A promising Zn-Ti layered double hydroxide/Fe-bearing montmorillonite composite as an efficient photocatalyst for Cr(VI) reduction: Insight into the role of Fe impurity in montmorillonite

Zn-Ti layered double hydroxide/montmorillonite (ZTL/MT) and Fe-doped ZTL/MT (ZTL/Fe@MT20%) were prepared as photocatalyst for Cr(VI) reduction. The ZTL/Fe@MT20% exhibited the highest photocatalytic activity, suggesting that a combination of ZTL and Fe-bearing MT could enhance photocatalytic activity by suppressing the recombination of photogenerated electron-hole pairs. On the basis of electronic-structure calculations, a photocatalytic mechanism for Cr(VI) reduction and the effect of Fe³⁺ on the structure of MT in ZTL/MT composites is proposed by density functional theory approach, whereby Fe impurities in MT generate a new mid-gap state that induce a Z-scheme heterojunction with ZTL, leading to reduced recombination of the photogenerated charge carriers in ZTL. These results establish that MT can not only act as a support material but also play an important role for photocatalytic reaction when Fe impurities are present in its structure.