Efficient visible-light-driven water remediation by 3D graphene aerogel-supported nitrogen-doped carbon quantum dots

Accepted Manuscript Title: Efficient visible-light-driven water remediation by 3D graphene aerogel-supported nitrogen-doped carbon quantum dots Authors: Shao-Hai Li, Ru Wang, Zi-Rong Tang, Yi-Jun Xu PII: DOI: Reference:

S0920-5861(18)30892-7 https://doi.org/10.1016/j.cattod.2018.10.060 CATTOD 11723

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

3 September 2018 8 October 2018 24 October 2018

Please cite this article as: Li S-Hai, Wang R, Tang Z-Rong, Xu Y-Jun, Efficient visible-light-driven water remediation by 3D graphene aerogelsupported nitrogen-doped carbon quantum dots, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.10.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Efficient visible-light-driven water remediation by 3D graphene aerogel-supported nitrogen-doped carbon quantum dots

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College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350116, P. R. China

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Shao-Hai Lia,b, Ru Wanga, Zi-Rong Tang*a and Yi-Jun Xu*a,b

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350116, P. R. China

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*To whom correspondence should be addressed.

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E-mail: [email protected]; [email protected]

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Graphical abstract

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A facile and efficient hydrothermal method is utilized to assemble nitrogen-doped CQDs (NCQDs) onto graphene aerogel (GA) for reduction of toxic Cr (VI) in aqueous solution under visible light illumination. This work could

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extend the promising applications of CQDs as an efficient visible-light-responsive photosensitizer for water

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purification and beyond.

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Highlights 3D nanostructured all-carbon composite has been fabricated (53 characters)

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Nitrogen-doped CQDs (NCQDs) exhibits enhenced visible light adsorption compared to undoped CQDs

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(85 characters)

Graphene aerogel (GA) acts as a scaffold for the immobilisation of hydrophilic NCQDs (73 characters)

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NCQDs/GA shows synergetic effect for the reduction of Cr (VI) under visible light irradiation (81 characters)

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Abstract Carbon quantum dots (CQDs) is a promising material for photosensitization applications but substantially suffers from the tedious product separation in solution and poor photosensitive efficiency in the visible light range. Herein, a facile and efficient hydrothermal method is utilized to assemble nitrogen-doped CQDs (NCQDs) onto graphene aerogel (GA) for reduction of toxic Cr (VI)

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in aqueous solution under visible light illumination. The photoactivity results suggest that GA skeleton endows NCQDs with a strongly enhanced photoactivity compared to that over blank NCQDs. Photoelectrochemical results demonstrate that the introduction of GA skeleton facilitates

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more efficient separation and transfer of photogenerated charge carriers under visible light irradiation. Moreover, as compared with N-free counterparts, the broadened light absorption of NCQD resulting from the ameliorated nanocrystallinity and the nitrogen dopant in graphitic structure play an

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important role in the reduction of Cr (VI) reaction under identical conditions. It is believed that this

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work could extend the promising applications of CQDs as an efficient visible-light-responsive

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photosensitizer for water purification and beyond.

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Keywords: visible light; water purification; nitrogen-doped; CQDs; graphene aerogel 1. Introduction

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Quantum dots (QD) photocatalysts have attracted widespread attention for various environmental and energy applications because of their high surface areas and short charge-transfer pathways [1-6].

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In particular, carbon quantum dots (CQDs) with low cost, earth abundant and a wide tunability of properties have recently emerged as a promising low-cost light absorber for photosensitization applications [7, 8]. However, the low photosensitization efficiency of CQDs are the one of current

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bottlenecks, which limits their applications in photocatalysis. Moreover, due to the high dispersion of water-soluble CQDs, it generally needs the tedious processes to separate and recycle the CQDs from liquid-phase reaction system. To circumvent these issues, significant efforts have been devoted to develop unique and highly efficient photocatalysts by integrating CQDs with conductive carbon supports [9], semiconductors [10-12], molecular catalysts [13], and so on. Though these hybrid heterogeneous photocatalysts improved the catalytic performance, the overall catalytic efficiency is 3

still unsatisfactory. Furthermore, most of these applied CQDs have a weak response in the visible light range, resulting in low photosensitization efficiencies owing to their intrinsic large bandgaps. Therefore, it is still of urgency to develop a highly efficient and recyclable visible-light-response CQDs-based composite photocatalyst for practical applications.

Recently, it has been suggested that doping carbon nanomaterials with heteroatoms can

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effectively induce charge delocalization and tune the work function of carbon [14, 15]. The dopants such as nitrogen (N) atom, having a comparable atomic size and five valence electrons for bonding

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with carbon atoms, has been widely used for chemical doping of carbon nanomaterials [16-18]. In particular, the introduction of N into CQDs, namely NCQDs, have been developed as a potential alternative to conventional organic dye photosensitizers for fabricating NCQDs-based composite

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towards visible light photocatalytic applications [19-22]. Three-dimensional (3D) graphene aerogel

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(GA) can be assembled from graphene oxide building blocks through a facile one-step hydrothermal

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co-assembly strategy. The formation of 3D spatial interconnected structure in the interior of GA makes it an ideal candidate as a support to graft other hydrophilic nanomaterials [23]. In addition, the

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macroscopic bulk appearance of 3D GA-based composites makes them easily and conveniently

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recyclable photocatalyst candidates [24]. Based on above considerations, in the work, we developed a cost-effective and highly efficient NCQDs photocatalyst for the reduction of Cr (VI) in aqueous solution under visible light irradiation with the assistance of 3D porous GA. The hydrophilic NCQDs

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were derived from the thermal condensation of biotic aspartic acid, which were efficiently immobilized onto the graphene network due to the π-π interaction through hydrothermal treatment.

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With the synergistic effects arising from NCQDs and porous GA, the obtained NCQDs/GA (NCGAs) composite catalyst exhibited superior catalytic activity and recyclability for visible light-driven

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reduction of Cr (VI) in aqueous solution.

2. Experimental section 2.1. Materials Aspartic acid (C4H7NO4), citric acid (C6H8O7), sodium hydroxide (NaOH), triethanolamine (C6H15O3N) and potassium dichromate (K2Cr2O7) were all obtained from Sinopharm Chemical 4

Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification. The deionized (DI) water used in the experiment was from local sources. 2.2. Preparation of nitrogen-doped carbon quantum dots (NCQDs) Nitrogen-doped carbon quantum dots (NCQDs) were prepared by a modified literature procedure. In detail, aspartic acid (5 g) was thermolyzed in furnace under air at 280 °C for 100 h. Then the

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brown high-viscosity liquid of carboxylic acid capped NCQDs was stirred with DI water and NaOH solution (5 M) to dissolve. More NaOH solution (5 M) was subsequently added to neutralize the acidic NCQDs to pH 7 resulting in an orange-brown solution of sodium carboxylate capped NCQDs.

2.3. Preparation of undoped carbon quantum dots (CQDs)

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Finally, the product was isolated as brown powder by freeze-drying.

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Undoped carbon quantum dots (CQDs) were prepared by a modified literature procedure. In detail, citric acid (40 g) was thermolyzed in furnace under air at 180 °C for 40 h. Then the brown

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high-viscosity liquid of carboxylic acid capped CQDs was stirred with DI water and NaOH solution

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(5 M) to dissolve. More NaOH solution (5 M) was subsequently added to neutralize the acidic CQDs

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to pH 7 resulting in an orange-brown solution of sodium carboxylate capped CQDs. Finally, the

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product was isolated as brown powder by freeze-drying. 2.4. Preparation of graphene oxide (GO)

Graphene oxide (GO) was prepared from natural graphite powder by a modified Hummers

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method. The details were presented in the Supporting Information.

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2.5. Preparation of nitrogen-doped carbon quantum dots/graphene aerogel (NCGA) composites The preparation of 3D nitrogen-doped carbon quantum dots/graphene aerogel (NCGA) composites was based on a facile one-step hydrothermal method. In a typical procedure, the NCQDs

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were first dissolved in DI water to produce aqueous solutions with a concentration of 3 mg mL–1. Then 0.5, 1, 1.5, or 2 mL of the prepared NCQDs aqueous solution were added into 6 mL of 5 mg mL–1 GO solution to obtain NCQDs/GO mixtures. DI water was added into the above mixtures to a volume of 12 mL. After being stirred for 1 h, the mixture was transferred into a 50 mL Teflon-lined autoclave and subjected to hydrothermal treatment at 180 °C for 12 h. The as-prepared NCQDs/graphene hydrogel (NCGH) was taken out with a tweezer, washed several times with 5

deionized water, and then treated by freeze-drying. The resulting samples with different loadings of NCQDs were labeled as NCGA5, NCGA10, NCGA15 and NCGA20, respectively. The blank graphene aerogel (GA) and undoped carbon quantum dots/graphene aerogel (CGA) were synthesized via the same procedure without the addition of NCQDs and with the addition of CQDs instead of NCQDs as references, respectively.

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2.6. Characterizations The morphology of the samples was determined by field emission scanning electron microscopy (FESEM) on a Hitachi New Generation cold field emission SEM SU-8000 spectrophotometer.

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Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected using a JEOL Model JEM 2010 EX microscope at an accelerating voltage of 200 kV. A Nanoscope IIIA system was used to measure the atomic force microscopy

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(AFM) spectra. Samples were dispersed on mica plate for the test. Nitrogen adsorption–desorption

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isotherm and the Brunauer–Emmett–Teller (BET) surface areas were collected at 77 K using

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Micromeritics ASAP2020 equipment. The crystal phase properties of the samples were analyzed

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with a powder X-ray diffractometer (Bruker D8 Advance) using Ni-filtered Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 5° to 80° with a scan rate of 0.02° per second. X-ray photoelectron

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spectroscopy (XPS) was performed on a Thermo Scientific ESCA Lab 250 spectrometer, which was made of a monochromatic Al Kα as the X-ray source, a hemispherical analyzer, and a sample stage

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with multiaxial adjustability to obtain the surface composition of the samples. All of the binding energies were calibrated by the C1s peak at 284.6 eV. Attenuated total reflection Fourier-transform

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infrared spectroscopy (ATR-FTIR) was carried out on a Nicolet Nexus iS50 FT-IR spectrometer. UV−vis diffuse reflectance spectra (DRS) were recorded on a Cary-500 UV−vis−NIR spectrometer in which BaSO4 powder was used as the internal standard to obtain the optical properties of the

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samples over a wavelength range of 200-800 nm. Zeta-potential in deionized water was determined by dynamic light scattering analysis instrument (Zetasizer 3000HSA) at room temperature. Photoluminescence (PL) spectra for NCQDs and CQDs solution were analyzed on Hitachi F4600 spectrophotometer. The photoelectrochemical measurements were performed in a three-electrode quartz cell with an electrochemical workstation (Autolab, PGSTAT204). A Pt plate was used as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. The working 6

electrode was prepared on fluorine-doped tin oxide (FTO) glass that was cleaned by ultrasonication in ethanol for 30 min and dried at 60 °C. Typically, 5 mg of the sample powder was ultrasonicated in 0.5 mL of N, N-dimethylformamide (DMF) to disperse it evenly to get slurry. The slurry was spread onto the FTO glass, whose side part was previously protected using Scotch tape. After air-drying, the working electrode was further dried at 100 °C for 2 h to improve adhesion. Then the Scotch tape was unstuck, and the uncoated part of the electrode was isolated with epoxy resin. The exposed area of

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the working electrode was 0.283 cm2. The photocurrent measurements were carried out in 0.2 M aqueous Na2SO4 solution (pH = 6.8) without additive. The cathodic polarization curves were

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obtained using the linear sweep voltammetry technique with a scan rate of 0.2 mV. Electrochemical impedance spectroscopy (EIS) measurement was carried out in the presence of 0.5 M KCl solution containing 0.01 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) by applying an AC voltage with 5 mV

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amplitude in a frequency range from 1 Hz to 85 kHz with the open circuit potential conditions.

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2.7. Photocatalytic measurements

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The photocatalytic reduction of Cr (VI) over the as-synthesized samples was performed in an

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aqueous solution under the visible light irradiation. A NCGA sample and 60 μL of triethanolamine (TEOA, sacrificial agent) were added into 40 mL of 10 mg L–1 aqueous solution of Cr (VI) in a

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quartz vial. The pH value of this system was about 8.5. A commercial 300W Xe lamp (PLS-SXE300/300UV, Beijing Perfectlight Co., Ltd.), emitting visible light (λ > 420 nm) with an

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energy output of 0.7 W cm–2 was placed at a distance of approximate 10 cm from the reactor. Before visible light illumination, the above suspension was kept in the dark for 2 h to establish adsorption–

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desorption equilibrium between the photocatalysts and the reactants. During the reaction process, 2 mL of sample solution was collected at a certain time interval and analyzed on a UV–vis spectrophotometer (UV-1750, Shimadzu Co.). The normalized temporal concentration changes (C/C 0,

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where C0 is the initial concentration after the establishment of adsorption desorption equilibrium before irradiation and C is the concentrations at a certain time during the photocatalytic reaction) of Cr (VI) during the photocatalytic reduction process were calculated from the change in the Cr (VI) absorbance at ca. 371 nm at a given time interval in light of their proportional relationship.

The recycling test of the synthesized aerogel catalyst was done as follows. Typically, when a 7

photocatalytic cycle was finished, the aerogel catalyst was separated from the reaction solution using a tweezer, rinsed, and then directly employed in the next photocatalytic cycle. Between the two cycles, it was unnecessary to perform tedious centrifugation, sonication, and drying processes. We regenerated the NCGA composites via a simple re-hydrothermal treatment of the used NCGA photocatalysts with fresh NCQDs.

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3. Results and discussion

In this study, N-doped CQDs (NCQDs) were synthesized by a one-step pyrolysis of aspartic acid in

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air at 280 °C (Scheme S1) [21]. Atomic force microscopy (AFM) image and corresponding height statistical distribution in Fig. 1a and b indicate that the average topographic height of the CQDs is mostly 2 nm. The synthesized NCQDs feature particle size with a mean diameter of 2.3 nm, as seen

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from transmission electron microscopy (TEM) image in Fig. S1 (Supporting Information). To

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demonstrate the merits of the NCQDs, the undoped-CQDs (CQDs) synthesized in a similar fashion

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through thermolysis of small molecule citric acid in air at 180 °C. The as-synthesized products show fairly uniform diameter about 2 nm (Fig. S2, Supporting Information) [13]. More importantly, the

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similar particle size excludes the effect of quantum size effect on the comparison of photoactivity

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over the corresponding composites. The powder X-ray diffraction (XRD) spectrum of NCQDs shows a broad peak centered at around 25° with an enhanced nanocrystalline graphitic structure than that of citric acid-based CQDs (Fig. S3a, Supporting Information), corresponding to the (002) reflection of

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hexagonal graphitic carbon [21]. The optical properties of the NCQDs and CQDs were investigated by UV−vis diffuse reflectance spectra (DRS). The NCQDs show enhanced absorption in the visible

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light region (Fig. S3, Supporting Information), which could be derived from the heteroatom nitrogen doping defects [25]. The synthesized NCQDs and CQDs both possess well dispersion with favorable negative charges in water, as were confirmed by Zeta potential measurement (Fig. S4, Supporting

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Information). Moreover, the NCQDs solution shows darker brown color than that of the CQDs solution with the identical concentration, proving the potential visible light-absorbing ability. The excitation dependent photoluminescence (PL) spectra of NCQDs and CQDs were further measured using excitation wavelengths progressively ranging from 360 to 500 nm with the interval of 20 nm, as shown in Fig. S5 (Supporting Information). The PL results suggest that NCQDs exhibit superior 8

visible light response than CQDs, which thus could be efficient photosensitizers in photoredox reactions. Furthermore, using GO as the precursor, Fig. 1c and the inset show the formation of 3D spatial interconnected structure in the interior after hydrothermal treatment and the monolith appearance of blank graphene aerogel (GA), implying the potential as a platform to graft the NCQDs to tackle with the issue of separation from reactant solution owing to the hydrophilicity [26].

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After one-step hydrothermal co-assembly, the macroscopic appearance of as-prepared NCQDs-GA (NCGA) cylinder is about 0.8 cm in height and 1.3 cm in diameter (inset of Fig. 1d),

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appearing larger than the blank GA. Meanwhile, the corresponding scanning electron microscopy (SEM) image shows that the microstructure of the NCGA15 composite possesses larger networks than the blank GA does. Simultaneously, it has been observed that the NCGA samples possess

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gradually enlarged macroscopic appearance and spatial network with the increase of NCQDs content

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(Fig. S5, Supporting Information). This could be attributed to the increased encapsulation of water

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by surface oxygenated groups of NCQDs during the hydrothermal process. The morphology and structure properties were further examined by TEM characterizations. Fig. 1e shows the typical TEM

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image of the NCGA15 composite, manifesting the 3D interconnected framework composed of

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graphene nanosheet as building blocks. As shown in Fig. 1f, the high-resolution TEM (HRTEM) image of the NCGA15 confirms the uniform loading of NCQDs onto the surface of graphene nanosheets in GA and indicates the formation of intimate interfacial contact between GA and

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NGQDs. Moreover, the size of NCQDs in NCGA15 possesses no apparent change after

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hydrothermal treatment compared to that of blank ones (Fig. S1, Supporting Information).

The crystal and chemical structures of as-prepared samples were analyzed by powder X-ray diffraction (XRD). As exhibited in Fig. 2a, GO features a peak at 9.4° corresponding to the (002)

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diffraction. The calculated interlayer space of GO (0.87 nm) is much larger than that of graphite (0.34 nm), due to the oxygen-containing functional groups attached on both sides of GO sheet and intercalated water molecules [27]. After the hydrothermal process, the peak almost disappears and a broad peak around 24.5° are observed in the GA and the NCGA composites, which indicates the effective reduction of GO. Moreover, a close view of the (002) diffraction peak from the NCGA 9

composites indicates a steady shift toward lower lattice space value upon increasing the amount of NCQDs (Fig. 2b), owing to the π-π interaction with the graphitized NCQDs [28]. In addition, the surface area and pore structure of the NCGA15 composite and blank GA were investigated. The nitrogen adsorption–desorption isotherms of NCGA15 and pure GA exhibit characteristic type IV property with a typical hysteresis loop of mesopores in Fig. S7 (Supporting Information). The decreases in the amount of N2 sorption and corresponding surface areas were observed for NCGA15

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(131.0 m²/g) compared to GA (289.3 m²/g). It is speculated that NCQDs occupied a portion of the interior space of mesopores in GA structure. Nevertheless, the enriched macropores and aggregated

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pores are beneficial for introducing photoenergy and reactants into the interior space of NCGA15 photocatalyst [29]. Furthermore, the chemical structures of the samples were investigated by Fourier-transform infrared spectroscopy (FTIR). Fig. 2c shows that the representative FTIR spectra

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peaks of the oxygen-containing functional groups of GO including the bands at 1037 cm−1 (C-O) and

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1725 cm−1 (C=O), which are significantly attenuated after hydrothermal treatment, indicating the

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reduction of these functional groups [30-32]. Unfortunately, the characteristic peaks (the symmetric

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discerned due to the low loading content.

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and antisymmetric stretches of the carboxylate group) of NCQDs in NCGA15 can not be clearly

X-ray photoelectron spectroscopy (XPS) was further employed to obtain elemental composition information of the samples. The survey XPS spectra show the surface of NCQDs and NCGA15 are

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mainly composed of C, O, and N (Fig. S8a, Supporting Information), further demonstrating the successful introduction of NCQDs in GA. As displayed in Fig. 2d, the C 1s spectra of NCQDs and

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NCGA15 can be divided into five peaks and the one at 284.6 eV is corresponding to the sp2-hydridized carbon (C=C/C-C) [25]. The peak at 285.4 eV is related to the bonds of C-N, originating from the introduction of nitrogen atoms. And the other three peaks at 286.1, 288.1 and

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291.2 eV are associated with C-O, C=O and shake up peaks [33]. It should be noted that blank GA and NCGA15 with the low content of NCQDs both show noticeable decreased peak intensity of oxygen containing group compared with GO (Fig. S9, Supporting Information), revealing the efficient reduction of GO, which is consistent with the results of FTIR spectra. The N 1s spectra in Fig. 2e can be deconvoluted into three peaks located at 398.4, 399.6, and 401.3 eV, which are 10

attributed to pyridinic-N, pyrrolic-N, and quaternary-N atoms, respectively [34, 35]. Compared with bare NCQDs, the slight shift to higher binding energy of N 1s in NCGA15 could be attributed to the decrease of the sodium carboxylate groups of NCQDs during the hydrothermal co-assembly processes (Fig. S8b, Supporting Information) and the bonding interactions due to the introduction of reduced graphene oxide sheets with delocalized π electrons. The above characterizations indicated

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the successful formation of NCGA composites.

The photocatalytic activity of the NCGA composites was investigated by the photoreduction of

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hexavalent chromium (Cr (VI)) aqueous solution under visible light irradiation, which represents a promising and significant treatment for the detoxification of industrial waste waters. As shown in Fig. 3a, as a benchmark, 34.9% of Cr (VI) can be removed over pristine NCQDs within 2 h of visible

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light irradiation (λ > 420 nm), manifesting that NCQDs can serve as the photosensitizer towards the

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reduction of Cr (VI). Moreover, after the immobilization onto the GA, 90.6% of Cr (VI) metal ions

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have been reduced over NCGA15, which is higher than the 40% reduction ratio achieved over CGA15 under identical conditions. By contrast, blank experiments performed without visible-light

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irradiation (Fig. S10, Supporting Information) reveal no observed reduction of Cr (VI), confirming that the reaction is typically a photocatalysis-driven process and exclude the physical adsorption

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effect. Owing to the presence of triethanolamine (TEOA) as sacrificial agent, CrO42− anion is the mainly existence form of Cr (VI) in alkaline solutions [36]. Therefore, the photosensitization

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reduction reaction can be expressed as follows: CrO42− + 8H+ + 3e− → Cr3+ + 4H2O. Although the photoactivity of NCGA15 decreases after the first recycle and keeps unchanged among the last two

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recycle times, the photoactivity is still comparable to that of CGA15 (Fig. 3b). Previous works have established that the reactive excited states of doped and undoped CQDs in the photocatalytic reactions may lead to the reluctant deactivation [9, 13, 21, 37]. However, the detailed deactivation

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mechanism is still not clear. Considering that good stability is as vital as activity for the application of photocatalytic materials, we tried to regenerate the NCGA composites via a very simple re-hydrothermal treatment of the used NCGA photocatalysts with fresh NCQDs. As displayed in Fig. 3b, four successive photoactivity tests have proven the photoactivity recovery of the NCGA15 composite by this simple efficient method. 11

To understand the origin of the enhanced photocatalytic performance over the NCGA composites, the photoelectrochemical characterizations of the samples were investigated. Fig. 3c shows transient photocurrent responses of the NCGA15, CGA15 and the blank NCQDs under intermittent visible light irradiation. The NCGA15 composite shows obviously enhanced photocurrent responses as compared to the blank NCQDs. This result suggests that 3D networks of GA can promote the

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separation of electron–hole pairs generated from the photoexcited NCQDs [38, 39]. Fig. 3d shows the polarization curves of NCGA15, CGA15 and the blank NCQDs under visible light irradiation,

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which not only further confirms the more efficient separation and transport of photoexcited electron-hole pairs over the NCGA15 than the pristine NCQDs, but also implies that nitrogen doping in NCQDs can facilitate the generation of photogenerated electrons [9, 21, 25]. To study the electron

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transfer process on the electrode and at the contact interface between electrode and electrolyte, the

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electrochemical impedance spectroscopy (EIS) of the samples was measured. As shown in Fig. 3e,

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the Nyquist plot of the NCGA15 electrode possesses a more depressed semicircle at high frequency, demonstrating a more efficient interfacial electron transfer over the NCGA15 in the reduction of Cr

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(VI) [40]. The above photochemical results together suggest that the combination of NCQDs and GA

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is able to synergistically promote the more efficient separation and transfer of carriers photogenerated from the excitation of NCQDs and provide more active sites for the enhanced

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performance of surface photoredox reaction under visible light irradiation (λ > 420 nm).

On the basis of the above discussions, a possible mechanism for photocatalytic reduction of Cr

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(VI) over the NCGA composites can be proposed as follows (Fig. 3f). Upon the visible light irradiation (λ > 420 nm), the electrons in the highest occupied molecular orbital (HOMO) of NCQDs are photoexcited to the lowest unoccupied molecular orbital (LUMO), leaving the holes in the

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HOMO captured by electron donors (TEOA). The photogenerated electrons subsequently are apt to transfer to the surface of 3D GA owing to the matched energy level [21, 38], thereby reducing poisonous Cr (VI) to nontoxic Cr (III). The GA with interconnected 3D network can provide multidimensional electron transport pathways to improve the separation efficiency of photoexcited electrons. In addition, the broadened absorption ability endows the NCQDs with enhanced photon 12

capture capabilities, thus contributing more photoexcited electrons. Therefore, such a synergetic action in the NCGA results in obviously higher photocatalytic activity than that of bare NCQDs and CGA composite under identical conditions.

4. Conclusion In summary, we have successfully fabricated metal-free 3D NCGA hybrids via a facile and efficient

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hydrothermal process, in which NCQDs are dispersedly immobilized onto the GA structure network due to the π-π interaction. The optimal synthesized NCGA composite significantly outperforms the

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blank NCQDs for visible-light-driven reduction of Cr (VI) in water. Additionally, as compared with N-free counterparts, NCQDs derived from the condensation reaction of aspartic acid show remarkably improved photosensitive efficiency under identical conditions. The results of

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spectroscopic characterizations coupling with photoelectrochemical studies reveal that the enhanced

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photocatalytic performance of NCGA composites are attributed to the synergetic effect between the

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extension of visible light absorption of NCQDs and high electronic conductive network of GA intimately contacted with NCQDs, markedly facilitating the generation, separation and transfer of

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photogenerated carriers. This study highlights new opportunities for the design of efficient metal-free

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Acknowledgements

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visible-light photocatalysts for various photoredox applications.

The support from the National Natural Science Foundation of China (NSFC) (U1463204, 21173045,

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21872029), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Rolling Grant (2017J07002), the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment

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(NO. 2014A05), and the 1st Program of Fujian Province for Top Creative Young Talents is gratefully acknowledged.

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Fig. 1. a, b) Atomic force microscopy (AFM) image and corresponding height statistical distribution

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of NCQDs. c, d) Scanning electron microscopy (SEM) images of blank GA and the NCGA15. Insets

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are the corresponding photographs. e, f) Transmission electron microscopy (TEM) and

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high-resolution TEM (HRTEM) images of the NCGA15 composite.

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Fig. 2. a) Powder X-ray diffraction (XRD) patterns of GO and the NCGA samples with different loading amount of NCQDs. b) The corresponding enlarged patterns of the square indicated in a). c)

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Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) spectra of power NCQDs, GO and NCGA15. d, e) X-ray photoelectron spectroscopy (XPS) of C1s and N1s of

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NCQDs and NCGA15, respectively.

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Fig. 3. a) Photocatalytic reduction of Cr (VI) aqueous solution over the as-prepared NCGA

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composites with different loading amount of NCQDs, blank NCQDs and CGA15 composite under visible light irradiation (λ > 420 nm). b) Regeneration photocatalytic tests of the NCGA15 for

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photocatalytic reduction of Cr (VI) aqueous solution under visible light irradiation (λ > 420 nm). c-e)

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Transient photocurrent responses, polarization curves and electrochemical impedance spectroscopy (EIS) curves of the NCGA15, CGA15 and blank NCQDs under visible light irradiation (λ > 420 nm). f) Schematic illustration of the proposed reaction mechanism for photocatalytic reduction of Cr (VI)

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over 3D NCGA composites under visible light irradiation (λ > 420 nm).

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