Design Of Ferrite-based Composites For Photocatalysts And Electrochemical Capacitors

In recent years, more and more high-performance catalysts and electrode materials have been applied to photocatalysis and energy storage due to the increasingly worse energy and environment issues. Since their intrinsic shortcomings, single component catalysts and electrode materials cannot meet the requirement of high-performance application. By contrast, multicomponent nanocomposites can exhibit enhanced performance resulting from synergetic effects among all components. In this dissertation, a series of ferrite-based nanocomposites have been designed and synthesized via facile methods, their photocatalytic performances under visible light irradiation and electrochemical performances for supercapacitors are investigated. The main contributions of this dissertation are described as follows:1) Synthesis and photocatalytic activities of ferrite-multiwall carbon nanotube (MWNT) nanocompositesMagnetically recyclable and highly photocatalytic CoFe2O4-MWNT and NiFe2O4-MWNT photocatalysts with differing MWNT content have been successfully prepared via a one-step hydrothermal method. By taking the high conductivity of MWNT, the photo-induced electrons and holes can been separated effectively, resulting in enhanced photoactivity. Meanwhile, due to the magnetic properties of ferrite, the as-prepared ferrite-MWNT nanocomposites are magnetically recyclable. The as-prepared ferrite-MWNT exhibited high photodegradation activities of phenol, o-nitrophenol (ONP), p-nitrophenol (PNP), and picric acid (PA) under ultraviolet light irradiation, and methylene blue under visible light irradiation.2) Synthesis, adsorption properties, photocatalytic activities of ferrite-polyaniline (PANI) nanocompositesA cobalt ferrite (CoFe2O4)-PANI photocatalyst is successfully prepared by in-situ oxidative polymerization. The PANI layers are coated on the surfaces of CoFe2O4 nanoparticles. The excellent magnetic properties of CoFe2O4 are maintained in the composite to some extent, and therefore the photocatalyst can be separated easily by an external magnetic field. Due to the synergetic effects between CoFe2O4 and PANI, the CoFe2O4-PANI nanocomposites showed improved photodegradation of methyl orange under visible light irradiation, compared with pure PANI or CoFe2O4. A significant adsorption can be observed in the case of the anionic dyes and neutral dyes because the negatively charged groups or the electron-rich groups of these dyes undergo chemical interactions with the positively charged backbone of polyaniline (PANI). Such an adsorption process followed pseudo-second-order kinetic model, and the adsorption isotherms fitted well with the Langmuir equation. It is interesting that due to electrostatic attraction, the anionic dyes containing negatively charged groups can’t easily gain access to the positively charged backbone of PANI, giving high photodegradation rates, while, the electrostatic repulsion between cationic dyes and PANI can lead to very low photodegradation rates.3) Synthesis and photocatalytic activities of ferrite-cadmium sulfide (CdS) nanocompositesCdS is one of the most important high-performance photocatalysts, while pure CdS is susceptible to producing photocorrosion, the photoactivity is remarkable decreased after several cycles. Besides, the separation of CdS after reaction is difficult. Regarding the above problems, we report a facile strategy to fabricate CdS-MFe2O4 (M= Zn, Co) nanocomposite with differing ferrite content via a two-step hydrothermal method and demonstrate its application as a magnetically recyclable photocatalyst with enhanced visible-light-driven photocatalytic activity and photostability. The photocatalytic activities of as-prepared CdS-MFe2O4 photocatalysts are evaluated by the degradation of Rhodamine B (RhB) and 4-chlorophenol (4-CP) in aqueous solution under visible-light irradiation. The synergic effects of CdS and ferrites can reduce the recombination probability of photogenerated electron-hole pairs and enhance the charge separation efficiency, leading to high photocatalytic performance and remarkable inhibited photocorrosion.4) Synthesis and photocatalytic activities of ternary titania-cobalt ferrite-polyaniline nanocompositeBased on the binary nanocomposites, we further designed and fabricated magnetically recyclable ternary titania-cobalt ferrite-polyaniline (P25-CoFe2O4-PANI) photocatalysts with differing P25/CoFe2O4 ratio via in-situ oxidative polymerization, and studied their adsorption properties and degradation of dyes under visible light irradiation. The pseudo-second-order and Langmuir models are found to be most suitable for describing the adsorption of methyl orange (MO) onto the photocatalysts. The photocatalytic activity of P25-CoFe2O4-PANI is evaluated by the degradation of various dyes under visible light irradiation, and the results show that the ternary P25-CoFe2O4-PANI photocatalyst exhibits high photocatalytic activity due to the good adsorption capacity of the hybrid and the introduction of P25, which can further improve the separation of the light-induced electron-hole pairs. The degradation of anionic dyes is much more effective than that of cationic dyes due to the negatively charged groups of anionic dyes undergo electrostatic attraction with the positively charged backbone of PANI, and such an effective adsorption helps in promoting the degradation.5) Synthesis and electrochemical energy storage properties of ternary ferrite-graphene-polyaniline nanocompositeTernary CoFe2O4-graphene-PANI and MnFe2O4-graphene-PANI nanocomposites are designed and fabricated via a facile two-step approach:ferrite nanoparticles dispersed on graphene sheets are achieved by a hydrothermal method, followed by coating with PANI through in-situ polymerization process. Electrochemical measurements demonstrate that the specific capacitance of the resulting ternary ferrite-graphene-PANI hybrid exhibited significantly higher capacity than those of pure ferrite, graphene and PANI, or binary ferrite-graphene, ferrite-PANI and graphene-PANI hybrids. The extraordinary electrochemical performance of the ternary ferrite-graphene-PANI hybrid can be attributed to its well-designed nanostructure and the synergistic effects of the individual components.6) Synthesis and electrochemical lithium storage properties of zinc manganate-graphene nanocompositeZnMn2O4-graphene hybrid nanosheets are synthesized via a facile two-step method for greatly enhanced lithium storage capability. The hybrid 2D nanosheets are composed of ultrafine ZnMn2O4 nanocrystals with a mean diameter of ～4nm attached to and well dispersed on the surface of reduced graphene oxide sheets.The hybrid nanosheets based anode offers a high capacity, excellent rate capability,and long-term cyclability. Moreover, when tested in a temperature range of ～0-60 ℃, the designed anode can maintain high discharge capacities.