# Synthesis and Surface Characterization ofγ-MnO2 Nanostructures

Authors: Chengxiang Liu; Jing Wang; Jiarui Tian; Lan Xiang
Journal: Journal of Nanomaterials
 (2013)

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## Abstract

A facile method was developed to synthesizeγ-MnO2 with different structures and surface properties in this paper. γ-MnO2 was prepared by oxidation of MnSO4 with Na2S2O8 at 90°C for 2.0 h. γ-MnO2 formed at initial pH 1.0 (M1) and 8.5 (M2) was composed of MnO1.93·0.23H2O and MnO1.96·0.18H2O, respectively. The higher ratio of pyrolusite in M2 (Pr=43.90%) than that in M1 (Pr=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 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 Zn2+ ions.


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## Body

## 1. 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 Zn2+ by γ-MnO2 in Zn/MnO2 alkaline battery has been studied widely with the aim of improving the efficiency of Zn/MnO2 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 [3–5].γ-MnO2 possesses a varying structure and composition, with the De Wolff disorder intergrowth of ramsdellite (1 × 2 tunnel) and pyrolusite (1  ×  1 tunnel) [6]. As shown in Figure 1, there are two types of oxygen atom coordination in γ-MnO2 [7]: the planar coordination occurred in pyrolusite (1  ×  1 tunnel) and ramsdellite (1 × 2 tunnel), and the pyramidal coordination occurred only in ramsdellite (1 × 2 tunnel). The common formula for γ-MnO2 is [6]
(1)(Mn4+)1-x-y(□)x(Mn3+)y(O2-)2-4x-y(OH-)4x+y,
where x and y represent the molar fraction of the cation vacancies (□) and Mn3+ ions, respectively.Figure 1
Structure ofγ-MnO2.Many researchers have studied the control of the structures and the surface properties of MnO2. The former work showed that the surface properties of MnO2 were connected closely with the crystallographic structures as α-, β-, γ-, δ-, and amorphous MnO2 [8–10]. In the case of γ-MnO2, 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 γ-MnO2 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 γ-MnO2.In this work, a facile method was developed to synthesizeγ-MnO2 with different structures and surface properties, using MnSO4 as the raw material and Na2S2O8 as the oxidant. The influence of the initial pH on the formation of γ-MnO2 was investigated and the relationship between the structure and the surface properties was discussed.


## 2. Experimental

### 2.1. Experimental Procedure

In a typical experiment, certain amounts of NaOH (4.0 mol·L−1) and H2SO4 (1.0 mol·L−1) were used to adjust the initial pH of the MnSO4 solution (0.17 mol·L−1) to 1.0 and 8.5, respectively. Certain amount of Na2S2O8 fine powder was then added to the MnSO4 solution, keeping the molar ratio of Mn2+ to Na2S2O8 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 γ-MnO2 sample. Then the γ-MnO2 was mixed with 0.025–1.5 mol·L−1 ZnSO4 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 Zn2+.


### 2.2. 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 CuKα radiation (λ = 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 Zn2+ was analyzed by the EDTA titration method.


## 2.1. Experimental Procedure

In a typical experiment, certain amounts of NaOH (4.0 mol·L−1) and H2SO4 (1.0 mol·L−1) were used to adjust the initial pH of the MnSO4 solution (0.17 mol·L−1) to 1.0 and 8.5, respectively. Certain amount of Na2S2O8 fine powder was then added to the MnSO4 solution, keeping the molar ratio of Mn2+ to Na2S2O8 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 γ-MnO2 sample. Then the γ-MnO2 was mixed with 0.025–1.5 mol·L−1 ZnSO4 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 Zn2+.


## 2.2. 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 CuKα radiation (λ = 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 Zn2+ was analyzed by the EDTA titration method.


## 3. Results and Discussions

### 3.1. Structure, Morphology, and Composition ofγ-MnO2

Figure2 presents the XRD patterns of the γ-MnO2 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 γ-MnO2 phase was detected in both cases. The 2θ 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 γ-MnO2 samples can be expressed by the Pr value which presents the molar fraction of pyrolusite in γ-MnO2. According to the models proposed by Chabre and Parmetier [12], the Pr values for M1 and M2 were calculated according to the following equations:
(2)Pr=0.602·δ(DW)-0.198·δ2(DW)+0.026·δ3(DW),δ(DW)=2θ(110)-δ(Tw)-21.808(indegrees),δ(Tw)=-0.0054·Tw-8.9×10-5·Tw2(indegrees,whenTw<55%),Tw=100-68.17·Δ2θ(Δ2θ:peaksplittingbetween(221)and(240)),
where Tw is the Twinning index of γ-MnO2, varying from 0 for perfect ramsdellite (1 × 2 tunnel) structure to 100 for fully twinned structure; δ(Tw) is the Theoretical shift of line 110 in XRD analysis produced by microtwinning; DW is the Logogram for De Wolff; δ(DW) is the Shift of line 110 in XRD analysis corrected for microtwinning; and Pr values were 24.86% for M1 and 43.90% for M2. The lower Pr in M1 than that in M2 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.XRD patterns ofγ-MnO2 formed at initial pH 1.0 (M1) and 8.5 (M2).
(a)
(b)Figure3 shows the SEM image of the γ-MnO2 samples. M1 appears to be the assembled nanorods with a diameter of 30–100 nm and a length of 1-2 μm, 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 γ-MnO2 samples. Both of M1 and M2 possessed the polycrystalline structures with defects due to the intrinsic De Wolff disorder intergrowth.SEM ((a)-(b)) images ofγ-MnO2: (a) M1, (b) M2.
(a)
(b)Figure 4
TEM ((a)-(b), (e)-(f)) images and SAED ((c)-(d)) patterns ofγ-MnO2. (a), (c), (e) M1; (b), (d), (f) M2.Figure5 shows the TG curves of the γ-MnO2. 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 MnO2 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 MnO2 [13–15]:
(3)2MnO2⟶Mn2O3+0.5O2↑.
Figure 6 shows the XPS spectra of the γ-MnO2. The binding energy (BE) values of Mn 2p3/2 and 2p1/2 for M1 and M2 were 642.14/654.14 eV and 642.21/654.19 eV, respectively, quite similar with those of MnO2, indicating that the major oxidation valence of Mn was +4 [16]. The peak splittings of Mn 3s, which can reveal the detail oxidation valence of Mn, for M1 and M2 were 89.77/85.25 eV and 89.73/85.26 eV, respectively. AOS (average oxidation state) was calculated using the following equation [17]:
(4)AOS=8.956-1.126ΔEs(ΔEs:peaksplittinginMn3s).
The ΔEs 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 3.86 and 3.92, respectively. Considering together the analysis results of XPS and TG, the formulation for M1 was deduced as MnO1.93·0.23H2O and MnO1.96·0.18H2O for M2.Figure 5
TG curves ofγ-MnO2.XPS spectra ofγ-MnO2.
(a)
(b)The different valences of Mn in M1 and M2 may be related to the different solution pH in the formation processes ofγ-MnO2. Mn2+ was oxidized to γ-MnO2 by Na2S2O8 via the following route:
(5)MnSO4+Na2S2O8+2H2O⟶γ-MnO2+Na2SO4+2H2SO4.
Compared with the solution with an initial pH 1.0, the solution with an initial pH 8.5 favored the oxidation of Mn2+, producing γ-MnO2 sample (M2) with higher oxidation valence.


### 3.2. Surface Property ofγ-MnO2

Figure7 shows the zeta potential of the γ-MnO2 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 7
Variation of zeta potential ofγ-MnO2 with pH.Figure8 shows the adsorption of Zn2+ by the γ-MnO2 samples normalized by surface area. The specific surface areas (BET) of M1 and M2 are 14.1 m2·g−1 and 39.4 m2·g−1, respectively. With the increase of the initial Zn2+ concentration from 0.025 mol·L−1 to 1.0 mol·L−1, the adsorption of Zn2+ 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 Zn2+, which should be attributed to its lower zeta potential.Figure 8
Adsorption of Zn2+ by γ-MnO2.


### 3.3. Relationship between Structure and Surface

Figure9 shows the schematic drawing for Mn/O atoms on γ-MnO2 surface. The surface properties of the γ-MnO2 samples were closely connected with their structures. Containing more 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 γ-MnO2. 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.Figure 9
Schematic drawing for adsorption model of Mn/O atoms onγ-MnO2 surface.Based on the structure-surface analysis, the adsorption of Zn2+ is closely connected to the ratios of pyrolusite (1  ×  1 tunnel) and ramsdellite (1  ×  2 tunnel) in the structure of γ-MnO2, which is affected by the initial pH in the synthesis process. Tis speculated that the adsorption of Zn2+ and other divalent ions as Cu2+, Pb2+, Cd2+, and so forth on γ-MnO2 should be quite similar.


## 3.1. Structure, Morphology, and Composition ofγ-MnO2

Figure2 presents the XRD patterns of the γ-MnO2 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 γ-MnO2 phase was detected in both cases. The 2θ 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 γ-MnO2 samples can be expressed by the Pr value which presents the molar fraction of pyrolusite in γ-MnO2. According to the models proposed by Chabre and Parmetier [12], the Pr values for M1 and M2 were calculated according to the following equations:
(2)Pr=0.602·δ(DW)-0.198·δ2(DW)+0.026·δ3(DW),δ(DW)=2θ(110)-δ(Tw)-21.808(indegrees),δ(Tw)=-0.0054·Tw-8.9×10-5·Tw2(indegrees,whenTw<55%),Tw=100-68.17·Δ2θ(Δ2θ:peaksplittingbetween(221)and(240)),
where Tw is the Twinning index of γ-MnO2, varying from 0 for perfect ramsdellite (1 × 2 tunnel) structure to 100 for fully twinned structure; δ(Tw) is the Theoretical shift of line 110 in XRD analysis produced by microtwinning; DW is the Logogram for De Wolff; δ(DW) is the Shift of line 110 in XRD analysis corrected for microtwinning; and Pr values were 24.86% for M1 and 43.90% for M2. The lower Pr in M1 than that in M2 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.XRD patterns ofγ-MnO2 formed at initial pH 1.0 (M1) and 8.5 (M2).
(a)
(b)Figure3 shows the SEM image of the γ-MnO2 samples. M1 appears to be the assembled nanorods with a diameter of 30–100 nm and a length of 1-2 μm, 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 γ-MnO2 samples. Both of M1 and M2 possessed the polycrystalline structures with defects due to the intrinsic De Wolff disorder intergrowth.SEM ((a)-(b)) images ofγ-MnO2: (a) M1, (b) M2.
(a)
(b)Figure 4
TEM ((a)-(b), (e)-(f)) images and SAED ((c)-(d)) patterns ofγ-MnO2. (a), (c), (e) M1; (b), (d), (f) M2.Figure5 shows the TG curves of the γ-MnO2. 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 MnO2 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 MnO2 [13–15]:
(3)2MnO2⟶Mn2O3+0.5O2↑.
Figure 6 shows the XPS spectra of the γ-MnO2. The binding energy (BE) values of Mn 2p3/2 and 2p1/2 for M1 and M2 were 642.14/654.14 eV and 642.21/654.19 eV, respectively, quite similar with those of MnO2, indicating that the major oxidation valence of Mn was +4 [16]. The peak splittings of Mn 3s, which can reveal the detail oxidation valence of Mn, for M1 and M2 were 89.77/85.25 eV and 89.73/85.26 eV, respectively. AOS (average oxidation state) was calculated using the following equation [17]:
(4)AOS=8.956-1.126ΔEs(ΔEs:peaksplittinginMn3s).
The ΔEs 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 3.86 and 3.92, respectively. Considering together the analysis results of XPS and TG, the formulation for M1 was deduced as MnO1.93·0.23H2O and MnO1.96·0.18H2O for M2.Figure 5
TG curves ofγ-MnO2.XPS spectra ofγ-MnO2.
(a)
(b)The different valences of Mn in M1 and M2 may be related to the different solution pH in the formation processes ofγ-MnO2. Mn2+ was oxidized to γ-MnO2 by Na2S2O8 via the following route:
(5)MnSO4+Na2S2O8+2H2O⟶γ-MnO2+Na2SO4+2H2SO4.
Compared with the solution with an initial pH 1.0, the solution with an initial pH 8.5 favored the oxidation of Mn2+, producing γ-MnO2 sample (M2) with higher oxidation valence.


## 3.2. Surface Property ofγ-MnO2

Figure7 shows the zeta potential of the γ-MnO2 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 7
Variation of zeta potential ofγ-MnO2 with pH.Figure8 shows the adsorption of Zn2+ by the γ-MnO2 samples normalized by surface area. The specific surface areas (BET) of M1 and M2 are 14.1 m2·g−1 and 39.4 m2·g−1, respectively. With the increase of the initial Zn2+ concentration from 0.025 mol·L−1 to 1.0 mol·L−1, the adsorption of Zn2+ 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 Zn2+, which should be attributed to its lower zeta potential.Figure 8
Adsorption of Zn2+ by γ-MnO2.


## 3.3. Relationship between Structure and Surface

Figure9 shows the schematic drawing for Mn/O atoms on γ-MnO2 surface. The surface properties of the γ-MnO2 samples were closely connected with their structures. Containing more 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 γ-MnO2. 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.Figure 9
Schematic drawing for adsorption model of Mn/O atoms onγ-MnO2 surface.Based on the structure-surface analysis, the adsorption of Zn2+ is closely connected to the ratios of pyrolusite (1  ×  1 tunnel) and ramsdellite (1  ×  2 tunnel) in the structure of γ-MnO2, which is affected by the initial pH in the synthesis process. Tis speculated that the adsorption of Zn2+ and other divalent ions as Cu2+, Pb2+, Cd2+, and so forth on γ-MnO2 should be quite similar.


## 4. Conclusions

A facile method was developed to synthesizeγ-MnO2 with different structures and surface properties. γ-MnO2 was prepared by oxidation of MnSO4 with Na2S2O8 at 90°C for 2.0 h. γ-MnO2 formed at initial pH 1.0 (M1) and 8.5 (M2) was composed of MnO1.93·0.23H2O and MnO1.96·0.18H2O, respectively. The ratio of pyrolusite in M2 (Pr = 43.90%) was higher than that in M1  (Pr = 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 Zn2+ ions.



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