Developing a Novel Layered Boron NitrideâCarbon Nitride Composite

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Developing Novel Layered Boron Nitride-Carbon Nitride Composite with High Efficiency and Selectivity to Remove Protonated Dyes from Water Yong Guo, Ruxia Wang, Peifang Wang, Lei Rao, and Chao Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05150 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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ACS Sustainable Chemistry & Engineering

Developing Novel Layered Boron Nitride-Carbon Nitride Composite with High Efficiency and Selectivity to Remove Protonated Dyes from Water Yong Guoa, Ruxia Wanga, Peifang Wanga*, Lei Raob, Chao Wanga a. Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Xikang road No1, Gulou district, Nanjing, Province Jiangsu, 210093, P.R. China. E-mail: [email protected] b. College of Mechanics and Materials, Hohai University, Xikang road No1, Gulou district, Nanjing, Province Jiangsu, 210093, P.R. China.

KEYWORDS: Boron nitride nanoparticle • Carbon nitride • Layer composite • Adsorption • Selectivity

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ABSTRACT：Layered boron nitride (BN)-carbon nitride (CN) composite has been successfully synthesized by calcinating the mixture of BN nanoparticle and urea at 600 oC, in which CN is formed from the thermal polymerization of urea during the calcination process. TEM analyses confirm that BN-CN composite has layered structure, being different from the morphologies of BN nanoparticle and CN. FTIR, XRD and NMR results support that the layered BN-CN composite is most likely formed via the dehydration reaction between -OH groups in BN nanoparticle and -NH2 groups in CN during the calcination process, which link BN nanoparticles and CN together to form the layered composite. The layered BN-CN composite can high efficiently remove neutral red (NR) and Malachite Green (MG) from water (NR removal quantity: 1350.1 mg/g, concentration: 220 mg L-1; MG removal quantity: 1040.6 mg/g, concentration: 120 mg L-1), whereas the layered BN-CN composite has low efficiency to remove Methyl orange (MO), Methylene blue (MB) and Crystal violet (CV) from water. The removal quantities for MO, MB and CV are all just around 60 mg/g with concentration at 120 mg L-1. NR and MG are all the protonated dyes and their positive charges are from the bound H+. The layered BN-CN composite has high affinity for H+, which can make the pH of deionized water increase to 8.89 from the initial 6.82. This may be responsible for the high removal efficiency of the layered BN-CN composite for NR and MG. Furthermore, the layered BN-CN composite can selectively remove NR (or MG) from NR-MO and NR-MB solutions (or MG-MO and MG-MB solutions).


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Introduction Hexagonal Boron nitride (BN) is a structural analogue of graphite, which has a wide variety of applications since it has many remarkable properties such as chemical inertness, thermal stability, thermal conductivity and electric insulation.1-3 There is a polarity in the B-N bond, leading to that BN has also good adsorption performance for hydrogen4 and organic pollutants.4-5 For enhancing the removing performance of BN for pollutants, many kind of modifications have been performed. For example, mesoporous BN with surface area (1,427m2/g) is synthesized by calcinating the mixture of boron trioxide and guanidine hydrochloride at 1,100 oC under H2/N2 atmosphere, which show high adsorption performance for organic solvent, oil and dyes.6 Chemical treatment can increase the quantity of oxygen-containing groups in BN, and this further increases the adsorption performance of BN for cationic dyes, being due to the electronic attraction and hydrogen bonding interaction.7 Exfoliating BN into layered nanomaterial via mechanical 8 or thermal treatment 9 can also enhance their adsorption ability for dyes since functional groups (such as -OH) will be formed during the exfoliation process and surface areas are increased as well. 8-10 So far, to develop BNbased material for water cleaning is still the hot topic in the research field of synthesizing water cleaning materials. 5-12 Combining BN with other material together is a common strategy to form new type of nanomaterial with enhanced performance. For example, graphene/BN nanomaterial has been synthesized, which shows unusual physical properties (such as commensurate–incommensurate transition).13-14 With urea and h-BN porous nanosheets as precursors, carbon nitride (CN) is synthesized on the surface of BN nanosheets to produce CN/BN nanomaterial and this nanomaterial can more effectively to produce H2 from water under light irradiation than g-C3N4.15 Graphene/BN/graphene nanomaterial shows a good thermoelectric transport performance.16 It is


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well accepted that the exfoliated BN from both mechanical and thermal treatment all have functional groups (such as -OH) on its surface.8-10 The amine functionalized BN can link the carboxylated graphene together with organic molecules as linker to form nanomaterial with sandwich structure, in which the amine functionalized BN and the carboxylated graphene are all bonded with the organic linker by using their -NH2 and -OH groups to react with the functional groups in the organic linker.17 Our previous study shows that BN nanoparticles can be synthesized with diboron trioxide and urea as precursors via oxygen-limited method at 1100 oC, and this BN nanoparticle has -OH groups.18 g-C3N4 has been widely studied since of its excellent photoelectric and photocatalytic properties.19-20 It is well known that g-C3N4 and carbon nitride (CN) with low polymerization degree can be synthesized by calcinating the precursors containing N elements (such as urea, melamine), which all have residual -NH2 groups.19-20 From our previous study, new type of metalfree photocatalyst can been prepared based on the dehydration reaction between the -NH2 groups in CN and 1,2,4,5-benzene tetracarboxylic dianhydride.21 Infrared-response photocatalyst can also be synthesized based on the interaction between the -NH2 groups in melamine and the oxygencontaining groups (such as -OH groups) in carbon quantum dot.22 This promotes us to consider if novel BN-CN material can be synthesized based on the dehydration reaction between the -OH groups in BN nanoparticles and the -NH2 groups in CN. Dyes are widely used in the coloration of many materials, such as paper, leather, and textiles. Most of dyes are organic salts containing aromatic rings and easily dissolve in water. After discharging into water bodies, they are harmful for aquatic organisms.23 Adsorption is a common used method to remove the contaminants from water since of its simplicity, high treatment efficiency and low cost.24-29 For example, zwitterionic polyacrylonitrile-polyethylenimine can


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effectively adsorb anionic Methyl orange (MO) or cationic Methylene blue (MB) from MO/MB mixture solution at different pH conditions. 30 Konjac glucomannan/graphene oxide hydrogel can effectively remove MO and MB from waste water. 31 Activated carbon has been widely applied to remove dyes from the dye wastewater.

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Fe3O4 hollow nanospheres have been used to adsorb

Neutral red (NR) from water with adsorption quantity of 105 mg/g.33 At the present time, most of studies just focus on the adsorption of single dye from waste water.6, 31-33 In real situation, dyes coexist in wastewater. It is necessary to investigate the adsorption removal of dyes by adsorbent when these dyes coexist in water. Herein, a novel layered BN-CN composite (BN-30-600) has been successfully synthesized by calcinating the mixture of BN nanoparticle and urea at 600

oC.

FTIR and

1H

NMR

characterizations support that the layered BN-30-600 composite is most likely synthesized via dehydration reaction between -OH groups in BN nanoparticles and -NH2 groups in CN. This composite is different from the reported CN/BN photocatalytic material.15 In that work, BN porous nanosheet and urea are the precursors, and TEM characterizations show that CN/BN photocatalytic material has the same porous nanosheet structure as that of BN porous nanosheet.15 The layered BN-30-600 composite in this work is a white sample with adsorption edge at 390 nm and band gap around 3.8 eV, which is not photocatalyst. But, it can effectively remove Neutral red (NR) and Malachite green (MG) from water (NR removal quantity: 1350.1 mg/g, concentration: 220 mg L1;

MG removal quantity: 1040.6 mg/g, concentration: 120 mg L-1); whereas it has low efficiency

to remove MO, MB and Crystal violet (CV) from water. The removal quantities for MO, MB and CV are all just around 60 mg/g. Furthermore, the layered BN-30-600 composite can selectively remove NR (or MG) from NR-MO and NR-MB solutions (or MG-MO and MG-MB solutions). Thus, BN-30-600 composite is not an analogue of the CN/BN photocatalytic material.15 It is well


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known that many nanomaterials have functional groups. 17-18 Thus, nanomaterials with functional groups most likely can react each other to produce new type of composite and this work may provide insight for the development of this field. The goals of this work are listed as the following: (1) clarifying the formation mechanism of the layered BN-30-600 composite; (2) clarifying the removal mechanism of NR by the layered BN-30-600 composite; (3) investigating the removal of NR by layered BN-30-600 composite when NR is mixed with other dyes (MO, MB or MG).

Experimental section Reagents Diboron trioxide (B2O3, analytical grade), urea (analytical grade), hydrochloric acid (HCl, analytical grade), methyl orange (MO, analytical grade), methylene blue (MB, analytical grade), neutral red (NR, analytical grade), crystal violet (CV, analytical grade) and malachite green (MG, analytical grade), were purchased from the Sinopharm Chemical Reagent limited corporation, P. R. China. Preparation of BN nanoparticle with oxygen-limited method The procedure synthesizing BN nanoparticle was same as that in our pervious work.18 Typically, 10 g B2O3 and 20 g urea were first grinded in mortar for 30 minutes to make sure that they were mixed together in uniform way. Then, these mixtures were put into alumina crucible and covered with crucible lid. After that, the crucible was put in the muffle furnace, with the followed calcination under 1100 oC for 4 hours. During the calcination process, an anoxic condition in the inside of the crucible was acquired since the thermal reaction of B2O3 and urea under high-temperature treatment would release gas, which resulted in that oxygen was driven out


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from the inside of the crucible. That was why this method was called as oxygen-limited method. The heating rate was 8 oC/min. Then, these mixtures were washed with HCl (1.2 mol L-1) to remove the unreacted B2O3 and other impurities with the following drying in oven at 80 oC for overnight. This white sample was named as BN. Preparation of CN samples 40 g urea was put into alumina crucible and covered with crucible lid. After that, the crucible was put in the muffle furnace and treated under 400 oC for 4 hours and the heating rate was 8 oC/min.

This obtained white sample was named as CN400. It was true for CN500 and CN600,

which were synthesized with the same synthesis procedure of CN400 except for that the calcination temperatures were 500 oC and 600 oC, respectively. Preparation of BN-x-600 samples BN nanoparticle and urea were first grinded in mortar with the mass ratio (1:10) for 30 minutes. Then, these mixtures were put into the alumina crucible and covered with crucible lid. After that, the crucible was put in the muffle furnace and treated at 600 oC for 2 hours with the heating rate of 8 oC/min. The obtained white sample was named as BN-10-600. It was true for BN-0-600, BN20-600, BN-30-600, BN-40-600 and BN-60-600, which were synthesized with the same synthesis procedure of BN-10-600 except for that the mass ratios between urea and BN were 0, 20, 30, 40 and 60, respectively. For BN-30-400 and BN-30-500, the synthesis procedure was same as that of BN-30-600 except for these calcination temperatures were 400 oC and 500 oC, respectively. BNW-30-600 was synthesized in the same procedure of BN-30-600 except for that BN was washed with deionized water until the supernatant was in neutral state. BN-CN400-600 was synthesized in the following procedure: BN nanoparticles and CN400 were firstly mixed together with the mass ratio (1:4) for 30 minutes, then, the mixture was put in


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an alumina crucible and covered with crucible lid. After that, the mixture was put in the muffle furnace with the subsequent calcination at 600 oC for 2 hours. ‘The yield of calcinating urea to produce CN400 was about 14%. In the synthesis of BN-30-600, the ratio between BN and urea was 1:30. Thus, the ratio between BN and CN400 was chosen as 1:4 when BN-CN400-600 was synthesized. Characterization of samples The morphologies of BN, CN400, CN500, CN600, BN-0-600, BN-10-600, BN-20-600, BN30-600, BN-30-400, BN-40-500, BN-CN400-600, BNW-30-600 and BN-60-600 were investigated with transmission electron microscope (JEM-200CX, Japan) and scanning electron microscope (Hitachi-S4800, Japan). The X-ray diffraction (XRD) patterns of BN, BN-0-600, BN30-600 and CN600 were collected with the X-ray Diffractomer radiation (X'TRA, Switzerland). XPS characterizations of BN, CN600, BN-0-600 and BN-30-600 were performed with photoelectron spectroscopy (PHI 5000 VersaProbe, Japan). Surface areas of BN, CN400, CN500, CN600, BN-0-600, BN-10-600, BN-20-600, BN-30-600, BN-40-600, and BN-60-600 were determined with surface analyzer (HD88, ASAP2020 micropore analyzer, USA). The Fourier transform infrared spectra (FTIR) of BN, CN400, CN500, CN600, BN-0-600, BN-10-600, BN20-600, BN-30-600, BN-40-600 and BN-60-600 were acquired using infrared spectrometer (Nexus 870 FT-IR instrument, USA). The 1H and 13C NMR spectra of BN, CN600 and BN-30600 were determined with solid state nuclear magnetic instrument (JNM-ECZ600R, Japan). The removal quantities of NR, MO, MB, MG and CV by BN-30-600 were determined via UVvisible spectrometer based on the change of their characteristic UV peaks before and after the removal experiments. The standard curves of NR, MO, MB, MG and CV had been done and their concentration had a good linear correlation with their characteristic UV peaks since these R2 values


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were all 0.999 (please see Figure S1 and S2). Removal experiments For investigating the influence of urea dosage on the removal performance of BN-based composites for NR, the removal experiments of NR by BN, CN400, CN500, CN600, BN-0-600, BN-10-600, BN-20-600, BN-30-600, BN-40-600, BN-60-600, BNW-30-600 and BN-400-600 samples had been performed. The detailed procedure was as following: 5 mg sample was put into the glass beaker with 30 ml NR solution (concentration: 200 mg L-1), followed by stirring for 480 minutes at room temperature under dark condition. Then, this stirring was stopped for about one minute to make sure that the adsorbent was sinked to the bottom of the glass beaker. After that, 5 ml supernatant was collected from the beaker and centrifuged for the following characterization via UV-visible spectrometer to determine the removal quantity of NR by different samples. All of these removal experiments were all done three times to make sure that the obtained results were reasonable. It was true for the following removal experiments. The adsorbed amount (qt) was calculated with the following formula: (C0  Ct )V m -1 where C0 and Ct (mg L ) were the initial NR concentration and the NR concentration at time (t) qt 

respectively, V (L) was the volume of the NR solution, and m (g) was the mass of the adsorbent. The removal kinetic of NR by BN-30-600 was investigated in the same procedure mentioned above, except for that the concentration of NR was 120 mg L-1 and the volume was 55 ml. For making sure that the obtained results were reasonable, eleven removal experiments of NR by BN30-600 had been done at the same time. Then, these experiments were stopped at 2 min, 10 min, 30 min, 60 min, 120 min, 180 min, 240 min, 300 min, 360 min, 420 min and 480 min, respectively. The supernatants in these experiments were taken and characterized with UV-visible spectrometer


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to determine the removal quantities of NR by BN-30-600 at different time interval. The removal kinetic of NR by BN-30-600 was simulated with the following two formulas: 1). The pseudo-first-order kinetic model equation:

lg(qe  qt )  lg qe  k1t 2).The pseudo-second-order kinetic model equation: t 1 t   2 qt k2 qe qe where qe was the adsorbed amount of NR by BN-30-600 at equilibrium (mg/g), qt was the adsorbed

amount of NR by BN-30-600 at time t (mg/g), k1 was the pseudo-first-order rate constant (min-1), and k2 was the pseudo-second-order rate constant (g mg-1 min-1). The removal isotherm of NR by BN-30-600 was investigated in the following procedure: 5 mg sample was put into the glass beaker with 55 ml NR solution (concentration: ranged from 100 mg L-1 to 220 mg L-1), followed by stirring for 480 minutes at room temperature under dark condition. Then, this stirring was stopped for about one minute to make sure that the adsorbent was sinked to the bottom of the glass beaker. After that, 5 ml supernatant was collected from the beaker and centrifuged for the following characterization via UV-visible spectrometer to determine the removal quantities of NR at different concentration by BN-30-600. The removal isotherm of NR by BN-30-600 was simulated with the following two formula 1) the Langmuir model equation： Ce Ce 1   qe qm qm K L where Ce was the remaining concentration of NR at equilibrium (mg L-1), qe was the adsorbed

amount of NR by BN-30-600 at equilibrium (mg/g), qm was the theoretical maximum adsorption capacity (mg/g) of BN-30-600 for NR, and KL was the Langmuir isotherm constant (L mg-1). 2) the Freundlich model equation:


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1 ln qe  ln K F  ( ) ln Ce n where qe was the adsorbed amount of NR by BN-30-600 at equilibrium (mg/g), Ce was the

remaining concentration of NR at equilibrium (mg L-1), KF was the Freundlich constant indicative of the relative adsorption capacity of the adsorbent (mg/g)(L mg-1)1/n, and n was the Freundlich constant indicative of the intensity of the adsorbent. The recycle experiment of NR removal by BN-30-600 was also investigated in the same procedure of NR removal isotherm by BN-30-600. The concentration of NR was 120 mg L-1. After the removal experiment, BN-30-600 was taken out with the subsequent washing by HCl solution (0.1mol/L), NaOH solution (0.1mol/L) and water in turn for three times. BN-30-600 sample was reused four times to evaluate it’s recycle performance for NR removal. The removals of MO, MB, MG and CV by BN-30-600 were also investigated in the same procedure of NR removal isotherm by BN-30-600, except for that the dye was MO, MB, MG, CV, respectively, and their concentrations were 120 mg L-1. The treatment of NR-MO solution by BN-30-600 was investigated in the same procedure of NR removal isotherm by BN-30-600. Both NR and MO concentration were all 120 mg L-1 and the volume was 55 ml. It was true for the treatment of NR-MB, MG-NR, MG-MB and MG-MO solutions by BN-30-600. Computational section For understanding the removal mechanism of NR by BN-30-600, m062x/6-31g(d,p) method 34

in Gaussian 09 program 35 had been adopted to calculate the sizes of MO anion, MB cation, NR

cation as well as the adsorption energies between BN-30-600 and MO anion, MB cation, NR cation. Full atom models were adopted for MO anion, MB cation and NR cation. A model containing 36 boron atoms, 34 nitrogen atoms, 3 oxygen atoms and 22 hydrogen atoms was constructed to


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simulate the structure of BN-30-600 (Figure. S3). Given that ions were involved, solvent effect had been considered in all optimizations by using PCM model 36 with water as solvent.

Results and discussion Characterization section From Figure 1(a) and Figure S4, BN exists in large aggregates, which are composed of many nanoparticles with sizes around 20-30 nm. There are two peaks (~3184 cm-1 and 1197 cm-1) in the FTIR spectrum of BN nanoparticle (Figure S5), which are assigned to the vibration modes of O-H bond.37 This maybe from that BN nanoparticles are synthesized via oxygen-limited method rather than under inert atmosphere. It has reported that thermal treatment of BN in air can make BN become hydroxylated.9, 37 Interestingly, BN-30-600 sample has layered structure (Figure 1(b) and Figure S6), which is different from the morphology of BN nanoparticle. As we know, it is first time to report that BN nanoparticle can be converted into the layered material. The number of layer is a very important index for two dimensional nanomaterials.10, 38 From Figure.S7, the layered thickness of BN-30600 sample is around 3 nm, suggesting that the layer of BN-30-600 is not too thick. But, the numbers of layer can not be determined since no obvious lines can be found at the edge of BN-30-600. From these yellow circles in Figure 1(b), the layered structure of BN-30600 is most likely from the overlap of these small layered chips. Thus, the layered structure of BN-30-600 is incomplete, which explains well why HRTEM characterization for BN30-600 can not found the numbers of layer. So, BN-30-600 is not a good two dimensional nanomaterial, but is a layered composite.


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Figure.1 (a) TEM image of BN nanoparticle; (b) TEM image of BN-30-600 sample; (c) TEM image of BN-0-600 sample; (d) TEM image of CN600 sample.

For deep understanding this fascinating phenomenon, a series of control experiments have been performed. From Figure 1(c), BN-0-600 still exists in aggregate, but it is looser and smaller than that of BN (Figure 1(a)), being due to that thermal treatment decomposes the huge BN aggregate into smaller one.9,37 Thus, just calcinating BN nanoparticles without the attendance of urea can not produce the layered composite. From Figure 1(d) and Figure S8, CN400, CN500 and CN600 are all not the layered material.39-41 CN600 also exists in large aggregates, which are composed of many thick strips with size around 100 nm (Figure 1(d)). Thus, just calcinating urea without the attendance of BN nanoparticle can also not produce the layered material.


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Scheme 1. The proposed formation mechanism of the layered BN-CN composite (BN-30-600) with BN nanoparticle and urea as precursors.

The morphologies of BN, CN400, CN500, CN600 and BN-30-600 strong promote us to consider that BN nanoparticles is linked together by the new-formed CN from the thermal polymerization of urea in the radial direction since this is only way to convert nanoparticle into layered composite. Depositing CN on the surface of BN nanoparticle in axial direction can just produce big particle and can not produce the layered material. For example, the reported CN/BN photocatalytic material is formed by synthesizing CN on the surface of BN porous sheet, which has same porous sheet structure as the BN porous sheet.15 It has reported that metal-free photocatalyst can been prepared based on the dehydration reaction between the -NH2 groups in CN and 1,2,4,5-benzene tetracarboxylic dianhydride.21 Infrared-response photocatalyst can also be synthesized based on the interaction between the -NH2 groups in melamine and the oxygencontaining groups (such as -OH group) in carbon quantum dot.22 In addition, it is well known in organic chemistry that two molecules can be bound together to form a new molecule with the formed C-N bond via the dehydration reaction between the C-bound -OH group in one molecule and the C-bound -NH2 group in another molecule.42 FTIR characterization for BN nanoparticle


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has confirmed the existence of -OH groups (Figure. S5). It is well accepted that carbon nitride (CN) with low polymerization degree (such as melam, melem, heptazine-based fragment) and gC3N4 all have the residual -NH2 groups.19, 21-22, 39-41 Thus, the layered BN-30-600 composite is most likely synthesized by the dehydration reaction between the -OH groups in BN nanoparticle and the -NH2 groups in CN, which links BN nanoparticle and the new-formed CN together in radial direction to produce the layered composite (please see the possible formation mechanism in Scheme.1). The new formed CN is from the thermal polymerization of urea during the calcination process. A CN/BN photocatalytic material has been synthesized by calcinating the mixture of BN porous nanosheet and urea at 550 oC, in which CN is deposited on the surface of BN porous nanosheet and TEM characterization shows that CN/BN photocatalytic material has the same porous nanosheet structure as the BN porous nanosheet.

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In that work, the used BN porous

nanosheet is synthesized under nitrogen atmosphere. 15 It has reported that BN material synthesized under inert atmosphere have not -OH groups. 9, 37 Thus, BN porous nanosheet in that work can not react with the new-generated CN during calcination process since it has not -OH group. This leads to that CN can just deposit on the surface of BN porous nanosheet and CN/BN photocatalytic material has similar porous nanosheet structure to the BN porous nanosheet. 15 According to Figure 2(a), obvious peaks around 3180 cm-1 are found in the FTIR spectra of CN400, CN500 and CN600, supporting the existence of -NH2 groups in these samples. 19, 21-22, 3941

CN500 and CN600 all have the typical stretching vibration peaks (1640 cm-1, 1396 cm-1 and

769 cm-1) of heptazine-derived repeating units in g-C3N4.39-41 CN400 also have these peaks, but their intensities are weaker than that in CN500 and CN600. In addition, the FTIR peak of CN400


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in the range between 3000 cm-1 and 3400 cm-1 is also different from that of both CN500 and CN600.

Figure.2 (a) FTIR spectra of CN400, CN500 and CN600; (b) Images of BN-30-600, CN400, CN500 and CN600 (c) XRD spectra of BN, BN-0-600, CN600 and BN-30-600; (d) FTIR spectra of BN, BN-0-600, BN-30-600 and CN600; (e) FTIR spectra of BN, BN-0-600, BN-10-600, BN20-600, BN-30-600 BN-40-600 and BN-60-600; (f) 1H NMR spectra of BN, BN-30-600 and CN600.

All of these suggest that CN500 and CN600 have the structure of graphitic carbon nitride, while CN400 just contains CN with low polymerization degree (such as melem, melam and heptazinebased fragment).39-41 This is further supported by the colors of these samples (Figure 2(b)): CN400 (white), CN500 (yellow) and CN600 (yellow). It is well accepted that the color of g-C3N4 is yellow.15,

19, 39-41

BN-30-600 is white as well, suggesting that CN (such as melam, melem,


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heptazine-based fragment) rather than g-C3N4 most likely participates in the formation of the layered BN-30-600 composite during the calcination process. This is easily to accept since CN is first formed and then followed by the formation of g-C3N4 during the thermal polymerization of urea.40 The new-formed CN can directly react with BN nanoparticle based on the dehydration reaction between its -NH2 groups and the -OH groups of BN nanoparticles. From Figure 2(c), CN600 has the typical XRD peaks (27.5 oC and 13.1 oC) of g-C3N4,19, 39-41 but this two peaks can not be found in the spectrum of BN-30-600, further supporting that CN rather than g-C3N4 participates in the formation of the layered BN-30-600. BN and BN-0-600 all have the typical XRD peaks (26.6 oC and 41.8 oC) of boron nitride,9, 18, 37 in which the intensities of these two peaks in the latter are lower than that in the former. This maybe due to that the size of BN-0-600 is smaller and thinner than that of BN (Figure 1(a) and 1(c)). For BN-30-600, the intensity of the typical XRD peak at 26.6 oC become very weak and that at 41.8 oC disappear, being from that BN30-600 is of the layered structure.8, 37 If the proposed mechanism is right, this FTIR peak intensities of -OH groups in BN nanoparticles will decrease after the reaction since most of these -OH groups have been reacted. From the two red ellipses in their FTIR spectra (Figure 2(d)), the intensities of -OH peaks(3184 cm-1 and 1197 cm-1) in BN-30-600 are obvious weaker than that in BN-0-600 and BN. The FTIR peak of -NH2 group in BN-30-600 is also less obvious than that in CN600. These results suggest that -OH groups in BN nanoparticle and -NH2 groups in CN are involved in the formation of the layered BN-30-600 composite. This supports that the proposed formation mechanism of BN-30600 is reasonable (Scheme. 1). The two typical vibration peaks (1640 cm-1 and 769 cm-1) of heptazine-derived repeating units in CN600 39-41 are also found in the FTIR spectrum of BN-30600 (please see the two purple rectangles in Figure 2(d)), implying that heptazine-derived


17


repeating units are one of the components in BN-30-600. For further identifying the role of -OH groups in BN nanoparticles in the formation of the layered BN-30-600 composite, the FTIR spectra of BN-10-600, BN-20-600, BN-40-600 and BN-60-600 have compared as well. From Figure 2(e), the intensities of -OH peaks (3184 cm-1 and 1197 cm-1) gradually become weaker from BN to BN60-600 with the increased dosage of urea, while the two peaks (1640 cm-1 and 769 cm-1) of heptazine-derived repeating units become stronger from BN to BN-60-600 with the increased dosage of urea. The FTIR peak intensities of -NH2 groups in these samples are also less obvious than that in CN600 (Figure 2(a) and 2(e)). This further supports that the -OH groups in BN nanoparticle and -NH2 groups in CN participate in the formation of the layered BN-30-600 composite (Scheme. 1). XPS characterization is also adopted to investigate the N1s peaks of the N atoms in BN, CN600, BN-0-600 and BN-30-600. From Figure S9(a), The N1s peaks of the B-bound N atoms in BN nanoparticle and BN-0-600 are 397.8 eV and 397.5 eV, respectively. The size of BN-0-600 is smaller than that of BN nanoparticle, which may result in the red-shift of N1s peak from BN to BN-0-600.8, 43 BN-30-600 has layered structure, which may further result in the red-shift of the N1s peak from the B-bound N. Thus, the N1s peak at 397.4 eV in the XPS spectrum of BN-30600 is attributed to the B-bound N. The N1s peak of the C-bound N in CN600 is also around 397.5 eV. From Figure S9(a), the N1s peaks of B-bound N in BN is 397.4 eV, and that of C-bound N in CN is 397.5 eV. This suggests that it is not easy to investigate the reaction between BN and the new-formed CN via XPS characterization since just -OH and -NH2 groups are involved in that reaction as well as the N1s peaks of B-bound N in BN and C-bound N in CN are too closed. However, the percent (15.75%) of B element in BN-30-600 is lower than that (44.07%) in BN,


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while the percent (43.93%) of C element in BN-30-600 is higher than that (6.54%) in BN (Figure S9(b)). This supports that BN and the new-formed CN have been successfully combined together. NMR characterization is a powerful method to investigate the reaction that hydrogen and carbon elements are involved. According to Figure S10, the 13C NMR signals in BN-30-600 (157.2 ppm and 165.3 ppm) and CN600 (157.2 ppm and 165.3 ppm) are identical, which are assigned to the two type of carbon elements in carbon nitride.44 This suggests that the C elements in CN does not involved in the reaction between BN and the new-formed CN. This is consistent with the proposed formation mechanism (please see Scheme 1): just -NH2 in the new-formed CN and -OH groups in BN are involved in the reaction to form the layered BN-30-600 composite. From figure 2(f), 1H NMR signal (10.4 ppm) in the 1H NMR spectrum of CN600 is assigned to -NH2 groups in carbon nitride 45 and this signal has disappeared in the 1H NMR spectrum of BN-30-600. The 1H NMR signal (8.0

ppm) in the 1H NMR spectrum of BN is attributed to the -OH group in BN,46-47

which also disappears in the 1H NMR spectrum of BN-30-600. Thus, 1H NMR characterizations for BN, CN600 and BN-30-600 support that -NH2 groups in CN and -OH groups in BN are involved in the formation of BN-30-600 composite (Scheme 1). This is consistent with the FTIR characterizations for BN, CN600 and BN-30-600 (Figure 2(d)). Furthermore, the 1H NMR signal at 5.0 ppm in the 1H NMR spectrum of CN600 is assigned to the N-bound H, in which the N atom is bound with two carbon atoms (please see Figure 2(f)). A new 1H NMR signal appears at 5.2 ppm in the 1H NMR spectrum of BN-30-600 (Figure 2(f)). It is most likely from the contribution of the N-bound H in BN-30-600, in which the N atom is bound with one carbon atom and one boron atom (please see Figure 2(f) and the red circle in Scheme.1). Thus,

1H

NMR

characterizations for BN, CN600 and BN-30-600 also support that the proposed formation mechanism of BN-30-600 is reasonable (Scheme. 1).


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The influence of urea dosage on the morphology of BN-CN nanomaterial is also investigated. From Figure 3(a), BN-10-600 is consist of plates with size around hundred (yellow circles) or

Figure. 3(a) TEM image of BN-10-600 sample; (b) TEM image of BN-20-600 sample; (c) TEM image of BN-40-600 sample; (d) TEM image of BN-60-600 sample.

thousand nanometer (red circle), which are obvious larger than that of BN (Figure 1(a)) and different from that of CN600 (Figure 1(d)). This may be from that the dehydration reaction between -OH groups in BN nanoparticle and -NH2 groups in CN links BN nanoparticle together to form the plate structure. For BN-20-600, thick layered structures have already appeared (Figure 3(b)), and the layered structure of BN-40-600 is better than BN-20-600 (Figure 3(c)). However, BN-30-600 has a better layered structure than BN-40-600 (Figure 1(b)) and Figure 3(c)). The layered structure in BN-30-600 become thicker again in BN-60-600 (Figure 3(d)). These


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interesting phenomena suggest that 1:30 is the optimum mass ratio between BN nanoparticle and urea for synthesizing the layered BN-CN composite. This phenomena may be explained in the following reasons: (1) the new-formed CN from the thermal polymerization of urea can use itsNH2 groups to react with the -OH groups of BN nanoparticle during calcination process, which will link BN nanoparticle together to form the layered BN-CN composite; (2) the new-formed CN can also continue the polymerization and pack together during calcination process. When the dosage of the used urea are not too much, the quantity of the new-formed CN from the thermal polymerization of urea are also not too much. Just part of BN nanoparticles are linked together by the new-formed CN, which result in the formation of the huge plate (BN-10-600 and BN-20-600). When the dosage of the used urea are too much, the quantity of the new-formed CN from the thermal polymerization of urea are too much as well. After these available -OH groups of BN nanoparticles are used out, the excessive CN will continue the polymerization and deposit on the surface of the layered BN-CN composite, which make that the layered composite (BN-30-600) become thicker again (BN-40-600 and BN-60-600). Just when dosage of the used urea are appropriate, the new-formed CN can react with all BN nanoparticle to form the layered BN-CN composite (BN-30-600). Thus, 1:30 is the optimum mass ratio between BN nanoparticle and urea for synthesizing the layered BN-CN composite. Since BN nanoparticle, CN and BN-30-600 are all difficult to dissolve in water or organic solvent, it is hard to remove the residual BN nanoparticle and CN from the as-synthesized BN-30-600. Adopting the optimum mass ratio (1:30) between BN nanoparticle and urea is the best way to make sure that the layered BN-CN composite is the main product. Since the discussions above has proposed that the new-formed CN reacts with BN nanoparticle to form the layered composite, the direct reaction between BN nanoparticle and CN400 is also


21


investigated with the same procedure as that of BN-30-600 to see if layered composite can be formed. From Figure S11, this sample has not the layered structure, being due to that CN400 has condensed structure (Figure S8) and just part of -NH2 groups in CN400 are available to react with the -OH groups in BN nanoparticles. When urea is used, the new-formed CN from the polymerization of urea is smaller and thinner than CN400, which easily react with BN nanoparticles to form the layered BN-CN composite. Thus, urea rather than CN400 is good precursor to produce the layered BN-CN composite. The mixtures of BN and urea with ratio 1:30 are also calcinated at 400 oC and 500 oC to investigate the temperature effect on the morphology of the BN-CN composite. According to Figure S12(a), BN-30-400 has plate structure, while the plate structure are further converted into the layered structure at 500 oC (BN-30-500, Figure S12(b)). From Figure S12 and Figure 1(b), BN-30-600 has the best layered structure among BN-30-400, BN-30-500 and BN-30-600, implying that 600 oC is the optimal temperature to synthesize the layered BN-CN composite. This maybe from the following two reasons: (1) higher temperature means that more powerful energies can be acquired to overcome the energy barrier of the dehydration reaction between -NH2 groups in CN and the -OH groups in BN nanoparticle, which will further favor the cross-linking of BN nanoparticles by CN; (2) higher temperature also facilitates the thermal exfoliation of BN nanoparticle into thinner and smaller one (Figure 1(a) and 1(c)), which will be further favor to the formation of the layered BN-CN composite. In addition, the released gas (such as NH3) from the thermal polymerization of urea during the calcination process may also exfoliate the BN aggregate into small pieces. For confirming this deduction, BN has mixed with (NH4)2CO3 together with the mass ratio (1:30), then followed by the calcination at 600 oC. It is well known that (NH4)2CO3 will completely decompose into CO2 and NH3 during the calcination process. From Figure S13, it is


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clearly to see that the the aggregated structure of BN has also been decomposed into small pieces when BN is mixed with (NH4)2CO3 together with the following calcination at 600 oC. Thus, the released gases from the thermal polymerization of urea during the calcination process also make contribution for exfoliating the aggregated BN into small BN pieces. It is well known that g-C3N4 will be decomposed when the temperature is high above 600 oC.19 For example, the reported CN/BN photocatalytic material is synthesized by depositing CN on the surface of BN nanosheet at 550 oC.15 Thus, 600 oC is chosen as the optimal temperature in this work to synthesize the layered BN-CN composite. BN nanoparticle is first washed with HCl (1.2 mol L-1) to remove the possible impurities, then followed by the directly mixing with urea to synthesize the layered BN-CN composite. The effect of the residual HCl in BN on the morphology of the layered BN-CN composite has been evaluated. From Figure S14, BNW-30-600 has large and aggregate structure rather than layered structure. This is most likely attribute to the following reason: the new-formed CN from thermal polymerization of urea can not only use their -NH2 groups to react with the -OH groups in BN nanoparticle to form the layered BN-CN composite, but can also continue the polymerization to form the condensed structure (please see the morphology of CN600 in Figure. 1(d) and CN400, CN500 in Figure. S8).19-20,

39-40

Further polymerization will be become dominant if the

polymerization speeds of urea and CN are too fast during calcination process. This will make that the new-formed CN have not enough time to react with BN nanoparticle, and CN will directly deposit on the surface of BN nanoparticles to form the large aggregate (Figure S14). It is well known that acid can catalyze many kind of organic reaction.48-50 The residual HCl in BN nanoparticle may catalyze the dehydration reaction between the -OH groups in BN nanoparticle and the -NH2 groups in CN as well as hinder the polymerization of CN by reacting with these -


23


NH2 groups in CN. Further experiments have been performed to confirm the existence of the residual HCl in BN-30-600. Firstly, BN-30-600 is washed with deionized water 0 time, 2 times

Figure. 4 (a) The pH values (blue color) of the supernatants from centrifuging BN-30-600-0, BN30-600-2 and BN-30-600-4 solutions as well as the zeta potential values (red color) of BN-30-6000, BN-30-600-2 and BN-30-600-4; (b) BET results of CN400, CN500, CN600, BN, BN-0-600, BN-10-600, BN-20-600, BN-30-600, BN-40-600 and BN-60-600 samples; (c) Removal results of NR by BN, BN-0-600, BN-10-600, BN-20-600, BN-30-600, BN-40-600 and BN-60-600 (dosage: 5mg, NR concentration: 200 mg L-1, volume: 30ml); (d) Removal results of NR by CN400, CN500, CN600, BN-CN400-600 and BNW-30-600 (dosage: 5mg, NR concentration: 200 mg L-1, volume: 30ml).


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and 4 times, in which these samples are named as BN-30-600-0, BN-30-600-2 and BN-30-600-4, respectively. Then, the zeta potentials of BN-30-600-0, BN-30-600-2 and BN-30-600-4 are characterized. From Figure 4(a), the zeta potentials of BN-30-600-0, BN-30-600-2 and BN-30600-4 are -14.3 eV, -17.5 eV and -29.3 eV, respectively. Thus, washing will make the zeta potential of BN-30-600 become negative (from -14.3 eV to -29.3 eV). This is most likely from that the adsorbed H+ by BN-30-600 are washed off, which results in that the zeta potential of BN-30-600 become negative. Thus, the zeta potential results of BN-30-600-0, BN-30-600-2 and BN-30-6004 support that there are residual HCl in BN-30-600. Furthermore, the supernatants are acquired by putting BN-30-600-0, BN-30-600-2 and BN-30-600-4 into deionized water for 20 mins with the following centrifugation. The pH values of these supernatants are detected as well. From Figure 4(a), the pH values of these supernatants from BN-30-600-0, BN-30-600-2 and BN-30-600-4 solutions are 5.65, 6.18 and 6.66, respectively. Obviously, the pH values of these supernatants from BN-30-600-0, BN-30-600-2 and BN-30-600-4 solutions are increased with the increase of washing times (from 5.65 to 6.66). This is most likely due to that the residual HCl quantity in BN30-600-0 is largest, then followed by BN-30-600-2, BN-30-600-4. Thus, the supernatant from BN30-600-0 is more acidic (5.65) than that from BN-30-600-2 (6.18) and BN-30-600-4 (6.66). So, the pH values of these supernatants from BN-30-600-0, BN-30-600-2 and BN-30-600-4 solutions also support that there are residual HCl on the surface of BN-30-600. BN-30-600 has the best layered structure among BN, BN-10-600, BN-20-600, BN-30-600, BN-40-600 and BN-60-600 samples (please see Figure 1(b) and Figure 3), suggesting that BN-30600 may also has the largest surface area among them. From Figure 4(b), BN-30-600 has the largest surface area (132.16 m2.g-1) among BN (11.70 m2.g-1), BN-0-600 (15.81 m2.g-1), BN-10600 (37.97 m2.g-1), BN-20-600 (52.02 m2.g-1), BN-30-600, BN-40-600 (82.20 m2.g-1), BN-60-600


25


(65.03 m2.g-1), CN400 (19.76 m2.g-1), CN500 (36.61 m2.g-1) and CN600 (70.66 m2.g-1). In addition, the surface area of BN-30-600 is also larger than the sum (82.36 m2.g-1) of BN nanoparticles (11.70 m2.g-1) and CN600 (70.66 m2.g-1). From SEM images of BN, BN-10-600, BN-20-600, BN-30600, BN-40-600 and BN-60-600 (Figure S15 and S16), the surface of these samples become more porous from BN to BN-30-600, then gradually become less porous again from BN-30-600 to BN60-600, explaining well why BN-30-60 has the largest surface area. It is well known that the modified BN materials have good adsorption performance for pollutants.4-8 Thus, the removal performance of these samples for pollutants are first investigated. NR is cationic dye, which has been widely used to evaluate the adsorptive or catalytic performance of the as-synthesized materials.51-54 Thus, NR is chosen in this work to evaluate the removal performance of NR by BN, BN-0-600, BN-10-600, BN-20-600, BN-30-600, BN-40-600, BN-60600, CN400, CN500, CN600, BN-CN400-600 and BNW-30-600 samples. From Figure 4(c), BN30-600 has the largest removal quantity (1180.9 mg/g) for NR, being obvious larger than that of BN (20.4 mg/g), BN-0-600 (22.4 mg/g), BN-10-600 (249.4 mg/g), BN-20-600 (295.1 mg/g), BN40-600 (998.7 mg/g) and BN-60-600 (758.0 mg/g). From Figure 4(d), the removal quantities of CN400, CN500, CN600, BN-CN400-600, BNW-30-600 for NR are 1.82 mg/g, 2.05 mg/g, 2.32 mg/g, 23.87 mg/g and 20.51 mg/g, respectively. The removal quantities of these samples for NR are also less than that of BN-30-600 for NR. It is well known that g-C3N4 has good photocatalytic performance under visible light irradiation.19-22 From Figure 2(b), BN-30-600 is a white sample. Furthermore, the adsorption edge of BN-30-600 is around 390 nm (Figure S17) and the band gap of BN-30-600 is about 3.8 eV (Figure S18). This is different from the reported CN/BN photocatalytic material, which has bandgap around 3.0 eV.15 Thus, BN-30-600 is not a good


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photocatalytic material under visible light irradiation and herein we do not investigate its photocatalytic performance. Dyes removing section Then, BN-30-600 is chosen for further investigating its removal performance and mechanism for NR. According to Figure. 5(a), the removal quantity of NR by BN-30-600 is up to 1129.4 mg/g in 30 min and the largest removal quantity is 1270.1 mg/g in 8 hours when NR concentration is 120 mg L-1 and volume is 55 ml. From Figure S19 and Table S1, the correlation coefficient (R2) values are 0.4368 for pseudo-first-order model, and 0.9997 for pseudo-second-order model. Thus, pseudo-second-order model is suitable for describing the kinetic removal of NR by BN-30-600. The influence of NR concentration on its removal by BN-30-600 is invested as well. From Figure. 5(b), the removal quantities of NR by BN-30-600 increase to 1350.1 mg/g from 980.9 mg/g when the concentrations of NR range from 100 mg L-1 to 220 mg L-1. The correlation coefficient value (R2) is 0.999 for Langmuir model, and 0.6512 for Freundlich model (Figure. S20 and Table S2,), suggesting Langmuir model is suitable for describing the isotherm removal of NR by BN-30-600. It has reported that the adsorption quantity of Fe3O4 hollow nanospheres for NR is 105 mg/g.33 The removal quantity of NR by halloysite nanotubes and the modified cetylpyridinium bromide hectorite are 65.45 mg/g 51 and 393.70 mg/g,52 respectively. Thus, BN-30-600 is a good water-cleaning material for NR removal with respect to these reported materials. 33, 51-52 The adsorption quantities of BN with surface area about 1427.0 m2.g-1 for MB and basic yellow are 313 mg/g and 556 mg/g, respectively. 6 The surface area of BN-30-600 is 132.16 m2.g-1 (Figure 4(b)), but the removal quantity for NR by BN-30-600 is 1350.1 mg/g when NR concentration is 220 mg L-1. Thus, surface area can not explain well why BN-30-600 has so high removal quantity for NR. It is well known that NR is cationic dye and the positive charge is from the bound H+ with


27


N atom in NR (Figure S21).51-52 BN-30-600 has negative surface when pH of solution is larger than 2.8 (Figure S22) and the pH values of NR solution during removal process after the addition of BN-30-600 into the NR solution increase to 6.9 from the initial 3.8 in 30 min, followed by the

Figure 5. (a) The kinetic removal of NR by BN-30-600 (dosage: 5mg, NR concentration: 120 mg L-1, volume: 55ml); (b) the isotherm removal of NR by BN-30-600 (dosage: 5mg, NR concentration: 100 mg L-1 to 220 mg L-1, volume: 55ml); (c) the removal of MO by BN-30-600 (dosage: 5mg, MO concentration: 120 mg L-1, volume: 55ml); (d) the removal of MB by BN-30600 (dosage: 5mg, MB concentration: 120 mg L-1, volume: 55ml).

vibration between 6.5 and 7 (Figure S22). Thus, electrostatic attraction is favor the removal of NR by BN-3-600. For confirming this point, MO (anionic dye) and MB (cationic dye) 53-55 have also been adopted for further evaluating the removal mechanism of BN-30-600 for NR. The removal


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quantities of MO and MB by BN-30-600 are just 55.9 mg/g and 54.0 mg/g, respectively (Figure 5(c) and 5(d)). This is much less than the removal quantity (1157.3 mg/g) of NR by BN-30-600 when the NR concentration is 120 mg L-1, suggesting that electrostatic attraction is not the main driving force for NR removal by BN-30-600. The previous reports show that the size of pollutants also play very important role in their adsorption by the adsorbents.55-56 From Figure S24, NR cation has the smallest size since its length and width are 12.2 Å and 4.9 Å, being smaller than that of MO anion (14.5 Å and 4.3 Å) and MB cation (14.2 Å and 5.4 Å). This is consistent with the calculated volumes of NR cation (188.4 cm3/mol), MO anion (208.6 cm3/mol) and MB cation (206.8 cm3/mol), respectively. However, the size difference among NR cation, MO anion and MB cation is still difficult to explain well why the removal quantity of NR (1157.3 mg/g) by BN-30600 is nearly 20 time larger than that of MO (55.9 mg/g) and MB (54.0 mg/g) by BN-30-600. The adsorption energies also play very important role in their adsorption by the adsorbents.55-56 Herein, a BNO model containing 36 boron atoms, 34 nitrogen atoms, 3 oxygen atoms and 22 hydrogen atoms were constructed to simulate the structure of BN-30-600 (Figurer. S3) since the following reasons: (1) FTIR results shown that -OH groups in BN-30-600 were not two much; (2) the previous work 9 suggested that O atom may be inserted into the lattices of BN; (3) CN was not a good adsorbent (Figure 4(d)) and the modified BN materials are good adsorbent.6-7 The calculated adsorption energies between BN-30-600 and NR cation, MO anion, MB cation are also calculated, which are 39.1 Kcal/mol, 37.2 Kcal/mol and 35.2 kcal/mol, respectively (Figure. S25). The difference of adsorption energies between BNO model and NR cation, MO anion, MB cation are also not too much, which is difficult to explain why the removal quantity of NR by BN-30-600 is nearly 20 time larger than that of MO and MB by BN-30-600. According to Figure S25(c), the distance between the N-bound H+ and the N atom in NR is 1.62 Å, while the distance between the


29


N-bound H+ in NR and the O atom in BNO model is 1.04 Å (please see the red circle in Figure S25(c)), implying that the N-bound H+ in NR may transfer to the O atom in BNO model during the removal process. A NR-BNOH model has been constructed, in which the proton of NR has

Figure.6 (a) the calculated structure of NR-BNOH model, in which proton is transferred from N atom of NR cation to O atom in the BNO model. BNO model is constructed to simulate the structure of BN-30-600; (b) NR solution with different pH values at starting time; (c) NR solution with different pH values three hours later; (d) The removal of CV solution by BN-30-600 for 8 hours (dosage: 5mg, CV concentration: 120 mg L-1, volume: 55ml); (e) The removal of MG solution by BN-30-600 for 8 hours (dosage: 5mg, MG concentration: 120 mg L-1, volume: 55ml).

been transferred to the O atom in BNO model. The calculated results show that the energy of NRBNOH is 5.5 Kcal/mol lower than that of NR-BNO (Figure 6(a) and Figure S25(c)), suggesting that proton transfer from NR to BN-30-600 is favorable in energy. NR is organic hydrochloride and the positively H+ is located at one N atom in NR (please see the red circle in Figure S21). NR


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molecule will precipitate from water after the positively H+ is lost since the organic NR molecule without H+ will be very difficult to solve in water. For identifying this point, NR solution with pH at 2.38, 4.21, 6.21, 7.89, 8.18 and 10.31 are prepared (Figure 6(b)). Three hours later, NR precipitates from solution with pH at 7.89, 8.18 and 10.31 (Figure 6(c)). This is due to that H+ in NR will be lost via the reaction with the OH- anion in the basic solution, which decrease the solubility of NR in water. MO does not precipitate from water when the pH values of MO solution are 2.45, 4.31, 6.53, 8.38 and 10.55 (Figure S26). This maybe from that MO is one kind of organic sodium salt and the negative charge in MO anion is not from OH- (Figure S24(a)). MB also does not precipitate from water when the pH values of MB solution are 2.41, 4.17, 6.35, 8.31 and 10.35 (Figure S27). MB is methythioninium chlorid and its positive charge is not from H+ (Figure S24(b)). From Figure. S22, BN-30-600 has negative surface when the pH of solution is larger than 2.8. For NR solution, the initial pH is 3.8. After the addition of BN-30-600 into NR solution, the pH value increases to the value between 6.5 and 7.0 (Figure S23). According to Figure 6(b), NR does not precipitate from water in the removal experiment since the pH value of solution is still below 7. Thus, BN-30-600 is most likely to remove NR by attracting H+ in NR. The high affinity of BN-30-600 for H+ has been confirmed. The pH value of the deionized water can be increased to 8.89 from the initial 6.82 after the addition of BN-30-600 into deionized water. For confirming this interesting removal mechanism, another two cationic dyes, MG and CV, have been further adopted to check the removal mechanism of NR by BN-30-600. CV is one kind of organic chlorides and positive charge is distributed in its organic skeleton (please see Figure S28(a)). But, the positive charge of MG (oxalate salt) is from H+ (please see the red circle in Figure S28(b)). If the proposed removal mechanism is right, BN-30-600 should also has high removal quantity for MG and low removal quantity for CV. From Figure 6(d), just a little CV is removed


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by BN-30-600 and the removal quantity is 70.3 mg/g. But, MG is nearly completely removed by BN-30-600 (please see Figure 6(e)). The removal quantity of MG by BN-30-600 is 1040.6 mg/g. This supports that the proposed removal mechanism of NR by BN-30-600 is reasonable and BN30-600 can effectively removing these protonated dyes from water by attracting the H+ in these protonated dyes. The solubility of CV and MG at different pH values also investigated. According to Figure S29, CV does not precipitate from water when the pH values of solution are at 2.26, 4.21, 6.27, 8.02 and 10.14, respectively. This is similar to that of MO and MB, in which their dissolubility are also not influenced by the change of pH values. The result about the solubility experiment of MG at different pH values is very interesting. From Figure S30, MG also does not precipitate from water when the pH values of solution are at 2.28, 4.01, 6.05, 8.23 and 10.48, respectively. NR and MG are all the protonated dyes, but their solubilities in alkaline condition are different. This is most likely from that the anion in MG is oxalate ion (Figure 28(b)) and the anion in NR is chloride ion (Figure S21). Oxalate ion can dissolve in water and is also organic specie, which can help its organic counterpart to dissolve in water after its organic counterpart loses H+ in alkaline condition. But, chloride ion is inorganic species, which is not easy for it to help its organic counterpart to dissolve in water after its organic counterpart loses H+ in alkaline condition. The difference between MG solutions with pH value smaller than 7 and MG solutions with pH value larger than 7 is that the color of the latter is a little different from that of the former (Figure.S30). This is most likely attributed to that the molecular structure of MG is changed when the pH of solution changes, which results in that the absorption peak of MG solution for light is different in acidic condition and alkaline condition. Similar phenomenon can also be found in the solubility experiment of MO at different pH values. In Figure S26, the left one is the MO with different pH values at starting


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time and the right one is the MO solution with different pH values after three hours later. One can see that the color of MO solution changes when the pH of solution changes. This is from that the pH change of MO solution results in the change of MO structure, which further leads to the absorption change of MO for light. From Figure. 6(b) and 6(c), NR can also be removed by using alkaline to adjust the pH of NR solution. However, this does not mean that using alkaline to remove NR is better than using the layered BN-30-600 composite. For example, sodium cations will be left in water when sodium hydroxide is used to remove NR from waste water. Furthermore, alkaline water is also harmful to the environment and human being. Acid (such as hydrochloric acid) is necessary used to adjust the pH of alkaline water into neutral. This results in that chloride anion is left in water as well. It has reported that the excess sodium cation and chloride anion are harmful to the bird,57 freshwater mussels 58 and oreochromis niloticus fingerlings.59 So, these sodium cations and chloride anions should also be removed from water and this will increase the cost of purifying wastewater with alkaline. When the layered BN-30-600 composite is used to remove NR from water, it does not has the problem mentioned above using alkaline to remove NR from water. This is because that no sodium cations are left in water and no acid is needed to be used for adjusting the pH of NR solution since the pH of NR solution after the removing experiment is between 6.5 and 7.0 (Figure. S23). In addition, BN-30-600 composite can be used many times, which will further decrease the cost using it to remove NR from water. Furthermore, MG is also the protonated dye and can be removed by BN-30-600 composite in high efficiency. But, alkaline can not remove MG from water since MG does not precipitate from water in alkaline condition. Thus, BN-30-600 composite is a good water cleaning material for removing these protonated dyes from water.


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The recycle performance of BN-30-600 for NR removal is also investigated. From Figure S31, the removal efficiencies of NR by BN-30-600 are 92% in the first time, 81% in the second time, 75% in the third time, and 72% in the fourth time, respectively. The removal quantity of NR by

Figure.7 (a) The removal of NR-MO solution by BN-30-600 for 8 hours (dosage: 5mg, NR concentration: 120 mg L-1, MO concentration: 120 mg L-1, volume: 55ml); (b) the removal of NRMB solution by BN-30-600 for 8 hours (dosage: 5mg, NR concentration: 120 mg L-1, MB concentration: 120 mg L-1, volume: 55ml); (c) the removal of MG-MO solution by BN-30-600 for 8 hours (dosage: 5mg, MG concentration: 120 mg L-1, MO concentration: 120 mg L-1, volume: 55ml); (d) the removal of MG-MB solution by BN-30-600 for 8 hours (dosage: 5mg, MG concentration: 120 mg L-1, MB concentration: 120 mg L-1, volume: 55ml)


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BN-30-600 decrease fast from the first time to the second one, being from that some NR are located in the deep pores of BN-30-600 and are not easily washed off. This will further hinder the interaction of BN-30-600 with H+. But the removal quantities of NR decrease slowly from the second time to the fourth time. Furthermore, the removal quantity of NR by BN-30-600 in the fourth time is still 862.5 mg/g, suggesting that BN-30-600 still is a good material to remove NR from waste water. In real dyeing wastewater, dyes coexist. Our previous studies show that the adsorption behaviour of chlorpyrifos by biochar in chlorpyrifos-atrazine solution is different from that in chlorpyrifos solution.56 Thus, it is necessary to investigate the removal efficiency of BN-30-600 for wastewater containing more than one kind of dye. From Figure. 7(a), a broad peak around 478 nm is found in the UV-vis spectrum of NR-MO solution, implying that there is an interaction between NR and MO since the UV peak of MO in this range is 467 nm (Figure 5(c)) and the UV peak of NR in this range is 524 nm (Figure S1). This is easily accepted because electrostatic attraction exists between the cationic NR and the anionic MO, which leads to the appearance of the broad peak around 478 nm in the UV-vis spectrum of NR-MO solution. After the addition of BN-30-600 into NR-MO solution for 8 hours, the UV peak at 478 nm of NR-MO solution shifts to 464 nm, which can be attributed to the characteristic UV peak of MO and the characteristic UV peak of NR at 524 nm is not found. All of this suggest that BN-30-600 can selectively remove NR from NR-MO solution and the remained dye in NR-MO solution is mainly MO. This can be further supported by the color (orange) of the NR-MO solution after the removal experiment, which is similar to that (orange) of MO solution (Figure 7(a) and Figure S26). The intensity of UV peak at 464 nm in the UV-vis spectrum of NR-MO solution after treatment by BN-30-600 is weaker than that in the UV-vis spectrum of MO after treatment by BN-30-600 (Figure 7(a) and Figure 5(c)),


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implying that the removal quantity of MO by BN-30-600 is also increased when MO and NR coexists. This is most likely from that the adsorbed NR can further adsorb some anionic MO. The treatment of NR-MB solution by BN-30-600 is investigated as well. According to Figure. 7(b), the UV-vis spectrum of NR-MB solution is nearly the overlap of the UV-vis spectra of both NR and MB (Figure 5(d) and Figure S1). For example, the peaks at 614 nm and 663 nm (Figure 7(b)) in the UV-vis spectrum of NR-MB solution are attributed the characteristic peaks of MB (Figure 5(d)), and the peak at 535 nm is attributed to the characteristic peak of NR (Figure S1). This maybe from that MB and NR are all cationic species, in which the interaction between them is weak and can not obviously influence the adsorption peaks of NR and MB in solution. After the addition of BN-30-600 into NR-MB solution for 8 hours, the peak at 535 nm obvious decreases, but the peaks at 614 nm and 663 nm just decrease a little. This implies that BN-30-600 prefers to remove NR from NR-MB solution and the remained dye in NR-MB solution is mainly MB. This can also be further supported by the color (blue) of the NR-MB solution after the removal experiment, which is similar to that (blue) of MB solution (Figure 7(b) and Figure S27). The removal of BN-30-600 for MG-MO and MG-MB solutions are also investigated. From Figure. 7(c), the interaction between MG and MO is not very strong since the UV peak at 617 nm can be assigned to MG (Figure 6(e)). This is most likely from that MG is not a planar molecule (Figure S28(b)), which inhibits its interaction with MO. After the addition of BN-30-600 into MGMO solution for 8 hours, the UV peak at 617 nm disappears and the UV peak at 432 nm shifts to 465 nm. All of this suggest that BN-30-600 can selectively remove MG from MG-MO solution and the remained dye in MG-MO solution is mainly MO. This can be further supported by the color (orange) of the MG-MO solution after the removal experiment, which is similar to that (orange) of MO solution (Figure 7(c) and Figure S26). According to Figure. 7(d), the UV-vis


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spectrum of MG-MB solution is nearly the overlap of the UV-vis spectra of both MG and MB (Figure 5(d) and Figure 6(e)). For example, the peaks at 615 nm and 661 nm (Figure 7(d)) in the UV-vis spectrum of NR-MB solution are attributed the characteristic peaks of MB (Figure 5(d)), and the peaks at 618 nm and 425 nm are attributed to the characteristic peak of MG (Figure 6(e)). This maybe from that MG and MB are all cationic species and the interaction between them is weak. After the addition of BN-30-600 into MG-MB solution for 8 hours, the peaks at 618 nm and 425 nm obvious decreases, but the peaks at 615 nm and 661 nm just decrease a little. Thus, BN30-600 prefers to remove MG from MG-MB solution and the remained dye in MG-MB solution is mainly MB. The color (blue) of the MG-MB solution after the removal experiment is similar to that (blue) of MB solution (Figure 7(d) and Figure S27), which also supports this inference. The removal of MG-NR mixed solution by BN-30-600 has also been investigated. From Figure S32, Figure 6(e) and Figure S1, the interaction between NR and MG is not strong since UV-vis spectrum of MG-NR mixture solution is nearly the overlap of MG and NR. This maybe from that MG and NR are all cationic dyes and the electrostatic repulsion prevent them packing together. In the UV-vis spectrum of MG-NR mixture solution, the absorption peak at 531 nm is assigned to the typical absorption of NR (Figure S1), while the absorption peak at 619 nm is assigned to the typical absorption of MG (Figure 6(e)). From Figure S31, BN-30-600 has a higher removal efficiency for NR than that for MG (70% vs 35%), suggesting that BN-30-600 prefer to remove NR from the NR-MG mixture solution. The result is very interesting. For understanding this experimental phenomenon, the molecular structures of MG and NR have been compared. From Figure S21, NR just has two methyl groups that bound with N atom and the rest part is of planar structure. But for MG, one aromatic ring is vertical to the other two aromatic rings except for the methyl groups bound with two N atoms


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(Figure 28(b)). In addition, the interaction between oxalate anion and MG cation may also weaken the removal of BN-30-600 for MG. Thus, the structure difference for MG and NR is most likely responsible for the higher removal of BN-30-600 for NR than that for MG when MG and NR are mixed together. Conclusion In summary, the layered BN-CN composite (BN-30-600) has been successfully synthesized by using BN nanoparticle and urea as precursors. TEM, FTIR and 1H NMR characterizations support that the layered BN-30-600 composite is most likely formed via the dehydration reaction between -OH groups in BN nanoparticle and -NH2 groups in CN. BN-30-600 can effectively remove the protonated dyes (NR and MG), while has weak adsorption for MO, MB and CV. This high removal quantity is from that BN-30-600 has high affinity for H+ bound with the N atoms in NR and MG. Furthermore, BN-30-600 can selectively remove NR from NR-MO and NR-MB mixed solutions as well as MG from MG-MO and MG-MB mixed solution. When NR and MG coexists, BN-30600 prefers to remove NR from the MG-NR mixed solution. As we know, this is first time to synthesize BN-based layered composite with BN nanoparticle as precursor and also the first time to report the selective removal of the protonated dyes (NR and MG) by water cleaning materials. This will be helpful to develop new type of BN-based material for water cleaning and deep understand the removal mechanism of dyes by the water cleaning materials.

Corresponding Author * E-mail: [email protected]


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Supporting Information: The TEM images of BN, BN-30-600, BN-CN400-600, BNW-30-600, BN-30-400, BN-30-500, SEM Figurers of BN, BN-10-600, BN-20-600, BN-30-600, BN-40-600 and BN-60-600, the FTIR and Zatal potential results of BN-30-600, the kinetic and isotherm parameters of NR removal by BN-30-600, the results of molecular simulation about the seizes of NR cation, MO anion and MB cation as well as the interaction energies between BN-30-600 and NR cation, MO anion, MB cation etc, were all provided in supporting information.

Notes The authors declare no competing financial interest

Acknowledgements We are grateful for the grants for Project supported by the National Science Funds for Creative Research Groups of China (No.51421006), the Key Program of National Natural Science Foundation of China (No. 91647206), the National Major Projects of Water Pollution Control and Management Technology (No. 2017ZX07204003) ， the National Key Plan for Research and Development of China (2016YFC0502203) ， the Fundamental Research Funds for the Central Universities (2015B25314), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Layered boron nitride (BN)-carbon nitride (CN) composite has been first time synthesized, which can selectively remove the protonated dyes from wastewater.


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Developing a Novel Layered Boron NitrideâCarbon Nitride Composite

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