Plasma-assisted construction of waterfall-type IEF in N-TiO2/WO3 S-scheme heterojunction for efficient visible-light-driven degradation of Cl-VOCs

Abstract

Constructing S-scheme heterojunctions with a robust internal electric field (IEF) to enhance the photocatalytic degradation of chlorinated volatile organic compounds (Cl-VOCs) presents a significant challenge. Herein, an innovative S-scheme heterojunction of N-doped titanium dioxide (TiO2) and tungsten trioxide (WO3) with abundant oxygen vacancies (OVs) was synthesized and manipulated for the first time via an eco-friendly two-step plasma. The charge transfer pathway between TiO2 and WO3 was analyzed using UV–Vis DRS, XPS, UPS, and EPR measurements, confirming the successful formation of the S-scheme heterojunction. Interestingly, two novel types of IEF, stream-type and waterfall-type were proposed for the first time to distinguish the IEF strength before and after regulation. Under visible light, 5NTW, with the optimal ratio of 4.66 at% nitrogen and 5 wt% WO3, achieved the highest degradation efficiency and carbon dioxide mineralization rate, 95.4% and 94.1% for chlorobenzene, respectively. The performance enhancement was attributed to the fact that N-doping modifies the electronic structure and work function of TiO2, enhancing the Fermi level difference (ΔEf) with WO3. Meanwhile, the plasma treatment roughened the surface topography of the catalyst and increased the content of OVs, which serve as charge traps and bolster active sites. These synergies led to a transformation from a stream-type IEF of TW to a waterfall-type IEF of 5NTW. KPFM, zeta potential tests, and DFT calculations confirmed that the IEF strength and the number of electron transfers in the waterfall-type IEF are 3.17 and 2.04 times greater, respectively, than those in the stream-type IEF. This strategy transcends the limitations of previous work, offering a novel perspective on the integrated optimization of photocatalysts for superior performance and also further broadens the application prospects of nonthermal plasma technology.