The Role of Catalyst Adhesion in ALD-TiO2 Protection of Water

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

The Role of Catalyst Adhesion in ALD-TiO2 Protection of Water Splitting Silicon Anodes Robert Tang-Kong,† Roy Winter,‡ Ryan Brock,† Jared Tracy,† Moshe Eizenberg,‡ Reinhold H. Dauskardt,† and Paul C. McIntyre*,† †

Department of Materials Science and Engineering, Stanford University, Stanford 94305, California United States Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel

‡

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/25/18. For personal use only.

S Supporting Information *

ABSTRACT: Atomic layer deposited titanium dioxide (ALD-TiO2) has emerged as an effective protection layer for highly efficient semiconductor anodes which are normally unstable under the potential and pH conditions used to oxidize water in a photoelectrochemical cell. The failure modes of silicon anodes coated with an Ir/ IrOx oxygen evolution catalyst layer are investigated, and poor catalyst/substrate adhesion is found to be a key factor in failed anodes. Quantitative measurements of interfacial adhesion energy show that the addition of TiO2 significantly improves reliability of anodes, yielding an adhesion energy of 6.02 ± 0.5 J/m2, more than double the adhesion energy measured in the absence of an ALD-TiO2 protection layer. These results indicate the importance of catalyst adhesion to an interposed protection layer in promoting operational stability of high efficiency semiconducting anodes during solar-driven water splitting. KEYWORDS: adhesion, failure mode, water oxidation, isotope, atomic layer deposition

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INTRODUCTION With the increasing adoption of renewable energy in power grids worldwide, effectively harnessing solar power is a matter of critical importance. A potential solution to the problem of intermittent and geographically nonuniform insolation is solardriven production of fuels to store solar energy when the sun is not shining and to facilitate distribution of such energy using existing infrastructure for transportation of liquid fuels.1,2 Oxidation of water on a stable anode is a key component in electrochemical synthesis of fuels, as water is a convenient source of protons and electrons required for production of either hydrogen or hydrocarbon precursors (e.g., CH3OH) on a cathode.3 There are two major paradigms for such systems: (1) an integrated photoelectrochemical cell (PEC) in which a photoabsorber is electrically coupled to efficient catalysts for water oxidation and proton/CO2 reduction; and (2) separate photovoltaic and electrochemical cells (“PV+electrolyzer”) which are wired together. There is considerable debate about the relative merits of these two approaches. Analysis of the costs of solar-to-hydrogen systems have provided estimates ranging from ∼$3−10 kg−1 for integrated PEC’s4,5 and ∼$5−8 kg−1 for PV+electrolyzer systems.6,7 These projected costs are similar and the ranges are overlapping. PV+electrolyzer systems have the practical advantage of utilizing off-the-shelf technologies such as commercial silicon solar cells and polymer electrolyte membrane (PEM) electrolyzers. However, PEM electrolyzers operate at high current densities (1−2 A/cm2) and use large loadings of expensive noble metal catalysts.8 Advantages of PEC’s are (1) the potential simplicity of an © XXXX American Chemical Society

integrated design, (2) their compact foot-print, and (3) their ability to accommodate a wide variety of catalysts by operating at electrolysis current densities better matched to solar photocurrent densities. In this report, we investigate the stability of simple integrated cells using silicon photoanodes to oxidize water. Thin film titanium dioxide layers synthesized by atomic layer deposition (ALD-TiO2) has allowed for the protection of highly efficient but oxidatively unstable photoanode materials such as silicon and GaAs.9−11 Due to its stability over a wide range of pH and potentials12 TiO2 is, in principle, able to protect photoanodes in all water splitting conditions.12 Prior reports on ALD-TiO2 protection layers clearly demonstrate the improved stability of protected semiconductors compared to otherwise identical catalyst and photoanode combinations without the protection layer.9,11 Generally, failure of ALD protected photoanodes is attributed to chemical attack of the underlying substrate due to a failure of the protection layer to block oxidation, with few published works addressing other degradation pathways.9,13,14 Literature reviews on photoelectrochemical systems have suggested a variety of degradation mechanisms,15−18 but interactions at a catalyst− protection layer interface have not been studied in detail. Thus, to develop a more complete understanding of protected photoanode stability, and to identify methods for avoiding Received: August 8, 2018 Accepted: October 8, 2018

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DOI: 10.1021/acsami.8b13576 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces early failure of devices in operation, it is important to explore the catalyst−protection layer interface and the role it plays in device durability. In this work, we explore the role of water oxidation catalyst adhesion in the stability of ALD-TiO2 protected silicon anodes. We observe localized catalyst adhesion failures of protected silicon anodes under conditions in which oxidative corrosion of the silicon substrate through the overlying catalyst, TiO2 and interfacial SiO2 layers does not occur based on depth profiling analysis of oxygen tracer isotope penetration. Quantitative work of adhesion measurements indicate a significantly greater debonding energy for TiO2/Ir interfaces than for the SiO2/Ir interface which is present in the absence of the TiO2 protection layer. These results indicate the importance of catalyst adhesion to an interposed protection layer in maximizing the operational stability of high efficiency semiconducting photoanodes during water splitting.

Figure 1. Electrochemical characterization of 2 nm Ir/6 nm TiO2/2 nm SiO2/p+Si anodes in pH 7 sodium phosphate buffer used in isotope studies. Cyclic voltammograms before and after chronoamperometry (CA) testing at an applied potential of 1.1 V RHE for 6 h show clear degradation of oxygen evolution activity.

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RESULTS AND DISCUSSION Anode failure often involves many different, mutually dependent failure modes. Failure of unprotected anodes during water splitting can occur by oxidative corrosion of the semiconductor, which, in the case of silicon, results in the growth of an extremely resistive SiO2 layer.9,19 However, in some reports showing increased SiO2 layer thickness after water oxidation testing, depth profiles reveal partial or complete loss of the oxygen evolution catalyst layer.9 To first investigate the ability of ALD-TiO2 to block oxidation of an underlying silicon anode during water oxidation, oxygen isotopes were employed in the form of H218O. Isotopically labeled water was added to an aqueous phosphate buffer solution in a ratio of one part (99%) H218O to four parts electrolyte, creating a solution with an 18O concentration 2 orders of magnitude above the background (0.2%).20 This solution was used to perform electrolysis in a three-electrode measurement in a covered Teflon cone cell under chrono-amperometry conditions at an applied potential of 1.1 versus RHE. Samples driven to the point of failure (classified as “degraded” when cyclic voltammograms showed a reduction in activity of ≥50% compared to the pristine device) were investigated. Anode structures consisting of an Ir oxygen evolution catalyst layer, an ALD-TiO2 protection layer and a p+silicon substrate were used in measurements performed in the dark to investigate failure modes unrelated to irradiation. Figure 1 shows cyclic voltammograms of the isotope-exposed and control devices before and after failure. After stressing the devices to failure, the samples were examined by time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and 18O relative concentration profiles were calculated by comparing the 16O and 18O signals. (Figure 2) ToF-SIMS profiling (Figure 2) demonstrates that, under the chrono-amperometry test conditions employed, oxygen does not penetrate through the TiO2 layer. Knock-on effects make quantification of the exact depth distribution of elemental species difficult. During sputtering, atomic collisions induce mixing of the surface atoms and ultimately distort the measured depth profile. This effect was mitigated as much as possible by minimizing the primary beam energy and maximizing the beam particle mass/charge ratio.21 However, evidence of knock-on effects can still be observed in the apparent diffusion of Ir into the sample. TEM cross sectional examination of similarly treated samples does not provide evidence of interdiffusion of iridium and TiO2 after anode

Figure 2. ToF-SIMS profile showing 18O concentration throughout the 2 nm Ir/6 nm TiO2/2 nm SiO2/p+ Si sample. Thick TiO2 used to aid in resolution of layers. Both control (dark orange dotted) and enhanced (lighter solid orange) 18O profiles are shown. 18O incorporated by performing chrono-amperometry at 1.7 V for 8 h in pH 14 sodium hydroxide solution. [18O] trace represents the quantity: counts(18O)/(counts(18O)/+ counts(16O)).

fabrication and subsequent electrochemical testing. (see Supporting Information (SI)) Iridium water oxidation catalysis involves cycling among several iridium oxidation states and incorporation of oxygen to form an IrOx surface layer,22−24 which produces the high 18O concentration in the topmost portion of the sample. The first sputter cycles in a ToF-SIMS experiment often provide unreliable composition data due to a combination of surface effects such as sputter equilibration and surface deposits.25 Nevertheless, the significant incorporation of 18O during oxidation of isotopically labeled water suggests that the iridium catalyst layer is not sufficient to block oxygen from reaching underlying layers in this structure. Focusing on the TiO2 region of the depth profile, we see an increase of ∼0.05% compared to the natural isotopic abundance of 18O (0.20%) observed across most of the titanium oxide thickness. This enhancement is extremely small, and cannot be differentiated from possible sputtering beam knock-on of 18O from electrochemically formed IrOx layers. The SiO2 layer in Figure 2 originates from the vendor-supplied “chemical” oxide layer present on the as-received silicon substrate. If loss in activity were due to penetration of oxidative species through B


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affected area. This is possibly due to the initiation of flaws by the compression seal of the electrochemical cell. Evolved gas bubbles were also found to accumulate in this region during electrochemical testing, suggesting that local generation of gas bubbles might accelerate catalyst loss, possibly by cavitation erosion.27,28 Catalyst loss far from these border regions is still observed, as observed in Figures 4 and 5. Catalyst loss was confirmed on a wider scale by a simple redeposition of the iridium catalyst layer. X-ray photoelectron spectroscopy (XPS) was used to confirm both iridium loss and the recovery of iridium signal after remetallization. An iridium film of nominal 2 nm thickness was deposited by electron beam evaporation in both the original metallization and remetallization after device failure. Samples that exhibited the most significant reduction in current under anodic potentials (e.g., Figure 3) showed nearly complete catalyst loss, as confirmed by XPS (see SI). Water splitting activity was restored almost exactly to original levels after the tested anode surface was recoated with Ir/IrOx catalyst (Figure 6), suggesting that the degradation was unrelated to thickening of the SiO2 interface layer via oxidation of the underlying silicon substrate. Surface carbon concentration detected by XPS is consistent with adventitious species deposited during handling, which implies surface contamination did not significantly contribute to loss of activity. Iridium/iridium oxide is reported to be one of the most stable water oxidation catalysts available, able to operate in aqueous electrolytes across the full range of pH.9,29,30 Given that catalyst cohesive failure is unlikely, these results suggest adhesive failure at the catalyst/TiO2 interface. Understanding such early delamination events is important for optimizing the stability of any water splitting anode with a catalyst coating. Even considering the effects of cell geometry, TiO 2 protected anodes are significantly more stable than those without the ALD protection layer. Control samples without a TiO2 protection layer were found to fail within 1 h of continuous water oxidation in pH 7 phosphate buffer. As before, failure was defined as the point where cyclic voltammograms showed a reduction in activity of ≥50% compared to the pristine device. These failed control devices had no iridium present on their surfaces. In order to quantify the effect of the ALD-TiO2 layer on the catalyst/substrate adhesive strength, we used ex-situ bending adhesion energy measurements. While such tests do not perfectly replicate water splitting conditions, they can provide a useful upper bound on the adhesion of the iridium catalyst layer to the anode. Under water splitting conditions, corrosive environments may accelerate catalyst-anode contact failure and subsequent delamination, such as in stress corrosion cracking.31,32 Sample geometries for the adhesion measurements used are shown schematically in Figure 7. To measure the adhesion energy of the catalyst to the silicon substrate, the four point bend (4PB) geometry was employed. In this configuration, a crack is initiated in the top beam and opposing loads introduced to induce a constant moment. The experimental procedures are described in more detail by Birringer et al.33 Briefly, the imposed bending moment propagates a crack into the thin film stack which spreads into the weakest interface, allowing for measurement of its interfacial adhesion energy. This method proved effective for silicon samples without an ALD-TiO2, due to the very low SiO2/Ir interface strength. ALD-TiO2 protected anodes proved much more difficult to

the protection layer, this SiO2 layer should grow in thickness, which is not observed in the ToF-SIMS data (see SI). Further characterization of failed anodes suggested that the primary cause of reduced activity was catalyst loss. Iridium (oxide) catalysts are reported to sustain water oxidation for extended periods of time.26 Nevertheless, AES elemental mapping revealed losses of Ir in some failed anodes. These samples exhibited more drastic failure than the isotope study samples, showing nearly no capacity for water oxidation after stability testing (Figure 3). Figures 4 and 5 show an extreme

Figure 3. Electrochemical characterization of 2 nm Ir/2 nm TiO2/2 nm SiO2/p+Si used in Auger elemental maps. Cyclic voltammograms in pH7 sodium phosphate buffer in water after testing under chronoamperometry conditions at an applied potential of 1.15 V versus NHE for 2 h showing almost complete activity loss after stability testing (red) when compared to the initial activity (black).

Figure 4. Low magnification SEM image of electrochemically affected area showing delamination leading to catalyst loss. Largest circle represents the affected/unaffected boundary (dashed white line), with smaller patches of catalyst loss appearing inside the affected area. Sample is 2 nm Ir/2 nm TiO2/2 nm SiO2/p+ Si, tested in pH 7 phosphate buffer solution for 2 h.

example of iridium catalyst loss, with a region of the electrochemically affected area showing complete delamination of the iridium catalyst layer. Delamination events were limited to the electrochemically affected area defined by the test cell, and no catalyst loss was observed outside of this affected area. A Teflon cone cell was used to define the electrochemically affected area by pressing a 5 mm diameter, circular opening onto the sample. All electrochemistry takes place within this 0.196 cm2 area (see SI for further details). Delamination appears to be enhanced at the point of contact between the cone cell and sample, at the border of the electrochemically C


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Figure 5. Elemental map of electrochemically affected area generated by scanning Auger electron spectroscopy. (a) SEM image of examined region prior to collection of spectra. (b−d) Scanning Auger elemental maps for Ir, Ti, and O, demonstrating clear regions of iridium delamination, revealing underlying TiO2.

of fracture mode mixity,34,35 the effect of phase angle can cause 4PB tests to slightly overestimate the opening mode fracture energy, implying the SiO2/Ir adhesion energy may actually be lower than measured.36,37 The adhesion energies for the SiO2/Ir and TiO2/Ir interfaces were measured to be 2.6 ± 0.4 J/m2 and 6.0 ± 0.5 J/m2 respectively (Figure 8). Without TiO2, iridium films debond relatively easily from the underlying SiO2 surface, as expected for a noble metal/SiO2 interface. These adhesion energies rarely exceed 1 J/m2. (Cu/SiO2: 0.5−1.0 J/m2,38−40 Ni/SiO2: 0.87 J/m2,38 Pt/SiO2:1.15 J/m241). Titanium dioxide has previously been recognized as an effective adhesion layer,42,43 The measured work of adhesion, 6.02 J/m2, of the TiO2/Ir interface is significant, comparing favorably to other strong systems which have adhesion energies around 5.0 J/m2 (SiO2/TiN: 5.0 J/m2, Al/Cu: 4.3 J/m2, Cu/Cr: 5.0−5.3 J/ m2)44 In some tests with clear evidence of catalyst loss, despite losing less than 50% of the active area (Figure 4), current densities measured after stability tests steadily decreased to less than 10% of original values (Figure 3). It is possible that, while IrOx/Ir catalyst delamination occurred locally due to effects such as surface contamination of the ALD-TiO2 surface prior to OER catalyst layer deposition, operation compromised electrical contact of the TiO2/Ir interface without completely removing the catalyst layer. Furthermore, failure by delamination was found to occur most often at the edges of the active area defined by the pressure contact of the Teflon cone cell. In order to reduce such delamination events, great care must be taken to limit the population of stress concentration points to avoid initiating flaws which compromise photoanode endurance during water splitting. For the sake of simplicity this work

Figure 6. Electrochemical characterization of 2 nm Ir/2 nm TiO2/2 nm SiO2/p+Si anode used in the remetallization study. Water splitting conducted in pH 7 phosphate buffer solution in water after testing under chrono-amperometry conditions at an applied potential of 1.6 versus NHE for 3 h. Initial samples (Ir catalyst metallization 1) were tested until >50% degradation of oxygen evolution activity, after which a 2 nm thick Ir catalyst layer was redeposited onto the sample by e-beam evaporation. The same affected area was tested again (metallization 2), and showed nearly identical initial degradation behavior.

delaminate, and thus required an approach that could more precisely target the strong TiO2/Ir interface. To achieve this, a dual cantilever beam (DCB) test geometry was employed. Critically, a weak gold layer was deposited between the Ir and TiO2 layers, allowing the interface of interest to be targeted directly (Figure 7b). In both test cases, delaminated structures were examined by XPS to confirm the identity of the debonded interface. Although 4PB and DCB introduce different amounts D


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Figure 7. Sample geometries used for adhesion measurements. (a) and (b) describe the device layers and crack propagation paths for four point bend (4PB) and dual cantilever beam (DCB) experiments, respectively. Layer thicknesses are not to scale. Basic load geometries are described for (c) 4PB and (d) DCB experimental setups, with loads and crack initiation devices for both.

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EXPERIMENTAL SECTION

Silicon substrates. Heavily boron-doped (100) p-type silicon wafers (ρ = 0.001−0.002 Ω-cm, thickness 500 μm) were used as conductive silicon substrates for electrochemical measurements. The 4” diameter wafers were used as received with a 1.5−2 nm vendor chemical oxide layer, as confirmed through ellipsometry. Samples for adhesion experiments were fabricated on similar wafers (3” diameter, ρ = 0.001−0.005 Ω-cm, thickness 355−405 μm), and also used as received with a 1.5−2 nm vendor chemical oxide layer. Back Contact and Catalyst Deposition. TiO2 was deposited onto the as-received silicon substrates by ALD. Twenty nm Pt back contacts and 2 nm Ir catalyst layers were deposited by e-beam evaporation following ALD. Atomic Layer Deposition (ALD). All TiO2 films were deposited by ALD in a custom built reactor. Tetrakisdimethylamido titanium (TDMAT) was used as the titanium source and water vapor as the oxygen source. The silicon substrates were held at 170 °C during the entire deposition. The chamber details are described elsewhere.46 Electrochemical Characterization. All electrochemical measurements were made using a bored (5 mm diameter, 0.196 cm2 area) Teflon cone cell pressed against the front side of the anode to contain electrolyte and define the electrode area. A reference electrode (Ag(s) | AgCl(s) sat. KCl) and counter electrode (Pt wire, 1 mm diameter) were used, and all potentials measured using either a Biologic or WaveNow potentiostat at room temperature. Cyclic Voltammograms were recorded at a scan rate of 100 mV s−1. Stability measurements were performed using a peristaltic pump to circulate solution at 1 mL s−1. Ten mM Ferri-Ferrocyanide in 1 M KCl in water used as a nonaqueous redox couple. Water splitting was performed in either 1 M H2SO4 in water, 1 M NaOH in water, or 1 M sodium phosphate buffer (NaH2PO4/Na2HPO4). Isotope Incorporation. Heavy water (H218O, 97%) was added to standard electrolyte in a ratio of 1:4 (1 mL H218O to 5 mL electrolyte solution). This value was chosen in order to exceed environmental concentrations of heavy oxygen (0.2%) by several orders of magnitude. Auger Electron Spectroscopy (AES) Mapping. AES maps were collected using a PHI 700 Scanning Auger Nanoprobe. Electron beam settings of 10 kV, 10 nA were chosen for all maps, with a 2 point acquisition method and a resolution of 256 pixels X-ray Photoelectron Spectroscopy (XPS). XPS characterization was performed on a PHI Versaprobe III Scanning XPS Microprobe. Chamber base pressure was typically on the order of

Figure 8. Adhesion energy values for the SiO2/Ir and TiO2/Ir interfaces. SiO2/Ir work of adhesion was characterized by four-point bend, and TiO2/Ir by dual cantilever beam methods. Error bars denote the standard deviation in measured energies. Data reduction details are provided in the SI.

focuses on the behavior of dark anodes, though the effect of adhesion applies equally well to illuminated systems. It is possible that additional degradation mechanisms will become important when performing water oxidation in the light, such as photocorrosion of sensitive substrates.45 These results illustrate the importance of catalyst adhesion for the stability of semiconductor photoanodes during water oxidation. While significant attention has been given to developing stable catalysts and protecting active substrates, the interface between the two is often the first point of failure. ALD-TiO2 is not only effective as an oxidation barrier under oxygen evolution reaction conditions, but also allows for an exceptionally strong catalyst/substrate interface. The measured adhesion energy of 6.0 J/m2 surpasses some of the strongest thin film interfaces. Future efforts to prolong lifetimes of water splitting devices will inevitably require consideration of the catalyst−substrate adhesion as a major factor in the endurance of photoelectrochemical devices. E



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10−7 Torr. X-ray power was 500W, with a beam diameter of 200 μm at an angle of 45°. Survey scans used a pass energy of (225 eV) and energy step of 0.8 eV with 20 ms/step, whereas high resolution scans used a pass energy of (55.0 eV) and energy step of 0.1 eV with 50 ms/ step Time of Flight (ToF) SIMS Analysis. Elemental depth profiling was performed by Roy Winters at the Technion − Israel Institute of Technology. Ion beams used were 15 keV Bi1+ for analysis (50 × 50 μm), and 1 keV Cs for sputtering (600 × 600 μm). Adhesion Measurements. Four Point Bend (4PB) samples were fabricated in a similar fashion to samples used for electrochemical measurements. 25 nm Ti and 175 nm Al layers were deposited above the 2 nm Ir film to ensure epoxy layers did not contact the iridium layer. 4PB tests were only used for control samples and as such had no ALD TiO2. After metal depositions, 353ND epoxy was applied and two wafers bonded together, polished sides facing each other. The structure was then cured at 100 °C for 1 h and subsequently diced into beams. Structure: p+ Si/2 nm SiO2/2 nm Ir/25 nm Ti/175 nm Al/Epoxy/ p+ Si 4PB tests were performed by relating applied load to the strain energy release rate as follows:

G=

*E-mail: [email protected]. ORCID

Robert Tang-Kong: 0000-0003-4583-460X Reinhold H. Dauskardt: 0000-0003-3989-362X Notes

The authors declare no competing financial interest.

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Where P is the load, L is the spacing between inner and outer loading lines, b is the beam width, h is the half thickness, and E and v are the elastic modulus and Poisson’s ratio of the bulk substrate, respectively. A plateau in the load−displacement curve implies propagation of the crack along an interface. Dual cantilever beam samples were fabricated with ALD TiO2. Before Ir deposition, a 5 mm wide, 5 nm thick gold strip was deposited on TiO2 using a shadow mask. Above this, 4 nm Ir, 25 nm Cr, and 175 nm Al were deposited. A thicker Ir layer was used to prevent cohesive failure within the Ir film and Cr was used to avoid ambiguity in postdelamination characterization. 353ND epoxy was used here as well, and samples again diced into beams. Beams were diced in half on one side of the gold strip to allow weak adhesion to initiate a crack. Five mm tabs were epoxied to the end of the beam with the weak layer. Structure: p+ Si/2 nm SiO2/2 nm TiO2/5 nm Au (strip)/4 nm Ir/ 25 nm Cr/175 nm Al/Epoxy/p+ Si Loads in the DCB configuration were related to the strain energy release rate as follows: G=

12P 2a2 b2Esh3

ACKNOWLEDGMENTS

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REFERENCES

(1) Yao, T.; An, X.; Han, H.; Chen, J. Q.; Li, C. Photoelectrocatalytic Materials for Solar Water Splitting. Adv. Energy Mater. 2018, 8 (21), 1−36. (2) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 2017, 1.0003 (3) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37−38. (4) Keable, J.; Holcroft, B. Photoelectrochemical Hydrogen Production, 1st ed.; van de Krol, R., Grätzel, M., Eds.; Electronic Materials: Science & Technology; Springer US: Boston, MA, 2012; Vol. 102. (5) Pinaud, B. a.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6 (7), 1983. (6) Newman, J.; Hoertz, P. G.; Bonino, C. A.; Trainham, J. A. Review: An Economic Perspective on Liquid Solar Fuels. J. Electrochem. Soc. 2012, 159 (10), A1722−A1729. (7) Trainham, J. A.; Newman, J.; Bonino, C. A.; Hoertz, P. G.; Akunuri, N. Whither Solar Fuels? Curr. Opin. Chem. Eng. 2012, 1 (3), 204−210. (8) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901−4934. (9) Chen, Y. W.; Prange, J. D.; Dühnen, S.; Park, Y.; Gunji, M.; Chidsey, C. E. D.; McIntyre, P. C. Atomic Layer-Deposited Tunnel Oxide Stabilizes Silicon Photoanodes for Water Oxidation. Nat. Mater. 2011, 10 (7), 539−544. (10) Scheuermann, A. G.; McIntyre, P. C. Atomic Layer Deposited Corrosion Protection: A Path to Stable and Efficient Photoelectrochemical Cells. J. Phys. Chem. Lett. 2016, 7 (14), 2867−2878. (11) Hu, S.; Shaner, M. R.; Beardslee, J. a.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 Coatings Stabilize Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation. Science 2014, 344 (6187), 1005−1009.

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Here the parameters are the same except for Es being the plane strain modulus and a being the crack length. Adhesion energy values were disregarded until a crack length of 5 mm was reached, due to the influence of tabs on the crack propogation dynamics. All adhesion tests were performed using a thin-film cohesion testing system (Delaminator DTS, Menlo Park, CA). For both 4PB and DCB experiments, debonding interfaces were confirmed by either XPS or AES.

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We thank T. Carver for metal e-beam evaporations. R.T. thanks Prof. Christopher Chidsey for countless insightful discussions and advice, as well as group members O. Hendricks, W. Tan, C. S. Tan, and A. Scheuermann. R. T. also thanks C. Hitzman and J. Jamtgaard of Stanford Nano Shared Facilities for guidance with XPS, AES, and SIMS. R.T. thanks A. Meng and M. Braun for preparation and acquisition of TEM cross section images. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152. This work was partially supported by National Science Foundation program CBET-1336844 and by seed funds from the Stanford Institute for Materials & Energy Sciences at SLAC. ToF-SIMS experiments were supported by the U.S.Israel Binational Science Foundation.

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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/acsami.8b13576. Electrochemical cell schematics, ToF-SIMS Oxide Thickness, TEM Cross Section, remetallization XPS characterization, TiO2 pourbaix diagram, 4 point bend data, Dual Cantilever Beam data (PDF) F


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The Role of Catalyst Adhesion in ALD-TiO2 Protection of Water

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