Revisiting the Polyol Synthesis of Silver Nanostructures: Role of

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Revisiting the Polyol Synthesis of Silver Nanostructures: Role of Chloride in Nanocube Formation Zhifeng Chen, Tonnam Balankura, Kristen A. Fichthorn, and Robert M. Rioux ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08019 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Revisiting the Polyol Synthesis of Silver Nanostructures: Role of Chloride in Nanocube Formation Zhifeng Chen,a Tonnam Balankura,a Kristen A. Fichthorn,a,b,* Robert M. Riouxa,c,* aDepartment

of Chemical Engineering, Pennsylvania State University, University Park, PA

16802 (USA) bDepartment

of Physics, Pennsylvania State University, University Park, PA 16802 (USA)

cDepartment

of Chemistry, Pennsylvania State University, University Park, PA 16802 (USA)

Corresponding Author R. M. Rioux ([email protected]) K. A. Fichthorn ([email protected])

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ABSTRACT

Chloride (Cl-) is often used together with polyvinylpyrrolidone (PVP) in the polyol synthesis of Ag nanocubes. In the literature, shape control is attributed predominantly to the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets; whereas the role of Cl- has not been well studied. Several hypotheses have been proposed regarding the role of Cl-; however, there is still no consensus regarding the exact influence of Cl- in the shape-controlled synthesis of Ag nanocubes. To examine the influence of Cl-, we undertook a joint theoretical-experimental study. Experimentally, we examined the influence of Cl- concentration on the shape of Ag nanoparticles (NPs) at constant H+ concentration. In the presence of H+, in-situ formed HNO3 etches the initially formed Ag seeds and slows down the overall reduction of Ag+ which promotes the formation of monodisperse Ag NPs. Ex-situ experiments probed the evolution of Cl- during the growth of Ag nanocubes, which involves the initial formation of AgCl nanocubes, their subsequent dissolution to release Cl-, which adsorbs onto the surfaces of single crystal seeds to impact shape evolution through apparent thermodynamic control. The formation of cubes is independent of the source of AgCl, indicating temporal control of the Cl- chemical potential in solution leads to high-yield synthesis of Ag nanocubes. Increasing the concentration of Cl- alone leads to a progression in shape from truncated octahedra, to cuboctahedra, truncated cubes and ultimately cubes, directly demonstrating the importance of Cl- in Ag NP shape control. We used ab initio thermodynamics calculations based on density-functional theory to probe the role of Cl- in directing shape control. With increasing Cl chemical potential (surface coverage), calculated surface energies  of Ag facets transition from 111 < 100 to 100 < 111 and predict Wulff shapes terminated with an

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increasing (100) contribution, consistent with experimental observations. The combination of theory and experiment is beneficial for advancing understanding of nanocrystal formation. Keywords: silver nanocubes, chloride, polyol synthesis, shape control, ab initio thermodynamics

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Solution-based methods have been widely employed for the synthesis of metal nanoparticles (NPs) to control the size,1–5 shape1,6–8 and composition,1,3,9–11 which sensitively determine behavior for various applications in catalysis,12–16 optics,17–21 magnetics22–25 and electronics.6,26,27 Various shapes of Ag NPs have been successfully synthesized such as cubes,7,28–37 nanowires,7,38–43 octahedra44–49 and plates.50–55 Promising applications of Ag nanocubes in photonics, catalysis and sensing drive further studies of their synthesis to achieve more uniform shapes and sizes. Sun and Xia reported the polyol method for shape-controlled synthesis of Ag nanocubes,28 in which the Ag precursor (AgNO3) is reduced by ethylene glycol at elevated temperatures in the presence of poly(vinylpyrrolidone) (PVP). Modifications of the original Ag nanocube synthesis method using additives such as Cl-/O2,56 HCl,30 Na2S/NaHS,33,37,57 Fe3+ or Fe2+ species29 and Br-34 to achieve monodisperse Ag nanocubes have been reported. More recently, utilizing a combination of Cl-, SH- and Br-, sub-15 nm Ag nanocubes with sharp edges have been synthesized.35 Various hypotheses regarding the role(s) of PVP have been proposed in literature based on the observations of the impact of PVP-related variables on the shape of the final products.38,47,58–62 The prevailing hypothesis is the face-blocking theory in which PVP binds more strongly to the (100) facets compared to the (111) facets of silver.38,47,58–62 The proposed preferential binding of PVP leads to the retention of Ag(100) facets at the expense of Ag(111) facets, resulting in Ag nanostructures terminated mainly by (100) facets. The interplay between theory and experiment has been valuable in resolving the origins of cubic Ag NPs. Theoretically, Qi et al. predicted Wulff shapes of PVP-covered Ag nanocrystals in solution and found the preferred Wulff shapes are truncated octahedra.63 Even though their calculations predict PVP binds more strongly to Ag(100) than to Ag(111),59,62,64,65 this binding preference is not large enough to favor cubes as the preferred thermodynamic shape.63 Qi et al. 4


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employed molecular dynamics simulations to investigate the growth rates of Ag(100) and Ag(111) facets with various densities of adsorbed PVP oligomers. By incorporating these facet growth rates into the kinetic Wulff construction, they predicted {100}-faceted Ag cubes formed for high PVP surface coverage and long oligomer chains, indicating PVP promotes kinetic nanocube shapes.61,66 More recently, one of our groups developed a quantitative analysis, with both experiments and theory, of the preferential binding of PVP binding towards various Ag NP shapes and only a 2-fold difference was found for the equilibrium adsorption constant for PVP with a molecular weight of 55 kg/mol to Ag(100) versus Ag(111) facets. The 2-fold difference is not nearly enough to produce cubes via thermodynamic control, also indicating the possibility of kinetic control.67 While these studies are consistent with experiments that demonstrate nanocube growth in the absence of additives can be directed by PVP alone,47 numerous studies have demonstrated that more robust nanocube syntheses can be achieved in the presence of additives. 29,30,33–35,37,56,57

In a recent study, Kim et al. combined experiment and first-principles density-

functional theory (DFT) to show that a facet-dependent synergy between chloride and cappingmolecule (hexadecylamine) adsorption can lead to the growth of Cu nanowires with high aspect ratios – again likely by kinetic means.68 This study demonstrated the powerful role that halides can play in directing shape control.

Here, we use first-principles DFT and experiment to

demonstrate chloride can be highly beneficial for producing Ag nanocubes in polyol synthesis. Cl- is often used together with PVP during Ag nanocrystal synthesis.30,31,35,56 However, it is still not clear if Cl- influences the final shape of the Ag NPs. A primary challenge in the deduction of its role in shape control in characterizing how Cl- evolves and speciates during synthesis. Various hypotheses regarding the role of Cl- have been proposed in the literature but generally lack experimental support. Work by Xia’s group reported that Cl-/O2 etch the initially formed twinned 5


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seeds, which leads to the formation of monodisperse Ag nanocubes.56 Schuette et al. reported initially formed AgCl acts as a heterogeneous nucleant for the growth of Ag nanowires as suggested by the observation of nanowires emanating from AgCl nanocubes.39 Peng et al. showed the solid-phase transition of AgCl nanocrystals to Ag NPs employing in-situ XRD.32 Sangaru et al. employed high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDX) to demonstrate Ag nanocube surfaces are passivated by chloride and they concluded that a thin layer of AgCl is essential for the stabilization of the cubic morphology.69 More recently, Zhou et al. used Cl- for the synthesis of Ag nanocubes in aqueous solution.31 In their study, the role of Cl- was attributed to the initial formation of AgCl octahedra. After light exposure and with ascorbic acid as a reducing agent, AgCl reduced to Ag nuclei and evolved to single-crystal seeds. Zhou et al. also proposed that Cl- acts as a specific capping agent toward Ag(100) to control NP shapes.31 The use of halides, especially bromides are ubiquitous in the colloidal nanoparticle synthesis literature, as emphasized by Ghosh and Manna70 in a recent review. Of particular relevance to this work is the study by Peng and co-workers,71 who found that Pd cubes were preferred over octahedra at high bromine concentrations. Using XPS and ICP analysis, they confirmed a shape evolution from cubes to octahedra was accompanied by a decrease in the surface density of Br- on Pd nanocrystals. As we will discuss below, we reach similar conclusions for the synthesis of Ag nanocubes in the presence of Cl-. With a number of proposed roles of Cl-, such as an etchant, a heterogeneous nucleant and selective adsorbate on Ag(100), it is apparent there is still no agreement regarding the exact role of Clduring Ag nanocube synthesis. We studied the influence of Cl- on the shapes of Ag NPs experimentally by varying the amount and source of Cl- added at constant H+ concentration. Ex6


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situ experiments documenting the role of Cl- during the entirety of the 28 h synthesis are presented. We used ab initio thermodynamics calculations to probe the impact of the solution-phase Cl chemical potential/surface concentration of Cl on the predicted Wulff shapes of Ag NPs. RESULTS AND DISCUSSION Influence of Cl- concentration on the shapes of Ag NPs We tested various hypotheses regarding the role of Cl- proposed in the literature30–32,39,69 by examining the influence of Cl- concentration at constant H+ concentration on the shapes of Ag NPs. Im et al. reported HCl promotes the formation of sharp and monodisperse cubes in the polyol synthesis of Ag nanocubes.30 To provide greater resolution of the role of HCl, we distinguish the separate roles of H+ and Cl- by introducing each ion from independent sources. We initially carried out three control synthesis experiments – (i) pure EG; (ii) 3 mM HNO3 in EG and (iii) 3 mM NaCl in EG as a replacement for HCl during Ag NP synthesis while keeping all the other parameters identical. After synthesis, SEM images were taken to examine the shape of the Ag NPs formed under the various modified synthetic conditions. The comparison among these control experiments demonstrated the importance of Cl- in controlling the cubic shape and the importance of H+ in controlling the monodispersity of the shapes. In pure EG, irregular and polydisperse Ag NPs formed (Figure 1A), confirming the importance of HCl addition for the formation of monodisperse Ag nanocubes. With the addition of 3 mM HNO3, the product was predominantly truncated octahedra (Figure 1B). As shown in Figure S1, the truncated octahedra exhibited various orientations and different extents of truncation. Fast Fourier transform (FFT) of TEM images confirm the crystalline nature of the Ag nanoparticles with Ag(111) and Ag(220) reflections indexed (Figure S2). This is consistent with a recent 7


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theoretical study by Qi et al. demonstrating a truncated octahedron is the predicted thermodynamic Wulff shape for Ag NPs in the presence of PVP and in the absence of Cl-.63 With the addition of 3 mM NaCl, the resulting Ag products (Figure 1C) consist of Ag nanowires, nanocubes and irregular shapes. The presence of Ag nanocubes in the products confirmed the importance of Clin shape control. However, nanowires and irregular NPs are also observed, which is likely due to the lack of H+ to form in-situ HNO3 serving as oxidative etchant to etch twinned Ag seeds, as proposed in the literature.30 In-situ formed HNO3 etches the Ag seeds and slows Ag NP growth, promoting the formation of monodisperse Ag NPs. In the absence of H+, both monocrystalline and twinned Ag seeds form and grow, which leads to various shapes of Ag NPs in the final products. We observed a color change of the solution within 5 min of NaCl addition to a yellow color, indicative of a significant fraction of Ag seeds formed, while it takes 20 h for the color change to occur with the addition of HCl or HNO3 (see details in Supporting Information). We obtained uniform Ag nanocubes only for the case when both H+ and Cl- from HCl addition were present (Figure 1D).

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Figure 1. SEM images of Ag NPs synthesized with different additives added: (A) pure EG (1 mL); (B) 3 mM HNO3 in EG (1 mL); (C) 3 mM NaCl in EG (1 mL); (D) 3mM HCl in EG (1 mL).

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Since the etching of the initially-formed Ag seeds appears important for the formation of monodisperse cubes, a constant concentration of H+ (from 3 mM HNO3) was maintained during syntheses while using NaCl as the Cl- source in order to independently control the concentration of Cl-. With the addition of 3 mM HNO3 and 0.03 mM NaCl in EG, cuboctahedra formed (Figure 2A), whereas with the addition of 3 mM HNO3 and 0.3 mM NaCl in EG, truncated cubes formed (Figure 2B), implying there was insufficient Cl- to stabilize Ag cubes. As shown in Figure 2C, with the addition of 3 mM HNO3 and 3 mM NaCl, Ag nanocubes formed (similar to the case of experiments employing 3 mM HCl). These results demonstrate Na+ has no consequence for the formation of cubes and more importantly, a sufficient Cl- concentration is required to stabilize Ag cubes. If all of the solution-phase Cl- adsorbs on the Ag NPs, the calculated Cl to surface Ag ratio (Cl/Ags) is 0 for truncated octahedra, 0.006 for cuboctahedra, 0.06 for truncated cubes and 0.6 for cubes based on the amount of Cl- added, size and shape of Ag NPs (see details of calculation in Supporting Information). The increase in Cl- concentration led to shape evolution from truncated octahedra to cuboctahedra, truncated cubes and eventually cubes.

Xia and co-workers

demonstrated when the concentration of PVP is below a critical value ([PVP] = 0.146 mM and [PVP]/[Ag] = 0.005), cubic seeds grow to cuboctahedra or octahedra due an insufficient passivation of the Ag(100) facets by PVP.47 The experimental protocol in this work uses a much higher PVP concentration ([PVP] = 36.75 mM and [PVP]/[Ag] = 1.5) and comparable sizes of Ag nanoparticles to previous work by Xia; therefore, the truncation of Ag nanoparticles in our case is not due to the inadequate passivation of Ag(100) facets by PVP. Rather, the increased fraction of Ag(100) facets with the addition of more Cl- suggested Cl- promotes stabilization of Ag(100) facets. Zhou et al. proposed the role of Cl- as a selective capping agent to Ag(100) facets to control the shape of Ag NPs – though this was not in a polyol synthesis.31 Our DFT calculations demonstrate 10


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Cl- adsorbs more strongly to Ag(100) than Ag(111) facets (see calculation details in Supporting Information), consistent with other literature studies using DFT.72,73 More recently, Qi et al. employed a multi-scheme thermodynamic integration method74 to predict that the truncated octahedron is the prevailing shape for Ag NPs in the absence of Cl- utilizing a thermodynamic Wulff construction,63 matching our experimental observations in the absence of Cl-. To probe the role of Cl- in dictating Ag nanocrystal shapes, we further carried out ab initio thermodynamics calculations to examine the influence of Cl on the surface energies of Ag facets and predicted Wulff shapes.

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Figure 2. SEM images of Ag NPs synthesized with different additives added: (A) 3 mM HNO3 + 0.03 mM NaCl; (B) 3 mM HNO3 + 0.3 mM NaCl; (C) 3 mM HNO3 + 3 mM NaCl.

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Wulff shape predictions from ab initio thermodynamics In Figure 3, we plot the surface energy (γhkl) for Ag(100) and Ag(111) as a function of 𝜇𝐶𝑙 with predicted Wulff shapes shown for selected values of 𝜇𝐶𝑙. As 𝜇𝐶𝑙 increases, the area fraction of Ag(100) facets increases, and the Wulff shape becomes more cubic, consistent with experimental observation that Ag NP shapes evolve from truncated octahedra to cubes with increasing Clconcentration (Figure 2). For 𝜇𝐶𝑙 < -3.71 eV, the surface energies of bare Ag(100) and Ag(111) are lower than those of surfaces with adsorbed chlorine. Bare Ag surfaces are thermodynamically favored in this regime and the predicted Wulff shape is a truncated octahedron. As 𝜇𝐶𝑙 reaches -3.06 eV, the surface coverage of Cl is 0.5 ML on Ag(100), 0.33 ML on Ag(111) and the surface energy of Ag(100) becomes less than the surface energy of Ag(111). At this point, the predicted Wulff shape begins to transition from a truncated octahedron to a cuboctahedron. When we reach 𝜇𝐶𝑙 = -2.68 eV, the Wulff shape is predicted to be a cube with slightly truncated Ag(111) corners, as shown in Figure 3. A cube, completely expressed with Ag(100) facets, is predicted when 𝜇𝐶𝑙 approaches -2.63 eV. The vertical red line at 𝜇𝐶𝑙 = -2.68 eV in Figure 3 represents the lower limit at which the formation of bulk AgCl is thermodynamically favored over Ag surfaces with adsorbed Cl. To confirm the formation of AgCl at higher 𝜇𝐶𝑙, we further increased the Cl- concentration during the synthesis (3 mM HNO3 and 30 mM NaCl). The products consist of a mixture of AgCl and Ag NPs as confirmed by XRD (see Figure S3B in the Supporting Information), since we have a much higher amount of Ag than Cl in the synthesis. SEM demonstrates the Ag NPs are cubic at higher Cl- concentrations (Figure S3C).

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The observed evolution of the Wulff shape from a truncated octahedron, to a cuboctahedron, and to a cube as 𝜇𝐶𝑙 increases, is consistent with the experiment observations presented in Figure 2. The Wulff construction demonstrates chlorine adsorption alters the surface energies of Ag facets (i.e., γ(100) < γ(111)) such that the preferred shape in sufficient concentration of Cl- is cubic. These results indicate a thermodynamic tendency for Ag nanocubes to form in the presence of chlorine.

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Figure 3. Surface energies 𝛾 of Ag(100) and Ag(111) for different surface coverages of Cl as a function of the solution-phase Cl chemical potential 𝜇𝐶𝑙. Each line represents one of the Cl coverages considered – details are given in the Supporting Information. Colored regions of the plot represent the minimum 𝛾 on each surface: minimum surface energies on Ag(100) are shown with solid lines and those for Ag(111) are shown with dashed lines. Wulff shapes are shown for several values of 𝜇𝐶𝑙. The vertical red line at 𝜇𝐶𝑙 = -2.68 eV denotes where the formation of bulk AgCl is thermodynamically favored over chlorine-adsorbed Ag surfaces.

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Ex-situ experiments showed the initial formation of AgCl nanocubes and their subsequent dissolution In order to explore how Cl- speciates between solution and solid product during the 28 h synthesis of Ag nanocubes, we carried out ex-situ experiments to examine the products obtained at different times during the synthesis. Ag nanocubes were synthesized employing the HCl method (see details in experimental section) and products were removed at different times (5 min, 1 h, 21 h, 23 h, 24 h and 28 h) for SEM characterization (Figure 4). The initial injection of AgNO3 led to the formation of a white precipitate within the first 5 min. The SEM image (Figure 4A) of the 5 min product demonstrates the formation of large cubic nanoparticles with edge length distribution of 253 ± 45 nm (Figure S4A). SEM-EDS (Figure S5) suggest these nanocubes are AgCl with Ag and Cl elemental maps demonstrating overlapping signals. These AgCl nanocubes were unstable under the electron beam and small Ag NPs were generated on the surface of AgCl nanocubes within a few seconds of electron beam exposure, consistent with previous observations.31,39,75 A clear change in morphology was observed after exposing AgCl nanocubes to the electron beam for one minute (Figure S6). However, it is not clear from SEM and EDS whether the products at 5 min are a mixture of Ag and AgCl or pure AgCl. The XRD pattern in Figure 5 clearly demonstrates the product collected at 5 min is pure AgCl. Ion chromatography (IC) was further employed to quantify the amount of Cl- in AgCl by dissolving these initially formed AgCl nanocubes in an ammonium hydroxide solution. IC quantification confirmed all injected Cl- precipitated as AgCl and no Cl- remained in solution (see details in Table S1). However, there is still excess AgNO3 in solution since the Ag to Cl mole ratio is 94:1. 16


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Between 5 min and 20 h, the solution color remained transparent implying negligible growth of the Ag NPs during this period. With H+ from HCl and NO3- from AgNO3, HNO3 formed in-situ to etch the initial formed Ag seeds. This observation is consistent with the results of our control experiments in Figure 1 and Xia’s hypothesis regarding the role of H+.30 SEM images of the products after 1 h synthesis time (Figure 4B) demonstrated the AgCl nanocubes were of similar size as the 5 min sample since AgCl is not soluble in HNO3. After 20 h, the solution color began to evolve from yellow to grey. SEM images from 21 to 24 h demonstrate the growth of small Ag nanocubes (10 to 50 nm) along with the dissolution of AgCl nanocubes (see the pits on the AgCl nanocubes in Figures 4C – E). The final products at 28 h (Figure 4F) are Ag nanocubes with an edge length of 74 ± 10 nm (Figure S4B). The maximum localized surface plasmon resonance (LSPR) peak red shifted as the reaction time increased from 21 to 28 h, consistent with the size increase observed with SEM (Figure S8A). The major LSPR peak has a linear dependence on the Ag nanocube edge length, consistent with previous UV-vis studies of Ag nanocubes (Figure S8B).76–78 SEM-EDS (Figure S9) confirmed the large nanocubes at 23 h were AgCl with Ag and Cl elemental mapping data overlapping. However, the Cl atomic percent decreased compared to the 5 min AgCl nanocubes (Table S2), suggesting dissolution of AgCl nanocubes led to the release of Cl-, which is consistent with the observation of pits on the AgCl NPs. The most direct evidence for the growth of Ag nanocubes along with dissolution of AgCl nanocubes is found in the XRD of the products obtained at 23 h (Figure 5). After 23 h, both AgCl and Ag(0) peaks were present in the diffractogram, consistent with the SEM and EDS data.

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At 28 h, only Ag diffraction peaks were detected by XRD (Figure 5), indicating the complete dissolution of AgCl to release Cl-. AgCl has a low solubility constant in EG similar to water at 25 C (Ksp = 1.77×10-10);79 however, at the reaction temperature of 140 °C, AgCl dissolves slowly to release Ag+ and Cl-. We measured a solubility constant, Ksp of 1.04×10-9 for AgCl powder (see details in experimental section). The released Ag+ was reduced to Ag at elevated temperature and shifted the equilibrium of AgCl dissolution to the right. However, the measured solubility constant at 140°C should be representative of the intrinsic solubility because the low concentration of Ag (relative to the concentration of Ag used in the cube synthesis) should limit Ag+ reduction to colloidal Ag products. With the dissolution of AgCl, a question arises as to the fate of the dissolved Cl- after synthesis. TEM-EDS data (Figure 6) of the final Ag nanocubes (28 h) confirmed a Cl signal on the Ag nanocube surfaces, demonstrating Cl- adsorbs on the surfaces of the Ag nanocubes.

This

observation is in agreement with the recent study by Sangaru who demonstrated a Cl passivation layer exists on the Ag nanocubes.69 The HR-TEM reported thickness of the AgCl layer was 2.3 nm. However, in our case, we did not see any lattice mismatch along the edge and center of the Ag nanocubes and the measured d-spacing is 0.202 nm, corresponding to Ag(200) as shown in Figure S10. The amorphous section outside of the Ag nanocube is a carbon layer, probably due to PVP.78 Based on the size of the Ag nanocubes and the amount of Cl and Ag added in our synthesis, the Cl/surface Ag ratio was calculated as 0.6 (see details in Supporting Information). This indicates half monolayer coverage of Cl on the Ag nanocubes, which explains why we only detect the Cl signal through EDS and not from HRTEM. The existence of a Cl signal on the Ag nanocube surfaces is further confirmed by XPS (Figure S11). We note the ab initio thermodynamics calculations (cf., Figure 3) predict a half ML coverage of Cl on Ag(100) in the regime for which 18


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the Wulff shape is a cube with truncated corners. To demonstrate that the final products are stable, Ag nanocubes synthesized after 28 h were aged at 140 °C in EG for 5 days. As shown in Figure S12, the cubic shapes are maintained, confirming long-term stability.

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Figure 4. SEM images of products sampled at different times synthesized by the HCl method: (A) 5 mins; (B) 1 h; (C) 21 h; (D) 23 h; (E) 24 h; (F) 28 h.

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Figure 5. XRD pattern of the products after 5 min, 23 h and 28 h. α represents peaks for AgCl and β represents peaks for Ag.

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Figure 6. (A) HAADF-STEM image of a Ag nanocube at t = 28 h. (B) EDS mapping of Ag nanocube at t = 28 h. (C) Line scan plot across the Ag nanocube from edge to center indicated in the white box in panel B. 22


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Externally sourced AgCl confirms the role of Cl- and importance of controlled dissolution on shape evolution It is apparent that Cl- governs shape control of Ag NPs from the ex-situ observations, which demonstrate high Cl- concentrations leads to the formation of AgCl nanocubes, which ultimately serve as a source of Cl- for the synthesis of Ag nanocubes. It appears a slow, controlled release and corresponding reduced concentration (relative to the initial concentration) of Cl- is required. We examined if the source of AgCl is critical in the synthesis of Ag nanocubes. Pre-synthesized AgCl nanocubes were added (in lieu of HCl) during the Ag cube synthesis. AgCl nanocubes were synthesized employing the HCl method with 3 mM HCl addition and the reaction stopped after 5 min rather than 28 h. The synthesized AgCl nanocubes (Figure S13A) were centrifuged with ethanol five times to remove PVP, unreacted AgNO3 and EG. These AgCl nanocubes were dried, dispersed in 3mM HNO3 in EG (1 mL) solution and added to the Ag NP synthesis as a replacement for 1mL of 3 mM HCl. As shown in Figure 7A, truncated cubes formed. This was probably due to the loss of AgCl nanocubes during the washing steps, reducing the total amount of Cl- introduced during the synthesis. We scaled the synthesis of AgCl nanocubes 10-fold with the addition of 30 mM HCl and stopped the reaction after 5 min. As shown in Figure S13B, aggregated AgCl NPs formed. These AgCl NPs were cleaned and dissolved in 1 mL of 3mM HNO3 in EG solution and used as a replacement for 1mL of 3mM HCl during the synthesis of the Ag nanocubes. As shown in Figure 7B, Ag nanocubes formed, confirming the external source of AgCl nanocubes yielded the same product as the in-situ generated AgCl nanocubes. Ag nanocube formation follows sequentially after AgCl nanocube formation and subsequent dissolution.

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Figure 7. SEM images of Ag NPs synthesized with different additives added: (A) 3 mM HNO3 + AgCl nanocubes (formed with 3 mM HCl condition); (B) 3 mM HNO3 + AgCl NPs (formed with the 30 mM HCl condition); (C) 3 mM HNO3 + 4.3 mg AgCl (equal to the amount of AgCl generated when 30 mM HCl was used); (D) 3 mM HNO3 + 43 mg AgCl (10-fold increase in the amount of AgCl generated when 30 mM HCl was used).

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To determine if the shape of the AgCl templated the formation of Ag cubes, we replaced the presynthesized AgCl nanocubes with a commercial AgCl powder. During the synthesis, 4.3 mg of AgCl powder (equal to the amount of AgCl generated when 30 mM HCl was used) and 1 mL 3mM HNO3 replaced the 3 mM HCl solution used during the synthesis. As shown in Figure 7C, truncated Ag nanocubes formed, implying the source of AgCl is not critical. However, because of the larger particle size of the AgCl powder (Figure S14), the dissolution rate of bulk AgCl in EG under reaction temperature is slower than AgCl nanocubes and led to truncated cubes instead of cubes.80,81 To prove that a critical concentration of Cl- (~0.5 ML equivalent) is necessary, we increased the amount of AgCl powder (43 mg) with 1 mL 3 mM HNO3 in EG (1 mL). As shown in Figure 7D, Ag nanocubes formed. To rule out other existing hypotheses regarding AgCl in Ag NP synthesis, control experiments were designed. Zhou et al. observed in the presence of light, AgCl can be reduced to Ag nuclei follow by their evolution into single crystal seeds and then Ag nanocrystals in aqueous solution.31 The influence of light was ruled out here by conducting the polyol synthesis of Ag nanocubes in the dark. As shown in Figure S15A, uniform Ag nanocubes formed in the absence of light. Cui’s group has reported that the addition of AgCl promoted the formation of silver nanowires.82 Schuette et al. proposed the role of AgCl as a heterogeneous nucleant for the growth of Ag nanowires.39 Without stirring, AgCl settled at the bottom of the glass vial and even with poor contact between the AgCl and Ag precursor, Ag nanocubes still formed (Figure S15B). This suggested AgCl is probably not acting as a heterogeneous nucleant for Ag nanocube synthesis. The discrepancy regarding the influence of AgCl in Cui’s and Schuette’s synthesis and the current work is most likely related to a dominating kinetic effect, due to the much higher PVP/Ag precursor and reaction temperature used by Cui and Schuette’s groups compared to our work. The 25


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impact of the reducing power of PVP has been suggested by Xiong and co-workers.83 The external source of AgCl and control experiments confirmed the role of AgCl as a Cl- source and with sufficient Cl chemical potential through controlled release, Ag nanocubes are obtained.

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Overall growth process of Ag nanocubes implicates the complex role of ClWith the role of H+ to slow down the net reduction rate of Ag+ and Cl- to control the shape thermodynamically, the 28 h growth process of Ag nanocubes can be summarized in four stages as shown in Scheme 1. (i) The initial formation of AgCl nanocubes as shown in reaction (1). 𝐴𝑔 + + 𝐶𝑙 ― →𝐴𝑔𝐶𝑙(𝑠)

(1)

Since Ag+ is in excess, all of the added Cl- forms AgCl with excess Ag+ remaining in EG solution. The initial formation of AgCl NPs is consistent with observations by Schuette et al.39 and Zhou et al.31 However, Zhou et al. observed at higher Cl-/Ag+ ratio than our experimental conditions, the initially formed AgCl were octahedra. Ma et al. showed at low Cl-/Ag+ ratio, AgCl(100) has lower surface energy whereas at high Cl-/Ag+, AgCl(111) is more favorable.84 Zhang et al. demonstrated by adjustment of the molar ratio of Ag+ to Cl-, reaction temperature and time, various shapes of AgCl NPs could be obtained.85 (ii) Reduction of AgNO3 to Ag seeds and their subsequent etching by in-situ formed HNO3 as shown in reactions (2) and (3). 𝐴𝑔 + 𝑟𝑒𝑑𝑢𝑐𝑖𝑛𝑔 𝑎𝑔𝑒𝑛𝑡𝐴𝑔(𝑠)

(2)

3 𝐴𝑔(𝑠) + 4 𝐻𝑁𝑂3 →3 𝐴𝑔 + + 3 𝑁𝑂3 ― + 𝑁𝑂 (𝑔) + 2𝐻2𝑂 (𝑙)

(3)

The Ag+ in solution is reduced at elevated temperature to form Ag seeds. However, with both H+ and NO3- available, HNO3 forms in-situ and etches the initially formed Ag seeds. The combination of reactions (2) and (3) forms a dynamic and reversible redox for Ag/Ag0. The initial etching of Ag seeds is consistent with previous studies on the role of HCl in promoting monodisperse cubes.30 27


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(iii) The dissolution of AgCl along with Ag+ reduction, as shown in reactions (4) and (5). 𝐴𝑔𝐶𝑙 (𝑠)↔𝐴𝑔 + + 𝐶𝑙 ―

(4)

𝐴𝑔𝐶𝑙 (𝑠)𝑟𝑒𝑑𝑢𝑐𝑖𝑛𝑔 𝑎𝑔𝑒𝑛𝑡𝐴𝑔(𝑠) + 𝐶𝑙 ―

(5)

After the consumption of H+, reaction (3) cannot proceed whereas reaction (2) continues in the presence of excess Ag+. At the same time, the large AgCl nanocubes dissolve, releasing Ag+ and Cl- ions (reaction (4)). The released Ag+ participates in reaction (2) whereas Cl- adsorbs on the surface of newly formed Ag seeds to stabilize the Ag(100) facet. Although the dissociation constant for AgCl is low (Ksp = 1 × 10-9 in EG at 140C), the reaction of Ag+ and Cl- drives the equilibrium of AgCl dissociation to the right. The net reaction is shown in (5). AgCl is a source for the slow release of Cl- which selectively adsorbs on Ag(100) facets to control the shape. This is consistent with a more recent study by Zhou et al. where they hypothesized the role of Cl- as specific capping agent toward Ag(100).31 The strong binding of Cl to Ag(100) compared to Ag(111) (see the Supporting Information) reverses the surface energy of Ag facets, so that γ(100) < γ(111) for sufficiently large Cl coverages and leads to cubic Ag nanocrystals. (iv) The continued growth of Ag nanocubes along with the disappearance of AgCl. With the initially formed Ag seeds stabilized by Cl-, small Ag seeds continue to grow due to the reduction of excess Ag+ along with the dissolution of AgCl to release Ag+ and Cl-. This is consistent with our ex-situ observations that small Ag nanocubes grow larger while AgCl disappears (Figure 4C-F). Eventually, growth of the Ag nanocubes ceases after 28 h due to the complete consumption of Ag+ in solution. Cl- evolves from the initial AgCl nanocubes to Ag nanocubes with a half monolayer of adsorbed Cl-. This is consistent with the literature results on the phase transition from AgCl to Ag as observed from in-situ XRD32 and the detection of Cl on 28


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Ag nanocubes through HRTEM.69 AgCl serves as a Cl- source and the growth of Ag nanocubes comes from the reduction of excess Ag+ in solution. Overall, our proposed 4-stage scheme unifies various literature studies on the role of Cl- in Ag nanocubes formation and further demonstrates the complex evolution of Cl- throughout the 28 h growth process.

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Scheme 1. Schematic of 28 h growth process of Ag nanocubes with four identified stages: (i) initial formation of AgCl nanocubes; (ii) reduction of AgNO3 to Ag seeds and their subsequently etching by in-situ formed HNO3; (iii) dissolution of AgCl along with AgNO3 reduction and (iv) the continued growth of Ag nanocubes along with AgCl disappearance.

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CONCLUSION We revisited the polyol synthesis of Ag nanocubes in the presence of HCl to conduct a detailed mechanistic study of the influence of H+ and Cl- concentration independently on the shape of Ag NPs, along with ab initio thermodynamics calculations to posit the thermodynamically-preferred Wulff shapes. Together along with ex-situ experiments, we demonstrated the formation of the Ag nanocube product is controlled by the slow release of Cl- during synthesis that adsorbs and ultimately stabilizes (100) facets. H+ forms in-situ HNO3 to control the kinetics of AgNO3 reduction whereas Cl- adsorbs more strongly on Ag(100) than Ag(111) to control the shape of Ag NPs. Increase in the concentration of Cl- leads to shape control of Ag NPs from truncated octahedra, to cuboctahedra, truncated cubes and cubes, consistent with ab initio thermodynamics calculations. Upon adsorption of Cl- on Ag facets, the surface energies of bare Ag facets are reversed, so that γ(100) < γ(111) and cubes are preferred thermodynamically. The evolution and speciation of Cl- during the growth is complex, initially reacting quantitatively with Ag+ to form large AgCl cubes, which subsequently decompose to generate a Cl- chemical potential favoring the formation of reduced Ag cubes with a 0.5 ML of Cl on their surface. Synthesis of Ag cubes was independent of the solid source containing chloride as long as the release of chloride into solution provided a Cl- chemical potential commensurate with 0.5 ML coverage of Cl-. Our work revisited the polyol synthesis of Ag nanocubes and unified previous literature studies on the role of Cl- in Ag nanocubes synthesis. The complex role of ions during colloidal nanoparticle synthesis demonstrated in this work is most likely relevant in other systems where shape control is achieved due to the presence of halides during solution-phase synthesis of nanomaterials.

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METHODS Chemicals and Materials. PVP (Mw = 55,000 g/mol), silver nitrate (99.9%), silver chloride powder, sodium chloride, hydrochloric acid, nitric acid and ammonium hydroxide solution (28% NH3 in H2O) were all purchased from Sigma Aldrich and used without further purification. Ethylene glycol (lot number 9300-03) was purchased from J.T. Baker. Deionized (DI) water with a resistivity of 18.2 MΩ·cm was obtained from a Milli-Q purification system. Pre-cleaned glass vials (catalog number 89093-836) were purchased from VWR. Synthesis of Ag Nanocubes (HCl method). The silver nanocube synthesis recipe was modified based on Xia’s paper.30 In a typical experiment, 5 mL EG was placed in a 20 mL glass vial and loosely capped. The EG solution was preheated to 140°C in an oil bath at a stirring rate of 360 rpm. After 1 h, 1 mL of 3 mM HCl in EG solution was injected. Ten minutes after the addition of 3 mM HCl, 3 mL of 94 mM AgNO3 in EG solution and 3 mL of 147 mM PVP concentration (based on monomer) in EG were simultaneously injected from two separate plastic syringes at 45 mL/h using a syringe pump. The vial was kept loosely capped for 20 h and then tightened. After 8 h the vial was removed from the oil bath. In control experiments, 1 mL 3 mM HCl in EG was replaced by (i) 1 mL 3 mM HNO3 in EG, (ii) 1 mL 3 mM NaCl in EG, (iii) 1 mL 3 mM HNO3 and 3 mM NaCl in EG, (iv) 1 mL 3 mM HNO3 and 0.3 mM NaCl in EG, (v) 1 mL 3 mM HNO3 and 0.03 mM NaCl in EG, (vi) 1 mL 3 mM HNO3 with pre-synthesized AgCl nanocubes in EG, (vii) 1 mL 3 mM HNO3 in EG with commercial AgCl powder. HNO3 should be added to EG with care since mixing concentrated HNO3 with EG can be explosive. Instrumentation. Scanning electron microscopy (SEM) images were taken using a Zeiss Sigma variable pressure (VP) field emission SEM at a voltage of 3 kV and operated with a secondary 32


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electron (SE2) detector or an In-lens Detector. Synthesized samples were washed twice with ethanol to remove EG and PVP and then redispersed in ethanol. 5 µL of the sample was drop cast on a silicon wafer for SEM imaging. Edge lengths for >100 NPs were measured to quantify the size distribution during the various syntheses. SEM energy dispersive X-ray spectroscopy (EDS) images were taken using an Oxford detector with Aztec software. Transmission electron microscopy (TEM) images were taken using a FEI Talos equipped with HAADF (high angle annular dark field) and superX EDS (energy dispersive X-ray spectroscopy) at an accelerating voltage of 200 kV. Ag products were dispersed in ethanol and drop cast on a TEM grid for imaging. X-ray powder diffraction (XRD) pattern was collected on a PANalytical Empyrean diffractometer with Cu Kα radiation at 45 kV and 40 mA. Fixed slit para-focusing geometry was utilized with 0.04 radians soller slits, 10 mm beam mask, a 0.25° divergence and a 0.5° anti-scatter slit on the incidence side. Divergent optics included a 0.25° anti-scatter slit, 0.04 radians soller slits, and a nickel filter. A PIXcel detector in 1D scanning mode with PSD length of 3.35° was used. The samples were washed five times with ethanol and dried on a zero background silicon support before measurements. The concentration of Cl- was measured by a Dionex ICS2500 ion chromatography (IC) system equipped with an IonPac AS18 4 × 250 mm anion exchange column and an IonPac AG18 4 × 50 mm guard column. KOH (39 mM) was used as the eluent and the run time was set to 15 min per sample. The solubility constant (Ksp) of AgCl powder in EG at 140C was measured by using an Agilent 700 series inductively coupled plasma optical emission spectroscopy (ICP-OES). 25 mg AgCl powder was added into 10 mL EG and heated at 140C to release Ag+ and Cl-. After three 33


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weeks, 5 mL of the clear solution was sampled and added with 100 mL ammonium hydroxide solution for Ag+ concentration measurement from ICP-OES. The solubility constant of AgCl was calculated based on the measured Ag+ concentration. X-ray photoelectron spectra of Cl were collected on a Kratos Analytical Axis Ultra DLD spectrometer, equipped with a monochromatic Al-Kα excitation source (hν = 1486.6 eV), a delay-line detector system and a hemispherical sector analyzer. UV-vis spectra were taken using a Shimazu UV-3600 spectrophotometer. 100 µL of Ag NP solutions removed at different time were diluted in 3 mL ethanol and their absorbance spectrum (350 – 800 nm) was measured. Theoretical calculations. For the density functional theory (DFT) calculations, we used the Vienna Ab Initio Simulation Package (VASP)86–88 employing the Perdew-Burke-Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA)89 and projector augmented wave (PAW) pseudopotentials.90 Our periodic supercell consists of chlorine atoms adsorbed on one side of an Ag slab with 6 atomic layers and a vacuum spacing equivalent to 22 Ag layers. As discussed in the Supporting Information, the lateral dimensions of our supercell are variable and depend on the Cl coverage studied. The bottom three Ag layers are held fixed in their bulk positions, while all other atomic degrees of freedom are relaxed until the forces are less than 0.01 eV/Å using the conjugate gradient algorithm.91 A dipole correction is applied to cancel out the fictitious dipole interaction between periodic images along the surface normal direction.92,93 The plane-wave cutoff energy is set to 400 eV. Additional details on the DFT calculations are provided in the Supporting Information. We include solvation effects using an implicit solvent model from the VASPsol implementation,94,95 an additional module to VASP. The implicit solvation model describes the electrostatic, cavitation, and dispersion interactions between a solute and solvent. Since the 34


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electrostatic interaction is dependent on the solvent relative permittivity, we set the relative permittivity for our calculations to be 22.6 (dimensionless), the relative permittivity of ethylene glycol (EG) at 413 K.96 The equilibrium shape of a crystal can be determined by the Wulff construction.97,98 To predict Wulff shapes, we calculated the surface energy, 𝛾 as a function of the chlorine chemical potential for a wide range of chlorine surface coverages. As we elaborate in the Supporting Information, 𝛾 is a linear function of the solution-phase chlorine chemical potential 𝜇𝐶𝑙. Surface energies are computed for both Ag(100) and Ag(111) because they represent the crystallographic faces present in the experimentally observed shapes (octahedron, cuboctahedron and cube). We considered 20 different chlorine surface configurations/coverages for Ag(100) and 23 for Ag(111). Details of the surface-energy calculations and chlorine surface coverages are presented in the Supporting Information. At each value of 𝜇𝐶𝑙, the surface coverage with the minimum surface energy represents the equilibrium Cl surface coverage. These equilibrium surface energies are used in the Wulff construction to predict the thermodynamic shapes of the Ag nanocrystals as a function of 𝜇𝐶𝑙.

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ASSOCIATED CONTENT Supporting Information. Edge length distribution of AgCl and Ag nanocubes, SEM and EDS data for the Ag products at 5 min and 23 h, AgCl morphology changes under electron beam, XRD of the products synthesized at 5 min, IC quantification of Cl- amount in AgCl precipitates, UV-vis spectrum of the Ag products at different time, SEM EDS quantification for the Ag products at 5 min and 23 h, HRTEM image of the Ag nanocubes at 28 h, Calculations of the Cl to surface Ag ratio, XPS of the sample synthesized with no Cl-, 3 mM Cl- and 30 mM Cl-, SEM image of the Ag nanocubes after aging at 140°C for 5 days, HRTEM and FFT pattern of the Ag products synthesized with 1 mL 3 mM HNO3, Comparison of the Ag products synthesized with HNO3 with geometry model, Image and XRD of the products synthesized at 3 mM HNO3 and 30 mM NaCl, SEM images of the presynthesized AgCl nanocubes, SEM image of the commercial AgCl powder and SEM images of the Ag products synthesized in dark and without stirring, details of density functional theory calculations, surface coverage explored in theoretical calculations, binding energy calculations and surface energy calculations.

This material is available free of charge via the internet at

http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author R. M. Rioux ([email protected]) K. Fichthorn ([email protected]) ACKNOWLEDGMENTS

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This work is funded by the Department of Energy, Office of Basic Energy Sciences, Grant DEFG02-12ER16364.

T. B. acknowledges training provided by the Computational Materials

Education and Training (CoMET) NSF Research Traineeship (grant number DGE-1449785).

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