Diffusion-Induced Shape Evolution in Multinary Semiconductor

Letter pubs.acs.org/JPCL

Diffusion-Induced Shape Evolution in Multinary Semiconductor Nanostructures Gyanaranjan Prusty, Amit K. Guria, Biplab K. Patra, and Narayan Pradhan* Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata, India 700032 S Supporting Information *

ABSTRACT: The classical mechanism of crystal growth for architecting different nanomaterials in solution, although widely studied, is mainly restricted to binary semiconductor systems. However, this method is not applicable to multinary nanomaterials, which have multivalent cations possessing different reactivity under identical reaction conditions. Hence, the shape architectures of these nanostructures, which require a more sophisticated approach, remain relatively unexplored compared to those of binary semiconductors. Owing to the importance of the multinary materials, which are emerging as excellent green materials for both light harvesting and light emission, we investigated the diffusion-rate-controlled formation of ternary AgGaSe2 nanostructures and studied their heterostructures with noble metals. Controlling the changes in the rate of diffusion of the Ag ions resulted in the formation of tadpoleshaped AgGaSe2 ternary nanostructures. In situ study by collecting a sequential collection of samples has been carried out, and the conversion of amorphous Ga-selenide to crystalline AgGaSe2 has been monitored. In addition, heterostructures of tadpole AgGaSe2 with noble metals, Au and Pt, were designed, and their photocatalytic behaviors were studied.

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chemistry, are coming up, which creates strong motivation in investigating the deeper synthetic mechanism that gives rise to the nanoheterostructures. However, only a few such heterostructures have been reported to date,32−39 and more needs to be fabricated with a variety of multinary systems to find out the new associated materials properties. We follow here a smart approach of a sacrificial-seedmediated, diffusion-controlled, tadpole-like shape evolution protocol for ternary AgGaSe2 nanostructures where changes in the rate of diffusion of the seed cations determine the shape of the ternary material. Details of the formation process have been monitored in situ to find the route of diffusion of the active Ag(I) ions from seed Ag2Se to amorphous Ga-selenide nanostructures, and the impact of a successive decrease of the rate of diffusion of Ag ions on the shape evolution has also been established. Further, heterostructures with Pt and Au are designed via in situ reduction of metals on the surface of the nanotadpole, and the contrasting difference in their catalytic activity toward degradation of organic dye has been studied and reported. A tadpole shape has a wide head and a skeletal diameter, which gradually tapers toward the tail. This type of structure typically forms via successive changes in the growth rate during the reaction along the fastest growth direction and is typically controlled by reducing the effective monomer concentration in

ultinary semiconductor nanostructures composed of multivalent cations are emerging as a new class of electronic material for both light-absorbing as well as lightemitting applications. Recent development suggests that these are also superseding the Cd- and Pb-based semiconductor quantum dots both in photovoltaic and light-emitting device performances.1−9 However, despite decades of investigations in understanding the growth mechanism of semiconductor nanostructures, the formation protocols of these multinary systems are not well known. The well-studied classical mechanism of nucleation followed by growth is mostly restricted to binary semiconductors but not appropriate to multinary structures.10−21 The major hurdle in building such nanostructures stems from the reactivity differences of these multivalent cations. Hence, like the binary systems, the classical approach of homogeneous nucleation and successive growth is failed here. To date, the only protocol developed for the fabrication of ternary systems is the diffusion-controlled mechanism where the secondary, but compatible, metal ions are controllably inserted into the lattice of preformed binary seed nanocrystals.1,3,4,6−9,22−31 In such cases, the nanocrystals typically retain the shape and phase of the seed crystals.3 Hence, a generic mechanism for these nanostructures has yet to be established and must be investigated further. In addition, nanoheterostructures, which constitute these multinary semiconductors and noble metals, are also not well known.32−39 These materials are emerging as a new class of greener photocatalytic materials, which can rapidly separate the photogenerated charge carriers.36−39 Recent literature reports reveal that new materials in this category, designed with new © XXXX American Chemical Society

Received: May 26, 2015 Accepted: June 9, 2015

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DOI: 10.1021/acs.jpclett.5b01091 J. Phys. Chem. Lett. 2015, 6, 2421−2426

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Figure 1. (a−d) TEM and HRTEM images of tadpole-shaped AgGaSe2 nanostructures. (e,f) HAADF-STEM images of the tadpoles and (g−i) elemental mapping of Se, Ga, and Ag, respectively.

the reaction system.40 However, such a mechanism is confined to binary systems, and even if such growths are reported for some ternary systems, their origin has not been illustrated.41,42 Here, ternary AgGaSe2 tadpoles are designed by introducing amorphous structures of Ga-selenide to the preformed seeds of crystalline Ag2Se. During annealing, the ternary AgGaSe2 semiconductor, which nucleates on a particular facet of Ag2Se, continues to grow until the tadpole shape is obtained. However, because of the chemical transformation of Ag(0) to Ag2Se proceeds faster, we used the Ag(0) particles as the source and then injected the Se precursor and Ga(III) salt successively into the reaction flask. Figure 1a−f shows the transmission electron micrscopy (TEM), high-resolution transmission electron micrscopy (HRTEM), and high-angle annular dark field scanning transmission electron microscopy (HAADFSTEM) images of the 98.41 ± 10 nm (length) × 34.85 ± 7 nm (head diameter) tadpole-shaped nanostructures obtained from a typical synthesis procedure. The elemental mapping results (Figure 1g−i) of the region shown in the HAADF image reveal that the materials are composed of homogeneously distributed Ag, Ga, and Se ions; the homogeneous distribution of Ag is especially important. These results also indicate that the materials are indeed AgGaSe2 and not a mixture of a complex structure or a mixture containing Ag2Se. Energy-dispersive Xray spectroscopy (EDAX) from different samples are obtained from various regions of the TEM grids support the similar composition ratio (see the Supporting Information, Figure S1). Figure 2a reveals a close correspondence of the X-ray diffraction (XRD) patterns of the nanostructured AgGaSe2, and the peaks resemble the tetragonal phase of the bulk AgGaSe2. However, some additional peaks correspond to a small amount of cubic phase of AgGaSe2, and the orthorhombic phase of Ag2Se is also seen in the XRD, which have insignificant intensities and also are not reproducibly observed in repeated reactions. The fast Fourier transform (FFT) pattern (Figure 2c), which is obtained from the HRTEM image shown in Figure 2b, also concurs with that of the tetragonal phase of AgGaSe2. The (020) plane and two of the planes belonging to the {112} family, obtained from the selected area, suggest that the tadpole is viewed along the [021] direction of the

Figure 2. (a) Powder XRD pattern of AgGaSe2 nanotadpoles, (b) HRTEM image of a part of the tadpole, and (c) selected area FFT pattern.

tetragonal phase. Moreover, the HRTEM image indicates that the tadpole grew along the [112] direction, as evidenced by the intense (112) peak in the XRD pattern. These results are indicative of the single-crystalline nature of the nanostructured ternary AgGaSe2 material. The tadpole shapes of the ternary nanostructures exhibited unidirectional growth. We determined whether this corresponded to the conventional 1D growth mechanism, where alternative charged species were adsorbed onto the surfaces along one direction and reacted. For this determination, samples were collected from the reaction system periodically and analyzed in the microscope. At first, the Ag(0) particles were formed (Figure S2a, Supporting Information) upon 2422

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Figure 3. (a,b) TEM images of the intermediate sample at two different magnifications. The inset in the right panel of (b) provides a magnified view of tadpole formation of the Ag2Se−AgGaSe2 nanostructure. (c) Schematic model of AgGaSe2 tadpole formation from the Ag2Se sacrificial seed.

Figure 4. Atomic models of (a) Ag2Se, (b) AgGaSe2, and (c) amorphous Ga-selenide. The diffusion of Ag ions from Ag2Se to amorphous Gaselenide via AgGaSe2 is indicated by the arrows.

heating AgCl in alkylamine,43 and then, those were selenised to Ag2Se binary semiconductor nanostructures (Figure S2b, Supporting Information). Next, the presence of noncrystalline Ga-selenide initiated the diffusion of Ag(I) ions from Ag2Se. Early stage samples, which were collected after 15 min of Ga(III) introduction, contained a mixture of Ag2Se spherical particles, nucleated tadpoles, and some noncrystalline materials (Figure 3a and b). This observation indicated that the tadpoles grew via nonconventional 1D growth, which eventually resulted in tadpoles of AgGaSe2. Furthermore, the homogeneous distribution of Ag ions, as revealed by the elemental mapping, suggested that the Ag2Se particles were sacrificed during the formation of the ternary tadpoles. A schematic model showing the formation of AgGaSe2 from the sacrificial seed Ag2Se is shown in Figure 3c. Moreover, Ga-selenide remained amorphous (see the XRD in Figure S3, Supporting Information) while, for the same reaction conditions, a control reaction was performed without silver. This indicated the exclusive role of Ag in introducing Ga(III) ions into the lattice and forming the ternary structures. However, as the reaction progressed, the accompanying gradual shape change resulted in the diffusion of Ag ions from Ag2Se, which in turn transformed amorphous Ga-selenide to crystalline ternary AgGaSe2. Typically, diffusion of the secondary cations into the lattice of the seed crystals is more favorable3,44,45 (than diffusion from Ag2Se), and this diffusion mostly retains the original shape of the seed nanocrystals. However, in this work, the spherical dots of Ag2Se changed to tadpole ternary semiconductors, where Ag ions were diffused out from seed Ag2Se. During continuous annealing, increasing numbers of Ag ions penetrated the already formed AgGaSe2 and diffused out toward the outer amorphous Ga-selenide, which was then converted to crystalline Ga-selenide owing to

this diffusion. A schematic model showing Ag ion diffusion from the Ag2Se seed into Ga-selenide via AgGaSe2 is shown in Figure 4. Further, it is well established that above 140 °C, the phase of Ag2Se changes from orthorhombic to cubic,46 in which cation vacancies are present in the lattices, and it behaves like a superionic conductor.43,47 Therefore, Ga(III) may be inserted into the Ag2Se lattice, thereby facilitating outward growth. However, the shape of the tadpole eliminated this possibility. In the case of superionic conductor catalysts, the expected solution−solid−solid type growth should lead to nanorods or nanowires that retain constant width during growth.43,47 Such morphologies, however, were not observed in this work. Core/ shell type growth was also not obeyed as the ternary AgGaSe2 did not nucleate in multiple facets of the seed Ag2Se instead of on one particular facet. Upon nucleation of AgGaSe2, the diffusion of Ag ions continued until all of the facets were consumed, resulting in nanostructures composed solely of the ternary structure. The tadpole shape, which has a wide head and a narrow tail, of the ternary AgGaSe2 nanostructure stems from the rate of diffusion of the Ag ions. As mentioned previously, Ag ions diffused from Ag2Se to Ga(III)-selenide via ternary AgGaSe2, and owing to the high amount of Ag available initially, the corresponding rate of diffusion was expected to be higher than that of other stages. However, increasing amounts of Ag ions were consumed as the reaction progressed, and hence, the rate of diffusion decreased gradually. This in turn resulted in a reduced rate of formation of AgGaSe2, thereby changing the shape to that of a tadpole. An atomic model showing the diffusion-rate-controlled change in the length of the ternary structure and the corresponding changes in the diffusion rate of Ag ions into the Ga-selenide amorphous structures are shown 2423

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AgGaSe2 and Pt−AgGaSe2 are shown in Figure S5 (Supporting Information). Post-synthesis deposition of either Pt or Au resulted in nonepitaxial connections of these elements to the surface of the AgGaSe2 tadpoles. Because Se2− and alkylamines are both reducing agents, Au and Pt ions are in situ reduced to Au(0) or Pt(0) on the ternary nanocrystal surfaces. These metaldecorated nanostructures, although well established for SnS,48 PbS,49 and Cu2ZnSnS4 (CZTS),32,35 represent here the firstever reductions performed for ternary AgGaSe2. We investigated the photocatalytic properties of these nanostructures further and found that, under 365 nm irradiation (6 mW), the well-studied methylene blue (MB) dye was degraded by Pt− AgGaSe2 only. AgGaSe2 and, interestingly, Au−AgGaSe2 also could not degrade the MB under similar reaction conditions. Figure 7 shows the kinetics of the reduction with AgGaSe2, Au−AgGaSe2, and Pt−AgGaSe2 (see Figure S6, Supporting Information). The control reaction in the dark suggested that the degradation ceased even for Pt−AgGaSe2, and this confirmed the photocatalytic activity of the heterostructures where AgGaSe2 acts as a photosensor. The degradation is feasible because electron transfer from AgGaSe2 is facilitated by the noble metal Pt. This has been widely discussed in several recent reports.34,48,50,51 However, the contrasting result for the inactivity of Au−AgGaSe2 can be assumed here due to the less metallic nature of small size Au particles, which could not efficiently transfer the photogenerated electron. The absorption of the Au−AgGaSe2 nanostructures does not show the characteristics of the surface plasmon or its shifting in the heterostructures (Figure S7, Supporting Information). This further suggests that the carrier transportation is hindered here in and as a result, the dye degradation is drastically slowed down. This plausible mechanism is supported by the band positions of bulk AgGaSe2 and Au cluster sp states (Figure S8, Supporting Information).48,52,53 Similar behavior has also been observed for other nanoheterostructures where the size of Au remains less than 3 nm, though the heterostructures having more than 5 nm Au again catalyzed the reaction.48,54 In this case, we just provide here the comparative study with Au and Pt where one works and the other does not. However, details of the product isolations remained beyond this work as such degradations are widely reported in the literature for a huge number of other materials. Experimental results for the dye degradation are provided in the Supporting Information. In conclusion, we described the growth mechanism of sacrificial-seed-induced formation of a ternary AgGaSe2 semiconductor where changes in the rate of the diffusion of Ag ions during the reaction determine the shape of the nanostructures of the material. Further, these nanostructures are coupled with noble metal Pt as well as Au. For the small size metal particles, it is further established that Pt-coupled heterostructures act as superior photocatalysts and facilitate the organic dye degradation where small size Au particles that behave more semiconductor-like cannot perform the catalysis. These nanostructures are brand new, and the mechanism proposed in this work should improve the fundamental understanding of developing different shapes of ternary materials and their heterostructures, which were lagging behind those of binary semiconductor nanomaterials. We believe that these findings will elucidate the growth mechanism of shape architectures of multinary nanomaterials, which have ideal band gaps for absorbing solar light, and may be promising candidates for use in various photocatalytic applications.

schematically in Figure 5a and b, respectively. The seed Ag2Se was completely dissolved, and hence, the length of the tadpole

Figure 5. (a) Schematic atomic model of the change in the rate of diffusion of Ag ions from Ag2Se to Ga-selenide for the formation of tadpole-shaped AgGaSe2. (b) Schematic of the successively reduced rate of Ag ion (arrows) diffusion into the Ga-selenide nanostructures.

could be considered as being directly proportional to the Ag ions present in the sacrificial seeds. Further, keeping eyes on new metal−ternary heterostructures, we designed here Pt−AgGaSe2 and Au−AgGaSe2 nanostructures via in situ reduction of metal ions on the surface of the multinary semiconductors. This method is considered a moderate and even efficient process of designing surface-decorated metal−semiconductor heterostructures.32,48 In the case of Au, the biphasic synthetic protocols at room temperature were followed, where the nanocrystals were immersed in an organic solvent and Au(III) ions in aqueous phase. Stirring, without the addition of any external reducing agents, resulted in the attachment of ∼3 nm of Au particles to the surface of the AgGaSe2 tadpoles. However, the hightemperature (>100 °C) reaction resulted in the formation of individual Au particles, without accompanying heterostructures (Figure S4, Supporting Information). On the other hand, for coupling with Pt, the reaction was performed at 200 °C because the Pt precursor, Pt(acac)2, decomposes only at relatively high temperatures. Figure 6 shows the TEM images of the Au- and Pt-coupled AgGaSe2 nanostructures. The noble metal particles decorated the surfaces of the tadpoles, whose dimensions were retained even with this decoration. The XRD patterns of Au−

Figure 6. TEM images of (a,b) Pt−AgGaSe2 and (c) Au−AgGaSe2 tadpole-shaped heteronanostructures obtained at two different resolutions. 2424

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Figure 7. (a,b) Schematics of degradation of MB under irradiation using different heterostructures. (c) Change in the absorbance (OD) during irradiation for AgGaSe2 and Au- and Pt-coupled AgGaSe2 nanotadpoles.

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ASSOCIATED CONTENT

* Supporting Information S

Additional figures and tables supporting the results, including materials used, synthesis details, characterization, EDAX data, UV−vis absoption spectra, powder XRD patterns, TEM images, and a schematic presentation of band positions, are provided. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01091.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS DST of India (Project DST/SJF/CSA-01/557 2010-2011) is acknowledged for funding. REFERENCES

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DOI: 10.1021/acs.jpclett.5b01091 J. Phys. Chem. Lett. 2015, 6, 2421−2426