Synthesis and Catalytic Properties of Binary/Core- Shell Coinage Metal(s) Nanostructures

Abstract

The work presented in this thesis enlightens the significance of coinage bimetallic (BM) nanocomposites (NCs) over their monometallic counterparts. Main emphasis has been given to the synthesis of BM NCs in the form of core@shell alloy and hollow nanostructures by varying the nature of core, shell composition and composition, their characterization, and application to catalysis and co-catalysis. This whole work is divided into six chapters which are described below:

Chapter 1: Introduction and Literature: The first chapter provides the brief introduction on the need of BM NCs over monometallic NPs which have been emerging as important catalyst due to the synergistic effects between the two metals. A brief description of preparation strategies for core@shell, alloy and hollow BM NCs and various techniques used for the characterization of optical, electrokinetic, catalytic and co-catalytic properties of BM NCs relative to their monometallic counterparts have also been incorporated.

Chapter 2: Improved Catalytic Activity and Surface Electro-kinetics of Bimetallic Au-Ag Core-Shell Nanocomposites: This chapter demonstrates the preparation of core@shell NCs of Au@Ag and Ag@Au for measuring their catalytic activity and electro-kinetic properties relative to their respective monometallic counterparts. A significant blue-shift (530 to 408 nm) and red-shift (420 to 550 nm) of the surface plasmon (SP) band for Au@Ag and Ag@Au NCs, respectively, was observed due to increased size of binary composites depending on the nature of core and shell material. The thickness of the deposited Ag shells varied from ~3-10 nm on Au core leading to the formation of Au@Ag NCs. On the other hand, the Ag core served as a sacrificial template, where Ag@Au NCs was converted to hollow Ag-Au alloy shells (~15 nm) because of the galvanic reaction between them due to the difference in their redox potential. Further, an increased zeta potential of resulting Au@Ag (+57.8 mV) and hollow Ag-Au alloy shell (-20.13 mV) NCs in comparison to monometallic Au (-6.13 mV) and Ag nanospheres (-5.74 mV) was found due to surface passivation with aqueous AgNO3 and AuCl4- solution, respectively. These BM NCs exhibited ~2 times higher catalytic activity than the monometallic nanoparticles depending on shell thickness and the core of the respective metals for the nitrobenzene and 1,3-dinitrobenzene reduction.

Chapter 3: Preparation and Characterization of Different Shapes of Au-Ag Bimetallic Nanocomposites for Enhanced Physicochemical Properties: This section reports the preparation and characterization of BM Au@Ag NCs consisting, Ag shell of varied thickness (2-20 nm) and Au nanospheres (Au NS) and Au nanorods (Au NR) as cores for the selective catalytic reduction of nitro-organics. A significant blue-shift in the SP band of Au NS (529 nm) and Au NR (700 and 538 nm) to 400 nm, and 522 and 412 nm, respectively, was found with an increased thickness of Ag shell that led to a notable color change. The measured zeta potential of Au NS (+26 mV) and Au NR (+22.4 mV) were also considerably increased due to the surface passivation with an Ag-shell over Au particles. It revealed that the catalytic reduction of 1,3-dinitrobenzene by the anisotropic AuNR@Ag BM NCs produced a highly selective formation of 3-nitroaniline (74%) relative to 1,3-phenylenediamine by bare Au NR (64%), Au NS (45%) and AuNS@Ag NCs (76%). Whereas 53-60% aniline was obtained from the reduction of nitrobenzene by Au@Ag compared to 34-41% yield by bare Au NPs. Thus, it was derived that the surface structural properties for the selective catalytic reduction of nitro-organics could be significantly tuned by varying the shape of the Au-core and Ag-shell thickness.

Chapter 4: Morphological and Physicochemical Properties of Ag-Au Binary Nanocomposites Prepared by Different Surfactants Capped Ag Nanoparticles: This chapter demonstrates the influence of surfactants of different chemical nature passivizing the Ag nanoparticles (Ag NPs) on the morphology and physicochemical properties of Ag-Au BM NCs. The Ag NPs were synthesized using: polyvinylpyrrolidone (PVP), cetyltrimethylammoniumbromide (CTAB) and Triton X-100 (TX), followed by the deposition of Au on their surface. TEM analysis revealed the formation of hollow Ag-Au shells (~15 nm) and mixed solid Ag-Au alloys (~20-25 nm) using PVP and CTAB-Ag NPs, respectively as the reaction template. In contrast, the porous-hollow aggregates of Ag-Au NCs (~16-22 nm) were evolved during the reaction between Au3+ and TX-Ag NPs due to the difference in reaction rates between the Au3+ ions and various surfactants capped Ag NPs. As a result, these diverse morphologies of BM NCs exhibited a significant variation in SP band, color, hydrodynamic size and zeta potential as compared to their monometallic Ag NPs. For example, a SP band of PVP-Ag NPs (488 nm) gradually red-shifted to 550 nm with the addition of Au3+ with notable color change from green to characteristic blue color indicating the composition change from Ag to Au rich. Therefore, the catalytic activity of various Ag-Au BM NCs was found to be ~2 times higher than the monometallic Ag NPs for the reduction of different nitro-aromatic compounds attributed to the electronic effect at Ag-Au interface and their morphology.

Chapter 5: Comparative Co-catalytic Effect of Monometallic and Bimetallic Core-Shell Nanocomposites on Titania Photocatalysis by Visible Light: This chapter signifies that the BM coinage metal NCs deposited on TiO2 possess the ability to absorb visible light in a wide wavelength range and are also expected to display the new properties due to synergy between two distinct metals. As a result, they reveal highest level of activity than the monometallics deposited on TiO2. In this respect, the core@shell, (Cu@Ag and Ag@Cu), inverse core@shell (Cu@Au and Au@Cu) BM NCs and their monometallics modified TiO2 have been relatively investigated for the optical absorption, emission, charge carrier dynamics, surface structural morphology and photocatalytic activity under visible light irradiation. A significant blue-shift in the SP band of Cu@Ag and Cu@Au and a red-shift in Ag@Cu and Au@Cu with notable color change were observed due to their composition change and morphology. Further, the TEM analysis also revealed the formation of eccentric core-shell Cu@Ag and uniform core@shell Ag@Cu and Au@Cu BM NCs which were deposited on TiO2, evidenced by diffuse reflectance spectroscopy, EDX, photoluminescence and time resolved spectroscopy. The visible light irradiation on core@shell BM-TiO2 promoted ~3 times higher reduction of 3-nitroacetophenone and 1-chloro-3-nitrobenzene and ~2 times higher degradation of salicylic acid as compared to monometallic-TiO2. This can be attributed to decreased work function of resulting Cu-Ag and Cu-Au BM NCs (ca. 3.8-4.6 eV) relative to their individual particles (4.3-5.3 eV) which decreased the height of Schottky barrier created at core@shell BM-TiO2 heterojunction. As a result, this led to the efficient electron transfer from the BM NCs to TiO2, resulting in enhanced photocatalytic activity than the monometallic-TiO2. Chapter 6: Influence of oxidative etching of Au nanostructures by KMnO4 on its surface morphology, electro-kinetic properties and improved catalytic activity: This section reports the impact of oxidative etching of Au NS and Au NR by KMnO4 on their surface morphology, electro-kinetic properties and catalytic activity. A significant blue-shift of the SP bands for Au NS (536 to 527 nm) and Au NR (679 to 532 nm) were observed, due to their size and shape alterations after oxidative dissolution. TEM analysis also revealed the formation of various irregular Au nano-morphologies such as spheres (~4-7 nm), low aspect ratio rods (2.6) and spheroids (~13 nm) of narrow size distribution after KMnO4 etching. As a result, the hydrodynamic diameter of Au NS (~41 nm) and Au NR (~109 nm) were reduced to ~4 nm and ~34 nm, respectively. The oxidative dissolution of Au0 by KMnO4 occurred via its oxidation to Au3+ ions as confirmed by the measured electrode potential, E0(Au0/Au3+) = -0.90 V by cyclic voltammetry with significant increase in the zeta potential and conductance values. The etched Au NPs being smaller in size and of higher surface to volume ratio resulted in ~2 fold higher catalytic activities for the reduction of p-nitrophenol and p-nitrobenzoic acid as compared to bare unetched Au nanostructures.