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Pt Nanoparticle Collisions Detected by Electrocatalytic Amplification and AFM Imaging: NP Collision Frequency, Adsorption, and Random Distribution at an UME Surface Cesar A Ortiz-Ledón, and Cynthia G. Zoski Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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Analytical Chemistry

Pt Nanoparticle Collisions Detected by Electrocatalytic Amplification and AFM Imaging: NP Collision Frequency, Adsorption, and Random Distribution at an UME Surface César A. Ortiz-Ledón and Cynthia G. Zoski* Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico, 88003

Corresponding Author * C.G. Zoski; Telephone: (575) 646-5292; Fax: (575) 646-2649; Email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT We demonstrate good agreement between the theoretical and experimental collision frequency of individual Pt NPs undergoing collisions at a Au UME (5 µm radius) using electrocatalytic amplification provided by 15 mM hydrazine in 5 mM PB (pH7) over 100 to 300 s. DLS measurements demonstrated that Pt NP aggregation in this solution had the least impact on NP diffusion coefficient and concentration values, which are directly proportional to collision frequency. We show that the smaller, uniform current steps are indicative of NPs of metallic radii in agreement with those determined by TEM, with corresponding larger NP diffusion coefficient and concentration, in agreement with DLS results. These contribute to the larger NP collision frequency observed experimentally. Using AFM imaging, we show good agreement between the number of NPs imaged on the UME surface and the number of NP collisions that led to their adsorption, a spherical NP shape with a metallic radius size distribution comparable to that determined by TEM, and a random NP distribution on the UME surface. Through the Pt NP electroactive surface area, we show that all NPs on the UME surface after collision are attached and electrochemically active. Collectively, these results demonstrate for the first time that within experimental error, every NP collision is successful and occurs through a sticking mechanism. Thus, collision experiments can be used to prepare small NP ensembles on a UME (i.e., UME-NPEs). In electrocatalysis, such UME-NPEs bridge the gap between classical ensemble studies on large platforms and isolated single NP investigations.


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INTRODUCTION Electrocatalytic amplification (EA), developed in the Bard group, has been used to study collisions of individual nanoparticles (NPs) on relatively inert ultramicroelectrodes (UMEs).1- 8 The detectable NP size using this method typically ranges from 4 to 70 nm in radius. In NP collision studies, a UME is biased at a potential where an inner sphere reaction9 of interest is kinetically slow and displays a minimal current response. When a NP that can catalyze the inner sphere reaction at that same potential collides and sticks to the UME surface, it behaves as a nanoelectrode and produces a step-like increase in the faradaic current relative to that of the UME. In such NP collision experiments, one can learn a lot from the collision frequency, and from the height and shape of the faradaic response. Because the collision of a NP onto a UME is stochastic, diffusion-limited mass transfer to the electrode is represented by an average collision frequency given by eq.(1)2,6,8 fdif = 4DNPCNPaNA

(1)

where fdif is the collision frequency governed by diffusion of the NP to the UME, DNP is the diffusion coefficient of the NP, CNP is the NP concentration, a is the radius of the UME, and NA is Avogadro’s number. In the model proposed by Bard, on which eq.(1) is based, all NPs are assumed to collide and stick at the electrode, with a very small total coverage. From eq.(1), one can obtain the concentration or the diffusion coefficient of the NPs. The height of the current step under mass transfer control is given by2,6 iss = 4π ln(2)nFDH CH rNP

(2)

where iss is the steady-state current for the NP, DH is the diffusion coefficient of the reactant (e.g., hydrazine, proton, etc.), n is the stoichiometric number of electrons, F is Faraday’s constant, CH is the reactant concentration, and rNP is the NP radius. Thus, from collisions



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information about the size of the NPs with known D and concentration of the reactant is also possible. However, this approach has not been routinely applied in determining the kinetics and catalytic properties of single particles for several reasons. First, the NP synthesis often results in a distribution of sizes and shapes. Relatively monodisperse samples of NPs are necessary to make a direct electrochemical comparison. Second, the current response in the amperometric i-t transient often decays over time, which implies a deactivation process. Third, NPs tend to aggregate in solution with time. Other complex effects on catalysis, e.g., heterogeneities on electrode surfaces, adsorption of molecular impurities, and capping agent, also exist. Of these, NP aggregation is especially problematic because it leads to a decrease in both NP concentration and diffusion coefficient, thus leading to a smaller collision frequency than predicted by eq.(1). The earliest discrepancy between the calculated and experimental collision fdif was reported by the Bard group, for collisions of citrate capped Pt NPs of ~3.6 nm diameter in 15 mM hydrazine + 50 mM phosphate buffer (PB, pH7.5), at a 5 µm radius Au UME.2 Experimentally, they found fdif = 0.4 s-1 in a 25 pM Pt NP solution, corresponding to DNP of ~ 1 × 10-8 cm2/s, which is ~10 times smaller than the DNP calculated from the Stokes-Einstein relationship based on the hydrodynamic radii of the NPs 2 and from reported values.10,11,12 The authors attributed this decrease in DNP to possible NP aggregation as well as to the possibility that only a small number of collisions result in NP sticking to the UME surface. They indicated that their observations were closer to a sticking frequency, which depends on an adsorption rate constant (i.e., kads (cm s-1)) rather than a collision frequency.2,6 Different electrode materials and treatments, different NP capping agents and UME surface modification (e.g., with a selfassembled monolayer (SAM)) were used to study factors influencing sticking frequency. The frequency decreased when a long chain alkyl thiol SAM was used as the NP capping group.


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Electrode surface modification with a SAM affected the frequency of collisions and totally blocked them for long chain SAMs.13 These results indicated that collision frequency is sensitive to kads and may also be a function of the location on the surface and the interaction between electrode surface and NP. The maximum collision frequency, eq.(1), results when kads is large.6 More recently, Koper reported collision frequencies for citrate capped Pt NPs of ~3.8 nm diameter in 10 mM hydrazine + 10 mM PB (pH8) solution, at a 2000 µm2 Au lithographic UME that were much smaller than values reported in the literature and those predicted from eq. (1).14 Cyclic voltammetry (CV) of the Pt-modified Au UME showed a signal characteristic of Pt, and electron microscopy showed various degrees of aggregated NPs after collision experiments were performed in the presence of hydrazine. Koper attributed the reduced collision frequency to this aggregation through a decreased effective NP concentration. Stevenson recently reported studies on the mechanism of colloidal instability and its effect on Pt NP collisions on a spherical cap Hg/Pt UME by EA.15,16 In contrast to the EA staircase response that typically results from consecutive NP collisions, a so-called “blip” response occurred that was attributed to Hg poisoning of the Pt NP catalytic surface which blocked hydrazine oxidation.17 Collisions of 50 nm diameter citrate-stabilized Pt NPs were observed over a 12 min time frame in 10 mM hydrazine in PB (pH7.8) ranging from 5 to 50 mM.15,16 The NP collision frequency was found to increase as the PB concentration was decreased to 5 mM. NP tracking analysis (NTA) measurements, acquired between 100 s and 12 min, showed that the least aggregation was observed in hydrazine solutions in 10 mM and 5 mM PB, with NP diameter and concentration changes that were smaller than those in the higher PB concentrations. These results provided evidence that each NP collision produced a current at a frequency predicted by eq. (1).



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Colloidal instability and its effects on the interpretation of NP collisions have also been reported for other NP collision systems. For example, EA based on the photocatalytic oxidation of methanol and subsequent charge transfer due to collisions of agglomerates of dye-sensitized TiO2 NPs in a colloidal suspension on a fluorine-doped tin oxide (FTO) UME was recently reported.18 NP collisions based on the direct oxidation (i.e., the so-called “nano-impact” collisions) of Ag NPs colliding on individual carbon microfibers distributed randomly in nonconductive epoxy, were used in distinguishing between agglomeration and aggregation in colloidal suspensions of electrolyte concentrations ranging from 0 to 2.5 M KCl.19,20 Here we demonstrate that the predicted (eq.(1)) and experimental frequency of individual Pt NPs of 16 ± 2 nm radius undergoing collisions over 100, 200, and 300 s after injection at a 5 µm radius Au UME with EA amplification provided by 15 mM N2H4 in 5 mM PB (pH7) are in agreement. We used DLS to explore the extent of Pt NP aggregation on NP diffusion coefficient, concentration, and hydrodynamic and metallic NP diameters in water and in PB ranging from 5 to 50 mM in the absence and presence of 15 mM N2H4. In EA collisions, the smaller, uniform current steps in 5 mM PB compared to those in 10-50 mM PB were indicative of NPs of metallic radii in agreement with that determined by TEM due to the smaller degree of NP aggregation, with a corresponding larger NP diffusion coefficient and concentration, in agreement with DLS results, and leading to the larger experimental NP collision frequency. Using AFM imaging, we show good agreement between the number of NPs imaged on the UME surface and the number of NP EA collisions that led to their adsorption, a spherical NP shape with a metallic radius size distribution comparable to that determined by TEM, and a random NP distribution on the UME surface after collision experiments, in contrast to that found for synthetic beads which have been reported to be distributed at the UME edge.21 Optical tracking of collisions in real time of


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insulating beads at a UME surface, and of nano-impact collisions of Ag NPs have also been reported as a way to visualize NPs on a UME surface.22,23 Through the Pt NP electroactive surface area, we further confirmed that all NPs on the UME surface after collision were attached and electrochemically active. Collectively, these results demonstrate for the first time, that within experimental error, every collision is successful and occurs through a NP sticking mechanism, as predicted by the Bard model for EA NP collisions.6 EXPERIMENTAL SECTION Reagents. All chemicals were of analytical grade and used as received. Potassium chloride (KCl), sodium phosphate dibasic anhydrous (Na2HPO4) (99.5%) and sodium borohydride (NaBH4) were obtained from Fisher Scientific, sodium phosphate monobasic (NaH2PO4) from EM Science, and ferrocenemethanol 97%, sodium citrate, ascorbic acid, citric acid, hydrazine solution 35% (N2H4) and hexachloroplatinic acid solution 8% (H2PtCl6) from Sigma-Aldrich. Water (18.2 MΩ cm) from an Integral3 Milli-Q system (Millipore Co., Bedford, MA) was used. Argon gas was ultrahigh purity grade. Electrodes. Au and Pt UMEs were fabricated by heat-sealing 10 µm diameter wire (Goodfellow, USA) in borosilicate glass capillaries ( 2.0 mmOD; 1.16 mmID, Sutter Instrument) under vacuum. 24, 25 Details are reported in the Supporting Information. Instrumentation. A CHI 660B electrochemical workstation and a CHI Faraday cage were used in electrochemical experiments. Filters in the CHI software were turned off prior to NP collision experiments. AFM images were recorded using a Bruker Dimension Icon Microscope with Scanasyst, 26 as reported in the Supporting Information.



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NPs were also characterized using a Hitachi H-7650 TEM by drop casting 3 µL of a calculated 7.9 nM NP stock suspension (DLS measured 7.2 ± 0.2 nM) from the NP synthesis onto a 200 mesh carbon film coated TEM grid (Electron Microscopy Sciences). The same volume of NP stock suspension was used to analyze NP on freshly peeled mica by AFM. A Malvern Zetasizer Nano S (Malvern Instruments, Malvern UK), based on a 633 nm laser with a 1730 back scatter function, was used to determine NP size, diffusion coefficient, and concentration in solution using a 12 mm square glass cuvette with a round aperture. NP synthesis. Citrate capped NPs were synthesized following a seed-mediated growth procedure for NPs of 32±3 nm diameter.27 Details are reported in the Supporting Information. DLS measurements. DLS measurements were used to determine the size of NPs (1) in the absence of aggregation (i.e., in DI water) for comparison with TEM and AFM measurements and (2) under solution conditions used in the NP collision experiments in order to investigate the effect of PB concentration and the presence of hydrazine on NP aggregation and thus on NP size and diffusion coefficient in the collision measurements. Details are reported in the Supporting Information. NP Collisions. Hydrazine oxidation was used to detect Pt NP collisions on a Au UME. This oxidation occurs at less positive potentials on Pt than on Au, which provides a wide potential window over which collisions attributed to a single Pt NP can be observed.3,5 Details are reported in the Supporting Information. Pt NP Number and Area on Au UMEs after Collisions. Experiments in which the number of NPs on the UME surface and the Pt NP area were determined and compared to the NP collisions counted experimentally were performed in 5 mM PB (pH7.00) + 15 mM N2H4, where


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NP aggregation based on DLS measurements was less than in more concentrated PB. Details are reported in the Supporting Information. RESULTS AND DISCUSSION NP characterization. NP size was determined using TEM, AFM, and DLS as described in the Experimental Section. The NP size distributions shown in Fig.1 are in good agreement with each other and with the targeted size of 32±3 nm of the NP synthesis method. TEM measurements showed an NP diameter of 33±4 nm and NPs that were of spherical shape (Fig. S2(a)). These results were in agreement with a size distribution of 33±4 nm from AFM imaging using AFM Peak force tapping in air (Fig. S-2(b)). DLS measurements performed on calculated 10 pM NP solutions in DI water showed a size distribution of 34±12 nm. The broader size distribution compared to TEM and AFM is attributed to the formation of Pt NP aggregates inherent to NP solution-based measurements. DLS: Pt NP aggregation investigation. DLS was used in exploring the effects of PB concentration and 15 mM hydrazine on Pt NP aggregation in solution. Pt NPs are negatively charged with a citrate capping agent which stabilizes the NPs, thus preventing aggregation (Fig. S-3).28,29,30 DLS measurements were made on calculated 10 pM solutions of Pt NPs in DI water and in 5, 10, 25, and 50 mM PB (pH7.00). Table S-1 lists values for NP hydrodynamic diameter and diffusion coefficient, NP concentration, and polydispersity index (PDI) (an indication of the degree of aggregation) determined by the DLS software as an average measurement over all NPs in solution. The diffusion coefficient is based on the light scattered from the Pt NPs in each PB concentration. The DLS software uses the determined NP diffusion coefficient and the Stokes-



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Einstein equation,31 assuming a spherical particle, to calculate an average hydrodynamic radius, rH, based on all NPs in solution: rH = kBT/6πηDNP

(3)

where kB is Boltman’s constant, T is temperature, and η is the viscosity of water. The NP hydrodynamic diameter, dH, includes the metal NP core diameter and hydrated layers due to the citrate capping agent and counter ions from solution (Fig. S3(a)). The metal NP core diameter, dMNP, is obtained from an algorithm based on the Mie theory in the DLS software, and can be compared directly with TEM measurements (Fig. 1) of 33 ± 4 nm. DLS also determines NP concentration as vol % of the sample, which is converted into molarity. Solutions in which there is no aggregation (i.e., monodisperse) have PDI values on the order of ≤ 0.1; PDI values > 0.1, indicate a polydisperse solution.32 Table S-1 shows that significant NP aggregation occurs in 50 mM PB, in agreement with both Koper and Stevenson.14,16 Similar values of DLS determined parameters in 5, 10, and 25 mM PB are indicative of NP size distribution and indicate the absence of aggregation, in agreement with Stevenson.16 In DI water, the DLS determined hydrodynamic NP diameter is similar to the metallic NP diameter and the NP concentration was found to be 10 pM, as prepared. We also examined the effect of 15 mM N2H4 in PB on NP aggregation, which best represents the NP environment during the electrochemical NP collision experiments. Table S-2 shows an increase in the NP hydrodynamic and metallic core diameters, and PDI, with a corresponding decrease in the NP diffusion coefficient and concentration values in PB ranging from 10 to 50 mM, compared to the baseline values shown in Table S-1. NPs in 5 mM PB were impacted the least by the presence of 15 mM N2H4. Effects that may account for the N2H4 induced aggregation of Pt NP have been reported.14,16,33,34,35


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The pH of the PB (pH7)/hydrazine solutions in Table S-2 was measured after adding hydrazine. The pH values were found to range from 7.6 to 8.6. We did not see a significant shift in potential between the different PB (pH7)/15 mM N2H4 solutions, as shown in Fig. S-4. Thus, at a potential of -0.1 V vs Ag/AgCl, the limiting current on the first wave of the N2H4 oxidation on Pt is of a magnitude which is significantly different from the current on Au (≈ 0 A). NP EA collisions: theory vs. experiment. Collisions of single Pt NP were first performed using NP calculated concentrations of 2.5, 5, 10 and 20 pM in a 25 mL solution containing 5 mM PB (pH7.00) + 15 mM N2H4 in order to confirm linearity between frequency and NP concentration as predicted by eq.(1). The 5 mM PB concentration was used due to the smallest degree of NP aggregation found in this solution (Table S-2). NP collisions were recorded by applying -0.1 V vs. Ag/AgCl (KCl sat’d) to a Au UME for 100 s followed by injection of an aliquot of a NP stock solution corresponding to a specific calculated NP concentration. NP collisions were then identified from the amperometric i-t curves (Fig. S-5(a)) and counted over a time frame of 300 s after injection. A frequency was obtained from these experiments by dividing the number of NP collision events over time. A linear dependence of frequency on NP concentration was found (Fig. S-5 (b), dashed line), as reported previously.5 Fig. S-5(b) also shows the corresponding theoretical collision frequency (i.e., solid line) based on eq. (1), for comparison. From these studies, the calculated NP concentration of 10 pM was selected for further collision investigations. This NP concentration permitted observation of the classic oxide absorption peaks in 0.1 M H2SO4 which were not possible at the lower concentrations. Higher NP concentrations resulted in increased observation of aggregated NPs, as shown in Fig. S-5(b). Pt NP EA experiments (Fig. 2) were carried out in 15 mM N2H4 in PB ranging from 50 mM (Fig. 2(a)) to 5 mM (Fig. 2(d)). The NP collision frequency increased as the PB concentration



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decreased and more negative currents were achieved in 5 mM PB over a time frame of 300 s after NP injection. The smaller current steps in 5 mM PB compared to those in 10-50 mM PB indicate collisions of smaller NPs due to the smaller degree of NP aggregation, in agreement with DLS results (Table S-2). The smaller degree of NP aggregation indicates a larger NP diffusion coefficient and concentration, thus leading to the larger NP collision frequency observed in 5 mM PB (Fig. 2(d)). The NP collision frequency, fdif , at a UME under diffusion control is defined by eq. (1). The NP experimental collision frequency using EA and calculated as fEA = number of NP collisions/time

(4)

was used in eq. (1) for NPs in each PB to calculate a value for DNP based on respective CNP values determined from DLS measurements. This DNP value was then compared to that obtained from DLS. These results are tabulated in Table 1 together with the DLS determined CNP values. Corresponding DLS determined values for the hydrodynamic and metallic NP diameters and the PDI are tabulated in Table S-3. The DNP calculated from eq. (1) based on experimental frequencies are in good agreement with those measured by DLS, within experimental error. This significant finding indicates that eq. (1) accurately describes EA NP collision frequencies based on measureable parameters that include the UME radius and NP concentration and diffusion coefficient. In addition to transport by diffusion, migration could also play a role in 5 mM PB and lead to an enhanced NP flux to the UME surface and invalidate eq.(1).7,8 To investigate whether migration contributes to the NP flux, NP collision experiments were carried out under open circuit conditions in the absence of an electric field, where NP collisions were detected by potential rather than current steps.6, 36 From the results shown in Fig. S-6, a frequency of


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0.34±0.02 s-1 was found, in good agreement with that determined using EA based on current measurement (Fig. 2(d)) and reported in Table 1. Thus migration does not contribute to NP flux in the collisions carried out in 5 mM PB. AFM imaging: NP number and distribution. AFM imaging was used to determine the number of NPs and their distribution on the AFM UME surface after collision experiments, and to provide information regarding the shape of the adsorbed NPs (Figs. 3(a) and (c)). The number of NPs on the UME surface was then compared with the number of collisions used in calculating the frequency. AFM was also used to investigate the AFM UME surface before (Fig. S-7(c)) and after (Fig. 3(a)) the collision experiments. The diameter of the AFM-UME, 9.6 µm, was calculated from the limiting current (-1.30 nA) of a steady-state CV (Fig. S-7(a)) recorded in 1 mM ferrocenemethanol/ 0.1 M KCl,37 ilim = 4nFDCa

(5)

where D = 7.0 ×10-6 cm2 sec-1.38,39 This diameter agrees with AFM results (Fig. S-7(b)) where a distance of 9.5 µm was found from the height/distance profile along the UME diameter (red line in Fig. S-7 (c)). Height/distance profiles were also used to monitor the surface roughness of the AFM UME. Typically, the roughness was on the order of ± 5 nm. The large oscillations at distances of ≈1 µm and ≈10.5 µm, correspond to the glass/metal seal. Since the NPs in 5 mM PB/15 mM N2H4 were on the order of 31±3 nm in diameter (Table S-3) and are significantly larger than the ± 5 nm roughness, they can be distinguished on the AFM UME surface. Fig. 3(b) shows EA NP collisions over a time period of 300 s after NP injection. The corresponding NP distribution on the Au UME surface is shown in the AFM image in Fig. 3(a) and the NP size distribution of 31±15 nm diameter is shown in Fig. 3(e).



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The collision frequency determined from the number of NP collisions through EA on the UME (eq. 4) can be compared to that determined through AFM from the number of NPs counted on the UME surface after the collision experiments, over the collision time measured after NP injection as fAFM = number of deposited NPs/time

(6)

Table 2 shows that these frequencies are in good agreement with each other, thus confirming that the number of NP detected electrochemically and microscopically are comparable. AFM image (Fig. 3(a)) shows that the NPs are distributed randomly across the UME surface, rather than at the edge where the current density is highest. This NP behavior, to our knowledge, has not been demonstrated previously, and is in contrast to that found for synthetic beads which were found to be distributed at the UME edge.21,22 The good agreement between the number of EA collisions with the number of NPs counted on the UME surface (Table 2) demonstrates that every collision is successful, within experimental error, and that each collision occurs through a NP sticking mechanism, as proposed by Bard et. al.6 Figs. S-8 to S-10 show results from two additional NP collision experiments including 2 and 3D AFM images, NP size distributions, and cross-section profiles. Comparison of the AFM images in these figures illustrate the varying degrees of NP aggregation that occurs as a NP moves through the solution and adsorbs on the UME surface. The degree of aggregation can be observed through the NP position and size distributions on the UME surface using AFM imaging and cross-section profiles of individual NPs and collections of NPs. NP collisions at times of 100 s after injection were also performed, as shown in Figs. 4 and S-12, and Table S-4. Here, no visible aggregates could be found on the UME surface after this short collision time, resulting in a much narrower size distribution than found for the longer 300s


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experimental collision time, and an average rNP value in good agreement with the TEM and AFM images of Fig. 1. Thus, the number of NPs colliding on the UME surface can be controlled by adjusting the duration of the collision experiment. Additionally, the shorter collision time significantly reduces NP size effects due to aggregation as shown by comparison of AFM results in Figs. 3, 4, S-8, and S-12. The average EA collision frequency (Table S-4) was found to be 0.34 ± 0.05 s-1 (representing a total of 103 collisions or an average of 34 ± 5 collisions per experiment), in good agreement with that found for the 300 s collision experiments (Table 1). Using this average fEA and the CNP value from Table 1, a DNP value of 8.3 ± 0.7 × 10-8 cm2/s was found, in good agreement with that for the 300 s collision experiments. From AFM images, the average AFM collision frequency was found to be 0.35 ± 0.06 s-1 (representing a total of 104 NPs or an average of 35 ± 6 NPs per experiment). These results are also in agreement with the EA results. Using Fig.S-12 data (left), Fig. S-13 shows an example of identifying and counting NP collisions from current step (a,c) and derivative (b,d) vs time scans and NPs on the UME surface through AFM imaging (e ). Pt NP Electroactive Surface Area. To confirm that all NPs on the UME surface were attached after a collision experiment, we determined the Pt NP electroactive surface area. Fig. 5 (a) shows the NP collisions over 200 s after NP injection, while Fig. 5(b) shows the AFM image of the NPs on the UME surface. Comparison of the CV’s in 0.1 M H2SO4 under Ar of a bare Au UME before (blue line) and after (red line) NP collisions is shown in Figs. 5 (c,d), where a positive reversal potential of 1.3 V was chosen to maximize the formation of Pt oxide, thus minimizing the formation of Au oxide in comparison to that shown in Fig. S-14. The selection of this reversal potential also avoided the oxygen evolution reaction (OER) which starts ≈1.3 V vs. Ag/AgCl for Pt compared to Au with an onset potential of ≈ 1.55 V vs. Ag/AgCl. The Au/Pt 15 ACS Paragon Plus Environment


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NP UME does not develop the classic pronounced hydrogen adsorption/desorption peaks observed at a continuous Pt UME (Fig. S-14) due to the small amount of Pt deposited on the Au surface.40,41 However, the increase in current above that at the bare Au UME in the hydrogen adsorption/desorption potential regions indicates the presence of Pt NPs. The Pt NP modified Au UME also shows enhanced cathodic and anodic currents in potential regions (~ 0.5 V vs Ag/AgCl) corresponding to Pt oxide reduction and formation typical of a continuous Pt UME (Fig. S-14) and in comparison to the bare Au UME. Thus, the Pt oxide reduction peak was used to calculate the electroactive surface area, AESA, of Pt NPs on the Au UME surface through the charge calculated from integration of the background-subtracted current under the peak and division by the constant 420 µC cm-2 and scan rate.42 The AESA determined for the electrode shown in Fig. 5(b) was (3.1 ± 0.1)× 10-9 cm2 (Table S-5) and is within experimental error of the average AESA of (2 ± 1) × 10-9 cm2 calculated from the electroactive surface area expression for a collection of spherical NPs = 4

(7)

where rNP is the average NP radius found from the histogram (Fig. 5(e)) of the NPs in the AFM image (Fig. 5(b)) and N is the corresponding number of NPs on the UME surface also determined from the AFM image. The results from two additional representative electrodes (Table S-5) are shown in Figs. S-15 (b) and S-16 (b) where AESA values of (2.4 ± 0.1) × 10-9 cm2 and (2.8 ± 0.1) × 10-9 cm2 respectively were determined from the Pt oxide reduction peak; these values are within experimental error of the average AESA of (2 ± 1) × 10-9 cm2 calculated from eq. (7). The good agreement between the number of NP collisions and the number of NPs counted on an electrode surface from corresponding AFM images, between the EA and AFM collision frequencies, and between the NP electroactive surface area calculated from the Pt oxide 16 ACS Paragon Plus Environment

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reduction peak and that using eq. (7) based on the average NP radius determined from the AFM histograms and the number of NPs counted on the AFM image indicate that each NP colliding at the UME surface attached to the surface and contributed to the total Pt NP electroactive surface area. The distribution of the average NP diameter determined from the AFM histograms, is up to three times wider than that shown for the TEM and AFM histograms (Fig. 1) using NP colloidal solutions drop cast onto a grid or mica surface, but of approximately the same magnitude as was found for the DLS histogram. This is consistent with NP aggregation as discussed earlier. Representative collision experiments, shown in Figs. S-17 to S-19, were also performed for a collision time of 300 sec after NP injection. This longer collision time resulted in a NP average diameter distribution (Table S-6) which was up to five times wider than that shown for the TEM and AFM histograms in Fig.1. There was also less agreement between the number of NP collisions and the number of NPs counted on an electrode surface from corresponding AFM images, between the EA and AFM collision frequencies, and between the NP electroactive surface area calculated from the Pt oxide reduction peak and that using eq. (7) based on the average NP radius determined from the AFM histograms and the number of NPs counted on the AFM image. This is consistent with a larger degree of NP aggregation in solution, as can be seen on the AFM images, and compared with the 200 s collision time. Experiments using a collision time of 100 sec after NP injection were also performed where, as discussed in the previous section (Fig. 4, Fig. S-12, Table S-4), the diameter of the NPs and their distribution determined from the AFM images of the UME surface after collision was found to be similar to that determined by TEM (Fig. 1). This indicates that little aggregation occurred in solution over the 100 s time frame compared to the 200 s and 300 s collision times. The



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average number of NPs attached to the UME surface through the collisions (Table S-4), 34 ± 5, was not large enough to collectively create an area where a Pt oxide peak could be distinguished from the background current of the continuous Au UME shown in Fig. S-14. Therefore, we were unable to obtain a quantitative AESA from these experiments. However, an average rNP value of 15±2 nm was evaluated from AFM images Fig.4(a,c) and Fig. S-12 (a) where the NP distribution across the UME surface could also be visualized. These results demonstrate that the AESA calculated from the Pt oxide reduction is in good agreement with that calculated using the average rNP, based on the size distribution and the number of attached NPs found from AFM images for each electrode. Additionally, collision experiments can be used to isolate electrochemically active NPs on a UME, as was also recently shown using tunneling ultramicroelectrodes (TUMEs) to capture a single NP.43 Moreover, the impact of NP aggregation on deposited NPs can be minimized by decreasing the collision time. Thus, NP EA collisions can be used to prepare small NP ensembles on a UME (i.e., UMENPEs). In electrocatalysis, such UME-NPEs bridge the gap between classical ensemble studies on large platforms and isolated single NP investigations.40,44 CONCLUSIONS We have demonstrated that the predicted and experimental frequency of individual Pt NPs undergoing a collision at a Au UME with detection by EA are in agreement. DLS measurements were used to explore the extent of Pt NP aggregation on NP diffusion coefficient and concentration values, which are directly proportional to the collision frequency. Using DLS measured NP concentration in 5 mM PB (pH7)/15 mM N2H4 and the corresponding experimental collision frequency in the collision frequency equation, led to the calculation of a NP diffusion coefficient which was in good agreement with that measured by DLS. This


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agreement provided confirmation of the collision frequency equation predicted by the Bard model of NP collisions. Each individual NP collision led to a staircase i vs t response, which indicated that a colliding NP adsorbed on the UME surface. AFM imaging showed that the colliding NPs adsorbed randomly across the UME surface. The number of NPs imaged by AFM agreed with the number of NP collisions. Histograms from the AFM images, in comparison to those from TEM images, showed that the average NP size was unchanged after adsorbing to the UME surface. The duration of the collision experiments after NP injection affected the distribution from the mean value of the NP histograms, with collision times on the order of 100 s being closer to that found from TEM images while collision times on the order of 300 s resulted in deviations from the mean value up to five times due to larger, aggregated NPs diffusing to and adsorbing on the UME surface. The Pt NP electroactive surface area was determined from the Pt oxide reduction peak of a cyclic voltammogram through the charge calculated from integration of the background subtracted current under the peak. This area agreed with that calculated from the mean NP radius of the AFM histograms and the number of NPs counted on the AFM images, thus confirming that all NPs on the UME surface after a collision experiment were attached and conductive. Collectively, these results demonstrate for the first time that within experimental error, every NP collision is successful and occurs through a NP sticking mechanism, as predicted by the Bard model. These results also demonstrate that small ensembles of NPs can be prepared on a UME (i.e., UME-NPEs) using collision experiments. Such UME-NPEs offer advantages in



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electrocatalysis related to the ability to access kinetics under high mass transport, steady-state diffusion which is difficult to achieve at macro-NPEs.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (C.G.Z., CHE-1408608) and the Mexican Council of Science and Technology (CONACYT) (C.A.O.L., doctoral fellowship No. 359346). Supporting Information AFM setup, experimental details, and additional figures and tables of NP collision data, AFM images, image analysis, and Pt NP area determination, as noted in the text. This material is available free of charge via the internet at http://pubs.acs.org.


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Table 1. DNP values measured using DLS and those calculated from experimental collision frequencies (eqs. (1), (4)) using Pt NP concentrations measured by DLS. The calculated NP concentration was 10 pM. Each value represents nine independent measurements. The total number of NP EA collisions observed for 300 s after NP injection are reported for each PB. DLS measurements for dH, dMNP, and PDI are reported in Table S-3. PB/15 mM N2H4 50 mM 25 mM 10 mM 5 mM

DLS DNP/10-8 cm2/s 2.0 ± 0.2 2.3 ± 0.2 4.8 ± 0.1 7.2 ± 0.1

DLS CNP/ pM 2.1 ± 0.3 1.8 ± 0.3 1.7 ± 0.4 3.5 ± 0.5

EA fEA/ Hz 0.040 ± 0.004 0.060 ± 0.009 0.13 ± 0.01 0.34 ± 0.02

EA #Collisions 108 162 351 918

EA DNP /10-8 cm2/s 1.6 ± 0.2 2.8 ± 0.5 6±1 8.2 ± 0.8

Table 2. Comparison of the average frequency and the corresponding total and average number of NPs on the UME surface determined using AFM with the NP EA frequency determined for 300 sec after injection and the corresponding total and average number of NP EA collisions. Each value represents six independent measurements. PB/15

AFM

AFM

AFM

EA

EA

EA

mM N2H4

fAFM/ Hz

# NPs

Avg. # NPs

fEA/ Hz

# Collisions

Avg. # Collisions

5 mM

0.35±0.06

629

105±19

0.36 ±0.05

644

107±16



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FIGURE LEGENDS

Figure 1. Pt NP size distributions obtained using TEM, DLS, and AFM of NP colloidal solutions in ultrapure water. TEM and AFM measurements were made from a NP colloidal solution drop cast on a C-coated grid and mica, respectively. DLS measurements were made on nominal 10 pM NP solutions in DI water.

Figure 2. NP collisions of 33 nm diameter Pt NPs of 10 pM nominal concentration in PB (pH7) + 15 mM N2H4 on a Au UME. PB: (a) 50 mM; (b) 25 mM; (c) 10 mM; (d) 5 mM. UME potential: -0.1 V vs. Ag/AgCl (KCl sat’d). Data acquisition: 50 ms.

Figure 3. EA NP collisions on a Au UME for 300 s after NP injection. (a) AFM image of the UME surface after NP collisions. (b) NP collisions from a nominal 10 pM Pt NP solution in 5 mM PB (pH7) + 15 mM N2H4. (c) Zoomed image of NPs from (a). (d) Height profiles of circled NPs in (c). (e) NP size distribution from AFM image (a).

Figure 4. EA NP collisions on a Au UME for 100 s after NP injection. (a) AFM image of the UME surface after NP collisions. (b) NP collisions from a nominal 10 pM NP solution in 5 mM PB (pH7) + 15 mM N2H4. (c) Zoomed image of NPs in (a). (d) Height profiles of circled NPs in (c). (e) NP size distribution from AFM image (a).

Figure 5. Pt NP electroactive surface area determination on a Au UME before (blue) and after (red) NP EA collisions for 200 s after NP injection. (a) NP collisions from a nominal 10 pM Pt NP solution in 5 mM PB (pH7) + 15 mM N2H4. (b) AFM image of the UME surface after NP collisions in (a). (c) CVs of bare Au and Pt NP/Au. Scan rate = 0.05 V/s. Solution: 0.1 M H2SO4 under argon. (d) Zoomed Pt oxide reduction/formation region from (c ). (e) NP size distribution from AFM image (b).


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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