Formation of nano-structured core–shell micro-granules by evaporation induced assembly

Abstract

Nano-structured spherical micro-granules of core–shell morphology have been realized by utilizing the contrasting interfacial interaction of two different types of nano-particles with liquid solvent. By enforcing evaporation induced assembly, a hydrophobic core has been wrapped inside a hydrophilic envelope consisting of correlated nano-particles. This is realized by a one step, fast and facile technique of spray-drying. The evaporation of water in a radially outward direction from mixed-suspension droplets enforces the hydrophobic component to travel towards the core and the hydrophilic component to reside at the surface forming a shell. Mapping the coherent neutron and X-ray scattering length density into reciprocal space, the structure as well as inter-particle correlation in such micro-granules has been characterized over a wide range of wave-vector transfers. Scattering results have been complemented with electron microscopy. Significant enhancement in specific surface-area due to core–shell morphology has been observed by gas adsorption technique. Treating the granules with hydrofluoric acid, the silica shell has been etched to unwrap the meso-porous carbon core. This demonstrates that the hydrophobic component indeed forms the nano-structured core inside the hydrophilic nano-structured shell. In view of the unique characteristics of these synthesized core–shell nano-structured micro-granules, a potential application of such granules has also been discussed.

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1. Introduction

Self-assembly is a fascinating “bottom-up” approach in realizing organized nano-structures in larger length scales with all possible dimensions.^1–14 By controlling direct and/or indirect interactions among the nano-particles, such an assembly method can be tuned to attain the desired morphology of 3D granules with correlated nano-structures.^8,10–14 It becomes more interesting when such granules are comprised of nano-particles of different natures. In nano-composite granules, tuning of internal morphology by self-assembly remains a challenging task. Controlling the nature of spatial distribution (e.g. Janus granules^15–18 or core–shell granules^19–22) of different nano-particles over internal granular structure is an important aspect as far as the technological relevance of the composite granules is concerned. The basic principle of controlling the distribution of internal nano-structure in a particular fashion by self-assembly is to exploit various thermodynamic forces intelligently and to choose the building blocks judiciously.⁸ For example, to synthesize a nano-composite core–shell granule, consisting of hydrophilic–hydrophobic components, the internal distribution can be controlled by utilizing the contrasting interfacial interaction of the two types of nano-particles with water. In fact, evaporation induced assembly^23–39 can be employed to exploit such competing interfacial interaction of nano-particles with liquid water. The opposite polarity of water-affinity of hydrophilic and hydrophobic components in course of evaporation can lead to spatial separation of two components. Based on such simple but novel approach, evaporation induced assembly in tiny colloidal droplets can be used as a one-step method to synthesize hierarchical micro-granules having core–shell morphology. It needs to be mentioned that existing techniques in synthesizing core–shell nano-structured micro-granules are multi-step processes and most of them require chemical reactions to coat the core with shell unlike the proposed method.^40–42

In a bulk suspension-mixture of hydrophilic and hydrophobic nano-particles, both types of particles remain dispersed in spatially random manner. However, as soon as the fluid starts evaporating out of the droplet, a competition builds up between the opposite interfacial interactions of the nano-particles with solvent. The water-loving component prefers to move along with water molecules whereas, water-repelling component tries to propel against the motion of water molecules. This interfacial force may dominate over other short range attractive molecular forces depending on thermodynamic environment. Due to spatial separation, the hydrophobic particles assemble near the core of the droplet, while hydrophilic particles assemble near boundary of the droplet. Eventually, the nano-structured core gets encapsulated inside a nano-structured shell. Spray drying^26–39 technique has been used to implement the above mentioned interfacial phenomenon in realizing core–shell silica-carbon micro-granules with correlated nano-structured carbon enclosed inside correlated nano-structured silica shell. Carbon^43–45 and silica⁴⁶ nano-particles play the role of hydrophilic and hydrophobic components, respectively. It is noteworthy that the absence of such competitive interfacial phenomenon would have led to formation of micro-granules comprising a non-preferential random distribution of silica and carbon nano-particles. This unique one-step fast and facile synthetic technique avoids using any precursor for chemical reaction.

To determine the unique features of such synthesized spray-dried micro-granules, physical characterization is worthy to carry out. Small-angle neutron scattering (SANS)⁴⁷ and small-angle X-ray scattering (SAXS)^48,49 techniques have been employed to characterize the mesoscopic structure of silica coated carbon micro-granules. The hierarchical nano-structure of the grains as well as the correlation among the nano-particles has been probed over a wide wave-vector transfer range. Field emission scanning electron microscopy (FESEM) has been used to directly image the micro-granules in order to corroborate the results inferred from scattering experiments. Further, to probe the effective surface area of spray-dried meso-porous micro-granules, which is an important parameter for adsorption properties, Brunauer–Emmett–Teller (BET) surface area analysis^50–52 has been employed. Apart from the synthesis and physical characterization of the granules, we have extended our work to find a potential application of such synthesized micro-granules.

2. Experimental

2.1. Materials

For synthesizing the silica coated carbon micro-granules, commercially purchased carbon black (CB) nano-particles powder (from M/s. Hi-Tech Carbon of grade N-330, used without any further purification) has been used as carbon source. Colloidal suspension of silica nano-particles, obtained from LUDOX® SM30, M/s. Sigma-Aldrich, has been used as hydrophilic component. Polyvinylpyrrolidone (PVP), a water soluble polymer has been used to coat the carbon particle surface in order to make a well dispersed solution of carbon black nano-particles. Laboratory de-ionized distilled water is used to prepare all the solutions.

2.2. Preparation methods

2.2.1. Preparation of colloidal suspension. CB is difficult to disperse in water. PVP solution has been used as the dispersing medium to prepare the colloidal suspension of CB nano-particles. Firstly, CB powder has been added to distilled water and then the solution has been kept under ultrasonic agitation for 1 hour. After the ultrasonic treatment, the CB dispersion has been poured drop by drop into PVP solution which was kept under continuous stirring. Then, the silica suspension has been added to the mixture. The final solution has been allowed to disperse well by keeping it under magnetic stirring for further 1 hour. The resultant dispersion consists of 1 wt% silica, 0.5 wt% CB and 0.1 wt% PVP.

2.2.2. Preparation of nano-composite micro-granules by spray-drying. The nano-composite micro-granules of carbon and silica have been prepared through evaporation induced assembly process in following manner. The mixed colloidal suspension consisting of silica, CB and PVP has been spray dried using a spray dryer (LU 228, Labultima, Mumbai, India).^31–33,39 The spray dryer is mainly composed of a cylindrical drying chamber of 22.5 cm diameter and 60 cm length. The droplets have been generated by using a compressed air spray nozzle. The maximum droplet size was found to be ∼20 μm as measured by the GRIMM 1.108 aerosol spectrometer. The dispersion has been fed at a constant rate of 2 ml min⁻¹ to the nozzle for the atomization of the liquid using a peristaltic pump. The inlet temperature and the aspiration value have been kept at 175 °C and 50 m³ h⁻¹, respectively. During passing the drying chamber, the liquid component of the droplets evaporates out and this induces the assembly among the nano-particles. The evaporation induced assembly through spray drying results into dried solid powders which was collected from cyclone separators. Another colloidal suspension consisting of CB and PVP (i.e., without the silica component) has been spray dried.

2.3. Characterization techniques

2.3.1. Small-angle neutron and X-ray scattering. Small-angle scattering technique has been used to map the coherent scattering length density fluctuation into scattering angle space (Fourier space) because the pertinent length scale, present in the micro-granule, ranges from 1 to 1000 nm — accessible length scale of these techniques. Both neutron and X-ray have been used as probe in two different Q regions. To access the lower Q domain (i.e. 0.003–0.173 nm⁻¹), ultra small-angle neutron scattering (USANS) experiments have been performed using the double crystal based medium resolution SANS facility (MSANS) installed at GT lab, Dhruva reactor, Mumbai, India.^53,54 The instrument was calibrated with respect to the high resolution USANS instrument S18 of Institute Laue–Langevin in Grenoble, France.⁵⁵ MSANS of Dhruva consists of a non-dispersive (1, −1) setting of two silicon (111) single crystals, which were used as monochromator and analyser. The monochromator allows a monochromatic neutron beam of wavelength (λ) 0.312 nm (Δλ/λ ∼ 1%) to fall upon the sample. The spray dried powder sample was kept in an aluminum can of ∼2 cm diameter and then mounted on a sample holder with circular slit of 1.5 cm diameter. The scattered neutron intensities were recorded as a function of wave-vector transfer Q (=4π sin(θ)/λ, where 2θ is the scattering angle and λ (=0.312 nm) is the incident neutron wavelength). The SANS data were corrected for background, transmission and instrument resolution⁵⁶ effects prior to further analysis. The higher Q regime (i.e. 0.1–2.5 nm⁻¹) nano-structure has been investigated by performing small-angle X-ray scattering (SAXS) using a laboratory based instrument having Cu-K_α X-ray source (wavelength 1.54 Å). The spray dried powder sample has been mounted in between two X-ray transparent polyamide films. The scattered X-ray intensities have been recorded by a 2D detector. The sample to detector distance has been kept fixed at ∼1.07 m. The transmission corrected radial averaged data have been processed for further analysis. Combining these two scattering techniques i.e. SAXS and USANS, three decades of wide Q range have been scanned in order to unveil the hierarchical nano-structure of the spray dried granules.

2.3.2. Field emission scanning electron microscopy. In complementary to scattering technique, direct imaging of the micro-granules has been performed using Carl-Zeiss high resolution field-emission scanning electron microscope (FESEM). SEM micrographs help us to understand the assembly of the nano-structures and also validate the scattering model assumed to fit the scattering profiles.

2.3.3. Dynamic light scattering and zeta-potential measurement. Dynamic light scattering (DLS) and zeta-potential measurement have been performed (using Horiba Scientific SZ-100 instrument at 90° and 173° scattering angles with solid state laser of 532 nm wavelength) to measure the hydrodynamic size of the colloidal particles and the stability of colloidal dispersion, respectively.

2.3.4. Brunauer–Emmett–Teller (BET) surface area analysis. Surface BET theory^50–52 has been applied by allowing physical adsorption/desorption of nitrogen gas molecules on surface using Thermo Scientific™ Surfer instrument. BET is a well established and important analysis technique for the measurement of effective specific surface area offered by a material. Analyzing the adsorption–desorption isotherms one can determine the amount of gas molecules adsorbed to a surface and can apply BET theory to evaluate the specific surface area.

3. Results and discussion

3.1. Characterization of spray-dried granules

Scattering signal originating from the density fluctuations in condensed matter system at small scattering angle has been analysed to unveil the morphological details such as, particle size and shape, particle size distribution, and inter-particle correlations by implementing an appropriate micro-structural model. It is worth mentioning that there is no match with SAS technique while probing the hierarchal structure and correlation of assembled nano-particles in spray-dried granules. Fig. 1 depicts the combined USANS and SAXS profiles of spray dried and raw CB powders. The inset of Fig. 1 shows the corresponding Porod plot (i.e., I(Q)Q⁴ vs. Q)^48,49 revealing two widely separated Porod regions (shown in shade in inset of Fig. 1) vis-à-vis mesoscopic structure. It is evident from the Porod plot that the scattering intensity at relatively higher Q is primarily dominated by the density fluctuations in form of individual nano-particles, which are the building blocks of micro-granules. The overall micro-granules which are bigger in length scale leave their signature at relatively lower Q. Before going to detail analysis of the SAS data model fitting, we would like to illuminate a basic feature of the spray dried nano-composite as observed from the scattering profiles. The SAS profiles are normalized at lowest accessible Q value (i.e., 0.003 nm⁻¹). If the SAS profile of raw CB (i.e. before spray drying) is compared to that of spray-dried CB + PVP sample, it is clearly evident that some extra scattering feature is present in case of raw CB. This extra scattering intensity is coming from the agglomerated mesoscopic structure of carbon nano-particles. However, this broadening in scattering intensity vanishes for spray-dried CB + PVP micro-granules. The raw CB has an irregular agglomerated structure, as shown in Fig. 2(a) and (b). van der Waals attraction among individual CB nano-particles leads to a dendritic structure. Such ramified structure gives rise to extra scattering in corresponding length scale domain. The SAS profile for raw CB has been fitted with a micro-structural model comprising of individual CB nano-particles (average radius ∼13 nm and polydispersity index ∼0.54) having sticky hard sphere⁵⁷ type interaction and a bigger structure in micron length scale domain. The local volume fraction (f) of aggregated CB nano-particles has been found to be ∼0.37 indicating a relatively loose binding of CB nano-particles. In order to break the agglomerated structure of CB, the dispersion has been ultrasonicated for one hour. The hydro-dynamic radius distribution of raw CB, from DLS measurement, immediately after ultrasonication, has been found to be bimodal (shown in ESI; Fig. S1†). Both bigger and smaller particles exist simultaneously in the colloidal dispersion. The mechanical agitation breaks the agglomeration into individual nano-particles but subsequently the nano-particles start aggregating. The average radius of the smaller particle has been found to be ∼20 nm whereas, the average radius of the bigger particles has been ∼947 nm (Table 1). The smaller size corresponds to the individual carbon nano-particles and the bigger size is coming from the agglomerated structure. It implies that aqueous dispersion of CB nano-particles is not stable. If the CB dispersion is added in PVP solution drop by drop, then PVP polymer coats either individual or some group of CB nano-particles and that makes the zeta potential high enough (∼−46.5 mV) to restrict the nano-particles from agglomeration. The hydrodynamic radius after PVP coating has been detected to be ∼72 nm (Table 1), much smaller compared to that of agglomerated raw CB. The PVP polymer coats approximately eight CB nano-particles together forming a unit of nano-particles of radius ∼72 nm. Thus, a colloidal suspension of CB nano-particles in solution of PVP has been prepared. Cartoon in Fig. 3 demonstrates each steps of the above mentioned process of breaking the agglomeration. Dispersion of PVP coated CB nano-particles has been spray-dried to solid powder following scattering measurement. The drop in intensity of SAS profile (Fig. 1) indicates no formation of irregular agglomerate, rather evaporation induced assembly leads to a compact 3D jamming of individual unit of nano-particles. The local volume fraction of CB nano-particles inside PVP wrapper has been increased to 0.53 indicating more compactness compared to previous case. The SAS fitting parameters are given in Table 1. After preparation of colloidal suspension of CB in PVP solution, a commercial suspension of silica nano-particles (average radius ∼5.5 nm having ∼0.16 standard-deviation in size distribution) has been added. Addition of silica nano-particles does not disturb the stability of the mixed colloidal suspension as observed from the zeta-potential measurement (∼−49.8 mV). The colloidal mixture of silica nano-particles and PVP coated CB nano-particles has been spray-dried to nano-composite solid powder. The humps in the scattering profile (Fig. 1) of spray-dried silica in large Q region are originating from the sticky hard sphere interaction among the well correlated silica nano-particles. The SAS profile has been fitted with a mathematical model considering four distinguished contributions. The total scattering intensity can be written as:

Fig. 1 Combined small-angle scattering (SAS) profiles (open motifs denote USANS and solid motifs denote SAXS data) of raw CB powder and spray-dried powders. The inset figure depicts the corresponding Porod representations (i.e., I(Q)Q⁴ vs. Q plot) of SAS profiles. The solid lines show the fit of the SAS profiles.

Fig. 2 FESEM pictures reveal the nano-structure and micro-granules of carbon black as well as silica coated carbon powder. (a) & (b) show the agglomerated porous carbon nano-structure before spray-drying. (c) & (d) depict the regular spherical spray-dried micro-granules of silica coated carbon. (e) & (f) show the micro-granules after partial and full HF etching, respectively.

Table 1 Values of physical parameters estimated from mathematical SAS model fitting to the combined SANS and SAXS profiles as well as results obtained from DLS and BET measurements. OR ≡ outer radius & IR ≡ inner radius

Sample name	Fitting parameters of small-angle scattering profiles					DLS & zeta potential measurements		BET measurements
	Avg. radius (rav) (nm)		Polydispersity index (σ)	Volume fraction (f)	Inverse stickiness (τ)	Hydrodynamic radius (nm)	Zeta potential (mV)	Surface area (m^2 g^−1)
Raw CB	13.3 ± 0.4		0.54	0.37	0.08	20 & 947 (Bi-modal)	+25.5	68
CB + PVP	CB	13.2 ± 0.4	0.54	0.53	2.0	72	−46.5	42
	PVP	60 ± 3 (OR) 39 ± 4 (IR)	0.61	0.48	0.10
CB + PVP + silica	CB	13.1 ± 0.4	0.58	0.53	2.0	—	−49.8	210
	PVP	61 ± 2 (OR) 38 ± 4 (IR)	0.64	0.48	0.10
	Silica	5.5 ± 0.1	0.16	0.66	0.001

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Fig. 3 Schematic showing the breaking of dendritic structure of aggregated CB nano-particles into smaller units and then coating of PVP polymer for preparing colloidal suspension.

Here, I₁(Q) and I₂(Q) originate from the individual CB and silica nano-particles, respectively. I₃(Q) contribution is coming from the PVP polymer which coats CB nano-particles. I₄(Q) represents the scattering contributions of Porod region of overall micro-granules of micron length scale. Scattering from both CB and silica nano-particles have been interpreted in the context of polydisperse spherical density fluctuation with log-normal distribution under local monodisperse approximation.⁵⁸ In the above equation, C_js denote the product of number density of nano-particles and contrast factor (i.e., the square of the scattering length density difference between the particles and the matrix). For the spherical type of particles, as in the cases of CB and silica nano-particles, the form factor P(Q,r) for a particle with radius ‘r’ can be written as:

S(Q,r) represents the inter-particle structure factor which is a measure of the correlation among the nano-particles. Sticky hard sphere⁵⁷ type of interaction has been considered for CB and silica. D(r) represents the particle size distribution function which is assumed to be normalized log-normal distribution in the present case and is expressed as:

where μ and σ are two controlling parameters representing median and polydispersity index, respectively and are related to the mean radius of the particle . v(r) represents the volume of a particle of radius ‘r’. The value of the fitting parameters have been tabulated in Table 1.

During the drying process of the nebulizer sprayed liquid micro-droplets, the liquid water evaporates out in radially outward direction. A competing interfacial interaction develops between the hydrophobic and hydrophilic nano-particles because of opposite water-affinity. If the water evaporation rate is kept sufficiently slow then the evaporation favours the movement of hydrophilic particles with water towards the outward direction while in opposite, the hydrophobic particles try to settle near the core of the spherical droplet. The opposite movements of the hydrophilic and hydrophobic particles during slow evaporation drying leads to a composite nano-structured micro-granules comprising of hydrophobic carbon particles inside the envelope of silica. According to Einstein–Stokes relation of diffusion co-efficient (D), D = K_BT/6πηr, where ‘K_B’ represents the universal Boltzmann constant, ‘T’ is the temperature, ‘η’ is the viscosity and ‘r’ represents the radius of the colloidal nano-particle. The smaller particle has greater diffusion co-efficient than that of bigger particle. In the colloidal mixture of nano-carbon and nano-silica, the average radius of carbon nano-particles (∼13 nm) is greater than that of silica nano-particles (∼6 nm). This means it is less likely for carbon nano-particles to diffuse faster than silica nano-particles and occupy the core area unless there exists other driving forces. The water affinity plays an important role to alter the diffusion of the colloids during drying. The interfacial interaction of water loving silica colloids drives them with water molecules while hydrophobic carbon prefers to occupy water-depleted area. The schematic in Fig. 4 illustrates the above mentioned mechanism. Thus, silica coated carbon micro-granules, schematic shown in Fig. 5, have been synthesized by one step facile process of evaporation induced assembly. The FESEM micrographs in Fig. 2(c) and (d) show the spray-dried spherical micro-granules of silica coated carbon. The silica surface is evident from the FESEM images. The silica surface coating enwraps the carbon nano-structure inside making it invisible. Evaporation induced assembly of nano-particles through slow spray-drying leads to regular spherical micro-granules.

Fig. 4 The schematic representation of spray-drying process when a micro-droplet passes through the hot chamber. The liquid evaporates in radially outward direction from the surface, which induces the suspended nano-particles to assemble together. It is the opposite affinity of carbon and silica with their liquid interfaces that results into a core–shell nano-composite structure.

Fig. 5 The schematic demonstrates how a final spray-dried silica-carbon nano-composite micro-granule looks like in 3D and its cross-sectional projection. Well correlated silica nano-particles make a nano-porous shell enclosing meso-porous carbon inside the core. The interstices of silica nano-particles provide a water transport channel towards carbon core.

To verify the existence of carbon nano-structure inside the layer of silica coating, the synthesized spray-dried nano-composite powder has been etched with concentrated hydrofluoric acid (HF). HF eats up the silica by chemical reaction and hence if the carbon nano-structure is indeed present inside the silica coating, we should be able to unwrap the carbon nano-structure after removing the shell of silica layer. The HF etching has been done in two steps. 1% concentrated HF has been used for partial etching of silica and 4% concentrated HF has been used for complete etching. Fig. 6 depicts the SAXS profiles of partially etched and completely etched silica-carbon nano-composite micro-granules along with the SAXS profile of that sample before etching. It is evident from the scattering intensity plot that the signature of correlated silica nano-particles decreases after partial HF etching and then completely vanishes after treatment of higher concentrated HF for sufficiently long duration. HF etching of the spray-dried micro-granules leaves only the carbon meso-porous structure behind. The scattering intensity scale reduces due to lowering in number density of silica nano-particles because intensity is directly proportional to number density. Direct evidence of HF etching can be obtained from the microscopic images of FESEM in Fig. 2(e) and (f). Fig. 2(e) depicts some micro-granules of rugged surface of a thin layer of silica enclosing the carbon inside, after treatment of lower concentration of HF for short duration. Micrograph in Fig. 2(f) shows that the complete etching of silica layer discloses the carbon core which was under the silica envelope. The carbon meso-porous structure does exist after complete HF etching. The energy dispersive X-ray spectroscopy (Fig. S2; given in ESI†) also reveals that HF etching removes the silica leaving the carbon behind. It is evident from the SAXS and FESEM results that silica forms a spherical shell which wraps the carbon nano-structure core inside. The average radius of the core and shell can be estimated from the knowledge of local volume fraction obtained from SAS results. It has been found that the average radius of the carbon core is ∼1.6 μm and that of silica shell is ∼150 nm.

Fig. 6 SAXS profiles show how the silica nano-particles are getting removed with HF etching. After full HF etching, the correlated silica nano-particle signature hump is completely vanished leaving only the CB particles. The solid lines show the fit of the SAXS profiles.

The available specific surface area of spray-dried micro-granules is also an important parameter to be examined as per as the adsorption property of carbon is concerned. The adsorption–desorption isotherms of nitrogen gas for raw CB, spray-dried PVP coated CB and for spray-dried silica coated CB powders have been provided in ESI; Fig. S3.† The BET measurement on raw CB agglomerated structure gives the specific surface area ∼68 m² g⁻¹ (Table 1). After spray-drying of CB with PVP coating, the available specific surface area (∼43 m² g⁻¹) slightly decreases. This decrement in specific surface area can be explained in terms of breakage of agglomerated structure. In case of dendritic structure of raw CB, nitrogen gas can access the area of all loosely jammed CB nano-particles. Whereas, after coating of PVP polymer, nitrogen gas can hardly access the surfaces of CB nano-particles penetrating the PVP polymer. PVP enwraps 8–9 CB nano-particles together and then the whole unit makes a compact jamming during spray drying. Thus, the effective specific surface area gets reduced due to transformation from agglomerated structure to compact jamming of PVP coated CB nano-particles through spray-drying. However, when silica makes a spherical shell enclosing the PVP coated CB nano-structure, it becomes a completely different scenario. The spherical shell of silica presents the open surface area of individual silica nano-particles of ∼5.5 nm radius to the nitrogen gas. In addition to this, the nitrogen gas gets access to enter inside the spherical shell through the interstitial nano-pores. The nitrogen gas can be trapped in between the available area of silica shell and meso-porous carbon core. Due to this core–shell structure of silica-carbon nano-composite, the effective surface area (∼210 m² g⁻¹) is increased to greater than three times of that of raw CB. The high specific surface area of this novel nano-composite micro-granule can enhance the adsorption efficiency of carbon.

At this juncture, we would like to clarify that the nature of distribution of hydrophilic–hydrophobic particles inside a spray-dried nano-composite granule will depend not only on interfacial interaction but also on the charge and size of the colloids depending on the respective strength of the individual factor. For example, if electrostatic interaction dominates, hydrophobic and hydrophilic nano-particles with opposite charge may agglomerate faster during drying and hence, their opposite interfacial interaction with water may fail to enforce the complete spatial separation of hydrophobic and hydrophilic components. The size of the colloids has also an important role to play because diffusion co-efficient is inversely proportional to the size of the colloids. Significantly large size of the (hydrophobic) nano-particles can make the diffusion too sluggish to occupy the water depleted region during drying of the liquid. The opposite interfacial interaction of hydrophobic–hydrophilic nano-particles with water should be able to override other unfavorable conditions in order to form core–shell morphology. Thus, the formation of core–shell nano-structured micro-granules, utilizing the contrasting interfacial interaction of hydrophilic and hydrophobic components with their liquid interfaces, is not limited to silica and carbon nanoparticles alone if other appropriate physicochemical parameters can be properly optimized. In order to verify the versatility of this fast and one-step method, we have investigated a different colloidal mixture system apart from carbon-silica nano-composite, whose experimental details and results are provided in ESI.†

3.2. A potential application of synthesized micro-granules

Keeping in mind that the synthesized nano-structured core–shell micro-granules are comprised of hydrophobic carbon core encapsulated inside the hydrophilic silica shell, we have attempted for a possible application of the granules. Nano-carbon is an efficient material for adsorbing various organic compounds from water to maintain odour and taste of drinking water⁵⁹ and hence the nano-carbon can be utilized in drinking water filtration process. However, carbon being hydrophobic in nature, it is difficult to incorporate it directly to a hydrophilic ultra-filtration membrane. Thus, if the carbon nanostructures are wrapped by hydrophilic silica, such composite micro-granules can be incorporated into an ultra-filtration polymeric membrane to possibly enhance the filtration efficiency of the membrane. Besides, the shell of highly correlated nano-silica consists of interstitial nano-pores and these interstitial nano-pores can serve as water transport channels towards the carbon core. Considering such unique characteristics, the synthesized novel micro-granules have been casted in ultra-filtration polysulfone membrane and the performance of composite membrane has been carried out. The intactness of the incorporated micro-granules in the membrane has been testified by small-angle X-ray scattering. The scattering profiles of virgin polysulfone membrane as well as spray-dried granule incorporated composite membrane have been shown in Fig. 7, along with the SAXS profile of spray-dried granules. The change in functionality in scattering intensity profile of nano-composite embedded membrane, in comparison to bare membrane, clearly clarifies the incorporation of spray-dried granules. The position of the characteristic hump of silica in SAXS profile (Q ≈ 0.6 nm⁻¹) remains exactly same for the virgin spray-dried powder granules and the nano-composite membrane. This clearly suggests the intactness of the spray-dried micro-granules after incorporation into the membrane. The most important aspect of this incorporation is accomplishing enhanced water permeability through the composite membrane without sacrificing any separation characteristics. In fact, the incorporation of silica coated carbon granules makes the rejection of polyethylene oxide of very high molecular weight of 10⁵ (PEO-100k) from water almost 100% (see Table 2), which has a huge impact as per as membrane separation characteristics are concerned. The enhancement of water permeability (by ∼18%) is the result of available interstitial pore channels and large specific surface of meso-pores present in the micro-granules. It is interesting to note from contact angle measurement (see Table 2) that the silica surface of micro-granules increases the hydrophilicity compared to bare polysulfone membrane and hence, it can improve the usage of the ultra-filtration membrane in water purification. The composite membrane, in comparison to standard polymeric membrane, can be utilized to gain the benefits of adsorbance properties of meso-porous carbon combining with its own ultra-filtration separation characteristics and, in addition, with enhanced flux.

Fig. 7 SAXS profiles of casted polysulfone membrane with and without spray-dried powder. The SAXS intensity of solid spray-dried silica coated carbon powder has also been depicted for comparison.

Table 2 The composition of membrane casting solution and performances of polysulfone ultra-filtration membrane with and without incorporation of spray-dried powder

Membrane name	Composition of casting solution			Water contact angle (°)	Pure water permeability (L m^−2 h^−1)	PEO-100k separation (%)
	Polymer (g)	Solvent (g)	Spray-dried granule (g)
PSf-NMP	18	82	0	72.8 ± 0.7	148.8 ± 14.6	85.0 ± 3.8
PSf-NMP-silica coated carbon	18	82	3	68.7 ± 2.5	176.2 ± 18.4	97.5 ± 1.2

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4. Conclusions

In summary, utilizing the competitive phenomenon of interfacial interaction, evaporation induced assembly has been used as a fast and one-step process to realize correlated nano-structured core–shell micro-granules of hydrophobic carbon enclosed inside hydrophilic silica shell. It has been unveiled that evaporation of water from a droplet in radially outward direction drags the water-loving silica towards peripheral surface whereas, hydrophobic carbon tries to settle in water depleted core region. In absence of such competitive interfacial interaction carbon nano-particles, being larger in size vis-à-vis having smaller diffusion co-efficient, would have been resided either near peripheral surface or made a random spatial distribution with silica nano-particles. Opposite water-affinity during evaporation of water plays a driving role to alter the normal diffusion. Etching of silica surface by hydrofluoric acid treatment reveals the meso-porous carbon core which confirms that correlated nano-structured carbon indeed gets enclosed by correlated silica envelope. The formation of core–shell morphology enhances the effective surface area more than three times in comparison to that of virgin agglomerated nano-structure. The hydrophilic shell of highly correlated nano-silica, on the core of hydrophobic meso-porous carbon provides a wetting surface as well as interstitial nano-pores. Hydrophilic coating around carbon helps in incorporation of such porous granules in polymeric ultra-filtration membrane to exploit the adsorption properties of nano-carbon for removing various organic compounds from water. The presence of interstitial nano-pores in nano-structured silica shell serve as water transport channels towards the core of the granules. The incorporation of silica coated carbon granules into polymeric membrane makes the rejection of PEO-100k from water almost 100% and even enhances the water permeability by ∼18% which implies a significant impact as per as membrane separation characteristics are concerned. Use of such silica-carbon composite membrane may make a household water filter substantially compact as the need for separate charcoal column, used in a standard filter, will become redundant. Thus, the present work succeeds to demonstrate the synthesis and characterization as well as potential water filtration application of such novel composite micro-granules by incorporating into polymeric ultra-filtration membranes.

Acknowledgements

AD would like to thank Mr Indresh Yadav and Dr V. K. Aswal of SSPD for DLS measurements. AD and DS thankfully appreciate the support of Dr M. Krishnan of GAMD, BARC in obtaining the FESEM micrographs.

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Footnote

† Electronic supplementary information (ESI) available: Graphs of DLS measurement, energy dispersive X-ray spectroscopy measurement, BET measurement and micrographs of silica-SDS spray-dried micro-granules have been provided in supplementary information. See DOI: 10.1039/c5ra15650e

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