Tunable Room-Temperature Synthesis of Coinage Metal

Jan 11, 2017 - Phase-pure and highly monodisperse coinage metal chalcogenide (Ag2E, Cu2–xE; E = S, Se) QDs are readily synthesized from the direct r...
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Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N‑Heterocyclic Carbene Synthons Haipeng Lu and Richard L. Brutchey* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *

ABSTRACT: We present a new toolset of precursors for semiconductor nanocrystal synthesis, N-heterocyclic carbene (NHC)-metal halide complexes, which enables a tunable molecular platform for the preparation of coinage metal chalcogenide quantum dots (QDs). Phase-pure and highly monodisperse coinage metal chalcogenide (Ag2E, Cu2−xE; E = S, Se) QDs are readily synthesized from the direct reaction of an NHC-MBr synthon (where M = Ag, Cu) with alkylsilyl chalcogenide reagents at room temperature. We demonstrate that the size of the resulting QDs is well-tailored by the electron-donating ability of the L-type NHC ligands, which are further confirmed to be the only organic capping ligands on the QD surface, imparting excellent colloidal stability. Local superstructures of the NHC-capped Ag2S QDs are observed by TEM, further demonstrating their potential for synthesizing monodisperse ensembles and mediating self-assembly.

1. INTRODUCTION Semiconductor nanocrystals, or quantum dots (QDs), are attractive functional materials for photovoltaics, photocatalysis, displays, and biomedical applications because of their uniquely tunable optoelectronic properties.1−5 In particular, the coinage metal chalcogenide (i.e., Ag2E, Cu2−xE; E = S, Se) QDs have gained significant attention resulting from their band gaps (Eg = 0.15−2.0 eV), low toxicity (as compared to Cd- and Pbcontaining semiconductors), high absorption coefficients, NIR surface plasmon oscillation, and NIR emission.6,7 Coinage metal chalcogenide QDs have proven to be useful for photovoltaics,8 bioimaging,9 thermochromics,10 NIR absorption,11 and thermoelectrics.12,13 Moreover, these QDs also have served as parent platforms for other chalcogenide-based ternary (CuInS2, Cu2SnS3)14 and quaternary (Cu2ZnSnS4)15 semiconductor nanocrystals via cation exchange or as reaction intermediates. Conventional synthetic routes to these coinage metal chalcogenide QDs involve inorganic salts (e.g., AgBr, CuCl, AgNO3) that are solvated at elevated temperatures by long-chain organic ligands (e.g., fatty carboxylic acids, amines, phosphines, thiols).9,16−19 Nanocrystal size and morphology is often controlled by the divergent stages of nucleation and growth through the LaMer mechanism in so-called hotinjection syntheses.20,21 While these high-temperature synthetic approaches have been extensively explored for many years, there are only a limited number of reports on the roomtemperature synthesis of coinage metal chalcogenide QDs.22−24 Preparation of functional QDs under room-temperature conditions is extremely tempting from the perspective of cost and energy efficiency;22,25 however, it still remains a challenge to control and tune QD size and size distribution under ambient conditions, as compared to high-temperature syntheses. © XXXX American Chemical Society

Prasad’s group has recently reported a room-temperature synthesis of covellite phase CuS nanocrystals using CuCl2 dissolved in oleylamine reacted via multiple injections of (NH4)2S.22 Well-defined CuS nanocrystals were synthesized by very carefully tailoring the number of (NH4)2S injections, the amount of (NH4)2S per injection, and the time interval between each injection. On the other hand, the preparation of phase-pure digenite Cu1.8S nanocrystals with high monodispersity under ambient conditions remains unreported, and more importantly, there is no general synthetic approach that gives monodisperse coinage metal chalcogenide (i.e., Ag2E, Cu2−xE; E = S, Se) nanocrystals at room temperature. At the origin of this challenge is the lack of precursors with appropriate solubility and reactivity that enable the room-temperature synthesis of colloidal coinage metal chalcogenide QDs with well-defined composition and morphology. In the search for synthetic conditions that might afford colloidal coinage metal chalcogenide QDs under ambient conditions, we were inspired by the recent examples of metal nanocrystal syntheses from N-heterocyclic carbene-metal halide complexes (NHC-MX n),26−32 as these might serve as precursors with potentially tunable reactivity for QD synthesis. In this article, we present a new synthetic strategy to coinage metal chalcogenide QDs under ambient conditions with various types of NHC ligands. NHCs are a useful and versatile class of L-type (neutral) σ-donating ligands, and a wide assortment of NHC-metal complexes have been explored in organometallic chemistry.33−36 With the chemical stability and wide structural tunability of NHC ligands, NHC-metal complexes may bring Received: December 14, 2016 Revised: January 10, 2017 Published: January 11, 2017 A

DOI: 10.1021/acs.chemmater.6b05293 Chem. Mater. XXXX, XXX, XXX−XXX

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0.1 M solution. THF is used here since (TBDMS)2Se does not completely dissolve in ODE at room temperature. Caution! (TBDMS)2Se is volatile, and it can react with moisture in air rapidly producing toxic H2Se gas! (TBDMS)2Se was stored at −20 °C in a N2f lushed glass vessel. (TBDMS)2Se/THF solution was prepared using dry and air-f ree Schlenk techniques. The dissolved (TBDMS)2Se solution was added with a 2:1 (M:Se) stoichiometry into the stirring NHCMBr solution. The reaction mixture changed from colorless to dark red within 1−5 min of addition depending on the different precursors, indicating the formation of metal selenide QDs. After 60 min, the metal selenide QDs were purified by precipitation from excess acetone and redispersed in toluene. Unreacted precursors or agglomerated QDs were discarded by centrifugation. 2.3. Characterization. UV−vis−NIR spectroscopy was carried out on a PerkinElmer Lamba 950 spectrophotometer equipped with a 150 mm integrating sphere, using a quartz cuvette for liquid samples. Thermogravimetric analysis (TGA) measurements were made on a TA Instruments TGA Q50 instrument, using sample sizes of ca. 5 mg in an alumina crucible under a flowing nitrogen atmosphere. TGA samples were prepared by drying the colloid under flowing nitrogen at 80 °C for ∼120 min, followed by lightly crushing the solid with a spatula prior to analysis. FT-IR spectra were acquired from pressed KBr pellets on a Bruker Vertex 80. Pressed pellets were made of dried metal chalcogenide QDs (∼2 mg) in a dry KBr matrix (∼100 mg). Solution 1D and 2D 1H, 13C, 1H−13C HSQC NMR spectra were collected at ambient temperature on a Varian 500 spectrometer (500 MHz in 1H) with chemical shifts represented in units of ppm. All spectra were referenced to the residual solvent peaks (5.33 ppm for CD2Cl2, 7.26 ppm for CDCl3, 7.00 ppm for toluene-d8). NMR samples were prepared by drying the QD colloid in a vacuum oven (∼60 °C), and hexanes were added to disperse the QDs, separating them from NHC-metal complexes since these precursors do not dissolve in hexanes. After drying the QD/hexanes suspension in a vacuum oven, the QDs were redispersed in deuterated solvents. QD dispersions were filtered through a 0.45 μm filter before NMR measurements. For molecular complexes, 32 scans with a delay time of 1 s were taken for each sample, and the data are presented as averages of those scans. For QD samples, 64 scans with a delay time of 30 s were taken to allow complete relaxation between pulses for 1H spectra.40 Typical NMR samples had a QD concentration in the range of 10−30 μM. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Ultra X-ray photoelectron spectrometer with an analyzer lens in hybrid mode. High-resolution scans were performed using a monochromatic aluminum anode with an operating current of 6 mA and voltage of 10 kV using a step size of 0.1 eV, a pass energy of 40 eV, and a pressure range between 1−3 × 10−8 Torr. The binding energies for all spectra were referenced to the C 1s core level at 284.8 eV. Powder X-ray diffraction (XRD) data were collected using a Rigaku Ultima IV diffractometer in parallel beam geometry (2 mm beam width) using Cu Kα radiation (λ = 1.54 Å). Samples were prepared by drop-casting onto zero-diffraction, single crystal Si substrates. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-2100 microscope at an operating voltage of 200 kV, equipped with a Gatan Orius CCD camera. Samples for TEM analysis were prepared from dilute purified QD suspensions deposited on 400 mesh carbon-coated copper grids (Ted Pella, Inc.). Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed on the nanocrystal samples using a Thermo Scientific Icap 7000 series ICP-OES. QDs were prepared by drying 10 mg of solid, digesting in concentrated HNO3, and diluting with ca. 99 mL of 10% HNO3. The atomic ratios were calculated by averaging 3 independently synthesized QD samples.

new opportunities to colloidal nanocrystal synthesis. For instance, by tuning the steric and/or electronic properties of NHC ligands, one should, in theory, be able to tailor the kinetic parameters of synthesis,37 leading to various particle sizes and/ or shapes, as well as tunable surface functionality assuming the NHC is the only viable ligand in the synthesis.32 Thus, far, NHC-metal complexes have only been utilized in the direct preparation of metal nanocrystals (i.e., Au, Pd, Pt, Ru).26−32 The size,26 polydispersity,30 and catalytic activity38 tunability of the resulting NHC-metal nanocrystal constructs has been demonstrated with a variety of NHC ligands. These NHCmetal synthons have only recently been explored for the synthesis of QDs; however, this was only achieved through the chalcogenization of an NHC-metal nanocrystal intermediate.39 NMR spectroscopy and DFT calculations revealed that the NHC ligands are more effective ligands in terms of imparting colloidal stability for metal chalcogenide nanocrystals as compared to their metal nanocrystal counterparts. Herein, we develop a one-step, room-temperature synthesis of monodisperse Ag2E and Cu2−xE QDs from NHC-MBr precursors (M = Ag, Cu) with excellent reproducibility. The QD size dependence with respect to electronic and steric parameters of the NHC ligands is investigated, giving kinetic control over the QD synthesis from the molecular precursor rather than time or temperature. This route to coinage metal chalcogenide QDs opens the door to semiconductor nanocrystal synthesis from a wide array of organometallic NHC complexes.

2. EXPERIMENTAL SECTION 2.1. Materials. Reagents and solvents were purchased from commercial sources and used as received, unless otherwise stated. Benzimidazole (C7H6N2, 99%), imidazole (C3H4N2, 99%), 1-bromotetradecane (C14H29Br), 1-bromodecane (C10H21Br), 1-bromooctane (C8H17Br), 1-bromohexane (C6H13Br), 1-bromoethane (C2H5Br), silver(I) oxide (99+%), copper(I) oxide (99.9%), potassium carbonate (K2CO3, anhydrous, 99%), 1,4-dioxane, bis(trimethylsilyl)sulfide ((TMS)2S, technical grade), tert-butyldimethylchlorosilane (TBDMSCl, or tBuMe2Si-Cl, >98%), sodium (sticks, in mineral oil, 99%), and selenium powder (−200 mesh, 99.999%) were purchased from Alfa Aesar. 1-Octadecene (ODE, technical grade, 90%), 4,5-diphenylimidazole (C15H12N2, 98%), and 4,5-dichloroimidazole (C3H2Cl2N2, >98%) were purchased from Sigma-Aldrich. ODE was dried under vacuum overnight. Reactions involving air- or moisture-sensitive compounds were conducted under a nitrogen atmosphere by using standard Schlenk techniques. 2.2. Synthesis. Detailed syntheses of benzimidazolium salts, imidazolium salts, (TBDMS)2Se, NHC-AgBr, and NHC-CuBr complexes are described in the Supporting Information. Synthesis of Metal Sulfide (Ag2S, Cu2−xS) QDs. NHC-MBr (0.2 mmol) was first dissolved in 4 mL of dried CH2Cl2 under nitrogen at room temperature. (TMS)2S was diluted with dried ODE to give a 0.1 M solution, which was rapidly added with a 2:1 (M:S) stoichiometry into the stirred NHC-MBr solution. ODE is used here as a well-known noncoordinating solvent. Caution! (TMS)2S is extremely reactive, and it can react with moisture in air rapidly producing H2S gas! (TMS)2S/ODE solution was thus prepared using dry and air-f ree Schlenk techniques. The reaction mixture changed from colorless to dark red within 1−5 min of addition depending on the precursors, indicating the formation of metal sulfide QDs. Aliquots were taken for UV−vis−NIR absorption and TEM analysis. After 60 min, the metal sulfide QDs were purified by precipitation with excess acetone and redispersed in toluene. Unreacted precursors or agglomerated QDs were discarded by centrifugation. Synthesis of Metal Selenide (Ag2Se, Cu2−xSe) QDs. NHC-MBr (0.2 mmol) was first dissolved in 4 mL of dried CH2Cl2 under nitrogen at room temperature. (TBDMS)2Se was dissolved in dried THF to give a

3. RESULTS AND DISCUSSION 3.1. Synthesis of NHC-Capped Ag2S QDs. It has previously been shown that the reaction of alkylsilyl chalcogenides ((RSi)2E; E = S, Se) with metal halides in solution yields bulk metal chalcogenides under ambient conditions.41 This metathesis reaction can be explained by the hard and soft (Lewis) acid and base (HSAB) principle B

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Chemistry of Materials (Scheme 1), with alkylsilyl chalcogenides, therefore, being highly reactive toward transition metal halides. Here, we arrest Scheme 1. Synthesis of (a) NHC-AgBr Complex and (b) NHC-Ag2S QDs

Figure 1. Characterization of the NHC-Ag2S QDs after a 60 min reaction: (a) powder XRD pattern; (b) UV−vis−NIR absorption spectrum; (c) size and size distribution (300 counts for each sample) from three different synthetic batches; (d−f) TEM micrographs. Inset in (f) is an HRTEM image showing d(1̅21) = 0.26 nm.

distribution over multiple batches (Figure 1c). The growth of well-defined QDs may be attributed to the steric hindrance provided by the long alkyl chains at the N-substituents of the NHC ligands. In comparison, bulk Ag2S was immediately obtained if (TMS)2S was reacted with AgNO3 without any organic ligands, indicating that the NHC ligands are, not surprisingly, playing a critical role in QD formation. Moreover, as compared to the conventional organic ligands, such as oleic acid or oleylamine (Figure S2), the NHC ligands also provide superior competence in preparing monodisperse Ag2S QDs under ambient conditions. 3.2. The Role of NHC Ligands: Tailoring the Steric and Electronic Properties. To further understand the role and examine the potential of NHC ligands in controlling QD morphology, we subsequently varied the alkyl chain length of the N-substituents, aiming to study the morphology dependence on the steric bulk of the NHC ligands while keeping the electronic properties the same. Four additional benzimidazolebased NHC-AgBr complexes with varying n-alkyl chain lengths at the N-substituents (Figure 2), 2-b-AgBr (R = C10H21), 3-b-

growth in this reaction to yield coinage metal chalcogenide QDs by the introduction of NHC ligands with long-chain nalkyl groups at the N-substituents. The starting NHC-AgBr complex can be readily prepared from the reaction of the NHC-bromide salts and Ag2O (Scheme 1a). The NHC-AgBr complex bromo[1,3-(ditetradecyl)benzimidazol-2-ylidene] silver(I) (1-b-AgBr) with Nsubstituents of C14H29 was first employed to investigate the synthesis of Ag2S QDs under ambient conditions. Upon reaction of 1-b-AgBr with (TMS)2S in dichloromethane, a dark red colloidal suspension formed immediately at room temperature, indicating a fast nucleation process. We propose that the reaction proceeds through NHC-stabilized silver sulfido complexes (such as NHC-AgSAg-NHC, Scheme 1b), as similar silver and copper phenylchalcogenolate complexes have been previously observed,42 followed by the nucleation of the monomer species. Upon completion, the resulting QDs were confirmed by powder X-ray diffraction (XRD) to be phase-pure monoclinic Ag2S (PDF no. 00-014-0072, Figure 1a). Reaction progress was monitored by UV−vis−NIR absorption and TEM analysis (Supporting Information, Figure S1). The UV−vis− NIR spectra display featureless absorption into the NIR for the Ag2S QDs, with no observable absorption difference after 5 min. TEM analysis shows highly monodisperse, spherical QDs, with a gradual growth and focusing of particle size over the reaction from 7.8 ± 0.8 nm (5 min), to 8.2 ± 0.7 nm (30 min), to 10.3 ± 0.6 nm (60 min) (Figure S1). These observations suggest fast nucleation with a slower stage of nanocrystal growth. The as-prepared Ag2S QDs possess an excellent size distribution (standard deviation/mean diameter, σ/d = 6%) after a 60 min reaction (Figure 1d−f). High-resolution TEM (HRTEM) reveals the highly crystalline nature of Ag2S QDs with lattice fringes corresponding to d(1̅21) of 0.26 nm, with the QDs appearing to be polycrystalline with some degree of crystal strain in most particles (Figure 1f). A Ag/S ratio of ca. 2.06 was obtained from ICP-OES, revealing the expected chemical composition of QDs. Moreover, the monodisperse Ag2S QDs can be synthetically reproduced with identical size and size

Figure 2. Structures of benzimidazole- and imidazole-based NHCAgBr precursors. C

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Chemistry of Materials Table 1. Summary of Ag2S Products from Different NHC-AgBr Precursors

a

precursor

R

TEP (cm−1)a

d (nm)

products

1-b-AgBr 2-b-AgBr 3-b-AgBr 4-b-AgBr 5-b-AgBr 1-i-AgBr (4,5-H) 2-i-AgBr (4,5-Ph) 3-i-AgBr (4,5-Cl)

C14H29 C10H21 C8H17 C6H13 C2H5 C14H29 C14H29 C14H29

2057 2057 2057 2057 2057 2054 2048 2059

10.3 ± 0.6 9.7 ± 0.6 9.2 ± 1.0 9.6 ± 1.0 N/A 7.9 ± 0.5 5.6 ± 0.4 10.5 ± 1.6

colloidal QDs colloidal QDs with aggregates colloidal QDs with aggregates colloidal QDs with aggregates bulk precipitate colloidal QDs colloidal QDs colloidal QDs

TEP values are estimated based on calculated results on the corresponding NHC ligands with methyl groups at both N-substituents.43,44

NHC ligands (with lower TEP values) yield smaller Ag2S QDs with a more monodisperse size distribution under otherwise identical reaction conditions. Both 1-i-AgBr (4,5-H) and 2-iAgBr (4,5-Ph) give Ag2S QDs with excellent size distribution (σ/d = 6%), whereas 3-i-AgBr (4,5-Cl) results in very poor control of size uniformity (Figure 3). This may be attributed to

AgBr (R = C8H17), 4-b-AgBr (R = C6H13), and 5-b-AgBr (R = C2H5), were prepared, and Ag2S QDs were synthesized via the same reaction between j-b-AgBr (j = 1−5) and (TMS)2S at room temperature for 1 h. Powder XRD results reveal that all the NHC-AgBr precursors give phase-pure, monoclinic Ag2S (Figure S3). However, compared to complex 1-b-AgBr, which only generates colloidal Ag2S QDs, complexes 2-b-AgBr, 3-bAgBr, and 4-b-AgBr all yield a mixture of colloidal Ag2S QDs and aggregated precipitates of Ag2S, while complex 5-b-AgBr results in only bulk Ag2S. The purified Ag2S QDs prepared from complexes 2-b-AgBr, 3-b-AgBr, and 4-b-AgBr possess a mean diameter of 9.7 ± 0.6, 9.2 ± 1.0, and 9.6 ± 1.0 nm, respectively (Figure S4, Table 1). It is thus concluded that the size of Ag2S nanocrystals does not depend strongly on the steric properties of the N-substituents on the NHC ligands, with the exception of the shortest ethyl group (R = C2H5) not being able to arrest growth and giving bulk Ag2S. The weak dependence of Ag2S QD size on the alkyl chain length may be explained by the fact that n-alkyl groups on the Nsubstituents can fold away from the QD surface, and thus do not provide additional steric stabilization after a certain chain length. The aggregates formed from j-b-AgBr (j = 2−4) likely result from poorer colloidal stabilization with shorter chain lengths as compared to 1-b-AgBr. These results suggest that long alkyl chains (C > 2) at the N-substituents of the NHC ligands provide enough steric hindrance to arrest QD growth and provide varying degrees of colloidal stabilization; however, the Ag2S QD size and size distribution do not correlate with alkyl chain length on the N-substituents of the NHC ligands. Given the lack of positive correlation between the NHC ligand steric properties with resulting QD size, we sought to investigate the effect of tuning the electron-donating properties of the NHC ligands. Imidazole-based NHC-AgBr complexes, 1i-AgBr (4,5-H), 2-i-AgBr (4,5-Ph), and 3-i-AgBr (4,5-Cl) (Figure 2) with significantly different electron-donating properties, were synthesized. In these complexes, the R groups at the N-substituents are all C14H29, ensuring an identical steric parameter for these NHC ligands. Following the same reaction protocol, phase-pure Ag2S was obtained from these imidazolebased NHC-AgBr precursors, as identified by powder XRD. Interestingly, the mean diameter of the resulting Ag2S QDs displays a linear correlation with the Tolman electronic parameter (TEP)45,46 of the ligand, which is a commonly employed merit of electron-donating ability.47 We estimated the TEP values of these synthesized NHC ligands based on the calculated results for the corresponding NHC ligands with methyl groups at both N-substituents.43,44 This is a fair estimation, as the size of R groups has minimal effects on the σdonating ability44,48 and all the j-i-AgBr (j = 1−3) NHC ligands possess the same C14H29. The more electron-donating

Figure 3. (a−c) TEM micrographs of Ag2S QDs prepared from imidazole-based NHC-AgBr precursors: 1-i-AgBr (4,5-H), 2-i-AgBr (4,5-Ph), and 3-i-AgBr (4,5-Cl), respectively. The insets on the upper left corner of each micrograph are the size and size distributions of the corresponding QDs (300 counts each). The inset on the upper right corner of (b) is a TEM image showing local superstructures of Ag2S QDs prepared from 2-i-AgBr (4,5-Ph). (d) Plot of Ag2S QD mean diameter as a function of calculated TEP43,44 of the NHC ligands.

the slower reaction kinetics between the initial nanocrystal nuclei with monomers in 1-i-AgBr (4,5-H) and 2-i-AgBr (4,5Ph).35 Since the NHC ligand is an almost pure σ-donor in NHC-coinage metal complexes, as revealed previously by Ghosh and co-workers,49 the more electron-donating NHC ligands would result in stronger NHC−Ag bonds. On the other hand, the more electron-withdrawing NHC ligands lead to a weaker NHC−Ag bond,35 giving poorer control over QD size uniformity. Interestingly, local QD superstructures are observed for the Ag2S QDs prepared from 2-i-AgBr (4,5-Ph) (inset in Figure 3b), which additionally confirms the ability of this synthetic approach to generate uniform, monodisperse Ag2S QDs. D

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Chemistry of Materials 3.3. Characterization of Surface NHC Ligands. The asprepared Ag2S QDs from complex 1-b-AgBr exhibit excellent colloidal stability in nonpolar organic solvents (e.g., toluene, tetrachloroethylene), with no noticeable precipitates after 6 months. As reported in our previous paper,39 such colloidal stability suggests a covalent interaction between the neutral NHC ligands and surface Ag(I) cations. 1H NMR was first applied to confirm the presence of NHC ligands on the QD surface. The same set of proton resonances was observed for the Ag2S QDs as for the molecular NHC-AgBr precursor (Figure 4a), with significantly broadened peaks in the case of

presence of Br is not associated with protonated benzimidazolium bromide as a proton resonance at ∼11.6 ppm was not observed in the 1H NMR spectrum of the purified QDs. Thus, we attributed the observed Br to unreacted NHC-AgBr precursors associated with the surface, or surface passivation by Br− to charge balance excess Ag+ cations. FT-IR spectra display characteristic bands for imidazolium ring stretching at 1560 cm−1 (s, sym) and 1467 cm−1 (s, sym)52 in both the NHC-AgBr precursor and Ag2S QDs (Figure S6). The quantity of NHC ligands on the Ag2S QDs can be estimated from TGA data. A single 7.3% mass loss event at 250−300 °C is assigned to the decomposition and/or loss of NHC ligands on the surface of the Ag2S QDs (Figure S6),30,53 and the number of NHC ligands is calculated to be 346 per nanocrystal on average (for d = 10 nm, Supporting Information), giving a surface NHC density of ∼1 NHC/nm2. This NHC surface density agrees well with our previous report.39 It has been posited that long Nalkyl chains bend outwardly from the surface due to steric hindrance, with the distance between the end corners of the bent tetradecyl groups being ∼1 nm.26 Therefore, a surface coverage of 1 NHC/nm2 (1 nm × 1 nm) seems feasible. 3.4. Synthesis of Colloidal Cu2−xS, Ag2Se, and Cu2−xSe QDs. To demonstrate the generality of this synthetic approach, the same chemistry was extended to copper sulfide (Cu2−xS) and silver and copper selenide (Ag2Se, Cu2−xSe) QDs using the 1-b-MBr precursor (M = Ag, Cu). Following the same procedure, Cu2−xS nanocrystals can also be obtained at room temperature. Similar reaction kinetics (i.e., fast nucleation and slower nanocrystal growth) are observed by UV−vis−NIR absorption and TEM analysis (Figure S1). Powder XRD reveals that the as-synthesized product crystallizes as phase-pure, cubic digenite Cu1.8S (PDF no. 00-056-1256, Figure 5a). It was found

Figure 4. Characterization of surface NHC ligands on Ag2S QDs. (a) 1 H NMR spectra of NHC-AgBr and NHC-Ag2S QDs in CD2Cl2 (from 1-b-AgBr). Resonances from 0.5 to 8 ppm are assigned accordingly. Solvent impurities are indicated by Δ (toluene), ∗ (CH2Cl2), and • (H2O). (b, c) 1H−13C HSQC spectra of NHC-AgBr and a colloidal suspension of Ag2S QDs in CD2Cl2, respectively. (d−f) Highresolution XPS spectra of Ag 3d, S 2p, and N 1s in Ag2S QDs, respectively.

the colloidal QDs. The broadening results from a slower tumbling rate of the surface ligands, therefore indicating coordination of the NHC ligands to the Ag2S QD surface.50 The identity of the surface bound NHC ligands was also investigated by a 1H−13C HSQC experiment, showing almost identical HSQC cross-peaks for the Ag2S QDs and the NHCAgBr complex (Figure 4b,c). NHC binding to the QD surface was further corroborated by XPS, which displayed a N 1s peak at a binding energy of 401.1 eV (Figure 4f). Furthermore, XPS revealed peaks at the expected binding energies for Ag 3d (3d3/2 at 373.5 eV and 3d5/2 at 367.5 eV) and S 2p (2p1/2 at 161.9 eV and 2p3/2 at 160.8 eV), consistent with the composition and expected oxidation states in the Ag2S QDs.51 It is also worth noting that XPS shows that Br is present on the surface of the Ag2S QDs (Figure S5). The atomic Br/Ag ratio was quantified by XPS to be ∼13% for purified Ag2S QDs, and therefore, the surface is not entirely shelled with Br. Additionally, the

Figure 5. Characterization of NHC-Cu2−xS QDs after a 60 min reaction: (a) powder XRD pattern; (b) UV−vis−NIR absorption spectrum exhibiting a broad LSPR peak; (c−e) TEM micrographs of Cu2−xS QDs. Inset in (e) is an HRTEM image showing d(220) = 0.20 nm.

that the phase does not change with different nominal sulfur stoichiometry; for example, adding (TMS)2S with a 1:1 (Cu:S) ratio yields the same digenite Cu1.8S phase instead of CuS, suggesting that no oxidation/reduction reactions are involved in the reaction. The UV−vis−NIR absorption spectrum displays a characteristic localized surface plasmon resonance (LSPR) peak at ∼1412 nm (Figure 5b). This is in line with the E

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stability, with no observable aggregates or decomposition (such as silver plating of the glass container, commonly observed for Ag2Se QDs16) over 6 months. It is also worth noting that monodisperse colloidal Ag2Se or Cu2−xSe QDs with sub-10 nm diameters have never been prepared under ambient conditions to the best of our knowledge.

nonstoichiometric nature of Cu2−xS nanocrystals, as revealed by XPS quantification giving a Cu:S ratio of ca. 1.6. TEM micrographs show a mostly spherical morphology in the resulting Cu2−xS nanocrystals, with a minority population possessing anisotropic shapes such as cubes, rods, and triangles after a 60 min reaction (Figure 5c−e). The average size of the as-synthesized Cu2−xS nanocrystals is 8.8 ± 0.8 nm (σ/d = 9%). An HRTEM image shown as the inset to Figure 5e reveals that Cu2−xS appears to be single crystalline with a lattice spacing of 0.20 nm for the (220) planes. Colloidal Ag2Se and Cu2−xSe QDs can also be synthesized using the same NHC-MBr precursors with (TBDMS)2Se.41 (TBDMS)2Se is much easier to handle compared to (TMS)2Se since it is a solid and less sensitive to both moisture and light. Both Ag2Se and Cu2−xSe QDs can be obtained after a 60 min reaction between the NHC-MBr precursor and (TBDMS)2Se. The UV−vis−NIR spectra exhibit featureless absorption into the NIR for Ag2Se and a characteristic LSPR peak at ∼1512 nm for the Cu2−xSe QDs (Figure 6b). Powder XRD reveals that the

4. CONCLUSIONS In conclusion, we have presented a tunable room-temperature synthesis of coinage metal chalcogenide QDs using NHC-MBr synthons. Phase-pure and highly monodisperse Ag2E and Cu2−xE (E = S, Se) QDs with excellent reproducibility are obtained from the reaction between NHC-MBr complexes and alkylsilyl chalcogenides. The mean size of the QDs can be tailored by tuning the electronic parameter of the NHC ligands; however, the QD size is not correlated with the linear n-alkyl chain length on the N-substituents of the NHCs. A linear relationship is observed between the mean diameter of the Ag2S QDs and the TEP value of the NHC ligand, demonstrating that more electron-donating NHC ligands result in smaller QDs. This synthetic approach thus allows for QD size control from the molecular precursor, rather than reaction time or temperature. With the rich library of NHC structures and NHC-metal complexes, this synthetic route provides new inspiration for (1) novel surface functionalization of QDs with NHC ligands, and (2) low-temperature, tunable synthesis of other binary or ternary metal chalcogenide (e.g., SnS, SnSe, ZnS, ZnSe, CuInS2) QDs using NHC-metal complexes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05293. Synthesis of NHC-metal bromide complexes, TEM images, XRD, XPS, FT-IR, TGA and NMR spectra (PDF)

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Figure 6. (a) Powder XRD patterns of Ag2Se (peaks are assigned based on calculated tetragonal Ag2Se phase16) and Cu2−xSe QDs. (b) UV−vis−NIR absorption spectra of Ag2Se and Cu2−xSe QDs. (c, d) TEM micrographs of Ag2Se and Cu2−xSe QDs, respectively. Insets are the HRTEM images of Ag2Se and Cu2−xSe QDs.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Richard L. Brutchey: 0000-0002-7781-5596

as-prepared Ag2Se QDs crystallize in a metastable tetragonal phase (a = 0.706 nm and c = 0.498 nm)54 instead of the thermodynamically preferred orthorhombic phase (PDF no. 00-024-1041). It has been previously observed that Ag2Se nanocrystals can be kinetically trapped in this metastable phase, which is not observed in the bulk.16,55 Our XRD pattern agrees very well with literature reports for the tetragonal Ag2Se QDs.10,16,55−57 TEM analysis reveals a spherical, monodisperse morphology for the Ag2Se QDs with a mean diameter of 9.7 ± 0.8 nm (σ/d = 8%), and HRTEM reveals a lattice spacing of 0.24 nm (Figure 6c), which is consistent with the calculated d(220) = 0.25 nm of the tetragonal phase.16 The as-prepared Cu2−xSe QDs crystallize in the expected cubic Cu1.8Se phase (Figure 6a, PDF no. 01-071-6181) and possess an quasispherical morphology with an average diameter of 10.1 ± 1.2 nm (σ/d = 12%) (Figure 6d). HRTEM analysis suggests that the QDs are single crystalline, with observed lattice spacings of 0.33 nm corresponding to the (111) planes of Cu1.8Se. These Ag2Se and Cu2−xSe QDs both possess excellent colloidal

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE− FG02−11ER46826.

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REFERENCES

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