Preparation Of TiO₂/Graphene 3D Hydrogel Electrodes For Photoelectrocatalytic Removal Of Organic Contaminants

Titanium dioxide(TiO2)is widely recognized as the great potential and powerful material in photocatalytic degradation of organic pollutants.To overcome the disadvantage of easy inactivation and aggregation of TiO2 nanoparticles,enhance the separation efficiency and photocatalytic oxidation activity for low concentration pollutants,it is necessary to fix TiO2 nanoparticles on adsorbents.Three-dimensional graphene hydrogel is synthesized utilizing two-dimensional graphene sheets,which not only maintains the excellent properties of graphene,but also exhibits high surface area,accessible pore volume,high strength and conductivity at extremely low density.Graphene hydrogel as emerging materials has been widely used as promising supports for catalysts.Through the combination of TiO2 nanoparticles and graphene hydrogel,contaminant molecules can be adsorbed and enriched on graphene hydrogel and subsequent in situ catalytic degradation by photocatalysts.Here NF-TiO2/graphene hydrogel(NF-TGH)electrode with three-dimensional network structure was designed and fabricated successfully via the chemical reduction self-assembly method.By applying external voltage,photoelectrocatalytic(PEC)degradation ability of methyl orange(MO)and phenol over NF-TGH electrode was studied.The optimal content of TiO2 in NF-TGH is 80%.The PEC degradation rate of MO over TGH electrode was about 81%,which is much higher than that using TiO2 thin film electrode(40%).Because the TGH has well-defined and interconnected pore structure which TiO2 nanoparticles distribute evenly on,providing more adsorption and catalytic activity sites,organic pollutants were adsorption-enriched and in-situ photocatalytic oxidized.Besides,graphene acting as an electron-acceptor material effectively hinder the electron/hole pair recombination.By applying a 1.5 V bias voltage,the PEC degradation rate was improved more than 30%.This phenomenon indicates bias voltage can drive the photo-generated electrons generated from hydrolysis electrode to the cathode,thus effectively minimizes photo-generated electron/hole pairs and increases the concentration of h+and OH on the surface of TGH electrode.The PEC degradation reaction of phenol and MO had an optimum voltage.When the voltage was too high,the PEC performance was reduced gradually,which was mainly ascribed to the fact that the formation of polymers and the oxygen evolution reaction occurred on the TGH inhibiting the further degradation of organic pollution.Comparing with the photolysis,photocatalytic and electrocatalytic degradation of Phenol,PEC showed the best degradation rate and mineralization yield with 53%TOC removal.More intermediate products,such as p-hydroquinone and benzoquinone,were generated during photolysis and photocatalytic reaction.Radical scavengers experiment suggested ·OH and h+radials work as the main reactive species for the PEC decomposition of phenol.In order to further enhance the adsorption-enrichment and in-situ PEC degradation ability of graphene hydrogel,titanium dioxide nanotubes(TNTs)was synthesized by hydrothermal method and loaded on graphene hydrogel.The effect of different TiO2 morphology on adsorption/PEC degradation of organic pollution was explored.First,the effects of reaction temperature,sodium hydroxide concentration and additives on the morphology,crystal structure and photocatalytic activity of the materials synthesized by hydrothermal method were compared.TNTs with rutile and anatase crystal structure were prepared in a low concentration(3 M)sodium hydroxide solution with the assistance of ethylene glycol.TNTs had larger adsorption capacity as well as kept the good photocatalytic activity of TiO2 precursors.Benefiting from the similar crystal structure and electronic properties of stannic oxide(SnO2)and TiO2,SnO2/TNTs heterojunction was synthesized and SnO2 nanoparticles adhere on the TNTs surface.SnO2/TNTs/graphene hydrogel(NF-STGH)electrode fabricated successfully via the chemical reduction self-assembly method and was used for adsorption/PEC degradation of MO and phenol.The optimal content of SnO2 in SnO2/TNTs is 2%.When the NF-STGH electrode was irradiated,electrons generated in the conduction band of TNTs will migrate into that of SnO2 nanoparticles.And holes generated in the valence band of SnO2 nanoparticles will transfer to that of TNTs.Therefore,the separation efficiency of photogenerated electrons and holes was increased in the SnO2/TNT heterojunction.The adsorption/PEC degradation rates of MO(68%)and phenol(90%)over NF-STGH electrode were higher than that using NFTNTs/rGH electrode(63%and 79%).Compared with TiO2 nanoparticles,TNTs have higher specific surface area and stronger adsorption capacity,due to its onedimensional structure.So NF-STGH exhibited better adsorption capacity of phenol than NF-TGH.The adsorption/PEC degradation rate of phenol kept above 80%after 5 recycles,indicative of the excellent adsorption/PEC activity and stability of the NFSTGH electrode.