The increasing demand of hydrogen fuel production has promoted the development of effective oxygen evolution reaction (OER) photocatalysts used for water splitting. Brownmillerite (BM) -type Ca₂FeCoO₅ (CFCO) is one of the most excellent catalysts for OER comparable with noble metal oxides [1]. On this stage of study, CFCO was synthesized by a conventional sol-gel method, and the particle size was about several dozen nanometers, which is too large for a cocatalyst. In this study, we synthesized CFCO ultrafine particles with less than 10 nm diameters by a reverse micelle (RM) method on TiO₂ nanoparticles and investigated the photocatalytic activity for OER [2].
An aqueous solution containing metal nitrates (Ca:Fe:Co=2:1:1) was mixed with cyclohexane and NP-6 as a surfactant at 283 K to obtain a RM solution. Another RM solution was also prepared from cyclohexane, NP-6 and 10% TMAH aqueous solution. These RM solutions were mixed and stirred at 283 K. Anatase TiO₂/cyclohexane and ethanol was added. The formed precipitates were collected, dried at 383 K and calcined at 873 K in air. The photocatalytic activity was evaluated under UV light irradiation in an aqueous solution containing AgF as an electron scavenger.
The half-spherical ultrasmall particle with ~5 nm was found on the TiO₂ surface, and the spacing of lattice fringes in it was 0.26 nm, corresponding to the (133) face of BM-type CFCO (Fig. 1a). The particle size of CFCO synthesized by the RM method without TiO₂ was about 30 nm, suggesting that presence of TiO₂ support played a role to get or keep the CFCO ultrasmall particles during the calcination. The photocatalytic activity of TiO₂ for OER was improved by loading of CFCO ultrasmall particles with 5 nm, whereas the activity decreased by loading of CFCO with more than 15 nm (Fig. 1b).

[1] E. Tsuji, T. Motohashi, et al., ChemSusChem 2017, 10, 2864.
[2] E. Tsuji, et al., Part. Part. Syst. Charact. 2020, in press.

Fig. 1 (a) A high resolution TEM image of CFCO ultrasmall particles on TiO<sub>2</sub> nanoparticles synthesized by the RM method and (b) Time courses of oxygen photo-evolution over 0.5 wt % CFCO (5 and 15 nm) /TiO<sub>2</sub>, together with that of bare anatase TiO<sub>2</sub> nanoparticles.

Fig. 1 (a) A high resolution TEM image of CFCO ultrasmall particles on TiO₂ nanoparticles synthesized by the RM method and (b) Time courses of oxygen photo-evolution over 0.5 wt % CFCO (5 and 15 nm) /TiO₂, together with that of bare anatase TiO₂ nanoparticles.

Ryosuke Nanbu

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