J. Phys. Chem. 1996, 100, 8927-8939
8927
Coupled Composite CdS-CdSe and Core-Shell Types of (CdS)CdSe and (CdSe)CdS Nanoparticles Yongchi Tian, Theresa Newton, Nicholas A. Kotov, Dirk M. Guldi, and Janos H. Fendler* Department of Chemistry, Syracuse UniVersity, Syracuse, New York 13244-4100 ReceiVed: July 14, 1995; In Final Form: NoVember 18, 1995X
Addition of H2S and H2Se, in different proportions, to aqueous Cd(ClO4)2 solutions containing (NaPO3)6 led to the formation of coupled composite CdS-CdSe nanoparticles. Conversely, the sequential introduction of different amounts of H2S and H2Se (or H2Se and H2S) into aqueous Cd(ClO4)2 solutions containing (NaPO3)6 produced core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles. These structural assignments were based on spectroscopic (absorption, transmission, excitation, and emission), electrochemical, pulse radiolytic, and electron microscopic measurements. Conditions were adjusted (by adding excess cadmium and hydroxide ions) for the maximization of excitonic fluorescence. Two emission bands were observed in the coupled and in the core-shell type mixed semiconductor nanoparticles. The first one, centered around 470 nm, was attributed to the 1se-1sh excitonic emission of CdS. The second, centered around 560 nm, was proposed to arise from charge transfer of CdS to the coupled composite CdS-CdSe and core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles.
Introduction The beneficial electrical, optical, and electrooptical properties of semiconductor nanoparticles and nanoparticulate films have prompted vigorous activities in this area.1 Innovative chemical,2 colloid chemical,1-3 and electrochemical4-6 methods have been developed for the preparation of relatively monodispersed semiconductor nanoparticles and nanoparticulate films.1,2 Particularly significant has been the reported preparation of such size-quantized composite semiconductors as CdS/Cd(OH)2,7 CdSx/CdSe1-x,8 CdS/Te1-x,8 CdSex/Te1-x,8 CdS/TiO2,9,10 TiO2/ ZnO,11 ZnS/CdSe,12 ZnSe/CdSe,13 CdS/PbS,14 CdS/ZnS,15 TiO2/ Cd3P2,16 CdS/HgS,17 and CdS/HgS/CdS,18 which often demonstrated excitonic fluorescence and interparticle electron transfer. Preparation and characterization of CdS-CdSe nanoparticles are the subject of the present report. Similarities between the CdS and CdSe lattices and the suitability of their bandgaps for superlattice construction19 dictated the choice of this semiconductor pair for the present investigation. By paying careful attention to the experimental conditions, it has been possible to prepare coupled composite CdS-CdSe and core-shell type [(CdS)CdSe and (CdSe)CdS] nanoparticles. The nanoparticles prepared here have been characterized by their absorption, transmission, and emission spectra as well as by electrochemical, pulse radiolytic, and electron microscopic measurements. Special attention has been paid to excitonic fluorescence since it provides information on the surface states of the mixed nanoparticles. Experimental Section 1. Materials. Cadmium perchlorate (Cd(ClO4)2, Alfa), sodium hexametaphosphate ((NaPO3)6, Aldrich, 96%), hydrogen sulfide (H2S, Matheson, 99.5%), hydrogen selenide (H2Se, Matheson, 99.5%), and sodium hydroxide (NaOH, Aldrich, 99.99%) were used as received. Water was purified by a Millipore Milli-Q system, containing a 0.22 µm Millistack filter at the outlet. 2. Preparation of Coupled Composite CdS-CdSe and Core-Shell Types of (CdS)CdSe and (CdSe)CdS Nanoparticles. Four different types (I, II, III, and IV) of mixed CdSX
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)01965-4 CCC: $12.00
CdSe nanoparticles were prepared in a 2.0 L, three-necked, round-bottomed flask, equipped with a pH electrode and a gas inlet-outlet system through a septum. All four preparations commenced with the bubbling of a 1.0 L aqueous 2.0 × 10-4 M Cd(ClO4)2 and 2.0 × 10-4 M (NaPO3)6 solution by highpurity argon (Cryogas Gas Corp.) for about 20 min and by adjusting its pH to 9.1 () starting pH) by aqueous 1.0 M NaOH. The resultant solution is hereinafter referred to as the stock solution. The injection of stoichiometric amounts of H2S and H2Se gases, premixed in a syringe, to the stock solution under vigorous stirring constituted the type I preparation. Precipitation of the mixed CdS-CdSe colloids, depending on the H2S and H2Se ratio (v/v), was completed within 5-20 min (monitored by absorption spectrophotometry). The pH of the dispersion (∼4.5 after the completion of the reaction) was raised to 11.0 by the addition of aqueous 1.0 M NaOH. Aqueous 1.0 M Cd(ClO4)2 solution was then added dropwise to the dispersion until the fluorescence emission of the semiconductor nanoparticles increased to its maximum value. Type II preparation involved the sequential addition of stoichiometric amounts of H2S and H2Se (or H2Se and H2S) to the stock solution. Before the addition of the second gas, the dispersion was allowed to incubate at room temperature for different lengths of time. Aqueous 1.0 M Cd(ClO4)2 solution was then added dropwise to the dispersion until the fluorescence emission of the semiconductor nanoparticles increased to its maximum value. In the type III preparation, the 1.0 L stock solution was divided into two 500 mL portions. CdS nanoparticles were made in the first portion by the addition of stoichiometric amounts of H2S. Additional H2S was added then stepwise until the development of CdS absorption leveled off (ca. 10% excess of the stoichiometric amount of Cd2+) to ensure the exhaustion of all Cd2+ ions present at this point. Unreacted H2S was removed by extensive (1 h) argon bubbling. The pH was adjusted to 11, and aqueous 1.0 M Cd(ClO4)2 solution was added dropwise to the dispersion until the fluorescence emission of the semiconductor nanoparticles increased to its maximum value. © 1996 American Chemical Society
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This 500 mL fluorescent CdS dispersion was added to the second 500 mL portion of the stock solution. This dilution resulted in a substantial (ca. 40%) reduction of the fluorescence intensity of the CdS nanoparticles. Stepwise addition of stoichiometric amounts of H2Se, monitored by absorption and emission spectroscopic measurements, was expected to result in the formation of CdSe-coated CdS nanoparticles. Aqueous 1.0 M Cd(ClO4)2 solution was then added dropwise to the dispersion until the fluorescence emission of the semiconductor nanoparticles increased to its maximum value. Type IV preparation was identical to that described for the formation of CdSe-coated CdS nanoparticles (type III preparation), except that H2Se was added first and H2S second. Type IV preparation was expected to produce CdS-coated CdSe nanoparticles. Aqueous 1.0 M Cd(ClO4)2 solution was then added dropwise to the dispersion until the fluorescence emission of the semiconductor nanoparticles increased to its maximum value. 3. Steady-State Spectroscopy. Absorption spectra were taken on a Hewlett-Packard 8452A diode array spectrophotometer. Excitation and emission spectra were taken on a Spex fluorolog instrument equipped with a Tracor Northern TN 6500 rapid scan spectrometer detection system. Emission spectra were corrected for excitation intensity fluctuation. Corrected excitation spectra were obtained by measuring the intensity of the fluorescence emission (If) at a certain wavelength as a function of the excitation wavelength. The excitation light beam was split by a quartz plate, and one portion of it was allowed to impinge onto a quantum counter (using a 2.0 M ethylene glycol solution of Rhodamine B as the energy converter; the intensity, Ii, was monitored by a Hamamatsu R508 photomultiplier).20 The corrected excitation spectra, S(λ), was obtained by substituting the observed values into eq 1:
S(λ) ) If(λ)/I0(λ)
(1)
where I0 ) Ii × 10-AL, A is the absorbance at the given wavelength, and L is the optical path length of the sample. 4. Fluorescence Lifetime Measurements. Fluorescence decay times were determined by means of a Hamamatsu C4334 streak camera system. Samples were excited by 355 nm, 3.5 pJ, 500 ps laser pulses at 5 kHz, generated by a Quantronix 4000 Nd:YAG (Q-switched, mode-locked, pulse-picked, and frequency-tripled by a BBO crystal), and the decay data were fed into a personal computer and fitted to the two-parameter Kohlraush equation:21
I(t) ) I0 exp[-(t/τ)β]
(2)
where I0 is the fluorescence intensity at t ) 0 and the parameter β (0 < β < 1) is related to a distribution of decay times that are serially linked to the lifetime (τ), which represents the peak of distribution. 5. Transmission Electron Microscopy. Samples for transmission electron microscopy (TEM) were prepared by dropping a small drop of the semiconductor nanoparticle dispersion onto a carbon-coated copper grid and allowing the water to evaporate. The TEM images were taken on a JEOL 2000 FX transmission electron microscope operating at 200 kV. 6. Cyclic Voltammetry. Cyclic voltammetry was performed in a three-electrode cell by means of an EG&G Princeton Applied Research potentiostat/galvanostat (Model 273 interfaced with an IBM PC-XT computer). Silver and platinum/iridinium wires (Aldrich) were used as the working and counter electrodes, respectively. A standard saturated calomel reference electrode (Fisher) was employed. Cyclic voltammetric (CV) measure-
ments of the aqueous nanoparticle dispersions were performed without any additional supporting electrolyte since the ionic stabilizers and counterions provided sufficient conductivities. Indeed, addition of 1.0 × 10-2 M NaCl (as a supporting electrolyte) was shown not to alter the important features of the CV curves. The working silver electrode was cleaned between experiments by a fine sandpaper, then thoroughly polished with a mild abrasive (paper), and finally rinsed by water and ethanol. CV measurements were taken in air-saturated and deoxygenated solutions at room temperatures. The potentials are reported with respect to SCE. CdS (or CdSe) nanoparticles were found to attach themselves, upon scanning the potential, to the silver electrode, forming a thin film which could be characterized by cyclic voltammetry in the semiconductor nanoparticle dispersions in an electrochemical cell. The cyclic voltammograms in the nanoparticle dispersions revealed the presence of sharp peaks at -1.15 ( 0.3 V for CdS and -1.54 ( 0.05 V for CdSe and at various potentials for mixed CdS(CdSe) nanoparticles. The scanning of the potential was performed in a cyclic manner until the peak reached the ultimate height (12-15 cycles for CdS and 20-30 cycles for CdSe). Characteristics of the peaks were found to depend on the size of the particles. They shifted to more positive potentials and broadened as the particle sizes increased. Importantly, cyclic voltammograms remained observable even subsequent to transferring the electrodes into an aqueous 0.01 M NaCl solution. Polishing the silver electrode resulted in the removal of the semiconductor coating and thus in the disappearance of the cyclic voltammograms due to the nanoparticles. The nature of the current responsible for the CV peaks was investigated by electrochemical means. The dependence of the height of the peaks on the scanning speed revealed the faradaic and diffusion-controlled character of the observed current. The nonfaradaic contribution, that is, charging at the interface and the accumulation of electrons on the attached semiconductor nanoparticles, can be neglected due to the slow (20 mV/s) scan speed. Noble metals have a broad polarization window, i.e., the range of electrode potentials for which the current passing through the electrode is small and independent of the potential. Once the semiconductor particles form a film onto the silver electrode, a number of processes (such as catalytic reduction of water, cadmium, and polyphosphate ions) occur at the newly formed semiconductor interface. Regardless of the type of reactions, redox process on nanoparticles can occur only if the electrode potential reaches the lowest unoccupied energy level of nanoparticles (conduction band) from which electrons can be passed to the solution. If the electrode potential does not reach this level, electrons will reside predominantly on the metal, and no reduction of the nanoparticle surface will occur. At this point it is essential to note that for bulk semiconductor electrodes the position of the lower edge of the conduction band (on absolute energy scale) is determined by the crystal structure inside the semiconductor film and the electron potential. The position of the conduction band in bulk may be shifted up or down by applying a voltage, whereas the energy of the conduction band at the interface is pinned by the surface states and does not change with alteration of the electrode potential. This results in band bending near the surface of bulk semiconductor electrodes. The region of band bending extends 50500 nm inside the semiconductor electrode. Hence, in 3-10 nm diameter nanoparticles the positions of energy levels are determined only by the interface and the structure of the surface. This conclusion represents the key difference between bulk and nanoparticle electrochemistry. Consequently, nanoparticles are
Composite CdS-CdSe Nanoparticles
J. Phys. Chem., Vol. 100, No. 21, 1996 8929
very sensitive to the structure of the particle solution interface and to the composition of their surface states. 7. Pulse Radiolysis. Dispersions containing (0.5-4.0) × 10-7 M22 coprecipitated CdS-CdSe or core-shell (CdS)CdSe [or (CdSe)CdSe] nanoparticles in the desired medium were freshly prepared before use and were irradiated after purging with N2. Pulse radiolysis experiments were performed with a Febetron Model 705 pulser (at the National Institute of Standards and Technology), which utilizes 50 pulses of 2 MeV electrons. The dose per pulse, determined by KSCN dosimetry, was equal to (1-3) × 106 M radicals. All experiments were carried out at room temperature (21 ( 1 °C). Other details on the pulse radiolysis apparatus and data acquisition and processing were described previously.23 All radiolytic investigations were conducted in nitrogensaturated, dilute aqueous dispersions containing methanol. This condition led to the production of reactive species (•H, •OH, e-aq) and molecular compounds (H+aq, H2, H2O2)
H2O f radiolysis f •H, •OH, e-aq, H+aq, H2, H2O2 (3) and to the conversion of the •OH and •H radicals into reducing species (by reacting with methanol):
CH3OH + •OH (•H) f •CH2OH + H2O (H2)
(4)
Results 1. Separately Produced CdS and CdSe Nanoparticles. Injection of H2Se gas into an aqueous Cd(ClO4)2 solution, containing (NaPO3)6 as the stabilizer, led to a light brownish coloration which deepened with time. Completion of color development generally required 15 min. In contrast, injection of H2S into an identical Cd2+ solution produced a light green color within 3-5 min. These observations are indicative of the faster growth of CdS than CdSe nanocrystallites. The absorption spectrum of the hexametaphosphate-stabilized CdS nanoparticles (see a in the upper box in Figure 1) corresponds to particles with average diameters of g4 nm.24 Presentation of the data in terms of transmittance is more informative (see a in the lower box in Figure 1) since it permits a better visualization of the absorption onset and affords the estimation of effective bandgap (Eg) from (σhν)2 ) (hν - Eg)C, where σ is the absorption coefficient and C is a constant. From the plot of (σhν)2 vs hν (see solid circles in the lower box in Figure 1), we calculated Eg to be 2.54 eV for the CdS nanoparticles prepared in this work (Table 1). Inspection of the absorption (see d in the upper box in Figure 1) and transmission spectra (see b in the lower box in Figure 1) indicated an absorption edge of 670 nm and a diameter between 5 and 6 nm for the hexametaphosphate-stabilized CdSe nanoparticles prepared in this work. Plotting (σhν)2 vs hν (see solid squares in the lower box in Figure 1) permitted the assessment of Eg to be 1.85 eV (Table 1). The obtained TEM images and histogram (Figure 2) led to a mean diameter of 5.0 ( 3.5 nm for the hexametaphosphate-stabilized CdSe nanoparticles prepared in this work. The emission spectrum of activated CdS nanoparticles contained a sharp peak with a maximum, near the absorption edge, at 470 nm which is assigned to excitonic fluorescence (see b in the upper box in Figure 1).7,23 For the sake of comparison, the fluorescence spectrum of nonactivated CdS particles (i.e., that due to prior addition of Cd2+ ions to the same CdS particles) is also shown (see c in Figure 1). The weak
Figure 1. (top) Absorption (a) and emission (b) spectra of activated CdS nanoparticles, prepared by the injection of stoichiometric amounts of H2S gas into an aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6, and the subsequent dropwise addition of aqueous 1.0 M Cd(ClO4)2 solution until maximum fluorescence is attained. For the sake of comparison, the fluorescence spectrum of CdS nanoparticles is also shown in the absence of activation (c). λex ) 380 nm, front-face illumination. Absorption spectrum of CdSe nanoparticles, prepared by the injection of stoichiometric amounts of H2Se gas into an aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6, is also shown (d). (bottom) Transmission spectra of CdS (a) and CdSe (b) nanoparticles (prepared as decried above). Data points of (σhν)2 vs photon energy, hν, are also included for CdS (circles) and CdSe (squares) nanoparticles. Lines interpolated to (σhν)2 ) 0 correspond to the direct bandgap of CdS (488 nm) and CdSe (695 nm) nanoparticles.
lower energy broad band, with an emission maximum of ca. 650 nm, is attributable to the recombination of trapped charge carriers.25 No fluorescence of the sodium hexametaphosphate-stabilized CdSe particles could be observed, even in the presence of excess cadmium ions. This implies that charge carrier traps on the CdSe surface cannot be removed by Cd(OH)2. This is hardly surprising since the luminescence of CdSe particles, having diameters larger than 5.0 nm, has been found to be highly dependent on surface-state structures.26 For example, the fluorescence efficiency of phosphine- or thiol-capped CdSe particles was found to be up to 90%27, while no significant fluorescence could be observed for hexametaphosphatestabilized CdSe.11 CdS (or CdSe) nanoparticles were found to attach themselves, upon scanning the potential, to a silver electrode, forming a thin film which could be characterized by cyclic voltammetry in an electrochemical cell (see Experimental Section). Typical cyclic voltammograms of CdS, CdSe, and CdS(CdSe) nanoparticles deposited onto silver electrodes are shown in Figure 3.28 Mixing separately prepared CdS and CdSe nanoparticle dispersions resulted in a series of absorption spectra which showed a progressive change from those indicated for pure CdS
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TABLE 1: Spectroscopic Parameters of CdS-CdSe, (CdS)CdSe, and (CdSe)CdS Nanoparticles preparation type
particles
absorption onset, nm
bandgap, eV
separately prepared separately prepared I I I I I II II II II III III III III IV IV IV IV
CdS CdSe CdS0.75-CdSe0.25 CdS0.60-CdSe0.40 CdS0.50-CdSe0.50 CdS0.25-CdSe0.75 CdS0.13-CdSe0.87 (CdS)0.50CdSe0.50 (5 min) (CdS)0.50CdSe0.50 (15 min) (CdSe)0.50CdS0.50 (5 min) (CdSe)0.50CdS0.50 (15 min) (CdS)0.75CdSe0.25 (CdS)0.66CdSe0.33 (CdS)0.25CdSe0.75 (CdS)0.10CdSe0.90 (CdSe)0.75CdS0.25 (CdSe)0.66CdS0.33 (CdSe)0.50CdS0.50 (CdSe)0.25CdS0.75
488 670 609 613 618 650 668 617 620 618 619 644 646 654 675 692 692 693 694
2.54 1.85
fluorescence maxima, nm 470 460 474 (sh) 480 (sh) 475 469 470 470 470 470
none 545 560 567 581 590 576 583 585 517
(200) (60) (20) (50)
650 (vb) 595 (b) 580 (vb) 575 (vb) 535 (vb) 583
Figure 3. Cyclic voltammograms recorded subsequent to the deposition of CdS (curve 1, taken as the 12th cycle), CdS0.25-CdSe0.75 (curve 2, taken as the 15th cycle), CdS0.50-CdSe0.50 (curve 3, taken as the 15th cycle), CdS0.75-CdSe0.25 (curve 4, taken as the 15th cycle), CdS0.13CdSe0.87 (curve 5, taken as the 12th cycle), and CdSe (curve 6, taken as the 24th cycle) nanoparticle dispersions (type I preparation) onto the silver working electrode. Recordings were taken on air-saturated samples at room temperature. Scan rates of the initial scans were 20 mV/s. Figure 2. TEM images and histogram of CdSe nanoparticles, prepared as described in the legend of Figure 1.
dispersions to those taken in pure CdSe dispersions and which had an isosbestic point at 470 nm (not shown). More importantly, the intensity of the CdS fluorescence emission decreased only to the extent that it had been diluted by separately prepared CdSe nanoparticle dispersions, as was evidenced by the zero slope in the apparent Stern-Volmer plot (Figure 4). Similarly, the excitation spectrum of a 1:1 mixture of separately prepared CdS and CdSe nanoparticles closely resembled the absorption spectrum that was obtained for the same dispersion (not shown). 2. Coprecipitated CdS-CdSe Nanoparticles (Type I Preparation). Transmission spectra of CdS-CdSe nanoparticles, coprecipitated by the addition of H2S and H2Se (premixed in different proportions) to the stock solution (a 1.0 L aqueous 2.0 × 10-4 M Cd(ClO4)2 and 2.0 × 10-4 M (NaPO3)6 solution, adjusted to pH ) 9.1 by aqueous 1.0 M NaOH), are shown in Figure 5. The absorption onsets of the different CdS-CdSe nanoparticles, evaluated from the transmission spectra, are
collected in Table 1 and illustrated in the insert in Figure 5. There is a pronounced isosbestic point between the spectra of pure CdS and CdSe and a progressive red shift with increasing amounts of H2Se. Furthermore, subtraction of the absorption spectrum of pure CdSe from the spectra obtained from mixed CdS-CdSe nanoparticles, prepared by the addition of H2S:H2Se ) 1:1 v/v, for example, reproduced the absorption spectrum due to pure CdS. The obtained TEM image and histogram showed a normal size distribution with a mean diameter of 4.0 ( 3.3 nm (Figure 6). Typical emission spectra of CdS-CdSe nanoparticles, coprecipitated by the addition of H2S and H2Se (premixed in different proportions) to the stock solution (a 1.0 L aqueous 2.0 × 10-4 M Cd(ClO4)2 and 2.0 × 10-4 M (NaPO3)6 solution, adjusted to pH ) 9.1 by aqueous 1.0 M NaOH), are shown in Figure 7. With increasing amounts of H2Se in the H2S-H2Se mixture, the emission maximum of the coprecipitated CdSCdSe dispersion was found to shift to longer wavelengths (see upper insert in Figure 7), and the fluorescence quantum efficiency was observed to increase slightly, up to 50% CdSe, and then decrease precipitously (see lower insert in Figure 7).
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Figure 4. Apparent Stern-Volmer plots for the quenching of CdS excitonic emission by CdSe nanoparticle dispersions (I0/I - 1 ) K[CdSe]): squares, CdS-CdSe dispersions coprecipitated by the addition of H2S:H2Se mixtures with increasing amounts of H2Se in the mixture (type I preparation); circles, separately prepared CdS and CdSe nanoparticles, mixed in different proportions. The emission intensities, I, were taken at the emission maxima (around 480 nm for CdS-CdSe and at 470 nm for the mixtures of separately prepared CdS and CdSe nanoparticles) and corrected for the concentration changes by dilution. λex ) 380 nm. Figure 6. TEM images and histogram of coupled composite CdS0.50CdSe0.50 nanoparticles, prepared as described in legend of Figure 5c.
Figure 5. Transmission spectra of CdS-CdSe nanoparticles prepared by the injection of stoichiometric amounts of premixed H2S:H2Se gases into an aqueous 2.0 × 104 M Cd(ClO4)2 solution containing 2.0 × 104 M Na(PO3)6 (type I preparation): a, H2S:H2Se ) 1:0 (v/v); b, H2S: H2Se ) 3:1 (v/v); c, H2S:H2Se ) 1:1 (v/v); d, H2S:H2Se ) 1:3 (v/v); e, H2S:H2Se ) 0:1 (v/v). A plot of absorption onset vs H2Se (%) in the mixed gases is shown in the insert.
Fluorescence activation of the coprecipitated CdS-CdSe nanoparticles by the addition of cadmium ions is illustrated in Figure 8. The fluorescence intensity of a given aqueous CdSCdSe dispersion was found to increase by the dropwise addition of aqueous 1.0 M Cd(ClO4)2. In most cases, maximum intensity of the fluorescence was reached when the solution contained 6.0 × 10-4 M excess cadmium ions. Addition of cadmium ions to CdS-CdSe nanoparticles, coprecipitated by the injection of H2S:H2Se ) 3:1 (v/v) to the stock solution, resulted in the initial appearance of a fluorescence band with a maximum at 545 nm. Increasing the added cadmium ion concentration slightly blueshifted the emission maximum and increased its intensity. Concomitantly, a new band appeared at 460 nm whose intensity also increased with added cadmium ions (Figure 8a). CdSCdSe nanoparticles, coprecipitated by the injection of H2S:H2-
Figure 7. Fluorescence emission spectra of CdS-CdSe nanoparticles prepared by the injection of 9.0 mL premixed H2S:H2Se gases into a 100 mL aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6 and the subsequent dropwise addition of aqueous 1.0 M Cd(ClO4)2 solution until maximum fluorescence is attained (type I preparation). The H2S:H2Se ratios are 1:1 (a), 1:3 (b), and 1.0:8 (C), λex ) 380 nm, 3.5 and 2.0 mm slits, front-face illumination. Upper insert is a plot of emission wavelength at maximum vs percentage of H2Se in the added H2S-H2Se mixture. Lower insert shows fluorescence quantum efficiency as a function of the percentage of H2Se in the added H2S-H2Se mixture. Quantum yields were assessed by comparing the intensity of a given fluorescence band with that due to rhodamine 6G (and assuming Q ) 100% for rhodamine 6G).
Se ) 1:1 (v/v) to the stock solution, fluoresced with an emission maximum of 580 nm, even prior to activation. Addition of cadmium ions also blue-shifted this emission band and increased its intensity; furthermore, it resulted in the development of a weak shoulder (rather than a well-defined peak!) at 480 nm
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Figure 9. Fluorescence decays from top to bottom of CdS0.50-CdSe0.50, CdS0.25-CdSe0.75, and CdS0.75-CdSe0.25 nanoparticles in aqueous dispersions, monitored at 580 nm.
Figure 8. Buildup of the fluorescence emission spectra by the dropwise addition of 0.73 M aqueous Cd(ClO4)2 to CdS-CdSe nanoparticles prepared by the injection of 9.0 mL of premixed H2S:H2Se gases into a 100 mL of aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6 (type I preparation): (a) H2S:H2Se ) 3:1 (v/v), added Cd(ClO4)2 ) 0, 4.5 × 10-5, 7.3 × 10-4, 1.4 × 10-3, 2.1 × 10-3, and 2.8 × 10-3 M (from bottom to top). (b) H2S:H2Se ) 1:1 (v/v), added Cd(ClO4)2 ) 0, 4.8 × 10-5, 9.6 × 10-4, 1.4 × 10-3, and 2.1 × 10-3 M (from bottom to top). (c) H2S:H2Se ) 1:8 (v/v), added Cd(ClO4)2 ) 0, 2.4 × 10-4, 7.3 × 10-4, and 2.4 × 10-3 M (from bottom to top). λex ) 380 nm, 3.5 and 2.0 mm slits, front-face illumination.
(Figure 8b). Significantly, addition of cadmium ions to CdSCdSe nanoparticles, coprecipitated by the injection of H2S:H2Se ) 1:8 (v/v) to the stock solution, only increased the intensity of the emission band at 590 nm without shifting its maximum or producing a shoulder or a second emission band (Figure 8c). Tentatively, we assign the emission maximum centered around 470 nm to CdS excitonic fluorescence and that peaking around 560 nm to CdSe excitonic fluorescence. Decays of the 580 nm emission for CdS0.75-CdSe0.25, CdS0.50-CdSe0.50, and CdS0.25-CdSe0.75 dispersions are illustrated in Figure 9. The calculated lifetimes (τ values) were found to decrease (τ ) 25.5, 33.9, and 41.0 ns), and the distribution parameters (β values) were found to increase (β ) 0.77, 0.86, and 0.94) with increasing amounts of CdSe in the CdS-CdSe coupled nanoparticles. Stern-Volmer plots for the quenching of the CdS excitonic emission (at 480 nm) by H2Se in CdS and CdSe nanoparticles are shown in Figure 4. Excitation spectra of CdS-CdSe nanoparticles, coprecipitated in different proportions and monitored at ∼470 and ∼580 nm, are shown in Figure 10. The former spectrum corresponds nicely to the absorption spectrum of CdS nanoparticles (the absorption spectrum of the CdS nanoparticles used here is also shown in Figure 10), and the latter matches the fluorescence maximum of the CdS particles well (see b in Figure 1). The cyclic voltammograms of coprecipitated CdS-CdSe nanoparticles revealed a steady shift in the position of the char-
Figure 10. Excitation spectra of CdS-CdSe coprecipitated (type I preparation) by the addition of H2S:H2Se ) 3:1 v/v (squares), H2S: H2Se ) 1:1 v/v (triangles), and H2S:H2Se ) 1:8 v/v (circles) and by the sequential formation (type II preparation) of (CdS)CdSe (diamonds). Spectra a were monitored at 480 nm, while spectra b were monitored at 570 nm. For sake of comparison the absorption spectra of pure CdS (solid line) and CdSe (broken line) are also redrawn.
acteristic peak from that due to CdS (-1.15 V) to that due to CdSe (-1.54 V) upon the increase of the Se/S ratio (Figure 3). Pulse radiolysis of coprecipitated CdS-CdSe particles (type I preparation) in nitrogen-saturated, aqueous dispersions containing 0.5 vol % methanol resulted in the formation of transients which decayed with half-lives in the order of 15 µs. The transients displayed very characteristic absorptions in the ultraviolet and near-visible range of the spectrum (Figure 11). Specifically, those formed in CdS0.75CdSe0.25 and CdS0.25CdSe0.75 dispersions had absorption maxima at 300 and 310 nm, respectively. Radiolysis, under the present experimental conditions, is known to result in the formation of methanol radicals, •CH2OH, and hydrated electrons, e-aq (reactions 3 and 4). The latter species absorbs strongly in the entire visible range, which renders the analysis of absorption changes above 400 nm to be less than straightforward. Although the reduced species is known to display strong bleaching around the bandgap position, the low concentration of the nanoparticle prevents their pulse radiolytic detection. Both e-aq and •CH2OH radicals could, in
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Figure 11. Differential absorption spectra obtained in aqueous nitrogen-saturated, 5.0 × 10-7 M22 CdS0.75-CdSe0.25 (9) and CdS0.25CdSe0.75 (b) dispersions, in the presence of 0.5 vol % methanol, 5 µs subsequent to the pulse. The insert shows a time profile of the absorption of (CdS0.75-CdSe0.25)•- at 300 nm.
principle, reduce the CdS-CdSe nanoparticles. However, no transient absorption with maxima around 300 nm appeared in N2O-saturated dispersions (N2O is known to scavenge e-aq) of the coprecipitated CdS-CdSe nanoparticles. •CH2OH radicals do not, therefore, reduce the CdS-CdSe nanoparticles, or the underlying reaction might be too slow to be detected. The identical rates of e-aq decay, observed at 700 nm, and buildup of the semiconductor anion radicals, monitored at any wavelength between 280 and 400 nm, further support the postulated mode of the reduction by e-aq. Thus, the transient spectra shown in Figure 11 can be attributed to the formation of CdS-CdSe anion radicals by electron attachment to the coprecipitated semiconductor nanoparticles:29
CdS-CdSe + e-aq f (CdS-CdSe)•-
(5)
This is in accord with a previous assignment of the transient absorption spectra (λmax ) 300 nm) to CdSe•-, formed in the electron transfer to colloidal CdSe particles.28,30 The transient spectrum due to (CdS0.75CdSe0.25)•- nanoparticles (Figure 11) is very much like the one monitored for the CdS•- nanoparticles. Similarly, the spectrum of (CdS0.25CdSe0.75)•- nanoparticles (Figure 11) resembles the (CdSe)•spectrum. The rate constant for the reaction of e-aq with pure CdS nanoparticles is quite similar to that with pure CdSe nanoparticles.28 Therefore, the site of the initial reduction is likely to be governed by the species (CdS or CdSe) whose concentration predominates in the nanoparticles prepared by H2S and H2Se coprecipitation. These results are explicable in terms of initial electron attachment to the semiconductor component which is present in higher concentration:29
CdS0.75CdSe0.25 + e-aq f CdS•-0.75CdSe0.25
(6)
CdS0.25CdSe0.75 + e-aq f CdS0.25CdSe•-0.75
(7)
3. Sequentially Precipitated (CdS)CdSe and (CdSe)CdS Nanoparticles (Type II, III, and IV Preparations). Transmission spectra of sequentially coprecipitated (CdS)CdSe are illustrated in Figure 12, and the assessed absorption onsets are collected in Table 1. Fluorescence spectra of sequentially precipitated (CdS)CdSe and (CdSe)CdS nanoparticles are compared in Figure 13. The
Figure 12. Transmission spectra of sequentially precipitated (CdS)CdSe. Portions of H2Se were injected into 1000 mL of argon-saturated aqueous solution containing 1.0 × 10-4 M preformed CdS colloids (type III preparation, as described in the Experimental Section) to give 0, 4.2 × 10-5, 1.26 × 10-4, and 2.94 × 10-4 M stoichiometric H2Se (from top to bottom).
spectrum of the (CdSe)CdS (see dotted line in upper box in Figure 13) is characterized by peaks at 470 and 590 nm, which originate in the excitonic CdS and (CdS)CdSe emissions. With an increase in incubation time, the spectrum of the shorter wavelength peak blue-shifted to 469 nm and the longer wavelength peak red-shifted to 583 nm (see dotted line in the lower box in Figure 13). These shifts can be ascribed to a size reduction of the CdS nanoparticles and to a concomitant increase of the CdSe particles. The fluorescence spectrum of the (CdS)CdSe particles showed a broad emission band with a maximum at ca. 520 nm (see dotted line in upper box in Figure 13). This band could be deconvoluted to two Lorentian peaks with maxima of 500 and 531 nm and half-widths of 31 and 48 nm (see lower box in Figure 13). Excitation spectra of the sequentially precipitated (CdS)CdSe nanoparticles (type II preparation) are quite similar to those obtained for their coprecipitated counterparts (Figure 10). Addition of H2Se to preformed CdS resulted in the formation of CdSe on the surface of the nanoparticles and in the decrease of the CdS excitonic fluorescence (type III preparation, Figure 14). Formation of more CdSe on the surface of CdS particles resulted in the appearance of an extremely weak excitonic emission due to CdSe (see spectrum d in Figure 14). Raising the pH to 11.0 and adding excess cadmium cations only restored the fluorescence to a barely perceptible level with an emission maximum of approximately 600 nm (see spectrum e in Figure 14). Transmission electron micrographs and histograms provided evidence for the formation of 6.0 ( 0.5 nm diameter (CdS)CdSe and 6.0 ( 0.5 nm diameter (CdSe)CdS particles (Figure 15). Decay of the 480 nm emission of the CdS nanoparticles (τ ) 12.2 ns and β ) 0.70) was found to accelerate upon the addition of H2Se (τ ) 9.86 ns and β ) 0.60), indicating an increased decay of the CdS exciton upon the addition of CdSe. Transmission spectra of sequentially coprecipitated (CdSe)CdS are presented in Figure 16, and the absorption onsets are collected in Table 1. Addition of the first increment of H2S to CdSe nanoparticles resulted in the appearance of a very broad emission peak, centered at 530 nm. Further addition of H2S sharpened the
8934 J. Phys. Chem., Vol. 100, No. 21, 1996
Figure 13. Fluorescence emission spectra of sequentially precipitated (CdS)CdSe and (CdSe)CdS nanoparticles (type II preparation). Solid line in upper box ) prepared by (i) the injection of 4.5 mL of H2S into a 1000 mL aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6, followed by (ii) 5 min incubation, (iii) injection of 4.5 mL of H2Se, and (iv) the subsequent dropwise addition of aqueous 1.0 M Cd(ClO4)2 solution until maximum fluorescence is attained. Dotted line in upper box ) prepared by (i) the injection of 4.5 mL of H2Se into a 100 mL aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6, followed by (ii) 5 min incubation, (iii) injection of 4.5 mL of H2S, and (iv) the subsequent dropwise addition of aqueous 1.0 M Cd(ClO4)2 solution until maximum fluorescence is attained. Solid line in the lower box ) prepared by (i) the injection of 4.5 mL of H2Se into a 100 mL aqueous 2.0 × 10-4 M Cd(ClO4)2 solution containing 2.0 × 10-4 M Na(PO3)6, followed by (ii) 15 min incubation, (iii) injection of 4.5 mL of H2S, and (iv) the subsequent dropwise addition of aqueous 1.0 M Cd(ClO4)2 solution until maximum fluorescence is attained. Dotted line in lower box ) fitting of curve a by two Lorentian peaks with maxima at 502 and 531 nm.
emission peak and ultimately formed an excitonic peak centered at 583 nm (Figure 17). Cyclic voltammograms of separately prepared CdS and CdSe and sequentially precipitated (CdS)CdSe and (CdSe)CdS nanoparticle dispersions are shown in Figure 18. The close resemblance of cyclic voltammograms of CdS and (CdSe)CdS nanoparticles is evident (Figure 18A). Sequentially precipitated (CdS)CdSe aggregates revealed two CV peaks: one of them is identical to CdSe particles (at -1.54 V) and the other one is close to the peak attributed to CdS but is substantially weaker and shifted to -0.98 V (Figure 18B). Pulse radiolysis of core-shell (CdS)CdSe and (CdSe)CdS nanoparticles (type II preparation) in nitrogen-saturated, aqueous dispersions containing 0.5 vol % methanol resulted in identical differential absorption spectra with broad ultraviolet onsets (Figure 19). The absence of absorption maxima around 300 nm in H2O-saturated dispersions (where •CH2OH is the sole reducing species) permits, in analogy with the corresponding process observed in coprecipitated CdS-CdSe nanoparticle dispersions (vide supra), the assignment of the transient spectra
Tian et al.
Figure 14. Fluorescence quenching of CdS nanoparticles. Portions of H2Se were injected into 1000 mL of argon-saturated aqueous solution containing 1.0 × 10-4 M preformed activated (type III preparation, as described in the Experimental Section) CdS colloids to give 0 (a), 4.2 × 10-5 (b), 1.26 × 10-4 (c), and 1.68 × 10-4 M (d) stoichiometric H2Se. Also shown in fluorescence recovery after the addition of excess Cd2+ ions (e). λex ) 380 nm, front-face illumination.
to the formation of semiconductor anion radicals:29
(CdS)CdSe + e-aq f [(CdS)CdSe]•-
(8)
(CdSe)CdS + e-aq f [(CdSe)CdS]•-
(9)
This conclusion is corroborated by the similar “apparent” rate constants determined for the reaction of e-aq with (CdS)CdSe [k ) (2-4) × 1012 M-1 s-1, see a plot of kobs vs (CdS)CdSe concentration22 in the insert in Figure 19] and with (CdSe)CdS [k ) (2-4) × 1012 M-1 s-1]. Furthermore, the rates of e-aq decay, observed at 700 nm, were found to be identical with the rates of the semiconductor anion radical buildup, monitored at any wavelength between 280 and 400 nm. The identity of the differential absorption spectra, obtained upon reductions of (CdS)CdSe and (CdSe)CdS, both core-shell composites, suggests an intraparticle electron transfer. Unfortunately, the resolution (∼microseconds) of the applied pulse radiolysis equipment used did not permit an investigation of this process. Interparticle electron transfer between differently sized CdS nanoparticles, dispersed in an aqueous solution, could be investigated, however. Pulse irradiation of deaerated solutions containing 5.7 × 10-7 M22 small (35-40 Å diameter) CdS nanoparticles resulted in the formation of transient absorption due to the formation of a corresponding anion radical:
small CdS + e-aq f small CdS•-
(10)
In the absence of other electron acceptors, small CdS•- decays on a time scale of 100 µs. The addition of various concentrations of large (60-65 Å diameter) CdS particles in the range (0.3-2.0) × 10-7 M22 resulted in an accelerated decay of the
Composite CdS-CdSe Nanoparticles
J. Phys. Chem., Vol. 100, No. 21, 1996 8935
Figure 16. Transmission spectra of sequentially precipitated (CdSe)CdS. Portions of H2S were injected into 1000 mL of argon-saturated aqueous solution containing 1.0 × 10-4 M preformed (type IV preparation, as described in the Experimental Section) CdSe colloids to give 0 (a), 1.26 × 10-4 (b), 2.1 × 10-4 (c), and 4.2 × 10-4 M (d) stoichiometric H2S (from top to bottom).
Figure 17. Increase of fluorescence upon the growth of CdS on preformed CdSe particles (type IV preparation). Portions of 0 (a), 1.0 (b), 2.0 (c), and 3.0 mL (d) of H2S were injected into 1000 mL of argon-saturated aqueous solution containing 1.0 × 10-4 M preformed (as described in the Experimental Section note, however, that the volumes there are 500 mL, it should be changed to agree with that given here) CdSe colloids. λex ) 380 nm, front-face illumination.
Figure 15. TEM images and histograms of core-shell type (CdS)0.50CdSe0.50 (a) and (CdSe)0.50CdS0.50 (b) nanoparticles, prepared as described in the legend of Figure 13.
radical anion. The observed rate constant, kobs, was found to be linearly dependent on the concentration of the large CdS nanoparticles, indicating an interparticle electron transfer:
small CdS•- + large CdS f small CdS + large CdS•- (11) The rate constant for reaction 11 was determined to be 6.5 × 1010 M-1 s-1. This reflects that the reaction is nearly diffusion controlled. The rate constant for intraparticle electron transfer within the (CdS)CdSe or (CdSe)CdS core-shell nanoparticles
is expected to be very much larger and, thus, undetectable with our system. This ultrafast reaction is expected to result in the distribution of the electron over the entire core-shell mixed semiconductor nanoparticles which manifests itself, of course, in an idential spectrum of the [(CdS)CdSe]•- and [(CdSe)CdS]•nanoparticles. Discussion 1. Structure of CdS-CdSe Nanoparticles Formed by Coprecipitation. The method used for the preparation of CdSCdSe nanoparticles by coprecipitation in the present work (see Experimental Section, type I preparation) could, a priori, lead to a number of different structures (A, B, and C in Figure 20). Theoretically, well-separated CdS and CdSe nanoparticles could form (see A in Figure 20). Alternatively, type I preparation may yield well-mixed CdS-CdSe nanoparticles which can be
8936 J. Phys. Chem., Vol. 100, No. 21, 1996
Figure 18. (A) Cyclic voltammograms recorded subsequent to the deposition of (CdSe)CdS (full line, taken as the 12th cycle) and CdS (broken line, taken as the 12th cycle) nanoparticle dispersions (type IV preparation) onto the silver working electrode. Recordings were taken on air-saturated samples at room temperature. Scan rates of the initial scans were 20 mV/s. (B) Cyclic voltammograms recorded subsequent to the deposition of (CdS)CdSe (taken as the 12th cycle) nanoparticle dispersions (type III preparation) onto the silver working electrode. Recordings were taken on air-saturated samples at room temperature. Scan rates of the initial scans were 20 mV/s. Positions of the cathodic peaks of pure CdS and CdSe are indicated by arrows.
Figure 19. Completely overlapping differential absorption spectra of two separate nitrogen-saturated dispersions of 3.0 × 10-7 M22 (CdS0.75)CdSe0.25 and 3.0 × 10-7 M22 (CdSe0.75)CdS0.25, in the presence of 0.5 vol % methanol, 10 µs subsequent to the pulse. The insert shows a plot of the observed first-order rate constant for the reaction of e-aq, kobs, as a function of (CdS)CdSe concentration, determined at 700 nm.
considered to be analogous to solid solutions (see B in Figure 20) or nanoparticles with pronounced domains of CdS and/or CdSe either within their matrices or coupled through their interfaces (see C in Figure 20). The presence of an isosbestic point in the transmission spectra, recorded in dispersions formed upon the introduction of premixed H2S and H2Se into aqueous cadmium ion solutions (Figure 5), and the observed gradual shift of the absorption onset and emission maximum to lower energies with increasing amounts of H2Se in the H2S-H2Se mixture (Table 1) are explicable by the formation of CdS-CdSe co-colloids rather than separate CdS and CdSe nanoparticles. The progressive decrease in fluorescence intensity of the particles, formed upon
Tian et al.
Figure 20. An oversimplified diagram of the possible structures of mixed CdS-CdSe nanoparticles. (A) Distinct and well-separated CdS and CdSe nanoparticles, (B) well-mixed (solid solutions) of CdS-CdSe nanoparticles, (C) nonideally mixed CdS-CdSe nanoparticles containing domains of CdS and CdSe, (D) sandwich CdS-CdSe nanoparticles, (E) CdSe encapsulated CdS, CdS-core CdSe-shell type nanoparticles (the cherry and its stone), and (F) CdS encapsulated CdSe, CdSecore CdS shell type nanoparticles (the cherry and its stone).
using gas mixtures which are increasingly richer in H2Se (Figure 7), is in accord with this interpretation. Upward bending of the Stern-Volmer plot (Figure 4) is not compatible with a quenching mechanism governed by the diffusion of the reacting partners to each other; it is, however, indicative of static quenching of the excited CdS by quenchers present in close proximity. Formation of separate CdS and CdSe nanoparticles (A in Figure 20) under the present experimental conditions would not have resulted in quenching of the fluorescence intensity at all (Figure 4). Taken together, these results provide compelling evidence for excluding the possibility of separate CdS and CdSe nanoparticle formation (A in Figure 20) upon the introduction of premixed H2S and H2Se into aqueous cadmium ion solutions. The substantial difference between the solubility products of CdS (Ksp ) 1.4 × 10-29) and CdSe (Ksp ) 1.4 × 10-35) is responsible for the thermodynamically preferred formation of the latter nanoparticle. Kinetics favors, however, the formation of CdS (see Results). Additionally and/or concurrently, the initially formed sulfides may be replaced by selenides:
CdS + Se2- f CdSe + S2-
(12)
These observations render the formation of well-mixed CdSCdSe nanoparticles (B in Figure 17) to be highly unlikely. The gradual shift of cathodic peaks for CdS-CdSe with increasing CdSe content from that observed for pure CdS to that found for pure CdSe (Figure 3) reflects the change in the composition of the coprecipitated particles which results in the change of the surface states with corresponding shifts in the energy of the conduction band with respect to SCE (see Experimental Section). An enrichment of the nanoparticle surface with Se atoms increases the adsorption of negatively charged stabilizers and, thus, pins the conduction band to a more negative potential, as seen in the CV curves (Figure 3). The gradual shift in the cyclic voltammetric peak position corresponds to the formation of mixed, rather than individual CdS and CdSe nanoparticles.
Composite CdS-CdSe Nanoparticles Taking all of our experimental results together leads us to describe the structures that are formed upon the addition of H2S and H2Se mixtures to aqueous Cd(ClO4)2 solutions in terms of nanoparticles which are likely to have pronounced domains of CdS and/or CdSe, either within their matrices or coupled to each other (C in Figure 20). For the sake of clarity, we refer to these structures as coupled composite CdS-CdSe nanoparticles with the understanding that a further distinction among the various possibilities (see the two extremes in box C, Figure 20) is not warranted at present. 2. Structure of Nanoparticles Formed by Sequential Precipitation of CdS and CdSe or CdSe and CdS. There are significant differences between the coprecipitated CdSCdSe (type I preparation) and sequentially formed (CdS)CdSe or (CdSe)CdS nanoparticles. Sequentially precipitated (CdS)CdSe or (CdSe)CdS nanoparticles, unlike their coprecipitated counterparts (type I preparations), do not have isosbestic points in their transmission spectra (see Figures 12 and 16). The addition of increasing amounts of H2Se to preformed CdS particles (type III preparation) shifted the absorption edge from 488 to 675 nm (Figure 12 and Table 1), indicating the growth of CdSe on CdS. Introduction of CdS onto preformed CdSe particles (type IV preparation) also affected the transmission spectrum. In particular, addition of H2S to preformed CdSe particles manifested itself in the development of a shoulder around 430 nm which sharpened and shifted into lower energies with increasing amounts of H2S added until a secondary absorption onset became apparent at 490 nm (Figure 16). It is necessary to recall that the formation of Cd(OH)2 on the surfaces of the initially precipitated nanoparticles was a necessary requirement for observing fluorescence. Similar fluorescence activation was reported for (CdSe)ZnS,12 (CdS)HgS,17 and (CdS)(HgS)CdS31 core-shell and double cores-shell type of semiconductor nanoparticles. In all these cases, however, the amount of Cd(OH)2 was significantly smaller than that would require even a monolayer coverage. Differences between the emission spectra of coprecipitated CdS-CdSe (Figure 8) and sequentially precipitated (CdS)CdSe or (CdSe)CdS (Figures 13 and 14) nanoparticles are most marked by the intensity of the lower energy excitonic emission. While the intensity of this emission is quite strong in the coprecipitated CdS-CdSe, it appears to be rather weak in (CdS)CdSe or (CdSe)CdS. The optical spectra are strongly indicative, therefore, of the formation of core-shell types of (CdS)CdSe and (CdSe)CdS in type II, III, and IV preparations. The satisfactory agreement between the calculated and the experimentally determined (TEM, Figure 15) diameters of (CdS)CdSe (calc ) 5.8 nm, expt ) 6.0 ( 0.5) and (CdSe)CdS (calc ) 6.1 nm, expt ) 6.0 ( 0.5) are in accord with the proposed coreshell structures (E and F in Figure 20). The close resemblance of cyclic voltammograms of CdS and (CdSe)CdS nanoparticles (Figure 18A) provides additional proof that the CdSe core is covered by a layer of CdS which, in turn, governs the electrochemical behavior of this core-shell semiconductor nanoparticle. Sequentially precipitated (CdS)CdSe nanoparticles revealed two peaks in CV (Figure 18B); one of them is identical to CdSe particles at -1.54 V, and the other one is close to the peak attributed to the CdS species, although it is substantially weaker and shifted to -0.98 V. The preservation of a small amount of the original CdS particles may be responsible for the presence of the weak CdS CV peak. Even in small populations, CdS particles have a greater propensity, than do CdSe particles, to attach to the silver electrodes (10-12 cycles were found to be necessary to obtain a fully developed CdS peak, as compared to the 25-30 cycles
J. Phys. Chem., Vol. 100, No. 21, 1996 8937 that were needed for the appearance of the peak due to CdSe particles; see Results). Indeed, the spontaneous formation of self-assembled films of thio compounds on silver substrates is well documented,1 while for selenium derivatives such a reaction is likely to be less facile. Thus, a weak but noticeable CdS peak may materialize even though the relative concentration of preserved CdS particles is very small. In summary, all of the obtained evidence indicates the preferential formation of core-shell type nanoparticles in the sequential precipitation of cadmium ions by H2S and H2Se (or H2Se and H2S). 3. Fluorescence. The emission spectra of CdS, CdSe, CdSCdSe, and (CdS)CdSe [or (CdSe)CdS] dispersions strongly depend on the sizes and size distributions of the particles and on their surface states.7,12,25,32-35 Typically, there are two emission bands: a sharp one near the absorption edge, due to excitonic fluorescence, and a broad lower energy band, attributable to the recombination of the trapped charge carriers. Conditions in the present work have been arranged for the maximization of excitonic fluorescence. This has been accomplished by activating the surface of the nanoparticles by the formation of a cadmium hydroxide layer which eliminates sites where radiationless decay may occur.7,25 The higher solubility product of Cd(OH2) (Ksp ) 5.3 × 10-15) than those of CdS (Ksp ) 1.4 × 10-29) and CdSe (Ksp ) 1.4 × 10-35) has necessitated the presence of sufficient concentrations of hydroxide ions (pH ) 10). Conversely, the much lower solubilities of CdS and CdSe than that of Cd(OH2) limits the amounts of cadmium hydroxide formed in the presence of S2- or Se2- ions to be negligible. The two excitonic emissions observed in the different CdSCdSe, (CdS)CdSe, and (CdSe)CdS nanoparticle dispersions are centered around 470 and 560 nm (Table 1). The former is observable in pure CdS nanoparticles as well as in CdS-rich CdS-CdSe, (CdS)CdSe, and (CdSe)CdS samples. This emission is due, therefore, to the 1se-1sh excitonic transition of CdS. The position of the longer wavelength emission band (centered around 560 nm, Table 1) corresponds to that reported for the 1se-1sh excitonic transition of CdSe.34,35 The possibility of having the exciton extend over the entire mixed semiconductor nanoparticles cannot be excluded, however. Indeed, excitons generated in core-shell type (HgS)CdS and (ZnS)CdSe particles were shwon not to be confined to either the core or shell, but they experienced the potential of the entire mixed semiconductor particle.17 Since the combined exciton diameters of CdS (∼7 nm) and CdSe (∼12 nm) are larger than the diameters of the composite size-quantized CdS-CdSe and (CdS)CdSe [or (CdSe)CdS] nanoparticles (6-7 nm, Figures 6 and 15), the exciton is likely to extend over the entire mixed nanoparticle. Modeling the optical spectra by solving the appropriate Schro¨dinger equations provided an insight into the structures of the excitons in the coprecipitated nanoparticles. As a first-order approximation, energies of the lowest excited state (E) in the two extreme cases (CdS and CdSe) are calculated from eq 1335-38
E)
[
p2π2 1 2R2 me
+
1
]
mh
-
1.8e2 2R
+
e2 n)1 R
∑an ∞
() F
R
2n
(13)
[where p is the Planck constant, R is the radius of the spherical particles, me is the effective mass of the electron (taken to be 0.19 for CdS and 0.13 for CdSe), mh is the effective mass of the hole (taken to be 0.80 for CdS and 0.45 for CdSe), 2 is the high-frequency dielectric constant (taken to be 5.7 for CdS and 8.3 for CdSe), and F is the effective distance between charges] and are plotted against the diameters of CdS and CdSe
8938 J. Phys. Chem., Vol. 100, No. 21, 1996
Figure 21. Calculated 1s-1s exciton energies for CdS and CdSe nanoparticles as functions of their diameters (eq 13). See Discussion for details.
nanoparticles in Figure 21. A shift of the emission maximum to lower energies with increasing particle size is immediately evident from these plots. Lines drawn along the observed emission maxima (see Table 1) for the coprecipitated CdSCdSe, as well as for the sequentially precipitated (CdS)CdSe and (CdSe)CdS nanoparticles, and along their mean diameters (determined by electron microscopy) cross between the curves calculated for the excitonic spectra of pure CdS and pure CdSe (see bold lines in Figure 21). It has been established that the heterostructured quantum dots possess novel electronic properties with their excitation energy lying between the core and the shell components.39 All of these observations have led us to attribute the observed longer wavelength emissions, therefore, to the 1se-1sh excitonic transition of the coprecipitated CdSCdSe or core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles. Assigning the origin of the longer wavelength excitonic emission is less than straightforward since no fluorescence could be observed in pure CdSe samples (Table 1). There are two possible sources of the excitonic transition in coprecipitated CdS-CdSe or in core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles. First, CdS nanoparticles may eliminate surface recombination sites more effectively than Cd(OH2) and thereby activate the CdSe excitonic fluorescence. A similar mode of activation was proposed for the enhanced CdS and ZnS fluorescence upon the addition of cadmium ions at high pH.7,12 Second, the observed excitonic fluorescence could be the consequence of charge transfer from the conduction band of CdS nanoparticles to the coprecipitated CdS-CdSe or coreshell type (CdS)CdSe [or (CdSe)CdS] nanoparticles. Lack of fluorescence in pure CdSe nanoparticles (Table 1), decreasing emission intensity and quantum yields with decreasing amounts of CdS in both the coprecipitated CdS-CdSe or the core-shell type (CdS)CdSe [or (CdSe)CdS] nanoparticles (Table 1 and lower insert in Figure 7), and the presence of a peak in the excitation spectra of CdS-CdSe and (CdS)CdSe which corresponds to the emission spectrum of CdS strongly favor the latter postulate. Analogous charge transfer was proposed for the core-shell type (CdS)HgS size-quantized semiconductor particles.17 Photoelectrons, formed upon the excitation of CdS nanoparticles, are rapidly trapped into shallow surface sites. Their detrapment into the conduction band and subsequent decay manifests in the appearance of the excitonic fluorescence observable around 470 nm. In the presence of neighboring CdSe particles, alternative decay pathways become available with
Tian et al. somewhat higher valence bands and somewhat lower conduction bands. Increasing the amount of the CdSe co-colloid (either in the composite or in the core-shell nanoparticles) progressively decreases the bandgap for the alternative pathway by raising its valency band energy level and by lowering its conduction band energy level. This, in turn, results in the initial appearance of a second excitonic band centered around 580 nm which, with increasing amounts of CdSe particles, becomes dominant (as seen in Table 1 and Figures 8, 14, and 17). The present system provides an example for coupled quantum dots in which the exciton energy can be tuned by altering the ratio of the two semiconductors. Electron attachment to coupled composite CdS-CdSe and core-shell type (CdS)CdSe and (CdSe)CdS nanoparticles has been demonstrated by pulse radiolysis. The sites of initial electron trapping are likely to be provided by the semiconductor surface states. Interestingly, there are remarkable differences between the coupled composite and core-shell type mixed semiconductors. The identity of the transient absorption spectra of the two core-shell type mixed semiconductor anion radicals, [(CdS)CdSe]•- and [(CdSe)CdS]•-, argues for a rapid electron transfer from some initial trapped sites to a location where it is able to spread over both CdS and CdSe (see Results section). Conversely, in coupled composite semiconductor nanoparticles the electrons are firmly trapped on the component which is present in higher concentration (see Figure 11 and Results section for CdS•-0.75CdSe0.25 and CdS0.25CdSe•-0.75). The absence of observable intraparticle electron transfer indicates better separated and larger domains of CdS and/or CdSe in the coupled composite than in the core-shell type mixed semiconductor systems. Alternatively, the electron may be located at the CdS-CdSe interface region in the core-shell nanoparticles. Conclusion The preparation and characterization of two different types of fluorescence-activated, two-component semiconductor nanoparticles have been the most significant accomplishments of the present report. Judicious colloid chemical manipulation of the solutions permitted the construction of coupled composite CdSCdSe and core-shell type (CdS)CdSe and (CdSe)CdS nanoparticles in controllable compositions and varying surface states. The effective combination of spectroscopic, electron microscopic, electrochemical, and pulse radiolytic techniques has allowed the characterization of the relatively complex nanoparticles reported here. Extension of these studies to other composite nanoparticles and the fabrication of two- and threedimensional lattice structures from them are the subject of our current intense scrutiny. Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged. T.N. thanks the National Science Foundation for support under the Research Experience for Undergraduates (REU) Program at Syracuse University during summer 1994. D.G. thanks the Alexander-von Humboldt Foundation for a “Feodor Lynen Forschungsstipendium”. We are also grateful to Pedi Neta for the use of his facilities at NIST and to Dr. Fiona Meldrum for TEM measurements. References and Notes (1) Fendler, J. H. Membrane-Mimetic Approach to AdVanced Materials; Advances in Polymer Science Series Vol. 113; Springer-Verlag: Berlin, 1994. (2) Weller, H. AdV. Mater. 1993, 5, 88. (3) Fendler, J. H.; Meldrum, F. C. AdV. Mater. 1995, 7, 607. (4) Hodes, G. Isr. J. Chem. 1993, 33, 95.
Composite CdS-CdSe Nanoparticles (5) Rajeshwar, K. AdV. Mater. 1992, 4, 23. (6) DeMattei, R. C.; Feigelson, R. S. In Electrochemistry of Semiconductors and Electronics; McHardy, J., Ludwig, F., Eds.; Noyes Publications: Park Ridge, NJ, 1992; p 1. (7) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (8) Grieser, F.; Furlong, D. N.; Scoberg, D.; Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Chem. Soc., Faraday Trans. 1992, 88 (15), 2207. (9) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett 1990, 174, 241. (10) Gopidas, K. R. M.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, 6435. (11) Spanhel, L.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 6632. (12) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Caroll, R. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (13) 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. (14) Zhou, H. S.; Sasahara, H.; Honma, I.; Komiyama, H. Chem. Mater. 1994, 6, 1534. (15) Youn, H. C.; Baral, S.; Fendler, J. H. J. Phys. Chem. 1988, 92, 6320. (16) Spanhel, L.; Henglein, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 1359. (17) Ha¨sselbarth, A.; Eychmu¨ller, A.; Eichberger, R.; Giersig, M.; Mews, A.; Weller, H. J. Phys. Chem. 1993, 97, 5333. (18) Schooss, D.; Mews, A.; Eychmu¨ller, A.; Weller, H. Phys. ReV. B 1994, 49, 17072. (19) Halsall, M. P.; Nicholls, J. E.; Cockayne, B.; Wright, P. J. J. Appl. Phys. 1992, 71, 907. (20) Ne´meth, S.; Jao, T.-C.; Fendler, J. H. Macromolecules 1994, 27, 5449. (21) Leung, L. K.; Meyer, G. J.; Lisensky, G. C.; Ellis, A. B. J. Phys. Chem. 1990, 94, 1214. Murphy, C. J.; Lisensky, G. C.; Leung, L. K.; Kowach, G. R.; Ellis, A. B. J. Am. Chem. Soc. 1990, 112, 8344. (22) Values given refer to the calculated concentrations of the semiconductor nanoparticles in a liter of dispersion. Semiconductor nanoparticle concentrations were calculated from the number of molecules (500-550)
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