SYNTHESIS, PHOTOPHYSICS AND PHOTOCATALYTIC ACTION OF SURFACE-CAPPED Q-CdS NANOPARTICLES

Abstract

Increased global consumption and demand of energy has led to a tremendous upsurge of interest in exploiting renewable sources of energy for the purpose. It is realized that among various renewable sources, the solar energy has vast potential because of its plentiful availability throughout the year. Initially, the main emphasis was laid on utilizing the solar energy for accomplishing the efficient photodecomposition of H2O to H2 and O2. To achieve this goal a large number of chemical systems have been synthesized and examined for their photocatalytic activity. This eventually led to the development of a variety of new materials involving transition metal complexes, semiconductor-based photoelectrochemical systems, suspensions and nanoparticles of semiconductors, which could be exploited as efficient photocatalyst(s). Researches on colloidal particles of metals and semiconductors have found tremendous scientific and technological applications in materials science, physics, chemistry, biology, optoelectronics, microelectronics and other areas. This area of research is now popularly known as the "Nanoscience and Nanotechnology". Among various semiconductor systems, CdS is one of the most widely investigated semiconductors. It has an absorption extending into the visible region. Amajor drawback associated with the naked Q-CdS is that it undergoes anodic photodissolution in the presence of 02. This problem has been minimized by synthesizing these particles in a variety of organized media and by modifying their surface by capping with both inorganics and organics. Lately, biological molecules-capped Q-CdS have been considered very promising for the restriction of the size of the cluster through chelation of these molecules with metal ion of the semiconductor and for the control of their electronic properties. The nature of surface capping agent has been found effective not only to control the structure ofthe photocatalyst but also the photophysics and reactivity of the photogenerated charge carriers. The first chapter presents a brief overview ofthe work done on the heterogeneous photocatalysis during the last two decades. The synthesis and mechanistic aspects of photocatalytic reactions induced by suspensions of Ti02 / CdS and colloidal semiconductors mainly involving CdS based systems have been presented. The influence ofcoupling oftwo semiconductors ofdifferent band gaps and capping ofsemiconductor nanoclusters by various inorganic and organic capping agents on the change in photocatalytic and electronic properties has been discussed. It also specifies the objectives of the present investigation. The second chapter deals with the experimental details of the used materials, equipment and techniques. Abrief account of the methodology used for the preparation and analysis of the reaction samples has also been included. The third chapter presents the synthesis, electronic and photocatalytic properties ofpurine-capped Q-CdS in aqueous basic medium in the presence ofexcess Cd2+. IR and NMR studies indicate core CdS binds purine through H-bonding interaction involving -OH of Cd(OH)2 and N(9) of purine. A further uptake of purine occurs through protonated N(7). The amount of purine controls the size of the clusters in a dynamic equilibrium. XRD analysis suggests the presence of CdS in its both cubic and hexagonal structural forms along with Cd(OH)2. These clusters exhibit size-dependent electronic properties. Purine binds to the deep defects involved in non-radiative transition and enhances the band gap emission. Emission decays in a complex process and consists of in three distinct time domain in sub-nanosecond and nanosecond range. Relaxation kinetics of the charge carriers monitored as a function of emission energy reveals a distribution of traps to varying depths on the surface of the particles. Their illumination results in the production of smaller particles. Thermolysis of these colloids induces the growth of particles by removing the weakly bound purine. The photophysics of these particles have been examined in the presence of certain indoles. The addition on indole does not modify electronic spectrum of purine-capped Q-CdS but it forms a fluorescing charge-transfer intermediate with illuminated CdS, which has an emissive peak at 495 nm. The intensity and the lifetime of this intermediate are enhanced initially with an increase in concentration of indole. In the presence of other indoles, the fluorescence is simply quenched in a dynamic process without forming any fluorescing intermediate. In all the cases the quenching of fluorescence, monitored by steady state and time-resolved methods, follows Stern-Volmer relationship and takes place with a rate constant of ~ 1010dm3 mol'1 s"1. The oxygenated purine-capped Q-CdS was highly photoactive. The photocatalytic oxidation of indole produces indigo with a quantum efficiency of 0.18. The sensitization of 3-methylindole undergoes C-C bond cleavage of pyrrole ring to yield products namely, 2-aminoacetophenone, 5-hydroxy-3-methylindole and 2-acetylformanilide. The fourth chapter gives an account of synthesis, photophysics and photocatalytic behavior of adenine-capped Q-CdS. Adenine-capped Q-CdS has been synthesized in aqueous basic medium. IR andNMR spectroscopy indicate an interaction between Q-CdS and adenine through Cd2+ of CdS. The amount of adenine controls the size of the clusters. Atypical 5xl0"3mol dm"3 of adenine produces nanoclusters with an IV onset of absorption and emission band at 2.8 and 2.35 eV, respectively. Adenine binds to the shallower traps and enhances the band gap emission. Thermolysis of these colloids leads to the production of larger CdS clusters with changed electronic properties. Relaxation kinetics reveals an increase in the average lifetime of charge carriers with a decrease in particle size. Illumination of these particles does not result in their photodissolution. This catalyst is, however, photoactive. The addition of indole(s) causes the quenching of its emission. The quenching follows dynamic behavior. From the Stern-Volmer plots, aquenching rate constant in case of indole has been computed to be (5.9±0.5)xl09 dm3 mof1 s'1 using steady state and fluorescence lifetime techniques. The illumination of reaction mixture containing oxygenated adenine-capped Q-CdS and indole produces indigo with aquantum efficiency of 0.03. It also exhibited photocatalytic activity for other substituted indole(s) in the presence of oxygen. The fifth chapter describes the preparation of 6-dimethylaminopurine stabilized CdS particles in aqueous medium at pH 11. The nucleation of the cluster is controlled by coordination of 6-dimethylaminopurine via. -N(CH3)2 group. Excess 6-dimethylaminopurine binds to the core structure through H-bonding involving N(9) proton. CdS capped with 6-dimethylaminopurine produced relatively small sized particles (~ 2nm) having anarrow size-distribution. These particles did not exhibit any Ostwald's ripening and its solid sample could be fully re-dispersed in aqueous basic medium. Thermolysis results in the growth of these particles but demonstrates a better photocatalytic activity. Relaxation kinetics reveals the surface of the particle to contain a large number of traps and the emission is originated by the presence of sulfur vacancies on the particle. The addition of indole(s) simply quenches the emission of CdS in a bimolecular collisional process at a diffusion-controlled rate. Irradiation of the oxygenated reaction mixture containing CdS and indole by light of A, > 360 nm results in the formation of indigo with a quantum efficiency of 0.1 whereas the sensitization of 3-methylindole results in the C-C bond cleavage of its pyrrole ring to yield 2-aminoacetophenone, 5-hydroxy-3-methylindole and 2-acetylformanilide. The sixth chapter presents a summary and discussion of the results presented in third, fourth and fifth chapters. Based on characterization and the analysis of electronic properties of purine(s)-capped Q-CdS, their structures have been proposed. Mechanisms of the photocatalytic action for different purine(s)-capped Q-CdS have been worked out by analyzing the dynamics / reactivity of photogenerated charge carriers in the absence and presence of indole(s)-02 redox couples. A comparison of their electronic and photocatalytic properties has been made.