From Triazine to Heptazine: Origin of Graphitic Carbon Nitride as a Photocatalyst

Abstract

Graphitic carbon nitride (g-CN) has emerged as a promising metal-free photocatalyst, while the catalytic mechanism for the photoinduced redox processes is still under investigation. Interestingly, this heptazine-based polymer optically behaves as a "quasi-monomer". In this work, we explore upstream from melem (the heptazine monomer) to the triazine-based melamine and melam and present several lines of theoretical/experimental evidence where the catalytic activity of g-CN originates from the electronic structure evolution of the C−N heterocyclic cores. Periodic density functional theory calculations reveal the strikingly different electronic structures of melem from its triazine-based counterparts. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy also provide consistent results in the structural and chemical bonding variations of these three relevant compounds. Both melam and melem were found to show stable photocatalytic activities, while the photocatalytic activity of melem is about 5.4 times higher than that of melam during the degradation of dyes under UV−visible light irradiation. In contrast to melamine and melam, the frontier electronic orbitals of the heptazine unit in melem are uniformly distributed and well complementary to each other, which further determine the terminal amines as primary reduction sites. These appealing electronic features in both the heterocyclic skeleton and the terminated functional groups can be inherited by the polymeric but quasi-monomeric g-CN, leading to its pronounced photocatalytic activity.

Carbon nitride materials: impact of synthetic method on photocatalysis and immobilization for photocatalytic pollutant degradation

“…For CN-C, the band gaps are at 2.8 eV. The presented data are in good agreement with the literature [47,49,50]. The CN-A materials show a band gap of ca.…”

A New Class of Carbon Nanostructures for High‐Performance Electro‐Magnetic and ‐Chemical Barriers

“…The C 1s spectra of the melamine and β ‐Ni(OH) 2 ‐Mel were fitted into two main peaks at 284.7 and 287.7 eV, which are assigned to the adventitious graphitic carbon (C ad ) and three‐coordinated carbon (C 3N ) in the triazine unit, respectively (Figure S3a,b , Supporting Information). [ 31 ] In addition, Ni 2p spectrum analysis confirmed that β ‐Ni(OH) 2 was formed in β ‐Ni(OH) 2 ‐Mel (Figure S3c , Supporting Information). As shown in the deconvoluted N 1s spectrum of the β ‐Ni(OH) 2 ‐Mel, two additional peaks of —N + H═ and NO 3 – were included in addition to the characteristic peaks corresponding to —NH 2 and —N═ arising from melamine.…”

“…Enlarged XRD patterns in the range from 32° to 70° exhibited the diffraction peaks of nickel‐related species, excluding the characteristic peaks of melamine (Figure 1b ). [ 31 ] As a result of the reaction between nickel nitrate and NaOH, the characteristic peaks of α ‐Ni(OH) 2 were observed at 33.8° and 60.3° in α ‐Ni(OH) 2 ‐Mel, which coincided with the (101) and (110) planes, respectively (JCPDS 22‐0444). After the aging step, β ‐Ni(OH) 2 ‐Mel exhibited different characteristic peaks at 33.7°, 38.8°, 52.8°, and 59.7°, which were assigned to (100), (101), (102), and (110) planes of β ‐Ni(OH) 2 , respectively (JCPDS 14‐0117).…”

“…As shown in Figure 2c, the N 1s spectrum of 𝛽-Ni(OH) 2 -Mel_330 was fitted into three main peaks at 398.2, 399.1, and 401.5 eV corresponding to pyridinic N, -NH 2 , and graphitic N, respectively, indicating the beginning of condensation of melamine. [31] For the sample at the second inflection point of 440 °C, the fitted N 1s spectrum exhibited additional peaks at 400.5 eV assigned to pyrrolic N. The condensation of melamine proceeded during the heat treatment, reducing the ratio of -NH 2 , while increasing the ratio of pyrrolic N and graphitic N. At the final temperature of 900 °C, 𝛽-Ni(OH) 2 -Mel_900 exhibited four peaks located at 398.2, 400.5, 401.6, and 404.1 eV, which can be attributed to pyridinic-, pyrrolic-, graphitic-, and oxidized N, respectively. [42] We further examined TEM to observe microstructural changes (Figure 2d-g).…”

A top-down design for easy gram scale synthesis of melem nano rectangular prisms with improved surface area

“…5,25 A new and distinctive rectangular prism morphology with an improved surface area was detected for the as-prepared melem, 5,8,26 which makes our study even more unique and novel. 5,8,27 It is well known that both morphology and specic surface area play important roles in affecting the photocatalytic activity of semiconductors. 22,28 Thus, our study not only provides a novel practical method for the preparation of nanostructured monomer melem, but also paves a new pathway for increasing its surface area.…”