Article pubs.acs.org/cm
Shape Focusing During the Anisotropic Growth of CuS Triangular Nanoprisms Su-Wen Hsu, Charles Ngo, Whitney Bryks, and Andrea R. Tao* NanoEngineering Department, University of California, San Diego, CA 92093, United States S Supporting Information *
ABSTRACT: Nanocrystals composed of CuS that exhibit semimetallic behavior are capable of supporting localized surface plasmon resonances in the near-infrared wavelengths. A major challenge in utilizing these nanocrystals for plasmonic applications is the ability to accurately control their nanoscale morphology and chemical composition, both of which are known to affect plasmon wavelength and amplitude. Here, we investigate the important role of halide ions in controlling the solvothermal synthesis of colloidal CuS nanocrystals. When oleylamine is used as a stabilizing surfactant, we find that the addition of halide ions to the reaction mixture instigates nanocrystal shape focusing in a process similar to Ostwald ripening, from nanodisks to faceted triangular prisms. We demonstrate that this shape focusing is likely to occur from the competition between CuS surface binding interactions with oleylamine ligands and halide ions and is the strongest for I− ions. We also demonstrate that this shape focusing, in combination with seedmediated growth, can be used to generate colloidal dispersions of CuS nanoprisms with narrow size distributions.
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INTRODUCTION Heavily doped metal chalcogenide nanocrystals are emerging as a new class of nonmetallic plasmonic materials that support localized surface plasmon resonances (LSPRs) at near-infrared (NIR) wavelengths.1−3 Specifically, nanocrystals composed of p-type copper chalcogenides (Cu2−xX with X = S, Se, Te) have been highly explored materials because these nanocrystals are self-doped where the concentration of hole carriers is determined by the number of copper vacancies. The LSPR response of these nanocrystals can thus be tuned by modulating copper deficiency by either (i) controlling nanocrystal phase and chemical composition during synthesis or by (ii) postsynthetic modification of the nanocrystals by redox or cation-exchange reactions.1,4−12 One of the most promising chalcogenides for LSPR excitation is CuS (covellite) due to its high carrier concentration and semimetallic behavior. CuS nanocrystals have been exploited for LSPR excitation in the near-infrared (NIR) region with potential applications in NIR chemo-photothermal therapy, bioimaging, and biosensing.13−18 Recently, Xie et al. reported that CuS nanodisks possess semimetallic behavior with a free hole-carrier density as high as 1022 cm−3.19 We observed that CuS nanodisks also exhibit strong plasmonic coupling, which is observed as a large redshift in the LSPR wavelength when the nanodisks are assembled into thin-films. This is attributed to the large holecarrier concentration and relatively high carrier mobility of CuS, which enables strong Coulombic interactions between neighboring nanodisks.7 For application in NIR plasmonics, faceted CuS triangular nanoprisms would be a highly desired nanostructure. Triangular nanoprisms have been the focus of extensive studies for both © XXXX American Chemical Society
Ag and Au nanoparticles. The high radius of curvature at nanoprism corners and plasmonic coupling between multiple nanoprisms through their corners leads to intense electromagnetic field localization effects.20−22 However, obtaining precise control of CuS nanocrystal size, shape, and composition remains a challenge. To date, a variety of synthetic methods have been utilized to prepare colloidal CuS nanocrystals, including sonoelectrochemical synthesis,8 the hot-injection method,19 sacrificial templating,23,24 biomolecule-directed synthesis,25 and solvothermal methods.8,11 Using the aforementioned synthesis methods can yield nanoparticle size distributions that vary by 200−300%. For example, Lou et al. synthesized Cu2−xS triangular nanoplates by solvothermal decomposition of a Cu dialkyldithiophosphate molecular precursor to yield triangular nanoplates. However, these nanoplates consist of a mixture of two compositions (Cu9S5 and CuS) and exhibit a high polydispersity in both size and morphology.11 This polydispersity is problematic because LSPR response depends critically on particle uniformity. The generation of triangular nanoprisms of CuS is also a challenge, and few methods are able to achieve these highly faceted nanostructures. Here, we present a method for synthesizing faceted CuS triangular nanoprisms that exhibit highly shape-dependent LSPR modes. We observe that halide anions (Cl−, Br−, I−) exert a significant role on the morphology of CuS (covellite) nanocrystals. Previously, halide anions were observed to Received: March 18, 2015 Revised: June 25, 2015
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DOI: 10.1021/acs.chemmater.5b01223 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. Nanoprisms synthesized using the seed-mediated process. (a) TEM image of seeds: CuS triangle prisms with edge length of e = 23.2 ± 2.8 nm. (b) TEM image of the first growth: e = 31.3 ± 4.2 nm. (c) TEM image of the second growth: e = 43.1 ± 7.7 nm. (d) TEM image of the third growth: e = 56.2 ± 10.3 nm. (e) Absorption spectra of CuS nanoparticles for different numbers of growth cycles: seed (black), first growth cycle (red), second growth cycle (blue), and third growth cycle (pink) solution.
Figure 2. (a) HRTEM image of triangle CuS nanoparticles with the basal plane parallel to the substrate, the ⟨001⟩ zone axis. Inset: Electron diffraction pattern taken along the ⟨001⟩ zone axis. (b) Analysis of TEM images indicates that the nanoprisms are close to equilateral triangles with edge-to-edge angles of 59.5 ± 0.31°. (c) TEM image of particles oriented in a side-by-side packed thin-film. (d) XRD spectrum of a thin-film with the basal plane parallel to the substrate (red line) compared to the powder sample (black), which shows that only the (00n) face, with n = even integer, occurred in the ordered thin-film.
min, and then added to a new reaction flask containing additional growth solution. This serial seeding process was continued until the desired copper sulfide nanoprism size was obtained. Figure 1 shows the results of three seeding cycles, where the copper sulfide nanocrystals are shown to grow larger and sharper with each cycle. We demonstrate that the nanoprisms can be nearly doubled in edge length (e) from 23.2 ± 2.8 to 56.2 ± 10.3 nm. This increase in size corresponds to a spectral red-shift in the dominant LSPR mode from 1050 to 1480 nm in the optical extinction spectrum of the colloidal dispersions. The size distribution increases (Supporting Information Figure S5) slightly with each growth cycle, as evidenced by an increase in the peak full-width half-max (fwhm) from 620 to 780 nm after three growth cycles. Figure 2a shows the transmission electron microscope (TEM) image and electron diffraction pattern of the copper sulfide triangular nanoprisms. Both indicate the basal plane of the triangular nanoprisms is the (001) plane, which is consistent with the basal plane of the CuS (covellite) disklike particles reported by Xie et al.19 This is further confirmed with a powder X-ray diffraction (XRD) pattern of nanoprisms that are oriented within a thin-film (Figure 2d). The nanoprisms are supported on a solid substrate and are packed side-by-side. The powder spectrum shows that the diffraction
mediate the growth of anisotropic nanostructures composed of Ag and Au, demonstrating that these anions cooperatively stabilize the low-index Ag/Au surfaces and preferentially enhance the growth in and along a given crystallographic direction during synthesis.20,21,26 Here, we observe that CuS nanoprisms with a narrow size distribution can be fabricated by a combination of halide- and seed-mediated syntheses. We also observe that halide anions mediate the Ostwald ripening processes of CuS nanocrystals, inducing simultaneous shapefocusing and size-defocusing. In the presence of halide anions, the formation of CuS triangular nanoprisms results from the ripening of disk-shaped nanocrystal intermediates.
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RESULTS AND DISCUSSION First, small copper sulfide (CuS, covellite) nanocrystals with a particle edge of 23.2 ± 2.8 nm (Figure 1a) were synthesized using a previously reported solution-based method.7 The only modification was the use of Cu(NO3)2 as the Cu source (instead of CuCl2). The reaction was maintained at a temperature of 140 °C for 45 min. These particles were then used as seeds in a one-pot reaction using a growth solution of CuCl2 and S powder dissolved in a mixture of oleylamine and 1-octadecene (see Experimental Methods for more details). The resulting nanocrystals were kept stirring at 120 °C for 45 B
DOI: 10.1021/acs.chemmater.5b01223 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. Halide ions enhance triangle nanoparticle formation. (a) Schematic showing triangle nanoparticle formation using a two phase method: octadecene (top layer) and pentanediol (bottom layer). (b) XRD spectra of nanoparticles synthesized with and without the presence of halide ions: without halide ions (black), with chloride (red), with bromide (blue), and with iodide (pink). All the nanoparticles possess covellite (CuS) crystal structure. (c) Absorption spectra of different CuS nanoparticles in 500−2000 nm: without halide ions (black), with chloride (red), with bromide (blue), and with iodide (pink). The TEM images show the morphology of the nanoparticle synthesis with no halide ion, chloride ion, bromide ion, and iodide ion in (d−g), respectively.
peaks corresponding to (00n), where n is an even integer, are dominant. This confirms that the basal plane of the triangular nanoprism is perpendicular to the c-axis of the covellite crystal structure. Statistical image analysis of TEM images indicate that the CuS nanoprisms adopt near equilateral triangular shapes with edge angles of ∼59.5 ± 0.31°. To understand the growth mechanism of the CuS triangular prisms, we used different halide ions to mediate anisotropic nanocrystal growth in a two-phase colloidal synthesis method. The top layer of the biphasic reaction mixture is comprised of dissolved S powder in a solvent mixture of octadecene and oleylamine. Within the hydrophobic phase, oleylamine and sulfur powder are known to form oleylammonium hydrosulfide. The bottom layer is comprised of a solution of Cu(NO3)2 and a sodium halide salt (NaX) with 1,5-pentanediol as the denser, immiscible hydrophilic phase. This two-phase solution was heated to 180 °C in an oil bath, and the reaction was carried out for 30 min. At the interface between the immiscible phases, a precipitation reaction takes place between the formed CuX complex and oleylammonium hydrosulfide.27 Small CuS seeds capped with oleylamine are formed at the interface. The
presence of the hydrophobic capping agent facilitates transfer into the hydrophobic octadecene layer. This is depicted in Figure 3a and is labeled as the “nucleation step”. In octadecene, these seeds grow into larger CuS triangular nanoprisms. This is depicted in Figure 3a, labeled as the “growth” step. Triangular nanoprisms are only observed when a halide ion, such as Cl−, Br−, or I−, is present in the hydrophilic pentanediol layer. Figure 3b shows the XRD pattern of CuS triangular nanoprisms synthesized with different halide ions, indicating that each reaction produces nanocrystals that possess the covellite crystal structure. In the absence of halide ions, only disk-like CuS nanoparticles are observed to form. Figure 3c shows the optical extinction spectra for the CuS nanoprisms produced using the various halide ions. The nanoprisms are dispersed in chloroform to give an isotropic colloidal suspension. Each colloidal suspension gives a spectrum that possesses one dominant peak, attributed to an in-plane dipolar LSPR mode.7,19 The in-plane quadrupole mode and the out-of-plane dipole mode are expected to overlap with the in-plane mode of the CuS nanoprisms,28 but do not significantly contribute to the observed spectral intensities of C
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Figure 4. Triangular nanoparticle ripening with extended reaction time. (a) Schematic shows that the triangle nanoparticles undergo ripening by extending the reaction duration, where small particles dissolve and redeposit onto larger particles. Also, the small particles become truncated. The TEM images show triangle nanoparticle ripening under three different reaction times: 15, 30, and 120 min in (b−d), respectively. The crystal structure remains covellite (CuS) during the extended reaction time. (e) Absorption spectra of different CuS triangle nanoparticles under different reaction times: 15 (black), 30 (red), 60 (blue), 90 (pink), and 120 (cyan) min.
focusing. Shape focusing is accompanied by size defocusing. The size distributions for nanoprism dispersions synthesized with Cl− and Br− appear bimodal based on TEM image analysis. With time, we observe dissolution of the unfaceted nanodisks (which grow smaller) and deposition onto faceted nanoprisms (which grow larger in edge length). To examine the shape focusing and size defocusing processes in more detail, we translated the two-phase colloidal synthesis described above to a single-phase synthetic method where nucleation is not limited by chemical transport to the solvent interface. The advantage of using a single-phase reaction mixture is that nucleation is expected to occur in a single, fast event similar to the well-known La Mer model,29 and the resulting size and shape distributions of the colloidal dispersion should reflect the rate and manner of nanocrystal growth. In the single-phase reaction, both CuCl2 and S powder are dissolved into a mixed solvent system of octadecene and oleylamine prior to heating. The CuS nanocrystals synthesized in the singlephase method also possess the covellite crystal structure but exhibit a narrower size distribution (28.9 ± 10.2 nm) in comparison to those produced by the two-phase method (42.8 ± 22.3 nm), as analyzed from TEM images (Supporting Information Figure S1).
the observed LSPR peak. Nanoprisms synthesized in the presence of Cl− possess a peak centered at λ = 1284 nm with a fwhm = 802 nm (red line), whereas nanoprisms synthesized in the presence of I− possess an LSPR peak at λ = 1060 nm with fwhm = 400 nm (pink line). This is consistent with TEM images that show larger nanocrystals and a larger size distribution for nanoprisms synthesized with Cl−. The particle size distribution analysis based on the TEM images in Figure 3e−g show that triangular prisms with edge lengths of e = 42.8 ± 22.3, 31.3 ± 12.3, and 14.3 ± 7.2 nm resulted from the addition of Cl−, Br−, and I− ions, respectively. It is likely that the size distribution of the CuS nanoprisms is different for the various halide ions due to differences in binding strength between the ions and the CuS surface. I− is expected to possess a stronger binding affinity because the covalent character of the bond between halide ions and metals (such as Au, Ag, and Cu) increases in the following order: Cl− < Br− < I−.20,26 This stronger binding affinity of I− is able to slow the ripening process by stabilizing the CuS surface, leading to nanoprisms with more homogeneous size distributions and shapes that resemble nanodisks produced without the addition of halide. In contrast, Cl− and Br− promote faster ripening and the formation of faceted nanoprisms, which we term shape D
DOI: 10.1021/acs.chemmater.5b01223 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 5. Size separation of triangle nanoparticles during ripening. (a) Absorption spectra of CuS triangle nanoparticles before and after size separation: the as-made nanocrystal dispersion separation (black line), the size-separated small particles (population 1, red line), and size-separated large particles (population 2, blue line). The TEM images show the size distribution and morphology of nanoparticles before and after size separation: (b) before separation, (c) population 2 with an average edge length of 142 ± 80.5 nm, (d) population 1 with an average diameter of ∼18.2 ± 8.6 nm.
We observe the occurrence of a shape-dependent ripening process when the colloidal nanocrystal dispersion is held at the reaction temperature for 2 h or longer. Initially, the nanocrystals are primarily composed of unfaceted, disk-shaped particles. After 15 min of ripening, the nanocrystals possess a narrow size distribution with e = 21.2 ± 4.2 nm, as analyzed from the TEM image in Figure 4b. After 30 min, the nanocrystals grow larger to e = 28.9 ± 10.2 nm (Figure 4c). After 120 min, the dispersion possesses a largely bimodal size distribution with larger faceted particles possessing edge lengths of e = 125 ± 70.2 nm and smaller unfaceted particles with average sizes of e = 16.7 ± 9.8 nm (Figure 4d, and the histograms in Supporting Information Figure S6). The large faceted particles adopt a triangular prismatic shape. XRD patterns (Supporting Information Figure S2) indicate that the CuS nanocrystals retain the covellite crystal structure throughout this ripening process. The triangular CuS nanocrystals undergo anisotropic growth, where their edge lengths are increased by ∼6× the original nanocrystal size during growth, but only a negligible change in prism thickness is observed by atomic force microscopy (Supporting Information Figure S3). This change in size distribution is further indicated by the optical extinction spectra for the colloidal dispersions (Figure 4e). After 15 min, the extinction spectrum shows one peak at λ = 1080 nm corresponding to the in-plane dipolar LSPR mode (black line). After 30 min, we observe the appearance of a peak centered at λ = 1135 nm and a lower intensity shoulder at λ = 835 nm (red line). After 120 min, the extinction spectrum shows a broad extinction response (cyan line) that we attribute to spectral overlap between the two LSPR modes associated with nanocrystals of two different sizes. To confirm this, we carried out size separation by centrifugation to obtain two distinct nanocrystal populations. The smaller nanocrystal
population is primarily composed of unfaceted nanodisks that are 18.2 ± 8.6 nm in diameter (Figure 5d) and gives rise to a single LSPR peak centered at λ = 1100 nm (Figure 5a, red line). The larger nanocrystal population is primarily composed of triangular nanoprisms that are 142 ± 80.5 nm in edge length (Figure 5c and histograms in Supporting Information Figure S7) and gives rise to an LSPR peak centered at λ = 1850 nm with a broad fwhm (Figure 5a, blue line). A small feature in the extinction spectrum is also apparent at around λ = 900 nm, which we attribute to the quadrupolar in-plane LSPR mode associated with the triangular nanoprism geometry.28,30 It is likely that this ripening process is a direct result of the competition between the CuS surface binding interaction with oleylamine ligands and the Cl− ions. In the absence of Cl−, oleylamine forms a stable ligand shell on the CuS surface and promotes the formation of unfaceted nanodisks. These nanodisks are stable and do not undergo any apparent ripening or further growth if the reaction is allowed continue for an extended period, indicating the stability of the oleylamine ligand shell on the nanodisk surface. Similarly, nanodisks are also the first shape that we observe to nucleate and grow in the presence of both oleylamine and Cl−. However, in the presence of Cl−, the nanodisks undergo shape focusing that promotes the growth of faceted triangular nanoprisms. We believe that strong metal-halide interactions promote the binding of Cl− ions to terminal Cu atoms that are exposed at the nanodisk surface, effectively disrupting the binding between oleylyamine and CuS. The strong metal-halide interaction can be observed by the formation of smaller nanoprisms when higher halide ion concentrations are used (Supporting Information Figure S8). This is further supported by the previous observations that oleylamine forms stable complexes with metal halide salts, such as AuCl.31 The formation of metal-halide bonds at the CuS surface potentially allows oleylamine to dynamically come on E
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chloroform and centrifuged at 7500 rpm for 7.5 min to remove any byproducts. Shape Focusing of Triangular Nanoprisms by Seed Mediated Growth. The CuS nanocrystals synthesized by a single phase method at 140 °C for 45 min were used as a seed solution to form the uniform nanoprisms with edges between 25 and 60 nm. The nanocrystal growth solution was prepared by dissolving 0.05 mmol copper(II) chloride in a mixed solvent (oleylamine 0.25 mL/1octadecene, 3.75 mL) in a glass vial. Next, the seed solution was dispersed in 1-octadecene (one tenth of the nanocrystals synthesized by the single phase method as seed) and added to the growth solution. The glass vial was placed in an oil bath at 120 °C for 75 min. The bright-blue solution turned to a dark green-blue solution and was then cooled to room temperature. Subsequently, 4 mL of ethanol was added, and the solution was centrifuged at 3000 rpm for 5 min to remove free oleylamine and 1-octadecene. The precipitate was redispersed in chloroform and centrifuged at 7500 rpm for 7.5 min to remove any byproducts. These CuS nanoparticles were used as “new seeds” for the next growth process, and the nanoprism size increased with further growth cycles. Materials Characterization. Nanoparticles were examined by transmission electron microscopy using an FEI Tecnai Sphera. To prepare TEM samples, we dispersed the nanoprisms in chloroform and then drop-cast this solution onto an air−water interface. The resulting nanoparticle film formed at the air−water interface is then transferred onto a TEM grid. This preparation encourages the nanoparticles to lie with their basal planes flat on the TEM grid for accurate size and shape measurements. UV−vis−NIR spectroscopy was carried out on a PerkinElmer LAMBDA 1050 UV/vis/NIR spectrophotometer and was used to characterize the surface plasmon resonance peaks of the CuS nanocrystals. For these optical measurements, the as-made powder was dispersed in carbon tetrachloride. We measured light transmission in the range of 800−3200 nm. To characterize the crystal structure of the synthesized nanoparticles, powder X-ray diffraction was carried out on a Rigaku RU200B diffractometer. Powder XRD samples were prepared by washing the as-made nanoparticles in ethanol and chloroform and drying them to a powder. Samples were dried onto a glass slide for XRD measurements.
and off the nanodisk surface, encouraging the shape focusing. This is also supported by temperature-dependent studies that indicate that a critical temperature of 160 °C is necessary for the ripening process to occur. At temperatures of 160 °C and above, the nanocrystals in the presence of both oleylamine and Cl− undergo ripening and develop bimodal size and shape distributions as indicated by the TEM images in Supporting Information Figure S4. Below this temperature, as-made CuS nanocrystals undergo no change in their size or size distribution, likely due to the activation energy required to disrupt oleylamine−CuS interactions.
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CONCLUSIONS We show that the formation of faceted CuS triangular nanoprisms is strongly mediated by the presence of halide ions, and that the size and shape distribution of the colloidal nanocrystal dispersion is dependent on the strength of this halide interaction with the nanocrystal surface. We observe that Ostwald ripening of a colloidal dispersion of CuS nanodisks is accompanied by shape focusing, where faceted nanoprisms grow larger in edge length at the expense of unfaceted nanodisks. This process only occurs in the presence of halide ions, which are capable of disrupting the binding interactions between oleylamine capping ligands and the CuS surface. This shape focusing effect enables the synthesis of CuS nanoprisms with tunable sizes, which may enable their use in light-focusing and enhanced spectroscopies that require LSPR excitation in the near-infrared region.
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EXPERIMENTAL METHODS
Chemicals. Copper(II) nitrate (Cu(NO3)2·2.5H2O, Fisher), sulfur powder (Sigma-Aldrich), copper(II) chloride (CuCl2, 97%, SigmaAldrich), sodium chloride (NaCl, Fisher), sodium bromide (NaBr, J.T. Baker), potassium iodide (KI, Fisher), oleylamine (C18H35NH2, 70%, Sigma-Aldrich), 1-octadecene (C18H36, 90%, Sigma-Aldrich), 1,5pentanediol (C5H12O2, >97%, Sigma-Aldrich), ethanol (CH3OH, Goldshield chemical), and chloroform (CHCl3, Fisher) were used. Two-Phase Synthesis of CuS Triangular Nanoprisms. A 0.15 M sulfur solution was prepared by dissolving 0.0192 g sulfur powder (0.6 mmol) in a 4 mL mixture of oleylamine/1-octadecene (volume ratio of 1:3) in a glass vial, and an orange solution formed after 5 min of ultrasonication. A 0.1 M copper nitrate solution with 0.1 mmol Cl− was prepared by dissolving 0.0928 g copper nitrate (0.4 mmol) and 0.2 mmol NaCl in a 4 mL mixture of water/1,5-pentanediol (volume ratio of 1:6). The copper nitrate solution was added to the sulfur solution, which forms the two phase solution with the hydrophilic copper nitrate solution as the bottom layer and hydrophobic sulfur solution as the top layer. Then, a glass vial was placed in an oil bath at 180 °C. The orange sulfur solution turned to a dark green-blue solution was then cooled to room temperature. Subsequently, 4 mL of ethanol was added, and the solution was centrifuged at 3000 rpm for 5 min to remove free oleylamine and 1-octadecene. The precipitate was redispersed in chloroform and centrifuged at 7500 rpm for 7.5 min to remove any byproducts. To study the effect of different halide ions on the reaction, we replaced NaCl with NaBr or KI. Single-Phase Synthesis of CuS Triangular Nanoprism. A 0.05 M copper chloride solution was prepared by dissolving 0.0267 g copper chloride (0.2 mmol) in a 4 mL mixture of oleylamine/1octadecene (volume ratio of 1:3) in a glass vial. A 0.0096 g (0.3 mmol) sulfur powder was added to the dark blue copper nitrate solution and stirred for 5 min. Then, a glass vial was placed in an oil bath at a temperature between 130 and 180 °C. The dark-blue solution turned to a dark green-blue solution and was then cooled to room temperature. Subsequently, 4 mL of ethanol was added, and the solution was centrifuged at 3000 rpm for 5 min to remove free oleylamine and 1-octadecene. The precipitate was redispersed in
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ASSOCIATED CONTENT
S Supporting Information *
XRD spectra, TEM images, AFM image, and nanoprism size histograms. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.5b01223.
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AUTHOR INFORMATION
Corresponding Author
*9500 Gilman Drive MC 0448, La Jolla, CA 92093-0448. Email:
[email protected]. Phone: (858) 822-4237. Fax: (858) 5349533. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported through a grant from the Office of Naval Research (Award No. N000141210574) and a grant from the National Science Foundation (CMMI, Award No. 1200850).
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DOI: 10.1021/acs.chemmater.5b01223 Chem. Mater. XXXX, XXX, XXX−XXX
