Synthesis of Coinage Metal Nanoparticles As Co-Catalysts for Tio2 Based Photocatalysts

Abstract

The work presented in this thesis enlightens the importance of coinage metal nanoparticles (NPs) as co-catalysts for the enhancement of TiO2 photocatalysis. Main emphasis has been given on synthesis of various shapes and sizes of coinage metal nanostructures such as rods, spheres, truncated triangles, wires, etc., and their implication as co-catalyst towards the improvement in the heterogeneous photocatalytic systems, which cannot be achieved through simple conventional approach. This whole work has been divided into eight chapters: Chapter 1: Introduction and Characterization Techniques The first chapter provides the brief introduction on the role of coinage metal nanoparticle as co-catalysts for the improvement of TiO2 photocatalytic reactions. Specific attention has been paid to the impact of catalyst-co-catalyst structure-activity interactions for enhancing the photocatalytic rate. A brief description of various techniques used for the characterization of optical, electrokinetic, co-catalytic and catalytic properties of metallic nanostructures and metal-TiO2 composites has also been incorporated. Chapter 2: Size and Shape Dependent Attachments of Au Nanostructures to TiO2 for Optimum Reactivity of Au-TiO2 Photocatalysis This chapter demonstrates the effects of AuNPs of various sizes and shapes on its co-catalytic activity imparted to TiO2 during photocatalytic oxidation of salicylic acid. The TiO2 photoactivity is remarkably improved with the decreasing size (9.5 ± 0.06 to 3.5 ± 0.25 nm) and increasing the surface to volume (S/V) ratio (0.629 to 1.95 nm-1) of spherical Au co-catalysts loading. The Au nanorod (aspect ratio = 2.8 ± 0.12 and S/V = 0.54 nm-1) attachment to TiO2 significantly decreased the photoactivity compared with the highly active quantum size (3.5 ± 0.25 nm) Au co-catalysts loading. The interaction of AuNPs of various morphologies with TiO2 induces the photoexcited charge transfer process in varied extent, leading to diverse photocatalytic activity. Zeta potential/surface charge and conductance measurement of aqueous dispersions of TiO2, AuNPs and salicylic acid was carried out to investigate the interaction among the various components in the photoreaction system. Chapter 3: Co-catalytic and Electro-kinetic Properties of Au Nanostructures Dispersed in Solvents of Varying Dipole Moments The influence of electrokinetic parameters; surface charge and zeta potential on the optical absorption band and co-catalytic activity of Au nanospheres (AuNS) dispersed in polar solvents i.e., methanol (MeOH), propanol (PrOH), dimethylformamide (DMF), and dimethyl-sulphoxide (DMSO) has been considered in detail here. The narrow intense surface plasmon resonance (SPR) absorption band of AuNS (520 nm) becomes broadened and a SPR band at > 970 nm is appeared, whose absorption intensity varies from 0.05 to 0.13 a.u with the extent of aggregation in solvents of varying dipole moment from 1.66 D to 3.96 D while no such alteration in SPR band occurs for AuNS dispersion in non-polar CCl4 as confirmed by DLS and TEM analysis. The effective electronic charge ca. +4306 µequ./l present over AuNS surface in water suspension is highly reduced to +512 µequ./l in photoreaction (TiO2 + salicylic acid + AuNS) mixture and the zeta potential (+50.32 mV) and conductance (1673 µS) of AuNS in water has been decreased to +24.06 mV and 1480 µS in methanol, respectively. The co-catalytic activity of aggregated AuNS (formed in polar solvents) varied in a diverse extent depending on its degree of agglomeration, where the non-aggregated aqueous AuNS + TiO2 mixture displayed higher degradation efficiency (r = 9.990×10-3 mM/min) of salicylic acid than that (r = 9.172×10-3 mM/min) of aggregated AuNS in MeOH + TiO2 mixture under UV light irradiation. Chapter 4: Electro-kinetic and Catalytic Sensitivity of Au Nanorods Agglomerated in Solvents of Varying Dipole Moment and Refractive Index In continuation of chapter-3, this chapter deals with the impact of dipole moment (1.66 to 3.96 D) and refractive index (1.333-1.422) of the dispersion solvent on the optoelectronic, adsorption, co-catalytic and catalytic properties of Au nanorods (AuNRs). AuNRs (Length ≈ 53 nm and Width ≈ 20 nm) also undergo agglomeration to a varied extent (size 50-180 nm) as a function of the dipole moment of the solvent, similar to AuNS aggregation. The electrostatic interaction of AuNRs with its surface adsorbed solvent dipoles leads to alteration of the their ionic state, absolute electronic charge and zeta potential (+49.79 mV in H2O, +8.99 mV in DMF and -4.65 mV in MeOH dispersion). This interaction distinctly modifies the adsorption behavior of polar molecules like p-nitrophenol and salicylic acid on AuNRs surface, as evidenced by the measured changes in their electro-kinetic parameters. As a result, we observe a substantial difference in catalytic and co-catalytic activities of AuNRs dispersed in various solvents (such as PrOH, DMSO, DMF, etc.) because the catalytic properties of AuNRs are strongly dependent on the type of solvent in which they are dispersed. The reduction of p-nitrophenol to p-aminophenol by AuNR-H2O exhibits the highest k = 5.28×10-2 min-1 as compared to the lowest k = 1.0×10-2 min-1 with AuNR in DMF. Similarly, the co-catalytic activity of AuNR-TiO2 mixture for the photo-oxidation of salicylic acid is found to be higher (k = 8.4 × 10-2 min-1) in H2O than in DMF (k = 5.2 × 10-2 min-1) due to the severe agglomeration of the NRs in the latter. Chapter 5: Improved Surface Properties and Catalytic Activity of Polymorphic Facets of Photoetched Au Nanostructures Formed by Variable Energy Laser Exposure Many anisotropic Au nanostructures with versatile crystal facets have been prepared in a single step by different energy (λ = 457.9, 488 and 514.7 nm) laser exposure and studied their optical, electrokinetic and catalytic properties. The surface morphology of photoetched AuNPs was monitored by variable energy laser beam follows an exponential (A = A0 e-kt) disintegration rate as a function of particles geometric size and shape. A variety of many irregular Au nanomorphology such as multifaceted spherical (5-20 nm), low aspect ratio rod, oval egg shaped, ice cubes (~7 nm), truncated triangle (~7-8 nm), pentagonal (~12 nm), nanocapsules, square types (~8 nm) and polygonal nanospheres (~8-12 nm) like particles were formed after laser exposure. The percentage decomposition of AuNRs (aspect ratio = 2.8) is found to increase as 59 % > 69% > 72% with increased energy of laser exposure; 2.41 eV (514.7 nm) > 2.50 eV (488 nm) > 2.71 eV (457.9 nm) as determined from the decrease in SPR band absorption during laser illumination. The formation of small clusters on laser irradiation results in the decrease of zeta potential from +49.79 to +12.71 mV and conductance values. Catalytic reduction of p-nitrophenol to p-aminophenol and m-dinitrobenzene to m-phenylenediamine with photofragmented Au nanostructures is ~ 15-20% is higher than bare Au particles. Chapter 6: Co-catalysis Effect of Different Morphology of Ag Nanostructures for the Photocatalytic Oxidation Reaction by Ag-TiO2 Photocatalysis This chapter highlights the comparative co-catalytic efficiency of different shapes of prepared AgNPs of a size much bigger as well as smaller than titanium for the Ag-TiO2 photocatalysis. Quantum sized Ag nanospheres (4_8 nm), nanorod (length 70-75 nm and width 30-38 nm), polygonal nanosphere (80-120 nm) and truncated triangles (side length 70-140 nm) are prepared by solvothermal process. The co-catalytic activities of these Ag nanostructures were investigated by mixing them with TiO2 for the photocatalytic degradation of aqueous salicylic (0.5 mM) and benzoic acid (0.5 mM) under UV light (125 W-Hg arc, 10.4 mW/cm2) irradiation. The Ag co-catalysis effect imparted to TiO2 follows as polygonal nanosphere > nanorod > truncated triangle > small nanosphere due to the formation of many Ag-TiO2 interfaces by a single large-sized Ag nanoparticle than smaller one. As the surface coverage of Ag particles by TiO2 decreases, the Ag-TiO2 photoactivity is decreased accordingly. The efficient adsorption of salicylic acid to TiO2 surface through -COOH and -OH groups render its higher photodegradation rate (1.8-2.7×10-2 µmol/min) than benzoic acid (1.5-2.5×10-2 µmol/min) having one chelating -COOH group. Zeta potential and conductance measurement of photoreaction mixture were carried out to investigate the ionic interaction-adsorption of reactant substrates over Ag-TiO2 surface. Chapter 7: Copper Nanostructures of Various Morphologies for Superior Catalytic and Co-catalytic Activities This chapter describes the improvement in the photocatalytic rate of TiO2 by utilizing different morphologies of CuNPs in comparison with bare TiO2. The Cu nanospheres of three different sizes ≈ 3-20 nm were synthesized employing long chain fatty acid surfactant (lauric acid) and polyvinyl pyrollidone as capping agent. Nanorods of length ≈ 600-700 nm and width ≈ 15-20 nm and nanowires with dimensions, length ≈ 4-6 μm and width ≈ 60-80 nm were prepared by solvothermal process and characterized by UV-vis spectrophotometer, DLS, SEM and TEM analysis. Both co-catalytic and catalytic activities of various Cu nanomorphologies were evaluated for the photooxidation of acetic acid by CuNPs-TiO2 mixture, and for the reduction of nitroaromatics (nitrobenzene, m- chloronitrobenzene and m-nitrotoluene), respectively. A significant increase in the catalytic reduction (30 to 50%) was found with decreasing size (20 to 3 nm) and increasing the surface to volume ratio (0.34 to 1.73 nm−1) of nanospheres, analyzed through UV-vis, HPLC and GC-MS measurements. The photocatalytic activity for acetic acid oxidation under UV irradiation and reduction reaction using varied morphologies was gradually improved with the increasing exposed surface area per particle as ≈ 1278295 nm2 for nanowires > nanorods (≈ 352154 nm2) and nanospheres (≈ 1076.02 nm2). Chapter 8: Comparative Co-catalytic Account of Coinage Metal Nanoparticles (Au, Ag and Cu) for TiO2 Photocatalysis under Visible Light Irradiation A comparative report on the nature of the coinage metal NPs (Au, Ag and Cu) co-catalysts for enhancement of the catalytic activity of TiO2 under visible light irradiation has been described here. Coinage metal nanospheres of similar sizes (3-5 nm) were prepared using sodium citrate as a capping agent and characterized through UV-vis spectrophotometer, DLS and TEM analysis. These NPs were impregnated onto TiO2 to produce metal-TiO2 nanocomposites, operating efficiently under visible light owing to the surface plasmon resonance effect of metallic particles. The comparative impact of Au, Ag and Cu-TiO2 nanocomposites was evaluated by utilizing them for the photodegradation of benzaldehyde and nitrobenzaldehyde. Ag-TiO2 was found to exhibit highest photoactivity relative to Au and Cu-TiO2 composites. Notably, very low resistance and thus high conductance in Ag-TiO2 (2.7×10-7 S) in contrast to Au (2.5×10-7 S) and Cu-TiO2 (2.7×10-7 S) composites highlights the importance of interface in conduction. The difference in work function, redox potential, e--h+ recombination time, oxidation state of the M-TiO2 nanocomposite dictates their varied co-catalytic activity.