# A New Method of Synthesizing Black Birnessite Nanoparticles: From Brown to Black Birnessite with Nanostructures

Authors: Marcos A. Cheney; Pradip K. Bhowmik; Shizhi Qian; Sang W. Joo; Wensheng Hou; Joseph M. Okoh
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

A new method for preparing black birnessite nanoparticles is introduced. The initial synthesis process resembles the classical McKenzie method of preparing brown birnessite except for slower cooling and closing the system from the ambient air. Subsequent process, including wet-aging at7∘C  for 48 hours, overnight freezing, and lyophilization, is shown to convert the brown birnessite into black birnessite with complex nanomorphology with folded sheets and spirals. Characterization of the product is performed by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and  N2  adsorption (BET) techniques. Wet-aging and lyophilization times are shown to affect the architecture of the product. XRD patterns show a single phase corresponding to a semicrystalline birnessite-based manganese oxide. TEM studies suggest its fibrous and petal-like structures. The HRTEM images at 5 and 10 nm length scales reveal the fibrils in folding sheets and also show filamentary breaks. The BET surface area of this nanomaterial was found to be  10.6 m2/g.  The TGA measurement demonstrated that it possessed an excellent thermal stability up to 400∘C.  Layer-structured black birnessite nanomaterial containing sheets, spirals, and filamentary breaks can be produced at low temperature  (−49∘C)  from brown birnessite without the use of cross-linking reagents.


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## Body

## 1. Introduction

There is an increasing interest in layer-structured materials due to their vast areas of applications, such as
catalysis, ion-sieves, and rechargeable batteries [1–3].
Birnessite-type manganese oxide, MnO2, is a layer-structured mineral
containing edge-shared MnO6 octahedral with a d-spacing of ca. 7Å,
which renders mobility of interlayer cations without a structural change [4]. Birnessite
nanoparticles thus have many potential applications, and the method for preparing them with
desired micro- and nanostructure has been a subject of intensive study.A
number of synthetic processes for preparing layer-structured brown birnessite with microscale structures have existed
since the early report of McKenzie [5], including oxidation of Mn(II) in basic solution [6], redox reaction
between Mn(II) and MnO4− [7], reduction of MnO4− using different routes
such as sol gel [8], reaction of HCl with MnO4− followed
by cationic exchange, and by
oxidation of Mn(II) using O2, K2S2O8,
and H2O2. Synthesis
methods for preparing black birnessite with nanostructures are relatively
scarce, although preparation of nanocrystalline MnO2 has been attracting special interest [9]. One
dimensional (1D) single-crystal nanowires of α-MnO2 and β-MnO2 have been synthesized via hydrothermal route using MnSO4 and
oxidizing agents, such as (NH4)2S2O8 or KMnO4 [10]. Nanowires of α-MnO2 are also synthesized using coordination polymers
[11–13]. Birnessite nanoparticles with hexagonal layer structure with dendritic
morphology are synthesized based on the reduction of KMnO4 in
dilute aqueous
H2SO4 with initial
stirring, followed by wet-aging time and air drying [14]. More recently, birnessite has also been
prepared by the electrochemical stimulation of Mn3O4 in
Na2SO4 solution to be used as electrochemical
supercapacitors [15, 16] and semiconductors [17]. Pillared birnessite
nanosheets have been prepared by an ion-exchange method [18]. Transition
metal-doped birnessite has also been prepared from surfactant-free nonaqueous
sol gel routes [19]. Nevertheless, birnessite structural details,
crystal chemistry, and morphological prediction remain elusive. Birnessite-type
manganese oxides are a two-dimensional (2D) layer-structure material and an
important precursor for the synthesis of tunnel-structure manganese oxides [20].
It forms by a regular distribution of Mn vacancies in
the MnO crystal matrix. The structure is generated by the removal of Mn atoms
from the corners and faces in a doubled unit cell of MnO [19].Some
of the aforementioned methods generate birnessite nanofibers with interesting and desirable nanostructures.
Synthesis of black birnessite
nanoparticles of inorganic clusters with complex architecture, such as sheets, spiral, and petal-like
morphology, however, is more difficult. Here,
we report the synthesis of such
particles from the conversion of brown birnessite (same batch) without
the use of chiral solvents, templates, or cross-linking reagents. The classical synthesis process of McKenzie [5]
is chosen with minor modifications to produce intermediate brown birnessite,
which is then converted to more crystalline black birnessite via a postprocess of slow cooling, cold
wet-aging, freezing, and
lyophilization.The
purpose of the current report is straightforward. As we report a new synthesis
method for black birnessite nanoparticles with an intriguing morphology, we show
that complex nanostructures can be created by purely physical treatments added
to otherwise brown-birnessite preparing processes. Cheney et al. [14] have shown
that a synthesis method for brown birnessite turns into that for black birnessite with dendritic nanostructure
by a simple preprocess of 10-minute stirring. In this study, we show the same
by a simple postprocess.


## 2. Experiment

### 2.1. Synthesis

The
synthesis can be divided into two steps: production of the intermediate brown
birnessite and conversion into black birnessite.
The first step is composed of a slow dropwise (86 drops/min) addition of
0.5 L of 2 M hydrochloric acid (HCl), with a vigorous stirring at 300 rpm, into a 4 L beaker containing a solution, prepared by dissolving 1.0 mol of KMnO4 (158 g) in a
2.5 L of 18 MΩ water, and then
heating at 95°C. Here, the beaker is covered with a large watch glass with a thermometer inserted through a protuberance of 1 inch in
diameter. Through the protuberance, HCl is added dropwise over a period of 2 hours using a separatory funnel
after which the thermometer is
removed. The beaker is then completely covered
with the watch glass, boiled for
10 minutes, and then wrapped with an aluminum
foil to prevent rapid cooling. The reaction produces chlorine gas, which keeps
air from entering the beaker during the
process. The reaction is
completed in 2 hours, producing
brown manganese oxide. After the stirring was stopped, particles began to
settle under a cloud of chlorine gas. The
coloration of the supernatant solution can
be detected visually.The second step of conversion to black
birnessite starts with a slow cooling of the beaker to room temperature. The mixture is left in the hood
overnight. Subsequently, the
mixture is wet-aged for 48 hours
at7°C. The brown slurry formed is then
centrifuged at 2 800 rpm, decanted, washed 13 times with 18 MΩ water, and placed in a standard freezer
overnight. The samples are then freeze-dried
(lyophilized) at 0.7 mbar pressure and −49°C for 3 days, after which they turned black.


### 2.2. Characterization

The manganese oxides are 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 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=5.43088Å). 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. Crystallography 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 at77°K using a Gemini 2370 (Micromeritics, GA, USA) surface and porosimetry
instrument. The surface area is
calculated from the Brunauer-Emmett-Teller (BET) analysis of N2 adsorption isotherm obtained after degassing and drying the sample at 200°C
for 24 hours [21].Transmission
electron microscopy (TEM) images are
obtained using a TecnaiG2 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 freeze-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 (TGA) measurements are
performed with a TA 2100 instrument in the range of 0 to600°C at a
heating rate of 20°C/min under nitrogen.


## 2.1. Synthesis

The
synthesis can be divided into two steps: production of the intermediate brown
birnessite and conversion into black birnessite.
The first step is composed of a slow dropwise (86 drops/min) addition of
0.5 L of 2 M hydrochloric acid (HCl), with a vigorous stirring at 300 rpm, into a 4 L beaker containing a solution, prepared by dissolving 1.0 mol of KMnO4 (158 g) in a
2.5 L of 18 MΩ water, and then
heating at 95°C. Here, the beaker is covered with a large watch glass with a thermometer inserted through a protuberance of 1 inch in
diameter. Through the protuberance, HCl is added dropwise over a period of 2 hours using a separatory funnel
after which the thermometer is
removed. The beaker is then completely covered
with the watch glass, boiled for
10 minutes, and then wrapped with an aluminum
foil to prevent rapid cooling. The reaction produces chlorine gas, which keeps
air from entering the beaker during the
process. The reaction is
completed in 2 hours, producing
brown manganese oxide. After the stirring was stopped, particles began to
settle under a cloud of chlorine gas. The
coloration of the supernatant solution can
be detected visually.The second step of conversion to black
birnessite starts with a slow cooling of the beaker to room temperature. The mixture is left in the hood
overnight. Subsequently, the
mixture is wet-aged for 48 hours
at7°C. The brown slurry formed is then
centrifuged at 2 800 rpm, decanted, washed 13 times with 18 MΩ water, and placed in a standard freezer
overnight. The samples are then freeze-dried
(lyophilized) at 0.7 mbar pressure and −49°C for 3 days, after which they turned black.


## 2.2. Characterization

The manganese oxides are 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 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=5.43088Å). 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. Crystallography 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 at77°K using a Gemini 2370 (Micromeritics, GA, USA) surface and porosimetry
instrument. The surface area is
calculated from the Brunauer-Emmett-Teller (BET) analysis of N2 adsorption isotherm obtained after degassing and drying the sample at 200°C
for 24 hours [21].Transmission
electron microscopy (TEM) images are
obtained using a TecnaiG2 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 freeze-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 (TGA) measurements are
performed with a TA 2100 instrument in the range of 0 to600°C at a
heating rate of 20°C/min under nitrogen.


## 3. Results and Discussion

### 3.1. Creation of Inorganic Nanosized Sheets and Spiral Clusters with Complex Architecture

The
synthesis method of McKenzie that
produces microscale brown particles resembles the first step of producing
intermediate brown birnessite in the new method described above. In the McKenzie method, 2 M of HCl is added more rapidly at 200 drops/min to 1 M of KMnO4, and the container is open to the air with
a less vigorous stirring of 150 rpm at 95°C. The solution is then cooled rapidly
to room temperature with no
aging time, and immediately centrifuged, washed, and lyophilized. The room temperature
and 7°C wet-aging and
overnight freezing are missing, which results in brown birnessite with
drastically different morphology. The missing processes thus are responsible
for the curved nanosheets with petal-like morphology and clover-like
nanoclusters of layer structure exhibited in the black birnessite produced with
the new method.The
synthesis consists of the reduction of permanganate ions by
concentrated hydrochloric acid (HCl) in acidic conditions in a closed system
and with aging time. In this reaction, permanganate is reduced to the lower
valence Mn(IV) species in acidic conditions via an intermediate state Mn(III) [22]
according to the following redox potential diagrams [23]:manganese(1)Mn→1.18Mn2+→−1.51Mn3+→−0.95MnO2→−2.26MnO42−→−0.56(Volts)MnO4−
chloride
(2)Cl−→−1.36Cl2→−1.62HClO→−1.63HClO2→−1.21ClO3−→−1.19(Volts)ClO4−.The reduction potentials suggest that the
manganic ion, Mn(III), produced by the reduction of permanganate ion by HCl, is
unstable in respect to the following decomposition [23]:
(3)2Mn(III)+2H2O→Mn(II)+MnO2+4H+.The Mn(II) produced is then oxidized to Mn(IV) by
permanganate or MnO2. This may be a reason why the XRD and XANES analyses (not shown)
showed no Mn(III) present in the sample. This result is rather remarkable because
nonbiogenic and biogenic manganese oxides are known to exhibit Mn in the +3
oxidation state probably due to (3) [24]. The production of K-birnessite
or H-birnessite containing only Mn(IV), via a chemical protocol, is indeed more
challenging. In biogenic manganese oxides, Mn(III) may be stabilized by organic complexing agents found in
cell material. In the new synthesis
method developed, it appears that
the Cl− ion does not
have a significant effect on stabilizing Mn(III), which otherwise would have been detected as a different phase in
the XRD. This is in spite of the high concentration of Cl− present
in the system. Also, in our method, the reaction between HCl and MnO4− does not seem to be complete,
as evidenced by the persistent
color of the solution at the end of the synthesis. This observation is
surprising because the
concentration of HCl is twice the concentration of MnO4− and, according to the redox potentials shown in (1) and (2) above, this reaction is likely to be
complete. Therefore, another
phenomenon may be involved in the inhibition of this reaction, and influences the amount of nanomaterial formed. We speculate that the cloud of
chlorine gas formed as a byproduct, which is accumulated in the headspace of the closed container, might have
influenced the reaction by limiting the presence of oxygen. The brown-colored
solid produced then is converted to black
solid through overnight freezing and subsequent lyophilization process
at 0.7 mbar pressure and −49°C for 3 days. This result is surprising because all methods of synthesis of, or conversion
to, black birnessites reported to
date are carried out at high (95°C) temperatures [24–28].


### 3.2. X-Ray Powder Diffraction

The phases formed by the new method described above are characterized by XRD in comparison with those prepared by the
McKenzie method [5]. Figure 1(a) shows the XRD patterns of nanosized
semicrystalline black birnessite obtained by the new method, along
with the analysis of the data using the Rietveld refinement. The XRD pattern of
brown birnessite obtained using the McKenzie method with no aging time is shown
in Figure 1(b). Although these graphs indicate that the profile parameter of
the diffraction peaks of black and brown birnessite are similar, it is seen that the crystallinity is increased with the aging time and the
diffraction peaks of black birnessite have
become more defined.X-ray powder diffraction of the synthesized manganese
oxide.
(a) Black birnessite produced by our method, and (b) brown birnessite produced
by the McKenzie method.
(a)(b)The XRD patterns of the black birnessite shown in Figure1(a) suggest that cold aging process and lyophilization affect the
crystallinity of final product. Based on XRD and Rietveld analysis, we can refine the lattice parameter of
birnessite to a=4.944(3)Å, b=2.834(2)Å, c=7.295(1)Å, β=97.8(1)Å, and the
space group C2/m is confirmed. Relative peak intensities
of (001), (002), (200)/(110), and (114)/(310) are calculated to a refinement residual (Rwp) of 4%. The
calculated XRD pattern further reveals the semicrystalline character of the
specimen and lattice planes of minor intensities that are not developed. The
layered structure of birnessite has a repeat distance of 7.23Å corresponding
to (001) lattice plane of black birnessite, as for the K-rich [29] or the
H-birnessite [24]. The peak profiles indicate crystal sizes in the range
of 14-15 nm, which is also confirmed by HRTEM. In
the case of the McKenzie method (open system and no aging time), the above
reaction produced specimens of lower crystallinity, as indicated by the relative peak intensities and the peak
profiles using Rietveld analysis (Figure 1(b)). The XRD pattern also shows a single phase with the strongest d-spacing (001) of 7.2Å, corresponding to brown birnessite. This
reaction is very sensitive to experimental conditions used. Minor modification
of the original procedure, that is, slow addition of HCl and with no cold aging
time, results in the formation of
nanosized crystals mixed with large crystals [29, 30]. This may be due to a rapid
and unrestricted reaction between Mn(III) and Mn(IV), which are formed by the oxidation of Mn(II) by
permanganate at low pH. The oxidation of Mn(II) by MnO4− is reported to be autocatalytic [16],
and can be influenced by the increased concentration of Mn(II) in solution from
(3). The repeated distances (d-spacing)
of 7.23Å and 7.20Å for both black and brown birnessites suggest that there is at least one
water layer present in the oxides with the former being a little more
hygroscopic.


### 3.3. Tem and Hrtem Studies

The nanostructures of both black and brown
birnessites are studied using
TEM. Figure2 shows the TEM images and petal-like distribution obtained for
nanosized black birnessite. The sheets and petal-like architecture are shown in Figures 2(a) and 2(b). High-resolution transmission electron microscopy (HRTEM)
images, shown in Figures 2(c) and 2(d), display the crystal structure of the
fibrils (010) direction. Figure 2(c) shows the edge of a sheet suggesting that
the sheets fold in a dough-like manner,
producing a plant stem-like appearance. This
micrograph also shows a defect in the arrangement as petiole (protuberance)
from the stem. The high-resolution image in Figure 2(d) shows lattice fringes
of the fibrils with a translation of 7-8Å which
correspond well with (001) lattice plane determined by XRD.Transmission electron microscopy (a) and (b) and high-resolution
transmission electron microscopy (c) and (d) images of the black birnessite
nanomaterial synthesized by our method.
(a)(b)(c)(d)The manganese oxides synthesized by the new method show a petal-like and
fibrous morphology of birnessite-type materials (Figures2(a)–2(c)). The TEM
reveals that the length of the clusters appears to be in the 300 nm range and
the width in the order of 200 nm,
suggesting nanosized clusters. The TEM also shows that curled architectures are produced at the top of the petals.
Formation of these clusters appears to depend on the cold-aging time and the temperature of lyophilization.Figure3
shows the TEM images and fiber distribution obtained for brown birnessite. The fiber architecture is represented
in Figures 3(a) and 3(b). Nanosized clusters can be observed with some twisting
in a coil-like manner. The HRTEM images shown in Figures 3(c) and 3(d) also suggest
an orientation of some of the fibers in (010) direction. However, the dominant
lattice fringes in the high-resolution images as shown indicate a repetition of
the (001) lattice plane (Figures 3(e) and 3(f)).Transmission electron microscopy (a) and (b) and high-resolution
transmission electron microscopy (c)–(f) images of the brown birnessite
produced by the McKenzie method.
(a)(b)(c)(d)(e)(f)In the case of brown birnessite nanoclusters,
the TEM images show a dendritic morphology in the 100 nm range, as shown in Figures3(a) and 3(b), which can be compared with echinate spines similar to the dandelion pollen grains, made from other inorganic materials reported by Hall et al. [30].
However, in this material, the dendrites are covered with a veil which can partly
be seen in the circled region in Figure 3(d) as single sheet. At 20 nm length
scale, sheets and coil-like morphologies are
also detected, and the veil, seen
at 100 nm, becomes more apparent,
suggesting that it is composed of
a small single sheet. It is interesting to learn that the dendritic
appearance observed in this image is actually caused by the folding of small
sheets. Figures 3(a), 3(b), and 1 also suggest that the nanoclusters of the birnessite appear crystalline and
link together to form larger clusters with no apparent crystallographic
orientation. To accurately characterize birnessite-type material, a combination
of complementary analytical tools like HRTEM and XRD are essential.HRTEM studies show that most of the black birnessite nanofibers are oriented in the (010)
direction (zone-axis direction),
and the lattice fringes reflecting the (001) lattice plane become visible in
Figures2(c) and 2(d). At 1 000 000-fold
magnification, the veins along the radial direction of the fiber are clearly
seen, indicative of layered
structure (Figure 2(d)). Additionally, some imperfections in the arrangement of
the fibers can also be seen in Figure 2(c). These imperfections suggest that
the sheets fold in a dough-like manner,
producing stem-like appearance of a plant. Furthermore, this micrograph also
shows a growth in the arrangement as petiole (protuberance) from the stem. This
result supports the observation that the formation of the petal-like sheets may
be due to the growth of a cluster of fibrous units along the c-axis and we speculate that
the high amount of structure defects of the semicrystalline material produce
peak broadening in the XRD. The development of a petiole is intriguing given
the fact that this kind of growth is seen only in living organisms such as
plants but not in inorganic materials made without templates. This work shows that
the production of the nanoscale fibers seems to be a key step for the formation
of petal-like morphology. 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 [31]
and the unreacted Cl− ion. The
sheets and petal-like architecture thus
can be produced by a growth of the corn rows on the surface of clusters
made of fibrils. The path then would be a branching direction (horizontal)
followed by a growth direction (vertical) of the subunits. The stacking of the
subunits in the vertical direction reaches a point that the units begin to fold
forming the petal-like architecture. 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 in Figures 2(a)–2(c). What is intriguing
is that the petal-like architecture observed in these inorganic nanomaterials is indeed similar to the
structures observed in biogenic manganese oxides [24] and in self-assembled
organic systems [32]. We support the view of Terada et al. [33] that the phenomenon that gives rise
to the observed coil and curved architectures in both inorganic and biological
materials may be related to the flexibility in the development processes of these
morphologies. A distortion in the development of the subunits promotes curved
sheets that tend to decrease their surface area when rolled up. This may be one explanation as to why the surface
area of black birnessite (10.6m2g−1), which
contains larger sheets, is lower than that of the brown birnessite (36.2m2g−1),
which contains much smaller sheets of dendritic morphology. The brown
birnessite produced by the McKenzie method consists of small clusters linked
together to form larger particles of micron size. The method reported here is
highly reproducible if the synthetic parameters such as temperature, stirring
rate, and the speed of the addition of one reactant into another are rigorously
controlled.


### 3.4. Thermal Stability of The Birnessite Nanofibers

The TGA measurements of the synthesized
materials using the new method and
the McKenzie method are shown in Figure4. The TGA profiles for both samples look similar and show that there are two weight losses in both
samples; the first consisting of about 10% in the range of 147–160°C and the second consisting of about 13% for the McKenzie method and
about 14% for the sample prepared using the
new method in the temperature range of 400–446°C. There is a slight gain in
mass observed in the range of 440–475°C. The thermal stability of the
samples seems unaffected by the
wet-aging process.Figure 4
Thermogravimetric analysis (TGA) of the synthetic manganese
oxides.
Black birnessite nanomaterial produced by our method (dashed line), and brown
birnessite produced by the McKenzie method (solid line).The first weight loss of 10% indicated
by the thermogram is due to loss of loosely bound water indicating  the samples are
hygroscopic. The second loss of 13% in the temperature range of 422 to443°C is attributed to chemically bound water
and oxygen from the oxide  [32].
The weight loss of the black birnessite
in this temperature range is slightly higher than the brown birnessite, suggesting that
this material is more hygroscopic and correlates well with a d-spacing of 7.22Å observed in the XRD.
The weight gained in the
temperature range of 443–463°C can be attributed to migration of the Mnlayer to the new vacant sites produced by the partial reduction of Mn4+ to
Mn3+ and subsequent oxidation of Mn3+
[32].


### 3.5. Surface Area

The surface areas estimated for the black and
brown birnessites prepared by the new and the McKenzie method, respectively, are
listed in Table1. The surface area of the black nanobirnessite-type
manganese oxide turns out to be
lower (10.6m2/g) than that
produced by using the McKenzie method. The aging time seems to affect the surface area.Table 1
Surface areas of standard material (Kaolinite) and synthesized nanosized black and brown birnessites.
SampleSurface aream2g−1Kaolinite—SRM lot# 19672-1814.9(±1)K-MnO2 (black birnessite synthesized by the developed method)10.6(±1)K-MnO2 (brown birnessite synthesized by the McKenzie method)36.2(±1)


## 3.1. Creation of Inorganic Nanosized Sheets and Spiral Clusters with Complex Architecture

The
synthesis method of McKenzie that
produces microscale brown particles resembles the first step of producing
intermediate brown birnessite in the new method described above. In the McKenzie method, 2 M of HCl is added more rapidly at 200 drops/min to 1 M of KMnO4, and the container is open to the air with
a less vigorous stirring of 150 rpm at 95°C. The solution is then cooled rapidly
to room temperature with no
aging time, and immediately centrifuged, washed, and lyophilized. The room temperature
and 7°C wet-aging and
overnight freezing are missing, which results in brown birnessite with
drastically different morphology. The missing processes thus are responsible
for the curved nanosheets with petal-like morphology and clover-like
nanoclusters of layer structure exhibited in the black birnessite produced with
the new method.The
synthesis consists of the reduction of permanganate ions by
concentrated hydrochloric acid (HCl) in acidic conditions in a closed system
and with aging time. In this reaction, permanganate is reduced to the lower
valence Mn(IV) species in acidic conditions via an intermediate state Mn(III) [22]
according to the following redox potential diagrams [23]:manganese(1)Mn→1.18Mn2+→−1.51Mn3+→−0.95MnO2→−2.26MnO42−→−0.56(Volts)MnO4−
chloride
(2)Cl−→−1.36Cl2→−1.62HClO→−1.63HClO2→−1.21ClO3−→−1.19(Volts)ClO4−.The reduction potentials suggest that the
manganic ion, Mn(III), produced by the reduction of permanganate ion by HCl, is
unstable in respect to the following decomposition [23]:
(3)2Mn(III)+2H2O→Mn(II)+MnO2+4H+.The Mn(II) produced is then oxidized to Mn(IV) by
permanganate or MnO2. This may be a reason why the XRD and XANES analyses (not shown)
showed no Mn(III) present in the sample. This result is rather remarkable because
nonbiogenic and biogenic manganese oxides are known to exhibit Mn in the +3
oxidation state probably due to (3) [24]. The production of K-birnessite
or H-birnessite containing only Mn(IV), via a chemical protocol, is indeed more
challenging. In biogenic manganese oxides, Mn(III) may be stabilized by organic complexing agents found in
cell material. In the new synthesis
method developed, it appears that
the Cl− ion does not
have a significant effect on stabilizing Mn(III), which otherwise would have been detected as a different phase in
the XRD. This is in spite of the high concentration of Cl− present
in the system. Also, in our method, the reaction between HCl and MnO4− does not seem to be complete,
as evidenced by the persistent
color of the solution at the end of the synthesis. This observation is
surprising because the
concentration of HCl is twice the concentration of MnO4− and, according to the redox potentials shown in (1) and (2) above, this reaction is likely to be
complete. Therefore, another
phenomenon may be involved in the inhibition of this reaction, and influences the amount of nanomaterial formed. We speculate that the cloud of
chlorine gas formed as a byproduct, which is accumulated in the headspace of the closed container, might have
influenced the reaction by limiting the presence of oxygen. The brown-colored
solid produced then is converted to black
solid through overnight freezing and subsequent lyophilization process
at 0.7 mbar pressure and −49°C for 3 days. This result is surprising because all methods of synthesis of, or conversion
to, black birnessites reported to
date are carried out at high (95°C) temperatures [24–28].


## 3.2. X-Ray Powder Diffraction

The phases formed by the new method described above are characterized by XRD in comparison with those prepared by the
McKenzie method [5]. Figure 1(a) shows the XRD patterns of nanosized
semicrystalline black birnessite obtained by the new method, along
with the analysis of the data using the Rietveld refinement. The XRD pattern of
brown birnessite obtained using the McKenzie method with no aging time is shown
in Figure 1(b). Although these graphs indicate that the profile parameter of
the diffraction peaks of black and brown birnessite are similar, it is seen that the crystallinity is increased with the aging time and the
diffraction peaks of black birnessite have
become more defined.X-ray powder diffraction of the synthesized manganese
oxide.
(a) Black birnessite produced by our method, and (b) brown birnessite produced
by the McKenzie method.
(a)(b)The XRD patterns of the black birnessite shown in Figure1(a) suggest that cold aging process and lyophilization affect the
crystallinity of final product. Based on XRD and Rietveld analysis, we can refine the lattice parameter of
birnessite to a=4.944(3)Å, b=2.834(2)Å, c=7.295(1)Å, β=97.8(1)Å, and the
space group C2/m is confirmed. Relative peak intensities
of (001), (002), (200)/(110), and (114)/(310) are calculated to a refinement residual (Rwp) of 4%. The
calculated XRD pattern further reveals the semicrystalline character of the
specimen and lattice planes of minor intensities that are not developed. The
layered structure of birnessite has a repeat distance of 7.23Å corresponding
to (001) lattice plane of black birnessite, as for the K-rich [29] or the
H-birnessite [24]. The peak profiles indicate crystal sizes in the range
of 14-15 nm, which is also confirmed by HRTEM. In
the case of the McKenzie method (open system and no aging time), the above
reaction produced specimens of lower crystallinity, as indicated by the relative peak intensities and the peak
profiles using Rietveld analysis (Figure 1(b)). The XRD pattern also shows a single phase with the strongest d-spacing (001) of 7.2Å, corresponding to brown birnessite. This
reaction is very sensitive to experimental conditions used. Minor modification
of the original procedure, that is, slow addition of HCl and with no cold aging
time, results in the formation of
nanosized crystals mixed with large crystals [29, 30]. This may be due to a rapid
and unrestricted reaction between Mn(III) and Mn(IV), which are formed by the oxidation of Mn(II) by
permanganate at low pH. The oxidation of Mn(II) by MnO4− is reported to be autocatalytic [16],
and can be influenced by the increased concentration of Mn(II) in solution from
(3). The repeated distances (d-spacing)
of 7.23Å and 7.20Å for both black and brown birnessites suggest that there is at least one
water layer present in the oxides with the former being a little more
hygroscopic.


## 3.3. Tem and Hrtem Studies

The nanostructures of both black and brown
birnessites are studied using
TEM. Figure2 shows the TEM images and petal-like distribution obtained for
nanosized black birnessite. The sheets and petal-like architecture are shown in Figures 2(a) and 2(b). High-resolution transmission electron microscopy (HRTEM)
images, shown in Figures 2(c) and 2(d), display the crystal structure of the
fibrils (010) direction. Figure 2(c) shows the edge of a sheet suggesting that
the sheets fold in a dough-like manner,
producing a plant stem-like appearance. This
micrograph also shows a defect in the arrangement as petiole (protuberance)
from the stem. The high-resolution image in Figure 2(d) shows lattice fringes
of the fibrils with a translation of 7-8Å which
correspond well with (001) lattice plane determined by XRD.Transmission electron microscopy (a) and (b) and high-resolution
transmission electron microscopy (c) and (d) images of the black birnessite
nanomaterial synthesized by our method.
(a)(b)(c)(d)The manganese oxides synthesized by the new method show a petal-like and
fibrous morphology of birnessite-type materials (Figures2(a)–2(c)). The TEM
reveals that the length of the clusters appears to be in the 300 nm range and
the width in the order of 200 nm,
suggesting nanosized clusters. The TEM also shows that curled architectures are produced at the top of the petals.
Formation of these clusters appears to depend on the cold-aging time and the temperature of lyophilization.Figure3
shows the TEM images and fiber distribution obtained for brown birnessite. The fiber architecture is represented
in Figures 3(a) and 3(b). Nanosized clusters can be observed with some twisting
in a coil-like manner. The HRTEM images shown in Figures 3(c) and 3(d) also suggest
an orientation of some of the fibers in (010) direction. However, the dominant
lattice fringes in the high-resolution images as shown indicate a repetition of
the (001) lattice plane (Figures 3(e) and 3(f)).Transmission electron microscopy (a) and (b) and high-resolution
transmission electron microscopy (c)–(f) images of the brown birnessite
produced by the McKenzie method.
(a)(b)(c)(d)(e)(f)In the case of brown birnessite nanoclusters,
the TEM images show a dendritic morphology in the 100 nm range, as shown in Figures3(a) and 3(b), which can be compared with echinate spines similar to the dandelion pollen grains, made from other inorganic materials reported by Hall et al. [30].
However, in this material, the dendrites are covered with a veil which can partly
be seen in the circled region in Figure 3(d) as single sheet. At 20 nm length
scale, sheets and coil-like morphologies are
also detected, and the veil, seen
at 100 nm, becomes more apparent,
suggesting that it is composed of
a small single sheet. It is interesting to learn that the dendritic
appearance observed in this image is actually caused by the folding of small
sheets. Figures 3(a), 3(b), and 1 also suggest that the nanoclusters of the birnessite appear crystalline and
link together to form larger clusters with no apparent crystallographic
orientation. To accurately characterize birnessite-type material, a combination
of complementary analytical tools like HRTEM and XRD are essential.HRTEM studies show that most of the black birnessite nanofibers are oriented in the (010)
direction (zone-axis direction),
and the lattice fringes reflecting the (001) lattice plane become visible in
Figures2(c) and 2(d). At 1 000 000-fold
magnification, the veins along the radial direction of the fiber are clearly
seen, indicative of layered
structure (Figure 2(d)). Additionally, some imperfections in the arrangement of
the fibers can also be seen in Figure 2(c). These imperfections suggest that
the sheets fold in a dough-like manner,
producing stem-like appearance of a plant. Furthermore, this micrograph also
shows a growth in the arrangement as petiole (protuberance) from the stem. This
result supports the observation that the formation of the petal-like sheets may
be due to the growth of a cluster of fibrous units along the c-axis and we speculate that
the high amount of structure defects of the semicrystalline material produce
peak broadening in the XRD. The development of a petiole is intriguing given
the fact that this kind of growth is seen only in living organisms such as
plants but not in inorganic materials made without templates. This work shows that
the production of the nanoscale fibers seems to be a key step for the formation
of petal-like morphology. 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 [31]
and the unreacted Cl− ion. The
sheets and petal-like architecture thus
can be produced by a growth of the corn rows on the surface of clusters
made of fibrils. The path then would be a branching direction (horizontal)
followed by a growth direction (vertical) of the subunits. The stacking of the
subunits in the vertical direction reaches a point that the units begin to fold
forming the petal-like architecture. 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 in Figures 2(a)–2(c). What is intriguing
is that the petal-like architecture observed in these inorganic nanomaterials is indeed similar to the
structures observed in biogenic manganese oxides [24] and in self-assembled
organic systems [32]. We support the view of Terada et al. [33] that the phenomenon that gives rise
to the observed coil and curved architectures in both inorganic and biological
materials may be related to the flexibility in the development processes of these
morphologies. A distortion in the development of the subunits promotes curved
sheets that tend to decrease their surface area when rolled up. This may be one explanation as to why the surface
area of black birnessite (10.6m2g−1), which
contains larger sheets, is lower than that of the brown birnessite (36.2m2g−1),
which contains much smaller sheets of dendritic morphology. The brown
birnessite produced by the McKenzie method consists of small clusters linked
together to form larger particles of micron size. The method reported here is
highly reproducible if the synthetic parameters such as temperature, stirring
rate, and the speed of the addition of one reactant into another are rigorously
controlled.


## 3.4. Thermal Stability of The Birnessite Nanofibers

The TGA measurements of the synthesized
materials using the new method and
the McKenzie method are shown in Figure4. The TGA profiles for both samples look similar and show that there are two weight losses in both
samples; the first consisting of about 10% in the range of 147–160°C and the second consisting of about 13% for the McKenzie method and
about 14% for the sample prepared using the
new method in the temperature range of 400–446°C. There is a slight gain in
mass observed in the range of 440–475°C. The thermal stability of the
samples seems unaffected by the
wet-aging process.Figure 4
Thermogravimetric analysis (TGA) of the synthetic manganese
oxides.
Black birnessite nanomaterial produced by our method (dashed line), and brown
birnessite produced by the McKenzie method (solid line).The first weight loss of 10% indicated
by the thermogram is due to loss of loosely bound water indicating  the samples are
hygroscopic. The second loss of 13% in the temperature range of 422 to443°C is attributed to chemically bound water
and oxygen from the oxide  [32].
The weight loss of the black birnessite
in this temperature range is slightly higher than the brown birnessite, suggesting that
this material is more hygroscopic and correlates well with a d-spacing of 7.22Å observed in the XRD.
The weight gained in the
temperature range of 443–463°C can be attributed to migration of the Mnlayer to the new vacant sites produced by the partial reduction of Mn4+ to
Mn3+ and subsequent oxidation of Mn3+
[32].


## 3.5. Surface Area

The surface areas estimated for the black and
brown birnessites prepared by the new and the McKenzie method, respectively, are
listed in Table1. The surface area of the black nanobirnessite-type
manganese oxide turns out to be
lower (10.6m2/g) than that
produced by using the McKenzie method. The aging time seems to affect the surface area.Table 1
Surface areas of standard material (Kaolinite) and synthesized nanosized black and brown birnessites.
SampleSurface aream2g−1Kaolinite—SRM lot# 19672-1814.9(±1)K-MnO2 (black birnessite synthesized by the developed method)10.6(±1)K-MnO2 (brown birnessite synthesized by the McKenzie method)36.2(±1)


## 4. Concluding Remarks

In
spite of all the research reported in the literature and in progress,
birnessite structural details, crystal chemistry, and morphological prediction
remain elusive. This is due, in part, to a lack of valid concepts and
principles necessary for reproducible size, surface area, and morphology
between synthetic batches.In
this spirit, a new synthesis method for black birnessite nanoparticles is
introduced.  The method first produces
intermediate brown birnessite via the classical McKenzie method for preparing brown birnessite
with minor modifications. The conversion of the brown to black birnessite is
accomplished by purely physical treatments of wet-aging, freezing, and lyophilization. This new synthesis method is notable at least
for the following two reasons. First, the method generates novel nanostructures
with sheets, spiral, and petal-like morphology. Secondly, while all synthesis or conversion methods for black birnessite
apply heating, the black birnessite here is prepared by cooling and
freezing.



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*Source: 763706-2008-12-21.xml*