Flower-like CoAl layered double hydroxides modified with CeO2 and RGO as efficient photocatalyst towards CO2 reduction

Article abstract of DOI:10.1016/j.jallcom.2021.160650

In this article, a simple yet effective in situ hydrothermal method has been developed to synthesize a series of CeO₂ and reduced graphene oxide (RGO) stepwise-doped CoAl-LDH (CACR) composites with CoAl-LDH/CeO₂ (CAC) as the intermediate. The smart design well integrates the photo-electrochemical merits of CoAl-LDHs and CeO₂ with conductive RGO to form an optimized formula with high light-harvesting ability, efficient charge separation, and rapid electron transfer. Owing to these features, the sample of CAC-5-R-10 containing 5 wt% CeO₂ and 10 wt% RGO, is found to be the most efficient one as the catalyst for CO₂ photoreduction with a high average CO evolution rate of 5.5 μmol·g^?1·h^?1 at 25 °C without sacrifice reagent or extra photosensitizer under ultraviolet light, which surpasses those of the single CeO₂, CoAl-LDHs, the intermediate of CAC, and many reported LDH-based counterparts. With the assistance of Mott–Schottky curves, the Z-scheme mechanism is proposed for the significantly enhanced photo-activity of the CAC-5-R-10. The present work provides a good sample for integrating LDHs with a second semiconductor and conductive RGO for collaborative optimization of photoelectric characteristics towards enhanced CO₂ photoreduction.

Full text of DOI:10.1016/j.jallcom.2021.160650

Journal of Alloys and Compounds 881 (2021) 160650
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Flower-like CoAl layered double hydroxides modified with CeO2 and RGO
as efficient photocatalyst towards CO2 reduction
Zhao Lia, Zhi Liub,⁎, Yang Lia, Qiu Wanga,⁎
a Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China
b Department of Chemistry, College of Science, Shantou University, Shantou, Guangdong 515063, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, a simple yet effective in situ hydrothermal method has been developed to synthesize a series
of CeO2 and reduced graphene oxide (RGO) stepwise-doped CoAl-LDH (CACR) composites with CoAl-LDH/
CeO2 (CAC) as the intermediate. The smart design well integrates the photo-electrochemical merits of CoAl-
LDHs and CeO2 with conductive RGO to form an optimized formula with high light-harvesting ability, ef-
ficient charge separation, and rapid electron transfer. Owing to these features, the sample of CAC-5-R-10
containing 5 wt% CeO2 and 10 wt% RGO, is found to be the most efficient one as the catalyst for CO2
photoreduction with a high average CO evolution rate of 5.5 μmol·g−1·h−1 at 25 °C without sacrifice reagent
or extra photosensitizer under ultraviolet light, which surpasses those of the single CeO2, CoAl-LDHs, the
intermediate of CAC, and many reported LDH-based counterparts. With the assistance of Mott–Schottky
curves, the Z-scheme mechanism is proposed for the significantly enhanced photo-activity of the CAC-5-R-
10. The present work provides a good sample for integrating LDHs with a second semiconductor and
conductive RGO for collaborative optimization of photoelectric characteristics towards enhanced CO2
photoreduction.
Received 11 March 2021
Received in revised form 8 May 2021
Accepted 30 May 2021
Available online 2 June 2021
Keywords:
CoAl-LDHs
CeO2 nanoparticles
RGO
CO2 photoreduction
Z-scheme mechanism
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
making their composition easily adjustable, allowing for great flex-
ibility in tuning the electron structure of these materials. In parti-
Photocatalytic technology is increasingly coming to be re-
cognized as a favorable alternative that can utilize limitless solar
energy to convert CO2 into hydrocarbon fuels, which has the po-
tential to overcome environmental issues and the energy crisis [1,2].
Generally, the main mechanism of CO2 photoreduction involves the
excitation of electron-hole pairs by appropriate photons, and the
thus extracted electrons are transported to the surface active sites
where reduction of the adsorbed CO2 molecules takes place [3,4].
However, the high energy barriers that frequently need to be over-
come render the process more difficult and illustrate the need for
research into practical methods to facilitate photoinduced electrons
towards CO2 photoreduction [5].
Among recently investigated photocatalysts [6,7], the layered
double hydroxides (LDHs) have attracted considerable attention in
photocatalytic CO2 reduction due to their superior properties such as
layered structure, high porosity, and facile synthesis [8,9]. Moreover,
the anions and metal cations of LDHs can be exchanged easily
cular, Co-based LDHs have become an excellent candidate for CO2
reduction due to its evenly distributed reductive Co2+ sites on the
LDH layers [10]. The typical CoAl-LDH photocatalyst has also been
developed in attempts to boost CO2 reduction, but the efficiency of
the pristine photocatalyst is severely limited due to high electrical
resistance and relatively poor charge separation [11]. However, re-
cent efforts using CoAl-LDH-based composite photocatalysts have
yielded significantly improved outcomes [12,13], and many methods
have been reported for modifying the CoAl-LDHs. For instance, Sa-
nati et al. used g-C3N4/CoAl-LDH as a new electrode material for
supercapacitor applications [14], while Jo et al. prepared a C dot/
CoAl-LDH photocatalyst for the highly efficient and selective re-
duction of CO2 with water to from CH4 under simulated-solar-light
illumination [15]. Chen’s group showed the superiority of the
Z-scheme heterojunction photocatalyst AgBr@Ag/CoAl-LDH, which
improved the outcome of the photocatalytic reduction of CO2 by
effectively promoting the separation of photogenerated electron-
hole pairs [16]. From these results, it can be inferred that introducing
semiconductors to LDHs to form a Z-scheme photocatalytic system
could lead to significant improvements on photocatalytic activity.
Therefore, more research in the design of a Z-scheme composite
⁎
Corresponding authors.
E-mail addresses: zhiliu@stu.edu.cn (Z. Liu), qiuwang@lnnu.edu.cn (Q. Wang).
https://doi.org/10.1016/j.jallcom.2021.160650
0925-8388/© 2021 Elsevier B.V. All rights reserved.

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
photocatalyst that incorporates CoAl-LDHs for the further improve-
ment of photocatalytic activities for CO2 reduction of is highly at-
tractive and desirable.
2.4. Preparation of CACRs
The procedure for the preparation of CoAl-LDH/CeO2/RGO can be
divided into two steps. First, the CoAl-LDH/CeO2 was prepared using
a hydrothermal method. Briefly, a known mass of CeO2 was dis-
persed in deionized water, to which Co(NO3)2·6H2O (0.006 mol), Al
(NO3)3·9H2O (0.002 mol), urea (0.04 mol), and NH4F (0.01 mol) were
dissolved. The resulting mixed suspension was transferred to a
Teflon-lined autoclave (100 mL) at 120 ℃ for 24 h. The precipitate
obtained was washed with deionized water until the washing’s pH
was neutral, then dried overnight at 60 ℃ in a vacuum. In this
manner, CoAl-LDH/CeO2 composites with CeO2 weight ratios of 1%,
3%, 5%, 8%, and 10% were obtained, which were denoted as CAC-1,
CAC-3, CAC-5, CAC-8, and CAC-10, respectively. Secondly, in order to
prepare the RGO containing ternary composite photocatalysts
(CACR), different ratios of RGO (5%, 10%, and 15%) were added to a
sample of the CAC composite with the optimal photocatalytic ac-
tivity of CAC-5 determined by preliminary evaluation, using the
method described above. Finally, the CoAl-LDH/CeO2/RGOs catalysts
with an RGO weight ratios of 5%, 10%, and 15% were obtained and
denoted as CAC-5-R-5, CAC-5-R-10, and CAC-5-R-15, respectively. In
addition, the pure CoAl-LDH was prepared to compare photo-
catalytic activities using the similar method, but without adding
CeO2 and RGO.
Cerium oxide (CeO2) is an n-type semiconductor exhibiting
properties of interest such as a wide band gap, nontoxicity, stability
against photo-irradiation, and chemical inertness. Its unique 4f
electronic configuration makes it suitable for the fabrication of
complex oxides or as a dopant to improve catalytic activity [17].
Therefore, the rational design of a Z-scheme photocatalyst in-
corporating dispersed CeO2 should convert the modified CoAl-LDHs
into a highly efficient photocatalyst for the reduction of CO2. While
previous research has used solid electron mediators, such as Au, Ag,
and others, to fabricate Z-scheme structures [18,19], the high cost of
these noble metals places a serious limitation on their practical
applications, so cheaper and more widely available materials would
be highly desirable as substitutes. With these considerations in
mind, a viable substitute appears to be reduced graphene oxide
(RGO), with its 2D structure, large specific surface area, and reported
superior performance [20]. Indeed, the introduction of RGO into
photocatalysts has been reported to improve the efficiency of pho-
togenerated electron-hole pairs [21]. It is therefore proposed to
combine the properties of CeO2 with RGO as an electron mediator to
construct a Z-scheme system with improved photocatalytic activity.
Bearing the aforementioned aspects in mind, herein, a series of
novel CeO2 and RGO-modified CoAl-LDH composites (denoted as
CoAl-LDH/CeO2/RGO with the abbreviation of CACR) is constructed
as the catalysts for CO2 photoreduction with improved mass transfer
efficiency and enhanced light-harvesting capacity and utilization of
photons. The crystal, morphological, and photoelectrical character-
istics of CACRs are investigated by various technologies. The optimal
sample of CAC-5-R-10 shows remarkably enhanced photocatalytic
performance for CO2 reduction. A Z-scheme CO2 photoreduction
mechanism of the CAC-5-R-10 is also proposed. This work provides a
new strategy and opportunity in the design of LDH-based photo-
catalysts for the efficient photocatalytic reduction of CO2.
2.5. Photocatalytic CO2 reduction
The photocatalytic activity of the synthesized samples is re-
flected by its CO2 reduction ability. For each test, 100 mL of deionized
water (H2O) is poured into a custom-made quartz reactor (300 mL),
followed by addition of 0.05 g of photocatalyst. The reactor is sealed,
and CO2 is flowed through the sample for 10 min, and then turn on
the ultraviolet (UV) light (200 W), which placed on the tops of quartz
reactor about 2.0 cm. The reaction temperature was monitored at 15,
25, 35 °C using cooling circulating-water. At 60 min intervals, 500 μL
of gaseous products from the reaction are sampled for measurement
and analyzed by a gas chromatograph (GC-7920) with a thermal
conductive detector and a flame ionization detector. To exclude the
possibility of carbonaceous component on the photocatalysts (RGO)
to contribute the hydrocarbon product(s), an additional isotopic la-
beling experiment with 13CO2 instead of 12CO2 as the raw gas is
conducted under the identical reaction conditions by combinative
analysis of gas chromatography-mass spectrometry (GC-7920 and
MS-5975).
2. Experimental
2.1. Chemicals and materials
Co(NO3)2 · 6H2O (ACS) was supplied by ALADDIN Reagent Co.,
Ltd., Al(NO3)3.9 H2O (AR), NH4F (GR), and urea (AR) were all pur-
chased from the Shanghai Chemical Reagent Company and used as
the precursors for the preparation of CoAl-LDHs. Ce(NO3)2·6H2O
(ACS grade, 99.95% purity) was supplied by ALADDIN Reagent Co.,
Ltd., Shanghai and used to synthesize CeO2. Polyethylene glycol 4000
(PEG, 4000) and ethylenediamine were purchased from Sinopharm
Chemical Reagent Co., Ltd and were used the synthesis of graphene
oxide (GO).
2.6. Characterization
The various characterizations of samples are presented in
Supporting Information.
3. Results and discussion
2.2. Preparation of CeO2
3.1. Structure and morphology characteristics
A certain amount of Ce(NO3)2·6H2O was grinded in an agate
mortar for 25 min. Next, the prepared Ce(NO3)2·6H2O powder was
placed in a crucible and calcined at 300 °C for 4 h, at a heating rate of
2 ℃ min−1. Finally, when the temperature had dropped to room
temperature, the sample was removed and ground to a fine con-
sistency.
The crystallinity of the synthesized samples are determined by
their X-ray diffraction (XRD) patterns, as shown in Fig. 1. As pre-
viously reported, the diffraction peak at 23.9° in Fig. 1a is attributed
to RGO of (002) crystal [23]. The peaks at 28.6°, 33.1°, 47.5°, and
56.4°, as observed in Fig. 1b, are ascribed to the cubic phase of CeO2
(JCPDS No. 43-1002), corresponding to the (111), (200), (220), and
(311) crystal planes [24]. The CoAl-LDHs exhibit seven diffraction
peaks at 11.5°, 23.2°, 34.4°, 39.0°, 46.5°, 60.0°, and 61.3° (Fig. 1c),
corresponding to the (003), (006), (012), (015), (018), (110), and (113)
crystal planes, respectively [25]. The four main diffraction peaks of
CeO2 are clearly visible in CAC-5 (Fig. 1d), indicating successful in-
corporation of CeO2 into the composite catalyst. For CAC-5, the
2.3. Preparation of reduced graphene oxide (RGO)
First, graphene oxide (GO) was synthesized via a modified
Hummers’ method [22] with a description of synthetic protocol in
Supporting Information. Then, the RGO was synthesized using
ethylene glycol and ethylenediamine for reduction.
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Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
presents the high overlaps of the undistinguishable crystal planes of
CeO2, RGO, and CoAl-LDHs, implying that the three components are
heterogeneous in structure rather than a simple physical mixture of
their separate phases. Such tight coupling is favorable for the se-
paration of photogenerated carriers, thus improving the photo-
catalytic activity.
e
d
(006)
Typical atomic force microscopy (AFM) analysis is performed to
determine the thicknesses of the as-prepared samples, as shown in
Fig. 4. The thickness obtained for the selected regions of CAC-5 is
3.7 nm (Fig. 4b2), much greater than that for the CoAl-LDHs (1.5 nm)
(Fig. 4a2). This may be the result of the high-degree of dispersion
exhibited by CeO2, which is deposited in large scale on the skeleton
of CoAl-LDHs during the hydrothermal reaction process. Meanwhile,
compared with the CoAl-LDHs and CAC-5, the CAC-5-R-10 has the
greatest thickness at 6.5 nm (Fig. 4c2) indicating that RGO and CAC-5
are closely joined together. Overall, the AFM measurements serve as
further evidence to confirm successful formation of the CAC-5-R-10.
The roughness and three-dimensional structure of the as-prepared
samples are further illustrated in Fig. 4a3–c3. On the basis of the
three-dimensional images and the calculated results, the CoAl-LDHs
is found to have the lowest roughness (Rb = 1.33), follows by CAC-5
(Rb = 2.45), while in contrast the CAC-5-R-10 (Rb = 5.51) has the
highest roughness. This result shows that incorporation of both CeO2
and RGO in CAC-5-R-10 leads to architectures with increased
roughness on the surface, in good accordance with the obtained AFM
images. Moreover, the specific surface areas (SSAs) of different
samples depend on their roughness, and therefore N2 adsorption/
desorption experiments are carried out (Fig. 5a).
The N2 adsorption–desorption isotherms and the corresponding
pore size distribution (PSDs) are depicted in Fig. 5a. It is found that
the CoAl-LDHs, CAC-5, and CAC-5-R-10 display the type-IV iso-
therms, indicating their essentially mesoporous structures, well
consistent with their average pore sizes listed in Table S1. However,
the PSDs of CAC-5 and CAC-5-R-10 show a backward tendency to
smaller mesorpores sizes (inset of Fig. 5a) accompanying with the
obvious decrease of peak intensity, suggesting the relatively low
proportion of mesopores of the two samples. The calculated SSAs by
the Barrett–Joyner–Halenda method of CAC-5 and CAC-5-R-10 are
19.3 and 9.9 m2 g−1, respectively, much lower than that of the CoAl-
LDHs (89.7 m2 g−1). With cage-like MIL-101 as substrate, Meng et al.
prepare a CdS quantum dots and carbon nanodots (CDs) co-im-
mobilized ternary photocatalyst (CD/CdS@MIL-101) for H2 evolution
by a one-step double solvents method. Compared with the pure MIL-
101, CdS@MIL-101, and CD@MIL-101, the CD/CdS@MIL-101 possesses
the lowest SSA but the highest photocatalytic activity for H2 evolu-
tion. They attribute the SSA decrease of CD/CdS@MIL-101 to the pore
blockage by the introduction of CDs and CdS [26]. Similar as the case,
the introduced CeO2 and RGO also scatter on the nanoflakes surface
of CoAl-LDHs (Figs. 2a, d, and e) and obstruct some of the inner
interstitial spaces, and thus leading to the decrease in SSA of the
corresponding samples. The following photocatalytic evaluations of
CO2 reduction exhibit the optimal activity of the CAC-5-R-10. These
results mean that high SSA does not correspond to high photo-
catalytic activity, and there is no necessarily relationship between
the two. To figure out this, CO2 adsorption-desorption experiments
are carried out to assess the CO2 adsorption capacity of the three
samples (Fig. 5b). In contrast to low absorbed amounts of CO2 of the
CoAl-LDHs and CAC-5, the CAC-5-R-10 shows a higher value, further
shedding light into the N2-sorption results that a photocatalyst with
a high SSA does not mean to absorb more CO2 molecules for en-
hanced photocatalytic activity. The suitable band structure for more
light absorption and utilization, efficient separation of photo-
generated electron-hole pairs, and strong interaction with CO2 mo-
lecules of the photocatalyst also plays critical roles on determining
the photocatalytic performances, especially for the multi-compo-
nent one.
(003)
(012)
(015)
(018)
(220)
(110)(113)
(311)
c
(111)
(200)
b
23.9°
a
10
20
30
40
2
50
(oC)
60
70
80
θ
Fig. 1. XRD patterns of (a) RGO, (b) CeO2, (c) CoAl-LDHs, (d) CAC-5, and (e) CAC-
5-R-10.
diffraction peak at 34.4° is stronger, which may indicate that CeO2 is
uniformly dispersed in the composite catalyst with a concomitant
increase in crystallinity, which is also conducive to greater absorp-
tion of radiation in the visible range. In Fig. 1e, the characteristic
diffraction peaks of the CoAl-LDHs, CeO2, and RGO can be seen, while
no other peaks from possible impurities are detected, indicating
successful formation of the CAC-5-R-10 ternary composite.
The morphological features of the prepared materials are studied
by SEM. The flower-like structure typical of layered double hydro-
xide is observed in the CoAl-LDHs, with 1 µm spherical agglomerates
(Fig. 2a) and numerous protruding thin 2D nanoflakes, which greatly
increase the number of active sites for the photocatalytic reduction
of CO2. Pure CeO2 consist of nanometer-sized particles (Fig. 2b),
while pure RGO is composed of extremely thin sheets (Fig. 2c), a
feature that is further revealed by wrinkles observed to be several
micrometers in length. As shown in Fig. 2d, the incorporation of
CeO2 affects the degree of dispersion of the CoAl-LDHs. Multiple
CeO2 assemblies emerge on the CoAl-LDHs surface, causing a certain
degree of agglomeration. During the hydrothermal reaction, as metal
ions in the precursors are hydrolyzed and gradually converted to the
CoAl-LDHs in the presence of urea, CeO2 nanoparticles are simulta-
neously broken down into smaller sizes and deposited onto the
CoAl-LDHs surface. As shown, the addition of CeO2 and RGO exert a
significant effect on the morphology of the CAC-5-R-10 composite
(Fig. 2e), transforming the hierarchical flower-like microsphere
structure agglomerate observed in the CoAl-LDHs into a more
rounded spherical-like structure. Further magnification of CAC-5-R-
10 (Fig. 2f) depicts clear morphological features, demonstrating that
CeO2 and RGO are deposited on the surface of CoAl-LDHs. Fig. 3 il-
lustrates the corresponding TEM images of the CAC-5-R-10. One can
clearly see an individual sphere-like structure with highly-over-
lapped central area and zigzag brims (Fig. 3a), which is well coin-
cident with its SEM observation (Fig. 2e). In the magnified image
(white circle zone of Fig. 3a), the CeO2 nanoparticles and RGO thin
sheets are well interspersed on the hexagonal nanoflakes of CoAl-
LDHs, as indicated by the orange arrows (Fig. 3b). Further HR-TEM
investigation (white circle zone of Fig. 3b) ascertains the co-ex-
istences of constitutive CeO2, RGO, and CoAl-LDHs, and the mea-
sured lattice spacing are ~0.318, 0.271 and 0.191 nm for the (111),
(200) and (020) crystal planes of CeO2, ~0.312 nm for the (002)
crystal plane of RGO, and ~0.268 nm for the (012) crystal plane of
CoAl-LDH, respectively (Fig. 3c). Moreover, the white circle zone
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Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Fig. 2. FESEM images of (a) the CoAl-LDHs, (b) CeO2, (c) RGO, (d) CAC-5, (e) CAC-5-R-10, and (f) partial amplified CAC-5-R-10.
3.2. Surface chemical composition and group analysis
the asymmetrical -OH stretching vibration [29]. The region from 1600 to
1300 cm−1 is attributed to the O–H bending vibration of physically ad-
sorbed water molecules [30]. There is little change in the FT-IR spectrum
of CAC-5 compares to the CoAl-LDHs, illustrating that the chemical
structure of CoAl-LDHs is virtually unchanged by the hydrothermal
treatment. However, a slight reduction in the intensity of the O-H ab-
sorption band is observed in the CAC-5-R-10, which may be due to
deposition of RGO species between the CoAl-LDHs nanosheets, resulting
in a decrease of interlayer water molecules.
The FT-IR spectra of the as-prepared samples are shown in Fig. 6. In
the CoAl-LDHs, the broad absorption peak at 3520 cm−1 is attributed to
the O-H stretching modes of interlayer water molecules and H-bonded
OH groups, while the weak peak at 1640 cm−1 corresponds to the
bending mode of water molecules. The strong peaks at 1380 and
766 cm−1 belong to the v3 vibrational and bending modes of CO32-, re-
spectively [27]. The absorption peaks from 800 to 500 cm−1 correspond
to the lattice vibrations present in the M-O and O-M-O (where M = Co,
Al) groups [28]. For pure CeO2, the wide peak at 3450 cm−1 is ascribed to
The chemical states and surface elemental composition of CAC-5-
R-10 are examined using XPS, as shown in Fig. 7. The survey XPS
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Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Fig. 3. TEM and HR-TEM images of the CAC-5-R-10 with increasing magnification.
spectrum of CAC-5-R-10 in Fig. 7a indicates that Co, Al, C, Ce, and O
are all present. The XPS spectrum of Co 2p (Fig. 7b) displays the spin-
orbit splitting of Co 2p into Co 2p1/2 (797.2 and 804.1 eV) and
Co 2p3/2 (781.9 and 787.6 eV), suggesting that both Co2+ and Co3+ are
simultaneously present [28]. The peak at 75.2 eV (Fig. 7c) in the
spectrum of Al 2p suggests the existence of Al3+ species present as
Al–OH [25]. The complex pattern of the C 1 s peak can be decon-
voluted into five peaks center at 284.6, 285.3, 286.8, and 290.4 eV,
which may be attributed to sp2-C, sp3−C, C−O, and O═C−O, respec-
tively, derive from the adsorbed organic molecules and CO32- groups
(Fig. 7d) [28]. The Ce 3d spectrum exhibits four peaks, as can be
observed in Fig. 7e. The peaks at 885.3 eV and 888.6 eV are ascribed
to the binding energies of Ce 3d5/2, while the remaining peak at
904.6 eV and 906.6 eV are ascribed to that of Ce 3d3/2, indicating a
configuration corresponding to Ce4+ present in the material [31,32].
The O1s XPS spectrum shows one peak at 532.4 eV (Fig. 7f), which is
assigned to the hydroxyl groups [33]. This result is due to the low
CeO2 content in CAC-5-R-10, so that most of the oxygen presents in
LDHs exists as interlaminar hydroxide.
3.3. Photo-electrochemical analysis
To understand the light absorption ability of the as-prepared
samples, the UV–vis DRS spectra of the CoAl-LDHs, CeO2, CAC-5, and
CAC-5-R-10 are record. As shown in Fig. 8a, the CoAl-LDHs exhibit
two distinct absorption bands, a broad band in the visible region
centers around 558 nm, and a sharper UV band around 300 nm.
Delamination shifts the middle band to ~520 nm, resulting in the
appearance of an additional absorption band around 670 nm. The
bands from 520 to 558 nm are indicative of the 4T1g(F) → 4T1g(P)
transition of Co2+ octahedrally coordinate by weak field ligands [34],
while the 670 nm band corresponds to a 3A2g(F) → 3T1g(F) transi-
tion arising from spin-orbit coupling [35]. The UV absorption may
have arisen from ligand → metal charge transfer at the CoAl-LDHs
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Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Fig. 4. AFM images, height profiles along the lines of AFM, and the corresponding three-dimensional models drawn from AFM images of CoAl-LDHs (a1–a3), CAC-5 (b1–b3), and
CAC-5-R-10 (c1–c3).
surface. These absorption features translate to an optical band gap of
2.10 eV (Fig. S1a), consistent with previous findings in the literature
[36]. CeO2 exhibits a typical absorption edge at about 427 nm, with a
calculated optical band gap of 2.84 eV (Fig. S1b). For the CAC-5, the
light absorption ability is significantly enhanced compares with the
CoAl-LDHs, the absorbance edge has been taken place distinct red-
shift, up to of 530 nm, the results could demonstrate that the en-
hanced light absorption is due to the introduction of CeO2 species. In
addition, it deserve to note that the DRS spectra of CAC-5-R-10 are
stronger than CAC-5. The significantly enhanced light absorption
prove that the introduce CeO2 and RGO play a crucial role in im-
provement of photocatalytic activity. Photoluminescence (PL) is
another well-known and widely accepted technique for in-
vestigating the separation efficiency of electron-holes in semi-
conductor [37–39]. As shown in Fig. 8b, a strong PL emission peak is
observed in CeO2 at around 480 nm (in accordance with the pre-
viously reported value) [40]. A strong PL emission peak is also ob-
served in the CoAl-LDHs, located nearby at around 490 nm [41].
Meanwhile, it is clear that the main emission peak of CAC-5 around
490 nm is of much weaker intensity compares to those of the CoAl-
LDHs and CeO2, suggesting that the separation rate of electrons/
holes and the photocatalytic activity is higher in CAC-5 compares to
the pure non-composite materials. Furthermore, the PL emission
peak in CAC-5-R-10 at around 490 nm is the weakest from among all
the samples. In general, a more intense spectrum indicates a higher
efficiency for the recombination of electrons-holes, while the op-
posite holds true for lower intensity spectra. Therefore, the PL
spectrum suggests that CAC-5-R-10 has the highest separation rate
of electron-holes. This illustrates that the charge transfer between
CoAl-LDHs and CeO2 is remarkably improved after the incorporation
of RGO and the decreased recombination of photogenerated elec-
trons and holes, which is the main reason for improving photo-
catalytic activity. To further figure out this point, TRPL decay spectra
are detected to measure the average lifetimes of the photogenerated
charge carriers. As shown in Table S2 and Fig. S2, the average life-
times of the entire decay in the CAC-5 and CAC-5-R-10 are 6.04 and
10.67 ns, respectively, quantitatively explaining the great advantage
of the incorporation of RGO on retarding the recombination of
charge carriers for efficient photocatalysis.
In order to gain further insights into the separation efficiency of
electron-hole pairs in the composite photocatalysts, photocurrent
tests of the three samples are further performed. In general, a higher
photocurrent indicates a higher efficiency of electron-hole separa-
tion, which further indicates
a higher photocatalytic activity
[42–44]. Fig. 9 shows the photocurrent–time curves of the as-pre-
pared samples with on–off intermittent UV irradiation cycles. The
weakest photocurrent is clearly shown on the CoAl-LDHs, indicating
that it has the lowest electron-hole separation rate. Meanwhile, the
6

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Fig. 5. (a) N2 adsorption-desorption isotherms and (b) CO2 adsorption-desorption isotherms of the CoAl-LDHs, CAC-5 and CAC-5-R-10.
7

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
counterpart, suggesting the crystal structure stability of CAC-5-R-10
during reaction. (Fig. S4). The samples obtained by overwhelming
introduction of CeO2 to CoAl-LDHs (CAC-8 and CAC-10) and RGO to
CAC−5 (CAC−5−R−15) greatly deteriorate the activity of the CAC and
CAC-5-R, whose yields of CO present striking decays compared with
those of the CAC-5 and CAC-5-R-10, respectively. This arises pre-
sumedly from the broad coverage of RGO to the CAC-5, inhibiting the
light absorption and utilization. Simultaneously, it can be clearly
found the O2 evolution with a yield of 14.7 μmol · g−1 for the CAC-5-
R-10. The stoichiometric ratio of CO to O2, in terms of CO evolution
rate to O2 evolution rate (ratio of μmol·g−1·h−1), is calculated to be
1.87, approaching to the theoretical value of 2. Accordingly, the de-
tected yield of H2 evolution is as low as to 1.6 μmol · g−1, well con-
firming that most of the photogenerated electrons from the valence
band (VB, H2O → 1/2 O2 + 2 H+ + 2e-) of the CAC-5-R-10 are involved
in the reaction for the reduction of CO2 but not for the reduction of
H2O. Furthermore, a couple of the blank evaluations without a
photocatalyst, photoirradiation (in dark) or under an Ar atmosphere
do not show the CO evolution, implying that the CO in this case is
indeed originated from CO2 photoredcution. Considering the fact
that the interlayer CO32− in the CoAl-LDH of CACRs are an underlying
carbon source contributing to the evolved CO, we perform an ion-
exchange treatment for the CAC-5-R-10 using a NaH2PO4 aqueous
Fig. 6. FT-IR spectra of (a) the CoAl-LDHs, (b) CeO2, (c) CAC-5, and (d) CAC-5-R-10.
CAC-5 composite displays a significantly improved photocurrent
with the intensity approximately 1.2 times higher than that of the
CoAl-LDHs. The highest photocurrent intensity is observed in CAC-5-
R-10, which is approximately 1.5 times higher than on the CoAl-
LDHs, confirming CAC-5-R-10 as a superior catalyst in terms of its
stronger light harvesting capacity and the highest separation effi-
ciency of electron-hole pairs. In addition, the EIS of as-prepared
samples (Fig. S3) also verify that the incorporation of CeO2 and RGO
can effectively accelerate the transfer of photogenerated carriers to
enhance the photocatalytic activity.
2−
solution (0.5 M) to remove the interlayer CO3 as much as possible,
and the treated CAC-5-R-10 is denoted as CAC-5-R-10-Dc. After the
photocatalytic activity evaluation, the amount of CO generated on
the CAC-5-R-10-Dc in 5 h is 25.2 μmol·g−1, indicating that most of the
evolved CO (91.6%) originate from the photoreduction of CO2, and
the interlayer CO32− in the CoAl-LDH has a minor contribution to the
production of CO. The carbon-13 isotopic labeling results of the CAC-
5-R-10-Dc in Fig. 10c disclose the origin of carbon in the generated
CO. Two signals at m/z = 29 and 32 in the MS are observed, which can
be assigned to 13CO from the used 13CO2 and O2 from H2O oxidation,
respectively. These results well verify that the production of CO is
exactly originated from the introduced 13CO2 excluding its potential
possibility from both oxidation of RGO and degradation of interlayer
CO32−, and further confirm the simultaneous reactions of CO2 re-
duction to CO and holes oxidation of H2O to O2. In order to in-
vestigated the contribution of thermal reduction to CO2
photoredcution, the yields of CO are also evaluated at two other
temperatures of 15 °C and 35 °C, respectively. As shown in Fig. 10d,
the average CO evolution rate of CO are 5.1 (15 °C) and 7.3 (35 °C)
μmol·g−1·h−1, respectively, presenting slight increment with the in-
creasing reaction temperature. It is commonly recognized that the
increase of temperature will greatly decrease the adsorbed amount
of CO2, thus leading to the activity decay of the used photocatalyst.
Evidently, our results prove the existence of the thermal reduction of
CO2 as a positive supplement to the photoreduction of CO2.
3.4. Photocatalytic CO2 reduction activities
The photocatalytic activities of the as-prepared samples as pho-
tocatalysts are evaluated by CO2 reduction with neutral H2O as re-
ducing agent at 25 °C under UV lamp irradiation in absence of any
sacrificial reagents or photosensitizers, in which the predominant
product is CO with negligible trace amount of CH4 for the different
samples. On the other hand, the reduction of CO2 competes with the
reduction of H2O which means that the splitting of H2O into O2 and
H2 would unavoidably proceed together with the reduction of CO2. It
would not be regarded as the photocatalytic reduction of CO2 if H2
was dominantly evolved as compared to the reduction products of
CO2. In order to affirm the former is the dominant reaction in our
case, we also monitor the evolution of H2. Fig. 10a compares the
photocatalytic activity among all the CAC samples. The CoAl-LDHs
without CeO2 exhibits limited activity. The total yield of CO is
2.91 μmol·g−1 after 5 h reaction with the corresponding average CO
evolution rate of 0.58 μmol·g−1·h−1. Meanwhile, trace amount of O2
and H2 evolutions are also detected. With the increasing CeO2 con-
tent in the CACs, the total yield of CO is significantly enhanced
during the time course compared with that of the CoAl-LDHs with
the better one (CAC-5) delivering a yield of CO of 11.5 μmol·g−1. After
RGO is incorporated into the CAC, the total yield of CO is further
boosted with the best one (CAC-5-R-10) conveying a yield of CO up
to 27.5 μmol·g−1 (Fig. 10b), which can be attributable to the positive
enhancements on absorbed amount of CO2 and generation of elec-
tron-hole pairs with rapid electrons transfer from CeO2 and RGO.
The corresponding average CO evolution rate is calculated to be of
5.5 μmol·g−1·h−1, which is superior to many reported LDH-based
counterparts under the similar evaluation conditions (Table S3).
Moreover, XRD analysis of the CAC-5-R-10 after the photocatalytic
reaction reveals that no significant change of the crystal structure
occurs after UV irradiation for 5 h compared with its original
3.5. Mechanism of photocatalytic CO2 reduction
Mott–Schottky curves are applied to determine the CB potential
of CeO2 and CoAl-LDHs (Fig. S5). It can be found that the
Mott–Schottky curves possess a positive slope at different fre-
quencies from 1500 to 2000 Hz, indicating that CeO2 is a typical
n-type semiconductor. The flat band potential derived from the in-
tercept of the tangent of the Mott–Schottky curves on the X-axis is
−0.43 V (vs Ag/AgCl, pH = 7). Generally, the CB potential of n-type
semiconductor is near the flat band potential. According to the
conversion formula of normal hydrogen electrode potential (NHE)
and Ag/AgCl electrode potential:
E +
(NHE) = E (Ag/AgCl) − E0
0.059 pH, where E0 (Ag/AgCl, at pH = 7) = 0.197 V, the CB potential
(ECB) of CeO2 is therefore determined to be −0.21 V (vs NHE, pH = 7).
The same algorithm is used to calculate the ECB of Co-Al LDHs to be
−0.62 V. Both are very close to the previously reported values [45,46].
Along with the values of the band gap (Eg) from UV−vis DRS for CeO2
8

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
O 1s
Co 2p
781.9
a
b
Co 2p
Ce 3d3
Ce 3d5
797.2
787.6
804.1
C 1s
Al 2p
1200
1000
800
600
400
200
0
810
805
800
795
790
785
780
775
Binding Energy (eV)
Binding Energy (eV)
C 1s
Al 2p
284.6
75.2
c
d
285.3
286.8
290.4
296 294 292 290 288 286 284 282 280
Binding Energy (eV)
80
78
76
74
72
Binding Energy (eV)
532.4
Ce 3d
O 1s
e
f
885.3
888.6
904.6
906.6
538
536
534
532
530
528
910
905
900
895
890
885
880
Binding Energy (eV)
Binding Energy (eV)
Fig. 7. XPS spectra of the CAC-5-R-10: (a) all survey spectrum, (b) Co 2p, (c) Al 2p, (d) C 1s, (e) Ce 3d, and (f) O 1s.
(2.84 eV) and CoAl-LDHs (2.10 eV), the VB potential (EVB) of CeO2 and
CoAl-LDHs can be converted into 2.63 and 1.48 V, respectively, fol-
lowing the formula of EVB = ECB + Eg [47]. Based on these, two pos-
sible CO2 photoreduction mechanisms, the type-II heterojunction
and Z-scheme, have been proposed for the optimal CAC-5-R-10, as
illustrated in Fig. 11. If the former works, the photogenerated
electron-hole pairs can be simultaneously excited on the CoAl-LDHs
and CeO2, during which the photoexcited electrons from the CB of
CoAl-LDHs tend to transfer to the CB of CeO2, meanwhile, the pro-
duced holes migrates correspondingly from the VB of CeO2 to the VB
of CoAl-LDHs. As a result, the photogenerated electrons in the CB of
CeO2 can not reduce CO2 to CO because the ECB of CeO2 (−0.21 V) is
9

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Fig. 8. (a) UV–vis DRS and (b) PL spectra of the CoAl-LDHs, CeO2, CAC-5, and CAC-5-R-10.
10

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
case. Therefore, we postulate the Z-scheme mechanism is feasible
for the present system as the following steps. After light irradiation
onto the CAC-5-R-10, the conventional generation of electron-hole
pairs and electron transition occur from VB to CB of the constitutive
CeO2 and CoAl-LDHs. With the assistance of RGO for conduction, the
electrons at the CB of CeO2 can rapidly transfer to the VB of CoAl-
LDHs and combine with the holes of it through the interface contact
of CeO2 and CoAl-LDHs. So the electrons at the CB of CoAl-LDHs are
effectively employed for reduction of CO2, which well meets the
photocatalytic Z-scheme mechanism. In order to further confirm
this, the electron spin resonance (ESR) data are collected to the
CoAl-LDHs
CAC-5
CAC-5-R-10
5x10-7
4x10-7
3x10-7
2x10-7
−
generated radicals (•O2 and •OH) with 5,5-dimethyl-1-pyrroline N-
oxide (DMPO) as spin-trapping agent to capture under UV−visible
light irradiation [48]. As shown in Fig. S6, the CoAl-LDHs exhibits
strong regular ESR signals for DMPO-•O2−, but the counterpart of the
CeO2 is invisible. Contrarily, there are significant ESR signals for
DMPO-•OH on the CeO2, and the counterpart of the CoAl-LDHs dis-
1x10-7
350
400
450
500
550
600
Time (sec)
appears. Such phenomena is attributed to the functions of the suf-
−
Fig. 9. Photocurrent response curves of the CoAl-LDHs, CAC-5, and CAC-5-R-10 under
UV light irradiation.
ficiently negative ECB of the CoAl-LDHs (ECB = −0.62 V < O2/•O2
=
=
−0.33 V vs NHE) and sufficiently positive EVB of the CeO2 (EVB
−
+2.63 V > H2O/•OH = +2.4 V vs NHE) [49] to generate •O2 and •OH
after the recombination of the electrons from CB of CeO2 with the
holes from VB of the CoAl-LDHs, in very good agreement with the
Z-scheme mechanism. Therefore, the Z-scheme charge transfer
process efficiently improves the separation of photoinduced
higher than that of the theoretical reduction potential of CO2/CO
(−0.53 V). Evidently, the type-II heterojunction mechanism deduced
easily from the Mott-Schottky plots conflicts with our results of the
CAC-5-R-10 with a high yield of CO, implying its unavailability in this
Fig. 10. CO yields of various photocatalysts under UV irradiation: (a) the CoAl-LDHs and CACs; (b) CAC-5 and CAC-5-Rs; (c) isotope analysis of 13CO with 13CO2 as the raw gas by
GC-MS for the CAC-5-R-10-Dc; (d) temperature-dependent photocatalytic activity of the CAC-5-R-10.
11

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Fig. 11. Proposed photoinduced carrier transfer mechanism over the CAC-5-R-10: (a) type-II heterojunction and (b) Z-scheme.
electrons and holes of the CAC-5-R-10 with superior activity for CO2
photoreduction. Base on the above discussion, the following in-
dividual steps in the mechanism of the reduction process are pro-
posed by the following equations:
the relationship between multi-component modified-LDHs and CO2
photoreduction behavior, providing a valuable attempt to overcome
the intrinsic deficiencies of LDH-based photocatalyst in photo-
catalysis.
CoAl- LDHs + hυ → Co-Al LDHs (e− + h+)
CeO2 + hυ → CeO2 (e− + h+)
(1)
(2)
CRediT authorship contribution statement
Zhao Li: Conceptualization, Methodology, Software, Data cura-
CeO2(e ) transferCo Al LDHs(e )
(3)
tion, Writing
Methodology, Investigation. Zhi Liu: Conceptualization, Resources,
Supervision, Writing review editing, Funding acquisition,
-
original draft. Yang Li: Conceptualization,
2H2O + h+ → 4H+ + O2 + 4e− (E0 = +2·4 V)
CO2 + 2H+ + 2e− → CO + H2O (E0 = −0·53 V)
(4)
(5)
-
&
Supervision. Qiu Wang: Validation, Supervision.
Declaration of Competing Interest
4. Conclusions
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
In this work, CeO2 and RGO are incorporated onto the flower-like
CoAl-LDHs to form catalysts with superior activity for CO2 photo-
reduction. Benefiting from the cooperation of the highly dispersed
CeO2 nanoparticles and conductive RGO with enhanced photo-
electric characteristics, the sample of CAC-5-R-10 exhibits both
high photocatalytic selectivity and activity towards CO2-to-CO
(5.5 μmol · g−1·h−1) under ultraviolet light, much higher than those of
the single CeO2, CoAl-LDHs, and the intermediate of CAC-5 coun-
terparts, respectively. More importantly, our findings demonstrate
Acknowledgements
We gratefully acknowledge the financial support to this work by
the National Natural Science Foundation of China (No. 21871124), the
Natural Science Foundation of Guangdong Province, China
(2021A1515010071), the start-up grant for talents under from
12

Z. Li, Z. Liu, Y. Li et al.
Journal of Alloys and Compounds 881 (2021) 160650
Shantou University (130-09419024), and fund for Innovative Talents
of Higher Learning Institutions of Liaoning Province (2018).
boosting photo-oxidation and reduction ability, Appl. Catal. B Environ. 250 (2019)
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Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2021.160650.
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