Enhanced photochemical CO2 reduction in the gas phase by graphdiyne

Article abstract of DOI:10.1039/d0ta02256j

Photocatalytic CO₂ reduction is promising for reducing the greenhouse effect and producing renewable energy, but still shows low activity and selectivity due to the ineffective utilization of photogenerated charge carriers and insufficient active sites for CO₂ adsorption and activation. Taking CdS nanocrystals as a model semiconductor, we demonstrate that graphdiyne, a new type of two-dimensional carbon allotrope uniquely formed from sp- and sp²-hybridized carbon, enhances CO₂ photoreduction over CdS with higher activity, selectivity, and stability in the gas phase without any sacrificial agent compared to graphene. Both experimental and theoretical results prove that the chemical bonding between graphdiyne and CdS and sufficient CO₂ adsorption sites due to the strong interfacial interaction-induced sulfur vacancies in CdS and more electron-deficient acetylenic linkages in graphdiyne lead to more efficient electron transfer and storage for the subsequent CO₂ reduction reaction. The excellent properties of graphdiyne make it promising for applications in solar energy conversion.

Full text of DOI:10.1039/d0ta02256j

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Zhenhai Wen et al.
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Enhanced Photochemical CO2 Reduction in the Gas Phase by
Graphdiyne
Shaowen Cao,*a Yajie Wang,a Bicheng Zhu,a Guancai Xie,bc Jiaguo Yu,*a and Jian Ru Gong*bc
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Photocatalytic CO2 reduction is promising for reducing greenhouse against activation during photocatalysis.4-6 Hence, the
effect and producing renewable energy, but still shows low activity employment of cocatalysts, for example, noble metals (Pt, Pd,
and selectivity due to the ineffective utilization of photogenerated Au etc.), earth-abundant metals (Cu, Co, Ni etc.), and enzymes,
charge carriers and insufficient active sites of CO2 adsorption and is necessary to accelerate the surface catalytic processes of CO2
activation. Taking the CdS nanocrystals as the model photoreduction.7 Nevertheless, noble metals and enzymes are
semiconductor, we demonstrate that graphdiyne, a new type of scarce and expensive; while non-noble metal-based cocatalysts
two-dimensional carbon allotrope uniquely formed by sp- and sp2- are less active and stable, which limit their practical applications.
hybridized carbon, enhances CO2 photoreduction over CdS with A promising candidate is the family of cost-effective
higher activity, selectivity, and stability in the gas phase without nanocabons (carbon dots, carbon nanutubes, graphene), which
any sacrificial agent compared to graphene. Both experimental and not only greatly promote the charge transfer, but also increase
theoretical results prove that the chemical bonding between the active sites.7 Especially, due to its excellent conductivity and
graphdiyne and CdS, and sufficient CO2 adsorption sites due to the electron mobility arising from its sp2 hybridized two-
strong interfacial interaction-induced sulfur vacancies in CdS and dimensional (2D) carbon atom network, graphene can serve as
more electron-deficient acetylenic linkages in graphdiyne, lead to an electron acceptor or transport channel to promote charge
more efficient electron transfer and storage for subsequent CO2 transfer and separation, thus improving photocatalytic
reduction reaction. The excellent properties of graphdiyne make it efficiency, which is even comparable to those high-performance
promising for applications in solar energy conversion.
noble metal cocatalysts.8-11 The ability of graphene to
accumulate photoelectrons is particularly attractive to increase
the kinetics of the multi-electron transfer process of CO2
photoreduction. In addition, improving CO2 binding to a
photocatalytic material is of vital importance to promote the
catalytic efficiency in surface CO2 conversion reactions.12,13
Photochemical conversion of CO2 into solar fuels is a clean,
carbon-neutral, and sustainable strategy to mitigate the global
energy and environmental issues simultaneously. In a general
process, solar light is harvested by
a semiconductor
photoabsorber to produce photogenerated charge carriers,
which subsequently trigger surface catalytic reactions to reduce
CO2 into CO and hydrocarbons by photoexcited electrons, while
to oxidize water to oxygen by photoexcited holes.1-3 However,
construction of an efficient photochemical system for CO2
reduction is still a challenge, because single-component
semiconductors are less effective in utilizing photoexcited
charges, and linear CO2 molecules are chemically rather inert
Graphdiyne (GDY),
a
stable synthetic 2D carbon
allotrope,14,15 shows interesting features similar to graphene
such as large surface area and high electron mobility, which
make graphdiyne a good host matrix for semiconductor
nanostructures, as well as an excellent electron acceptor or
electron transfer medium,16-19 promising for applications in the
fields of electrocatalysis,20-22 photocatalytic degradation,23-28
water splitting,29-31 and most recently CO2 photoreduction.32
More importantly, it should be noted that graphdiyne has a
highly π-conjugated structure composed of sp- and sp2-
hybridized carbon atoms, while graphene only has a purely sp2-
hybridized carbon framework. This enables the presence of
acetylenic linkages in graphdiyne, which are more electron-
deficient than ethylenic linkages and do not exist in graphene,
thereby graphdiyne can capture electrons more easily than
graphene.33,34 Moreover, the uniformly distributed pores
a. State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, 430070 Wuhan, P. R. China. E-mail:
swcao@whut.edu.cn; jiaguoyu@yahoo.com
b. CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and
Hierarchy Fabrication, National Center for Nanoscience and Technology, 100190
Beijing, P. R. China. E-mail: gongjr@nanoctr.cn
c. University of Chinese Academy of Sciences, 100049 Beijing, P. R. China
†Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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≡C–C≡C–) provide some CdS nanoparticles drop from the surface Voiefw gArrtiacplehOenlninee
among the three diacetylenic linkages (–C
abundant hollow sites for CO2 adsorption,35 while graphene under the same condition, as shown in Fig. S6. This observation
only possesses six-membered carbon rings with smaller void shows stronger interaction between CdS and graphdiyne,
space (Fig. S1). Also, the improved electronic properties indicating the good structural stability of CdGDY.
resulting from the diacetylenic links cause higher CO2
adsorption selectivity compared to graphene without such
links.36 These advantages in principle, make graphdiyne
promising to promote CO2 photoreduction over semiconductor
photoabsorbers.
DOI: 10.1039/D0TA02256J
As a proof of concept in this study, we demonstrate that
graphdiyne performs better than graphene as a cocatalyst, to
enhance CO2 photoreduction over the model semiconductor
photoabsorber, CdS nanocrystals, in the gas phase without any
sacrificial agent. Higher activity, selectivity, and stability of CO2
photoreduction can be achieved over graphdiyne-modified CdS
(denoted as CdGDY), as compared to graphene-modified CdS
(denoted as CdG). Such performance enhancement is attributed
to the chemical bonding between graphdiyne and CdS, and
sufficient CO2 adsorption sites due to the strong interfacial
interaction-induced sulfur vacancies in CdS and more electron-
deficient acetylenic linkages in graphdiyne, which essentially
improve the electron transfer and storage for subsequent CO2
reduction reaction.
CdGDY was synthesized by growing CdS nanocrystals in situ
Fig. 1. High-resolution XPS C 1s spectra of graphdiyne and CdGDY (a), GO and
CdG (b); S 2p spectra (c) and Cd 3d spectra (d) of CdS, CdG and CdGDY.
on the surface of graphdiyne, under a DMSO-involved
solvothermal condition. Here DMSO was used as both a solvent
and a reactant of S source. The composition of the composite
XPS analysis was performed to elucidate the chemical states
and interfacial interaction in the samples. As shown in Fig. 1a, C
1s spectra of GDY and CdGDY are deconvoluted into four peaks,
attributed to C=C (sp2-hybridized carbon, 284.5 eV), C≡C (sp-
hybridized carbon, 285.0 eV), C–O (epoxy or hydroxyl groups,
286.9 eV), and C=O (carboxyl or carbonyl groups, 288.8 eV).23
For both GDY and CdGDY, the peak area of sp2-hybridized
carbon is twice that of sp-hybridized carbon, which is consistent
with the structure of graphdiyne shown in the diagram in Fig.
S1. Three deconvoluted peaks of C 1s corresponding to GO (Fig.
1b) are centered at 284.8 eV (C=C), 286.8 eV (C–O), and 288.4
eV (C=O).41 Note that the relative peak intensity ratios of
oxygen-containing groups are much lower for CdGDY and CdG,
as compared to those for GDY and GO, suggesting increased
degree of reduction in the composites, which is beneficial for
improving electron mobility and conductivity. With regard to S
2p and Cd 3d spectra (Fig. 1c and d), pure CdS shows peaks of
S2− at 161.4 (2p3/2) and 162.6 eV (2p1/2),42 and Cd–S bond at
405.1 (Cd 3d7/2) and 411.9 eV (Cd 3d5/2).43 It is noteworthy that
both S 2p and Cd 3d spectra of CdG are shifted by 0.6 eV to
higher binding energy, and the C–O peak shifts to lower energy
by 0.5 eV, revealing the interfacial charge transfer from CdS to
graphene. Importantly, new peaks (404.5 and 411.3 eV)
originating from the Cd–O bond appear in CdGDY,44 together
with the Cd–S bond shifted to higher energy by 0.4 eV (due to
interfacial charge transfer). Moreover, the peak of C=C in
CdGDY shifts to lower energy by 0.4 eV. These results indicate
chemical bonding (Cd-O-C) between CdS and graphdiyne
accompanied by the interfacial charge transfer from CdS to
graphdiyne, which explains the stronger interfacial interaction
was elucidated from Raman spectra (Fig. S2), together with
graphene oxide (GO), graphdiyne, CdG, and CdS for comparison.
Particularly, Raman spectra of GO, graphdiyne, CdG, and CdGDY
show D peak at 1344 cm−1, a signature of disorder, and G peak
at 1585 cm−1, in-plane stretching vibrations of sp2-hybridized
carbon atoms, respectively. The result indicates the existence of
lattice defects and sp2 carbon network in all the samples.37,38
The characteristic peaks of acetylenic linkages in graphdiyne are
located at 1917 and 2170 cm-1. For CdS, CdG, and CdGDY, three
Raman peaks located at 296, 592 and 900 cm−1 correspond to
longitudinal optical (LO) phonon modes of first-order (1LO),
second-order (2LO) and third-order (3LO) of CdS,39,40 revealing
the existence of CdS. Transmission electron microscopy (TEM)
images show the 2D-layered structure of graphdiyne (Fig. S3a).
This morphology serves to prevent the aggregation of CdS
nanocrystals during the in situ formation process. As seen in Fig.
S3b–f, pure CdS shows significant aggregation to form spheres
with larger size, whereas fine CdS nanocrystals are seen
homogenously distributed on the surfaces of both graphdiyne
and graphene. The small size of CdS nanocrystals in the
presence of graphdiyne or graphene was also confirmed from
XRD data (Fig. S4). Diffraction peaks of CdGDY and CdG are
broader than pure CdS, which is attributed to the smaller
crystallite size. In addition, both CdGDY and CdG show much
higher specific surface area and larger pore volume than pure
CdS and the specific surface area of CdGDY is relatively larger
than that of CdG, as determined from N2 adsorption-desorption
measurement at 77 K (Fig. S5 and Table S1). It is noted here that
well-dispersed CdS nanoparticles can be observed on
graphdiyne surface even after 3-h continuous sonication, while
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between graphdiyne and CdS compared to that between transfer from CdS to graphdiyne through the interface.
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graphene and CdS. Note that the position of S 2p of CdGDY According to the calculated results from DFOigI.: 120d.1-0f,39th/De0TeAle0c2t2r5o6nJ
doesn't change obviously compared to that of CdS. This is transfer direction in CdG is similar to that in CdGDY. However,
because the formation of Cd-O bond, i.e., the introduction of O the electron density at the interface of CdG is much smaller than
would balance the electron density change in CdS with S that of CdGDY. Quantitative results from Bader charge analysis
vacancies, and thus leading to the unobvious change of the reveal that a charge of ~2.40 e is transferred from CdS to
binding energy.
graphene through the CdS(111)/graphene interface, while a
charge of ~3.86 e is transferred from CdS to graphdiyne through
the CdS(111)/graphdiyne interface. Our computational
modelling results indicate stronger electronic coupling and
faster interfacial charge transfer between CdS and graphdiyne
compared to those in CdG, which is consistent with the XPS
analysis.
Fig. 3. (a) EPR spectra of CdS, CdG, CdGDY, GDY and GO, (b) TRPL spectra,
(c) EIS and (d) transient photocurrent response curves of CdS, CdG, and CdGDY.
The interfacial interaction in CdS-graphdiyne was further
demonstrated by electron paramagnetic resonance (EPR)
Fig. 2. Top view (a,d) and side view (b,e) of the charge density difference, and
planar-averaged electron density difference along with Z direction (c,f) for
CdS(111)/graphdiyne and CdS(111)/graphene. The yellow and cyan regions measurements, shown in Fig. 3a. The EPR signals at 3436 Gauss
represent electron accumulation and depletion, respectively. The isosurface value
is 0.002 e/Å3.
(g =2.003) over GDY and graphene can be attributed to oxygen-
containing defects. These signals nearly disappear in the curves
of hybrid materials due to the low loading amount of carbon
materials and the reduction of oxygen-containing groups during
We further simulated the interfacial electronic interaction
the solvothermal process. The weak EPR signal at 3432 Gauss
between CdS and the two carbonaceous materials through DFT
(g=2.005) observed in CdS can be ascribed to sulfur
calculations. As indicated by the aforementioned XPS analysis,
the Cd−O−C bond formed between CdS and graphdiyne was not
found at the CdS/graphene interface. Modelling at this premise
(Fig. 2a and b), the calculated charge density differences in
vacancies.45,46 The intensity of this signal remains nearly
unchanged in the presence of graphene, but it is significantly
enhanced in the presence of graphdiyne. This enhancement is
due to the increase in sulfur vacancies, which are formed to
CdGDY are shown in yellow and cyan areas, representing
counter the charge imbalance resulting from the formation of
electron accumulation and depletion, respectively. At the
(Cd-O-C) chemical bonds at the interface. Sulfur vacancies can
Cd/GDY interface, the CdS surface consists mainly of the cyan
act as electron traps to promote the separation of
area and the graphdiyne surface is mainly filled by the yellow
photogenerated charge carriers47,48 and also improve the
area. Moreover, the planar-averaged electron density
difference at the Cd/GDY interface is also shown in Fig. 2c,
adsorption of CO2 molecules.49,50
Then, the free charge carrier density of the samples was
where positive and negative values represent electron
investigated by testing the band structure based on UV-vis
accumulation and depletion, respectively. The charge
distribution at the interface indicates that electrons mainly
diffuse reflectance spectra and Mott–Schottky (M–S)
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relationship. Fig. S7 suggests that the absorption edge of CdS is noted here that CdGDY shows a much higher CO2 adsorption
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(542 nm) undergoes a blue shift to 522 nm and 517 nm, in the capacity than CdG, which is mainly attribuDteOdI:t1o0.t1h03e9h/Do0llToAw02s2i5te6Js
presence of graphdiyne and graphene, respectively. The direct- between the electron-deficient acetylenic linkages and sulfur
transition bandgaps (Eg)of CdS in pure CdS, CdGDY, and CdG are vacancies serving as adsorption sites of CO2 molecules. In
thus calculated to be 2.29, 2.37 and 2.40 eV, respectively. The addition, the relatively larger specific surface area of CdGDY
absorption blue shift and bandgap increase are attributed to the than that of CdG can further increase the density of surface
quantum size effect of CdS nanocrystals in the composites, in adsorption sites for CO2 molecules.
agreement with the XRD results. Typical n-type semiconductor
The as-prepared photocatalysts were tested in
behavior of CdS was confirmed by the positive slope of the M– photocatalytic gas-phase CO2 reduction in the absence of a
S curve shown in Fig. S8.51 Normally, in n-type semiconductors, sacrificial agent. Control experiments were first performed and
the flat-band potential can be approximately regarded as the it was ascertained that there is no observable product in the
conduction band (CB) potential.52,53 It can be seen that the flat- absence of photocatalyst, CO2 or light irradiation. Table S3 lists
band potentials of CdS determined from the intercept on the X- the yields of the main products CO, CH4, and CH3OH, as well as
axis of the tangent to the M–S curves are –1.1 V (vs. Ag/AgCl, the important by-product, H2, over the different samples used
pH = 7) for all the samples. Hence, the CB potential of CdS is in this study. It is found that the presence of carbon enhances
calculated to be –0.49 V (vs. reversible hydrogen electrode, CO2 photoreduction activity of CdS by more than one order of
RHE). It is known that the density of charge carriers is inversely magnitude. It is interesting to note that better CO2
proportional to the slope of the M–S curves (see experimental photoreduction performance is achieved at a relatively low
details in Supporting Information). In the case of CdGDY, since loading of graphdiyne or graphene; the maximum CO2
the slope of the curve is much smaller than that for CdS and CdG, conversion rate is as high as 18.72 and 14.97 μmol h–1 g–1 for
we conclude that a higher density of charge carriers is present CdGDY and CdG, respectively (See Fig. 4b and Table S3), both of
in CdGDY, which is consistent with the afore-discussed DFT which are higher than or comparable as compared to some
calculation results.
typical results over graphene-based photocatalysts (Table
To elucidate the charge transfer properties of the as- S4).The better activity of CdGDY than CdG is attributed to the
prepared samples, time-resolved photoluminescence (TRPL) more efficient interfacial charge transfer and higher CO2
spectroscopic and photoelectrochemical measurements were adsorption capacity, originating from the stronger interaction at
conducted. The TRPL decay curves (Fig. 3b) are well fitted with the CdS/GDY interface as compared to the CdS/graphene
bi-exponential decay kinetics (Table S2). The short lifetime (τ1) interface, as well as the presence of more electron-deficient
and long lifetime (τ2) reflect the radiative and non-radiative acetylenic linkages with hollow adsorption sites in graphdiyne.
processes, respectively. In comparison with CdS, CdG shows Moreover, only a small amount of H2 was detected when using
both shorter lifetimes, indicating the relaxation of the CdG the different carbon-modified CdS. Importantly, the yield of H2
excitations via non-radiative decay channels, presumably by is much lower than the conversion of CO2, which demonstrates
rapid charge transfer from CdS to the surface of graphene. the high selectivity of CO2 photoreduction on
Moreover, the luminescence lifetimes of CdGDY are prolonged graphdiyne/graphene-supported CdS, as compared to
as compared to those of CdG and pure CdS, ascribed to the hydrogen evolution. It is noteworthy that CdGDY exhibits
defect-induced electron trap states. These defects can capture relatively better selectivity than CdG (see Table S3), which could
photogenerated electrons to effectively split excitons and also be attributed to the improved electronic properties
prevent their recombination,47,48,54 resulting in slower decay resulting from the diacetylenic links.36 Morever, higher
rate of luminescence, and thus longer lifetime. The average photocataltyic stability is found for CdGDY in comparison with
lifetime values (τm) are determined to be 0.50, 0.31, and 0.33 ns CdG under a 4-cycle run of photocataltyic tests (see Fig. 4c and
for CdGDY, CdG, and CdS, respectively. These results reveal that d). This is beccause the stronger interaction between CdS and
both the interfacial transfer and defect capture help to improve graphdiyne assures better structure stability of CdGdY. In
the charge separation and suppress the charge recombination, addition, O2 production is also detected (Fig. S10), which is the
although in different manners. And the results from the product of water oxidation for the accomplishment of the
corresponding steady-state PL spectra are consistent with those photoredox cycle. To confirm that carbon in the reaction
from the TRPL spectra (Fig. S9). Furthermore, the smaller radius products originates solely from CO2, gas chromatography–mass
of electrochemical impedance spectroscopic (EIS) Nyquist plots spectrometry (GC–MS) was employed to detect 12CO and 13CO
and larger photocurrent of CdGDY and CdG (Fig. 3c and d) when using isotope-labelled CO2. As shown in Fig. S11, when
indicate lower charge transfer resistance and improved charge using 13CO2 as the carbon source, the peak of 13CO (m/z=29) is
migration after combining the carbonaceous material with CdS, significantly stronger than that of 12CO (m/z=28), and vice versa.
and the charge transfer ability of CdGDY is better than that of These results prove that the carbon source for the
CdG.
The CO2 adsorption performance of the different samples is
photoreduction products is CO2.
traced in Fig. 4a, which shows that the introduction of
graphdiyne or graphene promotes CO2 adsorption on CdS. The
maximum adsorbed CO2 capacity at P/P0 = 1.0 for CdGDY, CdG
and pure CdS are 0.12, 0.08, and 0.07 mmol g
-1, respectively. It
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Acknowledgements
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This work was supported by the NatiDoOnaI:l10N.1a0t3u9r/Dal0TSAc0i2e2n5c6eJ
Foundation of China (51922081, 21773179, 51961135303,
51932007, and 21872043), the National Key Research and
Development Program of China (2018YFB1502001), the Natural
Science Foundation of Hubei Province of China (2017CFA031),
the Fundamental Research Funds for the Central Universities
(WUT: 2019-III-196) and the National Postdoctoral Program for
Innovative Talents (BX20180231).
Conflicts of interest
There are no conflicts to declare.
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Fig. 4. CO2 adsorption curves (a) of pure CdS, CdG and CdGDY; CH4, CH3OH
and CO evolution during photocatalytic CO2 reduction (b) over pure CdS, CdG
and CdGDY; cycling tests of photocatalytic CO2 reduction over CdGDY (c) and
CdG (d).
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In summary, using theoretical simulation and experimental
characterization, we identify the chemical bonding between
graphdiyne and CdS, which is stronger than the interface
interaction between graphene and CdS. The resultant sulfur
vacancies in CdS, together with the more electron-deficient
acetylenic linkages in graphdiyne, give rise to sufficient CO2
adsorption sites. Taking the advantages of such strong interface
coupling and improved CO2 adsorption, more efficient electron
transfer and storage can be achieved for subsequent CO2
reduction reaction. The obtained hybrid material of graphdiyne
modified CdS thus shows more efficient photocatalytic
reduction of wet CO2 to chemical fuels in the gas phase without
any sacrificial agent, with higher activity, stability and selectivity
in comparison with CdS/graphene photocatalyst. The unique
properties of graphdiyne make it a rising star material for the
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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA02256J
Graphdiyne enhances CO2 photoreduction over CdS with higher activity, selectivity, and
stability in the gas phase without any sacrificial agent compared to graphene.