Islam M. A.
Mekhemer‡
ab,
Mohamed M.
Elsenety‡
ac,
Ahmed M.
Elewa
a,
Khanh Do Gia
Huynh
a,
Maha Mohamed
Samy
bd,
Mohamed Gamal
Mohamed
bd,
Dalia M.
Dorrah
a,
Dung Chau Kim
Hoang
a,
Ahmed Fouad
Musa
a,
Shiao-Wei
Kuo
d and
Ho-Hsiu
Chou
*aef
aDepartment of Chemical Engineering, National Tsing Hua University, Hsinchu 300044, Taiwan. E-mail: hhchou@mx.nthu.edu.tw
bChemistry Department, Faculty of Science, Assiut University, Assiut, 71515, Egypt
cChemistry Department, Faculty of Science, Al-Azhar University, Cairo, 11884, Egypt
dDepartment of Materials and Optoelectronic Science, Center of Crystal Research, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
eCollege of Semiconductor Research, National Tsing Hua University, Hsinchu 300044, Taiwan
fPhotonics Research Center, National Tsing Hua University, Hsinchu 300044, Taiwan
First published on 27th March 2024
Photocatalytic hydrogen production through water splitting provides a promising route towards renewable energy generation. However, constructing photocatalytically active covalent organic frameworks with high charge separation remains challenging. Herein, we demonstrate for the first time the use of 2D imide–imine-based covalent organic frameworks as new photocatalysts for the hydrogen evolution reaction (HER) under visible light irradiation. The main achievement is incorporating donor and dual acceptors, including weak electron-deficient imine and strong electron-deficient imide groups within the 2D COF backbone that create favorable push–pull–pull intramolecular charge transfer to promote charge separation after photoexcitation. DFT and NBO calculations revealed the strong integration of donor and dual acceptors with a synergistic interplay enhancing spatial charge transfer and separation. The synthesized COFs show significantly high thermal stability >400 °C with a high energy barrier for degradation. Moreover, Py-DNII-COF exhibited a 104-fold enhancement in hydrogen evolution compared to TFPB-DNII-COF. Py-DNII-COF demonstrated excellent stability and hydrogen evolution of 625 μmol h−1 g−1 over 48 hours.
Thus, we synthesized 2D COFs of Py-DNII and TFPB-DNII frameworks, leading to the formation of imide–imine linkages. A new approach introduces dual electron-acceptor using push–pull–pull centers into the COF structure to enhance its electrical conductivity and facilitate efficient charge transport. The synthesized COFs were characterized using different spectroscopic techniques. The synthesis of these two-dimensional imide–imine COFs was achieved through a meticulous solvothermal method, ensuring precise control over their structural attributes. Thorough characterization using advanced spectroscopic techniques confirmed the successful implementation of the proposed COF structure. To better understand the electronic structure and the nature of push–pull–pull interactions, we employed Density Functional Theory (DFT) and Natural Bond Orbital (NBO) calculations. These calculations elucidated the strong synergistic interplay between the imide imine linkers, creating a favorable environment for spatial charge separation.
However, second order perturbation of the Fock matrix in NBO analysis provides a more intuitive picture of chemical bonding and allows for a clearer interpretation of electronic structure transition and charge transfer from donor to acceptor moieties. The results represent a donor–acceptor of most significant translation of the N lone pair of electrons in the imide-linkage to the π* orbital of both adjacent carbonyl groups with interaction energy of 51.42 and 51.26 kcal mol−1. However, the π-orbital of the –CN– of the imine group represents an interaction energy of 10.13 kcal mol−1 to the π* orbital of –C–C in the direction of the amide group and acceptor moiety, in contrast to the lower interaction energy of 8.21 kcal mol−1 for the π* orbital of –C–C in the direction of the pyridine moiety. In addition, there are multiple possible transitions from the bonding orbital to the antibonding orbital in the benzene rings due the conjugated system of the molecule, with interaction energy of 22
:
14 kcal mol−1. This finding shows that the first central position of imine-linkage might be more prone to withdraw (pull) electrons from the donor pyridine moiety to the second central position of imide (pull), facilitating charge separation on the whole COF. Furthermore, the ESP of the Py-DNII-COF provides insights into the distribution of electron density on the whole COF. The π electrons in the aromatic rings of the tetraphenyl pyrene contribute to the overall electron density of the molecule. However, the lower electron density could imply that the π electron density is more dispersed or delocalized, leading to a less concentrated region of electron density in this part of the molecule, which works as a donor moiety. This is in contrast to the higher electron density distributed on the imide groups of the dinaphthylimide (DNI), which will work as an acceptor moiety in the COF, thus facilitating the withdrawal of electron density from the donor to acceptor through the imide–imine linkage and creating a synergistic interplay within the COF structure. To investigate the hydrogen evolution potential of two synthesized COFs, DFT calculations were employed to determine the energy gaps and energy levels. The Py-DNII and TFPB-DNII COFs exhibited energy gaps of 1.80 eV and 2.69 eV, respectively, as illustrated in Fig. 1d and e. The DFT calculations indicate that the narrow energy gaps observed between the HOMO−1/HOMO and LUMO levels of Py-DNII suggest the likelihood of electronic transitions occurring under visible light, particularly at wavelengths of 457 nm and 688 nm. On the other hand, TFPB-DNII COF exhibits a larger energy gap, resulting in a blue shift in the maximum absorption wavelength, which is detected at 504 nm and 475 nm (Fig. 1e). Furthermore, examining the Py-DNII COF reveals HOMO and LUMO energy levels at −5.16 eV and −3.31 eV, respectively. This finding indicates a strong positive correlation with the potential for proton reduction to hydrogen, emphasizing the significance of these energy levels in the efficacy of the process.
The molecular structures of synthesized COFs were characterized via several spectroscopic techniques. The FTIR spectra of the monomers of DNI-2NH2, TFPB-4CHO, and their corresponding TFPB-DNII-COF are shown in Fig. 2a. The FTIR spectrum of DNI-2NH2 shows the following characteristic bands of ν(–NH2) at 3409 and 3316 cm−1, ν(>CO) at 1702 and 1645 cm−1, ν(C–C) at 1579, σ(C–H) at 980 cm−1 of the aromatic ring, and the stretching vibration of >C–N at 1246 cm−1.28–30 However, the FTIR spectrum of TFPB-4CHO shows ν(>C
O) at 1736 and 1697 cm−1, and the ν(C–H) at 2830 cm−1 of aldehyde groups, ν(C–C) at 1582 cm−1, and σ(C–H) at 1002 cm−1 of the aromatic ring. Thus, the disappearance of the –NH2 stretching vibration of DNII-2NH2 indicates that the amine groups were introduced in the formation of TFPB-DNII-COF. In addition, the existence and slight shifting in the peak position of the ν(>C
O) at 1705, and 1662 cm−1, ν(C–C) at 1579 cm−1 and ν(>C–N) at 1244 cm−1 confirm that the main monomers (aryl, carbonyl, and imine groups) are still a present in the synthesized DNII-TFPB-COF. As expected, the presence of the >C
N stretching vibration at 1602 cm−1 successfully confirms Schiff-base condensation reaction in the formation of TFPB-DNII-COF. Similar findings of vibrational bands were obtained for the formation of Py-DNII-COF, as shown in Fig. 2b.
The 13C NMR spectra provide robust structural evidence and support the successful formation of key chemical linkages and functional groups within the TFPB-DNII and Py-DNII COFs, as shown in Fig. 2c and d. These findings are pivotal in characterizing the precise composition and configuration of these COFs, facilitating a comprehensive understanding of their potential applications and properties. In particular, the appearance of the –CN– imine linkage was evident with a characteristic chemical shift at approximately 143 ppm, underscoring the successful Schiff-base formation within the COF structures. The chemical shift at approximately 163 ppm indicates the presence of carbonyl groups within the COFs. Additionally, the spectra exhibited chemical shifts within the range of 150–100 ppm, corresponding to the resonances of aromatic rings. This observation provides further evidence for COF structures, which are consistent with the anticipated structural elements based on the choice of monomers and synthetic strategy.
X-ray photoelectron spectroscopy (XPS) was employed to gain additional insights into the chemical composition of the synthesized COFs. In Fig. 2e, the survey spectra of TFPB-DNII-COF and Py-DNII-COF are presented, highlighting predominant peaks corresponding to the elements C, N, and O. X-ray photoelectron spectroscopy (XPS) further reveals the presence of C 1s and O 1s (Fig. S7†). The deconvolution of the C 1s signal generates peaks at 283.34, 284.53, 287.38, and 288.23 eV, attributable to the C–CC, C–N, C
O, and C
N bonds for both COFs, respectively (Fig. S7a and c†). The high-resolution O 1s spectrum display peaks at 530.31 and 531.32 eV assignable to the C–O and C
O bonds, respectively (Fig. S7b and d†). Furthermore, the high-resolution analysis provides detailed information about the nitrogen-containing functional groups present in the COFs, as shown in Fig. 2f. The N 1s spectra for both COFs reveal distinctive peaks at 397.4 and 399.1 eV, associated with the C–N bond and the imine linkage (C
N), respectively, which confirms the successful imide–imine linkage on the synthesized COFs.
The XRD pattern of Py-DNII-COF shows intense peaks at 2.55, 4.96, 12.86, 17.31, 22.83, and 24.48° (Fig. 3a), suggesting that the prepared Py-DNII-COF is a crystalline framework. In addition, the Pawley refinement method was used to simulate the optimum structure of Py-DNII-COF. The simulated PXRD pattern matches well with the experimental one, achieving lower residual values of Rwp (6.17%) and Rp (4.18%). The results imply that Py-DNII-COF mainly adopts the eclipsed AA stacking mode (Fig. 3b and c) of the P1 space group with lattice parameters of a (3.89 Å), b (36.74 Å), c (52.33 Å), α (135.07°), β (86.09°), and γ (79.92°). However, the atomic positions are recorded in Table S1.† Scanning electron microscopy (SEM) and high-resolution transmittance electron microscopy (HRTEM) images reveal fluffy aggregation of a needle-like structure for Py-DNII-COF, which could be the result of the strong π–π stacking interactions between adjacent layers with distance of about 4 Å, which is in a good agreement with the simulated XRD result.
In addition, the simulated XRD of TFPB-DNII-COF (Fig. S5†) reveals a relatively higher residual value of Rwp (8.89%). The Rp (5.98%) and the simulated model (of a space group P1, with lattice parameters of a (2.84 Å), b (35.11 Å), c (34.66 Å), α (44.29°), β (83.72°), and γ (89.71°)), show a twisted molecular geometry along the COF chains, presenting lower calculated pore width. However, the dihedral angle of 47° was measured from the optimized geometry structure, which shows that the geometry of the compound is not perfectly planar and exhibiting what is known as torsion or steric strain, which could be the reason behind a little higher residual compared to Py-COF. In addition, the SEM and TEM images of TFPB-DNII-COF (Fig. S6†) show the relatively high deformation and irregular shape of the crystals. These observations can impact the compound properties and reactivity, which could be the reason behind the lower activity of TFPB-DNII-COF in the experimental photocatalytic reduction.
Fig. S7† illustrates type (II) isotherms for Py-DNII-COF and TFPB-DNII-COF observed by N2 adsorption–desorption studies, which typically indicate COFs with microporous characteristics. Py-DNII-COF has a significantly higher Brunauer–Emmett–Teller (BET) surface area (183.00 m2 g−1) compared to TFPB-DNII-COF (45.00 m2 g−1). This suggests that Py-DNII-COF has a more extensive internal surface structure, possibly due to its specific structural features. In addition, Py-DNII-COF had an aperture that was spread at an average pore size of 4.1 nm and 7.4 nm (Fig. S8†), which are relatively large pores compared to TFPB-DNII-COF. Furthermore, the low sorption capacity and thus possibly obstructed pores of the TFPB-DNII-COF may either be attributed to their low crystallinity and the ill-defined stacking in the material, or due to residuals present in the pores, or both.37 Thermogravimetric analysis (TGA) conducted in this study was used to measure the weight loss of the material as a function of temperature. Py-DNII-COF and TFPB-DNII-COF were analyzed through TGA/Dr-TGA (differential thermal gravimetric analysis) with a heating ramp 20.00 °C min−1 to 800 °C under N2 gas to show the thermal degradation process of COFs (Fig. S9†) and better understand the mechanism of the degradation process.31 The synthesized COFs show a significantly high thermal stability of 420 and 450 °C for Py-DNII-COF and TFPB-DNII-COF, respectively. The Dr-TGA of Py-DNII-COF shows three steps of degradation. The 1st step is in the range of 350–420 °C, resulting in a weight loss of about 3% due to the removal of the adsorbed solvent molecules trapped inside the micro-pores of COF particles. This was followed by two extra steps at 515 °C and 635 °C with a mass loss of about 12% and 43%, respectively. In addition, TFPB-DNII-COF shows two steps of degradation at about 546 °C and 661 °C with mass loss of 39% and 60%, respectively. Comparing the Td5 and Td10 values (Table S2†) can give us an indication of how these materials react to initial thermal decomposition. TFPB-DNII-COF seems to resist initial decomposition slightly better but undergoes a more complete decomposition at high temperatures, as indicated by the lower char yield. In contrast, Py-DNII-COF starts decomposing at a slightly lower temperature but retains more structure, giving a higher char yield, which could suggest a better performance at high temperatures. Further, the Coats–Redfern model based on TGA data was used to estimate the thermodynamic parameters of a decomposition reaction, and the results are recorded in Table S3.† Regarding Py-DNII-COF, the energy barrier of 27386 J mol−1, 87
191 J mol−1, and 55
274 J mol−1 at 352 °C, 515 °C, and 635 °C of the three steps is required to overcome the decomposition process. Moreover, the positive enthalpy changes of 22
189 J mol−1, 80
640 J mol−1, and 47
725 J mol−1 indicate the endothermic reaction at all stages. In the case of TFPB-DNII-COF, it shows similar behavior to Py-DNII-COF in the decomposition process. Taken together, there is a decrease in the entropy in each step, while it is the reverse at higher temperatures, which can occur during reactions where gases are produced from COF materials. Furthermore, the change in Gibbs free energy implies that the decomposition reaction is not spontaneous under normal conditions. The increasing magnitude of the change in Gibbs free energy also suggests that any reaction becomes progressively less favorable unless continuously supplied with energy, which indicates a high thermal stability for the synthesized COFs.
Moreover, time-resolved fluorescence decay spectra (TRPL) were recorded to evaluate the excited-state lifetime (Fig. 4d). The weighted average lifetime (τavg) of Py-DNII-COF (9.12 ns) was longer than that of TFPB-DNII-COF (5.93 ns), further reflecting the enhanced separation efficiency and better photocatalytic potential.40 The electrochemical measurements, specifically photocurrent density responses and electro-impedance results, are illustrated in Fig. 4e and f. Additionally, the study of charge separation and migration behaviors involved the examination of transient photocurrents and electrochemical impedance spectroscopy (EIS). The transient photocurrent of Py-DNII-COF (Fig. 4e) exhibited a notable enhancement when compared to TFPB-DNII-COF, suggesting the rapid photoresponse of Py-DNII-COF. EIS measurements of the Nyquist plot (Fig. 4f) showed that Py-DNII-COF exhibits the smallest radius, suggesting its low interface charge-transfer resistance and high conductivity compared to TFPB-DNII-COF.
Noteworthy is the superior average hydrogen evolution rate of 577 μmol h−1 g−1 for AA, compared to 80 μmol h−1 g−1 and 40 μmol h−1 g−1 for TEA and TEOA, respectively. As anticipated, Py-DNII-COF exhibited a 104-fold enhancement in hydrogen evolution compared to the limited production (6 μmol h−1 g−1) observed in TFPB-DNII-COF (Fig. 5b). Additionally, the calculated AQY for Py-DNII-COF and TFPB-DNII-COF stood at 0.30% and 0.04% at 420 nm, respectively (Fig. S16†). Besides, we investigated the AQY of Py-DNII-COF using band-pass filters (420, 460, and 500 nm) corresponding to its visible light absorption (Fig. S17†). Extended photocatalysis up to 48 hours (Fig. 5c) using Py-DNII-COF revealed remarkable stability with an average hydrogen evolution of about 625 μmol h−1 g−1. Furthermore, Py-DNII-COF achieved good value of 377.5 μmol g−1 h−1 in H2O/AA. Notably, this value was consistent with that achieved in the presence of NMP as a co-solvent (625 μmol g−1 h−1) (Fig. S18†). This underscores water as the primary source of hydrogen, as opposed to the decomposition products of the COF.38 To validate our concept, we compared Py-DNII-COF to other COF-based materials (Table S4†). The phenomenon is illustrated in the proposed photocatalytic reaction mechanism (Fig. 5d). The D–A–A architecture with push–pull–pull phenomenon for Py-DNII-COF creates a directional flow of photogenerated electrons upon light absorption. The excited electron is propelled from the strong donor (pyrene) to the strong acceptor (imide bond) through weak acceptor (imine bond), effectively extending its lifetime and hindering the recombination of the hole remaining on the donor molecule by neutralizing it using ascorbic acid (AA) as a sacrificial reagent. Aligning with Zhen Li et al.‘s findings,41 Pt nanoparticles were found to bond with active sites within the COFs. Consequently, the uniform dispersion of Pt nanoparticles in Py-DNII-COF layers surface, forming Pt–O binding sites, played a pivotal role in enhancing effective charge separation and transfer, thereby elevating the overall efficiency of the photocatalytic system. These outcomes contribute to the broader landscape of materials science and renewable energy research, opening avenues for continued advancements in sustainable technology.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta01108b |
‡ Equal contribution. |
This journal is © The Royal Society of Chemistry 2024 |