In Situ Micro-Titration of Intermediates of Water Oxidation Reaction at

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

In Situ Micro-Titration of Intermediates of Water Oxidation Reaction at Nanoparticles Assembled Water/Oil Interfaces Shokoufeh Rastgar, and Gunther Wittstock J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03745 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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The Journal of Physical Chemistry

In Situ Micro-Titration of Intermediates of Water Oxidation Reaction at Nanoparticles Assembled Water/Oil Interfaces Shokoufeh Rastgar and 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 Sciences Institute of Chemistry 26111 Oldenburg Germany Fax +49-441-7983979 Email: [email protected]

Supporting Information

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ABSTRACT: Photo-driven water oxidation reaction (WOR) at nanoparticles assembled at polarized liquid-liquid interfaces is a possible realization of the anodic reaction for water splitting at chemically polarized liquid-liquid interfaces. The rational development of photocatalyst for the WOR requires their characterization under appropriate process conditions. A micropipette filled with an aqueous dispersion of nanostructured BiVO4 as a well-defined photoactive substrate is immersed into an immiscible organic solution containing perchlorate as common anion. Under illumination, hydroxyl radicals (OH•) are generated as adsorbed intermediates of the WOR under chemically controlled polarization. Combining the miniaturized liquid-liquid interface with a scanning electrochemical microscope allowed for application of the surface interrogation mode (SI-SECM) for quantitative assessment of adsorbed photo-generated intermediates. The loading of OH• intermediates per projected area of substrate (Γ = 3.33×10-5 mol m-2) and kinetics of their decay by a bimolecular reaction (k = 0.61×10-5 mol-1 m2 s-1) were determined with bare BiVO4 assembled at soft interface electrified by ClO4- transfer and irradiated by visible-light. This determination was possible without processing BiVO4 nanostructures to a solid electrode where particle-particle contacts and other structural features strongly modulate the overall outcome.

INTRODUCTION An interface between two immiscible electrolyte solutions (ITIES) is a defect-free (down to the molecular level), self-healing, renewable and conductive substrates for the preferential and controlled formation of molecular assemblies and nano-object layers that can be polarized.1–5 Similar to solid electrodes, the modifier layer(s) can be formed and/or characterized by charge (using either ion or electron transfer processes across the interface).6– 10

The potential drop across the interface can be controlled by the application of a potential 2 ACS Paragon Plus Environment

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difference across the interface either externally using a potentiostat or chemically by dissolving a common salt in both phases.11 In this regard, interfacial polarization could also effectively drive the interfacial photo-induced e-/h+ transfer reactions in a controlled way to produce molecular H2 in the presence of molecular photosensitizer,12–15 and of O2 in the presence of semiconductor nanomaterials assembled at the ITIES.16,17 The overall process may include heterogeneous reactions of adsorbed species (dyes as sensitizer or reaction intermediates) with dissolved redox species.18–20 The electric field at the interface of ITIES facilitates the spatial separation of photo-induced charge carriers and resulting reaction products on either side of the interface which partially restrict recombination processes.21 In our previous work,17 we have demonstrated the photoactivity of nanostructured BiVO4 particles, which were spontaneously assembled at the interface between butyronitrile and BNT water under a chemically controlled Galvani potential difference ( ∆ aq φ ) tuned by variation

of perchlorate concentration [ClO4-]aq in the aqueous phase. Perchlorate as a relatively hydrophilic common anion partitions between two phases and creates a voltage drop. In that paper, the redox kinetics of BiVO4 photocatalyst in photoinduced interfacial electron transfer (ET) reaction was characterized at the BiVO4|butyronitrile interphase with variable chemical polarization.17 The reaction occurs between electron in conduction band (CB) of photoexcited BiVO4 and [Co(bpy)3]3+ as a mediator and/or an electron scavenger soluble in the organic phase. O2 as a main product of the photogenerated hole transfer at the aqueous side of interface was also probed under variable interfacial polarization.17 Mechanistic studies of charge (e-/h+) transfer for the photoactivity of semiconductors (either O2 and/or H2 evolution) at polarized ITIES is of fundamental importance for the evaluation of interfaces in fluidic systems which may ultimately linked to a z-scheme for water splitting.1,2,4,8 Accordingly, the efficient coating of photoactive BiVO4 on the chemically polarized interface can also provide holes on the aqueous side which is potentially 3 ACS Paragon Plus Environment


applicable for driving oxidation of water under visible illumination. This energy-demanding WOR by holes on the aqueous side should embrace reactive oxygen species (ROS) such as H2O•, O• and OH• adsorbed on the BiVO4 surface.22 Understanding the photocatalytic WOR activity of BiVO4 requires the detailed knowledge of the reactions of these intermediates with active sites on surface which probably include adsorption, bond breaking and bond formation processes during the overall WOR.23–25 To achieve this goal, real-time evaluation of formation and decay kinetics of intermediates under working conditions opens up novel insight to the mechanistic pathways of photocatalytic WOR. The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) has been recently developed as a unique and direct technique to evaluate (photo)catalytic surfaces.26 It has been utilized to quantify the coverage of reactions intermediates and study of the kinetics of hydrogen- and oxygen-evolving catalysts at solid electrodes27–31 and photoelectrodes.32–35 This technique is based on monitoring of transient currents in the feedback mode of SECM. Previously, photogenerated OH• intermediates at W/Mo-BiVO4 solid electrodes were studied by applying potential and light pulses.32 Recently, SI-SECM has successfully been expanded in our group for redox titration of surface oxides in porous powdered materials such as nanoporous gold and carbon-supported Pt nanoparticles.36


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Figure 1. Investigation of polarized, microsized ITIES by SI-SECM using [Co(bpy)3]3+ as a titrant. Aqueous phase 1 mg mL-1 BiVO4 in 0.1 M NaClO4; organic phase 0.5 mM [Co(bpy)3]3+ in 0.1 M TBAClO4. The schematic is not to scale. The distance of the micropipette to the microelectrode is greatly expanded. For brevity, we denote [Co(bpy)3]3+ as Co3+ and [Co(bpy)3]2+ as Co2+.

Herein, we present the in situ evaluation of the surface of BiVO4 during water oxidation at the polarized ITIES by means of a novel application of SI-SECM technique. We identify and quantify the oxygen-containing adsorbates (mainly OH•) on the surface of BiVO4 at ITIES without the need for neither an external voltage source nor any contact electrodes to the photocatalyst (Figure 1). To this end, we introduce the SI-SECM mode with microsized ITIES at the micropipette (MP) orifice. The interface is chemically polarized by partitioning of ClO4- dissolving as a relatively hydrophilic common anion in the two liquid phases. To the best of our knowledge, this is the first report that an illuminated, semiconductor-sensitized ITIES is used to generate intermediates that are quantified in-situ by SI-SECM or amperometry in general.

EXPERIMENTAL SECTION 5 ACS Paragon Plus Environment


Chemicals. Sodium perchlorate (NaClO4, > 99%, Fluka), tetrabuthylammonium perchlorate (TBAClO4, Sigma-Aldrich) and butyronitrile (Sigma-Aldrich) were used as received. [Co(bpy)3](PF6)3 complex (as a SI-SECM mediator) as well as nanostructured BiVO4 (as semiconductor photocatalyst) were prepared according to the procedures described before.16 The TEM image of the synthesized BiVO4 is shown in Figure S1 in supporting information. Microelectrodes and micropipettes. 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 microelectrode (ME) served as quasireference (AgQRE), auxiliary and working electrodes in the organic phase. The Au ME was fabricated and polished as described before.37 In short, an Au wire (diameter 25 µm, Goodfellow, Cambridge, UK) was sealed into a borosilicate glass capillary (outer diameter/inner diameter = 1.5 mm/0.2 mm, 100 mm length) under vacuum. After connection to a Cu wire using silver-epoxy glue (EPO-TEK, John P. Kummer GmbH, Augsburg, Germany) and heat treatment inside the oven (at 60 oC for overnight), the ME was polished and shaped conically by a wheel with 180-grid Carbimet paper disks and micropolishing cloths with 0.3 µm and 0.05 µm alumina and rinsed with water before each experiment. The ME was sharpened to RG of 7.0, where RG is the ratio between the diameters of the glass sheath rglass and the Au wire rT = 12.5 µm. MPs (rs = 10 µm) were also fabricated from borosilicate glass capillaries using a single stage glass microelectrode puller (model PP-830, Narishige, Japan). Electrochemical Cell. The designed setup for implementation of SI-SECM in to the photoactive ITIES is shown in Figure S3A. The ME was mounted with a chromatography fitting to the bottom of SECM cell body made from polytetrafluroethylene. ME (bottom, facing up) and MP (top, facing down) were aligned to each other as shown in Figure S3B 6 ACS Paragon Plus Environment

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adjusted by moving the MP in x-y-z direction using piezoelectric motor (detailed in S4). In order to avoid crashing, the process was monitored with a video microscope mounted in the front of the electrochemical cell. An optical fiber was connected to the MP which also served as a wave guide to localize the illumination at the orifice where nanostructured BiVO4 was settled. The total power of the light source at the end of the pipette in the electrochemical cell was 0.788 mW for an illuminated area of 0.0254 cm2 giving a light intensity of 31 mW cm-2. In order to maximize incident light on BiVO4, the outer wall of MP was covered by silver ink. This caused consecutive reflection of the light inside the MP and avoided scattering of it at the entrance of MP. After initial positioning, the relative position between MP and ME including the tip-sample distance (dtip-sub) was fixed during the SI-SECM experiments. The synthesized BiVO4 (1 mg mL-1) was initially dispersed in 0.1 M aqueous NaClO4 electrolyte phase. The MP was filled with this suspension. The filled MP was immersed in butyronitrile phase containing 0.5 mM [Co(bpy)3](PF6)3 as redox mediator and 0.1 M TBAClO4 as supporting electrolyte (Figure 1). The liquid-liquid interface only formed at the opening of the pipette. The BiVO4 nanoparticles spontaneously assemble at the liquid-liquid interface driven by the surface charge of the particles and the equilibrium charges of the liquid electrolytes that share a common ion.16 The settling of the BiVO4 nanoparticles at the interface was observed by means of a microscopic camera as yellow coloration of the pipette orifice. Apparatus and Procedures. Chronoamperograms (CAs) were recorded using the homebuilt SECM instrument38 operated under SECMx.39 Positioning was performed with an x–y–z stepper motor system (Scientific Precision Instruments, Oppenheim, Germany). The ITIES was illuminated by a Xenon lamp (ARC light sources, 175 W, with UV filter, LOT Quantum Design, Germany) with an input current intensity of 8.5 A. The light was filtered to provide polychromatic light with wavelengths larger than 420 nm.



For recording the CAs, a 25-µm-diameter Au ME was positioned in the organic phase and biased at a potential for diffusion-controlled reduction of [Co(bpy)3]3+ to [Co(bpy)3]2+ (ET = -0.3 V vs. AgQRE, Figure S2). In order to perform SI-SECM experiments, the SECMx software was expanded to apply pre-defined potential and light pulses and switch the connection to electrodes.

RESULTS AND DISCUSSIONS Due to the transfer of ClO4- from the organic to the aqueous phase, the organic phase becomes charged positively and the aqueous phase attains a negative charge until the Galvani BNT potential difference ( ∆ aq φ ) at the interface reaches the equilibrated value according to the

Nernst-Donnan equation,40 which varies depending on the standard transfer potential ( BNT 0 ∆ aq φ ClO - ) of ClO4- and the ratio of the ClO4- bulk concentration in the aqueous and organic 4

BNT 0 phases, [ClO4-]aq/[ClO4-]org. ∆ aq φ ClO - was calculated to be 0.12 V vs. Ag/AgCl as 4

previously reported.17 The MP mechanically stabilizes the soft interface as a photoactive substrate for SI-SECM study. The MP is co-linearly positioned above a ME in a solution initially containing [Co(bpy)3]3+ as mediator in the organic phase as detailed in S4. By reduction, [Co(bpy)3]2+ (the titrant or reagent) is formed (Figure 1, (1)). It diffuses to the ITIES at the MP, then is oxidized by electron transfer to the photogenerated intermediates (the analyte or titrand, Figure 1, (2), (3)). The mediators diffuses back to ME (Figure 1, (4)) and delivers a transient feedback reduction current (called “positive feedback”) at the biased ME until all the adsorbed intermediates are consumed, after which the time-dependent current decays to the current controlled by hindered diffusion of mediator from bulk solution to the microelectrode (called “negative feedback”) which provides the detection of the endpoint. The charge represented by the current enhancement (the measured reagent quantity) is proportional to the 8 ACS Paragon Plus Environment

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amount of adsorbed OH• at BiVO4. The time between generation and detection pulses can be varied to allow the observation of decay kinetics of the adsorbed short-lived species. The SI routine was initiated by formation of radical intermediates at the polarized BiVO4 substrate illuminated for the time interval (τi) of 0.2 to 5 s while the ME is at open circuit potential (OCP, Figure 2, step (2)). After delay τd of 0.2 to 1 s in the dark and the ME at OCP (Figure 2, step (3)), a CA was recorded for the reduction of [Co(bpy)3]3+ at the ME at a potential ET = -0.3 V (vs. AgQRE) for 10 s. During the CA the electrogenerated [Co(bpy)3]2+ diffuses towards the ITIES with assembled BiVO4 and reacts chemically with the adsorbed OH• whereby the [Co(bpy)3]3+ is regenerated (titration step, Figure 2, step (4)). As a control experiment the same procedure was conducted under omission of the illumination step (Figure 2, step (2)).



Figure 2. SI-SECM mode of operation for the generation and detection of OH• on the BiVO4 surface at polarized ITIES. Below are the images related to the corresponding process taking place on each steps. For brevity, we denote [Co(bpy)3]3+ as Co3+ and [Co(bpy)3]2+ as Co2+. In Figure 3A, titration curves of Au ME (rT = 12.5 µm) were recorded for [Co(bpy)3]3+ reduction without and with light (background titration curve (1) and titration curves (2), respectively). In the background titration curves, the decaying current followed the Cottrellian behaviour of at a microelectrode.41 In the presence of light, the chemical reaction between the surface intermediates and the reduced form of the mediator causes enhanced reduction currents, which decreased with time and reached the background titration curve if all the surface-bound intermediates were consumed. The live time of photogenerated holes and conduction band electrons is very short in BiVO4 (~ 40 s).42 Since we use a delay time between the illumination and titration step that is longer than the hole life time, we assume 10 ACS Paragon Plus Environment

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that the oxidation of [Co(bpy)3]2+ occurs by surface-bound intermediates which is most likely OH•.32 This is contrast to a recently published SECM experiment in which [Fe(CN)6]4- was oxidized in a SECM feedback experiment during illumination of a macroscopic BiVO4 sample.43 During illumination both, adsorbed intermediates and holes, might be present in a steady-state concentration. By subtracting the background transient current from the titration curve, the net titration current is obtained. Its integration provides a charge Q (Figure 3B) from which the loading of photo-generated OH• intermediates (Γ) per projected geometric surface area of BiVO4 is obtained by equation 1 (Figure 3B, inset):33

Γ=

Q nFAproj

(1)

where n = 1 is the number of electron involved in titration reaction, F is the Faraday constant and Aproj is the projected area of BiVO4 substrate. The orifice of the MP (rs = 10 µm) is smaller than the Au ME (rT = 12.5 µm) and at dtip-sub = 3.0 µm, very high collection efficiencies are attained for the redox mediator (close to 100%).33,44 It means that approximately all titrant generated at the substrate can be fed back to the ME. Therefore, no correction is necessary for incomplete collection efficiency. With this well-defined setup, we assumed that the radius of projected substrate (rproj) equals the radius of the MP (rs).



Figure 3. (A) Redox titration CA curves for 1) negative feedback in dark (background curve) and for 2) positive feedback under irradiation (τi) for 5 s. (B) Integrated charge for subtracted titration curve resulting from (A); Inset is the amount of titrated OH• per surface area of substrate (ΓOH● ) vs. time. Aqueous phase 1 mg mL-1 BiVO4 in 0.1 M NaClO4; organic phase 0.5 mM [Co(bpy)3]3+ with 0.1 M TBAClO4; AuME with rT = 12.5 µm (RG = 7), ET = -0.3 V (vs. AgQRE) for reduction of [Co(bpy)3]3+ at the ME. Delay time (τd) was set to 0.2 s. In the first experiments, the limiting illumination time was found for which a saturation of the surface concentration of OH• occurs on the nanostructured BiVO4 using τd = 1 s between the end of illumination and the start of the interrogation step. The CAs are shown in Figure 4A. The titrated OH• increased and then leveled off for τi > 4 s (Figure 4B). The saturation loading of OH• on the BiVO4 surface corresponds to Γmax = 9.25×10-5 mol m-2 (τi > 5 s, τd = 1 s, Aproj = πrs2 = 3.14×10-10 m2 for MP with rs = 10 µm). The amount of photogenerated OH• in our system is about 3 times larger than values reported for the nanotubular TiO2 on solid supports (338 µC cm-2 or 3.5×10-5 mol m-2) under low-intensity irradiation33 and 6.5 times smaller than the value reported for BiVO4 (5.8 mC cm-2 or 6× 10-4 mol m-2).32 Those values 12 ACS Paragon Plus Environment

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refer to the geometric area and thus must change with the morphology of the semiconductor though the order of magnitude are in agreement. In addition, the details of illumination (intensity and spectral distribution) influence the result as it determines the steady state hole and adsorbate concentrations from which the decay commences.

Figure 4. (A) Titration CAs for τi equal to 1) 0 s, 2) 0.2 s, 3) 1 s, 4) 2 s, 5) 3 s, 6) 4 s, 7) 4.5 s and 8) 5 s; inset is a zoom-into the first 2 s of the CAs. (B) Titrated OH• (ΓOH● ) vs. illumination time τi. Aqueous phase 1 mg mL-1 BiVO4 in 0.1 M NaClO4; organic phase 0.5 mM [Co(bpy)3]3+ and 0.1 M TBAClO4; Au ME with rT = 12.5 µm (RG = 7), ET = -0.3 V (AgQRE); τd = 1 s. During the delay step, the decay of OH• intermediates occurs mainly by the recombination reaction under release of hydrogen peroxide (OH• + OH• → H2O2, Figure 2, step (3)). Therefore, titration CAs decreased at longer τd (Figure 5A), because OH• was consumed by the recombination reaction and eventually less [Co(bpy)3]3+ is regenerated by OH• at the ITIES. By integrating the transient signals in Figure 5A over time, the amount of titrated OH• can be plotted vs. τd. Obviously, the amount of OH• reduced with increasing τd (Figure 5B). 13 ACS Paragon Plus Environment


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As a control experiment, the influence of varying decay times on the transient redox curves for [Co(bpy)3]3+reduction was recorded without BiVO4 in the MP filling (Figure S5). The results show no meaningful changes of the transient signals with delay time confirming the necessity of BiVO4 to generate species that can be titrated. According to the previous reports for quantitative assessment of OH• on the surface of solid photoanode,32,33 we assume that the decay of OH• under formation of H2O2 follows a second order rate law according to the following equations:

d ΓOH• / dt = −k ΓOH•2

(2)

d ΓOH• / Γ OH•2 = −k dt

(3)

1/ Γ OH• = kt + 1/ Γ0

(4)

where k is the rate constant of OH• dimerization reaction, t = τd is the reaction time for generated intermediates between illumination and titration steps, ΓOH•(t) is the transient loading of OH• intermediates on BiVO4 per geometric area, Γ0 is the steady-state loading of OH• per geometric area at the end of the illumination step at t = 0. By plotting the reciprocal of ΓOH• over τd, Γ0 and k are determined from the slope and intercept of eq. 4, respectively (Figure 6). The plot of reciprocal surface concentration ΓOH•-1 (proportional to Q-1) vs. delay time agrees well with a second order rate low for the decay but not with a first order decay that might result from physical desorption of OH• as the significant decay channel.


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Figure 5. (A) Titration CAs with varying time delays (τd = 0.2-1 s) for 1) no light, 2) 0.2 s, 3) 0.3 s, 4) 0.4 s, 5) 0.5 s, 6) 0.6 s, 7) 0.7 s, 8) 0.8 s, 9) 0.9 s and 10) 1 s; inset is a zoom into the CAs for the first 1 s of the measurement. (B) Plot of ΓOH● vs. τd extracted from the CAs in (A); τi = 0.2 sec; aqueous phase 1 mgmL-1 BiVO4 in 0.1 M NaClO4; organic phase 0.5 mM [Co(bpy)3]3+ with 0.1 M TBAClO4; Au ME with rT = 12.5 µm (RG = 7), ET = -0.3 V (vs. AgQRE). Accordingly, Γ0 was obtained as 3.33×10-5 mol m-2 (or 322 µC cm-2) from the intercept of 0.3 mol-1 m2 and k amounted to 0.61×10-5 mol-1 m2 s-1 (or 60 µC-1 cm2 s-1) from the slope of 0.61 mol-1 m2 s-1.



Figure 6. Reciprocal ΓOH● vs. τd extracted from Figure 5. The experimental conditions are given in Figure 5.

CONCLUSION The SI-SECM technique has successfully been applied to ITIES with assembled photoactive semiconductor nanoparticles. A potential difference across the ITIES was maintained by the relatively hydrophilic common perchlorate anion in the two phases. This avoided the need of current collector electrodes in the two liquid phases for applying an external voltage. This is very important as it potentially allows the shrinking of the liquid phases and maximizing the area of the liquid-liquid interface per volume of the biphasic liquid system. While this work uses BiVO4 as a classical visible light-responsive WOR photocatalyst and the compositions of the two liquid phases were selected only as an example, the proposed methodology can be extended easily to the other polarized ITIES with other electrolyte solution compositions, other mediators and other photocatalysts differing by crystalline and electronic structures, morphology and doping levels. By technically coupling of soft interfaces to SI-SECM, several novel aspects have been demonstrated. Firstly, the MP was prepared with orifices smaller than the ME so that very high collection efficiencies for the reduced form of the redox mediator at the ME can be 16 ACS Paragon Plus Environment

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obtained. Such a well-defined area of MP with a known radius of its opening eliminated the uncertainty about the projected area of the substrate. On the other hand, it can optimally direct the light over the BiVO4 nanostructures which are settled at the opening of the capillary. Finally, the formation and decay of the OH• intermediates during photooxidation reaction at nanostructured semiconductor BiVO4 have been quantified in situ by monitoring the transient SECM currents after different delay times between photogeneration and detection. The integrated transient net current represents the quantity of oxidized intermediates chemically titrated by the reduced form of redox mediator produced at the ME.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: S1 Morphology of synthesized BiVO4, S2 Electrochemical characterization of [Co(bpy)3](PF6)3, S3 Setup for implementation of SI-SECM for photosensitized ITIES, S4 Positioning of the micropipette over the Au ME in x–y–z directions, S5 SI-SECM experiments at ITIES without BiVO4 photocatalyst

AUTHOR INFORMATION Corresponding Author: *E-mail:[email protected]. Fax: +49-441-798 3979. ORCID Shokoufeh Rastgar: 0000-0002-6012-3630 Gunther Wittstock: 0000-0002-6884-5515 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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In Situ Micro-Titration of Intermediates of Water Oxidation Reaction at

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