Microwave Synthesis of Co Doped ZnO Nanoparticles and Their Electrocatalytic Activity for Water Oxidation
Authors: Nishiumi Nobuyuki , Sun He , Shiroishi Hidenobu , Matsushima Yuta , Stadler Philipp , Yoshida Tsukasa 
Published: February 2020

Introduction: Fast and efficient conversion of renewable electricity to storable chemical energy is a subject of utmost importance for realization of sustainable society, and development of electrocatalysts out of non-precious elements is the key. Among various metal oxides previously tested, Co3O4 is known to be one of the best catalyst for oxygen evolution reaction (OER), essential for water splitting. Since water oxidation catalysis the surface phenomenon, nano-particles of Co3O4 and Co oxide clusters exhibit higher performance than bulky crystals. In this study, doping of Co ions into nanocrystals of zinc oxide (ZnO) has been achieved by microwave (MW) assisted hydrothermal reaction to form catalytic surface sites for OER combined with highly conductive and stable ZnO for flexible catalyst design and potential enhancement of the catalytic activity.

Experiments: Aqueous precursor solutions containing ZnCl2 and CoCl2 at various ratios in a total concentration of 0.2 M were basified with NaOH to ca. pH 13, and subjected to a 2.45 GHz MW to promote reaction at 160°C for 30 min. The products were characterized by FE-SEM, XRD and UV-Vis. Mesoporous electrodes of Zn1-x
Co
x
O (x = 0, 0.02, 0.05, 0.10, 0.20, 1) were fabricated by doctor blading method onto F-doped SnO2 glass substrate for electrochemical analysis in a 0.1 M KCl aqueous solution (pH 7.4). Evolved O2 has been analyzed by gas chromatography (GC).

Results and discussion: Minor addition of CoCl2 in the precursor solution turned product into green, exhibiting characteristic absorption peaks at around 560, 610 and 650 nm due to d-d transition of doped Co(II) ion along with a red shift of the bandgap absorption onset, and associated with morphological change of the nanoparticles and slight enlargement of the lattice constants of Wurtzite ZnO. Doping limit of about 10% has been suggested, as phase separation of Co(OH)2 is observed for x > 0.1.
Cyclic voltammograms measured at mesoporous ZnO, Zn0.95Co0.05O and Co3O4 electrodes are shown in Fig. 1. While pure ZnO is totally inactive for water oxidation, the Co-doped electrode exhibits an anodic hump at around +0.8 V associated with the oxidation of Co to be followed by a sharp rise of irreversible anodic current above ca. 1 V. The Co3O4 electrode shows even larger anodic peak at 0.8 V, for which, however, Co is reversibly reduced without showing strong rise of anodic current. No systematic increase of the catalytic current was observed by further increasing the x value. Potentiostatic electrolysis at 1.2 V (corresponding to η = 0.545 V) revealed constant anodic current of about 0.9 mA cm-2 for Zn0.95Co0.05O associated with formation of gas bubbled, which was confirmed as O2 by gas chromatography. Although the currently achieved catalytic activity is not satisfactory, the present result reveals new strategies in designing catalysts for OER. Bulk crystals of transition metal oxides are not needed, but reactive sites can be furnished by small doping of transition metals into chemically stable ZnO. Doping of various transition metals such as Mn, Fe, Ni and their alloys is expected to achieve further increase of catalytic activities.

Figure 1