Study On Preparation And Electrochemical Advanced Oxidation Process Of Three-Dimensional Porous Boron-Doped Diamond Film

Electrochemical advanced oxidation process(EAOP)is significant for water environment treatment and restoration because it can effectively degrade refractory organic pollutants by strong oxidizing radicals.The crux of construction of EAOP system is developing anodic materials with superior performance and stable microstructure.Three-dimensional(3D)porous boron-doped diamond(BDD)film,which not only favors excellent properties of BDD film,such as large oxygen evolution potential,high yield of radical and stable microstructure,but also exhibits high specific surface area and facile mass transfer channel,is regarded as an idea anodic material.However,as a result of uncontrollable pore size and high non-diamond carbon(NDC)content,the relevance of porous microstructure with EAOP performance and the influence mechanism of radicals on organic pollutants are still ambiguous for 3D porous BDD film.Based on these issues,the regulated deposition of BDD film,construction and adjustment of 3D porous microstructure,electrochemical properties and degradation mechanism are studied in this paper.The BDD films are controllably synthesized by regulating parameters of hot filament chemical vapor deposition technique.With elevation of methane concentration,deposition power and B/C ratio,the crystal size of BDD films transforms from micron to nano grade,combing with the increasing content of NDC.It’s proposed that high content of NDC is responsible for the shrink of oxygen evolution potential and enhancement of electrochemical activity of BDD films.Moreover,high CH4 concentration(5%),high deposition power or large B/C ratio(≥ 6000 ppm)is the essential condition of preparation of BDD/NDC composite film.The BDD/NDC composite films with three-dimensionally continuous distribution of NDC are controllably fabricated.The degradation process of organic pollutant by BDD/NDC composite films follows pseudo first-order reaction kinetics with the highest apparent rate constant of 1.35 × 10-2 min-1.This could be elaborated by the enhancement of direct oxidation of organic pollutant and the generation of SO4·through adsorption of H2SO4 and HSO4-on NDC surface.Unfortunately,the degradation efficiency of BDD/NDC composite films never exceeds that of highquality BDD film due to less oxidizing activity of electron than ·OH and the selectivity of SO4·.In addition,by integrating controllable deposition of BDD/NDC composite films and selective remove of NDC,the 3D porous BDD films with more than 92%of diamond,high doping level of more than 1.12×1022 atoms·cm-3,superior chemical corrosion resistance and stable microstructure are constructed.The pores,ranged from 10 nm to 650 nm,are interconnected and uniformly distribute in the 3D porous BDD films with channel-like shape.The 3D porous BDD films possess high oxygen evolution potential(1.75 V～1.95 V)and 2.37～2.76 times enhancement of electrochemical activity than that of planar BDD film.And the electrochemical activity can be further regulated by tailoring pore size.It is manifested that large double layer capacitance(17.54 mF/cm2)in diamond films,wide potential scan range of 2.6 V and near 100%capacitance retention are achieved for the 3D porous BDD film.These derive from the synergistic effect of the highly improved specific surface area,3D interconnected porous structure and good structure stability.Finally,the 3D porous BDD films are applied in the EAOP.The p-nitrophenol(PNP)with initial concentration of 330 mg/L is removed by 92.43%after 180 min using the 3D porous BDD film.The reaction rate is 2.37 times higher than that of planar BDD film and increased by reducing pore size.Indirect oxidation with radicals is the primary degradation mechanism for the 3D porous BDD films.The ·OH is the dominant radical and its concentration is boosted owing to the large specific surface area and accelerated mass diffusion in 3D porous microstructure,resulting in the fast PNP remove rate.SO4·can form by sing-electron transfer on BDD surface,leading to the remove of degraded intermediates of PNP.Besides,it’s found that the main oxidation reactions of PNP by 3D porous BDD films are detachment of nitro,abstraction of hydrogen atom and cleavage of aromatic ring.