DEVELOPMENT OF LAYERED AND HYBRID PEROVSKITES FOR PHOTOCATALYTIC WATER SPLITTING AND DECONTAMINATION

Abstract

The non-renewable energy reserves are depleting rapidly day by day due to overconsumption in one hand, while emission of CO2 is causing increasingly erratic climate patterns, on the other. In addition, wastewater from industries is often packed with organic pollutants, such as dyes, pharmaceuticals and phenols that contribute to chemical toxicity induced diseases vis-à-vis adversely affect the ecological diversity. Artificial photosynthesis using a semiconductor photocatalyst is considered to be one of the most suited sustainable solution to overcome these problems. The fundamental concepts behind photocatalytic hydrogen generation and organic pollutant degradation share commonalities with varying specialized needs [1,2]. Hydrogen generation through water splitting requires a semiconductor with appropriate band levels for proton reduction and water oxidation while for the degradation of organic pollutants, the generation of reactive oxygen species by semiconductor excitation is essential. Considering the abundance of 'visible' light in the solar spectrum amounting to ~ 46% of incoming solar energy, the development of visible-light-active photocatalysts is indispensable to utilize solar energy efficiently [3]. To create a promising photocatalyst, in addition to visible light absorption, other competing factors, such as enhanced light absorption, efficient charge separation, charge transport and desorption of products from the photocatalyst surface require considerable attention to optimize the efficiency of a photocatalyst. Various strategies are adopted separately for the generation of hydrogen, oxygen, and pollutant degradation to develop efficient photocatalysts. To improve the efficiencies of photocatalytic processes, the use of multiple oxides in the form of semiconductor heterojunction, oxide composites, or graphene to form graphene-composites has been investigated. But still, there is lack of systems that can meet practical needs with very high efficiency and low cost. Recently, dual-purpose photocatalysts were explored. A dualfunctional photocatalyst should have suitable position of band levels so that excited electrons can reduce the protons for the formation of H2, or electrons can generate superoxide radicals to break the organic molecule, and holes can oxidize the organic compounds, or oxidize water to oxygen. Among the various photocatalysts, the perovskites constitute a significant class with layered (2D) and three-dimensional (3D) structures amenable to the modification of band structures for particular photocatalytic reactions. In this regard, the role of Aurivillius perovskites in visible-light-driven photocatalytic activity is noteworthy [4,5]. Like other two-dimensional layered materials, Aurivillius perovskites also show several advantages, such as varying layer thickness, atom organization, open to surface modification, interaction with additional materials, and exciting crystal structure with internal electric fields. Aurivillius perovskites have great potential in photocatalysis due to the presence of a Bi2O2 layer (sandwiched between the perovskite slabs) that pushes the semiconductor's valence band upward resulting in a narrowing of band gap and also develops the internal electric field, which helps in charge separation. Various Aurivillius perovskite photocatalysts with multiple modification strategies have been reported to exhibit high photocatalytic performance [4,5]. Substitution or co-substitution at A- and B-site is one of the effective strategies to modify the optical, photophysical, and structural properties of these photocatalysts. The Aurivillius perovskites offer a new hybrid intergrowth phase formation with a structurally related Sillén phase. Due to the presence of shared Bi2O2 layers, self-assembling of these phases by heating with layered Aurivillius perovskites provides a novel method to manufacture new visible-light active photocatalysts with Sillén-Aurivillius intergrowth structures. Intergrowth of layered structures has the capability to modify the band structure to a significant extent and alter the photocatalytic properties. Sillén-Aurivillius intergrowth phases or hybrid perovskites, have recently been demonstrated as visible-light active photocatalysts possessing appropriate band levels for both water reduction and oxidation [6,7]. Single and two-layer Sillén Aurivillius phases were studied mainly for the water oxidation reaction, but three-layer perovskites were demonstrated for both water reduction and oxidation reactions. Moreover, their activity was improved by co-catalyst loading on their surfaces [8]. Although, few Sillén-Aurivillius intergrowth phases were prepared long ago but no photocatalytic studies were demonstrated when we undertook the study. However, these Sillén-Aurivillius perovskites have regained interest as evidenced by the recent publications on photocatalysis using these hybrid phases. The present work is devoted to the development of new compositions of two-layer and three-layer Aurivillius phases of titanates, niobates and titano-niobates. The three-layer Aurivillius phases were used with the Sillén phase to intergrow the Sillén-Aurivillius hybrid phases. The compounds were thoroughly characterized and employed for water splitting and decontamination studies. Based on their crystal and electronic structure, efforts have been made to establish the structure-property correlation for the enhanced photocatalytic activity over homologous series of compounds. The outcomes of these investigations are presented in the thesis. The thesis comprises of eight chapters. Chapter I gives an overview of various metal oxides, layered perovskites and metal oxyhalide systems applied in photocatalysis.Chapter II details all the characterization techniques used during the course of present investigation and their experimental procedures. The compounds were first synthesized by solid-state reactions employing simple metal carbonates/oxalates/oxides or oxychlorides. Reaction progress at each step and final phase of the compounds were determined using powder X-ray diffraction (P-XRD). Lattice parameters refinement, P-XRD pattern simulation, Rietveld refinement, and drawing of crystal structure were performed using POWDER CELL, FULLPROF and Diamond software, respectively. Morphological characterization, analysis of elemental composition and elemental mapping of the samples were done with field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) analyzer. Selected area electron diffraction (SAED) and lattice images were obtained using a high-resolution transmission electron microscope (HR-TEM). Data from UV-visible diffuse reflectance spectroscopy (UV-vis DRS) were converted into absorbance terms with the Kubelka−Munk (K-M) function and Tauc plot was used to estimate the band gap. Surface charge, photo-generated e-h+ pair lifetime and recombination were analyzed by ζ-potential measurements, Fluorescence lifetime spectroscopy (FLS) and photoluminescence (PL). The surface area of samples and surface chemical states of the elements were analyzed using BET and X-ray photoelectron spectroscopy (XPS). Electrochemical experiments such as Mott-Schottky, electrochemical impedance spectroscopy (EIS) and photocurrent density measurements were also performed to evaluate the band positions, charge-transfer resistance and charge transport properties. Photocatalytic experiments for the water splitting and decontamination were performed in an inner and top-irradiation-type reactor by irradiating with sunlight or 250W medium pressure mercury vapor lamp (MP-MVL). The role of reactive oxygen species and adsorption was also evaluated by the scavenger experiment, adsorption studies and infrared (IR) spectroscopy. Chapter III demonstrates an in-situ synergistic combination of photocatalyst and dye as a type- II heterojunction for the collective degradation of water-soluble dyes, pharmaceuticals, and phenolic pollutants. This combination helps to increase visible-light absorption and decreases the charge carrier recombination. For this, a series of visible-lightactive two-layer Aurivillius perovskites, Bi2.5A0.5Nb1.75Fe0.25O9 (A= Ca, Sr, Ba), was synthesized via solid-state reaction method. The presence of Fe induced doping energy levels is responsible for visible-light absorption. The semiconductors can efficiently photodegrade bisphenol (BPA), carbamazepine (CBZ), tetracycline (TC) and 4-chlorophenol (4-CP) individually as well as from a mixture in the presence of RhB. The photocatalytic degradation is driven by a synergistic effect of RhB adsorbed on the surface of the photocatalyst. The RhB adsorbed semiconductor acts as a type-II heterojunction, which not only enhances the light absorption by photosensitization effect but also improves charge separation, transfer and reactive oxygen species (ROS) generation. The strategy will be useful as a sustainable alternative for the degradation of multiple pollutants in wastewater by a unique and straightforward photoexcitation and sensitization process. In Chapter IV, dual-functional photocatalysis was explored. In this study, Fe was doped on the B sites of a wide bandgap semiconductor, Bi3TiNbO9, to make it a visible-lightactive photocatalyst with the new composition, Bi3Ti0.5Nb1.25Fe0.25O9. The photocatalyst efficiently photodegrades the RhB dye on irradiation with visible or UV light or their combination, while after photodeposition of platinum as a co-catalyst on Bi3Ti0.5Nb1.25Fe0.25O9, it displayed promising photocatalytic performance for hydrogen evolution with simultaneous degradation of RhB under UV-Vis light. The role of Pt cocatalyst to provide an active site for proton reduction is discussed. The role of RhB dye as a sacrificial agent to scavenge the holes in a water-splitting reaction is also discussed. This work provides a sustainable alternative system for hydrogen evolution with wastewater treatment. Chapter V describes a series of three layered Sillén-Aurivillius (SA) perovskites, Bi4AA′Ti2NbO14Cl (A, A′ = Sr, Ba), synthesized by solid-state reaction method. Our approach involves an innovative interchanging of Sr and Ba between Sillén and Aurivillius perovskite phases and examines the cation distribution in the SA intergrowth phase and its effect on the photocatalytic activity of the compounds. Rietveld structure refinement of the compounds using P-XRD data suggests preferred occupation of smaller cations in the perovskite block with subsequent migration of larger cations to the Bi2O2 layers or the Sillén block. When doped with 0.5 wt.% of Pt co-catalyst and illuminated by sunlight, the compounds with mixed cation distribution show good photocatalytic hydrogen evolution activity, especially with Bi4Ba[P]Sr[S]Ti2NbO14Cl, where migration of cations is observed between the Sillén and perovskite blocks. This study discusses the significance of cation distribution, site disorder, and their effect on the photocatalytic water splitting for these Sillén-Aurivillius compounds with the help of crystal and electronic structure analysis. In Chapter VI, the synthesis, characterization and photocatalytic activity of Sillén- Aurivillius hybrid perovskite, Bi5BaTi3O14Cl, with La doping at the Bi-site is discussed. The compounds are synthesized by the solid-state reaction method. All the Sillén-Aurivillius hybrids are visible-light absorbers with band gaps ranging from 2.66 to 2.87 eV and have demonstrated unusual photocatalytic activities for RhB and MB degradation. Among the photocatalysts studied here, Bi3La2BaTi3O14Cl showed enhanced activity because of sluggish recombination of photogenerated e––h+ pairs and superior dye adsorption. Hydrogen evolution activity is also improved with La doping and is doubled for Bi3La2BaTi3O14Cl. Furthermore, hydrogen evolution activity increased around twice when the La-doped compound underwent Al doping. The rising photocatalytic activity with La and Al co-doping has been described with the help of charge transfer resistance, charge recombination rate, crystal plane expansion, surface charge, and changes in morphology. Chapter VII includes general mechanistic insights into the photocatalytic removal of RhB dye from wastewater over Aurivillius perovskites. A complex interplay of several factors, including light absorption, carrier recombination, charge transport, lifetime, and surface adsorption, governs the efficiency of photocatalytic activity exhibited by the semiconductors. While many factors are intrinsic to the semiconductors linked to their unique electronic structure and composition, the overall activity is determined by the dominance of one or more of the factors over the others. The competing role of these factors in the light of cation disorder, octahedral distortion, band-level positioning, surface modification, and atmospheric conditions in layered Aurivillius perovskites is presented. The band bending effect is due to the internal electric field present in the non-centrosymmetric Aurivillius perovskite phase, which is beneficial to drive the separation of photoexcited charge carriers effectively. Oxygen vacancy plays a critical role in the photocatalytic degradation of emerging pollutants by enhancing the separation of photo-generated charges and providing active sites for the adsorption and degradation of target pollutants. In two different atmospheres, RhB shows two different photocatalytic mechanisms as degradation of RhB and conversion of RhB to Rh110 with simultaneous oxygen evolution over an Aurivillius perovskite, Bi3Ti0.5Nb1.25Fe0.25O9. The overall conclusions and future prospects of our current investigations are presented in Chapter VIII.