Article pubs.acs.org/cm
Symmetry Break and Seeded 2D Anisotropic Growth in Ternary CuGaS2 Nanocrystals Samrat Das Adhikari, Anirban Dutta, Gyanaranjan Prusty, Puspanjali Sahu, and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India S Supporting Information *
ABSTRACT: In the wurtzite phase, asymmetric growths of nanocrystals are typically observed along the polar [001] direction. Polarity driven layerwise alternative depositions of cations and anions on seed dots mostly drive such directional growths. However, while the reaction condition favored the asymmetric growth for 2D seeds, it breaks the symmetry of the basal plane and allowed the growth along one of the three symmetric directions. Considering Cu2S disks as seeds, the diffusion induced asymmetric growth leading to 2D tadpole shaped (2d-tadpole) ternary CuGaS2 nanostructures is reported here. The formation mechanism of these asymmetric unique ternary nanostructures is elaborately discussed with tailoring the growth patterns via tuning the interface or introducing a dopant. The entire formation process is also compared with standard tadpoles obtained from 0D Cu2S particles, and the parameters for obtaining such anomalous architectures were established. The most exciting chemistry tuned here is the exploration of specific reactivity of a dual sulfur source which led to highly monodisperse nanostructures observed self-assembled on microscopic grids. The entire study from the reaction chemistry to structural transformation are correlated and compared in both 2D and 0D seeded asymmetric growths leading along [100] and [001] directions, respectively.
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INTRODUCTION Anisotropic growths in semiconductor nanostructures which induce polarity, trigger self-assembly, delocalize charge carriers, and generate low energy stable facets remain in the interest of fundamental study of crystal growth in solution.1−12 Typically, these nanostructures were originated from seed particles and followed energetically favorable definite directional growth during the progress of the reaction.1,9,13−20 Mostly, the phase of the material, the facet energy, and factors including variable reaction parameters and monomer concentrations drive such asymmetric growths. Literature reports reveal that these are extensively studied for seed dots in various semiconductors, leading to different shaped nanostructures.4,21−26 However, beyond this or using shape variable seeds for further asymmetric growths, which typically required complex reaction chemistry, is limited. One of the important and ideal case is 2D seed nanostructures, which possess wide surface areas and high catalytic activity and have abilities of possible formation of wide area self-assembly and supercrystals,6,27−35 remain in the forefront of current research. Hence, study of the crystal growths in this domain is important and required further exploration. The ideal case to study the 2D seeded growth is Cu based nanocrystals. The hexagonal phase of these materials typically forms 2D disks, and they are widely known for their surface plasmon.6,27,31,34,36−41 In addition, these materials also served as the starting material for forming multinary semiconductors (I-III-VI and I-II-IV-VI) and their alloys which were known as © 2017 American Chemical Society
the only greener alternative work horse instead of Cd and Pb based nanomaterials.20,29,42−59 Even the anisotropic growths of these multinary nanostructures are extensively studied; but the seeded growth from 2D seeds leading to anisotropic structures via controlled directional growths is not explored. Coupling diffusion with a classical growth mechanism, herein, the symmetry loss in hexagonal Cu2S disks, and subsequent anisotropic growths leading to ternary 2D CuGaS2 nanostructures are reported. Instead of the most polar [001], the wurtzite phase nanostructures facilitated the anisotropic growth along the [100] direction. Moreover, as the diffusion rate changes with the progress of reaction, the growth ultimately led to 2D tadpole shaped structures, which remained unique for the ternary family of nanostructures. The entire growth pattern also compared with standard tadpole growth of CuGaS2 starting with 0D Cu2S hexagonal nanostructures. Initially, these 2D and 0D seeds were synthesized in alkylamine and octadecene (ODE) medium, which were further grown along [100] and [001] directions, respectively, via diffusion in the presence of trivalent Ga(III) ions. Successive samples were collected and microscopically analyzed for unveiling the involved chemistry. Roles of medium, ligands, sulfurizing agent, and other reaction parameters are established for understanding the different shapes and growth patterns on Received: May 1, 2017 Revised: June 1, 2017 Published: June 2, 2017 5384
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Figure 1. (a) Schematic presentation of formation chemistry of 2d-tadpoles and tadpoles of CuGaS2 nanostructures. (b) Wide area inverted TEM image of CuGaS2 2D tadpoles, and (c) wide area inverted TEM image of CuGaS2 tadpoles.
Figure 2. (a, b) TEM images of 2d-tadpoles in different resolutions. (c) Magnified TEM image showing vertical as well as horizontal appearance of 2d-tadpoles. (d) HRTEM image showing two 2D tadpoles. (e) HRTEM image of a single 2d-tadpole. (f) HRTEM and (g) corresponding selected area FFT pattern, respectively. (h) HRTEM showing the width of a 2d-tadpole. materials were purified using hot ethanol, followed by washing with chloroform and acetone as solvent−nonsolvent pair. Synthesis of Flattened Two-Dimensional Tadpole. 2d-tadpoles were observed undergoing flattening in the presence of Zn precursor. The synthesis followed similar procedures to the above 2d-tadpoles. However, in this case, the 0.01 mmol of Zn(acac)2 (2.6 mg) was taken along with 0.1 mmol of Cu(acac)2 (26 mg), 0.1 mmol of Ga(acac)3 (36 mg), 1 mL of DDT, and 1 g of HDA in the stock vial. Synthesis of CuGaS2 Tadpole Shaped Nanoparticles. Tadpole shaped CuGaS2 nanostructures were synthesized in nonpolar medium via 0D Cu2S mediated diffusion controlled growth. In a typical synthesis, 0.2 mmol of S powder (6.4 mg) and 4 mL of ODE were loaded in a three-neck round-bottom flask, purged Ar for 15 min at 60 °C, and the temperature of the reaction mixture was raised to 220 °C. In a separate vial, 0.1 mmol of Cu(acac)2 (26 mg), 0.1 mmol of Ga(acac)3 (36 mg), 1 mL of DDT, and 1 mL of ODE were loaded, capped with a rubber septa, and purged with Ar gas for 15 min. Once the solution became clear, it was injected into the reaction flask at 220 °C and annealed. The entire reaction for the nanocrystals growth ceased within 5 min, but the reaction was annealed for 30 min for better crystallization. Finally, the reaction was cooled down and the materials were purified using hot ethanol, followed by washing with chloroform and acetone as solvent−nonsolvent pair. Synthesis of Elongated Tadpole. For synthesizing elongated tadpoles, Zn(acac)2 was used as elongating agent and 0.1 mmol of Zn(acac)2 (26 mg) was additionally introduced to the reaction mixture
both 0D and 2D seeds. Further, by coupling with other materials or ions, their influences on the asymmetric growths are also established.
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EXPERIMENTAL SECTION
Materials. Copper(II) acetylacetonate (≥99.99%), gallium(III) acetylacetonate (≥99.99%), zinc(II) acetylacetonate hydrate (powder), 1-octadecene (ODE, 90% tech.), hexadecylamine (HDA, tech., 90%), 1-dodacanethiol (1-DDT, 98%), and S powder were purchased from Sigma-Aldrich. All the chemicals were used without further purification. Methods. Synthesis of CuGaS2 Two-Dimensional Tadpole Shaped Nanostructures. 2d-tadpoles of CuGaS2 were synthesized via Cu2S seed mediated diffusion controlled growths. This was performed using dual sulfur precursors, elemental S and alkyl thiols, and in the presence of alkylamine. In a typical synthesis, 0.2 mmol of S powder (6.4 mg) and 4 g of HDA were loaded in a three-neck roundbottom flask, purged with Ar for 15 min at 60 °C, and then the reaction temperature was raised to 220 °C. In a separate vial, 0.1 mmol of Cu(acac)2 (26 mg), 0.1 mmol of Ga(acac)3 (36 mg), 1 mL of DDT, and 1 g of HDA were loaded, capped with a rubber septa, purged with Ar gas for 15 min, and the mixture was kept ready as stock solution. Once the stock solution became clear, it was injected into the reaction flask at 220 °C and annealed at this temperature. The entire diffusion and growth process for the formation of CuGaS2 took approximately 15 min, but the reaction was annealed for an additional 30 min for better crystallization. Finally, the reaction was cooled down and the 5385
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Figure 3. (a) TEM, (b) inverted TEM, (c) and (d) HAADF-STEM, and (e−g) HRTEM images of CuGaS2 tadpole shaped nanostructures. (h) FFT pattern obtained from selected area of HRTEM image from panel (g). of CuGaS2 tadpoles. All other procedures were followed similar to tadpole nanostructures growth. Characterizations of Materials. The powder X-ray diffraction (XRD) study of the materials was carried out using a Bruker D8 Advance powder diffractometer with Cu Kα (λ = 1.54 Å) as the incident radiation source. Transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were recorded on a UHR FEG-TEM, JEOL JEM-2100F electron microscope using a 200 kV electron source, and also low-resolution TEM images were taken in a JEOL JEM-1400 plus using a 120 kV electron source. Microscopic study of the materials was carried out by drop-casting a drop of dilute nanocrystal solution in chloroform on a carbon-coated copper grid purchased from Ted Pella, and the grid was subsequently dried in air and stored in a desiccator.
2d-tadpoles - The Structural Analysis. Figure 2a−c (and Figure S1) shows wide area TEM images of obtained 2dtadpoles in different resolutions. Lengths of these tadpoles were 28 ± 5 nm, the width at the head part was tuned within 10−15 nm, and heights varied within 6−8 nm. Both self-assembled and individual tadpoles were seen in all the images, suggesting their monodispersity. Figure 2d−f presents HRTEM images of these 2d-tadpoles and a selected area FFT pattern from image (f) is depicted in Figure 2g. From the d-spacing of 0.33 nm and with 60° angles in between planes ((11̅0), (010), and (100)) suggested that these were in the wurtzite phase of CuGaS2 and viewed along the [001] direction. The wurtzite basal plane is symmetrical; however, the tadpoles were observed grown along in one of the three directions. For simplicity, the growth direction was marked along [100] throughout (Figure 2f). A different view of a 2d-tadpole is presented in Figure 2h, which is laterally aligned on the grid. The d-spacing of these planes was found to be 0.307 nm, which corresponded to the (002) plane of wurtzite CuGaS2. This confirmed that these tadpoles were not grown along the most polar and expected [001] direction; rather, the growth followed along one of the basal planes breaking the symmetry of the disk structure. In contrary, while the amine free reaction was monitored, the growth direction of the obtained tadpoles was observed different, and also both lengths and diameters were varied. Figure 3a,b shows the TEM and inverted TEM image of these tadpoles whose lengths were tuned within 35−40 nm, and width at their heads varied within 7−10 nm. Figure 3c,d presents HAADF-STEM images of these tadpoles in different resolutions. From these images, it was also noted that these nanostructures retained monodispersity like 2d-tadpoles. Figure 3e−g shows HRTEM images of these tadpoles, and Figure 3h presents the selected area FFT pattern from the HRTEM image. From analysis, it was observed that the growth direction of these tadpoles was along the polar [001] or “Z” direction, which was most expected for wurtzite phase nanostructures. Figure 4 shows the powder X-ray diffraction patterns of both 2d-tadpoles (upper panel) and tadpoles (bottom panel). Peak positions of both cases remained identical and matched with
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RESULTS AND DISCUSSION Tadpoles and 2d-tadpoles of CuGaS2. 2d-tadpoles of CuGaS2 were synthesized using acetylacetonate salts of both Cu(II) (which change to Cu(I) in reaction medium) and Ga(III) in the presence of dual sulfur precursors, alkylthiols and elemental S, in a mixture of alkylamine and 1-octadecene solvent or in pure alkylamine solvent. Elemental S was taken in the reaction flask along with the solvent/ligands, and cationic precursors mixed with alkylthiol were injected for the ternary nanostructures formation. After injection, the reaction mixture was further annealing for 30 min at 220 °C for obtaining highly monodisperse and very uniquely shaped 2d-tadpoles. A schematic presentation of the reaction protocol is shown in Figure 1a. However, if the same reaction was carried out in ODE without alkylamine (scheme in Figure 1a), highly monodisperse tadpole nanostructures of CuGaS2 were formed. Wide area representative TEM images (inverted) of typical 2dtadpoles and tadpoles are shown in Figure 1b,c, respectively. These were highly monodisperse, and such dispersity was optimized under specific reaction conditions (see the Experimental Section). However, among various reaction parameters, such as reaction medium, S source, and reaction temperature for these two reactions, were similar, but the only difference was the presence and absence of alkylamine, which lead to different shapes of nanostructures. 5386
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and tail with different materials. For identifying these two segments, HRTEM images were further analyzed. Selected area FFT patterns were obtained from the head part and from the entire structure, and then decoupled. Figure 5e,f presents the FFT patterns from the head (FFT 1) and entire nanostructure (FFT 2). The enlarge view of FFT 2 is shown in Figure S3, where the identified planes from different species are clearly distinguishable. Further identifying these two materials, HRTEM images were simulated. Figure 5g presents the simulated HRTEM image corresponds to the (110) plane of FFT 1 with a d-spacing of 0.311 nm, and this resembled Cu2S (110). Figure 5g was obtained from the simulation of (100), (010), and (11̅0) planes of FFT 2, whose d-spacing was 0.33 nm, and these resembled to respective planes of wurtzite CuGaS2. From these observations, the decoupled structures could be assigned as hexagonal Cu2S (Figure 5g) and wurtzite CuGaS2 (Figure 5h) for the head and tail parts, respectively. This clarified that, initially, Cu2S seed disks were formed, and then these were turned to 2d-tadpoles of CuGaS2. For further confirmation, a controlled reaction was performed by using purified Cu2S disks which, on Ga(III) treatment, also turned to 2d-tadpoles, but the dispersity and yield were never observed to be comparable to the in situ experiment (Figure S4). However, these images supported that the head part in the initial structure was Cu2S, which were the seeds for the tadpole formation. Beyond the fact that CuGaS2 2d-tadpoles were formed from a Cu2S disk, it was also observed that the diameter of the head part and also the length of the 2d-pole were higher in the final stage sample in comparison to the initial sample. This suggested that the 2d-tadpoles were grown not only along the either one of the three basal plane axes; rather, it confirmed that the nanostructures grew along all three directions, though at different rates. Accordingly, the growth mechanism is schematically presented in Figure 6. The HRTEM image of an intermediate sample showing high contrast at the center supported the fact that Cu2S was not entirely diffused along
Figure 4. Powder X-ray diffraction patterns for tadpoles and 2dtadpoles. For 2d-tadpoles (upper panel), arrows are shown for increasing and decreasing intensities of (100) and (101) planes, respectively, during growth. For tadpoles, the wurtzite Z-direction growth became prominent.
reported wurtzite phase of CuGaS2.58 For standard tadpoles which were obtained in amine free reactions, the (002) plane was seen as the highest intense peak, but in 2d-tadpoles, (100) became the intense one. These results supported with the growth directions derived from the HRTEM. Hence, it could be concluded here that, while the phase for both tadpoles remained identical even though their synthesis mediums were different, their growth directions were varied. The Formation Process. For understanding the formation pathways of these 2d-tadpoles, samples were collected from early stages of reactions and microscopically analyzed. Figure 5a shows the TEM, and Figure 5b−d presents HRTEM images of the nanostructures obtained after 2 min of injection of cationic precursors. Interestingly, from HRTEM images, lengths as well as diameters of these structures were observed shorter than the final 2d-tadpoles (Figure 2). Importantly, the contrast differences configured that these were heterostructures having head
Figure 5. (a) TEM image showing early stage (2 min) sample of 2d-tadpole formation. (b−d) HRTEM images of single nanostructure leading to formation of 2d-tadpoles. (e, f) Selected area FFT patterns obtained from area 1 and 2 of HRTEM image in (d), respectively. (g) Simulated HRTEM from the (110) plane of Cu2S of FFT 1. (h) Simulated HRTEM from (100), (010), and (11̅0) planes of CuGaS2 of FFT 2. 5387
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Figure 6. (a) HRTEM showing Cu2S retaining inside and shelled with CuGaS2. (b, c) Atomic models showing early stage formation of Cu2SCuGaS2 and the diffusion of central Cu2S. (d) The schematic model of transformation of Cu2S seed to CuGaS2 2d-tadpole via controlled growths along ⟨100⟩ directions. For modeling, Diamond software is used where wurtzite phase is defined in three coordinate system.
Figure 7. (a, b) TEM images in different resolutions. (c) HRTEM image of Zn incorporated flattened 2d-tadpole of CuGaS2. (d) Schematic presentation of a plausible growth pattern for the formation of these nanostructures.
lattice parameters, enabling the alloying of Zn with CuGaS2.61 Interestingly, with 10% Zn (with respect to total cations), the obtained 2d-tadpoles were flattened. The growth along [100] direction is reduced and instead relative growths along directions perpendicular to [010] and [11̅0] facets were enhanced. Figure 7a,b shows TEM images of Zn treated CuGaS2 2dtadpoles. 2D structures were reflected from both flat and sidewise fall of these nanostructures on the TEM grid. Magnified TEM image of a single 2d-tadpole is also presented in Figure 7c. The average diameter at the head part was observed ∼ 30 nm and length ∼ 35 nm. However, HAADFSTEM elemental mapping (Figure S5) indicated the presence of Zn and EDS showed only 4% of Zn (∼2% with respect of total cations) in these nanostructures (Figure S5). However, no separate nucleation of ZnS or any phase separation in these structures was noticed. Hence, these flattened 2d-tadpoles were treated as doped with Zn as Zn percentage remained low. It was assumed that, on Zn incorporation, the growth along [100] was restricted; rather, growths along two other symmetry planes were facilitated, turning the resultant structure flattened. However, with a greater amount of Zn, the ZnS was phase separated and L-shaped nanostructures were obtained (see Figure S6). Interestingly, these tadpoles had a significant asymmetric tail. As discussed by Scholes and co-workers,20 for such asymmetric growths before one layer was completed, the second layer was started growing and this continued until the diffusion ceased or the monomer concentration reduced to a critical level. This growth even accelerated when the length was increased and facilitated asymmetric secondary layer growths. Hence, toward
[100]; rather, the diffusion was confirmed in all three directions, though initially the elongation started along one direction. Figure 6b,c shows the atomic models for the diffusion process where it initially formed a coupled Cu2S-CuGaS2 structure and then turned to a core/shell type 2D structure before turning entirely to CuGaS2 2d-tadpole. Combining all of these steps together, the growth mechanism was proposed and is shown schematically in Figure 6c. The most striking observation was the limiting growth along [100] and further facilitating along [010] and [11̅0] directions, which were typically not observed in polar axis oriented asymmetric growth. This may be due to the nearly same facet energy of all of these planes, and also the polarity is low in comparison to (001). In addition, the low polarity along [100] did not exclusively drive the entire diffusion; rather, the system energy also helped triggering the growths along the other two symmetry directions. Tuning the 2d-tadpole Growth. From the above discussion, it was noted that the asymmetric growth of 2dtadpoles was due to controlled diffusions along three basal planes of seed disks. Further, to control the rate of growths along these facets, different reaction additives were introduced for surface passivation or change in surface energy. Typically, suitable ligands were used or other relevant reaction parameters were tuned for controlling the facet dependent growths. However, it was recently reported that metal ions also strongly influence the directional growth and change the shape of nanostructures.60 Hence, here, Zn was added as the additive dopants or alloying cations for observing the impact on 2D facet growths. The idea of introducing of Zn was due to the common phase formation under similar conditions and similar 5388
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Figure 8. TEM images of samples (a) collected at 2 min and (b) collected at 4 min. (c) HAADF-STEM image of the sample collected at 4 min during the formation of tadpoles in ODE medium. (d−f) HRTEM images of the nanostructures showing dual structures. (g) Schematic presentation of diffusion induced growth mechanism of CuGaS2 tadpole where the diameter of seed Cu2S retained at the head part of the tadpole during entire transformation.
Figure 9. (a) Wide area TEM image showing elongated ZnS-CuGaS2 tadpoles. (b) HAADF-STEM image of the same nanostructures. (c) Highresolution TEM image of ZnS-CuGaS2 tadpoles. (d) Selected area FFT patterns from area 1 and 2 marked in the HRTEM image. (e) Schematic presentation of the coupled structure. (f) Powder XRD pattern of these ZnS-CuGaS2 tadpoles. (g) Atomic model showing ZnS-CuGaS2 tadpoles viewed along [001] and [100] directions for upper and bottom panels, respectively.
the formation mechanism also established as diffusion controlled tapered growth.63 However, these were analyzed in detail for comparison with 2d-tadpole growths. The distinct observation noted was the overall diameter from the Cu2S seed to the head part of the tadpole, which retained the same during the transformation. This was an indication that the growth was fast as it was along the most polar [001] of the wurtzite phase and dominated for leading the tadpole structure. The proposed mechanism for such asymmetric growth keeping the head diameter of seed dots unchanged is shown schematically in Figure 8g. Tuning the Tadpole Growth. Similar to 2d-tadpole growth, Zn was also treated in the dot to tadpole conversion process for observing the impact on crystal diffusion on the asymmetric growth. However, as the growth along the polar [001] direction was faster (see the Experimental Section for reaction time) and more favorable, introduction of Zn could
the end, they attained an asymmetric nature. However, in 2dtadpoles without Zn, this was not prominent, but in the presence of Zn, as the length was shorter, the asymmetric tail became more prominent. The Tadpole Growths from Cu2S Dots. The nonpolar ODE medium reactions led to tadpoles of CuGaS2 which were observed to be originated from spherical Cu2S seed dots. TEM images obtained from successive samples collected from the reaction system are shown in Figure 8 (and also in Figure S7). The early sample showed only dots (Figure 8a), and next, these turned to dual structures (Figure 8b) which were clearly distinguishable from their contrast differences in the HAADFSTEM image in Figure 8c. The HRTEM images in Figure 8d−f also confirmed that these were coupled or dual nanostructures. Analysis confirmed that these were Cu2S-CuGaS2 and were intermediate in the formation of ternary CuGaS2 tadpoles. Similar reports were already investigated in the past19,62−65 and 5389
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Figure 10. (a) Schematic presentations of different reaction conditions for the formation of seeds and tadpole structures. The red part on the reaction process in each scheme represents the exclusive difference or the parameter which drives the resultant nanostructure. Columns with green background present the exclusive condition leading to highly monodisperse nanostructures. (b) Scheme showing the growth directions of 2d-tadpole and tadpole of wurtzite CuGaS2.
presence of elemental S, Ga(III) reacted immediately soon after the seeds formation, reducing the possibility of parallel Cu2S seed growths. In addition, the monodispersity was also obtained for Cu2S formation (Figure S9) in the presence of a dual S source, though post-treatment of Ga(III) did not retain the monodispersity. On the contrary, while the reaction temperature was increased to 280 °C, both 2d-tadpoles and tadpoles were formed even without the presence of elemental S (Figures S10 and S11). High temperature activates the reaction of Ga(III), and RSH remained the only S source. Under this condition, shapes were governed by the presence and absence of amine in the reaction mixture. However, monodispersity was compromised and nanostructures with wide size distributions were obtained. Hence, the optimized reaction conditions for obtaining highly monodisperse ternary tadpoles remained at 220 °C where elemental S was the active sulfur source for Ga(III). However, for Cu2S 2D platelets, RSH was the preferred S source in both reaction temperatures as these nanostructures were typically formed from Cu-thiolate complex decomposition (>180 °C).30,66,67 From the aspect of crystal growth in correlation with reaction chemistry, it was observed that only alkylamine in the reaction system triggered the 2D seed formation. Amines typically coordinate with cations on the wurtzite (001) facet and leave the anionic facet to grow. This leads to the formation of Cu2S disks, and this has already been reported investigated.39 Further growths to these disks only leave the option to grow along either or all of the equivalent facets (100), (010), and (110̅ ) which are parallel to the [001] direction. On the other hand, amine free systems led to dots where the polar (001) facet remained the dominant growth direction, leading to tadpole nanostructures.63 Schematic presentations for the contrasting directional growths for both shaped tadpoles is shown in Figure 10b. While ternary nanostructures are widely known for their efficient light harvesting abilities,43,57,68−72 these nanostructures were further explored for coupling with noble metals Au and Pt, which were known for boosting their photocatalytic activities.30 TEM images of different metal coupled nanostructures are shown in Figure S12 as possible exploration for their implementations in light harvesting. However, in this work, the chemistry of seeded disk induced 2d-tadpole formation,
not influence the resultant diameter of these tadpoles; rather, ZnS was grown and phase separated along the [001] direction. Tadpoles of binary−ternary heterostructures61 with more than 100 nm were the ultimate product. However, the dispersity here retained like previous 2d-tadpoles and tadpoles. Figure 9a,b presents the wide area TEM and HAADF-STEM images of these elongated coupled tadpole structures. HRTEM image shown in Figure 9c indicated the asymmetric segments, confirming the presence of phase separated materials. Selected area FFT pattern from two areas of HRTEM are presented in Figure 9d where one having a d-spacing of 0.307 nm corresponds to the (002) plane of ZnS and the other having a d-spacing of 0.313 represents the (002) plane of CuGaS2. A schematic presentation of the heterostructure is depicted in Figure 9e. The powder XRD pattern of the sample is shown in Figure 9f, which resembled the wurtzite phase of ZnS as well as CuGaS2. Combining both structures, the atomic models for the formation of ZnS-CuGaS2 viewed along [001] and [100] directions are shown in Figure 9g. Certainly, these structures are not exclusive ternary or Zn doped, but the resultant shape was elongated, which was started with seed Cu2S, followed by diffusion growth to CuGaS2 and then led to ZnS−CuGaS2 heterostructures. The dissimilarity here with 2d-tadpoles on Zn treatment was due to the fast growth, as stated above, that did not allow Zn to diffuse, but rather facilitated phase separation. This was observed for any amount of Zn treatment in the reaction mixture. Dual S Source and Formation of Highly Monodisperse Tadpoles. Key factors for exclusive formation of 2d-tadpoles and their monodispersity were the presence of alkylamine ligands and dual sulfidating agents (RSH and elemental S). Control reactions shown schematically in Figure 10 clarified that, at 220 °C, without elemental S, only Cu2S disks were formed (TEM image shown in Figure S8), and this did not depend on the presence or absence of Ga precursor. Similarly, in the absence of amine and in nonpolar solvent ODE, only dots of Cu2S were formed. Elemental S only triggered the reaction of Ga precursor forming CuGaS2 with either of the 2D or 0D seeds. Elemental S typically evolves H2S gas on heating with amine,66,67 and this remained the reactive S source to reaction with Ga(III) and facilitated the Cu diffusion from Cu2S seeds forming CuGaS2 nanostructures.63 This also helped in bringing monodispersity in the nanostructures as in the 5390
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their mechanistic growths, and comparisons with seeded dot induced standard tadpoles were mostly focused and the catalytic study of heterostructured materials left for future study.
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CONCLUSION In conclusion, the reaction chemistry triggering seeded diffusion controlled growths leading to tadpole shaped nanostructures of ternary CuGaS2 was investigated. The symmetry loss in 2D seeds of Cu2S on Ga(III) treatment was observed as a function of the solvent media and the polar ligands which suppressed the growth along [001] and facilitated predominantly one of the three planner directions. On the contrary, the seed with 0D Cu2S in the amine free system and nonpolar medium triggered the wurtzite polar [001] growth, leading to tadpoles. Just beyond the solvent media or ligands, treatment of the cation Zn also tuned the shape of both 2d- and tadpoles of CuGaS2. The foreign ions Zn restricted the growth of (100) and flattened the 2d-tadpoles. In contrary, the 0D seeded tadpoles which had faster growth led to heterostructures as both materials have identical d-spacing at the interface. Being multinary semiconductors are important for both lighting and photovoltaics, and also remained one of the few highly efficient greener materials, these findings on the fundamental understanding of 2D anisotropic growth would lead the materials further for their widespread applications.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01775. TEM and HAADF images for tadpoles, heterostructures with Au and Pt, mapping of Zn treated 2d-tadpoles (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Narayan Pradhan: 0000-0003-4646-8488 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS DST of India (project DST/SJF/CSA-01/557 2010-2011) is acknowledged for funding. S.D.A., A.D. acknowledge CSIR and G.P. (YSS/2015/001860/CS), P.S. to DST India for fellowship.
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