Comparison between the Quantum Yields of Compact and Porous

Jun 17, 2013 - The metallic contact and the foil edges were covered with Hysol 1C epoxy to produce an exposed area of 0.3–0.4 cm2. The contact wire ...
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Comparison between the Quantum Yields of Compact and Porous WO3 Photoanodes Karla R. Reyes-Gil,† Craig Wiggenhorn, Bruce S. Brunschwig, and Nathan S. Lewis* Beckman Institute and Kavli Nanoscience Institute, 210 Noyes Laboratory, 127-72, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: Ordered structures offer the potential for producing photoanodes with enhanced minority-carrier collection. To evaluate this approach to visible-light-driven oxidation in aqueous electrolytes, porous WO3 structures were synthesized by the potentiostatic anodization of W foil. The photoelectrochemical behavior of the porous WO3 photoanodes was compared to that of compact WO3 films. Relative to planar electrodes, the porous WO3 electrodes exhibited a 6-fold increase in photocurrent density, from 0.12 to 0.75 mA cm−2, under 100 mW cm−2 of simulated solar illumination. Spectral response measurements indicated that the porous electrodes exhibited internal quantum yields of ∼0.5 throughout most of the region of WO3 absorption. The external quantum yield of the porous WO3 films was a function of the angle of incidence of the light, increasing from 0.25 at normal incidence to 0.50 at 65° off normal. The porous WO3 films showed excellent stability against photodegradation. This work demonstrates that morphological control can improve the internal quantum yield of photoanodes in contact with aqueous electrolytes.

I. INTRODUCTION Control over the morphology of absorber materials, to facilitate orthogonalization of the directions of light absorption and charge-carrier collection, is attracting significant attention for application in solar energy-conversion devices.1−10 Microporous, macroporous, and nanoporous photoelectrode structures10−13 and arrays of semiconducting nanorods,14 nanotubes,7−9 and microwires3 are examples of systems that exploit this orthogonalization principle. In such systems, the materials have sufficient optical thickness to provide high optical absorption in one direction, while the structured, porous morphology allows for photogenerated minority carriers to be collected over relatively short distances in an orthogonal direction. Hence, in such structures, relatively impure materials, with low minority-carrier collection lengths, can produce high internal and external quantum yields for collection of photogenerated charge carriers. The application of the orthogonalization principle is especially interesting for photoelectrodes that can produce anodic current in aqueous electrolytes. Although many choices, including H2 and/or CO, CH3OH, CH4, or other reduced organic species, exist for production of a (reduced) fuel from the inputs of sunlight, CO2, and H2O, completion of the fuelforming cycle also requires the efficient production of O2 from H2O. Many metal oxide semiconductors have a potential at the top of the valence-band edge that is significantly positive of the thermodynamic potential for water oxidation, but such materials have large (Eg > 3 eV)15,16 and/or indirect band gaps that generally result in weak absorption of visible light. Additionally, most metal oxides have low charge-carrier © 2013 American Chemical Society

mobilities and/or short minority-carrier diffusion lifetimes, precluding collection of the photogenerated holes with high quantum yield in conventional planar device structures.13 For example, as early as in 1976, tungsten oxide, which absorbs in the visible region of the spectrum (Eg = 2.6 eV),17−28 is earth abundant, is nontoxic, and has a reduction potential for valenceband holes of >+3 V vs the normal hydrogen electrode (NHE), was used as a photoanode for oxidation in aqueous electrolytes. 17,18 However, these WO3 samples exhibited low efficiencies and low internal quantum yields. In this work, we have explored the use of structured, porous WO3 films as photoanodes in contact with aqueous electrolytes and have compared the photoelectrochemical behavior of ordered, porous WO3 photoelectrodes to that of compact WO3 films made under the same conditions and having nominally the same composition. Metal oxide nanostructures have been synthesized previously by the anodization of Ti,29 Fe,13 or W12,19,20,30,31 metal. A 300 nm thick nanoporous WO3 film synthesized by anodization exhibited 6 times higher incident photon to photocurrent efficiency than compact films, increasing the quantum yield for charge-carrier collection from 0.04 to ∼0.24, with the increase in current ascribed to the increased junction area of the semiconductor/liquid contact in such systems.19,32 The question of interest is whether it is possible to obtain high internal quantum yields for chargecarrier collection in photoanode materials by controlling the Received: March 13, 2013 Revised: June 15, 2013 Published: June 17, 2013 14947

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the WO3 foil attached to the exit port of the integrating sphere, with a tungsten foil used as a reference. The reflectance of the sample was defined as R = IF/I0, where IF is the reflected intensity and I0 is the reference intensity. The reflected intensity was not corrected for differences in scattering from the surface between sample and reference. C. Photoelectrochemical Measurements. For photoelectrochemical measurements, the back of the foil was polished to remove oxides. Conductive silver epoxy was used to attach a copper wire to the backside of the foil. The metallic contact and the foil edges were covered with Hysol 1C epoxy to produce an exposed area of 0.3−0.4 cm2. The contact wire was threaded through a glass tube, and the electrode was attached to the end of the tube through the use of Hysol 1C epoxy. The electrodes were allowed to cure for >12 h at 60 °C. The area of the electrode was measured by taking a digital image, using a flat bed scanner, of the electrode and of a calibration scale. The resulting images were analyzed using ImageJ software. A Teflon electrochemical cell with a quartz window, a platinum foil counter electrode, and standard calomel electrode (SCE) reference was used for all photoelectrochemical studies. Photoelectrochemical measurements were collected with bubbling O2 and with magnetic stirring. A 500 W Oriel Xe lamp with an Air Mass (AM) = 1.5 solar simulator optical filter set was used as an illumination source. The intensity of the Xe lamp was measured using a secondary standard calibrated Solarex photodiode, and the incident illumination intensity was adjusted by changing the position of the lamp relative to that of the electrochemical cell. The electrolyte was aqueous 0.10 M HCl (pH = 1.0) for all experiments, except for pH-dependence measurements (Figure S4), in which buffers (0.10 M) with different pH values were used as electrolytes, including acetate (pH = 4.0), phosphate (pH = 7.0), and carbonate (pH = 10) buffers. Electrochemical measurements were recorded using a BAS 100B electrochemical analyzer. Photocurrent densities were calculated as the difference between consecutive measurements of the current (corrected for the electrode area) recorded in the presence and absence of illumination, respectively. An Osram ELH-type 300 W tungsten/halogen lamp provided the illumination source for the long-term photoelectrode stability measurements. For the spectral response measurements, a 500 W Oriel xenon lamp was coupled to a Newport monochromator that had slits set to give a 10 nm exit illumination bandwidth. The intensity of the light exiting the monochromator was measured using a calibrated Si photodiode (United Detector Technologies model UV50) and a 10 Hz light chopper. The illumination spot (∼1 mm2 diameter) was significantly smaller than the electrode area (30−40 mm2). Except where specified otherwise, the incident light intensity quoted as striking the sample was not corrected for reflections from the electrode surface or the surfaces of the quartz windows. For a planar surface, the surface reflection is only very weakly dependent on the incident optical wavelength. By use of Fresnel’s law, IR/I0 = [(n1 − n2)]2, where IR and I0 are the reflected and incident intensities at each interface, and taking the refractive indexes of quartz, dilute aqueous solution, and WO3 to be 1.55, 1.33,34 and 2.135, respectively, the reflective losses were estimated to be ∼10% at normal incidence. The samples were biased at 0.93 V vs SCE (i.e., the formal potential for production of O2 from water under standard conditions at pH = 1.0). A rapid response for WO3 electrodes (going quickly to zero and then back to a positive current value) was observed

porosity and morphology of such systems. The goal is to make a photoanode material in which the minority-carrier collection distance in the electrode is shorter than the minority-carrier collection length, so that the photogenerated carriers can be collected before they can recombine.

II. EXPERIMENTAL SECTION A. Sample Preparation and Electrode Assembly. Porous WO3 layers were prepared by anodization of 0.25 mm thick, 99.95% tungsten foil (Alfa Aesar). Prior to anodization, samples of the foil were cut into pieces and sonicated for 15 min successively in water, acetone, and methanol. The W foils were then contacted on one side with a Cu plate, with the other side pressed against a Teflon cell that contained four circular cutouts (d = 0.79 cm). The total area of the W foil exposed to the solution was 1.96 cm2. The cell was sealed to the W foil by Viton O-rings. NaF was dissolved in either deionized water or in organic polar solvents, including ethylene glycol, formamide, and dimethyl sulfoxide, to provide the electrolyte for the anodization process. A power supply was connected between the W foil and a platinum gauze counter electrode (2.0 cm2) with the electrodes separated by a distance of 1 cm. After anodization, the oxidized W samples were rinsed with deionized H2O and dried under a stream of N2(g). The electrode potential, NaF concentration, and electrolysis time were varied to prepare differently anodized W films. Compact oxidized W films were prepared by anodization in fluoride-free electrolytes (1 M H2SO4) at 50 V for 5 h. After anodization, all of the WO3 samples were annealed in air for 2 h at various temperatures between 300 and 600 °C. B. Characterization. The thickness and morphology of the oxidized W films were determined by recording scanning electron microscopy (SEM) images at 60° and 0° off normal, respectively. All imaging was performed using a ZEISS 1550 VP field emission SEM. Surface images were acquired using the inlens detector at an accelerating voltage of 10 kV, a 4−6 mm working distance, and magnifications of 1000−150000×. The SEM images were analyzed, using ImageJ software, to determine the average film porosity, film thickness, and pore diameter. The film thickness was determined by scratching the film and measuring the distance in the SEM image from the exposed W foil to the top of the WO3. The measured distance was corrected for the 60° angle of tilt. The surface porosity was estimated by measurement of the total pore space evident in the SEM images. The effective absorption thickness of the films was estimated as the film thickness × (1 − film fractional porosity), where the fractional porosity of the film was estimated from SEM images assuming that the porosity was constant through the thickness of the film. Several areas of the film were analyzed using an Oxford INCA Energy 300 X-ray energy-dispersive spectrometer (EDS) system that was coupled to the SEM. All peaks in the EDS data were analyzed for elements having an atomic number ≥4. The number of iterations was 3, and SiO2 and W were used as standards for O and W, respectively. X-ray diffraction data were collected using a Philips XRD with Cu Kα radiation The chemical composition of the films was determined using an M-Probe X-ray photoelectron spectrometer (XPS) that has been described previously.33 Reflectance spectra were collected on a Shimadzu UV2101PC dual-beam spectrophotometer that was equipped with an integrating sphere. Reflectance spectra were measured with 14948

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Figure 1. SEM images of WO3 films synthesized by anodization of W foil in different concentrations of NaF (scale bar: 200 nm). The compact film (no NaF) was prepared in 1 M H2SO4(aq) at 50 V for 5 h. All of other films were prepared by anodization in NaF(aq) at 60 V for 9 h. At high NaF concentration (0.24 M), a yellow powder accumulated on the top of the film and covered the pores.

Figure 2. Top and 60° tilted SEM images of porous WO3 films after anodization times of 2, 5, 9, 15, and 24 h. All of the films were prepared by anodization of W foil in 0.10 M NaF(aq) at 60 V. Also shown are representative measurements of the distance from the top to the bottom of the film. To calculate the film thicknesses reported in Table 1, a large area was analyzed, and several measurements were taken, averaged, and corrected for the 60° angle of tilt.

Table 1. Properties of WO3 Films As Determined from SEM Imagesa anodization timea (h) 2 5 9 15 24 compact

pore diameter (nm) 70 120 160 180 200

± ± ± ± ±

5 11 15 14 18

distance between poresb (nm) 93 74 62 35 29

± ± ± ± ±

fractional porosity

9 6 7 7 5

0.39 0.45 0.50 0.58 0.67 0

± ± ± ± ±

0.06 0.03 0.04 0.03 0.06

thicknessc (μm)

effective absorption thicknessd (μm)

± ± ± ± ± ±

0.20 0.40 0.86 0.89 0.85 0.85

0.32 0.74 1.72 2.13 2.59 0.85

0.05 0.04 0.07 0.07 0.08 0.06

All porous films were synthesized in 0.10 M NaF(aq) at 60 V at various anodization times. bAverage edge-to-edge distance of WO3 between the pores of the film. cFilm thickness calculated by correcting the distance measured in SEM micrographs for the 60° angle of tilt of the samples. d Effective absorption thickness calculated as thickness × (1 − fractional porosity). a

at the 10 Hz chopping rate used in the experiments. For angledependent spectral response measurements, the sample was mounted on a manual rotation stage. Backscattering from the electrodes was used to determine when the electrode was at normal incidence relative to the direction of the incident illumination.

electrolyte (1 M H2SO4(aq)). Partial etching, but no pore formation, was observed in 0.020 M NaF(aq). Irregular pore formation was observed during anodization in 0.050 M of NaF, whereas a regular nanoporous structure that covered the entire surface was observed in 0.10 M NaF. At 0.24 M NaF, a film covered the porous structure, and yellow nanoparticles accumulated on the top of this film. No pores were observed when organic fluoride-containing electrolytes were used, regardless of the organic solvent (ethylene glycol, formamide, or dimethyl sulfoxide), NaF concentration (0.02−0.24 M), anodization potential (20−60 V), or anodization time (2−24 h). Hence, except where indicated otherwise, all of the results

III. RESULTS A. Morphology of the Porous WO3 Films. Figure 1 shows SEM images of the WO3 films that were obtained by anodic etching in electrolytes that had a series of increasing concentrations of NaF(aq). A compact film with no observable pores was obtained by anodization in the fluoride-free 14949

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reported herein were obtained using the porous WO3 films produced by anodization in 0.10 M NaF(aq). Figure 2 shows the effect of anodization time on the morphology of WO3 films that were obtained from anodization in 0.10 M NaF(aq). After 9 h of anodization, etching of the pore walls was observed, with loss of wall material between some of the pores. After 24 h of anodization, the pore diameter increased from 70 to 200 nm, and the thickness of the intervening WO3 between the pores decreased from 90 to 30 nm. As shown in the 60° tilted SEM views displayed in Figure 2, longer anodization times yielded thicker layers of oxidized W. For example, the film thickness increased from 0.3 to 2.6 μm when the electrolysis time was increased from 2 to 24 h. The film porosity also increased with anodization time, from 0.39 after 2 h of anodization to 0.67 after 24 h of anodization. The effective absorbance thickness of the film was estimated by multiplying the measured film height by (1 − film fractional porosity). The effective absorbance thickness did not change significantly when the anodization time was extended from 9 to 24 h (Table 1). When the porous WO3 layer was mechanically detached from its W foil base, a porous layer of thickness 2.5 μm was obtained (Figure 3). This freestanding material had pores on both sides of the film.

tures that have been annealed at various temperatures is not well documented. Figure 4 shows the effect of the annealing temperature on the structure of the porous WO3 films. Annealing of the films at temperatures below 400 °C produced little observable change in the film structure. At 500 °C, however, the pore walls started to break down, and by 600 °C, the organized porous structure had collapsed, creating an architecture similar to a network of nanoparticles. The thickness of this collapsed structure was significantly reduced from the thickness of the original, unannealed, anodized film. B. Composition and Crystal Structure of Porous WO3 Films. Energy-dispersive X-ray spectroscopic data on all of the porous and compact films confirmed the presence of W and O (Figure S1), but no signals from other elements were observed. XPS data showed adventitious C in addition to W and O. The compact and porous films exhibited mutually similar XPS behavior, with W 4f7/2, W 4f5/2, and O 1s peaks at 35.6, 37.7, and 531.0 eV of binding energy, respectively (calibrated using a C 1s energy of 284.6 eV) (Figure S2). These binding energy values are in good agreement with prior observations on WO3 films prepared by anodization of W.31 According to the NIST XPS database, the F 1s signal from NaF is found at ∼684.5 eV and the Na 2p signal is found at 24.7 eV. The XPS survey spectrum did not show any peaks in these regions, indicting a lack of detectable fluorine or sodium from the NaF electrolyte that was used in the anodization process. As expected, all of the compact and porous films showed an O/W atomic ratio of 3:1 by both EDS and XPS (Figures S1 and S2). Figure 5 shows the X-ray diffraction (XRD) patterns of porous WO3 films that had been annealed in air for 2 h as a function of the annealing temperature. No XRD peaks were detected for either the as-synthesized films or for films that were annealed in air at 300 °C for 2 h. After annealing at 400 °C, porous films showed three distinct WO3 peaks in the 23° < 2θ < 25° range. WO3 can adopt several different crystal structures (e.g., monoclinic, orthorhombic, and triclinic).36,37 Comparing the XRD patterns with the JCPDS files, the diffraction peaks corresponded to those of the monoclinic structure (JCPDS 43-1035), the most stable phase at room temperature and a photoactive phase of WO3.32,38 After annealing at 400 °C, the porous and compact WO3 films were oriented toward the (200) plane (Figure S5), while at 500 and 600 °C, a preference toward the (020) plane was observed. Taking the XRD peak intensities as a qualitative estimate of the crystallinity, Figure 5 shows that the crystallinity increased with increases in the annealing temperature.38 These results are in good agreement with other studies that have described the effect of annealing temperature on textured and smooth WO3

Figure 3. SEM images of a free-standing porous WO3 film obtained after removal from the W foil base: (a) 60° tilted view, (b) view of the underside of the film showing pores, and (c) a cross-sectional view of a film with a thickness of 2.5 μm. The film was prepared by anodization of a W foil at 60 V in 0.10 M NaF(aq) for 24 h, followed by mechanical detachment of the film from the W foil.

For photoelectrochemical applications, annealing of the films was required. The structural stability of organized nanostruc-

Figure 4. SEM images of porous WO3 films annealed for various temperatures in air for 2 h. Films were prepared by anodization of W foil at 60 V for 9 h in 0.10 M NaF(aq). 14950

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currents were negligible. At more negative potentials, electrochromic behavior appeared, and the dark currents increased. The data collected were therefore limited to the potential range in which the dark currents were essentially zero. Nonmonotonic behavior was observed on samples that were annealed at 300 °C (amorphous). For these samples, photocurrent increased from ∼0 to 0.70 V vs SCE but then decreased slightly from 0.70 to 1.30 V vs SCE. Normally, this “peak”, visible on the first cyclic voltammetric sweep, disappeared after several more cycles, and generally did not appear when scanning in the negative direction. Figure 7 displays the photoelectrochemical performance in 0.10 M HCl(aq) of a compact WO3 electrode and of porous Figure 5. Powder X-ray diffraction pattern for porous WO3 electrodes annealed at various temperatures. For reference, the XRD pattern for monoclinic WO3 is shown at the bottom of the figure. The films were prepared by anodization of W foil at 60 V for 9 h in 0.10 M NaF(aq).

films.39 In addition to the crystallinity, the optical and structural properties of WO3 also changed as a function of the annealing temperature.40 C. Photoelectrochemical Behavior under Simulated AM 1.5 Illumination. Figure 6 shows the photocurrent Figure 7. (a) Plot of the photocurrent density vs potential for porous WO3 electrodes prepared using various anodization times. (b) Plot of the photocurrent density divided by the absorption thickness, as a function of electrode potential, for porous WO3 electrodes that were produced by varying the anodization times. All of the films were synthesized by anodization of W foils at 60 V in 0.10 M NaF(aq). The behavior of a compact WO3 photoelectrode is also shown. The electrodes were immersed in 0.10 M HCl (pH = 1.0) and were exposed to 100 mW cm−2 of simulated solar illumination.

WO3 electrodes that were anodized for various times. The photocurrent densities of the porous electrodes increased with the anodization time, and all of the annealed porous electrodes exhibited significantly higher photocurrent densities than the compact electrode. Notably, the films that were anodized for 2 or 5 h yielded higher J values than the compact film (Figure 7b), even though such films had smaller effective absorbance thicknesses than the compact WO3 film (Table 1). For porous electrodes that had effective absorbance thicknesses similar to those of the compact electrode (i.e., for films that had been anodized for 9, 15, or 24 h), the porous electrodes displayed an approximately 5−6-fold increase in photocurrent density at the water oxidation potential. Although porous WO3 films prepared by anodization for 9, 15, and 24 h had mutually similar effective absorption thicknesses (Table 1), the electrodes with the higher porosity (24 h of anodization) exhibited the highest photocurrent density (J = 0.75 mA cm−2 at 1.23 V vs RHE). The fill factors were approximately 0.25 for the porous electrodes and 0.19 for the compact electrode. The onset potential, i.e., the potential at which significant anodic current density was observed, shifted slightly positive, by 50 mV, for porous electrodes relative to that observed for the WO3 compact electrodes. D. Stability under Photoanodic Conditions. Figure 8a shows the photocurrent as a function of time for a porous WO3 photoanode immersed in an acidic aqueous electrolyte (0.10 M HCl(aq), pH = 1.0). When the light was turned off, the current

Figure 6. Plot of photocurrent density vs potential for porous WO3 electrodes annealed at various temperatures. Films were prepared by anodization of W foil at 60 V for 9 h in 0.10 M NaF(aq). The electrodes were immersed in 0.10 M acetate buffer (pH = 4.0) and were exposed to 100 mW cm−2 of simulated solar illumination.

density as a function of the applied potential of porous WO3 electrodes in contact with a pH = 4 electrolyte (bubbling O2, 0.10 M acetate buffer, 100 mW cm−2 AM 1.5). The photocurrent density was calculated based on the projected area of the electrode, as opposed to the actual surface area. Unannealed electrodes, and those annealed at 300 °C, produced low photocurrent densities (J < 6 × 10−2 A cm−2 at 1.23 V vs RHE). The photocurrent density was largest for films that were annealed at 400 °C (J = 0.47 mA cm−2 at 1.23 V vs RHE), with the photocurrent density decreasing for films that were annealed at higher temperatures. Hence, all of the electrodes investigated in the experiments described below were annealed at 400 °C to preserve the fidelity of the porous structures. To facilitate direct comparison between the porous and compact films, photocurrent density vs potential data were also collected on compact electrodes that had been annealed at 400 °C. In the potential range of 0−1.20 V vs SCE, the dark 14951

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Figure 8. (a) Plot of the photocurrent at 0.93 V vs SCE as a function of time for a porous WO3 electrode in 0.10 M HCl(aq), exposed to 100 mW cm−2 of simulated solar illumination. (b) Plot of the current−potential data obtained at the beginning and at the end of the stability measurements. (c) SEM image of the electrode after the stability experiment.

rapidly went to zero, and the steady-state current was rapidly restored when the light was again turned on. The current was stable during the 8-day experiment (current variations at the end of the experiment were due to power fluctuations of the lamp). Approximately 2.2 × 10−3 mol of charge was produced during the 8-day run, compared with the ∼8.6 × 10−7 mol of WO3 in the porous electrode. Every morning and every evening during the 8-day experiment, current density vs potential (J−E) data were collected. As shown in Figure 8b, the J−E behaviors at the beginning and end of the stability experiment were almost identical, indicating that the performance did not significantly change after the electrodes were held under oxidizing conditions for ∼190 h. No changes were observable in the color or surface of the electrode from the beginning to the end of the experiment. SEM images of the electrode showed that the porous structure was unaffected by the long illumination time and by exposure to the acidic electrolyte (Figure 8c). E. Light Intensity Dependence. Figures 9a,b show the photocurrent density as a function of light intensity for porous and compact WO3 electrodes, respectively, in contact with 0.10 M HCl(aq). Negligible dark current was observed in the potential range 0−1200 mV vs SCE. To investigate the processes of electron transport and charge-carrier recombination, the photocurrent density at selected potentials was investigated as a function of the incident light intensity. At all potentials, the porous WO3 electrodes exhibited an approximately linear dependence of the current on the light intensity (Figure 9c). In contrast, compact electrodes did not exhibit a linear dependence of J on the light intensity; rather, the photocurrent density was linear relative to the square root of the light intensity (Figure 9d). F. Spectral Response Properties. Spectral response measurements were performed for compact films and for porous WO3 electrodes that had a fractional porosity of 0.50. The external quantum yield (ΦExt) was calculated as the number of electrons produced divided by the number of photons in the incident optical excitation. Figure 10A shows the external quantum yield of such photoanodes at 0.93 V vs SCE, as a function of the wavelength of illumination, in contact with 0.10 M HCl(aq). As expected, the WO3 electrodes exhibited photocurrent when illuminated with light of wavelength λ < 460 nm (2.7 eV). Figure 10A shows that for 310 < λ < 460 nm light, ΦExt was significantly enhanced by the porous structure, with the porous electrode showing a 5-fold increase in ΦExt at λ = 350 nm relative to the compact film. Diffuse reflectance spectra indicated that the porous film absorbed approximately half of the incoming UV light, while the compact film absorbed almost all of the incoming UV light (Figure S3). Although these electrodes had similar effective absorption thicknesses, the geometric filling fraction in the

Figure 9. Plots of photocurrent density vs potential for (a) porous and (b) compact WO3 electrodes at various light intensities. (c) Photocurrent density vs light intensity for porous WO3 electrodes held at various potentials. (d) Photocurrent density vs the square root of light intensity for compact WO3 samples held at various electrode potentials. The porous WO3 electrodes were prepared by anodization at 60 V for 9 h in 0.10 M NaF(aq). The electrodes were immersed in 0.10 M HCl (pH = 1.0) and were illuminated with a 500 W Oriel Xe lamp with an Air Mass (AM) = 1.5 filter set. The incident illumination intensity was adjusted by changing the position of the lamp relative to that of the electrochemical cell and was measured using a secondary standard calibrated Solarex photodiode.

optical plane of the porous structure was only ∼50%, while the compact film filled 100% of the optical plane at normal incidence. The internal quantum yield (ΦInt) was calculated using the observed absorption of the film and the value of ΦExt at each wavelength, by use of ΦInt = ΦExt/(1 − R), where R is the measured reflectance. For the porous electrode, ΦInt was constant at a value of ∼0.5 for λ < 420 nm, while the measured absorbance approached zero for λ > 420, yielding unreliable values for ΦInt. In contrast, for the compact electrode, ΦInt rose slowly as the illumination wavelength was decreased and only reached ∼0.4 at λ = 280 nm. The porous electrode thus produced a 6-fold increase in ΦInt at λ = 350 nm relative to the compact film. G. Determination of the Effective Minority-Carrier Collection Length. An important parameter for a semiconductor photoanode material is the minority-carrier diffusion length (Lp, the hole diffusion length, for an n-type semiconductor). The value of the diffusion length in a compact, planar semiconductor can be estimated by quantitative analysis 14952

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diffusion length is long enough to allow photogenerated minority carriers to reach the solution in the porous samples of interest in this work. For both the compact and porous films, the observed maximum values for ΦExt,corr were less than 1.0, whereas eqs 1 and 2 approach a plateau of 1.0 as α(W + Lp) becomes large. For the compact film, a fit of ΦExt,corr vs α(λ) to eq 1 yielded well-defined values of only one of the two parameters while the other parameter was small and/or poorly defined (Figure 11a). This behavior suggested that the two

Figure 10. Plot of the wavelength dependence of (A) the external quantum yield and (B) the internal quantum yield at the formal potential for water oxidation for compact (blue circles) and porous (red squares) WO3 electrodes in contact with 0.10 M HCl(aq) (pH = 1.0). The rise in the calculated internal quantum yield at λ > 400 nm is due to very low measured reflectance values at these wavelengths. The porous electrode was anodized at 60 V for 9 h in 0.10 M NaF(aq) and had a fractional porosity of 0.50, while the compact film was prepared by anodization of a W foil at 50 V for 5 h in water. The compact and porous films had comparable absorption thicknesses. The ΦExt values were not corrected for surface reflection losses, whereas ΦInt was calculated as ΦInt = ΦExt/(1 − R), where R is the measured reflectance of the sample.

Figure 11. Plot of the corrected external quantum yield, ΦExt,corr, vs the absorption coefficient, α, for (a) compact and (b) porous WO3 electrodes. The circles are the corrected experimental data while the green line is the fit to eq 1. The fits yield only the sum of the depletion distance plus the minority-carrier diffusion length. For compact samples, the sum of W + Lp was ∼0.02 μm−1, while for porous samples, the fit yielded an effective sum of ∼0.3 μm. In performing this analysis, the measured ΦExt values were corrected by 10% to account for the nominal surface reflection from planar WO3 films to produce values of ΦExt,corr at each wavelength.

provided that the wavelength dependence of the absorption coefficient, α(λ), is known.41 When negligible recombination at short circuit occurs in the depletion region, the quantum yield for planar electrodes is given by eq 1, when reflection and other losses, including contributions due to diffusion currents in the space-charge region, have been ignored42,43 ΦExt,corr

⎛ ⎞ 1 = 1 − e −αW ⎜ ⎟ ⎝ 1 + α LP ⎠

parameters were highly correlated and that only their sum was well-defined; hence, the sum, the effective, or apparent, minority-carrier collection length of the films, is denoted as Lcoll,p. For the compact film, Lcoll,p was ∼0.02 μm. Fitting of the data for the porous film yielded poor fits if all of the data were used because ΦExt,corr for these films plateaued at ∼0.3. Use of only the data for ΦExt,corr between 370 and 460 nm (α < 1 × 104 cm−1) (i.e., only the rise in the external quantum yield was used for the fit) resulted in an effective minority-carrier collection length, Lcoll,p, of ∼0.3 μm for a WO3 porous film (Figure 11b); however, the value was somewhat dependent on the wavelength range chosen for the fit. H. Angle-Resolved Photocurrent Measurement. The optical absorbance of a porous sample should be a function of the angle of the sample relative to the propagation vector of the illumination beam. Figure 12 shows ΦExt for compact and porous samples as a function of the angle of the incident light relative to the electrode surface normal, using an irradiation spot (∼1 mm2) much smaller than the sample area (30−40 mm2), in contact with 0.10 M HCl(aq). The value of ΦExt for a compact film was essentially independent of angle (Figure 12a).

(1)

and W is the width of the depletion layer. If αW ≪ 1, eq 1 reduces to ΦExt,corr ≈

α (W + L P ) 1 + α (W + L P )

(2)

where in both eqs 1 and 2 ΦExt,corr is the external quantum yield corrected for surface reflection and α is the wavelengthdependent optical absorption coefficient. The experimentally observed ΦExt value was corrected for the nominal ∼10% reflection due to the sample and the cell to yield ΦExt,corr, and values for α(λ) for WO3 were estimated from literature data (Table S1).44,45 The Gartner model was developed for a single crystal and does not rigorously apply to samples having a porous morphology. The analysis has therefore only been used to show qualitatively that the apparent, effective minority-carrier 14953

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∼ 460 nm, in agreement with the 2.6 eV band gap of WO3.17−28 Although the onset of the spectral response ΦExt was principally determined by the band gap, the efficiency with which incident photons were converted to current was dependent on the structure of the film. Even though the porous films absorbed only about half as much of the incoming light as the compact film (Figure S3), for λ > 300 nm, the ΦExt values of the porous WO3 films were higher than those of the compact WO3 films (Figure 10a). Furthermore, at all wavelengths, the ΦInt values of porous WO3 films were much higher than those of the compact films (Figure 10b). Notably, ΦInt was not corrected for the actual optical reflection losses of the porous film, suggesting that ΦInt for the porous film was higher than reported in Figure 10b. The hypothesis being challenged in this work is whether, for a given material prepared in a given way, orthogonalization can provide improved carrier-collection performance for solar energy-conversion applications in contact with liquid electrolytes. The spectral response data thus indicate that the porous geometry significantly increased the fraction of electron−hole pairs that were collected. Note that the system of interest relies on minority-carrier collection, driven by both drift and diffusion, as opposed to nanoporous electrodes used either with or without dyes in a dye-sensitized solar cell configuration, which are largely field-free in the solid and consequently rely on efficient majority-carrier collection. One of the factors that controls ΦExt is the rate of electron− hole recombination. The increase in ΦExt for the porous films relative to the compact films suggests that the rate of electron− hole recombination was slow enough in these films to allow all of the minority carriers (holes) to reach the solution interface and to allow the majority-carrier electrons to be collected at the back contact. This behavior is consistent with the observed dependence of ΦInt on λ and on the absorption coefficient of WO3. For WO3, the absorption coefficient increases significantly as the wavelength decreases (Table S1). The value of ΦInt for the porous film was constant at ∼0.5 for λ < 410 nm, at which point the absorption length (1/α) of the WO3 decreases from ∼10 to ∼0.02 μm (Table S1). Thus, in the porous films, the minority carriers can diffuse to reach the solution interface, suggesting that even when the penetration depth is 10 μm, the minority carriers in the porous film can still reach the solution interface and thus facilitate separation of most of the electron− hole pairs. Thus, the porous film exhibited excellent charge collection even when the light is expected to be absorbed throughout the film. This behavior is in contrast to the value of ΦInt observed for the compact film. At λ = 410 nm, ΦInt was less than 0.008 for the compact film, and ΦInt for such samples increased only slowly as λ decreased and α increased. For the compact film, only when the absorption length decreased to ∼0.2 μm (α > 5 × 104 cm−1, λ < 370 nm) did ΦInt begin to increase significantly. B. Dependence of the Photocurrent Density on Light Intensity for Porous WO3 Electrodes. For the porous films, the observed linear dependence of the photocurrent on light intensity is also consistent with the hypothesis that the porous film morphology imparted the films with an effective, apparent minority-carrier collection distance sufficient to compete with charge-carrier recombination such that all of the electron−hole pairs could be separated. A linear dependence of the photocurrent on light intensity has been related to a low interfacial electron−hole recombination rate47 and to favorable

Figure 12. Plot of the angle-resolved spectral response vs wavelength for (a) compact (anodization conditions: H2SO4, no NaF added, 50 V, 5 h), (b) porous electrodes of 50% porosity (0.05 M NaF(aq), 60 V, 9 h), and (c) porous electrodes of 67% porosity (0.10 M NaF(aq), 60 V, 24 h) in 0.10 M HCl(aq) (pH = 1.0). Angles were measured relative to the surface normal. All of the electrodes were annealed in air for 2 h at 400 °C. The ΦExt data were not corrected for optical surface reflection losses.

In contrast, ΦExt for the porous electrodes increased as the illumination angle increased (Figures 12b,c). The reduction observed in ΦExt at high angles of incidence and short wavelength was likely due to reflection and scattering losses. As expected, the magnitude of increase in the spectral response was dependent on λ. As λ decreased, the film absorption cross section increased due to the increase in the absorption cross section of WO3, and ΦExt increased. Figures 12b,c show that the wavelength dependence of the absorbance increase was a function of the porosity of the sample, and the maximum of the absorbance increase shifted to longer wavelength as the porosity of the electrode increased.

IV. DISCUSSION A. Internal Quantum Yields for Porous WO3 Electrodes. WO3 has been proposed for use in light-driven watersplitting devices as a photoanode.18,46 WO3 absorbs more of the solar spectrum than most other wide band-gap metal oxides and is stable in acidic medium. WO3 clearly does not have an optimal band gap for solar energy-conversion applications but is a useful model system to test the hypothesis of concern in this work. As shown in Figure 10, the onset of the spectral response of both the compact and porous WO3 electrodes occurred at λ 14954

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interfacial kinetics.24 For the porous electrodes, this linearity was an indication that, even at high light intensities, all of the photogenerated holes were collected. These results reinforce the suggestion that electron−hole recombination is not the limiting factor in these porous materials. In the case of the compact films, the deviation from linearity at higher intensities is indicative of significant electron−hole recombination.47 The existence of a nonlinear dependence of the photocurrent on light intensity has been observed previously for TiO2 singlecrystal photoanodes, implying that the major factor in determining the magnitude of the steady-state photocurrent in that system is the recombination rate of photogenerated holes and electrons.48 The photocurrent of the single crystal was approximately linearly dependent on intensity for incident intensities 2 mA cm−2 can be expected, which would be comparable to the photocurrent densities reported by Augustynski et al. for a 2 μm thick WO3 film consisting of a network of platelike highly crystallized nanoparticles (annealed at 550 °C).53 Such values are also comparable to the photocurrent densities recently reported for 3.3 μm thick nanocrystalline WO3 electrodes.32 Our work thus demonstrates the importance of the details of the nanostructured morphology on the photoelectrochemical performance of WO3 in contact with aqueous electrolytes. Further development of methods for achieving nearly complete light absorption is thus needed to obtain more efficient nanostructured photoelectrodes from this materials system.

†

K.R.R.-G.: Sandia National Laboratory, 7011 East Ave., Livermore, CA 94551. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the NSF Powering the Planet Center for Chemical Innovation, grant CHE-0947829, and by the Defense Advanced Research Projects Agency. Use of facilities at the Molecular Materials Research Center (MMRC) at Caltech is gratefully acknowledged. The authors also acknowledge the technical work of Miguel Ortiz from California State University, Los Angeles.

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(1) Shankar, K.; Basham, J. I.; Allam, N. K.; Varghese, O. K.; Mor, G. K.; Feng, X.; Paulose, M.; Seabold, J. A.; Choi, K.-S.; Grimes, C. A. Recent Advances in the Use of TiO2 Nanotube and Nanowire Arrays for Oxidative Photoelectrochemistry. J. Phys. Chem. C 2009, 113, 6327−6359. (2) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834−2860. (3) Boettcher, S. W.; Spurgeon, J. M.; Putnam, M. C.; Warren, E. L.; Turner-Evans, D. B.; Kelzenberg, M. D.; Maiolo, J. R.; Atwater, H. A.; Lewis, N. S. Energy-Conversion Properties of Vapor-Liquid-SolidGrown Silicon Wire-Array Photocathodes. Science 2010, 327, 185− 187. (4) Spurgeon, J. M.; Plass, K. E.; Kayes, B. M.; Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Repeated Epitaxial Growth and Transfer of Arrays of Patterned, Vertically Aligned, Crystalline Si Wires from a Single Si(111) Substrate. Appl. Phys. Lett. 2008, 93, 032112. (5) Kayes, B. M.; Spurgeon, J. M.; Sadler, T. C.; Lewis, N. S.; Atwater, H. A. Synthesis and Characterization of Silicon Nanorod Arrays for Solar Cell Applications. Proc. IEEE 2006, 221−224. (6) Reyes-Gil, K. R.; Spurgeon, J. M.; Lewis, N. S. Silicon and Tungsten Oxide Nanostructures for Water Splitting. Proc. SPIE 2009, 7408, 740826. (7) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Use of Highly-Ordered TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells. Nano Lett. 2006, 6, 215−218. (8) Beranek, R.; Tsuchiya, H.; Sugishima, T.; Macak, J. M.; Taveira, L.; Fujimoto, S.; Kisch, H.; Schmuki, P. Enhancement and Limits of the Photoelectrochemical Response from Anodic TiO2 Nanotubes. Appl. Phys. Lett. 2005, 87, 243114. (9) Kongkanand, A.; Dominguez, R. M.; Kamat, P. V. Single Wall Carbon Nanotube Scaffolds for Photoelectrochemical Solar Cells. Capture and Transport of Photogenerated Electrons. Nano Lett. 2007, 7, 676−680. (10) Chen, X.; Ye, J.; Ouyang, S.; Kako, T.; Li, Z.; Zou, Z. Enhanced Incident Photon-to-Electron Conversion Efficiency of Tungsten Trioxide Photoanodes Based on 3D-Photonic Crystal Design. ACS Nano 2011, 5, 4310−4318. (11) Maiolo, J. R.; Atwater, H. A.; Lewis, N. S. Macroporous Silicon as a Model for Silicon Wire Array Solar Cells. J. Phys. Chem. C 2008, 112, 6194−6201. (12) Mukherjee, N.; Paulose, M.; Varghese, O. K.; Grimes, C. Fabrication of Nanoporous Tungsten Oxide by Galvanostatic Anodization. A. J. Mater. Res. 2003, 18, 2296−2299. (13) Prakasam, H. E.; Varghese, O. K.; Paulose, M.; Mor, G. K.; Grimes, C. A. Synthesis and Photoelectrochemical Properties of Nanoporous Iron (III) Oxide by Potentiostatic Anodization. Nanotechnology 2006, 17, 4285−4291. (14) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. Comparison of the Device Physics Principle of Planar and Radial p-n Junction Nanorod Solar Cells. J. Appl. Phys. 2005, 97, 114302.

V. CONCLUSIONS WO3 has intrinsic properties such as its electronic configuration, solar absorption, and stability that make it a good candidate for photoanodic reactions in aqueous electrolytes. As for other metal oxides, electron−hole recombination is one of the major factors that limits the performance of WO3 as a photoanode. Porous WO3 photoanodes showed a significant improvement in photocurrent conversion efficiency compared to compact electrodes. Spectral response data demonstrated that the porous morphology enhanced the internal quantum yields and the effective minority-carrier diffusion lengths, consequently reducing the electron−hole recombination of WO3. In particular, angle-resolved spectral response studies showed that the performance of the porous WO3 photoanodes could be improved still further if the light absorption were increased. This work suggests that highly ordered porous morphologies can improve photogenerated carrier separation and collection, making nanostructured WO3 and related materials interesting candidates to drive the O2 production half-reaction in a prototypical first-generation integrated fuelproducing solar photoelectrochemical system.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption coefficients for WO3, an EDS spectrum for porous WO3, plots of photocurrent density vs potential for WO3 electrodes in electrolytes having various pH values, and XPS spectra, diffuse reflectance spectra, and powder XRD patterns for porous and compact WO3 films. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.S.L.). 14956

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