METHOD FOR DEPOSITING METAL OXIDE FILM IN LIQUID ENVIRONMENT

Abstract

A method for depositing a metal oxide film in a liquid environment is provided, and includes steps of: dissolving an oxidizing agent in solvent with hydrogen bond to form a solution, and placing a substrate into the solution for performing a deposition reaction to deposit a metal oxide hydroxide film on the substrate. The oxidizing agent is potassium permanganate, potassium chromate, or potassium dichromate, a reaction temperature of the deposition reaction ranges from 1 to 99 degrees Celsius, and a reaction pressure environment of the deposition reaction is an atmospheric pressure environment.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Taiwan Patent Application No. 107147031, filed on Dec. 25, 2018, and is partly disclosed in a thesis entitled “Redox-assisted multicomponent deposition of ultrathin amorphous metal oxides on arbitrary substrates: highly durable cobalt manganese oxyhydroxide for efficient oxygen evolution” on Oct. 7, 2018 completed by Ren-Huai Jhang, Chang-Ying Yang, Ming-Chi Shih, Jing-Qian Ho, Ya-Ting Tsai, and Chun-Hu Chen, and thus the disclosure of which is incorporated herein by reference.

FIELD OF INVENTION

The present disclosure relates to a method of producing a metal oxide film, and specifically to a method for depositing the metal oxide film by electroless plating in a liquid environment.

BACKGROUND OF INVENTION

Ultrathin multicomponent deposition (<10 nm) over large dimensions is of great interest to engineers and scientists, but it commonly suffers from island-like discontinuity and elemental segregation. Transition metal oxide thin films with uniform thickness and continuous coverage are shown to be essential in a wide range of modern devices and architectures, including flexible and wearable electronics.

Well-established chemical and physical depositions (e.g. chemical vapor deposition, evaporation, sputtering, atomic layer deposition, etc.) require a high standard of operation conditions (e.g. delicate chemicals, high vacuum/energy consumption, expensive instrumentation, etc.) but provide limited production scales. Solution processable deposition, due to its low-cost and easy operation, emerges to explore low temperature, massive-scale fabrication on substrates of low thermal-durability (plastics/soft materials) and complex 3D structures.

Many typical solution-processable depositions (e.g., drop-casting, sol-gel, spray/dip/spin coating, etc.) require pyrolysis to remove an organic residue and to promote film adhesion, however, they are not suitable for amorphous/metastable deposition and soft/flexible substrates. Electrodeposition may be considered as a substitute to avoid pyrolysis, but highly conductive substrates are generally needed. The drawbacks of pinhole formation, rapid deposition rates hindering ultrathin coatings, and inhomogeneous multi-element deposition due to varied deposition potentials for individual elements, also limit its control in active site formation and charge transport resistance for electrocatalysis.

Thin films of earth-abundant transition metal oxides with easy deposition are promising candidates to achieve efficient oxygen evolution reaction (OER) at a reasonable cost. Notably, studies have shown that amorphous transition metal oxides, including intermediate states present during electrocatalysis, possess greater activities than their crystalline forms.

However, since a pyrolysis step is commonly involved in solution processable deposition, that is not suitable for a soft plastic substrate, and an amorphous product cannot be obtained. Only a few examples of amorphous oxide coatings have been successfully reported, but their high resistivity also causes difficulties in electrocatalysis.

SUMMARY OF INVENTION

An object of the present disclosure is to provide a method for depositing a metal oxide film in a liquid environment and the method is implemented by depositing a multi-component metal oxide film on different substrates in the liquid environment in order to meet mass production requirements.

In order to achieve the above object, the present disclosure provides the method for depositing the metal oxide film in the liquid environment, including steps of: (S1) dissolving an oxidizing agent in a solvent with hydrogen bonds to form a solution; and (S2) placing a substrate into the solution for performing a deposition reaction to deposit a metal oxide hydroxide film on the substrate; wherein the oxidizing agent is potassium permanganate, potassium chromate, or potassium dichromate, a reaction temperature of the deposition reaction ranges from 1 to 99 degrees Celsius, and a reaction pressure environment of the deposition reaction is an atmospheric pressure environment.

In an embodiment of the present disclosure, in the step (S1), further includes a step of mixing a reducing agent and the oxidizing agent based on a molar ratio of the reducing agent to the oxidizing agent, in order to dissolve the oxidizing agent and the oxidizing agent in the solvent with hydrogen bonds to form the solution.

In an embodiment of the present disclosure, the reducing agent is selected from the group consisting of a divalent cobalt compound, a divalent iron compound, a divalent nickel compound, a divalent manganese compound, and a first transition metal ionic compound.

In an embodiment of the present disclosure, the molar ratio of the reducing agent to the oxidizing agent ranges from 9:1 to 1:3.

In an embodiment of the present disclosure, in the step (S1), further includes a step of adding an additive containing an anion into the solution, wherein the anion of the additive is selected from metal salt ions.

In an embodiment of the present disclosure, further including a step (S3) after the step (S2), wherein the step (S3) includes: causing the metal oxide hydroxide film to be calcined by a calcination process in a calcination temperature range and under a gas environment to produce a calcined metal oxide film, wherein the calcination temperature ranges from 250 to 800 degrees Celsius.

In an embodiment of the present disclosure, the gas in the gas environment is air in an atmospheric environment.

In an embodiment of the present disclosure, the gas in the gas environment is argon, nitrogen, or oxygen.

In an embodiment of the present disclosure, a duration of the calcination process ranges from 1 to 12 hours.

In an embodiment of the present disclosure, the substrate is selected from a group consisting of silicon crystal, carbohydrate, glass, nickel foam, metal, metal oxide, organic matter, organic polymer, carbon material, and glassy carbon electrode.

In an embodiment of the present disclosure, the solvent with hydrogen bonds is deionized water with an impedance of 18.2 MΩ·cm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing a method of depositing a metal oxide film in a liquid environment according to an embodiment of the present disclosure.

FIGS. 2-10 are schematic diagrams (1) to (9) that illustrate a deposition procedure and characterization of CMOH_acetate; wherein

FIG. 2 is a diagram of dipping a transparent FTO substrate into the aqueous reaction mixture of Co(OAc)₂ and KMnO₄;

FIG. 3 shows a diagram of aging FTO for 15 minutes (min.);

FIG. 4 shows a diagram of removing the FTO substrate after complete deposition (the darker contrast area), processes from FIGS. 2 to 4 are performed in sequence;

FIG. 5 shows a diagram of an SEM image;

FIG. 6 shows a diagram of AFM data of the coatings;

FIG. 7 shows a diagram of grazing-angle XRD patterns of thin and thick CMOH deposition by Co(OAc)₂ and CoSO₄;

FIG. 8 shows a diagram of an HR-TEM image of CMOH_acetate foils;

FIG. 9 shows a diagram of CMO_acetate after annealed at 500° C.; and

FIG. 10 shows a diagram of a TEM image of CMOH after 10000 cycles of OER tests;

wherein insets in FIGS. 8-10 are corresponding Fourier transform patterns of the CMOH area.

FIGS. 11-18 are schematic diagrams (1) to (8) that illustrate deposition condition tests; wherein

FIG. 11 shows a diagram of photographs of parallel deposition on six individual FTO substrates;

FIG. 12 shows a diagram of products of six coated FTO films from FIG. 11 with uniform coating contrasts;

FIG. 13 shows a diagram of Large scale deposition of CMOH with a highly ordered array over a 100 cm² FTO film;

FIG. 14 shows a diagram of CMOH deposition at room temperature (R.T.) and others, wherein arrows indicate the corners of the deposition areas;

FIG. 15 shows a diagram of transparency of CMOH coatings shown in FIG. 14;

FIG. 16 shows a diagram of CMOH adhesion tests by Scotch tape peeling for more than 100 cycles;

FIG. 17 shows a diagram of a cross-section profile of CMOH coating on a Si trench; and

FIGS. 18a-18f show diagrams of tests of arbitrary substrate deposition, wherein FIG. 18a shows a diagram of CMOH coated on 3D porous Ni foam with the comparison of bare Ni foam, FIG. 18b shows a diagram of homogeneous deposition on the cylinder-shape surface of screw pairs, FIG. 18c shows a diagram of CMOH coating with transfer of large-size complicated patterns onto a 100 cm² glass substrate by masking, FIG. 18d shows a diagram of the CMOH coated PET, where no appreciable cracking can be observed after bending and folding for 100 cycles, FIG. 18e shows a diagram of the CMOH coated curved wooden surface, and FIG. 18f shows a diagram of CMOH coated on the metallic surface of Cu foils.

FIGS. 19-24 are schematic diagrams (1) to (6) that illustrate spectroscopic and profile studies of CMOH; wherein

FIG. 19 shows a diagram of XPS data of Co 2p for uncalcined coating;

FIG. 20 shows a diagram of XPS data of Mn 2p for uncalcined coating;

FIG. 21 shows a diagram of XPS data of O 1s for uncalcined coating;

FIG. 22 shows a diagram of XAS spectra of Co K-edge;

FIG. 23 shows a diagram of XAS spectra of Mn K-edge; and

FIG. 24 shows a diagram of film composition profile.

FIGS. 25-29 are schematic diagrams (1) to (5) that illustrate QCM studies of film growth with different precursor recipes, wherein all tests were conducted under identical preparation conditions; wherein

FIG. 25 shows a diagram of a comparison of cobalt and manganese precursor-only cases with the two precursor mixed condition;

FIG. 26 shows a diagram of a comparison of different anions in the cobalt precursors;

FIG. 27 shows a diagram of the effect of additional ligands on CMOH growth;

FIG. 28 shows a diagram of a cross-sectional SEM image and EDXS mapping of CMOH prepared by CoSO₄; and

FIG. 29 shows a diagram of CVs of cobalt precursors with different anions and ligands.

FIGS. 30-33 are schematic diagrams (1) to (4) that illustrate MD simulated studies and analysis of CMOH growth; wherein

FIG. 30 shows diagrams (a) to (f) illustrating MD simulated studies of CMOH growth, wherein (a) and (b) are illustrations of simulation cells of the sulfate system and acetate system while the simulation is in progress after linking, removal of all the unreacted Co²⁺ and MnO₄⁻ ions, counter ions (OAc⁻, SO₄²⁻), and solvent (H₂O) was conducted to enable the clear presentation of (MnO₄)—Co complexes; (c) is an enlarged view of a (MnO₄)—Co complex as a colloidal precipitate in cells (a) and (b); (d) is an enlarged view of surface-linked (MnO₄)—Co complexes (CMOH film) deposited on the SnO₂ substrate in (e) the sulfate system and (f) the acetate system;

FIG. 31 shows a diagram of numbers of Co—O—Mn bonds of the colloidal complexes, analyzed from cells of (a) and (b) in FIG. 30;

FIG. 32 shows a diagram of numbers of Co—O—Mn bonds of the surface linked complexes, analyzed from (e) and (f) in FIG. 30; and

FIG. 33 shows a diagram of RDF analysis of Co²⁺ ions with O atoms of acetate (O_acetate) and sulfate (O_sulfate) ions.

FIGS. 34-39 are schematic diagrams (1) to (6) that illustrate electrocatalytic oxygen evolution studies of CMOH coatings on FTO; wherein

FIG. 34 shows a diagram of a comparison of amorphous CMOH and calcined CMO with benchmark RuO₂ recorded at 0.1 M KOH and an inset in FIG. 34 is a zoom-in plot;

FIG. 35 shows a diagram of the Tafel plot comparison of materials in FIG. 34;

FIG. 36 shows a diagram of current density-potential curves of CMOH coatings produced with varied Co-to-Mn ratios in the reaction mixtures;

FIG. 37 is a stability comparison between amorphous and crystalline coatings in i-t curves;

FIG. 38 is a stability comparison between amorphous and crystalline coatings in LSV cycle tests; and

FIG. 39 is a comparison of coatings with different cobalt precursors.

FIGS. 40-43 are schematic diagrams (1) to (4) that illustrate current density-potential curve comparisons of amorphous CMOH coated on typical substrates for the OER; wherein

FIG. 40 shows a diagram of a schematic diagram of a current density-potential curve comparison of amorphous CMOH coated on Ni foam;

FIG. 41 shows a diagram of a schematic diagram of a current density-potential curve comparison of amorphous CMOH coated on Cu foils;

FIG. 42 shows a diagram of a schematic diagram of a current density-potential curve comparison of amorphous CMOH coated on carbon cloth; and

FIG. 43 shows a diagram of a schematic diagram of a current density-potential curve comparison of amorphous CMOH coated on glassy carbon electrode (GCE).

FIGS. 44-47 are schematic diagrams (1) to (4) that illustrate the EDXS results of CMOH_acetate; wherein

FIG. 44 shows a diagram of SEM images of CMOH with the selected area highlighted by red dashed lines for mapping analysis;

FIG. 45 shows a diagram of the EDXS results show the signals of Co and Mn with the atomic ratios of Co/Mn=3.08; and

FIG. 46 and FIG. 47 show diagrams of mapping results of Co and Mn distribution corresponding to a dashed line area in FIG. 44.

FIG. 48 shows a schematic diagram that illustrates the Raman spectra of CMOH show a broad band at 599 cm⁻¹, indicating the presence of amorphous cobalt oxide, and the Raman signals of crystalline Co₃O₄ are shown for comparison, according to an embodiment of the present disclosure.

FIG. 49 shows a schematic diagram that illustrates the GIXRD pattern of the CMOH_sulfate after a calcination at 500° C., showing phases corresponding to Co₃O₄, according to an embodiment of the present disclosure.

FIG. 50 shows a schematic diagram that illustrates a characterization of CMOH_acetate cross-section under HR-TEM (also see FIG. 8). The label of I in the film area shows the EDXS signals with the majority of Co and Mn. The upper left inset shows the FFT patterns of I corresponding to an amorphous characteristic. The area labeled by II of FTO exhibits the strong Sn signal. The corresponding high resolution TEM images (the lower right inset) show a lattice corresponding to (110) of FTO. Ga signal is due to the ion-beam of Ga in FIB, according to an embodiment of the present disclosure.

FIG. 51 shows a schematic diagram that illustrates AFM results of CMOH_acetate show the film thickness around 11 nm prepared at 80° C. for 60 minutes.

FIGS. 52-54 are schematic diagrams that illustrate the XPS data of the crystalline CMO_acetate after annealing at 500° C.; wherein

FIG. 52 shows a diagram of the XPS data of Co 2p;

FIG. 53 shows a diagram of the XPS data of Mn 2p; and

FIG. 54 shows a diagram of the XPS data of O 1s.

FIGS. 55-56 are schematic diagrams that illustrate photographs of Co(OAc)₂-only and KMnO₄-only deposition on nonmental substrates of wood (see FIG. 55) and PET (see FIG. 56), wherein arrows in FIG. 55 indicate the boundary between deposition and deposition-free areas for comparison, and these results show no film formation by Co(OAc)₂-only deposition, while thin coating can be observed by KMnO₄-only deposition.

FIG. 57 shows a schematic diagram that illustrates a Faradaic efficiency test of CMOH_acetate films. After four hours oxygen evolution, the films exhibit the Faradaic efficiency of nearly 100%.

FIG. 58 shows a schematic diagram that illustrates the Tafel plots of CMOH_acetate samples prepared by different Co/Mn precursor ratios at 80° C. for 15 minutes, wherein the Tafel slopes are summarized in the following table.

FIG. 59 shows a schematic diagram that illustrates cobalt XPS data of the CMOH_7/1 (Co/Mn precursor ratio of 7/1), showing the high similarity to CMOH_3/1 (Co/Mn precursor ratio of 3/1), indicating that Co³⁺ is still the main species of the CMOH film.

FIG. 60 shows a schematic diagram that illustrates UV-vis spectra of CMOH_acetate deposited under varied deposition time at 80° C.

FIG. 61 shows a schematic diagram that illustrates a corresponding calibration curve of FIG. 60 with 550 nm absorbance.

FIGS. 62-65 are schematic diagrams that illustrate characterization of amorphous iron manganese oxide and ternary iron cobalt manganese oxide coatings on SiO₂/Si wafers, wherein the SEM results reveal that both these two coatings (iron manganese oxides in FIG. 62 and iron cobalt manganese oxide in FIG. 63) are highly smooth and crack-free, their EDXS results are respectively shown in FIG. 64 and FIG. 65 giving the corresponding compositions of Fe:Mn=2.39:1 and Fe:Co:Mn=1:2.11:0.77, and insets in FIG. 62 and FIG. 63 shows the photographs of the film appearance on FTO.

FIGS. 66a, 66b, and 66c are photographs showing samples resulting of a metal oxide calcined film at 400, 600, and 800 degrees Celsius, according to an embodiment of the present disclosure, respectively.

FIGS. 67a and 67b are photographs showing samples resulting of a sample of a substrate being different organic polymers, according to an embodiment of the present disclosure.

FIGS. 68a to 68c are photographs showing samples resulting of a sample of a substrate being different organic matters, according to an embodiment of the present disclosure.

FIGS. 69a to 69c are photographs showing samples resulting of a sample of a substrate being different carbon materials, according to an embodiment of the present disclosure.

FIG. 70 is a photograph showing a sample resulting of a substrate being a carbohydrate, according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description of the various embodiments is provided to illustrate the specific embodiments of the present disclosure. Furthermore, directional terms mentioned in the present disclosure, such as upper, lower, top, bottom, front, rear, left, right, inner, outer, side, surrounding, central, horizontal, lateral, vertical, longitudinal, axial, radial, uppermost or lowermost, etc., which only refer to the direction of drawings. Therefore, the directional terms used as above are for the purpose of illustration and understanding of the present disclosure, and are not intended to limit the present disclosure.

Please refer to FIG. 1, a method for depositing a metal oxide film in a liquid environment according to an embodiment of the present disclosure belongs to a metal oxide film production method, and may include following steps: (S1) dissolving an oxidizing agent in a solvent with hydrogen bonds to form a solution; and (S2) placing a substrate into the solution for performing a deposition reaction to deposit a metal oxide hydroxide film on the substrate; wherein the oxidizing agent is potassium permanganate (KMnO₄), potassium chromate (K₂CrO₄), or potassium dichromate (K₂Cr₂O₇), a reaction temperature of the deposition reaction ranges from 1 to 99 degrees Celsius (° C.), and a reaction pressure environment of the deposition reaction is an atmospheric pressure environment. The following examples illustrate some embodiments of the above method of the present disclosure, but are not limited as described here.

For example, the solvent with hydrogen bonds may be selected from water or the like, such as alcohol. It is understood that, water and alcohol both have hydrogen bonds and are mutually soluble, and the boiling points of water and alcohol are slightly different (such as water is about 100° C. and alcohol is about 78.4° C.). The choice of other available solvents is well understood by those of ordinary skill in the art and will not be described here. In the following, only water is taken as an example to illustrate the implementation in an aqueous environment. For example, the water may also be selected from deionized (DI) water with an impedance of 18.2 MΩ·cm to promote the reaction quality, but that is not limited as described here.

In an embodiment of the present disclosure, in the step (S1), an additive containing an anion is added into the solution, wherein the anion of the additive is selected from metal salt ions, such as the metal including cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn), vanadium (V), titanium (Ti), chromium (Cr), copper (Cu), and zinc (Zn). The salt may be a nitrate salt, a sulfate salt, an acetate salt, a halogen salt or the like of the above metal. Specifically, the anion of the additive may be selected to be any anion as below, such as acetate, sulfate, sulfite, nitrate, halogen anion, thiosulfate, hydrogen sulfate, sulfite, hydrogen sulfite, persulfate, arsenate, arsenate, borate, bicarbonate, carbonate, hydroxide, perchlorate, chlorite, hypochlorite, chlorate, nitrite, acetylacetonate or ethylenediaminetetraacetic acid, that is but not limited as described here. Thus, different concentrations of the solution, and different film growth times (such as 5 minutes to more than 24 hours) can be used to control the film thickness (<10 nm), so as to control different film thicknesses and growth rates by producers.

In an embodiment of the present disclosure, in the step (S1), a reducing agent and the oxidizing agent are mixed based on a molar ratio of the reducing agent to the oxidizing agent, in order to dissolve the oxidizing agent and the oxidizing agent in the solvent (such as water) with hydrogen bonds to form the solution. For example, the reducing agent may be selected from the group consisting of a divalent cobalt compound (Co²⁺), a divalent iron compound (Fe²⁺), a divalent nickel compound (Ni²⁺), a divalent manganese compound (Mn²⁺), and a first transition metal ionic compound. In addition, the molar ratio of the reducing agent to the oxidizing agent may be ranged from 9:1 to 1:3.

For example, the divalent cobalt compound may be selected from cobalt acetate (Co(CH₃COO)₂), cobalt sulfate (CoSO₄), cobalt nitrate (Co(NO₃)₂), cobalt chloride (CoCl₂) or acetoacetate cobalt. (C₁₅H₂₁CoO₆); the divalent iron compound may be selected from ferrous acetate (Fe(CH₃COO)₂), ferrous sulfate (FeSO₄) or ferrous nitrate (Fe(NO₃)₂); the divalent nickel compound may be selected from sulfuric acid Nickel (NiSO₄), nickel nitrate (Ni(NO₃)₂) or nickel chloride (NiCl₂); the divalent manganese compound may be selected from manganese acetate (Mn(CH₃COO)₂) or manganese sulfate (MnSO₄); and the first transition metal ionic compound may be selected from a compound of vanadium, titanium, chromium, copper, or zinc ions, but that is not limited as described here.

In an embodiment of the present disclosure, a reaction time of the deposition reaction may be ranged from 5 minutes to 24 hours (hr.), and the reaction time can be extended indefinitely according to requirements.

Some embodiments and test results of the above method embodiments of the present disclosure are further illustrated and described below, but that are not limited as described here.

To improve the intrinsic conductivity and reduce the charge transport barrier, achieving multicomponent metal oxide coatings with mixed valence and homogeneous distribution is a highly challenging, but effective strategy to enhance the electron hopping process and thus conductivities. Ultrathin, highly continuous deposition of amorphous multicomponent metal oxides is therefore an optimal and desirable model for OER electrocatalysts. As KMnO₄ is a strong stain reagent on various surfaces (e.g. fabrics, plastic, and even human skin), it is inspired to utilize this nature of KMnO₄ to achieve strong film adhesion on arbitrary substrates without pyrolysis treatment. Co(OAc)₂ and KMnO₄ interactions result in self-limited redox-coupled film growth governed by ligand coordination effects. For electrocatalytic OER applications, amorphous CMOH exhibits superior activities and durability to its crystalline counterpart and also benchmark RuO₂. Examples of the experimental part are presented as follows.

Preparation of CMOH Thin Films:

The reaction mixtures for deposition were prepared by dissolving cobalt precursors (i.e. Co(OAc)₂, CoSO₄, and Co(NO₃)₂) and KMnO₄ in deionized (DI) water (18.2 MΩcm) with a typical Co/Mn mole ratio of 3/1. As a substrate, we mainly used fluorine-doped tin oxide (FTO) glass obtained from Hartford Glass. FTO was rinsed with acetone, isopropyl alcohol (IPA), DI water, and 5.2 M HNO₃ under sonication for 10 minutes, followed by the exposure to O₂ plasma (25 W) for 20 seconds to complete the cleaning process. The deposition area is typically 0.5×0.5 cm², patterned by nail-polish oil masking. It also performed deposition on copper foil, Ni foam, carbon cloth, glassy carbon electrode (GCE), SiO₂/Si wafers, and glass. In a typical deposition, substrates were vertically placed in reaction mixtures of KMnO₄ and Co(OAc)₂ with 500 resale price maintenance (rpm) stirring at 80° C. for 15 minutes. The subscript of CMOH represents the anions of cobalt precursors used in the deposition. CMOH without specific subscript refers to Co(OAc)₂-deposition. After the deposition, the coatings were rinsed with DI water and the nail-polish mask was removed with acetone. The CMOH annealing was carried out at 500° C. for 1 hour under argon to obtain cobalt manganese oxide (CMO) films. The temperature-dependent CMOH deposition was carried out at room temperature, 50° C., 80° C., and 95° C.

Reaction mixtures with varied Co/Mn mole ratios were prepared (Co:Mn=1:3, 1:1, 3:1, 5:1, 7:1, and 9:1). For the redox deposition of iron manganese oxide coatings, Fe(OAc)₂ (Acros Organics) is used as the precursor with a Fe/Mn mole ratio of 3/1 in the reaction mixture. In the synthesis of ternary metal oxide films, Co(OAc)₂, Fe(OAc)₂, and KMnO₄ were mixed with a Fe/Co/Mn ratio of 1/2/1.

Electrochemical Measurements:

Electrochemical results were acquired using a three-electrode system on a CHI 614D Electrochemical Analyzer. FTO glass with CMOH coatings was used as the working electrode, where a Pt plate and Hg/HgO were used as the counter and reference electrodes, respectively. OER activities were evaluated by linear sweep voltammetry (LSV) with a scan rate of 5 mV s⁻¹ under 0.1 M KOH. All the overpotentials (η) were recorded at 10 mA cm⁻². The potentials presented herein are based on the reversible hydrogen electrode (RHE) following the equation:

ERHE=E_Hg/HgO+0.098+0 0.059×pH  (1)

Faradaic efficiency (FE) was obtained using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) to analyze the quantity of molecular oxygen. The FE was acquired from the ratio of O₂ measured/O₂ theoretical, where O₂ theoretical was integrated from the current-time (i-t) curve. A quartz crystal microbalance (QCM/CHI 401) was used to monitor the in situ growth of CMOH coatings at room temperature. The fundamental resonant frequency of QCM is 8 MHz. The weight change was calculated using the Sauerbrey equation:

Δf=−(2×f₀²×Δm)/[A(ρ_a×G_a)^1/2]  (2)

where f₀ is the fundamental resonant frequency of QCM, ρ_a is the density of quartz (2.648 g cm⁻³), G_a is the shear wave velocity of the quartz crystal (2.947×10¹¹ g cm⁻¹ s⁻²), and A is the active electrode area of QCM. For all the QCM measurements, Au/quartz substrates were first kept in DI water until frequency equilibrium is reached. Afterwards, Co and Mn precursors were carefully injected into the system to initiate coating growth. Pure Co(OAc)₂ and KMnO₄ were also tested in QCM as control samples. To study the effect of counter ions, cobalt precursors with different anions of Co(OAc)₂, CoSO₄, and Co(NO₃)₂ were used following the same deposition conditions. Sodium acetate (Acros Organics) was used as the source of the acetate anion.

Characterization:

Scanning electron microscopy (SEM) images were obtained using a FEI Inspect F50 and Zeiss Supra 55 Gemini with acceleration voltages of 10-20 kV. The X-ray photoelectron spectroscopy (XPS) measurements were done on a PHI 5000 VersaProbe. The film composition profile was studied by Arsputtering XPS with a removal rate of 3 nm min⁻¹. The grazing incident X-ray diffraction (GIXRD) was used to characterize CMOH thin coating with 1 degree (°) grazing angle on a Bruker D8 Advance diffractometer with a CuKα X-ray source. Field emission transmission electron microscopy (FE-TEM) images were collected with a FEI E.O Tecnai F20 G2 at 120 kV. TEM foils were prepared using a focused ion beam (FIB) using a SMI 3050. The CMOH/FTO samples were first coated with platinum and a subsequent carbon layer, followed by ion beam cutting and thinning. Samples were analyzed by energy dispersive X-ray spectroscopy (EDXS) under SEM and TEM. The Raman spectra were obtained using a WITec Confocal Raman Microscope with a 532 nm wavelength laser source. The CMOH samples were deposited on gold substrates to enhance the Raman signals via surface-enhanced Raman scattering. The X-ray absorption spectra (XAS) were collected at 17Cl in the National Synchrotron Radiation Research Center, Taiwan (NSRRC) with transmission mode. The roughness of CMOH films was analyzed by atomic force microscopy (AFM, Bruker Dimension Edge) with contact mode. The conductivity measurement was conducted using a four-point probe on a Quatek 5601Y Sheet Resistivity Meter. The UV-vis spectra were obtained with a Jasco V-630 UVvisible spectrometer. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were carried out with a PerkinElmer ELAN 6100 DRC Plus for elemental analysis. To determine Co/Mn ratios, CMOH samples were dissolved in a solution composed of HNO₃ (60%) and H₂O₂ (35%) with a 2:1 volume ratio. To study the elemental leaching issue, the OER electrolyte solution (0.1 M KOH) after 10 000 cycle sweeps was sampled to determine the contents of Co and Mn.

Simulation of CMOH Deposition Behavior:

Molecular dynamics (MD) simulations were carried out to investigate the growth of the CMOH □lm on the FTO surface. The cases of Co(OAc)₂ and CoSO₄ deposition were investigated. The composition of the MD cell in the acetate system includes 1500 Co²⁺, 3000 OAc⁻, 500 MnO₄⁻, 500 K⁺, and 2000 H₂O (solvent), while that of the sulfate system includes 1500 Co²⁺, 1500 SO₄²⁺, 500 MnO₄⁻, 500 K⁺, and 2000 H₂O. The crystalline tin oxide (SnO₂, 100×100×8 ų) substrate was established to imitate FTO glass for the deposition. All simulations were computed by using Material Studio software. COMPASS force field and NVT ensemble were adapted for the simulations. The density of the liquid phase in each system was set to be 1.0 g cm⁻³. The initial temperature of MD simulations was 298 K until a thermal equilibrium was reached; then the temperature was further increased to 353 K. This temperature setting corresponds to the real reaction temperature. The pair distances between Co²⁺ and Mn⁷⁺ (in MnO₄⁻) to O on the SnO₂ surface, as well as Co²⁺ to O in MnO₄⁻ (i.e. (MnO₄)—Co complexes), were analyzed. The metal cation-to-O distances shorter than 3.0 Å were recognized to be due to the bond formation for yielding precipitate. This linking process was repeated five times for every 75 picoseconds. The following examples illustrate results and discussion.

Deposition and Characterization of CMOH Coating:

The solution processed deposition of binary CMOH films was carried out in a single-step redox process under ambient conditions. The aqueous reaction mixtures were prepared by dissolving various Co_(II) precursors with KMnO₄ (as the metal-containing oxidant) without any additives (e.g. organic solvents, surfactants, polymers, etc.). To clearly demonstrate film deposition, transparent FTO was selected as the substrate as shown in FIGS. 2-10. The one-step CMOH deposition produced by the Co(OAc)₂ precursor (i.e. CMOH_acetate) can be accomplished by dipping pristine FTO in the reaction mixture, aging for a certain period of time, and removing it after complete deposition (FIGS. 2-4). Neither an inert atmosphere nor delicate operation was required. The uniformly dark contrast of deposition can be obtained with the homogeneous distribution of cobalt and manganese as proven by EDXS (FIGS. 44-47). The ICPMS analysis confirms the bulky composition of Co/Mn=2.92 (Table 1), similar to the selected-area composition of 3.08 acquired by EDXS.

TABLE 1
The elemental composition of CMOH films with varied deposition
times and different precursor Co-to-Mn ratios
	Deposition time
		1 min	5 min	15 min	30 min	60 min
	Co/Mn	3.03	2.87	2.92	2.86	2.91
	ratios of
	CMOH
	Co(II)-to-Mn(VII) precursor ratios
		3:1	5:1	7:1	9:1
	Co/Mn	2.92	2.95	4.46	5.72
	ratios of
	CMOH

Compared to other solution-based depositions, homogeneous binary elemental distribution generally requires specific reaction conditions due to potential mismatch in properties (e.g. hydrolysis rates, K_sp constants, thermal stabilities, etc.) between precursors. The fixed electron exchange stoichiometry dependent on the redox synthesis provides a reliable composition homogeneity for multi-precursor deposition. Different from typical dip-coating or polymer-assisted deposition, our procedure does not need thermal annealing to eliminate organic/polymer components and to consolidate coating adhesion, thus preserving the amorphous feature.

The SEM image (FIG. 5) of the CMOH film deposited on a SiO₂ wafer shows the smooth and highly continuous coating with a root-mean-square (RMS) value of 3.16 nm as acquired using AFM (FIG. 6). The GIXRD patterns of the samples deposited using Co(OAc)₂ and CoSO₄ precursors (CMOH_sulfate) show no diffraction peaks, indicating the amorphous features of CMOH coatings (FIG. 7). The Raman spectra of CMOH_acetate exhibit a broad band at 599 cm⁻¹ (FIG. 48), which is also in agreement with the presence of amorphous cobalt oxide. The signals of amorphous manganese oxide are difficult to recognize due to their relatively small amounts (<25%), significant peak broadening, and similar Raman wavenumbers to cobalt oxide. By annealing CMOHsulfate at 500 C for one hour, the coatings are shown to be crystalline corresponding to the spinel Co₃O₄ phase, denoted as cobalt manganese oxide (CMO_sulfate) (FIG. 49). To further verify the film crystallinity, the focused ion beam (FIB)-cut foils of CMOH_acetate were characterized by high-resolution TEM. The TEM image of uncalcined CMOH_acetate clearly shows amorphous features without any ordered lattice fringes (FIGS. 8 and 50), in agreement with the amorphous deposition shown in FIG. 7. The coating cross-section is highly continuous and pinhole/void-free with a major thickness of 6-10 nm, comparable with the thickness of 11 nm under AFM (FIG. 51). At this thickness, the reported coatings fabricated by physical deposition still remain discontinuous. The annealed CMO_acetate films exhibit crystalline lattices with a d-spacing of 0.244 nm, corresponding to the (311) plane of spinel Co₃O₄ (FIG. 9). The film conductivities are summarized in Table 2, showing that CMOH coatings exhibit the sheet resistance in the range of 7.4×107 to 13.0×107 (⋅□⁻¹). The annealed CMO generally displays a smaller sheet resistance (as low as 0.469×107 ⋅□⁻¹) than amorphous CMOH. CMOH_sulfate is slightly more conductive than CMOH_acetate. Different Co/Mn ratios show insignificant influence on the film resistance. The controlled samples of manganese oxide coatings possess a sheet resistance that exceeds measurement limits, suggesting that homogeneous binary oxide coatings exhibit a much lower sheet resistance than single oxides.

TABLE 2
The conductivity of CMOH and CMO coatings with varied
divalent cobalt precursors and Co-to-Mn ratios
Sheet resistance(Ω□−1) × 107
Co(OAc)2-precursor coating
Co:Mn = 3:1           13.0
Co:Mn = 7:1           11.5
Co:Mn = 3:1(annealed) 10.6
Co:Mn = 7:1(annealed) 8.95
CoSO4-precursor coating
Co:Mn = 3:1           7.41
Co:Mn = 7:1           7.40
Co:Mn = 3:1(annealed) 0.470
Co:Mn = 7:1(annealed) 0.649

Large Scale Fabrication and Properties of CMOH:

With the easy operation procedure, we attempted to achieve high throughput fabrication by parallel dipping of numerous substrates in one batch of the reaction mixture. FIGS. 11 and 12 show the success of parallel deposition to produce six individual, uniform and well-defined CMOH_acetate coatings on FTO, more efficient than batch-to-batch deposition such as spin coating. In addition, shape- and size-specific deposition can be controlled by masking techniques. FIG. 13 shows the realization of large-scale arrays of square-shaped deposition over 10×10 cm² defined by the resin-based masks. All the patterned units of CMOH are well defined in shapes, interval distance, and show highly similar contrast. The scalable and throughput redox deposition appears highly practical for massive production.

Notably, the as-coated CMOH also exhibits high visible-light transparency. By changing the deposition temperatures (FIG. 14), the coating contrasts become darker as the temperature increases indicating the formation of a thicker coating. The transparency (at 550 nm) of CMOH deposited at room temperature, 50° C., 80° C., and 95° C. was measured to be 99.2%, 98.4%, 97.4%, and 95.2%, respectively (FIG. 15). Room-temperature CMOH forms a thin, uniform, and barely visible coating (see the arrows in FIG. 14). This property becomes relevant as binary metal oxides recently gained attention as materials for transparent OER-active thin films. Moreover, CMOH exhibits a higher transparency than the reported binary FeNiO_x that is active for photoelectro-chemical cells (PEC) and sunlight-driven water splitting applications.

Film adhesion is a crucial concern particularly for low-temperature deposition. As shown in FIG. 16, we conducted peel-off tests with Scotch-tape on CMOH over 100 cycles. There were no appreciable film detachment or breaking observed. This strong adhesion is comparable to the annealed crystalline CMO samples, which allows CMOH to be directly used in the amorphous form. The step coverage studies show that CMOH coating can be deposited along the top, side wall, and base of SiO₂ trenches with a similar thickness of 9.1 nm, 9.5 nm, and 10.1 nm, respectively (FIG. 17); hence, homogeneous CMOH deposition strongly attached on 3D complex architectures and porous tunnels can be expected. As shown in FIGS. 18a-18f, it is further successfully demonstrated CMOH deposition on complex 3D structures (Ni foam) and cylinder-shape substances (screw pairs). On different types of surfaces, including rigid glass, soft plastics (polyethylene terephthalate, PET), wood, and metal foils (Cu), CMOH deposition exhibits strong adhesion and high homogeneity (FIGS. 18a-18f). In particular, the coating on PET can completely tolerate the bending strain without appreciable film breaking after 100 cycle tests under 180 folding, agreeing with the excellent adhesion and mechanical properties.

Redox Interaction:

To verify the underlying principles of CMOH formation, oxidation states of cobalt and manganese are investigated. In the XPS spectra (FIG. 19), the binding energy of Co 2p_1/2 at 796.2 eV, 2p_3/2 at 781.1 eV, and the satellite peaks at 790.8 eV reveals the presence of Co(III). The XPS data also show that signals from Mn 2p_3/2 and 2p_1/2 are 641.8 and 654.2 eV, respectively, where Mn³⁺ and Mn⁴⁺ are barely distinguishable (FIG. 20). In the spectra of O 1s, the strong hydroxide signals can be observed at 532.0 eV and a weak signal of O²⁻ at 530.0 eV (FIG. 21), showing similar ratios to metal oxyhydroxide species. After film annealing at 500° C., the hydroxide signals significantly decrease while O²⁻ signals are much stronger due to the conversion of amorphous oxyhydroxide to crystalline oxide (FIGS. 52-54). Therefore, the presence of Co_(III) and hydroxide/oxide signals indicates CoOOH-like amorphous CMOH. It is further conducted X-ray near edge structure studies of CMOH confirming the oxidation states of Co and Mn. The K-edge signals of Co (FIG. 22) are observed at 7728.6 eV, corresponding to the reported octahedral-coordinated Co_(III) at 7729 eV, which further agrees with the XPS data. The Mn K-edge spectra (FIG. 23) show the peak at 6563 eV, nearly identical to that of MnO₂ at 6562.2 eV instead of Mn_(III) at 6554.2 eV. The results, therefore, unambiguously confirm the presence of Co³⁺ and Mn⁴⁺ in CMOH coatings. As the elemental analysis highly agrees with ideal redox stoichiometry between Co²⁺ and Mn⁷⁺ as 3:1 (i.e. 2.86 to 3.03, see Table 1), it is evident that the redox-driven CMOH deposition can be expressed as Co_1−xMn_3x/4OOH. Therefore the net redox equation is shown as following:

9Co²⁺_(aq)+3MnO₄⁻_(aq)14H₂O_(I)→Co₉Mn₃O₂₆H_13(s)+15H⁺_(aq)  (3)

The film composition profiles of CMOH acquired by XPS (FIG. 24) exhibit uniform Co:Mn mole ratios (i.e. 3.30 by average) from top to bottom, corresponding well with the redox-coupled film growth as shown in equation (3). The depth profile studies show residue signals for sulfur and carbon to be less than 0.01%.

Coating Formation Process:

QCM is conducted to monitor the loading mass and film growth of CMOH on Au/quartz substrates in situ. 11,56 First, it is conducted control experiments of deposition with the precursor either Co(OAc)₂ or KMnO₄ only. The profiles of FIG. 25 show no increase of mass loading in the Co(OAc)₂-only case, while the appreciable film formation can be observed in the KMnO₄-only case. In addition, the deposition tests on carbon-based and transparent substrates also show that only KMnO₄-deposition contributes to film formation (see FIGS. 55-56). This further confirms that the strong oxidative staining ability of KMnO₄ is critical for film formation and immobilization on substrates. For the tests combining Co(OAc)₂ and KMnO₄, the QCM profiles indicate a much faster and greater increase in mass loading than the previous two control experiments, verifying the redox-coupled nucleation.

As Co²⁺ is the quantity-dominant species in the reaction mixture, attraction between the substrate-anchored MnO₄ anion and Co²⁺ cation could facilitate on-site redox interaction on the substrate surface to form CMOH coating. Despite the reported studies of cobalt oxyhydroxide preparation via the redox route (e.g. interaction of Co²⁺ and S₂O₈²⁻ to yield CoOOH), their thin film deposition has been rarely recognized Successful cobalt incorporation into the thin film form was first revealed in this work through redox interaction with KMnO₄. Theoretically, each Mn⁷⁺ would transfer charge directly to three neighboring Co²⁺ ions, giving the probability to construct interconnected networks holding multiple Co atoms with one Mn together through oxygen-bridged bonding. As a result, this network-like nucleation may favor the formation of continuous coating even at the ultrathin scale of several nanometers, rather than island-like, discontinuous deposition frequently observed in physical vapor deposition. Therefore, KMnO₄ is proposed to play the dual roles of both a surface-anchoring oxidant and a cobalt-fixation reagent in the binary oxide deposition process.

Effect of the Precursor Anion on Deposition:

To investigate the control parameters of the film thickness, notably, it is observed that film growth was highly dependent on the counterions of cobalt precursors. Under identical conditions, as shown in FIG. 26, CoSO₄ and Co(NO₃)₂ precursors exhibit the general trend that the CMOH thickness is proportional to the deposition time. Their deposition rates (0.059 μg min.⁻¹ for CoSO₄ and 0.086 mg mini for Co(NO₃)₂) are 6 to 9 times faster than that for Co(OAc)₂ (0.0097 μg min.⁻¹). In fact, the CoSO₄ and Co(NO₃)₂ deposition can continuously go beyond several hours to generate a much thicker coating. On the other hand, the Co(OAc)₂-deposition grew linearly at the first 50 minutes, and then became saturated at 60 minutes with the maximized loading mass of 2.97 μg cm⁻² (FIG. 26). This self-limiting phenomenon was also observed at different deposition temperatures of CMOH_acetate. The results clearly show that the deposition thicknesses produced vary depending on the precursor anions. As shown in FIG. 28, the cross-sectional SEM image of CMOH_sulfate for 2 hours deposition, featured with the homogeneous elemental distribution for 180 nm thickness with the absence of cracks or pinholes/voids. Together with the ultrathin thickness of CMOH_acetate, the film thickness can be controlled ranging from several nanometers up to submicrons via precursor anions. To lower the interfacial barrier of electrocatalysis, Co(OAc)₂ deposition is adopted to produce ultrathin CMOHacetate for the later OER studies.

To further investigate the effect of anions, it is carried out the control experiments by adding acetate ions to CoSO₄-deposition (FIG. 27). The addition of two acetate ion equivalents, whose quantity is comparable to that of Co(OAc)₂, yielded a nearly identical saturation time and coating mass to that of Co(OAc)₂— deposition. However, the addition of one acetate ion equivalent, which corresponds to half of that in Co(OAc)₂, behaved similar to that of CoSO₄-deposition but no saturation was observed. In addition, the loading mass lies in between those of CoSO₄ and Co(OAc)₂ deposition. These phenomena clearly indicate that the self-limiting film growth is due to the presence of the acetate anion. It is also observed similar film growth inhibition with the identical anion combination of Fe₂(SO₄)₃ and Ni(OAc)₂ to this case. Indeed, acetate anions may act as a buffer species to influence the pH conditions and also the deposition. Their pH variation studies were partially effective but comprehensive understanding of the saturation growth of binary iron nickel oxides remained uncertain. Since the acetate anion possesses the stronger intrinsic coordination capability than the other two, it is most likely that the ligand coordination effect may rationalize the anion influence.

To verify the coordination effect, the hexadentate ligand of ethylenediaminetetraacetic acid (EDTA) is added as the much stronger coordination ligand than acetate for comparison. No coating formation can be observed in the presence of EDTA (FIG. 27), indicating that its ability to coordinate with Co²⁺ significantly influences CMOH deposition. It is therefore compared the oxidation potentials of EDTA- and acetate-coordinated Co_(II) under the deposition conditions. The Co²⁺/Co³⁺ oxidation potentials in the presence of acetate and sulfate anions are 1.55 and 1.63 V, respectively (see (I) and (V) in FIG. 29). With the addition of the acetate ligand to Co sulfate (mole ratio 2 to 1, see (II) and (III) in FIG. 29), the Co²⁺/Co³⁺ oxidation potentials decrease which is eventually similar to that of Co(OAc)₂. On the other hand, the addition of a stoichiometric amount of sodium sulfate to Co(OAc)₂ shows no significant change of Co²⁺ oxidation peaks. By adding EDTA to CoSO₄, a drastic decrease in the Co²⁺/Co³⁺ oxidation potential to 0.67 V has been observed (see (IV) in FIG. 29). This oxidation potential drop for Co²⁺ should make the redox deposition even easier and more spontaneous. But the observed absence of CMOH indicates that the ligand effect governs the deposition growth rather than the redox potential changes. The EDTA ligand traps Co²⁺ tightly and may keep the coordinated Co²⁺ away from the substrate for film growth. Compared to EDTA, the relatively labile and weak coordination of acetate may result in the suppressed deposition rather than a complete stop. It is also likely that the addition of coordination ligands may establish a new equilibrium unfavorable for the CMOH deposition. Thus, the precursor anion effect could be mainly attributed to the ligand coordination ability. As a result, the precise control of the thickness can be feasible through the proper selection of ligand additives.

Simulation Study of CMOH Growth:

FIGS. 30-33 shows the MD simulation studies of (MnO₄)—Co complexes that are yielded after the redox reaction in sulfate (see (a) in FIG. 30) and in acetate (see (b) in FIG. 30) deposition. The different gray-level dots represent O, Co, and Mn, respectively. Two different (MnO₄)—Co complexes, one forms in solutions as a colloidal precipitate (see (c) in FIG. 30) while the other binds with SnO₂ substrates (see (d) in FIG. 30) as film deposition, were depicted and studied. The quantity of colloidal complexes in sulfate solution was observed to be larger than that in acetate solution.

By correlating the numbers of (MnO₄)—Co colloidal complexes formed versus simulation time (FIG. 31), it is found that the formation rates in the sulfate solution are more than those in the acetate solution. The saturated number of (MnO₄)—Co colloidal complexes in acetate solution is found to be 480. Since the total number of Co ions is 1500 at the initial stage, about one-third of Co ions in the acetate solution form the colloidal complexes. For ions in sulfate solution, the maximum number of colloidal complexes at the end of our simulation time is 780, that is, larger than one half of the number of Co ions.

Co ions are not only bonded to the oxygen in MnO₄, but also to the oxygen of the SnO₂ surface to form deposition (see (e) in FIG. 30 (sulfate system) and (f) in FIG. 30 (acetate system)). Similarly, the number of surface-linked complexes in sulfate solution was larger than that in acetate solution. Within the simulation time period, the number of surface-linked complexes in the acetate solution saturates but not in the sulfate case (FIG. 32). These simulation results agree well with the aforementioned experimental observation of ligand-governed CMOH deposition.

To further investigate the formation of the (MnO₄)—Co complex, we calculated the radial distribution function (RDF) of Co ions to O in sulfate (g_{Co—O(sulfate)}) and O in acetate (g_{Co—O(acetate)}). In FIG. 33, g_{Co—O(acetate)} has a peak appearing around 3.5 Å, while g_{Co—O(sulfate)} has no corresponding peak. This means that acetate anions closely surround Co ions with a high local density rather than being homogeneously dispersed in the whole solution cell. Such acetate-ion aggregation restricts the coordination of Co²⁺ to other oxygen atoms (e.g. O of SnO₂ substrates), and may also accumulate a great negative charge barrier to repulse away the MnO₄⁻ anion from facilitating redox interaction. The significantly inhibited CMOH deposition can also be explained to be due to change in ligands, from acetate to EDTA.

Electrocatalysis of the OER:

FIG. 34 shows a comparison of a linear sweep voltammogram (LSV) of amorphous CMOH_acetate and crystalline CMO_acetate deposited on FTO under 0.1 M KOH. No appreciable OER activities can be observed with bare FTO up to 1.9 V. The OER onset and over potentials (η at 10 mA cm⁻²) for amorphous CMOH are at 1.28 V and 390 mV, respectively. Crystalline CMO shows a higher onset potential than CMOH at 1.47 V with η of 460 mV. At an over potential of 400 mV, the current density of CMOH is 11.60 mA cm⁻², which is 4.7 times greater than that of CMO. As compared to benchmark RuO₂, CMOH has the smaller onset potential of 180 mV and an overpotential of 200 mV. In FIG. 35, CMOH exhibits the favorable OER kinetics with a smaller Tafel slope of 60.9 mV dec⁻¹ than that of CMO (72.8 mV dec⁻¹). The faradaic efficiency of CMOH was measured to be nearly 100%, indicating that no side electrochemical reaction occurred during the OER (FIG. 57). In general, the greater conductivities enable the higher electrocatalytic performance. Amorphous CMOH, despite its lower conductivity, exhibits higher OER performance as compared to crystalline CMO (Table 2), clearly showing the intrinsic OER superiority of amorphous materials over crystalline ones. The comparison of OER performance with the reported thin coating is summarized in Table 4.

Metal oxyhydroxide (e.g. CoOOH, NiOOH) has been identified as the activity species for the OER. Thin amorphous metal oxyhydroxides are commonly obtained from the electrochemical conversion of metal hydroxides as pre-catalysts during the OER, rather than produced by direct deposition. Electrochemical conditioning is needed to transform crystalline metal hydroxides to oxyhydroxides for enhanced OER activity. It is found that no appreciable electrochemical conditioning was needed for CMOH to enhance OER performance (FIG. 34) since it may be already in the form of oxyhydroxide.

To investigate the optimal composition, the coatings with varied Co/Mn ratios have been produced by changing the precursor ratios in the initial reaction mixtures (See Table 1). By increasing the contents of cobalt precursors, the coatings are generally produced with greater Co/Mn ratios. Due to the redox interaction, the Co/Mn ratios of coatings are shown to be less varied (i.e. 2.92-5.72) compared to those of the reaction mixtures ranging from 3/1 to 9/1. The Co/Mn precursor ratio of 7/1 yielded the most active coatings (CMOH_7/1) with the smallest onset potential among the others (FIGS. 36, 58 and Table 3). The cobalt XPS data of CMOH_7/1 (FIG. 59) show a high similarity to those of CMOH3/1, indicating that Co³⁺ is still the main species of the coatings, instead of Co²⁺. This might also suggest that Co3+ is the active species responsible for the OER rather than Mn sites. Compared to the ideal redox stoichiometry and binary compositions, the relatively lower quantity of Mn in CMOH_7/1 may suggest Mn⁴⁺ substitution by Co³⁺ yielding cation vacancies in the framework of films due to charge compensation. These defects in small quantity could increase material conductivity and improve catalytic activities as those observed in CMOH_7/1 (see Table 2), but large contents of defects can weaken the structure stability. The significant drop of activities at Co/Mn=9/1 is thus attributed to the observed incomplete film formation due to even higher cation vacancies that collapse the framework of CMOH.

Table 3 OER Performance

TABLE 3
OER performance
	Electrochemical performance
	Onset	Potential
	potential	(V)@	Tafel slope
Sample	(V)	10 mA cm−2	(mV dec−1)
Co:Mn = 1:3	1.50	1.73	64.0
Co:Mn = 1:1	1.47	1.71	62.9
Co:Mn = 3:1	1.43	1.68	62.0
Co:Mn = 5:1	1.41	1.67	61.2
Co:Mn = 7:1	1.28	1.62	60.9
Co:Mn = 9:1	1.45	1.77	77.8
CMO	1.47	1.69	72.8
RuO2	1.42	1.82	73.6
CMOH/Nickel foam	1.23	1.54	80.1
CMOH/Cu foil	1.25	1.64	78.1
CMOH/Carbon cloth	1.36	1.57	68.9
CMOH/GCE	1.44	1.65	63.3

TABLE 4
OER performance comparison between CMOHacetate
and the reported electrocatalysts
	Onset	Potential
	potential	@10	Electro-
Sample	(V)	mA cm−2	lyte	Reference
CoPi	1.64	N/A	0.1M	KPi	1
Ni0.9Fe0.1Ox	1.53	1.57	1M	KOH	2
Mn2O3	N/A	1.81	0.1M	KOH	3
FeCoNiOx	1.42	N/A	0.1M	KOH	4
Fe40Co40Ni20Ox	1.42	N/A	0.1M	KOH	5
Mn2O3	1.40	1.67a	1M	KOH	6
Mn/Co—CB	1.45	N/A	0.1M	KOH	7
CoOx—CoSe	1.50	N/A	0.1M	KOH	8
Cu3P	1.60	N/A	0.1M	KOH	9
CMOH	1.28	1.62	0.1M	KOH	The present
					disclosure

The OER stability tests of i-t curves (FIG. 37) were conducted at a current density of 10 mA cm⁻² for 60,000 seconds continuously. No appreciable drops (<2%) of current density for CMOH can be observed, while CMO shows the decrease of 18% of the current density. RuO₂ displays the more severe current density decrease of 67%, much larger than those of both CMOH and CMO. In addition, we compared cyclic LSV tests for 10,000 scans. Amorphous CMOH exhibits the nearly identical curves to the first run (FIG. 38), while crystalline CMO shows the current density decay of 16% with the increase of overpotential by 16 mV. Benchmark RuO₂ exhibits even greater current density decay of 18% and an increase of overpotential by 44 mV after the tests. In fact, both CMOH and CMO can be more stable than RuO₂ over the whole cycle tests. In the TEM characterization, the CMOH after the 10,000 cycle tests still remains amorphous without any crystalline features (FIG. 10), indicating that the exceptional oxygen evolution activities and stability were due to the amorphous characteristics despite the possible local structural rearrangement during the OER.

It is further conducted leaching studies by sampling the OER electrolyte solutions (0.1 M KOH) after 10,000 cycles. The ICP-MS data show that crystalline CMO coatings release both Co and Mn twice more than amorphous CMOH, which may explain the poor stability of CMO compared to CMOH over time (FIG. 37). The spinel phase has been recognized as an unfavorable structure to be transformed to oxyhydroxide. Well-crystalline spinel CMO with a rigid, ordered coordination environment may restrict the flexibility of structural alteration. The high lattice stress due to phase transformation increases the probability of irreversible bond breaking, leading to the observed Co and Mn leach. In contrast, the disordered amorphous CMOH is more structurally flexible to tolerate the greater degrees of structural rearrangement, including Co³⁺/⁴⁺ exchange in OER mechanisms.

It has studied the thickness effect on the OER by varying deposition time and with different precursors of Co(OAc)₂ and CoSO₄. As shown in FIG. 39, under the same deposition time, the Co(OAc)₂-coatings generally exhibit better activities than CoSO₄— coatings. The nearly identical OER activities between 15 minutes and 60 minutes Co(OAc)₂-coatings consist of the observed self-limiting growth and negligible difference in thickness (also see FIGS. 60-61). For the case of CoSO₄-coatings, the longer deposition time results in the weaker OER performance closely associated with their drastic thickness difference. As ultrathin coating facilitates charge transport in electrocatalysis, the thickness control of CMOH through the ligand coordination effect can be a promising route to manipulate OER performance.

OER on Various Substrates:

With the substrate-universal deposition and easy operation, it is tested ultrathin CMOH on various substrates commonly used for the OER, including metal foils (Cu foils), carbon cloth, 3D Ni foams, and a glassy carbon electrode (GCE). As shown in FIGS. 40-43, the electrochemical results generally show the superior OER enhancement compared to the uncoated substrates, suggesting (1) the strong interfacial contacts between CMOH and the substrates for electrocatalysis, and (2) the exceptional OER activity of amorphous CMOH coatings. Notably, the LSV curve for bare Ni foam shows the oxidation peak at 1.34 V, corresponding to the transformation of Ni_(II)/_(III). However, the CMOH coating on Ni foam does not reflect the Ni_(II)/_(III) signal, but it exhibits an OER onset potential of 1.234 V which is very close to the theoretical one (1.23 V) together with a small overpotential of 0.31 mV. The improved OER performance has been summarized in Table 3. Under a 3 V electrolysis setup made by a series connection of two commercial batteries (1.5 V for each), the operation video clips show vigorous O₂ bubbling solely from the CMOH coated area rather than the uncoated ones. Compared to CMOH-coated electrodes, the commercial pristine carbon rods and Pt wires are relatively weaker in O₂ production under the same conditions. These results confirm that CMOH is responsible for the oxygen evolution with the superior OER activity to the substrates, including highly conductive and electrocatalytically active copper foils and Ni foam.

Ternary Oxide Film Deposition:

Following the deposition principle above, it is explored diverse film compositions by replacing Co²⁺ with other transition metals, such as Fe²⁺. The preliminary results show the success of iron manganese oxide coatings with a Fe/Mn ratio of 2.39, suggesting the feasibility of various metal oxide combinations through the redox protocol. Furthermore, with the presence of both Co²⁺ and Fe²⁺ with KMnO₄, the ternary iron-cobalt-manganese oxide coatings on FTO have been successfully produced, in which their component ratios are similar to the precursor ratios (Fe:Co:Mn=1:2.11:0.77, see FIGS. 62-65). The metal ion fixation role of KMnO₄ has been observed again for both Co²⁺ and Fe²⁺. According to previous work, metal-containing oxidant KMnO₄ in the redox synthesis can potentially be replaced by other oxometallates (i.e. K₂Cr₂O₇), not limited to KMnO₄ only. The realization of diverse combinations of multi-component amorphous coatings can be thus systematically studied via this redox protocol.

In summary, the scalable, solution-processable protocols for multicomponent ultrathin metal oxide coatings capable of achieving pinhole-free, continuous, and substrate universal deposition. The redox-coupled film formation was proved critical for film growth, fixation, and homogeneous elemental distribution. As there is no more need for pyrolysis treatment, this protocol is a suitable alternative for amorphous deposition and substrates with low thermal durability. CMOH thickness and compositions can be controlled by means of ligand selection. This protocol might be useful for the fabrication of wearable semiconductor devices, such as gate material deposition. For oxygen evolution, the new exploration of multicomponent amorphous metal oxides (e.g. more than four different metals) can be pursued for even greater durability and efficiency. The high transparency and film integrity by the redox protocol may open a new avenue for light-assisted PEC applications.

In addition, in an embodiment of the present disclosure, the method further includes a step (S3) after the step (S2), wherein the step (S3) includes: causing the metal oxide hydroxide film to be calcined by a calcination process in a calcination temperature range and under a gas environment to produce a calcined metal oxide film, wherein a calcination temperature range can be from a phase transition temperature to a physical limit temperature of the material, such as the calcination temperature ranges from 250 to 800 degrees Celsius (for example, samples in FIGS. 66a, 66b, and 66c are implemented at 400, 600, and 800 degrees Celsius, respectively). In addition, the gas in the gas environment may be air in an atmospheric environment, argon (Ar), nitrogen (N), or oxygen (O₂), and a duration of the calcination process may be ranged from 1 to 12 hours.

In an embodiment of the present disclosure, the substrate may be selected from a group consisting of following object, such as silicon crystal board, organic polymer (for example, a plastic plate shown in FIG. 67a or rubber balloons shown in FIG. 67b, etc.), organic matter (for example, a wood shown in FIG. 68a, a plastic plate shown in FIG. 68b, rubber balloons shown in FIG. 68c, etc.), carbon material (for example, a graphene shown in FIG. 69a, a carbon cloth shown in FIG. 69b, a carbon/plastic composited substrate shown in FIG. 69c, etc.), carbohydrate (for example, biocellulose such as a wood shown in FIG. 70 or artificial leather, but that is not limited as described here), glass, nickel foam, metal, metal oxide, and glassy carbon electrode, but that is not limited as described here. For example, the glass may further be selected from a Fluorine-doped Tin Oxide (FTO) conductive glass plate or an Indium Tin Oxide (ITO) conductive glass plate, but that is not limited as described here.

Notably, the above embodiments of the present disclosure are particularly related to an electroless deposition of a metal oxide film on various substrates (especially a plastic organic substrate, such as polyethylene terephthalate, polyurethane, polymethyl methacrylate, polyethylene naphthalate, or polycarbonate) in a liquid environment.

The method has at least the following advantages: the metal oxide hydroxide film and the metal oxide film are still continuous in an ultra-thin state, only the thickness a few nanometers such as 5 nm is required to form a film.

In addition, the metal oxide hydroxide film and the metal oxide film have high activity and stability in an oxygen evolution reaction.

In addition, the metal oxide hydroxide film and the metal oxide film have strong adhesion to FTO conductive glass, ITO conductive glass, silicon wafer, wood, glass, nickel foam, plastic, metal substrate, carbon material, glass carbon electrode, and have low interface resistance to a conductive substrate.

In addition, the metal oxide hydroxide film can be uniformly coated on a substrate with a complex structure because of the high permeability of the solution and can be coated on low environmental tolerance (such as low pressure, high temperature, and insulator) substrates due to low temperature and room pressure reflection conditions.

In addition, the metal oxide hydroxide film is produced with adding no surfactant, no vacuum environment, no valuable equipment, low cost, and low pollution.

In addition, the metal oxide hydroxide film and the metal oxide film have characteristics, such as uniformly distributed elements (redox electronic measurement), flat surface, uniform thickness and excellent stepping coverage efficiency (solution permeability).

In addition, the metal oxide hydroxide film and the metal oxide film have good transparency and uniform appearance.

In addition, the metal oxide hydroxide film has flexible property.

In addition, the metal oxide hydroxide film and the metal oxide film can be used for large-scale coating and pattern transfer reproduction.

In addition, the metal oxide hydroxide film and the metal oxide film can precisely be controlled to a ratio between constituent metals.

In addition, the metal oxide hydroxide film has a structure belonging to the amorphous form.

The present disclosure has been disclosed in its preferred embodiments, and it is not intended to limit the disclosure, and those skilled in the art can make various changes and modifications without departing from the spirit and scope of the disclosure. Therefore, the scope of protection of the present disclosure is subject to the definition of the scope of the appended claims.

Claims

1. A method of depositing a metal oxide film in a liquid environment, comprising steps of: (S1) dissolving an oxidizing agent in a solvent with hydrogen bonds to form a solution; and(S2) placing a substrate into the solution for performing a deposition reaction to deposit a metal oxide hydroxide film on the substrate;wherein the oxidizing agent is potassium permanganate, potassium chromate, or potassium dichromate, a reaction temperature of the deposition reaction ranges from 1 to 99 degrees Celsius, and a reaction pressure environment of the deposition reaction is an atmospheric pressure environment.

2. The method of depositing the metal oxide film in the liquid environment as claimed in claim 1, wherein in the step (S1), further comprises a step of mixing a reducing agent and the oxidizing agent based on a molar ratio of the reducing agent to the oxidizing agent, in order to dissolve the oxidizing agent and the oxidizing agent in the solvent with hydrogen bonds to form the solution.

3. The method of depositing the metal oxide film in the liquid environment as claimed in claim 2, wherein the reducing agent is selected from the group consisting of a divalent cobalt compound, a divalent iron compound, a divalent nickel compound, a divalent manganese compound, and a first transition metal ionic compound.

4. The method of depositing the metal oxide film in the liquid environment as claimed in claim 2, wherein the molar ratio of the reducing agent to the oxidizing agent ranges from 9:1 to 1:3.

5. The method of depositing the metal oxide film in the liquid environment as claimed in claim 1, wherein in the step (S1), further comprises a step of adding an additive containing an anion into the solution, wherein the anion of the additive is selected from metal salt ions.

6. The method of depositing the metal oxide film in the liquid environment as claimed in claim 1, further comprising a step (S3) after the step (S2), wherein the step (S3) comprises: causing the metal oxide hydroxide film to be calcined by a calcination process in a calcination temperature range and under a gas environment to produce a calcined metal oxide film, wherein the calcination temperature ranges from 250 to 800 degrees Celsius.

7. The method of depositing the metal oxide film in the liquid environment as claimed in claim 6, wherein the gas in the gas environment is air in an atmospheric environment.

8. The method of depositing the metal oxide film in the liquid environment as claimed in claim 6, wherein the gas in the gas environment is argon, nitrogen, or oxygen.

9. The method of depositing the metal oxide film in the liquid environment as claimed in claim 6, wherein a duration of the calcination process ranges from 1 to 12 hours.

10. The method of depositing the metal oxide film in the liquid environment as claimed in claim 1, wherein the substrate is selected from a group consisting of silicon crystal, carbohydrate, glass, nickel foam, metal, metal oxide, organic matter, organic polymer, carbon material, and glassy carbon electrode.

11. The method of depositing the metal oxide film in the liquid environment as claimed in claim 1, wherein the solvent with hydrogen bonds is deionized water with an impedance of 18.2 MΩ·cm.