In this paper, we studied platinum (Pt)- and cobalt oxide (CoO
x)-loaded silver tantalate (AgTaO
3) for photocatalytic overall water splitting. Pt and CoO
x were loaded on AgTaO
3 by photodeposition, and the obtained photocatalysts were characterized by using powder X-ray diffraction (XRD), UV-visible (UV-vis) absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and microscope inspection. By the scanning transmission electron microscope (STEM) inspection, the diameters of Pt and CoO
x particles were found to be approximately 30 and 50 nm, respectively. When AgTaO
3 photocatalyst loaded with Pt and CoO
x nanoparticles was employed for water splitting, the activity was demonstrated to be 80 times higher compared with the bare AgTaO
3, indicating that the photodeposited Pt and CoO
x effectively functioned as the hydrogen (H
2) and oxygen (O
2) evolution cocatalysts, respectively. While the use of photodeposited CoO
x is often limited in photoelectrochemical water oxidation and photocatalytic half-reaction of water, CoO
x deposited on AgTaO
3 could be demonstrated to enhance the photocatalytic overall water splitting activity.
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