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Ultrasmall Particles of Cdse and Cds Formed in Nafion by an Ion-Dilution Technique

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Ultrasmall Particles of Cdse and Cds Formed in Nafion by an Ion-Dilution Technique Subscriber access provided by Univ. of Texas Libraries Ultrasmall particles of cadmium selenide and cadmium sulfide formed in Nafion by an ion-dilution technique Eugene S. Smotkin, R. Malcolm. Brown, L. K. Rabenberg, K. Salomon, Allen J. Bard, Alan. Campion, Marye Anne. Fox, Thomas E. Mallouk, Stephen E. Webber, and John M. White J. Phys. Chem., 1990, 94 (19), 7543-7549 • DOI: 10.1021/j100382a044 Downloaded from http://pubs.acs.org on January 26, 2009 More About This Article The permalink http://dx.doi.org/10.1021/j100382a044 provides access to: • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article Journal of Physical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 J. Pkys. Ckem. 1990, 94, 7543-7549 7543 period. The interaction between the phenyl groups is attractive when the phenyl ring configuration maximizes the porbital overlap between C2and C2,, and it is repulsive when the H2-H2, repulsion dominates. Thus, there is a narrow range of angles @ and a for which the barrier to DHP formation is low, as shown in Figure 2. When the repulsive interaction dominates, the symmetric phenyl twist couples with the ethylenic torsion, converting the initial phenyl twisting motion into ethylenic torsion leading to cis-trans isomerization. The quantum yield measurements by Fischer and co-workers imply that the branching ratio between cis-trans and DHP routes to nonradiative decay is 7030.' This 0' branching ratio is interpreted to be the fraction of configuration 95- 110' 125~ 140' space that leads to DHP formation during one phenyl twisting Phenyl-Ethylene Bend Angles (a) vibration. The nonradiative decav throueh ohenvl twistine Drowsed in Figure 2. Conceptual picture of SI potential surface for cis-stilbene in I. I. the symmetric phenyl twist angle and antral phenytethylene bend angle this work may be common to other molecules with vicinal, ste- coordinates. The position of the quilibrium configuration an the So rically hindered phenyl rings. For instance, Fox and co-workers surface ($ = 43O: a = 129O)" is marked by the shaded ellipse repre- found a strong dependence ofthe nonradiative decay rate on the senting a wave packet where the isomerization dynamics is initiated. The phenyl twist for substituted tetraphenylethylenes whose phenyl gray lines indicate two dynamical pathways for pieces of this initial wave rings were, to various degrees, constrained from twisting." Other packet. Most of the wave packet will stay in the cis-stilbene side of the molecules such as diphenylamine are also known to form stable surfaceand eventually end up in the perpendicular intermediate config- uration from which it decays radiationlessly to cis- and frons-stilbene. DHP-like photoproducts.2,22 From the viscosity dependence on A smaller part of the wave packet finds its way over to the left side of the nonradiative decay rate, phenyl twisting has been implicated the potential surface. where it becomes vibrationally excited dihydro- in triphenylmethane photochemistry?' Further experimental and phenanthrene (DHP). Some of this product decays to DHP while the theoretical investigation of cis-stilbene photwyclization may rest internally converts to cis-stilbene. provide a deeper understanding of nonradiative decay processes through the phenyl twisting motion. effect of the cyclobutene ring on the 151.7-cm-' mode shows that it has minor ethylenic torsional character. Finally. the observation Acknowledgment. We gratefully acknowledge support from of only two strongly coupled progressions with - 31-cm-' fre- the Institute for Molecular Science (H.P., K.Y., and Y.F.). quency for DBS is consistent with the bridging methylene group Acknowledgment is also made to the donors of the Petroleum strongly restricting motion along w, mode and strongly coupling Research Fund, administered by the American Chemical Society, o, and wo modes. for partial support of this work (J.H.F.). J.H.F. thanks IMS for Proposed Mechanism for cisSrilbene S, Stare Decay and its generous hospitality during two visits to Japan, at which time Isomerization. The observation and assignment of the DPC-4 part of the work was undertaken, and the Research Advisory fluorescence excitation spectra provides new information on the Board of the University of Nevada, Reno, for support during one cis-stilbene Sl-state potential energy surface from which it is of those visits. possible to reconcile formation of both trans-stilbene and DHP on a picosecond time scale. The gas-phase cis-stilbene Sl-state (30) Shultz. D. A.; Fox, M. A. J. Am. Chrm. Sm. 1989. 111. 6311. lifetime8 corresponds roughly to one symmetric twist vibrational (31) Ben-Amotz. D.; Harris. C. B. Chem. Phys, Leff. 1985, 119, 305. Ultrasmall Particles of CdSe and CdS Formed in Nafion by an Ion-Dilution Technique Eugene S. Smotkin,' R. Malcolm Brown, Jr.> L. K. Rabenberg? K. Salomon, Allen J. Bard,* Alan Campion, Marye Anne Fox, Thomas E. Msllouk, Stephen E. Webber, and John M. White Department of Chemistry, Uniuersity of Texas. Austin, Texas 78712 (Received: May IO, 1989; In Final Form: May 31, 1990) The transition of semiconductor chalcogenides from molecular CdX (X = Se, S) to bulk material has been observed in Nafion, a cation-exchange membrane. The two-phase structure of Nafion, consisting of a hydrophobic region and ionic clusters, has been utilized to form ultrasmall particles of CdS/Se by using a technique analogous to an inverted micelle microemulsion method, where the Cd ionlionomer cluster ratio is controlled by diluting the cadmium-exchange solution with an inert ion (Ca2+). The absorption onset for the cadmium chalcogenides formed can be tuned over a range of more than 3.5 eV by varying the Cd2+/Caz+ratio in the solution used to exchange the acidic form of the ionomer membrane. We report here the synthesis of ultrasmall particles of CdS and with an inert diluent ion (Ca*+) followed by dehydration and CdSe, with sizes from monomolecular species through larger reaction with H2S or H2Se. A large number of photoelectro- clusters to bulk materials, in the ion-exchange polymer Nafion chemical studies have been reported involving CdS (bandgap 2.4 as a matrix. These particles are formed by ccexchanging Cd2+ eV) particle suspensions.l-' CdS showing bulk properties can 'Prscnt address: Department of Chemistry. University of Hawaii, Hon- olulu, HA 96822. (I) Harbour. J. R.; Hair, M. L. J. Phys. Chem. 1977, 81. 1791. 'Department of Botany. The University of Teras at Austin. Austin. TX (2) Frank. S. N.;Bard. A. J. J. Phys. Chem. 1977.81, 1484. 78712. (3) Kalyanasudarm. K.; Borgarello, E.; Griitrel. M. Helo. Chim. Acta 'Department of Mechanical Engineering. The Univcrsity of Texas at 1981. 64, 362. Austin. Austin. TX 78712. (4) Matsumura. M.; Sam. Y.: Tsubomura, H. 3. Phys. Chrm. 1983,87, *Corresponding author. 3897. 0022-3654/90/2094-7543so2.50/0 0 1990 American Chemical Society 1544 The Journal of Physical Chemistry, Vol. 94, No. 19. 1990 Smotkin et al. also be formed in polymer matrices8-12 For example, when the I(CF2CF2)n-C F2CF -1m perfluorinated cation-exchange polymer Nafion was treated with I (OCF,CF--), OCF2S0,-M + an aqueous solution of Cd2+ followed by exposure to H2S or an I aqueous solution of sulfide ions, yellow CdS particles were pre- c F3 cipitated within the polymeric matrix.l0 This material could be used for the photocatalyzed reduction of methylvio1ogen,l0 and where n and m can be varied to produce materials of different in the presence of Pt (either chemically deposited throughout the ion exchange capacities and 1 is small. As in many ionomeric membrane or sputter deposited) and a sacrificial donor, photo- polymers, the ion-exchange sites in Nafion aggregate into clusters. chemical H2 formation was observed." A surface analysis of this Ion clustering in Nafion has been suggested by a number of semiconductor-incorporated polymer system showed that the CdS techniques, including mechanical and dielectric relaxation, NMR, had bulklike properties and particle sizes (1-2 pm).10-12Wang IR, transport experiments, electron microscopy, and X-ray and MahlerI3 showed that much smaller CdS particles could be studiesz6 Gierke and Hsu have derived a semiphenomenological formed in Nafion, if the H2S were reacted with dehydrated expression for the diameter of ionic clusters in Nafion resins that cadmium-exchanged Nafion in the gas phase. These quantum correctly describes the observed variation in cluster diameter with or Q-particle~'~had larger bandgaps than the bulk material (up water content, equivalent weight, and cation form.27 Their theory to 2.76 eV) and were identified with particles with diameters down also predicts that short channels connect adjacent clusters. A study to 50 A. of tracer self-diffusion coefficients for Na+ and Cs+ in Nafion Q-state semiconductors can also be prepared in a variety of by Yeager and Kipling support the Gierke For a fully matrices and media, including micelle microemulsions,I6 zeol- hydrated 1200 equiv wt polymer in the acidic form, the average ite~,~~~'~polymer~,~J~J~ ~olutions,~O-~~ glasses,24 and in Lang- cluster will contain approximately 70 ion-exchange sites and loo0 muir-Blodgett films.25 These methods produce isolated small water molecules in a spherical domain resembling an inverted particles. A key principle in these methods is the use of a matrix micelle with a diameter of about 40 A. As the polymer is de- to arrest Ostwald ripening. The method described here to produce hydrated, the cluster diameter and charge per cluster decreases. even smaller particles is based on several new features. Cd2+is It is believed that the spherical clusters are connected by short exchanged into the Nation membrane in the presence of a diluent channels approximately 10 in diameter in the swollen film.
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