PHOTOLUMINESCENCE OF COLLOIDAL CADMIUM SULPHIDE PARTICLES IN THE PRESENCE OF CERTAIN ANILINES AND INDOLES-STUDY OF CdS-SENSITIZED PHOTOCATALYTIC REACTIONS

Abstract

In recent years attention has been focused on the search of some solar energy conversion devices which could convert efficiently light energy to chemical or other usable form of energy. Various chemical approaches have been tried to achieve this objective. One of the attractive scheme among these is the photosplitting of water to produce simultaneously hydrogen and oxygen. In these systems, a variety of sensitizers involving transition metal complexes and semiconductors have been employed. Among transition metal complexes, tris(2,2'-bipyridyl)ruthenium(II) has been the most popular sensitizer. For the generation of hydrogen with this complex the basic scheme involved an electron transfer from the excited Ru + complex to an electron relay (MV +) to produce radical cation (MV+) which eventually reduced water to produce hydrogen. Ru(bpy)32+ + MV2+ > Ru(bpy)33+ + MV+ 2MV+ + 2H20 > 2MV2+ + H2 + 20H" The main problem in the above scheme has been the fast back reaction of the photogcnerated species in the initial step. It results in the low quantum yield of hydrogen production even in the presence of sacrificial donor. The yield could be improved by using certain redox catalysts and different heterogeneous interfacial systems. The study of photocatalytic reactions initiated by semiconductor suspensions has been a subject of long investigation. The discovery of Fujishima and Honda, of splitting water simultaneously to hydrogen and oxygen upon irradiating a titanium dioxide single-crystal electrode, has further prompted researches on photoindviced redox reactions sensitized by semiconductor materials. These systems are, however, poorly understood mechanistically. Such investigations could be very well accomplished using colloidal particles of nanometer dimension which have an added advantage that one may utilize optical and emission techniques for mechanistic analyses. In the present investigation we have employed colloidal cadmium sulphide as sensitizer to initiate redox reactions at its interface by the photogenerated electron-hole pair formed by irradiation. Cadmium sulphide has been widely explored as photocatalyst since its optical absorption extends into the visible region. Moreover, CdS colloids are stable for a long period, easy to prepare and characterize. In the case of CdS, some of the charge carriers are known to undergo radiative recombination. Any chemical or physical interaction between the additives and CdS might affect its electronic as well as emission behaviour. Hence for the mechanistic elucidation of these systems, the optical and luminescence changes of CdS have been monitored in the absence and presence of title substrates, together with the characterization of reaction intermediates and final products. The products of these reactions have been identified using chromatographic and spectroscopic techniques. These investigations have been restricted to the usage of steady state methods only due to nonavailability of the required time-resolved techniques. The present thesis comprises five chapters. The first chapter incorporates the background of the work being contributed in the above area of research and specifies the objectives of the present investigation. The second chapter describes experimental details viz., materials, equipments, methods and techniques used. It also includes the methodology, synthesis and characterization of colloidal particles of CdS and preparation of reaction samples. Studies on the mechanistic analysis of the CdS-sensitized reactions of anilines have been discussed in the third chapter. The photoluminescence of colloidal CdS has been examined in the presence of aniline, 2,6-dimethylaniline, N,N-diethylaniline and pyridine. Aniline and its substituted derivatives quench the photoluminescence of CdS and suppress its photoanodic dissolution. The presence of pyridine neither causes the quenching of emission nor prevents its photocorrosion. The effect of aniline on the photoluminescence of CdS has been examined at different pH. The quenching becomes more prominent in the acidic medium. The addition of 0.025 mM aniline at pH 3.7 and 0.15 mM of aniline at pH 10.6 to a colloidal solution of CdS reduces its emission intensity by about 50%. Anilines have been found to get adsorbed on the surface of CdS particles and these data largely follow the Langmuir adsorption isotherm. The degree of quenching of emission depends upon the extent of adsorption. The addition of aniline prior to the precipitation of CdS produces a surface modified catalyst with different electronic and emission properties from bulk CdS. The absorption band edge and emission band of the catalyst are gradually blue shifted with increasing aniline concentration. An aniline concentration of 0.015 M moves the onset of absorption and the luminescence peak to higher energy by 0.12 eV and 0.06 eV respectively. The CdS-sensitized reaction of aniline produces azobenzene with a quantum efficiency of 0.15. Silver containing CdS particles increase the quantum yield of azobenzene further by about 50%. These particles have higher luminescence intensity. Their luminescence is also quenched in the presence of aniline. At high concentrations of aniline, no photodissolution of the colloidal CdS could be detected. The fourth chapter presents a study on the mechanistic aspects of CdS-sensitized reactions of indoles. This chapter has been divided into two sections. The first section includes data on indole and the second section reports findings on substituted indoles. The presence of indole causes an increase in the green photo luminescence of CdS and simultaneously quenches its 660 nm emission. Indole is adsorbed on the surface of colloidal CdS and in the low concentration range the adsorption data follow the Langmuir isotherm. The CdS-sensitized oxidation of indole results in the formation of indigo.Tlie first monolayer of indole leads to the observed photophysical effect but does not contribute to the formation of indigo. The subsequent layer of adsorbed indole undergoes photooxidation to produce indigo. Its formation could not be detected at acidic pH and in an N2 atmosphere. A correlation between the extent of adsorption and the yield of the product is observed. Colloidal CdS produced in the presence of indole depicts very similar photophysical and photochemical behaviour. In the second part of this chapter a study of substituted derivatives of indoles viz., tryptophan, 2-MI and 3-MI is presented to verify the observations made with indole. The nature of photophysical changes and adsorption isotherms recorded with these systems were similar to those of indole. The products of CdS-induced photochemical reaction of 2-,and 3-MI are due to C-C bond cleavage of the pyrrole ring. The fifth chapter gives a brief account of general mechanistic behaviour of semiconductor induced photochemical reactions examined earlier. It furnishes a summary of the results of present investigations. A mechanism of the photocatalytic action of CdS is suggested.