CDs-Induced Polymorphous CaCO3Mineralization and Formation Mechanism

ZHANG Da-Qin JIA Zhi-Gang LUO Guang-Cheng WANG Hong-Zhou LI Chao-Yong WU Lin CHEN Qi-Hou

(School of Chemistry and Chemical Engineering,Anhui University of technology,Maanshan,Anhui 243002,China)

Abstract:Carbon dots(CDs)with various chemical structures can be obtained under the condition of different heat treat temperatures.The morphology and phase of CaCO3has been successfully controlled by the as-prepared CDs.Peanut-like,dumbbell-like and sphere-like particles were prepared in the presence of CDs.The CDs with changeable chemical structures,such as carboxyl group,aldehyde group,hydroxyl group,were deemed to be responsible for the variation in morphology.The as-obtained samples were characterized by scanning electron microscope(SEM),Fourier transform infrared spectrometer(FTIR),photoluminescence(PL)and X-ray diffraction(XRD).The growth mechanism of CaCO3twin sphere particles was investigated in detail.Ostwald ripening and oriented aggregation mechanisms were proposed to be the drive power of the final morphology formation in the presence of CDs.

Keywords:material science;crystal growth;carbon dot;synthesis method

0 Introduction

Calcium carbonate(CaCO3)with different crystal phases as a common biomineral has been intensively studied due to the wide distribution in nature and its important applications in many fields,such as rubbers,plastics,toothpastes,paints,and papers[1-6].The morphology and polymorph of CaCO3can be controlled by different approaches for various purposes.Inspired by the exquisite biomineralization process in nature,CaCO3has been synthesized using organic additives as the inducers for the past few years[7].Many insoluble matrices and soluble organic molecules as inducer,such as polymer[8-10],bovine carbonic anhydrase[11],casein[12],peptide monolayer[13],silk fibroin[14],imidazo-lium based ionic liquids(ILs)[15]and egg white lysozyme[16]as well as functional organic molecules,have been applied to control morphology,phase and growth pattern of CaCO3particles.The inducer usually contains biomineralization-mediating groups including nucleophilic and hydrogen-bonding acceptor groups to influence the mineral of CaCO3[17].A variety of novel morphologies have been obtained from microspheres,film,hollow particles,microrings to mesocrystals and complex aggregates,etc.

As we all know,carbon dots(CDs)bears hydrophilic carboxyl groups and the abundance of oxygen containing functional groups similar to biomolecules or polymers.Therefore,CDs are highly water-soluble due to the presence of hydrophilic functional groups,such as hydroxyl,carbonyl and carboxylate[18].The abundant hydrophilic functional groups also make CDs highly selective to adsorption of chemicals and metal ions.On the other hand,the growth of CacO3particles is greatly dependent on the functional group of the inducer due to the selective interaction with crystal surface.Therefore,it possible to utilize CDs to control the growth of CaCO3by mineralization process in facile condition.For example,fluorescent CDs and poly(acrylic acid)(PAA)were employed to control the crystallization and assembly of CaCO3[19].

However,CDs prepared at different condition have different chemical structure.We speculate that CDs prepared in different heating temperature as the inducer may have diverse influence for the formation of CaCO3.However,there are few reports about the different structures of CDs on the crystallization and growth process of CaCO3.In this study,we look critically to understand the influence of CDs′structure for the crystal-lization of CaCO3.The different CDs were firstly prepared by controlling the heating treatment temperature.CDs medicate the growing process of CaCO3and result in the formation of variable morphology and phases.A systematic investigation of the influence of CDs on the crystallization of CaCO3and the formation mechanism of the ultimate morphology were performed.

1 Experimental

1.1 Materials

Anhydrous calcium chloride(CaCl2,analytical grade),sodium carbonate(Na2CO3,analytical grade),and citric acid(C6H8O7,analytical grade)were purchased from China National Pharmaceutical Industry Corporation Ltd.All chemicals were used without further purification.All solutions were prepared with double distilled water.

1.2 Synthesis of CDs with different degree of carboxylation

CDs were prepared by an incomplete carbonization reaction of citric acid in air atmosphere.In brief,1 g of citric acid was first directly put into beaker(100 mL)and heated to the certain temperature for 10 h.The heat treatment temperature was set at 120,150,180 and 210℃,respectively.Accordingly,the asprepared CDs were named as CDs-120,CDs-150,CDs-180 and CDs-210.After cooling to room temperature,the as-obtained gel was dissolved in 2 mol·L-1NaOH solution and centrifuged(6 000 r·min-1for 30 min)to remove any aggregates.The pH value of CDs solution was set to 8.

1.3 Preparation of CaCO3particles mediated by CDs

CaCO3particles prepared in the presence of CDs.Firstly,20 mL of 0.1 mol·L-1CaCl2and Na2CO3solution were prepared with deionized water in beaker,respectively.Then,a certain volume of CDs solution was dropped into CaCl2solution under moderate stirring.Subsequently,20 mL of Na2CO3solution was gradually added into CaCl2solution.The mixture was moderately stirred for 2 h in room temperature(25℃)to complete the mineralization.After the reaction was over,the products were collected by filtration,washed with the double distilled water several times and then dried at 100℃in oven.The as-prepared samples are denoted as CaCO3-x-y,wherexandystand for the heat treat temperature of CDs and the volume of CDs solution taken in experiment,respectively.

1.4 Characterization

X-ray diffraction spectra(XRD)were collected on a Bruker X-ray powder diffractometer(advance D8)with CuKαradiation(λ=0.154 056 nm),and the operating voltage and current were 40 kV and 40 mA,with the 2θconfiguration and scanning the range of 10°to 70°range.The morphology and surface microstructure of CaCO3particles with CDs was observed by scanning electron microscope(SEM,Carl Zeiss NTS)at an accel-erating voltage of 5 kV.Fourier transform infrared spectra(FTIR)were obtained using a Nicolet 5700 spectrometer(Thermo Nicolet Corporation,US)in the range of 4 000～500 cm-1using KBr pellets at room temperature.Photoluminescence(PL)was collected on an F2500 spectrofluorophotometer(Shimadzu,Japan).Optical microscope(DM LB2,Leica Microsystems Ltd.,Germany)was used to characterize CaCO3morphology evaluation.

2 Result and discussion

2.1 FTIR characterization and photoluminescence behaviour of CDs

Up to now,the clear chemical structure of CDs is still controversial.Carbohydrates can be employed as precursor to synthesize CDs through the partial dehydration and dehydrogenation.Multistep organic reactions and varied intermediates are involved during the formation process of CDs.Therefore,the reaction conditions are critical for the chemical structures of the asprepared CDs.In our study,CDs with different chemical structures were prepared by heat treatment of citric acid at various temperatures.

TEM image and FTIR was used to characterize the chemical structure of the as-prepared CDs.As shown in Fig.1A,the as-prepared CDs-120 were main-ly spherical particles with the diameter below 10 nm,and a few of CDs were agglomerated in the larger clusters.Fig.1B shows FTIR spectra of the carbon materials of CDs-120,CDs-150,CDs-180 and CDs-210.The band at 3 400 cm-1is attributed to the O-H stretching vibration on the surface of the carbon structures.The signals at 1 206,1 400,and 1 720 cm-1are characteristic of C-O bending,C-H and C=O stretching vibration modes,respectively,suggesting the presence of oxygencontaining groups[20-23].The signal of C=O stretching indicates the presence ofsp2carbons within the CDs core.Several peaks at 750～880 cm-1as shown in Fig.1a represent the polycyclic aromatic hydrocarbons framework[24].It can be seen that some characteristic groups were reserved in the CDs.However,the signal at 1 723 attributed to C=O vibration of carboxylate as well as the signals at 1 206 and 1 400 cm-1became weaker with the increasing heat treat temperature for CDs(Fig.1a),CDs-150(Fig.1b),CDs-180(Fig.1c)and CDs-210(Fig.1d).The signa1 at 1 769 cm-1was attributed to C=O vibration of anhydrides gradually became distinct especially for CDs-210(Fig.1d).Furthermore,almost no obvious signal at 3 400 cm-1attributed to the O-H stretching vibration for CDs-210 can be observed(Fig.1d).These results indicate that the chemical structures of CDs undergo a notable variation with the increasing heat treat temperature.

Fig.1 TEM image of CDs-120 and FTIR spectra of of CDs-120(a),CDs-150(b),CDs-180(c)and CDs-210(d)

The internal composition and the structure of CDs control the photoluminescence behaviour.The fluorescence emission spectra of the CDs samples,which were excited by a single-wavelength of 400 nm,are shown in Fig.2.Aqueous solutions of CDs-120 exhibited blue emission centered at 440 nm(Fig.2a),CDs-150 exhibited green emission centered at 480 nm(Fig.2b),CDs-180 exhibited yellow emission centered at 520 nm(Fig.2c),and CDs-210 exhibited cyan emission centered at 500 nm(Fig.2d),respectively.The original photoluminescence of CDs depends on the chemical structures.Graphitic conjugated core,molecular fluorophores and the surface defect states related with the preparation condition have great influence on the photoluminescence behavior of CDs.CDs synthesized from citric acid possibility contain various fluorescent mo-lecular structures according to the difference of preparation condition[25].The fluorescence emission spectra of the as-prepared CDs samples at different heat treat temperatures reflect the difference chemical structures.In this study,CDs were synthesized by citric acid by bottom-up method.Citric acid molecules undergo a serious of dehydration and decomposition processes and finally carbonize to be CDs via heating.According to the reference,at lower temperature,small fluorophore molecules are first produced at lower temperature.Fluorophore molecules are gradually consumed with the increasing heating temperature.At the same time,carbon core and polymer clusters begin to be formed and the subsequent growth of core and cluster results in the production of CDs.That is to say,the structure of CDs and the functional group in the surface of CDs have changed with the heat treatment temperature although the chemical structure is not completely clear.

Fig.2 PL spectra of CDs-120(a),CDs-150(b),CDs-180(c)and CDs-210(d)excited by a single-wavelength of 400 nm

2.2 Effect of heat treat temperature of CDs on morphology of CaCO3particles

To the best of our knowledge,there are few reports about the influence of preparation condition of CDs for mineralization.Whether CDs with different chemical structures have influence on CaCO3growth.The influence of CDs prepared at different temperatures for morphology of CaCO3was investigated.SEM images of the samples and their corresponding magnification images are shown in Fig.3.Peanut-like morphol-ogy of CaCO3-120-9 particles with the length of about 5 μm was formed when CDs-120 was used(Fig.3a),and a small of pore structure can be seen in the surface of peanut-like morphology.The as-prepared CaCO3-150-9 and CaCO3-180-9 particles with CDs-150 and CDs-180 as the additive has the similar morphology as shown in Fig.3c and 3e,which was the twin-spheres with equatorial girdle.The size became larger with the length of 8～10 μm.When CDs-210 was added,the irregular CaCO3-210-9 particles with coarse surface and some sphere-like particles were obtained(Fig.3g).Fig.3b,3d,3f and 3h are the magnified images of Fig.3a,3c,3e and 3g,respectively.Rhombohedra crystals with step arrangement can be clearly seen from the surface of the asprepared particles,indicating that the resulting products were all built up of numerous small units.

The obtained particles were further characterized by FTIR spectra and XRD patterns as shown in Fig.4.In Fig.4A,the absorption peaks at 875 and 711 cm-1are characteristics of pure calcite[26].Furthermore,the weak shoulder peaks appearing at 1 625 cm-1in all samples can be ascribed to the C=O vibration of CDs,indicating that the CDs were composed into the samples.Obviously,the phase of CaCO3mediated by CDs was all calcite,and the phases of samples were also testified by XRD as shown in Fig.4B.The difference in morphology can be attributed to the changeable structure of CDs prepared in various heat treatment temperatures.

Fig.3 SEM images of CaCO3-120-9(a),CaCO3-150-9(c),CaCO3-180-9(e)and CaCO3-210-9(g)with the corresponding different magnification views(b,d,f,h)

Fig.4 FTIR spectra(A)and XRD patterns(B)of CaCO3-120-9(a),CaCO3-150-9(b),CaCO3-180-9(c)and CaCO3-210-9(d)

2.3 Effect of concentration of CDs-180 solution on morphology of CaCO3twin spheres

The effect of CDs-180 concentration for CaCO3growth was also investigated.As shown in Fig.5,cubeand sphere-like particles(CaCO3-180-3)were formed when the added volume of CDs-180 solution was 3 mL(Fig.5a).Sphere-like and twins-like particle(CaCO3-180-6)emerge while cube-like particles disappeared when 6 mL of CDs-180 solution was used(Fig.5b).With the increasing volume of CDs-180 solution(9 mL),more and more twin spheres particles(CaCO3-180-9)were produced and the ratio of sphere-like particle in the sample decreased correspondingly(Fig.5c).When 12 mL of CDs-180 solution was added in the reaction system,the product(CaCO3-180-12)was even twin sphere particles with clear equatorial girdle(Fig.5d).Moreover,the as-obtained twin sphere particles were all built by primary unit of CaCO3.

Fig.5 SEM images of CaCO3-180-3,CaCO3-180-6,CaCO3-180-9 and CaCO3-180-12

2.4 Morphology evolution of CaCO3twin spheres with reaction time in the presence of CDs-180

In order to clarify the formation process of the CaCO3twin spheres particles,the products at different growth periods were collected.As shown in Fig.6,fine particles were obtained at 2 min of the reaction(Fig.6a).With the increasing reaction time(10 min),sphere-like particles were formed besides of fine particles(Fig.6b).When the reaction time reached at 30 min,sphere-like particles and twin spheres particle were obtained(Fig.6c).The fine particle appeared at the initial stage completely disappeared.The final products were twin spheres particle of sphere after the reaction course of 90 min(Fig.6d).Moreover,the final twin spheres particles were constructed by primary units,which can be seen from the tough surface of particles of SEM images by careful observation(inset).

Fig.6 SEM images of CaCO3-180-9 at different growth periods

The phase obtained at different period was also analysized by FTIR spectra.The peaks at 875 and 742 cm-1were characteristic of pure vaterite.The absorption peaks at 875 and 711 cm-1are characteristics of pure calcite.As shown in Fig.7,three characteristic vibration bands at 1 475,1 418,and 866 cm-1of sample obtained at initial period confirmed the amorphous state of CaCO3(Fig.7a).The characteristic absorption peaks at 875 and 742 cm-1,and a split peak of 1 490 and 1 440 cm-1indicate the presence of vaterite in the sample obtained at 10 min(Fig.7b).Three peaks at 1 420,875,and 711 cm-1as shown in Fig.7c and 7d confirmed the presence of calcite in samples obtained at 30 and 90 min.Calcite is a typical and stable crystal-line phase,and amorphous,aragonite,and vaterite are metastable phases for CaCO3.FTIR results showed disappearance of the peak at 742 cm-1and the gradual appearance of peak at 713 cm-1with the extension of growth period,which indicated that the most thermodynamically unstable vaterite phase gradually disappeared and a mixture of vaterite and calcite phases formed.The weak shoulder peaks appeared at 1 625 cm-1in the final sample can be ascribed to the C=O vibration of CDs,indicating that CDs are present in the sample of vaterite and calcite.Pure calcite phase can be directly obtained when no CDs was added in the mixture.However,amorphous phase of CaCO3was firstly formed when CDs-180 was added in the reaction solution.It can be concluded that CDs inhibit the direct crystallization of CaCO3although no obvious peak ascribed to CDs can be obtained in the amorphous phase(Fig.7d).Furthermore,CDs participate in the phase transformation of CaCO3during the subsequent growth process.

Fig.7 FTIR spectra of CaCO3-180-9 at different growth periods

2.5 Live observation of CaCO3twin spheres with reaction time

Optical microscope was also used to record the formation of dumbbell-like CaCO3particles through the whole crystallization process.Fig.8 presents the light microscope images of CaCO3particles in the four different stages.Plenty of flocculent and fine particles were produced at the initial stage(Fig.8a).With the progression of the crystallization reaction,flocculent and fine particles gradually disappeared and twin spheres particles appeared in the visual field(Fig.8b).In the third stage,some dumbbell-like particles occur besides of sphere-like particles observed at the second stage(Fig.8c).In the last stage,the products were all twin spheres particles and almost no sphere-like particles can be observed.Furthermore,the final twin spheres particles show bright birefringence when light passes through object stage,implying that the crystals have a highly crystalline structure(Fig.8d).Few free CaCO3crystals were observed,indicating the nucleation and growth of crystal was largely mediated by CDs-180.

According to the above-mentioned results and analysis,the possible crystallization process was proposed.A possible growth process is illustrated as Scheme 1.Amorphous phase of CaCO3firstly appeared in the presence of CDs-180.Then,sphere-like vaterite phase formed with the consumption of the unstable state of amorphous phase.Subsequently,vaterite phase transform to calcite phase due to the unstable state of mesophase.The initially formed calcite with typical cu-bic shape supplies as seed or basement for subsequent growth of cubic calcite.The firstly formed calcite may serve as seed to the later crystallization of calcite.The edges and corners of cubic calcite are the preferential position for the growth of the late crystal in view of the demand of lower system energy.Thus,this growth mode results in the formation of dumbbell-like morphology.

Fig.8 Optical microscope images of CaCO3-180-9 at different growth periods

Scheme 1 Graphic presentation of the formation mechanism of calcium carbonate twin spheres

2.6 Growth mechanism of CaCO3twin spheres mediated by CDs-180

Ostwald ripening and oriented attachments are two kind of typical formation mechanism for crystal growth in solution[27].The former is characterized by the growth of bigger ones at the expense of the smaller crystals that formed at the initial stage and is in the metastable state in view of energy or phase.The smaller crystals are consumed and disappeare completely,and the bigger crystals appear in stable state.The latter is characterized by the growth of superstructure depending on aggregation of primary crystals.The primary crystals as basic unit exist in the superstructure to form ploycrystalline.The amorphous state of CaCO3,vaterite and calcite in solution display gradually stable state in turn.In our study,amorphous state of CaCO3is firstly formed and transforms metastable vaterite due to the unstable property and high subsequently.Vaterite subsequently continue to transforming calcite with the extention of reaction.From the SEM images,it can be seen that no primary particles can be observed for amorphous state and various.However,dumbbell-like CaCO3are constructed by primary particles.Therefore,we can conclude that the crystallization and growth of CaCO3particles in the presence of CDs-180 are driven by both Ostwald ripening and oriented aggregation mechanisms.The transformation of amorphous state to various is driven by Ostwald ripening.The transformation of vaterite to thermodynamically stable calcite is driven by oriented aggregation mechanism.CDs-180 play an important role during for the oriented aggregation of primary particles into ordered structures by capping on the surface of the primary particles.

2.7 Fluorescence property of CaCO3twin spheres

According to the above analysis,CDs not only mediate the crystallization of CaCO3and control the morphology of twin spheres,but also include into the CaCO3twin spheres.In view of remarkable fluorescent properties of CDs,one can imagine that the as-prepared twin spheres had photoluminescence behaviour upon irradiation with the special excitation wavelength.As shown in Fig.9,the fluorescent emission spectrum of CaCO3twin spheres presented a broad peak with the maximum at 500 nm upon excited at 400 nm and a blue shift(10 nm)compared with that of CDs-180,suggesting the interaction of CDs and CaCO3crystal.CaCO3twin spheres with fluorescent property can be anticipated in various filed,such as drug carrier[28].

3 Conclusions

In summary,CDs were employed to induce the mineralization of CaCO3and dumbbell-like particles were obtained.The heat treat temperature plays an important role for the formation of morphology.Various morphologies,such as peanut-like particle,dumbbell-like particle and sphere particle with superstructure were produced according to the change of CDs′inherent structure due to the different preparation temperature.Furthermore,the formation mechanism of dumbbell-like particle was investigated in detail.Ostwald ripening and oriented aggregation mechanisms drive the formation of dumbbell-like morphology in the presence of CDs-180,and possible growth process was suggested.The study supplies a new thinking that the morphology synthesis of inorganic particle can be controlled by CDs treated at different heat treat temperatures.The further research for other mineral particles is underway in our group.

Acknowledgments:This work is financially supported by National Natural Science Foundation of China (Grant No.20907001),University Natural Science Research Project of Anhui Province,China(Grant No.KJ2010A336),Student Research Training Program of Anhui University of Technology(Grants No.201810360208,201710360200).