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Langmuir 1997, 13, 3142-3149
Optical and Photochemical Properties of Nonstoichiometric Cadmium Sulfide Nanoparticles: Surface Modification with Copper(II) Ions Alex V. Isarov and John Chrysochoos* Department of Chemistry, The University of Toledo, Toledo, Ohio 43606 Received October 11, 1996. In Final Form: March 12, 1997X Nonstoichiometric cadmium sulfide nanoparticles ([Cd2+]/[S2-] ) 3) in 2-propanol were surface-modified with Cu2+ ions. Addition of copper(II) perchlorate to CdS nanoparticles leads to binding of copper ions onto the surface of the semiconductor, accompanied by rapid reduction of Cu2+ to Cu+, as confirmed by EPR and absorption spectra. Copper(II) perchlorate also quenches the recombination luminescence of CdS nanoparticles effectively. The quenching data obey a static interaction model, which confirms the binding of copper ions onto CdS. The latter was confirmed also by ultrafiltration and ICP spectroscopy. Copper ions bound onto the surface of CdS lead to formation of a new, red-shifted, luminescence band. The maximum of the new band is at 14 700 cm-1 compared to that of the original band at 17 900 cm-1. It is suggested that, at low copper ion concentrations, copper ions bound onto the surface of CdS nanoparticles exist as isolated Cu+ ions. They create a new energy level in the bandgap at about 1.2 eV below the conduction band, which is responsible for the new emission band (14 700 cm-1). Higher copper(II) perchlorate concentrations give rise to the formation of ultrasmall particles of CuxS (x ) 1-2) on the surface of CdS, which eventually lead to precipitation. Both isolated Cu+ ions and ultrasmall particles of CuxS quench the original recombination luminescence of CdS nanoparticles by facilitating e-/h+ nonradiative annihilation. The presence of copper ions bound onto the surface of CdS nanoparticles retard both the photocorrosion of the latter and the photodecomposition of copper(II) tetraphenylporphyrin induced by CdS nanoparticles. Appropriate mechanisms are presented.
1. Introduction Semiconductor nanoparticles have attracted significant interest in the past 10 years due to their potential applications in solar energy conversion, nonlinear optics, and heterogeneous photocatalysis.1 Surface modification of semiconductor nanoparticles may change their optical, chemical, and photocatalytic properties significantly.1i,j In general, surface modification of nanoparticles may lead to the following effects: (i) it may enhance their excitonic and defect emission by blocking nonradiative electron/ hole (e-/h+) recombination defect sites (traps) on the surface of the semiconductor (“activation” of fluorescence),2-4 (ii) it may enhance the photostability of semiconductor nanoparticles,2 (iii) it may create new traps on the surface of the nanoparticle leading to the appearance of new emission bands,5,6 (iv) it may lead to the formation of a layer of another semiconductor on the semiconductor nanoparticle (i.e., CdS/HgS and HgS/CdS,6 X
Abstract published in Advance ACS Abstracts, June 1, 1997.
(1) For reviews, see: (a) Henglein, A. Pure Appl. Chem. 1984, 56, 1215. (b) Brus, L. E. J. Phys. Chem. 1986, 90, 2555. (c) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (d) Henglein, A. Chem. Rev. 1989, 89, 1861. (e) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477. (f) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (g) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (h) Wang, Y. Acc. Chem. Res. 1991, 24, 133. (i) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (j) Kamat, P. V. Chem. Rev. 1993, 93, 267. (k) Wang, Y. Adv. Photochem. 1995, 19, 179. (l) Weller, H.; Eychmu¨ller, A. Adv. Photochem. 1995, 20, 165. (m) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (n) Kamat, P. V. Prog. Inorg. Chem. 1997, 44, 273. (2) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (3) Sooklal, K.; Cullum, B. S.; Angel, S. M.; Murphy, C. J. J. Phys. Chem. 1996, 100, 4551. (4) (a) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (b) O’Neil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356. (c) Johansson, K.; Cowdery, R.; O’Neil, M.; Rehm, J.; McLendon, G.; Marchetti, A.; Whitten, D. G. Isr. J. Chem. 1993, 33, 67. (5) Spanhel, L.; Weller, H.; Fojtik, A.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1987, 91, 88. (6) Ha¨sselbarth, A.; Eychmu¨ller, A.; Eichberger, R.; Giersig, M.; Mews, A.; Weller, H. J. Phys. Chem. 1993, 97, 5333.
S0743-7463(96)00985-7 CCC: $14.00
CdS/ZnS,7 ZnSe/CdSe,8 PbS/CdS,9 TiO2/SnO2 and SnO2/ TiO2,10 CdS/CdSe and CdSe/CdS,11 etc.) and in some cases to the formation of a three-layered structure-quantum dot quantum well like CdS/HgS/CdS,12,13 and (v) it may enhance the selectivity and efficiency of light-induced reactions occurring on the surface of semiconductor nanoparticles.1j,14,15 Surface modification of colloidal metal chalcogenides is commonly attained by the adsorption of metal ions onto the particle surface. The interactions of several cations with the surface of semiconductor nanoparticles, such as Mn,3 Ag,5,15 Zn, Cd, Hg, and Pb (see ref 1l and references therein) have been investigated. Interactions of semiconductor nanoparticles with metal ions can also be used to probe the dynamics of photoinduced interfacial electron transfer (i.e., trivalent lanthanide ions16). The effect of metal ions adsorbed onto the surface of semiconductor nanoparticles may be different from that expected in bulk semiconductors.17 Surface modification of cadmium chalcogenide single crystals with metal ions and in particular its effect upon the steady state photoluminescence of the semiconductor (7) Weller, H.; Koch, U.; Gutie´rrez, M.; Henglein, A. Ber. Bunsenges. Phys. Chem. 1984, 88, 649. (8) Hoener, C. F.; Allan, K. A,; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1992, 96, 3812. (9) (a) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. J. Phys. Chem. 1993, 97, 895. (b) Zhou, H. S.; Sasahara, H.; Honma, I.; Komiyama, H.; Haus, J. W. Chem. Mater. 1994, 6, 1534. (10) Bedja, I.; Kamat, P. V. J. Phys. Chem. 1995, 99, 9182. (11) Tian, Y.; Newton, T.; Kotov, N. A.; Guldi, D. M.; Fendler, J. H. J. Phys. Chem. 1996, 100, 8927. (12) Mews, A.; Eychmu¨ller, A.; Giersig, M.; Schooss, D.; Weller, H. J. Phys. Chem. 1994, 98, 934. (13) Kamalov, V. F.; Little, R.; Logunov, S. L.; El-Sayed, M. A. J. Phys. Chem. 1996, 100, 6381. (14) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (15) Kumar, A.; Kumar, S. Chem. Lett. 1996, 711. (16) (a) Lee, Y. F.; Olshavsky, M.; Chrysochoos, J. J. Less-Common Met. 1989, 148, 259. (b) Chrysochoos, J. J. Lumin. 1991, 48&49, 709. (17) Chandler, R. R.; Coffer, J. L. J. Phys. Chem. 1991, 95, 4.
© 1997 American Chemical Society
Surface Modification with Copper(II) Ions
have been also studied extensively.18,19 It was found that surface modification of CdS and CdSe single crystals by cations, whose sulfides or selenides were less soluble than those of Cd2+, led to an enhancement of the surface recombination velocity.18 In addition, Ellis and coworkers19 have shown that the photoluminescence of cadmium chalcogenides can be changed reversibly by the adsorption and desorption of various molecules. Photoluminescence quenching of semiconductors can be altered significantly by the pretreatment of the surface of the semiconductor under consideration with metal ions19d or metal coordination compounds.19f,h Heterojunctions of Cu2S/CdS have been used in solar cell applications.20,21 Furthermore, copper was found to be an efficient activator of luminescence in II-VI compound-based phosphors.22 In spite of this, very few studies dealing with the interactions of copper ions with CdS nanoparticles have been reported.7,23,24 The present work is devoted to the investigation of the interactions of Cu2+ ions with the surface of nonstoichiometric cadmium sulfide nanoparticles. Such interactions were probed in terms of the effect of surface modification upon the absorption, emission, and EPR spectra, as well as on the photostability and photocatalytic activity of the surface-modified semiconductor colloid. 2. Experimental Section Cadmium sulfide nanoparticles were prepared in 2-propanol at -78 °C, in the absence of any stabilizers, using well-established literature techniques of “arrested precipitation”.25 The preparation was carried out by a rapid mixing of a solution of 1.2 × 10-3 M Cd(ClO4)2‚6H2O in 2-propanol with a freshly prepared solution of Na2S in a mixture of 2-propanol-methanol (6:1) in the presence of an excess of NaOH. Both solutions were precooled and deaerated before mixing. Semiconductor nanoparticles prepared in this way had a [Cd2+]/[S2-] ratio equal to 3 (nonstoichiometric CdS nanoparticles); they were stabilized against precipitation by their electrostatic double layer repulsion.25 Such nanoparticles remained stable for at least a month at 0 °C. Their optical band gap energy (Eg) was calculated from plots of {R(hν)}2 vs hν(eV),26,27 where R(hν) is the absorption coefficient of the edge-to-edge absorption band of CdS nanoparticles, associated to direct transitions in the semiconductor (k ) 0 f k ) 0) and defined by the equation26
R(hν) ) {e2(2m*hm*e/(m*h + m*e))3/2/nch2m*e}(hν - Eg)1/2 (1) where m*h and m*e are the effective masses of the positive hole (18) (a) Benjamin, D.; Huppert, D. J. Phys. Chem. 1988, 92, 4676. (b) Rosenwaks, Y.; Burstein, L.; Shapira, Y.; Huppert, D. J. Phys. Chem. 1990, 94, 6842. (c) Bessler-Podorowski, P.; Huppert, D.; Rosenwaks, Y.; Shapira, Y. J. Phys. Chem. 1991, 95, 4370. (19) (a) Murphy, C. J.; Ellis, A. B. J. Phys. Chem. 1990, 94, 3082. (b) Lisensky, G. C.; Penn, R. L.; Murphy, C. J.; Ellis, A. B. Science 1990, 248, 840. (c) Murphy, C. J.; Ellis, A. B. Polyhedron 1990, 9, 1913. (d) Leung, L. K.; Komplin, N. J.; Ellis, A. B.; Tabatabaie, N. J. Phys. Chem. 1991, 95, 5918. (e) Zhang, J. Z.; Ellis, A. B. J. Phys. Chem. 1992, 96, 2700. (f) Moore, D. E.; Lisensky, G. C.; Ellis, A. B. J. Am. Chem. Soc. 1994, 116, 9487. (g) Kepler, K. D.; Lisensky, G. C.; Patel, M.; Sigworth, L. A.; Ellis, A. B. J. Phys. Chem. 1995, 99, 16011. (h) Moore, D. E.; Meeker, K.; Ellis, A. B. J. Am. Chem. Soc. 1996, 118, 12997. (20) Aperathitis, E.; Bryant, F. J.; Scott, C. G. Solar Cells 1990, 28, 261. (21) Encyclopedia of Semiconductor Technology; Grayson, M., Ed.; John Wiley & Sons: New York, 1984. (22) Chadha, S. S. In Solid State Luminescence. Theory, materials and devices; Kitai, A. H., Ed.; Chapman and Hall: London, 1993; p 159. (23) Ramsden, J. J.; Gra¨tzel, M. J. Chem. Soc., Faraday Trans. 1 1984, 80, 919. (24) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 301. (25) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (26) Pankove, J. J. Optical Properties in Semiconductors; Prentice-Hall: Englewood Cliffs, NJ, 1971. (27) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315.
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Figure 1. Absorption spectra of nonstoichiometric CdS nanoparticles (2 × 10-4 M CdS, 4 × 10-4 M excess Cd2+ and 2 × 10-4 M NaOH in 2-propanol) in the absence and presence of various amounts of Cu(ClO4)2 (as indicated). and the electron, respectively. Values of Eg, obtained at R(hν) f 0, were found to be about 3.05 eV (ranging from 3.0 to 3.1 eV for different preparations). Addition of Cu2+ ions to the colloidal solution of CdS nanoparticles was carried out using a solution of Cu(ClO4)2‚6H2O in 2-propanol. Ultraviolet and visible absorption spectra were recorded with a Hewlett-Packard 8452A diode array spectrophotometer and a Cary 5 spectrophotometer. Luminescence spectra were recorded with an Aminco-Bowman spectrophotofluorimeter, and EPR spectra were recorded using a Bruker ESP 300 E spectrometer. Ultrafiltration of CdS nanoparticles was carried out using inorganic membrane disposable syringe filters (Anotop 10, Whatman, cat. no. 02010H50U) and led to a retention of more than 95% of the nanoparticles, determined by the absorptivity of the filtrate at 360 nm. The determination of the concentration of copper ions in the filtrate was carried out by ICP spectroscopy (Perkin-Elmer plasma II emission spectrometer). Samples of CdS nanoparticles with or without copper ions were irradiated with an UV light source (Hg lamp) through a 10 cm water filter and an Oriel color glass filter (cat. no. 59810, λtrans ) 360 nm). The distance between the lamp and the sample was kept at 45 cm in all experiments. The photochemical transformations of the samples were monitored by UV/vis absorption spectroscopy. Copper(II) tetraphenylporphyrin (Cu(II)TPP), used in this study, was synthesized from purified H2TPP (chlorin free)28 and copper acetate, using standard literature preparation and chromatographic purification methods.29 All other chemicals used were of the highest purity available commercially.
3. Results 3.1. Interaction of CdS Nanoparticles with Copper(II) Ions. Addition of Cu2+ ions (Cu(ClO4)2) to CdS nanoparticles in 2-propanol has a profound effect upon the absorption spectrum of the latter. Figure 1 shows the absorption spectra of nonstoichiometric CdS nanoparticles in the absence and presence of added Cu(ClO4)2, ranging from 3 × 10-6 to 1 × 10-4 M. The absorption spectrum of CdS nanoparticles in the presence of copper(II) perchlorate is characterized by an absorption edge (threshold) shifted to longer wavelengths. The negligible absorptivity of CdS nanoparticles at λ > 450 nm increases considerably in the presence of Cu(ClO4)2. The absorptivity of CdS nanoparticles in the presence of Cu2+ ions becomes significant even at λ > 510 nm, which corresponds to the absorption threshold of bulk CdS (Eg ) 2.4 eV). Therefore, the increase in the absorptivity at longer wavelengths is not attributed to the growth of large CdS particles. The changes observed in the absorption spectra of nonstoichiometric CdS nanoparticles, in the presence of copper(II) perchlorate, are accompanied by quenching of the recombination luminescence of CdS (e-tr/h+tr) (Figure (28) Barnett, G. H.; Hudson, M. F.; Smith, K. M. Tetrahedron Lett. 1973, 2887. (29) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem. 1970, 32, 2443.
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Figure 2. Recombination luminescence spectra of 2 × 10-4 M nonstoichiometric CdS nanoparticles in 2-propanol at the following Cu(ClO4)2 concentrations: (a) 0 M, (b) 1 × 10-6 M, (c) 2 × 10-6 M, (d) 4 × 10-6 M, and (e) 1 × 10-5 M. The excitation wavelength was at 360 nm.
Figure 4. EPR spectra of 1 × 10-3 M Cu(ClO4)2 in 2-propanol before (a) and after (b) addition of 2 × 10-4 M of nonstoichiometric CdS nanoparticles. Spectra were recorded at 77 K.
Figure 3. EPR spectra of 5 × 10-5 M Cu(ClO4)2 in 2-propanol before (a) and after (b) addition of 2 × 10-4 M of nonstoichiometric CdS nanoparticles. Spectra were recorded at 77 K.
2). However, the emission band of CdS nanoparticles in the presence of Cu2+ ions is more complex than the original one. At very low [Cu2+] the luminescence band gets broader and exhibits two distinct components: the original emission of CdS nanoparticles and a red-shifted component. The presence of a new red-shifted component is manifested by the increase of the emission intensity at ν < 15 000 cm-1 after the addition of 1 × 10-6 M Cu2+ ions to CdS nanoparticles. At higher [Cu2+] the red-shifted component dominates at the expense of the emission band of CdS nanoparticles, although the overall emission intensity decreases. This latter red-shifted emission, which is obviously different from the original emission of CdS nanoparticles, is also quenched in the presence of higher concentrations of Cu2+ ions. The oxidation state of copper ions present in the sample (Cu2+/Cu+) was monitored by EPR spectroscopy since Cu2+ is a 3d9 ion (paramagnetic) and Cu+ is a 3d10 ion (diamagnetic). Figures 3 and 4 show the low-temperature (77 K) EPR spectra of copper(II) perchlorate in 2-propanol before and after the addition of CdS nanoparticles. At low concentrations of copper(II) perchlorate (less than 1
× 10-4 M) the EPR signal of Cu2+ ions disappears completely in the presence of CdS nanoparticles (Figure 3). At higher copper concentrations, a weaker Cu2+ EPR signal was observed in the presence of CdS (Figure 4). The disappearance or reduction of the EPR signal is most likely due to the reduction of Cu2+ to Cu+, since the latter ion is diamagnetic (3d10). An alternative explanation of the loss of the EPR signal is based on the decrease of the relaxation time T2 of Cu2+ upon adsorption onto the surface of CdS nanoparticles. This possibility will be discussed later on. To confirm the oxidation state of copper ions adsorbed onto CdS nanoparticles, we attempted to monitor the red and near-IR (600-1400 nm) absorption band of Cu2+, which is attributed to d-d transitions in Cu2+ ions (3d9). We could not detect this band in 1 × 10-4 M Cu2+ adsorbed onto 2 × 10-4 M CdS. However, a copper sulfide sol (5 × 10-4 M) in which approximately 25% of copper existed as Cu2+ ions (i.e. about 1.25 × 10-4 M) led to a broad red absorption band with λmax ) 920 nm.30 Therefore, a red absorption band should have been detectable in 1 × 10-4 M Cu2+ ions adsorbed onto CdS nanoparticles, particularly in the spectra recorded with a 5.0 cm pathlength. The spectral changes mentioned above occur quickly. Higher absorptivity, luminescence quenching, and reduction of Cu2+ to Cu + resulted immediately following the addition of copper(II) perchlorate into the solution of CdS nanoparticles. Further transformations of the sample took place more slowly, and they were found to depend on the concentration of copper(II) perchlorate added. The addition of 1 × 10-6 M to 1 × 10-5 M of copper(II) perchlorate appears to increase the stability of CdS nanoparticles stored at room temperature. On the other hand, copper(II) concentrations higher than 1 × 10-4 M led to the precipitation of CdS nanoparticles within 3-4 h after preparation. To determine the amount of copper adsorbed onto the surface of CdS nanoparticles, the samples were subjected to ultrafiltration through disposable membrane filters (30) Silvester, E. J.; Grieser, F.; Sexton, B. A.; Healy, T. W. Langmuir 1991, 7, 2917.
Surface Modification with Copper(II) Ions
Figure 5. Recombination luminescence spectrum of 2 × 10-4 M nonstoichiometric CdS nanoparticles in 2-propanol (λexc ) 360 nm). The points represent experimental data, corrected for the photomultiplier tube response; the dotted line represents a computer-generated Gaussian line shape fit. Table 1. Concentration of Copper Ions (M) in the Presence of 2 × 10-4 M CdS Nanoparticles in 2-Propanol before and after Ultrafiltration 2 × 10-5 4 × 10-5 6 × 10-5 before 1 × 10-5 ultrafiltration after
