CHAPTER II. THE CRITICAL ROLE OF ACID TREATMENT INDUCED
2.3 Results and Discussion
2.3.5 Oxidation states determination of the protonated and reassembled -MnO 2
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
Mn3+, and 640.0 eV to Mn2+.179,180 Since the main Mn 2p3/2 peak for all samples is located between 642.0 and 641.0 eV, and the shape of the Mn 2p3/2 peaks is obviously not symmetrical, which implies the coexistence of Mn3+ and Mn4+ in the MnO2 nanosheets. Moreover, studies of Biesinger et al181 showed that multiple peaks resolved under the Mn 2p spectrum can represent a single oxidation state. Thus, due to the complexity of oxidation states in Mn 2p spectrum as well as the lack of standardization, it is difficult to determine the manganese oxidation state only from the Mn 2p3/2 peak.
The splitting of Mn 3s peaks is often used to determine the oxidation state of Mn,182-184 where the electrons exchange interaction upon photoelectron ejection defines the magnitude of the splitting. The separation of peak energies for the electron exchange in the 3s-3d level of Mn is described by Equation (11).
ΔE = (2S + 1) K [3s, 3d] (11)
where ΔE is the separation of peak energies. S is the total spin of unpaired electrons in the 3s and 3d levels in the final states and K [3s, 3d] is the exchange integral between 3s-3d energy levels. Based on the above theory, lower valence of Mn will lead to wider splitting of the 3s peaks. Therefore, we are able to qualitatively compare the Mn3+
content in the three samples through study the Mn 3s peak energy separation as shown in Figure 47(b). The ΔE values obtained are 4.8 eV for the protonated MnO2, 4.9 eV for the pH = 4 treated reassembled MnO2, and 5.2 eV for the pH = 2 treated reassembled MnO2. The results indicate that lower pH treatment of the reassembled MnO2 nanostructures can lead to the formation of more Mn3+ in the MnO2 nanosheet. The protonated MnO2 exhibits a lower Mn3+ content compared with both the pH = 2 and 4 samples, despite being treated at lower pH. This apparent contradiction results from steric effects in the crystalline HxMnO2 that inhibit Mn occupation of the interlayer galleries.
Figure 47. XPS spectra in the (a) Mn 2p, and (b) Mn 3s regions corresponding to (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.
The oxidation state of the Mn ions in all samples was further investigated by using Mn K-edge X-ray absorption near-edge spectroscopy (XANES). The data were collected at Cornell High Energy Synchrotron Source (CHESS) at Cornell University, and the
range with two weak broad peaks at 6540-6545 eV, a main-edge range that has one inflection point A, and the resonance peak range B.98,185 The weak pre-edge peaks P and P correspond to the dipole-forbidden 1s→3d transition.172 All three samples exhibit higher intensity of peak P than for peak P, but the peak P is less intense as compared with the -MnO2 reference. This observation confirms the existence of mixed oxidation states of Mn3+/Mn4+ in the samples. Also, the higher intensity ratio of P to P for HxMnO2
compared with the pH = 2 and 4 samples indicates that it has less Mn3+, because the relative intensity of the peaks P/P is proportional to the average oxidation state.172
The main absorption range can be assigned to the dipole-allowed 1s→4p transition.
The associated edge energy is usually taken as the energy of the peak in the first derivative, which corresponds to the inflection point of the main edge in the XANES spectra (Figure 48(c)). Clearly, the main absorption edge (A) progressively shifts to lower energies with decreasing pH, implying lower pH progressively reduces Mn to the trivalent state. Also, the presence of the intense peak B for the nanosheet assemblies indicates that they are mainly comprised of edge-shared MnO6 octahedra.186 This observation further confirms that the δ-MnO2 lattice is not dissolved by equilibration in HCl at pH as low as 1.
The average oxidation state (AOS) of Mn was determined by establishing a linear relationship between the K-edge energy and Mn oxidation state (Figure 48(d)). The AOS of Mn is 3.59 for HxMnO2, 3.36 for the pH = 4 MnO2 nanosheet assembly, and 3.24 for the pH = 2 variant. The dependence of Mn valence on pH generally follows that reported earlier by Manceau and coworkers134 for bulk Na-saturated δ-MnO2 (birnessite) powders, where the Mn AOS varied from 3.81 at pH = 9 to 3.69 for pH = 3.
Figure 48. X-ray absorption measurements of the as-prepared three samples and reference Mn oxide materials. (a) XANES spectra of reference materials MnO, Mn3O4,
Mn2O3 and MnO2. (b) XANES spectra of protonated MnO2, pH = 2 and 4 treated reassembled MnO2. The reference materials of Mn2O3 and MnO2 from panel (a) are also
shown for ease comparison. (c) First derivative curves corresponding to the samples shown in panel (b). (d) Average oxidation state of Mn for the samples and standards
derived from the K-edge energy.
For the alkali-free MnO2 samples studied herein, we also find that treating the exfoliated MnO2 assemblies at lower pH increases the Mn reduction to a greater extent than found for crystalline NaxMnO2 by Manceau and coworkers.134 Furthermore, we find competing steric and thermodynamic effects by comparison of the proton-exchanged HxMnO2 to the exfoliated and re-assembled variants. The AOS of HxMnO2 treated at pH
< 1 is 3.59, whereas the exfoliated samples treated at higher pH have AOS values of 3.36
Mn4+, and of preventing displacement of Mn3+ to the sheet surface. Indeed, earlier work by Gaillot et al158 noted that during thermal reduction of Mn4+ 350 °C→ Mn3+ in crystalline K-birnessite, Mn vacancies were not formed despite the unfavorable in-sheet lattice strain due to Jahn-Teller distortions inherent to Mn3+. The interactions of neighboring in- plane Mn3+ and Mn4+ sites therefore contributes both strain and electrostatic-driven components to the energetics of δ-MnO2 defect equilibration.
2.3.6 Defects structure characterization of the protonated and reassembled -