MnO2 aerogels for highly efficient oxidative degradation of Rhodamine B

Article abstract of DOI:10.1039/c7ra04345g

High-efficiency oxidative degradation of Rhodamine B (RhB) is demonstrated with manganese dioxide (MnO₂) aerogels. The MnO₂ aerogels are fabricated by an ice-templating approach from MnO₂ nanosheet colloids, which have been synthesized by redox reaction between MnCl₂ and KMnO₄ in sodium dodecyl sulfate (SDS) aqueous solution. The obtained self-standing MnO₂ aerogels show a three dimensional (3D) structure of a percolating network with open pores ranging from hundreds of nanometers to tens of micrometers. The oxidative degradation efficiency of MnO₂ aerogels is compared with ultrathin MnO₂ nanosheets and commercial MnO₂ powder, and the effects of concentration of MnO₂ aerogels as well as pH on the degradation efficiency are also investigated. Typically, the MnO₂ aerogels show an excellent oxidative degradation performance of RhB (97.6% removed within 10 min) in acidic solution (pH 2.5), which can be attributed to the large open pores and high surface areas of the aerogels. Furthermore, the MnO₂ aerogels also exhibit good capability in the degradation of methylene blue (MB) under acidic conditions. It is believed that MnO₂ aerogels hold great promise for future applications in organic pollutant removal with virtues of high efficiency, low cost and environmental friendliness.

Full text of DOI:10.1039/c7ra04345g

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MnO2 aerogels for highly efficient oxidative
degradation of Rhodamine B†
Cite this: RSC Adv., 2017, 7, 30283
*
*
Yinxing Shang, Kongliang Xu, Yanan Tang, Jiayi Li and Zhenning Liu
Hang Sun,
High-efficiency oxidative degradation of Rhodamine B (RhB) is demonstrated with manganese dioxide
(MnO2) aerogels. The MnO2 aerogels are fabricated by an ice-templating approach from MnO2
nanosheet colloids, which have been synthesized by redox reaction between MnCl2 and KMnO4 in
sodium dodecyl sulfate (SDS) aqueous solution. The obtained self-standing MnO2 aerogels show a three
dimensional (3D) structure of a percolating network with open pores ranging from hundreds of
nanometers to tens of micrometers. The oxidative degradation efficiency of MnO2 aerogels is compared
with ultrathin MnO2 nanosheets and commercial MnO2 powder, and the effects of concentration of
MnO2 aerogels as well as pH on the degradation efficiency are also investigated. Typically, the MnO2
aerogels show an excellent oxidative degradation performance of RhB (97.6% removed within 10 min) in
acidic solution (pH 2.5), which can be attributed to the large open pores and high surface areas of the
aerogels. Furthermore, the MnO2 aerogels also exhibit good capability in the degradation of methylene
blue (MB) under acidic conditions. It is believed that MnO2 aerogels hold great promise for future
applications in organic pollutant removal with virtues of high efficiency, low cost and environmental
friendliness.
Received 18th April 2017
Accepted 1st June 2017
DOI: 10.1039/c7ra04345g
rsc.li/rsc-advances
a controlled preparation of hierarchical hollow MnO2 nano-
structures and their application in efficient oxidative degrada-
tion of Congo red.9 Yu Xin Zhang and co-workers have shown
Introduction
The environmental contamination caused by organic pollutants
has become a serious global problem with economic develop-
ment and population growth. Organic dyes, oen used in food
and textile industries, are major sources of environmental
contamination due to their high toxicity to aquatic creatures
and carcinogenic effects on human beings.1–3 Applications of
conventional physical and biological methods for the treatment
of organic dyes in surface and ground water are limited by
a series of factors, including high cost, tedious processes,4
demanding conditions and long degradation time, etc.
Recently, some novel techniques have been developed to
decontaminate surface and ground water, such as semi-
conductor photocatalysis and active metal oxide oxidation.5–8
Nevertheless, it still remains a grand challenge to further
elevate oxidative degradation efficiency.
a pH-dependent degradation of methylene blue (MB) by the
birnessite-type MnO2 nanosheets on berglass.10 Huihua Peng
and co-workers have reported MnO2 nanostructures of two
different crystalline (i.e. a-MnO2 and d-MnO2), which can be
uniformly deposited on diatomite to absorb and degrade
methyl orange. It is found the d-MnO2 nanosheets incur a faster
reaction rate.11 Recently, aerogels typically with macroscopic
sponge-like three dimensional (3D) architectures have exhibited
good performance in capacitors, lithium ion battery, catalysis,
toxicant absorption, etc. We have envisioned that MnO2 aero-
gels assembled by MnO2 nanosheets, which have larger surface
area and better crystalline than MnO2 powders, hold great
promise to improve the oxidative degradation of organics,
owing to their low density, large open pores and high surface
area. However, there is no relevant report yet.
Herein, we have developed a simple one-step approach to
synthesize ultrathin MnO2 nanosheets via the redox reaction
between MnCl2 and KMnO4 in the presence of sodium dodecyl
sulphate (SDS). Then self-standing MnO2 aerogels have been
prepared by freeze-drying the resultant nanosheet colloid and
used in the degradation of RhB directly. We have compared the
oxidative degradation efficiency of Rhodamine B (RhB) by the
obtained MnO2 aerogels with those of ultrathin MnO2 nano-
sheets and commercial MnO2 powder, and further investigated
the effects of concentration of MnO2 aerogel as well as pH on
Manganese dioxide (MnO2) has been increasingly utilized in
oxidative degradation due to its variable valence, abundance,
low cost, non-toxicity and little secondary pollution. In partic-
ular, MnO2 nanostructures of high specic surface area have
been known of superior property in the degradation of organic
pollutants. Jinbo Fei and co-workers have demonstrated
Key Laboratory of Bionic Engineering (Ministry of Education), College of Biological and
Agricultural Engineering, Jilin University, Changchun, Jilin Province, 130022, China.
E-mail: sunhang@jlu.edu.cn; liu_zhenning@jlu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra04345g
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the degradation efficiency. In addition, we have tested the analysis was performed using a Kratos-Axis spectrometer with
degradation of MB by MnO2 aerogels. The mechanism for the monochromatic Al Ka (1486.71 eV) X-ray radiation (15 Kv and 10
highly efficient oxidative degradation of organic dyes by MnO2 mA) and hemispherical electron energy analyzer. Total organic
aerogels is also discussed.
carbon (TOC) investigation was examined in Vario TOC cube
(Elementar, Germany).
Experimental
Materials
Oxidative degradation of RhB
The RhB aqueous solution with the concentration of 5 mg Lꢀ1
was prepared, the pH value of which is 5.9. The RhB aqueous
solutions with different pH values of 2.5 and 3.5 were obtained
by adjusting with H2SO4 solution (0.1 M). Then quantitative
MnO2 aerogel was added into 10 mL RhB solution under dark
condition at room temperature with magnetic stirring. At given
time intervals, the mixed solution of 0.8 mL was pumped by
a disposable syringe and ltrated quickly (lter head pore size:
0.22 micron) to remove MnO2 aerogel to quench the reaction.
Then, the ltrates were analyzed, using a SHIMADZU UV-2550
UV-vis spectrophotometer, by recording the variations in the
absorption band in the UV-visible spectra of RhB. Oxidative
degradation of RhB by ultrathin MnO2 nanosheets which were
obtained by drying ultrathin MnO2 nanosheet precipitate at
90 ꢁC as well as commercial MnO2 powder were also examined.
KMnO4 (A.R.), ethanol (A.R.) and concentrated sulfuric acid
(H2SO4, A.R.) were obtained from Beijing Chemical Works.
MnCl2 (C.P.) was obtained from Tianjin Fuchen Chemical
Reagents Factory. Sodium dodecyl sulphate (SDS, C.P.) and
Rhodamine B (RhB, A.R.) were purchased from Tianjin Guangfu
Fine Chemical Research Institute. Commercial MnO2 (A.R.) was
purchased from Tianjin Huadong Chemical Reagent Works. All
chemicals were used without any purication. Deionized water
(resistance > 18 MU cmꢀ1) was used throughout this study.
Synthesis of MnO2 colloid
MnO2 colloids were synthesized through a redox reaction
between KMnO4 and MnCl2 in the presence of SDS. First, SDS
aqueous solution (10 mM, 297 mL) was heated at 100 ꢁC for
15 min. MnCl2 solution (50 mM, 1.8 mL) was then added
dropwise into the above solution followed by the quick injection
of KMnO4 solution (50 mM, 1.2 mL) under rapid stirring to start
the reaction. The total volume of the solution was 300 mL,
where [SDS] ¼ 10 mM, [Mn2+] ¼ 0.3 mM, [MnO4ꢀ] ¼ 0.2 mM.
Aer the resultant mixture was continued heating for 3 hours,
the solution became colorless and the ultrathin MnO2 nano-
sheets formed had been precipitated. Then the precipitates
were washed repeatedly with ultrapure water and alcohol for
purication. At last, the puried MnO2 precipitates were redis-
persed in 10 mL of water and sonicated to yield homogeneous
MnO2 nanosheet colloids.
Result and discussion
Characterization of MnO2 nanosheets
A representative transmission electron microscopy (TEM) image
of the synthesized MnO2 nanosheets is shown in Fig. 1a, in
which transparent lamellar structure with abundant graphene-
like wrinkles and folds can be observed, displaying a typical 2
dimensional (2D) morphology of MnO2 nanosheets.13 The
lateral dimension of the nanosheets is mainly in the range of 80
to 200 nm. The thickness of MnO2 nanosheets is estimated as
1.14 nm as measured by AFM (Fig. S1†), in line with the re-
ported thickness for a single layer of MnO2 nanosheets.14 The
crystal structure of the as-prepared MnO2 nanosheets has been
examined by X-ray diffraction (XRD). As shown in Fig. 1b, the
XRD prole exhibits diffraction peaks at 2q ¼ 12.2ꢁ, 24.7ꢁ, 36.8ꢁ,
and 66.0ꢁ, which agree with the characteristic peaks of d-MnO2
(JCPDS no. 18-0802).15 Notably, the lattice spacing (d) of the
plane (001), corresponding to the prominent spike at 2q ¼ 12.2ꢁ,
is 0.725 nm (d001 ¼ 0.725 nm).16,17 The peaks at 2q ¼ 24.7ꢁ, 36.8ꢁ
and 66.0ꢁ, corresponding to d values of approximately 0.360,
0.244 and 0.141 nm, can be indexed to the (002), (100) and (110)
planes of d-MnO2, respectively.18
Fabrication of MnO2 aerogel
MnO2 aerogels were fabricated by an ice-templating approach
from ultrathin MnO2 nanosheet colloid. First, the ultrathin
MnO2 nanosheet colloid (1.35 g Lꢀ1) was frozen at ꢀ20 ꢁC for 12
hours to form brown ice-containing chunk.12 Then a freeze
dryer was used to remove the ice in the ice-containing chunk by
sublimation to get a self-standing MnO2 aerogel without
observable structure collapse or volume shrinkage.
Material characterization
Transmission electron microscopy (TEM) images were obtained
with a JEOL-2010 electron microscope operating at 200 kV.
Atomic force microscopy (AFM) under tapping mode was
carried out with a commercial instrument (Digital Instrument,
Nanoscope III, and Dimension 3000) at room temperature in
air. Scanning electron microscopy (SEM) images were collected
on a JEOL JSM-7500F eld emission scanning electron micro-
scope (FE-SEM). X-ray diffraction (XRD) was performed on
a SHIMADZU X-ray diffractometer (D/max rA, using Cu Ka
˚
radiation was at a wavelength of 1.542 A), and the data were
Fig. 1 (a) TEM image of the MnO2 nanosheets; (b) XRD profile of the
collected from 5ꢁ to 80ꢁ. X-ray photoelectron spectroscopy (XPS) MnO2 nanosheets.
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Fig. 3 XPS spectrum of the MnO2 aerogels. (a) For Mn2p: 641.5 and
652.8 eV correspond to Mn2p3/2 and Mn2p1/2 in MnO2; (b) for O1s: 528.5
and 531.8 eV correspond to O1s in the lattice of [MnO6] octahedra and
the interlayer H2O or H3O+.
obtained MnO2 aerogel: one in the lattice of [MnO6] octahedra
and the other in the interlayer H2O or H3O+.3
Fig. 2 (a–d) SEM images of the MnO2 aerogels at the same obser-
vation location but of different magnifications, which increase from
low (a) to high (d); inset of (a) is the photograph of the as-prepared
MnO2 aerogel.
Oxidative degradation of RhB
The oxidative degradation ability of the as-prepared MnO2 aer-
ogels was examined by the degradation of RhB, together with
controls of grinded MnO2 aerogel powder, MnO2 nanosheets
powder, commercial MnO2 powder (Fig. 4). The grinded MnO2
aerogel powder was used to see whether the characteristic
feature of the aerogels could be damaged by the degradation
reaction. As the degradation proceeded, the color of RhB solu-
tion quickly changed from pink to faint yellow and nally
turned colorless within 10 min (Fig. 4a, inset). As shown in
Fig. 4a, with the action of MnO2 aerogel, the characteristic peak
of RhB at the wavelength of 556 nm rapidly decreased accom-
panied with an evident blue shi from 556 to 498 nm. There are
two possible pathways for the degradation of RhB. One is
preceded by the generation of a carbon-centered radical with
Characterization of MnO2 aerogels
High-purity MnO2 aerogels were self-assembled from mono-
dispersed MnO2 nanosheets by fully manipulating the interac-
tions between MnO2 nanosheets via an ice-templating
approach.19 Specically, the colloids of ultrathin MnO2 nano-
sheets were rst made by the ultrasound-exfoliation of puried
layered MnO2 nanosheets.13,19 Subsequently, a 3D network was
formed by cultivating a colloid of MnO2 nanosheets at
a temperature of ꢀ20 ꢁC, which was conned by the growing ice
crystals. A free-standing cylindrical MnO2 aerogel (inset of
Fig. 2a) can be prepared through the sublimation of ice in
a freeze-dryer, which shows a biomimetic foam structure with
interconnected macro-pores visible to naked eyes. In order to
further characterize the micro-morphology of the obtained
aerogels, scanning electron microscopy (SEM) has been
employed, which revealed a 3D percolating network with open
pores ranging from hundreds of nanometers to tens of
micrometers (Fig. 2), conrming the successful self-assembly of
2D MnO2 nanosheets into macroscopic structure. As the
magnication increases from low (Fig. 2a) to high (Fig. 2d), it
becomes evident that the 3D network is mainly constituted by
2D akes and 1D rods. The average length of the 1D rods is
estimated as ꢂ30 mm (Fig. S2†), whereas the 2D akes show
a lateral dimension of ꢂ50 mm. These 1D rods usually possess
a prismatic shape with concaved sides, and three joined 1D rods
form a “Y-shaped” trident node, which can be found ubiqui-
tously in the SEM images.
X-ray photoelectron spectroscopy (XPS) was conducted to
determine the chemical composition of the obtained aerogels
(Fig. 3). Fig. 3a exhibits two peaks of Mn at 641.5 and 652.8 eV,
which can be indexed to Mn2p3/2 and Mn2p1/2 of MnO2,
respectively.16,20 No characteristic signal of Mn(II), Mn(III) and
Mn(VII) has been identied in the XPS spectrum. The XPS
spectrum for O1s is displayed in Fig. 3b, which shows two peaks
at 528.5 and 531.8 eV, representing two species of oxygen in the
Fig. 4 UV-vis absorption spectra of RhB aqueous solution (5 mg Lꢀ1
pH ¼ 2.5) after different durations with different kinds of MnO2 (0.5 g
ꢀ1): (a) MnO2 aerogel, (b) MnO2 nanosheets powder, and (c)
commercial MnO2 powder, insets are the corresponding photographs
of RhB aqueous solution during the degradation. (d) Plots of RhB
degradation with different kinds of MnO2: MnO2 aerogel (blue
triangle), grinded MnO2 aerogel powder (pink inverted triangle), MnO2
nanosheets powder (red circle), commercial MnO2 powder (black
square).
,
L
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the breakdown of the aromatic ring directly. The other is via the necessarily mean the full degradation. To this end, total organic
formation of a nitrogen-centered radical, which undergoes carbon (TOC) was measured to assess the level of organic removal.
a two-step reaction of N-deethylation and subsequent destruc- A TOC loss approximately of 92.1% was achieved aer 40 min of
tion of the backbone.21–25 In our case, the blue shi of the oxidative degradation of RhB by MnO2 aerogel (pH ¼ 2.5). The
characteristic peak to around 498 nm agrees with the N-dee- observed smaller loss of TOC compared to the disappearance of
thylation process. Meanwhile, the decoloration of the solution UV-vis absorption is common in the oxidative degradation of
and the decrease in the absorption peak indicate the destruc- organics by metal oxides, suggesting a multi-step degradation
tion of the conjugated xanthene structure.21,24,26,27 Therefore, the that takes longer time than the color change.30,31
oxidative degradation of RhB by our MnO2 aerogels is likely
Various concentrations of MnO2 aerogels were then exam-
through the second pathway. Simultaneously, the MnO2 aerogel ined for the degradation of RhB to obtain the optimal concen-
was reduced from Mn(IV) to Mn(II) by RhB and its derivatives.28 tration. As shown in Fig. 5a and b, the MnO2 aerogels at the
The commercial MnO2 powder and MnO2 nanosheets concentration of 1 g Lꢀ1 and 0.1 g Lꢀ1 resulted in similar blue
powder of same concentration (0.5 g Lꢀ1) were used in the shi and decrease of the characteristic RhB peak as those in the
decomposition of RhB for comparison. As seen in Fig. 4b, for case of 0.5 g Lꢀ1, and the RhB characteristic peak disappeared at
MnO2 nanosheets powder, the color change of RhB solution is 70 min, indicating the complete loss of RhB. Further compar-
slower than that of MnO2 aerogels, suggesting an increased ison in plots of RhB degradation shows that the concentration
activity of MnO2 nanosheets aer self-assembling into the aer- of MnO2 aerogel can indeed inuence the degradation rates of
ogel. In contrast, for the commercial MnO2 powder (Fig. 4c), the RhB. As shown in Fig. 5c, when the MnO2 aerogel concentration
pink color of RhB solution faded more slowly, and aer 40 min, increases from 0.1 g Lꢀ1 to 0.5 g Lꢀ1, the degradation ratio does
it still remained pale yellow. It should be noted that the char- increase from 54.6% to 97.6% at 10 min. However, a slight
acteristic peak of an intermediates of RhB degradation at the decrease of the oxidative rate to 86.8% was observed when the
wavelength of 498 nm rst rose and then fell (Fig. 4c), which is concentration of MnO2 aerogel further increased to 1 g Lꢀ1
also in line with the second two-step mechanism as discussed which may be attributed to the aggregation of MnO2 aerogel at
,
above.
an extremely high concentration. Therefore, we used the
In order to evaluate the degradation ability of these materials concentration of 0.5 g Lꢀ1 in further studies.32 It is also note-
quantitatively, the plots of RhB degradation with different kinds worthy that the oxidation ability of our MnO2 aerogels is supe-
of MnO2 were obtained by eqn (1),
rior to those of other MnO2-based and H2O2-based materials
(Table S1†).
A0 ꢀ A
D ¼
ꢃ 100%
(1)
It is known that in the degradation of RhB, the pH of the
solution also plays an important role. Therefore, the effect of pH
on the degradation efficiency was also examined (Fig. 5d–f). As
shown in Fig. 5d, at pH 3.5, a slight decoloration of RhB solu-
tion can be observed in 5 min, but aer 5 min the solution
remains unchanged, suggesting that the oxidation capability of
MnO2 aerogels has been attenuated under the increased pH
value. When the pure RhB dye (5 mg Lꢀ1 pH ¼ 5.9) without the
adjustment of pH value was tested (Fig. 5e), no evident discol-
oration and shi of the characteristic peak were observed with
extending time. Therefore, it is conceivable that the uctuations
in the plots of RhB degradation at pH of 3.5 and 5.9 were caused
by the adsorption removal of RhB molecules by MnO2 aerogels
(Fig. 5f), especially with the consideration of its 3D percolating
network with open pores.
Obviously, low pH value can improve the decomposition
efficiency of RhB. As a matter of fact, the redox reaction of d-
MnO2/Mn2+ is expressed as d-MnO2(s) + 4H+ + 2eꢀ / Mn2+(aq)
+ 2H2O, which has a standard reducing potential 4ꢁ equal to
1.29 V.13 The reducing potential of d-MnO2/Mn2+ can be
improved by low pH value, which means that low pH can
facilitate the oxidation process.
A0
in which D represents the decomposition ratio of RhB at each
designated time point during the reaction, and A0 and A are the
integrated area of the characteristic peak at 0 min and at each
designated time point, respectively. At 40 min, the decomposition
efficiency for MnO2 aerogel, MnO2 nanosheets powder and
commercial MnO2 powder are 100%, 99.0% and 70.3%, respec-
tively. The high oxidative degradation efficiency of MnO2 aerogel
and MnO2 nanosheets can be ascribed to their large active surface
areas, which afford higher probability for Mn(IV) to be reduced to
Mn(II) upon the contact with RhB. At 10 min, the oxidative
degradation efficiency of MnO2 aerogel and MnO2 nanosheets
powder are 97.6% and 89.2%, which are much faster than that of
commercial MnO2 powder (29.1%) (Fig. 4d). In the case of MnO2
aerogels, 2D MnO2 nanosheets are assembled into a 3D perco-
lating network with open pores, which not only prohibits the
aggregation of 2D MnO2 nanosheets but also allows easier
passage of RhB and its degradation products during the reaction,
leading to enhanced oxidative degradation rate. When the MnO2
aerogel was grinded into powder with agate mortar29 and used for
the decomposition of RhB, 99.5% of RhB was degraded within
40 min. Yet, the oxidative degradation efficiency at 10 min is only
77.2% (Fig. 4d). Thus, the 3D percolating network with open
pores, as that in MnO2 aerogels can indeed benet degradation
rate signicantly.
Based on the literature, it is believed that Mn(IV) is reduced to
Mn(II) in our case aer the reaction, which remains soluble in
the low pH solution.33 It has been reported that MnO2 can be
converted to Mn(II) under low pH condition, generating strong
oxidizing hydroxyl radicals (cOH).34 These radicals can subse-
quently react with a dye or other organic compounds to yield
a whole range of intermediates including radical and radical
As discussed above, RhB is mineralized stepwise via N-dee-
thylation and backbone destruction, instead of decomposition to
CO2 directly. Hence, the loss of RhB absorption doesn't
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Fig. 5 (a–c) UV-vis absorption spectra of RhB aqueous solution (5 mg Lꢀ1, pH ¼ 2.5) after different durations with MnO2 aerogels of different
concentrations: (a) 1 g Lꢀ1, (b) 0.1 g Lꢀ1, and (c) plots of RhB degradation at pH ¼ 2.5 with different MnO2 aerogel concentrations: 0.1 g Lꢀ1 (red
circle), 0.5 g Lꢀ1 (black square), 1 g Lꢀ1 (blue triangle). (d–f) UV-vis absorption spectra of RhB aqueous solution (5 mg Lꢀ1) after different durations
with MnO2 aerogels at the same concentration of 0.5 g Lꢀ1 but different pH: (d) pH ¼ 3.5, (e) pH ¼ 5.9, and (f) plots of RhB degradation at different
pH: pH ¼ 2.5 (black square), pH ¼ 3.5 (red circle), pH ¼ 5.9 (blue triangle). Insets in (a and b) and (d and e) are the corresponding photographs of
RhB aqueous solution during degradation.
which the degradation efficiency declined gradually. At 70 min,
86.7% of MB was decomposed and the corresponding TOC
removal was 59.5%. Together the excellent degradation of RhB
and MB indicate that the MnO2 aerogel holds great promise for
future applications in organic pollutant removal.
Conclusions
Fig. 6 (a) UV-vis absorption spectra of MB aqueous solution (5 mg Lꢀ1
,
In summary, high-efficiency oxidative degradation of RhB
solution has been demonstrated for MnO2 aerogels. The MnO2
aerogels were prepared via a facile synthesis of MnO2 nano-
sheets by redox reaction between MnCl2 and KMnO4 in the
presence of SDS, followed by an ice-templating approach from
MnO2 nanosheet colloids. The MnO2 aerogels possess 3D
percolating network with open pores, which not only prohibits
the aggregation of 2D MnO2 nanosheets but also allows easier
passage of RhB and its degradation products during the reac-
tion, leading to enhanced oxidative degradation efficiency
compared to those of the commercial MnO2 powder and MnO2
nanosheets. Moreover, the contribution from a favorable crys-
talline in the aerogels may also benet the reaction. Typically,
MnO2 aerogels show a high and rapid oxidative degradation of
RhB (97.5% removed within 10 min) in acid solution (pH 2.5).
Furthermore, MnO2 aerogels also exhibit good performance in
the degradation of MB. It is believed that MnO2 aerogel, as
pH ¼ 2.5) after different durations with MnO2 aerogels. Inset is the
corresponding photograph of MB aqueous solution during degrada-
tion. (b) Plot of MB degradation.
cations, leading to a compꢀlete mineralization with the forma-
35,36
tion of CO2, H2O, and NO3
.
Furthermore, we have also studied the oxidative degradation
of MB by our MnO2 aerogels. The color of MB solution changed
from blue to faint pink within 10 min, and the faint pink grad-
ually faded with time (Fig. 6a, inset). The UV-vis absorption
spectra of MB aqueous solution show three characteristic peaks at
the wavelength of 249 nm,37 290 nm38 and 663 nm,39 respectively
(Fig. 6a). According to previous reports,40 the peak at 663 nm
represents a blue shi because of the N-demethylation of MB, the
intermediates of which cause the pink color. With the reaction
continues, the peaks at 633 and 290 nm decrease simultaneously,
implying that the backbone of MB has been destructed. The peak
at 249 nm is ascribed to leuco-methylene blue (LMB), a secondary
reduced product of MB, which is colorless and stable in MB
aqueous solutions.39 The oxidative decomposition of LMB leads to
the loss of peak at 249 nm.
a
high-efficiency, cost-effective and environment-friendly
material, holds great promise for future applications in
organic pollutant removal.
Acknowledgements
The plot of MB degradation is made by integrating the area
under the characteristic peak at 663 nm. As illustrated in This work was supported by the National Natural Science
Fig. 6b, the decomposition surged up to 53.7% in 2 min, aer Foundation of China (NSFC, 21471067, 51402121), and
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