Design of Visible Light Driven Bismuth based Catalysts for Degradation of Organic Pollutants

This Ph.D. thesis deals with the synthesis, immobilization of bismuth-based nanostructures and their photocatalytic evaluation for degradation of organic pollutants mainly dyes, with the aim to optimize synthesis and photo-evaluation conditions by focusing the practical application of heterogeneous photocatalysis. Initially, various bismuth based single structures α-Bi2O3, Bi5O7NO3 and heterostructures α/β-Bi2O3, β-Bi2O3/Bi5O7NO3 were synthesized by a simple and scalable route i.e. thermal decomposition of precursor salt. Properties such as crystallinity, composition, morphology and optical parameters were tuned by simply varying the calcination temperature. Heterostructures of α/β-Bi2O3, β-Bi2O3/Bi5O7NO3 are well crystallized, formed stable composites (originated from single precursor salt) and eventually improved the stability of β-Bi2O3 (a metastable form) in the heterojunction structure. Single structures and heterostructures were evaluated for photodegradation of various dyes (differ in chemical structures and ionic behaviors) under visible and UV light. Test were conducted on single dyes or mixed solution of 2/3 dyes to assess the photocatalytic mechanism and kinetics when dealing the mixed effluent. From the obtained results, it was observed that α/β-Bi2O3 and β-Bi2O3/Bi5O7NO3 heterostructures have higher photocatalytic response due to efficient cascade of electrons and holes within the tuned heterojunction and band alignments. Moreover, different dyes interact differently with the photocatalyst and resulted in changed kinetics, while mechanism of degradation depended upon their ionic behavior. Furthermore, during degradation of mixed solution; dyes that have higher interaction (with photocatalyst) and low absorptivity preferentially degraded earlier. Afterwards, α/β-Bi2O3 were used to investigate and distinguish coexisting processes during photocatalysis: (i) intense adsorption, (ii) dye photobleaching and sensitization assisted photodegradation and (iii) partial or complete mineralization. It was found that some dyes with Azo (N=N) and sulphonic groups have intense adsorption over photocatalyst surface and discoloration could occur without photocatalysis. Further, it was revealed that under controlled conditions, the other coexisted processes hardly occur during photocatalysis. Moreover, indigo carmine (IC) dye was found appropriate for preliminary photo-evaluation because its discoloration/removal process could be directly associated with photocatalytic oxidation by analyzing some identified spectral changes in UV-vis absorbance spectrum. Moreover, it was highlighted that dye chromophoric groups react readily and are easily attacked by the originated photocatalytic reactive species and partially mineralized, while further degradation of resulted intermediates containing phenyl groups, became more difficult to oxidize or reduce to achieve complete mineralization. In addition, to investigate and identify the mechanism and the path of photodegradation of the investigated dyes, two approaches were used: 1) the photo-evaluation of dyes in presence of quenchers of hole, atomic oxygen and hydroxyl radical i.e. triethyl amine (TEA), P-benzoquinone (BQ) isopropanol (IP) and, respectively and 2) Evolution of O2 after water oxidation. From the experimental results it was observed, that the photocatalytic activity eventually reduced in presence of quenchers as they quench the originated reactive radical species. Moreover, evolved O2 during water splitting confirmed that electrons and holes are well separated and able to generate reactive oxygen and radical species for photodegradation and partial mineralization of dyes. Thereafter, the work was focused to tackle the challenges of powder photocatalyst recovery and to explore a competing route, i.e. immobilized fixed support. Glass, steel mesh and sintered silica were used for photocatalyst immobilization to solve the problems associated to photocatalyst recovery, mass limitation and low interaction of pollutants with fixed photocatalyst supports. The immobilization/deposition of β-Bi2O3 over each support, was achieved by pneumatic spray pyrolysis and subsequent calcination at 450 °C. During photo-evaluation of different β-Bi2O3 immobilized supports; deposited sintered silica exhibited higher activity and competing response to β-Bi2O3 powder. The improved activity of sintered silica was associated to the rough, porous and hydrophilic nature of silica that have facilitated in providing higher interaction of deposited β-Bi2O3 films with dye molecules. Furthermore, β-Bi2O3 deposited sintered silica exhibited improved performance for photodegradation and mineralization of various dyes of different chemical structures and ionic behaviors and cyclic stability up to 3 cycles. Then, the work was focused to obtain single structure ferromagnetic bismuth ferrite (BiFeO3) and its heterostructure (BiFeO3/Fe2O3/Bi2Fe4O9); as they have the advantage of easy magnetic separation from aqueous solution. The single structure BiFeO3 and its heterostructures were obtained by using Sol-Gel method, in which precursor solution, containing dissolved Bi(NO3)3:5H2O and Fe(NO3)3:9H2O were preheated and calcined at 500°C with and without addition of Polyethylene Glycol (PEG) and NaOH in the precursor solution. From the XRD and UV-vis DRS analysis it was observed that addition of PEG and NaOH, assisted to obtain single nanostructure BiFeO3, simply by enabling the particles polymerization and inhibiting the formation of other compounds like Fe2O3 and Bi2Fe4O9. It was revealed that single phase BiFeO3 is antiferromagnetic in nature and have very low photocatalytic response, due to the low energy band gap and high electron and holes recombination rate. On the other hand, BiFeO3/Fe2O3/Bi2Fe4O9 heterostructure displayed high magnetic saturation and exhibited improved photoactivity. This is due to a low electrons and holes recombination rate because of tuned band alignment and charge transfer within the heterojunction interfaces. Cyclic stability and photocatalytic performance of BiFeO3/Fe2O3/Bi2Fe4O9 were found almost similar during photodegradation of various dyes up to 3 cycles. At the end, detailed analyses of the efficient heterostructure α/β-Bi2O3 and promising β-Bi2O3 immobilized silica were made, for the evaluation of bulk single and mixed dye solutions under natural sunlight and at varying IC dye concentrations. It was found that the mechanism and the photodegradation kinetics were almost similar amongst lab conditions and during sunlight irradiation and for bulk solutions of single and mixed dyes. Moreover, the experienced phenomena of the degradation and achieved kinetic rate at varying IC concentration were almost alike for both α/β-Bi2O3 and deposited β-Bi2O3 sintered silica. These results revealed that deposited β-Bi2O3 sintered silica could have the promising potential over α/β-Bi2O3 or any other powder photocatalyst under solar light irradiation. Moreover, cyclic stability and the photoactivity of both α/β-Bi2O3 and deposited β-Bi2O3 silica were almost identical up to 3 cycles.