Laser-Induced Precursor Decomposition for Nanoparticle Modification

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C: Physical Processes in Nanomaterials and Nanostructures

Laser-Induced Precursor Decomposition for Nanoparticle Modification in Polymer Matrix Nanocomposites Fan Wen Zeng, Dajie Zhang, and James B. Spicer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05290 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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The Journal of Physical Chemistry

Laser-induced Precursor Decomposition for Nanoparticle Modification in Polymer Matrix Nanocomposites Fan W. Zeng,∗,† Dajie Zhang,‡ and James B. Spicer† †Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD, 21218, USA ‡Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA E-mail: [email protected]

Abstract This work investigates processes involved in the modification of palladium nanoparticles in polymer matrix nanocomposites (PMNCs) using continuous wave (CW) laser irradiation. Optical excitation of PMNCs containing palladium nanoparticles and palladium precursor resulted in localized decomposition of the palladium precursor and subsequent particle modification as well as particle formation. The photo-induced chemical reactions were performed under vacuum and at various background temperatures. Transmission electron microscopy analysis showed that the particle size and number density increased as the photoprocessing time increased, and the particle formation and growth rates increased as the background temperature increased. Calculations of temperature rise based on photothermal particle heating suggest that the temperature increase facilitates the diffusion of precursor as well as precursor decomposition.

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Photocatalytic interactions between particles and precursor as well as between particles and polymer were also considered since palladium is a well-known catalyst. The results here demonstrated the possibility of creating patterned nanocomposites with spatially-varying properties.

Introduction Polymer matrix nanocomposites (PMNCs) are multiphase materials consisting of a continuous phase made from a polymeric material and one or more nanoscale phases dispersed in the polymer matrix. The properties of PMNCs depend on the properties of the constituent phases, their relative amounts, and their distributions. In many cases, the continuous phase is used to protect the dispersed phases from undesired environmental effects and to provide underlying support for nanoparticles or other nanoscale structures. The chemical and optical behaviors of the embedded nanostructures determine the functionalities of PMNCs as long as the polymer matrix allows access to the nanocomponents. For example, photoactive nanoparticles should be grown in an optically transparent polymer matrix. Various PMNC systems have been designed for photochromic, 1–3 photocatalytic, 4–8 and gasochromic 9,10 applications. Regardless of the application, synthesis of PMNCs requires a way to incorporate functional nanoparticles into the polymer matrix. The common approaches for synthesizing PMNCs include direct mixing the uncured polymer with desired functional nanomaterials followed by the curing process 11–13 as well as using a polymer overlay to encapsulate nanostructures that are grown on a polymer surface. 14 While these methods are well-established, controlling the particle size and composition at specific locations in the polymer matrix can be difficult and generally requires the use of lithographic techniques. Even so, the lithographic processes are often time consuming and cannot actively control particle location within the matrix. Synthesizing nanoparticles at selective locations in a polymer matrix can be accomplished using laser-based methods. Fragouli et al. synthesized cadmium sulfide nanocrystals in a 2


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polymer matrix by diffusing a cadmium sulfide chemical precursor into a polymer and subsequently decomposing the precursor using a pulsed nanosecond ultraviolet laser. 15 Watanabe et al. used a similar laser photolysis approach to form gold and iron-platinum alloy nanoparticles in polyvinylpyrrolidone. 16 Femtosecond pulsed lasers have also been used to selectively grow silver 17 and lead sulfide 18 nanoparticles in polymer matrices via multiphoton processes. Unfortunately, these photo-induced unimolecular chemical reactions provide little control over particle size, and large nano-sized particles as well as aggregation of nanoparticles are often produced. To modify the composition of nanoparticles, previous studies have shown that core-shell nanoparticles can be made by decomposing the chemical precursors around the seed nanoparticles in the polymer matrices using femtosecond laser irradiation. 19,20 While all these studies demonstrated the concept of photonic production and modification of nanoparticles in polymer matrices, they rely on the use of pulsed laser light sources (high photon fluxes) that cause non-linear optical interactions with chemical precursors or seed nanoparticles. Controlling these non-linear optical processes can be difficult, and better control could be achieved using linear optical processes. A few studies have reported production and modification of metal nanoparticles using continuous wave (CW) laser irradiation. In particular, Bagratashvili et al. synthesized silver-polymer matrix nanocomposites by decomposition of silver precursors in fluoropolymer films using a 532 nm CW laser, but various photosensitizers were used to increase the optical interactions. 21 While adding photosensitizers has proven to be successful in some composite systems, the processing requirements may result in systems with undesirable performance. For example, the presence of the photosensitizer at the particle-matrix interface could alter the optical or chemical behaviors of the composite material. In the case of nanoparticle modification, Setoura et al. demonstrated CW-laser-induced morphological changes of a single gold nanoparticle, and they attributed the morphological changes to photothermal surface evaporation. 22 Hubenthal et al. applied a similar approach to alter the axial ratio of gold and silver nanoparticles using intense CW laser irradiation. 23 In both of these studies, the modification of nanoparticles was achieved

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by illuminating the nanoparticles with light that coupled to the localized surface plasmon resonance (LSPR) which results in an increase in particle temperature and subsequently surface evaporation. This type of modification is limited to particle size reduction, and the reported results were only demonstrated on the surface of a glass or quartz substrate. To the best of our knowledge, CW laser induced nanoparticle modification in polymer matrix nanocomposites has not been reported previously. In this work, we focused on synthesis and modification of palladium-polymer matrix nanocomposites since this type of composite systems has various potential applications, such as hydrogenation, methanol oxidation, and iodide sensing. 24–26 Results are presented that show in situ modification of palladium nanoparticles in fluoropolymers using a vapor infusion method along with CW laser irradiation. Specifically, palladium nanoparticles were synthesized in a fluoropolymer matrix by vaporizing a palladium chemical precursor, letting the precursor molecules diffuse into the polymer matrix, thermally decomposing the precursor, and finally forming nanoparticles from the decomposition products. These nanoparticles served as seed nanoparticles. To modify the size and number density of the palladium nanoparticles at selective locations, a second palladium precursor infusion process was performed, followed by CW laser irradiation. During laser exposure, various background temperatures were applied in order to explore particle modification processes. The change of particle size and number density were examined using transmission electron microscopy, and results were interpreted by considering the roles of precursor photolysis, photothermal particle heating, and photocatalytic interactions. In particular, temperature rises resulting from optical absorption of the palladium nanoparticles were calculated and were used as criteria for identifying the primary mechanisms involved in particle modification.

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Background and theory In a particle-precursor-polymer material system, there are various photo-induced mechanisms that could cause changes in palladium particle size and particle number density. If the optical excitation could directly interact with the precursor, the possibility of unimolecular decomposition (photolysis) of the precursor should be considered. If the changes are mediated by the palladium particles, the nature of the reactions should also be explored. Clearly, photothermal heating of the palladium nanoparticles could drive thermal decomposition of the nearby precursor. If the temperature increase is sufficiently high, the nanoparticle could melt and agglomerate subsequently, and the polymer matrix would likely deteriorate since the melting temperature of palladium is higher than the decomposition temperature of the polymer. Also, since palladium is a well-known catalyst, photocatalytic reactions should be considered. If the precursor is attached on the palladium particle, photocatalytic precursor decomposition could occur. If photocatalytic interaction occurs between the palladium particle and the polymer matrix producing reactive radicals, the precursor could decompose once it encounters the radicals. It is challenging to differentiate among the mechanisms associated with changes in particle size and particle number density, but it is obvious that the mechanism for precursor decomposition is important since it relates directly to the particle changes. In the following, we will present models for photothermal particle heating, and the model results will be examined in the results and discussion section to identify whether the main mechanism for precursor decomposition is a thermally-driven process.

Optical heating of nanoparticles A system with spherical symmetry consisting of a palladium nanoparticle of radius R and precursor molecules enclosed in a polymer matrix is considered. A CW laser is used to excite the nanoparticle, but it is assumed that it does not cause direct photolysis of the

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precursor. The optical energy deposition can be assumed to be uniform throughout the nanoparticle since the optical penetration depth in palladium is relatively large compared to the nanoparticle size. Also, the excitation can be considered to be composed of processes that involve photon-electron interactions, thermalization of conduction electrons, electronphonon interactions, and phonon-phonon coupling between the particle and the polymer matrix. 27–29 This type of phonon-phonon coupling results in energy transfer between the nanoparticle and its immediate surroundings. 28,30 If the precursor is within this immediate surrounding, coupling between particle phonon modes and vibrational state of the precursor would be critical to producing the desired precursor decomposition. 31 Unfortunately, detailed information related to precursor vibrational states involved in decomposition in this material system is not known, so temperature will be used as an ad hoc substitute for the occupation of various phononic and vibrational states involved in these interactions. Since the thermal conductivity of palladium is much higher than that of the polymer (72 vs. 0.1 W m−1 K−1 ), an equilibrium thermodynamic description can be used to describe the heating of the particle and the thermal diffusion into the polymer matrix. In particular, thermal diffusion in the polymer matrix and in the particle can be described, respectively, using the following equations: 32 ∂Tm 1 ∂ ∂Tm 2 = 2 r · κm , r>R ρm cvm ∂t r ∂r ∂r ∂Tp 1 ∂ ∂Tp 2 ρp cvp = 2 r · κp + S, r < R ∂t r ∂r ∂r

(1)

where ρ, cv , and κ are the material density, specific heat capacity, and thermal conductivity, respectively. The subscript m and p refer to matrix and particle, respectively. Temperature, time, radial distance from particle center, and heat source power density are represented by T , t, r, and S, respectively. In order to obtain solutions to these equations, boundary conditions at the particle-polymer interface must be specified. The nanoparticle environments in PMNCs is poorly understood. The near-particle environment likely has different properties

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than those of the neat polymer since polymer chains in this region reconfigure themselves in response to the nanoparticle, and continuity of energy transport and temperature at the particle boundary might not be easily specified. When the temperature distribution near a boundary is not known, the concept of interface resistivity is often applied. This interface resistivity simply assumes that the intervening material somehow resists heat flow or energy transport between two trans-boundary points, and the direct consequence of this modification of boundary condition is a temperature discontinuity at the interface boundary. 32 Since the nanoparticle likely transfers energy to its surroundings through some sort of conduction to precursor species along with coupling to phonon modes of polymer chains, interface resistivity might be a useful concept to invoke. However, attempts to capture related effects on energy transport at the interface are well beyond the scope of this work and, as a result, interface resistivity will be assumed to be 0. The boundary conditions at the particle-polymer interface will be assumed to be continuity of temperature and continuity of conductive heat flux. These equations are given as follows: Tm |r=R = Tp |r=R ∂Tp ∂Tm κm r=R = κp r=R ∂r ∂r

(2)

The material system will be assumed to have a uniform initial temperature distribution such that Tp = Tm when t = 0.

CW laser illumination Single nanoparticle heating At the very beginning of CW laser particle heating, the transient temperature profile of the nanoparticle is very different from that of the surrounding polymer matrix since the thermal diffusivity of the nanoparticle is much higher than that of the polymer—the thermalization inside the nanoparticle occurs much faster. Consequently, the temperature profile of the

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overall system can be considered to be governed by the evolution of the temperature profile in the polymer matrix. After the transient period, a steady state temperature profile will be reached for CW laser particle heating since the absorbed heat will be balanced by heat conduction to the polymer matrix. In the steady state regime, the two differential equations in Eq. (1) can be written as follows: κm ∂ 2 ∂Tm r = 0, r > R r2 ∂r ∂r κp ∂ 2 ∂Tp r = −S, r < R r2 ∂r ∂r

(3)

The solution for Eq. (3) has a simple form, and the temperature rise is given in the following: 32 σabs I , r>R 4πκm r σabs I κm r2 Tp = 1+ 1− 2 , 4πκm R 2κp R

Tm =

(4) r 453 K) and has been used extensively for study of chemical deposition. 34,35 Palladium nanocomposites (designated as Pd-PFA) were synthesized using a chemical vapor infusion technique. 36 The synthesis process was started by placing a 50 × 50 × 0.127 mm3 PFA film in a glass reaction vessel along with ∼15 mg of Pd(acac)2 powder dispersed around the inner wall of the vessel. The reaction vessel was evacuated to ∼160 mTorr in order to remove air from the vessel as well as volatiles from the PFA matrix. In this system, the sublimation or vaporization temperature and the decomposition temperature were determined to be approximately 413 K and 473 K, respectively. The reaction vessel was transferred to an oven and heated to 413 K for 2 hours to sublime or vaporize the precursor and allow the precursor to diffuse into the polymer. The temperature of the oven

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Figure 1: Optical transmission spectra for Pd-PFA and as-received PFA. was then raised to 473 K and held for 2 hours to induce precursor decomposition followed by nanoparticle formation. Optical transmission spectra shown in Fig. 1 for PFA and Pd-PFA were taken using a PerkinElmer Lamda 950UV/Vis spectrophotometer for wavelengths between 250 to 800 nm. While the as-received PFA film transmits well throughout the visible region, the presence of palladium particles produces a strong absorption in the ultraviolet (UV) region and a broad absorption in the visible region. Visual inspection of the Pd-PFA material indicates that it has a homogeneous light brown color and this suggests that the particles are uniformly distributed throughout the matrix. To verify the distribution and the presence of palladium nanoparticles, cross sections for transmission electron microscopy (TEM) imaging and energy dispersive X-ray spectroscopy (EDS) analysis were prepared using room-temperature diamond microtome methods, yielding sections approximately 100 nm thick. These sections were mounted on copper grids and imaged on a 100 kV FEI Tecnai 12 transmission electron microscope, and an Oxford EDS detector was used for the elemental analysis. ImageJ software was used to determine particle size, particle distribution, and average particle number density. ES Vision software was used to quantify the elemental composition. Figure 2 (a) shows a representative TEM image of Pd-PFA which shows that discrete nanoparticles having average radius of 1.6 nm are distributed throughout the bulk 10


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Figure 2: (a) Representative transmission electron micrographs of Pd-PFA. (b) Energy dispersive X-ray spectroscopy analysis for Pd-PFA. of the PFA. Figure 2 (b) shows an EDS spectrum that indicates a relatively small amount of palladium which is consistent with the estimated volume percentage of palladium (< 0.1%). The presence of carbon, oxygen, and fluorine are related to the PFA matrix. A second infusion process was performed by heating a vacuum reaction vessel that contained Pd-PFA as well as ∼10 mg of Pd(acac)2 at the precursor vaporization temperature for two hours. The resulting material is designated as Pd(acac)2 -Pd-PFA. Figure 3 shows the optical transmission spectra for Pd(acac)2 -Pd-PFA and Pd(acac)2 . The precursor is believed to be distributed uniformly in Pd(acac)2 -Pd-PFA since the material exhibited homogeneous color. The absorption characteristics of Pd(acac)2 -Pd-PFA are similar to that of Pd-PFA. However, the color of the Pd(acac)2 -Pd-PFA appeared to be slightly darker than Pd-PFA. This slight darkening is most likely related to the infusion of the palladium chemical precursor since it absorbed well in the ultraviolet region and no visible darkening was found for Pd-PFA in the same processing environment (without the presence of the chemical precursor). The Pd(acac)2 -Pd-PFA material was used as the starting material for CW laser processing, and six Pd(acac)2-Pd-PFA samples were synthesized in the same way.

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Figure 3: Optical transmission spectra for Pd(acac)2 -Pd-PFA and Pd(acac)2 .

CW laser processing and characterization Choosing an appropriate laser source is an important step for optical processing. In general, the characteristics of the excitation laser should be chosen to provide direct interaction with the particles or the precursor. The palladium precursor has a strong absorption below 350 nm (Fig. 3), and the LSPR of palladium nanoparticles is also located in the deep ultraviolet region. 37 However, an ultraviolet (UV) laser source was not chosen since UV lasers can produce photodegradation of fluoropolymers. 38,39 Fortunately, palladium nanoparticles exhibit moderate absorption in the visible region (Fig. 1). In this study, a 532 nm CW laser (Coherent DPSS) was used to produce a 1.32 × 104 W/m2 laser beam. Initially, a few control experiments were performed on Pd-PFA in order to identify potential processing conditions. In particular, Pd-PFA was treated with the laser in air and in vacuum (∼160 mTorr). After about 2 seconds of illumination at room temperature, a shallow deformation on the surface (dent) was found in the illuminated region when PdPFA was held in air, and a decomposed region (hole) was observed when the material was in an evacuated glass vessel. While the surface of the dent region appears to protrude along surface scratches (Fig. 4 (a)), the decomposed region was circular with a hole approximately 250 µm in diameter. It should be noted that the as-received PFA did not exhibit any 12


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Figure 4: Optical micrographs of Pd-PFA after 2 seconds of 532 nm CW laser irradiation: (a) in air and (b) in vacuum. The beam spot is ∼1 mm. The scale bars are 500 µm. physical transformation when the same laser irradiation conditions were used. These results indicate that some sort of photoinduced process cause the deformation and decomposition of the polymer matrix, and these could include bond breaking in the polymer chains as well as photooxidation when irradiating in air. 40,41 In order to avoid photodegradation of the polymer matrix, the laser beam size was expanded so that the average intensity was lowered to 307 W/m2 . The resulting beam did not induce any apparent physical changes to the Pd-PFA. Laser processing of Pd(acac)2 -Pd-PFA was performed in vacuum to avoid reactions with air. In a typical experiment, synthesis began by placing the sample in a glass reaction vessel. The vessel was evacuated to 160 mTorr. The CW laser beam was then directed to the Pd(acac)2 -Pd-PFA material. After more than 20 minutes of exposure at room temperature, a small dark spot appeared. This type of darkening is typically related to the changes of optical cross section of the nanoparticles and or the number density of nanoparticles. It is unlikely that the changes of the particle size and or particle number density were related to particle aggregation since this type of darkening was not observed in Pd-PFA when the same experimental conditions were used. The darkened region is the result of precursor decomposition and the subsequent particle formation. Figure 5 shows an optical image of the laser-processed material (left) as well as representative TEM images of the laser-processed region (top-right) and Pd(acac)2 -Pd-PFA (bottom-right). The average particle size and

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Figure 5: An optical image of Pd(acac)2 -Pd-PFA showing a small dark spot after 20 minutes of CW laser irradiation at room temperature (left). The scale bar is 5 mm. Representative TEM images show the distribution of palladium nanoparticles at the laser-processed region (top-right) and the region without optical treatment (bottom-right). The scale bars are 20 nm. size distribution of Pd(acac)2 -Pd-PFA are very similar to that of Pd-PFA, indicating the embedded nanoparticles are not affected by the second precursor infusion process. The average particle radius of the laser-processed region is ∼1.9 nm, and the particle number density is ∼3 times higher than the starting material (1.1 × 1016 cm−3 vs. 3.0 × 1016 cm−3 )

In order to accelerate the process, the background temperature was increased by placing the reaction vessel in a custom-made oven equipped with an optical window that allowed for laser processing. Increasing the background temperature presumably accelerates precursor decomposition and subsequent particle modification processes since polymer chain mobility as well as precursor diffusivity increase with temperature 42 —the time needed for the precursor molecules to diffuse into the precursor depletion zone would be reduced. 19 Also, it should be noted that direct measurement of the local temperature of the nanocomposites could not be performed during laser processing owing to the experiment setup—Pd(acac)2 Pd-PFA was enclosed in a reaction vessel and an oven. Figure 6 (a) shows an optical image

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Figure 6: Optical images of Pd(acac)2 -Pd-PFA showing dark spots after 2, 5, and 10 minutes of CW laser irradiation at background temperature (a) 353 K and (b) 433 K. Representative TEM images showing the nanoparticles at their corresponding locations. All the scale bars of the optical images are 5 mm, and all the scale bars of the TEM images are 20 nm. (c) Particle size distributions for different processing temperatures and irradiation times. (d) Average particle radius at various processing times and temperatures. The error bars represent ± one standard deviation. of Pd(acac)2 -Pd-PFA after laser exposure at a background temperature of 353 K. When the exposure time exceeded 2 minutes, a small dark spot started to appear. As the processing time increased, the size of the dark spot increased. Representative TEM micrographs are presented for the corresponding dark regions in Fig. 6 (a). Both particle size and particle number density increased with the processing time. When the exposure duration was 2, 5, and 10 minutes, the average particle radius was approximately 1.7, 2.0, and 2.9 nm, respectively, and the average particle number density increased to 2.3, 5.2, and 8.3 × 1016 cm−3 . Although a majority of the nanoparticles were isolated, there were a few regions where the particles seemingly grew into each other. When the background temperature was increased to 433 K, the dark spots on the optical images appeared to be roughly twice as big as those processed at 353 K for each irradiation time, and the average particle radius

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was approximately 2.1, 2.7, and 2.9 nm when the exposure time were 2, 5, and 10 minutes, respectively (Fig. 6 (b)). The average particle number density increased to 8.0 × 1016 cm−3 within 5 minutes of laser processing and reached ∼9.0 × 1016 cm−3 after 10 minutes. At high processing temperature, coalescence of nanoparticles was more noticeable and, sometimes, string-like particle structures could be observed. Figure 6 (c) shows that the particle size distributions broaden and shift to larger particle radius as the laser processing time increases, and Fig. 6 (d) indicates that higher processing temperatures lead to faster particle growth rates. It should be noted that at least two different laser-treated regions from two samples were chosen to perform the TEM analysis for each laser processing condition. The error bars in Fig. 6 (d) represent the standard deviation of the nanoparticle sizes measured from the laser-treated samples.

Results and discussion Precursor decomposition mechanisms Direct photolysis of precursor is unlikely based on the absorption characteristics of Pd(acac)2 at the laser wavelength used (Fig. 3). Nonetheless, this photo-induced interaction was examined since various metal precursors are photosensitive and no studies have been reported on the photolysis of Pd(acac)2 . Laser processing (1.32 × 104 W/m2 ) was performed on PFA matrix containing only Pd(acac)2 —this material was made by heating as-received PFA along with 10 mg of Pd(acac)2 in vacuum at 413 K for 2 hours. The transmission characteristics of the irradiated region remained unchanged after extended exposure. This result together with results obtained in Fig. 5 and 6 indicate that precursor decomposition was not related to photolysis but some type of particle-mediated reactions.

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Photothermal particle heating For the precursor-particle-matrix system considered here, the thermal excitation of the precursor follows the matrix temperature. The particle (and its surrounding matrix) temperature rise resulting from the CW laser heating can be estimated using Eq. (5). In order to use this equation, the particle absorption cross section σabs has to be determined beforehand. For very small spherical nanoparticles (R < 15 nm), the optical absorption cross section can be taken as: 46,47

kp4 |α|2 6π 4πR3 (p − m ) α= (p + 2m )

σabs = kp Im(α) −

(7)

where kp is the wavenumber, and p and m are the relative permittivity of the particle and the matrix, respectively. Using the values provided in Table 1 and considering the average particle radius equals to 1.6 nm, the absorption cross section was calculated to be 4.97× 10−19 m2 . It should be noted that calculations assumed bulk properties for palladium (particle) and PFA (polymer matrix). While both the thermal and optical properties vary with temperature, these are small for the temperature of interest. 48–50 Beyond the modeled approximation for the absorption cross section, an experimental-based value can be determined based on the UV-Vis results in Fig. 1 and using the Beer-Lambert law: 51

σabs =

A(λ) Ln10 Np L

(8)

where A is the absorbance and depends on the wavelength λ, and L is the pathlength or thickness of the material. While the UV-Vis extinction measurements take into account Table 1: Material Constants of PFA Matrix 43,44 as well as Palladium 45 and Specification for Laser Intensity and Wavenumber

p

m

-6.89 + 7.62 i 1.82

κp (W/(m*K)) κm (W/(m*K)) 72

0.195

17

Dm (m2 /s)

kp (1/m)

7.70 × 10−8

1.59 × 107


I (W/m2 ) 307


of both absorption and scattering, it is reasonable to assume the absorption contribution dominates for small nanoparticles since the absorption cross-section scales with γ 3 (the size parameter γ = 2πR/λ) and the scattering cross-section is proportional to γ 6 . Considering A = 0.66 (obtained from Fig. 1), L = 0.0127 cm (thickness of the nanocomposites), and Np = 1.75 × 1016 cm−3 (an average particle number density based on the TEM analysis), the absorption cross section was calculated to be 6.84× 10−19 m2 , which is similar to the estimate based on Eq. (7) (4.97× 10−19 m2 ). The average of these two absorption cross section values will be used to estimate the increase of temperature for the photo-excited particles. According to Eq. (5), a 1.6 nm palladium nanoparticle in a PFA matrix is expected to have an increase in temperature of only 4.9 × 10−8 K, and the time needed to heat the particle surface to the steady state temperature is less than 50 ps (τnano ). Even though the temperature rise for a single nanoparticle is quite small, the temperature rise of a system containing a large number of nanoparticles in close proximity is significantly higher since the heat source power density increases accordingly. For this material system, the palladium nanoparticles are uniformly dispersed, so the mean inter-particle distance can be estimated by performing Voronoi tessellation on the TEM micrographs and then equating the area of each Voronoi polygon to a circular region followed by averaging of the radii—the average distance among palladium nanoparticles ∆ is ∼23 nm. Based on this approximation, it takes less than 10 ns (τ0 ) for the temperature of the neighboring nanoparticles to overlap, and the temperature of the heated region starts to increase subsequently. At later times, the global temperature rise dominates the total temperature change, and the final steady state is reached in a few tens of seconds. According to Eq. (6) and taking LH = 4 mm (radius of the heated region), the steady state, global temperature rise in the nanocomposite in the irradiated region is ∼130 K. For the cases when the background temperatures were set to 433 and 353 K, the heated region is expected to have temperatures as high as 563 and 483 K, respectively. While these temperatures are not high enough to deteriorate the PFA matrix, they can readily decompose the palladium precursor since

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its decomposition temperature is 453 K. 34,35 When the precursor decomposition is driven by thermal activation, the decomposition rate depends on the probability for thermal decomposition of palladium precursor. 52 Higher temperatures generally yield higher precursor decomposition rates. As a result, faster particle formation can be expected at higher temperatures. This is supported by both the optical and the TEM images in Fig. 6 that show new particle formation and particle growth occurred noticeably faster at a background temperature 433 K than at 353 K. It should be noted that, based on the global temperature calculation, the photodegradation of Pd-PFA in Fig. 4 is caused by photothermal particle heating. The laser beam used in the control experiments was ∼50 times more intense than the one used for Pd(acac)2 -Pd-PFA (1.32 × 104 vs. 307 W/m2 ). According to Eq. (6) and having LH = 1 mm, this intense laser beam can induce a steady-state temperature rise ∼350 K in the irradiated region which can easily decompose the PFA matrix. The photodegradation difference occurred at different pressure (Fig. 4 (a) vs. (b)) is because heat conduction into the gas phase is slow for low pressure conditions. In the case when the background temperature was at room temperature, the irradiated region is expected to have a temperature of 428 K. While the probability for thermal decomposition of Pd(acac)2 at this temperature is rather low, 34 CW laser-induced darkening of Pd(acac)2 -Pd-PFA along with the increase of particle size and number density were observed (Fig. 5). As a result, it is unlikely that photothermal particle heating is the main mechanism for precursor decomposition when the processing temperature is low. In fact, previous studies have shown that catalytic effects played an important role in decomposition of Pd(acac)2 in the presence of palladium nanoparticles at similar temperature (453 K). 52 Photocatalytic processes Palladium is a well-known catalyst for a great number of chemical reactions, 53–57 and its photocatalytic behaviors have also been demonstrated in various studies which showed significantly higher reaction rates when conducted under illumination. 58,59 The mechanism for

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photocatalytic reactions consists of a series of electron excitation and electron transfer processes. Specifically, the absorption of visible light and UV light by palladium nanoparticles can excite electron interband transitions, and excited electrons at the surface of the nanoparticles can then transfer to species adsorbed on the nanoparticles. This type of electron transfer weakens the chemical bonds of the adsorbed species and facilitates the activation of chemical reactions. At elevated background temperatures and or high irradiation intensities, a large number of electrons of the palladium nanoparticles can be excited. Electron transfer and hence photocatalytic reactions can be readily achieved if the Pd(acac)2 reactants are physically attached to the seed nanoparticles. In this case, the growth of nanoparticles should be limited by the diffusion of chemical precursor onto the surfaces of the nanoparticles. When the temperature is above the glass transition temperature (Tg ) of the PFA matrix (∼363 K 43 ), the polymer chain mobility is sufficiently high to facilitate the diffusion of precursor; below Tg , diffusion of the precursor is likely hindered by the rigid polymer chains. 42 For laser processing of Pd(acac)2 -Pd-PFA at room temperature, the irradiated region is expected to have a temperature near 428 K. While this temperature cannot effectively cause thermal decomposition of precursor, the diffusion of the precursor increases and, as a result, the precursor has an enhanced probability to adsorb on the nanoparticles and subsequently undergo photocatalytic decomposition reactions. Comparing the nanoparticles in the laser-processed materials (Fig. 5 and 6) to the nanoparticles in Pd-PFA (Fig. 2), the laser-processed materials exhibit not only larger nanoparticles but also higher particle number density. At high background temperatures (433 K and 353 K), the formation of additional particles could be attributed to thermal decomposition of precursor followed by particle nucleation and growth. At a low background temperature (room temperature), neither photothermal particle heating nor photocatalytic precursor decomposition could explain the increase in particle number density. One possible interpretation is that photocatalytic reactions also occur between the photo-excited palladium nanoparticles and the PFA matrix. It is well-known that deep UV light with

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high photon flux can induce chain scission reactions and defluorination of the backbone of fluoropolymers. 40 While the laser light source used in this study is not capable of direct photolysis of the polymer, it is possible that photocatalytic interactions occur between the palladium nanoparticles and the PFA matrix producing active intermediates such as CFx and F radicals. 60 These radicals could react with the precursor molecules resulting in release of palladium atoms, and subsequent formation of nanoparticles especially in regions where the polymer bonds break (serving as heterogeneous nucleation sites).

Mechanism summary Three particle-mediated, precursor decomposition mechanisms have been proposed based on the sizes and geometries of nanoparticles obtained from the TEM micrographs as well as calculations of CW-laser induced temperature rises near particles. All three mechanisms likely facilitate precursor decomposition. At high background temperatures, precursor decomposition is dominated by thermal activation. At low background temperatures, photocatalytic reactions would most likely dominate. However, as the average particle size and particle number density increase, photothermal particle heating would become the primary contributor to precursor decomposition. While measuring the exact contribution of the proposed mechanisms to the overall precursor decomposition and particle modification processes is challenging, various results here indicate the dominant mechanisms under various processing conditions. For photothermal particle heating and photocatalytic precursor decomposition, the growth of palladium nanoparticles is limited by the thermal decomposition probability of palladium precursor and the diffusion of palladium precursor, respectively. For photocatalytic interactions with the polymer matrix, precursor decomposition would depend on the likelihood of producing reactive radicals and the probability of radicals encountering the precursor.

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Discussion In addition to the result interpretations based on the processing methods used, there are a few aspects that deserves further comment. Clearly, the increased size of palladium nanoparticles for materials photoprocessed over extended periods of time suggests that the surface of the palladium particles remain accessible to palladium atoms and palladium precursor so that the particles can grow. Also, the increased particle number density for longer processing times indicates that additional nucleation sites are available in the polymer matrix and that these are occupied given sufficient time. Moreover, the particle size differences between the existing nanoparticles and the newly-formed nanoparticles are minimal suggesting that the growth rate of the newly-formed nanoparticles is higher than that of the existing particles, and this is likely caused by the change of precursor accessibility to particles—the nearparticle polymer chains would likely reconfigure themselves more as the nanoparticles grow bigger and, consequently, impede the diffusion of the precursor more. Finally, the formation of particle strings is likely related to some kind of nanoparticle coalescence, but this is a relatively slow process and is most likely related to the microstructure of the polymer matrix.

Conclusion This work has reported the laser-based modification of nanoparticles in polymer matrix nanocomposites containing palladium nanoparticles and palladium precursor using continuous wave laser excitation. This excitation of the seed palladium nanoparticles resulted in decomposition of palladium precursor and subsequent growth of seed nanoparticles as well as formation of additional palladium nanoparticles. Transmission electron micrographs show that both nanoparticle size and particle number density increase as the processing time increases, and string-like nanostructures can be found within the polymer matrix after extended laser exposure. Estimates of temperature rises in the irradiated regions indicate

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that the temperature increase can cause precursor decomposition as well as facilitate the precursor diffusion. Photocatalytic processes were also considered, and it appears that both photothermal and photocatlytic reactions contribute to palladium precursor decomposition and subsequent particle formation as well as particle growth. Thermally-driven decomposition processes dominate at high temperatures while the catalytic processes likely dominate at low processing temperatures during early stages of particle formation and growth. The processing approach here demonstrates that it is possible to modify the size, number density and distribution of nanoparticles in polymer matrices using a continuous wave laser source. By regulating the irradiation period and intensity, interesting nanostructures can be created leading to the possibility of patterning nanocomposites with spatially-varying properties.

Conflicts of interest There are no conflicts to declare.

Acknowledgments The authors gratefully acknowledge support by the National Science Foundation through the CMMI Division under Award Number 1462151.

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Graphical TOC Entry Polymer Matrix Nanocomposite

: nanoparticle : precursor

Laser Illumination

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Laser-Induced Precursor Decomposition for Nanoparticle Modification

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