High Electron Mobility of Amorphous Red Phosphorus Thin Films.

Black phosphorus (BP) has been gathering great attention for its electronic and optoelectronic applications due to its high electron mobility and high ION/OFF current switching ratio. The limitations of this material include its low synthetic yield and high cost. One alternative to BP is another type of phosphorus allotrope, red phosphorus (RP), which is much more affordable and easier to process. Although RP has been widely used in industry for hundreds of years and considered as an insulating material, in this study, we demonstrate through field-effect transistors (FET) measurements that amorphous red phosphorus (a-RP) films are semiconductive with a high mobility of 387 cm2 V-1 s-1 and a current switching ratio of ≈103 , which is comparable to the electronic characteristics previously reported for BP. The films were produced via a thermal evaporation method or a facile drop-casting approach onto Si/SiO2 substrates. We also report a study of the oxidation process of the films over time and a method to stabilize the films via doping a-RP with metal oxides. The doped films retain stability for one thousand I-V cycles, with no signs of degradation.

Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation.

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni(2+) ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomic %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA cm(-2) . This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst ß-Ni(OH)2 (η≈550 mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.

Copper-Intercalated Birnessite as a Water Oxidation Catalyst.

We report a synthetic method to increase the catalytic activity of birnessite toward water oxidation by intercalating copper in the interlayer region of the layered manganese oxide. Intercalation of copper, verified by XRD, XPS, ICP, and Raman spectroscopy, was accomplished by exposing a suspension of birnessite to a Cu(+)-bearing precursor molecule that underwent disproportionation in solution to yield Cu(0) and Cu(2+). Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to ∼700 mV (at 10 mA/cm(2)) and 240 mV/decade, respectively, for birnessite without copper. Impedance spectroscopy results suggested that the charge transfer resistivity of the Cu-modified sample was significantly lower than Cu-free birnessite, suggesting that Cu in the interlayer increased the conductivity of birnessite leading to an enhancement of water oxidation kinetics. Density functional theory calculations show that the intercalation of Cu(0) into a layered MnO2 model structure led to a change of the electronic properties of the material from a semiconductor to a metallic-like structure. This conclusion from computation is in general agreement with the aforementioned impedance spectroscopy results. X-ray photoelectron spectroscopy (XPS) showed that Cu(0) coexisted with Cu(2+) in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu(2+) showed a reduction in water oxidation kinetics, further emphasizing the importance of Cu(0) for the increased activity of birnessite. The introduction of Cu(0) into the birnessite structure also increased the stability of the electrocatalyst. At a working current of 2 mA, the Cu-modified birnessite took ∼3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.

Coupled redox transformation of chromate and arsenite on ferrihydrite.

The redox chemistry of chromate (Cr(VI)) and arsenite (As(III)) on the iron oxyhydroxide, ferrihydrite (Fh), was investigated. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) were used to determine the composition of the adsorbed layer on Fh during and after exposure to solution-phase Cr(VI) and As(III). The individual exposure of Cr(VI) or As(III) on Fh resulted in the adsorption of the respective species, and there was no change in the oxidation state of either species. In contrast, exposure of Fh simultaneously to Cr(VI) and As(III) led to an adsorbed layer that was primarily Cr(III) and As(V). This redox transformation occurred over various experimental conditions at pH 3, 5, and 7 and in the presence or absence of O2, as demonstrated by in situ ATR-FTIR results. A similar redox transformation was not observed at a solution of pH 9, due to minimal Cr(VI) adsorption. Postreaction XPS showed that the majority of adsorbed arsenic existed as As(V) at pH 3, 5, and 7, while As(III) was the main species detected at pH 9. At pH 3 the redox chemistry between Cr(VI) and As(III) led to a As(V) product surface loading of ∼600 mmol/kg. Experiments performed in the absence of dissolved O2 resulted in less As(V) on the surface compared to experiments in which O2 was present for equivalent reaction times.

Discrimination of composite graphite samples using remote filament-induced breakdown spectroscopy.

Remote filament-induced breakdown spectroscopy (R-FIBS) using ultrashort laser pulses was used to measure the carbon/clay ratios between three graphite composites of different hardness at a standoff distance of approximately 6 m. Measurements using R-FIBS and femtosecond laser-induced breakdown (fs-LIBS) reveal similar selectivity and ability to excite emission. Comparison of the two stand-off techniques with optical microscopy and electron microprobe point detection confirmed the qualitative analysis capability of both femtosecond remote probing techniques. The R-FIBS technique produced more accurate results compared to fs-LIBS due to the intensity clamping nature of the filament ablation source. Measurement of the plasma temperatures for the metallic emission lines (approximately 8500 K) and the C(2) Swan lines (approximately 4500 K) suggest that the plasmas from different microdomains (clay and graphite) are not in equilibrium.