Tantalum Nitride Nanorod Arrays: Introducing Ni−Fe LayeredDouble Hydroxides as a Cocatalyst Strongly Stabilizing Photoanodesin Water SplittingLei Wang,†,⊥ Fabio Dionigi,‡,⊥ Nhat Truong Nguyen,† Robin Kirchgeorg,† Manuel Gliech,‡

Sabina Grigorescu,† Peter Strasser,‡ and Patrik Schmuki*,†,§

†Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremburg, Martensstr. 7, D-91058Erlangen, Germany‡The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division,Technical University Berlin, 10623 Berlin, Germany§Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia

*S Supporting Information

ABSTRACT: Ta3N5 nanostructures are widely explored as anodes for photoelectrochemical (PEC) water splitting. Althoughthe material shows excellent semiconductive properties for this purpose, the key challenge is its severe photocorrosion when usedin typical aqueous environments. In the present work we introduce a NiFe layered double hydroxide (LDH) cocatalyst thatdramatically reduces photocorrosion effects. To fabricate the Ta3N5 electrode, we use through-template anodization of Ta andobtain oxide nanorod arrays that then are converted to Ta3N5 by high temperature nitridation. After modification with ourcocatalyst system, we obtained solar photocurrents of 6.3 mA cm−2 at 1.23 VRHE in 1 M KOH, and an electrode maintains about80% of the initial activity for extended irradiation times.

■ INTRODUCTION

Photoelectrochemical (PEC) water splitting offers the capa-bility of harvesting the energy in solar radiation and transferringit directly into a chemical fuel in the form of hydrogen.1,2 Todesign a PEC system that optimally utilizes solar light, theappropriate choice of photoelectrode materials involves notonly the “right” optical absorption properties and relative bandedge positions to water but also chemical and photo-electrochemical stability in aqueous electrolytes.1 Metal oxides,such as Fe2O3,

3,4 TiO2,5,6 and BiVO4

7,8 have received wideattention as photoelectrodes for PEC light conversion.However, either due to nonoptimized energetics or otherintrinsic deficits such as very low carrier lifetime, these materialsshow theoretical and practical limits for an efficient use.9 Thus,considerable efforts have been focused on other semiconductormaterials with more appropriate band positions and improvedcharge carrier transport properties.9,10

Among the nonoxide semiconductors, (oxy) nitrides, namely,Ta3N5 and TaON, emerge as promising candidates for PEC

overall water splitting. The (oxy) nitride materials are able toreduce and oxidize water in the presence of appropriatesacrificial reagents under visible-light irradiation.11−16 Also as aphotoelectrode Ta3N5 has recently attracted intensive interest,because its maximum possible solar-to-hydrogen efficiency is ashigh as 15.9% under AM 1.5G irradiation.17−21 However,Ta3N5 is highly prone to photocorrosion in aqueous media. Tosolve the problem, efforts have been made on alleviatingaccumulation of photogenerated holes at the semiconductorsurface by suitable cocatalysts that facilitate water oxidation. Forexample, Ta3N5 has been modified with classic O2 evolutioncatalysts, such as IrO2. Although this measure indeed slowsdown photocorrosion, still a strong photocurrent decay isobserved with a decrease to ∼20% of the initial photocurrentvalue after illumination for 20 min.22,23 The modification of

Received: October 21, 2014Revised: March 9, 2015Published: March 9, 2015

Article

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© 2015 American Chemical Society 2360 DOI: 10.1021/cm503887tChem. Mater. 2015, 27, 2360−2366

Ta3N5 with Co(OH)x, another excellent water oxidationcatalyst (WOC), can lead to a high photocurrent (up to 5.2mA cm−2 at 1.23 VRHE), but the initially high photocurrent ismaintained only for less than 10 min.24,25 Also Co3O4decorated Ta3N5 electrodes were explored but still they showonly about 30% of the initial photocurrent after 2 hirradiation.26 Recently, Domen et al.27 reported a continuousproduction of oxygen for 100 min on a cobalt phosphate (Co-Pi) WOC modified Ba−Ta3N5 photoanode at a moderatepotential, 0.9 VRHE. The photoanode was measured in anaqueous solution of 0.5 M K2HPO4 electrolyte at pH = 13.Due to these findings it is clear that new and effective

cocatalysts are needed not only in order to reduce thephotocurrent-onset overpotential and enhance the overallenergy conversion efficiency but also to extend the long-termstability of the nitride.For this, one way considers mixed metal oxides of Fe, Co,

and Ni that were reported to have a high oxygen evolutionreaction (OER) activity as electrocatalysts in alkaline medium,often outperforming simple Co oxides and hydroxides.28−31

Recently, a nickel−iron layered double hydroxide (NiFe-LDH)−carbon nanotube electrocatalyst was reported thatprovides a higher OER electrocatalytic activity and stabilitythan commercial Ir-based catalysts in alkaline solutions.28,32

The key features of this catalyst are ultrathin nanoplatelets of ahighly OER-active NiFe-LDH structure.In the present work, we introduce the use of such a

crystalline NiFe-LDH film as a hole transfer catalyst on a Ta3N5photoanode, which leads to an outstanding stability of theelectrode against photocorrosion. The NiFe-LDH decoratedTa3N5 photoanode maintains about 90% of the initial activityafter 2 h irradiation. Moreover, combined with Co(OH)x andCo-Pi, the photoanode yields a photocurrent up to 6.3 mAcm−2 at a potential of 1.23 VRHE under AM 1.5G simulatedsunlight (100 mW cm−2) and maintains about 80% of the initialactivity after 2 h irradiation; this is, we believe, the highest long-term stable photocurrent of a Ta3N5 nanorod photoanodereported to date.

■ EXPERIMENTAL SECTIONMaterials. All chemicals were analytical grade and were used as

purchased without further purification. Solutions were prepared usinghigh purity water purified with a Barnstead purification system(resistivity ≥ 18.2 MΩ).Fabrication of Ta3N5 Samples. Vertically aligned Ta2O5

nanorods were grown on a Ti foil with a thickness of 0.1 mm(99.9% purity, Advent, U.K.) by through-mask anodization.22,33 TheTa foils were cut into a diamond saw and then cleaned by sonication inacetone and ethanol. An Al layer with a thickness of ∼1.3 μm wasdeposited on the Ta substrate by thermal evaporation (using a PLS500 evaporation system). The Al layer was anodized in 0.3 M oxalicacid at 40 V for 0.5 h at room temperature under stirring, to form aporous anodic alumina (PAA) mask. Subsequently, the nanochannelsof the PAA mask were widened by immersion into a 5% H3PO4aqueous solution at 60 °C for 90 s, 3 min, and 4 min. The masked Tasubstrates were then anodized in a 0.5 M H3BO3 aqueous solution atroom temperature under stirring. The anodizing voltage was rampedfrom 0 to 600 V at a rate of 0.05 V s−1 and held at 600 V for 1 h. TheTa2O5 nanorods embedded into the PAA mask were released byetching away the PAA mask with a 5% H3PO4 aqueous solution at 60°C for 4 h. To obtain Ta3N5 nanorod arrays, the as-grown Ta2O5nanorod arrays on the Ta foils were then heated in a quartz tubefurnace in a gaseous atmosphere of NH3 with a flow rate of about 200mL min−1 at 1000 °C for 2 h.

Decoration of Co(OH)x on Ta3N5 Nanorod Arrays. Before thewater splitting measurement, Co(OH)x and Co-Pi were deposited onthe Ta3N5 nanorod arrays as oxygen evolution catalysts. For theCo(OH)x decoration, the nanorod photoanodes were immersed in amixed solution of 0.1 M CoSO4 and 0.1 M NaOH with a ratio of 1:1for 30 min and then washed with DI water and dried in N2.

Decoration of Co-Pi on Co(OH)x/Ta3N5 Nanorod Arrays.Decoration of Co-Pi catalyst on the Co(OH)x/Ta3N5 nanorod arrayswas conducted by an electrodeposition method. For this theCo(OH)x/Ta3N5 nanorod layers were held potentiostatically at 1.0V versus Ag/AgCl for 8 min in a solution containing 0.5 mMCo(NO3)2 and 0.1 M KH2PO4 at pH = 7. Then the electrodes werewashed with DI water and dried in N2.

Decoration of NiFe-LDH on Ta3N5 Nanorod Arrays. For theprecursor solution, 186 mg of Ni(NO3)2·6H2O and 51.6 mg ofFe(NO3)3·9H2O were dissolved in 4.64 and 3.20 mL of DI water,respectively. The solution was dispersed in a mixture of 32 mL of N,N-dimethylformamide (DMF) and 60 mL of DI water, followed bysonication for 1 h. The Ta3N5 nanorod arrays were put in a 250 mLTeflon-lined autoclave filled with the precursor solution. The autoclavewas then heated for the solvothermal reaction to 120 °C for 15 h,followed by another solvothermal treatment at 160 °C for 2 h. Thenthe electrodes were washed with DI water and dried in N2.

Decoration of NiFe-LDH on Co-Pi/Co(OH)x/Ta3N5 NanorodArrays. To decorate the cocatalyst on the NiFe-LDH/Ta3N5 nanorodarrays, the samples were immersed in the mixed solution of 0.1 MCoSO4 and 0.1 M NaOH for 30 min, followed by washing with DIwater and drying in N2. Finally, the electrodes were further modifiedwith Co-Pi catalyst via electrodeposition method as described above(0.5 mM Co(NO3)2 and 0.1 M KH2PO4 at pH = 7 at 1 V versus Ag/AgCl for 8 min). The electrodes were washed with DI water and driedin N2.

Characterization. X-ray diffraction (X’pert Philips MPD with aPanalytical X’celerator detector, Germany) was carried out usinggraphite monochromized Cu Kα radiation (Wavelength 0.154056nm). Chemical characterization was carried out by X-ray photo-electron spectroscopy (PHI 5600, spectrometer, U.S.A.) using Al Kαmonochromatized radiation. A field-emission scanning electrodemicroscope (Hitachi FE-SEM S4800, Japan) was used for themorphological characterization of the electrodes. TEM electronmicroscopy was performed on a FEI Tecnai G2 20 S-TWIN withLaB6-cathode, 200 kV accelerating voltage, and resolution limit 0.24nm. The absorption of the samples was measured using a UV/visspectrometer (Lambda 950) using integrating spectra.

Photoelectrochemical and Electrochemical Measurements.The photoelectrochemical experiments were carried out undersimulated AM 1.5G (100 mW cm−2) illumination provided by asolar simulator (300 W Xe with optical filter, Solarlight; RT). One MKOH aqueous solution was used as an electrolyte. A three-electrodeconfiguration was used in the measurement, with the Ta3N5 electrodeserving as the working electrode (photoanode), an Ag/AgCl (3 MKCl) as the reference electrode, and a platinum foil as the counterelectrode. Photocurrent vs voltage (I−V) characteristics were recordedby scanning the potential from −0.3 to 0.8 V (vs Ag/AgCl (3 M KCl))with a scan rate of 10 mV s−1 using a Jaissle IMP 88 PC potentiostat.The measured potentials vs Ag/AgCl (3 M KCl) were converted tothe reversible hydrogen electrode (RHE) scale using the relationshipERHE = EAg/AgCl + 0.059 pH + E0Ag/AgCl, where EAg/AgCl is theexperimentally measured potential, and E0Ag/AgCl = 0.209 V at 25 °Cfor an Ag/AgCl electrode in 3 M KCl. The stability measurement forthe Ta3N5 electrode was conducted at 0.23 V (vs Ag/AgCl (3 MKCl)) in 1 M KOH as a dependence of time. For all experiments atleast three samples in the same condition were prepared andmeasured.

Photocurrent spectra were acquired at an applied potential of 0.23 V(vs Ag/AgCl (3 M KCl)) in 1 M KOH recorded with 20 nm steps inthe range of 400−600 nm using an Oriel 6365 150 W Xe-lampequipped with an Oriel Cornerstone 7400 1/8 m monochromator.The wavelength-dependent incident photon-to-current efficiency(IPCE) was calculated according to the following equation:

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λ= × × ×i pIPCE (%) ((1240 )/( )) 100ph in

where iph is the photocurrent (mA), λ is the wavelength (nm) ofincident radiation, and pin is intensity of the incident light irradiatingon the semiconductor electrode at the selected wavelength (in mW).Electrochemical impedance spectroscopy (EIS) was performed by

using of Zahner IM6 (Zahner Elektrik, Kronach, Germany).Measurements were performed by applying 1.23 VRHE at a frequencyrange of 100 000 to 0.1 Hz with an amplitude of 10 mV. Cyclicvoltammetry (CV) was performed with 3 cycles at a scan rate of 5, 20,50, and 100 mV s−1, respectively, under light and dark conditions.Mott−Schottky analysis was carried out in a DC potential range 0−1.7VRHE with a frequency of 1 Hz under dark conditions. All theimpedance and CV measurements were carried out in 1 M KOHsolution using Schott KL1500 electronic light source (Halogen lamp15 V, 150 W).Faradaic Efficiency. A two-electrode cell was used to measure the

Faradaic efficiency. A Ta3N5 nanorod photoanode and a Pt foil wereused as a working electrode and a counter electrode, respectively. TheTa3N5 nanorod photoelectrodes were biased at 1.23 VRHE in a 1 MKOH under AM 1.5G simulated sunlight. The cell was sealed and waspurged by N2 for 30 min, and no O2 and N2 was detected before theFaradaic efficiency measurement. Evolved oxygen gas was detected bya gas chromatograph (GCMS-QO2010SE, Shimadzu).

■ RESULTS AND DISCUSSION

Figure 1a−d illustrates the fabrication steps of the nanorodgeometry using a through-mask-anodization approach.22,33 First

we evaporated an aluminum layer on a Ta substrate. The Allayer was then anodized to form ordered porous alumina asdescribed in the Experimental Section. In a second anodizationstep, the Ta substrate was oxidized through the porous aluminamask under a high electric field forming Ta2O5 nanorods in aH3BO3 electrolyte. For adjusting the Ta2O5 nanorod diameter,pore-widening of the PAA was carried out using H3PO4 with apart of the specimens (that is, they were subjected to open-circuit dissolution before the second anodizing step). Due tothe nature of the through-hole anodization process, the anodicTa-oxide films formed are composed of an upper layer oftantala nanocolumns penetrating into the alumina pores and alower layer of continuous tantalum oxide underneath thealumina film. Figure 1e−h shows SEM images of the orderedTa2O5 nanorod arrays on the compact oxide layer. Clearly, acontinuous densely packed tantalum oxide nanorod layer, inthis case with a thickness of 330 nm, is obtained. The meandiameter of the columns is approximately 110 nm, which isapparently larger than that of the pores in the overlayingalumina film (Supporting Information Figure S1c). Underneaththe nanorod array, an ∼750 nm thick oxide layer is observed(Figure 1g). Supporting Information Figures S1 and S2 showthe effect of pore widening on the growth of nanorod arraysand the underneath oxide layers. The diameter and thickness ofthe columns change from 60 to 110 nm and 490 to 330 nm,respectively, when increasing the pore-widening time from 90 sto 4 min. The ratio between the nanorod array and compactoxide layer is decreased from 1:1 to 1:2, as the open pore-widening time is increased from 90 s to 4 min (SupportingInformation Figure S2). This is similar to the results originallyreported by Takahashi et al.,33 who ascribed this effect to thedielectric properties of the films formed by reanodizing throughthe widened pores. It is noteworthy that the pore wideningleads not only to a morphology change but also to a change inthe chemical composition of anodic tantala layers.33a

Figure 2a and Supporting Information Figure S3 show SEMimages of the Ta3N5 nanorod arrays after nitriding at 1000 °C

for 2 h. The diameter and thickness of Ta3N5 columns decreaseto ∼100 nm and ∼250 nm, respectively, due to the volumedecrease when Ta2O5 is converted to Ta3N5. The base oxidelayer underneath the nanorod arrays changes to a mesoporousnanoparticle layer (Figure 2a). X-ray diffraction (XRD)patterns (Figure 3a) show the presence of peaks that can be

Figure 1. (a−d) Schematic cross sectional views for the fabrication ofvertically aligned Ta2O5 nanorod arrays. (a) PAA mask formed by porewidening for 90 s after anodization; (b) Ta2O5 nanorod arraysembedded in the PAA mask of (a); (c) PAA mask formed by porewidening for 4 min after anodization; (d) Ta2O5 nanorod arraysembedded in the PAA mask of (c); (e, f) Top and (g, h) cross-sectional SEM images of the vertically aligned Ta2O5 nanorod arrays.Inset of (e) shows diameter distribution of Ta2O5 nanorod array.

Figure 2. (a, e) SEM cross-sectional images of (a) Ta3N5 and (e)NiFe-LDH/Ta3N5 nanorod arrays; (b−d, f) TEM images of (b)Ta3N5 nanorod (mark A), (c) porous subnitride Ta3N5 (mark B), (d)NiFe-LDH, and (f) NiFe-LDH/Ta3N5.

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assigned to Ta3N5 and Ta2N phases. The Ta2N phase is formedby nitridation of the interlayer that was originally formed duringthe through-mask anodization step.22 In Domen et al.’s work,27

barium (Ba) doping of the Ta3N5 electrodes drastically changedthe property of the oxide layer between the nanorod array andTa substrate. The nondoped oxide layer was reported tocontain Ta2N and Ta5N6; due to the Ta5N6 compound it wasdescribed to be much more resistive than a Ba doped layer,which contained only Ta2N. Thus, the activity of the nanorodphotoanode could be improved if Ta5N6 phase formation wassuppressed. However, in our work, only the Ta2N phase wasobserved for the undoped electrode, which was achieved byusing a high NH3 flow rate (200 sccm). In our case, thepresence of only the Ta2N subnitride phase provides already ahigh electrical conductivity that aids the electron transfer fromthe nanorods/porous layers to the Ta substrate.27

Supporting Information Figure S4a,b shows the SEM imagesof a Ta3N5 electrode after NiFe-LDH deposition using thehydrothermal method. The nanoplatelet size of NiFe-LDH istypically ∼400 nm (Figure 2d) and appears as nearlytransparent patches on the top surface of the Ta3N5 electrodes(Supporting Information Figures S4b and S5). The thickness ofthe nanoplatelets was estimated to be 10−20 nm (Figure 2f).The XRD pattern of the as-prepared NiFe-LDH (SupportingInformation Figure S6) is consistent with the typical profile ofLDH materials.28 X-ray photoelectron spectroscopy (XPS)corroborated the existence of both Ni and Fe in the hybridmaterial, as apparent from the high-resolution Ni 2p and Fe 2pspectra (Figure 3b,c). The O 1s spectrum (Figure 3d) is fittedwith two peaks at 531.9 and 529.4 eV, which are correspondingto hydroxyl and oxide species, respectively. The absence of the

Ta 4p peak in XPS (Figure 3e) is in line with almost a fullcoverage of Ta3N5 with the NiFe-LDH compared with the bareTa3N5 (Supporting Information Figure S7).In order to improve the PEC water splitting and to shift the

photocurrent onset potential to lower values, additionallyCo(OH)x combined with Co-Pi was employed as cocatalyst.Co(OH)x nanoparticles were deposited on the Ta3N5 nanorodarrays by an immersion method (see Experimental Section).After completion of a homogeneous Co(OH)x decoration, athin layer of Co-Pi nanoparticles was additionally deposited onthe Co(OH)x/Ta3N5 nanorod surface by electrodeposition,resulting in the formation of Co-Pi+Co(OH)x/Ta3N5 nanorodarrays. The optimum amount of Co-Pi nanoparticles forefficient water splitting can be adjusted by controlling theelectrodeposition time (Supporting Information Figure S8a).Figure 4 provides the current−potential curves of the

electrodes under AM 1.5G conditions in 1 M KOH electrolyte.For the Ta3N5 photoanode (Figure 4a), a considerable increasein dark current density occurs at ≈1.0 VRHE. As expected, theCo-Pi/Co(OH)x modified Ta3N5 electrode (Figure 4b) showsa better PEC performance measured under AM 1.5G light (100mW cm−2) compared with the bare Ta3N5 electrode in thetested potential range. Especially, the magnitude of thephotocurrent density of Co-Pi+Co(OH)x/Ta3N5 photoanodeis increased about 7-fold at a potential of 1.23 VRHE in the KOHelectrolyte.On the other hand, the Ta3N5 electrode modified with the

NiFe-LDH layer was also examined for its PEC response. InFigure 4c, for the surface modification with NiFe-LDH, theTa3N5 photoanode exhibits a water oxidation onset potential of0.9 VRHE and the photocurrent increases to 1.7 mA cm−2 at 1.23VRHE. Nevertheless, the highest effect is clearly reached using acombined approach, i.e., decorating the Ta3N5 electrode withthe cobalt cocatalysts and NiFe-LDH. A clear negative shift ofthe onset potential is observed, and a photocurrent of 6.3 mAcm−2 at 1.23 VRHE is reached (Figure 4d). The photocurrentdensity of the Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 at 1.23VRHE is 9 times higher than that of the bare Ta3N5, which is alsohigher than for the plain Co-Pi+Co(OH)x/Ta3N5 nanorod

Figure 3. (a) XRD patterns for Ta3N5 and NiFe-LDH/Ta3N5 nanorodarrays; (b−e) high-resolution (b) Ni 2p, (c) Fe 2p, (d) O 1s, and (e)Ta 4p spectra of NiFe-LDH/Ta3N5 nanorod arrays; (f) UV−vis diffusereflectance spectra of Ta3N5 and NiFe-LDH/Ta3N5 nanorod arrays.

Figure 4. Current−potential curves with chopped light illuminationonto (a) Ta3N5, (b) Co-Pi+Co(OH)x/Ta3N5, (c) NiFe-LDH/Ta3N5,and (d) Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 nanorod arrays underAM 1.5G simulated sunlight (100 mW cm−2). Conditions: 1 M KOHsolution (pH 13.6), 10 mV s−1 scan rate.

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arrays. The low onset potential and high photocurrent densityreflect a clearly higher photoconversion efficiency for the Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 electrode.We also considered the effect of nanorod/underlayer

dimensions on the PEC water splitting performance usingCo(OH)x/Ta3N5 electrodes, as shown in Supporting Informa-tion Figure S9a. The photoanode with large diameter rods(∼110 nm) and a thick oxide layer underneath (∼750 nm)yields a photocurrent density of 4.1 mA cm−2 at 1.23 VRHE,which is two times higher than for photoanodes with diametersof 60 and 90 nm, i.e., also corresponding to a smaller thicknessof the entire oxide layer. The increase in PEC activity with largediameter nanorod arrays can mainly be ascribed to theincreased thickness of the porous layer and less defectivenanorods.Overall, above-reported magnitudes of the photocurrent

density are among the best ever reported for Ta3N5. But aneven more significant effect observed in this work is the use ofthe NiFe system to induce a high photocorrosion stability.Figure 5 shows photocurrent versus time of Ta3N5, NiFe-

LDH/Ta3N5, Co-Pi+Co(OH)x/Ta3N5, and Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 electrodes for continuous illuminationunder AM 1.5G simulated sunlight at 1.23 VRHE for extendedtimes. The photocurrent of the bare Ta3N5 electrode rapidlydecays to a negligible level within minutes as the surface isoxidized. This can be ascribed to photocorrosion and someextent of dark corrosion (as widely reported in literature20−23).Photocorrosion is caused by an accumulation of photo-generated holes at the surface of Ta3N5. This, in turn, is dueto the poor hole transfer kinetics from the semiconductorvalence band to water. If Co(OH)x and Co-Pi as cocatalysts aredecorated on the Ta3N5 photoanode, a substantial enhance-ment in photostability is achieved compared to the bare Ta3N5.However, the photostability of Co-Pi+Co(OH)x/Ta3N5 elec-trode is still poor, in that only about 20% of the initialphotocurrent remained after 2 h irradiation. The decliningphotocurrent trend is similar to that of IrO2 modified Ta3N5.

22

In contrast, the NiFe-LDH/Ta3N5 electrode retains about 90%of the initial value even after 2 h of irradiation. Thephotocurrent increases, partially at the beginning stage, whichis attributed to a “surface charging” process. This phenomenonindicates that surface-reaching holes are initially stored, whichin turn increases a recombination flux of electrons at thesurface.26 XPS analysis after the stability test in SupportingInformation Figure S10 shows a weak Ta 4p peak, and some

cracks are also formed on the electrode surface during the PECwater splitting, evident in SEM as shown in SupportingInformation Figure S4c. Thus, the slight deactivation might beassociated with partial exposure of Ta3N5 to the electrolytecaused by these cracks. However, for the cobalt cocatalystsdecorated NiFe-LDH/Ta3N5 electrode, the activity only mildlydecreases in the initial 15 min but then exhibits a steadyphotocurrent of ∼5 mA cm−2 (about 80% of the initial activity)reliably for at least 2 h of irradiation. (Results for longerexposure times become less reliable and vary from sample tosample, which may reflect the finding that crack formation andpinholes in the cocatalyst layer may allow electrolytepenetration and thus a slow corrosion of the nitride phase.)Nevertheless, the results indicate that the NiFe-LDH layerefficiently prevents photocorrosion, restrains the recombinationpathways, and improves the kinetic transport of photogeneratedcharge carriers away from the Ta3N5 surface. The observedphotostability of Ta3N5 based systems at this high level of aphotocurrent density (∼5 mA cm−2) for several hours has notbeen achieved for any Ta3N5 nanorod arrays.In order to explore the interfacial properties between the

electrodes and the electrolyte, electrochemical impedancespectroscopy (EIS) measurements were carried out coveringthe frequency range of 105−0.1 Hz and using an amplitude of10 mV as shown in Figure 6a. The results were fitted using the

equivalent circuit,21,34 shown in Figure 6b, in which Rs is thesolution resistance, and the RC loop consists of a capacitanceCct and a resistance Rct, to characterize the charge transferbehavior across the electrode−solution interface. The fittedresults for Rct values yield 3893 Ω, 526.8 Ω, 514.9 Ω, and 155.6Ω for the Ta3N5, NiFe-LDH/Ta3N5, Co-Pi+Co(OH)x/Ta3N5,and Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 electrodes, respec-tively. That is, the Rct value of the Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 electrode is an order of magnitude smaller thanthat of the Ta3N5 electrode, confirming that the decorationwith cobalt- and NiFe-LDH- cocatalysts significantly reducesthe charge transfer resistance, thus reduces the recombination

Figure 5. Steady-state photocurrents of the bare Ta3N5 (black curve),NiFe-LDH/Ta3N5 (green curve), Co-Pi+Co(OH)x/Ta3N5 (redcurve), and Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 (blue curve) nano-rod arrays measured in 1 M KOH solution at 1.23 VRHE under AM1.5G simulated sunlight (100 mW cm−2).

Figure 6. (a) Electrochemical impedance spectra of the bare Ta3N5,NiFe-LDH/Ta3N5, Co-Pi+Co(OH)x/Ta3N5, and Co-Pi+Co(OH)x/NiFe-LDH/Ta3N5 nanorod arrays measured in 1 M KOH solution at1.23 VRHE; (b) an equivalent circuit model for the photoanodesaccording to refs 21 and 34. Rs represents the solution resistance andthe capacitance Cct and resistance Rct characterize the charge transferbehavior across the electrode−solution interface.

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of photogenerated carriers, and as a result enhances thephotocurrent.Therefore, we attributed the improved PEC performance for

NiFe-LDH/Ta3N5 mainly to the NiFe-LDH phase. Inparticular, the LDH/Ta3N5 junction facilitates charge transferand improves the water oxidation. In the cyclic voltammogramexperiments (Supporting Information Figure S11a−c), weobserve that the NiFe-LDH without the Ta3N5 electrodesshows high activity for OER catalysis and is superior to NiFeoxide, Ni(OH)x, or FeOx catalysts.

28,35 In the Mott−Schottkyanalysis (Supporting Information Figure S11d), the NiFe-LDHexhibits linear dependence with a positive slope from 0.2 to 0.5VRHE, which indicates a typical feature for an n-typesemiconductor. EIS further reveals that the decrease ofcharge-transfer resistance is correlated to the increase ofcorresponding capacitance, which indicates that surface-reach-ing holes are immediately transferred away from the nitride.

■ CONCLUSIONSUsing a through-mask anodization of Ta, regularly alignedTa2O5 nanocolumns were grown. Upon nitridation in NH3atmosphere at 1000 °C, the structures were converted to Ta3N5nanorod arrays that represent highly efficient anodes for PECwater splitting. By modification of the layers with NiFe-LDH,Co(OH)x, and Co-Pi as cocatalysts, we obtained solarphotocurrents of 6.3 mA cm−2 at 1.23 VRHE, which is thehighest value reported for Ta3N5 nanorod arrays (to the best ofour knowledge). More importantly, the electrode exhibits aremarkable stability against photocorrosion. A photocurrent of∼5 mA cm−2 can be steadily maintained over 2 h irradiation.The combination of catalytic activity, stability, and decorationmakes this new type of cocatalyst very promising to improvethe photoelectrochemical performance of photoanodes in thefield of energy conversion.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional SEM images, XRD spectrum, XPS survey spectrum,PEC water splitting, CV diagrams, IPCE, and UV−vis duffucerelectance spectra. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*(P.S.) E-mail: schmuki@ww.uni-erlangen.de.

Author Contributions⊥(L.W. and F.D.) The authors contributed equally to this work.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Dr. Lin Gan and the center for electron microscopyat the TU Berlin (ZELMI) for help with the TEM analysis. Theauthors would like to acknowledge DFG and the DFG clusterof excellence “Engineering of Advanced Materials” (EAM) forthe financial support.

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