High Energy {001} Polar Facet Exposed TiO2 Nano Single Crystals for Electrocatalytic Degradation of Organic Pollutants

Author:Liu Chang

Supervisor:yu han qing


Degree Year:2019





Electrochemical oxidation is widely used for water and wastewater treatment.Anodic degradation of refractory pollutants at low bias before oxygen evolution exhibits high current efficiency and low energy consumption.However,the lack of low-cost and high-performance electrode materials as well as the severe electrode fouling largely limits its practical applications.Titanium dioxide(TiO2)is a typical semiconductor and has been widely studied as an effective photocatalyst for water pollution control,but it is highly difficult to be used as an electrochemical catalyst due to its both low electric conductivity and weak electrocatalytic activity.In this dissertation,both polar-facet and defect engineering are adopted to enhance the electrochemical activity of TiO2 semiconductor.An effective electrode anti-fouling and a novel photochemical protecting strategy are proposed and validated for electrochemical pollutant degradation promoted by photochemical oxidation on facet-tailored {001}-exposed TiO2 single crystals.The system can stabilize the defective reactive sites on TiO2-x surface and sub-surface for long-term anodic oxidation.Based on this strategy,a novel and synergistic photo-assisted electrochemical system at low anodic bias is further constructed for efficient water and wastewater treatment.The main contents of this dissertation are listed as follows:1.Exploring the anodic mechanism of {001}-TiO2 single crystals.With the target TiO2,we controllably prepare the high-energy {001}-exposed single crystals as anodic material by virtue of the facet-engineering strategy.The electrochemical treatment of wastewater containing five typical p-substituted phenols by {001}-TiO2 is synergistically studied in terms of efficiency.The relationships between the anodic activity(Langmuir-Hinshelwood first-order reaction rate constant)with the initial surface concentration of phenols,the Hammett’s constant of p-substituted groups and phenol diffusion coefficient onto electrode are explored based on density function theory and electrochemical tests.The anodic mechanism of {001}-TiO2 is proposed by taking deep insight into the electrochemical characteristics of phenols oxidation.Experimental results indicate that the anodic oxidation of p-substituted phenols becomes more difficult with the increasing Hammett’s constant of phenols,while their degradation rates increase continuously with the initial surface concentration.Phenols are degraded mainly by surface-bound OH and direct electron transfer.These findings provide a new chance to degrade phenolic pollutants in wastewater and offer atomic-scale insights into the preparation,modification and application of TiO2-based anodic materials for electrochemical water treatment.2.Realizing the efficient electrochemical reduction of nitrobenzene on defective TiO2-x single crystals.The lack of promising cathodic material is the key challenge for electrochemical water treatment,herein we controllably prepare the defective {001}-exposed TiO2-x single crystals as cathodic material by virtue of the defect-engineering strategy.With the highly oxidation-resistant nitrobenzene as target pollutant,we systematically study the electrochemical reduction efficiency of defective TiO2-x single crystals.The degradation tests,electrochemical characterizations,radical scavenging and product analysis are carried out to explore the cathodic capacity and mechanism of TiO2-x.Results demonstrate that TiO2 can act as an excellent cathodic electrocatalyst when its crystal shape,exposed facet and oxygen-stoichiometry are finely tailored by the local geometric and electronic structures.With these findings,a more practical non-Pd cathodic electrocatalyst could be prepared and applied for electrocatalytic reduction of pollutants,and extend the applications of semiconducting TiO2 in environmental science and technology field.3.Developing the new electrode anti-fouling strategy.To resolve the electrode anti-fouling in pollutant conversion at low anodic bias,we prepare the high-energy{001}-exposed single crystals as anodic material,construct the novel UV photo-assisted electrochemical oxidation system and develop a new electrode anti-fouling strategy based on the bi-functional properties of both photochemical and electrochemical activity on the one single {001}-TiO2 single crystals.With the target bisphenol A and landfill leachate,the synergistic degradation and treatment capacities of photo-assisted electrochemical system are systematically investigated under both UV irradiation and anodic bias.The cyclic degradation tests,photochemical tests,electrochemical tests,radical scavenging and surface analysis are carried out to explore the oxidation capacity,anti-fouling characteristics and catalytic mechanism of {001}-TiO2 single crystals.Results demonstrate that electrode fouling from anodic polymers at low bias is greatly relieved by the free OH-mediated photocatalysis under UV irradiation,a clean electrode surface is kept during cyclic pollutant degradation,thus the efficient and stable degradation of bisphenol A and treatment of landfill leachate are accomplished.This work develops a brand new synergistic oxidation system for efficient and cost-effective pollutant degradation.4.Proposing a novel protecting approach for defective reactive sites.To stabilize the defective reactive sites on non-stoichiometric metal oxides,we construct a novel visible photo-assisted electrochemical oxidation system and develop a novel protecting approach for defective reactive sites based on the mild photochemical and electrochemical activation pathways on the one single TiO2-x single crystals.With the target bisphenol A,surface water,municipal and industrial wastewater,the synergistic degradation and treatment capacity of photo-assisted electrochemical system is systematically investigated under both visible/sunlight irradiation and anodic bias.The cyclic degradation tests,photochemical tests,electrochemical tests,radical scavenging and crystal analysis are carried out to explore the oxidation capacity,stabilizing characteristics and protecting mechanism of TiO2-x single crystals.Results demonstrate that the defective oxygen vacancies as surface reactive sites on TiO2-x electrode can be effectively stabilized by the mild non-band-gap excitation pathways under both visible light irradiation and moderate anodic polarization,also the ·OHfree-mediated photochemical anti-fouling capacity is observed.Under real sunlight irradiation,a 20-run cyclic test for BPA degradation demonstrates the excellent performance and stability of the synergistic system at low bias.This work provides a new opportunity to utilize the defective and reactive TiO2-x for efficient,stable and cost-effective electrochemical water treatment with the aid of its photo-and electro-chemical bi-functional properties.