The deposition of a TiO2–GO double layer, however, significantly improved a membrane0 s photocatalytic performance under not only UV but also sunlight irradiation. As clearly observed in Fig. 6(d), the MB photodegradation performances of the TiO2–GO membrane under UV and under sunlight were very similar, each being better than the respective performances of the TiO2-mod- ified (Fig. 6(b)) and GO-modified (Fig. 6(c)) membranes. This observation confirms that integration of TiO2 and GO decreases the band-gap energy of each individual semiconductor so that sunlight is able to activate electrons in these semiconductors and hence leads to photocatalytic activities.