Boron nitride nanomaterials with different morphologies: Synthesis, characterization and efficient application in dye adsorption

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Boron nitride nanomaterials with different morphologies: Synthesis, characterization and efficient application in dye adsorption Preeti Singla, Neetu Goel, Vinod kumar, Sonal Singhal

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Cite this article as: Preeti Singla, Neetu Goel, Vinod kumar, Sonal Singhal, Boron nitride nanomaterials with different morphologies: Synthesis, characterization and efficient application in dye adsorption, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2015.04.151 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Boron Nitride Nanomaterials with different morphologies: Synthesis, Characterization and efficient application in dye adsorption. Preeti Singlaa, Neetu Goela, Vinod kumarb and Sonal Singhala* a

Department of Chemistry, Panjab University, Chandigarh-160014, India b

Icon Analytical Equipment (P) Ltd., Mumbai, 400018, India.

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Abstract Novel boron nitride (BN) nanomaterials with unique and unprecedented properties such as semiconductor with constant band gap, high chemical stability, high thermal conductivity and excellent mechanical properties have been successfully fabricated in the present work in different morphologies as BN nanoparticles and BN nanosheets. Solid state high temperature chemical reactions were adopted for their engineering and various techniques like powder Fourier transform infrared spectroscopy, X-ray diffraction technique and high resolution transmission electron microscopy were employed to examine their morphology and other physicochemical properties. The FT-IR spectra revealed the presence of the B-N bond stretching and bending of B-N-B bond. The hexagonal structure of these BN nanomaterials was divulged from the XRD data, while their different morphology and size were depicted from the high resolution transmission electron microscopy. The nanoparticles are found to be spherical with average diameter of ~ 30 nm and the BN sheet is two dimensional with average thickness of ~ 0.90 nm. The lattice interplaner distance of 3.30 Å determined by high resolution transmission electron microscopy is in good agreement with that of calculated using XRD data. The high surface areas as calculated from the BET analysis are 120.68 m2g-1 and 50.02 m2g-1 for BN nanoparticles and BN nanosheets respectively. The present work also reports the adsorption behaviour of the synthesized BN nanomaterials towards the adsorption of two different dye molecules, namely, brilliant green and methyl orange. The adsorption conditions have been optimized by varying a number of experimental parameters such as initial dye concentration, adsorbent dosage, pH and time. The interactions of dye molecules with the adsorbate nanomaterials, nature of feasibility of adsorption process have been understood by performing calculations within the formalism of density functional theory at B3LYP/6-31G level of theory. Key Words: Boron nitride nanomaterials; Dye adsorption; High resolution TEM; Density functional theory

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1.

Introduction Boron nitride (BN) nanomaterials, isoelectronic and isostructural to carbon have

attained a significant place in the recent world of nanoscience owing to their high thermal stability, conductivity and mechanical strength [1-3]. Their semiconducting nature with wide band gap (~ 5.5 eV) irrespective of structural parameters [4,5] and chemical inertness towards oxidation make them suitable for a plethora of applications[6,7]. Such unique attributes have dragged the major concentration of scientists in the recent years for the fabrication of BN nanomaterials in different morphologies such as nanotubes [8], nanoparticles [9], nanoribbons [10], nanosheets [11] and nanofibers [12]. Apart from their remarkable compatibility in electronic and mechanical devices, BN nanomaterials have been explored for their potential as adsorption surfaces with sure recyclability that is facilitated by their chemical inertness [13-15]. Different morphologies of BN nanomaterials have been tested as adsorbent for the removal of dyes that are the most widely used colouring agents in various industries such as paper, plastic, textile, food and cosmetics along the development of industries and human society. The concentration of dyes in the industrial effluent has a major contribution in the environmental pollution that causes direct destruction of flora and funna [16-18]. The economical and simplistic adsorption technology for waste water treatment [19] has gained superiority over the dye degradation process that is made difficult by highly stable and complex structure of dyes. A few BN nanomaterials with different morphologies have been used as adsorbent for the removal of dyes, solvents and oils. Lei et al. fabricated the porous BN nanosheets that showed great adsorption towards oils, solvents and dyes [20]. Xue et al. developed mesoporous hexagonal boron nitride fibers for the fast and efficient removal of methylene blue from the waste water [21]. Boron nitride nanocarpets have also been synthesized by Zhang et al. that exhibit quick adsorption rate for the adsorption of methylene blue [22]. Lian et al. have fabricated BN ultrathin fibrous nanonets and employed their excellent performance for water treatment by ultrafast adsorption of methylene blue dye [13]. Several theoretical investigations suggest the favourable and efficient adsorption of a large number of molecules including gases [14,23-25], aromatic compounds [15,26,27], drugs [28,29] and various metals [30,31] on the surface of BN nanomaterials. However, the experimental efforts to validate the wide applicability of BN nanomaterials are still lacking. The limited study reported in this respect is pertaining to efficient adsorption of H2 gas over 3

the surface of modified BN nano structures [32] and successful removal of Arsenic from water by its efficient adsorption on the surface of Fe3O4 nanoparticles coated boron nitride nanotubes [33]. The present study provides a comprehensive understanding of the efficiency of BN nanomaterials as advanced adsorbent material for effectual adsorption and removal of organic pollutants from waste water by experimental elucidation, further supported by theoretical investigations. The BN nanomaterials have been synthesized in two different morphologies i.e. BN nanoparticles and BN nanosheets using solid state reaction. The efficacy of the synthesized nanomateials towards the adsorption of brilliant green (BG) and methyl orange (MO) dyes have been explored in detail by taking into account the impact of various factors such as initial dye concentration, pH, adsorbent dosage and time. Up till now, no work has been carried out on the morphology based study of BN nanomaterials for the adsorption of cationic and anionic dyes with different experimental parameters along with this work also validated the theoretical suggestions with the experimental results. The experimental conclusion about rapid and efficient adsorption of dyes on the nanomaterial surface is backed up by density functional theory (DFT) based theoretical studies that provide a thorough understanding about feasibility and nature of adsorption.

Experimental 2.1

Chemicals

Boric acid (H3BO3, 99.5%) was obtained from Fisher Scientific. Urea (CO(NH2)2, 99%), melamine (99%), Methyl Orange and Brilliant Green were purchased from Central Drug House (CDH) and used without further purification. Water was deionized using an ultrafiltration system (Milli-Q, Milipore). 2.2

Fabrication of BN nanomaterials

Different morphologies of BN nanomaterials; BN nanoparticles and BN nanosheets were successfully synthesized by solid state thermal annealing methods using different reacting materials in Nitrogen gas atmosphere. The detail procedures were discussed below. 2.2.1 Synthesis of BN nanoparticles To obtain the highly crystalline and pure BN nanoparticles, a commonly used solid state thermal annealing method was adopted for their fabrication [34]. Firstly the precursor was prepared by using boric acid and melamine as starting materials. Both the reacting materials were mixed well in pestle mortar in 1:1 molar ratio and then sintered at 200 ºC for 1 h and further for 2 h at 300 ºC in nitrogen atmosphere. The obtained precursor was then subjected

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to pulverization and then loaded in alumina boat. Afterwards the alumina boat was heated in furnace at 1400 ºC for 2 h in the presence of nitrogen gas atmosphere. The precursor was transformed into BN compounds with volatilization of H, O and C during annealing in the nano range. The final product was collected, washed with deionized water and then finally dried for characterization and use as adsorbent. 2.2.2 Synthesis of BN nanosheet Boric acid and Urea were used as starting materials for the synthesis of BN nanosheet [11]. To homogenize the reactants 1:12 molar ratio of boric acid to urea were dissolved in 40 ml of deionized water and heated at 65 ºC till the complete evaporation takes place. A homogeneous mixture of boric acid and urea obtained by this method was further heated at 900 ºC for 5 h in the nitrogen atmosphere to fabricate the BN nanosheets. White coloured product was formed at the end of reaction that was extracted, washed with deionized water and then dried. The details of the reaction are explained by the following equations: 2B(OH)3

∆

B2O3 + 3H2O

NH2CONH2

∆

NH3 + HNCO

B2O3 + 2NH2

∆

2BN + 3H2O

On the basis of these reactions, the mechanism of the fabrication of sheet like morphology of BN has also been proposed. The production of ammonia gas in the second step of the reactions might form large bubbles of atomically thin hexagonal BN that got flattened at high temperature and resulted in the formation of two dimensional sheet like morphology of BN i.e. BN nanosheets. As suggested by wang et. al [35] “Chemical Blowing” method leads to the formation of BN and Cx-BN nanosheets. 2.3 Physical measurements The Fourior transformed infra red spectra for all the BN nanomaterials were recorded on FT-IR spectrophotometer (Perkin Elmer) in the range of 4000 – 450 cm-1. Formation of BN compounds was confirmed by the powder X-ray diffraction (XRD) technique using PANalytical’s X’pert PRO spectrophotometer with Cu-Kα radiations. Particle size, morphology and elemental composition of all BN nanomaterials were determined using High resolution transmission electron microscopy (HRTEM) and Energy dispersive X- ray (EDX) analysis (FEI Tecnai, G2 F20) with an accelerating voltage of 200 keV. The pH meter of MAX with glass electrode was used for the pH measurements. UV-Vis spectra of dye solutions were measured by UV-Visible spectrophotometer (JASCO model V-530). The surface area of both the samples were determined by N2 adsorption using BET surface area

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analyzer ((11-2370) Gemini, Micromeritics, USA) that was operable in the temperature range of 10 ºC to 350 ºC. To remove the adsorbed gases and moisture, the samples were preheated at a temperature of 150 ºC for one hour. 2.4 Adsorption Procedure To explore the adsorbent properties of BN nanomaterials, adsorption studies were carried out for the removal of BG and MO dyes from the aqueous solutions. The chemical structure and basic properties of these dyes were given in Fig 1 and Table 1.The stock solutions of both dyes were prepared by dissolving the accurately weighted quantities of solid dyes in deionized water. In adsorption experiment, the 100 ml of dye solution with known concentration was taken in 250 ml glass beaker where a definite amount of adsorbent was dispersed using sonication. The dispersed solution was continuously stirred on magnetic stirrer at a constant speed of 500 rpm. After predetermined time intervals, the sample solutions were withdrawn and filtered using 0.45 µm nylon syringe filter to analyze the residual concentrations of dye. The initial and final concentrations in aqueous solutions were measured by using UV-Vis spectrophotometer at 625 nm and 463 nm for BG and MO respectively. Upon analyzing the UV-Vis data, the percentage of removal efficiency for both dyes by BN nanomaterials were calculated using following equation: %R =

(Co − Ct ) ×100 Co

Eq (1)

where % R (adsorption ratio) is the percentage of removal of BG and MO dyes (%). C0 is the initial concentration of BG and MO dye solutions (mg/L) and Ct is the dye concentration (after adsorption) at any time t (mg/L). The effect of experimental parameters such as initial dye concentration, pH, adsorbent dosage and time on the adsorption was discussed in detail. 2.5 Computational method Feasibility of the adsorption process and nature of interactions between adsorbent and adsorbate were examined by theoretical calculations within the DFT formalism using Becke, 3-parameter, Lee–Yang–Parr (B3LYP), hybrid functional [36,37] and 6-31G basis set as implemented in Gaussian 09 suit of programme [38]. A two dimensional h-BN nanosheet model consisting of 36 B and 36 N atoms in hexagonal manner was chosen as adsorbent where ends were saturated with hydrogen atoms to avoid boundary effects. The full geometry optimization and density of state (DOS) analysis was performed for bare and dye adsorbed BN nanosheet. GaussSum program was opted to study the DOS [39]. 6

2.

Results and Discussion

3.1 Characterization of adsorbents 3.1.1 Fourier transform infra-red (FT-IR) Spectroscopy The infrared analysis of all the samples of BN nanomaterials was carried out to confirm the presence of B-N bond. Both the samples were exposed to infrared rays resulted the FTIR spectra (shown in Fig. 2) with characteristic peaks of B-N bond. A small and broad frequency band at around 3200 cm-1 was attributed to the stretching vibration of O-H bond due to the presence of water. The most important information received from the spectra was the strong asymmetric band appeared at around 1370 cm-1 that attributed towards the stretching frequency of B-N bond and peak at around 760 cm-1 was ascribed to the bending of B-N-B bond as this was the only bond present in the BN nanomaterials [40]. Therefore, the presence of characteristic peaks of BN assured the formation of BN nanomaterials. 3.1.2 Powder X-ray Diffraction (XRD) Patterns The phase purity and crystal structure of BN nanomaterials were investigated using non-destructive powder XRD studies shown in Fig. 3. All the peaks indexed to (002), (101) and (004) lattice plane indicated towards the hexagonal type of crystal structure in both the BN nanomaterials (JCPDS file no. 00-034-0421). The sharpness of these peaks confirmed the formation of highly crystalline single phase BN nanomaterials with P63/mmc space group. Additionally the interplannar (002) distance obtained from the XRD pattern 3.3 Å was an another evidence in the favour of crystallanity of h-BN nanomaterials. The lattice parameters a = 2.5 Å and c = 6.6 Å were also deduced from this study confirming their hexagonal crystal structure. The crystallite size of these BN nanomaterials was calculated by evaluating the line broadening of the most intense (002) peak using the classical Scherrer equation [41].The crystallite sizes of both the BN nanomaterials were found to be ~13 nm and 15 nm for BN nanoparticles and BN nanosheets respectively. 3.1.3 Morphology, Size and composition of the nanomaterials HRTEM and EDX provide an insight vision of structure, morphology, elemental composition and particle size of the BN nanomaterials. HRTEM also depict the presence of specific lattice planes as shown by XRD analysis. Prior to analysis, the obtained samples were undergone ultra sonication for 1 h in ethanol to completely disperse the BN nanomaterials. Afterwards the disperse samples were loaded over carbon coated copper grid. A uniform and homogeneous distribution of BN nanoparticles was shown in Fig 4. TEM images with increase in magnification levels at 0.2 µm (Fig 4(a)), 100 nm (Fig 4(b)), 50 nm

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(Fig 4(c)) and 20 nm (Fig 4(d)), confirmed the spherical shape of the particles with size in the range of 20 to 35 nm. HRTEM of the BN nanoparticles shown in Fig 4(f) clearly indicated the lattice inter-planer distance 0.33nm that was believed to represent the (002) plane of hBN. This was in good agreement to the XRD results that depicted the inter-planer distance 0.33 nm correspond to the 002 plane. As the crystallinity of the BN nanoparticles remains intact at nanolevel thus they could also be referred as nanocrystals. The reconstructed image after filtering of HRTEM image (shown in Fig 5 (a)) was also reported in Fig 5(b) along with the profile of frame (Fig 5(c)) with average fringe width 0.33 nm. The arrangement of electron diffraction rings as well as white spots in the selected area electron diffraction (SAED) patterns (shown in Fig 5 (d)) added another confirmation to the crystalline nature of synthesized BN nanoparticles. The surface of the BN nanoparticles was studied using Scanning transmission electron microscopy (STM) shown in Fig 4 (e). The uniformly spread sheet like morphology was clearly observed by the TEM images of BN nanosheets at 100 nm (Fig 6 (a)) and 20 nm (Fig 6(b)). Upon measuring the thickness of the BN nanosheet by HRTEM image (shown in Fig 6(c)) that was around 0.8-0.9 nm, the number of layers were estimated between 2 to 3 by taking 0.3 nm inter-layer distance (shown in Fig 6(d and e)) into account. The SAED pattern (shown in Fig 6(f)) confirms the presence of crystalline nature in the BN nanosheet corresponding to the (002) plane. The purity and elemental composition of the BN nanomaterials were also discerned via simulation from the EDX analysis for both morphologies as BN nanoparticles (shown in Fig 5(e)) have B: N = 1: 0.9 and BN nanosheets have B: N = 1: 0.8. The peaks of Cu and C were also appeared as carbon coated copper grid was used as sample carrier. 3.1.4 BET Surface area analysis As the surface area plays a key role in the process of adsorption, therefore, it was evaluated using Brunauer–Emmett–Teller (BET) analysis. The outgassing of samples was carried at 150 ºC prior to nitrogen adsorption. The quantity of gas adsorbed was measured with respect to the relative pressure of the gas. The specific surface area and total surface area were calculated from the BET adsorption relation [42]. The relative pressure (P/Po), quantity adsorbed and other empirical values were reported in the Table 2 and 3 that were required for plotting the BET adsorption isotherm i.e. the plot of 1/[Q{(Po/P)-1}] vs. P/Po. The values of slope and intercept were noted from the plots (shown in Fig 7) that were observed to be 0.035811 g/cm³ STP and 0.000260 g/cm³ STP respectively for BN nanoparticles while 0.086310 g/cm³ STP and 0.000713 g/cm³ STP respectively for BN nanosheets. From these, the values of quantity of monolayer adsorbed and BET constant 8

were calculated. Furthermore, these values were used to calculate the total surface area (STotal) and specific surface area (SBET). STotal and SBET were observed to be 120.68 m2g-1 and 536.62 m2g-2 respectively for BN nanoparticles while 50.02 m2g-1 and 99.80 m2g-2 respectively for BN nanosheets. It was depicted from the results that the surface area of the BN nanoparticles was higher than that of the BN nanosheets. 3.1.5 Optical properties The effect of adsorption on the optical properties was investigated using diffused reflectance (DR) UV-Visible spectroscopy. The optical absorption coefficient near the band edge based upon the following equation:

αhν = A(hν − Eg)1/2

Eq (2)

where α, h, ν, Eg, and A are the absorption coefficient, Plank’s constant, light frequency, band gap, and proportionality constant respectively. The band gap values were examined from the Tauc plot i.e. plot of (αhv)2 versus photon energy (hν) after the adsorption of dye over the surface of BN nanomaterials are shown in Fig. 8. The band gap values were calculated by extrapolating the linear part of the curve obtained by plotting (αhν)2 vs hν to cut the energy axis. The estimated band gap values were found to be 2.72 eV and 3.02 eV for BG and MO adsorbed BN nanomaterials respectively. Remarkable reduction in the band gap of BN nanomaterials was observed after the dye adsorption. The semiconductor nature of the BN nanomaterials is transformed into the conductor with significant reduction in the band gap. 3.1.6 Zeta potential The surface charge is one of the important parameter that controls the type of adsorbate to get absorb. To fulfil this demand, Zeta potential analysis was performed for both the BN nanomaterials. By definition Zeta potential is the potential difference between the dispersion medium and the stationary phase of the medium attached to the dispersed particles. The values of the zeta potential reveal the type and value of charge on the dispersed particles. For BN nanomaterials, the medium was kept water at natural pH. A constant voltage (3.4 V) was applied between the electrodes placed in the dispersion medium and conductivities of both the BN nanomaterials were measured that was found to be 0.114 mS/cm and 0.123 mS/cm for BN nanoparticles and BN nanosheets respectively. From this zeta potential values were evaluated that are -37.6 mV and -59.0mV for BN nanoparticles and BN nanosheets respectively. The negative values of the zeta potential demonstrate the presence of negative charge on their surface. 3.2 Adsorption studies

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To explore the adsorbent properties of BN nanomaterials, adsorption of BG and MO dyes was investigated on the surface of BN nanoparticles and BN nanosheets. The UV-Vis adsorption spectra of BG and MO on the BN nanoparticles and BN nansheet were shown in Fig. 9 and 10 respectively that clearly indicate the amazing adsorbed properties of BN nanomaterials by rapid and high adsorption of dyes on their surface. Effect of various experimental factors such as initial dye concentration, adsorbent dosage, pH and time on the % removal was discussed in details. 3.2.1 Effect of initial dye concentration The removal efficiencies of BN nanoparticles and BN nanosheets were examined with the different initial dye concentrations for both BG and MO dyes. The dye concentration was varied from 0.01 mM to 0.025 mM of both dyes while other experimental parameters; adsorbent dosage (0.01 g/L), pH (6) and contact time (upto 60 min) were kept constant. The percentage of removal efficiency was observed to be decreased with increase in initial concentration of both dyes using BN nanoparticles (shown in Fig. 11 (a)) and BN nanosheet (shown in Fig. 12 (a)). This may be attributed towards the reduction in the adsorption sites which were responsible for the adsorption due to Van der Waals interactions between dye molecules and adsorbents. 3.2.2 Effect of adsorbent dosage In order to investigate the effect of adsorbent dosage on the percentage of removal efficiency for both adsorbents, a series of adsorption experiments were performed. The adsorbent dosages were varied form 0.25 g/L to 2.00 g/L at the initial dye concentration of 0.01 mM and solution pH of about 6 for both dyes. The percentage removal efficiency of both dyes with different concentrations of adsorbents; BN nanoparticles and BN nanosheets were shown in Fig. 11(b) and 12 (b) respectively. The results indicated by the figures depict the increase of percentage removal efficiency with increase in adsorbent dosage that can be explained on the basis of increase in the availability of the adsorption sites on the surface of adsorbents. 3.2.3 Effect of pH The adsorption studies are greatly affected by the solution pH that directly influences the surface charges and surface binding sites of the adsorbents and the structures of the dye molecule. Therefore, effect on the percentage removal efficiency by pH was evaluated by varying the solution pH from 3 to 8 using 0.1 M NaOH and 0.1 M HCl solutions accordingly and shown in Fig. 11 (c) and 12 (c). It was analysed from the figures that different trend was followed by both dyes unlike other parameters that mainly attribute towards the type of 10

charge on them. As BG is a cationic dye, carrying +ve charge therefore decrement in removal efficiency was occurred below pH 7. This might be due to the increase in competition between H+ and positively charged dye that lead to lesser availability of adsorption sites. In case of MO dye, having negatively charged moiety, opposite trend was found. The removal efficiency increased at low pH upto 4 that might be due to the reduction of negative charge by neutralizing with H+, but at pH below 4, increase in competition between dye and H+ leads to decrease in removal efficiency. The presence of negative charge on the surface of BN nanomaterials depicted by zeta potential suits well with the results of adsorption of differently charged dye molecules at varied pH. The appearance of electrostatic repulsion between negatively charged BN nanomaterials and anionic dye molecule results their lesser adsorption as compared to cationic dye molecule. Therefore, the adsorption of dyes is found to be very specific towards the charge present on the surface of adsorbate. 3.2.4 Effect on Time To evaluate the rate of adsorption, contact time is very important factor in adsorption studies. The ultrafast adsorption of dyes on the surface of BN nanomaterials was observed by Lian et al. [13]. Similar adsorption behaviour with high rate of adsorption was seen in our case where maximum adsorption takes place within 1 min while a slight increment was noticed after 60 min exposure shown in Fig 11 (d) and 12 (d). Therefore, we could place the BN nanomaterials in the category of ultrafast adsorbent that would be highly useful for quick and significant removal of harmful compounds. 3.3 Theoretical Studies The optimized structures of both the dye molecules and the BN nanosheet were obtained at DFT level of theory. The dye molecule was placed at varying distance from the BN nanosheet model in order to ascertain the best possible site of interaction between the adsorbate and the adsorbent. At the completion of the optimization procedure that did not impose any symmetry constraints, the hetero atoms of dye molecule were aligned towards BN sheet surface as shown in the Fig 12. 3.3.1 Adsorption energy and structural parameters An important characteristic of the adsorption process is the adsorption energy that is usually defined as: Eq (3)

∆Eads = Ecomplex − Eadsorbent − Eadsorbate

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where Ecomplex is the total energy of optimized geometry of dye adsorbed BN nanosheet. Eadsorbent is the energy of optimized BN nanosheet and Eadsorbate is the energy of optimized dye structures. By definition,

Eads <0 corresponds to exothermic nature of adsorption.

The adsorption energies for BG and MO calculated according to the above equation are -104.58 and -90.97 Kcal/mol respectively. These values are indicative of exothermic adsorption, and suggest the thermodynamic feasibility of the adsorption process. The adsorption energy is high for both the dyes, the cationic dye (BG) gets adsorbed by evolving higher energy. Another significant aspect in the process is the nature of interaction between the dye molecule and the BN nanomaterial surface and it can be deduced from the optimized geometries of the concerned molecules. The atomic site in the dye molecule, closes to the nanomaterial surface is H atom for BG and O atom for MO and it is observed to be at a distance of 3.18 Å for BG and 3.13 Å for MO (shown in the Fig. 13) in the optimized geometry of BN nanosheet/dye system. As per earlier suggestions, the covalent interactions are negligible at the separation of 2.5 Å [43,44]. Thus, the greater separation as observed in the present case, excludes the possibility of chemisorption through covalent interactions. The adsorption process in the present case is physical in natures dominated by Van-der Waals attractions. This adds further advantages to the process owing to its easy desorption and reusability as BN nanomaterials were stable at high temperatures. 3.3.2 Electronic properties

The electronic modulations were also scrutinized by exploring their band gap examined by their density of state (DOS) plots (shown in Fig. 14). The band gap of the bare BN nanosheet computed here, comes out to be 5.66 eV [45]. The decrement in the band gap was noticed after adsorption of dye molecule upto 2.75 eV and 3.24 eV for BG and MO respectively. These values are in good agreement with the experimentally observed values of the band gap of the dye adsorbed BN nanomaterials. The lowering in band gap is attributed to the appearance of new energy levels that represent the mixing of electronic states of the dye molecules and BN nanosheet.

Conclusions In this work, BN nanomaterials with different morphologies; BN nanoparticles and BN nanosheets were successfully synthesized using solid state high temperature annealing method. The formation of all the samples was confirmed by the FT-IR and XRD analysis where as the morphologies and particle size of them was revealed with the help of TEM. The application of these nanomaterials has been explored in the field of adsorption both

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experimentally and theoretically for the removal and adsorption of BG and MO dyes. Effect of various experimental parameters such as initial dye concentration, adsorbent dosage, pH and time on the percentage removal efficiency was also investigated for both dyes and adsorbents. With increase in the initial dye concentration the removal efficiency decreased due to lower availability of adsorption sites while with increase in adsorbent dosage the removal efficiency increased due to increase in the adsorption sites. The effect of pH was also studied that indicate the decrease in removal efficiency with decrease in pH for cationic dye i.e. BG while opposite has been observed in case of anionic dye i.e. MO. This was due to the presence of negative charge over the surface of BN nanomaterials. The change in removal efficiency as the function of time indicated the very fast adsorption of dyes on the surface of BN nanomaterials. The exothermic and physical nature of the adsorption process has been established by theoretical calculations. The decrement in band gap after adsorption, observed both theoretically and experimentally point towards the sensing capabilities of the BN based nanomaterials through electronic measurements.

Acknowledgements N.G. thanks University Grants Commission (U.G.C.), New Delhi under grant F.No.41342/2012 (SR) for financial support. S.S. Gratefully acknowledges financial grant from Council of Scientific and Industrial Research (C.S.I.R.) via grant No. 01(2499)/11/EMR-II. P.S. also thanks C.S.I.R. for the senior research fellowship.

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16. Y. Wong, J. Yu, Laccase-catalyzed decolorization of synthetic dyes, Water. Res. 33 (1999) 3512–20. 17. L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Self-Assembled 3D Flowerlike Iron Oxide Nanostructures and Their Application in Water Treatment, Adv. Mater. 18 (2006) 2426-2431. 18. J.B. Zimmerman, J.R. Mihelcic, S. James, Toward Understanding the Efficacy and Mechanism of Opuntia spp. as a Natural Coagulant for Potential Application in Water Treatment, Environ. Sci. Technol. 42 (2008) 4247-4254. 19. M.T Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: A review, Adv. Colloid Interface Sci. 209 (2014) 172-184. 20. W. Lei, D. Portehault, D. Liu, S. Qin, Y. Chen, Porous boron nitride nanosheets for effective water cleaning, Nat. Commun. 4 (2013) 1777-1784. 21. L. Xue, B. Lu, Z.S. Wu, C. Ge, P. Wang, R. Zhang, X.D. Zhang, Synthesis of mesoporous hexagonal boron nitride fibers with high surface area for efficient removal of organic pollutants, Chem. Eng. J. 243 (2014) 494-499. 22. X. Zhang, G. Lian, S. Zhang, D. Cui, Q. Wang, Boron nitride nanocarpets: controllable synthesis and their adsorption performance to organic pollutants, Cryst. Eng. Comm. 14 (2012) 4670-4676. 23. P. Singla, S. Singhal, N. Goel, Theoretical study on adsorption and dissociation of NO2 molecules on BNNT surface, Appl. Surf. Sci. 283 (2013) 881-887. 24. J. Beheshtian, A.A. Peyghan, Z. Bagheri, Detection of phosgene by Sc-doped BN nanotubes: A DFT study, Sens. Actuators B 171 (2012) 846-852. 25. R.J. Baierle, T.M. Schmidt, A. Fazzio, Adsorption of CO and NO molecules on carbon doped boron nitride nanotubes, Solid. State. Commun. 142 (2007) 49-53. 26. M. Noei, N. Ahmadsghaei, A.A. Salari, Ethyl benzene detection by BN nanotube: DFT studies, J. Saudi. Chem. Soc. 2013 In press.

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17

N

HSO4 -

Brilliant Green

N

O N N

S

N

O Na

O

Methyl Orange Fig 1. The Chemical Structure of Brilliant Green and Methyl Orange

18

100

BNNSheet BNNParticle

90 80

Transmission %

70 60 50 40 30 20 10 0 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Fig 2. FT-IR spectrum of all the morphologies of synthesized BN nanomaterials.

19

8000

(004)

(110)

4000

(112)

6000

(100) (101)

Relative Intensity

(002)

10000

(a)

2000

(b) 0 10

20

30

40

50

60

70

80

Angle 2θ Fig 3. XRD patterns of (a) BN nanoparticles and (b) BN nanosheets.

20

90

100

(a)

(b)

(c)

(d)

(e)

(f)

20 nm

Fig 4. TEM images of BN nanoparticles at (a) 0.2 µm, (b) 100 nm, (c) 50 nm and (d) 20 nm. STM image showing the surface of BN nanoparticles (e) and HRTEM showing the lattice interplaner distance and inset showing the FFT (f) fo the BN nanoparticles.

21

(a)

(b)

(c) 0.33 nm

(d)

(e)

Fig 5. HRTEM image of BN nanoparticles at 2 nm (a) and its reconstructed image after filtering (b) with profile of frame with average fringe width (c) have been shown along with SAED pattern (d) and Energy Dispersive X-Ray Spectra (e) of BN nanoparticles.

22

(a)

(b)

(c)

20 nm

(d)

0.33 nm

(e)

(f)

Fig 6. TEM image of BN nanosheets at (a) 100 nm, (b) 20 nm and (c) 5 nm showing the sheet width. HRTEM image (d) showing the lattice interplaner distance along with its reconstructed image after filtering (e) inset having the frame profile with average fringe width. Energy Dispersive X-Ray Spectra (f) inset having SAED pattern of BN nanoparticles.

23

0.012

(a)

1/[Q{(P/Po)-1}]

0.010

0.008

0.006

0.004

0.002 0.05

0.10

0.15

0.20

0.25

0.30

(P/Po) 0.024

(b)

1/[Q{(P/Po)-1}]

0.020

0.016

0.012

0.008

0.004 0.05

0.10

0.15

0.20

(P/Po)

0.25

Fig 7. The linear curve of the BET surface area of (a) BN nanoparticles and (b) BN nanosheets. 24

200

200

(a)

175 150

150

( α hν ν)2 (eV/cm2)

( α hν ν)2 (eV/cm2)

(b)

175

125 100 75 50

125 100 75 50 25

25

2.72 eV

0 1

2

3

4

hν ν (eV)

3.02 eV

0

5

6

1

7

2

3

4

hν ν (eV)

5

6

7

Fig 8. Plots of (αhν)2 vs hν showing band gap of (a) BG and (b) MO adsorbed BN nanomaterials.

25

0.55

0.25

0.50

MO

BG Pure dye 1 min 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min 50 min 60 min

Absorbance

0.40 0.35 0.30 0.25 0.20 0.15

Pure dye 1 min 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min 50 min 60 min

0.20

Absorbance

0.45

0.10

0.15

0.10

0.05

0.05 0.00 350

400

450

500

550

600

650

0.00

700

Wavelength (nm)

250

300

350

400

450

500

550

Wavelength (nm)

Fig 9. UV-Visible spectra of pure and adsorbed dyes: BG and MO on the surface of BN nanoparticles. 0.50

0.25

0.45

Absorbance

0.35

pure dye 1 min 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min 50 min 60 min

0.30 0.25 0.20 0.15

0.20

Absorbance

0.40

pure dye 1 min 5 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min 50 min 60 min

MO

BG

0.10

0.15

0.10

0.05

0.05 0.00 350

400

450

500

550

600

Wavelength (nm)

650

0.00

700

250

300

350

400

450

500

550

Wavelength (nm)

Fig 10. UV-Visible spectra of pure and adsorbed dyes: BG and MO on the surface of BN nanosheets.

26

100

100

(a)

80 70 60 50

BG

40

MO

30 20

80 70 60 50

BG

40

MO

30 20 10

10

0

0 0.005

0.01

0.015

0.02

0.025

0

0.03

Dye Concentration (mM)

0.005

0.01

0.015

0.025

0.02

Adsorbent Dosage (g) 100

100

(c)

(d)

90

Removal Efficiency (%)

90

Removal Efficiency (%)

(b)

90

Removal Efficiency (%)

Removal Efficiency (%)

90

80 70

BG

60

MO

50 40 30 20 10

80 70

BG

60

MO

50 40 30 20 10 0

0 2

3

4

5

6

pH

7

8

0

9

10

20

30

40

50

60

70

Time (min)

Fig 11. Effect of experimental parameters on the adsorption of BG and MO dyes on the BN nanoparticles: (a) initial dye concentration (b) adsorbent dosage (c) pH and (d) time.

27

100

(a)

90

Removal Efficiency (%)

Removal Efficiency (%)

100 80 70 60 50 40

BG

30

MO

20 10 0 0.005

0.01

0.015

0.02

0.025

(b)

90 80

BG

70

MO

60 50 40 30 20 10 0

0.03

0

Dye Concentration (mM)

0.01

0.015

0.02

0.025

Adsorbent Dosage (g)

100

100

(c)

(d)

90

Removal Efficiency (%)

90

Removal Efficiency (%)

0.005

80 70 60

BG

50

MO

40 30 20 10

80 70 60

BG

50

MO

40 30 20 10 0

0 2

3

4

5

pH

6

7

8

0

9

10

20

30

40

Time (min)

50

60

70

Fig 12. Effect of experimental parameters on the adsorption of BG and MO dyes on the BN nanosheets: (a) initial dye concentration (b) adsorbent dosage (c) pH and (d) time.

28

(b)

(a)

3.83 Å

3.13 Å

3.18 Å

Fig 13. Optimized geometries of dye adsorbed BN nanosheet complexes (a) Brilliant Green and (b) Methyl orange adsorbed BN nanosheet.

29

Eg = 5.66 eV

Energy (eV)

Eg = 2.53 eV

Energy (eV)

Eg = 2.66 eV

Energy (eV)

(e)

Eg = 2.75 eV

Energy (eV)

DOS (arb. units)

DOS (arb. units)

(d)

(c) DOS (arb. units)

(b) DOS (arb. units)

DOS (arb. units)

(a)

Eg = 3.24 eV

Energy (eV)

Fig 14. Density of state (DOS) plots of (a) Pristine BN nanosheet (b) Brilliant green (c) Methyl orange (d) BG adsorbed BN nanosheet and (e) MO adsorbed BN nanosheet complexes.

30

Table 1. Basic Properties of the Investigated Dyes Dye

Brilliant Green

Methyl Orange

Abbrrivation

BG

MO

Chemical Formula

C27H33N2HO4S

C14H14N3NaO3S

Molecular weight (g/mol)

482.64

327.33

Class

Triarylmethane

Azo

λmax (nm)

625

464

C.I

42040

13025

Charge

+1

-1

Table 2. Relative Pressure, Quantity of gas adsorbed and their empirical values calculated for BET plots for BN nanoparticles. P/Po

Q (cm3g-1STP)

Po/P

(Po/P)-1

Q{(Po/P)-1}

1/[Q{(Po/P)-1}]

0.04945 0.07696 0.10135 0.13189 0.15979 0.18762 0.21587 0.24376 0.27119 0.29907

25.5897 27.5116 28.9096 30.4847 31.8476 33.1827 34.5456 35.9174 37.3056 38.7732

20.222 12.994 9.867 7.582 6.258 5.330 4.632 4.102 3.687 3.344

19.222 11.994 8.867 6.582 5.258 4.330 3.632 3.102 2.687 2.344

491.8973 329.9750 256.3393 200.6557 167.4565 143.6803 125.4825 111.4324 100.2579 90.8717

0.002033 0.003031 0.003901 0.004984 0.005972 0.006960 0.007969 0.008974 0.009974 0.011005

Table 3. Relative Pressure, Quantity of gas adsorbed and their empirical values calculated for BET plots for BN nanosheets. P/Po

Q (cm3g-1STP)

Po/P

(Po/P)-1

Q{(Po/P)-1}

1/[Q{(Po/P)-1}]

0.04972 0.07736 0.10132 0.13208 0.16008 0.18801 0.21597 0.24408

10.4409 11.2958 11.9022 12.5855 13.1590 13.7059 14.2439 14.7779

20.111 12.927 9.869 7.571 6.247 5.319 4.630 4.097

19.111 11.927 8.869 6.571 5.247 4.319 3.630 3.097

199.5336 134.7289 105.5647 82.6982 69.0439 59.1922 51.7088 45.7673

0.005012 0.007422 0.009473 0.012092 0.014484 0.016894 0.019339 0.021850

31