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ZnS semiconductor nanoparticles
Journal of Colloid and Interface Science 297 (2006) 607–617 www.elsevier.com/locate/jcis
Photophysics and charge dynamics of Q-PbS based mixed ZnS/PbS and PbS/ZnS semiconductor nanoparticles Anil Kumar ∗ , Anshuman Jakhmola Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India Received 18 August 2005; accepted 14 November 2005 Available online 27 December 2005
Abstract The surface of ZnS and PbS has been modified by interfacing PbS on ZnS and ZnS on PbS nanoparticles. This produced core–shell nanocomposites ZnS/PbS and PbS/ZnS with tunable electronic properties. In both structures PbS particles are present in cubic form with an average diameter of about 6 nm. The addition of Pb2+ (3 × 10−4 mol dm−3 ) to Q-ZnS (1.5 × 10−4 mol dm−3 ) in the basic pH range produces sizequantized fluorescent PbS particles coated by metal hydroxides. In these particles the relaxation kinetics of charge carriers has been followed using a picosecond single-photon counting technique. At 1.5 × 10−4 mol dm−3 Pb2+ an interfacial relaxation of charge from ZnS to PbS phase could be observed in subnanosecond time domain. An increase in [Pb2+ ] from 2 × 10−4 to 1 × 10−3 mol dm−3 enhanced the average emission lifetime from 9.4 to 19.4 ns. Composite PbS/ZnS particles are produced at high [ZnS] only. These particles had emission lifetime in µs time range. The extent of charge separation and the dynamics of charge carriers could be manipulated by the surface modification of these nanostructures. © 2005 Elsevier Inc. All rights reserved. Keywords: Q-PbS; Photophysical properties; Q-ZnS/PbS; Q-PbS/ZnS; Nanocomposites
1. Introduction Synthesis of different nanostructured materials with tunable optical and photophysical properties has been a subject of extensive interest because of their great scientific and technological applications [1–7]. Over the past one-and-a-half decades a variety of colloidal particles of metal sulfides and metal oxides have been prepared and investigated for their changed electronic properties. In the recent past several quantized nanoheterostructures of semiconductors, viz., CdS–Ag2 S [8,9], CdS–TiO2 [10–13], CdS–ZnO [10], CdS–PbS [14], AgI– Ag2 S [15], CdS–HgS [16], CdS–ZnS [17], SnO2 –CdS [18], ZnO–ZnS [19], etc., have been synthesized and characterized. A number of promising applications of these materials in photophysics, photocatalysis, and photonics have been reported. Interest in PbS-based nanomaterials has been aroused because of their unique optical and emission properties, which * Corresponding author. Fax: +91 1332 273560.
E-mail address: [email protected] (A. Kumar). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.11.028
have tremendous applications in lasers [20], light-emitting devices [21,22], detectors [23], nonlinear optics [24–27], single electron devices [28], optical switches, optical amplification [27,29], telecommunication [29], etc. PbS is an indirect band gap semiconductor with a bulk band gap of about 0.41 eV. It has smaller effective masses of charge carriers, high value of dielectric constant, and a fairly high value of excitonic size. For these reasons it allows strong quantum confinement even in relatively large PbS quantum dots [30,31]. A number of methods have been used to modify the surface of these particles, which results in their changed optical, electronic, and structural properties [32–38]. Lately, exchange of cation(s), specifically Cd2+ for Ag+ , has been investigated to synthesize CdSe/Ag2 Se and CdS/Ag2 S nanocrystals of varied composition, size, and shape [39]. In the present work we have synthesized core–shell type structures of PbS nanoparticles, ZnS/PbS and PbS/ZnS, by interfacial exchange of Zn2+ for Pb2+ from ZnS and interfacing ZnS on PbS, respectively. In these materials ZnS and PbS are present in the wurtzite and face-centered cubic (fcc) forms, respectively. The optical and photophysical properties of these
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Fig. 1. Electronic spectra of ZnS (1.5 × 10−4 mol dm−3 ) containing 1.5 × 10−4 mol dm−3 excess Zn2+ in the presence of different amounts of Pb2+ (×10−4 mol dm−3 ): 0.0 (—); 0.08 (---); 0.3 (· · ·); 1.0 (−·−·); 1.5 (−··−··); 5.0 (·····) at pH 10.8. Inset: Absorption spectra of PbS (1.5 × 10−4 mol dm−3 ) at pH 10.8.
particles have been analyzed, and the dynamics of charge carriers has been monitored to understand these systems.
with a stand-alone ignitor and a 200-W mercury–xenon lamp. Solid samples of different photocatalysts were prepared by removing water on a Buchi Rotavapor RIIH.
2. Experimental 2.3. Preparation of mixed colloidal ZnS/PbS and PbS/ZnS 2.1. Materials Anhydrous zinc acetate (CDH), NaOH (BDH), lead acetate, and sodium hexametaphosphate (Qualigens) were of analytical grade. All these chemicals were used without any further purification. Nitrogen (purity >99.9%) was used for deoxygenating the prepared samples and oxygen (purity >99.9%) was bubbled during irradiation of the samples.
Colloidal solutions of ZnS [40] and PbS were prepared by injection of SH− into their respective deaerated salt solutions using HMP as stabilizer, following the earlier reported method. Mixed ZnS/PbS colloids were prepared by the addition of different amounts of lead acetate to the above-prepared Q-ZnS. PbS/ZnS was prepared by coating ZnS on PbS by adding varied amounts of zinc acetate, followed by SH− . The pH of all these solutions was maintained at 10.8 by adding dilute solutions of NaOH.
2.2. Equipment 2.4. Methodology Electronic spectra were recorded on a Shimadzu UV2100S spectrophotometer. Emission measurements were made on a Shimadzu RF-5301PC spectrofluorophotometer. Electron microscopy was performed on a Fei-Philips Morgagni 268D Digital TEM with image analysis system having variable magnifications up to 280,000×. X-ray diffraction patterns were recorded on a Philips DW 1140/90 X-ray diffractometer using the CuKα line of the X-ray source. IR spectra were measured on a Thermonicolet Nexus FTIR spectrophotometer. The fluorescence lifetime in the picosecond/nanosecond time domain was recorded on an IBH 5000U fluorescence spectrophotometer using a picosecond Tsunami titanium–sapphire modelocked laser as an excitation source. Some experiments have also been performed using a 295-nm diode laser as an excitation source. A Hamamatsu photomultiplier was used for the detection of fluorescence. Photolysis of different semiconductor systems was carried out on an Oriel photolysis assembly equipped
For recording transmission electron micrographs of different particles a small drop of the colloidal solution was adsorbed on carbon-coated copper G-200 grids. Grids were scanned at an accelerating voltage of 100 kV. The particle size was measured online using Analysis 3.2 from a soft imaging system. X-ray diffraction patterns of powders of various nanosystems were recorded by scanning these samples in a 2θ range of 10◦ –90◦ . In these experiments the acceleration voltage was set at 35 kV with 30 mV flux. IR spectra were measured in KBr and CsI media in the mid and far IR ranges, respectively. Fluorescence lifetime decay curves were analyzed kinetically using a multiexponential iterative technique from IBH. The goodness of the fit was adjudged by evaluating χ 2 from the plot of weighed residuals and autocorrelation functions. For photolysis experiment, solutions and glass filters were used to select the light in the desired wavelength range.
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Fig. 2. Electron micrograph(s) of PbS (a); ZnS containing excess Zn2+ (b); ZnS/Pb2+ (Pb2+ = 5.0 × 10−4 mol dm−3 ) (c); ZnS/Pb2+ (Pb2+ = 8.0 × 10−4 mol dm−3 ) (d); and PbS/ZnS (PbS = 1.5 × 10−4 mol dm−3 and ZnS = 5.0 × 10−4 mol dm−3 ) (e).
3. Results 3.1. Optical, structural, and photophysical behavior 3.1.1. ZnS/PbS (Pb2+ + ZnS) The addition of Pb2+ to the colloidal ZnS solution containing 1.5 × 10−4 mol dm−3 excess Zn2+ results in the shift of its optical band gap to a longer wavelength (Fig. 1). An increase in the amount of Pb2+ gradually shifted the band gap to the red and eventually to the NIR region. The absorption coefficient of the mixed particle is enhanced in the entire recorded spectral range from 200 to 800 nm. These particles depicted broad shoulders between 280–300 nm and 340–360 nm at high [Pb2+ ] and may be assigned as excitonic bands. These absorption features are similar to those reported earlier for PbS particles [27]. Electron micrograph(s) and size histogram(s) (not shown) of ZnS and the mixed colloids (ZnS/PbS) demonstrate that the average size of the mixed colloids having an excess of Pb2+ was relatively much smaller (6 nm) than that of pure ZnS (14 nm) and depicted a relatively narrow size distribution (2–10 nm). The electronic spectrum, as well as the size of the resulting particles in the mixed colloids at higher concentrations of Pb2+ , is very similar to those of pure PbS particles (inset, Fig. 1, and Fig. 2). IR spectral features of the mixed particles (not shown) were found to match literature data on the sum spectra of pure ZnS and PbS [41,42]. The structures of these material(s) in both pure and mixed colloids were analyzed by XRD (Fig. 3). A comparison of the 2d spacing of pure ZnS with its literature data reveals
it to be produced in wurtzite form [43]. It also showed the presence of Zn(OH)2 in orthorhombic shape (Fig. 3a). In the mixed colloids, at low concentrations of Pb2+ (1 × 10−4 mol dm−3 ), ZnS and PbS phases were detected to be present in wurtzite and face-centered cubic structures, respectively (not shown). At high concentrations of Pb2+ (>5 × 10−4 mol dm−3 ), ZnS phase disappeared completely and besides PbS, Zn(OH)2 and PbO/Pb(OH)2 phases were also detected. PbS and Zn(OH)2 /PbO are produced in the facecentered cubic and orthorhombic forms, respectively (Fig. 3b). Fig. 4 presents the fluorescence spectra of pure ZnS and the mixed colloids consisting of different amounts of Pb2+ added to ZnS having excess Zn2+ (1.5 × 10−4 mol dm−3 ). The fluorescence spectrum of ZnS exhibits a band at 460 nm (Fig. 4a), which is similar to that reported earlier [40]. Pure PbS particles under normal experimental conditions did not depict any emission band. In the presence of excess Zn2+ it, however, shows a very weak and broad emission peaking at 700 nm (not depicted in the figure). Changes in the fluorescence spectra of ZnS at low and high concentrations of Pb2+ are shown in Figs. 4a and 4b, respectively. These samples were excited by both 295- and 420-nm light. In the low concentration range of Pb2+ (1×10−5 to 1.5×10−4 mol dm−3 ), the excitation of these particles by 295-nm radiation results in the quenching of the emission due to ZnS with a regular change in the emission maxima ranging from 460 to 520 nm. About 1.5 × 10−4 mol dm−3 Pb2+ quenched the emission almost completely (Fig. 4a). The excitation of these particles by 420-nm light did not depict emission due to ZnS; however, a very weak new band is seen
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(a)
(b)
(c) Fig. 3. X-ray diffraction patterns of different colloids: ZnS containing excess Zn2+ (a); ZnS containing Pb2+ (Pb2+ = 5.0 × 10−4 mol dm−3 ) (b); and PbS/ZnS (c).
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(a)
(b) Fig. 4. Emission spectra of ZnS (1.5 × 10−4 mol dm−3 ) containing 1.5 × 10−4 mol dm−3 excess Zn2+ (a) in the presence of different amounts of Pb2+ (×10−5 mol dm−3 )—0.0 (—); 1.0 (---) 3.0 (· · ·); 6.0 (−·−·); 9.0 (−··−··); 15.0 (·····)—at pH 10.8, λex = 295 nm; (b) in the presence of different amounts of Pb2+ (×10−4 mol dm−3 ) at pH 10.8, λex = 420 nm. Inset. Emission intensity at 710 nm as a function of [Pb2+ ] upon excitation by 295-nm (") and 420-nm (Q) radiation.
in the red region. The addition of higher concentration of Pb2+ (>1.5 × 10−4 mol dm−3 ) resulted in the production of a new emission band at 680 nm, which is red-shifted gradually by the increased addition of Pb2+ (Fig. 4b) up to 1 × 10−3 mol dm−3 of Pb2+ . A further increase in the concentration of Pb2+ coagulates this colloidal solution within a few hours. The emission due to these particles is relatively weak, similar to that reported earlier for PbS particles [34]. This emission is quenched by SH− . The addition of about 1 × 10−4 mol dm−3 of SH− reduced the emission intensity by about 60%. A comparison of the fluorescence behavior of mixed colloids observed at relatively higher [Pb2+ ] (>1.5 × 10−4 mol dm−3 ) upon excitation by 295- and 420-nm wavelengths reveals that under both conditions the emission band is observed at around 710 nm. However, the excitation by 295-nm depicts smaller intensity of emission at 710 nm despite having higher absorption coefficients due to both ZnS and PbS at 295 nm (inset, Fig. 4b).
3.1.2. PbS/ZnS The absorption spectrum of PbS has an onset of absorption in NIR region, and is very similar to that of an indirect gap semiconductor [30]. Fig. 5 shows the electronic spectra of Q-PbS coated by ZnS at its different concentrations. The coating by ZnS at its low concentration did not exhibit any spectral change nonindicating the interaction between the two; however, at higher concentrations (>3.5 times that of PbS) it reveals a change in its optical behavior (Fig. 5, curve c). The absorption coefficient of the composite particles is decreased in the wavelength range 320–800 nm and depicts two shoulders at 300 and 330 nm. The electron micrograph of PbS/ZnS composite exhibits the presence of large (bright) as well as small size (dark) particles (Fig. 2e). Their XRD pattern demonstrates the presence of both ZnS and PbS phases in wurtzite and face centered cubic structural forms respectively (Fig. 3c). Apart to this it also exhibited
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posite particles ZnS/PbS and PbS/ZnS also underwent photodissolution by visible light with quantum efficiencies of 0.04 and 0.14, respectively. Interestingly, PbS particles coated by a layer of zinc and larger amounts of lead hydroxide become relatively more photostable toward UV/visible radiation and are corroded with a significant reduction in the quantum efficiency to 0.0006 (Table 1). These particles could be stored at 5 ◦ C for more than 6 months without any appreciable changes in their optical properties. 3.3. Dynamics of charge carriers
Fig. 5. Electronic and emission spectra of PbS (1.5 × 10−4 mol dm−3 ) coated with different concentrations of ZnS (×10−4 mol dm−3 ): 0.5 (a, a1 ); 3.0 (b, b1 ); 5.0 (c, c1 ) at pH 10.8, λex = 295 nm.
the presence of Zn(OH)2 in the orthorhombic form. Thus TEM and XRD studies demonstrate them to contain both ZnS and PbS phases. At low concentrations of ZnS (3 × 10−4 mol dm−3 ), the excitation of the layered PbS/ZnS particles by 295-nm causes emission centered at about 450 nm (Fig. 5, curve b1 ), which appears to arise due to the ZnS phase. The high concentration of ZnS (>3 × 10−4 mol dm−3 ), however, induces a new fluorescence band in the visible region at 560 nm (Fig. 5, curve c1 ), which is different to both ZnS and PbS. The excitation of these particles by light >320 nm, where pure ZnS does not have any absorption, reduced the emission intensity of the 560-nm band without causing any appreciable shift in the emission maximum. An addition of 3 × 10−4 mol dm−3 zinc acetate enhanced the intensity of this band by about threefold. Further addition of zinc results in its coagulation. 3.2. Effect of illumination on photodissolution of ZnS, PbS, and their composites The colloidal solution of ZnS containing extra Zn2+ (1.5 × 10−4 mol dm−3 ) and PbS undergoes dissolution upon illumination by light >300/400 nm, respectively (Table 1). The com-
(a)
Dynamics of charge carriers in pure colloidal ZnS and mixed colloids ZnS/PbS and PbS/ZnS was measured by exciting these particles with 295- and 420-nm radiation using the time correlated single photon counting technique. 3.3.1. ZnS/PbS (Pb2+ + ZnS) particles Some of the representative traces for the fluorescence decay obtained upon excitation of mixed colloidal ZnS/PbS solution with 295- and 420-nm light are shown in Figs. 6, 7, and 8, respectively. It may be seen that the fluorescence due to ZnS at 450 nm decays in three exponential processes and depicts an average lifetime τ of 3.5 ns (Table 2a). In steady state experiments it was noted that the addition of a small amount of Pb2+ to ZnS quenches its fluorescence drastically (Fig. 4a). In the presence of 2.5 × 10−5 mol dm−3 of Pb2+ only, τ is reduced to 0.42 ns (Fig. 6b). A further addition of Pb2+ makes the fluorescence decay much faster, which Table 1 Quantum efficiencies of photodissolution of different nanosystems in the presence of oxygen Nanoparticles
λirr (nm)
Quantum efficiency (Φ)
ZnS with extra Zn2+ PbS ZnS/PbS (Pb2+ = 5 × 10−4 mol dm−3 ) ZnS/PbS (Pb2+ = 8 × 10−4 mol dm−3 ) PbS/ZnS
>300 >400 >400 >400 >400
0.05 0.15 0.04 0.0006 0.14
(b)
Fig. 6. Fluorescence decay curves of ZnS (1.5 × 10−4 mol dm−3 ) with excess Zn2+ containing different amounts of Pb2+ (×10−5 mol dm−3 ): 0.0 (a); 2.5 (b). λex = 295 nm; λem = 450 nm; pH 10.8. Time calibration = 5.095 × 10−11 s/channel.
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(a)
613
(b)
Fig. 7. Fluorescence decay curves of ZnS (1.5 × 10−4 mol dm−3 ) with excess Zn2+ containing different amounts of Pb2+ (×10−4 mol dm−3 ): 3.0 (a); 5.0 (b). λex = 295 nm; λem = 710 nm, pH 10.8. Time calibration = 5.095 × 10−11 s/channel.
Fig. 8. Fluorescence decay curves of ZnS (1.5 × 10−4 mol dm−3 ) with excess Zn2+ containing different amounts of Pb2+ (×10−4 mol dm−3 ): 1.0 (a); 2.0 (b); 3.0 (c); 5.0 (d); 6.5 (e); 10.0 (f). λex = 420 nm; λem = 710 nm; pH 10.8. Time calibration = 5.095 × 10−11 s/channel.
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Table 2 Effect of addition of excess Pb2+ on the lifetime of Q-ZnS: [ZnS] = 1.5 × 10−4 mol dm−3 with excess Zn2+ = 1.5 × 10−4 mol dm−3 at pH 10.8 Pb2+ (mol dm−3 )
ZnS/Pb2+ lifetime Component 1 τ1 (ns)
0 2.5 × 10−5
3.0 × 10−4 5.0 × 10−4
2.0 × 10−4 3.0 × 10−4 5.0 × 10−4 6.5 × 10−4 1.0 × 10−3
Component 2 Emission %
0.125 (0.52) 0.0345 (2.92)
30.31
0.109 (1.68 × 10−3 ) 1.01 (2.28 × 10−3 )
16.26
0.0622 (2.02) 0.0712 (1.49) 0.0679 (1.55) 0.0708 (1.51) 0.0586 (1.78)
30.24
3.14
9.95
15.36 10.83 9.12 14.59
τ2 (ns)
Component 3 Emission %
(a) λex = 295 nm; λem = 450 nm 1.06 27.84 (5.69 × 10−2 ) 0.558 91.61 (6.19 × 10−3 )
χ2
τ3 (ns)
Emission %
7.47 (1.21 × 10−2 ) 7.09 (8.16 × 10−4 )
41.85
3.5
1.22
5.25
0.42
1.06
9.2
1.02
11.9
1.01
61.25
9.4
1.13
74.31
11.9
1.01
80.10
15.1
1.03
83.54
18.9
1.04
81.25
19.4
1.07
(b) λex = 295 nm; λem = 710 nm 0.873 83.74 (4.05 × 10−3 ) 13.1 90.05 (1.59 × 10−3 ) (c) λex = 420 nm; λem = 710 nm 1.68 8.52 (2.11 × 10−2 ) 1.54 10.34 (4.6 × 10−2 ) 1.58 9.07 (5.58 × 10−2 ) 2.13 7.34 (4.02 × 10−2 ) 2.19 4.17 (1.36 × 10−2 )
τ (ns)
15.0 (1.70 × 10−2 ) 15.7 (3.2 × 10−2 ) 18.6 (4.19 × 10−2 ) 22.4 (4.36 × 10−2 ) 23.8 (2.44 × 10−2 )
Note. Values in parentheses are pre-exponential factor corresponding to respective τ .
(a)
(b)
Fig. 9. (a) Fluorescence decay curves of PbS (1.5 × 10−4 mol dm−3 ) coated with ZnS (5 × 10−4 mol dm−3 ) at pH 10.8; (b) with excess Zn2+ (3 × 10−4 mol dm−3 ). λex = 295 nm; λem = 560 nm.
did not allow its measurement because of the limitation in time resolution of the equipment used. At higher concentrations of Pb2+ these samples were excited by both 295- and 420-nm light and the decay of fluorescence was monitored at 710 nm (Figs. 7 and 8). For a typical increase in Pb2+ from 3 × 10−4 to 5 × 10−4 mol dm−3 , the 295-nm excitation results in the enhancement of the average emission lifetime from 9.2 to 11.9 ns (Table 2b). When these particles were excited at 420 nm, the increased addition of Pb2+ from 2 × 10−4 to 1 × 10−3 mol dm−3 prolonged the emission lifetime gradually from 9.4 to 19.4 ns (Table 2c). Under the latter conditions the complexity due to simultaneous excitation of ZnS could be avoided and the fluorescence lifetime corresponded to only PbS particles.
3.3.2. PbS/ZnS particles When PbS/ZnS composites are excited by 295 nm, the fluorescence at 560 nm decays in three exponential processes (Fig. 9). The average lifetime of this emission, having different components with time constants of τ1 = 45.7 ns (α1 = 0.06), τ2 = 0.24 µs (α2 = 0.35), and τ3 = 0.86 µs (α3 = 0.59), is computed to be 0.77 µs. A further addition of 3 × 10−4 mol dm−3 Zn2+ did not influence its lifetime appreciably. In view of the relatively much shorter lifetimes of pure Q-ZnS and PbS, as observed above, these lifetimes are relatively much longer. It is thus interesting to note that the modification of the surface of ZnS by PbS, PbS by ZnS, and their composites by different amounts of metal hydroxides induce emission in the wide spectral range from 350 to 800 nm (Fig. 10).
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Scheme 1. Structure of PbS and ZnS linked through metal hydroxide
Fig. 10. Normalized fluorescence spectra of different core–shell particles containing Q-PbS. ZnS/PbS containing different amount of Pb2+ (×10−4 mol dm−3 ): 0.1 (—); 1.0 (---); 5.0 (−·−·); 8.0 (−··−··); and PbS/ZnS (·····) particles containing Q-PbS (1.5 × 10−4 mol dm−3 ) ZnS (5 × 10−4 mol dm−3 ).
4. Discussion 4.1. ZnS/PbS The formation of PbS at the interface of ZnS upon adding Pb2+ could be understood due to the low solubility product of PbS, which is more than five orders of magnitude smaller than ZnS [44]: ZnS + Pb2+ → PbS + Zn2+ . The production of nanosized PbS in the face-centered cubic form is revealed by transmission electron microscopy and X-ray diffraction analysis. The shifting of the onset of the absorption and emission to longer wavelengths upon increased addition of Pb2+ can be attributed to the formation of size-quantized PbS particles (Figs. 1, 4a, 4b). At intermediate concentrations of [Pb2+ ], changes in spectral behavior demonstrate the formation of nanocomposites comprising ZnS/PbS. A loss of prominence of the excitonic peak in the absorption spectrum (Fig. 1), during exchange of Zn2+ in ZnS for Pb2+ specifically at high [Pb2+ ] might have occurred due to a change in morphology from wurtzite in ZnS to cubic in PbS and a decrease in size of the resulting PbS particles. Such an unstructured absorption spectrum of PbS has earlier been assigned to wide size distribution and surface defect states [45,46]. A complete understanding of this aspect, though, requires detailed investigation of these systems. ZnS in wurtzite and PbS in face-centered cubic forms have a mismatch in lattice constants. PbS might be nucleating separately on the ZnS phase, but in solution the two are likely to be linked through –OH of Zn(OH)2 /PbOH2 , as shown in Scheme 1 as the presence of the latter has been found to be essential for the preparation of fluorescent particles. At low concentrations of Pb2+ , where PbS is present at the interface of ZnS, the fluorescence due to ZnS is quenched, possibly due to the interfacial transfer of charge carriers from the
excited ZnS to PbS. This argument is also supported by a drastic decrease in the intensity of fluorescence and fluorescence lifetime of ZnS (Figs. 4a and 6). The fast fluorescence decay with a time constant in the picosecond time domain may arise due to the high density of trap states at the interface. It may be pointed out that pure Q-PbS particles prepared under the above conditions were nonfluorescing and a small fluorescence in them could be induced by activation with excess Zn2+ only (Fig. 4b). The development of new emission band at longer wavelengths at higher concentrations of Pb2+ (>1.5×10−4 mol dm−3 ) (Fig. 4b) and an enhancement in emission lifetime with increasing Pb2+ (Fig. 8, Table 2) might be interpreted by the increased coating of Zn(OH)2 and Pb(OH)2 on the surface of these particles in the basic medium. These hydroxides possibly passivate the surface of PbS by eliminating the nonradiative sites located in the shallow traps and exhibit band-gap emission at 680 nm at mild concentrations of Pb2+ . A further coating of these hydroxides removes relatively deeper traps, which shifts the fluorescence to longer wavelengths along with an enhancement in its intensity (Fig. 4b). Such coating does not allow the trapped e− and h+ to cause the decomposition of the PbS particle in the presence of air, as is evidenced by a substantial decrease in the quantum efficiency of its dissolution (Table 1). The addition of SH− , however, removes the coated metal hydroxides surrounding the particles by making their corresponding sulfides, which makes them nonfluorescing. In an earlier pulse radiolysis study, metal hydroxides were suggested to remove the surface defects from the surface of PbS colloids [46]. A reduction in the intensity of emission at 710 nm upon excitation by 295-nm compared to that by 420-nm light could be understood by the simultaneous excitation of ZnS and PbS at this wavelength. The CB electron transported by excited ZnS might be annihilating the CB electrons on PbS (Fig. 11), which otherwise were involved earlier in the radiative recombination. In a recent report on polymer-capped PbS systems [45], the relaxation of the conduction band electrons was found to have decay time constants of 1.2 and 45 ps using femtosecond transient absorption spectroscopy, and was observed to be independent of probing wavelength, intensity, particle size, shape, and surface environment. A careful examination of the lifetime data on PbS particles (Table 2c) reveals that τ in the Q-PbS phase varied with the extent of lead hydroxide capping. Upon increased addition of Pb2+ , the time constant(s) corresponding to component 1 remains unchanged, component 2 shows a slight increase, and component 3 shows a significant increase, and these lie in the subnanosecond and nanosecond time domains, respectively. The initial decay in the subnanosecond time domain possibly arises due to fast relaxation of charge carriers, which is then followed by their slow relaxation into the shallow
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5. Conclusions
Fig. 11. Charge dynamics in mixed colloid particles ZnS/PbS.
and deep traps of the PbS phase occurring on the nanosecond and tens-of-nanoseconds time scales, respectively. These observations indicate the distribution of surface states at varied depths on the surface of the particle. Thus the dynamics of charge carriers in the above composite system(s) can be understood by their fast relaxation in the ZnS phase followed by relatively much slower relaxation in the PbS phase. This is a clear manifestation of the interfacial relaxation of charge carriers between these two particles. 4.2. PbS/ZnS system The electronic and emission behavior of the composite PbS/ZnS particles is very different compared to either pure ZnS and PbS nanoparticles or their additive behavior. Electron micrographs of these particles (Fig. 2e) is indicative of the formation of core–shell mixed particles in which each PbS particle is present as the core unit and is surrounded by a ZnS phase attached through –OH of Zn(OH)2 /Pb(OH)2 . To surround each core PbS particle one would require a much higher concentration of ZnS; it indeed required more than three times the [PbS] before any interaction could be seen between them by absorption or emission studies (Fig. 5). Further, since the resulting mixed particle could be induced by light >320 nm also, where the ZnS phase does not possess any absorption, this clearly indicates the formation of nanocomposites comprising PbS/ZnS. In view of the energetics of ZnS and PbS phases, the excitation of the ZnS particles in the shell of PbS/ZnS would cause both the electron and hole to be transported to PbS, as shown in Fig. 11. This thermodynamic drive would eventually cause both the electron and the hole to be confined to the core PbS. Since these particles underwent photodissolution with a fairly high quantum efficiency of 0.14 in the aerated solution, this suggests that under irradiation the trapped electron may be scavenged by O2 and the hole is able to dissolve PbS/ZnS molecules present at the surface. Increased fluorescence lifetime of these particles in µs time domain also supports the charge separation in this system. A possibility that the electron remains in the shell and the hole is transported to the core PbS cannot be ruled out.
In summary, nanocomposites of ZnS/PbS and PbS/ZnS have been prepared and characterized by XRD, IR, and electronic spectroscopy. The addition of a small amount of Pb2+ to ZnS produces size-quantized PbS particles at its interface, which quenches the emission due to ZnS without inducing any significant emission due to PbS, whereas at higher concentrations of Pb2+ it produces fluorescent Q-PbS particles coated with Pb(OH)2 /Zn(OH)2 . Relaxation dynamics of charge carriers is influenced by the extent of passivation of the surface. Increased coating of PbS by Pb(OH)2 enhances the lifetime of charge carriers. Nucleation of ZnS on PbS produces a core–shell structure, which induces the separation of charge and enhances the lifetime to the microsecond time domain. It is thus apparent that the optical, photophysical, and electronic properties of these nanoparticles can be manipulated by changing the surface environment, which may have important implications in the areas of photonics and photophysics. Acknowledgments The financial support of DST, New Delhi, for this work is gratefully acknowledged. A.J. is thankful to CSIR, New Delhi, for the award of JRF. Thanks are also due to the Director & Coordinator, NCUFP, Chennai, and Director, AIIMS, New Delhi for providing us the facilities of the single photon counter and TEM, respectively. We also thank M/s Horiba John Yvon–IBH Ltd., U.K., for recording two of our samples on their fluorescence lifetime system. References [1] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [2] A. Henglein, Chem. Rev. 89 (1989) 1861. [3] M. Bruchez Jr., M. Moronne, P. Gui, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [4] A. Hagfeldt, M. Grätzel, Chem. Rev. 95 (1995) 49. [5] P.V. Kamat, Chem. Rev. 93 (1993) 267. [6] J.Z. Zhang, J. Phys. Chem. B 104 (2000) 7239. [7] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [8] L. Spanhel, H. Weller, A. Fojtik, A. Henglein, Ber. Bunsen-Ges. Phys. Chem. 91 (1987) 88. [9] A. Kumar, S. Kumar, Chem. Lett. (1996) 711. [10] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632. [11] K.R. Gopidas, M. Bohorquez, P.V. Kamat, J. Phys. Chem. 94 (1990) 6435. [12] H. Fujii, M. Ohtaki, K. Eguchi, H. Arai, J. Mol. Catal. A: Chem. 129 (1998) 61. [13] A. Kumar, A.K. Jain, J. Mol. Catal. A: Chem. 165 (2001) 267. [14] H.S. Zhou, I. Homma, H. Komiyama, J.W. Haus, J. Phys. Chem. 97 (1993) 895. [15] A. Henglein, M. Gutierrez, H. Weller, A. Fojtik, J. Jirkovsky, Ber. BunsenGes. Phys. Chem. 93 (1989) 593. [16] A. Hasselbarth, A. Eychmüller, R. Eichberger, M. Giersig, A. Mews, H. Weller, J. Phys. Chem. 97 (1993) 5333. [17] A. Henglein, M. Gutierrez, Ber. Bunsen-Ges. Phys. Chem. 87 (1983) 852. [18] C. Nasr, S. Hotchandani, W.Y. Kim, R.H. Schmehl, P.V. Kamat, J. Phys. Chem. B 1 (1997) 7480. [19] J. Rabani, J. Phys. Chem. 93 (1989) 7707. [20] P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595.
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Photophysics and charge dynamics of Q-PbS based mixed ZnS/PbS and PbS/ZnS semiconductor nanoparticles Anil Kumar ∗ , Anshuman Jakhmola Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India Received 18 August 2005; accepted 14 November 2005 Available online 27 December 2005
Abstract The surface of ZnS and PbS has been modified by interfacing PbS on ZnS and ZnS on PbS nanoparticles. This produced core–shell nanocomposites ZnS/PbS and PbS/ZnS with tunable electronic properties. In both structures PbS particles are present in cubic form with an average diameter of about 6 nm. The addition of Pb2+ (3 × 10−4 mol dm−3 ) to Q-ZnS (1.5 × 10−4 mol dm−3 ) in the basic pH range produces sizequantized fluorescent PbS particles coated by metal hydroxides. In these particles the relaxation kinetics of charge carriers has been followed using a picosecond single-photon counting technique. At 1.5 × 10−4 mol dm−3 Pb2+ an interfacial relaxation of charge from ZnS to PbS phase could be observed in subnanosecond time domain. An increase in [Pb2+ ] from 2 × 10−4 to 1 × 10−3 mol dm−3 enhanced the average emission lifetime from 9.4 to 19.4 ns. Composite PbS/ZnS particles are produced at high [ZnS] only. These particles had emission lifetime in µs time range. The extent of charge separation and the dynamics of charge carriers could be manipulated by the surface modification of these nanostructures. © 2005 Elsevier Inc. All rights reserved. Keywords: Q-PbS; Photophysical properties; Q-ZnS/PbS; Q-PbS/ZnS; Nanocomposites
1. Introduction Synthesis of different nanostructured materials with tunable optical and photophysical properties has been a subject of extensive interest because of their great scientific and technological applications [1–7]. Over the past one-and-a-half decades a variety of colloidal particles of metal sulfides and metal oxides have been prepared and investigated for their changed electronic properties. In the recent past several quantized nanoheterostructures of semiconductors, viz., CdS–Ag2 S [8,9], CdS–TiO2 [10–13], CdS–ZnO [10], CdS–PbS [14], AgI– Ag2 S [15], CdS–HgS [16], CdS–ZnS [17], SnO2 –CdS [18], ZnO–ZnS [19], etc., have been synthesized and characterized. A number of promising applications of these materials in photophysics, photocatalysis, and photonics have been reported. Interest in PbS-based nanomaterials has been aroused because of their unique optical and emission properties, which * Corresponding author. Fax: +91 1332 273560.
E-mail address: [email protected] (A. Kumar). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.11.028
have tremendous applications in lasers [20], light-emitting devices [21,22], detectors [23], nonlinear optics [24–27], single electron devices [28], optical switches, optical amplification [27,29], telecommunication [29], etc. PbS is an indirect band gap semiconductor with a bulk band gap of about 0.41 eV. It has smaller effective masses of charge carriers, high value of dielectric constant, and a fairly high value of excitonic size. For these reasons it allows strong quantum confinement even in relatively large PbS quantum dots [30,31]. A number of methods have been used to modify the surface of these particles, which results in their changed optical, electronic, and structural properties [32–38]. Lately, exchange of cation(s), specifically Cd2+ for Ag+ , has been investigated to synthesize CdSe/Ag2 Se and CdS/Ag2 S nanocrystals of varied composition, size, and shape [39]. In the present work we have synthesized core–shell type structures of PbS nanoparticles, ZnS/PbS and PbS/ZnS, by interfacial exchange of Zn2+ for Pb2+ from ZnS and interfacing ZnS on PbS, respectively. In these materials ZnS and PbS are present in the wurtzite and face-centered cubic (fcc) forms, respectively. The optical and photophysical properties of these
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Fig. 1. Electronic spectra of ZnS (1.5 × 10−4 mol dm−3 ) containing 1.5 × 10−4 mol dm−3 excess Zn2+ in the presence of different amounts of Pb2+ (×10−4 mol dm−3 ): 0.0 (—); 0.08 (---); 0.3 (· · ·); 1.0 (−·−·); 1.5 (−··−··); 5.0 (·····) at pH 10.8. Inset: Absorption spectra of PbS (1.5 × 10−4 mol dm−3 ) at pH 10.8.
particles have been analyzed, and the dynamics of charge carriers has been monitored to understand these systems.
with a stand-alone ignitor and a 200-W mercury–xenon lamp. Solid samples of different photocatalysts were prepared by removing water on a Buchi Rotavapor RIIH.
2. Experimental 2.3. Preparation of mixed colloidal ZnS/PbS and PbS/ZnS 2.1. Materials Anhydrous zinc acetate (CDH), NaOH (BDH), lead acetate, and sodium hexametaphosphate (Qualigens) were of analytical grade. All these chemicals were used without any further purification. Nitrogen (purity >99.9%) was used for deoxygenating the prepared samples and oxygen (purity >99.9%) was bubbled during irradiation of the samples.
Colloidal solutions of ZnS [40] and PbS were prepared by injection of SH− into their respective deaerated salt solutions using HMP as stabilizer, following the earlier reported method. Mixed ZnS/PbS colloids were prepared by the addition of different amounts of lead acetate to the above-prepared Q-ZnS. PbS/ZnS was prepared by coating ZnS on PbS by adding varied amounts of zinc acetate, followed by SH− . The pH of all these solutions was maintained at 10.8 by adding dilute solutions of NaOH.
2.2. Equipment 2.4. Methodology Electronic spectra were recorded on a Shimadzu UV2100S spectrophotometer. Emission measurements were made on a Shimadzu RF-5301PC spectrofluorophotometer. Electron microscopy was performed on a Fei-Philips Morgagni 268D Digital TEM with image analysis system having variable magnifications up to 280,000×. X-ray diffraction patterns were recorded on a Philips DW 1140/90 X-ray diffractometer using the CuKα line of the X-ray source. IR spectra were measured on a Thermonicolet Nexus FTIR spectrophotometer. The fluorescence lifetime in the picosecond/nanosecond time domain was recorded on an IBH 5000U fluorescence spectrophotometer using a picosecond Tsunami titanium–sapphire modelocked laser as an excitation source. Some experiments have also been performed using a 295-nm diode laser as an excitation source. A Hamamatsu photomultiplier was used for the detection of fluorescence. Photolysis of different semiconductor systems was carried out on an Oriel photolysis assembly equipped
For recording transmission electron micrographs of different particles a small drop of the colloidal solution was adsorbed on carbon-coated copper G-200 grids. Grids were scanned at an accelerating voltage of 100 kV. The particle size was measured online using Analysis 3.2 from a soft imaging system. X-ray diffraction patterns of powders of various nanosystems were recorded by scanning these samples in a 2θ range of 10◦ –90◦ . In these experiments the acceleration voltage was set at 35 kV with 30 mV flux. IR spectra were measured in KBr and CsI media in the mid and far IR ranges, respectively. Fluorescence lifetime decay curves were analyzed kinetically using a multiexponential iterative technique from IBH. The goodness of the fit was adjudged by evaluating χ 2 from the plot of weighed residuals and autocorrelation functions. For photolysis experiment, solutions and glass filters were used to select the light in the desired wavelength range.
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Fig. 2. Electron micrograph(s) of PbS (a); ZnS containing excess Zn2+ (b); ZnS/Pb2+ (Pb2+ = 5.0 × 10−4 mol dm−3 ) (c); ZnS/Pb2+ (Pb2+ = 8.0 × 10−4 mol dm−3 ) (d); and PbS/ZnS (PbS = 1.5 × 10−4 mol dm−3 and ZnS = 5.0 × 10−4 mol dm−3 ) (e).
3. Results 3.1. Optical, structural, and photophysical behavior 3.1.1. ZnS/PbS (Pb2+ + ZnS) The addition of Pb2+ to the colloidal ZnS solution containing 1.5 × 10−4 mol dm−3 excess Zn2+ results in the shift of its optical band gap to a longer wavelength (Fig. 1). An increase in the amount of Pb2+ gradually shifted the band gap to the red and eventually to the NIR region. The absorption coefficient of the mixed particle is enhanced in the entire recorded spectral range from 200 to 800 nm. These particles depicted broad shoulders between 280–300 nm and 340–360 nm at high [Pb2+ ] and may be assigned as excitonic bands. These absorption features are similar to those reported earlier for PbS particles [27]. Electron micrograph(s) and size histogram(s) (not shown) of ZnS and the mixed colloids (ZnS/PbS) demonstrate that the average size of the mixed colloids having an excess of Pb2+ was relatively much smaller (6 nm) than that of pure ZnS (14 nm) and depicted a relatively narrow size distribution (2–10 nm). The electronic spectrum, as well as the size of the resulting particles in the mixed colloids at higher concentrations of Pb2+ , is very similar to those of pure PbS particles (inset, Fig. 1, and Fig. 2). IR spectral features of the mixed particles (not shown) were found to match literature data on the sum spectra of pure ZnS and PbS [41,42]. The structures of these material(s) in both pure and mixed colloids were analyzed by XRD (Fig. 3). A comparison of the 2d spacing of pure ZnS with its literature data reveals
it to be produced in wurtzite form [43]. It also showed the presence of Zn(OH)2 in orthorhombic shape (Fig. 3a). In the mixed colloids, at low concentrations of Pb2+ (1 × 10−4 mol dm−3 ), ZnS and PbS phases were detected to be present in wurtzite and face-centered cubic structures, respectively (not shown). At high concentrations of Pb2+ (>5 × 10−4 mol dm−3 ), ZnS phase disappeared completely and besides PbS, Zn(OH)2 and PbO/Pb(OH)2 phases were also detected. PbS and Zn(OH)2 /PbO are produced in the facecentered cubic and orthorhombic forms, respectively (Fig. 3b). Fig. 4 presents the fluorescence spectra of pure ZnS and the mixed colloids consisting of different amounts of Pb2+ added to ZnS having excess Zn2+ (1.5 × 10−4 mol dm−3 ). The fluorescence spectrum of ZnS exhibits a band at 460 nm (Fig. 4a), which is similar to that reported earlier [40]. Pure PbS particles under normal experimental conditions did not depict any emission band. In the presence of excess Zn2+ it, however, shows a very weak and broad emission peaking at 700 nm (not depicted in the figure). Changes in the fluorescence spectra of ZnS at low and high concentrations of Pb2+ are shown in Figs. 4a and 4b, respectively. These samples were excited by both 295- and 420-nm light. In the low concentration range of Pb2+ (1×10−5 to 1.5×10−4 mol dm−3 ), the excitation of these particles by 295-nm radiation results in the quenching of the emission due to ZnS with a regular change in the emission maxima ranging from 460 to 520 nm. About 1.5 × 10−4 mol dm−3 Pb2+ quenched the emission almost completely (Fig. 4a). The excitation of these particles by 420-nm light did not depict emission due to ZnS; however, a very weak new band is seen
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(a)
(b)
(c) Fig. 3. X-ray diffraction patterns of different colloids: ZnS containing excess Zn2+ (a); ZnS containing Pb2+ (Pb2+ = 5.0 × 10−4 mol dm−3 ) (b); and PbS/ZnS (c).
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(a)
(b) Fig. 4. Emission spectra of ZnS (1.5 × 10−4 mol dm−3 ) containing 1.5 × 10−4 mol dm−3 excess Zn2+ (a) in the presence of different amounts of Pb2+ (×10−5 mol dm−3 )—0.0 (—); 1.0 (---) 3.0 (· · ·); 6.0 (−·−·); 9.0 (−··−··); 15.0 (·····)—at pH 10.8, λex = 295 nm; (b) in the presence of different amounts of Pb2+ (×10−4 mol dm−3 ) at pH 10.8, λex = 420 nm. Inset. Emission intensity at 710 nm as a function of [Pb2+ ] upon excitation by 295-nm (") and 420-nm (Q) radiation.
in the red region. The addition of higher concentration of Pb2+ (>1.5 × 10−4 mol dm−3 ) resulted in the production of a new emission band at 680 nm, which is red-shifted gradually by the increased addition of Pb2+ (Fig. 4b) up to 1 × 10−3 mol dm−3 of Pb2+ . A further increase in the concentration of Pb2+ coagulates this colloidal solution within a few hours. The emission due to these particles is relatively weak, similar to that reported earlier for PbS particles [34]. This emission is quenched by SH− . The addition of about 1 × 10−4 mol dm−3 of SH− reduced the emission intensity by about 60%. A comparison of the fluorescence behavior of mixed colloids observed at relatively higher [Pb2+ ] (>1.5 × 10−4 mol dm−3 ) upon excitation by 295- and 420-nm wavelengths reveals that under both conditions the emission band is observed at around 710 nm. However, the excitation by 295-nm depicts smaller intensity of emission at 710 nm despite having higher absorption coefficients due to both ZnS and PbS at 295 nm (inset, Fig. 4b).
3.1.2. PbS/ZnS The absorption spectrum of PbS has an onset of absorption in NIR region, and is very similar to that of an indirect gap semiconductor [30]. Fig. 5 shows the electronic spectra of Q-PbS coated by ZnS at its different concentrations. The coating by ZnS at its low concentration did not exhibit any spectral change nonindicating the interaction between the two; however, at higher concentrations (>3.5 times that of PbS) it reveals a change in its optical behavior (Fig. 5, curve c). The absorption coefficient of the composite particles is decreased in the wavelength range 320–800 nm and depicts two shoulders at 300 and 330 nm. The electron micrograph of PbS/ZnS composite exhibits the presence of large (bright) as well as small size (dark) particles (Fig. 2e). Their XRD pattern demonstrates the presence of both ZnS and PbS phases in wurtzite and face centered cubic structural forms respectively (Fig. 3c). Apart to this it also exhibited
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posite particles ZnS/PbS and PbS/ZnS also underwent photodissolution by visible light with quantum efficiencies of 0.04 and 0.14, respectively. Interestingly, PbS particles coated by a layer of zinc and larger amounts of lead hydroxide become relatively more photostable toward UV/visible radiation and are corroded with a significant reduction in the quantum efficiency to 0.0006 (Table 1). These particles could be stored at 5 ◦ C for more than 6 months without any appreciable changes in their optical properties. 3.3. Dynamics of charge carriers
Fig. 5. Electronic and emission spectra of PbS (1.5 × 10−4 mol dm−3 ) coated with different concentrations of ZnS (×10−4 mol dm−3 ): 0.5 (a, a1 ); 3.0 (b, b1 ); 5.0 (c, c1 ) at pH 10.8, λex = 295 nm.
the presence of Zn(OH)2 in the orthorhombic form. Thus TEM and XRD studies demonstrate them to contain both ZnS and PbS phases. At low concentrations of ZnS (3 × 10−4 mol dm−3 ), the excitation of the layered PbS/ZnS particles by 295-nm causes emission centered at about 450 nm (Fig. 5, curve b1 ), which appears to arise due to the ZnS phase. The high concentration of ZnS (>3 × 10−4 mol dm−3 ), however, induces a new fluorescence band in the visible region at 560 nm (Fig. 5, curve c1 ), which is different to both ZnS and PbS. The excitation of these particles by light >320 nm, where pure ZnS does not have any absorption, reduced the emission intensity of the 560-nm band without causing any appreciable shift in the emission maximum. An addition of 3 × 10−4 mol dm−3 zinc acetate enhanced the intensity of this band by about threefold. Further addition of zinc results in its coagulation. 3.2. Effect of illumination on photodissolution of ZnS, PbS, and their composites The colloidal solution of ZnS containing extra Zn2+ (1.5 × 10−4 mol dm−3 ) and PbS undergoes dissolution upon illumination by light >300/400 nm, respectively (Table 1). The com-
(a)
Dynamics of charge carriers in pure colloidal ZnS and mixed colloids ZnS/PbS and PbS/ZnS was measured by exciting these particles with 295- and 420-nm radiation using the time correlated single photon counting technique. 3.3.1. ZnS/PbS (Pb2+ + ZnS) particles Some of the representative traces for the fluorescence decay obtained upon excitation of mixed colloidal ZnS/PbS solution with 295- and 420-nm light are shown in Figs. 6, 7, and 8, respectively. It may be seen that the fluorescence due to ZnS at 450 nm decays in three exponential processes and depicts an average lifetime τ of 3.5 ns (Table 2a). In steady state experiments it was noted that the addition of a small amount of Pb2+ to ZnS quenches its fluorescence drastically (Fig. 4a). In the presence of 2.5 × 10−5 mol dm−3 of Pb2+ only, τ is reduced to 0.42 ns (Fig. 6b). A further addition of Pb2+ makes the fluorescence decay much faster, which Table 1 Quantum efficiencies of photodissolution of different nanosystems in the presence of oxygen Nanoparticles
λirr (nm)
Quantum efficiency (Φ)
ZnS with extra Zn2+ PbS ZnS/PbS (Pb2+ = 5 × 10−4 mol dm−3 ) ZnS/PbS (Pb2+ = 8 × 10−4 mol dm−3 ) PbS/ZnS
>300 >400 >400 >400 >400
0.05 0.15 0.04 0.0006 0.14
(b)
Fig. 6. Fluorescence decay curves of ZnS (1.5 × 10−4 mol dm−3 ) with excess Zn2+ containing different amounts of Pb2+ (×10−5 mol dm−3 ): 0.0 (a); 2.5 (b). λex = 295 nm; λem = 450 nm; pH 10.8. Time calibration = 5.095 × 10−11 s/channel.
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(a)
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(b)
Fig. 7. Fluorescence decay curves of ZnS (1.5 × 10−4 mol dm−3 ) with excess Zn2+ containing different amounts of Pb2+ (×10−4 mol dm−3 ): 3.0 (a); 5.0 (b). λex = 295 nm; λem = 710 nm, pH 10.8. Time calibration = 5.095 × 10−11 s/channel.
Fig. 8. Fluorescence decay curves of ZnS (1.5 × 10−4 mol dm−3 ) with excess Zn2+ containing different amounts of Pb2+ (×10−4 mol dm−3 ): 1.0 (a); 2.0 (b); 3.0 (c); 5.0 (d); 6.5 (e); 10.0 (f). λex = 420 nm; λem = 710 nm; pH 10.8. Time calibration = 5.095 × 10−11 s/channel.
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Table 2 Effect of addition of excess Pb2+ on the lifetime of Q-ZnS: [ZnS] = 1.5 × 10−4 mol dm−3 with excess Zn2+ = 1.5 × 10−4 mol dm−3 at pH 10.8 Pb2+ (mol dm−3 )
ZnS/Pb2+ lifetime Component 1 τ1 (ns)
0 2.5 × 10−5
3.0 × 10−4 5.0 × 10−4
2.0 × 10−4 3.0 × 10−4 5.0 × 10−4 6.5 × 10−4 1.0 × 10−3
Component 2 Emission %
0.125 (0.52) 0.0345 (2.92)
30.31
0.109 (1.68 × 10−3 ) 1.01 (2.28 × 10−3 )
16.26
0.0622 (2.02) 0.0712 (1.49) 0.0679 (1.55) 0.0708 (1.51) 0.0586 (1.78)
30.24
3.14
9.95
15.36 10.83 9.12 14.59
τ2 (ns)
Component 3 Emission %
(a) λex = 295 nm; λem = 450 nm 1.06 27.84 (5.69 × 10−2 ) 0.558 91.61 (6.19 × 10−3 )
χ2
τ3 (ns)
Emission %
7.47 (1.21 × 10−2 ) 7.09 (8.16 × 10−4 )
41.85
3.5
1.22
5.25
0.42
1.06
9.2
1.02
11.9
1.01
61.25
9.4
1.13
74.31
11.9
1.01
80.10
15.1
1.03
83.54
18.9
1.04
81.25
19.4
1.07
(b) λex = 295 nm; λem = 710 nm 0.873 83.74 (4.05 × 10−3 ) 13.1 90.05 (1.59 × 10−3 ) (c) λex = 420 nm; λem = 710 nm 1.68 8.52 (2.11 × 10−2 ) 1.54 10.34 (4.6 × 10−2 ) 1.58 9.07 (5.58 × 10−2 ) 2.13 7.34 (4.02 × 10−2 ) 2.19 4.17 (1.36 × 10−2 )
τ (ns)
15.0 (1.70 × 10−2 ) 15.7 (3.2 × 10−2 ) 18.6 (4.19 × 10−2 ) 22.4 (4.36 × 10−2 ) 23.8 (2.44 × 10−2 )
Note. Values in parentheses are pre-exponential factor corresponding to respective τ .
(a)
(b)
Fig. 9. (a) Fluorescence decay curves of PbS (1.5 × 10−4 mol dm−3 ) coated with ZnS (5 × 10−4 mol dm−3 ) at pH 10.8; (b) with excess Zn2+ (3 × 10−4 mol dm−3 ). λex = 295 nm; λem = 560 nm.
did not allow its measurement because of the limitation in time resolution of the equipment used. At higher concentrations of Pb2+ these samples were excited by both 295- and 420-nm light and the decay of fluorescence was monitored at 710 nm (Figs. 7 and 8). For a typical increase in Pb2+ from 3 × 10−4 to 5 × 10−4 mol dm−3 , the 295-nm excitation results in the enhancement of the average emission lifetime from 9.2 to 11.9 ns (Table 2b). When these particles were excited at 420 nm, the increased addition of Pb2+ from 2 × 10−4 to 1 × 10−3 mol dm−3 prolonged the emission lifetime gradually from 9.4 to 19.4 ns (Table 2c). Under the latter conditions the complexity due to simultaneous excitation of ZnS could be avoided and the fluorescence lifetime corresponded to only PbS particles.
3.3.2. PbS/ZnS particles When PbS/ZnS composites are excited by 295 nm, the fluorescence at 560 nm decays in three exponential processes (Fig. 9). The average lifetime of this emission, having different components with time constants of τ1 = 45.7 ns (α1 = 0.06), τ2 = 0.24 µs (α2 = 0.35), and τ3 = 0.86 µs (α3 = 0.59), is computed to be 0.77 µs. A further addition of 3 × 10−4 mol dm−3 Zn2+ did not influence its lifetime appreciably. In view of the relatively much shorter lifetimes of pure Q-ZnS and PbS, as observed above, these lifetimes are relatively much longer. It is thus interesting to note that the modification of the surface of ZnS by PbS, PbS by ZnS, and their composites by different amounts of metal hydroxides induce emission in the wide spectral range from 350 to 800 nm (Fig. 10).
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Scheme 1. Structure of PbS and ZnS linked through metal hydroxide
Fig. 10. Normalized fluorescence spectra of different core–shell particles containing Q-PbS. ZnS/PbS containing different amount of Pb2+ (×10−4 mol dm−3 ): 0.1 (—); 1.0 (---); 5.0 (−·−·); 8.0 (−··−··); and PbS/ZnS (·····) particles containing Q-PbS (1.5 × 10−4 mol dm−3 ) ZnS (5 × 10−4 mol dm−3 ).
4. Discussion 4.1. ZnS/PbS The formation of PbS at the interface of ZnS upon adding Pb2+ could be understood due to the low solubility product of PbS, which is more than five orders of magnitude smaller than ZnS [44]: ZnS + Pb2+ → PbS + Zn2+ . The production of nanosized PbS in the face-centered cubic form is revealed by transmission electron microscopy and X-ray diffraction analysis. The shifting of the onset of the absorption and emission to longer wavelengths upon increased addition of Pb2+ can be attributed to the formation of size-quantized PbS particles (Figs. 1, 4a, 4b). At intermediate concentrations of [Pb2+ ], changes in spectral behavior demonstrate the formation of nanocomposites comprising ZnS/PbS. A loss of prominence of the excitonic peak in the absorption spectrum (Fig. 1), during exchange of Zn2+ in ZnS for Pb2+ specifically at high [Pb2+ ] might have occurred due to a change in morphology from wurtzite in ZnS to cubic in PbS and a decrease in size of the resulting PbS particles. Such an unstructured absorption spectrum of PbS has earlier been assigned to wide size distribution and surface defect states [45,46]. A complete understanding of this aspect, though, requires detailed investigation of these systems. ZnS in wurtzite and PbS in face-centered cubic forms have a mismatch in lattice constants. PbS might be nucleating separately on the ZnS phase, but in solution the two are likely to be linked through –OH of Zn(OH)2 /PbOH2 , as shown in Scheme 1 as the presence of the latter has been found to be essential for the preparation of fluorescent particles. At low concentrations of Pb2+ , where PbS is present at the interface of ZnS, the fluorescence due to ZnS is quenched, possibly due to the interfacial transfer of charge carriers from the
excited ZnS to PbS. This argument is also supported by a drastic decrease in the intensity of fluorescence and fluorescence lifetime of ZnS (Figs. 4a and 6). The fast fluorescence decay with a time constant in the picosecond time domain may arise due to the high density of trap states at the interface. It may be pointed out that pure Q-PbS particles prepared under the above conditions were nonfluorescing and a small fluorescence in them could be induced by activation with excess Zn2+ only (Fig. 4b). The development of new emission band at longer wavelengths at higher concentrations of Pb2+ (>1.5×10−4 mol dm−3 ) (Fig. 4b) and an enhancement in emission lifetime with increasing Pb2+ (Fig. 8, Table 2) might be interpreted by the increased coating of Zn(OH)2 and Pb(OH)2 on the surface of these particles in the basic medium. These hydroxides possibly passivate the surface of PbS by eliminating the nonradiative sites located in the shallow traps and exhibit band-gap emission at 680 nm at mild concentrations of Pb2+ . A further coating of these hydroxides removes relatively deeper traps, which shifts the fluorescence to longer wavelengths along with an enhancement in its intensity (Fig. 4b). Such coating does not allow the trapped e− and h+ to cause the decomposition of the PbS particle in the presence of air, as is evidenced by a substantial decrease in the quantum efficiency of its dissolution (Table 1). The addition of SH− , however, removes the coated metal hydroxides surrounding the particles by making their corresponding sulfides, which makes them nonfluorescing. In an earlier pulse radiolysis study, metal hydroxides were suggested to remove the surface defects from the surface of PbS colloids [46]. A reduction in the intensity of emission at 710 nm upon excitation by 295-nm compared to that by 420-nm light could be understood by the simultaneous excitation of ZnS and PbS at this wavelength. The CB electron transported by excited ZnS might be annihilating the CB electrons on PbS (Fig. 11), which otherwise were involved earlier in the radiative recombination. In a recent report on polymer-capped PbS systems [45], the relaxation of the conduction band electrons was found to have decay time constants of 1.2 and 45 ps using femtosecond transient absorption spectroscopy, and was observed to be independent of probing wavelength, intensity, particle size, shape, and surface environment. A careful examination of the lifetime data on PbS particles (Table 2c) reveals that τ in the Q-PbS phase varied with the extent of lead hydroxide capping. Upon increased addition of Pb2+ , the time constant(s) corresponding to component 1 remains unchanged, component 2 shows a slight increase, and component 3 shows a significant increase, and these lie in the subnanosecond and nanosecond time domains, respectively. The initial decay in the subnanosecond time domain possibly arises due to fast relaxation of charge carriers, which is then followed by their slow relaxation into the shallow
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5. Conclusions
Fig. 11. Charge dynamics in mixed colloid particles ZnS/PbS.
and deep traps of the PbS phase occurring on the nanosecond and tens-of-nanoseconds time scales, respectively. These observations indicate the distribution of surface states at varied depths on the surface of the particle. Thus the dynamics of charge carriers in the above composite system(s) can be understood by their fast relaxation in the ZnS phase followed by relatively much slower relaxation in the PbS phase. This is a clear manifestation of the interfacial relaxation of charge carriers between these two particles. 4.2. PbS/ZnS system The electronic and emission behavior of the composite PbS/ZnS particles is very different compared to either pure ZnS and PbS nanoparticles or their additive behavior. Electron micrographs of these particles (Fig. 2e) is indicative of the formation of core–shell mixed particles in which each PbS particle is present as the core unit and is surrounded by a ZnS phase attached through –OH of Zn(OH)2 /Pb(OH)2 . To surround each core PbS particle one would require a much higher concentration of ZnS; it indeed required more than three times the [PbS] before any interaction could be seen between them by absorption or emission studies (Fig. 5). Further, since the resulting mixed particle could be induced by light >320 nm also, where the ZnS phase does not possess any absorption, this clearly indicates the formation of nanocomposites comprising PbS/ZnS. In view of the energetics of ZnS and PbS phases, the excitation of the ZnS particles in the shell of PbS/ZnS would cause both the electron and hole to be transported to PbS, as shown in Fig. 11. This thermodynamic drive would eventually cause both the electron and the hole to be confined to the core PbS. Since these particles underwent photodissolution with a fairly high quantum efficiency of 0.14 in the aerated solution, this suggests that under irradiation the trapped electron may be scavenged by O2 and the hole is able to dissolve PbS/ZnS molecules present at the surface. Increased fluorescence lifetime of these particles in µs time domain also supports the charge separation in this system. A possibility that the electron remains in the shell and the hole is transported to the core PbS cannot be ruled out.
In summary, nanocomposites of ZnS/PbS and PbS/ZnS have been prepared and characterized by XRD, IR, and electronic spectroscopy. The addition of a small amount of Pb2+ to ZnS produces size-quantized PbS particles at its interface, which quenches the emission due to ZnS without inducing any significant emission due to PbS, whereas at higher concentrations of Pb2+ it produces fluorescent Q-PbS particles coated with Pb(OH)2 /Zn(OH)2 . Relaxation dynamics of charge carriers is influenced by the extent of passivation of the surface. Increased coating of PbS by Pb(OH)2 enhances the lifetime of charge carriers. Nucleation of ZnS on PbS produces a core–shell structure, which induces the separation of charge and enhances the lifetime to the microsecond time domain. It is thus apparent that the optical, photophysical, and electronic properties of these nanoparticles can be manipulated by changing the surface environment, which may have important implications in the areas of photonics and photophysics. Acknowledgments The financial support of DST, New Delhi, for this work is gratefully acknowledged. A.J. is thankful to CSIR, New Delhi, for the award of JRF. Thanks are also due to the Director & Coordinator, NCUFP, Chennai, and Director, AIIMS, New Delhi for providing us the facilities of the single photon counter and TEM, respectively. We also thank M/s Horiba John Yvon–IBH Ltd., U.K., for recording two of our samples on their fluorescence lifetime system. References [1] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [2] A. Henglein, Chem. Rev. 89 (1989) 1861. [3] M. Bruchez Jr., M. Moronne, P. Gui, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013. [4] A. Hagfeldt, M. Grätzel, Chem. Rev. 95 (1995) 49. [5] P.V. Kamat, Chem. Rev. 93 (1993) 267. [6] J.Z. Zhang, J. Phys. Chem. B 104 (2000) 7239. [7] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [8] L. Spanhel, H. Weller, A. Fojtik, A. Henglein, Ber. Bunsen-Ges. Phys. Chem. 91 (1987) 88. [9] A. Kumar, S. Kumar, Chem. Lett. (1996) 711. [10] L. Spanhel, H. Weller, A. Henglein, J. Am. Chem. Soc. 109 (1987) 6632. [11] K.R. Gopidas, M. Bohorquez, P.V. Kamat, J. Phys. Chem. 94 (1990) 6435. [12] H. Fujii, M. Ohtaki, K. Eguchi, H. Arai, J. Mol. Catal. A: Chem. 129 (1998) 61. [13] A. Kumar, A.K. Jain, J. Mol. Catal. A: Chem. 165 (2001) 267. [14] H.S. Zhou, I. Homma, H. Komiyama, J.W. Haus, J. Phys. Chem. 97 (1993) 895. [15] A. Henglein, M. Gutierrez, H. Weller, A. Fojtik, J. Jirkovsky, Ber. BunsenGes. Phys. Chem. 93 (1989) 593. [16] A. Hasselbarth, A. Eychmüller, R. Eichberger, M. Giersig, A. Mews, H. Weller, J. Phys. Chem. 97 (1993) 5333. [17] A. Henglein, M. Gutierrez, Ber. Bunsen-Ges. Phys. Chem. 87 (1983) 852. [18] C. Nasr, S. Hotchandani, W.Y. Kim, R.H. Schmehl, P.V. Kamat, J. Phys. Chem. B 1 (1997) 7480. [19] J. Rabani, J. Phys. Chem. 93 (1989) 7707. [20] P.T. Guerreiro, S. Ten, N.F. Borrelli, J. Butty, G.E. Jabbour, N. Peyghambarian, Appl. Phys. Lett. 71 (1997) 1595.
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