Anodization fabrication techniques and energy-related applications for nanostructured anodic films on transition metals

Nanoporous and nanotubular arrays

To avoid local acidification at the oxide-electrolyte interface, the Grimes group chose glycerol as the electrolytic medium^[24]. They prepared nanoporous anodic films on Fe foils by the potentiostatic anodization of Fe in glycerol containing HF, NH₄F and HNO₃ at 10 °C. Unlike the anodization currents observed for the anodization of Al^[1] or Ti^[10], the anodization current for Fe decreased monotonically with time during anodization, whereas the typical current-time behavior for the anodization of Al or Ti is characterized by an increase in the current amplitude after an initial rapid current drop. The Grimes group explored the influence of anodization bath temperature, electrolyte composition and applied voltage on the pore size of the as-anodized nanoporous structures. For example, nanoporous films with a barrier layer thickness of ~800 nm could be formed by anodization at 90 V for 2 h at 10 °C. Nevertheless, the as-anodized nanoporous films obtained in the electrolyte containing HF, NH₄F and HNO₃ in glycerol had relatively disordered porous structures with a typical pore depth of only ~500 nm. Interestingly, the authors obtained a highly ordered nanoporous structure by anodization at a constant voltage of 40 V at 10 °C when using 0.3 wt.% NH₄F in EG as an electrolyte. The cylindrical nanochannels of the obtained nanoporous anodic films on Fe substrates were very similar to those found in porous AAO^[1,25,26].

Following the pioneering work of Grimes and coworkers, high-aspect-ratio and relatively ordered nanoporous anodic films were later developed. The Schmuki group utilized NH₄F/EG-based electrolytes similar to those used in the fabrication of TiO₂ nanotube arrays to prepare relatively ordered nanoporous anodic films on Fe at 20 °C^[27]. The resulting porous layer had a significantly increased thickness (up to ~13 μm) compared with those obtained in the glycerol electrolyte. The resulting porous layer was confirmed to be essentially amorphous and the nanopores were most probably comprised of a relatively compact inner oxide shell and FeF₂-rich outer shell^[27]. Using Mössbauer spectroscopy at room to cryogenic temperatures, Jagminas et al. demonstrated that their as-prepared iron anodic films mainly consisted of lepidocrocite (γ-FeOOH) with some Fe(OH)₂ and/or FeF₂·4H₂O^[28]. In addition, Habazaki et al. also obtained porous anodic films on Fe in the same EG electrolytes containing NH₄F and H₂O by galvanostatic anodization instead of the commonly used potentiostatic anodization^[29-31]. Their studies showed that the porous anodic films could always be formed provided that anodization was performed galvanostatically at a constant current density of less than 10 mA cm^-2 at temperatures below 30 °C.

Schmuki and coworkers observed an interesting temperature effect, where anodization at higher temperatures could induce a morphological change from the porous layer to nanotube arrays under otherwise identical conditions^[27]. Later, LaTempa et al. reported the formation of anodic oxide nanotube arrays by the potentiostatic anodization of Fe in NH₄F/EG-based electrolytes at higher temperatures^[32]. The authors were able to produce iron oxide nanotube arrays using EG electrolytes containing 0.2-0.5 wt.% NH₄F and 2%-4% H₂O over a voltage range of 30-60 V by changing the electrolyte temperature. For instance, when the anodization of Fe foil was conducted at 50 V for 180 s in an EG electrolyte containing 0.3 wt.% NH₄F and 3% H₂O at a starting bath temperature of 60 °C, vertically oriented nanotube arrays with an average pore diameter of 55 nm and length exceeding 3 μm were obtained. It was suggested that the formation of nanotube arrays strongly depended on the electrolyte bath temperature and NH₄F concentration. However, at room temperature, Rangaraju et al. were also able to prepare nanotubular oxide films by anodization in a similar EG solution at 50 V for 15 min^[33,34]. The obtained nanotubes with a length of ~1.5 μm had an inner diameter of 54-100 nm and a wall thickness of 12-22 nm. Later work suggested that, for the anodization of Fe at room temperature for 30 min, increasing the water content or anodization voltage led to the transformation of nanotubes to nanoparticles or nanowires, respectively^[35].

Consequently, the formation of nanotubes was believed to be associated with the extended anodization times in addition to the electrolyte temperature. In reality, the stirring of the electrolyte^[28,36], the applied magnetic field^[36] and the rotation speed of the Fe electrode^[37,38] during anodization can also influence the nanostructures of anodic films on a Fe substrate. Of course, the purity of the Fe substrate also plays an important role in determining the formation of iron oxide nanostructures. Lee et al. investigated the anodization behavior of two Fe substrates with purities of 99.5% and 99.99% at room temperature, at 60 °C and under ultrasonication-assisted conditions, respectively^[39]. They found that there was a notable difference in the thickness of the obtained anodic films for the two Fe substrates regardless of which anodization conditions were used. Although there was no significant difference in the top morphologies for both substrates, the anodic films grown on the 99.99% purity Fe substrate were typically twice as thick as those grown on the 99.5% purity substrate for the same anodization time^[39].

Some fabrication techniques available for anodic TiO₂ nanotubes and porous AAO can also be used in the electrochemical anodization of Fe. For example, Fe anodization experiments were carried out using ultrasonic waves^[39-42]. Nanotubular array films on Fe foils could be grown by ultrasound-assisted anodization due to the introduction of additional energy^[39-41]. To prepare porous anodic iron oxide films with more ordered nanopores, the so-called “two-step anodization” process was also utilized in the anodization of Fe^[43-47]. In this process, an anodic film grown on the Fe foil by the first anodization process was removed to obtain a Fe surface textured with arrays of concave features. After the second anodization, a self-ordered porous anodic film was formed, apparently because these concave features guide the growth of nanopores. In addition to the usual potentiostatic and galvanostatic anodization, sinusoidally modulated current waveforms were also employed in the anodization of Fe^[48]. Interestingly, periodically structured porous anodic films were obtained by applying a sine-wave current or a voltage pulse to the Fe electrode^[49]. By changing the periodicity of the sine-wave current, the structural periodicity of the obtained anodic films and the film color could be tuned^[48]. The reported anodization conditions for the fabrication of nanoporous or nanotubular iron oxide arrays are listed in Table 1, along with their morphological parameters.

Formation mechanism

For the potentiostatic anodization of Fe, the current-time transients during anodization typically display three different stages, as shown in Figure 1A^[50], which are very similar to those observed for the anodization of Al or Ti^{[1,10,32,34,40,45,47,50,52]}. Therefore, it is presumed that the nanostructured anodic films on Fe were formed in a manner analogous to the growth of porous AAO or TiO₂ nanotube arrays. Following the suggested mechanism for the formation of anodic TiO₂ nanotubes, some authors proposed the possible formation mechanism of iron oxide nanopores or nanotubes formed by anodization in NH₄F/EG-based electrolytes^[47,50,59]. It is generally believed that the formation of iron oxide nanopores or nanotubes involves two electrochemical/chemical processes, namely, the field-assisted oxidation of Fe metal either to form iron oxides or Fe ions dissolved in the electrolyte and the chemical dissolution of iron oxides owing to etching by fluoride ions. A delicate equilibrium between the formation and dissolution of iron oxide films results in nanopore or nanotube formation.

Figure 1. (A) Current density transient during the anodization of Fe foil at 50 V in EG + 0.5 wt.% NH₄F + 3.0 wt.% water for 300 s at 60 °C. (B-G) FESEM images of anodic iron oxide films after 1, 5, 10, 30, 120 and 300 s, respectively. Insets show the corresponding cross-sectional image at each stage^[50]. Reproduced from Ref.^[50] with permission from Elsevier.

The current-time behavior shown in Figure 1A can be understood in terms of the above-described model. In NH₄F/EG-based electrolytes containing water, an initial oxide layer is first formed on the Fe substrate at the metal-electrolyte interface according to Equation 1 when an anodization voltage is applied. This process corresponds to a rapid decrease in current density, as shown in Figure 1A at the initial stage (stage I), as a result of the fast formation of the insulating iron oxide layer. Subsequently, localized chemical etching of the iron oxide can take place to form some disordered small pits in the oxide layer due to the formation of soluble fluoro complexes, according to Equation 2. In this stage (stage II, Figure 1A), a slight rise in current occurs since these small pits increase the surface area of the Fe anode. At the bottom of the small pits, the electric field strength is higher than at other locations due to the relatively thin barrier layer, thereby giving rise to the emergence and/or growth of pores. As anodization proceeds, the pores tend to share the anodization current equally, leading to the formation of an ordered nanoporous film. In this stage (stage III, Figure 1A), the anodization current stays almost constant, implying the establishment of an equilibrium between the formation and dissolution of iron oxides. With a further increase in the anodization time, transformation of the ordered nanoporous structures into nanotube arrays eventually occurs, largely due to the selective dissolution of the Fe fluoride-rich layer at the cell boundaries. Xie et al. investigated systematically the effects of anodization conditions on the morphology and growth behavior of anodic iron oxide films and presented the morphological evolution of anodic films during anodization, as depicted in Figure 1B-G^[50].

2Fe + 3H₂O → Fe₂O₃ (anodic) + 3H₂↑ (cathodic) (1)

Fe₂O₃ + 12F^- + 6H⁺ → 2[FeF₆]^3- + 3H₂O (2)

Recently, some researchers^[59,61] have argued that the formation of nanoporous anodic films on Fe can also be explained in terms of the electronic current and oxygen bubble model suggested by our group in the case of porous AAO or anodic TiO₂ nanotubes^[62-66]. This model emphasizes that the formation of nanopores is closely related to oxygen gas generation during anodization. The anodization currents (consisting of ionic and electronic currents) are not all consumed to form the oxide. During anodization, the ionic currents originating from the migration of oxygen/metal ions lead to oxide growth, while the electronic currents give rise to the evolution of oxygen gas at the bottoms of the pores. Furthermore, the generation of bubbles all over the Fe anode surface could be observed clearly during the Fe anodization. It was suggested that nanopores in the anodic films on Fe were generated by the flow of the material in the barrier layer from the pore bases towards the pore walls around the gas bubble under the pressure of bubbles and growth stresses.

Other nanostructures

In addition to the most commonly found nanoporous and nanotubular structures, the anodization of Fe in the NH₄F-H₂O-EG electrolyte system can also result in anodic films with other morphologies^[67,68]. Kang et al. synthesized vertically aligned iron oxide nanoflakes on the surface of a Fe foam by anodization at 80 V for 5 min at 25 °C with the assistance of ultrasonication in an EG electrolyte containing 0.25 wt.% NH₄F and 2 vol.% H₂O^[67]. The authors reported that electrolyte composition plays a critical role in influencing the final morphologies of anodic films. To demonstrate this concept, they conducted the anodization of Fe foil in four EG electrolytes with different compositions. An interesting phenomenon observed in their study was that, as shown in Figure 2, as the concentrations of NH₄F and H₂O increased, anodic iron oxide films with different nanostructures ranging from nanoflakes to nanotubes were obtained^[67]. Later, Ali et al. reported the synthesis of a three-dimensional nanoflower-like (composed of nanoflakes) hierarchical morphology of iron alkoxide by anodization. Anodization was performed at 50 V at room temperature for 1 h using an EG electrolyte containing 0.5 wt.% NH₄F and 0.2 wt.% H₂O^[68]. Compared to the frequently observed nanoporous and nanotubular structures, the only major difference in anodization conditions was the utilization of an EG electrolyte with a very low water content in their nanoflower fabrication. When the EG in this electrolyte was changed to glycerol, the anodization of Fe foil under identical conditions was found to induce the formation of nanotubes rather than nanoflowers. Therefore, the authors suggested that EG plays a pivotal role in obtaining the nanoflower-like morphology^[68].

Figure 2. SEM images of iron oxide nanostructures prepared by anodization of Fe foil at 50 V for 1 h using EG electrolytes of various compositions: (A) 0.125 wt.% NH₄F and 1.0 vol.% H₂O; (B) 0.250 wt.% NH₄F and 2.0 vol.% H₂O; (C) 0.500 wt.% NH₄F and 3.0 vol.% H₂O; (D) 1.000 wt.% NH₄F and 4.0 vol.% H₂O^[67]. Reproduced from Ref.^[67] with permission from Wiley.

Several investigations have sought to generate other nanostructured anodic films on Fe substrates in fluoride-free electrolyte systems^[69-71]. Sagu et al. employed a 10 mol L^-1 aqueous NaOH solution as the electrolyte for the anodization of steel^[69]. The surface layer of the anodized steel was confirmed to be Fe₂O₃ with a highly porous sponge-like structure. Recently, our group^[70,71] carried out the anodization of Fe using an aqueous oxalic acid solution. Interestingly, the anodic film grown on Fe exhibited a rod-like morphology, with the rods having relatively regular hexahedral shapes with feature sizes of 250-700 nm. FTIR and XRD analyzes showed that the as-anodized films essentially consisted of amorphous ferrous oxalate^[70].

Finally, it is worth emphasizing that, unlike anodic TiO₂ nanotubes, the nanostructures (nanopores and nanotubes) of as-anodized iron oxide films generally undergo a significant change after thermal annealing due to an amorphous to crystalline transition^{[31,36,38,46,50,53,55,56,68,72]}. To inhibit the morphological changes of the anodic iron oxide films as a result of the agglomeration of the hematite particles during thermal annealing, Jun et al. specifically devised a 10 nm-thick alumina layer deposited on the surface of nanopores as a shielding layer to maintain the vertically ordered nanoporous structure of the hematite^[44].

Applications

Iron oxides, like FeO, Fe₂O₃ and Fe₃O₄, have attracted intense research interest for various applications, including photocatalysis^[49], PEC water splitting^{[38,39,44,67]}, lithium-ion batteries^[45,46], supercapacitors^[51] and biomaterials^[54,73], because of their unique electrochemical/PEC properties, crystallographic polymorphism, earth abundance, eco-friendliness and low cost. Here, we focus on their applications in the PEC fields and electrochemical energy storage devices.

Nanoneedles, nanowires and nanotubes

As early as 1976, Shoesmith et al. thoroughly investigated the anodic oxidation of Cu in a LiOH solution using potentiostatic, galvanostatic and voltammetric sweep techniques^[87]. The main anodization product was demonstrated to be Cu(OH)₂, with the formation of CuO and Cu₂O phases also possible in terms of the anodization conditions. The authors also discussed the nucleation and growth mechanism of Cu(OH)₂ films. Furthermore, the acicular morphology of the resultant films was first observed in their pioneering work^[87]. In 2005, the Shi group utilized KOH-based electrolytes to form nanostructured anodic films on Cu under galvanostatic conditions^[88]. As shown in Figure 3, the typical anodic products exhibit a pine needle-like morphology, which was confirmed to be mainly Cu(OH)₂ with traces of Cu₂O. It is noteworthy that unlike the amorphous nature of the majority of anodic films on metals, the as-anodized products formed on Cu are crystalline. Figure 3 shows the morphological evolution of anodic films on Cu during anodization. After anodization for 1500 s, Cu(OH)₂ nanoneedles oriented roughly perpendicular to the Cu substrate covered the whole Cu surface [Figure 3D]. These nanoneedles had an average length of 15 μm with a typical diameter of 500-550 nm at their roots and 100-120 nm at their sharp tips. As the electrolyte temperature decreased, the number density of nanoneedles clearly increased, whereas their average length decreased correspondingly. This could be attributed to the easier nucleation of Cu(OH)₂ crystals at lower temperatures. Under otherwise identical conditions, scroll-like Cu(OH)₂ nanotubes were formed on Cu foil when the KOH concentration was increased to over 2.5 mol L^-1. Moreover, the current density also played an important role in governing the resulting anodic products. At a low current density of 0.5 mA cm^-2, black nanosheets or nanoflowers of CuO, together with a certain amount of Cu₂O, were generated. When the current density was increased to over 1 mA cm^-2, Cu(OH)₂ rather than CuO nanoneedles was obtained. The increase in current density facilitated the formation of Cu(OH)₂ and prevented the generation of the side product Cu₂O. The optimized anodization conditions for fabricating Cu(OH)₂ nanoneedles or nanotubes are listed in Table 2. In addition, Cu(OH)₂ nanoneedles can be readily converted to CuO nanoneedles by annealing at 200 °C for 3 h under a nitrogen atmosphere^[88].

Figure 3. SEM images of as-prepared Cu(OH)₂ nanoneedles formed at 1.5 mA cm^-2 in a 2.0 mol L^-1 KOH solution at T = 28 ± 2 °C for different anodization times: (A) 300 s; (B) 500 s; (C) 1000 s; (D) 1500 s^[88]. Reproduced from Ref.^[88] with permission from the American Chemical Society.

The formation of nanoneedles by the anodization of Cu in KOH or NaOH electrolytes was further confirmed in subsequent studies^[89-103]. Allam and Grimes found that the reaction of Cu foil in an aqueous KOH electrolyte was pH dependent^[89]. No film was formed on the Cu surface when the reaction was performed in KOH solutions with pH ≤ 10. Zhang and Wang explored the anodization of electrodeposited Cu films in a 3 mol L^-1 solution NaOH at different current densities^[90]. They found that the anodic films prepared at a lower current density were Cu₂O with polyhedral structures, while those prepared at a higher current density were Cu(OH)₂ with a randomly packed nanowire (or nanoneedle) structure. This was in reasonable agreement with the experimental findings of the Shi group^[88]. Moreover, different annealing environments and temperatures were found to result in the formation of different CuO and/or Cu₂O nanostructures when the Cu(OH)₂ nanoneedles were annealed^[92,104]. For instance, when anodized Cu foam was heated under a nitrogen atmosphere at 150 °C for 3 h and at 200 °C for 3 h, CuO nanowires were formed on the Cu foam. In contrast, when the anodized sample was heated in air at 200 °C for 15 min, CuO nanosheets were generated on the Cu foam^[91]. When the Cu foam was anodized at 30 mA cm^-2 in a 3 mol L^-1 aqueous KOH solution at 40 °C, leaf-like CuO/Cu₂O nanosheets were obtained directly^[105]. Recently, Anantharaj and coworkers found that a much denser Cu(OH)₂/CuO nanoneedle array could be obtained within just 80 s by the anodization of Cu foil using a conventional three-electrode system-based potentiostatic (at 0.864 V vs. a RHE) method in 1.0 M KOH^[106,107]. Increasing the pH of KOH electrolytes led to an increase in the average length of the Cu(OH)₂/CuO nanoneedles.

Nanoporous and leaf-like nanoarchitectures

Inspired by the fact that the anodization of metals like Ti in fluoride-containing media can lead to the formation of nanoporous/nanotubular structures, the Grimes group studied the anodization of Cu foil in halide-containing solutions in 2011 in an attempt to obtain nanoporous copper oxide films^[89]. They investigated the combined effect of KOH and NH₄Cl on Cu anodization and found that no porous structure could be obtained using NH₄Cl-based electrolytes. This result was also verified in a more recent study^[108]. The Grimes group^[89] discovered that a nanoporous structure appeared on the entire Cu surface after the Cu foil was anodized at 6 V for 300 s in an aqueous electrolyte containing 0.15 mol L^-1 KOH and 0.1 mol L^-1 NH₄F. When the KOH concentration was increased to 0.2 mol L^-1 while maintaining all other conditions, anodic products with porous microspheroids were obtained. However, when Cu thin films of ~450 nm thickness were anodized in the same aqueous electrolytes containing 0.1 mol L^-1 NH₄F under similar anodization conditions, the anodic products still showed nanowire structures^[109].

The Grimes group also showed that changing the electrolyte solvent from water to EG still did not produce the nanoporous/nanotubular structures like those found after Ti anodization^[89]. Nevertheless, the formation of leaf-like nanoarchitectures with a thickness of ~160 nm was observed upon the anodization at 30 V for 300 s of Cu foil in an EG electrolyte containing 0.15 mol L^-1 KOH, 0.1 mol L^-1 NH₄F and 3% H₂O. Increasing the KOH concentration to 0.2 mol L^-1 led to the growth of thicker oxide films of ~500 nm with the same leaf-like morphology^[89]. However, Jerez et al. obtained nanoporous mixed Cu₂O/CuO films when the Cu anodization was conducted at a relatively lower temperature (5 °C) in similar EG electrolytes^[110]. Furthermore, the current-time transients during potentiostatic anodization were similar to those observed for Ti anodization. Wang et al. found that the calcination temperature played a key role in determining the final morphology of the anodization products^[111]. Nanoporous Cu₂O films were formed by anodization in an EG electrolyte containing 0.75 wt.% KOH, 0.20 wt.% NaF and 3.0 wt.% H₂O. The formation of a dense array of CuO nanowires embedded within the surface of the Cu₂O layer was observed after the anodized films were annealed at 400 °C for 60 min.

Shu et al. also prepared leaf-like copper oxide nanosheets in the NaOH-NaCl-polyethylene glycol (PEG) aqueous electrolyte system^[112,113]. They found that higher anodization temperatures were required for generating copper oxide nanosheets in NaOH-NaCl-PEG electrolytes. No oxide film was observed on the Cu surface when the anodization temperatures were lower than 50 °C^[112]. Furthermore, they found that the addition of ammonium molybdate (AM) in the NaOH-NaCl-PEG solutions inhibited the Cu → Cu⁺ oxidation reaction, resulting in the formation of single-phase CuO nanosheet films. Typically, in an aqueous solution containing 1 mol L^-1 NaOH, 2.5 mol L^-1 NaCl, 1 g L^-1 PEG and 20 g L^-1 AM, single-phase CuO nanosheets were obtained directly on Cu foam when it was anodized at a constant current of 10 mA cm^-2 at 65 °C. These grown nanosheets were ~2 μm in size with a thickness of ~30-50 nm^[113].

Other nanostructures and anodization products

Since the aqueous solutions of carbonate salts of alkali metals are weakly alkaline, they have also been used as electrolytes for Cu anodization in addition to the commonly used NaOH or KOH solutions. Stępniowski et al. investigated the growth of nanostructured anodic films on Cu in 0.1 mol L^-1 K₂CO₃^[114-116], 0.1 mol L^-1Na₂CO₃^[117] and 0.01 mol L^-1 KHCO₃^[118] solutions, respectively. The resultant anodic films formed in these carbonate-based electrolytes typically had a nanorod-like morphology. Moreover, in a 0.5 mol L^-1 potassium oxalate solution, the Cu anodization led to the formation of copper oxalate and a CuO complex film with rough and porous microstructures^[119]. When the Cu anodization was completed in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, containing N-butyl-N-methylpyrrolidinium iodide^[120], or in a potassium iodide electrolyte^[121], crystalline cuprous iodide (CuI) thin films were obtained. Interestingly, after anodization in a 0.1 mol L^-1 Na₂WO₄ solution at 3 V for 5 min, anodic films with cauliflower-like structures appeared on the Cu surface, which were confirmed to be CuWO₄ and CuO composites^[122]. Similarly, Cu anodization at 10 V in a mixed aqueous solution containing 0.05 mol L^-1 NaH₂PO₄ and 0.05 mol L^-1 Na₂HPO₄ resulted in the growth of leaf-like copper phosphate films^[123]. The copper phosphate films showed high catalytic activity for the Fenton oxidative degradation of dyes. Moreover, copper(II) myristate films with a “star-like” structure were found to be formed by Cu anodization at a relatively high voltage (100 V) in a nonaqueous electrolyte (0.1 mol L^-1 myristic acid and 0.1 mol L^-1 tetraethylammonium tetrafluoroborate in methanol)^[124]. In particular, Lu et al. added 0.07 mol L^-1 cetyltrimethylammonium bromide (CTAB) into a 3 mol L^-1 KOH aqueous solution and explored Cu anodization in this electrolyte^[125]. Because CTAB served as a template for the formation of the nanofilms, graphene-like Cu(OH)₂ nanofilms were obtained by anodization. Consequently, graphene-like CuO nanofilms were successfully prepared after the common thermal treatment of Cu(OH)₂ films. Recently, Liu et al. fabricated tremella-like Cu_xO@Cu_xS nanosheets on copper mesh by anodization in a 0.5 mol L^-1Na₂S solution with a constant current of 5 mA at 0 °C and a subsequent annealing process at 200 °C in air^[126]. The synthesized organic-free tremella-like Cu_xO@Cu_xS nanosheets exhibited superhydrophilicity and underwater superoleophobicity.

Applications

Copper oxides (CuO and Cu₂O) have attracted significant attention for a wide variety of applications due to their relatively low cost, nontoxicity, abundance and high chemical stability. As indicated above, numerous research efforts have focused on tuning the nanostructure and function of copper oxide-based materials by anodization, which has led to the discovery of many fascinating properties and applications. In addition to the PEC and supercapacitor applications commonly encountered, copper oxide-based materials have found applications in sensors^{[43,109,119,127,128]}, pH-controllable water permeation^[93], superhydrophobic anticorrosion coatings^[94,95,129], oil/water separation^[122] and surface-enhanced Raman scattering substrates^[108].

Supercapacitors

Over the past decade, copper oxides have experienced a surge in interest for their application in supercapacitors owing to the rich variety of their electrochemical responses and nanostructures^[133]. Because copper oxides/hydroxides are grown in situ on Cu substrates by the anodization route, unlike powder-type active materials, they can be directly employed as electrodes in supercapacitors without using any binder. Moreover, nanostructured copper oxides/hydroxides with various morphologies can also be obtained by anodization, as described above. Consequently, copper oxides/hydroxides prepared by anodization have been studied extensively for use in supercapacitors^{[91,96,98-101,103,105,113,125,134]}.

Generally, nanostructured CuO, Cu₂O or Cu(OH)₂ films (e.g., nanowires and nanosheets) obtained by the anodization technique have produced cyclic voltammograms with ill-defined or broad redox peaks in alkaline solutions, although their capacitances were believed to originate from Faradaic redox reactions^{[91,98,103,105]}. Figure 4 displays the typical cyclic voltammograms of CuO nanowires, nanosheets and nanoflowers in a 6.0 mol L^-1 KOH solution. It has been proposed that the following redox reactions of these copper oxides/hydroxides in alkaline solution may be involved in the transition between Cu(I) and Cu(II) species:

Figure 4. Cyclic voltammograms of (a) CuO nanowires, (b) CuO nanosheets and (c) CuO nanoflowers recorded in a 6.0 mol L^-1 KOH solution at a scan rate of 20 mV s^-1. The inset shows the cyclic voltammograms of the Cu foam substrate and the CuO nanowires on the Cu foam electrode with the same geometrical area of 1 cm^2[91]. Reproduced from Ref.^[91] with permission from Elsevier.

½Cu₂O + OH^- ↔ CuO + ½H₂O + e^- (3)

½Cu₂O + ½H₂O + OH^- ↔ Cu(OH)₂ + e^- (4)

CuOH + OH^- ↔ CuO + H₂O + e^- (5)

CuOH + OH^- ↔ Cu(OH)₂ + e^- (6)

The broad redox peaks in their cyclic voltammograms, as illustrated in Figure 4, could be attributed to the overlap of the redox processes expressed in Equations 3-6^[91]. These nanostructured copper oxides/hydroxides in alkaline solutions can exhibit relatively high specific capacitances and good cycling stability. For example, leaf-like CuO-Cu₂O nanosheets on Cu foam showed a high capacitance of 1.954 F cm^-2 and excellent cycling stability without a decrease in capacitance after 5000 cycles^[105]. The anodized Cu(OH)₂ nanorods delivered a high capacitance of 260 F·g^-1 (at 5 mV s^-1) and retained 92.0% of the initial capacitance after 5000 charge-discharge cycles^[134]. However, these copper oxides/hydroxides had relatively narrower operating potential windows (e.g., from 0 to 0.4 V, Figure 4). This may be due to their low oxygen evolution potential in alkaline solutions. Furthermore, their coulombic efficiency was comparatively low^[105].

As mentioned above, Lu et al. successfully fabricated graphene-like CuO nanofilms on Cu foam by in-situ anodization^[125]. As shown in Figure 5A-C, the fabricated electrode was comprised of interconnected CuO nanofilms with open mesopores. Furthermore, the individual CuO nanofilms with a thickness of ~2 nm had a graphene-like flexible and folded structure [Figure 5D]. Unlike the aforementioned copper oxides/hydroxides, the CuO nanofilms displayed well-defined redox peaks [Figure 5E and F], corresponding to the transition between Cu(I) and Cu(II) species (Equation 3). The dramatically different electrochemical response was primarily attributed to the ultrathin graphene-like structure with short pathways for ion transport. Moreover, the CuO nanofilms exhibited a much higher specific capacitance (919 F·g^-1 at 1 A·g^-1) and good cycling stability (7% capacitance loss after 5000 cycles). In addition, they showed excellent photocatalytic activities. More recently, Gong et al. prepared needle-like Cu(OH)₂ nanowires on nanoporous Cu by anodization, with the cyclic voltammograms also exhibiting a pair of very obvious redox peaks^[103]. Their nanoporous Cu with uniform and continuous porous structures was fabricated by dealloying a Cu-Zr-Ni-Be quaternary amorphous alloy in mixed acid solutions. The reason why the needle-like Cu(OH)₂ nanowires showed distinct redox peaks may be related to the nanoporous Cu substrate.

Figure 5. Morphologies of CuO nanofilms: (A-C) SEM images taken at different magnifications; (D) TEM image. Cyclic voltammograms of (E) different electrodes at 10 mV s^-1 and (F) CuO nanofilm electrode at various scan rates. The inset of (A) shows an SEM image of the bare Cu foam^[125]. Reproduced from Ref.^[125] with permission from the American Chemical Society.

Nanoneedles, nanorods, nanowires and nanotubes

Similar to the case of Cu(OH)₂ nanoneedles discussed above, the first report on the anodization preparation of ZnO nanoneedles was also published by the Shi group in 2006^[144]. They demonstrated that highly oriented ZnO nanoneedle arrays could be prepared by the anodization of Zn foil in a saturated zincate [Zn(OH)₄^2-] solution at room temperature. The electrolyte used was a freshly prepared saturated aqueous zincate solution, which was prepared by adding 10 mL of 1.0 mol L^-1 Zn(NO₃)₂ into 10 mL of 4.0 mol L^-1 KOH with vigorous stirring and then removing the precipitate by centrifugation. Crystalline ZnO nanoneedle arrays were obtained by anodization in a freshly prepared 0.5 mol L^-1 zincate aqueous solution at 0.3 mA cm^-2 and 1.5 C cm^-2. As shown in Figure 6, these ZnO nanoneedles were aligned perpendicular to the Zn substrate surface and coated on the substrate uniformly and compactly, typically having a length of 550-650 nm with a tip diameter of 10-15 nm and a root diameter of 50-60 nm. They found that the concentration of zincate may play a key role in the formation of anodic ZnO nanoneedles. The nanoneedles were not formed in electrolytes with low zincate concentrations. The anodization of Zn in a pure aqueous KOH solution (2.0 M) only led to the dissolution of Zn in the solution^[144]. Subsequent studies also demonstrated that ZnO nanoneedles could be formed by anodization in a concentrated NaOH solution (4.0 M), whereas nanoporous ZnO films were obtained in 0.1 mol L^-1 NaOH (KOH) solution, since the zincate was formed in concentrated alkaline solutions^[145-147].

Figure 6. (A and B) SEM and (C and D) TEM images of as-prepared ZnO nanoneedles. The inset of (A) shows a magnified view. The inset of (D) shows the SAED and HR-TEM pattern of a ZnO nanoneedle^[144]. Reproduced from Ref.^[144] with permission from IOP Publishing.

Several years later, Hu and coworkers reported the rapid synthesis of ultralong ZnO nanowire films by anodization in a slightly basic solution^[148]. They found that high-aspect-ratio nanowire arrays of tens of microns in length and 30-100 nm in diameter can be formed on Zn foil rapidly by anodizing in an aqueous 5-50 mM KHCO₃ solution at room temperature. The as-anodized nanowires were demonstrated to be highly crystalline materials that may have the composition Zn(OH)_n(CO₃)_m and could be readily converted to ZnO nanowires by annealing at 250 °C for 1 h in air. Figure 7 shows the morphological evolution during anodization of Zn at 10 V. In the initial stages of anodization, the Zn foil was dissolved to form irregular, discrete pits over the surface of the metal [Figure 7a]. Furthermore, nanoflower-like products appeared inside the pits [Figure 7A]. As the anodization time increased, the morphology of the anodic films was transformed gradually from discrete nanoflower-like structures to a uniform film of aligned nanowires [Figure 7]. Numerous follow-up studies further revealed that the nanowire length and diameter and density of the nanoflower-like structures were directly proportional to the anodization time^[149-154]. In addition, nanowires can be grown at a rather high rate (~1.3 μm min^-1)^[149-154] in an aqueous KHCO₃ solution.

Figure 7. Morphological evolution of anodized Zn foils at 10 V for selected periods. (a-d) refer to 1, 5, 10 and 20 min anodization times, respectively. (A-D) show the corresponding magnified images^[148]. Reproduced from Ref.^[148] with permission from the American Chemical Society.

In addition to the aqueous KHCO₃ solution, other bicarbonate electrolytes, including NaHCO₃ and NH₄HCO₃, have also been utilized in the anodization fabrication of ZnO nanowires^[155-160]. Miles et al. thoroughly investigated the anodization behavior of Zn in aqueous KHCO₃, NaHCO₃ and NH₄HCO₃ solutions^[155]. They demonstrated that the growth of high-aspect-ratio nanowires by Zn anodization was possible using the three bicarbonate electrolytes. The authors found that, for the very thick nanowire films (> 50 μm), hierarchical three-dimensional nanowire structures instead of aligned nanowires were typically formed. Furthermore, the nanowire growth rates in NaHCO₃ and NH₄HCO₃ electrolytes were noticeably higher than those in the KHCO₃ solution. Under the optimized conditions, growth rates as high as 3.2 μm min^-1 could be attained, even well above those of AAO under high-field anodization conditions^[161-163]. It is noteworthy that short nanowire structures were also obtained in an aqueous NH₄F solution^[164] and in an electrolyte based on EG and NH₄F^[165].

In work by Katwal et al., it was observed that nanotubes appeared at the edges (< 5% of the area) of the Zn foil, although the foil was covered predominantly by nanowires [Figure 8A], while the Zn anodization was performed first at 10 V in a 6.8 mM NaHCO₃ solution, followed by a second anodization at 20 V in a 0.2 mMNaHCO₃ solution^[166]. Nevertheless, when the anodization was conducted only once in a mixed solution of 6.8 mM NaHCO₃ and 5.7 mM Na₂CO₃ at 10 V, the nanotube coverage improved remarkably to ~50%, as shown in Figure 8B and C. The obtained nanotube-nanowire hybrid architectures with a length of ~14 μm were formed after 10 min of anodization. The diameters of the nanotubes ranged typically from ~100 to ~300 nm and the pore diameters were from ~10 to ~200 nm [Figure 8D]^[166]. Similar results were obtained later in a mixed solution of NaHCO₃ and Na₂CO₃ by other authors^[167]. More recently, Batista-Grau and coworkers explored the influence of hydrodynamic conditions (from 0 to 5000 rpm) and the addition of ethanol or glycerol to the NaHCO₃ solution on the morphology of the products formed during the Zn anodization process^[168,169]. They showed that the hydrodynamic conditions during anodization facilitated the formation of ordered ZnO nanowires along the surface. A significant influence on the morphology, as a result of the addition of ethanol or glycerol, was observed. In glycerol-containing NaHCO₃ electrolytes, ZnO nanotubes, nanospheres and nanosponges were also formed in addition to nanowires^[169]. Furthermore, hierarchically structured self-organized ZnO/ZnS nanotubes can be synthesized by the anodization of Zn in a Na₂S- or (NH₄)₂S-based electrolyte containing a small amount of NH₄F^[170-172].

Figure 8. SEM images of a zinc oxide nanotube-nanowire mixed structure fabricated in an aqueous electrolyte containing (A) only NaHCO₃, in which < 5% of the substrate area was covered with nanotubes, and (B and C) both Na₂CO₃ and NaHCO₃. (D) TEM image showing a nanotube and a nanowire^[166]. Reproduced from Ref.^[166] with permission from the American Chemical Society.

Nanoporous films

Several reports have mentioned that, in oxalic acid electrolytes, nanoporous ZnO thin films were typically formed by the electrochemical anodization of Zn^[173,174]. Interestingly, ZnO porous structures on Zn metal were also obtained by Zn anodization just using deionized/distilled water as an electrolyte at room temperature^[175,176]. As the deionized water itself was slightly acidic (pH 5.8), the Zn anodization could be carried out in water without the need for the addition of any extra electrolyte^[175]. The morphologies of the ZnO thin films were found to be highly dependent on the anodization voltages^[176].

As described above, Zn anodization in a concentrated NaOH solution led to the formation of ZnO nanoneedles^[145]. In contrast, it was also reported that nanoporous anodic ZnO films could be produced in dilute NaOH solutions (< 1 M)^{[147,177-179]}. For example, a porous anodic film with a thickness of ~10 μm was obtained by anodization in a 0.1 mol L^-1 NaOH solution at 20 V for 90 min, which consisted of crystalline ZnO with a straight cellular structure similar to that of AAO^[177]. It was suggested that the addition of EG to the NaOH solution could improve the film properties and current efficiency of anodization^[177]. When a 1 mol L^-1 NaOH aqueous electrolyte was employed, relatively low anodization voltages (2 or 4 V) were adopted to fabricate nanoporous ZnO films. Furthermore, the prepared porous films had relatively disordered nanoporous structures in comparison with the films produced in a 0.1 mol L^-1 NaOH solution^[179]. Additionally, the Schmuki group found that Zn anodization in a fluoride electrolyte containing 2-methyl-1,3-propanediol at low temperatures (-2 to -30 °C) resulted in hierarchically structured self-organized porous films^[180]. The as-grown film was confirmed to be ZnF₂-rich amorphous ZnO.

Other nanostructures and anodization products

So far, it has been shown that the compositions of the electrolytes used in anodization play a particularly influential role in determining the nanostructures of anodic ZnO films. Apart from the commonly used NaOH/KOH and bicarbonate electrolytes, other electrolytes, such as silicate-based solutions^[181], H₂SO₄ solutions^[182], methanol solutions containing HF and water^[183], mixed solutions of (NH₄)₂SO₄ and NH₄Cl^[184], H₃PO₄/NH₄H₂PO₄ solutions^[185] and mixed solutions of (NH₄)₂SO₄ and NaOH^[186], could also be employed to make nanostructured anodic films by Zn anodization. In 2008, Kim and Choi utilized H₂SO₄ solutions in ethanol instead of in water to fabricate ZnO films by anodization^[182]. Unlike the formation of hexagonal etched arrays or the stair-like morphology of ZnO obtained in aqueous H₂SO₄ solutions, anodization in a H₂SO₄ solution in ethanol led to the formation of self-assembled ZnO stripes consisting of 800 nm polygonal flakes, which may be attributed to the water-selective dissolution of ZnO and the hydrodynamic flow of water by convection^[182]. In 0.1 wt.% HF solutions in a mixture of methanol and water, ZnO nanoflowers composed of hundreds of sheet-like nanopetals were found to be grown on a Zn substrate after a potentiostatic anodization of 15 min^[183]. In contrast, in a mixed solution of 0.2 mol L^-1 (NH₄)₂SO₄ and 0.2 mol L^-1 NH₄Cl, large-scale ZnO sunflower structures with a diameter of 400-500 μm were formed^[184]. In addition, the formation of leaf-like ZnO nanoflakes by Zn anodization in a mixture of (NH₄)₂SO₄ and NaOH was reported^[186]. Most interestingly, zinc phosphate microporous spheres with diameters of 5-23 μm were obtained by the potentiostatic anodization of Zn in a 0.2 mol L^-1 H₃PO₄ or NH₄H₂PO₄ solution at 1 V^[185]. More recently, Heo and coworkers reported a facile anodization method for synthesizing zinc ricinoleate (a deodorizing agent)^[187]. Zinc ricinoleate powders with a spherical morphology were prepared by the anodization of Zn foil in an electrolyte containing ricinoleic acid, water and ethanol (4:4:2 v/v).

Applications

Zinc oxide (ZnO) is a well-known piezoelectric material and also an n-type semiconductor with a wide direct bandgap (~3.37 eV). It offers many unique and promising properties, such as a large exciton binding energy (60 meV), high electron mobility, good photocatalytic activity, excellent thermal and chemical stability, large piezoelectric constant and easily modified electric conductivity^[188]. Therefore, nanostructured ZnO materials have been studied for many potential applications, including photoluminescence^{[142,146,189]}, gas sensors^{[173,190,191]}, biological applications^{[166,167,192]}, photodetectors^[151], control of surface wettability^{[152,183,193]}, photocatalysts^{[141,177,186,194-198]}, dye-sensitized solar cells^[149] and PEC water splitting^{[168-171,199,200]}.

Unsurprisingly, the majority of research efforts have focused on the PEC applications of ZnO due to its efficient PEC response. For example, the Schmuki group fabricated hierarchically structured self-organized ZnO nanotubes by Zn anodization in a sulfide-based aqueous electrolyte containing a small amount of NH₄F and studied their PEC water splitting performance^[170]. The highest photocurrent values (~1.7 mA cm^-2) were found for the fabricated ZnO nanotubes (containing a certain amount of ZnS) with a length of 3 μm annealed at 375 °C under an external bias of -0.14 V vs. Ag/AgCl in a sacrificial polysulfide electrolyte. They suggested that the excellent PEC properties could be ascribed to the sacrificial polysulfide electrolyte, which hindered the corrosion/photocorrosion of the ZnO/ZnS structure^[170]. Huang et al. demonstrated that anodized ZnO with different nanostructures exhibited markedly different PEC properties^[199]. The photoanode made of ZnO nanowires had a photocurrent density of 0.32 mA cm^-2 at 0.5 V vs. a SCE under 100 mW cm^-2 illumination, ~1.9 times higher than that of anodes made of nanosheets (0.17 mA cm^-2 under the same conditions). The superior PEC performance of nanowires probably resulted from single crystals with a preferred orientation of (002), higher carrier concentration and lower charge transfer resistance relative to polycrystalline nanosheets. Furthermore, the stability of the anodized ZnO samples was higher than that of other samples prepared by sol-gel or chemical bath processes^[199]. Sanz-Marco et al. synthesized ZnO/ZnS nanotubular structures by anodization in a glycerol-based electrolyte containing Na₂S and NH₄F under hydrodynamic conditions^[171]. The PEC properties of the resultant films were found to be heavily dependent on the anodization voltage and the electrode rotation speed during anodization. Optimal PEC performance was observed for samples anodized at 40 V and at 1000 rpm, which exhibited a 71% increase in photocurrent density for PEC water splitting in comparison to those anodized at 20 V and 1000 rpm^[171].

Very recently, as mentioned earlier, Batista-Grau et al. fabricated different nanostructured ZnO electrodes by anodization in NaHCO₃ and NaHCO₃ solutions containing ethanol or glycerol under hydrodynamic conditions^[168,169]. Their experimental results established that the obtained ZnO nanostructures presented high photocurrent density response during water splitting measurements in a sacrificial polysulfide electrolyte. In particular, the samples anodized at 5000 rpm in NaHCO₃ solutions showed a ~159% improvement in the photocurrent density response compared to those anodized under stagnant conditions^[168]. It was observed that when 10% v/v of ethanol or 25% v/v of glycerol was added to the aqueous NaHCO₃ electrolyte, the photocurrent density response for the electrodes obtained in the organic solvent containing an electrolyte under stagnant conditions was greatly increased. The photocurrent density response of the ZnO electrode anodized in a NaHCO₃ electrolyte containing 10% v/v ethanol (0.34 mA cm^-2) was twice that of the sample anodized under the same conditions in an aqueous NaHCO₃ electrolyte (0.17 mA cm^-2)^[168,169]. Finally, it should be mentioned that, unlike iron and copper oxides, ZnO materials have rarely been investigated as electrode material for supercapacitors, likely due to the absence of variable oxidation states for Zn. There is only one publication reporting the specific capacitance of anodized ZnO nanorods but without providing any other performance parameters^[154].