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Photoactive Zeolitic Imidazolate Framework as Intrinsic Heterogeneous Catalysts for Light-Driven Hydrogen Generation Sizhuo Yang, Brian Pattengale, Evgenii L Kovrigin, and Jier Huang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00540 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016
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ACS Energy Letters
Photoactive Zeolitic Imidazolate Framework as Intrinsic Heterogeneous Catalysts for LightDriven Hydrogen Generation
Sizhuo Yang, Brian Pattengale, Evgenii L Kovrigin, and Jier Huang* Department of Chemistry, Marquette University, Milwaukee, Wisconsin, 53201
AUTHOR INFORMATION Corresponding Author *Jier Huang (
[email protected])
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ABSTRACT Zeolitic Imidazolate Frameworks (ZIFs) with intrinsic photoactivity represent an excellent scaffold for a single-site photocatalytic center. However, their application in visiblelight driven photocatalysis is hampered by their light harvesting ability due to limited coverage in the realm of the solar spectrum and relatively low extinction coefficient. In this work, we report the first example of enhanced light harvesting property of ZIF-67 via energy transfer from a molecular photosensitizer, i.e. RuN3 complex, sensitized onto the surface of ZIF-67. Using transient absorption spectroscopy, we show that energy transfer occurs from the excited RuN3 to ZIF-67 with ~ 86.9% efficiency. More importantly, this RuN3/ZIF-67 hybrid system exhibits significantly enhanced photocatalytic activity for H2 production from water, which can be attributed to the efficient energy transfer from RuN3.
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ACS Energy Letters
Metal organic frameworks (MOFs) are an emerging class of nanoporous crystalline materials consisting of metal nodes coordinated by bridging organic linkers.1-8 Their inherent porous nature, large surface area, and tunable cavities have led to various applications including gas separation and storage,9-11 chemical sensing,12,
13
and heterogeneous catalysis.14-20 Zeolitic
Imidazolate Frameworks (ZIFs) are a subclass of MOFs which are particularly attractive for catalysis application due to their exceptional thermal and chemical stability.21, 22 Recent works have demonstrated their catalytic applications for a variety of reactions including organic transformations,23-27 as well as gas phase CO oxidation and hydrogenation.28, 29 Furthermore, by embedding photoactive guests into the structure, photocatalytic applications of ZIFs have been demonstrated for dye and phenol degradation and CO2 reduction. 30-33 While these examples evidently demonstrate the great promise of ZIFs in heterogeneous catalysis, ZIFs in these systems are largely treated as inert hosts for reaction substrates or/and catalytic active species, resembling the roles of zeolites in catalysis.34 In contrast to these studies, our recent findings show that the framework of ZIF-67 exhibits an intrinsic photochemical response, featured by multiple absorption bands in UV-Visible-Near IR region and an exceptionally long-lived excited state due to the formation of a charge separated state.35 While our studies largely promise the application of ZIFs as intrinsic photocatalytic materials, the efficiency for visible light driven photocatalysis of ZIF-67 is expected to be low using single component ZIF-67 because of the limited absorption in the solar spectrum due to relatively small extinction coefficients of Co2+ d-d transitions (~100-1000 mol·L-1·cm-1).36 One desirable strategy to improve its light harvesting ability is to expand the absorption spectrum of ZIF-67 by encapsulating a chromophore into the framework, such that broader region of the solar spectrum can be absorbed and relayed to ZIF-67 through energy transfer (ENT). Although this approach
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has been used to improve the light harvesting properties of MOFs,37-40 no such studies have been reported in ZIFs, yet enhancing their light absorption ability is critical for their photocatalytic and solar energy conversion application.
Figure 1. (a) UV-visible absorption spectra of ZIF-67, RuN3/ZIF-67, and RuN3/Al2O3 thin films. The inset shows the spectral overlap between the absorption spectrum of ZIF-67 and emission spectrum of RuN3. (b) Powder XRD patterns of ZIF-67 and RuN3/ZIF-67. The inset shows the cartoon of RuN3 sensitized ZIF-67. The yellow ball indicates the cavity in the framework. In this work, we fill this need for the first time and report the enhanced light harvesting and photocatalytic properties of ZIF-67 through ENT process from a sensitized chromophore, i.e. RuN3 (cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II)). We show that efficient ENT occurs from excited RuN3 to ZIF-67 using transient absorption (TA) spectroscopy. We also successfully demonstrate the enhanced photocatalytic activity of ZIF-67 for light-driven H2 generation from water in the presence of RuN3, which can be attributed to the enhanced light harvesting ability of ZIF-67 through ENT. RuN3 is chosen as a chromophore in this study because of its multifold benefits as a photosensitizer. First, it can absorb visible light < 550 nm with large extinction coefficient (ε534nm = 1.42x104 M-1cm-1)41 where ZIF-67 has negligible absorption, and thus compensates the absorption spectrum of ZIF-67 (Figure 1a). Second, the emission spectrum of RuN3 has overlap with the ZIF-67 absorption in near IR
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ACS Energy Letters
region attributed to the lower-lying d-d transition of Co2+ (4A2(F)-4T1(F), the inset of Figure 1a),35 which makes ENT process from RuN3 to ZIF-67 practically feasible. Finally, RuN3 is a widely used photosensitizer for solar energy conversion and has well-known optical properties, which can facilitate our optical studies in this work. ZIF-67 thin films were synthesized by following a direct-growth approach previously published.42 RuN3 is linked to ZIF-67 following the procedure for dye sensitized semiconductor oxide film reported elsewhere.43, 44 The details of the synthesis is provided in the supporting information (SI). Because the size of RuN3 is much larger than the aperture size of ZIF-67 (~ 3.4 Å),21 we believe that RuN3 molecules are directly attached to the surface of ZIF-67 thin film rather than being encapsulated within the cavities. The presence of RuN3 on ZIF-67 thin film is confirmed by both UV-visible and IR absorption spectra. Figure 1a compares the UV-visible spectra of ZIF-67 and RuN3/ZIF-67 films. Compared to the spectrum of ZIF-67 film (black plot, Figure 1a) which is featured by a spin-allowed d-d transition band centered at 585 nm, two additional absorption bands centered at 385nm and 535 nm were observed in the spectrum of RuN3/ZIF-67 film (blue plot, Figure 1a). These two spectral features are consistent with the metal-to-ligand charge transfer (MLCT) bands of RuN345 (red plot, Figure 1a) and can thus be attributed to the directly adsorbed RuN3 on ZIF-67 thin film. The adsorption of RuN3 on the surface of ZIF-67 film is believed to be through –COOH functional group in RuN3, as supported by the shift of C=O stretching mode in IR absorption spectra (Figure S1) upon binding.46 The crystal structures of ZIF-67 and RuN3/ZIF-67 were measured by powder X-ray diffraction (XRD). As shown in Figure 1b, the XRD patterns of RuN3/ZIF-67 strongly resemble the patterns for ZIF-67, both of which agree well with the literature results of ZIF-67 with SOD topology,47 suggesting that the crystal structure of ZIF-67 is retained after RuN3 binding.
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a)
fs-TA
1
b)
∆A (a.u.)
1ps 20ps 50ps 200 ps 1 ns 2 ns 4 ns
450
c)
500
550 600 650 Wavelength(nm)
∆A (a.u.)
0
RuN3/Al2O3@710nm RuN3/ZIF-67@710nm
0 RuN3/ZIF-67@515nm
-1
700
RuN3/Al2O3@515nm
0 40 80 1000 Time Delay (ps)
1
fs-TA
d)
RuN3/ZIF-67
0 1ps 20ps 50ps 200ps 1 ns 2 ns 4 ns
-1 450
500
e)
ns-TA
550 600 650 Wavelength(nm)
0.25
0.20
0
0 40 80
1000
585 nm
0 40 80 1000 Time Delay (ps)
700
1
f)
ns-TA
RuN3/ZIF-67
100ns 500ns 1µs 2µs 5µs 45µs
550 600 650 Wavelength (nm)
700
∆A (a.u.)
610 nm
0
450 500
610 nm
610 nm
RuN3/ZIF-67
-2
fs-TA
RuN3/ZIF-67
∆A (a.u.)
∆A (a.u.)
fs-TA
RuN3/Al2O3
∆A (a.u.)
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0 515 nm 585 nm
-1 0.0
0.1
1 Time (µs)
10
Figure 2. Femtosecond transient absorption (fs-TA) spectra of RuN3/Al2O3 (a) and RuN3/ZIF-67 (c) thin films following 410 nm excitation. (b) The comparison of excited state decay (710 nm) and ground state bleach recovery (515 nm) of RuN3 in RuN3/ZIF-67 and RuN3/Al2O3 thin films. (d) The fs-kinetic traces of RuN3/ZIF-67 thin film at 610 nm and 585 nm, demonstrating the formation of excited state ZIF-67. The rising component occurring in 610 nm kinetics was zoomed in (inset). (e) Nanosecond transient absorption (ns-TA) spectra of RuN3/ZIF-67 film. (f) The ns-kinetics traces of RuN3/ZIF-67 at 610 nm, 515 nm, and 585 nm. The ENT dynamics from RuN3 to ZIF-67 was examined by TA spectroscopy following selectively exciting RuN3 using 410 nm pump pulse. The excited state (ES) dynamics of RuN3 on Al2O3 thin film, representing a non-ENT and non-charge transfer model, was first examined to account for the intrinsic effect of porous surface on ES dynamics. As shown in Figure 2a, the femtosecond TA (fs-TA) spectra of RuN3/Al2O3 thin film consist of a negative band centered at 6 ACS Paragon Plus Environment
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ACS Energy Letters
525 nm and a broad absorption feature > 615 nm. These features have been observed previously49 and can be assigned to the photoinduced ground state bleach (GSB) and ES absorption of RuN3, respectively. The recovery of GSB (black open circles, Figure 2b) and the decay of ES (red open circles, Figure 2b) occur with same kinetics with the presence of an isosbestic point at 622 nm (Figure 2a), suggesting that the recovery of GS molecules from ES is the only relaxation process responsible for the observed ES dynamics. Compared to the fs-TA spectra of RuN3/Al2O3, dramatic difference was observed in the spectra of RuN3/ZIF-67 (Figure 2c). First, the recovery of GSB and the decay of ES absorption of RuN3 were simultaneously enhanced, which can be further clearly seen from their kinetic traces (blue and pink open triangles, Figure 2b). These features are accompanied by the formation of a new negative band centered at 589 nm and an absorption band centered at 606 nm. The formation of these new features is further confirmed from their kinetic traces at 585 nm and 610 nm, where a rising component was clearly observed in each kinetic trace at later time (> 1 ns) (Figure 2d and inset). Because these new features closely resemble the derivative-like feature observed previously in ZIF-67 after direct excitation of d-d transition,35 they can be attributed to the formation of ES of ZIF-67. We can exclude the possibility that ES of ZIF-67 is due to the direct excitation of ZIF-67 as negligible fs-TA signals were observed in ZIF-67 following 410 nm excitation under the same experimental conditions. The formation of ES ZIF67 due to photoexcitation of RuN3, along with the simultaneously enhanced GSB recovery and ES decay of RuN3, all support that ENT process occurs from excited RuN3 to ZIF-67 (Figure 3a). Because the only spectral features after ~100 ns observed in nanosecond TA (ns-TA) spectra of RuN3/ZIF-67 (Figure 2e) arise from ZIF-67, we can rule out the possibility of charge
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(electron or hole) transfer process from RuN3 to ZIF-67. As electron/hole transfer process leads to the formation of oxidized/reduced state of RuN3, the GSB of RuN3 should be expected in nsTA spectra if it occurs. The negligible contribution from the GSB of RuN3 in ns-TA spectra of RuN3/ZIF-67 thus suggests that ENT is the only process accounting for the spectral change of RuN3/ZIF-67 from RuN3/Al2O3. Meanwhile, the super long-lived derivative feature of ZIF-67 after ENT process from RuN3 (Figure 2e) is consistent with the formation of charge separated state with LMCT nature following direct excitation of d-d transition reported previously,35 suggesting that charge separation through LMCT also occurs after ENT process (Figure 3a). Consequently, the ENT rate from RuN3 to ZIF-67 can be directly measured by probing the kinetic traces of GSB recovery of RuN3, ES decay of RuN3, or the formation of ES ZIF-67. Due to the spectral overlap of ES ZIF-67 with RuN3 GSB and ES at where ES ZIF-67 occurs, the ENT time in this work was quantified by fitting kinetic traces of GSB of RuN3 using multiple-exponential decay functions. The minimum number of exponential components used to quantify the GSB kinetics was determined according to Akaike’s Information Criterion (See SI for details). The obtained average lifetimes for RuN3 GSB recovery are 0.86 ns and 5.7 ns for RuN3/ZIF-67 and RuN3/Al2O3, respectively. The quantum yield of ENT process can be estimated according to η = k ENT /( k ENT + k i ) ,37 where k ENT and ki are the rate constants for ENT and intrinsic GSB recovery of RuN3, respectively. The obtained quantum yield of ENT is ~ 86.9%, suggesting an efficient ENT process from RuN3 to ZIF-67.
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Table 1. Photocatalytic Hydrogen Production with ZIF-67 under different conditions Variables
Proton sourcea
RuN3 concentrationsb
Control experiment
Trials
H2 (µmol/g)
c
TEA·HCl (30 µmol)
2.2
