Studies on the Proficient Photocatalytic Applications of Versatile Semiconductor-Semiconductor Heterojunction System

Abstract

Several crucial characteristics must be met proficiently in order to develop a visible light active photocatalyst that can use sustainable solar energy efficiently. To begin with, the semiconductor material has to have a narrower band-gap so that it can absorb solar radiation over a wide spectrum. The semiconductor should also have a suitable valence and conduction band edge that has sufficient potential to carryout plentiful generation of both h+ for ˙OH radicals and e− for ˙O2 ̄. The disadvantage of these semiconductors, however, is that they cannot be used as solar energy- harvesting photocatalysts because of their wide band-gap, and photogenerated carrier recombination process, which renders the photocatalytic activity. Certain modifications to the semiconductor system have been made to improve the efficiency of their visible light responses to address this critical deficiency. Fabrication of a heterojunction system drastically reduces those concerns regarding the activity. Out of various heterojunction systems, semiconductor- semiconductor (S-S) heterojunction is most appropriate because of its easy band optimization and wider redox potential window results in versatile photocatalytic applications. Additionally, in photo-electrochemical processes, the semiconductor materials should have strong photo stability. The choice of semiconductor materials, morphological architecture, crystallinity, and surface properties have been taken into account when building an efficient and stable visible light- responsive photocatalytic system. Based on this, the following chapters describe the fabrication of different S-S heterojunction photocatalyst and their applications in diverse fields. Chapter 1 signifies a general introduction to photocatalysts, their unique properties, and their application in various fields. The use of safe unconventional water sources to increase water supply and improve water purification efficiency are both possible using visible light-responsive photocatalytic technology. This chapter mainly consists of underlying ideas that underpin photocatalysis in general and then about the many classes of photocatalytic materials, addressing various facets of their characteristics, including efficiency, stability, scalability, and cost. Recent advances are also reviewed in the design and fabrication of visible light-responsive photocatalysts using a variety of synthetic approaches, including the doping, dye sensitization, or heterostructure formation of conventional photocatalysts, as well as the significant efforts made in the exploration of novel visible light-responsive photocatalysts. The principles of heterogeneous photocatalysis, the photocatalytic pathways, and the distinctive characteristics of visible light-responsive photocatalysts are described in detail. Regarding the water treatment, the photocatalytic qualities of the resultant visible light-responsive photocatalysts are also discussed, i.e., on the subject of the photocatalytic disinfection, degradation, and removal of organic and inorganic contaminants. This chapter contains a summary of the present difficulties and future lines of inquiry in this developing field of study. Chapter 2 describes how a heterostructure is fabricated between the modified titania and g-C3N4 to separate the carriers efficiently. The nanostructured composite is created using a simple and affordable sol-gel procedure and a co-calcination approach. Photocatalytic studies were conducted after XRD, Raman, XPS, TEM, and PL analysis to determine their ideal dopant concentration and degree of doping. Electrochemical impedance analysis and UV-DRS are used to examine the influence on the band locations. The heterojunction band alignment promotes carrier mobility from the bulk to the active sites. The photogenerated electron and hole reserve the characteristic redox ability to generate both the ˙OH and ˙O2 ̄ through the Z-scheme mechanism. The persistent herbicide Bromoxynil was fragmented by the photocatalytic activity and also demonstrated improved photocatalytic H2 evolution. The improved photocatalytic performance for TiO2-Zr-N/g- C3N4 was attributed to g-C3N4 and Zr working together to extend the material's spectrum- absorptive nature into the visible region and NOx acting as a carrier mobilizer. The produced photocatalyst heterojunction creation not only made it easier to separate the photogenerated charge carriers but also preserved the oxidation and reduction abilities. Chapter 3 deals with the use of prospective materials for the degradation of organic pollutants from aqueous sources using stacked MOFs, a structural variation of the metal organic framework (MOFs), were examined. The development of a heterostructure photocatalyst with superior catalytic active sites and optoelectrical properties was optimized for the efficient mineralization of hazardous organic pollutants and the water splitting process. Through a simple hydrothermal process, a series of three-dimensional micro-rods mediator-free Z-scheme heterojunction photocatalysts were effectively produced. The morphology and composition show that the composite heterojunction materials have micro-rods with sparsely dispersed MOF spikes. 25- MOF/BVO stands out among these samples for photocatalytic H2 evolution and bromoxynil degradation effectiveness. The primary cause of the increased photocatalytic activity is thought to be the built-in electric field that facilitated carrier movement. Time-resolved fluorescence spectra and photoelectrochemical measurements provide to further support this. Additionally, it has been proven that the heterostructures adhere to a conventional Z-scheme charge transfer mechanism rather than a conventional Type-II heterojunction charge transfer mechanism based on the results of tests for free radical scavenging activity and EPR measurements. The ideal photocatalyst (25- MOF/BVO) displayed improved photocatalytic efficiency for the degradation of bromoxynil. Chapter 4 describes improved charge separation and migration performance in photocatalytic applications and how it has been extensively explored to fabricate binary p-n heterojunctions and the band alignment at the interface of the individual semiconductor photocatalyst. In this case, a straightforward wet chemical followed by a hydrothermal manufacturing technique was used to create binary p-n heterostructures. The tunable band structure of individual semiconductors with the work function (ϕ) witnessed a band bending at the space charge region. The bending at the interface induces a carrier concentration gradient and manifests a rectifying current transport diode. Different morphological and physicochemical methods verified the fabricated p-n heterostructures and carrier migration between n-type and p-type. The band banding at the interface, leading to a narrow depletion region, favours the tunneling of electron-hole pairs through a Z-scheme carrier transport mechanism. The electron-hole pair movement has further been confirmed by considering the band edge position after contact, photocatalytic scavengers, and the radical trapping experiment. The p-n heterojunction photocatalyst manifested H2 generation and endosulfan degradation efficiency. The p-n heterojunction photocatalyst displayed a higher current density with electron-hole migration efficiency, synergistically enhancing the catalytic activity through the interfacial space charge junction. Chapter 5 deals with the summary and conclusion as well as the future perspective of the work. The current study tackles the water remediation and energy demands of a growing population and increased industry. In this study, a series of different semiconductor heterojunction photocatalytic systems were designed and investigated for versatile photocatalytic applications. All the heterojunction photocatalysts were categorically classified based on their types (p-type or n-type) and aligned electron transfer process. The first observation investigated a doped n-n semiconductor heterojunction photocatalytic system with a Z-scheme carrier transfer mechanism. The band edge of TiO2 has been successfully tailored by doping Zr, which substantially replaced Ti from its lattice point and reduced the overall band threshold. To counterbalance the recombination process of individual semiconductor material (a consequence of the defect level TiO2), a heterojunction was fabricated with g-C3N4. The Physio-chemical evaluation showed an intimate architecture, suitable band edge position, low interfacial charge resistance, and reduced recombination rate. The redox potentials of resultant carriers could able to generate both ˙OH and ˙O2 ̄ through a direct Z-scheme mechanism, participating in the degradation of bromoxynil and H2 evolution process. Similarly, a semiconductor-based n-n semiconductor heterojunction system was developed with a Z-scheme carrier movement mechanism. A novel Mg-MOF-74/BiVO4 heterojunction hybrid with a hierarchical 3D micro-rod-shaped structure has been developed as a heterojunction photocatalyst using a simple hydrothermal process. The stratified heterostructure materials' unique structural and optoelectronic characteristics include high crystallinity, surface-exposed active sites, strong visible absorption, rapid charge carrier mobility, and excellent resistance to recombination. The scavenges and radical trapping experiment deduced a Z-scheme carrier transport system with a 93% bromoxynil decomposition and H2 generation (1.97 mmolg-1h-1) efficiency. Lastly, a visible light active binary Ag3PO4/Cu2O p-n heterojunction photocatalyst was developed for the rapid degradation of endosulfan along with the H2 evolution process. This synthetic approach ensured a uniform distribution of p-type Cu2O nanospheres (20-30 nm) over an n-type Ag3PO4 matrix through a nano-sized interfacial junction, which is confirmed by the morphological analysis. The built in potential developed at the p-n junction interface provides better carrier migration throughout the semiconductors, boosting visible light absorption capacity and higher exciton lifetime. The binary p-n heterojunction displayed splendid photocatalytic endosulfan degradation efficiency of 91% and hydrogen generation (HER) of 1017.8 μmolg-1.