High-Resolution Analysis of Photoanodes for Water Splitting by Means

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High-Resolution Analysis of Photoanodes for Water Splitting by Means of Scanning Photoelectrochemical Microscopy Felipe Conzuelo, Kirill Sliozberg, Ramona Gutkowski, Stefanie Grützke, Michaela Nebel, and Wolfgang Schuhmann Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03706 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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Analytical Chemistry

High-Resolution Analysis of Photoanodes for Water Splitting by Means of Scanning Photoelectrochemical Microscopy Felipe Conzuelo,a Kirill Sliozberg,a Ramona Gutkowski,a Stefanie Grützke,a Michaela Nebel,b Wolfgang Schuhmanna,* a

Analytical Chemistry – Center for Electrochemical Sciences (CES), Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany b Sensolytics GmbH, Universitätsstr. 142, D-44799 Bochum, Germany * Corresponding Author E-Mail: [email protected], Fax: +49 234 3214683

ABSTRACT: In pursuance of efficient tools for the local analysis and characterization of novel photoelectrocatalytic materials, several SECM-based techniques have been developed, aiming on the combined benefit of a local irradiation of the analyzed sample and a microelectrode probe for the localized electrochemical analysis of the surface. We present the development and application of scanning photoelectrochemical microscopy (SPECM) for the laterally resolved characterization of photoelectrocatalytic materials. Particularly, the system was developed for the photoelectrochemical characterization of n-type semiconductor-based photoanodes for water splitting. By using the tip microelectrode simultaneously for local irradiation and as an electrochemical probe, SPECM was capable to simultaneously providing information about the local photocurrent generated at the sample under irradiation and to detect the photoelectrocatalytically evolved oxygen at the microelectrode. In combination with a novel means of irradiation of the interrogated sample local analysis of semiconductor materials for light-induced water splitting with improved lateral resolution is achieved. conversion devices with improved efficiency. In this regard, scanning electrochemical microscopy (SECM) has proven to be a powerful tool for the analysis of a wide variety of surfaces and has been applied to the study and screening of different photocatalysts.4,6-8 SECM is a scanning probe technique used for the study of local electrochemical properties of a surface.9 In addition, SECM was applied as a rapid method for the evaluation of libraries of photocatalysts10 and is supposed to be capable of providing in-depth characterization of photoactive systems. The interrogation of photoactive materials under dark and controlled irradiation conditions is of great importance. Several studies have addressed the analysis of photoactive samples using SECM. At first, interrogation and characterization of semiconductor materials was performed using a conventional SECM set-up, working under global irradiation of the analyzed sample.8,11-15 However, under irradiation of the entire sample the recorded photocurrent is generated at the entire sample surface. Hence, local changes of photoelectrocatalytic activity cannot be derived therefore impeding the characterization of complex samples such as materials libraries. Moreover, in the case of performing SECM area scans under global sample irradiation, processes like degradation or photocorrosion of the analyzed sample can influence the response obtained at the SECM tip. This is similarly the case for the recently proposed SECCM platform for ultrasensitive photoelectrochemical imaging.16

KEYWORDS: scanning electrochemical microscopy; scanning photoelectrochemical microscopy; SECM; SPECM; in-situ oxygen detection; photoelectrocatalysis; BiVO 4 INTRODUCTION In search for renewable clean energies the development of more efficient energy conversion devices is of great importance. Of particular attention is the light-driven electrolysis of water, where sunlight acts as source of energy allowing the formation of H2 under simultaneous evolution of O2.1-3 With the aim to obtaining highly efficient devices for water splitting, catalysts are required which are able to decrease the overpotential necessary for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Particularly, photoelectrochemical systems based on semiconductor materials have shown high efficiencies for the conversion of solar energy to fuels or electricity. While the HER is a comparatively easily catalyzed process featuring a low overpotential, the OER requires a significant overpotential even with the best known electrocatalysts due to the four-electron/four-proton transfer process.4 Metals and metal oxides have been investigated as potential efficient electrocatalysts for photolelectrochemical water oxidation,5 capable to promote the multielectron OER driven by photon-generated holes.4 Evidently, a better understanding of the properties and electrochemical processes involved in photoelectrochemical water splitting will lead to the design and development of energy

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wafers coated with a 100 nm Pt layer. A target of elemental Ti (99.99 %) was used. The deposition was performed at a substrate temperature of 400 °C and a chamber pressure of 1.33 Pa. The flows of reactive gas (O2) and sputter gas (Ar) were adjusted to 90 and 30 sccm, respectively.

In order to overcome these issues, localized irradiation of the interrogated sample was introduced. A set-up for local illumination was developed in which an optical fiber of ~400 µm in diameter was used instead of a tip microelectrode in a conventional SECM set-up for the analysis of different semiconductor materials.6,10 The localized response at a semiconductor surface could be evaluated by measuring the photocurrent produced at the analyzed surface during the scan of the optical fiber along the sample. Yet, this strategy was only able to evaluate the photocurrent generated locally at the sample, losing important information related to the electrochemical properties of the interrogated sample surface with the SECM tip. With the aim of combining the advantages of localized illumination of the sample with SECM analysis of the surface, the set-up was further improved by Lee et al.17 using a commercial Au-coated optical fiber sealed in a glass capillary as SECM tip. By this, on the one hand the localized photocurrent response at the interrogated sample could be recorded. On the other hand, the current recorded at the SECM tip could be additionally evaluated providing valuable information about the reactivity of the analyzed samples.10,17,18 Examples of application include rapid screening of arrays of catalysts dropcoated on a BiVO4-modified FTO substrate. In this case, the Au ring electrode at the tip was electrochemically plated with Pt for the detection of O2 produced during water photooxidation.4,10,19-21 Moreover, analysis of Ta3N5 nanotube arrays modified with several electrocatalysts for photoelectrochemical water oxidation was performed.7 The comparably large dimensions of the ring electrode dictated by the size of the Au-coated optical fiber10 provided a rather low resolution in the currents recorded at the tip microelectrode. Moreover, the possibility of using ring-shaped electrodes and of a restricted number of materials like gold or platinum, limit the applications of the available scanning photoelectrochemical microscopy (SPECM) systems. We present a further improved SPECM set-up for the evaluation of photoactive samples with increased resolution, capable of high-sensitive multi-purpose photoelectrochemical measurements. By coupling the light source into the microelectrode capillary and using the glass sheath of the microelectrode as a light guide, microelectrodes of different sizes and materials can now be used for the interrogation of different samples. In this way, the proposed set-up is not limited by size, material and geometry of the microelectrode as in previously reported systems. The set-up was applied for the characterization of various photoanodes for water splitting. An important feature is that the system was proved to provide a substantially higher resolution of the currents recorded with the SPECM tip. This characteristic was exploited for the analysis of complex photoactive samples.

PREPARATION OF Bi-V-O SAMPLES The deposition of BiVO4 was performed on fluorine-doped tin oxide (FTO) substrates (Pilkington, TEC 8A, 2.3 mm). Before use, the FTO substrates were cleaned with acetone in an ultrasonic bath for 5 min and in a 0.1 M NaOH bath at 75 °C for 15 min. After rinsing with water, the substrates were dried under an argon flow. All measurements were carried out in a three-electrode set-up using a Ag/AgCl/3 M KCl (210 mV vs. NHE) as reference electrode and a platinum mesh as counter electrode. The preparation of BiVO4 films was performed following a previously reported electrodeposition procedure.23 Before deposition, the FTO substrates were modified with Ptnanoparticles by applying a potential pulse profile in a 0.4 mM H2PtCl6 solution with the purpose to decrease the overpotential for the electrodeposition of BiVO4. A no-effect potential of 0 V was applied for 1 s followed by a deposition potential of -1 V for 0.2 s. The potential pulse profile was repeated 50 times. For BiVO4 deposition, a 0.54 M HNO3 solution containing 35 mM VOSO4 and 10 mM Bi(NO3)3 was prepared, increasing the pH value to 5 by adding 2 M sodium acetate (VWR). After adjusting the pH value to 4.7 with concentrated HNO3, the electrodeposition was performed applying 1.9 V vs. Ag/AgCl/3 M KCl for 600 s at 70 °C using a heating bath circulation thermostat. In order to avoid the precipitation of Bi0, the counter electrode was separated from the rest of the solution by means of a membrane. Mo-doped BiVO4 samples were prepared by addition of 5 mM NaMoO4 to the precursor solution. For the preparation of samples with a thickness gradient, BiVO4 was deposited while slowly inserting the FTO substrate in the modification solution at a constant speed and simultaneously applying the anodic potential for deposition. After that, the substrate was turned 90° and inserted again in the Bi-V precursor solution containing 2 mM NaWO4 or 2 mM NaMoO4, creating a thickness gradient both in Bi and the doping metal (W or Mo, respectively) as indicated in Figure S-1 (Supporting Information). After deposition, the samples were annealed in air at 500 °C for 1 h at a heating ramp of 2 K/min. Finally, the samples were cleaned from impurities in 1 M KOH for 20 min. A thick layer of cobalt oxide (CoOx) used as light blocking coating was deposited on Mo-doped BiVO4 (Figure S-2; SI) from a 2 mM CoCl2 and 0.5 M KHCO3 pH 8.4 solution following a procedure as in 24. Briefly, the sample was mounted in an electrochemical cell with an opening of 0.5 cm in diameter on one side and a quartz window on the other side. To obtain a homogeneous and dense film of CoOx, the deposition was performed under illumination through the quartz window using a 150 W Hg-Xe lamp (Hamamatsu photonics). Additionally, a potential of 0.6 V (vs. Ag/AgCl/3 M KCl) was applied to facilitate the reduction reaction at the counter electrode.

EXPERIMENTAL SECTION All reagents were of analytical grade and used without further purification. All chemicals were purchased from SigmaAldrich if not otherwise indicated. PREPARATION OF Ti-O SAMPLES

SCANNING PHOTOELECTROCHEMICAL MICROSCOPY

Sharp-edged TiO2 samples were prepared by reactive magnetron sputtering as described previously.22 Briefly, sputter deposition was performed with a sputter system (AJA Int.) on Si

SPECM experiments were performed using a Ag/AgCl/3 M KCl double junction reference electrode, a Pt cylindrical mesh

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counter electrode and a Pt microelectrode. The SPECM set-up is schematically shown in Figure 1A. The set-up is based on a previously described SECM system,25 consisting of three stepmotor driven micrometer screws (Owis) for positioning of the

SECM tip in x-, y-, and z-directions, a bipotentiostat (PGU-BI 100, Jaissle), and a control software programmed using Visual Basic 6.0.

Figure 1. (A) Representation of the scanning photoelectrochemical microscopy set-up used for local interrogation of semiconductor-based photoactive samples. Inset, schematic diagram of the microelectrode used simultaneously for local illumination and as sensing probe. CE: Counter electrode, RE: Reference electrode, WE: Working electrode. (B) Schematic drawing showing the dimensions of the Pt disk microelectrode and glass sheath used for local illumination. (C) The analysis of semiconductor samples for water splitting under illumination was performed by collecting evolved oxygen at a Pt microelectrode polarized at an adequate potential for ORR. The tip microelectrode was used for localized irradiation of the interrogated substrate and simultaneously as a probe for the detection of evolved oxygen during water photooxidation at the semiconductor sample. A 150 W Hg-Xe lamp (LC8, Hamamatsu photonics) was coupled to the end of the tip microelectrode by means of a light fiber (HITRONIC® POF Simplex PE). In this way, the glass sheath of the microelectrode acts as an extended light-guide allowing focalizing the radiation on the interrogated area just below the microelectrode tip. The Pt microelectrode was fabricated following a procedure described elsewhere26 using 25 µm Ø Pt wire (Goodfellow). In order to allow an adequate amount of light to reach the analyzed surface, the microelectrodes were fabricated with an RG value of about 8 (Figure 1B). The top end of the glass capillary was sealed and polished for coupling of the light fiber (Figure 1A). The effective light intensity which reaches the sample surface was measured to be 0.35 mW which corresponds to a light intensity of 280 mW cm‒2. The sample was placed in a Teflon SECM cell sealed with an Oring exposing an area of about 1.5 cm². Before starting the analysis of the modified surfaces, the tip was positioned in close proximity to the analyzed sample by recording zapproach curves in air-saturated buffer. For this, the tip was polarized at -600 mV vs. Ag/AgCl/3 M KCl for the steadystate measurement of the oxygen reduction reaction (ORR). The tip was then approached to the sample surface while recording the tip current. In order to avoid any possible interference from the sample on the current recorded at the tip during the z-approach, the sample was kept unbiased, thus behaving as an electrochemically inactive surface leading to negative feedback approach curves. For distance control during the scan, z-approach curves were recorded at three x-, y- coordinates and the tilt of the sample surface was calculated. The zdistance was corrected using the calculated tilt plane at each

point of the scanned grid of an SECM array scan experiment. During the scan a given potential was applied to the interrogated sample and tip by a bipotentiostat. The tip was then positioned at a predefined distance from the sample and the surface was scanned. The duration of a full area scan was between 40 min and 3.5 h. The measured tip current and sample photocurrent produced a color-coded image of the scanned surface. RESULTS AND DISCUSSION ANALYSIS OF SEMICONDUCTOR M ATERIALS In order to prove the capability of the developed system for the interrogation of photocatalyst surfaces, different semiconductor materials for photoelectrochemical water oxidation were evaluated.

Figure 2. Difference in tip current recorded under light and dark during an area scan of BiVO4 deposited on an FTO substrate. Sample polarized at +200 mV vs. Ag/AgCl/3 M KCl (910 mV vs. RHE). A 25 µm Ø Pt tip polarized at -600 mV vs. Ag/AgCl/3 M

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KCl was used for ORR detection. Area scan at constant distance of about 20 µm from the surface. x,y grid increment: 20 µm. Electrolyte: 0.1 M phosphate buffer, pH 8.5.

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strate. (A) Stripe of Mo-doped BiVO4 of about 100 µm width; (B) Mo-doped BiVO4 deposited on the left region of the substrate; (C) Mo-doped BiVO4 deposited on the left region of the substrate with a CoOx catalyst deposited additionally on top of the bottom half of the sample. The schemes on the right side illustrate the composition of the modified surfaces. Sample polarized at +200 mV vs. Ag/AgCl/3 M KCl (910 mV vs. RHE). The scan was recorded with a 25 µm Ø Pt tip polarized at -600 mV vs. Ag/AgCl/3 M KCl for ORR. Area scan at constant distance of the SPECM tip about 20 µm from the surface. x,y grid increment: 10 µm. Electrolyte: 0.1 M phosphate buffer, pH 8.5.

As a result of the localized illumination through the microelectrode tip coupled to the irradiation source, we were able to obtain 2D-maps of local photocatalytic activity of the investigated sample. Moreover, information about the semiconductor behavior for water oxidation was derived by collecting locally evolved O2 at the tip (Figure 1C). For the evaluation of the tip current at each grid point of the scanned area, the dark current was subtracted from the current measured under illumination. The tip current was therefore only related to the collection of oxygen formed upon illumination of the sample preventing any contributions from dark reactions. Two different approaches were evaluated to obtain the difference plots for tip current and sample photocurrent. Either the steady state current at each grid point of the scan area was recorded under dark and illumination before moving the tip to the next x,ygrid point, or two successive full scans were recorded under light and dark conditions. No significant differences were observed using both approaches (data not shown); however, for long scans the second approach was preferred to avoid possible sample degradation of the applied bias potential. The results obtained proved the ability for the interrogation of the sample with a microelectrode probe while simultaneously collecting data from the photocurrent at the sample, where the tip microelectrode acts at the same time as source for localized irradiation of the evaluated surface and for the collection of generated species (i.e., evolved O2). As an example, Figure 2 shows the SECM surface scan image of a BiVO4 thin film deposited on an FTO substrate. The photocurrent increase was consistent with an increased water oxidation to O2 at local hot-spots of the deposited semiconductor.

In the area scan, the high resolution of the tip current allowed to detect regions of different catalytic activity on a uniform semiconductor layer as a result of a microscopic inhomogeneity of the sample. Further evaluation of the SPECM set-up was performed by the investigation of different Bi-V-O samples. First, a Mo-doped BiVO4 stripe of about 100 µm width deposited on an FTO substrate was analyzed. In Figure 3A a clear contrast is shown between the two different regions of the sample. With the SPECM tip located above the unmodified FTO a minimum cathodic current at the tip was measured, while higher cathodic O2 collection currents were recorded with the tip positioned above the Mo-doped BiVO4 stripe upon illumination. The edge of a Mo-doped BiVO4 layer deposited on FTO was also investigated (Figure 3B). The ORR tip current drastically increased when the microelectrode was positioned above FTO modified with the semiconductor as a consequence of the high activity for water splitting of the locally illuminated Mo-doped BiVO4 region. Moreover, a sample consisting of the edge of a Mo-doped BiVO4 layer deposited on FTO was additionally modified with a cobalt oxide (CoOx) water oxidation catalyst (Figure 3C). The modified sample comprised four different regions, including bare FTO, Mo-doped BiVO4-modified FTO, bare FTO with CoOx, and Mo-doped BiVO4-modified FTO modified with a CoOx layer. Higher cathodic currents were recorded with the Pt tip positioned above the left side of the scanned area, corresponding to the region modified with a deposited Mo-doped BiVO4 film. The maximum cathodic current was observed if the microelectrode was positioned above the region modified with both Mo-doped BiVO4 and CoOx at the bottom left of the analyzed area, where CoOx acts as electrocatalyst for water oxidation.4 In contrast, the CoOx on FTO area at the bottom right only exhibits a slight increase in cathodic tip current. Moreover, as expected, the unmodified FTO substrate provided the lowest tip current. The obtained results encouraged us to evaluate the limits in resolution of the tip current during illumination of a semiconductor. HIGH RESOLUTION SPECM A higher lateral resolution of the locally recorded photocurrents as well as ORR tip currents was expected as compared with previously suggested systems due to the small surface of the used SPECM tip. Considering the spatial resolution of a system as the smallest distance between two different areas of a sample which are represented in an SECM image by significantly different currents,27 the resolution was evaluated by scanning across the sharp edge of a TiO2 layer deposited by sputter coating on a Pt-modified silicon wafer. Figure 4 shows the corresponding high lateral resolution both for the sample photocurrent as well as the ORR current recorded at the

Figure 3. Difference in tip current recorded under light and dark for different semiconductor samples deposited on an FTO sub-

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SPECM tip. To exclude topography effects from the sample, this scan was recorded using the ORR current measured under dark as feedback signal for positioning of the microelectrode at a constant tip-to-sample distance during the whole experiment. During the scan the tip and sample currents were recorded at the end of sequential dark and light periods at each analyzed grid point and the difference in current under light and dark was calculated. The small dimensions of the SPECM tip allowed for a high-resolution collection of evolved O2 (Figure 4A), in contrast to a negligible difference in tip reduction current when the tip was positioned above the unmodified substrate. Due to the localized irradiation of the sample surface the photocurrent recorded at the modified sample could also be simultaneously precisely measured (Figure 4B). The local sample photocurrent is in very good agreement with the determined activity for O2 evolution as shown in Figure 4A. The TiO2 layer exhibited a thickness gradient along the y direction, therefore leading both to an increasing ORR tip current as well as an increasing photocurrent along the ycoordinates.

pends on the distance between the sample and the tip. This effect was evaluated by performing line scans over the sharp edge of the deposited semiconductor (Figure S-3; SI). The obtained results indicate that the maximum resolution was reached with the microelectrode tip positioned at short tip-tosample distances of up to 20 µm from the surface. Moreover, the resolution in the collection of evolved O2 with the tip microelectrode was evaluated by comparing the ORR obtained in a line scan performed over the edge of the semiconductor (Figure S-4; SI) providing as expected a lateral resolution limit determined by the tip radius27 of ∆x/r = 6.7. The versatility of the implemented set-up allows to easily increase the spatial resolution by simply decreasing the size of the SECM tip used for performing the scan. While for the examples shown here a 25 µm Ø Pt tip provided a sufficiently high resolution, the dimensions of the tip electrode can be decreased if needed for the analysis of more complex samples. ANALYSIS OF COMPLEX PHOTOACTIVE SAMPLES Further studies were performed by analyzing a sample comprised of a 5 mm diameter spot of photo-electrodeposited CoOx on previously electrodeposited Mo-doped BiVO4. In this case, the intentionally thick layer of CoOx acts as just as a blocking coating, preventing the radiation from reaching the underlying Mo-doped BiVO4 layer. The sample was polarized at a potential sufficient for light induced water splitting by the deposited Mo-doped BiVO4, but low enough to prevent direct water oxidation at the CoOx spot in the dark. Therefore, only regions of the sample with exposed Mo-doped BiVO4 are showing light-induced water oxidation. No detection of O2 is expected when the tip is positioned on top of the CoOx spot. The sample was first evaluated by recording line scans over the CoOx spot, with the tip positioned at different distances from the sample surface (Figure S-5; SI). For larger tip-tosample distances (> 20 µm), a lower ORR current is recorded at the tip when it was positioned on top of the CoOx spot, thus leading to a lower contrast among the different regions in the recorded line scan.

Figure 4. (A) Difference in tip current and (B) sample photocurrent recorded during an area scan of the edge of a TiO2 layer deposited on a Pt-modified silicon wafer. Sample polarized at 400 mV vs. Ag/AgCl/3 M KCl (1023 mV vs. RHE). The area scan was recorded using a 25 µm Ø Pt tip polarized at -600 mV vs. Ag/AgCl/ 3 M KCl for ORR. Scan at constant distance of about 10 µm from the surface. x,y grid increment: 25 µm. Electrolyte: 0.1 M phosphate buffer, pH 7.0.

Interestingly, as the tip is used simultaneously as source of irradiation, the resolution of the recorded photocurrent de-

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Bi-V with a thickness gradient in metal composition as illustrated in Figure S-1 (SI).

Figure 6. Difference in tip current (A, C) and sample photocurrent (B, D) recorded during the surface scan of W-Bi-V (A, B) and Mo-Bi-V (C, D) samples prepared with a thickness gradient in metal loading as indicated in Figure S-1. Sample polarized at 489 mV vs. Ag/AgCl/3 M KCl (1230 mV vs. RHE). The scan was recorded with a 25 µm Ø Pt tip polarized at -600 mV vs. Ag/AgCl/3 M KCl for ORR. Scan at constant distance of about 20 µm from the surface. x,y grid increment: 1000 µm. Electrolyte: 0.1 M borate buffer pH 9.0.

Figure 5. Tip current recorded during an area scan of a Mo-doped BiVO4 sample on FTO with a spot of about 5 mm Ø of photodeposited CoOx. (A) Scan without illumination, (B) scan with irradiation, (C) difference in tip current between (B) and (A). Sample polarized at 0.0 mV vs. Ag/AgCl/3 M KCl (980 mV vs. RHE). The scan was recorded with a 25 µm Ø Pt tip polarized at -600 mV vs. Ag/AgCl/3 M KCl for ORR. Scan at constant distance of about 20 µm from the surface. x,y grid increment: 100 µm. Electrolyte: 0.1 M KOH.

As previously demonstrated,28 the thickness of the deposited semiconductor layer is directly related to the deposition time in the precursor solution for the formation of the semiconductor layer in the modification of FTO substrates (Figure S-6; SI). The thickness gradient in metal composition of the modified samples was evaluated by EDX analysis of an FTO substrate modified with the W-Bi-V system. The obtained results presented in Figure S-7 (SI), for the different analyzed regions on the gradient line for W-Bi, show an increased deposition time directly related with a higher loading of semiconductor, as inferred from a higher intensity in the characteristic elemental peaks. Thus, confirming the presence of a thickness gradient in the prepared samples. For analysis of thickness-gradient samples with the SPECM set-up, the tip was scanned over a large area of the modified surfaces recording simultaneously the response at the sample under local illumination and the collected evolved O2. The differences in tip current and sample photocurrent after the analysis of both semiconductor samples is depicted in Figure 6, while the individual values obtained during the scan under light and dark conditions are presented in Figure S-8 (SI) for the W-Bi-V sample and in Figure S-9 (SI) for the Mo-Bi-V sample. Interestingly, the highest photoelectrocatalytic activity for light induced oxygen evolution is related to smaller thicknesses of the BiVO4 layer. At thick films both the tip current and sample photocurrent decreased, indicating a reduced production of molecular oxygen. This can be explained by the limiting impact of electron diffusion to the back contact for thick

The highest contrast was observed when the tip was positioned 20 µm above the interrogated sample. This distance was selected for performing area scans of the sample surface (Figure 5). In the dark, the current recorded at the tip was negligible, only allowing to visualize the CoOx spot due to small changes in the tip current caused by topographic effects (Figure 5A). Under irradiation, the semicircular spot of the deposited CoOx on top of the Mo-doped BiVO4 sample becomes visible. The O2 collection current recorded at the tip increases when the tip was positioned above the Mo-doped BiVO4 modified substrate, while a negligible current for oxygen collection at the tip was obtained when the microelectrode was positioned above the circular spot of CoOx (Figure 5B). In Figure 5C the difference in tip current recorded under light and dark is presented. A clear edge between the two different regions is observed, in accordance with the microscope image of the modified surface (Figure S-2; SI) and demonstrating the capability for the collection of evolved O2 with the SPECM tip used simultaneously for localized irradiation of the sample. The SPECM was additionally applied for the analysis of semiconductor materials with a thickness gradient (Figure 6). The two analyzed samples were prepared as described in the Experimental Section, comprising the systems W-Bi-V and Mo-

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layers of BiVO4, resulting in high rates of recombination of the photogenerated electron/hole pairs. Moreover, a higher activity was observed for the Mo-doped sample in comparison with the W-containing system, as can be inferred when the intensities in the recorded sample photocurrent and tip current are compared, suggesting an increased activity for Mo-doped samples under the same conditions.

ACKNOWLEDGMENTS

CONCLUSIONS

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The proposed SPECM set-up allows the local interrogation and analysis of photoactive samples, as shown exemplarily for the analysis of photoanodes for water splitting. The use of a microelectrode tip for simultaneous local irradiation of the analyzed sample and as SECM probe for the collection of electroactive species provides a high lateral resolution both, at the sample photocurrent and at the tip current. Moreover, as the glass sheath of the microelectrode is used as light guide, virtually all electrode materials and dimensions are possible to use. The high contrast obtained for the collection of evolved oxygen can be directly related with the sample photocurrent, making the developed system of particular interest for the screening of libraries of OER-catalysts bearing different catalyst loadings. The resolution of the system for the collection of electroactive evolved species was proved to be superior when compared to previous systems and it is expected that even a higher resolution could be attained by decreasing the surface of the Pt microelectrode, if necessary. Consequently, the implemented SPECM set-up is suggested as a powerful tool for highresolution analysis and characterization of different photoactive samples.

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This work was supported by the Cluster of Excellence RESOLV (EXC 1069) and in the framework of the SPP 1613 (SCHU929/12-1, 12-2) funded by the Deutsche Forschungsgemeinschaft (DFG).

REFERENCES

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ASSOCIATED CONTENT (15)

Supporting Information Supporting Information Available: Additional figures including a scheme for the prepared binary samples with thickness gradient, microscope image of CoOx deposited on Mo-BiVO4, photocurrents recorded while moving the microelectrode tip at different distances from the surface, evaluation of the resolution limit of the tip microelectrode, tip current plots recorded at different tip-tosample distance, SEM image for thickness gradient evaluation, EDX analysis of gradient W-Bi-V sample, tip and sample currents recorded under dark and light for W-Bi-V, tip and sample currents recorded under dark and light for Mo-Bi-V. (PDF) This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author

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*E-Mail: [email protected] Tel.: +49 234 3226200 Fax: +49 234 3214683 (25)

Author Contributions (26)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes

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The authors declare no competing financial interest.

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High-Resolution Analysis of Photoanodes for Water Splitting by Means

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