A Reduced Graphene Oxide Quantum Dot-Based Adsorbent for

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A Reduced Graphene Oxide Quantum Dot-Based Adsorbent for Efficiently Binding with Organic Pollutants Huawen Hu, Haiyan Quan, Biqi Zhong, Zheng Li, Yonghao Huang, Xiaowen Wang, Min Zhang, and Dongchu Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01799 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Applied Nano Materials

A Reduced Graphene Oxide Quantum Dot-Based Adsorbent for Efficiently Binding with Organic Pollutants Huawen Hu*† ‡, Haiyan Quan†, Biqi Zhong†, Zheng Li†, Yonghao Huang†, Xiaowen Wang† §, Min Zhang*†, and Dongchu Chen*† †

College of Materials Science and Energy Engineering, Foshan University, Foshan,

Guangdong 528000, China. ‡

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

Development, Guangzhou 510070, China. §

Nanotechnology Centre, Institute of Textiles and Clothing, The Hong Kong Polytechnic

University, Hong Kong SAR 999077, China KEYWORDS: graphene quantum dots, green chemistry, temperature-controlled sonication irradiation, reduced graphene oxide edges, quantum confinement, adsorption interactions

ACS Paragon Plus Environment

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ABSTRACT: This paper, for the first time, presents a novel, robust, and efficient highquality reduced graphene oxide quantum dot (GQD)-based adsorbent for addressing the alarming environmental pollution issues nowadays. Such GQDs were fabricated in a facile manner via a combined green chemistry (GC) and temperature-controlled sonication irradiation (TSI) process. The mechanism underlying the fragmentation of the reduced graphene oxide (RGO) sheets prepared by GC with vitamin C (VC) as the only reactant to regulate the microstructure of graphene oxide (GO) was clarified through various characterization techniques such as X-ray diffraction, Raman, Brunauer-Emmett-Teller specific surface area measurements, transmission electron microscopy, scanning electron microscopy, elemental mapping, Fourier transform infrared

spectroscopy,

X-ray

photoelectron

spectroscopy,

photoluminescence

spectroscopy, selected area electron diffraction, and energy dispersive spectroscopy. The GQDs were assumed to be generated via cutting the nanometer-size sp2 domains or clusters out of the RGO sheets along the defect sites decorated with oxygen groups and sp3 carbons. The adsorption of the methylene blue (MB) dye onto the prepared GQDs was fast and obeyed the pseudo-second-order kinetic model, while the Freundlich adsorption isotherm was more suitable to fit the experimental data, as a result of the preferential enrichment of the organic dye molecules onto the edges rather than the homogenous adsorption over the entire surface of GQDs. This could also be evidenced


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by the finding that the smaller was the lateral size of the RGO sheets, the stronger were the adsorption interactions with the MB dye. The dangling bonds with non-bonding electrons on the RGO edges were thus believed to play a significant role in the heterogeneous adsorption of the dye molecules through donor-acceptor charge transfer interactions. Impressively, the striking edge effect rendered the GQDs highly adsorptive toward the organic dye, with the maximum adsorption capacity calculated to be 827.5 mg g-1 for MB, outstripping most of the reported adsorbents. In addition, this GQDsbased adsorbent was found to be durable for many runs of repeated usage, and its universality was also demonstrated through efficiently binding with rhodamine B.

1. INTRODUCTION Graphene, a two dimensional (2D) single-atom carbon sheet in a honeycomb structure, has generated enormous excitement due to its fascinating structure and properties, such as the huge specific surface area, and extraordinary electronic, thermal and mechanical properties.1-4 To further meet the requirement of the nanoscale electronic device, optical, optoelectronic, sensing, biomedicine, surface-enhanced Raman scattering, and photocatalysis applications,5-9 it is desirable to further engineer the structure of 2D graphene sheets, e.g., further cutting 2D sheets into fragments or pieces with a nanoscale lateral size (less than 100 nm) for opening the zero bandgap of bulk graphene10. Such fragments have been defined as graphene quantum dots (GQDs) that exhibit exotic physicochemical properties related to quantum confinement and


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edge effects, and the GQDs represent a new class of zero-dimensional (0D) quantum dots10-13. GQDs with edge enrichment and small size usually possess different performance and functions compared to their 2D counterparts,7 e.g., edge-plane atoms have been demonstrated to exhibit much higher electron transfer rates compared to those in basal planes.14 Both “top-down” and “bottom-up” methods have been employed to produce GQDs.13, 15-17 The former has been reported to prepare GQDs through splitting from the carbon source (e.g., graphite, carbon nanotube, carbon fiber or even coal),10, 13, 16-20 while the latter involves organic synthesis via, for example, chemical vapor deposition.21 It is worth mentioning that “top-down” methods can start from cheap, naturally abundant, and environmentally safe graphite source as a precursor, and thus they are more intensively investigated due to their more economical, convenient and efficient nature in the synthesis of various GQDs in most cases as compared to “bottom-up” strategies, e.g., hydrothermal routes to blue-luminescent GQDs18 and to photocatalytically active GQDs with a broad visible absorption capacity,20 a supercritical water-assisted “topdown” formation of GQDs for photothermal treatments of tissues,22 an electrochemical approach to luminescent and electrocatalytically active GQDs doped with nitrogen for the oxygen reduction reaction,23 and an ultrasonic and chemical treatment approach for the

fabrication

of

activated

GQDs

with

enhanced

photo-luminescence

and

supercapacitance.14 To date, however, no reports can be found on the investigation of the quantum confinement and edge effects of GQDs on the adsorption of the environmental


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pollutants.24 Considering that energy crisis and environmental pollution have been listed in the humanity’s top 4 problems for the next 50 years,25 it is highly desirable to develop effective techniques to address the environmental pollution issues. Among the methods explored to tackle the environmental pollutants, adsorption processing is regarded as the most viable, cost-effective, and efficient one, and no secondary pollutants will be generated in the adsorption process.26-28 Although significant advancements in the exploration of 2D graphene sheets-based adsorbents for processing the environmental pollutants have been achieved,29-36 little attention has been paid to the synthesis of the 0D GQDs-based adsorbents for environmental remediation applications and even to the investigation of the effect of the intrinsic structure of the 2D graphene sheets on the adsorption performance. There are a large number of reports focusing on boosting the adsorption efficiency through cooperation with other functional compositions.37-43 This would impede the clarification of the origin of the adsorption interactions from graphene and its derivatives due to the foreign species (such as organic polymers2, 44 and inorganic nanoparticles45-47) that were strongly attached to their surface. Therefore, it is of significance to probe the effect of the molecular structure of naked graphene sheets on the adsorption interactions in order to fabricate highly-performing graphene-based functional materials, especially for environmental remediation applications. In this study, green chemistry (GC) was combined with temperature-assisted sonication irradiation (TSI) using a probe sonicator for the “top-down” formation of various naked graphene sheets including 2D graphene sheets, and the 0D GQDs with


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different lateral sizes. The mechanism for the fragmentation of graphene sheets and the formation of GQDs was also clarified by means of systematic characterizations and analyses. The GC-based process is eco-friendly, simple, low-cost, and scalable, only involving biocompatible vitamin C (VC) as a reactant for tuning the molecular structure of graphene oxide (GO). The structural composition of VC only includes C and O atoms, excluding the doping effects from heteroatoms (e.g., N atom contained in many reducing agents for GO such as commonly-used hydrazine48-50) on the adsorption performance of the product as generated by the reaction between GO and VC. The adsorption performance of these naked graphene sheets toward cationic organic dye was compared, in addition to clarifying their structural features. The commonly-used organic dyes were investigated including methylene blue (MB) and Rhodamine B (RhB). Efforts were also put to study the origin of the excellent adsorption performance of the resulting GQDs. The development of this new type of GQDs-based adsorbents with tunable adsorption properties paves the way for the fabrication and design of various functional materials with novel structures, and tremendous properties and functions for environmental remediation applications.

2. EXPERIMENTAL SECTION Materials. Graphite fine powder of the spectrographic purity was supplied by Tianheng Technology Co. Ltd. (Hong Kong SAR, China). L(+)-ascorbic acid (GR ACS ISO) was purchased from International Laboratory, USA. All of the other chemicals


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(analytical grade) were obtained from Xilong Chemical Co. Ltd. (Guangzhou, Guangdong, China) and used as-received.

Manipulating the molecular structure of GO through GC and TSI. The GO was firstly prepared (see detailed procedures in the Electronic Supporting Information (ESI)) and subsequently used to fabricate a series of graphene adsorbents on the basis of GC and TSI (Table S1 in the ESI). Typically, the G1 and G2 samples were firstly prepared via a chemical reaction between GO and VC in an aqueous solution at room temperature (RT, 25 ± 2 oC) and 60 oC, respectively. The prepared GO powder (0.2 g) was homogeneously dispersed in deionized (DI) water (200 ml) by ultrasonication, followed by the addition of the VC powder (2.0 g), and then the reaction was carried out by magnetic stirring at RT and 60 oC for 8 h, yielding the G1 and G2 samples, respectively. The other samples, including G3, G4, and G5, were obtained using the prepared G2 sample as a precursor. For example, the G3 sample was prepared by dispersing the prepared G2 powder (50 mg) into DI water (50 mL) via slightly shaking without involving TSI, forming a 1.0 mg/mL solution, and then by filtration and drying at RT. The G4 and G5 samples were prepared in a similar manner according to the above procedure for the preparation of G3, except that additional TSI was conducted for 20 and 80 min, respectively. Tests on the adsorption of methylene blue (MB). The commonly-used MB dye was selected as a model organic pollutant for the present adsorption study. The initial MB dye solutions at a series of concentrations (10, 20, 30, 40, 50, 60, 70 to 80 mg L-1) were


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prepared, and the dosage of the graphene adsorbent varied from 100, 200, 300, 400, 500, 600, 700 to 800 mg L-1. In order to study the adsorption kinetics, the concentration of the solute and the mass of the adsorbent were set as 40 mg L-1 and 100 mg L-1 respectively, and the time of adsorption was changed. For the measurement of the adsorption isotherms, the mass of the adsorbent and the equilibrium time were set as 200 mg L-1 and 24 h, respectively, and the concentration of the MB solute in the range from 10 to 80 mg L-1 was adopted. It was found that adsorption for 24 h was sufficient to reach an equilibrium state. In order to calculate the maximum adsorption capacity based on the Freundlich model, the concentration of the solute was set as 80 mg L-1, and the mass of the adsorbent was changed from 100 to 800 mg L-1. The adsorption experiment was performed in tightly-closed glass vials by adding a given amount of the prepared graphene powder as the adsorbent at RT in a static mode without any disturbance from external agitations. During the adsorption process, the supernatant was withdrawn at predetermined time intervals for the analysis of the residual concentration of MB using an ultraviolet/visible (UV/vis) spectrophotometer at 664 nm. All experiments were conducted at least twice, and the mean values were reported. The regeneration tests on the graphene adsorbent were carried out as follows: the sample recycled from the adsorption tests was re-dispersed into ethanol, and then intense magnetic stirring was conducted at RT for 30 min for the desorption of the MB dye. The recovered adsorbent was subsequently re-used in a new cycle, and a total of 5 runs of repeated usage were carried out. The adsorption amounts at equilibrium (qe)


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and adsorption efficiency (%) were estimated according to the following Equations (1) and (2), respectively.

qe = (C 0 - Ce) ×

V M

Adsorption efficiency (%) =

(1)

C0 - Ce ×100 C0

(2)

where C0 and Ce represent the initial and equilibrium concentrations (mg L-1) of the MB dye, respectively, V is the volume (L) of the solution containing the MB dye, and M is the dosage (g) of the graphene adsorbent. Characterizations. The surface morphologies, elemental mapping images and energy dispersive X-ray spectra (EDS) of the prepared graphene adsorbents were observed using a scanning electron microscope (SEM, Hitachi S4800, Japan). Fourier transform infrared (FTIR) spectra were recorded using an IRAffinity-1S spectrophotometer (Shimadzu, Japan). A probe sonicator (Branson Sonifier 450, 120 W continuous output), equipped with an ice-water base, was employed for the TSI process. The transmission electron microscopy (TEM) images of the prepared graphene adsorbents were captured using a transmission electron microscope (JEOL JEM 2010), and the beam energy adopted was 100~300 keV (depending on the magnification scale during the TEM measurements). The sample holder used was a computer-controlled, fully aligned, 5-axis motor stage in a side-entry mode (CompuStage). Ultraviolet-visible (UV/vis) spectra were recorded with a UV-3100 UV/vis spectrometer (Shimadzu, Japan). X-ray photoelectron spectra (XPS) were obtained from an XSAM800-XPS system (Kratos Ltd., Manchester, UK) using Ma-Kα Xray source. X-ray diffraction (XRD) analysis was conducted using a powder


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diffractometer (X'Pert-PRO, PANalytical), equipped with Cu-Kα radiation, a working voltage of 40 kV, and a working current of 40 mA. The Raman spectra recorded for the prepared graphene adsorbents were performed using a LabRAM HR800 spectrometer (excited with a laser of 488 nm). Brunauer–Emmett–Teller (BET) specific surface area was measured from N2 adsorption-desorption tests using a Micromeritics ASAP 2020 nitrogen adsorption apparatus (Norcross, GA, USA) at the liquid nitrogen temperature. Photoluminescence

(PL)

analysis

was

performed

using

a

Spex

Fluorolog-3

spectrofluorometer (Instruments SA, NJ, USA) equipped with a 450 W Xe light source and double excitation monochromators.

3.

RESULTS AND DISCUSSION Based on GC and subsequent TSI, various graphene adsorbents including 2D

graphene sheets and 0D GQDs with different lateral sizes were fabricated. The main content of this study is schematically shown in Figure S1 in the electronic supporting information (ESI). Significantly, the temperature was found to play a pivotal role in the chemical reaction between GO and VC, leading to a notable difference in the structures of the prepared G1 and G2 samples which were prepared at RT and 60 oC using GO and VC as the raw materials, respectively. Although it was reported that VC was a strong reducing agent for GO, with a reducing capability even comparable to the commonlyused hydrazine,51 it was found in this study that the temperature played a pivotal role in the effective reaction between GO and VC and the conversion of GO to a reduced


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graphene oxide (RGO). Figure 1 presents the XRD patterns of the various prepared samples. G1 showed similar XRD patterns to GO, with only a little difference in the diffraction peak indexed to the (001) crystallographic plane of GO. According to the Bragg’s equation nλ=2dsinθ (n=1, λ=1.541 Å, θ is the diffraction angle, and d is interplanar spacing), G1 exhibited a smaller d001 value than GO (8.5 vs. 9.5 Å), revealing that a chemical reaction between GO and G1 actually occurred albeit with a low reaction degree. We then raised the reaction temperature from RT to 60 oC and finally produced G2 as a counterpart to G1. The (001) diffraction band of GO completely disappeared for G2, and a broadband centered at approximately 23.5o was emerged, as a result of the diffraction of the graphenic (002) plane (d002 spacing=3.78 Å). This finding indicates that the chemical structure of GO can hardly be converted to that of RGO at RT using VC as the reducing agent, while the higher temperature (i.e., 60 oC) is able to induce a stronger chemical reaction between GO and VC, making a complete change in the crystalline structure of GO. The more effective chemical reaction between GO and VC at higher temperatures was because the internal hydrogen bonds formed between VC molecules themselves were more readily destructed at elevated temperatures.52 The freedom of VC molecules from the internal hydrogen bonding facilitated the reaction between GO and VC, and RGO was finally produced.


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Figure 1. XRD patterns of the prepared GO, G1, G2, G3, G4, and G5 samples. We subsequently employed the TSI technique to further engineer the structure of G2 as the precursor. Through tuning the TSI time, various samples were prepared, including G3, G4, and G5 involving TSI durations of 0, 20 and 80 min, respectively. It was noted from the XRD patterns (Figure 1) that the TSI processing did not significantly alter the crystalline structure of G2. The calculation of the d002 spacing revealed that the longer the TSI duration, the smaller the interplanar spacing, which can be an indication that the physical structure might be changed to some extent after the TSI process. The energy generated by ultrasound cavitation exerted a significant impact on the 2D graphene sheets, leading to the breakage of the C-O/C-C bonds on the backbone of the G2 sheets. The graphene sheets are more readily broken along the defect sites such as the C-O/C-C sites since less energy is required to break the C-O and C-C bonds (ca. 383


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and 357 kJ/mol respectively) as compared to the C=C bonds (ca. 579 kJ/mol).24 As a result, splitting the graphene sheets would suggest a loss of some structural defects (e.g., oxygen groups) at the same time, further restoring the in-plane structure of graphene sheets and debasing their hydrophilicity. The interplanar distance between the graphene sheets is hence decreased, in consistence with the higher graphitization of the graphene sheets with a lowered d002 spacing (Figure 1, and Figure S2 in the ESI). XPS survey spectra are shown in Figure S3 in the ESI, and the comparison plot of the O/C ratio among the various prepared samples is provided in Figure S4 in the ESI. The O/C ratio is slightly decreased for G1 as compared to that for GO, indicating that the reaction between GO and VC actually took effect even at RT, even though the reaction extent was low (GO and G1 exhibited similar high-resolution C1s XPS core-level spectra shown in Figure S5 in the ESI), in agreement with the results as obtained via XRD. The temperature (i.e., 60 oC)-assisted reaction between GO and VC led to the generation of the G2 sample exhibiting an O/C ratio markedly smaller than that for GO and G1, again demonstrating the critical role that the temperature played in the reduction of GO by VC. Although a long TSI duration of 80 min enabled a mild increase in the C1s peak signal relative to the O1s peak signal attributed to the cutting of the RGO sheets along the oxygen-laden defect sites, the TSI process, actually, did not significantly alter the O/C ratio when the G2 sheets with a high extent of reduction were employed as the precursor. As shown in the high-resolution C1s XPS core-level spectra (Figure 2), four peaks can be deconvoluted at approximately 284, 285, 288 and 289 eV, which can be assigned to C=C/C-C, C-O C=O, and HO-C=O, respectively. This result again


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demonstrates that the temperature-assisted GC process significantly reduced the oxygen functionalities and changed the structure of GO to that of RGO, while the subsequent TSI process exerted a slight impact on the concentration of the oxygen groups on the RGO planes. The oxygen group variation in all the prepared samples was also evidenced by the FTIR and EDS techniques, and the results are shown in Figure S6, S7, S8, S9, S10 and S11, and S12 in the ESI. The atomic percentages of the element O were tested to be 19.85, 18.31 and 17.15 at.% in the G3, G4 and G5 samples involving the TSI durations of 0, 20 and 80 min, respectively (Figure S11 in the ESI). The TSI-related mild reduction of the oxygen groups is assumed to result from the breakage of the RGO sheets into fragments along the structural defects (e.g., oxygen groups and sp3 carbons), in agreement with the XPS results.


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Figure 2. High-resolution C1s XPS core-level spectra of the prepared G1 (a), G2 (b), G3 (c), G4 (d), and G5 (e) samples.

To further validate the breakage of the graphene sheets after subjected to TSI, TEM images are provided (Figure 3). As a precursor for the TSI processing, the G2 sheets featured a large continuous film-like structure with a micrometer-sized lateral dimension. Without involving the TSI process, G1 and G3 also presented a large filmlike structure, similar to G2. Impressively, for the G4 sample involving the TSI for only 20 min, the big continuous film-like structure started to break into pieces with a lateral dimension of 340 nm on average. Concerning the G5 sample, the prolonged TSI processing for 80 min significantly fragmentized the graphene sheets into the GQDs with a mean lateral size of 60 nm. The high-frequency acoustic cavitation effect (as generated by TSI) caused the graphene sheets to be significantly cut along the defect sites into GQDs which were expected to possess unique physicochemical properties related to quantum confinement and edge effects in comparison with the 2D graphene counterpart.53 The clear lattice fringes as shown in the high-resolution TEM image of the GQDs indicated reduction of the graphene sheets to a high degree, and the adjacent lattice fringe distance was estimated to be 0.24 nm, indexed to the (1120) lattice fringes of graphene.13 Basically, the SAED patterns of all the prepared samples exhibited diffraction rings with ambiguous diffraction spots (Figure 3), revealing a loss of the long-range ordering


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between the graphene sheets.54 When one focused on the diffraction ring pattern for the G5 sample involving the longest sonication time, faint spots could be noted, implying that a further removal of the defect sites occurred and hence an in-plane structural restoration of the G5 sheets could also be imparted via TSI, in addition to the size reduction, which is consistent with the results obtained via XPS and EDS. Three observable diffraction rings can be associated with the graphenic (112), (101) and (002) planes.55 All of these revealed that the high-quality GQDs enriched with the edges of the RGO at a high extent of reduction were formed by the combined effect of fragmentation and in-plane structural restoration.


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Figure 3. TEM images of the various prepared samples, along with their SAED patterns; the size distribution histograms of the fragments as formed by the fragmentation of the


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micrometer-sized graphene planes via the TSI process are also provided for the G4 and G5 sample. SEM images are also presented for the prepared samples (Figure S13, S14, S15, S16 and S17 in the ESI). The continuous film-like structure exhibited under TEM for G1 was also seen under SEM (Figure S13 in the ESI), while a porous structure was noted for G2 and G3 (Figure S14 and S15 in the ESI respectively), different from the continuous filmlike structure shown in the TEM images of G2 and G3 (Figure 3). This difference can be attributed to a much higher extent of reduction for the G2 and G3 sheets relative to that for the G1 sheets, along with a restoration of the π-conjugated system and an enhancement of the hydrophobicity. The more hydrophobic G2 or G3 sheets readily aggregated in a disordered manner via hydrophobic and π-π stacking interactions, leading to the generation of the porous structure composed of these sheet-like moieties (exhibited under TEM) as building units. Similarly, the disorderedly aggregated graphene sheets with a high extent of reduction were also observed for G4 and G5 (Figure S16 and S17 in the ESI respectively). The RGO edges were more strikingly observed for the G5 sample (as white lines in Figure S17 in the ESI) with the highest number of the GQDs (Figure 3). The elemental mapping images are also presented in Figure S18 to S22 in the ESI, and in all the prepared samples only elements C and O could be detected, thereby excluding the doping effect from heteroatoms and hence providing evidence that no other elements than C and O were included in the prepared samples.


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Raman spectra are effective to demonstrate the existence of the structural defects of graphene, such as oxygen functionalities and RGO edges.14, 56 Two Raman absorption bands could be observed at approximately 1355 and 1590 cm-1, which are corresponding to D and G bands, respectively (Figure 4(a) and 4(b)). The latter results from the firstorder scattering of the E2g mode observed for sp2-hybridized carbon domains. and is normally ascribed to ordered graphitic carbons (schematically illustrated in Figure 4(c) as the area highlighted in blue), while the D band is indexed to the defect-containing carbons such as structural defects, amorphous carbons, or edges that can break the symmetry and selection rules.57 The intensity ratio of D to G band (ID-to-IG ratio) provides evidence of the variation of the structural defects and electronic conjugation state of graphene,20 and it can also be used to estimate the in-plane crystallite size La (i.e., sp2 clusters within the sheets) according to the following Equation (3).58-59 𝐿𝑎 (nm) = (2.4 × 10 ―10) λ4𝑙𝑎𝑠𝑒𝑟

𝐼𝐷 ―1

() 𝐼𝐺

(3)

where λlaser is the laser line wavelength. According to this equation, the average sizes of sp2 clusters within different samples were calculated and compared (Table S2 in the ESI). The average size of the sp2 clusters within GO sheets was calculated as 14.8 nm, while the GC-based reduction processing at room temperature (RT) slightly lowered the average size of the sp2 clusters. The decrease in the average size of the sp2 domains within the G1 sheets can be owing to the newly formed smaller sp2 clusters. This is also an indication that the mild reduction at RT based on GC can also mildly change the in-plane structure of GO sheets through the formation of a number of smaller sp2 clusters. The chemical reduction at the elevated temperature (i.e., 60 oC) based on GC led to the higher extent of


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reducing the average size of the sp2 clusters in the G2 sheets, revealing that more smaller sp2 clusters are formed. PL properties of the typical samples were also tested for the further demonstration of this finding, and it was noted from Figure S23 (in the ESI) that rather broad and weak PL emissions could be detected for the prepared RGO samples with a high degree of reduction, including the G2 and G5 samples. This can be well explained by the following description. The reduction of the GO sheets to a high extent based on temperature-assisted GC process led to the generation of the electrical transportation pathway as interconnected by the sp2 clusters in large quantities that were newly formed after the reduction process, and the fraction of localized/isolated sp2 sites significantly reduced due to the interconnectivity.60 The percolation among these sp2 configurations facilitated hopping of excitons to nonradiative recombination centers, consequently quenching the PL. Even for the GO sample prepared by a modified Hummers’ method, the negligible PL emission could be detected as well, which can be attributed to abundant surface states as a result of the surface functional groups such as O=C-OH and COH. These functional groups caused the vibration relaxation and non-radiative recombination in GO under excitation conditions, yielding negligible PL emission (Figure S23 of the ESI), in consistence with ref.61 G2 exhibited a much larger ID-to-IG ratio as compared to G1, while G1 showed only a slight increase in the ID-to-IG ratio relative to GO (Figure 4(b)), indicating the higher extent of the reduction of G2 sheets and the generation of a larger number of sp2 domains with a small average size,48 as also schematically presented in Figure 4(c). This again demonstrates the more effective reaction between GO and VC at 60 oC when compared to that at RT, yielding more small sp2 domains embedded within the sp3 matrix. Surrounded by defect sites bearing sp3 carbons and oxygen groups, the


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numerous small sp2 domains were subsequently cut out of the micrometer-sized graphene sheets through the TSI process. The gradual TSI processing stepwise broadened the D band, along with an increase in the ID-to-IG ratio, which resulted from the gradual enrichment of the RGO edges as generated by cutting the numerous small sp2 domains out of the graphene sheets along the defect sites in a stepwise manner during the TSI process (Figure 4(c)).

(c)

Figure 4. (a) Raman spectra of the various prepared graphene-based adsorbents including GO, G1, G2, G4, and G5. (b) Comparison plot of the ID-to-IG ratios for the


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different samples. (c) Schematic illustration of the mechanism underlying the fragmentation of the graphene sheets and the formation of GQDs, that is the formation of numerous small sp2 carbon domains embedded within an sp3 matrix and the subsequent cutting of these domains out. Three main steps were involved in the “topdown” synthesis of the GQDs including (i) the oxidation of graphite to yield GO based on the Hummer’s method, (ii) the chemical conversion of GO to RGO based on GC, and (iii) the cutting of the sp2 domains out of the RGO sheets along the defect sites (e.g., oxygen groups and sp3 carbons) and leading to the generation of GQDs based on TSI.

The combined GC and TSI process was demonstrated to be effective in the controllable fabrication of GQDs. We then put efforts on the clarification of whether the as-formed GQDs exhibited superior adsorption performance toward the MB dye as compared to the 2D graphene counterpart and whether the larger is the number of GQDs, the higher is the adsorption efficiency. Figure 5 presents the main results of the adsorption tests on the prepared samples. The standard calibration plot of the MB concentration (C) as a function of the UV/vis absorbance (A) at 664 nm was provided in Figure S24, exhibiting a good linearity. We then compared the adsorption efficiency of the prepared samples. We first measured the stability of the neat dye solution. It showed a high stability due to its recalcitrant nature, as evidenced by the almost overlapped UV/vis absorption lines that were measured for the starting MB solution without any adsorbents before and after allowed to stand for 40 min (see the leftmost UV/vis spectra and the corresponding photoimage shown in Figure S25 in the ESI).


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Due to its similar structure to that of hydrophilic GO, G1 showed a good dispersion in water (see the photoimage of an aqueous dispersion with G1 sheets in Figure S25 in the ESI), leading to a high baseline of the UV/vis absorption, as caused by the absorption from the well-dispersed G1 sheets themselves (Figure 5(a)). The 40 min of adsorption using the G1 sample as the adsorbent cannot significantly change the UV/vis absorption line, indicating a low efficiency in the adsorption of the MB dye onto the G1 sheets. Under both TEM and SEM, G1 sheets exhibited a continuous film-like structure, and thus the edge effect of the RGO would be very weak. The RGO edges possessed dangling bonds with non-bonding electrons, which were believed to facilitate the interactions with the electron-deficient MB dye molecules, thereby suggesting that the RGO edges could play a significant role in the present adsorption interactions. The dangling bond is defined as an unsaturated chemical bond, as generated by the termination of the crystal lattice at the crystal surface. In this respect, the termination of graphene sheets occurred at the edge. The dangling bonds bearing non-bonding electrons are reactive toward the electrondeficient cationic organic species through donor-acceptor charge transfer interactions because the non-bonding electrons are relatively weakly attracted by the atomic nucleus. In addition, like the structure of GO, abundant oxygen functionalities existed in the G1 sheets, and a little restoration in the in-plane conjugation structure had also been demonstrated in the G1 sheets. It has been widely reported that the sp2 domains have strong interactions with the aromatic molecules including the MB dye through π-π stacking interactions.62 The regeneration of the sp2 domains was thus expected to enhance the adsorption interactions with the MB dye. Indeed, the G2 sample, with a higher extent of reduction


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imparted via the temperature-assisted GC process, possessed the higher performance in the adsorption of the MB dye as compared to its counterpart G1 prepared by GC at RT. If oxygen groups are present on adsorbents, electrostatic and hydrogen bonding interactions are usually considered as a mechanism for the effective adsorption of environmental pollutants, but π-π stacking interactions have been demonstrated as a more important driving force for the adsorption of aromatic pollutants as far as graphene-based adsorbents are concerned.62 More importantly, we present here a new finding that the RGO edges with dangling bonds could also play a critical role in the adsorption interactions with the organic dyes bearing electron-deficient moieties. In Figure 5, the G3 sample exhibited a debased adsorption performance toward the MB dye as compared to G2. Considering the similar structural features of the G2 and G3 sheets, the lower adsorption performance of G3 relative to G2 can be ascribed to the aggregation of the G3 sheets, resulting in a lower BET specific surface area (97.6 m2 g-1) as compared to G2 (137.1 m2 g-1), as shown in Figure S26 and Table S3 in the ESI. It is worth pointing out that G3 was prepared using the G2 sample as the precursor through a simple process of mixing (in water) and subsequent filtration and drying without any sonication treatment, causing the G3 sheets to readily aggregate at the water/air interface due to water evaporation.63-64 For the G4 sample involving TSI for 20 min, its adsorption performance was obviously improved when compared to G2. It had been demonstrated that the size of G2 sheets was largely reduced from the micrometer to several hundred nanometers after subjected to the 20 min of TSI for the formation of the G4 sample. The improved adsorption performance is most likely due to the


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fragmentation of the graphene sheets and the enrichment of the edges, but the important parameter (i.e., specific surface area) should also be considered. The specific surface area of the G4 sample was measured to be 104.7 m2 g-1, even smaller than that of G2. This lowered specific surface area of G4 can be attributed to the cutting of G2 sheets along the defect sites (e.g., oxygen functionalities and sp3 carbons) and hence the removal of the structural defects on the G2 sheets. As a result, the aggregation of G4 sheets could more readily occur through π-π stacking interactions. Nevertheless, such aggregation of G4 sheets did not debase the adsorption interactions with MB as compared to G2, thereby revealing that the size or edge effect substantially promoted the adsorption interactions between the G4 sheets and the MB dye. Considering the G5 sample prepared by the longest TSI duration of treatment, it presented the best adsorption performance among all the prepared samples. G5 possessed a specific surface area (121.9 m2 g-1) larger than G4, but slightly lower than G2 as a precursor for the preparation of G4 and G5. The much longer TSI duration better prevented the aggregation of the G5 sheets even though fewer oxygen groups and better in-plane structural restoration existed in the G5 sheets resulting in stronger π-π stacking interactions as compared to that for the G4 sheets. All of these demonstrated that the RGO edges exerted a significant impact on the adsorption interactions with the MB dye. The RGO edges bearing abundant dangling bonds with non-bonding electrons could strongly bind with the cationic MB dye with electron-deficient functionalities through donor-acceptor charge transfer interactions. Therefore, this study provides evidence that the charge transfer interactions also need to be considered if to conduct an investigation on the interactions


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between edge-enriched graphene materials and organic dyes with electron-deficient moieties, even though most of the researches were limited to explain the adsorption results on the basis of the commonly-recognized electrostatic, hydrogen bonding, and π-π stacking interactions.2, 36, 40

Figure 5. UV/vis spectra recorded to monitor the adsorption process of the MB dye onto the prepared G1 (a), G2 (b), G3 (c), G4 (d) and G5 (e) samples (the time interval between the adjacent adsorption lines progressing from top to bottom was set as 5 min). (f) Comparison of the adsorption efficiency of the various prepared samples, including


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G1 to G5 for the removal of the MB dye. (g) Plot of qe as a function of Ce, together with the curves as obtained via fitting the experimental data with Freundlich and Langmuir isotherms. (h, i) Linearity fitting of the experimental data according to the Langmuir (h) and Freundlich (i) equations.

The adsorption kinetics and isotherms were also investigated for the optimized sample (i.e., G5). The adsorption kinetics were analyzed on the basis of the pseudo-firstorder and pseudo-second-order kinetic models which can be expressed as the following Equations (3) and (4), respectively.

ln(qe  qt )  ln(qe)  k 1t

(3)

t 1 t = + 2 qt k 2 qe qe

(4)

where k1 is the first order adsorption rate constant, k2 is the second order adsorption rate constant, t is the adsorption time (min), and qe and qt are the amounts of the MB dye adsorbed at equilibrium and at time t (min) respectively. The Langmuir isotherm assumes that the adsorption takes place on homogeneous surfaces in a monolayer manner, and no interactions are present between adsorbates on the plane of the surfaces, while the Freundlich equation represents empirical adsorption related to the adsorption on heterogeneous surfaces, e.g., pollutant enrichment at the edges. The equation of the Langmuir isotherm and Freundlich isotherm can be written as the following equations (5) and (6), respectively.


27


Ce 1 Ce  KL  qe q 0 q0

(5)

ln qe = ln K F + 1 / n ln Ce

(6)

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where Ce is the equilibrium concentration of the MB dye (mg L-1), qe is the equilibrium adsorption amount (mg g-1), q0 is the saturated adsorption amount (mg g-1), KL is the Langmuir adsorption constant which can indicate the adsorption strength. KF is the Freundlich adsorption constant associated with adsorption capacity, and n is an empirical parameter related to the adsorption intensity. Figure S27 and Table S4 in the ESI present the results of fitting the experimental data to the pseudo-first-order and pseudo-second-order kinetic models. It was found that the adsorption of the MB dye onto the G5 sample better followed the pseudo-second-order kinetic model relative to the pseudo-first-order kinetic model, as judged by the higher correlation coefficient value, R2 (0.996), implying a chemisorption process.65 On the other hand, the values of the correlation coefficient (R2) obtained from the fittings with Langmuir and Freundlich models were calculated as 0.739 and 0.955, respectively, confirming that the Freundlich isotherm was much more suitable to fit the experimental data than the Langmuir isotherm (Figure 5(g-i), and Table S5 in the ESI). These findings thus effectively demonstrate the heterogeneous enrichment of the MB dye around the edges, rather than the homogenous adsorption over the entire surface of the G5 sheets. According to the following Equation 7, the Freundlich maximum adsorption capacity (qmax) can be calculated.


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KF =

qmax C0 1 / n

(7)

where C0 is the initial concentration of the MB dye in the bulk solution (mg L-1), and qmax is the Freundlich maximum adsorption capacity (mg g-1).

The maximum adsorption capacity of G5 (qmax) was thus estimated to be 827.5 mg g-1, which is higher than most of the reported graphene-based adsorbents listed in Table 1. The extraordinary adsorption capacity can be due to the strong adsorption interactions by virtue of the edges of GQDs, and the donor-acceptor charge transfer interactions between the active RGO edges and the cationic dye species can outstrip the other kinds of interactions such as hydrogen bonding, electrostatic attraction, and even π-π stacking interactions. The regeneration tests were also conducted through five runs of repeated usage of the G5 sample, and it exhibited impressive recycling/reuse properties without a significant loss of the adsorption capacity over these five runs (Figure S28 in the ESI).

Table 1. Comparison of the adsorption kinetics, isotherms, and maximum adsorption capacity of the present adsorbent G5 with other reported graphene-based adsorbents for binding with the MB dye. Adsorbent

Maximum

Kinetic model

adsorption capacity

Isotherm model

Reference

(mg g-1)


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Reduced GO

153.85

Pseudo-secondorder

Langmui r

31

Exfoliated GO

17.3

Pseudo-secondorder

Langmui r

36

GO

243.9

Pseudo-secondorder

Langmui r

66

GO

714

NA

Freundlic h

67

GO–Fe3O4 composite

hybrid 167.2

NA

NA

68

GO–Fe3O4

190.1

NA

Freundlic h

69

Polyimide derived 926 laser-induced graphene

Pseudo-secondorder

Freundlic h

70

Agar/GO composite aerogel

578

Pseudo-secondorder

Langmui r

35

Konjac glucomannan/GO hydrogel

92.3

Pseudo-secondorder

Freundlic h

71

Montmorillonitepillared GO

250

Pseudo-secondorder

Langmui r

72

Polyethersulfone Enwrapped GO

62.5

pseudo-secondorder

Langmui r

73

Graphene-CNT hybrid

81.97

Pseudo-secondorder

Freundlic h

34

Magnetic graphene- 65.79 CNT composite

pseudo-secondorder

Langmui r

74

GO/calcium alginate composite

181.81

pseudo-secondorder

Langmui r

33

G5

827.5

Pseudo-secondorder

Freundlic h

This work


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Note：NA represents that the information is unavailable from the literature.

In addition to the MB dye, another target pollutant, i.e., RhB, was also tested using the optimized sample, G5, as the adsorbent to demonstrate the universality of the G5 sample in the highly efficient processing of the organic dye, and the results are presented in Figure S29 in the ESI. Although the efficiency of the G5 sample (bearing the largest number of RGO edges) in the adsorption of the RhB dye was a little lower than that of the MB dye, less than 1 h was needed to completely bind with the RhB dye (50 min for RhB vs. 40 min for MB). Such a slightly lower efficiency can be due to the size of RhB molecules that is a little larger than that of MB molecules (see Figure S30 in the ESI), causing a steric hindrance effect on the adsorption process. In addition, there existed an additional carboxyl group on the backbone of RhB molecules (Figure S30 in the ESI), which is probably unfavorable for the adsorption interactions with the dangling bonds on the edges of G5 sheets. All of these demonstrated that the prepared GQDs showed promising applications as a high-performance adsorbent for addressing the alarming environmental pollution issues.

4.

CONCLUSION This study has presented a novel, robust, and highly performing GQDs-based

adsorbent fabricated via a combined GC and TSI process. The RGO sheets that were produced by the temperature-assisted GC process can be broken into pieces by TSI, and


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their size can be well controlled by tuning the TSI duration. The Raman spectra reveal the generation of a larger number of sp2 domains with small sizes after the temperatureassisted GC process, which can be further cut out of the RGO sheets by TSI along the defect sites. As a consequence, the cutting of the RGO sheets into pieces results in the removal of part of the structural defects, accompanied by a further restoration of the inplane structure of graphene sheets. This indicates the generation of high-quality GQDs enriched with effective edges of the RGO. No obvious PL emission can be observed for the water dispersion of these GQDs due to their high extent of reduction and hence a lack of localized/isolated sp2 sites. Nevertheless, the RGO sheets with a nanoscale lateral size are demonstrated to have strong interactions with the MB dye, and the smaller is the lateral size the stronger are the adsorption interactions; this thereby reveals that the RGO edges can exert a significant impact on the adsorption interactions with the cationic MB dye through the donor-acceptor charge transfer interactions between the non-bonding electrons at the edges and the electron-deficient cationic moieties of the organic dye. A completely new insight into the adsorption interactions between graphene materials and cationic dyes is thus provided. Significantly, the optimized sample (i.e., G5) exhibits one of the highest adsorption capacity toward the MB dye (827.5 mg g-1) reported in the literature based on the Freundlich isotherm fitting well to the experimental data, which again implies the heterogeneous enrichment of organic pollutant onto the edges rather than the homogeneous adsorption over the entire surface of the GQDs. In addition, excellent recycling and reuse properties of the


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G5 sample are demonstrated, along with its universality in efficiently binding with the RhB dye.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publication website at DOI: Detailed formulation used for the synthesis of various graphene adsorbents ranging from G1 to G5, schematic illustration of the main content of this study, comparison plot of the d002 spacing based on the XRD results and Bragg’s law for the typical samples, XPS survey spectra of the prepared samples, comparison plot of the O/C ratio among various prepared samples on the basis of the XPS analysis, high-resolution C1s XPS corelevel spectra of the GO and G1 samples, FTIR spectra of the prepared samples, EDS spectra for the estimation of the weight and atomic percentages of the oxygen in the prepared samples, SEM and elemental mapping images of the prepared samples, summary of the IG/ID ratios and La values calculated for the typical samples on the basis of the Raman spectra, PL spectra of the typical samples including GO, G2 and G5, under excitation at a 360 nm wavelength, the standard calibration plot of the MB dye solution, photoimages of the test solutions with the MB dye after the adsorption process had been conducted for 40 min using various samples, N2 adsorption-desorption isotherms for the typical samples together with a table summarizing their BET specific surface area, a kinetic


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study on the adsorption of the MB dye onto the G5 sample, recycling and reuse test results for the G5 sample,

UV/vis spectra and photoimages used to monitor the

adsorption of the RhB dye using the G5 sample as the adsorbent, and chemical formulas of the MB and RhB dye molecules.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.-W. H.). *E-mail: [email protected] (M. Z.). *Email: [email protected] (D.-C. C.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We greatly appreciate the National Natural Science Foundation of China (51702050), the Featured Innovation Project of the Department of Education of Guangdong Province (2017KTSCX188), and the Open Research Foundation of Guangdong


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Provincial Key Laboratory of New and Renewable Energy Research and Development (Y807s31001).

ABBREVIATIONS GQDs, graphene quantum dots; GO, graphene oxide; RGO, reduced graphene oxide; GC, green chemistry; TSI, temperature-assisted sonication irradiation; VC, vitamin C; MB, methylene blue.

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