Photocatalytic activity of silicon-based nanoflakes for the decomposition of nitrogen monoxide

Article abstract of DOI:10.1039/c7dt01682d

The photocatalytic decomposition of nitrogen monoxide (NO) was achieved for the first time using Si-based nanomaterials. Nanocomposite powders composed of Si nanoflakes and metallic particles (Ni and Ni₃Si) were synthesized using a simple one-pot reaction of layered CaSi₂ and NiCl₂. The synthesized nanocomposites have a wide optical absorption band from the visible to the ultraviolet. Under the assumption of a direct transition, the photoabsorption behavior is well described and an absorption edge of ca. 1.8 eV is indicated. Conventional Si and SiO powders with indirect absorption edges of 1.1 and 1.4 eV, respectively, exhibit considerably low photocatalytic activities for NO decomposition. In contrast, the synthesized nanocomposites exhibited photocatalytic activities under irradiation with light at wavelengths >290 nm (<4.28 eV). The photocatalytic activities of the nanocomposites were confirmed to be constant and did not degrade with the light irradiation time.

Full text of DOI:10.1039/c7dt01682d

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Photocatalytic Activity of Silicon-based Nanoflakes for the
Decomposition of Nitrogen Monoxide
Received 00th January 20xx,
Accepted 00th January 20xx
Hiroshi Itaharaa,*, Xiaoyong Wub, Haruo Imagawaa, Shu Yinb, Kazunobu Kojimab, Shigefusa F.
Chichibub and Tsugio Satob
DOI: 10.1039/x0xx00000x
The photocatalytic decomposition of nitrogen monoxide (NO) was achieved for the first time using Si-based nanomaterials.
Nanocomposite powders composed of Si nanoflakes and metallic particles (Ni and Ni3Si) were synthesized using a simple
one-pot reaction of layered CaSi2 and NiCl2. The synthesized nanocomposites have a wide optical absorption band from
the visible to the ultraviolet. Under the assumption of a direct transition, the photoabsorption behavior is well described
and an absorption edge of ca. 1.8 eV is indicated. Conventional Si and SiO powders with indirect absorption edges of 1.1
and 1.4 eV, respectively, exhibit considerably low photocatalytic activities for NO decomposition. In contrast, the
synthesized nanocomposites exhibited photocatalytic activities under irradiation with light at wavelengths >290 nm (<4.28
eV). The photocatalytic activities of the nanocomposites were confirmed to be constant and did not degrade with the light
irradiation time.
www.rsc.org/
of Ca from the layered CaSi2 by the formation of CaCl2 as a
16, 17
driving force13, where CaSi2
has a layered structure with
Introducition
alternate stacking of a planar Ca layer and a monolayer Si
sheet composed of six-membered Si rings. The reaction of
CaSi2 and TaCl5 provides Ca-bridged siloxene nanoflakes with
thicknesses of ca. 15 nm (hereafter referred to as Ca-siloxene)
and a typical molar ratio of Ca:Si = 0.1:2. Ca-siloxene has a
structure composed of Ca atoms bridging adjacent Kautsky-
type siloxene10 monolayer sheets (six-membered Si rings
connected via O-Si-O). Ca-siloxene exhibits composition-
directed tunable light absorptivity15 and stable performance as
an anode for Li-ion batteries18. On the other hand, the reaction
The discovery of strong visible luminescence emitted from
porous silicon (Si)1 has stimulated research into the synthesis
of Si nanomaterials for optoelectronic applications. Si
nanomaterials are also expected to be applicable as
photocatalytic materials for hydrogen generation by water-
splitting2-7 and as anode materials for Li-ion batteries8, 9. Useful
nanomaterials are typically in the form of porous or hollow
particles and nanotubes that have free nanospaces and large
surface-to-volume ratios. Various synthetic routes for
photocatalytic Si nanomaterials have been investigated, such
as chemical etching of Si5, magnesioreduction of natural clay6,
molten-salt-induced exfoliation of natural clay7 and a self-
templating method by the co-precipitation of Si and a salt3. On
the other hand, conventional layered siloxene (Si6O3H6)10 is
known as a Si-based bulk material with a direct band gap11,
although photocatalytic applications have not been reported,
with the exception of a recent study on exfoliated siloxene
nanosheets12.
of CaSi2 and NiCl2 provides
a nanocomposite powder
composed of similar Si nanoflakes, Ni and nickel silicide (i.e.,
Ni3Si, Ni2Si or NiSi) particles. Excellent performance as an
anode for Li-ion batteries was observed for these
nanocomposites13, 14, where Ni and nickel silicide were
considered to function as the conductive medium.
In this study, we focused on the application of nanocomposites
derived from CaSi2 as a catalyst material for the photocatalytic
decomposition of nitrogen monoxide (NO), which is a typical
air pollutant. In contrast to TiO2, which is a representative
photocatalyst for the decomposition of pollutants19, 20, the
study of Si or siloxene for NO decomposition has not been
reported to date. Here we report on the photoabsorption and
photocatalytic properties of these nanocomposites composed
of Si nanoflakes and metallic particles.
We have developed a simple and scalable synthetic method
that provides nanomaterial containing agglomerates of Si-
based nanoflakes with slit-like nanopores13-15. The method
uses a solid-state exfoliation reaction of layered CaSi2 and
transition metal chlorides as raw materials. We have verified
that the formation of the nanoflakes is based on the extraction
a.Toyota Central R&D Labs., Inc., 41-1 Yokomichi Nagakute, Aichi 480-1192, Japan
b.Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Experimental
E-mail: h-itahara@mosk.tytlabs.co.jp
Electronic Supplementary Information (ESI) available: Measurement procedures of
photocatalytic activities using a continuous reactor. See DOI: 10.1039/x0xx00000x
The nanocomposites containing Si nanoflakes were prepared
by a previously described method13, 14. Reagents of CaSi2
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(99.5%, <32 μm, Rare Metallic Co.) and anhydrous NiCl2
(99.9%, Kojundo Chemical Lab.) were used as starting
materials. A mixture of ca. 200 mg of CaSi2 and NiCl2 (molar
ratio of 1:1) powder was loaded into a BN crucible under an Ar
atmosphere in a glove box. The crucible was placed inside a
stainless-steel cell (inner volume of ca. 10 cm3) and then
Results and Discussion
View Article Online
DOI: 10.1039/C7DT01682D
Figure 1 shows XRD patterns for the raw CaSi2 powder,
synthesized powder samples, and reference commercial
powders. The peaks attributed to CaSi2 were absent in the XRD
patterns for synthesized Ca-siloxene15 (Fig. 1(b)), and S-300,
400 and 500 samples (Figs. 1(c)-(e)). The XRD peaks in the
pattern for the S-300 sample were assigned to Si and Ni, while
those for S-400 and 500 samples were assigned to Si, Ni and
Ni3Si. Amorphous Si nanoflakes were also present in all the
samples, as described below (Fig. 2). This is consistent with the
previously reported results13, in which the Si nanoflakes were
formed by extracting Ca from layered CaSi2 and simultaneously
exfoliating the layered structure. At elevated synthesis
temperatures, a larger amount of Si crystallized with the
diamond c-Si structure and also formed Ni3Si by reaction with
Ni, but excess Si still remained as amorphous Si nanoflakes.
The amount of Ni and Ni3Si would be ~ 50 mol% because NiCl2
was not observed for the reaction products before washing.
Commercial Si powder was identified as c-Si with low
crystallinity (Fig. 1(f)). No crystalline phases could be assigned
for the commercial SiO powder (Fig. 1(g)); however, SiO was
heated at 300, 400 or 500 °C for 5 h, followed by cooling to
room temperature to produce samples denoted as S-300, 400
and 500, respectively. The reaction products were washed
with dimethylformamide (Wako Pure Chemical Industries, Ltd.)
to remove the CaCl2 by-product. Finally, the obtained powders
were dried in a vacuum at 80 °C for 5 h. As a reference sample,
Ca-siloxene was prepared by the method described in a
previous report15. The synthesis procedure was essentially the
same as that for the nanocomposites except that a different
chloride source (TaCl5: 99.99%, Kojundo Chemical Laboratory)
and washing solvent (anhydrous ethanol: Wako Pure Chemical
Industries, Ltd.) were used, and the synthesis temperature was
set at 215 °C. The nominal molar ratio of CaSi2:TaCl5 was 1:1.
The crystalline phases of the synthesized samples were
identified using powder X-ray diffraction (XRD; Rigaku RINT-
TTR). Microstructures were analyzed using scanning electron
microscopy (SEM; Hitachi High-Technologies SU3500) and
scanning transmission electron microscopy (STEM; JEOL, JEM-
2010FEF). Elemental mapping images of the synthesized
samples were obtained using SEM with energy-dispersive X-ray
(EDX; Horiba EX-370) spectroscopy. N2 adsorption isotherms
(Quantachrome Instruments Nova 3000) were measured to
determine the specific surface areas of the samples from
Brunauer-Emmett-Teller (BET) plots.
reported to be a mixture of nanosized elemental Si and
25
amorphous SiO2
.
Diffuse reflectance spectroscopy (Jasco V-670) measurements
were performed over a 2 mm diameter of each sample. The
reflectivity data were processed under the Kubelka-Munk
formalism21 and absorption edges were estimated from Tauc
plots22. The photocatalytic activity for NO decomposition was
determined using a continuous reactor as described in our
previous report23 (Fig. S1†). Here, the evaluation was followed
by the established procedures in the Japanese Industrial
Standard24. The photocatalytic activity of the samples was
characterized by the decrease in the concentration of NO gas
at the outlet of the reactor. The powder samples were packed
in the groove of the holder (20×20×0.2 mm3). A constant flow
(200 mL/min) of 1 ppm NO-50 vol% air (balance N2) was used
as a test gas. Prior to light irradiation, the test gas was flowed
continuously for 10 min to achieve a balance between
diffusion and adsorption. The wavelength of the irradiated
light was set to >290 nm (450 W high-pressure mercury lamp
equipped with Pyrex glass as a filter) or >400 nm with the
ultraviolet emission filtered out.
Figure 1. Powder XRD patterns for (a) CaSi2 (raw material), (b)
Ca-siloxene, synthesized (c) S-300, (d) 400 and (e) 500 samples,
and (f) commercial Si and (g) SiO.
Figure 2 shows the microstructure and N2 adsorption isotherm
for the S-400 sample. Particles with sizes less than 1 µm were
distributed on and around grains with sizes on the order of
micrometers (Figs. 2(a) and (b)). EDX elemental mapping
images (Figs. 2(c) and (d)) indicated that the particles were Ni-
rich, and Si was dominant in the grains. In addition, the grains
were agglomerates of the Si nanoflakes, as reported in
previous papers13, 14. Considering the XRD pattern shown in
Fig. 1(d), the particles were assigned to Ni and Ni3Si. The
average molar ratio of Ca:Si was 0.07:2 for the Si nanoflakes,
which is
As
a standard photocatalytic material, we evaluated a
commercial TiO2 powder (P25, Degussa; ST-01, Ishihara
Industry Co. Ltd.) composed of 70% anatase and 30% rutile.
Commercial Si (1-2
powders (<38 m, 99.9%, Kojundo Chemical Lab.) were also
µm, >97%, Strem Chemicals, Inc.) and SiO
µ
evaluated as Si-based reference samples.
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not contribute to the absorption. Accordingly, the absorption
View Article Online
DOI: 10.1039/C7DT01682D
around 2-3 eV is assigned to low crystallinity c-Si present in the
synthesized samples because both the amount of light
absorption (Fig. 3(b)) and the XRD peak intensities of Si (Fig.
1(d) and (e)) were increased with the synthesis temperatures.
The absorption of light with photon energies higher than 4 eV
would be attributable to the Si nanoflakes. For the S-300
sample with weak XRD peaks for Si, the Tauc plot in Fig. 4(b)
was similar to that for Ca-siloxene and light absorption due to
c-Si was not clearly observed.
Figure 2. (a,b) SEM images, SEM-EDX elemental mapping
image for (c) Si and (d) Ni, (e) STEM image and (f) N2
adsorption isotherm for the synthesized S-400 sample.
close to that of Ca-siloxene (0.1:2). The streak-like contrasts in
the STEM image (Fig. 2(e)) were Si nanoflakes with thicknesses
of ca. 10 nm, as reported previously13, 14, while the round
shaped dark contrasts in the image were Ni and Ni3Si particles.
Ni and Ni3Si particles with diameters of 5-50 nm were present
in contact with the Si nanoflakes. The N2 adsorption isotherm
(Fig. 2(f)) of the sample showed an IUPAC H3 type hysteresis
loop26, which is typically observed for materials with slit-like
pores. The gaps between the assembled Si nanoflakes acted as
slit-like nanopores for the adsorption of N2. The
microstructures of the S-300 and 500 samples were similar to
that of the S-400 sample.
Figure 3(a) shows diffuse reflectance spectra of the S-300–500
samples, Ca-siloxene, and the commercial Si and SiO powders.
Figures 3(b) and 3(c) show Tauc plots22 of Kubelka-Munk
absorption (K/S)21 as a function of photon energy (h
ν)
processed with the reflectance data under the assumptions of
direct and indirect transitions, respectively. The synthesized
samples (Ca-siloxene, S-300, 400 and 500 samples) exhibited a
broad absorption tail ranging from visible to ultraviolet
wavelengths. The absorption edge was ca. 1.8 eV under the
assumption of a direct transition, as shown in Fig. 3(b). The
commercial Si and SiO powders appeared to have identical
absorption edges (Fig. 3(b)). However, assuming an indirect
transition (Fig. 3(c)) resulted in absorption edges of ca. 1.1 and
1.4 eV for the Si and SiO powders, respectively, while
reasonable Tauc plots were not obtained for the synthesized
samples. These absorption edges are close to those of c-Si (1.1
eV) and amorphous Si (ca. 1.8 eV27, 28). The determined
absorption edge is consistent with that reported for SiO, which Figure 3. (a) Diffuse reflectance spectra, and (b,c) Tauc plots of
was considered to be a mixture of nanosized elemental Si and Kubelka-Munk absorption (K/S) as a function of photon energy
amorphous SiO2 25. The Tauc plot in Fig. 3(b) had a flexion (h
ν) under the assumption of (b) direct and (c) indirect
point at around 4 eV for the S-400 and 500 samples, which transition for reference samples (Si, SiO, Ca-siloxene) and
suggests that the samples have at least two constituents that synthesized samples (S-300, 400 and 500).
absorb the light. Metallic compounds of Ni and Ni3Si29, 30 would
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Figure 4 shows the photocatalytic NO decomposition ratio for materials in the current reactor (Fig. S1†) and VtiehwuAsrtitchleeOnNlinoe
DOI: 10.1039/C7DT01682D
the synthesized and reference samples. Approximately 20% of decomposition ratio was 40 % at highest. In addition, because
NO was decomposed by the ultraviolet rays, even without a it is a continuous reaction system, the decomposition ratio
photocatalyst23; therefore, the commercial Si and SiO powders strongly related to the flow rate and the concentration of NO
exhibited considerably low photocatalytic properties. On the gas. The characterization system in the present research was
other hand, Ca-siloxene and the S-300–500 samples showed similar to that of the Japanese Industrial Standard24. About 200
considerably high NO decomposition ratios. The formation of ml/min of NO gas was continuous introduced in the reactor,
singlet oxygen (1O2) was observed as one of the active species 40% might be considered as high decomposition ability. On the
for the single phase Ca-siloxene powders31, as is the case with other hand, the rapid response of photocatalytic reactions was
photocatalysis over TiO232. Thus, it is expected that the indicated in Fig. 5.
degradation products would be N2 and HNO3 as clarified for
TiO2 catalyst33, 34
.
S-300–500 samples showed higher NO decomposition ratio
than the single phase Ca-siloxene powders. Metallic particles
on photocatalyst materials are generally considered to
facilitate the separation of photoinduced charges35, 36. The
electrical resistivity of Ca-siloxene and the S-300–500 samples
measured using powder compacts were 1.9×107, 1.0×103,
1.0×103 and 1.1×103
show electrical resistivity at room temperature: of 6.9×10-6
and 9.4×10-5
cm, respectively, suggesting that most Si
Ω
cm, respectively. Ni37 and Ni3Si29 bulk
Ω
nanoflakes in the S-300–500 samples would contact Ni and
Ni3Si particles. The exact positions of conduction and valence
bands of the Si nanoflakes are hard to be determined by both
theoretical and experimental methods because Si nanoflakes
are powders with low crystallinity: e.g., the photoemission
yield spectroscopy measured in air gives only a rough
estimation of the ionization potentials. However, the
measured ionization potential and the absorption edge (Fig.3)
suggested that the valence band of the Si nanoflakes located
around 3 eV below the vacuum level while the Fermi level of
Ni locates further below (~4 eV). Thus, the photoinduced
charges in the Si nanoflakes could easily migrate to Ni.
Systematic study by changing the ratio of Si nanoflakes and
metallic particles would be necessary to prove the exact effect
of co-existing metallic phases as a future study.
Figure 4. Nitrogen monoxide decomposition ratio over
reference samples (TiO2, Si, SiO, Ca-siloxene) and synthesized
samples (S-300, 400 and 500). The dotted lines indicate the
blank data (ratio observed without a catalyst).
Under irradiation with light at wavelengths >400 nm (<3.1 eV;
light without ultraviolet), the S-400 sample and Ca-siloxene
both exhibited weak but observable photocatalytic activity.
The photocatalytic activity was reproducibly observed and was
independent of the synthetic batch. All of the S-300–500
samples had absorption edges of ca. 1.8 eV (ca. 690 nm, Fig.
3(b)), which indicates that these samples have similar potential
to function as photocatalysts with light irradiation of >400 nm
wavelength. The difference in the observed photocatalytic Figure 5. Nitrogen monoxide decomposition rate as a function
activities among the S-300–500 samples may originate from of the light (>290 nm) irradiation time for the synthesized
the different degree of charge separation associated with the sample S-400.
different mixing ratios of Si nanoflakes to metallic particles and
their particle sizes. These differences were evident from the A previous report38 on TiO2 photocatalysts prepared under
observed phases (Fig. 1) and the specific surface areas (see various synthetic conditions showed that the specific surface
inset Table in Fig. 4). Furthermore, photocatalytic NO area of the samples was an important factor for enhancement
decomposition under the light containing ultraviolet rays (>290 of the photocatalytic activity for NO decomposition. The S-
nm) for the S-400 sample did not degrade over time, as shown 300–500 samples prepared in the present work exhibited
in Figure 5. This is clearly different from the results reported comparable or higher photocatalytic NO decomposition ratios
for photocatalytic water-splitting over siloxene nanosheets, when compared to commercial TiO2 (Fig. 4), even though their
whereby the activity was rapidly degraded12. It is also noted specific surface areas were approximately 5 times smaller than
that a part of test gas passed without contacting the catalyst that of TiO2 (see inset Table in Fig. 4). Thus, the
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nanocomposites synthesized in this study have significant 8. U. Kasavajjula, C. Wang and A. J. Appleby, Journal of Power
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The photoabsorption properties and photocatalytic activity for
NO decomposition were examined for nanocomposites
containing Si nanoflakes and metallic particles derived from
layered CaSi2 as a raw material. The Si nanoflakes were ca. 10
nm thick and the metallic particles formed were Ni and Ni3Si.
The nanocomposite had a broad absorption tail ranging from
visible to ultraviolet wavelengths, due mainly to the Si
nanoflake content. The synthesized nanocomposites exhibited
high photocatalytic NO decomposition ratios that were far
superior to those of conventional Si and SiO powders. Under
light irradiation (>290 nm), the nanocomposites exhibited
comparable or higher activity than a commercial TiO2 powder
used as a standard photocatalytic material. The photocatalytic
activity of the synthesized nanocomposite for NO
decomposition was not degraded with time. It is considered
that the presence of metallic particles in the nanocomposite
may facilitate photoinduced charge separation. Here, we have
demonstrated that one-pot synthesis nanocomposites can
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DOI: 10.1039/C7DT01682D
A table of contents entry
Photocatalytic Activity of Silicon-based Nanoflakes for the
Decomposition of Nitrogen Monoxide
Hiroshi Itaharaa,*, Xiaoyong Wub, Haruo Imagawaa, Shu Yinb, Kazunobu Kojimab,
Shigefusa F. Chichibub and Tsugio Satob
a Toyota Central R&D Labs., Inc., 41-1 Yokomichi Nagakute, Aichi 480-1192, Japan
E-mail: h-itahara@mosk.tytlabs.co.jp
b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
The photocatalytic decomposition of nitrogen monoxide was achieved using one-pot synthesized
nanocomposite powders composed of Si nanoflakes and metallic particles.