Influence of Shape on the Surface Plasmon Resonance of Tungsten

Publication Date (Web): February 17, 2014. Copyright © 2014 American Chemical Society. *E-mail: [email protected]. Cite this:Chem. Mater. 26, 5...
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Influence of Shape on the Surface Plasmon Resonance of Tungsten Bronze Nanocrystals Tracy M. Mattox,† Amy Bergerud,†,‡ Ankit Agrawal,§ and Delia J. Milliron*,†,§ †

The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States § Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ‡

S Supporting Information *

ABSTRACT: Localized surface plasmon resonance phenomena have recently been investigated in unconventional plasmonic materials such as metal oxide and chalcogenide semiconductors doped with high concentrations of free carriers. We synthesize colloidal nanocrystals of CsxWO3, a tungsten bronze in which electronic charge carriers are introduced by interstitial doping. By using varying ratios of oleylamine to oleic acid, we synthesize three distinct shapes of these nanocrystalshexagonal prisms, truncated cubes, and pseudosphereswhich exhibit strongly shape-dependent absorption features in the near-infrared region. We rationalize these differences by noting that lower symmetry shapes correlate with sharper plasmon resonance features and more distinct resonance peaks. The plasmon peak positions also shift systematically with size and with the dielectric constant of the surrounding media, reminiscent of typical properties of plasmonic metal nanoparticles.

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INTRODUCTION Controlling the size and geometry of metal nanocrystal surfaces makes it possible to tailor these particles to achieve diverse surface plasmon resonance (SPR) properties, allowing select nanoparticles to be tuned for use in sensors,1,2 electronic circuits,3 and photonic devices.4 The influence of particle geometry and size on optical properties is documented for many different metal nanoparticle shapes, including cubes,5 octahedra,6 rods,7 and stars.8 Semiconductor nanocrystals can offer similar synthetic control over size and geometry. The shape of such nanocrystals dramatically influences their near-infrared (NIR) optical spectra. Changing the shape of a metallic particle alters the surface polarization, which influences how the surface interacts with light and thus the SPR. Particle size also has an effect on the optical properties of these materials, which is often described using Mie’s solution to Maxwell’s equation9 or the discrete dipole approximation.10 Though exact solutions only exist for spheres, spheroids, and infinite cylinders, changing the nanoparticle size of a wide variety of shapes is known to influence the SPR. For example, increasing the aspect ratio in nanorods causes the plasmon peak to red-shift in samples of gold11 as well as tungsten oxide.12,13 Increasing the size of silver triangles and plates also results in a red-shift of the SPR.14,15 Until recently, semiconductor nanocrystals had low free carrier concentrations and did not exhibit SPR phenomena. In metal oxide nanocrystals, substitutional doping with heterovalent metal ions has recently been used to introduce large free electron populations that support SPR in the infrared region. © 2014 American Chemical Society

For example, this phenomenon has recently been demonstrated in tin-doped indium oxide,16 aluminum-doped zinc oxide,17 indium-doped cadmium oxide,18 and niobium-doped anatase titania.19 In copper chalcogenide nanocrystals, large populations of free holes have been generated by inducing copper deficiencies,20 which yields similar infrared plasmon absorption features. However, these defect-doping strategies are limited with respect to the achievable carrier concentrations and therefore the spectral tuning range of the SPR. Tungsten oxide of the formula WO3−x (x < 0.33) is a reduced form of tungsten trioxide (WO3) that is widely understood to contain oxygen vacancies within the crystal lattice. These oxygen vacancies create changes in the charge states of tungsten,21 which influence absorption properties22 and can give rise to infrared SPR phenomena. However, the crystal structure of WO3 offers additional opportunity for introducing free electrons beyond the concentrations typically achievable by defect mechanisms. Unlike substitutional doping, in which a dopant atom replaces an atom within the crystal lattice, interstitial doping allows a new atom to be incorporated within the crystal structure as a result of atomic vacancies or defects within the lattice. As a result, it is possible for the nanocrystal to have a higher concentration of free electrons. The crystal structures of WO3 and WO3−x are composed of WO6 octahedra as the basic structural unit, creating oneReceived: September 13, 2013 Revised: February 4, 2014 Published: February 17, 2014 1779

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dimensional tunnels of variable sizes and shapes.23 These tunnels are highly selective for cations with ionic radii of 1.2 and 1.7 Å,24 so they are ideal for the incorporation of cesium atoms with ionic radii of 1.69 Å. Such cesium-doped tungsten oxides are known to have exceptionally strong NIR absorption;25 the cesium acts as an interstitial dopant, introducing a far higher population of free electrons than is achievable using lattice-site defects including heterovalent metals or vacancies. A class of interstitially doped tungsten oxides known as tungsten bronze has the formula MxWO3, where M is Li+, Na+, Cs+, or Rb+. Tungsten bronze materials retain a high transmittance of visible light while exhibiting high absorption of NIR light,25 making them ideal candidates for spectrally selective optical devices, such as electrochromic smart windows.26 Various methods have been used to synthesize MxWO3, such as solid-state synthesis,27 thermal plasma synthesis,28 and chemical transport.29 To our knowledge, this work represents the first colloidal synthesis of tungsten bronze and explains how shape evolution from pseudospheres to truncated cubes to hexagonal prisms of the same CsxWO3 crystal structure influences the SPR. X-ray diffraction (XRD) confirms that the synthetic procedures described here translate to synthesize RbxWO3, and using cerium and sodium as dopants also influences the NIR optical spectra (see Figures S1 and S2 in the Supporting Information). Given the preferred incorporation of ions, the size of cesium, and the exceptional NIR absorption of tungsten oxide doped with cesium, this exemplary tungsten bronze is the primary focus of our investigation.

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Optical spectra were collected on an ASD LabSpec Pro, and highresolution transmission electron microscopy (TEM) images were collected on a JEOL 2100 operated at an accelerating voltage of 200 kV. XRD modeling was performed using diffraction data for CsxWO3 (P6322 space group) in CrystalMaker, and Le Bail refinement was performed using the Generalized Structure Analysis System (GSAS).30 X-ray photoelectron spectroscopy (XPS) measurements were performed using a Physical Electronics PHI 5400 equipped with an aluminum X-ray source. All XPS spectra were calibrated to the C1s peak at 284.8 eV in order to correct for possible charging,31 and highresolution spectra of the Cs 3d5/2 and W 4f peaks were analyzed using the CasaXPS software package. Elemental analysis using flame atomic absorption (FLAA) and inductively coupled plasma optical emission spectrometry (ICP-OES) was performed by Galbraith Laboratories, Incorporated.32 Unless otherwise stated, optical spectra were collected in tetrachloroethylene (TCE).

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RESULTS AND DISCUSSION To evaluate the shape of the resulting nanocrystals, we surveyed the samples by low-resolution transmission electron microscopy (TEM). Absent any cesium doping, nanorods of WO2.72 were produced (Figure 1a). It has been observed in CeO2 and

EXPERIMENTAL SECTION

Materials. Cesium chloride (Aldrich), rubidium chloride (Aldrich), sodium chloride (Sigma-Aldrich), tungsten(IV) chloride (Strem Chemical), oleylamine (Aldrich), oleic acid (Sigma-Aldrich), acetone (BDH), ethanol (Aldrich), tetrachloroethylene (Sigma-Aldrich), dimethylformamide (Aldrich), and toluene (Aldrich) were used as received. Syntheses were performed under nitrogen flow in a 50 mL three-neck flask equipped with an air-cooled condenser. Synthesis of CsxWO3. First, 0.20 mmol (66 mg) tungsten(IV) chloride (WCl4), 0.12 mmol (20 mg) cesium chloride (CsCl), 0.60 mmol (0.20 mL) oleylamine (OlAm), and 19.0 mmol (5.2 mL) oleic acid (OlAc) were stirred at 300 °C for 120 min and resulted in a blue− green solution. The reaction was cooled to room temperature, and 0.5 mL of toluene was added to the blue−green solution. A 1:1 ratio of acetone to reaction mixture precipitated the product, and the mixture was centrifuged at 3800 rpm for 10 min. The blue−green precipitate was redispersed in 0.5 mL of toluene and precipitated again with 1 mL of acetone. Centrifuging for an additional 10 min at 3800 rpm resulted in clean nanocrystals of CsxWO3. The reaction yields a qualitatively similar product over a range of CsCl (0.05−0.18 mmol), while varying the amounts of OlAm and OlAc results in three different nanocrystalline shapes (pseudospheres, 1.5 mmol OlAm and 3.1−7.9 mmol OlAc; truncated cubes, 0.50−1.0 mmol OlAm and 19.0 mmol OlAc; hexagonal prisms, 0.60−1.5 mmol OlAm and 19.0−31.7 mmol OlAc). Synthesis of WO2.72. First, 0.20 mmol (66 mg) WCl4, 19 mmol (1.2 mL) OlAm, and 1.5 mmol (3.5 mL) OlAc were stirred at 300 °C for 120 min. The product was precipitated with 12 mL of acetone and centrifuged for 15 min at 3800 rpm. The blue pellet was redispersed in 0.5 mL of toluene and precipitated once more with 2 mL of acetone. Centrifuging for 20 min at 3800 rpm resulted in a clean, dark blue product with nanorod morphology. Characterization. XRD patterns were collected on a Bruker D8Discover X-ray diffractometer equipped with a GADDS area detector and operated at 40 kV and 20 mA at the wavelength of Cu Kα 1.54 Å.

Figure 1. TEM images with inset illustrated shapes of (a) WO2.72 rods and CsxWO3 (b) hexagonal prisms, (c) truncated cubes, and (d) pseudospheres. All scale bars are 10 nm.

Re2O3 that changing the ratio of OlAc to OlAm provides shape control as a result of different binding capabilities of the ligands and passivation onto the surface of the nanocrystal.33 In this work, it is clear that increasing the amount of OlAc produces more faceted shapes. When cesium is incorporated, we achieve different shapes depending on the ratio of OlAc to OlAm. These include CsxWO3 hexagonal prisms (Figure 1b), cubes (Figure 1c), and pseudospheres (Figure 1d). Hexagonal prisms result when the OlAc:OlAm ratio is 12.5−31, truncated cubes when the ratio is 5.2−12.5, and pseudospheres when the ratio is 2.1−5.2. An OlAc:OlAm ratio of less than 2.1 or greater than 31.3 results in mixed crystalline phases by XRD (see Figure S3 in the Supporting Information). Particle sizes were determined by TEM to be 16.0 ± 5.8 nm for the WO2.72 rods (measuring the long dimension), 4.6 ± 0.79 nm for the CsxWO3 1780

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pseudospheres, 20.4 + 2.4 nm for truncated cubes, and 13.2 ± 3.0 nm for hexagonal prisms. The truncated cube particles included a variety of smaller sizes (under 12 nm) besides the primary product that were omitted from size measurements (see Figures S4 and S5 in the Supporting Information). The faceted shapes observed by TEM suggest crystalline particles, and they are confirmed to be single crystals by highresolution TEM (Figure 2). Shown below are lattice-resolved

Figure 2. High-resolution TEM images of (a) WO2.72 rods and CsxWO3 (b) hexagonal prism, (c) truncated cube, and (d) pseudospheres. All scale bars are 5 nm.

Figure 3. Crystal structures of the ball-and-stick model of (a) WO3 and polyhedral models of (b) WO3, (c) WO2.72, and (d) CsxWO3.

TEM images of WO2.72 rods (Figure 2a) with a [−2 4 1] zone axis, a hexagonal prism viewed perpendicular to its largest face (Figure 2b) with a [−1 0 1] zone axis, and a truncated cube (Figure 2c) and pseudospheres (Figure 2d) with a [1 0 0] zone axis. The XRD pattern of the undoped tungsten oxide is consistent with monoclinic WO2.72 (see Figure S6 in the Supporting Information), supporting the assignment of the composition as WO3−x, where x is 0.28. The WO3−x structure is a reduced form of cubic WO3 that contains oxygen vacancies within the crystal lattice. These oxygen vacancies create a distribution of W formal oxidation states between 6+, 5+, and 4+,34 and the alterations in charge state are responsible for changes in absorption properties compared to WO3.22 This is apparent in our case by the dark blue color of the nanorods, which results from the tail of the strong infrared absorption crossing into the red edge of the visible. To better understand how NIR plasmonic characteristics can emerge in interstitially doped tungsten bronze nanocrystals, it is useful to consider the crystallographic implications of doping tungsten oxide. WO3 may be described as a modification of the perovskite-type ABO3 lattice in which the B site is occupied by W atoms and the A site is unoccupied,35 as seen in the ball-andstick model in Figure 3a. The oxygen vacancies within the lattice result in the formation of WO6 octahedron as the basic structural unit, and these vacancies are responsible for creating tunnels within the crystal structure (Figure 3b). WO2.72 is a reduced form of WO3 in which additional oxygen vacancies

distort the WO6 octahedral units and the local environments of the tungsten ions become heterogeneous (Figure 3c). Doping with Cs+ ions instead results in occupation of the open channels in the WO3 structure to form CsxWO3 (Figure 3d). All three shapes of cesium tungsten bronze were determined by XRD to be hexagonal CsxWO3, consistent with a Cs0.29WO3 reference pattern (see Figure S7 in the Supporting Information). Though XRD confirms that Cs is incorporated within the crystal structure, further analysis was performed to determine how the Cs ions are distributed throughout the nanocrystals. Doping near the surface of the nanocrystals was characterized by XPS (see Figure S8). The surface composition was dominated by oxygen and carbon, with a metals composition of 18.2% Cs and 81.7% W determined from normalized peak area measurements, i.e., x = 0.15 in CsxWO3. By contrast, the average doping content analysis was determined by ICP-OES and FLAA to be x = 0.09. These results suggest that the Cs distribution is radially nonuniform with higher doping levels near the surface of the nanocrystals. To investigate whether the NIR absorbance of our CsxWO3 nanocrystals can be ascribed to SPR or to absorption by local defect states,36 we observed how the absorbance peak shifts in solvents of varying refractive index (RI). The SPR is known to be sensitive to the RI of the surrounding environment, where increasing the RI of the solvent has a linear effect on the wavelength of the plasmon peak.15 Optical spectra of the same hexagonal prism sample were collected in tetrachloroethylene 1781

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possibility for doped semiconductor nanocrystals to exhibit shape-dependent plasmonic properties has been investigated. Results for vacancy-doped copper chalcogenide nanocrystals are conflicting. Cu2−xS is known to have tunable plasmons,43 and changing the aspect ratio of semiconductor Cu2‑xS nanodiscs has a strong influence on the NIR absorption.44,45a By contrast, Kriegel et al. found that the SPR of Cu2−xTe nanospheres, rods, and tetrapods has only a weak shape dependence.46 Changing the aspect ratio of phosphorus-doped silicon nanowires was also reported to yield a shape-dependent SPR.47 In the case of substitutionally doped metal oxides, Gordon et al. have recently published clear evidence of shapedependent SPR in indium-doped cadmium oxide spheres and octahedra.48 Noguez discusses SPRs for a series of silver nanoparticle shapes, explaining how the optical peaks change when increasing the number of truncations of a cube.49 That is, truncating a cube results in shapes with different numbers of planes or faces; cube (6 faces), truncated cube (14 faces), icosohedron (20 faces), and sphere (infinite faces). As the number of faces on the particle increases, three important trends are apparent in the resonance spectra:49 (1) the main/ largest resonance will blue-shift, (2) peaks with smaller wavelength resonances move closer to the main resonance and can be hidden, and (3) the width of the main resonance peak increases. We analyze the trends in optical spectra of our CsxWO3 nanocrystals by analogous deconstruction of their idealized shapes into the number of faces. The nanocrystals described here exhibit the same three trends that Noguez identified for silver nanoparticles, highlighting the similarity of physics between metals and doped semiconductors when looking at plasmons. Figure 5a contains flat projections of a hexagonal

(TCE), dimethylformamide (DMF), and acetonitrile (MeCN), with respective RIs of 1.51, 1.43, and 1.35 (Figure 4a). Two

Figure 4. Absorbance spectra of hexagonal prism CsxWO3 in solvents of varying refractive indices: TCE, DMF, and MeCN including (a) both absorbance peaks, (b) magnification of the shorter wavelength peak, and (c) a plot of the shorter wavelength peak displaying solvent refractive indices versus plasmon position.

distinct peaks were observed for hexagonal prism-shaped nanocrystals, as discussed below. Unfortunately, the longer wavelength peak is too broad to draw any definitive conclusions as to the peak shift. (We expect that inhomogeneous broadening, e.g., due to distributions of shape and size, may contribute to the width of the spectral features.) However, there is an obvious red-shift in the shorter wavelength peak as RI changes. (Figure 4b) This is further illustrated in the plot of RI versus wavelength (Figure 4c), which shows an obvious and systematic shift of the peak position to longer wavelengths as the RI of the solvent increases. This result supports the assignment of this shorter wavelength peak as plasmon resonance absorption, although it remains to be definitively ascertained whether the longer wavelength peak is also plasmonic in nature. Mie’s solution to Maxwell’s equation predicts a stronger dependence of the peak wavelength on RI than we observed experimentally (see Figure S10). This difference can be ascribed to the nonspherical shape of the particles as well as the contribution of the ligands attached to the surface of the nanocrystals.37 The connection between SPR and particle shape has been well studied for metal nanoparticles. For example, a wide range of gold nanoparticle shapes have been studied, including stars,38 dumbbells,39 and triangular and hexagonal plates.40 The influence of shape on SPR has also been studied in silver octahedra,41 rods, triangles, and plates.14,42 Recently, the

Figure 5. (a) Illustration of faceted shapes of nanocrystals and their flat projections: hexagonal prism (top) and truncated cube (bottom). (b) Absorbance spectra from top to bottom of CsxWO3 hexagonal prisms (red), truncated cubes (blue), pseudospheres (green), and WO2.72 rods (gray). Spectra have been offset for clarity.

prism and truncated cube, with 8 and 10 faces, respectively (a sphere has an infinite number of faces so is not depicted). The absorbance spectra of all three shapes of CsxWO3 are displayed in Figure 5b. The hexagonal prism has two distinct absorbance peaks, a less intense peak at 860 nm (peak 1) and a more intense peak at 1602 nm (peak 2). As the number of faces on the nanocrystal surface increases from 8 (hexagonal prism) to 10 (truncated cube) to infinity (sphere), the optical peaks reduce from 2 obvious, distinct peaks to 1. This is observed as the number of faces increase with a red-shift of peak 1 (respectively 860, 990, 1315 nm) and a blue-shift of peak 2 (respectively 1602, 1506, and 1315 nm). As the trend of increasing the faces of the particles progresses, peak 1 becomes 1782

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optical spectra. The high concentration of free electrons yields SPR peaks at far higher energy than those achievable by substitutional doping of metal oxides. Though the broadness of peak 2 makes its assignment as a plasmon inconclusive, the obvious shift of peak 1 in response to changes in solvent RI support its assignment as a plasmon. This is the first example in the literature explaining the expected connection between particle shape and resonance properties in interstitially doped semiconductor nanocrystals. Controlling nanocrystal geometry, size, and dielectric environment are necessary to systematically manipulate optical properties. The work presented here confirms that all three of these parameters influence SPR properties in tungsten bronze materials. The synthesis of hexagonal CsxWO3 as hexagonal prisms, truncated cubes, and pseudospheres demonstrates how increasing the number of faces on a nanocrystalline tungsten oxide surface causes the SPR peaks to shift toward one another, eventually becoming one peak.

less visible. In effect, peak 1 in the pseudosphere sample becomes indistinguishable from peak 2, so that both resonances occur under the envelope of one broader peak. These shape-dependent absorption spectra are compared to the spectra of WO2.72 nanorods in Figure 5b. The defect structure of oxygen-vacancy-doped tungsten oxide creates strong electron−phonon interactions which activate polarons that absorb visible and NIR light.50−52 Figure 5b shows the absorbance spectra of WO 2.72 with a broad peak at approximately 2100 nm. This spectrum is sharply contrasted with those that result from doping with cesium, which changes the crystal structure from monoclinic to hexagonal. Adachi has suggested that peak 2 in CsxWO3 is a shift of the polaron seen in WO2.72.53 However, given the change in crystal structure and increase in carrier density in CsxWO3, it is possible that both NIR absorption peaks in CsxWO3 are a result of the SPR of free electrons in the conduction band54 Indeed, the carrier concentration in our interstitially doped CsxWO3 nanocrystals appears to be substantially higher than what is observed so far in the literature on plasmonic semiconductor nanocrystals. Commonly studied plasmonic metals such as silver, gold, and copper have carrier densities in the range of 1022−1023cm−3.55 Doped plasmonic materials have substantially lower carrier densities, such as Cu1.85Se with a carrier density of 3.0 × 1021 cm−3 56 and indium tin oxide with a density of 1.9 × 1021 cm−3.37 The carrier density for our faceted CsxWO3 particles cannot be rigorously derived from their optical spectra without detailed electromagnetic modeling, but we approximated it by Drude theory to be as high as 5 × 1021 cm−3 (see Figure S9 in the Supporting Information). Figure 6a shows the absorbance spectra of a CsxWO3 pseudosphere sample, with aliquots of the same reaction taken at 5, 30, and 60 min. As the reaction progresses, the particle size increases from roughly 3 to 5 nm, and there is an obvious red-shift of the resonance peak from 1081 nm at 5 min to 1350 nm at 60 min. The same trend is seen for the CsxWO3 hexagonal prism in Figure 6b, where from 30 to 90 min the

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ASSOCIATED CONTENT

S Supporting Information *

X-ray diffraction data of RbxWO3, WO2.72, and mixed phases produced when changing OlAc:OlAm ratios beyond those stated in this publication; size histograms from high-resolution TEM of WO2.72 rods, and CsxWO3 hexagonal prisms, truncated cubes and pseudospheres; peak fit of pseudosphere SPR; theoretical prediction of SPR using Mie’s solution to Maxwell’s equation; absorbance spectra of WO2.72 doped with Rb, Na, and Ce. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare the following competing financial interest: D.J.M. has a financial interest in Heliotrope Technologies, a company pursuing commercial development of NIR electrochromic devices.

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ACKNOWLEDGMENTS The authors gratefully acknowledge helpful discussions with J. Urban, R. Buonsanti, and W. L. Queen. This work was performed at the Molecular Foundry, Lawrence Berkeley National Laboratory, and was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (DOE) under Contract No. DEAC02-05CH11231. D.J.M. was supported by a DOE Early Career Research Program grant under the same contract. A.B. was supported by a National Science Foundation Graduate Student Research Fellowship under Grant No. DGE 1106400, and additional support was provided by The University of Texas at Austin.

Figure 6. Absorbance spectra of progressing reactions for (a) spheres (black, 5 min; red, 30 min; blue, 60 min) and (b) hexagonal prisms (black, 30 min; red, 90 min).

peaks shift from 835 to 870 nm and 1840 to 1962 nm. These shifts in spectra follow the same qualitative trends reported in the literature for 5−40 nm gold rods (with fixed 2.4 aspect ratio)57 and 9−99 nm gold spheres.58

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CONCLUSIONS The high selectivity of the WO3 tunnels for species with a 1.7 Å size make them ideal for the incorporation of 1.69 Å Cs+ ions. Doping tungsten oxide with cesium drives the crystal structure to shift from monoclinic to hexagonal, and the free electrons introduced within the system create plasmon peaks in the 1783

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dx.doi.org/10.1021/cm4030638 | Chem. Mater. 2014, 26, 1779−1784