| Summary: | In an attempt to improve the photocatalytic activity of anatase TiO<sub>2</sub>, we developed a composite photocatalyst composed of hollow TiO<sub>2</sub> microspheres (hTS) and graphene. The hTS were prepared through a two-step hydrothermal process, where SiO<sub>2</sub> microspheres with desirable diameters of 100–400 nm were used as sacrificial templates. Accordingly, the effect of the hTS cavity size on the activity of the catalyst in wet CO<sub>2</sub> photoreduction (CO<sub>2</sub>PR) was studied. Furthermore, it was established that the hydrothermal pH value crucially influences the photocatalytic activity of the hTS photocatalyst, as well as its composition and microstructure. The hTS photocatalyst was also combined with graphene (0–90 wt%) to improve its photocatalytic activity. This study provides insight into the optimal microsphere diameter, hydrothermal pH value, and graphene/hTS<sub>x</sub> ratio required for designing hollow microsphere-based photocatalysts with enhanced CO<sub>2</sub>PR performances.
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