PHOTOPHYSICS AND PHOTOCATALYTIC BEHAVIOR OF Q-CdS-Ti02 IN THE PRESENCE OF CERTAIN AROMATICS

Abstract

There has been a tremendous upsurge of interest and activity in the understanding of photochemical processes during the last two decades. Photochemistry has found numerous applications in multidisciplinary areas of science and technology and works as one of the most important tools in present day life. The dwindling resources of conventional fuels have promoted the search for alternate sources of energy. Among various available options for renewable energy sources, solar energy is one of the most attractive and viable sources. During last one and half-decade semiconductors have widely been used as sensitizers to initiate photocatalytic reactions. Initially these researchers focussed on photosplitting of water, which though did not succeed but it prompted to use these materials for carrying out redox reactions at their interface. Among various semiconductor materials, CdS and Ti02 have been examined extensively. Cadmium sulphide has the bandgap of 2.4eV and absorbs light in the visible region. Besides, its preparation and characterization are relatively easy. It, however, undergoes photoanodic dissolution in the presence of 02. This problem has been minimized by coating of Cd(OH)2 layer around these particles. It reduced the quantum efficiency of its photodissolution from 0.04 to 0.002 under our experimental conditions. These particles, however, initiated the reactions of certain redox couples like heterocycles-02 only. On the contrary, Ti02 has a large bandgap (3.2eV) and absorbs light in UV region but it is cheap, non-toxic and photostable, which led its exhaustive applications in environmental cleanup. In order to make use of visible light to perform photochemical reactions, we have employed coupled photocatalyst comprising Cd(OH)2- coated Q-CdS and Ti02. The coupling of two semiconductors provides an interesting approach for obtaining an efficient separation of charge. The photocatalytic activity of the coupled semiconductors has further been improved by doping of each of the component by transition metal ions. The present thesis comprises of five chapters. The first chapter presents a brief overview of the work done on the heterogeneous photocatalysis during the last two decades. The mechanistic aspects of photocatalytic reactions induced by suspensions and colloidal semiconductors have been discussed with major emphasis on CdS and Ti02 based systems. Applications of doped and coupled semiconductors have been described in reference to improve the charge separation in these systems. It also narrates the objectives of the present work. The second chapter deals with the experimental details of the used materials, equipment, technique and methods. It describes the synthesis of the Cd(OH)2-coated Q-CdS and colloidal Ti02 particles. Methodology for the separation and analysis of the reaction samples has also been given. The third chapter presents preparation and characterization of coupled QCdS- Ti02 photocatalyst. The photophysical changes taking place by coupling of Cd(OH)2-coated Q-CdS with colloidal Ti02 have been examined. The coupling of Ti02 causes the quenching of the bandgap emission of CdS and the red emission is not affected appreciably. For a typical 0.22x10"3 mol dm"3 of Ti02 the average lifetime (<x>) of Cd(OH)2 -coated Q-CdS is reduced from 26.4 to 6.8 ns by trapping ofthe conduction band electron by Ti02. The illumination ofthe coupled catalyst by hi light of X> 400 run where Ti02 does not have any absorption results in the enhanced photodecomposition of CdS component. About 1.28xl0"3 mol dm"3 of Ti02 caused the decomposition of CdS particles with a quantum efficiency of 0.09. Cd(OH)2 - coated Q-CdS does not sensitize the reaction of indole, however, the coupled CdS-Ti02 catalyst in the presence of 02 photoinitiated this reaction efficiently. The photoactivity of the coupled semiconductors is suggested to increase due to chemical interaction between Cd(OH)2-coated Q-CdS and Ti02 which removes Cd(OH)2 layer to form possibly [CdS-Ti02(OH)2]. The photogenerated hole in the coupled catalyst, CdS(h+) forms an emissive exciplex with indole. The exciplex is long-lived and results in the decomposition of indole (<b=0.18) to yield indigo with a quantum efficiency of 0.08. The concentrations of both Ti02 and the redox couple control the extent of charge separation in the photocatalyst. The very similar photocatalytic effects of the coupled Q-CdS-Ti02 have been observed for the oxidation of 3- methylindole and aniline The fourth chapter gives an account of photophysics and photocatalytic behaviour of Ag+ -doped Cd(OH)2-coated Q-CdS and the coupled semiconductors consisting of Ag+-doped Cd(OH)2-coated Q-CdS - Ti02 and Ag+-doped Ti02 - Cd(OH)-coated Q-CdS. The latter two systems produce three component composite catalysts (CdS-Ti02-Ag2S). Doping of Ag+ to Cd(OH)2 -coated Q-CdS causes the quenching of bandgap emission of CdS and induces simultaneously the red emission. For a typical [Ag+] (2.0 xlO"5 mol dm"3) <t> of Cd(OH)2-coated Q-CdS at 462 nm decreases from 19.97 to 15.99 ns whereas at 650 nm it increases from 7.73 to 14.68 ns. Silver doped Cd(OH)2-coated Q-CdS particles were fairly photostable and underwent photodissolution with a quantum efficiency of <0.002 similar to that of undoped Cd(OH)2-coated Q-CdS. Both Ag+-doped Cd(OH)2-coated Q-CdS coupled with Ti02 and Ag+-doped Ti02 coupled with Cd(OH)2-coated Q-CdS cause the enhanced photodecomposition of CdS phase. The highest decomposition of Q-CdS is observed when Ti02 component is doped with Ag+ and then coupled to Cd(OH)2- coated Q-CdS («j» = 0.18). Unlike Ag+-doped stoichiometric Q-CdS, Ag+-doped Cd(OH)2-coated Q-CdS does not sensitize the reaction of indole-02 redox couple. Coupling of Ag+-doped Ti02 with Cd(OH)2-coated Q-CdS quenches the 462 nm emission of CdS completely and produces a new emission band at around 650 nm. The photogenerated hole CdS (h+) in the composite catalyst forms an emissive exciplex with indole and results in its decomposition to yield indigo. The photoinduced charge separation in this system was exactly two fold higher compared to that of undoped coupled photocatalyst (CdS-Ti02). The doping of Cu2+ in this composite system enhanced the photocatalytic efficiency for the oxidation of indole by about 40% only. On the contrary the doping of Mn2+ did not depict any catalytic action. Preliminary investigations with other additives also indicate the photocatalytic effect of Ag+ doped composite system. The fifth chapter furnishes a summary of the results of the present investigation. It includes the photophysics and photochemistry of Cd(OH)2 - coated Q-CdS, coupled Q-CdS -Ti02, and Ag+ doped coupled Q-CdS -Ti02 particles. The mechanistic aspects of the photochemical reactions sensitized by undoped and Ag+- doped coupled semiconductors have been discussed.