The Performance of Nanoparticulate Graphitic Carbon Nitride as an

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The Performance of Nanoparticulate Graphitic Carbon Nitride as an Amphiphile Jingsan Xu, and Markus Antonietti J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b11346 • Publication Date (Web): 13 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Journal of the American Chemical Society

The Performance of Nanoparticulate Graphitic Carbon Nitride as an Amphiphile Jingsan Xu*† and Markus Antonietti*‡ †

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Australia Max Planck Institute of Colloids and Interfaces, Potsdam, Germany Supporting Information

‡

ABSTRACT: Graphitic carbon nitride (g-C3N4), a polymeric semiconductor that finds potential applications in multiple areas, is shown to feature amphiphilic behavior. Since the feasibility of g-C3N4 aqueous colloids is well established, g-C3N4 can therefore be viewed as an effective and well-accessible colloidal amphiphile. Its activity at different interfaces (liquid-liquid, liquidsolid and liquid-air) is illustrated: g-C3N4 is able to stabilize Pickering emulsions formed by water and organic solvents, and also gas-filled g-C3N4 frameworks based on those emulsions are obtained by natural drying. Hydrophobic solid substances like graphite and carbon nanotubes are smoothly dispersed in water assisted by g-C3N4. Besides, network-like g-C3N4 membranes floating on water surface are created and can be readily transferred to substrates. These findings provide many opportunities for the processing of g-C3N4 containing functional materials and devices.

During the past few years, g-C3N4 has attracted growing attention in fields including catalysis,1 energy conversion,2-5 and optoelectronics6-8 as a metal-free semiconductor. This disordered graphitic phase is the most thermodynamically stable phase of carbon nitrides and is commonly synthesized through thermal condensation of nitrogen-rich organic molecules such as cyanamide, dicyandiamide or melamine.9 Generally, g-C3N4 is recognized to have a layered structure that is held together by van der Waals force, and each layer comprises of aromatic tri-s-triazine units connected by tertiary amines. In practice, primary and secondary amine groups are evidenced to exist owing to incomplete condensation of the precursors.10 In addition to the extensive studies in light-toenergy applications, the physicochemical properties of g-C3N4, typically in liquid phases, are also under investigation. g-C3N4 is found insoluble in water and most organic solvents, except that it can be dissolved in concentrated sulfuric acid but then structure will be destroyed.11 Moreover, g-C3N4 can be exfoliated into fewlayer nanomaterials by bath sonication in polar solvents that form hydrogen bonds with g-C3N4.12 In particular, water has been frequently used as an efficient medium for exfoliation. Well dispersed g-C3N4 colloids with high stability are acquired due to partial hydrolysis and the resulted electrostatic repulsion, and these g-C3N4 colloids show remarkable properties.13-18 For instance, ultrathin g-C3N4 nanosheets aqueous dispersion was used as the support to deposit Pt nanoclusters and the resulted materials showed remarkable activity for photoelectrocatalytic methanol oxidation.19 However, to date, the activity of g-C3N4 at liquidinvolved interfaces, including liquid-liquid interface as well as liquid-solid and liquid-air interfaces, has not been reported.

Amphiphiles are substances that possess both hydrophilic and hydrophobic moieties. Some amphiphilic molecules (e.g. phospholipids) play crucial roles in biological membranes. In terms of g-C3N4 with the structure as described above and schematically shown in Figure 1a, the conjugated basal plane is apparently hydrophobic while the edge groups could endow the compound with hydrophilicity (hydroxyl group are assumed to exist as well because of partial hydrolysis of edge terminating amine groups, such that the zeta-potential of the structures is always negative, -35 mV in the present case), which overall imply a possible amphiphilic feature of g-C3N4. In this sense, the performances and behaviour of g-C3N4 at different interfaces are to be explored. The g-C3N4 sample was synthesized by the approach of heating cyanuric acid-melamine supramolecular precursor in nitrogen based on previous paper.20 The g-C3N4 demonstrates nanosheetlike morphology with the size of several hundred nanometers (Figure S1) and the thickness in the range of 15~45 nm. Other physical properties are summarized in Table S1. Firstly, it is very interesting to note that emulsion of hexane in water can be generated by simple manual shaking of hexane in the presence of gC3N4 aqueous dispersion (left, Figure 1b). The g-C3N4 particles were mostly transferred into the emulsion phase. Such an emulsion stabilized by solid particles rather than molecular surfactants is known as a Pickering emulsion,21 but usually Pickering emulsions do not form with such great ease. We suppose that g-C3N4 tightly adsorbed at the surface of the dispersed droplets and effectively stabilizes them from coagulation and coalescence.22 Moreover, the emulsions became readily coloured when dyes were dissolved in the g-C3N4 suspension in advance (middle and right, Figure 1b), indicating the formation of oil-in-water (o/w) emulsion in the present case. The type of emulsion (o/w instead of water-in-oil) was further examined by ejecting the emulsion onto the hexane surface, wherein the emulsion remained as tight droplets until getting dried (Figure S2); on the contrary, the emulsion instantly dispersed (in 1 sec) when contacting with water phase (Figure S3). Besides, Tyndall effect was observed in the bottom aqueous phase, which we assign to the presence of the residual g-C3N4 nanoparticles. In the top emulsion, light undergoes multiple scatterings, and no beam can be seen (Figure 1c). We found out that Pickering emulsions also worked for other water-oil systems, wherein possible oil phases include chloroform, toluene, dichloromethane and n-heptane (Figure 1d), with density either higher or lower than water. This result demonstrates the generality of g-C3N4 as a solid emulsifier. We noticed that g-C3N4 showed lower ability to emulsify chloroform and dichloromethane than to emulsify the other organic solvents, probably due to the relatively high polarizability and high dielectric constant of the two compounds.

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Figure 1. (a) Schematic illustration of g-C3N4 framework with hydrophilic groups on the edge. (b) Picture of dyed water-hexane emulsions stabilized by g-C3N4. From left to right: pure water, dyed by methyl orange and red MX-5B. (c) Tyndall effect of the lower colloidal phase and the upper emulsion phase. (d) Emulsions formed between water and chloroform, toluene, dichloromethane and n-heptane, respectively (from left to right). In a next set of experiments, the effect of g-C3N4 concentration on the emulsification was explored. Figure 2a and b show the water-hexane mixtures with various g-C3N4 concentrations before and after emulsifying. As can be seen, the volume of the emulsion phase increased with the rise of the g-C3N4 amount. Notably, the sizes of the hexane droplets varied with the g-C3N4 concentration, as seen via the light microscope (LM, Figure 2c-g). Measurement of the sizes by counting randomly selected ~100 droplets demonstrates that the average diameters of the droplets declined from 134 to 82 µm, as the g-C3N4 concentration increased from 0.083 to 1 mg/ml (Figure 2h). This result is in good agreement with the typical behaviour of Pickering emulsion.22 In addition, it is worth mentioning that although the droplets in the present o/w emulsion (generated by gently shaking) have sizes much larger than those conventional Pickering emulsions (< 1 µm), they remained stable against coalescence for at least months under ambient environment, no matter the concentration of g-C3N4. This remarkable capability of the g-C3N4 emulsifier can be partly attributed to its shape-anisotropy down to a nanosheet-like texture, which creates locked, intertwined networks at the interface.23,24 Typically, organic molecules that consist of both hydrophilic head groups and hydrophobic tail groups are used as emulsifiers. For inorganic solid emulsifiers, chemical surface modification is required to achieve appropriate wettability. For instance, micro/nanosized silica particles, the most popular stabilizer for Pickering emulsions, are naturally highly hydrophilic so that it is necessary to modify the surface by grafting non-polar organic groups (e.g. silanes).25 However, the herein used g-C3N4 was simply prepared through a direct thermal condensation without any other chemical treatment, thereby being a promising emulsifier for large-scale applications. Importantly, we found that the g-C3N4 materials made under various conditions (different temperature, precursor and atmosphere) could all result in the generation of stable emulsions (Figure S4), indicating that g-C3N4 can function as a general and robust emulsifier. In addition, fluffy g-C3N4 frameworks can be obtained by drying the emulsions in air (Figure S5). The acquired material demonstrated high adsorption capacity over rhodamine 6G (Rh6G) while the uptake of methyl orange (MO) was less efficient (Figure S6). This selectivity should result from the electrostatic attraction/repulsion between gC3N4 and the dye molecules in aqueous solution, i.e. g-C3N4 and MO are negatively charged and Rh6G is positively charged.

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Figure 2. Pictures of water-hexane mixture (a) before and (b) after emulsification. LM images of the hexane droplet in the emulsion with different g-C3N4 concentration: (c) 0.083 mg/ml, (d) 0.167 mg/ml, (e) 0.333 mg/ml, (f) 0.667 mg/ml and (g) 1 mg/ml. The scale bars represent 100 µm. (h) Average size of hexane droplets versus g-C3N4 concentration. The surface activity of g-C3N4 is expected to depend on the pH value. We noticed that the optimal emulsification was already achieved in rather pure water (pH 6.4); any major pH variation led to suppress, or even no formation of emulsion (for very low and high pH, Figure S7). Additionally, the emulsion could be reversibly broken and regenerated by sonication and shaking, respectively (Figure S8a). This emulsification-breakdown cycle was carried out multiple times, and the emulsion still showed unchanged features and remarkable stability. Due to this amphiphilicity shown at the liquid-liquid interface, g-C3N4 is expected to be also capable of dispersing hydrophobic solids in aqueous phase. Graphite was selected as a model as it is very difficult to process in water, and the strong interaction of gC3N4 with graphene by polarization via charge transfer interactions is already well documented in the literature.5,26,27 As shown in Figure 3a (left), graphite alone could not be well processed even by long-time sonication and bulk flakes were obtained which sank to the bottom quickly. LM and SEM images show that the graphite sonicated in water consists of flakes with a very wide size distribution, with species as large as 100 µm (Figure 3b,e). In contrary, after sonication treatment for 4 h, the aqueous mixture of g-C3N4 and graphite turned into homogenous grey suspension. Although the suspension precipitated due to gravitation, no obvious phase separation was observed. The presence of g-C3N4 enabled obviously a direct and much improved dispersion of graphite (Figure 3c), demonstrated by the LM picture which shows high homogeneity of the graphite/g-C3N4 mixture. SEM image (Figure 3f) shows that graphite closely integrates with the g-C3N4, meanwhile having significantly decreased agglomerate size, down to a few micrometers. The graphite flakes are broken down more effectively in the presence of g-C3N4, as they band together in water and thus the ultrasound energy is more concentrated and better utilized. Graphite binds to g-C3N4 due to the partial hydrophobicity of g-C3N4 as well as the π-π interactions between the two conjugated materials, leading to better wettability and dispersion.28 In addition, the UV-vis spectra of the suspensions were measured and recorded (Figure 3d), exhibiting the typical light absorption of gC3N4 colloids and the spectrum of the graphite/g-C3N4 system, but much enhanced optical absorption over the whole wavelength range owing to the strong absorption of graphite and possible charge transfer coupled optical transitions at the materials interface.29 Photoluminescence (PL) spectra (Figure S9) display al-


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most no shift of the graphite/g-C3N4 composite than the single gC3N4 suspension, suggesting that although quenching occurred due to the materials interaction, no energy level structure was changed for g-C3N4. Importantly, we found out that g-C3N4 is also able to disperse carbon nanotubes (CNTs) in water. Following a similar procedure as with graphite, CNTs could be processed into hybrid aqueous suspension. It is well known that special surfactants,30 polymers31 or biomolecules32 are necessary to render CNTs water accessible; otherwise CNTs will remain heavily entangled even after long time sonication (Figure S10a,b). Here, with the assistance of gC3N4 the CNTs bundles were disentangled and individually adhered to the g-C3N4 layers as illustrated by SEM and TEM (Figure S10c,d). The stability of the g-C3N4 based suspensions was quantitatively evaluated by standing-time dependent UV-vis absorption measurement. The CNTs/g-C3N4 suspension showed much higher stability against sedimentation compared to the graphite/gC3N4 system (Figure S11), probably owing to the relatively large density/size of the graphite flakes. These results imply that gC3N4 is presumably effective for making a wider range of substances water processable, probably beyond carbon materials, such as metal microstructures or conjugated polymer particles.

(Figure 4c). Atomic force microscopy (AFM) image further confirms the very low roughness of the membrane over a relatively large scale (Figure 4d), suggesting that the membrane indeed transferred as such from the water surface.

Figure 4. (a) In-situ LM image of g-C3N4 network-like membrane on water surface. (b) LM, (c) SEM and (d) AFM image of the gC3N4 network transferred to glass.

Figure 3. Samples after sonication treatment. (a) Picture of the suspension of graphite (left) and graphite/g-C3N4 (right). LM images of (b) graphite and (c) graphite/g-C3N4 powders. Scale bars represent 100 µm. (d) UV-vis absorption spectra of g-C3N4 and graphite/g-C3N4 suspension (10 times diluted). SEM images of (e) graphite and (f) graphite/g-C3N4 powders, and the arrows indicate the graphite flakes. Finally, the behaviour of g-C3N4 at the water-air interface was examined. The g-C3N4 was first dispersed in hexane by wild sonication, and after standing for 1 h the supernatant phase containing smaller g-C3N4 nanoparticles was carefully spread onto the water surface dropwise with a syringe. The hexane instantly spread out and after complete vaporization, two-dimensional, network-like gC3N4 membranes could be obtained, which floated on the water surface, as illustrated by in-situ LM observation (Figure 4a). The presence of a two-dimensional foam pattern containing large voids inside the membranes should be assigned to the rapid evaporation of hexane and phase demixing throughout the process. The larger g-C3N4 aggregates were found all over the surface and did not sink thanks to the support of the as-formed overall membrane structure and the possibly coupled textural superhydrophobicity.33 Furthermore, these solid network layers could be readily transferred to substrates by dip coating. In the present case, a glass substrate was vertically moved down into water and then moved up to achieve deposition. LM image (Figure 4b) shows the flat, 2D feature of the transferred membrane. The different segments tended to arrange parallel along the marked arrows, following the direction of flow during the coating process. Height analysis by a profilometer exhibits that the membranes had a thickness in the range of 80~120 nm. SEM image shows the membrane islands were mostly smooth except for a few large g-C3N4 aggregates

The g-C3N4 floating on the surface of water was further analyzed using a Langmuir-Blodgett (LB) trough equipped with a Wilhelmy plate to detect the surface pressure. Obvious increase of surface pressure was observed when the system was compressed by carriers (Figure S12), indicating the repulsion and structural integrity of the g-C3N4 structure. On the other hand, g-C3N4 aqueous colloids (without floated material) did not show any surface pressure change during the compression. In summary, we describe, despite being commonly treated as highly water dispersible, the amphiphilic features of g-C3N4 which root in its intrinsic structural characteristics, i.e. hydrophobic conjugated framework and hydrophilic terminating groups. The effects of high polarizability and electronic interactions could also contribute. Therefore, g-C3N4 can act as a colloidal surfactant which stabilizes Pickering emulsions, disperses carbon nanostructures in water and adsorbs on the water surface as 2D foamy networks. Considering the ease of preparation, the high stability and semiconducting properties of g-C3N4, these findings offer remarkable opportunities for exploring large-scale, technological applications of g-C3N4 based materials, such as new photocatalysis/photoconversion systems.

ASSOCIATED CONTENT Supporting Information TEM, SEM, optical pictures, UV-vis, dye adsorption, PL and surface pressure measurements. The Supporting Information is available free of charge on the ACS Publications Website. AUTHOR INFORMATION

Corresponding Author [email protected] [email protected]

Notes The authors declare no competing financial interests.



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ACKNOWLEDGMENT J.X. thanks Australian Research Council (DE160101488) for financial support.

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The Performance of Nanoparticulate Graphitic Carbon Nitride as an

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