Brian M. Leonard

Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction

Molybdenum carbide has been proposed as a possible alternative to platinum for catalyzing the hydrogen evolution reaction (HER). Previous studies were limited to only one phase, β-Mo2C with an Fe2N structure. Here, four phases of Mo-C were synthesized and investigated for their electrocatalytic activity and stability for HER in acidic solution. All four phases were synthesized from a unique amine-metal oxide composite material including γ-MoC with a WC type structure which was stabilized for the first time as a phase pure nanomaterial. X-ray photoelectron spectroscopy (XPS) and valence band studies were also used for the first time on γ-MoC. γ-MoC exhibits the second highest HER activity among all four phases of molybdenum carbide, and is exceedingly stable in acidic solution.

Ligand Induced Circular Dichroism and Circularly Polarized Luminescence in CdSe Quantum Dots

Chiral thiol capping ligands L- and D-cysteines induced modular chiroptical properties in achiral cadmium selenide quantum dots (CdSe QDs). Cys-CdSe prepared from achiral oleic acid capped CdSe by postsynthetic ligand exchange displayed size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). Opposite CPL signals were measured for the CdSe QDs capped with D- and L-cysteine. The CD profile and CD anisotropy varied with size of CdSe nanocrystals with largest anisotropy observed for CdSe nanoparticles of 4.4 nm. Magic angle spinning solid state NMR (MAS ssNMR) experiments suggested bidentate interaction between cysteine and the surface of CdSe. Time Dependent Density Functional Theory (TDDFT) calculations verified that attachment of L- and D-cysteine to the surface of model (CdSe)13 nanoclusters induces measurable opposite CD signals for the exitonic band of the nanocluster. The origin of the induced chirality is consistent with the hybridization of highest occupied CdSe molecular orbitals with those of the chiral ligand.

Carbides of group IVA, VA and VIA transition metals as alternative HER and ORR catalysts and support materials

High surface area nano dimensional carbides of nine transition metals in group IV–VI have been synthesized using a salt flux method. Uniformity was maintained throughout the investigation, from synthesis method to electrochemical tests, so that a comparison can be made for the various carbides for their catalytic activities towards hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Catalytic activities are dependent on synthesis method which determines the properties of the catalyst, and electrochemical conditions. Maintaining uniformity throughout the investigation allows for a more balanced comparison of a family of materials. Activity of all nine carbides show increased HER activity compared to bare glassy carbon working electrode. Mo2C, WC, and V8C7 show particularly enhanced HER activity. Similarly, Mo2C, Cr3C2, and V8C7 have significant ORR activities. Using a wet impregnation method, dispersed platinum nanoparticles ranging between 3 and 5 nm were successfully deposited on the carbides. The Pt deposited carbides have as much as three times higher HER activity and four times higher ORR activity compared to commercially available Pt/C catalyst, and show enhanced stability under fuel cell conditions.

Crystal structure and morphology control of molybdenum carbide nanomaterials synthesized from an amine–metal oxide composite

Multiple phases of molybdenum carbide have been synthesized using a unique amine-metal oxide composite material. By combining molybdenum oxide and an amine, a templated precursor is formed which can be thermally decomposed to form molybdenum carbide with control over the structure and morphology of the nano-sized products.

Chirality Inversion of CdSe and CdS Quantum Dots without Changing the Stereochemistry of the Capping Ligand

L-cysteine derivatives induce and modulate the optical activity of achiral cadmium selenide (CdSe) and cadmium sulfide (CdS) quantum dots (QDs). Remarkably, N-acetyl-L-cysteine-CdSe and L-homocysteine-CdSe as well as N-acetyl-L-cysteine-CdS and L-cysteine-CdS showed mirror-image circular dichroism (CD) spectra regardless of the diameter of the QDs. This is an example of the inversion of the CD signal of QDs by alteration of the ligands structure, rather than inversion of the ligands absolute configuration. Non-empirical quantum chemical simulations of the CD spectra were able to reproduce the experimentally observed sign patterns and demonstrate that the inversion of chirality originated from different binding arrangements of N-acetyl-L-cysteine and L-homocysteine-CdSe to the QD surface. These efforts may allow the prediction of the ligand-induced chiroptical activity of QDs by calculating the specific binding modes of the chiral capping ligands. Combined with the large pool of available chiral ligands, our work opens a robust approach to the rational design of chiral semiconducting nanomaterials.

Orthogonal Reactivity of Metal and Multimetal Nanostructures for Selective, Stepwise, and Spatially-Controlled Solid-State Modification

Chemists rely on a toolbox of robust chemical transformations for selectively modifying molecules with spatial and functional precision to make them more complex in a controllable and predictable manner. This manuscript describes proof-of-principle experiments for a conceptually analogous strategy involving the selective, stepwise, and spatially controlled modification of inorganic nanostructures. The key concept is orthogonal reactivity: one component of a multicomponent system reacts with a particular reagent under a specific set of conditions while the others do not, even though they are all present together in the same reaction vessel. Using the chemical conversion of metal nanoparticles into intermetallic, sulfide, and phosphide nanoparticles as representative examples, the concept of orthogonal reactivity is defined and demonstrated for a variety of two- and three-component nanoscale systems. First, solution-phase reactivity data are presented and collectively analyzed for the reaction of metal nanoparticles (Ni, Cu, Rh, Pd, Ag, Pt, Au, Sn) with several metal salt and elemental reagents (Bi, Pb, Sb, Sn, S). From these data, several two- and three-component orthogonal systems are identified. Finally, these results are applied to the spatially selective chemical modification of lithographically patterned surfaces and striped template-grown metal nanowires.

Supramolecular ssDNA Templated Porphyrin and Metalloporphyrin Nanoassemblies with Tunable Helicity

Free-base and nickel porphyrin-diaminopurine conjugates were formed by hydrogen-bond directed assembly on single-stranded oligothymidine templates of different lengths into helical multiporphyrin nanoassemblies with highly modular structural and chiroptical properties. Large red-shifts of the Soret band in the UV/Vis spectroscopy confirmed strong electronic coupling among assembled porphyrin-diaminopurine units. Slow annealing rates yielded preferentially right-handed nanostructures, whereas fast annealing yielded left-handed nanostructures. Time-dependent DFT simulations of UV/Vis and CD spectra for model porphyrin clusters templated on the canonical B-DNA and its enantiomeric form, were employed to confirm the origin of observed chiroptical properties and to assign the helicity of porphyrin nanoassemblies. Molar CD and CD anisotropy gu2005factors of dialyzed templated porphyrin nanoassemblies showed very high chiroptical anisotropy. The DNA-templated porphyrin nanoassemblies displayed high thermal and pH stability. The structure and handedness of all assemblies was preserved at temperatures up to +85u2009°C and pH between 3 and 12. High-resolution transition electron microscopy confirmed formation of DNA-templated nickel(II) porphyrin nanoassemblies and their self-assembly into helical fibrils with micrometer lengths.

Catalyst supports for polymer electrolyte fuel cells

A major challenge in obtaining long-term durability in fuel cells is to discover catalyst supports that do not corrode, or corrode much more slowly than the current carbon blacks used in today’s polymer electrolyte membrane fuel cells. Such materials must be sufficiently stable at low pH (acidic conditions) and high potential, in contact with the polymer membrane and under exposure to hydrogen gas and oxygen at temperatures up to perhaps 120°C. Here, we report the initial discovery of a promising class of doped oxide materials for this purpose: Ti1−xMxO2, where M=a variety of transition metals. Specifically, we show that Ti0.7W0.3O2 is electrochemically inert over the appropriate potential range. Although the process is not yet optimized, when Pt nanoparticles are deposited on this oxide, electrochemical experiments show that hydrogen is oxidized and oxygen reduced at rates comparable to those seen using a commercial Pt on carbon black support.

Nanocrystalline Mo2C as a Bifunctional Water Splitting Electrocatalyst

Mo2C is a well‐known low cost catalyst for the hydrogen evolution reaction (HER), but the other water splitting half reaction, the oxygen evolution reaction (OER), has not been previously reported. To investigate both reactions and the origin of the catalytic sites, four synthesis methods were employed to prepare hexagonal Fe2N type Mo2C. A comparison of the HER activities in acidic and alkaline electrolyte and OER activities in alkaline electrolyte revealed that changes in synthesis route leads to morphological and surface composition variations resulting in different catalytic activities. In general, the trend in HER and OER activities show remarkably similar trends across the carbides synthesized via different routes irrespective of either electrolyte employed or reaction probed for electrocatalytic activities. Mo2C templated on multiwalled carbon nanotubes demonstrated the highest bifunctional catalytic activities, as well as superior electrochemical stability for both HER and OER.

Probing synergetic effects between platinum nanoparticles deposited via atomic layer deposition and a molybdenum carbide nanotube support through surface characterization and device performance

Platinum (Pt) supported on transition metal carbide (TMC) surfaces (Pt/TMC) has been the focus of significant research interest, due to the similar electronic and geometric structures between TMC and Pt. This paper focuses on a new type of electrocatalyst fabrication using high purity β-molybdenum carbide (referred to as Mo2C hereafter) nanotubes as a support and atomic layer deposition (ALD) as the Pt nanoparticle deposition technique (Pt/Mo2C). In particular, rotary ALD equipment was used to grow Pt particles from the subnanometer level to 2 to 3 nanometers by simply adjusting the number of ALD cycles in order to probe the interaction between the deposited Pt nanoparticles and Mo2C nanotube support. Lattice spacing analysis using high resolution transmission electron microscopy (HRTEM) images, combined with Pt binding energy shift in XPS results, clearly showed a strong interaction between Pt nanoparticles and the Mo2C nanotube support in all the resultant Pt/Mo2C samples. We postulate that this strong interaction is responsible for the significantly enhanced durability observed in constant potential electrolysis (CPE) testing. Of the three samples from different ALD cycles (15, 50 and 100), Mo2C nanotubes modified by 50 (1.07 wt% Pt loading) and 100 cycles (4.4 wt% Pt) of Pt deposition showed higher activity per Pt mass than commercial 20% Pt supported on carbon black. Finally, we report the nanoscale Pt/Mo2C catalyst performance in a device setting for different Pt loadings by applying it in a PEMFC anode.