Synthesis,Characterization,and Photocatalytic Performance Of Zn-Based Metal Oxide Nanocomposites

Recently,Zn-based metal oxides,have received much attention as photocatalysts as semiconductor photocatalysts due to their strong photocatalytic activity.In comparison to the single-phase semiconductor,the coupled semiconductor systems demonstrate even much higher photocatalytic activity.To further improve the photocatalytic activity of Zn-based metal oxides,several Zn-based composites were devised.The main results of this paper are as follows:（1）First,we devised a metal-organic framework（MOFs）-derived method to successfully prepare the hetero Zn O/Fe₂O₃ nanoflowers（NFs）by using the ZIF-8 as a precursor.The purpose of the design here is to combine the advantages of both hierarchical porous structure and heterojunction.The photocatalytic performance of the as-prepared Zn O/Fe₂O₃ NFs was evaluated in detail under UV light irradiation by choosing organic dyes of methylene blue（MB）and methyl orange（MO）as model pollutants,respectively.The resultant Zn O/Fe₂O₃ NFs exhibit much greater photocatalytic activity than single-phase Zn O and Fe₂O₃ as a photocatalyst for both MB and MO degradation.In particular,over 100%of MB may be photo-catalytically degraded in 90 min,when exposed to UV light via the hetero-NFs photocatalyst.The improved photocatalytic properties are most likely ascribed to the synergistic effects of the appropriate band alignment of Zn O and Fe₂O₃,as well as the large surface area and high light absorption property.Experiments using radical scavengers demonstrate that photogenerated holes,.OH and.O₂^-are critical in the photocatalytic degradation of organic dyes.As a result,a possible mechanism for the photocatalytic degradation of hetero-Zn O/Fe₂O₃ NFs towards dyes is presented.The work advances our understanding of how to develop and construct improved photocatalysts with heterojunction structures for photocatalytic applications.（2）Second,the photocatalytic activity of a semiconductor with a low band gap has been greatly suppressed due to the high recombination rate of photogenerated charge carriers.The most economic and efficient remedy is to construct a heterojunction to enhance the transfer/separation of interfacial charge carriers.Inspired by this,the rod-like Zn O/Zn Mn₂O₄ composite heterojunction photocatalysts are synthesized via a facile template-free microwave-assisted hydrothermal route.The photocatalytic activities of the as-prepared samples are evaluated by the photocatalytic degradation of MB and MO under UV light irradiation.The optimized Zn O/Zn Mn₂O₄ demonstrates enhanced photocatalytic efficiencies towards degradation of MB and MO,which are about 4.3 and3.0 times higher than those of pure Zn O,respectively.Furthermore,the composite could still maintain good photocatalytic activity for photocatalytic degradation of MB even after four recycles.The active species in the photocatalytic system are.O₂^-and h⁺,which can be confirmed by the trapping experiments.The enhanced photocatalytic activity of Zn O/Zn Mn₂O₄ composite may result from the formation of n-p junction,which supports the generation of.O₂^-and h⁺.More significantly,the intrinsic photocatalytic degradation mechanism of Zn O/Zn Mn₂O₄ composite towards organic dyes is tentatively proposed.（3）Third,Zn Fe₂O₄ microspheres were prepared via a microwave-assisted method.Simultaneously,the carbon quantum dots（CQDs）were also synthesized through a solid state approach.Hence,a facile impregnation-thermal method was developed to fabricate CQDs-decorated Zn Fe₂O₄ composites.The TEM and HRTEM images indicated the CQDs were distributed evenly on the surface of Zn Fe₂O₄ microspheres.The photocatalytic activities of the as-prepared CQDs/Zn Fe₂O₄ composites were evaluated by using MB and Rhodamine（Rh B）as the model dyes.Among all the as-prepared photocatalysts,0.30 CQDs/Zn Fe₂O₄ composite exhibited the most strong photocatalytic performance.Assisted by the UV light irradiation,nearly 100%of MB was photocatalytic degraded within only 60 min.,and the photocatalytic degradation of Rh B needed 80 min.Obviously,the CQDs have greatly improved the photocatalytic performance of Zn Fe₂O₄.Besides,controlled experiments with a variety of radical scavengers demonstrated that the primary participants in the degradation of organic pollutants were.O₂^-,.OH,and h⁺.A photocatalytic mechanism for the photocatalytic degradation of MB and Rh B over the CQDs/Zn Fe₂O₄ composite has been proposed.The experimental results indicate that CQDs/Zn Fe₂O₄ is an excellent photocatalyst for organic dye degradation.（4）Fourth,Zn Co₂O₄ microspheres were fabricated by a solvothermal method.Meanwhile,the bamboo shoot-derived CQDs were prepared through a hydrothermal route.And then a simple and economic impregnation-thermal method was applied to construct CQDs/Zn Co₂O₄ composite photocatalyst.The TEM and HRTEM images confirmed the success attachment of the CQDs to Zn Co₂O₄ microspheres.The photocatalytic activity of the as-prepared CQDs/Zn Co₂O₄ composite was examined by applying MB as a model dye.The experimental results presented that 0.7 CQDs/Zn Co₂O₄displayed the most strong photocatalytic performance in all the as-prepared samples.After the UV light irradiation of 50 min.,MB was almost completely decomposed.The great improvement in the photocatalytic activity of Zn Co₂O₄ further validated the significance of CQDs.Besides,the controlled studies with a variety of radical scavengers indicated that.O₂^-,.OH,and h⁺were the key components in the photocatalytic degradation of MB.Meanwhile,the photocatalytic degradation of MB over a CQDs/Zn Co₂O₄ photocatalyst has been tentatively presented.Over four consecutive cycles,the 0.7 CQDs/Zn Co₂O₄ photocatalyst maintained its high photodegradation rate and good photostability with no change in crystallinity.