Synthesis and Surface Characterization of γ-MnO 2 Nanostructures

A facile method was developed to synthesize γ-MnO 2 with different structures and surface properties in this paper. γ-MnO 2 was prepared by oxidation ofMnSO 4 with Na 2 S 2 O 8 at 90C for 2.0 h. γ-MnO 2 formed at initial pH 1.0 (M1) and 8.5 (M2) was composed of MnO 1.93 ⋅ 0.23H 2 O and MnO 1.96 ⋅ 0.18H 2 O, respectively. The higher ratio of pyrolusite in M2 (P r = 43.90%) than that in M1 (P r = 24.86%) indicated that compared with M1, M2 would absorb more protons since the planar oxygen atoms in pyrolusite were incompletely coordinated and liable to absorb the protons. Meanwhile, the higher oxidation valence of Mn in M2 than that in M1 revealed that theMn atoms inM2 were more liable to draw the electron density from the surface oxygen atoms in hydroxyl groups. The structural and compositional differences between M1 and M2 were the major reasons why M2 possessed a higher surface potential and a weaker ability to absorb Zn ions.


Introduction
Manganese dioxides are poor crystalline oxides exhibiting large ion adsorption capacities for the metal ions as Cu, Zn, Pb, and Cd [1], owing to their varying tunnel structures and large specific surface areas.The adsorption of Zn 2+ by -MnO 2 in Zn/MnO 2 alkaline battery has been studied widely with the aim of improving the efficiency of Zn/MnO 2 batteries or enhancing the recovery of Zn from the spent batteries [2,3].The adsorption of the metal ions on the surfaces of manganese dioxides arises from the surface hydroxyl groups originated from dissociative chemisorption of water molecules [3][4][5].
Many researchers have studied the control of the structures and the surface properties of MnO 2 .The former work showed that the surface properties of MnO 2 were connected closely with the crystallographic structures as -, -, -, -, and amorphous MnO 2 [8][9][10].In the case of -MnO 2 , even though the electrochemical protonation and lithiation have been extensively studied [11], the knowledge about the control of the structure, and surface properties is still limited.For example, it was reported that the morphology, the inner structure and the surface properties of -MnO 2 were connected by the presence of the surfactants as hexadecyl trimethyl ammonium bromide (CTAB) sodium dodecyl benzene sulfonate (SDBS) [3], and so forth or by the heating treatment [7].Up to now, little work was reported on the relationships between the structure and the surface properties of -MnO 2 .
In this work, a facile method was developed to synthesize -MnO 2 with different structures and surface properties, using MnSO 4 as the raw material and Na 2 S 2 O 8 as the oxidant.The influence of the initial pH on the formation of -MnO 2 was investigated and the relationship between the structure and the surface properties was discussed.

Planar oxygen
Planar oxygen Then the -MnO 2 was mixed with 0.025-1.5 mol⋅L −1 ZnSO 4 at room temperature, keeping the initial molar ratio of Mn to Zn as 1.8-0.03.The slurry was stirred (400 min −1 ) for 1.0 h and kept aging for 24.0 h and then centrifuged and the supernatant was collected for detection of Zn 2+ .

Analysis.
The morphology of the samples was examined by the high resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan).The structures and composition of the samples were identified by the X-ray powder diffractometer (D8 advance, Brucker, Germany) using CuK radiation ( = 0.154178 nm).The thermogravimetric (TG) analysis was carried out by the thermal analyzer (NETZSCH STA409, Germany).The solution pH was detected by pH meter (DELTA 320, METTLER-TOLEDO, China).The soluble Zn 2+ was analyzed by the EDTA titration method.would absorb more protons since the planar oxygen atoms in pyrolusite were incompletely coordinated and liable to absorb the protons.

Results and Discussions
Figure 3 shows the SEM image of the -MnO 2 samples.M1 appears to be the assembled nanorods with a diameter of 30-100 nm and a length of 1-2 m, while M2 is composed of the reunited spherical particles (D: 20-50 nm). Figure 4 shows the TEM image and the SAED patterns of the -MnO 2 samples.Both of M1 and M2 possessed the polycrystalline structures with defects due to the intrinsic De Wolff disorder intergrowth.
Figure 5 shows the TG curves of the -MnO 2 .The weight loss (4.57% for M1 and 3.63% for M2) between 30-400 ∘ C should be connected with the release of the absorbed water from the samples.The molar ratio of MnO 2 to water was then estimated to be 1 : 0.23 for M1 and 1 : 0.18 for M2.The weight loss between 500 and 550 ∘ C (7.04% for M1 and 7.36% for M2) should be attributed to the decomposition of MnO 2 [13][14][15]: Figure 6 shows the XPS spectra of the -MnO The Δ  was 4.52 eV for M1 and 4.47 eV for M2, then the oxidation valences of Mn in M1 and M2 were deduced as ( Compared with the solution with an initial pH 1.0, the solution with an initial pH 8.5 favored the oxidation of Mn 2+ , producing -MnO 2 sample (M2) with higher oxidation valence.

Surface Property of 𝛾-MnO 2 .
Figure 7 shows the zeta potential of the -MnO 2 samples formed at different initial pH.The negative zeta potentials of M1 and M2 in most of the experimental conditions revealed that their surfaces were negative charged owing to the adsorption of hydroxyl groups in most cases.The higher zeta potentials for M2 than those for M1 indicated that compared with M1, M2 was more liable to absorb H + .Figure 8 shows the adsorption of Zn 2+ by the -MnO 2 samples normalized by surface area.The specific surface areas (BET) of M1 and M2 are 14.1 m 2 ⋅g −1 and 39.4 m 2 ⋅g −1 , respectively.With the increase of the initial Zn 2+ concentration from 0.025 mol⋅L −1 to 1.0 mol⋅L −1 , the adsorption of Zn 2+ increased from 0.021 to 8.63 mmol⋅m −2 by M1 and 0.006 to 0.61 mmol⋅m −2 by M2.Compared with M2, M1 showed a stronger ability to absorb Zn 2+ , which should be attributed to its lower zeta potential.pyrolusite units, M2 possessed more planar coordinated oxygen atoms O(planar) both in the bulk and on the surface.According to the Multisite Complex Model (MUSIC) developed by Hiemstra et al. [18,19], the surface planar oxygen was coordinated incompletely with either one or two Mn atoms instead of three Mn atoms in the bulk, thus tending to absorb protons to become a neutralized state.The absorbed proton layer led to the increase of the surface potential of -MnO 2 .Thus compared with M1, M2 possessed more planar oxygen atoms O(planar) which would absorb more protons, thus showing a surface with higher potential.In addition, compared with Mn (+3.86) in M1, Mn (+3.92) in M2 was more liable to draw the electron density from the surface oxygen, thus decreased the basicity of the hydroxyl groups existed on the external surface and also increased the surface potential.

Relationship between Structure and Surface.
Based on the structure-surface analysis, the adsorption of Zn 2+ is closely connected to the ratios of pyrolusite (1 × 1 tunnel) and ramsdellite (1 × 2 tunnel) in the structure of -MnO 2 , which is affected by the initial pH in the synthesis process.Tis speculated that the adsorption of Zn 2+ and other divalent ions as Cu 2+ , Pb 2+ , Cd 2+ , and so forth on -MnO 2 should be quite similar.

Conclusions
A facile method was developed to synthesize -MnO 2 with different structures and surface properties.-MnO 2 was prepared by oxidation of MnSO 4 with Na 2 S 2 O 8 at 90 ∘ C for 2.0 h.-MnO 2 formed at initial pH 1.0 (M1) and 8.5 (M2) was composed of MnO 1.93 ⋅0.23H 2 O and MnO 1.96 ⋅0.18H 2 O, respectively.The ratio of pyrolusite in M2 (  = 43.90%)was higher than that in M1 (  = 24.86%).Compared with M1, M2 could absorb more protons since more planar oxygen atoms in pyrolusite were incompletely coordinated and liable to absorb the protons existed in M2.The higher oxidation valence of Mn in M2 than that in M1 revealed that the Mn     atoms in M2 were more liable to draw the electron density from the surface oxygen atoms in hydroxyl groups.The structural and compositional differences between M1 and M2 were the major reasons why M2 possessed a higher surface potential and a weaker ability to absorb Zn 2+ ions.
Figure 9   shows the schematic drawing for Mn/O atoms on -MnO 2 surface.The surface properties of the -MnO 2 samples were closely connected with their structures.Containing more

Figure 7 :
Figure 7: Variation of zeta potential of -MnO 2 with pH.

Figure 9 :
Figure 9: Schematic drawing for adsorption model of Mn/O atoms on -MnO 2 surface.
S 2 O 8 fine powder was then added to the MnSO 4 solution, keeping the molar ratio of Mn 2+ to Na 2 S 2 O 8 at 1 : 1.2.After being stirred (400 min −1 ) at 90 ∘ C for 2.0 h, the suspension was filtrated, washed with deionized water, and dried at 105 ∘ C for 12.0 h to get the -MnO 2 sample.
[12]2 .According to the models proposed by Chabre and Parmetier[12], the   values for M1 and M2 were calculated according to the following equations:  = 0.602 ⋅  (DW) − 0.198 ⋅  2 (DW) + 0.026 ⋅  3 (DW) ,  is the Twinning index of -MnO 2 ,varying from 0 for perfect ramsdellite (1 × 2 tunnel) structure to 100 for fully twinned structure; (  ) is the Theoretical shift of line 110 in XRD analysis produced by microtwinning; DW is the Logogram for De Wolff; (DW) is the Shift of line 110 in XRD analysis corrected for microtwinning; and   values were 24.86% for M1 and 43.90% for M2.The lower   in M1 than that in M2 indicated that compared with M1, M2 3.1.Structure,Morphology, and Composition of -MnO 2 .Figure2presents the XRD patterns of the -MnO 2 samples formed at varying initial pH.The samples formed at initial pH 1.0 and 8.5 were expressed as M1 and M2, respectively.Sole -MnO 2 phase was detected in both cases.The 2 values of the diffraction peaks of 110, 221, and 240 planes were 22.16 ∘ , 56.00 ∘ , and 57.19 ∘ for M1 and 22.49 ∘ , 56.20 ∘ , and 57.09 ∘ for M2, respectively.The slight difference of the XRD peaks for M1 and M2 was connected with the change of the structures.The structure difference of the two -MnO 2 samples can be expressed by the   value which presents the molar fraction of pyrolusite in -