Iridium oxide fabrication and application: A review

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Iridium oxide fabrication and application: a review Hansaem Jang , Jaeyoung Lee PII: DOI: Reference:

S2095-4956(19)30883-6 https://doi.org/10.1016/j.jechem.2019.10.026 JECHEM 997

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Journal of Energy Chemistry

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26 June 2019 15 October 2019 30 October 2019

Please cite this article as: Hansaem Jang , Jaeyoung Lee , Iridium oxide fabrication and application: a review, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.10.026

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Highlights  Iridium oxide fabrication methods are collected and discussed.  Characteristics of iridium oxide formed by a certain method are summarized.  It is suggested where to use iridium oxide formed by a certain method.

Review

Iridium oxide fabrication and application: a review Hansaem Janga, Jaeyoung Leea,b,* a

Electrochemical Reaction & Technology Laboratory (ERTL), School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, South Korea b Ertl Center for Electrochemistry and Catalysis, GRI, GIST, Gwangju 61005, South Korea *

Corresponding author. E-mail address: [email protected] (J. Lee). ABSTRACT Despite the scarcity and cost of iridium oxide, it is still the material of choice in numerous fields of science and applications, including capacitors, electrochromism, sensors, and various oxidation electrocatalysis (e.g., chlorine evolution reaction, detoxification, and oxygen evolution reaction). Such versatility is attributed to the distinct features of iridium oxides, such as their activity, biocompatibility, conductivity, and durability. The features and properties of iridium oxides are strongly dependent on the fabrication method. In this review, methodologies relating to the synthesis and fabrication of solid-state iridium oxides have been thoroughly collected and discussed. Structuring and crystallization techniques for iridium oxides are also noted. At the end of the review, the effects of utilizing a certain fabrication method on the characteristics of the iridium oxide product are recapitulated, together with the recommended application of the product in various fields. Keywords:

Iridium

oxide;

Iridium

dioxide;

Electrochromism; pH sensor; Oxygen evolution reaction

Electrode;

Electrocatalyst;

Hansaem Jang joined Prof. Dr. Jaeyoung Lee’s group at GIST in 2014 after completing an undergraduate course at the University of Seoul, South Korea. He began with investigating solid-oxide carbon fuel cells for his graduate studies and has since expanded his interest to water electrolysis. His current research focuses on iridium-based electrocatalysts for the oxygen evolution reaction.

Jaeyoung Lee is the vice-director of the Ertl Center for Electrochemistry and Catalysis and a full professor at the School of Earth Sciences and Environmental Engineering, GIST, South Korea. He obtained his Ph.D. under Prof. Gerhard Ertl (2001) from the Fritz–Haber–Institut der Max–Planck–Gesellschaft and FU Berlin, Germany. He was a senior scientist at RIST/POSCO (2002–2004) and at the Fuel Cell Research Center, KIST (2004–2007). His current research interests include fuel cells, electrocatalysis, ammonia decomposition, and water electrolysis.

1. Introduction Iridium is a tie-breaker if the game is to achieve a higher oxidation state [1]. The oxidation states of iridium range from –3 to +9 [2]. Thus, iridium compounds exist in a variety of oxidation states. To obtain iridium(IV) oxide, one can simply oxidize iridium(III) oxide with appropriate treatments to form iridium(IV) oxide. Iridium(III) oxide, in this case, is a precursor of iridium(IV) oxide. Meanwhile, other chemicals, such as iridium(III) hydroxide, can act as precursors of iridium(III) oxide, and there may be other precursors of iridium(III) hydroxide. Similarly, the preparation of iridium oxide generally involves multiple processes, which may intensify the ambiguity associated with determining which chemical species actually works as a precursor of the final product, iridium oxide. In this article, the process of iridium oxide formation, in which the starting material itself serves as a precursor without further treatment, is described as a “single-step process.” Otherwise, the process is denoted as a “multistep process.” These considerations are graphically outlined in Scheme 1.

Scheme 1. Schematic illustration of the iridium oxide fabrication methods: (a) iridium

complex as a starting material; (b) metallic iridium as a starting material; (c) reforming iridium oxide from one iridium oxide type to another; and (d) flowchart of a general iridium oxide fabrication process.

1.1. Nomenclature Since iridium oxide is a material of choice in various fields due to its unique and versatile properties, many names with different notations have been proposed to describe the fabricated iridium oxide structure. To summarize, the acronyms and abbreviations devised to refer to iridium oxide are tabulated in Table 1. In some cases, a single term represents multiple instances. For example, EIROF (Table 1) denotes two different kinds of anodically formed iridium oxide films. EIROF (or AEIROF) usually denotes an iridium oxide film grown by an anodic electrodeposition process using an iridium complex as a starting material. However, EIROF (or AIROF) can also denote an iridium oxide film that is formed by anodic activation using metallic iridium as the starting material. Thus, special care must be employed when interpreting these homonymous terms. In most cases, EIROF represents AEIROF unless otherwise designated as CEIROF. In Table 1, some terms are recommended for circumventing ambiguity. For example, TIROF may represent both TDIROF and TOIROF; thus, the use of TIROF is not recommended. Table 1. Reported notations for iridium oxide. Notationa

Original name or description

Remarksb

AEIROF

Anodically Electrodeposited IRidium Oxide Film

C/–/R

AIROF

Activated IRidium Oxide Film

C/H/R

AIROF

Anodic(ally) (grown/formed) IRidium Oxide Film

C/H/–

AIRONP

Anodic IRidium Oxide NanoParticle

–/–/–

CEIROF

Cathodically Electrodeposited IRidium Oxide Film

–/–/–

CIROF

IRidium Oxide Film by Chemical bath deposition

–/–/–

E-Ir

Electrochemically oxidized Iridium

–/–/–

EIROF

Electrochemically activated IRidium Oxide Film

–/H/–

EIROF

Electrodeposited IRidium Oxide Film

C/H/R

EIrOx

Electrodeposited Iridium Oxide

–/–/–

hIrO2

Iridium Oxide produced by hydrolysis

–/–/–

HIROF

Hydrous(Hydrated) IRidium Oxide Film

–/–/–

IrO

Iridium Oxide

–/–/–

IrO2ND

IrO2 NanoDot

–/–/–

IrO2NT

IrO2 NanoTube

–/–/–

IrOxNF

IrOx NanoFoil

–/–/–

IrOxNP

Iridium Oxide NanoParticles

–/–/–

IROF

IRidium Oxide Film

–/–/–

NIROF

Nanostructured IRidium Oxide

–/–/–

PRIROF

Periodic Reverse current electrolysis IRidium Oxide Film

–/–/–

SIRO

Sputtered IRidium Oxide

–/–/–

SIROF

Sputtered IRidium Oxide Film

C/–/R

T-IrO2

Thermally prepared IrO2

–/–/–

TDIROF

Thermally Decomposed IRidium Oxide Film

–/–/R

TIROF

Thermal(ly) (formed/treated) IRidium Oxide Film

C/H/–

TOIROF

Thermally Oxidized IRidium Oxide Film

–/–/R

a

Spelled and noted as found in the original text. Listed alphabetically. Marked with C (Consensus) if the term has reached a certain level of consensus on its usage. Marked with H (Homonym) if the term represents multiple instances with identical spellings. Marked with R (Representative) if the term is recommended as a representative of the homonyms or terms sharing a similar concept. b

1.2. Preparative routes and processes In a multistep process, a “preparative route” is adopted prior to the “iridium oxide formation” step (Scheme 1(d)). The final product of the preparative route is generally a precursor solution or iridium-coated substrate (Scheme 1(a, b)). A representative example of the latter is a process in which iridium is deposited on the target surface through a physical or chemical vapor deposition method (Scheme 1(b)). For the solution preparation (Scheme 1(a)), generally, the starting material is added to water to prepare an aqueous solution, followed by a number of chemical reactions. The post-treatment of an aqueous solution containing the starting material (i.e., an iridium complex) is optional, depending on the purpose of the solution. When not treated, the solution is generally prepared as a dispersion to be loaded on a substrate, serving as a precursor of iridium oxide, after which the loaded substrate can be dried and calcined in the presence of oxygen to form an IrO2 layer on top of the substrate. When treated, the ligand(s) of the starting material is (are) replaced, and this newly coordinated iridium complex works as an actual precursor. In most cases, the starting material is an iridium chloride or chloridoiridates: IrCl3, IrCl4, M3IrCl6, and M2IrCl6 (M = H, NH4, Na, K, etc.). The dissolution mechanisms of these complexes are quite different from each other; however, they share the same key reaction, i.e., hydrolysis. During the hydrolysis of the iridium complex, the oxidation state of the iridium core of the complex may be affected by a number of factors, such as the complex concentration, pH, O2 partial pressure, and temperature [3]. A mechanism of the iridium(IV) dioxide formation from chloridoiridate (IV) in solution is briefly explained in Scheme 2 [4].

Scheme 2. Chemical reaction flow chart of the formation of iridium(IV) dioxide [4]. For Scheme 2, the starting material (chloridoiridate (IV) ion) converts into hydroxidoiridates and aquairidates, and these complexes finally form iridium dioxide. In other words, the starting material undergoes a hydroxido/aqua complex mechanism. If an iridium complex with a different oxidation state is used as the starting material, (aqua/hydroxido)chloridoiridates can appear during the course of the precursor solution preparation [5–8]. Thus, the conversion of chloridoiridate to the precursor solution by varying only the pH can be said to be an “aqua/chlorido/hydroxido complex route.” Although in Scheme 2, the prepared ionic precursors (yellow and pale yellow solutions) are oxidized by O2 to produce IrO2, it is worth noting that such precursors can be converted to IrO2 by other processes, including electrodeposition and thermal oxidation. Besides the aqua/chlorido/hydroxido complex routes, many other routes are also proposed: an oxalato complex route, a sulfato complex route, a sulfito complex route,

a nitrato complex route, a nitrito complex route, an ethylene diamine tetraacetic acid (EDTA) complex route, a pentamethylcyclopentadienyl(Cp*) complex route, an acetylacetonate(acac) complex route, a dimethylsulfoxide(DMSO) complex route, etc. Notably, some of the aforementioned routes are claimed to be controversial. For example, in the case of oxalate addition, there are conflicting views regarding the role of oxalate during the synthesis of iridium oxide. One is that oxalate replaces the ligands of an iridium complex to form an oxalato complex, and the oxalato ligand is oxidized to CO2 during the electrodeposition process [9]. The other view is that oxalate does not coordinate with iridium to form an oxalato complex, but rather acts as a stabilizer in the solution [10]. Similar to the latter view on the role of oxalate, there are some cases, in which chemicals are added as stabilizers during the synthesis of iridium oxide. A stabilizer is often added to the solution to control particle growth or to impede rapid precipitation. Iridium oxide hydrosols can be produced using stabilizers, and the following are examples of stabilizers (if the first letter is capitalized, it is a commercial product): Carbowax

20M,

Polybrene,

citrate,

poly(styrenesulfonate),

and

poly(4-vinyl-N-methylpyridinium) and its ortho isomer [11]. Extensive research has been conducted on the role of surfactants. It has been reported that surfactants such as butylmalonate, malonate, and succinate can effectively lead to the formation of 2 nm particles [12]. Conversely, stabilizers such as acetate, citrate, glutarate, and phthalate are known to form a network of aggregated or fused nanoparticles [12]. Occasionally, iridium oxide particles may be capped (or stabilized) to be in a charged state. This allows the charged particles to be attracted to the oppositely charged surface, which is known as “electrostatic deposition” [13,14]. Phosphate capping can be used to prevent nanoparticle flocculation [15]. Moreover, generally, stabilizers can serve as templates

for pore structuring [16]. Other roles and examples of stabilizers are described below (see §4.4). There are other chemicals that may be added to the solution during the iridium oxide synthesis. Ascorbic acid may be introduced to an alkaline solution to avoid oxidation of the iridium complex by air [17]. Conversely, NaClO can work as an oxidizing agent [18]. Interestingly, H2O2 can serve as both a reducing agent and an oxidizing agent. Hydrogen peroxide can act as a reducing agent in acidic media, although it works as an oxidizing agent in alkaline media [10]. However, it should be noted that although H2O2 itself is acidic, it can cause the oxidation of IrCl3 [19]. Therefore, special care must be taken when determining the role of peroxide.

2. Iridium oxide formation Iridium oxide formation processes can be grouped according to various criteria, such as purpose, type, starting material, and driving force. The processes are generally employed to produce stand-alone iridium oxide particles or to coat iridium oxide on the target surface. The processes are mainly divided into dry (non-solution) and wet (solution) processes. The starting materials for the processes are mostly metallic iridium or iridium complexes. The driving force is usually provided by heat, radiation, or electricity. However, the actual iridium oxide formation process may not be easily distinguished as classified above. The reality is that the process can be conducted in a method not described above, or a mix of different cases. Therefore, the categories used in this article are based on the unique characteristics of each process for the formation of iridium oxide.

2.1. Substitution and solvolysis This section deals with the preparation of iridium oxide, in which the entire process is carried out in solution with the involvement of substitution and/or hydrolysis reactions (or generically, solvolysis [15]) as in Scheme 3. The conditions (c, c1, c2 in Scheme 3) may involve various energy forms, including heat, radiation, and electricity. These energy sources can trigger, accelerate, or induce the formation of iridium oxide. Electricity, as the driving force, is not discussed in this section but is discussed below (§2.4 & §2.5). Noteworthily, iridium oxide can also be produced by a single-step process (Scheme 3), while the preparative route is based on a multi-step process (§1.2).

Scheme 3. Schematic illustration of a substation and solvolysis process for the formation of iridium oxide. Note that the conditions (c, c1, c2) could be produced by one or several of the following: H+, OH−, Δ, hν, and chemicals (e.g., O2); aStarting material works as a precursor, where a preparative route is not employed; bThe starting material is not the precursor once a preparative route is employed.

2.1.1. Hydrolysis only Iridium oxide can be prepared by hydrolysis without further processing. Hydrolysis for the formation of iridium oxide can be initiated by adding a base to the solution and can be facilitated by heating the solution (c = OH− and/or Δ; in Scheme 3). Iridium oxide has long been prepared through this method since between the late 19th century and early 20th century [20,21]. This method appears to be a single-step process but can involve multiple reactions (c1 = OH− and/or Δ; c2 = OH− and/or Δ; in Scheme 3). Depending on its oxidation state, the starting material may undergo a redox reaction. Additionally, it may form an intermediate species, such as (chlorido)hydroxidoiridate. The following cases are select examples of a process, in which iridium oxide is produced only by hydrolysis in solution: (NH4)2[IrCl6] → IrO2·xH2O, K2[IrCl6]·3H2O → IrO2·xH2O, IrCl3·2.6H2O → IrO1.45(OH)1.10·1.5H2O.

2.1.2. Hydrolysis or oxidation with the addition of chemical or O2 The hydrolysis reaction for the formation of iridium oxide can be tuned by the addition of a chemical or O2 (c = chemical (with OH− and/or Δ); in Scheme 3). This method can involve multiple reaction steps (Scheme 3). The effect of acid addition is discussed below (§2.1.3). The following cases are select examples of a process, in which a chemical or gas is added to a solution to assist the hydrolysis or oxidation to produce iridium oxide: Na2[IrCl6]·3H2O → IrOx, Na2[IrCl6] → IrOx, M3[IrCl6] (M = Na, K) → IrOx, K2[IrCl6] → IrO2·xH2O. Stabilizers can be used to control the growth of colloidal iridium oxide particles. For example, iridium oxide hydrosols can be prepared from chloridoiridate(IV) or chloridoiridate(III) by hydrolysis with the aid of various stabilizers at ca. 80 ℃ [11,22]. An oxidizing agent or gaseous O2 can be added to the solution during the iridium oxide synthesis. The presence of air or an oxidizing agent such as Na2S2O8 is known to promote the oxidation of iridium colloids [23]. For example, it has been reported that when a colloidal iridium suspension is allowed to stand in air at 40–50 ℃, the absorption band of the oxidized iridium species increases [23]. More documentation on the use of oxygen for the formation of iridium oxide is available [24,25]. Occasionally, the use of O2 can be combined with the use of stabilizers to produce a colloidal suspension of the hydrated iridium dioxide [26]. Borohydride is known to play a dual role in the synthesis of iridium oxide nanostructures. It can reduce IrCl3 to metallic iridium at a temperature of 40 ℃ or higher [27]. Conversely, the use of borohydride under an ambient environment at 25 ℃ is responsible for increasing the pH of the solution, resulting in the hydrolysis of IrCl3 to IrO2 [27].

2.1.3. Hydrolysis and acid addition An acid may be added before or after the hydrolysis step. As described above, when iridium chloride is dissolved in water, the solution becomes acidic [10,28]. In other words, the purpose of acid addition prior to the hydrolysis step may be to induce the conversion of a starting material to another iridium complex, such as aqua(chlorido)iridates. However, the hydrolysis step is generally carried out prior to the acid addition (c1 = OH− and/or Δ; c2 = H+; in Scheme 3). The purpose of hydrolysis as the preceding step is also to induce the conversion of the starting material to another iridium complex. Various forms of iridium complexes can be produced. Generally, (chlorido)hydroxidoiridate is the goal [3,6]. Acid addition to this intermediate material results in the formation of iridium oxide, which is believed to be due to acidic condensation [29]. The following cases are select examples of a process in which iridium oxide is produced by hydrolysis and subsequent acid addition: H2[IrCl6]·6H2O → IrOx, K2[IrCl6] → (via [Ir(OH)6]2− →) IrOx·nH2O, K2[IrCl6] → IrOx. Iridium hydroxide can also be produced by hydrolysis and acid addition [17]. For example, generally, iridium hydroxide hydrate is precipitated by adjusting the pH of the iridium complex solution to ca. 8 using 0.1 M HClO4, and this solution can be prepared by the following procedure: first, a solution is prepared by treating H2[IrCl6]·xH2O with an aqueous solution of 0.038 M NaOH at 40 ℃; Subsequently, the solution is stirred and bubbled with N2 for a period of time and then cooled to room temperature; Lastly, ascorbic acid is added to the solution to prevent air oxidation prior to the acid addition step [17].

2.1.4. Chemical bath deposition Iridium oxide can be deposited on a substrate surface via “chemical bath deposition.” An iridium complex is used as a starting material, and it undergoes substitution and/or hydrolysis to afford a “chemical bath.” Iridium oxide is grown on a substrate placed in the chemical bath. For example, chloridoiridate can be converted to colloidal IrO2 through nitrito and hydroxido complex routes (§1.2), and a substrate can be immersed in the colloidal suspension and deposited with IrO2 [18,30]. The following cases are select examples of a process in which iridium oxide is produced by chemical bath deposition: Na3[IrCl6]·xH2O → (via a mixture of [Ir(NO2)4Cl2]3−, [Ir(NO2)3Cl3]3−, [Ir(OH)6]3−, and oligomer →) IrO2, Na3[IrCl6]·xH2O → IrO2, IrCl4·2H2O → IrO2, Ir(ClO2)x·yH2O → IrO2 (Note: although the authors [31] stated that iridium chlorite was employed, iridium chloride might have been utilized). HNO3 is an acid commonly used for acidic condensation to produce iridium oxide. In acidic chemical bath deposition, however, HNO3 together with H2O2 can act as complexing agents [32]. Conversely, in a basic chemical bath deposition, NaNO2 and NaOH are used for that purpose [32]. In an acidic chemical bath deposition, HNO3 can prevent the homogeneous nucleation of IrO2 by lowering the pH of the chemical bath. Consequently, an IrO2 film can be uniformly formed [32]. Occasionally, chemical bath deposition is used to coat particulate materials with IrO2 [31]. In addition, chemical bath deposition can be used to fabricate structured iridium oxide, when combined with a template removal technique [32,33].

2.1.5. Hydrothermal method Hydrolysis is often facilitated by heating to ca. 80 ℃ (§2.1.1). Solutions having a temperature of 100 ℃ or higher can also be used to prepare iridium oxide. This

method is considered a hydrothermal process. Although the use of a hydrothermal process is known to possibly yield crystalline IrO2, it may also result in the formation of iridium oxohydroxide [34]. A hydrothermal process requires a vessel designed for high operating pressures. In many cases, a polytetrafluoroethylene (PTFE)-lined vessel is used as a container. A hydrothermal process often involves a base addition step, although it may also be performed without base addition [35,36]. For hydrothermal processes, microwave-assisted reactors can be used to supply heat as well as conventional means, such as ovens or autoclaves. The following cases are select examples of the hydrothermal process for the formation of iridium oxide: H2[IrCl6]·nH2O → IrO2 (Although this process appears to be a “solvothermal process,” it is classified as a “hydrothermal process” according to the description of the original text [36]), H2[IrCl6]·6H2O → IrO2, IrCl3·3H2O → IrO2, K2[IrCl6] → IrOx(OH)y.

2.1.6. Photochemical method Irradiation plays various roles, as explained in Scheme 4. For instance, the previous section describes the effect of microwaves on hydrolysis [34]. This section deals with the photochemical hydrolysis used to produce iridium oxide (c = hν (with OH−, Δ, and/or chemicals), in Scheme 3). Radiolytic iridium oxide formation is discussed in the next section.

Scheme 4. Schematic illustration of the iridium oxide formation through radiation. Hydrolysis can be triggered photochemically and iridium oxide can be produced [37]. Both monochromatic and polychromatic light can be employed as the incident light [38]. Conventionally, ultraviolet irradiation results in the formation of iridium oxide nanoparticles [37]. For visible light, the maximum wavelength required to induce hydrolysis is reported to be near 500 nm [37,38]. Generally, the irradiation by 498 nm light does not result in hydrolysis [38]. Occasionally, the maximum wavelength is referred to as the threshold wavelength [37]. It has been reported that when certain chemicals are present in the solution, the maximum wavelength can be extended [38]. For example, the presence of Fe2O3 is known to induce hydrolysis upon irradiation by 498 nm light. This is because electron holes can be generated in the valence band of Fe2O3 by irradiation with light of 498 to 590 nm in the presence of Ir(III) [38]. The following case is a selected example of the photochemical process for the formation of iridium oxide: IrCl3·3H2O → IrOx.

2.1.7. Radiolysis

Radiolysis using γ-rays is another viable option for producing iridium oxide. Hydrolysis can be radiolytically induced, and iridium oxide hydrosols can be formed [22]. A mechanism for explaining the role of γ irradiation has been proposed. For example, during the γ-radiolysis of an aqueous chloridoiridate solution, water undergoes radiolytic decomposition to generate free radicals, and these free radicals dissociate chlorido ligands from the chloridoiridate [39]. Gamma irradiation can be conducted in two ways: steady-state irradiation and pulsed irradiation [40]. The following cases are select examples of a radiolytic process for the formation of iridium oxide: Na3[IrCl6]·3H2O → IrOx·nH2O, M3[IrCl6] (M = Na, K) → IrOx·nH2O, Na3[IrCl6] → IrO2·2H2O.

2.1.8. Non-aqueous method The starting material for iridium oxide synthesis can also be dissolved in a solution other than water or a solution containing water at the solute level (c = OH−, Δ, and/or chemicals in non-aqueous solution; in Scheme 3). This strategy includes a polyol method, a sol–gel method, and a solvothermal method. The cases, in which a non-aqueous process is combined with another process such as thermal decomposition, are not elucidated here but are discussed in the relevant section (§2.3.4). The following cases are select examples of a non-aqueous process for the formation of iridium oxide: IrCl3·xH2O → IrOx (polyol), IrCl3 → IrOx (polyol), Ir(acac)x → IrO2 (sol–gel), Na2[IrCl6] → IrO2 (sol–gel), K2[IrCl6] → IrO2 (solvothermal). Iridium oxide can be prepared via a polyol process. One study noted that amorphous iridium oxide nanoparticles can be produced through a polyol method [41]. However, another study using a similar process reported that iridium nanoparticles were initially formed. In the latter case, the formation of IrO2 was realized by an

additional electrochemical oxidation process [42]. A polyol reaction can be promoted by supplying heat using microwaves [43]. A sol–gel process can be utilized to deposit iridium oxide on the support material [44]. Alternatively, a sol–gel technique can also be used to produce IrO2 powder [45]. The mixed oxide gel of IrO2–TiO2 can be prepared by adding a titanium precursor to iridium alkoxide [45]. Generally, the mixed oxide gel is not formed when the iridium ratio is high [45]. In addition, the formation of iridium oxide using a sol–gel technique can be realized in other ways. For example, a gel of the support material can be decorated with iridium oxide using chloridoiridate [46].

2.2. Unique chemical process This section may have some similarities with other sections (§2.1 & §2.3). However, the processes listed in this section are categorized separately because they show overwhelming uniqueness. Both wet and dry processes are included in this section.

2.2.1. Oxidation or reduction of ligand of iridium complex by chemicals Certain oxidizing agents can convert an iridium complex to iridium oxide. Studies on the use of organometallic iridium complexes as water oxidation catalysts in the presence of cerium ammonium nitrate have been reported [47]. One of these studies reported that Ce(IV) caused the ligand oxidation and rapidly changed the iridium complex to IrOx or IrO2 nanoparticles [47]. Interestingly, certain reducing agents can partially convert an iridium complex to iridium oxide. Sodium borohydride is an example. In one study, IrCl3 was added to an anhydrous ethanoic solution of cetyltrimethylammonium bromide (CTAB), and sodium borohydride was added later as a reducing agent [48]. Here, CTAB was added

to reduce the surface tension and used as a surfactant [48]. The addition of sodium borohydride resulted in a powder of IrOx–Ir (nano-sized iridium oxide incorporated in a metallic iridium structure) [48].

2.2.2. Heterogeneous oxidation of metallic iridium by chemical A film of iridium oxide may grow on metallic iridium in a particular acidic solution. For example, once immersed in an oxygenated sulfuric acid solution at 30 ℃, a polished metallic iridium rod undergoes a chemical reaction, in which a film of iridium oxide grows on the surface [49]. This chemical reaction can occur under open-circuit conditions and is affected by the concentration of the acid [49]. If the concentration is higher than 0.5 M, the film is compact and multilayered [49]; otherwise, the film will hydrate and dissolve [49]. Similar processes can be performed without using a solution. For example, nitric oxide can convert an iridium electrode to iridium dioxide [50]. This conversion can occur not only in the open circuit but also in potentials more positive than −300 mV vs. an air electrode [50].

2.2.3. Photodeposition As described above (§2.1.6), the hydrolytic formation of iridium oxide can be assisted photochemically. However, the photochemical iridium oxide formation is not restricted to the hydrolysis method. For example, a photocatalytic water splitting process can lead to the formation of iridium oxide when the solution contains an iridium complex. In many cases, this phenomenon is called “photodeposition.” In one study, IrO2 was photodeposited on the surface of La-doped NaTaO3 using (NH4)2[IrCl6] or Na3[IrCl6] [51]. The photodeposition occurred only in the presence of

nitrate ions, in whose absence, metallic iridium was formed [51]. In another study, iridium(IV) oxide (IrOx) was photodeposited on the surfaces and edges of exfoliated nanosheets ([TBA, H]-Ca2Nb3O10) [52]. In this study, IrOx was prepared from a solution of K3IrCl6 and KNO3 by irradiating the solution with a Xe lamp for 1 h [52]. This study proposed a photodeposition mechanism, as shown in Scheme 5 [52].

Scheme 5. Schematic illustration of the steps involved in a photodeposition process: (1) generation of an electron–hole pair by ultraviolet irradiation; (2) charge trapping; (3) charge transfer to the sacrificial redox agent; and (4) nucleation and growth of nanoparticles (D = Electron donor; A = Electron acceptor). Images are obtained and relabeled from [52]. Prior to the photodeposition of iridium oxide, the iridium complex can be hydrolyzed [53]. For example, IrCl3·3H2O in carbonate buffer (pH 8.9) can be hydrolyzed with heating and stirring in the presence of Na2S2O8; upon irradiation of this hydrolyzed solution, IrOx nanoparticles can be photodeposited on the substrate [53]. In this case, (S2O8)2− acts as a sacrificial electron acceptor [53].

2.2.4. Photochemical conversion

The photodeposition of iridium oxide can be realized also from an iridium complex other than a liquid solution. In this case, “photochemical conversion (decomposition)” appears to be a more appropriate term. In one study, a substrate was prepared, coated with Ir(acac)3 dissolved in toluene, and then dried in a vacuum [54]. Thereafter, the substrate was irradiated (355 or 458 nm) to form iridium oxide [54]. It should be noted that, in this work, the formation of iridium oxide nanocluster was actually completed by calcination under O2 [54]. In another study, a substrate was coated with Ir(acac)3 dissolved in chloroform by spin-casting [55,56]. Thereafter, the substrate was irradiated with ultraviolet rays (≤ 254 nm) for 1 h to decompose Ir(acac)3 [55]. Consequently, the spin-cast of Ir(acac)3 was converted to an amorphous IrOx film [55]. The duration of the ultraviolet irradiation can be adjusted according to the spectroscopic signal [56]. In other words, the duration can be extended or shortened until the absorbance band(s) of an iridium complex disappears [56]. Occasionally, the photochemical conversion may be followed by a thermal decomposition process [57]. During the irradiation, iridium can be oxidized to iridium oxide. For example, iridium nanoparticles on carbon support (Ir/C) can be converted to iridium oxide nanoparticles (IrO2/C) through flashlight irradiation [58]. In this case, irradiation can be performed using a Xe flashlamp (wavelength range: 380–1000 nm) in a pulsed manner under ambient conditions [58].

2.2.5. Electroless deposition Similar to photodeposition (§2.2.3), electroless deposition utilizes the reactivity of the substrate surface. For example, the use of anodized gold surfaces can lead to electroless deposition. In one study, a solution of IrCl3·3H2O in 0.1 M NaOH was

prepared to immerse an anodized gold surface [59]. This work reported that [Ir(OH)6]2− was formed in the solution and converted to IrOx nanoparticles on the gold surface [59]. A mechanism has been proposed to explain this phenomenon: the surface of gold is converted to NaAu(OH)4 in NaOH solution, and NaAu(OH)4 causes the condensation of [Ir(OH)6]2−, resulting in the formation of iridium oxide [59]. Noteworthily, this process does not require an external energy supply. This is because the open circuit potential of the anodized gold surface is sufficient to induce various reactions, such as condensation of [Ir(OH)6]2− in alkaline medium [59].

2.2.6. Heterogeneous oxidation of metallic iridium by oxygen plasma The heterogeneous oxidation of metallic iridium by chemicals has already been discussed above (§2.2.2). In addition, the heterogeneous oxidation of metallic iridium by gaseous oxygen is discussed below (§2.3). However, there is another distinctive heterogeneous oxidation of metallic iridium to iridium oxide. Conventionally, when metallic iridium is exposed to an O2 plasma, it is converted to iridium oxide [60]. Spectroscopic studies have confirmed that metallic iridium can be converted to iridium oxide by O2 plasma treatment [60].

2.2.7. Ball milling of iridium complex Iridium oxide can be formed during the ball milling of iridium precursors. For example, iridium (IV) oxide nanoparticles (IrO2·xH2O) can be prepared by ball milling IrCl3·3H2O with NaOH for 30 min at room temperature [61]. Heat may be generated during the ball milling, but this process is not categorized as a “thermal dry process (§2.3).” This is because heat is not intentionally supplied by the operator.

2.2.8. Heterogeneous oxidation of metallic iridium by oxygen due to size effect When the sizes of the iridium particles are sufficiently small, the particles are known to be sufficiently active to be oxidized at low temperatures or even at room temperature. In one study, Ir/IrOx clusters were prepared from Ir4(CO)12 by chemical vapor deposition and were converted to IrO2 by treating the clusters in air at 30 ℃ for 24 h [62]. In another study, a support (multi-walled carbon nanotube) was prepared and loaded with IrCl3 [63], after which it was reduced using H2 at 400 ℃ for 2 h [63]. Subsequently, oxidation was carried out in air at 40 ℃ for 24 h to obtain ultrafine iridium oxide (IrO2@CNT) particles [63].

2.3. Thermal dry process Heat plays diverse roles in the fabrication of iridium oxide, as demonstrated in Scheme 6. Several terms can be employed to refer to a process that involves a heat treatment step. The following are examples: annealing, calcination, pyrolysis, thermolysis, etc. Notably, these terms are technically different from each other and cannot be used interchangeably in most cases. Thus, some technically uncontrolled terms have been reclassified in this section for clarity [64–68].

Scheme 6. Schematic illustration of the fabrication of iridium oxide through heating.

2.3.1. Heterogeneous oxidation of metallic iridium by gaseous oxygen with heating The simplest method of fabricating iridium oxide may be the direct oxidation of iridium. Iridium can be heated in the presence of gaseous oxygen or air and can be converted to iridium oxide if heated sufficiently [69]. Generally, iridium powder reacts with O2 at ca. 600 ℃ and converts into IrO2 [19]. When iridium is produced in nanoparticle form, oxidation can be carried out at reduced temperatures (cf. §2.2.8) [70]. In addition to iridium powder, iridium films can also act as a precursor [71,72]. For iridium films, thin IrOx begins to grow on the surface at 100–300 ℃, crystallizes at ca. 400 ℃ and is further oxidized at ca. 500 ℃ [73]. A preparative route may be employed to coat iridium at desired locations or to obtain particular iridium structures, as shown in Scheme 1(b). For example, iridium can be prepared by heat treatment (e.g., heating iridium chloride with reducing agents and stabilizing agents) or by electrodeposition (e.g., galvanostatic electrodeposition; potentiostatic electrodeposition).

Metallic iridium can be annealed to tune its physicochemical properties. However, annealing of iridium can also be employed as a method for producing iridium oxides. If metallic iridium is annealed in the presence of O2 at elevated temperatures, iridium oxide films begin to grow on the surface. In this case, a number of preparative routes for the formation of metallic iridium may be employed. For example, iridium can be prepared by pulsed laser deposition, e-beam processing, electrodeposition, chemical vapor deposition, and heat-assisted transport. Notably, an annealing process can lead to the formation of structured iridium oxides [74].

2.3.2. Annealing of iridium oxide by gaseous oxygen with heating Iridium oxide can be annealed in an inert or active atmosphere to modify its physicochemical properties, such as porosity and crystallinity (see also §3.6 & §4.1) [67,75,76]. In many cases, the annealing process is used in the presence of O2 to convert amorphous and/or nonstoichiometric IrOx to crystalline IrO2. In general, the crystallization of amorphous iridium oxide into rutile IrO2 is initiated at ca. 400 ℃ by thermal treatment under O2, and accelerated at ca. 500 ℃ and higher [77].

2.3.3. Oxidation of metallic iridium using chemicals as oxygen sources There is another method of converting metallic iridium to iridium oxide using heat treatment. Like O2 gas, chemicals can also be used as oxygen sources. For example, a layer of iridium oxide can be formed on the surface of metallic iridium when the metallic iridium is immersed in a specific chemical (e.g., NaOH) and then treated thermally (800 ℃) [78]. In this process, the key to a successful deposition is repetition [78]. This process can be modified by adding an additional step between the repetitions. For instance, a sample that has already been thermally treated can be

quenched using deionized water before the process is reinitiated [79].

2.3.4. Thermal decomposition of iridium complex Thermal decomposition of iridium complex in the presence of oxygen has been widely adopted as a means for preparing iridium oxide. This section discusses any thermal decomposition process carried out in the presence of O2, such as calcination, combustion, firing with O2, thermolysis with O2, and pyrolysis with O2 [80–83]. Noteworthily, basically, “pyrolysis” refers to a thermal decomposition reaction in an inert atmosphere, and “calcination” refers to a thermal treatment process in the absence or limited supply of air or O2. The thermal decomposition of iridium complex in the absence of oxygen can lead to the formation of metallic iridium. Thus, it is important to supply sufficient air or O2 to the thermal decomposition process, through which iridium oxide is synthesized. Occasionally, water-saturated air can also be used as the oxygen source [84]. Iridium(III) chloride is one of the most attractive precursors and has long been studied to understand its mechanism of iridium oxide formation by thermal decomposition. In an oxygen atmosphere, IrCl3·3H2O may be decomposed at 453 and 494 ℃ (Fig. 1) [85]. In an air atmosphere, IrCl3·3H2O may be converted to an intermediate Ir2O3 phase at 250–600 ℃ and then crystallized into IrO2 by annealing at 600 ℃ (Fig. 2) [67]. In an oxidative atmosphere (2% O2 + 98% Ar), IrCl3·3H2O may undergo dechlorination and reoxidation at 650–750 ℃ (Fig. 3) [86]. Specifically, dechlorination would occur in the temperature range of 550–700 ℃, and reoxidation would occur in the temperature range of 700–800 ℃ [86]. Other thermal decomposition studies using IrCl3·3H2O in either an inert (Ar) or reductive (2% H2 + 98% Ar) atmosphere have been documented elsewhere but are not discussed in this

article [86].

Fig. 1. Thermal analysis of IrCl3·3H2O in an oxygen atmosphere: (a) TGA and (b) DTG; (TG: thermogravimetry; DTG: derivative thermogravimetry; heating rate: 5 ℃ min−1; flow rate: 15 dm3 h−1 (250 cm3 min−1); isothermal stages: 400 ℃, 450 ℃, and 500 ℃). Plots are obtained, rearranged, and relabeled from ref. [85].

Fig. 2. Thermal analysis of IrCl3·3H2O in an air atmosphere: (a) TGA and (b) DTA; (TGA: thermogravimetric analysis; DTA: differential thermal analysis; heating rate: 10 ℃ min−1; flow rate: 100 cm3 min−1). Plots are obtained, rearranged, and relabeled from ref. [67].

Fig. 3. Thermal analysis of IrCl3·3H2O in an oxidative atmosphere: (a) TGA, (b) DTA, and (c) DTG; (2% O2 + 98% Ar; TG: thermogravimetric method; DTA: differential thermal analysis; DTG: derivative of TG; heating rate: 10 ℃ min−1). Plots are obtained, rearranged, and relabeled from ref. [86]. As IrCl3·3H2O can work as a precursor, other similar complexes are popularly selected as precursors (or starting materials); for example, iridium chlorides (e.g., IrCl3,

IrCl3·xH2O,

IrCl4,

IrClx)

or

chloridoiridates

(e.g.,

(NH4)2[IrCl6],

H2[IrCl6]·xH2O). In addition, other iridium complexes may be used; for example, Ir4(CO)12,

Ir(acac)3,

Ir(acac)x,

(Ir(C5H7O2)n)m,

Na6[Ir(SO3)4],

Ir

acetate,

[(ppy)2Ir(bpy)](CH3CH2OCH2CO2), Ir(OC2H5)3. Iridium complexes can be dispersed or dissolved in solution prior to the thermal decomposition. Although aqueous solutions may be considered first, the use of organic solvents is also quite common; for example, cyclohexane, dimethylformamide, dimethyl sulfoxide, ethanol, ethylene glycol, isopropanol, toluene, xylene, etc. Whether aqueous or non-aqueous, the solution can be treated with chemicals, such as an acid (e.g., HCl, citric acid, oxalic acid), a base (e.g., NaOH, ammonia), and/or another chemical (e.g., polyvinylpyrrolidone). As mentioned earlier, a preparative route can be adopted. The following are examples of viable preparative routes: hydrolysis, ligand substitution, photochemical deposition, self-assembly, and sol–gel. However, some applications of hydrolysis go beyond the scope of this section. For example, hydrolysis can assist the “colloidal method,” in which iridium oxide colloids are already produced and loaded onto the substrate surface prior to the thermal treatment (§2.3.2) [87,88]. A sol–gel process, followed by thermal decomposition, i.e., “thermal method,” is a quite attractive strategy for fabricating particulate iridium oxide or for placing iridium complexes on the substrate surface (including on a template [89]). In addition to the “sol–gel method,” other similar or related techniques have been reported; for example, the “polyol method” [41] and “Pechini method (polymeric precursor method)” [90]. The “sol–gel method” can be used to produce a film of iridium oxide, and the film can be dense or porous [84,89,91]. The following is an example of a sol– gel process used to produce iridium dioxide particles. At first, an anhydrous ethanolic solution of IrCl3·3H2O and sodium ethylate is converted to an ethoxide solution by

heat treatment under reflux in an inert atmosphere [92]. Subsequently, the ethoxide solution is mixed with hydrogen peroxide [92]. Thereafter, the mixed solution is hydrolyzed using aqueous ammonia solution [92]. Finally, the powdery product is thermally treated in air at elevated temperatures to obtain IrO2 particles [92]. When the thermal decomposition is used as a method of depositing iridium oxide on the substrate, the precursor should be prepared in a liquid state [93,94]. Once a precursor solution is ready, the substrate must first be wetted or loaded with the precursor solution. The following are examples of coating techniques: brushing, dip-coating, drop-coating, impregnation, painting, printing, spin-coating, spraying, etc. It is interesting to note that when a precursor is loaded using a spraying technique, the size of the IrO2 crystallites depends on the type of precursor; for example, IrCl3 is known to produce smaller crystallites than H2IrCl6 [95]. Structured iridium oxide can be produced via thermal decomposition aided by other technologies. The following technologies can be combined with thermal decomposition: electrospinning and template-assisted methods. The latter can be categorized into two methods: template-removal and template-coating. For the template-removal, the template is removed during the thermal decomposition at an elevated temperature, leaving only the iridium oxide. In general, this resulting iridium oxide exhibits a unique structure, such as a macroporous or mesoporous structure. Conversely, for the template-coating method, the support or substrate does not disappear after the treatment at an elevated temperature. Thus, the support or substrate remains coated with iridium oxide and exhibits a unique structure, such as a core– shell (e.g., support@IrO2) or supported (e.g., IrO2/substrate) structure. For instance, IrO2/C was successfully produced by the “incipient wetness method” [96]. Some studies have attempted thermal decomposition under unique conditions

and have proposed new names to represent the process. “Spray pyrolysis technique” refers to a method in which a precursor solution is sprayed onto a preheated substrate in air [67]. “Reactive spray deposition technology” refers to a method in which a precursor solution is splayed with rapid calcination at 1700–2000 ℃, followed by air-quenching [97]. “Solution combustion synthesis” refers to a method in which a precursor solution is subjected to combustion via an exothermic and auto-thermal reaction, and in some cases, nitrates can be added to the solution to act as an oxidizer [82]. “Evaporation-induced self-assembly” refers to a method by which ordered mesoporous IrO2 thin films can be formed using templates, spin-coating, and self-assembly [16].

2.3.5. Melt methods Originally, Adams’ fusion is a process invented to prepare platinum oxide from platinic precursor and sodium nitrate by fusion [98]. Interestingly, a modified Adams’ fusion method can lead to the formation of iridium oxide [99]. In one study, IrO2 was prepared from a mixture of H2[IrCl6] and NaNO3 in water by heating the mixture at 340 ℃ for 30 min [100]. In another study, IrO2 nanoparticles were prepared from a mixture of H2[IrCl6]·nH2O and NaNO3 in 2-propanol by drying the mixture and heating it at temperatures between 400 ℃ and 500 ℃ for 30 min [101]. A modified Adams’ fusion can be carried out with other precursors, such as K2IrCl6 [102]. Ir(acac)3 is used as a precursor to realize a chlorine-free process [103]. A modified Adams’ fusion can be employed to form structured iridium oxide particles. In one study, ultrathin IrO2 nanoneedles were prepared from IrCl3·xH2O together with NaNO3 and cysteamine by calcination [99]. This study reported that the iridium–cysteamine complex was first synthesized and then converted to IrO2 [99]. In

another study, a porous IrO2 catalyst was synthesized by combining “modified Adams’ fusion” and “template-removal” [104]. In this study, IrO2·La2O3 was first formed by heating a mixture of IrCl3·nH2O and La(NO3)3 at 320 ℃ for 15 min and at 450 ℃ for 30 min, after which it was converted to IrO2 by eliminating La2O3 using H2SO4 [104]. Molten salts other than nitrate melts can also be utilized. For example, a chloride melt can be employed. In this case, ultrafine iridium oxide nanorods can be prepared from a mixture of IrCl4, NaCl, and KCl by heating the mixture at 650 ℃ for 12 h [105]. In addition to the iridium complexes, metallic iridium can also be used as a precursor (cf. §2.3.3). In one study, an iridium oxide layer was grown on an Ir metal wire using a carbonate melt [106]. In this study, an Ir wire was initially placed in Li2CO3 powder and heated at 870 ℃ for 5 h in air atmosphere [106]. Thereafter, the carbonate was removed using dilute HCl to obtain a wire coated with a black iridium oxide layer [106]. In another study, an IrOx electrode was prepared from a metallic iridium bar [107]. In this case, an iridium bar was placed in a mixed powder of Li2CO3 and Na2O2 and heated at elevated temperatures to convert it to IrOx [107].

2.4. Electric field assisted process Electric energy plays diverse roles in the fabrication of iridium oxide, as demonstrated in Scheme 7. This section covers the processes through which an electric field contributes to the formation of iridium oxide. In this sense, less relevant processes such as the electrostatic deposition (§4.4) or electrochemical iridium oxide formation (§2.5) are discussed elsewhere.

Scheme 7. Schematic illustration of the fabrication of iridium oxide using electric energy.

2.4.1. Electroflocculation Electroflocculation can serve as a deposition technique through which colloidal iridium oxide particles dispersed in a suspension can be deposited as films on the electrode. In other words, an iridium oxide dispersion must be prepared in advance to carry out this process. The dispersion or colloidal suspension of iridium oxide particles is generally prepared from chloridoiridates by hydrolytic reactions. Consequently, colloidal iridium oxides (e.g., IrOx·nH2O colloidal nanoparticles [108]) are formed and collected on the surface of an electrode upon generation of an electric field (with the electric potential of ~1 V). Electric fields can be created using a variety of techniques, including the constant potential, dynamic potential (pulsed potential, sweeping, then constant potential), and constant current. In some studies, the term “electroflocculation” was used to describe a process in which iridium oxide was prepared from [Ir(OH)6]2− by applying an electric potential [109]. However, such processes have been reclassified as “electrochemical deposition (§2.5)” in this review, for clarity.

2.4.2. Electrophoresis Similar to electroflocculation, electrophoresis can serve as a deposition technique in which the iridium oxide nanoparticles present in a solution can be deposited as a film on the electrode. The iridium oxide nanoparticles must be prepared in advance to carry out this process. Iridium oxide nanoparticles are often prepared from chloridoiridates by hydrolytic reactions [110]. In electrophoresis, electric fields are used

to

attract

iridium

oxide

nanoparticles.

The

overall

processes

of

electroflocculation and electrophoresis appear similar; however, their operational details are different. That is, in electrophoresis, the electric field tends to be maintained for a relatively short time using a relatively high voltage range (~4 V) [110].

2.4.3. Formation of iridium oxide by electric pulses In fact, basically, an electroporation process is irrelevant to the formation of iridium oxides. However, it has been reported that the use of an electroporator can lead to the formation of iridium oxides. In one study, an aqueous solution of iridium(III) chloride hydrate was prepared, and the pH of the solution was adjusted to pH 7.5 using NaOH [111]. After further treatment of this solution, an electroporator was operated to apply a repetitive electric pulse of 1.05 kV for 1 min at a frequency of 0.5 Hz [111]. Thereafter, IrO2 was produced in the solution [111]. This study stated that the electric pulses oxidized Ir3+ ions, resulting in the formation of IrO2 [111]. The role and mechanism of the electric field for the formation of iridium oxide have not yet been proposed, although they have been explained in part in other literatures. In one study, TiO2 was prepared from titanium(IV) isopropoxide (TTIP) by

applying a pulsed electric field (using a square wave DC electric pulse of high voltage) [112]. This study stated that the pulsed electric field affected the nucleation of TiO2 particles [112]. It has been suggested that a pulsed electric field can locally generate high temperatures and high pressures in a solution [112]. These high temperatures and high pressures have been identified as being responsible for the tremendous nucleation in this study [112]. Although it is difficult to apply this phenomenon and mechanism directly to the iridium oxide formation process, it is believed that the hydrolysis (and/or electrolytic ligand oxidation) of an iridium complex can be accelerated due to the high temperatures and high pressures locally generated in the solution.

2.5. Electrochemical process An electrochemical iridium oxide formation can be achieved from metallic iridium or iridium complexes. Therefore, this section addresses not only the electrochemical activation process of metallic iridium but also the electrochemical deposition process from iridium complexes. In the electrochemical formation of iridium oxide, the term “anodic” should be interpreted with caution because it can describe both of the aforementioned cases.

2.5.1. Electrochemical activation of metallic iridium The surface of metallic iridium can be electrochemically converted to iridium oxide in the electrolyte by the application of electric energy. There are many denotations

of

this

process:

anodic

oxidation,

electrochemical

activation,

electrochemical oxidation, and electro-oxidation. The term “(electrochemical) activation” has been primarily used throughout this article for clarity.

Aqueous solutions are generally used as electrolytes, and water may be used as an oxygen source. H2SO4 solutions have widely been selected as electrolytes [113]. The pH of an electrolyte can be varied, and the solutions of the following chemicals can also be used as electrolytes: HClO4, phosphate buffer (pH 7), unbuffered saline (NaCl), phosphate-buffered saline, KOH, etc. Not only electrodes made of iridium but also electrodes coated with iridium can act as precursors and can be activated [114]. In the latter case, the following preparative route can be adopted to prepare the iridium coating: atomic layer deposition, arc plasma deposition, e-beam processing, electrodeposition, sol–gel, sputtering (RF sputtering, DC magnetron sputtering), the reduction of iridium oxide by heat and hydrogen, etc. Occasionally, a preparative route is adopted to prepare a particular structure made of (or coated with) iridium, iridium oxide, or the like. Electric energy can be applied using a variety of techniques, including triangular wave potential cycling (i.e., similar to cyclic voltammetry) and rectangular wave potential cycling (i.e., similar to pulse voltammetry). In general, the operating conditions of “potential cycling” are expressed as the number of cycles and the sweeping rate during the cycling process. However, “potential cycling” can also be expressed in terms of the frequency and total duration of the cycling process. The number of activation cycles can determine whether the product is Ir(OH)3 or IrO2, and can affect the electroactive surface area and charge density of the product [115]. The potential limits are generally determined and set according to the following rules: the lower potential limit is set higher than the potential value for the onset potential of the hydrogen evolution reaction (i.e., > 0.00 V vs. RHE), and the upper potential limit is set lower than the redox potential of iridium (cf. Pourbaix diagram or EH–pH diagram; Fig. 4). For example, a range of 0.05–1.20 V vs. RHE may be used

for the electrochemical activation of metallic iridium. Notably, the upper potential limit can determine the thickness and growth of the iridium oxide film [116]. Lastly, notably, if an electric potential is applied to metallic iridium at an extremely elevated temperature such as 1575 ℃, the metal undergoes electrochemical oxidation to form gaseous iridium oxide instead of solid iridium oxide [117].

Fig. 4. EH–pH diagram (Pourbaix diagram) for iridium and chlorine species. This plot is obtained, rearranged, and relabeled from ref. [118].

2.5.2. Electrochemical decomposition and substitution Iridium complexes can be electrochemically converted to iridium oxide on the electrode surface by decomposition or substitution upon application of electric energy. There are many denotations for this process: electrochemical decomposition, electrodeposition, electroplating, and electrolysis. The term “electrodeposition” has been primarily used throughout this article for clarity. The electrodeposition of iridium oxides can be divided into two types: anodic electrodeposition and cathodic electrodeposition. In the latter case, conventionally, the

formation of reduced amorphous iridium oxide (IrO2−δ) [9] or iridium oxide containing metal iridium tends to occur [119]. Nonetheless, the formation of iridium oxide is most likely to be performed by anodic electrodeposition. Therefore, in this section, the term “electrodeposition” refers to an anodic electrodeposition process unless otherwise specified. In the late 1980s, leading studies on iridium oxide electrodeposition were carried out. The following is the procedure suggested by Yamanaka: (i) dissolve 0.15 g of IrCl4·H2O in 100 mL of H2O with stirring for 30 min; (ii) add 1 mL of H2O2 (30 wt% in H2O) to the solution and stir for 10 min; (iii) add 0.5 g of C2H2O4·2H2O to the solution and stir for 10 min; (iv) adjust the pH of the solution to 10.5 using K2CO3; (v) allow the solution to stand for at least two days [9]. The role of oxalate remains a controversial topic, and it is not covered in this section. (cf. §1.2). To date, many electrodeposition operations are based on the pioneering procedure described above [9]. Generally, this procedure is modified and applied by the operator. The following modification examples have been reported: the modification carried out by changing the order of the addition of chemicals; by excluding the addition of one or more chemicals; by replacing the chemical(s) used in the above procedure with other related or similar chemicals (e.g., replacing oxalic acid with K2C2O4; K2CO3 with Li2CO3), etc. An iridium complex other than IrCl4·H2O (the above procedure) can also be used as a starting material. The following are examples: H2[IrCl6]·xH2O, IrClx, IrCl3, IrCl3·xH2O,

IrCl4,

IrCl4·xH2O,

Ir2(SO4)3,

K2[IrCl6],

K3[IrCl6],

Na2[IrCl6],

Na3[IrCl6]·xH2O, (NH4)3[IrCl6], etc. The starting material is often treated to convert it to another form of iridium complex that actually acts as a precursor. The following are actual reported precursors: [Ir(OH)6]2−, [Ir(OH)(OH2)Cl4]2−, [Ir(OH2)2Cl4]−, iridium

oxalate, etc. The above procedure employs a basic solution as an electrolyte. However, an acidic solution can also be used as the electrolyte [119]. In some studies, the addition of K2CO3 was omitted from the above procedure, suggesting that the solution was probably acidic [120,121]. The solutions of the following chemicals have been reported to be available: H2SO4; NaCl; KNO3; Li2CO3; KOH; NaOH; phosphate buffer (NaH2PO4 + Na2HPO4); a mixture of PdCl2, K2SO4, and KCl; a mixture of PdCl2 and Na2SO4; etc. Interestingly, in some studies, the electrolyte was prepared with only the starting material (iridium complex) in the solvent without adding any other chemical [119,122]. Once the electrolyte solution has been prepared, electrical energy must be applied for electrodeposition. Electric energy can be applied using a variety of techniques, including constant current, dynamic current, constant potential, dynamic potential (triangular wave potential cycling, rectangular wave potential cycling, the combination of the preceding two potential cycling techniques), the combination of some of the preceding techniques, etc. Electrodeposition on a structured substrate or under controlled conditions can lead to the formation of structured iridium oxides. Templates can also be used as substrates [123,124]. The following shapes of iridium oxide particles were obtained by electrodeposition: cauliflower-like film, colloidal film, dendrite, microneedle, nanoleaf, nanoparticles, nanotube, nanowire, three-dimensional structures, etc. Electrolytic ligand oxidation has been reported to convert an organometallic iridium complex to iridium oxide. For example, iridium oxides can be prepared from a pentamethylcyclopentadienyl iridium complex or a picolinate iridium complex by ligand oxidation [125–127]. The electric potential required in this case is known to be

generally higher than that in electrodeposition processes [125–127].

2.6. Vapor assisted process This section deals with the preparation of iridium oxides in which an iridium oxide precursor (or starting material) is present as a vapor or undergoes vaporization. This section primarily discusses deposition techniques, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), but also discusses the formation of stand-alone iridium oxides, where a vapor-phase precursor is employed.

2.6.1. Laser as an energy source Of the various PVD techniques, reactive sputtering (in the presence of O2) is most commonly used to deposit iridium oxides. However, other PVD techniques such as “pulsed laser deposition” may be used to fabricate an iridium oxide film. In pulsed laser deposition, a pure polycrystalline iridium target is ablated under ambient oxygen pressure, while the substrate is placed parallel to the target [128]. The temperature of the substrate during the deposition is the key to determining the crystallinity and structure of the resulting iridium oxide [128]. For the pulsed laser deposition, iridium vapor is used to form an iridium oxide film, as described above. Occasionally, iridium vapors generated by lasers can also be used to form IrO2 molecules [129]. In this case, the vapor of the iridium atom reacts with oxygen to produce IrO2 [129].

2.6.2. Vapor transport There are several documents on PVD-based technologies that utilize source materials (starting materials) other than metallic iridium to form iridium oxides, as

illustrated in Scheme 8. For example, a method called “direct-vapor-phase” employs IrO2 as a source material [130]. In this method, the IrO2 powder is placed in the center of a furnace and sufficiently heated in the presence of an oxygen flow [130,131]. Evidently, this process must be performed after the substrate has already been placed downstream [130,131]. The IrO2 species produced by this method has been reported to exhibit a micronanostructure [130]. In another study, a method called “vapor transport” also employed IrO2 as the source material, resulting in structured IrO2 particles [132].

Scheme 8. Schematic illustration of vapor transport as an example of physical vapor deposition. However, notably, the term “vapor transport” can also be used to denote some methods that employ gaseous IrO3 as a precursor. The following are examples: “chemical vapor transport” and “vapor phase transport” [133,134]. In these methods, IrO3 is generally generated from the reaction of a source material (e.g., metallic iridium [133] and iridium dioxide [134]) with O2 at elevated temperatures. The IrO3 species in the flow reaches the substrate lying downstream and begins nucleation to form IrO2 with the liberation of O2 gas [133,134]. The resulting IrO2 particles are known to exhibit various shapes, including pyramidal growth and nanowires [133,134]. In this method, the substrate should be positioned at a suitable distance from the source material so that the temperature would be sufficiently low to facilitate

epitaxial growth [133].

2.6.3. Reactive sputtering One of the most prevalent strategies for producing iridium oxide films is the “reactive sputtering” of metallic iridium in the presence of oxygen. In this method, O2 is added to the feed (carrier) gas [135], and the partial pressure of O2 determines whether the resulting deposit will be a metal or an oxide or a mixture of both [136,137]. The hysteresis pattern of the reactive sputtering according to the oxygen flow rate has been reported elsewhere [138]. Ar is generally used as the feed gas, [139]. Nevertheless, a plasma of pure oxygen can also be used [140]. Occasionally, the feed gas is humidified, and H2O can be present in the oxygen plasma [141,142]. The presence of H2O, albeit devoid of O2, can lead to the formation of iridium oxide [143]. The presence of H2 in the feed gas may foster competitive reduction/oxidation reactions, resulting in IrOxHy or IrO2 [144,145]. The temperature of the substrate is another key parameter. The substrate temperature varies from below 0 ℃ to elevated temperature. The changes in the substrate temperature have been reported to affect the crystallinity of the resulting iridium oxide [143,146,147]. In addition, the following factors can also affect the sputtering process: sputtering pressure and sputtering power [148]. Since metallic iridium is a conductor, both DC voltage (direct current; DC sputtering) [140] and AC voltage (radio frequency alternative current; RF sputtering) [141] can be used as an energy source. Occasionally, DC is applied in a pulsed manner [149]. Both DC and RF sputtering can be assisted using a magnetron as a strategy to improve efficiency (i.e., magnetron sputtering) [149,150]. An unbalanced magnetron can also be employed to perform unbalanced magnetron sputtering [136].

Reactive sputtering can be used to fabricate structured iridium oxides. The following shapes of iridium oxide were obtained using reactive sputtering: nanofoil, nanolayer of core–shell, nanoplatelet array, 1D nanocrystals, etc.

2.6.4. Chemical vapor transport Controlled CVD can lead to the formation of iridium oxide. For example, IrO2 nanowires were fabricated from the carbonyls of iridium (e.g., Ir(CO)2(hfdaNnPr)) by performing CVD in the presence of a sufficient oxygen partial pressure at elevated temperatures [151]. This study reported that metallic iridium was produced when the oxygen partial pressure was insufficient [151]. CVD can serve as a preparative route. For instance, Ir(CO)12 can be converted to an Ir/IrOx clusters through a two-step process [62]. First, Ir(CO)12 is deposited on a substrate (e.g., CNT) using a CVD method [62]. Subsequently, the deposit is calcined at 200 ℃ under Ar to form the Ir/IrOx clusters on the substrate [62]. As discussed above (§2.2.8), the ultrasmall iridium particles may be sufficiently active to be oxidized even at low temperatures, as low as room temperature. In this sense, the Ir/IrOx clusters on the CNT are converted to IrO2 by treatment in air at 30 ℃ for 24 h [62]. Metal–organic chemical vapor deposition (MOCVD) is another viable strategy for preparing iridium oxides. MOCVD can be carried out at elevated temperatures in the presence of oxygen and an appropriate precursor [152–154]. Metal–organic or organometallic iridium compounds can serve as an iridium oxide precursor. The following are examples of available precursors: (C6H7)(C8H12)Ir, (MeCp)Ir(COD), etc. MOCVD can be used to produce structured iridium oxides, such as nanorods and nanotubes [152–154].

2.6.5. Atomic layer deposition Atomic layer deposition (ALD) is based on the chemistry of CVD. Unlike conventional CVD, ALD requires self-limiting, stepwise, and repetitive deposition of at least two precursors (or reactants), as shown in Scheme 9. For example, when two precursors are used to deposit an iridium oxide film, one acts as an iridium source and the other serves as an oxygen source. In many cases, the former is typically called the “precursor” and the latter “reactant (or oxidant)”. The following combinations have been reported: Ir(acac)3 + O3, (MeCp)Ir(CHD) + O3, Ir(EtCP)(COD) + O2, etc. Whether the deposit will be iridium or iridium oxide depends largely on the deposition temperature and the partial pressure of the oxidizing agent (e.g., O2) [155]. In one study, the operating temperature of ALD was successfully lowered to as low as 100 ℃ [156]. This study showed that the temperature affected the crystallinity of the resulting IrO2 [156]. Plasma enhanced atomic layer deposition (PEALD) is another possible strategy for fabricating iridium oxides. PEALD can be used to prepare IrO2 nanodots from Ir(EtCP)(CHD) + O2 + H2 [157]. In this case, a mixed plasma of oxygen and hydrogen acts as the reactant [157].

Scheme 9. Schematic illustration of atomic layer deposition as an example of chemical vapor deposition: (a) functionalization of the substrate surface; (b) reaction of Precursor A with the surface; (c) purging both unreacted excess precursors and reaction by-products; (d) reaction of Precursor B with the surface; (e) purging both unreacted excess precursors and reaction by-products; and (f) repeating (b)–(e) until the desired thickness is achieved [158].

2.7. Reduction of high–valent iridium Iridium oxide, the iridium component of which exhibits an oxidation state higher than +4, can be reduced to IrO2. In addition, if the oxidation state of iridium in the iridium complex exceeds +4, the iridium compound can be converted to iridium(IV) oxide accompanied by a reduction reaction.

2.7.1. Reduction of iridium(>IV) oxide Generally, the “desublimation” of gaseous IrO3 produces solid-state IrO2 [159]. In a “vapor transport” process, the nucleation of IrO3 can lead to the formation of IrO2

(§2.6.2). Solid-state IrO4 is calculated to be unstable at room temperature with respect to IrO2(s) + O2(g); thus, the “unimolecular elimination” of O2 from IrO4 may produce IrO2 [160].

2.7.2. Reaction of iridium(>IV) compound It is worthwhile to point out a similar but different iridium oxide formation mechanism. For example, IrF5 is sensitive to moisture, and the reaction between IrF5 and H2O is known to produce IrO2·xH2O and HF [161].

2.8. Reconstruction and modification The removal of elements other than Ir and O from the iridate compound may result in the formation of iridium oxide. In addition, the controlled treatment of iridium alloys is known to yield iridium oxide.

2.8.1. Removal of elements other than Ir and O Iridate compounds in which cations other than iridium are replaced or removed will be converted to iridium oxide. The “deintercalation” of Li from LixIr2O6 can lead to the formation of Ir2O4 spinel [162]. In this case, the deintercalation can be initiated by immersing lithium iridate in an I2/acetonitrile solution at room temperature [162]. The “ion exchange” of K0.75Na0.25IrO2 in HCl can produce IrOOH [163]. In this case, both the precursor iridate and the resulting IrOOH may contain trivalent iridium on a triangular lattice [163]. The “leaching” of various iridates such as SrIrO3 and IrNiOx can be triggered during electrochemical testing, thereby resulting in the formation of iridium oxide

[164,165]. In addition, a variety of double-perovskite iridates such as Ba2PrIrO6, Ba2NdIrO6, Ba2YIrO6, and Sr2YIrO6 can be leached in HClO4 under open circuit potential conditions and can partially be converted to iridium oxide [166]. The “selective etching” of Os from IrOsOx can be electrochemically initiated at a potential of about 1.0 V vs. RHE [167]. This condition can cause the oxidation of Os to OsO42−, followed by the dissolution of OsO4, thus leaving IrOx [167]. The “transfer of alkali metal ions (M+)” from MxIrO2 can result in the formation of IrO2 with the liberation of M+ and e− [168].

2.8.2. Controlled treatment of iridium alloys As described above (§2.8.1), the “selective etching” of Os begins with an iridate compound during electrochemical reactions. However, a similar but different method can be attempted using an iridium–osmium alloy. The “fast-dealloying” of Ir25Os75 nanocrystals can be triggered by applying a constant current [169]. This process is known to lead to the formation of a core–shell structure consisting of a metallic iridium core and a nanoporous iridium oxide shell [169].

3. Structuring technique This section describes the techniques for shaping and crystallizing iridium oxides. In most cases, the iridium oxide structures in this section can be prepared as described in the sections above.

3.1. Use of templates Templates are useful for imparting special shapes or properties to iridium oxides. Templates can be used not only for shaping but also for patterning and pore-formation.

The use of a template for pore-formation can lead to the following structures [104]: mesoporous, macroporous, micrometer-sized porous, etc. When assisting the formation of structured iridium oxides, the template can be used generally in two ways. First, the template may not be removed after being coated with iridium oxide. This method can afford the following morphologies: dendrite, nanodot, nanomesh, nanopore, nanotube, nanowire, etc. Second, the template may be removed after being coated with iridium oxide, leaving only the structured iridium oxide. This method can afford the following morphologies: nanorods, nanotubes, etc.

3.2. Zero dimensional structure Zero-dimensional (0D) iridium oxide nanodots can be formed (Fig. 5). The following methods can be used to prepare 0D iridium oxides: plasma-enhanced atomic layer deposition [157], the annealing of iridium oxide nanolayers [76], etc.

Fig. 5. Example of a zero-dimensional iridium oxide structure: nanodot. (a) Cross-sectional HRTEM image, (b) plane view TEM image, and (c) image of a single core–shell structure [76].

3.3. One dimensional structure One-dimensional (1D) iridium oxide nanostructures can be divided into two

types: the iridium oxide grown on a substrate and the free-standing (stand-alone) iridium oxide structure (Fig. 6). The latter is formed in most cases by electrospinning [170–172]. However, it can also be produced by controlled the annealing process in the presence of oxygen [74]. In the case of the iridium oxide grown on a substrate, various techniques can be employed. Notably, when grown on a substrate, iridium oxide often exhibits a quasi-1D structure (i.e., as a cluster or an array). The following iridium oxide structures can be grown on the substrate (cf. method instructions in parentheses): nanocrystal (metal–organic chemical vapor deposition, reactive sputtering), nanoneedle (controlled combustion), nanorod (metal–organic chemical vapor deposition, controlled annealing, template-assisted chemical bath deposition, fusion by molten salt method), nanotube (metal–organic chemical vapor deposition, template-assisted electrodeposition, template-assisted chemical bath deposition), and nanowire (chemical vapor deposition, vapor transport. Note that the term “iridium oxide nanowire” may also be used to refer to an electron transfer matrix, as in bacterial nanowires.).

Fig. 6. Example of a one-dimensional iridium oxide structure: nanofiber. (a) SEM image of the precursor fibers; (b) SEM image of the IrOx nanofibers after annealing at 500 °C; (c) enlarged SEM image of IrOx nanofibers after annealing at 500 °C, and (d) the corresponding diameter distribution [170].

3.4. Two-dimensional structure Two-dimensional (2D) iridium oxide nanostructures can be divided into two types: independently grown iridium oxide (Fig. 7) and iridium oxide, grown on a substrate. The latter can be further divided into two types according to the growth orientation: parallel growth to the substrate surface (e.g., deposition) and non-parallel growth (e.g., vertical growth). The following iridium oxide structures include only 2D or quasi-2D morphologies grown by methods other than coating techniques (cf. method instructions in parentheses): nanofoil (reactive sputtering), nanoplatelet (controlled annealing, reconstruction by ion exchange, reactive sputtering), and nanosheet (exfoliation).

Fig. 7. Example of a 2D iridium oxide structure: nanosheet. (a, b) TEM images of IrO2 nanosheet and (c) selected area electron diffraction pattern of (b) [173].

3.5. Three-dimensional structure Three-dimensional (3D) iridium oxide structures vary (Fig. 8). The following 3D iridium oxide structures have been given specific names to describe their morphologies (cf. method instructions in parentheses): cauliflower-like film (electrodeposition), dendrite (template-assisted electrodeposition), flower-like rod (controlled annealing), micropillar (electrodeposition), nanogranules (modified Adams’ fusion), nanosphere as a shell (calcination, coating of iridium oxide colloids on the support, leaching), nanostructured arrays that are aggregated or not grown

individually (sol–gel followed by calcination, template-assisted atomic layer deposition, template removal, etc.), porous hierarchical structure (template-assisted calcination), and 3D ordered macroporous structures (template infiltration by capillary force, followed by annealing in air; immersing the template in the precursor solution, followed by heat treatment).

Fig. 8. Example of a 3D iridium oxide structure: porous hierarchical structure. (a) General view of the SEM images of the hierarchical catalysts calcined at 450 °C; (b) SEM image of the hollow broken spheres; (c) SEM image of the donut-like particles; (d) SEM image of the spheroidal particles, and (e) SEM image of the buckled particles. Images are obtained, rearranged, and relabeled from ref. [174].

3.6. Crystalline iridium oxide Iridium dioxide single crystals can be grown using molecular–beam epitaxy. For example, epitaxially grown IrO2 on single crystal rutile TiO2(110) shows a rutile IrO2(110) film [175,176]. An IrO2(110) film can also be generated by the reaction of Ir with O2. The film can be formed by exposing an Ir(100) sample to O2 at a surface temperature of 765 K [177]. This method is reported to afford a high-quality IrO2(110) surface with a stoichiometric surface termination [177]. Iridium oxide can be crystallized by applying sufficient heat during synthesis.

For example, if the substrate is heated, the iridium oxide film grown by reactive sputtering may exhibit crystallinity [147,178]. However, it is a more general crystallization strategy to apply sufficient heat after the synthesis of the amorphous iridium oxide. In other words, the post-treatment can convert amorphous iridium oxide to a crystalline phase (e.g., rutile) [17,28,179]. Iridium oxide can be hydrothermally crystallized using a hydrothermal reactor [36]. In addition, a hydrothermally prepared iridium oxide, albeit amorphous in bulk, could partially contain a crystalline structure, such as a hollandite motif [180]. It is interesting that the high-pressure phase transition of rutile-type IrO2 can lead to the formation of a pyrite-type structure of IrO2 [181,182]. The methods for the preparation of a crystalline iridium oxide are tabulated and summarized for clarity (Table 2). Table 2. Summary of the methods for preparing crystalline iridium oxides Target structure

Method

Details

Ref.

Crystalline IrO2

Reactive sputtering

Performing sputtering when the substrate is sufficiently heated

[147,178]

Crystalline IrO2 (rutile)

Heat-treatment

Exposing amorphous IrOx to sufficient heat

[17,28,179]

Crystalline IrO2 (pyrite)

High-pressure treatment

Exposing rutile IrO2 to a sufficiently high pressure for the transformation

[181,182]

Monocrystalline IrO2(110)

Heterogeneous oxidation of Ir

Exposing Ir(110) to O2 at a surface temperature of 765 K

[177]

Growing IrO2 epitaxially on a single crystal rutile TiO2(110)

[175,176]

Preparing a precursor solution of iridium complex with or without additional chemicals and applying sufficient heat to the solution

[36,180]

Monocrystalline Molecular-beam IrO2(110) epitaxy Partially crystalline IrOx

Hydrothermal synthesis

4. Reforming iridium oxide Iridium oxide can be further processed and converted into another form of iridium oxide, as shown in Scheme 1(c). The purpose of the further process can be divided into the following two categories: (i) to impart a specific quality on the iridium oxide, and (ii) to coat or deposit the substrate with iridium oxide. The latter can be a process in which iridium oxide formation and iridium oxide deposition occur in different batches or in the same batch. Furthermore, iridium oxide deposition in the same batch can be realized in the presence or absence of an external energy source.

4.1. Quality modification This category includes various technologies, such as crystallization and activation. Most of the cases have already been discussed above (cf. §2.3.2, §2.5.1, §2.6.2, §2.7.1, etc.). The following case has not been discussed. An IrO2 powder can be converted to a sinter of IrO2 by post-treatment [183]. Note that the vapor transport of iridium oxide (§2.6.2) shows both the features of this section and the following section (§4.2).

4.2. Deposition in a different batch This category includes common coating techniques. Particulate iridium oxide (powder, nanoparticle, nanotube, etc.) can be prepared in the form of ink. The ink is generally a dispersion [184] (e.g., suspension, colloid, gel, paste). The ink can be mounted onto the substrate by various loading techniques, including drop-casting, ink-jet printing, pipetting, screen-printing, and spraying. Subsequently, the solvent of the ink has to be evaporated or dried to leave only iridium oxide on the substrate.

Occasionally, iridium oxide can be placed on the substrate surface by a decal method [185]. Note that the vapor transport of iridium oxide (§2.6.2) shows the features of both this section and the preceding section (§4.1).

4.3. Deposition in the same batch by applying an external energy source The processes of this category can be performed using an electric field to collect the iridium oxide particles on the electrode surface. Electroflocculation (§2.4.1) and electrophoresis (§2.4.2) are good examples of this category.

4.4. Deposition in the same bath without an external energy source Self–assembly is an excellent example of this category. Self–assembly can be accomplished through the following two-step process. First, iridium oxide nanoparticles are prepared (most likely as a colloidal dispersion). Second, the substrate or support material is immersed in the dispersion. For example, a simple soaking process, in which the substrate material is placed within the iridium oxide colloidal dispersion, can cause the iridium oxide to be loaded on the substrate surface [186]. “Chemical bath deposition” (§2.1.4) is a good example of a self–assembly process. “Adsorption” is another good example of a self–assembly process. In this case, the iridium oxide nanoparticles are adsorbed onto the substrate surface [187]. Both physisorption [188] and chemisorption [189] can occur. Occasionally, iridium oxide nanoparticles are decorated with stabilizers, such as citrate [190]. The decoration can modify the interaction between the nanoparticles and the substrate surface, and can thus induce self–assembly [189]. For example, a monolayer of carboxylated alkanethiolate can be formed on the substrate surface by

treatment with a thiol solution, and this monolayer can tether iridium oxide nanoparticles [191]. This is known as an “attaching” process. Conversely, the adhesion of the iridium oxide on the bare substrate surface is an “adsorbing” process (or, as described earlier, “adsorption”) [191]. “Electrostatic assembly” can also lead to the immobilization of the iridium oxides on the substrate surface. The key to this case is the interaction of positively charged substrate surfaces with negatively charged iridium oxide particles [13]. In this case, the iridium oxide nanoparticles may be decorated with stabilizers, such as citrate [14].

5. Applicability and concluding remarks Thus far, several dozen methods of preparing iridium oxide have been discussed. Among them, the following four are the most adopted deposition techniques: thermal decomposition (as in §2.3.4), reactive sputtering (§2.6.3), electrodeposition (§2.5.2), and electrochemical activation (§2.5.1). These methods result in TIROF (specifically TDIROF), SIROF, EIROF (specifically AEIROF), and AIROF (denoting “Activated” IRidium Oxide Film; cf. §1.1), respectively. Hydrolysis is also of great importance in iridium oxide synthesis; however, it is not discussed in this section because it is commonly employed to produce powdery iridium oxide or to be followed by a subsequent deposition process, i.e., it is used as the preparative route. These films are subtly different from each other, having their own unique characteristics. However, there are common characteristics that make the use of iridium oxide attractive: activity, biocompatibility, conductivity, durability, etc. In general, hydrous iridium oxides exhibit enhanced electrochemical and electrochromic activity but are less stable and durable [192]. Conversely, anhydrous iridium oxides

are fit for biomedical and long-term applications, although they exhibit insufficient activity [73,193,194]. Thus, it is important to obtain iridium oxides that exhibit the intended properties, depending on the application. The general properties of iridium oxides imparted by a preparation method are summarized in Scheme 10 [73,195–199]. The hydrous iridium oxide is generally amorphous, and the crystalline form is anhydrous; however, this tendency is not constant. For instance, basically, SIROF is anhydrous and amorphous unless prepared at elevated temperatures. However, if that is the case, SIROF may be anhydrous and crystalline [137]. Another counterexample is the hydrothermally prepared hydrolytic iridium oxide. This iridium oxide can exhibit crystallinity, albeit it is a hydrolytic product [36].

Scheme 10. General characteristics of iridium oxides imparted by the respective deposition method. Note: OER stands for oxygen evolution reaction, and EC stands for electrochromism. The electric conductivity of iridium oxide may vary depending on the chemical states. In general, hydrous iridium oxide is known to be semiconducting when reduced; however, it is known to be metallic when oxidized [199]. Amorphous hydrous iridium oxide can convert into a crystalline anhydride upon heat treatment,

such as annealing [77]. The resulting anhydride may exhibit good conductivity. If the heat-treatment process leads to uniform crystalline IrO2 particles, the crystallization process can induce high conductivity [73]. However, it has also been reported that, in some cases, heat-treatment of hydrous or amorphous IrOx may result in the formation of crystalline IrO2 particles with increased charge transfer resistance [198]. This peculiarity could occur if the heat-treatment causes the growth of IrOx particles with grain boundaries [198]. If a change in the volume of one particle induced by the heat-treatment is dramatic enough to lose the contact points to the adjacent particles, the interfacial resistance between this particle and the adjacent particles increases, which decreases the electrical conductivity [200,201]. Therefore, the electron transport could become poor due to the heat-treatment, if the heat-treatment of IrOx causes its overgrowth together with grain boundaries and the separation between the particles [198]. However, it must be noted that in this case, a film of compact coating undergoes a heat-treatment process, causing the separation between the particles [198]; in other words, the separation between the particles is responsible for the decreased electron transport, while the particles themselves may possibly exhibit relatively enhanced electrical conductivity [73]. Iridium oxides are often applied to the following four technologies: biomedical electrode, electrochromism (EC), oxygen evolution reaction (OER), and pH sensor. Each technology requires certain characteristics of iridium oxides. For example, when iridium oxide is used in biomedical or clinical applications, stability must be seriously taken into consideration. Meanwhile, if iridium oxide is used in EC, OER, or pH sensors, the activity (or sensitivity) and stability must be well balanced. Based on such considerations, Table 3 recommends iridium oxide deposition methods for possible and suitable applications.

Table 3. Recommended applications of iridium oxide films deposited by different techniques.

Biomedical electrode

TIROF

SIROF

+

+++

Electrochromism Oxygen evolutiona

+

pH sensor

+b

a

+

EIROF

AIROF

+++

+++

++

+++

+++

in terms of activity for long-term use Please note that the charts of the expected characteristics and recommendation

b

are of a general view (Scheme 10 & Table 3). In other words, the same deposition technique could result in a different property for iridium oxide depending on the deposition condition. As discussed earlier, for instance, the properties of TIROF and SIROF may vary depending on the operational temperature, gas composition, gas partial pressure, etc., while the properties of EIROF and AIROF may change depending on the starting materials, electrolyte composition, potential application conditions, etc. Among various potential applications, the use of IrOx in the OER is of great interest; examples of such uses of IrOx in the OER as the anode catalyst can be found elsewhere [202–205]. The electrocatalytic activity of IrOx toward the OER requires active sites, such as coordinatively unsaturated sites and/or defects sites [176,200]. The formation of defects can be facilitated through the dissolution occurring at the surface [200], which may contribute to the electrocatalytic activity of IrOx toward the OER using the adsorbate evolution mechanism, in which O2 is produced from acid– base and/or direct-coupling reactions [166]. Interestingly, the lattice oxygen within non-stoichiometric hydrous IrOx is known to be involved in the OER, accompanying

the enhanced dissolution of Ir [166]. Conversely, the occurrence of the lattice participation mechanism is negligible, when stoichiometric thermally-treated IrO2 species are deployed as the OER catalysts [166]. Overall, the thermally prepared IrO2 particles exhibit better stability than activated IrOx, while the activity is better in the case of activated IrOx [206]. Similar to other platinum group metal oxide catalysts, IrO2 displays a trade-off relationship between the activity and stability toward the OER (Scheme 10) [206]. Noteworthily, the IrOx structures prepared from the leaching of SrIrO3 exhibit excellent activity together with appreciable stability, although such a particle is still fabricated on a lab-scale [164]. Nevertheless, the development and enhancement of this methodology is believed to be the future direction for the OER, since it addresses both the activity and stability, simultaneously. The preparation method highly affects the stability and the activity of IrOx toward the OER. Due to the cost and scarcity of iridium, the sparing use of it and the prevention of its loss are highly recommended. From this viewpoint, thermally prepared IrO2 appears to be beneficial as an anode catalyst for industrial water electrolyzers [207–211]. The operation parameters affect the physicochemical property of IrOx, thereby affecting its electrocatalytic activity toward the OER. The quality of the IrOx catalyst produced using a thermal decomposition technique, for instance, may vary according to the following operation conditions: type of the precursor, addition of chemicals or templates, heat-treatment temperature (HTT), hold time at HTT, and the partial pressure of O2 (cf. §2.3.4). In particular, the last three parameters must be taken into substantial consideration. Elevated HTT, or extended hold time at sufficient HTTs, with the adequate supply of O2 (cf. §2.3.4) generally results in the formation of stoichiometric crystalline IrO2 structures, which have high stability and are less active than amorphous IrOx toward the OER. However, IrO2

undergoes decomposition at the HTT of 1100 °C or higher [212]; therefore, the HTT for the IrO2 fabrication must be selected with special care. Considering that each fabrication method leads to different physicochemical properties of the fabricated IrOx, the fabrication method must be carefully selected according to the potential application of the iridium oxide (Scheme 10 & Table 3). For the selection, the following statistical information (percentage calculation based on Scopus refine results as of August 2019) on the popularity of a method chosen by the researchers may be also helpful: thermal treatment (27%) > electrochemical activation (24%) > electrodeposition (19%) > sputtering (15%). Please note that the denominator is the number of publications that contain either “iridium oxide” or “iridium dioxide” in the publication title, abstract, or keyword list, while the numerator is the number of publications that contain relevant keywords for each fabrication method together with any “iridium oxide” or “iridium dioxide” mentions in the publication title, abstract, or keyword list.

Acknowledgments This research was supported by the Technology Development Program to Solve Climate Change through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (2018M1A2A2063861). This work was also supported by the GIST Research Institute (GRI) grant funded by the GIST in 2019.

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Graphical Abstract Description Numerous and diverse methods exist for the fabrication of iridium oxides. This review categorizes iridium-oxide fabrication methods and recommends suitable methods according to the intended application of the iridium oxide.

Declaration of Interest Statement Iridium oxide still underlies much of energy-related chemistry, albeit scarce and expensive, due to its distinctively attractive features. For water splitting as a means of facilitating hydrogen production, operators may place it on the first priority as an anode electrocatalyst because it shows an optimal trade-off between activity and stability. Not only this but also other fields such as capacitor can consider it to be the material of choice. Since it has received much attention in a variety of fields for a long period of time, the fabrication method of it is accordingly suggested in a great amount. In this work, we thoroughly collected the reported and suggested methodologies for iridium oxide fabrication. Each method imparts unique property to the formed iridium oxide and thus can be selectively used for application according to desired property. The fabrication methodologies reported thus far are numerous and diverse and are therefore required to be grouped by specific criteria. This review categorizes the methodologies by characteristics of each method and summarizes expected characteristics of iridium oxide formed by a certain method. In addition, it is suggested where to use iridium oxide formed by a certain method because the property of the formed iridium oxide varies according to a method used for its fabrication.