Microwave-Assisted Solution–Liquid–Solid ... - ACS Publications

May 1, 2015 - John G. Hauck,. ‡. Peter C. Burns,. †,§. Paul J. McGinn,. ‡ and Chongzheng Na*. ,†. †. Department of Civil and Environmental ...
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Microwave-Assisted Solution−Liquid−Solid Synthesis of SingleCrystal Copper Indium Sulfide Nanowires Galyna Krylova,† Halyna Yashan,† John G. Hauck,‡ Peter C. Burns,†,§ Paul J. McGinn,‡ and Chongzheng Na*,† †

Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, United States ‡ Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, Indiana 46556, United States § Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Chalcopyrite copper indium sulfide (CuInS2) is an important semiconductor with a bandgap optimal for terrestrial solar energy conversion. Building photovoltaic and microelectronic devices using one-dimensional CuInS2 nanowires can offer directional conduits for rapid and undisrupted charge transport. Currently, single-crystal CuInS2 nanowires can be prepared only using vapor-based methods. Here, we report, for the first time, the synthesis of single-crystal CuInS2 nanowires using a microwave-assisted solution−liquid−solid (MASLS) method. We show that CuInS2 nanowires with diameters of less than 10 nm can be prepared at a rapid rate of 33 nm s−1 to more than 10 μm long in less than 10 min, producing a high mass yield of 31%. We further show that the nanowires are free of structural defects and have a nearstoichiometric composition. The success of MASLS in preparing high-quality tertiary nanowires is explained by a eutectic growth mechanism involving an overheated alloy catalyst.

1. INTRODUCTION Semiconductor nanowires are important anisotropic building materials for next-generation photovoltaic and microelectronic devices.1,2 Compared to thin films presently used in these devices, nanowires can offer directional conduits for rapid and undisrupted charge transport.3−5 Semiconductor nanowires are typically synthesized using vapor−liquid−solid (VLS)6,7 and solution−liquid−solid (SLS) methods.8 Both VLS and SLS use liquid metal catalysts to grow solid nanowires, with the distinction being that organometallic precursors are provided as a vapor in VLS but dissolved in a solution in SLS. The selection of vapor or solution as the synthesis environment can lead to unique advantages and disadvantages.9−11 In VLS, the vaporization of precursors often requires a comparably high temperature, producing high-quality nanowires with few defects. In SLS, the use of solution offers flexibility to precursor formulation. SLS is usually operated at a temperature below the solution’s boiling point, leading to an increased concentration of structural defects in nanowires that can cause charge trapping, scattering, and recombination.12−15 Solution-based synthesis methods have gained considerable attention with the potential for scale up in manufacturing. An urgent need for applying methods such as SLS is to improve nanowire quality by eliminating structural defects. Previously, high-quality silicon nanowires have been produced in a SLS solution heated to a temperature close to that used in VLS;16 © XXXX American Chemical Society

however, the high temperature vaporizes the reaction solution, creating high pressure in a sealed reactor (200 atm at 500 °C) that transforms the solution into a supercritical fluid. The supercritical fluid−liquid−solid or SFLS method has raised new challenges regarding synthesis safety and solvent decomposition.17 In addition, organometallic precursors needed to synthesize nanowires with complex compositions are unlikely to be stable under the supercritical conditions.18,19 Growing nanowires at a relatively high temperature but keeping the precursor-containing solution relatively cool requires establishing a temperature differential between the solution and the liquid catalysts suspended in it. Microwave heating is an effective technique for establishing temperature differentials in reactive solutions, utilizing the effect that different materials absorb microwaves differently.20 Microwave irradiation delivers energy through its alternating electromagnetic field. Solvent molecules are continuously forced to align themselves with the field through rotation, during which heat is generated by friction and collision (dielectric heating).21 In comparison, the alternating field can induce eddy currents on the surface of conductive nanomaterials, generating heat from electrical resistance (Joule Received: February 27, 2015 Revised: April 6, 2015

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heating).22 In both cases, the absorption of microwave energy can be described by the Beer−Lambert law: Ia = I0[1 − exp(−L/δ)], where I0 is the incident energy, L is the path length, and δ is the penetration depth. Accordingly, Ia increases with the decrease of δ. At the frequency used by most microwave operations (i.e., 2.45 GHz), δ is typically on the order of centimeters for solvents but micrometers for metals;23 therefore, between solvents and metal catalysts used in SLS, the latter is a much better microwave absorber and thus can be readily overheated. Previously, microwave-assisted (MA) synthesis has been mainly used to prepare nanoparticles;24−29 to our knowledge, the use of microwave irradiation in the SLS synthesis of nanowires has not been described in the literature. Here, we report the development of MASLS for synthesizing tertiary copper indium sulfide (CuInS2) nanowires. Chalcopyrite CuInS2 is an important semiconducting material with a bandgap of 1.55 eV, an optimal value for achieving the Shockley−Queisser limit of terrestrial solar energy conversion.30 Currently, single-crystal chalcogenide nanowires with sophisticated compositions like CuInS2 can be prepared only using vapor-based methods.31,32 We show that single-crystal CuInS2 nanowires with a near-stoichiometric composition can be produced at a rapid growth rate and a high mass yield by combining microwave irradiation with solution−liquid−solid synthesis.

for 1 h, which produced 0.8 g of [Cu(CH3CN)4]PF6. 0.6 g of [Cu(CH3CN)4]PF6 was dissolved in 10 mL of dichloromethane (CH2Cl2). A solution of triphenylphosphine (PPh3) was made by dissolving 0.835 g of PPh3 in 4 mL of CH2Cl2. The PPh3 CH2Cl2 solution was added dropwise to the [Cu(CH3CN)4]PF6 CH2Cl2 solution under nitrogen (N2) protection (performed in a glovebox). The reacting suspension was shaken for 20 h to complete Reaction 2. The suspension was filtered to obtain a clear solution of [(PPh3)2Cu(CH3CN)2]PF6. To synthesize the portion containing In and S, 0.477 g of sodium ethoxide (NaOC2H5) was dissolved in 7 mL of methanol (CH3OH) to obtain a clear light-yellow solution. A solution of thiophenol (HS-Ph) was made by dissolving 0.65 mL of HS-Ph in 6 mL of CH3OH. This solution was added to the NaOC2H5 CH3OH solution under N2 protection. The mixture was stirred for 20 h to complete Reaction 3 and form sodium phenylthiolate (NaS-Ph). A solution of indium chloride (InCl3) was made by dissolving 0.352 g of InCl3 in 4 mL of CH3OH. The InCl3 CH3OH solution was added dropwise to the NaSPh solution. The resulting mixture was stirred for 20 h under N2 protection to complete Reaction 4. The results was a clear deep-yellow solution of Na[In(S-Ph)4] with a small amount of white precipitate (NaCl byproduct) at the bottom, which was subsequently filtered out. To combine the Cu- and In-containing portions together, the [(PPh3)2Cu(CH3CN)2]PF6 CH2Cl2 solution was gradually added to the Na[In(S-Ph)4] CH3OH solution to initiate Reaction 5. The mixture was placed in an open vial and stirred for 3 days in a glovebox (under N2) to remove all volatile chemicals (drying from the solvents). The mixture was then dissolved in benzene. White precipitates of sodium chloride (NaCl) and sodium hexafluorophosphate (NaPF6) were removed using a 0.45 μm PTFE syringe filter. The filtrate was distilled under the vacuum of a Schlenk line with gentle heating (50 °C) to remove benzene and obtain a light-yellow SSP powder. 2.2. Synthesis of Copper Indium Sulfide Nanowires. CuInS2 nanowires were synthesized using MASLS. For comparison, nanowires were also synthesized using conventional SLS carried out with hot injection. To perform HISLS,19 we prepared two reaction solutions. The first solution was made by dissolving 0.2 mg of BiCl3 in 0.3 mL of acetone.33 The second was made by dissolving 0.34 g of SSP and 0.27 g of decylphosphonic acid (DPA) in 1 mL of trioctylphosphine (TOP). Synthesis was initiated by injecting both solutions into 6 mL of boiling TOP under N2 protection. The reactive mixture had a molar ratio of BiCl3/SSP/DPA/acetone/TOP = 0.002:1:4:13.3:52. The synthesis was terminated after 5 min by rapid cooling in acetone bath. During the injection, the solution temperature did not decrease below 270 °C. After injection, the temperature of the solution increased to 291 °C within 1 min due to the exothermal nature of the reactions. To perform MASLS, we first melted 0.97 g of trioctylphosphine oxide (TOPO) with 57 mg of SSP, 45 μL of TOP, and 48 μL of dodecanethiol (DT) in a thick-walled quartz reactor tube. Melting was conducted at 50 °C for 15 min while stirring in a N2-purged glovebox. The tube was capped, removed from the glovebox, and placed in a CEM Discover SP microwave reactor. Immediately before microwave irradiation was administered, 50 μL of 2 mM BiCl3 acetone solution was injected through the self-sealing cap. The final reaction mixture had a molar ratio of BiCl3/SSP/TOP/DT/acetone/TOPO = 0.002:1:2:4:13.6:50. To initiate synthesis, the reactor was programmed to heat the solution to 300 °C in 4.5 min (55 °C min−1). Once 300 °C was reached, the reactor was set to maintain the temperature for 5 min before the microwave irradiation was switched off. A constant temperature was maintained by measuring the solution temperature using an infrared sensor and adjusting the microwave power accordingly through a feedback algorithm. The reactor radiated microwaves at 2.45 GHz and had a maximum power of 300 W. The pressure inside the reactor was monitored by the top valve (IntelliVent), which also served to seal the vial cap. After reaction, the mixture was removed from the microwave reactor and promptly immersed into an acetone bath for rapid cooling. Five milliliters of toluene was added to the mixture upon cooling to prevent solidification of TOPO. The mixture containing nanowires was washed with toluene/ethanol solvents and centrifuged for

2. EXPERIMENTAL SECTION All chemicals used in this study were purchased from Sigma-Aldrich and were of analytical grade. Deionized (DI) water was generated on site using a Millipore system. 2.1. Synthesis of Single-Source Precursor (SSP). The key precursor of the synthesis was a binuclear single-source precursor (SSP) complex [(PPh3)2Cu][(μ-SPh)2In(SPh)2] (Ph = phenyl), which was synthesized following the protocol established for a similar complex containing Cu, In, and Se.19 The synthesis protocol involved the following reactions: CH3CN

Cu 2O + 2HPF6 ⎯⎯⎯⎯⎯⎯⎯→ 2[Cu(CH3CN)4 ]PF6 + H 2O

(1)

[Cu(CH3CN)4 ]PF6 + 2PPh3 CH2Cl2 /N2

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [(PPh3)2 Cu(CH3CN)2 ]PF6 + 2CH3CN CH3OH/N2

NaOC2H5 + HSPh ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NaSPh + C2H5OH CH3OH/N2

4NaSPh + InCl3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Na[In(SPh)4 ] + 3NaCl

(2) (3) (4)

and

[(PPh3)2 Cu(CH3CN)2 ]PF6 + Na[In(SPh)4 ] CH3OH/CH2Cl2 /N2

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [(PPh3)2 Cu][In(SPh)4 ] + 2CH3CN + NaPF6 (5) The general approach was to synthesize a portion of the complex containing Cu and then the portion containing In and S, separately. The two portions were then combined together. To synthesize the Cu-bearing portion, 0.41 g of copper monoxide (Cu2O) was added to 8 mL of acetonitrile (CH3CN) to form a 0.36 M suspension. 1.5 g of hexafluorophosphoric acid (HPF6) was mixed with 1.7 mL of DI water to make a HPF6 aqueous solution. The HPF6 aqueous solution was then added dropwise to the Cu2O CH3CN suspension, forming a colorless transparent solution. The mixture was stirred for 3 min at room temperature to complete Reaction 1. Insoluble brown precipitates of unreacted Cu2O were removed using a 0.45 μm PTFE syringe filter. The clear filtrate was dried under vacuum B

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Figure 1. Comparison of CuInS2 nanowires synthesized using hot-injection (HI; a, b, e−g) and microwave-assisted SLS (MA; c, d, e−g). (a, b, c, d) Transmission electron micrographs (scale bars: a, c, 200 nm; b, d, 100 nm). (e) Powder X-ray diffraction patterns (gray) and Rietveld refinement fits (black) using patterns of chalcopyrite (ICDD 01-085-1575) and wurtzite (ICDD 01-077-9495) standards. (f) Energy dispersive X-ray spectra (C, gray; O, cyan; Cu, blue; P, pink; S, green; In, red). Molar ratios are computed from the integrated areas of the main EDX peaks representing In, S, and P relative to the integrated area of the main Cu peak. (g) UV−vis−NIR absorption spectra (absorption edge highlighted in gray). nanowire recovery. The wash−centrifugation cycle was repeated four times. The synthesis was repeated twice to ensure consistency. 2.3. Nanowire Characterization. Nanowire structures were analyzed with a transmission electron microscope (TEM; Titan 80− 300, FEI) equipped with a high angle annular dark field (HAADF) detector for scanning TEM imaging. The TEM was operated at an accelerating voltage of 300 keV. Energy dispersive X-ray spectroscopy (EDS) of nanowires was performed using a scanning electron microscope (SEM; JEOL JSM-7500F) equipped with an EDS spectrometer (Thermo Fisher). The SEM was operated at an accelerating voltage of 30 keV. TEM samples were prepared by drop-casting a dilute solution of nanowires onto 15−25 nm thick carbon films or lacey carbon supported on 300 mesh copper grids (Ted Pella). SEM samples were drop-casted on silicon wafers. Powder X-ray diffraction (XRD; D8 Advance Davinci, Bruker) data was collected at a scanning rate of 0.007° min−1 with a Cu Kα radiation source (λ =1.5418 Å). XRD samples were deposited onto a glass slide. Phase compositions of nanowires were obtained from the Rietveld refinement of the XRD pattern using Jade 9.0 (Materials Data Incorporated). The experimental patterns were fitted to calculated diffraction patterns for structures of CuInS2 chalcopyrite (space group I4̅2d) and CuInS2 wurtzite (space group P63mc) using unit cell parameters and atom locations obtained from the CuInS 2 literature.34,35 The refinement was performed using pseudo-Voigt profiles with a third-degree-polynomial background correction. Adjustments were also made for the full width at half-maximum values of individual peaks. The absorption of solar light by nanowires was measured using a UV−vis−NIR spectrometer (Jasco V-670) equipped with an integrating sphere accessory. The photoluminescence (PL) spectroscopy of nanowires was measured using a spectrofluorometer (Horiba NanoLog) equipped with an InGaAs array detector (spectral range: 800−1700 nm). Samples were prepared by suspending nanowires in toluene. For PL measurement, a nanowire suspension was placed in an NMR tube and frozen in a liquid nitrogen cryogenic cell that was compatible with the NanoLog sample compartment. The PL spectrum was obtained at 77 K. Other analyses performed in this study included thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of precursor decomposition, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) of nanowire capping, and mass spectrometry (Thermo Star, Pfeiffer) measurements of vapors produced in MASLS. These analyses were performed following standard procedures.

3. RESULTS AND DISCUSSION The microwave-assisted solution−liquid−solid method for synthesizing CuInS2 nanowires was developed on the basis of the recent success of hot-injection SLS synthesis of copper indium selenide (CuInSe2) nanowires using a binuclear singlesource precursor.19 In HISLS, the precursor-containing solution is injected into a boiling solvent to initiate nanowire synthesis. Following this method, we have synthesized a control sample of CuInS2 nanowires using a similar precursor: [(PPh3)2Cu][(μSPh)2In(SPh)2] (Ph = phenyl; Figure S1), together with bismuth chloride (BiCl3) as catalyst precursor,33 decyphosphonic acid as stabilizer, and trioctylphosphine as solvent. The resulting HI nanowires are found to bend sharply and repeatedly along their lengths, as shown in Figure 1a,b, indicating the prevalence of planar defects. These observations are consistent with those reported for CuInSe2 nanowires previously.19 Irradiating the SSP/BiCl3/DPA/TOP solution used in HISLS in a microwave reactor does not produce CuInS2 nanowires. Instead, we find poorly crystallized phosphide nanowires (Figure S2), suggesting the interference of phosphorus from DPA and TOP decomposition, as observed in CuInSe2 synthesis.19 To prevent this interference, we replace DPA with sulfur-containing dodecanethiol (Figure S1). Although dodecanethiol has not been used in catalyzed SLS growth of nanowires, it is well-known as an effective stabilizer for nanostructure growth that does not involve catalysis.36,37 To prevent the breakdown of solvent molecules, we also replace most of TOP with trioctylphosphine oxide (TOPO; Figure S1), which has a similar structure but a higher boiling point (411 vs 291 °C for TOP). As shown in Figure 1c,d, the new precursor solution produces long and smooth nanowires under microwave irradiation at 300 °C for 5 min. Interestingly, injecting this solution into boiling TOP produces chalcocite Cu2S nanoparticles instead of CuInS2 nanowires (Figure S3),38 indicating the oversupply of sulfur by dodecanethiol. The unique solution compositions required for successful HISLS and MASLS synthesis of CuInS2 highlight the complexity of reactions and processes involved in SLS. In addition to defects, CuInS2 nanowires synthesized using MA and HI methods have different diameters, which are C

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estimated at 6.8(±2.8) nm (standard deviation in parentheses) for MA nanowires and 25.9(±10.4) nm for HI nanowires (Figure S4). The MA nanowires have a smaller diameter with a narrower standard deviation because small and uniform catalyst nanodroplets are formed under rapid microwave heating.39 Further measurements show that MA nanowires can grow to 10 μm within 5 min, which corresponds to a growth rate of 33 nm s−1. The mass yield of MICIS is approximately 31%. Both the growth rate and the mass yield are comparable to those obtained in hot-injection SLS.40 Powder X-ray diffraction reveals that the MA and HI samples contain both chalcopyrite and wurtzite phases, as shown in Figure 1e. Reference intensity ratio analysis indicates that the MA sample consists of 74.5(±0.7)% (by weight) chalcopyrite and 24.5(±0.4)% wurtzite, whereas the HI sample consists of 62.0(±8.7)% wurzite and only 38.0(±5.3)% chalcopyrite. Chalcopyrite is the thermodynamically stable phase of CuInS2 and thus the preferred phase for high-quality nanowire growth. Wurtzite is a metastable phase and often exhibits phase polytypism due to partial conversion to chalcopyrite.41 Energy dispersive X-ray spectroscopy reveals the presence of Cu, In, S, and P in both MA and HI samples, as shown in Figure 1f. The Cu/In/S molar ratio in the MA sample is 1.0:0.9:1.8, consistent with the expected stoichiometry of CuInS2 (for both chalcopyrite and wurtzite). The slightly deficient S content (ca. 5%) can be attributed to sulfur vacancies.42 The small amount of P is from the capping of nanowires by TOP (see below).19 In comparison, the HI sample is severely sulfur-deficient (ca. 33%), with a Cu/In/S molar ratio of 1.0:0.8:1.2. The large compositional deviation from the CuInS2 stoichiometry and the presence of a significant amount of P suggest that the HI sample contains impurity phases such as copper phosphate (Cu3P).19 Absorption spectroscopy performed for the ultraviolet, visible, and near-infrared range reveals a clear absorption edge for MA nanowires, as shown in Figure 1g, which is lacking for HI nanowires. According to the absorption edge, an optical band gap is estimated as Eg = 1.57 eV for MA nanowires, comparable to the band gap of bulk CuInS2 (1.55 eV).43 Further analysis using photoluminescence spectroscopy reveals that MA nanowires contain only intrinsic point defects. As shown in Figure 2, a broad band occurs between 1.1 and 1.5 eV, corresponding to emissions from radiative recombination of donor and acceptor defect pairs.43,44 Fitting with Lorentzian functions reveals two distinctive emission bands at hν1 = 1.47 eV and hν2 = 1.377 eV. We attribute hν1 to the recombination of sulfur vacancy (VS) and copper vacancy (VCu). The emission at hν2 is ascribed to the recombination of Cu-substituted In site (CuIn) or In vacancy (VIn) with a donor site having an energy level 0.073 eV below the CuInS2 conduction band. Potential candidates include In-substituted Cu site (InCu),45 interstitial In site (Ini),44 and interstitial Cu site (Cui).46 Because Cu is enriched over In in MA nanowires (cf. Figure 1f), the most likely donor is Cui. The ionization energies of the donor and acceptor pairs are related to Eg, hν1, and hν2 by hν1 = Eg − (EVS+ EVCu) + Ee and hν2 = Eg − (ECuIn/VIn+ECui) + Ee, as illustrated by the energy diagram. Here, EVS = 35 meV, EVCn= 100 meV, ECuIn/VIn= 73 meV, and ECui = 150 meV; Ee = 30 meV is the energy of Coulomb interaction between ionized donor and acceptor pairs.43,44,47 These results indicate that point defects in MA nanowires are intrinsic, not structural, in nature.

Figure 2. Photoluminescence spectrum of CuInS2 nanowires synthesized under microwave irradiation and measured at 77 K. Curves in the spectrum are least-squares fits to Lorentzian functions. Prominent emission bands (marked as hν1 and hν2) are rationalized using an energy diagram (not drawn to scale). See text for the definition of the energy levels.

The crystallinity of MA CuInS2 nanowires is further examined by inspecting 10 nanowires from different batches segment by segment using high-resolution transmission electron microscopy (HRTEM). As shown in Figure 3a,b, MA nanowires are free of linear and planar defects, with wurtzite nanoparticles and nanorods scattered in the vicinity of the nanowires (Figure S5). The complete separation of chalcopyrite and wurtzite phases suggests that the sample can be further purified by postsynthesis separation. Fast Fourier transform (FFT) of the HRTEM images, as illustrated in Figure 3c,d, reveals that nanowires are single-crystal chalcopyrite viewed in the [12̅2], [2̅12], and [114̅] directions. The spacing between lattice fringes perpendicular to the nanowire length is measured at 3.22(±0.05) Å, in good agreement with the value for bulk chalcopyrite CuInS2 (3.197 Å).48 HRTEM and FFT also suggest that the nanowires are terminated by (12̅2), (1̅22̅), (2̅12), (21̅2̅), (114̅), and (1̅1̅4) planes on six sides and by (112) and (1̅1̅2̅) planes on both ends, as illustrated in Figure 3e. The preferred growth of MA nanowires thus occurs in the [112] direction, which is consistent with the intensified (112) reflection observed by XRD (Figure 1e). In cubic close-packed chalcopyrite, there are 4 equiv ⟨112⟩ growth directions, which may switch from one to another under nonoptimal conditions in conventional SLS, thereby producing bent HICIS nanowires (Figure S6). The surfaces of MA nanowires are examined using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The examination reveals that the nanowires are capped by dodecanethiol and trioctylphosphine ligands. As shown in Figure 4a, XPS shows that the elemental ratio at the nanowire surface is Cu/In/S/P = 1:1.4:4.5:2.2. According to the stoichiometry of CuInS2, 47% of S measured by XPS is attributable to the nanowires, with the remaining contributed by surface-attached DT. We also attribute P to surface-attached TOP. Together, we have SDT/PTOP = (4.5 − 1 − 1.4):2.2 ≈ 1:1, indicating that the nanowire surface is covered by equal amounts of DT and TOP. As shown in Figure 4b, FTIR of the nanowires reveals broadened regions corresponding to the D

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Figure 3. Crystallinity of CuInS2 nanowires synthesized by MASLS. (a−c) Transmission electron micrographs. (d) Electron diffraction pattern of panel c obtained using Fast Fourier transform: O, simulated diffraction pattern; †, 024̅ , 02̅4, or 02̅4; ‡, 132̅, 312̅, 1̅16, or 11̅6. (e) Model representation. Scale bars: a, 100 nm; b, 5 nm; c, 2 nm; d, 2 nm−1.

Figure 5. Solidified catalyst nanoparticle attached to a nanowire created by acetone quenching. (a) Transmission electron micrograph. (b) Elemental profile of the solidified catalyst nanodroplet. Scale bar: 10 nm.

solidified catalysts have a glassy structure with a near-equal molar ratio of Bi and Cu without any appreciable amount of In or S remaining. The unique composition of the BiCu catalyst is likely a result of the balance between the dependence of melting point and that of microwave absorption on alloy composition (Figure S7).50,51 The finding of only BiCu alloy and CuInS2 nanowires as the two solid phases after quenching indicates that nanowire growth in MASLS conforms to a eutectic growth mechanism. As shown in Figure 6, the phase diagram of the eutectic system can be divided into four regimes. The first regime is the liquid melt of Bi, Cu, In, and S formed at temperatures bounded by the melting points of BiCu (847 °C)50 and CuInS2 (980 °C).52 The second regime consists of BiCu and CuInS2 solids formed at temperatures below the eutectic temperature Teu (Teu < 291 °C since HISLS operating near 291 °C can produce CuInS2

Figure 4. Capping of CuInS2 nanowires by DT and TOP molecules attached to the surface of CuInS2 nanowires. (a) X-ray photoelectron spectroscopy (XPS). (b) Fourier transform infrared spectroscopy (FTIR). Inset: Nanowire segment viewed from the side. The first three layers are expressed by the space-filling model, the middle three layers, by the S tetrahedron model, and the bottom three layers, by the Cu/In tetrahedron. Colors of the atoms: Cu, blue; In, red; S, green.

vibrations of the C−S bond of DT and the C−P bond of TOP, suggesting that DT and TOP are attached to the nanowire surface through S and P, respectively. We propose that DT is attached to Cu/In through the metal−thiol bond, whereas TOP is attached to S. Both Cu/In and S are exposed on the sides of CuInS2 nanowires in equal amounts, as illustrated by the inset of Figure 4b. To understand the mechanism of nanowire growth in MASLS, we examine the metallic catalyst after the reaction system is quenched with acetone. Although the catalyst liquid droplets usually fall off from nanowires during quenching and are thus difficult to identify,49 a few of them are found as nanoparticles still attached to nanowires. As shown in Figure 5a,b, microscopic and elemental analyses reveal that the

Figure 6. Phase diagram of the eutectic system consisting of BiCu and CuInS2. Teu: eutectic temperature. Tam: ambient temperature. C: CuInS2 concentration in liquid BiCu. Cs: solubility of CuInS2 in BiCu. Note that the diagram is not drawn to scale. E

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benzene vaporization (from SSP decomposition), as revealed by mass spectrometry (Figure S9). The lack of vaporization in the absence of BiCl3 suggests that vaporization does not occur at the solution temperature of 300 °C. When vaporization occurs, overheated hot spots must be present. With BiCl3, the candidates are the catalyst droplets.

nanowires, although they are of poor quality). Between the temperature limits, the eutectic system has two other regimes corresponding to a mixture of the liquid melt with one of the two solid phases: solid BiCu at low CuInS2 fractions (colored in yellow) and solid CuInS2 at high CuInS2 fractions (colored in green). Obviously, the last regime is where both MASLS and HISLS operate for the growth of CuInS2 nanowires. According to the phase diagram, the complete phase separation of BiCu and CuInS2 by quenching can be rationalized as follows. During nanowire growth, the liquid catalyst can be represented by a solid red circle in the regime representing the mixture of liquid melt and CuInS2 solid (green in Figure 6). Under this condition, the concentration of CuInS2 in BiCu (C) is greater than the solubility dictated by the liquidus boundary (Cs; open blue circle). The supersaturation is created by the continuous supply of Cu, In, and S from SSP decomposition at the catalyst−solvent interface. Quenching rapidly lowers the system’s temperature, which also cuts the supply of Cu, In, and S from SSP decomposition, as supported by TGA/DSC measurements (Figure S8). Consequently, the CuInS2 fraction in the catalyst quickly reduces until the system reaches the liquidus boundary. Further decrease of temperature drives the system along the liquidus boundary with minimal growth of CuInS2 until the eutectic point is reached, where the remaining CuInS2 separates from the catalyst as the catalyst is solidified into BiCu nanoparticles. The difference between C and Cs defines the supersaturation of CuInS2 in liquid BiCu: θ = ln(C/Cs), which drives both the nucleation and growth of CuInS2 nanowires, as shown in Figure 7a. Defect-free nanowires need to grow one layer at a time,

4. CONCLUSIONS We have successfully synthesized single-crystal CuInS2 nanowires using a microwave-assisted SLS method. The key feature of MASLS is the use of microwave irradiation to overheat the liquid catalyst, which, in turn, increases the solubility of Cu, In, and S and decreases the supersaturation of CuInS2. The low supersaturation ensures that two-dimensional crystal growth outcompetes crystal nucleation, leading to the layer-by-layer addition of CuInS2 to the end of the nanowire. In addition to the use of microwave irradiation, another important parameter of MASLS design is the use of a sulfur-based stabilizer such as dodecanethiol to prevent the interference from traditional phosphorus-based stabilizers. With further development, MASLS can become a generic method for synthesizing highquality semiconductor nanowires with compositions other than CuInS2 from solution. To optimize microwave-assisted solution-liquid−solid synthesis, potentially important parameters worth investigating in future studies include solution temperature, irradiation time, precursor concentrations, and microwave frequency and power, many of which are expected to be mutually correlated. Preliminary experiments performed by setting the solution temperature at 250 °C, below the temperature of 300 °C used in this study, yield nanowires deficient of indium, suggesting that 300 °C is required to thermally decompose the less reactive portion of SSP containing indium. Shortening and extending the irradiation time can produce nanowires shorter and longer than 10 μm, respectively; however, variation of the nanowire’s dimensions is intricately linked with precursor concentrations. Microwave frequency and power, which are often constrained by the design of microwave reactors, can affect the temperature differential between solution and catalyst. We are currently in the process of investigating the effects of these parameters.

Figure 7. Supersaturation-controlled growth of single-crystal CuInS2 nanowire in MASLS. (a) Schematic of the supply and consumption processes leading a steady-state supersaturation θ of CuInS2 in BiCu. f rxn: flux of Cu, In, and S provided by the decomposition of singlesource precursor (SSP). f n and fg: fluxes consumed by the nucleation and growth of nanowires. (b) Solution temperature (Ts) and system pressure (Ps) with and without the use of catalyst precursor BiCl3 in synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: Structures of single-source precusor, decylphosphonic acid, dodecanethiol, trioctylphosphine, and trioctylphosphine oxide. Figure S2: Poorly crystallized phosphide nanowires formed by irradiating 1 mL of reactive solution containing BiCl3, SSP, DPA, acetone, TOP at a molar ratio of 0.002:1:4:13.3:52 at 300 °C for 5 min. Figure S3: Amorphous phosphide and crystalline chalcocite particles formed by the hot injection SLS with 1 mL of the precursor solution used in MASLS synthesis, containing BiCl3, SSP, TOP, DT, acetone, and TOPO at a molar ratio of 0.002:1:2:4:13.6:50. Figure S4: Diameter histograms for CuInS2 nanowires synthesized under microwave irradiation and by hot injection. Figure S5: TEM, FFT, and truncated structure of wurtzite impurity produced by MASLS. Figure S6: Bending and twinning of CuInS2 nanowires synthesized by HISLS. Figure S7: Relationships of melting temperature and electrical conductivity with composition in the Bi−Cu alloy. Figure S8: Decomposition of single-source precursor analyzed by thermogravimetric analysis and differential scanning calorimetry. Figure S9: Mass spectrometry of

which requires the rate of lateral growth (fg) to be faster than the rate of nucleation (f n) so that only one nucleus exists for each layer of atoms added to the end of a nanowire. According to classical theory,53 growth outcompetes nucleation under low supersaturation with small θ values. We propose that microwave irradiation decreases supersaturation by overheating the catalyst and thus increasing the solubility of Cu, In, and S in the liquid catalyst. Although we cannot measure the catalyst temperature directly, measurements of the system pressure supports the presence of overheated catalyst droplets. Figure 7b compares the solution temperature (Ts) and the system pressure (Ps) with and without BiCl3 to form catalyst nanoparticles. While Ts remains similar in both cases, Ps reaches 2.2 atm in 8 min with BiCl3 but remains zero without BiCl3. With BiCl3, the buildup of Ps is the result of acetone and F

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vapors produced in MASLS. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00284.

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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 C.N. thanks the Department of Energy Office of Nuclear Energy’s Nuclear Energy University Programs, the National Science Foundation Environmental Engineering Program, and the Notre Dame Sustainable Energy Initiative for financial support. P.C.B.’s contribution was supported by the Energy Frontier Research Center Materials Science of Actinides. We thank the Center for Sustainable Energy at Notre Dame, the Notre Dame Integrated Imaging Facility, Argonne National Laboratory Center for Nanoscale Materials, and Horiba Instruments, Inc. (Edison, NJ) for assistance with instrumentation.

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