Article pubs.acs.org/JPCC
Charge Transfer Dynamics in CdS and CdSe@CdS Based Hybrid Nanorods Tipped with Both PbS and Pt Pazit Rukenstein,† Ayelet Teitelboim,‡ Michael Volokh,† Mahmud Diab,† Dan Oron,‡ and Taleb Mokari*,† †
Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva, Israel 8410501 ‡ Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot, Israel 7610001 S Supporting Information *
ABSTRACT: The synthesis of hybrid nanostructures that have specific properties has become a significant topic for construction of “smart” nanomaterials for various applications. Formation of hybrid nanostructures, particularly those combining metals and semiconductors, often introduces new chemical, optical, and electronic properties. Here, we show a simple solution phase synthesis of multicomponent heterostructures based on the growth of metal and semiconductor onto the tips of semiconductor nanorods, leading to the formation of a hybrid semiconductor/semiconductor/metal structure. The synthesis involves the reduction of Pt− acetylacetonate to achieve selective growth of a Pt metal tip onto one side of the CdS rod, followed by the thermal decomposition of Pb−bis(diethyldithiocarbamate) to grow a PbS nanocrystal onto the other tip of the nanorod. The band alignment between the two semiconductor components as well as the alignment with the Fermi level of the metal could support intraparticle charge transfer, which is often sought after for photocatalysis applications. Yet, we show, using femtosecond transient differential absorption spectroscopy (TDA), that carrier dynamics in such a hybrid system can be more complex than that predicted simply by bulk band alignment considerations. This highlights the importance of the design of band alignment and interface passivation and its role in affecting carrier dynamics within hybrid nanostructures.
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INTRODUCTION A hybrid nanostructure is composed of two or more distinct subcomponents in a particular spatial arrangement. Interfacing two or more materials in the nanoscale regime can provide new properties that are different from the individual components. In the past decade, the formation of hybrid nanostructures composed of two (or more) semiconductors,1−7 two metals,8,9 and a combination of metals and semiconductors10−16 has been reported. Heterostructures of two semiconductors can form two types (type I, type II) of interfaces based on their energy band alignment. Type I confines charge carriers in one material, often improving the optical emission properties,17−20 whereas type II leads to charge separation within the hybrid structure and thus often enhances the catalytic activity of the nanostructure.21−23 Furthermore, growth of a metal onto a semiconductor can not only lead to charge separation but also can significantly modify charge transfer dynamics from the structure and can thus enhance the photocatalytic activity of the semiconductor.24−27 Various groups have prepared anisotropic multicomponent heterostructure using colloidal solution-phase methods. Selective growth of metals onto the tips of semiconductor rods such as Au, 28 Pt, PtNi, PtCo, 29 and Co 30 was previously © XXXX American Chemical Society
demonstrated. The anisotropic growth was extended to semiconductor materials that were grown onto the tips of semiconductor rods like PdS on CdS,31 EuS on CdS or CdSe rods,32 and PbSe on CdSe or CdS rods.33 We reported recently the synthesis of several hybrid nanostructures of CdS−PbS, CdS−Ag2S, and CdS−Cu2−xS via thermal decomposition of the metal precursor.34 An alternative method for attaining hybrid nanostructures with similar compositions is the use of partial cation exchange.35,36 Most of the reported hybrid nanostructures comprise of two components as was mentioned above. Recently, the growth of multiple distinct materials onto the tips of semiconductor nanorods has become widespread due to the unique properties that may result from the formation of multiple types of interfaces. For example, selective growth of Au on one side and Ag2S on the other side of CdSe@CdS rods was reported by Chan and co-workers.37 They showed selective growth of Au on one tip and on the other tip they grew Ag2S by the cation exchange method. The number, size, and composition of the Received: April 25, 2016 Revised: June 28, 2016
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DOI: 10.1021/acs.jpcc.6b04151 J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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EXPERIMENTAL SECTION CdSe@CdS nanorods were synthesized according to previously published procedure.44,45 Briefly, CdO (60 mg, 0.47 mmol), octadecylphosphonic acid (ODPA, 290 mg, 0.86 mmol), hexylphosphonic acid (HPA, 80 mg, 0.48 mmol), and trioctylphosphine oxide (TOPO, 3.00 g) were degassed under inert atmosphere and then heated to 300 °C, and 1.80 mL of trioctylphosphine (TOP) was injected. The solution was further heated to 380 °C, and a solution of TOP:S with CdSe dots was injected (120 mg, 3.74 mmol of S in 1.8 mL of TOP mixed with 0.2 mL of ∼400 μM CdSe seeds). The solution was allowed to stand for about 6−8 min before cooling down to 60 °C, when toluene and methanol were added. The synthesis of the PbS−CdSe@CdS−Pt hybrid nanostructure is carried out in two steps. First, we grew the Pt onto one side of the nanorods (Figure 1c). The synthesis is based on the injection of 18 mg (0.04 mmol) of Pt−acetylacetonate mixed with colloidal CdS rods dispersed in dichlorobenzene into a solution of 44 mg (0.17 mmol) of 1,2-hexadecanediol, 0.2 mL of oleylamine, 0.2 mL of oleic acid, and 10 mL of diphenyl ether at 200 °C for 4 min. The product was separated and cleaned by adding toluene and methanol followed by centrifugation.29 In the second stage, PbS particles were grown onto the other tip of the rods by heating the redispersed CdSe@CdS−Pt rods (OD = 0.9 at the first excitonic peak) with 5 mg (0.01 mmol) of Pb−bis(diethyldithiocarbamate) in TOP solvent at 190 °C for 2 min (Figure 1d).34 Transient absorption (TA) of different samples was measured in toluene, using the doubled fundamental of an amplified 120 fs Ti:sapphire system at 400 nm as a pump source or an optical parametric amplifier (OPA) generated 520 nm pump, operating at 250 Hz. The pump was followed at different delay times with a white light pulse, generated by focusing a part of the 800 nm fundamental beam onto a 6 mm thick Ca2F plate, creating a white supercontinuum. The Ca2F plate was translated continuously in a circular motion to avoid thermalization, which would affect the Ca2F white spectrum. For each time delay, the signal was accumulated for 8 s for signal-to-noise ratio (SNR) improvement. The experiment was done in two modes. In the first mode, the pump was set to 400 nm, exciting the CdS. The bleach dynamics of the CdS was analyzed to see the effect of the presence of Pt and PbS on the tips of the CdS nanorods. CdS− Pt, CdS−PbS, and Pt−CdS−PbS hybrid nanostructures were compared to a reference sample of pure CdS. The 400 nm pump excites the CdS and the PbS, however as the extinction coefficient of the CdS is much higher, the effect is dominated by the CdS absorption. The samples’ OD at 400 nm was set to be the same, in order to have similar concentration of the nanostructures for all samples, to avoid artifacts. The excitation power of the 400 nm pump was set to 95 μW (134 μJ cm−2), determined by the saturation measurement of the CdS rods, finding the saturation pump intensity in which the excitation leads to a single exciton per rod on average. In the second mode, the pump was set to 520 nm, exciting only the PbS. Pumping at 520 nm shows the effect of PbS excitation on the CdS bleach at 475 nm. The samples’ OD at 400 nm was set to be the same, in order to have similar concentration of the nanostructures for all the samples, to be able to compare them, making sure no bleach is seen in the reference sample. Because of low SNR, resulting from the low excitation intensity (40 μJ cm−2), and the small cross section of
metal domains grown on seeded semiconductor nanorods have recently been shown to determine the overall catalytic properties of such structures.25,38−40 The systems discussed above demonstrate the promise of such hybrid materials in a variety of applications. Yet, the range of materials presently used is still rather small, and it is difficult to infer from the available data how physical properties such as charge separation dynamics depend on the choice of materials and their bulk properties. It is also unclear how modifications of the synthesis methods may affect the functionality of hybrid particles (if at all). The limited available data seems to imply the presence of some general design rules. For example, charge separation at the semiconductor−metal interface is dominated by electron transfer to the metal, and long-lived charge separation is expected if holes are trapped within the semiconductor.38,41−43 Here, we present a modular hybrid structure which enables us to study in detail the effect of a particular domain on the photophysics of the entire structure. Moreover, we expand the range of materials from which hybrid nanostructures are made to include infrared-absorbing IV−VI materials and test for the applicability of these rules in this hybrid system which appears to present only a small modification on previously studied structures. Our findings, outlined below, highlight the fact that these simple considerations are often insufficient to describe the properties of such complex systems. We first demonstrate the synthesis of PbS−CdSe@CdS−Pt (Figure 1d) and PbS−CdS−Pt (Figure S2d in the Supporting
Figure 1. TEM images of (a) CdSe@CdS, (b) CdSe@CdS−PbS, (c) CdSe@CdS−Pt, and (d) PbS-CdSe@CdS−Pt nanorods. All scale bars are 50 nm. Higher magnification images of each nanostructure are shown in Figure S1.
Information) hybrid nanostructures and then study the dynamics of photoexcited charge carriers of the PbS−CdS−Pt hybrid nanostructure via transient absorption spectroscopy, trying to relate it with the predicted band alignment of this system from bulk properties. Selective growth of Pt particles onto one side of the CdS rods followed by growth of PbS onto the other free tip was attained. B
DOI: 10.1021/acs.jpcc.6b04151 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C PbS, the spectrum was averaged out by 5 nm for each delay by convolving it. Lifetime analysis of the change in absorption (summed over 10 nm around the 475 nm bleach) as a function of the pump probe delay was done for all samples, using a biexponential fit model −ΔOD(t) = A1e−t/τ1 + A2e−t/τ2 + c. The transient spectra were noise subtracted using the average of the three first negative delay spectra and normalized. The complete synthetic experimental details of the nanostructures appear in the Supporting Information.
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RESULTS AND DISCUSSION In order to study the growth mechanism and the optical properties, seeded rods of CdSe@CdS were used as templates (instead of plain CdS rods) due to their intrinsically higher quantum yield and also because of the distinctive location of the CdSe seed relative to the CdS shell. It was shown that the CdSe seeds are usually located close to the Cd-rich facet which thus can be used as a structural marker as shown below. The synthesis of CdSe@CdS rods is based on forming CdSe seeds (ca. 3 nm in diameter) in hot organic solvent, followed by an anisotropic growth of CdS rods (see Supporting Information for the complete experimental details).44,45 Figure 1 and Figure S1 show transmission electron microscopy (TEM) images of CdSe@CdS (length 62 ± 5 nm, width 5 ± 1 nm), CdSe@CdS−PbS (PbS tip diameter 13.6 ± 1.8 nm), CdSe@CdS−Pt (Pt tip diameter 4.7 ± 1.1 nm), and PbS−CdSe@CdS−Pt hybrid nanostructures (a−d, respectively). The growth of PbS (Figure 1b) or Pt (Figure 1c) onto one tip of CdSe@CdS rods can be controlled by the concentration of the precursors of the Pb−bis(diethyldithiocarbamate) or Pt−acetylacetonate, respectively. This can be explained by different chemical composition of the two tips.33,46−48 The dissimilarity of the S and Cd atoms distribution along the [001] axis leads to the growth of either Pt or PbS particles on the S-rich facet, which is less passivated by the ligands. Further evidence of this hypothesis is provided below. Figure 1d shows the unique morphology of the PbS− CdSe@CdS−Pt hybrid nanostructure, where the Pt and PbS domains are selectively grown on different tips of the same CdSe@CdS rod. According to the synthesis described above, the Pt was grown first and then the PbS. The growth of the Pt tip onto one side of the nanorod hinders the growth of the other material on the same tip and facilitates the construction of this complex architecture. Thus, PbS particles grow on the free, Cdrich, end of the rods. This is, however, not a general phenomenon. When the growth of the Pt was preceded by the growth of PbS, Pt grew on the PbS tip rather than on the exposed tip of the rods as shown in Figure S3. A schematic illustration of the growth process of all the studied materials is depicted in Figure S4. Figure 2 presents high-resolution TEM (HRTEM) images and the corresponding energy dispersive X-ray spectroscopy (EDS) spectra of CdSe@CdS−Pt, CdSe@CdS−PbS, and PbS−CdSe@CdS−Pt hybrid nanostructures. The HRTEM images confirm that the CdSe@CdS rods, the PbS, and the Pt are crystalline. The growth axis of the wurtzite CdSe@CdS rods in all the hybrid nanostructures is along the ⟨001⟩ direction. The lattice spacing for PbS was measured and found to be 0.343 nm, which corresponds to the cubic (111) plane, and the lattice spacing for Pt is 0.227 nm, which corresponds to a (111) plane in a cubic lattice. The crystal structure of the
Figure 2. (a−c) HRTEM images of CdSe@CdS−Pt (a), CdSe@ CdS−PbS (b), and PbS−CdSe@CdS−Pt (c) hybrid nanostructures. The inset in (c) shows a high-magnification image of the rod near the Pt tip. All scale bars are 10 nm. (d−f) High-angle annular dark field (HAADF) images with the corresponding EDS elemental profiles of CdSe@CdS−Pt (d), CdSe@CdS−PbS (e), and PbS−CdSe@CdS−Pt (f) hybrid nanostructures. The elemental analysis includes profiles of the S-K (red), Se-L (orange), Cd-L (green), Pt-M (pink), and Pb-M (blue) lines. The red line on the HAADF image represents the line scan where the EDS profile was recorded.
hybrid nanostructures was confirmed by X-ray diffraction as shown in Figure S5. To verify the elemental distribution in the three hybrid nanostructures, an EDS line scan in STEM mode was performed on each structure (Figure 2d−f). The EDS results clearly show that the CdSe@CdS, Pt, and PbS domains are separated, and no interdiffusion was detected between the three parts. The apparent intensity of the Pb along the rod in Figure 2e,f is due to overlap between the Pb-M and S-K lines (ca. 2.3 eV). Figure 2d confirms that Pt grows more readily on the Srich facets. It shows that the Pt grew on the tip that is far from the CdSe core. Previous works have shown that the CdSe core is generally located near the Cd-rich tip, typically around 1/3 to 1/4 of the rod length from the edge, due to faster growth rate of the rods along the ⟨001⟩ axis relative to the other directions.44,45 It can be clearly seen in Figure 2d that the Se peak is located around 15 nm from the Cd rich facet, which is ∼1/3 of the rod length (45 nm). The Pt spectrum in Figure 2f clearly shows the spatial separation between the Pt particle and C
DOI: 10.1021/acs.jpcc.6b04151 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
such complex systems is femtosecond transient differential absorption spectroscopy. Using transient absorption and the fact that our heterostructures are composed of multiple domains with significantly different band gaps, we are also able to control (to some extent) the position of generated excitons via the use of different pump wavelengths. Transient absorption (TA) of the different samples was measured in toluene, using the doubled fundamental of an amplified 120 fs Ti:sapphire system at 400 nm as a pump source or an OPA generated 520 nm pump, operating at 250 Hz. To further facilitate the interpretation of the data, we chose to work here with a simpler nanostructure featuring a CdS nanorod instead of the seeded CdSe@CdS one. Lifetime analysis of the change in absorption (summed over 10 nm around the 475 nm CdS band edge bleach band) as a function of the pump probe delay was done using a biexponential fit model −ΔOD = A1 exp(−t/τ1) + A2 exp(−t/τ2) + c. The excitation power of the 400 nm pump was set to 134 μJ cm−2 aiming to excite, on average, a single exciton per rod as was determined by saturation measurement of the CdS nanorods. The experiment was performed in two modes. In the first, the pump was set to 400 nm directly exciting the CdS. The bleach dynamics of the pure CdS, compared to CdS−Pt, CdS− PbS, and PbS−CdS−Pt hybrid nanostructures, were analyzed. While the 400 nm pump excites both the CdS and the PbS, the extinction coefficient of the CdS is much higher; thus, the effect is dominated by the CdS absorption. In the second mode, the pump was set to 520 nm, exciting only the PbS. Figure 4 and Table 1 (see Table S1 with full error estimation data in the Supporting Information) summarize the bleach lifetime results for the CdS, CdS−Pt, CdS−PbS, and PbS− CdS−Pt hybrid nanostructures. The CdS−Pt sample exhibits fast decays with a dominant contribution of τ2 as can be seen by the fast decay amplitude ratio (∼55%) in Figure 4b (green
the rest of the structure. The PbS domain is located only on the tip (distance
