Article pubs.acs.org/JPCC
Morphology-Directed Catalysis with Branched Gold Nanoantennas Naiya Soetan, Holly F. Zarick, Christopher Banks, Joseph A. Webb, Greg Libson, Andrew Coppola, and Rizia Bardhan* Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States S Supporting Information *
ABSTRACT: We synthesized multibranched gold nanoantennas (MGNs) of two morphologies by varying the core-to-branch ratio. We compared their efficacy in catalytic reduction of p-nitrophenol (PNP) to p-aminiphenol (PAP). We observed that MGNs with shorter protrusions had a faster induction time and higher apparent rate constant, kapp, for PNP catalysis relative to the MGNs with longer protrusions. By examining the reaction as a function of temperature, we observed significantly lower activation energy for the MGNs with shorter protrusions (80 J/g) compared to MGNs with longer protrusions (200 J/g). The Langmuir−Hinshelwood model was used to fit the change in kapp as a function of increasing [PNP], which demonstrated more efficient PNP adsorption on the surfaces of MGNs with shorter protrusions. For the MGNs with longer protrusions, PNP adsorption is affected by the heterogeneity of the surface sites resulting in a lower adsorption coefficient. We attributed the improved efficiency of the MGNs with shorter protrusions to the presence of {100} and {110} crystal planes, which have a high density of atomic steps and kinks that promote higher catalytic activity for PNP degradation. MGNs with long protrusions are bound by low index {111} facets; the highly coordinated atoms of {111} reduce the adsorption efficiency of PNP.
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INTRODUCTION Anisotropic branched metal nanostructures have recently drawn tremendous interest because of their highly tunable morphology-driven catalytic, optical, and photothermal characteristics.1−8 The properties of these unique yet complex nanostructures can be tuned by modulating the core-to-branch ratio and the number density of branches with minimum alteration of the overall dimensions.6,7,9−11 This has significant implications particularly in catalysis because (1) the size of the nanostructure is a critical parameter that controls the rate of the reaction and the induction time. Therefore, maintaining the overall size while manipulating the shape to enable catalytic activity is highly desirable.12 (2) The branched morphology provides large surface area-to-volume ratio.2,5 (3) The core and protrusions of branched nanostructures are bound by distinct crystal planes.9,10 Therefore, by altering the morphology, the crystallographic facets with high catalytic activity can be engineered to drive rapid redox reactions. While branched nanostructures offer several advantages for catalysis, due to the complexity of their geometry, a simple model reaction is necessary to quantitatively assess their catalytic activity. In this model reaction, a single reactant should result in a single product in a well-defined controlled chemical reaction with no undesirable side products. The catalytic reduction of pnitrophenol (PNP) to its amino derivative p-aminophenol (PAP), via sodium borohydride (NaBH4) is a well-studied model reaction catalyzed only in the presence of metal nanostructures. PNP catalysis to PAP is a pseudo-first-order reaction where the BH 4 − ions adsorb to the metal nanostructure surface and transfer a hydrogen species to the nanocatalyst. The hydrogen species then reduce the nitro© XXXX American Chemical Society
phenolate ions, which are also adsorbed on the nanocatalyst surface, to PAP.13 As the degradation of PNP is easy to monitor, this reaction has been used to study the catalytic efficiency of numerous shape-, size-, and compositioncontrolled metal nanostructures.1,13−30 The transformation of PNP to PAP can be precisely monitored with a spectrophotometer by following the change in absorbance of PNP at 400 nm. Furthermore, a change in temperature does not alter the mechanism of PNP catalysis, allowing us to controllably measure the kinetics and activation energy of the reaction. In our recent work, we synthesized multibranched gold nanoantennas (MGNs) by a seedless aqueous synthetic route and demonstrated highly tunable optical properties from the visible to the near-infrared by changing the core-to-protrusion ratio.10 In this work, we show that the catalytic properties of MGNs are also tunable by precise modifications of their overall morphology. By modulating the branch length and branch density, the catalytically active crystallographic facets of MGNs are selectively exposed and the efficiency of PNP reduction to PAP is systematically assessed. As discussed in our previous work by Webb et al., MGNs consist of a near-spherical core bound by a mixture of {110} and {100} crystal planes surrounded by branches that are bound by {111} planes.9,10 Numerous studies have shown that nanostructures with sharp tips, edges, and corners drive rapid catalytic reactions because of the presence of valency-unsatisfied surface atoms that act as active sites for redox reactions.2,28,31−33 Therefore, we Received: February 4, 2016 Revised: April 22, 2016
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DOI: 10.1021/acs.jpcc.6b01238 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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(with the exception of the MGNs and NaBH4, which were always on ice to ensure their stability) were put on ice, the lab bench, or in a hot water bath. The temperature of the water bath (or water placed on ice or the lab bench) was monitored by placing a wire temperature probe in a glass sheath that made contact with the water but not with any of the reagents. We did this to avoid cross contamination of the reagents. Because NaBH4 had to be ice cold, a 240 mM stock solution was used instead of a 60 mM stock solution so that less volume of the solution could be added to the cuvette. PNP, water, and gold at one of the three temperatures were combined into a cuvette. Then, the cuvette was put into the UV−vis spectrophotometer, the NaBH4 was added, and the reaction was monitored less than 10 s after removing the cuvette from the hot plate, the lab bench, or the ice. We assumed that the temperature of the solution would remain constant long enough to determine the value of the kapp. 5. Calculation of the Average Surface Areas of the MGNs. The average surface area of the MGNs was calculated by modifying the method described by Mahmoud et al.33 First, standards of known concentrations of PNTP were freshly prepared by diluting the 0.1 mM stock solution of PNTP with water. The absorbance of the standards was used to create a standard curve of PNTP (Figure S3). Then, for both morphologies of MGNs, a fixed concentration was mixed with various amounts of a 0.1 mM stock solution of PNTP and was left on a shaker at 60 rpm for 3−4 h. The samples were centrifuged at 6000 rpm for 20 min, and the supernatant was collected while the pellet containing the MGNs conjugated with PNTP was discarded. This process of centrifuging and recovering the supernatant was repeated three times, before the absorbance of PNTP in the supernatant was recorded. The absorbance was correlated to the concentration using the standard curve, and the average surface area of the two morphologies of MGNs was calculated as shown in Figure S4.
predicted that MGNs with sharp and long protrusions will show a higher catalytic efficiency for PNP reduction. On the contrary, we observed that MGNs with shorter protrusions demonstrated a stronger adsorption affinity for PNP, higher reaction rate, faster induction time, and lower activation energy for PNP catalysis relative to the MGNs with longer protrusions. Here, we have performed detailed experiments by measuring the surface area of the two morphologies of MGNs, by modulating the particle density of MGNs, by varying the PNP concentration, and by changing the reaction temperature to investigate the factors that attribute to the higher catalytic activity of MGNs with shorter protrusions. We also used the Langmuir−Hinshelwood model to determine the adsorption coefficient of PNP and BH4− and to compare how the reaction rate of the two MGNs is influenced by the nonhomogeneous binding of reactants on the catalytically active surface sites.
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EXPERIMENTAL SECTION All reagents were purchased from Sigma-Aldrich, and aqueous solutions were prepared using ultrapure, Milli-Q water (18.2 Mohm). 1. Synthesis of Multibranched Gold Nanoantennas (MGNs). MGNs were synthesized as described in our previously published report.10 Briefly, stock solutions of 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) ranging from 80−330 mM were prepared and adjusted to a pH of 7.4 ± 0.1 using 1 M NaOH. The stock solution of HEPES was diluted by adding 3 parts water to 2 parts of the HEPES stock and mixing by inversion. For every 10 mL of the diluted HEPES solution, 100 μL of 20 mM HAuCl4 was added and then immediately mixed by gentle inversion. The solution sat at room temperature for 75 min to allow for MGN growth. Postsynthesis, the MGNs were stored for up to 1 week at 4 °C. 2. Preparation of p-Nitrophenol (PNP), p-Nitrothiophenol (PNTP), and NaBH4 Stocks. A stock solution of 2 mM p-nitrophenol (PNP, 98% pure) was prepared in aqueous media and sonicated for 10 min. The PNP stock was discarded after 8 h. To make the 0.1 mM solution of p-nitrothiophenol (PNTP), 80% purity PNTP and water were combined and sonicated for at least 30 min. The 60, 160, and 240 mM NaBH4 stock solutions were prepared no earlier than 15 min before the reaction with ice-cold water. 3. Nanoparticle Characterization. The absorbance of MGNs was characterized with a Varian Cary 5000 UV−vis− NIR spectrophotometer with dual-beam capabilities. Transmission electron microscopy (TEM) micrographs of MGNs were obtained with an Osiris TEM at 200 keV. Prior to preparation of TEM grids, MGNs were centrifuged at 6000 rpm and washed twice. To determine the concentration of MGNs, we used thermogravimetric analysis (TGA)18,34−39 to correlate the mass of the MGNs to the absorbance intensity of the MGNs at the wavelength of their plasmon resonance (Figure S5). 4. Kinetic Analysis of PNP Degradation. The reduction of PNP was monitored over time in a 1 cm path length polystyrene cuvette by measuring the absorbance of PNP at 400 nm13,14,24 with a Varian Cary 5000 UV−vis−NIR spectrophotometer. The 2 mM PNP stock was diluted to 0.14−1.8 mM in the cuvette, and MGNs were added so that the final concentration of MGNs in the cuvette was in between 1.5 and 3.5 μg/mL, followed by mixing with pipetting. Immediately before the scan, ice-cold NaBH4 with a concentration of either 60, 160, or 240 mM was added to the cuvette. The reagents
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RESULTS AND DISCUSSION Multibranched gold nanoantennas were synthesized as described in our previously published report by using 4-(2hydroxyethyl)-1-piperazineethanesulfonic acid, HEPES, as the shape-directing and reducing agent.10 The protrusion lengths of MGNs are controlled by the concentration of [HEPES] and [HAuCl4] and by the pH of the aqueous media. Here, we held [HAuCl4] and the solution pH constant but varied the concentration of HEPES to generate MGNs with short and long protrusions. The protrusion length of the MGNs increases with increasing HEPES concentration in the growth solution, where HEPES plays the role of both a capping and reducing agent.9,10 HEPES contains a tertiary amine in its piperazine ring that binds to the preferred crystallographic planes of Au surface with weak adsorption on the {111} planes. As the [HEPES]/ [Au3+] ratio is increased, more molecules are available to bind to Au and to further reduce the Au3+ ions enabling growth along the ⟨111⟩ direction, which increases the size of the MGNs and gives rise to longer protrusions. Representative transmission electron micrographs of MGNs of two different protrusion lengths are shown in Figure 1a, b. Additional TEM images are provided in the Supporting Information (Figures S1, S2). The protrusion lengths direct the overall morphology of the MGNs and control the plasmon resonance peak position (Figure 1c). Here, we observe that MGNs with shorter branch lengths have a bonding plasmon resonance at 680 nm, referred to as MGN680s, while those with longer protrusions have B
DOI: 10.1021/acs.jpcc.6b01238 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Morphological control of catalytic reaction was examined by monitoring the model reaction of PNP reduction to PAP in the presence of MGNs and NaBH4 (Figure 2a, b). The basic
Figure 1. Morphological and optical characterization of multibranched gold nanoantennas (MGNs). TEM of MGNs (a) with short protrusions and (b) with long protrusions. The scale bar is 50 nm in the inset of a. (c) Extinction spectra of the MGNs with plasmon resonance at 680 nm (short protrusions) and at 800 nm (long protrusions).
Figure 2. Spectral monitoring of PNP catalysis of (a) 680 nm MGNs and (b) 800 nm MGNs. (c) Apparent rate constant, kapp, obtained from the linear fit of PNP catalysis over time.
bonding plasmons at 800 nm, referred to as MGN800s. Both MGNs also demonstrate antibonding peaks at ∼520 nm contributed by the core plasmons. Plasmon resonance red-shifts with increasing protrusion lengths because of phase-retardation effects resulting from the larger size of the MGNs.10 The peak at 1100 nm for the MGN800s is due to a higher density of protrusions and results from plasmon hybridization between the multiple protrusions on the same nanoantenna.6,40,41 The tip-to-tip length of the MGN680s is 46 ± 3 nm, and the tip-totip length of the MGN800s is 55 ± 4 nm. Typically, MGN680s demonstrated a protrusion length of ∼13.5 nm, and MGN800s demonstrated a mixture of long branches of ∼30 nm and short branches of 12 nm as described in our previous work.10
environment induced by the BH4− ions results in the formation of nitrophenolate ions, which have a distinct absorption peak at 400 nm. NaBH4 is added in excess in this reaction; therefore, it is a reasonable assumption that its concentration remains relatively constant throughout the catalytic reduction.14 This implies the reaction order is determined only by the adsorbed PNP on the Au surface, allowing us to use pseudo-first-order reaction kinetics with respect to PNP.24,30 As the catalytic reaction proceeds, the 400 nm peak decreases and a 300 nm peak arises. The peak at 300 nm represents the generation of PAP. This observation is consistent with that reported C
DOI: 10.1021/acs.jpcc.6b01238 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. Apparent rate constant, kapp, for 680 nm MGNs and 800 nm MGNs at (a) different particle densities of MGNs and (b) different PNP concentrations performed at room temperature (21 °C). The error bars represent standard deviations obtained from three or more trials.
previously for other metal nanostructures.13,14,33,39 The 400 nm absorbance peak does not overlap with plasmon resonances of the MGNs enabling us to monitor the change in absorbance of PNP over time. The reaction rate and the induction time needed to initiate the catalytic reaction strongly depend on the shape and size of the nanocatalysts used; a clear difference is observed between the two morphologies of MGNs. To understand how the overall geometry of the MGN680s and MGN800s controls the kinetics of PNP conversion to PAP, we examined the correlation between ln(At/A0) versus time, where A0 is absorbance at t = 0, and we observed a linear behavior as expected for a pseudo-first order reaction (Figure 2c). The apparent reaction rate, kapp, for both MGNs morphologies was determined from the negative slope of the linear fits. The induction time (t0) and apparent rate constant for MGN680s were t0 = 0.0833 min and kapp = 1.60951 min−1 and for MGN800s were t0 = 0.253 min and kapp = 0.6252 min−1. These t0 values indicate that the PNP catalysis initiates rapidly on the MGN680s relative to the MGN800s; the kapp values indicate the reaction proceeds nearly 2 times faster on the MGN680s than on the MGN800s. This suggests the size and corresponding surface area of the MGNs, and the catalytically active crystallographic planes influence the adsorption of the reactants and likely correlate to the overall performance.14 The size of the metal nanostructures plays a dominant role in reaction rate; the rate constant is known to increase with a decrease in size because of higher surface area to volume ratio of smaller nanostructures.18,31,42 To understand the mechanism of catalytic progression on the two different morphologies of MGNs, we determined the surface area of MGN680s and MGN800s by following the method outlined by Mahmoud et al.33 The MGNs were functionalized with p-nitrothiophenol (PNTP), which binds to Au surface via thiol linker. By evaluating the amount of PNTP bound to the MGNs surface and by correlating to the standard curve generated for PNTP (see Figure S3), an adsorption isotherm was created and the surface area of the two MGNs was calculated (see Figure S4). To be consistent, we kept the masses of both MGNs similar by correlating the absorbance to the mass of gold obtained from thermogravimetric analysis (TGA, see Figure S5). Surprisingly, the surface areas of the two different MGNs were relatively similar; MGN680s had a surface area of 1.08 × 105 m2/g and MGN800s had a surface area of 1.19 × 105 m2/g. In simple isotropic nanostructures, such as spheres and cubes, the surface area is a function of its size. However, in complex
nanostructures, the anisotropy of the morphology impacts the overall surface area.3 This nearly equivalent surface area of the MGNs despite the variation in their size is likely attributable to the similar branch-to-core ratio of the MGNs. The MGN680s have a branch-to-core ratio of 0.964, and the MGN800s have a ratio of 0.857 for the short protrusions and 2.286 for the long protrusions. In the MGN800s, only one to two branches (out of five to seven total branches) grew longer than the remaining branches. Therefore, the observed differences in t0 and kapp for PNP catalysis cannot be correlated to surface area of the two MGNs. To further evaluate other parameters that contribute to the differences in the catalytic efficiency of MGN680s and MGN8000s, we also varied the particle density of MGNs and the concentration of PNP while maintaining the temperature at ambient conditions (see Figures S6 and S7). By varying the concentration of the MGNs, we determined the ideal regime where maximum catalytic conversion of PNP to PAP can be achieved with desirable kapp values (Figure 3a). In general, the higher the kapp, the faster the catalytic reaction and the higher the efficiency of the nanocatalyst utilized. The [PNP] in the reaction mixture was held constant at 0.131 mM, [NaBH4] was held at 30 mM, and particle density of MGNs was varied. We observed a systematic increase in kapp with an increase in the concentration of both MGN680s and MGN800s from 1.4 μg/mL to 2.8 μg/mL in the reaction solution. This clear trend in kapp suggests that as the amount of nanocatalysts increases, a larger number of surface sites are available for the adsorbed reactants improving the kapp. A decrease in kapp beyond 2.8 μg/mL MGNs indicates that the particle density of MGNs in the reaction solution was too high for the reaction to be considered first order. Aggregation of MGNs at higher concentrations decreases the overall surface energy available for PNP reduction resulting in less efficient conversion of PAP.43 Therefore, the optimal amount of nanocatalyst required for both MGN680s and MGN800s is ∼2.8 μg/mL. Table S1 in the Supporting Information provides all the individual values of kapp for all the MGNs concentrations shown in Figure 3a. To further understand the driving factors that contribute to the differences in catalytic properties of the two different morphologies of MGNs, we also altered the concentration of PNP while holding the amount of MGNs constant at 2.8 μg/ mL (see Table S2). As the concentration of PNP increased (Figure 3b) from 0.84 to 1.75 mM, the kapp of PNP catalysis decreased systematically for both the MGN680s and the MGN800s. The kapp depends on [PNP] because both D
DOI: 10.1021/acs.jpcc.6b01238 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C nitrophenolate and BH4− ions compete for active sites on the MGNs surface, allowing reactions to occur only for ions adsorbed on the Au surface. If most of the surface sites are occupied by a single species, the reaction will proceed relatively slowly, which is observed for increasing PNP concentration.14 A similar observation has been reported previously by other authors and can be well predicted by the Langmuir− Hinshelwood (LH) model.14,44 By fitting the change in kapp with increasing [PNP] to the LH model, we determined the true rate constant, k, of the reaction to ultimately understand the factors that contribute to the observed differences in the catalytic efficiency of the two morphologies of MGNs. The LH model is given by kapp =
Table 1. Values of the Coefficients Obtained by Fitting Langmuir−Hinshelwood Model to the Kinetics of PNP Degradation Shown in Figure 4 MGN680s
MGN800s
1.528 0.6475 9.039 0.413 1.29
0.592 0.6873 7.185 0.957 1.26
s−1). Finally, we also observed that the exponent n was lower for MGN680s (n = 0.413) relative to the MGN800s (n = 0.957) showing that the adsorption of PNP on MGN680s is less affected by the heterogeneity of the surface sites. These results indicate that the observed catalytic behavior of the two different morphologies of MGNs is likely driven by the different crystallographic planes exposed to the adsorbed reactants. The adsorption coefficient for BH4− was relatively similar for both MGNs, and m ≈ 1 for both MGNs as expected for classical Langmuir isotherm.14,39 The parameters obtained from the LH model fits suggest that the crystallographic planes likely play a strong role in the degradation of PNP on the surfaces of MGNs. The energy required for adsorption of reactants to different crystallographic planes drives the kinetics of reactions.46 It has been demonstrated that the energetic pathways required for adsorbates to bind to less coordinated atoms of high index facets such as {100} are relatively easier to bind to than highly coordinated atoms of low index facets such as {111}.18,47 The protrusions of MGN800s are bound by the {111} facets, and MGN800s have longer and more protrusions than MGN680s which explains the better efficiency of PNP reduction by MGN680s than MGN800s. We further examined the variation in the reaction rate with temperature for the two morphologies of MGNs to evaluate if the activation energies measured from these experiments will further support our hypothesis regarding crystal planes (Figure 5). The temperature of the PNP reduction to PAP was varied between 0 °C (on ice), 21 °C (ambient temperature), and 25 °C (on hot plate). Because the reaction occurs rapidly, the temperature remained relatively constant throughout the experiments. For both morphologies of MGNs, an increase in temperature correlated to an increase in the rate of degradation of PNP (Figure 5a, b), as evidenced by the magnitudes of the slopes. The kapp obtained from the slopes systematically improved with increasing temperatures (Figure 5c). The values of the kapp at each temperature are noted in the Supporting Information (Table S3). The kapp values were then correlated to the inverse temperature (Figure 5d), and using the Arrhenius equation, we determined the apparent activation energies (EA) and frequency factor (A) of PNP catalysis occurring on the surfaces of the two morphologies of MGNs. A notable difference in EA and A was observed for both morphologies. The EA for the MGN680s was 80 J/g (A = 1.2 × 103 min−1) which is significantly lower than the EA for the MGN800s at 200 J/g (A = 1.2 × 107 min−1). The observed trends in EA for the two different morphologies of MGNs support our earlier observation that PNP reduction to PAP is driven more efficiently by MGNs with shorter protrusions. The trends observed collectively from the LH model fits (Figure 4) and temperature studies (Figure 5) confirm our hypothesis that the observed differences in PNP
n k·S ·KPNP ·[PNP]n − 1 ·(K BH4 ·[BH4])m
(1 + (KPNP·[PNP])n + (K BH4 ·[BH4])m )2
Parameters k (10−5 mol m−1 s−1) KBH4 (L mol−1) KPNP (104 L mol−1) n m
(1)
where k is the true rate constant for the reaction of the adsorbed PNP, S is the total surface area of the MGNs, and KPNP and KBH4 are the adsorption coefficients for PNP and BH4−, respectively. The exponents, n and m, relate to the nonhomogeneous binding of PNP and BH4−, respectively, on the catalytic sites present on the MGNs surface.13,14 We performed the PNP reduction reaction at 30 °C as PNP is known to have a higher solubility in H2O at this temperature enabling more reliable kapp values with increasing [PNP].45 The particle density of both morphologies of MGNs was kept constant at 2.1 μg/mL, and NaBH4 was held constant at 7 mM for all reactions in these experiments. For simplicity, we assumed that the reaction is not diffusion limited as concluded previously by Wunder et al.;14 they observed a fast mass transport coefficient for the reduction of PNP to PAP on the surface of gold nanoparticles. The change in kapp with [PNP] with the LH model fits is shown in Figure 4, and the parameters
Figure 4. Kinetics of PNP degradation to PAP at 30 °C for 680 nm MGNs (blue) and 800 nm MGNs (red) fitted with the Langmuir− Hinshelwood model. The parameters obtained from these fits are shown in Table 1.
obtained from the fits are provided in Table 1. Notably, the adsorption coefficient of PNP, KPNP, was higher for MGN680s (KPNP = 9.039 × 104 L mol−1) relative to MGN800s (KPNP = 7.185 × 104 L mol−1) indicating that PNP likely has a stronger affinity for the surface of MGN680s. In addition, the reaction rate, k, was higher for MGN680s (k = 1.528 × 10−5 mol m−1 s−1) showing that the reaction occurs relatively rapidly on their surface relative to the MGN800s (k = 0.5922 × 10−5 mol m−1 E
DOI: 10.1021/acs.jpcc.6b01238 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Temperature dependence of PNP catalysis of (a) 680 nm MGNs and (b) 800 nm MGNs. (c) The apparent rate constant, kapp, as a function of temperature, and (d) Arrhenius law plot of kapp vs 1/T to calculate the activation energy, EA, from the linear slope.
facets on the MGN680s which are catalytically more active for PNP reduction relative to MGN800s which are dominated by the ⟨111⟩ family of planes. By examining the reaction as a function of temperature, we observed that the activation energy was significantly lower for MGN680s relative to MGN800s. Further, we varied the concentration of PNP in the reaction and fit the observed trends of kapp versus [PNP] with the Langmuir−Hinshelwood model. Notably, the adsorption coefficient of PNP was higher for MGN680s relative to MGN800s suggesting that PNP adsorbs more strongly to the surface of MGN680s and is less affected by the heterogeneity of the surface sites. Therefore, by modulating the morphology of nanostructures, the catalytically active crystal planes can be tuned to enable highly rapid and efficient catalytic reactions.
reduction are due to the catalytic reactivity of the different crystalline planes of the MGN680s and MGN800s. Because HEPES binds weakly to the {111} crystal planes of MGNs allowing protrusion growth along the ⟨111⟩ direction, the MGN800s with their longer protrusions are dominated by the {111} family of facets, whereas MGN680s with shorter protrusions are mostly bound by the {100} and {110} facets.10 The surface energy of Au, similar to many other face-centered cubic (fcc) metals, decreases in the order of {110} > {100} > {111} which implies that the ⟨111⟩ family of planes is the most stable and least reactive.9 Numerous studies have demonstrated that the {111} crystal planes are less catalytically active for PNP reduction to PAP than the {110} and {100} crystal facets since the latter has a high density of atomic steps and kinks.13,20,28−30,32,46,48 Crystal facets with a high number of kinks have atoms with lower coordination numbers which render them less stable. These unstable atoms are prime locations for the adsorption of reactants, and they promote catalytic conversion. This justifies the higher kapp, faster t0, higher KPNP, and lower EA observed for the MGN680s making them more catalytically active for PNP reduction relative to MGN800s.
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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.jpcc.6b01238. Additional TEM images, data used to calculate the surface area of each type of MGNs, thermogravimetric analysis, and values of the apparent rate constant for Figures 3 and 5 (PDF)
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CONCLUSIONS In this study we performed an in-depth analysis of the catalytic activity of two different morphologies of MGNs with varying core-to-branch ratios to understand the impact of crystallographic planes on the catalysis of PNP to PAP. The MGNs with the shorter protrusions (MGN680s) catalyzed the degradation of PNP more efficiently and demonstrated a higher kapp and shorter t0 than the MGNs with longer protrusions (MGN800s). We attributed this to the presence of {110} and {100} crystal
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 615-322-6905. Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.jpcc.6b01238 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by Vanderbilt University Startup Funds and NSF EPS 1004083. H. F. Z. acknowledges support from the Department of Education for Graduate Assistance in Areas of National Need (GAANN) Fellowship under grant number P200A90323. J. A. W. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under Grant Number 1445197. The authors acknowledge Eric Talbert and Will Erwin for helpful discussions.
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