Large, Mesoporous Carbon Nanoparticles with Tunable Architectures

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Large, Mesoporous Carbon Nanoparticles with Tunable Architectures for Energy Storage Nisa V. Salim, Srikanth Mateti, Pavel Cizek, Nishar Hameed, Jyotishkumar Parameswaranpillai, and Bronwyn L. Fox ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00213 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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ACS Applied Nano Materials

Large, Mesoporous Carbon Nanoparticles with Tunable Architectures for Energy Storage Nisa V Salim1,2ǂ*, Srikanth Mateti1, Pavel Cizek1, Nishar Hameed2, Jyotishkumar Parameswaranpillai3, Bronwyn Fox2 1Deakin

University, Institute for Frontier Materials, Geelong, VIC, AUS 3216 University of Technology, Faculty of Science Engineering and Technology Hawthorn, VIC, AUS 3122 3CUSAT, Department of Polymer science and Rubber Technology, Kochi, IN 682022 2Swinburne

ABSTRACT: We report the synthesis of giant carbon particles with spherical and interconnected porous morphologies. Direct organic-organic self-assembly between Phenolic resin and polystyrene-block-poly(ethylene oxide) (PS-b-PEO) was employed to prepare these highly ordered carbon particles. Mesoporous carbon nanoparticles with tuneable textural characteristics such as large surface area, uniform pore structure, and high thermal properties were achieved by simply adjusting the ratio between the block copolymer to phenol without using acid, base or activating agents. The synthesized nanoparticles possess a very high surface area of up to 832 m2 g−1, and ultra-small pore size, as small as 3 nm and exhibited excellent electrochemical performance when used as active electrodes for lithium ion batteries. Making use of the secondary interactions between the functional groups of the block copolymer and phenol, this study created tuneable nanoparticles with excellent surface area and uniform morphologies that can be used in target oriented applications in the energy storage area.

Key

words:

Mesoporous

carbon,

block

copolymer,

nanospheres,

performance, anode

Corresponding author: [email protected]; [email protected] ǂ Present address

ACS Paragon Plus Environment

electrochemical

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION Synthesis of mesoporous carbon nanospheres (MCNs) with well-defined pore arrangement is very attractive owing to their excellent properties such as large surface area, chemical stability and hydrophobicity. The outstanding properties of MCNs make them ideal candidates for a range of applications, include in supercapacitors, catalysis, separation and controlled drug release and cellular delivery and many more.1-5 The fabrication of uniform carbon particles with narrow distribution is the greatest challenge as most of the times such particles get agglomerated.5,6 MCNs with pore size less than 50 nm can enhance localization path for reaction substrates with moderate accessible surface area.6 The most effective chemical method for tuning the structure of MCNs is the template carbonization method, and this can be done in two ways – through hard templating and soft templating approaches.7 Hard templating methods use inorganic materials such as alumina, silica, zeolites etc.7 and have drawbacks with hazardous chemical pathways involved that affect product yields. Moreover, this is expensive and the aggregation and cross-linking tendency of the particles make it difficult to fabricate ordered mesoporous carbon nanoparticles. The soft templating methods are conceptually very straight forward that uses materials such as surfactants or block copolymers as templates. This method can offer easier production of larger quantities of materials over minimum synthetic stages without the use of harsh acidic or basic treatments to remove the sacrificial templates. Recently, many studies have been reported to fabricate mesoporous carbon nanoparticles with a facile control to tune pore size and structure at an accuracy of the angstrom level.8,9 There are many soft templating studies reported using organic macromolecules or biomass such as saccharides, by pyrolysis or chemical activation methods.10-12 However, porous materials synthesized with these methods exhibited disordered structures and have less porosity. Therefore, it is challenging to prepare highly ordered mesoporous carbon particles. Block copolymers (BCPs) on the other hand are ideal candidates for creating various ordered structures like spheres, cylinders, gyroid and other complex phases by self-assembly approach and thus can be used as feasible structure directing agents.13 Hydrothermal carbonization (HTC) is considered as a fast and more energy efficient thermochemical pre-treatment processes. Zhao et al conducted hydrothermal synthesis of carbon monoliths by templating block copolymers of Pluronic F127 and P123 with phenolic resol in the presence of a base catalyst.14 The resultant monoliths showed a surface area of 620 m2 g-1. Liu et al.15 reported the synthesis of ordered mesoporous carbon materials using


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citric acid as the catalyst, while disordered structures were obtained when Hydrochloric acid (HCl) was used as the catalysts at the same conditions. There are a few porous carbon studies reported by using glutamic acid, lysine and N-containing catalysts, in addition to the conventional inorganic compounds like HCl and Sodium hydroxide (NaOH).16 The fabrication of MCN with narrow distribution is the greatest challenge as most of the times such particles get agglomerated. Generally, a strong acid or base is required to induce heterogeneous nucleation and growth to synthesize MCNs. The key challenge to make MCNs is to induce the crosslinking reaction of the components by poly-condensation of toxic phenol-formaldehyde resins with the aid of catalysis of either inorganic acid or base by using large amount of solvents. There has been only a few studies reported on the synthesis of uniform, giant and hollow monodispersed carbon spheres by HTC of polymers due to the high surface energy of small particles.17 However, little work has been involved in the fabrication of tunable mesoporous carbon particles from phenol without the use of such strong acid or base. In this paper, for the first time, we have successfully developed a facile method for synthesizing tuneable carbon spheres with uniform porous strcutures without the use of strong acid or base or any other catalysts. MCN with large surface area, uniform pore size, and high thermal properties were achieved by simply adjusting the ratio between the block copolymer to phenol without using any activating agents. The concept is based on inter polymer interactions between the template and the carbon precursor, thus leading to a stable complex compared to analogous systems. Making use of the secondary interactions between the functional groups of the block copolymer and phenol, this study created highly stable, tuneable nanoparticles with excellent surface area and uniform morphologies that were further demonstrated for their target applications as electrodes for lithium ion batteries. In the present study, we used polystyrene-block-polyethylene oxide (PS-b-PEO) as the template and phenolic resin, with its high thermal conductivity and high carbon yield was used as the carbon precursor. The interaction between phenol to PS-b-PEO followed I0S0 self-assembly mechanism, where I0 represents the –OH moiety and S0 represents the hydrophilic parts of the block copolymer irrespective of I+XˉS+ mechanism carried out in the absence of electrostatic interactions (here Xˉ represents the counter ion). Hydrogen bonding interactions between the sacrificing polymer blocks and precursor play a key role to synthesize highly dispersed uniform size carbon spheres by hydrothermal methods. The addition of phenolic resin to PS-b-PEO enables the creation of porous carbon without the use of extreme acidic conditions. Here the introduction of phenol into PS-b-PEO leads to the formation of self-



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assembled micellar aggregates due to the complexation between PEO and phenol molecules. Our studies conformed the complexation between phenol and PEO and the repulsion between PS chains w.r.t the molar ratio of PEO to phenol manipulate the evolution of various micellar morphologies. Moreover, the size control of the MCNs are easy to implement by adjusting the ratio between [PEO] and [Phenol]. The inter polymer interactions leads to the creation of spherical, and interconnected porous carbon particles when different molar ratio of block copolymer to phenols were used. EXPERIMENTAL SECTION Preparation of nanostructured PS-b-PEO/Phenolic nanoparticles. The materials used in the

present

study

include,

polystyrene-block-poly(ethylene

oxide)

(PS-b-PEO),

tetrahydrofuran (THF), phenolic resins, and hexamethylenetetramine (HMTA). The PS-bPEO block copolymer was purchased from Polymer Source, Inc., with Mn (PS) = 59,000, Mn (PEO) = 31,000, and Mw/Mn = 1.03. The desired amount of PS-b-PEO di block copolymer was dissolved in minimum amount of Tetrahydrofuran (THF) followed by the addition of deionized water to make the complexes and thereby induce the self-assembly. Subsequently, as the addition of water continues, solvent quality becomes poor for the PS blocks. The block copolymer solution was then transferred into a dialysis membrane tube and dialyzed for two days to completely remove THF. A calculated amount of dissolved phenolic/HMTA (molar ratio of [Phenol]:[HMTA] was kept constant at 1:0.12), in water was added dropwise to the block copolymer mixture with continuous stirring and transferred into a Teflon-lined autoclave for hydrothermal synthesis. The curing reaction was conducted in an oven set at 160 °C for 5 hours. The PS-b-PEO/Phenolic resin nanospheres were collected from brownish/yellow water solutions by centrifugation after washing with distilled water. As collected samples were dried at 60 °C for 24 hours and later pyrolyzed in a tube furnace at rate of 5 °C/min at 400 °C for 1 hour and at 800 °C for 4 hours under N2. RESULTS AND DISCUSSION A schematic illustration of the synthesis of the highly ordered MCNs is detailed in Fig. 1. The MCNs were synthesised via a three stage process; (1) preparation of micellar complexes from PS-b-PEO block copolymer and phenolic resin (2) hydrothermal curing process of PSb-PEO/phenol complexes and HMTA and (3) carbonisation of PS-b-PEO/phenol/HMTA. Here phenol is used as the carbon precursor and PS-b-PEO block copolymer as the soft template and during blending, phenol makes hydrogen bonds with PEO blocks of the block copolymer.


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Fig. 1 Synthesis of MCNs from /PS-b-PEO/phenol complexes by hydrothermal process Formation of PS-b-PEO/phenolic complex micelles In stage 1, the micellar self-assembly of PS-b-PEO was performed in the solvent mixture of THF and water, leading to the complexation of the block copolymer. During dialysis, the phase inversion takes place forming cylindrical micelles, where PEO block of PS-b-PEO interacts with water forming the ‘shell’ and the PS block centred within the shell forming the ‘core’.18 To kinetically freeze the core-shell structure, an excess amount of water was added rapidly into the mixture.18 The SEM images of the polymer particles derived from aqueous solution of phenolic/HMTA with different molar ratio of PS-b-PEO block copolymer are shown in Fig. 2. From SEM, the neat PS-b-PEO block copolymer exhibits cylindrical morphology (Fig. 2a). It can be noted from the images that there is a ‘bulb’ like structures at the end of the cylindrical body forming dumbbell-like intermediates.



Fig. 2A SEM images of the (a) PS-b-PEO block copolymer and mix of PS-bPEO/phenolic with [PEO]/[Phenol] ratio (b) 0.5:1(c) 1:1 (d) 2:1

Fig. 2B TEM images of the (a) PS-b-PEO block copolymer and mix of PS-bPEO/phenolic with [PEO]/[Phenol] ratio (b) 0.5:1(c) 1:1 showing the neck formation. A morphological transformation from cylinders to spherical composite micelles was observed in PS-b-PEO/phenolic mixture by upon addition of phenol. The phenolic mixed PSb-PEO complexes showed a combination of spherical and rod like composite micelles as evidenced by Fig. 2b. The surface (or corona) of these spherical micelles is hydrophilic and has a distribution of OH groups and ether groups formed from the hydrogen bonded phenolPEO blocks that form the matrix and PS blocks phase seperate in the continuous phenol-PEO matrix. Plethora of studies have been conducted on the hydrogen bonding interactions and miscibility of PEO blocks with phenolic resin.19 Here, by complexing with phenolic, the block copolymer exhibited a noticeable morphology transition that proceeds through


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cylinders with ‘bulbs-’ released from the ends of the cylindrical body, i.e. the bulbs are progressively pinched off and spherical particles are released in order to reduce the interfacial energy of the cylindrical structures. The morphological transitions including the neck formation are detailed in the Figure 2B. Here, the cylindrical to spherical composite micelle transformation is induced by an effective corona volume enlargement and also the large interfacial curvature between hydrophilic/hydrophobic chains.20 Phenol/PS-b-PEO composite micelles have an average diameter of 260 nm. The size of the PS-b-PEO/phenol micelles increases progressively until the interfacial energy is exceeded by the core-chain stretching and corona repulsion.20a Here, the density of corona chains contains high hydrophilic content from the complexes thereby pushing the blocks of corona from each other to transform the structure to spherical particles (Fig. 2c and 2d). XRD patterns of the PS-bPEO/phenol/HMTA are illustrated in Fig. 3a. The reflections corresponds to 19° and 23° indicate the crystalline patterns of PEO blocks in the block copolymer. PEO also shows weak diffraction at 13.61° and 27.32° respectively. It can be seen that the addition of phenol has deteriorated the crystalline orientation of PEO blocks w.r.t various [PEO]/[Phenol] ratios. The decline in the crystalline peak of PEO blocks confirms the formation of strong intermolecular interactions with phenol chains. To further confirming the short-term order of the PS-b-PEO/phenol/HMTA mixtures, SAXS measurements were performed. As displayed in Fig. 3b; PS-b-PEO shows q/q* values of close-packed cylindrical structures, located at values of 1: √4: √9: ….√17. SAXS profiles of the PS-b-PEO/phenol/HMTA mixtures shows weak and broad peaks, demonstrating the declining of existing block copolymer long-range order which is in agreement with SEM images.



(a)

(b) 4

PS-b-PEO

[PEO]/[Phenol] = 2:1

9

I (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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13

PS-b-PEO

[PEO]/[Phenol] = 1:1

[PEO]/[Phenol] = 2:1 [PEO]/[Phenol] = 0.5:1

[PEO]/[Phenol] = 1:1 [PEO]/[Phenol] = 0.5:1

10

15

20

2 theta (deg)

25

30

0.0

0.1

0.2

q (1/nm)

Fig. 3 (a) XRD and (b) SAXS profiles of pure PS-b-PEO and PS-bPEO/phenol/HMTA mixtures Hydrothermal curing of PS-b-PEO/phenol/HMTA mixtures In stage 2, a calculated amount of aqueous phenolic-HMTA solution was added drop wise into PS-b-PEO/phenolic complexes and stirred for another 5 hours. Phenol to PEO molar ratios of the solution were adjusted as 0.5:1, 1:1 and 2:1 and were stabilized under hydrothermal process. After curing, a change in colour of solution was observed from milky white to brownish/yellow, confirming the formation of PS-b-PEO/phenol/HMTA particle formation. Under hydrothermal conditions, the PS-b-PEO/phenol composite micelles transformed to form spherical particles. As the temperature and pressure increases during the hydrothermal reactions, the decomposition of HMTA takes place and form formaldehyde and ammonia (Fig. 4, step I). As formed Ammonia, in turn, catalyzes the polymerization of phenol and formaldehyde, (Fig. 4, step II) and is followed by the aggregation of phenol networks and to form continuous and highly interconnected structures.


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I II

III

Fig. 4 Chemical reaction of Phenol and HMTA during HTC As the cross-linking of phenol progresses, PS-b-PEO/phenolic composite structures unite each other by carbon-carbon bonds to form micelle aggregates. When the reaction proceeds, a higher number of micelles adheres on the surface of the nuclei and transforms into solid particle structures. Here, composite structure of PS-b-PEO/phenolic corona and resin free PS core were gained, that later leads to the mesoporous structure after carbonisation. Fig. 5 shows the SEM and TEM images of PS-b-PEO/phenol/HMTA particles formed after hydrothermal process at various [PEO]/[phenol] ratio. All ratios of [PEO]/[phenol] show hydrogen bonded phenol/PEO matrix with phase separated PS domains. SEM and TEM images also confirm increase in particle size with an increase in BCP concentration. Here, the PS-b-PEO template supports phenol/HMTA polymerization yielding bigger spherical particles.



Fig. 5. SEM and TEM images of the particles formed from PS-b-PEO/phenolic posthydrothermal reactions when [PEO]/[Phenol] ratio (a) 0.5:1(b) 1:1 (c) 2:1 Formation of MCNs via carbonisation process After the hydrothermal process, the dried particles were carbonized and the MCNs prepared from [PEO]/[phenol] with 0.5:1, 1:1 and 2:1 are denoted as MCN-A, MCN-B, and MCN-C, respectively. During carbonisation, PS-b-PEO block copolymers act as sacrificing compounds that decompose and generate pores. All samples experienced shrinkage from the elimination of volatile compounds including non-carbon atoms and thus the particles become carbon- rich compounds after carbonization.22 The morphology and size of MCNs were confirmed using both SEM and TEM as shown in Fig. 6A. The particle size measured from SEM and TEM indicates that the diameter of the MCN-A, MCN-B, and MCN-C are 200-400 nm, 400-600 nm and 800-1000 nm, respectively. TEM images of the MCNs at higher magnifications are displayed in Fig 6B. The MCN-C prepared at a higher concentration of block copolymer exhibit interconnected porous structures (Fig. 6A (c)). The transformation from mesoporous nanospheres to interconnected porous spheres, with block copolymer content is due to the thermodynamic change of PS hydrophobicity and the selective swelling of EO blocks. Moreover, the addition of HMTA into phenol has enabled MCNs formation without using catalysts, leading to high-purity carbon materials.


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Fig. 6 SEM (left) and TEM (right) images of (a) MCN-A (b) MCN-B (c) MCN-C (d) magnified TEM image of MCN-C



DLS provides further information on the particle characteristics of MCNs such as hydrodynamic diameter (Dh) and its distribution as represented in Fig. 7. The measurements were conducted by dispersing the carbon particles in deionized water without any modifications. For all MCN solutions, the Dh and its distribution consist of a single peak, indicating the homogeneity of the particles. The intensity of the peak represents the relative accumulation of the carbon particles with size, i.e. Dh. As shown in Fig. 7. MCN-C shows an giant carbon particles, which agrees well with what was observed in SEM and TEM. The peak shifts towards higher Dh for MCNs with increasing block copolymer content (Dh values obtained from DLS studies for MCN-A, MCN-B, and MCN-C are 200-400, 600, and 8001000 nm, respectively). The results show that the size and pore diameter of hollow particles can be tuned by changing the composition of the PS-b-PEO with respect to phenol/HMTA carbon precursor. 30 PDI=0.159

25 PDI=0.059

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MCNs-A MCNs-B MCNs-C

20

15 PDI=0.098

10

5

0 10

100

1000

10000

Size (nm)

Fig. 7 DLS studies of MCNs The carbon phases of the particles were analysed using Raman spectroscopy and the results are summarized in Fig. 8a. All carbon particles display a G band at about 1580 cm-1 and a D-band at 1350 cm-1 corresponding to the vibration of graphitic sp2 bonded carbon atoms and sp3 carbon atom (related to the disordered structures in the carbon materials), respectively. The band intensity of G in comparison to D proposes a high degree of graphitization. As the high level of graphitization is related to the applied temperature, it is possible to improve the graphitic structures if the particles were carbonized at high temperature.23,24


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Fig. 8a Raman spectra of MCNs (b) XRD pattern of MCN-A

Relative intensity ratios of IG/ID is proportional to the degree of graphitization of the carbon.25,26 The IG/ID values obtained for carbon particles include, MCN-A (1.06), MCN-B (1.07), MCN-C (1.08) indicating the co-existence of amorphous as well as graphitic carbon. 25,26

The XRD pattern of the MCNs (Fig. 8b) shows two peaks around 25 and 44°, indicating

(002) and (101) planes of the amorphous carbon.27 The surface area and the pore volume of the MCNs were studied using nitrogen adsorption/desorption measurements, carried out by Brunauer–Emmett–Teller (BET).

Fig. 9 The N2 adsorption-desorption isotherms of MCNs As shown in Fig. 9 all the MCNs exhibited type IV hysteresis curve implies the presence of mesopores in the materials. With block copolymer content, surface area was calculated to be 626, 698 and 832 m2 g−1 respectively for the MCNs prepared from PS-b-PEO/phenolic precursors. MCN A and B show only single pore system compared to MCN C. The desorption curve displays a large hysteresis loop in the range of P/P0 =0.4−1 and are related to mesopores formed through the pyrolysis of PS-b-PEO block copolymers.



Sample

Pore Surface volume area, (cc/g (m2/g) STP)

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Pore diameter (nm)

MCN- A 626.8

0.89

3.2

MCN- B 698.6

0.71

3.3

MCN- C 832.6

0.61

3.4, 6.8

Table 1 Specific surface area calculated by BET mean mesopore diameter by BJH model with the adsorption branch and total pore volume by adsorption capacity. Table 1 shows the values corresponding to the pore volumes, pore diameter and surface. The specific surface area and pore volume were increased with block copolymer content. The small pore diameter refers to the formation of the space during the pyrolysis of block copolymer. Hence the average size of the MCNs can be varied by changing the EO/phenolic ratio in the solution. The isotherms in Fig. 9 show little uptake at low relative pressures (P / P 0 < 0.1), demonstrating the existence of micropores. The H3-type hysteresis loops at 0.4 < P / P 0 < 0.9 confirm the presence of interconnected mesopores.27 The uptake at high relative pressures (P/ P 0> 0.9) displays the presence of extra-large mesopores or macropores from the stacking interspace of the MCNs. The presence of high surface area can allow large material/electrolyte area, and diffusion of electrolyte to active sites with both less resistance and the volume change. As these carbon particles possess ample micro and meso pores with interconnected hierarchical porous structure for charge acceleration, MCNs are proved to be a promising electrode material for energy storage area. The electrochemical characterization of the MCNs is performed and is shown in Fig. 10.


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1400

1400

1000 800 600 400 200 0 0

10

20

30

40

50

60

MCNs-A MCNs-B MCNs-C

25 mA g-1

Discharge Capacity (mAh g-1)

MCNs-A (Charge) MCNs-A (Discharge) MCNs-B (Charge) MCNs-B (Discharge) MCNs-C (Charge) MCNs-C (Discharge)

1200

Specific Capacity (mAh g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70

-1

50 mA g

1200

75 mA g-1

100 mA g-1

125 mA g-1

1000

200 mA g-1

500 mA g-1 25 mA g-1

800 600 400 200 0 0

5

Cycle number

10

15

20

25

30

35

40

Cycle number

Fig. 10 (a) Lithium ion battery cyclic performance and (b) Rate capability studies of the MCNs As prepared MCNs are used as the active electrodes for lithium ion battery. The cyclic performance of the MCNs after 75 cycles exhibited values of 250, 251 and 503 mAh/g for MCN-A, MCN-B and MCN-C, respectively. Figure 10 displays that the capacity of the MCNs reduces gradually in the first 10 cycles due to the decomposition of electrolytes and the absence of irreversible contribution of solid-electrolyte interface (SEI) between the textures.7,28 The specific capacity of MCN-C were retained higher than the theoretical capacity calculated for graphite which is 372 mAh/g at 75 cycles mainly due to the inter connected and open mesopore structures. The rate capabilities of the MCNs were examined at various rates by changing current density from 25 mAg-1 to 500 mAg-1. The initial discharge capacities of the MCN-A, B and C at 25 mAg-1 were observed as 1301 mAh g-1, 800 mAh g-1 and 482 mAh g-1 respectively. The MCN-C still achieved a higher 1100 mAh g-1 when the current density was decreased to 25 mAg-1. MCN-Cs shows superior cell performance compared to MCN-A and B owing to their pore size and pore size distributions. The large surface area and porosity provide plenty accessible active sites for Li-ion intercalation and thus may lead to a high reversible specific capacity than the theoretical value for graphite.29 Therefore, the battery electrodes from carbon materials possess high rate capability, as the electrolyte can seep into the particles and thereby increase the contact area of the electroactive surface with the electrolyte. The presence of meso pores shortens the diffusion length of Li+-ion transport and enhances the kinetics.30 Hence, these materials can withstand high currents during charge/discharge cycling. The results suggested that MCNs formed from PS-b-PEO/phenol-HMTA system are promising candidates for Li ion battery materials.



We have successfully prepared a method for creating mesoporous carbon particles without use of acids or bases and the particles possess excellent surface areas and uniform morphologies that can be used in target oriented applications in the energy storage area. The most interesting characteristics of MCNs is that there is an obvious morphology transformation with respect to the composition leading to tuneable mesoporous structures. The morphology transformation is schematically represented in Fig. 11. Fig. 11a indicates the cylindrical morphology of the PS-b-PEO in water. Microscopy and X-ray studies have confirmed that PS-b-PEO block copolymer morphologies were changed from cylindrical structures to disordered spherical micelles with the addition of phenol. The driving force for the morphological transition is the effective miscibility of the PEO blocks in block copolymers with phenol, thereby forming the secondary interactions. It is already reported by Hamley,21 that phase separation takes place by the minority block in the PS-b-PEO is get separated from the majority chains, thereby form ordered nano structures. The volume fraction of the minority block (say ƒx) and block incompatibility contributes to the morphology and order of phase separated chains. By addition of phenol into PS-b-PEO, the volume fraction of PEO increases and the change in the [PEO]/[Phenol] ratio causes the transition of the cylindrical morphology to disordered structures. During the hydrothermal stage, PS-b-PEO/phenol micelles solidified to form polymer particles through cross-linking reactions with HMTA. The morphology of the as-formed polymer particles was determined by actual micelle morphology. Fig. 11b shows raspberry like particles formed under hydrothermal reactions at various PEO/phenol compositions. Here hydrogen bonded PEO/phenol forms the matrix of the particle and PS blocks get phase separated and imbedded in the matrix. The switching of morphologies by varying the block copolymer composition is determined by selective swelling of the PS phase and the hydrogen bonding interaction of non-swelling phenol-PEO phases. As such, the composition of PS-b-PEO is one of the factors of the resultant structure of MCNs (Fig. 11c). When the PEO/phenol ratio becomes 1:1, radius of the raspberry sphere becomes more definite and polydisperse (Fig. 11).


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Fig. 11 Schematic representation of MCNs formed from PS-b-PEO/Phenol-HMTA organic assembly It can be observed from SEM and TEM studies that all the three, MCN-A, MCN-B, and MCN-C, exhibit a well-established porous structure and maintained spherical morphologies. Moreover, particles with more block copolymer have larger pores and inter connected carbon networks and appeared as raspberry like structures. The average size of these porous carbon particles was increased with an increase in PS-b-PEO composition. When the PS-b-PEO is high, the hydrophobicity of PS blocks increases due to the solvent quality; and this can further swell the PS chains that eventually results in an increase in particle size. The thermodynamic of PS hydrophobicity and selective swelling of EO blocks can explain the transformation of morphologies from mesoporous nanospheres to interconnected porous spheres, with increasing amount of block copolymer. Moreover, the addition of HMTA into phenol enabled the MCNs formation without using any catalysts, leading to high-purity carbon materials. As the cross-linking of phenol progresses, PS-b-PEO/phenolic composite micelles connect with each other through carbon-carbon bonds to form micelle aggregates. When the reaction



proceeds, a higher number of micelles adheres on the surface of the nuclei and transforms into solid particle structures. Here, composite structure of PS-b-PEO/phenolic corona and resin free PS core are obtained, that later leads to the mesoporous structure after carbonisation. At higher block copolymer composition, the changes are dominated by the continuous growth of the polymer chains in the matrix, reslts an imbalance of the force. Moreover, aggregates change progressively from PEO to bonded PEO/phenol, then to the bonded PEO/phenol coexisted with excessive PEO-b-PS w r t hydrogen bonding interactions. So the microporosity is generated by pyrolysis of cross-linked phenol, hydrogen bonded PEO/phenol along with the removal of remaining PS-b-PEO. During the carbonization of block copolymer at high PEO/phenoxy content, leads the formation of interconnected microtunnels between the carbon spheres, leading to a transition from raspberry like spheres to an interconnected porous morphology. In short, hydrogen bonding interaction between PEO and phenol enables the organization of the carbon precursors and therefore, high hydroxyl density from phenol provides the greater driving force for the self-assembly interaction with the PEO blocks. The phase separation of the PS-b-PEO/phenolic resin in water is the key to the wide processing conditions for synthesizing mesoporous carbons based on phenolic resins. CONCLUSIONS We have fabricated novel, tuneable mesoporous carbon nanoparticles by utilising the hydrogen bonding interaction between PS-b-PEO block copolymer and phenolic resin. . Particles with highly ordered porous structure was obtained via hydrothermal carbonization without the addition of any catalysts for the preparation and stabilization of the MCNs. The addition of HMTA enabled the formation of formaldehyde and further catalysed the polymerization reactions. All MCNs produced by direct self-assembly process exhibited tuneable porosities with a high surface area of up to 832 m2 g−1. The interconnected porous structures formed from MCN-C demonstrated excellent electrochemical behaviour indicating promising application to use as an anode in lithium-ion batteries. Rate capabilities studies confirmed that porous carbon materials can withstand high currents during charge/discharge cycling. The cyclic performance of the MCNs after 75 cycles exhibited values of 250, 251 and 503 mAh/g for MCN-A, MCN-B and MCN-C, respectively. The initial discharge capacities of the MCN-A, B and C at 25 mA g-1 were observed as 1301 mAh g-1, 800 mAh g1 and 482 mAh g-1 respectively. MCN-Cs shows superior cell performance compared to MCN-A and B owing to their pore size and pore size distributions. The results suggested that


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MCNs formed from PS-b-PEO/phenol-HMTA system are promising candidates for Li ion battery materials. ACKNOWLEDGEMENTS The Author Nisa V. Salim would like to thank Veski -the Victoria Fellowship for providing the opportunity for travelling overseas and conducting the research. N.V Salim acknowledge financial support from Deakin University for Alfred Deakin Post-Doctoral Fellowship. N.H would like to thank Australian Research Council for a DECRA Fellowship funding (DE170101249) Supporting Information Available: Further experimental and characterization details, (i.e., X-ray diffraction analysis, Surface area measurements, Electron microscopy analysis, particle, Size distribution, Raman spectroscopy and Electrochemical performance) for presented materials. REFERENCES 1. Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F. A thermally re-mendable cross-linked polymeric material. Science 2002, 295, 1698– 1702. 2. Liu, J.; Qiao, S. Z.; Liu, H.; Chen, J.; Orpe, A.; Zhao, D.; Lu, G. Q. Extension of The Stöber Method to the Preparation of Monodisperse Resorcinol–Formaldehyde Resin Polymer and Carbon Spheres. Angew. Chem. Int. Ed. 2011, 50, 5947–5951 3. Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles. Nature 2001, 412, 169–172 4. Meng, Y.; Gu, D.; Zhang, F.; Shi, Y.; Yang, H.; Li, Z.; Yu, C.; Tu, B.; Zhao, D. Ordered mesoporous polymers and homologous carbon frameworks: amphiphilic surfactant templating and direct transformation. Angew. Chem. Int. Ed. 2005, 44, 7053–7059. 5. Liang, C. D.; Li, Z. J.; Dai, S.

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339x334mm (96 x 96 DPI)


Large, Mesoporous Carbon Nanoparticles with Tunable Architectures

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