Review pubs.acs.org/cm
Insights into the Seeded-Growth Synthesis of Colloidal Hybrid Nanoparticles James M. Hodges, James R. Morse, Julie L. Fenton, Jonathan D. Ackerman, Lucas T. Alameda, and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Colloidal hybrid nanoparticles integrate two or more nanocrystal domains into a single architecture that can have properties not found in, or enhanced relative to those of, the individual components. These hybrid nanomaterials are typically constructed using multistep seeded-growth reaction sequences, which are conceptually analogous to the total synthesis approaches used in molecular synthesis. Here, we discuss in detail the synthetic protocols that lead to the formation of three-component Ag−Pt−Fe3O4 and Au−Pt−Fe3O4 heterotrimers. These instructive model systems highlight the important synthetic details that underpin successful hybrid nanoparticle reactions. We provide detailed, step-by-step protocols for generating these materials, focusing on describing and rationalizing the key reaction parameters that need to be rigorously controlled to minimize unwanted nanoparticle byproducts. The importance of comprehensive analysis using a suite of materials characterization tools is highlighted, as such efforts are useful for diagnosing subtle chemical and morphological features that can lead to synthetic bottlenecks throughout the course of the reaction sequences. Finally, we offer strategies for circumventing these commonly encountered problems as well as insights that can lead to increased hybrid nanoparticle yields and improved sample-to-sample reproducibility. Although this work specifically details the synthesis of Ag−Pt−Fe3O4 and Au−Pt−Fe3O4 heterotrimers, these synthetic strategies and protocol guidelines are generally applicable to many other hybrid nanoparticle systems.
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yields.13,16−20 The studies that underpin this growing body of work have provided insights into the key synthetic parameters that lead to the successful formation of target hybrid nanoparticle systems. Here, we discuss the synthesis of two prototypical heterotrimers, Ag−Pt−Fe3O4 and Au−Pt−Fe3O4, which provide instructive reaction systems for highlighting the common synthetic bottlenecks encountered in hybrid nanoparticle synthesis. We provide detailed, step-by-step protocols for making these materials, with particular emphasis on the subtle synthetic parameters that must be rigorously controlled to generate hybrid nanoparticles with well-defined configurations in high yields. Additionally, we present strategies for troubleshooting the synthetic bottlenecks that are commonly encountered in seeded-growth protocols, which can be applied to a variety of two- and three-component hybrid nanoparticle systems. The importance of comprehensive characterization is also emphasized, as this is critical for identifying compositional
INTRODUCTION Colloidal hybrid nanoparticles, which integrate two or more nanocrystal domains into a single architecture through solidstate interfaces, offer greater tunability and enhanced functionality relative to those of one-component systems. Heterojunctions within these structures can allow electromagnetic coupling and synergistic properties, which are helping to advance applications in solar energy conversion,1−3 heterogeneous catalysis,4−6 magnetism,7,8 optoelectronics,9 cellular imaging,10 and theranostics.11,12 Typically, hybrid nanoparticles are synthesized through seeded-growth procedures, during which preformed nanocrystals are used as seeds for growing additional domains in subsequent reactions.13−15 These stepwise reaction sequences are conceptually similar to the multistep total synthesis approaches used in organic chemistry to construct large, complex molecules. However, the rules and guidelines that govern seeded-growth routes to hybrid nanoparticles are less understood, and for this reason, it can sometimes be challenging to synthesize colloidal hybrid nanoparticles with targeted configurations and desired properties. In recent years, our group has been developing a synthetic toolbox for constructing three- and four-component nanoparticle systems with precise domain configurations in high © 2016 American Chemical Society
Special Issue: Methods and Protocols in Materials Chemistry Received: July 8, 2016 Revised: September 2, 2016 Published: September 6, 2016 106
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highly sensitive to subtle changes in the reaction conditions, which can lead to problems with reproducibility if the conditions are not carefully controlled. Accordingly, in addition to providing strategies for troubleshooting this reaction, we offer an alternative approach that uses an Fe(acac)3 precursor, which produces high-quality Pt−Fe3O4 heterodimer seeds more consistently. In the final step of the reaction sequence, the Pt− Fe3O4 heterodimers (from either approach) are reacted with silver(I) acetate [Ag(C2H3O2) or Ag(OAc)] or gold(III) chloride trihydrate (HAuCl4·3H2O) in the presence of OLAM at 60 °C for 16 or 4 h, respectively. OLAM serves two roles in these reactions: as a reducing agent for converting the metal salts into zerovalent Ag0 and Au0 and as a surfacestabilizing ligand. The longer reaction time for the Ag system facilitates more uniform Ag domains on the Ag−Pt−Fe3O4 heterotrimer product, presumably through slower ripening processes for Ag versus Au. The final Ag−Pt−Fe3O4 and Au− Pt−Fe3O4 heterotrimer products are then precipitated, cleaned, and redispersed in hexanes for characterization. After each step of the reaction sequence, nanoparticle products are isolated and examined using a suite of characterization techniques, including powder X-ray diffraction (XRD), selected area electron diffraction (SAED), low- and highresolution transmission electron microscopy (TEM and HRTEM, respectively), and scanning transmission electron microscopy coupled with energy dispersive spectroscopy (STEM-EDS). The utility of STEM-EDS is highlighted, particularly for confirming the spatial distribution of the various chemically distinct nanoparticle domains. STEM-EDS is also useful for detecting subtle nanochemical details, such as unwanted FexOy shells on the Pt surfaces of Pt−Fe3O4 heterodimers that are often not detected using lower-resolution TEM techniques but that have a profound influence on the reaction outcome. Details regarding the utility of each characterization tool, including strategies for diagnosing subtle compositional ambiguities when high-resolution STEM-EDS analysis is not available, are outlined in Characterization.
and structural ambiguities that otherwise remain unknown yet are necessary for achieving successful reaction outcomes. The synthesis of both Ag−Pt−Fe3O4 and Au−Pt−Fe3O4 heterotrimers involves three-step reaction sequences, as shown schematically in Figure 1, where Ag or Au is added to
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EXPERIMENTAL OVERVIEW Colloidal nanochemistry, as outlined and discussed in detail elsewhere,21,22 is inherently complex in terms of synthetic implementation and reproducibility. It is well-known that subtle changes in reaction conditions, even those not initially anticipated to impact a reaction’s pathway or outcome, can dramatically impact the property-defining structural and compositional characteristics of the resulting nanoparticle products.23−25 These intricacies are compounded in multistep, seeded-growth reactions of hybrid nanoparticles, which rely on complex chemical environments and dynamic (and often poorly understood) heteronucleation and growth processes. Accordingly, synthetic details such as reagent purity, proper calibration of temperature probes, cleanliness of glassware and other equipment used in reaction setups, strict air-free conditions, and workup procedures need to be rigorously controlled to produce the desired hybrid nanoproducts in high yields. Reaction Setup. All of the reactions described in this protocol are performed using standard Schlenk line techniques, and a typical reaction setup is shown in Figure 2. We use threeneck flasks that allow for attachment to the Schlenk line, insertion of a thermometer adapter, and use of a septum that will allow for injection of precursors when necessary; disposable septa are used to avoid contamination. Colloidal nanocrystal
Figure 1. (a) Schematic showing the seeded-growth reaction sequence for generating Au−Pt−Fe3O4 heterotrimers, along with the corresponding TEM images of the (b) Pt, (c) Pt−Fe3O4, and (d) Au−Pt− Fe3O4 heterotrimer nanoparticles. All scale bars are 25 nm.
preformed Pt−Fe3O4 heterodimer seeds. In-depth protocols, including photos of reaction setups, detailed notes for each step, and suggestions for circumventing common problems, are found below. As a brief overview of a typical synthesis, Pt nanocrystal seeds are first synthesized by reducing platinum(II) acetylacetonate [Pt(C5H7O2)2 or Pt(acac)2] with Fe(CO)5 at 190 °C in 1-octadecene (ODE). In this hot-injection protocol, Fe(CO)5 is used to reduce the Pt2+ salt to generate Pt0, and importantly, only small amounts are used to minimize incorporation of Fe into the Pt nanoparticles and/or surface contamination by iron oxide (FexOy) species. In the second step of the reaction sequence that yields Pt−Fe3O4 heterodimer nanoparticles, the presynthesized Pt seeds are reacted at 315 °C with Fe(CO)5 in the presence of an ODE solution of oleic acid (OLAC) and oleylamine (OLAM), a surfactant mixture that is commonly used in colloidal nanoparticle synthesis. In this step, Fe(CO)5 and OLAC react in situ to form a putative Fe3+− oleate complex [Fe(C18H34O2)3], which thermally decomposes to form iron oxide, while OLAM and residual OLAC serve as surface-stabilizing agents. In our experience, this reaction is 107
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although inaccurate temperature readings have been observed when the PID (proportional−integral−derivative) parameters are not set correctly. Importantly, these parameters need to be tuned for each set of reaction conditions, including choice of solvent, volume of solvent, and temperature range, to deliver accurate temperatures and consistent ramp rates. Reactions can also be readily monitored using alcohol-based thermometers, which provide an inexpensive alternative to mercury thermometers and calibrated thermocouples, but they are limited to reaction protocols operating under 260 °C. The reaction temperatures that we report correspond to the temperature of the solvent as measured by the chosen thermometer or thermocouple/temperature controller system. Inconsistencies found in the literature, such as reports of reaction temperatures that exceed the boiling points of the solvents, are presumably the result of improper use of thermocouples and temperature controllers or reaction setups that use indirect temperature measurements, such as procedures that monitor the temperature of the sand in a sand bath setup. Technique. All reactions described in this protocol are degassed under vacuum at elevated temperatures, except when working with low-boiling point solvents, such as toluene. This ensures that the reaction environment is consistently free of air and water, which can cause unwanted oxidation of chemical reagents and newly formed nanoparticles and are therefore detrimental to colloidal synthesis. When possible, we prefer to use disposable syringes and needles in procedures that call for injection of chemical reagents into reaction solutions, such as the Pt nanoparticle synthesis described below, to reduce the number of sources of possible contamination. In all cases described below, nanocrystal products are precipitated and cleaned using appropriate solvents that decrease the colloidal stability of suspended nanoparticles (“antisolvents”), followed by centrifugation and then resuspension of the collected nanoparticles in hexanes for further use and characterization. Hexanes have a boiling point lower than those of other commonly used solvents, such as toluene, which is desirable for deposition onto TEM grids and XRD plates. For seededgrowth synthetic methods to be effective, the mass of nanoparticle seeds being added to the reaction solution must be known. Accordingly, prior to performing a seeded-growth process, a consistent way to analyze the nanoparticle concentration in a colloidal solution must be identified. In many cases, including for the Pt reaction described in the following protocol, nanocrystal mass yields are near unity (see Yields), and synthesized particles can be dispersed in a premeasured volume of solvent to yield a desired concentration with reasonable precision. Alternatively, the concentration of the nanoparticle suspensions can be determined using an analytical microbalance (see Reaction Protocol). In our experience, presynthesized nanoparticle seeds should be used within 1 week for subsequent seeded-growth reaction steps to obtain the heterodimer and heterotrimers in optimal yields. Yields. Seeded-growth approaches to the synthesis of colloidal hybrid nanoparticles are conceptually similar to the total synthesis methods used by organic chemists to construct large complex molecules. However, unlike in synthetic organic chemistry, conventions for reporting the yield of hybrid nanoparticle products are not well established, in part because “yield” can refer to various aspects of a nanoparticle sample, including the absolute yield and the morphological yield. In addition to these complexities, high-order hybrid nanoparticle
Figure 2. Photograph of a typical, fully assembled reaction setup, including labels showing the (a) inlet adaptor, (b) Liebig condenser, (c) 50 mL three-neck flask, (d) heating mantle, (e) thermometer adaptor, and (f) thermocouple, along with photographs of the unassembled components.
reactions often leave surfactant and metallic residues on glassware that can be detrimental to subsequent reactions, and accordingly, proper cleaning that completely removes organic and inorganic residues is critical. We clean our glassware using a sequence of acid and base baths, where flasks are first soaked in strong acid (such as 3 M HCl) for roughly 1 h, followed by a longer soak in a strong base bath (such as 1 M NaOH in a 50:50 water/ethanol mixture), usually overnight. The glassware is then scrubbed with soap and water and finally rinsed with deionized water and placed in a drying oven that is set to a minimum temperature of 100 °C for at least 30 min before being used. When working with noble metals, such as Pt and Au, we use a 10 min soak in aqua regia (a 1:3 HNO3/HCl solution) to fully remove these metals. Note that all of these cleaning solutions are highly corrosive and caustic, and as such, they must conform to established safety and usage guidelines and be handled carefully and properly by trained personnel. Schlenk lines, which can also be a source of unwanted contamination, are also cleaned regularly. Thermometers. As is the case with most aspects of colloidal nanochemistry, the reactions described below are extremely sensitive to temperature, and accordingly, accurately measuring the temperature of these reactions is critical. Mercury thermometers are highly accurate for monitoring colloidal nanoreactions, particularly at high temperatures, but because they contain mercury, they have been replaced with thermocouples connected to temperature controller units. Such systems offer an environmentally friendly alternative to mercury that can be equally effective when implemented correctly, 108
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for accessing a range of temperatures relevant to colloidal nanoparticle reactions. It is sold with a purity of 90%, and while the remaining 10% is presumed to be other long chain hydrocarbons, this has not been reported. Platinum(II) Acetylacetonate [Pt(acac)2, Pt(C5H8O2)2, 97% (Sigma-Aldrich, catalog no. 282782)]. Pt(acac)2 is a widely available Pt2+ reagent that is soluble in a variety of nonpolar solvents. Additionally, unlike other common Pt reagents, such as PtCl2, the acetylacetonate counteranion of the Pt(acac)2 platinum reagent does not bind strongly to nanocrystal surfaces and therefore does not play a significant role in nanoparticle growth. It is sold with a purity of 97%; the remaining impurities have not been reported. Pt(acac)2 is kept in a desiccator under vacuum to preserve its shelf life. The reagent is observed to become darker over time, and we find that the darker-colored reagent commonly produces Pt nanoparticles that are polydisperse in size and shape. Iron(0) Pentacarbonyl [Fe(CO)5, >99.99% (Sigma-Aldrich, catalog no. 481718)]. Fe(CO)5 is commonly used as an iron precursor and as a reducing agent in nanoparticle reactions. This liquid reagent is directly compatible with hot-injection syntheses and is also soluble in a variety of solvents. Fe(CO)5 is highly air sensitive and readily oxidizes; therefore, it is kept refrigerated in a glovebox under inert atmosphere to prevent decomposition. Silver(I) Acetate [Ag(OAc), Ag(C2H3O2), anhydrous, 99% (Sigma-Aldrich, catalog no. 204374)]. Ag(C2H3O2) is a widely available silver reagent that is soluble (with oleylamine) in toluene, a commonly used solvent in Ag nanoparticle reactions, and contains a relatively inert acetate counteranion. Ag(OAc) should be kept away from light to prevent photodegradation. Gold(III) Chloride Trihydrate [HAuCl4·3H2O, 99% (SigmaAldrich, catalog no. 520918)]. HAuCl4·3H2O is used as the gold reagent because of its high solubility in oleylamine, as well as its widespread availability and high purity. HAuCl4·3H2O is stored in a glovebox under an inert atmosphere to minimize exposure to light and moisture, which preserves its shelf life. Iron(III) Acetylacetonate [Fe(acac)3 or Fe(C5H8O2)3, 99% (STREM Chemical, catalog no. 26-2300)]. Fe(acac)3 is used as a reagent for iron oleate formation in Pt−Fe3O4 synthesis, offering a more stable alternative to Fe(CO)5. Hexanes (reagent grade). Hexanes, which is a readily available mixture of five structural isomers having the formula C6H14, is commonly used to disperse nanoparticles that are functionalized with nonpolar ligands. Hexanes has a boiling point lower than that of other comparable solvents and accordingly, evaporates quicker and makes it easier to deposit nanoparticles onto substrates for characterization. Isopropanol (IPA, reagent grade). IPA is an effective antisolvent for precipitating nanocrystals that are stabilized with OLAC and OLAM and dispersed in nonpolar solvents, such as hexanes and ODE. Toluene (reagent grade). Toluene is a widely available solvent that is used in the Ag−Pt−Fe3O4 synthesis because of its ability to dissolve (with oleylamine) Ag(OAc), a highly effective Ag nanoparticle precursor. Chemical Safety. Prior to running any chemical experiment, including the protocol listed below, one should thoroughly study the material safety data sheet (MSDS) for all chemical reagents to identify any potential dangers and health risks and also to help in the development of a plan to safely conduct the reactions. As is the case for all chemical reactions, including
systems, such as the three-component heterotrimers described below, can adopt multiple configurations with unique connectivities and distinct functionalities. The issue of yield reporting for colloidal nanoparticle systems has been discussed in detail elsewhere,21,26 and these “best practices” should also be routinely applied to hybrid nanoparticle systems. We recommend reporting as much yield information as possible and also explicitly identifying how each reported yield was obtained and what it corresponds to. For example, absolute yield can be determined by comparing the theoretical yield to the mass of total synthesized nanoparticles (see Reaction Protocol). Morphological yield (the fraction of the sample that has a particular morphology) can be estimated through TEM analysis of a statistically relevant number of particles on different parts of multiple TEM grids, assuming that corresponding bulk analyses are consistent. When morphological yields of nanoparticle products are being reported, it is good practice to perform Scherrer analysis on the peaks from corresponding powder XRD diffraction patterns to determine the size of the crystallites, which should corroborate the TEM and HRTEM data.27,28
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MATERIALS As noted by many other authors, batch-to-batch inconsistencies in chemical reagents are often found to hinder reproducibility attempts in colloidal nanoparticle reactions.29−31 In addition to being aware of these potential inconsistencies, it is generally good practice to purchase reagents with the highest possible purity, store them properly, use them prior to the onset of any type of chemical degradation, and implement purification protocols, as needed and available. Many commonly used surfactants in nanoparticle synthesis are purchased and used at low purities because of cost considerations. In some cases, further purification processes are used to remove unwanted impurities that can contaminate nanoparticle products and be detrimental to morphological yields. Reagents. Oleylamine [OLAM, C18H35NH2, technical grade, 70% (Sigma-Aldrich, catalog no. O7805)]. Oleylamine is an unsaturated primary amine that is structurally similar to 1octadecene and oleic acid, as they all contain an 18-carbon chain with a single double bond. As mentioned previously,32 oleylamine can serve as both a surface-stabilizing ligand and a reducing agent. While OLAM is sold as a technical grade chemical with a purity of 70%, the remaining 30% consists mainly of other primary amines (>98%), as well as unidentified chemical species that are likely other related amines. All reactions described in the protocol are highly sensitive to inconsistencies in oleylamine, as well as degradation that can occur over time. Oleic acid [OLAC, C18H34O2, technical grade, 90% (SigmaAldrich, catalog no. 364525)]. Oleic acid is an unsaturated carboxylic acid that is structurally similar to 1-octadecene and oleylamine, as mentioned above. Oleic acid is used as a surfacestabilizing ligand in the synthesis of Pt and Pt−Fe3O4 and as a precursor to iron oleate formation in the synthesis of Pt−Fe3O4 heterodimers. It is sold with 90% purity, and the remaining 10% is presumed to be other carboxylic acid species, although this has not been reported. 1-Octadecene [ODE, C18H36, technical grade, 90% (SigmaAldrich, catalog no. O806)]. 1-Octadecene is a long chain alkene that is structurally similar to oleylamine and oleic acid, as mentioned above, and it serves as a solvent in the reported syntheses. The boiling point of ODE is 315 °C, making it useful 109
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and workup, and troubleshooting suggestions can be found in Table S1.
colloidal nanoparticle syntheses of all kinds, it is imperative that good lab safety practices always be used. This includes, but is not limited to, the use of personal protective equipment (PPE), including safety goggles and gloves that are compatible with the chemicals being used, at all times. Only individuals properly trained in chemical safety, chemical reactivity, and the appropriate chemical manipulations should be performing these experiments. With this in mind, some chemicals used in these syntheses pose more of an immediate safety risk than others. The list below outlines some of the key safety considerations for these reagents. However, this list is in no way comprehensive, nor is it meant to replace in-depth consideration and study of the corresponding MSDS documents or the development of individual standard operating procedures for use of these reagents. Oleylamine (OLAM). Corrosive chemical reagent that can cause serious burns upon contact with skin or eyes. Always ensure that proper personal protective equipment, such as safety goggles, a lab coat, and appropriate gloves, is worn when working with this reagent. Iron Pentacarbonyl [Fe(CO)5]. Flammable and highly toxic reagent, which releases 5 equiv. of poisonous carbon monoxide gas during the Pt−Fe3O4 synthesis. Only use small volumes in well-ventilated areas. Inhalation must be avoided. Hexanes and Toluene. Flammable, neurotoxic reagents. Only use in well-ventilated areas. Avoid inhalation.
Figure 3. Photographs showing the Pt nanoparticle reaction (a) prior to and (b) after injection of Fe(CO)5, illustrating the yellow to black color transition that indicates the formation of Pt nanoparticles. Photographs show the color of the supernatant after the (c) first, (d) second, and (e) third centrifugation cycles.
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REACTION SETUP Unless otherwise noted, all stepwise reactions are conducted in 50 mL three-neck round-bottom flasks with 14/20 joints equipped with a rubber septum, magnetic stir bar, condensing column, and thermometer or thermocouple, as shown in Figure 2; 100 mL three-neck round-bottom flasks with 24/40 joints are an effective alternative for larger-scale reactions. For highboiling point solvents heated to or near reflux, such as 1octadecene, water does not need to circulate through the condensing column. We utilize silicone high-vacuum grease to ensure that all glass joints on our reaction setups are fully sealed. The setup is attached to the Schlenk line through a flow adaptor to the condensing column, and the entire apparatus is set inside an appropriately sized heating mantle. Use of a reliable heating device, such as a heating mantle or heating tape, ensures good conduction of heat between the heating apparatus and reaction vessel. Such reaction setups, with various modifications, are commonly used in the colloidal nanochemistry literature. In the Supporting Information, we include specific product information for the equipment and supplies that we use most frequently. However, it should be noted that, unless otherwise stated, appropriate substitutions can be made without negatively impacting the reaction or products formed.
(1) Add 100 mg of Pt(acac)2, 10 mL of ODE, 1 mL of OLAC, and 1 mL of OLAM to a 50 mL three-neck roundbottom flask. (2) Stir the solution using the magnetic stir bar to begin dissolving the Pt(acac)2, and then introduce vacuum to begin degassing. Note that complete dissolution will not occur until the reaction mixture is heated. Also, introducing vacuum too quickly or to a system that is not well sealed can lead to violent degassing and suction of solvents into the Schlenk line, particularly for solutions containing low-boiling point solvents. It is therefore important to ensure good connections between glass joints on the setup and introduce vacuum gradually. Proper Schlenk line training and technique is essential. After complete dissolution of the Pt(acac)2 precursor, the reaction solution should be a transparent yellow, as shown in Figure 3a. (3) To purge the reaction of water and other low-boiling point impurities, which can oxidize the Fe(CO)5 reagent added in step 5, degas the reaction mixture thoroughly by heating the solution to 120 °C under vacuum. Maintain this temperature under vacuum for 30 min. After degassing, refill the reaction atmosphere with an inert gas (e.g., Ar or N2) via the Schlenk line. After the reaction mixture is fully under an inert atmosphere, heat it to 180 °C in preparation for the hotinjection process described in step 5. (4) In a glovebox under an inert atmosphere, prepare a solution containing 10 μL of Fe(CO)5 and 0.5 mL of ODE in a septum-capped vial. Cap the vial, and remove it from the glovebox. In this step, the ODE serves to dissolve and dilute the Fe(CO)5 reactant and to facilitate easy reagent transfer in the hot-injection step. To prevent oxidation of the Fe(CO)5, it is imperative to keep this solution under an inert atmosphere until it is injected into the reaction mixture in the three-neck flask. We suggest using 10 μL of Fe(CO)5 for this step,
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REACTION PROTOCOL Synthesis of 5 nm Pt Nanoparticle Seeds. This procedure has been adapted from a literature report,27 which describes the synthesis of monodisperse Pt nanoparticles having average diameters of 3, 5, and 7 nm that can be selectively accessed through subtle modifications to the reaction procedure. Pt nanoparticle seeds with ∼5 nm diameters work well for the multistep heterotrimer syntheses outlined below. Accordingly, the following steps outline the procedure we use to generate ∼5 nm nanoparticles. Figure 3 provides images of a typical reaction mixture at significant points of the synthesis 110
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Chemistry of Materials although 10−30 μL is within the range of volumes reported in the literature and is therefore acceptable, although larger volumes of Fe(CO)5 will lead to an increase in the amount of Fe in the final product17 (see Table S1 for troubleshooting insights). (5) Before injecting the Fe(CO)5/ODE solution into the reaction, purge a plastic syringe (1−3 mL) fitted with a needle with an inert atmosphere to ensure that no oxygen or water is introduced into the reaction vessel. After purging, insert the needle of the syringe into the septum of the vial containing the Fe(CO)5. Pull the entire solution into the syringe. Swiftly inject the contents of the syringe into the reaction vessel. Swift injection ensures a temporally discrete nanoparticle nucleation event, which is critical to generating monodisperse nanocrystals, as described elsewhere.33,34 To minimize the pressure difference between the vial and the surrounding atmosphere, it is helpful to inject an inert gas into the vial (roughly 0.6 mL) prior to pulling the Fe(CO)5 solution into the syringe. Also, note that upon injection of the Fe(CO)5 solution, the reaction solution will turn black, as shown in Figure 3b, indicating the reduction of Pt2+ to Pt0 and the subsequent rapid formation of small Pt nanoparticles. (6) Immediately after injecting the Fe(CO)5 solution into the reaction flask, heat the solution to 190 °C. Leave the reaction mixture at 190 °C for 1 h, which is an appropriate time and temperature to facilitate slow growth of the Pt nanoparticles. During this time, the remaining Pt2+ is slowly reduced by OLAM to form Pt0, which adds onto the small Pt nanocrystals that are initially formed during the hot-injection step. Note that at this stage of the synthesis, it is imperative that the reaction temperature not exceed 200 °C, because above this temperature any remaining Fe species will decompose to form unwanted iron oxide impurities along with the isolated Pt nanoparticle product. (7) After 1 h at 190 °C, remove the heat source and allow the reaction mixture to cool to room temperature. Disconnect the reaction setup from the Schlenk line, detach the flask from the reflux condenser, and remove the rubber septum. Pour the reaction solution into clean centrifuge tubes, and then add IPA to give an approximately 5:1 IPA:reaction solution ratio, which represents an appropriate amount of IPA (a polar solvent that is slightly miscible in ODE and hexanes) to help precipitate the nanoparticles during centrifugation. Centrifuge the reaction solution to precipitate all of the nanoparticles. (We centrifuge at or above 10000 rpm for 3−5 min using a Beckman Coulter Allegra X-22 centrifuge with a FO630 rotor.) Remove the supernatant, and resuspend the nanoparticles using a small amount of hexanes. (When resuspending the particles in hexanes, it is helpful to sonicate the solution to ensure full dissolution of the nanoparticles before adding IPA and recentrifuging.) Add IPA and centrifuge the solution once more. Repeat this process until the supernatant is completely colorless, which signifies that the soluble impurities are removed; during the first cycle, the supernatant is yellow because of the small amount of unreacted Pt and Fe complexes that remain after the synthesis. Panels c−e of Figure 3 show pictures of typical supernatants for these rinsing cycles, including before, during, and after thorough washing, respectively. Finally, redisperse the precipitated particles in hexanes, and transfer to a scintillation vial for storage. (8) The as-synthesized Pt nanoparticles should then be initially characterized via powder XRD to confirm the formation of crystalline Pt nanoparticles and TEM to confirm
that high-quality, monodisperse nanoparticles of the anticipated average diameter were produced. Troubleshooting tips are provided in Table S1. It is important to begin with high-quality, monodisperse Pt seeds for the subsequent seeded-growth steps outlined below. Any deviations from the target morphology, size, and yield in this first step will be propagated in subsequent steps. (9) To approximate the concentration of Pt nanoparticles dispersed in the solvent, tare an aluminum weigh boat on an analytical microbalance. Sonicate the hexanes/nanoparticle suspension from step 7 to ensure complete dispersion of the nanoparticles. Next, drop-cast a known volume (15−20 μL) of the dispersion onto the weigh pan, and allow the hexanes to evaporate. Weigh the pan to measure the change in mass. Divide by the volume of deposited solvent to obtain the mass of nanoparticles per unit volume of hexanes solution. Repeat this measurement several times to confirm the reliability of the measurement. After a concentration has been determined, one or two drops of both OLAC and OLAM can be added to the original nanoparticle dispersion to facilitate longer-term colloidal stability. If excess OLAC and OLAM is added before the concentration is determined, this will impact the nanoparticle concentration value determined by a microbalance. Note that the absolute yield of this reaction is typically close to 50 mg of Pt nanoparticles, which is the maximal theoretical yield for a reaction that uses 100 mg of Pt(acac)2. If the collected particles are resuspended in 5 mL of the hexanes stock solution, the density will be approximately 10 mg/mL. However, this method assumes that all Pt nanoparticles are recovered during the centrifugation step. If the supernatant is not completely transparent after centrifugation, Pt nanoparticles are likely still in the supernatant, which will decrease the accuracy of this approximation. It is also worth noting that we assume the mass fraction contributed from surfacestabilizing ligands and trapped solvents to be negligible. Such contributions would lead to a higher calculated concentration. However, microbalance measurements have not provided yields greater than the theoretical maximum, suggesting that these contributions are indeed negligible. Regardless of surface coverage, it is important to maintain consistency in how nanoparticle concentrations are determined and used, to maintain reproducibility in subsequent hybrid-growth steps. Synthesis of Pt−Fe3O4 Nanoparticles Using Fe(CO)5. This protocol has been adapted from a literature report,35a which describes the synthesis of Pt−Fe3O4 “dumbbells” from Pt nanoparticle seeds and Fe(CO)5. Again, we choose to detail a synthetic protocol that is designed to yield a particular domain size (10 nm Fe3O4), but the details of this strategy can be generally applied to obtain a range of Fe3O4 sizes by changing the ratio of Pt seeds to the Fe precursor.35a It should noted that assignment of the Fe3O4 crystal phase is somewhat ambiguous, because it is difficult to distinguish this phase from γ-Fe2O3 using XRD, which has been discussed by other authors.35b Typically, the formation of γ-Fe2O3 nanocrystals is reported to require oxidation conditions harsher than those that our procedure requires, such as very high temperatures and/or additional oxidation agents.35b,c (10) Prepare a solution containing 1 mL of ODE and 0.5 mL of OLAM in a 20 mL septum-capped vial. To this vial add the appropriate volume of the as-prepared Pt nanoparticle dispersion to provide 10 mg of Pt. For example, if the Pt dispersion has a concentration of 5 mg/mL, add a total of 2 mL of the Pt nanoparticle dispersion. This amount of Pt seeds 111
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Chemistry of Materials
decomposition of the iron oleate, which leads to heterogeneous nucleation of FexOy onto the platinum seeds. (16) By the time the reaction mixture is cooled to 100 °C, the Fe oleate has already decomposed and deposited FexOy onto the Pt nanoparticle seeds, so an inert atmosphere is no longer required. Consequently, the reaction mixture can be exposed to air by disconnecting the Schlenk line from the condensing column. Gently bubble air into the reaction solution by inserting a long needle connected to a compressed air line into the rubber septum of the reaction mixture. Position the tip of the needle so that it is submerged in the reaction solution. Bubble air into the solution at 100 °C for 16 h. Bubbling air at 100 °C allows for complete oxidation of the iron and crystallization of the resulting iron oxide, producing Fe3O4 domains attached to the Pt. If no oxidation is performed, the iron portion of the heterodimer particles will be amorphous. The crystallinity of the Fe3O4 domain significantly impacts the magnetic response of the particles, which can be demonstrated with a permanent magnet, as shown in panels a (not oxidized) and b (oxidized) of Figure 4. Bubbling air into the reaction
coupled with the prescribed volume of Fe(CO)5 in step 12 provides an appropriate ratio of Pt seeds to Fe precursor, allowing for nucleation and growth of a single Fe3O4 domain of comparable size on the majority of Pt seeds. Cap and place this vial under vacuum at room temperature by penetrating the septum of the vial with a needle connected to a Schlenk line. Degas the vial for approximately 30 min to remove the hexanes from the solution. Refill the vial with inert gas by changing the channel on the Schlenk line. Once under inert gas at ambient pressure, remove the needle from the septum cap, and sonicate the vial to ensure that the Pt nanoparticles are fully dispersed. Reserve the vial until step 14. Note that many vacuum pumps contain valves or seals that may degrade if significant volumes of organic solvent are pulled though the Schlenk line. This should be considered before any vacuum system is used to remove low-boiling point solvents from reaction flasks. Our vacuum pumps are fitted with appropriate gas ballast valves, which prevent significant damage from exposure to most organic solvents. (11) Add 10 mL of ODE and 0.5 mL of OLAC to a standard reaction setup, seal with a septum, and begin to stir the solution. Gradually place the solution under vacuum, and heat the solution to 120 °C. Degas under vacuum at 120 °C for 30 min to ensure the removal of water and other low-boiling point impurities from the reaction solution, which can react with Fe(CO)5. Refill with an inert atmosphere using the Schlenk line to deliver an air-free environment at ambient pressure. It should be noted that previous studies have reported that water contamination can affect the morphology of the Fe oxide domains,35c and accordingly, thorough degassing is necessary to reproducibly generate targeted nanoparticle products in high yields. (12) While the reaction mixture is being degassed, prepare a solution of 0.5 mL of ODE and 70 μL of Fe(CO)5 in a 20 mL septum-capped vial in a glovebox under an inert atmosphere. Cap the vial, and then remove it from the glovebox. This provides accessibility to the proper amount of Fe(CO)5 under an air-free environment just as in step 4. (13) Obtain a 1 mL plastic syringe, fitted with a needle. Purge the needle and syringe as outlined in step 5. Inject the entire prepared ODE/Fe(CO)5 solution, which is the exact amount needed to form a complex with all of the OLAC in the reaction solution. Keep the reaction mixture at 120 °C for 5 min. This 5 min dwell time allows the Fe(CO)5 to form a complex with the OLAC to form iron oleate, which is the active precursor for iron oxide formation. Note that the boiling point of Fe(CO)5 is 103 °C, which is below the temperature of the reaction solution at the time of injection, and it has been observed that this can cause the Schlenk line to turn an orange color because of evaporation of the Fe reagent. As such, it is important not to overshoot the temperature in this step, to minimize undesired evaporation of Fe(CO)5 and a consequent decrease in Fe oleate concentration. (14) Five minutes after the previous injection, inject the solution containing the Pt seed particles in ODE and OLAM using a new 3 mL plastic syringe fitted with a needle and purged with an inert atmosphere. This step provides the seeds required for heterogeneous nucleation, as well as the OLAM, which is believed to decompose the iron oleate formed in situ. (15) Immediately after the addition of the Pt seed solution to the reaction mixture, steadily ramp the reaction temperature to reflux (315 °C), hold for 30 min, and then decrease to 100 °C. This step provides the thermal energy required to trigger
Figure 4. Photographs showing the response of Pt−Fe x O y heterodimer nanoparticles to a magnet (a) before and (b) after air had been bubbled through the synthesized sample. Postsynthesis oxidation leads to crystallization of the Fe3O4 domain, which is superparamagnetic and therefore responds to an external magnet.
mixture should be done slowly (
