Stable Water Oxidation in Acid Using Manganese-Modified TiO2

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Energy, Environmental, and Catalysis Applications

Stable Water Oxidation in Acid using Manganese-Modified TiO Protective Coatings 2

Georges Siddiqi, Zhenya Luo, Yujun Xie, Zhenhua Pan, Qianhong Zhu, Jason A Rohr, Judy J. Cha, and Shu Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05323 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

Stable Water Oxidation in Acid using ManganeseModified TiO2 Protective Coatings Georges Siddiqi,1,2 Zhenya Luo,1,2 Yujun Xie,2,3 Zhenhua Pan,1,2 Qianhong Zhu,1,2 Jason A. Röhr, 1,2

1

Judy J. Cha,2,3 Shu Hu1,2*

Department of Chemical and Environmental Engineering, Yale University, New Haven,

Connecticut 06520 (USA), 2 Energy Sciences Institute, Yale University, 810 West Campus Drive, West Haven, CT 06516 (USA), 3 Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520 (USA) KEYWORDS. Oxygen evolution, water oxidation, water splitting, acid stable, protective film, charge transfer, p-type oxide

ABSTRACT

Accomplishing acid-stable water oxidation is a critical matter for achieving both long-lasting water splitting devices and other fuel-forming electro- and photo-catalytic processes. Since water oxidation releases protons into the local electrolytic environment, it becomes increasingly acidic during device operation, which leads to corrosion of the photo-active component and hence loss in device performance and lifetime. In this work, we show that thin films of manganese modified titania, (Ti,Mn)Ox, topped with an iridium catalyst, can be used in a coating stabilization scheme for acid-stable water oxidation. We achieved a device lifetime of more than 100 hours in pH = 0

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acid. We successfully grew (Ti,Mn)Ox coatings with uniform elemental distributions over a wide range of manganese compositions using atomic layer deposition, and using x-ray photoelectron spectroscopy we show that (Ti,Mn)Ox films grown in this manner gives rise to closer-to-valenceband Fermi levels, which can be further tuned with annealing. In contrast to the normally n-type or intrinsic TiO2 coatings, annealed (Ti,Mn)Ox films can make direct charge transfer to a Fe(CN)63/4-

redox couple dissolved in aqueous electrolytes. Using the Fe(CN)63-/4- redox, we further

demonstrated anodic charge transfer through the (Ti,Mn)Ox films to high work function metals, such as iridium and gold, which is not previously possible with ALD grown TiO2. We correlated changes in the crystallinity (amorphous to rutile TiO2) and oxidation state (2+ to 3+) of the annealed (Ti,Mn)Ox films to their hole conductivity and electrochemical stability in acid. Finally, by combining (Ti,Mn)Ox coatings with iridium, an acid stable water-oxidation anode, using acid sensitive conductive fluorine-doped tin oxides, was achieved. Introduction Since most photo-active materials used for water oxidation, that is generation of free oxygen, corrode under acidic conditions, and since water oxidation naturally acidifies the local electrolytic environment, obtaining acid stable anodes, photo-anodes or photo-catalysts for water oxidation is a critical matter. For instance, driving a current density of 2.5 mA∙cm-2 across an anode in contact with a weakly basic electrolyte, pH = 9.2 buffer, the local pH at the anode surface decreased to pH < 7.

1-3

Photo-electrochemical (PEC) cells or devices that operate in strong bases do not suffer

from this challenge, but suffer from concerns from the poor stability of anion-exchange membranes which are necessary to prevent the generation of explosive mixtures of H2 and O2.4-5 Alternatively, a bi-polar membrane can be used to separate an acidic or neutral pH compartment from a basic compartment, which can host a water-oxidation photo-anode that is so far only stable


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in base for 100 hours.6-7 However, using the bi-polar membrane introduces additional ion-transport losses across the membrane leading to inefficient water splitting at the bi-polar interface, thereby lowering the overall solar-to-fuel conversion efficiencies. For this reason there is still a need for acid-stable, water-oxidation interfaces. Material stability during water oxidation in acid is uniquely challenging, not just for electrocatalysts but also for light-absorbing materials and their linkers to the catalysts. While a large library of electro-catalytic materials exist to facilitate water oxidation in base, such as earthabundant metals (Ni and Co) and Fe-based oxides, only Ir and Ru-based catalysts can survive water oxidation in acid.8-11 All early transition-metal oxides, chalcogenides, tin- and indium-oxide conductive supports, are not stable in acid.12-14 Water-oxidation photo-electrodes or photocatalysts present an even more complex issue regarding acid stability: in addition to the previous concerns of electro-catalysts, the electrochemical stability of their photo-absorbers limits the material selection to TiO2, SrTiO3, BiVO4, Ge3N4, Ta3N5 or GaN,15-19 preventing the use of visiblelight active photo-absorbers such as Si, GaAs or CdTe because they otherwise photocorrode.20-24 Recently, numerous protective coating strategies have emerged for stabilization of both photoabsorbers and conductive supports, with the most common coating being TiO2.22-29 Aside from extremely thin protective oxide layers allowing for tunneling, thicker TiO2 coatings (> 5 nm) that rely on charge transport through “leaky” defective states can so far only transfer charge to mid-tolow work function catalysts such as Ni and carbon. Spectroscopic and computational investigations of semiconductor/TiO2 interfaces,30 TiO2/liquid interfaces,31 and transport through TiO2 films,32 verify the existence of a defect band of Ti3+/Ti4+ intrinsic states. The intrinsic “leaky” TiO2 has defect bands that match with mid-tolow work function metals such as Ni and carbon (~ 4.5 eV), but is not aligned well to acid-stable


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water oxidation catalysts such as Ru, Ir, their oxides or molecular Ir complexes due to their high work functions (5.0 – 5.7 eV) of filled valence electronic states or highest occupied molecular orbital (HOMO) levels. 8-9, 11, 31, 33 Existing “leaky” TiO2 coatings prevent anodic charge transfer to acid stable Ir catalysts, making the semiconductor/TiO2/Ir configuration not yet suitable for water oxidation.22-25 To enable charge transfer to high work function catalysts, the TiO2 protective coatings may be modified to create in-gap defect states in the TiO2 with their energy levels closer to its valence band edge. Computational studies have indicated that mixed dopants such as Ni, Mn, or Co may produce extrinsic in-gap states nearer to the valence band edge of TiO2.34-38 Atomic layer deposition (ALD) has been demonstrated to be a viable route to forming a range of mixed oxide films such as indium tin oxide, ZnAlOx, TiO2-RuO2 alloy and Ti-Mn oxides.39-44 In addition to the ease of creating mixed oxides, ALD is also able to create conformal, pinholefree coatings. Alloying functional oxides such as RuO2 with TiO2 in an all-ALD process has been attempted to create highly conductive, high work function anodes.49 Beyond the ALD demonstration of these dimensionally stable anodes for industrial electrochemical oxidation processes, we intend to discover and design a TiO2-based coating that is alloyed with earthabundant elements, such as Ni, Sn and Mn, despite their acid instability when not alloyed. We have tried Ni- or Sn-modified TiO2 coatings (by mixing TiO2 with NiOx and SnO2 during ALD growth) that support acid-stable catalysts but so far their anodic charge transfer has not been satisfactory. Mn had been experimentally shown to introduce in-gap states close to the valence band, but not yet achieved with a range of controlled crystallinity and defect density; 45 and high Mn content (10-70%) (Ti,Mn)Ox films have been studied electrochemically by Pickrahn et al. as catalytic materials for water oxidation.51 However, to this date (Ti,Mn)Ox has not been extensively studied as a protective layer due to their poor acid instability.


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In this work we discuss the effect of modifying TiO2 protective coatings with Mn to allow for anodic charge transfer to high work function metals and to achieve the performance and stability needed for water oxidation in acid. We present the synthesis of thick (Ti,Mn)Ox ternary mixed oxide films (> 20 nm) by using ALD to extrinsically modify TiO2 coatings and preserve their robust chemical, electrochemical and mechanical stability in acid, with focus on charge transfer behavior to a surface-deposited co-catalyst material. We investigate the ranges of composition and processing conditions for chemical and anodic water-oxidation stability in acid and for facile charge transport to high work function water-oxidation catalysts. Recently, acid stable catalysts 4648

including crystalline nickel manganese antimonates,47 the individual elements of which are

otherwise unstable, showed acid stability of their crystalline oxide alloys, thus motivating our alloying strategy for acid stable coatings. We correlate the structural and chemical properties of the as-grown and air annealed (Ti,Mn)Ox thin-film coatings, providing an initial understanding of the factors that influence their acid stability and anodic charge transfer to metal catalysts.

Experimental Section Atomic Layer Deposition (Ti,Mn)Ox films were grown using ALD on both Si (boron doped, p-type, NA = 3×1019 cm-3, (100) wafers, Addison Engineering) and fluorine-doped tin oxide (FTO) substrates (TEC15, sheet resistance 15Ω/□, Hartford Glass Co.) for characterization and electrochemistry, respectively. All substrates were thoroughly cleaned immediately prior to deposition. The Si substrates were firstly cleaned with RCA SC-1 (a cleaning procedure abbreviated from Radio Corporation of American, Standard Clean 1), then etched using a buffered oxide etch (BOE, 10:1, Transene, Inc.), and finally cleaned again with RCA SC-2. The FTO substrates were cleaned using a standard procedure of


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ultra-sonication in ethanol for 30 min. After cleaning, all substrates were thoroughly dried using gaseous nitrogen. The growth of TiO2, MnO and (Ti,Mn)Ox films was performed using a commercial ALD system (Ultratech Fiji G2). We used tetrakis-dimethylamido-titanium (TDMAT, Sigma Aldrich, 99.999%) and bis-(ethylcyclopentadienyl)manganese (Mn(EtCp)2, Sigma Aldrich, 97%) as the Ti and Mn precursors, and water was used as the co-reactant to form TiO2 and MnO (+2 oxidation state verified by x-ray photoelectron spectroscopy), respectively. The ALD chamber was continuously purged under a constant flow of Ar gas (99.9997%) at a flow rate of 60 sccm, at 150 oC, and at a chamber pressure of approximately 10-1 Torr. The film thicknesses were controlled by the number of ALD cycles and the growth rate per cycle (see Table 1).22 Figure 1 illustrates the process for growing conformal films of ternary TiO2 modified by Mn and can be generalized to mixing other elements. The mixed (Ti,Mn)Ox films have been synthesized by alternating fixed-number subcycles of TiO2 deposition (one 0.06 s pulse of H2O then after 15 s one 0.25 s pulse of TDMAT) and one subcycle of MnO deposition (one 0.06 s pulse of H2O then after 15 s two 0.25 s pulses of Mn(EtCp)2), and by repeating these subcycles to achieve the desired film thickness. The growth conditions were modified from a combination of a recipe found in the literature and the manufacturer recipe: 51 for the Mn deposition, we found that the optimal condition was to deliver the heated Mn(EtCp)2 precursor (100 oC) using two 0.25 s pulses spaced 7 s apart, with the optimization process for the MnO ALD shown in Fig. S1.


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Figure 1. Schematics for the deposition of (Ti,Mn)Ox thin films, showing the pulsed ALD sequences using a fixed number of Ti subcycles with the number varied between 2 and 16 and one Mn subcycle. TDMAT pulses (purple waveform) and Mn(EtCp)2 pulses (red waveform) occur 15 s after the water pulses (black waveform). The Mn(EtCp)2 precursor delivery per subcycle consists of 2 × 0.25 pulses.

In order to find the range of Mn compositions of the (Ti,Mn)Ox films, for stability in acid, we grew mixed (Ti, Mn)Ox films by supercycles of TiO2 and MnO consisting of a (a = 2, 4, 8 or 16) subcycles of TiO2 and b (b = 1 or 8) subcycles of MnO (Fig. 1). The supercycle was then repeated to grow films with a target thickness of approximately 25 nm. The annealing time for the ALD grown films varied depending on the annealing temperature: 400 oC in air for 6 hours and annealing at 500 oC and 600 oC in air was shortened to 2 hours. The conductivity and stability of (Ti,Mn)Ox films increased universally after annealing above 400 oC. Metal deposition A Cressington 208 sputtering tool was used to deposit approximately 8 nm of iridium (Ir) onto the (Ti,Mn)Ox films (on both as-grown and annealed films). The Ir notation indicates the initial composition of as-deposited layers, whereas the Ir surface will become IrOx during water oxidation. In some cases gold (Au) was sputtered on top of the (Ti,Mn)Ox films at the same tool. Materials Characterization To determine the elemental composition of the film, (Ti,Mn)Ox, x-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI VersaProbe II Scanning XPS Microprobe equipped with a monochromated Al source. As-grown and annealed (Ti,Mn)Ox films were deposited onto silicon substrates, and all samples were measured under ultra-high vacuum.


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The value of the oxygen stoichiometry coefficient, indicated as x in the (Ti,Mn)Ox formula throughout the text, results from the measured composition and oxidation states of Ti and Mn. After an initial survey scan, region scans of O 1s, C 1s, Si 2p, Mn 2p, and Ti 2p orbital energy peaks were performed with a pass energy of 23.5 eV. The obtained data was analyzed using the CasaXPS software. Besides the XPS measurements probing the orbital energies, valence band XPS was performed using a pass energy of 11.75 eV. Fitting of the Mn 2p3/2 core-level spectra for MnO and (Ti,Mn)Ox before and after annealing was performed according to the procedure by Beisinger et al.: a multiplet of 5 peaks was used to fit to the Mn 2p3/2 peak, with an additional shake-up peak being fitted for the Mn2+ ion peak. This shake-up peak is caused by an intrinsic energy loss process, where a fraction of the photon energy excites the ion out of the zero energy loss state, appearing as a shoulder of the main peak.49 In order to ensure that the spacing between each peak remained constant, the full-width half maximum (FWHM) of each peak, along with the relative difference in binding energies between each of the peaks in the multiplet, was constrained to the reported values and listed in Table S2. 49 A Hitachi SU8230 UHR Cold Field Emission scanning electron microscope (SEM) was used to characterize the film thickness and morphology. Immediately prior to characterization, the samples were cleaved and the cleaved edge was mounted vertically to determine the film thickness from the sample cross-section. For more detailed structural analysis, cross-sectional and plan-view transmission electron microscopy (TEM) was performed on both the as-grown and annealed (Ti,Mn)Ox films using a FEI Tecnai Osiris operating at 200 kV, equipped with a quadrant energy dispersive X-ray detector. The cross-section TEM sample was prepared using a focused ion beam (FEI Helios), following standard lamella sample preparation procedures. UV-Visible (UV-Vis) absorption data was obtained by a Shimadzu scanning spectrometer with an integrating sphere


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attachment. Diffused reflection (R%) and transmission (T%) spectra were taken separately to calculate the absorption (A% = 1 – R% – T%). X-ray diffraction (XRD) data was obtained using a Rigaku SmartLab X-ray Diffractometer in a grazing incident XRD (GIXRD) mode, with an ω = 0.45 o and the range of 2θ = 20 – 70o. Electrochemistry Sulfuric acid (Sigma Aldrich, 0.25 M, 99.7%) was diluted to pH =1 as the electrolyte used during water oxidation, and concentrated sulfuric acid (Baker, ACS Grade, 95.0–98.0%) was used for the pH = 0 electrolyte. For the charge transfer measurements, an aqueous solution containing a Fe(CN)6 3-/4- redox couple was prepared using 0.2 M K3Fe(CN)6 (Fisher Scientific, 99.4%), 0.2 M K4Fe(CN)6 (Fisher Scientific, 99.4%) and 0.2 M K2SO4 (Sigma Aldrich, 99.0%). 18.2 MΩ∙cm H2O was obtained from a Millipore deionized water system. The charge-transfer behavior of the (Ti,Mn)Ox films were measured using cyclic voltammetry (scan rate of 20 mV∙s-1) by contacting the films with the Fe(CN)63-/4- redox couple in a threeelectrode setup. A Bio-Logic S200 potentiostat system connected the working electrode, an Ag/AgCl with a saturated KCl reference electrode, and a Pt mesh or a carbon rod as a counter electrode. To quantify the resistance through the (Ti,Mn)Ox films, electrochemical impedance spectroscopy (EIS) was used. The EIS measurements of the (Ti,Mn)Ox films were measured using the same cell as those used for the cyclic voltammetry. Besides the charge-transfer measurements, water oxidation behavior was also characterized in a three-electrode setup, with a carbon rod as the counter electrode and either a pH = 1 or pH = 0 H2SO4 solution as the electrolyte. For measuring the electrode stability in the acidic environment, a pump was used to force convection directly at the anode, and both chrono-potentiometry and chrono-amperometry were employed, targeting a


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current density of ca. 1 mA∙cm-2 beyond which transport in the electrolyte starts to affect the measurement. Oxygen Quantification The Faradic efficiency of O2 evolution of an FTO/(Ti,Mn)Ox/Ir electrode was quantified by comparing the amount of the generated O2 on the working electrode as a function of time to that on a separate Ir electrode as a measurement standard (under an applied anodic current of ca. 7 mA). Prior to the measurement, the electrochemical cell was purged in N2 for 20 min to obtain an oxygen-free environment, and the headspace was subsequently sealed at a constant volume. The concentration of O2 in the headspace was monitored by an oxygen probe (OX-NP, Unisense) controlled by a Unisense Microsensor Multimeter. Prior to the measurement, a three-point calibration for the oxygen probe was conducted in an O2-saturated environment, then an airsaturated environment, and finally in a N2-puraged environment. The oxygen leakage rate was calibrated by a 10-min measurement of O2 signal with the working electrode at open circuit. The O2 production was recorded for 30 min for both the FTO/(Ti,Mn)Ox/Ir working electrode and the Ir standard electrode, with the Ir standard benchmarked at 100% Faradaic efficiency under anodic currents to obtain the working electrode efficiency.8

Results Structural, Composition and Chemical Characterization The TiO2:MnO subcycle ratios, film growth rates and elemental compositions are listed in Table 1. XPS characterizations show that the films consist of Ti, Mn and O with trace carbon impurities (Fig. 2a). High energy resolution XPS scans of the Ti 2p, Mn 2p and O 1s regions are used to quantify the concentration of Mn present in the (Ti,Mn)Ox films (Fig. S2a, Table 1). The


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measured Mn concentration matches closely to the predicted Mn composition from the rule of mixtures (ROM) over a wide range, with minor deviation for 16-1 (Ti,Mn)Ox.44, 50 Mn ROM is calculated from the following formula: Mn ROM = αMn βMn ⁄[αMn βMn + αTi (1 − βMn )], where αM is the growth rate of bulk oxide M (TiO2 or MnO), βM is the fraction of the subcycles for bulk oxide M divided by all the subcycles in a supercycle. For the binary oxides, α 𝑇𝑖𝑂2 = 0.47 Å per cycle, α𝑀𝑛𝑂𝑥 = 0.97 Å per cycle. We further compared the range of XPS peak positions for the (Ti,Mn)Ox (Ti 2p3/2: 458.2 – 458.6 eV, Mn 2p3/2: 640.6 – 641.8 eV) to TiO2 (Ti 2p3/2: 458.8 eV), and MnO (Mn 2p3/2: 642.1 eV), and this shows that in all cases the binding energies of Ti and Mn in the (Ti,Mn)Ox coating are shifted to the lower binding energies compared to TiO2 and MnO (Table S1). No trends were observed for the O 1s peak. A closer examination of the Mn oxidation state is possible by the peak fitting procedure outlined in the Experimental Section,49 and the Mn 2p3/2 peak position, shape and fittings are indicative of Mn2+ being the primary species in (Ti,Mn)Ox films after ALD growth (Table S2, Fig. S2 – S4).

Figure 2. XPS and SEM characterizations of (Ti,Mn)Ox films. a) Representative survey scan of Sample 4-1, and b) Growth rate of ALD films observed by SEM cross sections compared to the


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expected growth rate for the films, Growth rate = Ticycles ∙ αTiO2 + Mncycles ∙ αMnOx , where α 𝑇𝑖𝑂2 = 0.47 per cycle and α𝑀𝑛𝑂𝑥 = 0.97 per cycle. The growth rates of the individual constituents are listed in Table 1. Sample 16-8 is omitted for clarity.

Table 1. Summary of the ALD cycle design, growth rates and elemental composition of (Ti,Mn)Ox films studied in this work. Mnat% d (Mn:(Mn+Ti), %)

Film thickness (nm)

1.36

51

25

4-1

1.43

37

25

1

8-1

3.16

26

25

16

1

16-1

7.08

18

25

16

8

16-8

9.90

43

25

1

0

TiO2

0.47

--

>20

0

1

MnO

0.97

--

>20

Subcycles TiO2 a

Subcycles MnO b

Sample ID

2

1

2-1

4

1

8

Growth rate (Å/supercycle) c

a) TiO2 subcycles consist of one pulse H2O followed by one pulse TDMAT, b) MnO subcycles consist of one pulse H2O followed by two pulses Mn(EtCp)2, c) growth rate per supercycle calculated by measuring film thickness via SEM cross sections of cleaved films and dividing by the number of supercycles, d) composition measured by XPS elemental quantification.

We further estimated the growth rate per supercycle for the mixed (Ti,Mn)Ox films by initially assuming the growth rate per cycle of MnO on MnO is the same as on TiO2 during the (Ti,Mn)Ox growth. From the growth rates of the individual TiO2 (α 𝑇𝑖𝑂2 = 0.47 Å per cycle) or MnO (α𝑀𝑛𝑂𝑥 = 0.97 Å per cycle) subcycles, the supercycle growth rate was estimated as: Growth rate = Ticycles ∙ αTiO2 + Mncycles ∙ αMnOx . The growth rates of (Ti,Mn)Ox films as measured from SEM cross


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sections (Fig. 2b) deviated from predicted in all composition ranges, which is commonly observed for ternary oxide ALD films.44, 50-51 Our hypothesis that this difference arises from the steric differences of the two metal-organic precursors, Mn(EtCp)2 and TDMAT. However, Mn compositions predicted via the rule of mixtures match XPS composition measurements, indicating that although the density of the film might change, the total atomic composition remains within predictions. Changes in the morphology, oxidation states and film compositions of the air-annealed (Ti,Mn)Ox films were investigated. SEM shows that the annealing temperature influences the morphology of the films. Sample 16-1 has a flat surface, typical of as-grown ALD films; however, as the sample is annealed from 200 to 500 oC, we observe changes in morphology at and above 500 oC, with sporadic bubbles appearing on the surface but no pinholes or cracks in the (Ti,Mn)Ox coatings (Fig. S5). The GIXRD data of Sample 16-1 before annealing indicates an amorphous film, while after annealing at 500 oC the observed broad peaks correspond to the rutile phase of TiO2 (Fig. 3). TEM of Sample 16-1 (Ti,Mn)Ox shows an amorphous film with uniform Ti and Mn elemental distribution (Fig. 4a-4d). The white contrast band at the Si/(Ti,Mn)Ox interface is a native oxide as the result of ALD growth, which is consistent with previous observations.22 After annealing at 500 oC for 1 hour, the amorphous thin film crystallized with a polycrystalline grain size of approximately 8 nm (Fig. 4e). XRD Scherrer analysis (assuming spherical crystals and a K factor of 1.47) indicates an average nano-crystallite size of 8.3 nm. This crystallite size approximately matches what is observed from the plan-view TEM (Fig. 4e). The selected area electron diffraction (SAED) pattern indicates the crystallized TiO2 has a rutile structure (Fig. 4f). EDX mapping shows that after annealing and crystallization, a uniform distribution of the chemical elements is maintained without phase segregation (Fig. 4g – 4h).


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XPS was used to analyze the (Ti,Mn)Ox surface after annealing. Compared to the as-prepared films, samples 2-1, 4-1, and 16-8 show increases in Mnat% after annealing (43, 10, and 21%, respectively) at the coating surface, while 8-1 and 16-1 show minimal change in composition after annealing (Fig. S2a, Table S1). The Ti 2p3/2 peak position before and after annealing shows no significant changes (Table S1). However, the Mn 2p3/2 peak shape and fitting reflects the change in oxidation state from Mn2+ to Mn3+ after annealing, with all samples but sample 16-1 being fit better with the Mn3+ fitting model (Table S2, Fig. S2-S4). Comparing all compositions of (Ti,Mn)Ox, there is a trend of a small positive shift in the Mn peak positions as more TiO2 is added to the supercycle (Fig. S2-S4). Comparing each composition of (Ti,Mn)Ox before and after annealing, the first peak position of the Mn multiplet fitting consistently shifted to higher binding energies (Table S2). Compared to samples annealed at 400 oC, sample 8-1 annealed at 500oC shows minimal changes in the overall Mn composition (Mnat% = 28%) and oxidation state (Mn3+, Fig. S7a). Similarly, sample 16-1 annealed at 600 oC shows minimal changes in composition (Mnat% = 22%), while the Mn 2p3/2 peak is best fit using a model for Mn3+ (Fig. S7b). To directly probe the effects of Mn alloying to (Ti,Mn)Ox’s electronic structures, we employed valence XPS. Valence XPS provides a more accurate means to analyze the bulk film’s valence band position relative to the Fermi level than ultraviolet photoelectron spectroscopy (UPS) which only probes the film’s surface; and the greater penetration depth of the XPS probe beam compared to the ultraviolet light of UPS reduce the effect of spurious surface states on the analysis of the valence band position.30-31 Comparing the valence XPS spectra of samples 2-1 and 16-1 (Ti,Mn)Ox to pure TiO2 and MnO samples, the Fermi levels of (Ti,Mn)Ox films lie between those of TiO2 and MnO, proving modification of the electronic structure of TiO2 by Mn (Fig. 5). The difference between the Fermi level (Ef) relative to the top of the valence band maximum (VBM) (ΔEEf-VBM)


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lies at 3.1 eV for TiO2 and 0.3 eV for MnO, whereas ΔEEf-VBM of samples 16-1 and 2-1 (Ti,Mn)Ox shifts to 2.8 eV and 1.2 eV, respectively, indicating that addition of Mn to TiO2 creates a new oxide the Fermi level of which is able to shift down closer to the valence band. Annealing the (Ti,Mn)Ox films at 500 oC leads to a downward shift of ΔEEf-VBM, decreasing to 1.7 eV for sample 16-1 and 0.7 eV for sample 2-1 (Fig. 5), respectively. For both as-grown and annealed films, UVVis absorption data show the bandgap of (Ti,Mn)Ox coatings stay at approximately 3.4 eV (Fig. S6), suggested by extrapolating the linear region of the plot to intersect with the axis of photon energy. Figure 5b simply visualize the (Ti,Mn)Ox Fermi levels.

Figure 3. GIXRD data of (100)-oriented Si, as-grown 16-1 prior to annealing and 16-1 after annealing at 500 oC. Diffraction peaks are labeled for rutile TiO2.


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Figure 4. TEM characterization of 16-1, a) – d) cross section before annealing, and e) – h) planview after annealing at 500 oC. a) bright-field TEM image, b) high-angle annual dark field (HAADF) STEM image, and c) – d) Mn and Ti EDX maps of 16-1. e) plan-view bright-field TEM image showing polycrystallinity, f) selected area electron diffraction pattern with rutile TiO2 indices shown, g) – h) Mn and Ti EDX maps.


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Figure 5. Electronic structure characterization of (Ti,Mn)Ox films. (a) Valence XPS spectra of MnO, TiO2, and samples 2-1 and 16-1 (Ti,Mn)Ox before and after annealing at 500 oC in air for 2 h. Spectra used Au as an internal standard for calibration of the Fermi level. (b) Fermi levels relative to band edge positions for 2-1 and 16-1 (Ti,Mn)Ox films before and after 500 oC annealing. Electrochemical charge transfer behavior ALD growth parameters and annealing conditions can be used to control of the composition and electronic properties of (Ti,Mn)Ox coatings. To further quantify these effects on the electronic properties, the charge-transfer behavior was probed electrochemically for the layered stack of FTO, (Ti,Mn)Ox and high work function metals. FTO was selected as a substrate because it is not only conductive but also extremely sensitive to corrosion in acid and allows us to test if the protective properties of TiO2 coatings can be successfully maintained following modification with Mn.22 An Ir catalyst was deposited to enable charge transfer and provide catalytic sites for water oxidation in acid. We first probed charge transfer by contacting the (Ti,Mn)Ox-coated FTO electrodes to a Fe(CN)63-/4- redox couple dissolved in a K2SO4 (aq) electrolyte, with and without an Ir catalyst. Figure 6 shows the charge-transfer behavior of 500 oC annealed (Ti,Mn)Ox films that are mainly studied for acid stable water oxidation. Both with and without Ir surface layers (Fig. 6a and 6b), FTO/(Ti,Mn)Ox electrodes annealed at 500 oC allowed for charge transfer to the Fe(CN)63-/4- redox couple. The separation of the cathodic and anodic charge transfer peaks in the cyclic voltammogram waves was measured to be between 0.42 – 0.45 V for the cases with and without Ir. However, these peak-to-peak separations are larger than the 0.12 V of a Pt disc electrode measured in the same electrolyte, indicating a resistive (Ti,Mn)Ox film.


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Charge transfer across (Ti,Mn)Ox films without Ir was investigated further as a function of annealing temperatures, and films annealed at 500 oC and 600 oC can transfer anodic currents to the Fe(CN)63-/4- redox couple without any catalysts (Fig. 6c). Comparison between the as-grown and annealed coatings highlights the effect of annealing temperatures on charge transfer; FTO/asgrown (Ti,Mn)Ox electrodes exhibit measurable charge transfer even with Ir and poor conductivity without Ir (Fig. 7a-7b). Contrasting the results of Fig. S8a to Fig. S8b shows that it is the high work function metals such as Ir and Au which enable the charge transfer of (Ti,Mn)Ox films that were annealed at 400 oC in air. Only in the cases where samples were annealed below 500oC was charge transfer assisted by having Ir on the surface.

Figure 6. Charge transfer to Fe(CN)63-/4- redox couple across the annealed (Ti,Mn)Ox films (scan rate 20 mV∙s-1). a) Electrochemical charge-transfer behavior of FTO/annealed (Ti,Mn)Ox/Ir electrodes, and b) behavior of FTO/(Ti,Mn)Ox electrodes without any catalysts. (Ti,Mn)Ox films in a) and b) were annealed at 500 oC. c) behavior of FTO/16-1 (Ti,Mn)Ox electrodes with no Ir catalyst annealed at temperatures of 400 – 600 oC. The measured open-circuit potential of Pt to Fe(CN)63-/4- is 0.31 V versus Ag/AgCl(sat). Behavior of electrodes annealed at 400 oC and 600 oC are shown in the Supporting Information.


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As-grown with Ir

As-grown without Ir

Figure 7. Charge transfer to Fe(CN)63-/4- redox couple across the as-grown (Ti,Mn)Ox films (scan rate 20 mV∙s-1). a) FTO/(Ti,Mn)Ox films with Ir catalyst for charge transfer, b) FTO/(Ti,Mn)Ox films without any catalyst, and c) charge transfer across a TiO2 interface, TiO2 annealed at 500 oC. The measured open-circuit potential of Pt to Fe(CN)63-/4- is 0.31 V versus Ag/AgCl(sat).

As mentioned above, ALD grown TiO2 and annealed TiO2 does not allow for anodic charge transfer to Fe(CN)63-/4- redox couples. However, the ability of (Ti,Mn)Ox to allow for anodic currents is in sharp contrast with the ALD amorphous “leaky” TiO2, which showed poor anodic conductivity even after annealing at 500 oC (Fig. 7c). EIS conveniently measures the resistance through the (Ti,Mn)Ox films by fitting equivalent circuitry (Fig. S9), without additional metal contacts deposited atop. The through-layer resistance of all the 500 oC annealed (Ti,Mn)Ox films (25 nm) gave a statistical value of 43.1 ± 3.1 Ω, which indicates a functioning conductive (Ti,Mn)Ox coating with no discernable difference at various compositions. With Ir deposited on the surface all the (Ti,Mn)Ox coatings the through-layer resistance of 46.2 ± 3.1 Ω did not vary significantly, as expected. In fact, deposition of Ir was found to reduce the charge-transfer resistance to the electrolyte to a negligible value according to


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the data fitting (Fig. S9 table). The total resistance of (Ti,Mn)Ox electrodes is considered to consist of through-layer resistance and charge-transfer resistance, and has a total upper bound of approximately 49.8 ± 5.6 Ω. Acid stable water oxidation To confirm if (Ti,Mn)Ox can function as a protective layer for FTO during OER in acidic conditions, the accelerated corrosion testing of lowest Mn content films such as 16-1 (Ti,Mn)Ox was first conducted in pH = 0 H2SO4, without any applied potential (Fig. S10). The change in film thickness as a function of time, when in contact with acid, was measured, and it was observed that films annealed at temperatures above 400 oC exhibited a lower rate of dissolution. Of the range of (Ti,Mn)Ox films synthesized in this work, 16-1 films are the focus of this study as they showed the greatest acid stability. The observed chemical stability of 500 oC annealed 16-1 (Ti,Mn)Ox films in acid highlights the importance of thermal treatment on film durability, and this is demonstrated further by testing the water oxidation performance of (Ti,Mn)Ox films in pH = 1. FTO/16-1 (Ti,Mn)Ox (25 nm, annealed at 400 oC)/8 nm Ir electrodes fail during water oxidation in pH = 1, under a constant current of 1 mA∙cm-2 (Fig. S11). Raising the annealing temperature of 16-1 deposited on FTO from 400 oC to 500 and 600oC yields films that are far more stable for acid-stable water oxidation. With minimal deactivation observed over 120 h of water oxidation, sample 16-1 annealed at 500 oC shows an average current density of approximately 2.0 mA∙cm-2 at an overpotential of 446 mV, and 16-1 annealed at 600 oC shows an average current density of approximately 1.1 mA∙cm-2 at an overpotential of 456 mV (Fig. 8a). In fact, sample 16-1 annealed at 500oC could survive for over 200 h in pH = 0 H2SO4, showing an average current density of 0.3 and 0.9 mA∙cm-2 at an


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overpotential of 352 and 437 mV, respectively (Fig. 8b). The stability of a 50 nm 8-1 film after annealing at 400 oC was also tested in pH = 1 electrolytes. While the film can survive 120 h of water oxidation, it starts to suffer a loss of activity after around 60 h (Fig. S12).

Figure 8. Stability of 16-1 (Ti,Mn)Ox films under chronoampometry in H2SO4, for a) FTO/16-1 (Ti,Mn)Ox film/Ir electrodes with the (Ti,Mn)Ox films annealed at the temperatures shown and tested in pH = 1 H2SO4, b) FTO/16-1 (Ti,Mn)Ox film/Ir electrodes with the (Ti,Mn)Ox films annealed at 500 oC and tested in pH = 0 H2SO4. At ca. 100 h, the applied potential was increased to 1.47 V vs. Ag/AgCl (1.67 V vs. RHE).

The morphology of the (Ti,Mn)Ox electrodes was compared before and after the 100 hour stability testing in acid. Minimal changes in morphology was observed (Fig. S13). XPS characterization of the films after water oxidation showed no significant change of the Ir surface coverage, with XPS signals of Ir only, and with no sign of Ti or Mn or any signals from the substrate. This is an indication that the coatings and substrates were not exposed (Fig. S14a), meaning good adhesion of Ir to the (Ti,Mn)Ox coating, giving further proof of minimal film dissolution. We show that FTO/Ir electrodes degrade during water oxidation, leaving behind a bare


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FTO surfaces due to FTO being acid sensitive, resulting in lift-off of the Ir layer (Fig. S14b). The Faradaic efficiency of FTO/16-1 annealed at 500 oC with Ir during water oxidation was measured to be 96%. Although bare (Ti,Mn)Ox films annealed at 500 and 600 oC can transfer charge to the Fe(CN)63-/4- redox couple, they can only undergo water oxidation at high overpotentials (Fig. S15).

Discussion Change of Oxidation States and Crystallinity after Annealing The main property changes occurring in (Ti,Mn)Ox films after annealing are two-fold: 1) a rise in the oxidation state of Mn, with the dominant species changing from Mn2+ (as-grown) to Mn3+ (annealed), and; 2) solid-phase crystallization, with rutile TiO2 nano-crystallites emerging from the original amorphous (Ti,Mn)Ox film (Fig. 3, 4, S2-4, Table S2). Prior to annealing, the XPS binding energies of Ti and Mn in the (Ti,Mn)Ox films are all lower than either TiO2 or MnO (Table S1). This indicates the formation of Mn-O-Ti bonds and suggests good contact between the alternating TiO2 and MnO layers.44, 52 After annealing at 400 oC, the Ti 2p3/2, Mn 2p3/2 and O 1s peaks all consistently shift to slightly higher binding energies than their respective peak positions before annealing. There is no observable difference of the position and shape of Mn 2p3/2 peaks for (Ti,Mn)Ox films annealed at 400 oC and at 500 oC. The Mn 2p3/2 peak fitting further shows that the oxidation state changes from 2+ to predominantly 3+ after annealing. Only annealed 16-1 (Ti,Mn)Ox is an outlier that shows a best fit for Mn2+ but also shows the negligible characteristics of Mn2+ “shake-up” peak (ca. 646 eV binding energy), which fully becomes Mn3+ after annealing at 600 oC (Fig. S7b, Tables S1-S2).


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Although annealing of the (Ti,Mn)Ox films in air leads to the formation of Mn-rich surfaces for high Mn-content (Ti,Mn)Ox films of 2-1, 4-1, and 16-8, surfaces of low Mn-content (Ti,Mn)Ox of 8-1 and 16-1 are largely unchanged (Fig. S2a). The formation of a Mn-rich surface layer among high Mn-content films is less of a concern for the charge-transfer study because those films (2-1, 4-1 and 16-8) are less interesting from the standpoint of stability. SEM of 16-1 shows that voids were formed after annealing at above 500 oC but they neither affect the stability of the film during water oxidation nor create pinholes or cracks in the (Ti,Mn)Ox coatings (Fig. 8 and S5). GIXRD and TEM consistently verified that crystallization of the film occurs after annealing at 500 oC with diffraction peaks corresponding to rutile TiO2 appearing in GIXRD, and crystallites being observable in plan-view TEM images (Figs. 3 – 4). The as-grown films show a uniform composition across the film thickness with no evidence of crystallinity. This indicate that in addition to oxidation state and morphological changes, the other observable change occurring to (Ti,Mn)Ox films after annealing is crystallization. Fermi Levels of (Ti,Mn)Ox The valence XPS spectra indicate that the position of the Fermi level relative to the valence band maximum (VBM), EEf-VBM, can be tuned both by the Mn concentration and thermal annealing of the film (Fig. 5). Valence XPS of as-grown 2-1 and 16-1 shows that their Fermi levels moved closer to the VBM for the more Mn-rich (Ti,Mn)Ox films. Valence XPS of 2-1 and 16-1 after annealing shows that, in both cases, the Fermi level is moved closer to the VBM; EEf-VBM of 2-1 decreases by 0.5 eV after annealing and 16-1 decreases by 1.1 eV (Fig. 5b). Thus, these observations indicate that both annealing the Mn-alloyed films and tuning the Mn content reduces the EEf-VBM. Notably, the 16-1 sample after annealing has comparable Fermi level with the 2-1


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sample before annealing. The 16-1 sample after annealing had shown anodic currents to high work function metals (Fig. 6b), but the 2-1 sample before annealing had no anodic currents observed (Fig. 7b). In fact, all as-grown (Ti,Mn)Ox films studied in this work showed negligible anodic currents to a high work function metals prior to annealing (Fig. 7a). Our observations agree with recently reported observations that p-type cobalt oxide protective coatings showed high anodic conductivity by lowering the coatings’ Fermi levels towards the valence band.28-29 Factors in addition to the (Ti,Mn)Ox Fermi levels are considered to be responsible for the anodic current conduction. The fact that the GIXRD and TEM measurements showed evidence of the formation of rutile TiO2 after annealing the (Ti,Mn)Ox coatings at 500 oC suggests that crystallization is also correlated with the anodic charge transfer across the film (Fig. 3). Also, as discussed above, there is a correlation between the crystallization from amorphous to rutile TiO2 and the change of the Mn oxidation states. At this stage, we cannot yet differentiate which of these effects enable the anodic charge transfer to Ir unless we could obtain amorphous (Ti,Mn)Ox films with Mn3+. However, we can so far conclude that the main mechanism controlling charge transfer across (Ti,Mn)Ox films to Ir is the Mn oxidation state in the (Ti,Mn)Ox films along with the film crystallinity. Surface Layers on (Ti,Mn)Ox for Anodic Charge Transport After annealing, the (Ti,Mn)Ox films crystallized into a matrix of crystalline rutile TiO2 alloyed with Mn3+ ions homogeneously. The role of Ir in enabling anodic charge transfer is directly apparent, as annealed films without surface Ir transport holes poorly (Fig. 6c). This observation once again supports the reported mechanism of charge transfer across in-gap defect states of thick (Ti,Mn)Ox films to the metallic surface layer.22, 32 The “leaky” hole transport observed in ALD


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grown TiO2 can be further extended to TiO2 films modified by Mn, which enables charge transfer to high work function metals. Just as the combination of Ni and intrinsic “leaky” TiO2 is essential for anodic conduction,24, 30 it is necessary to combine the surface layers of (Ti,Mn)Ox with metals or metal oxides to achieve the anodic charge transfer via their surface electronic states. We hypothesize that the mechanism for anodic charge conduction across the (Ti,Mn)Ox coating is not solely because the achieved lowering of the Fermi level of (Ti,Mn)Ox coatings enhances hole conductivity (Fig. 9), but also because the shift in Mn oxidation state from 2+ (as-grown) to primarily 3+ (annealed) creates a partially filled defect band that matches the electronic states of surface Ir layers (Table S2, Fig. S2 – S4). Figure 9 schematically explains the charge-transfer behavior to redox potentials of the Fe(CN)63-/4- redox couple: the Fermi level of (Ti,Mn)Ox is lower than the electrochemical potential of Fe(CN)63-/4- while that of intrinsic TiO2 is higher. For the (Ti,Mn)Ox, the defect band exists in the band gap as indicated by the XPS valence spectra (Fig. 5) and UV-Vis absorption spectra (Fig. S6). The optical absorption at sub-bandgap energies of < 3.4 eV results from electronic transitions from Mn-related defect states to the conduction band, and the Mn3+ states created after 500 oC annealing gave stronger sub-bandgap absorption than the asgrown films, indicating the increased density of defect states. The energy barrier for charge transfer across the (Ti,Mn)Ox coating to the Fe(CN)63-/4- electrolyte is so large that anodic currents should be unlikely. However, because we did observe anodic charge transfer, this is another indication of highly conductive defect states similar to what was observed in the ALD grown “leaky” TiO2. Furthermore, the Fermi level of annealed (Ti,Mn)Ox films and the energy level of their partially filled in-gap defect states match with the Ir Fermi energy. This charge-transfer route enables high anodic conductivity via Ir to redox potentials of the Fe(CN)63-/4- redox couple, and similarly, to O2/H2O whereas the original “leaky” TiO2 cannot.


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Figure 9. Comparison in band energy diagrams of TiO2 and annealed (Ti,Mn)Ox, their contacting Ir, and the redox levels of Fe(CN)63-/4- and O2/H2O in liquids. The Fermi energy level and filled electronic states of annealed (Ti,Mn)Ox match with the states of Ir catalysts, enabling to anodic current conduction to high work function metals. The (Ti,Mn)Ox films that were annealed at temperatures above 400 oC also show anodic charge transfer to a Fe(CN)63-/4- redox couple without Ir surface layers (Fig. 6). One possible explanation is that the surface of annealed (Ti,Mn)Ox films becomes Mn-rich, which introduces surface states and enables anodic conductivity similarly to deposited Ir. This explanation is supported by the reduced charge-transfer resistance (4.17 – 10.71 Ω) for annealed (Ti,Mn)Ox films according to the EIS analysis.(Fig. S9) Furthermore, the annealed 16-1 (Ti,Mn)Ox films without Ir can oxidize water in pH = 1 H2SO4 without losing its anodic conductivity and can maintain their electrochemical stability in acid even under oxidative potentials of 1.8 – 2.0 V vs. RHE (Fig. S15). Annealing (Ti,Mn)Ox films at 500 or 600 oC creates fundamental changes in their structure and


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electronic properties, which are not focus of this study and will be further investigated comprehensively. Acid Stability Annealing enhanced acid stability of crystalline (Ti,Mn)Ox coatings. Annealed 16-1 (Ti,Mn)Ox films can successfully function as protective coatings for corrosion-sensitive electrodes undergoing acidic water oxidation, despite changes in morphology after thermal treatment at above 500 oC (Fig. S5). These FTO/(Ti,Mn)Ox/Ir electrodes show relatively low overpotentials for water oxidation, 290 – 456 mV overpotential at 1 – 2 mA∙cm-2, because it allows for low-resistance conduction to, low-overpotential, acid stable Ir catalysts. These electrodes have also repeated shown to function in a pH = 1 electrolyte for over 120 h of reaction (Fig. 8a), and to operate in a pH = 0 electrolyte for over 200 h of water oxidation (Fig. 8b). While annealing at temperatures above 400 oC improves the chemical stability in acid under no applied potential (Fig. S10), 16-1 annealed at 400 oC can only withstand about 24 h of electrochemical water oxidation in pH = 1 H2SO4, before suffering a large increase in overpotential indicative of loss of coatings and catalysts (Fig. S11). However, 16-1 annealed at 500 and 600 oC exhibit both chemical and electrochemical durability, with minimal drop-off in current density during chronoamperometry (Fig. 8). When deposited on substrates that are sensitive to corrosion and dissolution in acid, the Ir catalysts may lose their water-oxidation activity for two reasons: Since the FTO substrates dissolve in acid when not protected, FTO/Ir electrodes, without a protective (Ti,Mn)Ox coating, can degrade, leaving behind a bare tin oxide surface (Fig. S14b); high Mn-content (Ti,Mn)Ox coatings may gradually dissolve in acid, also leaving behind a bare tin oxide surface. Due to the initial loss of the coatings layer, subsequent lift-off of the Ir catalysts can start from weak points and extend to the entire electrode area, causing an eventual dissolution of the entire coating. Certain crystallinity of


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(Ti,Mn)Ox films enables the observed chemical and electrochemical stability in acid, and such exceptional stability is correlated with high annealing temperatures, polycrystallinity and Mn oxidation states. These findings highlight that a new class of Mn-modified TiO2 films enable anodic charge transfer to acid stable, high work function water-oxidation metal co-catalysts, thus improving the stability of the electrodes materials for water oxidation in acidic media. Conclusions In this study we used an ALD supercycle technique to synthesize alternating cycles for TiO2 and MnO in various ratios to make homogeneous solid solutions of (Ti,Mn)Ox, and we characterized the material properties both as-grown and after annealing in air. Annealing is shown to be a critical component to unlocking both the charge transfer and protective capabilities of (Ti,Mn)Ox films: 1) the oxidation state of Mn changed from 2+ to 3+ across all Mn compositions; 2) the position of the Fermi level shifted towards the valence band maximum with Mn alloying; and 3) crystallization of as-grown amorphous films was correlated with improved acid stability. With the inclusion of increasing Mn content, along with post-synthesis annealing, the Fermi level shifted towards the valence band maximum because of Mn-related defect states. We showed successful charge transfer to Fe(CN)63-/4- redox couples, despite a large 2 eV barrier for hole transfer to the electrolytes caused by the very deep valence band edge of the protective coating. We also demonstrated the ability of (Ti,Mn)Ox films to conduct anodic current to high work function metals. While as-grown (Ti,Mn)Ox films dissolve in acid, their stability can be readily improved by either annealing or by growing more Ti rich films. We successfully demonstrated the utility of these films in protecting corrosion-sensitive FTO electrodes from electrolytes as acidic as pH = 0 (sulfuric acid) and in performing water oxidation in acid for over 200 h.


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ASSOCIATED CONTENT The following files are available free of charge. Addition SEM images, XPS data and summaries of XPS peak fittings, charge transfer experiments and water oxidation results are provided in the Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: +1-203-737-6521. Fax: +1-203-432-4387 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The TEM analysis by Y. Xie and J. J. Cha was supported by NSF EFMA 1542815. ACKNOWLEDGMENT The authors would like to thank Dr. Min Li at Yale’s Materials Characterization Core (MCC) for his invaluable help with SEM, XRD and XPS, Dr. Michael Rooks at the Yale Institute for Nanoscience and Quantum Engineering for the use of the Ir sputter coater and Prof. Gary Brudvig for the use of the UV-Vis spectrophotometer. We also thank the start-up support from the Tomkat Foundation. ABBREVIATIONS


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FTO, fluorine doped tin oxide; TDMAT, tetrakis-dimethylamido-titanium; Mn(EtCp)2, bis(ethylcyclopentadienyl)manganese; ALD, atomic layer deposition; SEM, scanning electron microscopy; XPS, x-ray photoelectron spectroscopy. REFERENCES (1) Jin, J.; Walczak, K.; Singh, M. R.; Karp, C.; Lewis, N. S.; Xiang, C. An Experimental and Modeling/Simulation-Based Evaluation of the Efficiency and Operational Performance Characteristics of An Integrated, Membrane-Free, Neutral pH Solar-Driven Water-Splitting System. Energy & Environmental Science 2014, 7 (10), 3371-3380, DOI: 10.1039/C4EE01824A. (2) Singh, M. R.; Papadantonakis, K.; Xiang, C.; Lewis, N. S. An Electrochemical Engineering Assessment of the Operational Conditions and Constraints for Solar-Driven Water-Splitting Systems at Near-Neutral pH. Energy & Environmental Science 2015, 8 (9), 2760-2767, DOI: 10.1039/C5EE01721A. (3) Haussener, S.; Xiang, C.; Spurgeon, J. M.; Ardo, S.; Lewis, N. S.; Weber, A. Z. Modeling, Simulation, and Design Criteria For Photoelectrochemical Water-Splitting Systems. Energy & Environmental Science 2012, 5 (12), 9922-9935, DOI: 10.1039/C2EE23187E. (4) Sata, T.; Tsujimoto, M.; Yamaguchi, T.; Matsusaki, K. Change of Anion Exchange Membranes in an Aqueous Sodium Hydroxide Solution at High Temperature. Journal of Membrane Science 1996, 112 (2), 161-170, DOI: 10.1016/0376-7388(95)00292-8. (5) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T.; Zhuang, L. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy & Environmental Science 2014, 7 (10), 3135-3191, DOI: 10.1039/C4EE01303D. (6) Wu, J.; Yuan, X. Z.; Martin, J. J.; Wang, H.; Zhang, J.; Shen, J.; Wu, S.; Merida, W. A Review of PEM Fuel Cell Durability: Degradation Mechanisms and Mitigation Strategies. Journal of Power Sources 2008, 184 (1), 104-119, DOI: 10.1016/j.jpowsour.2008.06.006. (7) Huang, C.; Xu, T. Electrodialysis with Bipolar Membranes for Sustainable Development. Environmental Science & Technology 2006, 40 (17), 5233-5243, DOI: 10.1021/es060039p. (8) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137 (13), 4347-4357, DOI: 10.1021/ja510442p. (9) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 2013, 135 (45), 1697716987, DOI: 10.1021/ja407115p. (10) Blakemore, J. D.; Schley, N. D.; Olack, G. W.; Incarvito, C. D.; Brudvig, G. W.; Crabtree, R. H. Anodic Deposition of a Robust Iridium-Based Water-Oxidation Catalyst From Organometallic Precursors. Chemical Science 2011, 2 (1), 94-98, DOI: 10.1039/C0SC00418A. (11) Sheehan, S. W.; Thomsen, J. M.; Hintermair, U.; Crabtree, R. H.; Brudvig, G. W.; Schmuttenmaer, C. A. A Molecular Catalyst for Water Oxidation That Binds to Metal Oxide Surfaces. Nature Communications 2015, 6, 6469, DOI: 10.1038/ncomms7469


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Table of Contents (TOC) Figure (Ti,Mn)Ox/Ir "leaky"TiO2/Ir

(Ti,Mn)Ox coatings CB eEf Defects VB

Ir

“leaky” TiO2 coatings CB Ef Defects

eIr

VB


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Stable Water Oxidation in Acid Using Manganese-Modified TiO2

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