synthesis has been successfully applied to Pd cubic seeds with different edge lengths in the range of 6-18 nm, with smaller seeds being more favorable for the formation of Ru shells with a flat, smooth surface due to shorter distance for the surface diffusion of the Ru adatoms. Self-consistent density functional theory calculations indicate that these unique fcc-structured Ru nanocages might possess promising catalytic properties for ammonia synthesis compared to hcp Ru(0001), on the basis of strengthened binding of atomic N and substantially reduced activation energies for N2 dissociation, which is the rate-determining step for ammonia synthesis on hcp Ru catalysts.

Coating Pt-Ni Octahedra with Ultrathin Pt Shells to Enhance the Durability without Compromising the Activity toward Oxygen Reduction.

We describe a new strategy to enhance the catalytic durability of Pt-Ni octahedral nanocrystals in the oxygen reduction reaction (ORR) by conformally depositing an ultrathin Pt shell on the surface. The Pt-Ni octahedra were synthesized according to a protocol reported previously and then employed directly as seeds for the conformal deposition of ultrathin Pt shells by introducing a Pt precursor dropwise at 200 °C. The amount of Pt precursor was adjusted relative to the number of Pt-Ni octahedra involved to obtain Pt-Ni@Pt1.5L octahedra of 12 nm in edge length for the systematic evaluation of their chemical stability and catalytic durability compared to Pt-Ni octahedra. Specifically, we compared the elemental compositions of the octahedra before and after treatment with acetic and sulfuric acids. We also examined their electrocatalytic stability toward the ORR through an accelerated durability test by using a rotating disk electrode method. Even after treatment with sulfuric acid for 24 h, the Pt-Ni@Pt1.5L octahedra maintained their original Ni content, whereas 11 % of the Ni was lost from the Pt-Ni octahedra. After 10 000 cycles of ORR, the mass activity of the Pt-Ni octahedra decreased by 75 %, whereas the Pt-Ni@Pt1.5L octahedra only showed a 25 % reduction.

Palladium@Platinum Concave Nanocubes with Enhanced Catalytic Activity toward Oxygen Reduction

We report the facile synthesis of Pd@Pt core–shell nanocubes with concave facets on the surface and their enhanced activities toward the oxygen reduction reaction. The success of this synthesis depends critically on our ability to manipulate both the deposition rate of atoms and the surface diffusion rate of the adatoms. In the first step, Pd nanocubes with side faces capped by Br− ions are transformed into concave nanocubes by seed‐mediated growth. The PdCl42− precursor is added in one shot, and the growth is conducted at 40 °C to accelerate deposition and decelerate surface diffusion. For the synthesis of Pd@Pt core–shell concave nanocubes, the PtCl42− precursor is added dropwise into a suspension of the Pd concave nanocubes and reduced by citric acid at 90 °C. As such, we can deposit an ultrathin Pt shell on the surface of each Pd concave nanocube. Despite their much larger size, the Pd@Pt concave nanocubes exhibit greater specific electrochemical surface areas than a commercial Pt/C catalyst, which implies the more efficient dispersion of Pt atoms on the surface. In addition to excellent durability, the catalyst based on Pd@Pt concave nanocubes shows enhanced catalytic activities toward the oxygen reduction reaction.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals

Significance Controlling the shape of colloidal metal nanocrystals is central to the realization of their diverse applications in catalysis, photonics, electronics, and medicine. Here, we demonstrate that autocatalytic surface reduction can be employed to enable the formation of metal nanocrystals with well-controlled and predictable shapes through seed-mediated growth. Our quantitative analysis suggests that the kinetics of autocatalytic surface reduction is highly sensitive to the atomic structures on the surface of the seed, leading to different growth rates for different sites on the seed and eventually resulting in the evolution of nanocrystals into different shapes. The mechanistic insights into autocatalytic surface reduction obtained in this work can be extended to other systems involving nanocrystals with different compositions, facets, and structures. The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Facile Synthesis of Pd@Pt3−4L Core−Shell Octahedra with a Clean Surface and thus Enhanced Activity toward Oxygen Reduction

The presence of a capping agent or stabilizer in the synthesis of colloidal metal nanocrystals will compromise their performance if employed as electrocatalysts. Herein we demonstrate the synthesis of Pd@Pt3–4L core–shell octahedral nanocrystals with greatly enhanced activity toward the oxygen reduction reaction by eliminating the use of any capping agent or stabilizer. This was achieved by employing Pd octahedral seeds with well‐defined {1 1 1} facets and by dispersing them on a carbon black support prior to Pt deposition. Upon optimization of the reaction conditions, Pt ultrathin shells could be conformally deposited on the Pd octahedral seeds in a layer‐by‐layer fashion without involving self‐nucleation or island growth for the Pt atoms. The as‐obtained octahedral Pd@Pt3–4L/C catalyst exhibited a specific activity 50 % greater than that of a reference sample prepared in the presence of a polymer stabilizer such as poly(vinyl pyrrolidone). The polymer‐free catalyst also showed 5‐fold enhancement in specific activity if benchmarked against a commercial Pt/C catalyst.

Synthesis of Palladium Nanoscale Octahedra through a One‐Pot, Dual‐Reductant Route and Kinetic Analysis

Shape-controlled synthesis of colloidal metal nanocrystals has traditionally relied on the use of an approach that involves the reduction of a metal precursor by a single reductant. Once the concentration of atoms surpasses supersaturation, they will undergo homogeneous nucleation to generate nuclei and then seeds, followed by further growth into nanocrystals. In general, it is a grand challenge to optimize such an approach because the kinetic requirement for nucleation tends to be drastically different from what is needed to guide the growth process. In this work, we overcome this difficulty by switching to a dual-reductant approach, in which both strong and weak reductants are added into the same reaction solution. By controlling their amounts to program the reduction kinetics, the strong reductant only regulates the homogeneous nucleation process to generate the desired seeds, and once consumed, the weak reductant takes over to control the growth pattern and thereby the shape of the resulting nanocrystals.

Enhancing the Tactile and Near-Infrared Sensing Capabilities of Electrospun PVDF Nanofibers with the Use of Gold Nanocages

Owing to its piezoelectric and pyroelectric properties, poly(vinylidene fluoride) (PVDF) has been extensively explored for applications related to tactile sensing, energy harvesting, and thermal imaging. However, PVDF cannot be directly used to detect light because of its weak absorption in the visible and near-infrared (NIR) regions, preventing effective conversion from light to heat and then electrical signal. In this work, we address this issue by incorporating Au nanocages (AuNCs) into PVDF nanofibers during electrospinning. The strong and tunable optical absorption associated with AuNCs makes them an effective transducer for converting light to heat and then electrical signal. The presence of AuNCs and the strong electric field inherent to electrospinning both promote the formation of the ferroelectric β phase for maximal piezoelectric and pyroelectric conversions. With the incorporation of AuNCs, the electrospun PVDF nanofibers show enhanced capabilities for tactile and NIR sensing. While the voltage output under the tactile force is increased by 12.6-fold relative to the case of pristine PVDF nanofibers, a voltage output of 7.2 V is achieved when the hybrid device is subjected to the on/off cycles of NIR irradiation by an 808-nm diode laser at a power density of 0.2 W/cm2.

A facile, robust and scalable method for the synthesis of Pd nanoplates with hydroxylamine as a reducing agent and mechanistic insights from kinetic analysis

Nanocrystals lined with stacking faults have received much attention in recent years due to their typical anisotropic, plate-like geometry and their perplexing formation mechanism. In this report, we introduce a simple and reliable method for the scalable production of Pd nanoplates containing stacking faults. The success of our protocol was reliant on the use of hydroxylamine as a reducing agent, which allowed the nucleation and growth of well-defined Pd nanoplates with an average edge length of 24.5 ± 6.6 nm and a thickness of 4.7 ± 0.4 nm. We conducted a kinetic analysis to validate the importance of an appropriate initial reduction rate in determining the formation of seeds lined with stacking faults. To demonstrate the robustness of this synthesis, we conducted a set of control experiments under different experimental conditions such as acidity, temperature, and chemical environment and demonstrated that Pd nanoplates could be obtained as final products in all scenarios. We further extended the batch-based synthesis to a continuous flow reactor, moving one step closer towards high-volume production. Taken together, this method offers both simplicity and reproducibility for the synthesis of Pd nanoplates, which will enable future mechanistic studies and applications.