Controllable Synthesis And Performance Study Of Diamond Via Hydrogen Etching Graphite

Graphite reserve in our country is huge,accounting for about 70%of the world.However,the value of graphite is low due to the large reserves.How to transform graphite into products with higher value has recently been a focus issue.As known,diamond and graphite are both elementary carbon materials.While diamond has a relatively high price and commercial value,due to the scarce natural reserves,excellent performance,and wide range of uses.Therefore,instead of exporting abundant graphite raw material at low prices,it is better to use cheap graphite materials to prepare diamond materials,which not only enhances the value of graphite raw material,but also increases the output of diamond material.The commonly used methods to produce diamond with graphite are high pressure and high temperature method and detonation method.There are some drawbacks for growing diamond in the above methods.In recent years,a popular method for preparing diamond is chemical vapor deposition（CVD）,whose condition is gentle with low pressure,and environmentally-friendly.The traditional CVD carbon source is methane in hydrogen plasma.While in this paper,based on CVD method,the solid carbon source is used.Different from the HPHT,detonation method and traditional CVD method,this method is called hydrogen etching graphite to convert diamond.The method can prepare many kinds of diamond materials,including diamond film,particles and boron-doped diamond material by controlling source,substate types and nucleated surface.The mechanism is explained by the theory of non-equilibrium thermodynamics and opical emission was detected during diamond growth.When using silicon wafers with seeded surface pretreatment as the substrate,diamond films can be prepared,with growth rate of 5μm/h.The emission spectrum of the plasma was monitored during the growth process.There is a variety of hydrocarbon radicals,such as H_α,H_β,H_γ,H₂,and C₂ group and CH group in the plasma.Among them,C₂ and CH group are precursors for diamond growth.When graphite plates are used as the carbon source,the concentration of C₂ groups and CH groups will decrease as the growth time increases.The use of graphite powder instead of graphite plates as a carbon source can solve the problem of the reduction of hydrocarbon radicals.The process is mainly composed of four steps:the generation of microwave-excited hydrogen plasma,the formation of hydrogen plasma etching graphite to form hydrocarbon radicals,the formation of diamond nuclei,and the growth of diamond nuclei.Hydrogen plasma etching graphite to form hydrocarbon radicals is a key step.Both the hydrogen ions and hydrogen atoms in the hydrogen plasma will bombard the carbon atoms on the graphite surface and destroy the carbon-carbon bonds through adsorption,thereby detaching the carbon atoms.The graphite surface combines with hydrogen to form hydrocarbon radicals.When graphite is used as the carbon source and the substrate,the diamond powders can be prepared.The diamond powders are easily separated from the surface of the substrate,which is convenient for collection.The aggregated diamond particles can be dispersed by mechanical method,which is favorable for analyzing their particle size distribution.With the increase of the hydrogen flow rate,the diamond powders show a clearer crystal morphology,with the crystal orientation mainly（111）.The diamond purity also increases significantly with the increase of the hydrogen flow rate.Adding nitrogen or argon in hydrogen can obtain diamond powders with nanocrystalline morphology.The addition of oxygen is favorable to improve diamond purity.The contact angle of diamond particles can reach 146.9°,higher than single crystalline diamond（100°）.The diamond particles prepared by this method has better hydrophobicity because of their micro-nano and aggregated structure.On the surface nucleated graphite substrate,mesh chain diamond/graphite materials can be prepared.The hydrophobicity of graphite can be improved from hydrophilic to hydrophobic,and the water contact angle is up to 135.2°.At last,the hydrogen etching graphite method can also be used to prepare boron-doped diamond（BDD）films.A solid boron source and graphite powder are mixed and pressed into tablets as the raw material for preparing boron-doped diamond.During the preparation of BDD,emission lines related to CH groups,C₂ groups and BH compounds were observed in the plasma spectrum.The surface of the prepared BDD is modified with metal nanoparticles（Au,Ni）,and the modified BDD electrode has a good detection effect on the concentration of dopamine and glucose.The detection range of Au/BDD electrode for dopamine is 5-1000μM,the sensitivity is～125μAm M^-1cm^-2,and the detection limit is～0.83μM.The Au/Ni/BDD electrode has a detection range of 0.02-9 m M for glucose.In the glucose concentration range of 0.02-2 m M,the sensitivity is 157.5μAm M^-1cm^-2;in the range of 2-9 m M,the sensitivity is 61.2μAmM^-1cm^-2,and the detection limit is 0.0026 mM.