Thermal stability of polymeric carbon nitride (PCN)-Al2O3–ZrO2 nanocomposites used in photocatalysis

Polymeric carbon nitride (PCN) was recently found to have extensive applications in the field of photocatalysis. Knowledge about thermal stability of PCN nanocomposites is crucial for this application and designing the final product. In this work, the thermal stability of PCN-Al2O3–ZrO2 nanocomposites was investigated. PCN nanocomposites were obtained in two steps: (1) microwave hydrothermal synthesis of co-precipitated AlOOH and ZrO2 precursors, followed by drying; (2) mixing the nanopowders with melamine powder and annealing in air in a tube furnace at 400, and 450 °C. The PCN nanocomposites were examined by attenuated total reflection technique of Fourier transformed infrared spectroscopy. Also, the evolved gas analysis was performed combining differential scanning calorimetry and thermogravimetry coupled with mass spectroscopy and FTIR. The results show that only PCN-Al2O3–ZrO2 nanocomposite obtained at 400 °C is stable from room temperature up to 490 °C and during thermal decomposition, in one step releases ammonia (NH3), cyanic acid (HNCO), water (H2O), and carbon dioxide (CO2). The limitation of the PCN-Al2O3–ZrO2 thermal stability and performance is AlOOH–ZrO2 used as a nanocomposite component.


Introduction
Recent research of scientists from different scientific fields are focused on manufacturing composites and heterojunctions containing carbon nitride called by graphitic carbon nitride (g-C 3 N 4 ) [1], melon or polymeric carbon nitride (PCN) [2]. This material is usually obtained from the pyrolysis of urea, cyanamide, thiourea, and melamine [2]. PCN is characterized by optical bandgap of 2.7 eV, possess a 2D crystal structure and great chemical stability [1]. Due to physical and chemical properties of PCN it was used in the fields of photocatalysis, electrochemistry and photo-electrochemistry. PCN based compounds have attracted attention due to their promising applications to clean up air and water pollution by degradation of organic pollutants.
It was discovered recently that by addition of a metal oxide (ZnO, ZrO 2 , TiO 2 , Al 2 O 3 , etc.) to PCN, it is possible to form a heterostructured composite. Such composite was characterized by long-term photostability, as well as thermal and mechanical stability [4,5].
Analysis of literature shows that there is a lot of ambiguity as far as melamine thermal behavior. Synthesis of PCN from melamine was reported to occur in the broad temperature range from 400 to 700 °C [2,6,7]. During heating of melamine up to ~ 320 °C condensation reactions with evolution of NH 3 take place. Above 360 °C during condensation of melamine and its reaction with residual cyanamide (NH 2 CN) melem (triaminotri-s-triazine, triamino-heptazine, C 6 N 7 (NH 2 ) 3 ) is formed. Melem was found to condense and form 1D chains of aminelinked heptazine units above 520 °C ([C 6 N 7 (NH 2 )(NH)] n -polymeric carbon nitride (PCN)) [2]. Melon, consist from layers made from 1D chains of NHbridged melem (C 6 N 7 (NH 2 ) 3 ) monomers [8]. Praus et al. [6] recommend a higher than 600 °C temperature for PCN production from melamine. Authors [6] found, that at lower annealing temperature such as 550 °C and/or 575 °C PCN formation takes place, with the empirical formula C 6 N 9 H 3 . At higher temperature this material decomposes. Melamine annealed at 600 °C leads to C 6 N 9 H 2 formation (which is very close to PCN) with the maximal C/N molar ratio of 0.68. This value indicates incomplete condensation of amino groups of PCN [6]. PCN is known as the most stable composition under ambient conditions among the family of carbon nitrides [2]. It was found [8] that the empirical PCN composition dependence on preparation method. Authors summarized that that melon structure is a mixture of molecules of different shapes and sizes. If so, PCN structure has rather amorphous character [8].
In previous work, it was demonstrated [7] that by using a tube furnace and applying the fast 50 °C/min heating rate during melamine and AlOOH-ZrO 2 annealing favorable PCN formation can be achieved at 400 °C. This temperature of PCN-nanocomposite formation is lower than reported in literature [7]. It was postulated that lower PCN formation temperature was probably due to the interaction of the melamine with the AlOOH/γ-Al 2 O 3 and ZrO 2 nanoparticles (NPs). The presence of AlOOH/γ-Al 2 O 3 and ZrO 2 NPs provide high specific surface areas and morphology beneficial for the formation of a PCN layer. The PCN nanocomposite was characterized by a 3 eV band gap and showed significant photocatalytic ability for degradation of common pollutants.
Simultaneous thermal analysis is widely used in investigation of various nanocomposites thermal behavior [9][10][11][12][13]. DSC-TG-FTIR-QMS method is applied to identify gases released during the thermal treatment of different materials [9,10,13,14]. In the literature there is lack of complete studies about thermal decomposition of PCN or PCNnanocomposites with analysis of released gases. Miller et al. [12] reported that during thermogravimetric analysis NH 3 is evolved from precursors such as melamine above approximately 450 °C, with C 2 N 2 and CxNyHz species. However, detailed analysis of PCN-materials using EGA have not been conducted. Systematic and complementary DSC-TG-QMS-FTIR experiments performed for PCNnanocomposites provide knowledge about PCN based materials which will be crucial for their synthesis performance investigation and their further application. Therefore, the aim of the work is to (1) find optimum synthesis conditions for PCN-Al 2 O 3 -ZrO 2 nanocomposite photocatalysts; to (2) investigate the type of emitted gases during temperature program; and to (3) gain knowledge about interdependency between nanocomposite components.

Materials and methods
The microwave synthesis of nanopowder mixtures containing boehmite (AlOOH) with 20 mass% of ZrO 2 is described in details in previous works [7,9,10]. The PCN nanocomposites were made by hand mixing of AlOOH-20 mass%ZrO 2 nanopowder in zirconia mortar with 20 mass% of melamine (Sigma-Aldrich, CAS Number 108-78-1 (99%)). Further, prepared powder was annealed in a tube furnace in two temperatures: 400 °C and 450 °C for 5 h, in air. The heating rate applied was 50 °C min −1 .
The surface morphology of the investigated materials was observed using a scanning electron microscope (Zeiss, model Ultra Plus, Zeiss, Oberkochen, Germany).
All powders after synthesis were examined by a Fourier transform infrared spectrometer (Bruker Optics, Tensor 27, Bruker BioSpin GmbH, Rheinstetten, Germany) equipped with ATR accessory and gas cell connected to STA device. The ATR-FTIR spectra were recorded in the 4000-400 cm −1 range, at room temperature. The resolution of spectra and their accuracy were as follows: 4 cm −1 and 1 cm −1 , respectively.
Thermal stability of PCN-nanocomposites was investigated using DSC-TG-QMS-FTIR set of devices. The experiments were performed on a STA 449 F1 Jupiter by Netzsch equipped with SiC furnace. All tests were carried out with a heating rate of 10 °C min −1 from room temperature up to 1000 °C, with constant flow of air (60 mL min −1 ). The released gases during thermal heating of PCN-nanocomposites were detected by a mass spectrometer (QMS 403C Aeolos, Netzsch) and FTIR (Bruker Optics, Tensor 27,) coupled in line with the STA instrument. In order to avoid condensation of eventual melamine inside the equipment, maximum 20 mg of powder was placed into crucible for each measurement. Figure 1 shows FTIR-ATR spectra of pure as-synthesized mixture of AlOOH and 20 mass% ZrO 2 nanopowder (further denoted as AlOOH-20 mass% ZrO 2 nanopowder, which will be treated as reference sample), as well as PCN-nanocomposites annealed at 400 and 450 °C. The reference material was described by author previously [7,9] and is characterized by strong bands 3099, 3311 cm −1 which belong to asymmetric and symmetric O-H stretching vibrations from (O)Al-OH. Bands 1070, 1170 cm −1 are assigned to (HO)-Al=O asymmetric stretching and the O-H bending, respectively. The strong band at ~ 737 cm −1 can be attributed to Al-O-Al framework. AlOOH-20 mass% ZrO 2 in melamine presence and after annealing at 400 and 450 °C does not show any visible vibrations associated to AlOOH, ZrO 2 or Al 2 O 3 . In previous work [7] it was discussed already that PCN was identified only in the nanocomposite prepared at 400 °C. The confirmation of this fact was presence of PCN characteristic band at approximately 800 cm −1 [8]. It was found that -NH 2 stretching vibrations of crystalline melamine (C 3 N 3 (NH 2 ) 3 ) at ~ 3475 and 3417 cm −1 are similar to spectrum for melem (C 6 N 7 (NH 2 ) 3 ), containing the C 6 N 7 central heptazine unit [12].  [7]. Nanocomposite obtained at 450 °C ( Fig. 2b) represents morphology typical for Al 2 O 3 -ZrO 2 nanoparticles mixture. Figure 3 presents thermal behavior of as-synthesized AlOOH-20 mass%ZrO 2 , AlOOH-20 mass%ZrO 2 annealed at 400 °C, and melamine. All presented results refer to PCNnanocomposite thermal decomposition. Figure 3a shows thermal behavior (DSC-TG) of pure melamine which fully decomposes at approximately 350 °C, which is in agreement with literature [6,15,16]. Figure 3b shows thermal behavior for sample of pure AlOOH-20 mass% ZrO 2 and transformation of AlOOH to Al 2 O 3 . The endothermic transition with peak at 300 °C can be associated to gibbsite transforming to χ-Al 2 O 3 [17]. Further, exothermic peak at 410 °C can be explained by crystallization followed by boehmite (γ-AlOOH) transformation to γ-Al 2 O 3 [17]. Tha all phase transformations are associated with OH − groups detachments. The total mass loss of this sample was 18 mass%. The phase transitions of AlOOH-ZrO 2 materials in relation to annealing temperatures were discussed before [7,9,9,18]. Thermal analysis results for AlOOH-20 mass%ZrO 2 nanopowders annealed at 400 °C are presented at Fig. 3c, d. DSC curve shows exothermic event at ~ 230 °C (Fig. 3c) which is related to CO 2 and H 2 O release (Fig. 3 d) and may be associated with crystallization process of γ-Al 2 O 3. At 400 °C annealing temperature AlOOH should already transforms to Al 2 O 3 . However, it is clear that nano powder after thermal treatment at 400 °C still contains some AlOOH phase (boehmite or diaspore [17]). The broad endothermic transition with onset at 430 °C (T peak = 490 °C) is caused by further decomposition of AlOOH residues and phase transformation to alumina. The total mass loss for this material was 11 mass% related mainly to OH − groups release.

Results
The DSC-TG curves of PCN-Al 2 O 3 -ZrO 2 nanocomposites prepared at 400 °C and 450 °C are presented in Fig. 4. Nanocomposite prepared at 400 °C is characterized by narrow exothermic event in the temperature range: form 480 to 550 °C, while the sample prepared at 450 °C shows broad exothermic peak in the range 400-600 °C. The temperature stability of the nanocomposite prepared at 450 °C is similar to the reference material (Fig. 3a). The total mass loss observed during thermal decomposition was 19 and 9.5% for nanocomposite annealed at 400 and 450 °C, respectively.  (Fig. 5a, b). Sample prepared at 450 °C during thermal decomposition released CN − /HCN (m/Z = 26, 27) in addition to the gases listed above and rather did not evolved NO x . It should be noticed, that material obtained by annealing of AlOOH-ZrO 2 and melamine at 450 °C is less stable than Wavenumbers/cm −1 Trasmittance/a.u. the one annealed at 400 °C. Five different maxima can be distinguished (Fig. 5c, [11]. The cyanic acid (HNCO) bands were found at 1095 and at 3474 [11]. Figure 7 presents scheme of the PCN nanocomposites thermal decomposition prepared based on results from DSC-TG-QMS-FTIR experiment. Upon TG values analysis it can be concluded that as-synthesized AlOOH-ZrO 2 nanopowders loses 18 mass% of mass during thermal  treatment, while this material annealed at 400 °C loses only 11 mass%. It may lead to conclusion that while AlOOH-ZrO 2 is mixed with 20 mass% of melamine and annealed at 400 °C ~ 11 mass% mass loss may origin form AlOOH-ZrO 2 and only ~ 8 mass% from PCN. This means, that final PCN-nanocomposite may contain only 8 mass% of PCN. It expected then, that nanocomposite produced at higher temperature (450 °C) will contain even less melamine condensation products then its counterpart prepared at 400 °C.

Discussion
It was shown recently [7] that the PCN-nanocomposite obtained at 400 °C has very promising photocatalytic properties. The temperature 400 °C was surprising for PCN formation since according to literature [6] melamine undergoes transition to PCN above 500 °C. In this case the PCN formation at 400 °C takes place due to the presence of nanosized metal oxide in the nanocomposite and rapid heating (50 °C min −1 ). Additionally, in the heterogeneous surface processes that may not occur easily in a pure melamine used by others or in different experimental procedure [1]. As a result, the conversion temperatures of melamine to PCN presented in literature are much higher due to the need to overheat the system for a transformation to occur. It is postulated that in this work PCN formation from melamine takes place at or around the true transition temperature. It can also be assumed that the formation of PCN on the surface of the nanopowder may be accelerated by water release during the phase transition AlOOH to Al 2 O 3 . Based on results presented in Fig. 3, it is clear that thermal stability of PCN-nanocomposite depends on the stability of AlOOH-ZrO 2 which is present in nanoscale form. Annealing of AlOOH-ZrO 2 with melamine at 400 °C causes several chemical reactions during that process. At 400 °C AlOOH-ZrO 2 nanopowder undergoes already endothermic reaction due to water release (Fig. 3b), while melamine itself should already decompose (Fig. 3a). Thus, thermal stability of PCN-nanocomposite is the same as Al 2 O 3 -ZrO 2 stability (while comparing Figs. 3c and 4a) with the maximum of peak at ~ 500 °C.
Nanocomposite prepared at 450 °C does not show any presence of PCN but various melamine decomposition products what was described before [7]. Also, the amount of melamine decomposition products is lower than in case of nanocomposite prepared at 400 °C. Once the DSC curve of AlOOH-ZrO 2 is analyzed (Fig. 3b), it confirms lower stability of material in comparison to PCN-nanocomposite obtained at 400 °C. At 450 °C, AlOOH-ZrO 2 water released already (~ 300 °C, Fig. 3b) and material started crystallizing to form γ-Al 2 O 3 . It appears that exceeding crystallization temperature of Al 2 O 3 in AlOOH-ZrO 2 nanopowder influences dramatically PCN formation on Al 2 O 3 -ZrO 2 surface.
The annealing of the mixture containing AlOOH, ZrO 2 and melamine at 400 °C leads to a rather rapid process of bubbling off excess melamine, NH 3 , NO x and H 2 . In the same time, each solid particle of ZrO 2 and AlOOH/Al 2 O 3 act as nucleation centers for this bubbling process [7]. It is possible that, smaller and sharper particles may facilitate controlled interconversion of melamine at lower temperature and creating PCN layer around AlOOH and ZrO 2 particles. Evolved gas analysis showed different thermal stability of investigated materials. Nanocomposite prepared at 450 °C starts gradually decomposing already at 94 °C, while PCNnanocomposite obtained at 400 °C is stable from room temperature up to ~ 490 °C (when the final transformation AlOOH to Al 2 O 3 takes place). Decomposition of this material causes following gases release: ammonia (NH 3 ), water (H 2 O), cyanic acid (HNCO), carbon dioxide (CO 2 ), nitric oxides (NO, NO 2 , N 2 O). These findings are in agreement with Praus et al. [6] who performed elemental analysis of the melamine condensation products at different temperatures. They showed [6] that all melamine condensation products contain nitrogen and hydrogen.
The evolution of gaseous carbon-containing species may indicate the fact that PCN does not exist in ideal C 3 N 4 stoichiometry, which could be assumed as result when NH 3 component will be removed from the N-rich compound. Instead, the stoichiometry of PCN materials seem to be close to C 2 N 3 H, corresponding to melon (C 6 N 7 (NH)(NH 2 )) or hydromelonic acid (C 6 N 7 (NCNH) 3 ) [12].

Conclusions
The thermal stability of PCN-nanocomposites was studied for the first time using the DSC-TG-FTIR-QMS method. Designed experiments allowed confirming optimum synthesis conditions for PCN-Al 2 O 3 -ZrO 2 nanocomposite photocatalysts. PCN-nanocomposite obtained at 400 °C was found to be thermally stable from room temperature up to ~ 490 °C. At 500 °C, full decomposition of the material takes place with evolution of ammonia (NH 3 ), water (H 2 O), cyanic acid (HNCO), nitric oxides (NO, NO 2 , N 2 O).
In contrast to 400 °C route, nanocomposite prepared at 450 °C was thermally unstable with gradual decomposition starting already below 100 °C. The main difference in the type of emitted gases during thermal decomposition of the investigated nanocomposites was the presence of HCN/CN − . This compound was released in addition to NH 3 , H 2 , H 2 O, and HNCO in the case of nanocomposite obtained at 450 °C. The presence of HCN/CN − indicates poor stability of the nanocomposite produced at 450 °C.
The obtained results allow to conclude that in case of PCN-nanocomposites usage of AlOOH-ZrO 2 as a composite components mixture play a major role. The results showed, that obtained in microwave synthesis AlOOH-ZrO 2 uniformly mixed nanopowder undergoes several transitions of AlOOH to Al 2 O 3 . The final phase transition at 490 °C leads to complete decomposition of PCN. Even being close to this temperature (like in a case of nanocomposite annealed at 450 °C) is not beneficial for further thermal stability and performance of the nanocomposite. This could be explained by changing of morphology and internal rearrangements of nanoparticles leading to breaking continuity of PCN layer on Al 2 O 3 -ZrO 2 .
In the future, the mechanism of the PCN-nanocomposite formation will be investigated. However, currently it can be assumed that the formation of PCN in the nanocomposite is initiated by the disintegration of AlOOH and Al 2 O 3 formation at approximately 400 °C in the presence of AlOOH, ZrO 2 and melamine mixture.