Elsevier

Science Bulletin

Volume 66, Issue 3, 15 February 2021, Pages 265-274
Science Bulletin

Article
Forming electron traps deactivates self-assembled crystalline organic nanosheets toward photocatalytic overall water splitting

https://doi.org/10.1016/j.scib.2020.08.009Get rights and content

Abstract

Most biological photoredox reactions occur in sophisticated molecular assemblies consisting of highly organized light-harvesting moieties and catalytic centers. Mimicking these prototypes by creating supramolecular assemblies could be a potentially viable approach toward artificial photosynthesis. Although self-assembled organic materials are known to carry out water splitting reactions, developing self-assembled organic materials for photocatalytic overall water splitting still remains a critical challenge. Herein, we first demonstrate that crystalline organic nanosheets assembled from linear oligo(phenylene butadiynylene) (OPB) are able to catalyze overall water splitting under visible light irradiation. Further investigations reveal that the photocatalytic activity of self-assembled organic structures is closely related to the crystalline structure along with the corresponding electronic structure. Structural disorders in OPB nanosheets and extrinsic factors such as adsorbed water molecules will induce the formation of electron traps which can make the OPB nanosheets thermodynamically unfavorable for photocatalytic overall water splitting. The deactivation mechanism unveiled in this study provides crucial insights into the assembling of artificial organic materials for future solar-to-chemical energy conversion.

Introduction

Plants and algae utilize complex noncovalent supramolecular systems consisting of crucial components such as light-harvesting antennas and reaction centers to convert CO2 and H2O into carbohydrates while releasing O2 [1], [2], [3]. Inspired by the natural photosynthetic machinery, developing artificial photocatalytic assemblies represents a promising route toward producing solar fuels [4], [5], [6]. Until now, various organic chromophores are assembled into ordered nanostructures which can direct photoexcited energy to catalytic sites for photocatalytic water splitting [7], [8]. The numerous noncovalent interactions and vastly available molecular motifs offer fascinating opportunities in developing self-assembled organic materials for water splitting [9]. However, current supramolecular systems still rely on using sacrificial electron donors or acceptors to generate either H2 or O2 from water [5], [7], [8], [9], [10], [11]. The decomposition of pure water into H2 and O2 with 2:1 in molar ratio using self-assembled organic materials via one-step photoexcitation still remains elusive.

In self-assembled organic materials, the crystalline structure is a key feature to tailor charge transport over long distances and the resulting optoelectronic properties [12], [13]. Therefore, controlling the packing order in self-assembled structures is of primary importance for photocatalytic applications. However, structural disorders or other extrinsic sources in self-assembled organic materials will inevitably give rise to energetic disorders or traps which are known to be detrimental for the performance of functional devices such as organic field-effect transistors, polymer light-emitting diodes, and solar cells [14], [15], [16]. On the other hand, in photocatalytic reactions, the in-gap traps can capture photogenerated charge carriers and act as recombination centers [17], [18]. Consequently, the electronic structures and charge transfer efficiencies, both of which are essential factors in regulating photocatalytic activities, should be carefully examined in self-assembled organic materials. It is thus crucial to understand the underlying principles in using self-assembled organic materials with highly ordered structures for photocatalysis-related applications.

Linear conjugated oligomers with tunable electronic structures can be conveniently assembled into crystalline nanostructures [19], [20], [21], [22], [23], [24]. Herein, for the first time, we report that two-dimensional (2D) crystalline nanosheets can be assembled from linear oligo(phenylene butadiynylene) (OPB) via π-π interaction. We also demonstrate that, by both computational and experimental investigations, these self-assembled 2D crystalline nanosheets exhibit a bandgap and an electronic structure favorable for photocatalytic overall water splitting. More importantly, we further reveal that electron traps located ~ 0.30 eV below the lowest unoccupied molecular orbital (LUMO) level of the self-assembled OPB nanosheets will form during photocatalytic water splitting and thus deactivate the photocatalytic activity. Using density functional theory (DFT) calculations, we confirm that structural disorders such as parallel chain offset or chain twisting with small angles and extrinsic factors such as adsorbed water molecules will induce the formation of shallow electron traps in self-assembled organic nanosheets. The existence of the electron traps makes the overall water-splitting reaction thermodynamically unfeasible. Only after eliminating the traps can these self-assembled OPB nanosheets catalyze photocatalytic overall water splitting again. Our findings unveil a critical deactivation mechanism in using self-assembled structures for photocatalysis and provide important insights toward the realization of low-cost, self-assembled organic systems for future artificial photosynthesis by harnessing sunlight and water.

Section snippets

Synthesis of crystalline Oligo(phenylene butadiynylene) nanosheets

2.5 mL TMEDA was slowly added to 200 mL Dimethylformamide (DMF) with a magnetic stirring bar (1000 r min−1). Then, 10 mg CuBr was dissolved in above DMF solution. After ~ 10 min, 5 mL DMF solution dispersed with 20 mg 1,4-Diethynylbenzen was added dropwise into the above DMF solution at 20 °C (note that 1,4-Diethynylbenzen is prone to oxidation). The reaction was proceeded for 10 h in the dark at 20 °C. Since this is an oxidative coupling reaction, a bulb containing O2 gas could be inserted in

Results and discussion

In this work, the conjugated OPB was synthesized by the Glaser coupling of 1,4-diethynylbenzene in DMF (Fig. 1a). Since there are no solubilizing side chains in 1,4-diethynylbenzene, the synthesized OPB is insoluble in common organic solvents [33]. Therefore, as the reaction proceeded, the synthesized OPB tended to aggregate together through the strong π-π interaction and precipitated as yellow solids. The optoelectronic structure of the synthesized OPB is the most critical factor in achieving

Conclusion

In conclusion, we show that 2D crystalline organic nanosheets via strong π-π interaction can be obtained using the reaction-induced self-assembly approach. More importantly, the self-assembled 2D crystalline organic nanosheets can catalyze overall water splitting under visible light irradiation. To the best of our knowledge, the self-assembled OPB nanosheets are the first supramolecular materials that are active toward photocatalytic overall water splitting. We further reveal that in-gap

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgments

We thank Prof. Jun Luo at the Center for Electron Microscopy at Tianjin University of Technology for acquiring TEM images. The in situ DRIFTS and SRPES experiments were carried out at the Infrared Spectroscopy and Microspectroscopy Endstation (BL01B) and the Photoemission Endstation (BL10B) in the National Synchrotron Radiation Laboratory (NSRL). This work was supported by the National Key R&D Program of China (2017YFA0207301, 2016YFA0200602, and 2018YFA0208702), the National Natural Science

Author contributions

Lei Wang, Hao Cheng, and Hangxun Xu contributed to the materials syntheses and characterizations along with the photocatalytic water splitting tests. Jia Liu and Qun Zhang performed the ultrafast transient absorption measurements. Haiyun Wang and Xiaojun Wu carried out the DFT calculations. All authors discussed the experimental results and revised the manuscript.

Lei Wang received his Ph.D. degree in Polymer Chemistry and Physics in 2019 from University of Science and Technology of China (USTC). He is now working as a postdoctoral research fellow with Profs. Hangxun Xu and Yujie Xiong at USTC. His current research interest focuses on the design and synthesis of functional conjugated polymers for photocatalytic water splitting and CO2 reduction.

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    Lei Wang received his Ph.D. degree in Polymer Chemistry and Physics in 2019 from University of Science and Technology of China (USTC). He is now working as a postdoctoral research fellow with Profs. Hangxun Xu and Yujie Xiong at USTC. His current research interest focuses on the design and synthesis of functional conjugated polymers for photocatalytic water splitting and CO2 reduction.

    Xiaojun Wu is currently a Professor at University of Science and Technology of China (USTC). He received his B.S. and Ph.D. degrees from the USTC in 2000 and 2005, respectively. From 2005 to 2010, he worked as a postdoctoral research fellow in the University of Nebraska–Lincoln. In 2010, he joined the School of Chemistry and Materials Sciences as a Professor. His research interest focuses on computational investigations of low-dimensional materials using first-principle calculations.

    Qun Zhang received his Ph.D. degree in Chemical Physics from the University of Science and Technology of China (USTC) in 1999. After seven-years of postdoctoral training with Prof. Moshe Shapiro at the Weizmann Institute of Science, Israel (2000–2003), and Profs. Moshe Shapiro and John W. Hepburn at the University of British Columbia, Canada (2003–2007), he joined the Hefei National Laboratory for Physical Sciences at the Microscale (HFNL). His current research interest focuses on using ultrafast spectroscopic methods in investigating nanomaterials and condensed-phase molecular systems.

    Hangxun Xu received his B.S. degree from University of Science and Technology of China (USTC) in 2006 and his Ph.D. degree from University of Illinois at Urbana-Champaign in 2011. Then he worked as a postdoctoral associate with Prof. John A. Rogers from 2011 and 2013. He returned to USTC as a Professor at the Department of Polymer Science and Engineering in 2013. His primary research interest focuses on the synthesis of functional polymers for applications in energy conversion and flexible electronics.

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    These authors contributed equally to this work.

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