Controlled synthesis of hierarchical MnO2 microspheres with hollow interiors for the removal of benzene

Article abstract of DOI:10.1039/c4ra01146e

Three types of MnO₂ microspheres, viz., hierarchical hollow β-MnO₂ microspheres consisting of uniform nanorods, hierarchical double-walled hollow β/α-MnO₂ microspheres assembled by two-categorical nanorods, and hierarchica

Full text of DOI:10.1039/c4ra01146e

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Controlled synthesis of hierarchical MnO2
microspheres with hollow interiors for the removal
Cite this: RSC Adv., 2014, 4, 26796
of benzene†
*
Dongyan Li, Jun Yang, Wenxiang Tang, Xiaofeng Wu, Lianqi Wei and Yunfa Chen
Three types of MnO2 microspheres, viz., hierarchical hollow b-MnO2 microspheres consisting of uniform
nanorods, hierarchical double-walled hollow b/a-MnO2 microspheres assembled by two-categorical
nanorods, and hierarchical hollow a-MnO2 microspheres constructed by nanorods and nanowires, have
been synthesized by
a facile hydrothermal method without employing any templates, catalysts,
surfactants or calcinations. A number of characterization techniques, including X-ray diffraction (XRD),
field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS),
transmission electron microscopy (TEM), nitrogen adsorption–desorption measurements, and X-ray
photoelectron spectroscopy (XPS) are used to characterize the formed hierarchical products, and to
address the following critical issues: (i) effect of the precursor concentration; (ii) the role of HCl for the
formation of the hierarchical hollow structure; and (iii) the possible mechanism accounting for the
formation of the hierarchical hollow microspheres. The hierarchical hollow MnO2 microspheres can be
used as catalysts for the catalytic oxidation of benzene, of which the hierarchical hollow a-MnO2
microspheres exhibit the highest catalytic ability.
Received 9th February 2014
Accepted 6th June 2014
DOI: 10.1039/c4ra01146e
www.rsc.org/advances
hierarchical hollow microspheres consisting of NiS nanorods,11
hierarchical hollow Ca10(PO4)6(OH)2 microspheres constructed
by nanorods,12 hierarchical hollow microspheres assembled by
1 Introduction
Hierarchical hollow structures have attracted signicant atten-
tion due to their wide applications in catalysts,1,2 water treat-
ments,3,4 cells,5,6 and drug delivery.7 Hierarchical hollow
structures could combine the advantages of pristine building
blocks with the unique properties of hollow structures. More
importantly, this type of materials might possess new physico-
chemical properties arising from their secondary architecture.8
The morphology, size, and assembling way of the building
blocks have great inuence on the physicochemical properties
and potential applications of the hollow structures. One-
dimensional (1D) nanostructures usually exhibit unique elec-
trical, optical, magnetic, mechanical, and thermal properties,
and can be used as building blocks for the fabrication of
complex structures.9,10 The hollow structures assembled by 1D
nanostructure might be expected to possess unique physico-
chemical properties and to have wide potential applications. To
date, several 1D nanostructure-based hollow structures have
been synthesized by template or self-assemble methods, such as
titanate nanotubes,13 hierarchical MnS hollow sphere consist-
ing of the nanorod arrays,14 hollow nestlike a-Fe2O3 spheres
fabricated by nanorod subunits,15 et al. The conventional
template methods involve the complicate process for the
removal of sacricial templates, which may damage the struc-
tural integrity of the nal products. Hence it is desirable to
develop facile and environmentally friendly methods for the
preparation of hierarchical hollow structures using the self-
assembly of 1D nanostructures.
Manganese oxide has aroused increasing attention because
of its structural exibility, novel physicochemical properties,
and wide applications in catalyst,16,17 electrochemical super-
capacitor,18,19 microwave adsorption,20 and water treatment.21,22
To date, the hierarchical hollow MnO2 structures consisting of
1D nanorods23,24 or 1D nanotubes20,25 have been synthesized.
Studies have shown that the hierarchical MnO2 assembled by
1D nanostructure show excellent performance. For example,
Wang et al.26 reported that the hierarchical hollow a-MnO2
nanostructures with urchin-shape had a specic capacitance of
123 F gꢀ1 with superior rate capability. In addition, the hierar-
chical hollow microspheres composed of MnO2 nanotubes
obtained by them delivered a discharge capacity as high as
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing, China 100190. E-mail: yfchen@mail.ipe.ac.cn;
Fax: +86-10-6254 2803; Tel: +86-10-8254 4896
† Electronic supplementary information (ESI) available: EDS analyses, nitrogen
adsorption–desorption isotherms, pore characterizations, XRD patterns, SEM
images, XPS spectra and results of the nanostructures in this study. See DOI:
10.1039/c4ra01146e
983 mA h gꢀ1 during the 30th cycle at a current density of
300 mA gꢀ1
.
Fu et al.28 obtained the hierarchical MnO2
27
microspheres assembled by nanorods, which have high activity
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˚
(about 72% conversion) for the aerobic oxidation of benzyl radiation source (l ¼ 1.5418 A), which was operated at 40 kV
alcohol to benzaldehyde and high selectivity. However, almost and 30 mA. The morphology of the product was observed by
all of the reported hierarchical hollow MnO2 structures are eld-emission scanning electron microscopy (FESEM, JSM-
single-wall and assembled by one category of 1D nano- 6700F) operated at the accelerating voltage of 10 kV and trans-
structures. Hence, it is signicant to develop simple methods mission electron microscopy (TEM, JEOL JEM-2010) with an
for the fabrication of the hierarchical hollow MnO2 with double- accelerating voltage of 200 kV. Elemental compositions of the
wall or assembled by various categories of 1D nanostructure. sample was examined with energy dispersive spectroscopy
The complexity in morphology and structure maybe endow (EDS) attached to the FESEM. N2 adsorption–desorption
MnO2 new physicochemical properties for more potential isotherms were measured on Autosorb-1, Quantachrome at
applications.
77 K. The X-ray photoelectron spectroscopy (XPS) characteriza-
Herein, we report a facile method, which is based on the tions were carried out on ESCALab220i-XL.
hydrothermal reaction, for the controlled preparation of hierar-
chical hollow MnO2 microspheres consisting of 1D nanostructure.
2.4 Catalytic evaluation
Hierarchical hollow b-MnO2 microspheres consisting of nano-
rods, hierarchical hollow a-MnO2 constructed by nanorods and
nanowires, and hierarchical double-walled hollow b/a-MnO2
microspheres assembled by two-categories of 1D nanostructure
can be obtained by simply varying the concentrations of the
precursors. To the best of our knowledge, the reports on the
preparation of hierarchical hollow a-MnO2 and double-wall hier-
archical hollow b/a-MnO2 microspheres composed by two-cate-
gories of 1D nanostructure are still very limited. In this work, we
aim at addressing the following critical issues: (i) effect of the
precursor concentration; (ii) the role of HCl for the formation of
hierarchical hollow structure; and (iii) the possible mechanism
accounting for the formation of the hierarchical hollow micro-
spheres, upon the detailed characterizations of the hierarchical
products. In addition, the application of the as-prepared hierar-
chical hollow MnO2 microspheres as catalysts for the oxidation of
benzene will also be investigated in this report.
Analogous to the work we reported earlier,29 the catalytic oxidation
of benzene was performed in a quartz tubular xed bed reactor
(internal diameter (i.d.) ¼ 7 mm) at atmospheric pressure. 0.1 g of
catalyst (40–60 mesh) was loaded between two quartz wool plugs.
The reaction mixture consisting of air/benzene (1000 ppm of
benzene) owed at a rate of 100 mL minꢀ1 (weight hourly space
velocity (WHSV), 60 000 mL g ꢀ1 h ꢀ1). The reactions were carried
out in the range of 373–673 K with intervals of 20 K. The products
of the catalytic reaction were analyzed by an online gas chroma-
tography (Agilent 6890) equipped with a thermal conductivity
detector (TCD) and ame ionization detector (FID).
3 Results and discussion
Fig. 1 shows the XRD patterns of the samples prepared by
hydrothermal method with different concentration of precur-
sors. All the diffraction peaks of S1 can be indexed to b-MnO2
2 Experimental
2.1 Materials
˚
˚
(JCPDS 24-735, a ¼ 0.440 A and c ¼ 2.874 A). The XRD pattern of
S3 is in accord with the structure of a-MnO2 (JCPDS 44-0141, a ¼
˚
˚
9.784 A and c ¼ 2.863 A). The features of a-MnO2 (JCPDS
44-0141) and b-MnO2 (JCPDS 24-735) are observed in the
diffraction peaks of S2, indicating that both b and a-MnO2 are
present in the phase of S2. The results suggest that the
Manganese(II) sulfate (MnSO4$H2O 99.0%), potassium persul-
fate (K2S2O8 99.5%), concentrated hydrochloric acid (HCl 36%)
anhydrous ethanol (99.5%) and manganese dioxide (MnO2
85%) from Beijing Chemical Works.
2.2 Synthesis of the hierarchical hollow MnO2
In a typical hydrothermal method, 0.048 mol of MnSO4$H2O and
0.012 mol of K2S2O8 were dissolved in 60 mL of distilled water,
followed by adding 2 mL of HCl (36%) to form a homogeneous
solution, which was then transferred into a 100 mL of Teon
autoclave. The autoclave was sealed and heated at 140 ꢁC for 24 h.
Aer cooling down to room temperature, the obtained precipi-
tates were ltered and washed several times with distilled water
and anhydrous ethanol. The precipitates were dried in an oven at
80 ꢁC for 12 h and labeled as S1. By increasing the volume of HCl
(36%) to 4 mL or decreasing the amount of K2S2O8 to 0.006 mol,
while maintaining same for other conditions, the samples labeled
as S2 or S3 was obtained, respectively.
2.3 Characterization
The phase of product was characterized by X-ray powder
Fig. 1 XRD patterns of the samples obtained by a hydrothermal
diffraction (XRD, Philips X'Pert PRO MPD) with Cu Ka as the method with different concentrations of precursors.
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crystalline of the product could be facilely controlled by simply
altering concentration of precursors.
The detailed structures of the samples are further charac-
terized by TEM images, as presented in Fig. 3. The contrast
The FESEM images of the samples are shown in Fig. 2. When between the light center and the black edge further reveals the
2ꢀ
2 mL of HCl and 0.012 mol of S2O8 ions were used in the hollow structure of the sample S1 (Fig. 3a). The high-resolution
reaction system (conditions for S1), hierarchical hollow b-MnO2 TEM (HRTEM) image of one of the nanorods shows that the
microspheres with diameter of ꢂ7 mm are obtained (Fig. 2a). lattice spacing is about 0.31 nm, which is consistent with the
The cavities of the microspheres and the hemisphere in Fig. 2a interplanar distance of the [110] planes of b-MnO2 (Fig. 3b). For
reveal the presence of hollow structure in the microspheres. The the sample S2, no intensive contrast indicating the presence of
detailed structure of the hemisphere is shown in Fig. 2b, which hollow of the structure is found due to the cavity space is limited
reveals that the diameter of the inner cavity is ꢂ4.5 mm and the and the shell sphere is thick (Fig. 3c). The fringes distance of
thickness of the shell is ꢂ1.5 mm. The shell consists of uniform one of the nanorods in the HRTEM image is calculated to be
nanorods with width of ꢂ60 nm. In the presence of 4 mL of HCl 0.26 nm, which agrees well with the spacing between the [031]
and 0.012 mol of S2O82ꢀ ions in the reaction system (conditions planes of a-MnO2 (Fig. 3d). The TEM image of the sample S3
for S2), the product is composed of uniform microspheres with shows strong image contrast between the dark edge and the
diameter of ꢂ7 mm (Fig. 2c). Further observation of a broken pale center, which further conrms the formation of the hollow
microsphere discloses obviously that the microspheres have structure (Fig. 3e). HRTEM analysis reveals a lattice spacing of
hierarchical double-walled structure (Fig. 2d). The outer surface one of the nanorods is 0.26 nm, which is well in accordance
of the microsphere consists of nanorods with width of ꢂ200 nm with the [031] planes of a-MnO2 (Fig. 3f).
and ꢂ30 nm. The thickness of the outer shell and inner shell are
Moreover, the EDS spectra and the atomic ratios of the
ꢂ2.5 mm and ꢂ700 nm, respectively. The diameter of the inner elements are shown in Fig. 4 and Table S1 of the ESI.† For these
2ꢀ
cavity is ꢂ1.5 mm. When 2 mL of HCl and 0.006 mol of S2O8
three samples, the signals of Mn and O are obviously observed
ions was employed in the reaction system (conditions for S3), in the EDS spectra and the atomic ratio of Mn/O is about 1 : 2, in
the uniform hierarchical microspheres as-prepared are according with atomic ratio in MnO2. The observed Cu and C
composed of nanorods and nanowires (Fig. 2e). Closer inspec- signals are induced by the carbon-coated copper grid, which is
tion of a single sphere reveals that the widths of nanorods and
nanowires are ꢂ100 nm and 20 nm, respectively (Fig. 2f) The
broken microspheres indicate that the microsphere has hollow
interior and the diameter of the cavity is ꢂ500 nm.
Fig. 3 TEM images (a, c and e) and HRTEM images (b, d and f) of
Fig. 2 SEM images of sample S1 (a and b), S2 (c and d), and S3 (e and f) sample S1 (a and b), S2 (c and d), and S3 (e and f) obtained by a
obtained by a hydrothermal method.
hydrothermal method.
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Three peaks around 1.6 nm, 2.7 nm and 31.1 nm are observed
from the pore size distribution of S2. The pore size distribution
of S3 exhibits three peaks around 1.7 nm, 3.0 nm and 17.2 nm.
The Brunauer–Emmet–Teller (BET) specic surface areas for S1,
S2, and S3 are 14.21 m2 gꢀ1, 20.09 m2 gꢀ1, 31.76 m2 gꢀ1
,
respectively. The results indicate that S2 and S3 assembled by
two categorical nanostructures possess more types of pores and
larger surface area than those of S1 assembled by one categor-
ical nanostructures. This is important for expanding the
application range of hollow MnO2 microspheres.
To further investigate the effect of HCl on the formation of
the hierarchical hollow structure, we carried out the experi-
ments with different amount of HCl in the reaction system,
while maintaining the other reaction conditions same as those
for preparation S1. In the absence of HCl in reaction system,
uniform b-MnO2 nanorods with width of ꢂ100 nm (labeled as
S4) are obtained (Fig. 6a). The hierarchical hollow structure is
not observed in the absence of HCl. In the presence of 6 mL of
HCl in the reaction system, chrysanthemum nanostructure
MnO2 is formed. The chrysanthemum nanostructure consists of
nanowires with width of ꢂ60 nm and length ꢂ3 mm (Fig. 6b).
The results reveal that HCl is essential for the formation of
hierarchical hollow microsphere. In addition, the effect of the
concentration of S2O82ꢀ ions on the morphology of the product
was also investigated. The reaction conditions are same as those
Fig. 4 EDS spectra of sample S1 (a), S2 (b), and S3 (c) obtained by a
hydrothermal method.
used as the support of the sample. The EDS spectra of S2 and S3
indicate the existence of K element. Table S1† indicate the
atomic ratios of K in S2 and S3 are 0.5% and 2.5%, respectively.
b-MnO2 consists of narrow [1 ꢃ 1] channels with the size of
[0.23 nm ꢃ 0.23 nm] in the crystal structure.30 The voids within
the channels are too small for K ions. Therefore, K is not
detected in S1. The crystal structure of a-MnO2 is composed of a
series of [2 ꢃ 2] and [1 ꢃ 1] tunnels. The size of [2 ꢃ 2] tunnel is
[0.46 nm ꢃ 0.46 nm], which is large enough for the location of K
ions in the middle of the tunnels. Hence K irons are detected in
sample S2 and S3.
2ꢀ
for preparation S1 besides the concentration of S2O8 ions.
2ꢀ
When the concentration of S2O8 ions is 0.15 mol Lꢀ1, the
hierarchical hollow shell–core structures are obtained. The
shell consists of nanorods and nanowises distributed randomly
among the nanorods (Fig. 6c). As the concentration of S2O82ꢀ is
The pore structures of the samples are characterized by
nitrogen adsorption–desorption measurements. The nitrogen
adsorption–desorption isotherms are shown in ESI Fig. S1.†
Hysteresis loops can be observed in the isotherms of three
samples, indicating the presence of mesopores in all samples.
The pore size distributions determined by the Barrett–Joyner–
Halenda (BJH) method are displayed in Fig. 5. The pore size
distribution of S1 shows two peaks around 2.4 nm and 30.7 nm.
Fig. 6 SEM images of the samples as-synthesized in the absence of
HCl (a), in the presence of 6 mL of HCl (b), at 0.15 mol Lꢀ1 of S2O8
2ꢀ
Fig. 5 The pore size distribution of the as-prepared hierarchical ions (c), at 0.05 mol Lꢀ1 of S2O82ꢀ ions (d), at 0.6 mol Lꢀ1 of Mn2+ ions
sample S1 (a), S2 (b), and S3 (c) by a hydrothermal method.
(e), at 0.6 mol Lꢀ1 of Mn2+ ions (f), at 0.4 mol Lꢀ1 of Mn2+ ions.
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decreased to 0.05 mol Lꢀ1, the chrysanthemum nanostructure
consisting of nanowires with width of ꢂ50 nm and length of ꢂ4
mm are obtained instead (Fig. 6d). The results indicate that the
morphology of the product can also be tuned by changing the
2ꢀ
concentration of the S2O8 ions. Furthermore, the concentra-
tion of Mn2+ ions has also apparent inuence on the
morphology of the product. When the concentration of Mn2+
ions was decreased to 0.6 mol Lꢀ1, hierarchical core–shell
microspheres with diameter of ꢂ10 mm are obtained (Fig. 6e).
The surface of the hierarchical core–shell microspheres consists
of nanorods with width of ꢂ40 nm and 150 nm. As the
concentration of Mn2+ ions is 0.4 mol Lꢀ1, the product is
composed of nanorods with width of ꢂ100 nm (Fig. 6f).
2ꢀ
Decreasing the concentration of S2O8
ions leads to the
formation of the chrysanthemum nanostructure, which is
analogous to the inuence of increasing the concentration of
2ꢀ
HCl. Decreasing the concentration of Mn2+ and S2O8 ions
results in the formation of the hierarchical shell–core micro-
sphere. The hierarchical structure is composed of two categor-
ical nanostructure, the hierarchical double-wall or shell–core
structure can be obtained by controlling the concentration of
the HCl, S2O82ꢀ, or Mn2+ions in an appropriate range.
XRD patterns of the samples prepared with different
concentration of the precursor are shown in the ESI Fig. S2.† In
the absence of HCl in the reaction system, b-MnO2 is synthe-
sized (ESI Fig. S2a†). As the amount of HCl is changed to 6 mL,
the crystal phase of the products is converted into a-MnO2 from
b-MnO2 (ESI Fig. S2b†). In the presence of 0.15 mol Lꢀ1 of
2ꢀ
S2O8 ions in the reaction system, b-MnO2 is obtained (ESI
2ꢀ
Fig. S2c†). As the concentration of S2O8 ions in the reaction
system is 0.05 mol Lꢀ1, a-MnO2 is generated (ESI Fig. S2d†).
When the concentration of Mn2+ is 0.6 mol Lꢀ1 or 0.4 mol Lꢀ1
,
the peaks of the products could be indexed to b-MnO2 (ESI
Fig. S2e and S2f†). Increasing the concentration of HCl gives
rise to the formation of a-MnO2, which is analogous to the effect
2ꢀ
of decreasing the concentration of S2O8 ions.
To understand the formation mechanisms of the hierarchical
double-walled hollow structure, a series of time-dependent
experiments were carried out, the SEM images of the products are
shows in Fig. 7. When the reaction time is 5 min, the solution was
clear pink in color and no precipitate was formed. When the
Fig. 7 SEM images of the products obtained at 140 ꢁC and at 15 min (a
and b), 1 h (c and d), 2 h (e and f), 6 h (g and h) and 2 h (c and d), 6 h (e
and f), and 10 h (i and j).
reaction time is prolonged to 15 min, the product consists of ꢂ5 mm (Fig. 7g). Magnied image shows that the close-packed
nanowires (Fig. 7a). There are a lot of interspaces between nano- nanowires tend to combine together at the top to form an
wires, and the width of nanowires is ꢂ50 nm (Fig. 7b). When the increasing nanorod (Fig. 7h). Aer 10 h of hydrothermal treat-
reaction time is 1 h, the spheres are formed as dominant product ment, the diameter of microspheres is up to ꢂ6 mm (Fig. 7i).
(Fig. 7c). The nanowires are converted into nanorods with width of Closer observation indicates that the increasing nanorods with
ꢂ130 nm (Fig. 7d). The width of the nanorods is increased width of ꢂ200 nm have been formed and the width of the
comparing to that of the nanowires obtained at 15 min. The nanorods at bottom is larger than that on top (Fig. 7j). When the
generation of MnO2 is dominant during this stage, which result in reaction time is prolonged to 24 h, the widths of the nanorods
the width of the nanorod to increase. As reaction time reaches 2 h, tend to uniform from the bottom to top (Fig. 2a). The hollow
self-assembled mcirospheres with diameter of ꢂ2.5 mm are spheres composed of two categories of nanorods are nally
formed (Fig. 7e). The surface of sphere consists of nanowires with formed.
width of ꢂ50 nm (Fig. 7f). With the formation of the MnO2, the
On the basis of the above observations, the formation
reaction between MnO2 and HCl enhanced. When the reaction process and mechanism of the hierarchical MnO2 could be
rate of the HCl erosion is higher than that of the growth of MnO2, proposed and illustrated by Scheme 1.
the width of MnO2 nanorods gradually decreased. When the
The hydrothermal chemical reactions in this system could be
reaction time arrives at 6 h, the diameter of microspheres reaches described as following:
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Scheme 1, resulting in the formation of the hierarchical struc-
ture consisting of two kind nanorods with different width.
2ꢀ
When the amount of S2O8 ions is 0.012 mol is decreased to
0.006 mol (conditions for S3), the reaction is followed the step
C3 in Scheme 1, inducing the formation of the hierarchical
structure consisting of nanorods and nanowires.
To investigate the potential application of the samples for
abatement of volatile organic compounds (VOCs), the sample
S1, S2, S3 and S4 were examined as catalyst for the catalytic
oxidation of benzene. As comparative experiments, the catalytic
property of commercial MnO2 was also evaluated. The catalytic
performance of these four samples for benzene oxidation is
shown in Fig. 8. Fig. 8 is shown that the catalytic activities of the
samples obtained in our experiments are clearly higher than
commercial MnO2 and the catalytic abilities of the samples for
benzene oxidation decrease with the order of hierarchical
hollow a-MnO2 microspheres > hierarchical two-wall b/a-MnO2
microspheres > hierarchical hollow b-MnO2 microsphere >
b-MnO2 nanorods > commercial MnO2. The products of cata-
lytic reaction are only CO2 and H2O and the products from
incomplete oxidation of benzene are not detected. The image of
the commercial MnO2 is shown in Fig. S5,† indicating that the
commercial MnO2 consists of agglomerated nanoparticles.
Fig. S6† indicates the XRD pattern is complex and it can not be
indexed to any type of pure manganese oxide, which indicated
that the commercial MnO2 is a physical mixture. The purity of
the commercial MnO2 is greater than 85 wt%, which can explain
the complexity of the XRD pattern. The surface area of the
commercial MnO2 is 5.956 m2 gꢀ1. Low purity and low surface
area may lead to their low catalytic ability. The results show that
for the catalytic oxidation of benzene, the a-MnO2 has better
catalytic ability than that of b-MnO2, and the hierarchical
hollow structure also has better catalytic performance than that
of nanorods with same crystal type (b-MnO2). The activity of the
catalysts is associated with their crystalline structure, oxygen
species, surface area, and morphology, etc. Usually, catalysts
with a higher surface area show a better catalytic activity.
Scheme 1 Schematic illustration to show the formation of hierarchical
MnO2.
MnSO4 + K2S2O8 + 2H2O/MnO2Y+ K2SO4 + 2H2SO4 (1)
MnO2 + 4HCl/MnCl2 + Cl2[ + 2H2O
(2)
In the initial stages of the reaction, the oxidation–reduction
reaction (1) takes place rstly. Once the manganese oxide
formed, reaction (2) would occur and then Cl2 microbubbles are
created. Cl2 microbubbles could serve as an aggregation center
and a self-generated so-template. Driven by the minimization
of interfacial energy, the small manganese oxide particles
aggregate around the gas–liquid interface between the micro-
bubbles, analogous to the case (step A) reported previously.30,31
These small clusters would serve as nuclei for MnO2 to grow
into nanowires (step B).32,33 With the growth of nanowires, the
close-packed nanowires tend to combine together on the top to
form a increasing nanorod under oriented attachment function.
At the same time, the eroding process starts the center of the
microsphere because of the existence of a large amount of HCl
and gas–liquid equilibrium.34 The nanowires located on the
outside would serve as the starting points for the subsequent
crystallization process and then the nanorods and nanowires
grow larger and the hierarchical double-walled hollow structure
is formed (step C). The morphology of the product is dependent
on the competition between the growth of MnO2 and the HCl
erosion, or in other word, the relative reaction rate of reaction
2ꢀ
(1) and (2). When the concentration of HCl and S2O8 ions is
relatively high, it is difficult for the close-packing nanowires to
combine together because of the abundance Cl2, which leads to
the nanowires formed. The more Cl2 formed in the system, the
more nanowire generated in the microsphere. Increasing the
rate of reaction (2) is benecial to form a-MnO2 nanowires and
decreasing the relative rate of reaction (2) is helpful for the
formation of b-MnO2 nanorods. Hierarchical hollow structure
MnO2 could only be obtained in an appropriate relative rate of
reaction (1) and (2). The results of the effect of the concentration
of the precursor agree with the proposed mechanism. When the
amount of S2O82ꢀ ions is 0.012 mol and the amount HCl is 2 mL
(conditions for S1), the reaction is followed the step C1 in
Scheme 1, leading to the formation of hierarchical structure
consisting of b-MnO2 nanrods. When the amount HCl is up to
4 mL (conditions for S2), the reaction is followed the step C2 in
Fig. 8 Activities of the sample S1 (black), S2 (red), S3 (blue), S4 (green)
and commercial MnO2 (pink) in the catalytic oxidation of benzene.
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However, surface area is not determinant factor.32 MnO2 can
form several polymorphs because the [MnO6] octahedral units
can be linked in different ways.36 The a-type is constructed from
double chains of [MnO6] octahedra, which form [2 ꢃ 2] and [1 ꢃ
1] tunnels, while b-MnO2 have a [1 ꢃ 1] channel in the structure
being composed of individual chain of the [MnO6] octahedral
units.33,36 The longer diameter of the benzene is 0.27 nm, which
is larger than [1 ꢃ 1] tunnels ([0.23 nm ꢃ 0.23 nm]). It is diffi-
cult for benzene to enter into the [1 ꢃ 1] tunnels. [2 ꢃ 2]
([0.46 nm ꢃ 0.46 nm]) tunnels are large enough for the
accommodation of benzene. Therefore, both the active sites on
the surface and in the tunnels of a-MnO2 are available for
catalytic oxidation of benzene, while the active sites in tunnels
of b-MnO2 are not available for catalytic oxidation of benzene.
Hence, a-MnO2 offers more active sites than those of b-MnO2,
resulting in higher catalytic ability for benzene oxidation. It is
well-known that the catalytic activity of a transition metal oxide
is associated with its defect nature and the concentration of
oxygen species.35 XPS was employed to investigate the chemical
state of oxygen. The O 1s spectra of all samples and the results
were shown in ESI Fig. S3† and summarized in ESI Table S3,†
respectively. The binding energies at 529–530 eV and 531–
532 eV could be assigned to the lattice oxygen (Oa) and defective
oxides or surface adsorbed oxygen (Ob), respectively, while the
binding energy higher than 533 eV is associated with adsorbed
molecular water (Og).37–39 For the samples with b-type crystal
structure, S1 has better catalytic performance than that of S4.
The higher catalytic performance of S1 might be associated its
higher Oa proportion than that of S4.40 The proportion of Oa in
S2 is the highest among the samples, however, S2 does not
display the highest catalytic activity for the benzene oxidation
reaction. The crystal phase of S2 is a mixture of b- and a-type,
and hence its catalytic ability is between S1 with b-type and S2
with a-type. The results indicate that the crystal phase of MnO2
is a dominating factor while the lattice oxygen only plays an
assistant role in affecting the catalytic ability for the benzene
oxidation. In order to investigate the stability of the catalyst, we
carried out the catalytic ability cycle test of the most active
catalyst (S3). The catalytic ability cycle test was carried out for
four times under the catalyst test condition. Fig. S7† reveals that
the XRD patterns of the catalyst aer four catalyst cycles can be
indexed to a-MnO2 (JCPDS 44-0141). The intensity of the peaks
is increased aer the four catalyst cycle, indicating the degree of
crystallinity of the sample is increased. The SEM images and
nitrogen adsorption–desorption isotherms of the catalyst aer
catalytic ability cycles were shown in Fig. S8 and S9,† respec-
tively. Fig. S8† shows that the catalyst aer four catalyst cycles
consists of nanorods and nanowires. There is no obviously
change in morphology of the catalyst. The surface area of the
sample aer catalytic ability cycle test is 39.84 m2 gꢀ1, which is
higher than that of S3. Fig. S9† indicates the pores of the
4 Conclusions
In summary, hierarchical hollow b-MnO2 microspheres consist-
ing of nanorods, hierarchical double-walled hollow b/a-MnO2
microspheres assembled by two-categorical nanorods, and hier-
archical hollow a-MnO2 microspheres constructed by nanorods
and nanowires have been synthesized by a facile hydrothermal
synthetic method. The crystal form and morphology of the
product can be easily controlled by simply altering the concen-
tration of the precursors. HCl is essential for the formation of
hierarchical hollow microspheres. The signicant factor for
forming hierarchical hollow MnO2 was the relative rate of
formation and consumption reaction of MnO2. The growth
mechanism of hollow MnO2 spheres can be attributed to oriented
attachment and etching process. The hierarchical hollow MnO2
microspheres have potential applications in VOC elimination.
Their catalytic abilities for benzene oxidation decreased with the
order of hierarchical hollow a-MnO2 microspheres > hierarchical
two-wall b/a-MnO2 microspheres > hierarchical hollow b-MnO2
microspheres > b-MnO2 nanorods.
Acknowledgements
We thank National High Technology Research and Develop-
ment Program 863 of China (no. 2012AA062702) and Strategic
Priority Research Program of the Chinese Academy of Sciences
(Grant no. XDB0505000) for funding supports.
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