CVD Growth Of Doped Diamond And Surface Microstructure Preparation And Its Properties

Diamond has excellent structural and chemical stability.Boron doped diamond（BDD）is electrically conductive.Boron doped diamond electrodes have many excellent electrochemical properties and have broad application prospects in the electrochemical field.But at present,the research on the aplication of diamond based fuel cell catalyst is still very little.Catalyst is the most critical part of fuel cells.Traditional catalyst for the fuel cells is Pt.But Pt is too expensive,not suitable for large-scale applications.Nitrogen doped carbon nanomaterials are excellent fuel cell catalysts,which may be used to replace Pt.In this work,boron doped diamond films are prepared by microwave plasma enhanced chemical vapor deposition（MPCVD）and then nitrogen doped carbon nanosheets are epitaxially grown on the films.The composite films obtained are used as fuel cell catalysts.The main work and results are as follows:During the process of BDD preparation the influence of CH₄ concentration,temperature,boron source,boron doping methods,carrier gas flow rate and boron/carbon atom ratio（B/C）on the properties of BDD is studied.The results show that with the increase of the concentration of CH₄,the content of nondiamond carbon increases and the quality of diamond decreases.In the temperature range of 890-930℃ the quality of the obtained diamond is the best,beyond which the content of nondiamond carbon increases.NH₃·BH₃ or B2O3 is used as boron source to prepare the BDD.NH₃·BH₃ or B2O3 is introduced into the reaction chamber by dissolving in ethanol using H₂ as the carrier gas.BDD thin films with high conductivity was obtained by this method.After doping,the resistance of the diamond is reduced from MΩ/mm to 20 Ω/mm.For the epitaxial growth of nitrogen doped carbon nanosheets（NDCNSs）on BDD thin films,N2 and NH₃ are used as nitrogen sources.The influence of temperature,concentration of carbon source,concentration of nitrogen source and nitrogen doping methods on the structure and nitrogen content of the NDCNSs is studied.The results show that the optimum growth temperature range of the carbon nanosheets（CNSs）is 1050-1090℃,beyond which the diamond substrates can not be completely covered by the CNSs.For the in situ nitrogen doping process,if the carbon source concentration is constant,etching rate of the CNSs increase with the increase of the concentration of nitrogen source.When the nitrogen source concentration reaches a certain value,the CNS can not be deposited.By in situ nitrogen doping and post treatment nitrogen doping in plasma,NDCNS can be prepared successfully.Heat treatment in ammonia atmosphere can increase the total content of N in the NDCNSs,and alter the proportion of different N types.The cyclic voltammetry and rotating disk electrode are used to characterize the catalytic activity of the composite films.Samples heat treated in NH₃ atmosphere have the best catalytic activity with an oxygen reduction peak potential of-0.003 V in alkali media.