DEVELOPMENT OF NEW LAYERED OXIDE AND OXYHALIDE TITANATES FOR SOLAR PHOTOCATALYSIS

Abstract

Release of organic pollutants by various industries is of serious concern due to their harmful environmental and health effects. Development of novel semiconductor photocatalysts for rapid degradation of these pollutants by either complete mineralization or converting them into non-toxic fragments using solar energy may possibly render a sustainable solution. After the discovery of water-photolysis on TiO2 semiconductor electrode by Fujishima-Honda in 1972, the world has seen an enormous amount of research on semiconductor photocatalysis that continue to grow even today, although, the use of TiO2 and many other oxide semiconductors under solar irradiation is limited because of their UV active nature. Considering the abundance of ‘visible’ sunlight amounting to ~ 43% of incoming solar energy, there is an urgent need for the development of visible-light-active photocatalysts for efficient utilization of solar energy. Therefore, semiconductor photocatalysis driven by visible-light has emerged as an exciting area of research to address potential issues related to clean fuel production and environmental remediation. Hydrogen production by sunlight-driven water splitting is considered as a renewable way of clean fuel generation. Water disinfection by degradation of anthropogenic organic pollutants from industrial wastewater is indispensable for a cleaner environment. Ideally, an efficient sunlight-driven photocatalyst can find niche applications in some of the emergent areas of energy and environment. Toward the development of visible-light-active photocatalysts, several approaches are adopted. For tuning the band gap, a metal or non-metal ion doping in either binary or complex oxides are being widely investigated. Besides, a solid-solution formation between a narrow and a wide band gap material has been exploited for the suitable tuning of band gap. Moreover, for improving upon the efficiencies of photocatalytic processes, use of multiple oxides in the form of semiconductor heterojunction, oxide composites or with graphene to form graphene-composites etc. have been investigated. Often, many of these systems have drawbacks with respect to high solution stability, resistance to photocorrosion and photoleaching, high efficiency and recyclability under visible-light/solar radiation. In quest for robust and efficient photocatalysts, research thrusts in single bulk semiconductors together with their nanostructured counterparts are in the rise. Among these, the perovskites constitute a major class both with the layered (2D) and ii three-dimensional (3D) structures. In this regard, the role of layered perovskites in visible-lightdriven photocatalytic activity is noteworthy. A large body of research dealt with the formation and manipulation of layered perovskites for optimization and discovery of diverse properties including photocatalysis. It is observed that many oxides and oxyhalides studied for visible-light photocatalysis contain Bi3+ and among them the Aurivillius phases are noteworthy. The presence of Bi is known to push up the absorption band edge in the visible region and its dispersed valence band helps to generate high mobility holes. Recently, interest in these Aurivillius phase semiconductors has grown due to their superior activity as single bulk and nanostructured oxide photocatalysts. The most simple members of the Aurivillius family, namely, Bi2MoO6, Bi2WO6 (n = 1) and their solid-solution phases have been investigated for their photocatalytic activity under visible-light irradiation. Although, few reports on visible-light active photocatalysis over higher order members (n = 2, 3, 4) of the Aurivillius series were available, more complex systems remained largely unexplored. Recently, four (n = 4) and five-layer (n = 5) Aurivillius phases, Bi5Ti3FeO15 and Bi6Ti3Fe2O18, respectively, have drawn attention for their ferroelectric, magnetic, optical and photocatalytic properties. Moreover, the rare earth (La, Sm, Gd and Dy) substituted Bi6Ti3Fe2O18 were investigated for their magnetic and magnetoelectric properties. Recognizing the effect of cation disorder in the stability of Aurivillius phases and recent reports on importance of ferroelectricity in photocatalysis, we have undertaken a systematic exploration of La substitution in the four- and five-layer Aurivillius phases towards phase formation, structure, band gap and photocatalytic activity. This has further been extended to develop new series of five-layer Aurivillius perovskites by both A and B-site cation co-substitution. Moreover, a strategy toward band gap alteration for developing new visible-light active semiconductors have been devised via intergrowth of Aurivillius oxides with Sillén phases forming a new series of hybrid layered perovskites. All the compounds were thoroughly characterized and their activities toward dye degradations were evaluated. Finally, for a better insight into the photocatalytic processes and mechanism of degradation detail studies involving energy level diagrams have been carried out. The outcomes of the present investigations are presented in the thesis consisting of seven chapters. Chapter-1 gives an overview of various metal oxides and composite systems applied in the area of photocatalysis. iii Chapter 2 gives the details of all the characterization techniques used, during the present course of investigation, along with their experimental procedures. The compounds were first synthesized by solid-state reactions employing simple metal carbonates / oxalates / oxides or oxychlorides. The progress of reactions and formation of final products were monitored by powder X-ray diffraction (PXD) and the morphological and compositional characterizations were carried out by Field Emission-Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Spectroscopy (EDS) analysis. Further, the optical properties were studied by UV-vis Diffuse Reflectance Spectroscopy (UV-vis DRS) and the band gap energies were estimated by Tauc plots. Photocatalytic activity tests were performed following standard procedures under sunlight-irradiation using model dye solutions. For this, the catalyst-dye suspensions were equilibrated under stirring in the dark prior to photocatalytic degradations. In all cases, a control experiment was always performed in absence of catalysts to assess selfdegradation of the dyes involved under the same experimental conditions. To obtain deep insights into the degradation mechanisms and reactive species involved, various scavenger tests were carried out. To elucidate the role of adsorption, surface charge and e- h recombination processes, dye adsorption, -potential and photoluminescence (PL) studies were carried out. The details of PXD, FE-SEM, EDS, UV-vis DRS, PL and -potential techniques are discussed in this chapter. Chapter 3 describes a systematic exploration on the solid-state synthesis and characterization of four-layer Aurivillius perovskites, Bi5-xLaxTi3FeO15 (x = 1-3), together with theirs photocatalytic activity toward Rhodamine B (RhB) degradation under sunlight-irradiation. We report for the first time, to the best of our knowledge, the solid-state synthesis and characterization of Bi5-xLaxTi3FeO15 (x = 1, 2). All the compounds crystallize in the orthorhombic A21am space group. The analysis of lattice parameters obtained by least-squares refinement shows a slight increase in the c-parameter as x varies from 0 – 2 in the Bi5-xLaxTi3FeO15 series, while the difference between a and b-parameter reduces and become nearly equal in Bi3La2Ti3FeO15. This clearly indicates a decrease in orthorhombic distortion and a shift toward tetragonal structure with progressive replacement of lone-pair active Bi3+ by La3+. This has been attributed to the diminishing effect of out-of-center distortion which possibly leads to an increase in the c- parameter in the series with increasing La-substitution. The PXD simulation studies on iv Bi3La2Ti3FeO15 indicated preferential occupation (~ 67-75 %) of La in A-site of the central perovskite layer and the residual La were distributed over the terminal perovskite layers with a minimal occupancy over the [Bi2O2]2+ layers. UV-vis DRS data revealed that the compounds are visible light absorbing semiconductors with principal band gaps ranging from ~ 2.62 – 2.71 eV. Photocatalytic activity studies by RhB degradation under sunlight-irradiation showed that these layered oxides are very efficient photocatalysts in mild acidic medium. The activity of the catalysts toward RhB degradation is comparable or higher than many of the single-phase and composite catalysts with Aurivillius and perovskite structures in the bulk or nanostructured form reported recently. An enhanced charge separation rendering slow e–-h+ recombination and efficient transport of photogenerated holes are attributed to high efficiency of the bulk catalysts irrespective of the degradation mechanisms involved. Moreover, the large positive -potential led to enhanced dye adsorption and consequently a high rate of degradation in the acidic medium. Scavenger test experiments clearly indicated the dominant role of holes in the degradation process involving the Aurivillius layered oxides reported here. The positioning of VB and CB edges with respect to potentials of ●OH/H2O, O2/O2 ●– and HOMO-LUMO levels of RhB clearly corroborated with the scavenger test results indicating no major role of hydroxyl species in the degradation. In Chapter 4, we report the solid-state synthesis, characterization and photocatalytic activity studies of five-layered Aurivillius perovskites, Bi6–xLaxTi3Fe2O18 (x = 0, 1). The samples of Bi6–xLaxTi3Fe2O18 (x = 0, 1) were synthesized by conventional solid-state reactions. On Lasubstitution, the in-plane ‘b’ parameter remains nearly the same while a slight contraction in the in-plane ‘a’ and expansion in the ‘c’ parameter was noticed. These are attributed to complex interplays between cation disorder and concomitant octahedral tilting distortions. The SEM images show homogeneous plate-like morphology as expected and often observed for layered compounds. The crystallites are mostly agglomerated and composed of plate-shaped crystallites with irregular sizes ranging from few hundred nanometers to a few micrometers. The UV-vis DRS established the compounds as visible-band-gap semiconductors. La-substitution appeared to help in the suppression of photogenerated e––h+ recombination as evidenced by PL spectra. The photocatalytic RhB degradation in the acidic medium under sunlight-irradiation indicated Bi5LaTi3Fe2O18 as the most active catalyst among the two five-layered Aurivillius compounds v reported here. The enhanced activity of Bi5LaTi3Fe2O18 is attributed to the effective separation of photogenerated e––h+ pairs and improved dye adsorption in the acidic medium. The predominant role of photogenerated holes in the RhB degradation has been established by reactive species trapping experiments. The conduction and valence band alignment of Bi5LaTi3Fe2O18 with respect to potentials of ●OH/H2O, O2/O2 ●– and HOMO–LUMO levels of RhB supports the involvement of h+ and formation of O2 ●– as reactive species. Furthermore, a large positive ζ- potential has been ascribed to improved dye adsorption which ultimately led to an enhanced photocatalytic activity and efficient COD removal by the catalysts. Moreover, the compounds are reusable and stable under the acidic medium even after five consecutive cycles of degradation without any noticeable loss in activity. Chapter 5 deals with the synthesis and characterization of new five-layer Aurivillius perovskites, Bi5ATi4FeO18 (A = Ca, Sr, Pb), and their photocatalytic activity toward selective dye degradation under sunlight. The compounds are synthesized for the first time by solid-state reaction. The UV-vis DRS confirmed that all the compounds have visible band gaps ranging from 2.61 – 2.72 eV. These catalysts exhibited highly selective degradation of Methylene Blue (MB) from aqueous mixture of MB and RhB in alkaline as well as neutral medium. The improved photocatalysis of Bi5SrTi4FeO18 is attributed to a sluggish recombination of photoinduced e––h+ pairs and enhanced adsorption of dye on the catalyst surfaces. Scavenger tests indicated h+ and O2 ●– as active species involved in the photocatalytic degradation. The position of valence and conduction band edges of Bi5SrTi4FeO18 with respect to potentials of ●OH/H2O, O2/O2 ●– and HOMO–LUMO levels of RhB and MB also supports the role of h+ and O2 ●– radicals in the sunlight-driven photocatalysis. The photocatalytic cycle tests and postcatalysis PXD analysis confirmed that the catalysts are reusable and stable. In Chapter 6, we have devised a new strategy to make visible-light harvesting compounds from UV active compounds by designing new Sillén-Aurivillius intergrowth phases, namely, Bi5-xLaxBaTi3O14Cl (x = 0 – 2). For the synthesis of these hybrid phases a multi-step solid-state reaction strategy was adopted, where two pre-synthesized compounds, namely, Bi4-xLaxTi3O12 (x = 0 – 2) and BiBaO2Cl, were further reacted to prepare the hybrid phases. All the Sillén-Aurivillius hybrids are visible-light absorbers with band gap ranging from 2.66 to 2.87 eV and exhibit selective degradation of MB in neutral and alkaline media, where both RhB and vi MB degradation is noticed in acidic medium with mixed RhB-MB dye solutions. Furthermore, these compounds showed excellent photodegradation of individual RhB and MB at different pH conditions. Among the three catalysts, Bi3La2BaTi3O14Cl showed enhanced activity because of sluggish recombination of photogenerated e––h+ pairs and superior dye adsorption. Based on the band edge position of catalysts, HOMO-LUMO levels of dyes and scavenger test results, ●OH, O2 ●– and h+ are perceived as active species at pH 2, whereas O2 ●– and h+ are realized as active contributors at pH 7 and 11. These catalysts are reusable and stable after five photocatalytic cycles without showing any appreciable loss of activity. The overall conclusions and future prospects of our current investigations are presented in Chapter 7. The present work has tremendous potential and gives insight for the development of various new types of layered oxides for sunlight-driven photocatalysis not only in the area of dye pollutant degradation but many other organic pollutant degradation, selective degradation, wastewater treatment and recovery of precious chemicals, to name a few.