Preparation Of Cd_1-xZn_xS Based Nanojunctions For Photocatalytic Hydrogen Production Under Visible Light

Due to the growing global energy crisis and environmental problems, photocatalytic splitting of water into hydrogen, a clean highly effecient and renewable energy substituting for the fossil fuels, has aroused wide public concern. Over the past decades, although significant efforts have been made to develop new catalysts for improving the catalytic efficiency, further commercial development and mass production of catalyst are still restricted by the high cost of catalysts and low efficiency. The semiconductor nanojunctions have been designed to improve interfacial charge transfer and effective charge separation of the electron/hole (e/h) pairs driven by an internal electrostatic field formed in the junction region. Three junctions have been designed in this paper.1. metallic MoO2/Zn0.5Cd0.5S heterojunctionAs semiconductor-based nanoheterostructures play decisive roles in current electronics and optoelectronics, the introduction of active heteroj unctions can afford new and improved capabilities that will enable the use of converting solar energy into chemical energy. A novel metal/semiconductor MoO2/Zn0.5Cd0.5S heterojunction has been designed and prepared in such a way to significantly enhance photocatylytic hydrogen production efficiency. The photocatalytic activity of as-prepared MoO2/Zn0.5Cd0.5S for hydrogen generation from water under visible-light irradiation (λ≥420 nm) is measured.MoO2/Zn0.5Cd0.5S hybrid nanoparticles (NPs) have a higher photocatalytic activity than Zn0.5Cd0.5S even without the noble metal cocatalyst. The results show that the rate of H2 evolution over annealed MoO2/Zn0.5Cd0.5S is about 13 times higher than that of Zno.5Cdo.5S alone. It is noted that annealed MoO2/Zn0.5Cd0.5S sample shows higher photocatalytic than simply mixed MoO2/Zn0.5Cd0.5S, implying the strong coupling at the interface of MoO2 and Zn0.5Cd0.5S facilitates electrons transfer from the conduction band of Zno.5Cdo.5S to metallic MoO2, thus promoting the separation of photogenerated electrons and holes. MoO2 (2 wt%)/Zn0.5Cd0.5S heterostructured photocatalyst calcined at 673 K. achieves the optimal overall activity for H2 evolution. The junctions formed between metallic MoO2 and semiconductor Zn0.5Cd0.5S by calcination play a key role in high photocatalytic water splitting to hydrogen. Our study demonstrates that metallic MoO2 is an excellent H2 evolution cocatalyst, which could be used as cocatalyst for other semiconductors with improved performances.2.1T-LixMoS2Cd0.5Zn0.5S heterojunctionmetallic 1 T-LixMoS2 utilized as a novel cocatalyst for Cdo.5Zno.5S photocatalyst. The obtained LixMoS2/Cd0.5Zn0.5S hybrids show excellent photocatalytic performance for H2 generation from aqueous solution under splitting visible light illumination (λ≥ 420 nm) without precious metal cocatalysts. It turns out that a certain amount of intercalating Li+ions ultimately drives the transition of MoS2 crystal from semiconductor triagonal phase (2H phase) to metallic phase (IT phase). The distinct properties of 1 T-LixMoS2 promote the efficient separation of photo-excited electrons and holes when used as cocatalyst for Cdo.5Zno.5S photocatalyst. As compared to 2H-MoS2 nanosheets only having edge active sites, photoinduced electrons not only transfer to the edge sites of 1T-LixMoS2, but also to the plane active sites of 1T-LixMoS2 nanosheets. The content of LixMoS2 in hybrid photocatalysts influences the photocatalytic activity. The optimal 1T-LixMoS2 (1.0 wt%)/Cd0.5Zn0.5S nanojunctions dispaly the best activity for hydrogen production, achieving a hydrogen evolution rate of 769.9μmol h-1, with no use of noble metal loading, which is about 3.5 times higher than that of sole Cdo.5Zno.5S, and 2 times higher than that of 2H-MoS2 (1.0 wt%)/Cdo 5Zn0.5S samples. Our results demonstrate that Li+-intercalated MoS2 nanosheets with high conductivity, high densities of active sites, low cost, and environmental friendliness, are a prominent H2 evolution cocatalyst that might substitute for noble metal for potential hydrogen energy applications.3. zincblende/wurtzite (ZB/WZ) heterophase Cd1-xZnxS nanojunctionWe report the synthesis and demonstration of an efficient charge-separated zincblende/wurtzite (ZB/WZ) heterophase Cd1-xZnxS nanojunction by solvothermal method in a mixed solution of diethylenetriamine (DETA) and distilled water. L-cysteine is selected as a sulfur source and a protecting ligand for the stabilization of ZB/WZ homojunction. The optimal ternary chalcogenate Cd0.7Zn0.3S elongated nanocrystals (NCs) without any cocatalyst loading show an unprecedented highest visible light photocatalytic activity with the H2 production rate of 3.13 mmol h-1 and a record of apparent quantum efficiency (AQY) of 65.7% at 420 nm. This is the best visible light photocatalyst ever reported for photocatalytic hydrogen production without any cocatalysts. The charge separation efficiency, as a critical role in enhancing photocatalytic activity for the hydrogen production, is significantly improved. Highly efficient charge separation with prolonged carrier lifetime is driven by the internal electrostatic field originated from the type-Ⅱ staggered band alignment at the ZB/WZ junctions. Besides, the strong binding between L-cysteine ligand and Cd1-xZnxS elongated nanocrystals protects and stabilizes NCs, and L-cysteine ligand at the interface could trap holes from Cd1-xZnxS NCs, while photogenerated electrons transfer to Cd1-xZnxS catalytic sites for proton reduction. Our results demonstrate that Cd1-xZnxS ZB/WZ heterophase junctions stabilized by L-cysteine molecules can effectively separate charge carriers and achieve highly visible light photocatalytic hydrogen production. This work could provide a new insight into the design and fabrication of advanced materials with homojunction structures for photocatalytic applications and optoelectronic devices.