Article pubs.acs.org/JPCC
Extending the Limit of Low-Energy Photocatalysis: Dye Reduction with PbSe/CdSe/CdS Core/Shell/Shell Nanocrystals of Varying Morphologies under Infrared Irradiation Chaewon Pak,† Ju Young Woo,† Kangha Lee,† Whi Dong Kim,† Youngjae Yoo,‡ and Doh C. Lee*,† †
Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡ Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Korea S Supporting Information *
ABSTRACT: We demonstrate photocatalytic reduction of methylene blue in the infrared region, using PbSe/CdSe/CdS core/shell/shell heterostructure nanocrystals (HNCs) with type II or quasi-type II band offsets. Varying deposition rates of the CdS shell result in nanocrystals of diverse morphologies ranging from spheres to pyramids to tetrapods. The faceted shapes enable the selective growth of Au tips, which help increase photocatalytic activity since the Au tips serve as an electron sink. Comparative studies reveal that the photocatalytic activity appears to correlate with the integral overlap of electron and hole wave functions. Tetrapod-shaped HNCs with Au tips show considerably higher photocatalytic activity for the reduction of methylene blue than sphere or pyramid-shaped HNCs of equivalent composition arrangement.
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procedures.9−11 In addition, the metal chalcogenides within the strong confinement regime exhibit discrete energy levels and size-tunable energy gaps, which allow the design and engineering of band gap in metal−semiconductor hybrid structures. Banin and colleagues reported visible-range photocatalysis of methylene blue reduction using CdSe and CdS nanorods with metal tips.8,12 However, there are very few studies that show infrared-active photocatalysis based on colloidal quantum dots. Key results from previous reports are summarized in Table 1. Recent studies on colloidal synthesis of NCs have pushed the envelope by enabling the growth of more sophisticated heterostructures such as core/shell geometry with exquisite size and shape control.18−21 This delicate control of core/shell heterostructures is important because size and morphology can directly affect exciton dynamics.18,22−25 For instance, Lee et al. observed ultralong exciton lifetimes (∼80 μs) when PbSe/ CdSe/CdS core/shell/shell heterostructure nanocrystals (HNCs) were synthesized to have type II or quasi-type II band offsets.22 Spectroscopic analysis unveiled that reduced overlap integral of electron and hole wave functions resulted in prolonged carrier recombination time. Furthermore, it was shown that a simple change in reaction conditions could produce a radically altered structure, e.g., tetrapods, resulting from the growth of the CdS arms.22 The directionality introduced by the arms can greatly facilitate contact to a
INTRODUCTION Exploiting low-energy photons in the solar spectrum is of paramount significance in the field of photocatalysis. TiO2, the most widely used photocatalyst, absorbs light only in the ultraviolet region, despite superiority in factors such as longterm stability and low cost. As about 40% of solar energy reaching the Earth's surface lies in the near-infrared region, harnessing the near-infrared photons is evidently an important thrust in photocatalyst development. Naturally, research efforts have focused on modifying TiO2 to make it absorb light of lower energy, mostly in the visible region.1−3 One approach involves doping particulate TiO2 with impurity atoms. For example, Asahi et al. reported that TiO2 powder absorbs visible light (420 nm >420 nm 420 nm 400−705 nm
CdSe/Pt nanorods CdSe nanorods/gold nanodumbbells PbSe/TiOx CdSe/CNT CdSe/graphene ZnS/Au Cu2S/Au Cu2S/Pd4S
catalyst
methylene blue methylene blue rhodamine 6G methylene blue methylene blue thimine methylene blue methylene blue
>400 nm
CdS, Cu2S, CdSe, and TiO2,CNT complex
methylene blue
785 nm
Au-tipped PbSe/CdSe/CdS NCs
methylene blue
46% (1 h) 64% (2 h) 30% (1300 min) 55% (4 h) 20% (4 h) 81% (75 min) 60% (3h) 100% (30 min) CdS: ∼100% (3 h) Cu2S: 40% (3 h) CdSe: 40% (3 h) TiO2,CNT complex: negligible increase of photocatalytic activity Au-tipped tetrapods: 60% (4 h) Au-tipped pyramids: 23% (4 h)
ref 9 12 13 10 11 14 15 16 17
this study
Pietryga et al.26 PbSe NCs with 6 nm diameter were reacted with Cd cations and the Pb atoms at the rim of the NCs were replaced with Cd. For the growth of a CdS outer shell on top of PbSe/CdSe core/shell NCs, we adopted a modified version of the procedure described by Lee et al.22 The reaction temperature was varied from 170 to 240 °C to control the final morphology of PbSe/CdSe/CdS NCs. (170 °C: tetrapod; 200 °C: pyramid; 240 °C: sphere). After alternating nine injections of Cd-oleate and sulfur source, the reaction was quenched. Then, NCs were precipitated by adding 30 mL of methanol and centrifuging the mixture. The NCs were redispersed in hexane and centrifuged again with 20 mL of methanol. Gold Tip Growth. We adopted two different approaches to obtain Au-tipped PbSe/CdSe/CdS NCs: organic phase and aqueous phase.12,27 Organic phase: 4.5 mg of HAuCl4·3H2O, 43 mg of DDA, and 27 mg of DDAB were dissolved in 5 mL of toluene and sonicated until a bright yellow solution was obtained. This gold precursor was injected dropwise to the toluene solution of PbSe/CdSe/CdS HNCs while the mixture was vigorously stirred. Ten minutes after injection, the reaction was quenched. The mixture was centrifuged with methanol and collected HNCs were dissolved in hexane. Aqueous phase: PbSe/CdSe/CdS HNCs capped with oleic acid were treated with an excess of MUA to form a stable suspension in aqueous solution. Three milligrams of AuCl3 dissolved in 4 mL of water was added to the NC solution under vigorous stirring for 2 h. The Au-tipped HNCs were collected by centrifugation and dissolved in methanol/water (4:1) solution. Photocatalytic MB Reduction. To evaluate the photocatalytic effect of gold tip attached PbSe/CdSe/CdS NCs, photoreduction of methylene blue (MB) was carried out. A 530 nm laser (power: 25 W) and a 35 W Xe lamp were used as the visible light source in an experiment to see the photocatalytic effect on the dye reduction. A bandpass filter for 655 nm was used to select illumination wavelength. A 785 nm laser was also used to see photocatalytic activity in the infrared region. All experiments were carried out at room temperature. Various kinds of photocatalysts were used including Au-tipped and bare PbSe/CdSe/CdS sphere, pyramid, and tetrapod HNCs, PbSe NCs, PbSe/CdSe NCs to compare photocatalytic efficiency. In a typical experiment, 1.5 mg of the MUA-capped NCs and 5 μM of MB were dissolved in 3 mL of methanol/water (4:1) solution. Methanol acted as the hole scavenger and the pH of
charge collecting network or catalyst sites. This nanostructural motif of PbSe-based HNCs opens up a new possibility for infrared-based photocatalysis. By growing catalytic metal particles selectively at the sites of HNCs where electrons are localized, one could achieve even further charge separation. In other words, the photogenerated charges in HNCs remain separated for longer times and the presence of the metal tips would promote electron migration to the metal particles, resulting in enhanced photocatalysis. In this paper, we examine photocatalytic activity of the Autipped PbSe/CdSe/CdS core/shell/shell HNCs for the reduction of methylene blue (MB). By changing reaction temperature, we successfully synthesized HNC structures of various morphologies: spheres, pyramids, and tetrapods. Au nanoparticles were grown at reactive sites, e.g., the tips or corners, on the PbSe/CdSe/CdS HNCs, for the photocatalytic study, as the metal tips are likely to serve as electron sinks escalating the electron−hole separation in the HNCs. The present study of photocatalysis of MB reduction focuses on the effect that morphological changes have on the catalytic activity. We compared sphere-, pyramid-, and tetrapod-shaped core/ shell/shell HNCs in photocatalysis. Although electron and hole are separated for longer time in a spherical HNC than in a tetrapod according to spectroscopic analysis in an earlier study,22 the results in the present study indicated that photocatalytic efficiency does not necessarily follow the trend of exciton lifetimes. On the basis of observations of photocatalytic efficiency, we extended the photocatalysis into the near-infrared range (785 nm).
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EXPERIMENTAL SECTION Chemicals. The following chemicals were used without additional treatment: lead oxide (PbO, Aldrich, 99.999%), trioctylphosphine (TOP, Aldrich, 97%), oleic acid (Aldrich, 90%), phenyl ether (TCI, 99%), diisobutylphosphine ((iBu)2PH, Strem, 97%), selenium (Se, Aldrich, 99.999%), cadmium oxide (CdO, Aldrich, 99.999%), sulfur (S, Aldrich, 99.98%), octadecylamine (Aldrich, 97%), 1-octadecene (Aldrich, 90%), gold chloride trihydrate (HAuCl4·3H2O, Aldrich, >99.9%), dodecylamine (DDA, Aldrich, 98%), didecyldimethylaluminium bromide (DDAB, Aldrich, 98%), and mercaptoundecanoic acid (MUA, Aldrich, 95%). Synthesis of PbSe/CdSe/CdS HNCs. We first prepared PbSe/CdSe core/shell NCs, following a procedure described by 25408
dx.doi.org/10.1021/jp309371n | J. Phys. Chem. C 2012, 116, 25407−25414
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Figure 1. Schematic description of the synthesis of PbSe/CdSe/CdS HNCs and Au-tipped PbSe/CdSe/CdS HNCs with varying morphologies. For simplicity, organic ligands surrounding the surface of HNCs and Au particles were left out.
Figure 2. TEM images of (a−c) PbSe/CdSe/CdS HNCs and (d−f) Au-tipped heterostructures: (a, d) sphere, (b, e) pyramid, and (c, f) tetrapod NCs.
the solution was set to 7. The solution was kept in darkness for 30 min to reach absorption equilibrium. Three milliliters of the aqueous HNC solution in a cuvette was irradiated with a laser or Xe lamp. Four milligrams of photocatalysts was used when the 785 nm laser was the light source for photocatalysis. The UV−visible spectrum of MB solution was monitored as a function of illumination time. Gold nanoparticles 2 nm in diameter, PbSe NCs 3 nm in diameter, and PbSe/CdSe core/ shell NCs before CdS shell deposition were also used for the control experiments. Characterization. UV−vis and IR spectra were recorded with a UV/vis spectrometer (Shimadzu UV3600) and photoluminescence (PL) emission spectra were obtained with a HORIBA FL3-2IHR spectrometer. Transmission electron microscopy (TEM, Philips Tecnai F20 (300 K)) and energy dispersive X-ray spectroscopy (EDX) were used to analyze the
morphology and atomic composition of the products. An X-ray diffractometer (XRD, D/MAX-IIIC (3 kW)) was used to examine the crystalline structure of the nanoparticles.
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RESULTS AND DISCUSSION The preparatory step of growing PbSe/CdSe/CdS HNCs with controlled morphology and predictable energy level alignment is crucial for the synthesis of photocatalysts. As shown in Figure 1, PbSe NCs passivated with oleic acid and oleylamine were first prepared via an arrested precipitation approach, and the CdSe shell was subsequently coated by reacting the PbSe NCs with the Cd-oleate complex in toluene at 75 °C for 72−96 h. Cd2+ ions diffuse into the PbSe crystalline lattice and replace Pb2+. The cation exchange results in increased photoluminescence efficiency and enhanced chemical and spectral stability. Because PbSe and CdSe crystals have cubic crystalline 25409
dx.doi.org/10.1021/jp309371n | J. Phys. Chem. C 2012, 116, 25407−25414
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Figure 2d−f shows TEM images of the gold nanoparticlecoated PbSe/CdSe/CdS HNCs, with the Au nanoparticles being 2.4 nm in size dissolved in organic solvent. Au particles grew selectively at the tips of pyramid- and tetrapod-shaped HNCs as the tips of the edged structures are more reactive. On the other hand, spherical HNCs showed no such selectivity as shown in Figure 2d. The XRD patterns of the samples, before and after reacting pyramidal HNCs with the gold precursor, also indicate that Au particles are formed and they are crystalline (Figure S2, Supporting Information). After the Au reaction, the characteristic peak of gold evolved at 2θ = 38°, which corresponds to (111) planes for the FCC gold crystal. Furthermore, quantitative analysis points most of the Au tips on the HNCs as crystalline. That is, the areas under the peaks of pyramidal HNCs and Au have the ratio of 13.5:1, very close to the volumetric ratio estimated from the TEM images in Figure S2a (∼15:1) (Supporting Information). The TEM analysis also revealed that the single Au-tipped pyramid has dspacings of 2.1 and 3.1 Å corresponding to the (200) planes of FCC Au and (101) planes of wurtzite CdS, respectively. TEM images in Figure S3a−c (Supporting Information) also show that Au-HNC hybrid particles prepared by the “aqueous” approach did not undergo noticeable morphological changes during Au growth. Analysis of “organic”-grown Au-HNCs showed no morphology change either. To investigate the effect of morphology of HNCs on photocatalysis, we chose the reduction of MB as the test-bed reaction. The reduction potential of MB is −0.11 V and methanol was used for the hole scavenger.27,31,32 The PbSe/ CdSe/CdS HNCs with varying morphologies, with or without Au on the surface, were mixed with methanol and MB and placed under irradiation at different wavelengths: 530, 655, and 785 nm. The reaction progress of MB reduction was monitored by tracking the change in intensity of the peak characteristic of MB. Since the concentration of MB in the mixture is proportional to its absorbance, the concentration drop can be recorded by measuring absorption at different times of the catalytic reaction, as MB reduces to leucomethylene blue (MBH). To calculate the concentration drop accurately, absorbance contribution from the HNCs was subtracted from the data for MB and nanoparticle mixtures. Absorption spectra from an example batch (Au-tipped pyramidal HNCs reacted at 530 nm) are shown in Figure 3. The absorption peak at 653 nm, which represents MB, diminished as the MB reduces to MBH. In the presence of the pyramidal Au-HNCs, the MB concentration dropped by 59% after irradiation at 530 nm for 2 h. When Au tips were attached, MB reduction proceeded more rapidly. The Fermi energy level of Au accounts for the enhancement of photocatalytic activity. As described in Figure 4, the photogenerated hole is confined in the PbSe core and the electron from a photogenerated exciton can migrate over to Au, as the energy level by Au serves as an “electron sink”.12,33 The localization of holes in the PbSe core leads to the long-lasting charge carriers resulting from the minimization of the backward recombination, and thus facilitates the photocatalytic reaction.34 Furthermore, the presence of Au raises the possibility of separation of electron and hole, which otherwise would recombine more readily through either radiative or nonradiative processes.35 Photoluminescence (PL) spectra of the pyramidal HNCs with or without Au tip also hint at the charge transfer from PbSe to Au. Emission from the PbSe core of PbSe/CdSe/ CdS HNCs peaked around 1200 nm with a quantum yield
unit cells with very close lattice parameters (6.12 and 6.05 Å, respectively),26 the strain at their interface is very small and the resulting PbSe/CdSe core/shell NCs tend to have few interface defects. In addition, since CdSe-CdS colloidal heterostructure growth is well studied, it is beneficial to have the CdSe shell for the deposition of the CdS layer.28,29 The outermost CdS shell layer was coated through a successive ionic layer adsorption and reaction (SILAR) approach by injecting Cd-oleate and S-ODE complexes into the phenyl ether solution of PbSe/CdSe NCs.28 A simple change in the temperature of CdS injection results in diverse geometries of PbSe/CdSe/CdS HNCs: spherical HNCs grow at 240 °C, nanopyramids at 200 °C, and nanotetrapods at 170 °C. Figure 2a−c shows transmission electron microscopy (TEM) images of PbSe/CdSe/CdS core/shell/shell HNCs with morphologies ranging from spheres to pyramids to tetrapods. In a way, the pyramidal shape can be considered an intermediate between the nanospheres and the nanotetrapods. Since the dimensions of the PbSe/CdSe NCs are the same for all the samples (PbSe is 3.3 nm in diameter and the CdSe shell is 1.3 nm in thickness), the evolution of the different morphologies is attributed entirely to the kinetics of Cd and S atom deposition. As discussed in previous studies, growing Au on the surface of quantum dots or quantum rods helps increase the photocatalytic activity of semiconductor nanomaterials.12,14,15 The Au tip serves as a reservoir for photogenerated electrons, which facilitates interfacial charge transfer and consequently promotes photocatalytic performance.30 We adopted this motif for the PbSe/CdSe/CdS HNCs. Two key factors were considered in the growth of Au particles on the HNCs: (i) crystallinity of Au and (ii) colloidal stability of the resulting heterostructure. To test the premise, we attempted aqueous and organic approaches and compared the results obtained by the two different routes. Two to three nanometer Au nanoparticles grow at the tips of pyramid and tetrapod-shaped HNCs and on the surface of spherical HNCs regardless of the approach. The “organic” approach refers to Au particle growth in a nonpolar solvent, e.g., toluene. Since the method does not involve detrimental steps like ligand exchange, the resulting AuHNC heterostructure was very stable. As the photocatalysis takes place in aqueous solution, the Au-HNC hybrid needs to be soluble in water. Therefore, the Au-HNC structures prepared in the organic approach had to be ligand-exchanged to mercaptoundecanoic acid (MUA). The ligand-exchanged NCs, however, flocculated easily and the dispersion became unstable. We often observed gold tips detaching from the HNCs after the ligand exchange. As a result, their photocatalytic activity worsened very quickly, losing most of its catalytic function within a minute. In the “aqueous” method, capping ligands on the HNCs were exchanged to MUA and the water-soluble HNCs were then mixed with gold chloride precursors for Au to grow. MUA coated HNCs are much more soluble in aqueous solution than MUA coated Au-HNCs prepared by the organic approach. Because AuCl3, the gold precursor, is acidic, the surface ligands are likely to detach from the surface, resulting in gradual flocculation. In fact, the HNCs prepared this way noticeably precipitate within 30 min of photocatalytic investigation. To address this problem, we titrated the solution with tetramethyl ammonium hydroxide pentahydrate until neutral pH was reached, and the heterostructures remained soluble for a longer time (3−4 h). 25410
dx.doi.org/10.1021/jp309371n | J. Phys. Chem. C 2012, 116, 25407−25414
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outermost shell of CdS. If it is thin enough, the hole wave function can “leak” to the outside surface of the shell. In the pyramid-shaped core/shell/shell structure, the distance from the core surface to a basal plane is thinner than that to a pyramid tip, whereas the CdS shell of spherical HNCs has relatively uniform thickness that blocks the hole from moving out of the core. Consequently, geometric consideration points to the possibility that a hole wave function can reach the plane of the pyramidal HNC through tunneling, which becomes possible only when the barrier is thin enough. In our sample, the CdS thickness was 0.2 nm toward the planes and 1.5 nm to the tip; therefore, it would be much easier for the hole to reach the basal planes than to reach the CdS tip surface.18,38 To confirm that the energy level description accurately reflects the trend in photocatalytic performance, we prepared samples with slight variations from the Au-tipped PbSe/CdSe/ CdS core/shell/shell HNCs. For example, to testify that the photocatalysis results from the heterostructuring in the PbSe/ CdSe/CdS HNCs, PbSe NCs, PbSe/CdSe core/shell NCs, Au nanoparticles, and a mixture of the PbSe/CdSe/CdS HNCs and Au nanoparticles were prepared as control samples. Figure 5a shows the relative concentration of MB (C/C0) plotted against irradiation time (min) under different reaction conditions. For a fair comparison, 1.5 mg of NCs and 3 mL of methanol/DI water (4:1, v:v) solution of MB were mixed in a cuvette and irradiated with a 530 nm laser (power = 25 W). When no nanoparticles were mixed with MB, the relative concentration remained nearly unchanged (
