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Upon calcination, Ca2+and Zn2+ cations are incorporated into the St¨ober nanopar- ticles. Previously, it has been demonstrated that incorporating Ca2+ with this method produces spherical nanoparticles [161]. Investigations into the structure of the silicon matrix look at how this incorporation has affected the silicon structure. Firstly, as seen in Table 5.2, the NC of the undoped St¨ober particle has increased from 3.68 in the non-calcined samples to 3.84 in the calcined samples. This indicates that during calcination some hydroxyls were removed from the structure forming a more connected network. For the undoped particle, the 17O Echo and MQMAS measurements (Figure 5.5(a)) resembles a BO resonance, as shown in the undoped non-calcined spectra (Figure 5.2(a)), further indicating a well connected network.

The 29Si spectra of the doped samples shows that upon 10 mol% Ca ad- dition, an identical network connectivity as the un-calcined version (Table 5.2) is maintained. However, the network connectivity is still lower than the calcined un- doped sample, indicating that although hydroxyls are removed from the network during calcination, as Ca enters the St¨ober network, it acts as a network modifier and decreases the NC. Further evidence can be concluded from the observation of

Figure 5.3: The29Si MAS NMR spectra of calcined St¨ober particles for (a) undoped, (b) 10 mol% Ca, (c) 10 mol% Zn and (d) 20 mol% Zn. The individual deconvolutions are shown in blue and the sum of these are marked in red.

a Q1 Si site. The identification of thisQ1 site was made with the use of cross po- larisation (CP) NMR, from 1H, to enhance the resonance (Appendix: Figure B.1). Further evidence of Ca incorporation is detected, in the 17O MAS NMR measure- ments, with the observation of a small Ca non-bridging resonance (NBO) appearing at 100 ppm (Figure 5.5). While the proportion of the NBO site is small compared to the BO resonance (20 ppm), this is to be expected with such little Ca incorpo- ration. These characteristic resonances are mirrored in the MQMAS data where the 10 mol% Ca sample for the NBO exhibits little CS or Qis broadening. This is

characteristic of a Ca BO observed previously in the literature [168].

Similarly to the 10 mol% Ca sample, doping the St¨ober nanoparticle with 10 mol% Zn results in a lower NC than the undoped sample. However, this effect is significantly larger in the 10 mol% Zn sample, with a significant increase inQ2 and Q3 structural units observed, resulting in a more significant NC decrease than that of the 10 mol% Ca nanoparticles. From this we can infer that the Zn nanoparticle is having a larger disruptive effect on the St¨ober nanoparticle than the introduction of Ca. This network disruption continues in the 20 mol% Zn sample, where a further reduction of the Q4 site is observed resulting in a NC of 3.36. This drop in NC is comparatively similar in scale to the reduction seen from the undoped to the 10

Figure 5.4: The network connectivity (NC) determined by 29Si MAS NMR for the Ca and Zn St¨ober particles plotted against there respective cation (mol%) insertion.

mol% Zn sample. Upon measuring a range of Ca and Zn samples and plotting the NC against cation content (Figure 5.4), this effect becomes more apparent. Both the Ca and Zn series observe a linear relationship of NC when the cation dopant concentration is increased, although significantly more disruption is observed for Zn. The 17O MQMAS data for the 10 and 20 mol% Zn samples (Figure 5.5) highlights a second resonance, at δiso = 20 ppm, hidden underneath the bridging

oxygen resonance. Upon overlaying the 1D17O spectra (Figure 5.6(a)) this becomes even more apparent. At first glance this would appear to be a non-bridging oxygen resonance, however, shifted due to the similar 17O NMR shifts of ZnO and SiO2 [176, 178].

Further investigation using the method developed by Eck, Smith and Kohn to observe hydroxyls, a peculiarity becomes apparent [177]. Through the use of fast recycle delays and liquid (non-selective) pulses we can suppress the BO, while observing the hydroxyls in the nanoparticles. As shown in Figure 5.7, by employing a selective excitation scheme we see the characteristic static lineshape of the Si BO’s in the undoped sample [177]. However, when the ‘non-selective’ scheme is applied to the undoped nano particle, the static BO resonance is observed in an anti-phase position while observing a sharp resonance, previously assigned as a hydroxyl group, is selectively excited [177]. This resonance is also clearly observed in the 10 mol% Ca nanoparticles. However, upon applying this technique to the Zn systems, this hydroxyl resonance is not observed. While, the 10 mol% Zn St¨ober nanoparticle appears to only contain a BO resonance, for 20 mol% Zn a second anti-phase site is apparent in both the selective and non-selective static 17O spectra. This likely results from the Zn NBO resonance observed in the MQMAS data (Figure 5.5).

In the HRTEM images of the 10 mol% Zn St¨ober nanoparticles (Figure 5.8(a) & (b)), evidence for crystalline areas is observed. These are not apparent in the

Figure 5.5: The 17O MQMAS spectra of calcined St¨ober particles with Hahn-echo projections for (a) undoped, (b) 10 mol% Ca, (c) 10 mol% Zn and (d) 20 mol% Zn

Figure 5.6: Overlay of17O Hahn-echo data of the (a) calcined St¨ober nanoparticles and (b) calculated 17O CASTEP data.

Figure 5.7: Selective and non-selective 17O static echo NMR of the calcined St¨ober particles (a) undoped, (b) 10 mol% Ca, (c) 10 mol% Zn, and (d) 20 mol% Zn.

St¨ober particles featured in the previous work by Greasley et al. [161], where Ca was fully incorporated into the nanoparticle network. These areas mean, that unlike Ca, Zn does not incorporate into the glass network, but forms a separate phase. As this is only observed in the Zn samples, we can conclude that it is a Zn containing crystallite. This has been previously seen by investigations into similar materials, which suggest that Zn disrupts the amorphous nature of the silicon glasses form- ing zinc tetrahedra [179]. Through measurements of the atomic spacing from the HRTEM results (Figure 5.8(c) & (d)), determination of the lattice constant of this crystallite phase can be achieved giving d-spacings of 2.70 and 2.82A. Various zinc silicate [180–183] and zinc oxide [184] minerals were compared to these values, and a comparable d-spacing was found in the Zn2SiO4–II phase. A list of the most promi- nent d-spacing’s for these minerals are included in the Appendices (Table B.1).

To confirm this, CASTEP has been used to calculate the theoretical 17O NMR shifts for these minerals which can be compared with experimental data. The calculated lineshapes resulting of the various Zinc silicates [180–183] and Zinc Oxides [184], are shown below the measured 17O NMR data (Figure 5.6(b)). The calculated 17O parameters for the minerals shown in Figure 5.6(b) are included in the appendix in Table B.2. From these calculated values, we can see that the II and IV forms of Zn2SiO4 both agree with the experimentally measured shifts.

Figure 5.8: The HRTEM images of calcined 10 mol% Zn St¨ober particles at (a) showing the distribution throughout the entire particle, (b) high resolution of the boxed area showing amorphous and crystalline areas, (c) high resolution of the crys- talline area indicating the obtained d-spacings, (d) diffraction pattern of crystalline area within nanoparticle.

However, Zn2SiO4–IV is made up of multiple 17O environments (Appendix B.2) and no evidence for this is observed for in the MQMAS results (Figure 5.5). It can be concluded that crystallite observed in the 10 and 20 mol% St¨ober particles are likely to be a Zn2SiO4-II phase.

5.4

Conclusions

The structure of St¨ober nanoparticles has been successfully probed through a combi- nation of solid state NMR, CASTEP and HRTEM. Using H217O to label the St¨ober particles it has been possible to follow the synthesis steps. From this we can observe the characteristic BO resonance in the Ca containing St¨ober particle suggesting that the addition of Ca acts as a network modifier. This agrees with the obtained 29Si MAS NMR which indicates a reduced NC from undoped St¨ober particles.

In the Zn containing St¨ober particles we can observe the evidence of crys- talline regions forming inside the nanoparticle. These crystalline regions have been attributed to the formation of Zn2SiO4–II phases inside the particle. This crys- tallisation might also explain why there is a decrease in network connectivity for Zn with respect to Ca nanoparticles. Since the crystalline Zn is surrounded by the silica network we are likely to see a greater amount of low Q units due to the increased surface area. Further work will be needed to explore methods of integrating Zn into the St¨ober network without crystallisation.