SYNTHESIS AND STUDIES OF NEW TITANATES, NIOBATES AND TUNGSTATES WITH LAYERED STRUCTURE

Abstract

Transition metal oxides (TMOs) have constituted an interesting class of inorganic solids that have received considerable attention of solid-state and material chemists for the past several decades. The TMOs with perovskite and related structures have been extensively studied due to their diverse physicochemical properties and applications together with the compositional variations that the flexible perovskite structure can tolerate. Perovskite oxides exhibit a wide range of structural, physical, chemical and electronic properties that are not only of fundamental interest but have useful technological applications. Layered perovskites represent a special class of layered materials that contain ABX3 perovskite as their basic building block, where A refers to cations in a cubooctahedral cavity of twelve X anions (where X = O, halogen, N, S, and/or H) and B refers to a cation in a BX6 octahedron. The two most common structural classes of layered oxide perovskites that contain ion-exchangeable cations are Ruddlesden–Popper (R-P) phases, A2[An‒1BnO3n+1], and Dion–Jacobson (D-J) phases A[An‒1BnO3n+1]. Aurivillius phases, Bi2O2[An‒1BnO3n+1], represent another class of layered perovskites in which a covalent network of (Bi2O2)2+ occupies the interlayer galleries. While the three-dimensional perovskites readily form with the 3d transition metals of dn configurations, the layered perovskites are mostly composed of d0 metal cations of Ti, Nb, Ta, Mo or W. Thus, layered perovskites with complete 3d transition metal layers are infrequent especially with the D-J and Aurivillius phases, while a plenty of them exist for the R-P phases. In the pursuit of interesting physical and chemical properties in the layered perovskites, many researchers have devoted their efforts for the incorporation of transition metals with partially filled d orbitals in the layered perovskites, which initially contained only d0 transition metals in the perovskite layer. While the perovskite structure can support the replacement of B-site cations with a large variety of transition (3d, 4d & 5d) and p-block elements, the range of such replacements within the perovskite blocks of the layered structure is limited. Moreover, due to the lack of a suitable charge compensating isovalent or aliovalent/heterovalent cation combination, the choice of transition metals are also limited. Moreover, the transition metal incorporation at the perovskite block is possible only in a narrow range of compositions, where the structural integrity can be retained without the formation of any secondary competing phases. It is perceived that the layered perovskite oxides containing mostly d metals might exhibit ii interesting magnetic, ferroelectric and catalytic properties if they are replaced partly with transition metals having unpaired d-electrons in their orbital. For example, incorporation of magnetic transition metals in the B-site of Aurivillius phases have given rise to interesting photocatalytic and magnetic materials. Exchange of interlayer alkali cations by 3dn transition metals of R-P and D-J oxides have given rise to interesting materials with ferroelectric, ferromagnetic, antiferromagnetic and photocatalytic properties. Furthermore, the R–P and D–J phases have received adequate attention due to their rich interlayer chemistry. The environmental pollution is a serious problem in today’s world, with water contamination being one of the biggest concern worldwide. Removal of harmful matters from wastewater and detoxification of pollutants in surface water, and groundwater are key environmental issues before the humankind. Therefore, removal of pollutants, including organic dyes, from the aqueous medium is essential and techniques, such as; chemical oxidation and adsorption are mostly employed for this purpose. However, the above methods are energy intensive. The emergent technique of photocatalytic degradation can be considered as less energy intense and environment friendly for complete mineralization of dye pollutants into CO2 and H2O using semiconductors. The report of Fujishima and Honda in 1972 on the photo-electrochemical water splitting with TiO2 (Eg = 3.2 eV) photo-anode under UV-light have triggered intense research activity on semiconductor photocatalysis. But, in most of the cases the photocatalysis reactions were restricted to the UV-radiation. For sustainable photocatalysis, it is important to make use of the visible portion (nearly ~ 46 %) of solar radiation. Toward the development of visible-light-active photocatalysts, several approaches are adopted. A metal or non-metal ion doping in binary or complex oxides, solid-solution formation between a narrow and a wide band gap material etc. have been exploited for the suitable tuning of band gap. Most of the Bi-containing layered perovskites (Aurivillius phases) have exhibited catalytic activity under visible-light irradiation. However, there are only few reports on the visible-light-active photocatalysis involving double-layer Aurivillius phases. Recently, interest in these Aurivillius phase semiconductors has grown due to their superior activity as single-phase oxide photocatalysts. Therefore, we envisaged the study of coupled-substitution of La and transition metals (Cr, Mn, Fe, Co) in the two-layer Aurivillius niobates and Sillén-Aurivillius hybrid titanates and tungstates (both one- and two-layer). The study also reports effects of transition metal incorporation on the structure, band gap, iii photocatalytic activity and magnetic properties. The outcome of the investigations are presented in the thesis. Chapter-2 provides the details of general solid-state synthesis and all the characterization techniques used throughout the course of this investigation along with their experimental procedures. The compounds were synthesized by solid-state reactions employing metal carbonates, oxalates, oxides or oxychlorides as starting materials. The formation of products at every stage of the synthesis were monitored by powder X-ray diffraction (P-XRD) analysis. The morphology and chemical composition of the compounds were characterized by Field Emission-Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray Spectroscopy (EDS) and High-Resolution Transmission electron microscopy (HR-TEM) analysis. X-ray photoelectron spectroscopy (XPS) was employed for the determination of the oxidation states of the elements. Further, the optical properties were investigated by UV-vis Diffuse Reflectance Spectroscopy (UV-vis DRS) and the band gap energies were estimated with the help of Tauc plots. The photocatalytic activity of the compounds were tested by following standard procedures using model organic pollutants in aqueous medium and natural sunlight or 250 W medium/high pressure (MP/HP) mercury vapour lamps (MVLs) as radiation source. Scavenger tests were carried out for the detection of reactive species involved in the degradation process. Dye adsorption, zeta potential and photoluminescence (PL) studies were carried out to understand the surface charge effects on adsorption and relative e‒‒h+ recombination effects across a homologous series of catalysts. The experimental details of P-XRD, FE-SEM, EDS, HR-TEM, XPS, Raman, FT-IR, UV-vis DRS, PL, EIS and Zeta potential techniques are discussed in this chapter. In Chapter-3, the solid-state synthesis, characterization and photocatalytic studies of a series of lead-free double-layer Aurivillius niobates, LaBi2Nb1.5M0.5O9 (M = Cr, Mn, Fe, Co) is reported. The compositions are designed by adopting a heterovalent coupled substitution strategy starting with the parent SrBi2Nb2O9. Rietveld structure refinements of the compounds using P-XRD data suggests formation of 3dn transition metal incorporated double-layer Aurivillius niobates. The compounds form in the non-centrosymmetric orthorhombic A21am space group and are isostructural with the parent. UV-vis DRS data demonstrates optical absorption in the visible region with absorption edges extending up to ~ 650 nm and with estimated band gaps ranging from 2.252.94 eV confirming their sunlight-active nature. While all the compounds show Curie-like paramagnetism in the 5-300 K temperature range and the transition metals in them adopt iv high-spin (HS) configurations, the Mn compound stabilizes with a low-spin (LS) configuration, in contrast to others. The stabilization of the LS configuration for the Mn (3d4) compound occurs with an eg  t2g electron redistribution due to the suppression of the first-order Jahn-Teller (FOJT) distortion of MnO6 octahedra by the dominating second-order Jahn-Teller (SOJT) distortion of the Nb5+O6 (4d0 Nb). The compounds exhibit photocatalytic Rhodamine B (RhB) degradation at pH 2 within 50 110 minutes under natural sunlight-irradiation and remain stable after five consecutive cycles of photocatalytic degradation maintaining the activity of the catalysts largely intact in the cycles, as exemplified by cycles of photocatalytic degradation studies followed by the post-catalytic P-XRD and XPS analysis of LaBi2Nb1.5Cr0.5O9. However, the RhB degradation time reduces to 15-30 min when irradiated with a 250 W MP-MVL. It is believed that the heterovalent coupled-substitution strategy adopted herewith will open up possibilities for transforming many other UV-active layered niobates into sunlight-active compounds without using toxic Pb or expensive Ag, while the paramagnetic nature of the compounds will be helpful in post-catalytic magnetic separation of the catalysts. Chapter-4 deals with the synthesis and characterization of new double-layered Aurivillius perovskite titanates and their photocatalytic activity toward dye degradation. The compounds, Bi3-xLaxTi1.5W0.5O9 (x = 0, 1) and Bi3-xLaxTiW0.67Fe0.33O9 (x = 0, 1), are prepared by conventional solid-state reactions. The composition of the Fe-substituted two-layered Aurivillius titanate, Bi3-xLaxTiW0.67Fe0.33O9 (x = 0, 1), is designed by a coupled substitution strategy involving Fe, Ti and W in Bi3-xLaxTi1.5W0.5O9 (x = 0, 1). The Fe-substitution resulted in a decreased band gap of the compounds (Eg ~ 2.63 & 2.72 eV) from that of the parent, Bi3Ti1.5W0.5O9 (~3.0 eV). Fe-substitution appeared to help in the suppression of photogenerated e––h+ recombination as evidenced by the PL spectra. While Bi3-xLaxTi1.5W0.5O9 (x = 0, 1) show negligible to little RhB degradation in the acidic aqueous medium under sunlight irradiation, Bi3-xLaxTiW0.67Fe0.33O9 (x = 0, 1) show complete RhB degradation under similar conditions. It appears that poor sunlight absorption and high e––h+ recombination in Bi3-xLaxTi1.5W0.5O9 (x = 0, 1) lead to little or negligible photocatalysis. Moreover, La-substituted compound showed inferior activity as compared to its Bi-analog in contrary to the results observed in other layered Aurivillius titanates where La-substitution resulted in the enhanced photocatalytic activity. This is attributed to the higher zeta potential and consequent adsorption in the Bi-analog as compared to the La-analog of the double-layer titanate. The catalysts are reusable and remain stable after v five photocatalytic cycles of RhB degradation without showing any noticeable loss of activity. The paramagnetic nature of the compounds may be helpful in the post catalytic magnetic separation of the compounds from the solution phase. In Chapter-5, the details of solid-state synthesis, characterization and photocatalytic activity studies of Sillén−Aurivillius phases, LaBi3W0.67M0.33O8Cl (M = Mn, Fe) are reported. The La-substituted Sillén−Aurivillius phases, LaBi3W0.67M0.33O8Cl (M = Mn, Fe), are synthesized for the first time by solid-state reactions. The P-XRD data indicate that the compounds, LaBi3W0.67M0.33O8Cl (M = Mn, Fe), crystallize in the Cm2m space group as adopted by the parent Bi4W0.67Mn0.33O8Cl. The refined lattice parameters of the compounds indicate slight contraction in the c-parameter, while the in plane a and b parameters remained nearly the same on La substitution. Both the compounds exhibit paramagnetic behaviour between 5 and 300 K indicating absence of any magnetic phase transitions in the temperature interval. The UV-vis DRS data established the compounds as visible-light-active semiconductors with the band gap ranging from 2.11-2.41 eV. The photocatalytic activity of the new Sillén−Aurivillius phases is investigated by way of degradation of organic pollutants (RhB and MO) under sunlight irradiation. The compounds show collective degradation of RhB and MO in addition to the photodegradation of individual RhB and MO in the acidic aqueous medium (pH 2). Moreover, the compounds are reusable and stable in the acidic medium even after five consecutive cycles of degradation without any noticeable loss in the activity as evidenced by the cycle tests and post catalytic P-XRD and XPS analyses. Scavenger tests indicate h+ and ●OH radicals as active species that take part in the photocatalytic degradation of RhB and MO. Chapter-6 deals with the intercalative removal of a pollutant dye, Congo Red, using a three-layer Dion-Jacobson (D-J) perovskite, KCa2Nb3O10. During the course of this investigation involving transition metal incorporated Dion-Jacobson perovskite compositions derived from KCa2Nb3O10, it is realized that doping of nearly 16 % of transition metals at the Nb-site do not result in the formation of phase pure D-J compounds. However, during photocatalytic experiments with the parent KCa2Nb3O10, excellent Congo Red (CR) removal efficiency in the acidic aqueous medium is observed. Detail investigation of the removal process of CR employing KCa2Nb3O10 have revealed intercalation of CR in the interlayer space of the layered perovskite with concomitant expansion of the lattice along the c-axis. Moreover, the process is partly reversible as demonstrated by the de-intercalative release of the CR dye in the alkaline aqueous vi medium. Analysis of KCa2Nb3O10 before and after CR removal by P-XRD, FTIR, EDS, and Raman studies support the intercalative removal and de-intercalative release of the dye. EDS data also indicate ~35-40% proton exchange in the parent KCa2Nb3O10 during the dye removal process. Accordingly, an acid-base complexation between the protons of the in-situ formed proton-exchanged layered perovskite and the amine group of the CR is attributed for the intercalative removal of the dye from the aqueous medium. In the basic medium, a strong complexation of the proton with the basic OH frees the amine groups of the dye, which leads to the release of the CR molecules from the interlayer space of the layered perovskite. The above mechanism of CR removal is in line with earlier reports of Methylene Blue intercalation on protonated-layered perovskites and our study with the protonated forms of the Dion-Jacobson perovskite. The overall conclusions and future prospects of the current research is presented in Chapter-7. The idea of coupled-substitution may be extended in many series of layered perovskites to generate a large number of semiconducting compounds with tenable band gap and interesting photocatalytic and magnetic properties. The substitution of 4d and 5d transition metals can also be carried out in the layered titanates, niobates and tungstates with appropriate valence adjustments as the 4d and 5d elements are generally not stable at lower oxidation states that are easily adopted by 3d transition metals. Since the present work demonstrated efficient photocatalytic dye degradations and the reported compounds contain various transition metals, alkali/alkaline earth metals and bismuth/lanthanum, they may well be investigated for other environmental pollutant degradations and photocatalytic reactions, such as, water splitting, CO2 reduction and organic conversions. Thus, the present work hold considerable potential and provide insight for the development of sunlight active semiconductors with enhanced activity.