Solid State Synthesis And Photocatalytic Performance Of Novel One-Dimensional ZNS-Carbon Nanocomposites

ZnS nanocrystal is one of the most important wide-band-gap semiconductors as a result of the rapid generation of electron-hole pairs under irradiation and the highly negative reduction potentials of excited electrons. Carbon nanomaterials such as carbon quantum dots (CQDs), carbon nanofibers (CNFs) and graphene have unique optical properties, large surface areas, good thermal/chemical stability and low toxity. Recently, much effort has been made to combine carbon nanomaterials with ZnS for enhanced performance in environment and energy applications. However, their practical applications face several obstacles:(1) complicated chemical procedures, high cost and low production yield, (2) difficulties in controling the size and distribution of ZnS as well as the coupling interaction with carbon, which has great influence on catalytic activity and stability, (3) hard separation/reuse from reactor. In this study, we report the in situ solid synthesis of novel one-dimensional (1D) ZnS-CQDs, ZnS-CNF, Ag2S-ZnS-CNF, Ni-ZnS-CNF nanocomposites by using organic-inorganic layered metal hydroxides as precursor. These nanocomposites were used as photocatalyst for the degradation of methylene blue and H2 evolution. The main results are as follows:1. We successfully synthesized ZnS-CQDs and ZnS-CNF nanofibers through an in situ solid synthesis process using layered zinc hydroxide intercalated with benzoate (Zn(OH)BA) nanofiber as precursor. This method is simple, economical, efficient and requires no organic agents, surfactant or additional template, which is readily for large scale synthesis. The resulting composites show uniform ID fiber-shaped morphology and possess high crystallinity, purity, mesoporous structure and large surface area. ZnS-CQDs nanofibers prepared at low temperature are composed of ZnS and CQDs nanoparticles both less than 10 nm, while in the ZnS-CNF nanofibers prepared at higher temperature, ZnS nanoparticles less than 15 nm are uniformly embedded in CNF matrix. The intimate interaction between ZnS and carbon can promotes the electron transfer and enhances the visible light absorption of the photocatalysts. As a result of the unique sructural and optical features, the ZnS-C nanofibers exhibited excellent photocatalytic activity, stability, and good recyclability for degradation of MB.2. Ternary Ag2S-ZnS-CNF nanofiber photocatalyst are prepared via cation exchange reaction between the ZnS-CNF and AgNO3. Small ZnS and Ag2S nanoparticles with intimate interface are uniformly dispersed on CNF. The composition of Ag2S-ZnS-CNF can be controlled by tune the initial concentration of AgNO3. In Ag2S-ZnS-CNF nanofibers, the CNF can serve as electron collector and transporter as well as reduction active site for H2 production; while the Ag2S can function as a cocatalyst for assisting hole transfer. In this case, the photo-generated electrons and holes can be efficiently separated to result in higher photocatalytic H2 production rate (7498 μmol·h-1g-1) than reported photocatalysts. This work shows a simple way to enhance the photocatalytic H2 production activity of ZnS under the synergetic effect of CNF and Ag2S, and provides new insight for the design and development of other high-performance ternary composite photocatalysts.3. Magnetic Ni-ZnS-CNF nanofibers were prepared through an in situ solid synthesis process using benzoate intercalated layered bimetallic Zn-Ni hydroxide (Zn1-xNix(OH)BA) nanofiber as precursor. In the composite, small ZnS and Ni nanoparticles of 10-15 nm with direct contract are uniformly embedded in CNF. Ni/ZnS/CNF nanofibers showed high efficiency, magnetic recyclability, and good cycling stability for the degradation of methylene blue under visible light. The synergetic effect of Ni and CNF contribute to the high performance of the photocatalyst, where Ni serves as excellent electron acceptor and facilitates magnetic recovery of photocatalyst from solution, while CNF acts as sensitizer to extend the optical absorption of ZnS as well as protects ZnS and Ni nanoparticles from aggregation and oxidation. Furthermore, the Ni content in the composite can be tuned by varying the Ni2+ molar ratios in precursor, and the influence of Ni content on charge separation efficiency were investigated.