N doped carbon quantum dots modified defect-rich g-C3N4 for enhanced photocatalytic combined pollutions degradation and hydrogen evolution

Colloids and Surfaces A 591 (2020) 124552

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N doped carbon quantum dots modified defect-rich g-C3N4 for enhanced photocatalytic combined pollutions degradation and hydrogen evolution

T

Haiping Liua,b, Jing Liangc, Shuai Fua, Li Lia, Jiehu Cuib, Penghui Gaod, Fengying Zhaoa, Jianguo Zhoua,e,* a

School of Environment, Key Laboratory of Yellow River and Huai River Water Environment and Pollution Control (Ministry of Education), Henan Engineering Laboratory of Environmental Functional Materials and Pollution Control, Henan Normal University, Xinxiang, 453007, Henan, China b Key Laboratory of Environment Functional Materials, Zhengzhou University of Aeronautics, Zhengzhou, 450002, China c Henan Ecological and Environmental Monitoring Center, Zhengzhou, 450002, China d College of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, 453007, Henan, China e Key Laboratory of Green Chemical Media & Reactions (Ministry of Education), Xinxiang 453007, Henan, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: g-C3N4 N-doped carbon dot Sensitization Combined pollutions H2 generation

Exploiting metal-free cocatalysts is of huge interest for photocatalytic technology in environmental wastewater remediation and water splitting fields. The N doped carbon quantum dots (NCDs) modified defect-rich g-C3N4 (DCN) was prepared through impregnation method. The structural, optical and electronic properties of NCDs/ DCN were systematically investigated. The NCDs/DCN with outstanding visible light absorption, narrowed gap and electron transfer ability were conducive to combined pollutions photocatalytic removal and simultaneous H2 production and rhodamine B (RhB) degradation. The resulting NCDs/DCN performed much higher removal efficiency in different combined pollutions systems containing OFL/Cr (VI), BPA/Cr (VI) and CIP/Cr (VI) than in single system owing to the synergistic effect between organic pollution oxidation and Cr (VI) reduction. The freeradical quenching experiments, spectral technology and electron spin resonance (ESR) confirmed that Cr(VI) can be reduced by the conduction band (CB) electrons, and the h+ and O2%− radicals dominated OFL degradation. The optimized NCDs/DCN displayed excellent simultaneously photocatalytic H2 production activity and RhB degradation with the H2 evolution rate of 3.68 μmol‧h−1‧g−1 and 100% RhB removal efficiency. The

⁎ Corresponding author at: School of Environment, Key Laboratory of Yellow River and Huai River Water Environment and Pollution Control (Ministry of Education), Henan Engineering Laboratory of Environmental Functional Materials and Pollution Control, Henan Normal University, Xinxiang, 453007, Henan, China. E-mail address: [email protected] (J. Zhou).

https://doi.org/10.1016/j.colsurfa.2020.124552 Received 25 November 2019; Received in revised form 15 January 2020; Accepted 2 February 2020 Available online 03 February 2020 0927-7757/ © 2020 Published by Elsevier B.V.

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photocatalytic activity of simultaneous combined pollutions removal and synergistic H2 generation and RHB degradation provides a promising strategy for real wastewater remediation applications and energy utilization.

1. Introduction

of the visible light adsorption region [16,17]. A series of semiconductors photocatalyst such as NCDs/Bi4O5I2 [33], NCDs/BiOBr [34], NCDs/g-C3N4 [35], NCDs/Bi2O3 [36] and NCDs/TiO2 [37] have been reported for excellent photocatalytic activity. Based on the distinctive characteristics of small size, electronic capture and up-conversion of NCDs, the coupling of DCN and NCDs may efficiently reduce the recombination of photoexcited charge carriers and improve the charge transfer. Furthermore, due to synergistic effect between OFL (BPA,CIP) oxidation and Cr(VI) reduction, NCDs/DCN is envisioned to have high separation efficiency of charge carriers and improved photocatalytic activity towards combined pollutions (OFL/Cr(VI), BPA/Cr (VI) and CIP/Cr (VI)) systems, as well as simultaneous H2 generation and RHB degradation system. Here, NCDs/DCN heterojunction was synthesized by a facile impregnation method. Due to the interfacial effect, NCDs/DCN with more defects and faster photogenerated electron-hole separation exhibited better photocatalytic combined pollutions removal and simultaneous H2 evolution and RhB dagradation under visible light. XPS and FT-IR were employed to reveal the interfacial charge interaction between DCN and NCDs. The structure and phychemical properties were investigated by various characterization. The free-radical quenching experiments, spectral technology and ESR were performed to confirm the existence and contribution of active species. Furthermore, the mechanisms of synergic effect of organic pollutions oxidation and Cr(VI) reduction, as well as simultaneous H2 generation and RhB degradation were explored.

With the increasing populations and development, the growing societal demand for power and energy is one of the most important issues. Environmental pollutions such as ofloxacin (OFL), bisphenol A (BPA), ciprofloxacin (CIP), and hexavalent chromium (Cr(VI)) with high toxicity and accumulation definitely hamper the sustainable development of the world [1–6]. Moreover, the Cr(VI) and hazardous organics are often discharged at the same time, which is unfavorable for water treatment due to antagonistic effect [7]. Among various renewable energy, photocatalytic water splitting for H2 production has received widespread attention and is considered as an effective way to solve the global energy crisis utilizing solar energy due to its clean, renewable, carbon-free, and high energy density [8]. As for photocatalysts, the absorption efficiency of solar light, charge-transfer resistance, and recombination inhibition of photogenerated e−/h+ pairs are vital factors for their photocatalytic activity [9–12]. Therefore, it is crucial and essential to explore highly efficient multiple-effect photocatalysts for water environmental mediation and energy utilization. Recently, g-C3N4 has attracted tremendous attentions due to its relatively narrow band gap, proper band edges, cost effectiveness and excellent chemical stability [13–15]. However, a large amount of structural defects including unreacted amino and/or cyano groups exist in the pristine g-C3N4 due to incomplete polymerization, acting as recombination centers for electron-hole pairs during photocatalytic reactions, and thus significantly reduce the charge separation efficiency and decrease the photocatalytic activity. Defect engineering has been reported to exhibit a positive role in the improvement of photocatalytic activity for g-C3N4 [16–20]. The appropriate regulation of the surface defects can trap electrons or holes to inhibit the recombination of photogenerated charge carriers and promote the transfer of these trapped charge carriers [21]. More importantly, the surface defects can also serve as the active sites to adsorb more organic pollutants, and thus finally meliorate the photocatalytic performance and promote the photocatalytic efficiency. Various approaches have been applied to introduce defects (carbon/nitrogen vacancies) by controlling polymerization temperatures [22], adding additives, pre or post-treatment [23,24] and heating in Ar/H2 atmosphere [25,26]. The defective gC3N4 synthesized in hydrogen gas atmosphere displayed narrow band gap, increased visible light absorption and low rate of electron–hole recombination [21]. N-deficient g-C3N4 prepared via molten salt posttreatment approach, accelerated polycondensation and deamination degree of g-C3N4, exhibiting improved visible light harvesting capability and separation efficiency of charge carriers [24]. However, the photocatalytic efficiency of defective g-C3N4 is still need to be improved urgently. Hence, constructing defective g-C3N4-based composite for effective multiple-effect photocatalytic application and investigating the relation between improved photocatalytic properties and structure is of great significance. Carbon quantum dots (CQDs) has attracted significant attention in bioimaging, drug delivery, sensors, photocatalytic solar-energy conversion, semiconductor photocatalysis fields due to their intense photoluminescence, excellent photostability, and high biocompatibility [27,28]. CQDs can not only act as environmentally friendly photosensitizer due to upconversion photoluminescent properties, and as an electron acceptor or transporter to direct the flow of photogenerated charge carriers, but also serve as a semiconductor to generate electrons and holes, which lead to enhanced light absorption and photocatalytic activity of the composites [29–32]. Recently, NCDs have been intensively studied due to their improved electron transfer and extension

2. Experimental 2.1. Chemicals Urea (CH4N2O), citric acid, hydrochloric acid (HCl, 37 wt%), ciprofloxacin (CIP), bisphenol A (BPA), ofloxacin (OFL) and rhodamine B (RhB) were purchased from Aladdin Chemical Technology Co. Ltd., (Shanghai, China). All chemicals used in this work are analytical grade and used without any further purification. 2.2. Synthesis of NCDs NCDs were synthesized via a hydrothermal method [24]. In a typical procedure, 3.0 g of citric acid and 1.0 g urea were dissolved in 15 mL of DI water. The mixture was then transferred to a Teflon-sealed autoclave and heated at 180 °C for 5 h. After natural cooling, the obtained brown solution was centrifuged at 10,000 rpm for 30 min to remove large particles and the suspension was dialyzed by a dialysis bag (cutoff molecular weight: 10000 Da) for 72 h. The obtained NCDs solution was rotated evaporation to obtain the NCDs powder. 2.3. Preparation of NCDs/DCN nanocomposites The porous defective g-C3N4 nanosheets (DCN) were synthesized by our previously reported methods [23]. The NCDs/DCN composites were synthesized by a solvent evaporation approach. The as-synthesized gC3N4 powder (0.2 g) and a certain amount of NCDs were dispersed in 60 mL anhydrous ethanol with ultrasonic shaking for 1 h, followed by vigorous stirring for 10 h at 80 °C until the complete solvent evaporation. The dried samples were ground and further heated under N2 flow at 300 °C for 1 h to strengthen the interaction between the DCN and NCDs. The weight ratios of NCDs to DCN were varied from 0 to 2.0 % and noted as x% NCDs/DCN, where x = 0, 0.5, 1, 1.5 and 2 2

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degradation were carried out in a closed LabSolar-IIIAG photocatalytic online analysis system with 0.05 g of photocatalyst suspended in a 50 mL solution containing 0.5 mg RhB in a Pyrex glass reaction cell (Beijing Perfect Light Technology Co., Ltd, China). The reaction cell was connected to a gas-closed system with a gas-circulated pump. Visible light (λ > 420 nm) was generated by a 300 W Xe lamp (PLS-SXE300, Beijing Perfect Light Technology Co., Ltd, China) combined with a UVcutoff filter. The evolved H2 after each one hour was analyzed using an online gas chromatograph (GC9790II, Zhejiang Fuli Analytical Instrument Co., Ltd China) equipped with a thermal conductivity detector.

representing different weight ratios of NCDs at 0 wt%, 0.5 wt%, 1.0 wt %, 1.5 wt% and 2.0 wt%, respectively. 2.4. Photocatalytic activity evaluation 2.4.1. Photocatalytic combined pollutions degradation The photocatalytic activities of all the prepared samples were evaluated by the degradation of combined pollutions (OFL/Cr(VI), BPA/Cr(VI) and CIP/Cr(VI)) under visible light (500 W xenon lamp with a UV-cutoff filter, λ > 420 nm), which was conducted in an XPA7 photochemical reactor (Xujiang Machine Factory, Nanjing, China). Typically, 40 mL aqueous solution containing 10 ppm OFL and 10 ppm Cr(VI) (pH 5.5) was being constantly stirred for 30 min in dark to reach adsorption/desorption equilibrium before illumination. During photoreaction, 3 mL of the suspension was withdrawn from the reactor at a scheduled interval and then filtered after centrifugation. The concentrations of Cr(VI) in the supernatant were measured by diphenylcarbazide (DPC) method with UV–vis spectrophotometer (T6, Beijing optical spectrometer company, China) [38]. The detailed measurement of Cr(VI) was shown in Supporting Information. The photocatalytic BPA/Cr(VI) and CIP/Cr(VI) degradation system were performed as the same treatment. All experiments were repeated three times. 2.4.2. Photocatalytic H2 evolution and RhB degradation The simultaneously photocatalytic H2 evolution

and

2.5. Sample characterizations The X-ray diffraction (XRD) patterns of samples were recorded via an X'Pert 3 Powder diffractometer system equipped with a Cu Kα radiation (λ = 0.15406 nm). Fourier transform infrared spectra (FTIR) were recorded on a NEXUS spectrometer with a KBr pellet ranging from 500 to 4000 cm−1. The morphology of samples were investigated by transmission electron microscopy (HRTEM, JEM-2100). XPS spectra were collected on an ESCALA 260Xi system using a monochromatic Al Kα X-ray source. Light absorption (reflectance) spectra were recorded by UV–vis–NIR diffuse reflectance spectrum (UV–vis DRS, Shimadzu Corporation, UV-3900, Japan). The specific surface areas and pore volume of the samples were evaluated by N2 adsorption-desorption on a BELSORP-miniII (Japan). The photoluminescence (PL) spectra and TR-

RhB

Fig. 1. (a) XRD patterns, (b) FTIR spectra, (c) room-temperature EPR spectra and (d) N2 sorption isotherms of the samples. 3

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PL decay cures of samples were measured by Hitachi F-7000 fluorescence and FLS980 time-domain fluorescence spectrophotometer. The detailed photoelectrochemical measurement, Cr(VI) determination method, radical scavenger experiment and electron spin resonance (ESR) measurements were in supplementary materials.

integral structure of DCN as the host. The EPR measurements (Fig. 1c) show that the NCDs/DCN samples exhibit an excellent electron delocalization property. The DCN shows one single Lorentzian line with a g value of 2.0067, which is induced by the unpaired electrons on the carbon atoms of the heptazine rings within p-bonded nanosized clusters [41]. The spin intensity progressively enhances with the increased loading amounts of NCDs, indicating the promoted delocalization of the sole electrons, which is contributed to the improved separation and migration efficiency of photogenerated charge carriers for NCDs/DCN samples. In detail, the remarkably facilitated charge transfer from DCN to NCDs at interface can create more unpaired spin density in heptazine units. The specific surface area is an important factor impacting the photocatalytic activity of photocatalysts. N2 sorption isotherms are measured for DCN and 1.0 % NCDs/DCN and shown in Fig.1d. Both samples display type IV physisorption isotherms with a H3 hysteresis loop, signifying capillary condensation occurring in mesopores and silt-shaped pores resulting from plate-like particles [42]. The specific surface area of 1.0 % NCDs/DCN (51.77 m2/g) is smaller than that of DCN (85.58 m2/g), corresponding to the previous reports [37,43,44], which is probably attributed to that the loading of NCDs occupied part of spaces of DCN [37]. The XPS survey spectra in Fig. S1 show the existence of carbon (C1 s), nitrogen (N1 s), and oxygen (O1 s) on the surface of the 1.0 % NCDs/DCN. Both the C1 s and N1 s XPS spectra confirm the basic heterocycle structure of g-C3N4. Five peaks located at 293.7, 289.3, 288.3, 285.9 and 284.8 eV in the high-resolution XPS spectra of C1 s (Fig. 2a), can be ascribed to π excitation, sp2-hybridized C atom attached to N in the aromatic ring (N = C-(N)2), CeNH species on the edge of aromatic ring, C]N or CeO, CeC bond with sp2 orbital or adventitious carbon, respectively [45]. Obviously, compared with DCN, the enhanced OeC]O might be caused by the carboxyl groups (COOH) on surface of NCDs, and the enhanced peak area ratio of OeC]O/Ctotal indicates the assembly of NCDs/DCN by π-π stacking interaction between NCDs and DCN [30]. The high-resolution N 1s spectra can be deconvoluted into four peaks at approximately 404.5, 401.3, 400.1 and 398.7 eV (Fig. 2b), which are assigned to π excitation, amino functional groups (CeNeH), bridging N atoms in N-(C)3, and sp2-hybridized N atom attached to C in the aromatic ring (N-(C)2), respectively [35]. Notably, the shift of C and N towards low binding energy further confirms the interfacial interactions between NCDs and DCN. The morphology and microstructure of the prepared NCDs and 1.0 % NCDs/DCN are observed by TEM. As shown in Fig. 3a, the NCDs with

3. Results and discussion 3.1. Characterization The crystal structures of the as-prepared samples are demonstrated by their XRD patterns. As shown in Fig. 1a, two distinct diffraction peaks at around 12.8° and 27.6° are found for all the samples, which can be ascribed to the (100) reflection presenting in-plane structural packing and (002) inter-planar graphitic stacking typical diffraction peaks of g-C3N4. Notably, compared with DCN, the (002) diffraction peak of NCDs/DCN samples progressively shift towards the low angle, and the (002) diffraction peak of 1 % NCDs/DCN shifts from 27.35° toward 27.07°, based on Bragg's Law (2dsinθ=nλ), the estimated corresponding lattice distance increases from 0.167 to 0.169 nm. It is probably that the intense interaction between g-C3N4 and NCDs results in lattice distance augment. However, no observed characteristic peaks of graphite carbon is probably due to the low dosage of NCDs in composite. The FT-IR spectra of all samples in Fig. 1b exhibit several typical absorption bands corresponding to the characteristic structure of gC3N4. The absorption bands located at 1200−1600 cm−1 is related to the stretching modes of aromatic CeN heterocycles. The peak at ca. 807 cm−1 represents the breathing mode of the triazine units. While the broad band at 3000−3500 cm−1 belongs to the NeH vibration due to the surface uncondensed amine groups. Notably, the remarkably reduced peak intensity at 1200−1600 cm-1 of NCDs/DCN samples as compared to DCN were probably ascribed to the less ordered packed hydrogen-bond cohered long strands of polymeric melon units in the layers of DCN nanosheets after coupling of NCDs [39]. The less ordered effect is probably attributed to the expanded layer spacing, which results from the interaction between DCN and NCDs. In addition, the decreased amounts of deficiency of nitrogen due to the decreased DCN may be the other factor, which was similar as previous report [40]. The intensity of the peak around 2170 cm−1 increases, corresponding to an asymmetric stretching vibration of cyano groups. Whereas, the characteristic peaks of NCDs are hardly found in all the NCDs/DCN photocatalysts, signifying the low doping levels of NCDs and the unchanged

Fig. 2. The high-resolution XPS spectra of (a) C 1s, (b) N 1s in DCN and 1 % NCDs/DCN. 4

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Fig. 3. (a) TEM and (b) HRTEM image of NCDs, TEM image of (c) DCN and (d) 1 % NCDs/DCN. The inset of b and d are the lattice spacings of DCN and 1 % NCDs/gC3N4.

NCDS/DCN are estimated to be 2.52 eV and 2.49 eV, respectively. Compared to DCN, the narrow band gap of 1 % NCDS/DCN is beneficial for the absorption of the sunlight [56]. To gain a better understanding of energy band structures of the DCN and 1 % NCDS/DCN, VB-XPS spectra are employed to determine the VB positions. In Fig. 4c and 4d, the VB of DCN and 1 % NCDS/DCN are calculated to be 1.86 and 2.26 eV, according to Ec = Ev − Eg, the CB positions were subsequently estimated to be −0.66 eV and -0.23 eV, respectively. Therefore, the schematic illustration of band structure alignments in Fig. S2 manifests that 1 % NCDS/DCN can satisfy the thermodynamic requirements for organic pollutions degradation, Cr(VI) reduction and water splitting for H2 evolution. The migration and separation efficiency of photogenerated electronhole pairs are investigated by PL and TR-PL spectra. In Fig. 5a, the similar broad emission band centered at 440–455 nm appears on all the samples under the excitation wavelength at 375 nm. Compared with the highest PL peak of g-C3N4, the emission intensity of NCDs/g-C3N4 samples substantially decreases, indicating that the recombination of photo-induced electrons-holes is efficiently suppressed after modification by appropriate amount of NCDs. Among NCDs/DCN composites, 1 % NCDs/DCN exhibits the lowest PL intensity, suggesting the least

diameter of 2–3 nm are uniformly distributed without visible aggregation. TEM image in Fig. 3c displays lamellar porous structure of DCN, which is induced by the released gases (NH3 and HCl) generated from the thermal polymerization of precursor [23]. The HRTEM images of NCDs reveal the lattice spacing of 0.22 nm (Fig. 3b), corresponding to the (100) in-plane lattice spacing of graphite [46]. As displays in Fig. 3d, NCDs are observed on the surface of DCN, and the corresponding lattice spacing of (100) demonstrate that the NCDs have been successfully deposited onto the surface of DCN to form NCDs/DCN composites. As shown in Fig. 4a, the characteristic UV–vis absorption peaks of NCDs solution at about 270 nm and 350 nm are attributed to the π-π* and n-π* transition of nanocarbon particles [46]. The optical properties of samples were investigated by UV–vis DRS. As shown in Fig. 4b, the DCN has an innate absorption limit at around 460 nm, while the optical absorption edges of NCDs/g-C3N4 samples show gradually red shift with the increasing amount of NCDs. The red shift is probably due to the photosensitizer role and up-converted PL property of NCDs, indicating the important sensitization role of NCDs in enlarging the visible light absorption range. The optical band gap (Eg) of the DCN and 1 % NCDS/ DCN are calculated by the Tauc equation. The Eg of DCN and 1 % 5

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Fig. 4. (a) UV–vis absorption spectra of NCDs, (b) UV–vis diffuse reflectance spectra, VB-XPS spectra of (c) DCN and (d) 1 % NCDS/DCN.

electron lifetimes 1 % NCDs/DCN (208.5 μs versus 135.3 μs of DCN) also demonstrates effective interfacial charge transfer and efficient separation of photo-generated charge carriers.

charge recombination. Simultaneously, the enlarged fluorescence lifetime of 1 % NCDs/DCN (12.31 ns versus 4.04 ns for DCN) exhibits the enhanced separation efficiency of photogenerated charge. The photoelectrochemical performance of NCDs/DCN samples and DCN are investigated by transient photocurrent response, EIS Nyquist and Bode patterns to study the generation of the photoinduced charge carriers and the interfacial transfer process. EIS Nyquist under dark and visible light (Fig. 5c and 5d) display that arc radius decrease firstly and then increase with increased NCDs content. The smallest arc radius and Rct (interfacial charge transfer resistance, Table S1) of 1 % NCDs/DCN among all the samples manifests the most effective separation of the photogenerated electron-hole pairs and faster interfacial charge transport. For the 1.5 % NCDs/DCN, excessive NCDs may serve as recombination centers for charge carriers or shade the light (shading effect), leading to a low photocatalytic activity [47]. The transient photocurrent can reflect the separation efficiency and the migration rate of the photoinduced charge carriers. As shown in Fig. 5e, the transient photocurrent response over several on-off cycles exhibit quick and reproducible photocurrent response of DCN and NCDs/DCN samples. 1 % NCDs/DCN possesses the highest photocurrent value, indicating the greatest separation efficiency of photoinduced electronhole pairs. Lifetime (τ) of injected electrons (Fig. 5f) is further utilized to confirm the efficient charge carriers transfer by equation: τ = 1/2πf, (f is the inverse minimum frequency) [48,49]. The extended injected

3.2. Photocatalytic evaluation 3.2.1. The photocatalytic degradation activities Prior to photocatalytic degradation experiment, the adsorption experiments of Cr(VI) over the DCN and 1.0 % NCDs/DCN in dark are performed (Fig. S3) and manifest that the adsorption-desorption equilibrium can be obtained between DCN or 1.0 % NCDs/DCN and Cr(VI) in first 30 min. The photocatalytic performances of the catalysts towards OFL and Cr (VI) alone are examined and shown in Fig. 6a and 6b. The degradation efficiency exhibits firstly increasing and then decreasing trend with the increased amounts of NCDs, among which 1 % NCDs/DCN has the highest degradation efficiency with 69.2 % and 51.8 % removal efficiency towards OFL and Cr(VI), respectively. The decreased efficiency of 2 % NCDs/DCN is ascribed to the shade effect of excessive NCDs on the surface of DCN. Therefore, 1 % NCDs/DCN is chosen to be the optimized photocatalyst to further explore the photocatalytic performance. The photocatalytic activity of 1 % NCDs/DCN is assessed by simultaneous degradation combined pollutants of OFL and Cr (VI), BPA and Cr (VI), CIP and Cr (VI) systems. The CIP and BPA have 3.1 % and 6

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Fig. 5. (a) PL spectra, (b) TR-PL spectra (the inset is 3D TR-PL), (c) EIS in dark, (d) EIS under visible light (e) transient photocurrent response, (f) bode patterns of all samples.

7

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Fig. 6. Photocatalytic activities of the as-prepared photocatalysts for (a) OFL removal, (b) Cr(VI) reduction. Photocatalytic activities of 1 % NCDs/DCN for (c) OFL and Cr(VI), (d) BPA and Cr(VI), CIP and Cr(VI) combined systems. The impact of (e) different amounts of Cr(VI) and (f) different initial pH on Photocatalytic activities of 1 % NCDs/DCN for OFL and Cr (VI) system.

51.7 % under the same condition. Notably, in the combined systems, 75.2 % of OFL and 92.6 % of Cr(VI), 47.4 % of BPA and 53.8 % of Cr (VI), 75.8 % of CIP and 59.8 % of Cr(VI) can be simultaneously removed within 120 min, respectively, displaying the obviously higher removal efficiency than single pollutant system. The electrophilic reactive oxygen species have been reported to preferentially oxidize

17.7 % removal efficiency under visible light irradiation without photocatalyst (Fig. 6d), indicating the slightly photo-chemically stability of CIP and photosensitization of BPA, respectively. As shown in Fig. 6c and d, in single pollutant system, the degradation efficiency of OFL, BPA and CIP are 69.1 %, 34.4 % and 52.1 % after 120 min under visible light irradiation, respectively, while the removal efficiency of Cr (VI) is 8

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and decreased removal of OFL. At low pH, HCrO4− or Cr2O72− are the major Cr(VI) species, the positively charged 1 % NCDs/DCN surface facilitates the absorption of Cr(VI) and the photocatalytic Cr(VI) reduction happens according to Eq. (2) due to the positive reductive potential of (Eθ(Cr2O72−/Cr3+) = 1.232 V vs NHE) [53]. At pH = 5.5, the photocatalytic system achieves the optimized removal of OFL and Cr(VI) with 75.2 % and 92.6 % removal efficiency, respectively. Simultaneously, the positively charged 1 % NCDs/DCN facilitates the desorption of Cr(III) from the surface.

aromatic compounds with electro donating groups on position with high electron density, and thus OFL is more easily oxidized than BPA and CIP [50]. The enhanced photocatalytic activity of 1 % NCDs/DCN in combined systems can be attributed to the promoted photogenerated electrons and holes separation efficiency, which results from the simultaneously photocatalytic oxidation and reduction process. The results indicate that 1 % NCDs/DCN can be applied to other combined systems and exhibit excellent photocatalytic activity. The effect of initial Cr(VI) concentration on photocatalytic activity of 1 % NCDs/DCN is also investigated (Fig. 6e) [51]. At the initial Cr (VI) concentration of 20 ppm, 19.1 % of OFL can be removed within 120 min (versus 66.9 % and 75.2 % at initial Cr(VI) concentration of 5 and 10 ppm, respectively), and Cr(VI) removal efficiency is 57.7 % (versus 62.8 % and 92.6 % at initial Cr(VI) concentration of 5 and 10 ppm, respectively). The 10 ppm Cr(VI) can achieve the highest removal efficiency of OFL and Cr(VI) in the combined systems, and the excessive amounts of Cr(VI) decrease removal efficiency. The effect pH of reaction system is an important factor for photocatalytic degradation efficiency [52]. Firstly, the pH can change the surface charge properties of 1 % NCDs/DCN, herein impact the photocatalytic reactivity. Secondly, the pH can alter the molecular structure of the OFL and Cr(VI). OFL contains various functional groups and is an amphoteric substance which has two pKa values (pKa1 = 5.98, pKa2 = 8.00). The effect of different initial pH on the photocatalytic activity of 1 % NCDs/DCN was investigated under visible light irradiation (Fig. 6f). Both OFL and Cr (VI) removal efficiency enhance with the increased acidity. At initial pH 7.5, 51.8 % of OFL is degraded and 53.5 % of Cr(VI) is reduced within 120 min. In the neutral conditions, OFL mainly shows weak cationic feature, 1 % NCDs/DCN with negative charge surface will cause strong electrostatic attractions towards OFL, and result in the increased removal of OFL. However, Cr(III) tends to precipitate on the surface of 1 % NCDs/DCN in the form of Cr(OH)3 according to Eq. (1) and occupies the active sites, resulting in the inhibited absorption and decreased photocatalytic activity, and thus the simultaneous removal efficiency were both decreased. While 24.9 % of OFL and 98.9 % of Cr(VI) removal at pH 3.5 indicates the lower OFL degradation activity of 1 % NCDs/DCN under acidic condition, probably due to the effect of proton concentration in the reaction system. At initial pH range of 2.0–4.0, the content of cationic OFL species (OFL+) is dominant [3]. Based on the isoelectric point (pHzpc) of 1 % NCDs/ DCN (Fig. S4), 1 % NCDs/DCN with positive charge surface causes strong electrostatic repulsion attractions, causing inhibited absorption

CrO42− + 4H2O + 3e− = Cr(OH)3 + 5OH− Cr2O7

2−

+ 14H

+

−

+ 6e = 2Cr

3+

+ 7H2O

(1) (2)

The typical recycle experiments under visible light irradiation are performed to test the stability and reusability of 1 % NCDs/DCN. As shown in Fig. 7a, no significant decrease in photocatalytic performance is observed after recycling 4 times either on BPA oxidation or Cr(VI) reduction, which confirms efficient photocatalytic performance and potential application in actual water remediation of 1 % NCDs/DCN photocatalyst. In order to further study the process, XPS of the used 1 % NCDs/DCN were performed. As shown in Fig. 6b, the high-resolution XPS spectrum of 1 % NCDs/DCN after reaction time of 120 min demonstrates overt Cr element on the surface of used 1 % NCDs/DCN. Notably, two peaks located at 576.8 and 586.4 eV in the spectrum of Cr 2p are attributed to Cr 2p 3/2 and Cr 2p 1/2 of Cr(III), respectively [54], indicating the less toxic Cr(III) converted from highly toxic Cr(VI) after photocatalytic reduction. 3.2.2. Simultaneously photocatalytic H2 generation and RhB degradation The excellent photocatalytic activity and multiple-function of 1 % NCDs/DCN are also assessed by simultaneously photocatalytic hydrogen production and RhB degradation. As shown in Fig. 8a, 1.0 % NCDs/g-C3N4 shows the excellent photocatalytic H2 generation datum of 626.93 μmol g−1 h−1, which is about 6.7 times higher than that of DCN. The right amount of NCDs contribute to both a better visible-light acquisition and division of electron-hole pairs. In addition, the lone pair electrons of nitrogen atoms in amine are more prone to interacting with holes, guaranteeing that more photoelectrons are released to combine with H+ for hydrogen evolution [55]. 3.3. Proposed photocatalytic mechanism The effects of various radical scavengers such as TEMPOL, IPA and

Fig. 7. (a) Cycling performance of 1 % NCDs/DCN for OFL and Cr (VI) system. (b) high-resolution XPS spectra of Cr 2p in 1 % NCDs/DCN after four Cycling reactions. 9

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Fig. 8. (a) H2 evolution of DCN and 1.0 % NCDs/DCN and (b) UV–vis spectra of 1.0 % NCDs/DCN towards photocatalytic RhB decolorization before and after the simultaneous photocatalytic performance.

with shorter wavelengths, but also up-convert long wavelengths to shorter wavelengths in solar light. Therefore, the quantity and formation speed of photo-excited e−−h+ pairs are elevated and succedent excitation of system to form photoinduced carriers was promoted, and thus the increased utilizing efficiency in the visible light is achieved. Besides, as an electron reservoir and donor in photocatalytic process, NCDs can easily trap electrons from the conduction band of semiconductor and donate them to reduction reaction. As a result, NCDs can promote separation and transportation of photogenerated carriers, and thus enhance the photocatalytic activity of 1 % NCDs/DCN. Liu et.al have reported a direct z-scheme heterojunction of NCDs/ TNS [37]. Similarly, based on the more negative CB position of NCDs (-1.99 V) than that of DCN (-0.66 V) [37,57], 1 % NCDs/DCN follows the direct z-scheme charge transfer mechanism. The CB excited electrons of DCN would combine with the VB holes of NCDs, reserving the e− and h+ on the CB of NCDs and VB of DCN, respectively. Based on the active–free radical capturing and EPR experiment results, a probable mechanism for the photocatalytic reduction of Cr(VI) and oxidation of OFL over 1 % NCDs/DCN is proposed (Fig. 10). 1 % NCDs/DCN possesses porous ultrathin nanosheet structure, which means high specific surface area and numerous active sites for the reaction process. The synergetic effect of VN and NCDs can broaden the visible light response region of the photocatalyst, and thus promote the solar energy utilization efficiency. Under visible light irradiation, the 1 % NCDs/ DCN could be excited to generate conduction band electrons and valence band holes (Eq.(1)). Subsequently, the photogenerated electrons in the conductor band could directly reduce Cr(VI) to Cr(III) (Eqs. (7 and 8)) and react with the dissolved oxygen to produce O2%− and migrate to Cr(VI) to form its reduced state of chromium. In addition, OFL could be oxidized by the photogenerated holes and the produced oxidized species, such as %OH, O2%− and H2O2. The generation of the photogenerated electron-holes, radicals and reaction process can be list as equations (Eqs. (1–10)). In 1 % NCDs/DCN photocatalytic reaction system, Cr(VI) and OFL could act as electron acceptor and donor, reacting with the photogenerated CB electrons and photoexcited holes, respectively. These two processes can promote the separation rate of photogenerated electron-holes pairs, resulting in much more CB electrons for Cr(VI) reduction and VB holes for OFL oxidation. As a result, the recombination of electrons and holes was suppressed and the photocatalysis redox reaction cycles unceasingly. The better

EDTA-2Na are added into 1 % NCDs/DCN system to investigate the impact of active species O2%−, •OH and h+ on the photocatalytic OFL and Cr(VI) simultaneous removal performance [51,56]. As Fig. 9a displays, an obvious suppression on the OFL removal (47.4 %) is observed when 2 mM IPA is added, indicating the mainly factor of •OH in the photocatalytic reaction process. With the addition of TEMPOL, the degradation of OFL (56.1 %) was depressed to some extent. However, no obvious degradation change (68.3 %) with the adding of EDTA-2Na, demonstrated that O2%− active species played a conspicuous role in OFL degradation. These results suggested that the •OH radicals played a major role in the photodegradation of OFL. Accordingly, a slight change of Cr(VI) (44.2 % and 41.6 %) removal efficiency was detected when adding TEMPOL and IPA. However, the Cr(VI) removal efficiency (94.7 %) was enhanced when EDTA-2Na was added, which was probably ascribed to the consumption of photoinduced holes by the scavengers and thereby enhanced the efficient separation of photogenerated electron-hole pairs. The generation of ROS over the 1 % NCDs/DCN composite under visible light irradiation is probed by DMPO and TEMPOL spin-trapping ESR technique to elucidate combined pollutions degradation mechanism. As shown in Fig. 9b-d, no ESR signal is observed in the dark. Upon visible light irradiation, four-line ESR signal with intensity ratio of 1:1:1:1 and 1:2:2:1 and triplet signal are clearly observed, which are characteristic of DMPO-O2%−, DMPO-•OH and TEMPOL-1O2 adduct. Since more negative valance band potential of the photogenerated holes in the valance band of DCN (EVB = +1.86 V, vs. NHE) than •OH radicals (E (•OH/OH−) = +1.99 V, vs. NHE), the observed •OH radicals were speculated to be generated from the O2%− radicals by photochemical reaction. The gradually enhanced intensity of radical signals in both catalysts with time prolonged indicated the important role of O2%−, •OH and 1O2 radicals in degrading combined pollutions. Notably, the higher formation rates of O2%−, %OH and 1O2 radicals in 1.0 % NCDs/DCN systems than these in DCN were in well consistent with the observed combined pollutions degradation rate, demonstrating that more ROS reacted in the 1 % NCDs/DCN system and subsequently promoted pollutions degradation than in the DCN system. Compared to DCN, the improved photocatalytic performance of 1 % NCDs/DCN reveals the favorable role of NCDs in the enhancement of photocatalytic activity. Due to photosensitive and upconversion property, NCDs can not only aid 1 % NCDs/DCN system to capture photons 10

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Fig. 9. (a) Quenching experiment of active-species. ESR spectra of (b) DMPO-•OH adducts in aqueous solution, (c) DMPO-%O2− adducts in methanol solution and (d) TEMPOL-1O2 recorded with DCN and 1.0 % NCDs/DCN systems.

photocatalytic performance of 1 % NCDs/DCN can be attributed to the enhanced light absorption property, facilitated separation and transportation of charge carriers, and the promoted consumption of holes. In the same way, upon irradiation, the photogenerated electrons of DCN are transferred to NCDs through Mott-Schottky junction at the interface of DCN and NCDs, and react with protons or water molecules to generate H2. Simultaneously, RhB, as sacrificial electron donors, can be oxidized and degraded directly by photogenerated holes with the high oxidizing power. Using RhB as an electron donor in H2 generation process signifies the dual-function effect of 1 % NCDs/DCN photocatalyst. 1 % NCDs/DCN + hv → 1.0 % NCDs/DCN (h+ + e−)

(1)

O2 + e− →O2•−

(2)

O2 + 2H+ + 2e− → H2O2

(3)

2O2•−

(4)

O2•−

+ 2H −

+e

H2O2 + e Fig. 10. Schematic photocatalytic mechanism for the 1.0 % NCDs/DCN in OFL/ Cr6+ system under visible light irradiation.

HCrO4 Cr2O7 11

→ O2 + H2O2

+

−

2−

−

+ 2H

+

→ H2O2

•

(5)

−

→ OH + OH +

+ 7H

+ 14H

+ 3e

+

−

+ 6e

→ Cr

−

(6) 3+

→ 2Cr

+ 4H2O 3+

+ 7 H2O

(7) (8)

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h+ + OFL/BPA/CPI → degradation products O2•−/ H2O2/•OH + OFL/BPA/CPI → degradation product

(9) [4]

(10)

[5]

4. Conclusion In this work, NCDs-coupled defective g-C3N4 was designed and prepared through solvent evaporation and calcination method. The coupling effect between upconversion effect of NCDs and defect effect of DCN achieves enhanced visible light response, improved separation and suppressed recombination of charge, and thus improve the photocatalytic activity. The optimized 1.0 % NCDs/DCN exhibits prominently photocatalytic degradation efficiency towards combined pollutions, as well as simultaneous H2 generation and RhB degradation. The enhanced photocatalytic efficiency is attributed to the dual function of oxidation and reduction process in the same system, leading to the efficient separation of charge carriers and the more generated electrons and holes for redox reactions. The charge transfer mechanism and photocatalytic mechanism are investigated by active species capture experiments and EPR technology, and the mainly impact of h+ and O2•− in oxidation process and e− for reduction process is confirmed. The multiple functions of NCDs/DCN provide the strategy for design effective photocatalysts for simultaneous water environmental mediation and energy utilization.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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CRediT authorship contribution statement [15]

Haiping Liu: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Jing Liang: Software, Resources, Investigation. Shuai Fu: Formal analysis, Data curation. Li Li: Methodology, Software. Jiehu Cui: Investigation, Formal analysis. Penghui Gao: Data curation, Supervision. Fengying Zhao: Project administration, Supervision. Jianguo Zhou: Writing - review & editing, Resources, Funding acquisition.

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Acknowledgements [19]

This work is supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (2015ZX07204-002), Science and Technology Innovation Talents Program of Xinxiang City (CXRC17001), The National Natural Science Foundation of China (21771165).

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Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2020.124552.

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References [1] Y. Gong, X. Zhao, H. Zhang, B. Yang, K. Xiao, T. Guo, J. Zhang, H. Shao, Y. Wang, G. Yu, MOF-derived nitrogen doped carbon modified g-C3N4 heterostructure composite with enhanced photocatalytic activity for bisphenol A degradation with peroxymonosulfate under visible light irradiation, Appl. Catal. B Environ. 233 (2018) 35–45. [2] X. Xu, X. Ding, X. Yang, P. Wang, S. Li, Z. Lu, H. Chen, Oxygen vacancy boosted photocatalytic decomposition of ciprofloxacin over Bi2MoO6: oxygen vacancy engineering, biotoxicity evaluation and mechanism study, J. Hazard. Mater. 364 (2019) 691–699. [3] P. Chen, L. Blaney, G. Cagnetta, J. Huang, B. Wang, Y. Wang, S. Deng, G. Yu,

[24]

[25]

[26]

[27]

12

Degradation of ofloxacin by perylene diimide supramolecular nanofiber sunlightdriven photocatalysis, Environ. Sci. Technol. 53 (2019) 1564–1575. Q. Zhang, M. Ao, Y. Luo, A. Zhang, L. Zhao, L. Yan, F. Deng, X. Luo, Solvothermal synthesis of Z-scheme AgIn5S8_Bi2WO6 nano-heterojunction with excellent performance for photocatalytic degradation and Cr(VI) reduction, J. Alloy. Comp. 805 (2019) 41–49. I. Ibrahim, A. Kaltzoglou, C. Athanasekou, F. Katsaros, E. Devlin, A.G. Kontos, N. Ioannidis, M. Perraki, P. Tsakiridis, L. Sygellou, M. Antoniadou, P. Falaras, Magnetically separable TiO2/CoFe2O4/Ag nanocomposites for the photocatalytic reduction of hexavalent chromium pollutant under UV and artificial solar light, Chem. Eng. J. 381 (2019), https://doi.org/10.1016/j.cej.2019.122730. Y. Yan, M. Yang, C. Wang, E. Liu, X. Hu, J. Fan, Defected ZnS/bulk g–C3N4 heterojunction with enhanced photocatalytic activity for dyes oxidation and Cr (Ⅵ) reduction, Colloids Surf. A: Physicochem. Eng. Asp. 582 (2019), https://doi.org/10. 1016/j.colsurfa.2019.123861. J. Li, X. Wang, G. Zhao, C. Chen, Z. Chai, A. Alsaedi, T. Hayat, X. Wang, Metalorganic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions, Chem. Soc. Rev. 47 (2018) 2322–2356. Z. Lou, S. Kim, M. Fujitsuka, X. Yang, B. Li, T. Majima, Anisotropic Ag2S-Au triangular nanoprisms with desired configuration for plasmonic photocatalytic hydrogen generation in Visible/Near-Infrared region, Adv. Funct. Mater. 28 (2018) 1706969. T. Guo, K. Wang, G. Zhang, X. Wu, A novel α-Fe2O3@g-C3N4 catalyst: synthesis derived from Fe-based MOF and its superior photo-Fenton performance, Appl. Surf. 469 (2019) 331–339. J. Li, C. Xiao, K. Wang, Y. Li, G. Zhang, Enhanced generation of reactive oxygen species under visible light irradiation by adjusting the exposed facet of FeWO4 nanosheets to activate oxalic acid for organic pollutant removal and Cr(VI) reduction, Environ. Sci. Technol. 53 (2019) 11023–11030. L. Jiang, J. Li, K. Wang, G. Zhang, Y. Li, X. Wu, Low boiling point solvent mediated strategy to synthesize functionalized monolayer carbon nitride for superior photocatalytic hydrogen evolution, Appl. Catal. B Environ. 260 (2020), https://doi.org/ 10.1016/j.apcatb.2019.118181. Y. Guo, C. Li, Y. Guo, X. Wang, X. Li, Ultrasonic-assisted synthesis of mesoporous gC3N4/Na-bentonite composites and its application for efficient photocatalytic simultaneous removal of Cr(VI) and RhB, Colloids Surf. A: Physicochem. Eng. Asp. 578 (2019), https://doi.org/10.1016/j.colsurfa.2019.123624. L. Jiang, X. Yuan, Y. Pan, J. Liang, G. Zeng, Z. Wu, H. Wang, Doping of graphitic carbon nitride for photocatalysis: a reveiw, Appl. Catal. B Environ. 217 (2017) 388–406. J. Huang, J. Liu, J. Yan, C. Wang, T. Fei, H. Ji, Y. Song, C. Ding, C. Liu, H. Xu, H. Li, Enhanced photocatalytic H2 evolution by deposition of metal nanoparticles into mesoporous structure of g-C3N4, Colloids Surf. A: Physicochem. Eng. Asp. 585 (2020), https://doi.org/10.1016/j.colsurfa.2019.124067. F. Chang, J. Zheng, F. Wu, X. Wang, B. Deng, Binary composites WO3/g-C3N4 in porous morphology: facile construction, characterization, and reinforced visible light photocatalytic activity, Colloids Surf. A: Physicochem. Eng. Asp. 563 (2019) 11–21, https://doi.org/10.1016/j.colsurfa.2018.11.058. D. Zhang, Y. Guo, Z. Zhao, Porous defect-modified graphitic carbon nitride via a facile one-step approach with significantly enhanced photocatalytic hydrogen evolution under visible light irradiation, Appl. Catal. B Environ. 226 (2018) 1–9. D. Zhao, C.L. Dong, B. Wang, C. Chen, Y.C. Huang, Z. Diao, S. Li, L. Guo, S. Shen, Synergy of dopants and defects in Graphitic Carbon Nitride with exceptionally modulated band structures for efficient photocatalytic oxygen evolution, Adv. Mater. 31 (2019) e1903545. J. Cao, J. Zhang, Xa. Dong, H. Fu, X. Zhang, X. Lv, Y. Li, G. Jiang, Defective boratedecorated polymer carbon nitride: enhanced photocatalytic NO removal, synergy effect and reaction pathway, Appl. Catal. B Environ. 249 (2019) 266–274. H. Che, L. Liu, G. Che, H. Dong, C. Liu, C. Li, Control of energy band, layer structure and vacancy defect of graphitic carbon nitride by intercalated hydrogen bond effect of NO3− toward improving photocatalytic performance, Chem. Eng. J. 357 (2019) 209–219. Y. Jiang, Z. Sun, C. Tang, Y. Zhou, L. Zeng, L. Huang, Enhancement of photocatalytic hydrogen evolution activity of porous oxygen doped g-C3N4 with nitrogen defects induced by changing electron transition, Appl. Catal. B Environ. 240 (2019) 30–38. Q. Tay, P. Kanhere, C.F. Ng, S. Chen, S. Chakraborty, A.C.H. Huan, T.C. Sum, R. Ahuja, Z. Chen, Defect engineered g-C3N4 for efficient visible light photocatalytic hydrogen production, Chem. Mater. 27 (2015) 4930–4933. Z. Zhu, H. Pan, M. Murugananthan, J. Gong, Y. Zhang, Visible light-driven photocatalytically active g-C3N4 material for enhanced generation of H2O2, Appl. Cataly. B Environ. 232 (2018) 19–25. H. Liu, S. Ma, L. Shao, H. Liu, Q. Gao, B. Li, H. Fu, S. Fu, H. Ye, F. Zhao, J. Zhou, Defective engineering in graphitic carbon nitride nanosheet for efficient photocatalytic pathogenic bacteria disinfection, Appl. Catal. B Environ. 261 (2020), https://doi.org/10.1016/j.apcatb.2019.118201. J. Liu, W. Fang, Z. Wei, Z. Qin, Z. Jiang, W. Shangguan, Efficient photocatalytic hydrogen evolution on N-deficient g-C3N4 achieved by a molten salt post-treatment approach, Appl. Catal. B Environ. 238 (2018) 465–470. P. Niu, L.-C. Yin, Y.-Q. Yang, G. Liu, H.-M. Cheng, Increasing the visible light absorption of graphitic carbon nitride (Melon) photocatalysts by homogeneous selfmodification with nitrogen vacancies, Adv. Mater. 26 (2014) 8046–8052. Y. Kang, Y. Yang, L.C. Yin, X. Kang, L. Wang, G. Liu, H.M. Cheng, Selective breaking of hydrogen bonds of layered carbon nitride for visible light photocatalysis, Adv. Mater. 28 (2016) 6471–6477. H. Yu, R. Shi, Y. Zhao, G.I. Waterhouse, L.Z. Wu, C.H. Tung, T. Zhang, Smart

Colloids and Surfaces A 591 (2020) 124552

H. Liu, et al.

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42]

[43]

utilization of carbon dots in semiconductor photocatalysis, Adv. Mater. 28 (2016) 9454–9477. Z. Mu, J. Hua, S. Kumar Tammina, Y. Yang, Visible light photocatalytic activity of Cu, N co-doped carbon dots/Ag3PO4 nanocomposites for neutral red under green LED radiation, Colloids Surf. A: Physicochem. Eng. Asp. 578 (2019), https://doi. org/10.1016/j.colsurfa.2019.123643. H. Wang, Z. Wei, H. Matsui, S. Zhou, Fe3O4/carbon quantum dots hybrid nanoflowers for highly active and recyclable visible-light driven photocatalyst, J. Mater. Chem. A 2 (2014) 15740–15745. H. Zhang, L. Zhao, F. Geng, L.-H. Guo, B. Wan, Y. Yang, Carbon dots decorated graphitic carbon nitride as an efficient metal-free photocatalyst for phenol degradation, Appl. Catal. B Environ. 180 (2016) 656–662. J. Zhang, Y. Ma, Y. Du, H. Jiang, D. Zhou, S. Dong, Carbon nanodots/WO3 nanorods Z-scheme composites: remarkably enhanced photocatalytic performance under broad spectrum, Appl. Catal. B Environ. 209 (2017) 253–264. K. Lei, M. Kou, Z. Ma, Y. Deng, L. Ye, Y. Kong, A comparative study on photocatalytic hydrogen evolution activity of synthesis methods of CDs/ZnIn2S4 photocatalysts, Colloids Surf. A: Physicochem. Eng. Asp. 574 (2019) 105–114. M. Ji, J. Di, R. Chen, Z. Chen, J. Xia, H. Li, N-CQDs accelerating surface charge transfer of Bi4O5I2 hollow nanotubes with broad spectrum photocatalytic activity, Appl. Catal. B Environ. 237 (2018) 1033–1043. J. Di, J. Xia, M. Ji, B. Wang, X. Li, Q. Zhang, Z. Chen, H. Li, Nitrogen-doped carbon quantum Dots/BiOBr ultrathin nanosheets: in situ strong coupling and improved molecular oxygen activation ability under visible light irradiation, ACS Sustain. Chem. Eng. 4 (2015) 136–146. Y. Zhou, L. Zhang, W. Huang, Q. Kong, X. Fan, M. Wang, J. Shi, N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced photocatalytic H 2 evolution under visible light, Carbon 99 (2016) 111–117. Y. Sun, Z. Zhang, A. Xie, C. Xiao, S. Li, F. Huang, Y. Shen, An ordered and porous Ndoped carbon dot-sensitized Bi2O3 inverse opal with enhanced photoelectrochemical performance and photocatalytic activity, Nanoscale 7 (2015) 13974–13980. F. Wang, Y. Wu, Y. Wang, J. Li, X. Jin, Q. Zhang, R. Li, S. Yan, H. Liu, Y. Feng, G. Liu, W. Lv, Construction of novel Z-scheme nitrogen-doped carbon dots/{0 0 1} TiO2 nanosheet photocatalysts for broad-spectrum-driven diclofenac degradation: mechanism insight, products and effects of natural water matrices, Chem. Eng. J. 356 (2019) 857–868. Z.A. Zakaria, Z. Zakaria, S. Surif, W.A. Ahmad, Hexavalent chromium reduction by Acinetobacter haemolyticus isolated from heavy-metal contaminated wastewater, J. Hazard. Mater. 146 (2007) 30–38. P. Niu, L. Zhang, G. Liu, H.-M. Cheng, Graphene-like carbon nitride nanosheets for improved photocatalytic activities, Adv. Funct.Mater. 22 (2012) 4763–4770. Q. Wang, W. Wang, L. Zhong, D. Liu, X. Cao, F. Cui, Oxygen vacancy-rich 2D/2D BiOCl-g-C3N4 ultrathin heterostructure nanosheets for enhanced visible-light-driven photocatalytic activity in environmental remediation, Appl. Catal. B Environ. 220 (2018) 290–302. Y. Li, R. Jin, Y. Xing, J. Li, S. Song, X. Liu, M. Li, R. Jin, Macroscopic foam-like holey ultrathin g-C3N4 nanosheets for drastic improvement of visible-light photocatalytic activity, Adv. Energy Mater. 6 (2016) 1601273. X. Meng, Z. Li, H. Zeng, J. Chen, Z. Zhang, MoS2 quantum dots-interspersed Bi 2WO6 heterostructures for visible light-induced detoxification and disinfection, Appl. Catal. B Environ. 210 (2017) 160–172. J. Liu, Y. Xu, Y. Song, J. Lian, Y. Zhao, L. Wang, H. Ji, H. Li, Graphene quantum dots

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

13

modified mesoporous graphite carbon nitride with significant enhancement of photocatalytic activity, Appl. Catal. B Environ. 207 (2017) 429–437. B.Y. Yu, S.-Y. Kwak, Carbon quantum dots embedded with mesoporous hematite nanospheres as efficient visible light-active photocatalysts, J. Mater. Chem. 22 (2012) 8345–8353. Z. Xie, Y. Feng, F. Wang, D. Chen, Q. Zhang, Y. Zeng, W. Lv, G. Liu, Construction of carbon dots modified MoO3/g-C3N4 Z-scheme photocatalyst with enhanced visiblelight photocatalytic activity for the degradation of tetracycline, Appl. Catal. B Environ. 229 (2018) 96–104. F. Wang, P. Chen, Y. Feng, Z. Xie, Y. Liu, Y. Su, Q. Zhang, Y. Wang, K. Yao, W. Lv, G. Liu, Facile synthesis of N-doped carbon dots/g-C3N4 photocatalyst with enhanced visible-light photocatalytic activity for the degradation of indomethacin, Appl. Catal. B Environ. 207 (2017) 103–113. Y. Li, Z. Liu, Y. Wu, J. Chen, J. Zhao, F. Jin, P. Na, Carbon dots-TiO2 nanosheets composites for photoreduction of Cr(VI) under sunlight illumination: favorable role of carbon dots, Appl. Catal. B Environ. 224 (2018) 508–517. L. Liu, L. Ding, Y. Liu, W. An, S. Lin, Y. Liang, W. Cui, A stable Ag3PO4 @PANI core@shell hybrid: enrichment photocatalytic degradation with π-π conjugation, Appl. Catal. B Environ. 201 (2017) 92–104. C. Liu, H. Huang, L. Ye, S. Yu, N. Tian, X. Du, T. Zhang, Y. Zhang, Intermediatemediated strategy to horn-like hollow mesoporous ultrathin g-C3N4 tube with spatial anisotropic charge separation for superior photocatalytic H2 evolution, Nano Energy 41 (2017) 738–748. X. Tian, P. Gao, Y. Nie, C. Yang, Z. Zhou, Y. Li, Y. Wang, A novel singlet oxygen involved peroxymonosulfate activation mechanism for degradation of ofloxacin and phenol in water, Chem. Commun. (Camb.) 53 (2017) 6589–6592. H. Wang, Q. Li, S. Zhang, Z. Chen, W. Wang, G. Zhao, L. Zhuang, B. Hu, X. Wang, Visible-light-driven N2-g-C3N4 as a highly stable and efficient photocatalyst for bisphenol A and Cr(VI) removal in binary systems, Catal. Today 335 (2019) 110–116. S. Chen, W. Lu, J. Han, H. Zhong, T. Xu, G. Wang, W. Chen, Robust three-dimensional g-C3N4@cellulose aerogel enhanced by cross-linked polyester fibers for simultaneous removal of hexavalent chromium and antibiotics, Chem. Eng. J. 359 (2019) 119–129. Y. Deng, G. Zeng, Z. Zhu, M. Yan, Y. Zhou, Y. Liu, J. Wang, Simultaneous photocatalytic removal of Cr(VI) and 2,4-diclorophenol under visible light irradiation by phosphorus doped porous ultrathin g-C3N4 nanosheets from aqueous media_Performance and reaction mechanism, Appl. Catal. B Environ. 203 (2017) 343–354. Z. Fang, Q. Li, L. Su, J. Chen, K.-C. Chou, X. Hou, Efficient synergy of photocatalysis and adsorption of hexavalent chromium and rhodamine B over Al4SiC4/rGO hybrid photocatalyst under visible-light irradiation, Appl. Catal. B Environ. 241 (2019) 548–560. Z. Zhong, Z. Hu, Z. Jiang, J. Wang, Y. Chen, C. Song, S. Han, F. Huang, J. Peng, J. Wang, Y. Cao, Hole-trapping effect of the aliphatic-amine based electron injection materials in the operation of OLEDs to facilitate the electron injection, Adv. Electro. Mater. 1 (2015) 1400014. X. Wang, M. Hong, F. Zhang, Z. Zhuang, Y. Yu, Recyclable nanoscale zero valent iron doped g-C3N4/MoS2 for efficient photocatalysis of RhB and Cr(VI) driven by visible light, ACS Sustain. Chem. Eng. 4 (2016) 4055–4063. G. Gao, Y. Jiao, F. Ma, Y. Jiao, E. Waclawik, A. Du, Carbon nanodot decorated graphitic carbon nitride: new insights into the enhanced photocatalytic water splitting from ab initio studies, Phys. Chem. Chem. Phys. 17 (2015) 31140–31144.