I-III-VI2

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Lattice Strain and Ligand Effects on the Formation of Cu2−xS/I-III-VI2 Nanorod Heterostructures through Partial Cation Exchange You Zhai, Joseph C. Flanagan, and Moonsub Shim* Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: In the synthesis of anisotropic colloidal nanocrystal heterostructures, the interplay between many complicating factors such as interfacial chemistry, lattice strain, and coordinating ligands can make precise control over spatial distribution of composition extremely challenging. However, understanding how each complicating factor contributes to the growth mechanism can lead to otherwise difficult-to-achieve or unique structures and the means to tune their electronic/optical properties. Here, we report on the effects of lattice strain and the choice of ligands on the formation of Cu2−xS/IIII-VI2 colloidal nanorod heterostructures through partial cation exchange starting from Cu2−xS nanorods. Lattice strain can induce alternating Cu2−xS/CuGaS2 segments along a colloidal nanorod if CuGaS2 can nucleate easily from the sides of the nanorods. The choice in coordinating ligands can alter this preference to favor tip nucleation, in which case the resulting heterostructure has CuGaS2/Cu2−xS/CuGaS2 rod/rod/rod geometry. In the less strained CuInS2 case, superlattice-like alternating segmentation does not occur but the ligand induced difference in the preference of where nucleation initiates can still lead to distinct heterostructure morphologies. These results demonstrate how surface accessibility varied by the choice of ligands can be exploited synergistically with the driving force that creates interfaces to provide synthetic control over nanoscale heterostructure formation.

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INTRODUCTION

Because of the high cation mobility in the solid state, Cu2−xE (E = S, Se, or Te) has been examined most frequently for cation exchange reactions in nanoscale materials.15 Whether Cu2−xE NCs have been used as starting materials, intermediates, or part of desired products, cation exchange involving them has allowed the synthesis of a variety NCs and their heterostructures with unique shapes/morphologies. Examples include nanoplatelets of Cu2−xE based quaternary alloys,16 CuInS2 hollow nanoplatelets,17 and CuInSe2/CuInS2 dot-in-rod heterostructures.13 In the synthesis of CuInS2 and related NRs, where the initial formation of Cu2−xS seeds is necessary for subsequent CuInS2 growth, there is likely to be some degree of cation exchange as well.18,19 Although there have only been a limited number of studies, a combination of cation exchange and catalytic growth starting from Cu2−xS nanorods (NRs) has led to NRs and NR heterostructures (NRHs) of unusual morphologies.8,20 With respect to Cu2−xS/I-III-VI2 heterostructures achieved to date, the majority of the reports have been on CuInS2, but comparison to other I-III-VI2 and related compounds can give insights on how factors such as lattice mismatch might influence heterostructure formation through cation exchange. Lattice strain is, in fact, known to play an important role in determining the location and the degree of cation exchange.

Nanocrystal heterostructures (NCHs) consisting of two or more distinct crystalline phases within a single particle can bring together complementary properties in unique and synergistic ways to impart new capabilities greater than the sum of its components. The emergence of such materials, especially with anisotropic shapes, is paving the path to nextgeneration photocatalysts, optoelectronics, plasmonics, and medical imaging technologies among many potential applications.1−5 However, the increasing number of components and interfaces can escalate complexity in synthesis. In addition to the difficulties associated with the synthesis of singlecomposition colloidal nanocrystals, synthetic strategies must now address even more complicated issues. These challenges are analogous to a combination of those seen in total synthesis of complex organic molecules (e.g., multiple steps), polymer synthesis (e.g., polydispersity/structural diversity), and heteroepitaxy (e.g., lattice mismatch/strain).1−10 While there have been many synthetic approaches to anisotropic NCHs, each approach is often limited to a small number of specific compositions, shapes, and morphologies. In this sense, utilizing cation exchange may be especially enabling and appealing. In addition to impurity doping and alloying of NCs, various NCHs have been achieved through partial cation exchange and related approaches.8,9,11 Furthermore, NCHs achieved through other methods can be converted to different compositions using cation exchange.11−14 © 2017 American Chemical Society

Received: June 9, 2017 Revised: July 3, 2017 Published: July 5, 2017 6161

DOI: 10.1021/acs.chemmater.7b02392 Chem. Mater. 2017, 29, 6161−6167

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Chemistry of Materials

Figure 1. Representative dark-field STEM images of Cu2−xS NRs (a) and Cu2−xS/CuGaS2 (b, c) and Cu2−xS/CuInS2 (d, e) NRHs after partial cation exchange using t-DDT as the only ligand at 160 °C (for Cu2−xS/CuGaS2 NRHs) or at 140 °C (for Cu2−xS/CuInS2 NRHs) at the indicated reaction times. Insets are the schematics of the NRHs. Visible/near-IR spectral evolution during the cation exchange of Cu2−xS/CuGaS2 (f) and Cu2−xS/CuInS2 (g) NRHs at the indicated reaction times. Spectra are offset for clarity.

of Cu2−xS/CuInS2 starting from Cu2−xS NCs.18,19 A combination of cation exchange and catalytic growth has also been extended to obtain several unusual shapes in colloidal Cu2−xS/ ZnS heterostructures.8 In both of these systems, the lattice mismatch with Cu2−xS (most often seen in the chalcocite and djurleite phases) is relatively small: 1.0% for CuInS2 (along a direction perpendicular to the rod axis) and 3.3% for ZnS (along a direction) for wurtzite structures compared to hexagonal chalcocite. Here and throughout, lattice mismatch is defined as the absolute value of the difference divided by the average of the two lattice parameters. CuGaS2, with a much larger lattice mismatch of 5.5% (along a direction), represents an interesting comparison. Indeed, dark-field STEM images show that Cu2−xS NRs (Figure 1a) convert to heterostructures of strikingly different morphologies when partial cation exchange is carried out with Ga3+ (Figure 1b,c) compared to when it is carried out with In3+ (Figure 1d,e). The corresponding bright-field images are shown in the Supporting Information (Figure S1). In both cases, Cu2−xS NRs were first purified and allowed to undergo the cation exchange reaction using metal acetylacetonate as the cation source in the presence of t-DDT as the ligand. Metal acetylacetonate and t-DDT were mixed with Cu2−xS NRs and heated to the reaction temperature. For the Ga3+ case, which was carried out at 160 °C, cation exchange initiates at several places within a NR, ultimately leading to segmented heterostructures with several alternating Cu2−xS/CuGaS2 units. As indicated by the larger diameter compared to the CuGaS2 parts, there also appears to be a slight radial growth or restructuring of the remaining Cu2−xS segments at the end of the reaction. In contrast, CuInS2 nucleates usually at a single site per NR, growing mainly to rod/rod/rod structures with CuInS2 in the middle. We note that cation exchange appears to be faster for In3+ and, therefore, was carried out at a slightly lower temperature of 140 °C to be able to observe initial stages of the reaction. Continued reaction at 160 °C (Figure S1g) maintains the rod/rod/rod structure with slight elongation with a small degree of tapering toward the center of the rod. The evolution of visible/near-IR absorption features during the partial cation exchange reactions is shown in Figure 1f,g.

Superlattices within NRs have been achieved through partial cation exchange of CdS to Ag2S.21 Strain arising from the lattice mismatch between the two materials limits growth of Ag2S segments, resulting in a kinetically trapped superlattice structure rather than a thermodynamically more stable structure that minimizes interfacial area.22 However, there are many other factors that contribute to how and where cation exchange initiates and propagates. For example, superlattice-like Cu2S/ Ag2S nanowires have been shown to arise from twin boundaries, causing cation exchange to initiate at multiple locations in the Cu2S nanowires,23 distinct from how CdS/Ag2S superlattice NRs were achieved. Hence, there are many other factors, such as defects and the roles of coordinating ligands, that need to be better understood in order to take full advantage of the wide variety of nanoscale materials that can be achieved through cation exchange. Here, we report on the partial exchange of Cu cations in Cu2−xS NRs with Ga3+ or In3+ to achieve Cu2−xS/I-III-VI2 NRHs with a focus on the interplay between lattice strain and ligand steric hindrance. Lattice strain can lead to superlattice-like heterostructures with multiple alternating segments of Cu2−xS/CuGaS2 upon partial cation exchange with Ga3+, whereas, due to less lattice strain, mostly single CuInS2 domain heterostructures form on Cu2−xS NRs when In3+ exchanges partially with Cu+. However, the choice of ligands added to the reaction, in particular, the degree of their steric hindrance, can affect where cation exchange initiates and therefore alter the resulting morphology. Branched ligands allow cation exchange to initiate at multiple locations on the sides of the NRs, allowing superlattice-like structures with alternating Cu2−xS/CuGaS2 segments, but linear thiols and amines cause preference for cation exchange at the tips of Cu2−xS NRs, leading to CuGaS2/Cu2−xS/CuGaS2 rod/rod/rod structures. In the less strained case of CuInS2, superlattice-like multiple segmentation does not occur, but the branched-tolinear ligand change causes the same nucleation preference to change from sides to the tips of the NRs.

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RESULTS AND DISCUSSION Partial cation exchange, sometimes along with catalytic growth, has previously been used to achieve colloidal heterostructures 6162

DOI: 10.1021/acs.chemmater.7b02392 Chem. Mater. 2017, 29, 6161−6167

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Figure 2. Powder XRD of Cu2−xS/CuGaS2 (a) and Cu2−xS/CuInS2 (d) NRHs after partial cation exchange at 180 °C for 10 min using trioctylphosphine oxide as the ligand with t-DDT addition at 180 °C. (ICDD #s: djurleite Cu1.94S 00-023-0959; wurtzite CuInS2 97-016-3489; wurtzite CuGaS2 peaks from simulation, ref 27). Representative selected-area electron diffraction (SAED) patterns (after partial cation exchange using trioctylphosphine oxide and t-DDT at 180 °C for 10 min) and HRTEM images (after partial cation exchange using t-DDT as the only ligand same reaction conditions as Figure 1) of Cu2−xS/CuGaS2 (b, c) and Cu2−xS/CuInS2 (e, f) NRHs. Observed crystallographic planes and corresponding d-spacing are identified. In (f), the Cu2−xS fringes parallel to the rod axis correspond to (001), but spacing for (004) is shown for the sake of comparison. Figure S4 shows directly measurable (004) planes with the same spacing. Insets in (b) and (e) are the magnified view of the diffraction spots corresponding to the lattice planes perpendicular and parallel to the rod axis as indicated.

composition assignment. As shown by powder XRD and highresolution TEM (Figure 2), both Ga3+ and In3+ partial cation exchange cases lead to NRHs consisting of djurleite Cu2−xS with wurtzite I-III-VI2 components. Although the initial Cu2−xS NRs (Figure 1a) start mostly in the stoichiometric chalcocite phase, they become slightly Cu-deficient at elevated reaction temperatures and change to djurleite phase with low-index facets upon cooling to room temperature as expected.24 Although the chalcopyrite phase is thermodynamically more stable in the bulk, the wurtzite structure of I-III-VI2 compounds has been reported quite frequently in the nanocrystalline form.18,19,27 Interestingly, the interface between Cu2−xS and CuInS2 phases is mostly perpendicular to the rod axis, whereas partial cation exchange to CuGaS2 often leads to more irregular interfaces. We suspect this difference to arise from the larger lattice mismatch between Cu2−xS and CuGaS2. Selected-area electron diffraction (SAED) patterns of single Cu2−xS/CuGaS2 and Cu2−xS/CuInS2 NRHs (Figure 2b,e) show that the epitaxial relationships are Cu1.94S (046)//I-III-VI2 (110) in the radial direction and Cu1.94S (800)//I-III-VI2 (002) in the axial direction. The differences in the spacing between these two pairs of planes, (046)/(110) and (800)/(002), respectively, are 5.0% and 7.4% for Cu2−xS/CuGaS2 and 0.7% and 4.3% for Cu2−xS/CuInS2. High-resolution TEM images of Cu2−xS/CuGaS2 and Cu2−xS/CuInS2 also support these epitaxial relationships (Figure 2c,f). Schematics of the S sublattice at the (800)/(002) interface (planes perpendicular to the rod axis) for the two cases are shown in Figure 3. As expected, the Cu2−xS/CuInS2 case can be seen to have a very little mismatch with S atoms of the two

The initial Cu2−xS NRs start with very little or no near-IR absorption because they are mainly in the chalcocite Cu2S phase that cannot support localized surface plasmon resonance (LSPR) as previously reported.24 As the cation exchange reaction initiates at an elevated temperature, weak, but noticeable, near-IR LSPR appears around 1600 nm for both Ga3+ and In3+ cases. This appearance of LSPR is the result of Cu2−xS NRs undergoing a transition to the djurleite phase (Cu1.94S).24 Postsynthesis reduction of Cu chalcogenide nanocrystals has previously been reported to reduce the LSPR and improve fluorescence,25,26 and similar treatments may facilitate altering and exploiting the optical properties of these NRHs. For reaction conditions that allow more CuInS2 to form, there is a concurrent appearance of an absorption shoulder near ∼780 nm (Supporting Information, Figure S2) for the exchange with In3+. This feature corresponds to and is in the expected wavelength range for the band edge absorption of CuInS2. Corresponding band edge absorption for CuGaS2 expected at ∼2.3 eV (∼540 nm) is not observed in Figure 1f, presumably due to it overlapping with the Cu2−xS band gap transitions in that spectral region. Trioctylphosphine oxide (TOPO) can be added to the partial cation exchange reactions for improved stability of the final products without significantly altering the achievable structures (Figure S2). In the dark-field STEM images (Figures 1 and S2), the darker parts are CuGaS2 (or CuInS2) and the lighter parts correspond to Cu2−xS, which is opposite to what is observed in the bright-field images. The composition at different regions of each NRH from EDS measurements (Figure S3) as well as the high-resolution TEM images (discussed below) supports this 6163

DOI: 10.1021/acs.chemmater.7b02392 Chem. Mater. 2017, 29, 6161−6167

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Although the different degree of lattice strain causes singly or multiply segmented structures, it is interesting to note that partial cation exchange with Ga3+ and In3+ both favor nucleation of I-III-VI2 material mainly from the sides rather than the tips of the initial Cu2−xS NRs. This observation is surprising in that the tips with larger curvature are expected to be more exposed for chemical reactions to take place.17,29 That is, passivating ligands may pack more tightly and cover larger, flatter facets on the sides of the NRs better than the curved tips. However, the ligands passivating the surfaces of Cu2−xS NRs (as well as facilitating solvation of Cu ions in solution) are rather bulky t-DDT that may not be able to pack very closely on the NR surface even if it is flat. Therefore, the sides of the NRs may physically be more accessible for cation exchange to initiate. The introduction of TOPO does not change this situation since TOPO is also quite bulky. If such a steric hindrance effect were important, then linear molecules with terminal surface binding groups may lead to different morphologies; i.e., cation exchange may be induced to initiate only or mostly at the tips. To this end, 1-DDT was used in place of t-DDT. Figure 4 shows TEM images corresponding to the case of partial cation exchange of Cu2−xS NRs to Cu2−xS/CuGaS2 NRHs in the presence of excess 1-DDT. We do indeed see that cation exchange takes places mostly at the tips. However, the reaction is slow and both the temperature and the time of the reaction had to be increased to see a sufficient degree of exchange. For example, cation exchange for 30 min at 200 °C using 1-DDT led to only about 10−20% of each NR being converted to CuGaS2 (Figure 4), whereas more than 50% of each NR was converted with 10 min of reaction at 160 °C when using t-DDT (Figure 1c). The higher thermolysis temperature of 1-DDT (≳200 °C) than that of t-DDT (∼180 °C) on the Cu2−xS NR surface24,30,31 may have some contribution to this difference. However, given that the reaction temperatures are slightly below the respective thiols’ thermolysis temperatures, we anticipate the main reason for the sluggish reaction when using 1-DDT to be its better packing on the NR surface hindering cation exchange, especially on the sides of the NRs, and therefore providing preferred reactivity at the tips. With tDDT, on the other hand, cation exchange can occur more readily and can initiate both at the tips and on the sides of the NRs.

Figure 3. Schematics of the S sublattice alignment at the heterointerface perpendicular to the rod axis for Cu2−xS/CuGaS2 (a) and Cu2−xS/ CuInS2 (b) NRHs and parallel to the rod axis for Cu2−xS/CuGaS2 (c) and Cu2−xS/CuInS2 (d) NRHs. The black solid lines outline the cross sections of the unit cells of djurleite Cu1.94S and wurtzite I-III-VI2. Three atomic planes of Cu2−xS and I-III-VI2 were intentionally overlapped in (c) and (d) to illustrate differences in lattice mismatch. Note that the rod axis is along a direction of djurleite and c direction of wurtzite. Schematics were generated using VESTA.28

crystals overlapping well. A much larger mismatch is easily visualized for the Cu2−xS/CuGaS2 case. Hence, we can anticipate lattice strain to be an important factor that determines the morphologies of the resulting NRHs. That is, similar to what has been reported in CdS/Ag2S, the large lattice mismatch between Cu2−xS and CuGaS2 may be driving the formation of superlattice-like alternating segments in Cu2−xS/ CuGaS2 NRHs.21 The lattice strain can prevent different CuGaS2 domains to merge together, which would otherwise reduce the interfacial area.22 However, the CdS/Ag2S system starts with nucleation of Ag2S particles that protrude from the sides of the initial CdS NRs that merge and grow, whereas, in our case, each nucleated CuGaS2 domain appears as wedges in the cross-sectional view (e.g., Figure 1b) and grows to the opposite side of the NR with occasional merger with other CuGaS2 domains. In the case of CuInS2, a lower degree of lattice strain allows the single nucleated domain to grow more easily.

Figure 4. Representative bright-field TEM (a), dark-field STEM (b), and HRTEM images (c) of Cu2−xS/CuGaS2 NRHs after partial cation exchange using 1-DDT as the only ligand at 200 °C for 30 min. The darker part is Cu2−xS and the lighter part is CuGaS2 in the bright-field images, and the contrast difference is reversed in the dark-field images. Inset is the shape schematic. Observed crystallographic planes and the corresponding d-spacing are identified in the HRTEM image. 6164

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Chemistry of Materials Attempts to carry out similar cation exchange with In3+ using 1-DDT as the only ligand led to the formation of gels similar to what had previously been reported in the synthesis of CuInS2 NRs32 and made it difficult to purify and characterize the resulting products. At higher reaction temperatures, gel formation can be avoided and the results are consistent with enhanced cation exchange reactions at the tips where mainly rod/rod structures are obtained (Figure S5). However, the reaction temperature being slightly higher than the 1-DDT thermolysis temperature may complicate the process due to reactive S reagent in the reaction mixture. To further examine the effects of ligand steric hindrance, primary and secondary amines were compared. A chosen alkyl amine was first mixed with the Cu2−xS NRs and Ga(acac)3 in ODE and heated to the reaction temperature, at which point tDDT was added to initiate the cation exchange reaction. During this heating-up period, we expect the amines to replace the native ligands on the NR surface. We therefore expect the main role of t-DDT here to be extracting Cu+ ions into the solution, whereas different steric hindrance of the amines should lead to different packing density on the surface of Cu2−xS NRs, at least at the early stages of the reaction. Attempts to carry out the reaction without the addition of tDDT led to very little to no cation exchange and, in some cases, to separate nucleation of what appeared to be Ga 2 O 3 nanoparticles. Figure 5 compares the results of using different amines with t-DDT to initiate/enhance cation exchange. Similar to t-DDT only cases, bulkier secondary amines (dioctylamine and bis(2-ethylhexyl)amine) led to multiply

segmented heterostructures. Primary amines (oleylamine and octadecylamine), which can presumably pack at a higher density on the sides of NRs, led to cation exchange mainly at the tips similar to the 1-DDT only case. When dioctylamine, which should allow nucleation of CuGaS2 from the sides of the NRs, was used with linear chain 1-DDT, which should promote cation exchange at the tips, a significant amount of cation exchange still initiated from the sides of the NRs (Figure S6), indicating that the surface was mainly covered by the amines and that their steric effects rather than those of thiols dictated the location of the cation exchange reaction. Hence, the steric hindrance of the surface capping molecules can be exploited to control the morphology of resulting NRHs. The emerging picture of the reaction mechanism from the above observations is summarized in Scheme 1 and is as Scheme 1. Schematic of How Steric Hindrance of Capping Ligands, Which Can Dictate Preferred Nucleation Sites, and Lattice Strain, Which Can Cause Multiply Segmented or Single-Domain Structures, Affect Formation of NRHs from Partial Cation Exchangea

a Blue represents Cu2−xS, orange represents CuGaS2, and red represents CuInS2.

follows. The steric hindrance of the ligands on the surface of the NRs can dictate whether or not the side facets are readily accessible for cation exchange. For the case of linear ligands that can pack densely on the surface, cation exchange preferentially initiates at the tips where the ligand coverage is sparser. Bulky ligands that cannot pack densely allow surface access, leading to the loss of such spatial preference. When linear ligands that allow preferential tip nucleation are used, the resulting morphology is qualitatively the same rod/rod/rod structures with initial Cu2−xS remaining in the middle, independent of degree of lattice strain in the resulting heterostructures. There can be subtle differences due to strain, e.g., smaller tip domains for the large lattice strain case of CuGaS2 and rod/rod heterostructures or complete conversion arising from fast growth of better lattice matched CuInS2 from the tip. When bulky ligands are used, the side facets of the initial Cu2−xS NRs are sufficiently accessible to cation exchange and there is a higher probability of nucleation of the second phase somewhere in the middle of the NRs. In these cases, large lattice strain hinders separately nucleated CuGaS 2

Figure 5. Representative dark-field STEM images of Cu2−xS/CuGaS2 NRHs after partial cation exchange at 180 °C for 10 min using (a) dioctylamine, (b) bis(2-ethylhexyl)amine, (c) oleylamine, and (d) octadecylamine as the ligand with t-DDT added at 180 °C to initiate the reaction. Chemical structures of different alkyl amines are shown by the images. 6165

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diffraction (SAED) and high-resolution (HR)TEM images were recorded using a JEOL 2100 TEM. Samples were prepared by dropping the purified NRH solution in chloroform onto carbon-coated gold grids in order to avoid Cu interference for EDS measurements. A low background beryllium holder was used for EDS measurement. Powder X-ray diffraction (XRD) patterns were recorded with a Siemens-Bruker D5000 diffractometer. Samples were prepared by drop-casting a concentrated solution in chloroform on a low background quartz substrate. Visible/near-IR extinction spectra of the NR solutions in tetrachloroethylene were recorded at RT with a Varian Cary5G UV/vis/NIR spectrophotometer.

domains from coalescing and results in multiply segmented superlattice-like heterostructures, whereas smaller lattice strain allows the nucleated CuInS2 region to grow into a larger domain, resulting in the inverted rod/rod/rod structure with CuInS2 in the middle.

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CONCLUSIONS We have shown that it is the combination of large strain from lattice mismatch and side facet accessibility to cation exchange reaction that leads to the spontaneous formation of multiply segmented, superlattice-like Cu2−xS/CuGaS2 NRHs. Varying the surface capping molecules can change the accessibility of the NRs’ side facets, providing a means to control where cation exchange initiates, which in turn dictates the morphology of the final NRHs. In the better lattice matched case of CuInS2, the same ligand control over the location of cation exchange can be achieved but a much smaller degree of lattice strain facilitates the growth of singly nucleated CuInS2 domains, and therefore, superlattice-like NRHs do not form. These insights on how the interplay between lattice strain and surface ligand choice leads to variations in NRH morphology should help to pave the path to rational design of complex NC heterostructures with tailored properties.

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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.7b02392. Additional TEM and STEM images of various NRHs achieved using different synthetic conditions, additional visible/near-IR extinction spectra, and point EDS measurement results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

EXPERIMENTAL SECTION

Chemicals. Copper(II) nitrate hydrate (Cu(NO 3) 2 ·xH2O, 99.999%), tert-dodecanethiol (t-DDT, 98.5%), 1-dodecanethiol (1DDT, ≥98%), gallium(III) acetylacetonate (Ga(acac)3, 99.99%), indium(III) acetylacetonate (In(acac)3, ≥99.99%), trioctylphosphine oxide (TOPO, 99%), oleylamine (70%), octadecylamine (99%), dioctylamine (97%), bis(2-ethylhexyl)amine (99%), and octadecene (ODE, 90%) were purchased from Sigma-Aldrich. All chemicals were used without purification. All syntheses were carried out using the standard Schlenk technique. Synthesis of Cu2−xS NRs. Following a previously established procedure,8 a solution of 0.5 mmol of Cu(NO3)2·xH2O and 2.5 mmol of TOPO in 5 mL of ODE was degassed at ∼80 °C for 30 min and then heated up to 180 °C under N2 in 3 min. At 120 °C, 2.5 mL of tDDT was added. The reaction was allowed to proceed for 5 min once 180 °C was reached and cooled down to room temperature (RT). The NRs were then precipitated from the reaction mixture by adding methanol and centrifuged for ∼5 min. The NRs were then redissolved in chloroform for characterization or in ODE for NRH synthesis. Synthesis of Cu2−xS/CuGaS2 and Cu2−xS/CuInS2 NRHs. A solution of 0.1 mmol (unless otherwise noted) of Ga(acac)3 or In(acac)3 and the entire batch of Cu2−xS NRs (prepared and purified as described above) in 5 mL of ODE was degassed at ∼80 °C for 30 min. A 0.5 mL portion of t-DDT (or 1-DDT) was then added, and the reaction mixture was heated to the reaction temperature as indicated under N2. The reaction was allowed to proceed at this temperature for the indicated amount of time before the heating mantle was removed. For examining the effects of different ligands (i.e., alkylamines and TOPO), 2.5 mmol of the chosen ligand was added to the reaction mixture containing the metal precursor and Cu2−xS NRs in ODE and degassed at ∼80 °C for 30 min. The reaction mixture was then heated to the reaction temperature as indicated under N2. Once this temperature was reached, 0.5 mL of t-DDT (or 1-DDT) was added dropwise over an ∼30 s period, and the reaction was allowed to proceed at this temperature for the indicated duration before the heating mantle was removed. Aliquots of reaction mixture were taken during growth and precipitated with methanol and then redissolved in chloroform. This purification step was repeated three times before structural, compositional, and optical characterization. Characterization. Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energydispersive X-ray spectroscopy (EDS) were carried out using a JEOL 2010 FETEM with an Oxford EDS detector. Selected-area electron

ORCID

You Zhai: 0000-0001-5421-9372 Moonsub Shim: 0000-0001-7781-1029 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported in part by the U.S. NSF (Grant No. 1507170). Experiments were carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities at University of Illinois.

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ABBREVIATIONS NCHs, nanocrystal heterostructures; NRs, nanorods; NRHs, nanorod heterostructures; Cu2−xS, copper sulfide; LSPR, localized surface plasmon resonance; Cu(NO3)2, copper(II) nitrate; t-DDT, tert-dodecanethiol; 1-DDT, 1-dodecanethiol; Ga(acac)3, gallium(III) acetylacetonate; In(acac)3, indium(III) acetylacetonate; TOPO, trioctylphosphine oxide; ODE, octadecene; TEM, transmission electron microscopy; STEM, scanning transmission electron microscopy; EDS, energydispersive X-ray spectroscopy; SAED, selected-area electron diffraction; HRTEM, high-resolution transmission electron microscopy; XRD, powder X-ray diffraction

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REFERENCES

(1) Tongying, P.; Plashnitsa, V. V.; Petchsang, N.; Vietmeyer, F.; Ferraudi, G. J.; Krylova, G.; Kuno, M. Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures. J. Phys. Chem. Lett. 2012, 3, 3234−3240. (2) Tongying, P.; Vietmeyer, F.; Aleksiuk, D.; Ferraudi, G. J.; Krylova, G.; Kuno, M. Double Heterojunction Nanowire Photocatalysts for Hydrogen Generation. Nanoscale 2014, 6, 4117−4124.

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DOI: 10.1021/acs.chemmater.7b02392 Chem. Mater. 2017, 29, 6161−6167

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DOI: 10.1021/acs.chemmater.7b02392 Chem. Mater. 2017, 29, 6161−6167