Synergistic effect of citric acid and carbon dots modi ﬁ ed g-C 3 N 4 for enhancing photocatalytic reduction of Cr(VI)

Carbon dot (CD)-modi ﬁ ed graphitic carbon nitride (g-C 3 N 4 ) photocatalysts were synthesized through a one-step homogeneous thermal pyrolysis. The synergetic effect of citric acid (Cit) and g-C 3 N 4 /CDs for high-performance visible light Cr(VI) photocatalytic reduction had been investigated. Cit was not only acted as a hole scavenger, but might also form surface charge transfer complexes (CTC) with g-C 3 N 4 which delivered electrons on the Highest Occupied Molecular Orbital (HOMO) of Cit to the conduction band (CB) of g-C 3 N 4 . CDs decorated on g-C 3 N 4 could provide channels for the preferential transfer of electrons on CTC to the CB of g-C 3 N 4 as well as improved separation of the charge carriers. Owing to these synergistic effects, g-C 3 N 4 /CDs displayed much higher photocatalytic performance for the reduction of Cr(VI), which was 1.89 times higher than g-C 3 N 4 . Moreover, the synergetic photocatalytic reduction mechanisms of aqueous Cr(VI) were proposed to elucidate the active species formation and photogenerated electron transfer. The results suggested that the in situ generated hydrogen peroxide (H 2 O 2 ) dominated the reduction of Cr(VI). The addition of Cit could trigger the in situ generation of H 2 O 2 and the decorated CDs further enhanced the reaction. This work demonstrated the role of widely existed Cit on the photocatalytic reduction of Cr(VI) in natural aquatic environment.


GRAPHICAL ABSTRACT INTRODUCTION
Chromium contamination in aquatic ecosystems is a critical issue owing to natural processes and the escalating worldwide use in many industrial and manufacturing processes, such as mining, electroplating, leather tanning, and pigmentation (Patnaik et al. ). Generally, chromium mainly exists as nontoxic trivalent chromium (Cr(III)) and highly toxic hexavalent chromium (Cr(VI)) in the aqueous environment (Rong et al. ). Cr(III) is a necessary trace element for human beings and has low solubility and toxicity in the environment (He et al. ). Further, Cr(VI) is a highly toxic contaminant that can greatly threaten human health (Xiao et al. ). Therefore, the reduction of Cr(VI) to Cr(III) is an effective way for the removal of Cr(VI) in aqueous solutions (Huang et al. ).
In recent years, the photocatalytic reduction technique, especially using visible light-driven catalysis, has drawn intensive attention due to its high efficiency, low cost, and nontoxicity (He et al. ). Various types of photocatalysts have been developed for the photocatalytic reduction of Cr(VI) under visible light irradiation. Among them, g-C 3 N 4 has been widely studied as a metal-free semiconductor photocatalyst in the field of environmental photocatalysis due to its facile preparation, nontoxicity, low cost, and high thermal and chemical stability (Chen et al. ). However, the efficiency of photocatalytic reduction of aqueous Cr(VI) over g-C 3 N 4 is poor in practical applications due to its low surface area, high recombination rate of the photogenerated charge carriers and bulk-layered structure, which limits the surface migration of photogenerated carriers and increases the mass transfer resistance in the photocatalytic reaction it has been proven that decorating g-C 3 N 4 with CDs can efficiently facilitate the separation of photoinduced electron holes and could improve the photoelectrochemical performance (Wang et al. ). Recently, CD-decorated g-C 3 N 4 has been employed for an efficient photocatalyst for H 2 evolution (Wang et al. ), the degradation of dyes (Zhang et al. ), and PPCPs (pharmaceutical and personal care products) (Xie et al. ). However, there are few reports on CD-doped g-C 3 N 4 for highly efficient photocatalytic reduction of aqueous Cr(VI).
Carboxylic acid (CA) is a type of important natural organic matter. In the process of microbial degradation of root secretions and natural organic matter, CA is mainly released as a reaction intermediate or an end product ( Jiang et al. ). CA has one or more carboxylic groups, such as malic acid, citric acid and formic acid (Gu et al. ; Jiang et al. ). It is found that the reactions of direct electron transfer from CAs to Cr(VI) are very slow, so that the half-lives of these reactions are very long. However, the reactions for Cr(VI) reduction can be catalytically facilitated with the addition of surface-bound/dissolved metals, such as TiO 2 (Wang et al. ), Fe(II)/Fe(III), and Mn(II) (Mu et al. ). In such processes, CA not only serves as electron donors but also accelerates the electron transfer between the weak electron donors and Cr(VI) (Sun et al. ). Nevertheless, only a few studies have reported the synergetic performance and mechanism in the presence of CA in the g-C 3 N 4 photocatalytic system.
In this study, g-C 3 N 4 /CD composites were successfully fabricated via a simple precursor pretreatment strategy using dicyandiamide co-pretreated with citric acid (Cit) as the raw material (Qu et al. ). The efficiency of the g-C 3 N 4 /CD composite on the photocatalytic reduction of Cr(VI) was quantified, and the effects of CDs were also evaluated. Moreover, Cit, which is a common CA, was induced as a green sacrificial agent in the g-C 3 N 4 photocatalytic system.
The roles of the citric acid were studied and the synergetic mechanism in the presence of the citric acid of enhanced photocatalytic activity was deduced. The study shows the potential for the remediation of both Cr(VI)-bearing effluents and natural Cr(VI)-contaminated waters under solar light.
Preparation of catalysts g-C 3 N 4 was prepared by directly heating dicyandiamide (Ong et al. ). Typically, 20 g of dicyandiamide was added to an alumina crucible with a cover and then heated to 550 C at the heating rate of 5 C per minute in a muffle furnace. After heating for 3 h, the sample was allowed to cool down to room temperature, and the final product was obtained.
The g-C 3 N 4 /CD composites were prepared by a one-step homogeneous thermal method (Qu et al. ). In typical synthesis, 20 g of dicyandiamide and a certain quality of citric acid were added to an alumina crucible with a cover and then heated to 550 C at the heating rate of 5 C per minute in a muffle furnace. After heating for 3 h, the sample was allowed to cool down to room temperature, and the final product was obtained. The g-C 3 N 4 /CD composites were prepared with different amounts of CDs (0.005, 0.05, 0.075, and 0.1 wt%) and were denoted to be CNC1-4, respectively.
The pure g-C 3 N 4 sample was simply denoted as CN.

Experimental procedures
The photocatalytic activities of the synthesized catalysts were diffraction peaks at 27.5 and 13.0 were observed from the CNC photocatalyst XRD pattern which were well indexed to the pure g-C 3 N 4 (JCPDS Card No.87-1526). However, the intensity of the (002) peak of CNCs was obviously higher than that of CN. These results indicate that the crystal structure of CN was well preserved (Jiang et al. ) and the crystal structure of CNCs tended to be more stable. It is noted that no diffraction peaks of CDs can be observed in the CNCs due to the low content of CDs.   It can be seen that CN exhibited a strong emission peak at around 450 nm, which suggested the rapid recombination of photoinduced electron-hole (e À -h þ ) pairs. After the introduction of CDs, the intensity of the emission peak was significantly suppressed, suggesting that the separation of h þ and e À was improved by CDs modification. This further confirmed the role of CDs as an electron acceptor. The results suggest that CDs can improve the utilization of visible light and facilitate the separation of electron-hole (e À -h þ ) pairs, which would benefit the photocatalytic performance of the CNC composites under visible light.

Photoeletrochemical measurements
Electrochemical experimentation was conducted to further investigate the photoinduced charge transfer and separation behavior. As shown in Figure 5

Photocatalytic activity
The photocatalytic activity of the samples was evaluated by the degradation of Cr(VI) in the presence of citric acid under visible light irradiation. As shown in Figure 6(a), dark adsorption-desorption equilibrium between Cr(VI) and photocatalysts was obtained within 30 min, yet almost no obvious adsorption was observed. The poor adsorption capacity of CN or CNCs for Cr(VI) might be due to the weak electrostatic attraction between the anionic chromate species (HCrO 4 À and/or Cr 2 O 7 2À ) and the negative charge on the surface of CNCs at pH 3. It can be clearly seen that most of the CNCs showed higher photocatalyitic activity than that of pure CN and the photocatalytic activity of CNCs increased first and then decreased with an increase in the content of CDs. The zero-order rate constants k of CNCs were calculated to be 0.0057, 0.006, 0.0108, 0.0089 and 0.004 min À1 , respectively (Figure 6(b)). In addition, CNC2 showed the highest photocatalytic reduction efficiency of Cr(VI), as the zero-order rate constant (k) for CNC2 was calculated to be 0.0108 min À1 , which was 1.89 times higher than that of CN. mg L À1 ) were introduced to capture · OH, · O 2 À , e À , and H 2 O 2 , respectively. In addition, nitrogen gas was introduced to remove dissolved oxygen gas in the reaction solution to weaken the role of O 2 . As shown in Figure 7(a), the efficiency of Cr(VI) reduction was significantly decreased by adding CAT and Na 2 S 2 O 8 . Meanwhile, the Cr(VI) reduction efficiency decreased to some extent when · OH and · O 2 À were trapped. The k values for Cr(VI) reduction decreased from 0.0108 to 0.0068, 0.0079, 0.0051, and 0.0025 min À1 by adding TBA, BQ, Na 2 S 2 O 8 , and CAT, respectively (Figure 7(b)). On the contrary, the Cr(VI) removal efficiency was slightly enhanced when the solution was bubbling N 2 (k ¼ 0.0121 min À1 ), attributed to the decreased electron transport to O 2 , highlighting the role of e À in this system.
In sum, all of the active radicals mentioned above played an important role in the photocatalytic reduction of Cr(VI), especially for the H 2 O 2 and e À .
As a dominant active oxidative species in the Cr(VI) photoreduction system, the concentration of hydrogen spin-trapping EPR spectra for DMPO À · OH in water and DMPO À · O 2 À in methanol solution and (d) DMPO spin-trapping EPR spectra for DMPO À · O 2 À in DMSO solution.
No significant H 2 O 2 was detected in pure CNC2 or CN systems. About 0.14 mM of H 2 O 2 was generated in the CNC2- Firstly, photoexcited semiconductors produced e À and h þ (Equation (1) Figure 7(c) and 7(d) (Buettner ) showed signals of · O 2 À in the g-C 3 N 4 and g-C 3 N 4 /CD suspensions, confirming the oxygen reduction reaction for the production of H 2 O 2 . While in the CNC2-Cit system, the production of H 2 O 2 was not completely suppressed under the N 2 atmosphere (Figure 8(b)). Moreover, it can be seen in Figure 7(a) that the generation of H 2 O 2 played a more important role than e À and · O 2 À . All the results demonstrated that the oxygen reduction reaction was not the only route for H 2 O 2 generation and the water oxidation reaction might also exist simultaneously in this photocatalytic system. The poor production of H 2 O 2 in the CNC2 or CN systems might be due to the fast charge recombination caused intrinsically by the π-π-conjugated electronic system and the However, as a common electron donor, the addition of The interaction between H 2 O 2 , Cr(VI), and citric acid was also investigated by adding 0.14 mM H 2 O 2 instead of CNCs under visible light. As displayed in Figure 10( Figure S5, the EPR test results and the reaction activity diagram can correspond. In the reaction between Cr(VI) and H 2 O 2 , Cr(VI) was reduced to multi-peroxochromate(V) by H 2 O 2 (Equations (8) and (9)) and the formed peroxochromate(V) species spontaneously re-converted to Cr(VI) via the disproportionation reaction or was directly reduced to Cr(III) (Equations (10) and (11) tary Material S10). It was found that · OH can be generated from the interaction between Cr(VI) and H 2 O 2 , but · OH disappeared after the addition of citric acid (Wang et al. , ). This indicates that the addition of citric acid might inhibit the generation of · OH (Equation (10)) and promote the direct reduction of Cr(V) to Cr(III) (Equation (11) reduction by H 2 O 2 , which could be reduced to H 2 O 2 by e À to further promote the reaction. [ In consideration of the above evidence, the mechanism of photocatalytic Cr(VI) reduction over CNCs in the presence of citric acid is proposed, as illustrated in Figure 11.

Reusability
The photocatalytic stability is very important for practical applications. Hence, the cycling stability of CNC2 was measured. As seen in Figure 12, CNC2 exhibited inappreciable loss of photocatalytic activity after four-cycle reaction processes. Furthermore, the CNC2 before and after the reaction was tested by XPS (Liu et al. ). It was found that the ratio of elements in CNC2 changes before and after the reaction, as shown in Supplementary Material,

CONCLUSIONS
In summary, a novel visible light-induced, CD-decorated g-C 3 N 4 catalyst has been successfully prepared. CD modification greatly enhances the photocatalytic activity of g-C 3 N 4 , while an excess amount of CDs blocks the light adsorption. Therefore, CNC2 exhibits the best photocatalytic performance, which is 1.89 times higher than g-C 3 N 4 based on the zero-order rate constant (k) for Cr(VI) photoreduction in the presence of citric acid. The impregnation of CDs into g-C 3 N 4 not only broadens the absorption spectrum of g-C 3 N 4 , but also acts as charge carriers for photogenerated electrons (e À ). Moreover, it is found that the generation of H 2 O 2 is a crucial step in the photocataly-