Generalized One-Pot Synthesis of Copper Sulfide, Selenide-Sulfide

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Generalized One-Pot Synthesis of Copper Sulfide, Selenide-Sulfide, and Telluride-Sulfide Nanoparticles Pearl L. Saldanha, Rosaria Brescia, Mirko Prato, Hongbo Li, Mauro Povia, Liberato Manna, and Vladimir Lesnyak* Department of Nanochemistry, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy S Supporting Information *

ABSTRACT: Here we report a facile approach to synthesize copper chalcogenide (Cu2−xS, Cu2−xSeyS1−y and Cu2−xTeyS1−y) nanocrystals without employing hotinjection, at moderate reaction temperatures (200−220 °C) and free of phosphines. Scaling up of the synthesis yields monodisperse nanoparticles without variations in their morphology. We have observed the formation of alloyed copper selenidesulfide and telluride-sulfide nanocrystals due to the incorporation of sulfur by using 1-dodecanethiol as a ligand along with oleic acid. The materials obtained possess localized surface plasmon resonances in the near-infrared region, which are demonstrated to be widely tunable via a controlled oxidation generating copper vacancies. Copper sulfide nanoparticles with well-defined initial chalcocite crystal phase were subjected to oxidation followed by structural characterization. Structural rearrangement of the oxidized chalcocite Cu2−xS crystal lattice to roxbyite by aging is proven to release the copper vacancies. Further oxidation again can create new copper vacancies in the roxbyite lattice, however its structure does not evolve into covellite CuS. These findings suggest that besides nonstoichiometry (i.e., the value of x) induced by oxidation, crystal structure is an important factor responsible for plasmonic properties of copper chalcogenide nanocrystals. Furthermore, successful water solubilization of Cu2−xTeyS1−y nanoparticles with preservation of their plasmon band has been realized via a ligand exchange approach employing a mPEG-SH stabilizer.

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INTRODUCTION Colloidal synthesis of semiconductor nanoparticles has long been focused on II−VI, III−V and IV−VI compounds, such as chalcogenides of zinc, cadmium, lead, mercury, as well as indium phosphide and arsenide, on which a considerable degree of control upon their size and shape, and hence tunability of their optical and electronic properties has been achieved. These materials exhibit a great potential for electronic and optoelectronic applications.1 However, in some cases, implementation of these nanoparticles is hindered by the presence of toxic elements, like Cd, Hg, Pb, etc., which can be released into the environment or directly into a human body through their biomedical application as a result of their decomposition. This issue has attracted the attention of researchers toward copper chalcogenides (i.e., CuxS, CuxSe, and CuxTe (x = 1−2)) which in addition to excitonic properties, characteristic for the other classes of semiconductor nanocrystals (NCs), possess also plasmonic properties,2−5 typically inherent to metal nanoparticles (e.g., Au, Ag, Cu). Plasmonic behavior in the case of copper chalcogenides is the result of the collective oscillation of holes, opposite to metals possessing free electrons, and is induced via oxidation that leads to the creation of Cu vacancies. This combination of electronic and photonic modes in one nanosized entity without phase boundaries, in contrast to various metal-semiconductor heterostructures,6,7 opens up new opportunities for their application in light harvesting, nonlinear optics and quantum information processing.8 To date, many synthetic protocols © XXXX American Chemical Society

have been reported for preparing Cu chalcogenide NCs with controllable size, shape, and composition, most of which are based on the hot-injection approach9−16 or cation exchange reactions.17−19 In addition, the water-organics two-phase approach for synthesizing Cu2S NCs in autoclave20 and the solventless synthesis of monodisperse Cu2S particles of various shapes21,22 should be mentioned. Indeed, such conditions as hot injection,12,23−25 multiple stock solution preparation,26 high reaction temperatures,14 use of phosphines,16,23,27 and cation exchange from Cd-based NCs employing a large excess of substituting cation precursors17,18,28 complicate up-scaling of the synthesis. Therefore, development of facile, reproducible, and easily up-scalable synthesis of copper chalcogenide nanomaterials is still a challenging task, the accomplishment of which should provide new possibilities for their wide application. In this work, we report a facile one-pot, thermal, non-hotinjection approach for the synthesis of binary Cu2−xS, and ternary Cu2−xSeyS1−y and Cu2−xTeyS1−y NCs with a special emphasis on their structural and optical characterization. We note that, to the best of our knowledge, there are only few works published on the direct synthesis of copper telluride NCs exhibiting optical properties similar to sulfide and selenide families.5,23 In addition, for the first time, we demonstrate a Received: October 28, 2013 Revised: January 8, 2014

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dx.doi.org/10.1021/cm4035598 | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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were performed on a Kratos Axis Ultra DLD spectrometer, using a monochromatic Al Kα source (15 kV, 20 mA). Wide scans were acquired at analyzer pass energy of 160 eV. High resolution narrow scans were performed at constant pass energy of 10 eV and steps of 0.05 eV. The photoelectrons were detected at a takeoff angle of Φ = 0° with respect to the surface normal. The pressure in the analysis chamber was maintained below 7 × 10−9 Torr for data acquisition. The data were converted to VAMAS format and processed using CasaXPS software, version 2.3.15. The binding energy (BE) scale was internally referenced to the C 1s peak (BE for C−C = 284.8 eV). Data fitting was performed assuming Gauss-Lorentz profiles with a Shirleytype background. For S 2p doublets, a 1.2 eV energy splitting and a 2:1 intensity ratio between the 3/2 and 1/2 components were fixed. For Se 3d doublets, a 0.86 eV energy splitting and a 3:2 intensity ratio between the 5/2 and 3/2 components were fixed. For Se 3p doublets, a 5.75 eV energy splitting and a 2:1 intensity ratio between the 3/2 and 1/2 components were fixed. Powder X-ray Diffraction (XRD) Analysis. XRD patterns were recorded on a Rigaku SmartLab 9 kW diffractometer. The X-ray source was operated at 40 kV and 150 mA. The diffractometer was equipped with a Cu source and a Göbel mirror to obtain a parallel beam and to suppress Cu Kβ radiation (1.392 Å). To acquire data a 2θ/Ω scan geometry was used. The samples were prepared by drop casting concentrated NC solutions onto a zero background silicon substrate. The PDXL software of Rigaku was used for phase identification. UV−Vis−NIR Spectroscopy. Extinction spectra of all NC samples were measured in 1 cm path length quartz cuvettes using a Varian Cary 5000 UV−vis−NIR spectrophotometer. A dilute dispersion of NCs in tetrachloroethylene (TCE) which ensures no interference from the solvent in NIR extinction measurements was prepared inside a nitrogen filled glovebox. Oxidation of the NC colloids was monitored directly in the cuvette after each addition of an aliquot of a freshly prepared 0.1 M solution of CAN in methanol or ethanol into a NC solution. Elemental Analysis. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis performed on aiCAP 6000 spectrometer (ThermoScientific) was used to quantify the composition of the NCs. The samples were decomposed in aqua regia (HCl/ HNO3 = to 3/1 (v/v)) prior to measurements.

direct employment of the Te powder in the colloidal synthesis without preparation of a trioctylphosphine-Te solution. Therefore, the advantages of this technique over the existing synthetic protocols are the absence of hot injection, moderate reaction temperatures (200−220 °C), and a phosphine free synthesis that yields fairly monodisperse nanoparticles having tunable optical properties. In addition, we demonstrate up-scalability of the synthesis without variations in the morphology of the NCs formed. Investigations of their crystal structure combined with optical spectroscopy revealed a rearrangement of the lattice upon oxidation, providing a further insight into the processes responsible for the plasmonic behavior of these nanomaterials.

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EXPERIMENTAL SECTION

Materials. Copper(II) acetylacetonate (Cu(acac)2, 99.99%), 1dodecanethiol (DDT, ≥98%) and oleic acid (OA, 90%) were purchased from Sigma-Aldrich, selenium powder (Se, 99.99%), tellurium powder (Te, 99.999%), and sulfur powder (S, ≥99%) from STREM chemicals, ethanol (anhydrous, 99.9%), toluene (anhydrous, 99.8%), chloroform (anhydrous, 99.95%), methanol (anhydrous, 99.9%), and cerium(IV) ammonium nitrate (NH4)2Ce(NO3)6 (CAN, >98.5%) from Carlo Erba reagents. All chemicals were used as received without further purification. Synthesis of Cu2−xS, Cu2−xSeyS1−y, and Cu2−xTeyS1−y (x, y > 0) NCs. The synthesis was performed using a standard Schlenk line technique. In a typical procedure, 0.4 mmol of Cu(acac)2 (105 mg) and 0.2 mmol of chalcogen source (i.e., 6.4 mg of S for Cu2−xS, 16 mg of Se for Cu2−xSeyS1−y, and 26 mg of Te for Cu2−xTeyS1−y) were mixed with 2 mL of DDT and 4 mL of OA in a three-neck round-bottom flask and degassed under vacuum (pressure of 2−5 × 10−2 Torr) at 80 °C for 1 h. Then, the flask was backfilled with nitrogen and heated up to 220 °C in the case of Cu2−xS and Cu2−xTeyS1−y NCs and up to 200 °C for Cu2−xSeyS1−y (4−5 min to reach the temperatures specified). Cu2−xS and Cu2−xSeyS1−y reaction mixtures were stirred for 30 min at 220 and 200 °C, respectively. In the case of copper telluride-sulfide, the heating mantle was removed immediately after reaching 220 °C. A dark-brown solution in the case of Cu2−xSeyS1−y and Cu2−xTeyS1−y, and yellow-brown solution in the case of Cu2−xS were obtained, with trace amount of unreacted chalcogen powder on the bottom of the flask. The flask was cooled to room temperature and the solution was transferred to a nitrogen filled glovebox where it was washed 2−3 times by precipitating in ethanol and redissolving in chloroform (for Cu2−xS) or toluene (for Cu2−xSeyS1−y and Cu2−xTeyS1−y). The NCs in toluene (chloroform) were further centrifuged at 1000−2000 rpm for 1 min to separate larger particles formed as a byproduct. The growth process was monitored by taking aliquots of the reaction mixture at different time intervals and temperatures. Transmission Electron Microscopy (TEM). The samples were prepared by dropping dilute NC solutions on carbon-coated 200 mesh copper grids, and a JEOL JEM 1011 microscope operating at 100 kV accelerating voltage was used for measurements. High resolution TEM (HRTEM), high-angle annular dark-field (HAADF) scanning TEM (STEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were performed with a JEOL JEM-2200FS microscope equipped with a field-emission gun working at 200 kV, a CEOS spherical aberration corrector of the objective lens allowing for a spatial resolution of 1 Å. The chemical composition of the NCs was determined by EDS, performed in STEM-HAADF mode, using a Bruker Quantax 400 system with a 60 mm2 XFlash 6T silicon drift detector (SDD). For HRTEM analyses the NC solutions were drop-cast onto copper grids covered with an ultrathin amorphous carbon film, whereas for STEMEDS analyses, they were deposited onto carbon-coated aluminum grids and the measurements were carried out using a holder with a beryllium cup. X-ray Photoelectron Spectroscopy (XPS). The samples were prepared in a nitrogen filled glovebox by drop-casting NC colloids on a graphite substrate (HOPG, ZYB quality, NT-MDT) and then transferred to the XPS setup avoiding air-exposure. Measurements

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RESULTS AND DISCUSSION A simple synthetic method consisting in heating up a copper salt and a chalcogen powder (elemental S, Se or Te) dispersed in a mixture of DDT/OA (1/2 vv) resulted in fairly monodisperse Cu2−xS, Cu2−xSeyS1−yand Cu2−xTeyS1−y nanoparticles, as is evident from TEM images presented in Figure 1. Their corresponding size distributions are shown in Figure SI1 in the Supporting Information. Spontaneous self-assembly of copper selenide-sulfide NCs on a TEM grid proves their especially narrow size distribution (see Figure SI2 in the Supporting Information). To demonstrate reproducibility of the approach on a larger scale, we have performed the synthesis of Cu2−xSeyS1−y NCs scaled up to 150 mL with doubled concentration of Cu(acac)2 and Se (i.e., 5.25 and 0.8 g, respectively) without any appreciable variation in the size, size distribution (cf. their average diameter of 6.4 ± 1.3 nm with that one of small batch particles 6.4 ± 1 nm) and shape of the NCs observed, as displayed in Figure SI3 (see the Supporting Information). This would be particularly advantageous for the commercialization of the synthesis, where higher process rate with lower amount of solvent will lead to the lowering of the production cost. The reaction yield of the NCs was up to 50%, the main byproducts being unreacted chalcogen powder as well as large particles. By varying the time of the reaction in the synthesis of copper sulfide and selenide-sulfide NCs, we could easily tune the size of the particles in the range from 2.8 ± 0.6 nm (obtained at 180 B



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Figure 2. XPS narrow scans on (a) Cu 2p1/2 and 2p3/2 (with binding energies of 952.2 and 932.3 eV, respectively), Cu LMM (at binding energy of 569.5 eV, corresponding to a kinetic energy of 917.1 eV) and S 2p (161.7 eV) peaks of Cu2−xS NCs; (b) Cu 2p1/2 and 2p3/2 (952.2 and 932.4 eV, respectively), Se 3d (54.2 eV), Se 3p (160.3 eV), and S 2p (161.8 eV) peaks of Cu2−xSeyS1−y NCs; (c) Cu 2p1/2 and 2p3/2 (952.2 and 932.4 eV, respectively), Te 3d3/2 and 3d5/2 (583.0 and 572.6 eV, respectively), and Cu LMM (569.3 eV), Te 4s (169.2 eV), and S 2p (161.6 eV) peaks of Cu2−xTeyS1−y NCs.

Figure 1. Left: TEM images of (a) Cu2−xS, (b) Cu2−xSeyS1−y, and (c) Cu2−xTeyS1−y NCs with diameters of 9.7 ± 3, 6.4 ± 1, and 6.5 ± 1.5 nm, respectively. The scale bar is 50 nm. The insets show their electron diffraction patterns. Right: Corresponding HRTEM images of selected individual particles.

°C) to 9.7 ± 3 nm reached after 30 min of heating at 220 °C for Cu2−xS, and from approximately 3 ± 0.5 to 7.8 ± 2.5 nm by heating at 200 °C for Cu2−xSeyS1−y. The reaction temperatures specified are optimal conditions for the syntheses: at lower values, we observed a slower growth and lower yield, whereas at higher temperature, we observed destabilization and aggregation of the particles. Because a prolonged heating of a reaction mixture in the synthesis of Cu2−xTeyS1−y NCs leads to the aggregation of the particles, the size controlling parameter in this protocol was an ultimate temperature reached via a rapid (approximately 40 °C/min) heating. Thus, NCs with sizes of 3 ± 1.0, 5.5 ± 1.3, 6.5 ± 1.5 and 8.5 ± 1.7 nm have been obtained by heating the reaction mixture up to 200, 210, 220, and 230 °C, respectively. Fast heating of the reaction mixture in the synthesis of Cu2−xSeyS1−y NCs up to 220 °C also yielded spherical 7.5 ± 2.5 nm particles, which with prolonged time formed large aggregates losing their colloidal stability in solution. Analysis of the positions of Cu 2p peaks together with the Auger Cu LMM peaks of high-resolution XPS spectra of Cu2−xS, Cu2−xSeyS1−y, and Cu2−xTeyS1−y NCs reveals that copper is present as Cu(I) in all samples,16 as no Cu(II) satellites were detected (see Figure 2). The S 2p spectrum of the Cu2−xS particles can be decomposed into two doublets (see Figure 2a), indicating two different S species, one at 161.5 eV that is typical for sulfides, and one at 162 eV, a value already reported for DDT bound to Cu.29 According to the XPS results, the thiol constitutes approximately 60% of the total sulfur content of the NC surface (i.e., outermost 5 nm). Similarly, in the case of Se in copper selenide-sulfide particles, two doublets are required to fit the experimental data (see Figure 2b). The most intense one is assigned to selenide ions

Se2−, the less intense also might belong to selenide in a different chemical environment as well as to diselenide formed because of a partial oxidation of the sample.11 In addition to Se peaks, quite intense signal from sulfur was observed in the energy region where S 2p and Se 3p peaks overlap. The presence of a large content of sulfur was revealed also in Cu2−xTeyS1−y nanoparticles, where two different components with binding energy values consistent with sulfides and thiols were found. In all samples, an excess amount of copper as compared to the stoichiometric composition has been quantified by XPS, whereas the elemental analysis performed by EDS with averaging among several particles gives the following compositions for copper sulfide, selenide-sulfide and telluridesulfide respectively: Cu2S, Cu1.83Se0.43S0.57, and Cu1.74Te0.53S0.47. ICP also showed 0.1 ≤ x ≤ 0.5 in Cu2−xS(Se, Te) NCs. Because XPS provides information about the surface of a sample, comparison of the values obtained suggests that the surface of the particles is enriched with Cu. According to these results, alloyed copper selenide-sulfide Cu2−xSeyS1−y and telluride-sulfide Cu2−xTeyS1−y structures are formed because of incorporation of sulfur ions generated by the decomposition of DDT.30,31 However, it is necessary to point out that DDT is not an efficient sulfur source for the synthesis of Cu2−xS NCs, because we found that there is no nucleation of copper sulfide via interaction between Cu(acac)2 and DDT under the synthesis conditions: the mixture of the Cu precursor, OA, and DDT remains transparent even after heating up to 250 °C. Additional sulfur contribution with binding energy typical for thiols comes from Cu−S−C12H25 bonds on the surface of the particles, as in the case of copper sulfide mentioned above. C



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Interestingly, we demonstrate the synergetic effect of OA and DDT that may activate the chalcogen, which is the key to the formation of monodisperse Cu chalcogenide NCs. The synthesis performed in solely DDT without addition of OA provides Cu2−xTeyS1−y NCs with a broader size-distribution (see Figure SI4 in the Supporting Information), as compared to those obtained in DDT-OA mixture. At the same time, no reaction between Cu(acac)2 and Te takes place in OA in the absence of DDT. It is known that chalcogens can be dissolved at elevated temperatures in organic solvents, which can guarantee the formation of high-quality NCs.14 In our case, only sulfur may react in a liquid phase due to its relatively low melting point (115 °C32). Se and Te powder cannot be dissolved in solely DDT or OA. Moreover, the melting temperature of tellurium is 450 °C,32 which is far above the reaction temperature used. A value of 200 °C set in the synthesis of Cu2−xSeyS1−y particles is also insufficient to melt selenium powder (mp = 221 °C32). These findings prove that DDT plays a key role in the synthesis mediating interaction between copper and the chalcogen, although additional control experiment on heating up Te powder and DDT did not reveal any interaction between them in the absence of the Cu-precursor, as follows from the comparison of absorbance spectra of DDT and the mixture of DDT and Te presented in Figure SI5 (see the Supporting Information), where no remarkable shift of the DDT absorption maximum is observed. Overview structural characterization of the NCs was performed by means of XRD analysis. Although, as it has been mentioned in the literature, the identification of copper chalcogenide crystallographic phases is particularly difficult since they exist in a wide variety of compositions and crystal structures,8,23,24 certain patterns can be discerned for the samples obtained. For example, several phases exist for bulk copper sulfide with composition close to the ratio Cu/S = 2: low (α) and high (β) chalcocite, djurleite, digenite, anilite.33 By this, a slight change in temperature or composition can lead to the formation of one or the other crystal structure. The same is applicable also to copper selenide nanoparticles, as it was demonstrated by Hessel et al.26 As seen from Figure 3, the highest intensity peaks observed for the Cu2−xS sample are attributable to the hexagonal Cu2S high chalcocite phase (PDF #01−073−6919, space group P63/mmc, a = b = 3.96 Å, c = 6.72 Å), while for the ternary alloyed materials most of the diffraction peaks can be assigned to the same phase, but with a shift toward smaller Bragg angles. In both materials, Cu2−xSeyS1−y and Cu2−xTeyS1−y, an overall ∼5% volume dilation of the cell in the hexagonal structure is found (a = b = 4.02 Å, c = 6.81 Å). This lattice dilation can be ascribed to the partial substitution of S2− anions by the larger Se2− and Te2− anions into the Cu2S β-chalcocite crystal lattice. A β-chalcocite-type room temperature modification exists for Cu2Te (ICSD #77055), but an analogous structure was reported for binary Cu2Se only as a metastable phase for nanosized crystals28 and for molecular clusters.34 The Cu2−xSeyS1−y NCs prepared with the synthesis protocol reported here were stable under electron beam irradiation, suggesting this as a possible direct path to approach the β-chalcocite-type Cu2Se metastable structure. Some peaks of as-synthesized Cu2−xS NCs correspond to monoclinic low chalcocite, i.e., low temperature phase with correspondingly lower symmetry (see Figure SI6 in the Supporting Information).

Figure 3. XRD patterns of Cu2−xS, Cu2−xSeyS1−y, and Cu2−xTeyS1−‑y NCs. For the Cu2−xS XRD patterns of as synthesized (bottom) and aged (top) nanoparticles are displayed. The line diffraction patterns show (from top to bottom) the bulk Cu7S4 roxbyite (PDF card #01− 023−0958), Cu2S chalcocite low (00−023−0961) and high (01− 073−6919), Cu2Se berzelianite (01−088−2043), Cu2.86Te2 cubic (01−071−4334), α-Cu2S cubic (01−084−1770).

Electron diffraction patterns acquired from ensembles of NCs confirm the coexistence of high and low chalcocite phases in copper sulfide particles (see Figure SI7 in the Supporting Information). Analysis of individual NCs by HRTEM shows the single-crystal structure of the copper sulfide particles, some of them exhibiting the β-chalcocite (see Figure 1) and some others − the α-chalcocite phase, as demonstrated in Figure SI8a in the Supporting Information. Under similar synthetic conditions (temperature 180 °C, ratio DDT/OA = 1/2) Freymeyer et al. obtained tetragonal chalcocite particles,35 most probably due to a different ratio between Cu and S used in the synthesis (Cu/S = 1/2). After several months of storage of a colloidal solution of Cu2−xS NCs in chloroform, their chalcocite crystal phase transformed to monoclinic roxbyite having composition Cu7S4 (i.e., Cu1.75S), most probably due to a partial oxidation of the particles by traces of air or other impurities in the solvent. At the same time, no remarkable crystal phase transformations have been observed for selenidesulfide and telluride-sulfide, presumably because of their higher stability in solution. Note that copper sulfide NCs were dissolved in chloroform, whereas the others (Cu2−xSeyS1−y and Cu2−xTeyS1−y) in toluene. For Cu2−xSeyS1−y particles, some peaks conform to the cubic berzelianite phase of Cu2Se. By HRTEM, we have observed an uneven distribution of S and Se in copper selenide-sulfide NCs. We assume that particles with a larger than average content of sulfur (i.e., S/Se > 1.3) possess high chalcocite crystal structure, whereas NCs with a larger content of selenium (i.e., S/Se < 1.3) have berzelianite phase (see Figure SI8b in the Supporting Information), although the particles obtained are perfectly uniform. Here we also remind that the separate nucleation of copper sulfide is excluded under the synthesis conditions, as is mentioned above. In the case of copper telluride-sulfide, apart from the high chalcocite-type phase, cubic alpha-Cu2S and D



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rearrangement of the structure which is possible instead on a longer time scale. Upon oxidation, copper sulfide NCs preserve their size and shape (see Figure SI10 in the Supporting Information for TEM images of Cu2−xS NCs before and after oxidation), indicating that they only undergo a stoichiometric and corresponding crystal structure transformation. Cu2−xSeyS1−y nuclei and small particles at the initial stage of the growth do not show any localized surface plasmon resonance, whereas after 5 min of the reaction as-prepared NCs exhibit a broad band with maximum at approximately 1700 nm which intensifies and blue-shifts to ∼1300 nm via oxidation (see Figure 4b). As could be expected, the oxidation of smaller particles did not lead to the prominent plasmon intensification (see Figure SI11 in the Supporting Information), similarly to small metal NCs, due to the surface scattering of free carriers.39 Analyzing literature data, one can find a significant variation of the plasmon band positions for nonstoichiometric (oxidized) Cu2−xSe nanoparticles. For example, 1100 nm centered plasmon for Cu1.81Se NCs of 15 nm in diameter had been observed by our group in a previous work.40 It was found that the pristine (reduced) particles and the oxidized ones were both cubic berzelianite copper selenide assuming that the oxidation process forms cation vacancies in the lattice without altering the actual structure of the crystal. In addition, we were able to exclude the creation of a copper oxide layer on the surface of NCs which could be responsible for their plasmonic behavior. On the other hand, Kriegel et al. have measured plasmon band with maximum at ∼1350 nm for 12 nm Cu1.8Se particles. It is interesting to note that in that case stoichiometric tetragonal phase Cu2Se NCs were found to gradually transform into nonstoichiometric, cubic Cu1.8Se phase upon oxidation in air.5 Moreover, the authors suggested that Cu species form a thin layer at the NC surface, presumably in the form of CuO or as a monolayer of Cu(II) atoms bound to surface ligands, i.e., Cu(II) does not leave the particles during their oxidation. On the basis of these data, we may assume that plasmonic properties (positions and intensities of the localized surface plasmon resonances) of copper chalcogenide NCs strongly depend not only on their sizes, shapes and surrounding media (as in the case of plasmonic metal particles), stoichiometry (oxidation level), but also on their crystal structures, which is in agreement with observations of Dilena et al.15 and Wang et al.41 made on alloyed Cu2−xSeyS1−y particles. Likewise, crystal structures of pristine particles restrict their transformations upon oxidation to phases with a symmetry and lattice parameters close to those of pristine particles, preserving anion sublattice. To prove this assumption, we have added the oxidizing agent to the aged Cu7S4 sample exhibiting a single phase structure. Comparison of the XRD patterns of the particles before and after addition of CAN suggests a preservation of the roxbyite crystal lattice (see Figure SI9a in the Supporting Information). Furthermore, light absorption of the aged sample (Cu7S4) presented in Figure SI9b in the Supporting Information is very weak in the NIR region. This suggests a low amount of Cu-vacancies and thus a perfect crystal structure, despite the apparent nonstoichiometry (x = 0.25) of this sample, indicating that structural rearrangement from chalcocite to roxbyite releases the copper vacancies. As seen from the spectra, the addition of the oxidizing agent to the aged sample leads to the evolution of its plasmon band. Thus, a quick oxidation again creates vacancies via extraction of copper but does not rearrange the lattice, since its further trans-

Cu2.86Te2 with slightly different lattice parameters, due to their mixed composition, can be distinguished. As well as bulk copper sulfide and selenide, telluride can undergo several phase transformations at elevated temperatures,36−38 which indeed should be more pronounced and facilitated in NCs. Slight variations in the lattice constants for different particles are observed in both copper selenide-sulfide and telluride-sulfide samples (see Figure SI8b, c in the Supporting Information). Thus, the analysis of the crystal structure corroborates the formation of alloyed particles. All three types of copper chalcogenide NCs exhibit plasmonic properties, as is displayed in Figure 4, where

Figure 4. Absorption spectra of (a) Cu2−xS, (b) Cu2−xSeyS1−y and (c, left) Cu2−xTeyS1−y NCs as synthesized and after partial oxidation by stepwise addition of the oxidizing agent (0.1 M solution of CAN in ethanol or methanol) indicated by arrows; absorption spectra of differently sized as synthesized Cu2−xTeyS1−y NCs (c, right). Note that sharp peaks in the NIR region arise from traces of ethanol or methanol and that a gap at 800 nm is due to a detector changeover.

absorbance spectra of Cu2−xS, Cu2−xSeyS1−y and Cu2−xTeyS1−y NC colloids as-synthesized as well as partly oxidized via a stepwise addition of 0.1 M solution of (NH4)2Ce(NO3)6 in methanol or ethanol are demonstrated. As-synthesized particles possess localized surface plasmon resonances of remarkable intensity, suggesting the formation of Cu vacancies already during the synthesis and purification of the samples. As seen from Figure 4a, Cu2−xS has a very broad band in its absorption centered at ca. 2400 nm, which intensifies and shifts to shorter wavelengths upon oxidation, ultimately to approximately 1270 nm in 18 h after the addition of the last portion of the oxidizing agent. According to results of the XRD analysis, as in the case of the aged sample mentioned above, the lattice of the oxidized one can be attributed to the roxbyite (cf. patterns of the initial and oxidized samples shown in Figure SI9a in the Supporting Information). As seen from the pattern, quick oxidation by addition of the cerium salt (CAN) does not allow a perfect E



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Furthermore, taking into account this small alteration of the plasmon band upon oxidation and consequently a reasonable stability of the NCs, we have performed a successful transfer of Cu2−xTeyS1−y particles from toluene into water applying a lowmolecular-weight methoxypoly ethylene glycol terminated with HS-group (mPEG-SH) as a stabilizer in the ligand exchange procedure (see the Supporting Information for details). This compound has recently been demonstrated to impart inherent amphiphilicity to CdTe44 and Au45 nanoparticles making them soluble in media of different polarity, and moreover inducing their penetration through live cell membranes.46 As shown in Figure SI12 (see the Supporting Information), Cu2−xTeyS1−y NCs retain their plasmon band without a remarkable change of its position (maximum at approximately 800 nm) and shape after ligand exchange and dissolution in water, resembling the behavior of mPEG-SH-capped Au nanoparticles, which preserve their plasmon in different media.45 In addition, mPEG-SH ensures a high colloidal stability of the particles in water preventing their aggregation. This efficient water solubilization of copper telluride-sulfide NCs has a potential to realize cation exchange reactions in aqueous media, similarly to a recently demonstrated ion exchange on CdTe NCs in water,47 which might be used not only for a synthesis of alloyed as well as completely exchanged materials based on the initial NC matrix but also for a detection of different cations dissolved in water by monitoring plasmon band alterations. Moreover, the position of the Cu2−xTeyS1−y plasmon band perfectly fits an optical window from ∼600 to 1000 nm where the absorption coefficient of a living tissue is at a minimum, 48 in contrast to Cu 2−xS and Cu 2−x Se y S 1−y possessing plasmon bands in the spectral region where water absorbs strongly (starting from approximately 1150 nm its transmittance measured in a 1 cm cuvette is ca. 35%).49 Thus, such amphiphilic copper telluride-sulfide particles after permeation into living cells may be subjected to a NIR laser irradiation acting as a photothermal agent, as recently was demonstrated by Li et al.23 We point out that our procedure of water solubilization is much simpler and less laborious than those used for the water transfer of Cu2−xSe26 and CuTe23 NCs employing amphiphilic polymer encapsulation, which involves several steps, including cross-linking of the polymer shell.50 These two concepts, viz. cation exchange in aqueous media and photothermal treatment, will be developed in our future work.

formation to the next member of copper sulfide family with larger x, i.e., covellite CuS (x = 1), would require a complete rebuilding of the anion sublattice, which is hardly possible at room temperature due to a larger size and a lower diffusivity of lattice anions.19,42 Therefore, during the oxidation process, the degree of variability of the x value (stoichiometry) is restricted by the crystal phase. All the processes observed are summarized in the scheme of Figure 5.

Figure 5. Simplified scheme of the copper sulfide crystal lattice transformation from hexagonal chalcocite to monoclinic roxbyite structure through the formation of vacancies (white circles), along with the evolution of its surface plasmon resonance.

This kind of structural dependency provides an additional degree of freedom to control plasmonic properties of these materials. We notice that the reactivity of an oxidizing agent may also regulate the stoichiometry of the materials, i.e., x value. For instance, an extended to 18 h oxidation by (NH4)2Ce(NO3)6 results in a further intensification and shift of the plasmon band in Cu2−xS NCs (see Figure 4a), indicating generation of new cation vacancies. Furthermore, as has been demonstrated by Liu et al., the localized surface plasmon resonance in copper sulfide and selenide NCs can be tuned by altering the capping ligand.12,43 Thus, by addition of OA during synthesis the plasmon band red-shifts that is attributed by the authors to the effect of deprotonated carboxylate group trapping surface holes, thereby reducing effective carrier concentration. Indeed, more detailed investigations of crystal structure transformations should be performed on all three chalcogenides, employing in situ observations by means of XRD, HRTEM, electron diffraction, and EDS coupled with optical spectroscopy. Cu2−xTeyS1−y NCs as-synthesized show a broad localized surface plasmon resonance in the region between 600 and 1000 nm. As seen from Figure 4c, resonances of the particles intensify with increasing sizes, which is a typical behavior of the plasmonic feature. However, they do not exhibit the blue shift with increasing size that would be expected from a potential increase of a number of free carriers in the particles. Thus, 3 nm NCs have the plasmon band maximum at approximately 780 nm (not well resolved), the 5.5 nm band at 740 nm, 6.5 nm at 820 nm, and 8.2 nm at 802 nm. This deviation from a general trend is probably due to a slight alteration of x in the NCs obtained at different temperatures. In general, we note that despite of the alloyed composition, the particles, both Cu2−xSeyS1−y and Cu2−xTeyS1−y, exhibit their localized surface plasmon resonances in the regions characteristic for the corresponding pure binary compounds reported in the literature (for oxidized Cu2−xTe NC colloid, a peak with approximately 760 nm maximum has been observed previously5), which is in accordance with results obtained for alloyed Cu2−xSeyS1−y nanoparticles with varied y, previously reported by our group.15 As in the case of copper sulfide and selenide-sulfide, oxidation of copper telluride-sulfide NCs leads to a blue shift of their plasmon band and to its slight increase in intensity (see Figure 4c).

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CONCLUSIONS In this work, we have developed a common synthesis approach to three copper chalcogenide NCs: Cu2−xS, Cu2−xSeyS1−y and Cu2−xTeyS1−y, possessing spherical shapes, with sizes tunable in the range from 2−3 to 10 nm. This approach has an advantage over the existing ones as it does not involve hot-injection and does not use phosphines, requires quite low reaction temperatures, and is easily up-scalable. The characterization of the materials obtained has revealed that copper selenide and telluride exhibit alloyed composition with inclusion of sulfur: Cu2−xSeyS1−y and Cu2−xTeyS1−y. Despite their mixed structure, the samples show optical properties, in particular the position of their plasmon bands, similar to those reported for pure binary copper chalcogenides, i.e., approximately 1800 nm (1270 nm after a prolonged oxidation), 1300 and 800 nm for nonstoichiometric copper sulfide, selenide-sulfide, and telluridesulfide, respectively. Moreover, successful water solubilization of Cu2−xTeyS1−y NCs with preservation of their plasmonic F



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properties has been realized via the ligand exchange employing mPEG-SH stabilizer.

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ASSOCIATED CONTENT

S Supporting Information *

Histograms displaying size distributions of Cu 2−x S, Cu2−xSeyS1−y, and Cu2−xTeyS1−y NCs. TEM images demonstrating a spontaneous self-assembly of Cu 2−x S and Cu2−xSeyS1−y NCs. TEM images of Cu2−xSeyS1−y NCs obtained from scaled up synthesis as well as of Cu2−xTeyS1−y NCs synthesized in DDT. Absorbance spectra of DDT and the mixture of DDT with Te powder after heating. Crystal structures of high and low chalcocite Cu2S unit cells. Electron diffraction patterns of Cu2−xS, Cu2−xSeyS1−y and Cu2−xTeyS1−y NCs. HRTEM images of individual Cu2−xS, Cu2−xSeyS1−y and Cu2−xTeyS1−y particles, crystal structures of which differ from the corresponding dominant phases. XRD patterns of assynthesized Cu2−xS NCs, oxidized by addition of CAN, aged by storage and additionally CAN-oxidized aged sample. Absorbance spectra of Cu7S4 NC colloid before and after a stepwise addition of CAN. TEM images of Cu2−xS NCs before and after oxidation. Absorbance spectra of small size Cu2−xSeyS1−y NCs before and after their oxidation. Details of the ligand exchange on Cu2−xTeyS1−y NCs. Absorbance spectra and TEM images of Cu2−xTeyS1−y NCs before and after the ligand exchange and water solubilization. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Dr. A. Dubavik (TU Dresden, Germany) for the synthesis of mPEG-SH ligand. V.L. acknowledges the support by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme under the grant agreement n. 301100, project “LOTOCON” and the grant agreement n. 240111 (ERC Grant NANO-ARCH).

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, this article was published ASAP on January 16, 2014, without all of the corrections. The corrected article was published ASAP on January 17, 2014.

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Generalized One-Pot Synthesis of Copper Sulfide, Selenide-Sulfide

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