Article pubs.acs.org/JPCC
In Situ Self-Assembly of Silver Nanoparticles Boris B. Bokhonov,*,† Marat R. Sharafutdinov,† David R. Whitcomb,‡ and Lilia P. Burleva‡ †
Institute of Solid State Chemistry, Siberian Branch, Russian Academy Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Carestream Health, Inc., 1 Imation Way, Oakdale, Minnesota 55128, United States
‡
S Supporting Information *
ABSTRACT: We have found that the thermal decomposition of [Ag(O2CnH2n−1)]2, where n = 14, 16, 18, and 22, produces the ordered 3D nanostructure (supracrystals) directly during the chemical reaction, which makes it possible to obtain self-assembled structures in large quantities without requiring additional processing steps. The process of self-assembling of silver nanoparticles is characterized by TEM and in situ time-resolved SAXRD and WAXS data. The calculated unit-cell parameters of the fcc nanostructures formed correlate with the aliphatic chain length: these are 9.55 nm for silver myristate (n = 14), 9.8 nm for silver palmitate (n = 16), 10.46 nm for silver stearate (n = 18), and 12.5 nm for silver behenate (n = 22). Structural and morphological characteristics of supracrystals formed in situ during the thermal decomposition of silver carboxylates are found to be dependent on the sample heating rate. The self-assembled ordering of the silver nanoparticles was observed to begin at ∼180, and above ∼280 °C the supracrystalline structures begin to decompose.
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One of the first-reported ordered nanostructure arrays were obtained for gold nanoparticles with alkyl thiols, which were produced from evaporation of the reduction products of [AuCl4]− in the presence of thiols, CnH2n+1SH.8 Other procedures for obtaining ordered nanostructures include photolytic or templating procedures using inorganic or bioorganic structures.9−11 In addition to the development of synthetic methods for preparing ordered ensembles of 2D and 3D nanostructures, the need for analytical methods that are capable of studying the kinetics of the self-assembly of these systems is a serious dilemma. It is clear that new analytical methods are needed that can be applied to the study of the self-assembly process and that can follow the evolution of the nanostructure and morphology of 2D and 3D structures as they form. One of the most informative methods of X-ray diffraction is timeresolved synchrotron radiation.12 This in situ X-ray diffraction method has been applied for the first time for the investigation of the self-assembly of nanoparticles,13,14 which provides a new probe of the change in the structural parameters of silver nanoparticle ensembles during the process of self-assembly. Recently, X-ray diffraction from synchrotron radiation was used to study the self-assembly process of gold nanoparticles, while the solvent evaporated from a colloidal solution.15,16 This technique made it possible to establish the driving forces of the face-centered cubic (fcc) nanostructure formation. Lee et al.17 evaluated the difference between randomly packed nanocrystal, 3D periodic films, and faceted 3D colloidal crystals using the
INTRODUCTION Self-assembled structures having diverse compositions and morphologies are common for a multitude of inorganic materials.1 They are widely distributed in nature2−4 and are currently one of the most popular research topics. It is necessary to use not only modern analytical methods but also simpler and more efficient methods for preparation of the nanostructure that needs to be developed to further develop an understanding of the structural and morphological changes occurring during the self-assembly of nanoparticles as they form 2D or 3D ordered nanostructures.5 In addition, it is also of great scientific importance because it may be considered to be a model of fundamental crystallization processes in which atoms or molecules are “replaced” by nanoparticles. Currently, the structures most studied in detail are the 2D and 3D colloidal crystals having different dielectric, semiconductor, and metallic nanoparticles.6 There are two main approaches to the synthesis of 2D- and 3D-ordered nanocrystals.7 In the “evaporation-driven” method, the solvent evaporates slowly from the colloidal solution to the point at which the nanocrystal component reaches a certain threshold. At that point, the system makes a transition from the disordered to ordered state. Correspondingly, the second synthetic method is called a “destabilization-driven” selfassembly, occurring at a slow diffusion rate to the nucleation sites that enables large, 3D, organized crystals to form. In both cases, the methods for construction of self-assembled structures require separate stages of particle preparation to achieve uniform size and dimensions, which further requires stabilization by surfactants. The most popular surfactants are typically long, alkyl chain thiols and fatty acids, for example, oleic acid, which forms a ligand shell around the nanoparticle. © 2014 American Chemical Society
Received: February 12, 2014 Revised: April 20, 2014 Published: May 9, 2014 11980
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hexane) containing a surfactant was carried out by drying the resulting colloidal dispersion on various substrates. In many cases, these substrates were amorphous carbon films deposited on copper grids for electron microscopic analysis. Such a method is not very suitable for large-scale preparation of ordered structures. The results obtained in this paper, as discussed later, provide the experimental data that confirm the possibility of carrying out a silver nanoparticle self-assembly process in one step directly from the chemical reaction of the thermal decomposition of silver carboxylates.
time-resolved, small-angle X-ray scattering technique as well as estimating the possibility of manipulation of interparticle spacing by thermal treatment. In addition, an in situ liquidphase transmission, electron microscopy has been reported to follow the drying, mediated-nanoparticle self-assembly in real time.18 Obviously, the simplest and most convenient way to form ordered structures based on nanosized particles would be to create a single-step method that combines the formation of uniform-sized and -shaped particles and their self-organization into a periodic structure. In our opinion, a single-step method would be a chemical reaction where the products would produce metal nanoparticles (having the desired composition, size and morphology) and organic products having surface active properties. From our experience, and as shown in the results later, these conditions are satisfied by the thermal decomposition reaction of silver salts of saturated carboxylic acids having the general formula [Ag(O2CnH2n−1)]2, where n = 14, 16, 18, and 22. More specifically, it is well known that heating long-chain silver carboxylates, which have layered structures, produces several phase transitions in the solid, through various liquidcrystalline states, followed by thermal decomposition at temperatures above 180 °C. The products of thermal decomposition are metallic silver, saturated carboxylic acid, unsaturated paraffin wax, as well as gaseous CO2,19,20 as shown by the following equation:
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EXPERIMENTAL SECTION Silver carboxylates, [Ag(O2CnH2n−1)]2 were prepared according to the exchange reaction between the sodium salts of the corresponding fatty acids and silver nitrate by the method described elsewhere.21−23 In situ investigation of the structure changes during the thermal decomposition of silver carboxylate crystals were carried out in an X-ray diffractometer with the synchrotron radiation source used at the Institute of Nuclear Physics Siberian Branch of the Russian Academy of Science. Powders of silver carboxylate crystals were applied to an aluminum foil sheet and placed in a special holder for sample heating (Supplementary Figure S1 in the Supporting Information). An OD-3 detector28 was used to record the X-ray diffraction patterns. The OD-3 detector possesses the following distinctive features: • acquisition rate up to 10 photon/s with efficiency up to 50% • it is parallax-free from 0 to 30° (depending on the focal distance of the cathode strips) • photon coordinate resolution in the direction transverse to the beam, σ = 100 μm. The experimental setup allowed for both transmission and reflection data from the vacuum chamber between the same and detector. The monochromatic radiation wavelength was 1.5215 Å. TEM investigations of decomposed [Ag(O2CnH2n−1)]2 crystals were carried out using a JEM-2000FXII microscope at an accelerating voltage of 200 kV. For preparation of suitable TEM samples, the original products of thermal decomposition of silver carboxylates were directly (without dissolution, dispersion, or the use of any solvents) placed on a Cu TEM grid previously covered with holey carbon film. During the study of the self-assembled silver nanoparticles in the electron microscope, no significant changes in structure and morphology of the investigated samples could be observed. The change in the silver carboxylate properties in the decomposition process was also recorded by differential thermal analysis (DTA) of initial and decomposed silver carboxylates.
2CnH 2n + 1COOAg → 2Ag + CO2 + CnH 2n + 1COOH + CnH 2n
During the chemical transformation process at the higher temperatures, the carboxylic acid and paraffin products are in liquid form. As we21−23 and others20,24 have previously published, the thermal decomposition of long-chain silver carboxylates produce silver particles having nanometer dimensions. Similarly, Yamomoto et al.25 have shown that monodisperse silver nanoparticles, ranging from 2.6 to 4.4 nm, are formed upon chemical reduction of the long-chain silver carboxylates by triethyl amine. Moreover, subsequent extraction of the silver nanoparticles with toluene, followed by deposition on a carbon film, results in the formation of 2D ordered structures. Jiang et al.26 have shown that 2D arrays of Ag nanoparticles were prepared by treating palmitate-stabilized Ag NPs with the as-synthesized Ag(palmitate) or ribbon-shaped Ag(palmitate) templates. Thus, the published data show that the thermal decomposition of silver carboxylates forms nanoscale particles of silver, which, using colloidal chemistry techniques, can be assembled into ordered 2D nanostructures, and the organic reaction product (long-chain carboxylic acid) may serve as a surface-active substance. It should also be noted that the silver nanoparticles are one of the most intensively studied metals, which provides additional support for the possibility of forming 2D and 3D self-assembled nanostructures.27 It should be emphasized that in all cases (including silver carboxylates) the synthesis and subsequent process of silver nanoparticle self-assembly occurs in a two-stage process. The first step is the reduction of silver in solutions containing surface active agents of various types used in metal nanoparticle synthesis. In the second step, the process of self-assembly of a silver nanoparticle dispersion (generally in
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RESULTS AND DISCUSSION For confirmation of the self-assembly process and in situ examination of the changes in the structural and morphological characteristics of the silver nanoparticles produced by the thermal decomposition of silver carboxylates, we used an in situ small-angle and wide-angle X-ray diffraction method (SAXRD and WAXS) based on synchrotron radiation and transmission electron microscopy. 11981
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Figure 1. In situ synchrotron X-ray diffraction patterns during thermal decomposition of silver stearate (heating rate of 20 °C/min). (a) Small-angle X-ray diffraction patterns of phase transitions and decomposition of silver stearate during heating from 20 to 280 °C. (b) Wide-angle X-ray diffraction patterns of phase transitions and decomposition of silver stearate during heating from 20 to 280 °C. Diffraction reflections correspond to metallic silver. (c) Selected small-angle X-ray diffraction patterns of the products of silver stearate decomposition at 250 °C indexed as a cubic face centered structure with the unit-cell parameter a = 10.46 nm.
Table 1. Variation of the Unit-Cell Parameter of the FCC Structure with Temperature temperature (°C) unit-cell parameters (nm)
230 10.36
235 10.37
240 10.38
245 10.41
250 10.43
In Situ Time-Resolved SAXRD and WAXS. We found that the structural changes occurring in the heating process of silver carboxylates [Ag(O2CnH2n−1)]2 (where n = 14, 16, 18, and 22) are all of the same type (Supplemental Figures S2−S4 in the Supporting Information). Most changes in the structural characteristics of the representative silver carboxylates were obtained under conditions where the heating rate of the samples was 20 °C/min. As an example, Figure 1a shows the changes in X-ray diffraction patterns of silver stearate at small diffraction angles 2θ = 0.5−8°, directly during the heating process from 20 to 280 °C. When silver stearate is heated to ∼180 °C, the changes in the X-ray diffraction pattern can be assigned to create transformations of various liquid crystalline phases, which were examined in more detail in our previous work.20−22 After reaching a temperature of ∼180 °C, all diffraction reflections related to the silver stearate phase were completely gone, which is attributed to the transition to an isomorphic liquid. A further increase in temperature from 180 to 220 °C is characterized by the appearance of a wide diffraction reflection in small angles of the peak maxima at 2θ = 1.42°. As the temperature increases from 220 to 280 °C, an increase in the intensity and a decrease in the half-width of this
255 10.43
260 10.44
265 10.45
270 10.46
275 10.46
280 10.46
peak is observed (Supplementary Figure S5 in the Supporting Information), which also appears at diffraction reflections 2θ = 1.66, 2.35, 3.64, and 4.09°. Heating silver stearate from 280 to 300 °C leads to a decrease in intensity of the diffraction peaks until they completely disappear. Carrying out in situ X-ray studies of the thermal decomposition of silver carboxylates in the diffraction angles 2θ = 32 − 46° (Figure 1b) showed that during heating the Xray wide-angle reflections (111) and (200) are due to metallic silver, the intensity of which increases according to the increase in temperature. Calculations of the crystallite size from the diffraction data show that the size of the silver nanoparticles remains practically unchanged between 230 and 270 °C (4.6 nm) but significantly increases upon further heating (Supplementary Figure S6 in the Supporting Information). Comparison of changes in the diffraction pattern of the silver carboxylates during heating above 180 °C in the area of small and wide angles indicates that the appearance of the diffraction reflections 2θ = 1.42, 1.66, 2.35, 3.64, and 4.09° is related to the formation of silver nanoparticles in the thermal decomposition process. The analysis of the diffraction patterns observed during decomposition in the temperature range of 200−280 °C 11982
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Figure 2. Morphology and structure of self-assembled silver nanoparticles. (a) SEM image of supracrystals formed after silver stearate decomposition at 250 °C. (b) TEM image of a self-assembled nanostructure from 4.5 nm truncated silver nanoparticles. (c) SAED pattern from a self-assembled nanostructure. (d) Magnified and indexed as an fcc structure ([110] zone axis), the part of the SAED pattern close to the electron beam. (e) TEM image of self-assembled nanostructures from 4.5 nm silver nanoparticles with stacking faults.
showed that they correspond to the fcc (Figure 1a and Supplementary Figures S2−S4 in the Supporting Information). The calculations show that the unit-cell parameter of the fcc structure increases from 10.36 to 10.46 nm as the temperature increases from 230 to 280 °C (Table 1). At the same time, the intensity of the Bragg reflections increases, while the half-width decreases with increasing temperature (Supplementary Figure S5 in the Supporting Information), which favors a gradual increase in the size of the crystalline units of the emerging fcc phase. It should be noted that similar diffraction patterns were obtained in the study of self-assembly of colloidal silver and cobalt nanoparticles into fcc structures.29,30 The calculated unit-cell parameters of the fcc nanostructure formed during thermal decomposition of silver carboxylates (Figure 1c and Supplementary Figures S3b, S4b, and S5b in the Supporting Information) do not change significantly: these are 9.55 nm for silver myristate (n = 14), 9.8 nm for silver palmitate (n = 16), 10.46 nm for silver stearate (n = 18), and 12.5 nm for silver behenate (n = 22). There is some correlation between the length of the hydrocarbon chain31 of carboxylic acids and the calculated fcc parameter of the nanostructural cell forming during the process of thermal decomposition of silver carboxylates, which, we propose, corresponds to the presence of adsorbed molecules of carboxylic acids on the silver
nanoparticle surface. The possibility of saturated carboxylic (lauric and myristic) acids adsorbing on the surface of silver nanoparticles has been previously noted.32 Electron Microscope Investigation. For investigation of the morphology and structure of individual and ordered ensembles of silver nanoparticles, we have carried out an electron microscope investigation of the thermally decomposed silver carboxylates. We found that after thermal decomposition the silver carboxylate samples generally formed supracrystals with a triangular-shaped morphology several micrometers on a side, more than 300 nm thick (Figure 2a). Similar morphology of supracrystals from spherical Au nanocrystals coated with dodecanethiol molecules was observed by Pileni.33 We also found that the internal structure of the samples at thermal decomposition depends on the thermal decomposition temperature. When decomposed at 200−260 °C, the product consists mainly of periodic, self-assembled nanostructures from 4.5 nm silver nanoparticles (Figure 2b). The selected-area electron diffraction pattern (SAED), in the area close to the electron beam, which was obtained from the assembled nanostructures (Figure 2c), indicates the presence of a longrange order. A similar diffraction pattern has been previously reported by Kiely et al.34 for gold nanoparticles assembled into a hexagonal structure. However, in our case, the observed diffraction pattern corresponds to a cubic silver nanoparticle 11983
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Figure 3. Destruction of self-assembled nanostructure from 4.5 nm silver nanoparticles during heating. (a) TEM image of self-assembled nanostructure from silver nanoparticles after heating to 270 °C. The formation of silver nanoparticles on the sides of the supracrystals was observed. (b) TEM image of a self-assembled nanostructure from silver nanoparticles after heating to 290 °C. (c) SAED pattern from self-assembled nanostructure of silver nanoparticles after heating to 290 °C. (d) Silver nanoparticles that were formed after heating to 320 °C. (e) SAED pattern from silver nanoparticles that were formed at 320 °C.
ordering and can be correspondingly indexed (Figure 2d). The electron microscopic studies also showed that self-assembling nanostructures formed during thermal decomposition of silver carboxylates contain different degrees of nanostructure order. In some cases, self-assembled nanostructures contain domains and stacking faults (Figure 2e). We also found that the internal structure of the samples at thermal decomposition depends on the thermal decomposition temperature. When decomposed at 200−260 °C, the product consists mainly of periodic, self-assembled nanostructures from 4.6 nm silver nanoparticles (Figure 2b). The SAED pattern, in the area close to the electron beam, which was obtained from the assembled nanostructures (Figure 2c), indicates the presence of a long-range order. A similar diffraction pattern has been reported previously by Kiely et al.34 for gold nanoparticles assembled into a hexagonal structure. However, in our case, the observed diffraction pattern corresponds to a cubic silver nanoparticle ordering and can be correspondingly indexed (Figure 2d). The electron microscopic studies also showed that self-assembling nanostructures formed during
thermal decomposition of silver carboxylates contain different degrees of nanostructure order. In some cases, self-assembled nanostructures contain domains and stacking faults (Figure 2e). Analysis of the electron microscope images allows us to conclude that the silver nanoparticle facets in the self-assembled structure have the same orientation. This structure is nearly identical to the assembled structure from evaporation of a colloidal solution containing silver nanoparticles having surface active agents, such as thiolates (SR), where R = CnH2n+1, n = 4, 6, 8, and 12, previously published by Wang et al.35,36 According to the results obtained in Wang et al., the silver nanocrystals were a truncated cube or octahedra and were oriented along the [110] crystal lattice, self-assembled nanoparticles. The authors proposed the observed orientation ordering of the nanocrystals to be the result of selective adsorption of thiolate molecules on different facets of the nanocrystal. The thiolate molecules tethered on the facets of the nanocrystals are likely to interpenetrate, forming interdigitative bonds. One can assume that in our case, with the decomposition of silver carboxylate, the liberated saturated carboxylic acid plays a decisive role in 11984
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Figure 4. Morphological and structural characteristics of self-assembled silver nanostructures at a heating rate of 10 °C/min. (a) Small-angle X-ray diffraction patterns of phase transitions and decomposition of silver stearate while heating from 20 to 280 °C. (b) Selected small-angle X-ray diffraction patterns of the products of silver stearate decomposition at 250 °C indexed as a cubic face centered structure. Bragg reflections of the supracrystals formed from heating at 10 °C/min are broader (the (111) and (020) peaks of the fcc supracrystal overlap) compared with those formed at 20 °C/min heating rates (Figure 1a,c). (c) SEM image of thin film of product formed after silver stearate decomposition at 250 °C. (d) TEM image of a self-assembled nanostructure from silver nanoparticles after heating to 250 °C.
the orientation process ordering during self-assembly. The carboxylic acids, just as the thiolates, can selectively adsorb on certain faces of silver nanocrystals. It should be noted that the size of the primary nanoparticles in self-assembling nanostructures formed during thermal decomposition was typically 4 to 5 nm for all of these silver carboxylates, regardless of the chain length of the silver carboxylate being decomposed. We found that the samples thermally decomposed above 260 °C produce some morphological changes of assembled nanoparticles. Figure 3a is an image of an ordered structure formed at 270 °C. As seen from the Figure, the self-assembled nanostructures in this stage contain larger, irregularly shaped silver particles at the edges, the size of which are larger than the silver nanocrystal facets in a self-assembled structure. The sample decomposed at 290 °C contains large particles (30−50 nm) at the edges and at the surface of self-assembled nanostructure (Figure 3b). The SAED pattern of this structure (Figure 3c) is characterized by continuous ring reflections from the polycrystalline fcc silver nanoparticles as well as small-angle diffraction from the self-assembled nanostructures. Samples decomposed at 320 °C show that the self-assembled nanostructure completely disappears. The silver particles have become considerably larger and randomly distributed (Figure 3d), and the electron diffraction corresponds to polycrystalline face-centered metallic silver without small-angle diffraction from the self-assembled nanostructures (Figure 3e).
It should also be noted that the stability of an assembled structure formed by the thermal decomposition of silver carboxylates observed in the temperature range from 180 to 260 °C is substantially lower than that for thiolate-stabilized, silver nanoparticle,37 self-assembled structures, which remain stable even when heated to 700 °C. This difference in thermal stability of the stabilizing molecules could be explained by the formation of stronger adsorption of the thiolate layers on the surface of the silver nanoparticle; however, such a significant difference in the temperature ranges of stability affect the accuracy of the sample temperature in the process of heating the column of the electron microscope. Control of Morphology and Structure of Silver Supracrystals Formed during in Situ Thermal Decomposition (Dependence on Heating Rate). After the temperature stability range of supracrystals formed from in situ decomposition of silver carboxylates and the crystal structure parameters as a function of aliphatic chain length have been determined, it would be useful to identify other factors affecting the morphology and structure of the supracrystals. Previous studies of supracrystal crystallization from colloid solutions clarified the main factors influencing the structure and morphology of the growing supracrystals. Among these factors, the most important are the crystallization kinetics, the ratio of the metal nanoparticles and surfactant, the phase composition 11985
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Figure 5. (a) Structure of the unit cell of the silver nanoparticle-based supracrystal composed of truncated octahedral nanoparticles according to ref 35. Projections of the unit cell on (b) (110) and (c) (101) planes.
of the colloid solution, the shape and size of the metal nanoparticles, and the temperature of the substrate on which supracrystals precipitate. We propose that for the unique process of in situ nanoparticle self-organization into supracrystals described in this paper the heating rate may be the critical factor. This proposal is based on the analysis of the weight losses of the silver carboxylate crystals during heating at different heating rates (10 and 20 °C/min). Slower heating causes greater weight losses. Thermogravimetric investigations (Supplementary Figure S7 in the Supporting Information) showed that independent of the heating rate thermal stability increases as the length of the aliphatic chain of the silver carboxylates increases; that is, the longer the chain, the higher the onset temperature of thermal decomposition. The weight losses measured at different heating rates show similar trends: the higher the heating rate, the lower the weight loss. Thus, silver steareate heated to 270 °C at 10 °C/min loses 27% of its weight, while heating at 20 °C/min to the same temperature results in a weight loss of only 3.5%. When the sample is heated up to 280 °C, its weight losses are 40 and 6% at heating rates of 10 and 20 °C/min, respectively. Similar comparisons for the other carboxylates are given in Table S1 in the Supporting Information. The formation of a carboxylic acid during decomposition of a silver carboxylate is confirmed by the existing literature data and our FTIR and DSC measurements (Supplementary Discussion in the Supporting Information). The dependence of weight loss on heating rate can be explained by the partial evaporation of the carboxylic acid formed as a result of silver carboxylate decomposition: a slower heating rate simply allows more of the carboxylic acid to evaporate. Using time-resolved in situ X-ray diffraction studies (Figure 4a,b), we have shown that Bragg reflections of the supracrystals formed from heating at 10 °C/min are broader compared with those formed at higher heating rates (Figure 1a,c). Thus, while the (111) and (020) peaks of the fcc supracrystal are resolved in the XRD pattern from heating at 20 °C/min (Figure 1c), they overlap when heated at 10 °C/min (Figure 4b). Such a change in the diffraction patterns can be related to both the size of the supracrystals and the presence of various lattice defects. Electron microscopy results are similar: heating at 20 °C/min results in the formation of triangular supracrystals several micrometers on a side and more than 300 nm thick (Figure 2a), at 10 °C/min a product, in which no crystalline faceted structure is seen (Figure 4c). The internal structure of the supracrystals formed at 10 °C/min is substantially different from that of the supracrystals obtained at 20 °C/min. The size of the ordered regions at 10 °C/min is much smaller; in addition, high-angle boundaries and disordered regions are
observed (Figure 4d). Consequently, the effect of the heating rate on the supracrystal formation drawn from the differences in the weight losses is also supported by the dependence of both morphological and structural features of the supracrystals on the heating rate. It is clear that more detailed investigations are needed to better understand what factors are responsible for the morphological and structural features of the supracrystals formed in situ by thermal decomposition. However, it is difficult to perform these studies due to the changing composition during self-assembly. That is, when crystallization of supracrystals occurs from the conventional process of colloid solution evaporation, the size and shape of the initial metal nanoparticles are predetermined and do not change and the composition of the system can be varied by choosing different ratios of the nanoparticle and surfactant concentrations and even different components. It should also be noted that the effect of the surfactant evaporation rate on the structure of supracrystals formed from 5 nm silver nanocrystals has been described by Courty et al.38 In the self-assembly of nanoparticles by thermal decomposition the concentrations of the components are predefined by the stoichiometry of the initial reactant, which is a carboxylate with a general formula CnH2n+1COOAg. As the decomposition reaction proceeds, the size, shape, and concentration of the metal nanoparticles as well as the concentration of the surfactant (carboxylic acid) change continuously. In a sense, the observed self-assembly effect during thermal decomposition of silver carboxylates is a “lucky” case in which several parameters pertaining to the process align in a way that makes it possible to form supracrystals by a chemical reaction route. We believe that although the ratio of silver and carboxylic acid is predetermined by the initial stoichiometry of silver carboxylates, it is possible to influence the self-assembly process by using mixtures of carboxylates and carboxylic acids with varied concentrations of the components. Carboxylic acids, whose composition does not match the carboxylic group of the silver carboxylate, can be also used. A change in the stoichiometry and chemical composition of the components may affect the self-assembly process of metal nanoparticles and, correspondingly, the structure of the supracrystals. Further investigations are needed to evaluate these opportunities. Structure and Formation Stages of Supracrystals Formed by Thermal Decomposition of Silver Carboxylates. The adsorption of carboxylic groups on the crystallographic planes of silver undoubtedly plays an enormous role in the formation, growth, and, ultimately, self-assembly of silver nanoparticles. A large number of experimental and theoretical studies have been published on the structure of the adsorbed layers and their distribution over the surface of the nano11986
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Figure 6. Schematic representation of the self-assembly process of silver nanoparticles during silver carboxylate thermal decomposition.
formation of a supracrystal. As previously noted, one molecule of carboxylic acid is obtained from two silver atoms formed as a result of decomposition. For silver stearate, this corresponds to 46.15 wt % of Ag and 53.85 wt % of C. If we assume that the silver nanoparticles forming the supracrystal are truncated octahedra and all silver atoms are coordinated by the surfactant molecules, the metal and carbon compositions correspond to 59.14 wt % Ag and 40.86 wt % C. These calculations show that the amount of the stearic acid formed during decomposition is sufficient to involve all silver surface atoms. However, the experimentally determined carbon content in the supracrystals is lower (
