Rando Saar

Stabilizer-free silver nanoparticles as efficient catalysts for electrochemical reduction of oxygen

In this work we demonstrated the potential of the He+5% H2+1% N2 plasma jet treatment for the synthesis of surfactant-free silver nanoparticles (Ag NPs) with narrow size distribution. The obtained colloidal solutions of electrostatically stabilized Ag NPs do not show any agglomeration for several months. Apart from an atomic thin oxide layer and the relatively weakly bound OH- ions, the surface of Ag NPs can be considered as stabilizer-free. The surface charge (characterized by the zeta potential) of Ag NPs in solution was measured by electrophoretic light scattering technique. Plasmonic band position and width in the UV/VIS extinction spectra was utilized for the assessment of Ag NPs size distribution. Highly concentrated Ag NPs were uniformly deposited on the surface of the glassy carbon (GC) electrodes by vacuum-drying technique. The deposition process was monitored with a digital camera attached to a microscope. The assemblies of Ag NPs on the electrode surface were characterized by scanning electron microscopy. The Ag NP/GC catalysts were electrochemically tested in alkaline solution using the rotating disk electrode method. The Ag NP/GC electrodes exhibited high electrocatalytic activity toward the oxygen reduction reaction (ORR) in 0.1M KOH solution, indicating their potential applicability as cathode materials for alkaline fuel cells.

Shape Restoration Effect in Ag–SiO2 Core–Shell Nanowires

The combination of two different materials in a single composite core-shell heterostructure can lead to improved or even completely novel properties. In this work we demonstrate the enhancement of the mechanical properties of silver (Ag) nanowires (NW) achieved by coating them with a silica (SiO2) shell. In situ scanning electron microscope (SEM) nanomechanical tests of Ag-SiO2 core-shell nanowires reveal an improved fracture resistance and an electron-beam induced shape restoration effect. In addition, control experiments are conducted separately on uncoated Ag NWs and on empty SiO2 shells in order to gain deeper insight into the peculiar properties of Ag-SiO2. Test conditions are simulated using finite-element methods; possible mechanisms responsible for the shape restoration and the enhanced fracture resistance are discussed.

Oxygen Barrier Properties of Al2O3- and TiO2-coated LDPE Films

Current work focuses on establishing optimal growth conditions for covering commercial grade polymer blown films with a gas barrier oxide layer. As opposed to regular solid state substrates, low density polymers host an abundance of free space in the form of pores and voids between large polymer chains (MW > 1000) and a heterogeneous crystallized phase. In the following we will show how the structural peculiarities of the polymer matrix influence the ALD process for growing TiO2 and Al2O3 gas blocking layers on top of the LDPE substrate. Possible causes of low gas barrier performance, thermal expansion coefficient differences, are proposed and commercial process additives, such as slip or antioxidants, are suggested. GRAPHICAL ABSTRACT

Synthesis of highly-active Fe–N–C catalysts for PEMFC with carbide-derived carbons

Proton exchange membrane fuel cells (PEMFC) offer a viable alternative to internal combustion engines, but highly performing stacks still require large amounts of platinum-based catalysts. Fe–N–C catalysts have recently emerged as potential substitutes. Carbide-derived carbon (CDC) can be designed to have various pore size distributions (PSD), in the microporous and/or mesoporous domains, which can be used for defining the number and/or accessibility of active sites in Fe–N–C catalysts based on the CDC. In this work, we compare two sets of Fe–N–C catalysts derived from two different CDCs, one with most frequent pore size of 8.5 A, (CDC-2) and another one with most frequent pore sizes at 7.8 and 30 A (CDC-1). The CDC-based Fe–N–C catalysts show excellent half-wave potential for oxygen reduction reaction (ORR) of 0.81 V vs. RHE in 0.5 M H2SO4. This work presents the first study of CDC-based catalysts in a PEMFC, where the performance of the CDC-2 based catalyst rivaled that of the best Fe–N–C materials in the literature. The catalyst derived from CDC-2 showed ca. 5 times higher activity at 0.8 V vs. RHE than the one derived from CDC-1. We show that the residual presence of boron in CDC-1 is the main reason for the lower activity of CDC-1 derived catalysts, leading to the formation of iron boride instead of ORR-active FeNxCy moieties. Higher Fe contents were investigated for CDC-2, but lead to unmodified activity, which is explained from Mossbauer spectroscopy measurements by the increasing formation of ORR-inactive Fe species at high Fe content. In summary, we demonstrate the excellent potential for CDC materials to be used in catalyst design and also identify some key issues that may arise from the possible residual presence of secondary atoms from the starting carbide.

Structure-Dependent CO2 Gas Sensitivity of La2O2CO3 Thin Films

Rare earth oxycarbonates are potential candidate materials for constructing simple and low-cost chemiresistive sensors for monitoring carbon dioxide (CO2) gas in the living and working environment for personal comfort and health reasons. Also, measurement of CO2 concentrations is needed in many industrial processes. Specifically, sol-gel made nanoparticles of Nd and La oxycarbonates have been studied previously as novel CO2 gas sensor materials. In this paper, pulsed laser deposition of La oxycarbonate (La2O2CO3) thin films was studied and structural properties of obtained thin films were characterized. Also, CO2 gas sensing ability of synthesized films was evaluated. The films deposited under CO2 partial pressure in various conditions were all Raman amorphous. In situ or ex situ annealing procedure at high CO2 partial pressure was needed for obtaining crystalline La2O2CO3 films, whereby hexagonal and monoclinic polymorphs were obtained in ex situ and in situ processes, respectively. Sensor structure, made using in situ process, was sensitive to CO2 gas and showed relatively fast response and recovery characteristics.