Plasma-Induced Vacancy Defects in Oxygen Evolution Cocatalysts on

Oct 16, 2018 - Interface-defect engineering is proven to be an efficient and facile strategy to enhance the solar water oxidation activity of Ta3N5, w...
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Research Article Cite This: ACS Catal. 2018, 8, 10564−10572

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Plasma-Induced Vacancy Defects in Oxygen Evolution Cocatalysts on Ta3N5 Photoanodes Promoting Solar Water Splitting Lei Wang,*,† Beibei Zhang,† and Qiang Rui† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

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ABSTRACT: Surface recombination is a critical issue for tantalum nitride (Ta3N5)-based photoanodes in solar water splitting application. The efficient cocatalysts (Ni-, Fe-, and Cobased) have been developed to promote the electron−hole separation and transportation but still have limited success in some cases. Herein, we studied the Ar plasma-induced etching strategy on the pristine Ta3N5 nanotubes and Co(OH)xdecorated Ta3N5 nanotubes. The Ar plasma can not only destroy the recombination center (TaO) in the interface between the Ta3N5 and electrolyte, resulting in a fast charge transfer, but also most importantly generate more oxygen vacancies with a high ratio of Co2+/Co3+ and produce a higher surface area in the Co(OH)x cocatalyst. The more active sites on Ta3N5 and abundant oxygen vacancies on cocatalyst synergetically contribute to the enhanced solar water splitting activity, which give rise to a fast water oxidation reaction in the interface. The resulting photoanode shows double the performance improvement under AM 1.5G sunlight conditions. Interface-defect engineering is proven to be an efficient and facile strategy to enhance the solar water oxidation activity of Ta3N5, which highlights the advantages of the plasma-etching strategy for establishing the highly active cocatalysts on photoanodes in terms of the conversion of solar energy into chemical energy. KEYWORDS: tantalum nitride, plasma-induced defects, Co2+/Co3+, oxygen vacancies in cocatalyst, solar water splitting

1. INTRODUCTION Solar water splitting is a promising way to alleviate the energy crisis for conversion solar energy into chemical energy in the form of storable hydrogen. Previous research on solar water splitting has focused on the preparation of metal oxides, which are responsive to ultraviolet light.1−5 Recently, tremendous efforts have put toward the search and design of visible-light responsive semiconductors. Tantalum nitride (Ta3N5) has received significant attention for solar water splitting due to its suitable band gap of ∼2.1 eV and energy levels (positions of conduction band minimum and valence band maximum), which lead to a major part of visible light absorption over solar water splitting. Its theoretical maximum efficiency of light conversion is expected to be 16% under AM 1.5G simulated sunlight.6−9 For the desired solar water splitting performance of photoanodes to be obtained, the interface between semiconductor and electrolyte is critical for separating the photogenerated charges and then transferring the separated charges across the interface for the chemical reactions, which is the key to the performance of photoanodes. Recently, various oxygen evolution reaction (OER) cocatalysts, e.g., FeOOH,10 NiFe-layered double layer (NiFe-LDH),11 Co(OH)x,12,13 and Co-Pi,14 have been successfully deposited to extract photogenerated holes in the Ta3N5 and avoid recombination of electrons and holes. For instance, Li et al.15 reported one of the most photoactive Ta3N5 photoanodes, which was modified with five different layers (protective/hole storage/inorganic © XXXX American Chemical Society

catalyst/two successive molecular catalyst layers). Domen et al.16 had success in stabilizing the Ta3N5 photoanode by the introduction of the Co-Pi/GaN bilayer, and Co-Pi could be replaced by a more active cocatalyst. More recently, Wang and colleagues17 reported that the formation of the Ta−O−Co interface between Ta3N5 and Co(OH)x has improved the surface energetics and kinetics and further the solar water oxidation performance of Ta3N5. Nowadays, among the OER cocatalysts, cobalt-based cocatalysts are the most effective cocatalysts on the Ta3N5based photoanodes for improving solar water splitting performances.11a,13,14 Although the introduction of cobaltbased cocatalysts has obtained some encouraging results, these bulk cocatalysts are still less active for OER with relatively lower surface areas. Recently, tuning oxygen vacancies of metal oxides/hydroxides remarkably alters the catalytic activities in the electrocatalytic water splitting.18,19 For example, the proper ratio control of Co2+/Co3+ in the Co3O4 catalyst resulted in a significant modification of its catalytic property.20 Furthermore, plasma treatment technique on electrocatalysts or semiconductors introduces oxygen vacancies intentionally for solar water splitting.21−24 Wang et al.21 successfully exfoliated the bulk CoFe layered double hydroxide (LDH) sheets into ultrathin nanosheets by nitrogen plasma. Nitrogen doping and Received: August 4, 2018 Revised: September 29, 2018

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Figure 1. (a) Schematic illustration for synthesis of Ar-plasma-treated Ta3N5 NTs and Co(OH)x/Ta3N5 NTs. (b−g) TEM images of (b,c) Ta3N5 NTs, (d) Ar-plasma-treated Ta3N5 NTs (Ta3N5(A)), (e) Co(OH)x/Ta3N5, (f) Co(OH)x/Ta3N5(A), and (g) Ar-plasma-treated Co(OH)x/ Ta3N5(A) NTs (Co(OH)x(A)/Ta3N5(A)). Insets of (c), (d), and (e) show SAED patterns of Ta3N5, Ta3N5(A), and Co(OH)x, respectively.

eliminate the depletion layer on the interface, exposing more surface sites for a continuous charge-transport pathway, but also most importantly can etch the Co(OH)x OER cocatalyst with abundant oxygen vacancies, further enhancing the PEC performance of Ta3N5 photoanodes. The higher ratio of Co2+/ Co3+ in cocatalyst could lead to a remarkable alternation on its catalytic activity for fast charge separation and transfer across the interface for chemical reactions of photoanodes, providing a highly efficient photoanode configuration in PEC systems.

defects were introduced into exfoliated ultrathin LDHs nanosheets. The water-plasma etching was further used to exfoliate these CoFe LDH nanosheets.22 The water plasma not only destroyed the electrostatic interactions between the host metal layers and interlayer cations but also produced multivacancies in the ultrathin LDH nanosheets, contributing to the enhanced electrocatalytic activity for OER. Additionally, Delaunay and co-workers24 reported an air plasma treatment to produce oxygen vacancies on ultrathin hematite nanoflakes. Incorporated oxygen vacancies activated the electrochemically active sites on the nanoflakes and further promoted the photoactivity of photoanodes. These reports involved bulk defects in electrocatalysts or photoelectrodes; there are few studies involved in defects of cocatalysts decorated on photoelectrodes in photoelectrochemical (PEC) water splitting. Namely, it is unclear how the defects induced in active/ inactive cocatalysts control the interfacial reaction and promote the solar water splitting performance of semiconductors. In this work, we introduced a plasma-etching strategy on the Co(OH)x cocatalyst-decorated Ta3N5 nanotubes (NTs). The Ar plasma treatment can not only effectively

2. EXPERIMENTAL SECTION 2.1. Preparation of Ta3N5 Nanotube Arrays. Highly ordered Ta2O5 NTs were formed by anodizing Ta foil (99.95%, Alfa Aesar) in a two-electrode electrochemical cell by using a Pt sheet as counter electrode according to the literature.25 Prior to anodization, the Ta foils were cleaned by sonication in ethanol for 10 min followed by drying in air. The anodization experiments were performed at 60 V at room temperature in a sulfuric acid electrolyte consisting of 0.8 wt % NH4F and 13.6 vol % deionized water. The maximum current density is chosen at 0.1 mA cm−2. The first anodized Ta2O5 10565

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conversion from Ta2O5 to Ta3N5.11a Figure 1a illustrates the synthesis process of Ar-plasma-treated Ta3N5 NT arrays. The Ta3N5 NT arrays were etched by Ar-plasma using a plasma cleaner at an energy of 30 keV, and the resulting photoanode is referred to as Ta3N5(A). In this case, no change in morphology is observed upon the Ar-plasma etching by scanning electron microscopy (SEM) (Figure S2). The high-resolution transmission electron microscopy (HRTEM) images of reference sample and Ta3N5(A) NTs are shown in Figure 1b−d. In Figure 1c, a marked lattice spacing of 0.51 nm is clearly detected, which corresponds to the (200) crystal planes of Ta3N5. An ultrathin layer with a lattice spacing of 0.26 nm, which matches the (111) face of TaO, is found in the outer surface of pristine Ta3N5, consistent with the results reported by Li et al.15 and Takanabe et al.26 The existing defect of TaO may cause charge recombination in the interface during water splitting.15,26 Furthermore, it is clearly seen in Figure 1d that this layer was removed after Ar-plasma treatment, and the Ta3N5(A) sample shows an amorphous layer on the outer surface toward structure disorder. The selected area electron diffraction (SAED) pattern of Ta3N5(A) NTs (inset of Figure 1d) exhibits a blurred halo apart from the regular diffraction rings. Additionally, cobalt-based cocatalyst Co(OH)x was decorated on the Ta3N5 NTs without and with Ar-plasma treatment as illustrated in Figure 1a. From Figure 1e, TaO and Co(OH)x layers are observed for the cocatalyst-decorated pristine Ta3N5, and a layer with a thickness of 2−5 nm is covered on the top surface of etched Ta3N5(A) as shown in Figure 1f. The Co(OH)x possesses a defective and polycrystalline structure (inset of Figure 1e and Figure S3). We further etched the Co(OH)x/Ta3N5(A) NTs, and the sample denoted as Co(OH)x(A)/Ta3N5(A) (Figure 1g) shows a similar layer compared to that of the nontreated Co(OH)x/Ta3N5(A) NTs (Figure 1f). The Ar-plasma-treated samples were then investigated for PEC performances in 1 M KOH electrolyte under AM 1.5G illumination. Figure 2a shows the linear sweep voltammetry (LSV) scans for the Ta3N5NTs arrays without and with Arplasma treatment. It is obvious that Ar-plasma treatment has a strong effect on solar water splitting performance. The Ta3N5(A) photoanode shows a cathodic shift of onset potential and an improved photocurrent (1.25 mA cm−2 at 1.23 VRHE) relative to those of the pristine NTs (0.3 mA cm−2 at 1.23 VRHE). For the photoresponse to be enhanced further, additional Co(OH)x was introduced as a cocatalyst on the NT samples. After decoration with Co(OH)x, the Co(OH)x/ Ta3N5 photoanode displays an obviously negative shift in the onset potential and an enhanced photoresponse compared with the pristine Ta3N5 over the entire potential range. The photocurrent density of Co(OH)x/Ta3N5 is increased up to 3.5 mA cm−2 at 1.23 VRHE, which is 10-times higher than that of the pristine Ta3N5 (0.3 mA cm−2 at 1.23 VRHE). Furthermore, the Co(OH)x-treated Ta3N5(A) photoanode exhibits a much better enhanced PEC activity up to 4.5 mA cm−2 at 1.23 VRHE. The performance improvement of Co(OH)x/Ta3N5(A) could be ascribed to the decreased recombination center in the interface between Ta3N5 and Co(OH)x. Even more remarkably, the beneficial effect of Ar plasma is much more pronounced for the Ar-plasma-treated Co(OH)x/Ta3N5(A), reaching to 7.2 mA cm−2 at 1.23 VRHE. We additionally investigated the influence of Ar-plasma treatment time on the photoresponses of Ta3N5(A) and Co(OH)x(A)/Ta3N5(A) electrodes, as shown in Figures S4

NTs were removed ultrasonically in deionized water to remove the initial layer in which the underlying Ta was explored. The prepared Ta foil was carried out in the fresh electrolyte at 60 V for 3 min to form an open top anodic Ta2O5 NT. Then, the Ta2O5 NTs were put into the tube furnace (HF-Kejing, OTF1200X) and annealed in NH3 atmosphere at 1000 °C for 1 h to obtain Ta3N5 NTs. Co(OH)x cocatalyst was decorated on the Ta3N5 NTs. The Ta3N5NTs samples were immersed in a solution of 0.1 M CoSO4 and 0.1 M NaOH for 20−30 min at 40 °C, washed with deionized water, and dried in air. Synthesis of Ta3N5 and Co(OH)x/Ta3N5 by Ar Plasma. Ta3N5 and Co(OH)x/Ta3N5 samples were treated using Ar plasma (GD-J25B) with a power of 30 keV, and the treatment time was 2, 5, and 10 min, respectively. For comparison, the Co(OH)x/Ta3N5 was also treated under O2 plasma conditions. 2.2. Photoelectrochemical Performance. The photoelectrochemical experiments were carried out using a Chi760e electrochemical workstation (Huakeputian Technology Beijing) under simulated AM 1.5G (100 mW cm−2) illumination provided by a solar simulator (300 W Xe with optical filter). A three-electrode configuration was used in the measurement with a photoanode as the working electrode (photoanode), Ag/AgCl (3 M KCl) as the reference electrode, and platinum foil as the counter electrode. Linear sweep voltammetry (LSV) was prepared by scanning the potential from −0.7 to 0.6 V (vs Ag/AgCl (3 M KCl)) in 1 M KOH. The measured potentials vs Ag/AgCl (3 M KCl) were converted to the reversible hydrogen electrode (RHE) scale. Electrochemical impedance spectroscopy (EIS) measurements were performed at a frequency range of 105 to 0.01 Hz with an amplitude of 10 mV under AM 1.5G illumination. The electrochemical surface area was measured from capacitance measurements by cyclic voltammetry (CV) between 0.14 and 0.24 VRHE in 1 M KOH solution in the dark with scan rates of 20, 40, 60, 80, and 100 mV s−1, respectively. For the stability of plasma-treated Co(OH)x, the electrode was performed on CV tests for 1000 cycles in 1 M KOH solution with a scan rate of 100 mV s−1. 2.3. Characterization. X-ray photoelectron spectroscopy (PHI 5600, spectrometer, USA) using Al Kα monochromatized radiation was conducted to characterize the chemical element. X-ray diffraction (X’pert Philips MPD with a Panalytical X’celerator detector, equipped with graphite monochromized Cu Kα radiation) was performed to investigate the crystal structure. The UV/vis spectrometer (Lambda 950) using an integrating sphere was used to record the optical properties. The electron paramagnetic resonance (EPR) spectra were recorded using a JES-FA200 spectrometer at low temperature (−8 °C). A field-emission scanning electrode microscope (FE-SEM, Hitachi S-4800) and highresolution transmission electron microscopy (HRTEM, EI Tecnai G2 20 S-TWIN) were performed to characterize the morphology and nanostructure.

3. RESULTS AND DISCUSSION The self-organized Ta2O5 NTs layers from Ta foils are formed by chemical anodization in a H2SO4-based electrolyte (see Experimental Section for details).25 These layers (Figure S1) are comprised of tubes with ∼120 nm diameter and ∼11 μm length. The Ta2O5 NT samples were then converted to Ta3N5 NTs by NH3 annealing at 1000 °C. After nitridation, the diameter and length of Ta3N5 NTs decrease to ∼100 nm and ∼8 μm, respectively, owing to the volume decrease in the 10566

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For the electronic properties of the tubes after modification to be investigated, the impedance measurement was carried out. The results show that Ar-plasma treatment has a great influence on the electronic quality, leading to a decrease in the charge-carrier-transfer resistance to the electrolyte. To an extent, it can be attributed to amorphization of Ta3N5 structure caused by Ar-plasma etching, which strongly affects the tubes in view of the electrical property (Figure 2c and Table S1). Furthermore, for the Co(OH)x(A)/Ta3N5(A) electrode, Arplasma treatment leads to the lowest charge resistance (79.02 Ω), which is ascribed to the defects of cocatalyst and incorporation of oxygen vacancies in Co(OH)x (Figure S6). Despite the improved PEC performances and electrochemical properties, no obvious change is observed on the UV−visible (UV−vis) absorption spectra of corresponding electrodes as observed in Figures S7 and S8. For understanding the structural change of the NTs induced by Ar-plasma, X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used. As shown in Figure 3a, XRD patterns of the tube layers show the Ta3N5 and Ta2N phases. The Ta2N phase is composed of nitridation of the interlayer between the NT layer and Ta substrate that was initially formed during the anodization.13 After Ar-plasma treatment, the intensity of Ta3N5 peak is decreased clearly, indicating the reduction of length of structure coherence. It is attributed to the amorphization

Figure 2. (a) Linear-sweep voltammogram (LSV) curves of Ta3N5 NTs, Ar-plasma-treated Ta3N5 NTs (Ta3N5(A)), Co(OH)x/Ta3N5, Co(OH)x/Ta3N5(A), and Ar-plasma-treated Co(OH)x/Ta3N5(A) NTs (Co(OH)x(A)/Ta3N5(A)) in 1 M KOH under AM 1.5 G (100 mW cm−2) illumination. Line, in light; short dot, in dark. (b) Incident photon-current conversion efficiencies (IPCEs) and (c) electrochemical impedance spectroscopy (EIS) of corresponding samples.

and S5. All the Ar-plasma-treated NTs exhibit enhanced photocurrents, which strongly suggest that the photogenerated holes in the tubes can reach plasma zone: the defects interface and react there to form O2. Figure 2b shows the solar photocurrents of electrodes without and with Ar-plasma treatment calculated by incident photon-current conversion efficiencies (IPCEs). Over the range from 400 to 600 nm, Ar plasma treatment for the electrodes without and with Co(OH)x modification leads to the enhanced IPCEs compared to those of the nontreated electrodes. The Co(OH)x(A)/ Ta3N5(A) photoanode exhibits a significantly enhanced IPCE value: 70.4% at 400 nm. The increase in the photoresponse after Ar-plasma etching is attributed to the change in surface physical structure and energetics after modification of surface chemical structure.18

Figure 3. (a) XRD patterns; (b) Raman spectra of Ta3N5 NTs, Arplasma-treated Ta3N5 NTs (Ta3N5(A)), Co(OH)x/Ta3N5, Co(OH)x/Ta3N5(A), and Ar-treated Co(OH)x/Ta3N5(A) NTs (Co(OH)x(A)/Ta3N5(A)); (c−f) high-resolution XPS of (c) Ta 4p, (d) N 1s, (e) O 1s, and (f) Co 2p spectra of corresponding samples. 10567

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fitted to Co3+ (781.2 eV) and Co2+ (782.5 eV) to probe the electronic states of Co atoms. From Figure 3f, the atomic ratio of Co2+/Co3+ on Co(OH)x(A)/Ta3N5(A) (0.86) is higher than that of Co(OH)x/Ta3N5(A) (0.62) by comparing the area under the fitted curve (Table S2), indicating more Co2+ concentration present in the plasma-treated Co(OH) x accompanied by more surface oxygen vacancies induced by Ar plasma. The oxygen vacancies can be further vertified by the O 1s spectra (Figure 3e). The O 1s spectrum in Co(OH)x/ Ta3N5(A) is fitted with three peaks at 532.3, 531.6, and 530.6 eV. The peaks at 532.3 eV and 530.6 eV are associated with hydroxyl (Oabs) and oxide species (Olat), respectively, whereas the peak at 531.6 eV is attributed to the defect sites with oxygen vacancies (Ovan). The atomic ratio of Ovan/Oabs(OOH) on the Co(OH)x(A)/Ta3N5(A) (1.50) is higher than that of Co(OH)x/Ta3N5(A) (0.83) (Table S3). After plasma-treated Co(OH)x, the peak at 530.6 eV exhibits a strong signal, which is attributed to the reduction of Co(OH)x from surface oxygen defect species.18 The XPS spectra of Co 2p and O 1s regions thus confirm that Co3+ is partially reduced to Co2+ after etching treatment, producing more oxygen vacancies. Meanwhile, the oxygen vacancies induced by the plasma-etching in Co(OH)x possibly improve the electronic conductivity and generate more electrochemically active sites to further enhance the electrocatalytic activity of cocatalysts and PEC performance of Ta3N5 photoanodes. Overall, the TEM, Raman, and XPS results suggest that a mainly structural effect of Ar-plasma is the creation of active states. Such a feature has been increasingly recognized as a key factor for establishing a highly efficient Ta3N5-based photoanode system. The N2 sorption isotherms (Figure 4a) were conducted for the bulk Co(OH)x and Co(OH)x(A). The Brunauer− Emmett−Teller (BET) surface area of Ar-plasma Co(OH)x (7.62 m2 g−1) is higher than that of pristine Co(OH)x (5.81 m2 g−1). The relative electrochemical surface areas (ECSA) of Co(OH)x and Co(OH)x(A) were determined from the capacitance regions of cyclic voltammetry as shown in Figure 4b and Figure S9. The Co(OH)x(A) sample has 1.08-times the surface area of that of the Co(OH)x sample. These results demonstrate that the plasma treatment increases the surface area of Co(OH)x, resulting in abundant active sites of etched cocatalyst for PEC activity. Furthermore, in the electrocatalytic activity, the onset potential for the Co(OH)x(A) electrode shifts to the negative direction compared to that of the pristine one (Figure S10a), confirming the better electrocatalytic performance for OER on Ar-plasma-treated Co(OH)x. The stability of Co(OH)x(A) was performed as shown in Figure S10b. There is no obvious decay of the activity based on the polarization curves after 1000 cycles. These results are consistent with the literature reported by Wang et al.,18 suggesting better stability in the long term for the Ar plasma treatment. For the effect of oxygen vacancy on the electronic structure of Co(OH)x to be clarified further, electron paramagnetic resonance (EPR) spectroscopy, which is highly sensitive to orbital electron distribution in materials, was examined as shown in Figure 4c. The local atomic and electronic structure of Co is significantly influenced by the oxygen vacancies via the electron extraction effect compared to that of nontreated Co(OH)x owing to the disordered Co local structure. Connected with the XPS results (Figure 3f), the Co(OH)x exhibits an axial EPR signal with g = 2.06 arising from the Co3+,35 whereas for the etched Co(OH)x, a large Co 2+ feature becomes more visible accompanying an

and/or discretization of crystallites.27 Additionally, the Co(OH)x(A)/Ta3N5(A) sample shows a similar intensity of peak compared to that of Ta3N5(A). Raman spectroscopy, which is more sensitive in distinguishing the crystal phase by analysis of lattice dynamical properties,28−30 was used to further confirm the phase composition of the corresponding samples. From Raman spectrum of the plain Ta3N5 NTs (Figure 3b), typical Ta3N5 Raman signal can be observed. The Ar-plasma-treated Ta3N5 shows a slight intensity decrease in the position of the Raman band, which is in line with amorphization.13 Moreover, the Co(OH) x (or Co(OH) x (A))-treated Ta 3 N 5 (or Ta3N5(A)) NT samples exhibit an obviously positive shift compared to the pristine NTs. This shift is possibly due to a phonon confinement effect by the surface decoration.31 For further detecting the surface chemical composition in electrodes, XPS was carried out on the pristine and plasmatreated samples. The Ta 4p, N 1s, O 1s, and Co 2p XPS spectra of corresponding samples are given in Figure 3c−f. In Figure 3c and d, the binding energies for Ta 4f7/2 and 3f5/2 and N 1s are determined for all the Ta3N5 NTs without and with Ar-plasma treatment (without and with Co(OH)x). After Arplasma treatment on the Ta3N5, the results are close to those in the literature reported by Hashimoto32 and Li et al.15 A shift (0.1 eV) to higher binding energy of Ta 4f in Ta3N5(A) is observed, indicating the lower defect densities existing on the surface of Ta3N5(A),15,33 and meanwhile, a shoulder peak at 28.08 eV appears associated with the O−Ta−N peak.17 O 1s is obtained for the Ta3N5 sample (Figure 3e), which is due to the TaO layer from the outer surface. After plasma-etching, interestingly, the O 1s peak exhibits an increased intensity compared to that of the pristine sample. The layer of TaO on Ta3N5 should be removed after etching. However, a possible source of oxide species can be detected at the surface of the plasma-treated sample. The initial oxide film has been removed, and the pristine Ta3N5 would react with oxygencontaining reactants in the gas phase. Concerning this possibility, the oxygenated compound exists in the gas phase of the treatment chamber. This needs to be further clarified and investigated. After further decoration with polycrystalline Co(OH)x on Ta3N5(A), the Co(OH)x/Ta3N5(A) sample is shifted to a higher binding energy (0.4 eV) in Ta 4f (Figure 3c) and N 1s (Figure 3d), which is consistent with previous reports.13 Further etching of Co(OH)x leads to the opposite direction shift in binding energy of Ta 4f and N 1s. This means that cocatalyst decoration or plasma-etching cocatalyst would induce the change in the electronic properties of Ta 4f and N 1s. The Co 2p XPS spectra of Co(OH)x/Ta3N5(A) and Co(OH)x(A)/Ta3N5(A) are given in Figure 3f. The peaks of Co 2p3/2 and Co 2p1/2 are located at 781.5 and 797.3 eV, respectively. For the Co(OH)x/Ta3N5(A), the Co 2p XPS spectrum exhibits two satellite peaks centered at 786.5 and 803.2 eV, which is attributed to the Co2+ oxidation state. In the case of Co(OH)x(A)/Ta3N5(A), it exhibits an increased intensity of Co 2p satellite peaks compared to that of Co(OH)x/Ta3N5(A), mentioning that a portion of Co3+ ions is reduced to Co2+ in terms of generating oxygen vacancies.18,20 On the other hand, the shift of Co 2p1/2 peak toward higher binding energy direction also indicates the reduction of Co3+ to Co2+; it has strong electron-transport performance and relatively rich oxygen vacancies for Co(OH)x(A)/Ta3N5(A), which are consistent with the literature.34 For the surface properties to be explored further, the Co 2p3/2 spectra were 10568

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Co(OH)x leads to an increased ratio of Co2+/Co3+ with more oxygen vacancies (Figure 3e and f). On the other hand, from the cyclic voltammetry curve of Co(OH)x (Figure 5a), the

Figure 5. (a) Cyclic voltammetry curve of Co(OH)x; (b) photocurrent densities at 1.23 VRHE with various scan cycles of Co(OH)x/ Ta3N5 NTs; (c,d) LSV curves of Co(OH)x/Ta3N5 NTs. The NT sample was scanned from first to fourth cycle with a scan rate of 10 mV sec−1.

voltammogram in the potential from 0.7 to 1.6 VRHE shows a pair of current peaks; peak I is due to the oxidation of Co(OH)2 to CoOOH, and peak II is ascribed to the reduction of CoOOH to Co(OH)2.36 According to this, the decay in the photoresponse (photocorrosion) for the Co(OH)x/Ta3N5 electrode can be partially ascribed to the decreased ratio of Co2+/Co3+ in Co(OH)x cocatalyst (Figure S12). One kind of Ta3N5 NT with a thick initial layer on top of NTs,13a which is different from the open NTs in this work, was prepared. The nontreated Co(OH)2/Ta3N5 NTs electrode was scanned from 0.6 to 1.6 VRHE for several cycles. Interestingly, in the first cycle, the photoanode shows a relatively lower photoresponse with a high spike; upon further scanning from the second to fourth cycle, it exhibits a gradually increased photoresponse, 1.34 (1st) to 3.50 mA cm−2(4th) at 1.23 VRHE (Figure 5b and c). The photocurrent transient ratio value (isteady/iinitial) is increased from 0.43 (1st) to 0.85 (4th) (Figure 5d). The shape of the photocurrent transient reflects on the density and energy of trapping states, that is, the time delay of carriers at the interface.37 Wang and co-workers17 thought of a unique formation of Ta−O−Co bonds between Ta3N5 NTs and Co(OH)2. The new Ta−O−Co interface allowed a remarkable

Figure 4. (a) N2 sorption isotherms, (b) comparison studies of capacitive current density as a function of scan rate, and (c) EPR spectra of Co(OH) x and Ar-plasma-treated Co(OH)x (Co(OH)x(A)).

associated decrease in the intensity of g = 2.06. Indeed, this decrease indicates the reduction of Co3+ to Co2+. Above all, serious recombination of the heterojunction (Ta3N5/TaO) undergoes a random charge accumulation on the interface, and the formation of a depletion layer inhibits charge transfer. The introduction of Ar-plasma could remove this depletion layer and lead to low defect densities on the surface of Ta3N5 NTs, generating a fast charge-transport pathway (Figure S11).15 Furthermore, Co(OH)x cocatalyst has a mixture of Co2+/Co3+, and the relative population of Co2+ and Co3+ is correlated to the oxygen vacancies. Etching 10569

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interfacial contact area, improving the electrocatalytic activity of cocatalyst (Figure S10) and charge transfer of photoanodes (Figure 2c). Herein, the plasma-induced etching strategy not only changes the electronic structure of Ta3N5, but also leads to the formation of oxygen vacancies on Co(OH)x. The synergistic effect of depletion of recombination center on the interface and high oxygen vacancies of Co(OH)x result in superior PEC performances of Ta3N5 photoanodes.

increase in the photovoltage due to the improved charge separation at the semiconductor/liquid interface along with an improvement in the time-course stability of the photoanode by hindering the bond formation of Ta−O−Ta. However, we also ascribe one of the reasons according to a previous study.38 Upon scanning from 0.6 to 1.6 VRHE, Co(OH)x cocatalyst in the initial layer would oxidize to CoOOH from Co(OH)2, with more Co3+ covering on the top surface; then, the Co2+ from the inside part would move to the outer side to balance the Co2+/Co3+. Upon a second scanning, Co2+ would lead to more holes transferring to the interface for oxidation reactions. Up to the fourth cycle, the Co2+ ions are consumed, and finally, more Co3+ ions are accumulated on the surface, leading to the following decay of photoresponse (Figure S13). This finding further confirms that the increased ratio of Co2+/Co3+ in etched Co(OH)x cocatalyst leads to a significantly enhanced solar water splitting performance. The effect of oxygen vacancies in this work was further clarified in Co(OH)x cocatalyst for PEC performance (Figure 6). The Co(OH)x(A)/Ta3N5(A) electrode was treated under

4. CONCLUSIONS In summary, the present work demonstrates that a high-energy Ar-plasma treatment in Ta3N5 NTs can decrease the recombination center between the semiconductor and the electrolyte. Further plasma-etched Co(OH)x cocatalyst on Ta3N5 NTs could produce more oxygen vacancies in cocatalyst, significantly promoting the charge separation and transfer. The increased active sites and multivacancies synergistically result in enhanced PEC activities. Compared to the pristine Ta3N5 NTs, such etched NTs photoanode exhibits a 10-fold increase in the photocurrent under AM 1.5G sunlight condition. Furthermore, the resulting Ta3N5 NTs after etched Co(OH)x decoration reaches 7.2 mA cm−2 at 1.23 VRHE. This work provides a novel strategy to etch Ta3N5 and produce oxygen vacancies on cocatalysts simultaneously as a highly efficient photoanode configuration in solar water splitting systems.

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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/acscatal.8b03111.

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Figure 6. LSV curves of Ar-plasma-treated Co(OH)x/Ta3N5(A), O2plasma-treated Co(OH) x /Ta 3 N 5 (A), O 2 -plasma-treated Co(OH) x (A)/Ta 3 N 5 (A), and Ar-plasma-treated Co(OH) x (O)/ Ta3N5(A). The Co(OH)x/Ta3N5(A) samples were treated under O2 or Ar plasma conditions for 5 min.

Additional SEM, XRD, Raman, XPS data, LSV curves, and CV curves and data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lei Wang: 0000-0001-7449-2763 Notes

O2-plasma conditions, and the photocurrent was decreased from 7.2 to 5.8 mA cm−2 at 1.23 VRHE. The same phenomenon was also observed using O2-plasma on Co(OH)x in which the photocurrent of Co(OH)x(O)/Ta3N5(A) is remarkably reduced to 0.35 mA cm−2. The decrease in photoresponse is due to the filling oxygen vacancies, leading to a decreased ratio of Co2+/Co3+ (Figure S14). On the contrary, as the Co(OH)x(O) cocatalyst was further treated in Ar plasma, it shows a remarkable enhancement in photocurrent, reaching 4.4 mA cm−2 at 1.23 VRHE. According to literature reports, Wang et al.18 performed the plasma-engraved Co3O4 nanosheets with oxygen vacancies through proper control of the Co2+/Co3+ ratio. The engraved Co3O4 exhibited a higher current density and lower onset potential with a high ratio of Co2+/Co3+. Liu et al.39 confirmed that Co2+ was the active site for OER, promoting the formation of cobalt oxyhydroxide as active sites for OER. Therefore, in this work, the high PEC activity of Co(OH)x(A)/Ta3N5(A) could be attributed to high Co2+ population with the active sites for OER. The relatively higher surface area (Figure 4a) and electrochemical surface area (Figure 4b) lead to the higher electrode/electrolyte

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by National Natural Science Foundation of China (51802320) and a start-up funding from Lanzhou Institute of Chemical Physics (LICP).

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REFERENCES

(1) Zhang, P.; Zhang, J.; Gong, J. Tantalum-based Semiconductors for Solar Water Splitting. Chem. Soc. Rev. 2014, 43, 4395−4442. (2) Kaneko, H.; Minegishi, T.; Domen, K. Recent Progress in the Surface Modification of Photoelectrodes toward Efficient and Stable Overall Water Splitting. Chem. - Eur. J. 2018, 24, 5697−5706. (3) Hou, J.; Wu, Y.; Cao, S.; Liang, F.; Lin, Z.; Gao, Z.; Sun, L. In Situ Phase-Induced Spatial Charge Separation in Core-Shell Oxynitride Nanocube Heterojunctions Realizing Robust Solar Water Splitting. Adv. Energy Mater. 2017, 7, 1700171. (4) Narkeviciute, I.; Chakthranont, P.; Mackus, A. J. M.; Hahn, C.; Pinaud, B. A.; Bent, S. F.; Jaramillo, T. F. Tandem Core-Shell SiTa3N5 Photoanodes for Photoelectrochemical Water Splitting. Nano Lett. 2016, 16, 7565−7572.

10570

DOI: 10.1021/acscatal.8b03111 ACS Catal. 2018, 8, 10564−10572

Research Article

ACS Catalysis (5) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (6) Zhang, P.; Wang, T.; Zhang, J.; Chang, X.; Gong, J. Bridging the Transport Pathway of Charge Carriers in a Ta3N5 Nanotube Array Photoanode for Solar Water Splitting. Nanoscale 2015, 7, 13153− 13158. (7) Hajibabaei, H.; Zandi, O.; Hamann, T. W. Tantalum Nitride Films Integrated with Transparent Conductive Oxide Substrates via Atomic Layer Deposition for Photoelectrochemical Water Splitting. Chem. Sci. 2016, 7, 6760−6767. (8) Wang, C.; Hisatomi, T.; Minegishi, T.; Nakabayashi, M.; Shibata, N.; Katayama, M.; Domen, K. Thin Film Transfer for the Fabrication of Tantalum Nitride Photoelectrodes with Controllable Layered Structures for Water Splitting. Chem. Sci. 2016, 7, 5821−5826. (9) Seo, J.; Takata, T.; Nakabayashi, M.; Hisatomi, T.; Shibata, N.; Minegishi, T.; Domen, K. Mg-Zr Cosubstituted Ta3N5 Photoanode for Lower-Onset-Potential Solar-Driven Photoelectrochemical Water Splitting. J. Am. Chem. Soc. 2015, 137, 12780−12783. (10) Liu, G.; Shi, J.; Zhang, F.; Chen, Z.; Han, J.; Ding, C.; Chen, S.; Wang, Z.; Han, H.; Li, C. A Tantalum Nitride Photoanode Modified with a Hole-Storage Layer for Highly Stable Solar Water Splitting. Angew. Chem., Int. Ed. 2014, 53, 7295−7299. (11) (a) Wang, L.; Dionigi, F.; Nguyen, N. T.; Kirchgeorg, R.; Gliech, M.; Grigorescu, S.; Strasser, P.; Schmuki, P. Tantalum Nitride Nanorod Arrays: Introducing Ni-Fe Layered Double Hydroxides as a Cocatalyst Strongly Stabilizing Photoanodes in Water Splitting. Chem. Mater. 2015, 27, 2360−2366. (b) Haleem, A. A.; Majumer, S.; Perumandla, N.; Zahran, Z. N.; Naruta, Y. Enhanced Performance of Pristine Ta3N5 Photoanodes for Solar Water Splitting by Modification with Fe-Ni-Co Mixed-Metal Oxide Cocatalysts. J. Phys. Chem. C 2017, 121, 20093−20100. (12) Li, M.; Luo, W.; Cao, D.; Zhao, X.; Li, Z.; Yu, T.; Zou, Z. A Cocatalyst-Loaded Ta3N5 Photoanode with a High Solar Photocurrent for Water Splitting upon Facile Removal of the Surface Layer. Angew. Chem., Int. Ed. 2013, 52, 11016−11020. (13) (a) Wang, L.; Nguyen, N. T.; Zhou, X.; Hwang, I.; Killian, M. S.; Schmuki, P. Enhanced Charge Transport in Tantalum Nitride Nanotube Photoanodes for Solar Water Splitting. ChemSusChem 2015, 8, 2615−2620. (b) Wang, L.; Zhou, X.; Nguyen, N. T.; Hwang, I.; Schmuki, P. Strongly Enhanced Water Splitting Performance of Ta3N5 Nanotube Photoanodes with Subnitrides. Adv. Mater. 2016, 28, 2432−2438. (14) (a) Li, Y.; Takata, T.; Cha, D.; Takanabe, K.; Minegishi, T.; Kubota, J.; Domen, K. Vertically Aligned Ta3N5 Nanorod Arrays for Solar-Driven Photoelectrochemical Water Splitting. Adv. Mater. 2017, 25, 125−131. (b) Li, Y.; Zhang, L.; Torres-Pardo, A. J.; GonzálezCalbet, M.; Ma, Y.; Oleynikov, P.; Terasaki, O.; Asahina, S.; Shima, M.; Cha, D.; Zhao, L.; Takanabe, K.; Kubota, J.; Domen, K. Cobalt Phosphate-modified Barium-doped Tantalum Nitride Nanorod Photoanode with 1.5% Solar Energy Conversion Efficiency. Nat. Commun. 2013, 4, 2566. (15) Liu, G.; Ye, S.; Yan, P.; Xiong, F.; Fu, P.; Wang, Z.; Chen, Z.; Shi, J.; Li, C. Enabling an Integrated Tantalum Nitride Photoanode to Approach the Theoretical Photocurrent Limit for Solar Water Splitting. Energy Environ. Sci. 2016, 9, 1327−1334. (16) Zhong, M.; Hisatomi, T.; Sasaki, Y.; Suzuki, S.; Teshima, K.; Nakabayashi, M.; Shibata, N.; Nishiyama, H.; Katayama, M.; Yamada, T.; Domen, K. Highly Active GaN-Stabilized Ta3N5 Thin-Film Photoanode for Solar Water Oxidation. Angew. Chem., Int. Ed. 2017, 56, 4739−4743. (17) He, Y.; Ma, P.; Zhu, S.; Liu, M.; Dong, Q.; Espano, J.; Yao, X.; Wang, D. Photo-Induced Performance Enhancement of Tantalum Nitride for Solar Water Oxidation. Joule 2017, 1, 831−842. (18) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem. 2016, 128, 5363−5367.

(19) Gao, R.; Li, Z.; Zhang, X.; Zhang, J.; Hu, Z.; Liu, X. CarbonDotted Defective CoO with Oxygen Vacancies: A Synergetic Design of Bifunctional Cathode Catalyst for Li-O2 Batteries. ACS Catal. 2016, 6, 400−406. (20) Wang, Y. C.; Zhou, T.; Jiang, K.; Da, P. M.; Peng, Z.; Tang, J.; Kong, B. A.; Cai, W. B.; Yang, Z. Q.; Zheng, G. F. Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes. Adv. Energy Mater. 2014, 4, 1400696. (21) Wang, Y.; Xie, C.; Zhang, Z.; Liu, D.; Chen, R.; Wang, S. In Situ Exfoliated, N-doped, and Edge-Rich Ultrathin Layered Double Hydroxides Nanosheets for Oxygen Evolution Reaction. Adv. Funct. Mater. 2018, 28, 1703363. (22) Liu, R.; Wang, Y.; Liu, D.; Zou, Y.; Wang, S. Water-PlasmaEnabled Exfoliation of Ultrathin Layered Double Hydroxide Nanosheets with Multivacancies for Water Oxidation. Adv. Mater. 2017, 29, 1701546. (23) Ma, L.; Chen, S.; Pei, Z.; Li, H.; Wang, Z.; Liu, Z.; Tang, Z.; Zapien, J. A.; Zhi, C. Flexible Waterproof Rechargeable Hrbrid Zinc Batteries Initiated by Multifunctional Oxygen Vacancies-Rich Coblat Oxide. ACS Nano 2018, 12, 8597−8605. (24) Zhu, C.; Li, C.; Zheng, M.; Delaunay, J.-J. Plasma-Induced Oxygen Vacancies in Ultrathin Hematite Nanoflakes Promoting Photoelectrochemcial Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 22355−22363. (25) (a) Wang, L.; Mazare, A.; Hwang, I.; Schmuki, P. Tantalum Nitride Nanotube Photoanodes: Establishing a Beneficial Backcontact by Lift-off and Transfer to Titanium Nitride Layer. Electrochem. Commun. 2016, 72, 27−31. (b) Wang, L.; Mazare, A.; Hwang, I.; So, S.; Nguyen, N. T.; Schmuki, P. Synthesis of Freestanding Ta3N5 Nanotube Membranes and Flow-through Visible Light Photocatalytic Applications. Chem. Commun. 2017, 53, 11763− 11766. (26) Nurlaela, E.; Ould-Chikh, S.; Harb, M.; del Gobbo, S.; Aouine, M.; Puzenat, E.; Sautet, P.; Domen, K.; Basset, J.-M.; Takanabe, K. Critical Role of the Semiconductor-Electrolyte Interface in Photocatalytic Performance for Water-Splitting Reactions Using Ta3N5 Particles. Chem. Mater. 2014, 26, 4812−4825. (27) Liu, N.; Häublein, V.; Zhou, X.; Venkatesan, U.; Hartmann, M.; Mackovic, M.; Nakajima, T.; Spiecker, E.; Osvet, A.; Frey, L.; Schmuki, P. Black” TiO2 Nanotubes Formed by High-Energy Proton Implantation Show Noble-Metal-co-Catalyst Free Photocatalytic H2Evolution. Nano Lett. 2015, 15, 6815−6820. (28) Yuliati, L.; Yang, J.-H.; Wang, X.; Maeda, K.; Takata, T.; Antonietti, M.; Domen, K. Highly Active Tantalum(V) Nitride Nanoparticles Prepared from a Mesoporous Carbon Nitride Template for Photocatalytic Hydrogen Evolution under Visible Light Irradiation. J. Mater. Chem. 2010, 20, 4295−4298. (29) Tenne, D. A.; Soukiassian, A.; Xi, X. X.; Choosuwan, H.; Guo, R.; Bhalla, A. S. Lattice Dynamics in BaxSr1‑xTiO3 Thin Films Studied by Raman Spectroscopy. J. Appl. Phys. 2004, 96, 6597. (30) Fu, G.; Yan, S.; Yu, T.; Zou, Z. Oxygen Related Recombination Defects in Ta3N5 Water Splitting Photoanode. Appl. Phys. Lett. 2015, 107, 171902. (31) Xu, C. Y.; Zhang, P. X.; Yan, L. Blue Shift of Raman Peak from Coated TiO2 Nanoparticles. J. Raman Spectrosc. 2001, 32, 862−865. (32) Hashimoto, S.; Tanaka, C.; Murata, A. Formulation for XPS Spectral Change of Oxides by Ar Ion Bombardment: Application of the Formulation to Ta2O5 System. J. Surf. Analysis 2006, 13, 14−18. (33) Arshi, N.; Lu, J.; Joo, Y. K.; Yoon, J. H.; Koo, B. H. Effects of nitrogen composition on the resistivity of reactively sputtered TaN thin films. Surf. Interface Anal. 2015, 47, 154−160. (34) Zhang, J.-J.; Wang, H.-H.; Zhao, T.-J.; Zhang, K.-X.; Wei, X.; Jiang, Z.-D.; Hirano, S.-I.; Li, X.-H.; Chen, J.-S. Oxygen Vacancy Engineering of Co3O4 Nanocrystals through Coupling with Metal Support for Water Oxidation. ChemSusChem 2017, 10, 2875−2879. (35) McAlpin, J. G.; Surendranath, Y.; Dincǎ, M.; Stich, T. A.; Stoian, S. A.; Casey, W. H.; Nocera, D. G.; Britt, R. D. EPR Evidence 10571

DOI: 10.1021/acscatal.8b03111 ACS Catal. 2018, 8, 10564−10572

Research Article

ACS Catalysis for Co(IV) Species Produced During Water Oxidation at Neutral pH. J. Am. Chem. Soc. 2010, 132, 6882−6883. (36) Elumalai, P.; Vasan, H. N.; Munichandraiah, N. Electrochemical Studies of Cobalt Hydroxide-an Additive for Nickel Electrodes. J. Power Sources 2001, 93, 201−208. (37) (a) Picos-Vega, A.; Ramirez-Bon, R. Simulation of Transient Transport in Amorphous Multilayers. J. Non-Cryst. Solids 2002, 299− 302, 439−443. (b) Wang, L.; Lee, C. Y.; Kirchgeorg, R.; Hildebrand, H.; Müller, J.; Spiecker, E.; Schmuki, P. A Significant Cathodic Shift in the Onset Potential of Photoelectrochemical Water Splitting for Heamtite Nanostructures Grown from Fe-Si Alloys. Mater. Horiz. 2014, 1, 344−347. (38) Grigorescu, S.; So, S.; Yoo, J. E.; Mazare, A.; Hahn, R.; Schmuki, P. Open Top Anodic Ta3N5 Nanotubes for Higher Solar Water Splitting Efficiency. Electrochim. Acta 2015, 182, 803−808. (39) Wang, H.-Y.; Hung, S. F.; Chen, H.-Y.; Chan, T.-S.; Chen, H. M.; Liu, B. In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36−39.

10572

DOI: 10.1021/acscatal.8b03111 ACS Catal. 2018, 8, 10564−10572