Carrie L. Donley

The recent discovery of molybdenum oxynitride (MoO2–x/Mo2N) being an effective plasmonic catalyst demonstrated potential for light-driven ammonia synthesis under ambient pressure. However, its preparation requires high-temperature ammonia annealing to introduce lattice nitrogen, increasing both cost and process complexity. Here, we investigate molybdenum oxide (MoO3–x), the precursor to synthesize MoO2–x/Mo2N, as a catalyst for ammonia synthesis. We show that molybdenum oxide undergoes continuous nitrogen incorporation during the reaction, accompanied by enhanced catalytic activity. Time-dependent X-ray photoelectron spectroscopy reveals the emergence of nitrogen-containing surface environments. Thermal treatment is sufficient to induce this evolution, which approaches a relatively stable state after several hours. Notably, both molybdenum oxide and pre-synthesized MoO2–x/Mo2N develop similar nitrogen-containing surface chemical environments under reaction conditions. These findings demonstrate that the catalyst surface dynamically evolves during ammonia synthesis and that nitrogen incorporation is closely associated with enhanced catalytic activity.

This work performs a comprehensive investigation of the key electronic structure parameters pertinent to (opto)electronic device applications for a wide-bandgap organic-inorganic hybrid semiconductor β-ZnTe(en)0.5, using an array of complementary tools. The measurements yielded Fermi level of 0.80eV above the valence band maximum from X-ray photoelectron spectroscopy and work function of 4.59eV from Kelvin probe force microscopy, which together resulted in ionization energy of 5.39eV and electron affinity of 1.84eV or the conduction band minimum at 2.75eV above EF, using a bandgap of 3.55eV from photoluminescence. Ultraviolet photoelectron spectroscopy offered a consistent work function of 4.60eV. By integrating these findings, a complete energy band diagram for β-ZnTe(en)0.5 has been obtained. Furthermore, thermal contact probe revealed p-type conductivity of β-ZnTe(en)0.5 without intentional doping. This study facilitates the practical applications of this material, a prototype of a group of unique hybrid semiconductors, by providing the much-needed band structure parameters.

Dye-sensitized photoelectrosynthesis cells traditionally combine a photosensitizing dye to harvest light and a catalyst to generate chemical fuels on a semiconductor. In this work, a photoactive catalyst capable of both light absorption and fuel formation, [Cp*Ir(4,4'-Y₂-bpy)Cl][Cl] (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, Y = CH₂PO₃H₂), is anchored to a mesoporous tin-doped indium oxide (ITO) electrode and facilitates photoelectrochemical H₂ evolution in water without the need for additional photosensitizers or sacrificial reductants. Spectroelectrochemistry indicates a single-site H₂ evolution mechanism involving charge injection to ITO, in contrast to the bimetallic mechanism observed in solution. Cyclic voltammetry and variable-potential chronoamperometry under illumination probe competing pathways via interfacial electron transfer between the Ir hydride excited state and the conductive ITO electrode. A Marcus theory framework provides reorganization energies for competing interfacial electron transfer pathways from potential-dependent quantum yield measurements. The light-driven H₂ evolution catalysis on conducting oxide proceeds with high Faradaic efficiency and current densities comparable to photoelectrodes utilizing p-type semiconductors. By uncovering the principal electron transfer pathways that govern photocatalytic efficiency, this study establishes design principles for hybrid molecular photoelectrocatalysts.

We installed molecular CO2 reduction (CO2R) catalysts directly onto Si (photo)electrodes. The highly reactive M(5-azido-1,10-phenanthroline)(CO)3X (where M = Mn or Re, X = Br or Cl) complexes readily bubbled when dissolved in polar organic solvents, in both the presence and absence of an ultraviolet light source. When placed on hydrogen-terminated Si (H-Si) and native silicon oxide (SiOx), similar amounts of the complex were attached to the surface under illumination (367 nm, 50–200 mW/cm2) or in the dark. Surprisingly, these films revealed submonolayer coverages instead of the multilayered structures we expected. DFT analyses support monolayer formation, showing that the triplet-state nitrene of the complex is more energetically favorable than the singlet state. Using controlled-potential electrolysis experiments, we showed that Re- and Mn-containing films on pSi photoelectrodes generated small amounts of CO when exposed to 1 atm of CO2 and 1 sun illumination. These amounts of CO were an order of magnitude greater than control surfaces, producing 5.59 × 10–7 mol CO/h for Re(az-phen) and 7.83 × 10–7 mol CO/h for Mn(az-phen) films. Much of the charge passed at the pSi electrodes was consumed by the competing hydrogen evolution reaction, which we attribute to the low molecular coverage and the presence of native oxide on the electrode surface after attachment. This work demonstrates the feasibility of reacting azide-containing ligands with Si surfaces. Still, it highlights the need for alternative ligand structures and reaction conditions to form multilayer films.

Sulfur-based cathodes are attractive next-generation energy storage materials because of their high theoretical capacity, earth-abundance, and low cost; however, their practical application is hindered by polysulfide dissolution and poor cycle stability. Here, we report a nanoparticle-aggregated, porous, and amorphous Mo3S13–graphene oxide (GO) chalcocarbogel as a high-capacity and cycle-stable electrode material for lithium sulfide batteries. The chalcocarbogels are synthesized via a scalable, room-temperature, acid-free solution process, enabling precise control over the Mo3S13-to-GO ratio. Structural characterizations, including synchrotron pair distribution function (PDF), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) analyses, reveal an amorphous framework dominated by short-range Mo–Mo, Mo–S, S–S, C-S, and Mo–O coordination environments, suggesting ultrathin GO acts as a reactant to form a complimentary structures closely resembling the molecular structure of Mo3S13-xOx clusters and are connected to GO through C-S and Mo-O interactions. Electrochemical investigation of the Li/Mo3S13-GO cell demonstrates a remarkably high discharge capacity of 986 mAh g−1 at C/3 and retains 391 mAh g−1 even after 500 cycles. Additionally, the cell delivers a high reversible capacity of 433 mAh g−1 when the current is increased from C/3 to 1C, indicating excellent rate performance. The enhanced electrochemical performance originates from the synergistic coupling of amorphous Mo–S clusters with GO, which improves charge transport, GO-mediated percolating electrical conductivity, strengthens polysulfide anchoring, and maintains structural integrity during cycling. These findings highlight the effectiveness of cluster-based amorphous chalcocarbogels in regulating polysulfide chemistry and provide a general strategy for designing durable, high-capacity sulfur electrodes.

Controlling the morphological parameters of extended covalent organic frameworks remains challenging and represents an important yet often elusive metric of consideration. Typically, carbon nitride materials possess local ordering but remain largely amorphous in terms of their long-range order and orientation. This study probes the synthesis of a crystalline carbon nitride, poly(triazine imide) lithium bromide which possesses an atomically-precise extended structure, and demonstrates its exfoliation into a two-dimensional hexagonal sheet-like morphology. Furthermore, a previously unreported carbon nitride material, poly(triazine imide) copper bromide, or PTI-CuBr, was developed through an additional flux-assisted cation-exchange process and is shown to retain its internal Cu cations during solvothermal exfoliation. Characterization by dynamic light scattering and high-angle annular dark-field scanning electron microscopy reveals the morphological changes and captures the high aspect ratio of the thin carbon nitride sheets with <10 nm thickness while maintaining hundreds of nm in width. Additional characterization by energy-dispersive spectroscopy and X-ray photoelectron spectroscopy confirms that the Cu:Br:N molar ratio was maintained within the extended layers throughout the exfoliation process. This top-down synthesis approach differs from typical methods that isolate thin sheets for subsequent metal-cation coordination and illustrates the importance of maintaining oxygen-free conditions to minimize copper clustering. Thus, this new approach is demonstrated to provide a consistent and more homogeneous occupancy of the PTI pore spaces throughout the carbon nitride framework.

Covalently attaching molecular catalysts to semiconductor surfaces yields promising hybrid photoelectrode architectures for reducing CO₂ to higher-value carbon products. Polymeric molecular catalyst films have higher loading densities than their monolayer counterparts, promising greater rates of solar fuel production. Using photoassisted diazonium electrografting, multilayered films of a Re(apbpy)(CO)₃Cl CO₂-reduction catalyst were attached to low-doped p-type Si (pSi). Parallel characterization of the newly formed films with ellipsometry, XPS, and ICP-MS revealed that catalyst loading increased with increasingly negative applied grafting potentials (V_graft), providing us an experimental test bed to study the effects of film thickness on photocatalytic performance. Controlled-potential electrolysis experiments showed enhanced CO evolution rates on all photoelectrodes with increasingly negative applied potentials (V_app), with thicker films exhibiting the greatest rates of enhancement. Competitive proton reduction reactions at the Si surface were not strongly linked to V_app but dependent on film thickness, with thicker films showing increased CO-to-H₂ production ratios.