Anisotropic and Multicomponent Nanostructures by Controlled

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Anisotropic and Multi-Component Nanostructures by Controlled Symmetry Breaking of Metal Halide Intermediates Alexander E. Kossak, Benjamin O. Stephens, Yuan Tian, Pan Liu, Mingwei Chen, and Thomas J Kempa Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05090 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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Anisotropic and Multi-Component Nanostructures by Controlled Symmetry Breaking of Metal Halide Intermediates Alexander E. Kossak,

†,^

Benjamin O. Stephens,

†,^

Yuan Tian,

Thomas J. Kempa*

†

‡,¬

◊

Pan Liu, Mingwei Chen,

‡,¬

,†,‡

Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States

‡

Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, United States

¬

Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

◊

State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, PR China

KEYWORDS: anisotropic, multi-component, nanostructures, symmetry-breaking, synthesis

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ABSTRACT: We propose and validate herein a solution-phase synthetic strategy relying on in situ photo-stimulation and reduction of metal-halide intermediates to yield complex anisotropic and multi-component nanostructures. Exposure of AgBr nanoparticles to ultraviolet light and LArginine forms dimers composed of crystalline Ag and AgBr nanophases. The Ag nanoparticle nucleates at and grows from a single point on the surface of the AgBr phase, and the interface connecting these phases is atomically sharp. The complex nanostructures are generated at greater than 80% yield and are highly monodisperse in morphology and in size. The high crystallinity of the nanophases arises from an apparent solid–solid crystallization process and is unusual considering the nearly 40% lattice mismatch between Ag and AgBr. Such structures may be used to interrogate photocatalytic mechanisms or to construct more complex materials.

TEXT: Materials structured at the nanoscopic, mesoscopic, and microscopic levels have played an important role in defining scientific and technological advances within the areas of lowdimensional physics, energy conversion and storage, photonics, electronics, biosensing and, most 1–4

recently, meta-materials.

Materials which are anisotropic or contain multiple component

phases are of major interest because they can: (1) encode multi-functional properties, (2) contain prescribed interfaces at regions of broken structural symmetry, and (3) function as building blocks for complex materials. Although a number of pioneering methods

5–16

have yielded

several anisotropic and/or multi-component geometries (e.g. wires, rods, tetrapods), synthetic challenges still limit control of material morphology and composition at the nanoscale.


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Here we propose and validate a new strategy for solution-phase synthesis of anisotropic and multi-component nanostructures that involves controlled structural symmetry breaking of metalhalide nanoparticle intermediates. A canonical solution synthesis of nanostructures involves several, and often concerted, steps: (1) precursors are reduced and/or associate to form a small 17

cluster, and (2) Ostwald ripening,

driven by further reduction and/or deposition of atomic

species onto the nascent cluster, yields a nanostructure whose size and morphology can be 6,13

modulated by a ligand (Figure 1a). Though rod/wire-like structures

7,11,12,14–16

and other complex

are attainable, the majority of products from such reactions are isotropic

and single-component in nature. In these reactions, the simultaneous occurrence of multiple interacting processes (e.g. nucleation, precursor reduction, precursor–cluster adsorption) complicates spatiotemporal control over nucleation.

Figure 1. (a) Scheme showing canonical approach for solution phase nanostructure synthesis, which predominantly yields isotropic nanoparticles. (b) Scheme showing our proposed strategy, which can yield anisotropic and multi-component nanostructures. To improve control over nanostructure morphology, we conceived a synthetic approach, which seeks to define nucleation events more selectively in space and time (Figure 1b). Our process 18–25

involves 2 key steps: (1) preparation of ligand-stabilized metal-halide nanoparticles,

which 3

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encapsulate precursors of a secondary nanophase to be appended to the metal-halide, and (2) site-specific nucleation and growth of the secondary nanophase from the metal-halide when it is irradiated in the presence of a reductant (Figure 1b). Our approach intends to accomplish two things. First, it intends to spatially restrict and regulate the flux of precursors, which undergo reduction during Step 2 to form the secondary phase, by encapsulating them within the lattice of the metal-halide. We note that Ag-halides have relatively large lattice constants (5.5–5.8 Å) and 26

are known to accommodate metal cations at interstitial positions within their lattices.

Second,

our approach intends to temporally restrict nucleation by using a photo-induced reduction strategy to trigger secondary phase growth at a specific point in time.

Figure 2. (a) Powder XRD data of the reaction product and of a neat sodium oleate powder. Published Bragg peaks for Ag and AgBr are shown in lower panel.

27

(b) UV-vis spectra for

AgBr particles before and after their exposure to UV light in the presence of L-Arginine. Inset: Photograph of the samples. 4 ACS Paragon Plus Environment

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To validate our proposed strategy, we set out to prepare AgBr nanoparticles, which could serve as the metal-halide nanoparticle intermediates. Reaction of aqueous precursor and ligand solutions under ambient conditions for 24 hr yielded a light gray solution. A dark gray powder was isolated from this solution by centrifugation at 13,000 × g. X-ray diffraction (XRD) data collected on this powder reveal well-defined Bragg peaks, which correspond to those reported

27

for crystalline AgBr (Figure 2a). Furthermore, while some low-angle diffraction peaks matching those of sodium oleate are present in the sample, there is no evidence for a crystalline Ag bulk phase. Fourier-transform infrared (FT-IR) data collected on the sample reveal vibrational bands at 1516 cm

–1

–1

and 1406 cm , which can be assigned to the asymmetric and symmetric stretching –

28

modes, respectively, of the COO group of the putative oleate ligand

(Supporting Information,

S1). We note that the wavenumber difference between these asymmetric and symmetric modes is 29

not consistent with bidentate coordination of carboxylate to a metal atom,

though we are

investigating further the detailed nature of ligand binding to the metal-halide intermediate. Together, the above data attest to preparation of phase-pure AgBr particles whose surfaces appear to be bound by oleate ligand. Next, we demonstrated the photo-induced conversion of AgBr particles into more complex structures. Addition of a 1 M solution of L-Arginine to a solution containing AgBr particles followed by irradiation with 254 nm ultraviolet (UV) light generates a light purple colloidal solution whose UV-visible absorption spectrum features a peak centered at 370 nm (Figure 2b). This absorption feature is not present in a solution containing phase-pure AgBr particles before irradiation. Furthermore, the peak wavelength of this feature falls within the characteristic 360– 410 nm surface plasmon resonance band for Ag nanoparticles.

14,15

Taken together, these data 5

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attest to the formation of a distinct structural phase after exposure of AgBr particles to UV light and L-Arginine. We characterized the dimensions, composition, and crystal structure of the products resulting from exposure of AgBr to UV light and L-Arginine using conventional transmission electron microscopy (TEM), aberration-corrected scanning transmission electron microscopy (CsSTEM), and energy dispersive X-ray spectroscopy (EDS). Simultaneous acquisition of a highangle annular dark-field (HAADF) Cs-STEM image and EDS elemental composition maps of one of the products (Figure 3a) identifies that the structure in question is a dimer composed of two distinct and fused nanoscale phases: the lower contrast phase in the HAADF image is composed of both Ag and Br and the higher contrast phase is composed of Ag.

Figure 3. (a) Clockwise: HAADF-STEM image, EDS composite map of Ag and Br signal, EDS map of Br, and EDS map of Ag for a single Ag|AgBr dimer. (b) Bright-field TEM image illustrating several dimers. (c) Cs HAADF-STEM image of a single dimer. (d) Cs HAADFSTEM image of the Ag-AgBr interface.


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These data are consistent with our UV-vis absorption data (Figure 2b), which provided evidence for the formation in situ of a Ag phase, but further show that the two phases are in fact fused into one dimeric nanostructure. We refer to this dimer as Ag|AgBr. To assess the crystallographic nature of the Ag|AgBr dimers, bright-field TEM images, of which a representative is shown in Figure 3b, encompassing 95 dimers were collected. A statistical analysis of these dimers revealed that the average diameter of the Ag and AgBr phases is 15.8 ± 3.7 nm and 19.2 ± 3.9 nm, respectively. High-resolution HAADF Cs-STEM imaging (Figure 3c,d) of a single dimer reveals several features. First, the Ag and AgBr phases have measured lattice spacings of 2.33 Å and 3.26 Å, respectively. These distances are within 2.1% of the [111] interplanar d-spacings of bulk crystalline Ag (2.38 Å) and AgBr (3.33 Å).

27

Second,

the two phases are fused at a sharp and uniform interface formed by the [111] plane. Taken together, these data show that our solution-phase synthetic protocol renders dimers, which are composed of mutually crystalline Ag and AgBr phases. This is a rather unusual result given (1) +

the presumed solid–solid crystallization process involving reduction of interstitial Ag cations to 0

Ag at the AgBr surface, and (2) the large 40% lattice mismatch between Ag and AgBr.


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Figure 4. (a) Numerical yield of 4 distinct observed nanostructure morphologies as a function of pH. (b) Representative TEM images (electron dose: ~7 electrons s

–1

–2

Å ) of individual

nanostructures taken immediately following exposure of AgBr nanoparticles to UV light in the presence of L-Arginine for the length of time indicated in white text. (c) UV-vis absorption spectra of AgBr nanoparticle samples exposed to UV light in the presence of L-Arginine for the length of time indicated in the legend.


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We next established the role of pH in dictating nanostructure yield and morphology. Nine Ag|AgBr dimer syntheses were performed between pH = 8.4 and pH = 10.0, inclusive, in 0.2 pH increments. Between 104 and 132 structures at each pH condition were assayed via bright-field TEM images. The numerical yield of each distinct observed structure (dimer, multimer, Ag nanoparticle, and other morphology) showed a significant dependence on pH (Figure 4a). The dimer yield exhibits a maximum of 84% at pH 9.2. Measurements of the diameters of the Ag and AgBr phases (Supporting Information, S2) within the dimers over the 8.4–10.0 pH range were also performed. From pH 9.0 to 9.4, the average diameters of the Ag and AgBr phases differ by less than 12% and 8%, respectively (Supporting Information, S3). Furthermore, from pH 9.0 to 9.4, the dispersion (expressed as standard deviation) in the average diameters of the Ag and AgBr phases exhibits the following range of values: σAg = ± 3.1–3.7 nm and σAgBr = ± 3.6–5.2 nm, respectively. At pH 9.4 σAg = ± 3.4 nm and σAgBr = ± 3.6. For pH values less than 9.0 and greater than 9.4, the spread in average diameters, especially of the Ag phase, and the spread in dispersions is significantly larger. Together these data show that the yield and size consistency of the dimers can be maximized through synthesis at pH values between 9.0 and 9.4. Finally, we examined how the Ag nanoparticle phase nucleates from and grows relative to the AgBr phase. To do so, we studied the time evolution of dimer growth under UV exposure with L-Arginine, and also under exposure to an electron beam. We obtained TEM images of nanostructures resulting from the exposure of solutions containing AgBr particles and LArginine to UV light between 1–10 s (Figure 4b). In tandem, UV-vis spectra of these solutions immediately following their exposure to UV light were collected (Figure 4c). All TEM images were collected at low electron fluxes of ~7 electrons s

–1

–2

Å

in order to minimize electron beam


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induced reduction. Within the 2 nm spatial resolution of our measurement, we observed 2 distinct stages of dimer growth. The first stage occurs between 1 s and 4 s. During this stage, the AgBr phase adopts a cubic shape with a small protrusion attributable to the nascent Ag phase, which grows from one of the cube vertices. AgBr nanoparticles have a spherical shape prior to UV irradiation (Figure 3b and Supporting Information, S4a). We attribute their shape change +

under UV light to reduction of interstitial Ag cations near the AgBr surface by photogenerated 24,25,30,31

electrons.

This effect is responsible for the formation of a “latent image” within

traditional photographic emulsions.25 The second stage occurs for exposures greater than 4 s. Within 6–7 s a dimer composed of nearly equal and spherical components of Ag and AgBr is formed, and this species is consistent with the dimers reported elsewhere in this paper. For UV exposure longer than ~6 s, the Ag phase continues to grow until a nearly 20 nm protrusion is visible at 10 s. UV-vis absorption spectra (Figure 4c) reveal a steady increase as a function of time in the amplitude of the 370 nm peak, which was previously assigned (Figure 2b) to the surface plasmon resonance response of the growing Ag phase. We also examined dimer growth under electron beam irradiation in order to compare the dimer growth kinetics to those observed under UV exposure with L-Arginine, and also to gain further insight into the apparent solid-solid crystallization process occurring in our metal-halide system. Under electron beam exposure for 18 s we again observe single-site nucleation of the Ag phase (Supporting Information, S4a). We note that others have observed a similar effect of electron beam irradiation on metal-halide and other nanomaterials.

32

However, instead of a 2-step

process, we observe that expansion of the cross-sectional area of the Ag phase proceeds in concert with contraction of the AgBr phase (Supporting Information, S4b), and that there is a


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saturation in the rate of growth of the Ag phase after ~10 s of electron beam irradiation (dose: ~7 electrons s

–1

–2

Å ). We also note that there is no apparent damage to the AgBr phase after several

minutes of exposure to the electron beam and that the Ag phase grows away from the surface of the AgBr phase. In light of the data obtained under UV and electron beam irradiation of AgBr, we propose that +

growth of the Ag phase is initiated and maintained by efflux of interstitial Ag cations from the 24,25,30,31

bulk AgBr lattice

to the AgBr surface, followed by their subsequent reduction at the

Ag|AgBr interface. The reducing equivalents are either photogenerated electrons excited by UV light or electrons delivered by the TEM beam. The silver cation flux to the surface is likely +

driven by continued depletion of interstitial Ag cations at the surface as they become reduced to 0

30

Ag surface atoms.

0

Continued deposition of Ag at the Ag|AgBr interface, the stationary

growth front, pushes the Ag phase outward to form the dimer. Future analysis will determine the detailed interaction of oleate and L-Arginine with the metal-halide intermediate and the role these interactions play in regulating single-site symmetry breaking and the high yield of our dimers. Our work demonstrates the solution-phase synthesis of high-quality, nano-structured, and multi-component dimers from photo-triggered structural symmetry breaking involving metalhalide nanoparticles. The growth of Ag phases from metal-halides has been the subject of previous work.

18–25

25

For example, “latent image” formation +

emulsions stems from reduction of interstitial Ag

in traditional photographic 0

cations to small Ag

clusters by

photoelectrons generated under UV illumination of metal halides. More recently, researchers


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have examined the catalytic generation of H2 and catalytic photodegradation of organic pollutants by Ag clusters generated randomly on the surfaces of metal-halides or by the metal18–21

halides themselves.

Our work differs from these efforts, because it focuses on controlled

synthesis of complex nanoscale heterostructures from metal-halide particles. Significantly, our synthetic approach produces well-defined dimers owing to controlled single-site symmetry breaking. Furthermore, our nanostructured dimers are formed at high yields and exhibit low size dispersities. Structures analogous to the dimers we have prepared are promising candidates for study of photocatalytic mechanisms due to the ability of defining prescribed and high-quality interfaces between optically and catalytically active components. Efforts are also underway to use multi-site structural symmetry breaking to create nanoscale building blocks of hitherto unrealized structural complexity.

ASSOCIATED CONTENT: Supporting Information. Contents: Methods and Supporting Figures S1–S4. This material is available free of charge at http://pubs.acs.org. AUTHOR INFORMATION: Corresponding Author * Thomas J. Kempa ([email protected]). ORCID: 0000-0002-1672-8325 Author Contributions


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^ These authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors declare no competing financial interests. ACKNOWLEDGMENT: T. J. K. thanks The Johns Hopkins University (JHU) for financial support. The authors thank Tyrel M. McQueen at JHU for the use of powder X-ray diffraction facilities.

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