Electrodeposition of Zn-Co-Terephthalate MOF and Its Conversion to Co-Doped ZnO Thin Films

Abstract

Energy security and sustainability are the most important challenges of mankind for their survival towards the 22nd century. While renewable electricity by solar and wind has already become so cheap, technologies for its conversion to easily storable chemical energies are urgently needed. Electrocatalysts not dependent on noble metals therefore need to be developed.We have recently succeeded in microwave (MW)-assisted hydrothermal synthesis of Co-doped ZnO nanoparticles, which exhibit catalytic activity towards water oxidation superior to Co3O4, although the electrode prepared by paste coating faced problems of its stability due to imperfect connection of the particles [1]. Electrodeposition can overcome this problem since well crystallized, stable and perfectly conducting ZnO thin films can be obtained. However, it is not at all straightforward to dope cobalt because of the large different of solubility (K sp = 5.92 × 10-15 and 3 × 10-17 for Co(OH)2 and Zn(OH)2, respectively), so that almost pure ZnO is solely electrodeposited even in the presence of Co2+ ions in the bath.Taking advantage of structural flexibility of metal-organic framework (MOF) could be a possible solution to achieve co-precipitation of Zn and Co. We have recently achieved cathodic electrodeposition of Zn-terephthalate (TPA) MOF thin films by addition a TPA into the ZnO electrodeposition bath [2]. Layered MOF structure was self-assembled during precipitation by bridging of double-layered Zn hydroxide with TPA.Herein, we report successful electrodeposition Zn-Co-TPA MOF precursor thin films and its conversion to oxides to yield Co-doped ZnO. Compositional and structural characterization as well as their electrocatalytic activities for oxygen evolution reaction (OER) are discussed.Potentiostatic electrolysis at an F-doped SnO2 (FTO) coated glass rotating disk electrode (RDE, ω = 500 rpm) was carried out at -1.0 V (vs. Ag/AgCl) in an O2-saturated aqueous electrolyte (70ºC) containing 5 mM ZnCl2, 200 μM TPA, and 0,5 mM CoCl2, for 30min. The film was annealed at 450℃ for 1 hour, under air.A white film electrodeposited in the presence of CoCl2 turned into green film after annealing, as expected for the formation of Co-doped ZnO. (Fig. 1a). X-ray diffraction pattern (XRD, Fig. 1b) indicates electrodeposition of Zn-TPA MOF and their conversion of ZnO after annealing both in the presence and absence of Co. The absorption spectra of the films on the other hand show clear difference between the samples, for which the one with Co and after annealing exhibit clear red-shift of onset for bandgap absorption and characteristic triple peaks at about 560, 610 and 650 nm arising from d-d transition of doped Co(II) ions (Fig. 1c). These features were commonly found for the Co-doped ZnO nanoparticles by MW reaction, so that similar or superior OER catalysis can be expected for the sample prepared by electrodeposition in this study. Figure 1