Structural Diversity of Anodic Zinc Oxide Controlled by the Type Of

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Reviews ChemElectroChem doi.org/10.1002/celc.202100216

Zinc Anodizing: Structural Diversity of Anodic Zinc Oxide Controlled by the Type of Electrolyte Katja Engelkemeier,*[a, c] Aijia Sun,[a, c] Dietrich Voswinkel,[b, c] Olexandr Grydin,[b, c] Mirko Schaper,[b, c] and Wolfgang Bremser[a, b]

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Anodic zinc oxide (AZO) layers are attracting interdisciplinary The article gives an overview of the different possibilities of research interest. Chemists, physicists and materials scientists anodic treatment, whereby the voltage and the current type are are increasingly devoting attention to fundamental and the main distinguishing criteria. Presented is the electrolytic application-related research on these layers. Research work oxidation (anodizing) and the electrolytic plasma oxidation focuses on the application as semiconductor, corrosion protec- (EPO). The electrolytic etching is also a process of anodic tor, adhesion promoter, abrasion protector, or antibacterial treatment. However, it does not produce AZO layers, but rather surfaces. The structure and crystallinity essentially determine a degradation of the zinc layer. The review article shows the the properties of the AZO coatings. The type and concentration parameters used so far (electrolyte, current type, current of the electrolyte, the applied current density or voltage as well density, voltage) and points out the influence on the formation as the duration time enable layer structures of structural variety. of AZO structures in dependency to the used electrolyte.

The European parliament voted in favor of a general ban on 1. Importance of Zinc Anodization for the use of the carcinogenic chrome (Cr) VI compounds.[12,13] Corrosion Protection and Further Applications Alternative surface treatment methods are required, thus anodizing is becoming more attractive as a method of surface Zinc is essential as an protector layer primarily for steel treatment.[14] materials and is used for car bodies,[1] fence elements, or joining In the 1960s, the international lead zinc research organ- elements[2] like screws, nuts, and nails. However, zinc coatings ization inc. (ILZRO, USA) was engaged in zinc anodizing and tend to stain, tarnishes quickly and is susceptible to atmos- galvanized coatings. The treatment process was commercially pheric corrosion.[3] Under atmospheric conditions, zinc materials introduced by the Allied Kelite Product Division (AKPD) under exhibit a certain corrosion resistance in comparison to steel and the trade name “Iridizing”. Granted patents are those of A. G. form a smooth, compact, and weather-resistant protective layer. White[15] (1961) and M. M. Wright[16] (1967). They describe their The layer consists of zinc oxide (ZnO), zinc hydroxide, and process as “anodic treatment under alternating current (AC)”

carbonate (Zn(CO3)·Zn (OH)2), known as white rust. However, instead of direct current. The specified parameters for the the native protective layer, less than 20 nm thickness, is not anodization treatment: Voltages between 50 V and 250 V, sufficient for long-term corrosion protection. Furthermore, the current densities between 37.6 mAcmÀ 2 (35 AftÀ 2) and natural oxide layer is not scratch-resistant.[4] 48.4 mAcmÀ 2 and temperatures between 65.6 °C and 82.2 °C. The international lead and zinc study group (ILZSG) has The process of anodic treatment takes place at high voltages statistically captured, the demand for zinc increased up to 4 and is a process of plasma electrolytic oxidation (PEO).[17] percent between 2013 and 2018.[5] Even the increase in global The process generates corrosion protective coatings with a zinc prices did not affect the demand. The worldwide export of thickness of up to 300 μm. However, commercial acceptance is galvanized steel doubled between 2001 and 2017.[6–8] marginal. Reasons are high costs associated with the process in Produced were up to 247,000 tons of zinc, primarily in terms of equipment, the energy-inefficient process design, and demand by automotive and construction companies.[9] Galvan- the high-risk potential due to excessive clamping voltages of ized steels are furthermore attractive components in hybrid the AC power supply. Furthermore, the application of the materials. However, the smooth zinc surface does not provide process recommends sodium hydroxide and potassium any possibilities for mechanical interlocking. Surface treatments hydroxide based electrolytes (pH 5 to pH 11.4), phosphate ions must be applied to adapt the zinc surface to the requirements and chromate ions (Cr(VI), Cr(III).[18] of the hybrid material.[10] It is known that specific structures of The zinc anodizing process MIL-A-81801[19] (1971), devel- AZO increase the adhesive strength between the steel and the oped by the ILZRO and financed by the US military, also works fiber composite component.[11] with voltages of up to 200 V. Layers of up to 80 μm in thickness can be realized through the addition of chromate and phosphate compounds. The AZO layers have a salt water [a] K. Engelkemeier, A. Sun, Prof. W. Bremser resistance of above thousand hours and have an increased Paderborn University [20–22] Department of Chemistry, ‘Coatings, Materials & Polymers’ abrasion resistance compared to the base metal. 33098 Paderborn, Germany Anodizing is an alternative option to increase the corrosion E-mail: [email protected] resistance and wear properties of zinc and galvanized layers, as it [b] D. Voswinkel, Dr. O. Grydin, Prof. M. Schaper, Prof. W. Bremser [18] Paderborn University can generate protective oxide layers. Researchers are increasingly Department of Chemistry, ‘Chair of Material Science’ turning to basic research on zinc anodizing, with a focus on the 33098 Paderborn, Germany semiconductor functionalities. Zinc oxide is a wide bandgap II–IV [c] K. Engelkemeier, A. Sun, D. Voswinkel, Dr. O. Grydin, Prof. M. Schaper Institute for Lightweight Design with Hybrid Systems (ILH) semiconductor material (3.37 eV at 300 K). It is used in application [23–25] [26,27] Paderborn University areas such as microelectronics, optoelectronics, sensor 33098 Paderborn, Germany technology[28–30] and photocatalysis.[31,32] Zinc oxide surfaces also © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This have antibacterial properties that make them interesting for is an open access article under the terms of the Creative Commons [33,34] Attribution License, which permits use, distribution and reproduction in any biotechnical and biomedical applications. medium, provided the original work is properly cited.

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In an earlier review article, Zaraska et al. discussed the oxidation (PEO),[15,45–48] and electrolytic etching.[49] The Electro- anodic formation of zinc oxide nanostructures.[35] They dis- lytic oxidation is known as anodizing and PEO is commonly cussed the formation of different structures and showed the referred to as micro-arc oxidation (DC-current) or iridizing (AC- impact of specific applied parameters and electrolytes. The current). In all cases, the workpiece forms the anode. Figure 1 amount of literature on anodizing has increased in recent years shows a short overview of the three methods. Influenced is the and allows correlations between the structure formation and anodic formation of zinc oxide by the type of method, type of the anodizing parameters depending on the electrolyte. This electrolyte, concentration and pH value of the electrolyte, the article focuses on the anodizing process in different electrolytes applied current density or potential, the type of current (direct and shows the relationship between the AZO formation in current or alternating current), the current mode (pulsed or various electrolytes depending on other anodizing parameters. constant), the duration time, additives and its topography. Anodizing of zinc products leads to further possibilities for application such as wear protection for tools,[36] sensor technology,[37] automotive and aircraft construction,[38–40] bio- 2.1. Electrolytic Oxidation (Anodizing) medical and medical technology.[41–43] The anodic treatment is typically applied below the dielectric breakdown. Figure 2 shows the oxidation process at the 2. Types of Zinc Anodizing interphase between the electrolyte and the zinc anode. The affinity of zinc to oxygen is exploited to increase the thickness Tree types of anodic treatment. The methods of anodic treat- of the oxide layer. The anodic formation of the oxide layer is ment include electrolytic oxidation,[44] plasma electrolytic less established for zinc than for aluminum. Zinc tends to have

Katja Engelkemeier studied chemistry at the Dr. Olexandr Grydin gained a Ph.D. degree at Paderborn University from 2009 to 2014 and the National Metallurgical Academy of Ukraine is completing her Ph.D. this year at the Chair (Ukraine) in 2004. He habilitated to a Dr. tech. of Material Science (LWK) in the Faculty of sc. at the same university in 2014. Since 2014, Mechanical Engineering. Since 2019, she has he is working as a senior engineer at the been employed as a junior research group Department of Materials Science of the Pader- leader in the Faculty of Natural Sciences in the born University (Germany). He has expertise in working group Coatings, Materials and Poly- twin-roll casting of light metals, in particular mers at the Paderborn University and is high-strength aluminum alloys, and metallic actively involved as a specialist group repre- clads such as aluminum and steel, thermome- sentative at the Institute for Lightweight chanical treatment of metallic materials, sur- Construction with Hybrid Systems (ILH). She is face treatment of metals for advanced bond- currently focusing on the development of ing properties. corrosion-resistant coatings and developing Prof. Mirko Schaper obtained a Dr.-Ing. degree thermo-reversible crosslinkers for hybrid light- in 2005 from the Leibniz Universität Hannover weight components. (Germany). In 2010 he was awarded a Dr.-Ing. Aijia Sun studied chemistry at Qingdao Uni- habil. degree from the same university. Since versity of Science and Technology (QUST) in 2013, he is a professor at the Paderborn Qingdao (China) from 2016 to 2019 and has University (Germany), where he heads the been studying chemistry at the Paderborn Department of Materials Science. His research University since 2019 as part of the Chinese- interests include additive manufacturing proc- German Campus (CDC). She is currently a esses, in particular selective laser melting, student assistant in the working group Coat- development of high-strength metallic and ings, Materials and Polymers at the University composite materials, thermomechanical treat- of Paderborn, where she is writing her bach- ment of metals. elor‘s thesis. Prof. Wolfgang Bremser received his Ph.D. in Dietrich Voswinkel studied mechanical engi- 1991 from the University of Mainz (Germany) neering from 2010–2014 at the South West- and then worked at BASF Coatings in Münster phalia University of Applied Sciences and from until 2003. Since 2003, he has been the head of 2015–2018 at the Paderborn University. He the Coatings, Materials and Polymers working has been working on his Ph.D. at the Chair of group as a professor in the Technical Chemistry Material Science (LWK) of the Faculty of Department of Paderborn University. W. Brems- Mechanical Engineering at the Paderborn er is Vice-Rector of the Chinese-German Cam- University since 2019. He is currently working pus (CDC) and is Board Member of the on the optimization of surface properties for Organization Committee Coatings & Science the bonding of different materials in the Conference (OCCS) and of the Indonesian-Ger- hybrid material. man-Polymer-Research Center (IGPR) of the Industrie Transportgesellschaft Brandenburg (ITP).

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Figure 1. Overview of the three methods of zinc anodizing and zinc layers.

Figure 2. a) Schematic representation of the anodic oxidation process. b,c) SEM images of resulting anodic oxide layers observed by Dong et al. in a 0.1 M NaOH electrolyte. After 1 h nanoporous layers between 5 V–7 V can be observed (b) and at 12 V cellular column structures of nanorods (c). Adapted from Ref. [52] with permission. Copyright (2016) Royal Society of Chemistry.

a higher degree of structural variety of anodic oxidation layers centimeters away from the anode. They also attributed the than observed for aluminum, which typically shows an ordered formation of the anodic oxide layer to a dynamic competitive open-pore column structure. Dong et al. investigated the layer reaction between zinc oxidation, chemical etching, and field- formation mechanism at the zinc anode in a 0.1 M sodium induced dissolution.[50–52] hydroxide (NaOH). They applied a voltage between 5 V and The process of anodization is controlled by the balance 12 V (1 h) and have observed two structures, a porous structure between oxidation and dissolution of the zinc surface. When at about 9 V, as well as a nanocolumnar structure at about 12 V the dynamics of oxidation and dissolution rate reach equili- (Figure 2a, b). A titanium mesh was as a cathode, placed three brium (steady state), a critical oxide film thickness is formed. It

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is assumed that the difference in structure is affected by In potassium hydroxide (KOH), four regions of the current- redundant hydroxide ions at the interface of electrolyte and voltage curve could be recognized during zinc anodizing: initial anode. The electrolyte etches the nanocrystalline zinc oxide dissolution (I), active dissolution (II) of the zinc layer, the pre- into a globular structure. An electric field occurs upon the passive region in which the first oxide nuclei are formed (III), surface of the zinc anode after switching on the power source. and the passive region in which the oxide layer is formed (IV). That leads to a higher current density on the surface of the zinc Additives like metal silicates are useful to reduce the applied anode. The structure formation is supposed to be affected by current, and shift the region of zinc dissolution and passivation the migration of oxygen-containing anions such as O2À and towards more negative potentials.[54] OHÀ to the zinc surface [Eqs. (1), (2)]. An excess of hydroxide ions is reached by adding metal hydroxides like sodium or potassium hydroxide to the electrolyte. Zinc oxide is formed 2.2. Electrolytic Plasma Oxidation (EPO) due to the positive polarization of the zinc surface and its affinity to oxygen.[52,53] EPO works above the dielectric breakdown. The commonly high voltage of about 100 V leads to plasma that results from spark Zn þ O2À ! ZnO þ 2eÀ (1) discharges at the interface between electrolyte and anode surface. Locally high temperatures lead to the transformation of À þ À Zn þ OH ! ZnO þ H þ 2e (1) the metallic surface region into a ceramic zinc oxide layer.[15,45,57] The structure formation takes place in three steps. In the The final layer structure is due to the chemical etching that first step, a thin oxidic barrier layer occurs. Due to the increasing starts due to H+ ions [Eq. (3)] and the electrochemical etching current resistance the voltage increases rapidly in this step. In [(Eq. (4)] that takes place at the oxide-electrolyte interface at the second step, the oxide layer partially is degraded, which the bottom of the oxide layer. The relevant chemical reactions leads to a lower continuous increase in voltage. When the are as follows:[52] dielectric breakdown is reached, small white transition points appear. The rate of layer formation decreases as the rate of þ 2þ ZnOðsÞ # þ2 H ! Zn þ H2O (3) dissolution increases. In the third step, the voltage is approx- imately constant. This area is characterized by yellow to orange À ZnOðsÞ # þOH $ ZnOOH (4) colored sparks and is known as the “micro-arc step” due to spark discharges.[46] Nanoparticles result due to anisotropic chemical etching of The formation of the oxide layer shown in Figure 4, depends zinc oxide crystals on the surface of the anodic layer.[52,53] on the applied voltage range, the composition of the metal/ Other electrolytic systems can influence the layer structure. oxide/electrolyte-interface as well as on the reaction rate of However, the fundamental mechanisms are the same. The AZO forming [Eq. (5)] and dissolution processes [Eqs. (6), (7)].[48] kinetics of dissolution, oxidation, and the forming of by- À À products can be affected by adding additives to the electrolytic Zn þ 2 OH ! ZnO þ H2O þ 2e (5) bath.[52] AZO layers preferably were synthesized in alkaline À 2À À electrolytes. Zinc anodization in alkaline electrolyte has exten- Zn þ 4 OH ! ZnðOHÞ4 þ 2e (6) sively studied.[48,54–56] The growth kinetic in alkaline media can þ À þ ! ð Þ 2À be explained particularly well with the voltage-current diagram ZnO 2 OH H2O Zn OH 4 (7)

in Figure 3. À 1 À 2 OH ! =2 O2 þ H2O þ 2e (8)

Figure 3. Schematic representation of the current potential curve for zinc in a 1 M KOH without and with an additive like observed for a 0.1 M sodium [54] metasilicate (Na2SiO3).

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À 2 À 2 Figure 4. a,b) SEM-imageÀ ofÀ Zn microtipsÀ fabricatedÀ on a polycrystalline Zn foil in a NH4Cl/H2O2. Parameters are 55 mAcm , 30 s (a), 28 mAcm , 60 s (b). c) The side faces 01À 1 0, 11 2À 0, 01 1 1 and 11 2 1 of the pyramidal structure are parallel to the (0001) plane. The highest resolution rate is therefore for the crystal planes 01 1 1 and 11 2 1. Adapted from Ref. [49] with permission. Copyright Elsevier (2007).

2.3. Electrolytic Etching

The electrolytic etching is a dissolution process of the metallic surface in an etching solution under applying an electric current. The dissolution can lead to various structures. The crystallographic surfaces of crystals have different surface energies and therefore dissolve with different kinetics.[58–60] Kuan et al. investigated the behavior of electrolytic zinc etching in a peroxide-containing ammonium chloride solution and found highly ordered arrangements of single-crystalline pyramidal zinc microtips due to anisotropic etching. To under- stand the formation of the pyramidal structure, a look at the crystal orientation is necessary (Figure 5).[49] The electrolytic etching starts with the formation of an oxide layer[61,62] and its dissolution by forming metallic com- 2+[63] 2À plexes like Zn(NH3)4 or tetrahydroxozincate (Zn(OH)4 ) [Eqs. (9)–(12)]. Anodic reaction:

Zn ! Zn2þ þ 2eÀ (9) Figure 5. a) SEM images of the electrolytically galvanized surface before 2þ À anodization. b) Shear strength between a steel and CFRP laminate. c) SEM Zn þ 2 OH ! ZnO þ H2O (10) image of the highly porous AZO layer on the galvanized steel surface. Digital þ 2þ þ microscopic image in the right corner show the inherent coloring of the AZO ZnO þ 4 NH4 ! ZnðNH3Þ4 þ H2O þ 2 H (11) layer after 600 s anodizing time in a 0.2 M KOH electrolyte at 50 mAcmÀ 2.d) Shear strength between an anodized galvanized steel and CFRP laminate. Adapted from Ref. [11] with permission. Copyright Springer Nature Switzer- Cathodic reaction: land AG (2018).

À À H2O2 þ 2e ! 2 OH (12)

3. Influence of Electrolyte anodization in different electrolytes. An overview of the most important literatures is summarized in Table 1. Structural diversity of AZO layers. AZO layers show a remarkable Distilled water as electrolyte. Distilled water has a low structural diversity from porous layers to structures with electrical conductivity of less than 10 μWÀ 1 mÀ 1 and therefore is complex architecture. The structure often determines the unsuitable as an electrolyte for anodizing, as it is inefficient in properties and the associated application possibilities. Achiev- terms of time. However, Shetty et al.[70] and Voon et al.[64,71] able are various structures such as porous layers,[52,64,65] carried out anodizing in deionized water, applying constant platelets,[66,67] cellular structures,[52] needle arrays,[67] spheres,[68] voltages till 9 V of up to 12 h. Both observed a nanoporous AZO nanorod arrays,[68] sponge structures,[69] and geometric layer. While Shetty et al. anodized their zinc sheets between 6 h structures.[69] The structure formation depends primarily on the and 12 h at 1 V to 9 V, sheets were anodized by Voon et al. type and concentration of the electrolyte, the voltage, and the between 10 V and 30 V for 120 h. Shetty et al. investigated the current. Following is the comparison of literatures based on the behavior of the current density in the aqueous electrolyte

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Table 1. Overview and summary of the current literature and the applied anodizing parameters.

Year Electrolytic conditions tA J U Structure Ref. Type Amount [mAcmÀ 2] [V]

Water (DI-H2O)

2012 – 6 h–12 h – 1–9 Fine-walled fractal to porous [70] 2014 120 h [64], 2017 [71] Potassium hydroxide (KOH)

1959 KOH 0,356 M – 150–300 – Crystalline structure or porous [72] structure 1969 KOH 0,356 M 7 min, 150–300 – Porous films [73] 8 min 1983 KOH 0.1 M to 2.0 M 60 min 150 1–60 V [74] 2006 KOH 1 M, 900 s–3600 s 0.3 – Nanoneedles [68]

Zn(NO3)2 10 ml 4 M, 10 ml 2014 KOH, 0.025 M 900 s to – 5–10 Hierarchical nanorods [76]

NH4Cl 0.05 M 5400 s KOH 0.1 M – 3–15 Nanoporous coating [65]

2015 KOH, NaAlO2, Na2SiO3 0,005 M–0,5 M 1800 s to 300 Porous monolayers and multi- [48] layers 2019 KOH 0.2 M 30 s–600 s 10–50 – Homogeneous nanoporous thin [11] films 2020 KOH 1 gLÀ 1 180 s, 360 s, 75, 100, 125, 450 Dense, porous structure [57] À 1 Na3PO4 NaAlO2 8 gL 720 s 150, *pulse ratio (Inhomogeneous in thickness) À 1 12 gL ton : toff 1 ms :9 ms Sodium hydroxide (NaOH)

1999 NaOH 30 gLÀ 1 1 s–160 s - 1 V to 160 V – [20] À 1 Na2B4O7 90 gL Na-silicate 180 gLÀ 1 2007 NaOH 0,1 M 3600 s 20–40 Nanoparticle [80] 2008 NaOH 0.2 M 90 min 20 V Columnar porous ZnO [77] 2009 NaOH 1 M–6 M 900 s–3600 s 0,83–8,33 – Nanowires, nanoflakes, hierarch- [67] ical floral needle structures (nanoflowers) 2011 NaOH, 0.2 M n.b. – 25, 30 Porous layers [14]

Additive: NaNO2 Borax NiSO4 ·9 H2O 2013 NaOH 0.1 M, 60 s, 3600 s 1, 40 Nanoneedle array [69] 1 M NaOH 0,025 M 900–5400 s 5–10 Groove and subsequent growth [75] NH4Cl 0,05 M of ZnO 2016 NaOH 0.1 M, 0.3 M, 0.5 M 3600 s 10 Pitting of ZnO [91] NaOH 1 M 3600 s À 0.7 to 1.0 Nanorods, nanoplates [80]

Zn(NO3)2 0,25 M NaOH 0,05 M–0,2 M 1800 s–5400 s 5–40 Column structure, nanoporous [52] layers NaOH, 0,04 M–0,3 M 1800 s 4–50 – [92] KOH 0,3 M 2018 NaOH 0,025 M 5400 s 10 Nanorods, nanosheets, [93]

NH4Cl 0,05 M hierarchical structures NaOH 0,075 M 5400 s Pulsed current Nanoparticulate layers, 1 D [81] structures made of nanowires 2019 NaOH 1 M 1800 s 2, 4 Nanoporous [82]

Sodium sulfate (Na2SO4)

2020 Na2SO4 1.0 M 120 s 0,03 Nanorod array [37]

À Carbonate based (CO3 )

2010 KHCO3 0,005 M–0,05 M 60 s–7200 s – 5 – 50 Array of floral structures [83] made of nanorods

2012 KHCO3 0,005 M 600 s–3600 s – 10 – [84] 2013 KHCO3 0,1 M, 60 s, 3600 s – 1, 40 Array of mircroflowers consisting [69] 1 M of nanowires, nanoplate array, linked or interpenetrated structures

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Potassium hydroxide (KOH)

2016 NaHCO3 Ethanol 1–100 mM 1 s–1800 s – 10 Star like structures exist and [94] 10:1 formed in holes on the surface À 2017 NaHCO3 60 s–1200 s – 1, 40 Array and hierarchical flower [35] structures of nanowires

+ Ammonium based (NH4 )

2011 (NH4)2SO4 0,1 M–0,2 M 300 s–2400 s – Microplatelets (NH4)2SO4 [87] NH4Cl 2013 NH4F 0,0125 M–0,025 M *0,3 mV/s Hierarchical nanotubular struc- [88] Na2S 0,1 M–0,2 M tures 2014 NH4F 0,5 g Dauer bis 0 V 2 Nanorods and nanoporous [89] Glycerin 90 ml (20%) layers DDW 10 ml

2015 NH4HCO3, 0,05 M–0,2 M 60 s À 7200 s 1–10 Nanorods [90] 2018 NaOH 0,025 M 5400 s 10 Nanoplates [95]

Oxalic acid based (C2H2O4) 2008 0.3 M, 0.5 M, 1 M – – 10 Nanoporous thin film [96] 2013 0.1 M, 1.0 M 60 s, 3600 s 1, 40 Pitting of the ZnO layer [69] 2014 0.3 mol dmÀ 3 2 min 100 Cellular structure [77] 2016 0.1 M, 0.3 M, 0.5 M 3600 s 10 nanoporous structures [91]

Sulfuric acid (H2SO4) 2010 0,2 M 3600 s 3, 10 Nanostripes [79] Hydrochloric acid (HCl) 2013 HCl 0,1 M, 1 M 60 s, 3600 s 1, 40 Sheet like structures, microplates [69] Hydrogen fluoride (HF) 2010 HF 50 vol.% / 30% 30 s–1800 s 9.7, 16.7 Sheets, walled structures, [78] methanol, spheres, 1 mass% HF porous structures

Phosphoric acid (H3PO4) 2013 H3PO4 0,1 M, 1 M 60 s, 3600 s 1, 40 Microflakes [69] Nitric acid (HNO3)

(pH 5.8) with increasing anodic treatment time of up to 12 h components. The anodizing layer absorbed the polymer matrix using a constant voltage of 3 V. of the CFRP, thus providing adhesion.[11] A decrease of 80% of current density in the first 600 s was Voltages between 50 V and 300 V in 0.005 M to 0.5 M KOH observed (from 7.25 μAcmÀ 2 to 5.80 μAcmÀ 2). Afterward, the solutions lead to the formation of AZO layers characterized by current density increases again linearly during the anodic cracks running under the surface parallel to it. The thicknesses treatment time (from 15 μAcmÀ 2 after 2.5 h) and then with a of the achieved layers are between 5 μm and 50 μm. Rocca smaller slope linearly up to a value of about 20 μAcm-2 (after et al. showed the evolution of current density with increasing 12 h). The decrease in current density in the first 600 s is due to anodizing time. At 50 V, the current density decreases rapidly the formation of AZO. The subsequent increase in current and reaches a stable value below ten minutes anodizing time. density is indicated to zinc oxide dissolution[64,70,71] (Figure 6a). Oxygen releases at the anode surface and decreases Potassium hydroxide (KOH) as electrolyte. Often anodizing with time. The anodic oxide layer (greyish color) is partially or plasma electrolytic oxidation of zinc in potassium hydroxide destroyed again by the strong oxygen generation. A duration of electrolytes leads to columnar or granular porous structure. 15 minutes will lead to up to 6 μm thick layers. (Figure 6c). At Porous structure is obtained in pure KOH solutions of the 200 V, anodizing starts with a strong oxygen generation on the concentration range between 0.05 M and 2 M.[11,48,57,65,72–74] anode surface. Therefore, the current density is very high (> In 2018, hot-dip galvanized steel was anodized in a 0.2 M 400 mAcmÀ 2). After 6 minutes of duration, the current density KOH electrolyte to get nanoporous zinc oxide layers. Layer decreases below 100 mAcmÀ 2. Thus, homogeneously dispersed thicknesses of up to 500 nm were achieved using current micro arcs appeared on the anode surface. The high voltages densities between 10 mAcmÀ 2 and 50 mAcmÀ 2 and an anodiz- also lead to a higher conversion rate of zinc into zinc oxide, ing duration of up to 600 s. During anodizing, the voltage allowing higher layer thicknesses of up to 50 μm (Figure 6b). remained below 26 V. The AZO layers which show a change in Rocca et al. showed that the oxide layer consists of zinc oxide coloration from beige to brown, gain free surface energy of up and zincate.[48] to 65 mNmÀ 1. This is 71% higher than observed for pure zinc Similar layers to the previously mentioned anodic oxide surface. Such anodizing layers joined with CFRP increase the layers were also shown by the studies of Blawert et al.; they interlaminar shear strength up to 65% (from 11 MPa to 17 MPa) generated AZO layers at current densities between 75 mAcmÀ 2 (Figure 5). Adhesives were not used to join these both and 150 mAcmÀ 2 (up to 450 V).[57]

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Figure 6. a,b) Cross-sectional images of AZO layers (top) and SEM images of the surface structure (bottom), produced in 0.05 M KOH. AZO layer produced in a 15 minute process at 50 V (a) and (200 V) (b). c,d) Schematic representation of the potentiodynamic scan of zinc in KOH. Current density with increasing anodizing time for the anodic current at 50 V and 200 V (c). Behavior of the current density with increasing applied voltages with different KOH concentrations (d). Adapted from Ref. [48] with permission. Copyright Elsevier (2015).

Further, structures found like nanoneedles[68] and other Nanoporous structures, were prepared by Dong et al. in a hierarchical structures[75] gained with adding additives such as 0.1 molar NaOH electrolyte that were anodized for one hour at ammonium chloride or zinc nitrate. voltages between 5 V and 9 V. They attribute the formation of Wu et al. performed the anodization in a 4 M KOH solution the porous nanostructure to the insufficient voltage supply, complementing 10 ml of a 1 M zinc nitrate solution as an according to which the field-controlled transport of the oxygen- additive. Adding additives like zinc nitrate results in the containing ions to the substrate surface is inhibited. Hydroxide excessive presence of zinc ions (Zn2+) in the electrolyte. That ions attack the formed zinc oxide, resulting in the porous leads to promote crystal growth. In this way, an array of nanostructure (Figure 2).[52] needles is formed.[68] Mika et al. produced such a nanoporous (dark) layer in a [82] An array of hierarchical nanorods is obtained by Ravan- 1 M NaOH electrolyte between 2 V and 4 V (tA = 30 minutes). À bakhsh et al. in a 25 mM KOH solution containing ammonium Bicarbonate (HCO3 ) as electrolyte. In bicarbonate-based chloride (0.05 M) as additive.[76] electrolytes, primary hierarchical structures of ZnO nanoneedles Sodium hydroxide (NaOH) as electrolyte. Anodic treatment are observed in a floral structure (Figure 7). The needles can in sodium hydroxide also leads to porous AZO layers.[14,52,77–79] reach a length of up to 100 μm when zinc is anodized at a However, Sreekantan et al. observed other structures like nano- voltage of 10 V for 2 hours.[69,83–85] wires, nanoflakes, hierarchical floral structures consisting of Ramirez et al. observed the formation of hierarchical nano- nanorods in a concentration range of 1 M to 6 M below needle floral-kind arrays in 0.1 M to 1 M bicarbonate electrons. 10 mAcmÀ 2.[67] After anodization time of 1 min at 10 °C, hierarchical structures NaOH concentrations of 0.1 M and 1 M, respectively (1 V– are already visible. The floral structures that form in a 0.1 M 40 V, 60 s–3600 s) lead to an array of nanoneedles.[69] solution have a length of 1.5 μm with needles a few nano- Sreekantan et al., fabricated complex zinc oxide structures meters in thickness (Figure 7 a). In a one molar electrolyte, the in 1 M to 6 M NaOH electrolytes. Crystalline zinc oxide in rod flower-like structures are much larger (up to 5 μm) and the and plate like structure was otherwise observed when zinc thickness of their needles increases up to 30 nm (Figure 7 b). nitrate was added to the electrolyte as zinc species[80] or Anodization time of 1 hour leads to the formation of long zinc ammonium chloride.[75,81] In the case of potentiostatic anodiza- oxide needles forming a dense array (Figure 7c).[69] tion, the growth mechanism is controlled by the formation of a Hu et al. observed the same formation of homogeneous nanoscale passivation layer (as is also known for aluminum needle arrays with a length of up to 30 μm long. However, they

anodization) on which the zinc oxide nanorods are subse- produced such needle-arrays in 5 mM to 50 mM KHCO3 using a quently grown by field diffusion in a “bottom-up” process.[80] voltage of 10 V for up to 20 minutes (Figure 7d).[83]

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Zaraska et al. also received such structures formed in a tubular structures in ammonium fluoride and sodium sulfate.[88] bicarbonate electrolyte. They investigated the correlation Beedri et al.,[89] Miles et al.[90] and Ravanbakhsh et al.[75,76] between layer thickness and stress as well as the growth rate observed the formation of nanorods in electrolytes such as which increases exponentially (Figure 7e–g).[35] sodium fluoride, hydrogen carbonate and ammonium chloride + Ammonium based (NH4 ). In ammonium-based electrolytes, (Figure 8f–g). Miles et al. observed the formation of a densely various layer structures can be observed after anodizing. Zhao packed nanorod structure after 30 minutes of anodization[90] et al. observed nanoblades and sunflower-like structures on (Figure 8h). However, the surface properties of the formed zinc foils anodized in electrolytes of ammonium sulfate structures were not further investigated. [87] (NH4)2SO4) and ammonium chloride (Figure 8a–e). Shrestha Acidic based electrolytes. The studies on AZO layers, made et al. observed the formation of compact passivation layers of in the alkaline pH range are more extensive than those in the

Figure 7. SEM images of AZO layers produced in bicarbonate electrolyte. a,b) Hierarchical structures and arrays consisting of nanowire produced in KHCO3 electrolytes: 0.1 M, 1 V, 60 s, 10 °C (a), 1 M, 1 V, 60 s, 10 °C (b). Adapted from Ref. [69] with permission. Copyright Royal Society of Chemistry (2013).

c,d) Nanowire array produced in a 5–50 mM KHCO3 electrolyte at 40 V for 3600 s. Reprinted from Ref. [83] with permission. Copyright (2010) American Chemical Society. e,f) Diagram of the anodic layer thickness versus anodizing potential with anodizing time (e), Cross sectional SEM view of a hierarchical nanoneedle flower array (f), average growth rate versus anodizing potential (g). Reproduced from Ref. [86] with permission. Copyright Royal Society of Chemistry (2017).

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Figure 8. Production of AZO nanosheets and sunflower structures in ammonium-based electrolytes. a–c) SEM image of nanosheets produced in 0.1 M

(NH4)2SO4,+0.1 M NH4Cl at 1 V for 600 s (a), SEM images of the sunflower structure produced in 0,2 M (NH4)2SO4,+0,2 M NH4Cl at 1 V for 40 min (b) and 0.2 [87] MNa2S+0.025 M NH4F+10 vol.% ethanol at 5 °C (c). Adapted from Ref. [87] with permission. Copyright Elsevier (2011). c–e) SEM-images of porous ZnO (0.025 M NH4F) (e), SEM images of density packed nanorods (0.2 M Na2S+0.025 M NH4F) (f) and 0.2 MNa2S+0.025 M NH4F+10 vol.% ethanol at 5 °C (g). Adapted from Ref. [88] with permission. Copyright Elsevier (2013).

acidic. That is due to the high stability of zinc oxide in an (HCl), nitric acid (HNO3) and orthophosphoric acid (H3PO4).

alkaline environment. Research results show that zinc can be Flake-like structures preferably were produced in a 0.1 M HNO3

anodized in acidic based electrolytes. Ramirez-Canon et al. and a 0.1 M HCl and nanoporous in a 0.1 M H2C2O4 and in a 1 M [79] investigated anodizing in oxalic acid (H2C2O4), hydrochloric acid H3PO4. (Figure 9a–c)

Figure 9. AZO layers produced in acidic electrolytes like a) HNO3 (1 M, 1 V, 1 h, 10 °C), b) H3PO4 (1 M, 1 V, 60 s), c) HCl (0.1 M, 40 V, 1 h, 10 °C). Adapted from Ref. [69] with permission. Copyright Royal Society of Chemistry (2013). d) Representative AZO layers prepared in ethanolic HF (0 vol.% methanol, 1 mass% HF, 9.7 V, 900 s), e) HF (0 vol.% methanol, 1 mass% HF, 9.7 V, 30 s). Adapted from Ref. [78]. Copyright Elsevier (2010).

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AZO layers can also be produced in ethanolic hydrofluoric Table 2. EPO parameters for the production of densely packed anodic acid, as He et al. showed in their study. The anodizing time has layers from zinc oxide in different colors.[18] a decisive influence on the structure formation. In addition to Parameter Green Grey Anthracite Brown porous structures (Figure 9d) and sidewall structures (Figure 9e), Power supply AC 1–3 Phase flake-like sponge structures and particulate clusters can be Current density [mA cmÀ 2] 43 formed.[78] Starting voltage [V] 70 Terminal voltage [V] 200 105 100 Temperature [°C] 150–180 pH 6.8–7.2 12.6–13.0 12.4–12.8 4. Colors of AZO Layers Anodizing time [min] 9 10 8

If zinc is anodized, this may lead to a change in color. Chen et al. investigated the electronic structure of dark oxide layers produced on zinc in a ten-minute electrochemical reaction in

a 0.1 M Na2CO3 at 1.2 V by electrochemical treatment. They observed the formation of a compact grayish AZO layer on the zinc surface. If the AZO layer is reduced, the coloring takes on a darker brownish color. Crystalline hexagonal zinc oxide flakes are formed on the surface because of the electrochemical reduction. Both layers were then examined concern- ing the structural properties using Raman spectroscopy and X- ray diffraction (XRD). Raman spectra show the typical bands À 1 high À 1 for both layers: A1(TO)�380 cm , E2 �440 cm . A1(LO) À 1 high �555 cm ) for wurtzite type zinc oxide. However, the E2 mode is less in intensity for the reduced sample. AZO layers

show a higher intensity of the A1(LO) mode than the reduced layer.[97–105] By photoluminescence spectroscopy, the presence of Figure 10. a) Inherent coloration of AZO layers fabricated in 0,1 M to 2 M INGAP states were observed. This leads to an increased KOH medium in lower voltage region below 10 V and higher voltage region absorption of light in the spectral range. At room temperature, between 10 V and 60 V at 26 °C according to Ref. [74]. b) Coloration observed À 3 zinc oxide naturally has a bandgap of 3.37 eV.[106,107] Observed by in a 0,1 moldm KOH medium in lower voltage region between 4 V and 10 V. Adapted from Ref. [65] with permission. Copyright The Authors (2020). was the presence of a donor level below the bottom of the c) AZO layers from a 0.2 M KOH electrolyte prepared at constant current conduction band. The donor level reduces the bandgap to a density of 50 mAcmÀ 2 after 30 s (11 V), 120 s (14 V), 300 s (19 V), and 600 s width of 1.8 eV. Valence electrons could be stimulated into the (26 V) anodization times. Adapted from Ref. [11]. Copyright Springer Nature Switzerland AG (2018). conduction band, which also results in an increase in electrical conductivity. The visible light with energy between 1.59 eV and 4 eV is sufficient for this. If all wavelengths of visible light are absorbed, the AZO layer appears black. If red light with a Imam et al. observed two colors in various sodium silicate- wavelength between 640 nm and 780 nm (1.59 eV to 1.93 eV based mixed electrolytes called “good” and “bad” coatings. It photon energy) is absorbed, the AZO layer color appears was indicated that “bright silver” AZO layers exhibit a strong green Assumed is in general that the coloring is a synergistic adhesion while the “uniform dark” color leads to less effect of the structure and electron structure of the AZO layer, adhesion.[109] its defect density, and excess oxygen atoms (e.g. interstitial From a 0.2 M electrolyte at a voltage of <30 V (at constant oxygen).[105,108] current density of 50 mAcm2), beige to brownish colorations of Coloration at higher voltage (>100 V; EPO/MAO). Inherently the AZO layers was observed. Above all, the time factor plays a colored AZO layers are observable at higher voltages (> 100 V; significant role. It also seems to make a difference whether the EPO-Process) in KOH electrolytes. Besides black layers, brown, anodization is carried out at constant voltages or constant anthracite, and grey layers are producible using alternating currents. Contrary to the expectation of producing white layers current. Table 2 summarizes the parameters that must be in a 0.2 M KOH electrolyte according to,[74] up to dark brown applied to achieve specific coloration via the EPO process.[18] layers are obtained at the finish voltage of 26 V. In contrast, at Coloration at low voltage (<100 V, DC). In 1983 Nanto et al. the finish voltage of 11 V, beige-colored AZO films are observed the coloration of zinc in 0.1 M to 2 M potassium produced.[11] hydroxide in a voltage range from 1 V to 60 V. Figure 10 shows the black coloration produced below 7 V. Voltage between 9 V to 60 V led to white AZO. A peeling off was observed in the 5. Conclusions non-hatched area.[74] Mika et al. produced porous AZO in sodium hydroxide. Numerous studies report the zinc anodizing and zinc coatings. These AZO layers also show a nanoporous structure.[82] In summary this article gave a brief overview of the historical

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development and commercialization of anodic treatment since [16] M. M. Wright, G. Guttman, Zinc anodizing and zinc-bade alloys and the 1960s and has shown the types of anodic treatment. products thereof, 1967. [17] H. Hofmann, J. Spindler, Verfahren in der Beschichtungs-, Oberflächen- Depending on the type of electrolyte the resulting AZO technik, 3. Aufl., Hanser, München, 2015. structures are shown. The extensive studies previously pub- [18] S. Jakobson, D. Crotty, R. Griffin, D. Phipps, E. Rubin, Met. Finish. 1998, lished literatures were summarized to give a quick overview. 96, 114–118. [19] Organization, International Lead Zinc Research, Military Specification: There is a need to investigate the correlation between the Anodic Coatings for zinc and zinc alloys. structural formation, electrolyte, and other anodization parame- [20] H. C. Shih, F. Y. Shyr, J. W. Hsu, S. C. Chung, S. L. Sung, Plat. Surf. Finish. ters. Many small studies can lead to an understanding of the 1999, 104–107. [21] W. Wilkowski, Galvanotechnik 1988, 381–384. overall understanding of the zinc anodizing, its kinetics, layer [22] T. R. Tan, J. R. Cheng, J. H. Wang, J. G. Duh, H. C. Shih, Surf. Coat. structures, and related properties. Nanoporous layers produced Technol. 1998, 194–199. in KOH and NaOH seem to be particularly promising for use as [23] B. Kim, S.-B. Cho, International Symposium on Microelectronics 2018, 2018, 447–451. corrosion protectors or adhesive promoter. They are homoge- [24] A. F. Lotus, Y. C. Kang, R. D. Ramsier, G. G. Chase, J. Vac. Sci. Technol. B neous, compact, and firmly adhering. AZO layers synthesized in 2009, 27, 2331. bicarbonate are attractive as semiconductor layers due to the [25] B. Fang, J. C. She, J. Liu, N. S. Xu, in: 2009 22nd International Vacuum Nanoelectronics Conference, IEEE, 2009, 299–300. type of crystalline structure. [26] M. Godlewski, E. Guziewicz, K. Kopalko, G. Łuka, M. I. Łukasiewicz, T. Krajewski, B. S. Witkowski, S. Gierałtowska, Low Temp. Phys. 2011, 37, 235–240. [27] T. Pauporté, D. Lincot, Electrochim. Acta 2000, 45, 3345–3353. Acknowledgements [28] R. Martins, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, M. Bender, N. Katsarakis, V. Cimalla, G. Kiriakidis, J. Appl. Phys. 2004, 96, 1398–1408. The authors would like to thank to the German Federal Ministry [29] Y. Zhang, K. Yu, D. Jiang, Z. Zhu, H. Geng, L. Luo, Appl. Surf. Sci. 2005, for Economic Affairs and Energy (BMWi) for funding the project 242, 212–217. [30] H.-M. Lin, S.-J. Tzeng, P.-J. Hsiau, W.-L. Tsai, Nanostruct. Mater. 1998, 10, “AnodZnSteel” and to the European Regional Development Fund 465–477, DOI: 10.1016/S0965-9773(98)00087-7. (EFRE) and the State of North Rhine-Westphalia for funding the [31] Y. Li, G. W. Meng, L. D. Zhang, F. Phillipp, Appl. Phys. Lett. 2000, 76, research project HyOpt (http://hyopt.de/) and the lead partner 2011–2013. [32] C.-Y. Tsay, K.-S. Fan, S.-H. Chen, C.-H. Tsai, J. Alloys Compd. 2010, 495, Jülich (PTJ) which made the development of the review article 126–130. possible. Furthermore, the authors are thankful to publishers for [33] H. Mirzaei, M. Darroudi, Ceram. Int. 2017, 43, 907–914. the permission to adapt the Figures in this Review. Open access [34] J. Jiang, J. Pi, J. Cai, Bioinorg. Chem. Appl. 2018, 2018, 1062562. [35] L. Zaraska, K. Mika, M. Zych, G. D. Sulka, in: Nanostructured Anodic funding enabled and organized by Projekt DEAL. Metal Oxides, Elsevier, 2020, 385–414. [36] M. Bestetti, A. Da Forno, P. L. Cavallotti, P. Gronchi, F. Barlassina, Transactions of the IMF 2010, 88, 57–62. [37] A. Mateen Tantray, M. A. Shah, Chem. Phys. Lett. 2020, 747, 137346. Conflict of Interest [38] Y. Wu, W. Zhao, W. Wang, L. Wang, Q. Xue, Sci. Rep. 2017, 7, 1344. [39] M. Paz Martínez-Viademonte, S. T. Abrahami, T. Hack, M. Burchardt, H. The authors declare no conflict of interest. Terryn, Coating 2020, 10, 1106. [40] A. K. Sharma, Thin Solid Films 1992, 208, 48–54. [41] S. Chen, Y. Li, Y. F. Cheng, Sci. Rep. 2017, 7, 5326. Keywords: anodic zinc oxide (AZO) · anodizing, electrolytic [42] M. V. Diamanti, B. Del Curto, M. Pedeferri, J. Appl. Biomater. Biomech. 2011, 9, 55–69. oxidation · micro-arc-oxidation · structural diversity · [43] H. Dong, J. Zhou, S. Virtanen, Appl. Surf. Sci. 2019, 494, 259–265. nanoporous layers · nanocrystalline layers [44] T. P. Dirkse, J. Electrochem. Soc. 1955, 102, 497. [45] M. M. Wright, G. Guttman, Patent 1967, United States. [46] A. Fattah-alhosseini, K. Babaei, M. Molaei, Surf. and Interfaces 2020, 18, 100441. [47] S. Stojadinović, N. Tadić, R. Vasilić, Surf. Coat. Technol. 2016, 307, 650– [1] D. Santos, H. Raminhos, M. R. Costa, T. Diamantino, F. Goodwin, Prog. 657. Org. Coat. 2008, 62, 265–273. [48] E. Rocca, D. Veys-Renaux, K. Guessoum, J. Electroanal. Chem. 2015, 754, [2] Alistair Torrance, dengarden, 2019. 125–132. [3] M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, [49] C. Y. Kuan, J. M. Chou, I. C. Leu, M. H. Hon, Electrochem. Commun. 2007, NACE International, Houston, 1974. 9, 2093–2097. [4] X. G. Zhang, Corrosion and Electrochemistry of Zinc, Springer US, [50] J. P. O’Sullivan, G. C. Wood, Proc. R. Soc. London Ser. A 1970, 317, 511– Boston, MA, 1996. 543. [5] T. Reuters, Average price for zinc * worldwide from 1960 to 2018 (in [51] V. P. Parkhutik, V. I. Shershulsky, J. Phys. D 1992, 25, 1258–1263. US dollars per ton), 2019. [52] J. Dong, Z. Liu, J. Dong, D. Ariyanti, Z. Niu, S. Huang, W. Zhang, W. Gao, [6] World Steel Association, World Steel in Figures 2007, 2007. RSC Adv. 2016, 6, 72968–72974. [7] World Steel Association, Word Steel in Figures 2014, 2014. [53] G. S. Huang, X. L. Wu, Y. C. Cheng, J. C. Shen, A. P. Huang, P. K. Chu, [8] World Steel Association, World Steel in Figures 2019, 2019. Appl. Phys. A 2007, 86, 463–467. [9] Statista Dossier, Dossier Zink – Statistiken zu Zink, 2019. [54] Y.-C. Chang, Electrochim. Acta 1996, 41, 2425–2432. [10] A. A. Camberg, K. Engelkemeier, J. Dietrich, T. Heggemann, Lightweight [55] S. Stankovic, B. Grgur, N. Krstajic, M. Vojnovic, J. Serb. Chem. Soc. 2003, des worldw 2018, 11, 24–29. 68, 207–218. [11] K. Engelkemeier, C. Mücke, K. P. Hoyer, M. Schaper, Adv. Compos. [56] E. Lilov, V. Lilova, C. Girginov, S. Kozhukharov, S. Nedev, A. Tsanev, D. Hybrid Mater. 2019, 2, 189–199. Yancheva, V. Velinova, Mater. Chem. Phys. 2020, 239, 122081. [12] H.-J. Alfort, J. Oberfl. Techn. 2016, 56, 68–71. [57] C. Blawert, S. A. Karpushenkov, M. Serdechnova, L. S. Karpushenkava, [13] REACH, Entry No. 47. M. L. Zheludkevich, Appl. Surf. Sci. 2020, 505, 144552. [14] M. A. Imam, A. M. M. Moniruzzaman, in: Proceeding 18 International [58] B. W. Batterman, J. Appl. Phys. 1957, 28, 1236–1241. Corrosion Congress, Perth, WA. [59] R. Heimann, W. Franke, R. Lacmann, J. Cryst. Growth 1972, 13–14, 202– [15] White, AG, Patent 1961, United States. 206.

Wiley VCH Dienstag, 18.05.2021 2199 / 204431 [S. 13/15] 1 Reviews ChemElectroChem doi.org/10.1002/celc.202100216

[60] R. Lacmann, Berichte der Bunsengesellschaft für physikalische Chemie [86] L. Zaraska, K. Mika, K. Syrek, G. D. Sulka, J. Electroanal. Chem. 2017, 801, 1987, 91, 1313. 511–520. [61] A. J. Bard, L. R. Faulkner, Electrochemical methods, 2. Aufl., Wiley, [87] J. Zhao, X. Wang, J. Liu, Y. Meng, X. Xu, C. Tang, Mater. Chem. Phys. Hoboken, NJ, 2001. 2011, 126, 555–559. [62] M. Lai, D. J. Riley, Chem. Mater. 2006, 18, 2233–2237. [88] N. K. Shrestha, K. Lee, R. Hahn, P. Schmuki, Electrochem. Commun. [63] R. E. Lobnig, D. J. Siconolfi, L. Psota-Kelty, G. Grundmeier, R. P. 2013, 34, 9–13. Frankenthal, M. Stratmann, J. D. Sinclair, J. Electrochem. Soc. 1996, 143, [89] N. Beedri, Y. Inamdar, S. A. Sayyed, A. Shaikh, S. Jadkar, H. Pathan, 1539–1546. ChChT 2014, 8, 283–286. [64] C. H. Voon, N. Tukemon Tukiman, B. Y. Lim, U. Hashim, S. T. Ten, M. N. [90] D. O. Miles, P. J. Cameron, D. Mattia, J. Mater. Chem. A 2015, 3, 17569– Derman, K. L. Foo, M. K. M. Arshad, in: 2014 IEEE International Confer- 17577. ence on Semiconductor Electronics (ICSE2014), IEEE, 2014, 420–423. [91] S. Gilani, M. Ghorbanpour, A. Parchehbaf Jadid, J. Nanostruct. Chem. [65] R. Masuda, D. Kowalski, S. Kitano, Y. Aoki, T. Nozawa, H. Habazaki, 2016, 6, 183–189. Coating 2020, 10, 1014. [92] M. Remešová, L. Klakurková, L. Čelko, L. Sládková, D. Jech, J. Kaiser, SSP [66] S. W. Ng, F. K. Yam, L. L. Low, K. P. Beh, M. F. Mustapha, E. N. Sota, S. S. 2016, 258, 399–402. Tneh, Z. Hassan, Optoelectron. Adv. Mat. 2011, 89–91. [93] A. Ravanbakhsha, F. Rashchib, M. Heydarzadeh Sohic, M. Dezfoolian, in: [67] S. Sreekantan, L. R. Gee, Z. Lockman, J. Alloys Compd. 2009, 476, 513– Proceedings of the 5th International Conference on Nanostructures 518. (ICNS5). [68] X. Wu, G. Lu, C. Li, G. Shi, Nanotechnology 2006, 17, 4936–4940. [94] C. F. Mah, K. P. Beh, F. K. Yam, Z. Hassan, ECS J. Solid State Sci. Technol. [69] A. Ramirez-Canon, D. O. Miles, P. J. Cameron, D. Mattia, RSC Adv. 2013, 2016, 5, M105–M112. 3, 25323. [95] A. Ravanbakhsh, F. Rashchi, M. H. Sohi, Rasoul Khayyam Nekouei, [70] A. Shetty, K. K. Nanda, Appl. Phys. A 2012, 109, 151–157. Mohammadreza Mortazavi Samarin, in: Materials Science. [71] C. H. Voon, B. Y. Lim, K. L. Foo, U. Hashim, L. N. Ho, S. A. Ong, [96] P. K. Basu, N. Saha, S. Maji, H. Saha, S. Basu, J. Mater. Sci. Mater. Electron. NANOASIA 2017, 7. 2008, 19, 493–499. [72] N. P. Fedot’ev, S. J. Griliches, Electropolishing, anodizing and electrolytic [97] W.-B. Cai, D. A. Scherson, J. Electrochem. Soc. 2003, 150, B217. pickling of metals, Draper, Teddington, 1959. [98] Y. Y. Tay, T. T. Tan, M. H. Liang, F. Boey, S. Li, Appl. Phys. Lett. 2008, 93, [73] H. W. Dettner, Handbuch der Galvanotechnik 1969. 111903. [74] H. Nanto, T. Minami, S. Takata, J. Mater. Sci. 1983, 18, 2721–2726. [99] S. Permogorov, A. Reznitsky, Solid State Commun. 1976, 18, 781–784. [75] A. Ravanbakhsh, F. Rashchi, M. H. Sohi, R. K. Nekouei, AM+R 2013, 829, [100] P. P. Lottici, C. Razzetti, Solid State Commun. 1978, 25, 427–430. 347–351. [101] G. J. Exarhos, S. K. Sharma, Thin Solid Films 1995, 270, 27–32. [76] A. Ravanbakhsh, F. Rashchi, M. Heydarzadeh Sohic, M. Dezfoolian, in: [102] K. G. Saw, K. Ibrahim, Y. T. Lim, M. K. Chai, Thin Solid Films 2007, 515, Proceedings of the 5th International Conference on Nanostructures 2879–2884. (ICNS5) in Kish Island, Iran. [103] M. Guo, P. Diao, S. Cai, Appl. Surf. Sci. 2005, 249, 71–75. [77] S. Ono, Y. Kobayashi, H. Asoh, ECS Trans. 2008, 13, 183–189. [104] H. Liu, G. Piret, B. Sieber, J. Laureyns, P. Roussel, W. Xu, R. Boukherroub, [78] S. He, M. Zheng, L. Yao, X. Yuan, M. Li, L. Ma, W. Shen, Appl. Surf. Sci. S. Szunerits, Electrochem. Commun. 2009, 11, 945–949. 2010, 256, 2557–2562. [105] Y. Chen, P. Schneider, B.-J. Liu, S. Borodin, B. Ren, A. Erbe, Phys. Chem. [79] S. J. Kim, Y.-T. Kim, J. Choi, J. Cryst. Growth 2010, 312, 2946–2951. Chem. Phys. 2013, 15, 9812–9822. [80] M.-C. Huang, T. Wang, B.-J. Wu, J.-C. Lin, C.-C. Wu, Appl. Surf. Sci. 2016, [106] V. Srikant, D. R. Clarke, J. Appl. Phys. 1998, 83, 5447–5451. 360, 442–450. [107] V. A. Coleman, C. Jagadish, in: Zinc Oxide Bulk, Thin Films and [81] M. Khanlarian, M. Heshmat, F. Rashchi, A. Ravanbakhsh, AIP Conf. Proc. Nanostructures, Elsevier, 2006, 1–20. 2017, 20047. [108] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, ACS [82] K. Mika, R. P. Socha, P. Nyga, E. Wiercigroch, K. Małek, M. Jarosz, T. Appl. Mater. Interfaces 2012, 4, 4024–4030. Uchacz, G. D. Sulka, L. Zaraska, Electrochim. Acta 2019, 305, 349–359. [109] M. A. Imam, M. Moniruzzaman, A. Mamun, Proceeding 18 International [83] Z. Hu, Q. Chen, Z. Li, Y. Yu, L.-M. Peng, J. Phys. Chem. C 2010, 114, 881– Corrosion Congress 2011, 1–8. 889. [84] Y.-T. Kim, J. Park, S. Kim, D. W. Park, J. Choi, Electrochim. Acta 2012, 78, Manuscript received: February 18, 2021 417–421. Revised manuscript received: April 5, 2021 [85] J. Park, K. Kim, J. Choi, Curr. Appl. Phys. 2013, 13, 1370–1375. Accepted manuscript online: April 14, 2021

Wiley VCH Dienstag, 18.05.2021 2199 / 204431 [S. 14/15] 1 REVIEWS

Think anodic: This review provides K. Engelkemeier*, A. Sun, D. Voswinkel, an overview of the different possibil- Dr. O. Grydin, Prof. M. Schaper, ities of anodic treatments of zinc Prof. W. Bremser oxide, in which the voltage and 1 – 15 current type are the main distinguishing criteria. We summarize the Zinc Anodizing: Structural Diversity parameters used so far (electrolyte, of Anodic Zinc Oxide Controlled by current type, current density, the Type of Electrolyte voltage) and highlight the influence on the formation of anodic zinc oxide (AZO) structures in dependency to the used electrolyte.

Wiley VCH Dienstag, 18.05.2021 2199 / 204431 [S. 15/15] 1