Ahmad M. Mohammad

Oxidation of silicon nanowires

Silicon nanowires have received attention for nanoscale electronic devices and chemical and biological sensors. The thermal oxide grown on the silicon nanowires could be used in a variety of devices, so the oxidation of the silicon nanowires is investigated in this work. Silicon nanowires with an average radius of 37nm were grown for these experiments using the vapor-liquid-solid technique with Au to mediate the growth. Etching of the Au tips from the silicon nanowires was performed prior to oxidation to avoid local accelerated oxidation at the nanowire tip. Oxidation was performed at 700°C for 1–121h and at 650 and 750°C for 4h in O2, and the oxidized nanowires were examined by transmission electron microscopy. Depending on the conditions for oxidation, an oxide shell as thin as 6nm was observed, or the entire nanowire was oxidized. The kinetics of oxidation differ from those of a planar silicon wafer and are discussed in this work.

Novel fuel blends facilitating the electro-oxidation of formic acid at a nano-Pt/GC electrode

This paper addresses the promoting effect of the electrooxidation of formic acid (FAO) at a nano-Pt/GC electrode in the presence of selected low molecular weight alcohols (R–OH) as blending components. That is, blending FA with different molar ratios of methanol (MeOH), ethanol (EtOH), ethylene glycol (EGOH) and isopropanol (PrOH) resulted in a significant enhancement of the direct FAO to CO2 (desired pathway) with a concurrent depression of the amount of CO produced from the “non-faradaic” dissociation of FA. Moreover, a favorable negative shift of the onset potential of the direct FAO peak at the nano-Pt/GC electrode is observed. Fuel utilization (FU = amount of charge consumed during the oxidation process per mole of fuel) and the turnover number (TON = number of FA molecules oxidized per platinum site per second) are significantly enhanced as well for FAO in the various FA/R–OH blends compared to pure FA. That is, the use of equimolar amounts of FA with either EtOH, MeOH, EGOH or PrOH resulted in a facile FAO at the nano-Pt/GC electrode of about 9, 7, 5 and 4 times higher FU compared to pure FA, respectively. Similar increase of TON is observed as well. The blending component is believed to adsorb at the Pt surface sites and thus disfavor the “non-faradaic” dissociation of FA to CO. Additionally; it might induce the CH-down adsorption orientation of FA, thus favoring FAO to CO2. The enhanced oxidation activity indicates that this fuel blend is a promising fuel system.

Electrocatalysis of Formic Acid Electro-Oxidation at Platinum Nanoparticles Modified Surfaces with Nickel and Cobalt Oxides Nanostructures

The present study proposes a novel promising binary catalyst for formic acid electro-oxidation (FAO); the main anodic reaction in direct formic acid fuel cells (DFAFCs). The catalyst is basically composed of two metal oxides of nickel and cobalt nanostructures (i.e., NiOx and CoOx) assembled onto a platinum nanoparticles (PtNPs)−modified glassy carbon (Pt/GC) electrode. Actually, FAO proceeds at bare Pt surfaces in two parallel routes; one of them is desirable (called direct or hydrogenation) and occurred at a low potential domain (I p d ). While, the other (undesirable) involves the dehydration of formic acid (FA) at low potential domain to produce a poisoning intermediate (CO), which next be oxidized (indirect, I p ind ) at a higher potential domain after the platinum surface becomes hydroxylated. Unfortunately, the peak current ratio (I p d /I p ind ) of the two oxidation routes, which monitors the degree of the catalytic enhancement and the poisoning level, stands for bare Pt surfaces at a low value (less than 0.2). Interestingly, this ratio increased significantly as a result of the further modification of the Pt/GC electrode with NiOx \( \left({I}_{\mathrm{p}}^{\mathrm{d}}/{I}_{\mathrm{p}}^{\mathrm{ind}}=3\right) \), CoOx \( \left({I}_{\mathrm{p}}^{\mathrm{d}}/{I}_{\mathrm{p}}^{\mathrm{ind}}=4\right) \) and a binary mixture of both \( \left({I}_{\mathrm{p}}^{\mathrm{d}}/{I}_{\mathrm{p}}^{\mathrm{ind}}=15\right) \). This highlights the essential role of these in promoting the direct FAO, presumably via a mediation process that ultimately improved the oxidation kinetics or through a catalytic enhancement for the oxidation of the poisoning CO at the low potential domain of the direct FAO. The effect of the deposition order of NiOx and CoOx on the catalytic activity was addressed and fount influencing. The addition of CoOx to the catalyst was really important, particularly in improving the catalytic stability of the catalyst towards a long-term continuous electrolysis experiment, which actually imitates the real industrial applications.

On the Catalytic Activity of Palladium Nanoparticles-Based Anodes Towards Formic Acid Electro-oxidation: Effect of Electrodeposition Potential

In this investigation, the catalytic activity of palladium nanoparticles (PdNPs)-modified glassy carbon (GC) (simply noted as PdNPs/GC) electrodes towards the formic acid electro-oxidation (FAO) was investigated. The deposition of PdNPs on the GC substrate was carried out by a potentiostatic technique at different potentials and the corresponding influence on the particles size and crystal structure of PdNPs as well as the catalytic activity towards FAO was studied. Scanning electron microscopy (SEM) demonstrated the deposition of PdNPs in spherical shapes and the average particle size of PdNPs deposited at a potential of 0 V vs. Ag/AgCl/KCl(sat.) was the smallest (ca. 8 nm) in comparison to other cases, where the deposition proceeded at higher potentials. The electrochemical measurements agreed consistently with this, where the highest surface area of PdNPs was calculated similarly for the deposition carried out at 0 V vs. Ag/AgCl/KCl(sat.). Interestingly, the X-ray diffraction (XRD) analysis revealed a similar dependency of the PdNPs crystal structure on their particle size and distribution. The deposition of PdNPs at 0 V vs. Ag/AgCl/KCl(sat.) seemed exhibiting the best crystallinity. From the electrocatalytic point of view, the activity of the PdNPs/GC electrode towards FAO decreased with the deposition potential of PdNPs, which influenced consequently the particle size, shape, and/or crystallographic orientation of PdNPs.

SPECTROPHOTOMETRIC QUANTIFICATION OF PEROXONE

A simple spectrophotometric method is proposed for the simultaneous quantification of the ozone (O3) and hydrogen peroxide (H2O2) mixture, the so-called peroxone. This method is based on the measurements of the absorbance of the produced by the oxidation of I– by peroxone. The oxidation rates of I– by O3 and H2O2 are largely different and pH-dependent and, moreover, the oxidation of I– by H2O2 can be accelerated by a molybdate catalyst. In addition, the present method was successfully applied at concentrations as low as 10 µM for the analysis of a real peroxone sample prepared by electrolysis of water.

Electro-Oxidation of Formic Acid, Glucose, and Methanol at Nickel Oxide Nanoparticle Modified Platinum Electrodes

The current study presents a comparison for the electro-oxidation of formic acid (FA), glucose (GL), and methanol (ME) at nickel oxide nanoparticles (NiOx) modified electrodes. The modification with NiOx was pursed onto a bare glassy carbon (GC) and Pt-modified (Pt/GC) electrodes electrochemically, and the catalytic activity was measured in 0.3 M NaOH. Cyclic voltammetry (CV), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) are all used to provide a concrete characterization of the prepared electrodes. A catalytic enhancement of GL oxidation (GLO) and ME oxidation (MEO) was observed at the NiOx-modified GC (NiOx/GC) electrode, while the same electrode did not show any activity towards FA oxidation (FAO), revealing that FAO is substrate dependent. On the other hand, assembling NiOx onto the Pt/GC electrode assisted in improving the catalytic activity of all reactions (GLO, MEO, and FAO). The catalytic enhancement observed at the NiOx/Pt/GC electrode for GLO, MEO, and FAO was not only confined in the large increase of the oxidation current but also in a negative shift in the onset potential of the oxidation reaction. We believe NiOx could successfully play an essential role in this catalytic enhancement, presumably via participation in these reactions in a way facilitating the charge transfer or providing the oxygen atmosphere necessary for promoting an oxidative removal for unwanted poisoning species.

The Origin of Electrocatalytic Activity of Gold Nanoparticles Modified Pt-Based Surfaces Towards Formic Acid Oxidation

Recently, direct formic acid fuel cells (DFAFCs) have received much attention in both industry and academia, due to their unique properties. Despite of their broad benefits, DFAFCs have two major drawbacks that limit its lifetime and efficiency; the poor electrocatalytic activity (due to CO and Halides poisoning) and stability of the Pt-based electrodes. Herein, the electrocatalytic activity, stability and tolerance against poisoning species (CO and Halides) of Pt-based electrode (Pt/GC) towards formic acid (FA) oxidation; essential anodic reaction of DFAFCs, are shown to increase via interrupting the Pt surface with gold nanoparticles (AuNPs). Electrochemical measurements show that gold nanopartciles (AuNPs) modified Pt/GC (Au/Pt/GC) electrode supports a significant enhancement on the direct FA oxidation to CO2 (the dehydrogenation pathway). On the other hand, the oxidative treatment of GC (GCox) in acidic medium results in 2 times increases on the catalytic activity of unmodified and AuNPs modified Pt electrodes towards direct FA oxidation to CO2 compared to un-oxidized GC electrode. This significantly enhanced activity of AuNPs modified Pt/GC catalysts can be attributed to noncontiguous arrangement of Pt sites in the presence of the neighbored AuNPs, which promotes direct oxidation of FA to CO2 and retards the adsorption of CO at Pt surface. Moreover, AuNPs modified Pt/GC catalyst has satisfactory stability and show high tolerance against halides poisoning.

Electroanalysis of a Ternary Disinfectant Mixture

The analysis of a ternary mixture containing ozone (O3), sodium hypochlorite (NaClO), and hydrogen peroxide (H2O2), in their coexistence was simultaneously performed using a potentiometric method. In this method, the change in the open circuit potential of a Pt indicator electrode dipped in a potential buffer containing I−/ upon the addition of the ternary mixture is measured. The analysis was based on the different reaction kinetics of the three oxidants with I−. The kinetics of the reaction of O3 and I− is about three orders of magnitude faster than that of sodium hypochlorite, and the reaction of hydrogen peroxide and I− is negligible compared with those of O3 or hypochlorite ion and I− unless a molybdate catalyst is added. Several factors, including the iodide concentration, pH, and the molybdate concentration were investigated to optimize the analysis and achieve a reasonable separation among the reactions of the three oxidants and I−. A theoretical model was developed to compare with the experimental results and a reasonable correlation was obtained.

Template-assisted growth of rhodium nanowire contacts to silicon nanowires

Nanoscale materials are attracting growing interest due to their fascinating properties compared to the corresponding bulk materials, such as high effective surface area, catalytic activity and quantum confinement. Silicon nanowires, in particular, received much of this interest due to their promising applications in nano-electronics and nano-optoelectronics. Template synthesis, which involves a deposition into the nanopores of a nanoporous template, is considered among the easiest and least expensive approaches to fabricate nanowires with uniform diameters over a large area. More interestingly, it opens an opportunity to confine a nanocontact in the templates pores via a sequentional deposition of different materials. Herein, we describe a templatebased approach to fabricate metallic nanowire contacts to silicon nanowires. The metallic part of the nanowire (herein is rhodium) was deposited electrochemically within the pores of the template. However, the other (silicon) part was grown using the metal-catalysed vapour-liquid-solid (VLS) mechanism. The influence of the growth parameters on the structural quality of the nanowire was addressed.