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Structural Investigation of Graphitic Carbon Nitride Via XRD and Neu- Tron Diffraction Federica Fina†, Samantha K

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Structural Investigation of Graphitic Carbon Nitride Via XRD and Neu- Tron Diffraction Federica Fina†, Samantha K Structural investigation of graphitic carbon nitride via XRD and Neu- tron Diffraction Federica Fina†, Samantha K. Callear‡, George M. Carins†, John T. S. Irvine†* † School of Chemistry, University of St Andrews, St Andrews, KY16 9ST, Scotland, United Kingdom ‡ ISIS Neutron and Muon Facility, STFC, Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0QX, Unit- ed Kingdom ABSTRACT: Graphitic carbon nitride (g-C3N4) has, since 2009, attracted great attention for its activity as a visible light-active photocatalyst for hydrogen evolution. Since it was synthesized in 1834, g-C3N4 has been extensively studied both catalytically and structurally. While its 2D structure seems to have been solved, its 3D crystal structure has not yet been confirmed. This study attempts to solve the 3D structure of graphitic carbon nitride by mean of x-ray diffraction and of neutron scattering. Initially, various structural models are considered and their XRD patterns compared to the meas- ured one. After selecting possible candidates as g-C3N4 structure, neutron scattering is employed to identify the best model that describes the 3D structure of graphitic carbon nitride. Parallel chains of tri-s-triazine units organized in layers with an A-B stacking motif are found to describe the structure of the synthesized graphitic carbon nitride well. A misa- lignment of the layers is favorable because of the decreased π-π repulsive inter-layer interactions. INTRODUCTION due to the highly disordered character of the material an Graphitic carbon nitride (g-C N ) is one of the several accurate description of its 3D structure had only been 3 4 20-21 allotropes of the carbon nitrides family. It was synthesised suggested. for the first time by Berzelius and named “melon” by Liebling in 1834.1 It is a polyconjugated semiconductor composed of carbon and nitrogen atoms and it is charac- terised by a layered graphitic-like structure.2 The fully polymerised form of g-C3N4, characterised by a C:N ratio of 0.75, cannot be practically obtained. Therefore, the material presents a hydrogen content of 1-2 %, which var- ies depending on the synthesis procedure.3 Graphitic car- bon nitride can be easily synthesised via solid state syn- thesis from cheap materials such as melamine, it is insol- uble in most solvents and shows great stability in extreme conditions (pH = 0 and pH = 14).2, 4 Due to these charac- Figure 1 Structures considered representative of g-C3N4: a) teristics, it has recently attracted great attention for dif- triazine based and b) tri-s-triazine based. ferent catalytic applications, in particular photocatalysis To the best of our knowledge the most recent work on 2, 4 21 for hydrogen evolution from water. Even though widely the 3D structure of g-C3N4 is that of Tyborsky et al.. In studied as a catalyst, to date, the g-C3N4 crystal structure their work x-ray diffraction modelling is employed to has not been fully solved. The first examples of structural solve the structure of graphitic carbon nitride. However, investigation date back to the early 20th century.5-7 Since due to the short range order of the material which pre- then two main 2D structures have been suggested: a tria- vents proper structure refinement, x-ray diffraction alone zine-based (Figure 1a)8-11 and a tri-s-triazine-based is not sufficient. Neutron scattering on the other hand one(Figure 1b)12-15. Many investigations have been carried can provide additional information useful for the struc- out in order to confirm which of the two structures could ture determination of g-C3N4. 12-13, 16-20 16, 19 describe g-C3N4. By 2009 , with the aid of elec- Neutrons interact with the nuclei of the atoms instead tron diffraction,16, 19-20 and solid state Nuclear Magnetic of the electrons and can therefore offer complementary 16 Resonance, the 2D structure of g-C3N4 synthesised by information to XRD analysis regarding the atomic struc- thermal condensation of small organic polymers was con- ture of materials..22 Additionally due to the size of the sidered solved. The structure of g-C3N4 was confirmed as neutrons, neutron scattering is very sensitive to hydrogen a tri-s-triazine based polymeric structure. 16, 19 However, and other light elements such as carbon. Due to the op- posite sign of the scattering lengths of H and D,23 negative phitic carbon nitride was packed into a flat plate TiZr peaks are produced in the first case and positive in the container that was chosen because of the opposite sign of second. A comparison of labelled and not-labelled mate- the scattering length of the two elements, giving an over- rials allows for direct identification of atomic correlations all null scattering.24 The container had a wall thickness of involving hydrogen. Neutron scattering can therefore be a 1 mm and an inner aperture of 1 mm. The beam size was 3 suitable analytical technique for g-C3N4 and can offer cm x 3 cm. A Vanadium plate was used as a standard to complementary information to the XRD analysis. In the normalize the data. Measurements of the background and present work, we employ structural models proposed in container were also collected and used to correct the raw the literature, we create some new ones and generate data. The data normalisation and corrections were carried their theoretical XRD patterns. These are then compared out using GudrunN.24 PDFGui was employed to generate to the measured XRD pattern of the synthesised graphitic calculated PDFs of the crystal structures using scale and carbon nitride. After narrowing down the possible crystal damp-ing factors that were fitted to the experimental da- structures, the most promising ones are compared with ta. The atomic positions, lattice parameters and isotropic the results from the neutron scattering analysis. It is displacement parameters were inputted as per the crystal found that the material synthesised for this study is best structure data and were kept fixed.25 PDFGui outputs a described by tri-s-triazine layers with an off-set in their residual curve (the difference between the model and the alignment. To the best of our knowledge no report of data) and also the weighted R-factor (Rw) which is a Neutron Scattering on g-C3N4 exists in the literature and measure of how accurately the model represents the data this work brings the structural characterisation of this (the smaller the value the better the fit). active catalyst a step forward. Elemental Analysis and density measurement. The elemental analysis was acquired with Carlo Erba EXPERIMENTAL Flash 2000 Elemental Analyser, configured for wt.% CHN. Material synthesis. Graphitic carbon nitride was syn- Density measurements were carried out with a Mi- thesised by thermal polycondensation of Melamine (Sig- cromeritics, AccuPyc 1340 Gas Pycnometer. ma-Aldrich, 99.9 %) at 500 °C for 15 h. The precursor was placed in a closed alumina crucible and heated to tem- RESULTS perature with a rate of 5 °C/min. After the synthesis it was Photocatalytic performance. Graphitic carbon ni- ground to fine powder. To synthesize the deuterated tride is tested for photocatalytic hydrogen evolution un- g-C3N4 the same synthetic procedures was applied but der visible light. Triethanolamine (TEOA) is employed as Melamine-d6 was employed as the starting material. sacrificial agent to promote the production of hydrogen. Photocatalytic performance evaluation. Meas- The material without the aid of a co-catalyst shows an urement of the photocatalytic hydrogen evolution were evolution rate at the steady state of less than 1 μmol·h-1 performed in a custom-made photocatalytic reactor with under the described conditions. However, after loading of top irradiation through a quartz window. In a typical ex- 1 wt.% of platinum as the co-catalyst the performance periment 0.1 g of catalyst (1 g·L-1) were suspended in reaches a rate of 22 μmol·h-1. This results are consistent 100 mL of a 10 v.% solution of triethanolamine (TEOA)2 with what has been reported in the literature for graphitic employed as sacrificial agent. The inner atmosphere was carbon nitride,2 showing that the material subject of this purged with Argon gas to remove any trace of air. Top study is an active photocatalyst for hydrogen evolution. irradiation was carried out with a 250 W iron-doped met- XRD analysis. The x-ray diffraction pattern of g-C3N4 al halide ultraviolet-visible lamp (≥ 290 nm; UV Light synthesized by thermal polycondensation of melamine is Technology Limited) with a cut-off filter (≥ 420 nm; Boro- illustrated in Figure 2. The characteristic peaks associated 2, 14 silicate Coated Glass, UQG Optics Ltd) to block with g-C3N4 can be identified. The main one at UV-irradiations. Hydrogen evolution was monitored for 2θ = 27.4 ° is reported as the (002) plane, equivalent to a an experimental duration of 20 hours using a gas chro- d-spacing of 0.326 nm. This is usually ascribed to the dis- matograph (Agilent 3000 Micro Gas Chromatograph). tance between the layers of the graphitic material.2, 14 The X-ray diffraction. The microstructure was investigat- peak at 2θ = 13.00 ° is attributed to the (100) plane, with a ed by powder X-Ray Diffraction (XRD) using an Empyre- d-spacing of 0.680 nm. This is due to the intra-layer an PANalytical series 2 diffractometer with a Cu Kα radia- d-spacing.2, 14 tion source (λ = 1.5406 Å). The theoretical patterns of the The measured XRD pattern is initially compared to the modelled structures were generated using the software crystal structure proposed by Teter and Hemley8 in 1996 FullProf. In order to remove all symmetry operators dur- (Figure 2).
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