Epitaxial Growth And Electrochemical Properties Of Diamond/graphite Composite Nanosheets

In recent years,with the development of material science,the preparation and application of new structure carbon nanomaterials have attracted wide attention.The free-standing 2D carbon nanomaterials with high surface area,conspicuous edge,and interlaced porous morphology,have been applied in various fields,such as electronic devices,optoelectronic devices,biosensors,energy conversion and storage,and so on.Compared with fullerenes,carbon nanotubes,and graphene,the study on the standing and oriented two-dimensional(2D)carbon nanomaterials is not enough.At present,only a few studies have been synthesized the free-standing diamond/graphite composite nanosheets(DGCNSs)with a disordered structure.There are still many problems in the fields of the DGCNSs,such as control on position and orientation,preparation process,and growth mechanism.In this dissertation,the DGCNSs arrays were epitaxially grown on(110),(111),(001),and(113)planes of diamond single crystals substrates by microwave plasma chemical vapor deposition(MPCVD).We revealed the growth mechanism and established a growth process of the DGCNSs arrays.We also grew the DGCNSs on the large area polycrystalline diamond films and boron-doped diamond(BDD)films.The electrochemical biosensing performance of the DGCNSs and the oxygen reduction reaction(ORR)electrocatalytic performance of nitrogen doped DGCNSs(NDGCNSs)were studied.DGCNSs were epitaxially grown on single-crystal diamond substrates by MPCVD.DGCNSs grew following strict rules,forming highly ordered arrays on diamond single crystals.We observed three groups of the DGCNSs on(111)plane and four groups on(110),(001),and(113)planes of diamond substrates.The array structure of the DGCNSs is dependent on the crystal orientation of the applied diamond substrates.The geometrical relationship of the DGCNSs with the(110),(111),(001),and(113)planes of diamond substrates was the same as that of the {111} planes with the(110),(111),(001),and(113)crystal planes in FCC crystal structure,respectively.This result indicates that the DGCNSs are grown by in-plane epitaxy of diamond {111} crystal planes.The effects of the growth temperatures growth time,and CH4 concentration on the structure of the DGCNSs were investigated systematically.It was found that the growth of the DGCNSs arrays occurred in the temperature range of 1085-1115 °C and CH4 concentration range of 7.0-22.2%.The DGCNSs form different inclination angles with the substrates and those with larger inclination angles grew preferentially,forming the parallelogram arrays for the DGCNSs on(110)diamond substrate and parallel-line arrays on(113)diamond substrate.The density,growth rate,and thickness ratio of the graphite layers to the inner diamond layer increases with increasing the CH 4 concentration,while the thickness of the DGCNSs decreases with increasing the CH4 concentration.When the CH4 concentration increases to 20.3%,the average thickness of the DGCNSs decreases to 14 nm.Polycrystalline diamond films were deposited by MPCVD.The effects of the growth temperature and CH4 concentration on the surface morphology and crystal quality of diamond films were investigated.The optimum growth conditions of the diamond films were determined as follows: growth temperature of 900 °C,CH 4 concentration of 2%.We used the polycrystalline diamond films as the substrate for growing the DGCNSs.The effects of the crystal quality of substrate,CH 4 concentration,and negative bias on the structure of DGCNSs were investigated.It was found that the crystal quality of diamond films and negative bias have pronounced effects on the structure of the DGCNSs arrays.The ordered DGCNSs arrays grow on the {111} and {001} facets of grains with a high crystal quality.The selective growth for DGCNSs occurred under a negative bias of-60 V,forming parallel-line arrays on the {001} facets.The structure of the DGCNSs and the interface between the DGCNSs and diamond substrate were characterized by TEM.It was found that the DGCNSs consist of the inner diamond nanosheets and the surface graphitic layers.Both parts of the DGCNSs grow epitaxially from the diamond {111} planes.Finally,we inferred the growth mechanism of the graphite layers and their effets on the growth of the inner diamond.BDD films were prepared by MPCVD using B2O3 as the boron source and ethanol as the carbon source.The effects of the boron source concentration and H2 carrier gas flow rate on the morphology and crystal quality of the BDD films were investigated.The results indicate that the boron source concentration and carrier gas flow rate had a significant influence on the morphology and crystal quality of the BDD films.The BDD films with a high quality and conductivity were deposited at the boron source concentration of 5000 ppm and carrier gas flow rate of 20 sccm.We used the BDD films as the substrate for growing the DGCNSs.The performance of electrochemical biosensors of the DGCNSs for ascorbic acid(AA),dopamine(DA),and uric acid(UA)was investigated.It was found that the DGCNSs grown at higher CH4 concentration show high electrocatalytic activity for AA,DA,and UA.The range of linear detection of the DGCNSs in AA,DA,and UA solutions is from 1 ?M to 1.2 m M.The detection limit in AA,DA,and UA solutions is 0.6,0.3,and 0.6 ?M,respectively.When AA,DA,and UA coexists in solution,DGCNSs are still able to distinguish and detect these molecules.NDGCNSs were prepared by a nitrogen doping process for application as the ORR catalysts for fuel cells.The effects of nitrogen doping process on the structure,nitrogen doping forms,and ORR electrocatalytic activity of the NDGCNSs were investigated.The concentration range of nitrogen atom for NDGCNSs doped in NH3 gas at different temperature is 1.2%-1.92%,and the NDGCNSs doped at 650 °C possess the highest nitrogen concentration.It was found that the nitrogen doping forms changes with the doping temperature and lower temperature favors the formation of the pyridinic nitrogen.Nitrogen doping could effectively improve the ORR electrocatalytic performance of the NDGCNSs,and the NDGCNSs doped at 650 °C in NH3 gas exhibits higher electrocatalytic activity for ORR,which is comparable to the commercial Pt/C.The NDGCNSs also show excellent stability with the relative current at 81.2% after 100 h and good methanol resistance.