Electrochemical screening of tungsten trioxide - nickel oxide thin film

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Electrochemical screening of tungsten trioxide - nickel oxide thin film combinatorial library at low nickel concentrations Jun-Seob Lee, Cezarina Cela Mardare, Andrei Ionut Mardare, and Achim Walter Hassel ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.8b00117 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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ACS Combinatorial Science

Electrochemical screening of tungsten trioxide - nickel oxide thin film combinatorial library at low nickel concentrations

Jun-Seob Lee,†,‡,§ Cezarina Cela Mardare,§,||, Andrei Ionut Mardare,*,†,§ and Achim Walter Hassel†,§,||

†Competence

Centre for Electrochemical Surface Technology (CEST), Viktor Kaplan Str. 2,

2700 Wiener Neustadt, Altenberger Str. 69, 4040 Linz, Austria ‡Department

of Metallurgy and Advanced Materials Engineering, Changwon National

University, 20 Changwondaehak-ro, 51140 Changwon, S. Korea §Institute

for Chemical Technology of Inorganic Materials (TIM), Johannes Kepler University

Linz, Altenberger Str. 69, 4040 Linz, Austria ||Christian

Doppler Laboratory for Combinatorial Oxide Chemistry at TIM, Johannes Kepler

University Linz, Altenberger Str. 69, 4040 Linz, Austria

Corresponding authors: Assoc. Prof. Dr. Andrei Ionut Mardare, Prof. Dr. Achim Walter Hassel E-mail: [email protected], [email protected]

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ABSTRACT

The electrochemical behavior of a tungsten trioxide-nickel oxide (WO3-NiO) thin film library was investigated using scanning droplet cell microscopy (SDCM) in 0.1 mol dm–3 sodium perchlorate (NaClO4) solution. The WO3-Ni film library was deposited by thermal coevaporation on an indium tin oxide (ITO)-coated glass substrate in an atomic Ni concentration range from 2.8 to 15.6 at.%. After an oxidation/crystallization heat treatment, the Ni was oxidized and the crystal structure of WO3-NiO was transformed from monoclinic WO3 (3.5 at. % Ni) to cubic WO3 (up to 7.1 at.% Ni) and again to monoclinic WO3 when the Ni amount increased (> 11.8 at.%). Proton (H+) intercalation (cathodic reaction) and deintercalation (anodic reaction) into the WO3-NiO mixed phases was induced. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) analysis revealed that the WO3-NiO film has ntype bilayer capacitive property, with the outer capacitive layer having a higher defect density than the inner capacitive layer. With a Ni concentration of 7.1 at.%, the WO3-NiO film was the most defective in the library. Introduction of the Ni cation into the WO3 network was associated with changes of the semiconducting properties of the film.

Keywords: thin film combinatorial library; metal oxides; scanning droplet cell microscopy (SDCM); electrochemistry

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INTRODUCTION

As the use of fossil fuels has increased, environmental pollution has become an increasingly important issue. Therefore, the development of sustainable energy sources is of paramount importance over fossil fuels with limited availability. Solar fuel, which uses solar energy to synthesize chemicals, has the advantage of utilizing existing infrastructure such as Liquefied Petroleum Gas (LPG) stations. Much attention has been paid to hydrogen in solar fuels because hydrogen has the advantage of producing fuel from water. Photoelectrochemical water splitting (PWS) on semiconducting surfaces using solar irradiation is of great interest to scientists and engineers looking to produce hydrogen and oxygen gas from water.1 Since titanium dioxide (TiO2) was suggested as the first semiconducting oxide for PWS and platinum (Pt) was suggested for oxygen and hydrogen generation1, a large number of transition metal oxides such as titanium (Ti),2,3 iron (Fe),4,5 and tungsten (W) oxide6,7 have been reported to be of interest due to their electrochemical stability in acidic or alkaline aqueous solutions. Since photoelectrochemical properties are strongly associated with the efficiency of PWS on transition metal oxides, it is important to control doping elements concentrations in the oxides. Although the introduction of impurity levels in the oxides using hydrothermal8-10 and evaporation11-13 techniques has been reported in many studies, it is still challenging to control a compositional gradient of dopants in the oxides. The combinatorial co-evaporation method for fabricating thin oxide film libraries is a promising technique to ensure a compositional gradient of metallic and/or oxide materials as coatings. Using this technique, many types of combinatorial libraries, such as metal-metal14 and oxide-oxide15 libraries have been generated. The transition metal oxide tungsten trioxide (WO3) is an n-type semiconducting oxide that is more promising for PWS than TiO2 because of its narrower band gap (approximately 2.8 eV for WO3 vs. 3.2 eV for TiO2). The bottom

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levels of the conduction (CB) and the top level of valence bands (VB) of WO3 are more positive than the reduction potential of hydrogen (0 V vs. standard hydrogen electrode potential, SHE in pH 0) and the oxidation potential of oxygen (1.23 VSHE), while the bottom level of the CB of TiO2 is more negative than the reduction potential of hydrogen. Under sufficient solar irradiation, less photon energy is required to excite electrons from the VB into the CB of WO3 than in TiO2. Various studies concerning the improvement of WO3 efficiency toward hydrogen and/or oxygen generation by doping transition metal cations such as chromium (Cr),16 iron (Fe),17,18 cobalt (Co),19,20 or nickel (Ni) have already been conducted.21,22 Particularly, nickel oxide (NiO), which has a p-type semiconducting property, is doped into WO3 in order to introduce active sites for hydrogen generation and improve the photocatalytic activity. It has been reported that NiO has a higher suppression of charge recombination of electrons and holes than that of Fe-, Co-, or Cr-oxide.23 The addition of the metal cations to WO3 changes the efficiency of photocatalytic processes by changing the semiconducting properties of WO3. Thus, it is important to investigate in detail the properties of WO3 in order to understand the photocatalytic process of PWS on WO3-NiO. In our previous research, a WO3-NiO thin film library was fabricated using a combinatorial co-evaporation method with a composition gradient of an atomic concentration range of Ni (cNi) from 3.8 to 61.7 at.% in WO3.24 The photocurrent was measured using photoelectrochemical scanning droplet cell microscopy (PE-SDCM). The anodic current during the irradiation with ultra violet (UV) light with a wavelength of 405 nm peaked at a cNi of 6.2 at.% and then drastically decreased with increasing cNi in the WO3. It was suggested that the photoelectrochemical reaction on WO3-NiO is related to its semiconducting properties. Since the electrochemical behavior at cNi of approximately 7 at.% in WO3 is important in order to understand the photoelectrochemical properties of the WO3-NiO thin film library, it 4


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is necessary to study in detail both the electrochemical and semiconducting properties of the library with a higher compositional resolution than previously reported. In this study, a WO3NiO thin film library was fabricated using the combinatorial co-evaporation method with the maximum cNi of 15.6 at.%, and investigations were focused on the electrochemical behavior using an SDCM. EXPERIMENTAL PROCEDURES

Thin film library fabrication The WO3-Ni compositional spread was obtained by co-evaporation from WO3 (99.9%, Fluka Analytical) and Ni (99.0%, chemPUR) individual sources. These were realised by W boats that were positioned at a distance of 120 mm from the substrate. Borosilicate glass sputter-coated with indium tin oxide (ITO, 15 Ω/sq., Kintec Co.) was cut in 26 × 76 mm2 pieces to be used as substrates for the WO3-Ni compositional spread. The ITO-substrates were rinsed in three steps with acetone, ethanol and ultra-pure water (18.2 MΩ cm), and then dried in N2 flow before use. Amounts of WO3 and Ni (in the range of 1 g) were independently heated in vacuum to their evaporation temperatures (due to Joule effect) by direct current sources (3.3 kW, Keysight Technologies). In this process, up to 350 and 220 W were necessary for WO3 and Ni, in order to obtain stable evaporation rates of 600 and 5 pm s‒1, respectively. These very different deposition rates were pre-calculated for obtaining very low amounts of Ni mixed in vapor phase with WO3. Quartz crystal microbalances (QCMs, Inficon) monitored in-situ the deposition rates and the power applied to each deposition source were adjusted in real time for keeping the deposition conditions stationary. The vacuum chamber base pressure of 10‒5 Pa increased to 10‒3 Pa during co-deposition as a result of the evaporating material. The final thickness of the WO3-Ni thin film library was approximately 300 nm. 5



Heat treatment and surface characterization In order to obtain the final WO3-Ni library, a heat treatment of the as-deposited WO3-xNi library was conducted in the air for 18 h at 673 K with a heating rate of 3 K min−1 followed by natural cooling in the furnace. The heat treatment re-oxidizes the deposited WO3-x that lost some oxygen due to partial decomposition during the evaporation process as indicated by the subscript x in the formula.25 In parallel, this heat treatment oxidizes metallic Ni to NiO in the as-deposited film. After the heat treatment, surface morphology and an atomic elemental composition mapping of the WO3-NiO thin film library was carried out using a Zeiss 1540-XB scanning electron microscope (SEM; Oxford Instruments). The composition of the film was characterized by X-ray photoelectron spectroscopy (ThetaProbe; Thermo Scientific™). The XPS measurements were conducted with monochromatic AlKα radiation (power: 100 W) and a spot diameter of 400 μm. No charge compensation or reference was used. Before the measurements, each addressed spot was sputtered (Ar+) at an etching rate of 0.05 nm s–1 (calibrated on SiO2) for 60 s in order to remove the surface contamination. Due to the very different nature of the atomic species in the library (as compared to SiO2), a true superficial sample sputtering is expected, in the monolayer scale. Survey scans were taken over a wide energy range from 0 to 1400 eV, and quantitative analysis was performed by numerical integration of W4f, O1s, and Ni2p3/2 peaks. Following the XPS quantification, an atomic concentration of W and Ni in the WO3-NiO compositional spread is designated as cW and cNi, respectively. X-ray diffraction experiments (Philips X’Pert Pro, CuKα radiation) in BraggBrentano and grazing incidence geometries were performed at dedicated locations along the WO3-NiO thin film in a 2θ range from 10 to 90° with a scan rate of 0.002 ° s−1.

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Electrochemical setup and measurements A scanning droplet cell microscope (SDCM) was used to perform electrochemical measurements along the compositional spread. The SDCM microfluidic channels system was fabricated by 3D printing (ProJet 6000 stereo, 3D Systems). A detailed description of the fabrication procedure of the SDCM is given in a previous study.26 A circular area of 0.03 cm2 at the tip of the SDCM body was used to expose the investigated spot to the electrolyte. The SDCM was controlled by an X-Y-Z stage in order to address the desired coordinates on the WO3-NiO thin film library. A force sensor ensures that the SDCM tip contacts the library surface at a constant force of 800 ± 20 mN in order to avoid electrolyte leakage. A potentiostat (B08036, Ivium) was used to perform electrochemical measurements in a three-electrode configuration with the SDCM. The counter, working and reference electrodes were a Pt wire, the thin film library itself, and an Ag/AgCl electrode in 3 mol dm–3 NaCl solution with a nanoporous glass frit bridge, respectively. As electrolyte for electrochemical experimentation, 0.1 mol dm–3 NaClO4 solution was used at room temperature without deaeration. Into this, 0.1 mol dm–3 HClO4 was added for adjusting the final electrolyte pH to 3.3. For all compositions (all values of cNi) within the W-Ni-oxide thin film library, the open circuit potential (OCP) analysis, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and/or Mott-Schottky (M-S) analysis were performed. After monitoring the OCP for 600 s, cyclic voltammetry was carried out in a potential range between −1.0 and 1.0 VAg/AgCl/3M NaCl with a scan rate of 0.05 V s‒1. Additionally, after monitoring the OCP for 600 s, the spots on the working electrode (thin film library) locally addressed by the SDCM were polarized to 0.5 VAg/AgCl/3 M NaCl for 600 s, and then EIS, or M-S analysis was promptly conducted. When performing EIS measurements, a perturbation of ± 0.01 V in a frequency range from 104 to 10–2 Hz was superimposed to the electrode potential. The M-S analysis was 7



conducted at 0.1 or 5 Hz with a perturbation of ± 0.01 V and stepwise-shifted potentials from 1.0 to −1.0 VAg/AgCl/3 M NaCl. The EIS and M-S experimental data were fitted using commercial software (IviumSoft, Ivium).

RESULTS AND DISCUSSION Composition of W-Ni-oxide film Before electrochemical screening the WO3-NiO film library, a compositional mapping was performed. Figure 1 shows the compositional profile of W and Ni as measured by XPS along the WO3-NiO film library. The amount of O is absent due to the uncertainty induced through sputtering the film surface. The W concentration decreases with the increase of the Ni amount from the left side (position, X = 0 mm) to the right side (X = 76 mm) of the library. Before the quantitative XPS analysis, Ar sputtering of the surface was performed for removal of possible contaminants. This was done carefully in a calculated depth range of one monolayer for avoiding the possible compositional change of the sample due to unevenly removal of surface species. The observed W and Ni compositional distribution along the library is a direct result of the different deposition rates used for WO3 and Ni, combined with individual atomic/molecular spatial distribution profiles dictated by the deposition geometry and molecular masses. The desired compositional zoom in the low Ni amount region had as a result reaching the lowest limit of the Ni atomic distribution profile. Low amounts of Ni up to 4 at.% were measured along the first quarter of the library (from left to right in Figure.1), with a compositional gradient of below 1 at.% cm-1. This value doubled past the middle of the library when the cosine law (of the deposition angle) governing the atomic distribution on the substrate surface becomes relevant. A maximum Ni concentration of 15.6 at.% was measured

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at X = 71.5 mm. The compositional profile of W follows closely the same pattern in a complementary fashion. When compared to a compositional mapping via energy dispersive X-ray spectroscopy (EDX, not shown here) the XPS mapping from Figure 1 is slightly different only for X > 50 mm. Above this abscissa value the XPS map indicates a slight compositional flattening. The EDX data resembles very well the results from our previous study but the penetration depth is much larger as compared to the XPS currently used.24 If EDX analyses characteristic X-rays from as deep as several µm, XPS only analyses photoelectrons from the very first several nm on the surface. This may lead to concluding that a superficial layer along the library for X > 50 mm may have a slightly different composition as compared to the bulk composition of the entire thin film. In-depth slight compositional changes are commonly observed by XPS in thin film libraries for both metallic and oxidized, light and heavier species.27,28

Microstructure of W-Ni oxide film Diffractograms acquired by XRD along the WO3-NiO library after the heat treatment are shown in Figure 2(a), together with the pattern of the ITO coated glass that acted as a conductive substrate and with the reference patterns from the ICDD database for monoclinic WO3 (PDF #01-083-0950) and cubic WO3 (PDF #00-041-0905). The Miller indices from main reflections of both WO3 polymorphs are also indicated in the figure. In the upper site of the diffractograms the positions from main reflections of NiO (PDF #00-047-1049) and NiWO4 (PDF # 00-015-0755) phases are indicated by arrows. In the right-hand side of the figure, the corresponding Ni concentrations (complementary W) as provided by both EDX and XPS measurements are shown. Both concentration values are presented because on one hand the XRD measurements provide the information from the entire depth of the film, thus

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the EDX value are more relevant, whereas for all the other characterization, including the grains visualization by microscopy, the outer surface plays the main role, therefore the XPS values are shown. As a general observation, beside ITO peaks, reflections from crystalline WO3 emerge along the entire library. No reflections that can be attributed to spurious phases (e.g. NiO) or mixed oxide phases (e.g. NiWO4) can be observed. When comparing the peaks positions for cubic and monoclinic WO3, it is clear that the discrimination between these two phases is not trivial due to the fact the main peaks are very near to each other (2θ ≈ 24 and 34°). The monoclinic WO3 phase (m-WO3) is the stable crystallographic polymorph at room temperature,29 but a higher symmetry (cubic) can be stabilized by introduction of impurities.30 Edge-sharing octahedras caused by oxygen deficiency in WO3 were found responsible for the change of symmetry from lower ordered to cubic. Careful analysis of the peaks positions from both Figure 2(a) and 2(b) (plot of the diffractograms in restricted 2θ ranges for increased visibility of peaks position) reveals that for the lowest Ni concentration (3.5 at.%), the main peak from 2θ ≈ 24.3° is in between the main reflections from monoclinic and cubic WO3 (cWO3), but much closer to the former. With increasing the Ni content up to 5.3 at.%, there is a continuous shift of this peak towards lower angles (2θ ≈ 24.1°), and therefore it becomes closer to the main peak from c-WO3. Analyzing other peaks positions (i.e. at 2θ ≈ 34.2°, 49.7°, 55.8° and 61.6°) it can be observed that within this concentration range (3.5 - 5.3 at.% Ni) the peaks are at first (for 3.5 at.% Ni) closer to the peaks of m-WO3, and with the increase in the Ni concentration they are shifting towards the peaks from c-WO3. Therefore it can be inferred that the m-WO3 phase is most probably present for the sample containing approximately 3.5 at.% Ni, and that this amount of Ni is not enough to stabilize the c-WO3. Additionally, the peaks relative intensities and their Miller indices indicate the texturing of the film, with strong preferential orientation in [200] and [202] directions. The presence of preferentially oriented

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m-WO3 and not of the c-WO3 phase is supported by the reflection present at 29.1°, which can be found only for the m-WO3 phase, while no reflection belonging to c-WO3 is present around this 2θ value. With increasing the Ni concentration to 5.3 at.%, the peaks shift towards c-WO3 phase, indicating a transition from the m-WO3 to c-WO3. The presence of Ni within this concentration range (≈ 4 - 5.5 at.% Ni) has a similar effect to the presence of oxygen vacancies due to the lower oxidation number of Ni2+ as compared to W6+. Consequently, the formation of a structure close to the c-WO3 was promoted.. A similar effect was reported in thin films containing low concentrations of Ti in WO3 (10 at.% Ti and 90 at.% W).30

The appearance of an additional peak at 2θ = 23.2° for cNi = 5.9 at.% indicates the incipience of transition from cubic towards polycrystalline monoclinic phase. The coexistence of both cubic and monoclinic phases extends over a compositional region of cNi from approximately 5.9 to 10.6 at.%, and for cNi higher than 10.6 at.% only stable monoclinic WO3 reflections were identified. It was proposed that for cNi in the range of 4 - 5.5 at.%, the Ni2+ replaces W6+ into the atomic network and the number of Ni atoms is sufficiently high to lead to the formation of a cubic structure. With the increase of cNi in the WO3 above this threshold (5.9 at.%), the formation of the stable monoclinic structure is promoted. Therefore, it can be inferred that Ni2+ is no longer replacing W6+ into the WO3 network. More likely, an additional Ni-containing oxide phase segregates concomitant with the formation of monoclinic WO3 phase. From the NiO-WO3 phase diagram,31 the only oxide phases that can form are NiO, WO3 and NiWO4. Literature reports of two additional phases, Ni0.19WO4 and Ni0.24W1.3O4, can also be found.32 Bragg-Brentano operating geometry of the X-ray system did not reveal other oxide phases besides WO3 and the ITO layer underneath WO3. In order to assess only the oxide surface to investigate a possible formation of segregated phases, grazing incidence 11



X-ray diffraction measurements were performed over larger area, in selected regions along the library at different incidence angles ranging from 0.3 to 1° (patterns are not shown here). The NiO reflections were not identified in any compositional region, as indicated by the position markings (disks) placed in Figure 2 at 2θ of approximately 37, 43 and 63°. These measurements also did not show any additional reflections. Therefore, even though the formation of a mixed Ni(-W)-O phase cannot be excluded (positions indicated by rhomboids in Figure 2(a)), it can be inferred that if present, its amount is extremely low or it is not crystalline, thus being absent from the diffractograms. Figure 3 presents a tableau of images acquired at different cNi along the WO3-NiO film library surface (concentrations obtained from XPS are shown in the upper right side of each image). Severe modifications of the surface appearance can be observed with the increase of cNi in the WO3. For the lowest cNi (3.4 at.%) the surface comprises of small crystals with the size in the range of tens of nm and the overall aspect indicates a porous structure. With the increase of cNi (7.1 – 11.2 at.%), large round crystals (100 - 150 nm) develop on the surface, and concomitant the structure of the underlying layer becomes much more structured. These round grains covering the surface do not grow bigger with the increase of cNi in the WO3 and image analysis revealed that these grains surface coverage does not vary severely within this concentration spread. For even higher cNi than 14 at.%, the crystals on the surface are present in a decreased number, but their size appears to be much larger (> 250 nm) than for lower cNi in the WO3. In the same time, the underlying layer appears very structured in what seem to be separated domains that comprise of very fine grains. The changes observed in the microstructure in different compositional regions correlate well with the phases found in the XRD patterns. For cNi = 3.5 at.% (Figure 3a), interconnected crystals forming a porous structure are present, and the diffractogram shows the presence of textured m-WO3. For the next analyzed point (cNi = 3.9 at.%), the outer 12


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surface structuring is visible with the formation of roundish larger crystals. In this composition region, the peaks in the XRD pattern showed a strong shift toward c-WO3. Therefore, it can be inferred that the large roundish crystals emerging on the surface belong to c-WO3.In the compositional region approximately between 7 and 11 at.% Ni, the XRD revealed the shift of the main peak (2θ ≈ 24.1°) of c-WO3 toward m-WO3 (2θ ≈ 24.3°) together with the gradual development of m-WO3 peaks from (2θ ≈ 23.1 and 23.6°). The surface microstructure is composed of large crystals covering on underlying layer. The correlation reveals that the large crystals are consists solely of the c-WO3 phase stabilized by a defined Ni content, and simultaneously m-WO3 develops as the underlying layer. This is also supported by the observation that the coverage of these crystals does not change severely with the increase of Ni content, since the stabilization of c-WO3 is restricted to a defined Ni concentration, and above this level, m-WO3 develops together with an additional Ni containing phase. For even higher Ni concentrations (< 14 at.%), only a few large crystals cover the surface and only the monoclinic WO3 was detected. It can be concluded that the few very large crystals contain the Ni-stabilized c-WO3 crystals and most of the film consists now of m-WO3. Since no strong Ni enrichment in the grains was found in the chemical composition analysis, as well as no Ni-containing phases in the XRD24, it can also be inferred that the large crystals from the present study are composed only Ni stabilized c-WO3, with the underlying layer retaining the Ni atoms. Possible Ni-containing phase segregation is difficult to be proven at this nanometer scale, but as seen in the insets from Figure 3(c) – (i), some small nanoparticles are present on the surface of the structured grains of the underlying layer. Corroborating their presence with the formation of the stable m-WO3 phase it might be presumed that these nanoparticles belong indeed to a Ni-containing oxide segregated via diffusion from the monoclinic WO3.

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Cyclic voltammetry of WO3-NiO library Figure 4 (a) shows the open-circuit potential (OCP) of WO3-NiO film as a function of cNi in contact with 0.1 mol dm–3 NaClO4 after 600 s. The OCP shifts strongly in the negative potential direction from 0.57 to 0.23 VAg/AgCl/3 M NaCl when cNi reaches 11.2 at.%. Then, the OCP only slightly increases with the cNi increase, indicating that doping levels of Ni cations in WO3 structure may affect the electrochemical properties of the mixed oxide film. Figure 4 (b) shows cyclic voltammograms of the WO3-NiO film library at various Ni concentrations. The anodic and cathodic current peaks are both present at approximately –0.5 VAg/AgCl. The anodic and cathodic reactions are related to deintercalation and intercalation of protons (H+) into WO3:6,7,16-22

WO3 + nH+ + ne− = HnWO3

(1)

The color of the WO3 film changes from transparent to deep blue during the anodic reactions (electrochromic reaction), while the color of the WO3 film changes from deep blue to transparent during the cathodic reaction. The anodic and cathodic reactions at the higher and lower potentials of 0.8 and –0.5 VAg/AgCl/3

M NaCl

are associated with O2 and H2 gas

generation, respectively. When cNi increases in the WO3, both anodic and cathodic currents decrease, indicating that the deintercalation and intercalation reaction rate from the solution to the oxide film becomes slower when cNi increases in the film. Figure 4 (c) shows the transient electrical charge (Q) during the CV for the WO3-NiO films as a function of cNi change. The Q value increases and decreases during the anodic and cathodic potential regions, respectively. The final Q value after the CV scan at 0.2 VAg/AgCl/3 M NaCl shows a negative value below – 50 µC irrespective of the cNi in the WO3 film. The values of the anodic current peak observed during the CV experiments are presented as a compositional mapping in Figure 4(d) for 14


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precise observation of behavioral trends. Since the anodic peak current also gradually decreases with an increase of cNi in the WO3 film, it can be inferred that the presence of Ni cations in WO3 could hinder the electrochemical protons intercalation and deintercalation reaction in the WO3 film.

Semiconducting properties of WO3-NiO films Figure 5 (a) presents selected Bode plots measured along the WO3-NiO film library as a function of cNi. The equivalent circuit used is depicted in the inset of Figure 5 (a) and shows that the WO3-NiO film has a bilayer capacitive property, with defined outer and inner capacitive layers. This conclusion is a direct consequence of the two-time constants apparent in the Bode plots from Figure 5 (a). The Rs is the solution resistance, CPE is the constant phase element, and Rinner and Router are charge transfer resistances of the inner and outer layers, respectively. The impedance of CPE ZCPE is expressed by:

1

𝑍CPE =

(𝑗𝜔)𝛼𝑌0

,

(2)

where j is the imaginary number, ω is the angular velocity, Y0 is the CPE constant and α is the depression angle. The α value lies between 0.5 and 1. When α = 1, the CPE describes an ideal capacitor. The CPE can be substituted with the interfacial capacitance C:33

𝐶 =

𝑌0

1 𝛼

1 (𝑅 s

+

1

𝑅𝑙𝑎𝑦𝑒𝑟)

(𝛼 ― 1) 𝛼

,

(3)

The capacitance of the film/electrolyte interface consists of the space charge layer capacitance CSC and the capacitance of the Helmholtz layer CH that are connected in series: 15



1 𝐶

=

1 𝐶SC

1

+ 𝐶H.

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(4)

The values of R and C are shown in Figures 5 (b), and (c), respectively. For all cNi in the WO3 film, the values of R is higher for the inner capacitive layer than for the outer capacitive layer indicating that the inner capacitive layer of the film is less conductive than the outer capacitive layer, irrespective of the cNi in the film. The values of Router, and Rinner decrease when cNi increases up to 11.2 at.%. Then, those values gradually increase for cNi up to 15.6 at.%. However, the values of Rs are almost invariable because the electrochemical or chemical reaction on the film or ITO-substrate is too low to change the solution’s conductivity. The C values increase with the maximum at cNi of 8.5 at.%. Afterward, the C gradually decreases with increasing cNi up to 15.6 at.% in the film. The series of changes to R and C indicate that the outer and inner capacitive layers become conductive when cNi is lower than 11.2 at.%. However, those layers become less conductive when cNi in the WO3-NiO film increases above 11.2 at.%. The capacitance of the space charge layer was measured at 10‒1 and 5 Hz, as indicated by the capacitance shown in Figure 5 (a). These frequencies are in the region dominated by a capacitive response. Figures 6 (a) and (b) show Mott-Schottky (M-S) plots of the WO3-NiO film vs. the change of cNi after the polarization at 0.5 VAg/AgCl/3 M NaCl for 600 s at 10‒1 or 5 Hz, respectively. For all cNi in the WO3-NiO film, positive slopes are found at potentials from 0.7 to 0.4 VAg/AgCl/3

M NaCl.

The positive slope of the M-S plot demonstrates that the library

consists of n-type semiconductors, regardless of the cNi in the film. The Mott-Schottky equation of an n-type semiconductor is defined as follows:

16


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1 𝐶2SC

1

1

2

2

= (𝐶 ― 𝐶H) = (𝜀𝜀0𝑒𝑁D)(𝐸 ― 𝐸𝑓𝑏 ―

kT 𝑒)

,

(6)

where ε is the dielectric constant of pure WO3 of 44.7,34 ε0 is the vacuum permittivity constant, e is the elementary charge, ND is the donor density, Efb is the flatband potential, k is the Boltzmann constant, and T is the absolute temperature. The thickness of the space charge layer dsc is expressed as follows:

𝑑SC =

𝜀𝜀0 𝐶SC

,

(7)

The value of dsc calculated by the M-S result is shown in Figure 7. The value of Csc in the WO3-NiO film measured at 5 Hz appears higher than the one measured at 10‒1 Hz, independent of the cNi in the film, indicating that the dsc at 5 Hz is thinner than the one at 10‒1 Hz. Both dsc values measured at 10‒1 and 5 Hz sharply decrease approximately from 3 to 1 nm when the cNi increases from 3.4 to 8.5 at.%. Then, the dsc gradually increases to approximately 3 nm along with the increase of cNi to 15.6 at.% in the film. Since the film thickness of the WO3-NiO film determined during the deposition process with quartz crystal microbalances was approximately 300 nm and comprises of both outer and inner layers, the dsc of several nanometers is too thin to discuss the thickness of space charge layers corresponding to bilayers in the WO3-NiO film. Figures 8 (a) and (b) show Efb and ND in the WO3-NiO film as functions of cNi. The Efb at 10‒1 Hz is higher than that at 5 Hz. This means that the outermost capacitive layer in the WO3-NiO film is positioned at a higher electronic energy level as compared to the other capacitive layer in the film. The Efb gradually shifts to the negative potential direction when cNi increases from 3.4 to 15.6 at.% and then remains at 0.1 and 0.05 VAg/AgCl/3 M NaCl at 10‒1 and 5 Hz, respectively. This indicates that the structure 17



Page 18 of 42

and chemical composition of the film could change as cNi increases in the film. For all cNi in the WO3-NiO film, the ND at 5 Hz is approximately twice as high as that at 10‒1 Hz and the value of ND sharply increases when cNi increases between 3.4 and 8.5 at.%. Afterward, it gradually decreases as cNi increases to 15.6 at.% in the film. This means that the donor levels are mainly introduced in the outermost capacitive layer of the WO3-NiO film, and the film becomes less defective when cNi increases. Since the donor in an n-type oxide is presented as an interstitial metal cation (Mi••) or vacancy of an oxygen anion (VO ″ ), interstitial Ni cations (Nii••) or VO″in the WO3 layers are thought to be the reason for the change of ND in the WO3. The semiconducting properties such as bandgap and carrier mobility of WO3-NiO film are strongly related to its photoelectrochemical water splitting (PWS) capability. In order to improve the PWS efficiency, Ni cations are doped as impurity levels or co-catalysts by providing indirect charge carrier paths in WO3 or active hydrogen generation sites during photoirradiation, respectively.21,22 The optimized photocurrent on the WO3-NiO film was found at a Ni concentration of 6.2 at.% in a previous study24. In this study, it has been found a significant change in semiconducting properties as well as electrochemical behavior, at Ni concentrations in the range of 5-11 at.% along the WO3-NiO library. This is an indication that the

photoelectrochemical

behavior

is

related

to

semiconducting

properties.

The

photoelectrochemical study will be studied in the near future.

CONCLUSIONS The electrochemical behavior of a WO3-NiO film library was investigated using the SDCM technique. The WO3-NiO film library was deposited via co-evaporation of WO3 and Ni with an atomic concentration gradient of Ni (cNi) ranging from 3.4 to 15.6 at.% (complementary W). The crystal structure of WO3-NiO transformed from monoclinic cNi >

18


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3.5 at.% ) to cubic WO3 when cNi increased from 3.5 to 7.1 at.%, and above this concentration m-WO3 phase was found most probably together with an additional Ni-containing oxide as indicated by microscopy. The electrochemical chromic reaction rates of proton intercalation and deintercalation decreased with increasing Ni concentration in the WO3-NiO film. Electrochemical impedance spectroscopy and Mott-Schottky analysis performed as compositional screening revealed that an n-type bilayer capacitive property with more defective outer than inner capacitive layers was formed irrespective of cNi in the WO3-NiO thin film library. The donor density in the outermost capacitive layer of the WO3-NiO film peaked at 7.1 at.% Ni, followed by a steep decrease when the Ni amount increased to 15.6 at.% in the film. The Ni cation doped in the WO3 is directly related to the changes of the semiconducting properties of the WO3.

ACKNOWLEDGEMENTS

The support from the Competence Centre for Electrochemical Surface Technology GmbH (CEST) within the scope of the strategic project UPPER is gratefully acknowledged. The financial support by the Austrian Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology, and Development for the Christian Doppler Laboratory for Combinatorial Oxide Chemistry (COMBOX) is gratefully acknowledged. The authors are especially grateful to Mr. Michael Pichler and Dr. Jan Philipp Kollender for their assistance of the SDCM fabrication and the electrochemical measurements. This research was supported by Changwon National University in 2018-2019.

19



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Figure Captions: Figure 1 XPS composition at each point of the WO3-NiO film library.

Figure 2 (a) XRD pattern measured in Bragg-Brentano geometry on the WO3-NiO film at different concentrations of Ni. and (b) restricted 2θ ranges to enable the observation of peaks shift; the red line passing though the peaks tips is solely for eyes guidance.

Figure 3 SEM images of the WO3-NiO film at different atomic concentrations of Ni.

Figure 4 (a) Open circuit potential, (b) cyclic voltammograms (CV) with a scan rate of 0.05 V s–1 in 0.1 mol dm–3 NaClO4 solution of pH 3.3, (c) transient of electric charge consumed during CV, and (d) anodic peak current obtained from CV as a function of cNi in the WO3-NiO film.

Figure 5 (a) Bode plots of the WO3-NiO film with a change of cNi polarized at 0.5 VAg/AgCl/3 M NaCl

600 s in 0.1 mol dm–3 NaClO4 of pH 3.3. Solid lines are the fitting curves with an

equivalent circuit shown in Figure 5(a). The solid and hollow symbols are impedance and degree of phase shift, respectively. (b) solution resistance Rs and charge transfer resistance R, and (c) interfacial capacitance C from curve fitting plots as a function of cNi in the film.

Figure 6 Mott-Schottky (M-S) plot of the WO3-NiO film with a change of cNi after the polarization at 0.5 VAg/AgCl/3 M NaCl for 600 s at (a) 10‒1 or (b) 5 Hz in 0.1 mol dm–3 NaClO4 solution of pH 3.3.

24


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Figure 7 The thickness of the space charge layer, dsc, of the WO3-NiO film obtained at 10‒1 or 5 Hz as a function of cNi in the film.

Figure 8 (a) flatband potential, EFB, and (b) donor density, ND, of the WO3-NiO film as functions of cNi obtained from the Figure 6.

25



84x38mm (300 x 300 DPI)


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XPS composition (corresponding to surface composition) at each point of the WO3-NiO film library. 289x217mm (300 x 300 DPI)



2(a) XRD pattern measured in Bragg-Brentano geometry on the WO3-NiO film at different concentrations of Ni. 254x162mm (300 x 300 DPI)


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2(b) restricted 2θ ranges to enable the observation of peaks shift; the red line passing though the peaks tips is solely for eyes guidance. 254x151mm (300 x 300 DPI)



SEM images of the WO3-NiO film at different atomic concentrations of Ni. 271x203mm (300 x 300 DPI)


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4(a) Open circuit potential, 289x217mm (300 x 300 DPI)



4(b) cyclic voltammograms (CV) with a scan rate of 0.05 V s–1 in 0.1 mol dm–3 NaClO4 solution of pH 3.3 289x217mm (300 x 300 DPI)


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4(c) transient of electric charge consumed during CV 289x217mm (300 x 300 DPI)



4(d) anodic peak current obtained from CV as a function of cNi in the WO3-NiO film 289x217mm (300 x 300 DPI)


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5(a) Bode plots of the WO3-NiO film with a change of cNi polarized at 0.5 VAg/AgCl/3 M NaCl 600 s in 0.1 mol dm–3 NaClO4 of pH 3.3. Solid lines are the fitting curves with an equivalent circuit shown in Figure 5(a). The solid and hollow symbols are impedance and degree of phase shift, respectively 289x217mm (300 x 300 DPI)



5(b) solution resistance Rs and charge transfer resistance R 289x217mm (300 x 300 DPI)


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5(c) interfacial capacitance C from curve fitting plots as a function of cNi in the film 289x217mm (300 x 300 DPI)



6(a) Mott-Schottky (M-S) plot of the WO3-NiO film with a change of cNi after the polarization at 0.5 VAg/AgCl/3 M NaCl for 600 s at (a) 10‒1 or 289x217mm (300 x 300 DPI)


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6(b) Mott-Schottky (M-S) plot of the WO3-NiO film with a change of cNi after the polarization at 0.5 VAg/AgCl/3 M NaCl for 600 s at (b) 5 Hz in 0.1 mol dm–3 NaClO4 solution of pH 3.3. 289x217mm (300 x 300 DPI)



The thickness of the space charge layer, dsc, of the WO3-NiO film obtained at 10‒1 or 5 Hz as a function of cNi in the film 289x217mm (300 x 300 DPI)


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8(a) flatband potential, EFB, 289x217mm (300 x 300 DPI)



8(b) donor density, ND, of the WO3-NiO film as functions of cNi obtained from the Figure 6 289x217mm (300 x 300 DPI)


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Electrochemical screening of tungsten trioxide - nickel oxide thin film

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