Graphitic carbon nitride (g-C$_3$N$_4$) has garnered interest as a versatile photocatalytic platform owing to its tailorable electronic architecture; however, its solar-to-chemical conversion is bottlenecked by high exciton binding energies and slow charge-carrier transport. To circumvent these impediments, the construction of ``molecular junctions'' within the conjugated polymeric scaffold enables atomically precise modulation of spatial electron configurations. Unlike conventional heterostructures relying on physical contact, molecular junctions employ robust covalent bridging, facilitating molecular orbital hybridization and $\pi$-conjugation. This review summarizes recent advances in molecular-junction-functionalized g-C$_3$N$_4$ photocatalysts, focusing on built-in electric field (BIEF) induction and its effects on charge-carrier dynamics. Molecular junctions are categorized into homojunctions exploiting structural polymorphisms and heterojunctions incorporating donor–acceptor moieties or single-atom sites. Structural asymmetry engenders steep potential gradients, mitigating exciton binding and promoting unidirectional charge migration. State-of-the-art BIEF characterization techniques—including Kelvin probe force microscopy (KPFM), density functional theory (DFT), and ultrafast transient absorption spectroscopy—are systematically examined. Finally, the catalytic efficacy of these molecular junction paradigms is assessed across solar-driven applications, including photocatalytic hydrogen evolution, overall water splitting, and hydrogen peroxide synthesis. The review concludes by outlining key bottlenecks and future directions, emphasizing atomically precise synthesis and operando characterization.

Two-dimensional nanomaterials exhibit excellent physical barrier properties, which can effectively enhance the corrosion resistance of waterborne epoxy coatings. Herein, we report a facile strategy for preparing a multi-component synergistic anti-corrosion coating, where two-dimensional graphitic carbon nitride (g-C₃N₄) and high-entropy layered double hydroxides (HE-LDHs) are integrated into a waterborne epoxy matrix via magnetic-ultrasonic synergistic dispersion. The resulting HE-LDHs/g-C₃N₄-epoxy coating exhibits exceptional corrosion resistance for Q235 steel. Electrochemical impedance spectroscopy (EIS) and polarization curves showed that when the mass ratio of g-C₃N₄ to HE-LDHs was 1:1, the resulting coating (PCN-LDH-1.0) maintained a coating resistance of 5.48 × 10⁵ Ω·m² after 28 days of immersion in 3.5% NaCl solution, which was five orders of magnitude higher than that of pure waterborne epoxy coating. Meanwhile, the corrosion current density was reduced by four orders of magnitude, from 5.83 × 10⁻¹ A·m⁻² to 1.68 × 10⁻⁵ A·m⁻². After 30 days of salt spray testing, no rust, blistering or adhesion loss was observed on the coating surface. These enhanced performances by addition of g-C₃N₄ and HE-LDHs were attributed to the combined effects of the tortuous diffusion pathways. Additionally, the PCN-LDH-1.0 coating retained excellent mechanical properties, including a pencil hardness of 3H and the highest adhesion grade. This study provides a facile method for preparing high-performance waterborne anti-corrosion coatings.

Transparent self-cleaning coatings for photovoltaic (PV) glass require an effective balance among contaminant removal, optical transparency, and environmental durability. In this study, octadecyltrichlorosilane-modified silica (OTS-SiO₂) coatings were fabricated on glass substrates using a simple spray-deposition method, and the effects of initial SiO₂ suspension concentration on surface morphology, wettability, droplet mobility, self-cleaning performance, and optical transmittance were systematically investigated. Increasing the SiO₂ concentration promoted the development of particulate surface structures, enhanced water repellency, and reduced contact angle hysteresis, accompanied by improved droplet mobility and dust-removal performance. The OTS-SiO₂-15 coating, prepared using a SiO₂ concentration of 15 mg mL⁻¹, achieved a water contact angle of 140 ± 1.6°, a sliding angle of 3.9 ± 1.5°, a dust-removal efficiency of 91.6%, and a visible-light transmittance of 90.6 ± 0.3%. Further increases in SiO₂ concentration provided only limited improvements in droplet mobility and cleaning performance while substantially reducing optical transparency. Under mechanical impact, both sliding angle and contact angle hysteresis increased despite the retention of relatively high water contact angles, indicating deteriorated droplet mobility. When applied as photovoltaic cover glass, OTS-SiO₂-15 maintained an initial power conversion efficiency (PCE) comparable to that of uncoated glass (8.41% versus 8.43%). After 40 days of outdoor exposure, the normalized PCE decreased by 4.6% for the coated configuration, compared with 10.8% for uncoated glass. These results demonstrate that spray-deposited OTS-SiO₂ coatings can provide a practical balance among droplet mobility, self-cleaning performance, and optical transparency, supporting their potential application in photovoltaic glass protection.