Revealing the Dynamic Formation Process and Mechanism of Hollow

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Revealing the Dynamic Formation Process and Mechanism of Hollow Carbon Spheres: from Bowl to Sphere’s Shape Xin Liu, Pingping Song, Jiahui Hou, Bo Wang, Feng Xu, and Xueming Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04634 • Publication Date (Web): 30 Dec 2017 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Sustainable Chemistry & Engineering

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Revealing the Dynamic Formation Process and Mechanism of

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Hollow Carbon Spheres: from Bowl to Sphere’s Shape

3

Xin Liu, Pingping Song, Jiahui Hou, Bo Wang, Feng Xu and Xueming Zhang*

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Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, 35 Qinghua

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East Road, Haidian District, Beijing, P. R. China, 100083.

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*Corresponding author

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E-mail: [email protected] (Xueming Zhang). Tel. and Fax: +86-01062336189.

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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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ABSTRACT

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Hollow carbon spheres are attracting great attention due to their great potential utilizations in

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drug delivery, energy storage and catalysis. However, the formation process and mechanism of

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the hollow carbon spheres were still unclear. Herein, we chose glucose as carbon precursor,

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double surfactants PEO–PPO–PEO triblock copolymers and sodium oleate as soft template, the

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synthesis process of hollow carbon spheres was investigated in the coupling of soft templating

29

method and hydrothermal carbonization system by regulating the reaction time. A dynamic

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formation process of the hollow carbon spheres was identified based on the results from SEM and

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TEM images, in which it experienced three evolution stages including hollow carbon bowls,

32

capsules and spheres. In addition, the formation mechanism was also presumed: During the

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synthesis process, the double surfactant interacted with each other to act as the soft template, and

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the glucose underwent hydration, polymerization and aromatization stages. When the

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concentration of aromatic compounds reached the critical supersaturation, the nucleation took

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place from a point and extended outward gradually along the interface to widen and thicken the

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carbon shell, resulting in different morphologies’ hollow structured carbon particles were formed

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successively by controlling the reaction time. Furthermore, the resultant hollow structured carbon

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particles were stable and uniform, and we made preliminary explorations on their biochemical

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and electrochemical performance.

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KEYWORDS: hollow carbon spheres, glucose, soft templating method, hydrothermal

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carbonization

43 44

INTRODUCTION 2


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Hollow carbon spheres (HCSs) have attracted increasing attention in recent years. Owing to

46

their unique structure and properties such as encapsulation ability, high surface-to-volume ratio,

47

surface functionality, excellent thermal and chemical stabilities1-3, they have been widely studied

48

for adsorption/separation, catalysis, drug delivery, synthesis template, micro-reactor, energy

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storage and conversion applications4-10.

50

Much effort has been devoted to the synthesis of HCSs by various approaches. The most

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commonly and widely used method is the template-assisted methods11-13. Generally, the

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template-assisted methods can be divided into hard templating method and soft templating

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method according to the types of core templates used. In the hard templating method, some

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specially prepared rigid particles are employed as core templates. When the carbon shells around

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the cores are generated, the templates are generally sacrificed by high temperature, dissolution, or

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acid/alkaline etching, resulting in the formation of the hollow carbon structure14. This method has

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the advantages of better control of the morphology and the size of products, while it generally

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includes the drawbacks of multi-steps, time-consuming and non-environmentally friendly 15-18. In

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comparison, the soft templating method, based on the self-assemble of the precursor molecules

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and the colloidal systems as the core-template, is more convenient. Organic additives, block

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copolymers, surfactants, nanoemulsion droplets or gas bubbles always act as the core-templates,

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which can be easily consumed or removed in the subsequent process, such as pyrolysis or

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extraction19,20. However, the precursors in the soft-template method usually tend to polymerize at

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low temperatures. Additionally, many carbon precursors are highly dependent on fossil-based

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compounds or carbonaceous polymers (ie. benene, phenol, ethylene, resorcinol, formaldehyde,

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dihydroxybenzoic acid, cokes, polystyrene, polyacrylonitrile)13,21-23, which are generally 3



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corrosive, toxic or expensive.

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Considering the environmental concern and economic value, it is preferable to prepare HCSs

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from renewable resources in a highly efficient and environmental friendly way24,25. Hydrothermal

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carbonization (HTC), as a facile, low-cost and environmentally friendly technique, is used for

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preparation of functional carbon materials, and the obtained carbon materials are considered as

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sustainable alternatives to traditional carbons6,24,25. The hydrothermal carbonization process has a

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couple of merits, including the use of renewable resources (either from isolated carbohydrates or

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crude plants), facile instrumentation and techniques, and a high energy and atom economy7,26.

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However, the main problems of HTC carbons are their intrinsic low porosity and conductivity6,27,

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and it has been reported that the template-assisted methods is a powerful tool to generate the

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porous materials28,29. Notably, coupling the template-assisted method with the hydrothermal

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carbonization process has shown powerful capability in controlling the synthesis of various

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carbonaceous nanostructures with special morphology and property7,29,30.

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In a previous study by Sun and Li4, monodisperse colloidal carbon microspheres were prepared

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from aqueous glucose solutions by hydrothermal synthesis, and encapsulated noble-metal

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nanoparticles were also obtained by this procedure. In addition, in Wang et al. work17, the

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formation of carbon structure was investigated based on the hydrothermal treatment of glucose on

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amino-functionalized surfaces of silica-based templates, in which the templates were inversely

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replicated. After heat treatment and aqueous hydrofluoric acid treatment, the well-developed

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porous shell hollow carbon spheres were obtained and they were applicable to catalyst supports.

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Recently, Wang et al.23 used PEO-PPO-PEO triblock copolymers (P123) and sodium oleate (SO)

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as double surfactants and 2,4-dihydroxybenzoic acid and hexamethylenetetramine as the polymer 4


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precursors to synthesize uniform hollow polymer spheres (HPSs) based on hydrothermal

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treatment. Moreover, PtCo bimetallic nanoparticles within the hollow carbon spheres

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((PtCo@HCS-500) were prepared, which showed outstanding catalytic performance in the

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hydrogenolysis of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF).

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Although some researches focused on the synthesis of hollow carbon spheres and their

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potential utilizations, few studies aimed at investigating their formation process and mechanism,

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especially under the coupling of soft templating method and hydrothermal carbonization system.

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Therefore, further investigation into the evolution states and forming mechanism of hollow

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carbon spheres is required. In this paper, Glucose, an inexpensive, nontoxic and sustainable

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carbon-rich biomass material, was chosen as the carbon precursor. The biocompatible and

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commercially available double surfactants PEO-PPO-PEO triblock copolymers (P123) and

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sodium oleate (SO) were used as the soft template. Since no hazardous chemicals involved in the

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synthesis process, and the coupling of soft templating method and hydrothermal carbonization

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approach was simple and facile, it was believed that our study met the concept of sustainable and

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green chemistry. A dynamic formation process of the hollow carbon spheres was studied by

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regulating the reaction time. Depending on the reaction time, this process experienced the

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dynamic formation of hollow carbon bowls (HCBs), capsules (HCCs), and spheres (HCSs). The

106

formation mechanism of hollow carbon structures has also been assumed, which would be

107

valuable for illustrating the hollow carbon spheres growth in the hydrothermal carbonization and

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soft templating method synergistic system. Additionally, the resultant hollow structured carbon

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particles (HCPs) were stable and uniform, and their performances in potential applications for

110

biomedical materials and supercapacitors were simultaneously evaluated. 5



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111 112

EXPERIMENTAL SECTION

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Chemicals

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The PEO–PPO–PEO triblock copolymers (P123) was purchased from Aldrich. Sodium oleate

115

(SO) and glucose were purchased from Beijing Ouhe Technology Co., Ltd and Sinopharm

116

Chemical Reagent

117

polytetrafluoroethylene (PTFE) were supplied from Beijing Lanyi Chemical Products Co., Ltd.

118

All the chemicals above were analytical grade and used without further purification.

119

Synthesis of hollow carbon spheres (HCSs)

Co.,

Ltd,

respectively.

Other

chemicals including graphite

and

120

Typically, 0.016 mol glucose was dissolved in 40 mL deionized water. Then, 20 mL of an

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aqueous solution containing 0.12 mmol SO and 0.0075 mmol P123 was added. After stirring for

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30 min at room temperature, the solution was transferred into a 75 mL autoclave and

123

hydrothermal treated at 160 °C for different reaction times (3, 6, 12, 18 and 24 h). Then the dark

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brown products were collected after centrifugation and washed several times with deionized

125

water, and dried at 60 °C overnight. Based on the different morphologies of the obtained particles,

126

we named them as hollow carbon bowls (HCBs), capsules (HCCs), and spheres (HCSs),

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separately. And all of these were collectively called the hollow structured carbon particles

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(HCPs).

129

Synthesis of carbonized hollow carbon spheres (CHCSs)

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The HCSs sample was undergone a temperature-programmed calcination, in which the specific 6


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program was set as follows. The HCSs were heated to 200 °C and maintained for 30 min, then

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calcined to 600 °C and held for 1 h, subsequently the HCSs were further heated to 900 °C and

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kept for 1 h. Whole process was performed under N2 atmosphere with a heating rate of 5 °C/min.

134

The obtained black powder was named as carbonized hollow carbon spheres (CHCSs).

135

Characterization

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Scanning electron microscopy (SEM) micrographs were taken with a MERLIN VP compact field

137

emission scanning electron microscope (ZEISS, Germany) using an accelerating voltage of 15 kV.

138

Transmission electron microscopy (TEM) and high resolution transmission electron microscopy

139

(HRTEM) micrographs were recorded on JEM-2100F (JEOL, Japan) operating at 100 kV with an

140

energy dispersive X-ray spectrometer (EDX). The dynamic light scattering (DLS) and zeta

141

potential measurements were measured with a zeta meter (Zeta Master, Malvern Instruments,

142

UK). The crystal structure of samples was recorded on an X-ray diffractometer (Smartlab, Rigaku,

143

Japan) with Cu Kα radiation (λ=0.154 nm). The Raman spectra were performed on a Raman

144

Spectrometer (JY HR-800, HORIBA Ltd., France) at room temperature. Fourier-transform

145

infrared (FT-IR) spectra were measured by a Perkin Elmer Spotlight 400 imaging system (Perkin

146

Elmer, UK). Surface tension measurements were obtained on a contact angle meter SL200KS

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(Kino, USA). The thermogravimetric analysis (TGA) was carried out using a TG-DTA 7300

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analyzer (SⅡ, Japan) from 20 to 900 °C in N2 flow of 50 mL/min at a heating rate of 10 °C/min.

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The N2 adsorption−desorption isotherms and pore size distribution analysis were performed by

150

using a physisorption analyser (SSA-7000, Builder, China).

151

Cytotoxicity evaluation

7



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The cytotoxicity of HCBs was evaluated by a cell counting kit-8 (CCK-8) assay. Mouse

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fibroblast-cells (L929) were seeded at a density of 4 × 103 cells per well in 180 mL culture

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medium and incubated for 24 h. Then, the cells were treated with different concentrations of

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HCBs solution at 37 °C in a humidified incubator with 5% CO2 for 24 h. Then, 20 µL of CCK-8

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solution was added to each well and incubated for 1 h at 37 °C. The absorbance value was

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measured with an infinite M200 microplate spectrophotometer (Tecan, Switzerland) at 450 nm

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wavelength. The cell viability was expressed as percentage of absorbance relative to control, and

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the control was obtained in the absence of HCBs.

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Electrochemical characterization

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The electrode material was a mixture of the active material, graphite and PTFE with the mass

162

ratio of 85:10:5. At room temperature, alcohol was added to the electrode material and stirred

163

continuously to form homogeneous slurry8. Then the slurry was coated on the nickel foam and

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pressed under 10 MPa for 5min. After being dried to evaporate the solvent, the working electrode

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was obtained.

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The electrochemical measurements were performed by using two-electrode test system

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consisting of two symmetric CHCSs-based electrodes in 6 M KOH electrolyte. All the

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electrochemical measurements were carried on Autolab 302N electrochemical workstation

169

(Metrohm Ltd, Switzerland) at room temperature. The cyclic voltammetry (CV) was obtained in

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the voltage range between 0 and 1 V at scan rates from 10 to 200 mV/s. Galvanostatic

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charge−discharge (GCD) measurements were carried out at current density from 0.1 to 5 A/g in

172

the voltage range between 0 and 1 V. The Nyquist plot was performed at open circuit potential

8


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over the frequency range from 0.01 to 100 KHz with an amplitude of 5 mV, and it could be fitted

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to an equivalent circuit including an electrolyte resistance (Rs), charge transfer resistance (Rct),

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Warburg element (Zw) and pseudocapacitance (Cp)31.

176 177

The specific capacitance of the symmetric supercapacitor was determined from the GCD measurements using the equation: ଶ୍×∆୲

178

C୫ = ∆୚×୫

(1)

179

where I (A) is the discharge current, ∆t (s) is the discharge time, ∆V (V) is the voltage change

180

(excluding the iR drop) within the discharge time, and m (g) is the mass of the active materials on

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the two electrodes.

182 183

The energy density (E, Wh/kg) and power density (P, W/kg) derived from the GCD curves were calculated by the following equations: (∆୚)మ

184

E = C୫ × ଼×ଷ.଺ ୉

185 186

P = ∆୲

(2) (3)

The controlled experiment with the pure graphite as the active material was tested according to

187

the above method.

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RESULTS AND DISCUSSION

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Our research aimed at revealing the formation process and synthesis mechanism of the hollow

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carbon spheres (HCSs) using glucose as the carbon precursor and coupling both the soft

191

templating method and hydrothermal carbonization process. The double surfactants

192

PEO-PPO-PEO triblock copolymers (P123) and sodium oleate (SO) were chosen as the soft

193

template. The P123 is known to have strong interaction with ionic surfactants, leading to the 9



194

formation of mixed micelles in aqueous media32. As shown in the Figure S1, the dynamic light

195

scattering (DLS) results showed the sizes of SO and P123 micelles in H2O were around 150 nm

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and 25 nm, respectively. When mixing the P123 with SO in the solution, the micelle size

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dramatically decreased to 3.5 nm. It was also noted that the critical micelle concentration of

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mixed micelles was less than that from pure SO (Figure S2), which confirmed the formation of

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P123/SO micelles due to the strong interaction between P123 and SO.

200 201

Figure 1. (a−e) TEM and (f−j) SEM images of the HCPs prepared with different reaction times.

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To understand the formation and evolution process of the HCSs, we characterized the physical

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morphology and structure of the HCPs obtained at different reaction times from 3 to 24 h by

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TEM and SEM images (Figure 1). The images in Figures 1a and 1f revealed that the bowl-like

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HCPs began to take shape even though the thin shell did not have enough mechanical strength to

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retain the plump structure (3 h). For the samples synthesized for 6h (Figures 1b and 1g), the

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morphology of bowl-like HCPs were more regular, but deflated structure and fragments still

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existed. When the reaction time prolonged to 12 h (Figures 1c and 1h), HCBs with diameter

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around at 400 nm and shell thickness of about 60 nm formed eventually, which became more

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uniform and monodisperse. After 18 h reaction, the HCCs with an opening formed as shown in

10


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Figures 1d and 1i, the average diameter was between 500 and 550 nm and shell thickness was in

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the range of 80-100 nm. Finally, the uniform HCSs with diameter of 550-600 nm and shell

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thickness of about 180 nm were formed after hydrothermal treatment for 24 h as shown in

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Figures 1e and 1j. Obviously, we could conclude that the diameter of hollow void first increased

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and then decreased and shell thickness increased with the increase of the reaction time, which

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indicated that the swelling of the nanoemulsion template and the polymerization in the core kept

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happening during the hydrothermal carbonization process.

218 219

Figure 2. TEM and SEM images of the corresponding morphologies synthesized with different molar ratios of P123

220

and SO: (a) 0:16 (without P123), (b) 0.5:16, (c) 2:16, (d) 4:16, (e) 1:0 (without SO).

221 222

Figure 3. TEM and SEM images of the corresponding morphologies synthesized with different amounts of glucose (a)

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and (c): 0.008 mol, (b) and (d): 0.032 mol.

224

In order to investigate the effect of different molar ratios of P123 and SO and amounts of

11



225

glucose on the morphologies of HCSs, some contrast experiments were also carried out as shown

226

in Figures 2 and 3. It was clearly noted that many small spherical carbon particles with around

227

180 nm diameter and some large HCCs were formed with the absence of P123 in the reaction

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system (Figures 2a and 2f). However, when only the P123 was added, the carbon particles with a

229

wide size distribution from 80 nm to 2 µm could be obtained (Figures 2e and 2j). As the ratios of

230

P123 to SO were 0.5:16 (Figures 2b and 2g) and 1:16 (Figures 1c and 1h), the relatively regular

231

HCBs formed, which indicated the P123 interacted with SO and served as morphology controller.

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The successive increasing of P123 (2:16) resulted in the formation of larger HCBs with uneven

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shell thickness as shown in Figures 2c and 2h. However, too much P123 (Figures 2d and 2i) led

234

to the formation of much bigger bowl-like HCPs and the size distribution was very broad, which

235

might be attributed to the instability of the emulsions in hydrothermal process23.Therefore, it

236

could be concluded that the optimal molar ratio between P123 and SO was 1:16 in our study.

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What’s more, we also adjusted the amount of carbon precursor (glucose) as shown in Figure 3.

238

When the amount of glucose was reduced to 0.008 mol, the deflated HCBs were formed with 300

239

nm in average diameter and 80 nm in shell thickness (Figures 3a and 3c). With increasing of

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glucose to 0.032 mol, the HCSs formed with uneven size and shell thickness as depicted in

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Figures 3b and 3d.

12


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242 243

Figure 4. FT-IR spectra of the glucose, HCBs and HCSs.

244

The FT-IR spectra corresponding to HCBs, HCSs and glucose raw material are shown in

245

Figure 4. It was clear that nearly no differences between HCBs and HCSs samples were observed.

246

The bands at approximately 2900 cm-1 corresponded to stretching vibrations of aliphatic C-H33.

247

And the band at 1000–1460 cm-1 were assigned to C-O stretching vibrations in hydroxyl, ester or

248

ether and O-H bending vibrations34. Notably, the intensities of the bands corresponding to the

249

hydroxyl groups (3000–3700 and 1000–1450 cm-1)35 in the samples became weaker compared

250

with the raw glucose, and new vibration band of C=O groups appeared at 1707 cm-1, which was

251

attributed to the dehydration reactions. Meanwhile, the new vibration peaks at 1615, 1510, and

252

1450 cm-1 unambiguously indicated the existence of benzene ring structure in the samples36,37,

253

and the bands at 875–750 cm-1 were assigned to aromatic C-H out-of-plane bending vibrations38.

254

Based on the above analysis, it was demonstrated that the aromatization of glucose occurred

255

during hydrothermal treatment. Additionally, the zeta-potential of HCSs was negative value

256

(-18.4 mV), which also confirmed the hydrophilic groups attached to the carbon framework39.

257

The presence of hydrophilic groups would greatly improve the stability of HCPs for enhancing 13



258

their dispersions in aqueous media, and be beneficial for the introduction of additional functional

259

groups or materials during hydrothermal reaction or post functionalization, resulting in further

260

improvement in the physical and chemical properties of HCPs7, 40,41.

261 262

Scheme 1. Schematic illustration of the formation process of HCSs.

263

A possible mechanism and formation process of HCSs were proposed as illustrated in Scheme

264

1. Firstly, the SO and P123 as the double surfactants formed mixed micelles due to the

265

hydrophobic interaction between the alkyl chains of SO and PO blocks in P123 in aqueous

266

media23, 42. As the temperature increased and time prolonged, the oleate would convert to oleic

267

acid and form oleic acid emulsion core43. Meanwhile, the glucose interacted with

268

PEO–PPO–PEO triblock copolymer template through the hydrogen bond between hydroxyl

269

groups and EO repeating units20, 44, and the glucose underwent dehydration reactions and led to

270

the formation of soluble products, such as furfural-like compounds (i.e. 5-hydroxymethyl furfural,

271

furfural) and some organic acids (i.e. formic, acetic and lactic acids)4, 38, 45. These acids lowered

14


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272

the pH of the system and acted as the catalysts for the subsequent further dehydration and

273

degradation reaction26,29,36. In addition, the PPO segments became more hydrophobic led to the

274

swelling of the nanoemulsion and the PEO segments contributed to their stability during this

275

stage18, 44. The subsequent reaction stage has been denoted as the “polymerization” stage, which

276

may be induced by the intermolecular dehydration, fragmentation, aldol condensation and

277

resulted in the formation of soluble polymers. Subsequently, the aromatization of polymers

278

occurred. When the concentration of aromatic compounds reached its critical supersaturation, a

279

burst nucleation took place and grew towards the interface of the emulsion droplets and water,

280

leading to the formation of carbon shell. Based on the SEM and TEM images from different

281

reaction time, the nucleation started from a point and extended outward gradually along the

282

interface to widen and thicken the carbon shell until the final size and morphology were achieved.

283

And a dynamic formation process of hollow carbon spheres was identified, which experienced

284

three evolution stages including hollow carbon bowls, capsules and spheres.

285 286

Figure 5. Cell viability of L929 cells in the presence of different concentration of HCBs.

287

To explore the potential application of the HCPs in biochemical application, the cell counting

288

kit-8 (CCK-8) assay was used to evaluate the cytotoxicity of the HCPs to living cells. As shown 15



289

in Figure 5, take the HCBs as an example, the L929 cell viabilities against samples were

290

estimated to be greater than 80% after 24 h incubation with the concentration ranged from 10 to

291

1000 µg/ mL, which indicated the excellent biocompatibility, low toxicity and safety in vitro and

292

in vivo applications of the as-prepared products46-47. Additionally, the HCPs had quite uniform

293

shell thickness and easily controlled void size, also possessed appropriate surface functional

294

groups, which may be helpful in realizing identical behavior in biochemistry and clinical

295

diagnostics prospects36, 48.

296 297

Figure 6. (a) TEM images of CHCSs and (b-d) Element mapping overlap, C and O elements in CHCSs.

298

The carbonized hollow carbon spheres (CHCSs) were obtained by a temperature-programmed

299

calcination of HCSs as depicted in the experimental. As shown in Figure 6a, the original

300

morphology of HCSs were still remained after high-temperature treatment. The elemental

301

mapping images (Figures 6b-6d) showed that all the expected elements, including carbon and

302

oxygen elements, could be detected and matched well with CHCSs’ structure. In order to

303

investigate whether the soft template core in the HCSs was removed after hydrothermal process

304

or not, the pure oleic acid, P123 and HCSs were analysed by thermogravimetric (TG) analysis

305

(Figure S3). It was shown that the onset of thermal degradation of pure oleic acid and P123 was

306

around 200 °C, which was higher than the hydrothermal process (160 °C). Moreover, the

307

maximum rate of weight loss was observed at the second pyrolysis stage, in which over 96%

16


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308

weight was pyrolyzed between 200 and 400 oC. Therefore, it was tentatively concluded that the

309

soft template core was still retained after hydrothermal process, while it was completely removed

310

in

311

adsorption–desorption isotherms and pore size distribution of CHCSs are shown in Figure S4.

312

According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the

313

N2 adsorption–desorption isotherms of the CHCSs (Figure. S4a) were identified as type I,

314

supporting the microporous nature of the interconnected microporous carbon framework49. The

315

CHCSs displayed a specific BET surface area of 871.02 m2/g and a pore volume of 0.59 cm3/g,

316

respectively. As shown in Figure S4b, the pore size distribution concentrated in micropores region,

317

the existence of micropores was beneficial for increasing the effective contact area between

318

electrolyte ion and electrode49,50. On the contrary, if the pores are too small to allow easy access

319

of electrolyte ions, they will not contribute to double-layer capacitance and will weaken the

320

ability to store charge51. Additionally, the hollow core in CHCSs can further shorten the ion

321

diffusion distance and accelerate the ion response rate18.

the

temperature-programmed

carbonization

process23.

322 323

Figure 7. (a) Raman spectra and (b) XRD pattern of CHCSs and HCSs.

17


What’s

more,

the

N2


324

The Raman spectra of CHCSs and HCSs (Figure 7a) showed two distinct peaks at 1587 cm-1

325

(G-band) and 1341 cm-1 (D-band), which were attributed to in-plane vibrations of crystalline

326

graphite and disordered amorphous carbon, respectively52. Both the D band and G band of HCSs

327

displayed a broad shape and relative low intensity, indicating the high content of amorphous

328

carbon. While the D band and G band of CHCSs showed sharper and high intensity peaks, which

329

indicated the improved graphitization degree after calcination53. The IG/ID intensity ratio of

330

CHCSs was calculated to be 1.04, indicating that the CHCSs was partially graphitized54. In

331

addition, the weak broad 2D peak observed at ~2800 cm-1 was scribed to the second-order zone

332

boundary phonons55. This kind of carbons with partial graphitization is highly desirable for the

333

application as electrode materials56. Figure 7b showed the XRD patterns of CHCSs and HCSs.

334

Before the carbonization, the XRD pattern of HCSs showed a broad peak at 2θ = 21.30°, which

335

was attributed to highly disordered and low crystallined carbon atoms. After carbonization, two

336

obvious broad diffraction peaks (around at 24° and 44°) were observed in the XRD pattern of

337

CHCSs, which were assigned to the stacking carbon layer structure (002) and ordered graphitic

338

carbon structure (100), suggesting the carbonization degree increased after carbonization process

339

and coincided with the Raman result57,58. Additionally, an obvious increase in the intensity at the

340

low-angle region of the XRD pattern also demonstrates a high porosity in the sample as

341

mentioned above49,59.

18


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342 343

Figure 8. (a) CV curves at various sweep rates and (b) GCD curves at different current densities for CHCSs.

344

The electrochemical performance of the CHCSs was measured with symmetric two-electrode

345

system in 6 M KOH solution. Obviously, comparing with the pure graphite electrode (Figure S5),

346

the performance of CHCSs electrode has been improved significantly. The cyclic voltammetry

347

(CV) curves in Figure 8a showed nearly symmetrical rectangular shapes in the range from 10 to

348

200 mV/s and displayed a more rectangular shape at a low scan rate. The existence of Equivalent

349

Series Resistance (Figure S6) and pore structure may cause the slight deviation of the CV

350

curves53. The galvanostatic charge/discharge (GCD) profiles (Figure 8b) exhibited nearly linearly

351

symmetric triangular shape and the columbic efficiency can still reach up to 94% when the

352

current density was 1 A/g, indicating electrode’s charge–discharge process was highly reversible.

353

The slight curvature shown on the lines might be ascribed to a certain degree of pseudocapacitive

354

behavior of the electrode due to the existence of the oxygen-containing groups as revealed in the

355

EDX and FT-IR results60. The specific capacitance of CHCSs was calculated as 116 F/g at 0.1 A/g，

356

and the CHCSs displayed an energy density of 4.03 Wh/kg and a power density of 25.1 W/kg

357

under the same condition. These results indicated an efficient double layer and fast ion transport

358

within the working electrodes was formed and the CHCSs possessed the potential in 19



359

supercapacitor application.

360 361

CONCLUSION

362

In summary, the formation process and mechanism of hollow carbon spheres were investigated

363

based on the coupling the soft templating method with hydrothermal carbonization process.

364

During this process, the double surfactant interacted with each other, resulting in forming mixed

365

micelles and acting as the soft template. Glucose underwent dehydration reactions and led to the

366

formation of soluble products, such as furfural-like compounds and some organic acids. These

367

acids lowered the pH of the system and acted as the catalysts for the subsequent further

368

dehydration and degradation reaction. Then the soluble derivatives went through polymerization

369

and aromatization stages. When the concentration of aromatic compounds reached its critical

370

supersaturation, a burst nucleation took place from a point and extended outward gradually along

371

the interface to widen and thicken the carbon shell until the final size and morphology were

372

achieved. By regulating the reaction time, a dynamic formation process of hollow carbon spheres

373

was identified, in which it experienced three evolution stages including hollow carbon bowls,

374

capsules and spheres. Furthermore, the resultant hollow structured carbon particles were stable

375

and uniform, and showed good performance in potential applications for biochemistry materials

376

and supercapacitors.

377 378

379

ACKNOWLEDGMENT

We are grateful for the financial support of this research from the Fundamental Research Funds 20


Page 20 of 31

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380

for the Central Universities (No. 2016JX03), National Key Research and Development Program

381

of China (2017YFD0601004) and Natural Science Foundation of China (No. 31470606).

382 383

384 385

Note

The authors declare no conflict of interest including any financial, personal or other relationships with other people or organizations.

386 387

ASSOCIATED CONTENT

388

Supporting Information

389

The size distribution and surface tension measurements of the P123, SO and P123/SO

390

micelle/emulsion in H2O. The TG and DTG curves of P123, oleic acid and HCSs. N2

391

adsorption-desorption isotherms and pore size distribution of CHCSs. Some supplementary

392

electrochemical results for pure graphite and CHCSs electrodes. The Supporting Information is

393

available free of charge on the ACS Publications website.

394 395

AUTHOR INFORMATION

396

Corresponding Author

397

*Tel. /Fax: +86-01062336189. E-mail: [email protected].

398

399 21



Page 22 of 31

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(59) Zhu, Y.; Murali, S.; Stoller, M. D.; Ganesh, K.; Cai, W.; Ferreira, P. J.; Pirkle, A.; Wallace, R.

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Oh, M. Flexible, Highly Graphitized Carbon Aerogels Based on Bacterial Cellulose/Lignin:

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TOC graph “For Table of Contents Use Only”

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589 590 591

Synopsis: The dynamic formation process from bowl to sphere’s shape and mechanism of hollow

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carbon spheres were revealed.

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Revealing the Dynamic Formation Process and Mechanism of Hollow

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