Preparation, Characterization And Photocatalytic Properties Of Graphene-based Semiconductor Nanocomposites

Graphene is a single layer of sp2-bonded carbon atoms with unique properties, such as hightheoretical specific surface area, excellent electrical conductivity, exceptional transparency andadsorption activity, which make it quite suitable for using as a photocatalyst support material.TiO2has been intensely studied as a promising semiconductor photocatalyst for uses in cleanenergy conversion and pollutant treatment, owning to its high redox potential, cheapness,nontoxicity and chemical stability. However, the absence of visible-light absorption and the fastrecombination of photogenerated electron-hole pairs in TiO2nanoparticles result in its poorphotocatalytic activity, thus the modified technology and seeking for novel visible-lightphotocatalyst have been the urgent affairs at present. Moreover, Ag3PO4with high photocatalyticperformance is regarded as a significant breakthrough in the field of visible-light-drivenphotocatalysts, but suffering from the problems of serious photocorrsion and poor stability undervisible-light irradiation. Considering the synergetic effect of graphene with TiO2or Ag3PO4,graphene-based semiconductor nanocomposites are proved to be an effective way to improve thephotocatalytic activity and stability of these photocatlysts.With graphene oxide (GO) or reduced graphene oxide (RGO) as a facile substitute ofgraphene, this paper makes a great effort to the study of preparation, characterization andphotocatalytic properties of TiO2/RGO, Ag3PO4/RGO/Ag and TiO2/Ag3PO4/GO nanocomposites.The main results and conclutions are listed as follow:(1) The GO sample with various oxygen-containing groups on the surface was obtained bythe modified Hummers method. A facile one-step hydrothermal method was adopted tosynthesize TiO2/RGO nanocomposite with Ti(SO4)2and GO as starting materials in anethanol/water solvent. The GO was efficiently reduced into RGO during the hydrothermalprocess and anatase TiO2nanoparticles with a size of about10nm were well dispersed on theRGO sheets; with incremental RGO mass ratio in the nanocomposites, the photocatalyticdegradation rate of TiO2/RGO samples increased in a large degree, especially the degradationrate of TiO2/RGO-4sample was about5.6times that of bare TiO2nanoparticles; the excellentphotocatalytic performance of TiO2/RGO nanocomposite was arised from the synergetic effect ofTiO2and RGO, including: the RGO sheets with excellent electron trapping and transportationproperties can suppress the recombination of photogenerated electron-hole pairs in TiO2; thechemical bonds between TiO2and RGO result in a relative narrow bandgap and a broad lightharvest in the nanocomposite; the coexistence of RGO can enhance the adsorption ability for organic molecule and facilitate photocatalytic reaction on the surface of TiO2.(2) The Ag3PO4/RGO/Ag nanocomposite was synthesized by a simple photo-assistedreduction treatment from Ag3PO4/GO precursor. With the help of photo-generated electrons inAg3PO4, GO sheets with pre-absorbed Ag+on the surface can be reduced into RGO sheetsdecorated with well dispered Ag nanocrystals; the body-centered cubic phase of Ag3PO4nanoparticles with a diametere of200nm were enwrapped tightly by RGO sheets; comparedwith bare Ag3PO4, the Ag3PO4/RGO/Ag sample displayed a5times higher degradation rateunder visible-light irradiation and was also demonstrated to be more stable in repeateddegradation experiments; the improved photocatalytic activity and stability were mainlyattributed to the synergetic effect of Ag3PO4, RGO and Ag nanocrystals, including: thephoto-generated electrons can be rapidly transferred from the conduction band of Ag3PO4toRGO, preventing Ag3PO4from the photocorrosion; the electrons can be transferred along the Ï€-Ï€graphitic network of RGO and finally captured by Ag nancrystals, the special electron transferchannels of Ag3PO4â†'RGOâ†'Ag results in the efficient separation of electron-hole pairs; besidesimproving the electronic conductivity of RGO, the plasmonic Ag nanocrystals can react with O2to form O2radicals, which are beneficial to the decomposition of organic molecule.(3) The synthesis of TiO2/Ag3PO4/GO nanocomposite involved in situ growth of Ag3PO4nanoparticles onto graphene oxide (GO) sheets to form Ag3PO4/GO heterostructure and thendeposited TiO2nanocrystals on the surface of Ag3PO4by hydrolysis of Ti(SO4)2atlow-temperature hydrothermal condition.The Ag3PO4nanoparticles with a size of about200nmwere enwrapped tightly by GO sheets and well decorated with anatase TiO2nanoparticles with asize of about10nm; the degradation rate of TiO2/Ag3PO4/GO sample is about4times that ofbare Ag3PO4nanoparticles under visible light irradiation; It is suggested the photo-generatedelectrons on Ag3PO4can be quickly transferred to GO, while the photo-generated holes can betransferred to the valence band of TiO2since the the valence band of TiO2is high than that ofAg3PO4. The dual transfer channels for electron-hole pairs at the interface TiO2/Ag3PO4/GOresult in efficient carriers separation, leading to an enhanced photocatalytic activity and stability.Additionally, the fabrication of novel TiO2/Ag3PO4/GO heterostructure can significantly reducethe loading of noble metal Ag from77wt%in bare Ag3PO4to55wt%in the nanocomposite.Therefore, graphene-based semiconductor nanocomposites provide a promising way forimproving the photocatalytic performance of semiconductor photocatalysts and the study in thisfield has important significance in the use of clean energy and environmental protection.