Article pubs.acs.org/cm
Low Temperature Synthesis and Surface Plasmon Resonance of Colloidal Lanthanum Hexaboride (LaB6) Nanocrystals Tracy M. Mattox,† Ankit Agrawal,‡ and Delia J. Milliron*,†,‡ †
The Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, United States McKetta Department of Chemical Engineering, The University of Texas at Austin, 200 East Dean Keeton Street, Austin, Texas 78712, United States
‡
S Supporting Information *
ABSTRACT: Lanthanum hexaboride (LaB6) nanocrystals, with an ∼1000 nm wavelength localized surface plasmon resonance ideal for interacting with solar near-infrared radiation, have been synthesized for the first time in a relatively low temperature flask reaction using sodium borohydride as both boron source and “solvent”. Furthermore, the incorporation of isophthalic acid as a ligand allows the nanocrystals to disperse, permitting direct incorporation into polymer matrices including poly(methyl methacrylate) and polystyrene, suitable for composites and coatings.
well-known to produce plasmonic effects tunable by changing vacancy concentration in addition to shape and size. Similarly, oxygen vacancies in tungsten oxide nanocrystals can give rise to localized surface plasmon resonance in a similar spectral region (down to 900 nm) to the LaB6 nanocrystals reported here.13 Recent examples of copper chalcogenides show absorbance in similar regions, such as copper telluride (900−1300 nm)14,15 and copper sulfide Cu2‑xS (900−1700 nm).16 Until now, chemical synthesis of LaB6 required extreme reaction conditions. Our method uses a modest temperature and atmospheric pressure yet yields uniform, phase-pure NCs. This raises an interesting question as to what chemical processes constrain crystal growth. The reaction pathway involved in this new synthesis is not known in detail, but the overall equation (below) was described by Yuan et al.,17 who used a high temperature (1200 °C) synthetic route:
Nanocrystals (NCs) exhibiting localized surface plasmon resonance (LSPR) properties offer new possibilities for manipulating light-matter interactions to achieve a wide array of optical effects. The ability to tune the LSPR through particle shape, size, and the surrounding environment makes these particles ideal for applications in sensors1,2and photonic devices.3,4 Though LSPR properties of noble metals have been well explored, alternative materials with a high extinction in the near-infrared (NIR) and high transmittance in the visible3,5,6 give new opportunities for photonics and smart window coatings.7 For example, coatings based on NCs with NIR LSPR could control solar heat gain in cars and buildings without compromising visibility. For effective interaction with solar NIR, either a window coating or solar conversion device should interact with photons within the most intense NIR band, ideally 750−1200 nm.8 This region of the spectrum is difficult to access in degenerately doped wide band gap metal oxide NCs, such as tin-doped indium oxide (ITO) or aluminum-doped zinc oxide (AZO), that have recently been investigated for their lower energy (λ > 1500 nm) LSPR properties.9−11 This limitation arises from the practical limits of electron concentration achievable by substitutional doping. The LSPR of LaB6 NCs lies within this important spectral region with an exceptionally high absorbance at ∼1000 nm.8 We report here the first synthesis of colloidally dispersed LaB6 NCs using a surprisingly low temperature and exhibiting a prominent absorption peak in the NIR ascribed to the LSPR. Stoichiometric LaB6 is plasmonic owing to a finite density of states at the Fermi level.12 However, the free carrier density is lower than in an elemental metal since there is effectively one free electron per formula unit, so the LSPR energy lies in the NIR. Cu vacancies in copper chalcogenides in particular are © XXXX American Chemical Society
LaCl3 + 6NaBH4 → LaB6 + 3NaCl + 12H 2 + 3Na
LaB6 is traditionally made at high temperatures (>1500 °C) or high pressures through the solid-phase reaction of lanthanum metal or lanthanum oxide with elemental boron.18−20 Such approaches offer limited opportunity to control crystallite size and shape and do not result in a solventdispersible material. Nanosized morphologies are instead made by grinding bulk LaB6 using milling methods,21,22 which provide limited control and introduce unavoidable contaminants. LaB6 NCs have alternatively been synthesized under high Received: June 17, 2015 Revised: September 2, 2015
A
DOI: 10.1021/acs.chemmater.5b02297 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials pressures in an autoclave23 or by chemical vapor deposition,24−26 but these methods yield poor shape and size uniformity. No previously reported methods have resulted in nanoscale particles suitable for direct integration into optical materials, devices, or coatings through solvent dispersion. We report here the first LaB6 synthesis not requiring extreme temperatures or elevated pressures. We show it is possible to tune the LSPR by controlling NC size, and that ligand incorporation makes NCs dispersible and ready for use in optical coatings. LaB6 NCs were prepared in a round-bottom flask by stirring only sodium borohydride (NaBH4) and lanthanum chloride (LaCl3) at 360 °C for 1 to 6 h under argon using a magnetic glass-coated stir bar. Once the reaction temperature was reached, the white powdered mixture became slurry-like in appearance. Within 20 min the mixture turned dark gray, becoming completely black with a slight blue tint after 1 h. The reaction progresses similarly with the addition of isophthalic acid, which we hypothesized could act as a surface-coordinating ligand. The NCs were collected and treated with hydrochloric acid to convert sodium metal to sodium chloride (NaCl) and then washed with water to remove NaCl. If these washing procedures were omitted, Na or NaCl could be found by powder X-ray diffraction (XRD), validating that the overall reaction indicated above is also applicable to our lower temperature reaction. The LaB6 NCs were characterized by transmission electron microscopy (TEM), XRD, and optical absorption spectroscopy. Their surface functionalization was investigated by attenuated reflection-Fourier transform infrared spectroscopy (HATR-FTIR). Analogous synthetic procedures were shown to yield CeB6 and CaB6 NCs (Figure S1), although our investigation focused primarily on LaB6 and its optical properties. These mild reaction conditions yield nanometer sized NCs of LaB6. The average NC size increases with reaction time, while the shape observed by low-resolution TEM remains isotropic (Figure 1). Size distribution broadens with reaction time, but progressive crystal growth is apparent in TEM as well as XRD (Table S1). Reaction times of 1, 3, and 6 h result in respective particle diameters of 2.10 ± 0.31 nm, 3.40 ± 0.50 nm, and 4.73 ± 1.20 nm (Figure S2). Larger LaB6 particles form at higher temperatures, but given the greatly increased size distribution we focus only on those made at 360 °C. Including isophthalic acid in the reaction limits the tunability of particle size (Figures S2 and S5), with 1 and 5 h reactions resulting in respective particle diameters of 2.51 ± 0.50 nm and 2.80 ± 0.43 nm. XRD patterns confirm the cubic phase of our LaB6 NCs (Figure S6). As reaction time increases the diffraction peaks narrow (Figure S7), which is consistent with increasing particle size (Equation S1, Table S2). In high-resolution TEM, the single crystalline nature of the individual particles is apparent. A variety of crystal orientations were observed and lattice spacings, derived from fast Fourier transforms of the images, were assigned to various low-index d-spacings of the common cubic phase of LaB6 (Table S3). Elemental analysis by energy dispersive X-ray spectroscopy confirmed the composition of 1:6 La:B over the entire range of reaction times investigated regardless of the inclusion of isophthalic acid in the reaction medium (Table S4). In addition to forming uniform LaB6 NCs under mild conditions, our approach yields the first colloidally dispersible LaB6 when isophthalic acid is included in the reaction medium. The elevated temperature and presence of NaBH4, a well-
Figure 1. TEM images of ligand-free LaB6 NCs with reaction times of A) 1 h, B) 3 h, C) 6 h, D) 1 h in high resolution and ligand-bound LaB6 NCs with reaction times of e) 1 h and f) 5 h.
known reducing agent, pose a challenge for organic ligand selection. Though NaBH4 is typically unreactive toward carboxylic acids it does react with arene-bound carboxyl groups, converting −COOH to −CHOH. Isophthalic acid, an arene with two carboxyl groups, has a melting point of about 340 °C and was selected as the ligand for our reaction. Heating isophthalic acid in the presence of NaBH4 should produce 1,3phenylenedimethanol (Figure 2A) with a boiling point of 155 °C. We hypothesized that forming this diol in situ allows some fraction of the molecules to bind to the LaB6 surface, capping the nanocrystals instead of leaving as a volatile byproduct and creating a dispersible product. Using the ligand-bound LaB6 reaction conditions but omitting the LaCl3 results in the product boiling off. The off-white residue that forms in the condenser was collected and analyzed by FTIR. As seen in Figure 2B, bands indicative of NaBH4 are present at approximately 2260 and 1100 cm−1, which is in agreement with recent literature reports.27,28 The IR bands in the residue and unreacted ligand are similar, except the residue lacks the CO band at 1680 cm−1 and there is a very broad peak at ∼3500 cm−1 that can be assigned to −O−H stretching. These results support that isophthalic acid converts to the diol when heated with NaBH4. The surfaces of ligand-free and ligand-bound LaB6 were also characterized by FTIR (Figure 2C). There are B−H stretches and bends observed at about 1200 and 2500 cm−1, which may arise from some surface hydridation. The IR bands indicative of LaB6 are also present near 800 cm−1, 1000 cm−1, and 1450 B
DOI: 10.1021/acs.chemmater.5b02297 Chem. Mater. XXXX, XXX, XXX−XXX
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of 17.0% for ligand-bound LaB6 and only 9.52% for the ligandfree LaB6. MS analysis of the evolved gases also revealed contributions from ligand decomposition not present in the ligand-free sample, with a significantly strong signal at 29 amu (corresponding to CH2CH3), supporting the presence of surface-adsorbed organic ligands in the ligand-bound LaB6. Having developed synthetic control over LaB6, we correlated changes in NC size with optical properties arising from the LSPR. LaB6 NCs were suspended in tetrachloroethylene (TCE) and the absorbance spectra measured via diffuse reflectance. As NC size increases a red-shift of the LSPR occurs, where NCs with approximate sizes of 2.1, 3.4, and 4.7 nm have respective LSPR peaks of 1080, 1170, and 1250 nm (Figure S8). The same trend is seen in ligand-bound LaB6, where increasing the NC size from roughly 2.5 to 2.8 nm results in a plasmon shift from 1090 to 1220 nm (Figure S8). These shifts in spectra follow the same qualitative trends of metal NCs reported in the literature, such as 5−40 nm long gold rods (with fixed 2.4 aspect ratio)32 and 9−99 nm diameter gold spheres.33 The Lorentz−Drude model was used to predict the particle size dependent dielectric function by incorporating surface scattering, a dominant scattering mechanism for particle sizes less than 5 nm. Using effective medium theory and the obtained dielectric function, the absorbance was calculated both for ligand-bound and ligand-free nanocrystals. Ligand-bound NCs were modeled using core−shell geometry, with LaB6 NCs as the core surrounded by a very thin shell of ligand. For the detailed mathematical formulae, see Equations S2−S8 in the SI. All the parameters used in the models are listed in Table S5. The measured size scaling trend in the LSPR absorbance peak for ligand-free LaB6 NCs in TCE is similar to the calculated trend (Figure 3), where increasing particle size results in a slight red shift of the LSPR. Though the same trend exists in ligand-bound samples, the measured SPR is red-shifted relative to predicted values, suggesting the ligands do not influence the optical properties as much as expected. The measured values in both ligand-bound and ligand-free cases were found to be redshifted compared to the calculated values, which is consistent
Figure 2. A) Expected conversion of isophthalic acid to 1,3phenylenedimethanol under reaction conditions, B) FTIR spectra of residue from reaction involving only NaBH4 and isophthalic acid (top), unreacted NaBH4 (middle), and unreacted isophthalic acid (bottom), and C) FTIR of ligand-free LaB6 (top) and ligand-bound LaB6 (bottom).
cm−1.29−31 Stretching modes of the LaB6 were confirmed by Raman spectroscopy (Figure S3); a detailed Raman study will be reported in a future publication. Unfortunately, the absence of the ligand’s CO stretch (1690 cm−1) and presence of the −OH bending mode at 1420 cm−1 are obscured by the IR bands of LaB6 itself. However, a broad hump-like shoulder at ∼3500 cm−1 is observed in the ligand-bound LaB6 FTIR but not in the ligand-free sample, and this band’s position matches well with the −O−H stretching band observed when reacting only NaBH4 and isophthalic acid. This result supports that the colloidal stability of the ligand-bound nanocrystals is associated with in situ formation of the diol that then coordinates with the LaB6 surface. To further analyze the surface functionalization of the NCs, thermogravimetric mass spectrometry analysis (TGA-MS) was performed on ligand-bound and ligand-free LaB6 NCs, and a notable difference was observed between the two samples (Figure S4). TGA showed a weight loss between 55 and 188 °C
Figure 3. Calculated and measured (via diffuse reflectance) LSPR peak positions of ligand-free and ligand-bound LaB6 NCs. C
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the surrounding environment. Unlike ligand-bound NCs in TCE, measured and theoretical LSPR absorbance are close (Table S7). Dispersing LaB6 in a polymer matrix increases the interparticle distance, eliminating the aggregation that is likely prevalent in TCE suspensions and causing a blue-shift in the LSPR.36 In summary, we have successfully synthesized LaB6 NCs in a flask for the first time without the use of high temperatures or pressures. The ability to control the particle size and to introduce ligand functionalization makes it possible to uncover size and dispersion effects on the optical properties due to SPR effects. Using an isophthalic acid ligand during the synthesis also makes it possible to incorporate LaB6 NCs into sol gel and polymer matrices, which may be useful for optical coatings.
with limited aggregation of the particles. Red shifting of LSPR spectra upon aggregation is well-known in metallic nanoparticles, and, furthermore, Mendelsberg et al.34 reported similar red shifting of the plasmon resonance in closely packed films of ITO plasmonic NCs. The NC dispersibility makes it possible to directly incorporate LaB6 into various solid matrices for potential use in optical coatings. Previously, Jiang et al. embedded milled LaB6 NCs in organosilicate glass derived from tetraethoxyorthosilane (TEOS) to create coatings with minimal scattering, but this required large amounts of a stabilizing agent which could compromise long-term stability.35 Our LaB6 prepared with isophthalic acid is readily dispersible in acetone and can be codissolved with polymers without the need for additional stabilizers. Figure 4A shows the absorbance spectra
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02297. Detailed experimental procedures, XRD patterns, size histograms, Raman spectra, TEM images, d-spacings, EDX results, peak fits, absorbance spectra, calculations for theoretical predictions, calculated SPR (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors have given approval to the final version of the manuscript. Funding
This work was completed in part at the Molecular Foundry, Lawrence Berkeley National Laboratory, a user facility supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (DOE) under contract no. DE-AC02-447 05CH11231. D.J.M. was supported by a DOE Early Career Research Program grant and additional support is acknowledged from the Welch Foundation (F-1848). Notes
The authors declare the following competing financial interest(s): D.J.M. declares a financial interest in Heliotrope Technologies, a company pursuing commercial development of electrochromic windows.
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Figure 4. Extinction spectra (offset) of ligand-bound LaB6 NCcontaining films coated on glass, collected in transmission mode including a) NCs of two particle sizes 2.51 ± 0.50 nm and 2.80 ± 0.42 nm embedded in TEOS-derived glass and b) 2.80 ± 0.42 nm NCs embedded in PMMA, polystyrene, and TEOS-derived glass.
ACKNOWLEDGMENTS The authors gratefully acknowledge helpful discussions with J. Urban and R. Buonsanti. We also acknowledge Inwhan Roh for his assistance measuring film thickness.
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of two sizes of LaB6 in sol gel films. The LSPR red-shifts as particle size increases, with respective peaks at 627 and 710 nm for 2.51 and 2.80 nm NCs. Embedding LaB6 NCs in various polymers resulted in clear, pale yellow composites. Figure 4B shows the absorbance spectra of LaB6 embedded in PMMA, TEOS, and polystyrene, with respective SPR peaks at 672, 710, and 735 nm (Figure S9). Films were prepared on glass by spin coating, with all polymer-containing films having a similar film thickness of approximately 560 nm (Table S6). This result demonstrates the sensitivity of LSPR to particle size as well as
ABBREVIATIONS
LaB6, lanthanum hexaboride; NC, nanocrystal; SPR, surface plasmon resonance; NaBH4, sodium borohydride; XRD, X-ray diffraction; TEM, transmission electron microscopy; TEOS, tetraethoxyorthosilane; NaCl, sodium chloride; NIR, nearinfrared; FTIR, Fourier transform infrared spectroscopy; HATR, Horizontal Attenuated Total Reflectance; ITO, tindoped indium oxide; AZO, aluminum-doped zinc oxide; PMMA, poly(methyl methacrylate) D
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