Manganese dioxide modified silicon nanowires and their excellent catalysis in the decomposition of methylene blue

Article abstract of DOI:10.1063/1.3682486

A redox between hydrofluoric acid and ammonium fluoride-treated silicon nanowires and potassium permanganate solution was investigated. The results showed that MnO₂ nanoparticles might grow on the surface of silicon nanowires, which was confirmed with the transmission electron microscope. These MnO₂ modified silicon nanowires were employed as catalysts in the decomposition of methylene blue using sodium borohydride as the reducing agent, which exhibited excellent catalysis with its reaction rate 6 times larger than the unsupported MnO₂.

Full text of DOI:10.1063/1.3682486

Manganese dioxide modified silicon nanowires and their excellent catalysis in the
decomposition of methylene blue
Weiwei Gao, Mingwang Shao, Li Yang, Shujuan Zhuo, Shiyong Ye, and Shuit-tong Lee
Citation: Applied Physics Letters 100, 063104 (2012); doi: 10.1063/1.3682486
View online: http://dx.doi.org/10.1063/1.3682486
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APPLIED PHYSICS LETTERS 100, 063104 (2012)
Manganese dioxide modified silicon nanowires and their excellent catalysis
in the decomposition of methylene blue
1
,2
1,a)
1,2
Li Yang, Shujuan Zhuo, Shiyong Ye,
1
2,a)
Weiwei Gao, Mingwang Shao,
and Shuit-tong Lee
3
1
Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano
and Soft Materials Laboratory (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, People’s Republic
of China
2
Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science,
Anhui Normal University, Wuhu 241000, People’s Republic of China
3
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science,
City University of Hong Kong, Hong Kong SAR, China
(Received 4 December 2011; accepted 17 January 2012; published online 6 February 2012)
A redox between hydrofluoric acid and ammonium fluoride-treated silicon nanowires and
potassium permanganate solution was investigated. The results showed that MnO nanoparticles
2
might grow on the surface of silicon nanowires, which was confirmed with the transmission
electron microscope. These MnO modified silicon nanowires were employed as catalysts in
2
the decomposition of methylene blue using sodium borohydride as the reducing agent, which
exhibited excellent catalysis with its reaction rate 6 times larger than the unsupported MnO2.
VC 2012 American Institute of Physics. [doi:10.1063/1.3682486]
ꢁ
ꢂ1
Silicon nanowires (SiNWs) have various potential
ning rate of 0.05 s was applied to record the pattern.
Transmission electron microscope (TEM) was captured with
a FEI Tecnai F20 transmission electron microscope, with an
accelerating voltage of 200 kV. X-ray photoelectron spec-
troscopy (XPS) was recorded on a VGESCALAB MK II
x-ray photoelectron spectrometer, using non-monochromatized
Al Ka x-ray as the excitation source. UV-vis absorption
spectra were obtained using a PerkinElmer Lambda 750
spectrophotometer at room temperature under ambient con-
ditions. Inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) was taken with ICP-AES analyzer
(Vista MPX).
1–3
applications.
ductors, they have many unique properties, such as large sur-
Being one of the most prevalent semicon-
4
face-to-volume ratio, stability to atmospheric environment,
5
6
and ease of modification.
Currently, one obvious challenge is to utilize these 1D
silicon nanostructures as silicon mats to anchor metal oxide
materials with potential application in different fields.
Manganese oxides have attracted considerable research
interest due to their distinctive physical and chemical proper-
7
8
ties and wide applications in catalysis, magnetic storage,
10–12
9
ion exchange, electrode materials,
13
and sensors.
Among them, manganese dioxide (MnO ) has been consid-
SiNWs (0.01 g) were obtained by using the oxide-
2
15
ered as the promising one for its excellent catalytic activity.
For the above advantages of SiNWs and manganese
dioxide, we developed simple strategy to synthesize MnO2/
SiNWs. The entire reactions proceeded at room temperature
within 3 min without adding any organic auxiliary agent.
assisted growth method, and then etched with 10 mL 5%
HF aqueous solution for 10 s to remove their outer oxide
layer to become hydrogen-terminated SiNWs (H-SiNWs).
The H-SiNWs were rinsed with distilled water and immersed
in 15 mL 0.1 M KMnO aqueous solution. When the color of
4
The obtained MnO /SiNWs were employed as catalysts in
2
the SiNWs changed from yellow to gray black, it meant that
KMnO was reduced to MnO by the H-SiNWs and the
MnO /SiNWs were obtained. The fabrication of MnO /
the decomposition of methylene blue (MB) using sodium
borohydride (SB) as a reducing agent. Compared to other
4
2
2
2
14
report, MnO /SiNWs showed exhibited high catalytic
2
SiNWs was illustrated in Fig. 1.
activity in the decomposition of methylene blue.
Potassium permanganate (KMnO ) and manganese (II)
The preparation of unsupported MnO2 nanoparticles
with average diameter of 25 nm was based on the redox reac-
tion between MnCl ꢀ4H O and KMnO . KMnO (0.25 mL,
4
chloride tetrahydrate (MnCl ꢀ4H O) supplied from National
2
2
2
2
4
4
Pharmaceutical Group Chemical Reagent Co. Ltd. were used
directly without any further purification. SB and MB were
purchased from Alfa Aesar. Other chemicals used were of
analytical grade, and the corresponding solutions were pre-
pared using double-distilled water.
0.2 M) and MnCl ꢀ4H O (0.25 mL 0.3 M) were added into
2
2
50 mL water, which was sonicated for 1 h to obtain the
unsupported MnO2 nanoparticles. This process may be
expressed as 2MnO4 þ 3Mn2þ þ 2H O ¼ 5MnO þ 4 H .
2
þ
2
2
The phase purity of the as-prepared products was deter-
mined by a Shimadzu XRD-6000 x-ray diffractometer
equipped with Cu Ka radiation (k ¼ 0.15406 nm). A scan-
a)
Authors to whom correspondence should be addressed. Electronic mail:
mwshao@mail.ahnu.edu.cn and ahnuysy@yahoo.com.cn.
FIG. 1. (Color online) Schematic for the fabrication of MnO
2
/SiNWs.
0003-6951/2012/100(6)/063104/3/$30.00
100, 063104-1
VC 2012 American Institute of Physics
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063104-2
Gao et al.
Appl. Phys. Lett. 100, 063104 (2012)
Yet, if the reaction time was too long, the produced MnO2
nanoparticles will grow up to become very big nanoflowers.
To analyze the elemental composition as well as the
chemical-bonding environment of MnO /SiNWs, XPS mea-
2
surement was conducted on MnO /SiNWs (Fig. 3). Figure
2
3
(a) shows the full spectrum of MnO /SiNWs using C1s as
2
reference at 284.6 eV. No peaks of other elements except C,
O, Si, and Mn are observed. The core spectra in the Mn2p
region of MnO /SiNWs show the presence of Mn2p and
2
3/2
Mn2p1/2 peaks at 642.2 and 653.9 eV (Fig. 3(b)), which may
2 2
FIG. 2. (a) XRD pattern of the MnO /SiNWs; (b) TEM image of MnO /
SiNWs.
16
be attributed to MnO as the previous reports.
2
To investigate MnO /SiNWs’ catalysis, they were
2
employed as catalysts in the reduction of MB in the presence
of SB. The UV-vis spectrophotometer was employed to moni-
tor the decomposition of methylene blue (Fig. 4). Figure 4(a)
reveals that there exist linear relations between the logarithm
of CMB and time under different volume of SB. Simultane-
ously, it can be observed that the reaction takes place very fast
In a typical catalytic process, SB was dissolved with
ice-cold distilled water and preserved in an ice-water bath.
At the same time, 100 ll (1 ꢃ 10ꢂ3 M) of MB solution was
mixed with defined amount double-distilled water and
purged with N gas for 5–6 min to remove all dissolved
2
oxygen. Then, the as-prepared MnO /SiNWs (0.001 g, the
2
using MnO /SiNWs as catalysts, and the reduction rate
2
ꢂ4
amount of containing MnO is 5.5 ꢃ 10 g determined with
increased with the amount of SB increased. It is apparent that
the reduction is a first-order reaction to the concentration of
MB. The reduction rate of MB may be usually expressed as
2
ꢂ2
ICP-AES) catalysts and 1 ꢃ 10 M (50, 75, 100, 125,
or 150 ll) of freshly prepared SB were added. The progress
m
n
of the reaction was monitored using
spectrophotometer.
a
UV-vis
r ¼ kC C mol/min. When the reaction order to concentra-
SB MB
tion of MB is confirmed, the reaction order to concentration of
SB may be calculated in Fig. 4(b). Figure 4(b) shows the rela-
tionship of reaction rate vs CSB, which is proportional to the
square root of CSB obtained from the slope of Fig. 4(b). The
linear relationship may be expressed as LogK ¼ 4.59964
þ 0.99417 LogCSB. The reduction is also a first-order reaction
to the concentration of SB judged from this equation. By com-
bining with Figs. 4(a) and 4(b), the reduction rate of MB can
The phases of the prepared MnO /SiNWs were investi-
2
gated by the XRD analysis as shown in Fig. 2(a), which dis-
played no other characteristic peaks except MnO and Si.
2
ꢁ
The peaks at 2h around 28, 47, 56, 76, and 88 correspond to
the (111), (220), (311), (331), and (422) diffraction peaks of
Si. And, the calculated cell parameter is a ¼ 0.54296
6
0.0006 nm, which matches the value of face-centered
cubic silicon a ¼ 0.5430 nm (JCPDS card No. 27-1402). The
be expressed as r ¼ 111.5 CSB CMB mol/min.
Figure 4(c) shows UV-vis spectra of the reduction by
ꢁ
ꢁ
diffraction peaks at 36 and 65 may be indexed as (100) and
(
(
110) diffraction planes of hexagonal MnO , respectively
2
MnO
5 ꢃ 10 M, which reveals that the reduction takes place
2
/SiNW catalysts under the SB concentration of
ꢂ4
JCPDS card No. 30-0820).
The TEM image in Fig. 2(b) shows the morphologies of
fast. In order to have a comparison, a series of UV-vis spec-
MnO /SiNWs. When KMnO was reacted with H-SiNW for
2
tra were collected using 0.001 g unsupported MnO
(Fig. 4(d)) in the same reduction system. Overall, it demon-
strates high and excellent catalysis of MnO /SiNWs. In order
to further highlight the outstanding catalytic activity of
MnO /SiNWs, the linear relations between the logarithm of
MB and time were accomplished in different catalyst condi-
tion. And error bar was made for three repeated trials of each
time point using MnO /SiNW catalysts (Fig. 5). The reaction
rate of MnO /SiNW catalysts is 6 times larger than that of
2
catalysts
4
3 min, the deposited MnO particles are spherical with the
2
average diameter of 10 nm as shown in Fig. 2(b). After
2
KMnO was reacted with H-SiNW for 5 min, we found that
4
the formation of MnO nanoflowers with average diameter
2
2
of 150 nm wrapped on surface of SiNWs.
This phenomenon was resulted from the reaction
between KMnO4 and H-SiNWs, according to following
reaction:
C
2
2
MnO , and 60 times that of SiNWs or no catalysts obtained
2
KMnO4 þ H-SiNWs ! MnO2=SiNWs:
(1)
from Fig. 5. The above results obviously indicate that the
FIG. 3. (a) Wide scan XPS full spectrum of MnO
SiNWs; (b) core spectrum of Mn2p.
2
/
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063104-3
Gao et al.
Appl. Phys. Lett. 100, 063104 (2012)
FIG. 4. (Color online) (a) Curves of concentration
ꢂ2
vs time at various amount of SB (1 ꢃ 10 M);
(b) curve of reaction rate vs concentration of SB;
and UV-vis spectra of the reduction under the SB
ꢂ4
concentration of 3 ꢃ 10 M by (c) MnO
2
/SiNWs
2
and (d) unsupported MnO .
excellent catalytic activity of MnO /SiNW catalysts. The
2
MnO /SiNWs was prepared by solution approach. Com-
2
significant catalysis of MnO /SiNW catalysts is promising,
2
pared to other methods, solution method is simple, fast and
cost effective. At the same time, the MnO /SiNWs were
which may find wide applications in the catalytic field.
With regard to the catalytic mechanism of the reduction,
the MnO nanoparticles with an intermediate redox potential
2
employed as catalysts in the decomposition of methylene
blue using sodium borohydride as the reducing agent. The
results indicated that MnO /SiNWs possessed the excellent
2
value of the donor-acceptor partner plays an important role
during the electron-transfer step and acts as an electron trans-
fer system. In our experiment, MnO nanoparticles with an
2
catalytic activity, which might find potential application in
the catalytic field.
2
average diameter of 10 nm have a large surface-to-volume
ratio. It is possible to adsorb a great quantity of MB onto the
nanoparticle surface, which may bring high activity to the
as-prepared catalysts. Whereas SiNWs act as a carrier to sup-
The project was supported by the National Natural Sci-
ence Foundation of China (21071106), Specialized Research
Fund for the Doctoral Program of Higher Education
(20103201110016), and the National Basic Research Pro-
gram of China (973 Program) (Grant No. 2012CB932903).
port MnO nanoparticles, the nanoparticles are kept from
2
congregating because they are fixed by the SiNWs, which
makes it possible for them to have high catalytic efficiency.
Although unsupported MnO nanoparticles also revealed this
1
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0
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FIG. 5. (Color online) Curves of MB concentration vs time using MnO /
SiNW catalysts, unsupported MnO
concentration of 5 ꢃ 10ꢂ4 M.
2
, SiNWs, and no catalysts under the SB
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