Controllable Synthesis Of Ga₂O₃and Ge-based Photocatalytic Materials And Their Performances In Degradation Of Organic Pollutants

Environment pollution and energy shortage are one of important factors that restrictthe sustainable development of human society and economy. Photocatalysis caneffectively and directly decompose the organic pollutant at mild temperature andpressure with light and semiconductor photocatalysts. Photocatalysis are dependent onthe semiconductor photocatalysts and their microstructures including the morphology,size and pore structure have an important influence on the photocatalytic propertiesand recycle. Thus, the development of new and efficient photocatalysts has attractedgreat attention in recent years. In this work, we have successfully synthesized severalnovel photocatalysts and adjusted their microstructures by changing the preparationconditions. Moreover, we discussed the formation process of the photocatalysts indetai. Then, the as-prepared photocatalysts showed enhanced photocatalytic propertiesfor the degradation of pollutants in environment and the related mechanism was alsostudied. The main research work is as follows:(1) Zn2GeO4hollow spheres were successfully fabricated by a hydrothermalapproach using a zinc hydroxide carbonate spheres as template. The compositions,microstructures, specific surface area and light adsorption of Zn2GeO4hollow sphereswere characterized by XRD, XPS, SEM, FESEM, HRTEM, N2adsorption-desorptionand UV-vis DRS. Moreover, we calculated the band structure and density of states ofZn2GeO4semiconductor by the CASTEP software based on the density functionaltheory. The calculated results suggested that Zn2GeO4is a direct semiconductor. Thevalence band and conduction band of Zn2GeO4semiconductor are composed of O2s,O2p, Ge4s, Ge4p and Zn3d hybrid orbital and Ge4s, Ge4p and Zn4s hybridorbital, respectively. Meanwhile, we investigated the role of the reaction temperatureand time for the formation of the Zn2GeO4hollow spheres. The Zn2GeO4hollowspheres can be obtained at200°C. The building units of the Zn2GeO4hollow spherescould gradually transform nanoparticles into nanobundles with the increase of thereaction time, which was attributed to the splitting crystal growth mechanism. Thephotocatalytic activity and photostability of Zn2GeO4hollow spheres were examinedby the degradation of metronidazole solution.(2) A one-step and low-temperature route synthesized at ambient pressure was usedfor the preparation of Ag2ZnGeO4hollow spheres and their compositions, element chemical state, microstructures, specific surface area and light adsorption werecharacterized. The calculated results suggest that Ag2ZnGeO4is an indirectsemiconductor. The valence band and conduction band of Ag2ZnGeO4semicondutoris composed of hybridized O2s, O2p, Zn3d, Ge4s, Ge4p and Ag4d hybrid orbitaland Zn4s, Ge4s, Ge4p and Ag5s hybrid orbital, respectively. Moreover, weinvestigated the role of the dose of NH3·H2O and AgNO3and reaction time for theformation of the Ag2ZnGeO4hollow spheres. The Ostwald ripening was responsiblefor the formation mechanism of Ag2ZnGeO4hollow spheres. In addition, the reactionkinetics and the active species during the photocatalytic degradation of acid red Gsolution over the Ag2ZnGeO4hollow spheres, bulk Ag2ZnGeO4and ref-Ag2ZnGeO4were also discussed.(3) Cd2Ge2O6/CdS core-shell composites were prepared by ion-exchange method.FESEM, HRTEM, EDX and XPS analyses proved that the CdS particles existed onthe surface of the Cd2Ge2O6nanorods and formed the core-shell, which was beneficialto the separation of the photogenerated electrons and holes. The visible-lightabsorption properties and absorption edges of the Cd2Ge2O6/CdS composites werecharacterized by UV-vis DRS results. The electron transportation in theCd2Ge2O6/CdS composites and the active species during the photocatalyticdegradation of acid red G solution were investigated by theory calculation andquencher experiments. The enhanced photocatalytic activity of the Cd2Ge2O6/CdScomposites was mainly attributed to the synergic effect from well-matched bandpotentials and1D core-shell structure.(4) Mixed phase α-Ga2O3/β-Ga2O3microrods were synthesized by calcining theGaOOH template. The composition, crystal phase and microstructures were alsocharacterized. Compared to the single α-Ga2O3and β-Ga2O3, the mixed phaseα-Ga2O3/β-Ga2O3had an enhanced photocatalytic property for the degradation ofmetronidazole solution. The heterojunctions were formed in the mixed phaseα-Ga2O3/β-Ga2O3, which were beneficial to the separation of the photogeneratedelectrons and holes. Moreover, the recycle experiments showed that the mixed phaseα-Ga2O3/β-Ga2O3microrods had excellent photostability and could be separatedeasily from the reaction system.