In Situ-Fabricated 2D/2D Heterojunctions of Ultrathin SiC/Reduced Graphene Oxide Nanosheets for Efficient CO2 Photoreduction with High CH4 Selectivity

Article abstract of DOI:10.1002/cssc.201802088

Photoreduction of CO₂ into fuel molecules such as CH₄ represents a promising route to simultaneously explore renewable energy and alleviate global warming. However, the implementation of such a process is hampered by low product yiel

Full text of DOI:10.1002/cssc.201802088

Accepted Article
Title: In-situ fabricated 2D/2D heterojunction of ultrathin SiC/RGO
nanosheets for efficient CO2 photoreduction with high CH4
selectivity
Authors: Cheng Han, Yongpeng Lei, Bing Wang, and Yingde Wang
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In-situ fabricated 2D/2D heterojunction of ultrathin SiC/RGO
nanosheets for efficient CO2 photoreduction with high CH4
selectivity
Cheng Han,[a] Yongpeng Lei,[b] Bing Wang[a] and Yingde Wang*[a]
Abstract: Photoreduction of CO2 into fuel molecules like CH4
Currently, metal-free compound photocatalysts comprised
represents a rosy perspective in simultaneously exploring renewable of nonmetal elements, such as carbon, silicon, nitrogen and
energy and alleviating global warming. However, the implementation boron, are attracting more and more attentions.[6-11]
of such a process is hampered by low products yield and poor
selectivity. Herein, a 2D/2D heterojunction of ultrathin SiC/RGO have superior sustainability and cost effectiveness. As
nanosheets was in-situ fabricated for efficient and selective representative, silicon carbide (SiC), with a proper band gap for
Semiconductors of this type are less susceptible to erosion,
a
photoreduction of CO2. Ultrathin SiC is preferred to suppress serious visible-light absorption (~2.4 eV for 3C-SiC and ~3.0 eV for 6H-
charge recombination in bulk phase, thus providing more energetic SiC), has been developed for wide photocatalytic application in
electrons. The robust 2D/2D heterojunction allows fast transfer of organic synthesis,[12-14] water splitting,[15-21] and CO2
energetic electrons from SiC to RGO. Combining the vital role of reduction.[22,23] For example, in
RGO in facilitating CO2 activation, the optimized SiC/RGO exhibits photocatalytic CO2 reduction in aqueous solution, Inoue and co-
an electrons-transfer-rate of 58.17 μmol h-1 g-1 towards CO2 workers[24] reported that the photoexcited electrons in more
a
pioneering study of
reduction, 2.7-folder of that of pure SiC (20.25 μmol h-1 g-1). More negative conduction band (CB) of SiC had much stronger
interestingly, about 92% of the transferred electrons from SiC are reduction ability to reduce CO2. The high reduction potential of
devoted to generating CH4 (6.72 μmol h-1 g-1). Such high efficiency SiC also makes it promising for deep photoreduction of CO2 into
and selectivity are mainly contributed by the densely accumulated CH4.[25] However, most of the reported SiC-based photocatalysts
energetic electrons within RGO, which facilitates the eight-electrons are in micro or sub-micro scale with poorly controlled
process to produce CH4. Our work will inspire the design of catalyst- nanostructure, thus suffering from serious charge recombination
cocatalyst system for efficient and selective photoreduction of CO2.
in the bulk phase. Recently, ultrathin semiconductors have been
reported to shorten the diffusion length of photoexcited carriers,
which helps to substantially suppress the bulk electron-hole
recombination.[26-28] To make full use of the above-mentioned
potentials of SiC, ultrathin SiC is highly desired for efficient and
selective photoreduction of CO2.
Introduction
The excessive consumption of fossil fuels brings increasing
concerns on the energy deficiency and environmental problems.
More and more carbon dioxide (CO2) is released into the
atmosphere, which becomes the main reason for climate change.
Inspired by natural photosynthesis, photocatalytic reduction of
CO2 into hydrocarbon fuels like methane (CH4) appears to be an
ideal strategy to simultaneously develop renewable energy and
alleviate global warming.[1-5] Nevertheless, the state-of-the-art
technology is far from practical implementation due to low
overall efficiency. The exploration of low-cost, highly active and
selective photocatalysts is urgently required to advance the
application of CO2 photoreduction.
Very recently, the strategy of carbon template-deriving SiC
has been developed to regulate the nanostructure of SiC by
inheriting the morphology of carbon template.[29-33] Using this
method, it is possible to obtain ultrathin SiC on the 2D surface of
graphene or reduced graphene oxide (RGO).[34] Coincidentally,
the coupling of graphene or RGO with various semiconductors
such as TiO2,[35] Cu2O,[36] g-C3N4,[37] ZnV2O6,[38] and CdS,[39] has
been well proved as one of the most feasible ways to improve
artificial
photosynthesis
including
photocatalytic
CO2
reduction.[40-43] For instance, Yu et al.[39] reported that RGO-CdS
composite exhibited an 10-folder enhancement of CH4
generation rate than that of pure CdS. Besides the facilitated
charges separation contributed by RGO, they also concluded
that the presence of RGO would promote the adsorption and
activation of CO2 due to the π-π conjugation interaction, thereby
accelerating the photocatalytic reduction of CO2. Likewise, a
heterojunction between ultrathin SiC and RGO nanosheets is
specially anticipated which can fast transfer energetic electrons
from SiC into active sites on RGO to participate in deep
reduction of CO2 into CH4.
[a]
Mr. C. Han, Dr. B. Wang and Prof. Y. Wang
Science and Technology on Advanced Ceramic Fiber and
Composites Laboratory, College of of Aerospace Science and
Engineering, National University of Defense Technology, Changsha
410073, China.
E-mail: wangyingde@nudt.edu.cn
ORCID:0000-0002-1131-238X
[b]
Prof. Y. Lei
State Key Laboratory for Powder Metallurgy & School of
Aeronautics and Astronautics, Central South University, Changsha
410083, China.
Herein, we realize the robust 2D/2D heterojunction of
ultrathin SiC/RGO nanosheets by in-situ fabrication of ultrathin
SiC on the 2D surface of RGO, which serves as both the carbon
source and morphology template. The synergistic effects of
ultrathin SiC and RGO nanosheets are integrated into SiC/RGO
Supporting information for this article is given via a link at the end of
the document.
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heterojunctions which can facilitate the separation of
photogenerated charges,[44] provide large surface area for CO2
adsorption, and expose abundant catalytic sites for CO2
reduction. Accordingly, the optimal SiC/RGO heterojunctions
exhibit outstanding photocatalytic performance for reducing CO2
into CH4 and CO, with an electrons-transfer-rate of 58.17 μmol h-
template and carbon source. The residual content of RGO was
tuned by varying the holding time of step 2 from 30 to 180 min.
Thereafter, thermal gravimetric analysis (TGA) was used to
identify the real content of RGO in the prepared SiC/RGO-x
samples, where x represented the mass ratio of RGO. Seen
from Figure 1b, an abrupt mass loss occurs between 550 oC
1
o
g-1, 2.7-folder of that of pure SiC (20.25 μmol h-1 g-1). Besides
and 800 C for all the samples with RGO component, indicating
the improvements in photocatalytic activity, it is also of great
significance to tune the selectivity of highly reduced
hydrocarbons in the products.[45-47] Although different products
have been obtained using carbon cocatalysts, such as CO,[48]
CH4,[49] CH3OH,[38] HCOOH[50] even C2H4,[51] fewer of these
researches paid attention to the products selectivity and thus the
understanding of the catalytic selectivity and mechanism
remains unclear. In our case, we interestingly find that a higher
CH4 selectivity is achieved by less proportion of RGO. This is
contributed by the densely accumulated energetic electrons
within RGO cocatalyst, which facilitates the eight-electrons
process to produce CH4. Our work provides fresh insights into
the design of catalyst-cocatalyst system, which will inspire more
efficient and selective photocatalysts towards CO2 reduction.
the burning of carbon in air. The mass fraction of residual RGO
was estimated to be about 38.2, 25.0, 11.3 and 5.8%,
corresponding to a holding time of 30, 60, 120 and 180 min,
respectively. For comparison, pure SiC was prepared by
eliminating the carbon component from SiC/RGO-5.8 (step 3 in
Figure 1a), the TGA of which presents no obvious weight loss.
The X-ray diffraction (XRD) spectra were used to reveal
the crystal structure of the synthesized samples and shown in
Figure 1c. The broad peak of RGO at about 25.1o can be
attributed to the (002) plane of the carbon structure. The
SiC/RGO heterjunctions and pure SiC show obvious peaks at
35.6°, 41.2°, 60.2°, 72.1° and 75.5°, matching well with the (111),
(200), (311) and (222) crystal planes of the cubic SiC phase
(JCPDS, No. 29-1129), respectively. Moreover, with longer
duration for SiC growth, the residual content of RGO decreases.
Accordingly, the broad peak at 25.1o of the SiC/RGO-x samples
emerges less clearly from SiC/RGO-38.2 to SiC/RGO-5.8,
indicating that RGO was gradually consumed during the
carbothermal reduction. Meanwhile, the intensity of SiC peaks
rises correspondingly, indicating an increase of the crystal size
by prolonging the reaction time. These results also indicate the
integration of RGO and SiC in the SiC/RGO heterojunctions.
Results and Discussion
As schematically illustrated in Figure 1a, ultrathin SiC was
in-situ grown on the surface of RGO to construct 2D/2D
heterojunctions of SiC/RGO via a vapor-solid reaction,[30-33]
during which the RGO nanosheets acted as both the 2D
Figure 1. (a) The synthesis process for SiC/RGO heterojunctions and pure SiC. (b) TGA curves and (c) XRD spectra of SiC/RGO
with various proportion of RGO. Inset image in (b) is given to show how to calculate the weight loss. The bottom pattern in (c)
presents the crystal data of 3C-SiC from No. 29-1129 JCPDS card.
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Figure 2. TEM images of (a) RGO, (b) SiC/RGO-38.2 and (c, d) SiC/RGO-11.3. (e) EDS results from the selected two points marked
with triangle and rectangle in (d). (f) HRTEM image and (g ) AFM image with (h) corresponding height profile of SiC/RGO-11.3. (i)
TEM image of pure SiC.
The morphology and microstructure of RGO, SiC/RGO
heterojunctions, and pure SiC were investigated using
transmission electron microscope (TEM) and atomic force
microscope (AFM). Bare RGO in Figure 2a exhibits a 2D
nanosheet morphology with micrometer long wrinkles. After
short time reaction (30 min), the obtained SiC/RGO-38.2
presents an obvious islands-in-sea type morphology (Figure
2b). We infer that the edged or defective structure on the RGO
substrate would firstly react with Si atoms to obtain SiC. By
prolonging the reaction duration, the obtained SiC/RGO-11.3
shows an obvious layered structure with an intimate and large
area contact (Figure 2c and d). Furthermore, the composition
of the heterojunctions was studied by Energy Dispersive X-Ray
Spectroscopy (EDS) analyzed at the selected points from
Figure 2d. The results in Figure 2e shows that Si and C peaks
appear at the rectangle-marked area with an atom ratio of
0.59:1, while only C is observed at the triangle-marked area,
directly proving the covering ultrathin layer to be SiC and the
substrate nanosheets to be RGO. The partial conversion of 2D
RGO into the heterojunction of ultrathin SiC/RGO nanosheets is
much favorable to transfer the energetic electrons from SiC to
RGO cocatalyst. The intimate junctions are also demonstrated
by the high-resolution TEM (HRTEM) image in Figure 2f. The
clear lattice fringes with a d-spacing of 0.25 nm are well
observed corresponding to the (111) plane from the covering
SiC layers. Fewer layers carbon with distance of 0.35 nm are
also distinguished, contributed by the wrinkled RGO layers in
the substrate. AFM was performed (Figure 2g) to analyse the
height profile of SiC/RGO heterojunctions. Typically, the height
values of 7.3 nm and 5.2 nm in Figure 2h can be ascribed to
SiC/RGO, demonstrating the ultrathin morphology of the whole
heterojunctions. Their accompanied lower levels of 4.4 nm and
3.5 nm are considered as the height of RGO nanosheets.
These results reveal that the 2D/2D heterojunctions of ultrathin
SiC/RGO nanosheets are successfully fabricated. After
eliminating the RGO substrate from SiC/RGO, we also acquire
pure SiC for comparison. As can be expected, pure SiC exhibits
a morphology of ultrathin and non-continuous pieces (Figure
2i), which further confirms the 2D/2D nanostructure of SiC/RGO
from another side.
To examine the chemical composition and elemental
status in the SiC/RGO heterojunctions, X-ray photoelectron
spectroscopy (XPS) and Raman measurements were
conducted (Figure 3). The XPS spectrum of C1s for RGO
(Figure 3a) can be fitted into two main peaks at 284.4 and
285.1 eV, which can be ascribed to the sp2 bonded carbon (C-
C) and epoxy/hydroxyl (C-O), respectively. Another smaller
peak is also distinguished at about 288.3 eV (inset image),
attributed by the carbonyl (C=O) or carboxyl (O-C=O).[16,39]
While for the C1s spectrum of SiC/RGO-11.3, the peaks
inherited from residual RGO at 284.4 and 285.1 eV are well
distinguished. Moreover, the peak at 283.2 eV is assigned to
sp3 bonds of C-Si, which is also observed in pure SiC by
comparison. The peak at 284.1 eV in pure SiC is attributed to
Si3C-O surface fragments, introduced by the HF/HNO3
treatment.[52] As for the Si 2p spectrum of SiC/RGO in Figure
3b, two main peaks at 101.1 and 101.9 eV should be ascribed
to Si-C, and surface Si-OH, respectively. The presence of
surface Si-OH bonds is beneficial for enhancing CO2
absorption.[53] For the Si2p spectrum of pure SiC, besides the
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two peaks of Si-C (101.1 eV) and surface Si-OH (101.9 eV),
another peak at about 102.5 eV belongs to silicon oxocarbide
(C2SiO2 fragments), contributed by the absorbed oxygen, which
is usually observed in SiC nanomaterials.[52,54]
increase of ID/IG from RGO to SiC/RGO reveals that the defect
sites were consumed during the SiC growth process. It is
reported that graphene with relatively fewer basal plane defects
possesses higher electrical conductivity and mobility, and thus
contribute to a more superior photocatalytic CO2 reduction
activity when used as cocatalyst.[58]
Raman spectra were further employed to analyse the
residual carbon in the prepared SiC/RGO heterojunctions. As
presented in Figure 3c, two peaks at 791 cm-1 and 943 cm-1
are recorded in both the samples of SiC/RGO and pure SiC,
which can be attributed to the TO and LO phonon of β-SiC,
respectively.[55,56] The spectra of bare RGO exhibits three
carbon-related peaks at 1335, 1571 and 2674 cm-1,
respectively. The 1335 cm-1 (D band) one is commonly
associated with disordered sp2 carbon and the 1571 cm−1 (G
band) one is resulted from the in-plane vibration of sp2 carbon
atoms. The 2D band caused by the double resonant Raman
scattering with two-phonon emissions is also observed.[31] All
the typical peaks of pure SiC and RGO are observed in the
spectra of SiC/RGO, further proving the existence of SiC and
RGO. Moreover, the D band and 2D band peaks in SiC/RGO
have slight blue shift compared with RGO, indicating the
interfacial charge transfer between the intimate heterojunction.
Generally, the relative intensity ratio of D and G band (ID/IG) can
be used to evaluate the degree of defects in carbon
materials.[39,57] In Figure 3c, the ID/IG value of RGO and
SiC/RGO-11.3 is 0.68 and 0.76, respectively, which implied a
good quality of sp2 domains upon reduction at 1400 oC. The
The light absorption property of the prepared
photocatalysts is evaluated by UV-Vis diffuse reflectance
spectra (DRS) and presented in Figure 3d. Bare RGO absorbs
a wide range from UV to visible-light, determined by the black
nature of carbon nanomaterials (see inset the photography).
For pure SiC, its spectrum exhibits an absorption edge at about
480 nm, attributed by its bandgap absorption. As presented in
Figure S1, the calculated bandgap (2.59 eV) here is a bit larger
than the reported 3C-SiC (2.4 eV) in bulk phase, originated
from the ultrathin layered structure with quantum confinement
effect. Compared with pure SiC, the SiC/RGO-x samples show
significant enhancement of absorption in the visible light region,
which is in accordance with the color change from bright green
to greyish-green (insets of Figure 3d). The absorption
improvement attributed by the residual RGO is favored for
utilization of incident light. Moreover, there may exist some
black body heating around the layered heterojunctions, which
will speed up the diffusion of reactants and products, and help
to enhance the photocatalytic performance.[49,59]
Figure 3. XPS spectra of (a) C 1s, (b) Si 2p and (c) Raman spectra for RGO, SiC/RGO-11.3 or pure SiC.(d) UV-vis diffuse
reflectance spectra of RGO, various SiC/RGO-x heterojunctions and pure SiC. Inset in (d) shows digital photographs of typical
samples.
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Figure 4. (a) Nitrogen adsorption-desorption isotherms with corresponding pore size distribution curves and (b) CO2 adsorption
isotherms for the sample of SiC/RGO-11.3 and pure SiC.
The porous structure of catalysts, determining the number
of active sites and the transport pathways for reactant and
product molecules, plays important roles in the photocatalysis
reaction. Therefor, the pores structure and surface area were
carbonaceous residues contributed to the yields of CO or
CH4,[64] a contrast test in Ar instead of CO2 was performed
under identical conditions using the optimized sample. As
shown in Figure S2, no CO or CH4 was detected under Ar
atmosphere, implying that there was negligible interference
from the carbonaceous residues. This is reasonable,
investigated
using
nitrogen
adsorption-desorption
measurements for the sample SiC/RGO-11.3 and pure SiC.
Similar shape of the two isotherms in Figure 4a demonstrates
that the SiC/RGO heterojunctions have inherited the
nanosheets structure of RGO template. The pore size
distributions (inset in Figure 4a) indicate that the samples have
wide pore size distributions from 2 to over 100 nm, confirming
the existence of large mesopores and some macropores. The
SiC/RGO heterjunctions have larger specific surface area (90.1
cm2 g-1) than that of pure SiC (71.1 cm2 g-1) due to the
presence of some RGO whose theoretical specific surface area
is 2600 m2 g-1. A photocatalyst with high specific surface area
can accommodate more surface active sites and facilitate the
transport of charge carriers, which is beneficial for enhancing
the photocatalytic CO2 reduction.[60] It is believed that the π-π
conjugation interaction between CO2 and RGO would promote
the adsorption and subsequent activation of CO2 molecules.[43]
Actually, we proved such an important role of RGO using the
CO2 adsorption profiles in Figure 4b, which exhibit that the
SiC/RGO heterojunctions have higher CO2 adsorption capacity
compared with that of SiC.
considering the synthesis of SiC/RGO involved
a high-
temperature carbothermal process and the subsequent sample
treating did not use any organics. Control experiments were
also carried out under CO2 or Ar without adding H2O (Table S2).
The obviously decreased or negligible CO or CH4 yield
confirmed the vital role of H2O by providing protons for CH4 or
CO generation (Figure S3 and S4). These contrast results
demonstrated that the carbon products of CO and CH4
originated from the CO2 and H2O. The photocatalysis stability of
the optimized SiC/RGO sample was also investigated. As
shown in Figure S5, there is no obvious loss of the CH4 and
CO generation rates in three successive cycles. Moreover, the
XRD spectra in Figure S6 reveal that the composition and
structure of SiC/RGO-11.3 did not change after three cycles
test in comparison with the fresh sample. Therefore, it is proved
that the optimized SiC/RGO have a good stability in both the
structure and performance for photocatalytic reduction of CO2
under UV-visible irradiation.
Of note, bare RGO is not active for the conversion of CO2
to CH4 under the photocatalysis conditions employed (Figure
S7). Therefor in the SiC/RGO heterojunctions, the SiC phase
serves as the source of photogenerated electrons while the
RGO helps to fast transfer energetic electrons for subsequent
CO2 reduction. A lower RGO content in the SiC/RGO will not
provide adequate electrons trapping sites to suppress the
recombination of charge carriers; or otherwise, a higher RGO
content means a relatively low content of SiC, which will
definitely result in a decreased number of photogenerated
electrons. Thus it is reasonable that a suitable ratio of SiC with
RGO is crucial for optimizing photocatalytic CO2 reduction,
which is a similar observation to previous studies of graphene-
based photocatalysts.[35-39]
Photocatalytic CO2 reduction performance was evaluated
in the presence of H2O vapor, involving
a
gas-solid
heterogeneous reaction. CO and CH4 were detected as the
main products in our reaction system (Figure S2), in line with
the previous reports using similar method.[25,61-63] The
generation rates of CO, CH4, and active electrons are
summarized in Figure 5a. The active electrons’ rate represents
the rate of photogenerated electrons that participate in CO2
reductions (details see Table S1), reflecting the real charge
transfer efficiency of a specified photocatalysts.[47] Pure SiC
exhibits an average CH4 evolution rate of 2.25 μmol h-1g-1 and
CO evolution rate of 1.13 μmol h-1 g-1, respectively. Its active
electrons’ rate is 20.25 μmol h-1g-1. Surprisingly, by remaining
certain amount of RGO, the active electrons’ rate increases
obviously from SiC to SiC/RGO, achieving a maximum of 58.17
μmol h-1 g-1 with 11.3 wt% of RGO. To make it clear that if some
From Figure 5a, it is also noticeable that the proportion of
RGO has important effects on both the activity and selectivity. A
higher CH4 selectivity is achieved by a less proportion of RGO.
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Figure 5. (a) Photocatalytic generation rate of CH4, CO and active electrons with various samples.(b) ratio of CH4 in products
depending on the relative electrons density on RGO. (c, d) illustrated mechanism of the electrons density-dependant CH4 selectivity
in SiC/RGO heterojunctions with (c) low electrons density and (d) high electrons density on RGO. The active electrons’ rate in (a)
represents the rate of photogenerated electrons that participate in CO2 reductions and is calculated by the equation: r(active
electrons) = 8*r(CH4) + 2*r(CO).
Thermodynamically speaking, CH4 generation from CO2
photoreduction should be easier than CO formation because
the reduction potential for CH4/CO2 (-0.24 V vs. NHE) is less
negative than that for CO/CO2 (-0.52 V vs. NHE). But the
generation of one CH4 molecule involves the participation of
eight electrons (CO2 + 8e− + 8H+ → CH4 + 2H2O), while for one
CO generation it needs only two electrons (CO2 + 2e− + 2H+ →
CO + H2O). In our case, the energetic electrons in all the
samples come from the same CB band of SiC, thus they should
have a same reduction potential. Accordingly, a sufficient
supply of energetic electrons should be considered as the main
parameter that determines CH4 selectivity in this context. To
make this point clear, we calculate the relative electrons density
on RGO cocatalyst by using the active electrons’ rate to divide
the mass ratio of RGO in SiC/RGO-x (Table S1). Consequently,
a positive correlation is found between the CH4 ratio selectivity
and the relative electrons density on RGO, plotted in Figure 5b.
CH4 generation, as schematically illustrated in Figure 5c and d,
respectively. We expect our understanding of electrons density-
dependent CH4 selectivity here may instruct the development of
efficient and selective CO2 reduction photocatalysts especially
when 2D nanosheets are used as cocatalysts, such as
graphene, MXene, or transition metal disulfides.
The Mott-Schottky plots, transient photocurrent responses,
electrochemical
impedance
spectra
(EIS)
and
photoluminescence (PL) emission spectra were further
characterized to gain insights into the charge transfer dynamics
in the typical samples. The CB potential of pure SiC and
SiC/RGO heterojunction is measured to be about -1.58 V and -
1.41
V using Ag/AgCl reference electrode (Figure 6a),
corresponding to -1.38 V and -1.21 V vs NHE (pH = 7.0, the
same below), respectively. Both of them are far more negative
than the reduction potential of CO2/CH4 (-0.24 V) and CO2/CO
(-0.58 V). Combining the bandgap result of pure SiC (2.59 V),
its valance band (VB) potential is estimated to be 1.01 V (-1.58
V + 2.59 V), more positive than the oxidation potential of
O2/H2O (0.82 V). Hence, the photogenerated electrons from
SiC with strong reduction potential is energetically favorable to
trigger the conversion of CO2 into CO and CH4, especially the
In other words,
a high proportion of RGO in SiC/RGO
heterojunctions will share sparse energetic electrons on RGO,
facilitating the two electrons process for CO generation;
otherwise, a low proportion of RGO will accumulate dense
energetic electrons, promoting the eight electrons process for
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Figure 6. (a) Mott-Schottky plots, (b) transient photocurrent responses, (c) Nyquist plots measured in 0.5 M Na2SO4 (pH = 7.0)
solution and (d) room-temperature PL spectra for the sample of SiC and SiC/RGO-11.3.
deeply-reduced product of CH4.[25] Seen from the transient
photocurrent results in Figure 6b, SiC/RGO-11.3 exhibits a
much higher current than pure SiC under irradiation, confirming
the better charge transfer within the heterojunctions.
Specifically, the curve of pure SiC witnesses a saltatory sharp
at the beginning of each irradiation, induced by the immediately
excited carriers and their subsequent fast recombination. While
for SiC/RGO-11.3, the photocurrent is firstly generated by the
stored electrons from RGO rather than directly from SiC,
resulting in a gradual increase (decrease) when the light is on
(off).[39,49] As evidenced by the EIS Nyquist plots in Figure 6c,
densely accumulated energetic electrons on RGO cocatalyst
would facilitate the eight-electrons process to produce CH4.
Conclusions
To sum up, the heterojunctions of ultrathin SiC/RGO
nanosheets with intimate 2D/2D contact have been in-situ
fabricated by using RGO nanosheets as both the carbon source
and morphology template. Ultrathin SiC is much preferred to
provide energetic electrons with strong reduction potential. The
robust 2D/2D heterojunction allows fast transfer of energetic
electrons from SiC to RGO. The content of residual RGO was
easily tuned in the synthesis process, which had vital roles in
promoting both the activity and selectivity of SiC in
photocatalytic CO2 reduction. A higher CH4 selectivity was
SiC/RGO-11.3 shows
a lower electrochemical impedance
compared to that of pure SiC, which is preferred for the
transport of photogenerated electrons.[59] Accordingly, under
irradiation, the electrochemical impedance of SiC/RGO-11.3
further diminished due to the excitation of abundant free
carriers. Furthermore, room-temperature PL spectroscopy were
performed to reveal the separation-recombination fate of
photoexcited charges.[27,65] As depicted in Figure 6d, the PL
emission intensity of SiC/RGO heterojunctions decreased
dramatically in comparison with pure SiC, suggesting that the
carrier recombination rate was hindered effectively. These
photoelectrochemical observations and PL results credibly
demonstrate that the electron-hole separation efficiency is
dramatically enhanced in the robust 2D/2D heterojunction of
ultrathin SiC/RGO nanosheets, which certainly contributes to
the improved photocatalytic CO2 reduction. Particularly, the
achieved by
a
less proportion of RGO with densely
accumulated energetic electrons, which facilitates the eight-
electrons process to produce CH4. Our findings provide fresh
insights into the design of catalyst-cocatalyst system, which will
inspire more efficient and selective photocatalysts towards CO2
reduction.
Experimental Section
Synthesis of 2D/2D heterojunctions of SiC/RGO
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10.1002/cssc.201802088
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GO was prepared in our laboratory from natural graphite flakes
electrochemical workstation (CHI660e, Shanghai). The working
electrode was fabricated on fluorine-doped tin oxide coated
(FTO) substrates by a facile drop-casting method (0.4 mg cm-2).
Insulate epoxy resin was used to protect the boundary of the
FTO glass and leave a fixed area of 1*1 cm2. After air drying,
the working electrode was further dried at 100 oC for 2 h to
improve adhesion. A Pt wire and a Ag/AgCl electrode was used
as the counter electrode and reference electrode, respectively.
For the measurement of transient photocurrent responses, a
300 W Xe lamp (CEL-HXF300, Beijing Aulight Co.,) was used
as the source of simulated solar light to provide chopped
irradiation from a horizontal quartz window in the cell. The
electrochemical impedance spectra (EIS) measurement was
carried out in the same system and recorded over a frequency
from 100 mHz to 1 MHz with an amplitude of 5 mV at 0.5 V bias
potential. The Mott-Schottky (MS) plots were measured with a
frequency of 1000 Hz and amplitude of 10 mV in 0.5 M Na2SO4
aqueous solution (details see Supporting information).
(>99.8%, Alfa Aesar) using
a modified Hummers method
(details see Supporting information).[66] All other reagents were
analytical grade and purchased from Sinopharm Chemical Co.,
Ltd. (Shanghai, China). The in-situ fabrication process of
SiC/RGO heterojunction is graphically shown in Figure 1a with
the setup illustrated in Figure S8. In a typical procedure, 50 mL
GO solution (5 mg mL-1) was heated to 180 °C for 12 h in a
Teflon-lined stainless steel autoclave. After the hydrogel was
freeze-dried overnight, the obtained RGO monoliths was
directly loaded on a porous graphite sieve in a corundum
crucible, at the bottom of which excessive Si (3-fold mass of
RGO) powder was added previously. Then, the crucible was
heated to 1,400 °C (5 °C·min–1) under Ar atmosphere with a
holding time of 30-180 min. The SiC was in-situ grown on the
surface of RGO via a vapor-solid reaction between Si atoms
and solid carbon template. Thereafter, the products were
treated with 1:3 HF/HNO3 aqueous solutions to remove the
unreacted Si, washed repeatedly with water and then dried at
70 oC. The finally obtained samples were remarked as
SiC/RGO-x, where x represents the mass ratio of RGO in the
SiC/RGO heterjunctions and could be measured by subsequent
thermal gravimetric analysis (TGA). To prepare pure SiC as
comparison, SiC/RGO (obtained after 180 min reaction) was
calcined in air at 700℃ for 1 h to remove the RGO substrate
and then treated with HNO3/HF to remove surface SiOx. Bare
RGO was also treated with identical conditions to compare with
SiC/RGO heterjunctions when characterized.
Photocatalytic CO2 reduction experiment
Photocatalytic reduction of CO2 in the presence of H2O vapor
was carried out as a gas-solid heterogeneous reaction in a
home-made cylinder reactor with a quartz plate cover (Figure
S9). Typically, for each test, 30 mg of the powdered sample
was dispersed into some deionized water with 10 min
sonication to get a homogeneous suspension, which was then
transferred into a petri dish. After evaporation at 120 oC, the
catalyst was deposited as a thin film. The petri dish with
catalyst was then placed in the reactor (60 mL), in which there
is a glass holder to support the the petri dish. After sealing the
cover of quartz plate, the photoreactor was vacuumed at -0.1
MPa (relative pressure) for 20 min, and CO2 (99.995%) was
subsequently introduced into the reactor. The vacuum-filtration
process was repeated 5 times. After that, 100 μL ultrapure H2O
was injected into the reactor via the silicone rubber septum and
dispersed homogeneously on the bottom. A 300 W Xe arc lamp
(CEL-HXF300, Beijing Aulight Co., Ltd.) was used as the
irradiation source (10 cm away from the reactor ) to provide UV-
visible light with intensity of 0.86 W cm-2 measured by a CEL-
NP2000 optical power meter.Each test was typically performed
under 4 h irradiation. Certain amount of gases mixture would be
Characterization
The residual mass ratio of RGO in SiC/RGO heterojunctions
was measured by TGA in a PerkinElmer Pyris 1 machine with a
heating rate of 10 oC min−1 in air from room temperature to
o
1000 C. The crystal structure was firstly characterized by X-ray
diffraction (XRD) spectra, recorded on a Bruker AXS D8
Advance device using Cu-Kα radiation (λ = 1.5418 Å). The
morphology of prepared samples was observed using Titan G2
60-300 high resolution transmission electron microscope (HR-
TEM) equipped with Energy Dispersive X-Ray Spectroscopy
(EDS) analyzer. The thickness was further measured by atomic
force microscope (AFM) observation on a Bruker Multimode 8
machine. The elemental status were analyzed by X-ray
photoelectron spectroscopy (XPS), conducted on a Thermo
Scientific ESCALAB 250Xi machine with an Al Kα source.
Ultraviolet-visible (UV-vis) light diffuse reflectance spectra were
collected on a U-4100 Hitachi spectrophotometer. N2 and CO2
adsorption isotherms were performed on an ASAP2020M
apparatus. The samples were thoroughly degassed in vacuum
at 200 °C for 10 h, and then measured at 77 K and 273 K to
determine N2 and CO2 adsorption, respectively. The Brunauer-
Emmett-Teller (BET) surface area was evaluated from N2
adsorption data. Barrett-Joyner-Halenda (BJH) method was
chosen to determine the pore size distributions. Raman
measurements were conducted on Bruker RAM II with a laser
wavelength of 514 nm. Photoluminescence (PL) emission
taken and analyzed by
a gas chromatography (SP6890)
equipped with a flame ionized detector (FID) and a thermal
conductivity detector (TCD). The produced gases were
calibrated with a standard gas mixture and the identity was
determined by the retention time.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (51773236 and 61701514), the
Natural Science Foundation of Hunan Province (2018JJ3603)
and the Research Project of NUDT (ZK-16-03-32).
spectra were obtained on
spectrometer under excitation of 300 nm.
All of the the electrochemical experiments were
performed in a standard three-electrode system using an
a Hitachi F-7000 fluorescence
Keywords: photocatalysis • CO2 reduction • heterojunction •
silicon carbide • CH4 selectivity
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10.1002/cssc.201802088
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FULL PAPER
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10.1002/cssc.201802088
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Entry for the Table of Contents
FULL PAPER
In the described 2D/2D heterojunction
of SiC/RGO, energetic electrons
would be fast transferred from
Cheng Han, Yongpeng Lei, Bing Wang,
Yingde Wang*
Page No. – Page No.
ultrathin SiC and densely accumulate
on RGO for efficient and selective CH4
generation from CO2 photoreduction.
In-situ fabricated 2D/2D
heterojunction of ultrathin SiC/RGO
nanosheets for efficient CO2
photoreduction with high CH4
selectivity
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