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Author: Salim, Nisa V. ; Mateti, Srikanth; Cizek, Pavel;Hameed, Nishar; Parameswaranpillai,Jyotishkumar; Fox, Bronwyn

Title: Large, mesoporous carbon nanoparticles withtunable architectures for energy storage

Year: 2019Journal: ACS Applied Nano MaterialsVolume: 2Issue: 3Pages: 1727-1736URL: http://hdl.handle.net/1959.3/448261

Copyright: Copyright © 2019 American Chemical Society.The accepted manuscript is reproduced inaccordance with the copyright policy of thepublisher.

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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 2Swinburne University of Technology, Faculty of Science Engineering and Technology Hawthorn, VIC, AUS 3122 3CUSAT, Department of Polymer science and Rubber Technology, Kochi, IN 682022

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, electrochemical performance,

anode

Corresponding author: nisavs@deakin.edu.au; nsalim@swin.edu.au

ǂ Present address

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 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-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-b-PEO

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 PS-b-

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.

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-b-

PEO/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-b-

PEO/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 PS-

b-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 phenol-

PEO 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 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-b-PEO/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.

Fig. 3 (a) XRD and (b) SAXS profiles of pure PS-b-PEO and PS-b-

PEO/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.

10 15 20 25 302 theta (deg)

Inte

nsity

(a.u

.)

[PEO]/[Phenol] = 0.5:1

[PEO]/[Phenol] = 1:1

PS-b-PEO

[PEO]/[Phenol] = 2:1

(a)

0.0 0.1 0.2

[PEO]/[Phenol] = 0.5:1

[PEO]/[Phenol] = 1:1

[PEO]/[Phenol] = 2:1

PS-b-PEO

q (1/nm)

I (a.

u.)

√13

√9

√4

(b)

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.

I

II

III

Fig. 5. SEM and TEM images of the particles formed from PS-b-PEO/phenolic post-

hydrothermal 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.

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 800-1000

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.

10 100 1000 10000

0

5

10

15

20

25

30

PDI=0.098

Inte

nsity

(a.u

.)

Size (nm)

MCNs-A MCNs-B MCNs-C

PDI=0.059

PDI=0.159

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

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.

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.

Sample Surface area, (m2/g)

Pore volume (cc/g STP)

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

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.

0 10 20 30 40 50 60 700

200

400

600

800

1000

1200

1400

Cycle number

Spec

ific

Cap

acity

(mAh

g-1

)

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

0 5 10 15 20 25 30 35 400

200

400

600

800

1000

1200

1400

25 mA g-1

500 mA g-1200 mA g-1125 mA g-1

100 mA g-175 mA g-1

50 mA g-1

Cycle number

Dis

char

ge C

apac

ity (m

Ah g

-1)

MCNs-A MCNs-B MCNs-C 25 mA g-1

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).

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 g-1 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 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.

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