Hydrothermal synthesis of γ-MnOOH nanorods and their conversion to MnO2, Mn2O3, and Mn3O4 nanorods

Article abstract of DOI:10.1016/j.jallcom.2015.05.078

Single crystal γ-MnOOH nanorods were successfully prepared by employing a hydrothermal process based on the direct reaction between potassium permanganate (KMnO₄) and ethylene glycol (EG). It was found that the reaction time and temperature play important roles in the structural and morphological modifications. Our results show that the γ-MnOOH nanorods are formed through a lamellar growth, assembly and ripening process. Moreover, MnO₂, Mn₂O₃, and Mn₃O₄ nanorods were obtained by the subsequent calcination of γ-MnOOH precursors under different temperatures and atmospheres. These one-dimensional manganese oxide and oxyhydroxide nanorods are expected to have possible novel properties and wide applications.

Full text of DOI:10.1016/j.jallcom.2015.05.078

Accepted Manuscript
Hydrothermal synthesis of γ-MnOOH nanorods and their conversion to MnO2,
Mn O , and Mn O nanorods
2
3
3
4
Leilei Lan, Quanjun Li, Guangrui Gu, Huafang Zhang, Bingbing Liu
PII:
S0925-8388(15)01371-7
DOI:
http://dx.doi.org/10.1016/j.jallcom.2015.05.078
JALCOM 34194
Reference:
To appear in:
Journal of Alloys and Compounds
Received Date:
Revised Date:
Accepted Date:
17 March 2015
30 April 2015
11 May 2015
Please cite this article as: L. Lan, Q. Li, G. Gu, H. Zhang, B. Liu, Hydrothermal synthesis of γ-MnOOH nanorods
and their conversion to MnO , Mn O , and Mn O nanorods, Journal of Alloys and Compounds (2015), doi: http://
2
2
3
3
4
dx.doi.org/10.1016/j.jallcom.2015.05.078
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Hydrothermal synthesis of γ-MnOOH nanorods and their conversion to
MnO , Mn O , and Mn O nanorods
2
2
3
3
4
a,b
b
a
b
b
Leilei Lan , Quanjun Li* , Guangrui Gu* , Huafang Zhang , and Bingbing Liu
a
Department of Physics, College of Science, Yanbian University, Yanji 133002, P. R.
China
b
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R.
China
*
Corresponding author email: liquanjun@jlu.edu.cn (Q. Li), grgu@ybu.edu.cn (G. Gu).
Abstract
Single crystal γ-MnOOH nanorods were successfully prepared by employing a
hydrothermal process based on the direct reaction between potassium permanganate
(
4
KMnO ) and ethylene glycol (EG). It was found that the reaction time and temperature
play important roles in the structural and morphological modifications. Our results show
that the γ-MnOOH nanorods are formed through a lamellar growth, assembly and
ripening process. Moreover, MnO
2
, Mn
2
O
3
, and Mn
3
4
O nanorods were obtained by the
subsequent calcination of γ-MnOOH precursors under different temperatures and
atmospheres. These one-dimensional manganese oxide and oxyhydroxide nanorods are
expected to have possible novel properties and wide applications.
Keywords: Hydrothermal synthesis; Manganese oxides; Thermal conversion;
Nanostructures.
1

1
. Introduction
Manganese oxides and oxyhydroxides have attracted considerable interest due to their
outstanding structural flexibility combined with unique physical and chemical properties,
which provide wide-spread applications in catalysts, batteries, magnetic materials, ion
exchange, etc [1-5]. A large number of manganese oxides are available owing to the
different oxidation states of manganese (II, III, and IV). Among the series of manganese
oxides, the most commonly known manganese oxides MnO
2
, Mn
2
O
3
, and Mn
3
4
O have a
range of applications. MnO is a favorable material for electrochemical supercapacitors
2
because of its satisfactory charge storage performance and natural abundance [6,7]. It
.
also can be used in neutral aqueous electrolytes, unlike NiOOH and RuO
2
xH O, which
2
only can be used in alkaline or strong acidic electrolytes, thus causing serious
environmental problems [8,9]. Polymorphs of Mn have proved to be inexpensive,
2
O
3
environment-friendly catalysts for oxidation of organic pollutants and decomposition of
nitrogen oxides from waste gases [10,11]. On the other hand, the high disinfection
capacity for bacteria and marked cytotoxic effects also make Mn
2
3
O becomes an
alternative material for the development of new bactericides [12]. As a promising
cataluminescence sensing material with inexpensive and great environmental
compatibility, Mn
compounds [13]. Mn
reduction reactions [14], what is more important is that the catalytic applications of
3
O
4
shows excellent sensing characteristics for volatile organic
3
O
4
is also known to be an effective catalyst in various oxidation and
2

Mn
3
4
O have been extended to the combustion of organic compounds at temperatures of
the order of 373-773 K [15]. For MnOOH, it can be used as the precursor to prepare
intercalation compounds and lithium manganese oxides for rechargeable lithium batteries
[
16]. Furthermore, MnOOH shows extraordinary adsorption capacities for scavenging
numerous trace elements form aquatic environments [17].
In recent years, with the development of nanomaterials, morphologies and sizes of the
nanomaterials have been regarded as critical factors that may bring some novel and
unexpected properties [18-20]. Therefore, many studies have been done to develop
various methods for the synthesis of manganese oxide nanostructures. Up to now,
manganese oxide nanostructures in the shapes of nanorods/nanowires [21,22],
nanoparticles [23], nanotubes [24], nanoflowers [25,26], nanobundles [27], nanosheets
[
28], and other structures [29,30] have been synthesized. Such as, the needle-like MnO
nanorods with excellent pseudocapacitive behavior have been prepared by a sol-gel
method [31]. Li et al. synthesized Mn nanocrystals with various sizes and shapes by a
hydrothermal route [32]. In addition, the Mn nanotubes/nanofibers were prepared by
2
3
O
4
2
O
3
liquid-phase catalysis method [33]. Generally, those methods lead to only one oxidation
state of manganese as the final product. So, it is of great value to generate several
manganese oxides in a controlled way. Of late, a two-step approach has attracted
considerable interest, which provides an effective approach to obtain manganese oxides
with different oxidation states. This method comprises two steps: the first stage is
3

synthesize the solid precursors containing manganese, the second stage is heat treatment
of the precursors under different annealing parameters (i.e., atmosphere, time and
temperature), which can make the precursors convert to various manganese oxide
nanostructures. It not only can easily control the morphologies and oxidation states of
manganese by changing thermal treatment conditions, but also can show the relationship
between structures of the precursors and the target products clearly. At present, the
MnCO
3
[34,35], manganese oxalate [36], manganese oxyhydroxide [37], manganese
[39] have been demonstrated to be effective precursors to
acetate [38], and MnO
2
synthesize manganese oxides. However, there are few people reported the formation
mechanism of single-crystal γ-MnOOH nanorods and further study on their thermal
conversion into various manganese oxide nanorods is needed.
In this paper, we report a facile hydrothermal method to synthesize γ-MnOOH
nanorods by using potassium permanganate (KMnO ) and ethylene glycol (EG). The
4
structures and morphologies of the γ-MnOOH nanorods are characterized, and
corresponding formation mechanism is discussed. We also found that the reaction time
and temperature play important roles in the formation of the γ-MnOOH nanorods. The
tetragonal phase MnO
2
, cubic phase Mn
2
O
3
, and tetragonal phase Mn
3
4
O nanorods were
obtained by the annealing treatment of the γ-MnOOH precursors at different atmospheres
and temperatures.
2
. Experimental section
4

All the reagents in this study were of analytical grade and used without further
purification. In a typical synthesis, 0.75 mmol KMnO and 0.6 mL ethylene glycol (EG)
4
were dispersed in 44 mL distilled water. The mixed solution was stirred for 5 h and then
transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed
o
and heated at 120 C for 10 h, then cooled to room temperature naturally. The products
were collected by centrifugation and washed several times with absolute ethanol and
distilled water to remove the excessive reactants and byproducts, then finally dried in a
o
vacuum at 50 C for 12 h. Various manganese oxides could be obtained by calcining the
precursors under different conditions. MnO
2
nanorods were prepared by treating the
and Mn nanorods were obtained by
o
γ-MnOOH at 400 C for 4 h in air. Mn
2
O
3
3
O
4
o
calcining the γ-MnOOH at 700 C for 5 h in air and argon atmosphere, respectively.
X-ray diffraction (XRD) patterns were collected on a Rigaku D/max-RA X-ray
diffractometer using Cu Ka radiation (λ = 0.15406 nm). The Raman measurements were
carried out using a Raman spectrometer (Renishaw inVia) with a 633 nm He-Ne laser line
as excitation. Transmission electron microscopy (TEM) images, high-resolution
transmission electron microscopy (HRTEM) images, selected area electron diffraction
(
SAED) patterns and energy disperse spectroscopy (EDS) images were collected with a
JEM-2200FS transmission electron microscope performed at 200 kV. The samples for
TEM observation by dispersing the solid powder in ethanol ultrasonically and then
dropping the solution onto carbon coated copper grids. Thermal gravimetric and
5

differential thermal analysis (TG-DTA) were carried out on a Linseis STA PT1600
o
-1
thermal analyzer apparatus with a heating rate of 10 C min under air and argon
atmosphere, respectively.
3
. Results and Discussion
XRD patterns for the products synthesized at 120 C for 10 h are shown in Fig. 1. All
the diffraction peaks can be indexed to the monoclinic phase of γ-MnOOH, which are
consistent with the standard data from JCPDS card NO. 41-1379 (space group P2 /c14; a
0.5300 nm, b = 0.5278 nm, c = 0.5307 nm). Moreover, there are no diffraction peaks
o
1
=
corresponding to impurities, indicating the high purity of the as-obtained products. The
samples were also analyzed by Raman spectra, as shown in Fig. 2. Nine Raman-active
-1
modes at 218, 258, 282, 360, 389, 492, 530, 558, and 623 cm are observed, which
match well to the previous studies on γ-MnOOH [40,41]. These results demonstrate
success in synthesizing the γ-MnOOH through a simple, low-temperature, and
hydrothermal synthesis route.
o
The morphologies and microstructures of products synthesized at 120 C for 10 h
were further examined by TEM and HRTEM. From Fig. 3(a) and (b), we could see the
products have rod-like morphologies, and these nanorods have an average diameter of
3
0-120 nm and length of 1-6 micrometers. Fig. 3(c) shows a single nanorod and the
corresponding SAED pattern. The SAED pattern demonstrates that the nanorod is a
single crystal with a growth direction of [101]. Fig. 3(d) shows the HRTEM image of Fig.
6

3
(c). The measured spacings of the lattice planes are 0.252 nm and 0.264 nm, which are
consistent with the (111) and (020) planes of γ-MnOOH, respectively. The HRTEM
analysis further indicates that the high quality γ-MnOOH naorods are prepared
successfully.
To further investigate the formation process of the γ-MnOOH nanorods, intermediates
with different hydrothermal reaction times were collected and observed by XRD, TEM,
o
and HRTEM. Fig. 4 shows the XRD patterns of the products prepared at 120 C with
various reaction times. Form Fig. 4, we could see that all the diffraction peaks of XRD
patterns can be indexed to the γ-MnOOH. In the case of products prepared at 2 h, the
peaks are broad and low. There is an increase in crystallinity with increasing the reaction
time from 2 h to 10 h as evidenced by the appearance of sharper peaks. Further increasing
the reaction time to 22 h, the peaks have no obvious change. The XRD analysis indicates
that the well-crystallized γ-MnOOH is finally formed when the reaction time beyond 6 h.
o
Fig. 5 shows a series of TEM and HRTEM images of the products prepared at 120 C
with various reaction times, which reveal the process of the morphological transformation
in detail. From the Fig. 5(a), it can be clearly seen that there are many folds on the
lamellae at the beginning of the synthesis process. According to HRTEM image (the
insert of Fig. 5(a)), these folds have an average size of 2-5 nm in thickness, and they
connect with reticulate structures. As shown in the Fig. 5(b), there is an interesting
change in the morphologies of the samples synthesized at 1.5 h, which displays that part
7

of the previously lamellar structures grow to small nanorods with an average size of 5 nm
in diameter and lengths up to 20-100 nm. Prolong the reaction time to 2 h (Fig. 5(c)),
more and more oriented attachment of the small nanorods are formed. In addition, the
SAED pattern (the inset of Fig. 5(c)) indicates that the products are poorly crystallized.
This result is in agreement with that of XRD (Fig. 4(a)). Further increasing the reaction
time to 3 h (Fig. 5(d)), all the lamellar structures disappeared, some large nanorods are
formed as a result of the oriented assembling of several small nanorods along their side
surfaces, and the products comprise a large number of nanorods with a diameter range of
5
-100 nm and lengths up to several hundreds of nanometers. Prolong the hydrothermal
reaction time to 10 h (Fig. 5(e)), the nanorods have larger diameter with an average size
of 30-120 nm and length of 1-6 micrometers, and the well-crystallized γ-MnOOH
nanorods are formed finally. According to the XRD, TEM, and HRTEM analysis of
samples taken at different stages of hydrothermal reaction, the formation of the
γ-MnOOH nanorods can be divided into several steps. At the beginning of the reaction,
γ-MnOOH nuclei are formed as a sequence of the oxidation-reduction reaction between
KMnO
KMnO
4
4
and EG. This is indicated by the phenomenon that when the EG is mixed with
solutions, the color of the latter change from purple to dark black and precipitate
appeared. At the same time, EG also as a nonionic surfactant, which has multiple
coordination sites and ordered chain-structures, it shows less chelating ability and its
polymerization under basic condition is probable. EG acts as a binding agent between
8

initial γ-MnOOH nuclei and results in lamellar structures [42,43]. The lamellar structures
are in a metastable state at hydrothermal system, and much evidence demonstrates that
lamellar structures, regardless of whether they are artificial or natural, have a strong
tendency to form wires/rods [44,45]. Further prolonging the hydrothermal reaction time,
the growth kinetics of γ-MnOOH nuclei will be modified, which finally leads into the
anisotropic growth of the crystals, and these lamellar structures may redissolve then grow
into small nanorods. These small nanorods are not in thermodynamic equilibrium, so
Ostwald ripening dictates the growth and recrystallization. As a result, the small nanorods
combine with each other, the larger and longer nanorods are formed from the oriented
attachment of small nanorods along their side surfaces thus reducing the surface energy.
Finally, the well-crystallized and uniform γ-MnOOH nanorods are formed. The schematic
illustration of the growth process for γ-MnOOH nanorods is shown in Fig. 5(f).
In order to future study the effects of the reaction temperature on the final products, a
series of experiments with different reaction temperatures were carried out (the reaction
o
o
time (10 h) was kept constant). When the reaction temperature is 120 C and 150 C, the
XRD patterns show that all diffraction peaks can be perfectly indexed to the γ-MnOOH
o
(
Fig. 6(a) and (b)). Further increasing the reaction temperature to 180 C (Fig. 6(c)), it is
observed that the XRD patterns of the products exhibit a mixed phase of both Mn
JCPDS card NO. 80-0382, marked by #) and γ-MnOOH (JCPDS card NO. 41-1379,
marked by *). Fig. 7 shows the TEM and EDS images of the products synthesized at
3
O
4
(
9

different reaction temperatures. Fig. 7(a) and (b) show the TEM images of products
o
o
obtained at 120 C and 150 C, respectively, in which the nanorods are observed.
However, the mixture rod-like and particle-like structures are observed in the samples
o
synthesized at 180 C (Fig. 7(c)). Fig. 7(d) presents the EDS pattern of the particle-like
o
structures synthesized at 180 C. In the EDS pattern, Mn, O, C and Cu elements are
detected. Among them, the signals for Cu and C elements are derive from the carbon
coated copper grid. The atomic ration of manganese to oxygen is 0.734, which is very
close to 3:4. It is consistent with the stoichiometry of Mn
products of particle-like structures are mainly Mn . These results indicate that the
γ-MnOOH nanorods can be prepared at low temperature, while coexistence of Mn
particle-like structures with γ-MnOOH can be observed in the final products at higher
reaction temperature. At low reaction temperature, the KMnO is reduced by EG with a
3
4
O , which demonstrates that the
3
O
4
3
O
4
4
slow reaction rate then γ-MnOOH nanorods are formed. The reduction, nucleation and
growth are all accelerated with increasing reaction temperature, smaller nanorods
dissolve progressively to supply for the growth of the large ones, particle-like crystals are
formed. At the same time, some of γ-MnOOH will dehydrate to produce Mn
nanoparticles. This process is similar to the morphological conversion of the Fe and
BiPO structures [46,47]. The results indicate that the reaction temperature also has
certain influence on the formation of γ-MnOOH.
It is well-known that metal hydroxides can convert to the corresponding metal oxides
3
O
4
2
O
3
4
1
0

by dehydration under thermal annealing [48,49]. The annealing parameters (i.e.,
atmosphere, time and temperature) are fundamental to control the oxidation states and
morphologies of the final products. Therefore, we studied the thermal-annealing behavior
of γ-MnOOH under air and argon atmosphere, respectively. The TG-DTA curves of
γ-MnOOH under air and argon atmosphere are shown in Fig. 8. In Fig. 8(a), the TG curve
shows three weight loss steps under air. The first weight loss (2.9 %) can be attributed to
the removal of adsorbed water from the nanorods. The second weight loss is associate to
3
+
4+
the dehydroxylation of the γ-MnOOH followed by oxidation of Mn to Mn with
formation of MnO , the weight loss of corresponding parts is 1.3 %, which is close to the
theoretical value of 1.1 %. The third (10.2 %) weight loss can be attributed to the MnO
change into Mn , and theoretical value of weight loss is 9.1 %. In the DTA curve, the
endothermic peak at 578 C is corresponding to the transformation point of MnO
converts to Mn . However, the TG-DTA curves under argon atmosphere are different.
2
2
2
O
3
o
2
2
O
3
As shown in Fig. 8(b), the TG curve shows two weight loss steps under argon atmosphere.
The first weight loss (1.0 %) is associated to the removal of adsorbed water. The second
weight loss is 13.1 %, which may be γ-MnOOH decomposes into Mn
3
4
O , the weight loss
is close to the theoretical value of 13.3 %. In the DTA curve, the endothermic peak at 580
o
C is corresponding to the transformation point of γ-MnOOH converts to Mn
3
O
4.
In order to obtain various manganese oxide nanostructures, we calcined the
γ-MnOOH precursors under different conditions. Fig. 9 shows the XRD patterns of the
11

products prepared by calcining the γ-MnOOH precursors under different conditions. In
Fig. 9(a), all diffraction peaks of the products prepared by calcining the γ-MnOOH
o
precursors at 400 C for 4 h in air can be indexed to tetragonal phase MnO
2
, which are in
good agreement with standard data (JCPDS card NO. 72-1984, marked by *). From Fig.
(b) and (c), we could see that Mn (JCPDS card NO. 71-0636, marked by #) and
Mn (JCPDS card NO. 80-0382, marked by +) were obtained by calcining the
9
2
O
3
3
O
4
o
γ-MnOOH at 700 C for 5 h under air and argon, respectively.
The Raman spectroscopy affords useful supplements to XRD for the transformation
from γ-MnOOH to MnO
spectra of MnO show one main peak at 665 cm , which is attributed to A1g mode and
regarded as the signature for identifying MnO [41]. As shown in Fig. 10(b), three
Raman-active modes of Mn at 305, 396, and 690 cm are observed, this result
matches fairly well to the previous study on Mn [50]. Fig. 10(c) exhibits the Raman
spectra of Mn , in which three peaks at 320, 375, and 657 cm are observed, in
agreement with literature data for Mn [51].
2
, Mn
2
O
3
, and Mn
3
O
4
. As shown in Fig. 10(a), the Raman
-
1
2
2
-1
2
O
3
2
O
3
-1
3
O
4
3
O
4
According to our experimental results, the possible chemical reactions can be
formulated as follows:
o
Air/400 C
4
2
1
γ-MnOOH + O
2
 4MnO
2
+ 2H
2
O
(1)
(2)
(3)
o
Air/700 C
γ-MnOOH  Mn
2
O
3
+ H
2
O
o
Ar/700 C
2γ-MnOOH   4Mn
3
O
4
+ 6H
2
O + O
2
1
2

The morphologies and microstructures of these manganese oxides by calcining the
γ-MnOOH precursors were further investigated by TEM and HRTEM (Fig. 11). It can be
clearly seen from these TEM images that the obtained manganese oxides maintain the
rod-like morphologies of the γ-MnOOH precursors. These MnO
2
, Mn
2
O
3
, and Mn
3
O
4
nanorods have a diameter range of 30-120 nm, 40-130 nm, 40-130 nm, and lengths of 1-5
μm, 1-3 μm, and 1-3μm, respectively. To get more structural information about these
manganese oxide nanorods, the products are observed with HRTEM and shown in Fig.11
(
c), (f) and (i). From the Fig.11 (c), (f) and (i), it is observed that the interplanar distance
of the lattice fringes is 0.310 nm, 0.272 nm, and 0.492 nm, corresponding to the (110)
planes of MnO , (222) planes of Mn , and (101) planes of Mn , respectively. This
2
2
O
3
3
O
4
demonstrates that high quality manganese oxide nanorods could be obtained by calcining
the γ-MnOOH precursors under different conditions.
4
. Conclusions
In this paper, we have developed a facile hydrothermal method for the preparation of
γ-MnOOH nanorods with the use of potassium permanganate (KMnO
glycol (EG) as raw materials. Calcination of the γ-MnOOH under different heat treatment
conditions leads to the formation of MnO , Mn , and Mn nanorods, which indicates
4
) and ethylene
2
2
O
3
3
O
4
that γ-MnOOH is an ideal precursor for further synthesis of various manganese oxide
nanostructures. Based on the TEM results, the formation mechanism of γ-MnOOH
nanorods through a lamellar growth, assembly and ripening process was proposed. We
1
3

also noted that the reaction time and temperature make important effect on the formation
of the γ-MnOOH nanorods. These nanostructured manganese oxides are expected to have
possible novel properties and wide applications.
Acknowledgments
This work was supported financially by the National Basic Research Program of
China (2011CB808200), the NSFC (11374120, 10979001, 51025206, 51032001,
5
1272224, and 21073071), and the Cheung Kong Scholars Programme of China.
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8

Figure captions
o
Fig. 1 XRD patterns of the products synthesized at 120 C for 10 h.
o
Fig. 2 Raman spectra of the products synthesized at 120 C for 10 h.
Fig. 3 TEM and HRTEM images of the γ-MnOOH. (a) Low- and (b) high-magnification
TEM images, (c) the TEM image of a single nanorod and the corresponding SAED
pattern (the inset), and (d) the HRTEM image of (c).
o
Fig. 4 XRD patterns of the as-synthesized products at 120 C with different reaction times:
(
a) 2 h, (b) 3 h, (c) 6 h, (d) 10 h, and (e) 22 h.
o
Fig. 5 TEM and HRTEM images of the products synthesized at 120 C with different
reaction times: (a) 0 h (simply stirring, without hydrothermal process), (b) 1.5 h, (c) 2 h,
(
d) 3 h, and (e) 10 h. (f) Schematic illustration of the growth process for γ-MnOOH
nanorods.
Fig. 6 XRD patterns of the products synthesized at 10 h with different reaction
o
o
o
temperatures: (a) 120 C, (b) 150 C, and (c) 180 C.
Fig. 7 TEM images of the products synthesized at 10 h with different reaction
o
o
o
temperatures: (a) 120 C, (b) 150 C, and (c) 180 C. (d) The EDS pattern of the
o
particle-like nanostructures synthesized at 180 C.
Fig. 8 TG-DTA curves of γ-MnOOH nanorods under (a) air and (b) argon atmosphere.
Fig. 9 XRD patterns of the as-prepared products by calcining the γ-MnOOH precursors
o
o
under different conditions. (a) 400 C for 4 h in air, (a) 700 C for 5 h in air, and (c) 700
1
9

o
C for 5 h in argon.
Fig. 10 Raman spectra of the products by calcining the γ-MnOOH precursors. (a) MnO
b) Mn , and (c) Mn
Fig. 11 TEM and HRTEM images of the products by calcining the γ-MnOOH precursors.
a-c) MnO , (d-f) Mn , and (g-i) Mn
2
,
(
2
O
3
3
O .
4
(
2
2
O
3
3
4
O .
2
0