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Cite this: Energy Environ. Sci., 2013, 6, 3229 Received 10th May 2013 Accepted 19th August 2013

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Post-synthesis modification of a metal–organic framework to construct a bifunctional photocatalyst for hydrogen production† Tianhua Zhou,a Yonghua Du,b Armando Borgna,b Jindui Hong,a Yabo Wang,a Jianyu Han,a Wei Zhanga and Rong Xu*a

DOI: 10.1039/c3ee41548a www.rsc.org/ees

To construct photocatalytically active MOFs, various strategies have recently been developed. We have synthesized and characterized a new metal–organic framework (MOF-253-Pt) material through immobilizing a platinum complex in 2,20 -bipyridine-based microporous MOF (MOF-253) using a post-synthesis modification strategy. The functionalized MOF-253-Pt serves both as a photosensitizer and a photocatalyst for hydrogen evolution under visible-light irradiation. The photocatalytic activity of MOF-253-Pt is approximately five times higher than that of the corresponding complex. The presence of the short Pt/Pt interactions in the framework was revealed with extended X-ray absorption fine structure (EXAFS) spectroscopy and low temperature luminescence. These interactions play an important role in improving the photocatalytic activity of the resulting MOF.

Introduction The search for alternatives to fossil fuels has prompted growing interest in utilizing low-carbon or carbon-neutral energy sources. The splitting of water into H2 under visible light has attracted global attention recently with the aim to develop efficient ways for direct conversion of solar energy to chemical energy.1 Extensive efforts have been devoted to achieving this goal using various types of materials including metal oxides and metal complexes.2 Research efforts so far have been primarily focused on improving the photocatalytic activity through adequate handling of the electronic and optical properties of the materials or molecules.3 Development of new photocatalytically functional materials is still one of the major challenges. As a kind of porous materials, metal–organic frameworks (MOFs) comprised of metal building units and organic bridging a

School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459. E-mail: [email protected]; Fax: +65 6794 7553

b

Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 † Electronic supplementary information (ESI) available: General experimental procedures, materials, and instrumentations; full synthesis and characterization are available in the supplementary information. See DOI: 10.1039/c3ee41548a

This journal is ª The Royal Society of Chemistry 2013

Broader context Metal–organic frameworks (MOFs) are one of the most important functional porous materials. Due to tunable chemical and physical properties at the molecular level, MOFs are attractive compounds for further development as photocatalytically functional materials. However, most MOFs are not capable of harvesting visible light or lack catalytic sites for photocatalytic H2 production. Herein, we described a new bifunctional MOF253-Pt by post-synthetic modication of 2,20 -bipyridine-based MOF with platinum ions, which acts both as a photosensitizer and a photocatalyst for reduction of water into H2 under visible light irradiation. Furthermore, its photocatalytic performance is superior to that of the corresponding complex. This work provides useful insights for future design and synthesis of new functional MOF for solar energy conversion.

ligands have been recognized as potential promising candidates as photocatalysts. As compared to traditional zeolite-like materials, which have been extensively investigated in both liquid and gas photocatalysis, MOFs containing organic functionalities allow systematic tuning of their chemical and physical properties through modication of the compositions.4 Although a variety of functional MOFs have been developed, the applications of MOFs have been mainly conned to gas separation, storage and catalytic transformation of organic molecules.5 Only a few studies can be found reporting their applications in photocatalytic degradation of organic compounds.6 Attempts to obtain robust MOFs capable of photocatalytic conversion of solar energy into chemical energy have met with only limited success. Particularly, most of the MOFs are not able to absorb visible light or lack catalytic sites for photocatalytic H2 production. For the synthesis of photocatalytically active MOFs, two common strategies have been employed. One of the approaches is introduction of the light absorbing organic building units, such as porphyrins7 and 2aminoterephthalate,8 into MOFs in order to harvest visible light. Another approach is doping MOFs with light-absorbing metalcomplexes.9 In these systems, Pt nanoparticles are oen employed as the catalyst for photocatalytic H2 evolution. We hypothesize that inactive MOFs could be turned into a

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bifunctional photocatalyst, which can serve both as a photosensitizer and a hydrogen-evolution catalyst, through coordination of the available functional group in the bridge to active metal ions inserted. Herein, we demonstrate that inserting platinum ions into a MOF can afford a new bifunctional MOF for efficient photocatalytic H2 evolution without additional catalyst and dye.

Experimental Photocatalytic activity measurement Visible light driven H2 evolution reactions were conducted in a closed gas circulation and evacuation system tted with a top window Pyrex cell. In a typical reaction, the photocatalyst was added into 100 mL of a mixed solution containing triethanolamine (TEOA) under vigorous stirring. A concentrated aqueous HNO3 solution (69 wt%) was used to adjust the pH of the TEOA solution from 7.0 to 11.0. The light source was a 300 W xenon lamp with a cut-off lter (l > 420 nm). The reaction cell was kept at around 20 C with a cooling water jacket. The produced H2 was detected using online gas chromatography. Different band-pass lters (centered at 420, 440, 460, 480, 520 nm) were equipped when conducting reactions under photons of different wavelengths and collecting quantum efficiency (QE) results. The amount of H2 produced in the rst 2 h was used to calculate quantum efficiency using the equation below. The number of photons from irradiation was measured using a photodiode. QE ¼

2 the number of evolved hydrogen molecules 100% the number of incident photons

Results and discussion Al(OH)(bpydc) (MOF-253, bpydc ¼ 2,20 -bipyridine-5,50 -dicarboxylic acid) was chosen as a prototype because it exhibits high surface area, high chemical and thermal stability as well as a rigid framework structure (Fig. 1).10 Importantly, MOF-253 has accessible 2,20 -bipyridine units in the framework which allow post-synthetic modication to develop photocatalytically active porous materials. MOF-253 was synthesized according to the procedure previously reported.10,11 The phase purity of the obtained solid was independently conrmed by powder X-ray diffraction (XRD) displayed (Fig. 2) and elemental analysis (ESI†). The as-synthesized MOF-253 was de-solvated under a dynamic vacuum at 200 C for 24 h and further modied with cis-Pt(DMSO)2Cl2 in CH3CN. The bulk crystal sample of Al(OH)(bpydc)$0.5PtCl2 (MOF-253-Pt) (Fig. 1) was analyzed by elemental analysis, powder XRD (Fig. 2), infrared spectroscopy (IR) (Fig. S1†), N2 sorption measurements (Fig. S2†), and thermogravimetric analysis (TGA) (Fig. S3†). The atomic ratio of Pt/ Al in MOF-253-Pt was measured to be 0.5 by inductively coupled plasma mass spectrometry (ICP-MS) analysis (ESI†). The permanent porosity of the de-solvated MOF-253 and MOF-253-Pt was analyzed by N2 sorption experiments at 77 K. As shown in Fig. S2,† the sorption isotherm indicates that micropores are retained, even aer incorporation of platinum. The

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Fig. 1 Model structure of MOF-253-Pt, through post-synthetic modification of MOF-253 with PtCl2. Key: Cyan octahedron represents Al atoms, while yellow, green, red, blue, and blank circles represent Pt, Cl, O, N, and C atoms, respectively; H atoms are omitted for clarity.

Langmuir surface areas of MOF-253 and MOF-253-Pt were calculated to be 1237 and 527 m2 g1, respectively. A single data point at a relative pressure of 0.994 gave the total specic pore volumes of 0.563 and 0.333 cm3 g1 for MOF-253 and MOF-253Pt, respectively. The surface area and pore volume of MOF-253 are in good agreement with the recently reported values.11 The lower surface area and smaller pore volume of MOF-253-Pt are attributed to partial blocking of the open pores in MOF by Pt(bpydc)Cl2 complexes,11 rather than the collapse of the framework. As shown in Fig. 2, the peaks of XRD of MOF-253-Pt correspond well to those of MOF-253, indicating that the framework structure is maintained upon metal coordination, although their peaks shi slightly compared with those in the simulated XRD pattern. This is likely a result of the disorder in the crystal structure.11 To conrm the local coordination

Fig. 2 Powder X-ray diffraction patterns of simulated MOF-253, methanolexchanged MOF-253 and MOF-253-Pt.


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Communication environment of the Pt atom upon insertion within the framework, extended X-ray absorption ne structure (EXAFS) spectroscopy was performed. As shown in Fig. 3, the experimental ˚ Fourier transform spectrum shows a small peak at around 1.5 A attributed to a Pt–N bond and a strong peak at approximately ˚ corresponded to the Pt–Cl bond. Furthermore, an addi2.0 A ˚ was observed, which might have tional peak at around 3.6 A resulted from the short non-covalent Pt/Pt interactions.12 The inverse FT spectrum of the two peaks from the covalent bonds was tted by phase-shi correction with the contributions from two single scattering paths between Pt and neighboring N and Cl atoms. The best-t results indicate the Pt–N distance of 1.956 ˚ whereas the distance of the Pt–Cl bond is 2.301 A. ˚ These two A, bond distances are in good agreement with the structural parameters derived from the crystal structure of the similarly coordinated complex.13 By insertion of metal salts within MOF-253, we expect that the as-synthesized MOF-253-Pt can act as a solid-state photosensitizer. As shown in Fig. 4, compared to an absorption edge at 380 nm of MOF-253, the absorption edge of MOF-253-Pt is obviously red-shied. Furthermore, in agreement with its bright yellow color, MOF-253-Pt shows an extra absorption band centered at about 410 nm with the absorption edge extended to around 650 nm. These features are attributed to the binding of PtCl2 to bipyridine. The low-energy absorption in MOF-253-Pt is due to the metal-to-ligand (PtII / bipyridine p*) charge transfer (MLCT) transition.14 Upon excitation, MOF-253-Pt dispersed in CH3CN displays intense emission owing to ligand-centered (LC) transition. The emission can be quenched by TEOA (Fig. S4†). The electrochemical properties of MOF-253 and MOF253-Pt deposited on a glassy carbon electrode were also investigated. Fig. S5† shows the cyclic voltammograms (CV) of a MOF-253-Pt electrode in a 0.1 M Bu4NPF6/CH3CN solution at room temperature at a scan rate of 100 mV s1. Upon coordination of Pt atom to the nitrogen atoms of bipyridine, the reduction potential of the bipyridine is signicantly shied to

Fig. 3 Pt L3-edge EXAFS Fourier transform (FT) spectrum and filtered EXAFS spectrum of the first two peaks (inset) of MOF-253-Pt. Solid lines and dashed lines represent the observed experimental data and the best fitting results, respectively.



Fig. 4 UV-vis spectra of MOF-253, MOF-253-Pt and Pt(bpydc)Cl2 as well as the quantum efficiencies of hydrogen evolution for MOF-253-Pt at different wavelengths in a CH3CN/H2O (v/v, 1 : 1) system. The inset shows the colors of the samples.

more positive values due to the stabilisation of the lowest unoccupied molecular orbital (LUMO, p*), suggesting that the reduction electrons are possibly located on the bipyridine ligand.15 Furthermore, light irradiation of MOF-253 and MOF253-Pt lm coated on indium-tin-oxide glass in an electrolyte containing TEOA (at a constant potential of 0.35 V vs. SCE) under visible-light irradiation indicates that the trend of photocurrent response of MOF-253-Pt is consistent with the absorption spectrum (Fig. S6 and S7†). In contrast, MOF-253 did not show photocurrent response under the same conditions. In addition, no signicant photocurrent response was observed in the absence of TEOA. The results suggest that MOF-253-Pt can absorb visible light and the photo-generated carriers can be efficiently separated and transferred in the TEOA solution. The photocatalytic activity of MOF-253-Pt for H2 evolution was evaluated using MOF-253-Pt (0.53 mM based on Pt) in the presence of 15 vol% TEOA as a sacricial electron donor in water at pH 8.5 under visible-light irradiation (l > 420 nm). Fig. 5 shows that H2 production increased stepwise with time upon light irradiation, and then gradually reached a maximum at around 30 h. Control experiments showed that no H2 evolution was observed under the same experimental conditions, without the addition of MOF-253-Pt or TEOA in the solution. There was also no H2 detected from the TEOA solution under visible-light irradiation using (i) MOF-253 and Pt nanoparticles, (ii) MOF-253 and cis-Pt(DMSO)2Cl2, or (iii) MOF-253 alone. Furthermore, no H2 formation occurred in the dark, suggesting the photocatalytic nature of the reaction. When the TEOA aqueous solution was replaced by the acetate–EDTA buffer system (pH ¼ 5.0) (Fig. S8†), which is oen employed in a photocatalytic H2 evolution system containing platinum complexes,2b only a small amount of H2 was detected. To further determine the effect of pH on the H2 evolution rate, the reaction was performed in the pH range of 7.0–11.0 under the conditions described in Fig. S9 and S10.† The optimum pH was found at around 8.5 which is similar to the observation in other related

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Fig. 5 Photocatalytic H2 evolution over MOF-253-Pt (0.53 mM based on Pt) in H2O, MOF-253-Pt (0.53 mM based on Pt), Pt(bpydc)Cl2 (0.53 mM based on Pt) and MOF-253 (1 equivalent MOF-253-Pt) in CH3CN/H2O (v/v, 1 : 1), respectively. Other reaction conditions: 15% TEOA (v/v), pH 8.5, 100 mL solution, light source: a 300 W Xe lamp with a 420 nm cut-off filter.

photocatalytic systems.16 The rate of H2 evolution decreases under both more acidic and more basic reaction conditions. At lower pH values, the protonated TEOA is a weak electron donor. At higher pH, the formation of a hydride-diplatinum(II, III) species (a postulated reaction intermediate) is unfavourable and the driving force for H2 formation from water is weakened. To understand the photocatalytic H2 evolution mechanism, the concentration of MOF-253-Pt was varied. As shown in Fig. S11,† the H2 evolution rate signicantly depends on the concentration of Pt inserted in MOF-253-Pt. Increasing the concentration of MOF-253-Pt increases the reaction rate and the overall amount of H2 evolved. At lower catalyst concentrations, the initial rate of H2 evolution has a rst order dependence on the catalyst concentration. However, at higher catalyst concentrations of 0.53 mM Pt inserted in MOF-253-Pt and above, the rate of H2 evolution deviates from the linear relationship (Fig. S12†). The results suggest that the rate may also depend on the extent of photon absorption by MOF.17 Based on the present observations, the photocatalytic reaction mechanism was proposed as shown in Scheme 1. Upon light irradiation, the MOF in the presence of TEOA rst generates a one-electronreduced species MOF*(3MLCT) and holes. The MOF*(3MLCT) species is reductively quenched to form MOF_ with the electrons stored on the bpy_ ligands, which is consistent with the results from cyclic voltammetry. The reduced MOF_ further forms a Pt(III)-hydride intermediate via a proton-coupled electron transfer (PCET). The intermediates contribute to the formation of the hydride-diplatinum(II, III) intermediate by the synergistic effect of the neighbouring anchored Pt(bpy)Cl2 complex on the framework, which results in H2 production by a heterolytic coupling pathway.2b The postulated path is in accordance with the linear dependence of the rate of photocatalytic H2 evolution on the catalyst concentration. Moreover, the EXAFS analysis indicates that there are probably the short Pt/Pt interactions in the frameworks as discussed above. The short Pt/Pt interactions destabilized the ds* highest-occupied


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Scheme 1 Proposed reaction mechanism for the photocatalytic H2 evolution over MOF-253-Pt under visible-light irradiation.

molecular orbital (HOMO) originating from the 5dz2 orbital of Pt(II) centers, which contribute to the formation of reaction intermediates.2b To verify this, we further investigated the luminescence of MOF-253-Pt at 77 K. As shown in Fig. 6 and S14,† MOF-253-Pt exhibits strong emission at 77 K, whereas it is absent in the room-temperature emission spectrum. The emissive state could be assigned as a metal-to-ligand charge transfer state (3MLCT) band [ds* / p*(bpy)] and the metal– metal-to-ligand charge-transfer (3MMLCT) transitions,12,18 indicating the presence of short Pt/Pt interactions due to thermal lattice contraction shortening of the Pt/Pt spacing upon cooling to 77 K.12 In photocatalytic H2 evolution systems, the selection of the solvent is also of considerable importance. To investigate the inuence of the solvent, CH3CN was oen employed as a cosolvent.19 As shown in Fig. 5, a signicant improvement in H2 evolution was observed in the presence of the co-solvent

Fig. 6 Emission of MOF-253-Pt at 77 K (a) and at room temperature (b); emission of MOF-253 at room temperature (c); and excitation of MOF-253-Pt at room temperature (d). All samples were dispersed in Ar degassed CH3CN (lex ¼ 355 nm).


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Communication (CH3CN/H2O at 1 : 1, v/v). The highest quantum efficiency was measured to be 1.63% at 440 nm (Fig. 6), which is slightly higher than those obtained for MOF based photocatalysts reported under visible light irradiation.7,9b The photocatalytic activity also depends on the concentration of the TEOA sacricial reagent (Fig. S15†). At the constant catalyst concentration, with a decrease in the concentration of TEOA from 15% to 5%, the amount of H2 evolved is reduced by 80%, indicating that the quenching efficiency of the excited state decreased when the concentration of TEOA decreased. The results further supported the fact that TEOA is involved in the reductive quenching of the excited state of MOF-253-Pt. To make a comparison, the monomer complex Pt(bpydc)Cl2 was also studied under the same photoreaction conditions. As shown in Fig. 5, the amount of H2 production based on MOF-253-Pt is 4.7 times of that obtained by the complex Pt(bpydc)Cl2. The enhanced photocatalytic H2 evolution activities of MOF could be attributed to the interactions of shortly separated Pt/Pt pairs12 and more efficient electron transfer within the porous framework containing the metal-complex,20 as well as slowing down of the decomposition of the anchored Pt(bpy)Cl2 complex within the MOF.9 Furthermore, the catalytic activities obtained are competitive with those reported for Pt-based MOF systems.7,21 Finally, to probe whether the photocatalytic activity resulted from platinum nanoparticles possibly produced during the photocatalytic process, mercury experiment was carried out, which has been oen used to capture the colloidal metal particles.22 Photocatalytic H2 evolution over MOF-253-Pt was evaluated in the presence of mercury. As shown in Fig. S16,† the mercury present does not signicantly affect the photocatalytic activity of MOF-253-Pt. In addition, in the absence of an additional photosensitizer, the photocatalytic performance of MOF253-Pt depends on the pH, solvent as well as the concentrations of the catalyst and TEOA. These results rule out the origin of photocatalytic H2 evolution from the formation of platinum nanoparticles, which was further veried by the absorption and uorescence spectra of MOF-253-Pt aer photocatalytic reaction (Fig. S20†).

Conclusions In conclusion, we have described an approach to construct photocatalytically active MOF by inserting a platinum salt within the framework. The modied MOF serves both as a photosensitizer and as a photocatalytic H2 evolution catalyst. Although the precious metal was employed, importantly, we realized that the photocatalytic H2 evolution can occur without an additional co-catalyst and photosensitizer over such a bifunctional MOF obtained through post-synthetic modication of inactive MOF. This work also clearly demonstrates that the concept of inserting metal salts in a porous framework can be employed successfully as a strategy for designing and constructing new types of photocatalysts for H2 production. Further studies are in progress to design robust and efficient MOF photocatalysts incorporated with earth-abundant transition metal ions for visible light-driven H2 evolution and CO2 reduction.



Acknowledgements This work was supported by Singapore National Research Foundation (NRF) through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Programme, and Institute of NanoSystems Interface Sciences and Technologies (INSIST), Nanyang Technological University, Singapore. We also thank W. N. Zhang and Prof. Dr F. W. Huo from MSE in NTU for their help with the N2 sorption experiments, J. Z. Chen for photocurrent experiment, and F. Zheng for photoluminescence experiment from SCBE in NTU.

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