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Overall water splitting into H2 and O2 driven by visible light is a friendly route to convert solar energy into renewable and green hydrogen energy. Until now, numerous efforts have been devoted to develop inorganic semiconductor-based photocatalysts, while the types of organic photocatalysts for overall water splitting are still limited. In recent years, covalent organic frameworks (COFs) are an emerging type of crystalline and porous photocatalysts have shown excellent visible-light-driven photocatalytic activity for hydrogen evolution reaction (HER) in the presence of sacrificial electron donor due to its remarkable merits including: 1) Excellent visible light absorption ability and structural stability; 2) Porous structures with ordered pores favoring mass transfer and exposing active sites; 3) Structural diversity providing great opportunity to tune the band structures; 4) Regulable electron separation ability by designing donor-receptor molecule module at molecular level. However, the overall water splitting activity of COFs has not been revealed, and most of them only drive hydrogen and/or oxygen evolution half-reactions separately.

In this work, we designed a series of β-ketoamine COFs with the same topology and similar atomic composition as a model system to investigate the effect of atoms at diverse positions on overall water splitting activity (Figure 1). Three β-ketoamine COFs with and without Bpy N sites were synthesized and in-situ incorporated ultra-small Pt nanoparticles (NPs) co-catalyst into the pores of COFs nanosheets (COFs-NS).

Figure 1. The brief development of organic photocatalysts for overall water splitting and related photocatalysts in this work.

As a result, both COFs containing bipyridine (Bpy) segment, Pt@TpBpy-NS and Pt@TpBpy-2-NS, show the overall water splitting H2 and O2 production activity, while phenyl-structured Pt@TpBD-NS just drive H2 evolution half-reaction. The optimal visible-light-driven H2 and O2 amounts of Pt@TpBpy-NS are 9.9 and 4.8 μmol in 5 hours, respectively, while 3.1 and 1.4 μmol for Pt@TpBpy-2-NS (Figure 2). According to a series of comparisons of photocatalytic overall water splitting activity and the structural diversity of these three COFs-NS, we can infer that bipyridine structure is essential for overall water splitting and the overall water splitting activity also closely related to the position of N sites in the heterocyclic ring. Meanwhile, these comparisons also prove that the nanosheet morphology of COFs and the architecture with ultra-small Pt NPs encapsulated in the pores of COFs are crucial for the high overall water splitting activity of the resultant materials. Furthermore, the stability test shows that Pt@TpBpy-NS has excellent stability.

Figure 2. Photocatalytic performances of COFs-NSOverall water splitting activities comparison over Pt@COFs-NS. The error bar represents the standard deviation of the measurements. b Wavelength dependent AQY of overall water splitting for Pt@TpBpy-NS. The error bars indicate the incident wavelength with a full width at half maximum of 15 nm. c O2 evolution half-reaction rate over Pt@COFs-NS. d Overall water splitting activities comparison over TpBpy-COFs based catalysts. e Five-times cycle overall water splitting test of Pt@TpBpy-NS and f Mass spectra of Pt@TpBpy-NS in the photocatalytic reaction of H2O and H218O.

In order to reveal the charge transfer path and reactive active sites of a series of β-ketoamine COFs for overall water splitting, the systemic theoretical calculations based on density functional theory (DFT) were carried out (Figure 3). Based on the obvious ICT mode of TpBpy-NS, the photogenerated electrons transferred to the O sites on Tp segments would induce subsequent HER, the holes accumulations on C sites on Bpy segments would induce the OER process. Meanwhile, more deep insight for the overall water splitting mechanism suggests that the electrons transfer from Bpy to Tp section is more efficient in TpBpy-NS than that in TpBpy-2-NS, and a C2d are the optimal OER path.

Figure 3. DFT calculations and proposed schematic mechanism of TpBpy-NS. a UV-Vis absorption spectra of TpBpy-NS compared with TD-DFT calculated fragment. b The TD-DFT calculated electronic transition of TpBpy-NS. c The possible process of HER on Tp segment and OER via dual-site process on Bpy segment in TpBpy-NS. d The comparison of calculated Gibbs free energy change for C2d paths of OER for TpBD-NS, TpBpy-2-NS and TpBpy-NS at pH = 7.