| Carbon nanomaterials(CNMs),including fullerenes,carbon nanotubes,graphene,and new member carbon nanospheres(carbon spheres)and carbon quantum dots(carbon dots),have attracted much attention owing to their unique structures and physiochemical properties.Among them,carbon spheres and carbon dots are featured by simple preparation process,low production cost and rich surface functional groups,and have been widely used in electrochemistry,catalysis,adsorption,medicine and other fields.The biomass hydrothermal method is a green,sustainable and low-cost route for synthesis of CNMs.This low-temperature hydrothermal method uses biomass(e.g.sawdust,leaves and bagasse)or biomass structural units(e.g.sugar,glucose,amino acids,cellulose and starch)as raw materials.The resulting carbon spheres and carbon dots have abundant surface functional groups,which facilitate the subsequent direct application or surface modification.However,the current method for synthesis of CNMs is limited by low monodispersity,large size,and low controllability.The process of synthesizing carbon spheres and carbon dots by the hydrothermal method accords with the laws of nucleation and nuclear growth.The monodispersity of carbon spheres synthesized by this method is low,mainly due to the slow rate of glucose carbonization.This study was aimed to accelerate the nucleation of carbon spheres and carbon dots,forming a rapid continuous burst nucleation and uniform growth,and to effectively improve the monodispersity and size controllability of carbon materials.In the hydrothermal process,the increase of pressure can intensify the degree of water ionization,raise the concentrations of acid/base catalysts in the reaction system,and accelerate the carbonization.Based on the above principle,we designed a universal external pressure-assisted method to synthesize monodisperse carbon spheres and carbon dots.This simple and green method avoided the introduction of impurities and can be carried out at mild pressure(0.5-3.0 MPa)and controllable carbonization rate.Furthermore,the effects of acid/base catalysts and pressure-assisted methods on the carbonization rate of glucose and chitosan and the synthesis of monodisperse carbon spheres and carbon dots were studied.The mechanism of regulating hollow bowl-shaped carbon spheres by external pressure was explored.(1)In the hydrothermal synthesis of monodisperse carbon spheres,the addition of acid(HCl)/alkali(NaOH)catalyst accelerated the glucose carbonization and shortend the nucleation time.The size distribution of carbon spheres synthesized at pH 4 in an acidic system narrowed and decreased,while the carbonization rate slowed down and the size distribution widened when the pH further reduced to 3,which was due to the inhibition of glucose carbonization rate by excessive acids.Under the alkaline system,the synthetic carbon spheres at pH 11 were more homogeneous and the size was also smaller compared with the acidic system,which was because the alkaline system was more favorable for the aldol reaction of cyclic ketone and aromatic ketone/aldehyde.Moreover,the nucleation time was shortened and the carbon sphere uniformity was improved.At the same time,due to the limitation of raw materials,more raw materials were involved in nucleation rather than nuclear growth,so the carbon spheres in the alkaline system were smaller in size.The monodisperse carbon spheres were further carbonized and sulfonated into solid acid catalysts,which were used in the esterification reaction of levulinic acid and n-butanol.The solid acid catalysts exhibited excellent catalytic activity and stability,and the esterification rate reached 93%.The above results verify that the use of acid(HCl)/alkali(NaOH)as a catalyst is an effective way to improve the uniformity of carbon spheres.However,this method will leave some Cl-or Na+ impurities on the surface of carbon spheres,which limits its application in biomedicine and other fields where the required purity of carbon spheres is high.(2)Monodispersed carbon spheres were synthesized by controlling the hydrothermal carbonization rate of glucose under external pressure.The mechanism of the influence of external pressure on the uniformity of carbon spheres and the size reduction was systematically analyzed from the three aspects,including the hydrothermal carbonization rate of glucose,the formation rate of 5-hydroxymethylfurfural(HMF),and the nucleation rate of HMF.The temporal changes of color and pH of the glucose solution showed that pressure can significantly accelerate the hydrothermal carbonization of glucose and a higher pressure led to the faster carbonization.Analysis of the produced amount of HMF implied that pressure can accelerate the HMF generation and increase the product yield.From the temporal changes of color and pH of the HMF solution,it was known that pressure can also quicken the nucleation.Therefore,by introducing an applied pressure,the glucose carbonization can be effectively accelerated and the nucleation time can be shortened,which facilitated the synthesis of monodisperse carbon spheres.Similarly,the above mechanisms can still explain how pressure reduced the size of the carbon spheres and accelerated the HMF nucleation under the external pressure,leading to the rapid and continuous burst of carbon nuclei,and HMF raw materials were more involved in nucleation rather than nuclear growth,so the size of carbon spheres was hard to grow.The traditional hydrothermal carbon spheres(THCSs)and pressure hydrothermal carbon spheres(PHCSs-2.0 MPa)were used for the adsorption of Pb2+ and common paper dyes Mb,RB,and Mo.The results show that PHCSs-2.0 MPa had smaller particle size,more surface functional groups and better adsorption effect than THCSs.The proposed pressure method can effectively improve the uniformity of carbon spheres and avoid the introduction of impurities into acid/base catalysts.This new method has great potential for synthesizing other monodisperse nanoparticles.(3)In the synthesis of bowl-like carbon spheres by the traditional hydrothermal method-soft template method,external pressure was applied to control the morphology,particle size,size distribution and carbon shell thickness of carbon spheres.The formation mechanism of monodisperse carbon spheres was studied,and the problem of size non-uniformity of bowl-like carbon spheres was solved.The pressure mainly affected the size of vesicles,and without pressure,the inhomogeneity of vesicles led to that of carbon spheres.Under the application of 1.0 MPa pressure,the vesicles were compressed in volume,so their particle size was narrowed and reduced,which led to the smaller and more uniform particle size of pressure bowl-like hollow carbon spheres(PBHCSs-1.0 MPa).When the external pressure raised to 2.0 MPa,the excessive pressure caused the fusion and aggregation of vesicles.At the escape stage,the carbon shells cannot maintain the bowl-like morphology and eventually form a larger shriveled balloon.When the vesicles were small,because of the large surface energy,more glucose hydrolysate can be adsorbed to form a carbon shell,so the carbon shell with a smaller size was thicker.PBHCSs-1.0 MPa was carbonized and used in electrode materials and dye adsorption.PBHCSs-1.0 MPa exhibited excellent electrochemical properties,which was because its unique window-opening structure can promote ion transport and its ultra-thin bowl-like walls can reduce the length of ion diffusion.The adsorption of Mb was dominated by the physical adsorption of the pores,and the adsorption effect was remarkable.The pressure was successfully extended to the hydrothermal method-soft template method for the synthesis of bowl-like carbon spheres,which provided a broader research idea for pressure hydrothermal synthesis of nanomaterials with more types and shapes in the future.(4)To solve the size non-uniformity of chitosan-derived carbon dots that were synthesized by the traditional hydrothermal method,external pressure was used into the preparation of carbon dots.The key mechanism of pressure during the carbon dot synthesis was discussed from two aspects:the effects of pressure on carbon dot size and fluorescence intensity.It was found pressure can significantly improve the uniformity of carbon dots and reduce particle size.The particle size and size distribution of carbon dots both minimized at the pressure of 3.0 MPa.When the pressure further increased to 4.0 MPa,agglomeration occurred between carbon points under high pressure,resulting in larger size and wider particle size distribution.External pressure can also significantly intensify the fluorescence of carbon dots.For example,the quantum yield of pressure carbon dots(PCDs-3.0 MPa)was 26.7%compared to only 12.5%for traditional carbon dots(TCDs).This was because the carbonization of chitosan can be accelerated under pressure,so that more smaller and more-uniform carbon dots were produced in shorter time.The reduction in the size of carbon dots enlarged the specific surface area and increased the number of functional groups(e.g.amino groups).Also the presence of more amino groups introduced more defects on the surface of carbon dots,resulting in the enhanced fluorescence intensity.In the optical response to pH,the fluorescence intensity was strongly sensitive to pH,as the fluorescence intensity maximized at pH 3 and minimized at pH 11.The carbon dots were used in the optical response to temperature,and its fluorescence intensity decreased with the temperature rise.Moreover,the fluorescence intensity changed in an approximately linear way between 10 and 50℃,which was expected to be used for human body temperature sensors.which is expected to be used for human body temperature sensors.The carbon dots fluorescent inks were used in paper-base materials,almost no writing in daylight and strong blue fluorescence under ultraviolet light,and its specificity can be used in anti-counterfeiting.The excellent performance of pressure in the synthesis of carbon dots further proves that pressure is also suitable for preparation of uniform carbon dots,indicating the new method is universal for synthesis of monodisperse CNMs. |
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