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Standardizing Size- and Shape-Controlled Synthesis of Monodisperse Magnetite (Fe3O4) Nanocrystals by Identifying and Exploiting Effects of Organic Impurities † † † ‡ † † † § Liang Qiao, Zheng Fu, Ji Li, , John Ghosen, Ming Zeng, John Stebbins, Paras N. Prasad, † and Mark T. Swihart*, † § Department of Chemical and Biological Engineering and Institute for Lasers, Photonics, Biophotonics, University at Buffalo (SUNY), Buffalo, New York 14260, United States ‡ MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China

*S Supporting Information

ABSTRACT: Magnetite (Fe3O4) nanocrystals (MNCs) are among the most-studied magnetic nanomaterials, and many reports of solution-phase synthesis of monodisperse MNCs have been published. However, lack of reproducibility of MNC synthesis is a persistent problem, and the keys to producing monodisperse MNCs remain elusive. Here, we define and explore synthesis parameters in this system thoroughly to reveal their effects on the product MNCs. We demonstrate the essential role of benzaldehyde and benzyl benzoate produced by oxidation of benzyl ether, the solvent typically used for MNC synthesis, in producing monodisperse MNCs. This insight allowed us to develop stable formulas for producing monodisperse MNCs and propose a model to rationalize MNC size and shape evolution. Solvent polarity controls the MNC size, while short ligands shift the morphology from octahedral to cubic. We demonstrate preparation of specific assemblies with these MNCs. This standardized and reproducible synthesis of MNCs of well- controlled size, shape, and magnetic properties demonstrates a rational approach to stabilizing and expanding existing protocols for nanocrystal syntheses and may drive practical advances including enhanced MRI contrast, higher catalytic selectivity, and more accurate magnetic targeting. KEYWORDS: magnetite, nanocrystal, crystallization, mechanism, shape evolution

18 14 agnetite (Fe3O4) nanocrystals (MNCs) have ex- methods include high-temperature decomposition, hydro- tensive applications in magnetic resonance imaging,1 thermal,19 solvothermal,20 and sol−gel21 methods, among 2 3 drug/gene delivery, biomimetic materials, ferro- which high-temperature decomposition has been the most flM4 fi 5 6,7 14,22 uidics, eld-directed assembly, data storage, energy successful in producing monodisperse MNCs. Hyeon et al. storage,8 and catalysis.9 Uniformity of MNCs’ size, shape, and 10 developed a method of preparing highly monodisperse MNCs crystalline structure is crucial in many applications, e.g., from Fe(CO) (iron pentacarbonyl).22 However, the volatility MNCs’ size correlates to their effect on relaxivity in MRI,1 5 and toxicity of Fe(CO) have driven a search for methods using MNCs’ facets exhibit selective activity in catalysis,11,12 and 5 other precursors. Sun et al. developed a high-temperature MNCs’ high saturation magnetization guarantees accuracy in magnetic targeting of drugs or genes.2 Therefore, stable and decomposition approach using Fe(acac)3 (iron(III) acetylacet- − 14 standardized methods of producing monodisperse MNCs of onate) to prepare MNCs with diameters ranging 6 12 nm. In ’ specific size and shape are of essential importance. A diverse Sun s approach, benzyl ether (BE, a.k.a. dibenzyl ether) array of MNC synthesis methods has been developed,13 including bottom-up methods such as solution-phase syn- Received: April 20, 2017 thesis,14 aerosol synthesis,15 and laser pyrolysis,16 and top-down Accepted: June 9, 2017 methods such as ball milling.17 Solution-phase synthesis Published: June 9, 2017

a Scheme 1. Schematic Depiction of the Changes Made in the Synthesis Formulae To Achieve a Stable and Eﬃcient Synthesis

aHigh-temperature decomposition synthesis of MNCs with (a) BE and OBE, produced by reaction of BE with oxygen; (b) mixture of BE, BB, and BA, which were identiﬁed as components of OBE; (c) BE and BA; and (d) ODE and BA.

Figure 1. TEM atlas of MNCs synthesized by high-temperature decomposition. BE + OBE (letter (A) and a blue border), BE + BA + BB (letter (B) and a green border), BE + BA (letter (C) and a yellow border), and ODE + BA (letter (D) and an orange border) were used as solvent. Arrows are labeled with the parameter changed: T, time; HR, heating rate; AD, additive; DL, diol; SL, solvent; OM, oleylamine, noted as OAm elsewhere; and OA, oleic acid. Single arrows indicate an increase in the parameter, and double arrows indicate a switch of the parameter. The width of each panel equals 50 nm.

6371 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article provides a non-coordinating environment in which the (DL) between (l) 1,2-tetradecanediol (TDD) and (k) 1,2- organometallic precursor transforms, accumulates, supersatu- hexadecanediol (HDD). Arrows ⟨1⟩−⟨29⟩ in Figure 1 are rates, and consolidates into the MNCs. In our implementation categorized and analyzed to reveal each parameter’seffect. of this method, we realized that BE’s susceptibility to oxidation Additive Effects. The primary parameter being investigated in air had a major influence on the resulting MNCs. Based on is the additives (arrows ⟨1⟩, ⟨2⟩, ⟨6⟩, ⟨11⟩, ⟨12⟩, ⟨15⟩, ⟨16⟩, this observation, we hypothesized that the products of BE’s ⟨17⟩, and ⟨28⟩), including OBE, BA, and BB. An increase of oxidation can influence the monomers’ behavior and thus can additive(s), either OBE (⟨1⟩, ⟨2⟩) or BA/BB (⟨12⟩, ⟨17⟩, be used to tune the MNCs’ morphology. Therefore, we ⟨28⟩), increases the average size and promotes faceting of the analyzed the oxidized BE by GC-MS, identified the key MNCs. In the case of arrow ⟨16⟩, increasing BA narrows the products, simplified the formula to use only the most important size distribution. Switching the additive from artificial OBE to oxidation product, and then replaced BE with a more stable BA (⟨6⟩) or from BA + BB to BA alone (⟨15⟩) increases the solvent to design a series of efficient and repeatable high- average size. Arrow ⟨11⟩ indicates a change in two additives, temperature decomposition formulas that can produce MNCs from Figure 1(B5) 0.5 mL BA and 0.5 mL BB to (B6) 1.0 mL ranging from 4 to 55 nm in size, with controlled morphologies, BA and 1.5 mL BB. In this case, although the amount of including tetrahedra, octahedra, tetradecahedra, cubes, and additives increased, and the average size decreased, suggesting stars. We further studied their assembly into chains, rings, that increasing the amount of BB by a greater amount than BA clusters, monolayers, and multilayers to shed light on the actually decreases average size. The clear effect of BA and less fundamental interactions of the MNCs. clear effect of BB led us to reduce the additives to BA without BB. RESULTS AND DISCUSSION Ligand Effects. The second most-studied parameter was Synthesis of MNCs in Various Solvents. The synthesis of the identity and amount of ligands. BE, ODE, oleylamine MNCs here evolved through four stages (Scheme 1), (OAm), oleic acid (OA), HDD, and TDD can all coordinate to 3+ respectively studying the effects of (a) oxidation of BE, (b) Fe ions. However, the interactions of BE and ODE with both replication with the major products of oxidized BE, (c) precursor ions and MNCs are relatively weak, so they are 23 simplification of the formulas to essential components, and (d) usually considered as non-coordinating solvents. Among replacement of oxidation-sensitive chemicals with more stable other ligands, OAm and OA are monodentate, while HDD and ones. Figure 1 presents the atlas of the as-prepared samples in TDD are bidentate; OA and OAm both have an 18-carbon tail the form of transmission electron microscopy (TEM) images. with a central double bond, but differ in their terminal When prepared with fresh BE, the resulting MNCs were functional group, while HDD and TDD are similar diols; and relatively polydisperse (Figure 1A1). However, we observed OA and OAm behave contrarily, while HDD and TDD have that naturally oxidized benzyl ether (OBE, benzyl ether opened similar effects. Therefore, OA and OAm are discussed as one and stored in the air for several weeks) produced MNCs with a subgroup of ligands and HDD and TDD as another. much narrower size distribution. Artificial OBE, produced by OA and OAm serve as ligands to MNCs, but function bubbling air through fresh BE at 50 °C, yielded similar results contrarily. OAm decreases the activity of the monomers and (Figure S3). GC-MS analysis of both natural and artificial OBE yields smaller MNCs (⟨20⟩, ⟨23⟩), while OA does the opposite (Tables S2 and S3) revealed the major degradation products as (⟨27⟩). benzaldehyde (BA) and benzyl benzoate (BB). Equations S1 Two similar diols, TDD and HDD, were studied intensively and S2 in the Supporting Information illustrate the overall (arrow ⟨3⟩, ⟨4⟩, ⟨5⟩, ⟨10⟩, ⟨13⟩, ⟨18⟩, ⟨19⟩, ⟨20⟩, ⟨23⟩, ⟨26⟩, reactions that produce these products. To quantify the effect of and ⟨27⟩). Regardless of which diol was used, increasing the BA and BB, fresh BE, BA, and BB were blended to simulate amount of diol (⟨4⟩, ⟨5⟩, ⟨10⟩, ⟨13⟩, ⟨19⟩, and ⟨26⟩) decreases OBE (Figure S4). Ultimately, BA was identified as the primary the average MNC size, consistent with prior reports.24 component responsible for narrowing the size distribution, so Increasing the amount of shorter diol TDD (⟨3⟩, ⟨4⟩, ⟨13⟩) the solvent was reduced to BE (as major solvent) and BA (as or switching the diol from (C4) HDD to (C2) TDD (⟨18⟩) additive) (Figure S5). Finally, the original solvent, BE, was can reduce the mean size and promote the growth of {111} replaced with air-stable 1-octadecene (ODE), with BA as a planes, i.e., shift the morphology from octahedral to cubic and stable and efficient additive (Figure S6). The whole then to star-like (stellated cubic). Excessive TDD dramatically optimization process unfolded stepwise as illustrated in Scheme limited growth, producing small dots without well-defined 1. facets in some cases (⟨10⟩, ⟨26⟩). Parameters such as reaction duration, heating rate, degassing Solvent Effects. Solvents used here are BE and ODE. The atmosphere, solvent species, ligand species and amount, and switch from BE to ODE (⟨7⟩, ⟨21⟩, ⟨22⟩, and ⟨25⟩) not only additive species and amount are discussed in Figures S3−S6. stabilized the formulas but also decreased the average size, The monodispserse samples’ size distribution histograms are which is advantageous in many biomedical applications.25 presented in Figure S1. A demonstration of the repeatability of Though BE and ODE are “non-coordinating” solvents, they these formulas is given in Figure S2. Figure 1 summarizes the show different levels of affinity with the monomers and MNCs. observed changes, mapping out the dependence of the MNC Alkenes usually have lower polarity than ethers. In this case, morphology on these parameters. Blue labels indicate samples ODE has lower polarity than BE, shows weaker affinity, reduces produced in BE + OBE throughout the manuscript; green monomer activity, and thus yields smaller MNCs for a fixed labels BE + BA + BB; orange labels BE + BA; and red labels precursor concentration.26 ODE + BA. Samples are connected by arrows, denoted by small Temperature Effects. We also investigated effects of angle brackets, “⟨1⟩”. The direction of the single arrows heating rate and reaction duration at maximum temperature, indicates an increase in the value, and double arrows indicate keeping the reaction temperature fixed at 280 °C, near the the switch of a parameter, e.g., arrow ⟨1⟩ indicates an increase upper limit imposed by boiling. The temperature rate (⟨8⟩, ⟨9⟩, of the additives (AD), and arrow ⟨3⟩ indicates a switch of diol ⟨14⟩, ⟨29⟩) of one-pot heating methods is important in

6372 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article determining the concentration of nuclei. A slower rate allows a wider time window for nucleation,27 consumes more monomers during nucleation, and yields smaller MNCs. Two rates, 2.7 °C/min and 4.6 °C/min, were used here. The duration was around 90 min. Extending the reaction from 90 to 110 min (⟨24⟩) had little effect on the MNC’s size or shape. The growth of NCs in high-temperature decomposition methods is usually complete in minutes,28 so capturing the final morphology of MNCs is a more practical means of varying size and shape than trying to quench the reaction to trap MNCs during size and shape evolution. Tuning MNC Size. Here, we summarize the correlation of the synthesis parameters and the MNCs’ mean size by arrows (increasing ↑ or decreasing ↓): temperature rate, ↑; TDD/ HDD, ↓; OA, ↑; OAm, ↓; BA, ↑; BB, ↓. The results strongly suggest a positive correlation of solvent polarity with the average NC size, i.e., solutions of higher polarity yields larger MNCs. In general, functional group polarity trends are acid > alcohol > aldehyde > amine > ether > alkane.29 For the reagents here, the ranking of polarities should be OA > TDD > HDD > BA > OAm > BB > BE > ODE. BE, BA, and OA are more polar than ODE, BB, and OAm, respectively; meanwhile, BE, BA, and OA increase the average size of MNCs, whereas ODE, BB, and Figure 2. Schematic illustration of the proposed growth model for OAm decrease it. MNCs. (a) Primary particle enclosed by undetermined facets; (b) Tuning MNC Morphology. HDD, TDD, OA, and OAm tetrahedron dominantly enclosed by {111}; (c) octahedron can coordinate to the Fe3+ ion to form different monomer dominantly enclosed by {111}; (d) tetradecahedron (truncated species. Figure 1(A2), (A3), (C5), (C6), (C8), and (C9) shows cube/octahedron) dominantly enclosed by {100} and {111}; (e) well-defined octahedra bounded by {111} planes, while Figure cube dominantly enclosed by {100}; (f) star dominantly enclosed − by {100} with extruded ⟨111⟩ vertices; (g) star with more extruded 1(A4), (A5), (B1), (B5 B9), (C1), (C2), (C4), (C7), (D2), ⟨ ⟩ fi 111 vertices. The color gray stands for {100} planes in both the (D3), (D4), and (D6) shows well-de ned cubes with {111} outer ring and 3D models; green, {110}; and yellow, {111}. planes grown out of existence. The octahedral samples were produced using HDD, while cubic samples were generated using shorter TDD or a large amount of even shorter BA. This planes, i.e., monomers move from the solvated form of higher 34 comparison suggests that the shorter ligands, TDD or BA, chemical potential to the planes of lower chemical potential. promote the formation of cubic MNCs by growing {111} Thus, monomers deposit on {100}, {110}, and {111} planes planes to extinction. simultaneously at rates governed by the activation barriers for Rationalizing Observations with an MNC Formation each reaction according to eq 1, where km,i,{hkl} is the rate Model. Production of MNCs of dot, tetrahedron, octahedron, constant of the deposition of monomer i on {hkl} planes; A is Δ tetradecahedron, cube, and star morphologies was shown in the pre-exponential factor; Em,i,{hkl} is the activation energy for Figure 1 (and Figures S3−S6). Formulas are listed in Table S1, the deposition of monomer i on {hkl} planes; R is the universal and size distributions are plotted in Figure S1. Their formation gas constant; and T is temperature. −Δ mechanism can be rationalized by connecting these morphol- kA=×e ERTmi,,{ hkl }/ ogies into a shape evolution model (Figure 2, Video S1). The mi,,{ hkl } (1) MNCs in this study are all characterized as magnetite (Figure Because all facets are exposed to the same monomers, the 8), in which {111} planes are the most densely packed with the rate constants determine the deposition rates. Given that all lowest chemical potential and lowest reactivity among the low- facets are thermodynamically allowed to grow, stable {111} index planes, i.e., {100}, {110}, and {111} planes.30,31 The facets with the largest activation barrier for deposition will grow {100} planes, on the contrary, are the least densely packed with the slowest, so the primary particles retain the {111} facets and the highest relative chemical potential and highest reactivity. grow into either octahedra (Figure 2c) or tetrahedra (Figure μ The chemical potentials of low-index planes rank as {100} > 2b). The essential difference between the octahedron and μ μ {110} > {111}. The tricolor ring in the schematic illustration tetrahedron is the number of facets in the {111} family, i.e., the (Figure 2) represents the landscape of chemical potential of the octahedron is enclosed by (111), (11̅ 1̅ ),̅ (111),̅ (11̅ 1),̅ (111),̅ monomers in the solution, descending clockwise. The end of (111̅ ),̅ (111),̅ and (111̅ )̅ and the tetrahedron by (111), (11̅ 1),̅ the gray arc marks the chemical potential of {100} planes; (111̅ ),̅ and (111̅ ).̅ ’ μ green, {110}; and yellow, {111}. As the monomer s activity falls on the green arc, i.e., {100} > μ μ μ In the one-pot heating method employed here, the precursor m > {110} > {111}, the growth of {100} stops, but the growth (Fe(acac)3) dissolves and, upon heating, transforms into of {110} and {111} continues, so the octahedron grows into a monomers,32 which then accumulate, supersaturate, and tetradecahedron with reduced {110} and {111} facets (Figure nucleate into MNCs (Figure 2a).33 After rapid nucleation, 2d). μ μ μ μ ’ the order of chemical potentials is m > {100} > {110} > {111}, As the monomer s activity moves down to the yellow arc, μ μ μ μ μ where m is the chemical potential of the monomers. following {100} > {110} > m > {111}, only {111} planes can Subsequent diffusional growth is driven by the difference in grow, and the amount of growth depends on the amount and chemical potential between the monomers and the crystal length of the remaining monomers as discussed in the synthesis

6373 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article

Figure 3. Magnetic dipolar interaction-dominated assemblies of MNCs. Row (a) corresponds to assemblies of ∼30.2 nm polydisperse octahedra: (a1) single-stranded chain, (a2) double-stranded chain, (a3) cross assembly, (a4) ring assembly, and (a5) 3D illustration of an octahedron. Row (b) corresponds to assemblies of ∼47.7 nm stars: (b1) single-stranded chain, (b2) double-stranded chain, (b3) cross assembly, (b4) ring assembly, and (b5) 3D illustration of a star. Row (c) corresponds to assemblies of a mixture of ∼12.4 nm and ∼44.8 nm cubes: (c1) single-stranded chain, (c2) double-stranded chain, (c3) cross assembly, (c4) ring assembly, and (c5) 3D illustration of a cube. Row (d) corresponds to assemblies of ∼33.4 nm tetradecahedra: (d1−d4) tetradecahedra with descending magnification, and (d5) 3D illustration of a tetradecahedron. In row (e), panel (e1) higher and (e2) lower magnification image of ∼30.2 nm octahedra, (e3) monolayer cluster and (e4) multilayer cluster of ∼55.3 nm cubes, and panel (e5) 3D illustration of a cube. Blue labels denote BE + OBE as solvents; green, BE + BA + BB; orange, BE + BA; and red, ODE + BA. Scale bars equal 50 nm unless labeled otherwise. section. The promotion of the growth of {111} planes by When the {111} planes shrink during growth, tetradecahedra shorter monomers, such as TDD and BA, can be explained by become cubes (Figure 2e), then stars (Figure 2f), and then stars the steric effect of surface ligands, which is present to some with even sharper ⟨111⟩ vertices (Figure 2g),36 if sufficient extent on all facets, but is more important on the most densely short monomers remain to allow growth to proceed through 35 μ μ packed {111} facets. For the densely packed {111} planes, these stages by maintaining m,i > {111}. μ the ligands form a more crowded forest and thus exhibit a In eq 2, m,i is the chemical potential of monomer i in μ⊖ stronger steric barrier to incoming monomers. The larger solution; m,i is the chemical potential of monomer i in pure Δ fi γ monomers face higher energy barrier ( Em,i,{111} in eq 1)to liquid state (or other properly de ned reference state); m,i is ffi χ expel the crowded ligands and deposit, so only the shorter the activity coe cient of monomer i; and m,i is the molar monomers are able to deposit on the {111} planes. Moreover, concentration of monomer i. The activity of monomer i is the the monomers are complexes of iron with ligands, and therefore product of its activity coefficient and molar concentration. the presence of longer ligands in the synthesis formula also According to eq 2, by tuning the composition of the solution, ff χ ffi implies larger e ective monomer diameter, which also the concentration ( m,i) and activity coe cients of monomers γ ’ contributes to the steric barrier to deposition. In addition, the ( m,i) can be adjusted, and the monomers activities vary larger hydrodynamic diameter will decrease the monomer accordingly. As for the average size, higher polarity of ligands diffusion coefficient, which would slow the growth rate, and means stronger affinity with Fe3+ ion, enables higher monomer make the growth rate of different facets more nearly equal, activity, and therefore generates larger NCs. As for the under conditions for which monomer diffusion limits the morphology, the growth of the {111} planes can be adjusted growth rate. by varying the shorter monomer’s activity, so the morphology

6374 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article

Figure 4. Hexagonal superlattice of ∼17.1 nm octahedral MNCs. (a) TEM image; (b) SEM image; (c) SAED pattern corresponding to panel (a); (d) FFT pattern of panel (a); (e) HRTEM of a single MNC in the superlattice; and (f) 3D reconstruction of an MNC. of the product MNCs can be controlled. For example, if the matching the longest ligands (OA/OAm) present in all remaining monomers are not sufficiently short to overcome the samples. The saturation magnetization was measured (Figure steric barrier of ligands on {111} planes, the long monomers 7). The critical size of the MNCs produced here, from eq 3, was will remain inert in solution, which means the monomers’ ∼27 nm. Therefore, assembly of MNCs larger than 27 nm is μ μ equilibrium activity is trapped between {110} and {111}, i.e., expected to be dominated by magnetic dipolar interaction, μ μ μ μ {100} > {110} > m > {111}, and the resulting MNCs would forming localized chains, rings, and clusters (Figure 3), while remain octahedral. assembly of MNCs smaller than 27 nm should be dominated by van der Waals interactions, forming extended mono- or μ =+μγχ⊖ mi,, miRT ln mi ,, mi (2) multilayers (Figures 5 and 6). Strings, Rings, and Clusters. Template-Free Self-Assembly of MNCs. MNCs of Figure 3 presents strings, various size, shape, and dispersity can spontaneously form a rings, and clusters of MNCs larger than the critical size. The wide array of assemblies. Here, we discuss the correlation 30.2 nm MNC octahedra in Figure 3 row (a) can assemble into between MNC morphology and the resulting assemblies. In all (a1) single-stranded chains, (a2) double-stranded chains, (a3) cases, the assembly process is initiated by increasing MNC crosses, and (a4) rings. To minimize the system energy, the octahedral MNCs align with the preferred direction of the concentration during gradual solvent evaporation. Due to ⟨ ⟩ 40 meso- and microscale effects (such as evaporation-driven magnetization, i.e., 111 axes of magnetite. The 47.7 nm ff cubes form similar structures (Figure 3(b1−b4), but the cubic convection and the Marangoni e ect), the local concentration ⟨ ⟩ landscape transforms within a single experiment. MNCs cannot align along 111 axes due to their shape; short- range van der Waal forces bring their {100} facets into contact. The major interactions between MNCs consist of repulsive ’ forces, i.e., short-range isotropic steric repulsion, and attractive The MNCs in Figure 3 row (c) have a bimodal size forces, i.e., long-range anisotropic magnetic dipolar interaction distribution (histogram in Figure S1). In this case, only the 37 MNCs of ∼44.8 nm, larger than the critical size, assemble into and short-range isotropic van der Waals interaction. The ∼ dipolar parameter, λ, describing the competition of magnetic strings, rings, and clusters, while the 12.4 nm MNCs are dipolar interaction and van der Waals interaction, is defined as randomly distributed. The assembly of the oversize NCs cannot induce assembly of the undersize NCs, as predicted by the ratio of the magnetic dipolar energy to the thermal energy, 39 eq 3:38 simulation. With increased initial MNC concentration in the colloid, 3 2 μ2 πdMs 3 clusters form instead of the chain/ring assemblies, which /D 26 2 fi U 3 ()6 π dM signi cantly enhances the long-range dipolar attraction and λ ==D = = s 3 elongates the assemblies’ morphology. Figure 3(d1−d4) shows EkT kT 36kTD (3) B B B ∼33.4 nm tetradecahedral MNC assemblies at descending where λ is the magnetic dipolar parameter; U is the magnetic magnifications. The tetradecahedral MNCs periodically align dipolar energy; E is the thermal energy; μ is the particle along the ⟨111⟩ directions to form elongated rods (Figure magnetic moment; D is the overall diameter of the magnetic 3(d1) and (d2)) and then arrange into dendritic branches particle, i.e., the core diameter plus twice the length of the (Figure 3(d3) and (d4)). The quasi-fractal structure implies ’ ff surface ligands; kB is Boltzmann s constant; T is temperature; d di usion-controlled aggregation, i.e., the guest MNCs attach to fi 41 is the core diameter; and Ms is the saturation magnetization of the growing structure at the rst point of encounter. Similarly, the particle. concentrated colloidal octahedra can form assemblies with 3D The critical value of dipolar parameter is about 3, marking periodicity (Figure 3(e1) and (e2)). Although the assembly in the balance of magnetic dipolar interaction and van der Waals (e2) is rather thick, periodic vacancies still allow the beam to interaction.39 The length of the surface ligands is set as 1.8 nm, come through. In Figure 3(e3) and (e4), ∼55.3 nm truncated

6375 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article

Figure 5. 2D superstructures of MNCs, part I. Row (a) binary monolayer of ∼3.9 nm tetrahedra and ∼2.0 nm dot; row (b) hexagonal monolayer of ∼5.1 nm octahedra; row (c) hexagonal multilayer of ∼6.8 nm octahedra; row (d) hexagonal monolayer of ∼14.0 nm octahedra; row (e) hexagonal monolayer of ∼17.1 nm octahedra; row (f) hexagonal multilayer of ∼17.1 nm octahedra. The first column shows low- magnification TEM images; second column close-up TEM images; third column FFT patterns; fourth column SAED patterns; and fifth column 3D illustrations of the superlattices. Blue labels denote BE + OBE as solvents; green, BE + BA + BB; orange, BE + BA; red, ODE + BA. Scale bars equal 50 nm. cubes cluster into localized periodic structures. In (e3), the measured as 2.53 Å, matching {110} planes. So combining local concentration is not high enough for the formation of Figure 4c,e, the orientation of the single crystalline MNCs was clusters. In (e4), the higher local concentration allows the reconstructed in (f). Given the magnetite’s preferred axes of second layer to stack atop the first. Similar to Figure 3 rows (b) magnetization are ⟨111⟩ directions,40 the MNCs in Figure 4a,b and (c), the cubic MNCs cannot align along the ⟨111⟩ have their magnetic dipoles aligned to minimize the system directions due to their cubic morphology. energy. Monolayers and Multilayers. Figures 5 and 6 present In Figure 4a, some MNCs show higher contrast than the rest extended monolayers and multilayers of MNCs below the because of their different orientation, which can be explained by critical size (∼27 nm). The hexagonal monolayer of ∼17.1 nm the SAED pattern in Figure 4c. The diffraction spots of {220} octahedral MNCs is laid out in Figure 4 to facilitate the and {440} planes are associated with the [111] zone axis. understanding of Figures 5 and 6. Both TEM (Figure 4a) and Diffraction spots (11̅ 3)̅ and (113)̅ are associated with zone axis scanning electron microscopy (SEM) (Figure 4b) images show [332], which has a 10.02° angle with [111]. Diffraction spots rigorous periodicity of the hexagonal monolayer. The SAED (311),̅ (131̅ ),̅ (311̅ ),̅ and (113)̅ are associated with zone axis pattern in Figure 4c indicates the collective zone axis as [111]. [112], which has a 19.47° angle with [111]. Slight variations of In the high-resolution transmission electron microscopy the upright orientation lead to the “darker” MNCs in Figure 4a (HRTEM) image (Figure 4e), the interplanar spacing d is and the weaker diffraction spots in Figure 4c. Figure 4d, the fast

6376 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article

Figure 6. 2D superstructures of MNCs, part II. Row (g) random monolayer of irregularly shaped dots; row (h) vortical monolayer of quasi- cubes; row (i) irregular reticular multilayer of cubes; row (j) cubic monolayer of ∼20.0 nm cubes; (k) complex multilayer of ∼20.0 nm cubes; and (l) cubic monolayer of ∼16.5 nm stars. First column shows low-magniﬁcation TEM images; second column close-up TEM images; third column FFT patterns; fourth column SAED patterns; and ﬁfth column 3D illustrations of the superlattices. Blue labels denote BE + OBE as solvents; green, BE + BA + BB; orange, BE + BA; and red, ODE + BA. Scale bars equal 50 nm.

Fourier transform (FFT) pattern of (a) has highly periodic random orientation of the MNCs. The {220}, {113}, {004}, spots indicating the collective orientation and order of the and {440} rings, in (a4) and other SAED patterns in Figures 5 MNCs. Based on the collective zone axis [111], the crystalline and 6, were respectively labeled in green, orange, gray, and directions of the MNCs are labeled by arrows in Figure 4d. So green again. Figure 5 row (b) shows a hexagonal assembly of to summarize Figure 4, octahedral MNCs, preferentially ∼5.1 nm octahedral MNCs. The FFT pattern in (b3) indicates oriented along the [111] zone axis, form a rigorously hexagonal the periodicity of the monolayer, while the SAED pattern in superlattice with both positional and orientational order. The (b4) implies the random orientation of the MNCs in (b1). same analytic approach can be applied to the other assemblies Figure 5 row (c) shows the multilayer assembly of ∼6.8 nm in Figures 5 and 6. octahedral MNCs. In panels (c1) and (c2), the MNCs have Additional 2D assemblies of MNCs are exhibited in Figures 5 assembled into multilayers with 3D periodicity, leaving the and 6. Figure 5 row (a) demonstrates a binary superlattice of interstices visible under TEM. The SAED pattern in (c4) ∼3.9 nm tetrahedra and ∼2.0 nm dots of random orientations. reveals the collective zone axis as [101]. Given the octahedral The dots are in hexagonal arrangement, and the tetrahedra ﬁll MNCs in a monolayer (e.g., Figure 5 row (e)) have collective in the interstices. The ordered bright spots in the FFT pattern zone axis [111], we conclude that the MNCs in the ground (a3) conﬁrm the high periodicity of the superlattice. The SAED layer rotate from [111] to [101] to accommodate the incoming pattern in (a4) shows polycrystalline features, suggesting MNCs to form another hexagonal layer on top, with the

6377 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article magnetic dipoles aligned to lower the system energy. Figure 5 Similarly, the second column of MNCs in the second layer row (d) shows an irregular hexagonal superlattice of corresponds to the bright spots labeled with ②.InFigure 6(k4), polydisperse MNCs. Without rigorous periodicity (Figure the {220} bright spots of orientations ① and ② coincide with 5(d1) and (d2)), the FFT pattern in (d3) is not as sharp as the {220} spots of the ground layer, indicating the alignment of Figure 5(b3) and (e3). The SAED pattern in (d4) indicates the the magnetic axes of MNCs in the second layer and the ground random orientations of the MNCs. So the monodispersity of layer, which is predicted by simulation5 and confirmed by the the building blocks is indispensable for the formation of a TEM image (Figure 6(k5)). The FFT pattern in (k3) shows rigorous superlattice. Figure 5 row (e) summarizes the contents periodic bright spots of this complex multilayered super- of Figure 4 in the context of other assemblies. Figure 5 row (f) structure, indicating the great periodicity. shows a multilayer assembly of ∼17.1 nm MNCs. The The building blocks in Figure 6 row (l) are cubes with superlattice in (f1) is beyond the penetration depth of the extruded ⟨111⟩ vertices, referred to as stars. Though the MNCs electron beam, but the close-up in (f2) reveals the periodic are uniform, the extrusions hinder the formation of a strict vacancies. The SAED pattern in (f4) has similar major bright superlattice. As indicated by the SAED pattern (l4), the MNCs spots as (c4), but with more random spots associated with mis- share the zone axis [001], but instead of having identical oriented MNCs in the assembly. orientation, they rotate slightly around the zone axis and In Figure 5, MNC building blocks are mainly octahedra elongate the bright spots in (l4) into short curves. Also the enclosed by {111}, and the resulting superlattices show fuzzy lines in FFT pattern reflect the limited orientational hexagonal features. In contrast, the MNCs in Figure 6 are ordering of the building blocks. mostly cubic, and the resulting superlattices are based on a Characterization of Magnetization and Crystallinity. cubic motif. In Figure 6 row (g), the polydisperse dots can Figure 7 presents the magnetic hysteresis loops of the MNCs at tessellate the substrate but cannot form periodic structures. The FFT pattern (g3) has no ordered bright spots, and the rings in the SAED pattern indicate the random orientation of the MNCs. Quasi-cubic MNCs in Figure 6 row (h) can form vortical structures. Though the overall structure does not have long-range order, adjacent MNCs can align their (100) facets. Because the MNCs are uniform in neither size nor shape, the alignment of (100) planes cannot propagate on the substrate, and therefore the periodicity breaks down as the assembly expands. The FFT pattern in (h3) does not show periodicity. The SAED pattern in (h4) indicates random orientation of these quasi-cubic MNCs. Starting from a more concentrated colloid of MNCs, irregular multilayer structures form. In Figure 6(i1) and (i2), the ground layer is a random tessellation of MNCs, but the second layer is discrete and reticular. The concentration and monodispersity of the MNCs do not meet the requirement of the Kirkwood−Alder transition (disorder to 42 ff order transition), and the Marangoni e ect may drive the Figure 7. Magnetic hysteresis loops. The MNCs of 15 nm dots, 20 43 particles into rings due to the uneven surface tension. The nm cubes, 27 nm octahedra, and 33, 40, and 55 nm tetradecahedra FFT pattern in (i3) indicates no long-range order. The SAED were measured at 305 K. The zoom-in inset is to emphasize the pattern in (i4) indicates random orientations of the MNCs. trend. The 3D illustration in (i5) was rotated 10° off the zone axis [001] to deliver a clear view, as were the illustrations in Figure 305 K by vibrating sample magnetometry (VSM). The 6 (j5), (k5), and (l5). saturation magnetization of 15 nm dots, 20 nm cubes, 27 nm Monodisperse cubic MNCs can form rigorous cubic octahedra, and 33, 40, and 55 nm tetradecahedra increase from assemblies (Figure 6 row (j)). In (j1) and (j2), most of the 69.5, to 73.0, to 74.5, to 76.4, to 82.5, and eventually to 82.7 ∼20.0 nm cubic MNCs share the [001] zone axis. The FFT emu/g, exceeding the value of bulk maghemite (76.0 emu/g)44 pattern in (j3) indicates the periodicity of the superlattice. and approaching that of bulk magnetite (84.5 emu/g).14 The Because the MNCs in the superlattice share an identical inset in Figure 7 zooms in on the saturation magnetization of all orientation, the SAED pattern in Figure 6(j4) exhibits compact samples to emphasize the trend. Colloidal MNCs can approach bright spots associated with zone axis [001]. Figure 6 row (k) the bulk material’s saturation magnetization but are unable to shows a complex multilayer. The ground layer of MNCs form a match it.25 First, the surface spin canting phenomenon is non- square lattice similar to Figure 6(j1) and remain that negligible at the size scale of these particles. Second, the ligands throughout the assembling. However, the magnetic field of on the surface of the MNCs add mass of non-magnetic material the ground layer guides the orientation of the MNCs forming and may affect the magnetization of the core. Finally, the the second layer. In the second layer, the particles’ centroids fit samples may form a decomposed (nonmagnetic) layer during into a cubic lattice, but the building blocks have two storage and handling. All three effects are surface-related, and orientations with collective zone axis [101]. In Figure 6(k5), the specific surface area is inversely correlated with the size of the structure of the second layer is reconstructed based on the MNCs, so the deviation of saturation magnetization from that SAED pattern in (k4). The first column of MNCs in the second of bulk magnetite decreases with increasing MNC size. layer in (k5) corresponds to the bright spots labeled with ① in Because we were mainly concerned with saturation magnet- the SAED pattern. The dihedral angle of ① (040) and ① (131) ization here, we did not make detailed measurements at low is measured as ∼25°, matching the theoretical value 25.24°. fields to precisely determine coercivity of the MNCs. The

6378 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article

Figure 8. XRD characterization of MNC samples. (A) XRD patterns of MNCs made in (a) BE, (b) BE and OBE, (c) BE, BA, and BB, (d) BE and BA, and (e) ODE and BA; (B) standard reference of magnetite; (C) standard reference of maghemite; (D) unit cell of magnetite and its conversion to maghemite; and (E) tabulated lattice parameters of magnetite and maghemite. coercivity of phase-pure MNCs36 and critical size below which BA, and ODE + BA) to elucidate their effects on the they exhibit superparamagnetism40 have been previously synthesis. studied in some detail. In the near room-temperature (305 (3) The polarity of the solvent was found to be effective in K) measurements made here, all samples exhibited measurable controlling the mean size of the MNCs, while the activity coercivity, i.e., were not strictly superparamagnetic, with values of short monomers was necessary for the shift from up to about 100 Oe, as expected for magnetite. octahedral to cubic shape. MNCs prepared in various solvents were also characterized (4) With successful synthesis of various MNCs, a model, by XRD (Figure 8). As illustrated in panel (D), magnetite based upon the chemical potential of the solution and γ (Fe3O4) can convert to maghemite ( -Fe2O3) in the presence crystal planes, was proposed to rationalize the shape of oxidizers such as dissolved O2. The dotted circles in panel evolution of the MNCs. (D) indicate the vacancies in magnetite and the corresponding (5) A wide array of assemblies including chains, rings, O-occupied sites in maghemite. The permeation of oxygen clusters, monolayers, and multilayers of these MNCs atoms into the crystal lattice induces stronger cohesion, and were prepared and were analyzed to provide insights into therefore the volume of the unit cell of maghemite45 is 2.3% the intrinsic interactions of the magnetic NCs at the smaller than that of magnetite.46 Thus, peaks in the XRD nanoscale. Selective preparation of MNC superlattices is pattern of maghemite (Figure 8C) shift rightward compared to enabled by choosing the MNCs of right size, shape, and those of magnetite (Figure 8B). As indicated by the droplines dispersity as building blocks. across Figure 8A−C, all five patterns in panel (A) have peaks at ° ° ° ° ° We believe this study can shed light on facet-selective 30.0 , 35.5 , 43.0 , 57.0 , and 62.6 , matching the standard of catalysis, fundamental magnetics research, crystallization theory, magnetite in panel (B). Combined with the observation of and, more importantly, the increasing number of applications of saturation magnetization exceeding bulk maghemite, the crystal MNCs in biomedical contexts where reproducibility and phase of the MNCs is therefore magnetite, regardless of the stability of the synthesis is paramount, e.g., enhancing MRI variations in the synthesis formulas. contrast, improving magnetic hyperthermia therapy, and enabling more accurate magnetic targeting in magnetophoretic CONCLUSIONS drug and gene delivery. Finally, the general approach We have demonstrated the rational standardization of the demonstrated in these experiments can serve as an example of an approach to stabilizing and expanding existing protocols synthesis of MNCs ranging from 4 to 55 nm with for reproducible nanocrystal syntheses. monodispersity of both shape and size. Key points are as follows: METHODS (1) By analyzing the oxidized solvents, isolating the effective Chemicals. Iron acetylacetonate (Fe(acac)3) 99+% from Acros components, simplifying the formulas, and employing Organics, and BE 99%, BA 99+%, BB 99%, ODE 90%, HDD 90%, more stable reagents, the initial unstable formula was TDD 90%, OAm 70%, and OA 90% from Sigma-Aldrich. developed into a series of methodical formulas. Characterization. TEM images and SAED patterns were taken (2) Parameters such as heating rate and the amount and with a JEOL 2010 microscope, operating at 200 kV. SEM of the nanostructures was performed with a Carl Zeiss Auriga crossbeam identity of additives and ligands were studied across focused ion beam scanning electron microscope. XRD patterns were different solvents (BE, BE + OBE, BE + BA + BB, BE + obtained with a Rigaku Ultima IV with Cu Kα X-ray source.

6379 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381 ACS Nano Article

Magnetometry measurements were conducted using the VSM mode of (5) Singh, G.; Chan, H.; Baskin, A.; Gelman, E.; Repnin, N.; Kral, P.; a Quantum Design Physical Property Measurement System model Klajn, R. Self-Assembly of Magnetite Nanocubes into Helical 6000 (PPMS) with a field up to 10 kOe at 305 K. Samples were dried, Superstructures. Science 2014, 345, 1149−1153. loaded into gelatin capsules with wax, sealed, and fixed in a clear (6) Xiong, Y.; Ye, J.; Gu, X.; Chen, Q. W. Synthesis and Assembly of diamagnetic plastic straw. Magnetite Nanocubes into Flux-Closure Rings. J. Phys. Chem. C 2007, Synthesis of MNCs. Precursor (Fe(acac)3), ligands (OAm, OA, 111, 6998−7003. 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Sub-20 nm-Fe3O4 Square and Sortable table detailing conditions and outcomes for all Circular Nanoplates: Synthesis and Facet-Dependent Magnetic and syntheses (XLSX) Electrochemical Properties. Chem. Commun. 2014, 50, 15952−15955. (12) Geng, B. Y.; Ma, J. Z.; You, J. H. Controllable Synthesis of Single-Crystalline Fe3O4 Polyhedra Possessing the Active Basal Facets. AUTHOR INFORMATION Cryst. Growth Des. 2008, 8, 1443−1447. Corresponding Author (13) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Synthesis of *E-mail: swihart@buffalo.edu. Monodisperse Spherical Nanocrystals. Angew. Chem., Int. Ed. 2007, 46, − ORCID 4630 4660. (14) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Liang Qiao: 0000-0002-6345-3504 Wang, S. X.; Li, G. Monodisperse MFe2O4 (M = Fe, Co, Mn) Mark T. Swihart: 0000-0002-9652-687X Nanoparticles. J. Am. Chem. Soc. 2004, 126, 273−279. Author Contributions (15) Tang, Z.; Nafis, S.; Sorensen, C.; Hadjipanayis, G.; Klabunde, K. 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6381 DOI: 10.1021/acsnano.7b02752 ACS Nano 2017, 11, 6370−6381