Bioinspired Modification of TiO2 and g-C3N4 for Enhanced Photocatalytic Activities

Abstract

The rapid growth of industrial technologies has resulted in extended pollutant emissions and an energy shortage. In view of long-term development, it is essential to seek an economical, green, and effective method to remove existing pollutants and investigate substitute new energies to reduce hazardous emissions at their source. Photocatalysis is a promising approach to achieving solar energy conversion. Maximizing the utilization of solar irradiation and advancing photocatalytic efficiency are hot issues. The basic mechanism of photocatalysis can be summarized as follows: under solar irradiation, the excited electrons in the valence band (VB) of photocatalysts transfer to the conductive band (CB), leaving holes in the VB. Electrons and holes participate in redox reactions and generate free radicals such as ·OH and O2-·. These powerful radicals further degrade organic pollutants into H2O, CO2, and other organic residues, or participate in the water-splitting process for hydrogen evolution. However, most semiconductor photocatalysts face the obstacle of limited surface area and rapid recombination rate of photoinduced electron-hole pairs. Various methods such as element doping, morphology modulation, and heterostructure designing have been proven effective in resolving these problems. However, the requirement of strong acids, toxic chemicals, and harsh conditions in these processes might raise other environmental concerns. Inspired by nature, biomineralization and biomimetic modifications of photocatalysts have drawn attention due to their mild conditions and morphology direction. Additionally, the rich content of inorganic materials (such as carbon, nitrogen, and phosphorus) can provide in-situ element doping, which has been confirmed to narrow down the band gap and increase the separation rate of photoexcited charge carriers, thereby enhancing photocatalytic performance. Lysozyme has been confirmed as a promising biotemplate for the mineralization of TiO2. It is also an ideal candidate for incorporating negatively charged molecules such as polystyrene (PS) beads to form a reliable protein-polymer biotemplate with enriched organic components under ambient conditions. This protein-polymer template not only acts as the core to induce the nucleation of TiO2 but also introduces extra nitrogen as a dopant into the TiO2 lattice. Additionally, this protein-polymer template constructs a porous structure after the calcination process, which enlarges the surface area of TiO2. Therefore, the lysozyme-polystyrene templated N-doped TiO2 with increased surface area presents great photocatalytic degradation performance of Rhodamine B, a widely discharged organic dye pollutant, under simulated solar irradiation. Graphitic carbon nitride (g-C3N4) is a promising metal-free semiconductor with non-toxic properties and abundant availability in nature. Its moderate band gap (2.7-2.8 eV) and remarkable thermal and chemical stability make it suitable for various applications, particularly in solar water splitting. However, the 3D structure of bulk g-C3N4, characterized by π-π stacking and van der Waals forces between interlayers, poses a challenge in effectively separating photogenerated electron-hole pairs, thereby negatively impacting H2 generation performance. To address this limitation and improve both solar light absorption and the recombination rate of photogenerated carriers, a simple and environmentally friendly approach has been proposed. In this study, a deformed g-C3N4 structure was successfully modified with biological molecules by decorating a 14-mer peptide onto the porous g-C3N4 (CN). The modified CN exhibited a modified morphology and improved visible-light absorption, resulting in superior photocatalytic H2 production. It achieved a generation rate of 2018.4 μmol g-1 h-1 without the need for a co-catalyst (such as Pt), which is approximately 14 times higher than that of pristine CN (140.8 μmol g-1 h-1). Experimental along with theoretical computations have identified the deformation of CN structure caused by the electrostatic interaction between the positive-charge amine groups and the negative-charge edge N atoms. This interaction contributed to electron redistribution and delocalization, leading to the establishment of an electrical field and electronic modulation. Consequently, it enhanced charge separation and optimized the free energy of the reaction intermediates. This work presents a new approach for designing excellent bio-based, metal-free g-C3N4-based catalysts for visible-light photocatalytic H2 generation.