[en] The Pd-Au multiwall carbon nanotubes (MWCNTs) supported catalyst prepared by polyol method was found to be catalytically active material in direct formic acid fuel cell electrooxidation reaction and during cyclic voltammetry (CV) measurements. The surface of the catalyst after calcination and reduction treatments was investigated previously. The effect of Ar ion bombardment modification on the calcinated catalyst was studied by electron spectroscopy. The chemical and structural changes were deduced from the X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) spectra. The XPS quantitative and qualitative analysis of different chemical forms of atoms indicated that the sputtering causes an increase of Pd surface content, accompanied by decreasing content of N, O, C sp², and Au (due to preferential sputtering of Au) and significant increase of C sp³ amount. A decrease was also observed in the surface content of (i) carboxyl groups, (ii) water, and (iii) Pd oxide (mainly less stable PdO₂). Analysis of XPS inelastic background shape by QUASES indicates that Pd crystallites phase is covered by PdO_x + C_amorphous overlayer of decreasing thickness after sputtering. Namely, thickness of PdO_x overlayer decreased from 8.2 to 3.5 nm and the thickness of amorphous carbon overlayer located on the top increased from 2.3 to 3.5 nm. Significant increase of C sp³ content results from carbon nanotube (CNT) π bonds damaging, whereas decrease of PdO₂ content from lower stability of PdO₂. Larger thickness of C_amorphous overlayer, in contrary to PdO_x, may result from higher preferential sputtering of oxygen than carbon atoms and cracking of the MWCNTs structure by Ar⁺ ions. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] Highlights: ► XP-EELS and REELS loss spectroscopy of MWCNT, ox-MWCNT, Pd/MWCNT, Pd–Au/MWCNT. ► Analysis of REELS π + σ energy loss peak, surface and bulk C sp²/sp³ bonds. ► MWCNT oxidation: attachment of functional groups to the surface of the MWCNT. ► Composites preparation and calcination: amorphous carbon at the surface. ► Reduced calcined Pd and Pd–Au MWCNT catalysts: attachment of functional groups. -- Abstract: The X-ray PhotoElectron Energy Loss Spectroscopy (XP-EELS) and Reflection Electron Energy Loss Spectroscopy (REELS) were used for analysing surface layers of “as-received” and functionalised multiwall carbon nanotubes (MWCNT), and MWCNT decorated with Pd and Pd–Au particles after calcination/reduction. The decorated MWCNT were previously applied as catalysts in a reaction of formic acid electrooxidation. These spectroscopies, used as complementary methods of structural surface analysis, provide information on the energy position, intensity and full width at half maximum of the quasi-elastic peak and inelastic π and π + σ energy loss peaks. Analysing the π + σ energy loss peak, the bulk and surface C sp²/sp³ components can be separated. Functionalisation of MWCNT, catalyst reduction and Ar⁺ ion sputtering increase the C sp³ content in comparison to the “as-received” MWCNT and calcined catalysts. The intensity ratios of surface and bulk C sp³ and sp² components evaluated from the REELS π + σ energy loss peak indicate: (i) functionalisation leads to attachment of functional groups to the MWCNT surface, (ii) calcined catalysts show an amorphous carbon overlayer at the surface and (iii) reduction of calcined catalysts leads to increasing C sp³ hybridisations.

[en] Palladium catalysts deposited on carbon nanotubes are used as the anode material in a direct formic acid fuel cell (DFAFC). In this paper we propose a simple method for enhancing the catalytic activity of Pd deposited on carbon nanotubes for formic acid electrooxidation by using ammonia solution. Multiwall carbon nanotubes (MWCNTs) used as the catalyst support are oxidized with concentrated nitric acid for 6 and 50 h. During the oxidation polynuclear aromatic compounds (PACs) are formed. We propose the use of ammonia solution after the oxidation process to remove the PACs from the carbon nanotubes surface. The properties of differently functionalized MWCNTs are studied and the results show that the use of ammonia solution removes the PACs formed during the oxidation process. The oxidation time of the carbon nanotubes has also a significant effect on the properties of the obtained catalysts. The palladium catalysts deposited on MWCNTs oxidized for a shorter time and rinsed with ammonia solution exhibit the highest initial activity in electrooxidation of formic acid in DFAFC (P = 216 mW mg_Pd⁻¹).

[en] Purified and functionalized in boiling concentrated (68%) HNO₃ acid the oxidized multiwall carbon nanotubes (ox-MWCNTs) under thermal treatment from RT to 630 °C and at 350 °C time dependent (1-4 h) were investigated using the surface sensitive electron and mass spectroscopy methods. Mass spectroscopy indicates significant desorption of H₂ and H₂O to about 300 °C. Higher H₂ desorption rate from RT up to about 100 °C is most likely caused by decomposition of organic acid impurities included within a bundle and in channels of the ox-MWCNTs after their functionalization by HNO_3. In the range of 100-300 °C part of the detected H₂, accompanied by desorption of CO₂, may origin from desorbed water. Above 300 °C, the small amount of desorbing H₂O may result from transformation of carboxylic groups into carboxylic acid anhydride. Significant desorption of CO₂ starting from 150 °C may result from decomposition of carboxylic groups, whereas desorption of CO starting at about 300 °C from decomposition of acid anhydride groups created from carboxylic groups during thermal dehydration. Desorption of CO and CO₂ at about 470 °C may be due to decomposition of hydroxyl O-H and carbonyl C=O groups. Above 600 °C mainly decomposition of C=O groups takes place and results in small desorption of CO. Time dependent (1-4 h) thermal treatment of ox-MWCNTs at 350 °C shows in XPS spectra decreasing amount of C-O in carboxyl groups and increasing amount of C=O in carbonyl and acid anhydride groups arising from carboxyl groups decomposition. Between 350 °C and 470 °C the higher desorption rate of CO₂ than CO indicates significant decomposition of carboxyl and carboxyl anhydride groups. At 350 °C the dynamic changes are indicated by the energy, intensity and full width at half maximum (FWHM) of the π → π^* interband transition and π loss peak, and quasi-elastic peak FWHM. During 4 h at 350 °C no C sp² reconstruction is observed. For the applied procedure of MWCNTs oxidation, large amount of water and some organic acid impurities, resulting from the MWCNTs oxidation, remain in the CNTs channels, interstitial channels between tubes and at nanotubes surface.

[en] The purification and functionalization of commercial multiwall carbon nanotubes was investigated. Carbon nanotubes (CNT CO., Ltd, Korea) were treated with boiling concentrated HNO₃ under a reflux condenser for about 50 h at 120^oC in order to purify and oxidize the raw material. The oxidized multiwall carbon nanotubes were rinsed with deionized water until stabilization of the filtrate pH. Measurement techniques included elemental analysis (CHN), scanning electron microscopy with energy dispersive X-ray spectrometer, inductively coupled plasma mass spectrometry, Fourier transform: infrared spectroscopy and thermal analysis. With the measurement techniques used the following information was obtained: CHN analysis provided information about the quantitative composition of the following elements carbon, hydrogen, nitrogen, scanning electron microscopy imaging provided information on shape, thickness and length of the nanotubes, energy dispersive X-ray spectrometry analysis of information about surface atomic composition of the quantitative analysis, inductively coupled plasma mass spectrometry quantitative analysis of the atomic composition (metals, especially Fe, Al), the Fourier transform infrared studies provided information about qualitative analysis of surface functional groups C_xO_yH_z (COOH, OH, COO) and thermal gravimetric-differential thermal analysis - quantitative analysis of thermal decomposition products. It was found that oxidation leads to the removal of amorphous carbon and forms mainly carboxylic functional groups linked to the nanotubes. The Fourier transform infrared spectra indicate the presence of some other structures, like ketone (quinone), acid anhydride, ether and epoxy groups. Nitric acid treatment also effectively removes aluminum oxide catalyst and iron catalyst from commercial multiwall carbon nanotubes. (authors)

No abstract available

[en] An in situ synthesis of ZnS and CdS quantum dots (QDs) in an aqueous solution of sodium hyaluronate (Hyal) produced foils emitting light on excitation with a UV light. The wavelength of emission was only slightly QDs size and more QDs concentration dependent and reached up to ∼320 nm in the case of ZnS and ∼400-450 nm in the case of CdS. Nanoparticles remained as non-agglomerated 10-20 nm nanoclusters. CdS/Hyal and ZnS/Hyal-QDs biocomposites were characterized using photoluminescence (PL), IR spectrometric techniques, and Transmission Electron Microscopy (TEM). The absolute molecular weights, radii of gyration, R_g, and thermodynamic properties of the obtained foils are given. Electric resistivity studies performed for the hyaluronic foil in the 100-1000 V range have revealed that the hyaluronate foil has very weak conducting properties and QDs only insignificantly affect those properties as QDs practically did not interact with the foil. Size exclusion chromatography showed a decrease in the molecular weight of the hyaluronate after generation of QDs in its solution, particularly in the lower molecular fraction of the hyaluronate. The generation of CdS QDs was more destructive for the polysaccharide matrix.

[en] Highlights: • Graphene oxide by ozonation and cold glow air plasma from 8 nm graphene platelets. • XPS, REELS for characterising the surface chemical composition and the structure. • C sp³ and carbon–oxygen groups content smaller contrary to wet chemical methods. • REELS shows decreasing number of graphene layers: platelets > ozone > plasma. • Oxidation separates graphene layers via incorporating interlayer oxygen-H₂O groups. - Abstract: Graphene oxide was prepared from commercial graphene powder (G) platelets of 8 nm thickness by oxidation in ozone (G-O3) and low-energy (cold) glow air plasma in a pink region (G-PP). X-ray photoelectron spectroscopy (XPS) was applied for chemical characterisation of the atomic content and for obtaining quantitative information on C hybridisation. Reflection electron energy loss spectroscopy (REELS) was used for characterising the surface structure including the content of surface and bulk carbon atoms of sp² and sp³ hybridisation. The G-O3 and G-PP graphene oxide samples contain a low amount of C sp³ bonds and carbon–oxygen groups in comparison to graphene oxide prepared by the “wet” chemical methods. Oxidation of commercial graphene platelets powder, especially in air plasma, leads to increasing surface C sp³ and significantly bulk C sp³ contributions, indicating intercalation by oxygen groups. The intensity ratio of the REELS π + σ C sp² bulk to the C sp² surface energy loss peaks, decreasing in the order G>G-O3>G-PP, indicates exfoliation of layers in G-O3 and G-PP by oxygen functional groups and water with decreasing average number of layers in graphene oxide nanostructures due to oxidation. Although, the “wet” chemical methods are more effective for oxidation leading to a larger amount of C sp³ and oxygen groups, the proposed methods of oxidation by ozonation and in air plasma are inexpensive, safe, effective, environmentally friendly and do not result in toxic chemical waste products

[en] The oxidized multiwall carbon nanotubes, their temperature modification in the range from RT to 630 ^oC are studied. The oxidation proceeds in aqueous solution of the concentrated HNO₃. The oxidized multiwall carbon nanotubes are investigated using the infrared and electron spectroscopy methods. The oxidation creates mainly carboxyl, hydroxyl and carbonyl groups attached to multiwall carbon nanotubes, which can be detected by the infrared spectroscopy and at the surface by the electron spectroscopy methods. Thermal modification under ultra-high vacuum conditions leads to decreasing content of water, oxygen and carbon groups proceeding with different rates, increasing content of C sp² bonds (e.g. carbon nanotubes reconstruction), surface atoms ordering and changes of surface atomic density.

[en] Highlights: • Graphene oxide (FL-GOc) and reduced graphene oxide (FL-RGOc): XRD, TEM, XPS, REELS. • FL-GOc: stacking nanostructure—22 × 6 nm (DxH), 0.9 nm layers separation (XRD). • FL-RGOc: stacking nanostructure—8 × 1 nm (DxH), 0.4 nm layers separation (XRD). • Reduction: oxygen group degradation, decreasing distance between graphene layers. • Number of graphene layers in stacking nanostructure: 6–7 (FL-GOc), 2–3 (FL-RGOc). - Abstract: The commercial and synthesised few-layer graphene oxide, prepared using oxidation reactions, and few-layer reduced graphene oxide samples were structurally and chemically investigated by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron spectroscopy methods, i.e. X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS). The commercial graphene oxide (FL-GOc) shows a stacking nanostructure of about 22 × 6 nm average diameter by height with the distance of 0.9 nm between 6-7 graphene layers, whereas the respective reduced graphene oxide (FL-RGOc)—about 8 × 1 nm average diameter by height stacking nanostructure with the distance of 0.4 nm between 2-3 graphene layers (XRD). The REELS results are consistent with those by the XRD indicating 8 (FL-GOc) and 4 layers (FL-RGOc). In graphene oxide and reduced graphene oxide prepared from the graphite the REELS indicates 8–11 and 7–10 layers. All graphene oxide samples show the C/O ratio of 2.1–2.3, 26.5–32.1 at% of C sp³ bonds and high content of functional oxygen groups (hydroxyl—C-OH, epoxy—C-O-C, carbonyl—C=O, carboxyl—C-OOH, water) (XPS). Reduction increases the C/O ratio to 2.8–10.3, decreases C sp³ content to 11.4–20.3 at% and also the content of C-O-C and C=O groups, accompanied by increasing content of C-OH and C-OOH groups. Formation of additional amount of water due to functional oxygen group reduction leads to layer delamination. Removing of functional oxygen groups and water molecules results in decreasing the distance between the graphene layers