Nitrogen-doped carbon quantum dots-decorated 2D graphitic carbon nitride as a promising photocatalyst for environmental remediation: A study on the importance of hybridization approach

Journal of Environmental Management 255 (2020) 109936

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Research article

Nitrogen-doped carbon quantum dots-decorated 2D graphitic carbon nitride as a promising photocatalyst for environmental remediation: A study on the importance of hybridization approach Ru Xuan Seng a, Lling-Lling Tan b, *, W.P. Cathie Lee b, c, Wee-Jun Ong d, Siang-Piao Chai b a

School of Engineering and Physical Sciences, Heriot-Watt University Malaysia, Jalan Venna P5/2, Precinct 5, 62200, Putrajaya, Malaysia Multidisciplinary Platform of Advanced Engineering, Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, 47500, Bandar Sunway, Selangor, Malaysia c Entropic Interface Group, Engineering Product Development, Singapore University of Technology and Design, 487372, Singapore d School of Energy and Chemical Engineering, Xiamen University Malaysia, Selangor Darul Ehsan, 43900, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Keywords: Nitrogen-doped carbon quantum dots Graphitic carbon nitride Methylene blue degradation Hydrothermal

Growing concerns of water pollution by dye pollutants from the textile industry has led to vast research interest to find green solutions to address this issue. In recent years, heterogeneous photocatalysis has harvested tremendous attention from researchers due to its powerful potential applications in tackling many important energy and environmental challenges at a global level. To fully utilise the broad spectrum of solar energy has been a common aim in the photocatalyst industry. This study focuses on the development of an efficient, highly thermal and chemical stable, environmentally friendly and metal-free graphitic carbon nitride (g-C3N4) to overcome the problem of fast charge recombination which hinders photocatalytic performances. Nitrogen-doped carbon quantum dots (NCQDs) known for its high electronic and optical functionality properties is believed to achieve photocatalytic enhancement by efficient charge separation through forming heterogeneous interfaces. Hence, the current work focuses on the hybridisation of NCQDs and g-C3N4 to produce a composite photocatalyst for methylene blue (MB) degradation under LED light irradiation. The optimal hybridisation method and the mass loading required for maximum attainable MB degradation were systematically investigated. The optimum photocatalyst, 1 wt% NCQD/g-C3N4 composite was shown to exhibit a 2.6-fold increase in photocatalytic activity over bare g-C3N4. Moreover, the optimum sample displayed excellent stability and durability after three consecutive degradation cycles, retaining 91.2% of its original efficiency. Scavenging tests were also performed where reactive species, photon-hole (hþ) was identified as the primary active species initiating the pollutant degradation mechanism. The findings of this study successfully shed light on the hybridisation methods of NCQDs which improve existing g-C3N4 photocatalyst systems for environmental remediation by utilising solar energy.

1. Introduction In recent years, the issue of industrial wastewater pollution has gained attention around the world since these effluents pose significant health and environmental issues. According to Ryder, 2017, 56% of the global freshwater is released into the environment by the form of in­ dustrial waste effluents which sums up to around 2212 km3 annually. It is also estimated that 17–20% of industrial wastewater pollution arises from the dye industry (Akpan and Hameed, 2009). These textile dyes were found to result in severe threats when discharged into the water

course due to a few reasons: The presence of hazardous chemicals (sulphur, nitrates, soaps, heavy metals) produces highly toxic effluents in addition to its carcinogenic property, which poses risks to human consumption. Moreover, the colloidal matter present in the coloured water increases turbidity and leads to foul smell. Furthermore, the environmental issues caused by dye pollutants brutally impact the ma­ rine life as it diminishes the oxygen transfer rate due to lower light penetration into water (Kant, 2012). Up till now, various physical techniques have been applied for wastewater treatment including sedi­ mentation, flocculation, reverse osmosis, ion exchange and

* Corresponding author. E-mail address: [email protected] (L.-L. Tan). https://doi.org/10.1016/j.jenvman.2019.109936 Received 25 September 2019; Received in revised form 15 November 2019; Accepted 25 November 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.

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ultrafiltration (Kumar and Pandey, 2017). However, the traditional methods were deemed ineffective due to their long reaction time on top of the secondary wastes produced which end up in landfills (Akpan and Hameed, 2009). Besides, due to an increasing concern in environmental sustainability, alternative resources to non-renewable energy have gained immense interest by researchers to treat wastewater via a greener route. Hence, in recent years, heterogeneous photocatalysis has har­ vested tremendous research interest due to its prevailing potential ap­ plications in tackling numerous important energy and environmental challenges at a global level, especially in dye degradation where various recent studies proved the success in utilising photocatalysis for dye removal in wastewater (Houas et al., 2001; Silva et al., 2006; Zhang et al., 2016). A common issue with the clear majority of photocatalytic materials is that the fast charge recombination and limited light harvesting prop­ erties have hindered their potential in commercial applications. Thus, the new and emerging zero-dimensional (0-D) carbon quantum dots (CQDs) have shed light into the industry due to their advantageous photocatalytic properties. CQDs are quasi-spherical particles with an average diameter of 2–10 nm (Qin and Zeng, 2017) comprising of amorphous to nanocrystalline cores with typical graphite carbon (sp2 carbon) or graphene fused by diamond-like sp3 hybridized carbon in­ sertions (Li et al., 2016). It is widely known that CQDs possess excellent up- and down-conversion photoluminescence (PL) property (Ke et al., 2017), high electronic and optical functionality, low toxicity as well as being inexpensive. Owing to these properties, CQDs have been commonly used in sectors including biosensing (Liu et al., 2015), bio­ logical labelling and imaging (Hsu et al., 2013) and electrocatalysis (Li et al., 2012b). Nevertheless, CQDs are found to contain some flaws as well whereby they have low photosentisation efficiency and low recy­ clability when used in liquid phase reaction systems as CQDs cannot be easily separated from solutions. To date, heterogeneous doping of CQDs using nitrogen (Edison et al., 2016; Ren et al., 2017), sulphur (Wang et al., 2015) and phosphorous (Zhou et al., 2014) atoms have found to be able to improve photocatalytic performances. Among all, nitrogen-doped CQDs have been most commonly studied due to its high quantum yield (QY), reduced band gap due to increase in electron density and extended absorption wavelength (Wang et al., 2015). Research interest in the photocatalytic industry have been focused on utilising the advantages of CQDs while improving on their downside (Rao et al., 2019). Therefore, multiple studies have been carried out to utilise CQDs’ attractive properties by coupling them with semi­ conductors. For example, CQDs had been used to form composites such as CQD/TiO2, CQD/ZnO and CQD/CdS (Hu et al., 2018; Muthulingam et al., 2015; Zhang et al., 2013), all of which showed enhanced photo­ catalytic properties and activities toward pollutant/dye degradation such as methylene blue (MB), methyl orange (MO) and Rhodamine B (RhB) over pure CQD. However, the main issue of these semiconductor composites is the risk of toxic metals leaching during the photocatalytic process (Zhang et al., 2016). On the contrary, a metal-free polymeric semiconductor material-graphitic carbon nitride (g-C3N4) was researched extensively for the past few years due to its excellent pho­ tocatalytic properties (Fu et al., 2018). From previous researches, it was noticed that it is highly thermal and chemical stable, environmentally friendly (Yan et al., 2009), inexpensive and of high abundance on earth (Zhang et al., 2012b). Apart from these, the band gap of this material is at 2.6 eV (Wang et al., 2008; Yu et al., 2015), thus allowing it to absorb light in the visible region, resulting in enhanced sunlight utilisation as compared to other semiconductors such as TiO2 which only allows ab­ sorption in the ultra-violet (UV) spectra. Recent studies have also found that g-C3N4 exhibit excellent efficiency in CO2 reduction, hydrogen production (Han et al., 2015; Wang et al., 2008) and pollutant degra­ dation (Yan et al., 2009). However, the pristine g-C3N4 suffers from a few drawbacks which include fast charge recombination (Zhang et al., 2012a) and small specific surface area (Zhang et al., 2012b), which greatly hinder its application in the photocatalytic industry. Therefore,

many studies were done to modify the material either by metallic-doping (Ag, Pt) or semiconductor-doping (TiO2, ZnO) to reduce its overall band gap, the recombination of photoinduced electrons and holes, and increase the surface area. Nevertheless, a metal-free solution was critical which led to the design and development of g-C3N4/CQDs composites (He et al., 2018, 2019; Phang and Tan, 2019). A study by Li and co-workers demonstrated that the CQDs functioned as electron reservoirs to trap photoexcited electrons, which in turn led to enhanced charge separation efficiencies (Li et al., 2016). It should be noted that although the hybridisation of CQDs and g-C3N4 had been investigated, the actual charge transfer mechanism of photogenerated electron-hole pairs for MB degradation still lack detailed research. Besides that, the hybridisation methods used to fabricate these composites differ from one research paper to another, with most of them focusing on enhancing the composites’ photocatalytic activities over pure CQDs and pristine g-C3N4. To the best of our knowledge, there has yet to be any work that thoroughly investigates the photocatalytic mechanism of nitrogen-doped carbon quantum dots (NCQDs)/g-C3N4 for MB degra­ dation under LED light irradiation. No paper has also studied the best approach to carry out the hybridization process between NCQDs and g-C3N4. Therefore, in this work, various synthesis methods to prepare the NCQDs/g-C3N4 composite material were systematically investigated and compared. In addition, the photocatalytic mechanism of NCQDs/g-C3N4 for the degradation of MB under LED light was also studied. The selected precursors for CQDs and g-C3N4 were citric acid (CA) and urea respec­ tively, while the nitrogen heteroatom was donated from urea, which is an inexpensive and abundantly available material. Among the coupling methods studied were mechanical mixing, thermal polymerisation and the hydrothermal approach, with the latter being identified as the most effective hybridisation technique. Therefore, in subsequent studies whereby the optimum loading of NCQDs on polymeric g-C3N4 was investigated, the hydrothermal treatment was used to perform the coupling process. In overall, the as-synthesised photocatalyst compos­ ites are expected to be developed as a robust means to address various energy and environmental-related issues by means of a greener and more sustainable route. 2. Material and methods 2.1. Materials Citric Acid (CA, 99.5%), urea (99.5–100.5%), p-Benzoquinone (BQ, 98%), Isopropyl alcohol (IPA, 98%) were purchased from R&M Chem­ icals. Methylene Blue (MB, 98.5%) was purchased from HmbG Chem­ icals. Triethanolamine (TEOA, 100%) was purchased from QRe C. Distilled water was used in all experiments. All chemicals were of analytical reagent grade and were used as received without further purification. 2.2. Synthesis of bulk NCQD 3 g of citric acid and 1 g of urea were combined and dissolved in 15 ml of distilled water. Then, the combined mixture was transferred to a Teflon-sealed autoclave reactor and heated at 150 � C for 4 h and allowed to cool to room temperature. The dark brown solution obtained was centrifuged at 12,000 rpm for 20 min to eliminate larger particles and the supernatant was collected. The collected supernatant was placed in a crucible and dried in an oven overnight at 90 � C to obtain the dark brown NCQD powder. The powder was then stored in solid form as the bulk NCQD. 2.3. Synthesis of bulk g-C3N4 The synthesis of g-C3N4 was based on previous reported methods 2

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using urea as a precursor with slight modifications (Ong et al., 2017; Thurston et al., 2017). First, 10 g of dried urea was placed in a covered 50 ml porcelain crucible and heated to 550 � C at a ramping rate of 5 � C/min in a furnace for 3 h. Then, the resulting solid was cooled to room temperature overnight before grounding it into powder. The beige-coloured powder was collected and stored as the bulk g-C3N4.

was then subjected to a hydrothermal treatment in a Teflon-sealed autoclave at 120 � C for 4 h. After allowing it to cool to room tempera­ ture, the suspension was collected and transferred to a vacuum filter. The solid sample was collected and washed two times with distilled water to obtain the filter cake. Next, it was dried in an oven overnight at 70 � C. The above procedures were repeated with different mass loadings of NCQDs (0.5, 1.0, 2.0, 5 wt%). The synthesised samples were labelled as 0.5 wt% NCQD/g-C3N4, 1 wt% NCQD/g-C3N4, 2 wt% NCQD/g-C3N4 and 5 wt% NCQD/g-C3N4 respectively. The hydrothermal route to synthesise NCQD/g-C3N4 is depicted in Fig. 1.

2.4. Synthesis of NCQD/g-C3N4 Three different synthesis routes were investigated in this study to determine the optimum approach in preparing NCQD/g-C3N4. The routes for synthesising the composite photocatalyst were mechanical mixing, thermal polymerisation and the hydrothermal method. To determine the optimum synthesis approach, all photocatalysts derived from the three methods will be compared using their degradation effi­ ciency against MB dye.

2.5. Characterisation of photocatalysts The surface morphology of the optimum photocatalyst (1 wt% NCQD/g-C3N4) was observed using Transmission Electron Microscopy (TEM) (FEI Tecnai G2 20S-TWIN) with an accelerating voltage of 200 kV. A Field-Emission Scanning Electron Microscopy (FESEM) equipped with an Energy Dispersive X-Ray (EDX) was used for the surface morphology, microstructural and elemental analysis of 1 wt% NCQD/gC3N4 using the Hitachi SU8010 instrument. The crystal structures of the as-prepared photocatalysts were observed using Powder X-Ray Diffrac­ tion (XRD) by the Bruker D8 Discover. Fourier Transform Infrared (FTIR) spectra of NCQD, g-C3N4 and 1 wt% NCQD/g-C3N4 were measured using the Nicolet iS10 Fourier Transform Infrared Spectrom­ eter. UV–Vis diffuse reflectance spectra (DRS) was measured using the Agilent Cary-100 UV–Visible Spectroscope at a wavelength of 200–800 nm whereas the Photoluminescence (PL) Spectra was obtained using a Fluorescence Spectrometer (Perkin Lamer LS55). The samples’ textural properties were analysed using a Micromeritics 3Flex surface area and porosimetry analyser. The photocatalysts were vacuum degassed at 200 � C for 4 h. The linear portion (P/Po) of the Brunauer–Emmett–Teller (BET) model was used for calculating the surface area of the prepared samples. The desorption branch of the Barret–Joyner–Halenda (BJH) model was employed to calculate the pore diameter and volume of the samples.

2.4.1. Mechanical mixing of NCQD/g-C3N4 photocatalyst composite First, 1 g of bulk g-C3N4 powder was mixed with a measured mass of NCQDs and ground together to form a uniform powdered mixture by using a pestle and mortar. The sample was labelled as 1 wt% NCQD/gC3N4 (Mech). 2.4.2. Thermal polymerisation of NCQD/g-C3N4 photocatalyst composite A measured mass of bulk NCQDs was dissolved in 80 ml of water and ultrasonicated for 15 min to avoid agglomeration and to allow better dispersion over g-C3N4 in the subsequent step. Then, 12 g of urea was added and dissolved in the NCQDs solution followed by mechanical stirring. Following that, a uniform darker beige solution was obtained and transferred into a beaker. The solution was then vapourised at 90 � C until a semi-dry solid was obtained. The resultant solid was transferred into a 50 ml porcelain crucible and heated at a ramping rate of 15 � C/ min at 550 � C for 2 h in a muffle furnace. After allowing the sample to cool to room temperature, the resultant solid was collected and ground into powder before it was stored. The sample was labelled as 1 wt% NCQD/g-C3N4 (Poly).

2.6. Photoelectrochemical measurements

2.4.3. Hydrothermal synthesis of g-C3N4/NCQDs photocatalyst composite 1 g of bulk g-C3N4 was dispersed in 90 ml of distilled water. The resulting solution was ultrasonicated for 30 min to exfoliate and delaminate the layered structure of g-C3N4. Next, a measured mass of bulk NCQDs was dispersed in 10 ml of distilled water. Ultrasonication was applied to the solution for 15 min before combining it with the gC3N4 solution. The resulting mixture was stirred vigorously at room temperature for 30 min to produce a uniform suspension. The mixture

Photoelectrochemical characterisation was conducted using an electrochemical workstation (CHI 6005E, Chenhua Instruments, China) in a quartz cell with a standard three-electrode system. A platinum plate and a saturated Ag/AgCl were used as the counter and 0.003 mg of catalyst was dispersed in C2H5OH for 10 min using an ultrasonicator and drop-casted onto 1 cm2 of FTO substrate. The film was dried in the oven to obtain the g-C3N4 and 1 wt% NCQD/g-C3N4 thin film electrodes. 1.0

Fig. 1. Schematic illustration of the hydrothermal route for NCQD/g-C3N4 composite synthesis. 3

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M Na2SO4 aqueous solution was used as an electrolyte. The Transient Photocurrent Response results were measured with the lamp on and off, under a constant applied bias of 0.35 V. Electrochemical Impedance Spectra (EIS) were measured over a frequency range of 0.1–105 Hz at an applied potential of 0.35 V vs. Ag/AgCl with an alternating current perturbation signal of 0.01 V under visible light irradiation (λ > 400 nm). Mott–Schottky plots were acquired over a potential range from 1.0 to 1.0 V.

2.7. Photocatalytic activity measurements The photocatalytic activities of the as-prepared hybrid photo­ catalysts were tested in the degradation of methylene blue (MB). In a typical run, 50 mg of sample was dispersed in 100 ml of MB solution (10 mg/L) in a glass beaker. The resulting mixture was stirred in the dark for 30 min to ensure establishment of an adsorption-desorption equilibrium in a photocatalytic reactor. The equilibrated solution was then irradi­ ated under LED lamp (4410R–18W) and 5 ml of aliquots were collected

Fig. 2. TEM images of (a) g-C3N4 nanosheets and (b) 1 wt% NCQD/g-C3N4 showing NCQs embedded as small black dots ranging from 2 to 10 nm on surface. FESEM images of 1 wt% NCQD/g-C3N4 under (c) low- and (d) high magnifications. (e) EDX analysis of 1 wt% NCQD/g-C3N4. (f) Elemental mapping for Carbon (C), Nitrogen (N) and Oxygen (O). NCQDs dissolved in water under the irradiation of (g) visible- and (h) UV-light. 4

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at a 20 min interval for 180 min. The collected sample was then centrifuged at 12,000 rpm for 3 min to remove the photocatalyst com­ posite. The supernatant was analysed in the UV–Vis spectrophotometer for liquids (Hach DR 6000) to record the maximum absorption band (664 nm) as a function of time. The kinetic rate constant of the MB degradation was calculated using Langmuir-Hinshelwood model as shown below (Tyagi et al., 2016): ln

Co ¼k�t C

(30.81%) respectively (see Inset of Fig. 2(e)). Besides that, the EDX elemental mapping conducted also validated the existence of C, N and O as depicted in Fig. 2(f). The element C was derived from the amorphous carbon in the g-C3N4 sheets and the use of citric acid during the synthesis of NCQDs. The N element observed from the analysis stemmed from the N-doped CQDs and the tri-s-triazine patterns of g-C3N4, while the low O content could be attributed to the acidic species (–COOH) in the NCQDs. EDX analysis showed a high content of Si, which was due to the prep­ aration of the sample for FESEM imaging. In the process, the photo­ catalysts were dispersed in ethanol and ultrasonicated for 5 min before being etched onto a Si/SiO2 wafer. As shown in Fig. 2(g), the NCQDs were highly dispersive in water and formed a light brown solution with no visible solid particles or agglomeration. When the NCQD solution was placed under the illumi­ nation of UV light, a strong blue fluorescence emission was observed as shown in Fig. 2(h). The emission of blue fluorescence under UV light is one of the unique properties of CQDs and similar results were reported in various studies where CQDs were synthesised following a hydrothermal route (Alam et al., 2015; Janus et al., 2019). The luminescence mech­ anism of the NCQD could be attributed to the surface functionalisation of the material as without it the emission of light would not take place (Sun et al., 2018). The blue luminescence was ascribed to the hydroxyl groups on the surface of NCQD, which was later confirmed using FTIR. This phenomenon of surface passivation could result in the reduction of radiative combination of e /hþ pairs due to trapped electrons in the surface states, leading to enhanced photocatalytic degradation rates. The XRD patterns of pure NCQD, g-C3N4 and NCQD/g-C3N4 with different mass loadings of NCQD are shown in Fig. 3(a). Two diffraction peaks at 13.2� (0.672 nm) and 27.4� (0.325 nm) were observed for gC3N4 and the hybrid composites, which corresponded to graphite (C) [PDF #01-089-8491]. The lower angle diffraction peak at 13.2� could be indexed to the (100) diffraction plane, which resulted from the in-plane structural packing of the tri-s-triazine units. On the other hand, the stronger diffraction peak at 27.4� corresponded to the (002) plane, which was closely related to the characteristic interplanar stacking structure of the conjugated aromatic system (Wang et al., 2018b). The XRD patterns of the NCQD/g-C3N4 hybrid composites were similar to that of pure g-C3N4, which suggested that there was no disruption to the structural identity of g-C3N4 when NCQDs were added. However, it should be noted that none of the characteristic peaks of NCQD was observed in the NCQD/g-C3N4 samples (see Fig. 3(b)), which could be due to the low mass loading of NCQD on the hybrid composite. Similar observations were also reported in other studies where CQDs were coupled with various semiconductor materials, such as MoS2 composites (Zhao et al., 2016) and g-C3N4/TiO2 (Pan et al., 2018) nano-heterojunctions. The chemical structures of pure NCQD, g-C3N4 and 1 wt% NCQD/gC3N4 were studied using FTIR analysis (Fig. 3(c)). As shown in the Figure, pure NCQD exhibited several peaks which represented oxygen­ ated functional groups, including 1043 cm 1 (-C-O-C-), 1541 cm 1 – O) and 1358 cm 1 (-COOH) (Ong et al., 2017; Wang et al., 2018a). (C– The hydrophilic property of these polar groups enhanced the stability and solubility of the NCQD, which could lead to improved electron transfer capabilities (Wang et al., 2017). For non-oxygenated groups, the peak at 1181 cm 1 was due to the -C–NH–C vibration while the broad – N vi­ peaks at 1341 cm 1 and 1655 cm 1 were caused by C–N and C– brations, respectively (Shen et al., 2017). In comparison to pure g-C3N4, there was no visible change to the absorption peaks when NCQD was incorporated. Hence, it could be confirmed that the chemical framework of the as-prepared composite photocatalyst was not altered when pure g-C3N4 was coupled with the NCQD through the hydrothermal route. In addition, the sharp peak at 800 cm 1 observed in these two samples belonged to the breathing modes of the tri-s-triazine units (Li et al., 2016). The number of -C-N heterocycles were clearly observed from 1200 cm 1 to 1620 cm 1, with peaks located at 1205 cm 1, 1308 cm 1, 1381 cm 1, 1535 cm 1 and 1610 cm 1 (Ong et al., 2017). The wider

(1)

Moreover, the photodegradation efficiency was calculated as well by using the equation below (Hu et al., 2018; Zhang et al., 2018): Degradation efficiency ¼

C

Co � 100% Co

(2)

where Co ¼ initial MB concentration, C ¼ MB concentration at irradia­ tion time, t, and k ¼ kinetic rate constant of photocatalyst composite. The photocatalytic rector was set-up as shown in Fig. S2 where 0.1 g of the as-prepared catalyst was mixed in 10 mg/L of MB solution and stirred under LED light irradiation. All experiments were repeated twice to ensure data accuracy and reproducibility. 2.8. Trapping experiments for radicals and holes The effects of various reactive species such as holes (hþ), hydroxyl (�OH) and superoxide (�O2 ) radicals on the degradation of MB were studied to elucidate the photocatalytic mechanism in the NCQD/g-C3N4 system. Scavenging reagents including triethanolamine (TEOA), ben­ zoquinone (BQ) and isopropyl alcohol (IPA) were employed in this study. The concentration of each scavenger in the reaction system was held constant at 10 mmol/L. The analysis procedure was identical to the photocatalytic degradation experiment discussed in Section 2.7. 3. Results and discussion 3.1. Characterisation of the photocatalysts TEM and FESEM were used to study the structures and morphologies of the as-synthesised photocatalysts. Fig. 2(a) shows the TEM image of pure g-C3N4, where two dimensional laminar platelet-like structures could be clearly seen. Besides that, pores were observed on its surface with diameters ranging from 16 to 33 nm. Following the addition of NCQDs, no changes to the surface morphology of g-C3N4 were observed (see Fig. 2(b)), which is consistent with those reported in literature (Ong et al., 2017; Wang et al., 2018b). Black dots with diameters ranging from 2 to 10 nm could be distinguished, which indicated the successful doping of NCQDs on g-C3N4. Fig. 2(c–d) show the FESEM images of the 1 wt% NCQD/g-C3N4 hybrid composite. Some degree of agglomeration could be seen, which could have occurred during the coupling process of NCQD and g-C3N4. Fig. 2(d) showed a clearer image of the crystallites of g-C3N4. In addition, the spatial networks and mesoporous structures of the composite could also be observed (circled in white in Fig. 2(d)). The incorporation of NCQDs was believed to have resulted in the formation of larger spatial networks, which was in line with another study, which showed that an increase in CQD loading led to enlarged macroscopic appearances and spatial networks in the composite sample (Li et al., 2012a; Wang et al., 2018a). Therefore, it can be confirmed that the hybrid composite of NCQD/g-C3N4 was successfully synthesised through the hydrothermal treatment as depicted in Fig. 1. The textural properties (surface area, pore diameter, pore volume) of g-C3N4 and 1 wt% NCQD/g-C3N4 are provided in the Supplementary Information (Fig. S1). EDX analysis was carried out for the 1 wt% NCQD/g-C3N4 sample to investigate its elemental composition. The data showed that the sample was composed of C, N, O and Si with the atomic and weight percentages of 31% (45.06%), 18.85% (23.49%), 0.58% (0.63%) and 49.57% 5

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Fig. 3. (a) XRD spectra of pure NCQD, g-C3N4 and NCQD/g-C3N4 composites with different NCQD mass loadings. (b) Comparison of XRD spectra between pure NCQD and 1 wt% NCQD/g-C3N4. (c) FTIR spectra of pure NCQD, pure g-C3N4 and 1 wt% NCQD/g-C3N4.

peak at 3200 cm 1 was due to the stretching of the –NH vibrational modes whereas the peak at 2918 cm 1 was attributed to the –OH stretching (Hu et al., 2018). These functional groups stemmed from the presence of adsorbed hydroxyl radicals and amino acids of the samples through the thermal polymerisation process of urea to g-C3N4. However, it should be noted that there was an obvious shift in the peaks from 2913 cm 1 to 2918 cm 1 in the g-C3N4 and 1 wt% NCQD/g-C3N4 samples. This observation resulted from the hydrophilic interaction of the hy­ droxyl group from the precursors of the as-prepared composite sample, which increased the interactions of the -C-OH bonds and reduced the length, thereby causing an increase in the interstitial spacing of the structure. The diffuse reflectance spectra of all prepared samples were recorded using a UV–Vis spectrophotometer (see Fig. 4(a)). The absorption edge of pure g-C3N4 was observed to be at approximately 420 nm. As the loading of NCQD increased, the absorption edge of the hybrid composite was found to be slightly red-shifted to 430 nm. This confirmed that the coupling of NCQDs led to an improvement in the light absorption capability of g-C3N4 (Zhang et al., 2017). The inset of Fig. 4(a) also showed the darkening of the composite’s colour from light beige to brown as the mass loading of NCQDs increased. In addition, the optical band gaps, Eg of all samples were investigated using the Tauc equation. A graph of ahv0:5 against photon energy, hv (also known as the Tauc plot) was plotted in Fig. 4(b). Following the addition of NCQD on g-C3N4, the band gap energy was shown to have reduced from 2.5 to 2.2 eV. This indicated that the intrinsic charge generation of the NCQD/g-C3N4 photocatalyst composite heterojunction arises from the injection of excited electrons from the valence band (VB) to the conduction band (CB) of g-C3N4. Owing to the insignificant change in the composites’

absorption edge, it can be deduced that the NCQD served merely as an electron transfer conduit. Furthermore, the Fermi level of NCQD ( 0.3 eV) is significantly lower than g-C3N4 ( 0.61 eV), thus proving that the electron transfer from g-C3N4 to the electron conduit NCQD can occur easily (Wang et al., 2018b). To further investigate the light harvesting properties of the asprepared photocatalysts, pure NCQD was tested under the irradiation of different wavelengths, as shown in Fig. 4(c). As the excitation wavelength increased from 300 to 400 nm, an obvious redshift with gradually reduced intensity of emission peaks were observed. According to Qin and Zeng (2017), this phenomenon is known as radiative recombination where different emissions can be initiated at various excitation wavelengths. Since the CQDs were doped with N, the latter resulted in the trapping of more electrons in the surface states, leading to reduced charge carrier recombination. Consistent with other studies published in literature, the NCQD was found to possess down-converted photoluminescence (Qin and Zeng, 2017; Zhang et al., 2017). To date, the exact mechanism underlying the PL emission properties of NCQDs has yet to be fully understood. However, it has been widely observed in several studies that the different emission wavelengths were associated with the distribution of particle size as well as the surface energy traps (Shen et al., 2017; Tyagi et al., 2016). It should also be noted that the PL spectra of the NCQD showed two emission peaks for each excitation wavelength. According to Chava et al. (2017), several different func­ tional groups attached to the surface of the CQD possessed different states of energy levels which led to a series of emissive traps and hence two emission peaks. Therefore, the addition of NCQD to pristine g-C3N4 could lead to significantly enhanced photocatalytic performances. This was further validated by performing a second PL test for 1 wt% 6

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Fig. 4. (a) UV–Vis diffusion reflectance spectra and (b) the corresponding Tauc plot of pure g-C3N4, NCQD, and NCQD/g-C3N4 composites with different NCQD mass loadings. (c) Down-converted photoluminescence of NCQD with different excitation wavelengths. (d) PL spectra of pure g-C3N4 and 1 wt% NCQD/g-C3N4. Inset of Fig. 4(a) shows the changes in the composites’ colour as the mass loading of NCQD increased. Inset of Fig. 4(b) summarises the band gap energies of all samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

NCQD/g-C3N4 and comparing it with bare g-C3N4 (see Fig. 4(d)). From the Figure, the hybrid composite of 1 wt% NCQD/g-C3N4 showed a reduction in peak intensity as compared to bare g-C3N4. This signified that the doping of NCQD improved electron transfer properties, thus leading to reduced electron-hole recombination which is in good agreement with previous studies (Ong et al., 2017; Wang et al., 2017).

From Fig. 5(c), the mechanically mixed sample (1 wt% NCQD/gC3N4 (Mech)) exhibited a rate constant, k of 1.478 � 10 3 min 1, which was just 1.12 times higher than that of pure g-C3N4. Therefore, it can be deduced that mechanically combined NCQD/g-C3N4 showed negligible improvement in the photocatalytic activity, which could be caused by loosely connected heterojunctions as reported elsewhere (Ong et al., 2017). Furthermore, there was no obvious change in the colour of the 1 wt% NCQD/g-C3N4 (Mech) powder, which suggested that the NCQDs had not been successfully impregnated onto the g-C3N4 sheets. In com­ parison to mechanically mixed composites, the NCQD/g-C3N4 hybrid prepared via the thermal polymerisation route (1 wt% NCQD/g-C3N4 (Poly)) demonstrated an increase in photocatalytic activity. The sample showed a rate constant of 2.445 � 10 3 min 1, which was 1.86 folds higher compared to pure g-C3N4. In the thermal polymerisation route, both the NCQD solution and urea were ultrasonicated and stirred at room temperature. The resulting mixture was then vaporised at 90 � C to form a semi-dry paste before being heated in a muffle furnace at 550 � C for 2 h. According to a previous study, it was reported that when CQDs were heated at temperatures above 450 � C, the sample would undergo about 50% weight loss due to the loss of adsorbed water and decar­ boxylation of functional groups on its surface. Moreover, the heating process was also reported to caused serious aggregation of the CQDs (Guo et al., 2015). Therefore, the 1 wt% NCQD/g-C3N4 (Poly) hybrid composite prepared through this method was expected to follow a similar trend, which resulted in its low photodegradation rate and effi­ ciency. Among the synthesis procedures studied, the hydrothermal route was found to be the most effective method to perform the hybridisation of NCQD/g-C3N4. As can be seen from Fig. 5(c), the hybrid sample prepared via this method displayed a significantly higher rate constant of 5.061 � 10 3 min 1, which was 3.85 times higher than that of pure

3.2. Evaluation of photocatalytic activities toward the degradation of MB The photocatalytic efficiencies of the as-synthesised samples were studied in the degradation of methylene blue (MB) under LED light irradiation. In the present work, the (1) best method to hybridise NCQD and g-C3N4 and (2) the optimal mass loading of NCQD on the hybrid composite were systematically investigated. Fig. 5 summarises the photocatalytic performances of all samples toward MB degradation after 3 h of reaction. Control experiments were also carried out in the absence of the photocatalyst and light irradiation. In both cases, no appreciable dye degradation was observed, thus confirming the indispensability of the photocatalyst and the excitation source. 3.2.1. Effect of synthesising method towards photocatalytic activity Three different synthesis methods were studied to perform the hybridisation of NCQD and g-C3N4: (1) Mechanical mixing, (2) Thermal polymerisation and (3) Hydrothermal treatment. As can be seen from Fig. 5(c), the degradation efficiencies of 1 wt% NCQD/g-C3N4 prepared through mechanical mixing, thermal polymerisation and hydrothermal treatment were 21.01%, 30.99% and 54.60% respectively. Based on the results obtained, it is evident that 1 wt% NCQD/g-C3N4 displayed different efficiencies toward MB degradation despite having the same mass loading of NCQDs. 7

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Fig. 5. (a) Absorbance curve against time for 1 wt% NCQD/g-C3N4. (b) Linear photocatalytic MB degradation over pure g-C3N4 and NCQD/g-C3N4 with different NCQD mass loading. (c) Kinetic rate constant and degradation efficiency of all samples toward MB degradation under LED light irradiation. Inset of Fig. 5(a) shows the gradual change in the colour of MB with irradiation time. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

g-C3N4. Thus, it was concluded from this work that the hydrothermal route was most effective in preparing the NCQD/g-C3N4. This method was hence employed in the preparation of the hybrid composites for the subsequent study, where the effect of NCQD mass loading on the pho­ tocatalytic performance of the hybrid composite was investigated.

masking of the surface pores, thus impeding the absorption of visible light to induce charge generation. Moreover, excess NCQDs in the composite material could also have competed with g-C3N4 for the ab­ sorption of light, leading to lower photocatalytic performances. As shown in Fig. 5, the optimal mass loading of NCQDs on g-C3N4 is 1 wt%, with the assumption that the reaction undergoes pseudo-first order ki­ netics. The enhancement in photocatalytic efficiency towards MB degradation could be associated to the reduction in electron-hole recombination due to improved electron migration from the NCQDs to g-C3N4. This clearly demonstrated the importance of determining the optimum loading in heterojunction systems to harness their full poten­ tial in photocatalytic usage such as pollutant degradation, H2 generation and CO2 reduction. Fig. 5(a) shows the absorbance curve of the MB solution under LED light irradiation in the presence of 1 wt% NCQD/g-C3N4. The results showed that the absorbance curve at a maximum intensity of 664 nm decreased as the irradiation time increased. This was further validated by the gradual decolourisation of MB with irradiation time, as shown in the inset of Fig. 5(a). Initially (t ¼ 0 min), the solution was dark blue in colour but it gradually faded into a pale blue solution by the end of the reaction (t ¼ 180 min). These results further attested the effectiveness of the as-synthesised hybrid composite toward dye degradation. equation (S1–S11) describing the suggested photodegradation mechanism over NCQD/g-C3N4 are listed in the Supplementary Information (Houas et al.,

3.2.2. Effect of NCQD mass loading towards photocatalytic activity The mass loading of NCQD in the hybrid composite was varied (0.5, 1, 2 and 5 wt%) to study its effect on the photocatalytic performance toward MB degradation under LED light irradiation. Based on the results obtained, the photodegradation efficiencies of the as-synthesised sam­ ples demonstrated the following trend: 0.5 wt% NCQD/g-C3N4 < 5 wt% NCQD/g-C3N4 < 2 wt% NCQD/g-C3N4 < 1 wt% NCQD/g-C3N4. Among all samples, 0.5 wt% NCQD/g-C3N4 exhibited the lowest photocatalytic performance, achieving a rate constant of just 1.15 � 10 3 min 1. The highest photodegradation efficiency was achieved over 1 wt% NCQD/gC3N4, which displayed a rate constant of 5.061 � 10 3 min 1. It was observed that a further increase in NCQD mass loading to 2 and 5 wt% resulted in a reduction in the photocatalytic efficiency. Similar obser­ vations were also reported in other papers published in literature, where an excessively high NCQD loading could cause a shielding effect and weaken the absorption efficiency of the nanocomposite (Li et al., 2016; Ong et al., 2017; Pan et al., 2014; Zhang et al., 2016). Likewise, exces­ sive loading of NCQDs on the surface of g-C3N4 could contribute to the 8

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2001; Umrao et al., 2015).

between capacitance space charge region and applied potential. From Fig. 6(c), a positive tangent slope was obtained for both samples, which implied that they exhibited the characteristics of an n-type semi­ conductor which was in agreement with other studies (Bu, 2014; Zhang et al., 2010). Next, the flat band potential (Efb) of the photocatalysts were obtained by extrapolating the curve to the x-intercept where C 2 ¼ 0. It can be seen from Fig. 6(c) that the flat band potentials of g-C3N4 and 1 wt% NCQD/g-C3N4 were approximately 0.5 eV and 0.42 eV vs. Ag/AgCl, respectively. It is evident that there was a positive shift in the flat band potential following the incorporation of NCQD with g-C3N4. The equation below was used to convert the flat band potentials to vs NHE, where EAgCl ¼ 0.197 eV, and the pH was obtained to be around 6.8 (An et al., 2014).

3.3. Photoelectrochemical characterisation of the NCQD/g-C3N4 hybrid composite Transient photocurrent response experiments were carried out to further elucidate the photoinduced charge transfer mechanism. Fig. 6(a) depicts the transient photocurrent response curves of pure g-C3N4 and 1 wt% NCQD/g-C3N4 with multiple on-off visible light irradiation. The current density measured for pure g-C3N4 was relatively low, which could be attributed to weak electron separation efficiency. A marked improvement in current density was observed for the 1 wt% NCQD/gC3N4 composite, with a 6-fold enhancement compared to bare g-C3N4. In the hybrid sample, electron migration took place from g-C3N4 to the NCQDs, effectively hindering the recombination of electron-hole pairs (Zhang et al., 2017). This observation was in good agreement with the PL test, which was discussed earlier in Section 3.1 (Fig. 4(d)). In addi­ tion, electrochemical impedance spectra (EIS) was also conducted to study the interfacial charge transfer properties of the as-developed photocatalysts. In Nyquist plots, the magnitude of the semi-circular curve gives a measurement of the charge transfer resistance of the electrode surface, where a smaller arc radius indicates lower resistance (Leelavathi et al., 2014). As shown in Fig. 6(b), the arc resistance (-Z”) of pure g-C3N4 was relatively higher compared to the hybrid composite, which indicated that it possessed poor charge transfer capacity. A lower resistance was observed for the 1 wt% NCQD/g-C3N4 composite, which implied that the incorporation of NCQD contributed to the effective separation of electron-hole pairs. The faster interfacial charge migration at the heterojunction once again justified that NCQDs could serve as an effective electron storage and conduit to improve the optical properties of g-C3N4. Mott-Schottky analysis was conducted to investigate the relationship

Efbðvs NHEÞ ¼ EfbðpH¼0 vs Ag=AgClÞ þ EðAgClÞ þ 0:059pH

(3)

Using Equation (3), Efbðvs NHEÞ for g-C3N4 and 1 wt% NCQD/g-C3N4 were calculated to be 0.1 eV and 0.18 eV respectively. Since the con­ duction band minimum (Ecb) for n-type semiconductors are 0.3 eV more negative than the Efb, they were calculated as 0.2 eV and 0.12 eV for g-C3N4 and 1 wt% NCQD/g-C3N4 vs NHE, respectively. The valence band potential, Evb of both materials were then calculated using Equa­ tion (4). A summary of the results obtained was tabulated in Fig. 6(d). EVB ¼ ECB þ Eg

(4)

On the other hand, by using the x-intercept of the tangent to the Mott-Schottky plot, the carrier charge density, ND was calculated using Equation (5). 0 1 ND ¼

C 2 B B dE C qεε0 @d C12 A

(5)

Fig. 6. (a) Transient photocurrent response, (b) electrochemical impedance spectra and (c) Mott-Schottky plots of g-C3N4 and 1 wt% NCQD/g-C3N4. (d) Energy band potentials of g-C3N4 and 1 wt% NCQD/g-C3N4. 9

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Equation (2). As shown in Fig. 8(b), the degradation efficiency decreased from 54.60% to 26.6%, 49.7% and 34.9% respectively, which verified that hþ, �O2 and �OH were active species in the MB degradation process. It is evident that the inhibitory effect of TEOA was the highest, indicating that hþ played the most significant role in the photocatalytic reaction. In particular, the valence band potential of the photocatalyst is insufficient to oxidise water to generate hydroxyl radicals �OH (H2O/ � OH ¼ þ2.33 eV) which is required in the degradation of MBþ. It can however, be confirmed that the generation of �OH proceeds in a pathway where elementary oxygen is reduced to form �O2 and subsequently �OH via the pathway described in Equations S5-S8. Based on the results, hþ played a major role in the photocatalytic process because it was responsible for the generation of Hþ, OH and �OH (see Equation (S2 – S4)). Since the potential of hþ on the surface of NCQD/g-C3N4 was enough to oxidise hydroxyl radicals in water to form hydroxyl radicals (OH /OH� ¼ þ1.99 eV) (Wang et al., 2017), these reactive species took part in the degradation of MB ions (MBþ) to produce the degradation products, as described in Equation (S9 – S11). Based on the scavenging test results, the photocatalytic mechanism underlying the degradation of MB over the as-prepared 1 wt% NCQD/gC3N4 composite was proposed as shown in Fig. 9. Upon light irradiation, electrons were excited from the VB to the CB of g-C3N4, leaving behind holes on the VB. Since the energy level of the NCQD is lower than the CB of the g-C3N4 ( 0.20 eV) (Wang et al., 2017), the photoinduced elec­ trons will rapidly migrate from the surface of g-C3N4 to the NCQD. Moreover, owing to its high electron storage capacitance and electron conductivity, the electrons could easily be trapped in the NCQD (Hu et al., 2018; Ong et al., 2017; Pan et al., 2018). This enabled the efficient separation of electron-hole pairs as confirmed by EIS analysis, to hinder charge recombination and improve the photocatalytic performance. Moreover, the π π interactions on the surface of the composite could facilitate the adsorption of MB, leading to improved photodegradation efficiencies. Next, the electrons from the CB reacted with molecular oxygen since the CB position of the composite is more negative than the O2/�O2 potential, to form superoxide anion radicals (�O2 ), which in turn took part in the oxidation of MB (Wang et al., 2018b). At the same time, the hþ in the VB of the g-C3N4 will migrate to the surface, consequently reacting with the hydroxyl species to form active hydroxyl radicals (�OH). This is plausible as the VB position of the composite (þ2.23 eV) is more positive than the OH /OH� potential (þ1.99 eV). Finally, the reactive species (�O2 , OH�, hþ) on the surface of the as-synthesised photocatalyst composite will react with MB molecule to form the degradation products.

where q ¼ electronic charge ¼ 1.602 � 10 19 C; ε ¼ electrostatic con­ stant (5.25 for g-C3N4); ε0 ¼ permittivity in vacuum ¼ 8.85 � 1014 Fcm 2. The second term in the equation denotes 1/slope of the MottSchottky plot, hence the Nd of g-C3N4 and 1 wt% NCQD/g-C3N4 were calculated as 1.44 � 1021 cm 3 and 1.29 � 1021 cm 3 respectively. The results indicated that an improved spatial separation of e /hþ pairs could be obtained by doping NCQD on g-C3N4. From this, it can be concluded that the increase in photocatalytic degradation of MB could be attributed to the enhanced charge mobility and electrical conduc­ tivity of the composite which was in well agreement with the EIS and photocurrent tests. 3.4. Recyclability and stability tests As is well understood, to be able to put the as-synthesised photo­ catalyst to its full potential, the stability and durability play a crucial role in determining its practicality. To evaluate these aspects, recycla­ bility tests for the optimal sample (1 wt% NCQD/g-C3N4) were con­ ducted under the same conditions as before. However, the difference is that after each cycle (180 min), the 1 wt% NCQD/g-C3N4 composite was removed from the solution through vacuum filtration, followed by ovendrying to regenerate the photocatalyst. To ensure uniformity for each cycle, the loss of catalyst was compensated by a reduced volume of MB solution equivalent to 0.1 g catalyst: 100 mL 10 mg/L MB solution. The results of the recyclability test are shown in Fig. 7(a), where the degradation efficiency of the recycled catalyst reduced from 54% to 51.2% and 49.3% after the second and third cycles, respectively. The results suggested that the optimal sample exhibited excellent stability and durability towards the degradation of MB, retaining an activity of 91.2% after three successive tests. Moreover, the chemical structure of the recycled 1 wt% NCQD/g-C3N4 exhibited no obvious change when compared to a fresh sample, as evidenced in their respective FTIR spectrum (see Fig. 7(b)). These observations could be attributed to the π π stacking interactions between the NCQDs and g-C3N4 sheets, which led to the excellent stability and durability of the 1 wt% NCQD/gC3N4 photocatalyst (Wang et al., 2018b). 3.5. Scavenging tests and proposed photocatalytic mechanism over NCQD/g-C3N4 Scavenging tests were conducted to investigate the mechanism and reactive species (RS) behind the degradation of MB using the asprepared photocatalysts. The chemicals used for the scavenging tests were triethanolamime (TEOA, 10 mmol/L), benzoquinone (BQ, 10 mmol/L) and isopropyl alcohol (IPA, 10 mmol/L) as hþ, �O2 and �OH scavengers, respectively. A linear degradation curve of ln C/Co vs time was plotted in Fig. 8(a) to obtain the kinetic rate constant, k using Equation (1), while the degradation efficiency was calculated using

4. Conclusion In summary, NCQDs were successfully incorporated onto g-C3N4 to produce highly dispersed NCQD/g-C3N4 hybrid composites for the degradation of MB under LED light irradiation. The successful

Fig. 7. (a) Stability test for the recycled 1 wt% NCQD/g-C3N4. (b) FTIR for recycled and fresh 1 wt% NCQD/g-C3N4 sample. 10

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Fig. 8. (a) Linear degradation curve against irradiation time for scavenging tests, (b) Summary of MB degradation efficiencies and kinetic rate constants over 1 wt% NCQD/g-C3N4 in the scavenging tests.

Fig. 9. Schematic of photocatalytic mechanism of MB degradation over NCQD/g-C3N4.

11

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modification of pristine g-C3N4 was verified by characterization studies such as TEM, FESEM, EDX, XRD and FTIR to observe the change in its structure and morphology. UV–Vis diffuse reflectance analysis also showed an improvement in the composite’s optical properties, where the photocatalyst exhibited higher absorption intensity with NCQD mass loading. In the present work, three different synthesis methods were studied to perform the hybridization of NCQD and g-C3N4. Among them, the hydrothermal route was deduced to be the most effective, as the resulting composite showed the highest photocatalytic efficiency toward MB degradation. Besides determining the most effective hybridization method, the mass loading of NCQD on the hybrid composite was also systematically studied. 1 wt% NCQD/g-C3N4 was found to be the opti­ mum photocatalyst, giving the highest degradation efficiency of 54.6% and a first order kinetic rate constant of k ¼ 5.061 � 10 3 min 1, which was 3.85 folds higher compared to pristine, un-doped g-C3N4. The enhanced photocatalytic properties of the optimum composite were believed to stem from the synergistic effects of (1) reduced electron-hole recombination rates, (2) optimised heterojunction interface between NCQD and g-C3N4, and (3) Increased charge density to facilitate rapid charge migration. The aforementioned phenomena were elucidated through photoelectrochemical measurements, which included transient photocurrent response, EIS Nyquist and Mott-Schottky plots. The incorporation of NCQD improved the optical properties of the pristine urea derived g-C3N4 by promoting effective charge transfer as well as increasing the mobility of charges by acting as an electron storage to promote the separation of electron-hole pairs owing to its high electron storage capacitance and electron conductivity. In addition, the reactive species responsible for the photodegradation process were also investi­ gated in this study. It was concluded that all three species (�O2 , OH�, hþ) were involved in the reaction as reduced degradation efficiencies were observed in their absence. Based on these results, a plausible photo­ catalytic mechanism for MB degradation over 1 wt% NCQD/g-C3N4 was proposed. Finally, the hybrid composite was also shown to exhibit excellent durability and stability through recyclability tests, where the photocatalyst retained 91.2% of its original efficiency with no disruption to its chemical structure. In conclusion, the present study has success­ fully shed light on the development of efficient NCQD/g-C3N4 hybrid composites toward pollutant degradation under visible light irradiation. The as-synthesised metal-free photocatalysts are expected to be devel­ oped as a robust means to address various energy- and environmentalrelated issues, which are two of the biggest challenges faced by mod­ ern society today.

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Declaration of competing interest None. Acknowledgement This work was funded by the Ministry of Higher Education (MOHE) Malaysia under the Fundamental-Research Grant Scheme (FRGS) (Ref no: FRGS/1/2018/TK02/HWUM/03/2). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2019.109936. Credit author statement Ru Xuan Seng: Methodology, investigation, Writing – Original Draft, Lling-Lling Tan: Conceptualization, visualization, supervision, funding acquisition, W.P. Cathie Lee: Formal analysis, Wee-Jun Ong: Writing – Review & Editing, Siang-Piao Chai: Resources, Writing – Review & Editing. 12

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