Supporting Information for: Transient Electrocatalytic Water Oxidation in Single Nanoparticle Collision Fan Zhang, Peter A. Defnet, Yunshan Fan, Rui Hao, and Bo Zhang * Department of Chemistry, University of Washington, Seattle, WA 98195-1700 zhangb@uw.edu Contents: S1. Videos S2. Silver Nanoparticle Syntheses S3. Summary of Collision Results for Different NPs and Carbon Electrode S4. Characterization of Ag NPs S5. Cyclic Voltammetry of a Gold Ultramicroelectrode S6. Current-time Traces of 40 nm Ag NPs under Different ph at 1.1 V S1
S1. Videos Video 1: A video (playing at 7 frames/s) showing the dynamic collision/oxidation process of a 110 nm Ag nanoparticle on a 20-nm-thick gold film electrode at 0.9 V (corresponding to Figure 7 in the main article). The video was recorded with a frame rate of 19.8 Hz and an exposure time of 50 ms. S2. Silver Nanoparticle Syntheses The 110 nm silver nanoparticles were prepared following a citrate reduction method. 1 All glassware was first rinsed with aqua regia (3:1 HCl: HNO 3 ) prior to use to remove any adsorbed metal or organic contamination. Caution! Aqua regia is extremely corrosive and may result in explosions or skin burns if not handled with extreme caution. Following the aqua regia treatment, all glassware was heavily rinsed with DI water. A 250 ml 3-neck round bottom flask was fitted with a condenser and the remaining 2 necks were closed with stoppers. 100 ml of aqueous 1 mm AgNO 3 was heated to a boil. While stirring, 2 ml of 1% (w/w) trisodium citrate was added in two 1 ml aliquots over a 20 second period. The solution was refluxed for 1 hour, and thereafter cooled to room temperature. The nanoparticles were isolated by centrifugation at 8000 rpm for 30 minutes. The supernatant was removed and the particles were redispersed in 10 mm trisodium sodium citrate solution. The solution was sonicated for 5 minutes to encourage redispersion. This cycle of centrifugation, redispersion, and sonication was repeated for a total of 3 times prior to nanoparticle collision experiments. The 110 nm silver nanoparticles were characterized with nanoparticle tracking analysis as shown in Figure S2. The 60 nm silver nanoparticles were prepared following a seed-growth method. 2 Silver seeds approximately 15 nm in diameter were first synthesized, and grown to 60 nm using 19 repeated growth steps. Prior to use, all glassware was first rinsed with aqua regia to remove any adsorbed metal or organic contamination. Silver seeds approximately 15 nm in diameter were synthesized by heating 100 ml of an aqueous solution containing 5 mm trisodium citrate and 0.05 mm tannic acid solution to a boil in a 3- neck round bottom flask. The flask was fitted with a condenser and the remaining 2 necks were closed with stoppers. While stirring, 1 ml of 25 mm AgNO 3 was added to the solution turning it dark brown. The solution was refluxed for 1 hour and thereafter cooled to room temperature. The nanoparticle seeds were isolated by centrifugation at 8490 rpm for 30 minutes. The supernatant was removed and the particles were redispersed in 2.2 mm trisodium citrate solution. The solution was sonicated for 5 minutes to encourage redispersion. This cycle of centrifugation, redispersion, and sonication was repeated a total of 3 times before continuing. The silver nanoparticle seeds were grown to approximately 60 nm using a series of 19 growth steps. In each individual step, 19.5 ml of seed solution and 16.5 ml of DI water were added to a 100 ml 3-neck round bottom flask. The flask was fitted with a condenser and the remaining 2 necks were closed with stoppers. The solution was heated to 90º C, and 500 µl of 25 mm trisodium citrate, 1.5 ml of 2.5 mm tannic acid, and 1 ml of 25 mm AgNO 3 were individually added with a delay of about 1 min each. The solution was stirred at 90º C for 25 minutes and thereafter was cooled to room temperature. S2
The resulting nanoparticle solution was used as the seeds in the next growth step. To prepare the 19 th generation for nanoparticle collision experiments, the particles were isolated by centrifugation at 6000 rpm for 30 minutes. The supernatant was removed, and the particles were redispersed in 2.2 mm trisodim citrate solution. The solution was sonicated for 5 minutes to encourage redispersion. This cycle of centrifugation, redispersion, and sonication was repeated a total of 6 times, until the supernatant was clear, indicating that the excess tannic acid had been removed. The 60 nm silver nanoparticles were characterized by TEM (Figure S1). S3
S3. Summary of Collision Results for Different NPs and Carbon Electrode Table S1. Statistics for collision events on a gold UME at 1.1 V. ph of Ag NP solution is 7.8. NP diameters are 40, 60 and 110 nm. NP Number Q/fC SD Q/fC i τ /pa SD i τ /pa τ/ms Ratio of diameter/nm of events peaks with i τ 40 94 1114 910 235 111 102 ~100% 60 71 9580 9208 577 324 955 ~100% 110 50 31073 30930 773 423 2250 ~100% Table S2: Statistics for collision events of 60 nm Ag NPs on a gold UME at different ph and potentials. Potential/V Number of events ph Q/fC SD Q/fC i τ /pa SD i τ /pa τ/ms Ratio of peaks with i τ 0.6 45 5.2 274 265 ~0 37 7.8 243 224 ~0 51 9.7 395 318 93 61 23 84% 1.0 62 5.2 5879 9004 210 144 921 89% 89 7.8 4350 3944 281 243 850 92% 71 9.7 8835 10429 403 273 951 ~100% Table S3. A summary of 40 nm Ag NP collisions on a carbon fiber microelectrode at 1.1 and 1.0 V. Potential/V Electrolyte Number of events Q/fC SD Q/fC i τ /pa SD i τ /pa τ/ms Ratio of peaks with i τ 1.1 KNO 3 104 808 884 172 91 337 ~100% KCl 35 56 72 0 1.0 KNO 3 118 323 286 149 92 59 72% KCl 60 123 174 0 S4
S4. Characterization of Ag NPs Figure S1. Example TEM image (a) and size distribution (b) of the 60 nm citrate-capped Ag NPs used in NP collision experiments. (a) (b) The 60 nm Ag NPs prepared in-house were characterized by imaging with transmission electron microscopy (TEM). Ag NPs were drop-cast onto a carbon-coated Formvar copper TEM grid (Ted Pella) and imaged with an FEI Tecnai G2 F20 TEM operating at 200 kv (Figure S1a). NP sizes were measured using ImageJ. The histogram of the NP size distribution was created using Origin 8.5 (OriginLab) (Figure S1b). The particles were approximately spherical with an average diameter of 61 ± 13 nm. Using this size and the amount of Ag ions used in the synthesis, an upper limit for the particle concentration in the stock solution was calculated to be 1.3 10 11 NPs/mL. S5
Figure S2. Size distribution of the 40 nm (a), 60 nm (b) and 110 nm (c) citrate-capped Ag NPs used in NP collision experiments from Nanoparticle Tracking Analysis (NTA, NANOSIGHT). As shown in Figure S2a, commercial 40 nm Ag NPs show a main peak at 41 nm on distribution curve, which is in good agreement with its label. A small peak at 132 nm can be explained by further analysis. Although distribution curve of 60 nm NP show a peak at 60 nm and a shoulder at about 110 nm (Figure S2b), the latter is missing in TEM result (Figure S1b). The result suggests that the concentration of larger NPs tend to be overestimated by this light-scattering technique, as they scatter more 3,4. As a result, real average diameter of 40 nm and 60 nm NPs should match with their names. For 110 nm NPs, its distribution curve shows a main peak at 103 nm, as well as three subpeaks at larger values, suggesting this NP solution is more polydispersed than 40 and 60 nm NPs. As discussed above, it s highly possible that ratio of NPs with diameter larger than 178 nm is overestimated. Considering this, we estimate the mean diameter of NPs in Figure S2c to be 110 nm. Using this size and the amount of Ag ions used in the synthesis, an upper limit for the particle concentration in the stock solution was calculated to be 1.4 10 10 NPs/mL. S6
S5. Cyclic Voltammetry of a Gold Ultramicroelectrode Figure S3. A cyclic voltammogram at 50 mv/s of a bare 12.7-µm Au UME measured in 1 mm FcMeOH, 0.2 M sodium sulfate aqueous solution. S7
S6. Current-time Traces of 40 nm Ag NPs under Different ph at 1.1 V Figure S4. Current-time traces at 1.1 V of 30 pm 40 nm Ag NPs at a 12.7-µm Au UME in ph 5.2 (a) and ph 9.7 (b) solution. Here the tail current height is higher at ph 5.2 than ph 9.7, which is different from 1.0 V case (Table 2 and Table S2). The contradictory can be explained by a possible mechanism. Faster decay of tail current at ph 9.7 suggests Ag/Ag 2 O NP probably quickly desorbs from Au electrode after collision. As discussed in main text, at 1.1 V potential under ph 9.7, oxygen evolution reaction has fastest rate among all conditions in this work. Therefore, it s reasonable to assume that fast oxygen formation at gold electrode surface somehow leads to quicker desorption of Ag NPs, decreasing formed Ag 2 O amount, thus reducing tail current originating from Ag 2 O catalyzed water oxidation. S8
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