Determination Of COD Based On The Electrode Of A Boron-doped Diamond (BDD) Film And Fabrication Of TiO₂-BDD Heterojunction Photocatalysts

As a versatile electrode material,boron-doped diamond(BDD)possesses many advantages such as wide electrochemical window,low voltammetric background current,high level of mechanical strength arid corrosion resistance.Therefore,BDD has been investigated widely in the fields of electrochemical wastewater treatments and the electroanalysis of pollutants.As for water pollution,chemical oxygen demand(COD)is an important measure of organic pollution in water.Unfortunately,the conventional dichromate method for COD tests includes several inherent drawbacks.More importantly,the conventional method is difficult to be conducted automatically.The electrochemical method developed in recent years could overcome many disadvantages involved in the conventional method.However, the lack of proper electrode materials might limit the widespread application of this method in practice.Considering what mentioned above,the environmentally friendly material,BDD, was adopted as an electrode to determine COD by amperometric method.Meanwhile,a flow injection COD testing system was also emplored.Additionally,the photocatalytic technique with TiO₂ as a photocatalyst has been studied frequently in wastewater treatments because TiO₂ is easily available,nontoxic,inexpensive and chemically stable.However,TiO₂ also suffers severe constraints in practical applications due to the bottleneck of poor quantum yield that is normally caused by the rapid recombination of photogenerated electrons and holes. Among the ways to settle this problem,the application of heterojunction materials as photocatalysts is a good method because the internal electrostatic field of heterojunctions can provide a potential driving force to reduce the recombination of photogenerated charge carriers.Consequently,the p type material of BDD was tried combining with the n type material of TiO₂ to form a heterojunction.Higher photocatalytic ability can be expected when this heterojunction is used as a photocatalyst.In this dissertation,some works were carried out as follows:(1)The BDD film was prepared on a titanium sheet in a hot filament chemical vapor deposition(HF CVD)reactor.The characterization of SEM,.Raman spectrum and XRD demonstrated that the BDD film was compact,continuous,and uniform.The crystallites were about 0.5-1.5μm with no evident cracks or extensive pitting.The characteristic peak of diamond in Raman spectrum appeared clearly and the diffraction peaks of diamond(111), (220)and(311)in XRD pattern were observed.Additionally,the results investigated in cyclic voltammetry indicated that the BDD electrode was electrochemically stable with a wide electrochemical potential window and a low voltammetric background current.It was believed that the BDD film was a good electrode material.(2)The amperometric determination of COD was conducted by using a BDD electrode. The optimum pH value and applied potential were 2 and 2.5 V(vs.SCE),respectively.The linear range was 20-9000 mg·L^-1and the detection limit was 7.5 mg·L^-1COD(S/N=3).The performance of the BDD electrode still kept stable after over 400 COD tests during a period of 20 days.Under the optimized testing conditions,the proposed method was successfully employed to determine the COD of synthetic water samples and real samples.As compared with the conventional dichromate method,the amperometric method presented here could be carried out easily and rapidly(＜5 min).Moreover,the electrode material of BDD was environmentally friedly and no toxic chemicals were used in COD tests.Consequently,the problem of secondary pollution could be avoided efficiently.(3)A flow injection analysis(FIA)system for COD measurements was established successfully based on a flow injection apparatus with BDD as an electrode.The optimum applied potential,flow rate of carrier,electrolyte and pH value were 2.8 V(vs.Ag/AgCl),2.5 mL·min^-1,0.25 M Na₂SO₄ and 6.5,respectively.Under the optimized testing conditions,the COD measurements for synthetic water samples and real wastewater samples could be well completed by using the proposed FIA System.The linear range and the detection limit were 2-150 and 1.0 mg·L^-1(S/N=3),respectively.The FIA system still performed stably after over 600 COD tests during a period of 25 days.In addition,the proposed method and the conventional method consisted well with each other in terms of the COD tests for both synthetic and real samples.The application of FIA system could not only shorten the analysis period but also cut down the dosage of reagents.Meanwhile,a better reproducibility could be obtained.Furthermore,the electrode material adopted in the FIA system was environmentally friendly,and no toxic chemicals were used in the process of COD determination,which was undoubtedly beneficial to the avoidance of secondary pollution.More importantly,the COD measurement could be conducted automatically,which would promot the development of an environmentally friendly,in situ,and on-line COD testing system.(4)A TiO₂-BDD heterojunction photocatalyst was fabricated via two phase of CVD processes.That is,this heterojunction material was prepared first by depositing a BDD film on a titanium sheet in a HF CVD reactor,followed by covering a layer of TiO₂ in a metal-organic CVD system.The morphology and structure were characterized by using SEM, TEM,and Raman spectrum and XRD,respectively.The results demonstrated that the composite of TiO₂-BDD appeared to be continuous and uniform,and the crystal phase of TiO₂ was anatase.Besides,theâ… -â…¤plot detected by a semiconductor characterization system indicated that a heterojunction was formed between the BDD substrate and TiO₂.In the measurement of surface photovoltage(SPV),the response of the heterojunction in UV region was stronger than that of TiO₂ deposited on a titanium sheet(TiO₂-Ti),suggesting that the heterojuncti0n could facilitate the separation of photogenerated electrons and holes.An azo dye C.I.reactive yellow 15(RY15)and Cr(â…¥)were selectedto investigate the photocatalytic ability of TiO₂-Ti and TiO₂-BDD heterojunction.The results indicated that the heterojunction photocatalyst exhibited higher photocatalytic ability than TiO₂-Ti in the degradation of systhetic wastewater samples either containing only RY15 or only Cr(â…¥)or both pollutants. This good photocatalytic ability might be benefited from the reduced recombination of photogenerated charge carriers due to the existence of heterojunction.At the same time,a distinct synergic effect was observed in the simultaneous oxidation of RY15 and reduction of Cr(â…¥).This mutual promoting effect could faciliated the efficient degradation of pollutants. Additionally,the effects of pH and gas sparging on photocatalysis were also studied.An acidic condition was beneficial to the photocatalytic oxidation of RY15 and the photocatalytic reduction of Cr(â…¥).Oxygen sparging promoted more or less the reactions of both photocatalytic oxidation and reduction.At last,the possible photocatalytic mechanism was discussed.