Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction

Developing improved catalysts for the oxygen evolution reaction (OER) is key to the advancement of a number of renewable energy technologies, including solar fuels production and metal air batteries. In this study, we employ electrochemical methods and synchrotron techniques to systematically investigate interactions between metal oxides and noble metals that lead to enhanced OER catalysis for water oxidation. In particular, we synthesize porous MnO_<i>x</i> films together with nanoparticles of Au, Pd, Pt, or Ag and observe significant improvement in activity for the combined catalysts. Soft X-ray absorption spectroscopy (XAS) shows that increased activity correlates with increased Mn oxidation states to 4+ under OER conditions compared to bare MnO_<i>x</i>, which exhibits minimal OER current and remains in a 3+ oxidation state. Thickness studies of bare MnO_<i>x</i> films and of MnO_<i>x</i> films deposited on Au nanoparticles reveal trends suggesting that the enhancement in activity arises from interfacial sites between Au and MnO_<i>x</i>

Co₁₁Li[(OH)₅O][(PO₃OH)(PO₄)₅], a Lithium-Stabilized, Mixed-Valent Cobalt(II,III) Hydroxide Phosphate Framework

A new metastable phase, featuring a lithium-stabilized mixed-valence cobalt­(II,III) hydroxide phosphate framework, Co_11.0(1)Li_1.0(2)[(OH)₅O]­[(PO₃OH)­(PO₄)₅], corresponding to the simplified composition Co_1.84(2)Li_0.16(3)­(OH)­PO₄, is prepared by hydrothermal synthesis. Because the pH-dependent formation of other phases such as Co₃(OH)₂­(PO₃OH)₂ and olivine-type LiCoPO₄ competes in the process, a pH value of 5.0 is crucial for obtaining a single-phase material. The crystals with dimensions of 15 μm × 30 μm exhibit a unique elongated triangular pyramid morphology with a lamellar fine structure. Powder X-ray diffraction experiments reveal that the phase is isostructural with the natural phosphate minerals holtedahlite and satterlyite, and crystallizes in the trigonal space group <i>P</i>31<i>m</i> (<i>a =</i> 11.2533(4) Å, <i>c =</i> 4.9940(2) Å, <i>V =</i> 547.70(3) ų, <i>Z =</i> 1). The three-dimensional network structure is characterized by partially Li-substituted, octahedral [M₂O₈(OH)] (M = Co, Li) dimer units which form double chains that run along the [001] direction and are connected by [PO₄] and [PO₃(OH)] tetrahedra. Because no Li-free <i>P</i>31<i>m</i>-type Co₂(OH)­PO₄ phase could be prepared, it can be assumed that the Li ions are crucial for the stabilization of the framework. Co L-edge X-ray absorption spectroscopy demonstrates that the cobalt ions adopt the oxidation states +2 and +3 and hence provides further evidence for the incorporation of Li in the charge-balanced framework. The presence of three independent hydroxyl groups is further confirmed by infrared spectroscopy. Magnetization measurements imply a paramagnetic to antiferromagnetic transition at around <i>T</i> = 25 K as well as a second transition at around 9–12 K with a ferromagnetic component below this temperature. The metastable character of the phase is demonstrated by thermogravimetric analysis and differential scanning calorimetry, which above 558 °C reveal a two-step decomposition to CoO, Co₃(PO₄)₂, and olivine-type LiCoPO₄ with release of water and oxygen

Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons

Aromaticity profoundly affects molecular orbitals in polycyclic aromatic hydrocarbons. X-ray core electron spectroscopy has observed that carbon 1s−π* transitions can be broadened or even split in some polycyclic systems, although the origin of the effect has remained obscure. The π electrons in polycyclic systems are typically classified in the Clar model as belonging to either true aromatic sextets (similar to benzene) or isolated double bonds (similar to olefins). Here, bulk-sensitive carbon core excitation spectra are presented for a series of polycyclic systems and show that the magnitude of the 1s−π* splitting is determined primarily by the ratio of true aromatic sextets to isolated double bonds. The observed splitting can be rationalized in terms of ground state energetics as described by Hückel, driven by the π electron structure described by Clar. This simple model including only ground state energetics is shown to explain the basics physics behind the spectral evolution for a broad set of polycyclic aromatic hydrocarbons, although some residual deviations between this model and experiment can likely be improved by including a more detailed electronic structure and the core hole effect

Scalable Low-Cost Fabrication of Disposable Paper Sensors for DNA Detection

Controlled integration of features that enhance the analytical performance of a sensor chip is a challenging task in the development of paper sensors. A critical issue in the fabrication of low-cost biosensor chips is the activation of the device surface in a reliable and controllable manner compatible with large-scale production. Here, we report stable, well-adherent, and repeatable site-selective deposition of bioreactive amine functionalities and biorepellant polyethylene glycol-like (PEG) functionalities on paper sensors by aerosol-assisted, atmospheric-pressure, plasma-enhanced chemical vapor deposition. This approach requires only 20 s of deposition time, compared to previous reports on cellulose functionalization, which takes hours. A detailed analysis of the near-edge X-ray absorption fine structure (NEXAFS) and its sensitivity to the local electronic structure of the carbon and nitrogen functionalities. σ*, π*, and Rydberg transitions in C and N K-edges are presented. Application of the plasma-processed paper sensors in DNA detection is also demonstrated

Mn₃O₄ Supported on Glassy Carbon: An Active Non-Precious Metal Catalyst for the Oxygen Reduction Reaction

In this work, we explore the interplay between manganese oxide (MnO_<i>x</i>) nanomaterials and a glassy carbon (GC) support in catalyzing the oxygen reduction reaction (ORR) in an alkaline environment. Initially, we characterize the ORR activity of bare GC electrodes as a function of heat treatments in air, and find that ORR activity increases with increasing temperature up to 500 °C. Modification of GC with size-selected 1 nm MnO_<i>x</i> nanoparticles prior to the 500 °C heat treatment yields a highly porous GC (pGC) structure, devoid of MnO_<i>x</i>. This pGC sample exhibits the highest ORR performance of the bare carbon electrodes reaching an onset potential of 0.75 V vs the reversible hydrogen electrode (RHE) and a complete 2-electron reduction of oxygen to peroxide. Having established ORR activity of bare GC electrodes, we deposit size-selected 14 nm MnO nanoparticles onto the GC and pGC electrodes and then incite phase changes in MnO through heat treatments in air. Electrochemical characterization of the resulting electrodes reveals that MnO nanoparticles offer no improvement in the ORR onset potential over bare GC or pGC and only slightly increase the number of electrons transferred. By contrast, thermal oxidation of MnO nanoparticles to Mn₃O₄ at 500 °C, confirmed by Mn L-edge X-ray absorption spectroscopy, results in an improved ORR onset potential of 0.80 V and a 4-electron reduction of oxygen. Thus at low overpotentials, where GC and pGC were inactive for the ORR, MnO_<i>x</i> sites must contribute to all steps of the reaction. The catalyst’s estimated specific activity of 3700 μA·cm^–2_cat at 0.75 V compares favorably with specific activities of Pt/C as well as the best nonprecious metal catalysts. This establishes Mn₃O₄ as another MnO_<i>x</i> phase with high activity for the ORR

Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation

Much is known about the rate of photoexcited charge generation in at organic donor/acceptor (D/A) heterojunctions overaged over all relative arrangements. However, there has been very little experimental work investigating how the photoexcited electron transfer (ET) rate depends on the precise relative molecular orientation between D and A in thin solid films. This is the question that we address in this work. We find that the ET rate depends strongly on the relative molecular arrangement: The interface where the model donor compound copper phthalocyanine is oriented face-on with respect to the fullerene C₆₀ acceptor yields a rate that is approximately 4 times faster than that of the edge-on oriented interface. Our results suggest that the D/A electronic coupling is significantly enhanced in the face-on case, which agrees well with theoretical predictions, underscoring the importance of controlling the relative interfacial molecular orientation

Influence of Dopant Distribution on the Plasmonic Properties of Indium Tin Oxide Nanocrystals

Doped metal oxide nanocrystals represent an exciting frontier for colloidal synthesis of plasmonic materials, displaying unique optoelectronic properties and showing promise for a variety of applications. However, fundamental questions about the nature of doping in these materials remain. In this article, the strong influence of radial dopant distribution on the optoelectronic properties of colloidal indium tin oxide nanocrystals is reported. Comparing elemental depth-profiling by X-ray photoelectron spectroscopy (XPS) with detailed modeling and simulation of the optical extinction of these nanocrystals using the Drude model for free electrons, a correlation between surface segregation of tin ions and the average activation of dopants is observed. A strong influence of surface segregation of tin on the line shape of the localized surface plasmon resonance (LSPR) is also reported. Samples with tin segregated near the surface show a symmetric line shape that suggests weak or no damping of the plasmon by ionized impurities. It is suggested that segregation of tin near the surface facilitates compensation of the dopant ions by electronic defects and oxygen interstitials, thus reducing activation. A core–shell model is proposed to explain the observed differences in line shape. These results demonstrate the nuanced role of dopant distribution in determining the optoelectronic properties of semiconductor nanocrystals and suggest that more detailed study of the distribution and structure of defects in plasmonic colloidal nanocrystals is warranted

Synergistic Role of Dopants on the Morphology of Alloyed Copper Chalcogenide Nanocrystals

The presence of antimony, as a dopant in the colloidal growth reaction for CuIn_1–<i>x</i>Ga_<i>x</i>S₂ (CIGS) nanocrystals, causes end-to-end fusion of nanorod pairs into nanodumbbells at high yield. The influence of the dopant on shape is indirect; antimony catalyzes the incorporation of gallium, which is found in high concentration at the junction between the fused nanorods

Sensitivity of X‑ray Core Spectroscopy to Changes in Metal Ligation: A Systematic Study of Low-Coordinate, High-Spin Ferrous Complexes

In order to assess the sensitivity and complementarity of X-ray absorption and emission spectroscopies for determining changes in the metal ligation sphere, a systematic experimental and theoretical study of iron model complexes has been carried out. A series of high-spin ferrous complexes, in which the ligation sphere has been varied from a three-coordinate complex, [L^tBuFe­(SPh)] (<b>1</b>) (where L^tBu = bulky β-diketiminate ligand; SPh = phenyl thiolate) to four-coordinate complexes [L^tBuFe­(SPh)­(X)] (where X = CN^tBu (<b>2</b>); 1-methylimidazole (<b>3</b>); or <i>N</i>,<i>N</i>-dimethylformamide (DMF) (<b>4</b>)), has been investigated using a combination of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES) spectroscopies. The Fe K XAS pre-edge and edge of all four complexes are consistent with a high-spin ferrous assignment, with the largest differences in the pre-edge intensities attributed to changes in covalency of the fourth coordination site. The X-ray emission spectra show pronounced changes in the valence to core region (V2C) as the identity of the coordinated ligand is varied. The experimental results have been correlated to density functional theory (DFT) calculations, to understand key molecular orbital contributions to the observed absorption and emission features. The calculations also have been extended to a series of hypothetical high-spin iron complexes to understand the sensitivity of XAS and XES techniques to different ligand protonation states ([L^tBuFe^II(SPh)­(NH_<i>n</i>)]^3–<i>n</i> (<i>n</i> = 3, 2, 1, 0)), metal oxidation states [L^tBuFe­(SPh)­(N)]^<i>n</i>− (<i>n</i> = 3, 2, 1), and changes in the ligand identity [L^tBuFe^IV(SPh)­(X)]^<i>n</i>− (X = C^4–, N^3–, O^2–; <i>n</i> = 2, 1, 0). This study demonstrates that XAS pre-edge data have greater sensitivity to changes in oxidation state, while valence to core (V2C) XES data provide a more sensitive probe of ligand identity and protonation state. The combination of multiple X-ray spectroscopic methods with DFT results thus has the potential to provide for detailed characterization of complex inorganic systems in both chemical and biological catalysis

Effect of Backbone Chemistry on the Structure of Polyurea Films Deposited by Molecular Layer Deposition

An experimental investigation into the growth of polyurea films by molecular layer deposition was performed by examining trends in the growth rate, crystallinity, and orientation of chains as a function of backbone flexibility. Growth curves obtained for films containing backbones of aliphatic and phenyl groups indicate that an increase in backbone flexibility leads to a reduction in growth rate from 4 to 1 Å/cycle. Crystallinity measurements collected using grazing incidence X-ray diffraction and Fourier transform infrared spectroscopy suggest that some chains form paracrystalline, out-of-plane stacks of polymer segments with packing distances ranging from 4.4 to 3.7 Å depending on the monomer size. Diffraction intensity is largely a function of the homogeneity of the backbone. Near-edge X-ray absorption fine structure measurements for thin and thick samples show an average chain orientation of ∼25° relative to the substrate across all samples, suggesting that changes in growth rate are not caused by differences in chain angle but instead may be caused by differences in the frequency of chain terminations. These results suggest a model of molecular layer deposition-based chain growth in which films consist of a mixture of upward growing chains and horizontally aligned layers of paracrystalline polymer segments