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Article

Role of Oxygen Concentration in Reactive Sputtering of RuO2 Thin Films: Tuning Surface Chemistry for Enhanced Electrocatalytic Performance

by
Swapnil Nalawade
1,
Ebenezer Vondee
1,
Mengxin Liu
2,
Ikenna Chris-Okoro
2,
Sheilah Cherono
2,
Dhananjay Kumar
2 and
Shyam Aravamudhan
1,*
1
Department of Nanoengineering, North Carolina A & T State University, Greensboro, NC 27401, USA
2
Department of Mechanical Engineering, North Carolina A & T State University, Greensboro, NC 27411, USA
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(5), 417; https://doi.org/10.3390/cryst15050417
Submission received: 7 April 2025 / Revised: 23 April 2025 / Accepted: 27 April 2025 / Published: 29 April 2025
(This article belongs to the Special Issue Advanced Materials for Applications in Water Splitting)
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Abstract

:
Developing active electrocatalysts for water splitting is a great challenge due to slow four-electron transfer oxygen evolution reaction. Here, we report the effect of variable oxygen concentrations in sputtered RuO2 thin films on electrochemical performance. The impact of Ar/O2 ratios on the structural, chemical, and optical properties of sputtered RuO2 films is systematically investigated. The as-deposited amorphous RuO2 showed higher catalytic activity as compared to its annealed crystalline counterparts. The X-ray photoelectron spectroscopy results showed controlled stoichiometry with 20% oxygen. The electrochemical measurements of the RuO2 deposited with a 4:1 Ar:O2 ratio showed superior performance in cyclic voltammetry, linear sweep voltammetry, and Tafel slope. Transformation of as-deposited amorphous RuO2 into polycrystalline films is observed at 400 °C of annealing temperature. Film thickness is increased with increasing O2 concentration during deposition. This study highlights that sputtered RuO2 thin films with varying oxygen concentration during deposition can influence the electrocatalytic activities in water-splitting applications.

1. Introduction

Electrochemical water splitting has attracted much attention as an efficient pathway for carbon-free hydrogen production [1,2]. In general, transition metal oxides are used as catalysts to split water into its constituents of hydrogen and oxygen via hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. In recent years, ruthenium dioxide (RuO2) has been an extensively researched material in a variety of applications, such as electrochemical water splitting [3,4,5,6], a supercapacitor [7], pH sensing [8,9], and a coating on Li ion batteries [10]. However, water splitting is hindered by the sluggish kinetics of OER (2H2O− > O2 + 4H+ + 4e− in acid, 4OH− + O2 + 2H2O + 4e− in base) [11]. RuO2 demonstrates exceptional catalytic activity and facilitates an efficient four-electron transfer mechanism required for OER, making it a highly reliable catalyst for water oxidation.
Numerous studies have documented the development of RuO2 nanostructured electrodes for the oxygen evolution reaction (OER), achieved through a variety of synthetic approaches. Lee et al. demonstrated the OER activity of rutile RuO2 nanoparticles (NPs), which were prepared by synthesizing Ru metal NPs and subsequently thermally oxidizing them. These oxide NPs, approximately 6 nm in size, exhibited greater stability under OER conditions compared to commercial Ru/C catalysts in both acidic and basic electrolytes [12]. Studies on RuO2(110)/Ru(0001) anodes revealed that the OER exhibited a higher onset potential in HCl compared to H2SO4, driven by competition with the kinetically favorable chlorine evolution reaction (CER). While localized pitting and hydrous RuO2 formation occurred, the RuO2(110) layer largely maintained its thickness. Degradation was less pronounced in HCl, attributed to the reduced efficiency of RuO2 oxidation due to CER dominance [13]. Doria et al. examined the impact of RuO2 film thickness on the electrochemical and catalytic performance of Ti/RuO2 anodes synthesized using 2-hydroxyethyl ammonium acetate (2HEAA). Thicker coatings increased charge transfer resistance but significantly extended service life, with the thickest anode in NaCl lasting 48.6 times longer than the thinnest. Despite variations in thickness, all anodic electrodes achieved over 94% degradation of Reactive Black 5 dye within 30 min, with thinner coatings offering a more efficient synthesis route while maintaining high catalytic efficiency [14]. Liu et al. prepared a core shell structure of anatase TiO2 nanowires coated with rutile RuO2 nanoparticles using cyclic voltammetry electrodeposition. The electrochemical performance of this composite is enhanced for OER as compared to the Ti/RuO2 electrode. This was attributed to the increased electrochemical surface area due to support from one-dimensional TiO2 nanowires [15]. This strategy of creating composites such as bimetallic materials and heterojunctions of different transition metal oxides has been employed by many researchers to lower the overpotential for HER and OER and to enhance the stability of the catalyst in different electrolytes [16,17,18].
The catalytic performance of an electrode is highly influenced by its local surface structure, as the stability and reactivity of reaction intermediates are shaped by the specific adsorption sites. The reaction activity of an amorphous electrode differs significantly from that of crystalline electrodes, given the distinct nature of their local surface structures [19]. This raises the intriguing prospect of catalytic behavior of amorphous electrodes for oxygen evolution reaction. Interestingly, in catalytic research, amorphous alloy catalysts are often observed to exhibit higher activity and selectivity than their crystalline counterparts [20]. This difference in reactivity between amorphous and crystalline forms is frequently attributed to a higher concentration of coordinatively unsaturated sites, which facilitates reactant adsorption more readily than on equivalent crystalline catalysts. RuO2 thin films represent optimal nanostructures for the oxygen evolution reaction (OER) in water splitting, owing to their exceptional uniformity, reproducibility, and precise stoichiometric control. Reactive sputtering is a widely recognized deposition technique for RuO2 thin films, offering precision in controlling film thickness, uniformity, and compositional adjustments. Sputtering not only offers scalability for industrial applications, but also enables fine-tuning of film properties by modulating deposition parameters, such as gas composition [21,22]. Specifically, the ratio of oxygen to inert gases (e.g., argon) in the sputtering process significantly influences the resulting film’s surface characteristics, chemical composition, and electrochemical behavior [23]. Oxygen concentration during sputtering, for example, affects the oxidation state of Ru, which in turn plays a critical role in dictating the film’s catalytic efficiency and surface stability. Understanding these dependencies is essential for optimizing RuO2 thin films tailored to meet specific functional requirements.
In this study, we prepared amorphous RuO2 films with varying oxygen concentrations in the sputtering deposition and annealed them at 400 °C. We then compared the effect of different O2 concentrations of as-deposited and annealed RuO2 on the surface and electrochemical properties. The as-deposited RuO2 films were observed to be more catalytically active than their annealed counterparts. The annealed RuO2 showed rutile crystalline structure and redox peak as characteristics of RuO2 (110). The insights from this work contribute to a growing body of knowledge aimed at advancing RuO2-based materials for energy applications, offering a framework for enhancing catalytic performance and durability through precise control of sputtering parameters. The as-deposited RuO2 showed better catalytic performance as compared to their annealed counterparts.

2. Experimental Details

2.1. Deposition of the RuO2 Films

RuO2 thin films were deposited on SiO2/Si substrates using a reactive RF magnetron reactive sputtering system (K. J. Lesker). A 4-inch diameter ruthenium (Ru) target with a purity of 99.99% was used as the sputtering source. Argon (99.99%) and oxygen (99.99%) were introduced into deposition chambers through separate mass-flow controllers. Prior to deposition, the SiO2/Si substrates underwent ultrasonic cleaning in Piranha solution with a 3:1 mixture of sulfuric acid and hydrogen peroxide for 10 min followed by rinsing with deionized (DI) water. The substrates were dried with nitrogen (N2) gas and subjected to an O2 plasma treatment for 10 min with 60 W RF power to remove residual organic contaminants and activate the substrate surface. The substrates were loaded into the sputtering chamber, and the chamber was evacuated using a roughing pump to reach a pressure below 10 mTorr, followed by a turbo pump to achieve a base pressure of less than 5 × 10−6 Torr. Pre-sputtering was performed to remove the oxide layer. For the reactive sputtering of RuO2, the total gas flow (Ar + O2) was kept constant at 18 sccm, while varying the ratio of Ar to O2 at 1:1 (50% O2), 2:1 (33% O2), 3:1 (25% O2), and 4:1 (20% O2). Table 1 shows the labelling of the samples deposited, and the same labelling is used throughout the paper. The chamber pressure during deposition was maintained at 5 mTorr using a throttle valve. The deposition was performed at an RF power of 100 W, with the substrates kept at room temperature, and the deposition time for each gas ratio was 10 min. The annealing of the films was carried out in a Thermo fisher tube furnace at 400 °C for two hours in an open-air environment to synthesize crystalline films.

2.2. Characterization of the Films

The chemical composition and oxidation states of the deposited RuO2 films were analyzed using Thermo Scientific Escalab Xi + X-ray photoelectron spectroscopy (XPS) (Waltham, MA, USA) equipped with monochromatic Al Kα (1486.6 eV). The setup employed a 54.7° source-to-analyzer angle to capture photoelectrons emitted at 90°. Electron kinetic energy was assessed using a hemispherical analyzer, set to pass energy of 30 eV for high-resolution measurements and 50 eV for broader survey scans. The surface morphology was studied using Asylum Research MFP-3D (Oxford Instruments, Abingdon, UK) atomic force microscopy (AFM) in tapping mode with aluminum-coated silicon cantilever. X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements were carried out using a Rigaku Ultima IV diffractometer fitted with a Cu Kα radiation source. An XRR (Bruker AXS Inc., Madison, WI, USA) and J. A. Woollam iSE Spectroscopic ellipsometer (SE) (J. A. Woollam Co., Inc., Lincoln, NE, USA) with the CompleteEASE software package (version 6.73) were utilized to estimate the thickness. The electrocatalytic performance of the sputtered RuO2 thin film samples was tested by a standard three-electrode system, which was controlled by an electrochemical working station (Biologic, Inc., (Angers, France)). 0.1 M alkaline solution was prepared by dissolving KOH (99.99% purity, Sigma-Aldrich, (St. Louis, MO, USA)) into distilled water (DI, 18.2 MΩ cm). N2 was injected into the KOH solution for 30 min to create saturated N2 circumstance before each electrochemical examination. A platinum wire (Pine) and Ag/AgCl (Gamry) with saturated KCl solution were utilized as counter and reference electrodes, respectively. All potential values displayed in this report were converted to the reversible hydrogen electrode (RHE) standard by the equation ERHE = E(Ag/AgCl) + E0 + 0.0592*pH where EAg/AgCl and Eo are the obtained potential values and the standard potential of an Ag/AgCl versus a normal hydrogen electrode (0.197 V vs. NHE), and the pH value was measured based on the 0.1 M KOH solution. Electrochemical impedance spectroscopy (EIS) was implemented under an oscillation circuit with a 10-mV amplitude at open-circuit potential (OCP), and the frequency was limited between 200 KHz and 0.5 Hz. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were performed under the potential range from 0 to 1.51 V with the 200 mV/s scan rate. Copper wires were connected to all sputtered RuO2 thin film samples with the assistance of silver paint (Ted Pella, (Redding, CA, USA) Leitsilber 200). The edge, backside, and conjunct points were totally converted by epoxy glue (Omegabond 101).

3. Results and Discussion

The XPS core-level spectra of Ru3d and O1s of all the as-deposited RuO2 thin films are shown in Figure 1a and Figure 1b, respectively. The binding energy peak for Ru3d5/2 is located at 280.3 eV and falls into the range from 280.2 eV to 280.8 eV associated with the Ru4+ oxidation state [24,25]. The spin splitting between Ru3d5/2 and Ru3d3/2 was kept constant at 4.1 eV for all fitting. Satellite peaks associated with Ru3d5/2, and Ru3d3/2 are observed at higher binding energy. These satellite peaks are usually observed at ~1.8 to 1.9 eV above the photoemission line. As RuO2 thin films are metallic in nature, asymmetrical peak fitting algorithm is used. It is a very complex procedure to deconvolute the Ru3d3/2 peak as it overlaps with the carbon C1s peak. However, by giving tight constraints while fitting, C1s is comparably observable by setting FWHM in the 1 to 1.4 range. The C1s peak was deconvoluted at 284.9 eV, and the metal carbonate (CO3−) peak was fitted at 287.3 eV. For the RuO2 films with 50% (sample S50) and 33% (sample S33) oxygen, there was another peak associated with RuO3 at the binding energy of 283.2 eV. The existence of metastable RuO3 is also supported by Eichler et al. who mentioned in their studies that the formation of RuO3(s) at low temperatures occurs in non-equilibrium conditions [26,27]. Many researchers have supported the existence of RuO3(s) as a surface species along with RuO2 at the binding energy 283.2 ± 0.5 eV [28,29]. We assume that these Ru6+ species are formed at room temperature deposition due to higher O2 concentration, and, interestingly, these species were absent for lower O2 concentration of 25% (sample S25) and 20% (sample S20). So, later oxygen concentration films are assumed to be more stoichiometric films as the oxidation state was Ru4+. There was no peak found below a binding energy of 280 eV, which is usually associated with Ru metal. This confirmed that supplied oxygen was enough to oxidize Ru and form RuO2, except in the case of S50 and S33. Figure 1b shows the oxygen 1s fitting, which consisted of different peaks at 529.3 eV, 530.6 eV, 532 eV, and 533 eV associated with lattice oxygen (Olattice), adsorbed hydroxyl species (OHads), oxidized form of carbon (CO3−), and adsorbed water species (H2O), respectively. With an increase in oxygen concentration, the area intensity of the O1s peak has been observed to be increased in an XPS survey [30].
Figure 2 shows the morphological image of the as-deposited thin films using AFM. The root mean square (RMS) surface roughness (SR) was observed to be increased from ~1 nm to ~2 nm with the increase in O2 concentration from 20% to 50% (from sample S20 to sample S50). This increase in RMS values of SR might be due to more oxygen incorporation into the film. The thickness of the as-deposited RuO2 films was estimated by using spectroscopic ellipsometer and X-ray reflectometry (XRR). The optical properties of RuO2 thin films with varying pressures deposited on SiO2/Si substrates were investigated using a J. A. Woollam spectroscopic ellipsometer at a fixed-incident angle of 65°. The thickness of the as-deposited RuO2 thin films was also estimated in the operating spectral range from 400 nm to 1000 nm. Calibration of the ellipsometer was done using a standard 25 nm SiO2/Si wafer provided by J. A. Woollam to ensure the accuracy of the measurements. From the measured values of Ψ and Δ, the optical constants, refractive index (n) and extinction coefficients (k) were determined via optical modeling. Prior to measurements for RuO2, the optical constants for SiO2/Si substrates were measured.
As RuO2 is observed to be absorptive in nature over the range of 400 to 1000 nm, the B-spline layer is introduced to analyze absorption with an initial guess of the thickness values (starting from 50 nm) [31]. The B-spline was employed to fit the Ψ and Δ by using Kramers–Kronig model to maintain consistency between the resulting dielectric constants [32]. Then, this layer was parameterized using the Tauc-Lorentz oscillator model for a more physical fit and to minimize the mean squared error [33]. Figure 3a and Figure 3b show the spectra of the refractive index (n) and extinction coefficient (k) of as-deposited RuO2 films at varying pressures, respectively. The extinction coefficient (k) increasing at the lower wavelength suggested increased absorption for the films with a lower concentration of oxygen. All the samples showed absorption behavior; however, 20% O2 (S20) and 25% O2 (S25) showed highest absorption in the UV region. The measured thickness of the films using the above-mentioned film stack in the spectroscopic ellipsometer showed dependency on oxygen concentration. For the sample with 50% O2 (S50), the measured thickness was higher compared to other films at lower O2 partial pressure. The X-ray reflectivity (XRR) profiles in Figure 4a reveal distinct fringe patterns with varying oscillation frequencies corresponding to different O2 pressures. An increase in O2 pressure resulted in a greater number of oscillations, signifying a corresponding increase in film thickness. Additionally, the XRR profiles indicated an increase in surface roughness values at 33% (sample S33) and 50% (sample S50) O2 pressures, as evidenced by the diminishing oscillations at higher angles. The slope of the curve corroborates the increased roughness with O2 concentration also observed in AFM analyses, further validating the pressure-dependent surface morphology changes [34]. The thickness values of the RuO2 films estimated by SE and XRR are shown in Figure 4b. The SE estimated higher thickness than XRR, however, the trend of increasing thickness with O2 concentration is observed in both SE and XRR data [35,36]. This is likely due to the dependency of SE on optical modelling which can overestimate thickness. Elevating the oxygen partial pressure from 20% to 50% during the sputtering of RuO2 led to an increase in the deposited film thickness [37,38]. All the as-deposited RuO2 films were amorphous for all the pressures, as no peak was found associated with the crystal structure of RuO2 in the XRD. This might be due to the fact that the depositions were executed at room temperature. We annealed all the as-deposited RuO2 samples to see the effect of annealing on crystallinity and ultimately on electrochemical properties. As-deposited RuO2 with 33% O2 (sample S33) at 300 °C and 400 °C showed the transformation from amorphous RuO2 to rutile RuO2 with (110) and (101) orientations [39]. The XRD patterns of all the annealed samples showed no significant variation in peak positions; only two representative spectra with 33% O2 (sample S33) and 20% O2 (sample S20) are shown in Figure 5 to demonstrate the structural consistency among the samples.
We now describe the electrocatalytic properties of the amorphous as-deposited RuO2 and annealed crystalline RuO2. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and linear sweep voltammetry (LSV) were obtained in a three-electrode system in an N2 saturated electrolyte of 0.1 M KOH (pH~13). Figure 6a presents the Nyquist plot of amorphous RuO2, illustrating the contact resistance values across the samples. The bias voltage during EIS measurements was 0V versus open-circuit potential, and oscillation voltage was kept at 10 mV. The difference in the solution resistances displayed was under a reasonable range. For the deconvolution and Nyquist plot data analysis, a Randle equivalent circuit with a constant phase element was used. The solution resistance before and after fitting is tabulated in the Nyquist plot and observed to be well matched. The Nyquist response showed that the impedance was in the range from 60 ohm to 120 ohm [40]. Cyclic voltammetry was performed to investigate the OER activity measurement of as-deposited RuO2 in prepared alkaline media at 200 mV/s, as shown in Figure 6b. RuO2 with a 20% O2 (sample S20) concentration exhibited oxidation current density at 0.5 mA/cm2 at the earliest onset potential of ~1.25 V vs. RHE. From the CV and LSV curves, RuO2 with a 4:1 (Ar:O2) ratio (sample S20) also displayed a superior catalytic performance, and at a benchmark potential of 1.43 V vs. RHE, it achieved a higher current density than other samples, suggesting improved charge transfer kinetics, a higher electrochemically active surface area, and less energy consumption under water electrocatalysis than the other samples. The amorphous RuO2 showed broad redox peaks in a potential range of 1 V to 1.35 V vs. RHE [41]. Current scientific research has figured that RuO2 does not have eminent catalytic activity in the HER process, and hence, the potential window was set more towards positive voltages and the main focus was on OER [25]. Electrocatalytic performance and kinetic behavior were measured by LSV and the Tafel slope, respectively, as shown in Figure 7a and Figure 7b, respectively. We observed that current density is increased with decreasing oxygen pressure from 50% to 20%. The onset potential of water oxidation increases with increased oxygen under a current density of 0.5 mA/cm2.
The lowest Tafel slope value (79.8 mV/dec) among all electrodes tested, indicating the fastest reaction kinetics in the OER process, appeared in the RuO2 sample with 20% O2 (sample S20). The trend of an increasing Tafel slope was observed with increasing O2 concentration, which indicated the requirement of higher energy input increasing for the OER process. This suggests that electrochemical performance can be improved by controlling the oxygen content in the film by optimizing the argon-to-oxygen ratio, which may tune stoichiometry in the RuO2 thin films. To obtain more crystallized films, the as-deposited sputtered RuO2 were annealed to further detect the relations between electrocatalytic behavior and crystallinity of the as-deposited thin film. The water electrocatalytic performance of the annealed films was also evaluated. The Nyquist plot in Figure 8a shows the increase of the contact resistance after annealing, and the values are covered in the range from 85 ohm to 150 ohm.
The annealed RuO2 thin film with 20% O2 concentration still showed outstanding electrocatalytic activity compared to the other samples. The onset potential of the annealed 20% O2 RuO2 at a current density of 0.5 mA/cm2 was ~1.39 V vs. RHE, and the potential value increased with enhancement of the O2 concentrations (see Figure 8b). The redox peaks for annealed RuO2 were observed in the potential window of 1.1 V to 1.45 V vs. RHE.
The redox peaks became significant after annealing, which could be attributed to increased crystallinity of the RuO2 films. We observed that the redox peaks appeared at lower potential vs. RHE in the as-deposited samples as compared to the annealed samples. This could be due to the higher electrochemical activities observed in amorphous RuO2 than in crystalline RuO2. Additionally, the corresponding potential value on different samples was higher than their amorphous counterparts due to less activity of crystalline surfaces as compared to amorphous surfaces [33]. RuO2 with 33% O2 turned out to be rutile RuO2 where the prominent orientation was (110). The CV showed a great influence on this orientation and had a redox peak at ~1.38 V vs. RHE prior to catalyzing OER at ~1.46 V vs. RHE. The RuO2 (110) surface comprises two distinct types of Ru sites: a coordinatively unsaturated site (CUS) that is not capped by oxygen atoms and a bridge site (BRI) that is coordinated to six oxygen atoms. This is the reason why Ru CUS has been considered as the active site for water oxidation [42,43]. The observed redox peaks correspond to the deprotonation of surface-bound OH* species, resulting in the generation of adsorbed O* intermediates [44]. Recent first-principles calculations have revealed the presence of two distinct redox features in the cyclic voltammogram of single crystal RuO2 (110). With increasing potential, hydrogen atoms adsorbed at bridging oxygen (BRI) sites are progressively desorbed, predominantly contributing to the first CV peak, while coordinatively unsaturated sites (CUS) provide a minor contribution. Once hydrogen desorption from all BRI sites is complete, the residual hydrogen atoms on CUS are rapidly released within a narrow potential range, resulting in the sharp second peak observed in the CV [45]. The redox peaks observed in the amorphous RuO2 were very broad and appeared in the long potential window as compared to crystalline RuO2. There was a significant shift in redox peaks towards lower potential for the annealed RuO2 deposited with 25% O2 and 20% O2. Figure 9a illustrates the polarization curves of the annealed RuO2 samples. From the LSV curves of catalysts in alkaline media, it can be observed that the annealed RuO2 with a 4:1 (Ar:O2 ratio) shows the highest current density, illustrated by the superior catalytic activity toward the OER than the other RuO2 with a different Ar:O2 ratio. The OER electrocatalytic kinetics can be determined from the Tafel slope, as shown in Figure 9b. The Tafel slope of 97 mV/dec was observed for the annealed RuO2 with 20% O2 concentration, smaller than the other annealed samples with varying Ar:O2 ratios. For annealed RuO2 with 25% O2 (110 mV/dec), with 33% O2 (120mV/dec), and 50% O2 123 mV/dec, suggesting more favorable OER kinetics for annealed RuO2 with 20% O2, the Tafel slope was observed to be increased as compared to the amorphous films, and this could be attributed to lower activity of crystalline films [46]. Table 2 shows the onset potential for amorphous RuO2 and crystalline RuO2 at a current density of 0.5 mA/cm2 with varying O2 concentrations. It is noted that reverse-direction LSV measurements could further help in understanding pre-OER redox features from true OER onset and can be considered in future work to enhance mechanistic resolution.
By utilizing the non- Faradaic region of the CV plot, specific capacitance values of the annealed RuO2 films were measured. The CV curves in the non-Faradaic region, as shown in Figure 10a–d, were highly symmetric and reversible for the annealed RuO2 samples, and the non-Faradaic area was distinct compared to the amorphous RuO2 samples. Double-layer capacitance (Cdl) is calculated using the formula Cdl = i/ν, where i is the average of the anodic current density and cathodic current density measured from the non-Faradaic region, and ν is the scan rate. The linear relation between average current density and scan rate is plotted, and the slope of this line is considered as the double-layer capacitance. Then, specific capacitance Cs is calculated by measuring the geometric area of the films and by the formula, Cs = Cdl/A, where A is the geometric area of the respective thin film. By measuring the geometric areas of these films, the highest specific capacitance shown in Figure 11 (992 μF/cm2) was obtained for the annealed RuO2 with 20% O2, suggesting a superior ability to form and sustain an electric double layer at the electrode–electrolyte interface and its optimized surface area; and the lowest (146 μF/cm2) was obtained for the annealed RuO2 with 50% O2. The annealed RuO2 with 25% O2 also showed a good value of specific capacitance (967 μF/cm2). This indicated that the annealed RuO2 with 20% O2, possessed more accessible areas and better ionic interaction leading to capacitive behavior [47,48]. Figure 12 illustrates the cyclic voltammetry curves comparing current densities of as-deposited amorphous RuO2 deposited as a function of varying O2 concentration and its annealed counterpart at 400 °C. The as-deposited films exhibited higher current densities than their annealed counterparts. The amorphous material system has long-range disordered structures, which lead to superior electrocatalytic activity compared to their crystalline counterparts. Amorphous structures are rich in active sites due to dangling bonds, coordinatively unsaturated sites, and abundant defects, which could improve the diffusion property that facilitates the transportation of reactants [49,50,51,52]. Deka et al. [41] have studied amorphous RuO2 for OER using operando O K edge XAS and attributed the enhanced activity to the strong lattice oxygen activation for amorphous RuOx.

4. Conclusions

The electrocatalytic activity of as-deposited sputtered RuO2 thin films for oxygen evolution in a 0.1 M KOH aqueous solution was investigated. RuO2 thin films were deposited using sputtering at varying oxygen pressure from 50% O2 to 20% O2. Our XPS analysis showed that the as-deposited RuO2 thin films with 25% and 20% O2 had good stoichiometry, and there was no RuO3 phase observed. All the films were amorphous in nature; however, annealing transformed it into crystalline films at 400 °C. RMS surface roughness from AFM was higher at higher O2 pressure. The electrochemical performance showed superior activity for the RuO2 films deposited at 20% O2 pressure. This performance was observed to be decreased with an increase in O2 concentration during deposition. The as-deposited amorphous films showed higher activity than their crystalline counterparts. The specific capacitance for annealed RuO2 with 20% O2 concentration exhibited the highest capacitance of 992 μF/cm2. The amorphous RuO2 deposited with 33% O2 transformed to rutile polycrystalline RuO2, which showed redox peaks prior to water oxidation. The Tafel slope exhibited a reduction in films sputtered at the lowest O2 pressures, strongly indicating that RuO2 films sputtered under lower oxygen pressures offer enhanced electrochemical performance.

Author Contributions

Conceptualization, S.N., E.V. and S.A.; methodology, S.N, M.L. and S.C.; formal analysis, S.N., M.L. and I.C.-O.; resources, M.L., D.K. and S.A.; data curation, S.N., E.V. and M.L.; writing—original draft, S.N.; writing—review and editing, M.L., D.K, and S.A.; supervision, D.K. and S.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Center for Electrochemical Dynamics and Reactions on Surfaces (CEDARS), an Energy Frontier Research Center supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under grant #DE-SC0023415. Portions of the work utilized resources at the Joint School of Nanoscience and Nanotechnology, part of the National Nanotechnology Coordinated Infrastructure (NNCI), funded by the National Science Foundation (Grant ECCS-2025462).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this article.

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Crystals 15 00417 g001
Figure 1. Core-level spectra of (a) Ru3d and (b) O1s of the samples deposited by varying oxygen concentration from 50% to 20%.
Figure 1. Core-level spectra of (a) Ru3d and (b) O1s of the samples deposited by varying oxygen concentration from 50% to 20%.
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Crystals 15 00417 g002
Figure 2. AFM images of the RuO2 thin films deposited at (a) 50% O2, (b) 33% O2, (c) 25% O2, and (d) 20% O2.
Figure 2. AFM images of the RuO2 thin films deposited at (a) 50% O2, (b) 33% O2, (c) 25% O2, and (d) 20% O2.
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Crystals 15 00417 g003
Figure 3. The spectra for (a) refractive index, n, and (b) extinction coefficient, k, of the RuO2 thin films deposited at varying O2 pressure.
Figure 3. The spectra for (a) refractive index, n, and (b) extinction coefficient, k, of the RuO2 thin films deposited at varying O2 pressure.
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Crystals 15 00417 g004
Figure 4. (a) X-ray reflectivity profiles of sputtered RuO2 with varying oxygen pressure and (b) measured thickness by spectroscopic ellipsometer and XRRs.
Figure 4. (a) X-ray reflectivity profiles of sputtered RuO2 with varying oxygen pressure and (b) measured thickness by spectroscopic ellipsometer and XRRs.
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Crystals 15 00417 g005
Figure 5. XRD spectra of RuO2 annealed at 400 °C with as-deposited 33% and 20% O2 concentrations.
Figure 5. XRD spectra of RuO2 annealed at 400 °C with as-deposited 33% and 20% O2 concentrations.
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Crystals 15 00417 g006
Figure 6. (a) Nyquist plot with bias voltage 0 V vs. open-circuit voltage and (b) cyclic voltammetry of all the as-deposited sputtered RuO2 with varying O2.
Figure 6. (a) Nyquist plot with bias voltage 0 V vs. open-circuit voltage and (b) cyclic voltammetry of all the as-deposited sputtered RuO2 with varying O2.
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Crystals 15 00417 g007aCrystals 15 00417 g007b
Figure 7. (a) LSV curves, (b) Tafel slope of all the as-deposited sputtered RuO2 with varying O2.
Figure 7. (a) LSV curves, (b) Tafel slope of all the as-deposited sputtered RuO2 with varying O2.
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Crystals 15 00417 g008
Figure 8. (a) Nyquist plot and (b) cyclic voltammetry of all annealed RuO2 samples at 400 °C.
Figure 8. (a) Nyquist plot and (b) cyclic voltammetry of all annealed RuO2 samples at 400 °C.
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Crystals 15 00417 g009aCrystals 15 00417 g009b
Figure 9. (a) LSV curves, (b) Tafel slope of all annealed RuO2 at 400 °C.
Figure 9. (a) LSV curves, (b) Tafel slope of all annealed RuO2 at 400 °C.
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Crystals 15 00417 g010aCrystals 15 00417 g010b
Figure 10. CV curves in non-Faradaic region for annealed RuO2 at 400 °C deposited with varying O2 concentrations.
Figure 10. CV curves in non-Faradaic region for annealed RuO2 at 400 °C deposited with varying O2 concentrations.
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Crystals 15 00417 g011
Figure 11. Plot of capacitive current densities versus scan rate for all annealed RuO2 catalysts, along with the extracted double-layer capacitance (Cdl) values.
Figure 11. Plot of capacitive current densities versus scan rate for all annealed RuO2 catalysts, along with the extracted double-layer capacitance (Cdl) values.
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Crystals 15 00417 g012
Figure 12. Comparison of cyclic voltammetry curves of as-deposited amorphous RuO2 with varying O2 concentrations and its counterpart annealed at 400 °C.
Figure 12. Comparison of cyclic voltammetry curves of as-deposited amorphous RuO2 with varying O2 concentrations and its counterpart annealed at 400 °C.
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Table 1. Sample labelling of the deposited RuO2 thin films according to conditions.
Table 1. Sample labelling of the deposited RuO2 thin films according to conditions.
Sample NameAr:O2 RatioSample Description
S501:1RuO2 deposited with 50% oxygen concentration
S332:1RuO2 deposited with 33% oxygen concentration
S253:1RuO2 deposited with 33% oxygen concentration
S204:1RuO2 deposited with 33% oxygen concentration
Table 2. Onset potential at 0.5mA/cm2 of as-deposited and annealed RuO2 with varying O2.
Table 2. Onset potential at 0.5mA/cm2 of as-deposited and annealed RuO2 with varying O2.
Samples (Ar:O2)Onset Potential at Current Density of 0.5 mA/cm2
As-Deposited RuO2
(V vs. RHE)		Annealed RuO2 400 °C
(V vs. RHE)
S50 (1:1)1.371.54
S33 (2:1)1.331.53
S25 (3:1)1.281.45
S20 (4:1)1.251.39
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Nalawade, S.; Vondee, E.; Liu, M.; Chris-Okoro, I.; Cherono, S.; Kumar, D.; Aravamudhan, S. Role of Oxygen Concentration in Reactive Sputtering of RuO2 Thin Films: Tuning Surface Chemistry for Enhanced Electrocatalytic Performance. Crystals 2025, 15, 417. https://doi.org/10.3390/cryst15050417

AMA Style

Nalawade S, Vondee E, Liu M, Chris-Okoro I, Cherono S, Kumar D, Aravamudhan S. Role of Oxygen Concentration in Reactive Sputtering of RuO2 Thin Films: Tuning Surface Chemistry for Enhanced Electrocatalytic Performance. Crystals. 2025; 15(5):417. https://doi.org/10.3390/cryst15050417

Chicago/Turabian Style

Nalawade, Swapnil, Ebenezer Vondee, Mengxin Liu, Ikenna Chris-Okoro, Sheilah Cherono, Dhananjay Kumar, and Shyam Aravamudhan. 2025. "Role of Oxygen Concentration in Reactive Sputtering of RuO2 Thin Films: Tuning Surface Chemistry for Enhanced Electrocatalytic Performance" Crystals 15, no. 5: 417. https://doi.org/10.3390/cryst15050417

APA Style

Nalawade, S., Vondee, E., Liu, M., Chris-Okoro, I., Cherono, S., Kumar, D., & Aravamudhan, S. (2025). Role of Oxygen Concentration in Reactive Sputtering of RuO2 Thin Films: Tuning Surface Chemistry for Enhanced Electrocatalytic Performance. Crystals, 15(5), 417. https://doi.org/10.3390/cryst15050417

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