Quantitative Analysis of Different Formation Modes of Platinum

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Quantitative Analysis of Different Formation Modes of Platinum Nanocrystals Controlled by Ligand Chemistry Xi Yin,† Miao Shi,† Jianbo Wu,† Yung-Tin Pan,† Danielle L. Gray,‡ Jeffery A. Bertke,‡ and Hong Yang*,† †

Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, 206 Roger Adams Laboratory, 600 South Matthews Avenue, Urbana, Illinois 61801, United States ‡ George L. Clark X-ray Facility, University of Illinois at Urbana−Champaign, 505 South Matthews Avenue, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: Well-defined metal nanocrystals play important roles in various fields, such as catalysis, medicine, and nanotechnology. They are often synthesized through kinetically controlled process in colloidal systems that contain metal precursors and surfactant molecules. The chemical functionality of surfactants as coordinating ligands to metal ions however remains a largely unsolved problem in this process. Understanding the metal−ligand complexation and its effect on formation kinetics at the molecular level is challenging but essential to the synthesis design of colloidal nanocrystals. Herein we report that spontaneous ligand replacement and anion exchange control the form of coordinated Pt−ligand intermediates in the system of platinum acetylacetonate [Pt(acac)2], primary aliphatic amine, and carboxylic acid ligands. The formed intermediates govern the formation mode of Pt nanocrystals, leading to either a pseudo two-step or a one-step mechanism by switching on or off an autocatalytic surface growth. This finding shows the importance of metal−ligand complexation at the prenucleation stage and represents a critical step forward for the designed synthesis of nanocrystal-based materials. KEYWORDS: Nucleation and growth, nanocrystal, ligand, complexation, platinum

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thermodynamic equilibrium of nanocrystal shapes.2,11,25−31 For instance, it was suggested that the deposition rate of adatom is affected by the hopping frequency of primary amine on the facets of metal nanoparticle.5 In another case, oleic acid was shown to be able to modify the surface energy at the solid−liquid interface of PbS nanocrystals and their thermodynamic equilibrium shape.11 Despite their tremendous importance, the roles of these ligands, in terms of quantifiable coordination chemistry are poorly understood, because the complexation between surfactants and metal precursors and its effects on the nucleation and growth processes are complicated. In recent years, efforts have been made to elucidate the chemical roles of ligands in the synthesis of nanomaterials. Early work investigated the formation of metal−oleate complexes from metal ions and OA ligand, and their thermal decomposition into monodispersed nanocrystals.13,32 Alivisatos and co-workers studied the molecular mechanism of precursor evolution in the synthesis of colloidal group II−VI semiconductor nanocrystals, and identified some key reaction mechanisms.33 More recently,

ucleation and growth are fundamentally important processes that govern universally the formation of condensed matters in both natural and human-made processes. Understanding these processes is essential for the rational design of functional nanocrystals (NCs) with well-defined composition, shape and size for the applications in catalysis, medicine and nanotechnology.1−10 In practice, thermodynamic and kinetic controls over the nucleation and growth have become popular tools in the design of nanocrystals.7 With recent advances in in situ characterization techniques, nucleation and growth of nanomaterials have been intensively re-examined at nanometer scale.3−6,11−21 Corresponding kinetic models were refined to account for the observed nucleation and growth of nanocrystals during chemical reaction and the collision of primary nanoparticles,1,22,23 addressing some limitations of the classical LaMer model that was originally developed for the precipitation phenomenon in supersaturated solution.24 The molecular level understanding on this process however is still limited. Formation of metal nanocrystals in solution phase is often accompanied by chemical reactions of metal precursors in a confined environment. Surfactants such as oleylamine (OAm) and oleic acid (OA) are able to strongly affect the size and shape of metal nanocrystals. These surfactants adsorb on metal surfaces, subsequently modify the surface energy and affect the © 2017 American Chemical Society

Received: June 29, 2017 Revised: August 15, 2017 Published: September 5, 2017 6146

DOI: 10.1021/acs.nanolett.7b02751 Nano Lett. 2017, 17, 6146−6150

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Nano Letters

different molar ratios of OA:Pt(acac)2. The Pt NCs evolved from uniform quasi-spherical to quasi-cubic shape over time. We did not observe new nuclei formation during the growth stage (>2 min). The constant number density of Pt NCs and their uniform morphology enabled further quantitative analysis by calculating the volumetric size of individual Pt NC based on its 2D projected area in TEM micrographs and performing statistical analysis. Figure 1b shows the time evolution of nanoparticle size distributions, measured in projected area. Figure 1c shows the increase of the averaged volumetric size of nanoparticles as a function of reaction time. We observed that the increased molar ratio of OA:Pt(acac)2 resulted in the decrease of the formation rate of Pt NCs, as indicated by the time for color change of the solution (Supporting Information, Figure S1). The product showed increased size with the increased molar ratio of OA:Pt(acac)2 as the result of kinetically controlled growth.7 The particle size and formation rate were sensitive to the OA:Pt(acac)2 molar ratio in a narrow range with OAm in large excess, suggesting there could be a chemical interaction introduced by OA. To probe the interactions between Pt(acac)2 and OAm or OA, we examined the reaction system using UV−vis spectroscopy (Figure 2a,b) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI−TOF MS) (Figure 2c−e). Figure 2a shows the UV−vis spectra of Pt(acac)2/OAm/ OA mixture after being preheated. We observed significant changes at the wavelengths of 274 and 312 nm with varied OA:Pt(acac)2 molar ratios. These peaks were attributed to the

new clues were discovered in various synthesis systems for metal nanocrystals and semiconductor quantum dots, which strongly suggest the important roles of metal−ligand intermediates in the nucleation and growth processes.34−42 For instance, formation of palladium carbonyl acetate intermediate and its competitive adsorption were shown to be important for the anisotropic growth of palladium nanosheets.37,43,44 Another recent study quantitatively analyzed the different formation pathways of Pd nanoparticles from various Pd halides.45 Despite all these studies, molecular structures of many metal−ligand intermediates are still unidentified. The molecular level understanding of these intermediates on the kinetics and growth of nanocrystals are not quantitatively analyzed, highlighting the challenging need to fill the knowledge gap between the metal−ligand complexation and the nucleation and growth processes.1,2 In a typical synthesis of Pt NCs using Pt(acac)2 as the precursor in the presence of OAm and OA under carbon monoxide (CO),46−48 the average particle size increased monotonically with the increase in molar ratios of OA:Pt(acac)2 in the excess of OAm (e.g., [OAm] ≫ [OA]). Figure 1a shows the morphology of Pt NCs sampled during the synthesis, with

Figure 2. Changes in UV−vis spectra and MALDI−TOF MS due to the formation of Pt-ligand complexes. (a) UV−vis absorption spectra of the reactant mixture of Pt(acac)2 and OAm at various OA:Pt(acac)2 molar ratios. (b) UV−vis spectra of acacH and its reaction products with OAm and OA. (c−e) MALDI−TOF MS of Pt(acac)2, mixture of Pt(acac)2 with OAm, and mixture of Pt(acac)2 with OAm and OA. R−NH2 stands for OAm.

Figure 1. Size evolution of platinum nanocrystals during ligand-assisted synthesis. (a) TEM micrographs of Pt nanoparticles synthesized in OAm at various OA:Pt(acac)2 molar ratios, sampled during the reaction. (b) Time evolution of particle size distribution measured in projected area. (c) Average volumetric size of Pt NCs as a function of reaction time. 6147

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Nano Letters π−π* transition in acac− ligand (λmax= 274 nm) and its amino ketone form with OAm (acac−OAm, λmax= 312 nm).49,50 The peak assignments were also verified by UV−vis spectroscopy of acetylacetone (acacH), OAm and OA mixture (Figure 2b). The UV−vis spectra of OAm and OA:OAm mixture are shown in Figure S2 for reference. The results suggest that adding OA could catalyze the aminolysis reaction between OAm and acac− ligand from Pt(acac)2, resulting in the formation of acac−OAm. The UV−vis study shows that a maximum peak intensity at 312 nm could be reached at high molar ratio of OA:Pt(acac)2, indicating the complete conversion of acac− into acac−OAm. In addition, the UV−vis spectra of the reaction mixtures exhibited different trends when OA:Pt(acac)2 molar ratio was varied during the synthesis of Pt NCs (Supporting Information, Figures S3−S5), suggesting the occurrence of different reactions. Figure 2c shows the MALDI−TOF MS of pure Pt(acac)2, which can be ionized in the forms of dimers or trimers but maintains the Pt-acac coordination. The main peak at m/z = 687.106 was assigned to [Pt2(acac)3]+. In the mixture of Pt(acac)2 and OAm, the amine ligand substituted acac− and formed oleylamineplatinum(II) acetylacetonate, which was ionized as [Pt(OAm)2(acac)]+ (Figure 2d, m/z = 801.625). This observation suggests that OAm tended to coordinate with Pt2+ and spontaneously replaced acac− ligands. With the addition of OA, the [Pt(OAm)2(acac)]+ peak disappeared, with no significant peaks of Pt complex remaining (Figure 2e). Under this condition, acac− was released and reacted with OAm to form byproducts showing two peaks at m/z = 350.350 and m/z = 599.682, in the forms of (C23H45NO− H+) and (C41H80N2−H+), respectively. This result indicates that different Pt complexes formed but were difficult to be detected by MALDI−TOF MS. Nevertheless, our data show that Pt(acac)2 precursor was converted into different coordination complexes at varied molar ratio of OA:Pt(acac)2. To determine the structures of Pt−ligand complexes and understand the ligand−metal complexation during the prenucleation stage, we conducted single crystal X-ray diffraction analysis. Since Pt(acac)2 and OAm or OA do not readily form crystals due to the disordered nature of long hydrocarbon chains in solution, butylamine (BAm), acetic acid (HOAc) and propanoic acid (PA) were used instead to facilitate the crystallization of Pt complexes, while maintaining similar chemical functionalities. Three Pt complex crystals were obtained, including tetrakis(butylamine)platinate acetate ([Pt(BAm)4]2+(OAc−)2, 1), tetrakis(butylamine)platinate propionate ([Pt(BAm)4]2+(PA−)2, 2) and tetrakis(butylamine)platinate acetylacetonate ([Pt(BAm)4]2+(acac−)2, 3). Figure 3 shows the structures of 2 and 3, and Figure S6 shows the structure of 1, as identified from the XRD analysis. In all cases, four amine groups coordinated with Pt2+ ions to form [Pt(BAm)4]2+ in square planar symmetry, which is chargebalanced with acac− or carboxylate anion (Figure 3a,b, Figure S6). With the addition of carboxylic acid, the acac− anions were replaced by carboxylate anions. These findings suggest a ligand replacement mechanism in the system of Pt(acac)2, amine and carboxylic acid, as illustrated in Figure 3c. When Pt(acac)2 dissolves in primary amine, amines coordinate with Pt2+ and replace acac− ligands, leading to the formation of Pt(amine)4(acac)2. With the addition of carboxylic acid as a proton donor, acac− reacts with protonated amine and forms an amino ketone, while Pt(amine)4(carboxylate)2 forms. The complete conversion of Pt(acac)2 requires at least 2 equiv of carboxylic acid. When OA:Pt(acac)2 molar ratio is equal to 4:1, Pt precursors might be in the form of Pt(OAm)4(OA)2. This

Figure 3. Structures of platinum−ligand complexes. (a, b) Thermal ellipsoid plots of [Pt (BAm)4]2+ cations with propionate (PA−) and acac− anions, as identified from X-ray diffraction analysis of 2 and 3, respectively. Key: platinum, silver; carbon, gray; oxygen, red; nitrogen, blue (ellipsoids set at 50% probability; H atoms omitted for clarity). (c) Proposed reactions between Pt(acac)2, amine, and carboxylic acid during the prenucleation stage.

mechanism may determine the real form of Pt precursor and affect the formation kinetics and mechanisms of Pt NCs. On the basis of the proposed forms of Pt complexes, we developed one-step, two-step, and mixed growth models to quantitatively analyze the formation kinetics of Pt NCs. Figure 4a shows the data analysis for the formation kinetics of Pt nanoparticles at different OA:Pt(acac)2 ratios. When the ratio

Figure 4. Kinetic analysis of ligand-controlled synthesis of Pt nanocrystals. (a) Kinetic data under three reaction conditions and the curve fitting to different kinetic models. Inset shows the enlarged sigmoid curve when the OA:Pt(acac)2 molar ratio was 0:1. (b) Proposed prenucleation interactions between metal precursors and ligands, which determine the growth mechanisms of Pt nanocrystals. 6148

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Nano Letters increases from 0:1 to 4:1, the curves switch from a fast, sigmoid shape to a slow, near-linear shape. We used a model describing a two-step mechanism (I) of pseudoelementary reactions to fit the data. This model is a modification of the original Finke−Watzky model.22 It describes a slow, first-order reduction (eq 1, rate constant k1) with a fast, autocatalytic surface growth (eq 2, rate constant k2), where A is a Pt precursor, B is the Pt in nanocrystals, and Bsurface is the autocatalytic surface of Pt nanocrystals. Thus, the differential eq 3 describes the yield (y) of Pt nanoparticles, in which k′2 is the apparent rate constant obtained from fitting the data in a convoluted form of k2 with other parameters, including a geometric factor (F), the number density of nanocrystals (n), the density of bulk Pt (ρ), and initial concentration ([A]0). k1

k2

A + Bsurface → 2B

(2)

dy = k1(1 − y) + k′2 (1 − y)y 2/3 dt

(3)

k 2Fn1/3 ρ2/3

[A]0 2/3

(4)

The fitting results suggest that the Pt(acac)2:OAm system follows the pseudo two-step mechanism (I), where k1 is equal to 0.0034 ± 0.0001 min−1, k′2 is equal to 0.96 ± 0.02 min−1, and the adjusted R2 is 0.99531. The system with OA:Pt(acac)2 molar ratio of 4 had a slow first-order reaction with k1 equal to 0.0022 ± 0.0002 min−1, and the autocatalysis reduction appears to be inhibited with a small value of k′2 equal to 0.002 ± 0.002 min−1 and the adjusted R2 equal to 0.9906. When k′2 is small, the model can be approximated to a pseudo one-step mechanism (II), with k1 equal to 0.0025 ± 0.0005 min−1, and the adjusted R2 is equal to 0.98873. Combining the fitted kinetic data and observed Pt− ligand complexations, it appears that Pt(OAm)4(OA)2 is reduced more slowly than Pt(OAm)4(acac)2. The plausible reason is that the change in ligand complexation switched off the autocatalytic surface growth step and resulted in a much slower growth. When OA:Pt(acac)2 became 1.7, Pt(OAm)4(acac)2 and Pt(OAm)4(OA)2 could coexist. The data show mixed characters of the above two modes: an initial burst of fast growth, followed by a slow growth. These formation modes should be modeled by different kinetics with other Pt complexes in the solution. Therefore, we further developed the following mixed mechanism (III) to describe this mode of formation: k1

A1 → B

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k2

k3

A1 + Bsurface → 2B

[A]0 2/3

(9)

ASSOCIATED CONTENT

(6)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02751.

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Supplementary text on experimental details and data analysis, eqs S1−S24, Figures S1−S9, and Table SI (PDF)

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where A1 and A2 represent two different forms of Pt precursor in solution, which may correspond to Pt(OAm)4(acac)2 and Pt(OAm)4(OA)2, respectively. The yield (y) of Pt nanocrystals can be calculated by numerically solving the following differential equations with an initial value of y0 equal to 0:

AUTHOR INFORMATION

Corresponding Author

*(H.Y.) E-mail: [email protected]. ORCID

dy = k1[100% − y − C exp(−k 2t )] + k 2C exp(−k 2t ) dt + k′3 y 2/3 [100% − y − C exp(−k 2t )]

ρ2/3

S Supporting Information *

(5)

A2 → B

k 3Fn1/3

where C is the percentage of Pt precursor in the form of A2. By fitting the kinetic data at OA:Pt(acac)2 molar ratio of 1.7:1 using eq 8, we obtained an optimized rate constant of k1 to be 0.05 ± 0.02 min−1, k2 to be 0.005 ± 0.001 min−1 and k′3 to be 0.72 ± 0.16 min−1, with constant C to be 0.79 ± 0.02. The adjusted R2 is 0.992. More detail on the kinetic data analysis is described in the Supporting Information, Figures S7−S9. Figure 4b summarizes the proposed reaction pathways that show the effect of ligand on the formation modes of Pt nanocrystals. In this proposed reaction system, Pt(acac)2 forms two main types of complexes and undergoes three different pathways to form Pt nanocrystals. For the system with only Pt(acac)2 and OAm, Pt(OAm)4(acac)2 complex forms. This Pt(OAm)4(acac)2 reaction intermediate has two possible pathways to form Pt nanoparticles. One is through the reduction in solution to generate Pt(0) species, which contributes to both the initial nucleation and the subsequent growth of Pt nanocrystals. The other is via the reduction on the Pt surface through a surface autocatalytic reaction, which also contributes to the continuous formation of Pt NCs. The addition of OA results in the formation of a complex with Pt(acac)2, possibly Pt(OAm)4(OA)2. With this complex, the formation kinetic of Pt NCs appears to be dominated by a pseudo first order ratedetermining step. Thus, the formation mode of Pt nanocrystal can be controlled through the change of ligands used in solutions. In summary, we have presented evidence on the prenucleation complexation between an organometallic precursor and ligands in the Pt(acac)2/OA/OAm system for a kinetic-controlled synthesis of Pt nanocrystals. Our data reveal the likely structures of Pt−ligand complexes and the ligand replacement reactions. On the basis of the experimental data, we developed kinetic models for quantitative analysis of the formation mode of Pt nanocrystals. We found that the mode of formation may follow a pseudo one-step or two-step mechanism, by switching on or off an autocatalytic surface growth via the choice of ligands. The connection between metal−ligand complexation and formation modes of metal nanocrystals is established. These findings indicate the importance of metal−ligand interactions at the prenucleation stage and represent a critical step forward in the designer synthesis of nanocrystal-based materials.

(1)

A→B

k′2 =

k′3 =

Danielle L. Gray: 0000-0003-0059-2096 Jeffery A. Bertke: 0000-0002-3419-5163 Hong Yang: 0000-0003-3459-4516

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Nano Letters Author Contributions

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X.Y. and H.Y. conceived the idea and designed the experiments. X.Y. conducted the experiments and data analysis. M.S. contributed to the synthesis of Pt nanocrystals. M.S., J.W. and Y.-T.P contributed to data analysis. D.L.G and J.A.B. conducted X-ray crystallographic analysis. X.Y. and H.Y. cowrote the paper. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest. CCDC 1484407, 1484408, and 1484409 contain the supplementary crystallographic data for 2, 3, and 1 respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ structures.

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ACKNOWLEDGMENTS This work is supported in part by the NSF (CHE-1213926). The Materials Chemistry Laboratory at the University of Illinois was supported in part by the NSF (CHE 95-03145 and CHE 0343032). TEM was carried out in the Frederick Seitz Materials Research Laboratory Central Research Facilities, UIUC. We thank Catherine J. Murphy, Edmund G. Seebauer, and Damien Guironnet for helpful discussion. We also thank Steven A. Warren, Andrew M. Baker, and Ling Lin for their assistance. Y.T.P. is grateful for the fellowships from Ministry of Education of Taiwan and the Dow Chemical Company.

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DOI: 10.1021/acs.nanolett.7b02751 Nano Lett. 2017, 17, 6146−6150