Carol Korzeniewski

Comparing Graphene-TiO2 Nanowire and Graphene-TiO2 Nanoparticle Composite Photocatalysts

We demonstrate that uniform dispersion of TiO(2) on graphene is critical for the photocatalytic effect of the composite. The hydrothermal method was employed to synthesize TiO(2) nanowires (NW) and then fabricate graphene-TiO(2) nanowire nanocomposite (GNW). Graphene oxide (GO) reduction to graphene and hybridization between TiO(2) NWs and graphene by forming chemical bonding was achieved in a one-step hydrothermal process. Graphene-TiO(2) nanoparticle (NP) nanocomposite (GNP) was also synthesized. Photocatalytic performance and related properties of NP, NW, GNP, and GNW were comparatively studied. It was found that by incorporation of graphene, GNP and GNW have higher performance than their counterparts. More importantly, it was found that NWs, in comparison with NPs, have more uniform dispersion on graphene with less agglomeration, resulting in more direct contact between TiO(2) and graphene, and hence further improved electron-hole pairs (EHPs) separation and transportation. The adsorbability of GNW is also found to be higher than GNP. The result reveals that the relative photocatalytic activity of GNW is much higher than GNP and pure NWs or NPs.

Elementary steps in the oxidation and dissociative chemisorption of ethanol on smooth and stepped surface planes of platinum electrodes

Infrared spectroscopy is applied to map pathways in the electrocatalytic oxidation of ethanol at platinum electrodes. Reactions at Pt(111), Pt(335) and polycrystalline platinum are studied for ethanol at 1–4 mM in 0.1 M HClO4. Isotopic labelling is used to track the fate of carbon atoms on the alcohol and methyl groups across a range of potentials from the hydrogen adsorption region to the leading edge of the oxide region. For the CC bond cleavage pathway, routes to the stable adsorbed carbon monoxide intermediate depend upon the applied potential. In the hydrogen adsorption region, adsorbed CO originating from the carbon on the alcohol group is detected, while more positive potentials are required to oxidize the methyl group. For the direct oxidation pathway, all surfaces catalyze the four-electron route to acetic acid. Strong vibrational bands from adsorbed acetic acid are detected during ethanol oxidation for the first time, and the appearance of these features correlates with a region of reaction inhibition in the voltammetry. In all cases, reactions progress to a greater extent at the Pt(335) and polycrystalline electrodes.

Effects of thermal activation on the oxidation pathways of methanol at bulk Pt-Ru alloy electrodes

Abstract The competition between pathways that lead to adsorbed CO and CO 2 during the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO 4 on two bulk Pt–Ru alloys (10 at.% Ru ( X Ru ≈0.1) and 90 at.% Ru ( X Ru ≈0.9)) was investigated for temperatures in the range of 25–80°C. On the high Ru content alloy studied ( X Ru ≈0.9), the dissociative chemisorption of methanol was inhibited below 70°C; the faradaic current for methanol oxidation was low, and only small quantities of adsorbed CO and CO 2 were detected with infrared spectroscopy between 0.2–0.8 V (vs. RHE). At 80°C, strong infrared bands from CO 2 and adsorbed, atop coordinated CO were observed over the potential ranges of 0.4–0.8 V and 0.2–0.8 V, respectively. The infrared measurements are consistent with the observation that bulk, high Ru content alloy electrodes appear passivated toward methanol oxidation below 70°C. On the low Ru content alloy studied ( X Ru ≈0.1), the methanol surface chemistry was similar to that of pure, polycrystalline Pt, but the electrode was more poison resistant than Pt. For both alloys, the persistence of strong adsorbed CO bands and rapid CO 2 production between 0.4–0.8 V suggests CO functions as a reactive species with high steady-state coverages at these potentials.

Ethylene glycol electrochemical oxidation at platinum probed by ion chromatography and infrared spectroscopy

Abstract Wet chemical methods were used in combination with in situ infrared spectroscopy to identify and quantify soluble products formed during ethylene glycol electrochemical oxidation on platinum. A small volume electrolysis technique allowed reactions to be performed in samples of 150 μl in contact with a small, bead-type single crystal electrode. Following reaction, 50–100 μl aliquots were assayed for aldehydes and carboxylic acids. Glycolaldehyde (HOCH 2 CHO), and glycolic (HOCH 2 COOH) and oxalic ((HOOC) 2 ) acids were detected following the 360 s electrolysis of 0.2 M ethylene glycol in 0.1 M HClO 4 at potentials in the range 0.0–0.6 V (vs. SCE). Determination of the carboxylic acids by ion chromatography aided the assignment of carboxylate vibrational bands in in situ infrared spectra.

Vibrational coupling as a probe of adsorption at different structural sites on a stepped single-crystal electrode.

Adsorption of carbon monoxide at step and terrace sites on a Pt(557) ≡ Pt(s)-[6(111) × (100)] electrode was detected with infrared spectroscopy. Vibrational coupling between adsorbates provided insights into the assembly of molecules at the different structural sites. The intermolecular coupling was weak at low coverages as CO ordered along the steps. For coverages between 40 and 70% of saturation, separate bands assignable to CO on steps and CO on terraces appeared. Coupling across this coverage range was markedly weaker on Pt(557) than on the structurally related Pt(335) ≡ Pt(s)-[4(111) × (100)] electrode surface. The results indicate that, after the steps fill, CO populates the terraces on Pt(557) at random rather than by ordering in alignment with the steps. At coverages below saturation, vibrational bands assignable to CO molecules at step and terrace sites are affected differently by changes in electrode potential. The potential-induced spectral changes for the terrace CO bands are similar to those of Pt(111)/CO, but the step CO bands show deviations from this trend at hydrogen adsorption potentials.

Infrared Spectroscopy in Electrochemistry: New Methods and Connections to UhV Surface Science

Abstract This review discusses recent advances in the use of infrared spectroscopy in electrochemistry. The central focus is on analytical factors that affect the ability to derive structural information from infrared spectra of molecular adlayers. The effects of vibrational coupling and dielectric screening are emphasized. Applications of real-time polarization modulation and step-scan Fourier transform infrared spectroscopy are also presented.

Transmission infrared spectroscopy as a probe of Nafion film structure: analysis of spectral regions fundamental to understanding hydration effects.

Transmission infrared spectroscopy was applied to investigate properties of the perfluorosulfonated polymer Nafion. Measurements were made on thin films formed by casting the polymer from solution onto ZnSe windows. Effects of water vapor permeation were studied. A complex band structure between 1350 and 1100 cm−1 was analyzed qualitatively by fitting the region to Gaussian functions. Features associated with vibrational modes of –CF2 and –SO3− groups were identified and observed to be sensitive to film hydration. The intensities of bands for the –SO3− modes increased with film hydration, while bands assignable to –CF2 modes decreased. The results were applied to interpret infrared difference spectra of Nafion and shed light on the complicated features that appear. Vibrational bands for water were also examined. In partially hydrated films, the stretching mode of the free –OH group for interfacial water present in pores and channels of the polymer and bands for hydrated proton clusters were detected.

Tailoring cobalt doped zinc oxide nanocrystals with high capacitance activity: factors affecting structure and surface morphology

Highly crystalline zinc cobaltite (ZnCo2O4) nanocrystals were successfully synthesized through an epoxide driven, sol–gel method using Zn(NO3)·6H2O and CoCl2·6H2O as precursors. The crystal phase, morphology, specific surface areas, porosity, and capacitance activity of the prepared materials were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), gas sorption techniques, and cyclic voltammetry, respectively. Results reveal that the synthesized nanocrystals are ∼4 nm in diameter. Electron microscopy studies illustrate significant changes brought on by varying the solvent and epoxide. Gas sorption analyses detail high specific surface areas (>200 m2 g−1) and porosities of the as prepared and annealed samples. Cyclic voltammetry experiments show that these zinc cobaltite nanocrystals have exceptional capacitance (∼700 Fg−1) and excellent cycle durability making them an excellent electrode material for supercapacitors.

Synthesis of PtCu3 bimetallic nanoparticles as oxygen reduction catalysts via a sonochemical method

We report a sonochemical synthesis of homogeneous PtCu3 nanoparticles. Ultra-sonication during reduction in a non-aqueous solution is compared with synthesis under identical conditions in the absence of sonication (to form a Rieke alloy). X-ray diffraction (XRD) measurements suggest that the sonochemical procedure produces an amorphous, uniformly alloyed nanomaterial having a composition consistent with the PtCu3 stoichiometry, while the Rieke alloy is polyphasic. Energy dispersive X-ray (EDX) analysis indicates that the composition of the sonochemically prepared PtCu3 material reflects the nominal values. EDX and XRD analyses also provide evidence for the inhibition of oxide formation on sonochemically prepared PtCu3 nanoparticles, but oxide is readily apparent in the Rieke alloy. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the sonochemically prepared sample show particles with diameters of ∼2 to 3 nm. As-synthesized PtCu3 particles were activated using an electrochemical de-alloying procedure to prepare an oxygen reduction electrocatalyst. The de-alloyed catalyst consisted of a Pt-rich surface layer, over a core indicated as having a Pt3Cu composition. The de-alloyed sample exhibited ∼3 to 6 fold enhancements in oxygen reduction reaction (ORR) activity when compared to commercial Pt catalysts.

Structural analysis of sonochemically prepared PtRu versus Johnson Matthey PtRu in operating direct methanol fuel cells

Sonochemically prepared PtRu (3 : 1) and Johnson Matthey PtRu (1 : 1) were analyzed by X-ray absorption spectroscopy in operating liquid feed direct methanol fuel cells. The total metal loadings were 4 mg cm(-2) unsupported catalysts at the anode and cathode of the membrane electrode assembly. Ex situ XRD lattice parameter analysis indicates partial segregation of the Ru from the PtRu fcc alloy in both catalysts. A comparison of the in situ DMFC EXAFS to that of the as-received catalyst shows that catalyst restructuring during DMFC operation increases the total metal coordination numbers. A combined analysis of XRD determined grain sizes and lattice parameters, ex situ and in situ EXAFS analysis, and XRF of the as-received catalysts enables determination of the catalyst shell composition. The multi-spectrum analysis shows that the core size increases during DMFC operation by reduction of Pt oxides and incorporation of Pt into the core. This increases the mole fraction of Ru in the catalyst shell structure.