The UV–Vis Tipiracil spectra of TiO2, CdS/TiO2, CdSe/TiO2 and CdSe/CdS/TiO2 NTAs are shown in Fig. 7. The absorption edge of TiO2 NTAs is about 380 nm, corresponding to the band gap of 3.2 eV of anatase TiO2. After co-modification of CdS and CdSe NPs, there is a wide absorption band in the visible light ranging from 400 to 600 nm. CdSe/TiO2 NTAs show stronger visible light absorption than CdS/TiO2 NTAs, because the band gap of CdSe is narrower than the band gap of CdS. Co-modification of CdS and CdSe NPs on TiO2 NTAs forms a cascade structure, which makes the CdSe/CdS/TiO2 NTAs achieving the highest absorption by comparison with single modified CdS/TiO2 or CdSe/TiO2 NTAs. It is obvious that CdS and CdSe NP co-modified TiO2 NTAs have complementary and enhanced effects in the light harvest. The broad absorption peaks of TiO2, CdS/TiO2, CdSe/TiO2 and CdSe/CdS/TiO2 NTAs may be attributed to the sub-band gap state of the special tube structures  and .