1

-13C

Fig. 5.16 Scheme of Hartmann-Hahn match for the system 1H-13C

was obtained by performing solid-state 113Cd NMR measurements of the samples before and after the annealing. In Fig. 5.17, we report the 113Cd CP MAS spectra of the precursor/polystyrene films before (a) and after (b) the annealing. In the spectrum (b) the signal at 715 ppm arises from the CdS nanoparticles formed after 10 min of annealing. These measurements directly probed the presence of CdS and some unreacted precursor, and other reaction products (signal at 648 ppm) as well. Generally, depending on the type of nanoparticles, by appropriate choice of the observable NMR nucleus one can follow the formation reactions of the nanoparticles as well.

1000 800 600 PPm

Fig. 5.17 113Cd CP MAS spectra of cadmium-(bis)thiolate/polystyrene films before (a) and after (b) annealing in air at T = 300 °C. Spectra are carried out at 44.366 MHz on a Bruker ASX-200 spectrometer. Spin rate 8 kHz; p/2 pulse width 4 ms; recycle delay 3 s; contact time 15 ms; number of scans 32,000. Chemical shifts are reported with respect to Cd(ClO4)2 6H2O. Reprinted from [32], supporting information, copyright (2005), with permission from Elsevier

1000 800 600 PPm

Fig. 5.17 113Cd CP MAS spectra of cadmium-(bis)thiolate/polystyrene films before (a) and after (b) annealing in air at T = 300 °C. Spectra are carried out at 44.366 MHz on a Bruker ASX-200 spectrometer. Spin rate 8 kHz; p/2 pulse width 4 ms; recycle delay 3 s; contact time 15 ms; number of scans 32,000. Chemical shifts are reported with respect to Cd(ClO4)2 6H2O. Reprinted from [32], supporting information, copyright (2005), with permission from Elsevier

The second important point concerns the purity of the obtained nanocomposite samples. The presence of undesired species in the final sample prompted us to optimize the thermolytic process for obtaining enhanced quality CdS/PS nanocomposites. Thus, we performed 113Cd CP MAS spectra of samples annealed in different atmospheres (see Fig. 5.18). Depending on the annealing conditions, the intensity of the CdS peak at 715 ppm increases from air to nitrogen, reaching the maximum value in the vacuum-annealed sample (Fig. 5.18c). These data qualitatively identify annealing under vacuum as the treatment that produces the best quality sample.

The great chemical shift dispersion of 113Cd (800 ppm) with respect to the other spin (-1/2) nuclei, such as 1H (12 ppm), 29Si (200 ppm) and 13C (300 ppm), indicates that the shielding of the 4 13Cd nuclei may possibly be most sensitive to the local environment [58]. However, 4 13Cd solid-state NMR spectra need both long signal averaging and the use of cross-polarization (CP) [59, 60] and magic angle spinning (MAS) techniques to overcome problems such as low natural abundance, low gyro-magnetic ratio and large spin-lattice relaxation times of these nuclei. Thus, when applied in studying the in situ synthesis of organic-inorganic nanocomposites, solid-state cadmium NMR spectroscopy is a valid tool for a qualitative assessment of the principal features of the material (formation of nanoparticles and purity of the composite). Its only limit is the concentration of the inorganic part which should be at least 10% wt/wt.

5.5.1 13C onpolymer

The organic counterpart of hybrid nanocomposites often plays a role of matrix serving as dispersion media for nanoparticles precursors (in situ synthesis) or as a physical

Fig. 5.18 113Cd CP MAS spectra of cadmium-(bis)thiolate/polysty-rene films annealed in (a) air; (b) nitrogen; (c) vacuum. Reprinted from [33], copyright (2006) with permission from Elsevier

Fig. 5.18 113Cd CP MAS spectra of cadmium-(bis)thiolate/polysty-rene films annealed in (a) air; (b) nitrogen; (c) vacuum. Reprinted from [33], copyright (2006) with permission from Elsevier

obstacle to particles agglomeration. However, in some cases the polymeric component interacts with the inorganic counterpart through molecular mechanisms.

It is well established that solid-state 13C NMR spectroscopy is a powerful method to characterize completely polymers and probe the nature of the molecular interactions between the organic and inorganic phases of polymer-based nanocomposites.

In our work, CdS/polystyrene nanocomposites were studied by 13C NMR in the solid state. The first step was the assignment of polystyrene and of the precursor cadmium-(bis)thiolate resonances. In Fig. 5.19a . we reported the [ 3C CP MAS NMR spectrum of Cd(SR)2 as synthesized, and compared the spectra of Cd(SR)2/ PS samples before (b) and after (c) the annealing in air. In the spectrum of Fig. 5.19b, the resonance at 127.7 ppm is due to the protonated aromatic carbons C.-C6 of polystyrene, while the signal at 146 ppm is from the non-protonated carbon C1. Ca and the Cb carbons of polystyrene resonate at 39.8 and 45 ppm, respectively. The resonances of cadmium thiolate are found in the 12-40 ppm region: the terminal methyl C12 at 13.6 ppm and the methylene C11 bound to C12 at 22.6. The comparison of the spectra in Fig. 5.19b, c evidences that all the resonances of the polystyrene matrix are not affected by the thermal treatment, while weak signals due to reaction by-products are observed in the 12-30 ppm region. Since 13C CP MAS experiments

Fig. 5.19 13C CP MAS spectra of (a) cadmium-(bis)thiolate as synthesized and cadmium-(bis)thiolate/polystyrene film before (b) and after (c) the annealing in air. Reprinted from [33], copyright (2006), with permission from Elsevier

Fig. 5.19 13C CP MAS spectra of (a) cadmium-(bis)thiolate as synthesized and cadmium-(bis)thiolate/polystyrene film before (b) and after (c) the annealing in air. Reprinted from [33], copyright (2006), with permission from Elsevier

are not quantitative, evaluating the amount of the unwanted substances cannot be performed by simply integrating the 12-30 ppm resonances. Therefore, according to the procedure reported in the following paragraph, the cross-polarization dynamic process was investigated for all the samples. As an example, we report the correlation between the intensity of the carbon resonances C1 and Ca of polystyrene, and C11 of thiolate, and the contact time t for a CdS/PS sample annealed in air (see Fig. 5.20). It is worth to note that the T1p ^ H) value (120 ms) of the resonance C11 is definitely much longer than the corresponding ones (9, 10 ms) obtained for the resonances C1 and Cb of polystyrene. Therefore, in order to obtain a semiquantitative analysis, SPE spectra were carried out. It is well known that the simplest experiment for obtaining quantitative measurements in solid-state high-resolution NMR is the Single-Pulse excitation (SPE) experiment. A single p/2 pulse is applied to excite the carbon signal that is recorded in the presence of MAS and dipolar decoupling (DD). The recycle delay RD between successive scans must be chosen to ensure that the longitudinal nuclear magnetization has recovered to equilibrium following the preceding pulse.

The 13C SPE NMR spectra of the CdS/PS sample obtained by annealing in different conditions are shown in Fig. 5.21: (a) air, (b) nitrogen and (c) vacuum; only the 0-60 ppm region is shown. We compared the integral of some signals of polystyrene resonating in the 36-60 ppm range (region A) with the ones resonating in the 12-30 ppm range (region B). In all the spectra, the integral of region A was

Fig. 5.20 Correlation between the area of the carbon resonances C1? Ca and Cn and the contact time t. Lines through experimental points have been obtained by fitting the experimental data to (5.14). Reprinted from [33], copyright (2006), with permission from Elsevier

normalized to 1. In this way, it was possible to quantitatively evaluate the amount of the impurities in all the samples. In the case of annealing in air, the integral of region B is found to be 0.8. When the annealing is conducted in vacuum, the integral decreases down to 0.4, whereas an intermediate value of 0.6 is found in the sample annealed in nitrogen. To summarize, the 1 3C NMR semi-quantitative analysis confirmed the 113Cd NMR qualitative results, and established that the best CdS/PS nanocomposites, with a low amount of impurities, are obtained when annealing the samples in vacuum.

5.5.2 13C CP MAS Cross-Polarization Dynamic

Because 13C intensities depend on the cross-polarization rates which may be different for different carbon atoms [57, 61], CP MAS spectra are not quantitative. The rate of the process strongly depends on the number of the abundant spin I near the dilute spin S and on their distance from S. The question arises when we wish to obtain a quantitative analysis of the intensity of resonances due to carbon nuclei of different types.

In simple cases, the problem can be solved by investigating the cross-polarization dynamic [54] .

The kinetic of the cross-polarization dynamic is described by the equation [61]:

0 0

Post a comment