Confined Electrochemical Behaviors of Single Platinum Nanoparticles Revealing Ultrahigh Density of Gas Molecules inside a Nanobubble

0.055 ± 0.001) × 10 −9 m 2 /s; and the value is approximately 2 order of magnitude less than that for bulk water. These results suggested that the oxygen molecules inside the nanobubble remained in a very stable state and mainly moved inside the spherical cap.


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Figure S1
Characterization of C UME. S3

Figure S3
The verification of current spikes S5

Figure S5
Contact angles of glass carbon plates after hydrophilic pretreatment S7

Figure S6
Electrochemical behaviors of individual PtNPs on hydrophilic C UME S8

Figure S7
Cyclic voltammograms of Pt UME in HClO4 and H2O2 S9

Figure S9
Electrochemical behaviors of individual PtNPs in HClO4 with PEG S12

Figure S10
Zeta potential distribution of PtNPs S13

Figure S11
DLS profile of PtNPs S14

Figure S13
TEM images of PtNPs before and after nanocollision experiments S17

Figure S15
The rate of H2O2 decomposition at PtNPs S21

Figure S16
The nanobubble formation time at PtNPs by UV-vis spectra S22

Figure S17
The nanobubble formation time at PtNPs by nanocollision electrochemistry S23

Figure S18
MD simulations of oxygen molecules diffusion coefficient S24

Table S1 and S2
Model parameters and potential functions S27 A disk UME surface was observed from the microscopic image of C UME (Figure S1a).Furthermore, we estimated the radius of the C UME (r), using the limiting current by the equation S1:

S3
Where   is the Boltzmann constant (1.38×10 -23 J• K -1 ), i is the mass-transfer controlled limiting current, n is the number of electrons involved in the electrochemical reaction (n = 1 for oxidation of ferrocenylmethanol), F is the Faraday constant (9.64853 × 10 4 C• mol -1 ), D is the diffusion coefficient of ferrocenemethanol as a reacting species (7.6 × 10 -6 cm 2 s -1 ), and C is the bulk concentration of the reacting ferrocenylmethanol solution (0.9 mM).From the limiting current, the radius of C UME was calculated to be 3.5 μm (Figure S1b).To confirm whether the dynamic interaction of NP-electrode affects the current responses, we further investigated the electrochemical behaviors of individual PtNPs at a C UME with the hydrophilic pretreatment.

S4
The hydrophilic pretreatments of the glass carbon thin slice or carbon fibers were carried out in a round-bottom flask with 3:1 (v/v) mixture of concentrated H2SO4/HNO3 solution for 15 min sonication at 60 ℃.S1, S2 Figure S5 showed that the contact angle decreased from 77˚ to 42˚ after the glass carbon thin slices were treated with H2SO4/HNO3 solution.The contact angle measurements indicated that a pretreated carbon surface with superior hydrophilicity was obtained.This is because that the oxygenated groups were introduced after the pretreatment with the concentrated H2SO4/HNO3 solution, resulting in the local hydrophilicity of the carbon surface.Gaussian fits.The data were obtained from a large population of proton reduction events of individual PtNPs (more than 1000 events).

S8
To exclude the interaction effect between the NP and the electrode during the electrochemical measurements, we further examined the electrocatalytic proton reduction of individual PtNPs in a 50 mM HClO4 solution containing 2 mM H2O2 at a H2SO4/HNO3-pretreated C UME (Figure S5).We observed current traces with an amplitude of 18.41 ± 0.13 pA and a duration of 0.23 ± 0.01 ms (Figure S6), which were close to the experimentally measured results at an untreated C UME.This suggests that the hydrophilic pretreatment of C UME does not affect the current responses of PtNPs at the electrode interfacee, thus excluding the interaction effect of NP-electrode to the current signals.and with (red) 5 mM H2O2.

S9
When PtNPs are dispersed in the solution, only decomposition of H2O2 occurred on PtNPs, resulting in the formation of oxygen bubble on the surface of PtNPs.As the particles approach the electrode surface, hydrogen evolution reaction takes place on the PtNPs.To study the competing effect of H2O2 decomposition on the current amplitude for hydrogen evolution reaction, we examined the voltammetric response of a 5 μm Pt UME in 50 mM HClO4 solution with and without H2O2.In previously obtained results, the reduction reactions of O2 and H2O2 on individual PtNPs were recently reported.S3-S5 However, experimentally, negligible differences were observed at a 5 μm Pt UME in 50 mM HClO4 solution regardless of H2O2 or a lack thereof.As can be seen in Figure S7, a cathodic current can be observed at the 5 μm Pt UME at the potential from +0.40 V vs.
Ag/AgCl wire, in which the reduction of H2O2 occurred.However, cyclic voltammograms performed in argonsaturated 50 mM HClO4 solution without (black) and with (red) 5 mM H2O2 showed nearly perfect overlap in H + reduction currents, indicating H2O2 reduction contributed negligibly to currents at potentials negative of -0.30V vs. Ag/AgCl wire.This is because that electrochemical reduction of H2O2 was probably inhibited by H adsorption at Pt, which occurred before H2 evolution.S6,S7 Therefore, the decomposition and reduction of H2O2 contributed negligibly to HER current at potentials more negative than the H-adsorption region (< -0.30V vs. Ag/AgCl wire).To investigate the reduction of O2 nanobubble on a single PtNP, we employed the microjet collision method during the electrochemical measurements.This method used a pressure-driven flow to transport PtNPs from a glass micropipette onto a detecting C UME placed ∼10 μm away (Figure S8a).In brief, a glass micropipette with ∼5 μm diameter orific (prepared by pulling borosilicate glass capillary tubes) was filled with PtNPs in target solution, connected to a Femtojet microinjector, and attached to a micropositioner so that the orifice was

S11
dipped in the electrolyte solution.The C UME was then connected to a separate micropositioner directly opposite from the micropipette's position.The positions of the micropipette and the C UME were adjusted until both were in focus using a 20× objective, and were approximately 10 μm apart facing each other.
Firstly, we attempted to transport PtNPs in 10 mM NaClO4 solution from a glass micropipette onto a detecting C UME in 50 mM HClO4 solution containing 10 mM NaClO4.Rapid, consistent, and repeatable single-particle collision events in HClO4 solution were observed for the proton reduction with a current height of 47.10 ± 0.24 pA (Figure S8b), which is closed to the current signals obtained from our free diffusion collision experiment.The similarity of these two current signals confirmed the feasibility of our microjet collision method.On this basis, we used the microjet collision method to transport PtNPs in 5 mM H2O2 solution from a glass micropipette onto a detecting C UME in 50 mM HClO4 solution containing 10 mM NaClO4.As expected, the current amplitude of electrochemical signals decreased to 19.83 ± 0.11 pA (Figure S8c).This is because that PtNPs in 5 mM H2O2 generated the oxygen nanobubbles which partially blocked the reactive area of PtNPs and consequently inhibited the reduction reaction of protons.In additon, the decreased current indicated that the formed bubble-particle agglomerations lasted long stability during the microject process; that was, O2 nanobubble still existed on the surface of PtNPs after the microinjection of bubble-particle agglomerations in 50 mM HClO4 solution.
To study the reduction of O2 nanobubble on single PtNPs, we further examined the amperometric response of individual bubble-particle agglomerations by microjeting PtNPs in 5 mM H2O2 solution from a glass micropipette into 10 mM NaClO4 solution.Significantly, a smooth curve was observed in the amperometric current-time curve at -500 mV vs. Ag/AgCl wire (Figure S8d).We speculated that the O2 nanobubble may not be reduced on the PtNP at a submillisecond scale because the adhesion of nanobubble on PtNP caused an increase of interfacial ohmic drop and resulted in an impeded mass transfer inside nanobubble.Gaussian fits.The data were obtained from a large population of proton reduction events of individual PtNPs (more than 1000 events).
To probe the PEG effect to PtNPs, we investigated the electrochemical behaviors of individual PtNPs in 50 mM HClO4 solution with and without 1.4 mg mL -1 PEG in absence of H2O2.In a 50 mM HClO4 solution, a characteristic single peak was observed with a duration of 0.15 ± 0.01 ms and a current amplitude of 46.32 ± 0.28 pA, corresponds to HER process of individual PtNPs (Figure S9a).When 1.4 mg/mL PEG was added into 50 mM HClO4 solution, single peak still appeared, showing a duration of 0.15 ± 0.01 ms and a current amplitude of 46.36 ± 0.40 pA.It is worth noting that, almost the same electrochemical responses were observed for individual PtNPs in 50 mM HClO4 solution with and without 1.4 mg mL -1 PEG in absence of H2O2 (Figure S9b), indicating PtNPs were not affected by PEG during our electrochemical measurement process.We performed Zeta potential measurement to reflect the magnitude of electrostatic interaction between dispersed PtNPs in different solution conditions.Figure S10a  To further demonstrate the monodispersed state of PtNPs in different solutions, we estimated the collision frequency of PtNPs based on the Shoup-Szabo equation.S12 In this work, we counted the number of collision signals for 100 seconds to estimate the averaged experimental collision frequencies in different solutions.In theory, the number of collision events ( ̂ ) as a function of time (t) can be estimated by the integration of the diffusive flux towards a disc electrode with respect to t:

S13
where   is the disc radius of electrode (3.5 μm) and  * is the particle number concentration (6.02×10 16 m - 3 ).Using a Taylor expansion, F(τ) can be approximated to: and  is a dimensionless time parameter, which is defined as: The diffusion coefficient of spherical PtNPs (  ) can be calculated by the Stokes-Einstein equation as follows: where   is the Boltzmann constant (1.38×10 -23 J•K -1 ),  is the temperature (298 K),  is the viscosity of the solution (1 mPa•s).As a result, the calculated frequencies are approximately 146 Hz and 145 Hz for the 100 pM PtNPs at a 7.0 μm diameter C UME in 50 mM HClO4 solution without H2O2 and with 0.5 mM H2O2 and 1 mg•mL -1 TBP, respectively (Figure S12).For a dumbbell-like bubble-particle agglomeration, the diffusion coefficient ( − ) can be expressed in terms of the duration ratio of individual PtNPs before and after nanobubble formation as given in eq S6: where  =0 (= 0.15 ms) is the duration of the current traces in 50 mM HClO4,  − is the duration of the current traces of bubble-particle agglomeration in the presence of 0.5 mM H2O2 without (= 0.19 ms) and with 1.4 mg•mL -1 PEG (=0.23 ms).Correspondingly, the calculated frequencies ar approximately 106 Hz and 86 Hz (Figure S12).The experimentally observed current transient frequencies in above solution are close to the theoretical value in 100 pM PtNPs based on usual variation in the stochastic measurements, which suggests that the experimental conditions do not significantly influence the particle monodispersion.Moreover, TEM images show the narrow size distributions with the average diameters of 3.2 nm and 3.1 nm before and after the single entity electrochemical measurements, respectively, indicating the Pt NPs were well monodispersed during the entire electrochemical processes (Figure S13).These findings indicate that the above experimental conditions do not significantly influence the monodispersion of Pt NPs during the electrochemical measurements.

S19
Despite considerable efforts have been devoted in a rich literature, small size and optical transparency make interfacial nanobubbles challenging to be probed with sufficient spatial and temporal resolution.S13 Micrometer-or submicrometer-sized bubbles can be probed by techniques, such as atomic force microscopy S14 and dark-field S15 or fluorescence microscopy.NPs via dark field microscopy (DFM).S18 However, due to the intrinsic weakness of plasmonic property in small size PtNPs, PtNPs less than 30 nm are invisible under DFM.Thus, it is challenging to directly visualize the nanobubble generated around 3 nm PtNPs in our work.In order to provide evidence of the nanobubble being attached to the PtNPs, we adopted a plasmonic antenna strategy to corroborate the generation of nanobubbles.S19,S20 Here, the gold nanorods (AuNRs) with excellent plasmonic property were served as the signal conduction unit and the platform for loading high catalytic components (small PtNPs).Hence, we employed the hybrid plasmonic antenna structure (AuNR@PtNPs) where 3 nm PtNPs modified on AuNR surface to monitor the nanobubble formation in H2O2 solution.
As shown in Figure S14a, the AuNRs had a cylindrical shape, and the length of these AuNRs was about 100 nm.Significantly, a fairly uniform morphology with highly-dispersed 3 nm PtNPs densely decorated on the surface of the AuNR.Firstly, we immobilized of AuNR@PtNPs on a cleaned indium tin oxide glass slide.A drop of 50 mM HClO4 solution without or with 10 mM H2O2 was added to the ITO glass slide modified by AuNR@PtNPs.Compared with the scattering spectra in 50 mM HClO4 solution, a brightness diming was significantly observed under the DFM after adding 10 mM H2O2 into 50 mM HClO4 (Figure S14b).Meanwhile, when AuNR@PtNPs was in 50 mM HClO4 solution, the measured scattering spectrum was found to peak at 690 nm.After adding 10 mM H2O2 into 50 mM HClO4 for 5 min, the scattering spectrum exhibited a blueshift to 650 nm and an obviously decreased intensity (Figure S14c).This is because that when the O2 nanobubbles were generated around PtNPs, it induced decreasing in refractive index of the medium around PtNPs from 1.33 to 1.00 and caused the blueshift of the scattering spectrum of AuNR@PtNPs.S21,S22 The observed blueshift of S20 40 nm was in good agreement with the blueshift obtained from a Mie theory for the same particle embedded in water compared with that in air.S23 In addition, previous reports showed the formation of metal oxide on NPs, and that the scattering spectra of NPs would exhibit a redshift.S24 Therefore, the blueshift of AuNR@PtNPs scattering spectra in 50 mM HClO4 with 10 mM H2O2 also excluded the formation of Pt oxide on PtNPs.
To verify that the brightness change in DFM and the scattering peak difference were due to O2 nanobubbles surrounding the PtNPs, we further investigated DFM images and scattering spectra of individual AuNRs in 50 mM HClO4 without and with 10 mM H2O2 as control.Notably, DFM image and scattering peak were both unchanged regardless of 10 mM H2O2 or a lack (Figure S14d,e).All experimental results evidenced that nanobubble was attached to the PtNPs by H2O2 decomposition.The concentration of H2O2 in the solution was determined by a spectrophotometric method based on Beer-Lambert law as previous work S25 :

S21
where A is the measured absorbance, ε is the wavelength-dependent molar extinction coefficient ((ε  2  2 ) = 43.6M -1 • cm -1 at 240 nm),  is the pass length and  is the concentration of H2O2.In order to obtain absorbance in an analytically meaningful range, H2O2 was diluted to in a range of 1 -4 mM with ultrapure water.Figure S15a shows a broad absorbance peak at 240 nm in the UV spectra.The calibration curve with a good correlation coeffin cient (R 2 = 0.99987) indicates that the measured absorbance at 240 nm can be used for adequately converting the absorbance readings to the concentration of H2O2 (Figure S15b).This is because that the formation of stable O2 nanobubble on the PtNPs surface after ~300 s incubation in 2 mM H2O2 solution.That is, the formation time of the stable nanobubble from single NP electrochemical experiment is consistent with the measured result from UV-vis spectra (Figure S16), thus further confirming the reliability of stable nanobubble formation time in this work.In the study of Wang et al.S26 , the surface of a nanobubble was kinetically stable against high internal pressures and the gas-water interface had great diffusive resistance.According to Kim et al.S27 , the experimental diffusion coefficient of the gas inside 10 nm nanobubble was 0.5×10 −18 m 2 /s, which was eight orders of magnitude smaller than the bulk diffusion coefficient.Therefore, the confined diffusion rate of O2 molecules inside nanosized bubble (a curvature radius of 3.66 nm) should be considered in this case.Here, we used MD simulations to investigate the diffusion rate of O2 molecules inside nanobubble with high laplace pressure.The catalytic generation of nanobubbles on a PtNP was simulated in this work.As shown in Figure

S25
S18a, the initial configuration of the model size was 6.0 nm*6.0 nm*6.0 nm, and the ratio of water molecules to oxygen molecules was 70:1.MD simulations were performed using the open source software LAMMPS (2020 version) in this work.All of the parameters and potential functions for bonded and nonbonded interactions during the simulations were given in Tables S1 and S2.The structure was optimized first, and then the relaxation time was 2 ns under the isobaric-isothermal (NPT) environment, temperature was set at 298 K, and pressure was 1 atm (atmospheric pressure).After 1 ns, the configuration of the simulation demonstrated that the O2 molecules in the aqueous solution will adsorb on the surface of PtNP (Figure S18b), indicating that O2 molecules can aggregate on the surface of PtNP.
Due to the intrinsic defects of MD method (limitation of time and space scale), it was difficult to simulate the formation of hemispherical bubbles according to such irregular adsorption.Therefore, we simplified the simulation configuration and directly built a spherical cap-shaped oxygen nanobubble model formed at the PtNP surface (Figure S18c).The nanobubble was stabilized on the hemispherical surface constructed by hydrophobic Lennard−Jones particles; the basal diameter and height of the fitted spherical cap were about 3.0 and 7.0 nm.Then the positions of oxygen molecules inside the nanobubble and dispersed inside the surrounding water were recorded as a function of the simulation time.Specifically, we monitored every oxygen molecule in the last 1 ns to describe the stabilized configuration.The trajectory was saved completely in 100 frames.
And the mean square displacements of oxygen molecules in nanobubble and in water were recorded for every frame and were shown in Figure S18d.The diffusion coefficients of the oxygen molecules were estimated in the linear region of 300−1000 ps.Accordingly, the diffusion coefficient of the oxygen molecules dispersed in water was estimated to be (3.37 ± 0.01) × 10 −9 m 2 /s, which is comparable to the result reported in bulk water (2.10 × 10 −9 m 2 /s) at 298 K.However, the diffusion coefficient of the oxygen molecules inside the nanobubble was estimated to be (0.055 ± 0.001) × 10 −9 m 2 /s; and the value is approximately 2 order of magnitude less than that for bulk water.These results suggested that the oxygen molecules inside the nanobubble remained in a very stable state and mainly moved inside the spherical cap.
Table S1.Model parameters and potential functions of all non-bonded interactions.Note: The optimized parameters of H2O and O2 are adopted from S29 and S30.

Figure S1 .
Figure S1.Characterization of C UME. (a) Microscopic image of C UME.(b) Cyclic Voltammetry of C UME in 0.9 mM ferrocenylmethanol solution at a scan rate of 100 mV s -1 .

Figure S8 .
Figure S8.(a) Schematic illustration of microjet collision system.The current-time curves and the

Figure S13 .
Figure S13.TEM images and size distributions of the Pt NPs before (a) and after (b) the electrochemical

Figure S15 .
Figure S15.The rate of H2O2 decomposition at PtNPs.(a) The absorbance spectra of H2O2 at various

Figure S18 .
Figure S18.MD simulations of oxygen nanobubbles.(a) Snapshot of the initial configuration in the S16,S17Recently, Xu et al. reported the visualization of individual nanobubbles at hundreds of nanometers by observing the scattering intensity change of individual noble metal

Table S2 .
Model parameters and potential functions of all bonded interactions.