New Avenue for Appendage of Graphene Quantum Dots on Halloysite

Research Article pubs.acs.org/journal/ascecg

New Avenue for Appendage of Graphene Quantum Dots on Halloysite Nanotubes as Anode Materials for High Performance Supercapacitors Akhilesh Babu Ganganboina,†,§ Ankan Dutta Chowdhury,‡,§ and Ruey-an Doong*,‡ †

Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101 Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan ‡ Institute of Environmental Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan S Supporting Information *

ABSTRACT: Graphene quantum dots (GQDs) are a newly developed graphene family with good electrical conductivity and high theoretical capacitance, while halloysite nanotubes (HNTs) are naturally occurring layered mineral materials containing high active sites for energy storage support. The combination of HNTs and GQDs can offer a new strategy on the fabrication of eco-friendly electrode materials for high performance supercapacitor applications. Herein, an environmentally friendly GQD-HNT nanocomposite is fabricated in the presence of (3aminopropyl)-triethoxysilane to provide increased charge storage sites as well as to allow for the fast charge transport for supercapacitor application. Morphological and surface analytical results show that 5−10 nm GQDs are homogeneously distributed on the surface of APTES-coated HNTs via amide linkage. This new and novel layered nanocomposite can provide accessible electroactive sites and low resistance to accelerate the electrons and electrolyte ion transport, resulting in excellent specific capacitance and high energy density. The specific capacitances of 363−216 F/g at current densities of 0.5−20 A/g are obtained. In addition, the GQD-HNTs exhibit excellent energy density of 30−50 Wh/kg. Results obtained in this study clearly demonstrate the feasibility of using GQD-HNTs as alternative energy storage materials with increased charge storage sites and fast charge transport for high energy density supercapacitor applications. KEYWORDS: Halloysite nanotubes (HNTs), Graphene quantum dots (GQDs), Covalent attachment, Supercapacitor, Energy density

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INTRODUCTION The accelerating demand on environmental conservation and energy-driven applications has reinforced research interest in energy storage and conversion from alternative energy sources. The excessive use of fossil fuels has caused a serious problem in the environment and the utilization of renewable energy resources has been becoming increasingly critical.1 Therefore, the development of environmentally friendly and less expensive electrical energy storage devices is essential for many appliances. Supercapacitors are some of the promising devices for next generation energy storage systems which can offer an inherently high power density, high Coulombic efficiency, rapid charge/discharge rate, and very long service lifetime.2,3 These devices are electrochemical energy storage devices which can store and release energy by reversible adsorption/desorption processes of ions and fast faradaic redox reactions at the interfaces between electrode materials and electrolytes.4,5 Several carbon based materials including activated carbons,6,7 graphene,8−10 and carbon nitride11 have been used as electrode materials for supercapacitors. Although the theoretical capacitance value of these materials is relatively high, the specific energy density of carbon-based supercapacitors is usually low because of their limited accessible storage sites. © 2017 American Chemical Society

Therefore, the development of composites by combining the layered structures and carbon materials as the active electrode materials for high capacitance of supercapacitors has received considerable attention.12,13 Halloysite nanotubes (HNTs) are naturally occurring layered aluminosilicate clay minerals consisting of multiple alumina/ silica layers with a gibbsite-like array of aluminol groups (Al− OH) on the internal surface and siloxane groups (Si−O−Si) on the external surface.14 This uniqueness in layered structure has resulted in a negatively charged external surface and a positively charged internal lumen, which has attracted high interest in surface functionalization for novel applications such as adsorption,15 drug delivery,16 and bone implantation.17 In addition, the large surface area as well as high porosity makes the HNTs promising materials for electrochemical energy storage. However, only limited studies have been conducted on halloysite-based nanomaterials for supercapacitor applications because of the difficulty in surface functionalization.18,19 Huang et al. have synthesized halloysite-polyaniline-poly(sodium-pReceived: February 2, 2017 Revised: April 11, 2017 Published: May 1, 2017 4930

DOI: 10.1021/acssuschemeng.7b00329 ACS Sustainable Chem. Eng. 2017, 5, 4930−4940

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Preparation of Graphene Quantum Dot Coated Halloysite Nanotubes (GQD-HNT) Nanocomposites and Their Supercapacitor Applications

minerals, and GQDs can be fabricated by pyrolysis of carbon materials, making this nanocomposite eco-friendly. However, the fabrication of GQD-modified HNT nanocomposites (GQD-HNTs) for supercapacitor application has rarely been reported. Herein, a new class of environmentally friendly nanocomposites by combining HNTs and GQDs as efficient and stable electroactive materials for high performance of supercapacitor application is developed. Scheme 1 illustrates the fabrication of GQD-HNT nanocomposites and supercapacitor application. The HNTs were first functionalized with (3aminopropyl)-triethoxysilane (APTES) followed by the addition of GQDs prepared by the pyrolysis of citric acid to form GQD-HNT nanocomposites. The prepared GQD-HNT nanomaterials exhibit excellent specific capacitance, high energy density, and good cycling stability over 5000 cycles. To the best of our knowledge, this is the first report on the fabrication of GQD-HNT nanocomposites through covalent attachment as the electroactive nanomaterials for potential supercapacitor application. Results obtained in this study can provide an alternative to fabricate eco-friendly electrode materials with an increased charge storage site and fast charge transport for supercapacitor applications.

styrenesulfonate)-polyaniline as electroactive materials and found that the specific capacity of surface modified halloysite was 137 F/g at a current density of 0.5 A/g.18 By adding the conducting polymers, the halloysite/polypyrrole nanocomposite exhibits 522 F/g capacitance at 5 mA/cm2.19 Liu et al. have used reduced graphene oxide (rGO) coated HNTs as the supercapacitor material, and a capacitance of 27.2 F/g at 0.2 A/ g was obtained.20 These results clearly indicate the potential of using HNT-based nanomaterials as the electrode materials for supercapacitor applications. However, the inherently high resistance of these aluminosilicate clay minerals hampers their electrochemical performance. Therefore, the development of a novel strategy on fabrication of HNT-based nanomaterials with fast electron and electrolyte ions transport is thus needed. Graphene quantum dots (GQDs), the most newly developed member of the graphene family, belong to zero-dimensional nanomaterials with the unique properties of both graphenes and quantum dots.21 GQDs have attracted extensive attention on biomedical and sensing applications because of the excellent physicochemical and electrochemical properties such as high specific surface area, good electrical conductivity, and excellent dispersion in various solvents.17 Several studies have used GQD-based nanomaterials as the electrode materials for electrochemical application.22−25 Chen et al. have developed GQD-3-dimensional graphene composites for supercapacitor application and found that the optimal specific capacitance was 268 F/g at a current density of 1.25 A/g.23 Liu et al. have fabricated GQDs//GQDs symmetric microsupercapacitors which showed a specific capacitance of 534.7 μF/cm.22 It is noteworthy that the graphene family is a good conductive material which can exhibit high performance as a supercapacitor material.24,25 These exquisite properties make GQDs a promising nanomaterial to combine with HNTs as the active electrode material for high capacitive performance. The HNTs provide suitable pore texture and layered structure for electrons and electrolyte ions transport, while GQDs can improve the electrical resistance of nanocomposites and specific capacitance. Most importantly, HNTs are naturally occurring layered clay

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EXPERIMENTAL SECTION

Chemicals. Citric acid, halloysite nanotubes (HNTs), (3-aminopropyl)-triethoxysilane (APTES), sodium dihydrogen phosphate, disodium hydrogen phosphate, and N-(3-(dimethylamino)-propyl)N′-ethylcarbodiimide hydrochloride (EDC) were purchased from Sigma-Aldrich. N-Hydroxysuccinimide (NHS) was obtained from Alfa Aesar. Analytical grade of toluene and other reagents were used as received without further purification. All of the solutions were prepared by using bidistilled deionized water (18.2 MΩ cm) unless otherwise mentioned. The water used throughout the experiments was purified through a UV treated Rephile water system. Synthesis of GQDs. GQDs were synthesized by direct pyrolysis of citric acid as reported earlier with minor changes where 2 g of citric acid was added into a 100 mL round-bottomed flask and heated to 200 °C by using a heating mantle in an oil bath.26,27 The citric acid was

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Figure 1. (a) TEM image of homogeneously distributed GQDs. Inset is the HRTEM image of single GQD, (b) histogram of GQD particle size distribution, and (c) Raman spectrum of GQDs. liquated and turned yellowish after 5 min. Then, an orange color appeared after heating for 35 min, depicting the formation of GQDs. The obtained orange liquid containing GQDs was then added drop wise into 100 mL of 10 g/L sodium hydroxide (NaOH) solution under vigorous stirring. After the adjustment of pH to 8.0 with NaOH, the obtained solution of GQDs was dialyzed in a 1 kDa dialysis bag for 24 h (dialysate was replaced every 8 h) to remove the unreacted chemicals and preserved at 4 °C for further use. APTES Coating onto HNTs. HNTs were coated with APTES by using the previously reported method where HNTs were pretreated by washing with distilled water and drying at 100 °C for 10 h.14 APTES was first dissolved in dry toluene before the addition of HNTs, and then the suspension was sonicated for 30 min. The mixture was refluxed at 120 °C for 20 h under constant stirring to get the APTEScoated HNTs. The solid phase in the resultant mixture was filtered, washed with fresh toluene to remove the excess APTES, and then dried overnight at 70 °C for further use. Functionalization of HNTs with APTES-Coated GQDs. The GQD-HNTs were prepared by coupling the amine groups of APTEScoated HNTs with the carboxylic acid groups of GQDs using the standard EDC/NHS reaction at room temperature and pH 6.28 Briefly, 3 mg of EDC and 4 mg of NHS were added into 10 mL of as-prepared GQDs solution under vigorous stirring. The APTES-coated HNTs were then added to the above solution and then reacted for 4 h under stirring. The fluorescence intensity was measured at regular intervals to monitor the attachment of GQDs onto APTES-coated HNT. The solid phase in the resultant mixture was filtered after reaction and washed with DI water to remove the unattached GQDs, and then dried overnight at 80 °C for further use. In addition, 100 μg/mL GQD solutions were prepared at pH 6 and then mixed with the APTEScoated HNTs in the absence of EDC and NHS. After 2 h of reaction, the fluorescence intensity was measured from the supernatant solution to understand the effect of physical mixing on the attachment of GQDs to APTES-coated HNTs. Characterization of GQD and GQD-HNTs. The morphology of GQDs and GQD-HNTs were characterized using JEOL JEM-2010 high-resolution transmission electron microscopy (HRTEM) at 300 kV. X-ray photoelectron spectroscopy was performed with an ESCA Ulvac-PHI 1600 photoelectron spectrometer from Physical Electronics using Al−Kα radiation photon energy at 1486.6 ± 0.2 eV. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.5406 Å), and the Fourier transform infrared (FTIR) spectra of HNT-based

nanomaterials were determined by using a Horiba FT720 spectrophotometer to confirm the change in functional groups and the attachment of GQDs onto APTES-coated HNTs. Raman spectra of GQDs were determined by a Bruker Senterra micro-Raman spectrometer equipped with an Olympus BX 51 microscope and DU420-OE CCD camera. The thermal properties of HNT and GQD based nanomaterials were examined by thermogravimetric analysis using a Mettler Toledo DSC/TGA 3+ Stare system in air. Nitrogen adsorption−desorption isotherms were determined at 77 K using a Micromeritics ASAP 2020 analyzer. Prior to the adsorption isotherm, samples were outgassed under a high vacuum at 150 °C. The Brunauer−Emmett−Teller method was utilized to calculate the specific surface areas and pore texture of HNT- and GQD-based materials. Electrochemical Measurements. All of the electrochemical experiments including cyclic voltammogram (CV), galvanostatic charge−discharge, and electrochemical impedance spectroscopy (EIS) were performed by an Autolab PGSTAT 302 N electrochemical test system (Metrohm Autolab B.V.) by a conventional three-electrode system using 1 M Na2SO4 as the electrolyte. The active materialcoated carbon paper electrode was used as a working electrode, while Ag/AgCl and platinum sheet electrodes were used as the reference and counter electrodes, respectively. In addition, the electrode material powders were mixed with 5% Nafion and 10% carbon black to form a homogeneous paste and then drop-cast onto carbon paper. The quality of the electroactive material and electrode was examined by galvanostatic charge−discharge experiment in triplicate. The standard deviation of specific capacitance of GQD-HNTs at 6 A/g after 100 cycles was within 1%, showing the good quality and stability of electroactive materials used in this study. The specific capacitance (Cs), energy density (E), and power density (P) were calculated from the discharge curves based on the entire amounts of active materials and the following standard equations:

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Cs = Q /ΔV

(1)

Cs = I/m(ΔV /Δt )

(2)

E=

1 C(ΔV )2 2

(3)

P=

E Δt

(4) DOI: 10.1021/acssuschemeng.7b00329 ACS Sustainable Chem. Eng. 2017, 5, 4930−4940

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Figure 2. (a) Fluorescence intensity of GQDs before and after the formation of GQD-HNTs, (b) HRTEM image of GQD-HNTs (inset is the GQD-HNTs with lower magnification), (c) fringes of GQDs on the HNT surface from the selected area of panel b, and (d) lattice spacing of GQD (0.21 nm) on the GQD-HNT surface from the selected line of pnael c.

change in fluorescence intensity of as-prepared GQDs and GQD-HNTs. The GQDs emit strong fluorescence at 460 nm after irradiation of UV light at 360 nm, which is in good agreement with our previously reported results.27 After the addition of 50 mg of APTES-coated HNTs into the GQD solutions to form GQD-HNT nanocomposites, the fluorescence intensity reduces significantly. This phenomenon is mainly attributed to the fact that most GQDs are attached onto the surface of APTES-coated HNTs and that the metal oxide framework of HNTs hinders the π−π conjugation of GQD structures, resulting in the quenching of fluorescence intensity. In addition, the fluorescence intensity of GQDs was also monitored by physically mixing with APTES-coated HNTs in the absence of EDC/NHS to understand the effect of chemical bonding between GQD and APTES on structure stability. As shown in Figure S1b (Supporting Information), only a slight decrease in fluorescence intensity of GQDs in the presence and absence of APTES-coated HNTs is observed, clearly indicating the lower effect of physical mixing on the fluorescence intensity of GQDs. This result also proves that the decrease in fluorescence intensity of GQD-HNTs is mainly attributed to the successful attachment of GQDs onto the surface of APTEScoated HNTs. As the GQDs contain plenty of carboxylic functional groups, they can undergo the covalent linkage with the flanking amino groups of APTES coated on the HNTs, resulting in the strong attachment of GQDs onto the HNT surfaces. The surface morphology and GQD distribution on the GQD-HNTs are characterized by electron microscopic images. Figure S2 (Supporting Information) shows the low magnification TEM and SEM images of as-received HNTs and GQD-

where Cs is specific capacitance, Q is total charge storage, V is applied potential window, I is current density, m is total mass of active materials on electrode, t is time, E is energy density, and P is the power density of the electrode materials.

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RESULTS AND DISCUSSION Characterization of Graphene Quantum Dots (GQDs). The morphology of GQDs, synthesized by the pyrolysis of citric acid, was characterized initially. The TEM image in Figure 1a clearly shows the homogeneous distribution of GQD nanoparticles. The fringe in the single GQD lattice is clearly observed, and the fringe distance is calculated to be 0.21 nm (inset of Figure 1a), which corresponds to the (100) plane of defect-free single crystal graphene.27,29 The particle size distribution of GQDs shown in Figure 1b lies in a narrow window range of 5−11 nm with the average lateral size, determined by the histogram, of 9 ± 0.5 nm (n = 70). The Raman spectrum of GQDs is also carried out to confirm the structure of graphene. As shown in Figure 1c, the characteristic peaks at 1236 and 1586 cm−1, which can be assigned as D and G bands of carbons, are clearly observed.30 In addition, the GQD solution is colorless in visible light and emits bright blue fluorescence light after irradiation with 360 nm UV light, showing the excellent photoluminescence property of asprepared GQDs (Figure S1a, Supporting Information). Characterization of GQD-Modified HNTs (GQD-HNTs). To fabricate the GQD-based halloysite nanotubes (GQDHNTs), APTES was first coated onto the surface of HNTs (APTES-coated HNTs) through silanization, and then, GQDs were covalently anchored onto the surface of APTES-coated HNTs in the presence of EDC/NHS. Figure 2a shows the 4933


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Figure 3. (a) XRD patterns, (b) TGA, (c) BET surface area, and (d) FTIR spectra of HNTs, APTES-coated HNTs, and GQD-HNTs. For BET surface area, the Y-axis of as-received HNTs and APTES-HNTs were shifted 100 and 55 cm3/g-STP, respectively, for clarity.

HNT nanocomposites. It is clear that the surface morphology of GQD-HNTs changes obviously after modification with APTES and GQDs. The HNT surface covers with an irregular shape of thin film, indicating the successful modification of HNTs with APTES. However, the GQD nanoparticles are hard to observe in the figure, presumably due to the ultrasmall particle sizes of GQDs. When using high magnification of HRTEM images shown in Figure 2b, the distribution of GQDs is remarkably homogeneous and condensed onto the HNT surfaces. In addition, the fringe of GQD-HNTs is also deciphered (Figure 2c), and the distance of 0.21 nm clearly matches the characteristic lattice plane of GQDs (Figure 2d).27 The XRD patterns of original HNTs show major peaks at 11.8°, 20.1°, 24.6°, 35.1°, 35.8°, 38.4°, 54.5°, and 62.4° 2θ (Figure 3a), which are the characteristic peaks of as-received HNTs (JCPDS 29−1487).17 In addition, the XRD patterns of as-prepared GQDs show two broad peaks at 10.5° and 24.9° 2θ (Figure S3, Supporting Information).31 After the surface coating with APTES, the XRD patterns of ATPES-coated HNTs shows the identical peaks in comparison with those of as-received HNTs, indicating that the bonding of APTES onto HNTs does not alter the crystallinity of HNTs. An additional peak at 24.7° 2θ is also observed after the functionalization with GQDs, confirming the attachment of GQDs onto APTEScoated HNTs. Figure 3b shows the thermogravimetric analysis (TGA) of HNT-based nanomaterials including as-received HNTs, APTES-coated HNTs, and GQD-HNTs. The thermal behaviors of all of the HNT-based nanomaterials follow a similar trend, and the total weight losses are found to be 15, 20, and 21.8% for as-received HNTs, APTES-coated HNTs, and GQDHNTs, respectively. For as-received HNTs, the slight weight loss at 50−200 °C is due to the loss of water molecules, while the major decrease in weight loss at 450−540 °C is associated with dehydroxylation of structural Al−OH groups of HNTs.14

Different from the as-received HNTs, an obvious weight loss in the temperature range of 350−470 °C is observed for APTEScoated HNTs and GQD-HNTs, which is mainly attributed to the decomposition of oxygenated and hydrogenated functional groups of organic functional groups.32 Moreover, the thermogram of GQD-HNTs shows an additional 1.8% weight loss in comparison with that of APTES coated HNTs, which can be assigned as the attached amounts of GQDs on GQD-HNTs surface. In this study, excess amounts of GQDs were added to allow the maximum attachment of GQDs onto the APTEScoated HNTs during the synthesis process and the attached amount of GQDs is highly dependent on the active sites on the surface of APTES-HNTs. Therefore, the obtained value of 1.8 wt % of GQDs onto APTES-HNT is the maximum amount after equilibrium. Although the GQD content can be increased by the increase in APTES amounts or by stacking of GQDs, the morphology and electrochemical performance of GQD-HNTs will be changed and then alter the electrochemical performance because of the increase in thickness and stacking effect of GQDs. Figure 3c shows the N2 adsorption−desorption isotherms of as-received HNTs, APTES-coated HNTs, and GQD-HNTs. The as-received HNTs show a typical type VI isotherm with a hysteresis loop in the relative pressure (P/P0) range of 0.6− 0.95, clearly indicating the mixture of meso- and macroporous nature of as-received HNTs.33 The BET specific surface area of as-received HNTs is found to be 45.4 m2/g, and the average pore size is 14.8 nm. Addition of APTES decreases the specific surface area and average pore diameters to 25.3 m2/g and 14.4 nm, respectively, which is mainly attributed to the surface coating and deposition of the organic moiety of APTES on the surface. In addition, the surface area of GQD-HNTs further decreases to 19.4 m2/g with a similar average pore size of 14.3 nm, and the hysteresis loop slightly shifts to 0.7−0.95, 4934


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Figure 4. (a) Survey scan, deconvoluted XPS of (b) Si 2p, (c) N 1s, and (d) C 1s of as-received HNTs, APTES-coated HNTs, and GQD-HNT nanocomposites.

GQDs, the N 1s peak appears at ∼400 eV, and the peak intensity of C 1s increases, clearly indicating the successful attachment of APTES and GQDs onto HNT surfaces.35 It is noteworthy that the survey scan of as-synthesized GQDs shows strong peaks of C 1s (284.6 eV) and O 1s (530.9 eV) (Figure S4, Supporting Information). After peak deconvolution of the C 1s peak of as-prepared GQDs, several peaks including CC, −COOH, CO, and C−O are observed, clearly indicating the existence of several carboxyl functional groups on the surface of GQDs. The O 1s peak contributes 69.6% to the total elements in HNTs and then decreases to 55.7% after the addition of APTES. This may be attributed to the removal of the −OH group from the outer surface of HNTs, resulting in the decrease in atomic ratio of the O element. After functionalization with GQDs on APTES-coated HNTs, the oxygen percentage slightly increases to 60.2%, which indicates the successful addition of the carboxylic group of GQDs to APTES-coated HNTs. It is noteworthy that the intensity of Al and Si peaks remains unchanged after the attachment of GQDs, which means that the functionalization of HNTs with GQDs mainly occurs on the outer surface of HNTs.17 The peak deconvolution on XPS of Si 2p spectra was further performed to elucidate the possible interaction between APTES-coated HNTs and GQDs after the functionalization. The deconvolution of the Si 2p peak in Figure 4b shows two peaks at 102.85 and 103.5 eV, which can be assigned as the Si− O and Si−OH groups, respectively.34 In addition, the binding energies of Si−O and Si−OH slightly shift to 102.95 and 103.6 eV, respectively, after the addition of APTES, presumably due to the formation of APTES-HNT bonding. The shift in the Si 2p peak to the high binding energy after the coating of APTES

suggesting the formation of amide linkage to block some mesopores after the attachment of GQDs. The FTIR spectra illustrated in Figure 3d show the that strong absorption peak at 1032 cm−1 is considered as the stretching vibration of external Si−O groups in HNTs.15 The other three characteristic peaks at 912, 550, and 471 cm−1 correspond to the O−H deformation vibration of inner Al−OH groups.34 After the formation of GQD-HNT nanocomposites, all the characteristic peaks of HNTs remain the same, indicating that the maintenance of layered structures of HNTs after the surface functionalization with GQDs. In addition, a new absorption peak at 1105 cm−1 appears,34 which indicates a small distortion of Si−O structure on the outer surface of HNTs caused by the functionalization of HNTs by APTES.26 It is noteworthy that the FTIR spectrum of as-synthesized GQDs (Figure 3d) clearly shows the peaks at 1728 and 3200 cm−1, confirming the presence of a carboxyl functional group in GQDs.27 However, the lower amounts (1.8 wt %) of GQDs onto HNT surface and the large contribution of HNTs in the GQDs-HNT nanocomposites inhibit the appearance of other functional group peaks of GQDs in the GQD-HNT FTIR spectrum. Therefore, the deconvolution of XPS of GQD-HNTs is further performed to confirm the formation of bonding after the addition of GQDs. The survey scan of as-received HNTs clearly shows the characteristic peaks of Al 2p (75.1 eV), Si 2p (103.1 eV), Al 2s (119.6 eV), Si 2s (153.4 eV), C 1s (284.4 eV), and O 1s (531.6 eV) (Figure 4a).33 The small peak of C 1s in as-received HNTs is mainly attributed to the carbonaceous contamination from ambient air. In addition, no N 1s peak is observed in the survey scan of HNTs. After the functionalization with APTES and 4935


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Figure 5. (a) CV curves and (b) specific capacitances of GQD-HNT nanocomposites at various scan rates ranging from 5 to 100 mV/s, (c) charge− discharge curves, and (d) specific capacitances of GQD-HNT nanocomposites at various current densities between 0.5 and 20 A/g.

potential window of 0−1 V to understand the electrochemical behavior of GQD-HNTs. Figure 5a shows the CV curves of the GQD-HNTs electrode in 1 M Na2SO4 electrolyte solution at various scan rates ranging from 5 to 100 mV/s. The CV curves of GQD-HNTs exhibit a nearly ideal rectangular shape, and no obvious redox peaks are observed, indicating the typical characteristic of electrical double-layer capacitors. In addition, the CV curves still retain a good rectangular shape even at a scan rate of 100 mV/s, depicting the superior rate capability and excellent electrochemical reversibility of GQD-HNTs.37 The GQD-HNTs shows excellent capacitive behavior, and the specific capacitance of 323 F/g at a scan rate of 5 mV/s is obtained (Figure 5b). The specific capacitance of GQD-HNTs decreases gradually upon the increase in scan rate, and the specific capacitance decreases to 186 F/g, which corresponds to the 58% retention, when the scan rate increases to 100 mV/s. In addition, the CV curves of as-received and APTES coated HNTs at a scan rate of 5 mV/s were also carried out and compared. The specific capacitances are found to be 16 and 14 F/g for as-received HNTs and APTES-coated HNTs, respectively (Figure S5, Supporting Information). This result clearly reflects the fact that the drastic enhancement of capacitance of GQD-HNTs is mainly contributed by the existence of conducting GQDs. Hassan et al. have recently prepared a GQD film on glassy carbon electrode for supercapacitor application, and a specific capacitance value of 108 F/g was obtained.38 In this study, the specific capacitance of GQD-HNTs is much higher than GQD data reported23 and that of modified HNTs,18,39 clearly indicating that the GQDHNT nanocomposites are superior electroactive nanomaterials for supercapacitor application. Figure 5c shows the galvanostatic charge−discharge measurements in 1 M Na2SO4 solution in the potential window of 0−1

is mainly attributed to the fact that the coupling reaction between the ethoxy group of APTES and hydroxyl group of HNTs makes the nanocomposites more electronegative because of the Si−O−CAPTES bond and subsequently results in the positive shift in binding energy. After the functionalization of GQDs, the binding energy of the Si−O group further shifts to the high binding energy of 103.1 eV, which is attributed to the formation of amide linkage between GQDs and APTES-coated HNTs. The N 1s spectra of the APTES-coated HNT and GQDHNTs are further deconvoluted. As shown in Figure 4c, two peaks centered at 399.2 and 401.8 eV are observed in N 1s spectra, which are the characteristic peaks of N−H and C−N structures, respectively.36 The binding energy of N 1s shifts from 399.2 eV in APTES-coated HNTs to 399.4 eV in GQDHNTs along with the decreasing peak intensity, indicating the slight oxidation of N−H bond contributed from the reaction of -NH2 with the carboxyl group. The C−N peak shows the reverse trend after the attachment of GQDs because of the formation of the amide bond between -NH2 of APTES and −COOH of GQDs.35,36 This phenomenon is also noticed in the deconvoluted C 1s peak shown in Figure 4d. For GQDHNTs, the C−N peak slightly shifts from 285.6 to 285.5 eV, and an additional C−O peak centering at 288.3 eV is observed after the addition of GQDs to APTES-coated HNTs, confirming the presence of the amide group produced from the successful coupling reaction between the carbonyl group of GQDs and amino group of APTES coated HNTs. In addition, the CC peak at 284.5 eV is observed, which can be assigned as the sp2 structure of GQDs. Electrochemical Measurements. After the surface characterization of HNT-based nanomaterials, cyclic voltammetry (CV) was performed in a three-electrode cell system in a 4936


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ACS Sustainable Chemistry & Engineering V at various current densities ranging from 0.5 to 20 A/g. The charge−discharge curves show the symmetric nature and high capacitance, suggesting the low IR drop of GQD-HNT nanocomposites. Figure 5d shows the specific capacitance of GQD-HNTs as a function of applied current density. The electroactive nanomaterials deliver excellent capacitances of 363, 338, 294, 272, 258, 246, 233, and 216 F/g at 0.5, 1, 2, 4, 6, 8, 10, and 20 A/g, respectively. Similar to the capacitance of GQD-HNTs at various scan rates, a 60% capacitance can be retained at a high current density of 20 A/g, indicating that the GQD-HNT nanocomposite is stable over a wide range of current density. In addition, no obvious IR drop is observed, depicting that the GQD-HNTs have a small internal resistance and efficient extraction of stored energy.39 HNTs are layered structures which can provide meso- and macropores for rapid transfer of electrolyte ions. Because of the poor conductivity, the as-received HNTs exhibit low capacitance. Several modification methods including the carbonization of negatively charged external surface with chitosan40 and the addition of cationic surfactants to obtain the hydrophobic shell41 have been attempted to improve the electrochemical performance of HNTs. The surface functionalization with -NH2 groups was also reported for further modification on HNT surface for various applications.21 On the other hand, graphene based nanomaterials such as reduced graphene oxide (rGO), 42 graphene nanosheet,43,44 and GQDs23,45 have been used for energy applications in different matrixes. A previous study has immobilized APTES-coated HNTs onto the surface of rGO sheets as an electrode material.20 The specific capacitance of 22.73 F/g was obtained at a current density of 100 mA/g. In this study, the surface functionalization of naturally available HNTs with GQDs for supercapacitor application is explored, and the specific capacitances of 363−216 F/g at 0.5−20 A/g prove that the GQD-HNT nanocomposites are superior electroactive electrode materials for supercapacitor application. After conjugation with GQDs having −COOH groups as well as a conductive graphitic layer throughout the HNTs, the layered and porous GQD-HNT nanocomposites offer abundantly accessible electroactive sites for electrolyte ions and electrons to rapidly transport during the charge−discharge processes46 and subsequently result in the significant enhancement of electrochemical performance to achieve the expected high capacitance performance. The energy and power densities are important parameters for supercapacitor application. Figure 6 shows the Ragone plot of GQD-HNT nanocomposites and the comparison of the electrochemical performance with previously reported data based on a 3-electrode system.47−51 The GQD-HNT nanocomposites provide an energy density of 50.03 Wh/kg with a power density of 0.23 kW/kg at 0.5 A/g and can maintain the energy density of 30.1 Wh/kg at a power density of 10.12 kW/ kg when the current density increases to 20 A/g. In the energy density range of 30.2−40 Wh/kg (Inset of Figure 6), the electroactive material shows relatively high power density, which is one of the most important behaviors for supercapacitor applications. It is noteworthy that the 2-electrode system more resembles the real supercapacitor system for calculation of energy density. However, the electrochemical performance of the 3-electrode GQD-HNTs cell used in this study is in good agreement with the reported data shown in Figure 6. It is also clear that the electrochemical performance of GQD-HNTs is relatively higher than the most characteristic

Figure 6. Ragone plot (power density vs energy density) of GQDHNT nanocomposites and the comparison with reported references. Gr, graphene; Ppy, polypyrrole; and Gr-sh,: graphene sheet.

range of recently reported supercapacitors including Ppy/ MoS2,47 N-Gr,48 graphene sheet,49 and NiO-Ni50 and Ni(OH)2.51 In addition, the result obtained in this study is comparable to the asymmetric highly ordered mesoporous carbon//MnO2 supercapacitors,6 indicating that the surface functionalization of HNTs with GQDs dramatically enhances the capacitive performances of nanocomposites. The cycling performance of electroactive material is one of the most significant parameters for the evaluation of electrochemical performance. Figure 7a shows the cycling stability of GQD-HNT nanocomposites over 5000 cycles at a constant current density of 6 A/g. The initial specific capacitance is found to be 258 F/g at 6 A/g and then slightly decreases to 251 F/g after the first 100 cycles. The specific capacitance can maintain at 221 F/g, which corresponds to 88% of the original capacitance, after 5000 consecutive charge−discharge cycles. The good cycling stability of the developed nanocomposites is mainly attributed to the defect free GQDs, which form strong covalent attachments with HNTs. These results clearly indicate that GQD-HNT nanocomposites are excellent and stable electrochemical materials which can be used to meet the requirements of energy storage devices. In addition, an independent experiment was performed to understand the long cycling stability of GQD-HNT nanocomposites after 10000 cycles at 6 A/g. As shown in Figure S6 (Supporting Information), the cycling curve can maintain good stability during the first 5000 cycles. However, further cycling results in the decrease in specific capacitance, which can be attributed to the high current density and decay of carbon paper electrode support. Several studies have used HNT- or GQD-based nanocomposites for supercapacitor applications and found that 81−98% of capacitance can be retained after 1500−5000 cycles for GQD-based nanocomposites (Table 1).9,10,22,23,45 In addition, the combination of metal oxide with HNTs also shows reasonable capacitance retention of 81−95% after 50− 8600 cycles.20,39,52,53 These results clearly indicate that the stability study of 5000 cycles for GQD- and HNT-based nanomaterials is reasonable. For further application, more studies on the long cycling stability of GQD-HNTs are needed. Figure 7b shows the Nyquist plot of GQD-HNT nanocomposites and as-received HNTs. It shows a straight line in the low frequency region and an obvious semicircle in the high frequency region.54,55 Usually the high frequency region is related to the charge/ion transfer resistance between the GQDHNT electrode materials and electrolytes, which is influenced 4937


Research Article


Figure 7. (a) Cycling stability of GQD-HNT nanocomposites at 6 A/g for 5000 cycles and (b) Nyquist impedance plot of the as-received HNTs (black line) and GQD-HNT nanocomposites (red line). The insets in panel b are the equivalent electrical circuit and enlarged Nyquist impedance spectra in the frequency region of 7−10 Ω at 0.2 V.

HNT has a short ion diffusion path. This can facilitate the efficient access of electrolyte ions to the GQD surface. The magnitude of equivalent series resistance (Rs) is 7.8 Ω for asreceived HNTs and 6.7 Ω for GQD-HNTs, which indicates that the bulk electrolyte transport resistance for both materials is low. In addition, the Rct value of 5.5 kΩ for GQD-HNTs is significantly lower than that of the as-received HNTs (31.9 kΩ), presumably attributed to the increased electrical conductivity in the presence of GQDs. The high value of Rct for as-received HNTs is mainly due to the inherently poor conductivity in the neutral Na2SO4 electrolyte. In addition, the n value of as-received HNTs is 0.88 and slightly increases to 0.92 for GQD-HNTs, which represents the enhancement of roughness after GQD attachment.

Table 1. Comparison of Cycle Stability Measurement for Specific Capacitance Retention for GQD Based Materials Using Different Matrices type of material GQD based materials

HNT based materials

a

materials

a

GQD//GQD Ppy/GQD GQD/PANI GQD/ Graphene PANI/GQD nanofiber HNT/rGO C/MnO2 (HNT template) NiCo2O4− HNT NiCo2S4− HNT//NG GQD-HNT

scan rate/ current density

no of cycles

capacitance retention (%)

1 V/s 0.05 V/s 1 V/s 5 A/g

5000 2000 1500 5000

93 98 97 90

9 10 22 23

1 A/g

3000

81

45

1 V/s 80 mV/s

50 1200

85 81

20 39

10 A/g

8600

95

52

10 A/g

1700

83

53

6 A/g

5000

88

this work

reference

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CONCLUSIONS In this study, we have successfully developed a new class of GQD-HNT nanocomposites for supercapacitor applications. GQDs can be tightly attached to the surface of HNTs through the formation of amide linkage from the carboxylic groups of GQDs and amine groups of APTES coated HNTs. By utilization of the high charge storage sites and fast ion transport characteristics, GQD-HNT nanocomposites exhibit excellent specific capacitance and energy density for supercapacitor applications. The specific capacitances of 363−216 F/ g at current densities of 0.5−20 A/g in the potential window of 0−1 V are obtained in 1 M Na2SO4 electrolyte solution. In addition, the GQD-HNTs exhibit excellent specific energy and power densities of 30−50 Wh/kg and 0.23−10.12 kW/kg, respectively. The high cycling stability of 88% is also achieved after 5000 cycles at a current density of 6 A/g. Results obtained in this study are the first report on the utilization of GQDHNTs as a promising energy storage material for high performance supercapacitor applications, which can provide an alternative to using environmentally benign HNT based layered materials for the fabrication of supercapacitors with high energy density.

Ppy, polypyrrol; PANI, polyaniline; NG, nitrogen-doped graphene.

by the material porosity and thickness.56,57 Moreover, the semicircle diameter of GQD-HNT nanocomposites in the high frequency region is smaller in comparison with the as-received HNTs, which indicates the high electrochemical performance of the GQD-HNT electrode and high ion transport rates to the pores of GQD-HNTs. The equivalent circuit model shown in the inset of Figure 7b indicates the contribution of capacitive and resistive elements of the electrode materials. Table 2 illustrates the electrochemical parameters of GQD-HNTs and HNT electrodes obtained from the EIS fitting. The Warburg curve is quite short, which gives an indication that the GQDTable 2. Electrochemical Parameters of GQD-HNT and HNT Electrodes Obtained from Impedance Analysis Based upon the Proposed Equivalent Circuit materials

Rs (Ω)

CCPE (mF)

n

ω (Ω)

Rct (kΩ)

χ2

HNTs GQD-HNTs

7.8 6.7

0.318 0.312

0.88 0.92

0.149 0.134

31.9 5.5

0.012 0.013

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00329. 4938


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Fluorescence intensity of GQDs under UV and visible light irradiations; change in fluorescence of GQD-HNTs before and after physical mixing; TEM and SEM images of HNTs and GQD-HNTs at lower magnification; XRD and XPS spectra of as-prepared GQDs; comparison of CV curves of GQD-based materials; and the cycling stability of GQD-HNT nanocomposites for 10,000 cycles (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +886-3-5726785. Fax: +886-35725958. E-mail: [email protected]. ORCID

Ruey-an Doong: 0000-0002-4913-0602 Author Contributions §

A.B.G. and A.D.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology (MOST), Taiwan for financial support under grant numbers MOST 1042221-E-009-020-MY3 and 105-2113-M-009-023-MY3. A.D.C. is acknowledges MOST, Taiwan for financial support.

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New Avenue for Appendage of Graphene Quantum Dots on Halloysite

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