Tailored g-C3N4 Architectures for High-Performance Photodetection, Ultra-Trace Heavy Metal Sensing, and CO2-to-Methanol Photocatalysis

Graphitic carbon nitride (g-C3N4) is a metal-free 2D semiconductor widely recognized for its low cost, chemical stability, and visible-light activity, offering a sustainable platform for next-generation energy and sensing technologies. This dissertation establishes a materials engineering framework to tailor g-C3N4 at structural, electronic, and interfacial levels for high-performance multifunctional applications, including optoelectronics, environmental sensing, and solar fuel conversion. In the first part of this work, a nitrogen (N) self-doped g-C3N4 (g-C3N4+) was synthesized and coupled with carbon quantum dots (CQDs) to create a 2D/2D p–n heterostructure for visible (Vis) -light organic photodetector (OPD). The N self-doping enriches the conduction band density of states and narrows the bandgap (to ~1.2 eV), while CQDs provide size-tunable band alignment and fast carrier transport. Under zero bias, this OPD delivered ultrahigh alternating current performance across the Vis spectrum—achieving a specific detectivity of 4.6 × 1018 Jones, responsivity of 1.43 × 107 A W⁻¹, and external quantum efficiency of 43 × 107 % at an optical intensity of 3.56×10-4 mW/cm2 and a wavelength of 405 nm while delivering competitive performance at 532 nm and 635 nm as well. Frequency-domain analysis revealed symmetric rise and decay times (~2.9 ms) at high modulation frequencies and stable signal generation even at subzero temperatures, underscoring the potential of this platform for low-power, high-frequency photodetection in harsh environments. Building on this optoelectronic characteristic in the second part of this work, a dual-functionalized CQD-(NH2-COOH)/g-C3N4 heterostructure was developed for ultrasensitive and selective detection of hexavalent chromium (Cr6+) in water. CQD-(NH2-COOH), produced from spent coffee grounds via a one-step ultrasonic process, were non-covalently assembled onto g-C3N4 nanosheets to construct a 2D/2D hybrid interface with covalent, hydrogen-bonding, and π–π interactions. This interfacial engineering introduced mid-gap bands that broadened emission bandwidth and enhanced fluorescence quenching efficiency, enabling trace-level Cr6+ detection down to 70 pM, well below the WHO guideline of 96 pM, across a wide dynamic range (0.1 nM–100 µM) in the presence of 12 competing ions. This sustainable sensing approach demonstrates the ability of engineered CQD-(NH2-COOH)/g-C3N4 to achieve a portable, low-cost, field-deployable environmental sensor. To advance the material platform toward energy conversion, a 2D cobalt (Co) and N doped heptazine-based g-C3N4 (Co-g-C3N4+) photocatalyst was designed for visible-light-driven artificial photosynthesis of methanol from carbon dioxide (CO2). In this work, both heptazine- and triazine- based g-C3N4 framework, as well as effect of single and dual doping, were systematically investigated. The heptazine-based-g-C3N4 framework was selected over triazine structures due to their ~30 kJ mol⁻¹ higher thermodynamic stability, which enhances long-term durability, and offers a more extended π-conjugation network for charge transport. The heptazine units’ larger pore sizes and altered electronic configuration facilitate stronger CO2 adsorption and activation, while cobalt dopants enrich active sites offering variable oxidation states that also promote the adsorption of CO2. Nitrogen doping created localized midgap-band and enhances the visible light absorption and improves electron mobility. This catalyst achieved a methanol production rate of 2-3 mmol g-1h-1 and methanol yield of 27.72 %, highlighting its promise for solar-driven carbon recycling and sustainable fuel production. Together, these three parts demonstrate a cohesive design strategy for multifunctional g-C3N4-based nanostructures, linking electronic band structure engineering, heterointerface design, and surface catalytic modification.