Low-temperature Solution-based Synthesis of SnO2 Nanostructures and Their Photocatalytic Properties

Author:Wang Jing

Supervisor:fan hui qing


Degree Year:2017





With the process of global industrialization and population growth,environmental protection has become one of the most concerned issues in the current society.Semiconductor photocatalysts has the advantages of reduced speed,non-toxic and harmless,and can completely degrade organic pollutants.SnO2 is a unique material of widespread technological applications,particularly in the field of photocatalyst,gas sensors,dye-sensitized solar cell and a transparent electrode.In the present work,novel structured SnO2 with controllable morphologies were desighed and prepared by one-step low-temperature synthesis processing.Meanwhile,the properties and applications of SnO2 nanomaterials in the photocatalytic degradation of organic matter were studied from the aspects of morphology control,metal doping and noble metal modification.Novel walnut-like SnO2 spheres have been synthesized using a one-step hydrothermal reaction with SnCl2?2H2O and KOH as rawmaterials.These SnO2 spheres have uniform walnut-like appearance and are highly monodisperse.It is found that the concentration of KOH affects the morphology dramatically and thus the morphology of the products could be controlled by adjusting the amount of KOH.The reaction time-dependent experiments indicate the walnut-like structure was formed via Ostwald ripening process.The adsorption energies between SnO2 nanoparticles and the alkali metal cations are different.Because of its smaller adsorption energies,KOH is benefit to form a sphere-like morphology.The walnut-like SnO2 spheres exhibited excellent photocatalytic properties by degradation of methylene orange,which was mainly attributed to its unique morphology and good dispersion stability.SnO2 nanosheets and hollow microspheres have been selectively synthesized via a simple hydrothermal method,using neither template nor surfactant.As a hydrolysis agent,urea may kinetically control the nucleation and the growth of crystals.The insufficient dissolved oxygen in the solution directly affects the oxidation of divalent Sn ions.Based on reaction time-dependent experiments,the formation of SnO2 hollow structure can be rationally expressed as Ostwald ripening process.The key factor in the preparation of sheet-like SnO2structure is relatively insufficient oxygen in solution.Compared with bulk single-crystal SnO2materials,the Raman lines of the as-prepared SnO2 samples exhibit measurable line shifts and broadening.This behavior may be ascribed to the decrease of SnO2 crystal size.There is a slight red shift in the band gap values of both SnO2 samples if compared with the bulk SnO2,which may be due to the number of oxygen vacancies,which causes formation of defect states within the band gap.The optical and electrochemical experiments show that the photo-excited electrons and holes of SnO2 hollow microspheres and SnO2 nanosheets possess the enough redox potentials required for photocatalytic degradation.Compared with SnO2 hollow microspheres,SnO2 nanosheets exhibit high photocatalytic activity and the main active species are hydroxyl radicals.The beteer catalytic activity of SnO2 nanoplates are ascribed to their unique 2D structure and the exposed high energy plane(001).Forthermore,SnO2nanoplates have a larger BET surface which is favorable for semiconductor photocatalysts.The flower-like SnO2 nanorods and Zn-doped SnO2 porous microspheres have been synthesized by a solvothermal method.The structure and optical properties of the products were characterized and analyzed.The SnO2 nanorods can be prepared without template or surfactant,indicating the morphology dictated by the crystal symmetry and the surface energy in the aqueous environment.The doping of Zn ions lead to the change of SnO2 microstructure,the distortion of SnO2 lattice,and the increasing of the aspect ratio of SnO2 nanorods,which indicates that the doping of Zn ions will change the surface energy of SnO2 and affect the morphology of SnO2 nanocrystals.Based on the optical studies,the substitutional doping of Zn influenced not only electronic structural modifications but also oxygen vacancies.This is confirmed by XPS and electrochemical tests.the Zn2+ions lead to the shift of SnO2 valence band and conduction band,and reduces the band gap of SnO2.The Zn-doped SnO2 porous microspheres exhibited excellent efficiency of photocatalytic activity,which could be ascribed to their abundant oxygen vacancies,large specific surface area,and porous structure.The flower-like SnO2 nanosheets with exposed high energy(002)surface have been synthesized via one-step low-temperature process with cetyl trimethyl ammonium bromide(CTAB)or sodium dodecyl benzene sulfonate(SDBS)assist.When the surfactant’s concentration is low,the surfactant can be absorbed on the crystal surface and induce the sheet-like structures.When the surfactant’s concentration is high,the surfactants form micelles and induce the irregular structures.The size and shape of nanosheets prepared with CTAB or SDBS is different,which may be attributed to the different interactions between the functional groups and crystals.The flower-like SnO2 with CTAB-assisted exhibits a good photocatalytic activity,which may ascribed to its large specific surface area providing more reactive sites and its thin thickness favering the carriers transfer.Ag/SnO2 heterostructures were successfully prepared by loading Ag onto the flower-like SnO2.Based on the photoluminescence spectra,PL decay spectra and transient photocurrent response spectra,Ag loading can significantly restrain the recombination of photon-generated carrier and passivate the band edge of SnO2.In addition,the electrochemical test shows that the Ag loading not only greatly increases the carrier concentration,but also changes the electronic band structure in SnO2.Compared with pure SnO2,5%Ag/SnO2 exhibts better photocatalytic activity and the rate constant of the degradation enhance 44%and 43%in UV and visible light,respectively.However,if the amount of Ag is too excessive,the photocatalytic performance of10%Ag/SnO2 decrease slightly.The pure Sn3O4 was successfully synthesized with PVP assist.The concentration of precursor and PVP directly affected the structure and morphology of the products.The reducing propertiy of PVP is crucial for synthesizing pure Sn3O4.The concentration of PVP affects the self-assembly process of Sn3O4 nuclei,leading to the hierarchical structure and dispersed nanosheets,respectively.With XPS analysis,the valence bands of Sn3O4 and SnO2 are found to be different within 04eV.Due to its unique band structure and appropriate bandgap(2.75eV),Sn3O4 possess a strong visible light absorption,indicating it can be used as a visible light photocatalyst.Furthermore,the flower-like Sn3O4 exhibite better photocatalytic activity than that of dispersed Sn3O4 nanosheets,and show good photocatalytic activity to methyl orange(MO)and rhodamine B(RhB)under visible light irradiation.