A comparative study of band gap engineered in-situ and ex-situ MWCNTs/TiO2 heterostructures for their enhanced photocatalytic activity under visible light

Organic pollutants, such as various types of organic dyes coming out from the textile industries, are polluting surface and groundwater resources alarmingly and posing a threat to aquatic ecosystems. So, the demand for visible-light-driven high-performance photocatalysts having high activity and structural stability is a need of an hour. TiO2 has been one of the well-known and most studied semiconductor photocatalysts for decades. But its low electron-hole pair symbolscript recombination time reduces its efficiency, and the large band gap restricts its use as a visible-light-driven photocatalyst. To overcome these limitations of TiO2, herein, we have reported an in-situ and ex-situ MWCNTs modified TiO2 heterostructure nanocomposites photocatalyst and established a compara-tive study in terms of their ability to degrade methylene blue (MB) dye under visible light irradiation. The as-synthesized in-situ CNTs-TiO2 nanocomposite and ex-situ CNTs-TiO2 nanocomposite were characterized struc-turally, morphologically, compositionally, and optically through various characterization techniques such as XRD, RAMAN, SEM, XPS, FTIR, and UV-Vis diffuse reflectance spectroscopy. The result reveals the band gap tuning in the in-situ and ex-situ CNTS-TiO2 nanocomposites as a result of increasing MWCNTs concentration. The in-situ CNTs-TiO2-2 nanocomposite has high degradation efficiency (94% in 150 min) and stability due to smooth and strong chemical interactions between the MWCNTs and TiO2, while ex-situ CNTs-TiO2-20 with 10 times more MWCNTs concentration (by weight) as compared to MWCNTs concentration in in-situ CNTs-TiO2-2, exhibits degradation efficiency of 89% in 150 min. The possible degradation mechanism to degrade MB dye has also been put forward.