Direct Exfoliation of gC 3 N 4 with Addition of NaOH for Enhanced Photocatalytic Cr ( VI ) Reduction

A simple, effective and environmental-friendly method was adopted for enhancing the photocatalytic activity of g-C3N4 in the reduction of aqueous Cr(VI) under visible-light irradiation. The enhancement was achieved via treatment of g-C3N4 in organic solvent with addition of NaOH particles by ultrasonic process for two hours. The results demonstrated that the treated g-C3N4 exhibited much higher photocatalytic activity than pristine g-C3N4 in the reduction of Cr(VI) . Under visible light irradiation for 120 min, the reduced ratios of Cr(VI) with the initial concentration of 50 mg/L in the presence of the treated g-C3N4and pristine g-C3N4 were 100% and 37.1%, respectively. With the addition of fulvic acid, Cr(VI) was efficiently removed at 40 min. Based on the characterization results of the structures and other physiochemical properties of the treated g-C3N4 and pristine g-C3N4 by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV Vis diffuse reflectance, the possible reasons responsible for the enhanced photocatalytic activity of the treated g-C3N4 were proposed. The yield and mechanism of different exfoliation methods were compared by semi-quantitative method.


Introduction
Graphite-like Carbon nitride (g-C3N4) is a metal-free polymer with a band gap of about 2.7 eV.It is nontoxic, stable, and easy synthesized from rich and cheap CN-containing precursor [1].In addition, it has been proved to have photocatalytic activity responsive to visible light in many chemical reactions [2][3][4][5][6][7].Thus, g-C3N4 is regarded as a kind of promising photocatalyst for water splitting and degradation of organic pollutants under sunlight irradiation.Bulk g-C3N4 synthesized by the traditional thermal condensation polymerization shows low photocatalytic activity for its large particle size, small surface area and high recombination rate of photogenerated carriers [8].So it is necessary to explore an effective and environmentally friendly method for improving the photocatalytic activies.
Bulk g-C3N4 has a layered structure with properties similar to that of graphite in which the C-C bond is replaced by a strong covalent C-N bond, and the layer is connected by a weak van der Waals force.g-C3N4 can be easily peeled off into monolayer or layers of nanosheets due to its graphite-like structure.It's reasonable that monolayered g-C3N4 nanosheets have excellent photocatalytic properties.For example, Wang firstly stirred it with strong acid (37% HCl) for 3 hours, reporting a reversible protonation of carbon nitride [7]; Liu and his colleagues developed a heat treatment to reduce its size [9].From these results, the treated C3N4 showed much higher photocatalytic activities than the bulk g-C3N4 under visible light irradiation.
Notably, this method still has some disadvantages, such as the formation of the interface defects during the calcination process, the reduction of the photo absorption ability, the relatively thick nanosheet, and the poor yield (< 6%).These disadvantages for the photocatalytic reaction are critical.Although the exfoliation of high quality C3N4 2D nanosheets with low defects could also be achieved mechanically on a small scale, a simple liquid exfoliation method would allow the formation of 2D nanosheets in large quantities.Inspired by the graphene stripping method, some researchers have developed some liquid stripping methods to fabricate few-layered C3N4 nanosheets [10,11].Unfortunately, the concentration of the as-prepared C3N4 nanosheets suspension is very low(anosheets su [11], and the monolayered nanosheet is limited [10], and the atomic structure of the plane during stripping maybe also seriously damage.Therefore, it's desired to develop a novel, facile, and rapid route to fabricate the C3N4 nanosheets.
In order to develop a simple, environmentally friendly and high-yield stripping method, NaOH were added into the organic solvent for liquid exfoliation of bulk g-C3N4 to produce monolayered C3N4 nanosheets for the first time, which improved the original liquid exfoliation process.On the basis of this, The photocatalytic activities of the untreated C3N4 and C3N4 nanosheets were compared by the reduction of Cr(Ⅵ) under visible-light (ed ies of liation tion of bulk g-C, 14</Yeardifferent organic acid.Moreover, the possible reasons responsible for the enhanced photocatalytic activity of the treated g-C3N4 were proposed based on the characterization results of the structures and other physiochemical properties of untreated C3N4 and C3N4 nanosheets.

Preparation of g-C 3 N 4 nanosheets
The bulk g-C3N4 was prepared by polymorization of melamine molecules in a muffle furnace, melamine was heated at 600℃ for 4 h under air condition with a ramp rate of about 2.3℃/min in detail.The ultrathin g-C3N4 nanosheets were obtained by liquid exfoliating of as-prepared bulk g-C3N4 in organic solvent (DMF).In detail, 300 mg of untreated C3N4 was added into 250 ml of DMF and 50 mg of sodium hydroxide (NaOH) was added, and then sonicated for 2 h after magnetic stirring, the resulting solution was centrifuged and the supernatant was dried in air at a constant temperature, referred to as g-C3N4-DMF/NaOH.g-C3N4-DMF was obtained without the addition of NaOH for comparison.
Photocatalytic activities of the samples (60 mg) were tested by the reduction of Cr(Ⅵ) with the initial concentration of 100 mg/L K2Cr2O7 aqueous solution under visible-light (ueous solu irradiation, with the addition of 1.0 mL of 100 mg/mL organic acid solution (citric acid, fulvic acid, oxalic acid) as sacrificing reagent.The concentration of Cr( Ⅵ ) was measured by using diphenyl carbazide spectrophotometry(GB 7467-87).

Characterization of physiochemical properties of g-C 3 N 4 nanosheets
Fig. 1 and 2 showed the SEM images of g-C3N4 obtained from different methods at different magnification.g-C3N4-DMF/NaOH sample still maintained loose and irregular tissue-like 2D nanosheet morphology at different magnification (Fig. 1c, Fig. 2 ), although a small portion of C3N4 nanosheets was stacked.Its morphology was quite different from that of the untreated g-C3N4 with a typical layer structure stacked layer by layer (Fig. 1a), g-C3N4-DMF had a thicker layered structure and more layers was stacked without the addition of NaOH.There are no obvious layered structure and obvious shadows observed at g-C3N4-DMF (Fig. 3d, e, f).Smaller particle sizes mean that their photogenerated electrons and holes move from the site to the solid-liquid interface at a shorter distance, thereby reducing the recombination probability of their photogenerated carriers.[12,13]  The ultra-thin thickness of g-C3N4-DMF/NaOH was further verified by the AFM image.As shown in Fig. 4, the representative AFM image shows a uniform thickness of about 0.38 nm.This thickness agrees well with the theoretical values of single-layer carbon nitride (0.324 nm) [14,15].This can be used as a strong evidence for the presence of g-C3N4 nanosheets in the presence of monatomic layers in mixed solvents.
Fig. 4 AFM image of g-C3N4-DMF/NaOH.with the interplanar spacing of 0.676 nm, the 3-s-triazine ring structure of g-C3N4; the diffraction peak at 27.3 ° corresponds to g (002) crystal plane with the interplanar spacing of 0.324 nm, which is the accumulation of g-C3N4 aromatic structure, corresponding to the layer spacing of g-C3N4 [16].As shown in the figure, the peak height of the diffraction peak at 27.3 ° decreases gradually from bottom to top, indicating that the intergranular stacking structure of g-C3N4 and the periodic arrangement of the layers are destroyed, indicating that g-C3N4 of is successfully stripped [12].

g-C 3 N 4 nanosheet production and exfoliation mechanism
Fig. 8 shows the g-C3N4 nanosheet suspension (3000 rpm low-speed centrifuged supernatant) with different exfoliation methods.For the convenience of description, the obtained in DMF or IPA were recorded as g-C3N4-DMF, g-C3N4-IPA and so on.As shown in the right-hand centrifuge tube of Figure 8a, the g-C3N4 nanosheets were dispersed to a maximum concentration, showing milky white (about 1.2 mg/ml).
In contrast, pure DMF stripping showed a relatively low concentration (Figure 8a, left), showing a pale white (about 0.3 mg/ml).The experimental results showed that the addition of NaOH increased the yield of g-C3N4 nanosheets.This phenomenon verifies a fact that dispersed concentration is maximized when the energy of exfoliation is minimized.The energy of the exfoliation can be expressed by the enthalpy of mixing (ΔHmix, unit volume), calculated from the empirical formula shown below (Eq 1) [18,19]: Where V is the volume of the suspension, δ is the square root of the surface energy, T is the average thickness of the nanosheets, and φ is the volume fraction of the nanosheets.The Hildebrand-Scratchard formula indicates that the enthalpy of mixing is dependent on the surface energy balance of C3N4 and solvent.For C3N4, the surface energy can be defined as the energy per unit area to overcome the energy required for the van der Waals force to separate the two pieces.
From the formula, we hope that the surface energy (δ_solvent) of the solvent matches the surface energy of the carbon nitride (δ_nanosheet) and obtain more nanosheets at the minimum energy cost.For a mixed system, the surface energy can be adjusted by simply changing their composition.In this case, NaOH is not soluble in DMF, but it effectively changes its surface energy, thereby increasing the yield of g-C3N4 nanosheets, which is consistent with the literature [20].
As shown in Figure 8a, b, c, we also studied the effect of NaOH on the yield of g-C3N4 nanosheets for several other organic solvents.The centrifugal tubes on the left side of each panel were centrifuged at 3000 rpm without adding NaOH, the right side of the supernatant is the one with the addition of NaOH.From the figure it is clearly seen the left side of the clarity in the clear, the right side of the milky white.After the collection, it was found that the addition of NaOH in different organic solvents for g-C3N4 nanosheet stripping could increase the yield, which was consistent with the results of the addition of NaOH in the organic solvent to increase the yield of graphene nanosheets [20].
Fig. 8 C3N4 sheets dispersed in organic solvents with or without addition of NaOH.

Conclusions
In conclusion, monolayer C3N4 nanosheets have been successfully prepared by a new, simple solvent intercalation method.The poor solvent could be changed into good solvent by adding NaOH particles for efficiently exfoliating bulk g-C3N4 into C3N4 nanosheets.Importantly, the concentration of C3N4 nanosheets could be increased for 4 times by simply adding NaOH particles into the solvent.The as-prepared C3N4 nanosheets with monolayer thickness would be endowed a new band structure and superior photocatalytic properties, such as favorable band structure, higher visible light absorption efficiency, lower photo-generated electron-hole pair recombination efficiency, higher specific surface area, and low surface defects.More intuitive to say, the high degradation efficiency of Cr(Ⅵ) was obtained by the as-prepared C3N4 nanosheets.The removal efficiency of Cr(Ⅵ) is twice as high as that of g-C3N4-DMF, which is 6 times for bulk g-C3N4.g-C3N4-DMF/NaOH can be directly added to the organic acid leaching Cr(Ⅵ) wastewater for photocatalytic reaction, Cr(Ⅵ) can be completely removed (120 min) under three different organic acid conditions.

Fig. 5
Fig. 5 showed the XRD patterns of g-C3N4 nanosheets.It can be seen from the

Fig. 6a showed
Fig. 6a showed the UV-Vis diffuse spectrum of g-C3N4.A typical semiconductor

Fig. 7
Fig. 7 (a) the photocatalytic reduction of Cr(VI) using different samples; (b) the effect of organic acids on the photocatalytic reduction of Cr(VI); Cr(VI initial concentration: 50 mg/L; organic acid concentration: 1 mg/L; 300 mW Xe lamp; l > 400 nm).