A Synthetic Exploration of MetalâSemiconductor Hybrid Particles of

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A Synthetic Exploration of Metal-Semiconductor Hybrid Particles of CuInS

Emil A. Hernandez-Pagan, Alice D. P. Leach, Jordan M. Rhodes, Suresh Sarkar, and Janet E. Macdonald Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03142 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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Chemistry of Materials

A Synthetic Exploration of Metal-Semiconductor Hybrid Particles of CuInS2 Emil A. Hernández-Pagán,†,‡ Alice D. P. Leach,†,‡ Jordan M. Rhodes,† Suresh Sarkar,† and Janet E. Macdonald*,†,¶ † ¶

Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, USA Vanderbilt Institute for Nanoscale Science and Engineering, Vanderbilt University, Nashville, Tennessee 37235, USA

ABSTRACT: The ternary copper chalcogenide semiconductor nanoparticles have gained much attention as their optical properties make them ideal candidates for many applications ranging from photovoltaics to bioimaging. While their synthesis is well documented, there have been few reports on the synthesis of ternary copper chalcogenide-metal hybrid nanoparticles, which can further expand the list of potential applications through synergistic properties. To this end, Pt-CuInS2 hybrids have been synthesized by a two-step approach in high boiling organic solvents. The hybrid nanostructures were characterized employing TEM, XRD, UV-Vis spectroscopy, and EDS mapping. We find that during hybrid nanoparticle synthesis under conditions modified from typical Pt nanoparticle reaction schemes a near complete shell of Pt forms on the semiconductor nanoparticles. Careful control of the reactivity of the Pt and precursor through choice of organic reducing agent and Pt coordinating ligands was successfully used to obtain controlled and isolated domains on the semiconductor nanoparticles. This strategy was further extended for the synthesis of Pd-CuInS2 hybrids.

1. Introduction Over the past two decades research efforts in the field of nanocrystal synthesis have led to the development of synthetic pathways that allow precise control of the nanocrystal size, shape, and composition. Recently, considerable effort has been focused on the synthesis of hybrid nanostructures which requires a higher level of control. Hybrid nanostructures combine distinct and often disparate material components into a single particle, which can lead to bifunctionality and synergistic properties. In particular, semiconductor-based hybrids have garnered much interest as promising architectures for applications such as sensing, bioimaging, photovoltaics, and photocatalysis.1-4 Building on knowledge from single component nanocrystals, most of the semiconductor hybrid systems synthesized and studied have been based on II-VI semiconductors. While these semiconductors have been the cornerstones for development of hybrid structures, the high toxicity of Cd and Pb is a major drawback for many applications. Cu-based semiconductors have attracted considerable attention as alternatives to the II-VI materials in hybrid structures due to their optical properties, low toxicity, and high elemental abundance.5 A key feature is the localized surface plasmon resonance exhibited by these semiconductors due to the oscillations of holes in Cu-deficient off-stoichiometries, which has been shown to enhance the photovoltaic6 and electrocatalytic7 properties. The synthesis of Cu-based semiconductor hybrids has been achieved following diverse strategies.8-14 For example, Au-Cu2S dimers have been synthesized by phase segregation of AuCu alloy nanoparticles upon addition of sulfur.8 The synthesis of core-shell Au-Cu2S and Au-Cu2-xSe dimers was achieved by nucleation of the semiconductor phase on Au nanoparticles;9 while Ru caged Cu2S hybrids were obtained through selective growth of Ru on the edges of the facets of

the Cu2S seeds.10 Recently, both Yu et al.11 and Dilsaver et al.12 reported on the synthesis of hybrids of the quaternary semiconductor Cu2ZnSnS4 with Pt and Au. The hybrids were formed using Cu2ZnSnS4 as seeds for the growth of the metal phase. As with other systems,9, 15-19 the hybrids showed enhanced photocatalytic properties with regards to hydrogen evolution and dye degradation relative to the pristine nanoparticles. One of the most studied semiconductors of the Cu family is copper indium disulfide (CuInS2). CuInS2 has a direct band gap of 1.5 eV, photostability, high tolerance to offstoichiometries, and can also exhibit the localized surface plasmon resonances observed in other members of the family.20 These properties make CuInS2 an ideal component for a semiconductor-metal hybrid photocatalyst system. To date, only CuInS2-Au hybrid nanostructures have been synthesized.21 Au possesses a strong surface plasmon resonance and high photothermal conversion efficiency that make these hybrids ideal candidates for biological applications.22, 23 However, they are not as suitable for photocatalysis as Au is not as catalytically active as Pt. In addition to catalytic activity, control of the metal catalyst loading on the semiconductor is of critical importance. Excess loading can prevent efficient light absorption16 and hinder effective separation of the photogenerated charge carriers. The large amount of metal present can also impact the ability to modify the semiconductor surface with ligands that can render the hybrids water-soluble. Herein we report our studies towards the synthetic control of the placement and loading of Pt domains on wurtzite CuInS2 with the ultimate goal of photocatalytic application. The wurtzite structure was selected over both the chalcopyrite and zinc blende structures due to its anisotropic nature, which al-

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lows for the possibility of enhanced orthogonal charge propagation within the material.24, 25 Single or multiple metal domains could be obtained based on the seeding material and by controlling the reactivity of the Pt precursor through choice of organic reducing agent and coordinating ligands. This strategy was further extended to synthesize CuInS2-Pd hybrid nanostructures.

2. Experimental Section 2.1 Synthesis of Pt nanoparticles. A synthesis was developed based on the work of Sun et al.26 Standard Schlenk line techniques were used throughout with N2 as the inert gas. Oleylamine (0.2 mL), oleic acid (0.2 mL), 1,2-hexadecanediol (0.16 mmol), and diphenyl ether (10.0 mL) were added to a 3neck flask. The mixture was degassed under vacuum at 80oC for 30 min. The temperature was then increased to 210oC under N2. Pt(acac)2 (0.043 mmol) dissolved in diphenyl ether (1.0 mL) was swiftly injected and the color changed from yellow to black within 30 sec. The reaction was allowed to proceed for 10 min. The reaction was then cooled to room temperature. The particles were precipitated with methanol followed by centrifugation and removal of the supernatant. This process was repeated three times. The nanoparticles were stored as a stable black solution in hexanes. 2.2 Synthesis of CuInS2 nanoparticles. CuInS2 nanoparticles were synthesized following the procedure reported by Korgel et al.27 CuCl (0.5 mmol), InCl3 (0.5 mmol), thiourea (1.0 mmol), and oleylamine (10.0 mL) were loaded into a 25 mL 3neck flask containing a stir bar. The mixture was placed under vacuum at 60oC for 30 min, then heated to 245oC under N2 and held at this temperature for 1 hr. After cooling to room temperature, the particles were precipitated with ethanol followed by centrifugation and removal of the supernatant. Three cycles were performed of suspension in hexanes: oleylamine (20:1, by volume) followed by precipitation by excess ethanol, centrifugation and removal of the supernatant. Finally, the nanocrystals were stored in hexanes: oleylamine. 2.3 Pt-CuInS2 hybrids. Pt nanoparticles in hexanes were transferred to a 25 mL 3-neck flask containing CuCl (0.5 mmol), InCl3 (0.5 mmol), oleylamine (10.0 mL) and a stir bar. The mixture was placed under vacuum at room temperature to remove the hexanes. Then the mixture was degassed at 60oC for 30 minutes. In a single neck flask with a stir bar and a septum, thiourea (1.0 mmol) was dissolved in oleylamine (1.2 mL) by heating to 175oC under N2. Once the thiourea was dissolved the temperature was decreased to 50oC. The flask containing the metal precursors was heated to 200oC under N2. At this temperature, thiourea was injected and the growth of hybrid nanoparticles proceeded for 1 hr. The flask was cooled to room temperature and the particles were precipitated with ethanol followed by centrifugation and removal of the supernatant. Three cycles were performed of suspension in hexanes: oleylamine (20:1, by volume) followed by precipitation by excess ethanol, centrifugation and removal of the supernatant. Finally, the nanocrystals were stored in hexanes: oleylamine. 2.4 CuInS2-Pt hybrids 1,2-hexadecanediol. Oleylamine (0.2 mL), oleic acid (0.2 mL), 1,2-hexadecanediol (0.16 mmol), and diphenyl ether (10.0 mL) were loaded into a 25 mL 3-neck flask containing a stir bar. The mixture was placed under vacuum at 80oC for 30 min. In a vial, Pt(acac)2 (0.043 mmol) was dissolved in diphenyl ether (1.0 mL) with addition of 1.0 mL of the CuInS2 nanoparticles stock solution.

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The flask containing 1,2-hexadecanediol was heated to 210oC under N2 and the Pt(acac)2-nanoparticle mixture was swiftly injected. After 10 min the heating mantle was removed and the flask was allowed to cool to room temperature. The particles were precipitated with ethanol followed by centrifugation and removal of the supernatant. Three cycles were performed of suspension in hexanes: oleylamine (20:1, by volume) followed by precipitation by excess ethanol, centrifugation and removal of the supernatant. Finally, the nanocrystals were stored in hexanes: oleylamine. 2.5 CuInS2-Pt hybrids TOP. 2.0 mL of the CuInS2 nanoparticles stock solution, oleic acid (0.2 mL), and oleylamine (10.0 mL) were loaded in a 25 mL 3-neck flask containing a stir bar. The mixture was placed under vacuum at 80oC for 30 min. In a vial with a septum, Pt(acac)2 (0.039 mmol) was dissolved in a mixture of trioctylphosphine (TOP) (17.0 µL) and diphenyl ether (1.0 mL). Under N2, the temperature of the flask containing the CuInS2 nanoparticles was increased to 210oC. At this temperature, the Pt solution was swiftly injected. After 5 min the heating mantle was removed and the flask was cooled to room temperature. The particles were precipitated with ethanol followed by centrifugation and removal of the supernatant. Three cycles were performed of suspension in hexanes: oleylamine (20:1, by volume) followed by precipitation by excess ethanol, centrifugation and removal of the supernatant. Finally, the nanocrystals were stored in hexanes: oleylamine. 2.6 Characterization. Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS) were performed on a Phillips CM20 microscope operating at 200kV. High resolution TEM (HRTEM) and EDS mapping data were collected using a FEI Tecnai Osiris™ digital 200 kV S/TEM system equipped with ChemiSTEM EDS. XRD measurements were performed using a Scintag XGEN-4000 X-ray diffractometer with a CuKα (λ = 0.154 nm) radiation source. The resulting diffraction patterns were then visually compared to data from the JCPDS database and literature examples to determine the structure.28 The absorption spectra of nanoparticle samples were collected from 300 – 1200 nm on a UVvisible spectrophotometer (Jasco V-670). Samples were measured in solution with hexanes as the solvent. 2.7 I-V measurements. Measurements were carried out using a Gamry Series G-300 potentiostat with the PHE200 software package in the linear sweep voltammetry and chronoamperometry modes. A simple device was fabricated, based on the work of Wang et al.,29 by drop-casting a concentrated solution of the nanoparticles on a glass substrate with ITO film on either edge as conductive electrodes. The current was then measured using linear sweep voltammetry from -0.5 - 0.5 V with a scan rate of 0.01 Vs-1 and a 1 mV voltage step. This measurement was performed both in the dark and under illumination by a 500 W Xe lamp with the solar spectrum. Chronoamperometry measurements were then taken using a fixed voltage of 0.5 V and a light on-off cycle with 20 sec intervals.

3. Results and Discussion The first route taken to synthesize hybrid nanostructures was based on Pt seeds serving as nucleation sites for the semiconductor, which we denote as Pt-CuInS2 (Fig. 1a). The Pt seeds were prepared by the reduction of Pt(acac)2 with 1,2hexadecanediol in a mixture of oleylamine and oleic acid (Fig. S1a).26 The Pt seeds were transferred to a flask containing the


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CuCl and InCl3 precursors in oleylamine. To initiate the hybrid growth, thiourea dissolved in oleylamine was swiftly injected at 200oC. Figures 1b-c show TEM images of the PtCuInS2 hybrid nanostructures. This synthetic path yields hybrid dimers with CuInS2 domains of 5-20 nm in size and platelike morphology. The formation of more complex structures, such as trimers, was not observed. For ~2 % of the Pt particles, growth of a CuInS2 domain was not observed. An aliquot of the reaction taken 30 seconds after the injection of thiourea suggest that the hybrid growth proceeds via heterogenous nucleation (Fig. S1b). The UV-Vis spectrum of the heterostructured nanoparticles showed an optical transition analogous to that of pristine CuInS2 nanoparticles (Fig. S1d). Figure 1d illustrates a high angle annular dark field (HAADF) STEM image showing the separate domains. EDS mapping indicated the presence of Cu, In, S, and Pt (Fig. 1e). Furthermore, XRD analysis of the hybrids revealed the crystal structure of the CuInS2 domains to be wurtzite (Fig. S1c). The wurtzite structure is of particular interest for photocatalysis as its anisotropy allows for orthogonal charge propagation.25 This scheme was further employed to synthesize Pt-CuInS2 hybrids with bulletlike CuInS2 domains by replacing the metal chlorides with Cu (I) and In (III) acetates and a mixture of dodecanethiol/tertdodecylmercaptan20 in place of thiourea (Fig. S2). While this synthesis yielded Pt-CuInS2 hybrids, the polydispersity of the nanostructures motivated us to investigate a different approach.

seeds, which we denote as CuInS2-Pt. CuInS2 seeds were synthesized in a single pot reaction with CuCl, InCl3, and thiourea, in oleylamine (Fig. 2b). These seeds had the wurtzite structure (Fig. S3) and were again plate-like in morphology, but more uniform in size (~ 18 nm). The growth of the Pt domains was carried out by injecting a mixture of Pt(acac)2 and CuInS2 seeds into a flask that contained oleylamine, oleic acid, and 1,2-hexadecanediol, in diphenyl ether at 210oC. Herein, 1,2-hexadecanediol acts as a reducing agent, while oleylamine and oleic acid are stabilizing ligands. The TEM image (Fig. 2c) shows that the CuInS2 seeds are decorated with multiple Pt domains on the edges of the plateshaped CuInS2 seeds. In addition to the strong imaging contrast of Pt, EDS was also employed to confirm the presence of Pt domains (Fig. S4). A similar result was obtained when bullet-like CuInS2 were used as seeds (Fig S5). It should be noted that under these reaction conditions homogenous nucleation of Pt nanocrystals was also observed, but these could be removed through centrifugation. The hybrids also formed at shorter reaction times, as well as at injection temperatures up to 20oC lower (Fig. S6). Upon further examination of the CuInS2-Pt hybrids it was noticed that in addition to the Pt domains there appeared to be a Pt shell surrounding the CuInS2 seeds. a) CuInS2

+ Pt(acac)2

b)

Oleylamine Oleic Acid 1,2-hexadecanediol Diphenyl ether 210o C c)

Pt CuInS2

Figure 2. a) Schematic of the synthesis of CuInS2-Pt from CuInS2 nanoparticles/1,2-hexadecanediol and TEM images of b) the CuInS2 nanoparticles used as seeds and c) the CuInS2-Pt hybrids. Scale bars are 20 nm for both images.

Figure 1. a) Schematic showing the synthesis of Pt-CuInS2 from Pt nanoparticles. b, c) TEM images of the Pt-CuInS2 hybrids. Scale bars are 20 nm and 5 nm, respectively. d) High angle annular dark field STEM image and e) EDS mapping of the hybrids. Scale bars are 10 nm.

Figure 2a illustrates a schematic of an alternate and complementary route to Pt-CuInS2 in which the hybrid nanostructures are synthesized by nucleation of the Pt domains on to CuInS2

To elucidate if the CuInS2 seeds were covered with a Pt shell, aliquots taken during the reaction at 1, 5, and 10 minutes were analyzed by HAADF-STEM and EDS mapping. HAADF allows us to exploit the high imaging contrast of Pt relative to CuInS2 since the technique is sensitive to Z2. After 1 min, HAADF and EDS mapping (Fig. 3a,d) show the presence of a patchy Pt shell covering the CuInS2 seeds. After 5 min the shell coverage increases, however Pt domains are not observed, protruding from the particle (Fig. 3 b,e). The shell has a ring-like appearance in EDS, as the shelling plane is parallel to the plane of observation at the nanoparticle edges. At 10 min, the CuInS2 are decorated with a Pt shell and between three and five discrete, protruding domains (Fig 3 c,f) reminiscent of the Stranski-Krastinov growth mode.4 The UVVis spectra (Fig. 3g) of the different aliquots are in agreement with these observations. The spectrum obtained with the 1 min aliquot shows a defined optical transition resembling that of the CuInS2 seeds, consistent with low Pt coverage. On the other hand, due to the higher Pt coverage, the 5 and 10 min


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show a washed out band gap absorption onset from the presence of additional Pt.

Figure 4. a) Schematic diagram showing treatment of hybrids with neocuproine, b, c) TEM images showing Pt shell left behind after the hybrids were reacted with neocuproine. Scale bars are 5 nm. d-h) EDS mapping of the Pt shell. Scale bars are 10 nm.

Figure 3. a-c) HAADF images and d-f) corresponding EDS mapping for aliquots taken after 1, 5, and 10 minutes of the hybrids synthesized with HDD. Scale bars are 20 nm. g) UV-Vis spectra of the hybrids at the different time intervals.

While some applications may require the chemical robustness imparted to the CuInS2 covered by a complete Pt shell, a photocatalyst would require exposure of both the Pt and CuInS2 domains to the solution for electron and hole charge transfer. To determine if the Pt shell was complete or patchy, an aliquot of the final hybrid nanostructures was dissolved in chloroform and reacted with an excess of neocuproine. It has been shown that neocuproine can preferentially bind and leach Cu+ from copper sulfides.10 Figure 4 shows TEM images of the hybrid nanostructures after reaction with neocuproine. The leaching of the Cu+ leaves behind a Pt shell forming a rattle-like structure. Analysis of the core lattice fringes gave lattice spacing in the range of 0.33-0.34 nm, close to that of the (100) plane for wurtzite CuInS2 and the (109) plane for tetragonal In2S3. However, given that the neocuproine treatment is done at room temperature, it is unlikely that a transformation from the original wurtzite CuInS2 to tetragonal In2S3 would occur with retention of crystallinity. From the standpoint of a photocatalyst, the rattle-like structure indicates the Pt is patchy and there is CuInS2 exposed to the solution. Although the Pt shell is patchy, its presence can decrease light absorption due to scattering of photons limiting the efficiency of a photocatalytic system.16 Thus we sought additional routes that would limit the Pt growth on the surface of the CuInS2 to create discrete domains of Pt rather than a shell.

The morphology of the hybrid structures can be tuned by the ligands present in the reaction, as well as the ratio thereof. For instance, in the synthesis of FePt-PbS hybrids reported by Lee et al. the resulting nanostructures exhibit a core-shell morphology in the presence of oleic acid/oleylamine.30 In contrast, they could obtain dumbbell morphology when only oleic acid was present in the reaction. Another example is that reported by Baaziz et al. for Co-Fe based nanoparticles.31 In their work, the reaction of a combination of Fe- and Co-stearate in the presence of oleylamine lead to CoxFe1-xO@CoyFe3-yO4 coreshell nanoparticles. When they replaced oleylamine with oleic acid, the product was CoFe2O4 nanoparticles. With this in mind, a systematic study was carried out to determine the influence of the ligands on the synthesis of CuInS2-Pt hybrids. In this study, the hybrids were synthesized in the presence of only one or two of the three components (1,2-hexadecanediol, oleylamine, and oleic acid) under otherwise identical conditions as those reported in the experimental section. The results can be broken down as follows. In the three cases where oleylamine was present, oleylamine/oleic acid, oleylamine/1,2-hexadecanediol, and oleylamine (Fig. 5a, b, and f, respectively) the Pt growth is indiscriminate and agglomeration of the particles can be observed. In contrast, in the presence of oleic acid/1,2-hexadecanediol or only oleic acid (Fig. 3 c and e) the Pt growth is attenuated and aggregates were not observed. When the reaction was done with only 1,2hexadecanediol present, single Pt domain growth was predominately observed with significant particle aggregation (Fig. 5d). These results suggest that oleylamine in addition to being a ligand also serves as a co-reducing agent to 1,2hexadecanediol.32 On the other hand, oleic acid seems to bind more strongly to the nanoparticles than oleylamine, providing colloidal stability. Therefore, it is established that under our experimental conditions the presence of the three components is essential for the synthesis of higher quality hybrids with colloidal stability. A similar result was reported by Shevchenko and coworkers in their synthesis of CoPt3 nanocrystals with 1,2-hexadecanediol, where the combination of 1adamantenecarboxilic acid and hexadecylamine yielded narrow size distribution and stability against aggregates.33


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a)

Oleylamine Oleic Acid

c)

Oleic Acid 1,2-hexadecanediol

e)

Oleic Acid

b)

Oleylamine 1,2-hexadecanediol

from Yang et al.34 and Ortiz et al.35 in which TOP-based precursors were employed to control the nucleation and morphology of Pd and Pt nanoparticles.

d) 1,2-hexadecanediol

f)

Oleyamine

Figure 6. a) Schematic showing the synthesis of CuInS2-Pt from CuInS2 nanoparticles with the complex TOP:Pt2+ as the precursor. TEM images of the resulting CuInS2-Pt. Scale bars are b) 20nm and c) 5 nm.

Figure 6a shows a scheme of the reaction pathway with TOP: Pt2+ as the precursor. When the TOP:Pt2+ molar ratio was 1:6, Pt growth did not occur at all. On the other hand, when the molar ratio was 1:1, the reaction yielded CuInS2-Pt hybrids. It is evident that the CuInS2 seeds lower the energy barrier for nucleation serving as nucleation sites given that the reaction temperature is 210oC and homogenous nucleation was not observed in the control experiment until 225oC.

a)

b)

c)

d)

Figure 5. TEM images of CuInS2-Pt hybrids synthesized with only a) oleylamine/oleic acid, b) oleylamine/1,2-hexadecanediol, c) oleic acid/1,2-hexadecanediol d) 1,2-hexadecanediol, e) oleic cid, and f) oleylamine. All scale bars are 20 nm.

In seeded hybrid growth, ideal conditions should be such that nucleation of the second material only occurs via heterogeneous nucleation. To test this, the reaction containing the three components was executed in the absence of CuInS2 seeds. Nucleation of Pt nanoparticles was observed 30 seconds after injection of the Pt(acac)2. It became clear that attenuation of the reactivity of the Pt precursor was required to obtain dimeric hybrids with discrete Pt patches rather than a shell. To slow down the nucleation of Pt, the Pt precursor, formed in situ, was changed. Pt2+ is a very soft Lewis acid, therefore a soft Lewis base would bind stronger than the acetlyacetontate and oleylamine/oleic acid present in the original methodology, which are hard Lewis bases. Taking this into consideration, we chose to utilize the much softer trioctylphosphine (TOP) as the coordinating ligand in the Pt precursor solution. In addition, a second method was employed to limit the Pt reactivity; the 1,2-hexadecanediol was removed from the synthesis leaving the weaker reducing agent oleylamine. As a control, the reaction was carried out without CuInS2 seeds. The TOP:Pt2+ precursor solution was injected into a flask containing oleic acid and oleylamine at 210oC and after 10 minutes the mixture remained colorless. The temperature was slowly increased and around 225oC the color changed to black indicating nucleation of Pt nanoparticles. The higher temperature required for nucleation relative to the Pt(acac)2 confirms that the reactivity of the Pt has been suppressed. This is in agreement with reports

Figure 7. a) HAADF image and b) corresponding EDS mapping of the hybrids synthesized with the complex TOP:Pt2+ as the precursor. Scale bars are 20 nm. c, d) TEM images of the hybrid nanostructures after being reacted with neocuproine. Scale bars are 5 nm.

TEM images show that the CuInS2 seeds are decorated with a single Pt domain (Fig. 6b-c), resembling an island or VolmerWeber like growth mode.4 The presence of Pt was confirmed by EDS (Fig. S3). The UV-Vis spectrum of these hybrids (Fig. S7) was comparable to that of the seeds. Although 1,2-


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hexadecanediol was not present in this reaction, the key difference is the Pt precursor. In the systematic study discussed previously, it was shown that with Pt(acac)2 and oleylamine/oleic acid the reaction yields agglomerated hybrids with multiple Pt domains (Fig. 5a). To validate if the controlled nucleation hindered the formation of a Pt shell the CuInS2-Pt hybrids were analyzed with HAADF and EDS mapping. The HAADF and EDS mapping images in Figure 7a-b clearly show discrete Pt domains and no shell formation. Additionally, TEM images (Fig.7c-d) of hybrids reacted with neocuproine corroborate that the seeds are in fact shell-free as the leaching of Cu+ leads to a dimer morphology, instead of the rattles observed with the more reactive Pt conditions. CuInS2-Pd hybrid nanostructures were also synthesized by replacing the Pt precursor with TOP:Pd2+ (1:1) under otherwise similar conditions. Contrary to the Pt hybrids, TEM images show the formation of between three and five Pd domains (Fig. 8 a-b). Further when the Pd hybrids were examined using EDS mapping, it was apparent that some smaller nanoparticles had undergone cation exchange (Fig. S9), as the sulfur regions overlapped with the Pd regions. The distinct behavior of these two CuInS2-metal hybrids is attributed to the lower reduction potential of Pd2+ complexes (EΘ = 0.07 V), when compared to those of Pt2+ (EΘ = 0.14 V). Similar reactivity and cation exchange has been observed for Pd in the CdS-PdX family of hybrid nanoparticles and by Ye et al. for ZnS-CuInS2.14, 18 In these cases, the Pd-S species was identified as Pd4S. It is possible that this species also exists in CuInS2-Pd samples prepared here, however, Pd-S reflections were not of high enough intensity to confirm this via XRD (Fig. S10). A low angle reflection at ~ 22° was observed in good agreement with a reflection seen for the Pd2.8S phase. However, higher angle peaks matching this pattern were not observed so a definitive phase identification is not possible. a)

b)

Figure 8. a, b) TEM images of the resulting CuInS2-Pd. Scale bars are a) 20nm and b) 5 nm.

For applications such as bioimaging and photocatalysis, it is important that the nanostructures are able to undergo ligand exchange processes to modify the surface with a particular functional group or to impart water solubility. In order to make the hybrid nanostructures water soluble, a ligand exchange using 11-mercaptoundecanoic acid (MUA) was attempted. Briefly, an aliquot of the hybrids was precipitated with ethanol followed by centrifugation and removal of the supernatant. The hybrids were re-dispersed in chloroform with excess MUA followed by the addition of water with pH 11. After vigorous agitation, the phases were allowed to separate (Fig. S11). When this process was carried out with the shellless CuInS2-Pt hybrids synthesized with the TOP:Pt2+ precursor, the hybrids transferred from the organic to the aqueous layer. The phase transfer to the aqueous layer indicates the

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hybrids underwent a successful ligand exchange. The same result was observed with pristine CuInS2 seeds and Pt-CuInS2 hybrids. In contrast, the CuInS2-Pt hybrids synthesized with Pt(acac)2/1,2-hexadecandiol remained in the chloroform layer (Fig. S11). This result suggests that the Pt shell present on these hybrids alters the surface chemistry sufficiently to prevent ligand exchange, perhaps requiring a more aggressive ligand exchange methodology based on Pt surface chemistry.

Figure 9: a, b) Current versus voltage plots of CuInS2 and CuInS2 nanostructures. Inset: Schematic diagrams of the corresponding nanoparticles. c, d) Current against time for On-Off cycles for CuInS2 and CuInS2-Pt nanostructures, respectively. Inset c): Schematic diagram of the device.

To determine the efficacy of using these nanostructures in photocatalytic and photovoltaic application, a photoelectrical study was carried out on hybrid CuInS2-Pt nanoparticles prepared with HDD and bare CuInS2 seeds (Figure 9).36 A simple device was fabricated, based on the work of Wang et al.,29 by drop-casting a concentrated solution of the nanoparticles on a glass substrate with ITO films on either edge as the conductive electrodes. (Figure 9c. inset). Chronoamperometry measurements (Figure 9 c.-d.) indicated that the hybrid nanoparticles had higher stability with no decrease in observed current over 260 s, while bare nanoparticles showed a significant decrease. Hybrid nanoparticles also displayed higher photocurrent than the corresponding bare nanoparticles. While this difference is not large, we believe that the hybrid device had a smaller number of nanoparticles deposited on it, as the concentration of the dropcast solutions were set to be of equal absorbance at 400 nm and we anticipate additional absorbance from the Pt. This suggests that hybrid nanoparticles exhibit superior carrier transport and stability with respect to bare CuInS2, demonstrating their potential use in both photovoltaic and photocatalytic application.

4. Conclusion We have presented several schemes for the synthesis of CuInS2-Pt hybrid nanoparticles. When Pt nanoparticles were employed as seeds for the growth of CuInS2, the hybrids obtained had a dimer configuration. Alternatively, hybrids were synthesized from CuInS2 seeds with 1,2-hexadecanediol as the reducing agent, and oleylamine/oleic acid as stabilizing ligands. A systematic study revealed that the presence of all three components is important to obtain stable hybrids. Further-


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more, we found that this reaction pathway yields hybrids consisting of a Pt shell and over growth of Pt domains. The presence of the Pt shell altered the surface chemistry, preventing these hybrids from undergoing ligand exchange. The behaviour of the Pt was controlled by rational selection of the Pt precursor. Utilization of TOP:Pt2+ as precursor, attenuated the reactivity and allowed for single Pt domain growth, resulting in dimeric hybrid nanostructures that could be rendered water soluble through ligand exchange. Additionally the different reactions reported were employed to synthesize hybrids with bullet-like CuInS2 domains and extended for the synthesis of CuInS2-Pd hybrids. Hybrids were synthesized with Pt domains on both the basal and perpendicular edge planes. It has been determined that the mass of the hole and electron are different for wurtzite CuInS2 in the a-/b- and c-directions,37 thus the position of these domains should facilitate the advanced design of nanostructures for photocatalytic function. A complete investigation of this phenomenon and detailed characterization of the optical and photocatalytic properties of the reported hybrid nanostructures is currently in progress.

ASSOCIATED CONTENT Supporting information. Additional synthetic detail. Additional TEM images and XRD for Pt-CuInS2 hybrids. TEM images, UVVis spectra and XRD for CuInS2 bullet-Pt hybrids. EDS spectra. TEM images for additional kinetic studies of CuInS2-Pt. EDS mapping and XRD for CuInS2-Pd hybrids. Pictures of ligand exchange. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions ‡These authors contributed equally. All authors have given approval to the final version of the manuscript.

Funding Sources NSF Career Award Sus-ChEM-1233105; NSF EPS-1004083; United States-Israel Binational Science Foundation grant 2012276; Vanderbilt Institute of Nanoscale Science and Engineering; Department of Education fellowship P200A140248.

ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under CHE-1253105 and EPS-1004083, United States-Israel Binational Science Foundation (BSF) grant number 2012276, and the Vanderbilt Institute for Nanoscale Science and Engineering (VINSE). JMR thanks the Department of Education graduate assistance in areas of national need fellowship P200A140248.

ABBREVIATIONS CuInS2, Copper Indium Disulfide; TEM, Transmission Electron Microscopy; EDS, Energy Dispersive X-ray Spectroscopy; HAADF, High Angle Annular Dark Field; STEM, Scanning Transmission Electron Microscopy; XRD, X-ray Diffraction; TOP, trioctylphsophine; acac, acetylacetonate; HDD, 1,2hexadecanediol; MUA, 11-mercaptoundecanoic

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