Article Cite This: Langmuir XXXX, XXX, XXX-XXX
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Rapid and Controlled In Situ Growth of Noble Metal Nanostructures within Halloysite Clay Nanotubes Taha Rostamzadeh,† Md Shahidul Islam Khan,† Kyle Riche’,† Yuri M. Lvov,‡,§ Anna V. Stavitskaya,§ and John B. Wiley*,† †
Department of Chemistry and Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana 70148, United States ‡ Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana 71272, United States § Gubkin Russian State University of Oil and Gas, Moscow 119991, Russia S Supporting Information *
ABSTRACT: A rapid (≤2 min) and high-yield low-temperature synthesis has been developed for the in situ growth of gold nanoparticles (NPs) with controlled sizes in the interior of halloysite nanotubes (HNTs). A combination of HAuCl4 in ethanol/toluene, oleic acid, and oleylamine surfactants and ascorbic acid reducing agent with mild heating (55 °C) readily lead to the growth of targeted nanostructures. The sizes of Au NPs are tuned mainly by adjusting nucleation and growth rates. Further modification of the process, through an increase in ascorbic acid, allows for the formation of nanorods (NRs)/ nanowires within the HNTs. This approach is not limited to golda modified version of this synthetic strategy can also be applied to the formation of Ag NPs and NRs within the clay nanotubes. The ability to readily grow such core−shell nanosystems is important to their further development as nanoreactors and active catalysts. NPs within the tube interior can further be manipulated by the electron beam. Growth of Au and Ag could be achieved under a converged electron beam suggesting that both Au@HNT and Ag@HNT systems can be used for the fundamental studies of NP growth/attachment.
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(anticorrosion agents, antioxidants, and flame retardants), nanoreactors, and drug loading and delivery systems as well as in catalysts.1,5,9−15 The construction of HNT nanocomposites based on the attachment/formation of noble metals/metal oxide nanoparticles (NPs) within the inner sites or on the outer surfaces of HNTs may lead to the development of advanced nanostructures and thus applications arising from the favorable features of HNTs combined with complementary properties of NPs.4,8,16,17 Wang et al. have reported the attachment of TiO2 NPs onto the surface of HNTs via a one day solvothermal approach and demonstrated that these nanocomposites exhibit superior pH sensitivity and photocatalytic activity for the degradation of organic pollutants.18 Ru/HNT nanocomposites, developed by Wang et al. through a wet impregnation method, showed catalytic activity for the decomposition of ammonia.19 Pd NPs deposited on the salinized HNTs were also shown to enhance catalytic performance.20 Recently, Fu et al. reported Au/HNT nanocomposites produced via a deposition/precip-
INTRODUCTION Halloysite nanotubes (HNTs), chemical formula of Al2(OH)4Si2O5·nH2O, n = 1−2, possess unique properties including bio- and environmental compatibility and abundancy.1−4 These features are not found in other forms of tubular structures such as carbon nanotubes. HNTs are naturally formed by revolution of kaolin aluminosilicate sheets and exist predominately in the form of hollow tubular structures.1,5 Their average length and inner and outer diameters were found to be in the range of 500−800, 12−15, and 50−60 nm, respectively,5 though these projected values may vary depending on different types of available halloysites and their crystallization processes as well as geological occurrences.6 The inner diameters of the HNTs are further adjustable via acid etching.7 Inner and outer surfaces of HNTs have different compositions with aluminol (Al−OH) groups in the interior and silanol (Si−OH) groups on the exterior; this different inside/outside chemistry of halloysite is also unique among nanotubes.8 Natural availability in thousands of tons, low price, as well as exclusive compositional and morphological features of HNTs have, therefore, attracted several research groups to investigate their applications in myriads of fields including functional polymeric composites doped with HNTs loaded with chemical inhibitors © XXXX American Chemical Society
Received: July 10, 2017 Revised: October 5, 2017
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DOI: 10.1021/acs.langmuir.7b02402 Langmuir XXXX, XXX, XXX−XXX
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solution color rapidly changed from light yellow to slightly brownish after 3 min of stirring, indicating the formation of silver NPs. The final solution was kept stirring at the same temperature (55 °C) for another 20 min. The obtained dark Ag@halloysite NPPs were washed with ethanol/toluene and separated with multiple centrifugation steps. The final product was then redispersed in toluene and drop-cast on a TEM grid. Electron Beam Modification of Metal NPs. Ag and Au NPs located inside HNTs could also be modified/grown under the influence of TEM electron beam. For sample preparation, Ag@HNT or Au@HNT NPPs (synthesized using no ascorbic acid) were dropcast on copper TEM grids (200 mesh) and dried for 5 min at 70 °C in air before being loaded into the TEM. The electron beam was then converged (0.8 μA/cm2) on the area of interest, and the samples were imaged after being exposed to the converged electron beam for different time intervals. Characterization. A TEM (JEOL 2010) equipped with an EDAX genesis energy dispersive spectroscopy (EDS) system, operated at an accelerating voltage of 200 kV and an emission current of 109 μA, was used for all the sample observation/characterization. X-ray diffraction (XRD) data of powder samples were obtained on a Philips X’Pert PW 3040 MPD equipped with a curved graphite monochromator; the instrument was operated at 45 kV accelerating voltage and 40 mA current.
itation technique and investigated their use in the oxidation of benzyl alcohol.13 Formation of Ag nanorods (NRs) inside the HNTs via thermal decomposition of silver acetate and vacuum cycling has also been reported; however, the yield of this process was extremely low with only 4−5% of such core−shell structures formed.21 Although a few synthetic methods for the formation of noble metals on the surface or within the interior of different types of nanotubes have been reported, their fabrication routes are usually slow and require several steps, high temperatures, and special instrumentation.21−24 Moreover, there is no report on the formation of noble metal nanostructures inside HNTs with controlled size and morphology. We report a simple strategy for the rapid in situ growth of Au NPs in high yield and of controlled size and morphology within the interior of HNTs. A modified version of this approach has also been shown to be effective in the synthesis of Ag@HNT nanocomposites. The use of electron beam irradiation to modify noble metals in these core−shell, peapod-like nanosystems has been also exploited.
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EXPERIMENTAL SECTION
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Materials. HAuCl4·3H2O (gold(III) chloride trihydrate), oleic acid (99.8%, OAc), and oleylamine (>70%, OAm), silver nitrate (99.99%), and toluene (99.8%, anhydrous) were obtained from Sigma-Aldrich. LAscorbic acid (99+%) was purchased from Alfa Aesar. HNTs were provided by Applied Minerals Inc, NY and contained ca. 97% clay nanotubes. Rapid In Situ Growth of ∼6.7 ± 1.7 nm Au NPs within HNTs. Au@halloysite nanocomposites were synthesized using commercially available HNTs as templates. HNTs (15 mg), HAuCl4·3H2O (22 mg), ethanol (2 mL), toluene (2 mL), OAc (0.5 mL), and OAm (0.5 mL) were mixed inside a 20 mL glass vial and sonicated for 1 min. The mixture was magnetically stirred and heated to 55 °C. Ten milligrams of ascorbic acid was then added to the stirring mixture after which the solution color rapidly changed from light yellow to yellow and, after 2 min of stirring, to a dark purple solution indicating the formation of gold NPs. The obtained dark Au@halloysite nanopeapods (NPPs) were washed with ethanol/toluene and separated using a centrifugation step. To separate the free Au NPs from the peapod structures, the obtained product was dispersed in 10 mL of toluene and further centrifuged (1000 rpm, 30 s). Then, the supernatant, which mainly consists of free NPs, was removed, and another 10 mL of toluene was added to the centrifuge tube that consisted mainly NPPs. This step was repeated three times. The separated product was then redispersed in toluene, sonicated for 10 s, and drop-cast on a transmission electron microscope (TEM) grid. The samples were then kept at 70 °C for 5 min in air before analysis. In Situ Growth of ∼15.8 ± 3 nm Au NPs inside HNTs. To increase the size of Au NPs to ∼15.8 nm, the same experimental conditions used for the formation of ∼6.7 Au NPs inside HNTs were applied, except that no ascorbic acid was added to the stirring mixture. In this case, the solution color gradually changed and a dark purple solution was obtained after 12 h stirring at 55 °C. The obtained dark products were then washed with ethanol/toluene and separated using multiple centrifugation steps. Rapid In Situ Growth of Au NRs/Nanowires (NWs) inside HNTs. For the rapid formation of Au NRs/NWs within HNTs, the same experimental conditions used for the formation of ∼6.7 nm Au NPs inside HNTs were applied, but the amount of ascorbic acid was increased to 150 mg. The final solution was again kept stirring at 55 °C for 2 min. Rapid In Situ Growth of Ag NPs/NWs inside HNTs. To synthesize Ag@halloysite NPPs, HNTs (15 mg), silver nitrate (22 mg), ethanol (2 mL), toluene (2 mL), OAc (0.2 mL), and OAm (0.2 mL) were mixed inside a vial and sonicated for 1 min. The mixture was then heated to 55 °C and magnetically stirred for 1 min. Fifty milligrams of ascorbic acid was then added to the stirring mixture; the
RESULTS Gold Peapods. The in situ formation of gold NPs inside the HNTs readily occurred. Au NPs@halloysite NPPs were prepared via a rapid (2 min) and low-temperature (55 °C) synthetic approach when adding 10 mg of ascorbic acid reducing agent to the homogeneous and stirring mixture of HNTs, HAuCl4·3H2O, toluene, ethanol, OAm, and OAc. It is expected that fast loading of the gold ions and other components into the wettable clay nanotube interior was assisted by strong capillary pressure.1 The rapid formation of gold NPs within HNTs can be followed simply by looking at the color change of the solution with respect to reaction time. As shown in Figure 1, after adding ascorbic acid to the stirring mixture, the color of the solution rapidly changes. Within 2 min, the initial orange color becomes light yellow and finally a dark purple, consistent with the formation of Au NPs. TEM images of HNTs used in this study are shown in Figures 2a and S1. This rapid synthetic approach resulted in the in situ growth of Au NPs inside the HNTs, leading to the formation of high-quality NPPs with high NP loading (Figure 2b−d). The filling fraction of NPs to the empty space of HNTs was estimated to be >65%. Au NPs were also formed outside of the HNTs and directly within the liquid as shown in the TEM images of the product obtained before the purification step (Figure S2a,b). As expected, when the HNTs were eliminated from the synthesis media, the product inclusively consists of spherical-like Au NPs that formed rapidly (≤2 min) within the liquid (Figure S2c,d). The TEM images of the NPPs product obtained after the purification process (centrifugation and separation), however, show that the product mainly consists of uniform Au NP chains located inside the HNTs (Figure 2b−d). In situ grown gold NPs were generally spherical in shape. They have a fairly narrow size distribution, where the majority of NPs are in the size range of 5−9 nm; from an evaluation of over 200 NPs in a series of TEM images, an average size of 6.7 ± 1.7 nm was determined (Figure S3). In typical NPPs with a uniform arrangement of NP chains, interparticle distances were estimated to be ∼2.5 ± 0.2 nm; these values are comparable to our previously reported studies on the in situ growth of gold NPs within hexaniobate nanoscrolls (NScs), which further B
DOI: 10.1021/acs.langmuir.7b02402 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. (a) TEM images of HNTs used as the template for the in situ growth of NPs; (b−d) different magnification TEM images showing of ∼6.7 nm Au NPs@HNTs obtained when 10 mg of ascorbic acid was used. Arrows in (d) highlight approximate NP diameters (6.5−7.5 nm) and the approximate spacing between NPs (∼2.5 nm).
Figure 1. Color change of the solution: (a) before adding ascorbic acid and (b−i) after adding ascorbic acid for different time intervals.
confirms the presence of OAm on the surface of NPs as the capping agent.25 In some cases, however, the Au NPs were not as uniform and had reduced or increased interparticle distances (Figures 2b−d, S4, and S5). Formation of multiple rows of Au NP chains in a single HNT was also observed (Figure S4). In situ growth within HNTs with different lengths as well as different inner/outer diameters was also seen (Figure S5); here, we see that when there is an increase in the inner diameters of HNTs, the number of in situ grown Au NPs decreases (Figure S6). To investigate the effect of molar ratios of surfactant to solvent on the formation of Au NPs, the amount of solvent was increased from a total of 4 mL (2 mL of ethanol and 2 mL of toluene) to 20 mL (10 mL of ethanol and 10 mL of toluene). The amounts of surfactants and reducing agents were kept constant (0.5 mL of OAc, 0.5 mL of OAm, and 10 mg ascorbic acid). The TEM images of the product obtained after 12 h of the reaction showing the formation of NPPs are shown in Figure 3. It can be seen that the size of directly grown spherical Au NPs inside the HNTs was slightly increased to an average size of ∼6.7 to ∼7.9 nm (Figure S7). Interparticle distances in a chain of Au NPs (Figure 3d) were found to be around ∼4 ± 0.4 nm. The excess of solvent also influenced the nanostructure growth outside the HNT; the formation of NRs and aggregated NPs occurred with this increase in the solvent (Figure 3). The size of Au NPs could further be increased up to an average of ∼15.8 nm. By not including ascorbic acid in the reaction but maintaining all other reaction components (HNTs
Figure 3. (a,b) Low magnification TEM images of ∼7.9 nm Au NPs@ HNTs obtained when the amount of total solvents was increased from a total of 4 to 20 mL (10 mg of ascorbic acid was used); (c,d) higher magnification TEM images of the same sample.
(15 mg), HAuCl4·3H2O (22 mg), toluene (2 mL), ethanol (2 mL), OAm (0.5 mL), and OAc (0.5 mL)) with a reaction time of 24 h, these larger NPs could be accessed. The TEM images in Figure 4 as well as the size distribution histogram shown in Figure S8 indicate that NPs do not have a narrow size C
DOI: 10.1021/acs.langmuir.7b02402 Langmuir XXXX, XXX, XXX−XXX
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though the presence of aggregated Au NPs, free Au NPs, as well as those grown inside the HNTs were also realized. Comparable forms of nanostructures were obtained when using 75 mg of ascorbic acid, (Figure S9), though the aspect ratio of NRs and fraction of NRs to NPs were not as high as when 150 mg of ascorbic acid were used. The formation of Au NRs in hexaniobate NScs was shown in our previous study upon an increase in the reaction time.25 To investigate the HNT’s role as the template in the formation of NRs/NWs, the same reaction conditions were examined without the presence of HNTs. In this case, only Au NPs were observed (Figure S10). A schematic summarizing the different reaction conditions needed to construct the various nanostructures, NPs, NRs, and NWs, in HNTs is shown in Figure 6. The XRD patterns of the
Figure 4. (a,b) Low magnification TEM images of ∼15.8 nm Au NPs@HNTs obtained when no ascorbic acid was used; (c,d) higher magnification TEM images of the same sample showing the formation of bigger NPs. The inset in (d) shows EDS data for Au NPs@HNTs. Cu peaks is from the TEM grid.
distribution where the size of spherical-like Au NPs varies between 8 and 26 nm. NRs and NWs could also be rapidly grown within the HNTs. When the amount of ascorbic acid used as the reducing agent was increased to 150 mg, NRs/NWs were produced in less than 2 min. The TEM images (Figure 5) of the product obtained after the separation process show the formation of NRs/NWs,
Figure 6. Schematic showing the in situ growth of gold NPs/NRs inside HNTs with respect to different reaction conditions.
Figure 7. XRD patterns of (a) ∼6.7 nm Au NPs@HNT, (b) ∼15.8 nm Au NPs@HNT, (c) Au NRs@HNTs, (d) HNTs used as template, and (e) Au reference pattern.
various composites (Figure 7) indicate the presence of both HNTs and gold. The peak broadening seen in the gold reflections, (111), (200), and (220), is consistent with different particle sizes, with the ∼6.7 NPs in the Au NPs@HNT sample (Figure 7a) exhibiting the greatest breadth.
Figure 5. (a,b) Low magnification TEM images of Au NR@HNTs obtained when the amount of ascorbic acid was increased to 150 mg; (c,d) higher magnification TEM images of the same sample showing the formation of NRs and aggregated NPs. D
DOI: 10.1021/acs.langmuir.7b02402 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Silver Peapods. A modified synthetic approach has also been applied for the rapid formation of Ag nanostructures within the interior of HNTs. While silver nanostructures are readily produced within the HNTs, unlike gold, they are polydispersive in nature. The TEM images of the obtained products show the presence of both NPs and NWs within the HNTs. The quality and number of in situ grown NPs (Figure 8) are not as high as in the Au@HNT samples. Ag NPs with
Figure 8. TEM images of (a,b) Ag NPs@HNTs and (c,d) Ag NRs/ NWs@HNTs. The inset in (b) shows a top view along the HNT axis and that Ag NPs are clearly located inside HNT.
Figure 9. TEM images showing the impacts of the electron beam on the in situ growth of NPs and formation of NRs. (a) TEM image of Ag@HNT, (b) TEM image of the same sample after the marked area in (a) was exposed to a focused electron beam (109 μA) for 1 min. (c,d) TEM images of the Au@HNT sample exposed to a focused electron beam for different time intervals; (c) right after the Au@HNT sample was exposed to electron beam; (d−f) respectively after 1, 3, and 10 min of focused electron beam exposure. Images (c−e) have the same magnifications, whereas (f) represents a higher magnification image of the understudied area.
different sizes ranging from 2.8 to 14 nm can be seen (Figure 8a,b). Formations of Ag NWs up to 570 nm long can also be observed (Figure 8c,d). Interestingly, an HNT pointing in the direction of the electron beam provides a top view of the formed NPPs (Figure 8b, inset); this directly shows that NPs are located inside the HNTs rather than being attached to the surface. The XRD patterns of the sample after the separation step also confirm the formation of crystalline Ag@HNT nanocomposites (Figure S11). Electron Beam Modification of Metal NPs. Ag and Au NPs within the HNTs can be manipulated with focused electron irradiation. Ag@HNT and Au@HNT samples were dispersed on copper TEM grids and dried for 5 min at 70 °C before being loaded into the TEM. Ag@HNTs, before exposure to the converged electron beam, are shown in Figure 9a. After 1 min of converged electron beam exposure on the shown area in Figure 9a, the formation of Ag NRs from the union of smaller Ag NPs can be observed (Figure 9b). Similar experiments were also performed on Au@HNTs. As the sample was being exposed under focused electron beam for different time intervals, a clear growth of NPs can be observed within the HNTs as the adjacent NPs merge together. Figure 9c,d was imaged when the electron beam was not converged on a small area. One can clearly notice that the smaller Au NPs were rapidly, within 1 min, attached to the bigger Au NPs (Figure 9d). The electron beam was then converged on the sample (Figure 9d). After 3 min of exposure time, initial
attachment of two short NRs presented by a red arrow was detected (Figure 9d,f). Further growth of NRs was achieved after 10 min of irradiation (Figure 9f); two short NRs join with each other (represented by a red arrow) and a small NP and an NR also merge (green arrow). The trace of the converged electron beam on the TEM grid can clearly be seen from Figure 9f.
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DISCUSSION Several properties and applications of naturally available tubule nanoclays, including their use in catalysis, drug loading, delivery and release systems, and water treatment, can benefit from rapid and fast functionalization/modification of inner/outer surfaces using noble metals.8,10,19,20,26−29 Since 2011, some of us have developed a number of different approaches for the formation of NPP composite materials such as solvothermal encapsulation,30−32 solvent evaporation,31 and in situ growth of NPs.25,33 These techniques allow different classes of NPs to be loaded within the hollow space of tubular structures. This in E
DOI: 10.1021/acs.langmuir.7b02402 Langmuir XXXX, XXX, XXX−XXX
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through the capillary force.22,24,42 Our TEM observations also showed mainly the presence of more NPs with smaller diameters within HNTs (Figure S6); this could indicate that during the NP growth, that diffusion of additional gold ions within the HNT is limited relative to those free NPs forming outside the HNT. The entrance of solvents into capillary tubes depends on various parameters such as radius of the nanotube, surface tension, and the viscosity of the solvent.24,42 Though there are some fundamental studies on the capillary forces at the nanoscale based on the formation of second compounds/ structure in the cavity of carbon nanotubes,22,24,43,44 the fabrication/investigation approaches are complicated and usually require high-temperature synthesis methods with multiple steps. Using our developed synthesis strategy for the formation of NRs/NPs/NWs in the cavity of tubular structures, one may consider further investigation on capillary forces at the nanoscale by looking at the rapid formation of noble metal nanostructures at low temperatures inside the cavity of nanotubes while varying, for instance, morphological features of nanotubes, surface tension, and viscosity of solvents. Careful attention to the experimental conditions provides effective synthetic routes for size and morphological controls of noble metal (Au/Ag) nanostructures grown within HNTs. Crystalline Au NPs in high yields with an average size of ∼6.7 nm were successfully formed within the HNTs (Figure 2) in as little as 2 min after 10 mg of ascorbic acid were added to the stirring mixture of HNTs (15 mg), gold(III) chloride trihydrate (22 mg), ethanol (2 mL), toluene (2 mL), OAc (0.5 mL), and OAm (0.5 mL). When the amount of solvent was increased from 4 to 20 mL and 10 mg of ascorbic acid was used, the reaction time as well as the average NP size was increased to 12 h and ∼7.9 nm, respectively (Figures 3, and S7). These differences in the reactivity are likely related to the influence of the amount of solvent on nucleation and growth rates;45 by increasing the amount of the solvent, the reducing ability of synthesis media and, therefore, gold atom generation rates were decreased, resulting in the formation of fewer nuclei. Because the amount of the precursor was kept constant, the presence of fewer nuclei led to the formation of bigger Au NPs. As previously discussed, after the nucleation step, metal species need to diffuse from the bulk liquid to the surface of growing nuclei/crystals and get adsorbed.39 The migration rates of Au atoms or ions to the surface of formed nuclei/growing crystals is decreased by an increase in the amount of the solventthis resulted in an increase in the reaction time from 2 min to 12 h. The sizes of NPs were further increased to an average of ∼15.8 nm for a reaction time of 24 h, when the ascorbic acid was eliminated and the volume was set at 4 mL of solvents (Figures 4 and S8). Similarly, this can be directly related to the decreased nucleation/migration rate, which results when only OAm, a weak reducing agent, is used.46,47 Formation of Ag/Au NRs/NWs within HNTs was also successfully performed. While the formation mechanism of Au NRs/NWs within HNTs when the amount of ascorbic acid was increased to 150 mg (Figure 5) is not entirely clear, we believe that the adsorption/interactions of the surfactant with the surface of growing crystals located within HNTs were affected upon an increase in the amount of ascorbic acid in such a way that NPs have a tendency to be grown together possibly during either oriented attachment or coalescence processes.39,48 This is also evidenced by granular structures of NRs (Figure 5c,d), which appear to be fused NPs and are likely formed because of NP−NP attachments. In the case of oriented attachment, NP
turn leads to new properties arising from NP−NP and NP− NT/NSc interactions.30−33 The formation of NP chains within the tubular structure, moreover, provides a system for the fundamental studies of NP−NP interactions.34,35 Though we have previously demonstrated the in situ growth of Au NPs within hexaniobate NScs through a 24 h reaction,25 the rapid formation of halloysite-based NPPs in as little as 2 min is new. This is especially exciting for technological applications because of the fact that HNTs are biocompatible, cheap, and readily available.36 This rapid in situ growth of precious metal NPs, Ag and Au, within tubular structures can further facilitate the development of bifunctional NPPs25 and provide a system for the investigation of NP interactions Rapid in situ growth of precious metal NPs directly inside the HNTs was effectively achieved through a simple and lowtemperature (55 °C) synthetic approach utilizing the HNTs as nanoreactors/templates. For making both Ag@HNTs and Au@HNTs, the synthesis media consisted of noble metal ions, surfactants (OAm and OAc), as well as reducing agents (ascorbic acid with OAm or OAm by itself). The presence of miscible polar/nonpolar solvents is also necessary for the uniform dispersion of Chinese HNTs as practical hydrophilic components, gold ions, as well as surfactants. As depicted in Figure 1a, after the sonication step, the obtained mixture was highly stable and homogeneous, which is necessary for the formation of uniform nanostructures. Practically, this homogeneity could not be reached by using either toluene or ethanol as the only solvent. After adding ascorbic acid to the stirring solution, reduction of noble metal ions to atoms occurs rapidly, and when the concentration of atoms reaches to a minimum concentration needed for nucleation, NPs readily grow.37−39 The growth of nuclei happened by the migration of noble metal atoms to the surface of growing crystals, leading to the formation of noble metal NPs.37−40 The presence of surfactants is also needed for the shape control as well as preventing the aggregation of growing NPs.38,40,41 Our TEM characterization after the separation step showed the presence of Au NPs mainly inside the HNTs (Figures 2−4). The existence of NPs outside the HNTs, however, suggests that some portions of Au NPs were also formed directly within the solution. In other words, nucleation and growth of precious metal NPs can happen either directly inside the solution/liquid as homogeneous nucleation sites or on heterogeneous nucleation sites within HNTs.37 One might also expect the possibility that preformed NPs could diffuse into the HNTs; we found, however, that by stirring a mixture of preformed NPs and HNTs, NPPs were not formed. Direct formations of NPs within HNTs necessitate the presence of solution/liquid containing noble metal ions into the interior of HNTs. Our estimations for the in situ formation of an NPP (e.g., volume 15% filled with Au NPs) showed that the volume of the solution that should enter into the inner site of a single HNT is over a thousand times more than the interior volume of an HNT; this indicates that a rapid and permanent flow-supply of gold atoms/ions from a bulk solution into the tubes is needed for the direct formation of gold NPs within the HNTs (for details on estimation, see Supporting Information, page S14). The formation of nearly uniform-sized NPs throughout the interior of HNTs (Figure 2b−d) indicates the presence of a continuous rapid flow of solution, which provides similar diffusion/migration rates of gold atoms/ions to the surfaces of growing crystals evenly across different sites of HNTs (ends vs the center). Liquid or solutions containing metal ions/atoms likely enter into the interior of HNTs F
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nanomaterials were mainly adjusted by changing the amount of ascorbic acid used as the reducing agent. The resulting unique nanocomposite structures may be considered as nanoreactors with a concise space confinement as well as unique catalytic systems whose active NPs have been protected by halloysite tube walls. A modified version of this synthetic approach can also be applied to the formation of Ag NPs and NRs within the HNTs. We have also shown that adjacent metal NPs can be connected using converged electron beam and that our NPP systems can be used for the fundamental studies of NP growth/attachment without any requirement of liquid cells.
attachments can happen through a common crystallographic facet to minimize the interfacial energy. For the coalescence, though, there is no specific preference observed for the attachment and crystal facets are randomly positioned between domains.39,48 The role of HNTs as a template for the direct formation of NRs/NWs can be further highlighted considering two clear observations: the presence of Au/Ag NRs/NWs only inside the HNTs (Figures 5 and 8) and the formation of only NPs when using the same experimental conditions but without the presence of HNTs (Figure S10). The oriented attachments/coalescence of NPs are most likely encouraged when they are located inside HNT templates. Formation of Ag NPs/ NWs using a modified procedure also confirmed the versatility of our approach for the formation of other metal/noble metal nanostructures within HNTs. Methods for the rapid formation of other noble metal NPs such as Pd within HNTs using a similar approach, but stronger reducing agents as well as using solvothermal encapsulation and solvent evaporation, are currently being sought. Another interesting aspect of this work was the observation of in situ electron beam assisted growth of Au/Ag NPs located within HNTs. To fully understand the formation of NPs during nucleation and growth processes, real-time observation is essential. Though there are several fundamental studies on the formation of NPs using TEM, they mainly involve liquid cells and are based on the formation of NPs within the liquid.49−53 Many strategies for using in situ electron microscopy via liquid cell and their application is reviewed by de Jonge et al.52 For example, Zheng et al. investigated the real time in situ growth of platinum NPs utilizing liquid cell and observed the growth of NPs by monomer attachments and coalescence of NPs.53 Our results, however, showed that the presence of noble metal NP chains within HNTs provides a system for direct observation of NP growth inside HNTs using a converged electron beam; this approach does not require a liquid cell. Rapid attachment of smaller NPs and formation of bigger NPs in Au@HNTs NPPs as well as attachment of similarly sized NPs using the focused electron beam at their interface led to the formation of NRs in Au/Ag@HNT NPPs (Figure 9). Formations of Pt3Fe NRs through the attachment of NPs presented in a liquid cell were previously reported by Liao et al.54 Li and Zhang have also reported studies on the increase in Ag NP sizes with respect to electron irradiation times.55 One possible reason for the in situ growth of NPs located within HNTs can be the ability of focused electron beam to act as a source of energy/heat and its influence on the sintering/melting of two adjustment NPs leading to the formation of bigger particles.55 Converged electron beam might also reduce any remaining noble metal ions to atoms,54,56,57 which could still exist within the HNTs and in the vicinity of the already formed NPs. Electron beam can also transport noble metal atoms to the interface of the NPs allowing further growth.39 Our noble metal@HNT systems may, therefore, serve as reaction vessels for the observation of atomic resolution TEM images, and these in turn would allow one to investigate important questions related to NP−NP interactions, NP or NR growth, and interfacial relaxations.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02402. TEM images of HNTs; TEM images of the Au NP formation in the absence of HNTs and within HNTs; size distribution histograms of Au NPs with varying amounts of ascorbic acid; and XRD patterns of Ag@ HNTs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
John B. Wiley: 0000-0002-6954-6752 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support from the National Science Foundation CHE-1412670 is gratefully acknowledged. T.R. also thanks the International Precious Metals Institute (IPMI) for support through the Gemini Graduate Student Award. A.V.S. and Y.M.L. thank support by the Ministry of Education and Science of the Russian Federation (grant 14.Z50.31.0035).
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ABBREVIATIONS NPs, nanoparticles; NScs, nanoscrolls; NTs, nanotubes; NPPs, nanopeapods; NRs, nanorods; NWs, nanowires; OAm, oleylamine; OAc, oleic acid
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REFERENCES
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CONCLUSIONS In summary, a rapid (≤2 min) and low-temperature synthetic approach (55 °C) has been introduced to adjust the in situ growth of Au nanostructures within HNTs with controlled size and morphology. The size and dimensions of the Au G
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