Graphene oxide-assisted production of carbon nitrides using a solution process and their photocatalytic activity
Introduction
Graphitic carbon nitride (g-C3N4) composed of triazine- or tri-s-triazine-based expanded networks is the most stable form of the several allotropes of carbon nitride under ambient conditions [1], [2], [3]. g-C3N4 and its derivatives have shown great promise as metal-free catalysts and/or supports for various photocatalytic reactions due to their suitable band structures [4], [5], [6]. However, their practical applications are limited by the lack of cost-effective mass production. g-C3N4 and its derivatives are generally synthesized by condensation or polymerization of monomers at high temperature and high pressure. For example, thermal treatment of precursors such as cyanamide, dicyandiamide, and melamine at 400–500 °C produces g-C3N4 [7], [8], [9], [10], [11]. Montigaud et al. reported the generation of g-C3N4 containing pendant amino groups by solvothermal condensation of melamine with cyanuric chloride [12]. Similarly, condensation of 2-amino-4,6-dichlorotriazine under 1–1.5 GPa at 500–600 °C generated a close-to-crystalline g-C3N4 derivative [8]. The development of cost-effective solution-based processes for the synthesis of g-C3N4 derivatives at low temperatures remains a challenge.
Herein, we report the graphene oxide-assisted generation of g-C3N4 using a one-pot aqueous solution process at a low temperature of 100 °C. The reaction of an aqueous homogeneous colloidal suspension of graphene oxide with cyanamide under reflux at atmospheric pressure produces hybrid materials (CGH) composed of g-C3N4 and graphene-based nanoplatelets. These metal-free hybrids show good photocatalytic performance for the decomposition of organic pollutants using one sun solar light illumination.
Chemically modified graphene nanoplatelets have been used in various hybrid materials for nanocatalysts due to their excellent electrical properties, high surface area, and solution processibility [13], [14], [15], [16]. Graphene oxide (G–O) obtained by sonication of graphite oxide (GO), which was in turn produced by oxidation of graphite, has been extensively used to generate a variety of graphene-based nanoplatelets as precursors in solution-based processes [13], [17], [18], [19], [20], [21], [22], [23]. G–O features a wide range of oxygen functional groups on its basal planes and edges and disperses well in water as individual single-layered nanoplatelets [24], [25], [26]. In this work, we mix an aqueous homogeneous colloidal suspension of G–O with cyanamide (Fig. 1a, see Section 2 for details), which is a common precursor for the production of g-C3N4 [7], [27], [28], [29], [30], [31], [32], [33]. Although it has been reported that high temperature (up to 1000 °C) treatment of film samples composed of pre-mixed G–O and cyanamide produces N-doped graphene-based nanoplatelets [34], the solution-based reactivity of G–O and cyanamide has not yet been investigated.
Section snippets
Production of a set of g-C3N4/rG–O hybrid samples
GO was synthesized from natural graphite (SP-1, Bay Carbon, MI) using the modified Hummers method, as reported previously [13]. GO powder was loaded in a round-bottom flask filled with purified water (1 mL per 3 mg of GO). A homogeneous colloidal suspension of graphene oxide was sonicated using a Bransonic® 8510 ultrasonic cleaner (250 W) until it became clear without visible GO particles. The specified amount of cyanamide powder (99%, Sigma–Aldrich), i.e., 68 (1), 270 (2), 540 (3), and 2180 mg (4)
Results and discussion
A brown suspension of G–O nanoplatelets and cyanamide was stirred under reflux for 4 days. After filtration, residual cyanamide was removed by washing with water. As shown in Fig. 1a, the resultant hybrid materials were obtained as a gray mixture composed of black powders (i.e., reduced G–O, henceforth referred to as rG–O) and white solids (i.e., g-C3N4, see below for chemical analyses). The color of the hybrids (CGH) varied from black (1) to gray (4) as the amount of cyanamide increased (see
Conclusion
We synthesized novel hybrid materials composed of g-C3N4 and rG–O using a solution-based process involving the reaction of an aqueous suspension of graphene oxide with cyanamide under reflux. The production of the hybrids was confirmed by XRD, XPS, FT-IR, TGA, and elemental analyses. During the reaction, simultaneous reduction of graphene oxide and graphene oxide-assisted generation of g-C3N4 occurred. This is the first report of the generation of graphene-based hybrids containing g-C3N4 using
Acknowledgments
S.P. thanks Busan Center, Korea Basic Science Institute (KBSI) for the XPS analysis. This work was supported by INHA University and grants (Code No.2011-0031630) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT & Future Planning, Korea and from the International Cooperation of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the Ministry of Knowledge Economy, Korea. S.O.K. acknowledges the
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