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Synthesisofcarbonxerogelparticlesandfractal-likestructures

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Chemical Engineering Science 64 (2009) 1536 -- 1543

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Chemical Engineering Science

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Synthesis of carbon xerogel particles and fractal-like structures

Chandra S. Sharmaa, Manish M. Kulkarnia, Ashutosh Sharmaa,∗, Marc Madoub

aDepartment of Chemical Engineering and DST Unit on Nanosciences, Indian Institute of Technology, Kanpur 208016, UP, IndiabDepartment of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697, USA

A R T I C L E I N F O A B S T R A C T

Article history:Received 1 September 2008Received in revised form 12 December 2008Accepted 14 December 2008Available online 25 December 2008

Keywords:Carbon xerogelParticleFractalEmulsionSurfactantMorphology

A variety of dense and open-architecture amorphous carbon xerogel microspheres and folded fractal-like structures were synthesized by sol–gel polycondensation of resorcinol with formaldehyde ina slightly alkaline aqueous solution. Carbon structures were obtained by inverse emulsification ofresorcinol–formaldehyde (RF) sol in cyclohexane containing a non-ionic surfactant Span-80, followed byits pyrolysis at 1173K in nitrogen. We have investigated the effects of synthesis parameters includingstirring time, resorcinol/catalyst (R/C) ratio and surfactant concentration on the structures. The averageparticle size of the carbon microspheres could be modulated from 5 to 46�m by increasing the stirringtime from 2 to 7h and by varying the R/C ratio from 0.2 to 500. Particles agglomerated as the R/C ratioincreases above 100. Increase in the surfactant concentration from 1% to 4% (v/v) produced smallerspherical particles with narrower size distribution. Further increase in the surfactant concentration from10% to 50% (v/v) produced branched and folded fractal-like structures of large external area. Thus, RFsol-based precursor chemistry can be easily tuned to produce a spectrum of desired carbon particlemorphologies with potential applications in printing technology, adsorbents, resonance-based solar cells,thermal detectors and carbon-based micro-electromechanical devices (C-MEMS).

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Synthesis and characterization of carbon microspheres and otherstructures of carbon have been of interest in recent years by manyresearchers (Al-Muhtaseb and Ritter, 2003; Horikawa et al., 2004;Job et al., 2004, 2006; Kim et al., 2006; Matos et al., 2006; Pekala,1989; Pekala and Alviso, 1992; Pekala et al., 1994, 1998; Pol et al.,2004; Shen et al., 2005, 2006; Tamon and Ishizaka, 1998; Tonanonet al., 2003; Xu et al., 2005; Yamamoto et al., 2002), especially in thecontext of carbon-basedmicro-electromechanical devices (C-MEMS).The patterned as well as non-patterned carbon electrodes in differ-ent forms (graphitic/non-graphitic) are used extensively in recharge-able batteries and fuel cells (Hasegawa et al., 2004; Park et al., 2007;Pekala et al., 1994, 1998;Wang et al., 2006). There aremany other po-tential applications of engineered carbon structures yet to be fully ex-plored. One example is the use of spherical carbon particles for forcemeasurement by attaching it to the cantilever tip of an atomic forcemicroscope (AFM). The understanding of the interaction between theparticles and surfaces by the direct measurement of force is an use-ful method to study the effects of surfactant adsorption in variouscontext like particle detachment, deinking of film from paper, etc.

∗ Corresponding author. Tel.: +915122597026; fax: +915122590104.E-mail address: ashutos@iitk.ac.in (A. Sharma).

0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2008.12.013

In addition to its passive use as a probe particle on AFM tip, it canalso be used actively by using its electrical conductivity for scanningtunneling microscopy.

Although new precursors and new solvents have been proposedmore recently (Pol et al., 2004; Shen et al., 2006; Xu et al., 2005),most of the organic gels are still prepared by sol–gel polyconden-sation of resorcinol (1,3-dihydroxybenzene) with formaldehyde asproposed initially by Pekala (1989). These gels find applications aselectrode materials for electric double-layer capacitors, rechargeablebatteries, adsorbents, etc. The increasing popularity of these gels ismainly due to the low-cost of the process and their unique and con-trollable properties. The sol–gel process allows better control of thecomposition, homogeneity and structural properties of the resultingmaterials.

After sol–gel polymerization, the gel can be dried in three differ-ent ways before its carbonization. The original process of producingthe resorcinol–formaldehyde (RF) aerogels by Pekala (1989) is rathertime consuming and expensive because of the use of supercriti-cal drying. A number of investigators, e.g., Al-Muhtaseb and Ritter(2003), Kim et al. (2006), Tonanon et al. (2003), Yamamoto et al.(2002) envisaged freeze drying which leads to the formation of RFcryogels as an alternative to supercritical drying. However, the poretexture of cryogels is often heterogeneous and production of mono-lithic structures is difficult. It may be noted that aerogel and cryogelcarbon particles are highly porous and nearly spherical in shape as

C.S. Sharma et al. / Chemical Engineering Science 64 (2009) 1536 -- 1543 1537

reported by most of the researchers, Al-Muhtaseb and Ritter (2003),Horikawa et al. (2004), Kim et al. (2006), Pekala (1989), Pekala andAlviso (1992), Pekala et al. (1994, 1998), Tamon and Ishizaka (1998),Tonanon et al. (2003), Yamamoto et al. (2002). Particle morpholo-gies with high external surface areas such as dendritic and fractal-like structures have not been obtained. This may have been partlybecause of relatively low surfactant concentrations employed in theinverse emulsification step prior to drying.

In contrast to highly porous aerogels and cryogels, very few stud-ies have been conducted on organic xerogels that are made by re-moving the solvent from the gel structure by drying conventionallywith nitrogen or air under normal conditions (Al-Muhtaseb andRitter, 2003). The capillary pressure of the liquid inside a pore leadsto the collapse of the gel-network during oven drying, which re-sults in a dense polymeric structure called xerogel (Al-Muhtaseb andRitter, 2003; Pekala and Alviso, 1992). This structure is carbonized inan inert atmosphere (pyrolysis) to produce the corresponding car-bogel. Recently, Job et al. (2004, 2006) and Matos et al. (2006) havereported the synthesis of the porous carbon xerogels also.

The aim of the present study is to produce nearly non-porous RFcarbon xerogel particles of varying morphologies from spherical tofractal-like structures of high external surface area. Towards this end,we devise several control strategies for the modulation of particleshape and size. In particular, we varied the surfactant concentrationand the catalyst concentration over a wider range compared to theprevious studies on aerogels and cryogels.

Micro-particles of RF xerogel were synthesized by inverse emul-sion polymerization followed by air-drying. Carbon micro-particleswere obtained by pyrolysis of the RF xerogel structures at a hightemperature in an inert atmosphere. The nature of C–C bondswas established from its Raman spectra, while the shape and mi-crostructure of the carbon particles were studied using scanningelectron microscopy (SEM). The surface area was measured usingBrunauer–Emmett–Teller (BET) nitrogen adsorption–desorption.We investigated the effect of stirring time of RF sol which alsocontrols the apparent viscosity of the solution during the inverseemulsion polymerization. The effect of alkaline catalyst used inthe RF polycondensation reaction on the final material was alsostudied. Finally, we demonstrate that by changing the surfactantconcentration, the shape of the carbon particles can be tailored fromdense spherical to open fractal-like structures. This method is alsosuitable for the production of nearly mono-dispersed, multi-scalecarbon particles and fractal-like structures over large areas or in thebulk form.

Here, it is worthwhile to note that fractals are disordered systemswhose disorder can be described in terms of non-integral dimension(Pfeifer and Obert, 1989). The fractal structures obtained in this studyare random in nature and self similar at certain length scales. Wehave calculated fractal dimension of these structures as a measureof their surface irregularities.

2. Experimental

2.1. Synthesis of spherical RF xerogel particles

RF hydrogels were synthesized by the polycondensation of re-sorcinol (99% purity) with formaldehyde (37% w/v; stabilized by11–14wt% methanol) in water (W), in the presence of a basic cata-lyst. Potassium carbonate (99.0% purity) was used as the basic cat-alyst in this study. All these chemicals were obtained from threedifferent sources (Sigma Aldrich, USA; Qualigens Fine Chemicals, In-dia and Loba Chemie, India) to ensure the reproducibility of thecarbon structures and to exclude the possibility of artifacts. It wasfound that the chemicals obtained from the different sources notedabove did not produce any variability in the results. The typical

results shown here are for the chemicals obtained from QualigensFine Chemicals and Loba Chemie.

Resorcinol was added to formaldehyde and the mixture wasstirred for 15min to get a clear solution. Potassium carbonate wasdissolved in ultra pure milli-Q water to form a separate solution.The two solutions were then mixed and stirred continuously for30min until the colorless RF sol changed to golden yellow. Theresorcinol to formaldehyde molar ratio was 0.50, and resorcinolto water molar ratio was 0.037. It is important to note here thatreported resorcinol to water molar ratio does not take into accountthe water present in the formaldehyde solution. Various resorci-nol/catalyst (R/C) molar ratios of 0.2, 1.0, 10, 25, 50, 100 and 500were chosen to study the effect of catalyst on the size and shapeof particles. Spherical RF hydrogel particles were obtained by in-verse emulsion polymerization route. A viscous RF sol (1ml) wasadded slowly to 50ml of cyclohexane (high performance liquidchromatography grade) and agitated in the presence of a surfactant,non-ionic sorbitan monooleate, Span-80. The same surfactant witha hydrophile–lipophile balance (HLB) of nearly 4.3 was obtainedfrom three different sources (Sigma Aldrich, USA; SD Fine Chemi-cals, India; and Loba Chemie, India) to establish the robustness ofthe results. The results shown here are for the surfactant obtainedfrom Loba Chemie, unless otherwise noted.

The RF solution was dispersed as spherical droplets throughoutthe cyclohexane. The emulsion droplets assumed different shapesand sizes depending on the stirring time and the amount of sur-factant. Surfactant concentration (v/v; vol. of surfactant/vol. of totalsolution) was varied from 1% to 50%. The suspension was stirred atroom temperature. Stirring time of the suspension was varied from1 to 24h to investigate its effect on the particle size distribution offinal product. Particle size could be measured by rapidly (5–30min)desiccating the RF hydrogel particle suspension on quartz sub-strates to partially evaporate the solvent and arrest the movementof particles. Subsequently, samples were dried in subcritical condi-tions by heating in oven at 333K for 12h to obtain the RF xerogelparticles.

2.2. Pyrolysis of RF xerogel particles

After drying, the quartz substrates with the RF xerogel particleswere placed in a quartz boat and heated to 1173K under inert ni-trogen (N2) atmosphere in a tubular, high temperature furnace forcarbonization of the polymer. The rate of heating was programmedat 7.5K/min, while the N2 gas flow was kept constant at 0.3 l/min.Once the maximum temperature was reached, it was kept constantfor 60min. The furnace was then cooled to room temperature inabout 10h to obtain RF derived carbon xerogel particles. The inertatmosphere was maintained by purging N2 gas until the furnace at-tained the room temperature.

2.3. Characterization of carbonized RF xerogel particles

A confocal micro-Raman microscope (CRM 200, WiTec, Germanywith � = 543nm) was used to record the Raman spectra of the py-rolyzed RF xerogel particles. This technique allowed recording of theRaman spectra of individual particles to characterize the types ofbonds between the elements constituting the material. Specific sur-face area of the RF carbon xerogel particles was calculated by theBET from the adsorption isotherms of nitrogen. Samples were outgassed for 1h at 393K. The pore size distribution was estimated byapplying the Barrett–Joiner–Halenda (BJH) to the desorption branchof the isotherms.

The surface roughness, size and shape of the RF carbon xero-gel particles were studied using SEM (Quanta 200, FEI, Germany).All samples were first sputter coated with a thin layer of Au–Pd to

1538 C.S. Sharma et al. / Chemical Engineering Science 64 (2009) 1536 -- 1543

reduce the surface charging of the non-conducting quartz substratesduring electron beam scanning. In addition, the particles were alsoimaged using Samsung SDC 4304PA camera fitted to Leica opticalmicroscope at different magnifications. These images were then ana-lyzed using Leica-QWIN software (QWIN_32) to measure the particlesize distribution (PSD). Fractal dimension was measured by two dif-ferent implementations of the box counting method (Fractal Analy-sis System of NARO, Japan and Fractalyse of ThéMA Research Centre,France).

3. Results and discussion

Micro-Raman spectroscopy was used to ensure that pyrolysis ofthe RF-particles completely carbonizes the material. A typical Ra-man spectrum of the pyrolyzed particles (Fig. 1) shows two broadpeaks centered at about 1340 and 1590 cm−1, which are associatedwith the vibrations of sp2 carbon atoms with dangling bonds. Theintensity ratio of D to G band is calculated to be 0.8, which indicatesan amorphous carbon structure with a high content of lattice edgesand plane defects (Pol et al., 2004).

BET adsorption isotherm of nitrogen was used to measure thespecific surface area of the RF carbon xerogel particles. For exam-ple, the specific surface area was found to be 11.91 ± 0.48m2/g inthe case of R/C = 25, surfactant concentration of 4% (v/v) and stirringtime of 5h. This is about two orders of magnitude smaller than thesurface areas obtained for highly porous RF carbon aerogels, cryogelsand xerogels. Thus, the RF-based carbon xerogel microspheres pro-duced in our work are rather dense and non-porous. The pore vol-ume was measured to be 0.02 cm3/g, which is also significantly lessthan the reported values for RF carbon aerogel, cryogels and recentlyreported porous xerogel. Nitrogen adsorption isotherm results ruleout the possibility of any macro or meso pores in these structures.However, there may be few micropores which are undetectable bynitrogen.

Thus, the porosity and surface areas of the particles synthesizedhere are markedly different from RF gel derived carbon aerogels,cryogels and xerogels as well, studied earlier (Al-Muhtaseb andRitter, 2003; Horikawa et al., 2004; Job et al., 2004, 2006; Kim et al.,2006; Matos et al., 2006; Pekala, 1989; Pekala and Alviso, 1992;Pekala et al., 1994, 1998; Shen et al., 2005; Tamon and Ishizaka,1998; Tonanon et al., 2003; Yamamoto et al., 2002). The additionof more amount of surfactant as discussed later also does not seemto have a significant change on either specific surface area or porevolume which is also observed by Matos et al. (2006).

Fig. 1. Raman spectrum of RF-based carbon xerogel particles.

3.1. Effect of stirring time

The optical micrographs in Fig. 2 show the variation in the sizeof RF carbon xerogel particles as a function of the stirring time of RFsol emulsion in cyclohexane. As the emulsion stirring time increasedfrom 2 to 7h, the mean diameter of the RF carbon xerogel particlesincreased from 5 to 46�m. However, there is no apparent changein the spherical shape of the particles. For a stirring time of 2h,(Fig. 2a), most of the particles have diameters in the range of 5–15�mand form very compact clusters. This could be because of the incom-plete gelation of the RF sol and its subsequent stiffening during the2h stirring time, which leaves the surface soft with reactive groupsthat promote permanent sticking of particles. With increasing stir-ring time, the number density of the particles in emulsion decreasedand their size increased. This is shown in the representative figures:Fig. 2b where particle size ranges from 10 to 35�m and Fig. 2c showparticles of 12 to 46�m diameter.

The observed increase in particle diameter with increased stir-ring time may be understood as follows. Immediately after additionof RF sol to the cylcohexane–surfactant mixture, the viscosity of thesol is very low at the beginning of its polymerization. The drop-breakage occurs by the action of viscous shear forces and turbulentpressure fluctuations. On the other hand, drop-coalescence of ran-domly colliding droplets is promoted by the inter-droplet turbulentand squeezing forces such as the capillary pressure and the van derWaals attractive force (Gabler et al., 2004; Horikawa et al., 2004;Narsimhan, 2004). Initially, the low viscosity of the dispersed phasepromotes more drop breakage resulting in smaller diameter parti-cles. With increased stirring time, drop-coalescences become moresignificant and thus larger diameter particles are formed. The vis-cosity of the sol also gets increased with time in the gelation pro-cess as studied by Horikawa et al. (2004). The growth of particlesshould however become sluggish with increased viscosity and nofurther change in their shape and size is expected once gelation setsin (5–7h).

3.2. Effect of catalyst concentration

Potassium carbonate was used in this study as a basic catalystfor the polymerization reaction, which occurs by the catalyst initi-ated ionization of resorcinol and addition reaction of resorcinol withformaldehyde. This polymerization of resorcinol leads to formationof small clusters of cross linked polymer which subsequently aggre-gate and link together to complete the gelation.

Al-Muhtaseb and Ritter (2003), Pekala (1989), Pekala and Alviso(1992), Pekala et al. (1994, 1998) and Shen et al. (2005) reportedearlier that the R/C ratio plays a major a role in deciding the sizeand porosity of the carbon aerogels and cryogels. Job et al. (2004)have linked this ratio with another parameter, pH of the solution andstudied the effect of pH on the porous texture of xerogels. Here, it isto be noted that the role of the catalyst is usually to adjust the pH ofthe solution only. But, as pH changes continuously throughout theexperiment, it is more applicable to use the R/C ratio as a controllingparameter.

One of the aims of our study was to optimize the synthe-sis conditions to obtain nearly mono-disperse, spherical densecarbon xerogel particles in the range of 1–50�m diameter thatare required for different applications. For example, non-porousspherical particles of 5–20�m diameter are especially suitable toprepare the colloidal probes for the study of interaction of car-bon with a variety of materials by attaching them to AFM can-tilevers. The colloidal probes thus prepared provide control overthe exact nature, shape and size of the tips which is not possi-ble with regular cantilevers that have pyramidal tips for imagingpurposes.

C.S. Sharma et al. / Chemical Engineering Science 64 (2009) 1536 -- 1543 1539

Fig. 2. Optical micrographs of the RF-based carbon micro-spheres obtained at R/C = 25, 1% (v/v) Span-80 concentration with different stirring times: (a) 2h; (b) 5h; (c) 7h.

Table 1Characteristic sizes of RF carbon xerogel particles synthesized at different R/C ratioswith 5h stirring time.

R/C ratio Mean particle diameter (�m) Standard deviation

0.2 0.9 0.11 3.8 1.125 18.1 6.8100 57.9 19.7

Towards this end of size control, we varied the R/C molar ratiofrom 0.2 to 500. Its effect on the particle size is summarized inTable 1.

At very low R/C molar ratio (high catalyst concentration), smallparticles in the range of few hundred nanometers to few micronswere obtained. For R/C = 0.2, mean diameter of the particles wasaround 900nm, which increased to 18�m for R/C ratio of 25. More-over, the mean size of the particles can be further increased to 58�mby increasing the R/C ratio from 25 to 100. However, it was observedthat the shape of the particles does not remain exactly spherical atmuch higher values of R/C ratios (R/C > 100). The time of stirring waskept constant at 5h to observe the effect of catalyst concentration,unless specified otherwise. The lower R/C ratio limit (0.2) is relatedto the saturation value of potassium carbonate in water.

A large value of the catalyst ratio R/C means the lower amountof catalyst which results in a longer gelation time. At larger valueof R/C ratio, there would be fewer nucleation sites in the sol for thegrowth of clusters because of the lower amount of catalyst. Thus,these clusters will form less branched network andwill persist longerin the nucleation regime (Job et al., 2004). It leads to formationof bigger particle sizes at lower catalyst concentrations. Also, thegelation time increases at low catalyst concentration, thus givingmore opportunity for droplet coalescence. At higher catalyst loading,the individual clusters will be highly branched with increased crosslinking and thus will be less stable. Thus, these clusters have verylittle time to grow and coalesce before the onset of gelation, whichfreezes the droplet shape and size resulting in smaller particles.

It is worthwhile to note that unlike previous studies (Job et al.,2004, 2006; Pekala, 1989; Pekala and Alviso, 1992; Pekala et al., 1994,1998; Shen et al., 2006), the effect of R/C ratio on porosity is in-significant. This is in contrast to the effect of R/C ratio in significantlymodulating the porosity of aerogels and cryogels. Interestingly, theeffect of R/C ratio in xerogels is to greatly alter the external surfacearea by changing the particle-morphology from spherical to moreopen, fractal-like structures. We thus investigated a wide range ofR/C ratio varying from 0.2 to 500 to characterize its influence on theparticle-morphology in xerogels. It was observed that the tendencyof particles to aggregate increases with stirring time and with theR/C ratio as illustrated in Fig. 3.

For R/C = 10, increased stirring time does not greatly affect theaggregation pattern as the stirring time was increased from 5 to 24h

(Fig. 3a and b). However, at higher R/C ratios it is clearly seen thatthe particles initially lose their identity and finally coalesce to form alayered fractal-like structure as the stirring time was also increasedto 24h (Fig. 3c and d).

The effect of R/C ratio is also observed on the shape of the parti-cles formed as shown in Fig. 4. As the R/C value was increased from25 to 100, the gelation time also increased. The radius of gyrationis directly related to the gelation time (Tamon and Ishizaka, 1998).It increases with the gelation time and then approaches a constantvalue. Thus, it was observed that at a higher value of R/C ratio, distor-tion of particle shape from spherical becomes prominent and tubu-lar shapes are preferred. This has also been supported by the smallangle X-ray scattering data analysis devised by Tamon and Ishizaka(1998) to study the structure of RF solution.

3.3. Effect of surfactant concentration

As discussed above, the morphology of particles and their aggre-gate shapes can be altered by increasing the catalyst concentration.However, it was observed that the viscosity of the RF sol rapidlyincreases at the onset of gelation depending on the catalyst concen-tration. Therefore, it is often difficult to control the size and surfacemorphologies of the resulting carbon structures just by changingthe catalyst concentration and an additional control mechanismwould offer greater flexibility in the engineering of different par-ticle shapes, size and aggregate morphologies. In what follows, wediscuss the results of modulating the interfacial tension and com-position of the continuous phase in the inverse emulsification. Thisis in contrast to the earlier discussion on modification of the dis-persed phase properties by catalyst concentration. The properties ofthe oil–water interface and the continuous phase are best changedby altering the concentration of the non-ionic surfactant (Span 80)used in our study. Span 80 has a low HLB value of 4.3 and is thusdominantly hydrophobic.

The surfactant concentration in cyclohexane was varied from 1%to 50% (v/v). At low surfactant concentrations, the mean drop sizedepends strongly on the surfactant concentration (Jiao and Burgess,2003; Kim et al., 2006; Tcholakova et al., 2004; Tonanon et al., 2003).The mean size of our particles in this “low-surfactant-regime” wasreduced from 10 to 5�m by increasing the surfactant concentrationfrom 1% to 4% (v/v) for stirring time of 5h as shown in Fig. 5a and b.Also, it was observed in Fig. 5b that number density of the particlesincreased at higher concentration, 4% (v/v).

Increase in the surfactant concentration from 1% to 4% (v/v) re-duces the RF sol–cyclohexane interfacial tension, which aids in theformation of a large number of inverted micelles. As a result, RF soladded in the cyclohexane gets dispersed in smaller drop volumes.Also, due to a higher concentration of surfactant molecules presenton the RF sol–cyclohexane interface, the possibility of coalescenceof the droplets during stirring is reduced because of the steric hin-drance and surface elasticity induced stabilization of the emulsion.

1540 C.S. Sharma et al. / Chemical Engineering Science 64 (2009) 1536 -- 1543

Fig. 3. SEM images showing aggregation and coalescence of RF-based carbon microspheres at Span-80 concentration 1% (v/v): (a) R/C = 10, stirring time = 5h; (b) R/C = 10,stirring time = 24h; (c) R/C = 25, stirring time = 24h; (d) R/C = 500, stirring time = 24h.

Fig. 4. Optical micrographs showing the transition to non-spherical shapes of RF-based carbon xerogel micro-particles at Span-80 concentration 1% (v/v), stirring time = 5hdue to change in the value of R/C ratio: (a) R/C = 25; (b) R/C = 50; (c) R/C = 100; (d) magnified view of (c).

C.S. Sharma et al. / Chemical Engineering Science 64 (2009) 1536 -- 1543 1541

Fig. 5. Optical micrographs showing the effect of surfactant concentration on particle size at R/C = 25, stirring time 5h in “low surfactant concentration region”: (a) 1% (v/v)Span-80; (b) 4% (v/v) Span-80.

Higher surfactant concentration should thus produce smaller size ofthe carbon particles. Another important feature observed was a nar-rower PSD of the resulting micro-spheres. Such a narrow PSD hasnot been easily obtained in the synthesis of RF gel particles by in-verse emulsification (Kim et al., 2006; Tamon and Ishizaka, 1998;Horikawa et al., 2004; Tonanon et al., 2003).

The effect of surfactant concentration on particle-size in RF de-rived carbon cryogels has been reported in few studies (Kim et al.,2006; Tonanon et al., 2003). Matos et al. (2006) have studied the ef-fect of surfactants on the porosity of carbon xerogels and reportedthat addition of non-ionic surfactant does not influence the mor-phology and porous texture. However, all these studies are confinedto the low surfactant regime discussed above in the case of xero-gels. In what follows, we discuss the influence of higher surfactantconcentration on particle-morphology.

Further increase in the surfactant concentration from 10% to 16%(v/v) leads to more viscous cyclohexane–surfactant and RF sol dis-persion mixture. Increased visco-elasticity of the dispersion and re-duced interfacial tension now facilitate shear deformations duringagitation, rather than immediate breakage of the droplets. Increasedsurfactant concentration first produces deformed oval shapes as par-tially supported by Matos et al. (2006). The increase in the surfactantconcentration causes substantial aggregation of inverted micelleswhich decide the micellar shape. With the increase in aggregationnumber, the morphology can no longer be considered to be spheri-cal, and for large aggregation numbers, oblate, vesicles and bilayersare generally envisioned (Bourret and Schecter, 1988). For very largeaggregation numbers, these distorted oval shapes eventually trans-form to increasingly fibrous, layered structures shown in Fig. 6a. Byincreasing the surfactant concentration in the range of 33–50% (v/v)and by controlling the time of stirring, carbon structures with nee-dle like features and large surface areas were obtained, as shown inFig. 6b–l. At such super-micellar concentrations, arrangement of sur-factant molecules into ordered structures becomes favorable leadingto liquid crystalline phases such as the lamellar phase (Miller andNeogi, 2007). Folding and breakup of the large bilayer sheets of thelamellar phase by stirring induced shear may form the forked andfolded fractal-like structures shown here. There is as yet no data onthe phase behavior of the sheared surfactant solutions used in thisstudy. Further work with small-angle neutron scattering techniquewould be required to elucidate the phase behavior.

Symmetrically branched structures at different size scales span-ning an order of magnitude are shown in Fig. 6b–e. Mathematicalquantification of these structures is done by calculating fractal di-mension which was found to be in the range of 1.60 ± 0.10.

Some of the aggregated needle structures resemble highly folded“carbon flower” type structures with larger external surface area(Fig. 6f and g).

As mentioned earlier that surfactant from three different suppli-ers was used to ensure the robustness of the results. Layered struc-tures consisting of small particles as shown in Fig. 6a were alsoformed by using the surfactant from the other two sources (Fig. 6hand i). Similarity of these structures between three different surfac-tant sources can also be observed in Fig. 6j–l. Synthesis conditionsin these cases remain the same as mentioned.

These carbon structures resembling aggregated needles or“flowers” and “bushes” obtained at high surfactant concentrationare therefore fractal-like and may have potential applications due totheir higher external surface area. The present study reports for thefirst time the synthesis of highly folded, fractal-like, high externalsurface area carbon structures through sol–gel polycondensation. Inmany electrochemical energy conversion devices, such as batteries,fuel cells as well as sensors, where large surface area is advantageous,Park et al. (2007) have shown that the fractal networks can proveto be a more favorable architecture. We are currently exploring thepossible ways to integrate these RF gel-based fractal-like structureswith three-dimensional high aspect ratio, carbon micro electrodearrays. As fractal-like structures have better absorption and scatter-ing properties than those of volume equivalent spheres (Min et al.,2006), they are potential radar-absorption materials and may beused in resonance-based solar cells and thermal detectors as well.

4. Conclusions

RF derived dense carbon xerogels have been synthesized bysol–gel polycondensation followed by its inverse emulsification ina non-ionic surfactant. The effects of various synthesis parametersincluding the surfactant concentration, R/C ratio and stirring timehave been studied. It was found that the average particle size of thecarbon microspheres could be controlled by modulating the rateof coalescence of the RF sol droplets in emulsion. Dense, sphericalamorphous carbon particles with average diameter in the range of 5to 46�m were obtained as the stirring time increased from 2 to 7h.Decrease in the catalyst concentration, by increasing the R/C ratiofrom 0.2 to 500, produced larger carbon microspheres. However, agreater tendency for the agglomeration of particles was observedat low catalyst concentration (larger R/C ratio) and longer stirringtime. The BET surface area measurements confirmed the formationof dense polymeric carbon structures, as the total pore volume wasvery small compared to the reported values for the RF aerogel andcryogel particles.

The most interesting control of the carbon particle size and mor-phology could be affected by changing the surfactant concentration.Increase in the surfactant concentration from 1% to 4% (v/v) leadsto smaller and more mono-dispersed spherical particles. Highersurfactant concentrations engender markedly non-spherical shapes.

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Fig. 6. SEM images of carbon “bushes” and “flowers” type fractal-like structures obtained at high surfactant concentrations: (a) 16% Span-80, 2h stirring; (b) 33% Span-80,1h stirring; (c) higher magnification of (b); (d) 33% Span-80, 2h stirring; (e) higher magnification of (d); (f) 50% Span-80, 1h stirring; (g) 50% Span-80, 2h stirring. Images(a)–(g) are obtained using the surfactant from Loba Chemie at R/C = 25. Images (h) and (i) are obtained at R/C = 25, 2h stirring at 33% Span-80 using the surfactant fromSigma Aldrich and SD Fine chemical respectively. Images (j)–(l) are obtained at R/C = 10, 2h stirring with 33% Span-80 using the surfactant from Loba Chemie, Sigma Aldrichand SD Fine chemical, respectively.

An interesting example is the formation of increasingly fibrous,fractal-like structures as the surfactant concentration was varied inthe range of 16–50% (v/v). The resulting highly folded carbon struc-tures consisting of aggregated “needles” and bearing large externalsurface areas resemble “bushes” and “flowers” in their morphology.The fractal dimension of these carbon structures was measured tobe 1.60 ± 0.10.

The methodology presented here offers the ease of synthesis, nar-row size distribution of spherical particles and a tight control on thecarbon particle size and shape by tuning the reaction parameters.Possibility of producing high external surface area, highly folded,fractal-like carbon xerogel structures may also lead a new direc-tion in the study of xerogels. These carbon micro-particles of variousmorphologies can be utilized in a host of scientific and technological

C.S. Sharma et al. / Chemical Engineering Science 64 (2009) 1536 -- 1543 1543

applications. For example, we are currently using the dense carbonmicro-spheres as probe particles to study the interaction forces be-tween carbon and a variety of surfaces as a model of carbon deposi-tion and removal. The interaction between such model particles andfabric substrates provided fundamental understanding of the forcesand their manipulation to aid removal which is important for deter-gency. It also is of importance in textile and paper industry wherefiller particles are added to improve the printability of paper, modifyfabric properties like smoothness and water repellency. Another ex-ample being pursued is the use of the high surface area fractal-likecarbon structures as an electrode material in the micro-batteries andfuel cells. Fractal electrodes can minimize internal resistance whichis a key to achieve efficient power transfer. Other than MEMS, thesehigh surface area fractal-like structures may find potential applica-tions in radar-adsorption materials, resonance-based solar cells andthermal detectors also.

Notation

RF Resorcinol–formaldehydeR/C resorcinol to catalyst molar ratiov/v volume of surfactant/volume of total solution

Acknowledgments

This work is supported by Indo-US center for Advanced and Fu-turistic Manufacturing funded by Indo-US Science and TechnologyForum, New Delhi, by the DST Unit on Nanosciences at IIT Kanpurand by a Grant from the DST (IRHPA Grant). Helpful discussions withGenis Turon are acknowledged. Ideas and support received from Prof.Amitabha Ghosh are gratefully acknowledged.

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