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Investigating the dispersion behavior in solvents, biocompatibility and use as support for highly efficient metal catalysts of exfoliated graphitic carbon nitride Miguel Ayan-Varela, Silvia Villar-Rodil, Juan Ignacio Paredes, José M Munuera, Ana Pagán, Abel Lozano-Perez, Jose Luis Cenis, Amelia Martinez-Alonso, and Juan M.D. Tascon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06974 • Publication Date (Web): 14 Oct 2015 Downloaded from http://pubs.acs.org on October 22, 2015

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ACS Applied Materials & Interfaces

Investigating the dispersion behavior in solvents, biocompatibility and use as support for highly efficient metal catalysts of exfoliated graphitic carbon nitride

M. Ayán–Varela,a S. Villar–Rodil,a,* J.I. Paredes,a J.M. Munuera,a A. Pagán,b A. A. Lozano–Pérez,b J.L. Cenis,b A. Martínez–Alonso,a J.M.D. Tascóna

a

b

Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain

Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario (IMIDA), C/ Mayor 1, 30150 La Alberca (Murcia), Spain

*Corresponding author: E-mail address: [email protected] (S. Villar-Rodil)

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Abstract The liquid-phase exfoliation of graphitic carbon nitride (g-C3N4) to afford colloidal dispersions of two-dimensional flakes constitutes an attractive route to facilitate the processing and implementation of this novel material towards different technological applications, but quantitative knowledge about its dispersibility in solvents is lacking. Here, we investigate the dispersion behavior of exfoliated g-C3N4 in a wide range of solvents and evaluate the obtained results on the basis of solvent surface energy and Hildebrand/Hansen solubility parameters. Estimates of the three Hansen parameters for exfoliated g-C3N4 from the experimentally derived data yielded yielded δD ≈ 17.8 MPa1/2, δP ≈ 10.8 MPa1/2 and δH ≈ 15.4 MPa1/2. The relatively high δH value suggested that, contrary to the case of other two-dimensional materials (e.g., graphene or transition metal dichalcogenides), hydrogen-bonding plays a substantial role in the efficient interaction, and thus dispersibility, of exfoliated g-C3N4 with solvents. Such an outcome was attributed to a high density of primary and/or secondary amines in the material, the presence of which was associated to incomplete condensation of the structure. Furthermore, cell proliferation tests carried out on thin films of exfoliated g-C3N4 using murine fibroblasts suggested that this material is highly biocompatible and noncytotoxic. Finally, the exfoliated g-C3N4 flakes were used as supports in the synthesis of Pd nanoparticles, and the resulting hybrids exhibited an exceptional catalytic activity in the reduction of nitroarenes. Keywords: graphitic carbon nitride, colloidal dispersion, graphitic hybrid materials, metal nanoparticles, catalytic reduction 1 Introduction The isolation of graphene in 2004 and the subsequent discovery of its exceptional physical properties1,2 have aroused in more recent years great research interest in twodimensional (2D) materials in general. 3 The reduced dimensionality of 2D materials introduces unique electronic, mechanical or optical properties that are frequently absent from their bulk counterparts, making them promising candidates for many applications in different fields, such as electronics, photonics, chemical sensing and biosensing, energy conversion and storage or catalysis.4 Indeed, there is currently enormous interest in the development of a plethora of 2D materials, including transition metal dichalcogenides, transition metal oxides or hexagonal boron nitride, with novel and interesting properties. In this context, top-down approaches involving exfoliation of the parent layered material are one of the most useful strategies towards their mass preparation, a prerequisite for their large-scale practical implementation.5 Particularly, methods based on the exfoliation of layered materials in the liquid phase to yield colloidal dispersions of 2D nanosheets are very much sought after, as they boast a number of advantages. 6 Apart from potentially affording scalable production of the nanosheets, such methods facilitate their chemical modification by wet techniques and their processing into different macroscopic materials, such as thin films, composites or hybrid materials. Recently, graphitic carbon nitride (g–C3N4), a polymeric semiconductor that is the most stable allotrope of carbon nitride,7 has become the focus of significant attention amongst layered materials due to its prospective application in many relevant technological fields, such as catalysis, photocatalysis, energy conversion, chemical sensing and biosensing, or biomedicine.8–17. g–C3N4 possesses a graphite-like layered structure based on 2D sheets of tris-s-triazine units connected by tertiary amines as


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basic building block,7 as shown in Fig. 1, and is commonly synthesized by polycondensation of nitrogen-rich organic precursors (e.g., cyanamide, melamine or urea).7, 18 , 19 This layered material possesses high chemical and thermal stability, withstanding mild acids, bases and organic solvents20 as well as temperatures up to 600 ºC in air17 without significant degradation. However, g–C3N4 in bulk form exhibits several features that limit its use in many of its most relevant potential applications, such as low specific surface area, high recombination rate of photogenerated charges, 21 , 22 and a medium-scale bandgap. Some strategies have been attempted to overcome these disadvantages, such as doping with heteroatoms, 23 , 24 controlling its nanometer-scale morphology or structure,25,26 or combining it with other materials to form composites,27,28 or hybrids.29 As recent theoretical studies have predicted, and in line with what is usually observed for other layered materials, exfoliation of g–C3N4 to yield 2D nanosheets should also lead to improved and/or emerging properties compared with its bulk counterpart. 30 For instance, the probability of recombination of photogenerated charge carriers is reduced, the bandgap enlarged, the specific surface area increased and the catalytic activity improved when g–C3N4 is exfoliated into thin nanosheets.31,32 Similar to the case of graphene and other 2D materials,33,34,35, amongst the possible methods to obtain g–C3N4 nanosheets31–41 exfoliation in the liquid phase assisted by sonication 37 to give colloidal dispersions is particularly attractive, as it introduces little or no chemical and structural modification to the material, making it the production method of choice towards many purposes. However, at present only a few studies on the liquid-phase exfoliation and colloidal dispersion of g–C3N4 have been reported in the literature, being mostly qualitative in nature and limited to a reduced number of solvents.37, 42 – 45 Owing to the practical relevance of exfoliated g–C3N4, a more quantitative and comprehensive knowledge of its colloidal dispersibility, in the vein of that previously attained for different types of graphene as well as other 2D materials, would be highly desirable, as this would facilitate its manipulation and processing towards many different potential applications. Here we report a quantitative study on the dispersion behavior of exfoliated g–C3N4 in a wide range of solvents, evaluated on the basis of their surface energy and Hildebrand/Hansen solubility parameters. As could be expected from the singular characteristics of g–C3N4, which will be discussed below, the colloidal dispersion behavior of this material turned out to be complex and differed from that previously reported for other 2D materials, (e.g., graphene and transition metal dichalcogenides). We also demonstrate that the exfoliated g–C3N4 nanosheets are excellent supports for the growth of noble metal (Pd) nanoparticles (NPs) and the resulting g–C3N4–Pd NP hybrids possess an exceptional catalytic activity, as demonstrated by the reduction of 4nitrophenol (4–NP) and 4-nitroaniline (4–NA) with NaBH4 taken as model reactions. Finally, g–C3N4 is generally thought to be a biocompatible substance with potential use in biomedicine, but to the best of our knowledge no biocompatibility studies have yet been reported for materials based on exfoliated g–C3N4. As an initial biocompatibility assessment, we also describe here the culture of the murine fibroblast cell line L–929 on thin films of exfoliated g–C3N4, the results suggesting that the material lacks significant cytotoxicity.



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2

Experimental section

2.1 Exfoliation and dispersion behavior of g–C3N4 in solvents A commercial g–C3N4 powder obtained from Carbodeon Ltd. (chemical purity ≥99.5%, particle size >30 µm) was used as starting material. All the solvents were provided by Sigma-Aldrich and used as received. The effect of different processing parameters on the liquid-phase exfoliation and dispersion of g-C3N4 was investigated, but the typical procedure employed for every g–C3N4/solvent combination was as follows: 30 mg of g– C3N4 was added to 10 mL of the corresponding solvent, the mixture was sonicated for 4 hours at a power of 20 W L-1 in an ultrasound bath cleaner (JP Selecta Ultrasons system, 40 kHz), subsequently centrifuged at 500g for 10 min (Eppendorf 5424 microcentrifugue), and finally the top ~75% of the resulting supernatant volume was collected for further analysis. To quantitatively assess the efficiency of each solvent to exfoliate and colloidally stabilize g-C3N4, we estimated the amount of material in dispersion by measuring its optical absorbance through a double beam Heλios α spectrophotometer (Thermo Spectronic). The measured absorbance is known to be proportional to the dispersed concentration according to the Lambert-Beer law: A/L = αC, where A is the absorbance determined at a given wavelength, L is the optical path length, α is the extinction coefficient of the dispersed material at such a wavelength and C is the dispersion concentration. In the present work, the absorbance was measured at a wavelength of 660 nm because it is well within the range (>300 nm) where all the tested solvents are completely transparent and also because it is frequently the wavelength of choice in the literature for quantitative studies of the dispersion behavior of many 2D materials (e.g., graphene). 46 α was estimated on the basis of g-C3N4 dispersions prepared in the following solvents: water, isopropanol, dimethyl sulfoxide, and γ– butyrolactone. Aliquots of the corresponding g–C3N4 dispersions were taken and sequentially diluted to obtain a series of dispersions with different concentrations, the absorbance of which was measured. The remaining known volume of the as-prepared dispersions was vacuum-filtered through alumina membrane filters and dried overnight in a vacuum oven at 80 ºC, allowed to cool down in a desiccator and finally weighed to calculate its original concentration. Calibration curves relating measured absorbance and g–C3N4 concentration for the selected solvents could then be plotted, the slopes of which yielded α. A mean value of α (660 nm) = (5.4±0.2) × 102 mL mg-1 m-1 was determined for g–C3N4. We note that the value of α was very similar for all the tested solvents. 2.2 Synthesis and catalytic activity of exfoliated g–C3N4–Pd NP hybrids Exfoliated g–C3N4–Pd NP hybrids were obtained through the following procedure: 5 mL of 0.34 mM PdCl2 in ethanol was added to 5 mL of an aqueous dispersion of exfoliated g–C3N4 (0.2 mg mL-1). The mixture was subsequently allowed to react at 60 ºC for 90 minutes under magnetic stirring. The resultant product was purified by two cycles of sedimentation via centrifugation (20000g, 10 min) and re-suspension in milliQ water. The catalytic performance of the g–C3N4–Pd NP hybrids was evaluated on the basis of two model reactions, namely, the reduction of the nitroarenes 4–NP and 4–NA to 4–aminophenol (4–AP) and p-phenylenediamine (p-PDA), respectively, using NaBH4 as a reducing agent in water at room temperature. To this end, the reaction progress was monitored by following the change in intensity of one of the optical absorption bands characteristic of the substrate. Specifically, we measured absorbance at 400 nm to monitor the reduction of 4–NP to 4–AP, and at 380 nm to follow the reduction of 4-NA to p-PDA.47 An excess of NaBH4 was added to the reaction mixture ACS Paragon Plus4Environment

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(NaBH4:nitroarene molar ratio of 600:1) to ensure a constant concentration of the reducing agent during the reaction. In particular, mixed aqueous solutions containing 0.06 mM of nitroarene, 36 mM of NaBH4, and 1.9-5.7 µg mL-1 of g–C3N4–Pd NP hybrid with a Pd concentration of 2-6 µM was freshly prepared and hand-shaken for a few seconds before measuring its absorbance at the wavelength specified above at 0.25 s time intervals. 2.3 Cell viability/cytotoxicity of exfoliated g-C3N4 Murine fibroblasts (L-929 cell line) were chosen to evaluate the biocompatibility of exfoliated g-C3N4, as these fibroblasts are highly stable, fast-growing and commonly used in cell culture studies for cytotoxicity assessment. Graphene oxide (obtained from Graphenea) was also assayed for comparison. The fibroblasts were seeded at a density of 5000 cells/well onto 48–well tissue culture plates made of polystyrene that were precoated with a thin film of exfoliated g–C3N4. The culture medium used was Dulbecco´s modified Eagle´s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), penicillin (100 U mL-1) and streptomycin (100 mg mL-1), and incubation was carried out at 37 ºC in a 7.5% CO2 atmosphere. This medium was carefully replaced every two days during cell growth. Prior to fibroblast seeding, the culture plates were treated with 500 µL of pure FBS at 37 ºC for 2 h to facilitate the initial adhesion of the cells. Cell viability and proliferation tests were carried out by means of the MTT assay (SigmaAldrich), based on the enzymatic reduction of the tetrazolium dye 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) to its insoluble formazan derivative. Assays were conducted 1, 2 and 5 days after seeding of the fibroblasts, performing each experiment in triplicate. To this end, each well from the seeded culture plates was incubated in 500 µL of MTT solution (1 mg mL-1) at 37 ºC and 5% CO2 atmosphere for 4 h. 48 Then, the MTT solution was removed, 200 µL of dimethyl sulfoxide were added to solubilize the formazan crystals, and finally the absorbance of this reaction product was measured with a BMG FLUOstar Galaxy microplate reader (MTX Lab Systems, Inc.) at 570 nm and the reference wavelength of 690 nm. 2.4 Characterization techniques The samples were characterized by UV-vis absorption spectroscopy, inductively coupled plasma-mass spectrometry (ICP–MS), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE–SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and elemental analysis. UV-vis absorption spectra were recorded with a Heλios α spectrophotometer (Thermo Spectronic). ICP–MS analysis was performed with a 7500ce instrument (Agilent) equipped with an octopole collision/reaction cell to destroy interfering ions. FTIR spectroscopy was carried out in transmission mode with a Nicolet 8700 spectrometer (Thermo Scientific), using a KBr pellet with sample concentration of ~1 wt%. The recorded spectrum was the result of coadding 64 interferograms obtained at a resolution of 4 cm-1. XPS measurements were accomplished on a SPECS system at a pressure of 10-7 Pa with a non-monochromatic Mg Kα X–ray source operated at 11.81 kV and 100 W. Bulk g–C3N4 specimens for XPS analysis were prepared by pressing the powder into pellets by means of a hydraulic press, whereas exfoliated g–C3N4 samples were prepared by drop-casting aqueous dispersions onto a metallic sample-holder pre-heated at ~50-60 ºC, which was then allowed to dry. The g–C3N4 samples exhibited a charging effect amounting to a binding energy shift of ~2 eV that was corrected a posteriori, using the carbon impurity (adventitious carbon) as a reference. FE-SEM images were acquired on a Quanta FEG ACS Paragon Plus5Environment


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650 system (FEI Company) operated at 30 kV, whereas TEM imaging was performed with a JEOL 2000 EX-II instrument operated at 160 kV. Specimens for TEM were prepared by mixing the sample suspension with an equal volume of ethanol and dropcasting ~40 µL of the resulting mixture onto a copper grid (200 square mesh) covered with a carbon film (Electron Microscopy Sciences), which was then allowed to dry. AFM imaging was performed under ambient conditions with a Nanoscope IIIa Multimode apparatus (Veeco Instruments) in the tapping mode of operation. Rectangular silicon cantilevers with nominal spring constant and resonance frequency of about 40 N m-1 and 250-300 kHz, respectively, were employed. Samples for AFM were prepared by drop-casting a small volume (~20-40 µL) of diluted g–C3N4 dispersion (~0.02-0.05 mg mL-1) onto either freshly cleaved highly oriented pyrolytic graphite (HOPG) or mica substrates, and allowing it to dry at room temperature. The elemental composition of the starting g–C3N4 powder was determined in a LECO CHN2000 elemental analyzer. 3

Results and discussion

3.1 General characterization of bulk and exfoliated g-C3N4 Ideally, g-C3N4 should consist of stacked sheets made up exclusively of carbon and nitrogen atoms connected by strong covalent bonds, forming tris-s-triazine units as building blocks that in turn are linked to each other by way of tertiary amines. However, actual synthetic approaches usually yield incomplete condensation of the structure, leading to the presence of primary and secondary amines (see Fig. 1). The latter groups should be relatively abundant, as they are expected to be present not only along the periphery of the individual sheets (a natural consequence of the finite particle size) but also at internal edges within the sheets. Furthermore, because such groups are able to establish hydrogen bonds, interlayer interactions in g-C3N4 should incorporate not only weak dispersive (van der Waals) forces, as is typical for many well-studied layered materials (graphite, transition metal dichalcogenides, etc), but also a significant hydrogen bonding component. As will be discussed below in the light of the obtained results, this feature should make the colloidal dispersion behavior of exfoliated g–C3N4 quite different to that commonly observed for other 2D materials, such as graphene, hexagonal boron nitride or MoS2, where interlayer interactions are dominated altogether by van der Waals forces. The starting, bulk g–C3N4 material was characterized by means of FE–SEM, elemental analysis, FTIR spectroscopy and XPS. As demonstrated by FE–SEM imaging (Fig. 2a), the g–C3N4 powder investigated here consisted of a collection of particles with layered morphology (see inset to Fig. 2a) and significant polydispersity in lateral size (from a few to a few tens of micrometers), a characteristic that has been sometimes attributed to uneven temperature distributions during the synthesis of this compound.49 According to the results of elemental analysis, the material comprised 34.5 wt. % C and 63.3 wt. % N, yielding a C/N atomic ratio of ~0.64 compared with a value of 0.75 for ideal, fully condensed g–C3N4, as well as 1.7 wt. % H, which confirmed its incomplete condensation. Such a considerable amount of H (equivalent to ~19 at. %) is generally reported in the literature for g–C3N4 materials7 and suggests the above-mentioned primary and secondary amine groups to be present in significant numbers. The coexistence of the expected nitrogen-containing heterocycles and uncondensed amine groups in the starting material was substantiated by FTIR spectroscopy (Fig. 2b). Specifically, doubly degenerated bands characteristic of s–triazine ring vibrations


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appeared at 1573 and 1542 cm-1 (quadrant stretching) as well as at 1460 and 1405 cm-1 (double semicircle stretching), together with a band at ~809 cm-1 (out-of-plane ring bending by sextants).50 Furthermore, primary amines were detected by the appearance of bands at 3246 cm-1 (asymmetric N-H stretching) and 3180 cm-1 (symmetric stretching), as well as a strong NH2 scissoring deformation band at 1639 cm-1.50 XPS analysis indicated that only C and N were present in the starting material (survey spectrum not shown), with the exception of some O impurity (

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