Methods for Electrochemical Synthesis and Photoelectrochemical

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Methods for Electrochemical Synthesis and Photoelectrochemical Characterization for Photoelectrodes Gokul V. Govindaraju,† Garrett P. Wheeler,† Dongho Lee, and Kyoung-Shin Choi*

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Department of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States ABSTRACT: Electrodeposition is a widely used technique for electrode synthesis in various applications. Because of its low synthesis cost and easy scalability, electrodeposition is particularly attractive for the production of semiconductor and catalyst electrodes for use in solar fuel production. For researchers who are interested in learning about or utilizing electrodeposition, the current paper describes detailed methods for electrodeposition, which include procedures for preparing electrodes and plating solutions, determining deposition conditions, and performing electrodeposition. Postdeposition treatments that can be used to prepare electrodes of more diverse compositions and photodeposition procedures that can be used to place catalyst layers on semiconductor electrodes are also provided. The methods are described using the synthesis and modification of photoelectrodes as an example, but most principles and procedures explained in this paper are general and can be applied to electrodeposition of various electrodes. In addition to methods for electrochemically preparing photoelectrodes, methods for photoelectrochemical characterization, which include light setup and calibration, photoelectrochemical characterization, and efficiency calculations, are described along with rationale for each setup and procedure. This will improve understanding and performance of various experimental procedures used for photoelectrode evaluation.

1. INTRODUCTION Electrodeposition has been traditionally regarded as a synthesis method used mainly for metal plating. However, the scope of electrodeposition has been significantly broadened in recent years and is now one of the major synthetic tools for the production of a variety of inorganic electrodes. The types of materials that can be electrodeposited include oxides, hydroxides, phosphates, chalcogenides, and III-Vs as well as metals and metal alloys.1 Additionally, electrodeposited films can be thermally or chemically treated to produce films with more complex compositions (e.g., ternary compounds) and unique morphologies, which further expands the types of materials that can be prepared as high quality film-type electrodes via electrodeposition. Electrodeposited materials are directly grown on conducting substrates, ensuring good electrical contact and continuity. This feature is particularly advantageous for producing and investigating electrodes for use in electrochemical and photoelectrochemical applications. Compared to gas phase depositions (e.g., atomic layer deposition and organometallic vapor phase epitaxy), which require specific vacuum and/or heating conditions as well as the use of specialized precursors, solutionbased electrodeposition performed under ambient conditions is inexpensive and easily scalable. This makes electrodeposition more attractive for producing semiconductor and catalyst electrodes for use in energy related applications, such as solar fuel production, where the production cost of the electrode is a critical factor to consider.1 We have recently reviewed various electrodeposition conditions developed to produce photoelectrodes and catalysts for use in water splitting photoelectrochemical cells (PECs).1 © 2016 American Chemical Society

This review paper along with basic textbooks about electrodeposition can provide a foundation for using electrodeposition for materials synthesis and development.1−3 However, these resources may not necessarily provide the specific details required for electrodeposition experiments. Ideally, research papers on electrodeposition should offer experimental methods to reproduce the results contained, but detailed procedures and the rationale behind the chosen procedure are often omitted due to space limitations. For the same reason, research papers on the investigation of photoelectrodes frequently do not explain the measurement setup and the procedures used to perform photoelectrochemical measurements in detail, although these explanations can be useful for new researchers in the field to reproduce and better understand the procedures. To address the aforementioned issues, the focus of this paper is to provide detailed and practical information on the procedures used for the electrodeposition and photoelectrochemical characterization of photoelectrodes that are used in our group. We first describe our routine procedures for preparing electrodes and plating solutions, determining deposition conditions, and performing electrodeposition. Then we discuss postdeposition procedures, which can be applied to the as-deposited electrodes to form electrodes of more diverse compositions or morphologies, and photodeposition procedures that can be used to deposit catalysts Special Issue: Methods and Protocols in Materials Chemistry Received: August 18, 2016 Revised: October 31, 2016 Published: November 16, 2016 355

DOI: 10.1021/acs.chemmater.6b03469 Chem. Mater. 2017, 29, 355−370

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Chemistry of Materials on photoelectrodes. Although the synthesis methods described in this paper are written for the preparation of photoelectrodes, most procedures and principles discussed here can be generally applied to the electrodeposition of electrodes and catalysts for use in other applications. After explaining procedures for the preparation of photoelectrodes, we describe methods used in our group for photoelectrochemical characterization of the photoelectrodes, including light setup and calibration, photoelectrochemical characterization, and efficiency calculations. We explain the rationale behind each setup and procedure so that a better understanding of various experimental processes used for photoelectrode evaluation, and, therefore, more accurate measurements and reporting can be achieved. We hope that the methods described in this paper are useful to the electrodeposition and solar fuel communities.

FTO and ITO also exhibit differences in their chemical stability. ITO is known to be chemically unstable in acidic conditions (pH < 5) due to the slow dissolution of In3+, but it is stable at moderate and basic pH conditions.8,9 FTO is chemically stable between pH 0 and pH 14 and, thus, it is better suited for depositions in acidic solutions, especially when a longer deposition time is required.8 Both FTO and ITO are anodically stable but they can be reduced under cathodic conditions (i.e., reduction of Sn4+ or In3+).10,11 The reduction of FTO and ITO can be observed by a color change to gray. The exact potential where the reduction of ITO or FTO occurs varies depending on the composition of the plating solution (e.g., pH). Therefore, for cathodic deposition, it is recommended to examine the electrochemical behaviors of ITO and FTO in the plating solution of choice using linear sweep voltammograms (LSVs) or cyclic voltammograms (CVs) to determine the potential window where these substrates are electrochemically inert. Noble metals such as Pt and Au are often used as the WE to deposit photoelectrodes due to their wide windows of chemical, thermal, and electrochemical stability. For use as the WE, they are usually prepared as thin films using sputter coating or ebeam evaporation on glass slide substrates using Ti or Cr as the underlying adhesion layers. Other metals commonly used as the WE include stainless steel, Cu, and Ti. Because these metals are inexpensive, their foil forms can be used directly. However, these metals are not inert against thermal oxidation and may be used as a WE only when the photoelectrode films are crystalline as-deposited or when crystallinity can be achieved by postdeposition annealing under inert atmospheres (Ar or N2). Another important factor to consider when choosing a WE is the adhesion and morphology of the photoelectrode film. The nucleation, growth, and adhesion of the deposited film can be affected considerably by the type of WE. Therefore, the optimum WE may be determined after examining the adhesion and morphology of the desired material using various WEs. The type of semiconductor junction created between the WE and the photoelectrode is another criterion for selecting the proper WE. The substrate selected as the WE for the deposition of a photoelectrode will serve as the back contact of the photoelectrode. It is desirable to form an Ohmic contact between the WE (back contact) and the photoelectrode being synthesized in order to facilitate the transport of majority carriers to the counter electrode (CE) during PEC operation. Therefore, the Fermi level of the WE, which should be comparable to that of the film to be deposited on the WE, can be another consideration for the selection of a WE. The Ohmic nature at the WE/photoelectrode interface may be tested by first depositing a metal contact on the surface of the photoelectrode, which forms an Ohmic contact at the photoelectrode/metal interface, and then obtaining solid state I−V characteristics of the WE/photoelectrode/metal cell. 2.1.2. Counter Electrode (CE). CEs are typically materials that are electrochemically inert over a wide range of potentials such as Pt or carbon-based electrodes. It is always useful to check the potential applied to the CE vs the reference electrode (RE) during electrodeposition, which can be done using a multimeter, to evaluate the CE reaction and ensure that the CE reaction does not directly affect the deposition on the WE. For example, when a Pt CE is used and the potential applied to the CE is sufficiently positive to dissolve Pt, the resulting Pt ions may be codeposited with the photoelectrode material on the WE, altering the performance of the photoelectrode.12 If the

2. ELECTROCHEMICAL SYNTHESIS OF PHOTOELECTRODES In this section, we describe our procedures for the selection and preparation of electrodes, preparation of plating solutions, electrodeposition, and postdeposition processes to modify asdeposited films and to add catalyst layers. 2.1. Selection and Preparation of Electrodes. 2.1.1. Working Electrode (WE). Various factors need to be considered when choosing a proper working electrode (WE) for the preparation of a photoelectrode. The WE must be chemically and electrochemically inert, meaning that it should not react with the plating solution and that it should not be oxidized or reduced during electrodeposition. (Preparation of photoelectrodes by anodization of a substrate used as a WE is an exception and is not considered in this paper.) Most asdeposited films require postdeposition annealing processes, either to form a desired phase or to increase the crystallinity of the as-deposited film. Therefore, the WE should also be inert under the necessary postdeposition annealing conditions so as not to form an undesired interfacial layer between the WE and the photoelectrode film during annealing, which may affect or interfere with the photoelectrochemical performance of the photoelectrode. Transparent conducting oxides (TCOs), such as fluorinedoped tin oxide (FTO) or tin-doped indium oxide (ITO), are frequently used as the WE for preparing photoelectrodes. This is because they allow for the measurement of photocurrent using back-side illumination as well as front-side illumination. This is advantageous because a comparison of the photocurrent generated by front-side and back-side illumination can provide useful information regarding charge transport properties of the photoelectrode.4−7 The use of a TCO WE to deposit photoelectrodes also allows for facile measurements of absorption spectra of the photoelectrode in transmittance mode. Selection between FTO and ITO as the WE depends mainly on the annealing conditions necessary for the preparation of a specific photoelectrode. ITO maintains its sheet resistance to about 350−400 °C before its conductivity begins to decrease. FTO has a much higher thermal stability and can be heated up to about 650 °C before a significant conductivity loss occurs. ITO and FTO have similar optical transmissions, ranging between 80 and 90% depending on the synthetic route taken. However, ITO has a notably higher cost than FTO because of the relative scarcity of its primary component, In. 356


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2.1.4. Preparation of WE. Prior to use, the substrate upon which the material will be deposited must be thoroughly cleaned to ensure uniform growth of the target material with good adhesion to the substrate. For the various types of substrates, different types of cleaning processes are employed, which are described well in other references.14,15 The typical size of the WE used in our group is 1 cm × 2.5 cm (Figure 1a). After the substrate/foil is cut to this desired

presence of Pt impurities affects the properties of the photoelectrode, but the presence of Pt in the deposited film is not identified, incorrect conclusions can be made. If the anodic dissolution potential of a Pt CE vs RE is identified in a given plating solution, monitoring the potential applied to CE vs RE during electrodeposition allows for adjusting the deposition condition to prevent undesired CE reactions. The surface area of the CE should, at a minimum, match the surface area of the intended WE so as not to limit the rate of the electrochemical reaction occurring at the WE. 2.1.3. Reference Electrode (RE). The choice of RE should depend on the composition and pH of the plating solution. Our group typically uses a silver/silver chloride (Ag/AgCl) RE (in 4 M KCl) when an aqueous medium is used and the pH is not higher than 13. When leaking of Cl− ions from the RE into the plating solution can complicate the deposition (e.g., Cl− forming insoluble salts with Ag+ or Pb2+ ions in the solution or affecting the morphology of the film), the use of a double junction RE should be considered. A double junction Ag/AgCl RE contains an inner and outer chamber. The inner chamber contains a AgCl covered Ag wire and 4 M KCl filling solution saturated with AgCl used as the reference couple. This inner chamber is connected by a frit to an outer chamber that is filled with a secondary inert solution, such as K2SO4 or KNO3, to minimize the leakage of Cl− into the plating solution. When a strongly basic plating solution (pH > 13) is required, a Hg/HgO RE (in 1 M NaOH) is more suitable, as the Ag/ AgCl RE is unstable under these conditions due to the precipitation of Ag+ to Ag2O.8 Such precipitation may occur within the frit, in the plating solution, or on the Ag/AgCl electrode itself, which reduces the stability and kinetics of the reference couple. When nonaqueous solvents are used for plating solutions (e.g., formamide, acetonitrile, propylene carbonate, and dimethyl sulfoxide), our group typically uses a Ag/Ag+ RE with a filling solution containing 0.1 M AgNO3 in the same solvent as the plating solution.13 A regular Ag/AgCl RE in aqueous 4 M KCl solution may still be used for deposition in nonaqueous media if leakage of water into the nonaqueous plating solution does not have a significant effect on the electrodeposition. It should be noted that an aqueous RE in nonaqueous media may suffer from a non-negligible liquid junction potential at the water/nonaqueous solvent interface as well as a lower solubility of KCl in nonaqueous media, resulting in the solidification of KCl at the junction (e.g., pores in the frit).13 REs should be stored in vials of their filling solution to ensure that the composition of the filling solution within the RE does not change, as the dilution or contamination of the filling solution may affect the redox potential of the RE. The use of a RE that can measure the WE potential accurately and consistently is of utmost importance for obtaining reproducible deposition results. Therefore, it is recommended that before each use of the RE, the potential of the RE is tested against a new RE (never used for electrochemical experiments) of the same kind, which serves as a benchmark electrode, immersed in 4 M KCl; the open circuit potential (OCP) between the two REs immersed in the same solution should be zero if the RE is reading accurately. Drifts of more than a few millivolts from zero or constant fluctuation of the potential reading requires cleaning of the frit or the inner chamber, replacement of the filling solution, or replacement of the RE.

Figure 1. Photographs of (a) an FTO substrate (1 cm × 2.5 cm), (b) Cu tape applied to the FTO, (c) Teflon tape covering the surface of the Cu tape, (d) immersion of a prepared FTO WE in a plating solution, showing the desired position of the solution/air interface on the WE, and (e) an FTO WE with additional masking to prevent the edge effect.

size, Cu electrical tape with the back side containing conductive adhesive (3M Company) is attached to the WE to enable connection to the potentiostat lead (Figure 1b). The tops of the electrodes are then covered with Teflon tape (J.V. Converting Company, Inc.) (Figure 1c). When the WE is immersed in the plating solution, the position of the WE is adjusted so that the solution/air interface is located within the region wrapped with the Teflon tape (Figure 1d). A deposition made at or near the solution/air interface can differ significantly from depositions made in the bulk solution, resulting in uneven thickness, composition, and adhesion of the film. Therefore, the presence of the Teflon tape mask gives a greater likelihood of uniform depositions on the exposed area of the WE. Additionally, the Teflon tape protects the Cu tape from making contact with the electrolyte, which may result in undesired current generation directly from the Cu. The procedures explained above are also used when metal foil or e-beam deposited metal films are used as WE or CE. When metal foils are used as the WE, regulation of the deposition occurring on the front side is facilitated by also covering the back side of the WE with Teflon tape or other electrochemically inert coatings to prevent a double-sided deposition. 357


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Chemistry of Materials The edges of the WE may need to be masked when film uniformity suffers seriously from the edge effect. The edge effect is a result of higher current density generated at the edges of the WE due to greatly enhanced mass transport around the edges of the electrode, as opposed to the central region of the WE.16,17 When the effect is serious, the film deposited at the edge regions can be much thicker, can have a different morphology, and can even have a different composition due to the serious deviation of the current density generated in these areas. When these phenomena are observed, masking the edges of the film can provide better control over current densities across the film, and, therefore, the deposition of more uniform and reproducible films becomes possible. In our group, an electroplating tape designed to be inert in most common chemical and solvent systems (3M Company) is cut to the dimensions of the WE with a hole of an appropriate size punched out of the center and is then attached to the WE surface (Figure 1e). 2.2. Preparation of Plating Solutions. When plating solutions are prepared, purified water is used because any impurities in the plating solution can affect the nucleation and growth processes of the film. Impurities in the plating solution can also be incorporated into the film and affect the properties of the film considerably. Even if the change caused by the impurity is favorable, any unrecognized presence of impurities may lead to incorrect conclusions or results that may not be easily reproduced by others. Our facilities provide water treated by reverse osmosis and deionization, and we purify this water further using a 3-module water purification system (Barnstead E-pure water purification systems) where all ionic impurities and nonionic particles with a diameter larger than 0.2 μm are removed. The purity level of the output water is monitored by the resistivity of the water and is maintained to be ≥18 MΩ·cm. (The maximum resistivity of water free from ionic impurities obtainable with this purification system is 18.2 MΩ·cm at 25 °C.) When thorough removal of organics is critical, an additional filter composed of activated carbon can be added to the purification system. Before plating solutions are prepared, the purified water is purged with an inert gas such as Ar or N2 to remove dissolved O2 or CO2 that may be present. Dissolved O2 can oxidize easily oxidizable metal ions in the plating solution (e.g., Fe2+) prior to electrodeposition, decreasing the concentration of the metal ions and resulting in undesirable bulk precipitation in the plating solution (e.g., FeOOH). Furthermore, dissolved O2 is electrochemically active (e.g., O2 + 4H+ + 4e− → 2H2O) and can compete with a reduction reaction that should occur at the WE during cathodic deposition. If the dissolved O2 appears to have a serious detrimental effect on the deposition, continuous purging with N2 throughout the deposition can be employed. Dissolved CO2 can decrease the pH of the plating solution by forming carbonic acid and serves as a source of pH variability. The plating solution can contain electrochemically active species, which are oxidized or reduced during electrodeposition, and various electrochemically inactive species that play other important roles. In the case of metal plating, the electrochemically active species and the species that are deposited as films are identical. However, for deposition of metal oxide-based compounds, the electrochemically active species and the species that are deposited as films may be different. For instance, electrodeposition of BiOI can be performed using a plating solution composed of Bi(NO3)3, KI, and p-benzoquinone. In this case, the electrochemically active species is p-benzoqui-

none, but it is not the species that is deposited on the WE. Instead, Bi3+ from Bi(NO3)3 and I− from KI are incorporated into the film during deposition. The deposition utilizes the local pH increase at the WE due to the reduction of p-benzoquinone (eq 1), which lowers the solubility of Bi3+ at the WE and induces the precipitation of Bi3+ as BiOI.

When the metal ion sources are not soluble in a solution of a desired pH, complexing agents can be used to solubilize the metal ions. For example, for the deposition of BiOI, I− serves as the complexing agent for Bi3+ to form soluble BiI4− species because Bi3+ is not very soluble in an aqueous plating solution with pH ≥ 3. Although Bi3+ is soluble in strongly acidic solutions, the use of a strongly acidic plating solution is not feasible as the more acidic solution would also result in the dissolution of the BiOI deposits. Complexing agents can also be used to shift the reduction potential of a metal ion in the negative direction. When two metal ions with drastically different reduction potentials need to be codeposited to form alloys, a complexing agent, which can bind more strongly to the metal ions with a more positive reduction potential, can be used to make the reduction potentials of the two metal ions more comparable.2 Having a plating solution with high ionic strength is critical, as the movement of electrons through the external circuit during deposition needs to be coordinated with the movement of ionic species in solution. Therefore, the solution conductivity directly affects the deposition current. Also, when the solution is too resistive, the potentials measured between WE and RE will not be accurate due to the considerable uncompensated IR drop in solution. Insufficient ionic strength in solution can also affect the formation of the electrical double layer at the electrode surface and cause the electrical double layer structure to stretch out into the solution. This has the effect of decreasing the potential field available for interfacial electron transfer at the electrode−solution interface.16,17 Therefore, when the ionic strength achieved by electrochemically active species and other species serving various purposes (e.g., complexing agents, metal sources) is low, electrochemically inactive ionic species, referred to as the supporting electrolyte, need to be added to ensure proper solution conductivity. Various inorganic or organic species can also be added to affect the film growth rate, growth direction, and morphology.2 When experimental sections for electrodeposition processes are described, it is highly recommended that the composition of the plating solution and the role of each component are clearly described. Additionally, the pH of the plating solution should be reported to ensure facile reproduction of the reported deposition conditions. 2.3. Deposition Modes. The deposition potential and deposition current cannot be controlled independently; only one of the two can be manipulated or held constant during deposition. A deposition carried out under constant potential is called potentiostatic deposition whereas a deposition carried out under constant current is called galvanostatic deposition. Potentiostatic deposition typically employs a three-electrode setup and applies a constant potential between the WE and the RE during electrodeposition. Because the potential applied to the WE can be controlled precisely, it is easy to deposit a pure 358


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Chemistry of Materials single phase material on the WE even when multiple electrochemical reactions with reduction potentials close to one another are possible in the given plating solution. As the deposition continues and the concentration of the electrochemically active species in solution decreases, a corresponding decrease in the deposition current is typically observed. Galvanostatic deposition applies a constant current between the WE and CE during electrodeposition and is compatible with a two-electrode setup that does not require a RE. A twoelectrode cell can be setup using a potentiostat by connecting both the lead for RE and the lead for CE to the CE. In this case, the potential recorded by the potentiostat is the potential between the WE and CE, the cell potential. During galvanostatic deposition, the cell potential gradually increases over time to keep the deposition current constant as the activity of the electrochemically active species decreases. This may trigger the initiation of other reactions that can compete with the desired reaction and cause the deposition of undesirable phases and loss of Faradaic efficiency for the desired reaction. Therefore, during galvanostatic depositions it is useful to monitor the change in the WE potential against the RE to ensure that the deposition potential remains within the range where only the desired reaction occurs. To monitor the potential of the WE against a RE, galvanostatic deposition can be carried out using a threeelectrode setup and the potentiostat can read the potential between the WE and RE during the deposition. If the potential of the WE against the CE as well as the potential of the WE against the RE needs to be monitored, a multimeter can be used to read the potential difference between the WE and the CE. Galvanostatic deposition is generally advantageous when it is critical to keep the deposition rate constant in order to achieve a specific morphology or obtain certain properties of the desired phase and when the desired reaction is the only reaction that can occur in the expected deposition potential range. 2.4. Setup of an Electrochemical Cell. A typical threeelectrode setup in an undivided cell used in our group is shown in Figure 2. The RE should be placed as close as possible to the WE in order to minimize the effects of the uncompensated solution resistance, which prevents precise control over and reading of the WE potential. When a planar WE and CE of similar size are used, the CE is best placed parallel to the WE so that uniform distribution of current density throughout the WE can be achieved, which ensures the uniformity of the deposited film. Because the distance between the WE and CE affects the deposition current, the distance between the WE and CE should be kept constant to ensure reproducibility. In our cell setup, the typical distance between the planar WE and CE is ∼1 cm. If the CE reaction is found to generate products that may interfere with the electrodeposition at the WE, a separator that prevents the flow of these species to the WE chamber may be advised. 2.5. Selecting Deposition Conditions. For potentiostatic deposition, appropriate deposition potentials can be most rationally determined by obtaining and examining a LSV using the chosen plating solution and WE to identify a potential range where only the desired reaction can occur. The standard reduction potentials for the reaction of interest can be used to predict appropriate deposition potentials.18 However, because the standard reduction potential values do not offer any information as to how much kinetic overpotential is necessary to initiate the electrochemical reaction, predictions based solely

Figure 2. Typical three-electrode cell setup with an undivided cell for potentiostatic deposition: (a) an FTO WE, (b) an e-beam evaporated Pt CE, and (c) a Ag/AgCl double junction RE. Regular beakers are typically used as undivided cells but a square cell made of quartz is used for this photo to show the arrangement of the electrodes more clearly.

on thermodynamic data can often fail in identifying proper deposition potentials. We will use the electrodeposition of BiOI as an example to explain how LSV data can be used to determine deposition conditions. BiOI is deposited from a plating solution containing 0.03 M Bi(NO3)3·5H2O (Sigma-Aldrich, ≥98.0%), 0.3 M KI (Sigma-Aldrich, ≥99.0%), and 0.067 M p-benzoquinone (Sigma-Aldrich, ≥98%) in a mixed solvent solution made up of 50 mL water and 20 mL ethanol, using the reduction of pbenzoquinone to initiate the deposition. Ethanol was added to increase the solubility of p-benzoquinone, and the roles of the other components were explained above in the section describing the preparation of the plating solution. The pH of the final solution was adjusted to 3 by adding nitric acid. The reduction of p-benzoquinone to hydroquinone generates OH− (eq 1) at the surface of the WE, causing Bi3+ stabilized as [BiI4]− to precipitate out as BiOI on the WE. The major competing reaction for the deposition of BiOI is the reduction of Bi3+ to Bi (eq 2). Therefore, it is necessary to identify a deposition potential that can reduce only p-benzoquinone but not Bi3+ in order to deposit pure BiOI. Bi 3 + + 3e− → Bi

E° = 0.308 V

NO3−

−

−

(2)

+ H 2O + 2e → NO2 + 2OH

−

o

E = 0.01 V (3)

In this case, an LSV is first obtained from a plating solution that does not contain p-benzoquinone (Figure 3, blue) in order to identify the reduction potential for the deposition of Bi metal. The reduction onset observed in this solution may also be due to the reduction of nitrate as Bi(NO3)3 was used as the Bi3+ source. To observe the reduction of nitrate separately from the reduction of Bi3+, a control solution containing nitrate without Bi3+ can be prepared and an additional LSV can be obtained. As an example, an LSV obtained from a solution containing 0.09 M KNO3 and 0.3 M KI is shown in Figure 3 359


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Elevated deposition temperature can also improve the crystallinity of the deposited phase or provide a new window of potential to deposit a phase that may not be deposited at RT. Elevated temperature can also be used if the solubility of any of the components that need to be dissolved in the plating solution is limited at RT. Therefore, temperature is an important deposition condition, and its effect on electrodeposition should be carefully examined. Because temperature can affect both the thermodynamics and kinetics of the desired reaction and its competing reactions, when selecting deposition conditions at different temperatures, an LSV at each temperature should be examined. 2.6. Postdeposition Treatments. When electrodeposition is combined with postdeposition chemical or annealing processes, even more diverse phases can be obtained as filmtype electrodes. Therefore, our group develops various ways to use electrodeposited films as precursor films to form desired phases using postdeposition chemical or annealing processes. This has significantly broadened the type of photoelectrodes that can be prepared with desired morphologies and high quality contacts to the substrate, which may not be easily obtained by any other synthesis method.1 In this section, we describe our typical postdeposition annealing procedure for the conversion of electrodeposited BiOI to BiVO4 as an example.19,20 The extremely thin plate morphologies of BiOI crystals that are vertically grown from the WE during electrodeposition can be converted to high surface area nanoporous BiVO4 films using postdeposition chemical/ annealing processes (Figure 4). The resulting morphology, which cannot be easily achieved by direct deposition of BiVO4, is highly favorable for increasing electron−hole separation efficiency.19

Figure 3. LSVs of FTO WEs in a solution containing Bi(NO3)3 and KI (blue), a solution containing KNO3 and KI (red), and a solution containing Bi(NO3)3, KI, and p-benzoquinone (black). Nitric acid was used to adjust the pH of all the solutions to 3.

(red). (This control solution has the same pH and nitrate concentration as the plating solution mentioned above.) This result shows that nitrate reduction occurs at about −0.5 V vs Ag/AgCl and confirms that the reduction onset shown in the blue LSV is in fact due to the reduction of Bi3+. When p-benzoquinone is added to the BiOI plating solution and an LSV is obtained, the onset of the reduction current occurs at a much more positive potential, which can be attributed to p-benzoquinone reduction (Figure 3, black). The LSVs obtained with and without p-benzoquinone can be compared to determine the potential range in which pbenzoquinone reduction can occur without interference from Bi3+ reduction, which is between −0.2 and +0.2 V vs Ag/AgCl. After this analysis, several potentials within this range can be chosen to perform trial potentiostatic depositions and determine an optimum deposition potential based on film homogeneity, adhesion, and morphology. For galvanostatic deposition, a proper current density to deposit a desired phase may not be easily deduced from LSV data. This is because the J−V relationship shown in an LSV, which is obtained during potential sweeping, cannot provide information about the variation of potential while applying a constant deposition current. Because the deposition current is affected significantly by kinetic factors and the precise setup of a cell (e.g., positions of the electrodes), even explicitly following galvanostatic conditions reported in the literature can often fail in reproducing the reported result. The best approach when choosing galvanostatic deposition conditions is to first identify proper potentiostatic deposition conditions to obtain a desired film. The deposition current density should then be monitored during potentiostatic deposition to identify a proper current density to be used for the galvanostatic deposition with the given setup and conditions. During galvanostatic deposition, the deposition potential applied to the WE versus the RE can be monitored to ensure that the deposition potential remains within the range to deposit the pure desired phase, which is identified by the LSV analysis. Temperature is another key factor that can affect the kinetics of electrodeposition. Many electrodeposition experiments that are performed at room temperature (RT) can result in the formation of desired phases with reasonable deposition rates. However, if the reaction kinetics are slower than desired, deposition at elevated temperature can be considered to increase the deposition current by improving the reaction kinetics. The deposition rate should be carefully optimized because when the film is deposited too quickly film uniformity and adhesion may suffer.

Figure 4. SEM images of (a) BiOI and (b) nanoporous BiVO4 electrodes.

To convert BiOI to BiVO4, a vanadium source is added to the BiOI film by drop-casting a dimethyl sulfoxide (DMSO) solution containing 0.2 M VO(acac)2 (Sigma-Aldrich, 98%) using a micropipette. The typical volume of the solution placed on a 1.0 cm × 1.3 cm size film is 75−100 μL, which can completely cover the electrode surface. The top and side view photographs of BiOI films covered with this vanadium solution are shown in Figure 5a,b. The choice of solvent type containing the vanadium source is important to properly wet the BiOI surface. For example, we found that because the surface of BiOI is hydrophobic, DMSO can more easily penetrate between the BiOI plates than water, which can be confirmed by comparing the contact angles formed by a DMSO solution and an aqueous solution on the BiVO4 film.19 Wettability is important because any unwetted surface may not get sufficient access to vanadium when it is 360


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Although V2O5 is much more soluble than BiVO4 in 1 M NaOH, BiVO4 can also gradually dissolve in this solution. Therefore, the dissolution time is optimized such that it is sufficient to dissolve V2O5 thoroughly while minimizing the loss of BiVO4. Using excess vanadium precursor ensures the formation of more uniform BiVO4 compared to using a stoichiometric amount of vanadium. Therefore, when we need to incorporate a new component into an electrodeposited film via a postdeposition annealing process, we always prefer to dropcast a solution containing excess of the new component if we can identify conditions to remove the excess component without destroying the desired phase. 2.7. Deposition of Catalysts on Photoelectrodes. It is often necessary to deposit catalyst or passivation layers onto a photoelectrode. Because the solution-based electrodeposition method allows for a conformal coating, electrodeposition can be particularly advantageous when these additional layers need to be deposited onto a photoelectrode with complex, high surface area nanostructures as shown in Figure 4b. Electrodeposition to add additional layers on a photoelectrode can be performed using the same procedures described above. The only additional requirement is that because the photoelectrode is now being used as the WE, it is critical to choose deposition conditions where the photoelectrode is stable. For this purpose, the chemical stability of the photoelectrode should first be examined by immersing the photoelectrode in the desired plating solution. If its stability is confirmed, an LSV of the photoelectrode should be obtained in the plating solution to identify a window of potential where the photoelectrode is electrochemically inert. (To identify unambiguously current due to the reduction or oxidation of the photoelectrode alone, the LSV should be performed in a plating solution that does not contain the electrochemically active species.) If the photoelectrode is inert under the deposition conditions necessary for the addition of the extra layer, deposition can proceed using the procedures described above. If not, a different condition to deposit the desired layer should be developed. For the deposition of catalysts, photoelectrochemical deposition, which utilizes photogenerated charge carriers in a photoelectrode for a desired redox reaction, is often used. The difference between electrodeposition and photoelectrochemical deposition is illustrated in Figure 6. During electrodeposition, electrons produced during an oxidation reaction at the anode are consumed by a reduction reaction at the cathode with the application of an appropriate potential between the WE and CE

Figure 5. Photographs of (a) top view and (b) side view of a BiOI electrode covered with a DMSO solution containing a vanadium source, (c) BiVO4 electrode with excess V2O5 obtained after heat treatment, (d) removal of V2O5 by soaking in 1 M NaOH solution while stirring, and (e) a pure BiVO4 electrode.

added as a solution. The boiling point of the solvent is also an important factor, as vanadium may be even more uniformly distributed on the BiVO4 surface if the solvent remains as a liquid up to a higher temperature during the annealing process. We tested various solvents containing a vanadium source to choose an ideal vanadium solution to produce BiVO4 with a uniform distribution of vanadium and the desired film morphology. The BiOI film covered with the VO(acac)2 solution is transferred to a furnace and is annealed in air at 450 °C for 2 h, using a ramp rate of 2 °C/min. During the annealing process, BiOI is decomposed into Bi2O3 and I2, which sublimes. At the same time, evaporation of the solvent containing the vanadium source results in the formation of a V2O5 layer on the Bi2O3 surface, and Bi2O3 and V2O5 react to form BiVO4. For this type of solid state reaction, where the diffusion of ions is not as easy as in the solution phase, we intentionally add excess vanadium so that BiOI has a sufficient amount of V2O5 to react with. This results in a final BiVO4 film that contains excess, unreacted V2O5 (Figure 5c). Any excess V2O5 can be easily removed by immersing the electrode in 1 M NaOH solution for 30 min while stirring, resulting in a pure BiVO4 film (Figure 5d,e).

Figure 6. Schematic diagrams comparing (a) electrochemical and (b) photoelectrochemical reactions. The magnitude of the applied bias (Eappl) is shown against the RE in a three-electrode setup. 361


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The thickness of the FeOOH layer can be adjusted by varying the deposition time. The optimum thickness can be found by checking the J−V plots of BiVO4 while gradually increasing the thickness of FeOOH. As the thickness of the FeOOH layer increases, photocurrent for water oxidation also increases gradually. However, if the FeOOH layer becomes too thick, the photocurrent starts to decrease because the low conductivity of FeOOH limits the photocurrent generation. The optimum deposition condition is to form the thinnest possible FeOOH layer that completely covers the BiVO4 surface. The resulting electrode will show the highest and most sustainable photocurrent generation for water oxidation. In our previous study, we showed that although FeOOH forms a clean interface with electrochemically prepared BiVO4 that can minimize interfacial recombination, FeOOH alone is limited in its ability to improve the photocurrent and photostability of BiVO4 for solar water oxidation.6,19 The same procedure described above can be used to deposit an additional catalyst layer on top of the FeOOH layer.19

(Figure 6a). For photoelectrochemical deposition using a photoanode, photogenerated holes in the valence band (VB) are used for an oxidation reaction at the photoanode surface, whereas electrons that have been excited from the VB to the conduction band (CB) of the semiconductor are transferred to the cathode for a reduction reaction (Figure 6b). Photoelectrochemical deposition utilizing photogenerated minority carriers (holes for a photoanode and electrons for a photocathode) on the photoelectrode surface has the advantage of depositing catalyst species where the minority carriers are readily available.21 This means that oxygen evolution catalysts (OECs) photoelectrochemical deposition on a photoanode using holes can more readily utilize holes for water oxidation, and hydrogen evolution catalysts (HECs) photoelectrochemical deposition on a photocathode using electrons can more readily utilize electrons for water reduction. In this section, we describe our procedure to photoelectrochemically deposit FeOOH on BiVO4 as an example for photoelectrochemical deposition of a catalyst layer on a photoelectrode.19,22 FeOOH can be anodically deposited using the oxidation of Fe2+ to Fe3+ in a 0.1 M FeSO4·7H2O solution (pH 4.2−4.3) as shown in eq 4.23 Fe 2 +(aq) + 3OH− → FeOOH(s) + H 2O + e−

(4)

Fe 2 +(aq) + h+ + 3OH− → FeOOH(s) + H 2O

(5)

3. ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL CHARACTERIZATION In this section, we describe in detail our methods for photoelectrochemical characterization of photoelectrodes for use in solar water splitting. We also discuss practices that are recommended or discouraged for these characterization methods, based on our experiences. 3.1. Light Setup. Two types of light setup are used in our group for routine photoelectrochemical characterization. The first setup shown in Figure 7 can mimic the sunlight and also offer flexibility for varying the light intensity, removing IR light

The electrochemical onset potential for Fe2+ oxidation in this solution is 1.6 V vs RHE.22 Because the valence band maximum (VBM) of BiVO4 is 2.4 V vs RHE, the photogenerated holes in the VB of BiVO4 have sufficient overpotential to drive the oxidation of Fe2+, and, therefore, deposition of FeOOH on BiVO4 can occur (eq 5). The rate of photoelectrochemical deposition of FeOOH can be controlled by the light intensity and the potential applied to the WE, as well as by the concentration of Fe2+ in the plating solution. The light intensity affects the number of electron− hole pairs generated in a photoelectrode, and the potential applied to the photoelectrode affects the band bending, and, therefore, the number of electron−hole pairs separated. Fast deposition rates can often result in the formation of uneven films that contain pinholes. Therefore, a slow deposition rate is necessary to form a thin and uniform conformal coating of FeOOH. For the photoelectrochemical deposition of FeOOH on BiVO4, a 300 W Xe arc lamp with an AM1.5G filter, neutral density filters, and a water filter (IR filter) was used as the light source.19 The light was illuminated through the FTO contact (back-side illumination) because back-side illumination generates more photocurrent in the case of our BiVO4. The light intensity at the FTO surface was calibrated to be 1 mW/cm2 to allow for a slow photoelectrochemical deposition. A threeelectrode system was used and an external bias of ca. 0.25 V vs Ag/AgCl (4 M KCl), which is the open circuit potential (OCP) of the BiVO4 immersed in the plating solution in dark, was applied to the BiVO4 WE. This deposition potential (i.e., the OCP of BiVO4 in the dark) does not define any unique condition, rather we empirically determined that this condition is suitable to slowly photooxidize Fe2+ under the given illumination conditions. When it is necessary to increase the deposition rate, either the light intensity can be increased or a more anodic bias can be applied.

Figure 7. Photograph of a light setup used for photoelectrochemical characterization: (a) the lamp housing holding an ozone free 300 W xenon arc lamp, (b) the condensing lens, (c) the IR filter, (d) the light chopper, (e) the spacer, (f) the filter holder containing neutral density filters and an AM1.5G filter, which is directly connected to the fiber optic attachment inside the dark box, (g) the optical fiber cable, (h) the collimating lens, and (i) the custom-built dark box; (j) transmittance spectrum of an AM1.5G filter. 362


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one sun solar illumination. Because of this reason, when the intensity of the light is calibrated using a reference solar cell, the presence of an IR filter does not affect the spectral distribution and intensity of the calibrated light with energy ≥Eg of the reference solar cell as long as the IR filter does not cut out any light with energy ≥Eg of the reference solar cell. Of course, the use of an IR filter decreases the total intensity of the calibrated light because the intensity contribution from the IR light is removed. However, this does not affect the evaluation of photoelectrodes with a bandgap ≥Eg of the reference solar cell. When an IR filter is used, the use of a GaAs reference cell (Eg = 1.4 eV) instead of a Si reference cell (Eg = 1.1 eV) is recommended because the IR filter may remove a small portion of light with energy ≥Eg of Si. For the light setup that does not contain an IR filter, either a Si or a GaAs reference solar cell can be used. However, because the spectral distribution of the calibrated AM1.5G, one sun light from the xenon arc lamp is still not exactly the same as that of the AM1.5G solar spectrum,24 the use of a reference solar cell that has a Eg close to the Eg of the photoelectrode to examine may provide the most accurate evaluation.26 When the light intensity needs to be measured directly in watts, a radiometer (International Light, IL1700) with a thermopile detector (International Light, SED623) can be used. The thermopile detector uses the thermoelectric effect to convert heat from the photons in the range of 200−4000 nm into electrical energy and, therefore, allows for the measurement of the light intensity in watts. Figure 8a,b shows schematic PEC setups for front-side and back-side illumination. For front-side illumination, the light

or UV light, illuminating selective areas, and chopping light with a desired frequency. The role of each part of this setup is discussed in detail below. The lamp housing (Figure 7a) holds an ozone free 300 W xenon arc lamp (UXL-302-0, Ushio Inc. Japan) and a mirror that reflects light back toward the exit port. The condensing lens (Figure 7b) focuses all of the randomly oriented light that enters the exit port toward the fiber optic assembly. The light then passes through a water IR filter (Figure 7c) that filters out wavelengths of light greater than 950 nm. The water in the IR filter is cooled by a water jacket, which circulates cooling water around the water filter. The light generated by a xenon arc lamp contains more intense IR light than solar illumination.24,25 The use of an IR filter prevents heating of the electrode and the electrolyte during photoelectrochemical deposition or photoelectrochemical characterization, which can last for several hours or days. The light chopper (Figure 7d) is used to block light from reaching the sample at a chosen frequency for alternating measurements of dark current and photocurrent. The enclosed spacer (Figure 7e) located past the light chopper helps to achieve the proper focal length from the condensing lens to the fiber optic cable without exposing light to the surroundings. The spacer then fits flush into the custom-built dark box, which can completely block ambient light for dark measurements. The spacer on the inside of the dark box connects to a filter box that holds neutral density (ND) filters and the global airmass 1.5 (AM1.5G) filter (Figure 7f). The ND filters are used to decrease the light intensity without affecting the spectral distribution. To achieve the proper intensity of light, a series of ND filters of different strengths can be combined to mimic precisely the intensity of the sun. The AM1.5G filter is used to simulate the solar spectrum at sea level at a zenith angle of 48.2% on a clear day by eliminating portions of the ultraviolet spectrum, and attenuating the high intensity spikes of light generated from the xenon arc lamp in the 800− 1000 nm region, which do not exist in the real solar spectrum (Figure 7j).24,26,27 The filter box has an attachment that holds a fiber optic cable (Figure 7g). A fiber optic cable allows for the light to be directed in any direction, which permits measurements that would be difficult to perform when lined up with the lamp. The collimating lens (Figure 7h) realigns the light that leaves the fiber optic cable to achieve uniform illumination. The diameter of the collimated beam is ∼0.3 cm. The second setup is used to offer uniformly collimated AM1.5G, one sun illumination of a larger area (1.5 in. × 1.5 in.), which is suitable to test larger scale optimized photoelectrodes. This setup (Newport, LCS-100) uses a 100 W xenon arc lamp that contains an AM1.5G filter and a collimating lens in the lamp housing to output collimated light. 3.2. Light Calibration. The intensity of the light after passing through the IR and AM1.5G filters is adjusted using neutral density filters. The intensity of the light is calibrated to be one sun (100 mW/cm2) using a NREL-certified GaAs reference solar cell (PV Measurements); neutral density filters are used to match the short circuit current density of the reference cell to the value reported in the cell’s certification details. It should be noted that when the intensity of the light is calibrated to be one sun using a reference solar cell, this does not mean that the total intensity of the calibrated light in the entire wavelength range (UV−vis−IR) is truly one sun. Instead, it means that the number of photons having energy equal to or greater than the bandgap energy (≥Eg) of the reference solar cell in the calibrated light is the same as that in the AM1.5G,

Figure 8. Diagrams of a three-electrode setup for (a) front-side illumination and (b) back-side illumination, (c) a photograph of an undivided quartz cell used for photoelectrochemical characterization, and (d) a transmittance spectrum of the 3 mm thick quartz cell shown in panel c.

intensity is calibrated to be one sun at the surface of the photoelectrode after the light passes through the quartz cell. For back-side illumination, the light intensity is calibrated to be one sun at the surface of the FTO substrate before the light passes through the FTO. The quartz cell used for our photoelectrochemical measurements is shown in Figure 8c. Using a cell made of quartz minimizes the amount of light absorbed by the cell. Also, the sides where the light passes through the cell are made flat to minimize the influence of the 363


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illumination. Another possibly confusing term is the photovoltage gain for a half reaction used for solar fuel generation such as water oxidation or water reduction. This term represents the potential difference between the photocurrent onset and the thermodynamic potential for water oxidation or water reduction measured in a three-electrode cell. These terms are often used without explaining their definitions. Therefore, knowing the differences between these terms will help identify which term is being referred to, based on the context and the experimental conditions, and can minimize possible confusion while reading literature. 3.3.2. Mott−Schottky Plots. EFB values can also be measured using the Mott−Schottky equation shown in eq 6 where Csc is the capacitance of the space charge layer, εr is the relative permittivity of the semiconductor, ε0 is the permittivity of a vacuum, A is the surface area, e is the charge of an electron, Ndopant is the majority carrier density, k is the Boltzmann constant, T is the temperature, E is the applied potential vs RE, and EFB is the flat-band potential vs RE.26,28−31 A Mott− Schottky plot is obtained by plotting 1/Csc2 vs E.

cell on the light path. The transmittance of a 3 mm thick quartz plate used for the construction of our homemade cell is shown in Figure 8d. The photoelectrode is typically masked to expose only an area smaller than the size of the illuminated beam. This ensures that the area of the electrode contributing to the generation of photocurrent and dark current is the same. 3.3. Determination of Flatband Potential (EFB). Flatband potential (EFB) is one of the most important factors that govern the photoelectrochemical properties of a photoelectrode. Here we describe the three most commonly used techniques to determine the EFB. All of the measurements used to determine the EFB utilize a three-electrode setup and EFB is reported against a RE. The electrolyte solution used for the measurement should be the same as the solution used for photoelectrochemical water splitting reactions, as the solution pH and adsorption of ions on the photoelectrode surface affect the EFB value.28 In general, more than one method described here should be used to check the reliability of the results, as each method has limitations, especially for the measurement of the EFB values of photoelectrodes that are polycrystalline and contain various surface states/defects.26 3.3.1. Illuminated Open Circuit Potential (OCP). When a photoelectrode is illuminated under open circuit conditions to generate electron−hole pairs, the electrons and holes move in opposite directions because of the electric field within the space charge layer; the minority carriers move toward the photoelectrode/electrolyte interface whereas the majority carriers move away from the interface. These movements create an electric field that opposes the electric field already present in the space charge layer. As a result, the open circuit potential (OCP) measured against the RE under illumination shifts from the OCP in the dark toward the EFB. When the intensity of light increases, the electric field generated by the movement of photogenerated carriers can increase and reach the point where it can completely compensate for the electric field in the space charge layer. At this point, the space charge layer and, therefore, the band bending can be completely eliminated, and the OCP measured will equal the EFB of the photoelectrode.26,28 This means that the EFB can be determined by measuring the OCP of a photoelectrode under illumination with increasing light intensity until the OCP reaches a plateau. The experimental setup for the measurement of the illuminated OCP requires the immersion of a photoelectrode and a RE in the electrolyte of interest and reading the potential between them using a multimeter or an OCP measurement function of a potentiostat. Because the potential is measured under open circuit conditions, the presence of a CE is not required. When the light intensity is varied, the OCP should be measured after its value is stabilized. This method may not work if the photoelectrode is not photostable under open circuit conditions or if the photoelectrode has a large number of defect sites that can allow the recombination of electrons and holes to occur at a high rate, making it impossible to remove band bending completely. Also, if the semiconductor undergoes Fermi level pinning, with increasing light intensity, the OCP will continue to shift and a plateau will not be achieved.26 The illuminated OCP should not be confused with the open circuit voltage (Voc) measured with a two-electrode cell under illumination. Although the former shows the WE potential vs RE under illumination and open circuit conditions, the latter shows the change in the OCP of the WE caused by

⎛ 1 2 kT ⎞ ⎜E − E ⎟ = FB − e ⎠ Csc 2 εrε0A2 eNdopant ⎝

(6)

The linear region of the Mott−Schottky plot gives three distinct pieces of information. First, the type of doping can be determined by the sign of the slope; n-type materials have a positive slope whereas p-type materials have a negative slope.26,28−30 Second, the EFB can be calculated from the xintercept, EFB − (kT/e), which is obtained from the extrapolation of the linear region of the plot.26,28−30 Because kT/e is only 0.0257 V at RT, the x-intercept is often reported as EFB. Third, the majority carrier density can be obtained from the magnitude of the slope.26,28−30 It should be noted that the electrode area (A) included in the slope in eq 6 is not the geometric surface area but the actual surface area of the electrode. Therefore, unless a photoelectrode with a flat surface is used, the geometric area of the electrode should not be used to calculate Ndopant. When two electrodes with identical surface morphologies are compared, although the actual surface areas are not known, the slopes of the Mott−Schottky plots may still be used to compare qualitatively their relative carrier densities in the two electrodes. To obtain Mott−Schottky plots, Csc is measured using electrochemical impedance spectroscopy in a three-electrode cell at varying potentials. Because the exposed geometric area of the electrode, which contributes to the actual surface area of the electrode exposed (i.e., actual surface area = geometric surface area × surface roughness factor), affects the Csc value, we mask the photoelectrode to ensure that we always expose the same geometric area of the electrode to the electrolyte to obtain more reproducible data. This also prevents the contribution of the photoelectrode near the edges of the electrodes, which may have slightly different thicknesses than the central region. When the film is uniform, exposing a larger geometric area of the photoelectrode, which provides greater magnitude measurements of the space layer capacitance, will result in a more reliable estimation of EFB. The potential should be varied from an initial potential value that ensures sufficient band bending to the direction of decreased band bending. In other words, the potential should be moved toward the negative direction for ntype semiconductors and toward the positive direction for ptype semiconductors.26,30 At each potential that Csc is 364


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surface reaching minority carriers can contribute to photocurrent generation with no loss to surface recombination. Therefore, as long as band bending exists to separate electron− hole pairs and the minority carriers reach the surface, photocurrent should be observed. This means the potential where the photocurrent disappears (i.e., photocurrent onset potential) can be considered to be the EFB, where the bands become completely flat and no minority carriers can reach the surface. It should be emphasized that the photocurrent onset potential can be approximated to be the EFB only when the photocurrent is obtained using hole or electron acceptors with ϕinj = ∼1. For most photoelectrodes that are not catalytic for water oxidation or reduction, their photocurrent onset for water oxidation and reduction usually deviates significantly from the true EFB. This is because when the rate of interfacial charge transfer is slower than the rate of surface recombination, photocurrent is ceased when all the surface reaching minority carriers are lost to surface recombination before band bending is completely eliminated. In this case, the photocurrent onset potential cannot be reported as the EFB. The photocurrent onset potential can be obtained by measuring a J−V plot in a three-electrode cell using chopped illumination, which allows for the measurement of the photocurrent and dark current in a single LSV scan. For an accurate onset potential measurement, a slow potential scan rate and a high light chopping frequency are needed. If the electron or hole acceptor has a ϕinj of ∼1, transient photocurrent should be negligible even when the potential approaches very near the EFB because surface recombination causing transient photocurrent should be negligible.34 For example, photocurrent of BiVO4 for the oxidation of sulfite, which is often used as a hole acceptor with ϕinj of ∼1 in pH 7− 9 solutions, is shown in Figure 10, where no significant

measured, a sinusoidal AC voltage of various frequencies with a typical amplitude of 5−10 mV is applied, and the amplitude and phase angle of the current are measured.31,32 From this data, impedance can be calculated and Csc can be determined by applying a proper equivalent circuit model.26,30,31 Most commercially available electrochemistry software provides Mott−Schottky plots based on the impedance measurement using the most basic circuit model shown in Figure 9a.33 This simplified circuit can be used only under

Figure 9. Simplified equivalent circuits considering only the contribution of (a) the space charge layer and (b) the surface states as well as the space charge layer (R, resistor; C, capacitor).

several assumptions. First, the photoelectrode behaves ideally and has no other layers on the surface. Second, the impedances of the solution, bulk of the photoelectrode, and back contact to the photoelectrode are either negligible or accounted for. Third, the double layer capacitance is considerably larger than Csc and can be ignored. Fourth, the photoelectrode does not contain surface states or the measurements are performed at high frequency conditions where the surface state capacitance (Css) can be considered zero.26,30 This means that interpretation of the impedance using the simplified circuit model shown in Figure 9a would result in incorrect assessment of Csc, EFB, and Ndopant if any of the assumptions made above are not valid. The Csc should be independent of the frequency by which the AC voltage is modulated. Therefore, if the x-intercepts or the slopes of the Mott−Schottky plots show a frequency dependence, Csc cannot be determined in a straightforward manner using the simplified circuit and the resulting EFB and Ndopant may not represent the true EFB and Ndopant of the photoelectrode. Therefore, it is very important to obtain Mott− Schottky plots using multiple frequencies and report EFB and Ndopant only when identical x-intercepts and slopes are measured at multiple frequencies. If a frequency dependence is observed, the dependence may be due to the presence of surface states. (An equivalent circuit including the effect of surface states is shown in Figure 9b.) In this case, higher frequencies should be used for capacitance measurements so that the contribution of the surface states to the capacity can be negligible and the equivalent circuit can be simplified to that shown in Figure 9a.26,30 When the use of various high frequencies results in different slopes but the same x-intercept, the results may still be used to determine EFB, but not Ndopant, if the EFB results agree well with EFB values obtained from other methods. 3.3.3. Photocurrent Onset. The EFB of a photoanode (or a photocathode) can also be estimated by a J−V plot obtained using a three-electrode system where photocurrent is generated using hole acceptors (or electron acceptors) with fast interfacial charge transfer kinetics such that the charge injection yield (ϕinj) is ∼1. (The ϕinj is the fraction of the surface reaching minority carriers that are injected into the solution species.) In this case, the photocurrent onset potential can be considered to be the EFB of the photoelectrode. The assumption being made here is that with hole or electron acceptors with ϕinj = ∼1, all

Figure 10. J−V plot of BiVO4 for sulfite oxidation obtained in a phosphate buffer solution (pH 7) containing 1.0 M Na2SO3 under AM1.5G illumination.

transient photocurrent is observed even when the potential approaches very near the onset potential.19,22,35−38 (It should be noted that kinetics for sulfite oxidation is pH dependent and its ϕinj decreases in strongly basic conditions (pH ≥ 13)). The onset potential can also be measured by comparing two individually obtained LSVs, one measured under illumination and the other measured in the dark, if LSVs can be obtained reproducibly for multiple scans. In addition to sulfite, H2O2, whose rate constant for oxidation was reported to be 10−100 times greater than that of water, has been used as a hole acceptor, especially in basic solution with photoanodes that are oxidatively stable in an 365


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Chemistry of Materials H2O2 environment, such as α-Fe2O3.39−41 Electron acceptors that have been used to investigate photocathodes include methylviologen, H2O2, and O2.42−44 It should be noted that even if the oxidation or reduction kinetics of these species are faster than those of water and their photocurrent onset potentials shift toward the EFB of the photoelectrode, unless ϕinj is close to 1 very near the EFB, their photocurrent onset potentials may still deviate from the true EFB. 3.4. Photocurrent Density−Potential (J−V) Measurement. Photocurrent density-potential (J−V) characteristics can be obtained using either a three-electrode setup or a twoelectrode setup, and the results obtained from the two conditions offer different information. The J−V measurement obtained using a three-electrode setup shows the photocurrent response of a photoelectrode whereas the Fermi level of the photoelectrode is varied with respect to the RE. In a threeelectrode setup, the performance of the CE does not affect the photocurrent generation by the WE. Therefore, the observed J−V characteristics are solely determined by the nature and performance of the photoelectrode. The J−V measurement in a three-electrode cell enables determination of the onset potential against the RE and photocurrent densities at any reference potentials. For the characterization of photoelectrodes for use in water splitting, the potential is typically reported against the reversible hydrogen electrode (RHE). This is very useful as the photoelectrode performance is shown against the thermodynamic water reduction potential at the specified pH, which allows for easy calculation of the photovoltage (i.e., the difference between the photocurrent onset and the thermodynamic redox potential) as well as an easy comparison of the performances of photoelectrodes operated in various pH conditions. Therefore, it is highly recommended to report J− V plots and any other photoelectrochemical properties against the RHE. The conversion of the potential measured against the Ag/AgCl RE to the potential against the RHE is shown in eq 7.

measuring photocurrent density and the absorption spectrum of the photoelectrode. Also, the J−V measurements obtained with hole or electron acceptors with ϕinj = 1 provide the upper limit of the J−V performance that can be achieved by the photoelectrode for water oxidation or reduction when an efficient OEC or HEC is added on the photoelectrode surface. The J−V plots can also be measured using a two-electrode cell. In this case, the potential is varied between WE and CE, and the photocurrent is presented as a function of the potential applied between WE and CE. The photocurrent generated using a two-electrode cell is affected by the performance of the CE as well as the performance of the WE, which is a key difference from the three-electrode case. In other words, the overall reaction of the cell affects the photocurrent generation in a two-electrode cell, not just the half-cell reaction of the photoelectrode. Therefore, any experiment involving device characterization or the determination of device efficiency, such as the power conversion efficiency or solar-to-hydrogen efficiency, must use a two-electrode setup. The photocurrent density measured without any external bias applied between the WE and CE is called the short circuit photocurrent density (Jsc), which is an important characteristic of a photoelectrode. Some researchers use the term, zero-bias photocurrent to refer to the short circuit photocurrent measured with zero bias between the WE and CE. However, the term zero-bias photocurrent creates confusion because it can also mean the photocurrent obtained at zero bias between WE and RE. The photocurrent obtained at zero bias between WE and RE is fundamentally different from Jsc, and it should not be reported as Jsc. 3.5. Photocurrent Density−Time (J−t) Measurement. When new or improved photoelectrodes are developed, it is important to examine their stability. A J−t measurement, which shows the change of photocurrent density over a period of time, is the most commonly used stability test used in our group. If the photoelectrode can split water without external bias, the Jsc should be monitored over time. If the photoelectrode cannot split water without external bias, an appropriate potential should be applied during the J−t measurement. The initial photocurrent density of the J−t plot obtained at a certain applied potential should match well with the photocurrent density shown in the J−V plot at the same potential if the J−V plot is recorded with a slow scan rate to minimize charging current. (Sweeping of the potential is always accompanied by the generation of a non-Faradaic charging current that is proportional to the scan rate and the electrode area.) If the photocurrent densities do not match, the origin of the discrepancy needs to be investigated. Because the J−t measurement provides an additional opportunity to confirm the results shown in the J−V plot, the absolute current densities of the J−t plot should be reported, as opposed to reporting the relative percent decay. When the J−t measurement is performed for several hours or days, it is important to monitor and correct for the evaporation of water as well as the pH and temperature changes of the electrolyte, as these factors can considerably affect the photocurrent generation. 3.6. Efficiency Calculations. 3.6.1. Solar-to-Hydrogen (STH) Efficiency. Solar-to-hydrogen (STH) efficiency is the most important efficiency value to report when studying a water splitting PEC. It gives the most direct measurement of a material’s ability to evolve hydrogen gas from solar water

E (vs RHE) = E (vs Ag/AgCl) + EAg/AgCl (reference) + 0.0591 V × pH

(7)

(EAg/AgCl (reference) = 0.1976 V vs NHE at 25°C)

In addition to investigating J−V characteristics for water oxidation/reduction, we often perform J−V measurements using hole and electron acceptors with fast interfacial charge transfer kinetics using a three-electrode cell for several reasons. First, as discussed previously, the onset potential can be used for determining EFB values. Second, when considering a material’s performance for photoelectrochemical processes, it is useful to discuss the performance of the photoelectrode in terms of efficiencies for three separate processes: light absorption, charge separation, and interfacial charge injection as shown in eq 8.39,45,46 JPEC = Jabs × ϕsep × ϕinj

(8)

In this equation, JPEC is the measured photocurrent density. Jabs is the photon absorption rate expressed as current density and can be calculated assuming 100% APCE using the absorption spectrum of the photoelectrode. ϕsep is the yield of photogenerated carriers that reach the surface, and ϕinj is the yield of the surface reaching minority carriers that are injected into the solution. Therefore, when JPEC is measured with hole or electron acceptors with ϕinj = 1, ϕsep can be easily calculated by 366


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conditions. This is because the CE reaction, which is the hydrogen evolution reaction on the Pt surface, happens to be the same as the reaction occurring at the RHE, and the potential difference between the WE and CE can be similar to the potential difference between the WE and RE under certain operating conditions. However, the ABPE obtained from a three-electrode cell, even in this specific case, can be very different from the true ABPE depending on the operating conditions. Therefore, the true ABPE should be measured and reported using a two-electrode setup. Another common mistake made is to calculate ABPE using photocurrent obtained with hole or electron acceptors. The 1.23 V used in the calculation of the output power density is only for a water splitting PEC. Therefore, photocurrent generated by any other reaction should not be used for the equation written for a water splitting PEC. One important factor when comparing ABPEs is the cell type (divided vs undivided cell), as the presence of a divider can affect the ABPE for the same photoelectrode; when a divided cell is used, more potential between the WE and CE is required to generate the same current density due to a potential drop across the divider (e.g., frit, membrane). Therefore, reporting and checking the cell type when comparing ABPE values will allow for more meaningful comparison. 3.6.3. Incident Photon-to-Current Efficiency (IPCE) and Absorbed Photon-to-Current Efficiency (APCE). Incident photon-to-current efficiency (IPCE) provides wavelengthdependent external quantum yields of a photoelectrode. IPCE typically shows the percentage of incident photons that are converted to photocurrent by a photoelectrode as a function of wavelength. Calculating IPCE requires the measurements of photocurrent density and light intensity at each wavelength (Pmono) using the equation shown below (eq 12).19,24

splitting. Equations 9 and 10 are two different forms of the STH efficiency equation.26,47 STH Efficiency (%) =

(mmol H 2 /s) × (237 kJ/mol) Ptotal (mW/cm 2) × Area (cm 2) (9)

STH Efficiency (%) =

|Jsc (mA/cm 2)| × (1.23 V) × ηF Ptotal (mW/cm 2) (10)

In eq 9, the numerator is the rate of hydrogen production multiplied by the change in Gibbs free energy per mol of H2 at 25 °C, whereas the denominator is the power of the illumination (Ptotal), which should be 100 mW/cm2 for one sun, multiplied by the geometric illuminated area.26 Equation 10 allows for the use of Jsc to calculate the STH efficiency by multiplying it by the thermodynamic potential for water splitting (1.23 V) and the Faradaic efficiency for hydrogen evolution (ηF).26,47 The STH efficiency can be reported only when the PEC requires no external bias to split water. If a bias is necessary for overall water splitting, the applied bias photonto-current efficiency (ABPE) metric is required. 3.6.2. Applied Bias Photon-to-Current Efficiency (ABPE). When a photoelectrode coupled with a metal CE in a PEC does not have proper Fermi level and band alignment to allow for the majority carriers transferred to the CE to complete the opposite half reaction, a bias is required to drive the work function of the counter electrode to the proper potential.26,28 When photocurrent for water splitting is generated with the aid of an external bias, an applied bias photon-to-current efficiency (ABPE) is a valid device efficiency value to report (eq 10).26,28,47 ABPE (%) =

|Jph (mA/cm 2) × (|1.23 − Vb|) (V)| Ptotal (mW/cm 2)

IPCE (%) = (11)

The ABPE is calculated by dividing the output power density by the input power density of the device. The input power density is the power density of incident light, which is 100 mW/cm2 for AM1.5G solar irradiation. The output power density for a water splitting PEC is given as the product of the photocurrent density (Jph) and the thermodynamic potential required for water splitting, 1.23 V. However, because an external bias is applied to facilitate water splitting, the amount of applied bias (Vb) between the WE and CE is subtracted from 1.23 V. It is important to note that the ABPE calculation requires J− V measurements using a two-electrode cell so that both the Jph and the Vb are measured between the WE and CE to represent accurately the device performance for overall water splitting. A mistake commonly made is to use the potential between the WE and RE as Vb to calculate the output power density. The potential between the WE and RE, which does not provide any information about the bias applied between the WE and CE during photocurrent generation, cannot be used for evaluating device performance. For a three-electrode PEC composed of a photoanode as the WE for water oxidation, a Pt CE for water reduction, and the RHE as the RE, the ABPE calculated using the Vb as the potential difference between the WE and RHE can be similar to the true ABPE calculated using a two-electrode cell composed of the photoanode and a Pt CE under certain operating

|Jph (mA/cm 2)| × 1239.8 (eV × nm) Pmono (mW/cm 2) × λ (nm)

(12)

For IPCE measurements, our group uses light from a 150 W Xe arc lamp passed through an AM1.5G filter and neutral density filters. Monochromatic light is generated by an Oriel Cornerstone 130 monochromator with a 10 nm bandpass, which is directly attached to the filter box. The power density of light is measured by a radiometer (International Light, IL1700) with a Si photodiode detector (International Light, SED033), which can detect lower levels of light than a thermopile detector. When the photoelectrode cannot split water under the short circuit condition, IPCE is measured using a threeelectrode setup while applying an appropriate potential against the RE. When the measured IPCE of a photoelectrode is weighted by the number of photons available at each wavelength of the solar spectrum48 and integrated across all wavelengths, the resulting integrated photocurrent density should match exactly with the photocurrent density of the photoelectrode measured at the same bias using white light, if the white light used in the laboratory is truly well calibrated to mimic the solar irradiation. Therefore, measuring IPCE and calculating integrated photocurrent density to compare with the photocurrent densities obtained from the J−V and J−t measurements with white light can be a useful way to check the calibration of the white light. Absorbed photon-to-current efficiency (APCE) shows the percentage of absorbed photons utilized for photocurrent generation. To calculate APCE, accurate absorbance data from 367


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Chemistry of Materials a UV−vis spectrophotometer is required. When the absorbance of a photoelectrode is obtained using transmittance mode, surface-scattered or surface-reflected photons can be included as absorbed photons, leading to an overestimation of the absorbance. Measurement of absorption spectra using an integrating sphere with a photoelectrode on a TCO substrate positioned at the center of the integrating sphere allows for measuring both the transmitted and the surface scattered photons to determine absorbance accurately. Using the absorbance data, a light harvesting efficiency (LHE) at each wavelength can be calculated using eq 13.19,26 LHE = 1 − 10−A(λ)

(13)

where A(λ) is the absorbance at a given wavelength. By dividing IPCE at each wavelength by the LHE at the corresponding wavelength, APCE can be calculated (eq 14).19,26 APCE (%) =

IPCE (%) LHE

Figure 11. An air-tight, two compartment cell used for oxygen detection: (a) copper wire lead to the CE, (b) Pt mesh as the CE, (c) N2 inlet for the CE chamber, (d) N2 outlet for the CE chamber, (e) copper wire lead to the WE, (f) photoelectrode as the WE, (g) RE, (h) O2 sensor probe, (i) N2 inlet for the WE chamber, and (j) N2 outlet for the WE chamber.

(14)

3.6.4. Faradaic Efficiency. Faradaic efficiency reports the percentage of the charges passed during an electrochemical reaction that are used for a desired reaction (i.e., O2 production, H2 production). Therefore, in order to calculate Faradaic efficiency for O2 or H2 evolution of a photoelectrode, the amount of H2 and O2 produced are needed in addition to the photocurrent density. The Faradaic efficiency can be obtained by dividing the actual amount of H2 and O2 produced by the expected amount of H2 and O2 based on the total charge passed during the water splitting reaction using eqs 15 and 16. The methods for H2 and O2 detection are discussed in the following section. FE of O2 (%) =

4 × nO2 (mol) × F (C mol−1) Charge Passed through WE (C)

Figure 11 shows a custom built, airtight, three-electrode cell, which has two compartments divided by a glass frit, used for the detection of O2. We found that the gas crossover through the pores of the frit is negligible for the photocurrent and time scales typically used in our experiments. The cell is made of quartz to minimize light absorption, and the side where light passes through is flat in order to minimize the alteration of the light path. Rubber septa are used to seal off all openings of the cell. The oxygen probe is inserted into the WE chamber through the septum along with the WE and Ag/AgCl RE (BASi RE-5B). The WE is held by an alligator clip connected to a Cu wire that penetrates the septum. The CE chamber contains Pt mesh as the CE, also connected to a Cu wire by an alligator clip. The electrolytes in both chambers are first thoroughly purged with N2 gas before the cell is sealed with the rubber septa. Then, the head space of the sealed cell is purged with N2 gas for 1 h with a metal syringe needle placed through the septa and connected to a dry N2 source serving as the inlet for N2 gas. A short needle in the septa serves as the outlet for N2 gas. During N2 purging, the O2 sensor should show a drop in the O2 content of the headspace from ∼20% to ∼1%. (The O2 content does not drop below 1%, and this background level of O2 is subtracted from O2 generated during the reaction.) After purging, the needles serving as gas inlets/outlets are removed, and the existing holes in the septa are sealed with epoxy. Before O2 production is started, the cell is intentionally left undisturbed for 30 min, and the O2 content is monitored as a leak test. If the cell is completely sealed, the O2 content remains at ∼1%. If the cell leaks, a gradual increase in O2 content will be observed, which warrants checking the seals of the cell. After the cell passes the leak test, O2 measurement can be initiated by illuminating the photoelectrode and recording the photocurrent density at a desired potential. If the photoelectrode cannot generate sufficient O2 that can be reliably detected by the O2 sensor, the intensity of the light may be increased to improve the signal-to-noise ratio and obtain more reproducible data. With more intense light, the use of an IR filter is recommended to prevent the IR light from affecting photocurrent generation (e.g., by heating the photoelectrode and the electrolyte). To convert the mole % of O2 into the number of moles (nh) in the headspace, the number of moles of total gas in the

× 100 (15)

FE of H 2 (%) =

2 × n H2 (mol) × F (C mol−1) Charge Passed through WE (C)

× 100 (16)

3.6.5. Detection of H2 and O2. An airtight cell is needed for gas detection. Ideally, a divided cell should be used to prevent product cross over. For example, if an undivided cell is used, O2 produced at the anode can be reduced at the cathode competing with the water reduction reaction, which will decrease the amount of O2 detected while simultaneously lowering the Faradaic efficiency for H2 production. Gas chromatography (GC), mass spectrometry (MS), or GC−MS can be used for the detection of H2 and O2. For O2 detection alone, we use an oxygen sensor (Ocean Optics fluorescencebased oxygen sensor, FOSPOR-R 1/16″) more frequently as it allows for in situ monitoring of O2 concentration during water splitting without any additional sampling processes. The protocols for using GC and MS for H2 and O2 detection have been well documented in the literature.26 Therefore, we discuss here only our procedure for detecting O2 using an oxygen sensor. The fluorescent probe of the oxygen sensor measures the O2 content in the headspace as mole %, which can be converted to micromoles. O2 dissolved in solution, which cannot be detected by the sensor, can be calculated using Henry’s Law once the O2 concentration in the headspace is determined. 368


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Chemistry of Materials headspace (nh,total) is first calculated using the ideal gas law (eq 17).

nh,total =

PV RT

(3) Glasstone, S. The Fundamentals of Electrochemistry and Electrodeposition; Franklin Publishing Company, INC.: NJ, 1960. (4) Liang, Y.; Tsubota, T.; Mooij, L. P. A.; van de Krol, R. Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes. J. Phys. Chem. C 2011, 115, 17594−17598. (5) Lindquist, S. E.; Finnström, B.; Tegnér, L. Photoelectrochemical Properties of Polycrystalline TiO2 Thin Film Electrodes on Quartz Substrates. J. Electrochem. Soc. 1983, 130, 351−358. (6) Park, Y.; Kang, D.; Choi, K.-S. Marked enhancement in electronhole separation achieved in the low bias region using electrochemically prepared Mo-doped BiVO4 photoanodes. Phys. Chem. Chem. Phys. 2014, 16, 1238−1246. (7) Barnes, P. R. F.; O’Regan, B. C. Electron Recombination Kinetics and the Analysis of Collection Efficiency and Diffusion Length Measurements in Dye Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 19134−19140. (8) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon Press: Oxford; New York, 1966. (9) Benck, J. D.; Pinaud, B. A.; Gorlin, Y.; Jaramillo, T. F. Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte. PLoS One 2014, 9, e107942. (10) Armstrong, N. R.; Lin, A. W. C.; Fujihira, M.; Kuwana, T. Electrochemical and surface characteristics of tin oxide and indium oxide electrodes. Anal. Chem. 1976, 48, 741−750. (11) Kirkov, P. The electrochemistry of the tin oxide semiconductorI. the establishment of mechanisms at polarized n-type tin oxide. Electrochim. Acta 1972, 17, 519−532. (12) Kulesza, P. J.; Lu, W.; Faulkner, L. R. Cathodic fabrication of platinum microparticles via anodic dissolution of a platinum counterelectrode: Electrocatalytic probing and surface analysis of dispersed platinum. J. Electroanal. Chem. 1992, 336, 35−44. (13) Inzelt, G.; Lewenstam, A.; Scholz, F. Handbook of Reference Electrodes; Springer: Berlin; New York, 2013. (14) Handbook of Electrochemistry; Zoski, C. G., Ed.; Elsevier: Amsterdam; Boston, 2007. (15) Schlesinger, M.; Paunovic, M. Modern Electroplating; Wiley: Hoboken, NJ, 2010. (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (17) Pletcher, D. A First Course in Electrode Processes; Royal Society of Chemistry: Cambridge, 2009. (18) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2 ed.; National Association of Corrosion Engineers: Houston, 1974. (19) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (20) McDonald, K. J.; Choi, K.-S. A new electrochemical synthesis route for a BiOI electrode and its conversion to a highly efficient porous BiVO4 photoanode for solar water oxidation. Energy Environ. Sci. 2012, 5, 8553−8557. (21) Steinmiller, E. M. P.; Choi, K. S. Photochemical deposition of cobalt-based oxygen evolving catalyst on a semiconductor photoanode for solar oxygen production. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 20633−20636. (22) Seabold, J. A.; Choi, K.-S. Efficient and Stable Photo-Oxidation of Water by a Bismuth Vanadate Photoanode Coupled with an Iron Oxyhydroxide Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 2186−2192. (23) Spray, R. L.; Choi, K. S. Photoactivity of Transparent Nanocrystalline Fe2O3 Electrodes Prepared via Anodic Electrodeposition. Chem. Mater. 2009, 21, 3701−3709. (24) Doescher, H.; Young, J. L.; Geisz, J. F.; Turner, J. A.; Deutsch, T. G. Solar-to-hydrogen efficiency: shining light on photoelectrochemical device performance. Energy Environ. Sci. 2016, 9, 74−80. (25) Newport. 200−500 W Xe and Hg(Xe) research Arc Lamp Sources. https://www.newport.com/f/xe-and-hgxe-research-arc-lamp-

(17)

Then, nh is obtained using the following equation. nh = nh,total × mole % of O2 in headspace

(18)

The number of moles of O2 dissolved in the electrolyte solution (ns) is calculated using Henry’s Law, which states that the molar concentration of dissolved gas (cs) is proportional to its molar concentration in the gas phase (c h ). The proportionality factor is called the Henry’s law constant (HCC = cs/ch), and the value is 3.2 × 10−2 for O2 in water at 298.15 K.49 Then, using the definition of molarity (molarity (c) = moles (mol)/volume (v)), the number of moles dissolved in the electrolyte (ns) and the total number of moles of O2 in the system (nO2) can be determined using eqs 19 and 20. c n ns = cs × vs = s × ch × vs = 0.032 × h × vs ch vh (19) nO2 = nh + ns

(20)

4. SUMMARY Because of the simplicity and versatility of electrodeposition, it is expected that electrodeposition will be employed for materials synthesis even more widely in the future. In this paper, we described methods used in our group for electrochemical synthesis in detail and provided explanations for each process, which may be useful for researchers who are interested in learning or employing electrodeposition for their research. We also described methods for photoelectrochemical characterization of photoelectrodes for use in water splitting. By explaining the rationale behind each procedure, we intend to help improve the understanding and performance of various experimental processes used for photoelectrode evaluation. Recommended practices, as well as practices that should be avoided for these characterization methods, are explained. The methods described in this paper will be useful for more effectively designing experimental conditions to obtain accurate and reproducible results for electrochemical synthesis and photoelectrode evaluations.

■

AUTHOR INFORMATION

Corresponding Author

*K.-S. Choi. E-mail: [email protected]. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) under the NSF Center CHE-1305124. REFERENCES

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Methods for Electrochemical Synthesis and Photoelectrochemical

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