DOI: 10.1002/chem.201403800

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& Covalent Organic Frameworks

A Covalent Organic Framework–Cadmium Sulfide Hybrid as a Prototype Photocatalyst for Visible-Light-Driven Hydrogen Production Jayshri Thote,[a] Harshitha Barike Aiyappa,[a] Aparna Deshpande,[a] David Daz Daz,[b] Sreekumar Kurungot,*[a] and Rahul Banerjee*[a]

Abstract: CdS nanoparticles were deposited on a highly stable, two-dimensional (2D) covalent organic framework (COF) matrix and the hybrid was tested for photocatalytic hydrogen production. The efficiency of CdS-COF hybrid was investigated by varying the COF content. On the introduction of just 1 wt % of COF, a dramatic tenfold increase in the overall photocatalytic activity of the hybrid was observed. Among the various hybrids synthesized, that with 10 wt %

Introduction Covalent organic frameworks (COFs) belong to a topical class of porous crystalline materials synthesized from lightweight elements, such as C, B, O, N, and Si.[1] A range of high-surfacearea COFs have been specifically designed and used for gas adsorption, storage, optoelectronic, and energy-storage devices.[2] However, these materials have found limited applications in areas such as catalysis owing to their poor chemical stability[3] until recently, wherein this problem has been countered using a combined reversible and irreversible Schiff base reaction.[4] Additionally, such stable COFs carry the virtue of the presence of heteroatom in the framework apart from extensive p–p conjugation along the layers, which has popularized them as ideal aromatic platforms for anchoring active nanoparticles, and therefore improving their catalytic activity.[5] Currently, a number of photocatalysts based on highly porous materials, such as MOFs,[6] C3N4 sheets,[7] and polymers are employed as [a] Dr. J. Thote,+ H. B. Aiyappa,+ A. Deshpande, Dr. S. Kurungot, Dr. R. Banerjee Physical/Materials Chemistry Division CSIR-National Chemical Laboratory Dr. Homi Bhabha Road, Pune-411008 (India) Fax: (+ 91) 20-25902636 E-mail: [email protected] [email protected] [b] Prof. Dr. D. Daz Daz Institut fr Organische Chemie, Universitt Regensburg Universittsstrasse 31, 93040 Regensburg (Germany) and IQAC-CSIC, Jordi Girona 18-26, 08034, Barcelona (Spain) [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403800. Chem. Eur. J. 2014, 20, 15961 – 15965

COF, named CdS-COF (90:10), was found to exhibit a steep H2 production amounting to 3678 mmol h 1 g 1, which is significantly higher than that of bulk CdS particles (124 mmol h 1 g 1). The presence of a p-conjugated backbone, high surface area, and occurrence of abundant 2D hetero-interface highlight the usage of COF as an effective support for stabilizing the generated photoelectrons, thereby resulting in an efficient and high photocatalytic activity.

photosensitizers and support for the loading of active nanoparticles resulting in an improved catalytic activity. However, most of these materials show limited performance owing to the poor water stability, as in the case of MOFs and C3N4 sheets, and poor crystallinity as in the case of polymers, which affects their photocatalytic activity. With this perspective, crystalline and stable COF seem to be an attractive support matrix[5] for the loading of nanoparticles owing to its remarkable stability in both acidic (9 m HCl) and basic (9 m NaOH)[4] mediums along with high surface area and porosity. Moreover, the 2D crystalline nature of COF is reported to enhance the charge mobility on its surface owing to the presence of p arrays in comparison to the 1D or 3D polymers.[8] Although COFs are known to be semiconductors with ability to show absorption in visible light,[9a] their potential as photocatalysts, and in particular for water splitting, remains hardly explored.[9b] To date, numerous nanoparticle 2D hybrid (for example, ZnS/ CdSe/CdS/MoS2graphene, TiO2)-based photocatalysts have proven to be prospective candidates for H2 production owing to their tunable size and band structure.[10] However, most of these 2D nanosheets suffer from acute limitations, such as high shielding effect,[11] poor visible light absorption,[12] and low stability. In contrast, in case of COFs, the stability is attributed to the reactions involving irreversible keto–enol tautomerism, ultimately leading to the formation of the dominant keto form. These forms have high stability in neutral, acid, and base pH conditions, which makes them ideal catalyst supports. Herein, we have employed a highly stable COF (TpPa-2)[7] as a support matrix for anchoring CdS nanoparticles (Scheme 1). The p-conjugated 2D crystalline framework of COFs could possibly help in an efficient charge transfer, thereby stabilizing the nanoparticles and ultimately suppressing the recombination of

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Scheme 1. Representation of a) Synthesis of the covalent organic framework (COF) TpPa-2 using a previously reported procedure.[4] b) CdS-COF hybrid formation by hydrothermal synthesis of CdS nanoparticles on the COF matrix.

CdS photogenerated holes and electrons. As expected, the CdS-COF hybrids showed enhanced photocatalytic H2 evolution as compared to the bulk CdS. The FTIR spectra of the synthesized hybrids resembled that of the COF (Figure 1 a). The spectra showed the presence of characteristic bands corresponding to the stretching frequency of the C=C (1578 cm 1) and C N (1255 cm 1) bonds of parent COF, which remained unaltered after hybrid formation. This indicates that the COF is highly stable to the hydrothermal conditions on the course of the deposition of CdS nanoparticles. In case of the hybrids, the PXRD peaks at 2q values 26.5, 44, and 52.18 correspond to (111), (220), and (311) planes of CdS nanoparticles that indicate the presence of CdS nanoparticles in cubic phase (Figure 1 b). Intense diffraction peaks corresponding to the (100) plane of COF were not observed in the hybrids owing to the relatively lower amount and inherently poor diffraction intensity of the COF in comparison to that of CdS. The diffraction peaks of hybrids were broad and intense. This reflects the reduction in the particle size of CdS on hybrid formation with COF. The BET surface area of the bulk CdS was found to be 93 m2 g 1 (Figure 1 c). Upon hybrid formation, the surface area of the hybrids was found to increase with the increasing content of COF (up to 280 m2 g 1), which could be attributed to the contribution effect of the supporting COF matrix (866 m2 g 1). The increase in the specific surface area Chem. Eur. J. 2014, 20, 15961 – 15965

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possibly boosts the number of surface active sites apart from promoting easier charge transport. HRTEM images of COF showed a sheet-like structure, formed as a result of p–p stacking of COF layers (Figure 1 d). In absence of the support material (COF here), the particle size of bulk CdS was found to be very large (> 200 nm) with prominent agglomeration (Supporting Information, Figure S7). However, upon addition of COF, the hybrid showed uniform distribution of CdS along with reduction in the particle size (20– 25 nm; Figure 1 e). The HRTEM image of the dispersed CdS nanoparticles in CdS-COF (90:10) displayed observable lattice fringes, thereby suggesting the formation of well-defined crystal structure (inset of Figure 1 e). The lattice spacing is about 0.35 nm, corresponding to the (111) plane of CdS nanoparticles, as observed in its PXRD pattern. The SEM micrographs of COF confirmed the formation of sheet like layers owing to extensive p–p stacking interactions, while CdS-COF (90:10) showed well-distributed CdS nanoparticles on the COF sheet matrix (Supporting Information, Figure S8). The UV/Vis diffuse reflectance spectrum of COF showed a broad absorption band over the visible region (Figure 1 f). It could be observed that upon addition of COF, the absorption edge of the hybrid CdSCOF (90:10) shifted towards larger visible light domain (> 535 nm) in comparison with that of the bulk CdS spectra. This was also supported by an observable change in the color of

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Figure 1. a) FTIR and b) PXRD spectra; c) N2 adsorption isotherms of the synthesized hybrids along with bulk CdS and COF materials; d) and e) TEM images of COF and CdS-COF (90:10) hybrid; inset: lattice fringes of CdS nanoparticles in CdS-COF (90:10) hybrid; f) UV-DRS spectra of bulk CdS, COF, and CdS-COF (90:10) hybrid.

CdS that is, yellow to reddish brown on addition of COF (Scheme 1). The shift in the absorption spectra confirmed the improved visible light absorption of CdS-COF hybrids. The loading of COF in CdS-COF was confirmed by EDAX and CHNS analysis (Supporting Information, Table S1). The percentage concentration of C, H, N, and S in all of the hybrids was found to match quantitatively with the added amount of COF. The EDAX analysis carried out at different spots on the hybrid surface confirmed that the concentration of Cd and S are in agreement with the desired loading amount. The thermal stability of the hybrids was determined using thermo gravimetric (TGA) analysis (Supporting Information, Figure S9). The COF was found to be thermally stable up to 350 8C, while the bulk CdS showed thermal degradation from 220 8C. The thermal stability of the hybrid was found to increase on the introduction of COF. This further confirmed the improved stability of CdS on hybrid formation with a stable COF. The contact angle study showed a low contact angle of 108, which confirmed the hydrophilic nature of the hybrid (Supporting Information, Figure S10). To examine the photocatalytic behavior of the CdS-COF hybrids, visible-light-driven H2 evolution experiments were carried out using 0.5 wt % Pt as a co-catalyst, lactic acid as the sacrificial agent, and a 400 W xenon arc lamp with a UV cut-off filter (l  420 nm) as a visible light source. Under visible-light illumination, the as-synthesized bulk CdS showed a photocatalytic hydrogen production of 128 m mol h 1 g 1 (Figure 2 a). Remarkably, upon addition of just 1 wt % of COF, a dramatic increase in the H2 production with 1320 mmol h 1 g 1 was obChem. Eur. J. 2014, 20, 15961 – 15965

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served, which is about ten times that of the bulk CdS. The poor activity of plain COF (28 mmol h 1 g 1) could be attributed to its band gap (2.52 eV), which is sufficiently large to split water during band gap excitation, yet small enough to cover the entire visible-light spectrum (Supporting Information, Figure S12). Moreover, the stacking of such COFs decreases the number of sheets exposed and their availability for hydrogen production. Among all the synthesized hybrids, CdS-COF (90:10) was found to display the photocatalytic activity with 3678 mmol h 1 g 1 of H2 production. The hydrogen production increased to a certain extent (up to 10 wt % of COF) and then eventually decreased on further addition of the support matrix. It is noteworthy that the hybrids differ only in terms of weight percentage of COF added. Thus the detriment in the photocatalytic activity is likely a consequence of shielding effect (as observed in case of graphene-based hybrids[11]), wherein an excess of COF (a poor visible-light absorber) now coversthe visible-light-active CdS particles. This decreases the photocatalytic activity of such hybrids as they can no longer absorb light efficiently. The photocatalytic activity of CdS-COF (90:10) hybrid was found to be consistent without any indication of deactivation for the next three consecutive cycles of 4 h each (Figure 2 b) with an apparent quantum yield of 4.2 % at 420 nm. The control experiments under dark conditions showed negligible H2 production, which unambiguously confirmed that, the catalytic activity of the hybrid materials is triggered by light. Furthermore, the TEM study confirmed that the CdS-COF hybrid is stable up to three consecutive cycles with

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Figure 2. a) Comparison of visible-light-induced H2 production using as-synthesized hybrid along with bulk CdS and COF; b) stability test of the displayed photocatalytic activity; c) Mott–Schottky plots of COF, bulk CdS, and CdS-COF (90:10) hybrids; d) photoluminescence spectra of bulk CdS and CdS-COF (90:10) hybrid at 460 nm excitation wavelength; e) energy-level diagram of the hybrid system.

CdS nanoparticles still dispersed on the COF matrix (Supporting Information, Figure S11). To study the effect of the coupling interfaces in CdS-COF (90:10) hybrid, Mott–Schottky (MS) measurements were performed under dark conditions using impedance measurements (Figure 2 c). The experiment was carried out in real H2 producing conditions (that is, water– lactic acid solution). Thin films of the materials, namely bulk CdS, COF, and CdS-COF (90:10) hybrid, were prepared by the doctor-blade technique on a FTO plate and were used as the working electrode. The flat band potential measured for CdS ( 0.64 V, vs. NHE) is higher than that of COF ( 0.54 V, vs. NHE), implying that CdS has a higher Fermi level in comparison to COF. In the case of the CdS-COF (90:10) hybrid, a flat band potential of 0.59 V vs. NHE was obtained. Upon hybrid formation, the flat band potential of COF was found to be elevated and a new Fermi level was established between those of CdS and COF. This implies a high probability of transfer of generated photoelectrons on illumination of visible light from CdS to COF.[13a] In support to the conclusions extracted from the MS plots, the photoluminescence (PL) study of CdS-COF(90:10) at 460 nm excitation showed decrease in the peak intensity in comparison to that of bulk CdS, a characteristic feature of efficient suppression of electron–hole recombination (Figure 2 d).The optical density (OD) was maintained at 0.11 to normalize the concentration throughout the experiment. The MS and PL studies thus indicate a possible charge transfer from Chem. Eur. J. 2014, 20, 15961 – 15965

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CdS to COF on hybrid formation, thereby decreasing the chances of recombination of the photogenerated electron–hole pair.[13b] Thus the maximum photocatalytic activity of CdS-COF (90:10) can be attributed to the presence of high surface and abundantly available ordered 2D heterointerface of COF, which facilitates the charge stabilization of generated CdS photoelectrons on the support COF matrix.

Conclusion High and efficient photocatalytic activity of CdS nanoparticles towards hydrogen production has been achieved on their hybrid formation with a stable COF. The COF support was observed to improve the photostability of the deposited CdS nanoparticles apart from effective electron–hole pair recombination suppression thereby resulting in a remarkably high activity compared to that of bulk CdS. This study demonstrates an effective amalgamation of organic and inorganic materials within a single hybrid resulting in an improved activity compared to each of the constituents. Thus, we believe that this study highlights the authenticity of usage of such highly stable COF 2D semiconductor sheets as an effective support towards development of more efficient and stable visible-light-active photocatalysts for H2 production.

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Full Paper Acknowledgements J.T. acknowledges CSIR, New Delhi, India, for a CSIR-Nehru Post-doctoral Fellowship and H.B.A. acknowledges UGC, New Delhi, India, for SRF. R.B. acknowledges CSIR’s Five Year Plan (CSC0102) for funding. Financial assistance from DST (SB/S1/IC32/2013) is acknowledged. Keywords: charge transfer · covalent organic frameworks · hybrid materials · hydrogen production · photocatalysis [1] a) H. M. El-Kaderi, J. R. Hunt, J. L. Mendoza-Corts, A. P. Ct, R. E. Taylor, M. O’Keeffe, O. M. Yaghi, Science 2007, 316, 268 – 268; b) E. L. Spitler, B. T. Koo, J. L. Novotney, J. W. Colson, F. J. Uribe-Romo, G. D. Gutierrez, P. Clancy, W. R. Dichtel, J. Am. Chem. Soc. 2011, 133, 19416 – 19421; c) E. L. Spitler, M. R. Giovino, S. L. White, W. R. Dichtel, Chem. Sci. 2011, 2, 1588 – 1593; d) F. J. Uribe-Romo, C. J. Doonan, H. Furukawa, K. Oisaki, O. M. Yaghi, J. Am. Chem. Soc. 2011, 133, 11478 – 11481; e) J. F. Dienstmaier, D. D. Medina, M. Dogru, P. Knochel, T. Bein, W. M. Heckl, M. Lackinger, ACS Nano 2012, 6, 7234 – 7242; f) E. L. Spitler, J. W. Colson, F. J. Uribe-Romo, A. R. Woll, M. R. Giovino, A. Saldivar, W. R. Dichtel, Angew. Chem. Int. Ed. 2012, 51, 2623 – 2627; Angew. Chem. 2012, 124, 2677 – 2681; g) E. L. Spitler, W. R. Dichtel, Nat. Chem. 2010, 2, 672 – 677; h) X. Feng, X. Ding, D. Jiang, Chem. Soc. Rev. 2012, 41, 6010 – 6022. [2] a) S. S. Han, H. Furukawa, O. M. Yaghi, W. A. Goddard, J. Am. Chem. Soc. 2008, 130, 11580 – 11581; b) M. G. Rabbani, A. K. Sekizkardes, Z. Kahveci, T. E. Reich, R. Ding, H. M. El-Kaderi, Chem. Eur. J. 2013, 19, 3324 – 3328; c) H. Furukawa, O. M. Yaghi, J. Am. Chem. Soc. 2009, 131, 8875 – 8883; d) S. Ding, W. Wang, Chem. Soc. Rev. 2013, 42, 548 – 568. [3] a) L. M. Lanni, R. W. Tilford, M. Bharathy, J. J. Lavigne, J. Am. Chem. Soc. 2011, 133, 13975 – 13983; b) Y. Du, K. Mao, P. Kamakoti, P. Ravikovitch, C. Paur, S. Cundy, Q. Li, D. Calabro, Chem. Commun. 2012, 48, 4606 – 4608. [4] a) S. Kandambeth, A. Mallick, B. Lukose, M. V. Mane, T. Heine, R. Banerjee, J. Am. Chem. Soc. 2012, 134, 19524 – 19527b) S. Kandambeth, D. B. Shinde, M. K. Panda, B. Lukose, T. Heine, R. Banerjee, Angew. Chem. Int. Ed. 2013, 52, 13052 – 13056; Angew. Chem. 2013, 125, 13290 – 13294c) S. Chandra, S. Kandambeth, B. P. Biswal, Bi. Lukose, S. M. Kunjir, M. Chaudhary, R. Babarao, T. Heine, R. Banerjee, J. Am. Chem. Soc. 2013, 135, 17853 – 17861. The improvement in the surface area of the COF

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Received: June 3, 2014 Published online on October 13, 2014

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A covalent organic framework-cadmium sulfide hybrid as a prototype photocatalyst for visible-light-driven hydrogen production.

CdS nanoparticles were deposited on a highly stable, two-dimensional (2D) covalent organic framework (COF) matrix and the hybrid was tested for photoc...
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