Synthesis of graphitic carbon nitride crystals by thermal vapor condensation on Cu surface

Abstract

Graphitic carbon nitride (g-C3N4) is an allotrope of carbon nitride characterized by a layered structure composed of triazine or tri-s-triazine units, N-containing heterocycles. The layers form a π-conjugated planar structure connected by N- bridges and are stacked through interlayer van der Waals forces, resembling a graphite-like carbon nitride skeleton. Unlike graphene, g-C3N4 possesses a direct band gap of 2.7 eV, offering semiconductivity in contrast to graphene's zero-band gap. Its unique features, such as semiconductivity, suitable band positions, high thermal and chemical stability, along with high photo-responsiveness, make it as a highly promising 2D material. g-C3N4 is typically synthesized through the thermal condensation of precursor molecules containing C and N, such as cyanamide, dicyandiamide, and melamine. However, g-C3N4 produced by thermal condensation and thermal vapor condensation (TVC) has powder form with randomly oriented small crystalline domain. It makes g-C3N4 challenging for various applications. To address this limitation, active research is underway to synthesize highly oriented g-C3N4 single crystals. To addresses this limitation, this thesis suggests a method for synthesizing g-C3N4 crystals through a TVC on Cu(111). TVC is a growth method of g-C3N4 on substrate directly through the gas phase thermal condensation. In the TVC system, the selection of substrate is important because it directly affects the nucleation and growth of the deposited film. We chose Cu(111) as a substrate which has high affinity for melamine molecules, a representative precursor of g-C3N4. Also, by unifying the crystal faces into Cu(111), highly oriented g-C3N4 crystals were obtained through the TVC method. Synthesized g-C3N4 crystals were characterized by Scanning electron microscope (SEM), Transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). Chapter 1 provides the basic knowledge essential for understanding this thesis. Chapter 1.1 introduces the fundamental information regarding structures and properties of g-C3N4. Chapter 1.2 describes the conventional synthetic methods of g-C3N4, thermal condensation and thermal vapor condensation (TVC). Chapter 1.3 presents characterization methods of g-C3N4, such as XRD, TEM, Raman spectroscopy, and XPS. Chapter 2 describes a method for synthesizing highly oriented g-C3N4 crystals using Cu(111) as a substrate through TVC. Through XPS and TEM, we confirmed that g-C3N4 Crystals were successfully synthesized. Also, we observed the growth of g-C3N4 crystals. During the temperature increase, an amorphous CN layer is first created on the copper, followed by g-C3N4 crystals. When the melamine precursor is heated in a sealed Pyrex ampoule containing Cu(111) foil, the pronounced strong affinity between the melamine and Cu results in a massive melamine vapor near the Cu surface. This heightened concentration of melamine facilitates rapid polymerization, leading to the crystalline g-C3N4. Additionally, the presence of single-oriented Cu(111) surface support to alignment of g-C3N4 crystals in one direction.