Visible-Light Photoreduction of CO2 in a Metal-Organic Framework: Boosting Electron-Hole Separation via Electron Trap States

Article abstract of DOI:10.1021/jacs.5b08773

It is highly desirable to convert CO₂ to valuable fuels or chemicals by means of solar energy, which requires CO₂ enrichment around photocatalysts from the atmosphere. Here we demonstrate that a porphyrin-involved metal-organic framework (MOF), PCN-222, can selectively capture and further photoreduce CO₂ with high efficiency under visible-light irradiation. Mechanistic information gleaned from ultrafast transient absorption spectroscopy (combined with time-resolved photoluminescence spectroscopy) has elucidated the relationship between the photocatalytic activity and the electron-hole separation efficiency. The presence of a deep electron trap state in PCN-222 effectively inhibits the detrimental, radiative electron-hole recombination. As a direct result, PCN-222 significantly enhances photocatalytic conversion of CO₂ into formate anion compared to the corresponding porphyrin ligand itself. This work provides important insights into the design of MOF-based materials for CO₂ capture and photoreduction.

Full text of DOI:10.1021/jacs.5b08773

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Communication
Visible–Light Photoreduction of CO2 in A Metal–Organic Framework:
Boosting Electron–Hole Separation via Electron Trap States
Hai-Qun Xu, Jiahua Hu, Dengke Wang, Zhaohui Li, Qun Zhang, Yi Luo, Shu-Hong Yu, and Hai-Long Jiang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08773 • Publication Date (Web): 04 Oct 2015
Downloaded from http://pubs.acs.org on October 6, 2015
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Visible–Light Photoreduction of CO2 in A Metal–Organic Framework:
Boosting Electron–Hole Separation via Electron Trap States
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HaiꢀQun Xu,†,‡,♯ Jiahua Hu,†,§,♯ Dengke Wang,ǁ Zhaohui Li,ǁ Qun Zhang,*,†,§ Yi Luo,†,§ ShuꢀHong Yu,†,‡
and HaiꢀLong Jiang*,†,‡
†Hefei National Laboratory for Physical Sciences at the Microscale, ‡CAS Key Laboratory of Soft Matter Chemistry, Collabꢀ
orative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, §Synergetic Innovation Center
of Quantum Information and Quantum Physics, Department of Chemical Physics, Hefei Science Center of CAS, University of
Science and Technology of China, Hefei, Anhui 230026, P. R. China
ǁResearch Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemꢀ
istry, Fuzhou University, Fuzhou 350002, P. R. China
Supporting Information Placeholder
light contributes to ~43% of solar energy, it is rather imperative to
develop visibleꢀlightꢀresponsive photocatalysts.1g
ABSTRACT: It is highly desirable to convert CO2 to valuable
fuels or chemicals by means of solar energy, while this practically
requires CO2 enrichment around the photocatalysts from the atꢀ
mosphere. Here, we demonstrate that the photoactive porphyrinꢀ
involved metal–organic framework (MOF), PCNꢀ222, can selecꢀ
tively capture and further photoreduce CO2 with high efficiency
under visibleꢀlight irradiation. Mechanistic information gleaned
from ultrafast transient absorption spectroscopy (combined with
timeꢀresolved photoluminescence spectroscopy) has enabled eluꢀ
cidation of the relationship between the photocatalytic activity
and the electron–hole separation efficiency in PCNꢀ222. It is
found that the presence of a deep electron trap state in PCNꢀ222
effectively inhibits the detrimental, radiative electron–hole reꢀ
combination. As a direct result, PCNꢀ222 exhibits significantly
enhanced photocatalytic conversion of CO2 into formate anion
compared to the corresponding porphyrin ligand. This work proꢀ
vides important insights into the design of MOFꢀbased materials
for CO2 capture and photoreduction.
To integrate CO2 capture and its photocatalytic conversion usꢀ
ing solar energy, metal–organic frameworks (MOFs),3 a relatively
new class of porous materials, could be a judicious choice. MOFs
have been extensively studied over the past two decades and
demonstrated to possess promising applications in many fields,4–6
especially in CO2 capture,7 thanks to their high porosity and well
tunable interaction with CO2 molecules. In addition, previous
studies have revealed that metal clusters in MOFs can behave as
inorganic semiconductor quantum entities, and the organic linkers
could serve as antenna to activate these metal clusters upon phoꢀ
toexcitation,8 thus making “MOFꢀbased photocatalysis” possible.
Some MOFs have recently been developed for photocatalytic
water splitting,9 whereas reports on photocatalytic CO2 reduction
using MOFs10 or MOFꢀbased composites11 are still very rare.
Meanwhile, the catalytic mechanism largely remains unclear.
Given the versatile and tailorable characters of MOF structures,
the utilization of solar energy by MOFs, especially in the visible
region, could be feasible by judiciously selecting metal ions and
organic linkers. However, the current modulation of light adsorpꢀ
tion over MOFs is limited to a narrow range by tethering the ligꢀ
and with one or two additional amine groups.10b,c To efficiently
harvest visible light, it is desired to develop MOFs with absorpꢀ
tion covering the visible region as wide as possible.
The heavy dependence on fossil fuels for energy results in excesꢀ
sive CO2 emissions and global warming. Tremendous efforts have
been made to address this challenge. Currently, carbon capture
and sequestration/separation (CCS) technology from postꢀ
combustion effluents is an accepted working approach. The capꢀ
tured CO2 is injected into underground geological formations to
achieve permanent sequestration. Despite this, the catalytic transꢀ
formation of the captured CO2 into highꢀvalue chemicals and/or
fuels would be more desirable instead of simply storing it as a
waste. To this end, the chemical reduction of CO2 with solar enerꢀ
gy becomes one of the most promising solutions. In the past decꢀ
ades, diverse semiconductors, metalꢀincorporated zeolites, and
metal complexes have been exploited for CO2 photoreduction.1
Among them, the molecular systems are mostly limited to expenꢀ
sive noble metal catalysts.1h The TiO2ꢀbased photocatalysts have
been intensively investigated owing to their stability and availaꢀ
bility with low cost.1j,2 Unfortunately, their efficiency for CO2
photoreduction is not satisfactory partly because of the weak CO2
adsorption on the catalyst surface. In addition, most of the reportꢀ
ed photocatalysts are active only in the ultraviolet (UV) region.
Given that the UV light only accounts for ~4% while the visibleꢀ
Bearing these in mind, we have employed a very stable mesoꢀ
porous zirconium–porphyrin MOF, PCNꢀ222 (also called MOFꢀ
545) based on tetrakis(4ꢀcarboxyphenyl)porphyrin (H2TCPP),12
for effective integration of CO2 capture and CO2 reduction under
visibleꢀlight irradiation in the presence of triethanolamine
(TEOA) as an sacrificial agent. The H2TCPP ligand in PCNꢀ222
behaves as a visibleꢀlightꢀharvesting unit and the high CO2 uptake
might facilitate the enrichment of CO2 molecules around the cataꢀ
lytic Zr6 centers, thereby enhancing the photocatalytic efficiency.
Furthermore, interrogations by means of ultrafast spectroscopy
(together with timeꢀresolved photoluminescence spectroscopy),
for the first time on a MOF photocatalyst, have allowed us to
unveil the underlying mechanism. We have found that the emerꢀ
gence of a type of deep electron trap state in PCNꢀ222 enables
effective suppression of the detrimental electron–hole recombinaꢀ
tion. Hence, PCNꢀ222 is capable of supplying longꢀlifetime elecꢀ
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trons for the photoreduction of captured CO2 molecules with
Vis spectrum for the H2TCPP ligand suggest that it is able to act
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much higher activity than that of the H2TCPP ligand.
as a harvesting unit for visible light (Section 4).
HCO3-
(a)
The PCNꢀ222 in needle shape, formulated as Zr6(µ3ꢀ
OH)8(OH)8(TCPP)2, was prepared from ZrCl4 and H2TCPP acꢀ
cording to the documented method (Supporting Information, Secꢀ
tion 2, Figure S1).12 PCNꢀ222 exhibits a 3D network based on Zr6
clusters connected by H2TCPP ligand and features ultraꢀlarge 1D
channels with a diameter of 3.7 nm (Figure 1a and S2). Despite its
high N2 sorption at 77 K and the large BET surface area (ca. 1728
m2/g, Figure S4), PCNꢀ222 adsorbs little N2 at 273 or 298 K. In
contrast, it possesses high CO2 uptake of 14, 35 and 58 cm3/g at
308, 298 and 273 K at 1 atm, respectively, (Figure 1b). This sugꢀ
gests the presence of sound interaction between PCNꢀ222 frameꢀ
work and CO2 molecules. The calculated CO2 heat of adsorption
is 23.6 kJ/mol, implying that PCNꢀ222 can be readily regenerated
with low energy penalty. These features render PCNꢀ222 a suitaꢀ
ble material for CO2 enrichment in a N2ꢀrich atmosphere.
HCOO-
-
CO32
(b)
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Figure 2. (left) The amount of HCOO– produced as a function of
the time of visibleꢀlight irradiation over (a) PCNꢀ222, (b)
H2TCPP, (c) no PCNꢀ222, (d) no TEOA, and (e) no CO2. The
reaction with each photocatalyst (50 mg) in the MeCN/TEOA
(10:1 v/v, 60 mL) solution was irradiated using a Xe lamp filtered
to produce light in the range of 420–800 nm. (right) The 13C NMR
spectra for the product obtained from the reaction with (a) 13CO2
or (b) 12CO2.
The photocatalytic reduction of CO2 over PCNꢀ222 was conꢀ
ducted with TEOA in CH3CN, as solvent due to its good solubiliꢀ
ty for CO2 and possible cationꢀsolvating property beneficial for
the electron transfer to CO2,11c,14 under visibleꢀlight irradiation
(Figure 2 left). Remarkably, PCNꢀ222 exhibits significant photoꢀ
catalytic activity for CO2 reduction. The HCOO– anion was conꢀ
tinuously produced and the amount increased up to 30 ꢁmol in 10
h, a value much higher than those over UiOꢀ66ꢀNH2 and MILꢀ
125ꢀNH2 under similar conditions.10b,c No other products can be
detected in both gas and liquid phases (Figure S8 and S9), sugꢀ
gesting that the catalyst is highly selective toward the conversion.
To examine the stability of PCNꢀ222, recycling experiments have
demonstrated that no noticeable change occurs to the yield rate of
HCOOꢀ during the 3 runs of reaction (Figure S10). Moreover,
powder Xꢀray diffraction profile further demonstrates retained
structure of PCNꢀ222 after reaction (Figure S11). In contrast, no
HCOO– was detected in the absence of either PCNꢀ222 or TEOA,
demonstrating their critical roles in the reaction. Moreover, no
HCOO– can be generated when the reaction was carried out in the
dark, demonstrating a truly photocatalytic behavior (Figure 2 left).
To verify the origin of HCOO–, N2 instead of CO2 as a reactant
was introduced into the reaction system under otherwise similar
conditions. Only a trace amount (0.14 ꢁmol) of HCOO– was obꢀ
tained after 10 h. Furthermore, the isotopic 13CO2 was employed
in the photocatalytic reaction and the product was identified by
13C NMR spectroscopy to confirm the origin of HCOO–. The 13C
NMR spectrum clearly gave a peak at 164.8 ppm corresponding to
HCOOꢀ when 13CO2 was introduced,10b while such signal cannot
be detected when charging 12CO2 only (Figure 2 right, S12 and
S13). The results unambiguously demonstrate that the produced
HCOO– anion comes from CO2 indeed. On the basis of the above
observation, it is safe to conclude that PCNꢀ222 is very active and
capable of selectively converting CO2 to HCOO– under visibleꢀ
light irradiation. To unveil the mechanism behind the CO2 photoꢀ
reduction over PCNꢀ222, electron spin resonance (ESR) studies
have been conducted and reveal that ZrIII ions are generated by
electron transfer (Section 5, Figure S14). We thus propose that,
upon irradiation, the H2TCPP in PCNꢀ222 would behave as an
antenna to absorb visible light to its excited state, which then
transfers electrons to the Zr oxo clusters for the reduction of CO2
to HCOO– in the presence of TEOA as the electron donor (Section
5, Scheme S1).10b During this process, TEOA might be oxidized
to its aldehyde form (Scheme S2, Figure S15).
Figure 1. (a) View of the 3D network of PCNꢀ222 featuring large
channels running through the cꢀaxis. (b) CO2 and N2 sorption
isotherms of PCNꢀ222 at different temperatures. (c) UV–Vis specꢀ
tra of H2TCPP and PCNꢀ222. (d) Mott–Schottky plots for PCNꢀ
222 in 0.2 M Na2SO4 aqueous solution (pH = 6.8). Inset: Energy
diagram of the HOMO and LUMO levels of PCNꢀ222.
The UV–Vis spectrum of PCNꢀ222, which well inherits the feaꢀ
ture of the H2TCPP ligand, shows a broad, strong absorption in
the region of 200–800 nm (Figure 1c). This allows for promoting
the electrons of PCNꢀ222 to an excited state upon visibleꢀlight
irradiation. To elucidate the semiconductor character of PCNꢀ222
and its possibility for subsequent CO2 reduction upon visibleꢀlight
excitation, the Mott–Schottky measurements on PCNꢀ222 were
conducted at frequencies of 500, 1000, and 1500 Hz (Figure 1d).
The positive slope of the obtained Cꢀ2 values (vs. the applied poꢀ
tentials) is consistent with that of the typical nꢀtype semiconducꢀ
tors. The intersection point is independent on the frequency and
the flat band position (Vfb) determined from the intersection is
approximately –0.60 V vs. Ag/AgCl (i.e. –0.40 V vs. NHE). Since
it is generally accepted that the bottom of the conduction band
(LUMO) in nꢀtype semiconductors is approximately equal to the
flatꢀband potential,13 the LUMO of PCNꢀ222 is estimated to be –
0.40 V vs. NHE. With the bandgap energy (Eg) of PCNꢀ222 estiꢀ
mated to be 1.75 eV from Tauc plot (Figure S5), its valence band
(HOMO) is then calculated to be 1.35 V vs. NHE (Figure 1d).
Given the more negative potential of LUMO in PCNꢀ222 than the
reduction potential of CO2 to formate (−0.28 V vs. NHE), it is
theoretically feasible for the photocatalytic reduction of CO2 to
generate formate over PCNꢀ222. The MottꢀSchottky plot and UVꢀ
For comparison, H2TCPP was also employed as a photocatalyst
to reduce CO2 under similar conditions. In contrast, only 2.4 ꢁmol
of HCOO– was produced after 10 h (Figure 2 left, Table S1), reꢀ
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vealing that the great enhancement of photocatalytic activity can
in the nanosecond domain,16 we resorted to timeꢀresolved PL
spectroscopy.17 In Figure 3c and 3d, we compare the PL kinetics
of H2TCPP and PCNꢀ222 at the 712 nm emission (i.e., the maxiꢀ
mum for the lowerꢀlying PL band excited at 515 nm; refer to Figꢀ
ure S16). The PL lifetime of H2TCPP is determined to be 11.32 ±
0.03 ns. Interestingly, similarly to the TA case, two lifetimes with
sizable components were also observed for the PL of PCNꢀ222: τ1
= 0.38 ± 0.02 ns (46%) and τ2 = 1.63 ± 0.03 ns (54%); or 1.07 ±
0.03 ns on average.
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be achieved by assembling the H2TCPP ligand into MOFs. Notaꢀ
bly, the steadyꢀstate photoluminescence (PL) measurement (Figꢀ
ure S16) indicates that the transformation from H2TCPP to PCNꢀ
222 results in pronounced PL emission quenching, implying
greatly suppressed radiative electron−hole recombination in PCNꢀ
222 relative to H2TCPP, in line with the observed higher photoꢀ
catalytic activity of PCNꢀ222 than that of H2TCPP.
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In an attempt to gain deeper understanding on the efficacy of
photoexcited charge separation in PCNꢀ222, we resorted to ultraꢀ
fast transient absorption (TA) spectroscopy, a robust tool for
tracking in real time the charge carrier dynamics in nanosysꢀ
tems.11b,15 In our TA measurement, a femtosecond visible
pump/whiteꢀlight continuum (WLC) probe scheme was adopted
(see Supporting Information for details of the pump–probe experꢀ
iments). The center wavelength of the pump pulses was chosen at
500 nm (i.e., 2.48 eV), which can effectively photoꢀinduce an
interband transition in the semiconductorꢀlike PCNꢀ222 system.
Figure 3a shows the transient TA spectra recorded at different
probe delays ranging from 0.2 to 3000 ps, all of which manifest as
negative absorbance changes (ꢂA) due to groundꢀstate bleaching
by noting that the WLC probe (400−490 nm) is blueꢀshifted relaꢀ
tive to the 500 nm pump. As the WLC probe yields essentially the
same TA kinetics in this spectral region, we show here a typical
one taken at 430 nm (Figure 3b). Within the probeꢀdelay limit of
our pump–probe spectrometer (ca. 3 ns), the ꢂA recovery conꢀ
verges to an asymptote (dashed line in Figure 3b) with a nonꢀzero
value of ca. –1.2 mOD. The multiꢀexponential fitting to this asꢀ
ymptotic recovery gives rise to two characteristic time constants,
i.e., τ1 = 18 ± 1 ps (47%) and τ2 = 190 ± 11 ps (53%). Notably, the
nearly perfect parallelism between the asymptote and the ꢂA = 0
line (dashedꢀdotted line in Figure 3b) suggests that the eventual
recovery to ꢂA = 0 (a process that should take place)15 features an
extremely long lifetime (τ3), probably a few tens to hundreds of
nanoseconds or even microsecond long.
By combining the above TA and PL observations, we illustrate
the pertinent mechanisms underlying the photoexcited dynamics
involved in the two systems (Figure 4). For the molecular system
of H2TCPP (left panel), the relevant picture is quite straightforꢀ
ward: The 500 nm laser pulse launches an electronic transition
from the ground state S0 to a certain excited state Sn, followed by
rapid intramolecular deactivation processes to the lowestꢀlying
excited state S1 as dictated by Kasha’s rule.18 As a result, the PL
from S1 to S0 with a lifetime of ca. 11 ns was detected. On the
other hand, in the semiconductorꢀlike PCNꢀ222 system (right
panel), the existence of two distinct lifetimes for the PL emissions
monitored at ca. 712 nm (i.e., 1.74 eV, slightly below the 1.75 eV
bandgap) suggests that the radiative electron–hole recombination
could originate from two trap states (acting as PL emission cenꢀ
ters)16,17 that are located in the very close proximity of the conꢀ
duction band bottom (depicted as a whole in Figure 3d). These
two, near bandꢀedge, trap states receive and accumulate the phoꢀ
togenerated electrons transferred from the conduction band botꢀ
tom in a biꢀexponential relaxation manner, reflecting their differꢀ
ent trap depths. In terms of the biꢀexponential component proporꢀ
tions, the two electronꢀtrapping processes (47% for τ1 and 53% for
τ2) seem to nicely coincide with the PL emissions (46% for τ1 and
54% for τ2), providing collateral evidence for the validity of our
assignment. As compared to the S1 state of H2TCPP, the two trap
states of PCNꢀ222 are relatively shortꢀlived (ca. 1 ns on average
for PCNꢀ222 vs. ca. 11 ns for H2TCPP), implying relatively shalꢀ
low depths. Nevertheless, the eventual recovery (to ꢂA = 0) feaꢀ
turing an extremely long lifetime τ3 most likely correlates to a
third trap state that possesses a rather deep depth. The detrapping
of electrons from this deep trap state turns out to be so slow that
its corresponding radiative electron–hole recombination is draꢀ
matically suppressed, which reasonably explains the absence of
the longꢀlifetime component in the timeꢀresolved PL kinetics
(refer to Figure 3d). We thus conclude that, as a direct result of
the transformation from H2TCPP to its MOF architecture (i.e.,
PCNꢀ222), such a unique, deep trap state emerges and enables
effective accumulation of longꢀlived, photogenerated electrons
ready for the photocatalytic reduction of CO2 molecules captured
by PCNꢀ222.
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Figure 3. (a) The transient TA spectra of PCNꢀ222 registered at
different probe delays (pump at 500 nm). (b) The representative
TA kinetics of PCNꢀ222 taken at the probing wavelength of 430
nm. In (a) and (b), the TA signal is given in absorbance change
(ꢂA) in the unit of mOD (OD: optical density). The timeꢀresolved
PL kinetics for (c) H2TCPP and (d) PCNꢀ222, recorded with exciꢀ
tation at 515 nm and emission at 712 nm.
Figure 4. The mechanisms underlying the photoexcited dynamics
involved in H2TCPP (left) and PCNꢀ222 (right). Left panel: S0, S1,
and Sn denote the electronic ground state, the first electronically
excited state, and a certain highꢀlying electronically excited state
reached by the 500 nm photon, respectively. Right panel: VB
(CB) denotes the valence (conduction) band.
The observed two components in the picosecond domain (τ1
and τ2) may reflect the electron dynamics associated with the
electron trap states that are energetically located within the
bandgap of PCNꢀ222. Considering that the lifetimes of such trap
states (characterizing the electronꢀdetrapping rates) are typically
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341, 974. (d) Cook, T. R.; Zheng, Y.ꢀR.; Stang, P. J. Chem. Rev. 2013,
In summary, the photocatalytic reduction of CO2 under visibleꢀ
light irradiation has been realized, for the first time, over a photoꢀ
active porphyrinꢀbased semiconducting MOF of PCNꢀ222, which
exhibits much better activity than that of the H2TCPP ligand. The
high CO2 uptake capacity in the MOF might allow its better interꢀ
action with CO2 in MeCN, thus benefiting its photocatalytic conꢀ
version. Moreover, as demonstrated by ultrafast transient absorpꢀ
tion and photoluminescence spectroscopy, the emergence of an
extremely longꢀlived electron trap state in PCNꢀ222 brings about
substantial suppression of the detrimental electron–hole recombiꢀ
nation, thereby boosting the efficiency of the CO2 photoreduction.
The current study highlights the great potential and advantages in
the assembly of photoactive molecular system, such as porphyrin,
into MOFs for CO2 photoreduction. This work not only enables us
deeper understanding of the electronꢀtransfer mechanism involved
in MOFs, but also would stimulate further studies toward develꢀ
oping more efficient visibleꢀlightꢀresponsive MOFs for CO2 capꢀ
ture and photoreduction utilizing solar energy.
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ASSOCIATED CONTENT
Supporting Information
Experimental details and discussions for material synthesis and
characterizations. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
jianglab@ustc.edu.cn (H.ꢀL.J.); qunzh@ustc.edu.cn (Q.Z.).
Author Contributions
♯These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We are grateful to the reviewers for their insightful comments and
valuable suggestions. This work was supported by the 973 proꢀ
gram (2014CB931803, 2010CB923300, 2014CB239303), the
NSFC (21371162, 51301159, 21173205, 91127042, 21273035),
the CAS (XDB01020000), the Recruitment Program of Global
Youth Experts, and the Fundamental Research Funds for the Cenꢀ
tral Universities (WK2060190026, WK2340000063).
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