Plasma-assisted simultaneous reduction and nitrogen, sulfur co

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Plasma-assisted simultaneous reduction and nitrogen, sulfur codoping of graphene oxide for high-performance supercapacitors Yuanling Miao, Yulong Ma, and Qi Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05838 • Publication Date (Web): 17 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Sustainable Chemistry & Engineering

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Plasma-assisted simultaneous reduction and

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nitrogen, sulfur co-doping of graphene oxide for

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high-performance supercapacitors

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Yuanling Miao, †, ‡, § Yulong Ma, †, ‡ and Qi Wang*, †, ‡, §

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†

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Physics, Chinese Academy of Sciences, 350 Shushanhu Road, Hefei 230031, Anhui, China

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‡ University

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§Collaborative

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Institutions, Soochow University, Suzhou 215123, China

Key Laboratory of Photovoltaic and Energy Conversation Materials, Institute of Plasma

of Science and Technology of China, 230026 Hefei, China Innovation Center of Radiation Medicine of Jiangsu Higher Education

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KEYWORDS: Plasma treatment, Reduced graphene oxide, Sulfur/nitrogen co-doped,

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Supercapacitors

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Corresponding Author: *Qi Wang, Email: [email protected]

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ORCID: Qi Wang: 0000-0003-3594-2244

ACS Paragon Plus Environment

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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: Nitrogen and Sulfur co-doped reduced graphene oxide (RGO) was prepared

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with the assistance of an environmentally benign and facile route using an Inductive

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Coupled Plasma (ICP) source. This novel gas-phase process avoids high temperature, toxic

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solvents and can minimize re-aggregation of graphene flakes. The plausible mechanism

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of the plasma treatment to achieve simultaneous doping and reduction of graphene oxide

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is discussed. Morphology and component characterization indicate that oxygen-

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containing functional groups were effectively removed, meanwhile N and S elements were

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uniformly doped into the graphene nanosheets. Electrochemical performance of the

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plasma treatment electrode is better than that of the hydrothermal prepared electrode

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and graphene oxide (GO), which has a high specific capacitance of 307.4 F g-1 at 1 A g-1

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and retains 83% of the capacitance after 10,000 cycles at 4 A g-1 in 6 M KOH. In addition,

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the symmetric supercapacitor device was fabricated and the energy density of the plasma

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treated sample is 9.33 Wh Kg-1 at the power density of 125 W Kg-1 and remains 7.36 Wh Kg-1 at

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10 KW Kg-1 in 6 M KOH. Hence, the plasma-assisted technique is an effective route to

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introduce heteroatom and the as-prepared N, S co-doped graphene-based material

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exhibits great potential for high-performance supercapacitors.

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INTRODUCTION

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With the rapid consumption of fossil energy and the increase of energy demand,

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researchers are forced to explore sustainable energy and efficient energy storage


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devices.1-2 Supercapacitors have attracted great research interests owing to the

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advantages of ultra-high power density, fast charge/discharge rate, long cycle life and

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environment friendly, which combine the superiority of conventional capacitors and

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batteries.3-5 According to the reaction mechanism, the supercapacitors can be divided into

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electrical double layer capacitors (EDLCs) and pseudocapacitors.6 The EDLCs electrode

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materials, mainly are carbon materials such as activated carbon,7 carbon nanotubes8 and

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graphene9 which has the outstanding advantages of high cycle stability, high conductivity

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and low cost compared with the pseudo-capacitive materials.10 Although these merits of

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EDLCs electrode make them easier to be commercially used, the relatively mediocre

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capacitance is an urgent problem to be solved.11 Therefore, considerable works have been

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done on controlling over the microstructure and component, for example, preparation of

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electrode materials with three-dimensional (3D) structures,12 ideal porous structure,13

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doping heteroatoms,14 which can affect ion-accessible surface area, ionic transport rate,

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conductivity and electrochemical activity, thereby enhancing capacitive performance.15

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As a good candidate for electrode material of EDLCs, graphene is an ideal two-

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dimensional crystal with high specific surface area, excellent electrical conductivity and

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stability, which has been used widely in catalyst, energy conversion and storage fields.16-

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19

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crumpled graphene sheets21 and porous structure22 to obtain larger specific surface area

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and better capacitance in recent years. Besides the morphological control, doping of a

Researchers have built hierarchical three-dimension (3D) graphene hydrogels,20


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heteroatom in the graphene frameworks is also a effective and simple way to tailor the

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capacitive performance of graphene.23 Due to the modified intrinsic electrical nature, the

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fast access of electrolytes and ions to the electrode, heteroatoms can improve the

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electrochemical performance of conductive materials24-27. As is known, N-doped carbon

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nanostructures show greater electron mobility introducing chemically active sites.28 S-

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doped graphene is of particular attention which is expected to have a wider band gap23.

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Recently, co-doping can further tune the properties of graphene compared with the solely

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doping because of the synergistic effect between doped species.29 Despite its favourable

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doping effect, the previous synthesis methods for obtaining nitrogen and sulfur co-doped

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graphene include hydrothermal method and carbonization which require multiple

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chemical reactions and high temperatures.30-32 In addition, most of the precursors such as

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benzyl disulfide, nitrobenzenamine and carbon disulfide are toxic which are adverse to

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the environment.33-37 More importantly, the existing methods involved with wet-chemical

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method usually result in re-aggregation inevitably due to the strong π–π interaction and

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van der Waals interaction.20,

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economical, facile and environmentally friendly method to prepare co-doped graphene-

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based electrode materials for supercapacitors.

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Thus, it is quite significant to continually develop an

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Plasma treatment has been demonstrated as an environmentally friendly, energy-saving

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and novel strategy to modify the morphology and component of nanomaterials.39 As the

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fourth state of matter, plasma may produce highly reactive species (ionized species,


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electron, atomic excited state, etc.) which can mediate the surface,40 tunable pore size

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distribution,41 surface roughness and wettability of the materials.42-43 It is noted that

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plasma can also be applied to obtain heteroatom-doped graphene.44-45 In our previous

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work, H2 plasma treatment was successfully applied to reduce Pt ions and graphene oxide

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(GO), meanwhile NH3 and H2 plasma were used to obtain reduced metallic Pt and N-

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doped reduced graphene oxide (RGO).46-49 Moreover, Wang et al. reported the exfoliated

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N-doped graphene from GO by low-temperature plasma.50 In another study, N-doped

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graphene quantum sheets were prepared one-step from monolayer graphene by nitrogen

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plasma.51 For another instance, the graphene with a doping level of 3 at% were obtained

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by treating with ammonia radio frequency (RF) discharge plasma.52 Though doping is

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achieved through plasma treatment based on the previous reports, until today, the

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simultaneous reduction and doping of graphene with minimal re-aggregation by plasma

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method have rarely been reported, especially co-doped with nitrogen and sulfur. Hence,

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these methodologies for simultaneous reduction and doping of graphene, which provide

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the possibility of achieving less aggregation and higher doping content, deserve to be

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further developed.

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Herein, we report a facile process for the simultaneous reduction and N, S co-doping of

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GO by employing a low-temperature plasma-assisted technique. As a comparison, N, S

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co-doped graphene by hydrothermal treatment (H-NS-RGO) and GO were also prepared.

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Ultimately, the new route for N, S co-doped graphene by plasma treatment (P-NS-RGO)


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is the best performing with several advantages. First, the method is simple which can be

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accomplished one-step with effectively removed oxygen-containing functional groups on

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the surface of GO and homogeneous doping of N and S. Second, the prepared material

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shows a relatively high doping content with porous structure. Third, the process is gas-

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phase reaction, thus minimize the re-aggregation of graphene nanosheets and the low-

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temperature plasma is energy saving, the moderate raw materials are environment

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friendly. In addition, the N, S co-doped graphene was utilized as the electrode of

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supercapacitors and exhibited enhanced specific capacitance, excellent stability and

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conductivity in 6 M KOH.

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EXPERIMENTAL SECTION

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Synthesis of GOs and nitrogen, sulfur-containing precursor. GOs were synthesized

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based on the modified Hummers method.53 In detail, 1.5 g of flake graphite powder (325-

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mesh) was added to the Erlenmeyer flask, and 50 mL of concentrated sulfuric acid (H2SO4,

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98%) was slowly poured in. Under an ice water bath (< 5 °C), 6 g of potassium

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permanganate (KMnO4) was slowly added and reacted for 2 hours. After the low

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temperature reaction was completed, the temperature was raised to 35 °C and kept for 1

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hour. Then, slowly added deionized water with the temperature rising to about 85 °C and

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kept for 30 minutes. After the reaction was completed, hydrogen peroxide (H2O2, 30%)

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was added dropwise until no bubbles were generated. Finally, it was washed and

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centrifuged with hydrochloric acid (HCl, 5%) and a large amount of deionized water to


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remove the excess sulfuric acid. The solution was then treated by ultrasonic and freeze-

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dried for 24 hours to gain fluffy powder. The nitrogen and sulfur-containing precursor

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was prepared by dissolving 90 mg GO powder prepared above in 30 mL deionized water,

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thoroughly mixing, ultrasonic for 1 hour, and then adding 450 mg thiourea as doping

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reagent under stirring. Subsequently, consecutively stirred for another 1 hour so as to fully

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mixed. Then freeze-dried overnight and obtained a fluffy three-dimensional flocculent

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product.

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Plasma treatment. The whole device is mainly composed of the gas source, the

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discharge chamber, the plasma discharge device and the vacuum system. H2 and Ar are

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used as gas sources, and the gas flow rate is controlled by the flow meter. The discharge

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chamber is quartz glass tube with a diameter of 80 mm, and the inductive coupling coil is

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wound around the tube, the discharge power is controlled by a RF power supply. The

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vacuum system consists of a mechanical pump, and the ultimate pressure can reach 0.1

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Pa. During the plasma treatment process, the sulfur-containing precursor prepared above

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was placed as thin as possible in the quartz boat, and the chamber was sealed followed

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by vacuuming to below 0.3 Pa. Then H2 and Ar were introduced into the chamber,

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controlling their flow rate of 30 sccm and 15 sccm (H2: Ar = 2:1), respectively. After the

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period of stabilization, opened the discharge device and set the output power of 150 W,

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after 15 minutes, the sample was taken out. After stirring well, the thin layer was

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rearranged and placed in the chamber for another 15 minutes with the same parameters.


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Considering that the plasma mainly affects on a very thin layer of the sample surface, the

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discharge was divided into two times in order to make the sample and the plasma fully

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contact. Figure 1 presents the schematic diagram of the plasma treatment process for the

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P-NS-RGO. In order to obtain the optimum experimental results, different experimental

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parameters such as output power, the ratio of H2 to Ar, and treatment time were discussed

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below.

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Figure 1. Schematic diagram of the plasma treatment process for the P-NS-RGO

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Preparation of nitrogen and sulfur co-doped reduced graphene oxide by

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hydrothermal method. First, 90 mg GO obtained by the modified Hummers method was

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taken in a small beaker, added 30 mL of deionized water, and then ultrasonic for 1 hour

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to obtain a homogeneously mixed GO dispersion. Then, 450 mg of thiourea was added

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slowly while stirring. After thoroughly mixing, the mixture was added to a 50 mL teflon-

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lined reaction kettle, placed in an oven to set a reaction temperature of 180 °C for 12

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hours, and after the reaction was completed, it was washed by deionized water and then

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sufficiently freeze dried.


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Electrochemical measurement. The active material, acetylene black and the

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polytetrafluoroethylene (PTFE, 60 wt%) dispersion was ground and uniformly mixed in a

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mortar at a mass ratio of 7:2:1 in isopropanol to obtain a slurry with the appropriate

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concentration. The slurry was then uniformly coated on the one side of the nickel foam with

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the area of 1cm *1 cm, and then dried in a vacuum oven at 50 °C overnight. The mass of the

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active material on the nickel foam is about 1.5~2 mg. The dried electrode was pressed at 10

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MPa for 10 seconds to complete the preparation of the electrode. Electrochemical

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measurement was performed on an electrochemical workstation (CHI 660E, CH

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Instruments, China) using the conventional three-electrode system. An Ag/AgCl electrode

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was served as the reference electrode. The platinum wire electrode was served as the

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counter electrode, and the prepared electrode was tested as the working electrode in the

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electrolyte of 6 M KOH. The electrochemical performance of the working electrode was

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studied by cyclic voltammetry (CV) at different scanning rates and galvanostatic

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charge/discharge (GCD) at different current densities. The specific capacitance can be

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calculated by equation (1) and (2) according to the CV and GCD, respectively.

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C = (∫I dV) / (mν ΔV)

(1)

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C = (I Δt) / (m ΔV)

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Where I is the current and current constant in equation (1) and (2), respectively, dV is the

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potential differential, ΔV is the range of potential, ν is the scan rate, m is the mass of active material

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and Δt is the time of discharge. The Nyquist plot was obtained by electrochemical impedance

(2)


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spectroscopy (EIS) which performed with the frequency from 100 KHz to 0.01 Hz under ac

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perturbation of 5 mV.

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The symmetric supercapacitor in two-electrode system of coin cell was fabricated using

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cellulosic separator NKK TF 4030 and 6 M KOH as the electrolyte. The slurry (active material:

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acetylene black: PTFE = 8:1:1) was coated on the one side of the nickel foam in the area of

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1cm*1cm with the same mass loading (appr.1.5-2 mg) for the both electrodes. The CV, GCD and

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EIS tests were performed on the electrochemical workstation (CHI 660E, CH Instruments, China).

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The cycling stability test was operated on the CT2001A battery test system (LANHE, Wuhan,

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China). The specific capacitance (Cs) of the single electrode was calculated by the equation (3),

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the energy density (E) and power density (P) of the cell was obtained by the equation (4) and (5).

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Cs = 2(I Δt) / (m ΔV)

(3)

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E = Cs ΔV2/ (7.2*4)

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P = 3600E/Δt

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Where I is the current constant, Δt is the discharge time, m is the mass loading on the single

(4) (5)

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electrode, ΔV is the operation potential of the cell.

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RESULTS AND DISCUSSION

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OES Characterization. Optical emission spectroscopy (OES) was applied to diagnose

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the species excited in the plasma, providing an important basis for deriving possible

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reaction mechanisms and screening for optimal processing parameters (gas ratio and

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discharge power).


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Figure 2. Optical emission spectra of H2/Ar plasma under (a, b) different gas ratios of

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H2/Ar and (c, d) discharge power. (b, d) are magnified peaks of Hα.

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Figure 2(a) and (c) show OES of different power and H2/Ar gas ratios, respectively. They

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both have four distinct peaks, corresponding to the peak of atomic hydrogen Hα at 656.3

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nm and the Hβ at 486.1 nm.54 The corresponding enrichment peak near 603 nm is an

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unionized H2 molecule, and between 700 and 800 nm assigned to Ar.55 Among three

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excited states of atomic hydrogen, Hα is the most abundant species known as the Balmer

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series, which can be easiest observed by OES.56 Therefore, the Figure 2(b) and (d) which

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indicate the relative intensity of the Hα under different parameters are specifically

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discussed. It can be seen from the Figure 2(b) that the presence of Ar can contribute to

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the output of H radicals due to the ionization of Penning,57 so the mixture of H2 and Ar


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was selected as the gas source, when the radio of H2/Ar was 2:1, the intensity of Hα

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reached its maximum. With the increase of power, the peak intensity of Hα is gradually

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enhanced in the Figure 2(d). Considering that the power is too strong, the structure of the

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material may be damaged and the temperature of the reaction chamber would be

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increased, so the discharge power of 150 W was adopted. Similarly, so did the treatment

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time, it is not difficult to infer that with the increase of treatment time, the degree of

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reduction for GO will be also increased as well as the chamber temperature. Therefore,

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considering the damage to the material structure, the temperature of the chamber and

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the electrochemical performance, the parameter of H2/Ar = 2:1, power of 150 W and

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treatment time of 30 min were adopted.

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During the process, the H2 and Ar mixed gas enter the inside of the vacuum chamber,

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under the influence of ICP, the excited reactive radicals and energetic particles diffused

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along the induction coil and then effectively treated on the surface of the samples.

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Physical bombardment and chemical reactions are co-existence, which has a promoting

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effect on doping atoms and removing the oxygen functional groups on the surface of the

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GO.56 Specifically, the glow plasma discharge generate a variety of reactive species, such

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as radicals, ions (Ar+ and H+), and neutral molecules (H2 and Ar). The H+, H2 and hydrogen

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radical main play the role of reduction, the Ar+ and Ar contribute to the ionization of H2

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and its higher energy particles have a synergistic effect on the reaction process. As the

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reaction goes, oxygen-containing functional groups and sulfur-containing precursor may


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generate species of NH2·, H· and OH· and then interaction each other to form H2O and

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NH3 thus induce interconnected pores.58 The Hα peaks in Figure 2(b) illustrate that when

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pure Ar existed in the system, there are also has the intensity of Hα, which further testify

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the H containing species can be induced from material rather than H2. Furthermore, the

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transfer of the species excited by plasma causes the nitrogen and sulfur-containing

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dopants have an etching effect on the surface of the RGO, thereby increasing the doping

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content and controlling the reconfiguration of chemical bonds.

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Structure and component analysis. The morphology and structure of the obtained

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samples are investigated by scanning electron microscopy (SEM) and transmission

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electron microscopy (TEM) technique.

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Figure 3. SEM images of (a) P-NS-RGO and (b) H-NS-RGO. TEM images of (c) P-NS-RGO

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with SAED pattern and (d) H-NS-RGO. (e-h) elemental mapping of P-NS-RGO.

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Figure 3(a) and (b) show the SEM images of P-NS-RGO and H-NS-RGO, respectively.

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The surface of H-NS-RGO is rough with interconnected pore structure through and within

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the flakes while the P-NS-RGO has the significantly surface wrinkles and the folded of these

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flakes induce the bigger interspace between the cross-linked graphene sheets. It may be

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attributed to less re-aggregation of the graphene flakes during the gas-phase plasma

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treatment process. The TEM image of P-NS-RGO in Figure 3(c) exhibits the existence of

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typical wrinkled and the folded morphologies, the small black dots indicating the

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existence of some impurities (i.e. N, S atoms) in graphene. The bright discrete ring pattern

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from the selected area electron diffraction (SAED) in the inset indicates the polycrystalline

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nature of the sample. Figure 3(d) is a TEM image of H-NS-RGO, the obviously stacking of

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multilayer graphene sheets indicates the presence of aggregation. Furthermore, the SEM

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and TEM of GO are also shown in Figure S1, the more transparent with little winkles on the

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surface is observed compared with the P-NS-RGO, which indicates the important role of

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plasma to form special morphology and component. Eventually, the rough surface and

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porous structure of P-NS-RGO with less re-aggregation promote the ion-accessible

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surface area and mobility of ions between electrode and electrolyte, which are beneficial

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to improve capacitance performance. From Figure 3(e-h), elemental mapping confirmed

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the existence and uniformly distribution of C, O, N, S in P-NS-RGO sheets. Hence, the P-


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NS-RGO prepared by a simple plasma method compared with the H-NS-RGO obtained

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by hydrothermal method, possesses the richer porous structure, less re-stack of the

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graphene sheets and the S and N evenly distributed in the nanosheets, which efficiently

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improve the electrochemical performance.

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The specific surface area and pore size distribution of P-NS-RGO and H-NS-RGO were investigated using N2 adsorption-desorption isotherms.

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Figure 4. (a) Nitrogen adsorption–desorption isotherms and the insert is pore size

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distribution of H-NS-RGO and P-NS-RGO. （ b ） XRD patterns (c) FTIR spectra and (d)

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Raman spectra of H-NS-RGO, P-NS-RGO and GO.


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Figure 4(a) displays a hysteresis loop of type H2 for P-NS-RGO while H-NS-RGO shows

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hysteresis loop of type H4, indicating the pore size distribution and porous structure of

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P-NS-RGO are more uniform and complex. The pore size distribution plot is inserted in

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Figure 4(a), which illustrates the typical mesoporous (appr. 4 nm) structure of the samples. The

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Brunauer-Emmett-Teller (BET) specific surface area of P-NS-RGO and H-NS-RGO are

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218.57 m² g-1 and 69.71 m² g-1, while the pore volume of them are 0.26 cm³ g-1 and 0.32

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cm³ g-1, respectively. Based on the analysis, it is reasonable to conclude that the larger

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specific surface area and more abundant porous structure were well-presented in the P-

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NS-RGO. And such a pore size distribution result can favorable for the rapid adsorption

10

and diffusion of ions, thus resulting in an excellent electrochemical performance. Figure

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4(b) shows the X-ray diffraction (XRD) peak spectra of GO, H-NS-RGO and P-NS-RGO. The

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obvious peak in 11° (d-spacing = 4.04 Å calculated by the Bragg equation) of GO owing

13

to the interlayer spacing of oxygen containing functional groups. After hydrothermal or

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plasma process, the diffraction peak of GO in 11° disappeared while shows the diffraction

15

peak at 24.6° (d-spacing = 1.85 Å) and 26° (d-spacing = 1.76 Å) for H-NS-RGO and P-NS-

16

RGO respectively, demonstrating that oxygen containing functional groups has been

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reduced to some extent.59 These much lower value of d-spacing after hydrothermal or

18

plasma treatment can be attributed to the recovery of a graphitic crystal structure.

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Furthermore, the diffraction peak of P-NS-RGO at 26° corresponding to the (002) is

20

weaker and broader than that of H-NS-RGO, suggesting that the P-NS-RGO was more


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disordered and lower graphitization degree due to the significantly increase of defects

2

and the modified pore structure after plasma treatment. Figure 4(c) shows the Fourier

3

Transform Infrared spectroscopy (FTIR) spectrums of GO, H-NS-RGO and P-NS-RGO. The

4

clear absorption peaks in the wave number of 1731 cm-1, 1614 cm-1 of GO can be indexed

5

as the C=O carboxyl group, C=C aromatic ring, while peaks at 1226-1058 cm-1 and 833

6

cm-1 belong to C-OH and epoxy base C-O-C stretching vibration peak, C-H out-of-plane

7

bending vibration.60 The broad peak at 3401 cm-1 is the hydroxyl absorption peak (OH).

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The peak of C=O carboxyl group at 1731 cm-1 evidently disappeared in H-NS-RGO and

9

P-NS-RGO, indicating the efficiently reduction of GO. Moreover, new peaks appear at

10

1587 cm-1, 1467 cm-1, 1126 cm-1 and 730-620 cm-1 are related to the bending vibration of

11

N-H, the stretching vibration of C-S, stretching vibrations of C–C and the C-H/O-H/N-H

12

out-of-plane bending vibration, indicating that N and S were successfully doped into

13

graphene sheets layer.61 Compared with the H-NS-RGO, P-NS-RGO has a more obvious

14

absorption peak intensity which indicates the plasma-assisted technique is more mild but

15

effective. In Figure 4(d) of Raman spectra for GO, H-NS-RGO and P-NS-RGO, D band in

16

1350 cm-1 is caused by the disordered carbon atoms and defects while G band in 1597

17

cm-1 is the characteristic peak of sp2 hybridization of carbon atoms.62 The tiny peak of H-

18

NS-RGO in 1657 cm-1 maybe due to the D' band from the intra-valley double resonance

19

process in case of defects, and will be merged with the G band in the high-defect graphite

20

system. Furthermore, the figure also shows the red shift of the G band, 1606 cm-1 for GO,


17


Page 18 of 45

1

1600 cm-1 for H-NS-RGO, 1586 cm-1 for P-NS-RGO, which is an symbolic feature of n-type

2

substitutional doping in carbon materials.63 Moreover, the intensity ratio of D and G band

3

(ID/IG) for P-NS-RGO (1.18) is larger than H-NS-RGO (1.11) and GO (0.87), which is usually

4

used to describe the order of graphitization degree. It is noted that the highest defective

5

result obtained by plasma treatment in comparison with H-NS-RGO and GO, which may

6

be attributed to the high level heteroatoms-doping, more defects introduced and the

7

etched pores structure after plasma treatment.

8

As shown in Figure 5, X-ray photoelectron spectroscopy (XPS) measurement were

9

performed to determine the level of N and S atoms introduced following by plasma

10

treatment and hydrothermal method. Figure 5(a) shows the survey scans for P-NS-RGO

11

and H-NS-RGO, from which a C 1s peak (∼285 eV), an O 1s peak (∼533 eV), a N 1s peak

12

(∼400 eV), and two S peaks (S 2p at∼164 eV, S 2s at ∼227 eV) can be clearly discerned.

13

Figure 5(b) shows the C 1s high resolution spectrum fitted with peaks at 284.75 eV, 285.7

14

eV, 287.3 eV, which related to C-C/C=C, C-O/C-S/C-N and C=O in P-NS-RGO, respectively.

15

The S 2p and N 1s spectra in Figure 5(c) and (d) further indicate the S and N co-doped in

16

P-NS-RGO. In detail, the S element is mainly in the form of -S=C=S-/-C=S- (162.4 eV), -

17

C-S-C- (164.1, 165.7 eV) and-C-SOx-C- (168.5, 169.5 eV)，the highest peak of -C-S-C-

18

indicates that the sulfur-containing group was successfully grafted into graphene.64 The

19

N element mainly includes pyridinic N (398.7 eV), pyrrolic N (400.15 eV) and quaternary

20

or graphitic nitrogen (400.85 eV) which can introduce chemically active sites and result in


18

Page 19 of 45 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


1

an increased pseudocapacitance.65 Furthermore, the high resolution scan of the C 1s, S 2p

2

and N 1s fitted peaks of H-NS-RGO are also showed in Figure S2(a) (b) and (c). The binding

3

energy of C 1s and N 1S peaks are almost as same as the P-NS-RGO while the S 2p peaks

4

in obviously difference. The S 2p peak intensity of H-NS-RGO at ~162.4 eV is way lower

5

and the content of –C-S-C- is higher than P-NS-RGO. It may be due to the plasma

6

treatment method is so mild that the more C=S in doping reagent (thiourea) is remained

7

and directly grafts on the surface of RGO. In addition, the atomic contents of P-NS-RGO,

8

H-NS-RGO and GO are illustrated in Figure S2(d) and Table S1. The P-NS-RGO obtained

9

the higher level contents for N (11.89 at%) and S (4.25 at%) than H-NS-RGO. The intensity

10

of the C 1s peak relative to the O 1s peak in H-NS-RGO (C/O ratio = 8.99) is higher than

11

P-NS-RGO (C/O ratio = 5.89) and GO (C/O ratio = 1.84), which indicates the oxygen-

12

containing functional groups were effectively removed by the plasma and hydrothermal

13

process. Meanwhile it is further demonstrated that the hydrothermal method is violent and

14

hyperthermal while plasma assisted method is mild but effective.

15


19


Page 20 of 45

1 2

Figure 5. (a) XPS survey spectra of P-NS-RGO and H-NS-RGO, (b) high resolution scan of

3

the C 1s, (c) S 2p and (d) N 1s fitted peaks of P-NS-RGO.

4

Electrochemical performance. The electrochemical performance of the P-NS-RGO for

5

supercapacitors electrode material was evaluated by CV and GCD measurements. Figure

6

6(a) shows the CV curves at different scan rate (10, 20, 50, 100, 150 and 200 mV s-1) with

7

the potential ranging from -1 to 0 V. These curves all have similar rectangular-like shapes

8

that indicate the electrical properties of the EDLC while the minor deviation of the curves

9

can be attributed to the pseudocapacitive effects introduced by the nitrogen and sulfur-

10

containing functional groups. Furthermore, the higher current response of the CV curves


20

Page 21 of 45 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


1

with the increasing scan rate which shows the excellent rate capability of P-NS-RGO

2

electrode.

3

Figure 6(b) shows the GCD curves of the P-NS-RGO electrode material at different current

4

densities (0.5, 1, 2, 4, 8, 10 and 20 A g-1) between the potential from -1 to 0 V. The curves exhibit

5

a triangular-like shape, indicating good capacitive performance, where the distortion at the

6

beginning of the discharge curve can be attributed to the chemical groups formed by the doping of

7

nitrogen, sulfur atoms and the internal resistance of the electrode material, the interface resistance

8

between electrode and electrolyte. The insert plot shows the situation at high current density

9

more clearly. The specific capacitances of P-NS-RGO electrode are 341.1, 307.4, 212.4,

10

176, 161.6, 155 and 152 F g-1 at the current density of 0.5, 1, 2, 4, 8, 10 and 20 A g-1,

11

respectively.

12


21


Page 22 of 45

1

Figure 6. (a) CV curves of P-NS-RGO electrode measured in 6 M KOH electrolyte at

2

different scan rates and (b) galvanostatic charge-discharge curves at different current

3

densities. (c) The specific capacitance at various scan rates and (d) the cycling performance

4

of the P-NS-RGO at the current density of 4 A g-1.

5

Figure 6(c) is the mass specific capacitance value at different scan rates, showing the

6

specific capacitance as a function of scan rate. P-NS-RGO shows a specific capacitance of

7

226.53 F g-1 at a scan rate of 10 mV s-1, which decreased to 156.08 F g-1 at 200 mV s-1.

8

Capacitance is usually higher at very low scan rates because ion enters and exists with

9

much longer time in the pores and surface to form EDLC. These performances illustrate

10

that the material has good capacitive performance. The cyclic stability of P-NS-RGO was

11

examined by the GCD process and it showed good cycle stability at 4 A g-1 in Figure 6(d).

12

It can be observed that the specific capacitance had obvious decline at first 2000 cycles,

13

and had good retention at later (83% after 10000 cycles).

14 15

In order to compare the electrochemical properties of GO, H-NS-RGO and P-NS-RGO, the CV, GCD, EIS tests were carried out between the three materials.


22

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1 2

Figure 7. Electrochemical performance of the P-NS-RGO, H-NS-RGO and GO in a three-

3

electrode setup: (a) The CV curves at 100 mV s-1. (b) The GCD curves at 2.0 A g-1. (c) The

4

specific capacitance at different current densities. (d) The Nyquist plots of the electrodes.

5

From the Figure 7(a), the CV plot were obtained at scan rate of 100 mV s-1 with the

6

potential ranging from -1~0 V. The much larger inside area of P-NS-RGO illustrated the

7

better capacitance performance than GO and H-NS-RGO, which did not introduce

8

nitrogen and sulfur or doped nitrogen and sulfur by hydrothermal method.

9

The GCD curves of GO, H-NS-RGO and P-NS-RGO at the current density of 2 A g-1 were

10

presented in Figure 7(b). P-NS-RGO shows the best capacitance of 212.4 F g-1 while 179.6


23


Page 24 of 45

1

F g-1 for H-NS-RGO and 75 F g-1 for GO at the current density of 2 A g-1. Based on their

2

specific capacitance at different current densities, Figure 7(c) shows the capacitance

3

decreased linearly with the increasing of the current density and the higher capacitance

4

retention at higher current density implies the good rate capability of the P-NS-RGO

5

electrode, thus demonstrating the plasma-treated nitrogen and sulfur co-doped sample

6

has better electrochemical behavior.

7

Figure 7(d) shows the Nyquist plots for the different electrodes to evaluate the charge

8

transfer behaviors and the resistance. EIS measurements show a straight line in the low-

9

frequency region that the higher slope of the line means electrode materials contact well

10

with the electrolyte and lower resistances and a conspicuous semicircle in the high-

11

frequency region.66 Generally, the series resistance (RS) and charge-transfer resistance

12

(RCT) can be reflected on semicircle. From the plot, the P-NS-RGO has the highest line

13

slope in low-frequency region and smallest semicircle in the high-frequency region means

14

smaller resistance and good conductivity, which facilitate the fast access of ions to

15

electrode. The specific capacitance of nitrogen and sulfur co-doped carbon-based

16

electrodes prepared by chemical methods with different carbon source, dopant and

17

electrolyte reported in the Table 1.

18

19

Table 1. Capacitance comparison of sulfur and nitrogen co-doped carbon-based three-

20

electrode system reported in the literatures.


24

Page 25 of 45 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


Carbon source

Dopant

Electrolyte

Capacitance/F g- Ref. 1

pnitrobenzenami ne

p-nitrobenzenamine

6 M KOH

73 (1 A g-1)

34

GO

thiocarbohydrazide

6 M KOH

141.1(0.3 A g-1)

60

of sulphuric acid

2 M KOH

180 (1 A g-1)

67

GO

L-cysteine

6 M KOH

203.9(0.5 Ag-1)

31

GO

L-cysteine

1 M KOH 0.5 M KCl

254 (1 A g-1)

68

glucose

thiourea

6 M KOH

277.1 (0.3 A g-1)

69

willow catkin

willow catkin

6 M KOH

298 (0.5 A g-1)

29

GO

1-amino-2-thiourea

1 M H2SO4

302.2 (5 mV s−1)

70

GO

thiourea

6 M KOH

303.7 (1 A g-1)

this study

oligomer pyrrole

1 2

The electrode of P-NS-RGO in this study performance remarkably. P-NS-RGO electrode

3

materials exhibit excellent electrochemical performance, which may be based on the

4

following reasons ， (1) The plasma-assisted method avoids the toxicity and re-

5

aggregation of the conventional chemical method, and makes the material structure more

6

stereoscopic. Meanwhile, the continuously introduced gas and the gas generated during

7

the reaction process increase the interconnected holes of P-NS-RGO and facilitate the

8

penetration of the electrolyte and the shuttle of ions. (2) Since the doping process

9

introduces many defects, the more defects, the easier the electrons are to be conducted,

10

the better capacitance properties of the electrode. (3) The introduction of the S atoms


25


Page 26 of 45

1

form the C-S bands in P-NS-RGO and provide more abundant active sites, which enhance

2

EDLC and pseudocapacitance.71 The pyrrolic N and pyridinic N also can provide

3

pseudocapacitance owing to the participation in the redox reactions in alkaline electrolyte.

4

72

5


26

Page 27 of 45 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


1

Figure 8. Electrochemical performance of the P-NS-RGO and H-NS-RGO electrodes measured

2

in a two-electrode system. (a) CV curves of P-NS-RGO and (b) H-NS-RGO at different scan rates.

3

(c) GCD curves of P-NS-RGO and (d) H-NS-RGO at different current densities. (e) Ragon plot.

4

(f) Cycling stability at the current density of 4 A g-1.

5

To further investigate the electrochemical performance of the P-NS-RGO and H-NS-RGO in

6

two-electrode system device, the coin cell was assembled with the two same electrodes in 6 M

7

KOH as shown in Figure S3. The CV curves of P-NS-RGO and H-NS-RGO obtained at different

8

scan rates are shown in Figure 8(a) and (b). All curves retain the typical rectangular shape without

9

large distortion from relative low scan rate of 10 mV s-1 up to 200 mV s-1, which indicated the fast

10

ion and charge transfer. P-NS-RGO has the stronger current response and more obvious distortion

11

of the curves compared with the H-NS-RGO, which demonstrated the better electrochemical

12

activity. It may be attributed to the lower interface resistance and higher level of N, S doping of

13

the P-NS-RGO. Figure 8(c) and (d) shows the GCD curves of the P-NS-RGO and H-NS-RGO at

14

different current densities. The triangular-like shape is observed, the specific capacitance of P-NS-

15

RGO is 268.8 F g-1 at the current density of 0.5 A g-1 and the value can be 81.8% retention with

16

the increase of the current density to 20 A g-1, while H-NS-RGO is 181.9 F g-1 at 0.5 A g-1 and

17

70.3% retention at 20 A g-1. It is noted that the specific capacitances in two-electrode system are

18

lower than three-electrode system and the CV and GCD curves are less distortion. The capacitance

19

of the symmetric supercapacitor equivalent to two serial capacitors, in theory, the capacitance of

20

the two capacitors should be equal and then the capacitance reaches the optimal value. However,

21

in a symmetric system include some redox reactions of the functional groups, the reaction will not

22

occur at the same time of the two electrodes. Thus, there inevitably induce capacitance loss due to

23

the partially inefficient pseudocapacitance. Ragon plot of the P-NS-RGO and H-NS-RGO is shown


27


Page 28 of 45

1

in Figure 8(e). The energy density of the P-NS-RGO is 9.33 Wh Kg-1 at the power density of 125

2

W Kg-1 and remained 7.36 Wh Kg-1 at 10 KW Kg-1, while the H-NS-RGO is 6.31 Wh Kg-1 at 125

3

W Kg-1 and remained 4.45 Wh Kg-1 at 10 KW Kg-1. Figure 8(f) shows the cycle stability of the

4

cell, after 10000 GCD cycles at the current density of 4 A g-1, the P-NS-RGO remains 91.3% of

5

the initial capacitance while the H-NS-RGO is 87.9% retention. The ragon plot and the cycle

6

stability test are further demonstrated the better electrochemical performance of the P-NS-RGO in

7

comparison with H-NS-RGO. The EIS tests of P-NS-RGO and H-NS-RGO are also performed

8

and the results show in Figure S4. The obviously lower semicircle of the P-NS-RGO than H-NS-

9

RGO reflects the lower conductivity and faster ion transfer.

10

CONCLUSION

11

In summary, we have developed a facile and novel plasma-assisted method to synthesis

12

nitrogen and sulfur co-doped reduced graphene oxide. The process of reduction and N,

13

S co-doping for graphene sheets occurred simultaneously. The proposed methodology

14

has significant advantages over other reported methods because it is gas-phase reaction

15

thus minimize the re-aggregation to some extent. Moreover, the low-temperature plasma

16

is energy-saving compared with high temperature carbonization and wet-chemistry

17

reaction. The constructed electrode with plasma treatment exhibits high specific

18

capacitance (307.4 F g-1) at 1 A g-1, good cycling performance, excellent conductivity and

19

outperforming to the electrode material based on the hydrothermal method. The

20

fabricated symmetric supercapacitor device also exhibits the satisfactory energy density

21

retention at high power density. Considering the simplicity of material synthesis process,


28

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1

the plasma-assisted technique might open up a new route for the facile synthesis of

2

nitrogen and sulfur co-doped reduced graphene oxide materials to apply in varied

3

applications. Therefore, the exact mechanism of low-temperature plasma treat for

4

materials and the wider applications are worthy of further investigation.

5

ASSOCIATED CONTENT

6

Supporting Information.

7

The Supporting Information is available free of charge.

8

The SEM and TEM images of GO, the XPS spectra of H-NS-RGO, the atomic % of the C, O, N,

9

S, the picture of two-electrode coin cell, the Nyquist plot of the fabricated symmetric

10

supercapacitor devices. (PDF)

11

Notes

12

The authors declare no competing financial interest.

13

ACKNOWLEDGMENT

14

We gratefully acknowledge the National Natural Science Foundation of China

15

(11575253), the Anhui Provincial Natural Science Foundation for Distinguished Young

16

Scholars of China (1608085J03), the Anhui Provincial key research and development plan

17

(1704a0902017), the Hefei Institutes of Physical Science, Chinese Academy of Sciences

18

(CASHIPS) Director’s Fund (YZJJ201505), the Youth Innovation Promotion Association of


29


Page 30 of 45

1

the Chinese Academy of Sciences (2015262), the Key Lab of Photovoltaic and Energy

2

Conservation Materials of Chinese Academy of Sciences (PECL2018QN005).

3

ABBREVIATIONS

4

GO, graphene oxide; RGO, reduced graphene oxide; ICP, Inductive Coupled Plasma;

5

EDLC, electrical double layer capacitors; 3D, three-dimensional; RF, radio frequency; H-

6

NS-RGO, nitrogen and sulfur co-doped reduced graphene oxide obtained by

7

hydrothermal method; P-NS-RGO, nitrogen and sulfur co-doped reduced graphene

8

oxide obtained by plasma treatment; CV, Cyclic voltammetry; GCD, Galvanostatic

9

charge/discharge; EIS, electrochemical impedance spectroscopy; OES, Optical emission

10

spectroscopy; SEM, scanning electron microscopy; TEM, transmission electron

11

microscopy; SAED, selected area electron diffraction; BET, Brunauer-Emmett-Teller; XRD,

12

X-ray diffraction; FTIR, Fourier Transform Infrared spectroscopy; XPS, X-ray

13

photoelectron spectroscopy; RS, series resistance; RCT, charge-transfer resistance.

14

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Synopsis: A facile, sustainable, effective plasma-assisted method was developed to

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84x24mm (300 x 300 DPI)


Plasma-assisted simultaneous reduction and nitrogen, sulfur co

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