| Over the last few decades,semiconductor-based photocatalysis has provoked tremendous research interest because of their potential applications in environment remediation and energy transformation.Zinc oxide(ZnO)has always been regarded as an efficient heterogeneous photocatalyst toward the elimination of hazardous pollutants in aqueous and gaseous phases,thanks to its various merits such as high intrinsic catalytic efficiency,high electron mobility,chemical stability and non-toxicity,together with its natural abundance in nature.Nevertheless,large-scale practical exploitations of bare ZnO semiconductor material is profoundly hampered by its relatively large band gap(3.37 eV)that is responsible for harvesting of ultraviolet light(?5%of the solar spectrum)but not in visible light(?43%of the solar spectrum).Besides,the rapid recombination of photogenerated holes and electrons also severely limits its photocatalytic applications.In order to overcome these critical bottlenecks,spherical ZnO materials with large specific surface area and excellent optical properties were prepared via a two-step method.Subsequently,physical and chemical modification approaches were utilized to construct ZnO inverse opal composite photocatalysts with uniform macroporous framework and visible light absorption.The relationship between structure and photocatalytic activity was explored in detailsIncipiently,ZnO colloidal nanoparticles were synthesized using a two-step route.The size of as-synthesized ZnO nanoparticles was controlled by adjusting the amount of seed solution added.Dynamic light scattering(DLS)and transmission electron microscopy(TEM)indicated that the size of as-synthesized ZnO samples was in the range of 60?140 nm with a polydispersity index below 0.05.On high-resolution TEM micrograph,it was noted that the ZnO nanoparticles were actually assembled by large amounts of tiny ZnO subunits.X-ray diffraction(XRD)and selected area electron diffraction(SAED)analysis revealed that the as-synthesized nanoparticles were polycrystalline structure.The size of the subunits,which was corroborated by Ultraviolet-visible(UV-vis)absorption spectroscopy,increased as the amount of seed solution added was reduced.The ultraviolet emission peak at 374 nm and the red emission peak at 651 nm were observed in the photoluminescence spectra(PL).In addition,the formation of ZnO nanoparticles follows core-shell growth mechanism.Then,monodisperse ZnO submicrometer spheres with average diameter of approximately 180 nm were prepared through a modified two-step method,followed by calcination at different temperatures.The influences of calcination temperature on structure,morphology and optical properties of as-prepared samples were studied by using Fourier-transform infrared(FT-IR)spectroscopy,XRD,field-emission scanning electron microscope(FE-SEM),N2 adsorption/desorption,UV-vis diffuse reflectance spectroscopy(UV-vis DRS)and PL techniques.UV-vis DRS results provided obvious evidence for the decrease in band gap energy of samples with increasing calcination temperature.PL spectra revealed calcination-dependent emission features,especially the ultraviolet emission intensity.In particular,the ZnO samples calcined at 400? exhibited the highest photocatalytic activity under UV light irradiation,bleaching methylene blue(MB)dye solution by about 99.1%within 70 min,ascribed to the large BET surface area and pore volume,high charge carrer separation and transfer efficiency,and great redox potential.Furthermore,it was found that the generation of hydroxyl radicals was good agreement with the MB degradation over ZnO products.Utilizing isopropanol as hydroxyl radical scavenger,·OH was determined to be the predominant reactive species during photocatalytic degradation of MB dye solution.Next,using porous spherical ZnO materials obtained via calcination at 400? as photocatalysts,photocatalytic activity of the ZnO products was assessed toward decolorization of rhodamine B(RhB)dye solution under simulated sunlight illumination.The influences of operational parameters on the photo-degradation reaction progress,including catalyst dosage,initial dye concentration,reaction temperature,pH value,and the addition of inorganic oxidants,transition-metal ions as well as inorganic anions were investigated systematically.Moreover,active species in the oxidation decomposition of RhB dye were pinpointed via introducting several scavengers into the photocatalytic reaction system and O2·-were ascertained to be the primary active species.The catalyst was found to be reusableSubsequently,highly ordered porous ZnO inverse opal(ZnO-IO)was manufactured via an auto-forced impregnation approach using self-assembled polystyrene(PS)spheres as colloidal crystal templates.ZnO-IO with deposited silver nanoparticles(Ag/ZnO-IO)was successfully fabricated based on a simple yet efficient photoassisted reduction route using an AgNO3 ethanol-water mixed solution at room temperature and characterized properly by means of various analytical techniques.Upon visible-light irradiation,the Ag/ZnO-IO composite catalyst exhibited higher photocatalytic activity than pristine ZnO-IO regarding the decolorization of RhB in aqueous solution.Such a significant photoactivity improvement was predominantly attributed to surface plasmon resonance effect of metallic silver nanoparticles,which could enhance the harvesting of visible light and improve the segregation and transfer of photoinduced charge carriers.The stability of the Ag/ZnO-IO photocatalyst was also investigated with respect to the decomosition of RhB.Furthermore,a plausible degradation mechanism of the Ag/ZnO-IO sample in the course of photocatalytic oxidation of organic pollutants was tentatively put forward from active species trapping experimentsFinally,Ag3PO4 nanoparticles were applied to deposit on ZnO inverse opals and form novel Ag3PO4/ZnO-IO composite photocatalyst with enhanced catalytic activity toward the decolorization of RhB under simulated sunlight irradiation.The photocatalytic conversion of RhB over Ag3PO4/ZnO-IO composite catalyst was around 1.4-fold higher than that of pristine ZnO-IO and maintained relatively high levels after five successive recycling runs.The notably boosted photocatalytic activity could be ascribed to the efficient separation and transfer of photogenerated electron-hole pairs as well as improved visible light harvesting capability Addirionly,the quenching experiments revealed that 02·-played the vital role in the RhB decolorization.The separation and transfer of photogenerated electrons and holes conform to Z-Scheme mechanism. |
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