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Enhancement of the graphitic carbon nitride surface properties from calcium salts as templates
Accepted Manuscript Enhancement of the graphitic carbon nitride surface properties from calcium salts as templates P. Gibot, F. Schnell, D. Spitzer PII:
S1387-1811(15)00412-6
DOI:
10.1016/j.micromeso.2015.07.026
Reference:
MICMAT 7231
To appear in:
Microporous and Mesoporous Materials
Received Date: 4 March 2015 Revised Date:
18 June 2015
Accepted Date: 24 July 2015
Please cite this article as: P. Gibot, F. Schnell, D. Spitzer, Enhancement of the graphitic carbon nitride surface properties from calcium salts as templates, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.07.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhancement of the graphitic carbon nitride surface properties from calcium salts as templates. P. Gibot*, F. Schnell, D. Spitzer
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Laboratoire des Nanomatériaux pour Systèmes Sous Sollicitations Extrêmes (NS3E), CNRS-ISL-UNISTRA UMR 3208, Institut franco-allemand de recherches de Saint-Louis
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(ISL), 5 rue du Général Cassagnou, BP70034, 68301 Saint-Louis, France
Type of article: short communication
*Corresponding author. Tel: +33 (0)3.89.69.58.77. E-mail: [email protected] (Pierre Gibot)
Co-authors: [email protected]; Tel.: +33 (0)3.89.69.50.75 [email protected]; Tel.: +33 (0)3.89.69.51.70
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ACCEPTED MANUSCRIPT Abstract A graphitic carbon nitride material with enhanced surface properties has been successfully synthesized from guanidine monohydrochloride used as carbon nitride precursor, and from calcium salts carbonate nanoparticles used as templates. The products were characterized by
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X-Ray Diffraction (XRD), chemical analysis, Fourier Transform Infra-Red spectroscopy (FTIR), nitrogen adsorption, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Ultra-Violet Visible spectroscopy (UV-Vis). The results show that
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the products adopt a graphitic structure with basal planes made of carbon and nitrogen atom species linked together by single and double bonds in an aromatic array (s-triazine, tri-s-
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triazine ring). As a function of the amount of the calcium-based template, a series of mesoporous materials was prepared which had specific surface areas ranging from 24.0 to 39.4 m²/g and coupled with pore volumes of 0.13 to 0.22 cm3/g. Instead of using the usual silica hard templates, our results show that using CaCO3 and Ca3(PO4)2 nanoparticles appears
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to be an encouraging solution for developing the surface properties of the carbon nitride. The carbon nitride is a promising candidate in the field of photocatalysis.
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Keywords: carbon nitride; calcium carbonate; calcium phosphate; mesoporous material;
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surface properties.
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ACCEPTED MANUSCRIPT 1. Introduction These days there is great interest in the graphitic carbon nitride, C3N4-g. This interest is thanks to its various properties that make it a reliable material with many potential applications in very different fields. Some of the advantages of the C3N4-g carbon nitride
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include its high thermal stability against oxidation in air (773 K), and its excellent chemical inertia toward both acid and alkali environment [1-2]. The electronic and optical properties of C3N4-g are also interesting. They include a small energy gap of 2.7 eV and a
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photoluminescence peak maximum at 420 nm, which can shift to higher values depending on the condensation degree and the packing between layers [3-4]. Among its potential
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applications, graphitic C3N4 seems to be appropriate as a gas storage material [5] and as a heterogeneous metal-free catalyst for Friedel-Crafts Reaction [6] or for NO decomposition [7]. More recently, its most recent potential and promising application is producing hydrogen by splitting water under visible-light irradiation [8]. Due to its smaller band gap, C3N4-g
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covers a wider spectrum that can be enlarged by specific doping [3]. Typically, carbon nitride powders are synthesized through the pyrolysis of carbon- and nitrogen-rich molecules (cyanamide, melamine, dicyandiamide, cyanuric species, guanidine
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salts) at relatively low temperatures (500-600°C) [9-27]. These methods yield materials with low specific surface areas (< 10m²/g) which limit using the materials in many potential
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applications in particular in the separation, selection and catalysis fields. There has been a lot of interest in enhancing the C3N4-g material’s surface properties such as specific area, porosity and pore diameter, and many teams have published papers on classical uses of templates to prepare porous carbonaceous materials such as silica in the form of spheres [6, 27] or matrices [17, 22, 28]. Zeolites (HZSM-5) [29], layered clays (saponite, montmorillonite) [30], and alumina porous coordination polymers [10] have also been employed successfully as hard templates. But in all these cases, using hydrofluoric acid (HF)
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ACCEPTED MANUSCRIPT or ammonium bifluoride (NH4HF2) is mandatory for template removal. We maintain that this point could hinder the industrialization of the processes. Few authors have suggested using soft templates such as non-ionic surfactants and amphiphilic block polymers (Triton X-100, Pluronic P123...) [14]. Proposing suitable (hard-) templates is a real scientific and industrial
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challenge for it is hard to scale up the production of these materials that exhibit tailored surface properties.
The present paper reports the synthesis of a graphitic carbon nitride material with enhanced
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surface properties obtained from pyrolysis of guanidine monohydrochloride (Gu.HCl) and nanosized calcium carbonate (CaCO3-nm) and calcium phosphate (Ca3(PO4)2-nm) particles
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used as g-C3N4 molecule precursor and as hard template agents, respectively. The main advantages of the calcium minerals are their low cost, their nanoscale (10-100 nm), commercial availability and their low chemical resistivity against diluted mineral acid (hydrochloric acid). Calcium carbonate has already been used successfully for generating
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mesoporous carbon, silicalite, titania and polymeric carbon nitride [31-35] and we find that it can act as a suitable template. In opposite to the work of Wang et al. [35], we used in our investigation nanosized calcium carbonate particles as template which should allow the
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preparation of carbon nitride materials with higher surface properties as observed with the use of silica spheres templates of 12 and 290 nm, respectively [6, 27]. Calcium phosphate, to your
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knowledge, has never been suggested as template for the synthesis of enhanced surface properties ceramics. Pristine products have been identified and characterized by various techniques, as described below.
2. Experimental section Guanidine hydrochloride (Gu.HCl, CH6N3Cl, NH2(=CH)NH2.HCl, 99%) was purchased from Alfa Aesar. Calcium carbonate (CaCO3, 97.5%) and calcium phosphate (Ca3(PO4)2, 98%)
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ACCEPTED MANUSCRIPT hard nanotemplates were supplied by SkySpring Nanomaterials Inc. Hydrochloric acid (HCl, 37 wt %) was purchased from Prolabo. All the reagents were used without further purification. 2.1. Synthesis of the graphitic carbon nitride material
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Typically, synthesizing high specific surface area graphitic carbon nitride powders occurred as follows: 4.00 g of Gu.HCl was dissolved in 30 mL of ethanol solution. 0.5 g and 1 g of calcium carbonate nanoparticles or 0.25 and 0.5 g of calcium phosphate nanoparticles were
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added to the solution obtained, and the resulting dispersions were magnetically stirred and sonicated for 30 min. The solvent was evaporated at room temperature until white powders
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were obtained. The powders were ground in a mortar, placed in a quartz crucible, and heattreated under argon atmosphere (100 mL/min) with a heating rate of 5 °C.min-1 up to 550°C (3 hours). During the experiments, ammonia gas (NH3) was released. The resulting monoliths were ground and washed with diluted hydrochloric acid (0.1M – 100 mL) to remove the
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templates. The dispersions were centrifuged and the precipitates were washed with distilled water and also once with acetone. Finally, the powders were dried overnight in an oven at 100°C. For clarity’s sake, the different samples were termed as C3N4_CaCX and C3N4_CaPx,
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where CaC and CaP symbolizes CaCO3 and Ca3(PO4)2, respectively and x their amount in the initial mixture. These terms do not reflect the exact chemical composition of the as-prepared
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samples. To evaluate the influence of the template on the porosity development of the carbon nitride material, a reference sample (C3N4) was prepared under the experimental conditions described above, but without the use of template nanoparticles. In that case, the carbon nitride phase yield is ca. 25%. 2.2. Characterization The X-Ray powder Diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer (Cu-Kα radiation) equipped with a Lynxeye detector and operating at 40 kV -
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ACCEPTED MANUSCRIPT 40 mA. The analysis was performed in the range of 2? = 20 - 80° with a 2θ step size of 0.02°. A chemical analysis (C, N, H, O and Cl) of the carbon nitride material (reference) was performed by CNRS laboratories (Vernaison and Nancy, France). For this purpose, the sample was burned under pressure with a helium/oxygen flow at 1050°C. The reaction
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products (CO2, H2O, NOx...) were separated on a chromatographic column and quantified by using a thermal conductivity detector. The Fourier Transform Infra-Red (FTIR) spectroscopic analysis of the samples was conducted by means of a Bruker Tensor 27 spectrometer. The
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spectra were collected in the Attenuated Total Reflection (ATR) mode and recorded in the wavenumber range of 4000 - 520 cm-1 by superposing 15 scans. Nitrogen sorption
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measurements were carried out with a Micromeritics SA 2020 surface area analyzer on the samples (-196.15°C), outgassed at 200°C for 6 hours under vacuum. The specific surface areas (SBET) were determined by using the Brunauer-Emmett-Teller (BET) method in the 0.05–0.25 relative pressure range. The pore size distributions were obtained by the Barrett-
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Joyner-Halenda (BJH) model from the desorption branch, whereas the total pore volumes (Vp) were estimated from the volume of nitrogen adsorbed at a relative pressure P/P0 of 0.989. The morphology of the samples, previously sprayed with a 5-nm-thick layer of gold (Desk II TSC,
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Denton Vacuum), was studied by Scanning Electron Microscopy (SEM) operating at 10 kV
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with a Nova NanoSEM 450 microscope (FEI). The Ultraviolet-Visible (UV-Vis) diffuse reflection spectrum of the sample was recorded on a CARY 5E spectrophotometer (Varian) in the 300-800 nm range.
3. Results and Discussion Figure 1 shows the X-ray diffraction patterns of the carbon nitride material. It shows material prepared without and with different amounts of calcium salts nanoparticles (C3N4_CaCx and C3N4_CaPx). As can be observed from the wide-angle PRD patterns, the nitrogen-based
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ACCEPTED MANUSCRIPT materials exhibit one obvious broad characteristic peak around 2Ɵ = 22 – 23° and another weaker peak around 2Ɵ = 13° (Figure 1(a)). These peaks are respectively attributed to the (002), (100) diffraction planes from the graphitic structure. Using the templates (CaCO3, Ca3(PO4)2), shows two major modifications in the corresponding diffractograms (Figures
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1(b), (c), (d) and (e)). First, there is a slight shift of the peak positions to greater angles probably due to a decrease in the interlayer distances. Second, there is a very weak decrease in the intensity of the (001) peak suggesting the ordering character of the structure for the
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material synthesized with the calcium-based hard template is less pronounced. As previously mentioned in [17], the presence of the porous agent in the mixtures before pyrolysis generates
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a confinement effect seen in these two structural evolutions.
Figure 1: X-ray diffraction patterns of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d)
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and C3N4_CaP0.5 (e).
From the elemental chemical analysis, the atomic compositions in carbon, nitrogen, hydrogen, oxygen and chlorine for the three samples were determined and gathered in the Table 1. From
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these results, empirical formulas were determined: C3.00N4.41H1.58O0.42Cl0.03, for the sample prepared without template and C3.00N4.36H2.32O0.81Cl0.004,, C3.00N4.47H3.55O1.12Cl0.008 for the
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materials synthesized with 0.5 and 1g of calcium carbonate nanoparticles and C3.00N4.50H2.03O0.55Cl0.003, C3.00N4.54H2.48O0.67Cl0.01 for the materials synthesized with 0.25 and 0.5g of calcium phosphate nanoparticles. The C:N molar ratios were deduced to be equal to about 0.66-0.69, which is lower than the ideal ratio (i.e. 0.75) for the g-C3N4. Regarding the weight loss recorded for each sample from the outgassing stage (7.07%, 12.41%, 15.41%, 6.42% and 7.49% for the C3N4, C3N4_CaC0.5, C3N4_CaC1, C3N4_CaP0.25 and C3N4_CaP0.5, respectively), prior to the nitrogen sorption measurements, and taking into hypothesis that the desorbed species are exclusively physisorbed surface water, the general formulas can be 7
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as
follows:
C3.00N4.41H0.74Cl0.03,
C3.00N4.36H0.77O0.03,
C3.00N4.47H1.49O0.09,
C3.00N4.50H1.25O0.16 and C3.00N4.54H1.11O0.17, respectively. The as-prepared samples can be considered to be graphitic carbon nitride materials with non-negligible hydrogenated nitrogen groups (NHx) as already reported in the literature [15]. These amino groups might be
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physisorbed on surface defects, as e.g., on the edge planes of the graphitic structure.
The Fourier Transform Infra-Red (FT-IR) spectra related to the different prepared products are displayed in Figure 2 and seemed to be typical of carbon nitride materials [10, 11, 13-21,
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23-26, 29]. For all spectra, the characteristic band of the bending vibration of a C-N bond (out-of-plane) in an aromatic ring is observed at 810 cm-1. This band is usually considered to
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be caused by an s-triazine C3N3 or a tri-s-triazine C6N7 ring, as in the
investigated sample.
The infrared bands ranging from 1200 to 1450 cm-1 are attributed to the bending vibration modes of the carbon-nitrogen simple bond (C-N) in s-triazine aromatic amines. As for the vibration band close to the latter infrared band, i.e. located at 1568 cm-1, that is associated
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with the bending vibration of the C=N bond in a similar environment. As largely reported in the literature, an incomplete polycondensation of the starting precursor which leads to carbon nitride phase formation is usually observed [10-26, 29]. This phenomenon, which is also
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observed in our case and for all the investigated samples, is suggested by the presence of some infrared bands described as follows: (i) those at 900 cm-1 and 1650 cm-1 related to the
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stretching vibrations of the C-N bond modes in a primary amine function (C-NH2); (ii) those in the 3000-3300 cm-1 region with characteristics of the N-H and O-H stretching vibrations, which are due to primary amine (-NH2) and water molecules adsorbed on the surface of the material; and (iii) the low-intensity band observed at 2160 cm-1 which can be attributed to the -C≡N, -N=C=O or -N=C=N- functions [11, 13, 17, 21, 23-25]. That last infrared band is particularly observable in the samples synthesized from the calcium carbonate template (Figures 3(b)-(c)). About these samples, no characteristic infrared vibration bands of CaCO3
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ACCEPTED MANUSCRIPT and Ca3(PO4)2 (Supporting information) were observed, thus confirming complete template removals according to the resolution limit of the technique.
Figure 2: FTIR spectra of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and
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C3N4_CaP0.5 (e).
Finally, as demonstrated by FTIR spectroscopy, the samples synthesized from the pyrolysis of
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Gu.HCl with or without calcium salts are composed of carbon and nitrogen atom species. These species are chemically linked together by single and double bonds in an array of the
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aromatic type (s-triazine or tri-s-triazine ring), as described for graphitic carbon nitride phases.
Figure 3(A) shows the nitrogen adsorption/desorption curves of the carbon nitride materials prepared with different calcium salts amounts. A type-IV isotherm with an H3 adsorption-
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desorption hysteresis loop, according to the IUPAC classification, was obtained for the asprepared powder with no porous agent (C3N4). For the samples synthesized from CaCO3 and Ca3(PO4)2 nanoparticles, similar isotherms are obtained, although the hysteresis loop
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increases with the amount of porous agent. In the high relative pressure region, the presence of the weak hysteresis is characteristic of a large cage-like structure corresponding to broad
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pore size distributions including mesopores (< 50 nm) and macropores (> 50 nm). The surface properties of the C- and N-rich samples, i.e. the specific surface areas, the pore size distributions, and the pore volumes, were all determined according to the methods reported above and gathered in Table 2. As shown in Table 2, increasing the amount of calcium carbonate or phosphate nanoparticles in the guanidine monohydrochloride aqueous solution from 0 to 1 g or 0.5 g, respectively enhances the surface area and pore volume of the nitrogenbased material from 12.9 m2/g and 0.06 cm3/g to 32.9 m2/g and 0.23 cm3/g with calcium carbonate and to 39.4 m2/g and 0.22 cm3/g with calcium phosphate. This demonstrates that the 9
ACCEPTED MANUSCRIPT CaCO3 and Ca3(PO4)2 nanoparticles are effective porous agents. This result can be explained as follows: the template nanoparticles take up a defined volume in the [carbon nitride/template nanoparticles] composites prepared after the calcination of the [guanidine monohydrochloride/template nanoparticles] mixtures. The higher is the amount of template
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nanoparticles, the bigger is the occupied volume by the nanotemplates. The removal of the template nanoparticles by an acid washing releases this volume thus increasing the surface properties of the carbon nitride materials (specific surface area, porosity).
The surface
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properties (specific surface area, pore volume) evolution is consistent with the weight loss recorded from the desorption stage discussed on top; the weight loss increases with the
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increase of the surface properties known to facilitate the physisorption of water molecules. The surface properties exhibited by the carbon nitride materials prepared by means of CaCO3 and Ca3(PO4)2 nanoparticles as templates also appear similar between themselves. However, to reach the better surface properties twice less of calcium phosphate is need. From the
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manufacturer’s data, both are characterized as nanosized particles; i.e. 15 – 40 nm, but whereas the calcium phosphate particles present a spherical morphology, a cubic like shape is observed for the calcium carbonate particles. Therefore, the contact surfaces are higher for the
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latter case than for the former case. The dispersion of the nanoparticles of template is then more difficult, even by sonication, for the calcium carbonate than for the calcium phosphate.
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As the surface properties are function of the nanostructuration [6, 27], the current result seems obvious. A mesoporous carbon nitride phase was prepared for the sample synthesized from 0.5 g of CaCO3 nanoparticles, whereas a higher amount (1 g) led to a meso-/macroporous sample being prepared. Because the CaCO3 nanoparticles are described by the manufacturer as having a size ranging from 15 to 40 nm associated with a specific surface area higher than 40 m2/g, the result was expected to be better. However, the particles exhibit a cubic morphology in a chain that increases the surface interactions between them and makes their
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ACCEPTED MANUSCRIPT dispersion very difficult, even by sonication. This general agglomeration tendency of the nanotemplates, especially as the dimension is small, also explains why the pore size distributions are large and not centred around the average size of the calcium-based particles (Figure 3(B)). The calcium salts/Gu.HCl composites prepared before pyrolysis, are clearly of
calcium
salts
nanoparticles
surrounded
by the
guanidine
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made of clusters
monohydrochloride molecule. Although desired, they are not composed of isolated calciumbased particles covered with the guanidine monohydrochloride molecule.
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Finally, in comparison with the Wang’s work [35] who has worked with submicron- and micronsized calcium carbonates particles, the surface properties of the carbon nitride
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materials discussed here are similar but in our investigation the amount of calcium carbonate template was divided by a factor ranging from eight to sixteen. Our work confirms and strengthens the interest of using nanostructured sacrificial templates for synthesizing ceramic
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materials with enhanced surface properties.
Figure 3: (A) Nitrogen gas adsorption isotherms and (B) BJH pore-size distributions for C3N4 (a),
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C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and C3N4_CaP0.5 (e). (The isotherms curves were
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shifted to the ordinate axis for a better legibility).
Figure 4 shows the SEM micrographs of the carbon nitride materials prepared by the pyrolysis of the guanidine hydrochloride precursor without (Fig. 4(a)) and with the calciumbased porous agent (Fig. 4(b) and (c)). Without a template, the carbon nitride sample exhibits an unspecific morphology, but a very airy architecture can be observed. With CaCO3 and Ca3(PO4)2 nanoparticles, similar global morphologies are verified. However, when the calcium-based templates are removed, a more pronounced surface roughness, a higher
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ACCEPTED MANUSCRIPT porosity with the presence of smaller particles are observed. This observation is in perfect alignment with the surface properties determined from the nitrogen adsorption measurements.
Figure 4: SEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium
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salts nanoparticles.
Figure 5 shows TEM pictures of the reference carbon nitride material (C3N4 – Figure 5 (a)) and two
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samples synthesized with calcium carbonate (Fig. 5 (b)) and phosphate nanoparticles (Fig. 5 (c)) used as templates for enhancing the surface properties of the cited first. After a meticulous analysis, it
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seems difficult to distinguish the samples between themselves taking into account the size and morphology of the different particles. The three samples are characterized by agglomerated particles with an indefinite morphology.
Figure 5: TEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium
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salts nanoparticles.
The diffuse reflection spectra of the reference carbon nitride (C3N4) and the carbon nitrides
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prepared with calcium carbonate (C3N4_CaC1) and calcium phosphate (C3N4_CaP0.5) are shown in Figure 6. The reference sample exhibits photoabsorption from ultraviolet to the
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visible light (absorption over 400 nm), with a band gap absorption edge extrapolated to 470 nm that corresponds to a band gap of ~2.7 eV, as expected for such material. For the samples prepared with the nanosized calcium carbonate and phosphate particles, i.e. C3N4_CaC1 and C3N4_CaP0.5, the maximum UV absorption shifts toward lower wavelengths (380 nm instead of 390 nm), and the visible range has a wider absorption (490 nm instead of 470 nm). It seems that the samples’ optical properties depend on the materials’ surface properties and/or particle sizes; a comprehensive study is currently in progress in our laboratory to find out.
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Figure 6: UV-Vis spectra of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of calciumbased templates.
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4. Conclusion In conclusion, it seems suitable to use nanosized calcium carbonate (CaCO3) and calcium phosphate (Ca3(PO4)2 particles as templates for preparing carbon nitride phases with
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interesting surface properties. For instance, the specific surface area (SBET) and porosity were gradually increased to reach values of 32.9 m2/g and 0.23 cm3/g, respectively with the use of
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CaCO3 nanoparticles and 39.4 m2/g and 0.22 cm3/g, respectively with the use of Ca3(PO4)2 nanoparticles. To reminder, without CaCO3 or Ca3(PO4)2 templates the values of the bulk carbon nitride were 12.9 m²/g and 0.06 cm3/g, respectively. More, the calcium carbonate and phosphate templates have one main advantage over classical silica hard templates: they can be
silica template removal.
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Acknowledgements
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removed easily with diluted hydrochloric acid instead of hydrofluoric acid as needed for the
The authors express their grateful thanks to L. Vidal (IS2M, University of Mulhouse, France),
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S. Adach (SRSMC, University of Lorraine, France) and J. Kokot and O. Muller (ISL, France) for the Transmission Electron Microscopy analysis, the elemental chemical analyses and UVVis measurements, respectively.
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[26] Long B, Lin J, Wang X, Thermally-induced desulfurization and conversion of guanidine thiocyanate into graphitic carbon nitride catalysts for hydrogen photosynthesis, J. Mater. Chem. A, 2014;2:2942-51.
[27] Hwang S, Lee S, Yu JS, Template-directed synthesis of highly ordered nanoporous graphitic
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carbon nitride through polymerization of cyanamide, Appl. Surf. Sci. 2007:253;5656-5659. [28] Vinu A, Ariga K, Mori T, Nakanishi T, Hishita S, Golberg D, Bando Y. Preparation and
1652.
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characterization of well-ordered hexagonal mesoporous carbon nitride, Adv. Mater. 2005:17;1648-
[29] Liu L, Ma D, Zheng H, Li X, Cheng M, Bao X, Synthesis and characterization of microporous carbon nitride, Microporous Mesoporous Mater. 2008;110:216-22. [30] Jiang G, Zhou C, Xia X, Yang F, Tong D, Yu W et al. Controllable preparation of graphitic carbon nitride nanosheets via confined interlayer nanospace of layered clays, Mat. Lett. 2010;64:271821. [31] Xu B, Peng L, Wang G, Cao G, Wu F, Easy synthesis of mesoporous carbon using nano-CaCO3 as template, Carbon, 2010;48:2361-80. [32] Yang G, Han H, Li T, Du C, Synthesis of nitrogen-doped porous graphitic carbons using nanoCaCO3 as template, graphitization catalyst, and activating agent, Carbon 2012:50;3753-65.
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ACCEPTED MANUSCRIPT [33] Zhu H, Liu Z, Wang Y, Kong D, Yuan X, Xie Z, Nanosized CaCO3 as hard template for creation of intracrystal pores within silicalite-1 crystal, Chem. Mater. 2008;20:1134–1139. [34] Gao LD, Luo LL,Chen JF, Shao L, Synthesis of Hollow Titania Using Nanosized Calcium Carbonate as a Template, Chem. Lett. 2005;34:138-139. [35] Wang J, Zhang C, Shen Y, Zhou Z, Yu J, Li Y, Wei W, Liu S, Zhang Y, Environment-friendly preparation of porous graphite-phase polymeric carbon nitride using calcium carbonate as templates,
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and enhanced photoelectrochemical activity, J. Mater. Chem. A; 2015;3:5126-5131.
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Table 1: Chemical atomic compositions of the different carbon nitride materials calculated by EA.
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Brunauer-Emmett-Teller model, (b) determined at P/P0 = 0.989.
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Table 2: Surface properties and chemical compositions of the different carbon nitride materials. (a)
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N at.%
H at.%
O at.%
Cl at.%
C3N4
31.77
46.75
16.78
4.41
0.3
C3N4_CaC0.5
28.58
41.57
22.13
7.68
0.03
C3N4_CaC1
24.70
36.79
29.20
9.25
0.06
C3N4_CaP0.25
29.75
44.60
20.15
C3N4_CaP0.5
28.03
42.44
23.20
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5.47
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6.24
0.09
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(m²/g)
(cm3/g)
C3N4
12.9
0.06
C3N4_CaC0.5
24.0
0.13
C3N4_CaC1
32.9
0.23
C3N4_CaP0.25
27.0
0.15
C3N4_CaP0.5
39.4
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Specific surface area (a)
Sample
0.22
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Table 2: Surface properties and chemical compositions of the different carbon nitride materials. (a)
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Brunauer-Emmett-Teller model, (b) determined at P/P0 = 0.989.
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Figure 1: X-ray diffraction patterns of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d)
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and C3N4_CaP0.5 (e).
Figure 2: FTIR spectra of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and C3N4_CaP0.5 (e).
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Figure 3: (A) Nitrogen gas adsorption isotherms and (B) BJH pore-size distributions for C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and C3N4_CaP0.5 (e). (The isotherms curves were
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shifted to the ordinate axis for a better legibility).
Figure 4: SEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium salts nanoparticles.
salts nanoparticles.
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Figure 5: TEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium
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Figure 6: UV-Vis spectra of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of calcium-
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ACCEPTED MANUSCRIPT Enhancement of surface properties of carbon nitride phases from CaCO3 template. Calcium carbonate is an alternative to the usual silica templates.
CaCO3 template is cheap and easily produced in nanosized dimensions (10-100 nm).
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This approach appears as a promising route to prepare highly porous g-C3N4.
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S1387-1811(15)00412-6
DOI:
10.1016/j.micromeso.2015.07.026
Reference:
MICMAT 7231
To appear in:
Microporous and Mesoporous Materials
Received Date: 4 March 2015 Revised Date:
18 June 2015
Accepted Date: 24 July 2015
Please cite this article as: P. Gibot, F. Schnell, D. Spitzer, Enhancement of the graphitic carbon nitride surface properties from calcium salts as templates, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.07.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhancement of the graphitic carbon nitride surface properties from calcium salts as templates. P. Gibot*, F. Schnell, D. Spitzer
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Laboratoire des Nanomatériaux pour Systèmes Sous Sollicitations Extrêmes (NS3E), CNRS-ISL-UNISTRA UMR 3208, Institut franco-allemand de recherches de Saint-Louis
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(ISL), 5 rue du Général Cassagnou, BP70034, 68301 Saint-Louis, France
Type of article: short communication
*Corresponding author. Tel: +33 (0)3.89.69.58.77. E-mail: [email protected] (Pierre Gibot)
Co-authors: [email protected]; Tel.: +33 (0)3.89.69.50.75 [email protected]; Tel.: +33 (0)3.89.69.51.70
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ACCEPTED MANUSCRIPT Abstract A graphitic carbon nitride material with enhanced surface properties has been successfully synthesized from guanidine monohydrochloride used as carbon nitride precursor, and from calcium salts carbonate nanoparticles used as templates. The products were characterized by
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X-Ray Diffraction (XRD), chemical analysis, Fourier Transform Infra-Red spectroscopy (FTIR), nitrogen adsorption, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Ultra-Violet Visible spectroscopy (UV-Vis). The results show that
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the products adopt a graphitic structure with basal planes made of carbon and nitrogen atom species linked together by single and double bonds in an aromatic array (s-triazine, tri-s-
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triazine ring). As a function of the amount of the calcium-based template, a series of mesoporous materials was prepared which had specific surface areas ranging from 24.0 to 39.4 m²/g and coupled with pore volumes of 0.13 to 0.22 cm3/g. Instead of using the usual silica hard templates, our results show that using CaCO3 and Ca3(PO4)2 nanoparticles appears
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to be an encouraging solution for developing the surface properties of the carbon nitride. The carbon nitride is a promising candidate in the field of photocatalysis.
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Keywords: carbon nitride; calcium carbonate; calcium phosphate; mesoporous material;
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surface properties.
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ACCEPTED MANUSCRIPT 1. Introduction These days there is great interest in the graphitic carbon nitride, C3N4-g. This interest is thanks to its various properties that make it a reliable material with many potential applications in very different fields. Some of the advantages of the C3N4-g carbon nitride
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include its high thermal stability against oxidation in air (773 K), and its excellent chemical inertia toward both acid and alkali environment [1-2]. The electronic and optical properties of C3N4-g are also interesting. They include a small energy gap of 2.7 eV and a
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photoluminescence peak maximum at 420 nm, which can shift to higher values depending on the condensation degree and the packing between layers [3-4]. Among its potential
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applications, graphitic C3N4 seems to be appropriate as a gas storage material [5] and as a heterogeneous metal-free catalyst for Friedel-Crafts Reaction [6] or for NO decomposition [7]. More recently, its most recent potential and promising application is producing hydrogen by splitting water under visible-light irradiation [8]. Due to its smaller band gap, C3N4-g
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covers a wider spectrum that can be enlarged by specific doping [3]. Typically, carbon nitride powders are synthesized through the pyrolysis of carbon- and nitrogen-rich molecules (cyanamide, melamine, dicyandiamide, cyanuric species, guanidine
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salts) at relatively low temperatures (500-600°C) [9-27]. These methods yield materials with low specific surface areas (< 10m²/g) which limit using the materials in many potential
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applications in particular in the separation, selection and catalysis fields. There has been a lot of interest in enhancing the C3N4-g material’s surface properties such as specific area, porosity and pore diameter, and many teams have published papers on classical uses of templates to prepare porous carbonaceous materials such as silica in the form of spheres [6, 27] or matrices [17, 22, 28]. Zeolites (HZSM-5) [29], layered clays (saponite, montmorillonite) [30], and alumina porous coordination polymers [10] have also been employed successfully as hard templates. But in all these cases, using hydrofluoric acid (HF)
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ACCEPTED MANUSCRIPT or ammonium bifluoride (NH4HF2) is mandatory for template removal. We maintain that this point could hinder the industrialization of the processes. Few authors have suggested using soft templates such as non-ionic surfactants and amphiphilic block polymers (Triton X-100, Pluronic P123...) [14]. Proposing suitable (hard-) templates is a real scientific and industrial
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challenge for it is hard to scale up the production of these materials that exhibit tailored surface properties.
The present paper reports the synthesis of a graphitic carbon nitride material with enhanced
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surface properties obtained from pyrolysis of guanidine monohydrochloride (Gu.HCl) and nanosized calcium carbonate (CaCO3-nm) and calcium phosphate (Ca3(PO4)2-nm) particles
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used as g-C3N4 molecule precursor and as hard template agents, respectively. The main advantages of the calcium minerals are their low cost, their nanoscale (10-100 nm), commercial availability and their low chemical resistivity against diluted mineral acid (hydrochloric acid). Calcium carbonate has already been used successfully for generating
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mesoporous carbon, silicalite, titania and polymeric carbon nitride [31-35] and we find that it can act as a suitable template. In opposite to the work of Wang et al. [35], we used in our investigation nanosized calcium carbonate particles as template which should allow the
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preparation of carbon nitride materials with higher surface properties as observed with the use of silica spheres templates of 12 and 290 nm, respectively [6, 27]. Calcium phosphate, to your
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knowledge, has never been suggested as template for the synthesis of enhanced surface properties ceramics. Pristine products have been identified and characterized by various techniques, as described below.
2. Experimental section Guanidine hydrochloride (Gu.HCl, CH6N3Cl, NH2(=CH)NH2.HCl, 99%) was purchased from Alfa Aesar. Calcium carbonate (CaCO3, 97.5%) and calcium phosphate (Ca3(PO4)2, 98%)
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ACCEPTED MANUSCRIPT hard nanotemplates were supplied by SkySpring Nanomaterials Inc. Hydrochloric acid (HCl, 37 wt %) was purchased from Prolabo. All the reagents were used without further purification. 2.1. Synthesis of the graphitic carbon nitride material
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Typically, synthesizing high specific surface area graphitic carbon nitride powders occurred as follows: 4.00 g of Gu.HCl was dissolved in 30 mL of ethanol solution. 0.5 g and 1 g of calcium carbonate nanoparticles or 0.25 and 0.5 g of calcium phosphate nanoparticles were
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added to the solution obtained, and the resulting dispersions were magnetically stirred and sonicated for 30 min. The solvent was evaporated at room temperature until white powders
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were obtained. The powders were ground in a mortar, placed in a quartz crucible, and heattreated under argon atmosphere (100 mL/min) with a heating rate of 5 °C.min-1 up to 550°C (3 hours). During the experiments, ammonia gas (NH3) was released. The resulting monoliths were ground and washed with diluted hydrochloric acid (0.1M – 100 mL) to remove the
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templates. The dispersions were centrifuged and the precipitates were washed with distilled water and also once with acetone. Finally, the powders were dried overnight in an oven at 100°C. For clarity’s sake, the different samples were termed as C3N4_CaCX and C3N4_CaPx,
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where CaC and CaP symbolizes CaCO3 and Ca3(PO4)2, respectively and x their amount in the initial mixture. These terms do not reflect the exact chemical composition of the as-prepared
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samples. To evaluate the influence of the template on the porosity development of the carbon nitride material, a reference sample (C3N4) was prepared under the experimental conditions described above, but without the use of template nanoparticles. In that case, the carbon nitride phase yield is ca. 25%. 2.2. Characterization The X-Ray powder Diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer (Cu-Kα radiation) equipped with a Lynxeye detector and operating at 40 kV -
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ACCEPTED MANUSCRIPT 40 mA. The analysis was performed in the range of 2? = 20 - 80° with a 2θ step size of 0.02°. A chemical analysis (C, N, H, O and Cl) of the carbon nitride material (reference) was performed by CNRS laboratories (Vernaison and Nancy, France). For this purpose, the sample was burned under pressure with a helium/oxygen flow at 1050°C. The reaction
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products (CO2, H2O, NOx...) were separated on a chromatographic column and quantified by using a thermal conductivity detector. The Fourier Transform Infra-Red (FTIR) spectroscopic analysis of the samples was conducted by means of a Bruker Tensor 27 spectrometer. The
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spectra were collected in the Attenuated Total Reflection (ATR) mode and recorded in the wavenumber range of 4000 - 520 cm-1 by superposing 15 scans. Nitrogen sorption
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measurements were carried out with a Micromeritics SA 2020 surface area analyzer on the samples (-196.15°C), outgassed at 200°C for 6 hours under vacuum. The specific surface areas (SBET) were determined by using the Brunauer-Emmett-Teller (BET) method in the 0.05–0.25 relative pressure range. The pore size distributions were obtained by the Barrett-
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Joyner-Halenda (BJH) model from the desorption branch, whereas the total pore volumes (Vp) were estimated from the volume of nitrogen adsorbed at a relative pressure P/P0 of 0.989. The morphology of the samples, previously sprayed with a 5-nm-thick layer of gold (Desk II TSC,
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Denton Vacuum), was studied by Scanning Electron Microscopy (SEM) operating at 10 kV
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with a Nova NanoSEM 450 microscope (FEI). The Ultraviolet-Visible (UV-Vis) diffuse reflection spectrum of the sample was recorded on a CARY 5E spectrophotometer (Varian) in the 300-800 nm range.
3. Results and Discussion Figure 1 shows the X-ray diffraction patterns of the carbon nitride material. It shows material prepared without and with different amounts of calcium salts nanoparticles (C3N4_CaCx and C3N4_CaPx). As can be observed from the wide-angle PRD patterns, the nitrogen-based
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1(b), (c), (d) and (e)). First, there is a slight shift of the peak positions to greater angles probably due to a decrease in the interlayer distances. Second, there is a very weak decrease in the intensity of the (001) peak suggesting the ordering character of the structure for the
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material synthesized with the calcium-based hard template is less pronounced. As previously mentioned in [17], the presence of the porous agent in the mixtures before pyrolysis generates
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a confinement effect seen in these two structural evolutions.
Figure 1: X-ray diffraction patterns of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d)
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and C3N4_CaP0.5 (e).
From the elemental chemical analysis, the atomic compositions in carbon, nitrogen, hydrogen, oxygen and chlorine for the three samples were determined and gathered in the Table 1. From
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these results, empirical formulas were determined: C3.00N4.41H1.58O0.42Cl0.03, for the sample prepared without template and C3.00N4.36H2.32O0.81Cl0.004,, C3.00N4.47H3.55O1.12Cl0.008 for the
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materials synthesized with 0.5 and 1g of calcium carbonate nanoparticles and C3.00N4.50H2.03O0.55Cl0.003, C3.00N4.54H2.48O0.67Cl0.01 for the materials synthesized with 0.25 and 0.5g of calcium phosphate nanoparticles. The C:N molar ratios were deduced to be equal to about 0.66-0.69, which is lower than the ideal ratio (i.e. 0.75) for the g-C3N4. Regarding the weight loss recorded for each sample from the outgassing stage (7.07%, 12.41%, 15.41%, 6.42% and 7.49% for the C3N4, C3N4_CaC0.5, C3N4_CaC1, C3N4_CaP0.25 and C3N4_CaP0.5, respectively), prior to the nitrogen sorption measurements, and taking into hypothesis that the desorbed species are exclusively physisorbed surface water, the general formulas can be 7
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as
follows:
C3.00N4.41H0.74Cl0.03,
C3.00N4.36H0.77O0.03,
C3.00N4.47H1.49O0.09,
C3.00N4.50H1.25O0.16 and C3.00N4.54H1.11O0.17, respectively. The as-prepared samples can be considered to be graphitic carbon nitride materials with non-negligible hydrogenated nitrogen groups (NHx) as already reported in the literature [15]. These amino groups might be
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physisorbed on surface defects, as e.g., on the edge planes of the graphitic structure.
The Fourier Transform Infra-Red (FT-IR) spectra related to the different prepared products are displayed in Figure 2 and seemed to be typical of carbon nitride materials [10, 11, 13-21,
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23-26, 29]. For all spectra, the characteristic band of the bending vibration of a C-N bond (out-of-plane) in an aromatic ring is observed at 810 cm-1. This band is usually considered to
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be caused by an s-triazine C3N3 or a tri-s-triazine C6N7 ring, as in the
investigated sample.
The infrared bands ranging from 1200 to 1450 cm-1 are attributed to the bending vibration modes of the carbon-nitrogen simple bond (C-N) in s-triazine aromatic amines. As for the vibration band close to the latter infrared band, i.e. located at 1568 cm-1, that is associated
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with the bending vibration of the C=N bond in a similar environment. As largely reported in the literature, an incomplete polycondensation of the starting precursor which leads to carbon nitride phase formation is usually observed [10-26, 29]. This phenomenon, which is also
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observed in our case and for all the investigated samples, is suggested by the presence of some infrared bands described as follows: (i) those at 900 cm-1 and 1650 cm-1 related to the
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stretching vibrations of the C-N bond modes in a primary amine function (C-NH2); (ii) those in the 3000-3300 cm-1 region with characteristics of the N-H and O-H stretching vibrations, which are due to primary amine (-NH2) and water molecules adsorbed on the surface of the material; and (iii) the low-intensity band observed at 2160 cm-1 which can be attributed to the -C≡N, -N=C=O or -N=C=N- functions [11, 13, 17, 21, 23-25]. That last infrared band is particularly observable in the samples synthesized from the calcium carbonate template (Figures 3(b)-(c)). About these samples, no characteristic infrared vibration bands of CaCO3
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Figure 2: FTIR spectra of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and
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C3N4_CaP0.5 (e).
Finally, as demonstrated by FTIR spectroscopy, the samples synthesized from the pyrolysis of
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Gu.HCl with or without calcium salts are composed of carbon and nitrogen atom species. These species are chemically linked together by single and double bonds in an array of the
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aromatic type (s-triazine or tri-s-triazine ring), as described for graphitic carbon nitride phases.
Figure 3(A) shows the nitrogen adsorption/desorption curves of the carbon nitride materials prepared with different calcium salts amounts. A type-IV isotherm with an H3 adsorption-
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desorption hysteresis loop, according to the IUPAC classification, was obtained for the asprepared powder with no porous agent (C3N4). For the samples synthesized from CaCO3 and Ca3(PO4)2 nanoparticles, similar isotherms are obtained, although the hysteresis loop
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increases with the amount of porous agent. In the high relative pressure region, the presence of the weak hysteresis is characteristic of a large cage-like structure corresponding to broad
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pore size distributions including mesopores (< 50 nm) and macropores (> 50 nm). The surface properties of the C- and N-rich samples, i.e. the specific surface areas, the pore size distributions, and the pore volumes, were all determined according to the methods reported above and gathered in Table 2. As shown in Table 2, increasing the amount of calcium carbonate or phosphate nanoparticles in the guanidine monohydrochloride aqueous solution from 0 to 1 g or 0.5 g, respectively enhances the surface area and pore volume of the nitrogenbased material from 12.9 m2/g and 0.06 cm3/g to 32.9 m2/g and 0.23 cm3/g with calcium carbonate and to 39.4 m2/g and 0.22 cm3/g with calcium phosphate. This demonstrates that the 9
ACCEPTED MANUSCRIPT CaCO3 and Ca3(PO4)2 nanoparticles are effective porous agents. This result can be explained as follows: the template nanoparticles take up a defined volume in the [carbon nitride/template nanoparticles] composites prepared after the calcination of the [guanidine monohydrochloride/template nanoparticles] mixtures. The higher is the amount of template
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nanoparticles, the bigger is the occupied volume by the nanotemplates. The removal of the template nanoparticles by an acid washing releases this volume thus increasing the surface properties of the carbon nitride materials (specific surface area, porosity).
The surface
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properties (specific surface area, pore volume) evolution is consistent with the weight loss recorded from the desorption stage discussed on top; the weight loss increases with the
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increase of the surface properties known to facilitate the physisorption of water molecules. The surface properties exhibited by the carbon nitride materials prepared by means of CaCO3 and Ca3(PO4)2 nanoparticles as templates also appear similar between themselves. However, to reach the better surface properties twice less of calcium phosphate is need. From the
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manufacturer’s data, both are characterized as nanosized particles; i.e. 15 – 40 nm, but whereas the calcium phosphate particles present a spherical morphology, a cubic like shape is observed for the calcium carbonate particles. Therefore, the contact surfaces are higher for the
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latter case than for the former case. The dispersion of the nanoparticles of template is then more difficult, even by sonication, for the calcium carbonate than for the calcium phosphate.
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As the surface properties are function of the nanostructuration [6, 27], the current result seems obvious. A mesoporous carbon nitride phase was prepared for the sample synthesized from 0.5 g of CaCO3 nanoparticles, whereas a higher amount (1 g) led to a meso-/macroporous sample being prepared. Because the CaCO3 nanoparticles are described by the manufacturer as having a size ranging from 15 to 40 nm associated with a specific surface area higher than 40 m2/g, the result was expected to be better. However, the particles exhibit a cubic morphology in a chain that increases the surface interactions between them and makes their
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ACCEPTED MANUSCRIPT dispersion very difficult, even by sonication. This general agglomeration tendency of the nanotemplates, especially as the dimension is small, also explains why the pore size distributions are large and not centred around the average size of the calcium-based particles (Figure 3(B)). The calcium salts/Gu.HCl composites prepared before pyrolysis, are clearly of
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Finally, in comparison with the Wang’s work [35] who has worked with submicron- and micronsized calcium carbonates particles, the surface properties of the carbon nitride
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materials discussed here are similar but in our investigation the amount of calcium carbonate template was divided by a factor ranging from eight to sixteen. Our work confirms and strengthens the interest of using nanostructured sacrificial templates for synthesizing ceramic
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Figure 3: (A) Nitrogen gas adsorption isotherms and (B) BJH pore-size distributions for C3N4 (a),
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C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and C3N4_CaP0.5 (e). (The isotherms curves were
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Figure 4 shows the SEM micrographs of the carbon nitride materials prepared by the pyrolysis of the guanidine hydrochloride precursor without (Fig. 4(a)) and with the calciumbased porous agent (Fig. 4(b) and (c)). Without a template, the carbon nitride sample exhibits an unspecific morphology, but a very airy architecture can be observed. With CaCO3 and Ca3(PO4)2 nanoparticles, similar global morphologies are verified. However, when the calcium-based templates are removed, a more pronounced surface roughness, a higher
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Figure 4: SEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium
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Figure 5 shows TEM pictures of the reference carbon nitride material (C3N4 – Figure 5 (a)) and two
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samples synthesized with calcium carbonate (Fig. 5 (b)) and phosphate nanoparticles (Fig. 5 (c)) used as templates for enhancing the surface properties of the cited first. After a meticulous analysis, it
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seems difficult to distinguish the samples between themselves taking into account the size and morphology of the different particles. The three samples are characterized by agglomerated particles with an indefinite morphology.
Figure 5: TEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium
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The diffuse reflection spectra of the reference carbon nitride (C3N4) and the carbon nitrides
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prepared with calcium carbonate (C3N4_CaC1) and calcium phosphate (C3N4_CaP0.5) are shown in Figure 6. The reference sample exhibits photoabsorption from ultraviolet to the
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visible light (absorption over 400 nm), with a band gap absorption edge extrapolated to 470 nm that corresponds to a band gap of ~2.7 eV, as expected for such material. For the samples prepared with the nanosized calcium carbonate and phosphate particles, i.e. C3N4_CaC1 and C3N4_CaP0.5, the maximum UV absorption shifts toward lower wavelengths (380 nm instead of 390 nm), and the visible range has a wider absorption (490 nm instead of 470 nm). It seems that the samples’ optical properties depend on the materials’ surface properties and/or particle sizes; a comprehensive study is currently in progress in our laboratory to find out.
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Figure 6: UV-Vis spectra of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of calciumbased templates.
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4. Conclusion In conclusion, it seems suitable to use nanosized calcium carbonate (CaCO3) and calcium phosphate (Ca3(PO4)2 particles as templates for preparing carbon nitride phases with
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interesting surface properties. For instance, the specific surface area (SBET) and porosity were gradually increased to reach values of 32.9 m2/g and 0.23 cm3/g, respectively with the use of
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CaCO3 nanoparticles and 39.4 m2/g and 0.22 cm3/g, respectively with the use of Ca3(PO4)2 nanoparticles. To reminder, without CaCO3 or Ca3(PO4)2 templates the values of the bulk carbon nitride were 12.9 m²/g and 0.06 cm3/g, respectively. More, the calcium carbonate and phosphate templates have one main advantage over classical silica hard templates: they can be
silica template removal.
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Acknowledgements
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removed easily with diluted hydrochloric acid instead of hydrofluoric acid as needed for the
The authors express their grateful thanks to L. Vidal (IS2M, University of Mulhouse, France),
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S. Adach (SRSMC, University of Lorraine, France) and J. Kokot and O. Muller (ISL, France) for the Transmission Electron Microscopy analysis, the elemental chemical analyses and UVVis measurements, respectively.
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Table 1: Chemical atomic compositions of the different carbon nitride materials calculated by EA.
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Brunauer-Emmett-Teller model, (b) determined at P/P0 = 0.989.
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Table 2: Surface properties and chemical compositions of the different carbon nitride materials. (a)
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N at.%
H at.%
O at.%
Cl at.%
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31.77
46.75
16.78
4.41
0.3
C3N4_CaC0.5
28.58
41.57
22.13
7.68
0.03
C3N4_CaC1
24.70
36.79
29.20
9.25
0.06
C3N4_CaP0.25
29.75
44.60
20.15
C3N4_CaP0.5
28.03
42.44
23.20
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6.24
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12.9
0.06
C3N4_CaC0.5
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0.13
C3N4_CaC1
32.9
0.23
C3N4_CaP0.25
27.0
0.15
C3N4_CaP0.5
39.4
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Specific surface area (a)
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Figure 1: X-ray diffraction patterns of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d)
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and C3N4_CaP0.5 (e).
Figure 2: FTIR spectra of C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and C3N4_CaP0.5 (e).
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Figure 3: (A) Nitrogen gas adsorption isotherms and (B) BJH pore-size distributions for C3N4 (a), C3N4_CaC0.5 (b), C3N4_CaC1 (c), C3N4_CaP0.25 (d) and C3N4_CaP0.5 (e). (The isotherms curves were
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Figure 4: SEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium salts nanoparticles.
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Figure 5: TEM images of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of the calcium
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Figure 6: UV-Vis spectra of C3N4 (a), C3N4_CaC1 (b) and C3N4_CaP0.5 (c) after removal of calcium-
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ACCEPTED MANUSCRIPT Enhancement of surface properties of carbon nitride phases from CaCO3 template. Calcium carbonate is an alternative to the usual silica templates.
CaCO3 template is cheap and easily produced in nanosized dimensions (10-100 nm).
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This approach appears as a promising route to prepare highly porous g-C3N4.
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