Synthesis of NiO/Mn<sub>0.3</sub>Cd<sub>0.7</sub>S and its decomposition of shale gas backflow wastewater producing hydrogen

Single photocatalysts generally suffer from issues such as easy recombination of photogenerated carriers and photocorrosion. The rational design of heterostructures has emerged as a critical strategy for enhancing the separation efficiency of photogenerated carriers and the photocatalytic activity of semiconductor photocatalysts. In this work, a novel p-n type NiO/Mn_0.3Cd_0.7S heterojunction composite photocatalyst is successfully prepared using a combination of hydrothermal and calcination methods. To thoroughly analyze the structure, morphology, and optical properties of the NiO/Mn_0.3Cd_0.7S composite, we employed a series of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet-visible spectroscopy (UV-vis DRS), etc. Additionally, 0.35 mol/L Na₂S·9H₂O and 0.25 mol/L Na₂SO₃ are used as sacrificial agents, with shale gas backflow wastewater serving as the reaction medium for hydrogen production experiments to evaluate the photocatalytic hydrogen production performance of the composite. The experimental results demondtrate that NiO and Mn_0.3Cd_0.7S are successfully combined to form a hererojunction, and the introduction of NiO significantly improved the visible light responsiveness of Mn_0.3Cd_0.7S. Specifically, the NiO/Mn_0.3Cd_0.7S composite with a NiO mass fraction of 15% (hereafter,15NOMCS) exhibits the best photocatalytic hydrogen production performance in shale gas backflow wastewater, with a hydrogen production rate of 20416.7 µmol/(g·h), which is approximately 1.9 times higher than that of the single sample of Mn_0.3Cd_0.7S. Moreover, after conducting five consecutive hydrogen production cycling experiments, the hydrogen yield of the 15NOMCS composite maintained 94.6% of the initial hydrogen production amount, demonstrating excellent stability and reusability of the composite in photocatalytic hydrogen production applications. Compared to pure Mn_0.3Cd_0.7S, the characterization results of 15NOMCS shows that the photoluminescence (PL) spectrum intensity at an excitation wavelength of 370 nm significantly decreases, while the photoelectric current response intensity markedly increases. These indicate that the recombination rate of photogenerated electron-hole pairs in the 15NOMCS photocatalyst is significantly reduced, leading to a higher separation efficiency of photogenerated carriers. According to the UV-visible diffuse reflectance spectra, the absorption edge wavelength of 15NOMCS is significantly red-shifted, and the bandgap width is reduced from 3.42 to 2.27 eV, enhancing the utilization of visible light by Mn_0.3Cd_0.7S and contributing to the remarkable improvement of its hydrogen production performance. These excellent photocatalytic hydrogen production performances are mainly attributed to the formation of a p-n type heterojunction between NiO and Mn_0.3Cd_0.7S, which facilitates the effective transfer of electrons from NiO to Mn_0.3Cd_0.7S and promotes the separation of photogenerated carriers. On this basis, we explain the possible photocatalytic mechanism using the Fermi level theory, illustrating how the alignment of energy bands facilitates charge separation and transfer. The formation of the p-n heterojunction creates an internal electric field at the interface, which drives the migration of photogenerated electrons and holes in opposite directions, thus enhancing the separation efficiency of charge carriers. This study demonstrates the great potential of rationally designed heterostructures in photocatalytic applications, providing valuable new ideas for further improving the photocatalytic performance of Mn_0.3Cd_0.7S. Additionally, by effectively utilizing shale gas flowback wastewater as a reaction medium, promotes the effective utilization of shale gas flowback wastewater in sustainable energy production, offering a sustainable method for hydrogen production.