Electroless deposition of RuO₂-based nanoparticles for energy conversion applications

Abstract

This study reports a delicate electroless approach for the deposition of RuO₂·nH₂O nanoparticles on the VO_x·mH₂O nanowires and this method can be extended to deposit RuO₂·nH₂O nanoparticles on various material surfaces. Electrochemical characterizations, including linear sweep voltammetry (LSV), electrochemical quartz crystal microbalance (QCM) analysis and rotating ring-disc electrode (RRDE) voltammetry, were carried out to investigate the growth mechanism. The deposition involves the catalytic reduction of dissolved oxygen by the V⁴⁺ species of VO_x·mH₂O, which drives the oxidation of RuCl₃ to proceed with the growth of RuO₂·nH₂O. This core/shell VO_x·mH₂O/RuO₂·nH₂O shows a better catalytic activity of the oxygen reduction reaction (ORR) than RuO₂·nH₂O, which is ascribed to the pronounced dispersion of RuO₂·nH₂O. Such an electroless approach was applicable to the preparation of a RuO₂-based nanoparticle suspension as well as the deposition of nanocrystalline RuO₂·nH₂O on other functional supports like TiO₂ nanowires. The thus-obtained RuO₂-decorated TiO₂ nanorods exhibit significantly an enhanced photoactivity toward photoelectrochemical water oxidation. The versatility of the current electroless approach may facilitate the widespread deployment of nanocrystalline RuO₂·nH₂O in a variety of energy-related applications.

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1. Introduction

With exceptional electrochemical and optoelectronic properties, ruthenium oxide (RuO₂) has garnered considerable attention for numerous technological applications. For example, hydrous ruthenium oxide (RuO₂·nH₂O) is considered to be one of the most promising electrode materials for supercapacitors due to its capability of driving reversible multi-electron transfer redox reactions with over-lapped potentials.^1–5 In electrocatalysis, RuO₂ can function as an active component for chlorine generation from HCl,⁶ catalytic CO₂ reduction^7,8 as well as the efficient catalyst for the oxygen and hydrogen evolution reactions.^9–11 Furthermore, promising potentials for photocatalytic water splitting,^12–15 sodium-ion batteries,¹⁶ aerobic oxidation of alcohols,¹⁷ nitrous oxide (N₂O) decomposition,¹⁸ photocatalytic CO₂ reduction,¹⁹ carbon-free H₂ production,²⁰ and room-temperature CO oxidation²¹ have also been demonstrated by exploiting the intrinsic properties of RuO₂. Recently the lithium–oxygen (Li–O₂) battery has attracted tremendous attention and RuO₂ plays a key role as bifunctional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) to reduce the overpotential of discharging/charging process.^22–25 However, the high cost of ruthenium precursors has hindered the large-scale deployment of RuO₂. Especially for the bulk, thick RuO₂ films, the relatively low specific capacitance resulting from the long diffusion lengths of ions and electrons has limited their practical applications.⁷ Thanks to the appealing attributes derived from the size effect, nanocrystalline RuO₂ with high specific surface area and enhanced kinetics of mass and charge transport can provide a promising solution to address the above issues.⁸ Consequently, countless efforts have been dedicated to the preparation of RuO₂ nanostructures with miscellaneous structural forms, e.g., nanoparticles and nanotubes, whose capacitive performance can be accordingly enhanced.

In view of the ease of handling and the feasibility of massive production, nanoparticles of RuO₂ have proven their prowess in energy storage^26,27 and energy conversion applications.^11,28,29 To achieve homogeneous distribution and thereby satisfactory performance, depositing RuO₂ nanoparticles on specific substrates with unique functionalities is desirable.^{17,25,30–34} For instance, carbon nanotubes were utilized as the template to grow highly dispersed RuO₂ with the size down to 1.35 nm.¹⁷ With the striking structural uniformity, the resultant composites exhibited high activity and excellent selectivity toward catalytic aerobic oxidation. Moreover, RuO₂ nanoparticles were deposited on TiO₂ nanoflowers to form nanocomposites with three-dimensional porous architectures, which displayed high power capacitive characteristics relative to the un-supported RuO₂.^30,31 The superiority was ascribed to the porous nature from the nanoflower backbones of TiO₂ and the connected electron pathways endowed by the well distributed RuO₂. Till now, a vast body of the literature has documented various synthetic approaches for the deposition of RuO₂ nanoparticles on functional substrates.³³ Most of them however involved complicated preparation processes, such as prolonged reaction time,^17,34 energy-consuming steps, for example, the need of post-annealing treatment at an elevated temperature,^9,24,35 or the most concerned issue, i.e. the use of large amount of ruthenium precursors,^17,22,23,34 which may further obstruct the practical utilization of nanocrystalline RuO₂. Hence, the creation of a facile, cost-effective approach to obtain highly dispersive RuO₂ with structural integrity is imperative more than ever before.

In this work, we developed a simple yet effective electroless method to deposit RuO₂·nH₂O nanoparticles on hydrated vanadium oxide (VO_x·mH₂O) nanowires. Here, VO_x·mH₂O nanowire arrays were employed as supports for RuO₂·nH₂O deposition. The nanowire backbones not only provided growth platform for homogeneous distribution of RuO₂·nH₂O deposits but also formed abundant structural interstices to create three-dimensional porosities, which is beneficial to the applications requiring high surface area and facile charge transport, such as electrochemical energy conversion, solar energy conversion, catalysis and sensing. The deposition was conducted at room temperature and completed within 1 h without the post-heat treatment. The resultant RuO₂·nH₂O crystallites had an average size of 1.9 nm and were homogeneously distributed on the VO_x·mH₂O nanowire surface with an ultrahigh density of 2.1 × 10¹³ particles per μm². The results of linear sweep voltammetry (LSV), quartz crystal microbalance (QCM) analysis and rotating ring-disc electrode (RRDE) voltammetry suggested that the V⁴⁺ species of VO_x·mH₂O nanowires catalyzed the reduction of dissolved oxygen, which drove the oxidation of Ru³⁺ to deposit RuO₂·nH₂O. Such a delicate electroless deposition process can be extended for the deposition of nanocrystalline RuO₂ on other oxides supports, for example TiO₂ nanowires, which has significant implications in the development of multi-functional nanocomposites for a variety of energy-related applications ranging from photoelectrochemical energy conversion to electrochemical energy storage.

2. Materials and methods

2.1. Preparation of RuO₂·nH₂O-decorated VO_x·mH₂O nanowires

VO_x·mH₂O nanowires were first grown on graphite substrates (1 × 1 cm) using the anodic deposition method.^36,37 The substrate deposited with VO_x·mH₂O nanowires was then immersed into a reaction solution (50 mL) containing 10 mM of RuCl₃·xH₂O and 10 mM of CH₃COONa. Upon steady stirring at room temperature for 1 h, RuO₂·nH₂O nanoparticles were deposited on the VO_x·mH₂O nanowire surface. The substrate was then taken out, rinsed with de-ionized water and dried at room temperature. To study the growth mechanism, the reaction solution of V⁴⁺ species was used and prepared by adding 0.2 g of VOSO₄ into 50 mL of deionized water. To prepare the reaction solution of V⁵⁺, a stoichiometric content of H₂O₂ was added along with 0.2 g of VOSO₄ in the 50 mL deionized water, which led to the complete oxidation of V⁴⁺ to V⁵⁺. The homogeneous growth of RuO₂ nanoparticle suspensions was conducted by employing the reaction solution containing 10 mM of RuCl₃·xH₂O, 10 mM of CH₃COONa and a given amount of V⁴⁺.

Rutile TiO₂ nanorod arrays were grown on fluorine-doped tin oxide (FTO) glass substrates using a hydrothermal method reported in the literature.^38,39 Concentrated HCl (15 mL) was diluted with deionized water (15 mL) and then mixed with titanium n-butoxide (Ti(OBu)₄, 0.5 mL) in a 100 mL beaker. The resulting clear solution was transferred to a Teflon-lined stainless-steel autoclave (40 mL in volume), where a clean FTO glass substrate was submerged into the solution. The sealed autoclave was heated in an electric oven at 150 °C for 5 h and then slowly cooled to room temperature. A white TiO₂ nanorod film was uniformly coated on the FTO glass substrate after cooling. The samples were thoroughly washed with deionized water and then annealed in air at 550 °C for 3 h to improve the crystallinity.

2.2. Characterizations

The morphology and dimensions of the samples were observed with a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800) and a field-emission transmission electron microscope (FE-TEM, Philips, Tecnai F20 G2). The elemental analysis of the samples was conducted with energy dispersive X-ray spectrometer (EDS), an accessory of TEM. The electrochemical quartz crystal microbalance (EQCM) analysis was performed on a commercially available Au/Ti-sputtered quartz electrode with an electrochemical workstation (CH Instruments, CHI 4051a). The chemical environment information was acquired with an X-ray photoelectron spectrometer (XPS, ULVAC-PHI, Quantera SXM) using Al Kα radiation (hν = 1486.69 eV) under high vacuum conditions. Electrochemical characterizations were carried out using an electrochemical workstation (CH Instruments, CHI 633A). To study the oxygen reduction reaction (ORR) kinetics, the RRDE voltammetry was employed and the potential values were represented with respect to the reversible hydrogen electrode (RHE). The measurements were conducted at 900 rpm with a scan rate of 5 mV s⁻¹. The photoelectrochemical measurements were performed and in 1.0 M NaOH under a three-electrode configuration where the Ru-based nanoparticles (NPs) decorated TiO₂ nanorods, Pt foil, and Ag/AgCl serve as the working, counter, and reference electrodes, respectively. The simulated sunlight irradiation was produced using a solar simulator (LCS-100 Solar Simulator) with an AM 1.5 global filter with the light intensity of 100 mW cm⁻².

3. Results and discussion

3.1. Structural investigations

Fig. 1(A) shows the SEM image of VO_x·mH₂O nanowires obtained from the anodic deposition process. These nanowires were bundled, mutually interlaced to form porous urchin-like structures. By virtue of this highly porous structural feature, the VO_x·mH₂O nanowires have been demonstrating promising potentials as the cathode of asymmetric Li-ion supercapacitors.³⁷ After reacted with RuCl₃ in the electroless reaction, the nanowires became thicker in diameter and transformed into coral-like structures with rounded ends. As clearly observed in Fig. 1(B), the drastic morphology change indicates that certain deposits should have grown on the nanowire surface.

Fig. 1 SEM images of (A) pristine VO_x·mH₂O and (B) RuO₂·nH₂O-decorated VO_x·mH₂O nanowires.

TEM observations were carried out to examine the microstructural details. As evident in Fig. 2(A), the nanowire surface was covered with a large quantity of nanoparticles. The deposited nanoparticles showed an average size of 1.9 nm and were uniformly distributed on the nanowire surface with an ultrahigh density of 2.1 × 10¹³ particles per μm². The corresponding TEM-EDS elemental maps in Fig. 2(B) verify that the deposited particles were composed of Ru and O, suggesting the formation of Ru oxide-based nanoparticles on the surface of VO_x·mH₂O nanowires.

Fig. 2 (A) TEM image and (B) TEM-EDS mapping results of RuO₂·nH₂O-decorated VO_x·mH₂O nanowires. Inset in (A) shows the enlarged image at the marked region.

3.2. Deposition mechanism

To manipulate the present electroless process for depositing Ru oxide-based nanoparticles, it is essential to understand the growth mechanism behind. For electroless deposition of metal oxides in the higher oxidation state in comparison with their precursors (e.g., RuO₂ deposited from RuCl₃), oxidizing agents are required to drive the metal ion oxidation. Since no specific oxidant was added in the current synthetic process, the interactions among the reaction species, i.e., RuCl₃ precursor, VO_x·mH₂O electrode, and dissolved oxygen molecules, might be responsible for the initiation of the Ru³⁺ oxidation. We first analyzed the oxidation kinetics of Ru³⁺ with electrochemical characterization methods and evaluated the possible involvement of VO_x·mH₂O electrode. Fig. 3(A) presents the LSV curves for two different electrodes, graphite substrates and VO_x·mH₂O nanowires, recorded in the electrolyte containing RuCl₃. For graphite substrate, a prominent anodic peak at 1.15 V vs. Ag/AgCl was noticed, which was attributed to the oxidation of Ru³⁺ into Ru⁴⁺. The Ru³⁺ oxidation peak remained eminent for the VO_x·mH₂O nanowires electrode, exhibiting a higher current density with a less positive potential relative to the peak on the graphite substrate. This phenomenon suggests that VO_x·mH₂O nanowires may facilitate the electrochemical oxidation of Ru³⁺ by improving the reaction kinetics. Note that VO_x·mH₂O nanowires were composed of mixed V⁴⁺ and V⁵⁺ oxy-hydroxyl species,^36,37 shown in Fig. S1.† To clarify the species responsible for facilitating the oxidation of Ru³⁺, we carried out the electroless reaction by using the reaction solution of V⁴⁺ or V⁵⁺ and compared the results with QCM. Fig. 3(B) displays the mass change of the QCM analyzer for the reaction solutions of V⁴⁺ and V⁵⁺. Also Fig. 3(B) includes the QCM result from the V⁴⁺ solution with N₂ purging to clarify the possible role of the dissolved oxygen in the electroless deposition of RuO₂. Several important features can be observed from Fig. 3(B). First, the electroless deposition was totally inhibited in the reaction solution of V⁵⁺, revealing that V⁴⁺ was the main species for facilitating the RuCl₃ oxidation. Since the composition of VO_x·mH₂O electrode was unchanged during the electroless deposition process, V⁴⁺ was considered to be a catalyst for RuCl₃ oxidation instead of an oxidant agent. Second, the deposition rate was hindered as the reaction solution of V⁴⁺ was purged with N₂, suggesting that the dissolved oxygen is the oxidizing agent in the catalytic oxidation of RuCl₃. Furthermore, when the RuCl₃ solution without V⁴⁺ was purged with air for 1 h or exposed to the ambient air for 7 days, no precipitation could be gathered. Accordingly, we propose that the V⁴⁺ species of VO_x·mH₂O nanowires catalyze the reduction of dissolved oxygen to drive the oxidation of RuCl₃ in order to proceed with the RuO₂·nH₂O deposition.

Fig. 3 (A) LSV curves of RuCl₃ oxidation for graphite substrates and VO_x·mH₂O nanowires recorded in the reaction solution. (B) QCM mass change on the electrode recorded from the reaction solutions containing V⁴⁺ and V⁵⁺, respectively. The result of performing N₂ purging was also included for comparison.

To manifest the above oxygen reduction contention, we investigated the electrochemical behavior of VO_x·mH₂O nanowires toward the ORR. It should be noted that the ORR in aqueous solutions occurs mainly through two pathways: the direct 4-electron reduction pathway from O₂ to H₂O, and the 2-electron reduction pathway from O₂ to H₂O₂.⁴⁰ For the 4-electron transfer pathway, the catalytic oxidation of Ru³⁺ was driven by the direct electron transfer from Ru³⁺ to the oxygen molecules adsorbed on the VO_x·mH₂O surface, which generated OH⁻ ions to enable the growth of RuO₂·nH₂O. As to the two-electron transfer pathway, highly oxidative H₂O₂ could be produced in the vicinity of the VO_x·mH₂O surface, subsequently oxidizing Ru³⁺ to proceed with the RuO₂·nH₂O deposition.^1,17 Interestingly, the concentration of proton at the surface of VO_x·mH₂O without potential bias was monitored after RuCl₃ was added (in Fig. S2†). The result shows that the concentration of proton increased suddenly at first and reached a plateau, which indicates that the ORR and RuO₂ deposition involve hydroxyl ions. Note that both 4-electron and 2-electron pathways of the ORR consume protons under the acidic circumstance to oxidize Ru(III) to form the Ru(IV) ions (i.e., decrease in proton concentration). Since every Ru(IV) ion will react with four OH⁻ to form Ru(OH)₄, which significantly increases the proton concentration, we cannot determinate the deposition mechanism from the sudden increase in the proton concentration. Accordingly, the RRDE method was employed to give a deep investigation. Fig. 4(A) shows the RRDE voltammograms on the GC electrodes deposited with VO_x·mH₂O in 0.5 M Na₂SO₄ electrolyte with pH = 3, a RuCl₃-free electrolyte. When the applied potentials were swept from 0.8 to 0.2 V vs. RHE, the disk current density continuously increased. The apparent electron transfer number (n) can be determined from the equation, , where I_D is the disk current, I_R is the ring current, and N is the ring collection coefficient (0.38). As shown in Fig. 4(B), the apparent electron transfer number of ORR varied from 2.6 to 3.9 for the RuCl₃-free electrolyte. In fact, the ORR on VO_x·mH₂O in the electrolyte containing RuCl₃ was studied as well. However, the result was not shown here because of the enormous possibility of direct reduction of Ru³⁺ into Ru metal according to the ruthenium Pourbaix diagram,⁴¹ which may lead to misunderstanding. The RRDE results manifest that the ORR catalyzed by VO_x·mH₂O is shifted from the 2-electron transfer pathway to the 4-electron transfer pathway with the application of more negative potentials. This phenomenon suggests that without the potential bias, the ORR should obey the 2-electron transfer pathway, resulting in the phenomenon of increasing the proton concentration.

Fig. 4 (A) The RRDE voltammograms and (B) the mean electron transfer number (n) of the ORR obtained from the GC disk electrode deposited with (1) VO_x·mH₂O/RuO₂·nH₂O, (2) VO_x·mH₂O, and (3) RuO₂·nH₂O in O₂-saturated 0.5 M Na₂SO₄ with pH = 3 at 5 mV s⁻¹ and a rotating speed of 900 rpm.

3.3. Applications of the catalytic redox deposition of RuO₂·nH₂O/VO_x·mH₂O

Fig. 4(A) shows the RRDE voltammograms on the GC electrodes deposited with RuO₂·nH₂O/VO_x·mH₂O and RuO₂·nH₂O in 0.5 M Na₂SO₄ electrolyte with pH = 3. The onset potentials of the ORR on RuO₂·nH₂O/VO_x·mH₂O and RuO₂·nH₂O are about 0.83 and 0.34 V, respectively. Note here that the onset potential of the ORR was defined as the potential where the current density of oxygen reduction reached 0.5 A cm⁻² g⁻¹. Clearly, the usage of VO_x·mH₂O nanowires as the catalyst backbone promotes the onset potential, probably due to a higher electrolyte-accessible surface area in comparison with RuO₂·nH₂O. Also note that the catalytic activity of RuO₂·nH₂O is extremely low at potentials positive than 0.4 V while a sharp increase in the disc current density was found in the less positive potential region. This phenomenon may be assigned to that the Ru(III)/Ru(IV) redox couple is the main active species catalyzing the ORR.^42–45 In Fig. 4(B), the mean electron transfer number, n, of RuO₂·nH₂O monotonously increases from ca. 2.6 to 3.9 with the negative shift in the electrode potential in Fig. 4(B). Surprisingly, the mean electron transfer number of the ORR on RuO₂·nH₂O/VO_x·mH₂O was approximately constant and very close to 4. This result reveals that RuO₂·nH₂O/VO_x·mH₂O is an excellent catalyst for the ORR, leading to the four-electron transfer pathway. From all the above results and discussion, the RuO₂·nH₂O/VO_x·mH₂O composite is a promising electrocatalyst of the ORR for the applications of H₂O₂ generation and fuel cells in weak acid media containing different supporting electrolytes.

3.4. RuO₂ suspensions and deposition on TiO₂ nanowires

To realize the growth mechanism, we further performed the electroless reaction in a reaction solution simply containing V⁴⁺ species and the RuCl₃ precursor. The synthesis was conducted under the ambient atmosphere for 6 h, which produced abundant nanoparticle suspensions in the bulk solution. The morphology and composition of resultant RuO₂ precipitates were characterized with SEM and XPS analyses. The SEM image in Fig. 5(A) shows that the precipitates are particles of 100–200 nm in size. From a detailed examination, these particles are considered to be aggregates of tiny nanocrystals with sizes of 50–80 nm. The corresponding XPS spectrum in Fig. 5(B) suggests the diverse chemical environment of ruthenium species within the precipitates. The deconvolution of Ru 3p peak yields three chemical states, RuO₂, RuO₂·nH₂O, and Ru(OH)₃, with the percentage determined to be 37.8, 40.6, and 21.6 at%, respectively. This illustration highlights the versatility of our electroless approach which not only deposits RuO₂·nH₂O nanoparticles on the VO_x·mH₂O nanowires but also produces RuO₂-based nanoparticle suspensions in the bulk solution.

Fig. 5 (A) SEM images and (B) XPS Ru 3p core-level spectra of RuO₂-based nanoparticle suspensions.

The feasibility in obtaining the highly dispersive and well suspending nanocrystalline RuO₂ is particularly appealing for stimulating the practical applications. As a demonstration, TiO₂ nanowires array was utilized as a frame in the electroless deposition of nanocrystalline RuO₂. Here the high-resolution TEM image was employed to inspect the dispersion of RuO₂ on the surface of TiO₂ nanowires and Fig. 6 shows that TiO₂ nanowire was covered with a 2 nm thin layer of nanocrystalline RuO₂ to form a core/shell nanostructure. The lattice fringes of TiO₂ frame with the interplanar spacing of 0.32 nm and 0.29 nm were observed, consistent with the d-spacing of (110) and (001) planes of rutile TiO₂.³⁸ On the other hand, the lattice fringes of RuO₂ nanoparticles with the interplanar spacing of 0.221 nm and 0.225 nm were obtained, corresponding to the d-spacing of (111) and (200) planes of rutile RuO₂, respectively.

Fig. 6 A high-resolution TEM image of a RuO₂-decorated TiO₂ nanowire.

The capacitive behavior of TiO₂ nanowires and TiO₂/RuO₂ core/shell structure was examined. For the comparison purpose, the electroless deposition of nanocrystalline RuO₂ on an FTO was conducted as well. From a comparison of CV curves of the core/shell TiO₂/RuO₂ and RuO₂-decorated FTO in Fig. 7(A), well-dispersed RuO₂ nanocrystallites promotes the utilization of active species, leading to higher specific capacitance of the core/shell TiO₂/RuO₂ in comparison with the RuO₂-decorated FTO electrode. Since RuO₂ has been widely recognized as an efficient oxygen evolution co-catalyst in photo-electrochemical water splitting,^46,47 the decoration of RuO₂ nanocrystallites onto TiO₂ may significantly enhance the photocatalytic activity of TiO₂ for water oxidation. Here, the photoelectrochemical measurements were conducted to evaluate the potential of the RuO₂-decorated TiO₂ nanowire for water splitting. Fig. 7(B) compares the LSV curves of pristine TiO₂ and RuO₂-decorated TiO₂ photoelectrodes for water oxidation. Evidently, RuO₂-decorated TiO₂ displays larger photocurrent densities in the entire potential widow than pristine TiO₂. This phenomenon was ascribed to the improved water oxidation kinetics on the decorated RuO₂.

Fig. 7 (A) CV curves of pristine TiO₂ and RuO₂-decorated TiO₂ for supercapacitors. (B) LSV curves of pristine TiO₂ and RuO₂-decorated TiO₂ photoanodes for water oxidation.

To extend the application of this electroless deposition, NiCl₂ was used as the precursor for Ni-based oxyhydroxide deposition and a precipitate was observed from the mixed solution of VOSO₄ and NiCl₂. In comparison with the precursor NiCl₂, the precipitate displayed different Ni 2p spectrum shown in Fig. S3.† Ni 2p_3/2 was further deconvoluted into two peaks cantered at 856.5 eV and 857.6 eV shown in Fig. S4,† which are corresponding to Ni(OH)₂ and NiOOH.⁴⁸ Again, this result demonstrates the oxidation of NiCl₂ to form nickel oxyhydroxide and nickel hydroxide. Based on the above preliminary test, this V⁴⁺-catalyzed electroless deposition presents an enormous potential in transition metal oxide deposition.

4. Conclusions

An effective electroless approach has been developed for the deposition of nanocrystalline RuO₂ onto various functional supports such as VO_x·mH₂O and TiO₂ nanowires as well as the preparation of RuO₂-based nanoparticle suspensions in the bulk solution. The success of this synthetic approach relied on the presence of V⁴⁺ species which catalyzed the oxygen reduction reaction to drive RuCl₃ oxidation for RuO₂ deposition. The versatility of the present electroless approach shall foster the advanced applications of nanocrystalline RuO₂ in a wide array of fields, which include electrochemical cells, room-temperature CO oxidation, and photocatalytic water splitting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Ministry of Science and Technology (MOST) of Taiwan under grants MOST 105-2221-E-007-127-MY3, 106-2221-E-007-089-MY3, and 106-2923-E-007-005.

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Footnote

† Electronic supplementary information (ESI) available: The concentration of proton at the surface of VO_x· mH₂O versus time, XPS spectra of VO_x, precipitate from solution contained VOSO₄ and NiCl₂, and NiCl₂. See DOI: 10.1039/c8ra07810f

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