Gennady Cherkashinin

Visible Light Photocatalysis with c-WO3–x/WO3×H2O Nanoheterostructures In Situ Formed in Mesoporous Polycarbosilane-Siloxane Polymer

In recent years, there have been significant efforts to find novel photocatalytic materials with improved properties. Thus, there is an active ongoing search for new materials that can operate at a broad range of wavelengths for photocatalytic reactions. Among photocatalytically active semiconductors, considerable attention has been given to tungsten oxide with a band gap of E(g) ≈ 2.6 eV, which provides the opportunity to harvest visible light. In the present work, we report on a one-step synthesis of c-WO(3-x)/WO3×H2O nanowhiskers dispersed in a hydrolytically stable mesoporous polycarbosilane-siloxane ([-Si(O)CH2-]n) matrix. The as-synthesized nanocomposites possess high photocatalytic activity for the degradation of methylene blue (MB) under visible light irradiation. The enhanced photocatalytic activity is due to (i) the reduction in the electron-hole recombination rate because of the reduced dimensions of nanowhiskers, (ii) more efficient consumption of photogenerated electrons and holes as a result of the high surface-to-bulk-ratio of the nanowhiskers, and (iii) better electron-hole pair separation due to the formation of c-WO(3-x)/WO3×H2O nanoheterostructures.

Binary Au/MWCNT and Ternary Au/ZnO/MWCNT Nanocomposites: Synthesis, Characterisation and Catalytic Performance

Gold nanoparticles of 10-24 and 5-8 nm in size were obtained by chemical citrate reduction and UV photoreduction, respectively, on acid-treated multiwalled carbon nanotubes (MWCNTs) and on ZnO/MWCNT composites. The shape and size of the deposited Au nanoparticles were found to be dependent upon the synthetic method used. Single-crystalline, hexagonal gold particles were produced in the case of UV photoreduction on ZnO/MWCNT, whereas spherical Au particles were deposited on MWCNT when the chemical citrate reduction method was used. In the UV photoreduction route, n-doped ZnO serves as the e(-) donor, whereas the solvent is the hole trap. All materials were fully characterised by UV/Vis spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and BET surface analysis. The catalytic activity of the composites was studied for the selective hydrogenation of alpha,beta-unsaturated carbonyl compound 3,7-dimethyl-2,6-octadienal (citral). The Au/ZnO/MWCNT composite favours the formation of unsaturated alcohols (selectivity=50% at a citral conversion of 20%) due to the presence of single-crystalline, hexagonal gold particles, whereas saturated aldehyde formation is favoured in the case of the Au/MWCNT nanocomposite that contains spherical gold particles.

Single-Source Magnetic Nanorattles By Using Convenient Emulsion Polymerization Protocols

A novel strategy to achieve easily scalable magneto-responsive nanoceramics with core/shell and nanorattle-type or yolk/shell architectures based on a ferrocene-containing polymer precursor is described. Monodisperse nanorattle-type magnetic particles are obtained by using convenient semicontinuous emulsion polymerization and Stöber process protocols followed by thermal treatment. The particles are characterized by TGA, TEM, WAXS, DLS, XPS, and Raman spectroscopy. Herein, established synthetic protocols widen opportunities for the convenient bottom-up strategies of various ferrocene-precursor-based spherical architectures for advanced ceramics with potential applications within fields of sensing and stimuli-responsive nanophotonics.

LiMO2 (M = Ni, Co) thin film cathode materials: a correlation between the valence state of transition metals and the electrochemical properties

The electronic properties of the LiMO2 (M = Ni, Co) thin film cathode materials grown by RF sputtering/co-sputtering are in situ studied by X-ray photoelectron spectroscopy (XPS). Stoichiometric Li1.0Co1.0O2 thin films deposited on a heated substrate at T = 500–550 °C reveal the Co3+ (t2g6eg0) ground state configuration in the low spin (LS) state. Stoichiometry of the Lix(Ni,Co)O2 films and the valence and spin states of the Ni ions depend strongly on the growth conditions. The electronic configuration of stoichiometric Li1.0Ni0.5Co0.5O2 is described as the Ni3+ (t2g6eg1) LS and Co3+ (t2g6eg0) LS states. The Li-deficient Lix<1.0(Ni,Co)O2 exhibits Ni2+ (t2g6eg2) in the high spin (HS) and Co3+ (t2g6eg0) in LS states. The reduction of the trivalent Ni ions to Ni2+ (t2g6eg2) with a HS state electronic configuration is related to the evaporation of Li2O at elevated substrate temperatures coupled to a loss of O2 due to an internal oxidation reaction of O2− lattice ions induced by the strongly oxidizing Ni3+ ions. Owing to the stable Co3+ (t2g6eg0) with a LS state electronic configuration, Li1.0Co1.0O2 thin films cycled to 4.2 V exhibit a very good electrochemical reversibility. Li1.0Ni0.5Co0.5O2 films annealed at the same temperature as for Li1.0Co1.0O2 manifest a broadening of the oxidation/reduction peaks of the cyclic voltammogram (CV) curves with a strong current drop after the first step of the electrochemical Li-deintercalation. The observed irreversibility of the Li-intercalation/deintercalation process is attributed to instability of the Ni3+ (t2g6eg1) ions. Temperatures of the deposition/annealing above 750 °C lead to the phase separation of the Lix(Ni,Co)O2 films, a strong Li deficiency, the occurrence of Co2+ (t2g5eg2) with HS ions and consequently a complete degeneration of the electrochemical cyclability.

Dissociative adsorption of H2O on LiCoO2 (00l) surfaces: Co reduction induced by electron transfer from intrinsic defects

Understanding the mechanism of the interaction of lithium ion conductors with water is crucial for both fundamental and technological points of view. Despite the generally accepted fact that water is one of main sources of the degradation of Li-ion recharge batteries, the physicochemical processes occurring at the water-lithium ion conductor interface are not fully understood. By using synchrotron X-ray photoelectron spectroscopy (SXPS) and O K- and Co L- X-ray absorption near edge structure (XANES), we evidence that H2O is dissociatively adsorbed on LiCoO2 thin film at room temperature resulting in the formation of OH groups and the accumulation of the negative charge at the surface accompanied by electron transfer to the initial empty Co3d (eg (*)) state. By considering the experimentally obtained energy diagram of the ionic conductor and water, direct charge transfer is not favorable due to a high difference in the chemical potential of the ionic conductor and electronic levels of the molecule. Here, we develop the model for the dissociative water adsorption which explains the electron transfer to LiCoO2 by using the atomistic approach. The model takes into account the intrinsic defects found on the surface (<2 nm depth) by using the depth resolved photoemission experiments and can be explored to other layered transition metal oxides to interpret the interaction of water with the surface of ionic conductors.

3D-ordered carbon materials by melt-shear organization for tailor-made hybrid core–shell polymer particle architectures

The melt-shear organization technique for tailor-made polystyrene-co-polyacrylonitrile (PSAN) shell and silica core particles is investigated yielding easy-scalable carbonaceous porous films after etching and appropriate thermal treatment. Monodisperse silica core particles are surface modified and transduced to a seeded emulsion polymerization for the preparation of processable well-defined core–shell PSAN particles. Melt-shear organization for particle alignment into a colloidal crystal structure is applied prior to the thermally induced crosslinking of the PSAN shell material, followed by etching and carbonization of the porous polymeric opal film. It is shown that polymer processing and applied thermal treatment protocols are crucial and capable of maintaining the pristine particle order in the free-standing carbonaceous films. The obtained films reveal hexagonally aligned pores as part of a conductive carbonaceous matrix. Conductivity and adjustable porosity are evidenced by conductive atomic force microscopy (C-AFM) and scanning electron microscopy (SEM) measurements, respectively. The herein developed melt-shear organization technique for a novel polymer-based carbonaceous particle precursor material is shown to be a potential platform for the preparation of scalable conductive materials. The route described here will be feasible for the preparation of doped and tailor-made conductive materials with a wide range of applications in the fields of electrodes, batteries, as well as sensing and photonic band gap materials.

Strain engineering in epitaxial La1−xSr1+xMnO4 thin films

We have synthesized epitaxial thin films of La1−xSr1+xMnO4 with x = 0.0 and x = 0.5 by pulsed laser deposition on NdGaO3 and LaSrAlO4 substrates with different lattice mismatch. X-ray analysis shows that these layered doped manganites can be grown fully strained allowing to tune the lattice degrees of freedom which otherwise are a function of chemical composition x. Since the crystal structure is strongly coupled to the magnetic, orbital, and charge degrees of freedom in the doped manganites, the demonstrated strain engineering is the base for an extrinsic control of, e.g., charge-orbital order.

Metallopolymer-Based Block Copolymers for the Preparation of Porous and Redox-Responsive Materials

Metallopolymers are a unique class of functional materials because of their redox-mediated optoelectronic and catalytic switching capabilities and, as recently shown, their outstanding structure formation and separation capabilities. Within the present study, (tri)block copolymers of poly(isoprene) (PI) and poly(ferrocenylmethyl methacrylate) having different block compositions and overall molar masses up to 328 kg mol-1 are synthesized by anionic polymerization. The composition and thermal properties of the metallopolymers are investigated by state-of-the-art polymer analytical methods comprising size exclusion chromatography, 1H NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. As a focus of this work, excellent microphase separation of the synthesized (tri)block copolymers is proven by transmission electron microscopy, scanning electron microcopy, energy-dispersive X-ray spectroscopy, small-angle X-ray scattering measurements showing spherical, cylindrical, and lamellae morphologies. As a highlight, the PI domains are subjected to ozonolysis for selective domain removal while maintaining the block copolymer morphology. In addition, the novel metalloblock copolymers can undergo microphase separation on cellulose-based substrates, again preserving the domain order after ozonolysis. The resulting nanoporous structures reveal an intriguing switching capability after oxidation, which is of interest for controlling the size and polarity of the nanoporous architecture.

Localized Gap States in LiCoO2 and Their Influence on the Transport Properties in Li-Ion Batteries

The electronic structure of Co2O3 and LiCoO2 is analyzed in a comparative study employing synchrotron radiation. We study the Co2p and O1s core levels, the X-ray absorption spectra at the Co2p and O1s absorption edges, as well as the valence band and conduction band states of Co2O3 and LiCoO2 by resonant photoelectron spectroscopy. We notice a metallic behavior of Co2O3 while LiCoO2 has a gap. We discuss our data in terms of a band diagram in which localized states around the Fermi energy are within the gap formed from covalent O2p-Co3d contributions.

Exploring redox activity in a LiCoPO4–LiCo2P3O10 tailored positive electrode for 5 V lithium ion batteries: rigid band behavior of the electronic structure and stability of the delithiated phase

The stability of the novel highly electronically conductive LiCoPO4–LiCo2P3O10 tailored polyphosphate is studied under electrochemical delithiation using quasi in situ depth-resolved synchrotron photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray diffraction. The cathode material exhibits a rigid band behavior of the electronic structure and is stable in its fully delithiated state without amorphization in air.