Sol-Gel Formation of Reticular Methyl-Silicate Materials by Hydrogen Peroxide Decomposition

A new method for the formation of reticular silicate and organically modified silicate is introduced. Monoliths were prepared by incorporating a few percent hydrogen peroxide in the sol-gel starting solution. For example, incorporation of 6–10% (v) hydrogen peroxide in base catalyzed sol-gel precursors of methyl-Ormosil yielded macroporous monoliths with a bi-modal pore size distribution. The average characteristic pore diameters were approximately 1.2 nm and 0.7 m, depending on the sol-gel precursors used and the preparation protocol. The specific surface area was approximately 160 m2/g, contributed mainly by the microporous structure. A similar preparation procedure without hydrogen peroxide yielded only fractured or powdery materials. Presumably, the decomposition of the hydrogen peroxide yielded microbubbles, which formed templates for the polycondensation reaction. SEM, nitrogen adsorption isotherms and small angle X-ray spectroscopy were used to characterize the reticular materials.     

Self-assembling two-dimensional nanophotonic arrays for reflectivity-based sensing

We propose a nanoplasmonic platform that can be used for sensing trace levels of heavy metals in solutions via simple optical reflectivity measurements. The considered example is a lead sensor, which relies on the lead-mediated assembly of glutathione-functionalized gold nanoparticles (NPs) at a self-healing water/DCE liquid | liquid interface (LLI). Capillary forces tend to trap each NP at the LLI while the negatively charged ligands prevent the NPs settling too close to each other. In the presence of lead, due to chelation between the lead ion and glutathione ligand, the NPs assemble into a dense quasi-2D interfacial array. Such a dense assembly of plasmonic NPs can generate a remarkable broad-band reflectance signal, which is absent when NPs are adsorbed at the interface far apart from each other. The condensing effect of the LLI and the plasmonic coupling effect among the NP array gives rise to a dramatic enhancement of the reflectivity signals. Importantly, we show that our theory of the optical reflectivity from such an array of NPs works in perfect harmony with the physics and chemistry of the system with the key parameter being the interparticle distance at the interface. As a lead sensor, the system is fast, stable, and can achieve detection limits down to 14 ppb. Future alternative recognizing ligands can be used to build sister platforms for detecting other heavy metals.

We propose a nanoplasmonic platform that can be used for sensing trace levels of heavy metals in solutions via simple optical reflectivity measurements at the liquid–liquid interface.     

Time-Resolved Product Observation for CO2 Electroreduction Using Synchronised Electrochemistry-Mass Spectrometry with Soft Ionisation (sEC-MS-SI)

The mechanistic understanding of electrochemical CO₂ reduction reaction (CO₂RR) requires a rapid and accurate characterisation of product distribution to unravel the activity and selectivity, which is yet hampered by the lack of advanced correlative approaches. Here, we present the time-resolved identification of CO₂RR products by using the synchronised electrochemistry-mass spectrometry (sEC-MS). Transients in product formation can be readily captured in relation to electrochemical conditions. Moreover, a soft ionisation (SI) strategy is developed in MS for the direct observation of CO, immune to the interference of CO₂ fragments. With the sEC-MS-SI, the kinetic information, such as Tafel slopes and onset potentials, for a myriad of CO₂RR products are revealed and we show the hysteresis seen for the evolution of some species may originate from the potential-driven changes in surface coverage of intermediates. This work provides a real-time picture of the dynamic formation of CO₂RR products.     

Mass transport and kinetics of electrochemical oxygen reduction at nanostructured platinum electrode and solid polymer electrolyte membrane interface

Oxygen reduction reaction (orr) at nanostructured Pt electrode in a flooded polymer electrolyte membrane fuel cell environment has been investigated using a nanoporous Pt–Nafion membrane composite microelectrode by means of steady-state voltammetry and chronoamperometry. The interfacial mass transport of dissolved oxygen is characterized by comparable diffusion coefficients and lower concentrations as compared with literature data obtained with a humidified membrane. The exchange current densities measured at the nanoporous Pt and membrane interface are higher than those reported for the orr in acidic solutions or at polycrystalline Pt and Nafion membrane interface, indicating the improvement of the orr kinetics. Increasing temperature substantially improves the orr kinetics and accelerates the diffusion of oxygen, as expected by their Arrhenius behavior. At the nanoporous Pt and membrane interface, the Tafel plot exhibits an unusual slope of around 240?mV?dec^?1 at high overpotentials. This Tafel slope doubling the value of 120?mV?dec^?1 normally reported for the orr in acidic media and at the polycrystalline Pt and membrane interface is a signature of non-uniform polarization of the nanoporous Pt electrode on the membrane which origins have been discussed.     

Electrochemical impedance studies of the undoping process of platinum phthalocyanine charge transfer microcrystals

Platinum phthalocyanine (PtPc) microcrystal films undergo three successive electrochemical oxidations. Each of these processes is associated with anion insertion or doping. The reverse process of anion insertion, undoping, has been investigated using electrochemical impedance spectroscopy and in-situ UV–vis spectroscopy. The impedance theory of conductive polymer films developed by Vorotyntsev et al. is applicable to this process. The kinetics of the undoping process depend upon the previous oxidative treatment, and thus the doping level. Three different states of the film can be demarcated, depending on the degree of oxidation (and thus the degree of doping) of the PtPc film. These are called the lightly doped, the conductive and the over-doped state, respectively. For lightly doped films, the film conductivity, the redox capacitance, the diffusion coefficient for charge transport and the rate of electrochemical reaction all decrease with decreasing potential. The film conductivity depends upon the concentration of free charge carriers. For the more highly doped conductive film, all of the above parameters are greatly enhanced, and the electrochemical reaction is accelerated and proceeds at a very high rate. The potential dependence of the redox capacitance and the diffusion coefficient depends on the type of anion. During undoping at 0 V, unusually high diffusion coefficients with a magnitude of order 10⁻² cm² s⁻¹ are observed and are attributed to the strong interactions between the electronic and ionic carriers during the phase transformation. For the over-doped film, undoping leads to an increase in the film conductivity and electrochemical reaction rate. The potential dependence of the redox capacitance and diffusion coefficients for charge transport implies strong interactions within the film. Hypsochromic shifts in UV–vis spectra with decreasing potential indicate conformational relaxation during the undoping process. SEM investigation confirms that the doped film swells during the de-doping process.     

The photoelectrochemistry of platinum phthalocyanine films in aqueous media

Platinum phthalocyanine (PtPc) films applied to glassy carbon (GC) and platinum surfaces have been shown to exhibit complex photoelectrochemistry in aqueous media, acting as an n-type semi-conducting material. Using chopped polychromatic radiation and a lock-in amplifier to detect the modulated photocurrent, photo-responses were measured for PtPc films on GC in a variety of background electrolytes. In all instances the photo-responses were anodic over the voltage ranges used. Two peaks were seen in each sweep direction for the experiments performed in sulfuric acid and sodium hydroxide solutions. The origin of the photocurrent has been ascribed to the photo-oxidation of water in the electrolyte by positive holes produced under illumination, which accumulate at the PtPc/electrolyte interface. Photocurrent transients were also recorded and indicated some recombination of excitons within the system.     

Nanostructured platinum as an electrocatalyst for the electrooxidation of formic acid

Nanostructured platinum prepared by the chemical reduction of hexachloroplatinic acid dissolved in aqueous domains of the liquid crystalline phases of oligoethylene oxide surfactants, was examined as an electrocatalyst for the electrooxidation of formic acid. The electrocatalytic properties of the catalyst combining highly specific surface areas and a periodic mesoporous nanostructure were accessed in sulfuric acid solution containing 0.5 mol dm⁻³ formic acid using cyclic voltammetry (CV) and chronoamperometry. The electrocatalytic activity of the material at 60 °C, is characterised by a mass activity of 8.6 A g⁻¹ and a specific surface area activity of 26 μA cm⁻² at 0.376 V (vs. RHE). The resistance to CO poisoning was found to depend upon electrode potential. At hydrogen adsorption potentials, the material is easily poisoned, while the material shows high resistance to CO poisoning at potentials positive of the hydrogen region. These facts suggest that the decomposition of HCOOH on the mesoporous platinum is likely to proceed through a dual-path mechanism and the high surface area material is a potential electocatalyst towards the electrooxidation of small organic molecules.     

Synthesis of highly active nanostructured PtRu electrocatalyst with three-dimensional mesoporous silica template

Nanostructured PtRu material has been successively synthesized via chemical co-reduction of hexachloroplatinic acid and ruthenium trichloride using three-dimensional (3D) hexagonal mesoporous SBA-12 silica as a solid template, and has been studied as an electrocatalyst toward methanol electro-oxidation. The ordered nanostructure of the PtRu particles has been disclosed by transmission electron micrographs and is characterized by regular pores of ca. 3.0 ± 0.3 nm in diameter separated by walls of ca. 3.0 ± 0.3 nm thick. X-ray diffraction and energy dispersive X-ray spectroscope studies indicate that the PtRu material comprises of complicated phases rather than a single alloy phase of Pt and Ru. The specific electrochemical surface area of the nanostructured powder measured using both CO and underpotential deposited Cu stripping techniques is 74–78 m² g^–1, higher than that of unsupported precious metal catalysts prepared using standard techniques. The combination of high surface area and periodic nanostructure of the templated PtRu makes it an interesting promising fuel cell electrocatalyst. This has been demonstrated by the high activity of the templated PtRu towards the methanol electrooxidation. Therefore the solid template route based on 3D mesoporous silica with controlled pore size and high pore interconnectivity provides an interesting alternative to produce promising high-surface-area electrode materials.     

Electrodeposition of highly alloyed quaternary PtPdRuOs catalyst with highly ordered nanostructure

Introducing palladium to traditional platinum-based alloy electrocatalysts offers a novel approach to develop highly efficient anode electrocatalysts for direct methanol fuel cells. In this communication, we report the preparation of thin-wall mesoporous quaternary PtPdRuOs alloy catalyst via electrochemical co-reduction of their chloride precursors all dissolved in aqueous domains of the liquid crystalline phases of an oligoethylene oxide surfactant. Scanning electron micrographs (SEM) reveal that the deposit is composed of uniform nanospheres with an average diameter of around 120 nm and the average mole composition of the metal elements is Pt₃₇Pd₃₃Ru₂₂Os₁₀. Transmission electron micrographs (TEM) disclose that the nanospheres have an ordered nanostructure which is characterized by periodic pores of 3.6 ± 0.4 nm in diameter separated by walls of 2.4 ± 0.4 nm in thickness. X-ray diffraction studies signal a highly alloying degree for the four metal components in the deposit. The specific electrochemical surface area of the nanostructured powder assessed using underpotential deposited Cu stripping technique is as high as 105 m² g^–1, much higher than that of unsupported precious metal catalysts prepared using standard techniques. These characters suggest that the quaternary PtPdRuOs alloy materials with high surface area and thin-wall mesoporous structure would be a novel class of promising electrocatalysts for direct methanol fuel cells.