PHOTOPHYSICAL AND PHOTOCATALYTIC BEHAVIOURS OF Q-CdS IN THE PRESENCE OF SOME HETEROCYCLES

Abstract

Over the last two decades the global energy requirement has grown exponentially owing to rapid industrialization and increased living standard Among the renewable energy resources, solar energy has drawn considerable attention due to its adequate availability in almost all parts of the world throughout the year Initially these researches were focussed on the photosensitized decomposition of water into elemental H2 and 02 For this purpose, a variety of sensitizers involving transition metal complexes, semiconductor based photoelectrochemical systems, suspensions and colloidal particles of semiconductors have been tried. Sens. +A hv » Sens.+ + A" (1) 2Sens ++H2O > 2Sens. +2H+ +112 02 (2) A- + H20 > A+ OH"+l/2H2 (3) The model system comprising tns (2,2' bipyndyl) ruthenium (II) as sensitizer and MV2' as a relay has been widely investigated for the generation of hydrogen. In order to prevent the fast back reaction of the photogenerated species produced in eq. 1, which reduces the quantum efficiency of H2 generation, anumber of methods involving the use of sacrificial donor, colloidal metals as catalysts, trapping ofthese materials in different matrices etc. have been employed. The simultaneous production ofoxygen has been obtained using Ru02 as a catalyst. (ii) In course of these investigations, a new field of heterogeneous photocatalysis initiated by semiconductors has developed in which novel redox reactions of a variety of organic and inorganic substrates have been studied at their interface. Mechanisms of these systems are, however, understood poorly. The primary photochemical events of these reactions could be examined kinetically by using nanosized particles of semiconductors. Photophysical changes taking place in the presence of the used redox couples have been exploited to understand and analyze the scheme of the reactions. Cadmium sulfide has been extensively investigated as a photocatalyst since its optical absorption extends into the visible region Besides this, its preparation and characterization are relatively easy A major drawback associated with the CdS system is its photoanodic dissolution Coating ofCd(OH)2 layer on the surface of these particles minimizes this shortcoming. In the present study Cd(OH)2-coated Q-CdS particles have been employed as photocatalyst to perform visible light induced redox transformations of biologically important molecules in the presence of oxygen at their interface. These particles have been found to be fairly photostable and have high luminescing efficiency and emission lifetime The change in photophysical behaviour of CdS in the presence ofdifferent additives has been used to analyze the nature of then interaction with the photocatalyst, estimate the distribution of the charge carriers on the surface of the particle and elucidate the operating reaction scheme. Thefirst chapter presents an overview of the work carried out in the area of heterogeneous catalysis during the last two decades. The mechanistic aspects of (iii) photocatalytic reactions initiated by suspensions and nanoclusters of semiconductors have been described with major emphasis on CdS system. It also specifies the objectives of the present investigation. The second chapter incorporates the experimental deta.ls about the used materials, equipment, techniques and methods It describes the synthesis and characterization of the Cd(OH)2-coated Q-CdS particles Abrief account of the methodology used for the preparation and analysis of the reaction samples has also been included. The third chapter deals with the photophysical and photocatalytic aspects of Cd(OI I)2-coated Q-CdS in the presence of tryptophan and some related compounds The possibility of complexation of Cd2' with tryptophan in the presence of the used stabilizer (HMP) and interaction between Cd(OH)2 -coated Q-CdS and tryptophan have been examined by electronic and emission spectroscopy. The absorption spectroscopy did not reveal any chemical or physical interaction of tryptophan with Q-CdS. However, tryptophan quenches the bandgap emission of these particles efficiently with aquenching rate constant of 2.8x10" dm1 moP1 s1. The possibility of complexation between Cd2' and tryptophan has been examined further by adding tryptophan to Cd2' solution in the presence of HMP prior to the precipitation of CdS. The addition of tryptophan in this manner induced the production of smaller CdS particles and demonstrated the size quantization effect. Strikingly, the photophysical changes observed in these experiments were similar to when tryptophan was added after the precipitation of CdS. An analogy in the photophysical behaviour in the two cases indicates that the quenching of emission (iv) is not caused due to the complexation of free Cd2' with tryptophan Instead it involves some weak interaction oftryptophan with Cd(OH)2-coated Q-CdS. The photophysical behaviour of Q-CdS has also been examined in the presence of the parent compound of tryptophan viz. indole and related C-3 substituted derivatives such as 3-methylindole, indole-3-acetic acid and N-acetyl tryptophan. The efficiency for quenching of bandgap emission by these substrates follows the order : tryptophan> N-acetyl tryptophan> indole -3-acetic acid > 3- methylindole ~ indole. These results suggest mainly the participation of -NH2 group at C-3 position in tryptophan in having an interaction with the -OH of Cd(OH)2 layer of the photocatalyst through hydrogen bonding. Emission lifetime of CdS has been found to decrease upon addition of tryptophan from which a quenching rate constant of 2.7x1011 dm3 mol"1 s"1 was calculated On the basis of these experiments the quenching of emission has been concluded to be dynamic. The Cd(OH)2-coated Q-CdS sensitized the photocatalytic decomposition of tryptophan in the presence of 02 ((^-tryptophan = 0.22) by visible light to produce 5-hydroxytryptophan as the major and kynurenine as the.-minor product of its oxidation. Thefourth chapter gives an account of the photophysical and photocatalytic properties of Q-CdS in the presence ofguanine, adenine and purine. The possibility of complexation of Cd2' with the substrate in the presence of HMP and the nature of surface interaction between the photocatalyst and the substrate have been examined using electronic spectroscopy. These results indicated absence of complexation between Cd2' and the additives under the used experimental (v) conditions These substrates are, however, adsorbed on the surface of the colloidal CdS particles. Guanine and adenine exhibit the adsorption behaviour of Type IV whereas the adsorption isotherm of purine is of Type V. Both guanine and adenine quench the bandgap em.ss.on of the CdS particles. About 2x10"' mol dm"3 of guanine quenches 50% of the bandgap em.ss.on of Cd(OH)2-coated Q-CdS particles whereas nearly an order of magn.tude h.gher concentrat.on of adenine was needed to bring the quench.ng of s.m.lar magn.tude. The quenching data have been found to follow Stern-Volmer relationship from wh.ch quench.ng rate constants for guan.ne and aden.ne were found to be 2.0xl0,a and 1.2x10" dm3 moP1 s\ respectively. In contrast, the presence of purine d.d not influence the em.ss.on behaviour of theQ-CdS particles significantly The addition of 5x10"4 mol dm3 of guan.ne to 3xl0"4 mol dm"3 of Cd2' in the presence of HMP pr.or to the precipitation of CdS results in the blue sh.ft of the onset of absorption of CdS by 0.09 eV. In th.s system, the luminescence quenching efficiency by guan.ne is increased by about 25% compared to when guanine was added after the preparation of CdS. Similar experiments with aden.ne result in more pronounced .nfluence on the onset of absorption of CdS and its em.ss.on quench.ng eff.c.ency. In these exper.ments a two fold .ncrease in [Cd2+] caused the exc.to.nc absorpt.on and em.ss.on to become relat.vely less prominent wh.ch indicates the formation of large number of traps of varying energies at the surface of these particles. The emission lifetime of Cd(OH)rcoated Q-CdS has been found to reduce in the presence of both guanine and adenine Interception of hole by these

substrates has been suggested to occur through hydrogen bonding interaction between -OH of Cd(OH)2 and certain functional groups of the additive. The illumination of the reaction mixture containing Q-CdS and guanine by visible light in the presence of 02 resulted in photocatalytic decomposition of guanine The quantum efficiency for its decomposition has been determined by HPLC to be 0.36. The photodecomposition of adenine has also been followed quantitatively and takes place with the quantum efficiency of 0.1. The irradiation of the CdS-purine reaction mixture results in the photoanodic dissolution of the photocatalyst similar to that observed in the absence of purine. The fifth chapter presents a brief account of the mechanistic behaviour of semiconductor induced photophysical and photochemical processes studied earlier. It also furnishes a summary of the results on photophysical and photocatalytic aspects of Cd(OH)2-coated Q-CdS in the absence and presence of tryptophan, guanine, adenine and related compounds. On the basis of these results, the nature of interaction of the additives with the photocatalyst and the mechanisms of their photocatalytic decomposition are suggested.