Bottom-up methods
8.3 Methods and materials
In the following, attempts to synthesise metallic Ti nanoparticles followed by deposition of Pt shells are described, along with the methods employed for the physical characterisation of the synthetic products.
8.3.1 General strategy for Ti@Pt nanoparticle synthesis
A core-mediated sequential-reduction method was developed for the synthesis of Ti@Pt nanoparticles in this work. The overall scheme is shown in Figure 8.6. Metallic Ti0 cores were first formed by chemical reduction of a Ti4+ precursor in anhydrous THF under inert atmosphere. A Pt2+ precursor was then added slowly for the controlled formation of Pt shells. Given the difficulties anticipated in selecting the FM growth mode for the Pt shell, a strategy for the reduction of Ti core surface energy was developed, which involved the in-situ grafting of organic ligands to the surface of Ti cores during their nucleation using diazonium chemistry.
Figure 8.6: Overall synthetic protocol employed in this work for the synthesis of Ti@Pt nanoparticles. First a Ti4+
precursor is reduced by LiBEt3H in the presence of a diazonium salt to produce ligand-stabilised metallic Ti cores,
8.3.2 Synthesis of diazonium compounds
Diazonium compounds are commonly employed as reactive intermediates in organic synthesis, and are characterised by the diazonium ion
N≡N
+, usually attached to an aromatic ring as in Figure8.7 (a).
It is well known that organic ligands can be grafted to a wide range of surfaces (including polymers, metals, metal oxides and carbon materials) by reduction of their diazonium derivatives [269]. Treatment of a diazonium salt (R-N2+) with a reducing agent results in the release of N2, leaving an extremely reactive radical (R●), which forms a covalent bond with any available substrate – in this case colloidal Ti nanoparticles undergoing nucleation and growth.
The functionalisation of metal nanoparticles with organic ligands via their diazonium derivatives has been demonstrated previously for Pd [270], Au [271], Pt [271] and Ti [272] nanoparticles. In all of these studies, the diazonium salt was present along with the metal precursor prior to the reduction step, and the product(s) were found to consist of metallic nanoparticles capped by covalently-bound organic ligands. Of particular relevance to this work is that by Ghosh et al., in which Ti nanoparticles capped by biphenyl ligands were prepared by reduction of TiCl4 with lithium triethylborohydride in the presence of biphenyldiazonium tetrafluoroborate [272]. Their product was shown to be metallic and was reported to be stable in air; unlike previous Ti nanopowders, which were found to be extremely pyrophoric [273]. The biphenylamine precursor employed by Ghosh et al. is a potent carcinogen and was unavailable for this study. The alternative compounds considered for this study are shown in Figure 8.7 (b-d).
Due to their inherent reactivity, diazonium salts were prepared immediately prior to use. The most common method for the preparation of diazonium salts is the oxidation of the analogous amine
Figure 8.7: Diazonium ligands used in this study for the stabilisation of metallic Ti nanoparticles
precursor in acidic aqueous solution, using sodium nitrite as in scheme (8.7). The counter ion X- for the diazonium salt is provided by the acid species in the reaction.
(8.7)
Because the diazotization is carried out in aqueous solution, it is important that the product can be isolated and dried thoroughly prior to use in the anhydrous synthesis of Ti nanoparticles. Diazonium halide salts are typically far too unstable to be isolated in dry form, and have been known to detonate upon solvent removal. Fortunately, the tetrafluoroborate salts are stable in crystalline form, and can be isolated and dried as required. Even so, they decompose readily on exposure to UV radiation and react with water, so care was taken to avoid prolonged exposure of the salt to light or moisture during work-up.
The three diazonium ligands were trialled in this work (Figure 8.7 (b-d)) were prepared according to the method described by Mirkhalaf and Schiffrin [271]. Amine precursors for each ligand (4- benzylaniline, 4-ethylaniline and 4-decylaniline) were purchased from Sigma Aldrich and used as- received. In a typical preparation, 2 mmol of the amine precursor was dissolved in 2 ml of a 50/50 v/v mixture of acetic and propionic acid (or hexanoic acid in the case of 4-benzylaniline), to which 2 ml 50 wt% aqueous solution of tetrafluoroboric acid (HBF4, Sigma) was added before cooling to 0ºC in an ice bath. One molar equivalent of an ice-cold 0.3 mM solution of NaNO2 was added dropwise under stirring. The mixture was stirred for 1hr and allowed to warm gently to ~10ºC. The coarse precipitate was filtered over a glass sinter and washed twice with cold fluoroboric acid, then twice with diethyl ether. The product, a colourless crystalline solid, was dried by vacuum desiccation over CaCl2. The presence of the diazonium moiety in the product was confirmed by FTIR and NMR spectroscopy. Samples for NMR were prepared by dissolving ~1 mg of the product in deuterated chloroform (CDCl3), and FTIR samples were prepared by evaporating a few drops of this solution from a KBr pellet.
8.3.3 Synthesis of unstabilised Ti-Pt nanoparticles
Early experiments on the synthesis of Ti@Pt nanoparticles were carried out using an adapted version of the method used by Abe et al. for the preparation of Pt3Ti alloy nanoparticles. Whereas Abe et al. mixed the Ti and Pt precursor prior to co-reduction, it was hoped that reduction of the Ti
precursor, followed sequentially by slow infusion and reduction of the Pt precursor might result in the desired core@shell product.
The experimental setup was as shown in Figure 8.8. Prior to each experiment, all glassware was cleaned by soaking in aqua regia, rinsed with HQ water followed by meticulous drying – first in an oven at 160ºC for several hours, and then with a hot air gun once the apparatus was assembled and under vacuum/N2 purge. The sodium naphthalide reducing agent was prepared by adding a freshly cut sliver of sodium metal (~15mg, washed with hexanes to remove residual mineral oil from storage) to a 1:1 stoichiometric quantity of naphthalene (reagent grade, Sigma) under N2 purge in a 3-neck reaction vessel. Dry THF (50 ml) was added via cannula transfer to the addition flask, and from there in portions to the Na/naphthalene mixture under stirring using a PTFE-coated magnetic follower. The reaction was allowed to stir overnight, whereupon a clear, dark green solution of sodium naphthalide was formed.
A THF adduct of TiCl4 (Ti(THF)2Cl4, Sigma) was used as received as the Ti precursor. In a typical synthesis, 20 mg Ti(THF)2Cl4 was dissolved in 10 ml dry THF under N2 purge in a dry flask. The solution was loaded into a dry, glass syringe without exposure to air and infused at 20 ml.min-1 through a PTFE transfer line into the sodium naphthalide solution under continuous stirring. The reaction was allowed to stir for several hours. The Pt precursor solution was then prepared in the same manner as the Ti precursor by dissolving the required amount of (1,5-cyclooctadiene)PtCl2 (99.9% metals basis, Sigma) in dry THF, and transferred into a syringe before infusing slowly at 3 ml.hour-1 into the pre-formed Ti core mixture. Experiments were performed with Pt:Ti molar ratios of 1:1 and 2:1 by adjusting the quantity of the Pt precursor.
To isolate the product, the solvent and excess naphthalene were removed by vacuum distillation. The solid residue was washed with hexane and methanol to remove by-products, with the solid product separated from the washing solvent by centrifugation after each step.
8.3.4 Synthesis of ligand-stabilised Ti-Pt nanoparticles
Functionalisation of Ti core surfaces with organic ligands was expected to reduce the Ti surface energy and promote overgrowth of Pt shells, as proposed in section 8.1.6 . Following the method of Ghosh et al. [272], Ti nanoparticles were synthesised by reduction of TiCl4 using lithium triethylborohydride (LiBEt3H) in anhydrous THF, in the presence of the diazonium salt prepared as described in section 8.1.6 . The LiBEt3H reducing agent (supplied by Sigma under the trade name Superhydride™) is slightly weaker than sodium naphthalide, with a standard reduction potential of approximately -2.2 V. However, it has been employed previously for the preparation of metallic Ti nanoparticles, and is a more convenient to use than sodium naphthalide.
The Ti precursor Ti(THF)2Cl4 was added to a dry, N2-purged 3-neck reaction flask along with a calculated quantity of either the 4-benzyl, 4-decyl or 4-ethyl diazonium salt. Experiments were performed at a diazonium:Ti ratio of 4:1, with a quantity of the Ti precursor calculated to yield 10 mg of metallic Ti in the product. Dry THF was added directly to the diazonium/Ti precursor mixture by cannula transfer, followed by stirring for 2 hours to form a clear, yellow solution. The reducing agent, supplied as a 1M solution of LiBEt3H in THF, was first diluted to 0.1M with dry THF, before loading a calculated volume of the solution into a dry glass syringe. To ensure complete reduction of the Ti4+ precursor and the diazonium salt, a 1.5-fold molar excess of LiEt3BH was used, based on the molar equivalents of hydride required to reduce the Ti4+, diazonium and (in the following step) Pt2+ species, calculated according to equation (8.1),
Figure 8.8: Diagram of apparatus used for synthetic work according to method of Abe et al.
n
LiBEt3H
=1.5(4 n
Ti4++n
R-N2 ++2 n
Pt2+
)
(8.8)where nLiBEt3H is the number of moles of hydride required for a reaction involving
n
Ti4+,n
R-N2 +and
n
Pt2+ moles of Ti4+, diazonium and Pt2+ species respectively.The required volume of 0.1M LiBEt3H solution was infused at 5 ml.hour-1 through a PTFE transfer line into the stirring Ti/diazonium solution, and the mixture was left stirring for several hours.