Matriz de factores

In order to protect unsaturated bonds from undesired polymerization in oxygen, reactions with 9-decen-1-ol, 2-allylphenol, and trimethylolpropane diallyl ether were carried out under argon. For the microwave-heated samples, 15 mg of TBA-LnNb NS were combined with 5 mL of pure organic reactants within reactor vessels, sealed in a glovebox under argon, and heated at 90 °C for 45 min (max 500 W). Alternative to these microwave-assisted reactions, similar exchange reactions were carried out with convection heating under argon in a glovebox; samples were heated on a hot plate with stirring for 2 d at 80 °C. The glovebox reactions were done as controls and in all cases yielded identical results to microwave reactions. The monomer-grafted sheets were washed with ethanol, then acetone, dried under vacuum at room temperature, and stored under argon in a glove-box.

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4.2.8 Characterization

A Philips X’Pert system equipped with Cu Kα radiation (λ = 1.5418 Å) and a curved graphite monochromator was used in continuous mode with a scan rate of 0.02 °/s to collect the X-ray powder diffraction (XRD) data. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out on a TA Instruments TGA-DSC SDT Q600 system in alumina pans under a dilute oxygen atmosphere (ca. 50% argon); samples were heated to 900 °C at a rate of 15 °C/min. Raman spectra were collected in a Thermo-Fisher DXR dispersive Raman spectrometer using the λ = 532 nm line with a spectral resolution of 3 cm-1_{. The thickness}

of the nanosheets was examined under an Asylum Research MFP-3D Atomic Force Microscope (AFM) working in the dual amplitude resonance tracking (DART) mode; nanosheet samples were observed as a dilute dispersion made by an ethanol drop cast onto a mica sheet.

Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) were taken at JEOL 2010 high-resolution microscope (200 keV) and FEI TECNAI G2 F30 FEG TEM (300 keV). For TEM measurements, a dilute dispersion of nanosheets in ethanol was drop cast onto a grid (carbon film coated fine mesh copper), and dried at room temperature for a few hours. Field emission SEM images of sample morphologies were obtained on a HITACHI S- 4800 FEG CRYO-SEM. For the FESEM sample preparation, either a dispersion of nanosheets in ethanol was drop cast on small pieces of aluminum foil, or a trace of the dried powder was mounted on a piece of double-sided carbon tape. The samples were lightly coated with gold and observed in 1-3 kV range.

4.3– Results

4.3.1 Exfoliation of HLnNb2O7

It has been found that microwave heating can be used to readily exfoliate layered

perovskites. A variety of conditions were investigated to examine the influence of reaction times and temperature, and in all cases nanosheets were efficiently obtained through exfoliation of the perovskite host: 2 h at 60 °C, 1 h at 80 °C, 30 min at 100 °C, and 15 min at 120 °C (halving the reaction times with every 20 °C increase in the exfoliation temperature). TEM images of the TBA-PrNb nanosheets synthesized under these different conditions are presented in Figure 4- 2a−d, demonstrating the efficient production of nanosheets in 15, 30, 60 and 120 minutes at temperatures ranging from 60 °C to 120°C. Evaluation of nanosheets across the entire TEM grid

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supports effective and essentially complete exfoliation in all the conditions above. It appears that nanosheets obtained at higher temperatures are on average relatively smaller in size and

generally more fragmented. Reactions carried out at 60 °C produced the largest nanosheets with some as large as few microns on an edge. The insertion of TBA+ _{ions into the perovskite}

galleries results in delamination of the layered structure and loss of crystallinity. XRD patterns of the reassembled nanosheets consistently confirmed the expected loss of crystallinity, and indicated a high-yield exfoliation when compared to the HPrNb2O7 host (Figure 4-3). The

exfoliation conditions employed in all the following results were chosen to be a 2 h reaction as presented in Figure 4-2e,f (1 h at 60 °C , ramped to 80 °C and heated for 1 h—as a single

reaction with two steps). This ensured a very efficient exfoliation without high fragmentation of

the nanosheets. Figure 4-4 provides the X-ray diffraction data for both TBA-LaNb and TBA- PrNb nanosheets obtained via this selected method, and compares them to the XRD patterns of the hosts. The high angle reflections are minimized with 0k0 reflections dominating the pattern. The first reflections related to 0k0 set of planes shift from about 8.5 ° for HLnNb2O7 to 4.5 ° for

TBA-LnNbNS, confirming the expansion in the interlayer spacings after the intercalation of TBA+. The first peak in the XRD pattern of TBA-LnNb NS is also broader than the very sharp first peak of the HLnNb2O7, suggesting the formation of nanostructures in the former.

Figure 4- 2: TEM images of the nanosheets obtained via various microwave conditions: (a) 2 h at 60 ˚C, (b) 1 h at 80 ˚C, (c) 30 min at 100 ˚C, (d) 15 min at 120 ˚C, (e,f) the main synthesis approach, consisting of a two-step heating method: 1 h at 60 ˚C - 1 h at 80 ˚C.

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The topology of individual nanosheets was further examined with AFM. The height contact-mode image and height profile from a TBA-PrNb nanosheet sample are provided in Figure 4-5. The average nanosheet thickness was found to be 1.4(3) nm based on 29 values measured from different height profiles of TBA-PN nanosheets. Considering the crystal structure of RbPrNb2O7 reported in the literature,140 the thickness of the PrNb2O7 slab is about 0.82 nm

(oxygen-oxygen distance across the slab). Taking into account the thickness of each slab, any assembly of two nanosheets is expected to be greater than 1.64 nm (two slabs as well as an

Figure 4- 3: XRD patterns of nanosheets obtained under various microwave exfoliation conditions, versus the host.

Figure 4- 4: XRD patterns of the layered perovskite hosts versus the exfoliated

111 interlayer spacing between adjacent organic

layers). This implies the delamination of the hosts to at most two layers – though most likely single layers in the current study.

FESEM images of TBA-LnNb2O7 (Ln =

La, Pr) nanosheets are presented in Figure 4-6 showing the existence of transparent nanosheets which stand individually or as assemblies of a few to multiple after being drop cast and dried. For nanosheets of a few layers, the thickness is small enough that underlying nanosheets can be observed. Interestingly, some small islands are

observed on the surface of these nanosheets in absolute focus; these spots are not evident in TEM images. Figure 4-7 presents the TEM images and selected area electron diffraction (SAED) patterns of TBA-LnNb (Ln = La, Pr) nanosheets. The SAED analysis was performed to ensure the intact delamination of the perovskite slabs without disturbing the order of the in-plane elements. The known body-centered orthorhombic unit cell parameters are a = 5.4941 Å, b =

Figure 4- 5: Height contact-mode AFM image and height profile for a TBA-PN NS sample.

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21.9901 Å, and c = 5.4925 Å for RbLaNb2O7 and a = 5.4534 Å, b = 22.012 Å, and c = 5.4549 Å

for RbPrNb2O7.139,140The delamination of the crystalline structure occurs in b direction so that

the SAED patterns are along the [010] zone axis. These SAED patterns can be indexed on a body-centered cell. The d value of the 200 reflection was then calculated (the wavelength of the electron beam was 0.0251 Å at 200 keV and the camera length was either 100 cm or 200 cm), which then allowed estimation of the a parameters; 5.71 Å and 5.68 Å for Ln = La and Pr, respectively. Similar calculations were then done for the 101 reflection to estimate the c

parameter (5.71 Å and 5.74 Å for Ln = La and Pr, respectively). These estimated a and c values are close to those of the starting material,48,142 and consistent with delamination of the layered host in b direction. EDS analysis on TBA-LnNb nanosheets under TEM (Figure 4-8) confirms that nanosheets retain both Nb and Ln. This investigation of thickness and atomic arrangement well proves that the exfoliation has efficiently taken place, maintaining the integrity of the slab composition as individual nanosheets.

Figure 4- 7: TEM images of TBA-LnNb nanosheets are presented as well as the SAED patterns from the specified portions (Ln = (a) Pr, and (b) La).

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As explained above, TBA-LnNb nanosheets are typically obtained by reacting HLnNb2O7 with TBAOH base. As a further study, exfoliation was also investigated using

various layered hybrids with terminal alkoxy-grafted organics (n-alkyl alkoxy-PrNb2O7 instead

of HPrNb2O7 acid exchanged hosts) to see whether alkoxy covalent bonds survive the

intercalation of TBA+ _{and nanosheets with alkoxy surface groups are obtained, or similar TBA-}

LnNb nanosheets are obtained regardless of the interlayer functionality of the exfoliating host. In all conditions, alkoxy groups were exchanged with TBA+ _{ions and the nanosheets exfoliated}

from the layered hosts, resulting in similar products as is seen starting from HLnNb2O7 (Figures

4-9 and 4-10). This suggests that the reactivity of the oxygen atoms present on the surface of the nanosheets is very similar to those present in the interlayer of the perovskite-based hybrids. The methods available for organic modification of the layered perovskites can then be readily applied in the same fashion to modify the surface of the nanosheets with a variety of organics containing hydroxyl or amine functional groups.

Figure 4- 8: Elemental analysis results for TBA-LnNb nanosheets: (a) Ln = Pr, (b) Ln = La.

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Figure 4- 9: TGA-DSC curves for n-pentoxy-PrNb2O7 and the

nanosheets produced from this host after reaction with TBAOH.

Figure 4- 10: Raman spectra of (a) RbPrNb2O7, (b) HPrNb2O7 and

(c-e) CnH2n+1PrNb2O7 (n = 1, 3, and 5 in c, d, and e, respectively), versus

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Table 4- 1: Summary of microwave-assisted surface modification reactions.