Synthesis of Au/Ag-CdS Nanostructures Effect of Size and Shape on Photocatalytic Activity

Abstract

This thesis presents a fine insight into many aspects of nanostructures and their applications in photocatalysis. Au/Ag − Cadmium sulfide (CdS) nanostructures viz., nanospheres, nanorods and nanowires of different dimensions have been synthesized to investigate the effect of size, shape, nature and spatial orientations of co-catalyst onto the change in photophysical, (absorbance, photoluminescence, relaxation lifetimes) and photocatalytic properties to achieve the best information in CdS nanostructures. The present thesis is divided into eight chapters: Chapter 1: Introduction The first chapter introduces the brief aspects and literature survey on semiconductor nanostructures with photocatalysis as an application point of view. Specific attention has been paid on cadmium sulfide (CdS) vis-à-vis their structure, morphology, their heterocomposites with coinage metals. Chapter 2: Synthesis and Experimental Techniques The second chapter gives a brief description of various techniques used for synthesis and characterize pure and Au/Ag-CdS nanocomposites. CdS nanostructures of different morphologies viz.; nanospheres, nanorods and nanowires have been synthesized by reverse micelles method, solvothermal and anodic alumina membrane (AAM) template techniques. Coinage metals were introduced onto CdS surface by means of in-situ deposition, photodeposition, impregnation and doping techniques. To understand the potential of CdS nanostructures, a deeper knowledge of their overall properties is required. Therefore, the synthesized materials have been characterized by UV-vis, Diffused absorbance, Photoluminescence and Time resolved spectroscopy, Scanning electron microscope (SEM), Field emission-electron microscope (FE-SEM), Low resolution (TEM) and High resolution (HRTEM) Transmission microscope, BET surface analyzer, X-ray diffraction study. Electrical properties (resistance, current, voltage and conductance) were studied by current-voltage (IV) characteristics curves. Photochemical reactions were tested under both UV and direct sunlight irradiations. Products and intermediates have been identified by UV spectroscopy, High performance liquid chromatography (HPLC), Gas chromatography-Mass spectroscope (GC-MS) and Gas chromatography (GC) measurements. Chapter 3: Enhanced Photocatalytic Activity of Coinage Metal-Cadmium Sulfide Nanorod Composites under Sun Light Irradiation It deals with the influence of size, shape and nature of co-catalyst, pH and concentration of solution, catalyst amount, and light intensity for optimum photooxidation of salicylic acid (0.5 mM) by CdS nanostructures. The bare CdS nanorod exhibited higher photoactivity as compared to low activity of bare CdS (~10 nm) nanosphere and Au & Ag photodeposition highly improved the CdS nanorod photoactivity compared with Cu deposition. The fluorescence emission of CdS nanorod at 479 nm is also quenched due to metals deposition. It is observed that Au-CdS-nanorod composites completely degrade salicylic acid within 2 h of sun light exposure. The significant effect of Au-CdS photocatalytic activity on the various sizes (3.5 & 2 nm) of Au deposits has been observed during salicylic acid photodegradation. Chapter 4: Fine-tuning the Photoluminescence and Photocatalytic Properties of CdS Nanorods of Varying Dimensions The effect of aspect ratio, crystallinity, band gap energy, surface area and Au/Ag loading on the various photoactive properties of CdS nanostructures have been discussed here. CdS nanorods having different aspect ratio viz., 17 (length ~170 nm and width ~10 nm) and 28 (length ~140 nm and width ~5 nm) has been synthesized by solvothermal technique at different time intervals (2–10 h). The photocatalytic activity for salicylic acid oxidation under UV irradiation is gradually improved with the increasing crystallinity of CdS nanostructures, length (140 < 170 nm), exposed area (2358 < 5722 nm2) per particle and decreasing surface area (158 < 122 < 76 m2g-1), surface to volume ratio (0.82 < 0.41 nm-1) and aspect ratio (28 < 17). The deposition of 1–2 wt% Au (~3.5 nm) and Ag (~1.8 nm) nanoparticles onto CdS drastically quenched the emission and enhanced the photocatalytic activity. Chapter 5: Study of Excited Charge Carrier’s Lifetime for the Observed Photoluminescence and Photocatalytic Activity of CdS Nanostructures of Different Shapes This chapter demonstrates the influence of the relaxation time of photoexcited charge species on the photoluminescence and photocatalytic activity for oxidation–reduction reactions by CdS nanostructures of different dimensions. CdS nanospheres (size ∼10 nm) and different aspect ratio (17 and 23) CdS nanorods have been prepared by two different techniques. CdS nanorods formed by autoclaving is found to be more lengthy, wider (length ∼170 nm and width ∼10 nm) and having better crystallinity than CdS nanorods (length ∼126 nm and width ∼5.5 nm) prepared by refluxing under similar conditions. Relaxation lifetime of photoexcited electron–hole pairs is measured to be 20, 24 and 116 µs for CdS nanosphere, shorter and longer CdS nanorod, respectively, seems to be responsible for the observed fluctuation in photoluminescence and photocatalytic activity. The photooxidation rate of salicylic acid (0.5 mM) and photoreduction of p-nitrophenol (0.2 mM) are significantly improved with increasing dimensions of CdS nanorods despite having a comparable surface area (81 and 76 m2 g−1) and CdS nanospheres (18 m2 g−1) exhibit poor photocatalysis. The better delocalization of charge species along the radial as well as longitudinal dimensions of CdS nanorods, higher crystallinity and delayed recombination time facilitate electrons or holes for active participation in the photoinduced reactions, and Au deposition always displayed superior photoactivity. Chapter 6: Highly Enhanced Photocatalytic Activity of Au Nanorod–CdS Nanorod Heterocomposites This chapter signifies the importance of different shapes of Au and CdS-nanoparticles for fabricating Au–CdS heterocomposites {prepared by mixing or impregnation of Au-nanosphere (9.1 nm), Au-nanorod (20 nm × 8.6 nm; L × W), CdS-nanosphere (10–12 nm) and CdS-nanorod (126 nm × 5.5 nm)} onto the photoluminescence and photocatalytic study. Second derivative absorption spectra’s exhibit precise onset at 441 nm (CdS-nanosphere) and 469.5 nm (CdS-nanorod) that are significantly red-shifted to 509 nm (Au-nanosphere–CdS-nanosphere), 485 nm (Au-nanosphere–CdS-nanorod), 520 nm (Au-nanorod–CdS-nanosphere) and 485.5 nm (Au-nanorod–CdS-nanorod) depending upon interfacial contact-area. XRD patterns reveal the hexagonal phase of pure and Au–CdS nanocomposites. Photoluminescence of CdS nanorods has been effectively inhibited by modifying its surface with Au-nanoparticles (0.002–0.04 wt%). Relaxation lifetime of photoexcited charge-carriers found to be improved ca. 12.5 µs for Au-nanosphere–CdS-nanorod and 34 µs for Au-nanorod–CdS-nanorod due to effective charge transfer kinetics at the interface, in contrast to prompt charge-recombination (2.7 µs) in CdS. Under UV light (10.4 mW/cm2) irradiation, Au-nanorod–CdS-nanorod exhibits the best photocatalytic activity for the oxidation of salicylic acid (86%, k = 9 × 10−3 min−1) and reduction of p-nitrophenol to p-aminophenol (53%) as a function of improved stability and better current–voltage (I–V) characteristics suitable for rapid charge transfer process during photoreaction. Chapter 7: Woollen Bun like CdS Microspheres Wrapped with Lengthy Nanowires Exhibit Superior Photoactivity for Dye Degradation Under Sunlight This chapter deals with the synthesis of woollen buns like CdS microspheres (2–5 µm) wrapped by lengthy, broader and crystalline hexagonal CdS nanowires (8–10 µm × 50–80 nm; L × W) via anodic alumina membrane to achieve the best photoactivity for the decomposition of rhodamine B (50 µM) and methylene blue (20 µM) dyes under sunlight (50 mWcm-2) irradiation in comparison to CdS nanospheres {cubic (~12 nm) and hexagonal (~15 nm) phase} and nanorods (170 nm × 10 nm). Higher crystallinity, least surface defects, better charge separation as evident from longer relaxation lifetime (3.6 ns) of photoexcited electron–hole pairs seems to be the cause for its outstanding performance and maximum (>80%) photoluminescence quenching. The largest surface area (1.42 µm2), more surface exposed CdS molecules (2.5 × 106) and higher dye coverage (4.9 × 105) per–CdS nanowire led to the drastic improvement in the photoactivity. Many intermediate products and a linear increase in CO2 production are quantified during rhodamine B degradation by CdS nanowires under different time periods of solar exposure. Chapter 8: Preparation of Coinage Metal Doped CdS Nanorods for Highly Improved Photocatalytic Oxidation and Reduction Processes This chapter introduce the doping of Au, Ag and Cu into CdS hexagonal nanorods with different mol ratio of metal/Cd2+ {M1−xCdxS (x = 0, 0.01, 0.02, 0.03, 0.05, 0.1)} by a facile solvothermal method. Results show that absorption and photoemission spectra of CdS exhibit a significant red-shift (506−532 nm) with luminescence quenching over 80% by varying the content and nature of dopant. Distortion in hexagonal crystal structure of CdS has been interestingly observed due to lattice mismatching of introduced coinage metal ions. The rate of photooxidation of salicyldehyde and photoreduction of nitroaromatics was remarkably influenced by the nature and concentration of doping metal. Many important parameters such as surface area, current-voltage characteristics, lattice distortion, magnetic properties, and crystallite size are correlated with photocatalytic properties.