Influence of high temperature synthesis on the structure of graphitic carbon nitride and its hydrogen generation ability

materials Article Influence of High Temperature Synthesis on the Structure of Graphitic Carbon Nitride and Its Hydrogen Generation Ability Emilia Alwin 1,2 , Kamila Koˇcí3, Robert Wojcieszak 2, Michał Zieli´nski 1, Miroslava Edelmannová3and Mariusz Pietrowski 1,* 1Faculty of Chemistry, Adam Mickiewicz University in Pozna´n, Uniwersytetu Pozna´nskiego 8, 61-614 Pozna´n, Poland; [email protected] (E.A.); [email protected] (M.Z.) 2Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181-UCCS-Unitéde Catalyse et Chimie du Solide, F-59000 Lille, France; [email protected] 3Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17 listopadu 15/2172, 70800 Ostrava, Czech Republic; [email protected] (K.K.); [email protected] (M.E.) *Correspondence: [email protected] Received: 19 May 2020; Accepted: 13 June 2020; Published: 17 June 2020   Abstract: Graphitic carbon nitride (g-C 3 N 4 ) was obtained by thermal polymerization of dicyandiamide, thiourea or melamine at high temperatures (550 and 600 ◦ C), using different heating rates (2 or 10 ◦ C min −1 ) and synthesis times (0 or 4 h). The effects of the synthesis conditions and type of the precursor on the efficiency of g-C 3 N 4 were studied. The most efficient was the synthesis from dicyandiamide, 53%, while the efficiency in the process of synthesis from melamine and thiourea were much smaller, 26% and 11%, respectively. On the basis of the results provided by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–vis), thermogravimetric analysis (TGA), elemental analysis (EA), the best precursor and the optimum conditions of synthesis of g-C 3 N 4 were identified to get the product of the most stable structure, the highest degree of ordering and condensation of structure and finally the highest photocatalytic activity. It was found that as the proton concentration decreased and the degree of condensation increased, the hydrogen yields during the photocatalytic decomposition of water–methanol solution were significantly enhanced. The generation of hydrogen was 1200 µmol g−1and the selectivity towards hydrogen of more than 98%. Keywords: graphiticcarbonnitride; hydrogengeneration; photocatalysis; melon; synthesisconditions 1. Introduction Graphitic carbon nitride (g-C 3 N 4 ) is the organic polymer, based on heptazine (tri-s-triazine) building blocks. Heptazine tectons are linked into chains of the melon type through the C − NH − C bonds or into layers of fully condensed carbon nitride through the C − N(C) − N bonds (Figure 1). Both forms, melon and fully condensed g-C 3 N 4 , make graphite-like layered structures. In contrast to graphite, g-C 3 N 4 is a semiconductor characterized by the band gap of ~2.7 eV [ 1 – 3 ]. Due to this property, g-C 3 N 4 can be active in many photocatalytic reactions with the use of visible light [ 3 – 7 ], but also as a heterogeneous catalyst [ 8 , 9 ] and due to its layered structure and high nitrogen content as a flame retardant [ 10 – 12 ]. Graphitic carbon nitride is usually obtained by the air–atmosphere pyrolysis of a few simple and cheap organic compounds, e.g., melamine, dicyandiamide, urea, thiourea and their mixtures [ 13 – 15 ]. Over the last few years, much effort has been focused on the search of methods for enhancement of photocatalytic activity of carbon nitride. Attempts have been made to increase its specific surface area (SSA) using the hard template method [ 11 , 16 – 21 ] and to reduce the energy Materials 2020,13, 2756; doi:10.3390/ma13122756 www.mdpi.com/journal/materials Materials 2020,13, 2756 2 of 19 gap by doping with heteroatoms [ 22 – 28 ] or exfoliation [ 29 , 30 ]. Although these methods have been proved as effective, they are multistage and thus expensive. It has been recently shown that SSA and band gap can be simply modified by suitably adjusting the conditions of condensation. Carbon nitride can be synthesized in a wide range of temperatures (500–650 ◦ C), time (1–10 h) and rate of temperature increase (2–20 ◦ C min −1 ). All these factors, together with the type of organic precursor used for the synthesis have impact on the SSA, morphology, and energy gap of carbon nitride, so on its photocatalytic properties. The literature provides just a few reports on comparative analysis of the influence of the condition of synthesis on the properties of carbon nitride [ 31 – 33 ]. Their authors proved the influence of the type of precursor and temperature of synthesis on the properties of g-C 3 N 4 , although the results have been sometimes ambiguous. However, they did not check the effect of different rates of temperature increase, while according to literature data they can be of significant consequences. For instance, g-C 3 N 4 obtained from melamine at a heating rate of 2 ◦ C min −1 (2 h, 550 ◦ C) has SSA of 15.4 m 2 g −1 [ 31 ], while at 10 ◦ C min −1 its SSA decreased to 11.2 m 2 g −1 [ 34 ]. Moreover, also the time of annealing at the target temperature has a considerable impact. Increase in the time of annealing from 2 to 3 h, at the rate of temperature increase of 10 ◦ C min −1 , results in the SSA increase from 11.2 to 14.0 m 2 g −1 [ 32 ]. Zhao et al. [ 33 ] have found that in the process of carbon nitride synthesis from dicyandiamide, increase in the annealing temperature from 550 to 600 ◦ C gives the increase in SSA from 18.1 to 30.6 m 2 g −1 . A comparison of literature data of SSA and band gap values of carbon nitride obtained from melamine and urea is given in Table 1. Materials 2020, 13, x FOR PEER REVIEW 2 of 19 specific surface area (SSA) using the hard template method [11,16–21] and to reduce the energy gap by doping with heteroatoms [22–28] or exfoliation [29,30]. Although these methods have been proved as effective, they are multistage and thus expensive. It has been recently shown that SSA and band gap can be simply modified by suitably adjusting the conditions of condensation. Carbon nitride can be synthesized in a wide range of temperatures (500–650 °C), time (1–10 h) and rate of temperature increase (2–20 °C min−1). All these factors, together with the type of organic precursor used for the synthesis have impact on the SSA, morphology, and energy gap of carbon nitride, so on its photocatalytic properties. The literature provides just a few reports on comparative analysis of the influence of the condition of synthesis on the properties of carbon nitride [31–33]. Their authors proved the influence of the type of precursor and temperature of synthesis on the properties of gC3N4, although the results have been sometimes ambiguous. However, they did not check the effect of different rates of temperature increase, while according to literature data they can be of significant consequences. For instance, g-C3N4 obtained from melamine at a heating rate of 2 °C min−1 (2 h, 550 °C) has SSA of 15.4 m2 g−1 [31], while at 10 °C min−1 its SSA decreased to 11.2 m2 g−1 [34]. Moreover, also the time of annealing at the target temperature has a considerable impact. Increase in the time of annealing from 2 to 3 h, at the rate of temperature increase of 10 °C min−1, results in the SSA increase from 11.2 to 14.0 m2 g−1 [32]. Zhao et al., [33] have found that in the process of carbon nitride synthesis from dicyandiamide, increase in the annealing temperature from 550 to 600 °C gives the increase in SSA from 18.1 to 30.6 m2 g−1. A comparison of literature data of SSA and band gap values of carbon nitride obtained from melamine and urea is given in Table 1. Figure 1. Structures of melam, melem, melon, and fully condensed graphitic carbon nitride. Table 1. Specific surface area (SSA) and band gap energy (EG) for carbon nitrides obtained under various conditions (literature data) Preparation Conditions 1 Surface Area (m2 g−1) Band Gap (eV) Reference Melamine Urea Melamine Urea 2/550/2 15.4 48.1 - - [31] 10/550/2 11.2 55.1 2.58 2.69 [34] 10/550/3 14.0 153.0 2.56 2.88 [32] 1 heating rate (°C min−1)/target temperature (°C)/time of annealing (h). Unfortunately, the relations between the above mentioned factors have not been studied in detail as of yet, so it is difficult to choose the optimum conditions of the synthesis of carbon nitride from a given precursor. Moreover, no information on the yield of the final product is available and it is known that the efficiency strongly depends on the synthesis conditions. We have undertaken a study on the influence of the synthesis conditions on the efficiency of carbon nitride, as well as its physicochemical and photocatalytic properties. Figure 1. Structures of melam, melem, melon, and fully condensed graphitic carbon nitride. Table 1. Specific surface area (SSA) and band gap energy (EG) for carbon nitrides obtained under various conditions (literature data) Preparation Conditions 1Surface Area (m2g−1)Band Gap (eV) Reference Melamine Urea Melamine Urea 2/550/2 15.4 48.1 - - [31] 10/550/2 11.2 55.1 2.58 2.69 [34] 10/550/3 14.0 153.0 2.56 2.88 [32] 1heating rate (◦C min−1)/target temperature (◦C)/time of annealing (h). Unfortunately, the relations between the above mentioned factors have not been studied in detail as of yet, so it is difficult to choose the optimum conditions of the synthesis of carbon nitride from a given precursor. Moreover, no information on the yield of the final product is available and it is known that the efficiency strongly depends on the synthesis conditions. We have undertaken a study on the influence of the synthesis conditions on the efficiency of carbon nitride, as well as its physicochemical and photocatalytic properties. Growing demand for energy and the necessity of protection of the natural environment stimulate the search for new environmentally friendly and cheap sources of energy. Carbon nitride as a Materials 2020,13, 2756 3 of 19 semiconductor is an excellent candidate for a photocatalyst in the reactions of photodecomposition of organic and inorganic compounds to get hydrogen [ 15 ]. Studies in the area have been initiated by the pioneering work of Wang et al. [ 6 ]. The carbon nitride activity in photocatalytic decomposition of water has proved rather low of just under 100 µ mol H 2 after 25 h of reaction. It has been improved by addition of 3% platinum to 250 µ mol H 2 after 25 h of reaction. The efficiency of hydrogen production can be improved by addition of some organic compounds (alcohols, acids) to water, these compounds act as the so-called sacrificial reagent [ 35 ]. On the other hand, methanol is one of the most popular solvents used in industry and its release to the environment is hazardous. One of the methods of its disposal could be its use as a sacrificial reagent in photodecomposition of a mixture of water and methanol [ 36 , 37 ]. This reaction was used in our study for evaluation of the photocatalytic activity of carbon nitride. 2. Materials and Methods 2.1. Preparation of g-C3N4 g-C 3 N 4 was prepared by the pyrolysis of dicyandiamide (Sigma-Aldrich, Darmstadt, Germany, 99%), thiourea (Sigma-Aldrich, 99%), and melamine (Sigma-Aldrich, 99%) in a semi-closed system according to a defined procedure. Typically, 4 g of each of above precursor was put into a quartz crucible (~50 mL) with a cover and heated at 550 or 600 ◦ C in a muffle furnace for 0 or 4 h with a heating rate of 2 or 10 ◦C min−1under ambient pressure in air (Table 2). Table 2. Synthesis conditions of g-C3N4 Symbol of Synthesis Conditions Precursor 1Heating Rate (◦C min−1)Temperature (◦C) Time of Annealing (h) 10/550/4 D 10 550 4 2/600/4 D 2 600 4 10/600/0 D 10 600 0 10/600/4 D 10 600 4 10/600/4 T 10 600 4 10/600/4 M 10 600 4 1D—dicyandiamide; T—thiourea; M—melamine. The crucible was allowed to remain in the furnace until it cooled down to room temperature. The obtained yellow material was ground into a fine powder in an agate mortar (Table S1). The variable parameters of the synthesis were: •heating rate (◦C min−1); •target temperature (◦C); •time of annealing (h); •type of precursor: dicyandiamide (D), thiourea (T), melamine (M). The product obtained is labelled as “precursor-heating rate/temperature/time”, e.g., D-10/600/4 means the dicyandiamide was heated with heating rate of 10 ◦ C min −1 , at 600 ◦ C for 4 h (Table 2). One of the most important parameters of the synthesis of carbon nitride—highlighted in this work—was the efficiency with which carbon nitride from a given precursor can be obtained. It was calculated as the ratio of the mass of the obtained carbon nitride (m CN ) to the mass of the precursor (mp) and expressed as a percentage E= mCN mp ×100% (1) 2.2. Characterization of g-C3N4 g-C 3 N 4 was characterized by SEM, XRD, FTIR, XPS, UV–vis, low temperature nitrogen adsorption (BET/BJH) techniques, EA, and TGA. The scanning electron microscope (SEM) images were taken using a FEI Helios NanoLab 660 (Thermo Fisher Scientific, Waltham, MA, USA) electron microscope. Materials 2020,13, 2756 4 of 19 XRD analysis was performed in the 2 θ range between 6 and 40 ◦ on a Bruker D8 Advance (Billerica, MA, USA) diffractometer by using Cu K α radiation. The distance d hkl for the sample was calculated based on the Bragg’s law d =n λ /2sin θ , where nis the integer, λ is the radiation wavelength λ =1.5418 Å, θ is thereflectionangleforthereflexhkl. Thecrystallitesizeofg-C 3 N 4 wascalculatedusingScherrerformula D=K λ / β cos θ , where D is the crystallite size (nm), K is the Scherrer constant (0.94), λ is the radiation wavelength λ =1.5418Å,andisfull width of (002) crystallitepeakathalfmaximum. X-ray photoelectron spectroscopy (XPS) analysis of the carbon nitrides was carried out with a Kratos Axis Ultra spectrometer (Kratos Analytical, Manchester, UK). The excitation source was monochromatized aluminium X-ray source (Al K α (1486.6 eV) operated at 10 mA and 15 kV. The charge referencing method used was the C (C, H) component of the C 1speak of adventitious carbon fixed at 284.6 eV. Spectroscopic data were processed by the CasaXPS ver. 2.3.17PR1.1 software (Casa Software Ltd., UK), using a peak-fitting routine with Shirley background and asymmetrical Voigt functions. FTIR analysis was performed using FTS 3000 Bio-Rad (Bio-Rad, CA, USA) spectrophotometer by KBr pellet over range from 400 to 4000 cm −1 . UV–vis spectra were recorded using a Jasco (Tokyo, Japan) model V-670 spectrophotometer. The surface area was determined by BET method using a Micromeritics (Norcross, GA, USA) model ASAP 2000 surface area and porosity analyzer (surface area was obtained from N 2 adsorption isotherms collected at 77 K). The elemental analysis was investigated on Flash 2000 exhaust-gas analyzer (Thermo Fisher Scientific, Waltham, MA, USA) by combustion on 900–1000 ◦ C. Also, the temperature resistant was examined by thermal gravimetric analysis on TGA 4000 (PerkinElmer Inc., Waltham, MA, USA) in N2atmosphere (gas flow −20 mL min−1) with the heating rate of 5 ◦C min−1. 2.3. Photocatalytic Activity Measurements Photocatalytic tests were performed in a batch mixed photoreactor (stainless steel, volume 348 mL). Reaction mixture contained 100 mL of 50% methanol in water with a photocatalyst (0.1 g) was saturated with helium, to purge the air and to saturate the solution. An 8 W Hg lamp (peak intensity at 254 nm wavelength; Ultra-Violet Products Inc., Cambridge, UK) was used as the irradiation source and was placed on a quartz glass window on the top of the photoreactor in horizontal position. The reactor was tightly closed and before the start of the reaction (switching on the UV lamp), a gaseous sample was taken (at time 0 h) through septum by syringe. All gaseous samples were analyzed by a gas chromatograph (Shimadzu, Kyoto, Japan) model Tracera GC-2010Plus equipped with BID (barrier discharge ionization detector). The reaction mixture was irradiated for certain time intervals (0–4 h) and samples were taken at 1, 2, 3, and 4 h for analysis. All measurements were reproducibly measured. Using 254 nm wavelength irradiation only three products (hydrogen, methane, and carbon monoxide) were detected from methanol/water photocatalytic splitting. 3. Results and Discussion 3.1. Efficiency of Synthesis and Surface Area of g-C3N4 In material chemistry, one of the crucial parameters is the efficiency of the synthesis. However, as far as the synthesis of carbon nitride is concerned this parameter is usually neglected. According to Zhang et al. [ 32 ] for g-C 3 N 4 it changes in a wide range from 1% when urea is used as a precursor to 44% for melamine. For that reason, these studies started with the determination of the efficiency of carbon nitride synthesis. Another important parameter is the specific surface area (SSA) of the material. Figure 2a illustrates the effect of the type of precursor on the efficiency and SSA of carbon nitride condensed in the conditions 10/600/4 (10 ◦ C min −1 , 600 ◦ C, 4 h). The highest efficiency, of 53%, was obtained for the synthesis from dicyandiamide. When melamine was the precursor, the efficiency was nearly twice lower, 26%, while the lowest efficiency, of 11%, was obtained for thiourea. However, the sample of g-C 3 N 4 produced from thiourea was characterized by the largest SSA, of 36 m 2 g −1 , when dicyandiamide was used the product’s SSA was 26 m 2 g −1 , while the lowest SSA, of 20 m 2 g −1 , was obtained using melamine as a precursor. These results are fully consistent with those reported Materials 2020,13, 2756 5 of 19 by Zhang et al. [ 32 ], who observed the SSA decrease in the sequence T >D>M, and those of Devthade et al. [ 34 ], who found that the samples of carbon nitride obtained from dicyandiamide have a larger SSA than those obtained from melamine. Materials 2020, 13, x FOR PEER REVIEW 5 of 19 sample of g-C3N4 produced from thiourea was characterized by the largest SSA, of 36 m2 g−1, when dicyandiamide was used the product’s SSA was 26 m2 g−1, while the lowest SSA, of 20 m2 g−1, was obtained using melamine as a precursor. These results are fully consistent with those reported by Zhang et al. [32], who observed the SSA decrease in the sequence T > D > M, and those of Devthade et al. [34], who found that the samples of carbon nitride obtained from dicyandiamide have a larger SSA than those obtained from melamine. Figure 2. (a) Influence of the precursor on the synthesis efficiency and the specific surface area of carbon nitrides synthesized under conditions 10/600/4 (10 °C min−1, 600 °C, 4 h); T–thiourea, D– dicyandiamide, M–melamine. (b) Influence of condensation conditions on the synthesis efficiency and specific surface area of carbon nitrides obtained by condensation of dicyandiamide. As follows from Figure 2a, the precursor that gives the product of the largest SSA provides the product with the lowest efficiency. When considering commercial applications of carbon nitride, the issue of synthesis efficiency should be a decisive factor. A reasonable compromise between the efficiency and SSA is the product obtained using dicyandiamide; the efficiency of 53% is economically justified, while the SSA of 26 m2 g−1 is acceptably large. Therefore, in further studies, the product obtained from dicyandiamide as a precursor was used to check the effect of the rate of temperature increase, target temperature and time of annealing on its properties. Figure 2b illustrates the impact of the conditions of synthesis on the efficiency and SSA of carbon nitride. For lower temperature of condensation (10/550/4) and the shorter time of annealing (10/600/0) the efficiency of the product is high, of 61% and 64%, respectively. Unfortunately, the SSA of the products obtained in these conditions is very low, of 11 and 15 m2 g−1, respectively. Increase in the temperature of synthesis from 550 to 600 °C gives a small decrease in the efficiency, from 61 to 53%, but an almost twofold increase in SSA, from 11 to 26 m2 g−1. A similar effect is observed if the time of annealing is increased from 0 h to 4 h, the efficiency decreases from 64% to 53%, while SSA increases from 15 to 26 m2 g−1. A similar effect of decreasing SSA with increasing temperature was observed by Papailias et al. [38]. Besides SSA, also the porous structure of the product is important. It can be characterized by the low-temperature nitrogen sorption isotherms. According to IUPAC classification, the isotherms of N2 adsorption/desorption of all synthesized samples of carbon nitride are of IVa type, characteristic of mesoporous materials, Figure S1. The shape of the hysteresis loop indicates the presence of slit shaped pores of nonuniform shape and sizes, characteristic of pores open at two ends or partly closed [22]. Based on the Barret–Joyner–Halenda theory (BJH), the pore size distributions were obtained, Figure S2, which permitted calculation of the average pore size and cumulative pore volume, Figure 3. For the samples of carbon nitride obtained from dicyandiamide and melamine, the cumulative pore volume and average pore size are similar, close to ~0.13 cm3 g−1 and ~15 nm respectively. For the sample obtained from thiourea, these parameters are higher, of 0.22 cm3 g−1 and 21.6 nm, respectively. Higher temperature of synthesis and higher ramp of temperature resulted in an increase in the pore volume and decrease in their size. The longer time of annealing at 600 °C resulted in an increase in Figure 2. ( a ) Influence of the precursor on the synthesis efficiency and the specific surface area of carbon nitrides synthesized under conditions 10/600/4 (10 ◦ C min −1 , 600 ◦ C, 4 h); T–thiourea, D–dicyandiamide, M–melamine. ( b ) Influence of condensation conditions on the synthesis efficiency and specific surface area of carbon nitrides obtained by condensation of dicyandiamide. As follows from Figure 2a, the precursor that gives the product of the largest SSA provides the product with the lowest efficiency. When considering commercial applications of carbon nitride, the issue of synthesis efficiency should be a decisive factor. A reasonable compromise between theefficiencyandSSAistheproductobtainedusingdicyandiamide; theefficiencyof53%iseconomically justified, while the SSA of 26 m 2 g −1 is acceptably large. Therefore, in further studies, the product obtained from dicyandiamide as a precursor was used to check the effect of the rate of temperature increase, target temperature and time of annealing on its properties. Figure 2b illustrates the impact of the conditions of synthesis on the efficiency and SSA of carbon nitride. For lower temperature of condensation (10/550/4) and the shorter time of annealing (10/600/0) the efficiency of the product is high, of 61% and 64%, respectively. Unfortunately, the SSA of the products obtained in these conditions is very low, of 11 and 15 m 2 g −1 , respectively. Increase in the temperature of synthesis from 550 to 600 ◦ C gives a small decrease in the efficiency, from 61 to 53%, but an almost twofold increase in SSA, from 11 to 26 m 2 g −1 . A similar effect is observed if the time of annealing is increased from 0 h to 4 h, the efficiency decreases from 64% to 53%, while SSA increases from 15 to 26 m 2 g −1 . A similar effect of decreasing SSA with increasing temperature was observed by Papailias et al. [38]. Besides SSA, also the porous structure of the product is important. It can be characterized by the low-temperature nitrogen sorption isotherms. According to IUPAC classification, the isotherms of N 2 adsorption/desorption of all synthesized samples of carbon nitride are of IVa type, characteristic of mesoporous materials, Figure S1. The shape of the hysteresis loop indicates the presence of slit shaped pores of nonuniform shape and sizes, characteristic of pores open at two ends or partly closed [ 22 ]. Based on the Barret–Joyner–Halenda theory (BJH), the pore size distributions were obtained, Figure S2, which permitted calculation of the average pore size and cumulative pore volume, Figure 3. For the samples of carbon nitride obtained from dicyandiamide and melamine, the cumulative pore volume and average pore size are similar, close to ~0.13 cm 3 g −1 and ~15 nm respectively. For the sample obtained from thiourea, these parameters are higher, of 0.22 cm 3 g −1 and 21.6 nm, respectively. Higher temperature of synthesis and higher ramp of temperature resulted in an increase in the pore volume and decrease in their size. The longer time of annealing at 600 ◦ C resulted in an increase in the cumulative pore volume with no change in their pore size, which suggests an increase in their number. Materials 2020,13, 2756 6 of 19 Materials 2020, 13, x FOR PEER REVIEW 6 of 19 the cumulative pore volume with no change in their pore size, which suggests an increase in their number. Figure 3. Cumulative pore volume and average pore diameter for carbon nitrides obtained from different precursors under different thermal synthesis conditions. 3.2. Morphological Characterizations (XRD and SEM) Powder X-ray diffraction patterns of carbon nitride samples obtained from different precursors in different conditions of annealing are shown in Figure 4. Thanks to the graphitic-like structure of carbon nitride it is possible to observe the reflections from the (002) crystal plane at ~27.7° 2θ which is related to the interlayer stacking of aromatic rings, and from the plane at ~13.3° 2θ (Figure 4) which is related to the in-plane structural packing motif of tri-s-triazine units in melon (separation between parallel melon chains) [39–42]. Figure 4. Influence of precursor on diffraction patterns of carbon nitrides obtained from various precursors under conditions of 10/600/4 (a) and influence of condensation conditions on diffraction patterns of carbon nitrides obtained from dicyandiamide (b). At first sight, the differences between diffractograms of the g-C3N4 samples obtained in different conditions and from different precursors are slight. However, careful analysis reveals differences in Figure 3. Cumulative pore volume and average pore diameter for carbon nitrides obtained from different precursors under different thermal synthesis conditions. 3.2. Morphological Characterizations (XRD and SEM) Powder X-ray diffraction patterns of carbon nitride samples obtained from different precursors in different conditions of annealing are shown in Figure 4. Thanks to the graphitic-like structure of carbon nitride it is possible to observe the reflections from the (002) crystal plane at ~27.7 ◦ 2 θ which is related to the interlayer stacking of aromatic rings, and from the plane at ~13.3 ◦ 2 θ (Figure 4) which is related to the in-plane structural packing motif of tri-s-triazine units in melon (separation between parallel melon chains) [39–42]. Materials 2020, 13, x FOR PEER REVIEW 6 of 19 the cumulative pore volume with no change in their pore size, which suggests an increase in their number. Figure 3. Cumulative pore volume and average pore diameter for carbon nitrides obtained from different precursors under different thermal synthesis conditions. 3.2. Morphological Characterizations (XRD and SEM) Powder X-ray diffraction patterns of carbon nitride samples obtained from different precursors in different conditions of annealing are shown in Figure 4. Thanks to the graphitic-like structure of carbon nitride it is possible to observe the reflections from the (002) crystal plane at ~27.7° 2θ which is related to the interlayer stacking of aromatic rings, and from the plane at ~13.3° 2θ (Figure 4) which is related to the in-plane structural packing motif of tri-s-triazine units in melon (separation between parallel melon chains) [39–42]. Figure 4. Influence of precursor on diffraction patterns of carbon nitrides obtained from various precursors under conditions of 10/600/4 (a) and influence of condensation conditions on diffraction patterns of carbon nitrides obtained from dicyandiamide (b). At first sight, the differences between diffractograms of the g-C3N4 samples obtained in different conditions and from different precursors are slight. However, careful analysis reveals differences in Figure 4. Influence of precursor on diffraction patterns of carbon nitrides obtained from various precursors under conditions of 10/600/4 ( a ) and influence of condensation conditions on diffraction patterns of carbon nitrides obtained from dicyandiamide (b). At first sight, the differences between diffractograms of the g-C 3 N 4 samples obtained in different conditions and from different precursors are slight. However, careful analysis reveals differences in the positions and full width at half maximum (FWHM) of the reflection from the (002) plane. The position of this reflection changed from 27.50 ◦ 2 θ for D-2/600/4 to 27.76 ◦ 2 θ for M-10/600/4 (Table S2). In the diffractograms of the samples condensed at 600 ◦C this reflection is shifted towards higher angles 2 θ . It is known that higher angles correspond to smaller interplanar distances, Figure 5a. Materials 2020,13, 2756 7 of 19 Materials 2020, 13, x FOR PEER REVIEW 7 of 19 the positions and full width at half maximum (FWHM) of the reflection from the (002) plane. The position of this reflection changed from 27.50° 2θ for D-2/600/4 to 27.76° 2θ for M-10/600/4 (Table S2). In the diffractograms of the samples condensed at 600 ° C this reflection is shifted towards higher angles 2 θ . It is known that higher angles correspond to smaller interplanar distances, Figure 5a. Figure 5. Interplanar distances (a) and crystallite sizes (b) of different g-C3N4 samples. The shortest distance, of 3.21 Å, indicating the highest degree of condensation and ordering of carbon nitride, was obtained for the samples obtained from dicyandiamide and melamine condensed in the conditions 10/600/4. The samples obtained at the lower temperature of synthesis, lower rate of temperature increase, and short time of annealing are characterized by a lower degree of condensation, as evidenced by greater interplanar distances (3.22–3.24 Å). It has been reported that the single layers in bulk g-C 3 N 4 are undulated, but could be planarized by heating at elevated temperature, which results in a denser stacking [43,44]. In the samples we studied, the heating at 600 ° C should lead to a denser packing and thus shorten the interlayer distance. In the layer structure of carbon nitride, the size of crystallites calculated from the Scherer formula corresponds to the number of condensed layers. According to Figure 5b it is smaller for the samples obtained from thiourea than in those obtained from dicyandiamide and melamine. As follows from our results that the higher condensation temperature and longer time of annealing at 600 ° C lead to increased size of crystallites so also increased number of condensed layers. Summing up, the XRD results indicate that the sample of carbon nitride obtained from thiourea is built of a smaller number of loosely arranged layers, in contrast to g-C 3 N 4 obtained from dicyandiamide or melamine, whose layers are densely packed and form thicker packets. Similarly, higher temperature and longer time of condensation lead to a denser packing and larger crystallites. The results of XRD studies and low-temperature nitrogen sorption were confirmed examination of SEM images, Figure 6, revealing higher crystallinity and lower porosity of the carbon nitride samples obtained from dicyandiamide and melamine than those of the sample from thiourea. Moreover, the samples annealed at 600 ° C show a more delicate, brittle, and thinner structure than those annealed at 550 ° C, which may be a result of degradation taking place at the higher temperature. Figure 5. Interplanar distances (a) and crystallite sizes (b) of different g-C3N4samples. The shortest distance, of 3.21 Å, indicating the highest degree of condensation and ordering of carbon nitride, was obtained for the samples obtained from dicyandiamide and melamine condensed in the conditions 10/600/4. The samples obtained at the lower temperature of synthesis, lower rate of temperature increase, and short time of annealing are characterized by a lower degree of condensation, as evidenced by greater interplanar distances (3.22–3.24 Å). It has been reported that the single layers in bulk g-C 3 N 4 are undulated, but could be planarized by heating at elevated temperature, which results in a denser stacking [ 43 , 44 ]. In the samples we studied, the heating at 600 ◦ C should lead to a denser packing and thus shorten the interlayer distance. In the layer structure of carbon nitride, the size of crystallites calculated from the Scherer formula corresponds to the number of condensed layers. According to Figure 5b it is smaller for the samples obtained from thiourea than in those obtained from dicyandiamide and melamine. As follows from our results that the higher condensation temperature and longer time of annealing at 600 ◦ C lead to increased size of crystallites so also increased number of condensed layers. Summing up, the XRD results indicate that the sample of carbon nitride obtained from thiourea is built of a smaller number of loosely arranged layers, in contrast to g-C 3 N 4 obtained from dicyandiamide or melamine, whose layers are densely packed and form thicker packets. Similarly, higher temperature and longer time of condensation lead to a denser packing and larger crystallites. The results of XRD studies and low-temperature nitrogen sorption were confirmed examination of SEM images, Figure 6, revealing higher crystallinity and lower porosity of the carbon nitride samples obtained from dicyandiamide and melamine than those of the sample from thiourea. Moreover, the samples annealed at 600 ◦ C show a more delicate, brittle, and thinner structure than those annealed at 550 ◦C, which may be a result of degradation taking place at the higher temperature. Materials 2020, 13, x FOR PEER REVIEW 8 of 19 Figure 6. SEM images of carbon nitrides synthesized from various precursors and under different condensation conditions, (a) D-10/550/4, (b) D-10/600/4, (c) T-10/600/4, (d) M-10/600/4. 3.3. Elemental Analysis The synthesized samples of carbon nitride were subjected to elemental analysis by determination of N, C, and H (Table 3). The contents of nitrogen, carbon and hydrogen are very similar in all samples. The C/N ratio varies from 0.686 to 0.699, while the C/H ratio–from 1.467 to 1.697, which gives the average molecular formula of C3N4.35H2.0. The C/N ratio is lower than the theoretical values of 0.75, however it is consistent with literature data [9,45,46]. Akaike et al. [42] have reported C/N = 0.67, which corresponds to the composition of melon. Miller et al., [45] using annealing at 600 °C have obtained samples of C/N = 0.67. Table 3. Elemental analysis of carbon nitrides obtained under various conditions and elemental composition determined from XPS analysis Synthesis Conditions Precursor Elemental Analysis XPS Atomic Composition C/N N/C C/H Theoretic Formula C/N Theoretic Formula N (at.%) C (at.%) H (at.%) 10/550/4 D 46.42 31.86 21.71 0.686 1.457 1.467 C3N4.4H2.0 0.688 C3N4.4 2/600/4 D 46.65 32.42 20.94 0.695 1.439 1.548 C3N4.3H1.9 0.708 C3N4.2 10/600/0 D 47.01 32.65 20.33 0.695 1.440 1.606 C3N4.3H1.9 0.712 C3N4.2 10/600/4 D 47.32 33.08 19.60 0.699 1.431 1.688 C3N4.3H1.8 0.702 C3N4.3 T 46.54 32.33 21.13 0.695 1.440 1.530 C3N4.3H2.0 0.674 C3N4.4 M 47.46 33.05 19.48 0.696 1.436 1.697 C3N4.3H1.8 0.687 C3N4.4 At a lower condensation temperature (500 °C) [46] the ratio C/N = 0.686, which corresponds to the formula C3N4.41. Low C/N ratio implies an excess of nitrogen in the structure, which means that the condensation was incomplete and evidences the presence of –NH2 and/or –NH– groups. With increasing degree of condensation, the contribution of nitrogen decreases, which leads to increasing C/N ratio that becomes closer to the theoretical value of 0.75. For the samples obtained from dicyandiamide, the temperature of synthesis, time and rate of temperature increase do not influence the C/N ratio, which is 0.69 for all samples. In addition, no significant influence of the type of precursor on the C/N ratio was detected; for the samples synthesized in the conditions 10/600/4 this ratio was 0.695–0.699. The condensation temperatures used, of 550 and 600 °C, were high enough to ensure almost the same elemental composition of the samples. According to literature, the structure Figure 6. SEM images of carbon nitrides synthesized from various precursors and under different condensation conditions, (a) D-10/550/4, (b) D-10/600/4, (c) T-10/600/4, (d) M-10/600/4. Materials 2020,13, 2756 8 of 19 3.3. Elemental Analysis The synthesized samples of carbon nitride were subjected to elemental analysis by determination of N, C, and H (Table 3). The contents of nitrogen, carbon and hydrogen are very similar in all samples. The C/N ratio varies from 0.686 to 0.699, while the C/H ratio–from 1.467 to 1.697, which gives the average molecular formula of C 3 N 4.35 H 2.0 . The C/N ratio is lower than the theoretical values of 0.75, however it is consistent with literature data [ 9 , 45 , 46 ]. Akaike et al. [ 42 ] have reported C/N=0.67, which corresponds to the composition of melon. Miller et al., [ 45 ] using annealing at 600 ◦ C have obtained samples of C/N=0.67. Table 3. Elemental analysis of carbon nitrides obtained under various conditions and elemental composition determined from XPS analysis Synthesis Conditions Precursor Elemental Analysis XPS Atomic Composition C/N N/C C/HTheoretic Formula C/NTheoretic Formula N (at.%) C (at.%) H (at.%) 10/550/4 D 46.42 31.86 21.71 0.686 1.457 1.467 C3N4.4H2.0 0.688 C3N4.4 2/600/4 D 46.65 32.42 20.94 0.695 1.439 1.548 C3N4.3H1.9 0.708 C3N4.2 10/600/0 D 47.01 32.65 20.33 0.695 1.440 1.606 C3N4.3H1.9 0.712 C3N4.2 10/600/4D 47.32 33.08 19.60 0.699 1.431 1.688 C3N4.3H1.8 0.702 C3N4.3 T 46.54 32.33 21.13 0.695 1.440 1.530 C3N4.3H2.0 0.674 C3N4.4 M 47.46 33.05 19.48 0.696 1.436 1.697 C3N4.3H1.8 0.687 C3N4.4 At a lower condensation temperature (500 ◦ C) [ 46 ] the ratio C/N=0.686, which corresponds to the formula C 3 N 4.41 . Low C/N ratio implies an excess of nitrogen in the structure, which means that the condensation was incomplete and evidences the presence of –NH 2 and/or –NH– groups. With increasing degree of condensation, the contribution of nitrogen decreases, which leads to increasing C/N ratio that becomes closer to the theoretical value of 0.75. For the samples obtained from dicyandiamide, the temperature of synthesis, time and rate of temperature increase do not influence the C/N ratio, which is 0.69 for all samples. In addition, no significant influence of the type of precursor on the C/N ratio was detected; for the samples synthesized in the conditions 10/600/4 this ratio was 0.695–0.699. The condensation temperatures used, of 550 and 600 ◦ C, were high enough to ensure almost the same elemental composition of the samples. According to literature, the structure of carbon nitride is stabilized already at ~520 ◦ C, so using 550 ◦ C and 600 ◦ C we obtained stable melon type structures of defined composition. 3.4. XPS Analysis Figure7presentsexemplaryXPSspectraofthe sample M-10/600/4. The survey spectrum(Figure7a) of M-10/600/4 showed that the sample was composed solely of C, N, and O, which may have originated from adsorbed water molecules [ 40 , 47 – 49 ]. The XPS spectra of the samples of carbon nitride obtained from dicyandiamide and thiourea were similar. For the sample obtained from thiourea, no signal from sulfur (S 2p) was detected, which means that the whole sulfur was removed upon thermal condensation Materials 2020, 13, x FOR PEER REVIEW 9 of 19 of carbon nitride is stabilized already at ~520 °C, so using 550 °C and 600 °C we obtained stable melon type structures of defined composition. 3.4. XPS Analysis Figure 7 presents exemplary XPS spectra of the sample M-10/600/4. The survey spectrum (Figure 7a) of M-10/600/4 showed that the sample was composed solely of C, N, and O, which may have originated from adsorbed water molecules [40,47–49]. The XPS spectra of the samples of carbon nitride obtained from dicyandiamide and thiourea were similar. For the sample obtained from thiourea, no signal from sulfur (S 2p) was detected, which means that the whole sulfur was removed upon thermal condensation Figure 7. XPS survey spectrum of carbon nitride obtained from melamine (M-10/600/4) and O 1s spectrum (insert) (a); C 1s spectrum (b) and N 1s (c). The C 1s XPS spectrum of M-10/600/4 (Figure 7b) contained two dominant signals located at 284.6 and 288.3 eV, corresponding to graphitic carbon (C−[C,H] adventitious carbon, AdC), and N−C=N in the triazine or heptazine rings, respectively. There was also a peak at 287.0 eV, which can correspond to nitrile species −C≡N [50–54]. In the spectrum of the sample obtained from melamine, M-10/600/4, the contribution of this peak in the C1s signal was only 1.0%. In the spectra of the other samples, the contribution of this peak varied from 0.8% for D-10/550/4 to 2.9% for D-2/600/4. In general, the contribution of this peak is greater in the samples synthesized at higher temperatures and for longer annealing time. Nitrile species may form upon carbon nitride annealing at high temperatures, as reported by Dante et al., [55], who observed that the thermal degradation of the graphitic carbon nitride occurs through cyan group formation. Similarly, Lau et al., [56] have observed the release of HCN already at 570–600 °C. The temperatures of condensation which we used (550 °C and 600 °C) and a long time of annealing (4 h) are high enough to make some of the heptazine rings undergo thermal degradation with formation of cyano group. This supposition is supported by the fact that the number of these groups is a bit higher in the samples annealed at 600 °C than at 550 °C. The N 1s (Figure 7c) spectrum has a complex structure with a pronounced signal at 398.8 eV assigned to pyridinic nitrogen in heptazine (or triazine) ring. The other peaks at 399.5, 400.4 and 401.3 eV are assigned to primary amine (−NH2), secondary amine (−NH−), and tertiary nitrogen, respectively. Based on the XPS spectra, the C/N ratio was calculated to be 0.674–0.712 (Table 3), which corresponds to the molecular formulas C3N4.4 to C3N4.2 and indicates an excess of nitrogen in the samples. These results agree with those of elemental analysis. As follows from XPS results, the samples studied contained nitrogen in the form of −NH2 and −NH− groups, which were also detected by FTIR spectra (Section 3.5.). The above results together with XRD data imply that the structure of carbon nitride is of melon ribbon type instead of fully condensed polyheptazine structure. 3.5. Structure Characterization (FTIR) The FTIR spectra of all carbon nitride samples (Figure 8) indicate a structure typical of graphitic carbon nitride. The bands in the range of 1400–1700 cm−1 corresponding to the characteristic ring stretching modes of C−N heterocycles were observed. The number and shape of the bands suggest Figure 7. XPS survey spectrum of carbon nitride obtained from melamine (M-10/600/4) and O 1s spectrum (insert) (a); C 1sspectrum (b) and N 1s(c). Materials 2020,13, 2756 9 of 19 The C 1s XPS spectrum of M-10/600/4 (Figure 7b) contained two dominant signals located at 284.6 and 288.3 eV, corresponding to graphitic carbon (C − [C,H] adventitious carbon, AdC), and N − C=N in the triazine or heptazine rings, respectively. There was also a peak at 287.0 eV, which can correspond to nitrile species − C ≡ N [ 50 – 54 ]. In the spectrum of the sample obtained from melamine, M-10/600/4, the contribution of this peak in the C1s signal was only 1.0%. In the spectra of the other samples, the contribution of this peak varied from 0.8% for D-10/550/4 to 2.9% for D-2/600/4. In general, the contribution of this peak is greater in the samples synthesized at higher temperatures and for longer annealing time. Nitrile species may form upon carbon nitride annealing at high temperatures, as reported by Dante et al., [ 55 ], who observed that the thermal degradation of the graphitic carbon nitride occurs through cyan group formation. Similarly, Lau et al. [ 56 ] have observed the release of HCN already at 570–600 ◦ C. The temperatures of condensation which we used (550 ◦ C and 600 ◦ C) and a long time of annealing (4 h) are high enough to make some of the heptazine rings undergo thermal degradation with formation of cyano group. This supposition is supported by the fact that the number of these groups is a bit higher in the samples annealed at 600 ◦ C than at 550 ◦ C. The N 1s (Figure 7c) spectrum has a complex structure with a pronounced signal at 398.8 eV assigned to pyridinic nitrogen in heptazine (or triazine) ring. The other peaks at 399.5, 400.4 and 401.3 eV are assigned to primary amine (−NH2), secondary amine (−NH−), and tertiary nitrogen, respectively. Based on the XPS spectra, the C/N ratio was calculated to be 0.674–0.712 (Table 3), which corresponds to the molecular formulas C 3 N 4.4 to C 3 N 4.2 and indicates an excess of nitrogen in the samples. These results agree with those of elemental analysis. As follows from XPS results, the samples studied contained nitrogen in the form of − NH 2 and − NH − groups, which were also detectedbyFTIR spectra (Section3.5). Theabove resultstogetherwithXRD data imply thatthe structure of carbon nitride is of melon ribbon type instead of fully condensed polyheptazine structure. 3.5. Structure Characterization (FTIR) The FTIR spectra of all carbon nitride samples (Figure 8) indicate a structure typical of graphitic carbon nitride. The bands in the range of 1400–1700 cm −1 corresponding to the characteristic ring stretching modes of C − N heterocycles were observed. The number and shape of the bands suggest the presence of tri-s-triazine (heptazine) subunits, which is also indicated by the doublet at 1576 cm −1 and 1543 cm−1[57]. The presence of these subunits is confirmed by the intense band at about 808 cm −1 assigned to the ring-sextant out-of-plane bending vibration characteristic of both triazine or heptazine ring systems [ 41 ]. The linkage of these ring systems by − NH − groups is shown by the absorption bands in the 1200 − 1400 cm −1 region that are characteristic of the C − NH − C units in melam and melon [ 7 ]. According to some authors [ 40 , 58 , 59 ], the presence of the bands at ~1209, 1232, and 1318 cm −1 indicates the presence of tertiary amine (C − NH − C) fragments, which indicates the formation of a more condensed carbon nitride polymer. It should be kept in mind that the C − N stretching peaks of tertiary amines are weak to medium in intensity, due to the lack of polarity of the C − N bond. Lotsch [ 41 ] has reported that the FTIR spectra of carbon nitride of the nanocrystalline melon structure, made of ribbons of linked heptazine units, show prominent FTIR features at 1206, 1235, and 1316 cm −1 . These peaks were correlated with characteristic modes associated with C − NH − C units, as found in melam. The spectra recorded in the present work showed the corresponding bands at 1205, 1238, and 1319 cm −1 , which proves that the structure of the obtained samples was practically identical to that of the samples studied by Lotsch [ 40 ]. High intensity of these peaks indicates a high number of the bridging − NH − groups. In general, the bands are intensive and well-resolved, which means that the polymeric material is well-ordered at a molecular level [57]. Materials 2020,13, 2756 16 of 19 4. 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