Characterization of Photoactivity of Nanostructured BiVO4 at Polarized

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Characterization of Photo-Activity of Nanostructured BiVO at Polarized Liquid-Liquid Interfaces by Scanning Electrochemical Microscopy Shokoufeh Rastgar, and Gunther Wittstock J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09550 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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The Journal of Physical Chemistry

Characterization of Photo-Activity of Nanostructured BiVO4 at Polarized Liquid-Liquid Interfaces by Scanning Electrochemical Microscopy

Shokoufeh Rastgar, Gunther Wittstock*

Carl von Ossietzky University of Oldenburg, School of Mathematics and Natural Sciences, Center of Interface Science, Institute of Chemistry, D-26111 Oldenburg, Germany

* corresponding should be address Prof. Dr. Gunther Wittstock Carl von Ossietzky University of Oldenburg School of Mathematics and Natural Science Institute of Chemistry 26111 Oldenburg Germany Fax +49-441-7983979 Email: [email protected]

Supporting Information

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ABSTRACT: Scanning electrochemical microscopy (SECM) has been used for recording the photo-induced charge transfer reactions at bismuth vanadate (BiVO4) photocatalyst nanostructures arranged at chemically polarized liquid/liquid (L/L) interfaces between an organic butyronitrile electrolyte and an aqueous electrolyte containing perchlorate as common anion. In addition, [Co(bpy)3](PF6)3 soluble in the organic phase was used as a probe of SECM analysis, but also as a sacrificial agent, which effectively improves the separation efficiency of photogenerated charge carriers at the BiVO4 modified L/L interface. The nanoassembled layer did not require contact to a current collector. Interfacial polarization effects on the photoinduced charge transfer reactions have been investigated by recording the regeneration of the reduced form [Co(bpy)3](PF6)2 at the BiVO4 particle layer in organic phase in SECM feedback approach curves. The kinetic parameters of the interfacial photoinduced electron transfer (ET) reaction have been analysed under chemically controllable galvanic potential difference at the L/L interface. The potential dependence of the photoinduced ET rate constants followed Butler-Volmer theory. The photogenerated oxygen species distribute between the aqueous and organic phases through the porous BiVO4 particle layer. It can be traced by a transient current for O2 reduction at a gold microelectrode positioned close to the particle layer in the organic phase and biased for diffused-controlled O2 reduction. The results represented a new approach to characterize semiconductor photocatalyst systems spontaneously assembled at the L/L interface and their use for water oxidation reaction (WOR) as a challenging step in overall water splitting process.


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INTRODUCTION Interfaces between two immiscible electrolyte solutions (ITIES) represent a soft, defect-free, polarizable liquid-liquid (L/L) interface that can serve as a platform for assembling nanostructured semiconductor photocatalysts. Powdered semiconductors are widely used for light absorption with generation of electron-holes pairs. If those charge carriers can undergo reactions at the particle-solution interface before they recombine, the energy of the photon can be converted to chemical energy.1-4 The arrangement of such particle at interfaces allows for facile control of the pathway and the efficiency of this charge transfer reactions by polarization of the interfaces either externally using a potentiostat or chemically by changing the composition or the concentrations of supporting electrolytes in the organic and aqueous phases.5, 6 Butyronitrile (BNT) was selected as organic phase in our “inverted” ITIES configuration. Butyronitrile (and other organic nitriles) could be excellent candidates for the “inverted” liquid/liquid electrochemistry due to their low toxicity and environmental impact (compared to haloginated solvents), lower density than water (ρBNT = 0.795 g cm-3), low miscibility with water (0.033 g/100 mL) and other suitable electrochemical features such as the potential window (~0.7 V) and high dielectric permittivity (ε = 20.70 at 294 K). Perchlorate ClO4- is contained as common ion in both phases. Due to the transfer of ClO4- from the organic to the aqueous phase, the organic phase is charged positively and the aqueous phase is charged negatively until the Galvani potential difference between the two 0 phases prevents a further net transfer of ions. The standard transfer potential ∆BNT aq φ ClO - of the 4

common ion, and the ratio of its bulk concentrations, [ClO4-]aq/[ClO4-]BNT, determine the 7, 8 Galvani potential difference ∆BNT aq φ according to the Nernst-Donnan equation (eq. 1).



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BNT 0 ∆ aq φ = φBNT − φaq = ∆ BNT aq φ ClO - − 4

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RT [ClO− 4 ]BNT , ln [ClO − 4 ]aq zi F

(

(1)

)

0 in which z is the ionic charge of the perchlorate zClO − = −1 . ∆BNT aq φ ClO - is related to the 4

4

BNT 0 BNT 0 Gibbs free energy ∆ aq G ClO - = − zClO − F ∆ aq φ ClO - of ClO4- transfer from the buytronitrile to 4

4

4

the aqueous phase. At low ClO4- concentrations (i.e., less than 10 mM),9 the ion transfer (IT) may become the rate-determining step. To avoid this situation, the concentration of ClO4- in butyronitrile was kept at 0.10 M in all experiments described here. The BiVO4 nanostructures are originally contained as suspension in the aqueous phase. They carry a net negative surface charge (Figure 1a). Interestingly, the electrostatic attraction between negatively charged BiVO4 nanostructures in the aqueous phase and the positively charged organic phase causes the BiVO4 nanostructures to assemble spontaneously at the L/L interface as thin layer (Figure 1b, as schematic, photograph in Figure S2 in Supporting Information (SI)). This unique feature of ITIES is fully desirable in design of artificial photosynthetic systems using a Z-scheme for an efficient water splitting reaction into molecular oxygen O2 and hydrogen H2. In view of the pressing need for devising new ways for meeting the growing energy demand with minimal environmental impact, such systems receive high attention in fundamental research. Similar to the charge transport properties between photosystem I and II in nature, the photoexcited electrons in the semiconductor can be captured by reagents at both sides of the L/L interface. This reaction may be supported by the potential drop across the L/L interface which can help to minimize undesired charge recombination processes within the semiconductor photocatalyst and to promote the pathway for the water oxidation reaction WOR (yielding O2) by separated holes at the aqueous side of photo-active layer (Figure 1c).


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Figure 1. Schematic representation of the investigation of nanoparticle assemblies at ITIES. a) suspension of 0.1 mg mL-1 BiVO4 nanostructures dispersed in aqueous NaClO4 solution; b) addition of an immiscible organic solution (TBAClO4 in butyronitrile) and transfer of ClO4ions to the aqueous phase driving BiVO4 to the interphase; c) establishing a potential drop over the ITIES and electron/hole separation within BiVO4 upon illumination; d) investigating of photo-induced electron transfer at BiVO4–modified ITIES using the SECM feedback mode. The upper phase consists of 0.5 mM [Co(bpy)3]2+ (as mediator) in 0.1 M TBAClO4 in butyronitrile; the lower phase contains 0.1 M NaCl and 0.01 - 1 M NaClO4 in water.

Bismuth vanadate (BiVO4) is a well-known photocatalyst for water oxidation with light absorption in the visible rang of the spectrum. However, the efficiency has remained far below the theoretical solar-to-hydrogen conversion efficiency, mainly due to the poor electron conductivity and the slow kinetics of oxygen evolution resulting in recombination of electron-hole pairs as the dominant process.10, 11 So far, many strategies have been developed 5 ACS Paragon Plus Environment


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to increase the efficiency of visible light-driven oxygen evolution at BiVO4 semiconductor powders.12-15 In our previous work,16 an ITIES has been proposed for assembling hyperbranched nanostructured BiVO4 which increases the available surface area for interfacial reactions but maintains short diffusion length of the photogenerated charge carriers to the interface. The particle layer does not need an external electronic contact nor does it depend on efficient particle-particle conductivity (as it would be the case for electrode coatings from sintered powders). Upon illumination, photogenerated holes and electrons diffuse to the interface and enter into reactions i) between conduction band (CB) electrons and [Co(bpy)3](PF6)3 in the organic phase and ii) between holes and water (i.e. water oxidation reaction, WOR) in the aqueous phase. The high surface area of nanosized BiVO4 crystals with a specific hyperbranched structure along with the defect-free nature of ITIES reduces the interfacial recombination of photo-excited electron-hole pairs by rapidly transferring electrons to reactants in the adjacent liquid phases. Specifically, we have shown that the presence of [Co(bpy)3](PF6)3 as electron scavenger can improve the rate of WOR at the interface between hyperbranched BiVO4 and the aqueous phase solution by removing the CB electrons.16 The electron transfer (ET) reactions occurring at the interface between BiVO4 and the organic and the aqueous phases can be sensitively followed by scanning electrochemical microscopy (SECM) in different working modes (Figure 1d). For that purpose, an amperometric microelectrode (ME) together with a reference and auxiliary electrode is inserted into the upper, organic phase. We have shown before that the light-dependent reduction of [Co(bpy)3]3+ to [Co(bpy)3]2+ at the interface between BiVO4 and the organic phase can be followed by the ME as an oxidation current for [Co(bpy)3]2+ originating from the photo-driven reaction at the ITIES.16 Here we expand this concept by determining the BNT change of rate constants with the Galvani potential difference ( ∆ aq φ ) adjusted by the ionic


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composition in both electrolyte phases. The Galvani potential difference drops at the interface and causes high field strengths close to the interface supporting the photoinduced reactions similar to a potentiostatic voltage control at a photoelectrode, but without the need for an external voltage source nor any contact electrodes. This provides new insights about redox kinetics of BiVO4 photocatalyst as a function of interfacial potential across the L/L interface without interfering effects resulting from the assembly of nanoparticles at a solid electrode surfaces, such as contact resistance, additional charge carrier recombination due to longer transport distances to the back contact, or back transfer at the back contact. It should be noted that the ME is part of a circuit used as an interrogation tool only and does not polarize the L/L interface. SECM in the feedback mode has been used before as a viable and powerful technique for the kinetic analysis of charge transfer reactions at ITIES.17-24 In this work, the SECM feedback mode is used to measure rate constants of the photo-induced electron transfer reactions at nanoparticle-modified ITIES. It should be emphasised that this scheme does not require particle-to-particle electron transfer as was studied with metallic nanoparticles at liquid-gas interfaces.25 Photo-generated molecular oxygen is detected in the substrate-generation/tip-collection mode of SECM very close to the ITIES in the organic phase before the enhanced oxygen concentration at the interfaces is diluted to the value in the bulk solution. Similar measurement strategies have been applied for detection of H2O226, 27 and H228 generated at ITIES (without nanostructures) under aerobic and anaerobic conditions, respectively. While BiVO4 serves only as an easily accessible example, the general approach can be expanded to other nanostructure, other ITIES and other mediators or electron scavengers. Therefore, understanding the potential-dependent rate of photo-induced ET at nanoparticles assembled at ITIES is necessary for the evaluation of such interfaces for fluidic systems for water splitting devices. Taken together, the concept of nanostructures assembled at ITIES 7 ACS Paragon Plus Environment


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plus the presented method for evaluation of electron transfer rate constants in such systems provide new opportunities for optimizing such systems with respect to the choice of semiconductors, composition of the two liquid phases with the ultimate goal of linking several interfaces to a Z-scheme29, 30 for water splitting.

EXPERIMENTAL SECTION Chemicals. Sodium perchlorate (NaClO4, > 99%, Fluka)), tetrabuthylammonium perchlorate (TBAClO4, Sigma-Aldrich) and butyronitrile (Sigma-Aldrich) were used as received. [Co(bpy)3](PF6)2 and [Co(bpy)3](PF6)3 complexes (as SECM mediators) as well as hyperbranched nanostructured BiVO4 (as semiconductor photocatalyst) were prepared according to the procedures described before.16. The SEM image of the synthesized BiVO4 is shown in Figure S1 in supporting information (SI).

Electrodes and Electrochemical Cells. For all the SECM experiments, a monopotentiostat µ-P3 (M. Schramm, Heinrich Heine University, Düsseldorf, Germany) was used in the threeelectrode configuration. An Ag wire, a Pt wire and Au ME served as quasi-reference (AgQRE), auxiliary and working electrodes in the organic phase. The Au ME was fabricated and polished as described elsewhere.31 An Au wire (diameter 25 µm, Goodfellow, Cambridge, UK) was sealed into a 1 mm Pyrex glass capillary under vacuum. The ME was polished and shaped conically by a wheel with 180-grid Carbimet paper disks and micropolishing cloths with 1.0 µm, 0.3 µm, and 0.05 µm alumina. The ME was sharpened to RG of 8 to 8.5, where RG is the ratio between the diameters of the glass sheath rglass and the Au wire rT. Before each experiment, the ME was polished with 0.3 and 0.05 µm alumina powder and rinsed with water. For SECM experiments, BiVO4 was initially dispersed in 0.01-1 M aqueous NaClO4 electrolyte phase (bottom). The L/L interface was established by


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adding the butyronitrile phase (top) containing 0.1 M TBAClO4 and 0.1-1 mM [Co(bpy)3](PF6)2 (or 0.5 mM [Co(bpy)3](PF6)3). 0 BNT 0 The quoted ∆BNT aq φ ClO - and the Gibbs free energy ∆ aq G ClO - transfer from BNT to the 4

4

aqueous phase in our system) were calculated and determined from a separate three-phase electrode systems formed by a droplet-modified electrode8 as described in SI-7.

Apparatus and Procedures. The SECM approach curves and photocurrent transients were obtained utilizing a home-built instrument32 operated under SECMx.33 For measuring ET by SECM approach curves, a 25-µm-diameter Au ME was placed in the organic phase (top) and biased at a potential for diffusion-controlled oxidation of [Co(bpy)3]2+ to [Co(bpy)3]3+ (ET = 0.45V vs. AgQRE, Figure S3 in ESI). Positioning was performed with an x–y–z stepper motor system (Scientific Precision Instruments, Oppenheim, Germany). Approach curves were obtained by moving the ME toward the ITIES and recording the ME current as a function of the distance d between the ME and the ITIES under illumination by a Xenon lamp (ARC light sources, 175 W, with UV filter, LOT Quantum Design, Germany). The lamp provides polychromatic light with wavelengths larger than 420 nm at an input current intensity of 8.5 A. When the ME approach the ITIES from the upper, organic electrolyte, a sharp decrease of currents for [Co(bpy)3]2+oxidation or oxygen reduction indicated touching of the interface and penetration into the aqueous electrolyte. This allowed a very accurate determination of the position of the interface.24, 34-36 For positioning of the ME for recording current transient, the approach curves were repeated after removal and cleaning of the ME. The approach was then stopped when the current level was reached just before the touch. Experiments with incidental contact of the ME to the BiVO4-modified ITIES are readily detected by the current drop. In addition, the ME was retracted after the experiment to a large distance. A CV was repeated to check for the absence of any changes to the CV recorded before the photoelectrochemical experiment. 9 ACS Paragon Plus Environment


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O2 as main product of hole transfer reaction was traced by recording of approach curves in the upper, organic phase while the ME was biased at potential of diffusion-controlled reduction of oxygen (ET = -0.8 V vs. AgQRE, see Figure S4) and the ITIES was illuminated by visible light. The ME was kept at a constant distance d = 5 µm from the ITIES in the organic phase for detection of photogenerated O2 while changing the illumination of the ITIES.

RESULTS AND DISCUSSION Study of photo-induced ET reaction. SECM feedback mode was used to investigate the interfacial electron transfer reactions from BiVO4 to [Co(bpy)3]3+ at the L/L interface (Figure 1d). [Co(bpy)3]2+ dissolved in organic phase was used as a redox mediator for recording approach curves in the SECM feedback mode. [Co(bpy)3]2+ is oxidized to [Co(bpy)3]3+ at the ME. The ME-generated [Co(bpy)3]3+ diffused to the L/L interface where it was photochemically reduced back to [Co(bpy)3]2+ by reaction with photoexcited electrons from BiVO4 (reaction 2). e-CB(BiVO4) + [Co(bpy)3]3+ [Co(bpy)3]2+

(2)

For brevity we denote the concentration of [Co(bpy)3]3+ as [Co3+] and that of [Co(bpy)3]2+ as [Co2+]. If we emphasize a bulk concentration, this is indicated by a subsequent *. By band-gap illumination of the interface, the generated photo-induced electron has quasiFermi levels close to the energies of the CB edge and holes have quasi-Fermi levels close to the upper edge of the valence band (VB) of BiVO4. The CB energy of BiVO4 is -0.185 V vs. Ag|AgCl at pH 037 and the redox potential of [Co(bpy)3]3+ in butyronitrile was determined as 0.175 V vs. Ag/AgCl using a droplet-modified electrode (details SI-7). Therefore, 10 ACS Paragon Plus Environment

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heterogeneous ET of CB electrons from BiVO4 to [Co(bpy)3]3+ is thermodynamically feasible. This reaction allows the ME to sense the charge carrier generation but also to produce the sacrificial electron acceptor that increases the lifetime of the holes remaining in BiVO4. The increased life time of holes improves the efficiency of WOR. The driving force for interfacial photo-induced ET reaction (χ) was determined by the thermodynamic energy BNT difference (∆E0′) and the Galvani potential difference (∆aq φ ) across the ITIES which was

BNT 0 determined by calculation of standard potential of ClO4- transfer ( ∆ aq φ ClO - ) from the 4

butyronitrile phase to the aqueous phase according to the Nernst-Donnan equation (as described in SI-7). As an ionic species, [Co(bpy)3]2+/3+ may transfer and partition across ITIES as was previously shown for Ru(III) complexes.5,

6

This was not significant for our

conditions. SECM approach curves showed that [Co2+] was essentially zero after the ME penetrated into the aqueous phase (Figure S5). For [ClO4-] > 0.01 M, the large access of ClO4- over [Co(bpy)3]2+ (0.1 to 1 mM) will additionally cause the charge transfer to be mainly carried by ClO4- within the time scale of the SECM experiments. The approach curves were plotted in normalized quantities IT vs. L, in which the ME current iT was normalized to the diffusion-controlled oxidation current iT,∞ for [Co(bpy)3]2+ at the ME in the dark and at quasi-infinite distance to the ITIES. The normalized distance L = d/rT was obtained from distance d and rT. Figure 2, curve 1 shows the approach of the ME in the dark when polarized to ET = 0.45 V (vs. AgQRE) for diffusion-controlled oxidation of [Co(bpy)3]2+ at the ME. Due to the lack of photogenerated CB electron in BiVO4, reaction 2 cannot occur at the interface of BiVO4 and organic phase. The current decreases when the ME comes close to the interface because the interface represents an obstacle for the diffusion of the hydrophobic [Co(bpy)3]2+ to the ME. The approach curve shows larger currents than the predicted for hindered diffusion,40 Since the partitioning of [Co(bpy)3]2+ into the aqueous phase is negligible (vide supra), we assume a deformation of the L/L interface upon close 11 ACS Paragon Plus Environment


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approach as a likely reason for this observation. Figure 2, curve 2 shows approach curve under illumination (λ ≥ 420 nm) and otherwise identical condition. Close to the ITIES the ME currents are enhanced compared to curve 1 because photogenerated CB electrons are available for reaction 2 which produces [Co(bpy)3]2+ at the ITIES, from which it diffuses to the ME and is re-oxidized there.

Figure 2. SECM approach curves in the feedback mode to BiVO4-modified ITIES; 1) without and 2) with illumination. Organic phase 0.1 mM [Co(bpy)3]2+ + 0.1 M TBAClO4 in butyronitrile; aqueous phase 0.1 M NaCl + 0.05 M NaClO4; rT = 12.6 µm (RG = 4), vT = 1 µm s-1, ET = 0.45 V (AgQRE) for oxidation of [Co(bpy)3]2+ at the ME.

In the following experiments, electron transfer kinetics were studied at the BiVO4 particlemodified ITIES by measuring approach curves towards that interface with various concentration of [Co(bpy)3]2+ and fixed [ClO4-] of 0.1 M in the aqueous and organic phases. The interface was illuminated from the bottom of the electrochemical cell (λ ≥ 420 nm). As can be observed in Figure 3A, the normalized ME current (IT = iT/iT,∞) grows with decreasing the [Co(bpy)3]2+ bulk concentrations when approaching the Au ME (rT = 12.5 µm, RG = 88.5) to the interface. The experimental approach curves were fitted by a least-square method 12 ACS Paragon Plus Environment

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to an analytical approximation by Cornut and Lefrou40 describing the finite, irreversible heterogeneous kinetics of first-order reactions (equations S1 to S7 in SI-5). The experimental curves fit well to these predictions. The obtained dimensionless, normalized apparent heterogeneous electron transfer rate constant κ for [Co(bpy)3]3+ reduction are given in Tables S3 to S5. The effective heterogeneous electron transfer constant keff [cm s-1] was obtained by eq. 3 from κ and the diffusion coefficient of [Co(bpy)3]2+ D = 4.9 × 10−6 cm2 s−1 in butyronitrile Isolution. It was determined from the steady-state, diffusion-controlled oxidation current of [Co(bpy)3]2+ at an Au ME (rT = 12.5). keff = κ D/rT

(3)

Figure 3. SECM approach curves in the feedback mode towards BiVO4-modified ITIES under illumination with different mediator concentrations. A) Experimental (symbol) and fitted (solid lines) curves; organic phase 0.1 M TBAClO4 in butyronitrile with different [Co2+]* of 2) 0.1 mM, 3) 0.25 mM, 4) 0.5 mM, 5) 1 mM; aqueous phase 0.1 M KCl with 0.1 M NaClO4 in water; rT = 12.50 µm, vT = 1 µm s-1, ET = 0.45 V (AgQRE) for oxidation of 13 ACS Paragon Plus Environment


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[Co(bpy)3]2+ at the ME. Curves 1) and 6) are calculated curves for diffusion-controlled feedback and hindered diffusion, respectively. B) Reciprocal effective heterogeneous first order rate constants keff vs. [Co2+]* in the organic phase for different [ClO4-]aq of 1) 0.01 M, 2) 0.1 M, 3) 0.5 M and 4) 1 M. Lines are fits based on keff-1/ (103 cm-1 s) = α + β [Co2+]*, (R2 = γ) where the extracted values of α, β and γ are 1) 0.084, 0.66 and 0.97; 2) 0.16, 2.22 and 0.97; 3) 0.29, 3.46 and 0.99; 4) 0.97, 4.19 and 0.98. The keff values were extracted from the approach curves in Figure 3A, 4A, 4B, S10A, S10B.

Despite being controlled by different mechanisms, the formal kinetics resembles previous experiments of mediator regeneration at dye-sensitized photoanodes in which an effective positive feedback is approached if the flux of charge carriers to the interface within the semiconductor can compete with the mediator flux from the ME to the interface, i.e. at low mediator concentrations and high illumination intensities.41, 42 The keff values summarize the influence of light absorption, recombination and regeneration of the reduced form of mediator. They can be calculated from a formal kinetic treatment (detailed in SI-6, eq. S9 to S35) leading to eq. 4. 1 rT 2 k rec Fr 2 [Co 2+ ]∗ = + T keff Ak redα (λ ) I 0 α (λ ) I 0

(4)

Here [Co2+]* stands for the bulk concentration of [Co(bpy)3]2+, krec and kred denote the charge recombination constant of BiVO4 and regeneration constant of the reduced form of mediator, respectively. In addition, A is the geometric area of the L/L interface, F is the Faraday constant, α(λ) is the linear absorption coefficient and I0 is the intensity of incident light. Figure 3B shows plots of reciprocal keff for different [Co2+]* and for different polarisation of ITIES caused by different [ClO4-]aq (data are summarized in Table S3). Since the intercept in eq. 4 is proportional to the ratio krec/kred, one obtains the relative increase of kred when applying more positive potentials (φBTN-φaq) across the interface by decreasing the [ClO4-]aq.


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The effect of the polarization of the interface by the transfer of the common ion (ClO4-) between organic and aqueous phases was studied by SECM approach curves in the feedback mode at fixed bulk concentration of the mediator [Co(bpy)3]2+ in the organic phase of 0.1 mM and 0.5 mM. The polarization was changed by varying [ClO4-]aq while the TBAClO4 concentration was fixed in the organic electrolyte (Figure 4). Obviously, stronger positive feedback can be observed for photo-induced regeneration of [Co(bpy)3]2+ by applying a more positive Galvani potential difference across the ITIES (φBNT-φaq) as [ClO4-]aq decreases (described in SI-7 and calculated in Table S2). This enhancement can be ascribed to the faster interfacial heterogeneous ET reaction between photo-excited CB electrons in BiVO4 and [Co(bpy)3]3+ by polarization of the interface.



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Figure 4. Experimental (symbol) and fitted (solid lines) SECM approach curves in the feedback mode towards BiVO4-modified L/L interface under illumination with [Co(bpy)3]2+ as mediator in the organic phase for [Co2+]* of (A) 0.1 mM and (B) 0.5 mM and polarizations by different [ClO4-]aq of 2) 0.01 M, 3) 0.1 M, 4) 0.5 M, 5) 1.0 M for A) and 2) 0.01 M, 3) 0.1 M, 4) 1.0 M for B); organic phase 0.1 M TBAClO4 in butyronitrile with 0.1 mM or 0.5 mM [Co(bpy)3]2+; aqueous phase 0.1 M NaCl and different concentrations of NaClO4; rT = 12.56 µm, vT = 1 µm s-1, ET = 0.45 V (AgQRE) for oxidation of [Co(bpy)3]2+ at the ME. Calculated curves for diffusion-controlled feedback and hindered diffusion are given for comparison (curves 1 and 7 in A, curves 1 and 5 in B). According to the Butler-Volmer model,23,

43, 44

the ET rate constant should depend

exponentially on the driving force χ for ITIES at low χ (eq. 5).  α nF χ  keff = k 0 exp    RT 

(5)


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Where k0 is the apparent standard heterogeneous electron transfer rate constant, α is the transfer coefficient, F is Faraday constant, n is the number of transferred electrons in the interfacial reaction (n = 1), R is the universal gas constant and T is the temperature. The driving force χ of interfacial photo-induced electron transfer is defined by23, 28, 42, 45, 46

χ = ∆E° '+ ∆aqBNTφ where ∆E ° ' = E ° ' Co(bpy) [

(6)

3

]3+/2+

− E °CB,BiVO4 is the difference between the formal potentials of

[Co(bpy)3]3+/2+ in the butyronitrile E° ' Co(bpy) [

3

and CB electrons in BiVO4 E°CB,BiVO4 .

]3+/2+

BNT Additionally, the Galvani potential difference across the ITIES ∆aq φ as described by

equation (1) should be a linear function of log[ClO4-]aq with a slope of -59 mV per concentration decade (SI-7, Figure S9B) and was calculated in Table S2 from known value of BNT 0 standard transfer potential of perchlorate ∆ aq φ ClO - determined using a three-phase droplet4

modified electrode. Thus, the dependence of the plot of lnkeff vs. χ for variable [ClO4-]aq should be linear with a slope proportional to the electron transfer coefficient α and the intercept related to the apparent standard electron transfer rate constant k° (eq. 5). In agreement with eq. 7, plots of lnkeff vs. χ increase linearly with applying higher positive BNT ∆aq φ governed by decreasing [ClO4-]aq from 1 M to 0.01 M for [Co2+]* of 0.1 mM and 0.5

mM (Figure 5). Therefore, the values of α = 0.38 and k° = 2.2×10-6 cm s-1 can be extracted for 0.1 mM [Co(bpy)3]2+ and α = 0.38, k° = 0.66×10-6 cm s-1 for 0.5 mM [Co(bpy)3]2+.

ln keff = ln k 0 +

α nF RT

χ

(7)

The results show that photo-induced electron transfer rates depend strongly on the polarization of the ITIES. The potential difference can be adjusted by the concentration ratio of the common ion (ClO4-) between aqueous and organic phases. This aids in finding suitable conditions in terms of interfacial polarization at which the photo-induced interfacial electron transfer is favourably affected. This might improve the charge separation process in WOR caused by photo-generated holes on the other side of excited BiVO4 nanoparticles.



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Figure 5. Tafel plots of lnkeff vs. χ for the approach curves of Figure 4 for [Co2+]* of 1) 0.1 mM and 2) 0.5 mM under polarization of ITIES using a fixed concentration of 0.1 M TBAClO4 in the organic phase and 0.01-1 M ClO4- in the aqueous phase. Lines are fits based on 1) ln[keff/(10-3 cm s-1)] = 15.12χ/V - 13.400 (R2 = 0.95) and 2) ln[keff/(10-3 cm s-1)] = 15.00 χ/V- 14.23 (R2 = 0.97) .

Photo-Induced Hole Transfer Reaction: Electrochemically Detection of O2. In order to study the WOR as the second reaction, SECM in sample-generation/tip-collection mode was applied to detect O2 as the main product of photo-catalytically WOR at the interphase between BiVO4 and the aqueous electrolyte. WOR is caused by holes in the valence band of BiVO4 and is energetically possible. O2 as the main product could be detected by approaching the Au ME to the illuminated ITIES from the organic phase while the ME was biased at a potential suitable for diffused-controlled oxygen reduction reaction (ET = -0.8 V vs. AgQRE). After formation of O2 in the aqueous phase, O2 will partition between the aqueous and organic phases. Diffusion of photo-generated oxygen through the permeable, nanostructured BiVO4 film allows the sensitive detection of O2 on the organic side of the 18 ACS Paragon Plus Environment

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ITIES before O2 is diluted in the bulk of the organic solution. While approaching to the interface, the O2 reduction current increases indicating the generation of O2 by WOR at the photoexcited BiVO4 at ITIES (Figure 6A, curves 2 to 5). In contrast, the ME current changed only negligibly during approach to the ITIES in the dark (Figure 6A, curve 1). The effect of interfacial WOR is also studied by recording approach curves for O2 reduction at the ME at varying [ClO4-]aq. As shown in Figure 6A, the current related to the photogenerated O2 increases gradually by decreasing [ClO4-]aq providing a more positive Galvani potential BNT difference ∆aq φ = φBTN - φaq. The Galvani potential difference will not influence the

distribution of the neutral molecule O2 between the organic and aqueous phase. Hence, the different currents must originate from different O2 generation rates. This in turn can be caused by the following three effects: i) increased driving force for WOR, ii) increasing the concentration of holes in BiVO4 accessible to the aqueous phase as a result of more efficient charge carrier separation caused by the controlled polarization of the L/L interface, and finally iii) the effect of interfacial polarization on the coverage of BiVO4 particles at the ITIES.



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Figure 6. SECM approach curves in the sample-generation/tip-collection mode for the reduction of photo-generated O2 at [ClO4-]aq of 1) 0.01 M (without light), 2) 1 M, 3) 0.1 M, 4) 0.05 M and 5) 0.01 M; organic phase 0.1 M TBAClO4 in buytronitrile with [Co3+]* = 0.5 mM; rT = 12.5 µm, vT = 1 µm s-1, ET = -0.8 V (AgQRE) for O2 reduction at the ME; B) Transient currents for reduction photogenerated O2 at [ClO4-]aq of 1) 0.1 M, 2) 0.05 M, and 3) 0.01 M and [ClO4-]BNT = 0.1 M. The hatched regions indicate the duration of illumination for curves 1) and 2), the grey-coloured part indicates illumination for curve 3); rT = 12.5 µm; ET = -0.8 V (AgQRE) for O2 reduction, d = 5 µm.

O2 can be electrochemically detected by recording photocurrent transients based on O2 reduction while the ME is positioned 5 µm away from the interface in the organic phase (Figure 6B). On one hand, the necessity of light for the photo-induced generation of oxygen is fully obvious. On the other hand, higher O2 reduction currents (relative to the background 20 ACS Paragon Plus Environment

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from dissolved O2 in both phases equilibrated with air) is observed by decreasing [ClO4-]aq confirming the effect of interfacial polarization on the photo-induced WOR at the interface of BiVO4 as the source of photo-induced holes.

CONCLUSION Herein, we introduced nanostructured BiVO4 particles spontaneously assembled at a chemically polarized L/L interface as an efficient photoactive layer providing electron-hole separation which is potentially applicable for driving electron transfer reactions in the presence of [Co(bpy)3]3+ as sacrificial agent in the organic phase. This reaction runs parallel to water oxidation reaction at the aqueous side of the interface using photo-generated holes. In our case the conversion of the sacrificial agent could be used to follow the overall kinetics of the conversion at BiVO4. Reversible sacrificial agents could also be used to couple two interfaces to design a water splitting Z-scheme system. By means of SECM, we probed the kinetics of interfacial photo-induced ET under potential bias tuned by variation of [ClO4-]aq as a relatively hydrophilic common anion in the organic and aqueous phases. Higher rate constants for photo-induced ET are extracted as the driving force of ET increases by decreasing [ClO4-]aq. Molecular oxygen was electrochemically detected as main products of WOR using photo-generated holes (after partitioning of O2 from the aqueous phase to the organic phase) in the sample-generation/tip-collection mode of SECM. Accordingly, the interfacial polarization also influences the rate of O2 production at the BiVO4/water interface. This can be caused by increasing the driving force for WOR, improving the charge carrier separation efficiency in BiVO4, or by favourably affecting the relative wetting of the BiVO4 nanostructures by the organic and aqueous electrolytes. The results are further supported by recording the transient current responses based on O2 reduction at the ME. These findings



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provide novel insight into new tuning possibilities of photocatalysts at chemically polarized L/L interfaces for the energy-demanding WOR within an overall water splitting systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. S1 experimental details of the liquid-liquid system, S2 electrochemical characterization of [Co(bpy)3](PF6)2, S3 details of electrochemical O2 reduction, S5 extraction of rate constants from SECM approach curves, S6 interpretation of the extracted rate constants, S7determination of the galvanic potential difference with experimental details.

AUTHOR INFORMATION Corresponding Author: *E-mail:[email protected]. Fax: +49-441-798 3979. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS S.R. thanks the Alexander von Humboldt Foundation of Germany for research fellowships.

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Characterization of Photoactivity of Nanostructured BiVO4 at Polarized

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