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Extraordinary Adsorption of Methyl Blue onto Sodium-Doped Graphitic Carbon Nitride Maciej Fronczak, Milena Krajewska, Katarzyna Demby, and Michal Bystrzejewski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03674 • Publication Date (Web): 04 Jul 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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The Journal of Physical Chemistry

Extraordinary Adsorption of Methyl Blue Onto Sodium-doped Graphitic Carbon Nitride Maciej Fronczak, Milena Krajewska, Katarzyna Demby, Michał Bystrzejewski*

Faculty of Chemistry, University of Warsaw, Pasteur 1 street, PL 02093 Warsaw, Poland Corresponding Author *(Michał Bystrzejewski) E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.

ABBREVIATIONS PDA, polydopamine; PEI, polyethylenimine; rGO, reduced graphene oxide; GO, graphene oxide; NPs, nanoparticles;

1 Environment ACS Paragon Plus


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ABSTRACT: Herein, the adsorption performance of sodium-doped graphitic carbon nitride in relation to the removal of methyl blue is investigated. The adsorbent was synthesized via the direct thermal polycondensation of cyanamide in the presence of sodium chloride. The inclusion of sodium in graphitic carbon nitride resulted in a substantial improvement of its adsorption capacity and adsorption kinetics. The maximum capacity for methyl blue was at least 8 times higher in comparison to commercial activated carbon and even 36 times higher than in the case of undoped material. The obtained adsorbents had very low porosity and the resultant high adsorption capacities, as determined from the experiments, pointed to the extraordinary adsorption. Moreover, the equilibrium of the adsorption process was reached at the contact time less than 5 minutes. The obtained adsorbent were thoroughly investigated by means of various physical and chemical analyses. Additionally, the regeneration studies of the spent adsorbents were carried out.


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1. Introduction Graphitic carbon nitride (g-C3N4) is a solid material which consists of s-triazine or s-heptazine units bonded by nitrogen in two-dimensional layers1. The research on this unique material is still developing because of its prospective applications, especially in photocatalysis and catalysis, which are described in details in recent reviews25

. Graphitic carbon nitride is a semiconductor with the band gap of 2.7eV3. The incorporation of alkali metals ions

into the structure of graphitic carbon nitride has been reported in a few papers, only6-9. The doped graphitic carbon nitride has, in comparison to undoped g-C3N4, improved visible light absorption performance6, increased hygroscopicity7 or enhanced photocatalyst properties8-9. Moreover, g-C3N4 is used to obtain composites with other semiconductors10-13 or magnetic NPs14-15. The synthesis of g-C3N4 is commonly carried out via the polycondensation of cyanamide, dicyanamide1, melamine16-18 and its derivatives19. It is also possible to obtain g-C3N4 from thiocyanates19 and urea18,20. The synthesis is usually carried out in a typical laboratory furnace, via combustion synthesis or solvothermal methods. All these methods are described in the following reviews2,3,21. There is a relatively small number of papers dealing with the adsorption properties of g-C3N4, both on unmodified and modified. The adsorption properties of graphitic carbon nitride were investigated in recent papers14-15,22-25. Wang et al. described the adsorption properties of a composite comprising of iron oxide NPs and g-C3N4 and found that this material efficiently removes phthalate esters and can be used for the solid phase extraction applications14. The high adsorption capacity of g-C3N4 was found in the removal of perfluorooctane sulfonate (ca. 300 mg·g-1) and perfluorooctanoic acid (ca. 175 mg·g-1)15. The uptake of Cu(II) and Pb(II) onto functionalized graphitic carbon nitride with melamine-based dendrimer amine was investigated by Anbia et al.22. The adsorption capacity values were ca. 200 mg·g-1 for both studied metal ions. On the other hand, the removal of small organic compounds (e.g. aniline) resulted in the adsorption capacity of 50 mg·g-1 23. The composite of g-C3N4 with TiO2 was applied to the removal of chromium (VI) ions. In this case the adsorption capacity was on a low level (14 mg·g-1)24. The presented work is focused on the synthesis of undoped and sodium-doped graphitic carbon nitride and evaluation of their adsorption properties. The adsorption studies involved the experiments both in kinetics and equilibrium conditions. Furthermore, some attempts were taken to study the regeneration of spent adsorbents.



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2. Experimental section Synthesis of undoped and sodium-doped graphitic carbon nitride Sodium-doped graphitic carbon nitride was synthesized as follows: 30g of cyanamide aqueous solution (50% wt) was mixed with sodium chloride. Please note, that the amount of cyanamide in this solution was 15g. Next, NaCl was added to this solution. The amount of NaCl was referred to the total mass of cyanamide and NaCl. The NaCl content was between 5 and 75 wt. %. As for example, the sample with 25% content of NaCl was obtained from 30 g of the starting solution and 5 g of NaCl. Then, the solution was diluted to 1 dm3 with distilled water (in order to dissolve all NaCl) and placed in a oven at 80 oC for 24 h. After evaporation of water the solid mixture was put into porcelain crucibles (with a lid) and placed in a steel retort. The retort had two connections for inlet and exhaust gas. The retort was placed in a muffle furnace and was continuously flushed with argon (2 dm3·min-1). The thermal polycondensation included the two-step heating program. First, the retort was heated with the heating rate of 20 oC·min-1 to 550 oC, and then it was isothermally annealed at 550 oC for 3 h. Afterwards, the furnace was cooled down naturally. The yellow powder product was present only in the crucibles. The as-obtained product was washed with distilled water (40 oC, portions of 500 cm3) until the test with silver nitrate aqueous solution (10 %wt) did not detect the chloride anions. The undoped graphitic carbon nitride was synthesized and purified in the same way. The material, methods of synthesis and methods of regeneration are the subject of the patent application (Poland) no. P.417123 (submitted in May 2016 to the Polish Patent Office). Characterization Elemental analysis was performed on a CHNS analyzer Elementar Vario EL III. The X-ray photoelectron spectra (XPS) were acquired using a PHI 5000 Versa Probe (ULVAC-PHI) spectrometer with monochromatic Al Kα radiation (1486.6 eV) as the X-ray excitation source. The low temperature nitrogen adsorption measurements were performed at 77 K (-196 oC) using an ASAP 2020 Surface Area and a Porosimetry Analyzer (Micromeritics). The morphology of the products was investigated using a scanning electron microscopy (Zeiss Merlin). The phase composition studies were conducted on a Bruker D8 diffractometer using a Cu K-α radiation in a 2θ angle range between 10 and 40. The FTIR spectra were recorded using a Thermo Fisher Scientific Nicolet spectrometer. A petite amount (ca. 1 mg) of each sample was mixed with 300 mg of KBr (FTIR grade), then, the mixtures were pressed into pellets using a laboratory hydraulic press. The spectra were acquired in a transmission mode in the range of 500-3000 cm-1 with the spectral resolution of 4 cm-1. Shimadzu UV-2401 PC spectrometer was used for the


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determination of equilibrium concentration of methyl blue. The measurement were carried out in quartz cuvettes (optical path length 1 mm). The pH (in the determination of the point of zero charge) was measured with an Elmetron CPI-505 pH/ion-meter associated with an Elmetron EPS-1 glass electrode. 3. Results and discussion Synthesis of and characterization of undoped and doped g-C3N4 Six experiments were performed, in which the content of NaCl was varied. The materials were named as X-CN, where X refers to the percentage of NaCl in the initial mixture of the precursor (cyanamide) and NaCl (Table 1). The conversion yield was evaluated for each synthesis. This parameter was calculated with Formula 1, and the conversion yield in each case was in the range between 35 and 46 % wt (Table 1). The highest conversion yield was found in the case of the synthesis of the undoped g-C3N4. The addition of NaCl to cyanamide resulted in a decrease of the conversion yield and was found to be in the range of 35-43%. Conversion yield =

∙ 100% (Formula 1)

Table 1. Conversion yields and textural characteristics of obtained materials. Sample 0-CN 5-CN 10-CN 25-CN 50-CN 75-CN

NaCl content (%) 0 5 10 25 50 75

Conversion yield (%) 46 37 39 35 43 40

SBET (m2·g-1) 19 20 8 8 1 6

Vmicro (cm3∙g-1) 0,0011 0,0018 0,0015 0,0013 0,0001 0,0009

Vmeso (cm3∙g-1) 0,0765 0,0422 0,0271 0,0179 0,0004 0,0185

Vtotal (cm3∙g-1) 0,0834 0,0541 0,0321 0,0217 0,0005 0,0407

Mean pore size (nm) 8,8 5,4 8,0 5,4 ≈1 13,6

The textural properties of the synthesized materials were evaluated from nitrogen adsorption/desorption isotherms at 77 K. The isotherms are shown in Supplementary Data (Figures S1-S6). The evaluated values of the specific surface area (SBET) are between 1 and 20 m2·g-1 (Table 1). The highest surface area was found for the undoped and 5CN materials. The materials obtained from the mixtures richer in NaCl have lower porosity. Because of very low surface area both the undoped and sodium-doped g-C3N4 cannot be regarded as porous materials. The micropore volume in all materials does not exceed 0.002 cm3·g-1. The corresponding pore size distribution curves are shown in Supplementary Data (Figures S7-S12). The undoped graphitic carbon nitride and most of the doped materials (excluding 50-CN) are characterized by the presence of mesopores, whilst the micropore volume is on a very low level. The mesopore volume in the doped graphtic carbon nitride (for samples synthesized with the content of NaCl greater than 5 %) is lower in comparison to the undoped material (Table 1). This observation obviously is associated



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with smaller specific surface area in the doped carbon nitride. The 50-CN material contains some micropores, however, their incremental pore volume is lower than 4·10-4 cm3∙g-1. This value explains the lowest specific surface area among the all studied samples. The chemical composition was evaluated via elemental analysis. The results are shown in Table 2. The elemental analysis shows that all studied materials contain hydrogen, and this is connected with the presence of amino (primary and secondary) groups in the peripheral parts of g-C3N4 layers19. The content of hydrogen is on a similar level for all products. The carbon to nitrogen ratio for the studied is in the range of 0.617-0.650, whilst for ideal C3N4 this ratio is 0.75. This finding shows that the products are nitrogen-rich materials, and the most possible form of the nitrogen is the amino group, which origins from the incomplete condensation of the heptazine units. Importantly, the chemical composition of the obtained materials follows the content of NaCl (Figure 1). The carbon and nitrogen content monotonically decreases with the increase of NaCl. This finding suggests that the additive (NaCl) is homogenously doped in graphitic carbon nitride.

Figure 1. Content of carbon, nitrogen and hydrogen vs. percentage of NaCl in the initial mixture of the cyanamide and NaCl Table 2. Chemical composition (data from elemental analysis)for the products (% at) Element

0-CN

5-CN

10-CN

25-CN

50-CN

75-CN

Carbon Nitrogen Hydrogen 100-(C+N+H) C/N ratio

37.61 60.09 2.26 0.04 0.626

35.86 58.11 2.29 3.74 0.617

34.05 54.42 2.31 9.22 0.626

32.05 50.58 2.56 14.83 0.634

30.36 46.58 2.58 20.49 0.652

29.01 44.61 2.77 23.26 0.650


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The undoped material (0-CN) and the material synthesized with 50 % addition of sodium chloride (50-CN), were investigated by X-ray photoelectron spectroscopy. The XPS survey spectra are shown in Figures 2 and 3. The undoped g-C3N4 contains mainly carbon and nitrogen, whilst the presence of oxygen originates from contaminations. The 50-CN sample additionally contains (as expected) sodium and traces of chlorine. The chemical composition evaluated from the XPS spectra is listed in Table 3. Interestingly, the inclusion of NaCl in the synthesis process results in the incorporation of sodium only. Importantly, the relative content of sodium is 100 times higher than the content of chlorine. The C1s, N1s, O1s, Cl2p and Na1s components are presented in Figures S13-S22. The components were deconvoluted and the peak energies are listed in Table 4. The C1s and N1s components are typical for graphitic carbon nitride18 and this findings confirm that the inclusion of NaCl also results in the material which has the structural features of g-C3N4. The position of the Na1s component in the 50-CN material is typical for sodium ion9 and this observation proves that sodium is incorporated in g-C3N4 as a cation. The total content of carbon and nitrogen derived from XPS spectrum for 50-CN material is higher (93.61%) than from the elemental analysis (76.93%). This difference is a consequence of the fact that XPS primarily probes the surface composition, whilst elemental analysis provides more accurate information about the real chemical composition. Table 3.Chemical composition (data from XPS analysis) for 0-CN material and 50-CN (% at) Element

0-CN

50-CN

Carbon Nitrogen Oxygen Sodium Chlorine

40.03 59.56 0.41 0 0

41.78 51.83 3.28 3.08 0.03



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Table 4. Chemical moieties found in 0-CN and 50-CN. Energy (eV) 284.60 287.89 398.36 399.62 400.76 403.88 405.48 531.58 533.00

0-CN Chemical moiety -C= (aromatic ring) N-C=N C=N-C N-CIII H-N-CII N-O N-O C-O-C C-N-O

Energy (eV) 284.60 288.07 286.19 293.31 398.53 399.80 400.92 404.11 405.28 531.01 532.57 535.40 1071.57

50-CN Chemical moiety -C= (aromatic ring) N-C=N C-OH π electrons C=N-C N-CIII H-N-CII N-O N-O C-O-C C-N-O H 2O Na+

CIII- tertiary carbon atom CII- secondary carbon atom

Figure 2. XPS survey spectrum for undoped graphitic carbon nitride (0-CN)

Figure 3. XPS survey spectrum for sodium-doped graphitic carbon nitride obtained with 50 % wt addition of NaCl to cyanamide (50-CN) The morphology of the products was investigated by scanning electron microscopy (Figure 4). The undoped sample comprises of sub-micrometre quasi-spherical particles connected together. The inclusion of NaCl changes


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the morphology. The materials synthesized with the addition of sodium chloride have similar morphological features. The samples are built of bended flakes (ca. 30-50 nm in thickness) and nanoparticles (ca. 100 nm in diameter). There are also some macropores within the bended flakes. The detailed analysis of the SEM images brings a conclusion that the surface of flakes has a 'glassy' appearance and the studied materials are non-porous. Importantly, there are no cubic or other regular (in shape) objects on the images. These objects are typical for crystalline particles (e.g. NaCl).

Figure 4. SEM images of a) 0-CN b) 5-CN c) 10-CN d) 25-CN e) 50-CN f) 75-CN

The X-ray diffractograms of all materials (Figure 5a) comprise of a strong reflection at ca. 28 deg, which is characteristic for the (002) reflection of graphitic carbon nitride3. The position of this feature is downshifted for the materials synthesized with the addition of NaCl. This observations proves that the interlayer distance in sodiumdoped g-C3N4 is higher in comparison to the undoped material. This is an expected finding, because of the presence of the additive (Na+) in the lattice. The XRD patterns for the samples synthesized with the addition of NaCl higher than 25 % contain additional reflections. These reflections are due to the presence of two crystalline phases: melamine and sodium azide (Figure 5b). Melamine is a product of cyanamide condensation3. Importantly, there are



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no reflections from NaCl (e.g. the strongest (200) peak at 32 deg). This finding shows that sodium incorporated in the g-C3N4 structure is present in a form crystallites which resemble the structure of sodium azide.

Figure 5. X-ray diffractograms of obtained materials. The FTIR spectra, shown in the Figure 6, have a few characteristic bands. The region between 1600-1200 cm-1 is the most interesting in relation to g-C3N4 materials26. The undoped materials have the bands located at 1667, 1597, 1506, 1434 and 1364 cm-1 and they originate from the stretching vibrations of heptazine units. Moreover, the peaks at 1292 and 1219 cm-1 are associated with the out-of-plane bending vibrations of heptazine rings, whilst the peak at 837 cm-1 is typical for the breathing mode of tri-s-triazine units and finally the band at 899 cm-1 is connected with the deformation mode of N-H bond. In the case of sodium-doped graphitic carbon nitride the bands characteristic for heptazine units are shifted, what suggests that sodium ions interact with heptazine units which are present in g-C3N4 layers. There band located at 2170 cm-1 corresponds to the azide units (N3-)27 and this features exists in the spectra of


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materials synthesized with the addition of NaCl, only. This observation is in agreement with X-ray diffraction patterns, which showed the presence of crystalline sodium azide.

Figure 6. FTIR spectra of obtained materials.

The point of zero charge (PZC) of the materials was determined using a method described in details by CardenasPeña et al.28. A sample of ca. 500 mg was suspended in a mixture of 5 cm3 of 0.01 M KOH, 15 cm3 of 0.1 M KNO3 and 30 cm3 of deionised water. After 1 h of shaking, 10 ml of the suspension was titrated with 0.01 M HNO3. The measurements were repeated twice and compared with blank solution. The titration curves are shown in Supplementary Data (Figures S23-S28). The point of zero charge was estimated at the point where the both titration curves are crossed. Table 5 shows the determined values of the point of zero charge for all products. The PZC of the undoped graphitic carbon nitride is 4.2. The inclusion of NaCl increases the PZC values. The samples obtained with the addition of 5, 10, 25 and 75 % of NaCl have the PZC higher than 7. The 50-CN material has an acidic character, because its point of zero charge is 5.6. There is no relation between the amount of added NaCl to the cyanamide during synthesis of the materials and the determined value of PZC. Therefore, it is very likely that graphitic carbon nitride synthesized with various addition of NaCl contains different fractions of primary and secondary amine groups. It is known that that various classes of amines have different dissociation constants.



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Table 5. Point of zero charge for studied materials. Sample 0-CN 5-CN 10-CN 25-CN 50-CN 75-CN

Point of zero charge (pH unit) 4.2 8.0 8.9 8.7 5.6 7.2

Studies on adsorption of methyl blue onto materials The adsorption experiments were carried out at room temperature (23±1 oC) and at pH=6. The initial concentration of the working solutions was between 10 and 1000 mg·dm-3. 50 mg of sodium-doped or the undoped material, was added to a plastic vial containing 50 ml of the working solution. The suspensions were shaken overnight. Importantly, the experiments were carried out in the dark in order to avoid any photocatalytic activity of g-C3N4. Afterwards, the materials were removed from the suspensions using a centrifuge (11000 rpm, diameter 25 cm, 30 minutes). The equilibrium concentration was evaluated from the UV spectra which were acquired in the visible wavelength range. The adsorption capacity (q) was calculated using the following Formula 2, where V is the volume of the solution, m is the mass of the added adsorbent, and c0 and ce are the initial and equilibrium concentration of the methyl blue solution, respectively. q =

("# $" )∙&

(Formula 2)

Figure 7 shows the adsorption isotherm of methyl blue onto the undoped material. The adsorption isotherms of this solute onto sodium-doped materials are presented in Figure 8. The obtained isotherms could not be fitted using Langmuir or Freundlich models. The adsorption isotherm of the undoped material is of type L2 according to the Giles classification29 and its shape is frequently observed in the adsorption of organic dyes onto porous and semi-porous materials. The sodium-doped adsorbents have different adsorption characteristics. Firstly, all of the doped materials have substantially higher adsorption capacity, which is between 200 and 360 mg·g-1 (Table 6). The observed values of the adsorption capacity exceed the performance of the undoped material (10 mg·g-1) by at least one order of magnitude. The highest adsorption capacity is found for the sample synthesized with 50% addition of sodium chloride. The shape of the adsorption isotherm for this sample is similar to the undoped material. Interestingly the graphitic carbon nitrides synthesized with other additions of NaCl have different shape of the adsorption isotherm. In these cases the


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isotherms are of type S2, i.e. the isotherm are convex at the equilibrium concentration range between 50 and 75 mg∙dm-3. This effect corresponds to a cooperative adsorption, i.e. at low concentration the interactions between methyl blue molecules are stronger than the attraction of methyl blue to the surface of the adsorbent. The equilibrium concentration at which the adsorbent is fully covered by the solute molecules also depends of the doping level. The undoped material is saturated with methyl blue at the lowest equilibrium concentration, i.e. 100 mg∙dm-3. The plateau on the adsorption isotherms is reached at the concentration of 200, 150, 200, 350 and 400 for 5-CN, 10CN, 25-CN, 50-CN and 75-CN samples, respectively. This finding shows that the incorporation of sodium lowers the affinity between the adsorbent and the solute.

Figure 7. Adsorption isotherm of methyl blue onto graphitic carbon nitride (0-CN).The solid curve is only to guide the eye.

Figure 8. Adsorption isotherms of methyl blue onto sodium-doped graphitic carbon nitride .The solid curves are only to guide the eye.



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Table 6. Maximum adsorption capacity of methyl blue onto sodium-doped graphitic carbon nitride materials Sample 0-CN 5-CN 10-CN 25-CN 50-CN 75-CN

Maximum adsorption capacity (mg·g-1) 10 200 275 250 360 200

The adsorption kinetics was studied for all samples. Figure 9 shows the adsorption kinetics curve for 50-CN sample. The kinetics curves for other materials are shown in Supplementary Data (Figures S29-S34). The experiments were carried out using the working solution with the concentration of methyl blue of 100 mg·dm-3. The temperature and pH were the same as in the case of equilibrium adsorption studies. The adsorption capacity was calculated also using Formula 2. As it follows from the kinetics curves the adsorption equilibrium is reached at the contact time between 5 and 60 minutes. (Table 7). Importantly, the time which is needed to reach the adsorption equilibrium depends on the content of NaCl and this time gradually decreases with an increase of the content of NaCl in the synthesis process. The adsorbents synthesized with the largest content of NaCl (50-CN and 75-CN) are characterized by the fastest kinetics, because the equilibrium is reached at the contact time lower than 5 minutes. The adsorption kinetics data along with the adsorption isotherms undoubtedly demonstrate that the doping of sodium substantially improves the adsorption capacity and the adsorption kinetics.

Figure 9. Adsorption kinetics curve of methyl blue onto sodium-doped graphitic carbon nitride (50-CN). The solid curve is only to guide the eye.


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Table 7. Time of reaching equilibrium of the adsorption process for each sample. Time of reaching equilibrium (min) 40-60 20-40 20-40 10-20

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