Preparation And Performance Of Carbon Nanotubes Modified Metal Sulfide Solution Composites

Environmental and energy problems become two main key global issues in the21st century. Semiconductor photocatalytic technology exhibits widely potential application in water and air purification and solar energy conversion. The photocatalytic technology includes energy photocatalytic (containing three areas: water splitting to generate hydrogen, dye-sensitized solar cells and reduction of carbon dioxide into organic fuel) and environmental photocatalytic (degradation of contaminants). Owing to the gradual exhaustion of fossil fuels (such as oil, coal and natural gas) and the increasing global environmental contamination caused by the burning of these fossil fuels, the development of new environmental-friendly and non-polluting renewable energy resource have been widely aroused great concern of the world. And solar energy will play an important role in the development of new energy sources, because it is clean, abundant and especially renewable; hydrogen has been widely considered as an ideal and attractive energy candidate in the future due to its high combustion energy and environmental friendship. Since two Honda and Fujishima’s first discovery of photocatalytic water splitting of TiO2electrodes in1972, sunlight excitation induced by the photocatalytic reaction is generally considered as an effective method to convert solar energy into hydrogen energy. Moreover, metal sulfides have been widely considered to be excellent visible-light photocatalysts which have been intensively used in the field of photocatalytic H2-production of water splitting due to their appropriate conduction band and valence band position. So, in order to further improve photocatalytic H2-production activity of metal sulfides, various modification methods have been adopted to fabricate the highly visible-light driven photocatalytic composites. The main keypoints could be summarized as follows:Firstly, a novel visible-light-driven multiwalled carbon nanotube modified Cdo.1Zn0.9S solid solution photocatalyst (CNT/Cd0.1Zno.9S) was prepared by a simple hydrothermal method. The prepared samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, N2adsorption-desorption isotherms and UV-visible absorption spectroscopy. The as-prepared samples exhibited enhanced photocatalytic H2-production activity under visible light. The CNT content had a great influence on photocatalytic activity and the optimum amount of CNT was determined to be ca.0.25wt%, at which the CNT/Cd0.1Zn0.9S composite displayed the highest photocatalytic activity with H2-production rate of78.2μmol/h and the apparent quantum efficiency (QE) of7.9%. Furthermore, the as prepared composiye shows higher photocatalytic activity without any noble metal cocataltsts and even exceed that of pure Cd0.1Zn0.9S by more than3.8times. The enhanced photocatalytic activity was due to CNT as an excellent electron acceptor and transporter, thus reducing the recombination of charge carriers and enhancing the photocatalytic activity. Furthermore, the prepared sample was photostable and no photocorrosion was observed after the photocatalytic recycles. Our findings demonstrated that CNT/Cd0.1Zn0.9S composites were a promising candidate for the development of high-performance photocatalysts in the photocatalytic H2production. This work shows a possibility for the utilization of low cost CNT as a substitute for noble metals (such as Pt) in the photocatalytic H2-production.Secondly, a series of novel visible-light-driven photocatalysts were designed based on photoinduced interfacial charge transfer through surface modification of Cd0.1Zn0.9S by Ni(OH)2. Ni(OH)2/Cd0.1Zn0.9S nanocomposite photocatalysts were fabricated by a facile-mild hydrothermal process in a mixed solution of Zn(Ac)2, Cd(Ac)2and thioacetamide, and followed by a precipitation method using Ni(NO3)2and NaOH aqueous solution as precursors. Even without Pt as the cocatalyst, the as-prepared Ni(OH)2/Cd0.1Zn0.9S photocatalysts reach a high H2-production rate of322.4μmol/h at Ni(OH)2loading content of1.25mol%and an apparent quantum efficiency (QE) of44.2%at420nm in Na2S/Na2SO3aqueous solution under visible-light irradiation, exceeding that of0.6wt%Pt/Cd0.1Zn0.9S sample (giving H2-production rate of183.6μmol/h at the same experiment condition). The reason for the enhanced photocatalytic H2-production mechanism can be attibuted to the synergistic effects of the following two factors:(1) The potential of Ni2+/Ni (Eo=-0.23V) is less negative than the CB (-0.45V) of Cd0.1Zn0.9S, meanwhile this potential is more negative than the reduction potential of H+/H2(Eo=-0.00V), which favors photoinduced interfacial charge transfer from the CB of Cd0.1Zn0.1S to Ni(OH)2and the reduction of partial Ni2+to Ni0. The function of Ni0is to act as an efficient cocatalyst for water reduction, thus enhancing the photocatalytic H2-production activity;(2) The in-situ formed NiS during the visible-light irradiation reaction, can accelerate the electrons transferring from the CB of Cd0.1Zn0.1S to NiS and H+which could be efficiently reduced to H2, and further improves H2-production activity. This work presents not only a possibility for substituting Ni(OH)2for noble metals (such as Pt) in the photocatalytic H2-production but also for deepening the understanding of the mechanisms of photocatalytic H2production.