Experiment on Size, Shape and Structures of Aggregates

Experiment on Size, Shape and Structures of Aggregates Introduction Dye sensitized solar cells (DSSC) have made considerable attention because of their unique features for conversion of free, unlimited solar energy into electricity by utilizing environmental friendly, inexpensive raw materials for low production cost1,2. DSSC composed of nanostructured, mesoporous semiconductors with wide-bandgap materials, to which the dye molecules (sensitizers) are adsorbed, a counter electrode and an electrolyte. In DSSC the dye molecule absorb visible light, and inject electron from photo-excited state of dye molecule to conduction band of semiconductor1,3 The performance of a dye-sensitized solar cell is mainly based on the structure of the visible light absorbing dye/ sensitizer. Applications of several natural carotenoid dyes with higher absorption coefficient in the visible region of the solar spectrum with higher conversion efficiency, is under investigations1,4. Carotenoids are C40 tertraterpenoid hydrocarbons consist of eight-C5 isoprene units5. The major characteristic feature of Car molecule is it comprises of extensive conjugated system of delocalized Ï€-electrons which makes them highly hydrophobic molecules. Therefore when these carotenoid molecules are dissolved in hydrated solvents they tend to form aggregates since the molecule is hydrophobic in nature. The surface arrangements and the aggregation behavior of the sensitizers (carotenoids) on semiconductor nanoparticles is greatly influenced the performance of carotenoid based artificial photosystems such as DSSC. It was stated (Wang et al.2006) that formation of dye aggregates suppresses the performance of DSSC6 since the device performance is influencing the nanoscale morphology of the aggregates. Therefore several researchers have focussed on the nanoscale organization of molecular aggregation in solutions of these conjugated molecules at atomic level resolution. Interchain agg regation behavior of a conjugated polymer (DP10-PPV) which used in opto-electronic applications such as pohotovoltaics, dissolved in two solvents having different qualities at different concentrations were studied by (Chen et al. 2009) using Small angle neutron scattering (SANS), revealed that the internal conformational structure of these network aggregates were differ in different solvents. A similar study was also carried out by Chen and coworkers for another semirigid conjugated polymer (DP6-PPV) in solutions7,8. Moreover it has been reported that, when carotenoid molecules are deposited on the surface of semiconductor often it forms H-shape aggregates5. Aggregation phenomena of natural and artificially modified carotenoids were proved that J- and H-aggregates are the possible geometries9. Apart from the formation of self-assembled aggregates in hydrated solvents of these carotenoid molecules, there is strong relationship between the structural conformation and the composition o f molecules in the solution with the ability of aggregation5. Polar carotenoids consist of –OH (hydroxyl groups) in the carotenoid structure promotes the formation of aggregates compared to the molecules without any functional groups, (Simonyi et al. 2003). Furthermore the position of the functional group may also significantly affect the type of aggregates form10. It was observed experimentally for the first time (Cheng et al. 2009), the crossover from swollen coils in semidilute regime to unperturbed coil at high concentrations11 with the variation of polymer radius of Gyration (Rg) of poly(methyl methacrylate) in chloroform as a function of polymer concentration (à Ã¢â‚¬ ¢). Perahia et al. used SANS data12to investigate how molecular solutions of poly(2,5-dinonylparaphenylene ethynylene)s (PPE) aggregates into large flat clusters. Ratnaweera et al. 2012 and Lodge and coworkers were studied self-assembly modes of several block copolymers in selective solvents using SANS13 -15. Therefore the studies of dye aggregates are really essential for future developments of carotenoid based electrochemical devices for solar energy conversion. Spectroscopic studies on carotenoids in hydrated solvents revealed that the aggregation behavior of carotenoids is significantly affecting the S0 →S2 electronic transition16 of neutral carotenoids. Therefore the solvent quality is one of the major factors for carotenoid solutions which govern the size of the aggregate. When the carotenoid molecules are uniformly dissolved in a good solvent at low concentrations to form a homogeneous mixture, carotenoids are exist as isolated molecules that are very far from each other17. Therefore the interactions between monomers are very weaker than the monomer-solvent interactions and polymer tends to swells and its size is larger than the ideal size13,18. Cheng coworkers stated11 that in the good solvent domain (T > ÃŽ ¸), the repulsions of excluded volume is greater which results Rg of the aggregates enhanced beyond its unperturbed size. Further this expansion effect is greater in dilute solutions since the volume fraction of polymer is very low compared to its overlapping concentration. When the concentration of the solution is increased, the attraction between monomers are stronger and the aggregates tried to collapses into globules wit h solvents inside it corresponds to a poor solvent. The size of the structural conformation of collapsed globule is smaller than the ideal chain17. For a polymer solution (NA = N and NB = 1) to be favorable for mixing or aggregation is depends on the Flory-Huggins equation which consist of two terms, the entropy and energy terms17. The energy change of binary mixing which depends on composition of the mixture is the main factor, for a polymer to be dissolved in a good solvent to make an equilibrium state of a homogeneous mixture of polymer solution. Entropy of mixing is small for polymer solutions and always positive hence promotes mixing, but the energy of mixing is either positive or negative which measures the attractive or repulsive interactions between species. The net attraction between species in the solution is important because it measure the value and the sign of the Flory interaction parameter (χ) and hence to know whether the given mixture consist of single-phase or separated into phases. In this research the small angle neutron scattering (SANS) data was used to investigate the quantitative experimental information on size, shape and structures of aggregates. Two carotenoid molecules, bixin and norbixin were selected in this study to characterize their structural information of monomeric and aggregated forms. The natural dyes, bixin and norbixin are belongs to the group of apo-carotenoids, extracted from annatto seeds of the Bixa orellana tree1. The chemical structures of cis-bixin (C25H30O4) and cis-norbixin (C24H28O4) is shown in fig. 1. (1) Gà ³mez-Ortà ­z, N. M.; Và ¡zquez-Maldonado, I. A.; Pà ©rez-Espadas, A. R.; Mena-Rejà ³n, G. J.; Azamar-Barrios, J. A.; Oskam, G. Solar Energy Materials and Solar Cells 2010, 94, 40. (2) Zhou, H.; Wu, L.; Gao, Y.; Ma, T. Journal of Photochemistry and Photobiology A: Chemistry 2011, 219, 188. (3) Xu, H.; Tao, X.; Wang, D.-T.; Zheng, Y.-Z.; Chen, J.-F. Electrochimica Acta 2010, 55, 2280. (4) Yamazaki, E.; Murayama, M.; Nishikawa, N.; Hashimoto, N.; Shoyama, M.; Kurita, O. Solar Energy 2007, 81, 512. (5) Landrum, J. T. Carotenoids : physical, chemical, and biological functions and properties; CRC Press: Boca Raton, 2010. (6) Wang, X.-F.; Koyama, Y.; Nagae, H.; Yamano, Y.; Ito, M.; Wada, Y. Chemical Physics Letters 2006, 420, 309. (7) Li, Y.-C.; Chen, K.-B.; Chen, H.-L.; Hsu, C.-S.; Tsao, C.-S.; Chen, J.-H.; Chen, S.-A. Langmuir 2006, 22, 11009. (8) Li, Y.-C.; Chen, C.-Y.; Chang, Y.-X.; Chuang, P.-Y.; Chen, J.-H.; Chen, H.-L.; Hsu, C.-S.; Ivanov, V. A.; Khalatur, P. G.; Chen, S.-A. Langmuir 2009, 25, 4668. (9) Auweter, H.; Benade, J.; Betterman, H.; Beutner, S.; Kà ¶psel, C.; Là ¼ddecke, E.; Martin, H.; Mayer, B. Pigments in food technology. Sevilla: Dep Legal 1999, 197. (10) Simonyi, M.; Bikadi, Z.; Zsila, F.; Deli, J. Chirality 2003, 15, 680. (11) Cheng, G.; Graessley, W. W.; Melnichenko, Y. B. Physical Review Letters 2009, 102, 157801. (12) Perahia, D.; Traiphol, R.; Bunz, U. H. F. The Journal of Chemical Physics 2002, 117, 1827. (13) Ratnaweera, D. R.; Shrestha, U. M.; Osti, N.; Kuo, C.-M.; Clarson, S.; Littrell, K.; Perahia, D. Soft Matter 2012, 8, 2176. (14) Lodge, T. P.; Hamersky, M. W.; Hanley, K. J.; Huang, C.-I. Macromolecules 1997, 30, 6139. (15) Lodge, T. P.; Bang, J.; Park, M. J.; Char, K. Physical Review Letters 2004, 92, 145501. (16) Alwis, D. D. D. H.; Chandrika, U. G.; Jayaweera, P. M. Journal of Luminescence 2015, 158, 60. (17) Rubinstein, M.; Colby, R. Polymers Physics; Oxford, 2003. (18) Halperin, A. Journal de Physique 1988, 49, 547.