Tin Ion Directed Morphology Evolution of Copper Sulfide

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Tin Ion Directed Morphology Evolution of Copper Sulfide Nanoparticles and Tuning of Their Plasmonic Properties via Phase Conversion Lihui Chen,† Masanori Sakamoto,*,‡,§ Mitsutaka Haruta,‡ Takashi Nemoto,‡ Ryota Sato,‡ Hiroki Kurata,‡ and Toshiharu Teranishi*,‡ †

Department of Chemistry, Graduate School of Science and ‡Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan § PRESTO, Japan Science and Technology Agency, Gokasho, Uji, Kyoto 611-0041, Japan S Supporting Information *

ABSTRACT: Copper-deficient copper sulfide (Cu2−xS) nanoparticles (NPs) have been investigated as important hole-based plasmonic materials because of their size, morphology, and carrier density-dependent localized surface plasmon resonance (LSPR) properties. Morphology and carrier density are two important parameters to determine their LSPR properties. Here, we demonstrate that the foreign metal ion, Sn4+, directs the growth of djurleite Cu31S16 from nanodisk to tetradecahedron along the [100] direction. To control the LSPR properties by tuning the carrier density, the djurleite Cu31S16 nanoparticles were pseudomorphically converted into more copper-deficient (higher carrier density) roxbyite Cu7S4 NPs by heat treatment in the presence of amine. The roxbyite Cu7S4 NPs exhibited a shorter and stronger LSPR peak while retaining the morphology of the djurleite Cu31S16 NPs.

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conductivity of Cu2−xSe film.11 Therefore, control over morphology and carrier density of the Cu2−xX NPs have been an important subject for application as plasmonic materials. For the morphology control of the Cu2−xX NPs, many synthetic routes have been proposed. For example, Cabot et al. reported that the CuTe nanocubes, plates, rods, or Cu2−xS polyhedral NPs were synthesized by tuning the reaction conditions, such as the reaction temperature, reaction time, and amount of reactant.10,13 Kolny-Olesiak et al. succeeded in the formation of Cu2−xS nanorods by replacing the well-used sulfur source, 1-dodecanethiol, with tert-dodecanethiol.14 Another attractive strategy for the morphology control of the Cu2−xX NPs is making use of the morphology direction ability of foreign metal ions. It was found that the foreign metal ions, such as tungsten,15 iron,16 and cobalt,17 can direct the formation of Pt-based nanocubes. This effect of metal ions can also be applied to the morphology control of metal chalcogenides. For instance, a foreign metal ion, Al3+, was reported to assist the morphology transformation of Cu2−xSe NPs from sphere to cube.12 However, the foreign metal iondirected morphology control has been rarely reported, especially for the Cu2−xS NPs. Unlike Cu2−xSe NPs with only cubic phase regardless of copper vacancy, the Cu2−xS NPs possess numerous crystal phases: monoclinic djurleite Cu1.94S,

INTRODUCTION Copper chalcogenide (Cu2−xX: X = S, Se, Te) nanoparticles (NPs) exhibit localized surface plasmon resonance (LSPR) owing to the collective oscillation of free holes generated by copper vacancies in valence band.1 Because of much lower carrier density (Nh ≈ 1021 cm−3) compared with noble metals (Ne ≈ 1023 cm−3), the Cu2−xX NPs show LSPR absorptions in the near-infrared (NIR) region.2 Recently, the LSPR absorption bands of Cu2−xS/Se NPs have been finely tuned by varying several factors, including size,3 shape,4 phase,5 and posttreatment such as oxidation, reduction,6 and interpartilce interaction.7 Our previous research showed that the LSPR absorption band of roxbyite Cu7S4 nanodisks was readily adjusted by plasmon coupling through their one-dimensional self-assembly. The in-plane plasmon coupling in nanodisk arrays caused the blue-shift of LSPR peak.7 Because of these tunable plasmonic properties, the Cu2−xX NPs have been used for biosensing, photothermal agents,8,9 surface-enhanced Raman scattering (SERS) sensor,10 and optical switcher in NIR region.11 For these plasmonic applications, morphology and carrier density are two crucial parameters to determine the LSPR properties of the Cu2−xX NPs. Cabot et al. showed that in SERS sensor the CuTe nanocubes exhibited higher enhancement factor than the nanoplates.10 They also demonstrated that Cu2−xSe nanocubes can maximize NP packing to form compacted films by making use of the convenient geometry, which is essential for technological applications.12 In addition, Riha et al. reported that an increase of carrier density resulted in significant enhancement of © XXXX American Chemical Society

Received: May 30, 2016 Revised: July 6, 2016

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DOI: 10.1021/acs.langmuir.6b02035 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. TEM images of copper sulfide NPs synthesized (a) with and (b) without 1.0 mmol SnCl4. Scale bars = 20 nm. (c) XRD patterns and (d) UV−vis-NIR absorption spectra of copper sulfide NPs synthesized with and without SnCl4. The NP sizes in (a) diameter, 23.7 ± 3.1 nm; thickness, 29.5 ± 3.2 nm and (b) diameter, 26.3 ± 5.7 nm; thickness, 11.5 ± 0.8 nm. Organics. Aluminum(III) chloride anhydrous (AlCl3, 98%), zinc(II) chloride (ZnCl2, 98%), 1-dodecanethiol (1-DT, C12H25SH, 98.0%), ethanol (99.5%), methanol, toluene, and chloroform were purchased from Wako Chemicals. All chemicals were used without any purification. Synthesis of Djurleite Cu31S16 Nanodisks and Tetradecahedra. The Cu31S16 nanodisks and tetradecahedra were synthesized by injecting the sulfur source into the mixture of metal precursors. CuCl (1.0 mmol), 5 mL of OAm, and 5 mL of ODE were mixed in a 50 mL three-necked flask with and without 1.0 mmol SnCl4·5H2O and the mixture was heated at 140 °C under reduced pressure. After 30 min, the mixture was backfilled with N2 and then 2 mmol 1-DT dissolved in 4 mL of OAm was rapidly injected. Afterward, the reaction temperature was raised to 240 °C and the mixture was kept for 60 min. After natural cooling to room temperature, 5 mL of toluene and 30 mL of ethanol were added to the mixture. After centrifugation at 8500 rpm for 5 min, the precipitate was dispersed in 5 mL of chloroform (CHCl3). The inadequately capped and/or largely agglomerated NPs were removed by centrifugation at 4000 rpm for 1 min. Finally, the synthesized NPs were stored in refrigerator. In order to study the influence of amount of SnCl4·5H2O, 0.5 or 1.5 mmol SnCl4·5H2O were mixed with 1.0 mmol CuCl, while other reaction conditions were maintained. In order to study the influence of tin valence on the final NPs, 1.0 mmol of SnCl4·5H2O was replaced by SnCl2·2H2O, while other reaction conditions were maintained. Seeded Growth of Djurleite Cu31S16 Tetradecahedra. The obtained Cu31S16 nanodisks were utilized as seeds for morphology evolution. Particularly, 5 mL of CHCl3 seed solution, 1.0 mmol CuCl, 5 mL of OAm, and 5 mL pf ODE were mixed in a 50 mL three-necked flask with or without 1.0 mmol SnCl4·5H2O. The synthesis and purification conditions are the same as above. The synthesized NPs were stored in 5 mL of CHCl3. Sn4+-treated Cu31S16 nanodisks were also used as seeds. For the treatment of nanodisks with the Sn4+, 5 mL of CHCl3 seed solution, 1.0 mmol of SnCl4·5H2O, 5 mL of OAm, and 5 mL of ODE were mixed in a 50 mL three-necked flask and degassed at 140 °C for 30 min. Then, the mixture was cooled to room temperature and purified by 5 mL of toluene and 30 mL of ethanol 2 times. The Sn4+-treated nanodisks were dispersed in 5 mL of CHCl3. Phase Conversion of Djurleite Cu31S16 Tetradecahedra to Enhance Their LSPR response. The Cu31S16 tetradecahedra

triclinic roxbyite Cu1.75S, cubic Cu1.8S diginite, orthorhombic anilite Cu1.75S, and hexagonal covellite CuS.18,19 The diversity of crystal phases would provide varieties of morphologies of Cu2−xS NPs. For the control over carrier (hole) density of the Cu2−xX NPs, an oxidation process has been reported as an effective approach to increase the carrier density of Cu2−xS/Se NPs. An extraction of copper atoms/ions by oxidizing agents such as air,2 iodine,20 and Ce(IV) complex6 leads to an increase of carrier density and thus a shorter and stronger LSPR peak. More effective approach to increase the carrier density is the phase conversion into more copper-deficient phase via thermal or chemical oxidation.21,22 However, the thermal oxidation often accompanies the morphology change of NPs, while the chemical oxidation requires long time to achieve the complete phase conversion. Here, we report the effect of tin ions on the morphology evolution of copper sulfide NPs. We discovered that the djurleite Cu31S16 nanodisks were transformed into djurleite Cu31S16 tetradecahedra by the morphology-direction effect of Sn4+, whereas they were transformed into the Cu−Sn−S ternary NPs by adding Sn2+. In order to tune the LSPR properties of djurleite Cu31S16 tetradecahedra by increasing the carrier density, these NPs were heat-treated in the presence of oleylamine. Consequently, the djurleite Cu31S16 phase was successfully converted into more copper-deficient, roxbyite Cu7S4 phase with retaining the morphology, which could not be achieved by a simple oxidation. The converted roxbyite Cu7S4 tetradecahedra exhibited a shorter LSPR peak with much higher intensity compared with the djurleite Cu31S16 tetradecahedra.

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EXPERIMENTAL SECTION

Chemicals. Copper(I) chloride (CuCl, 99%), tin(IV) chloride pentahydrate (SnCl4·5H2O, 98%), tin(II) chloride dihydrate (SnCl2· 2H2O, 98%), and octadecene (ODE) were purchased from SigmaAldrich. Oleylamine (OAm, 80−90%) were purchased from Acros B

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Figure 2. TEM images of the Cu31S16 NPs obtained at different reaction times. (a) 15, (b) 45, and (c) 90 min. Scale bars = 20 nm. Inset: SEM images. The NP sizes in (a) diameter (D), 16.1 ± 1.8 nm; thickness (T), 7.5 ± 0.8 nm. (b) D, 22.4 ± 1.4 nm; T, 20.1 ± 1.3 nm. (c) D, 25.2 ± 2.4 nm; T, 34.3 ± 1.7 nm. (d) Schematic of morphology evolution of the Cu31S16 NPs.

direction. We also investigated the influence of amount of Sn4+ on the morphology of the resulting Cu31S16 NPs (Figure S2). Smaller or larger amount of Sn4+ gave a mixture of various polyhedral NPs but not identical polyhedral NPs. Furthermore, we found that the valence of tin ion is crucial for morphology evolution of the Cu31S16 NPs. Compared with Sn4+, the resulting NPs synthesized in the presence of Sn2+ ion showed quite different physicochemical properties from the Cu31S16 NPs. The higher affinity of Sn2+ to 1-DT may lead to the incorporation of Sn2+ into the copper sulfide NP following HSAB theory to form the ternary Cu−Sn−S NPs (Figure S3). Although the obtained tetradecahedra and nanodisks have the same Cu31S16 phase, they showed different optical properties (Figure 1d). The Cu31S16 nanodisks synthesized without Sn4+ exhibit a strong LSPR absorption band in the near-infrared (NIR) region derived from the collective oscillation of free holes. On the other hand, the Cu31S16 tetradecahedra synthesized with Sn4+ have no LSPR absorption in the NIR region. It is proposed that even a trace amount of Sn4+ (XRF molar ratio Cu/Sn/S = 48.9:0.5:50.6) could effectively fill the copper vacancies or trap the free charge carriers.24 To clarify the morphology evolution mechanisms of the Cu31S16 NPs by the Sn4+, the electron microscopy of the intermediate was carried out by changing the reaction time at 15, 45, and 90 min. Figure 2a−c shows the TEM and SEM images of the Cu31S16 NPs obtained at different reaction times. At the initial stage, the disk-shaped NPs are formed. Then the morphology of the Cu31S16 NPs involves into the tetradecahedra probably due to the faster growth rate along the vertical direction of the disks than along the lateral directions. Figure 2d displays the schematic of the morphology evolution of the Cu 31 S 16 NPs. Because the Sn content in the Cu 31 S 16 tetradecahedra is quite low (0.5 mmol %), we propose that the Sn4+ assist the NP growth along the vertical direction of the nanodisks instead of capping on the lateral crystal planes (vide infra). Seeded Growth of Cu31S16 Tetradecahedra. To confirm the role of the Sn4+ on the morphology evolution of the Cu31S16 NPs, a seeded growth of the Cu31S16 tetradecahedra was carried out. The synthesized Cu31S16 nanodisks (seeds) were mixed with Sn4+ and CuCl in a three-necked flask. An injection of 1-DT to the mixture produced the tetradecahedra without any original nanodisks (Figure 3a), suggesting that the

synthesized from the seeded growth were utilized for phase conversion. Briefly, the Cu31S16 tetradecahedra in 5 mL of CHCl3, 5 mL of OAm, and 5 mL of ODE were mixed in a 50 mL three-necked flask and vacuumed at 140 °C for 40 min. After filling with N2, the mixture was cooled to room temperature and purified by toluene and ethanol. The obtained tetradecahedra were stored in CHCl3. Characterization. Transmission electron microscopy (TEM) were conducted with a JEM-1011 (JEOL) instrument at an accelerating voltage of 100 kV. The NP sizes were estimated by measuring 200 NPs in TEM images. Scanning electron microscopy (SEM) images were captured with a field emission S-4800 (FE-SEM, Hitachi) microscopy at an accelerating voltage of 5 kV. UV−vis-NIR (300−2500 nm) absorption spectra of the synthesized NPs were acquired in a 1 mm quartz cuvette in a U-4100 spectrophotometer (Hitachi). X-ray diffraction (XRD) patterns of the synthesized NPs were obtained on an X’Pert Pro MPD (PANalytical) instrument with CuKα radiation (λ = 1.542 Å) at 45 kV and 40 mA. X-ray fluorescence (XRF) analysis was performed with a JSX-3202C (JEOL) at 30 kV and 1 mA.

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RESULTS AND DISCUSSION Synthesis of Cu31S16 Nanodisks and Tetradecahedra. We developed a simple and convenient method for the morphology control of copper sulfide NPs in the presence of Sn4+ as a morphology-directing agent. Figure 1a,b shows the TEM images of the resulting copper sulfide NPs synthesized with and without SnCl4. We found that addition of SnCl4 greatly affects the final morphology of the copper sulfide NPs, that is, the tetradecahedral and disk-shaped copper sulfide NPs were formed with and without SnCl4, respectively. This indicates that Sn4+ and Cl− play a crucial role to control the morphology of the copper sulfide NPs. Both of the NPs show djurleite Cu31S16 phase with small roxbyite phase (solid green circles in Figure 1c. The unusual peak intensity ratio of (0 8 0) with (12 0 4) crystal planes of the nanodisks is derived from the preferential diffraction of X-ray by the (0 8 0) planes. To investigate the effect of Sn4+ and Cl− on the morphology evolution, the copper sulfide NPs were synthesized with AlCl3 and ZnCl2 in place of SnCl4 (Figure S1). These two metal chlorides did not lead to the formation of polyhedral NPs. Cl− has been reported as a morphology focusing agent in the synthesis of covellite CuS triangular prisms and this shape focusing is likely to occur from the competition between CuS surface capping by oleylamine and Cl−.23 However, this focusing effect of Cl− was not observed in the present study. Therefore, we concluded that the Sn4+ may assist the morphology evolution of the Cu31S16 NPs along a special C

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Figure 3. TEM images of (a) Cu31S16 tetradecahedra synthesized by the seeded growth method. The Cu31S16 nanodisk seeds treated (b) with and (d) without Sn4+. (c,e) Cu31S16 NPs obtained by the reaction of the corresponding seeds, CuCl, and 1-DT. Scale bars = 20 nm except for (f,g). (f,g) HRTEM and (h) fast Fourier transform (FFT) analysis of the Cu31S16 tetradecahedra in (c).

Sn4+ does not influence the formation of the nanodisks and only serves to direct the growth of the nanodisks along the vertical direction. Furthermore, we pretreated the seeds with Sn4+ (see Experimental Section for details). The Sn4+-treated seeds (Figure 3b) were then mixed with CuCl and an injection of 1-DT to the mixture gave the copper sulfide tetradecahedra (Figure 3c), which confirm the morphology directing ability of the Sn4+. When the Cu31S16 nanodisk seeds, which were not treated with Sn4+ were used for the seeded growth reaction (Figure 3d), only larger-sized copper sulfide nanodisks were generated (Figure 3e). The crystal phases of the copper sulfide NPs were retained after the seeded growth reactions for all cases (Figure S4). In order to confirm the growth direction of the Cu31S16 nanodisk, the HRTEM and FFT analysis were conducted (Figure 3f−h). The djurleite Cu31S16 phase with monoclinic crystal System, P21/n space group was selected for simple characterization. From the spots in FFT image, the Cu31S16 nanodisks are found to grow along the [100] direction. In a previous report on the morphology control of the Cu2−xSe NPs by the Al3+, the NPs grew isotropically, and no Al3+ could be detected on the surface or within the final NPs.12 At the present study, the EDS analysis reveals that the Cu31S16 nanodisks (seeds) pretreated with Sn4+ do not contain the element Sn on the surface of the nanodisks (Figure S5). XRF analysis of the Cu31S16 NPs (Figure 1a) also shows the detection-limit-level of the element Sn. Here, we came to the speculation that rather than adsorption of the Sn4+ on the specific crystal planes the Sn4+ affected the relative surface energy/reactivity between the vertical and the lateral crystal planes of the disk seed. In general, the (100) crystal planes show higher reactivity than the lateral ones in djurleite.14 Macdonald el al. reported that when the 1DT molecules are used as a sulfur source, “crystal-bound” thiols

located on high coordination sites become the terminal sulfur layer of the NP and the residual linear hydrocarbon chains serve as capping ligand.25 Therefore, the hydrocarbon chain capping significantly reduces the reactivity of the (100) crystal planes in disk seeds.14 Because of the larger density of sulfur and thus hydrocarbon chains on the (100) crystal planes compared with lateral ones13 in the seeded growth without Sn4+ treatment, the copper and sulfur sources (copper thiolate)26 may preferentially deposit on the lateral crystal planes (high surface energy) and produce larger-sized nanodisks. On the contrary, the Sn4+ may form strong bonding with 1-DTs, which promotes the extraction of 1-DT from (100) crystal planes. As a result, increased reactivity of the (100) crystal planes leads the formation of Cu31S16 tetradecahedra. Phase Conversion of Cu31S16 Tetradecahedra To Tune the LSPR Properties. The Cu31S16 tetradecahedra synthesized by the seeded growth method exhibited weak LSPR peak (Figure 4a). A previous report has shown that an air oxidation of freshly prepared Cu2S NPs resulted in a significant enhancement of LSPR response within several hours.2 However, the Cu31S16 tetradecahedra are insensitive to oxidation due to the large size, and the incompletely removed Sn4+ on the surface of NPs after purification may also hinder the extraction of copper by air oxidation. In fact, longtime air oxidation did not lead to an obvious blue-shift and enhancement of LSPR response (Figure 4a). The crystal phase was also retained after 10 days air oxidation (Figure 4b). The phase conversion by the heat-treatment in the presence of OAm was conducted to increase the carrier density to tune their LSPR response. As mentioned before, copper sulfides have a variety of crystal phases, some of which possess the similar sulfur anion sublattice.18 For example, monoclinic djurleite, triclinic roxbyite, and hexagonal covellite copper sulfide share quasiD

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Figure 4. (a) UV−vis-NIR absorption spectra and (b) XRD patterns of freshly prepared and air oxidized djurleite Cu31S16 tetradecahedra. (c) XRD patterns of the djurleite Cu31S16 tetradecahedra treated with ODE and ODE + OAm. Inset: TEM image of the Cu31S16 tetradecahedra after ODE + OAm treatment. Scale bar = 20 nm. (d) Unit cells of djurleite and roxbyite. Parameters of monoclinic djurleite unit cell: a, 26.89700 Å; b, 15.74500 Å; c, 13.56500 Å; α, 90.0000°; β, 90.1300°; γ, 90.0000°. Triclinic roxbyite: a, 13.40510 Å; b, 13.40900 Å; c, 15.48520 Å; α, 90.0215°; β, 90.0210°; γ, 90.0202°; a axis, [100] direction. Solid green sphere, copper cation, and solid yellow sphere, sulfur anion. UV−vis-NIR absorption spectrum of (e) Cu31S16 tetradecahedra treated with excess OAm and (f) Cu31S16 tetradecahedra before and after treatment with ODE or ODE + OAm.

copper−amine complexes after the treatment of Cu31S16 NPs with excess OAm (Figure 4e). The phase conversion is accompanied by a drastical change of LSPR peak (Figure 4f). That is, higher density of copper vacancies leads to a blue-shift, higher intensity, narrower bandwidth of the LSPR peak as well as an increased onset of bandgap absorption.6,7 To trigger the phase conversion, not only the generation of copper vacancy but also the rearrangement of copper cation and sulfur anion are essential.

hexagonal sulfur anion sublattice. Desnity functional theory calculation for the formation energy of djurleite, chalcocite, roxbyite, and anilite demonstrates that phase conversion between these copper sulfides is energy-favorable.27 An extraction of copper atoms/ions can proceed the phase conversion between these copper sulfides with similar anion sublattice. We demonstrated that the synthesized djurleite Cu31S16 tetradecahedra were transformed to more copperdeficient phase (higher carrier density) roxbyite Cu 7 S 4 tetradecahedra. The XRD patterns in Figure 4c show the successful phase conversion from djurleite Cu31S16 to roxbyite Cu7S4 (Cu28S16) without changing the particle morphology. From the experimental result that the crystal phase did not change in the absence of OAm, we propose that the distributions of cation and anion were slightly rearranged after the removal of copper atoms/ions by OAm at high temperature (140 °C) (Figure 4d). Copper ions/atoms are easily removed from the Cu2S lattice by amine molecules through the formation of copper−amine complexes.20,28 Indeed, we observed the absorption peak assigned to the

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CONCLUSIONS In conclusion, we demonstrated the Sn4+-directed morphology evolution of the Cu31S16 NPs from disks to tetradecahedra. The valence of tin ion is crucial for this morphology evolution. In the seeded growth, we discovered that the Sn4+ modified/ activated the vertical crystal planes of the Cu31S16 disk seeds instead of capping on lateral crystal planes. In the case of the Sn4+-treated disk seeds, the copper and sulfur source preferentially deposited on the vertical crystal planes to form the tetradecahedra. For further tuning of the LSPR properties, E

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(13) Li, W.; Shavel, A.; Guzman, R.; Rubio-Garcia, J.; Flox, C.; Fan, J.; Cadavid, D.; Ibáñez, M.; Arbiol, J.; Morante, J. R.; Cabot, A. Morphology evolution of Cu2−xS nanoparticles: from spheres to dodecahedrons. Chem. Commun. 2011, 47, 10332−10334. (14) Kruszynska, M.; Borchert, H.; Bachmatiuk, A.; Rümmeli, M. H.; Büchner, B.; Parisi, J.; Kolny-Olesiak, J. Size and Shape Control of Colloidal Copper(I) Sulfide Nanorods. ACS Nano 2012, 6, 5889− 5896. (15) Zhang, J.; Fang, J. A General Strategy for Preparation of Pt 3dTransition Metal (Co, Fe, Ni) Nanocubes. J. Am. Chem. Soc. 2009, 131, 18543−18547. (16) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. Synthesis of Monodisperse Pt Nanocubes and Their Enhanced Catalysis for Oxygen Reduction. J. Am. Chem. Soc. 2007, 129, 6974−6975. (17) Lim, S. I.; Ojea-Jiménez, I.; Varon, M.; Casals, E.; Arbiol, J.; Puntes, V. Synthesis of Platinum Cubes, Polypods, Cuboctahedrons, and Raspberries Assisted by Cobalt Nanocrystals. Nano Lett. 2010, 10, 964−973. (18) Chakrabarti, D. J.; Laughlin, D. E. The Cu-S (Copper-Sulfur) system. Bull. Alloy Phase Diagrams 1983, 4, 254−271. (19) Mumme, W. G.; Gable, R. W.; Petříček, V. THE CRYSTAL STRUCTURE OF ROXBYITE, Cu58S32. Can. Mineral. 2012, 50, 423−430. (20) Jain, P. K.; Manthiram, K.; Engel, J. H.; White, S. L.; Faucheaux, J. A.; Alivisatos, A. P. Doped Nanocrystals as Plasmonic Probes of Redox Chemistry. Angew. Chem., Int. Ed. 2013, 52, 13671−13675. (21) Hsu, S.-W.; Bryks, W.; Tao, A. R. Effects of Carrier Density and Shape on the Localized Surface Plasmon Resonances of Cu2−xS Nanodisks. Chem. Mater. 2012, 24, 3765−3771. (22) Saldanha, P. L.; Brescia, R.; Prato, M.; Li, H.; Povia, M.; Manna, L.; Lesnyak, V. Generalized One-Pot Synthesis of Copper Sulfide, Selenide-Sulfide, and Telluride-Sulfide Nanoparticles. Chem. Mater. 2014, 26, 1442−1449. (23) Hsu, S.-W.; Ngo, C.; Bryks, W.; Tao, A. R. Shape Focusing During the Anisotropic Growth of CuS Triangular Nanoprisms. Chem. Mater. 2015, 27, 4957−4963. (24) De Trizio, L.; Li, H.; Casu, A.; Genovese, A.; Sathya, A.; Messina, G. C.; Manna, L. Sn Cation Valency Dependence in Cation Exchange Reactions Involving Cu2‑xSe Nanocrystals. J. Am. Chem. Soc. 2014, 136, 16277−16284. (25) Turo, M. J.; Macdonald, J. E. Crystal-Bound vs Surface-Bound Thiols on Nanocrystals. ACS Nano 2014, 8, 10205−10213. (26) Wang, Y.; Hu, Y.; Zhang, Q.; Ge, J.; Lu, Z.; Hou, Y.; Yin, Y. One-Pot Synthesis and Optical Property of Copper(I) Sulfide Nanodisks. Inorg. Chem. 2010, 49, 6601−6608. (27) Ha, D.-H.; Caldwell, A. H.; Ward, M. J.; Honrao, S.; Mathew, K.; Hovden, R.; Koker, M. K. A.; Muller, D. A.; Hennig, R. G.; Robinson, R. D. Solid−Solid Phase Transformations Induced through Cation Exchange and Strain in 2D Heterostructured Copper Sulfide Nanocrystals. Nano Lett. 2014, 14, 7090−7099. (28) Broome, F. K.; Ralston, A. W.; Thornton, M. H. Complex Formation with High Molecular Weight Amines. II.1 A Spectrophotometric Study of the Dodecylamine−Cupric Acetate System. J. Am. Chem. Soc. 1946, 68, 849−851.

the djurleite Cu31S16 tetradecahedra were converted to more copper-deficient roxbyite Cu7S4 tetradecahedra by the heat treatment in the presence of OAm. Our method provides a versatile way to control the morphology and crystal phase of various copper sulfide NPs to create the novel plasmonic materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02035. TEM images, XRD patterns, and EDS analysis.(PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +81-774-38-3120. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b02035 Langmuir XXXX, XXX, XXX−XXX