Tailoring Chemical Composition To Achieve Enhanced Methanol Oxidation Reaction and Methanol-Tolerant Oxygen Reduction Reaction Performance in Palladium-Based Nanowire Systems

In this article, we address two key challenges in the development of electrocatalysts for direct methanol fuel cells by rationally tailoring the morphology and chemical composition of Pd-based nanowires (NWs) for enhanced performance. First, we have examined the morphology and composition-dependent performance of Pt_1–<i>x</i>Pd_<i>x</i> NWs toward the methanol oxidation reaction (MOR). Elemental Pt NWs were found to possess a significant morphology-dependent enhancement of nearly 3-fold in terms of peak MOR-specific activity over that of commercial Pt NP/C. In addition, tailoring the chemical composition in Pt_1–<i>x</i>Pd_<i>x</i> NWs can lead to measurable increases in MOR kinetics, which can be attributed to improved oxidation of formic acid and, potentially, increased selectivity for a direct, CO-free pathway. Second, we have explored the stability of ORR performance in the presence of measurable concentrations of methanol as a function of chemical composition in Pt_1–<i>x</i>Pd_<i>x</i> NWs and Pt-free Pd₉Au NWs. In the context of the Pt_1–<i>x</i>Pd_<i>x</i> NWs, a distinctive volcano-type dependence has been noted with respect to chemical composition, and on the basis of the MOR activities and methanol tolerant ORR behavior, Pt₇Pd₃ NWs have been highlighted as an optimal catalyst architecture. We have also analyzed the methanol tolerance in Pd₉Au NWs, which represents a highly active, durable Pt-free alternative to traditional Pt-based nanostructured catalysts. Herein, we have demonstrated that Pd₉Au NWs (0.42 mA/cm²) with no effective Pt content can outperform Pt-based nanostructures, such as Pt NWs (0.32 mA/cm²) and nanoparticulate Pt NP/C (0.24 mA/cm²) in the presence of 4 mM methanol/0.1 M HClO₄

Observation of Photoinduced Charge Transfer in Novel Luminescent CdSe Quantum Dot–CePO₄:Tb Metal Oxide Nanowire Composite Heterostructures

We report on the synthesis, structural characterization, and intrinsic charge transfer processes associated with novel luminescent zero-dimensional (0D) CdSe nanocrystal–one-dimensional (1D) CePO₄:Tb nanowire composite heterostructures. Specifically, ∼4 nm CdSe quantum dots (QDs) have been successfully anchored onto high-aspect ratio CePO₄:Tb nanowires, measuring ∼65 nm in diameter and ∼2 μm in length. Composite formation was confirmed by high-resolution transmission microscopy, energy-dispersive X-ray spectroscopy mapping, and confocal microscopy. Photoluminescence (PL) spectra, emission decay, and optical absorption of these nanoscale heterostructures were collected and compared with those of single, discrete CdSe QDs and CePO₄:Tb nanowires. We show that our composite heterostructure evinces both PL quenching and a shorter average lifetime as compared with unbound CdSe QDs and CePO₄:Tb nanowires. We propose that a photoinduced 0D–1D charge transfer process occurs between CdSe and CePO₄:Tb and that it represents the predominant mechanism, accounting for the observed PL quenching and shorter lifetimes noted in our composite heterostructures. Data are additionally explained in the context of the inherent energy level alignments of both CdSe QDs and CePO₄:Tb nanowires

Multifunctional Ultrathin Pd_<i>x</i>Cu_1–<i>x</i> and Pt∼Pd_<i>x</i>Cu_1–<i>x</i> One-Dimensional Nanowire Motifs for Various Small Molecule Oxidation Reactions

Developing novel electrocatalysts for small molecule oxidation processes, including formic acid oxidation (FAOR), methanol oxidation reaction (MOR), and ethanol oxidation reaction (EOR), denoting the key anodic reactions for their respective fuel cell configurations, is a significant and relevant theme of recent efforts in the field. Herein, in this report, we demonstrated a concerted effort to couple and combine the benefits of small size, anisotropic morphology, and tunable chemical composition in order to devise a novel “family” of functional architectures. In particular, we have fabricated not only ultrathin 1-D Pd_1–<i>x</i>Cu_<i>x</i> alloys but also Pt-coated Pd_1–<i>x</i>Cu_<i>x</i> (i.e., Pt∼Pd_1–<i>x</i>Cu_<i>x</i>; herein the ∼ indicates an intimate association, but not necessarily actual bond formation, between the inner bimetallic core and the Pt outer shell) core–shell hierarchical nanostructures with readily tunable chemical compositions by utilizing a facile, surfactant-based, wet chemical synthesis coupled with a Cu underpotential deposition technique. Our main finding is that our series of as-prepared nanowires are functionally flexible. More precisely, we demonstrate that various examples within this “family” of structural motifs can be tailored for exceptional activity with all 3 of these important electrocatalytic reactions. In particular, we note that our series of Pd_1–<i>x</i>Cu_<i>x</i> nanowires all exhibit enhanced FAOR activities as compared with not only analogous Pd ultrathin nanowires but also commercial Pt and Pd standards, with Pd₉Cu representing the “optimal” composition. Moreover, our group of Pt∼Pd_1–<i>x</i>Cu_<i>x</i> nanowires consistently outperformed not only commercial Pt NPs but also ultrathin Pt nanowires by several fold orders of magnitude for both the MOR and EOR reactions in alkaline media. The variation of the MOR and EOR performance with the chemical composition of our ultrathin Pt∼Pd_1–<i>x</i>Cu_<i>x</i> nanowires was also discussed

Probing Ultrathin One-Dimensional Pd–Ni Nanostructures As Oxygen Reduction Reaction Catalysts

An ambient, surfactant-based synthetic means was used to prepare ultrathin binary (<i>d</i> ∼ 2 nm) Pd–Ni nanowires, which were subsequently purified using a novel butylamine-based surfactant-exchange process coupled with an electrochemical CO adsorption and stripping treatment to expose active surface sites. We were able to systematically vary the chemical composition of as-prepared Pd–Ni nanowires from pure elemental Pd to Pd_0.50Ni_0.50 (atomic ratio), as verified using EDS analysis. The overall morphology of samples possessing >60 atom % Pd consisted of individual, discrete one-dimensional nanowires. The electrocatalytic performances of elemental Pd, Pd_0.90Ni_0.10, Pd_0.83Ni_0.17, and Pd_0.75Ni_0.25 nanowires in particular were examined. Our results highlight a “volcano”-type relationship between chemical composition and corresponding ORR activities with Pd_0.90Ni_0.10, yielding the highest activity (i.e., 1.96 mA/cm² at 0.8 V) among all nanowires tested. Moreover, the Pd_0.90Ni_0.10 sample exhibited outstanding methanol tolerance ability. In essence, there was only a relatively minimal 15% loss in the specific activity in the presence of 4 mM methanol, which was significantly better than analogous data on Pt nanoparticles and Pt nanowires. In addition, we also studied ultrathin, core–shell Pt∼Pd_0.90Ni_0.10 nanowires, which exhibited a specific activity of 0.62 mA/cm² and a corresponding mass activity of 1.44 A/mg_Pt at 0.9 V. Moreover, our as-prepared core–shell electrocatalysts maintained excellent electrochemical durability. We postulate that one-dimensional Pd–Ni nanostructures represent a particularly promising platform for designing ORR catalysts with high performance

Size- and Composition-Dependent Enhancement of Electrocatalytic Oxygen Reduction Performance in Ultrathin Palladium–Gold (Pd_1–<i>x</i>Au_<i>x</i>) Nanowires

In this report, we examine the composition- and size-dependent performance in hierarchical Pd_1–<i>x</i>Au_<i>x</i> nanowires (NWs) encapsulated with a conformal Pt monolayer shell (Pt∼Pd_1–<i>x</i>Au_<i>x</i>). The ultrathin Pd_1–<i>x</i>Au_<i>x</i> NWs are prepared by a solution-based method wherein the chemical composition can be readily and predictably controlled. Importantly, as-prepared Pd₉Au NWs maintain significantly enhanced oxygen reduction reaction (ORR) activity (0.40 mA/cm²), as compared with elemental Pd NW/C (0.12 mA/cm²) and Pt nanoparticles (NP)/C (0.20 mA/cm²), respectively. After the deposition of a Pt monolayer, a volcano-type composition dependence is observed in the ORR activity of the Pt∼Pd_1–<i>x</i>Au_<i>x</i> NWs as the Au content is increased from 0 to 30% with the activity of the Pt∼Pd₉Au NWs (0.98 mA/cm², 2.54 A/mg_Pt), representing the optimum performance. We note that the platinum group metal activity of the ultrathin 2 nm NWs (0.64 A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs (0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance

Utilizing Electrical Characteristics of Individual Nanotube Devices to Study the Charge Transfer between CdSe Quantum Dots and Double-Walled Nanotubes

To study the charge transfer between cadmium selenide (CdSe) quantum dots (QDs) and double-walled nanotubes (DWNTs), various sizes of CdSe–ligand–DWNT structures are synthesized, and field-effect transistors from individual functionalized DWNTs rather than networks of the same are fabricated. From the electrical measurements, two distinct electron transfer mechanisms from the QD system to the nanotube are identified. By the formation of the CdSe–ligand–DWNT heterostructure, an effectively n-doped nanotube is created due to the smaller work function of CdSe as compared with that of the nanotube. In addition, once the QD–DWNT system is exposed to laser light, further electron transfer from the QD through the ligand, specifically, 4-mercaptophenol (MTH), to the nanotube occurs and a clear QD size-dependent tunneling process is observed. The detailed analysis of a large set of devices and the particular methodology employed here for the first time allowed for extracting a wavelength and quantum dot size-dependent charge transfer efficiencya quantity that is evaluated for the first time through electrical measurement

Synthesis, Characterization, and Formation Mechanism of Crystalline Cu and Ni Metallic Nanowires under Ambient, Seedless, Surfactantless Conditions

In this report, crystalline elemental Cu and Ni nanowires have been successfully synthesized through a simplistic, malleable, solution-based protocol involving the utilization of a U-tube double diffusion apparatus under ambient conditions. The nanowires prepared within the 50 and 200 nm template membrane pore channels maintain diameters ranging from ∼90–230 nm with lengths attaining the micrometer scale. To mitigate for the unwanted but very facile oxidation of these nanomaterials to their oxide analogues, our synthesis mechanism relies on a carefully calibrated reaction between the corresponding metal precursor solution and an aqueous reducing agent solution, resulting in the production of pure, monodisperse metallic nanostructures. These as-prepared nanowires were subsequently characterized from an applications’ perspective so as to investigate their optical and photocatalytic properties

Highly Enhanced Electrocatalytic Oxygen Reduction Performance Observed in Bimetallic Palladium-Based Nanowires Prepared under Ambient, Surfactantless Conditions

We have employed an ambient, template-based technique that is simple, efficient, and surfactantless to generate a series of bimetallic Pd_1–<i>x</i>Au_<i>x</i> and Pd_1–<i>x</i>Pt_<i>x</i> nanowires with control over composition and size. Our as-prepared nanowires maintain significantly enhanced activity toward oxygen reduction as compared with commercial Pt nanoparticles and other 1D nanostructures, as a result of their homogeneous alloyed structure. Specifically, Pd₉Au and Pd₄Pt nanowires possess oxygen reduction reaction (ORR) activities of 0.49 and 0.79 mA/cm², respectively, which are larger than the analogous value for commercial Pt nanoparticles (0.21 mA/cm²). In addition, core–shell Pt∼Pd₉Au nanowires have been prepared by electrodepositing a Pt monolayer shell and the corresponding specific, platinum mass, and platinum group metal mass activities were found to be 0.95 mA/cm², 2.08 A/mg_Pt, and 0.16 A/mg_PGM, respectively. The increased activity and catalytic performance is accompanied by improved durability toward ORR

Generalizable, Electroless, Template-Assisted Synthesis and Electrocatalytic Mechanistic Understanding of Perovskite LaNiO₃ Nanorods as Viable, Supportless Oxygen Evolution Reaction Catalysts in Alkaline Media

The oxygen evolution reaction (OER) is a key reaction for water electrolysis cells and air-powered battery applications. However, conventional metal oxide catalysts, used for high-performing OER, tend to incorporate comparatively expensive and less abundant precious metals such as Ru and Ir, and, moreover, suffer from poor stability. To attempt to mitigate for all of these issues, we have prepared one-dimensional (1D) OER-active perovskite nanorods using a unique, simple, generalizable, and robust method. Significantly, our work demonstrates the feasibility of a novel electroless, seedless, surfactant-free, wet solution-based protocol for fabricating “high aspect ratio” LaNiO₃ and LaMnO₃ nanostructures. As the main focus of our demonstration of principle, we prepared as-synthesized LaNiO₃ rods and correlated the various temperatures at which these materials were annealed with their resulting OER performance. We observed generally better OER performance for samples prepared with lower annealing temperatures. Specifically, when annealed at 600 °C, in the absence of a conventional conductive carbon support, our as-synthesized LaNiO₃ rods not only evinced (i) a reasonable level of activity toward OER but also displayed (ii) an improved stability, as demonstrated by chronoamperometric measurements, especially when compared with a control sample of commercially available (and more expensive) RuO₂

Synthesis of Compositionally Defined Single-Crystalline Eu³⁺-Activated Molybdate–Tungstate Solid-Solution Composite Nanowires and Observation of Charge Transfer in a Novel Class of 1D CaMoO₄–CaWO₄:Eu³⁺–0D CdS/CdSe QD Nanoscale Heterostructures

As a first step, we have synthesized and optically characterized a systematic series of one-dimensional (1D) single-crystalline Eu³⁺-activated alkaline-earth metal tungstate/molybdate solid-solution composite CaW_1–<i>x</i>­Mo_<i>x</i>O₄ (0 ≤ “<i>x</i>” ≤ 1) nanowires of controllable chemical composition using a modified template-directed methodology under ambient room-temperature conditions. Extensive characterization of the resulting nanowires has been performed using X-ray diffraction, electron microscopy, and optical spectroscopy. The crystallite size and single crystallinity of as-prepared 1D CaW_1–<i>x</i>­Mo_<i>x</i>O₄:Eu³⁺ (0 ≤ “<i>x</i>” ≤ 1) solid-solution composite nanowires increase with increasing Mo component (“<i>x</i>”). We note a clear dependence of luminescence output upon nanowire chemical composition with our 1D CaW_1–<i>x</i>Mo_<i>x</i>O₄:Eu³⁺ (0 ≤ “<i>x</i>” ≤ 1) evincing the highest photoluminescence (PL) output at “<i>x</i>” = 0.8, among samples tested. Subsequently, coupled with either zero-dimensional (0D) CdS or CdSe quantum dots (QDs), we successfully synthesized and observed charge transfer processes in 1D CaW_1–<i>x</i>Mo_<i>x</i>­O₄:Eu³⁺ (“<i>x</i>” = 0.8)–0D QD composite nanoscale heterostructures. Our results show that CaW_1–<i>x</i>­Mo_<i>x</i>O₄:Eu³⁺ (“<i>x</i>” = 0.8) nanowires give rise to PL quenching when CdSe QDs and CdS QDs are anchored onto the surfaces of 1D CaWO₄–CaMoO₄:Eu³⁺ nanowires. The observed PL quenching is especially pronounced in CaW_1–<i>x</i>Mo_<i>x</i>O₄:Eu³⁺ (“<i>x</i>” = 0.8)–0D CdSe QD heterostructures. Conversely, the PL output and lifetimes of CdSe and CdS QDs within these heterostructures are not noticeably altered as compared with unbound CdSe and CdS QDs. The differences in optical behavior between 1D Eu³⁺ activated tungstate and molybdate solid-solution nanowires and the semiconducting 0D QDs within our heterostructures can be correlated with the relative positions of their conduction and valence energy band levels. We propose that the PL quenching can be attributed to a photoinduced electron transfer process from CaW_1–<i>x</i>­Mo_<i>x</i>O₄:Eu³⁺ (“<i>x</i>” = 0.8) to both CdSe and CdS QDs, an assertion supported by complementary near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements