Formation of OTS self-assembled monolayers at chemically treated titanium surfaces

Enhanced biocompatibility of titanium implants highly depends on the possibility of achieving high degrees of surface functionalization for a low immune response and/or enhanced mineralization of bioactive minerals, such as hydroxyapatite. In this respect, surface modification with Self Assembled Monolayers (SAMs) has a great potential in delivering artificial surfaces of improved biocompatibility. Herein, the effectiveness of common chemical pre-treatments, i.e. hydrogen peroxide (H₂O₂) and Piranha (H₂SO₄ + H₂O₂), in facilitating surface decontamination and hydroxylation of titanium surfaces to promote further surface functionalization by SAMs is investigated. The quality of the octadecyltrichlorosilane (OTS) based SAM appeared to strongly depend upon the surface morphology, the density and nature of surface hydroxyl sites resulting from the oxidative pre-treatments. Contrary to common belief, no further hydroxylation of the titanium substrate was observed after the selected chemical pre-treatments, but the number of hydroxyl groups available on the surface was decreased as a result of the formation of a titanium oxide layer with a gel-type structure. Further examinations by atomic force microscopy, infrared spectroscopy and X-ray photoelectron spectroscopy also revealed that mild oxidizing conditions were sufficient to remove surface contamination without detrimental effects on surface hydroxylation state and surface roughness. Furthermore, the adsorption of the alkylsiloxane molecules forming the SAM film is believed to proceed through hydrolysis at surface acidic hydroxyl groups rather than randomly. This site dependent adsorption process has significant implications for further functionalization of titanium based implants. It also highlights the difficulty of achieving an OTS based SAM at the surface of titanium and question the quality of SAMs reported at titanium surfaces so far.     

Dual-tuning the thermodynamics and kinetics: Magnesium-naphthalocyanine nanocomposite for low temperature hydrogen cycling

2,11,20,29-Tetra-tert-butyl-2,3-naphthalocyanine (TTBNc) was used as an alternative host to support magnesium (Mg) nanoparticles. After deposition and decomposition of dibutylmagnesium on TTBNc, Mg nanoparticles of around 4 nm supported on TTBNc were observed by TEM. These TTBNc stabilized Mg nanoparticles were found to absorb hydrogen at 100 °C and release hydrogen from 75 °C. The Mg-TTBNc material showed good hydrogen cycling properties and structural stability. Kinetic measurements showed fast hydrogen absorption within 2 min at 150 °C. The hydrogen desorption kinetics were slower at the same temperature but faster at 250 °C with 80% of the hydrogen desorbed within 1 h. Enthalpy and entropy for hydrogen uptake and release in Mg-TTBNc determined from PCT measurements were found to be of 52.7 ± 4.9 kJ mol^?1 H₂ and 107.8 ± 9.4 J K^?1 mol^?1 H_2, respectively. These values are much lower than those of bulk Mg.     

Effect of Nb2O5 on MgH2 properties during mechanical milling

Recently, it was shown that hydrogen absorption–desorption kinetics in magnesium were improved by milling magnesium hydride (MgH₂) with transition metal oxides. Herein, we investigate the role of the most effective of these oxides, Nb₂O₅ when added in larger volume fraction. The effect of Nb₂O₅ on magnesium crystalline structure, particle size and (ab)desorption properties has been characterised. Moreover, we report that pure MgH₂ can also show fast hydrogen sorption kinetics after a long milling time. The effects of Nb₂O₅ on MgH₂ sorption properties are rationalised in a new approach considering Nb₂O₅ as a dispersing agent, which helps reduce MgH₂ particle size during milling.     

Synthesis of borohydride nanoparticles at room temperature by precipitation

Borohydride (LiBH₄, NaBH₄ and KBH₄) nanoparticles were successfully synthesized by a modified ternary anti-precipitation method. A co-solvent was introduced to facilitate the formation of a microemulsion and modify the metastable zone to favour the precipitation of borohydride particles. In this process, the stabilising surfactant's carbon chain length was found to provide an additional mean to further tune particle size due to the resulting steric hindrance. The method reported here finally provides an effective and simple mean to prepare nanosized borohydride particles for hydrogen storage purposes, while minimising the amount of stabilising surfactant that can contaminate the hydrogen release.     

Formation of aluminium hydride (AlH3) via the decomposition of organoaluminium and hydrogen storage properties

Aluminium hydride (AlH₃) is a promising hydrogen storage material due to its competitive hydrogen storage density and moderate decomposition temperature. However, there is no convenient way to prepare/regenerate AlH₃ from (spent) Al by direct hydrogenation. Herein, we report on a novel approach to generate AlH₃ from the decomposition of triethylaluminium (Et₃Al) under mild hydrogen pressures (10 MPa) with the use of surfactants. With tetraoctylammonium bromide (TOAB), the synthesis led to the formation of nanosized AlH₃ with the known α phase, and these nanoparticles released hydrogen from 40 °C instead of the 125 °C observed with bulk α-AlH₃. However, when tetrabutylammonium bromide (TBAB) was used instead of TOAB, larger nanoparticles believed to be related to the formation of β-AlH₃ were obtained, and these decomposed through a single exothermic process. Despite the possibility to form α-AlH₃ under low conditions of temperature (180 °C) and pressure (10 MPa), TOAB stabilised AlH₃ was found to be irreversible when subjected to hydrogen cycling at 150 °C and 7 MPa hydrogen pressure.     

Destabilisation of complex hydrides through size effects

Nanoparticles of NaAlH4, LiAlH4 and LiBH4 were prepared by encapsulating their respective hydrides within carbon nanotubes by a wet chemical approach. The resulting confinement had a profound effect on the overall hydrogen storage properties of these hydrides, with NaAlH4 and LiAlH4 releasing hydrogen from room temperature, for example.     

Influence of particle size on electrochemical and gas-phase hydrogen storage in nanocrystalline Mg

Nanocrystalline Mg powders of different particle size were obtained by inert gas evaporation and studied during electrochemical and gas-phase hydrogen cycling processes. The samples were compared to dehydrided samples obtained by mechanical milling of MgH₂ with and without 2 mol% Nb₂O₅ as catalyst. The hydrogen overpotential of the pure Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced with smaller particle sizes reaching values comparable to samples with Nb₂O₅ additive. On the other hand gas-phase charging experiments showed the capacity loss with smaller particle sizes due to oxidation effects. These oxidation effects are different depending on the synthesis method used and showed a major influence on the hydrogen sorption kinetics.     

MgH2 with different morphologies synthesized by thermal hydrogenolysis method for enhanced hydrogen sorption

Magnesium hydride (MgH₂) with a range of morphologies has been synthesized via a simple hydrogenolysis route involving the decomposition of di-n-butylmagnesium. As the synthetic medium evolved from an inert atmosphere of argon to hydrogen pressure, the morphology shifted from rod like to small particles (25–170 nm). In cyclohexane, a solvent relative inert toward magnesium, smaller particles (15–50 nm) were formed. However in diethyl ether, which is more reactive toward magnesium, flakes organized in large microstructures were obtained. Remarkably in all cases β-MgH₂ was readily obtained with some residual carbon contamination. Hydrogen release from these structures occurred at a relatively low temperature (300 °C), with desorption kinetics faster or equivalent to that of ball milled magnesium. In particular, hydrogen desorption from the smallest particles of MgH₂ produced via the hydrogenolysis of di-n-butylmagnesium under hydrogen pressure or cyclohexane was impressive with the full desorption achieved in less than 10 min without any catalyst. These remarkable hydrogen storage properties are believed to result from an appropriate stabilization of the nanoparticles generated.     

Processing of strong and highly conductive carbon foams as electrode

Porous carbons were processed by the foaming of two-part polymer precursors with pre-loaded carbon powder (graphitic or amorphous), and then resin impregnation and carbonization to control both porosity and mechanical strength of the resulting foam. Electrical conductivity of the foams was improved by nickel-catalyzed graphitization. Different levels of graphitization were obtained for varied concentrations of nickel to the amorphous carbon foams. The presence of graphitic carbon improves the electrical conductivity by a factor of 50, compared to the amorphous counterparts. Electrochemical studies showed that graphitization of the amorphous structures increased the specific electrochemical surface area and electron transfer rate of the carbon electrodes.     

Nanoconfinement of borohydrides in CuS hollow nanospheres: A new strategy compared to carbon nanotubes

Herein, the nanoconfinement of LiBH₄ and NaBH₄ in a carbon (carbon nanotubes, MBH₄@CNT) and an inorganic support (CuS hollow nanospheres, MBH₄@CuS) is compared. Both led to drastic improvements in hydrogen storage properties, with hydrogen desorption occurring from room temperature, and the reversibility greatly enhanced. However, successive hydrogen desorption and absorption cycles for MBH₄@CNT led to a decrease in hydrogen storage capacity, most likely due to partial oxidation from oxygen-containing groups on the surface of the carbon nanotubes. In contrast, little to no decrease in capacity was observed for MBH₄@CuS, indicating that similar materials may be a more viable alternative for future nanoconfinement research.