pubs.acs.org/JPCL
Surface Ligand Population Controlled Oriented Attachment: A Case of CdS Nanowires Bhupendra B. Srivastava,†,‡ Santanu Jana,†,‡ D. D. Sarma,§ and Narayan Pradhan*,†,‡ †
Centre for Advanced Materials, and ‡Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, 700032 India, and §Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012 India
ABSTRACT Using two types of organic ligands having similar chemical structure but different physical properties and varying their dynamic population at the surface of zinc blende seed nanocrystals, self-assembled zinc blende semicircularshaped bent nanowires of CdS are synthesized via a colloidal synthetic approach. It is found that the hydrophobic tail interaction of long-chain ligands puts strain on these thin nanowires (< 2 nm diameter) and bend them to some extent, forming strained nanowires. SECTION Nanoparticles and Nanostructures
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crystals either fusing or growing faster in these open sites. Hence, the shape of the nanocrystals can be controlled by varying the mobility of different ligands during the crystal growth in solution. Using this physical binding principle of various organic ligands, different nanostructures like nanorods (ZnS),13 nanowires (CdSe),14 nanooctahedra (PbSe),15 flower-shaped nanocrystals (In2O3, ZnO, ZnSe, NiO),10,11,16 and so forth have recently been synthesized. When we applied this principle of adopting two ligands having similar chemical structure but different physical properties and varied their dynamic population at the surface of zinc blende seed CdS nanocrystals, these nanocrystals oriented attached along the [111] direction, and a new kind of zinc blende bent CdS nanowire was formed. Directional crystal growth in solution, which forms nanorods and nanowires, exists mostly in the wurtzite structure, where the most polar [001] axis remains the growth direction.4,7,17,18 However, zinc blende nanocrystals are less polar (along the [111] direction, Supporting Information Figure S1) and possess a minimum probability of directional growth like the wurtzite system, but alternatively, these have an additional favorable feature to form 1D nanostructures via oriented attachments.4,13 We report here the possibility of ligand-driven linear alignment of seed quantum dots and fusion of these dots to form nanowires by oriented attachment. Using a proper concentration ratio of two types of aliphatic organic acid ligands having identical functional groups but different chain lengths (i.e., different boiling points) and controlling different reaction parameters, zinc blende CdS dots, connected dots, and bent nanowires are synthesized via colloidal synthetic approach. In presence of an insufficient amount of high boiling ligands, nanocrystals become agglomerated to form flakes like structures having no definite shape,
rystal growth in solution, shape and size control, and chemical and physical properties of different semiconductor nanocrystals are highly related to the interface bonding between the inorganic core material and organic capping ligands.1-4 Capping ligands are generally organic molecules which prevent the agglomeration, help in dispersion, and also control the optical emission of these nanocrystals.1-3,5,6 Importantly, these ligands also control the growth rate as well as growth direction(s) during nanocrystal synthesis by controlling the rate of approach of monomers at the surface of nanocrystals.2,7-9 The binding strength of these ligands plays an important role as ligand removal can open selective crystal facets which can facilitate the growth of the nanocrystals, or the nanocrystals may undergo oriented attachment via these available exposed common crystallographic facets.4,10-12 A strong binding ligand hinders the crystal growth, but a weak binding ligand remains more flexible on the surface of the nanocrystals, allowing the nanocrystal to grow or for the easier approach of other nanocrystals in the case of oriented attachment.4,8,10 This binding strength depends on the coordinating ability, molecular weight or chain length of the ligand, and importantly the growth temperature of the reaction.8 For the reaction system containing more than one type of ligand having different binding strengths, nanocrystals can grow selectively from the weaker ligand binding facets. However, when the coordinating strengths of both ligands are same, that is, molecules having a similar functional group, the binding ability depends on the chain length of the ligands as it controls the temperaturedependent mobility of ligands and dynamic bonding at the interface of nanocrystals.8,10,11 For the case of two ligands having different boiling points (having different molecular weights) but identical functional groups, the shorter-chain ligands are more mobile, and with an increase of temperature, these become more dynamic at the nanocrystal surface;8 as a result, these might leave the surface nearly close to or higher than its boiling temperature. This opens the possibility of the
r 2010 American Chemical Society
Received Date: May 11, 2010 Accepted Date: June 2, 2010 Published on Web Date: June 08, 2010
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DOI: 10.1021/jz1006077 |J. Phys. Chem. Lett. 2010, 1, 1932–1935
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Figure 1. (a) A schematic presentation of possible alignment and attachment of CdS nanocrystals (sky blue spheres) having two types of organic ligands (L1 and L2). (b) The top panel is the high-resolution transmission electron micrograph (HRTEM) of a bent or semicircular nanowire showing the crystal strain at different places. The bottom panel is a model of the bent wire, showing stacking and disorder of crystal planes. The d spacing in the HRTEM picture is ∼0.31 nm, which matches with that of the (111) plane of zinc blende CdS.
Figure 2. Time dependence on TEM images of (a) CdS dots, (b) connected dots, and (c) the formation of bent or semicircular nanowires at 2, 6, and 20 min, respectively, using acetate and a decanoic acid mixture at 180 °C. Different additional large area TEM images are provided in Supporting Information Figure S2.
(or less, Figure 2a), and then, they become self-aligned in a linear 1D array (Figure 2b). These dots are observed in very early samples within seconds of injection of S precursors, but their long-range linear self-assembly can be noticed after 5 min of the reaction (Figure 2b). Further annealing at the same temperature changes these nanostructures to fragmented semicircular nanowires, as shown in Figure 2c. Initial small-size CdS nanocrystals are also reflected in their intense absorption peak at ∼370 nm in the UV-visible spectra, typically observed for magic size semiconductor nanocrystals2 (Figure 3a). The shoulder at above 400 nm might be due to larger-size CdS particles formed separately. The bent wires after annealing of connected dots also retain the absorption at ∼370 nm with reduced intensity, indicating that the width of the nanowires is similar to the diameter of the dots, and its reduced intensity indicates the merging of dots to form nanowires. The long-range linear self-assembly of nanodots, formation of semicircular or bent nanowires, and their self-assembly (small range) are observed due to the different chemical interaction strengths of two types ligands. It has been found that the acetate ligand has a greater probability to be removed
and with excess presence of these long chain ligands, seed nanocrystals grow three dimensionally, forming spherical CdS particles. Figure 1a shows the possible interactions of two moderate covalently bonded ligands (acetate and a long-chain fatty acid (C8H17COOH to C18H37COOH)) on the surface of nanocrystals and removal of the low boiling ligand at elevated temperatures, forming 1D linear association of dots via oriented attachment. Upon further annealing, these dots merge, forming a strained 1D nanostructure. The crystal strain and defects in the nanowires are shown in Figure 1b, where crystal planes get strained to form bent or semicircular nanostructures. Traditional colloidal synthesis of CdS nanocrystals has been carried out by using CdO19 and long-chain fatty acids in solvent 1-octadecene and injecting S (in ODE) at a desired temperature. Using one fatty acid or acids having closer boiling points (say C12H25 and C14H29 carbon chain acids), spherical CdS nanocrystals remained the only product. However, in the presence of acetic acid and long-chain fatty acids (or use of Cd-acetate and long-chain fatty acids) and at desired the reaction temperature (180 °C for acetate as a low boiling ligand), spherical CdS nanocrystals grow up to 2 nm
r 2010 American Chemical Society
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DOI: 10.1021/jz1006077 |J. Phys. Chem. Lett. 2010, 1, 1932–1935
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Figure 3. (a) UV-visible spectra of connected CdS dots and bent wires corresponding to Figure 2b (red line) and c (blue line). (b) XRD of CdS dots and bent or semicircular nanowires. In the case of nanowires, we have observed a 2-4° blue shift of all major peaks, which is expected due to the larger volume of crystal strains. ZB and WZ signify the zinc blende and wurtzite crystal phase of bulk CdS.
Figure 4. Formation of different nanostructures at different concentration ratios of long-chain fatty acids to short-chain acetate. The marked area with dotted line indicates the formation zone of bent or semicircular nanowires.
from the system at the temperature chosen for the nanowire formation (170-180 °C). In Supporting Information Figure S3, it has been shown that the interparticle distance (d1) between CdS dots in the linear self-assembled array is smaller than that between two neighboring arrays (d2). These distances are measured on the basis of average values from larger-area TEM images. This indicates the possibility of binding two different chain ligands at two different facets of CdS nanocrystals. The bent or semicircular nanowires, having planes with d spacings of 0.31 nm (for the (111) plane as shown in Figure 1b), suggest that the linear alignment of these seed dots is along the [111] direction. Hence, there is a smaller interspacing distance between CdS dots aligned in this direction, which are expected to have preferably bonded with small acetate ligands. The other remaining binding sites of these nanocrystals during the self-assembly as well as after the formation of nanowires should be with long-chain ligands. XRD of dots as well as bent wires (Figure 3a) also supports their zinc blende crystal structure. Hence, it can be concluded that these nanocrystals aligned in a 1D array are oriented attached along the [111] direction. Continuous annealing removes the ligands in between these aligned dots (preferably acetates) and helps them to merge to form fragmented strained nanowires. Considering the possibility of semicircular or bent nanowire formation using various fatty acids having different chain lengths in addition of acetates, we have carried out systematic variation of their concentration in the reaction mixture. Figure 4 shows the possible nanowire formation with the variation of the ratio of long-chain ligands to the acetate ligand. It is observed that with an increase in the carbon chain length of long-chain fatty acids, the optimum concentration required for the semicircular nanowire formation is reduced. Ligands on the surface of nanocrystals are always dynamic, and this increases with the rise of reaction temperature but decreases with an increase in the molecular weight of the ligands. Hence, at 180 °C, when the ligand carbon chain length increases from C8 to C18, their mobility in solution and dynamic bonding at the interface decreases. Therefore, to get a particular ratio of long-chain ligands to acetate at the interface needed for the
bent wire formation, a greater amount of shorter-chain fatty acid (C8) ligands compared to the longer-chain one (C18) is needed. However, when their ratio with respect to acetate increases to a certain optimum value (Figure 4), they replace the acetate ligand from the interface and allow the nanocrystals to grow in all possible directions, forming spherical CdS nanocrystals following a normal crystal growth process. However, when an insufficient amount of long-chain ligands is used, the short-chain ligand acetate remains on the surface of the nanocrystals, and its removal at high temperature causes agglomeration of CdS nanoparticles, forming irregular flakes like nanostructures (Supporting Information Figure S4). Hence, a balanced population of both short- and long-chain ligands is essential for oriented attachment and formation of such bent nanowires. Not only has the ratio of both ligands determined the shape of nanostructures, but also, the reaction temperature plays a crucial role. In Figure 4, the optimized reaction temperature is 180 °C, but higher temperature leads to spherical CdS nanocrystal formation, and lower temperature does not allow the small crystals to grow. Our preliminary report suggests that by using butyric acid as the short ligand, the optimized reaction shifts to higher temperature (∼220 °C), which is expected as butyric acid ligands need higher temperature than acetate ligands for their removal. The case of the bending of nanowires, which is reported in this Letter, is found to be related to the physical binding strength of the chosen high boiling ligands. When these carboxylate ligands are replaced with fatty amine ligands in this reaction, no such bent wires are observed; rather, spherical nanocrystals of CdS remain the only product. Similarly, when purified bent wires are annealed in fatty amines for ligand exchange or in an amine/acid mixture with an insufficient amine, these bent nanowires open up and are straightened (Supporting Information Figure S5). This indicates that carboxylates which binds firmly on the surface of nanocrystals compared to more mobile amine ligands play the crucial role for bending. Hence, for carboxylate ligands, their heads are bonded at the interface of the nanowires, which is expected to be stronger (compared to the amine), and at the same time, their tails interact via hydrophobic interactions.
r 2010 American Chemical Society
1934
DOI: 10.1021/jz1006077 |J. Phys. Chem. Lett. 2010, 1, 1932–1935
pubs.acs.org/JPCL
Due to the strong interfacial boding of acids, the interactions among their long-chain hydrophobic tails can put strain on the ultrathin nanowires and bend them to a certain extent, which is not possible for weak interfacial bonded amines. As the self-assembled bent nanowires have a different curvatures radius, we found difficulty in calculating the bending parameters with changes in chain lengths of different long-chain fatty acids (C8 to C18) as well as their exact concentration. The inner core nanowire in the self-assembly has been observed as having a lower radius of curvature than the outer ones. However, by changing different parameters, we have observed poor reproducibility of an exact or similar inner curvature. In conclusion, by varying the population of surface-capping ligands at the interface of CdS nanocrystals, self-aligned ordered nanostructures are synthesized through oriented attachment. Further annealing of these attached nanocrystals merges them together and forms ultrathin (
