Quantifying Visible-Light-Induced Electron Transfer Properties of

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Quantifying Visible-Light-Induced Electron Transfer Properties of Single Dye-Sensitized ZnO Entity for Water Splitting Hui Ma, Wei Ma, Jian-Fu Chen, Xiao-Yuan Liu, Yue-Yi Peng, Zhe-Yao Yang, He Tian, and Yi-Tao Long J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01623 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Journal of the American Chemical Society

Quantifying Visible-Light-Induced Electron Transfer Properties of Single Dye-Sensitized ZnO Entity for Water Splitting Hui Maa‡, Wei Maa‡, Jian-Fu Chenb, Xiao-Yuan Liua, Yue-Yi Penga, Zhe-Yao Yanga, He Tiana*, Yi-Tao Longa* a

Key Laboratory for Advanced Materials & School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China; b State Key Laboratory of Chemical Engineering Centre for Computational Chemistry & Research Institute of Industrial Catalysis, East China University of Science and Technology 200237, Shanghai, P. R. China. KEYWORDS: Single entity, Electron transport dynamics, Water splitting, Random walk numerical simulation ABSTRACT: Quantifying the photoinduced electron transfer properties of single entity is of paramount importance for clarifying the link between the photoelectrochemical performance and the specific properties of individual. Here, we successfully monitored the photoelectrochemical behavior of single dye-sensitized ZnO entity on a Au ultramicroelectrode with different TiO2 film thicknesses. Due to a trap-limited electron diffusion, a sub-millisecond photocurrent transient of an individual N719@ZnO associated with single-particle photocatalysis was observed. Furthermore, a Monte Carlo random walk numerical simulation (RWNS) model was developed to simulate the photoinjected electron transport dynamics and recombination in nanoparticulate TiO2 film. Our approach allowed the photocatalytic properties of N719 at the single molecule level to be quantified, and electron diffusivity and electron collection efficiency as a function of the film thickness was estimated by simulation analyses. Excellent agreement was obtained between the experimental results and theoretical simulations, indicating that the underlying photoinduced electron transfer processes can be reliably explored.

INTRODUCTION The increasing demand for renewable energy has motivated considerable effort to exploit the solar energy conversion properties of various semiconductor nanomaterials and molecular sensitizers in photocatalytic water splitting.1,2 Although TiO2 is used as a photocatalyst to oxidize water, producing oxygen, it is only active under UV illumination due to a wide bandgap.3 Recently, dye-sensitized oxide semiconductors have received much attention for visible-light water splitting, in which dye was considered as antenna to collect energy and then transferred electron to oxide semiconductor. The widely studied system is that of ruthenium-based dyes anchor to a wide bandgap TiO2, ZnO or SnO2 nanocrystallite. A range of analytical methods capable of investigating the photocatalytic prosperities and correlating performance of such water splitting process, including some of their dynamic behaviors, has been developed.4-7 However, the ensemble averaging obscure the link between the intrinsic photocatalytic performance and the specific properties of individual because of the interplay of inhomogeneity and heterogeneity.8 Identify individual contribution is central to photoelectrochemical performance. Thus, measuring the photocatalytic process of a single entity will enable the roles of nanoparticle (NP) and dye molecule to be quantified, resulting in high efficiency of photocatalyst by designing efficient structures.9-11 Single entity electrochemistry is a very powerful tool to immediately characterize and explore the structural and dynamic properties of freely diffusing individual entity, i.e., NP, vesicle, molecule, during the stochastic collision at an ultramicroelectrode (UME) because of its high sensitivity. In a typical

event, the colliding single entity mediates an electrochemical reaction on the surface of UME or directly reacts, thus offering the penetrating insights into the properties of individual entities.12-18 Recently, we investigated the motion trajectories of individual NPs using time-resolved current traces14 to identify the electrocatalytic efficiency of a single molecule.19 Upto-now, some efforts have been devoted to detecting the photoelectrochemical properties of individual semiconducting NPs20,21 and dye-sensitized TiO2 agglomerates22,23 by stochastic collision measurements. However, the stepwise changes of individual steps in the photocurrent were observed on the timescale of the second level that are assigned to irreversible particle-by-particle absorptions on the surface of UME. These methods exhibit sensitivity for detecting entities or agglomerates but have not reached the level of quantifying electron transfer of a single entity during photoelectrochemical reaction. Herein, we developed an ultrasensitive photoelectrochemical system that can real-time detect the photocurrent of a single dye-sensitized oxide semiconductor entity with picoampere (pA) and sub-millisecond sensitivity (Figure 1). In this work, highly efficient N71924 and ZnO semiconductor NPs11 were selected as the photosensitizer and electron continuity carrier for visible-light water splitting, respectively. Moreover, a Monte Carlo random walk numerical simulation (RWNS) model was simulated to obtain the electron transport dynamics and recombination in TiO2 film. Based on the high-resolution electrochemical measurements and the theoretical studies, our method can be used to quantify the photoinduced electron transfer process of single N719 sensitized ZnO (N719@ZnO) entity.

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Figure 1. Schematic representation of the single N719@ZnO entity photoelectrochemical measurement. (a) Individual N719@ZnO entity collisions in the photocatalytic water splitting process on a nanoparticulate TiO2@Au UME at +500 mV vs a Ag/AgCl wire, generating spike-like current transients under visible-light (λ > 400 nm Xe lamp). (b) Scanning electron microscopy (SEM) images of a nanostructured TiO2 film on a Au UME. (c) Cyclic voltammograms of a TiO2@ Au UME in a 1 mM ferrocene and 1 mM N719 acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TEAPF6) in the dark (black) and under visible-light (red) at a scan rate of 10 mV/s. (d) High-resolution transmission electron microscope (TEM) image of N719@ZnO NPs.

EXPERIMENT SECTION Chemicals and Materials. Dimethylsulfoxide (DMSO, HPLC grade), titanium diisopropoxide bis (acetylacetonate), di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2'bipyridyl-4,4'-dicarboxylato) ruthenium (II) (N719), tetrabutylammonium hexafluorophosphate (TEAPF6), and 5bromovaleric acid were purchased from Sigma-Aldrich (Milwaukee, USA). Dimethyl formamide (DMF, AR grade), tetrabutylammonium perchlorate (TBAP) and gallium tin alloy (99.99%) were purchased from Alfa Aesar (Lancashire, UK). Gold wire (diameter: 10 µm, 99.99%) was purchased from Goodfellow Cambridge Limited (UK). Millipore water (MilliQ system) was purified to a resistivity of 18.2 MΩ•cm with a UHQ II system (Elga). All electrodes for the electrochemical experiments were purchased from Shanghai Chenhua Co., Ltd., China. N2 (99.998%, purified) was obtained from Cryogenic Gases (Detroit, MI). ZnO NPs Synthesis. The ZnO NPs were synthesized according to the reported method.25,26 KOH (1.48 g) in 65 mL of methanol was dropped into a solution of Zn(Ac)2·2H2O (2.95 g) in 125 mL methanol at 60 °C over 10 min with vigorous stirring. The reaction solution was stirred for 2 hours at the same temperature, and then the nanocrystals were obtained after precipitation for an additional 2 hours. The synthesized ZnO nanocrystals were washed twice using methanol and redispersed in chloroform for further use. N719@ZnO NPs Preparation. 5-bromovaleric acid (0.5430 g) and 0.0181 g N719 were dissolved in 5 mL of DMF. The prepared ZnO nanocrystals in 5 mL of chloroform were mixed with different amounts of the N719 solution and stirred for 30 mins. Finally, the functionalized ZnO NPs were spun down, washed twice using methanol and re-dispersed in DMSO/water electrolyte solution.

Nanoparticulate TiO2@Au UME Fabrication. First, a Au UME was fabricated by using a heating coil puller. A 10-µm diameter Au wire was inserted in a borosilicate capillary (I.D. 2.0 mm, O.D. 1.16 mm) and sealed together with a heated resistor coil. A sharp Au UME with a desirable surface was obtained by polishing. Two types of TiO2 pastes yielding nanocrystalline TiO2 (20 nm, paste A) and microcrystalline TiO2 (400 nm, paste B) particles were brush-painted on the surface of a Au UME to form the transparent and the lightscattering layers, respectively. Each layer of paste A or paste B was coated on the Au UME, kept in a clean box for 3 min to allow the paste to relax and reduce the surface irregularity and then dried for 8 min at 125 °C. The Au UME coated with the TiO2 pastes was calcined at 500 °C for 30 min under a nitrogen flow. Then, the electrode was immersed into a 40 mM aqueous TiCl4 solution at 70 °C for 30 min. After washing with water and ethanol, the nanoparticulate TiO2-coated Au UME was sintered at 500 °C for 30 min. Finally, the sealed wire was then electrically connected to a larger wire using a conducting gallium tin alloy.

RESULTS AND DISCUSSION Photoelectrochemical Response of Single N719@ZnO Entity. We developed an ultrasensitive photoelectrochemical method to observe the collision events of individual N719@ZnO entities by combining a low-noise electrochemical measurement with simulated sunlight. The two-electrode electrochemical system was consisted of a 10 µm TiO2@Au UME as the electron collecting electrode and a Ag/AgCl wire reference electrode (Figure 1a). A micrometre-thick TiO2 semiconductor film was deposited as a paste and sintered on a 10µm diameter of Au UME (TiO2@Au UME) to produce electrical continuity and extend the electron diffusion to a submillisecond timescale (Figure S1).27,28 The SEM image exhibited a 4.4-µm-thick nanoparticulate TiO2 film compact deposit,


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Figure 2. Photoelectrochemical responses of individual N719@ZnO entities for visible-light water splitting. (a) Current-time curves on a TiO2@Au UME in the presence of 200 pM of N719@ZnO entities in a DMSO solution with and without water containing 50 mM TBAP. The red rectangle is a further enlarged view and representative time-resolved current traces of certain sections. (b) Schematic diagrams of the corresponding energy band diagrams. (c) Typical gas chromatographic traces of evolved oxygen in the detector cell containing N719@ZnO NPs in DMSO with (red) and without (black) water under visible illumination. Histograms of the transient duration (T, d), the integral charge (Q, e) for the individual N719@ZnO entity collision events and the turnover number (TON, f) of photocatalytic O2 evolution of a single N719 molecule per stochastic collision event. The number of collision events in each histogram is at least 1,000.

which indicated excellent electrical contact between the TiO2 NPs, resulting in the efficient collection of the injected electrons in the TiO2@Au UME (Figure 1b). Moreover, a typical UME steady-state voltammogram was observed for the TiO2@Au UME in a 1 mM ferrocene and 1 mM N719 acetonitrile solution, while there was a significant increase in the limiting current under visible-light (Figure 1c). The electrochemical results demonstrated the successful fabrication of a nanoparticulate TiO2 film with a substantially high photoconductivity and electron mobility. We further investigated the uniformity of film thickness at different areas. SEM images showed that the thickness deviation of TiO2 film was less than 0.2 µm, demonstrating the homogeneity (Figure S2). Typically, N719 molecules were bund on the surface of ZnO NPs due to the bidentate carboxylate linkages (Figure S3 and S4). The corresponding characterization patterns confirmed the successful attachment of N719 onto the ZnO NPs surface with a monodisperse size distribution of (4.0 ± 1.0) nm (Figure S4). In this study, unless otherwise stated, visible-light (λ > 400 nm) was used to illuminate the Au UME with a 4.4-µm-thick TiO2 film. Moreover, the external bias voltage of the TiO2@Au UME was optimized at +500 mV vs a Ag/AgCl wire quasireference electrode because the photocatalytic water splitting is thermodynamically unfavorable (Figure S6 and S7). Figure 2a shows the current-time responses of individual N719@ZnO entities on a TiO2@Au UME in DMSO solution containing 2.2 M water and 50 mM TBAP. The concentration of the monodispersed N719@ZnO NPs solution was selected to be 200 pM so that the probability of two particles interact-

ing with the TiO2@Au UME at the same time is very small. Negligible current transients were observed in the dark, which implied that the electron transfer reaction did not occur without photoexcitation. Under visible-light, the current response was a smooth curve prior to the injection of the N719@ZnO NPs, and after the injection significant current transients appeared at potential bias of +500 mV vs a Ag/AgCl wire, indicating that the photoinduced electron transfer process of individual N719@ZnO entities occurred. In this case, N719, i.e., the photosensitizer, absorbed visible-light to produce photoexcited electron-hole pairs, which is then separated and resulted in electrons injection into the ZnO conduction band (Figure 2b). At the collecting electrode, sub-millisecond photocurrent transients were recorded for individual N719@ZnO entity collisions, and the photogenerated electrons repeatedly interacted with a trapping/detrapping distribution as they moved through the TiO2 film in a random walk. Meanwhile, N719 was regenerated by the bias driven photoelectrochemical water splitting, which created the available electron flow. Usually, for O2 evolution catalysis, sacrificial reagents such as triethanolamine, methanol and ascorbic acid, are used.29,30 However, in our photoelectrochemical systems, the ultimate electron donor was water due to the absence of sacrificial donors.29,31 To verify this, a control experiment using an electrolyte solution without water was performed under identical experimental conditions. No photocurrent spike was observed under illumination without water acting as an efficient electron donor (Figure 2a). Additionally, the gas chromatographic analysis showed that the volume ratio of O2 to N2 dramatically in



Figures 3. Single entity characterization. (a) Plots of the collision frequency as a function of concentration of N719@ZnO entities for the experimental data (black) and the theoretical calculated results (red). TEM images and size distributions of the N719@ZnO NPs before (b) and after (c) the photoelectrochemical experiments. The data errors are based on three separate experiments.

creased to 34% in the presence of water (Figure 2c), and photocatalytic water splitting was shown to occur due to the detection of O2 photoproducts in the gas samples from the photoelectrochemical vessel headspace. Moreover, considering methanol is a commendable sacrificial agent that candonate electrons to N719, we also carried out the photoelectrochemical experiments with methanol instead of water, and similar photocurrent transients were observed (Figure S8). Additionally, the current transient was a smooth curve for the bare ZnO NPs in a DMSO/water electrolyte solution, no matter under visible-light or in the dark (Figure S9). To quantify the electron transfer process of single collision, we further enlarged the view of sections of individual photocurrent transients. Due to the millions of electron trapping/detrapping processes in the TiO2 film before the electrons reached the collecting Au UME, an asymmetric current spike with a sub-millisecond time scale was observed for an individual N719@ZnO entity. The most representative population of spike events comprised sharp durations of 0.3 ± 0.1 ms (Figure 2d) and peak current amplitudes of 22.8 ± 4.2 pA (Figure S10). Meanwhile, the effectively collected charge (Q) distribution at 3.3 ± 0.2 fC was obtained by integrating photocurrent signals (Figure 2e). A small population of Q larger than 10 fC was appeared due to the unavoidable agglomeration during the entire photoelectrochemical measurement. However, the number ratio of small agglomerates in collision events is insignificant, thus excluding the interference in the statistical analysis. For the spike transients, we inferred a single collision should occur in a very short dwell time on or at the nanoparticulate TiO2 film. During the slow trap-limited transport, the injected electrons were favored over recombination with oxidizing species in the electrolyte. Considering four electrons are required for the conversion of one oxygen molecule, we can calculate the turnover number (TON) for oxygen evolution of single N719 per collision at TiO2@Au UME from the measured transients by TON 1 4 where is the average number of N719 on a single ZnO NPs surface, and e is the elementary charge. We estimated that was associated with the surface area of ZnO NPs (S), as described by 4π , and the N71932 and ZnO diameters were17.6 Å and 4.0 nm, respectively. This allowed us to calculate the average number of N719 molecules on a single NP, which was approximately 21. The theoretically calculated number was consistent with the experimental result from UVvis spectra after dye removal using 0.1 M NaOH solution

(Figure S5).33 This estimation gave the maximum TON for oxygen evolution of 285.2 ± 11.6 for a single N719 molecule per typical collision in good agreement with the previous value of solar driven oxygen production over ruthenium-based sensitizers,34,35 uncovering the photocatalytic capability at the single molecule level (Figure 2f). Single entity collision events. Each transient was associated with a single entity collision with a TiO2@Au UME. This photoelectrochemical behavior is consistent with that of reported for a single NP stochastic collision by the diffusion of a particle to the electrode surface22,23. An estimation of the collision frequency of individual N719@ZnO entities to the TiO2@Au UME under a stochastic diffusion control, f, can be calculated by eq 2: 4 2 where is the concentration of N719@ZnO NPs, is the radius of the TiO2@Au UME, and is Avogadro’s number. The diffusion coefficient of the N719@ZnO NPs, , can be calculated using the Stokes-Einstein equation16 !" # 3 6π% where !" is the Boltzmann constant, # is the absolute temperature, % is the solution viscosity, and is the radius of the N719@ZnO NP. The diffusion coefficient of the N719@ZnO NPs according to eq 3 is 5.45 × 10-7 cm2 s−1. As a result, the calculated frequency is approximately 131 Hz for the 200 pM concentration of N719@ZnO NPs on a 10-µm diameter collecting electrode. The observed photocurrent transient frequency of 88 Hz is close to the theoretical value in 200 pM of N719@ZnO NPs based on usual variation in the stochastic measurements, which suggests that any illumination effects do not significantly influence the particle diffusion. Experimental collision frequencies linearly increased with the concentration of N719@ZnO NPs within error at a rate of 0.45 s-1 pM-1 in the range of 100-300 pM (Figure 3a). However, the detection frequency abruptly decreased at a high concentration of 372 pM, which was likely due to the aggregation of the NPs. Thus, the frequency of the photoelectrochemical events strongly depended on the concentration of the N719@ZnO NPs, indicating single entity collision events. Moreover, average size of 4.0 ± 1.0 nm and narrow size distributions were observed during the entire photoelectrochemical measurement process, showing the N719@ZnO NPs were well monodispersed, with a very low density (Figure 3b,c). These findings indicated that the observed photocurrent transients mainly correspond to the individual N719@ZnO entity collisions on the TiO2@Au UME.


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Figure 4. The thickness effect of the nanoparticulate TiO2 film. (a) Chronoamperometric profiles showing the photoelectrochemical responses of individual N719@ZnO entities for visible-light water splitting at TiO2@Au UMEs with different thicknesses in DMSO solution containing 2.2 M water and 50 mM TBAP. Close-ups of the representative current traces (right panel) and the SEM images of the corresponding film thicknesses (left panel). Plots of the variation of the maximum current (b), the integrated charge (c), and the frequency (d) of individual collision events versus the thickness of the TiO2 film. (e) Histograms of the effective electron number ( ) of a single N719 molecule at TiO2@Au UMEs with different thicknesses. The number of collision events in each histogram is at least 1,000. Error bars are the standard deviation from 3 independent experiments.

Thickness Dependence. To further elucidate the electron transport process in the TiO2 film, we investigated the photoelectrochemical behavior with varying TiO2 film thicknesses ranging from 0 to 9.4 µm (Figure 4a). Note that no current transient was observed for N719@ZnO NPs on a bare Au UME, which was probably due to the “fast” collisions of individual entities beyond the acquisition time of a single-point electrochemical measurement or the unbearable energy dissipation for electron transfer through the semiconductor-metal interface.36 As expected, slightly distinguished current traces were observed for N719@ZnO NPs at different thickness TiO2@Au UMEs (Figure 4a) because of the diverse diffusion and recombination processes of the photoinjected electrons in the various nanoparticulate TiO2 networks. Figure 4b displays the film thickness dependence of the maximum current (Figure S11), showing an initial linear increase before reaching a constant value within a typical variation. We attributed the small fluctuations in the current intensity in the range from 5.8 to 9.4 µm to the stochastic measurements of individual entity collisions. Both the integrated charge and collision frequency versus the TiO2 film thickness dependences were similar (Figure 4c,d). A clear increasing trend was observed as the film thickness increased from 3.1 to 4.9 µm. This trend could be due to the electron diffusion time increasing as the film thickness increases, resulting in the increased probability that single collision events can be distinguished. However, the trend decreased as the film thickness further increased from 5.8 to 9.4 µm, which suggested that the effective electron collection was negatively affected by an excess thickness. This is because that photoinjected electrons have a finite diffusion length (L) due to electron-hole recombination, and L is defined as the average distance that the electron can travel before recombina-

tion. To efficiently collect the electrons at the Au UME, the diffusion length of the electrons must be larger than the TiO2 film thickness (L > d).37 Thus, the use of an excessively thick film should be prohibited because the effective electron collection was decreased due to long diffusion length. Considering the electron recombination, we calculated the effective number of electrons ( ) for a single N719 per collision on the TiO2@Au UME as described by ⁄. The photoanode TiO2 film thicknesses ranging from 4.9 to 7.5 µm that have substantially high value for a single N719 molecule (Figure 4e). Our results clearly indicate the significance of controlling the TiO2 film thickness, not only for single entity but also for ensemble measurement, to produce efficient photoelectrochemical conversion. Electron Transport Dynamics in TiO2 Films. To provide a quantitative and predictive understanding of this system, we investigated the combined effects of electrons transport and recombination on the transient response of photoinjected electrons moving through the TiO2 networks to Au UME for the electron collection using a Monte Carlo RWNS method.38,39 A three-dimensional cylinder network of sites with 5 µm radius and 5.8 µm thickness (d) on the basis of cylindrical grid with each unit TiO2 NP (diameter, D of 20 nm) was simply built to simulate electrons transport dynamics after Monte Carlo RWNS cycles () = 100000) (Figure 5a). Electrons started from the outer surface (x = 0) of TiO2 film to the collecting Au UME (x = d) were collected. Considering insignificant heterogeneous recombination with oxidized species in our photoelectrochemical system, we gave each site in the TiO2 network at a certain unit transit time ( *+ ) and the recombination probability (,) for an electron to jump to another site in this RWNS simulation. The unit transit time between two TiO2 particles



Figure 5. Theoretical simulation of the photoinjected electron transport in a TiO2 film within trapping network. (a) Schematic diagram for the three-dimensional RWNS simulation of electron diffusion in a TiO2 film with the traps (black dots). The blue and red lines represent the transfer and the recombination of electrons, respectively. The simulated results of %- dependence on the recombination probability (,) in arbitrary unit at 5.8-µm-thick film (b) and the film thickness at 10-6 of , (c), respectively. The red lines are fitting results by eq 6. (d) The simulated current traces at different thicknesses of TiO2 film. (e) The relation between % and the TiO2 film thickness. (f) The linear dependence of *./0 versus 1 . Error bars are the standard deviation from 3 independent experiments.

follows *+ 2 ln 5 ∗ *7 , where *7 is an adjustable parameter that controls the time scale of the simulation and R is a random number distributed uniformly between 0 and 1.40 We have computed the distribution of transit time (t) when they reached Au UME by plenty of one-electron RWNS simulation calculations in the interval of Δt 0.01 ms), and the collision frequency (;) and transit time (t) of the electrons can be obtained by normalized the distribution of 1/ Ns*Δt . Therefore, the relation between ; vs t could be well-fitted (Figure S12a) by B ? ; .@ A C 4 * We further estimated the constants a and b to be 9.2 × 10-6 and 4.6 × 10-4 by fitting the distribution of ; vs *, respectively (Figure S12a). Due to the recombination process, the collection efficiency of electron (%- was defined as %- /7 , where the 7 is the initial injected charge from single N719@ZnO entity. After well enough numbers of Monte Carlo RWNS simulation cycles, %- can be achieved by integrating along the time * by F

% E ;1* 5 7

After well enough numbers of Monte Carlo RWNS simulation cycles at a given d of TiO2 film (5.8 µm) and , in arbitrary units (10-6),41 the theoretical simulations showed that % was highly dependent on the value of , and the film thickness (Figure 5b,c). According to fitting, we found both relations could be derived by the eq 6 (see Figure S12 of the Supporting Information for details) 1 % 6 K M N 1 H , ∗ I.J@ L

Together with the relation between % and , 7 can be obtained from eq 7: 7 7 K M N 1 H , ∗ I.J@ L By fitting the relation between the experimentally measured and film thickness 1 (Figure S12b), we can estimate 7 = 6.24 fC and p = 4.5 P10-6, respectively. Notably, the experiment results at 3.1 µm TiO2@Au UME significantly deviated and thus excluding it. This is because that the time-resolution of the amplifier is insufficient to measure the ultrafast electron transfer process, which well supported by the simulated results at this thin-thick film (Figure S12c). As a result, the current (Q) can be obtained by eq 8: ?7 B Q ; ∗ C ; ∗ 7 .@ A C 8 * where S is the total number of injected electrons. Considering the trap-limited electron diffusion in the TiO2 film, we also derived the time-dependent current equation using a threedimensional random-walk model (see Figure S13 of the Supporting Information for details) by eq 9: 42-44 KW 7 T1 U A ILXC 9 Q 6 T V.@ * .@ which is consistent with eq 8 and further demonstrates the reliability of our RWNS simulation model. As shown in Figure 5d, the theoretical current traces that initially quickly increased and decreased at a slower rate, showed good agreement with the experimental results in the asymmetric spike shape (Figure 4a). Evidently, the duration at the peak current (*./0 ) increased while the photocurrent decreased as the film thickness increased. For the individual dye-sensitized NPs,


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Journal of the American Chemical Society without considering the high-surface-area network for dyes adsorption, % decreased dramatically as the film thickness increased during the thicknesses varying from 4.9 to 9.4 µm (Figure 5e). The random walks of electrons determine the electron diffusivity . Provided that the first derivative of the maximum current over time is 0, can be calculated by eq 10: 1 10 10*./0 Thus, the square of the TiO2 film thickness (1 ) and *./0 have a linear relation with a slope of 10 . The *./0 value was obtained by a second-order-differential-based calibration method45,46 and showed a linear dependence on 1 (Figure 5f). Using the slope of the plot, was determined to be 5.1 ×10-4 cm2 s-1 in a manner consistent with the ensemble measurements in the range of 10-6-10-4 cm2 s-1.47 In addition, , of the electrons in arbitrary units can be formulated by eq 11:48 1 , [\ 11 1 H 0] ^ where _` is the apparent activation energy of the recombination process. From the theoretically calculated , 4.5 P 10Aa , the corresponding apparent _` was estimated to be 0.32 eV, which seems to be reasonable under the typical condition at room temperature (~0.3 eV).49,50 All results strongly demonstrated the reliability of our RWNS simulation model.

CONCLUSION In summary, we presented thorough experimental studies, supported by Monte Carlo RWNS simulations on a photocatalytic water splitting system composed of individual N719@ZnO entity collisions. Our approach provided a micrometre-thick TiO2 film coated Au UME that significantly extended the electron diffusion to a sub-millisecond timescale and allowed single entity photocatalysis to be observed, avoiding any inhomogeneous broadening and averaging effects that would occur in ensemble measurements. These findings allowed us to suggest single entity photoelectrochemical-based strategy to quantify the role of individual contributing components, resulting in photocatalytic devices with high-efficient solar energy conversion.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions ‡

H.M. and W.M. contribute equally to this work.

ACKNOWLEDGMENT This research was supported by the National Natural Science Foundation of China (21421004, 21327807, 21775043), the Program of Introducing Talents of Discipline to Universities (B16017), Innovation Program of Shanghai Municipal Education

Commission (2017-01-07-00-02-E00023), and the Fundamental Research Funds for the Central Universities (222201718001, 222201717003).

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Quantifying Visible-Light-Induced Electron Transfer Properties of

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