Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides Denis Foy, Gérard Demazeau, Pierre Florian, Dominique Massiot, Christine Labrugère, Graziella Goglio To cite this version: Denis Foy, Gérard Demazeau, Pierre Florian, Dominique Massiot, Christine Labrugère, et al.. Mod- ulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides. Journal of Solid State Chemistry, Elsevier, 2009, 182 (1), pp.165-171. 10.1016/j.jssc.2008.10.018. hal-00351466 HAL Id: hal-00351466 https://hal.archives-ouvertes.fr/hal-00351466 Submitted on 7 Apr 2009 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Foy et al Journal of Solid State Chemistry 1 Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides Denis FOY a, Gérard DEMAZEAU a, Pierre FLORIAN b, Dominique MASSIOT b, Christine LABRUGERE a,c, Graziella GOGLIO a,* a CNRS, Université Bordeaux, ICMCB, 87 avenue du Dr. A. Schweitzer, 33608 Pessac, France b CNRS UPR3079, CEMHTI, Université d'Orléans, 1D avenue de la recherche scientifique, 45071 Orléans cedex, France c CECAMA, Université Bordeaux, ICMCB, 87 avenue du Dr. A. Schweitzer, 33608 Pessac, France email addresses : [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected] * Corresponding author : Graziella GOGLIO [email protected] Fax: +33 5 40002761 Foy et al Journal of Solid State Chemistry 2 Abstract An hydrogenated nitrogen-rich graphitic carbon nitride, structurally related to the theoretical graphitic phase of C3N4, has been synthesized in a bulk well- crystallized form. This new material was prepared by thermal decomposition of thiosemicarbazide up to 600°C at ambient pressure under nitrogen flow. Its composition was determined by elemental combustion analysis. Powder X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and C13 MAS NMR characterizations were performed. This material can be schematically described with a two-dimensional framework and a composition close to C3N4.17H1.12. In this nitrogen-rich material, C3N3 voids are fully occupied by water molecules which are strongly trapped into the material. A loss of crystallinity associated with a modification of the thermal behaviour is observed when the amount of trapped molecules decreases in the graphitic material, order being damaged both between and in the graphitic planes. Keywords carbon nitride; graphitic materials; X-ray diffraction (XRD), Nuclear Magnetic Resonance (NMR); X-ray photoelectron spectroscopy (XPS); Thermogravimetric analysis (TGA); Triazine; Heptazine Foy et al Journal of Solid State Chemistry 3 1. Introduction Light element-based materials are of great interest owing to their specific physico- chemical properties (particularly hardness) mainly due to the strong covalency of the chemical bonds and to the three-dimensional character of the different structures they can adopt [1]. In this way, dense (3D) polymorphs with C3N4 composition were predicted to present very low compressibility and superharness [2-7]. More generally, it was claimed that C3N4 stoichiometry was not necessary to observe exceptional mechanical properties in carbon nitrides, the inescapable condition being the three-dimensionality of the atomic framework [8-9]. Until now many attempts were made to synthesize these 3D hypothetical carbon nitrides by means of various techniques either as bulk materials [9] or thin films [10]. Nevertheless they mainly led to the formation of amorphous or poorly crystallized non-stoichiometric materials or to small crystallites embedded in an amorphous matrix. Despite these direct elaboration processes, the elaboration of low- compressibility carbon nitrides can be issued from a conversion process. In this case, flux assisted conversion processes developed in case of diamond and cubic boron nitride synthesis could be performed, a low density variety being then needed as a precursor [11]. In this way, graphitic (2D) carbon nitrides can be considered as adequate candidates. Various 2D forms were theoretically suggested with either hexagonal or orthorhombic cells and different stackings of the graphene planes [4,7,12-13]. In these materials carbon is fully involved in C3N3 heterocycles, these triazine rings being bridged one to another by N-C bonds. This bonding nitrogen belongs or not to the cycles, depending on the model. Recently, several structural models based on the heptazine (tri-s-triazine) unit were also Foy et al Journal of Solid State Chemistry 4 proposed [14]. These graphitic materials derivated from the melon polymer are claimed more stable than their homologous with triazine units. So far graphitic carbon nitrides were widely elaborated. Whatever experimental conditions, it was evidenced that the C/N molar ratio was difficult to control as well as crystallinity which was often poor. All materials were also significantly hydrogenated. It was proposed that C/N molar ratios lower than 0.75 could be explained in terms of C3N3 triazine voids into the graphenes planes, which did not seem to affect drastically the thermal stability of the materials [9]. It appears important to understand then how crystallinity can be modulated in graphitic carbon nitrides because it will influence the conversion process, as well as additive elements such as hydrogen will do. We report here the original synthesis of a well-crystallized graphitic nitrogen-rich carbon nitride by thermal decomposition of an organic reagent. A special care was given to investigate carbon and nitrogen environments. A sulfur treatment during the thermal process allowed us to modulate the composition of the material, hydrogen and oxygen amounts being strongly influenced while the carbon-nitrogen network was unchanged. We discuss here the consequences of such evolutions on the crystallinity of the material. 2. Experimental 2.1. Raw-sample preparation In the typical procedure, 2 grams of thiosemicarbazide (CH5N3S Aldrich, 99%) were placed in a Pyrex boat, this manipulation being performed in a glove-box with flowing nitrogen gas because the organic reagent is hygroscopic. The tube was maintained during any transport under N2 atmospher and was introduced in a Foy et al Journal of Solid State Chemistry 5 tubular furnace under nitrogen flow. Temperature was slowly increased (1°C/min) up to several consecutive intermediate 1 h steps at 180, 250, 350 and 480°C and then finally maintained at 600 °C for 10 h. Then the sample was cooled down to room temperature (RT) naturally. The as-obtained orange powder is called raw- sample. 2.2. Modification of the chemical composition We performed previous experiments by thermogravimetric analysis coupled with mass spectroscopy. We observed that during the thermal decomposition of thiosemicarbazide under N2 atmosphere, a large part of sulfur departure from the material occurred below 300°C leading to the formation of H2S gaseous species. We then considered that adding sulfur below 300°C would enhance H2S formation during the decomposition process and then decrease residual hydrogen amount in the as-obtained material. As a consequence, thiosemicarbazide was heated as mentioned above up to 180°C then cooled to RT naturally. Sulfur was then intimately mixed with the resulting product in a molar ratio sulfur/thiosemicarbazide (taking here into account the initial amount of CH5N3S) equal to 2.2. The mixture was then placed in the Pyrex boat, introduced in the tubular furnace and submitted to the end of the thermal decomposition procedure (i.e. 1 h steps at 250, 350 and 480 °C, 10 h step at 600 °C, then cooling at RT). The as-obtained sample is called 180-sample. Finally, 250-sample was obtained according to the same principle, sulfur being here added in the same amount after the 250 °C step. 180-sample and 250-sample are powdered materials which exhibit the same orange colour than raw-sample. Foy et al Journal of Solid State Chemistry 6 2.3. Samples characterization X-ray diffraction (XRD) patterns were collected on a PANalytical X'Pert MPD X-ray diffractometer with Cu K radiation ( = 1.5418Å). Infrared (IR) spectra were recorded on a IR Paragon 1000 spectrophotometer pelleting the sample with KBr. Bulk elemental combustion analysis of C, H, N, O, S elements was performed on a Thermofisher Flash EA 112 analyzer. Thermogravimetric analysis was performed with a SETARAM TAG 2400 equipment using a platinum crucible under argon flow (50 mL/min). A coupling with THERMOSTAR mass spectrometer was used. Raw- sample was characterized by X-ray photoelectron spectroscopy (XPS) on a VG 220i-XL ESCALAB spectrometer equipped with a non-monochromatized AlK source. The analyzed area was about 250 µm in diameter. The pressure in the -7 chamber was in the range of 10 Pa. The spectra were fitted using the AVANTAGE program provided by THERMOFISHER SCIENTIFIC. 13C CPMAS NMR spectrum of raw-sample powder was recorded using a Bruker Avance 300 MHz spectrometer under a 7.0 T magnetic field with a magic angle rotation at 10 KHz. The 1H MAS NMR spectrum was recorded on the same apparatus with a magic angle rotation at 8 KHz and a Hahn echo pulse sequence (short Hahn echo time of 300 µs). The 13C and 1H chemical shifts were referenced to tetramethylsilane at 0 ppm. 3. Results and discussion 3.1. Characterization of raw-sample Elemental combustion analysis of raw-sample confirms that the material is a carbon nitride (General formula in Table I). The C/N molar ratio (0.72) indicates a Foy et al Journal of Solid State Chemistry 7 slight overstoichiometry in nitrogen.