The Study Of Effect Of Oxygen-containing Functional Groups On The Doping Effect Of Heteroatomic Doped Grapheme Oxide Quantum Dots

Graphene quantum dots(GQDs),as an important member of the family of carbon nanomaterials,have many excellent properties due to its unique quantum confining effect and boundary effect.The performance of GQDs prepared by different methods is very different.GQDs prepared by oxidizing and cutting large size carbon materials with strong oxidants have a large sp²hybrid carbon plane and many defects,and the edges and defects also contain a large number of oxygen-containing functional groups.GQDs prepared by this method have good water solubility,low biological toxicity and stable and bright photoluminescence(PL)properties.The band gap of quantum dots can be changed by heteroatomic doping,thus the PL of quantum dots can be regulated and the application range of quantum dots can be broadened.The heteroatomic doping is affected by many factors.Except for the doping conditions,it is also affected by the size of the quantum dots,and the degree of defects and the types and contents of oxygen-containing functional groups on the surface of the quantum dots.Graphene oxide quantum dots(GOQDs),which contain a large number of oxygen-containing functional groups on the surface of GQDs prepared by oxidative cracking with a strong oxidant.These oxygen-containing functional groups exist at the surface edges or defects of the quantum dots,which can make the quantum dots disperse well into the aqueous solution and have a great influence on the PL of the quantum dots.The study on the influence of oxygen-containing functional groups on the doping effect in the process of heteroatomic doping of GOQDs will provide experimental and theoretical supports for regulating the performance of GQDs.This paper mainly consists of the following contents:1.The preparation of GOQDs and reduced graphene oxide quantum dots(rGOQDs)Using concentrated nitric acid as oxidant,GOQDs were prepared by oxidative cracking VCX-72 conductive carbon black.In this method,3.5g GOQDs can be prepared at one time,which has bright fluorescence and a PL emission wavelength of 521 nm under a certain wavelength range of light excitation.The oxygen-containing functional groups of GOQDs were regulated by thermochemical reduction method,and the rGOQDs with different content oxygen-containing functional groups can be obtained.The content of oxygen-contaniing functional groups in the quantum dots can be changed by regulated the calcination temperature.Thermochemical reduction can well regulate the oxygen-containing functional groups inside the quantum dots.The results show that the carboxyl group,the main oxygen-containing functional group in the quantum dots,will be decomposed at 375?-450?,while the epoxy group is relatively stable.2.Study on the influence of oxygen-containing functional groups on the doping effect of nitrogen-doped graphene oxide quantum dots(NrGOQDs)The rGOQDs can be obtained by reduced GOQDs by thermochemical reduction method,and then the rGOQDs were doped with nitrogen to study the influence of oxygen-containing functional groups on the nitrogen doping effect of quantum dots.We selected three different methods for nitrogen doping of quantum dots.Firstly,the rGOQDs were doped with nitrogen by photochemical method.Using ammonia asthe nitrogen source,r GOQDS was illuminated by high-pressure mercury lamp under ammonia atmosphere,and the illumination time was set for 5min.The results show that the presence of carboxyl groups in the process of photochemical nitrogen doping can make the quantum dots doped with a large number of amides,and the carboxyl and epoxy groups will promote the incorporation of amino-like nitrogen(N-A).In addition,pyridine-like nitrogen(N-6),pyrrolic-like nitrogen(N-5)and graphitic-like nitrogen(N-Q)are difficult to be doped by photochemical methods for a short illumination time.The photochemical nitrogen doping has little effect on the structure of the quantum dots,and the incorporation of amide nitrogen and(N-A)will cause the red shift of the PL of the quantum dots.Secondly,the rGOQDs were doped with nitrogen by thermochemical method.Using ammonia as the nitrogen source,the quantum dots were heated at 550?in ammonia atmosphere for 1 h.The results show that the process of thermochemical nitrogen doping leads to the increase of structural defects of rGOQDs and the decrease of oxygen content.The quantum dots are mainly doped with N-6 and N-5,while the amide nitrogen and N-A are basically unable to be doped.This is because the temperature is too high in the process of thermochemical doping,the carboxyl group in the quantum dots will decompose,resulting in the inability to produce the amide group,while the N-A is easy to decompose at high temperature.Thirdly,nitrogen doped rGOQDs was carried out by hydrothermal method.With concentrated ammonia water as the nitrogen source,10 ml of concentrated ammonia water was added into a 100 ml PTFE reactor,and rGOQDs was placed in the tripod in the reaction kettle.The reactor were heated at 210?under closed conditions for 8h.The results show that the NrGOQDs have high contents of N-A,N-6 and N-5.Typically,the presence of carboxyl group will lead to the incorporation of amide group in the quantum dots.A small amount of N-Q will be doped by hydrothermal method in the presence of a variety of oxygen-containing functional groups.With the decrease of the carboxyl and oxygen content,amide nitrogen and N-Q decrease.By testing the PL of the quantum dots,it was found that the PL blue shift occurred,which was caused by the incorporation of N-6 and N-5 in the quantum dots.From the above results of the NrGOQDs doped by three different mothods,we can deduce that(i)the incorporation of amide type nitrogen requires the presence of carboxyl group in the quantum dot,and the incorporation of amide type nitrogen and N-A will lead to the PL red shift of the quantum dot;and(ii)the incorporation of(N-6)and(N-5)requires higher temperature,and the presence of oxygen-containing functional groups will promote doping;and(iii),the incorporation of(N-6)and(N-5)will lead to the PL blue shift of quantum dots;(?)when there are many oxygen-containing functional groups in the quantum dot,a small contents of N-Q will be doped into the quantum dot by hydrothermal method.3.Study on the influence of oxygen-containing functional groups on the doping effect of fluorinated graphene oxide quantum dots(F-rGOQDs)The rGOQDs were fluorinated by hydrothermal method.The steps were as follows:50 mg rGOQDs and 400 mg Xe F₂were placed in a PTFE reactor.The reactor was heated at 200?under closed conditions for 24 h,and the sample was vacuum dried for 3 h after the reaction.The results show that the structure of quantum dots does not change obviously during the hydrothermal fluorination process,and F atom is mainly doped in the way of substitution.When the rGOQDs have high content of oxygen,the doping content of F will be low or will be none.This result indicates that it is difficult for F atom to be doped into rGOQDs when the oxygen content is high.When the oxygen content in rGOQDs is low,the doping content of F will be high.The doping of F atom in rGOQDs will lead to the increase of the oxygen content of the quantum dots,and the increased oxygen content mainly exists in the form of stable epoxy and carbonyl groups.C-F bond will cause the blue shift of the PL of the quantum dots.4.Study on the influence of oxygen-containing functional groups on the doping effect of chlorine-doped reduced graphene oxide quantum dots(Cl-rGOQDs)Firstly,chlorine-doped graphene oxide quantum dots(Cl-GOQDs)was carried out by hydrothermal method,and the influence of hydrothermal reaction temperature on chlorine doping was explored.With concentrated hydrochloric acid as the chlorine source,40 m L of concentrated hydrochloric acid was added into the 400 m L Teflon reactor,and 50 mg of GOQDs were placed into the tripodium in the reactor.The samples were heated in the sealed container,and different Cl-GOQDs can be obtained by heating the samples at different temperature.Finally,the obtained samples were dried.The results show that the percentage of chlorine atom in Cl-GOQDs obtained at 230?and 250?is 4.42 at.%and 9.31 at.%,respectively.When the temperature of the hydrothermal reaction is below 230?,the doping content of chlorine atom in the quantum dots is very low or will be none.With the doping of chlorine atoms,the defects of quantum dots will increase significantly.In addition,the PL of the Cl-GOQDs will have a blue shift compared to that of GOQDs,which may be caused by the increase of the defects of the quantum dots and the decrease of oxygen content.Secondly,the rGOQDs were doped with chlorine atoms by hydrothermal method,and the hydrothermal doping temperature was set at 250?.The results show that the doping content of chlorine element is high,doping will lead to more defects in the quantum dots.The incorporation of chlorine atoms is not affected by the content of oxygen-containing functional groups in the quantum dots.