Mars-van-Krevelen mechanism-based blackening of nano-sized white semiconducting oxides for synergetic solar photo-thermocatalytic degradation of dye pollutants
Blackening (or enhancing the optical absorption in the visible) of nano-sized white semiconducting oxides (N-WSOs) is of significant importance for solar utilization. Here, we present a novel Mars- van-Krevelen mechanism-based method for blackening the N-WSOs via facile one-step heatingBlackening (or enhancing the optical absorption in the visible) of nano-sized white semiconducting oxides (N-WSOs) is of significant importance for solar utilization. Here, we present a novel Mars-van-Krevelen mechanism-based method for blackening the N-WSOs via facile one-step heating the N-WSOs with alcohols. Taking n-butanol-induced blackening of TiO2 (Anatase) as an example, the pristine TiO2 NP powders can be successfully blackened to form black TiO2 (B-TiO2) via heating with n-butanol at 300oC for 20 min. Technical analyses demonstrate that the B-TiO2 nanocrystals are wrapped with 2 nm thick disordered layer, which is rich in oxygen vacancies, Ti3+ and hydroxyl groups. Both theoretical and experimental results show that the B-TiO2 has much stronger optical absorption in the visible than the pristine TiO2. Further, the influence factors (including the heating temperatures, and alcohol types) and good universality of this blackening method are also demonstrated. A blackening principle based on Mars-van-Krevelen mechanism induced oxygen vacancy generation and hydroxylation-anchoring of oxygen vacancies has been proposed, and the mechanism can well explain all the phenomena observed in experiments. Importantly, such B-TiO2 shows hugely enhanced activity in solar photodegradation of dye pollutants. Under simulated solar irradiation, the degradation rate constant achieved by the B-TiO2 catalyst is 2.3 times of that of the pristine TiO2, showing an obvious enhancement. Further experiments reveal that the improved degradation activity is mainly attributed to the enhanced optical absorption in the visible and the synergistic effect of photothermal and photocatalysis. This study demonstrates a new and facile approach to blacken the N-WSOs for enhanced solar utilization.