Frontiers of Materials Science (v.6, #2)

From nanoparticles via microtemplates and milliparticles to deep-sea nodules: biogenically driven mineral formation by Xiao-Hong Wang; Ute Schloßmacher; Shun-Feng Wang; Heinz C. Schröder; Matthias Wiens; Renato Batel; Werner E. G. Müller (97-115).
Deep-sea minerals in polymetallic nodules and seamount Co-rich crusts are not only formed by mineralization but also by biologically driven processes involving microorganisms (biomineralization). Within the polymetallic nodules, free-living and biofilm-forming bacteria provide the matrix for manganese deposition, and in seamount Co-rich crusts, coccolithophores represent the dominant organisms that act as bio-seeds for an initial manganese deposition. These (bio)minerals are economically important: manganese is an important alloying component and cobalt forms part of special steels in addition to being used, along with other rare metals, in plasma screens, hard-disk magnets and hybrid car motors. Recent progress in our understanding of the participation of the organic matrices in the enrichment of these metals might provide the basis for feasibility studies of biotechnological applications.
Keywords: polymetallic nodule; biomineralization; bacteria; sustainable exploitation

Tribological properties of transition metal di-chalcogenide based lubricant coatings by Jun-Feng Yang; Braham Parakash; Jens Hardell; Qian-Feng Fang (116-127).
Transition metal di-chalcogenides MX2 (X = S, Se, Te; and M= W, Mo, Nb, Ta) are one kind of solid lubricant materials that have been widely used in industry. The lubricant properties of such lubricant coatings are dependent not only on microstructure, orientation, morphology, and composition of the coatings, but also on the substrate, the interface between substrate and lubricant coatings, and the specific application environment. In this review, the effects of parameters on tribological properties of such kind of lubricant coatings were summarized. By comparing advantages and disadvantages of those coatings, the special treatments such as doping, structural modulation and post-treatment were suggested, aiming to improve the tribological performance under severe test conditions (e.g. high temperature, oxidizing atmosphere or humid condition).
Keywords: transition metal di-chalcogenide; solid lubricant coating; tribological property

Sol-gel auto-combustion synthesis of spinel-type ferrite nanomaterials by Andris Sutka; Gundars Mezinskis (128-141).
Recent developments and trends of sol-gel auto-combustion method for spinel ferrite nanomaterial synthesis are briefly discussed and critically analyzed. The analysis of various parameters of reaction which could be used for better understanding of synthesis process and control of microstructure and property of spinel ferrite nanopowder products was the main objective of this review article. Special attention was paid to variety of particle size and phase purity. For these purposes the correlation between complexant, oxygen balance and combustion process chemical additives, as well as heating mechanism and atmosphere, was established. These results are relevant from standpoints of both application and processing of ferrites.
Keywords: ferrite; synthesis; sol-gel; auto-combustion; nanomaterial

Co-M (M= Co, Ni, Fe, Mn) layered double hydroxides (LDHs) were successfully fabricated by a hexamethylenetetramine (HMT) pyrolysis method. Composite electrodes were made using a self-assembly fashion at inorganic/organic surface binder-free and were used to catalyze oxygen evolution reaction. Water oxidation can take place in neutral electrolyte operating with modest overpotential. The doping of other transitional metal cations affords mix valences and thus more intimate electronic interactions for reversible chemisorption of dioxygen molecules. The application of employing LDH materials in water oxidation process bodes well to facilitate future hydrogen utilization.
Keywords: layered double hydroxide (LDH); oxygen evolution reaction; neutral; cobalt catalyst

A simple and controllable nanostructure comprising non-conductive poly(vinylidene fluoride) and graphene nanosheets for supercapacitor by Jing Sun; Ling-Hao He; Qiao-Ling Zhao; Li-Fang Cai; Rui Song; Yong-Mei Hao; Zhi Ma; Wei Huang (149-159).
An effective method was used to produce stable and homogeneous colloidal suspensions of highly reduced graphene oxide (RGO) in N,N-dimethylformamide (DMF) without the assistance of dispersing agents. According to the results of general characterization, relatively pure graphene sheets with the morphology of single layer or few-layer structure were obtained. Then nanocomposite powders of RGO and poly (vinylidene fluoride) (PVDF) were prepared by vacuum filtration of the mixed dispersions of both components. The nanocomposites exhibit a high-frequency capacitative response with small equivalent series resistance (ESR) at 0.4 Ω, a nearly rectangular cyclic voltammogram and possess a rapid current response as electrodes for supercapacitor in 5mol/L KOH electrolyte. Furthermore, after 600 galvanostatic charge/discharge cycles, the supercapacitor still performs a very high stability and efficiency of capacitance.
Keywords: reduced graphene oxide (RGO); poly(vinylidene fluoride) (PVDF); nanocomposite; supercapacitor

A hydroxyapatite (HA) coating was achieved on H2O2-treated carbon/carbon (C/C) composite through hydrothermally treating and induction heating deposited CaHPO4 coating in an ammonia solution under ultrasonic water bath. Then, this HA coating was placed in a NH4F solution and hydrothermally treated again to fabricate fluorinated hydroxyapatite (FHA) coatings for 24 h at 353, 373, 393 and 413 K, respectively. The structure, morphology and chemical composition of the HA and FHA coatings were characterized by SEM, XRD, EDS and FTIR, and the adhesiveness and chemical stability of these FHA coatings were examined by a scratch test and an immersion test, respectively. The results showed that the as-prepared FHA coatings contained needle-like or stripe-like crystals, different from those of the HA coating. As the fluoridation temperature rose, the adhesiveness of the FHA coating first increased from 34.8 to 40.9 N at a temperature between 353 and 393 K, and then decreased to 24.2 N at 413 K, while the dissolution rate of the FHA coating decreased steadily. The reasons for the property variation of the FHA coatings were proposed by analyzing the morphology, composition and structure of the coatings.
Keywords: carbon; hydroxyapatite (HA); coating; scratch test; dissolution behavior

In the present study we carried out the synthesis of β-cyclodextrin (β-CD) functionalized gold nanoparticles (AuNPs) using a microwave assisted heating method in alkaline media. Stable dispersion of β-CD stabilized AuNPs was obtained at an optimized pH of 10.5. At this pH value the deprotonated secondary hydroxyl group of β-CD shows the highest chelating affinity toward Pb2+ ions thereby inducing AuNP aggregation. The Pb2+ induced aggregation in β-CD-AuNP solution is monitored by both colorimetric response and UV-Vis spectroscopy. TEM, DLS and FTIR analyses were carried out to confirm the Pb2+ ion induced aggregation behaviour of β-CD-AuNPs under alkaline conditions. Furthermore at the experimental pH the response of the β-CD-AuNP system towards Pb2+ ions is selective when compared with other interfering metal cations.
Keywords: gold nanoparticle (AuNP); β-cyclodextrin (β-CD); colorimetric detection; metal ion

Exfoliation and dispersion of graphene in ethanol-water mixtures by Wei-Wei Liu; Bao-Yu Xia; Xiao-Xia Wang; Jian-Nong Wang (176-182).
Graphene has attracted much attention as a new nano-carbon for its unique structure and properties. However, production and dispersion of unfunctionalized graphene are still big challenges. Herein, we demonstrate a simple method for preparation and dispersion of such graphene with low cost and non toxicum. This approach is achieved by exfoliating graphite in an ethanol/water mixture and forming stable dispersion of mono- and few-layer graphenes. The ratio of ethanol/water in the mixture is found to be crucial to both the exfoliation and dispersion processes. Exfoliation in pure water or pure ethanol produces no graphene. This method avoids the conventional use of harsh oxidants and surfactants; therefore, the graphitic structure is well maintained without destruction. Benefiting from the use of ethanol and water, it can be easy to prepare transparent and conductive graphene films by vacuum filtering or spray method, and does not need special post-treatment to remove the impurity, which could be beneficial for potential applications in electronic, optic and energy areas.
Keywords: graphene; liquid-phase exfoliation; transparent conductive film; solubility parameter