Letter pubs.acs.org/JPCL
Monodisperse SnS Nanocrystals: In Just 5 Seconds Biplab K. Patra, Suresh Sarkar, Amit K. Guria, and Narayan Pradhan* Department of Materials Science and Center for Advanced Materials, Indian Association for the Cultivation of Science, 2A & 2B Raja S C Mullick Road, Kolkata, India 700032 S Supporting Information *
ABSTRACT: As per the classical growth mechanism, tuning the reaction parameters in the growth stage remains pivotal to control the shape, size, dispersity, and size distribution of the colloidal nanocrystals, but what would be the case when the growth is very fast and the nanomaterials are formed instantaneously? Certainly, it needs a different chemical protocol. We investigate here one of such cases: the formation of different shapes of SnS nanostructures. With proper programming of chemical reaction, highly monodisperse αSnS nanocubes and nanotetrahedrons are obtained within 5 s of the reaction. Furthermore, tuning the density of nucleation, the size of the nanostructures is tuned in a wide window. These two shapes of SnS are also explored for the study of photocatalytic dye degradation, and the facet-dependent rate for this photocatalytic activity has been compared.
SECTION: Physical Processes in Nanomaterials and Nanostructures
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material are already reported, and several applications are also documented in the literature.28−30,33−36 However, we especially focus here on the monodispersity, size tunability, and shape architecture considering their fast formation process, where growth stage cannot be brought under control for facet directive growth. With proper programming of the chemical reaction protocol, and exploring the alkylphosphine and alkylacid ligands as selective facet binders along with alkylamine ligand, tunable monodisperse and narrow size distributed SnS cube and tetrahedron in their most stable orthorhombic phase are achieved within just a few seconds (2 to 5 s) of the reaction. Furthermore, tuning the density of nucleation, the size of these structures is varied in a wide window. To our knowledge, this is the fastest ever reaction to produce such highly monodisperse nanostructures reported to date. To understand this fast formation process, we have studied and discussed the details of the chemistry of the formation of these structures. Furthermore, both shapes of the nanostructures are explored for surface-adsorption induced dye degradation under visible-light irradiation, and the rates for different nanostructures are compared. Figure 1a−d and Figure S1−S3 in the Supporting Information show the TEM images of ∼22 nm size of nanocubes obtained after 5 s from a typical reaction. This has been obtained by taking Sn salt (SnCl2) in the reaction flask along with coordinating alkylamine solvent and calculated amount of alkylphosphine and injecting S precursor (thiourea) at 150 °C. The obtained nanocrystals are highly monodisperse
aintaining monodispersity and size/shape uniformity of nanocrystals in solution is the prime goal in the colloidal synthesis. These are mostly achieved by controlling the reaction kinetics and tuning the interface ligand chemistry.1−4 As per the classical mechanistic approach, one of the most successful protocols is the decoupling of growth from nucleation process, which restricts the formation of new nucleation and helps in maintaining the monodispersity/narrow size distribution of the nanocrystals. Following similar mechanism, several high-quality semiconductor nanocrystals have already been reported.5−15 In addition, monitoring the growth process is especially important for tuning the size as well as the morphology of the nanostructures,3,4,16−24 but for the case of nanomaterials with higher formation constant, which either form instantaneously or are obtained from highly reactive precursors where the reaction proceeds in a much faster rate, controlling the reaction parameters during the growth remains practically difficult. In such cases, there is even no opportunity to study the growth kinetics or the ligand-binding chemistry. Hence, the classical mechanism cannot be applicable here in controlling the monodispersity and/or shape architecture of the nanostructures. Again, as growth cannot be controlled, the tunability in the size of the nanostructures further casts questions. Hence, even though many advances in the colloidal synthetic chemistry have been achieved, these issues have not yet been addressed. Certainly it requires a different chemistry that needs to be explored. Among the fast forming nanomaterials, we focus here on the synthesis of Sn(II)S, which is an important earth-abundant material and is useful in several leading applications, mostly in photovoltaics,25,26 Li ion storage,27−30 and photocatalysis.31,32 No doubt, a large number of the shapes and sizes of this © 2013 American Chemical Society
Received: October 23, 2013 Accepted: November 2, 2013 Published: November 3, 2013 3929
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Figure 1. (a−d) TEM images of ∼22 and (e−h) ∼18 nm SnS nanocubes. All of these images are obtained from the samples collected after 5 s of injection of S precursor to Sn solution in the reaction system.
(Figure S5 in the Supporting Information). Without trialkylphosphine (TBP) or oleic acid (OA), irregular shapes of SnS are formed (Figure S6 in the Supporting Information). Here the driving force for obtaining the specific shape is the presence of phosphine or acid ligands. In addition, the mode of injection of the S precursor also determines the monodispersity. Because the nucleation and growth are very fast here, a swift injection is critically important to avoid the new nucleation. However, to understand the nanostructure formation in this fast process, we have collected the intermediate sample of the nanocubes within 2 s of the reaction. Interestingly, it has been observed from the TEM image (Figure S7a in the Supporting Information) that the cubes are already formed within this short time range, but in addition, several tetrahedron shapes nanostructures are also noticed, and this suggests that the cubes are, in fact, formed via tetrahedron intermediate. Unfortunately, we could not collect the sample before the formation of the tetrahedron intermediate. Even for the acid treatment case, we have observed that all of the tetrahedrons are formed within 2 s of the reaction. Hence the formation mechanism for the tetrahedron shapes in either phosphine or acid treatment could not be achieved; rather we have investigated the formation of cubes from the tetrahedron shape intermediate. However, to understand more about this formation, we have analyzed the crystal structures and the growth patterns of both cubes and tetrahedron nanostructures. Figure 4 shows the powdered XRD of the cube- and tetrahedron-shaped samples obtained in 5 s of the reaction. The peaks are mostly matched with the orthorhombic phase of αSnS (JCPDS 83-1758). This phase of SnS is thermodynamically stable, and it possess the space group of Cmcm(63) and has the atomic parameters a = 4.148, b = 11.48, and c = 4.177 Å. The sample collected at 2 s also shows a similar XRD pattern. But to further understand the atomic arrangements, we have analyzed the HRTEM image of the nanostructure. Figure 5a shows the
and show narrow size distribution. Figure 1e−h shows the TEM images and Figure S4 in the Supporting Information shows HAADF images of the nanocubes collected from another set of reactions, and these are of ∼18 nm size. The difference here is the Sn-to-S ratio, and in this case the amount of S remains higher. However, the samples collected at the different time intervals from both sets of the reactions suggest that the growth is ceased within 5 s of S precursor injection. No further growth of these cubes has been observed even after annealing the reaction for 5 min. Interestingly, on changing the capping ligands alkylphosphine to alkylacid in similar reaction, monodisperse tetrahedron-shaped nanostructures are obtained instead of cubes. Figure 2 shows the TEM images of the
Figure 2. TEM images of ∼23 nm SnS nanotetrahedrons.
tetrahedron-shaped SnS (also see Figure S4 in the Supporting Information) harvested within 5 s of the reaction. Details of the synthetic process and fast collection technique are provided in the Supporting Information. To our knowledge, this is a rare case of semiconductor growth that completes very quickly and produces highly monodisperse nanostructures. Further varying the Sn to S ratio, nanocubes of ∼35, ∼65, and ∼80 nm are also obtained. (See Table 1 in the Supporting Information.) Figure 3a−c shows the TEM images of different sizes of nanocubes obtained in 5 s of the reaction. Tilting experiment has been carried out to confirm that these nanostructures are cubes rather than square platelets 3930
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Figure 3. (a−c) TEM images of ∼35, ∼65, and ∼80 nm SnS nanocubes obtained from different Sn to S ratio. (a) Cartoons of cubes are inserted showing their different orientations.
Figure 4. (a) Powdered X-ray diffraction pattern of ∼18 nm size SnS nanocubes and (b) ∼23 nm SnS tetrahedrons. XRD peak positions of both are overlapping, although the intensity of some peaks varies.
Figure 5. (a) HRTEM image of a single nanocube. (b) Selected area (marked in a) FFT pattern. (c) Enlarged HRTEM image showing (101), (040), and (131) planes and (d) the simulated HRTEM from the FFT pattern of panel b corresponding to (101) plane. (e) Models showing tetrahedron, formation of cube from tetrahedron, and cube shape.
HRTEM image of a single cube, and Figure 5b presents the selected area FFT of the same cube. The observed d-spacing of 0.291, 0.285, and 0.231 nm corresponds to (101), (040), and (131) planes of α-SnS, respectively. The zone axis here is [1̅01]. The (101) plane is further viewed from the enlarged HRTEM image presented in Figure 5c, where two Sn atoms remain up and two are in down planes. Similar observation has also been noticed in the simulated HRTEM in Figure 5d. All of these data support the fact that the cubes are of orthorhombic phase of SnS. Similarly, Figure S7b,c in the Supporting
Information shows the HRTEM image of the intermediate tetrahedron and the selected area FFT pattern, respectively. From the FFT, the d-spacing is observed to be 0.28 nm, which corresponds to the (130) plane of the tetrahedron. Figure S7d in Supporting Information shows the simulated HRTEM of the same plane. Similarly, the HRTEM image of the tetrahedron SnS obtained with acid treatment shows the (130) planes. (See the details in Figure S8 in the Supporting Information.) This further suggests that these tetrahedrons obtained separately 3931
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Figure 6. (a) Successive absorption spectra of methylene blue obtained during irradiation. These spectra are obtained successively at 0, 15, 30, and 60 min. (b) Change in the absorbance of the dye at 660 nm versus the irradiation time using the cubes and tetragon shapes of SnS. OD of SnS stock solution remains same in both cases.
hence they restrict the shape at the tetrahedron stage. However, even though the phosphine/acid helps in obtaining the cube/ tetrahedron shapes, the dispersity and tunability are mostly governed by the mode of nucleation. As the rate of growth does not keep a major value here, the stage of nucleation remains important, and the decoupling of the growth is absolutely not required. We have further explored both tetrahedron and cube shape structures as visible-light photocatalysts for the degradation of organic dye methylene blue. The photocatalytic activity of different nanomaterials for catalyzing organic reactions is widely known.42−44We study here the facet-dependent adsorption of the organic dye and its rate of degradation with different shapes of SnS nanostructures. This has been performed for the degradation of the methylene blue dye under Xe light irradiation. (See the Supporting Information.) It has been observed that tetrahedron SnS with exposed {130} facets serves as a better catalyst than the four {101} and two {040} facets of the cubes. Figure 6a shows the successive decrease in the absorption of methylene blue on irradiation using tetrahedron SnS nanocrystals. Figure 6b presents the change in absorption of methylene blue with irradiation time for both SnS tetrahedron and cube nanostructures. The results suggest that both shapes can catalyze the degradation of methylene blue but tetrahedron shapes are a better catalyst for this case. The difference in the rate is expected due to the change in surface energy that varies the dye adsorption and so also affects the carrier transformation for the degradation. In conclusion, we report here the nucleation-densitycontrolled formation of highly monodisperse and size/shapetunable SnS nanostructures with narrow size distribution in a fast chemical reaction. The protocol does not require us to monitor the growth process or to tune the parameters for size focusing or restricting the thermal ripening process as required in traditional classical growth process. The synthetic chemistry presented here provides important information to the community, suggesting that the monodispersity and narrow size distribution of the nanostructure can be obtained even if it is difficult to decouple the growth from the nucleation. This is highly important for the fundamental aspects of understanding the formation of nanocrystals in solution. Furthermore, different shapes of SnS nanostructures are explored for the photocatalytic dye degradation, and it has been observed that {130} facets are more favorable for dye adsorption and catalyzing the degradation.
either with acid treatment or as the intermediate of the cubes, have the same phase and d-spacing. Next, we have investigated the formation mechanism of the cube-shaped nanostructures from the tetrahedron intermediate. It is known that most of the semiconductor nanocubes under similar dimension are normally obtained due to selective facet growth of the polygon-shaped nanostructures,31,37−41but the case here follows the facets growth of tetrahedron structures. Tetrahedron has four facets, (130), (1̅30), (03̅1), and (03̅ 1̅) (Figure S9 in Supporting Information). Each of these facets has alternative arrangement of Sn and S atoms (Figure S10a in Supporting Information). Hence, these are more reactive facets that further grow until all of these facets vanish and new lowenergy six facets (101), (1̅01̅), (101̅), (1̅01), (04̅0), and (040) of the cubes are created (Figure S11 in Supporting Information). These facets contain both Sn and S atoms (See Figure S10b in Supporting Information for 101 plane) and are expected to be less reactive. This growth process remains here very fast until the stable cubic structure is formed. A typical model of such growth has been shown in Figure 5e. This type of growth is already established for pencil- or bullet-shaped nanostructures,37−41 but here we have observed it for cube. Furthermore, we discuss the size tunability of these cube shaped nanostructures and their unique size dispersity. It has been observed that changing the ratio of S to Sn, the density of nucleation is changed and so also is the size of the nanostructures. With fixed Sn concentration when S concentration is increased, a greater number of SnS nuclei is formed, and they consume more monomers. In this case, the size of the nanostructures remains smaller. Reduction of this S to Sn ratio reduces the number of nuclei and hence the nanostructures grow larger. Hence, while in the classical growth mechanism, the time-dependent growth mostly tunes the size, but here the density of nucleation determines the same, and growth stage keeps minimum importance on size tuning. It has been further observed that alkylphosphine or alkylacid plays crucial role for the evolution of the cube shaped nanostructures. TBP is the weaker ligand and thus cannot restrict the growth of reactive {130} facets of the intermediate tetrahedron, but once the cubes are formed, new low-energy facets are created, and in this stage the alkylphosphine ligands restrict further growth to faceted shapes. However, in pure fatty amine solvent, where amines are relatively weaker binders (than TBP), the six facets of the cube also grow further leading to irregular shape (Figure S6 in the Supporting Information). But acid ligands are stronger than phosphine or amine ligands, and 3932
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ASSOCIATED CONTENT
S Supporting Information *
Methods, instrumentation, and supporting figures. This material is available free of charge via the Internet at http://pubs. acs.org.
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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 is acknowledged for funding (DST/SJF/CSA-01/ 2010-2011). B.K.P. and S.S. acknowledge CSIR, and A.K.G. and N.P. acknowledge SPMF CSIR and DST swarnajayanti for fellowships.
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