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High-Performance Coral Reef-like Carbon Nitrides: Synthesis and Application in Photocatalysis and Heavy Metal Ion Adsorption Jeannie Z. Y. Tan,†,‡ Natalita M. Nursam,†,‡,⊥ Fang Xia,‡,§ Marc-Antoine Sani,∥ Wei Li,‡,▽ Xingdong Wang,‡ and Rachel A. Caruso*,†,‡ †
Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia Manufacturing, Commonwealth Scientific and Industrial Research Organization, Clayton, Victoria 3168, Australia § School of Engineering and Information Technology, Murdoch University, Murdoch, Western Australia 6150, Australia ∥ School of Chemistry, Bio21 Institute, The University of Melbourne, Melbourne, Victoria 3010, Australia ‡
S Supporting Information *
ABSTRACT: Synthesis of carbon nitrides (CNx) by refluxing under nitrogen exhibited mixed growth mechanisms of oriented attachment and Ostwald ripening, leading to the formation of coral reef-like microstructures from spherical agglomerates. Some phase transformation from β-phase to α-phase CNx occurred upon refluxing for 1.5 h, producing a biphasic CNx. The N content relative to C was determined from CHN elemental analysis, and the presence of CN and terminal groups (i.e., COOH and NH2) was consistent with the Fourier transform infrared, nuclear magnetic resonance, and X-ray photoelectron spectroscopic results. The sample refluxed for 2.0 h (CNx/2.0 h) had the highest surface area of 24.5 m2·g−1 and displayed enhanced adsorption capacities for methylene blue (MB) molecules and heavy metal ions Pb2+ (720 mg·g−1), Cd2+ (480 mg·g−1), and As(V) (220 mg·g−1), which was attributed to the presence of COOH functional groups. CNx samples had a negative surface charge that electrostatically attracted the cationic heavy metal ions as well as MB molecules for subsequent photodecomposition under visible-light illumination. The photocatalytic activity of CNx/2.0 h toward phenol, a common pollutant in aqueous waste, was also demonstrated and a possible photocatalytic route was proposed. KEYWORDS: carbon nitrides, photocatalysis, heavy metal, α phase, β phase, oriented attachment, Ostwald ripening
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INTRODUCTION Carbon nitrides, CNx, have been the subject of intensive research since the 1989 prediction by Liu and Cohen1 that carbon nitrides have the potential to be super-hard materials. A number of crystalline phases of C3N4, including graphene-like/ graphitic C3N4 (g-C3N4) and the dense phases, such as α, β, cubic, and pseudocubic C3N4, have been postulated theoretically.2,3 Furthermore, attractive properties, such as reliable chemical and thermal endurance, low density, water resistivity, and biocompatibility, make C3N4 one of the most promising materials for metal-free photocatalysts,4,5 organic light-emitting devices,6 and high-performance tribological coatings.7 Various methods have been reported to synthesize C3N4 with different crystalline phases, such as chemical vapor deposition, shock wave synthesis, and ionothermal condensation.8 For instance, α-C3N4 was fabricated by direct current (dc) arc discharge in ammonia gas,9 while g-C3N4 can be synthesized by simply calcining melamine,4 cyanamide,10 or other nitrogenrich precursors9−11 at 550 °C for 4 h. This thermal polycondensation process has been widely used to fabricate g-C3N4 as a metal-free photocatalyst, after it was first reported by Wang et al.4 Wet chemistry methods have also been developed to avoid high-temperature pyrolysis and to control © XXXX American Chemical Society
the microstructure of g-C3N4, for example, synthesizing g-C3N4 nanotubes via a benzene−thermal process12 and preparing wellcrystallized C3N4 via a solvothermal route.13 However, use of 1,3,5-trichlorotriazine and sodium azide as the reactants and carbon tetrachloride as the solvent in the solvothermal synthesis was not environmentally benign. The visible-light-driven photocatalytic activity of g-C3N4 can be further improved by increasing its specific surface area, as postulated by Wang et al.4 Templating methods have been used to produce porous g-C3N4.14,15 For instance, Thomas et al.14 used silica as the sacrificial template to fabricate mesoporous gC3N4. They found that g-C3N4 materials with higher surface area, less polycondensation, and functionalized termination edges were more efficient catalysts compared to g-C3N4 materials with a lower surface area and higher degree of polycondensation. Later, a partially hydrolyzed g-C3N4 with an NH2 group in the framework was prepared, and the material showed 8.6 times higher photocatalytic activity than untreated g-C3N4.16 Then, Li et al.17 reported the fabrication of a Received: September 10, 2016 Accepted: January 11, 2017
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DOI: 10.1021/acsami.6b11427 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
a pressure below 100 mTorr for at least 12 h prior to the measurements. The surface area was calculated by use of Brunauer− Emmett−Teller (BET) theory. Room-temperature powder X-ray diffraction (PXRD) was performed on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. Data were inspected by use of Bruker EVA with crystalline phases identified by comparison with the ICDD−JCPDS powder diffraction file database. In situ PXRD experiments were carried out at the Australian Synchrotron on the Powder Diffraction Beamline. The X-ray energy was 18 keV, and the wavelength (0.689 Å) was calibrated by use of a LaB6 standard (NIST SRM 660b). The methodology was similar to previous in situ PXRD studies.21,22 The precursor solution (LiNO3 and C3N3Cl3 dissolved in anhydrous diethylene glycol dimethyl ether) was injected into a quartz glass capillary (1 mm in outer diameter, 0.1 mm wall thickness, and 40 mm in length), which was then sealed into a custom-made stainless steel holder initially designed by Norby.23 External N2 pressure (3 MPa) was applied to the capillary during synthesis to prevent evaporation of the solvent. The loaded capillary was placed at the X-ray beam center and heated (10 °C·min−1) to the synthesis temperature (180 °C) by a hot air blower under the capillary. The temperature was sensed by a K-type thermocouple about 2 mm beneath the capillary and was calibrated by use of a KNO3 temperature standard. In situ PXRD patterns were collected during the synthesis with a position-sensitive MYTHEN detector over the 2θ range 1.5−81.5° with a time resolution of 2 min. X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Manchester, U.K.) with a monochromated Al Kα source at a power of 144 W (12 kV × 12 mA), and a hemispherical analyzer operating in fixed transmission mode with a standard aperture (analysis area 0.3 mm × 0.7 mm). Survey spectra were acquired at 160 eV. To obtain more detailed information about chemical structure, HR spectra were recorded from individual peaks at 20 eV pass energy, yielding a typical peak width for polymers of
