Design of functionalized mesoporous C₃N₅ for photocatalytic CO₂ reduction

The unrestricted usage of fossil fuels to meet the energy needs of households, transport, and industries has resulted in enormous CO₂ emissions. CO₂ pollution is extremely hazardous to living beings and can cause chronic health conditions. Global temperature rise has endangered environmental ecosystems, leading to abrupt weather conditions. Capturing CO₂ before it is released into the atmosphere and subsequent prompt conversion to value-added products has emerged as a key strategy to mitigate global warming. Efficient capture of CO₂ and its subsequent conversion are realized. The scale of CO₂ to CO conversion is enormous (industrial); therefore, suitable catalyst materials are required to catalyze the reaction. Among the numerous feasible catalytic reactions, photocatalysis employs abundantly available solar light and is considered one of the most sustainable and eco-friendly green strategies. Solar-assisted catalysis promises a scalable reaction without the cost of energy to catalyze the photochemical reaction converting CO₂ to various valuable products including CO, CH₄, CH₃OH, and HCHO. Semiconductors with suitable band gaps falling in the visible range can be photo-excited which populates electrons in the conduction band and holes in the valance band. These individual excited charge carriers such as electrons and holes, can efficiently participate in reduction and oxidation respectively before they recombine. Apart from metal oxides/sulfides, carbon nitrides have recently emerged as efficient solar-active photocatalysts, especially for CO₂ reduction. CNs have high physicochemical stability and can be scalably produced at an economic cost. The ease of material manipulation such as defect generation and surface-functionalization makes it worth to explore. Low electrical conductivity and consequential frequent exciton recombination in crystalline g-C₃N₄ give rise to limits the charge transfer. This has been a huge barrier for this material to be implemented for CO₂ reduction. Therefore, nanoarchitectonics of CNs to achieve a narrower band gap, and prompt charge transfer before exciton recombination takes place, is the urgent need of the hour. Material manoeuvres would help to suitably modulate its physicochemical properties and in engineering its band gap necessary for efficient coupling with light. It would further give rise to better structural/chemical stability during catalytic cycling. N-rich CNs, such as C₃N₅ with a band gap of ~2eV, are suitable for visible light-assisted photocatalysis. In addition, by virtue of their energetics during crystal growth and resulting crystal structures consisting of plenty of nitrogen atoms, containing excess electrons. Moreover, the edge orbitals at N-sites being efficiently electron-rich are useful for the chemisorption of CO₂ and can participate in CO₂ reduction as well. Low effective surface area and fast exciton recombination are limiting the candidature of pristine C₃N₅ in realizing its full potential as visible light photocatalysts for CO₂ reduction. In principle, introduction of mesoporosity can substantially increase the surface area and catalytically active sites, which can enhance the photocatalytic performance. Additionally, hetero-atom doping, or organic surface-functionalization can modify the physio-chemical properties of C₃N₅ such as bandgap energy, band position, conductivity, etc. The integration strategy of heteroatom doping and mesoporosity in C₃N₅ can enhance light absorption efficiency. Further, it can help in photo-induced charge separation and transfer efficiency. These features together are supposed to enhance conductivity, resulting in improved photocatalytic CO₂ conversion performance. Consequently, my PhD research aims to develop functionalized mesoporous C₃N₅ materials with high photocatalytic activity for CO₂ reduction reaction (CO₂RR). C₃N₅ and modified C₃N₅ were synthesized from an N-rich precursor, 3-amino-1,2,4 triazole via thermal polymerization and hard-templating method. The physicochemical properties of synthesized materials such as structure, functional group, nature of bonding, optical property, electrochemical behaviour, etc. were investigated using sophisticated analytical techniques. The optimized conditions for photocatalytic CO₂ reduction were investigated by comparing the efficiencies of modified C₃N₅ and bare C₃N₅. Additionally, theoretical calculation studies provided a better understanding of optimized modified C₃N₅ structures and their photocatalytic CO₂ conversion mechanism. Overall, the current findings have been recorded in the form of chapters in this thesis. The first chapter comprehensively introduces the present-day significance of the photocatalytic reduction reaction, providing a detailed understanding of the CO₂RR. The unique advantages of the photocatalytic CO₂RR were compared to other catalytic processes. The historical aspects of CO₂RR have been presented, and the mechanisms involved have been adequately addressed. Furthermore, this chapter reviewed various materials utilized as photocatalysts for CO₂RR, with a particular emphasis on CN materials as metal-free and eco-friendly photocatalysts. The second chapter offers a comprehensive, systematic review of CN-based hybrids for photocatalytic CO₂ reduction into valuable commodities. This review highlights the importance and requirement of CNs as a metal-free semiconducting photocatalyst materials. Various CN materials explored till date as CO₂RR photocatalysts were reviewed. The review encompassed over numerous strategies for manipulating CN through heteroatom doping, single-atom doping, hybridization with metal oxides/sulfides/metal complexes/etc. for boosted photocatalytic CO₂RR. Additionally, structural engineering of CN i.e. N-rich CN such as C₃N₅, C₃N₆, etc., was implemented to overcome the limitations of g-C₃N₄. Tuning physiochemical properties of CNs has been discussed in detail, such as engineering of band structure and improved charge separation efficiency in terms of functionalization of the CN. This chapter also addresses the knowledge gaps and current challenges in this field. Moreover, it provides an outlook for overcoming these challenges through the coupling of computational and AI-predicted approaches to photocatalytic CO₂RR. In the third chapter, mesoporous C₃N₅ materials were successfully synthesized via hard-templating method for photocatalytic CO₂ reduction. SBA-15, a mesoporous silica material with 2D hexagonal texture, was used as a hard template for generating mesoporosity in C₃N₅. The synthesized mesoporous C₃N₅ exhibited increased specific surface area up to ~319.8 m²/g compared to the bulk C₃N₅ (7.9 m²/g). While the crystal structure of mesoporous C₃N₅ was confirmed by XRD, the nature and coordination of C as well as N, and the surface functionalities were investigated by NEXAFS and FTIR, respectively. The porous morphology of C₃N₅ was investigated using HR-TEM and FE-SEM analyses. The introduction of porosity can change d-spacing, and interatomic distances, hence results in an engineered band structure. The porous nature of C₃N₅ with larger pores efficiently adsorbed CO₂ molecules in the photocatalytic reaction medium. Consequently, mesoporous C₃N₅ exhibited superior photocatalytic activity in reducing CO₂ to valuable products like CO in an aqueous medium. The photocatalytic activity of mesoporous C₃N₅ converting CO₂ to CO remarkably enhanced 7.2 times compared to bare C₃N₅. In the fourth chapter, we reported the synthesis of S-doped mesoporous C₃N₅ and demonstrated its photocatalytic CO₂RR activity. The surface of C₃N₅ was modified by S-doping, where some N-atoms might be substituted by S-atoms in the C₃N₅ lattice. This modification resulted in improved light-absorption efficiency and suppressed charge-carrier recombination kinetics, as evident from spectroscopic investigations. S-doped C₃N₅ exhibited increased photocurrent and reduced electrochemical impedance, indicating that photoinduced charge carriers swiftly transferred to the material surface due to enhanced conductivity. Additionally, 5% S-doped C₃N₅ demonstrated the highest activity for the conversion from CO₂ to CO. In order to enhance surface active sites, mesoporosity was further introduced in S-doped C₃N₅ by colloidal silica as a hard template. The band structure of C₃N₅ was engineered by S-doping in conjunction with the introduction of mesoporosity. PL spectroscopy revealed that the creation of mesoporosity in S-doped C₃N₅ surface further suppressed electron-hole recombination kinetics. The final mesoporous S-doped C₃N₅ exhibited 2.8 times and 8.5 times enhanced conversion efficiency from CO₂ to CO under visible-light irradiation compared to undoped C₃N₅ and undoped C₃N₄, respectively. In chapter five, the successful synthesis of mesoporous C-doped C