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

DEPOSITED Bi2O3 THIN FILMS ON SINTERED SILICA FOR PHOTOCATALYTIC WATER TREATMENT

Bismuth oxide (Bi2O3) thin films were deposited on sintered silica surface by the spray pyrolysis method using bismuth acetate as the precursor salt. Spray pyrolysis enables uniform film distribution on sintered silica. The films were characterized by X-ray diffraction (XRD) to investigate the obtained Bi2O3 phase, UV-vis diffuse reflectance spectroscopy (DRS) to obtain the energy band gap, and Field Emission Scanning Electron Microscopy (FESEM) to study the morphological structure. XRD pattern revealed that obtained phase of films is tetragonal β- Bi2O3, by using DRS results the calculated energy band gap is 2.4eV and FESEM images show porous flowery structures. Furthermore the heterogeneous photocatalytic activity of the films was investigated by introducing the deposited silica filters into organic dyes solution and irradiating in UV and visible light for photocatalyst activation. Bi2O3 thin films shown good photocatalytic activity showing that (i) spray pyrolysis is a promising deposition method and that (ii) deposited silica filters are suitable support to promote effective interaction of photocatalyst with dye molecules while avoiding the problem of photocatalyst powders suspension in dye solution. Improved photocatalytic activity in visible light proves Bi2O3 thin films for sustainable water treatment and related applications

Bismuth-based nanomaterials: Photo catalyst for water dye degradation

Growth of nano sized Bismuth oxy chloride (BiOCl) and Bismuth Oxide (Bi2O3) thin film on glass was achieved by Metal Organic Chemical Vapour Deposition (MOCVD) method. It was synthesized from solutions containing Bismuth chloride (BiCl3) in Acetone and Bi(NO3)3:5H2O in 1M HNO3.The BiCl3 solution was carried by nebulisation effect by Argon gas and Bi(NO3)3:5H2O by Oxygen in the reactor. The temperature degrades the chemical composition into nano thin films of desire particles on the glass surface. The expected reaction is mentioned below. NPs growth takes place on glass substrate at 600°C. Self assembly of BiOCl and Bi2O3 crystals were observed on the glass substrates. Obtained thin films were characterized by Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDS). Further, thin films of BiOCl and Bi2O3 were evaluated for photocatalytic study. BiOCl NPs thin film was tested against methyl orange dye and Bi2O3 thin film was tested against Acid blue 113 dye under UV light. Analysis results are interesting, 90 % of dye absorption was observed in both the films. As this is the preliminary expt. on growth and dye degradation study with both the films, the data is not comparable. The study only confirms the preliminary observation on dye degradation by UV-vis spectroscopy. Further, the detailed study is in a process

DEPOSITED Bi2O3 THIN FILMS ON SINTERED SILICA SURFACE FOR PHOTOCATALYTIC WATER TREATMENT

Increasing concerns on water pollution encourage to investigate new techniques for controlling associated problems with practical and sustainable approaches. Heterogeneous photocatalysis is reported as the green, sustainable and clean process by using metal oxide semiconductors in removing dyes, medicines and insecticides from water. In this study deposition of nano sized Bismuth oxide (Bi2O3 NPs) thin film on sintered silica discs(30mm dia) was achieved by spray pyrolysis technique, by spraying crystal clear solution (containing Bismuth Acetate (CH3CO2)3Bi dissolved in 1 Molar CH3COOH) onto the sintered silica substrate kept at 450 °C. Deposited disc were characterized by Field emission scanning electron microscopy (FESEM), X-ray Diffraction (XRD) for evaluation of obtained crystallinity and phases, diffused reflectance spectroscopy (DRS) to determine energy band gap possessed by films. Photocatalysis was performed under visible light to analyse the photocatalytic activity of deposited thin films

Evaluation of the photodiscoloration efficiency of β-Bi 2 O 3 films deposited on different substrates by pneumatic spray pyrolysis

Immobilization of β-Bi2O3 over corning glass and sintered silica discs was performed to evaluate a competing fixed support as an alternative to slurry based photocatalytic systems. Pneumatic spray pyrolysis technique was used to produce coatings of β-Bi2O3 at 450 °C. Coated substrates were characterized using X-ray diffraction (XRD) and UV-Vis diffused reflectance spectroscopy (DRS), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectrometry (EDS). The XRD analysis showed that the obtained crystalline structure of the films is tetragonal β-Bi2O3 for both glass and silica. DRS showed the broad absorbance spectra of the films, correlated to β-Bi2O3 with an energy band gap of 2.4 eV. FESEM showed that the morphology of the films was different according to the substrate. In the case of corning glass, random non-compact distribution of particleswas observed while over silica discs overlapped sheets of β-Bi2O3 were seen. This is probably due to differences in thewetting and evaporation rate of the sprayed droplets. During photodiscoloration of anionic indigo carmine (IC) dye solution, the β-Bi2O3 coated silica was found more effective than the coated glass, response associated to the larger interaction between the dye molecules and catalysts assisted by the roughness and porosity induced by the silica. Furthermore, photocatalytic evaluation of coated silica in discoloration of other two organic dyes: cationic rhodamine B (RhB) and anionic acid blue 113 (AB) revealed that adsorption and interaction of the dye molecules with coated films could vary due to the presence of different functional and branched groups that overall affect the photocatalytic process, the kinetic rate of discoloration and the TOC analysis. Finally, after the assessment of stability of films and suitability of silica, it was confirmed that silica discs are favorable as a photocatalyst support and last long usage for photodiscoloration of organic dyes

Efficient α/β-Bi2O3 composite for the sequential photodegradation of two-dyes mixture

A mixture of α/β-Bi2O3 and α-Bi2O3 powders were obtained by a simple solid state reaction–annealing route at 550 °C. The structure, optical properties and surface area of the commercial α and β-Bi2O3 and the synthesized α-phase and α/β-composite were well characterized by X-ray diffraction, diffuse reflectance spectra and N2 physisorption. The annealed sample at 550 °C showed 20% of β-phase, forming a heterojunction of α/β-Bi2O3 whereas annealing at elevated temperature (650 °C) lead to the α-phase. Optical properties showed that the presence of the β-phase is mainly responsible for narrowing the energy band gap. The photocatalytic activity of the commercial α and β-Bi2O3 and the synthesized α-phase and α/β-composite were investigated in degradation of single dyes, Indigo Carmine (IC) and Rhodamine-B (RhB) under both UV and visible light-induced photocatalysis. For the best photocatalyst, the photodegradation in a two-dye mixture solution was systematically studied considering the type of dye, the adsorption capacity of the samples and the behavior of dye photodegradation. The photocatalytic performance of α/β-Bi2O3 was comparatively much higher than the commercial α and β-Bi2O3, indicating that better performance of efficient charge separation and transfer across α/β-Bi2O3 composite was obtained. Possible mechanism of the single dye and two-dye mixture degradation was given by using α/β-Bi2O3 composite

Synthesis and characterization of Ce and Er doped ZrO2 nanoparticles as solar light driven photocatalysts

In this work, nanostructured rare-earth-doped zirconia was successfully synthesized, through a simple hydrothermal method, with a rare-earth oxide (REO)-based content varying from 0.5 to 5% molar (REO¼CeO2;Er2O3). The samples were characterized by using several techniques, such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy. The photoactivity of the samples was tested under irradiation with simulated solar light by both the spin trapping technique, using DMPO (5,5-dimethyl-1-pyrroline-N-oxide) as spin trapping agent, and by the photodegradation of Rhodamine B, a common textile dye. All the doped samples resulted more active than the bare zirconia, and the correlation between the physical-chemical prop- erties and the photoactivity of the materials has been investigated