5382
J. Phys. Chem. 1991,95, 5382-5384
utm value, ca. 2095 cm-I, for Pt( 1 1l)/CO(O,
0.6-0.7) at 300
K can be deduced from data in ref 11. The corresponding frequency comparison for the bridging up feature is complicated by the observed coverage sensitivity of vc0 at high 0 , in the uhv system.'Ib Nevertheless, the observed electrochemical uto value at 1.0 V vs SCE, 1865-1875 cm-' (Figure 3), is comparable to (within ca. 15 cm-' of) those obtained for the uhv system.11bThe ratio of integrated absorbances (AI/AF) of the terminal and bridging features in the Pt(1 11)-uhv system, ca. 6, is also roughly comparable to those observed for the electrochemical interfaces, ca. 5-7, at 1.O V (Table I). The bandwidths of the terminal vco feature at the R ( l 1 l)-solution interfaces, 11-14 cm-l (Table I), are nonetheless significantly larger than that obtained, ca. 7 cm-I, for the uhv system at 300 K.II' This difference most likely results from line-broadening effects associated with local solvation. The effect of varying the supporting-electrolyte cation, most simply TBA+ vs TEA+ in acetonitrile (Figure 4), can be understood in terms of a simple double-layer model. In the expected absence of a marked net orientation of interfacial solvent dipoles in the presence of saturated CO adlayers, the potential of zero charge (E,) of the Pt(1 1 1)/CO-nonaqueous interfaces should be close to that for the corresponding uhv surface, 1.O V vs SCE, since the latter interface is necessarily uncharged. To the extent that the observed spectral differences between TBA+ and TEA+ are due to variations in electrostatic field, this cation effect should eventually disappear at E, by ca. 1.0 V. Inspection of Figure 4 shows that this is indeed the case. Moreover, the progressively greater .bo discrimination observed in TBA+ and TEA+ as the potential decreases below 1.0 V, Le., the dissimilarities in the &,-E slopes (Figure 4, Table I), can be understood simply in terms of the differences in average electrostatic field resulting from the unequal crystallographicradii, r, of TBA+ (4.95 A) and TEA+ (4.0 A).Lz This field is induced ( I 1) (a) Schweizer, E.; Persson, B. N.J.; Tiishaus, M.; Hoge, D.; Brad. clhaw, A. M. Sur/. Sd. 1989, 213,49. (b) Penson, B. N.J.; Tiishaus, M.; Bradshaw, A. M. J . Chem. Phys. 1990,92,5034.
by the negative electrode charge and accompanying cationic charge, located largely at the outer Helmholtz plane (OH ) We take the 'thickness" of the CO adlayer, d m , to be 3.1 and assume the oHp is located at a distance (dm + rc)from the metal surface. If the electrostatic field is constant across this inner layer, and the CO molecules sense only the portion of the potential drop, then the variation of dvbo/dE with the electrolyte cation will be proportional to (dco re)-l. The ratio of dvco/dE values for TEA+ vs TBA+ predicted on this basis, 1.13, is in reasonable accordance with the experimental dvbo/dE ratios in acetonitrile, 1.1-1.15 (Table I). The same argument can account semiquantitativelyfor the larger dvko/dE values observed in water containing HClO, vs TEAP electrolytes (Figure 4), given the smaller hydrated radius of H30+.l2 The experimental findings, however, do not impact the muchdiscussed issue of the relative importance of potential-induced surfaceadsorbate charge sharing as distinct from electrostatic field effects upon the vc0 freq~encies.'~J~ Rather, they simply imply that dvco/dE (and hence dvco/d#M) is affected primarily by the component of 4M-Slying geometrically within the CO layer. Overall, then, the present findings provide unexpectedly straightforward support to the notion, espoused elsewhere," that a major and even predominant effect upon the structure of saturated CO layers is provided by the average surface potential drop. Such infrared spectral measurements for a wider range of ordered metal-nonaqueous-electrolyte systems are currently being pursued in our laboratory.
ll;
+
Acknowledgment. This work is supported by the National Science Foundation and the Office of Naval Research. (12) For a compilation, sce: Nightingale, Jr., E. R. J. Phys. Chem. 1959, 63, 1381. (13) This estimate of d,, from the edge of the Pt(ll1) surface plane to the periphery of the CO oxygen atom, is deduced by assuming PtC and C-O bond lengths of 1.9 and 1.2 A, a Pt atomic radius of 1.4 A, and an oxygen van der Waals radius of 1.4 A. (14) For example, sce: (a) Lambert, D. K. J. Chem. Phys. 1988,89,3847. (b) Anderson, A. B. J . Electroanal. Chem. 1990, 280, 37.
GaAs Nanocrystals Prepared in Qukrdine Hiroyuki UcYda,**tCalvin J. Curtis,* and Arthur J. Nozik* Solar Energy Research Institute, Golden, Colorado 80401 -3393 (Received: April 9, 1991; In Final Form: May 17, 1991)
GaAs nanocrystals were prepared in quinoline according to procedures previously published in the literature. However, we find that the optical absorption and photoluminence properties of GaAs colloids that we prepared in quinoline are dominated by molecular species that mask the optical properties of the GaAs nanocrystals. These species appear to be present both in solution and on the surface of the GaAs nanocrystals; they have not been identified but are believed to consist of quinoline oligomers and/or Ga-quinoline complexes.
Very interesting quantization effects in semiconductors studied in the context of photoelectrochemistry have been reported for quantum confinement in both one dimension (superlattices, multiple-quantum wells, and single-quantum wells)' and three dimensions (colloidal particles)? In particular, onedimensional GaAs quantum wells (labeled here as quantum films) have been studied extensively.'f One important effect that occurs with GaAs quantum films is the large reduction in the cooling rate of photogenerated hot electrons;' this effect has important implications for hot electron transfer processes in quantum well photoelec'Current address: Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565. Japan,
0022-3654/91/2095-5382S02.50/0
trodes.ld4 It is anticipated that the cooling rates of hot carriers in quantized particles will be wen slower than in quantum films.$ (1) (a) Nozik,A. J.; Thacker, B. R.; Turner, J. A.; Klem, J.; Morkoc, H.
Appl. Phys. Len. 1987, 50, 34. (b) Nozik, A. J.; Thacker, E. R.; Turner, J. A.; Peterson, M. W. J . Am. Chem. Soc. 1988,110,7630. (c) Nozik, A. J.; Tumer, J. A.; Peterson, M.W. J. Phys. Chem. 1988,92,2493. (d) Peterson,
M. W.; Turner, J. A.; Parsons, C. A.; Nozik, A. J.; Arcnt, D. J.; Van Hoof, C.; G.; H&, R.; Mor&, H. Appl. Phys. Lett. 1968,53,2666. (e) Zuhoski, S.P.; Johnson, P. B.; Ellis, A. B.; Biefeld, R. M.; Ginley, D. S.J . Phys. Chem. 1988,92,3961. (f) Parsons, C. A,; Peterson, M. W.; Thacker, B. R.; Turner, J. A,; Nozik, A. J. Ibid. 1990,91, 3381. (8) Parsons, C. A.; Thacker, B. R.; Szmyd, D. M.; Peterson, M. W.; McMahon, W. E.; Nozik, A. J. J . Chem. Phys. 1990, 93,7706.
@I 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5383
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Wavelength (nm) Figure 2. Optical abwrption spectra of solutions prepared in some control experiments (optical path length = 2 mm). Solid line: quinoline solution of GaC13 refluxed for 1 day. Ga concentration determined by atomic absorption was 0.53 mM. Dotted line: I-pm filtrate of bulk GaAs powder (0.5 mmol)-quinoline (20 mL) suspension after heating a t 315 OC for 40 h.
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films and particles, makes the preparation of quantized GaAs particles a very desirable objective. However, initial work has shown that it is very difficult to prepare monodispersed quantized GaAs particles that can be well characterized$*' Recently, Alivisatos and co-workers have reported* succtss in the synthesis of well-characterized GaAs nanocrystals in quinoline using a reaction developed by Wells et al.;9 this work represents the fmt preparation of redissolvable GaAs nanocrystals. However, we find that while GaCl, and As(SiMe&, indeed, react in quinoline at the reflux temperature to give GaAs nanocrystals, we also find that such colloids contain chemical species that interfere strongly with the optical measurements and seriously cloud their interpretation. Here, we report some additional properties of GaAs colloidal sols prepared in quinoline. By use of the same chemicals and the same methods employed in the literature? a mixture containing 6.5 mmol of GaCl, and 6.5 mmol of As(SiMe,), in 50 mL of quinoline was heated at reflux for 3 days. The resulting raw colloid was a turbid, dark red solution; a transparent reddish-brown solution was obtained at lower reactant concentration (ca. 0.3 mmol in 50 mL). We confirm the formation of GaAs nanocrystals in colloids prepared in quinoline using various recipes by transmission electron microscopy (TEM) and electron diffraction data; however, we find that the optical absorption characteristics of the resulting sols were the same irrespective of the reaction conditions. A series of ultrafiltrations of the GaAs colloids was performed under nitrogen atmosphere using Spectra/Por Type F (poly(vinylidene fluoride)) filters with molecular weight cutoffs (MWCO) of 5000 (average pore size d 15 A), 100O00 (d 100 A), and 1000000 (d 1 pm).Io The optical absorption spectra of the filtrates are shown in Figure la. The filtrates through the 1-pm and l W A pore-size filters exhibit similar absorption spectra that consist of a structured spectrum with two clear peaks observed at about 470 and 500 nm and a small shoulder at about 550 nm. The spectrum for the 15-A filtrate had less resolvable structure, but its general shape was similar to the other two filtrates. Judging from the particle size observed by TEM, the initial colloid
w5*pa" 4 750
575
Wavelength (nm) Figure 1. (a) Optical absorption spectra of a series of filtrates of GaAs colloids prepared in quinoline (optical path length = 2 mm): solid line, the raw colloid diluted to 1 / 5 was passed throu h a I-pm filter; dotted line, the I-pm filtrate was p a a d through a IO-! filter; dashed line, the 100-A filtrate was passed through a 15-A filter. (b) Difference absorption spectra of series of filtrates: difference between I-pm and 15-A filtrates (solid line), 100- and 15-A filtrates (dotted line), and I-pm and 100-A filtrates (dashed line). (c) Emission spectra for the various filtrates described above with excitation at 385 nm.
This prediction, as well as other important comparisons between the photophysical and photochemical properties of GaAs quantum (2) (a) Brus, L.IEEE J . Quantum Electron. 1986, QE-22, 1909; Noun J . Chim. lW, 11,123. (b) Stcigmald, M. L.; Brus, L. E. Annu. RN.Mater. Sci. 1989,19,471. (c) Henglein, A. Top. Curr. Chcm. 1988, 143, 115. (d) Williams, F.; Nozik, A. J. Nature 1984, 311,21. Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. J . Phys. Chem. 1985, 89, 397. (e) Nedeljkovic, J. M.;Nenadovic, M. T.; Micic, 0.I.; Nozik, A. J. Ibid. 1986, 90,12. (f) Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. lW4,80,4464. Rossetti, R.; Hull, R.; Gibson, J. M.; Brm, L. E. Ibid. 1985,82,552. (e) Peterson, M. W.; Nenadovic, M. T.; Rajh, T.; Herak, R.; Micic, 0. I.; Goral, J. P.; Nodk, A. J. J , Phys. Chcm. 1988,92, 1400. Rajh, T.; Vucemilovic, M. I.; Dimitrijevic, N. M.; Micic, 0. I.; Nozik, A. J. Chem. Phys. Lett. 1988, 143, 305. (h) Fojtik, A,; Weller, H.; Koch, V.; Hcnglein, A. Ber. Bunsen-Gcs. Phys. Chem. 1984,88,969. Fischer, C. H.; Weller, H.; Fojtik, A.; Lume-Pereira, C.; Janata, E. Ibid. 1986, 90,46. Henglein, A.; Fojtik, A.; Wellcr, H. Ibid. 1987, 91, 441. Wang, Y.; Herron, N. J . Phys. Chem. 1987, 91, 257. (i) Wang, Y.; Herron, N. Ibid. 1988, 92, 4988. Hilinski, E. F.; Lucas, P. A. J. Chem. Phys. 1988,89, 3435. Dannhauser, T.; ONeil, M.;Johansson, K.;Whitten, D.; McLendon, 0 . J . Phys. Chem. 1986,90,6074. (k) K o m n n , C.; Bahnemann, D. W.; Hoffmann. M. R. Ibid. 1988,92,5196; 1987, 91,3789. (I) Yoneyama, H.; Haga, S.; Yamanaka, S . Ibid. 1989, 93,4833. (m)Miyoshi, H.; N i p p , S.;Uchida, H.; Yoneyama, H. Bull. Chem. Soc. Jpn. 1990,63, 3380. (n) Uchida, H.; Ogata, T.; Yoneyama, H. Chem. Phys. Lett. 1990,173, 103. ( 0 ) Alivisatos, A. P.; Hams, A. L.;Levinos, N. J.; Steigerwald, M. L.; Brus, L. E. J . Chem. Phys. 1988, 89,4001. (p) Alivisatos, A. P.; Hams, T.D.; Jagaramany, A. Ibid. 1988,89, 5979. (3) (a) Dingle, R., Ed. Semlconducrors and Scmlmctals; Academic Pres: New York, 1987; Vol. 24. (b) J a m , M. Physics und Applications ofSemiconductor Microstructures; oxford University Press: Oxford, 1989. (c) IEEE 1. Quantum Electron. 1986. QE-22, 161 1-1920.
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(4) (a) Xu, Z. Y.;Tang, C. L. Appl. Phys. Lett. 1984, 44, 692. (b) Edelstcin, D. C.; Tang, C. L.; Nozik, A. J. Ibid. 1987,51, 48. (c) Nozik, A. J.; Parsons, C. A.; Dunlavy, D.; Keycs, B.; Ahrenkiel, R. Solid State Commun. 1990, 75,297. (d) Uchiki, H.; Arakawa: Y.; Sakag, H.; Kobayashi. T. Ibid. 1985,55,31 1 . (e) Uchiki, H.; Kobayashi, T.; Sakah, H.J . Appl. Phys. 1987, 62, 1010. (f) Tatham, M.; Taylor, R. A.; Ryan, J. F.;Wang, W. I.; Foxon, C. T. Solid-Stare Electron. 1988, 31, 459. (5) Nozik, A. J. To be published. (6) Byrne. E. K.; Parkanyi, L.; Theopold, K. H. Science 1988,241, 332. (7) Sandroff, C. J.; Harbison, J. P.; Ramesh, R.; Andrejcvo, M.J.; Hegde, M.S.;Hwang, D. M.;Chang, C. C.; Vogel, E. M. Science 1989, 245, 391. (8) Olshavsky. M. A.: Goldstein, A. N.: Alivisatos, A. P. J. Am. Chem. s&'.im,112,-9438. (9) Wells, R. L.: Pitt, C. 0.;McOhail, A. T.; Purdy, A. P.; ShaAcczad, S.;Hallock, R. B. Chem. Muter. 1989, 1, 4. (IO) Peterson, M.W.; Micic, 0.I.; Nozik, A. J. J. Phys. Chcm. 1988, 92, 4160.
5384 The Journal of Physical Chemistry, Vol. 95. No. 14, 1991
x.
reparation contained very few GaAs particles smaller than 20 Hence, the appreciable absorption displayed by the 15-A filtrate (where, essentially, no particles are present) suggests that molecular species are present in the original colloid and are responsible for a substantial part of the optical absorption. Furthermore, with decreasing average pore size of the filters, the spectra are not blue-shifted and their difference spectra (Figure lb) are quite similar in shape. Emission spectra (excitation at 385 and 500 nm) were also obtained for the three filtrates described above; all of these emission spectra were very similar (see Figure IC). Both the absorption and emission spectra of the filtrates are inconsistent with size quantization effects. Thus, although physical characteristics of the sols by TEM and electron diffraction confirm the presence of quantized GaAs nanocrystals, their optical properties are masked by molecular entities. For a blank experiment, GaC13was heated in quinoline to reflux without adding A S ( S ~ M The ~ ~ )yellow ~ color of the GaCI3blank solution turned to a clear, dark red color at reflux; the resulting spectrum (solid line in Figure 2) is exactly the same as that of the GaAs-quinoline colloid shown in Figure 1. The emission spectrum of the blank (not shown) was also the same as in Figure IC. When quinoline was heated at reflux without Ga present, a red color did not develop. Our results indicate that Ga(II1) and quinoline interact at reflux temperatures to produce a molecular species. Since GaC13 is a strong Lewis acid, two possibilities for molecular species can be anticipated; quinoline oligomers and Ga(1II)quinoline complex(ts). Ga(II1) may act as a catalyst to promote the formation of quinoline oligomers (which are reported to be red”); however, the polymerization of pure quinoline is also reported to be rather difficuIt.l2 On the other hand, pure or substituted quinolines are good complexing agents for Ga(III);”J‘ for example, 8hydroxyquinoline forms a chelating complex with Ga(II1) at a ratio of 1:3. Although, to our knowledge, the properties of Ga(II1) complexes formed by refluxing in e x m s quinoline have not been reported, we suggest that quinoline may form some chelating complex(es) with Ga(II1) since GaCIJ may be a catalyst for electrophilic substitution reactions on quinoline. We do not know at the present time whether quinoline oligomers are produced or ( I 1) Brennan, J. G. Private annmunication. (12) (a) Chiang, L. Y.;Swirczewski, J. W.J. Chem. Soc., Chem. Commun. 1991. 131. Ib) Chiann. L. Y.: Chianelli. R. R. I6ld. 1986. 1461. IC) Chiang, L.’Y. I6fd. 1987,3&. (d) Chiang, L.‘Y.; Stoked, J. P.;Johnson,D. C.; Goshom, D. P. Synth. Met. 1989, B, E483. (13) Patel, S. J.; Tuck, D. G. Can. J . Chem. 1969, 47, 229. (l4j (a) Biryuk, E. A.; Raviukaya, R.V. Zh. Anal. Khim. 1971,26,1752. (b) Bankoakis, J.; Krasmka, M.;Cera, L.;Lejcjs, J.; W i a . R. Law. PSR Zfnar. Akud. Vestis, Kfm. Ser. 1971,742. (c) Dymock, K.; Palenik, Gus J. J. Chem. Soc., Chem. CM”.1973,22, 884. (d) Ambulkcr, R. S.;Munahi, K. N. J . Indfun Chem. Soc. 1975,52,315. (e) Watanabe, K.; Kawagaki, K. Bull. Chem. Soc. Jpn. 1975, 48, 1812. (0 Akimov, V. M.;Busev, A. I.; Zarorina, E. V. Tr. Vses. Konf. Anal. Khfm. Neuodnykh Rasruora, Ikh Ffz.-Khfm.Suofsruam, 3rd 1974, 35. (g) Lctkeman, P.; Martell, A. E.; Motekaitb, R.J. J . Coord. Chem. 1%0, 10, 47. (h) Pcscher-Cluzeau, Y.; Desbarres, J.; Bauer, D. Soluenr Extr. Ion Exch. 1986, 4, 301.
Letters whether a Ga-quinoline complex(es) is formed. However, we believe that the optical properties of the GaAs colloids formed in quinoline solutions using the present synthesis are dominated by molecular species and not by GaAs nanocrystals. The results shown in Figures 1 and 2 also suggest that, in addition to the presence of molecular species in solution, the GaAs particles are also capped with these red entities. The strong interaction between Ga and quinoline was confirmed by other experiments. For example, the same red species were produced by heating bulk GaAs powder (99.9% average size 2.7 pm, purchased from CERAC) in quinoline at reflux or at higher temperature (315 “C) in a pressurized vessel. The absorption spectrum of this sample after 1-pm filtration is shown in Figure 2. Since particles smaller than 1 pm were not observed by TEM, the interaction of quinoline at surface Ga sites to form either Ga-quinoline complexes or polyquinoline is believed to be responsible for the formation of the colored species in this case. Reddish-purple powders were isolated from the original raw colloids either by removal of the solvent9 (powder A) or by filtration with a medium-porosity glass filter, followed by washing with fresh quinoline (powder B). Powder A was redissolvable in pyridine and quinoline9and produced the same absorption spectrum as Figure 2. Powder B was less soluble, probably due to loss of the capping species by washing. This material was partially soluble in quinoline when heated at reflux and showed spherical GaAs particles ranging from 30 to 200 A in diameter by TEM. However, the ultrafiltration experiments on these colloids also indicated that the GaAs particles obtained were covered with the same red molecular species. The red capping species on the powder surface were easily removed by sublimation at about 200 “C under vacuum, resulting in black GaAs powder with no solubility. In conclusion, we find that quinoline has a strong tendency to interact with Ga(II1) to form polyquinoline and/or Ga-quinoline complexes. These molecular species appear to be present both in solution and as capping moieties on the surface of the GaAs particles; the solubility of the GaAs nanocrystals may depend on the presence of the capping species. We believe the red molecular entities formed in the presence of Ga(II1) and quinoline dominate the optical properties of GaAs particles preserved in quinoline, and therefore, care must be taken in interpreting optical spectra. We are now investigating the optical behavior of GaAs nanocrystals prepared in other media, as well as in solid matrices.
Acknowledgmenr. We thank Kim Jones and John Goral for obtaining TEM pictures of the colloids and electron diffraction data and Dean Levi,John Connolly, and John Baker for obtaining emission spectra; we also acknowledge useful discussions with M. W. Peterson. We very much appreciate Prof. A. P. Alivisatos sending us preprints of his recent work. Hiroyuki Uchida was supported by the US.-Japan Cooperative Program on Photoconversion and Photosynthesis Research. Calvin Curtis was supported by the SERI Director’s Development Fund, and A. J. Nozik was funded by the US. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences.
