Photoinduced Silver Precursor Decomposition for Particle Modification

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Photoinduced Silver Precursor Decomposition for Particle Modification in Tungsten Oxide−Polymer Matrix Nanocomposites Travis J. DeJournett and James B. Spicer* Department of Materials Science and Engineering, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States ABSTRACT: A processing technique based on optical excitation of a polymer matrix nanocomposite containing a chemical precursor is presented to demonstrate the potential of this process for localized modification of nanoparticles in the bulk of the material. Femtosecond laser irradiation of a nanocomposite consisting of tungsten oxide nanoparticles in a fluoropolymer (tetrafluoroethylene-co-hexafluoropropylene) matrix resulted in decomposition of a silver precursor (vinyltriethylsilane(hexafluoroacetylacetonate)silver(I)). Transmission electron microscopy (TEM) results showed an increase in particle size as a result of photoprocessing with particles displaying core−shell type architectures. Optical absorption spectroscopy measurements indicated that silver was deposited in the bulk of the material and were consistent with absorption cross-section models for tungsten oxide core/silver shell nanoparticles. Estimates for silver shell volumes were found to correlate with the surface areas of the base tungsten oxide particles. Modeled results for photothermal temperature rises suggest that thermal processes are probably not responsible for precursor decomposition and the most likely mechanism is multiphoton, photocatalytic precursor decomposition at the surface of the tungsten oxide nanoparticles because tungsten oxide is a known photocatalyst. Models for precursor diffusion in the matrix after particle excitation are used to identify relevant time scales associated with precursor depletion and to provide additional insight into this in situ deposition process for particle modification.

1. INTRODUCTION Polymer matrix nanocomposites (PMNCs) can provide a range of interesting and potentially useful behaviors that depend largely on the nature of a constituent material having nanoscale dimensions. In many cases, the desired functionalities of PMNCs are dictated primarily by nanoparticles or other nanoscale structures that are integrated into the matrix. In these cases, the matrix primarily serves as a supporting medium, but it must also provide appropriate access to these structures to maintain the desired optical/chemical/electronic behaviors. For example, nanostructured systems designed for artificial photosynthesis,1,2 environmental photocatalysis,3−6 or photochromic7,8 applications have all been investigated with various implementations using polymer matrixes as the underlying support for various functional components. Challenges exist not only for the design and synthesis of these components but also for the processing required to incorporate them into matrixes in ways that maintain the desired functionalities. This can be accomplished using a variety of methods. A common approach is to disperse nanostructured components into uncured polymer and to fix these components in place by curing the polymer.9 This method can be accomplished easily using simple processing tools. Another approach is to create the nanostructured components on surfaces and to encapsulate them using a polymer overlayer.10 This maintains the distribution and orientation of components in the resulting composite film and allows for a high degree of control of component location.11 © 2014 American Chemical Society

The general processing approach reported here is to synthesize tungsten oxide nanoparticles directly in a polymer matrix12,13 and to modify these using a photoinitiated, chemical vapor deposition process. The photochromic response of tungsten oxide to ultraviolet light has been studied extensively, and various efforts have sought to extend this response into the visible region by incorporating additional materials into these systems. Even though details regarding the photochromic response of tungsten oxide are not completely settled, approaches to shifting this response to longer wavelengths include incorporating noble metals, semiconductors, or other metal oxides into the overall material structure.14,15 Methods for doing so vary depending on whether the base tungsten oxide is in the form of a thin film, is composed of nanoparticles (or other nanoscale structures), or is a component in a more complicated system. The base tungsten oxide PMNC used here displays a variety of photochromic behaviors depending on the processing used and, like other tungsten oxide-based systems, methods are needed to extend its spectral response to the visible region.16 To achieve this type of change in spectral response, in situ modification of nanoparticles in polymer matrixes using photonically induced chemical reactions has been pursued. Specifically, the nanocomposite is excited using femtosecond Received: February 12, 2014 Revised: April 5, 2014 Published: April 8, 2014 9820

dx.doi.org/10.1021/jp5015324 | J. Phys. Chem. C 2014, 118, 9820−9831

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precursor decomposition and permit nanoparticle formation. These processing steps were repeated a second time to increase particle size. The resulting material consisted primarily of the FEP polymer matrix loaded with discrete tungsten oxide nanoparticles distributed throughout the bulk of the material as indicated by the transmission electron micrograph (TEM) of an area of the film cross section shown in Figure 1a where particles of tungsten oxide appear to be randomly distributed in the matrix. Samples were prepared for TEM imaging using room-temperature diamond microtome methods, yielding sections approximately 100 nm thick, and were mounted on copper or nickel TEM grids. Imaging was performed on a 120 kV Philips EM 420 transmission electron microscope, and ImageJ software was used to analyze the micrographs for nanoparticle polydispersity. Higher magnification micrographs of particles in the film cross section are shown in Figure 1b,c. In Figure 1b, larger particles appear to be composed of collections of smaller particles with varying electron absorption cross sections. Figure 1c shows details of the larger particles more clearly. Variation in the opacity of particle structures is likely related to localized variations in the oxidation state of tungsten oxide because this affects the permittivity of the material. Figure 1c also reveals the presence of a large number density of smaller particles with radii in the 1−2 nm range. These particles do not appear at lower magnifications because they are relatively electron transparent and are also obscured by matrix structures. This directly affects estimates of the particle size distribution. The low volume fraction of tungsten oxide in this material prevents ready examination using X-ray diffraction, and two additional processing cycles were performed to increase particle size and volume fraction. A transmission electron micrograph of an interior cross section of this material is shown in Figure 2a. Again, the larger particles are highly structured and appear to be composed of smaller particles. Also, images of isolated, smaller particles are present in the micrograph but are partially obscured by matrix structure. X-ray diffraction measurements on this material were performed on a Philips X’Pert Pro X-ray diffractometer and are shown offset in Figure 2b along with corresponding measurements on the neat matrix. The broad peak centered near 25° is related to tungsten oxide and is consistent with previously reported measurements of noncrystalline tungsten oxide.20 The peak at 31−32° is related to ordering of CF3 side groups along the polymer chains of the matrix. 2.2. Materials, Optical Processing, and Characterization. Using the tungsten oxide nanocomposite produced using two cycles, subsequent processing was pursued to investigate the possibility of modifying particle characteristics by inducing chemical reactions in the near-particle region using photonic excitation. For this work, decomposition of a silver precursor (vinyltriethylsilane(hexafluoroacetylacetonate)silver(I) (99.9%Ag; STREM Chemicals Inc., Newbury, MA) was chosen because it would yield species distinct from the existing material. Also, incorporation of silver into this tungsten oxide system could alter its photochromic response to visible light; this type of effect has been reported in related systems.21 A variety of laser sources could be used to excite this system; one of the selection criteria used here was to maximize direct interactions with the tungsten oxide particles and minimize those with the matrix and precursor. Optical transmission spectra shown in Figure 3a for the matrix and for the nanocomposite were taken using a Cary 50 3.00 UV/vis

laser pulses to decompose a silver precursor in the near-particle environment to alter local chemical/electronic/optical properties. Both the chemical and optical accessibility of particles in this PMNC system permit this type of processing−diffusion of chemical precursors into regions surrounding tungsten oxide nanoparticles as well as the subsequent optical initiation of chemical reactions leading to silver deposition can be readily carried out. This modifies particle structure and properties in ways that affect overall material functionality. The work described here demonstrates this processing scheme, reports various characteristics of materials produced using this approach, and identifies physical/chemical processes that play important roles in the optical processing of tungsten oxide PMNCs. In particular, the roles of precursor photolysis, photothermal excitation, and photocatalytic decomposition are considered. As part of this analysis, photothermal temperature rises resulting from optical excitation of the nanoparticles are modeled and are used to assess the likelihood of thermally induced precursor decomposition. In addition, precursor diffusion in the matrix after particle excitation is considered to identify relevant time scales associated with precursor depletion and to provide insight into design of optimized processes for particle modification. The resulting nanocomposites are examined using transmission electron microscopy and optical absorption spectroscopy to provide additional insight into the characteristics of this processing approach.

2. EXPERIMENTAL METHODS Nanoparticles in the polymer matrix were synthesized in a twostep process. The first step is a type of thermally activated, in situ chemical vapor deposition process that yields tungsten oxide nanoparticles distributed throughout the bulk of the matrix. The second uses photonic excitation to induce decomposition of a silver precursor in the near-particle environment. These processing steps were carried out sequentially with the aim of producing particles that have tungsten oxide core/silver shell architectures. 2.1. Materials, Thermal Processing, and Characterization. The tungsten oxide nanocomposite was produced using a tungsten hexacarbonyl organometallic (W(CO)6; 99% W, 10RP and D ≈ 10−8 μm2/ns.54 Calculations using the full expression for nearparticle concentrations yield results that follow those obtained using eq 19 for times up to approximately 1 ms. Representative results using RP = 10 nm and ΔR = 1 nm are shown in Figure 10 for various distances from the particle center.

Figure 10. Fractional change in precursor concentration as a function of time for various radial distances outside a nanoparticle with a radius of 10 nm.

These results indicate that equilibrium concentrations in the particle environment are not achieved within 100 μs after excitation. In particular, at the particle surface, precursor concentrations exceed 0.999c0 at 100 μs, but even this small deviation from equilibrium has significant implications. If photoexcitation occurs at intervals less than 100 μs, then precursor does not fully populate the near-particle environment, resulting in altered locations and rates of decomposition even after short times corresponding to a small number of events. Indeed, if the interval between excitation events is 1 μs or less, then precursor decomposes as it reaches the particle surface (or near-particle zone); the excitation appears to be continuous from a diffusion viewpoint, and the distribution of various species in the reaction region will vary accordingly. This

5. CONCLUSIONS This work has reported the modification of nanoparticles in a tungsten oxide−polymer matrix nanocomposite containing a 9829

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silver precursor using femtosecond laser excitation. Optical excitation of the nanocomposite resulted in decomposition of precursor and subsequent deposition of silver in the nearparticle environment. Transmission electron micrographs show increased particle sizes associated with optically processed material. Expected temperature rises in near-particle regions indicate that photothermal processes are likely not responsible for precursor decomposition and that some other mechanism mediates the reaction, possibly a multiphoton, photocatalytic process. Models for precursor diffusion in the matrix indicate that decomposition occurs continuously as these species move into near-particle regions and that precursor depletion for particles in the nanocomposite interior can occur for long exposure times. On the basis of interpretation of micrographs as well as optical absorption spectra, silver deposition results in particles having optical spectra consistent with WO3−Ag core− shell structures. This overall processing approach demonstrates the potential for localized, in situ modification and growth of nanostructures in polymer matrixes and points to more sophisticated processing approaches that can be used to create multifunctional materials containing hierarchical structures over various length scales.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 410 516 8524. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Michael McCaffery and the Johns Hopkins Integrated Imaging Center for the invaluable contributions made to this work through TEM sample preparation and imaging.

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REFERENCES

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