Synthesis of CaCO 3 @C yolk – shell particles for CO 2 adsorption †

We report the synthesis of CaCO 3 @C yolk – shell particles with a microporous carbon shell through a selective etching method. The CaCO 3 @C exhibits an enhanced CO 2 adsorption, relative to the porous CaCO 3 nanoparticles or the porous carbon shell, with a capacity of 19.30 cm 3 STP g (cid:1) 1 (0.86 mmol g (cid:1) 1 or 31.64 cm 3 STP cm (cid:1) 3 sorbent) under ambient conditions (23 (cid:3) 1 (cid:4) C and 1 atm CO 2 ).

Yolk-shell nanoparticles are materials with nanoparticle cores inside hollow shells.2][3] Various yolk-shell nanoparticles (YSNs) with different chemical compositions have been reported such as metal NPs@SiO 2 , metal oxide@SiO 2 , metal NPs@C, metal NPs@metal oxide, metal NPs@polymer, SiO 2 @metal oxide, SiO 2 @C and polymer@ polymer 2,4-11 by using different synthetic methods, for example, a so templating method and selective etching methods.3][14] To enrich the YSNs library and meet the requirements of practical applications, synthesis of YSNs with a new composition is desirable.
1][22][23][24] Taking the advantages of yolk-shell structures, such as large void space for accommodating of guest molecules, different functionality of both core and shell, constructing yolk-shell particle with porous carbon shell would be very promising for design of CO 2 capture and conversion nanoreactors.Calcium-based materials (calcium oxide, calcium hydroxide, and calcium carbonate) have been proved as excellent sorbents for high temperature CO 2 capture, 25 however, their adsorption performance quickly declines with multiple reuse due to irreversible particle sintering and agglomeration at high temperatures. 26,27e herein report the rst example for the synthesis of CaCO 3 @C yolk-shell particles.The features offered by these particles are: high surface area and pore volume, basic calciumbased core for affinity of CO 2 , large void space for CO 2 storage, and microporous carbon shell 28 for preferential passage of small molecules with respect to larger sized molecules. 29These yolk-shell particles could nd application as a catalyst support (e.g., nano-metals) or in drug and gene delivery for biomedical applications, 30 or high temperature CO 2 capture.
As illustrated in Scheme 1, a four-step synthetic process was employed by using CaCO 3 particles as core materials.First, a silica layer was coated around the CaCO 3 nanospheres by a Stöber method to obtain CaCO 3 @SiO 2 core-shell particles.Next, the CaCO 3 @SiO 2 was coated with a RF resin via a modied Stöber method 28 to produce CaCO 3 @SiO 2 @RF core-shell-shell particles.This was followed by a carbonisation process under N 2 ow, which converted the RF resin into a microporous carbon shell to produce CaCO 3 @SiO 2 @C.Finally, the silica layer was removed with the treatment of hot concentrated NaOH.The detailed experimental procedures are presented in the ESI.† CaCO 3 nanospheres were prepared by the rapid mixing of solutions of CaCl 2 and Na 2 CO 3 containing surfactant poly(4-styrenesulfonic acid) sodium salt (PSS).The assynthesised CaCO 3 nanospheres have an average particle size of 450 nm as demonstrated by SEM image (Fig. 1a).The particle size distribution of the CaCO 3 spheres (Fig. 1b) further conrms the uniformity of the porous CaCO 3 particles.The successful preparation of CaCO 3 @SiO 2 as well as CaCO 3 @SiO 2 @C from CaCO 3 nanospheres was tracked and conrmed by TEM characterization as shown in Fig. 2a-d.Fig. 3a and b show the SEM and TEM images of the obtained CaCO 3 @C yolk-shell particles.The presence of a movable core inside a thin carbon shell can be easily identied from the SEM images via the particles with broken shells, exposing the core.The TEM image shows a yolk-shell particle with a porous CaCO 3 core and a golf ball like porous carbon shell.The size of the core and hollow space is ca.240 nm and 310 nm in diameter, respectively, and the carbon shell's thickness is around 10 nm.High angle annular dark eld scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDX) of the particles are shown in Fig. 3c-f, which indicates the formation of a carbon shell outside the calcium carbonate core.
XRD pattern of the CaCO 3 @C yolk-shell particles (Fig. 3g) further conrms that the main composition of the core is CaCO 3 .A small amount of CaO and CaSiO 3 (due to reaction of CaO with un-removed SiO 2 ) is also present.Interestingly, when the CaCO 3 precursor nanoparticles were subjected to the same calcination conditions as with the yolk-shell particles, a Ca(OH) 2 phase was formed (according to the XRD data, Fig. S1 †) due to the loss of CO 2 resulting from the continuous supply of fresh N 2 in the furnace at 600 C for a long period (4 hours).This suggests that the nanoporous SiO 2 layer prevent the escape of the large CO 2 molecules during carbonation of CaCO 3 @ SiO 2 @RF core-shell-shell particles.As a result, the retainment of CO 2 gas within the SiO 2 layer led to an expansion of the space between the nanocrystals inside the core, increasing its porosity by creating channels and bridges, compared to the CaCO 3 precursor nanoparticles (this can be seen by comparing Fig. 2a  and d).Furthermore, some of CaO formed at the edge of the core reacted with SiO 2 (that could not be removed by NaOH etching) to form CaSiO 3 .The porosity of the CaCO 3 @C yolkshell particles were measured by nitrogen sorption, which revealed a type IV isotherm indicating their mesoporous structures 31 (Fig. 3h).The pore size distribution curve in Fig. S2 † shows that the material is highly microporous to mesoporous in nature.The high microporosity of the yolk-shell particles is due to the carbon shell. 28The BET surface area, density and total pore volume of the CaCO 3 @C YSNs are 381 m 2 g À1 , 0.12 g cm À3 and 0.61 cm 3 g À1 , respectively, while those of the CaCO 3 precursor nanoparticles are 89 m 2 g À1 , 0.63 g cm À3 and 0.14 cm 3 g À1 , respectively (see Fig. S3 † for N 2 sorption isotherm and pore size distribution of the CaCO 3 precursor).
In order to control: (1) the hollow space inside shell and (2) the thickness of shell, the synthesis parameters such as silica precursor concentration and RF ratio were varied, respectively (see Fig. S4-S6 in ESI †).It was found that the general size of the hollow space and shell thickness increased when increasing the silica precursor concentration and the RF ratio, respectively.In addition, by increasing the etching time, as shown in Fig. 4, the hollow space increases and the porous CaCO 3 core gets more exposed due to the removal of the silica layer around the core.The ability to control these physical parameters is important in terms of improving the mechanical strength of the particles, and tuning its functionalities.
For further optimisation and conrmation of the core compositions, TGA of CaCO 3 precursor nanoparticles was performed as shown in Fig. S7, † from which it can be seen that the CaCO 3 decomposes to CaO between $600 C and $780 C. As a result, the effect of high temperature recalcination on the CaCO 3 @C yolk-shell particles was investigated.Fig. S8 † shows the XRD patterns for CaCO 3 @C samples recalcined under N 2 for 1 hour at 650 C, 700 C and 750 C, respectively.At 650 C, a CaO peak starts to appear and becomes more prominent at 700 C.However, when the temperature is further increased to 750 C, a CaSiO 3 phase forms due to reaction of CaO with remaining SiO 2 on the core surface.Hence it is shown that the phase and composition of the core can be changed from CaCO 3 to mixture of CaCO 3 and CaO, further to CaSiO 3 by simply tuning the recalcination temperature of the CaCO 3 @C yolkshell particles.The presence of CaO in the core increases its basicity, which can be an attractive characteristic for catalytic applications requiring basic conditions or high temperature CO 2 capture.
Developing a low cost and high efficient adsorbent for CO 2 capture is highly desirable in order to alleviate the crisis of climate change and greenhouse effect.Fig. 5 shows the CO 2 adsorption isotherms for the CaCO 3 precursor nanoparticles, Ca(OH) 2 (calcined CaCO 3 precursor) particles, CaCO 3 @SiO 2 @C and CaCO 3 @C yolk-shell particles.The original CaCO 3 shows a relatively low adsorption capacity (6.30 cm 3 STP g À1 or 45.00 cm 3 STP cm À3 sorbent at 23 AE 1 C and 1 atm CO 2 ) even at higher CO 2 pressures due to weak physisorption of the gas on the particles.The adsorption capacity of CaCO 3 @SiO 2 @C is  (d-f) EDX elemental mapping of carbon, calcium and oxygen respectively, (g) XRD analysis and (h) N 2 adsorption isotherm for the CaCO 3 @C yolk-shell particles.TEOS concentration ¼ 2 mL g À1 CaCO 3 , RF ratio ¼ 0.5 and etching time ¼ 3 hours.Fig. 2 TEM images for tracking the steps in synthesis of CaCO 3 @C: (a) CaCO 3 particle, (b) CaCO 3 @SiO 2 particle, (c) CaCO 3 @SiO 2 @RF particle and (d) CaCO 3 @SiO 2 @C particle.TEOS concentration ¼ 4 mL g À1 CaCO 3 , RF ratio ¼ 0.5.
even lower (4.50 cm 3 STP g À1 sorbent at 23 AE 1 C and 1 atm CO 2 ) due to the impervious layer of SiO 2 which hindered CO 2 penetration towards the CaCO 3 core; hence physisorption was mostly achieved on the surface of the carbon shell.The calcined precursor, Ca(OH) 2 particles had the lowest CO 2 adsorption capacity at 0.40 cm 3 STP g À1 sorbent (at 23 AE 1 C and 1 atm CO 2 ), probably due to the loss in surface area resulting from particles agglomeration at high calcination temperature.However, the improvement in the CO 2 uptake is obvious with the yolk-shell particles due to removal of the SiO 2 layer for CO 2 adsorption.The CaCO 3 core of CaCO 3 @C was more porous than the CaCO 3 precursor (as can be seen by comparing Fig. 2a and  d).Furthermore, the core-shell architecture allowed each CaCO 3 core to be completely surrounded by a CO 2 atmosphere for enhanced adsorption.The maximum amount of CO 2 adsorbed with CaCO 3 @C at ambient conditions (23 AE 1 C and 1 atm CO 2 ) is 19.30cm 3 STP g À1 sorbent (0.86 mmol g À1 or 31.64 cm 3 STP cm À3 sorbent).The volumetric capacity of CaCO 3 based sorbents is comparable with other adsorbents such as activated carbon, 15 much higher than high surface area zeolite 13X (with 5.00 cm 3 STP cm À3 ). 32Fig. S9 † shows the determination of the optimum etching time according to CO 2 adsorption isotherms.It was found that 3 hours of NaOH etching was enough to expose the CaCO 3 core for optimum CO 2 adsorption.The CO 2 adsorption amount is due to the physisorption, which follows the sequence of CaCO 3 @C > CaCO 3 precursor nanoparticles > CaCO 3 @SiO 2 @C > Ca(OH) 2 particles, indicating that the combination of CaCO 3 core, hollow space and carbon shells is favourable for high CO 2 adsorption at low temperature.

Conclusions
CaCO 3 @C yolk-shell particles have been successfully synthesised by a selective etching method.The yolk-shell structures exhibited enhanced CO 2 uptake of 19.3 cm 3 STP g À1 (0.862 mmol g À1 or 31.64 cm 3 STP cm À3 sorbent) at 23 AE 1 C under 1 atm CO 2 , compared with 6.3 cm 3 STP g À1 for the original CaCO 3 core, or 4.5 cm 3 STP g À1 for the porous carbon shell.This was due to the relatively high surface area and pore volume, as well as the hollow space of the yolk-shell particle.It was shown that the core composition of the yolk-shell particles can be varied from CaCO 3 , CaO, or CaSiO 3 by simply tuning the recalcination temperature.These yolk-shell particles with porous calcium-based materials core and microporous carbon shell make them potentially attractive materials for environmental remediation (SO x , NO x removal), biomedical applications and nanocatalysis.Further investigation on the high temperature adsorption of CO 2 , SO x is ongoing.