Metal-Doped Bipyridine-Linked Covalent Organic Framework Nanosheets As a Novel Platform for Photoelectrocatalysts

Abstract

Light-driven conversion of chemical substances has attracted much attention with respect to the photogeneration of valuable chemicals and the photodegradation of pollutants. Covalent organic frameworks (COFs), which are crystalline porous polymers, have attracted much attention as novel platforms for photo-energy conversion materials because of their unique physiological properties, including their nano-porous structure, high design flexibility and visible light absorption. In addition, as COFs possess abundant hetero atoms with a lone electron pair, such as N, O and S, they can support metal atoms via coordination bonds and exhibit various functions originated from supported metals. For example, our group has recently demonstrated metal-doped COFs (M-COFs) exhibited various unique elctrocatalytic functions depending on the metal species. Cu- and Ni-doped COFs serve as the electrocatalysts for oxygen reduction reaction (ORR) [1] and carbon dioxide reduction (CRR) [2], respectively. In this study, we newly synthesized various metal-doped bipyridine-linked COFs (bpy-COF) and evaluated their photoelectrocatalytic activity. First, the details in the photo-induced charge transfer process were investigated using photoelectrochemical action spectrum, photoluminescence and in situ spectroscopy by employing the ORR mediated by Cu sites doped in bpy-COF (Cu-bpy-COF) as a model system. We subsequently attempted to expand metal-doped COFs as the photo electrocatalysts for CRR toward the application in artificial photosynthesis. First, we physically characterized our Cu-bpy-COF using various X-ray technics. The X-ray diffraction (XRD) pattern of Cu-bpy-COF exhibited a peak at 2q = 3.5° and 26.5°, which are assignable to (100) and (001), respectively. These XRD patterns indicated that the COFs have the microporous structure. The surface elemental composites calculated by X-ray photoelectron spectroscopy (XPS) shows that the atomic ratio of Cu atom was 1.8 %, and Cu/N ratio was 0.13. Then, we analyzed Cu-2p XPs and XANES spectra to determine the oxidation state of Cu atoms. The Cu 2p3/2 XPS peaks generated by Cu-bpy-COF at 932.7 eV corresponded to the Cu(II). The Cu K-edge XANES spectra show that the absorption edges of the Cu-bpy-COF were 8980 eV, which is similar to that of CuO. These results indicate that that the Cu(II) oxidation state was dominant in Cu-bpy-COF. Then, we investigated the photoelectrochemical ORR property of Cu-bpy-COF. The photocurrent was measured under the monochromated light. Interestingly, the value of the cathodic photocurrent corresponded with ORR was drastically increased by the deposition of Cu in bpy-COF. The incident-photon-to-current-efficiency (IPCE) was also calculated. The resulting IPCE for Cu-bpy-COF clearly corresponded to the absorption spectrum of Cu-bpy-COF. These results clearly indicate that the photocurrent was originated due to the photo absorption of Cu-bpy-COF. Then, we investigated the detailed electron transfer mechanism of Cu-bpy-COF in the presence of O2. Photoluminescence and gas-phase in situ X-ray absorption spectra show that the photo-excited electrons in the bpy-COF film were transferred to oxygen via the Cu atom. The remained holes in the valence band reached to the ITO electrode, generating the cathodic photocurrent. At the conference, we will also present the photoelectrocatalytic CO2 reduction reactions (CRR) using M-COF for the application toward artificial photosynthesis. Reference: [1] K.Iwase, T.Yoshioka, S.Nakanishi, K.Hashimoto, K.Kamiya, Angew. Chem. Int. Ed. 2015, 54, 11068-11702. [2] P.Su, K Iwase, T.Harada, K.Kamiya, S.Nakanishi., Chemical Science. 2018, 9, 3941-3947. [3] K.Dey, M.Pal, K.C.Rout, S.Kunjattu, A.Das, R.Mukherjee, U.K.Kharul, R. Banerjee. J. Am. Chem. Soc. 2017, 139, 13083-13091.