Stabilizing Layered BiOBr Photoelectrocatalyst by Van Der Waals Heterojunction Strategy

The photoelectrocatalytic (PEC) hydrogen evolution reaction (HER) holds immense promise as a clean and sustainable method for hydrogen production. However, finding a suitable catalyst which is efficient, stable and scalable still remains an open challenge. BiOBr is a 2D layered material studied as photoelectrocatalyst because of its suitable band gap for light absorption and potential for up-scalable production. However, its application in HER is not commonly reported, because of instability in a cathodic PEC environment, driven by a strong tendency to reduction to metallic bismuth. To solve this problem, 2D MoS2 is used to induce the formation of a van der Waals (vdW) layered heterojunction (HJ) to stabilize the lattice of BiOBr during HER. By performing PEC HER with the HJs containing different ratios of MoS2, it is found that the HJ with 1 % MoS2 can increase the stability of BiOBr, while the one with 50 % MoS2 can even accelerate the reduction of BiOBr to metallic bismuth. DFT calculations reveal that the interface between BiOBr and MoS2 in the HJ with 1 % MoS2 tends to push active electrons on the sulfur atoms, thus favoring HER. On the other hand, in the 50 % HJ, active electrons are prone to react with BiOBr to induce reduction. In situ wide-angle X-ray diffraction (WAXD) on the MoS2/BiOBr HJs with 1 % and 50 % of MoS2 allows to track the phase change and the phase transfer speed of BiOBr during PEC HER. Interestingly, when the HJ is illuminated with UV light, a lower amount of BiOBr is reduced to Bi under negative potential, due to the presence of photogenerated holes reacting with the extra electrons derived from the negative bias and preventing the BiOBr photon absorber to be further reduced.