nanocrystals as the catalyst for electron-transfer reactions Narayanan and El-Sayed (2005). .... derstand how kinetic and thermodynamic influence seed formation. ..... Bonet, F., K. Tekaia-Elhsissen, and K. V. Sarathy, 2000: Study of interaction ...
PROJECT DESCRIPTION 1
Project Summary
Research objective
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Intellectual Merit
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Broader Impacts
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Introduction
Nanoscale materials have the potential to solve many of today’s biggest problems such as peak oil, the global water crisis, and the burden of cancer. Faraday’s discovery of colloidal ruby gold producing different colored solutions in 1857 Faraday (1857); Thompson (2007) has inspired generations of nanoscale science. Controlled synthesis of nanocrystals, namely quantum dots, was invented by Bawendi et al. in 1993 Hakimi et al. (1993); Murray et al. (2000). They paved the way for the utilization of well-defined nanocrystals in various fields ranging from heterogeneous catalysis Astruc (2008); Astruc et al. (2006) to photovoltaics Atwater and Polman (2010), DNA sequencing McNally et al. (2010), batteries Panniello et al. (2014), hydrogen storage Jena (2011); Ramos-Castillo et al. (2015), and cancer therapeutics Jain and Stylianopoulos (2010); Kim et al. (2010). Nanocrystals can be grown to specific sizes and shapes, but the question of what is the growth controlling mechanism remains elusive. This is important because numerous properties of metal nanocrystals are found to depend on their size Roduner (2006) and shape Xia et al. (2008). In catalysis for instance, tetrahedral Pt nanocrystals are more active than spherical and cubic Pt nanocrystals as the catalyst for electron-transfer reactions Narayanan and El-Sayed (2005). The controlling mechanism for various systems have been studied using both experimental and computational techniques. Experimentalists have employed techniques such as in situ transmission electron microscopy Liao et al. (2014); Woehl et al. (2014), X-ray photoelectron spectroscopy Gao et al. (2004); Park and Shumaker-Parry (2014); Huang et al. (1996); Kedia and Kumar (2012); Bonet et al. (2000), and variation of reaction parameters Personick and Mirkin (2013); Xia et al. (2012); Zeng et al. (2010); Zhang et al. (1996); Chang et al. (2011); Zhu et al. (2011) to elucidate the controlling mechanism. Theorists address this important question using techniques such as density functional theory Kilin et al. (2008); Al-Saidi et al. (2012); Saidi et al. (2013); Zhang et al. (2008) and molecular dynamics Zhou et al. (2014). However, previous work in the literature have not yet adequately addressed the mechanism of shape control in the solution phase. Despite much excellent experimental and theoretical work, there is no quantitative evidence in the facet selective adsorption of structure-directing agents and their role in shape control. Without such evidence, we are left with an incomplete description of the shape control mechanism that creates the condition for ill-informed reaction engineering for scale-up. To date, only a few syntheses have been scaled up to the gram-scale, and yet they still have poor quality control Jana (2005); Lohse et al. (2013). This study will remedy this gap in the literature by elucidating the role of structure-directing agents using molecular dynamics simulation, in which explicit solvent is computationally feasible and observations in the atomic resolution can be made. Using enhanced sampling methods, I will provide quantitative evidence that will support or refute the facet selective adsorption hypothesis and further examine its implications. The scope of my investigation is limited to the synthesis of colloidal metal nanocrystals, particularly silver (Ag). The polyol synthesis is a popular solution-phase synthesis of Ag nanocrystals
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Skrabalak et al. (2007). The typical reaction temperature is 150◦ C. In the polyol synthesis, ethylene glycol acts as both the solvent and the reducing agent. The source of Ag is from silver nitrate (AgNO3 ) that is dissolved in ethylene glycol. Ag seeds may be added in order to disentangle growth from nucleation, enhancing control of nanocrystal size and shape. Structure-directing agents, typically polyvinylpyrrolidone (PVP), are added to prevent aggregation of nanocrystals and to promote the formation of {100}-faceted nanocrystals. The reaction conditions such as temperature, the concentration of AgNO3 , and the molar ratio of PVP to AgNO3 is critical to the formation of different nanocrystal shapes. These shapes can range from cubes Xia et al. (2012); Zhang et al. (2010), triangular plates Lofton and Sigmund (2005); Liu et al. (2012), and five-fold twinned pentagonal wires Zhu et al. (2011); Zhang et al. (2008); Sun (2002a). It was hypothesized that PVP promotes the formation of {100} facets by preferentially binding to {100} facets over {111} facets Xia et al. (2012); Sun (2002a), also known as facet specific adsorption. Binding of PVP to Ag surfaces can be characterized by the potential of mean force (PMF) profiles, which can be calculated by umbrella sampling Torrie and Valleau (1977); K¨astner (2011). The PMF represents the free energy of a system as the function of one collective variable. Adsorption processes can be described by the free energy of the adsorbate as the function of the orthogonal distance from the surface where adsorption occurs. Binding energies can be obtained from the difference between the PMF of the adsorbate at the adsorbing state and at the solvent phase. In addition, kinetics of the adsorption process can be obtained from the energy barrier for adsorption in the PMF profile. The limitations of this method are the collective variable must be accurately chosen to represent the physical nature of the system and the multidimensional energy landscape is reduced to one dimension thus it may not give the complete description of the system.
2.1
Intellectual Merit
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Broader Impacts
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Desired Outcomes and Evaluation Criteria
We aim to develop a fundamental understanding of how Ag nanocrystals are grown to specific shapes, such as nanocubes Im et al. (2005), nanoplates Lofton and Sigmund (2005) and nanowires Tsuji et al. (2008a). Using molecular simulations, we hope to answer these intriguing questions: How are 100 facets stabilized in the growth of nanocubes? How are twinned nanocrystals formed and stabilized? How does structure-directing agents, foreign species, and solvent molecules contribute to shape selectivity? Our ultimate goal is to provide guidelines for engineers on designing syntheses that produces batches of mono-disperse metal nanocrystals. With the goals described in mind, our research objectives: 1. Compute the solution-phase binding energies of PVP to Ag(100) and Ag(111) surfaces to elucidate the role of structure-directing agents in shape-controlled synthesis. 2. Compare rate of Ag atom addition to Ag surfaces that are adsorbed by PVP to verify that {100} faceted nanocrystals can be favored by kinetic control growth. 3. Measure the influence of 2-pyrrolidone on PVP layers adsorbed on Ag surfaces to explain truncation of nanocubes in the presence of 2-pyrrolidone. 4. Quantify the interaction between Ag nanoplates in the oriented attachment process to clarify the mechanism of the process. 5. Calculate energy profiles of shape-specific seed formation in different environments to understand how kinetic and thermodynamic influence seed formation.
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Background
The unique properties of metals nanocrystals are enabling state-of-art technologies. Metal nanocrystals are clusters of metal atoms in a crystalline lattice with the size of 1 to 100 nanometers. Shapecontrol in noble metal systems, Ag in particular, is our interest. This is because the structureproperty relationship of Ag nanocrystals is evident, particularly in their capabilities in localized surface plasmon resonance. That is when light incident upon the surface, collective oscillation of free electrons (also known as plasmons) arises as light wave are trapped within nanocrystals smaller than the wavelength of light Petryayeva and Krull (2011). The geometric shape of the nanocrystal dictates the number of resonance peaks, their wavelengths, and the partitioning between scattering and absorption cross sections. This is because polarization of free electrons and charges distribution occurs over the nanocrystal surface. Among all metals, Ag exhibits the strongest plasmonic interaction with light Lu et al. (2009). Achieving tight shape-control can complement applications such as bioassays based on surface-enhanced Raman spectroscopy (SERS). For example, a localized surface plasmon resonance bio-chip can be used for real-time detection of insulin Hiep et al. (2008).
4.1
Synthesis of Metal Nanocrystals
Metal nanocrystals can be produced through vapor-phase Swihart (2003) and colloidal methods Tao et al. (2008). Examples of vapor-phase methods are inert gas condensation Wegner et al. (2002); Simchi et al. (2007), chemical vapor synthesis Lee et al. (2012); Ostraat et al. (2001), and flame spray pyrolysis Teoh et al. (2010). Vapor-phase methods require a high temperature (over 1000 ◦ C Smetana et al. (2005)), vacuum and expensive equipments. The scope of my investigation is limited to the chemical synthesis of colloidal metal nanocrystals. The colloidal method typically involves a metal salt precursor reduced in solution in the presence of a dispersant, which prevents the aggregation of nanoparticles and improve their stability. Advantages of the colloidal method are: 1) No specialized equipment required; 2) Solution-based processing is matured and readily available; 3) Parameters such as solvent, temperature, precursor concentration, surface capping and foreign species can be tweaked to make versatile nanocrystals; 4) Continuous process allow large yields of nanocrystals to be synthesized. The growth of colloidal nanocrystals can be roughly divided into three stages Xia et al. (2008): 1) nucleation, where individual metal atoms cluster together to form nuclei; 2) evolution of nuclei into seeds; 3) growth of seeds into nanocrystals. The difference between nuclei and seeds is the structure of nuclei can fluctuate but the seeds cannot. In a thermodynamic-control growth, the seed shape is known to define the nanocrystal shape. Although the process can be roughly defined, our current understanding of the evolution pathway is far from being able to visualize the atomistic details of how seeds nucleate and evolve into nanocrystals of specific shape. Colloidal methods to synthesize Ag nanocrystals include citrate reduction Wu et al. (2008); Lee and Meisel (1982), silver mirror reaction Yin et al. (2002), polyol synthesis Wiley et al. (2008); Sun (2002a), seed-mediated growth Pietrobon et al. (2009); Sun (2002a); Zhang et al. (2010), and light-mediated synthesis Pietrobon and Kitaev (2008); Jin et al. (2003); Zhou et al. (2008). The polyol synthesis is a common and successful synthesis route, in which nanocrystals with welldefined shapes and sizes can be obtained. In a self-seeding polyol synthesis, AgNO3 and PVP are dissolved in ethylene glycol. The mixture is typically refluxed at 150◦ C. The morphology and dimensions of the product were shown to depend on different aspects of the reaction conditions Sun (2002a). Irregular nanoparticles were obtained when the temperature is below 120◦ C or above 190◦ C. At the initial concentration of AgNO3 higher than 0.1 M nanocubes were formed, otherwise nanowires were the major product. Multiply twinned particles were obtained instead of single crystal cubes when the molar ratio of PVP to AgNO3 was increased from 1.5 to 3. It was shown that nanowires with higher aspect ratios can be obtained by reducing the molar ratio of PVP to
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AgNO3 Sun (2002a). The optimal molar ratio was found to be 1.5:1, noting that the concentration of AgNO3 is below 0.1 M. Although at a molar ratio of less than 1:1, the yield and monodispersity of nanowires were too unacceptably low. Higher degree of polymerization of PVP can also promote the formation of nanowires with high aspect ratios Sun (2002a). Irregular nanoparticles are formed when 1-ethyl-2-pyrrolidone, the monomer of PVP, is used in place of PVP. This observation demonstrates the importance of the steric effect of PVP that prevents aggregation between nanoparticles. Modifications of the polyol synthesis protocol by introducing foreign species enables a highly selective production of different nanocrystal shapes. In the synthesis of Ag nanocubes, trace amount of Na2 S can be added to increase the selectivity towards nanocubes Skrabalak et al. (2007); Siekkinen et al. (2006). The addition of Na2 S allows the formation of Ag2 S nanocrystals, which catalyzes the reduction of AgNO3 . It is proposed that the faster reduction process can limit formation of twinned Ag seeds Wiley et al. (2006), thus promoting the formation of nanocubes. Ag nanoplates can be obtained with yields as high as 90 % by substituting PVP with polyacrylamide Xiong et al. (2007). It has been reported that the amino groups of polyacrylamide can form complexes with metal cations Sari et al. (2006), greatly reducing the potential of the Ag/Ag+ pair, thus reduction rate of AgNO3 is significantly reduced. The reduced reduction rate favors the formation of nanoplates through kinetic control. Introducing Fe(II) ions into the system allows for the control over oxidative etching Wiley et al. (2005). The Fe(II) ions can remove adsorbed oxygen atoms from the surface of Ag nanocrystals and prevent their dissolution by oxidative etching. At a high concentration of Fe(II) ions, nanowires were obtained because dissolution of twinned seeds were inhibited. On the other hand, single-crystal nanocubes formation was favored at low concentration of Fe(II) ions because adsorbed oxygen atoms were only partially removed, and thus twinned seeds were selectively etched. It was proposed that replacing AgNO3 with CF3 COOAg as the new silver precursor can improve both the robustness and reproducibility of the polyol synthesis Zhang et al. (2010). The argument was that the trifluoroacetate group is more stable than the nitrate group. The nitrate group may decompose at an elevated temperature, making the synthesis more difficult to understand and control. High-quality Ag nanocubes with controllable dimensions were produced with CF3 COOAg in the presence of NaSH and HCl. Seed-mediated growth is essentially the polyol synthesis with nanoparticle seeds added to disentangle growth from nucleation. This method allows for a greater control over the final morphology. The two main categories of seed-mediated growth are homoepitaxial and heteroepitaxial growth. For a homoepitaxial growth, the seed crystal is composed of the same metal as the atoms being deposited. With either spherical or cubic seeds, Ag nanocubes with edge lengths controllable in the range of 30 to 200 nm can be synthesized Zhang et al. (2010). Tunable reaction parameters, additional to the self-seeding process, include molar ratio of Ag seeds to AgNO3 and concentration of Ag seeds added. Remarkable size control can also be achieved as demonstrated in the seed-mediated synthesis of Ag decahedrons Pietrobon and Kitaev (2008). Seeds of Ag decahedral particles can be grown in a controllable fashion by mixing in additional precursor solution and exposing the mixture to white light of a metal halide lamp for 20 h. These regrowth steps can be repeated to produce Ag decahedrons ranging from 40 to 120 nm. The decoupling of growth from nucleation allow the nanocrystal size to be controlled while fully preserving the monodispersity. For a heteroepitaxial growth, the seeds and the deposited atoms are chemically different. Difference in lattice constants between the two types of metals plays an important role in heteroepitaxial growth. For example, Au and Ag have a lattice mismatch of only 0.25% therefore Au seeds have been successfully used as template for Ag deposition. Synthesized shapes with the Au-Ag core-shell structure include triangular bifrustums Yoo et al. (2009) and nanorods Seo et al. (2008); Tsuji et al. (2006). When there is a large lattice mismatch, anisotropic growth are promoted because isotropic growth is inhibited by high strain energy. For example, Pt and Ag have a lattice mismatch of 4.15% thus Pt nanocrystals can be used as seeds for Ag nanowires growth Sun (2002a,b); Tsuji et al. (2008b).
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4.2
Mechanistic Studies of Shape Control
Nanocrystals synthesis can be viewed as a set of sequential and parallel chemical reactions. Each route of chemical reactions yields nanocrystals of different dimensions and morphology. The end product of chemical reactions can be controlled by a set of thermodynamic and kinetic parameters. For the synthesis of metal nanocrystals, thermodynamic parameters can include reduction potential and surface capping. The kinetic parameters can include concentration, mass transport, temperature, and the involvement of foreign species. These parameters dictates the shape of the free energy landscape and how well the system can minimize its free energy. The free energy landscape can be depicted by wells of free energy minima separated by free energy barriers, as shown in Figure 1. The thermodynamic product can be obtained if the system can overcome all free energy barriers and reach the global minimum. This is ideally achievable at high temperature or with long relaxation time after each sequential reaction. When the system is stuck in a local minimum, the kinetic product is obtained. In this case, the system do not have enough time or energy to cross the free energy barriers for the system to be at the global minimum. We can see that temperature plays an important role in determining whether the synthesis is controlled by thermodynamics or kinetics. It is interesting to point out that the thermodynamic product only depends on the final state, while the kinetic product depends on the exact process. Xia et al. have comprehensively reviewed the perspective between thermodynamic and kinetic products in the shape-controlled synthesis of colloidal metal nanocrystals Xia et al. (2015). The growth process of metal nanocrystals consists of atoms initially adding to a specific site on the nanocrystal surface (atom deposition) and migrating to the site lowest in surface free energy (surface diffusion). Atoms tend to deposit at the most active region with the highest surface free energy, such as corners, edges, and high-index facets. This is because it is energetically favorable to stabilize the most active sites of the nanocrystal surface. The shape of the nanocrystal will be determined by the relative magnitudes of the atom deposition rate (Vdeposition ) and the surface diffusion rate (Vdi f f usion ). The relative magnitudes can be concisely represented by the ratio between the rates for atom deposition and surface diffusion (Vdeposition /Vdi f f usion ) Xia et al. (2013). When Vdeposition /Vdi f f usion > 1, most adatoms stay at the high surface energy region since they do not diffuse fast enough. In this case, the synthesis is under kinetic control. Kinetically controlled shapes depend on the relative deposition rates to different facets of the nanocrystal, which the shape can be predicted by the kinetic Wulff construction Zhang et al. (2006). Various experimental parameters can influence the rates for atom deposition and surface diffusion. The rate at which metal atoms are available for deposition directly correlates with Vdeposition . In the colloidal synthesis, typically metal atoms are supplied through the reduction of a salt precursor by a reductant. Thus, the magnitude of Vdeposition can be influenced by the concentration of reagents, reaction temperature, additives that form coordination complexes with the metal ion, and the injection rate of the precursor solution. On the other hand, surface diffusion is a thermally activated process where there is a potential energy barrier to diffusion. The adatoms on the solid surface diffuses by a jumping or hopping mechanism Tringides (1997). Consequently, Vdi f f usion is mainly determined by the reaction temperature and the height of the potential energy
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barrier. The height of the potential energy barrier can be affected by factors such as the strength of bond between the surface atom and the adatom, the crystallographic plane of the surface, and capping agents surface passivation.
Figure 1: Free energy landscape of a hypothetical synthesis of nanocrystals illustrated in one-dimension by a parameter that represents the nanocrystals structure Xia et al. (2015). Through decades of research, different aspects of the shape control mechanism in colloidal metal nanocrystals synthesis have been gathered. Because of the complexity of the collodial synthesis, we are still far from being able to describe the mechanism in precise atomic details. Previous efforts of experimentalists and theoriests to decipher the shape control mechanism will be described respectively below. The scope of this review will be on the polyol synthesis of Ag nanocrystals in the presence of PVP as the structure directing agent. It has long been hypothesized that PVP might interact more strongly with the {100} facets than with the {111} facets because the presence of PVP allows {100} faceted Ag nanocrystals such as cubes Sun (2002b) and wires Sun (2002a) to be formed. The first evidence that PVP preferentially binds to {100} facets over {111} facets of Ag nanocrystals was given by Xia et al. Sun et al. (2003).
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Proposed Research
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Summary and Significance
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Broader Impacts
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Timelines
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