Modification of graphitic carbon nitride with hydrogen peroxide

nanomaterials Article Modification of Graphitic Carbon Nitride with Hydrogen Peroxide Petr Praus 1,2,* , Aneta Smýkalová1,2, Kryštof Foniok 1,2 and Vlastimil Matˇejka 1,2 1Department of Chemistry, VŠB-Technical University of Ostrava, 70800 Ostrava, Czech Republic; [email protected] (A.S.); [email protected] (K.F.); [email protected] (V.M.) 2 Institute of Environmental Technology, VŠB-Technical University of Ostrava, 70800 Ostrava, Czech Republic *Correspondence: petr[email protected]; Tel.: +420-59-732-1625 Received: 4 August 2020; Accepted: 1 September 2020; Published: 3 September 2020   Abstract: Graphitic carbon nitride (GCN) was synthetized by heating melamine and then it was thermally exfoliated for 1–3 h in air. Both bulk and exfoliated GCN nanomaterials were treated in the 10–30% aqueous solutions of H 2 O 2 for us to study their modification. The light absorption properties were observed by the reddish color and the red-shifts of their UV-Vis spectra. The content of oxygen increased and hydrogen peroxide was supposed to partially oxidize C-OH groups to C=O ones and to form C-O-C groups instead of edge C-NH-C ones. The GCN structure changes were not observed. However, a surface modification of the GCN materials was recognized by their changed photocatalytic activities tested by means of Acid Orange 7 (AO7) and Rhodamines B (RhB), zeta-potentials, and neutralization titration curves. Keywords: graphitic carbon nitride; hydrogen peroxide; modification; characterization 1. Introduction Graphitic carbon nitride has been intensively studied, especially since 2009 when a paper about its photocatalytic properties was published by Wang et al. [ 1 ]. It is a 2D semiconducting nanomaterial with interesting properties, such as high thermal, physical, chemical and photochemical stability [ 2 , 3 ]. Itabsorbsvisibleirradiationasaresult of the band gap energyof2.7eV.Thisnanomaterialispotentialfor a lot of applications, such as solar cells manufacturing [ 4 ], imaging, sensing of some compounds [ 5 – 9 ], etc. In addition, the special interest has been concentrated on its catalytic [ 10 ] and photocatalytic utilization [ 11 – 16 ]. However, there are some drawbacks associated with graphitic carbon nitride (GCN), such as fast recombination of photoinduced electrons and holes and low specific surface area. The doping of GCN with metals and non-metals enables tuning its band gap energy to enhance the absorption of visible light, facilitates the separation of photoinduced electrons and holes and improves other physico-chemical properties [ 17 – 21 ]. Especially, attention has been paid to its doping with environmental-friendly non-metals, such as S, O, P, and N. One of the procedures used for the GCN doping with oxygen is the formation of oxygen functional groups by the treatment of already prepared GCN with H 2 O 2 [ 22 – 29 ]. Hydrogen peroxide was also hypothesized to create some defects in the GCN structure [ 25 ]. The treatment with H 2 O 2 is easy but interpretation of the results is not unambiguous. Oxygen functional groups, such as N-O [ 22 ] and N-C-O [ 22 , 24 – 26 ], were supposed to be formed in the GCN structure. Other authors observed the formation of C-O-C [ 23 ] and C-OH groups [ 23 , 27 ] as well. Therefore, the aim of our work was to verify if H 2 O 2 is suitable for the doping of already synthetized GCN with oxygen and how the GCN physico-chemical properties can be changed. The bare and treated GCN nanomaterials with H 2 O 2 were studied by means of common solid-state characterization methods, such as UV-Vis diffuse reflectance (DR) spectroscopy, photoluminescence Nanomaterials 2020,10, 1747; doi:10.3390/nano10091747 www.mdpi.com/journal/nanomaterials Nanomaterials 2020,10, 1747 2 of 17 (PL) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), transmissionelectron microscopy (TEM) and scanning electron (SEM) and transmission electron microscopy (TEM), physisorption of nitrogen, and measurement of zeta potentials. The content of H, C, N was determined by elemental analysis (EA) and the content of oxygen was calculated up to 100%. Their photocatalytic activity was tested by the decomposition of Acid Orange 7 (AO7) and Rhodamines B (RhB). 2. Materials and Methods 2.1. Chemicals All chemicals used were of analytical-reagent grade. Melamine, Acid Orange 7, and Rhodamine B were obtained from Sigma-Aldrich (Darmstadt, Germany). NaOH and HCl were purchased from Lach-ner (Neratovice, Czech Republic). Distilled water was used for the preparation of all solutions and experiments. 2.2. Preparation of Bulk and Exfoliated GCN Bulk GCN was prepared in air by heating of melamine at 550 ◦ C for 4 h with the heating rate of 3◦C min−1 in a ceramic crucible with a lid (diameter 5 cm, 30 mL) in a muffle furnace. The crucible was cooled down to room temperature out of the furnace and then grounded in an agate mortar into a fine powder. The GCN exfoliation was also carried out in air by heating of the bulk GCN in a thin layer on a ceramic plate (diameter 8 cm, 50 mL) at 500 ◦ C in the muffle furnace for 1–3 h with the heating rate of 10 ◦ C min −1 . The ceramic plate with the product was cooled down to room temperature out of the furnace. The bulk GCN was labelled as Bulk and the GCN exfoliated for 1, 2, and 3 h were labelled as Ex1, Ex2, and Ex3. 2.3. Treatment of GCN Nanomaterials with Hydrogen Peroxide One gram of a GCN sample was placed in a 100 mL glass autoclave together with 50 mL of the 10%, 20%, and 30% hydrogen peroxide solution and then mixed for 5 min. After mixing, the autoclave was put into an oven at 150 ◦ C for 5 h. The resulting dark yellow nanomaterial was filtered, washed several times with deionized water, and dried overnight at 105 ◦C. 2.4. Elemental Analysis The analysis of C, N, and H in the GCN nanomaterials was performed by means of a CHNS 628 analyser (Leco Corporation, St. Joseph, MI, USA). The samples (100 mg) were burned in oxygen atmosphere at 950 ◦ C and product oxides were registered in an IR cells. The measurement range was of 0.2–60%. The content of oxygen was calculated up to 100%. The relative error of EA was 3–5%. 2.5. UV-Vis Spectrometry The UV-Vis DR spectra were registered using a spectrophotometer Shimadzu UV-2600 (IRS-2600Plus, Shimadzu, Kj ó to, Japan) at room temperature. Measured diffuse reflectance was transformed to Schuster-Kubelka-Munk’s function as follows F(R∞)=(1−R∞)2 2R∞ (1) where R∞is the diffuse reflectance of a semi-infinite layer. Nanomaterials 2020,10, 1747 3 of 17 2.6. Photoluminescence Spectrometry ThePLspectrawererecorded usingaspectrometerFLS920 (Edinburgh InstrumentLtd., Edinburgh, UK). The spectrometer was equipped with a 450 W Xenon lamp (Xe900). The excitation wavelength was 325 nm. The width of excitation and emission slits was 3 nm. 2.7. FTIR Spectrometry The FTIR spectra were recorded using a Nicolet iS50 device (Thermo Scientific, Waltham, MA, USA) and the KBr pellet technique was employed. A small amount of sample was mixed and homogenised with KBr (approximately 200 mg) and pressed at a pressure of 20 MPa to obtain a transparent tablet. The prepared sample was placed in a holder of a transmission attachment where spectra were collected in the wavenumber range of 500–4000 cm−1with the resolution of 2 cm−1. 2.8. X-Ray Diffraction Analysis The XRD patterns were recorded by means of a Rigaku SmartLab diffractometer (Rigaku, Tokyo, Japan) with a detector D/teX Ultra 250. A source of X-ray irradiation was a Co tube (CoK α , λ1 =0.178892 nm, λ2 =0.179278 nm) operated at 40 kV and 40 mA. The XRD patterns were recorded between 5 ◦ and 90 ◦ of 2 θ with the step size of 0.01 ◦ and speed 0.5 deg min −1 . The crystallite size Lwas evaluated using Scherrer’s equation for broadening B(2 θ ) (in radians) at a half maximum intensity (FWHM) of a diffraction band as follows B(2Θ)=Kλ LcosΘ(2) where λis the wavelength of X-rays, θis Bragg’s angle and the constant Kwas set at 0.9. 2.9. XPS Analysis The XPS analysis was performed by means of an X-ray Photoelectron Spectrometer ESCA 3400 (Kratos Analytical, Manchester, UK) with a base pressure in the analysis chamber of ~5.0 ×10−7Pa . Electrons were excited with an Mg K α radiation generated at 12 kV and 10 mA. For all spectra, the Shirley background was subtracted. Peaks in spectra concerning the sp 2 hybridized nitrogen (C=N-C) were set to 398.8 eV as a charge correction. 2.10. Physisorption of Nitrogen The physisorption of nitrogen was measured at − 196 ◦ C using a device SORPTOMATIC 1990 series (Thermo Scientific, Waltham, MA, USA). The obtained data were evaluated by means of the Brunauer–Emmett–Teller (BET) method. 2.11. SEM Analysis Microscopic investigations were performed using a scanning electron microscope (FEI Quanta 450, Thermo Fischer Scientific, Waltham, MA, USA) with a field emission gun and energy-dispersive X-ray spectroscopy (EDAX). The SEM micrographs were obtained using secondary electrons (SE) with an acceleration voltage of 20 kV and spot size 5. The samples were applied on SEM pins by the use of a carbon conductive double-tape with Al interlayer. 2.12. TEM Analysis Transmission electron microscopy was performed by means of a JEOL 2100 microscope with a LaB6 electron gun. The accelerating voltage of 160 kV was applied. Micrographs were taken by a camera Tengra (EMSIS). The samples were prepared by dispersion in ethanol and sonication for 5 min. One drop of this solution was placed on a copper grid with a holey carbon film and dried at room temperature. Nanomaterials 2020,10, 1747 4 of 17 2.13. Photocatalytic Experiments The photocatalytic activity of the GCN nanomaterials was investigated by means of the decomposition of AO7 and RhB in the concentration of 25 mg · L −1 and 10 mg · L −1 , respectively. In dark, into 150 mL of this solution 45 mg of each nanomaterial was added and stirred for 60 min to reach adsorption-desorption equilibria. Then, the suspension was irradiated with a LED source (420 nm). The samples of 2 mL were taken and absorbances at 485 nm for AO7 and 496 nm and 554 nm for RhB were measured using a UV-Vis spectrometer Helios (Thermo Scientific, Waltham, MA, USA). Their full spectra were recorded by a spectrometer Shimadzu UV-2600 (IRS-2600Plus, Shimadzu, Kjóto, Japan). 2.14. Statistic Calculations Statistical calculations were performed using the software packages QC.Expert (Trilobyte, Pardubice, Czech Republic) and XLSTAT 2018 (Addinsoft, Boston, MA, USA) at the α =0.05 significance level. 3. Results and Discussion The bulk GCN was synthetized from melamine in air and then it was exfoliated for 1, 2 and 3 h. These nanomaterials were treated with the 10%, 20%, and 30% solutions of hydrogen peroxide. The physico-chemical properties of all the GCN nanomaterials were studied using several solid-state characterization methods including photocatalysis. 3.1. Elemental Analysis The elemental analysis was performed for us to find the content of C, N, H and O in the GCN nanomaterials, see Table 1. The C/N ratios between 0.560 and 0.568 indicate that some insignificant structure changes occurred. However, the contents of oxygen after the treatment with H 2 O 2 were mostly higher than those before the treatment. It implies that hydrogen peroxide brought more or less additional oxygen into the GCN structures. The content of carbon and nitrogen can be reduced by decarboxylation [30] and the removal of amino groups [31], respectively, as a result of the treatment. Table 1. Elemental analysis of graphitic carbon nitride (GCN) nanomaterials. Nanomaterial C (wt.%) H (wt.%) N (wt.%) C/N O (wt.%) Bulk 34.93 1.72 61.47 0.568 1.88 Bulk 10 % H2O234.11 1.87 60.25 0.566 3.77 Bulk 20 % H2O233.21 1.92 58.86 0.564 6.01 Bulk 30 % H2O234.25 1.60 60.53 0.566 3.62 Ex1 34.14 1.79 60.30 0.566 3.77 Ex1 10 % H2O233.24 2.05 59.25 0.561 5.46 Ex1 20 % H2O234.13 1.73 60.54 0.564 3.60 Ex1 30 % H2O233.76 1.77 59.65 0.566 4.82 Ex2 34.22 1.79 60.57 0.565 3.42 Ex2 10 % H2O233.16 2.29 59.11 0.561 5.44 Ex2 20 % H2O233.59 1.86 59.50 0.565 5.05 Ex2 30 % H2O233.56 1.84 59.46 0.564 5.14 Ex3 33.85 1.83 59.97 0.564 4.35 Ex3 10 % H2O232.97 2.07 58.90 0.560 6.06 Ex3 20 % H2O233.03 1.85 58.60 0.564 6.52 Ex3 30 % H2O233.76 1.82 59.76 0.565 4.66 The formation of hydroxyl groups during their synthesis and exfoliation was explained by disclosing of double bonds -C=Nto -C(OH)-NH- [ 32 ]. Their oxidation to -C=O groups can be supposed as well. The carbonyl groups can act as chromophores causing the red-shifts of UV-Vis Nanomaterials 2020,10, 1747 5 of 17 absorption spectra, as discussed below. The similar effect of -C=O groups on the band gap energies was referred by Yang et al. [ 31 ]. However, the oxidation of –COH to C=O does not explain the increased amount of oxygen which was further investigated. 3.2. UV-Vis Spectrometry The UV-Vis spectra demonstrated in Figure 1were registered for us to observe the light absorption properties of the GCN nanomaterials. The reflectance decrease, which means the increase of light absorption, is remarkable for the GCN nanomaterials treated with H 2 O 2 . Additionally, their red-shifts were registered which can be attributed to the role of the created chromophores already mentioned. The photographs of these nanomaterials are displayed in Supplementary nanomaterials in Figures S1 and S2. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 18 The formation of hydroxyl groups during their synthesis and exfoliation was explained by disclosing of double bonds -C=Nto -C(OH)-NH- [32]. Their oxidation to -C=O groups can be supposed as well. The carbonyl groups can act as chromophores causing the red-shifts of UV-Vis absorption spectra, as discussed below. The similar effect of -C=O groups on the band gap energies was referred by Yang et al. [31]. However, the oxidation of –COH to C=O does not explain the increased amount of oxygen which was further investigated. 3.2. UV-Vis Spectrometry The UV-Vis spectra demonstrated in Figure 1 were registered for us to observe the light absorption properties of the GCN nanomaterials. The reflectance decrease, which means the increase of light absorption, is remarkable for the GCN nanomaterials treated with H2O2. Additionally, their red-shifts were registered which can be attributed to the role of the created chromophores already mentioned. The photographs of these nanomaterials are displayed in Supplementary nanomaterials in Figures S1 and S2. Figure 1. UV-Vis diffuse reflectance spectra of bulk (left) and exfoliated GCN nanomaterials (right). The optical band gap energies (hereinafter the band gap energy) were evaluated by plotting (F(R∞)·hν)1/2 against hν and then by means of the well-known Tauc´s procedure [33] considering indirect electron transition [34–38]. The evaluated band gap energies (Eg) are summarized in Table 2. Table 2. Band gap energy and specific surface area values of GCN nanomaterials. Nanomaterial Eg (eV) SSA (m2 g−1) Nanomaterial Eg (eV) SSA (m2 g−1) Bulk 2.65 12 Ex2 2.73 93 Bulk 10 %H2O2 2.59 11 Ex2 10 %H2O2 2.61 107 Bulk 20 %H2O2 2.59 9 Ex2 20 %H2O2 2.59 113 Bulk 30 %H2O2 2.59 14 Ex2 30 %H2O2 2.57 101 Ex1 2.71 58 Ex3 2.75 143 Ex1 10 %H2O2 2.60 83 Ex3 10 %H2O2 2.65 117 Ex1 20 %H2O2 2.60 93 Ex3 20 %H2O2 2.64 116 Ex1 30 %H2O2 2.56 98 Ex3 30 %H2O2 2.61 104 The Eg values of the GCN nanomaterials after the treatment were always lower than those before the treatment. It confirms the red-shits of light absorption mentioned above. The Eg increase due to exfoliation is not surprising and was already observed, e.g., [39]. It was documented by the increase of SSA given in Table 2. However, these data show that there is no significant correlation (Spearman r = 0.427) between the SSA and the content of oxygen which means that hydrogen peroxide did not affect the GCN structure. It was also observed by XRD, SEM, and TEM, as discussed below. Figure 1. UV-Vis diffuse reflectance spectra of bulk (left) and exfoliated GCN nanomaterials (right). The optical band gap energies (hereinafter the band gap energy) were evaluated by plotting (F(R ∞ ) · h ν ) 1/2 against h ν and then by means of the well-known Tauc ´ s procedure [ 33 ] considering indirect electron transition [34–38]. The evaluated band gap energies (Eg) are summarized in Table 2. Table 2. Band gap energy and specific surface area values of GCN nanomaterials. Nanomaterial Eg(eV) SSA (m2g−1)Nanomaterial Eg(eV) SSA (m2g−1) Bulk 2.65 12 Ex2 2.73 93 Bulk 10 % H2O22.59 11 Ex2 10 % H2O22.61 107 Bulk 20 % H2O22.59 9 Ex2 20 % H2O22.59 113 Bulk 30 % H2O22.59 14 Ex2 30 % H2O22.57 101 Ex1 2.71 58 Ex3 2.75 143 Ex1 10 % H2O22.60 83 Ex3 10 % H2O22.65 117 Ex1 20 % H2O22.60 93 Ex3 20 % H2O22.64 116 Ex1 30 % H2O22.56 98 Ex3 30 % H2O22.61 104 The E g values of the GCN nanomaterials after the treatment were always lower than those before the treatment. It confirms the red-shits of light absorption mentioned above. The E g increase due to exfoliation is not surprising and was already observed, e.g., [ 39 ]. It was documented by the increase of SSA given in Table 2. However, these data show that there is no significant correlation (Spearman r=0.427) between the SSA and the content of oxygen which means that hydrogen peroxide did not affect the GCN structure. It was also observed by XRD, SEM, and TEM, as discussed below. Nanomaterials 2020,10, 1747 6 of 17 3.3. Photoluminescence Spectrometry The PL spectra of the GCN nanomaterials are displayed in Figure 2. GCN is known to have delocalized electrons in its conjugated system, forming π * antibonding orbitals. Therefore, the PL spectra are mostly caused by the electron-hole transmission between π * conduction bands and lone pair valence bands [40]. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 18 3.3. Photoluminescence Spectrometry The PL spectra of the GCN nanomaterials are displayed in Figure 2. GCN is known to have delocalized electrons in its conjugated system, forming π* antibonding orbitals. Therefore, the PL spectra are mostly caused by the electron-hole transmission between π* conduction bands and lone pair valence bands [40]. Figure 2. Photoluminescence spectra of bulk (left) and exfoliated GCN nanomaterials (right). The photoluminescence intensity decrease corresponding to the treated GCN nanomaterials is remarkable. It is also obvious that when the higher concentration of H2O2 was used the lower PL was observed likely due to absorption of PL irradiation by the created chromophores. This is another indication that hydrogen peroxide modified the bulk and exfoliated GCNs. 3.4. X-Ray Diffraction Analysis The structure of the bare and treated GCNs was studied by XRD and only some selected diffractograms are presented in Figure 3. Two typical diffraction peaks at about 15° and 31–33° 2Θ correspond to the (100) and (002) diffractions. They are attributed to the hexagonal structure of graphitic carbon nitride (JCPDS 87-1526). The more intensive (002) diffraction peak corresponds to interlayer stacking of the (002) melem planes. The less intensive (100) one is attributed to the in-plane arrangement of nitrogen-linked heptazine units [41]. Some XRD characteristics are given in Table 3. The values of the interlayer distance d(002) were typical for CGN and can be found anywhere in the literature. The crystallite sizes were tested for their normality which was confirmed by the selected statistical tests: Shapiro-Wil (p = 0.290), Anderson-Darling (p = 0.206), Lilliefors (p = 0.420), JarqueBera (p = 0.705) and Kolmogorov-Smirnov (p = 0.544). This finding indicates that the crystallite sizes was not affected by the exfoliation, which provided non-diffracting nanosheets [39], as well as by the treatment with H2O2. Figure 2. Photoluminescence spectra of bulk (left) and exfoliated GCN nanomaterials (right). The photoluminescence intensity decrease corresponding to the treated GCN nanomaterials is remarkable. It is also obvious that when the higher concentration of H 2 O 2 was used the lower PL was observed likely due to absorption of PL irradiation by the created chromophores. This is another indication that hydrogen peroxide modified the bulk and exfoliated GCNs. 3.4. X-Ray Diffraction Analysis The structure of the bare and treated GCNs was studied by XRD and only some selected diffractograms are presented in Figure 3. Two typical diffraction peaks at about 15 ◦ and 31–33 ◦ 2 Θ correspond to the (100) and (002) diffractions. They are attributed to the hexagonal structure of graphitic carbon nitride (JCPDS 87-1526). The more intensive (002) diffraction peak corresponds to interlayer stacking of the (002) melem planes. The less intensive (100) one is attributed to the in-plane arrangement of nitrogen-linked heptazine units [ 41 ]. Some XRD characteristics are given in Table 3. The values of the interlayer distance d(002) were typical for CGN and can be found anywhere in the literature. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 18 Figure 3. X-ray diffraction (XRD) patterns of bulk (left) and exfoliated nanomaterials (right). Table 3. XRD characteristics of GCN nanomaterials. Nanomaterial 2 Theta (deg) FWHM (deg) L(002) (nm) d(002) (nm) Bulk 31.84 1.31 7.0 0.326 Bulk 10 % H2O2 32.11 1.32 7.0 0.323 Bulk 20 % H2O2 32.10 1.55 6.0 0.324 Bulk 30 % H2O2 32.60 1.37 6.7 0.319 Ex1 31.88 1.32 7.0 0.326 Ex1 10 % H2O2 31.99 1.05 8.8 0.325 Ex1 20 % H2O2 32.19 1.25 7.4 0.323 Ex1 30 % H2O2 32.60 1.21 7.6 0.319 Ex2 31.84 1.36 6.8 0.326 Ex2 10 % H2O2 32.05 1.01 9.1 0.324 Ex2 20 % H2O2 32.25 1.25 7.4 0.322 Ex2 30 % H2O2 32.62 1.13 8.2 0.319 Ex3 31.88 1.29 7.2 0.326 Ex3 10 % H2O2 32.07 1.01 9.1 0.324 Ex3 20 % H2O2 32.09 1.23 7.5 0.324 Ex3 30 % H2O2 32.63 1.16 8.0 0.318 Note: The 2 Theta values correspond to the (002) diffractions. 3.5. FTIR Spectrometry The structure of GCN nanomaterials was also studied by the FTIR spectrometry, see the spectra in Figure 4. They contained the typical bands within the areas A and B. The spectral bands in the area A are being attributed to the stretching vibrations of N-H bonds [42–46] and the bands in the area B are ascribed to the stretching vibrations of C=N and C-N bonds of heterocyclic rings. In addition, no C=O stretching vibrations typically located around 1700 cm−1 were observed likely due to their low content and/or overlap with the GCN stretching ones. Besides, the convincing presence of carbonyl groups in the FTIR spectra of the oxygen doped GCNs has not been clearly provided in the reviewed literature likely due to overlaps with the complex part B. The medium bands at 805 cm−1 and 809 cm−1 are explained by the breathing mode of triazine units. The spectral bands around 3500 cm−1 can be ascribed to the O-H stretching vibrations. Figure 3. X-ray diffraction (XRD) patterns of bulk (left) and exfoliated nanomaterials (right). Nanomaterials 2020,10, 1747 7 of 17 Table 3. XRD characteristics of GCN nanomaterials. Nanomaterial 2 Theta (deg) FWHM (deg) L(002) (nm) d(002) (nm) Bulk 31.84 1.31 7.0 0.326 Bulk 10 % H2O232.11 1.32 7.0 0.323 Bulk 20 % H2O232.10 1.55 6.0 0.324 Bulk 30 % H2O232.60 1.37 6.7 0.319 Ex1 31.88 1.32 7.0 0.326 Ex1 10 % H2O231.99 1.05 8.8 0.325 Ex1 20 % H2O232.19 1.25 7.4 0.323 Ex1 30 % H2O232.60 1.21 7.6 0.319 Ex2 31.84 1.36 6.8 0.326 Ex2 10 % H2O232.05 1.01 9.1 0.324 Ex2 20 % H2O232.25 1.25 7.4 0.322 Ex2 30 % H2O232.62 1.13 8.2 0.319 Ex3 31.88 1.29 7.2 0.326 Ex3 10 % H2O232.07 1.01 9.1 0.324 Ex3 20 % H2O232.09 1.23 7.5 0.324 Ex3 30 % H2O232.63 1.16 8.0 0.318 Note: The 2 Theta values correspond to the (002) diffractions. The crystallite sizes were tested for their normality which was confirmed by the selected statistical tests: Shapiro-Wil (p=0.290), Anderson-Darling (p=0.206), Lilliefors (p=0.420), Jarque-Bera (p=0.705) and Kolmogorov-Smirnov (p=0.544). This finding indicates that the crystallite sizes was not affected by the exfoliation, which provided non-diffracting nanosheets [ 39 ], as well as by the treatment with H 2 O 2 . 3.5. FTIR Spectrometry The structure of GCN nanomaterials was also studied by the FTIR spectrometry, see the spectra in Figure 4. They contained the typical bands within the areas A and B. The spectral bands in the area A are being attributed to the stretching vibrations of N-H bonds [ 42 – 46 ] and the bands in the area B are ascribed to the stretching vibrations of C=N and C-N bonds of heterocyclic rings. In addition, no C=O stretching vibrations typically located around 1700 cm −1 were observed likely due to their low content and/or overlap with the GCN stretching ones. Besides, the convincing presence of carbonyl groups in the FTIR spectra of the oxygen doped GCNs has not been clearly provided in the reviewed literature likely due to overlaps with the complex part B. The medium bands at 805 cm −1 and 809 cm −1 are explained by the breathing mode of triazine units. The spectral bands around 3500 cm −1 can be ascribed to the O-H stretching vibrations. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 18 Figure 4. Fourier transform infrared (FTIR) spectra of bulk (left) and exfoliated GCN nanomaterials (right). 3.6. XPS Study The presence of functional groups in the GCN nanomaterials was studied by XPS. Only spectra of the bulk and exfoliated (Ex3) GCNs before and after the treatment with 30% H2O2 are demonstrated in Figures 5–7. The nitrogen 1s peak (Figure 5) consisted of at least four signals at 398.8 eV, 400.0 eV, 401.4 eV and 404.2 eV [47]. The dominant part of this spectrum was the pyridinic (triazinic) NC2 nitrogen signal which corresponds to N atoms present at edges of the melem units. The peaks at 400.0 eV and 401.4 eV were ascribed to the NC3 nitrogen. The less intensive one was likely connected with bridging nitrogen atoms between melem structure cores NC3B; the more intensive one was ascribed to NC3C nitrogen in the centres of melem units. Both signals were also attributed to the N-O groups [23,48]. The fourth peak at 404.2 eV can be ascribed to –NH2 or =NH groups [43,49]. Figure 5. X-ray photoelectron spectrometry (XPS) spectra of N 1s of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). The carbon 1s spectrum (Figure 6) displays two peaks at 285.5 eV and 288.3 eV of binding energies. The peak at 288.3 eV can be attributed to the CN3 carbon typical [50], the peak of 285.5 eV corresponds to sp2 hybridized carbon of C=C or CN2 bonds [51,52]. The spectra of oxygen are displayed in Figure 7. Both peaks were found to be composed of two components. The components at the higher binding energy at 533.8 eV and 533.6 eV can be linked with oxygen [28,53] or water [54] adsorbed on the GCN surface or with the presence of O-N bond [55,56]. The second components at 532.4 eV and 531.8 eV should be ascribed also to adsorbed water Figure 4. Fourier transform infrared (FTIR) spectra of bulk ( left ) and exfoliated GCN nanomaterials ( right ). Nanomaterials 2020,10, 1747 8 of 17 3.6. XPS Study The presence of functional groups in the GCN nanomaterials was studied by XPS. Only spectra of the bulk and exfoliated (Ex3) GCNs before and after the treatment with 30% H 2 O 2 are demonstrated in Figures 5–7. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 18 Figure 4. Fourier transform infrared (FTIR) spectra of bulk (left) and exfoliated GCN nanomaterials (right). 3.6. XPS Study The presence of functional groups in the GCN nanomaterials was studied by XPS. Only spectra of the bulk and exfoliated (Ex3) GCNs before and after the treatment with 30% H2O2 are demonstrated in Figures 5–7. The nitrogen 1s peak (Figure 5) consisted of at least four signals at 398.8 eV, 400.0 eV, 401.4 eV and 404.2 eV [47]. The dominant part of this spectrum was the pyridinic (triazinic) NC2 nitrogen signal which corresponds to N atoms present at edges of the melem units. The peaks at 400.0 eV and 401.4 eV were ascribed to the NC3 nitrogen. The less intensive one was likely connected with bridging nitrogen atoms between melem structure cores NC3B; the more intensive one was ascribed to NC3C nitrogen in the centres of melem units. Both signals were also attributed to the N-O groups [23,48]. The fourth peak at 404.2 eV can be ascribed to –NH2 or =NH groups [43,49]. Figure 5. X-ray photoelectron spectrometry (XPS) spectra of N 1s of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). The carbon 1s spectrum (Figure 6) displays two peaks at 285.5 eV and 288.3 eV of binding energies. The peak at 288.3 eV can be attributed to the CN3 carbon typical [50], the peak of 285.5 eV corresponds to sp2 hybridized carbon of C=C or CN2 bonds [51,52]. The spectra of oxygen are displayed in Figure 7. Both peaks were found to be composed of two components. The components at the higher binding energy at 533.8 eV and 533.6 eV can be linked with oxygen [28,53] or water [54] adsorbed on the GCN surface or with the presence of O-N bond [55,56]. The second components at 532.4 eV and 531.8 eV should be ascribed also to adsorbed water Figure 5. X-ray photoelectron spectrometry (XPS) spectra of N 1s of Ex3 ( left ) and Ex3 30 % H 2 O 2 nanomaterials (right). Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 18 [27,28,53,57] or -OH groups [29,55,58,59] and the formation of N-C-O groups in the GCN lattice [26,28,29,32,48,60], respectively. The signal at 531.8 eV can be also attributed to the presence of C-OC or C=O [29,53]. It is obvious that the explanation of the O bonds in GCN is difficult and the literature references are not unambiguous. In addition, the N-C-O groups often correspond to the C-OH ones. Figure 6. XPS spectra of C 1s of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). A higher ratio of the signals at 531.8 eV and 533.6 of the Ex3 30 % H2O2 nanomaterial demonstrated in Figure 7 and the XPS results given above indicate that the following chemical reactions occurred: (i) oxidation of -OH groups to -C=O ones and (ii) formation of C-O-C groups instead of edge C-NH-C ones between two heptazine units, see Figure S3. This assumption is also supported by a little decrease of the nitrogen content after the H2O2 treatment, see Table 1. Figure 7. XPS spectra of O 1s of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). 3.7. SEM and TEM Analysis Morphology of the GCN nanomaterials was investigated by means of SEM. Only the bulk and 3h exfoliated nanomaterials before and after the treatment are demonstrated here, in Figures 8 and 9. The bulk GCNs were composed of bigger blocks than the exfoliated ones which had more disordered flake-like structures. However, the clear effect of H2O2 on the GCN morphology was not observed either for the bulk or exfoliated nanomaterials. The similar findings were obtained by the TEM analysis. Only the differences between the bulk and exfoliated materials and no effect of H2O2 were visible, see Figures 10 and 11. Figure 6. XPS spectra of C 1s of Ex3 (left) and Ex3 30 % H2O2nanomaterials (right). Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 18 [27,28,53,57] or -OH groups [29,55,58,59] and the formation of N-C-O groups in the GCN lattice [26,28,29,32,48,60], respectively. The signal at 531.8 eV can be also attributed to the presence of C-OC or C=O [29,53]. It is obvious that the explanation of the O bonds in GCN is difficult and the literature references are not unambiguous. In addition, the N-C-O groups often correspond to the C-OH ones. Figure 6. XPS spectra of C 1s of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). A higher ratio of the signals at 531.8 eV and 533.6 of the Ex3 30 % H2O2 nanomaterial demonstrated in Figure 7 and the XPS results given above indicate that the following chemical reactions occurred: (i) oxidation of -OH groups to -C=O ones and (ii) formation of C-O-C groups instead of edge C-NH-C ones between two heptazine units, see Figure S3. This assumption is also supported by a little decrease of the nitrogen content after the H2O2 treatment, see Table 1. Figure 7. XPS spectra of O 1s of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). 3.7. SEM and TEM Analysis Morphology of the GCN nanomaterials was investigated by means of SEM. Only the bulk and 3h exfoliated nanomaterials before and after the treatment are demonstrated here, in Figures 8 and 9. The bulk GCNs were composed of bigger blocks than the exfoliated ones which had more disordered flake-like structures. However, the clear effect of H2O2 on the GCN morphology was not observed either for the bulk or exfoliated nanomaterials. The similar findings were obtained by the TEM analysis. Only the differences between the bulk and exfoliated materials and no effect of H2O2 were visible, see Figures 10 and 11. Figure 7. XPS spectra of O 1s of Ex3 (left) and Ex3 30 % H2O2nanomaterials (right). Nanomaterials 2020,10, 1747 9 of 17 The nitrogen 1s peak (Figure 5) consisted of at least four signals at 398.8 eV, 400.0 eV, 401.4 eV and 404.2 eV [ 47 ]. The dominant part of this spectrum was the pyridinic (triazinic) NC 2 nitrogen signal which corresponds to N atoms present at edges of the melem units. The peaks at 400.0 eV and 401.4 eV were ascribed to the NC 3 nitrogen. The less intensive one was likely connected with bridging nitrogen atoms between melem structure cores NC 3B ; the more intensive one was ascribed to NC 3C nitrogen in the centres of melem units. Both signals were also attributed to the N-O groups [ 23 , 48 ]. The fourth peak at 404.2 eV can be ascribed to -NH2or =NH groups [43,49]. The carbon 1s spectrum (Figure 6) displays two peaks at 285.5 eV and 288.3 eV of binding energies. The peak at 288.3 eV can be attributed to the CN 3 carbon typical [ 50 ], the peak of 285.5 eV corresponds to sp2hybridized carbon of C=C or CN2bonds [51,52]. The spectra of oxygen are displayed in Figure 7. Both peaks were found to be composed of two components. The components at the higher binding energy at 533.8 eV and 533.6 eV can be linked with oxygen [ 28 , 53 ] or water [ 54 ] adsorbed on the GCN surface or with the presence of O-N bond [ 55 , 56 ]. The second components at 532.4 eV and 531.8 eV should be ascribed also to adsorbed water [ 27 , 28 , 53 , 57 ] or -OH groups [ 29 , 55 , 58 , 59 ] and the formation of N-C-O groups in the GCN lattice [ 26 , 28 , 29 , 32 , 48 , 60 ], respectively. The signal at 531.8 eV can be also attributed to the presence of C-O-C or C=O [ 29 , 53 ]. It is obvious that the explanation of the O bonds in GCN is difficult and the literature references are not unambiguous. In addition, the N-C-O groups often correspond to the C-OH ones. A higher ratio of the signals at 531.8 eV and 533.6 of the Ex3 30 % H 2 O 2 nanomaterial demonstrated in Figure 7and the XPS results given above indicate that the following chemical reactions occurred: (i) oxidation of -OH groups to -C=O ones and (ii) formation of C-O-C groups instead of edge C-NH-C ones between two heptazine units, see Figure S3. This assumption is also supported by a little decrease of the nitrogen content after the H2O2treatment, see Table 1. 3.7. SEM and TEM Analysis Morphology of the GCN nanomaterials was investigated by means of SEM. Only the bulk and 3h exfoliated nanomaterials before and after the treatment are demonstrated here, in Figures 8and 9. The bulk GCNs were composed of bigger blocks than the exfoliated ones which had more disordered flake-like structures. However, the clear effect of H 2 O 2 on the GCN morphology was not observed either for the bulk or exfoliated nanomaterials. The similar findings were obtained by the TEM analysis. Only the differences between the bulk and exfoliated materials and no effect of H 2 O 2 were visible, see Figures 10 and 11. Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 18 Figure 8. SEM images of bulk (left) and bulk 30 % H2O2 nanomaterials (right). Figure 9. SEM images of Ex3 (left) and Ex3 30 % H2O2 nanomaterials (right). Figure 10. TEM images of bulk (left) and bulk 30 % H2O2 nanomaterials (right). Figure 8. SEM images of bulk (left) and bulk 30 % H2O2nanomaterials (right). Nanomaterials 2020,10, 1747 16 of 17 38. Liu, X.; Jin, A.; Jia, Y.; Xia, T.; Deng, C.; Zhu, M.; Chen, C.; Chen, X. 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