MOF-based ternary nanocomposites for better CO2 photoreduction: Roles of heterojunctions and coordinatively unsaturated metal sites

Article abstract of DOI:10.1039/c7ta09192c

Semiconductors are the most widely used catalysts for CO₂ photoreduction. However, their efficiencies are limited by low charge carrier density and poor CO₂ activation. Towards solving these issues, a metal-organic framework (MOF)-based ternary nanocomposite was synthesized through self-assembly of TiO₂/Cu₂O heterojunctions via a microdroplet-based approach followed by in situ growth of Cu₃(BTC)₂ (BTC = 1,3,5-benzenetricarboxylate). With increased charge carrier density and efficient CO₂ activation, the hybrid ternary nanocomposite exhibits a high CO₂ conversion efficiency and preferential formation of CH₄. Systematic measurements by using gas chromatography, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, and time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy reveal that the semiconductor heterojunction and the coordinatively unsaturated copper sites within the hybrid nanostructure are attributable to the performance enhancements.

Full text of DOI:10.1039/c7ta09192c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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ARTICLE
MOF-based Ternary Nanocomposites for Better CO2
Photoreduction: Roles of Heterojunction and Coordinatively
Unsaturated Metal Sites
Received 00th January 20xx,
Accepted 00th January 20xx
Xiang He and Wei-Ning Wang†
DOI: 10.1039/x0xx00000x
Semiconductors are the most widely used catalysts for CO2 photoreduction. However, their efficiencies are limited by low
charge carrier density and poor CO2 activation. Towards solving these issues, a metal organicꢀframework (MOF)ꢀbased
ternary nanocomposite was synthesized through selfꢀassembly of TiO2/Cu2O heterojunction via a microdropletꢀbased
approach followed by inꢀsitu growth of Cu3(BTC)2 (BTC = 1,3,5ꢀbenzenetricarboxylate). With increased charge carrier
density and efficient CO2 activation, the hybrid ternary nanocomposite exhibits a high CO2 conversion efficiency and
preferential formation of CH4. Systematic measurements by using gas chromatography, photoluminescence spectroscopy,
Xꢀray photoelectron spectroscopy, and timeꢀresolved inꢀsitu diffuse reflectance infrared Fourier transform spectroscopy
reveal that the semiconductor heterojunction and the coordinatively unsaturated copper sites within the hybrid nanostructure
are attributable to the performance enhancements.
www.rsc.org/
photoexcited charge carriers between the metal oxides in the
12
heterojunction.10,
An added benefit of this type of
Introduction
heterojunctions is that the enhanced solar energy utilization can
be achieved due to the coupled narrow bandgap
semiconductors, such as Cu2O.16
Carbon dioxide (CO2) photoreduction is
a
promising
engineering approach to reduce atmospheric CO2 levels and
simultaneously convert CO2 into hydrocarbon fuels.1ꢀ9 Among
the numerous catalysts for CO2 photoreduction, semiconductors
have been studied intensively because of their low cost, easy
availability, nontoxicity, and exceptional chemical stability.2, 10
However, the semiconductorꢀbased CO2 photoreduction still
suffers from low efficiency and poor selectivity, mainly due to
low charge carrier density and weak ability to activate the
adsorbed CO2 molecules.
Extensive efforts have been directed to creating
semiconductor heterojunctions, many of which, however,
17
require complicated processes.10,
In addition, many
heterojunctions are fabricated in the form of “coreꢀshell”
structure, that is, one semiconductor layer is uniformly coated
on the other.10 This significantly limits the use of
heterojunctions, since in many cases, the surfaces of both
semiconductors are required to participate in the redox
reactions.12 Moreover, despite the promise of enhanced charge
transfer, the semiconductor heterojunctions still suffer from
poor CO2 adsorption and activation due to the low affinity of
CO2 molecules for the semiconductor surface.18 Rational design
of photocatalysts for efficient CO2 photoreduction is thus
highly desirable.
The low charge carrier density is mainly caused by inherent
fast electronꢀhole recombination. This issue may be remediated
by surface metalation,11 where the metals serve as electron
sinks to separate electronꢀhole pairs. Creating heterojunctions
of semiconductors with proper band alignments,12 such as
14
TiO2/Cu2O,10 Fe2O3/Cu2O,13 C3N4/Bi2WO6 and ZnO/CuO,15
is another effective approach to boost the transfer of
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On the other hand, metalꢀorganic frameworks (MOFs), as
Journal Name
an emerging class of highly porous materials, have received
increasing attention in CO2 photoreduction,19ꢀ23 mainly due to
their high surface area, tunable pore size, and rich surface
functionality. Given the exceptional gas adsorption ability of
MOFs, many current studies are focusing on designing
MOFs/semiconductor hybrids to enhance the gas uptake during
CO2 photoreduction.20, 24 For example, the hybrid nanorods of
Zn2GeO4/zeolitic imidazolate framework (ZIF)ꢀ8 exhibited 1.62
times higher CO2 photoreduction efficiency than that of bare
Zn2GeO4,25 attributed to the enhancement in the CO2 adsorption
capacity after the incorporation of ZIFꢀ8. Similarly, C3N4/ZIFꢀ8
composite nanotubes were reported to have a better CO2
photoreduction performance than the pristine C3N4, where the
enhanced CO2 adsorption capacity due to ZIFꢀ8 was the main
reason.24 Besides ZIFꢀ8, other MOFs were also used to increase
the CO2 adsorption ability of the catalysts.26
It should be noted that, adsorption is the first step in CO2
photoreduction. Whether and how the adsorbed CO2 molecules
are activated and then reduced within the MOF structure are
also of great importance to study. In particular, some MOFs
(e.g., Cu3(BTC)2, BTC = 1,3,5ꢀbenzenetricarboxylate) have
coordinatively unsaturated metal sites, which could act as
electron donors to activate the adsorbed gas molecules.27
However, little attention has been directed into this area.
Towards addressing the aforementioned longꢀstanding
issues in CO2 photoreduction (i.e., low charge carrier density
and inefficient CO2 activation), we herein report a rational
development of MOFꢀbased ternary nanocomposites composed
of TiO2/Cu2O heterojunctions and Cu3(BTC)2, where the roles
of the heterojunctions and MOF in charge transfer and CO2
activation are systematically explored. Specifically, the
nanocomposites were synthesized via rapid selfꢀassembly of
TiO2/Cu2O nanoparticles within microdroplets,28, 29 followed by
inꢀsitu growth of Cu3(BTC)2 on the TiO2/Cu2O surface, where
part of Cu2O serves as the sacrificial copper source. The overall
synthetic procedure of the TiO2/Cu2O/Cu3(BTC)2 ternary
nanocomposites is illustrated in Scheme 1 and detailed in
the incorporation of Cu2O (Fig. 1B) due to its broad light
absorption between 350 nm and 500 nm (Fig. S1E). After
partial conversion of Cu2O to Cu3(BTC)2, the color of the
Experimental
section.
The
unique
ternary
TiO2/Cu2O/Cu3(BTC)2 composite possesses heterojunctions
and abundant coordinatively unsaturated copper sites, which
results in not only increased charge carrier density but also
efficient activation of CO2 molecules, therefore leading to high
CO2 conversion efficiency and preferential formation of CH4.
nanocomposite changed to gray
magnification scanning electron microscopy (SEM) images
Fig. S2) show that all the three samples are relatively
(Fig. 1C). The lowꢀ
(
homogeneous. The highꢀmagnification SEM analysis reveals
that the asꢀsprayed TiO2 and TiO2/Cu2O particles have a
spherical shape (Figs. 1D and 1E), resulting from the one
droplet to one particle conversion principle.28 The existence of
Cu element in the TiO2/Cu2O composite was confirmed by
energy dispersive Xꢀray (EDX) analysis (Figs. 1G-I). Cu2O
nanocubes are also found in Fig. 1J, where free surfaces are
available for the subsequent growth of Cu3(BTC)2. After the
growth of Cu3(BTC)2, the spherical shape was largely
maintained (Fig. 1F). The octahedral Cu3(BTC)2 crystals are
clearly observed from both SEM (Fig. 1F) and transmission
electron microscopy (TEM) images (Fig. 1K). The existence of
Cu2O and Cu3(BTC)2 in the composite was further identified by
With systematic measurements and analyses,
a plausible
pathway of CO2 activation and subsequent reduction in this
ternary system was proposed. The outcome of this work
provides new insights in rational design of MOFsꢀbased hybrid
nanomaterials for efficient CO2 photoreduction.
Results and discussion
Materials Characterization. The representative samples, i.e., asꢀ
sprayed TiO2, TiO2/Cu2O, and TiO2/Cu2O/Cu3(BTC)2, were
first subjected to detailed characterization. As shown in the
digital images (Figs. 1A-C), the color change of the samples is
apparent. The white color of TiO2 (Fig. 1A) turned yellow with
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Xꢀray diffraction (XRD) analysis (Fig. 1L). For example, Cu2O
in the TiO2/Cu2O composite was confirmed by the peak of
(111) plane at 36.5° (see also Fig. S1D), the intensity of which
decreased after the growth of Cu3(BTC)2. The presence of
Cu3(BTC)2 in the ternary composite is evidenced by its main
XRD peak at 2θ = ~11.6°, corresponding to its (222) plane.21, 30
Besides, the Fourier transform infrared (FTꢀIR) spectra also
confirm the successful growth of Cu3(BTC)2, where intense
peaks were observed at 1375, 1444, 1560, 1620 and 1646 cmꢀ1,
assigned to carboxylate groups of Cu3(BTC)2.31, 32 It should be
noted that both XRD peaks of Cu2O and Cu3(BTC)2 are
relatively weak due to their low molar ratios. These were
intentionally set to minimize the influence of adsorption
variation (see Fig. S3), in order to better understand the roles of
TiO2/Cu2O heterojunction and Cu3(BTC)2 in the enhancements
of charge carrier density and CO2 molecule activation during
CO2 photoreduction. In addition, the valence states of Cu in the
ternary composite analyzed by Xꢀray photoelectron
spectroscopy (XPS) further confirms the coexistence of Cu2O
and Cu3(BTC)2 in the ternary system. As shown in Fig. 1N, two
major peaks were observed at 932.3 eV and 952.5 eV,
corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. The Cu
2p3/2 was further deconvoluted into two peaks centered 932.2
eV and 933.8 eV, originating from Cu+ in Cu2O and Cu2+ in
34
Cu3(BTC)2, respectively.33,
Furthermore, based on the
precursor components and the XPS data, the molar ratio of
TiO2, Cu2O and Cu3(BTC)2 in the ternary composite was
determined to be 20: 0.55: 0.45 (Supporting Information, S4).
CO2 Photoreduction Analysis. The CO2 photoreduction was
methane (CH4) (Figs. S4F
ꢀ
H
). Specifically, the CO and CH4
carried out in a continuousꢀflow mode. The analysis procedures
yields of the sprayed TiO2 were 72 and 33
respectively (Fig. 2). With the incorporation of Cu2O, the yields
increased to 155 and 85 mol/(h·g), respectively. In the case of
ꢁmol/(h·g),
35
were detailed in our previous studies21,
and are described
briefly in the Experimental section. Prior to CO2
photoreduction analysis, all samples were subjected to control
measurements under helium/water atmosphere to rule out the
possibilities of carbon contamination (Figs. S4A-E). The CO2
photoreduction analysis results indicate that pure Cu3(BTC)2
ꢁ
the ternary composite, the CO yield was increased more than 2ꢀ
folds, while the CH4 yield was almost 4 times higher than those
of TiO2. It’s worth noting that, besides the overall production
yields, the preferential formation of CH4 was also enhanced
along with the sample development. For example, the CH4/CO
ratio in the case of TiO2 was only 0.46. With the incorporation
of Cu2O and Cu3(BTC)2, the CH4/CO ratio was improved to
0.55 and 0.77, respectively (Fig. 2). Moreover, the Cu valence
states are relatively stable during the CO2 photoreduction
process, as evidenced by the similar Cu2p spectra of the ternary
composite before and after 7ꢀhour photocatalysis (Figs. 1N and
S6).
doesn’t serve as the photocatalyst for CO2 reduction (Fig. S5
)
due to the unfavorable charge separation,19, 21 while TiO2 and
the composite reduce CO2 to carbon monoxide (CO) and
There are several possible reasons for the enhanced CO2
photoreduction with the incorporation of Cu2O and Cu3(BTC)2,
including higher CO2 uptake, enhanced charge carrier density,
and improved CO2 activation. In this study, the interference
from the changes in the CO2 uptake capacity may be negligible
since the amounts of Cu2O and Cu3(BTC)2 in the
nanocomposites were controlled at very low percentages, which
ensures minimal improvement in CO2 uptake (Fig. S3). While,
the contributions from the increased charge carrier density and
facile activation of the adsorbed CO2 molecules are discussed
in detail as follows.
Analysis of Charge Carrier Density. To understand the roles of
the Cu2O and Cu3(BTC)2 in the increment of charge carrier
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As shown in Figs. 3A and S9, the asꢀsprayed TiO2
demonstrates a low charge carrier density as no apparent peak
at 455 nm was observed. With the incorporation of Cu2O, the
7HC peak is prominent (Fig. 3B). The strongest 7HC peak
intensity was found in Fig. 3C
,
where the ternary
nanocomposite was used. Based on the calibration results (Fig.
S8), the OH generated by TiO2, TiO2/Cu2O and
TiO2/Cu2O/Cu3(BTC)2 were quantified to be 0.032, 0.067 and
0.105
µmol/(gꢁh), respectively, which implies that the photoꢀ
induced holes in TiO2/Cu2O and TiO2/Cu2O/Cu3(BTC)2 were
2.09 and 3.28 times higher than those in TiO2.
The above results indicate that the decoration of TiO2 with
Cu2O and Cu3(BTC)2 significantly increased the charge carrier
density of the catalyst, which might be attributed to several
reasons, including the enhanced light absorption and promoted
electron/hole separation. The light absorption properties of the
three samples were assessed by the UVꢀVis absorption spectra
as shown in Fig. 4. Specifically, TiO2 only exhibits light
absorption in the UV region (< 400 nm). With the incorporation
of Cu2O, a new absorption peak was observed from 350 to 500
nm, consistent with the yellow color of the TiO2/Cu2O
composite. After the partial conversion of Cu2O to Cu3(BTC)2,
the ternary composite shows a broad absorption peak from 400
nm to 800 nm, suggesting the utilization of visible light for the
enhanced charge carrier density.
In addition to the light absorption, the band alignment
between TiO2 and Cu2O is also critical for the charge carrier
density. Herein, the method proposed by Kraut et al39 was
applied to figure out the precise band alignment between TiO2
and Cu2O. Specifically, XPS measurements were conducted to
acquire the coreꢀlevel energy and upper edge of valence band
(VB) (Figs. 5A-C), meanwhile, the information from the UVꢀ
density, quantitative measurements of charge carrier generation
were carried out. In this study, a convenient and reliable
photoluminescence (PL) technique using coumarin as a probe
molecule to detect and quantify photoinduced holes was
used.36ꢀ38 In this method, coumarin reacts with hydroxyl
radicals (·OH), produced from the reaction of photoinduced
holes with water molecules adsorbed on the photocatalyst
surface, to form highly fluorescent 7ꢀhydroxycoumarin (7HC),
which has a characteristic PL emission peak at around 455 nm
when excited at 350 nm (Figs. S7 and S8). In the current study,
all the three samples have similar water adsorption capacities
(
Fig. S10), thus the amount of the hydroxyl radicals can be
used to indicate the density of the holes.
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Vis analysis (Figs 4 and S1E) was used to derive the band gaps analysis.19 However, the photoinduced electrons can effectively
(BGs) (Fig. 5D). The valence band offset, VBO, could be transfer from semiconductors to the coordinatively unsaturated
determined from the following equation,
copper sites in Cu3(BTC)2, suppressing the electronꢀhole
recombination,19 which is in agreement with experimental
results of ·OH formation. The mechanism of the charge transfer
between semiconductor composites and Cu3(BTC)2 is
ꢀꢁꢂ ꢃ
ꢍꢇꢎꢏ
ꢍꢇꢎꢏ
ꢍꢇꢎꢏ
ꢍꢇꢎꢏ
ꢄꢅ
ꢐ ꢅ
ꢔ ꢐ ꢄꢅ
ꢐ ꢅ
ꢔ ꢐ
ꢆꢇꢈꢉꢊꢋꢌ
ꢆꢇꢈꢑꢒꢎꢌꢓꢉꢌ
ꢕꢖꢈꢉꢊꢋꢌ
ꢕꢖꢈꢑꢒꢎꢌꢓꢉꢌ
ꢉꢊꢘꢙꢊꢚꢖꢛꢌ
ꢆꢇꢈꢉꢊꢋꢌ
ꢉꢊꢘꢙꢊꢚꢖꢛꢌ
ꢕꢖꢈꢉꢊꢋꢌ
ꢗꢅ
ꢐ ꢅ
ꢜ
(1)
schematically shown in Fig. 5E
.
while, the conduction band offset, CBO, could be calculated
using the equation below based on the result from Eq. 1.
CO2 Molecule Activation Analysis. In addition to the enhanced
charge carrier density, the CO2 molecule activation also plays
an important role in the promotion of CO2 photoreduction
efficiency of the ternary nanocomposites. In the present work,
CO2 activation is evidenced by various intermediates formed
during the CO2 adsorption process (Fig. 6), which was analyzed
by timeꢀresolved inꢀsitu diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS). As shown in Fig. 6A, the
adsorbed water molecules are dominant on the pristine TiO2
surface (1640 cmꢀ1),41 along with small amount of bidentate
ꢝꢁꢂ ꢃ ꢁꢞꢆꢇ ꢠ ꢡ ꢀꢁꢂ ꢐ ꢁꢞꢕꢖꢠ
(2)
ꢟ
ꢟ
ꢍꢇꢎꢏ
ꢍꢇꢎꢏ
where
ꢅ
and
ꢅ
are the coreꢀlevel energies of
ꢆꢇꢈꢉꢊꢋꢌ
ꢕꢖꢈꢉꢊꢋꢌ
ꢍꢇꢎꢏ
pure bulk Cu2O and TiO2, respectively;
ꢅ
and
ꢆꢇꢈꢑꢒꢎꢌꢓꢉꢌ
ꢍꢇꢎꢏ
ꢅ
indicate the upper edges of VBs of Cu2O and TiO2,
ꢕꢖꢈꢑꢒꢎꢌꢓꢉꢌ
ꢉꢊꢘꢙꢊꢚꢖꢛꢌ
ꢉꢊꢘꢙꢊꢚꢖꢛꢌ
indicates the coreꢀlevel
ꢕꢖꢈꢉꢊꢋꢌ
respectively;
ꢅ
ꢐ ꢅ
ꢆꢇꢈꢉꢊꢋꢌ
energy differences in the TiO2/Cu2O composite.
With aforementioned information, the precise band
alignment in the TiO2/Cu2O heterojunction was obtained and
schematically shown in Fig. 5E. With a more negative
conduction band of Cu2O, the photogenerated electrons on
Cu2O can migrate to TiO2 for CO2 reduction. On the other hand,
the accumulated holes flow in the opposite direction for water
oxidation to release H+ and O2.40 The efficient transfer of
charge carriers between TiO2 and Cu2O highly remediates the
electronꢀhole recombination and contributes to the higher
charge carrier density.
carbonate (bꢀCO32ꢀ, 1325 and 1585 cmꢀ1)42, 43 and monodentate
ꢀ
bicarbonate (mꢀHCO3 , 1405 cmꢀ1)44
(Fig. 6B). With the
incorporation of Cu2O, the intermediates were dominated by bꢀ
2ꢀ
ꢀ
CO3 (1334 and 1543 cmꢀ1)43, 45 and monodentate HCO3 (1405
cmꢀ1 and 1464 cmꢀ1)44, 46
CO32ꢀ, mꢀHCO3 is more active and can easily be converted to
hydrocarbons.47 It’s worth noting that the percentage of mꢀ
(
Figs. 6C and 6D). Compared with bꢀ
ꢀ
ꢀ
HCO3 on TiO2/Cu2O is larger than that on TiO2, indicating the
When Cu2O was partially converted to Cu3(BTC)2, the
electronꢀhole separation was further enhanced. In particular,
Cu3(BTC)2 itself is unfavorable for charge separation as
demonstrated by the ultrafast transient absorption spectroscopy
enhanced activation ability of the catalyst with the
incorporation of Cu2O, which contributed to the increased CO2
photoreduction efficiency. Furthermore, after the partial
conversion of Cu2O to Cu3(BTC)2, the intermediates were
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ꢀ
dominated by CO2 (1677 cmꢀ1)42 and bridging bidentate
6D). After partial conversion of Cu2O to Cu3(BTC)2, the
amount of coordinatively unsaturated copper sites increased
significantly, leading to a bent CO2 configuration.50 The bent
configuration makes CO2 acting as an anionic molecule,
resulting in the localization of negative charge in the carbon
atom of the CO2 molecule. The terminal oxygen then gains
negative charge with the electron transferred from the
coordinatively unsaturated copper sites due to the charge
polarization.27 With the electrons transferred to the lowest
unoccupied molecular orbital (LUMO), the adsorbed CO2 can
formate (1563 and 1370 cmꢀ1)48
(
Figs. 6E and 6F). Notably,
ꢀ
ꢀ
CO2 and formate are even more active than mꢀHCO3 , which
can easily be transformed to the products upon
photoirradiation.49 The relative activities of the formed
intermediates agree well with the corresponding CO2
photoreduction performance (Fig. 2), which confirms the
importance of CO2 molecule activation in the CO2
photoreduction process in this work.
The formation of various intermediate species is related to
the abundance of the coordinatively unsaturated (or open) metal
sites in the catalysts. These open metal sites could induce
multipole moments in CO2 molecules and enhance the
electrostatic interactions between them, resulting in the
activation of CO2 molecules.50 As evidenced by the XRD
ꢀ 53
then be activated to CO2 . In the presence of protons derived
from the dissociative water adsorption (see Fig. S10C for more
information), the adsorbed CO2 would be activated to formate
(
Fig. 7B). In the meantime, the coꢀadsorption of water can also
help to stabilize the active intermediates through hydrogen
bonding.27
measurements
(Fig. S1D), the Cu2O nanoparticles are
The preferential adsorption sites for CO2 can be indirectly
assessed by observing the CO2 adsorption modes with the aid of
the DRIFTS analysis (Fig. 8). As shown in Fig. 8A, the
adsorbed CO2 molecules have two major characteristic peaks.
The peak centered at 2360 cmꢀ1 is attributed to the functional
groupꢁꢁꢁO=C=O adducts formed by the interaction of CO2 with
the functional groups.54 While, the peak centered 2340 cmꢀ1
with a frequency shoulder at 2330 cmꢀ1 originates from the υ3
mode of CO2 interacting with the coordinatively unsaturated
metal sites.55, 56 In addition, the peak area is associated with the
amount of adsorbed CO2. Thus, it is rational to estimate the
preferential adsorption sites for the CO2 molecules by using the
dominated by (111) facets, where twentyꢀfive percent of the
copper ions are coordinatively unsaturated.51 Compared with
Cu2O, Cu3(BTC)2 has more coordinatively unsaturated copper
sites, arising from the unique structure of the framework.52
Therefore, the incorporation of these two components brings
extra open metal sites to the system, which significantly
changes intermediate species during the CO2 adsorption
process. The formation mechanisms of two typical
ꢀ
intermediates (i.e., mꢀHCO3 and formate) are proposed as
follows and schematically shown in Fig. 7
.
In general, during the coꢀadsorption of CO2 and H2O on the
semiconductors, CO2 interacts with the coordinatively area ratios of the peaks centered at 2340 cmꢀ1 to those at 2360
unsaturated metal sites in a tilted linear configuration, while cmꢀ1. As shown in Fig. 8B, the highest peak ratios were found
H2O dissociates into hydroxyls (see Fig. S10 for more for the TiO2/Cu2O/Cu3(BTC)2 ternary nanocomposites,
information). The linear CO2 then reacts with hydroxyls to form indicating the existence of ample coordinatively unsaturated
ꢀ
mꢀHCO3
(
Fig. 7A). The incorporation of Cu2O led to slightly
copper sites in the structure, which contribute to the subsequent
charge separation and CO2 activation, and hence the
increased coordinatively unsaturated metal sites, and
subsequently enhanced the interaction of CO2 with the metal improvement of overall CO2 photoreduction performance.
ꢀ
sites, thus promoting the formation of mꢀHCO3 (Figs. 6C and
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Taken together, a CO2 photoreduction mechanism of using
the MOFꢀbased ternary nanocomposites is proposed in Scheme
and descried as follows. Initially, CO2 molecules interact with
the solution after 5 min and 10 min, respectively. The solution
was then aged for 30 min. All the above processes were done
with vigorous stirring at room temperature. After that, the Cu2O
nanocubes were collected and washed four times with DI water
via centrifugation and redispersion to ensure the complete
removal of residues. Characterization of asꢀsynthesized Cu2O is
2
various adsorption sites, including Ti, Cu and surface functional
groups. The presence of Cu2O and Cu3(BTC)2 in the ternary
system introduces abundant coordinatively unsaturated copper
sites, which can activate the adsorbed CO2 molecules to form
provided in the supporting information, S1. (2) Aerosol-
ꢀ
active intermediates, including formate species and CO2 . Upon
assisted synthesis of TiO2/Cu2O composites. TiO2/Cu2O
composites were assembled via the aerosol process as detailed
in our previous studies.28 Briefly, aqueous solution containing
TiO2 nanoparticles (0.05 mol/L, Degussa (Evonik) P25, see S1
for characterization) and Cu2O nanocubes (0.0025 mol/L) was
used as the precursor, from which microdroplets were
generated using a Collison nebulizer. The droplets were then
carried through a furnace (400 °C) carried by air (10 L/min).
During the flying process, the droplets underwent solvent
evaporation and selfꢀassembly of Cu2O and TiO2 nanoparticles.
The TiO2/Cu2O composites were collected in the downstream
of the furnace by a microꢀfilter. (3) In situ growth of MOF.
The asꢀprepared TiO2/Cu2O composite was then put into the
BTC solution (0.04 mol/L, VEtOH/VH2O = 9/1), which was then
subjected to vortex mixing for 0.5 min to partially convert
Cu2O to Cu3(BTC)2 and obtain TiO2/Cu2O/Cu3(BTC)2
composites. After that, the final product was immediately
collected and washed with ethanol four times to remove the
residuals via centrifugation/redispersion and then dried in the
vacuum at 50 °C.
light irradiation, these active intermediates are converted to
products with sufficient supply of electrons resulting from the
enhanced light absorption and the efficient charge separation.
In particular, the preferential formation of CH4 originates from
the existence of formate species, which are sequentially
reduced to CHOꢀ, CH3Oꢀ, and CH4 upon light irradiation
(
Scheme 2), where each step involves the transfer of two
58
electrons.57,
The proposed pathway requires only six
electrons, which is energetically favorable as compared to the
conventional eightꢀelectron process of direct reduction of CO2
7
ꢀ
to CH4.3, On the other hand, CO is generated from CO2
(
Scheme 2), which only requires one electron.1, Another
2
pathway where CH4 is formed with CO being the intermediate,
which usually occurs on the metalꢀcoated photocatalysts,35, 59, 60
is unlikely in this work, since the CO evolution in the ternary
system was not suppressed along with the promotion of CH4
formation (Fig. 2).
Conclusions
In summary, a MOFꢀbased ternary composite photocatalyst
(TiO2/Cu2O/Cu3(BTC)2) was designed towards enhanced CO2
photoreduction by increasing the charge carrier density and
facilitate CO2 molecule activation. Systematic CO2
photoreduction analyses were carried out to unravel the
mechanisms. The results show that the incorporation of Cu2O
and Cu3(BTC)2 not only significantly improved the overall CO2
photoconversion efficiency, but also led to the preferential
formation of CH4. The enhanced performance of the catalysts
stems from the increased charge carrier density and efficient
CO2 activation by coordinatively unsaturated metal sites as
verified and quantified by PL, XPS, and DRIFTS
measurements. This work demonstrates a novel strategy to
address low charge density and inefficient CO2 activation
issues, and meanwhile provides insights in the rational design
of MOFsꢀbased hybrid nanomaterials for CO2 photoreduction
and other applications.
Materials Characterization.
The morphologies and inner structures of the samples were
characterized by scanning electron microscope (SEM, Suꢀ70,
Hitachi) and transmission electron microscope (TEM, JEM
1230, JEOL), respectively. A PANalytical X’Pert Pro MPD Xꢀ
ray diffractometer equipped with a CuꢀKα radiation source (λ =
1.5401 Å) was used for the crystallinity determination. The
surface functional groups were analyzed by using a Fourier
transform infrared (FTꢀIR) spectrometer (Nicolet iS50, Thermo
Scientific). Xꢀray photoelectron spectroscopy (XPS)
measurements were carried out by using the Thermofisher
ESCALab 250. A UVꢀVisible spectrophotometer (Evolution
Experimental
Synthetic Procedures.
All chemicals were purchased from commercial suppliers
(SigmaꢀAldrich, VWR, Acros Organics) and used as received
without further purification. (1) Synthesis of Cu2O nanocubes.
The Cu2O nanocubes were synthesized as previously
published:61 1 mL CuSO4 solution (1.2 mol/L) was firstly added
into 400 mL deionized (DI) water. Then, NaOH (1 mL, 4.8
mol/L) and ascorbic acid (1 mL, 1.2 mol/L) were injected into
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220, ThermoFisher) was used to obtain the optical properties of helium gas flow (30 mL/min) at 150 °C for 30 min in order to
the samples.
further remove any residual hydrocarbons. After that, the heater
was turned off and the chamber was cooled down to room
temperature. Helium gas flow was stopped and CO2 gas flow
(10%, helium as the balance gas) was introduced into the
reaction chamber at a flow rate of 4 mL/min after passing
through the water bubbler. The IR spectra were recorded as a
function of adsorption time.
CO2 Photoreduction Analysis.
The CO2 photoreduction was carried out in a continuousꢀflow
mode and the analysis procedures were detailed in our previous
studies.21 In brief, CO2/H2O mixture was generated by passing
CO2 gas (purity > 99.99%, Praxair) through a water bubbler.
The gas mixture, serving as the reactants, was then introduced
into a homeꢀmade photoreactor loaded with catalysts. The
photoreactor consists of a cylindrical cavity made of stainless
steel (60 mm in diameter and 25 mm in depth) and a quartz
glass for the light to pass. The light illumination was provided
by a Xe lamp (450W, Newport). The reactor was first purged
with the gas mixture with a flow rate of 50 mL/min for 30 min.
The flow rate of the gas mixture was then reduced to 3 mL/min
and the lamp was turned on for the photoreduction process. The
effluent gases were analyzed by a gas chromatograph (GC,
Agilent 7890B) equipped with a CarboPLOT P7 column and a
thermal conductivity detector (TCD). For each sample, the
analysis was conducted three times to obtain the average
production yield. A liquid filter (6123NS, Newport) was used
to minimize the thermal effects. As reported in our previous
work where same apparatus was used,35 the reactor temperature
can be controlled within 32 °C with the aid of the filter, limiting
the thermal contributions to CO2 photoreduction. In order to
rule out any possibilities of carbon contamination, control
experiments were conducted with the same procedures using
helium gas (purity > 99.995%, Praxair) instead of CO2 (see Fig.
S4 for details).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This study was supported by the Donors of the American
Chemical Society (ACS) Petroleum Research Fund (57072ꢀ
DNI10), the National Science Foundation (CMMIꢀ1727553),
and the Virginia Commonwealth University (VCU) Presidential
Research Quest Fund. We thank Dr. Dawei Wang for TEM
measurements, which were carried out at the VCU Department
of Anatomy and Neurobiology Microscopy Facility, with
funding from the NIHꢀNINDS Centre core grant
(5P30NS047643).
Notes and references
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