Kinetic Evidence for Type-II Heterojunction and Z-Scheme Interactions in g-C₃N₄/TiO₂ Nanotube-Based Photocatalysts in Photocatalytic Hydrogen Evolution

Abstract

Composite materials based on g-C3N4 and TiO2 nanotubes have been synthesized as environmentally friendly photocatalysts with heterojunctions suitable for enhanced photocatalytic hydrogen production. Composites were prepared with various ratios (x = 0–1) of g-C3N4 and after chemical modification and exfoliation of bulk g-C3N4. Contact formation between g-C3N4 and TiO2 generally enhanced photoactivity, which caused the x-dependent changes in the photocatalytic hydrogen evolution rates of the g-C3N4/TiO2 compounds to vary following a volcano-shaped curve with the maximum rate at x ∼ 0.6 for all the compounds regardless of the pretreatment (bulk or modified) of g-C3N4. The modified g-C3N4-based composites showed higher photoactivities than the unmodified bulk g-C3N4 due to the high surface area. The major reason for the enhanced photoactivity with the volcano shape was attributed to the Z-scheme interaction at the heterojunction. Interestingly, detailed analysis of the kinetic H2 evolution rates of the composites with Pt cocatalysts only on TiO2 nanotubes further showed that the dominant type of interaction at the heterojunctions changed from the type-II heterojunction to the Z-scheme at x ∼ 0.1. It is inferred that structural diversity at the g-C3N4/TiO2 interfaces is the origin of the changes in the dominant type of interaction in the composites with increasing ratios of g-C3N4.

Graphitic Carbon Nitride Nanosheets Decorated with Zinc-Cadmium Sulfide for Type-II Heterojunctions for Photocatalytic Hydrogen Production

“…Upon visible-light irradiation, both ZnCdS and g -C 3 N 4 can be excited, and then electrons from the CB of g -C 3 N 4 can be transferred into the CB of ZnCdS, then reacting with H + for H 2 production. At the same time, the photo-induced holes of g -C 3 N 4 and ZnCdS can be used by oxidizing agents [ 30 , 31 ].…”

Visible Light Absorption Is Not Always Related to N-Doped TiO2 and High Photocatalytic Activity in Materials Synthesized by the Sol–Gel Method Using Urea and Ammonia as Precursors

“…V 0 can enhance the charge separation process since these species facilitate the charge redistribution around the g-C 3 N 4 /TiO 2 interface. , Thus, the S-scheme creates the conditions for a reductive photocatalyst (g-C 3 N 4 ) and an oxidative one (TiO 2 ), leaving the photoinduced electrons and holes in the most reductive and oxidative photocatalysts, respectively, and generating composite materials with high photocatalytic activity. S- or Z- schemes between g-C 3 N 4 are formed when the concentration of the metal-free semiconductor is high …”

“…However, forming type II heterojunctions when g-C 3 N 4 is found at low amounts in TiO 2 composites could be favorable . In this kind of g-C 3 N 4 /TiO 2 interaction, a built-in electric field is also created under irradiation due to the TiO 2 and g-C 3 N 4 Fermi level realignment allowing, in this occasion, the transfer of e – CB from the g-C 3 N 4 to the TiO 2 CB or SETOV and the photoinduced h + from the VB of TiO 2 to the VB of g-C 3 N 4 .…”

Urchin-like TiO2/CdS Nanoparticles Forming an S-scheme Heterojunction for Photocatalytic Hydrogen Production and CO2 Reduction

“…Another researcher simulated the kinetics of photocatalysis in Z-scheme heterojunctions by constructing Z-scheme TiO 2 @Ti 3 C 2 /MIS heterojunctions and g-C 3 N 4 /TiO 2 composites. 19,20 However, with the development of science, more scientifically supported step scheme (S-scheme) heterojunctions are proposed to replace the traditional Z-scheme heterojunctions. 21,22 To further improve the photocatalytic ability of TiO 2 , researchers recently studied a series of new catalysts for photocatalytic H 2 and CO 2 reduction reactions, such as 3D/ 2D TiO 2 @Ti 3 C 2 MXene/Bi 2 S 3 , 23 TiO 2 /C 3 N 4 , 24 3D ZnIn 2 S 4 / TiO 2 25 and so on.…”

“…Wang et al designed the Z-scheme heterostructure consisting of a covalent organic skeleton (COF) and an inorganic semiconductor TiO 2 , which has a great potential to achieve efficient photocatalytic reduction of CO 2 and exhibits a high conversion efficiency with a CO selectivity close to 100%. Another researcher simulated the kinetics of photocatalysis in Z-scheme heterojunctions by constructing Z-scheme TiO 2 @Ti 3 C 2 /MIS heterojunctions and g-C 3 N 4 /TiO 2 composites. , However, with the development of science, more scientifically supported step scheme (S-scheme) heterojunctions are proposed to replace the traditional Z-scheme heterojunctions. , …”