Preparation of CoreâShell CQD@PANI Nanoparticles and Their

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Functional Nanostructured Materials (including low-D carbon)

Preparation of Core-Shell CQD@PANI Nanoparticles and Their Electrochemical Properties Lingyun Li, Meng Li, Jing Liang, Xiao Yang, Min Luo, Lijun Ji, Yanling Guo, Hongfeng Zhang, Na Tang, and Xiaocong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00963 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Applied Materials & Interfaces

Preparation of Core-Shell CQD@PANI Nanoparticles and Their Electrochemical Properties Lingyun Li1, Meng Li1, Jing Liang1, Xiao Yang1, Min Luo1, Lijun Ji2,*, Yanling Guo1, Hongfeng Zhang1, Na Tang1,*, Xiaocong Wang1,* 1 College of Chemical Engineering and Material Science, Tianjin University of Science and Technology, Tianjin 300457, China 2 College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, China

Abstracts: In recent years, carbon quantum dots (CQDs) have been extensively investigated in many fields because of their un-comparable and unique properties. However, the application of CQDs in electrochemical field meets a big challenge due to their low specific capacitance. It is very important to improve the electrochemical performance of CQDs. In this study, a facile synthesis method was developed to synthesize CQD@PANI nanoparticles. The CQD@PANI nanoparticles were prepared via a precise quantitative adsorption polymerization method. TEM results showed the PANI shell thickness could be adjusted by controlling the addition amount of aniline. Cyclic voltammetry and galvanostatic charge/discharge results showed that the electrochemical properties of CQDs have been significantly improved by coating PANI. Keywords: carbon quantum dots; polyaniline; core-shell structure; electrochemical properties; cyclic voltammetry.

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ACS Applied Materials & Interfaces 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.

INTRODUCTION Carbon quantum dots (CQDs) have received increasing attention since the

accidental discovery by Xu et al1 for their unique properties such as the good electronic properties, low-cost, rich sources, low toxicity and good biocompatibility. And they have great potential applications in photocatalysts,2 organic photovoltaic devices,3 bioimaging4 and sensors.3 Carbon materials have high cyclic stability and chemical stability when used in electrochemical double-layer capacitors (EDLCs).5, 6 But, their specific capacity are very low because the available surface areas are limited.7 In contrast, pseudo-capacitors containing polymers show high specific capacitance behavior.8-10 Hence, considerable researches have been focused on combining the superior pseudo-capacitance of conducting polymers and the good stability of carbon materials to obtain electronic devices with better performance.11-14 Various conducting polymers including polyaniline (PANI), polypyrrole (PPy) and poly-(3,4-ethylenedioxythiophene) (PEDOT) have been developed as potential electrode materials for supercapacitors.15,

16

Among them, PANI is one of the best

choices due to its facile synthesis, promising electrochemical behaviors, good redox reversibility and stability in aqueous solutions.17-19 Recently, core-shell polymer composite materials have been attracted much attention because it can functionalize the cores with their special properties 20. Coating PANI onto carbon materials’ surface forming core-shell structured composites have been proven as an effective approach to improving the capacitance behavior of carbon materials.21 Therefore, various types of carbon materials including nanofibers,22 nanotubes16, graphene13 and porous carbon 21

have been used to prepare PANI composite electrodes. However, due to the small diameter of CQDs and the high molecular weight of 2


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polymers, it is difficult to synthesize core-shell polymer-coated CQDs because the shell is difficult to control. Zhang et al23 have reported they prepared core-shell CQD@PANI and used them for glucose and hydrogen peroxide detection. But, no direct evidence of core-shell structure was provided in the context. In this paper, CQD@PANI nano-composites with controlled shell thickness were successfully synthesized through precise quantitative adsorption polymerization using CQDs as a core and PANI as a shell. The composite materials combined the advantages of both PANI and CQDs, exhibiting excellent electrochemical performances. These remarkable electrochemical properties of carbon hybrid materials demonstrated that CQD@PANI composite materials should be a potential high-performance electrode substitute. 2.

MATERIALS AND METHODS

2.1. Materials Citric acid, sodium hydroxide, aniline and ammonium persulfate were purchased from Pharmaceutical Group Co., LTD.. All chemicals are analytical grade and used as received without further purification. 2.2. Synthesis of CQDs Water-soluble CQDs were prepared by pyrolysis of citric acid.24,

25

Different

from the literature, we changed the pyrolysis temperature from 180 °C to 220 °C, which greatly shortened the time of CQD products acquisition from 30 h to 2 h. 100 ml distilled water and a certain amount of 5 M NaOH were used to disperse the pyrolysis products and the pH was adjusted to 7. Finally, the products were isolated by freeze-drying. 2.3. Typical Synthesis of CQD@PANI 0.25 g of CQDs was dispersed in 250 ml distilled water. Five copies of the same 3



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CQDs solution were prepared. Then, the pH values of the solutions were adjusted to 2 with 1 M sulfuric acid. After that, aniline was added into the solutions at a mass ratio of 25%, 50%, 75%, 100%, and 150%, respectively. The solutions were kept for 2 h till aniline could be fully absorbed. Subsequently, an ammonium persulfate solution (APS, 0.5 M) was poured into the mixture as an oxidant (with a mole ratio of APS to aniline 1:1) under stirring until the solution changed to dark green. Finally, the products were separated by centrifugation and washed to neutral with distilled water and ethanol. 2.4. Characterization A JEOL JEM-2100 transmission electron microscope (TEM) was used to investigate the morphologies of the core-shell structured CQD@PANI composites. Ultraviolet–visible

(UV–vis)

spectra

were

recorded

on

an

UV-2550PC

spectrophotometer. Fluorescent effects of the products were observed by an F-280A Fluor spectrophotometer. Infrared spectra (IR) were obtained on an IS50 Fourier transform infrared spectrometer using KBr disks. A 6100 X-ray powder diffractometer was used to characterize the crystalline phase of these materials. Raman spectra were collected on a Kaiser’s RamanRxn2™ analyzer. 2.5. Electrochemical measurements The electrochemical performances of these samples were carried out on a CHI660D electrochemical work station at room temperature. The working electrode was the sample modified glassy carbon electrode (GCE). Pt plate was used as the counter electrode. Ag/AgCl electrode was selected as the reference electrode. The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) curves and electrochemical impedance spectra (EIS) were recorded in 1 M H2SO4 aqueous solution. 4


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3.

RESULTS AND DISCUSSION A schematic diagram of the synthesis procedure of the core-shell CQD@PANI

was shown in Figure 1. Then the products were isolated by freeze-drying. In the pyrolysis process, the color of citric acid gradually changed from colorless to brown. The obtained CQDs contained a large amount of –COOH reactive functional groups on their surfaces.30

O

Citric acid

O

NaOH

O

O

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HO

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Pyrolysis HO HO

HO

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HO

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O O

Carboxylate CQD

Carboxyl CQD

In-situ adsorption polymerization

H N

H N

N

N

NH3+

n

CQD@PANI

Figure 1. Schematic diagram of the preparation procedure of CQD@PANI nanoparticles.

When aniline monomers were added into the CQDs dispersion, they were preferentially adsorbed on the surface of CQDs as a result of electrostatic adsorption. And the shell-thickness of the CQD@PANI was depended on the aniline dosage. Figure 2 showed the HR-TEM images of the as-prepared CQD@PANI. The CQDs were nanoparticles of irregular shape centered at about 4 nm (Figure 2a). The well-resolved lattice fringes of graphitic carbon demonstrated an interlayer spacing of about 0.24 nm (the inset of Figure 2a). It could be found that the CQD@PANI nanoparticles were uniformly distributed and the shell thickness increased as the additive amount of aniline increasing (Figure 2a-d). When the mass ratio of aniline monomer to CQDs was 1:4, 1:2 and 3:4, the shell thickness was approximately 1 nm, 5



2 nm and 3 nm, respectively (inset of Figure 2b-d). The size distributions of CQDs and CQD@PANI nanoparticles were shown in Figure S1. From Figure S1, it could be found that the size distribution of the CQD@PANI particles became wider and the average particle size became larger with the increasing of aniline amount. At higher concentrations, the PANIs on the shell of different particles have more opportunities to connect each other. When the mass ratio of aniline monomer to CQDs was 3:4, the cross-linking between two or among three particles could be clearly observed. When the amount of aniline was increased to the ratio of 1:1, PANI fibers were formed and CQDs were embedded in the PANI fibers (Figure 2e). Further increased the amount of aniline to the ratio of 3:2, CQDs could not be observed because the diameters of PANI were larger and the PANI fibers intertwined with each other. These results confirmed that the morphologies of the CQD@PANI were depended on the additional amount of aniline.

Figure 2. The TEM images of CQD@PANI particles acquired from different mass ratios of 6


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aniline monomer to CQDs: (a) 0:1; (b) 1:4; (c) 1:2; (d)3:4; (e) 1:1; (f) 3:2.

The formation of CQD@PANI nanoparticles was proved by the UV-vis and photoluminescence (PL) spectra (Figure S2). The photos of CQDs and CQD@PANI nanoparticles under the irradiation of a 365 nm ultraviolet lamp were shown in Figure S3. The yellow green fluorescence appeared before polymerization (Figure S3a) and disappeared after polymerization (Figure S3b). The functional groups on the CQDs were confirmed by Fourier transform infrared spectra (FT-IR) (Figure 3). The broad adsorption peak at around 3296 cm-1 (Figure 3a) could be assigned to the vibration of -OH.26, 27 The doublet peaks at 1750 cm-1 and 1700 cm-1 were attributed to the stretching vibration of -C=O in -COO-28 and the stretching vibration of -C=O in –COOH, respectively. Compared the spectra of CQDs with citric acid (Figure 3a-b), it could be found that the citric acid molecules had been decomposed completely. Two peaks at 1557 cm-1 and 1392 cm-1 were attribute to the asymmetric and symmetrical stretching vibration of C-O in COO-,29 indicating that there were rich carboxy groups on the surface of CQDs. The signals of CQDs were concealed after being coated with PANI (Figure 3b-d). The broad peak at 3426 cm-1 in CQD@PANI (Figure 3c) and PANI (Figure 3d) were ascribed to the coupling stretching vibration of N-H and O-H.30 Two strong peaks at 1585 cm-1 and 1496 cm-1 in Figure 3c-d were ascribed to quinoid and benzenoid ring structures of PANI.12,

31

The bands at 1296 cm-1 and 1138 cm-1 were the stretching vibration of

C-N-C in PANI.32

7



Figure 3. FT-IR spectra of: (a) critic acid; (b) CQDs; (c) CQD@PANI (1: 1); (d) pure PANI.

In order to investigate the interaction between CQDs and PANI molecules, Raman spectra of pure PANI and typical CQD@PANI samples were collected and shown in Figure 4. From the characteristic peaks of pure PANI (Figure 4e), the peaks at 1170 cm-1, 1505 cm-1 and 1598 cm-1 were attributed to the C-H bending vibration, C=N stretching vibration and C=C stretching vibration of quinoid ring unit, respectively. The peak at 1226 cm-1 and the shoulder peak at 1645 cm-1 were ascribed to the C-H bending vibration and C-C stretching vibration of the benzenoid ring unit, respectively.33, 34 Different from pure PANI, all the above peaks of CQD@PANI (1:1) (Figure 4c) had a certain degree of shifting. The characteristic peak of C-H at 1170 cm-1 shifted to 1182 cm-1 and the characteristic band of C-N•+ at 1338 cm-1 shifted to 1343 cm-1, indicating the quinoid ring were converted to the benzenoid ring segments during doping process.35 The peak at 1505 cm-1 shifted to a higher wavenumbers of 1517 cm-1, indicating coupling vibration with C=N stretching vibration of imine.36 The characteristic peak of 1598 cm-1 shifted to 1609 cm-1, indicating that CQD@PANI was in a higher doping state,37 which was conducive to improve the electrochemical performance. 8


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Figure 4. Raman spectra of typical CQD@PANI samples acquired from different mass ratios of aniline monomer to CQDs: (a) 1:4; (b) 1:2; (c) 1:1; (d) 3:2; (e) 1:0.

Electrochemical properties of CQDs, CQD@PANI composites and pure PANI were evaluated through CV and GCD curves acquired in H2SO4 solution. Figure 5a showed the CV curves of CQDs, PANI and core-shell CQD@PANI nanoparticles. The result of CQD@PANI (1:1) presented the largest area surrounded by CV curve, suggesting that CQD@PANI (1:1) possessed a higher specific capacitance. This result might be aroused by the fast current response of the CQD@PANI (1:1) modified membrane electrode upon voltage reversal due to the excellent electrochemical performance of the CQD@PANI core-shell structure.38 The good oxidation/reduction response to the CQD@PANI (1:1) modified membrane electrode was related to the rapid extraction-insertion of electrons in the core-shell structure.39 This indicated that the synergistic effect between aniline and CQDs became stronger as the mass ratio of aniline to CQDs reached 1:1. Both pure PANI and CQD@PANI exhibited the characteristic relative redox peaks (at 0.31/0.23 V and 0.54/0.49 V). This could be used to estimate the reversibility of the electrode materials.6, 38 9



The CV curves for CQD@PANI (1:1) with different scan rates were shown in Figure 5b. Higher current response occurred as the scan rate increased to 0.5 V/s, illustrating its excellent rate capability. However, with the increase of the scan rates, it could be found that there was no distinct shift of oxidation and negative peaks, which suggested that CQD@PANI (1:1) had excellent reversibility. The calculation of specific capacitance in Figure 5c-d was based on the data of Figure 5a-b and the calculation method was described in the literature.45 It could be seen that the specific capacitance of CQD@PANI composites was obviously higher than that of pure CQDs. It was because CQDs belong to electron acceptor and PANI belongs to electron donor in the core-shell CQD@PANI nanoparticles. The π-π conjugated structure formed between CQDs and PANI was very conducive to electron transfer, which reduced the interface energy between CQDs and PANI significantly. Therefore, these composites had both Faraday pseudo-capacitance of PANI and double-layer capacitance of CQDs, which significantly enhanced the capacitance performance of the whole materials. It could also be observed that the specific capacitance increased with the increases of aniline dosage (Figure 5c). But when the mass ratio of aniline to CQDs was over 1:1, the specific capacitance decreased with the increases of aniline dosage. The possible reason was that, at high dosage of aniline, the specific surface areas of the composites decreased owing to the aggregation formation of PANI fibers, thus reducing the activation areas for the redox reaction. From Figure 5d, it could be found the specific capacitance decreased as the scan rate increasing.

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Current density (A/g)

20 15 10 5

80

a

CQDs 1:4 1:2 3:4 1:1 PANI 5:4 3:2

0 -5 -10 -15

40 20 0 -20 -40 -60

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Potential (V) vs. Ag-AgCl

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Specific Capacitance (F/g)


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90

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PANI

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Different Samples

150

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250

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350

400

450

500

Scan Rate (mV/s)

Figure 5. CV curves of typical samples at 100 mV/s (a), CQD@PANI (1:1) at different scan rates (b), and the corresponding specific capacitance (c, d).

Figure 6 showed the GCD curves of these typical samples. The CV (Figure 5a) and GCD results (Figure 6) confirmed that the specific capacitance of the core shell nanoparticles was related to the shell thickness, which was depended on the addition of aniline. As shown in Figure 6, the voltage drop (IR drop= 2.1 mV) of CQD@PANI (1:1) was smaller than those of other samples and exhibited good symmetry, indicating the CQD@PANI (1:1) had the best conductivity which not only could provide fast electron transfer but also ideal capacitive performance. The specific capacitance of CQDs, CQD@PANI (1:2), CQD@PANI (3:4), CQD@PANI (1:1) and PANI was calculated

40

to be 20.2 F/g, 121.5 F/g, 222.7 F/g,

215.2 F/g and 182.5 F/g, respectively. The increase of the specific capacitance from CQD@PANI (1:4) to CQD@PANI (1:1) could be attributed to the contribution of PANI. The specific capacitance of CQDs, CQD@PANI (1:2), CQD@PANI (3:4), CQD@PANI (1:1), CQD@PANI (5:4), CQD@PANI (3:2) and PANI at different 11



current densities, which were shown in Figure 6b. In addition, the current density increased with the increases of the amount of PANI, but it began to decrease when the amount ratio of PANI to CQDs increased to 1:1. Moreover, the specific capacitances of the PANI-CQDs composites were much higher than that of pure PANI. It was worth noting that the CQD@PANI (1:1) had the highest specific capacitance. Figure 6c showed the cyclic stability of CQDs, pure PANI and CQD@PANI nanoparticles at a current density of 1 A/g. The CQD@PANI (1:1) nanoparticles exhibited good cyclic performance with a specific capacity of 195.41 F/g after 500 cycles. It suggested that the CQD@PANI (1:1) nanoparticles had intrinsic merits for smooth H+ insertion/extraction owing to the core-shell structure and shell thickness. 40, 41

a

250

CQDs 1:2 3:4 1:1 5:4 3:2 PANI

0.5 0.4



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b

Time (s)


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CQDs PANI 1:2 1:1 3:2

200 150 100 50 0

0

100

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300

Cycle Number

400

500

Figure 6. GCD curves at 1 A/g (a), plot of specific capacitance vs. current density (b), and specific capacitance with various cycles (c) for CQDs, PANI and typical CQD@PANI samples acquired from different mass ratio of aniline monomer.

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EIS spectroscopy (Figure 7) was performed to investigate the electrode performance of the prepared materials and the kinetics involved. As could be seen from Figure 7, all samples had a straight line closed to 90 ° in the low-frequency region, while in the high-frequency region, only some of the samples had semi-circles. As it was well known that the slope of lines in the low-frequency region could be ascribed to the diffusion of the electrolyte ions in the electrode pores.42, 43 If the line was vertical, it was equal to an equivalent circuit consisting of a resistance in parallel with an ideal capacitor.44 However, the line was always inclination from the vertical which could be attributed to a non-homogeneity film.45 All these suggesting that the core-shell structure samples had the pure capacitive behavior and fast character of electrolyte ions than pure PANI.46 In addition, the short x-intercept which was equal to the solution resistance, and the small diameter of the semicircle which was equal to the electrode resistance.47

a

20

CQDs PANI 1:4 1:2 3:4 1:1 4:3 3:2

16000 12000

16

8000

b

CQDs PANI 1:4 1:2 3:4 1:1 4:3 3:2

18

-Z" (ohm)

20000

-Z" (ohm)

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Figure 7. EIS curves of CQDs, PANI and typical CQD@PANI samples acquired from different mass ratio of aniline monomer (a) and a larger version in different coordinate ranges (b).

4.

CONCLUSIONS In this paper, a facile synthesis approach of precise quantitative adsorption

polymerization to prepare CQD@PANI core-shell nanoparticles with enhanced electrochemical performance was demonstrated. The novel core-shell structured 13



quantum dots could enhance the electrochemistry performances of both PANI and CQDs. These CQD@PANI core-shell structured nanoparticles are promising electrode materials for application of high performance super capacitor. ASSOCIATED CONTENT Supporting Information Characterizations for the size distributions of CQDs and CQD@PANI nanoparticles (Figure S1); the UV-visible absorption (Figure S2a) and PL emission spectra (Figure S2b) of CQDs; the photos of CQDs (Figure S3a) and CQD@PANI (Figure S3b) particles under the irradiation of 365 nm ultraviolet lamp. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (N. Tang). *E-mail: [email protected] (X. Wang). *E-mail: [email protected] (L. Ji). ORCID Xiaocong Wang: 0000-0002-8486-0491 ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (No.21373150) and the Youth Innovation Foundation of Tianjin University of Science & Technology (No. 2015LG15).

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17



Table of Contents

O

Citric acid

O

NaOH

O

O

O O

O

HO

HO

O

Pyrolysis HO HO

HO

O

O

HO

O

O

HO

O

HO

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

Page 18 of 18

O O

Carboxylate CQD

Carboxyl CQD

In-situ adsorption polymerization

H N

H N

N

N

n

CQD@PANI

18


NH3+

Preparation of CoreâShell CQD@PANI Nanoparticles and Their

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