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Controllable synthesis of α- and β-MnO2: cationic effect on hydrothermal crystallization
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2008 Nanotechnology 19 225606
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 19 (2008) 225606 (7pp)
doi:10.1088/0957-4484/19/22/225606
Controllable synthesis of α - and β -MnO2:
cationic effect on hydrothermal
crystallization
Xingkang Huang, Dongping Lv, Hongjun Yue, Adel Attia1 and
Yong Yang2
State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry,
Xiamen University, Xiamen 361005, People’s Republic of China
E-mail: [email protected]
Received 2 October 2007, in final form 4 January 2008
Published 25 April 2008
Online at stacks.iop.org/Nano/19/225606
Abstract
α - and β -MnO2 were controllably synthesized by hydrothermally treating amorphous MnO2
obtained via a reaction between Mn2+ and MnO−
4 , and cationic effects on the hydrothermal
crystallization of MnO2 were investigated systematically. The crystallization is believed to
proceed by a dissolution–recrystallization mechanism; i.e. amorphous MnO2 dissolves first
under hydrothermal conditions, then condenses to recrystallize, and the polymorphs formed are
+
significantly affected by added cations such as K+ , NH+
4 and H in the hydrothermal systems.
+
+
The experimental results showed that K /NH4 were in competition with H+ to form
polymorphs of α - and β -MnO2 , i.e., higher relative K+ /NH+
4 concentration favoured α -MnO2 ,
while higher relative H+ concentration favoured β -MnO2 .
and observed the development of nanowires from flower-like
MnO2 particles obtained by the reaction between MnSO4 and
KMnO4 .
In addition, obtaining one polymorph from another was
another strategy; for example, Wei et al [33] successfully
transformed γ -MnO2 into α - and β -MnO2 at 140 and 170 ◦ C,
respectively. Ferreira et al [34] employed hydrothermal
transformation in a dodecylamine aqueous solution, from
δ -MnO2 into MnOOH, which can be subsequently annealed
at 300–500 ◦ C to form β -MnO2 . In addition, Shen et al [26]
reported a transformation from δ -MnO2 into other polymorphs
of MnO2 with (2 × 4), (2 × 3) and (1 × 1) tunnel structure at
pH = 13, 7 and 1, respectively; the pH value is believed to
affect the formation of polymorphs significantly.
Although great efforts have been made on controlling
the crystal type, size and morphology of MnO2 , a complete
understanding of the growth and transformation mechanisms
has still not been achieved. Therefore, some other factors
affecting the formation of polymorphic MnO2 should be
carefully studied. For instance, little attention has focused on
the effects of ions on the hydrothermal crystallization of MnO2 .
In this work, we aimed to synthesize different polymorphs
of MnO2 through a tunable process by adjusting the relative
1. Introduction
Manganese dioxide (MnO2 ) has many polymorphs, such
as α -, β -, γ -, and δ -MnO2 , and it is attracting a lot of
attention because of their applications in batteries [1–8],
ion exchange [9–13], catalysis [14–16] and oxidation
processes [17–19]. The performance of MnO2 is significantly
affected by the crystal type, size, and morphology, which,
therefore, have been investigated with great enthusiasm to be
able to synthesize controllably [20–30]. Polymorphs of MnO2
are significantly dependent on the synthetic conditions such
as reactant concentration, reaction time, temperature, etc. For
example, Wang et al [23] synthesized β -MnO2 hydrothermally
by a reaction between MnSO4 and (NH4 )2 S2 O8 , and obtained
α -MnO2 after the addition of (NH4 )2 SO4 . Via the same
reaction as Wang et al and with the assistance of silver foil
or silver cation catalyst, Li et al [31] prepared α -MnO2 at
room temperature, obtaining β -MnO2 hydrothermally at 80 ◦ C.
In addition, Subramanian et al [32] studied a hydrothermal
formation process of α -MnO2 at 140 ◦ C from 1 to 18 h
1 On leave from: Department of Physical Chemistry, National Research
Centre, El-Tahrir Street, Dokki 12622, Egypt.
2 Author to whom any correspondence should be addressed.
0957-4484/08/225606+07$30.00
1
© 2008 IOP Publishing Ltd Printed in the UK
Nanotechnology 19 (2008) 225606
X Huang et al
Figure 2. XRD patterns of samples synthesized hydrothermally in
the presence of 0, 0.1, 0.23, 0.25 and 0.27 M KCl for (a)–(e),
respectively.
Figure 1. XRD patterns of samples synthesized (a) at room
temperature according to equation (1), (b) obtained based on (a) and
subsequent hydrothermal treatment for 48 h at 180 ◦ C, and (c) after
addition of 4 M KCl based on (a) and subsequent hydrothermal
treatment for 48 h at 180 ◦ C.
(figure 1(b)).
However, after adding 4 M KCl into
the autoclave, we obtained α -MnO2 instead of β -MnO2
(figure 1(c)). These results were in agreement with the
work of Wang et al, in which they prepared β -MnO2
from a reaction between manganese sulfate and ammonium
persulfate; however, after deliberate addition of (NH4 )2 SO4 ,
they obtained α -MnO2 [23]. Since cations (e.g. K+ and NH+
4,
etc) are believed to be able to serve as inorganic templates
for the formation of α -MnO2 [35], we were inspired to
investigate cationic effects on the hydrothermal preparation of
α - and β -MnO2 . Therefore, such effects were subsequently
investigated systematically by varying the KCl concentrations
in the hydrothermal system. The formation of α and β
polymorphs was highly dependent on the concentration of
KCl; for example, at low concentration (from zero to 0.25 M)
we obtained β -MnO2 , while at high relative concentration
(0.27 M), we obtained α -MnO2 , as detected by XRD
(figure 2).
We anticipated that a mixture of α - and β -MnO2 may coexist with a medium concentration of KCl added; however,
XRD did not detect any mixture of α - and β -MnO2 , which
seemed unexpected to us. Since it is well known that the
crystallinity of β -MnO2 is usually much better than that of
α -MnO2 , we can suppose that some of β -MnO2 samples,
for example, those obtained after addition of 0.23–0.25 M
KCl, might contain impurity of α -MnO2 which, however,
was undetectable by XRD due to the low content and poor
crystallinity. Hence, these samples were investigated by SEM
and TEM techniques, to observe the morphology and phase
of the particles, respectively, to confirm exactly the phase of
MnO2 formed.
As seen in figures 3(c)–(e), some nanowires formed after
the addition of 0.23 M KCl, and their content in the samples
increased with concentration of the added KCl. Since a typical
β -MnO2 morphology is cuboid with a diameter of ∼650 nm
and a length of ∼2.7 μm (figure 3(a)), while α -MnO2 is
a nanowire with a diameter of ∼20 nm and a length of
+
concentration of some cations such as K+ , NH+
4 , and H ; and
to understand the effects of these cations on the polymorphism.
2. Experimental details
All the MnO2 were synthesized via a simple redox reaction:
3MnCl2 + 2KMnO4 + 2H2 O → 5MnO2 + 4HCl + 2KCl. (1)
Typically, 0.04 M of KMnO4 aqueous solution was added to
0.06 M of MnCl2 aqueous solution, stirred for 30 min (selective
cationic additives of KCl, NaCl, NH4 Cl and/or HCl were added
when necessary) then transferred into a 100 ml Teflon-lined
stainless steel autoclave, sealed and maintained for 48 h at
180 ◦ C. After being cooled at room temperature, the products
were filtered, washed, and finally dried at 120 ◦ C for 24 h. We
use concentration to describe the added chemicals; this was
calculated based on the final volume (i.e. 60 ml) of suspension
in the autoclave before hydrothermal treatment. All chemicals
used here were of analytical reagent grade.
X-ray diffraction (XRD) measurements were performed
on a PANalytical X’Pert diffractometer with Cu Kα radiation.
High-resolution transmission electron microscopy (HRTEM)
and selected area electron diffraction (SAED) studies were
performed on a Philips-FEI Tecnai 30 microscope. The surface
and morphology of the synthesized MnO2 were observed
with scanning electron microscopy (SEM) on a LEO 1530
microscope.
3. Results and discussion
3.1. Effect of K + concentration
The MnO2 obtained at room temperature, according to
equation (1), was amorphous (figure 1(a)), and it transformed
into well-crystallized β -MnO2 after hydrothermal treatment
2
Nanotechnology 19 (2008) 225606
X Huang et al
Figure 3. Typical SEM images of (a)–(b) β -, (c)–(d) α/β - and (e)–(f) α -MnO2 obtained in the presence of 0, 0.1, 0.23, 0.25, 0.27 and 4 M
KCl, respectively.
Figure 4. TEM image of sample obtained in the presence of 0.23 M KCl, (b)–(c) high-resolution images of nanowires selected randomly
from (a).
several micrometres (figure 3(f)), it could be deduced that such
newly developed nanowires are α -MnO2 . This conclusion was
subsequently confirmed by HRTEM (figure 4(b)). From the
lattice fringe image of a d -spacing of ∼0.70 nm (figure 4),
we could conclude that we might have α and δ polymorphs
while excluding β, γ and other polymorphs of MnO2 . Also,
from figure 4(c), the fringe image of a d -spacing of ∼0.50 nm
can exclude δ -MnO2 , and consequently these nanowires are
confirmed to be α -MnO2 only. These results suggest that we
succeeded in detecting a mixture of α - and β -MnO2 from the
samples obtained in the case of addition of 0.23 and 0.25 M
KCl. The above-mentioned polymorphs of MnO2 obtained at
different conditions are summarized in table 1 (Nos 1–7).
That hydrothermal synthesis produced α -MnO2 instead of
β -MnO2 was believed to be related to an increased reactant
concentration [23]. However, in our hydrothermal conditions,
when the concentration of reactants (equation (1)) increased
stoichiometrically, the products obtained were always β -MnO2
(figure 5; table 1, samples 1 and 12–14). Therefore, we
thought that the role of increased cationic concentration is
the principal one which induced the formation of α -MnO2
and not the increase of reactant concentration as suggested
by Wang et al [23]. In this work, to confirm this thought,
we added KCl deliberately to the reactants (equations (1)) and
we obtained α -MnO2 instead of β -MnO2 . To investigate this
further, we added 4 M NH4 Cl or NaCl instead of KCl before
the hydrothermal treatment, and the results showed that α and β -MnO2 were obtained in the case of 4 M NH4 Cl and
NaCl, respectively (table 1, samples 10 and 11). The lack of
formation of α -MnO2 on addition of NaCl makes us conclude
that cations rather than anions were responsible for forming
polymorphs of MnO2 since all the Cl− concentrations were
same in the three cases of adding 4 M KCl, NH4 Cl or NaCl.
Meanwhile, there are only two reports concerning
hydrothermal synthesis of α -MnO2 based on sodium
salts [28, 35]. However, it was formed in completely different experimental conditions, and the formation of α -MnO2
might be assisted by other cations such as Cr3+ [28], while the
other work [35] used Na+ to affect the transformation of polymorphic MnO2 ; α -MnO2 was successfully transformed from
δ -MnO2 obtained on the basis of sodium salt. However, this
α -MnO2 was obtained in a poor crystallized form, and the
(200) x-ray diffraction peak at ∼18◦ , which is one of the important peaks to index α -MnO2 , was absent.
3
Nanotechnology 19 (2008) 225606
X Huang et al
Table 1. Reaction conditions and assigned phases of MnO2 after hydrothermal treatment.
Reactants (M)
Additives (M)
No.
KMnO4
MnCl2
KCl
NaCl
NH4 Cl
HCl
MnO2 phases
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.01
0.02
0.08
0.1
0.4
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.015
0.03
0.12
0.4
0.6
0
0.1
0.23
0.25
0.27
0.5
4
0.5
0.5
0
0
0
0
0
4
4
0
0
0
0
0
0
0
0
0
0
4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
4
0
0
0
0
0
0
0
0
0
0
0
0
0
pH = 0.8
pH = 0.5
0
0
0
0
0
0
0
β
β
β/α a
β/α a
α
α
α
α/β b
β/α a
α
β
β
β
β
β/α c
β/α c
The β polymorph was the main phase while α was undetectable by XRD but
observed by SEM and TEM.
b
The α polymorph was the main phase while β was present as the minor phase as
detected by XRD.
c
The β polymorph was the main phase while α was present as the minor phase as
detected by XRD.
a
3.2. Discussion on formation mechanism of polymorphic
MnO2
Because of the difficulty in directly observing a hydrothermal
growth process of MnO2 , its growth mechanism is still
uncertain at present, although great efforts have been
made [26, 33, 36, 38]. For instance, Wang et al suggested that
the formation of α - and β -MnO2 undergoes a similar process
to a rolling mechanism and phase transformation from δ -MnO2
as an intermediate [36]. However, Wei et al [33] disagreed with
this proposal and believed it proceeded through a condensation
process. In our experimental results, the formation mechanism
of crystalline α - and β -MnO2 should agree with a condensation
reaction process because the samples, before hydrothermal
treatment, were amorphous and sphere-like with an average
diameter between 200 and 400 nm; and the amorphous MnO2
should dissolve first under hydrothermal conditions and then
condense again to recrystallize into β - and α -MnO2 with
diameters of ∼650 nm and ∼20 nm, respectively (figures 3(a)
and (f)). The polymorphs formed were greatly influenced by
cationic concentrations during the recrystallization processes.
Since β -MnO2 is well known as the thermodynamically most
stable structure of all the polymorphs of MnO2 , the formation
of α -MnO2 by addition of K+ means that the presence of
K+ may change their relative kinetic rates of crystal growth.
It is expected that higher K+ concentration means greater
chance for K+ to occupy the kink and step sites as favoured
growth places, and to serve as templates for the formation of
α -MnO2 . On the other hand, provided that MnO2 crystal is
composed of MnO6 octahedral units [1, 2], the formation of
α - and β -MnO2 by a dissolution–recrystallization process is
illustrated schematically in figure 6.
Figure 5. XRD patterns of samples obtained from different reactant
concentrations without addition of KCl: (a) 0.01 M
KMnO4 + 0.015 M MnCl2 , (b) 0.02 M KMnO4 + 0.03 M MnCl2 ,
(c) 0.04 M KMnO4 + 0.06 M MnCl2 , and (d) 0.08 M
KMnO4 + 0.12 M MnCl2 .
In contrast, it has been reported that α -MnO2 is easily
formed based on K+ and NH+
4 as both cations are believed to
stabilize the (2 × 2) tunnel structure of α -MnO2 [6, 32, 35–37]
The effective ionic radius of hydrated K+ (137 pm) is close
+
to that of NH+
4 (148 pm), while being larger than that of Na
(99 pm). The large size of the cations leads to the creation of
α -MnO2 successfully while a relatively small size (such as that
of Na+ ) was unsuccessful for the formation of α -MnO2 .
4
Nanotechnology 19 (2008) 225606
X Huang et al
Figure 6. Schematic illustration of crystal growth of α - and β -MnO2
from amorphous MnO2 , and the effects of K+ and H+ on this
process.
Figure 7. XRD patterns of samples synthesized hydrothermally
(a) in the presence of 0.5 M KCl, (b) and (c) by adjusting the pH to
0.8 and 0.5 based on (a), respectively.
According to a mechanism proposed by Shen et al
[26] to explain the δ -MnO2 transformation into other MnO2
polymorphs with (2 × 4) and (2 × 3) tunnel structure, MnO6
units in the layer sheets of δ -MnO2 tend to collapse from the
layer and then squeeze out the interlayer water molecules to
form a tunnel structure. In their report, the bond between
Na+ and hydrated H2 O molecules decreased with the decrease
in pH value and (2 × 4) and (2 × 3) tunnel structures,
consequently, formed at pH = 13 and 7, respectively. This
mechanism explaining the transformation from δ -MnO2 to
other polymorphs is reasonable; however, in our work, when
we try to apply this mechanism to explain the formation of the
(2 × 2) tunnel structure of α -MnO2 from amorphous MnO2 ,
it is not in complete agreement, as we obtained both α - and
β -MnO2 at the same pH (∼1) (table 1, samples 1–7), and the
bond between K+ and hydrated H2 O molecules, consequently,
should be the same under both conditions. The formation of
α -MnO2 only occurs with the addition of KCl.
The Shen mechanism explains the in situ transformation
from crystalline δ -MnO2 while ours explains the dissolution–
In
recrystallization process from amorphous MnO2 .
our dissolution–recrystallization mechanism, the added K+
significantly affected the hydrothermal growth of crystalline
MnO2 . Higher K+ concentration means more chance for
K+ to serve as the template for the (2 × 2) tunnel structure
(figure 6), which explains well why the α -MnO2 content in
the samples increased with the increased added amount of KCl
(figures 6(c)–(e)).
Figure 8. SEM images of samples obtained in the presence of 0.5 M
KCl and at (a) pH = 0.8 and (b) pH = 0.5.
3.3. Effect of H + concentration
The role of K+ in the hydrothermal crystallization of MnO2 ,
as discussed above, raises an important question: what is the
role of the KCl produced (equation (1)) in the formation of
β -MnO2 , since α -MnO2 is only obtainable after the addition
of KCl? Recalling equation (1), we found that HCl is formed
relative to KCl in the ratio of 2:1. This encouraged us to look
further at the role of H+ and its relation to K+ during the
formation of the MnO2 polymorphs.
We studied the H+ effect by adjusting the pH value,
based on a synthetic experimental condition in which we could
obtain α -MnO2 in the case of addition of 0.5 M KCl (pH
∼1). After decreasing the pH to 0.8 by deliberate addition of
HCl, β -MnO2 was observed again, although α -MnO2 was still
obtained as the main phase as detected by XRD (figure 7(b)),
which was also supported by SEM observation (figure 8(a)).
Further lowering the pH of the above system to ∼0.5 seemed to
result in a pure phase of β -MnO2 as observed by XRD analysis
(figure 7(c)); however, there were still some impurities of
5
Nanotechnology 19 (2008) 225606
X Huang et al
Figure 9. XRD patterns of samples obtained from different reactant
concentrations in the presence of 4 M KCl: (a) 0.04 M
KMnO4 + 0.06 M MnCl2 , (b) 0.1 M KMnO4 + 0.15 M MnCl2 , and
(c) 0.4 M KMnO4 + 0.6 M MnCl2 ; the asterisks present the two
characteristic reflection peaks of α -MnO2 .
Figure 10. (a) TEM image of β -MnO2 observed in the absence of
KCl, (b) its lattice fringe image, and (c) its corresponding SAED
pattern.
α -MnO2 as suggested by the SEM image (figure 8(b)). Further
attempts to obtain pure β -MnO2 by lowering the pH below
0.5 were unsuccessful, because this resulted in the dissolution
of the MnO2 in such high acidic medium. If dissolution of
MnO2 is not taken into account, it can be concluded that the
greatest H+ concentration helps to obtain pure β -MnO2 , which
is comparable to the highest K+ concentration applied to form
α -MnO2 . Hence, we emphasize the effect of H+ concentration
instead of pH value on the formation of MnO2 polymorphs.
The role of H+ is believed to impede the template action
+
of K in the formation of α -MnO2 . Although there is no direct
evidence that has indicated that H+ is necessary for template
formation of β -MnO2 , higher relative H+ concentration means
that K+ ions have a smaller chance to stay on kink or step
sites during the crystalline MnO2 growth process; also, it
can be deemed that K+ ions find it more difficult to occupy
tunnel sites to serve as templates for forming the (2 × 2)
tunnel structure of α -MnO2 . In fact, crystalline α -MnO2 can
be transformed hydrothermally to β -MnO2 in HCl aqueous
solution (not shown here). As a result, it is believed that
+
K+ /NH+
4 and H are in competition during the hydrothermal
crystallization of α - and β -MnO2 . This competition relation
is also supported by the other experiments where, although
both in the presence of 4 M KCl, α -MnO2 and β -MnO2 (main
phase) were obtained at low and high reactant concentrations,
respectively (figure 9; table 1, samples 15 and 16). According
to equation (1), the concentration of H+ increases to twice
that of K+ . As a result, greater reactant concentration means
higher H+ concentration, which lowers the chance of K+ to
serve as a template for α -MnO2 and consequently leads to
the formation of β -MnO2 via the dissolution–recrystallization
mechanism discussed above (figure 6). Note also that at the
calculated pH value of ∼0.1, we obtained a mixture of α and β -MnO2 (the latter as main phase) (table 1, sample 16),
which is apparently in disagreement with obtaining no product
when, in presence of 0.5 M KCl, we tried to lower the pH
to 0.1 to form a pure phase of β -MnO2 as mentioned above.
However, in the case of sample 16, a strong pungent, irritative
Cl2 -like smell was noticed when the seal of the autoclave bomb
was opened, which suggested that the MnO2 dissolution also
occurred, but not completely since the released Cl2 gas in the
sealed autoclave could re-oxidise Mn2+ to MnO2 . As a result
of such processes of dissolution and re-formation of MnO2 ,
both MnO2 precipitate and Cl2 gas were observed. On the
other hand, only 4.8 g MnO2 , 55% of the theoretical amount
of product was obtained in the case of sample 16.
In addition, it is worth mentioning that the cations
can have a dual effect in changing the polymorphs and
simultaneously drastically affecting the morphology of the
obtained particles of MnO2 . For example, in the absence
of KCl the β -MnO2 obtained exhibited a preferred growth
orientation along the [110] direction, as indicated by HRTEM
observation (figure 10). Note that we selected a small cuboid
rod to obtain a clear lattice image of β -MnO2 ; its HRTEM
and SAED patten indicated that this β -MnO2 was highly
crystalline, taking an orientation of a preferred [110] direction
which is in good agreement with the XRD pattern for the
bulk β -MnO2 (figure 1(b)). However, comparison of SEM
images between the two samples obtained in absence of
KCl and in the presence of 0.1 M KCl suggested that the
added KCl inhibited the preferred growth orientation along
the [110] direction of β -MnO2 (figures 3(a) and (b)), which
are also supported by their XRD characterization (figures 2(a)
and (b)). More details concerning the effect of different cations
on the morphologies of polymorphic MnO2 are still under
investigation.
6
Nanotechnology 19 (2008) 225606
X Huang et al
4. Conclusions
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α - and β -MnO2 were controllably synthesized by hydrothermally treating amorphous MnO2 obtained via a redox reaction between Mn2+ and MnO−
4 , and the effects of some cations
+
(K+ , NH+
,
and
H
etc)
on
the
hydrothermal crystallization of
4
MnO2 were studied systematically. The crystallization is believed to undergo a dissolution–recrystallization mechanism;
i.e., amorphous MnO2 dissolved first and then condensed to recrystallize under hydrothermal conditions, and the polymorphs
formed were significantly affected by these cation concentrations in the system. The opposite effects between K+ /NH+
4
and H+ , favouring α - and β -MnO2 , respectively, were investigated systematically for the first time. The experimental results
suggested that the formation of α - and β -MnO2 was highly
+
dependent on the relative concentration of K+ /NH+
4 and H ;
actually, they are in competition for the formation of α - and
β -MnO2 polymorphs, i.e., higher relative K+ or NH+
4 concentration favours α -MnO2 , while higher relative H+ concentration favours β -MnO2 .
Acknowledgments
Financial support from the National Basic Research Program
of China (973 Program) (Grant No 2007CB209702) and
the National Natural Science Foundation of China (Grant
nos 90606015 and 20473060) is gratefully acknowledged.
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