# The Effect of Stirring on the Morphology of  Birnessite Nanoparticles

Authors: Marcos A. Cheney; Pradip K. Bhowmik; Shingo Moriuchi; Mario Villalobos; Shizhi Qian; Sang W. Joo
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

The effect of mechanical stirring on the morphology of hexagonal layer-structure birnessite nanoparticles produced from decomposition ofKMnO4 in dilute aqueous H2SO4 is investigated, with characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and N2 adsorption (BET). Mechanical stirring during an initial stage of synthesis is shown to produce black birnessite containing nanofibers, whereas granular particulates of brown birnessite are produced without stirring. This is the first reduction synthesis of black birnessite nanoparticles with dendritic morphology without any use of organic reductant, and suggests that a particular morphology can arise from structural preferences of Mn in acidic conditions rather than particular organic reactants. These results enlighten the possibility of synthesizing nanoparticles with controlled size and morphology.


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## Body

## 1. Introduction

There are increasing interests in layer-structure materials due to their
potential applications in
catalysis, ion-sieves, and rechargeable batteries [1–3].  Birnessite,
a ubiquitous material present in diverse geological settings including ocean
nodules, ore deposits, and alteration crusts [4, 5], is a layer-structure
manganese oxide with edge-sharing MnO6 octahedra, and has been a
subject of intensive investigations due to its unique cation exchange capacity
[6], sorption [7], and redox properties [5, 8].
Synthetic production of birnessite and birnessite-like materials thus
has attracted great attentions, with recent focus on the structural or
morphology control [5, 9, 10].A
number of different synthetic processes can be used to prepare layer-structure brown birnessite of
micron size, including oxidation
of Mn(II) in basic solution [11]
using O2, K2S2O8, or H2O2,  redox reaction between Mn(II) and MnO4− [12], reduction of MnO4− by different routes such as sol gel [13],
reaction of HCl with MnO4− followed by cationic exchange.
All of these methods require addition of one reactant or another [14].  The structure characteristics of birnessite
produced, such grain size and morphology, vary with synthetic methods and in many cases with batch to batch even with identical methods of
preparation. Like most other
syntheses of micro and nanoparticles, the selection mechanism or conditions
leading to a particular choice of structure is not yet clearly understood.The
present study reports synthesis of
birnessite nanoparticles with hexagonal layer structure, based on the reduction
of KMnO4 in aqueous H2SO4, followed by wet-aging
time and air drying. Although this reaction has been studied intensively [15–18], the detailed synthetic conditions correlated with the
structural parameters of birnessite produced have not been reported. To date synthesis of black birnessite
nanoparticles, or nanofibers, with dendritic morphology by our direct method (reduction of KMnO4 by H2SO4) has not been reported, and it is speculated
that dendritic structures produced by other methods are caused by either organic [19] or inorganic [20] reactants. Here, we show that our method produces black birnessite nanoparticles
with dendritic morphology as well as the usual brown birnessite.  The black birnessite nanoparticles are
produced by stirring the solution during an initial stage of the synthesis. The
brown birnessite produced as observed by HRTEM correlates
well with an existing idealized structural hexagonal model for birnessite
constructed from extended X-ray absorption
fine structure (EXAFS) spectra. The size and
morphology of birnessite particles yielded, or the selection of a preferred
state of micro and nanostructure, thus are shown to depend in part on the
mechanical conditions of the reacting
solution.In the
following sections, we present the synthesis process of black and brown
birnessite particles, and show their morphological details using various state-of-the-art
characterization techniques
available.  Discussions on the
significance of the results and concluding remarks follow.


## 2. Experiment

### 2.1. Synthesis

The
preparation of birnessite is performed by reduction of KMnO4 in the presence of H2SO4 in a closed reaction vessel in
the absence of light. The
experimental procedure is a modification of the method used by Clark and
Coe [14] for preparation of MnO2.
All chemicals are analytical grade, and handled with extreme care to avoid any impurities. All pieces of
glassware are acid cleaned, and ultrapure water devoid of any
organic compounds is used, which is often
a critical issue in this type of reaction.
Our synthetic procedure involves preparation of acidified KMnO4 solutions
in two different initial processes for
solution homogenization, mechanical stirring followed by shaking only.  Both are then ensued by stationary aging in the dark.The first set of solutions are prepared by adding 100 mL of 18 MΩ
water into a 250 mL volumetric flask at room temperature, after which 0.69 mL of 18 M H2SO4 was added,
followed by addition of 9.9 g (0.063 mol) of KMnO4.  The volumetric flask containing the mixture is then filled to 250 mL mark with 18 MΩ
water, and then a magnetic stirring bar is
introduced. The solution is
then stirred for 10 minutes on a stirring plate at 300 rpm, at which state all solids are dissolved. The final solution is 0.25 M KMnO4 in 0.05 M H2SO4.
The same process is repeated to
prepare the other two acidified KMnO4 solutions in 0.01 M and 0.1 M H2SO4, which are properly adjusted for the
volumes of H2SO4 needed.
The second set of solutions is
prepared in the manner described above except that the stirring is omitted.  After the last step of filling the 250 mL volumetric flask containing the mixture to
the mark, the solution is then
shaken manually for 1 minute to further homogenize
the mixture.  Immediately after the
solutions are prepared, with or without stirring, 10 mL of
each solution is poured into 20 mL
polyethylene scintillation vials, capped,
and then aged stationary in the
dark for 1–20 days. Identical experiment is repeated using glass scintillation
vials to ensure that the result is not an
artifact of using the polyethylene vials.  For both
cases, the aging is performed in
the presence and absence of light. After the aging step, the vials are centrifuged at 4000 rpm for 10 minutes,
decanted, and  washed with 18 MΩ water 3 times to remove any unreacted permanganate.  The solid samples are then air dried at room temperature for three days before characterization.


### 2.2. Characterization Techniques

The manganese oxides were characterized by
X-ray powder diffraction (XRD) using a PANalytical X’Pert PRO X-ray diffractometer
with a CuKα radiation (40 KV, 40 mA), and an X’Celerator solid state
detector. The samples are prepared
by suspending the oxide and the standard reference material (SRM) in ethanol to
form a slurry. The oxide powders are
prepared on a low-background silicon sample holder with the addition of an
internal standard (NIST SRM 640c, a=5.43088⁢Å). The patterns are recorded at room temperature with
step-sizes of 0.008°, 2θ and 50 seconds. The phase constitutions are characterized using the International Center
for Diffraction Data (ICDD) base
for powder diffraction data. The crystal structure of the phase constitution is confirmed using Rietveld refinement
(Topas 2.1 Bruker AXS) and the International Crystal Structure Data (ICSD)
base.The specific surface areas of the synthesized
oxides are measured at 77 K using
a Gemini 2370 (Micrometrics) surface and porosimetry instrument. The surface
area is calculated by the Brunauer-Emmett-Teller (BET) [19] analysis of N2 adsorption
isotherm obtained after degassing and drying the sample at 200°C
for 24 hours.Scanning
electron microscopy (SEM) images are
obtained with the aid of a JEOL JSM-6700F field-emission SEM microscope.  The
samples for SEM analysis are
prepared by placing 3 mg of the air dried solid manganese oxide on a black tape and then coating with carbon prior to SEM imaging.Transmission electron microscopy (TEM)
images were obtained with a Tecnai G2 F30 S-Twin TEM
instrument.  The TEM operates at 300 KV
using a field emission gun in Schottky mode as an electron source. The samples
for TEM analysis are prepared by
placing 3 mg of the air dried solid manganese oxide in 10 mL of 2-propanol, and
sonication for 5 minutes for homogeneity.
One drop of the slurry is
deposited on a holey-carbon coated copper grid for analysis.The
thermogravimetric measurements (TGAs) are
performed with a TA 2100 instrument at a heating rate of 20°C/min
under nitrogen.


## 2.1. Synthesis

The
preparation of birnessite is performed by reduction of KMnO4 in the presence of H2SO4 in a closed reaction vessel in
the absence of light. The
experimental procedure is a modification of the method used by Clark and
Coe [14] for preparation of MnO2.
All chemicals are analytical grade, and handled with extreme care to avoid any impurities. All pieces of
glassware are acid cleaned, and ultrapure water devoid of any
organic compounds is used, which is often
a critical issue in this type of reaction.
Our synthetic procedure involves preparation of acidified KMnO4 solutions
in two different initial processes for
solution homogenization, mechanical stirring followed by shaking only.  Both are then ensued by stationary aging in the dark.The first set of solutions are prepared by adding 100 mL of 18 MΩ
water into a 250 mL volumetric flask at room temperature, after which 0.69 mL of 18 M H2SO4 was added,
followed by addition of 9.9 g (0.063 mol) of KMnO4.  The volumetric flask containing the mixture is then filled to 250 mL mark with 18 MΩ
water, and then a magnetic stirring bar is
introduced. The solution is
then stirred for 10 minutes on a stirring plate at 300 rpm, at which state all solids are dissolved. The final solution is 0.25 M KMnO4 in 0.05 M H2SO4.
The same process is repeated to
prepare the other two acidified KMnO4 solutions in 0.01 M and 0.1 M H2SO4, which are properly adjusted for the
volumes of H2SO4 needed.
The second set of solutions is
prepared in the manner described above except that the stirring is omitted.  After the last step of filling the 250 mL volumetric flask containing the mixture to
the mark, the solution is then
shaken manually for 1 minute to further homogenize
the mixture.  Immediately after the
solutions are prepared, with or without stirring, 10 mL of
each solution is poured into 20 mL
polyethylene scintillation vials, capped,
and then aged stationary in the
dark for 1–20 days. Identical experiment is repeated using glass scintillation
vials to ensure that the result is not an
artifact of using the polyethylene vials.  For both
cases, the aging is performed in
the presence and absence of light. After the aging step, the vials are centrifuged at 4000 rpm for 10 minutes,
decanted, and  washed with 18 MΩ water 3 times to remove any unreacted permanganate.  The solid samples are then air dried at room temperature for three days before characterization.


## 2.2. Characterization Techniques

The manganese oxides were characterized by
X-ray powder diffraction (XRD) using a PANalytical X’Pert PRO X-ray diffractometer
with a CuKα radiation (40 KV, 40 mA), and an X’Celerator solid state
detector. The samples are prepared
by suspending the oxide and the standard reference material (SRM) in ethanol to
form a slurry. The oxide powders are
prepared on a low-background silicon sample holder with the addition of an
internal standard (NIST SRM 640c, a=5.43088⁢Å). The patterns are recorded at room temperature with
step-sizes of 0.008°, 2θ and 50 seconds. The phase constitutions are characterized using the International Center
for Diffraction Data (ICDD) base
for powder diffraction data. The crystal structure of the phase constitution is confirmed using Rietveld refinement
(Topas 2.1 Bruker AXS) and the International Crystal Structure Data (ICSD)
base.The specific surface areas of the synthesized
oxides are measured at 77 K using
a Gemini 2370 (Micrometrics) surface and porosimetry instrument. The surface
area is calculated by the Brunauer-Emmett-Teller (BET) [19] analysis of N2 adsorption
isotherm obtained after degassing and drying the sample at 200°C
for 24 hours.Scanning
electron microscopy (SEM) images are
obtained with the aid of a JEOL JSM-6700F field-emission SEM microscope.  The
samples for SEM analysis are
prepared by placing 3 mg of the air dried solid manganese oxide on a black tape and then coating with carbon prior to SEM imaging.Transmission electron microscopy (TEM)
images were obtained with a Tecnai G2 F30 S-Twin TEM
instrument.  The TEM operates at 300 KV
using a field emission gun in Schottky mode as an electron source. The samples
for TEM analysis are prepared by
placing 3 mg of the air dried solid manganese oxide in 10 mL of 2-propanol, and
sonication for 5 minutes for homogeneity.
One drop of the slurry is
deposited on a holey-carbon coated copper grid for analysis.The
thermogravimetric measurements (TGAs) are
performed with a TA 2100 instrument at a heating rate of 20°C/min
under nitrogen.


## 3. Results

### 3.1. Formation of Dendritic and Spiral Clusters of Nanoscale

#### 3.1.1. Mass of the Solid MnO2 Formed During the Aging
Process (Stirring and No Stirring)

Figure1 shows  mass of solid MnO2 formed from the
decomposition of a 0.25 M KMnO4 solution in 0.1 M (solid
lines), 0.05 M (dashed lines), and 0.01 M (dotted lines) H2SO4 prepared by the two methods, with and
without stirring. It appears
that the rate of solid MnO2 formation and the yield increases with the
concentration of the acid in all cases.
It is to be noted that the
mass yields are the same regardless of stirring. We hypothesize that the mechanism to produce black birnessite by the
organic reductant may be similar to the one from stirring since both produce Mn2+ and then this species is oxidized by MnO4− to MnO2.Figure 1
Mass of MnO2 produced by
decomposition of 0.25 M KMnO4 in 0.1 M (solid lines), 0.05 M (dashed
lines), and 0.01 M (dotted lines) H2SO4 aqueous solution.
The solid and open circles represent, respectively, the results in the presence
and absence of stirring.


#### 3.1.2. X-Ray Powder Diffraction

The solids formed without stirring appear brown in color, while those with stirring
appear black. In either case there
is KMnO4 remnant, as evidenced by color in the
supernatant. The solid phases formed by the different methods are characterized by XRD.  Figure 2(a) shows the XRD patterns of
nanosized semicrystalline black birnessite obtained with the stirring process
along with the analysis of the data using the Rietveld refinement. The XRD
pattern of brown colored birnessite formed without stirring is shown in Figure 2(b). These patterns show that
the profile parameter of the diffraction peaks of black and brown birnessite is
of the same phase.  The diffraction peaks of the brown birnessite (see Figure 2(b))
seem better defined, indicating possibly a slight increase in
crystallinity and/or larger birnessite particles than the black birnessite.X-ray
powder diffraction of the synthesized manganese oxides. (a) Black birnessite produced with stirring; (b) brown birnessite produced without stirring. The gray line in the bottom of both (a)
and (b) is the pattern of the empty sample holder as a control.
(a)(b)


### 3.2. SEM Studies

The morphology of birnessite obtained by the
two methods is examined by
SEM.  Figure3 shows the SEM images
of black and brown birnessites with cotton ball-like and cloud-like particles, respectively, with diameters smaller than 1μm. At higher
magnification (see Figures 3(b) and 3(d)), the cotton-ball-like particles appear to link together to form
larger particles with well-defined patterns,
whereas the cloud-like particles appear as aggregates of individual
smaller particles with no particular shape.Scanning electron
microscopy (SEM) images of (a) and (b) black and (c) and (d) brown birnessite
nanomaterials.


### 3.3. TEM Studies

The nanostructures of black and brown
birnessites are studied by TEM.  Figure4 shows the TEM and HRTEM images of dendritic structure for the
nanosized black birnessite produced
with the stirring process.
The sheets and dendritic architecture are represented in Figures 4(a) and 4(b). High-resolution electron
microscopy (HRTEM) images in Figures 4(c)
and 4(d) display the crystal
structure of the fibrils [010] direction. Figure 4(c) shows the edge of a sheet and suggests that the sheets
overlap. The high-resolution image in Figure 4(d)
shows lattice fringes of the fibrils with a translation of 8-9Å, which correspond well with the (001)
lattice plane determined by XRD.(a) and (b) TEM and (c) and (d) HRTEM images of the black birnessite nanomaterial produced
with stirring.
(a)(b)(c)(d)Figure5 shows the TEM and HRTEM images of the brown birnessite produced without the stirring process. The
aggregated architecture is
represented in Figures 5(a) and 5(b). Nanosized aggregates are observed
with a discontinuous shape. The
HRTEM images shown in Figures 5(c) and 5(d) show orientations of
some of the fibrils in different direction. The dominant lattice fringes in the high-resolution images shown, however, indicate a repetition of the
(001) lattice plane. Figure 5(d)
also shows the spots of the individual atoms arranged in what appears to be a hexagonal
structure. Figure 5(e) is an
expansion of Figure 5(d) and is to be used to correlate the
existing idealized structural model, obtained from EXAFS, shown in Figure 5(f).TEM and HRTEM images of the brown
birnessite nanomaterial produced without stirring.
(a)(b)(c)(d)(e)(f)It is interesting to note that when stirring is
applied birnessite is formed not only on the bottom of the vial after centrifuging but also on the air-liquid interface in the form of a thin film.
Figure6 shows the
TEM images of this film. The developing
dendritic architecture is represented in Figures 6(a) and 6(b). The HRTEM images shown in Figures 6(c) and 6(d) suggest different orientation of some of the fibrils.(a) and (b) TEM and (c) and (d) HRTEM images of the film formed on the air-liquid interface.
(a)(b)(c)(d)


### 3.4. Thermal Stability

TGA measurements are shown in Figure7. The TGA profile for both black (see Figure 7(a)) and brown (Figure 7(b)) birnessite samples shows two similar weight losses. The first occurs with approximately 14% in the temperature range of 29–183°C, and the second occurs with approximately 4% in the range of 190–488°C.
A slight mass gain is observed for the brown birnessite in the range of
480–544°C.Thermogravimetric analysis (TGA) of the synthetic manganese
oxides. (a) Black birnessite, (b) brown birnessite.
(a)(b)


### 3.5. Surface Area

The specific surface areas of birnessite nanoparticles are listed in
Table1. The values are of the same order
as the ones obtained by other
reductants such as HCl and MnSO4 [14]. The aging time (not shown) appears to affect the
surface area as well.Table 1
Surface areas of the standard Kaolinite and synthesized birnessites.
SampleSurface area (m2/g)Nanopore area (m2/g)Kaolinite-SRM lot no. 19672-1815.3 (±1)2.2 (±1)Black birnessite58.4 (±1)4.3 (±1)Brown birnessite27.7 (±1)4.1 (±1)


## 3.1. Formation of Dendritic and Spiral Clusters of Nanoscale

### 3.1.1. Mass of the Solid MnO2 Formed During the Aging
Process (Stirring and No Stirring)

Figure1 shows  mass of solid MnO2 formed from the
decomposition of a 0.25 M KMnO4 solution in 0.1 M (solid
lines), 0.05 M (dashed lines), and 0.01 M (dotted lines) H2SO4 prepared by the two methods, with and
without stirring. It appears
that the rate of solid MnO2 formation and the yield increases with the
concentration of the acid in all cases.
It is to be noted that the
mass yields are the same regardless of stirring. We hypothesize that the mechanism to produce black birnessite by the
organic reductant may be similar to the one from stirring since both produce Mn2+ and then this species is oxidized by MnO4− to MnO2.Figure 1
Mass of MnO2 produced by
decomposition of 0.25 M KMnO4 in 0.1 M (solid lines), 0.05 M (dashed
lines), and 0.01 M (dotted lines) H2SO4 aqueous solution.
The solid and open circles represent, respectively, the results in the presence
and absence of stirring.


### 3.1.2. X-Ray Powder Diffraction

The solids formed without stirring appear brown in color, while those with stirring
appear black. In either case there
is KMnO4 remnant, as evidenced by color in the
supernatant. The solid phases formed by the different methods are characterized by XRD.  Figure 2(a) shows the XRD patterns of
nanosized semicrystalline black birnessite obtained with the stirring process
along with the analysis of the data using the Rietveld refinement. The XRD
pattern of brown colored birnessite formed without stirring is shown in Figure 2(b). These patterns show that
the profile parameter of the diffraction peaks of black and brown birnessite is
of the same phase.  The diffraction peaks of the brown birnessite (see Figure 2(b))
seem better defined, indicating possibly a slight increase in
crystallinity and/or larger birnessite particles than the black birnessite.X-ray
powder diffraction of the synthesized manganese oxides. (a) Black birnessite produced with stirring; (b) brown birnessite produced without stirring. The gray line in the bottom of both (a)
and (b) is the pattern of the empty sample holder as a control.
(a)(b)


## 3.1.1. Mass of the Solid MnO2 Formed During the Aging
Process (Stirring and No Stirring)

Figure1 shows  mass of solid MnO2 formed from the
decomposition of a 0.25 M KMnO4 solution in 0.1 M (solid
lines), 0.05 M (dashed lines), and 0.01 M (dotted lines) H2SO4 prepared by the two methods, with and
without stirring. It appears
that the rate of solid MnO2 formation and the yield increases with the
concentration of the acid in all cases.
It is to be noted that the
mass yields are the same regardless of stirring. We hypothesize that the mechanism to produce black birnessite by the
organic reductant may be similar to the one from stirring since both produce Mn2+ and then this species is oxidized by MnO4− to MnO2.Figure 1
Mass of MnO2 produced by
decomposition of 0.25 M KMnO4 in 0.1 M (solid lines), 0.05 M (dashed
lines), and 0.01 M (dotted lines) H2SO4 aqueous solution.
The solid and open circles represent, respectively, the results in the presence
and absence of stirring.


## 3.1.2. X-Ray Powder Diffraction

The solids formed without stirring appear brown in color, while those with stirring
appear black. In either case there
is KMnO4 remnant, as evidenced by color in the
supernatant. The solid phases formed by the different methods are characterized by XRD.  Figure 2(a) shows the XRD patterns of
nanosized semicrystalline black birnessite obtained with the stirring process
along with the analysis of the data using the Rietveld refinement. The XRD
pattern of brown colored birnessite formed without stirring is shown in Figure 2(b). These patterns show that
the profile parameter of the diffraction peaks of black and brown birnessite is
of the same phase.  The diffraction peaks of the brown birnessite (see Figure 2(b))
seem better defined, indicating possibly a slight increase in
crystallinity and/or larger birnessite particles than the black birnessite.X-ray
powder diffraction of the synthesized manganese oxides. (a) Black birnessite produced with stirring; (b) brown birnessite produced without stirring. The gray line in the bottom of both (a)
and (b) is the pattern of the empty sample holder as a control.
(a)(b)


## 3.2. SEM Studies

The morphology of birnessite obtained by the
two methods is examined by
SEM.  Figure3 shows the SEM images
of black and brown birnessites with cotton ball-like and cloud-like particles, respectively, with diameters smaller than 1μm. At higher
magnification (see Figures 3(b) and 3(d)), the cotton-ball-like particles appear to link together to form
larger particles with well-defined patterns,
whereas the cloud-like particles appear as aggregates of individual
smaller particles with no particular shape.Scanning electron
microscopy (SEM) images of (a) and (b) black and (c) and (d) brown birnessite
nanomaterials.


## 3.3. TEM Studies

The nanostructures of black and brown
birnessites are studied by TEM.  Figure4 shows the TEM and HRTEM images of dendritic structure for the
nanosized black birnessite produced
with the stirring process.
The sheets and dendritic architecture are represented in Figures 4(a) and 4(b). High-resolution electron
microscopy (HRTEM) images in Figures 4(c)
and 4(d) display the crystal
structure of the fibrils [010] direction. Figure 4(c) shows the edge of a sheet and suggests that the sheets
overlap. The high-resolution image in Figure 4(d)
shows lattice fringes of the fibrils with a translation of 8-9Å, which correspond well with the (001)
lattice plane determined by XRD.(a) and (b) TEM and (c) and (d) HRTEM images of the black birnessite nanomaterial produced
with stirring.
(a)(b)(c)(d)Figure5 shows the TEM and HRTEM images of the brown birnessite produced without the stirring process. The
aggregated architecture is
represented in Figures 5(a) and 5(b). Nanosized aggregates are observed
with a discontinuous shape. The
HRTEM images shown in Figures 5(c) and 5(d) show orientations of
some of the fibrils in different direction. The dominant lattice fringes in the high-resolution images shown, however, indicate a repetition of the
(001) lattice plane. Figure 5(d)
also shows the spots of the individual atoms arranged in what appears to be a hexagonal
structure. Figure 5(e) is an
expansion of Figure 5(d) and is to be used to correlate the
existing idealized structural model, obtained from EXAFS, shown in Figure 5(f).TEM and HRTEM images of the brown
birnessite nanomaterial produced without stirring.
(a)(b)(c)(d)(e)(f)It is interesting to note that when stirring is
applied birnessite is formed not only on the bottom of the vial after centrifuging but also on the air-liquid interface in the form of a thin film.
Figure6 shows the
TEM images of this film. The developing
dendritic architecture is represented in Figures 6(a) and 6(b). The HRTEM images shown in Figures 6(c) and 6(d) suggest different orientation of some of the fibrils.(a) and (b) TEM and (c) and (d) HRTEM images of the film formed on the air-liquid interface.
(a)(b)(c)(d)


## 3.4. Thermal Stability

TGA measurements are shown in Figure7. The TGA profile for both black (see Figure 7(a)) and brown (Figure 7(b)) birnessite samples shows two similar weight losses. The first occurs with approximately 14% in the temperature range of 29–183°C, and the second occurs with approximately 4% in the range of 190–488°C.
A slight mass gain is observed for the brown birnessite in the range of
480–544°C.Thermogravimetric analysis (TGA) of the synthetic manganese
oxides. (a) Black birnessite, (b) brown birnessite.
(a)(b)


## 3.5. Surface Area

The specific surface areas of birnessite nanoparticles are listed in
Table1. The values are of the same order
as the ones obtained by other
reductants such as HCl and MnSO4 [14]. The aging time (not shown) appears to affect the
surface area as well.Table 1
Surface areas of the standard Kaolinite and synthesized birnessites.
SampleSurface area (m2/g)Nanopore area (m2/g)Kaolinite-SRM lot no. 19672-1815.3 (±1)2.2 (±1)Black birnessite58.4 (±1)4.3 (±1)Brown birnessite27.7 (±1)4.1 (±1)


## 4. Discussion

Permanganate ions in an acidic medium decompose according to the following reaction [21]: (1)4MnO4−+12H+→4Mn2++5O2+6H2O.  Subsequently, the Mn2+ produced reacts with permanganate
ions to produce MnO2: (2)2MnO4−+3Mn2++2H2O→5MnO2+4H+. The overall reaction, (3)MnO4−+H+→MnO2+34O2+12H2O, shows that protons are reactants, thus explaining the increasing rate and  yield with
the acid concentration seen in Figure 1.
The released oxygen may combine with Mn2+ from (1) to produce MnO2 [22].  Permanganate reduction occurs via an (MnO3)2-SO4 unstable intermediate [18, 22, 23], which slowly transforms to the manganese heptoxide Mn2O7 as (4)(MnO3)2-SO4(aq)+H2O(l)→Mn2O7(s)+H2SO4(aq). The permanganate
ion can be reduced to the lower
valence MnO2 species in acidic conditions via an intermediate Mn3+ [16] according to the redox reactions and oxidation potential
diagrams [18]:
(5)(Volts)1.18−1.51−0.95−2.09−0.9Mn⟶Mn2+⟶Mn2+⟶MnO2⟶HMnO4−⟶MnO4−. The reduction potentials suggest that the manganic ion,
Mn(III), produced by the reduction of permanganate ion by H2SO4 is unstable with respect to the decomposition reaction [22–24]: (6)2Mn(III)+2H2O→Mn(II)+MnO2+4H+. The Mn(II) produced is then oxidized to Mn(IV) by
permanganate (2). This can be a clue to
the presence of traces of Mn(III) in the oxide. The contribution of SO42− on the stabilization of Mn(III) in sulfuric acid media has been discussed previously [25].  In our method, the
reaction between MnO4−, H2SO4, and
H2O does not seem to complete, as evidenced by the pink color of the
solution at the end of the synthesis. The Mn(III) produced in this reaction is
stable for months in acidic medium, such as in 4.5 M sulfuric acid. In our
system, the sulfuric acid maximum concentration is 0.1 M, which makes both the
Mn(III)-sulfate complex less stable [25]
and the oxidation reaction of Mn(II) by permanganate slow [15]. This phenomenon presumably is responsible
for the observed slow kinetics of MnO2 production at low molar
concentration of sulfuric acid (see Figure 1) and may explain why stirring has no effect on the total production of MnO2.XRD patterns of nanomaterials in Figure2
show four major peaks whose positions
and intensities seem to agree with
those reported by Villalobos et al. [26]
for acid birnessite. The products with and without stirring are thus named as
black and brown acid birnessites, respectively.
To confirm the lamellar structure, a
Rietveld refinement is done using
the ICSD crystallographic data base. The two peaks at 7.35 and 3.67Å
correspond to 001 and 002 basal reflections, respectively, indicating an excellent agreement with other
synthetic and natural birnessites.  The broad humps at 2θ=50–55° also look similar to those of acid birnessite reported by Villalobos et
al. [26].  Additionally, the refined peak parameters
indicate nanosize particles. One of the
main differences between the
birnessites obtained with and without
stirring lies in the size of the
particles, which is also evident
from specific surface area values shown in Table 1.  While both are nanosized, the
black acid birnessite appears to form smaller crystallites, as evidenced by the
slight less defined and lower-amplitude bands of its XRD pattern (see Figure
2), and by its larger specific surface area (see Table 1). We also note that without
stirring, the solids formed tend to attach
strongly onto the wall of both plastic and glass containers. There may be some
catalysis on the surface of the
containers, but it appears that colloids are formed first in solution and then
adsorbed onto the container surface.When stirring is applied initially during
the synthetic process, the product obtained is composed mainly of plate-like
fibrillar structures and black in color.  The low-magnification SEM images of the manganese
oxide synthesized show a raisin-like morphology with a diameter of less that 1μm (see Figure 3). At high magnification, the raisin-like crystal morphology
appears to consist of aggregates of plate-like crystals with diameters of ~0.5μm linked tightly together. This plate-like crystal morphology seems identical to those reported for natural birnessites
or their analogues obtained by different synthetic routes [5, 26].  This ubiquitous morphology indicates that in
acidic conditions with stirring, MnO2 has a structural shape
preference attributed to that of the small sized of
the building block (nanoscale embryo fibers). The product without the stirring process, on the other hand, seems highly
aggregated and brown colored. It is intriguing
to note that the 10 minutes
initial stirring in an otherwise mostly stationary aging reaction of several
days makes a considerable difference in the size and aggregation state of the
particles obtained.  SEM images of the
manganese oxide synthesized without
stirring show aggregates of cloud-like particles linked together with diameters
of  approximately 0.2μm. The aggregates
appear rough and discontinuous and do not show the plate-like crystal morphology seen in black birnessite.  It is
clear that one of the main reasons for the formation of birnessite with plate-like
morphology in black birnessite is due to the brief mechanical agitation
and not because of particular
organic reductants used in the synthesis of this class of materials [2].A closer look at the manganese oxide
synthesized with initial stirring shows a dendritic and fibrous morphology (see
Figure4). The TEM images reveal
that the length and width of the
clusters are in the order of 300 nm and 200 nm, respectively. They also show that curled architectures are produced at the top of the dendrites.
Formation of these clusters does not appear to depend on the aging
time.
Similar morphology has been
obtained using the same synthesis (immediately after the 10 minutes stirring) but
without aging time.HRTEM studies show that most of the black
birnessite nanofibers show (001) orientation; and the striations, evident along the
length, indicate that they are
arranged in different direction.  In 5 nm length scale, the veins along the
radial direction of the fiber are clearly seen, indicative of layered structure. This result supports the
observation that the formation of the dendritic morphology may be due to the
growth of a cluster of fibrous units along thec-axis, and that their small
size produces a peak broadening in the XRD.
Calculation of the d-spacing from
the HRTEM images, using pixel resolution, gives d=8-9Å,
compared to d=7.3Å obtained by the
XRD method. This difference is
attributed to the poorly crystalline nature of the birnessite obtained. What is
clear in black birnessite is that particles are made mainly of nanofibers
characterized by homogeneous surfaces and uniform in size. We thus attribute its fibrillar dendritic morphology and
formation to homogeneous nucleation. The production of the nanoscale fibers (embryos)
seems to be a key step for the formation of dendritic morphology, and mechanical agitation affects both the
morphology and the particle size.  In this case, the nuclei may form spontaneously
in the solution by stirring. The detailed mechanism for the formation of these
interesting shapes at low pH is not presently known. However, we speculate that
the formation of the sheets may be due to a structural control of crystal
growth guided by osmotic pressure [27]. It is known that fibers or rods attract
each other along their elongated sides forming clusters [28]. This
phenomenon is evident in the TEM images.
What is interesting is that
the dendritic architectures observed in these inorganic nanomaterials are indeed
similar to the structures observed using a variety of organic reductants [20], in biogenic manganese oxides [26] and in self-assembled organic systems
[29].  It may
explain why the surface area of
black birnessite, which contains
larger sheets, is higher (58.4 m2g-1) than that of the brown birnessite, which contains much smaller sheets and cloud-like morphology.  The black birnessite appears to be made up of
small clusters of fibers linking together to form larger highly porous particles
with high specific surface area.The
TEM image of the brown birnessite
in Figure5(a) shows cloud-like nanogranular
morphology in the 200 nm range,
similar to the aggregates produced from other materials, such as polymers [30].
It also suggests that the aggregates of brown birnessite are
semicrystalline linked together to
form larger clusters with no crystallographic orientation.  HRTEM studies of this brown birnessite show
particles composed of discontinuous nanofibers and individual atoms of Mn and O.
The nanofibers are oriented in
different directions, with their lattice fringes reflecting the (001) lattice plane. Individual atoms (spots) arranged in
hexagonal form are also visible in the same sample at 5 nm resolution (see Figures
5(d)–5(e)).  It is
difficult to obtain a reasonable d-spacing
value from the HRTEM images, using pixel resolution, probably due to the
discontinuous nature and poor crystallinity of the material.  However, due to
this discontinuity and the fact that the spots of the individual atoms can be
seen at 5 nm resolution, we are
able to correlate for the first time an idealized structural model for
birnessite, obtained by EXAFS spectroscopy, with our actual experimental
observations (see Figures 5(d)–5(f)).  The HRTEM image in Figure 5(e) clearly shows a hexagonal structural
arrangement of the brown birnessite sheets (see large hexagon drawn in).  Calculation of the equidistant magnitude
between observed adjacent “spots” in this hexagonal arrangement yields a value
of 4.8±0.2Å, from pixel resolution. Comparing this value with the idealized structural hexagonal model
proposed by Villalobos et al. [26]
from modeling of EXAFS spectra, we find a close correlation with the Mn-Mn
distance reported for second-shell neighbors around an Mn central atom of 4.99Å (see Figure 5(f)), which would
correspond also to second O-O neighbors in the sheet plane. Given this
assignment, the first Mn-Mn (or O-O) shell, corresponding to a 2.88Å distance
[26], would be represented by the
smaller hexagon depicted in Figures 5(e)–5(f).  If this interpretation is correct, then the
question remains as to why the hexagons observed are the ones from the second
Mn (or O) shell, and not those from the first Mn (or O) shell.  Further work needs to be done along these
lines.  It is clear from these images
that no mechanical agitation produced a higher percentage of larger particles, which are composed of nanofibers with
the individual atoms of Mn and O shown in hexagonal arrangement.The film, produced after a few days of aging on the surface of the solution mixture for the stirred suspension,
is characterized by XRD, and found to contain the same patterns
as those in Figure2(a). It is thus decided that the film material is black birnessite. The TEM and HRTEM images, shown in Figures 6(a)–6(d), also
suggest that this film material is indeed
the dendritic morphology observed for the material obtained from the bulk of the suspension (later
stage of aging), shown in Figure 4. Furthermore, these images suggest the film material to be black
birnessite with their fibrillar dendritic-like morphology in a developing stage. The HRTEM also shows transient embryos of nanofibers of
about 5 nm in size that we believe are the basis for nucleation growth. The
reason why these fibers are not destroyed by stirring may be due to the fact
that they are less dense at this stage of development and accumulate in the
surface of the mixture as a film,
thus surviving the stirring and
centrifuge processes. A possible mechanism for the formation of this
structures and shapes would be as follows: (1) initially, nanofibers of MnO2 form are forced to
collide into each other; (2) upon
collision, nucleation occurs in the surface of the particles; (3) once linked together, the particles
grow into larger particles through aggregation. Although
aggregation due to the van der
Waals attraction is reversible,
that due to the surface nucleation is not [30]. Energetically, this makes sense,
and we conclude that the acquired morphology observed in this work may be due
to minimization of energy between the embryos and the substrate.The TGA analysis shows that the birnessite nanoclusters prepared using the stirring and nonstirring
methods have similar thermal profiles. The thermograms show two weight losses.
The first loss of approximately 14% in the range of 29 to 183°C is due to loss of loosely bound water, indicating these samples are somehow
hygroscopic.  The second loss of approximately 4% in the temperature
range of 190 to 488°C is
attributed to chemically bound water and oxygen from the oxide [5].  There is
in fact a small third weight loss of about
0.5% for the sample obtained with
the stirring process (see Figure 7(a)) in the temperature range of 491–544°C, whereas a weight gain of  about 0.09%
is observed without the stirring in the temperature
range of 480 to 544°C (see Figure 7(b)).  It is
attributed to migration of the Mn layer to the new vacant sites produced by the
partial reduction of Mn4+ to Mn3+ and subsequent
oxidation of Mn3+ by the freed oxygen in the local microenvironment [5]. The
thermal stability of the samples appears unaffected by the wet-aging
process.


## 5. Condluding Remarks

Black and brown birnessite nanoparticles are produced by the reaction of KMnO4 in dilute aqueous
H2SO4 medium with
and without brief stirring, respectively,
followed by a stationary
aging at room temperature. Although the
molecular structures of these two birnessites
are similar, their morphologies are
drastically different.  The morphology of the brown birnessite, produced without the stirring process, correlates well with the theoretical hexagonal structural model of Villalobos et al. [26]. The morphology of the black birnessite,
produced with the stirring process, appears similar to that prepared by other
methods using organic reactants.  In this
study, black birnessites with dendritic morphology are produced for the first
time via a reduction reaction of permanganate by dilute H2SO4 and without using any organic reactant, thus showing that the dendritic
architecture is not induced by organic reactants.  It is notable that a brief stirring in an
early stage of the preparation can make a remarkable difference in the morphology of the product.  The stirring process also produces black
birnessite film on the air-liquid interface.
A quantitative analysis associated with the stirring time and rate is a
subject for future studies.



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*Source: 168716-2008-06-30.xml*