II. SYNTHESIS AND PROCESSING OF DEFECT ENGINEERED -MNO 2
2.4 R ESULTS AND D ISCUSSION
2.4.1 Controlling defects via equilibration in a pH controlled environment
the suspension in a pH controlled environment was first suggested by Ruetschi in the 1980s with 2 x 1 tunnel, γ-MnO2.52 More recently Manceau and coworkers developed a defect model for Mn3+ charged defects in layered -MnO2. Reduction of the Mn from 4+ to 3+
caused a Jahn-Teller distortion of the Mn3+O6 octahedron, and produced a surface Frenkel defect.56 Not all of the reduced Mn migrated to the surface of the sheet. As much as 50%
of the reduced Mn remained in the sheet.18
Using methods similar to those used by Manceau, et. al., surface Frenkel defect content was determined by performing PDF refinements on MnO2 floccules. Defect content was resolvable in PDFs because the MnL – MnL (in-sheet) distance is 2.89 Å, whereas MnL – MnIL (in-sheet to out of sheet) distance is 3.45 Å (Figure 18). Jahn-Teller distortion of Mn3+O6 octahedra also increases the average Mn – O bond length.
Figure 18. a) Surface Frenkel defect model for MnO2. b) PDF of MnO2 floccules.18 Partial PDFs derived from refinements of data for floccules provided excellent insight to individual contributing pair correlations for each peak (Figure 19). Termination ripples appeared at distances less than ~1.5 Å in all PDFs because Qmin and Qmax were non-ideal values, as usual. The typical transition metal – oxygen bond length is approximately 2 Å. In the case of -MnO2 floccules, the first peak was mainly composed of contributions from MnL – O (in-sheet) distances (●) at 1.90 Å with some contributions from MnIL – O (out of sheet) distances (▲) at 1.92 Å. MnIL – O contributed less to this peak because the MnIL species comprise only 19-27% of the total Mn in each sample. Out
of sheet MnIL – O average distances were longer than MnL – O distances because
not absolute occupancies for each site. The second peak in the PDF was the oxygen – oxygen correlation (□) at 2.45 Å.
Figure 19. A typical PDF of defect engineered MnO2 floccules with partial PDFs.
The third peak represents MnL – MnL (in-sheet) distances (♦) of roughly 2.89 Å.
There were no other contributing correlations to this peak. The fourth major peak near 3.45 Å has contributions from MnL – O, MnIL – O, and MnL – MnIL (out of sheet). About half of the intensity of this peak is a result of defect Mn contributions. Most of the information about defect Mn is gathered from this peak (*). The intensity of the MnL – MnIL peak at 3.45 Å decreases with increasing pH and implies lower defect content. The inverse trend is apparent in the MnL – MnL at 2.89 Å (Figure 20). Note the intensities of each PDF were normalized to the Mn – O peak, and thus these results assume no oxygen vacancies are present. Normalization of the Mn – O peak can only be applied if the number
1 2 3 4 5 6 7
G(r) (Å-2 )
r (Å)
MnL-MnL
MnL-MnIL MnL-O
MnIL-O
□
●
¨
▲
*
#
of Mn – O bonds in each sample is the same. Oxygen vacancies would alter the number of Mn – O bonds present in the sample.
Figure 20. PDFs of -MnO2 floccules equilibrated at a pH of 2, 4, and 9. The intensity of the peak at 2.89 Å (in sheet Mn – Mn distance) increased with increasing pH, while the intensity of the peak at 3.45 Å (out of sheet Mn – Mn distance) increased at lower pH values. This suggested an increase in the defect Mn with lowering pH.
Charged surface Frenkel defects in MnO2 floccules were studied as a function of drying method in freeze-dried and air-dried samples. Freeze-dried samples experienced -70 °C temperatures in vacuum where water in floccules was expelled through sublimation.
Air-dried samples experienced 60 °C under atmospheric pressure in a drying oven where water was removed by evaporation. Defect content as a function of degree of restacking was also studied.
The surface charge of a material is determined by its isoelectric point (IEP) where
1 2 3 4 5 6 7
-1.0 -0.5 0.0 0.5 1.0 1.5
G(r) (Å-2)
r (Å)
pH=2 pH=4 pH= 9
content at lower pH values causes an increase in the positive surface charge and forces Mn reduction for electrostatic charge balancing purposes.
Despite differences in drying conditions and degree of restacking among MnO2
nanosheet floccules, there was no significant change in the number of defects across samples equilibrated in the same pH environment (Figure 21). Reduction of Mn4+ to Mn3+
occurred when the nanosheet floccules were exposed to an acidic pH through adsorption of a H+ ion to the surface of MnO2. Drying method and restacking had no effect on charged defect content.
Figure 21. Mn surface Frenkel defect is invariant with pH for floccules with (upper) different degrees of restacking and (lower) between drying conditions.
1 2 3 4 5 6 7 8 9 10
10 15 20 25 30 35
less restacked more restacked
MnIL (%)
pH
1 2 3 4 5 6 7 8 9 10
10 15 20 25 30 35
air 60 °C freeze dried
MnIL (%)
pH
PDFs were refined with a model developed by Manceau and co-workers. Manceau et. al. found that highly crystalline MnO2 (birnessite) samples, with only Mn4+ layer cations possessed hexagonal symmetry.56 The model was slightly altered from a hexagonal crystal system to allow γ to refine. The γ lattice parameter was allowed to refine to account for disorder in the structure and only deviated slightly from the hexagonal, P3m1 space group.
Refined parameters with estimated standard deviations (ESDs) can be seen in Appendix section 9.1. ESDs were generated by PDFGUI; however, in some cases unphysical ESDs were reported including for some lattice parameters and atom positions. Atom positions for real space data with broad peaks modeled over 7 Å is likely not accurate to six significant digits (reported by PDFGUI). Reciprocal space methods are needed for such high accuracy atom positions.