Article pubs.acs.org/crystal
Photocorrosion-Assisted Transformation of Metal Selenide Nanocrystals into Crystalline Selenium Nanowires Whi Dong Kim,† Fábio Baum,† Dahin Kim,† Kangha Lee,† Jun Hyuk Moon,‡ 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 ‡ Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Korea S Supporting Information *
ABSTRACT: We report the transformation of several metal selenide nanocrystals (NCs), including PbSe, CdSe, ZnSe, and PbSe/CdSe core/shell NCs, in dimethyl sulfoxide (DMSO) in acidic solutions at room temperature. In this study, the DMSO solution of metal selenide NCs was mixed with nitric acid, which was used to adjust the pH of the solution. Upon mixing, the metal selenide NCs readily transformed into crystalline selenium (trigonal structure) nano- or microwires under ambient light, whereas little or no transformation occurred in the dark. Photocorrosion, where the photogenerated carriers within the NCs participate in the cleavage of the metal and selenium atoms, turns out to be responsible for the transformation. DMSO removes organic ligands on the NC surface and creates surface trap sites for photoinduced charge carriers. Then, nitric acid helps shift the reduction potentials, thereby promoting a “cathodic reduction”. In this sense, the photocorrosion rate can be controlled by several parameters, such as the absorption cross section of the selenide NCs and the pH. The diameter and shape of the resulting selenium wires help gauge the transformation rate and thus unveil the transformation mechanism.
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INTRODUCTION Central to the synthesis of nanomaterials in bottom-up routes is to control the decomposition or reduction rate of molecular precursors (e.g., organometallic salts).1,2 Typically, the precursors either thermally decompose to active atomic species upon heating in solution or chemically reduce into active monomers when reacted with reducing agents. The ensuing nucleation and growth of nanomaterials can be controlled by how fast the thermolysis or reduction occurs in solution. The size, composition, and morphology of the resulting nanomaterials, such as nanocrystals (NCs), nanowires (NWs), or nanorods, can be tuned with the exquisite control of the decomposition or reduction of molecular precursors.3−6 As first proposed in the seminal work by LaMer and Dinegar, the dependence of nanomaterial size on concentration in bottomup approaches underscores the importance of controlling the rate at which the active species are achieved from the precursors.7 More recently, colloidal nanoparticles were utilized as precursors in lieu of molecular organometallics for the growth of nanomaterials.8−13 For example, Kotov and co-workers reported a synthetic route for selenium (Se) and tellurium (Te) NWs using CdSe and CdTe quantum dots, respectively, as “particle precursors”.8 The CdSe and CdTe particles, whose surface was passivated with chemical ligands, underwent etching and dissolution when a strong metal complexing agent, such as ethylenediaminetetraacetic acid (EDTA), was introduced © 2014 American Chemical Society
into a solution of the particles. EDTA depletes the capping ligands on the surface of the nanoparticles and helps cleave the Cd cations from the Se or Te anions, which then nucleate Se and Te colloids and grow into crystalline NWs. The particle precursors would provide a well-controlled release of the active atomic species, as the release would be determined by how the atoms are bound in the particles. Herein, we examine various factors that can affect the transformation of metal selenide NCs into Se and expound the possibility of controlling not only the dimension of Se nanomaterials but also their shape by investigating the NC dissociation and subsequent Se NW growth upon reaction of the NCs with dimethyl sulfoxide (DMSO) in acidic conditions.
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EXPERIMENTAL SECTION
Chemicals. The following chemicals were used as received: lead oxide (PbO, Aldrich, 99.999%), oleic acid (OA, Aldrich, 90%), phenyl ether [(C6H5)2O, Tokyo Chemical Industry Co., 99.0%], cadmium oxide (CdO, Aldrich, 99.999%), trioctylphosphine (TOP, Aldrich, 97%), selenium (Se, Aldrich, 99.999%), zinc acetate (Zn(O2CCH3)2, Aldrich, 99.9%), nitric acid (HNO3, Fluka, 0.1 M in 0.1 N H2O), and dimethyl sulfoxide (DMSO, Aldrich, 99.5%). Synthesis of PbSe, CdSe, ZnSe, and PbSe/CdSe Core/Shell NCs. We prepared MSe (M = Pb, Cd, or Zn) NCs using a hot Received: November 27, 2013 Revised: January 15, 2014 Published: January 23, 2014 1258
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injection method.14 All synthetic procedures were carried out in a standard Schlenk line. For the PbSe NCs, 0.43 g of PbO, 1.8 mL of OA, and 2.67 mL of phenyl ether were loaded into a three-neck flask, degassed under vacuum at 80 °C for 30 min, and heated to 170 °C under Ar. Then, 0.21 g of Se was mixed in 1.32 mL of TOP inside a glovebox. The Se-TOP solution was rapidly injected into the flask containing the Pb-oleate in phenyl ether. After 35 s, we quenched the reaction by injecting cold toluene and subsequently immersing the flask into an iced water bath. The resultant products were collected from precipitation by centrifugation at 5000 rpm for 10 min after adding methanol and acetone. The resulting PbSe NCs were dispersed in toluene. To prepare the PbSe/CdSe core/shell NCs, the CdSe shell was created via a cation exchange process by mixing the Cd−oleate complex to PbSe NCs at an elevated temperature. A 0.5 g quantity of CdO, 3 mL of OA, and 8 mL of phenyl ether were loaded into a flask, and the mixture was heated to 250 °C under Ar to prepare an optically clear, colorless Cd−oleate complex. After cooling the solution to 75 °C, we injected it into a flask, in which 30 mg of PbSe NCs had been dispersed in 3 mL of toluene at 70 °C. The mixture was held at 70 °C for 2 days. The resulting NCs were collected as precipitates after centrifugation with methanol and acetone, which were added as antisolvents. In the case of CdSe and ZnSe NCs, the molar quantity of CdO and zinc acetate, respectively, were loaded instead of lead oxide to prepare the respective metal−oleate complexes. The reaction proceeded at 190 °C for 90 min for the ZnSe NCs and at 300 °C for 3 min for the CdSe NCs. Then, a stoichiometric ratio of the Se-TOP solution was injected as in the PbSe NC synthesis. Transformation of the NCs. NC solution (100 μL) in toluene (10 mg/mL) was blended with 2 mL of DMSO in ambient conditions. The pH of the solution was adjusted to between 3 and 6 by adding an appropriate amount of nitric acid. After mixing, the solution turned cloudy and then eventually colorless, and its precipitate was collected after centrifugation at 5000 rpm for 5 min. Characterization. The colloidal NCs and the transformation products were first examined using high-resolution transmission electron microscopy [HRTEM, JEM-3011 (300 kV), JEOL] and scanning electron microscopy [SEM, Nova230 (10 kV), FEI]. We used an X-ray diffractometer [XRD, D/MAX-RB (12 kW), Rigaku] to investigate the crystalline structure of the samples. Absorption spectroscopy of the NCs was recorded using a UV−vis spectrometer (UV3600, Shimadzu), and Fourier-transform infrared (FT-IR) spectra were collected using an FT-IR spectrometer (Alpha FT-IR, Bruker).
Figure 1. (a) Photograph of the PbSe NC solution mixed with DMSO before (left) and 10 min after (right) the nitric acid was introduced. The solution becomes colorless within 5 min after nitric acid is introduced. (b) TEM image of the 6.2 nm PbSe NCs (scale bar in inset is 2 nm). (c) XRD pattern of the product collected after the transformation process. Trigonal Se (t-Se) crystal planes are indexed. (d and e) TEM images of crystalline Se wires grown by mixing PbSe NCs with DMSO and nitric acid for 10 min.
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RESULTS AND DISCUSSION Transformation of PbSe Nanocrystals into Se Nanowires. A 6.2 nm PbSe nanocrystal (NC) solution in toluene (10 mg/mL) was mixed with DMSO for approximately 10 s, and then nitric acid was introduced into the mixture. The dark PbSe NC solution immediately turned brick red and then eventually colorless within 10 min at room temperature (Figure 1a). Upon introduction of nitric acid, the PbSe−DMSO mixture started flocculating, which implies that the PbSe NCs become colloidally unstable as a result of the detachment of the capping ligands. Interestingly, the resulting precipitates were crystalline selenium nanowires (NWs) (Figure 1, panels d and e). XRD analysis revealed that the NWs had the trigonal selenium (t-Se) crystalline structure (JCPDS card no. 06-0362). The (100)/ (102) and (101)/(102) plane ratios (2.75 and 4.72) of the t-Se NW samples in the experiment exceeds the value of bulk t-Se (1.57 and 2.86) by far, indicating that the growth direction in the Se NWs is predominantly ⟨001⟩15 (Figure 1c). HRTEM analysis revealed that the NWs exhibited crystalline lattice fringes of 5.0 Å, matching the ⟨001⟩ direction of t-Se (Figure 1e). We infer that the PbSe NCs are decomposed in the acidic solution and transformed into crystalline Se NWs. The
chemical and optical stability of the colloidal NCs, or lack thereof, are critical in using these nanomaterials in applications. For example, the use of colloidal NCs or quantum dots in photocatalysis warrants colloidal stability, which is significantly undermined by the acidic environment of the reactions.16 Then, central to understanding the colloidal stability of PbSe NCs is, conversely, to figure out how PbSe NCs are transformed by simulated, severe conditions. Effects of DMSO. Figure 2 shows the FT-IR spectra of the PbSe NC samples before and after DMSO was injected to the PbSe NC solution. The results suggest that the capping ligands (oleic acid) are desorbed from the NC surface when DMSO is introduced. The spectrum of PbSe NCs passivated with oleic acid shows vibrations at 2922, 2852, 1545, and 1403 cm−1, which are υasym (C−H), υsym (C−H), υasym (COO−), and υsym (COO−), respectively. These spectra indicate the presence of oleate on the surface of the synthesized PbSe NCs, while the FT-IR bands from oleic acid (C−H or COO−) completely vanish after the DMSO mixing.17 Therefore, DMSO removes the ligands from the NC surfaces before nitric acid is added. 1259
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responsible for the “corrosion” of the NCs.19 We refer this photoinduced surface etching as “photocorrosion”. Interestingly, when the mixing was carried out under Ar purge, the NCs underwent crystalline transformation far more slowly, although the DMSO mixing resulted in clustering of the NCs (Figure S3 of the Supporting Information). It is unclear how the reaction environment affects the reaction kinetics so significantly, yet the observation resembles a finding that surface-bound oxygen species facilitate anodic corrosion processes.20 Effects of Acidity. To elucidate why the photocorrosion process is facilitated in an acidic environment, we performed a series of DMSO mixing trials at varying pH values. At pH 3, the PbSe NCs rapidly turned into t-Se NWs in
