Letter pubs.acs.org/JPCL
Subnanometer Thin β‑Indium Sulfide Nanosheets Shinjita Acharya, Suresh Sarkar, and Narayan Pradhan* Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata, 700032 India S Supporting Information *
ABSTRACT: Nanosheets are a peculiar kind of nanomaterials that are grown twodimensionally over a micrometer in length and a few nanometers in thickness. Wide varieties of inorganic semiconductor nanosheets are already reported, but controlling the crystal growth and tuning their thickness within few atomic layers have not been yet explored. We investigate here the parameters that determine the thickness and the formation mechanism of subnanometer thin (two atomic layers) cubic indium sulfide (In2S3) nanosheets. Using appropriate reaction condition, the growth kinetics is monitored by controlling the decomposition rate of the single source precursor of In2S3 as a function of nucleation temperature. The variation in the thickness of the nanosheets along the polar [111] direction has been correlated with the rate of evolved H2S gas, which in turn depends on the rate of the precursor decomposition. In addition, it has been observed that the thickness of the In2S3 nanosheets is related to the nucleation temperature. SECTION: Physical Processes in Nanomaterials and Nanostructures We select here β-indium sulfide (β-In2S3) as the ideal candidate for the manifestation of the 2D growth among different inorganic nanomaterials. β-In2S3 is an n-type semiconductor with a band gap of 2.0−2.3 eV, which is the stable form at room temperature.26 The valence band resides at ∼6.3 eV, and the conduction band resides at ∼3.8 eV with respect to the vacuum level.26 It is well documented in the literature that its stable crystal structure predominantly allows the directional growth.27−31 Several reports on the fabrication of disks, sheets, rods, tubes of β-In2S3 in colloidal synthetic approach already exist in the literature.26−28,32−35 This is also proven as an efficient workhorse for several leading applications in recent days.29,30,36−39 Exploring the thermal decomposition of the single source precursor containing In and S at different reaction temperatures, the crystal growth along the thickness [111] of the 2D grown cubic β-In2S3 micrometer long nanosheets is monitored. This has been performed by controlling the decomposition rate of the single source precursor and monitoring the formation kinetics of these sheets. The precursor decomposition rate has been verified by measuring the rate of evolved H2S gas as side product with the progress of the reaction. Controlling the decomposition rate at different nucleation temperatures, the thickness of the nanosheets is varied. Finally, manipulating both kinetics and thermodynamic factors, β-In2S3 sheets with thickness corresponding to two atomic layers are synthesized. To our knowledge, this is one of the thinnest sheets ever reported among the inorganic semiconductor nanomaterials,
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anosheets are the unique material having dimensions in both nano- as well as in microscale.1−6 Two-dimensionally (2D) grown nanosheets are specifically important owing to their wide surface area essentially required for various potential applications.7−13 Nanosheets reflect the peculiar features in terms of both extremely high 2D anisotropy and molecular thickness. There have been significant research efforts to synthesize 2D nanosheets of various materials, including metals14−16 oxides,17−19 and chalcogenides.5,6,20,21 Unfortunately, the formation chemistry of these nanosheets in solution has not yet been widely explored. The crucial aspect of the formation of these 2D materials is the difference in the growth rate along the planar 2D direction and that of the third direction (thickness). The control over this direction growth that governs the thickness of the sheets has rarely been studied. However, the characteristic features of the nanosheets rely on their edge dimension and thickness; the controlling factors for which needs to be investigated. It is well-known that the high-temperature nucleation and growth of different nanostructures in solution are mostly driven by the thermodynamically and kinetically controlled parameters.22−25 In general, these processes are decoupled to achieve better control of the size and shape of the nanomaterials. A balance between these two parameters is essential to obtain the desired nanostructures. The nanosheets are unique materials where rapid growth along two planar directions is preferred, whereas the growth along the third direction (widely known as the polar direction) is restricted. Here, in this Letter, we focus on this controlled growth along this third direction considering both thermodynamic and kinetic parameters and obtain subnanometer thin nanosheets. © 2012 American Chemical Society
Received: November 5, 2012 Accepted: December 5, 2012 Published: December 5, 2012 3812
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Figure 1. (a,b) Plots of the amount of CuS formation per unit time versus reaction time at different reaction temperatures. CuS amount is directly proportional to the evolved H2S gas. Detailed procedure has been provided in the Supporting Information. Inset shows the cartoon models of heavily wrinkled nanosheets formed at two temperatures. (c) HAADF-STEM image of the sheet obtained at 320 °C, (d) HAADF-STEM image of the sheet obtained at 280 °C, and (e) HAADF-STEM image of the sheet obtained by slow heating protocol showing no flexibility and shows moiré patterns.
Figure 2. (a−d) TEM images showing the intrasheet folding and unfolding (opening) prepared at 320 °C. (a) A larger area view, (b) a typical unfolding image of a sheet. Image (b) is the enlarged area of the square marked portion in image (a). (c,d) Various images of the folded and opened area of sheets at different resolutions. (e−f) TEM images of In2S3 free floating sheets in different orientations prepared at 280 °C.
(at 100 °C or below) and swiftly injected to hot fatty amine solvent in another reaction vessel at different reaction temperatures ranging from 280 to 320 °C . On annealing, single-crystalline, thin free-floating sheets with different thicknesses are obtained within minutes (1−2 min of precursor
and details of its characterization and formation mechanism in solution have been analyzed. For the synthesis, the diethyldithiocarbamate salt of indium (In-DDTC) has been used as a single source precursor. Initially, a clear solution of In-DDTC in fatty amine is prepared 3813
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injection). Typical synthesis process for these nanosheets is schematically presented in the Supporting Information (Figure S1). The thermal decomposition temperature of In-DDTC is 265 °C.31 It is established that in the presence of alkyl amine, this precursor decomposes catalytically below this decomposition temperature. However, when the injection is carried out above this temperature, the rate of decomposition enhances, and rapid nucleation/growth of In2S3 nanostructures is observed. During the process, H2S gas continuously evolves out as a byproduct on decomposition of this metal−dialkyldithiocarbamate singlesource precursor.40 This helps us to measure the active monomers present in the solution during the progress of the reaction. The evolved H2S gas is periodically purged into Cu (II) solutions, and the amount of precipitated copper as copper sulfide has been measured using inductively coupled plasma (ICP). Figure 1 shows the rate of formation of H2S gas with reaction time for three sets of reactions carried out at three different reaction temperatures. For the high-temperature system (320 °C), it has been observed that the decomposition is completed within 1−2 min of the precursor injection, but it continues up to 4 min in the later case for reaction carried out at 280 °C. However, in a different scenario in Figure 1b where the precursor is loaded in fatty amine solvent and slowly heated to 280 °C, the complete decomposition extends up to 15 min. The morphologies of the nanosheets have been revealed by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM) images. Representative HAADF-STEM images of the samples collected at these three temperatures are shown in Figure 1c−e. It has been observed that in the first scenario (Figure 1c), where the synthesis is performed at 320 °C, wrinkled and folded sheets are obtained, more of such images are shown in Figure S2. However, for the second case, as shown in Figure1d (reaction at 280 °C), bent sheets are obtained, whereas for the slow temperature rise in one-pot reaction conditions (from room temperature to 280 °C), flat sheets are the final products in Figure 1e and the TEM image is shown in Figure S3. From the significant differences in the flexibility of these nanosheets, it can be assumed here that these sheets differ in their thickness. To understand more about the sheets, TEM images obtained from different sets of reactions are further analyzed. Figure 2a−d shows the TEM images of the sheets synthesized at 320 °C. These sheets are highly flexible manifested by their wrinkling and folding on the TEM grid. The image in Figure 2a (and 2b) clearly shows the folding and unfolding observed from a single layer sheet. However, the TEM images obtained at 280 °C (Figure 2e,f) do not show similar multilayer folding and opening, and are rather observed to be largely bent in nature. Both of these sheets are expected to be thinner in comparison to the flat sheets obtained in the slow heating reaction condition. These sheets (thickness >2 nm) with no flexibility are already reported.31 Further, to know the width of the flexible sheets, highresolution TEM (HRTEM) images of the multifolded sheets are analyzed. A typical such image has been shown in Figure 3a. From the enlarged view (inset in Figure 3a) it has been observed that each fold is ∼1.12 nm. To confirm this, intensity profiles of these folds are plotted, which are also found to be ∼1.12 nm (Figure 3b,c). Assuming that each fold obtained with 180 degree bending (simple folding of sheets) the thickness is
Figure 3. (a) TEM image showing folding of the nanosheet. Inset is an enlarged area (red square inside) with separated dashed bars. (b) HRTEM of the folded sheets. The d-spacing matches with (111) planes. (c) Line intensity scan of the square marked area in panel (b).
expected to be half this fold width, which is ∼0.6 nm. Similar thickness determination from the folding of the nanosheets observed in HRTEM images is already reported in the literature.41 However, for the sheets obtained at 280 °C, we were unable to measure the thickness because such type of folding has not been observed. However, the thickness of these sheets is expected to be higher than the sheets obtained at 320 °C, as these sheets are strained and not observed as folded on the TEM grid. Numerous trials to determine the exact thickness of these sheets from the atomic force microscopy (AFM) measurement were unsuccessful due to the extreme thinness of these sheets. Further, we have characterized the crystal structure of these sheets. The powder X-ray diffractions (XRD) pattern of these sheets has been shown in Figure 4a and the obtained peak positions matches perfectly with the cubic crystal structure of bulk β-In2S3 (JCPDS # 320456).31 Figure 4b shows an enlarged TEM image of two overlapped wrinkled sheets. The poor contrast of these nanosheets (in spite of being heavy metal sulfide) further suggests that these sheets are extremely thin. However, the selected area electron diffraction (SAED) of the sample (inset Figure 4b) indicates that these are crystalline in nature. Selected area fast Fourier transform (FFT) (inset Figure 4c) suggests these sheets are single crystalline. For better visibility of the planes, simulated HRTEM has been shown in Figure 4d. The planes having d-spacing 0.38 nm and their intercrossing at 60 degree reveals that these are cubic β-In2S3 and the planes are {022} planes viewed along ⟨111⟩ directions. The information furnished by both FFT and SAED analysis 3814
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Figure 5. (a) Schematic model of cubic β-In2S3 sheets with two and more atomic layers. (b) Plot of time versus control of crystal growth along different directions during the formation sheets at various reaction temperatures. This plot is the correlated plot of Figure 1a and 1b. The dotted line separates the basal planar ⟨110⟩ and the thickness [111] growth. The [111] axis growth is the thickness, and it is determined during the beginning at the nucleation stage of the formation of the nanosheets. Inset is a schematically shown 2D nanostructure with different axis identifications.
Figure 4. (a) Powder XRD pattern of In2S3 nanosheet obtained at 320 °C. (b) Representative TEM image of thin sheets showing wrinkling and overlapping. Inset is the SAED pattern. (c) HRTEM image and its inset is the selected area FFT. (d) Simulated HRTEM. (e) Model of cubic In2S3 viewed along the [111] direction.
support our claim. A typical atomic model obtained from ICSD 202353 data has been shown in Figure 4e, and it is viewed along the [111] direction. Since, d-spacing of the (222) plane is 0.62 nm, the thickness of ∼0.6 nm of the nanosheets can be expected to correspond to only one planar distance, i.e., with two atomic layers as clearly demonstrated in Figure 5a. Such thickness is also supported by the insignificant intensity of the (222) peak in the XRD pattern (Figure 4a). All these above results suggest that the thickness of the sheets can be controlled within a few atomic layers with proper manipulations of the reaction parameters. The planar dimension of the sheets obtained in all different reaction condition grow in micrometer range, but the major concern here is the growth control along the [111] direction which determines the width of the sheets. To study in detail, we have correlated the H2S evolution with the directional crystal growth of the sheets. The gas evolution is directly proportional to the rate of the precursor decomposition and the consequent crystal growth of the sheets. The growth period of the sheets is expected to continue until the gas evolution occurs, and the growth rate continues to increase until the gas evolution reaches the maxima. It is assumed that the monomer supply is a function of the precursor decomposition rate at different temperature; the decomposition at the maximum temperature would provide the monomers at the highest rate. For the reactions at different temperature, the crystal growth are related with the gas evolution and plotted with time (shown in Figure 5b). From these correlated plots we predict that sheets of 0.6 nm width are formed with faster rate of the crystal growth on injection of the precursor at 320 °C (highest temperature) in
comparison to others. However, importantly, it is essential to know whether the thickness growth along [111] direction is controlled at the nucleation stage or during the growth stage. In order to check this, a reaction is carried out at 320 °C, and within 1 min, it is cooled to 280 °C and annealed for 30 min more to study the rate of gas evolution and the crystal growth and it has been observed that the width remains same to that of the sheets obtained at 320 °C (Figure S4). All these observations clearly suggest that the nucleation temperature controls the growth along [111] of the nanosheet, and the growth along this axis is restricted to different extent depending on the different reaction temperatures (Figure 5b). In other words, we can safely predict that the thickness of the sheets is controlled at the nucleation stage and that the higher the nucleation temperature, the more restricted is the growth along the [111] axis, and the thinner is the sheet. The growth patterns for the planar ⟨110⟩ and thickness [111] directions have been shown in Figure 5b. However, the entire study has been carried out here using hexadecylamine as the capping agent. As surface ligands play a crucial role for the floating of the sheets in solution and also for stacking of multiple sheets, we have further tried to understand the role of these ligands for controlling the flexibility of the sheets. However, similar observations are also noticed for dodecylamine (DDA) and octadecylamine (ODA). For DDA, the temperature has been maintained at 240 °C, which is close to its boiling point, and similar thin sheets are observed (see Supporting Information). The nature of bending, folding, and unfolding observed here is in lesser extent. Hence, we can 3815
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assume here that the thickness of the sheets and their dispersion behavior in solution mostly depend on the reaction temperature. In addition, for our particular chosen reaction system, the precursor also shows the catalytic thermal decomposition in presence of alkyl amine. However, we have not considered this factor here, as it is more complicated and goes beyond the scope of our discussion. In conclusion, we report here the reaction kinetics for the control of thickness of 2D In2S3 nanosheets. Manipulation of different reaction parameters, the sheets with nearly two atomic layers thickness is designed, and to our knowledge these are one of the thinnest ever reported inorganic semiconductor nanosheets. As β-In2S3 materials are important in various leading applications in recent days, its thin free floating sheets has encouraging future as a substitute of CdS buffer layer in photovoltaics, and other related applications in the future.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures, instrumentations, schematic presentation of the synthesis of the nanosheets, and supporting TEM images of the nanosheets. This material is available free of charge via the Internet 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 CSIR and DST (Swarnajayanti, DST/SJF/CSA-01/2010-2011) of India are acknowledged for funding. DST unit of nanoscience at IACS is acknowledged for providing the TEM facility.
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