boosting visible light harvesting in p‐type ternary oxides

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Boosting Visible Light Harvesting in p-Type Ternary Oxides for Solar-to-Hydrogen Conversion Using Inverse Opal Structure

Yunjung Oh, Wooseok Yang, Jeiwan Tan, Hyungsoo Lee, Jaemin Park, and Jooho Moon*

P-type semiconductors based on ternary oxides have attracted wide interest owing to their earth-crust abundance and favorable optoelectronic proper-ties. Among the p-type ternary oxides, delafossite-phase CuFeO2 has received considerable attention because it has the potential to fully harness visible light (<800 nm) owing to its narrow bandgap (1.4–1.6 eV). Despite the favorable optoelectronic properties predicted by theoretical studies, CuFeO2 photocathodes have low quantum efficiency under visible light near the bandgap edge, which is a major bottleneck for efficient solar-to-hydrogen conversion. Herein, a novel method is presented for boosting visible-light harvesting in the CuFeO2 photocathode by employing an inverse opal structure as a periodic macrostructure. The periodic macroporous structure allows exceptional near-bandgap photon harvesting, particularly within the range of 600–700 nm, owing to the enhanced light absorption due to multiple scattering together with the short diffusion distance for minority carriers toward the electrolyte. After surface modification with a low-cost double hydroxide electrocatalyst, our CuFeO2-based photocathode exhibits a record-breaking photocurrent density of 5.2 mA cm−2 at −0.1 V with respect to the reversible hydrogen electrode for water reduction among p-type ternary oxide-based photocathodes.

DOI: 10.1002/adfm.201900194

Y. Oh, Dr. W. Yang, J. Tan, H. Lee, J. Park, Prof. J. MoonDepartment of Materials Science and EngineeringYonsei University50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201900194.

degrades the hole mobility in most metal oxides. Although p-Cu2O has demonstrated highly efficient water reduction owing to its favorable p-type conductivity through the hybridization of Cu 3d–O 2p orbitals,[3] it has an inherent limitation for visible-light absorption because of its relatively large bandgap (≈2.0 eV). In this regard, mul-tinary oxides, such as CuFeO2, CuBi2O4, CuNb3O8, and CaFe2O4, have attracted tremendous interest because the combi-nations of transition cations are capable of tuning the optoelectronic properties.[4–7] Among the ternary p-type oxides, Cu-dela-fossite CuFeO2 exhibits a small bandgap of 1.4–1.6 eV because the Fe 3d bands are located below the Cu 3d bands.[2,8] In addition, CuFeO2 is recognized as an attrac-tive candidate for water reduction owing to its good intrinsic stability in the basic electrolyte and large absorption coefficient (α ≈ 107 m−1).[8–11] Furthermore, its flat-band potential at ≈1 V versus the revers-ible hydrogen electrode (RHE) suggests the capability to develop a high photovoltage for the reduction of water to H2.[12]

However, despite these beneficial properties, CuFeO2 photocathodes exhibited performance far below the antici-pated value (15 mA cm−2) based on their optical bandgap.[12] Most importantly, CuFeO2 photocathodes generally suffer from poor visible-light harvesting, manifested by a low incident photon-to-current efficiency (IPCE) below 5% for hydrogen production in the wavelength of 600−800 nm.[10,13] The phenom-enon that the visible light is insufficiently utilized compared with the theoretical harvesting range, i.e., the optical bandgap, is commonly observed in other p-type ternary oxides. Because the photon flux of AM 1.5G accounts for the dominant portion of the spectral density in the visible-light region, ineffective utiliza-tion of visible light causes a critical loss of photocurrent density. As the low quantum efficiency of the ternary oxides is generally attributed to the weak absorption and poor charge-carrier trans-port near the absorption edge, simultaneous enhancement of both the absorption and carrier transport should be realized to boost the visible-light harvesting in p-type ternary oxides. Various attempts have been performed to improve the carrier transport in CuFeO2, such as tuning the electrical properties by doping[10,14] and enhancing the charge injection by forming a heterojunction.[13] For example, oxygen-intercalation treatment

Photocathodes

1. Introduction

The D4-type photoelectrochemical (PEC) tandem cell composed of a series-connected n-type semiconductor photoanode and a p-type semiconductor photocathode represents a promising target device for the cost-effective conversion of solar energy directly into fuels through water splitting.[1] Semiconductors based on transition-metal oxides such as WO3, Fe2O3, and BiVO4 are attractive n-type photoanodes for water oxidation owing to their low-cost and easy preparation.[2] In contrast to n-type metal oxides, it is difficult to achieve high-performance p-type oxide semiconductors because the strong localization phenomena of the O 2p orbital at the valence-band maximum

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improved the conductivity, resulting in 4.3-fold enhancement of the photocurrent density (1.3 mA cm−2 at 0.4 V vs RHE) with respect to those of untreated CuFeO2 photocathode.[10] Despite the enhanced photocurrent, most of the improvement resulted from the enhanced light harvesting in the near-ultraviolet (UV) region, while the IPCE value above 600 nm diminished dras-tically to <5%. Furthermore, the doping approach possibly resulted in a CuO secondary phase within the absorber, leading to the formation of trap sites for photogenerated electrons. All the previous studies indicate the necessity of developing a novel approach for absorbing more light in the visible region while enhancing the electrical properties. In this regard, sev-eral groups suggested the implementation of a ternary oxide nanostructure as a breakthrough solution to overcome these limitations.[4,10]

The nanostructuring strategies have already proven high effectiveness for improving the PEC performance because of the ability to capture more light and the enhanced charge transport arising from the unique optical and electrical properties.[15,16] In general, 1D building blocks, such as nanowires, nanorods, and nanohelixes, are synthesized via epitaxial growth, seed-oriented growth, and vapor–liquid–solid growth.[17,18] However, these structures are usually investigated only for simple binary oxides such as TiO2, WO3, and SnO2,

[19–22] whereas the complex growth mechanism and limited phase stability of multinary materials hinder preferential growth on certain crystallographic facets. Thus, several studies have been devoted to the nano-structuring of multinary materials using anodized aluminum oxide as a hard template.[23,24] However, the complex fabrica-tion processes—such as growth, transfer, and template removal are readily inapplicable to directly on PEC device. In addition to the 1D nanostructures, the inverse opal (IO) nanostructure was successfully demonstrated in n-type Fe2O3, WO3, and TiO2 photoelectrodes for high performance with an extraordinarily short length for carrier diffusion.[25–27,31] Moreover, 3D periodic structure of IO can significantly improve the light absorption as compared with 1D building blocks, owing to multiple scat-tering within the photoelectrode.[15,28–32] This IO metal oxide structure is often obtained using a two-step process: sol-infil-tration into the interstices of the latex-based opal-template and removal of the template via calcination. Notably, sol-infiltration has immense potential to produce various oxides through the simple composition control of the precursor sol solution.[15,33,34] Furthermore, during the oxidation of the sol precursor, the soft opal-template can be easily removed to form the IO structure. In this regard, the IO nanostructure platform is a promising candidate for implementing nanostructured phase-pure multi-nary oxides for PEC application.

Herein, we for the first time report IO-structured CuFeO2 as an effective material to boost visible-light harvesting. The IPCE of our well-designed macroporous CuFeO2 photocathode indicates a drastically improved visible-light harvesting ability, reaching 17.5% at 600 nm, which is an unprecedented result for a p-type ternary oxide-based photocathode. With electro-catalyst modification of the earth-abundant cobalt-iron layered double hydroxide (CoFe LDH), our IO-structured CuFeO2 photocathode exhibits a record-high photocurrent density of 5.2 mA cm−2 at −0.1 V versus RHE under 1-sun illumination, which is superior to the previously reported values for CuFeO2

as well as other ternary p-type oxide photocathodes. Our results clearly demonstrate the feasibility of practical solar-to-hydrogen conversion using the earth-abundant photocathode based on ternary oxides, which is achieved by overcoming the major bottleneck in the multinary oxide system.

2. Results and Discussion

Figure 1a shows the fabrication procedure for the IO CuFeO2. Prior to the formation of the IO structure, an ultrathin Cu–Fe oxide film was prepared to prevent the exposure of the bare fluorine-doped tin oxide (FTO) substrate. Then, an opal template composed of periodically assembled 350 nm polystyrene (PS) microspheres was produced using a well-known convective self-assembly method,[35–37] as shown in Figure 1a. This process yielded a hexagonally ordered PS opal template in long-range arrangement with eight to nine layers, as shown in Figure S1 (Supporting Information). The IO CuFeO2 photocathodes were fabricated using the as-prepared opal template via the sol-infiltration method. The opal template was submerged in a copper and iron sol precursor solution, followed drying in air for 5 h to achieve homogeneous impreg-nation and pore-filling (Figure 1a).[25,37] IO CuFeO2 photocath-odes were obtained via an oxidation and annealing process to form the delafossite phase.[9–11,38] Additionally, we produced a planar CuFeO2 photocathode by spin-coating an ethanol-based sol–gel solution (Figure 1a)[9,10] as a reference geometry to examine the effect of the photocathode architecture. The fabri-cation details are presented in the Experimental section.

To evaluate the geometric features of each photocathode, the microstructural evolution was monitored using scanning elec-tron microscopy (SEM). Figure 1b shows the top view of the IO CuFeO2, clearly displaying three-dimensionally ordered IO structures with a hexagonal skeleton. Furthermore, the black holes behind the walls in Figure 1b represent interconnected pores in the IO structure, confirming the 3D ordered hex-agonal close-packed array. The average pore size in Figure 1b is ≈275 nm, which represents 25% shrinkage compared with the diameter of the PS microsphere template in Figure S1 (Supporting Information), and the thickness of the CuFeO2 wall is ≈50 nm. The cross-sectional image of the IO CuFeO2 on the substrates confirms the periodically ordered macroporous structures with eight layers, as shown in Figure 1c. The thin CuFeO2 layer sandwiched between the FTO substrate and the IO CuFeO2 has a thickness of <90 nm. The low-magnification images in Figure S2 (Supporting Information) indicate that our IO CuFeO2 maintains its highly ordered skeleton nanostruc-ture in a long-range order. Cracks are observed in Figure S2a (Supporting Information), likely because of the PS template shrinkage during the annealing process. For comparison, we also examined the microstructure of the planar CuFeO2 photo-cathode. The planar CuFeO2 is composed of a particulate film whose size is <100 nm, and the particles are well connected, as shown in Figure 1d. In addition, as shown in Figure 1e, uni-form deposition of the copper/iron sol–gel ink was achieved as the uneven FTO was filled, yielding the planar structure of CuFeO2 with a thickness of 300 nm. Then, we evaluated the phase purity of both the planar and IO CuFeO2 deposited on

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Figure 1. a) Schematic of the fabrication procedure for the IO and planar CuFeO2 on FTO substrates. Microstructural features of the planar and IO CuFeO2 photocathodes. b,d) Top-view and c,e) cross-sectional SEM images of the IO and planar CuFeO2 on FTO substrates, respectively.



the FTO substrate, as shown in Figure S3 (Supporting Infor-mation). The X-ray diffraction (XRD) patterns for both CuFeO2 photocathodes exhibited diffraction peaks at 2θ = 31.26°, 35.8°, and 40.3°, which are attributed to the (006), (012), and (018) planes of the delafossite CuFeO2 crystal structure (JCPDS No. 0175-2146), respectively.[38] However, it should be noted that the XRD alone is insufficient to verify phase purity, and X-ray photoelectron spectroscopy (XPS) is an important technique for giving insight into secondary phases that could be present on the surface. XPS spectra for Cu 2p, Cu LMM, and Fe 2p were obtained to investigate the chemical states of both IO and planar CuFeO2 near the surface as shown in Figure S4 (Sup-porting Information). For both IO and planar CuFeO2, the Cu 2p spectra were consistent with the representative peaks for Cu(I), with two peaks at 932.4 and 952.4 eV, respectively. There were no detectable peaks for Cu(II) at CuFeO2 surface owing to the absence of the satellite peaks between 940 and 945 eV. It should be noted that it is almost impossible to differentiate Cu(I) from the Cu 2p signal. However, analysis of Cu LMM spectra provides a better way to distinguish between Cu(0) and Cu(I).[39] Cu LMM spectra revealed well-defined peak at 569.68 eV, implying that both IO and planar CuFeO2 did not contain Cu(0) on the surface of absorbers. Furthermore, there were two strong peaks at 745.78, and 711.58 eV with satellite peaks for IO CuFeO2, whereas planar CuFeO2 also presented nearly the same Fe 2p spectra. This clearly suggested the pres-ence of Fe(III) in CuFeO2, which is consistent with the pre-vious results for CuFeO2 absorber.[10,13] Therefore, XPS data in conjunction with XRD indicate that no impurity peaks cor-responding to either iron or copper compounds were detected. Phase constitution study indicates that the photocathodes con-tained phase-pure CuFeO2 regardless of the structures. Thus, we successfully synthesized phase-pure CuFeO2 delafossite photocathodes with a very distinct architecture.

To predict the visible-light absorption behavior with the introduction of the IO structure, finite-difference time-domain (FDTD) simulations were performed using a wave optics module. Although the light scattering of IO structures in the near-UV region has already been demonstrated for wide-bandgap materials (>2.0 eV, such as ZnO, WO3, and

TiO2),[25,37,39,40] the light-absorption boosting effect of ternary oxides for visible light has not been elucidated. The optical medium of CuFeO2 was provided by a previous report,[41] and the geometric domains were adopted from the microstruc-tural examination results of Figure 1b,c. Figure 2a shows the distribution of the magnitude of the electric field, as indicated by the color scale, at the surfaces of the planar and IO CuFeO2 when plane waves with three different wavelengths—400, 500, and 600 nm—are normally incident from the top. Strong light localization is observed in the IO CuFeO2, manifested by the complex distributions of the electric field, whereas the electric field in the planar CuFeO2 shows only waveguide-like peri-odic profiles caused by interference between the incident and reflected waves. Interestingly, the IO CuFeO2 exhibits a very high electric-field intensity under 600 nm light, which is even slightly higher than that at 400 nm, while the intensity of the electric field in the planar CuFeO2 drastically decreases with the increasing wavelength of the incident light. Considering that most ternary oxides cannot fully absorb light near the bandgap edge, resulting in absorption loss at 600−700 nm for CuFeO2, our simulation results provide an important clue for enhancing the visible-light absorption through light localization.

To verify the light-absorption enhancement due to the IO structure, diffusive reflectance and absorption spectra for the two aforementioned architectures were obtained using a UV–vis spectrometer, as shown in Figure 2b. In general, the reflectance is reduced by the absorption of the semicon-ductor; thus, the wavelength at which the reflectance decrease is correlated to the bandgap of the absorber layer. Therefore, the photocathodes exhibit the decreased diffusive reflec-tance near 800 nm in the direction of a higher photon energy (i.e., lower wavelength), which is related to the absorption from the bandgap of CuFeO2. The actual light absorption of the CuFeO2 photocathodes depending on the morphology was obtained by subtracting diffusive transmittance as well as diffu-sive reflectance from the unity. There was an evident absorption enhancement at 600–800 nm for the IO CuFeO2 compared with the planar CuFeO2, confirming that light is more intensively captured by the IO structure owing to the multiple scattering within the inverse opal structure, as expected in Figure 2a.

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Figure 2. a) Comparison of the simulated cross-sectional electric-field intensity, |E|, and the distribution of the electromagnetic waves at 400, 500, and 600 nm in the IO CuFeO2 and planar CuFeO2, respectively. The void diameter and wall thickness were set as 275 and 50 nm, respectively, in the simu-lation. The initial plane wave was illuminated from the top of the objects. The contour of the IO CuFeO2 wall is depicted as a white line in Figure 2a. b) Absorption and diffusive reflectance of the planar and IO CuFeO2 on FTO, respectively.



Therefore, according to our FDTD simulation and optical characterizations, the visible-light absorption can be improved by the IO structure alone, without any modification of the material. In addition, the bandgap of IO and planar CuFeO2 was determined by extrapolating the linear region of Tauc plots as shown in Figure S5 (Supporting Information). IO and planar CuFeO2 samples show the bandgap of 1.55 and 1.61 eV, respectively. The determined bandgaps of CuFeO2 with the two different types of structures are similar to the previously reported bandgap of CuFeO2.[9,11]

Next, the PEC water reduction activity of our ternary oxide photocathodes was tested. First, Mott–Schottky analysis indi-cated that the flat-band potential of the planar and IO CuFeO2 was 0.848 and 0.819 versus RHE (Figure S6, Supporting Information), respectively. These values agree with previous reports.[10,42] This result implies that the CuFeO2 photocath-odes, regardless of their structure, have a nearly identical thermodynamic ability to reduce water to H2. The photore-sponse properties of the CuFeO2 photocathodes were evaluated via linear sweep voltammetry in an argon-purged 1 m NaOH electrolyte, as shown in Figure S7 (Supporting Information). The photocurrent density of the planar CuFeO2 photocathode was 0.5 mA cm−2 at 0 V versus RHE, whereas the IO CuFeO2 photocathode exhibited PEC performance nearly twice as high, with a photocurrent density of 1.05 mA cm−2 at 0 V versus RHE. Notably, the IO-structured photocathode exhibited a significantly enhanced photoresponse property in the range of 0.2−0.7 V versus RHE. It should be noted that the thickness of our planar CuFeO2 was optimized as shown in Figure S8 (Sup-porting Information). Thinner CuFeO2 film based photocathode revealed much lower photocurrent density compared to the optimized device presumably due to insufficient light absorp-tion. More importantly, despite its higher light absorption, the decreased photocurrent was observed in thicker CuFeO2 film, implying that charge carrier transport in CuFeO2 is an impor-tant issue, which could be resolved by the IO structure. In addition to a better photocurrent, an evident shift of the onset potential was observed when the IO structure was adopted for the CuFeO2 photocathode. We defined the onset potential as the potential showing a cathodic photocurrent of 50 µA cm−2,

in accordance with the literature.[43] The onset potentials of the planar and IO CuFeO2 photocathodes were 0.12 and 0.65 V versus RHE, respectively.

Owing to the poor catalytic property of CuFeO2 toward hydrogen evolution, electrocatalysts are generally needed to achieve an efficient CuFeO2 photocathode for water splitting. CoFe LDH has received great attention recently as a low-cost catalyst that functions in a strongly basic solution.[20,44] Thus, an electrodeposition method was employed to uniformly deposit the CoFe LDH catalyst on the macroporous CuFeO2. Furthermore, LDH-based catalysts are often utilized in con-junction with highly conductive carbon materials to enhance the hydrogen evolution reaction (HER) activity.[44] The C60 layer was introduced to the CuFeO2 photocathodes via simple drop-casting, followed by the electrodeposition of CoFe LDH to construct an IO CuFeO2/C60/CoFe LDH photocathode. The SEM images shown in Figure S9 (Supporting Information) con-firm that the catalyst was uniformly deposited on the IO CuFeO2 without collapsing of the macroporous structure. The mor-phology of the IO CuFeO2 with the catalyst was further inves-tigated via scanning transmission electron microscopy (STEM) together with energy-dispersive X-ray spectroscopy (EDX), as shown in Figure 3a,b. The high-angle annular dark field (HAADF) image and EDX elemental mapping image clearly reveal the distribution of each layer (Figure 3b), illustrating the uniform CoFe LDH coating due to fast electrodeposition. In the HAADF image, the IO CuFeO2 appears brighter than the CoFe LDH nanosheets, as copper atoms have a higher atomic number than cobalt. The detailed elemental mapping images indicate that the copper has a very distinctive hexagonal honey-comb-like distribution in the IO CuFeO2, whereas the iron has a slightly obscured honeycomb structure. It is believed that the CoFe LDH catalyst contains iron; thus, the distribution of iron is significantly larger than that of copper. Likewise, elemental cobalt is well dispersed in the IO structure, confirming that the catalyst was uniformly deposited on the IO CuFeO2. Figure 3c,d shows high-resolution transmission electron microscopy (HRTEM) images of the IO CuFeO2/C60/CoFe LDH. Figure 3c clearly indicates the CoFe LDH nanosheets uniformly deposited on the IO CuFeO2 skeleton. The lattice spacing of 0.224 and

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Figure 3. a) STEM image and b) elemental maps. c) HRTEM image of IO CuFeO2/C60/CoFe LDH and d) high-magnification view of the yellow square area in (c). The white scale bars in (a–c) represent 200 nm.



0.284 nm agrees with the spacing of the (104) and (006) planes of CuFeO2,[11] whereas the lattice spacing of 0.251 nm is attrib-uted to the (012) planes of CoFe LDH.[45] Thus, the trans-mission electron microscopy (TEM) analysis suggests the successful formation of the CoFe LDH nanosheets on top of the IO CuFeO2 photocathodes. To compare the electrocatalyst-modified performance in argon-purged 1 m NaOH for different architectures, planar CuFeO2 photocathodes were also modified by the electrocatalyst, yielding planar CuFeO2/C60/CoFe LDH.

The C60/CoFe LDH electrocatalyst drastically improved the photocurrent generation—by a factor of 4.6—compared with the bare IO CuFeO2 photocathode without modification (Figure 4a). The planar CuFeO2 photocathodes also exhib-ited an enhanced photocurrent after modification. Specifi-cally, the IO CuFeO2/C60/CoFe LDH yielded a photocurrent of 4.86 mA cm−2 at 0 V versus RHE, which is more than twice the HER performance of the planar CuFeO2/C60/CoFe LDH photocathode. The photocurrent densities of the photocathodes

are listed in Table 1. Notably, our IO CuFeO2/C60/CoFe LDH photocathode exhibits a record-high photocurrent density (5.2 mA cm−2 at −0.1 V vs RHE) among CuFeO2-based photo-cathodes and outperforms most of the previously reported p-type ternary oxide-based photocathodes in the literature, as summarized in Table S1 (Supporting Information).[4–7,46] To investigate the charge transport/transfer behaviors for both IO and planar CuFeO2-based photocathodes, electrochemical impedance spectroscopy (EIS) measurements were carried out in argon-purged 1 m NaOH electrolyte under simulated solar illumination as shown in Figure S10 (Supporting Information). The Nyquist plot was fitted to a simple equivalent circuit model of one series resistance (Rs) and one resistor–capacitor circuit consisting of a resistance (R1) and a constant phase element (CPE1) in accordance with a diffusion-recombination model for a porous electrode[38] and the fitting parameter are listed in Table S2 (Supporting Information). According to the diffu-sion-recombination model, the resistance R1 obtained from the semicircular corresponds to the resistance for the charge trans-port and transfer. As both IO and planar based photocathodes have the same electrode/electrolyte interface properties, we can reasonably presume that the R1 could be predominantly influenced by charge transport properties. R1 was consider-ably reduced for IO CuFeO2-based photocathode, implying that the shorten diffusion length resulted from IO nanostructure facilitates charge transport to electrolyte/electrode interface. Therefore, the EIS analysis result implies that improvement in charge transport properties contributes to the enhanced photocurrent density. The observed enhancement in the charge transport properties is in good agreement with previous studies on nanostructured photoelectrodes.[25–27]

IPCE measurements were performed to investigate the wavelength-dependent efficiency of the planar and IO CuFeO2 after modification with C60/CoFe LDH at 0 V versus RHE in an argon-purged 1 m NaOH electrolyte, as shown in Figure 4b. The IPCE values monotonously decrease with the increasing wavelength for the planar CuFeO2/C60/CoFe LDH photocathode, falling to 4.8% at 600 nm from 25% at 400 nm, which is analogous to previous reports of CuFeO2 photocathodes.[9–13] Surprisingly, unlike the planar CuFeO2, the IO CuFeO2/C60/CoFe LDH photocathode shows no loss of quantum efficiency as the incident photon energy decreases from 400 to 700 nm, which has not been observed for ternary oxide-based photocathodes. These IPCE spectra indicate that the light harvesting near the bandgap edge is significantly improved by the IO structure. Specifically, the IO CuFeO2-based photocathode exhibited a high IPCE value of 17.5% at 600 nm, which is unprecedented among photocathodes,

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Figure 4. a) Linear sweep voltammograms of the IO and planar CuFeO2 photocathodes after modification by the electrocatalyst in an argon-purged 1 m NaOH solution (pH 13.5) under chopped illumination. The sweep rate was 8 mV s−1, and the scan was performed in the cathodic direction. b) IPCE spectra with respect to the wavelength for the planar and IO CuFeO2 photocathodes after modification of the C60/CoFe LDH biased at 0 VRHE in an argon-purged 1 m NaOH solution. Integration of the IPCE value with the solar AM 1.5 spectrum (gray line) yields a photocurrent value (dotted line) similar to the observed current under our simulated 1-sun illumination.

Table 1. Photocurrent densities of the CuFeO2 photocathodes in this study. The photocurrent density was measured under 1-sun illumination in a 1 m NaOH electrolyte at 0 V versus RHE.

Photocathode Photocurrent density [mA cm−2]

IO CuFeO2 1.05

IO CuFeO2/C60/CoFe LDH 4.86 (5.2 @ −0.1 V vs RHE)

Planar CuFeO2 0.51

Planar CuFeO2/C60/CoFe LDH 2.11



as shown in Table S1 (Supporting Information). Integrating the IPCE multiplied by the standard solar spectrum AM 1.5G provided an estimated photocurrent density of 4.4 and 1.99 mA cm−2 for the IO and planar CuFeO2 photocathodes after C60/CoFe LDH modification, respectively. These values agree well with the measured photocurrents under simulated solar illumination. Notably, the integrated photocurrent den-sity increases remarkably in the range of 600–700 nm for the IO CuFeO2-based photocathode. Therefore, we conclude that the remarkable performance of IO CuFeO2 compared with other ternary oxides predominantly originates from the drasti-cally improved IPCE over 600 nm.

This improved quantum efficiency can be explained by two factors: increased light absorption and enhanced charge transfer/extraction. Although the absorption spectra and FDTD simulations confirmed that enhanced light absorption is achiev-able through the multiple scattering of the periodic macroporous structure (Figure 2), the IPCE enhancement factor (the ratio of the observed value for the planar photocathode to that for the IO-based photocathode) at 600 nm is determined to be 3.5, whereas the absorption enhancement factor at 600 nm is only ≈1.2. This indicates that upon absorption of light at 600 nm, pho-togenerated carriers are eventually lost in the planar photo-cathode, whereas the photogenerated carriers in the IO-based photocathode are active for effectively driving the HER. This observation suggests that better light absorption alone cannot fully explain the improved quantum efficiency. In this regard, to understand the origin of the improved quantum efficiency near the band edge, the photogenerated carrier dynamics depending on the wavelength of the incident light are worth investigating.

Intensity-modulated photovoltage spectroscopy (IMVS) analysis is a useful technique for investigating the photo-generated carrier dynamics in fully working PEC devices. IMVS analysis determines the periodic modulations of the photovoltage in response to a small sinusoidal perturbation in the light intensity superimposed on direct-current illu-mination.[47,48] In the IMVS signal, a photovoltage plateau can be observed if there is a dynamic equilibrium between the generation and annihilation of photogenerated carriers within a certain range of the light modulation frequencies. As the light modulation frequencies decrease, the slow light perturbation allows the long-lived photogenerated carriers to transfer to the electrolyte side and induces the formation of inflection curves in the IMVS signal. By determining the frequency of the inflection point, the rate constant of charge transfer can be determined: a lower inflection frequency rep-resents slower charge transfer. According to previous studies on IMVS for PEC water splitting, the inflection point at a low frequency (≈1–10 Hz) is related to the rate constant of charge transfer.[49,50] Figure 5a,b shows the IMVS signals obtained from the IO and planar CuFeO2 by using 447- and 627 nm monochromatic light sources. Under 447 nm irradia-tion, inflection points were observed for both photocathodes, as indicated by the green arrows in Figure 5a. Furthermore, the phase Bode plots for both photocathodes are nearly iden-tical, which implies that the charge-transfer rates are similar. These results indicate that both photocathodes are capable of producing photogenerated electrons with sufficient energy to drive the HER reaction under relatively high-photon energy

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Figure 5. Photovoltage and phase measured via IMVS analysis for planar CuFeO2/C60/CoFe LDH and IO CuFeO2/C60/CoFe LDH photocathodes with monochromatic light at a) 447 nm and b) 627 nm. Green arrows represent the inflection point of voltage in IO structure due to fast charge transfer. c) Schematic diagram of the charge transfer for the different types of structures under different photon energy illumination.



light of 447 nm. On the other hand, with a 627 nm light source, the inflection point is not observed for the planar CuFeO2 photocathode, whereas the IO CuFeO2 photocathode exhibits a nearly identical inflection point to those of 447 nm, as indicated by the green arrow in Figure 5b. In addition, the phase Bode plots in Figure 5b show that the charge-transfer kinetics differ for the different architectures. These results indicate that the planar CuFeO2 photocathode has slower charge transfer than the IO CuFeO2 photocathode when lower-energy photons at 627 nm are shed. It is considered that the photogenerated electrons were not sufficiently transported to induce water reduction at the interfaces of the planar CuFeO2 photocathode, presumably owing to the insufficient energy and the long diffusion length; thus, the accumulated electrons generated a voltage, leading to a plateau of the photovoltage without an inflection point. In contrast, for the IO structure, the photogenerated electrons were readily transported to the electrolyte to induce the HER owing to the significantly shortened electron path, leading to fast charge transfer to drive HER. Therefore, the enhanced charge transfer observed in Figure 5b supports the improved IPCE for the IO struc-ture near 600–700 nm.The IMVS analysis with 627 nm monochromatic light indicates that the IO architecture facili-tates water reduction by enhancing the charge transfer, as depicted in Figure 5c, in addition to providing excellent near-bandgap photon harvesting. A photogenerated electron from relatively higher energy as 447 nm efficiently contributes to HER irrespective of structure. On the other hand, the IO structure generates more photogenerated carriers by multiple scattering than the planar structure in near-bandgap visible light as 627 nm, and the drive more electrons successfully to HER owing to the short diffusion length.

Stable operation of the photocathode is of fundamental interest for its potential application in PEC devices. Our IO CuFeO2 photocathode with C60/CoFe LDH modification exhib-ited stable operation over 1 h under argon-purged 1 m NaOH at 0.25 V versus RHE, as shown in Figure S11 (Supporting Infor-mation). The H2 produced from the water was also monitored under similar operation conditions by gas chromatography (GC) as shown in Figure S12 (Supporting Information). Figure S12 (Supporting Information) shows the time course curves for both H2 evolution from the photocathode with an active area of 0.3 cm2 and one-half of the electrons passing through the outer circuit (e−/2) as expressed by a solid line. By comparing detected hydrogen gas with the expected amount obtained by the photocurrent, the faradaic efficiency (i.e., the ratio of the rate of H2 evolution to that of e−/2) can be calculated. The faradaic efficiency was near unity (85%–100%), indicating that our IO CuFeO2-based photocathode efficiently produced H2 with the absence of the side reactions. The slightly lower fara-daic efficiency in the initial stage is presumably caused by the adhesion of the generated H2 on the surface of photocathode as generally observed in the literature.[51]

3. Conclusion

In summary, we proposed a novel strategy to boost visible-light harvesting via implementation of a periodic macroporous IO

structure on a CuFeO2 photocathode and achieved an IPCE of 17.5% at 600 nm. Optical analysis and simulation confirmed that our IO structure increases scattering within the absorber near the band edge, resulting in enhancement of the light absorption for IO CuFeO2 compared with planar CuFeO2. Furthermore, IMVS analysis indicated that the IO structure is advantageous for utilizing the visible-light-induced electrons to drive the HER by shortening the diffusion length. Owing to the synergetic effect of the enhanced light absorption and charge transfer, a remarkable photocurrent of 5.2 mA cm−2 at −0.1 V versus RHE was achieved, which is an unprecedented performance among ternary oxide-based photocathodes. Thus, our approach shows the universal possibility of being able to overcome the limitation of visible-light harvesting through the design of appropriate periodic structures on earth-abundant p-type ternary oxide.

4. Experimental SectionFabrication of Planar CuFeO2 Photocathodes: Planar CuFeO2

photocathodes were prepared using the sol–gel process.[10] Stoichiometric amounts of Cu(NO3)2·3H2O (99%, Sigma–Aldrich, St Louis, MO) and Fe(NO3)3·9H2O (98%, Sigma–Aldrich) were dissolved to 0.2 m in ethanol (Duksan Pure Chemicals Co., South Korea). After stirring for 2 h, ethylene glycol (Sigma–Aldrich) was added to the mixture, which was further stirred overnight. Copper/iron sol–gel ink was spin-coated (Spin-1200D, Midas, South Korea) on FTO-coated aluminum borosilicate glass (8 Ω sq−1, Wooyang GMS, South Korea) substrates at 3000 rpm for 1 min, and the spin-coated FTO was directly preannealed on a hot plate at 500 °C in air for 1 h. This cycle was repeated six times. Finally, the deposited electrode was kept in a microwave (UMF-04, Unicera, South Korea) for 2 h in an argon flow at 600 °C.

Preparation of Opal Templates: Opal templates were fabricated via the self-evaporation-induced self-assembly method on a preannealed copper/iron oxide layer. Similar to the synthesis of the planar CuFeO2, copper/iron sol–gel ink was spin-coated on an FTO substrate, the film was preannealed on a hot plate at 500 °C in air for 1 h, and the cycle was repeated four times. Monodisperse carboxylate-modified PS (2.5 wt%, Alfa Aesar, Heysham, Lancashire, UK) with a diameter of 350 nm was diluted to 0.1 wt% in deionized (DI) water. The preannealed copper/iron oxide film was lean vertically to a 10 mL vial containing the suspension of monodisperse PS spheres, and the vials were kept in a convection oven at 55 °C for 4 d. As the water evaporated and the meniscus kept down the substrate, capillary forces induced the ordering of the spheres on the surface of the copper/iron oxide substrate.

Fabrication of IO CuFeO2 Photocathodes: IO CuFeO2 photocathodes were synthesized via the sol–gel infiltration method. First, 0.2 m Cu(NO3)2·3H2O and 0.2 m Fe(NO3)3·9H2O were dissolved in DI water. The opal templates were immersed vertically in the aforementioned copper/iron solution. After 3 min, the templates were removed slowly and dried on the template surface for ≈5 h at room temperature. The resultant film was preannealed at 500 °C in a tube furnace and held for 2 h. The preannealed IO copper/iron oxide was transferred to a microwave (UMF-04, Unicera) and held for 2 h in an argon flow at 600 °C to convert the delafossite CuFeO2.

Deposition of C60/CoFe LDH Cocatalyst: The CuFeO2 photocathodes were immersed vertically in 1 wt% C60 (99.5%, Sigma–Aldrich) in cyclohexane. After 3 min, the photocathodes were removed slowly and dried on a hot plate at 120 °C. The CoFe LDH electrocatalyst was deposited on the CuFeO2 photocathodes via a fast electrodeposition method.[20] The CuFeO2 photocathode, a platinum coil, and a silver chloride electrode (E = +0.197 V vs RHE) were used as the working,

Adv. Funct. Mater. 2019, 29, 1900194



counter, and reference electrodes, respectively. The electrolyte for the electrosynthesis of CoFe LDH was obtained by dissolving Co(NO3)2·6H2O (98%, Sigma–Aldrich) and Fe(SO4)2·7H2O (99%, Sigma–Aldrich) in DI water with a continuous argon flow to prevent the oxidation of Fe2+. The potentiostatic electrodeposition was conducted at −0.973 V versus the reference electrode for 20 s. The resulting CoFe LDH-coated photocathode was rinsed thoroughly with DI water.

Characterization of Photoelectrodes: The phase evolution of the planar and IO CuFeO2 was determined using XRD (MiniFlex 600, Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 0.15406 nm) and XPS (K-alpha, equipped with monochromated Al Kα, Thermo). The microstructures of CuFeO2 photocathodes with and without the electrocatalyst were analyzed using field-emission SEM (JSM-7800, JEOL, Japan). TEM and elemental mapping were performed using a JEM-F200 (JEOL, Japan) equipped with an energy-dispersive X-ray spectrometer at an acceleration voltage of 200 kV. The diffusive transmittance (dT), diffusive reflectance (dR), and specular transmittance were recorded at room temperature using a UV–vis spectrophotometer (V-670, JASCO, Easton, MD, USA) equipped with an integrating sphere. The absorption was calculated by the formula of [absorption (%) = 100 − dR − dT], and the dR and dT spectra can be found in Figure 2b and Figure S13 (Supporting Information), respectively. The photoelectrodes were fabricated by securing a copper wire on the exposed electrically conductive parts of the FTO using silver paste, and the unnecessary parts of the electrode and the wiring parts were then covered with epoxy resin.

PEC measurements were conducted in a three-electrode configuration using a potentiostat (SI 1287, Solartron, Leicester, UK). A coiled platinum counter electrode and a silver chloride electrode (E = +0.197 V vs RHE) were used as the counter and reference electrodes, respectively. The PEC properties were measured under simulated sunlight (AM 1.5G, Newport Corp., Rochester, NY, USA) in a 1 m NaOH solution (pH 13.5). Linear sweep voltammetry was conducted using an Ag/AgCl reference electrode with an 8 mV s−1 sweep rate. EIS measurements were performed in the frequency range of 300 kHz to 0.1 Hz under simulated sunlight at 0 V versus RHE using a potentiostat combined with a frequency analyzer (SI 1260, Solartron, UK). The IPCE spectra of the CuFeO2-based photocathodes were measured under irradiation of monochromatic light at 0 V versus RHE (CIMPS-QE/IPCE, Zahner, Germany). IMVS measurements were performed using an electrochemical workstation (Zennium, Zahner) and a potentiostat (PP211, Zahner) with a 627 nm monochromatic light source; a modulation intensity of 10% was used. The frequency of modulation was swept from 1 kHz to 0.1 Hz. For H2 detection, all the cell compartments were thoroughly sealed in a quartz tube with a rubber septum to prevent any gas leakage. During irradiation, the headspace gas (100 mL) of the reactor was intermittently sampled using a gastight syringe and analyzed for H2 using a gas chromatogram (6500GC system, YL Instrument, Anyang, Korea) equipped with a pulsed discharge detector and molecular sieve column.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2012R1A3A2026417).

Conflict of InterestThe authors declare no conflict of interest.

KeywordsCu-delafossite CuFeO2, inverse opal, photocathode, ternary p-type oxide

Received: January 8, 2019Published online: February 4, 2019

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boosting visible light harvesting in p‐type ternary oxides

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