One-Step Hydrothermal Deposition of Ni:FeOOH onto Photoanodes

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One-Step Hydrothermal Deposition of Ni:FeOOH onto Photoanodes for Enhanced Water Oxidation Lili Cai,† Jiheng Zhao,† Hong Li,† Joonsuk Park,‡ In Sun Cho,†,§ Hyun Soo Han,† and Xiaolin Zheng*,† †

Department of Mechanical Engineering and ‡Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States § Department of Materials Science & Engineering and Energy Systems Research, Ajou University, Suwon 16499, South Korea S Supporting Information *

ABSTRACT: The realization of efficient photoelectrochemical (PEC) water splitting requires effective integration of earth-abundant active oxygen evolution catalysts (OECs) with diverse photoanodes. Although many good OECs have been investigated on conductive substrates under dark conditions, further studies are needed to evaluate their performance when integrated with photoanodes under illumination. Such studies will be facilitated by developing effective coating methods of OECs onto diverse photoanodes. Here, we report a one-step hydrothermal process that conformally coats various photoanodes with ultrathin Ni-doped FeOOH (Ni:FeOOH) OECs. The coated Ni:FeOOH, due to its unique open tunnel structure, tunable Ni doping concentration, and high coating/interface quality, lowers the onset potential of all of the photoanodes investigated, including WO3/BiVO4, WO3, α-Fe2O3, TiO2 nanowires, BiVO4 films, and Si wafers. We believe that this simple and yet effective hydrothermal method is a useful addition to the existing deposition techniques for coupling OECs with photoanodes and will greatly facilitate the scale-up of efficient PEC devices.

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BiVO4, Fe2O3, and WO3, and the coated photoanodes exhibited excellent PEC performance and stability.7,10,28−30 Despite the exciting progress in coupling active (Ni,Fe)OOH with photoanodes, the deposition techniques of (Ni,Fe)OOH onto photoanodes remain very limited. Almost all of those studies used either electrodeposition or photodeposition methods for depositing (Ni,Fe)OOH on photoanodes. Both electrodeposition and photodeposition methods are sensitive to the detailed structures of the photoanodes because both the local electrical field and photogenerated carrier concentrations are affected by the local morphology and light absorption of the photoanodes. As such, these two deposition methods may not be able to coat highly anisotropic and heterogeneous structures with uniform (Ni,Fe)OOH.31−36 Finally, it is difficult for both deposition methods to deposit (Ni,Fe)OOH with Ni and Fe in a single step as Ni and Fe ions have quite different redox potentials.10,19 Hence, more deposition methods of (Ni,Fe)OOH need to be developed to facilitate their coupling onto diverse photoanodes.

hotoelectrochemical (PEC) water splitting, that is, a process that uses sunlight to split water to produce hydrogen, holds promise for harvest and storage of renewable energy.1−3 One of the critical challenges that PEC water splitting faces is to accelerate the slow oxygen evolution reaction (OER) occurring at the photoanode and electrolyte interface.4−6 In response, many earth-abundant oxygen evolution catalysts (OECs) have been developed, ranging from cobalt phosphate (CoPi), to Fe-, Ni-, or Co-based oxides, to Ni- and Fe-based (oxy)hydroxides ((Ni,Fe)OOH).7−13 In particular, (Ni,Fe)OOH, proposed since the early 1980s,14 was recently benchmarked as among the most active earth-abundant OECs.15−18 In these studies, the electrocatalytic activities of (Ni,Fe)OOH have been mainly characterized on conductive substrates (e.g., fluorine-doped SnO2-coated glass, FTO), not on photoanodes.11−22 However, the observed high OER activities of (Ni,Fe)OOH on FTO may not be held when coupling (Ni,Fe)OOH onto photoanodes because the OER activity also depends on the interface and coating quality.10,23−27 Hence, for PEC devices, it is necessary to evaluate those OECs together with photoanodes. Recently, several pioneer studies successfully coupled (Ni,Fe)OOH onto several PEC photoanodes, including © XXXX American Chemical Society

Received: July 28, 2016 Accepted: August 24, 2016

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DOI: 10.1021/acsenergylett.6b00303 ACS Energy Lett. 2016, 1, 624−632

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Figure 1. (a) Schematic of the one-step hydrothermal process for depositing Ni:FeOOH OECs on anisotropic nanostructures, such as WO3/ BiVO4 core/shell NWs. An ultrathin Ni:FeOOH layer is conformally coated on the NWs with strong adhesion. The as-deposited Ni:FeOOH is amorphous in long-range but has a tunnel structure of β-FeOOH phase in short-range. SEM images show that the morphology of the WO3/ BiVO4 core/shell NWs (b) before and (c) after the hydrothermal deposition of Ni:FeOOH is indistinguishable.

photoelectron spectroscopy (XPS)). Then, the WO3/BiVO4 core/shell NW photoanodes were immersed in the precursor solution and kept there for 30−45 min at 100 °C inside of an oven. During this time, urea progressively decomposed and released OH−, which led to the gradual coprecipitation of Fe3+ and Ni2+ as (oxy)hydroxides onto the photoanode surface (Figure 1a) through heterogeneous nucleation and growth processes. After the deposition, the coated photoanodes were rinsed with DI water and air-dried. The same deposition process was also applied to other photoanodes investigated in present study, including BiVO4 thin films, WO3 NWs, α-Fe2O3 NWs, TiO2 NWs, and planar Si wafers (n-type, 1−4 Ohm·cm), and these photoanodes were prepared using previously reported methods.38,40−43 A key benefit of the hydrothermal deposition method is that it provides conformal deposition of Ni:FeOOH with tunable Ni concentration onto photoanodes in a single step (Figure 2). The scanning electron microscope (SEM) (Figure 1c) and low-magnification transmission electron microscope (TEM) (Figure 2a) images show the morphology of a Ni:FeOOHcoated WO3/BiVO4 core/shell NW array and one of the NWs, respectively. At these low magnifications, the morphology of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs is indistinguishable from that of the bare WO3/BiVO4 core/ shell NWs (Figure 1b), which suggests that the deposition of Ni:FeOOH by the hydrothermal deposition method has little impact on the morphology of the photoanodes. Closer examination of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs under higher magnification shows that the entire surface of the WO3/BiVO4 core/shell NW is covered by a uniform layer of tiny nanoparticles, for which panels b−d of Figure 2 correspond to the top, middle, and bottom parts of the NW in Figure 2a. Further magnified TEM images (Figure 2e−g) show that the Ni:FeOOH nanoparticles conformally coat the surface of the BiVO4 particle shell and the exposed WO3 NW core and even the kinked junction between them. Also, the thickness of the coated Ni:FeOOH nanoparticle layer is only about 5−10 nm throughout different parts of the NW. This conformal coating nature is attributed to the uniform nucleation and gradual growth of the Ni:FeOOH nanoparticles on the WO3/ BiVO4 core/shell NW during the hydrothermal process. Such

In this work, we report a simple one-step hydrothermal method that conformally deposits ultrathin Ni-doped FeOOH (Ni:FeOOH) nanoparticles (5 to 10 nm) as OECs onto diverse photoanodes, including WO3/BiVO4 core/shell nanowires (NWs), BiVO4 thin films, WO3 NWs, α-Fe2O3 NWs, TiO2 NWs, and planar Si wafers. Our hydrothermal deposition method dopes FeOOH with Ni during the growth to further enhance its OER activity. The hydrothermally deposited Ni:FeOOH has an amorphous matrix in the long-range with small embedded domains resembling the β-FeOOH phase structure, which is different from the previously reported γFeOOH or α-FeOOH phases formed by photodeposition or electrodeposition.7,9 The β-FeOOH phase was only recently applied as an OEC on α-Fe2O3 for PEC water splitting, but the addition of β-FeOOH did not lower the photocurrent onset potential of α-Fe2O3 photoanodes.37 In contrast, we found that β-FeOOH actually lowers the photocurrent onset potential and increases the charge-transfer efficiency of diverse photoanodes, and such an effect is even more prominent with Ni doping. In particular, the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs achieve a charge-transfer efficiency of 91% at 1.23 V vs RHE, which is comparable to the highest charge-transfer efficiency of 95% at 1.23 V vs RHE reported for photoanodes deposited with FeOOH-based OECs using photodeposition.10 The hydrothermal deposition process of Ni:FeOOH on the WO3/BiVO4 core/shell NW photoanode is shown in Figure 1a. The hydrothermal deposition recipe for Ni:FeOOH is adapted from a previous recipe for the hydrothermal growth of FeOOH with modification to enable Ni doping.38 Specifically, two separate solutions of 30 mM FeCl3 and 30 mM NiCl2 in deionized (DI) water were prepared. In both solutions, 45 mM urea was added to serve as the progressive OH− releasing agent. Next, appropriate amounts of the FeCl3 and NiCl2 solutions were mixed to prepare precursor solutions in 25 mL with [Ni2+]/[Fe3+] ratios ranging from 49 to 1249. It is critical to use a large [Ni2+]/[Fe3+] ratio in the precursor solution to incorporate Ni2+ into FeOOH because the solubility product constant of Ni(OH)2 (Ksp = 5.48 × 10−16) is much larger than that of FeOOH (Ksp = 2.79 × 10−39).39 Consequently, our precursor solutions lead to the growth of Ni:FeOOH with a [Ni2+]/[Fe3+] ratio ranging from 4 to 18% (estimated by X-ray 625

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Figure 2. TEM images of a single Ni:FeOOH-coated WO3/BiVO4 core/shell NW: (a) low magnification showing that the NW is composed of BiVO4 particles on top of a WO3 NW; (b−g) higher magnification showing that the surface and interface of top, middle, and bottom parts of the NW (a) are conformally covered by a layer of Ni:FeOOH (5−10 nm).

3c shows an asymmetric peak with a shoulder on the left. This peak can be fitted into two main constituent peaks centered at 530.1 and 531.2 eV, which correspond to O2− and OH−, respectively.9,19,46 The area ratio of O2− and OH− peaks of Ni:FeOOH on WO3/BiVO4 here is estimated to be 1.35 from the O 1s XPS spectrum. The higher O2− content than OH− is probably due to the contribution of O2− signal from the underlying WO3/BiVO4. To get more accurate O2− and OH− peak ratios for the deposited Ni:FeOOH, we further measured the O 1s XPS spectrum of Ni:FeOOH deposited on Ti foil (Figure S2), for which the area ratio of O2− and OH− peaks was estimated to be 0.94. This result is consistent with the theoretical 1 to 1 ratio between O2− and OH− in FeOOH. Overall, the XPS spectra analysis confirms that the as-deposited catalysts are indeed Ni:FeOOH. The crystallinity and phase of the Ni:FeOOH catalysts were further characterized using X-ray diffraction (XRD) and Raman spectroscopy. For the XRD measurement, a thick Ni:FeOOH film (∼500 nm) was deposited on top of WO3/BiVO4 core/ shell NWs to ensure that the amount of Ni:FeOOH was much higher than the XRD detection limit. As shown in Figure 3d, the XRD pattern of the Ni:FeOOH-coated WO3/BiVO4 NWs is identical to the bare WO3/BiVO4 NWs with peaks corresponding to WO3, BiVO4, and FTO only. No diffraction peaks corresponding to crystalline (α-, β-, or γ-) FeOOH phases were detected. The XRD result suggests that the asdeposited Ni:FeOOH catalysts are predominately amorphous in the long-range. The amorphous nature of the as-deposited Ni:FeOOH can also be seen in the HR-TEM images (Figure 2e−g) where Ni:FeOOH nanoparticles do not show obvious lattice fringes. We further investigated if the as-deposited Ni:FeOOH has any short-range order using Raman spectroscopy. For the Raman characterization, a Ni:FeOOH film about 100 nm thick was deposited on Ti foil to avoid strong

conformal coverage of Ni:FeOOH over the entire photoanode surface will improve both the water oxidation kinetics and stability of the photoanodes for PEC.44,45 The TEM images (Figure 2e−g) also show that the Ni:FeOOH layer has close contact with the core/shell NW that is beneficial for the good interface quality. Finally, such a thin Ni:FeOOH catalyst layer (5−10 nm) has little light blocking effect on the photoanodes. All of the above properties of hydrothermally deposited Ni:FeOOH allow it to be directly coupled to optimized photoanodes to enhance their PEC performance.x The chemical composition and oxidation state of the hydrothermally deposited Ni:FeOOH were examined by XPS. Figure 3a−c shows the high-resolution XPS spectra of the Fe 2p, Ni 2p, and O 1s regions for the as-deposited catalysts before the PEC test (the XPS spectra after the PEC test are shown in Figure S1, and there is no obvious change of chemical states for Ni:FeOOH before and after the PEC test). The Fe 2p XPS spectrum (Figure 3a) shows two major peaks, ∼ 711.2 eV for Fe 2p3/2 and ∼725.0 eV for Fe 2p1/2, and the binding energies of both peaks match well with the characteristics of Fe3+ in FeOOH.46−48 The Fe 2p XPS spectrum also exhibits two small satellite peaks located at higher binding energies, ∼ 719.0 eV for Fe 2p3/2, sat and ∼733.0 eV for Fe 2p1/2, sat, which can be ascribed to the charge-transfer or shakeup processes associated with Fe3+.49 These features of the Fe 2p XPS peaks indicate that the oxidation state of Fe in the as-deposited catalysts is 3+ and the Fe3+ ions exist in the form of FeOOH. The Ni 2p XPS spectrum (Figure 3b) has a small Ni 2p3/2 peak at ∼855.9 eV and an associated satellite peak at ∼861.6 eV. These two Ni 2p3/2 peaks suggest that the oxidation state of Ni is 2+ and Ni2+ ions are successfully incorporated in the as-deposited FeOOH catalysts.50−52 The atomic ratio of Ni/Fe in Ni:FeOOH was estimated by the area ratio of the Ni 2p3/2 and Fe 2p3/2 XPS peaks, and it is about 7.5%. The O 1s XPS spectrum in Figure 626

DOI: 10.1021/acsenergylett.6b00303 ACS Energy Lett. 2016, 1, 624−632

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Figure 3. XPS spectra of the (a) Fe 2p, (b) Ni 2p, and (c) O 1s regions for the hydrothermally deposited Ni:FeOOH. (d) XRD spectra of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs, the bare WO3/BiVO4 core/shell NWs, and the standard β-FeOOH (ICDD PDF # 04-0090709), α-FeOOH (ICDD PDF # 00-029-0713), and γ-FeOOH (ICDD PDF # 04-010-4300) references. (e) Raman spectra of the Ni:FeOOH on Ti foil and the bare Ti foil as reference.

layers (see Table S1, which summarizes the photocurrent densities and charge-transfer efficiencies at 1.23 V vs RHE, and the onset potential shifts for BiVO4-based photoanodes with Ni- and/or Fe-based OECs reported in the past 5 years). To evaluate the OER activity of Ni:FeOOH, Figure 4a also plots the J−V curves of the bare WO3/BiVO4 core/shell NWs measured in the same electrolyte with and without the addition of 0.5 M H2O2. Here, H2O2 serves as a hole scavenger with facile oxidation kinetics; therefore, the photocurrent measured with H2O2 represents the performance limit that a photoanode can achieve when the surface charge-transfer efficiency is 100%. Compared to the bare WO3/BiVO4 core/shell NWs, the Ni:FeOOH-coated core/shell NWs show about a 200 mV cathodic shift of photocurrent onset potential and steeper rising of photocurrent. Importantly, the photocurrent density of the Ni:FeOOH-coated core/shell NWs approaches the photocurrent limit measured with H2O2, and this indicates that the hydrothermally deposited Ni:FeOOH is indeed an effective OEC. To further quantify the effect of Ni:FeOOH on the interfacial charge-transfer process between the photoanode and the electrolyte, the charge-transfer efficiencies (ηtransfer) of the bare and Ni:FeOOH-coated WO3/BiVO4 core/shell NWs were calculated and compared in Figure 4b. In the low applied potential region (below 1.2 V vs RHE), the addition of Ni:FeOOH significantly increased the charge-transfer efficiency. For example, the addition of Ni:FeOOH increased the ηtransfer from 6 to 77% at the potential of 0.8 V vs RHE. In the high applied potential region (above 1.2 V vs RHE), the ηtransfer of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs saturated and reached as high as 91% at 1.23 V vs RHE. Both our ηtransfer and photocurrent density at 1.23 V vs RHE (4.5 mA/cm2) are comparable to the highest ηtransfer of 95% and photocurrent density of 4.5 mA/cm2 at 1.23 V vs RHE reported

background peaks from WO3 and BiVO4. The Raman spectrum of the freshly deposited Ni:FeOOH film on the Ti foil shows several broad peaks at around 310, 385, 417, 545, 673, and 720 cm−1 (Figure 3e, triangle symbols; the Raman spectra of Ni:FeOOH before and after the PEC test are compared in Figure S1d, showing no obvious difference in the peak positions). These peaks all match with the peak positions of β-FeOOH (note that the main Raman peaks for the α-FeOOH phase include 205, 247, 300, 386, 418, 481, 549, and 680 cm−1, while the main Raman peaks for the γ-FeOOH phase include 219, 252, 311, 349, 379, 528, and 648 cm−1, both of which have strong peaks below 300 cm−1 but do not have peaks at around 720 cm−1). Both α-FeOOH and γ-FeOOH have strong peaks below 300 cm−1 but no peaks around 720 cm−1, which is different with the peaks of our β-FeOOH.53 The broadening nature of these peaks indicates some long-range disorder of βFeOOH,9 which is consistent with the amorphous nature from the XRD and TEM results. Therefore, the combined XRD, TEM, and Raman characterizations suggest that the structure of the as-deposited Ni:FeOOH has an amorphous matrix in the long-range with some small embedded domains resembling the β-FeOOH phase structure. The effect of the hydrothermally deposited Ni:FeOOH catalysts (7.5% Ni doping) on the PEC performance of the WO3/BiVO4 core/shell NW photoanode was evaluated using linear sweep voltammetry under front-side illumination (AM 1.5 G, 100 mW/cm2) in a 0.5 M potassium phosphate buffer solution (pH = 7). The measured current density versus potential (J−V) curve of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs is shown in Figure 4a. The photocurrent density of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs reaches about 4.5 mA/cm2 at 1.23 V vs RHE, which is comparable to the state-of-art BiVO4-based photoanodes coated with the photodeposited FeOOH/NiOOH dual catalyst 627

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Figure 4. (a) Photocurrent density versus potential curves of the WO3/BiVO4 core/shell NWs with/without Ni:FeOOH OECs in a 0.5 M potassium phosphate electrolyte (pH = 7), and the uncoated NWs in the same electrolyte with H2O2 added. Solid lines: simulated AM 1.5 G illumination; dotted lines: dark. (b) Charge-transfer efficiency and (c) light absorption efficiency of the WO3/BiVO4 core/shell NWs with and without Ni:FeOOH OECs. (d) Stability and faradaic efficiency of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs in a 0.5 M potassium phosphate electrolyte buffered to pH 7 under simulated AM 1.5 G illumination (at 0.8 V vs RHE).

show that the WO 3 /BiVO 4 core/shell NWs are still conformally covered with Ni:FeOOH (∼5 nm), similar to those in Figure 2. This good stability again supports that the hydrothermal route deposits conformal and strongly bonded Ni:FeOOH OECs onto the surface of WO3/BiVO4 core/shell NWs. In addition, the faradaic efficiency of Ni:FeOOH-coated WO3/BiVO4 core/shell NWs for water oxidation to O2 (Figure 4d) was measured to be 96% after it reached steady state at 0.8 V vs RHE (note that the initial lower values of the faradaic efficiency are due to the fact that part of the evolved oxygen initially dissolves in the electrolyte and fills in the gas space of the reaction cell, instead of being detected by the gas chromatography; once the oxygen saturates in the electrolyte as well as in the gas phase, the faradaic efficiency reaches its steady-state value), which confirms that the as-deposited Ni:FeOOH indeed accelerates the kinetics for O2 evolution reaction. Previous studies showed that the mixed Ni−Fe oxyhydroxides have better OER activities than FeOOH when coated on conductive substrates under dark conditions in the absence of photoanodes and illumination.11,15 To test if the same observation holds when (Ni,Fe)OOH is coupled onto photoanodes, we varied the Ni doping concentration in Ni:FeOOH during the hydrothermal synthesis and compared their catalytic effects on planar BiVO4 thin films (∼50 nm, Figure 5a). Clearly, the amount of Ni doping affects the PEC performance of the Ni:FeOOH-coated BiVO4 film. For better comparison, their charge-transfer efficiencies were calculated and plotted as a function of the Ni doping concentration in Ni:FeOOH (estimated by XPS data) in Figure 5b. The chargetransfer efficiency first increases and then decreases as the Ni doping concentration increases, with an optimal achieved at

for the nanoporous BiVO4 photoanode coated with the FeOOH/NiOOH dual catalyst layers by the photodeposition method,10 and both values are better than most of the BiVO4based photoanodes coated with Ni- and/or Fe-based OECs by different deposition methods reported in the recent 5 years (see the summary and comparison in Table S1). Such high ηtransfer is ascribed to the conformal coverage of Ni:FeOOH on the WO3/ BiVO4 NW surface and their excellent interface quality. In addition to the good charge-transfer activity of Ni:FeOOH, its light absorption property is also an important factor to consider when coupling with photoanodes. The ideal OECs should absorb light as little as possible to avoid blocking light absorption by the photoanodes. Figure 4c shows that the wavelength-dependent light absorption of the Ni:FeOOHcoated WO3/BiVO4 core/shell NWs is almost the same as that of the bare NWs, and it is only slightly higher (∼3.5%) in the wavelength region of 500−570 nm. The nearly identical light absorption after the deposition of Ni:FeOOH is attributed to its very thin thickness (5−10 nm), as shown in Figure 2. As a result, front illumination, in contrast to back illumination,7,10 can be used for the Ni:FeOOH-coated WO3/BiVO4 core/shell NW photoanode, which is desirable for integration with photocathodes into a monolithic tandem PEC system. The stability of the Ni:FeOOH-coated WO3/BiVO4 core/ shell NWs was evaluated under AM 1.5G illumination in a 0.5 M potassium phosphate buffer solution at pH = 7 using a threeelectrode configuration. The photocurrent density over time was recorded while the potential of the photoanode was held at 0.8 V vs RHE. As shown in Figure 4d, the photocurrent of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs remains at around 2.7 mA/cm2 over the test period of 3 h with no sign of degradation. After the stability test, TEM images (Figure S3) 628

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Figure 5. (a) Photocurrent density versus potential curves of BiVO4 planar films (∼50 nm) with and without Ni:FeOOH (Ni/Fe ratios vary between 0.0 and 18.3% estimated from XPS data) in a 0.5 M potassium phosphate electrolyte buffered to pH 7 under AM 1.5 G illumination. (b) Charge-transfer efficiency of the Ni:FeOOH-coated BiVO4 film at 0.5 V vs RHE versus the Ni/Fe ratio in the as-deposited Ni:FeOOH. The overpotential of Ni:FeOOH on FTO in the dark versus the Ni/Fe ratio extracted from ref 19 is plotted together here for comparison.

Figure 6. Photocurrent density versus potential curves of (a) TiO2 NWs, WO3 NWs, and Fe2O3 NWs and (b) a planar n-type Si wafer with and without the Ni:FeOOH. Solid lines: simulated AM 1.5 G illumination; dotted lines: dark.

resembles that of the β-FeOOH phase, which is distinguished from the previously reported α- and γ-FeOOH OECs prepared by electrodeposition.7,9 β-FeOOH has an open square tunnel type of structure (0.5 nm × 0.5 nm in size, Figure 1a)57 that we think could facilitate the transport of electrolyte ions, water molecules, and evolved O2 gas through the catalyst layer, accelerating the water oxidation reaction. Second, according to previous studies, the ion-permeable catalysts such as NiOOH form adaptive Schottky junctions with light-absorbing semiconductors, which increases the photovoltage and fill factor.26 The same effect can be expected for Ni:FeOOH due to its tunnel structure and its associated ion permeability. Hence, we conclude that both the β-FeOOH phase open tunnel structure and the Ni doping are responsible for the observed high OER activity of the hydrothermally deposited Ni:FeOOH on diverse photoanodes. In summary, we have developed a one-step hydrothermal method for depositing Ni:FeOOH OECs onto diverse photoanodes to enhance their PEC water splitting performance. The hydrothermal deposition method conformally coats photoanodes with a 5−10 nm Ni:FeOOH thin layer with strong adhesion, leading to improved charge-transfer efficiency, little parasitic light absorption, and good stability under illumination. The hydrothermally deposited Ni:FeOOH has tunable Ni doping concentration and is amorphous in the longrange but resembles the open tunnel structured β-FeOOH phase in the short-range, and both of these chemical and structural factors contribute to its high OER activity. In particular, the Ni:FeOOH-coated WO3/BiVO4 core/shell NW

Ni/Fe = 7.5%. This trend is different from that of the previously reported Ni:FeOOH on a conducting substrate in the dark, which shows a monotonic overpotential decrease with increasing Ni concentration in Ni:FeOOH.19 This comparison demonstrates that the performance of OECs is affected by the presence of photoanodes. In other words, the OER activity determined for OECs on conductive substrates cannot be directly applied to an integrated photoanode/OEC system. For PEC water splitting, the design and optimization of the OEC need to be investigated together with photoanodes. We further investigated if the hydrothermally deposited Ni:FeOOH OECs have similar enhancement effects on other photoanodes, including WO3 NWs, α-Fe2O3 NWs, TiO2 NWs, as well as planar n-type Si. As shown in Figure 6a, with the addition of Ni:FeOOH, the photocurrents of TiO2, WO3, and α-Fe2O3 NWs all exhibit great cathodic shift in the range of 50−150 mV. The planar n-type Si photoanode (Figure 6b) without any OECs has negligible photoresponse due to its very poor surface water oxidation kinetics. After the deposition of Ni:FeOOH, the planar n-type Si photoanode shows an onset potential at about 1.0 V vs RHE, which is comparable to those of other OEC (NiOx or IrOx) coated n-Si photoanodes (1.0− 1.1 V vs RHE).54−56 These results demonstrate that the hydrothermal deposition can be used to deposit Ni:FeOOH OECs onto diverse photoanodes to improve their PEC performance. The good PEC performance of the hydrothermally grown Ni:FeOOH when coupled to diverse photoanodes has its structural reasons. First, the local structure of Ni:FeOOH 629

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ACS Energy Letters photoanode achieves a photocurrent density of 4.5 mA/cm2 and a charge-transfer efficiency of 91% at 1.23 V vs RHE under AM 1.5G, which is comparable to the state-of-art nanoporous BiVO4 photoanode with the FeOOH/NiOOH dual catalyst layers. We believe that the one-step hydrothermal deposition method for Ni:FeOOH is a valuable addition to the library of deposition techniques for coupling OECs with photoanodes, facilitating the optimization and scale-up of efficient PEC water splitting systems.

therefore, the ratio of photocurrent densities measured in H2O and H2O2 gives arise to the charge-transfer efficiency.7,59,60 The faradaic efficiency experiment was conducted in a sealed cell containing a 0.5 M potassium phosphate buffer solution (pH = 7) through which He carrier gas was flowed at a flow rate of 2.7 sccm. After purging the cell to remove oxygen, the potential of the photoanode was held at 0.8 V vs RHE under simulated AM 1.5 G illumination. Both the photocurrent and the oxygen gas concentration in the He carrier gas were monitored. The concentration of oxygen in the He carrier gas, as measured by gas chromatography (SRI Instruments), first increases and then gradually reaches a steady-state value once the oxygen content in the electrolyte and in the gas space of the cell stabilizes.

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EXPERIMENTAL METHODS Hydrothermal Deposition of Ni:FeOOH. Two separate solutions of 30 mM FeCl3 and 30 mM NiCl2 in DI water were prepared. In both solutions, 45 mM urea was added to serve as the progressive OH− releasing agent. Next, appropriate amounts of the FeCl3 and NiCl2 solutions were mixed to prepare precursor solutions in 25 mL with [Ni2+]/[Fe3+] ratios ranging from 49 to 1249. Then, the photoanodes were immersed in the precursor solution and kept there for 30−45 min at 100 °C inside of an oven. After the deposition, the coated photoanodes were rinsed with DI water and air-dried. The photoanodes investigated in the present study, including WO3/BiVO4 core/shell NWs, BiVO4 thin films, WO3 NWs, α-Fe2O3 NWs, TiO2 NWs, and planar Si wafers (n-type, 1−4 Ohm·cm) were prepared using previously reported methods.38,40−43 Material Charaterization. The morphology of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs was examined by a SEM (FEI Sirion XL30, 5 kV) and TEM (FEI Tecnai G2 F20 X-TWIN FEG, 200 kV). The chemical composition and oxidation state of Ni:FeOOH were analyzed by XPS (PHI VersaProbe), and the measured spectra were calibrated using the binding energy of adventitious carbon at 284.6 eV. The crystallinity and phase of Ni:FeOOH were characterized using parallel beam XRD (PANalytical XPert 2, 45 kV, 40 mA) and Raman spectroscopy (WITEC alpha500 Confocal Raman system). The wavelength-dependent optical absorption property of the bare and coated photoanodes was measured with an integrating sphere using illumination from a Xe lamp coupled to a monochromator (model QEX7; PV Measurements, Inc.). PEC Characterization. The PEC performance of the photoanodes was evaluated using a potentiostat (model SP-200, BioLogic) in a three-electrode compression cell under simulated solar light illumination (AM 1.5 G, 100 mW cm−2, solar simulator model 94306A, class AAA; Oriel). The photoanodes serve as the working electrode, and they were front-illuminated with an illuminated area of 0.26 cm2 defined by a mask. The reference and counter electrodes were a Ag/ AgCl (3 M NaCl) electrode and Pt foil, respectively. For WO3/ BiVO4 core/shell NW, BiVO4 film, and WO3 NW photoanodes, a 0.5 M potassium phosphate buffer solution (pH = 7) was used as the electrolyte. For Fe2O3 NW, TiO2 NW, and planar Si photoanodes, 1 M NaOH was used as the electrolyte. For all of the photoanodes, the photocurrent versus potential (J−V) curves were measured by sweeping the potential in the anodic direction at a scan rate of 50 mV s−1. The stability of the photoanodes was evaluated by measuring the photocurrent versus time (J−t) curves under a constant applied potential of 0.8 V vs RHE. During all of the measurements, the electrolyte was constantly purged with N2 to remove the dissolved oxygen. The charge-transfer efficiency was estimated as a function of the applied potential by adding 0.5 M H2O2 in the electrolyte as a hole scavenger.40,58 We assume that the oxidation kinetics of H2O2 is very fast and its charge transfer efficiency is 100%;

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00303. XPS and Raman spectra of Ni:FeOOH after the PEC test, O 1s XPS spectrum of Ni:FeOOH on Ti foil, TEM images of the Ni:FeOOH-coated WO3/BiVO4 core/shell NWs after the stability test, and table of literature comparison for BiVO4-based photoanodes with Ni- and/ or Fe-based OECs by different OEC deposition methods reported within the past 5 years (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

L.C. and X.Z. conceived of the study and wrote the manuscript. L.C. carried out the photoanode synthesis, catalyst deposition, material characterizations, and PEC measurements. J.Z. assisted in the preparation of the BiVO4 film and performed optical measurements. H.L. performed Raman measurements. J.P. performed TEM characterization of the photoanodes after the stability test. I.S.C. and H.S.H prepared Fe2O3 NWs. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Bay Area Photovoltaics Consortium (BAPVC) and The Global Climate and Energy Project (GCEP) at Stanford University. The authors acknowledge support from the Stanford Nano Shared Facility for material characterizations. The authors thank Dr. Guangxu Chen for assistance with the gas chromatograph measurements.

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REFERENCES

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