Tandem Core–Shell Si–Ta3N5 Photoanodes for ... - ACS Publications

Nov 22, 2016 - Ieva Narkeviciute and Thomas F. Jaramillo ... Pongkarn Chakthranont , Thomas R. Hellstern , Joshua M. McEnaney , Thomas F. Jaramillo...
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Tandem Core−Shell Si−Ta3N5 Photoanodes for Photoelectrochemical Water Splitting Ieva Narkeviciute,† Pongkarn Chakthranont,† Adriaan J. M. Mackus,† Christopher Hahn,‡,§ Blaise A. Pinaud,† Stacey F. Bent,† and Thomas F. Jaramillo*,†,‡,§ †

Department of Chemical Engineering and ‡SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94035, United States § SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park. California 94025, United States S Supporting Information *

ABSTRACT: Nanostructured core−shell Si−Ta3N5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta3N5 for photoelectrochemical water splitting. The core−shell devices were fabricated by atomic layer deposition of amorphous Ta2O5 onto nanostructured Si and subsequent nitridation to crystalline Ta3N5. Nanostructuring with a thin shell of Ta3N5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta3N5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta3N5 shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiOx and NiOx water oxidation cocatalysts were deposited onto the Si−Ta3N5 to yield active photoanodes that with NiOx retained 50−60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta3N5 photoanodes reported to date. KEYWORDS: Heterojunction, heterostructure, photoelectrochemical water oxidation, photoanode, oxygen evolution reaction, charge transport

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photodegradation, causing decreasing device performance due to surface oxidation.8 Various research groups have alluded to the poor charge transport properties of Ta3N5 causing measured efficiencies to be far below the theoretical maximum through different experimental and theoretical techniques.6,7,9,10 Ziani et al. report carrier lifetimes on the order of picoseconds and mobilities of ∼1.7 cm2 V−1 s−1, which are comparable to those of hematite, a well-studied photoanode material that suffers from poor electronic properties.7,11 With short charge carrier lifetimes,7 the thickness of the absorber film becomes an important criterion to consider because thick films could prevent the complete collection of generated charges. On the other hand, thin films suffer from insufficient light absorption. In recent PEC literature on metal oxides, nanostructuring via scaffolding has been an effective strategy for extracting photogenerated carriers by minimizing the charge transport distance to the semiconductor−liquid junction, while simultaneously absorbing a large portion of the incoming light by

hotoelectrochemical (PEC) water splitting is a clean and sustainable means of capturing solar energy and converting it to a storable chemical and fuel in the form of hydrogen.1 Historically, oxide semiconductors such as TiO2, Fe2O3, and WO3 have been heavily studied as photoanodes because of their stability in highly oxidizing conditions.1 In general, however, oxide semiconductors suffer from various limitations such as large bandgaps that prevent them from absorbing a significant portion of the solar spectrum, band edge positions that do not allow for unassisted overall water splitting, and poor charge transport properties that result in late onset and/or low photocurrent.2 Transition metal nitrides are a class of materials that have the potential to mitigate several of these limitations. In particular, tantalum nitride (Ta3N5) has emerged as an attractive material for PEC water splitting due to its favorable bandgap of 2.1 eV and band edge positions that straddle both the reduction and oxidation potentials of water. Because of its band energetics, Ta3N5 can theoretically split water spontaneously as a single photoelectrode with a maximum solar-tohydrogen efficiency as high as ∼15%.3−5 Holistically, the performance of Ta3N5 is hindered by three main challenges: (1) poor electronic properties, resulting in a high rate of charge carrier recombination,6,7 (2) low photovoltage, leading to a highly anodic photocurrent onset for water oxidation,5 and (3) © XXXX American Chemical Society

Received: August 13, 2016 Revised: November 4, 2016

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Figure 1. (a) An abbreviated process flow diagram of Si−Ta3N5 electrode fabrication. A Si wafer is first nanostructured using deep reactive ion etching to make black Si. Then Ta2O5 is conformally coated onto the Si using atomic layer deposition. Finally, the Ta2O5 is nitrided in a flow of ammonia to make Ta3N5. (b) A cross-sectional scanning electron micrograph of a finished Si−Ta3N5 electrode where the darker regions are the Si core and lighter regions are the Ta3N5 shell.

passivation steps in a flow of C4F8.30 With a high flow rate of C4F8 species, the deposited fluorinated polymer acts as a micromask, which protects Si from SF6 species, allowing a homogeneously distributed needle-like structure to form across the full wafer.31,32 The black Si produced is ∼2.3−2.5 μm in length and has base diameters ranging from 100 to 300 nm, which taper off toward the tips. To make high quality, crystalline Ta3N5, a conventional thermal nitridation of Ta2O5 to Ta3N5 process was used. First, atomic layer deposition (ALD) was done to coat a uniform layer of Ta2O5 on the black Si (Figure 1a). Using ALD to coat Ta2O5 is advantageous over chemical or physical vapor depositions because it ensures a uniform, conformal coating on top of the high-aspect ratio Si.33,34 To date, there are few reports of atomic layer deposition being used to fabricate Ta3N5 photoanodes 35 despite being a promising method for fabricating integrated, tandem PEC devices with Ta3N5. Immediately prior to Ta2O5 deposition, the black Si was cleaned using a modified RCA clean36 with the oxide removal step occurring last. A Ta2O5 ALD process was developed and performed at 200 °C using pentakis(dimethylamino)-tantalum and water as precursors.37,38 Saturating, linear ALD growth was observed with a growth rate of 0.5−0.6 Å/cycle (Figure S1). ALD at these conditions yields a stoichiometric, carbon-free film of Ta2O5 as confirmed by X-ray photoelectron spectroscopy (XPS) (Figure S2). The Ta2O5 was converted to Ta3N5 in a tube furnace in anhydrous ammonia flowing at a rate of 50 sccm for 8 h at 900 °C.39 Upon nitridation, the thickness of the Ta3N5 decreases compared to the thickness of Ta2O5 as measured by cross-sectional scanning electron microscopy because Ta3N5 has a higher density than Ta2O5 (Figure 2a).9 Figure 1b contains a cross-sectional scanning electron micrograph of a Si−Ta3N5 photoanode, demonstrating that the ALD and nitridation process produces uniform and conformal thin films on the black Si. The morphology of the Ta3N5 layer is similar to that of Ta3N5 films on Ta substrates.9,40 To understand the bulk and nanoscale structure of Ta3N5, Xray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), respectively, were performed. XRD confirms that a phase-pure, orthorhombic Ta3N5 (PDF 01-079-1533) forms from the amorphous ALD Ta2O5 (Figure 2b). The peak visible at 2θ = 33° is the forbidden Si 200 reflection.41 Ta3N5 is the desired, semiconducting phase that is able to form at these moderate nitridation temperatures without forming more reduced, metallic tantalum nitride phases like Ta2N, TaN, or Ta5N6. Further reduced tantalum nitride phases can be formed at higher nitridation temperatures

maintaining long optical path lengths.1,12−15 Furthermore, to reconcile the charge transport issues for both minority and majority carriers, “core-shell” or “host-guest” architectures have been employed to efficiently extract minority carriers from the “shell” photoabsorber at the semiconductor−electrolyte interface, while also improving the majority carrier collection in the “core” material serving as a charge collector.16−24 If the core is a semiconducting material then a tandem PEC system can be constructed.17,20−22,25 Hence, because Ta3N5 is hindered by its poor electronic properties that result in a late photocurrent onset for water oxidation, it would benefit from a tandem absorber device architecture. The photodegradation of Ta3N5 is a major challenge hindering its implementation in a PEC device. Ta3 N 5 undergoes oxidation by photogenerated holes to form Ta2O5 which acts as a hole-blocking layer.8 Self-oxidation can be slowed if water oxidation is kinetically favored, which can be facilitated by an oxygen evolution reaction (OER) cocatalyst that increases reaction kinetics and acts as a hole-scavenger.26 One approach to improve long-term photoanode performance is the synthesis of an active, stable and conformal catalyst coated onto Ta3N5. In this work, we aim to address and gain deeper understanding of the challenges associated with using Ta3N5 as a photoanode by designing and fabricating devices based on a nanostructured core−shell Si−Ta3N5 heterojunction (henceforth referred to as Si−Ta3N5). To our knowledge, this is one of the few demonstrations of a tandem Ta3N5 photoanode for water oxidation.27−29 Our devices were nanostructured to maintain efficient light absorption and improve carrier extraction, which resulted in improved photocurrent compared to planar devices. Furthermore, creating a heterojunction between n-Si and Ta3N5 enables a band alignment at the solid−electrolyte junction that improves onset potential. Lastly, CoTiOx and NiOx OER cocatalysts were coated onto the Si− Ta3N5 and investigated for activity and stability for PEC water oxidation. Specifically, CoTiOx resulted in a marked improvement in OER activity because it likely interfaces well with Ta3N5, while a more conformal NiOx catalyst was excellent for improving device stability. Overall, these core−shell Si−Ta3N5 devices hold promise for fabrication of efficient future Ta3N5 photoanodes. Figure 1a depicts an abbreviated process flow diagram for synthesizing Si−Ta3N5 electrodes. First, black Si was fabricated on single crystal n-type Si wafers by “pseudo-Bosch” deep reactive ion etching (DRIE), a process consisting of alternating cycles of isotropic etching steps in a flow of SF6 and surface B

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however, have shown an improvement in Ta3N5 device performance with a NbNx interfacial layer on Ta substrates.50 Because of its poor charge carrier mobility and short carrier lifetimes, Ta3N5 has short carrier diffusion lengths compared to its light penetration depth (1/α ≈ 160 nm at λ = 540 nm),39 and thus dense thin films exhibit poor performance.9 To examine the effects of nanostructuring on Si−Ta3N5 photoanodes, we synthesized samples with the same thickness of Ta3N5 on both a flat Si wafer and a nanostructured Si wafer. We then tested for photoelectrochemical ferrocyanide (Fe(CN)64−) oxidation in a three-electrode cell using a ferri/ferrocyanide sacrificial agent as an outer-sphere redox couple to probe the performance of the semiconductor photoanode decoupled from the sluggish kinetics of water oxidation. Moreover, the facile oxidation of ferrocyanide is a competing reaction with Ta3N5 photodegradation, allowing relatively stable photocurrent to be measured.8 The reversible potential for the ferri/ferrocyanide redox couple at these conditions is +0.98 V versus RHE, which is relatively close to the reversible potential for water oxidation and should therefore yield a similar degree of band bending at the semiconductor/electrolyte interface.51 A 1000 W Xe lamp was used for simulating an AM 1.5G spectrum for λ < 590 nm (above bandgap photons). Detailed experimental methods are provided in the Supporting Information. As seen in Figure 3a, a 10 nm thick film of Ta3N5 on black Si produces an order of magnitude more current than an equivalent 10 nm film of Ta3N5 on flat Si. This improvement is considerably higher compared to similar nanostructured core−shell devices in the literature that normally achieve a 2− 5-fold improvement in photocurrent.20,21,52 The higher photocurrent for nanostructured Si−Ta3N5 can be ascribed to enhanced light absorption and greater electrochemical surface area. Light absorption is enhanced for two principal reasons: the incident optical path length is increased and reflection off of the Si surface is minimized (Figure S3) due to light trapping effects.53 Photocurrent for the nanostructured devices is also higher due to an increase in the electrode surface area that provides more electrochemically active sites for ferrocyanide oxidation to occur. The Si−Ta3N5 devices have a surface area that is approximately 15 times greater compared to their planar counterparts (Figure S4). It is also noteworthy that very thin (10−30 nm) Ta3N5 absorber layers are producing high photocurrents on these high-aspect ratio Si supports, because the nanostructure enables efficient charge extraction while maintaining adequate light absorption due to optical effects, an effect observed for other semiconductor systems on nanostructured materials.19,21 In addition to the improved charge transport and absorption properties of the deposited Ta3N5 thin films, the nanostructured Si substrate can also improve the onset of photocurrent. In a tandem core−shell photoanode, absorption is a two-photon process where one photon is absorbed by the shell and a second photon, which is transmitted through the shell material, is absorbed by the core. If the conduction band minimum (CBM) and valence band maximum (VBM) of the shell semiconductor are lower in energy than that of the CBM and VBM of the core material, then the holes generated in the shell are used for the oxygen evolution reaction, electrons generated in the core are utilized for the hydrogen evolution reaction (HER) occurring at the counter electrode, and electrons generated in the shell and holes generated in the core annihilate one another to maintain charge neutrality.21 With this band alignment the electrons generated in the core

Figure 2. (a) Ta2O5 and Ta3N5 thicknesses as a function of Ta2O5 ALD cycles. A linear growth rate of Ta2O5 is observed and a reduction in thickness upon nitridation to Ta3N5 due to the higher density of the nitride phase. (b) X-ray diffractograms of Ta2O5 as deposited (gray) and after nitridation to Ta3N5 (green) compared to the reference patterns for Ta3N5 (red) (PDF 01-079-1533) and Ta2O5 (blue) (PDF 01-071-0639). (c) HRTEM of the Si−Ta3N5 interface without the presence of detectable SiO2.

due to a more reducing hydrogen environment formed by dissociation of ammonia.39,42,43 Additionally, when Ta3N5 is synthesized on a Ta substrate, reduced phases are likely to form due to diffusion of Ta atoms into the tantalum nitride film.39 From HRTEM imaging, it appears that there is a sharp interface between Ta3N5 and Si without the detectable presence of an interlayer of SiOx or SiNx (Figure 2c). However, it is possible that a thin layer of Si is converted to SiNx at these nitridation conditions44−46 but it is difficult to differentiate between a SiNx and Ta3N5 phase in the data at the interface of Si and Ta3N5. SiNx growth is likely limited to just a few monolayers because there is a 10−70 nm film of Ta2O5 that the N has to diffuse through, and SiNx growth is itself selflimiting.45,46 It is not immediately clear whether the absence of an interlayer would be detrimental or beneficial to the device, motivating further studies. On one hand, the SiOx or SiNx could function as a passivating, recombination layer at the Ta3N5−Si interface that has been shown in other core−shell PEC systems to improve performance.24,47,48 On the other hand, a thicker interlayer could prevent the transport of holes from Ta3N5 to the electrolyte due to the valence band alignment of the SiOx or SiNx and Ta3N5.5,49 Recent results, C

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cyclic voltammetry and open-circuit potential measurements (Figure S5). The open-circuit potential measurements are further confirmation that Si−Ta3N5 is a tandem device with ntype Si because the photovoltage with n-Si is greater than that of a device with n+-Si (Figure S5a). A depiction of the band energetics for this heterojunction is shown in Figure S6. To gain a deeper understanding of charge transport within the Ta3N5 film and to ascertain what thickness would yield optimal PEC performance for the tandem Si−Ta3N5, Ta3N5 was deposited on black Si in a range of thicknesses from 10 to 70 nm and tested in the ferri/ferrocyanide reversible redox couple. Figure 3c shows the trend in photocurrent with Ta3N5 thickness: as the thickness of the Ta3N5 shell increases, the photocurrent decreases. This result can be explained by the thickness-dependence of charge transport in Ta3N5. If the thickness of the Ta3N5 is greater than the minority carrier diffusion length, and holes are generated deep within the film, fewer holes can reach the semiconductor/liquid junction due to recombination, leading to a lower photocurrent.56 Thus, the extraction efficiency of charge carriers is likely higher with the thinner Ta3N5 shells, consistent with the trends in Figure 3c. These results were further corroborated with absorbed photon to current efficiency (APCE) measurements on flat samples with a similar analysis to the one reported by Klahr et al.56 Flat Ta3N5 on n-type Si as well as Ta3N5 on quartz were synthesized in the same manner as the nanostructured samples and tested for ferrocyanide oxidation at +0.98 V versus RHE at monochromated wavelengths of 425, 475, and 525 nm. UV−vis absorptance data for Ta3N5 on quartz is included in Figure S7. The APCE measurements show that thinner samples have higher APCE and therefore better charge carrier extraction (Figure 3d). Because the APCE is about equivalent for the 10 and 30 nm thick Ta3N5 samples and then decreases with increasing film thickness, we conclude that the minority carrier diffusion length of our Ta3N5 samples is roughly tens of nanometers. While trends in bulk charge carrier transport explain the trends in APCE, surface recombination can also play a role in device performance. Surface recombination would be expected to play a more prominent role with thinner films, which may explain why the 10 nm film shows slightly lower APCE than the 30 nm film.57 We note that the APCE at λ = 425 nm is lower than at the other wavelengths, which may result from the ferri/ferrocyanide solution absorbing light at this wavelength window.58 Overall, these studies involving ferrocyanide oxidation as a sacrificial probe of photoanode performance have allowed us to demonstrate that a nanostructured, core−shell n-Si−Ta3N5 device architecture facilitates efficient charge extraction. As the Si−Ta3N5 heterostructures are ultimately designed for use as photoanodes for PEC water splitting, we evaluated their performance for driving the water oxidation reaction, under illumination. Figure 4a shows linear sweep voltammograms (LSVs) of the nanostructured, core−shell n-Si−Ta3N5 (10 nm thickness) performing the OER in 0.1 M KOH (pH 13). For comparison, Figure 4a also shows LSVs involving sacrificial ferrocyanide oxidation. There is a significant difference in both the magnitude and the onset of photocurrent between ferrocyanide and water oxidation, as may be expected, because ferrocyanide oxidation is kinetically fast and requires less of an overpotential compared to the kinetically slower process of water oxidation.12,59 Poor kinetics for the OER are further evidenced by the photocurrent transients present in the chopped LSVs for bare Si−Ta3N5. Previous photoanode studies

Figure 3. A comparison of photoelectrochemical ferrocyanide oxidation of (a) Ta3N5 on flat Si and black Si shows a 10-fold improvement with the nanostructure. (b) Ta3N5 on nanostructured nSi and n+-Si shows a more cathodic onset with n-type Si. (c) Various thicknesses of Ta3N5 on nanostructured Si show that as the Ta3N5 shell thickens, the photocurrent decreases. (d) Absorbed photon to current efficiency (APCE) for ferrocyanide oxidation of different Ta3N5 thicknesses on flat Si substrates at +0.98 V versus RHE at wavelengths of 425, 475, and 525 nm exhibits decreasing APCE with increasing Ta3N5 thickness.

are higher in energy than those generated in the shell and hence the onset for the oxygen evolution reaction occurs at a more negative potential and thereby improves the overall photovoltage of the system.21,25,52 Even with the addition of water oxidation cocatalysts, Ta3N5 shows photocurrent onset at potentials of 0.6−0.8 V versus RHE, despite its reported flat band potential of −0.1 V versus RHE.5,7,43,54 Previous work on heterojunction Ta3N5 photocatalysts using TaON have shown improvement in current but not necessarily onset potential.27 In our work, to study whether there is a voltage contribution from n-type Si we constructed devices with the same thickness of Ta3N5 on two types of Si, n-Si and n+-Si. Testing these devices for ferrocyanide oxidation showed a 200 mV improvement in the onset of photocurrent for n-Si compared to n+-Si (Figure 3b), revealing a substantial increase in photovoltage due to a tandem photoanode device structure, consistent with previous studies.21,55 We define onset to be 0.001 mA/cm2, thus the onset for a device with n-Si is ∼0.1 V versus RHE whereas a device with n+-Si onsets at ∼0.3 V versus RHE. The difference in photocurrent onset is an appropriate indicator of device photovoltage as evidenced by good agreement between D

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device degradation at the semiconductor−liquid junction.67,69 Furthermore, since NiOx is a p-type semiconductor, it could efficiently transport minority carriers from the n-type photoanode.70,71 Figure 4a shows that the CoTiOx is an effective OER cocatalyst on Ta3N5, as the photocurrent for water oxidation approaches that of ferrocyanide oxidation at higher applied potentials, indicating that the photogenerated holes are more efficiently utilized for water oxidation compared to a bare electrode.12 The onset of photocurrent with CoTiOx for water oxidation is 0.6 V versus RHE (measured at 0.2 mA/cm2), shifted by approximately 0.4 V positive of that for ferrocyanide oxidation that can be ascribed to the additional overpotential required to drive water oxidation.72 This onset of water oxidation photocurrent is comparable to most state-of-the-art Ta 3 N 5 devices that onset at roughly 0.6 V versus RHE.54,63,73−76 The photocurrent of Si−Ta3N5 coated with CoTiOx is 2.6 mA at 1.23 V versus RHE, which is approximately 50% of the photocurrent of other highperformance devices in the literature with conventional PEC OER cocatalysts.54,73,77 Water oxidation with NiOx cocatalyst had a lower photocurrent and a more positive onset potential compared to CoTiOx. On the basis of dark electrochemical data on fluorine-doped tin oxide (FTO) substrates, CoTiOx is a better OER catalyst than NiOx at the loadings used for this PEC study (Figure S8), consistent with the previously reported overpotentials for CoTiOx and NiOx upon which the catalyst syntheses in this work were based.68,69 However, as the NiOx film is subjected to photoelectrochemical cycling, Fe impurities from the electrolyte are incorporated into the film and improve the activity of NiOx by decreasing the overpotential to drive the OER.78,79 In appropriate situations, that is, when there has been prolonged testing/cycling done with NiOx catalysts, we will refer to these catalysts at Ni(Fe)Ox. With Fe incorporation into the NiOx film, the onset shifts to more negative potentials and the photocurrent increases. The photocurrent onset with Ni(Fe)Ox is 0.74 V versus RHE and the photocurrent at 1.23 V versus RHE is 2.4 mA/cm2, though for the Si−Ta3N5 system the performance of Ni(Fe)Ox never quite reaches that of CoTiOx. There are several potential reasons why CoTiOx outperforms Ni(Fe)Ox in this particular system despite the known efficacy of Ni(Fe)Ox as an OER catalyst: (1) a more favorable interface exists between the CoTiOx and Ta3N5 (such effects have been seen for other semiconductor−catalyst systems in PEC water splitting);12 (2) the NiOx coating is very thin (only 1 nm) and therefore there are fewer OER active sites than with the thicker, rougher CoTiOx; and/or (3) the surface of Ta3N5 oxidizes during the NiOx ALD process because ozone, a highly oxidizing species, is employed. If the Ta3N5 surface oxidizes during NiOx ALD, then this surface tantalum oxide would behave as an insulating, hole-blocking layer and potentially impede efficient charge transfer between the semiconductor and catalyst.5 XPS confirms that indeed the Ta3N5 coated with NiOx has TaON and Ta2O5 phases present on the surface whereas bare Ta3N5 without NiOx deposition shows only Ta3N5 and TaON phases on the surface (Figure S9).5 Despite the TaON/Ta2O5 insulating layer observed after NiOx ALD, water oxidation still occurs under illumination, suggesting that charge is still able to pass either by tunneling or via defect states in the oxide layer. Future work will focus on the optimization of NiOx depositions with less harsh conditions as well as the development of a protective interlayer between the Ta3N5

Figure 4. (a) A comparison of water oxidation photocurrent of bare Si−Ta3N5 (10 nm Ta3N5 thickness) and Si−Ta3N5 coated with CoTiOx and Ni(Fe)Ox cocatalysts to kinetically facile ferrocyanide oxidation photocurrent. The dark current measured with both catalysts was less than 0.016 mA/cm2 over the tested voltage range. (b) The 24 h chronoamperometry measurements with 1 nm of Ni(Fe)Ox cocatalyst on Si−Ta3N5 at 1.00 and 1.23 V versus RHE show a 40− 50% decrease in peak photocurrent during testing. (c) IPCE measurements elucidating that Si is electrochemically accessible to electrolyte because the IPCE at λ > 600 nm is nonzero for an applied bias of 1.23 V versus RHE.

have attributed these transients to the accumulation of holes at the semiconductor−liquid junction due to poor OER kinetics,47,60 leading to charge carrier recombination occurring at the semiconductor−electrolyte interface rather than hole transfer across the interface for the OER. The poor kinetics for water oxidation on a Ta3N5 surface can be addressed by the presence of an OER cocatalyst, as shown in several reports in which improved activity and roughly 2 h of stability were achieved using cobalt-based catalysts.54,61,62 Recently, a stability record was set for Ta3N5 with a bilayer of Ni(OH)x and MoO3 as a “hole-storage layer” that enabled oxygen production with 88% Faradaic efficiency for 24 h.63 NiOx-based cocatalyst layers have also proven to improve stability and efficiency for Si and BiVO4 photoanodes.64−67 Thus, in an effort to stabilize and improve the efficiency of the core−shell n-Si−Ta3N5 heterostructures, we utilized a similar approach, employing two different catalysts, CoTiOx and NiOx. The CoTiOx and NiOx cocatalysts were deposited on Si− Ta3N5 by dipcoating from a sol−gel and by ALD, respectively. The CoTiOx sol−gel was prepared as previously reported.68 To coat Si−Ta3N5 with ∼1 nm of NiOx by ALD, nickelocene (NiCp2) and ozone precursors were used in a similar recipe to one previously reported.69 NiOx deposition by ALD can coat nanostructured absorber layers conformally to help prevent E

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with expectations based on previous studies of n-type Si coated with NiOx, as these photoanodes exhibit an onset of photocurrent at approximately 1.2 V versus RHE.64,66 The measurements at a higher potential indicate that the Si is electrochemically accessible. We hypothesize that Si is able to independently drive photocurrent either through the highly porous nature of the Ta3N5 film that directly exposes the Si or by shunts in the Ta3N5 film in which holes generated in Si can move along the grain boundaries of Ta3N5 to the electrolyte. This Si accessibility attests to opportunities in improving the Si−Ta3N5 system. In this work, tandem core−shell Si−Ta3N5 photoanodes were designed and fabricated for photoelectrochemical water oxidation. Nanostructured Si−Ta3N5 increases electrochemical surface area and incident light absorption due to light trapping effects that together enable higher photocurrents compared to planar devices of the same thickness. Furthermore, incorporation on n-type Si enables a band alignment at the heterojunction that shifts the photocurrent onset to more negative potentials. A minority carrier diffusion length on the order of tens of nanometers was inferred for Ta3N5, based on a thickness study of the Ta3N5 shell for PEC ferrocyanide oxidation. CoTiOx and Ni(Fe)Ox cocatalysts were added to Si− Ta3N5 to study PEC water oxidation; both demonstrated efficacy for driving the oxygen evolution reaction with Ni(Fe)Ox also providing stability to the Ta3N5 for longer time periods, commensurate with the more stable Ta3N5-based photoanodes reported to date. Overall, this core−shell dualabsorber design strategy can be applied to various charge transport limited semiconductors to improve both charge extraction and photocurrent onset, while the addition of surface cocatalysts improves water splitting efficiency and durability.

and NiOx to prevent surface oxidation. Overall, both the CoTiOx and the NiOx OER catalysts were shown to be effective coatings on Si−Ta3N5 photolectrodes, improving photoelectrochemical activity. On the basis of current synthetic methods, the Ta3N5 photoanodes perform better with the CoTiOx cocatalyst than with Ni(Fe)Ox. Photoelectrode efficiency is an important performance metric for PEC water splitting, as is long-term stability, a key consideration for the future commercialization of any device. The stability of Si−Ta3N5 photoanodes coated with CoTiOx and NiOx cocatalysts were compared and despite the lower activity of Ni(Fe)Ox-coated Si−Ta 3N5, the stability is significantly higher than that of the CoTiOx-coated Si− Ta3N5. Higher stability with Ni(Fe)Ox is likely due to a more uniform coating of the catalyst on nanostructured Si−Ta3N5, a characteristic of ALD compared to the sol−gel dipcoating method used for the CoTiOx layer. The formation of a surface Ta2O5 layer during NiOx ALD, while not ideal for charge transport, could also serve to improve stability. To assess the long-term stability of Si−Ta3N5 with Ni(Fe)Ox, chronoamperometry measurements were performed under illumination at both 1.0 and 1.23 V versus RHE (Figure 4b). Over the course of 24 h of continuous chronoamperometry, the photocurrent of these photoanodes decreased by only 40−50%. This is among the longest-term stability demonstrated for Ta3N5 photoanodes to date.54,61−63,73,77 However, the device activity does decrease during this time and optimizing the thickness of the NiOx layer could lead to greater stability in the future, because it is likely for pinholes to exist in a 1 nm coating that can cause device degradation over long-term testing. Furthermore, during electrochemical testing and Fe-incorporation, NiOx forms a NiOOH,78,80 which has been shown to be ion-permeable80 and thus OH− ions in solution could contact Ta3N5 and allow for the oxidation of Ta3N5. Indeed, Ta3N5 oxidation seems to be the cause of device performance degradation during stability testing as confirmed by XPS. Analysis of the Ta 4f region shows a higher proportion of Ta3N5 to TaON on fresh samples, whereas samples that had undergone stability testing had a higher ratio of TaON compared to Ta3N5 (Figure S10). The atomic ratio of Ni to Ta was similar between fresh and tested samples, which indicates that device degradation is not due to loss of the catalyst layer. Overall, we believe NiOx ALD is a promising route for stabilizing Ta3N5 as long as the integrity of the Ta3N5 layer can be sufficiently maintained during ALD and stability testing to achieve high activity, early photocurrent onset and stability. To study whether the water oxidation photocurrent observed in the LSVs is due to light absorption in the Ta3N5 or in the underlying Si, or both, incident photon to current efficiency (IPCE) measurements were performed using monochromated light at the aforementioned applied potentials of 1.0 and 1.23 V versus RHE. NiOx-coated Si−Ta3N5 electrodes were chosen for this measurement as they are more stable. Figure 4c shows that at an applied bias of 1.0 V, there is photocurrent only at wavelengths lower than 600 nm, consistent with the absorption edge of Ta3N5 (590 nm), indicating that this observed photocurrent can be attributed to hole generation occurring only in Ta3N5. However, at an applied potential of 1.23 V versus RHE photocurrent is being generated at wavelengths higher than 600 nm, which are lower in energy than the bandgap of Ta3N5 but higher in energy than the bandgap of Si (1.12 eV), indicating some contribution from Si as a photoabsorber at those longer wavelengths. This is consistent

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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/acs.nanolett.6b03408. Experimental details of Si−Ta3N5 synthesis, physical characterization, photoelectrochemical testing and cocatalyst deposition, ALD growth curves, Ta2O5 XPS, digital photographs of devices, roughness factor measurement, band diagrams, UV−vis absorptance, electrocatalysis, and Ta3N5 XPS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stacey F. Bent: 0000-0002-1084-5336 Thomas F. Jaramillo: 0000-0001-9900-0622 Author Contributions

All authors have given approval to the final version of the manuscript. Funding

CCI Solar Fuels CHE-1305124 Funding

NWO-Rubicon 680-50-1309 Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS This work was supported by the NSF under NSF Center for Chemical Innovation CHE-1305124 for Solar Fuels. A.J.M.M. was supported by The Netherlands Organization for Scientific Research (NWO-Rubicon 680-50-1309). Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and Stanford Nanofabrication Facility (SNF) at Stanford University. The authors thank Dr. Linsey Seitz for helpful discussions regarding CoTiOx and Dr. Adam Nielander for thoughtful conversations about photoelectrochemistry.

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