CHAPTER II. THE CRITICAL ROLE OF ACID TREATMENT INDUCED
2.3 Results and Discussion
2.3.4 Phases and microstructures of the reassembled -MnO 2 nanostructures
at pH = 2 and 4 solutions for 24 h, are shown in Figure 39(a) and (b). It can be clearly seen that both samples exhibit 3-D porous nanostructures and equilibration at different pH values have no apparent influence on their morphologies. The high-magnification SEM image shown in Figure 39(c) demonstrates the presence of scrolling at the edges, which is consistent with the TEM images and has been noted in other nanosheet studies.166,167 The observation of porous structures after reassembly highlights the usefulness of our ultrasonic-assisted exfoliation and titration-based flocculation procedures, where the exfoliation rate is greatly enhanced without sacrificing the porous nanostructures.
The X-ray diffraction patterns (Figure 40) for reassembled MnO2 (the XRD pattern for protonated MnO2 has also been included for comparison) demonstrate that phase purity was achieved during synthesis and that exfoliation and reassembly yields complex nanostructures as evidenced by peak broadening and asymmetry. After reassembly of the nanosheets, in-plane hk0 reflections remain discernible, indicating that the crystalline nature of the 2-D sheets is preserved. Further, the derived PDF is consistent both with literature168 and the calculated PDF of a single δ-MnO2 nanosheet (Figure 41); further
confirming the δ-MnO2 nanosheet motif is maintained. The broadening of the 00l basal reflections shows that although some sheet-to-sheet restacking occurs, the stacks are on the order of only 3-4 nm (estimated using the Scherrer equation), which is consistent with the AFM results. Shifts of the basal reflections result from increased water content in the reassembled nanosheet floccules compare to the proton-exchanged form.
Figure 39. SEM images of reassembled MnO2 nanostructures treated in (a) pH = 2 and (b) pH = 4 solution for 24 h. (c) high-magnification SEM image of reassembled MnO2
nanostructures treated in pH = 2 solution for 24 h. Scale bar, 500 nm (a-b); 50 nm (c).
Figure 40. XRD patterns of (a) protonated MnO2, (b) reassembled MnO2 treated in pH = 4 solution for 24 h, (c) reassembled MnO2 treated in pH = 2 solution for 24 h. Data
collected on APS 11-ID-B.
Figure 41. Comparison of the experimental PDF (reassembled MnO2, pH = 2) and the PDF calculated for a single δ-MnO2 sheet confirms the δ-MnO2 motif is maintained in
the exfoliated and reassembled nanosheet floccs.164
Nitrogen adsorption-desorption isotherms were used to quantify the specific surface area (SSA) of all specimens, and the results are shown in Figure 42. While protonated MnO2 showed no evidence of mesopores, both reassembled MnO2 nanostructures show
typical type IV isotherms (IUPAC classification) with distinct H3-type hysteresis loops, a result of open slit-like mesopores.169 The Brunauer Emmet and Teller (BET) specific surface areas (SSA) were 120 ± 0.4 m2/g for the pH = 2 sample and 144 ± 1 m2/g for the pH = 4 sample, and only 4.5 ± 0.1 m2/gfor HxMnO2, where the former values are roughly double the surface areas reported by Song et al98 for reassembled oxide nanosheets.
Together, the XRD, BET and microscopy shown above indicate that the re- assembled nanosheets have macro and mesopores, with the mesoporosity arising due to loose agglomeration of randomly oriented sheet clusters. The extent of exfoliation and/or restacking, typical sheet thicknesses determined by AFM and average crystallinity in the sheet stacking direction as determined by XRD are similar or better for our MnO2
specimens than those reported recently for MnO2, TiO2, Co3O4, ZnO, and WO3.167 Therefore, the typical structures in Figure 39(a) are of the form of edge-to-face assembled nanosheet booklets, with “wall thicknesses” of up to 4 nm. The high specific surface area of the MnO2 nanosheet assemblies facilitates infiltration of the electrolyte into the porous electrode to enhance the specific capacitance.3,15
During soft chemical processing, the surface and interlayer tetrabutyl ammonium hydroxide (TBAOH) and water content may vary with processing conditions. Figure 43 shows the thermogravimetric analysis of the protonated, pH = 2 and pH = 4 samples.
Clearly it indicates 9.5 wt.% H2O for HxMnO2 dried at 60°C (0.5 H2O per MnO2 formula unit), 12.3 wt.% (0.7 H2O per MnO2 formula unit) for the pH = 2 nanosheet assembly, and 15.5 wt.% (0.9 H2O per MnO2 formula unit) for the pH = 4 sample. The slightly higher water content in the pH = 4 vs. pH = 2 sample is a result of its slightly larger surface area.
Figure 43. TGA curves of (a) protonated MnO2, (b) reassembled MnO2 treated in pH = 2 solution for 24 h, (c) reassembled MnO2 treated in pH = 4 solution for 24 h.
The presence of structural water usually enables rapid proton or alkali cation transport within the interlayer, which is beneficial for increasing the charge storage properties.15 The presence of residual TBAOH in the reassembled samples was indicated by a small mass loss (~3 %, corresponding to 0.01 TBAOH per MnO2 formula unit.
Since one tetrabutylammonium molecule occupies an area of ~60 Å2, and one MnO2 unit
cell gives an surface area of 7 Å2, therefore the surface coverage of residual TBAOH on MnO2 nanosheet is about 9 % in the temperature region of 200-400 °C.170,171 At 400-600
°C all the samples show another mass loss corresponding to the evolution of oxygen from the lattice, leading to the formation of Mn3O4,172and loss of structural hydroxyls.173
Infrared (IR) spectroscopy (Figure 44) and X-ray photoelectron spectroscopy (XPS) (Figure 45) were also used to detect remnant TBAOH in the nanosheet assemblies after processing. As shown in Figure 44, all the sharp bands near 2980 cm-1 belong to C-H stretching vibrations in TBAOH molecules. Direct observation of nitrogen via the C-N vibration is apparent in a broad IR band near 2000 - 2300 cm-1 while the bands in the 950 - 1300 cm-1 region correspond to the C-N stretching vibration. The bands located at 1560 and 1600 cm-1 are a result of the N-H stretching vibration, and that at 1385 cm-1 is related to the N-O stretching vibration.174-177
Figure 44. FTIR spectra of (a) protonated MnO2; (b) reassembled MnO2 treated in pH = 4 solution for 24 h; (c) reassembled MnO2 treated in pH = 2 solution for 24 h (inset
shows the structure of the TBAOH molecule).
288 eV that can be attributed to carbon bounded to nitrogen,178 also confirmed the existence of TBAOH after exfoliation and flocculation processes.
Figure 45. XPS spectra of reassembled MnO2 treated in pH = 2 solution for 24 h. (a) XPS survey scan, (b) high resolution XPS N 1s and C 1s spectrum.
From the above TGA, FTIR and XPS analysis it can be seen that even after sufficient rinsing, the TBAOH molecules used for exfoliating the -MnO2 nanosheets still existed in the reassembled nanostructures. The potential effects of the remaining TBAOH molecules on the electrochemical performance have also been discussed. Figure 46 shows the infrared spectra of reassembled MnO2 treated in pH = 4 solution for 24 h, before and after one CV cycling in 1M Na2SO4 electrolyte at 50 mV/s scan rate. It can be
seen that the TBAOH is removed from the nanosheet assemblies after one charge- discharge cycle, since the IR bands corresponding to the TBAOH molecules have almost disappeared after cycling. Therefore, although it is not possible to completely remove the TBAOH molecules from the nanosheet assemblies by washing, its small surface coverage (~9 %, determined by TGA analysis and the geometry of the TBAOH molecules) and easy extraction during the first electrochemical cycle together suggest that it has limited if any influence on the measured electrochemical performance.
Figure 46. FTIR spectra of reassembled MnO2 treated in pH = 4 solution for 24 h, (a) before and (b) after one CV cycling in 1M Na2SO4 electrolyte at 50 mV/s scan rate.
2.3.5 Oxidation states determination of the protonated and reassembled -MnO2