Sodium naphthalide and Ti precursor solutions were prepared as described in section 8.3.3 . The colour of the naphthalide solution changed from dark green to dark brown upon addition of the clear, light brown Ti precursor solution, which was thought to indicate formation of metallic Ti nanoparticles. After stirring for 30 minutes, the mixture was found to contain a fine, dark brown precipitate. The precipitate was sampled for TEM imaging, but the sample turned white upon exposure to air, indicating oxidation of Ti to TiO2.
The apparent precipitation of Ti nanoparticles meant that the deposition of Pt shells was likely to be impossible in this case. Nonetheless, the Pt precursor was infused to the Ti/naphthalide mixture, whereupon the supernatant liquid turned dark brown. The coarse, dark brown Ti precipitate remained. The solvent was removed by vaccuum distillation, yielding a clear, colourless distillate and a mixture of dark brown and white residues in the reaction flask. The white residue – presumably naphthalene – was soluble in hexane and was removed in the first washing step. The brown residue was easily redispersed in MeOH.
The bright-field TEM images of the purified product in Figure 8.9 (a-b) show that nanoparticles were produced, but that they were rather agglomerated and appear to be dispersed over a crystalline substrate, thought to be TiO2. The size distributions in Figure 8.9 (c) reveals a slight increase in average particle size upon increasing the ratio of Pt:Ti from 1:1 to 2:1. Whilst this size increase is in keeping with an increase in Pt shell thickness, it does not prove that the product has a core@shell structure. HAADF-STEM imaging was performed in an attempt to discern the structure of the particles synthesised at Pt:Ti=2:1.
Figure 8.10 (a) shows a HAADF-STEM image in which the results of an EDX linescan across a single particle are superimposed. For a core@shell particle, the integrated intensities for the core and shell elements across the width of the particle should appear as illustrated in Figure 8.10 (b). The linescan in Figure 8.10 (a) may show the beginnings of such a pattern, but the analysis is hardly conclusive in identifying a core@shell structure. This analysis was performed close to the limits of the resolution and detection capabilities of the instrument, and longer integration times were precluded by image drift and beam-induced degradation of the sample.
XPS analysis of the purified products synthesised at Pt:Ti=1:1 and 2:1 revealed that Ti was in its +4 oxidation state in both cases. The Ti 2p3/2 and 2p1/2 photoelectron peaks in the spectra shown in Figure 8.11 are found at 458.3 eV and 464.5 eV respectively, which correspond well with
Figure 8.10: (a) HAADF-STEM image (x1.2 M) of a group of particles in the Pt:Ti=2:1 product, together with an EDX linescan across a single particle; (b) schematic showing the EDX linescan profile expected for a Ti@Pt core@shell particle.
Figure 8.9: (a-b) Bright-field TEM images (x 300 K) of the purified product synthesised at (a) Pt:Ti=1:1 and (b) Pt:Ti=2:1, showing agglomerated metallic nanoparticles; (c) particle size distributions for the products synthesised at Pt:Ti= 1:1 and 2:1.
reference data for TiO2[274]. The absence of peaks in the Ti 2p XPS spectra at 454.2 eV or 460.0 eV confirms that no metallic Ti was present in either sample[274].
On the basis of the sum of the evidence from TEM, HAADF-STEM and XPS analyses, it appears that the products produced via the method described in section 8.3.3 consist of metallic Pt nanoparticles dispersed on or within a TiO2 matrix. The agglomeration of the Ti cores prior to infusion of the Pt precursor precluded formation of Pt shells. Consequently, metallic Ti was either oxidised to TiO2 upon exposure to air, or leached from the product via the formation of titanium methoxide during methanol washing steps[243].
8.4.2 Synthesis of diazonium compounds
To prevent agglomeration of Ti cores and encourage the formation of Pt shells, subsequent experiments introduced capping ligands to the Ti cores during nucleation in an attempt to reduce their surface energy and provide steric stabilisation against agglomeration. Ligands were to be grafted to the Ti surface using diazonium chemistry, as described in section 8.3.4 . Being inherently unstable, the diazonium compounds were synthesised just prior to use according to the method described in section 8.3.2 .
The decyl- and benzyldiazonium products were isolated as white, crystalline solids, which were soluble in CDCl3 and MeOD respectively for NMR analysis. The synthesis of the ethyldiazonium salt was unsuccessful, as the reaction product was soluble in the washing solvent and could not be isolated by filtration.
The H1 NMR spectrum of the 4-decylaniline compound is shown in Figure 8.12 (a). The peaks at 7.3 ppm and 2.5 ppm are due to water and CHCl3 respectively, present in the NMR solvent. The
Figure 8.11: Ti 2p XPS spectra for products synthesised at Pt:Ti=1:1 and 2:1.
remaining peaks were all assigned to hydrogen atoms in the 4-decylaniline molecule: those at 0.75- 1.75 ppm are from hydrogen atoms in CH2 and CH3 groups in the decyl chain, the broad peak at 3.5ppm is due to NH2, and the doublets at ~6.7 and ~7.0 ppm are assigned to hydrogen atoms on the phenyl ring. In the H1 NMR spectrum of the diazotized product shown in Figure 8.12 (b), the positive shift of these phenyl doublets to ~7.6ppm and ~8.6ppm, together with the disappearance of the broad NH2 peak at 3.5ppm, is indicative of the formation of the diazonium compound[271]. Additionally, the presence of the diazonium moiety is confirmed by the peak at 2250 cm-1 in the FTIR spectrum in Figure 8.13, which is characteristic of the N≡N vibrational stretch [271-272].
Figure 8.12: H1 NMR spectra for (a) 4-decylaniline and (b) 4-decyl diazonium tetrafluoroborate. The solvent was