PEROVSKITE BASED HETEROSTRUCTURES FOR PHOTOCATALYTIC APPLICATIONS

Abstract

The burning of fossil fuels and unprecedented industrialization has resulted in enormous greenhouse gas emissions (GHGs). By trapping the heat from the sun, these GHGs have alleviated the global atmospheric temperature alarmingly, creating a worldwide climate emergency. It is imperative to say that the environmental impact of GHGs, notably CO2, which has about three-quarters of emissions, notoriously threatens our planet's weather, climate system, wildlife population, etc. In this regard, visible-light-assisted photocatalytic conversion of CO2 to fuels and hydrocarbons is one of the efficient ways to achieve carbon neutrality. Moreover, the issue of water pollution caused by industrial effluents in the form of non-degradable organic dyes can also be resolved via photocatalysis. As an advanced oxidation process (AOPs), photocatalytic reactions form robust reactive oxygen species (e.g., hydroxyl radical, superoxide radical, hydrogen peroxide, etc.), which mineralize the pollutants to carbon oxide, water, and stable mineral acids. Semiconductor heterogeneous photocatalysis spans a wide range of phenomena with energy and environmental impacts, namely hydrogen production, degradation of organic contaminants, CO2 reduction reaction (CO2RR), etc. The search for ideal photocatalysts with improved photoconversion efficiency has motivated the scientific community to explore a diverse range of semiconductors. Perovskite materials (both oxide and halide perovskites) secure a distinct position in the photocatalyst family due to their appropriate structural, physical, chemical, and electronic properties. They have been used due to their semiconducting and light-harvesting properties. However, the photocatalytic efficiency of standalone perovskite catalyst systems is limited because of the poor harmony between the light response range and redox ability. Additionally, ultrafast charge carrier recombination phenomena occurring on the photocatalyst surface are very challenging to control, limiting the catalyst performance. Heterojunction engineering is a counteractive measure to boost the standalone photocatalyst's performance due to its quintessential advantage of isolating electrons from holes. It suppresses the recombination by spatially separating charge carriers via modulating the energy level alignment between two semiconductors with different bandgaps. In this context, proper band alignment through critical material selection is required to form a heterojunction successfully. This thesis's research efforts are dedicated to overcoming the adverse scenarios of standalone perovskite photocatalysts and demonstrating multiple ways to utilize them via constructing heterojunctions for photocatalytic organic contaminant degradation and CO2 reduction. This work aims to explain the fundamental and applied science in photocatalysis, targeting material engineering to obtain heterojunction photocatalytic systems based on both oxide and halide perovskites. The photophysical behavior of the heterojunction photocatalysts is elucidated, and the underlying mechanism is studied. In the first work, oxide perovskite materials with an ABO3 structure were investigated for photocatalytic applications. This work shows the photocatalytic activity of KxNa(1-x)NbO3 – BaBiO3 (KNN-BBO) heterojunction material for the degradation of Rhodamine 6G organic dye is investigated. The materials are extensively characterized by X-ray diffraction (XRD), UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and N2 adsorption isotherms. The degradation efficiency of the organic contaminant under 1 SUN simulated sunlight is monitored using the spectral analysis from UV-Vis absorption spectroscopy. Electrochemical impedance spectroscopy (EIS) also observes the resistance to charge transfer. The effect of the sintering temperature on the photoinduced degradation activity is also included in our study. Unsintered KNN-BBO (UKB) composite material is the most efficient catalyst, with 84% removal efficiency compared to the sintered one (SKB). This is attributed to the reduced bandgap with staggered type band alignment, increased surface area, and surface oxygen vacancy states. The intimate interaction between KxNa(1-x)NbO3 and BaBiO3 is propitiously beneficial for charge transfer, thereby increasing efficacy. Together with the crucial findings of this work, a probable mechanism for enhanced photocatalytic activity has been proposed. The material focus is then shifted to halide perovskites in the subsequent work. Recently, metal halide perovskites with tunable bandgaps, considerable diffusion length, and abundant surface sites have drawn immense research interest for photocatalytic CO2 reduction reactions (CO2RR) and organic contaminant degradation. However, multistep solution-based methods of perovskite synthesis not only pose significant safety concerns but also introduce trap states in the perovskite, which negatively impact its properties/activity. This work targets developing heterojunctions via coupling a lead-free double perovskite Cs2AgBiBr6 and polymeric gC3N4 (GCN) through an environmentally non-hazardous manual hand-grinding synthesis approach. The photocatalytic CO2 reduction of these hybrid catalysts is investigated in isopropyl alcohol (IPA) using a 250W mercury vapor lamp as an irradiation source. A high average CO and CH4 yield of 12.14μmol/g/h and 8.85μmol/g/h are achieved for Cs2AgBiBr6-1GCN. These results exhibit a 3- and 9.8-fold improvement in CO and CH4 production compared to pristine Cs2AgBiBr6. Space charge limited current (SCLC) and XPS measurements prove that GCN incorporation reduces the trap density, increases carrier mobility, and induces strain in the perovskite. As a cumulative effect, the photocatalytic activity is improved in the heterojunction photocatalyst. The interfacial defect passivation at the heterojunction ensures a faster and smoother charge transport, as confirmed by EIS Nyquist plots, suggesting enhanced CO2 photoconversion activity. These results extend the use of lead-free halide double perovskites in the photocatalysis field for solar fuels, leveraging a solvent-free dry synthesis route to diversify the utilization of perovskite heterostructure photocatalysts. The previous project involving Cs2AgBiBr6-xGCN catalysts witnessed CO as the dominant product evolved during CO2RR. It is observed that tuning product selectivity during CO2RR in halide-perovskites is a rarely explored aspect of photocatalysis research. Considering this, we develop amorphous TiO2 (aTiO2) encapsulated Cs2AgBiBr6 double perovskite nanocrystal (NC) by room-temperature anti-solvent recrystallization method. Subsequently, we demonstrate the photocatalytic reduction of CO2 to CH4 (8.46μmol/g/h) and CO (5.72μmol/g/h) using this nanocomposite, where CH4 is the dominant product. Cs2AgBiBr6-aTiO2 nanocomposite exhibits an 11-fold enhancement in CH4 yield compared to pristine Cs2AgBiBr6 with prolonged stability of 16hours and higher selectivity of CH4 over harmful CO production. The product selectivity is attributed to the adventitious Ti3+ on the surface of perovskite, which accelerates the CO2 activation mechanism. Solvent-effect on product formation is also studied with ethyl acetate, acetonitrile, and dioxane. CH4 becomes the dominant product in all the cases, with an impressive evolution rate of 10.96μmol/g/h in acetonitrile only. Impedance spectroscopy and ultrafast femtosecond transient absorption spectroscopy were used to establish the mechanism of CO2 reduction. It was also confirmed that aTiO2 helps in a faster and smoother charge transport at the interface by passivating the surface defects of the perovskite nanocrystals. Our work provides a simple, highly efficient, and selective strategy for photocatalytic CO2 reduction using double perovskite-based nanomaterial. In summary, this thesis represents facile preparation methods for perovskite (both oxide and halide) heterostructure photocatalysts formation with particular attention to halide perovskite systems for photoinduced CO2 reduction reactions and organic contaminant degradation. The participant materials' structural, photophysical properties, and charge carrier dynamics have been explicitly discussed. The impact of parameters such as sintering temperature, vacancy, defect, and its passivation, co-catalyst amount, bandgap engineering, morphology, and thermodynamic regulation is established, which is beneficial to obtaining improved photocatalytic activity. The underlying photocatalysis mechanism of each photocatalyst is studied. The ultimate goal of this thesis is to provide a rational design of perovskite hybrid photocatalysts for successfully utilizing renewable energy to tackle environmental issues.