Hydrogen-terminated silicon nanowire photocatalysis: Benzene oxidation and methyl red decomposition

Article abstract of DOI:10.1016/j.materresbull.2011.08.027

Hydrogen-terminated silicon nanowires (H-SiNWs) were used as heterogeneous photocatalysts for the hydroxylation of benzene and for the decomposition of methyl red under visible light irradiation. The above reactions were monitored by GC-MS and UV-Vis spectrophotometry, respectively, which shows 100% selectivity for the transformation of benzene to phenol. A complete decomposition of a 2 × 10^-4 M methyl red solution was achieved within 30 min. The high selectivity for the hydroxylation of benzene and the photodecomposition demonstrate the catalytic activity of ultrafine H-SiNWs during nanocatalysis.

Full text of DOI:10.1016/j.materresbull.2011.08.027

Materials Research Bulletin 46 (2011) 2441–2444
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Hydrogen-terminated silicon nanowire photocatalysis: Benzene oxidation and
methyl red decomposition
a,b
a,c
a,
a,
c
a,c
Suoyuan Lian , Chi Him A. Tsang , Zhenhui Kang *, Yang Liu *, Ningbew Wong , Shuit-Tong Lee
a
Institute of Functional Nano & Soft Materials and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China
b
School of Chemical Engineering & Materials, Dalian Polytechnic University, Dalian 116034, China
c
Centre of Super Diamond and Advanced Films, City University of Hong Kong, Hong Kong, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Hydrogen-terminated silicon nanowires (H-SiNWs) were used as heterogeneous photocatalysts for the
hydroxylation of benzene and for the decomposition of methyl red under visible light irradiation. The
Received 2 April 2011
Received in revised form 24 June 2011
Accepted 23 August 2011
Available online 30 August 2011
above reactions were monitored by GC–MS and UV–Vis spectrophotometry, respectively, which shows
À4
1
00% selectivity for the transformation of benzene to phenol. A complete decomposition of a 2 Â 10
M
methyl red solution was achieved within 30 min. The high selectivity for the hydroxylation of benzene
and the photodecomposition demonstrate the catalytic activity of ultrafine H-SiNWs during
nanocatalysis.
Keywords:
A. Nanostructures
B. Crystal growth
C. Electron microscopy
D. Catalytic properties
ß 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Nanosized catalysts have attracted a large amount of attention
conductivity and the surface reactivity of SiNWs have been
studied extensively [9–11]. However, few reports exist in the
SiNWs catalytic research field [12,13]. Chen et al. reported on the
degradation of methyl red using H-SiNWs. The degradation rate of
methyl red increased with the amount of H-SiNWs and with the
ultrasonic agitation power [14]. Tsang et al. reported that metal-
modified (metal: Au, Cu) SiNWs were superior catalysts for the
selective oxidation of hydrocarbons and that SiNWs were a
powerful substrate support for nanocatalysts [15]. Megouda et al.
also reported high performance for H-SiNWs and SiNWs coated
with metal (Ag, Cu) nanostructures for the photodegradation of
rhodamine B under UV and visible light irradiation [16]. Porous Si
and Si nanoparticles have also been shown to be good photo-
sensitizers because of singlet oxygen generation in solution
[17,18]. In particular, ultrafine H-SiNWs have a quantum size
effect [19]. These strengths make them promising candidates for
photocatalysts. We thus show that H-SiNWs can be used as
excellent photocatalysts for the selective oxidation of benzene
and for methyl red decomposition.
because of their high surface areas and nanosize dimensions. They
are expected to have potential application in the development of
new catalysts. Photocatalysis is an important environmental and
energy related process [1]. Semiconductors are commonly used as
photocatalysts because of their wide ranging bandgaps. Among
them, TiO
2
has been studied the most and applied in chemical fuel
has a
synthesis and in selective oxidation [2]. However, pure TiO
2
high energy bandgap and, therefore, it absorbs UV light only [3].
Other species such as CdS suffer from photocorrosion and toxicity
[
4]. Therefore, the development of new photocatalysts with
controlled properties such as variable energy gaps with appropri-
ate surface modification is urgent to meet future requirements.
One-dimensional (1D) silicon nanowires (SiNWs) are very
important in many areas of science and technology because they
have unique semiconducting, mechanical and optical properties.
Additionally, they are compatible with conventional Si technolo-
gy [5,6]. Many methods have been developed for the preparation
of bulk quantities of SiNWs [7,8]. Properties such as photo-
luminescence, field electron emission, thermal and electronic
2. Experimental
2.1. Preparation of SiNWs
SiNWs were prepared by the thermal evaporation of pure SiO
*
Corresponding authors at: Soochow University, Box 32, 199 Ren-ai Road,
powder (Aldrich, 325 mesh, 99.9%) by the oxide-assisted growth
(OAG) method [20]. A carrier gas consisting of 95% Ar and 5% H
was introduced into a furnace that was maintained at a pressure of
Suzhou Industrial Park, Suzhou 215123, P. R. China. Tel.: +86 512 65880957;
fax: +86 512 65882846.
2
E-mail addresses: zhkang@suda.edu.cn (Z. Kang), yangl@suda.edu.cn (Y. Liu).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.08.027

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S. Lian et al. / Materials Research Bulletin 46 (2011) 2441–2444
1
0 Torr with a constant flow rate of 150 sccm. The furnace was
using an Agilent 6890 GC system coupled to an Agilent 5973
quadrupole mass-selective detector. UV–Vis measurements were
carried out with an HP Agilent 8453 UV-Vis spectrometer.
À1
heated to 1300 8C at a heating rate of 40 8C min
and it was
maintained at this temperature for 4 h. The surface oxide layer of
the as-grown SiNWs was then removed using a 5% HF aqueous
solution for 30 min and preserved in vacuum overnight.
3. Results and discussion
2.2. Photocatalytic reactions
3.1. Characterization of the SiNWs
The photochemical hydroxylation of benzene was performed in
Fig. 1 shows SEM images and EDS spectra of the as-grown
SiNWs and the H-SiNWs for the catalytic reactions. The SEM
images reveal that the diameters of the as-grown SiNWs and the H-
SiNWs are approximately 14–16 nm and 7–10 nm, respectively.
On the other hand, the EDS spectra of the samples (Fig. 1c, f) reveals
that the oxygen content decreases from a relatively high value to
approximately zero in the H-SiNWs after HF-treatment implying a
transformation from SiNWs to H-SiNWs.
a 100 ml round-bottomed flask equipped with a magnetic stirrer in
a constant temperature system at room temperature. A 15% H
solution was prepared by diluting 35% H (Aldrich, 35%) with DI
water. A 150 W halogen lamp was used as the visible light source.
2 2
O
2 2
O
0
9
.5 mg dried H-SiNWs were sonicated in 50 ml of benzene (VWR,
9.999%). A 20 ml 15% H solution was then added to the H-
2 2
O
SiNWs/benzene mixture. The mixtures were illuminated with a
halogen lamp under a constant temperature system for 12 h under
constant stirring. The samples were collected every 30 min and an
organic phase appeared at the end of the reaction. The phenol
content was determined by GC–MS.
3.2. Photocatalytic oxidation of benzene
The photochemical hydroxylation was carried out using
benzene to study the photocatalytic activity of the H-SiNWs.
Fig. 2 shows a plot of benzene conversion against reaction time. No
phenol is present in the first 30 min. From 1 to 4 h, benzene
conversion occurs at a slow rate and is approximately linear
between 4 and 7.5 h. The reaction finally becomes a first order
reaction after 8 h. In fact the reaction proceeded in several steps:
During the first 30 min the SiNWs were primarily hydrogen
terminated and catalytically inactive. Subsequently, the H-SiNWs
The photocatalytic decomposition of methyl red was carried out
in a 50 ml round-bottomed flask equipped with a magnetic stirrer
in a constant temperature system at room temperature. The
À4
2
 10 M methyl red solution was prepared by dissolving methyl
red powder (Aldrich, 99.99%) in a DI water-ethanol solution
O:C OH = 1:1, v:v). A mixture of 0.5 mg H-SiNWs and 20 ml
(H
2
2 5
H
of the methyl red solution was illuminated with same light source
under constant stirring. Samples were collected every 5 min. The
concentration of methyl red was determined spectrophotometri-
2 2 x
were gradually oxidized by H O to a Si/SiO core-shell structure,
cally at
.3. Characterization
Scanning electron microscopy (SEM) and energy dispersive X-
l
max = 490 nm.
which acted as a catalyst for benzene hydroxylation. Even if the
yield of phenol was low (only 8% conversion) its selectivity for
phenol production was nearly 100%. The results of the conversion
and selectivity were based on a GC–MS peak area analysis. Zhu
et al. used highly dispersed vanadium oxide supported on
mesoporous SBA-16 as a catalyst and obtained a conversion of
2.8% and a selectivity of 100% [21]. When amorphous carbon
powders or multiwalled carbon nanotubes were used as catalysts
the conversions were 2% and 6%, while the selectivities were 80%
and 98% [22]. From the data above we conclude that SiNWs are
2
ray spectroscopy (EDS) measurements were carried out using a
Philips XL30 FEG ESEM with an accelerating voltage of 30 kV. TEM
(transmission electron microscopy) measurements were per-
formed using a FEI/Philips Tecnai 12 BioTWIN TEM with an
accelerating voltage of 200 kV. A GC–MS analysis was performed
Fig. 1. SEM (a), enlarged SEM (b) and EDS (c) images of as-grown SiNWs; SEM (d), enlarged SEM (e) and EDS (f) images of H-SiNWs.

S. Lian et al. / Materials Research Bulletin 46 (2011) 2441–2444
2443
Fig. 2. Conversion of oxidation of benzene vs. reaction time.
Fig. 4. Variation of methyl red concentration against time.
excellent catalysts for the hydroxylation of benzene. Light is
necessary because no conversion was observed in the control
experiment that was carried out in the dark.
During the photochemical reaction the surface reaction is a
major reaction step while diffusion, the surface reaction and
migration toward and away from the surface of the SiNWs are
sequential steps. With constant stirring the diffusion effect is
eliminated and the surface reaction contributes predominately to
the overall rate of the reaction. The results are quite similar to the
Langmuir–Hinshelwood mechanism in gas phase kinetics. This
high selectivity for phenol formation is achieved. The reductive
photoelectrons lead to a low reaction conversion. In fact, the
conversion and selectivity are contradictory: high selectivity
results in low conversion and vice versa. The activity of the H-
SiNWs during the oxidation of phenol makes it a potential
photocatalyst for the decomposition of hydrocarbons and for water
disinfection.
3.3. Photocatalytic decomposition of methyl red
indicates that both H
SiNWs in a similar manner to gas phase heterogeneous catalytic
kinetics. By comparing these results to those from our previous
2
O
2
and benzene interact with the surface of
For the other catalytic system, the photodecomposition of
methyl red was carried out using H-SiNWs as a catalyst with the
same light source. Fig. 3 shows the UV–Vis spectrum of the methyl
red solution throughout the photocatalytic decomposition. The
methyl red peak at 524 nm decreases gradually and finally
disappears after 30 min. Fig. 4 is a plot of methyl red concentration
against time. It reveals a zeroth-order reaction during the first
5 min and it subsequently changes into a first-order reaction
because of a reduction in the methyl red concentration. It is
important to note that the shape and position of the methyl red
peak remains unchanged throughout the photodecomposition. As
indicated by the disappearance of the red color within 30 min the
high activity of the H-SiNWs during the photodecomposition of
methyl red makes it a potential catalyst for dye bleaching, which
would be useful for the treatment of industrial and commercial
report [23,24] for the oxidation of benzene to phenol by H
2
O
2
on a
a similar
Si/SiO core-shell structure the may have
x
H
2
O
2
reoxidation function on the H-SiNWs. A possible pathway for this
reaction includes the participation of the active oxygen species
2 2
that is generated from H O . Three active oxygen species are
possible: electrophilic, nucleophilic and free radical species. We
deduced that the electrophilic species oxene was present, as in our
previous report [24].
Based on the above results we can speculate on the mechanism
of the photocatalytic oxidation of benzene but further proof is still
required. The H-SiNWs are partially oxidized by H
2 2
O to form a Si/
SiO core-shell structure on which the H molecules decompose
x
2 2
O
into active oxygen species as they approach. Significantly, the
products are protected by the photoelectron reductive environ-
ment, which prevents phenols from further oxidation and thus
2 2
sewage. The as-grown SiNWs and H O treated SiNWs were also
used as catalysts to show their different behavior. We found that
they were both inert and no degradation of methyl red was
observed because of the reductive nature of the H-SiNWs.
4. Conclusions
In summary, H-SiNWs are ideal photocatalysts for the selective
hydroxylation of benzene into phenol and for methyl red
decomposition. Nearly 100% selectivity is achieved during the
hydroxylation of benzene, and the photodecomposition of methyl
red is completed within 30 min. By modifying the surfaces of the
H-SiNWs with various chemical species (Au, Pt, Pd and Cu), we
expect to produce novel catalysts.
Acknowledgements
This work is supported by the National Basic Research Program
of China (973 Program) (No. 2010CB934500), National Natural
Science Foundation of China (NSFC) (Nos. 21073127, 21071104,
20801010 and 20803008), A Foundation for the Author of National
Fig. 3. UV–Vis spectrum of methyl red at different time intervals of reaction.
Excellent Doctoral Dissertation of P R China (FANEDD) (No.

2
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