Photosensitizing Metal-Organic Layers for Efficient Sunlight-Driven Carbon Dioxide Reduction

Article abstract of DOI:10.1021/jacs.8b08357

Metal-organic layers (MOLs), a free-standing monolayer version of two-dimensional metal-organic frameworks (MOFs), have emerged as a new class of 2D materials for many potential applications. Here we report the design of a new photosensitizing MOL, Hf₁₂-Ru, based on Hf₁₂ secondary building units (SBUs) and [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) derived dicarboxylate ligands. After modifying the SBU surface of Hf₁₂-Ru with M(bpy)(CO)₃X (M = Re and X = Cl or M = Mn and X = Br) derived capping molecules through carboxylate exchange reactions, the resultant Hf₁₂-Ru-Re and Hf₁₂-Ru-Mn MOLs possess both [Ru(bpy)₃]²⁺ photosensitizers and M(bpy)(CO)₃X catalysts for efficient photocatalytic CO₂ reduction. The proximity of the MOL skeleton to the capping ligands (1-2 nm) facilitates electron transfer from the reduced photosensitizer [Ru(bpy)₃]⁺ to M^I(bpy)(CO)₃X (M = Re, Mn) catalytic centers, resulting in CO₂ reduction turnover numbers of 8613 under artificial visible light and of 670 under sunlight. MOLs thus represent a novel platform to assemble multifunctional materials for studying artificial photosynthesis.

Full text of DOI:10.1021/jacs.8b08357

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Communication
Photosensitizing Metal-Organic Layers for Efficient
Sunlight-Driven Carbon Dioxide Reduction
Guangxu Lan, Zhe Li, Samuel S Veroneau, Yuan-Yuan Zhu, Ziwan Xu, Cheng Wang, and Wenbin Lin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08357 • Publication Date (Web): 15 Sep 2018
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Photosensitizing Metal-Organic Layers for Efficient Sunlight-
Driven Carbon Dioxide Reduction
Guangxu Lan,1,† Zhe Li,1,2,† Samuel S. Veroneau,1 YuanꢀYuan Zhu,1 Ziwan Xu,1 Cheng Wang,2 and
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Wenbin Lin1,2,*
1Department of Chemistry, The University of Chicago, 929 E 57th Street, Chicago, Illinois 60637, United States
2College of Chemistry and Chemical Engineering, iCHEM, State Key Laboratory of Physical Chemistry of Solid Surface,
Xiamen University, Xiamen 361005, P.R. China
Supporting Information Placeholder
ability to generate CO with high selectivity and efficiency.32ꢀ36
ABSTRACT: Metalꢀorganic Layers (MOLs), a freeꢀstanding
Previous studies have demonstrated an improvement of photocataꢀ
lytic CO2 reduction efficiency by linking [Ru(bpy)3]2+ and
ReI(bpy)(CO)3Cl moieties in supramolecular systems.37ꢀ38 Herein
we report the design of the first photosensitizing MOL, Hf12ꢀRu,
built from Hf12 secondary building units (SBUs) and [Ru(bpy)3]2+
derived dicarboxylate ligands. SBU surface capping of Hf12ꢀRu
with ReI(bpy)(CO)3Cl or MnI(bpy)(CO)3Br derived monocarboxꢀ
ylic acids afforded multifunctional Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn
MOLs for efficient photocatalytic CO2 reduction. The proximity
of photosensitizing Hf12ꢀRu MOL skeleton to the capping monoꢀ
carboxylate ligands (1ꢀ2 nm) facilitates multiꢀelectron transfer
from photoexcited [Ru(bpy)3]2+* to MI(bpy)(CO)3X (M = Re, Mn)
catalytic centers, reaching 24ꢀhour turnover numbers (TONs) of
3,849 and 1,347, respectively (Scheme 1). Remarkably, Hf12ꢀRuꢀ
Re catalyzed sunlightꢀdriven CO2 reduction with a TON of 670 in
6 hours of daylight.
monolayer version of twoꢀdimensional metalꢀorganic frameworks
(MOFs), have emerged as a new class of 2D materials for many
potential applications. Here we report the design of a new photoꢀ
sensitizing MOL, Hf12ꢀRu, based on Hf12 secondary building units
(SBUs) and [Ru(bpy)3]2+ (bpy = 2,2′ꢀbipyridine) derived dicarꢀ
boxylate ligands. After modifying the SBU surface of Hf12ꢀRu
with M(bpy)(CO)3X (M = Re and X = Cl or M = Mn and X = Br)
derived capping molecules through carboxylate exchange reacꢀ
tions, the resultant Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn MOLs possess
both [Ru(bpy)3]2+ photosensitizers and M(bpy)(CO)3X catalysts
for efficient photocatalytic CO2 reduction. The proximity of the
MOL skeleton to the capping ligands (1ꢀ2 nm) facilitates electron
transfer from photoexcited [Ru(bpy)3]2+* to MI(bpy)(CO)3X (M =
Re, Mn) catalytic centers, resulting in CO2 reduction turnover
numbers of 8,613 under artificial visible light and of 670 under
sunlight. MOLs thus represent a novel platform to assemble mulꢀ
tifunctional materials for studying artificial photosynthesis.
Scheme 1. Schematic showing the synthesis of Hf12ꢀRuꢀM (M =
Re or Mn) via monocarboxylic acid exchange of Hf12ꢀRu and
sunlightꢀdriven CO2 reduction.
Metalꢀorganic frameworks (MOFs) have recently provided a
versatile material platform to study artificial photosynthesis and
photocatalysis,1ꢀ3 including proton reduction,4ꢀ7 CO2 reduction,8ꢀ12
water oxidation,13 and organic photocatalysis.14ꢀ17 Structural reguꢀ
larity and synthetic tunability of MOFs allow hierarchical integraꢀ
tion of multiple functional moieties, including photosensitizers
(PSs) and catalytic centers, to facilitate multiꢀelectron or radical
transfer in photoreactions.2, 18 However, due to intrinsically high
absorptivity of most PSs, only superficial layers of MOFs are
involved in photoreactions. Light scattering by MOF particles also
reduces photon utilization. Furthermore, photocatalytic efficiency
of MOFs is limited by restricted diffusion of radicals and other
energetic intermediates through MOF channels. By reducing one
dimension of MOFs to a single layer, metalꢀorganic layers
(MOLs) have recently emerged as a new class of functionalizable
2D materials for many applications.19ꢀ22 We believe that MOLs
can overcome the aforementioned issues for MOFs due to their
thinness and hold great promise for applications in artificial phoꢀ
tosynthesis and photocatalysis.
Sunlightꢀdriven CO2 reduction to energyꢀrich compounds repꢀ
resents a promising approach to overcome the shortage of fossil
fuels and to mitigate climate change.23ꢀ31 Among many photocataꢀ
lytic systems for CO2 reduction, combinations of [Ru(bpy)3]2+
(bpy = 2,2′ꢀbipyridine) as a PS and ReI(bpy)(CO)3Cl as a CO2
reduction catalyst have been most extensively studied due to their
Hf12ꢀRu MOLs were synthesized through a solvothermal reacꢀ
tion between HfCl4 and bis(2,2’ꢀbipyridine)[5,5’ꢀdi(4ꢀcarboxylꢀ
phenyl)ꢀ2,2’ꢀbipyridine]ruthenium(II) dichloride (H2LꢀRu), in
N,Nꢀdimethylformamide (DMF) at 80 °C with trifluoroacetic acid
(TFA) and water as modulators (Figure S1, SI). Hf12ꢀRu is proꢀ
posed as the first MOL constructed from Hf12 SBUs and laterally
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connected linear LꢀRu linkers to form an infinite 2D network of
(Figure S3, SI). Finally, Hf12ꢀRu showed similar absorption and
emission spectra as H2LꢀRu (Figure S5, SI), suggesting photosenꢀ
sitizing ability of Hf12ꢀRu.
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kagome dual (kgd) topology. Each Hf12 SBU is also vertically
capped by six TFA groups. Hf12ꢀRu thus has a formula of Hf12(ꢁ3ꢀ
O)8(ꢁ3ꢀOH)8(ꢁ2ꢀOH)6(TFA)6(LꢀRu)6. 2D Hf12ꢀRu MOL forms in
part due to stronger Hfꢀcarboxylate bonds in the lateral direction
than those in the vertical direction as we recently observed in
Hf12ꢀbased MOFs.39
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Figure 2. (a) NMR spectrum of digested Hf12ꢀRuꢀMBA. Yellow
and green stars correspond to LꢀRu ligands and MBA ligands,
respectively. TEM (b), EXAFS fitting (c), AFM topography (d)
and height profile (e) of Hf12ꢀRuꢀRe.
Hf12ꢀRu was modified with 2ꢀ(5'ꢀmethylꢀ[2,2'ꢀbipyridin]ꢀ5ꢀ
yl)acetic acid (HꢀMBA) ligands on the surface to afford Hf12ꢀRuꢀ
MBA by replacing weakly coordinating TFA groups on SBUs
Figure 1. TEM (a) and HRTEM (b) of Hf12ꢀRu. (c) PXRD pattern
of Hf12 MOLs in comparison to the simulated pattern.40 AFM
topography (d) and height profile (e) of Hf12ꢀRu. (f) Schematic
showing a Hf12 cluster capped by TFA groups with an estimated
height of ~1.7 nm.
1
with MBA ligands. The H NMR spectrum of digested Hf12ꢀRuꢀ
MBA gave an MBA to LꢀRu ratio of ~1:1 (Figure 2a and S8, SI),
indicating complete replacement of TFA groups in Hf12ꢀRuꢀMBA
to give a formula of Hf12(ꢁ3ꢀO)8(ꢁ3ꢀOH)8(ꢁ2ꢀOH)6(MBA)6(Lꢀ
Ru)6. Additionally, no signal corresponding to TFA was detected
in the 19F NMR spectrum of digested Hf12ꢀRuꢀMBA (Figure S9,
SI). PXRD studies indicated that Hf12ꢀRuꢀMBA exhibited the
same structure as Hf12ꢀRu (Figure 1c).
Transmission electron microscopy (TEM) imaging showed
Hf12ꢀRu as flat nanoplates of ~100 nm in diameter (Figure 1a).
The low contrast of the TEM images is consistent with the monoꢀ
layer nature of Hf12ꢀRu, which is confirmed by a nanoplate thickꢀ
ness of ~1.6 nm observed by atomic force microscopy (AFM,
Figure 1d, e and Figure S4, SI). This thickness is consistent with
the modeled height of Hf12 clusters capped with TFA groups (~1.7
nm, Figure 1f). Highꢀresolution TEM (HRTEM) imaging conꢀ
firmed the topological structure of Hf12ꢀRu with lattice points
corresponding to Hf12 SBUs and fast Fourier transform (FFT)
revealing a sixꢀfold symmetry (Figure 1b) that is consistent with
the projection of Hf12ꢀRu structure along the vertical direction.
The distance between adjacent lattice points in HRTEM images
was measured to be 2.7 nm, matching the distance between adjaꢀ
cent Hf12 SBUs. Moreover, the powder Xꢀray diffraction (PXRD)
pattern of Hf12ꢀRu matched well with that simulated from the
proposed Hf12 MOL structure (Figure 1c).40 All of these data supꢀ
port the proposed monolayer structure of Hf12ꢀRu. Furthermore,
the 1H NMR spectrum of digested Hf12ꢀRu showed all signals
corresponding to H2LꢀRu without any other aromatic signals,
which confirms the presence of only LꢀRu ligands in Hf12ꢀRu
Hf12ꢀRuꢀMBA was then metalated with Re(CO)5Cl or
Mn(CO)5Br to afford Hf12ꢀRuꢀRe with the catalytic center
Re(MBA)(CO)3Cl that is an analogue of ReI(bpy)(CO)3Cl or
Hf12ꢀRuꢀMn with the catalytic center Mn(MBA)(CO)3Br that is an
analogue of MnI(bpy)(CO)3Br. Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn
showed similar sizes and morphologies to Hf12ꢀRu by TEM (Figꢀ
ure 2b and Figure S15, SI) and maintained the same structure as
Hf12ꢀRu as indicated by PXRD (Figure 1c). AFM studies showed
that the thickness of Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn increased to
~3.7 nm (Figure 2d, e, Figures S14, and S16, SI), which is conꢀ
sistent with the height of Hf12 clusters capped with MBA ligands
(3.2ꢀ4.0 nm, Figure S19, SI). In Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn,
each catalytic center is adjacent to six LꢀRu ligands, which enaꢀ
bles multiꢀelectron transfer from LꢀRu to Re(MBA)(CO)3Cl or
Mn(MBA)(CO)3Br. Electron transfer was further facilitated by a
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short distance of 1.3 nm between each catalytic center and its
nearest LꢀRu ligand (Figure S20, SI).
confirms the important role of hierarchical organization of PSs
and Re or Mn catalytic centers in the MOLs in facilitating multiꢀ
electron transfer processes to drive photocatalytic CO2 reduction.
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Re and Mn coordination environments of Hf12ꢀRuꢀRe, Hf12ꢀRuꢀ
Mn, and Re(MeMBA)(CO)3Cl and Mn(MeMBA)(CO)3Br
The CO2 reduction product selectivity for Hf12ꢀRuꢀRe and Hf12ꢀ
RuꢀMn was examined by quantifying the amounts of HCOOH
generated using high performance liquid chromatography (HPLC)
(Figure S27 and Table S5, SI). Similar to previously reported
molecular systems,38, 41 Hf12ꢀRuꢀRe showed a ~98% selectivity
for CO, while Hf12ꢀRuꢀMn generated more HCOOH with a 82%
selectivity for CO.
{MeMBA
= methyl 2ꢀ(5'ꢀmethylꢀ[2,2'ꢀbipyridin]ꢀ5ꢀyl)acetate,
Figure S10ꢀ13, SI} were determined by Xꢀray absorption specꢀ
troscopy. Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn showed similar extended
Xꢀray absorption fine structure (EXAFS) profiles to
Re(MeMBA)(CO)3Cl and Mn(MeMBA)(CO)3Br (Figure S22,
SI), which were well fitted with their corresponding molecular
models [Re(bpy)(CO)3Cl for Hf12ꢀRuꢀRe and Mn(bpy)(CO)3Br
for Hf12ꢀRuꢀMn, Figure 2c, S23ꢀ26, SI]. Moreover, Hf12ꢀRuꢀRe
and Hf12ꢀRuꢀMn showed similar CO stretching vibrations and
characteristic UVꢀVis absorptions when compared to
Re(MeMBA)(CO)3Cl and Mn(MeMBA)(CO)3Br, respectively
(Figure S17ꢀ18, SI). These results indicate that Hf12ꢀRuꢀRe and
Hf12ꢀRuꢀMn exhibit the same Re and Mn coordination environꢀ
ments as molecular photocatalytic CO2 reduction catalysts
ReI(bpy)(CO)3Cl and MnI(bpy)(CO)3Br, respectively.
9
To confirm the structural stability of the MOLs following 24ꢀ
hour CO2 reduction, Hf12ꢀRu, Hf12ꢀRuꢀRe, and Hf12ꢀRuꢀMn were
centrifuged from the reaction suspensions. The recovered MOLs
showed the same PXRD patterns as freshly prepared MOLs (Figꢀ
ure 3c) with <1% leaching of Hf by inductively coupled plasma ꢀ
mass spectrometry. The recovered MOLs also showed similar CO
stretching vibrations and characteristic UVꢀVis absorptions to
freshly prepared MOLs (Figure S28, 29, SI). To further confirm
the photocatalytic durability of these MOLs, Hf12ꢀRuꢀRe was used
in 5 consecutive cycles of CO2 reduction, showing consistent
catalytic activity with a total TON of 8,613 (Figure 3d).
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4000
a
b
Hf12-Ru-Re + BIH
Hf12-Ru-Re + BNAH
Homo-Re + BIH
Hf12-Ru-Mn + BIH
Hf12-Ru-Mn + BNAH
Homo-Mn + BIH
1000
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3000
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1000
0
The CO2 reduction mechanism was investigated by photophysꢀ
ical and electrochemical studies. The phosphorescence spectra of
Hf12ꢀRu were measured with the addition of different equivalence
of BIH or Re(MeMBA)(CO)3Cl. As shown in the Figure S30, the
phosphorescence of Hf12ꢀRu was efficiently quenched by BIH but
not by Re(MeMBA)(CO)3Cl, indicating that the excited Hf12ꢀRu
was reductively quenched by BIH to generate [Hf12ꢀRu]ꢀ (Figure
S30, SI). Cyclic voltammetry (CV) and differential pulse voltamꢀ
metry (DPV) studies demonstrated that Hf12ꢀRu had similar reꢀ
Homo-Re + BNAH
Homo-Mn + BNAH
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Time (h)
Time (h)
c
d 2000
1500
1000
500
Hf12-Ru-Mn
Hf12-Ru-Re
Hf12-Ru
duction potential to H2LꢀRu (Ered = ꢀ0.89 V vs SCE, correꢀ
1/2
sponding to Hf12ꢀRu to [Hf12ꢀRu]ꢀ), which is negative enough to
reduce Re(MeMBA)(CO)3Cl (Ered = ꢀ0.34 V vs SCE, Figure
1/2
S31 and Table S7, SI). Moreover, we showed that the photocataꢀ
lytic efficacy of Hf12ꢀRuꢀRe system is sensitive to the distance
between LꢀRu photosensitizing center and the capping Re catalytꢀ
ic center (Figure S32ꢀ34 and Table S8, SI).
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2θ
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0
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Figure 3. Timeꢀdependent CO generation TONs of Hf12ꢀRuꢀRe
(a) and Hf12ꢀRuꢀMn (b) along with their homogeneous controls
H2LꢀRu plus Re(MeMBA)(CO)3Cl and H2LꢀRu plus
Mn(MeMBA)(CO)3Br. N = 3. (c) PXRD patterns of Hf12ꢀRu,
Hf12ꢀRuꢀRe, and Hf12ꢀRuꢀMn after photocatalysis. (d) Plots of CO
generation TONs for Hf12ꢀRuꢀRe in five consecutive runs (Section
S4.2, SI).
Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn provide an excellent opportunity
to test whether capping the surface of photosensitizing Hf12ꢀRu
with Re(MBA)(CO)3Cl or Mn(MBA)(CO)3Br catalyst could facilꢀ
itate multiꢀelectron transfer from photoexcited Hf12ꢀRu to catalytꢀ
ic Re or Mn centers to drive efficient CO2 reduction (Scheme 1).
The catalytic activities for visibleꢀlight driven (λ > 400 nm) CO2
reduction of Hf12ꢀRuꢀRe, Hf12ꢀRuꢀMn, and homogenous controls
H2LꢀRu plus Re(MeMBA)(CO)3Cl, or H2LꢀRu plus Mn(Meꢀ
MBA)(CO)3Br) were studied in an oxygenꢀfree CH3CN solution
with saturated CO2, triethanolamine (TEOA), and 1ꢀbenzylꢀ1,4ꢀ
dihydronicotinamide (BNAH) or 1,3ꢀdimethylꢀ2ꢀphenylꢀ2,3ꢀ
dihydroꢀ1Hꢀbenzo[d]imidazole (BIH) as sacrificial electron doꢀ
nor. The amount of generated CO was quantified by gas chromaꢀ
tography (GC) analysis of the headspace gas. The turnover numꢀ
ber (TON) [defined as n(CO)] reached 3849 (BIH) or 2092
(BNAH) for Hf12ꢀRuꢀRe and 1367 (BIH) or 240 (BNAH) for
Hf12ꢀRuꢀMn after 24 h irradiation (Figure 3a, b), which are supeꢀ
rior to previously reported molecular systems (Table S6, SI). In
comparison, H2LꢀRu plus Re(MeMBA)(CO)3Cl and H2LꢀRu plus
Mn(MeMBA)(CO)3Br exhibited modest TONs of <54 under the
same conditions. Greater than 70ꢀfold increase in catalytic activity
for Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn over their homogenous controls
Figure 4. Sunlightꢀdriven CO2 reduction TONs of Hf12ꢀRuꢀRe in
6 hours.
We next examined Hf12ꢀRuꢀRe catalyzed CO2 reduction upon
sunlight irradiation. The Hf12ꢀRuꢀRe reaction suspension was
prepared as above, and placed near a window inside a chemistry
laboratory at the University of Chicago. This suspension was
stirred from 11:00 am to 5:00 pm on Jun. 18th to Jun. 24th in 2018
(Figure S35, SI). Remarkably, Hf12ꢀRuꢀRe catalyzed CO2 reducꢀ
tion under sunlight irradiation with unprecedented efficiency,
reaching a TON of 670 in 6 h (Figure 4). The effectiveness of
Hf12ꢀRuꢀRe catalyzed sunlightꢀdriven CO2 reduction was underꢀ
standably weather dependent, with the highest TON of 670
achieved on sunny Jun. 18th, 2018 and the lowest TON of 138
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observed on rainy Jun. 22nd, 2018. To the best of our knowledge,
this work presents the first study of sunlightꢀdriven CO2 reduction
using a combination of [Ru(bpy)3]2+ as a PS and ReI(bpy)(CO)3Cl
as a CO2 reduction catalyst.
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In summary, we have synthesized the first MOL based on
readily functionalizable linear dicarboxylate ligands. The postꢀ
synthetic functionalization of photosensitizing Hf12ꢀRu MOL with
M(bpy)(CO)3X (M = Re or Mn) moieties via carboxylate exꢀ
change reactions afforded Hf12ꢀRuꢀRe and Hf12ꢀRuꢀMn MOLs
that possess both [Ru(bpy)3]2+ photosensitizers (PSs) and
M(bpy)(CO)3X catalysts for efficient photocatalytic CO2 reducꢀ
tion. We showed that multiꢀelectron transfer from photoexcited
[Ru(bpy)3]2+* to MI(bpy)(CO)3X (M = Re, Mn) catalytic centers is
greatly facilitated by the proximity of the photosensitizing MOL
skeleton to the capping CO2 reduction catalysts. The Hf12ꢀRuꢀRe
system exhibited high CO2 reduction turnover numbers (TONs) of
8,613 under artificial visible light and of 670 under sunlight. This
work thus provides a versatile synthetic strategy to multifunctionꢀ
al MOLs for studying artificial photosynthesis.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org. Synthesis and characterization of
MOLs, Xꢀray absorption spectroscopy, and photocatalytic CO2
reduction procedures.
AUTHOR INFORMATION
Corresponding Author
*wenbinlin@uchicago.edu
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank Yibin Jiang for PXRD simulation for Hf12ꢀRu. This
work was supported by National Science Foundation (DMRꢀ
1308229). Z. Li acknowledges financial support from the China
Scholarship Council and the National Science Foundation of Chiꢀ
na (21671162). XAS analysis was performed at Beamline 10ꢀBM,
Advanced Photon Source (APS), Argonne National Laboratory
(ANL). Use of the Advanced Photon Source, an Office of Science
User Facility operated for the U.S. Department of Energy (DOE)
Office of Science by Argonne National Laboratory, was supported
by the U.S. DOE under Contract No. DEꢀAC02ꢀ06CH11357.
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