Review of Anodic Tantalum Oxide Nanostructures: From Morphological Design to Emerging Applications

Anodization of transition metals, particularly the valve metals (V, W, Ti, Ta, Hf, Nb, and Zr) and their alloys, has emerged as a powerful tool for controlling the morphology, purity, and thickness of oxide nanostructures. The present review is focused on the advances in the synthesis of micro/nanostructures of anodic tantalum oxides (ATO) in inorganic, organic, and mixed inorganic–organic type electrolytes with critically highlighting anodization parameters, such as applied voltage, current, time, and electrolyte temperature. Particularly, the growth of ATO nanostructures in fluoride containing electrolytes and their applications are briefly covered. The details of the current– or voltage–time transient and its relation to the growth of the anodic oxide films are presented systematically. The main discussion revolves around the incorporation of various electrolyte species into the surface of ATO structures and its effects on their physicochemical properties. The latest progress in understanding the growth mechanism of nanoporous/nanotubular ATO structures is outlined. Additionally, the impact of annealing temperature (ranging from 400–1000 °C) and atmosphere on the crystalline structure, morphology, impurity content, and physical properties of the ATOs is briefly described. The common modification methods, such as decorating with other transition metal/metal oxide, heteroatom doping, or generating defects in the ATO structures, are discussed. Besides, the review also covers the most promising applications of these materials in the fields of capacitors, supercapacitors, memristive devices, corrosion protection, photocatalysis, photoelectrochemical (PEC) water splitting, and biomaterials. Finally, future research directions for designing ATO-based nanomaterials and their utilities are indicated.


INTRODUCTION
−5 Various synthetic procedures, such as hydrothermal, solvothermal, sol−gel, vapor phase deposition, chemical vapor deposition (CVD), electrodeposition, and electrochemical anodization, can be employed to fabricate nanostructures of TMO. 1,2However, among all of these synthetic strategies, electrochemical anodization is considered a simple, costeffective, and scalable method for producing TMO nanostructures with the potential to control their morphology and purity. 2,68][9][10][11][12][13][14]18 Tantalum can exist in either the +5 or +4 oxidation state, depending on the oxide phase, i.e., Ta 2 O 5 and TaO 2 , respectively. 1214 However, Ta 2 O 5 is considered thermodynamically the most stable state.The fabrication of Ta 2 O 5 , particularly by the anodization method, has been intensively investigated in different inorganic (H 2 SO 4 , H 3 PO 4 , NH 4 F, Na 2 SO 4 solutions), organic (oxalic acid, glycerol, ethylene glycol (EG)), and mixed inorganic−organic electrolytes (H 2 SO 4 + EG + NH 4 F) at various operating voltages (typically ranging from 10 to 200 V).13,14,18 The structure of anodic tantalum oxide (ATO) layers can vary from 0D to 3D, depending on the applied anodizing conditions, such as anodization voltage/current, anodization time, temperature, and pH of the electrolyte.Diverse morphologies, including compact layers, ordered porous structures, nonordered porous structures, nanotubes, and coral-like structures, have been observed in ATOs.18 Over the past two decades, continuous research efforts have sought to understand the anodic growth mechanism of nanoporous/tubular metal oxide structures.15−20 Typically, electrolytes containing fluoride ions with a suitable anodizing potential range are considered to promote the growth of nanoporous/tube structures, as observed for many valve metals, including Ti, Ta, and Zr.13−20 It is noteworthy that the formation mechanisms associated with nanoporous/tube structures of anodic titania (TiO 2 ) and Ta 2 O 5 have been intensively investigated in the previous decade.Various sophisticated analytical techniques, such as X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX) mapping, and radioactive labeling methods, have been applied to understand the growth mechanisms.25 Numerous studies have investigated surface doping or functionalization with various metals or nonmetals (e.g., B, Ti, Ni, and Co) and surface decoration with plasmonic materials like gold particles.These endeavors aim to tune the band gap energy for effective photo or photoelectrochemical catalysis.24,25,34,35 Additionally, many studies reported the incorporation of various electrolyte species (based on e.g., F, P, and S elements) into the anodic layers of Ta 2 O 5 , introducing new physicochemical properties that may broaden their range of applications.36−39 Efforts have also been directed toward creating defects in anodic Ta 2 O 5 films to modify their physical properties.40 For example, S. Xia et al. 40 demonstrated a lithium capacity of ∼480 mAh g −1 with cycling stability over 8000 cycles by inducing oxygen deficiency in the Ta 2 O 5 matrix.Therefore, different modification methods hold the potential to enhance the properties of anodic tantalum oxide, offering diverse avenues to study these effects.
In this paper, we systematically review the progress in the synthesis of doped and undoped structures of anodic tantalum oxides, post modification methods, and their characterization techniques.We explain how various types of oxide morphologies are generated under the influence of an electric field, consolidating dispersed information in one place and indicating possible future research directions relevant to anodic tantalum oxide structures.

TANTALUM SUBSTRATES FOR ANODIZATION
The methods of preparation of Ta substrates, along with their dimensions and purity, may vary depending on the research objectives and applications.In general, high purity of tantalum (∼99.9%) in the form of foil/sheet/disc/rod is an excellent choice for anodization experiments. 21,22,32,41Apart from that, Ta sputter-deposited or coated on glass/Si-wafer/metal substrates is commonly used and documented in various publications. 31,35A detailed list of the Ta substrates, including their dimensions and purity, can be found in Table 1.

PRETREATMENT OF TANTALUM SUBSTRATES BEFORE ANODIZATION
The pretreatment of tantalum substrates before anodization is an essential step to create a uniform, smother, and contaminant-free surface. 32,33,38In general, commercially available Ta substrates (e.g., foil/sheet/rod/disc) are coated with grease or oil to protect them from corrosion or aerial oxidation. 43,47,69The process of degreasing the metal surface involves ultrasonication of the substrate in a mixture of acetone and ethanol for few minutes, followed by washing with distilled water and drying in a vacuum or N 2 flow. 43,47Subsequently, various polishing methods (mechanical, chemical, and electrochemical) may be employed to remove the surface contaminants and oxides formed by areal oxidation.Mechanical polishing techniques may include rubbing the metal surface with various abrasive papers, such as emery papers (grade 2400) or SiC sandpaper of various grades, such as P500, P600, P800, P1200, and P2500 etc., 30,33,25 or polishing with a cloth coated with a diamond paste (1, 0.5, 0.25, and 0.125 mm) or alumina suspension slurries (0.1−0.05 μm particle size). 26,43,47,69These steps are crucial for minimizing the roughness and achieving a mirror finish to the metal surface.The details of chemical and electrochemical polishing of Ta substrates are collected in Table 2 and 3, respectively.

ANODIZATION OF TANTALUM
The formation of protective or decorative layers of anodic metal oxides through anodization has been conducted for many decades. 15,19,20−20 The electrochemical processes leading to the formation of anodic metal oxide structures are initiated by applying a suitable voltage between the two electrodes.−20 A generalized pictorial representation showing the anodization configurations and the formation of ATO structures is presented in Figure 1.

Different Types of Morphologies of Anodic
Ta 2 O 5 .−73,75,76,87,88,93−95 The anodizing potential was maintained below the dielectric breakdown potential, typically up to 300 V, resulting in the formation of a uniform, compact, and amorphous ATO layer.In 2005, I. Sieber et al. reported, for the first time, the formation of porous ATO layer by anodizing Ta foil in a 1 M H 2 SO 4 electrolyte containing a small amount of HF (0.1−3 wt %). 43,47The anodizing potential was ramped from E ocp to 20 V at 10 mV s −1 , resulting in an observed ATO layer thickness of about 130 nm (Figure 2a). 43The presence of fluoride ions in the electrolyte was found to promote the formation of porous ATO structures.
They predicted that during anodization, fluoride ions move inward into the ATO structure about 1.85 times faster than oxygen ions, and the mobility of fluoride ions is independent of the film thickness.The same group observed also a thickening (32 to 130 nm) of porous ATO layer in F − ion-containing electrolytes compared to F − ion-free electrolytes. 43It was indicated that the morphology and thickness of the oxide layer depend strongly on the applied potential, scan rate, and HF concentration.
Similarly, ATO layers with nanotube morphology can be fabricated using F − ion-containing electrolytes. 53,96H. A. El-Sayed et al. 53 demonstrated the formation of nanodimple and nanotube morphology of ATOs in a stirred mixture of concentrated HF and H 2 SO 4 (Figure 2b,c).The stirring of electrolytes during anodization is considered an important condition to obtain a well-patterned surface structure; otherwise, a smooth surface may be obtained.Subsequent studies indicated the dependence of the H 2 SO 4 to HF ratio to achieve ordered nanoporous/nanotube structures of ATO. 91,97,98Details of the anodizing conditions, including the composition of the electrolyte, potential/current, time, and other factors influencing the formation of compact or nanoporous or nanotubular structures, can be found in Table 4.

Incorporation of Species from
Anodizing Electrolytes.In this section, we will delve into significant contributions from the 20 th century to illuminate the earlier understanding of the growth mechanism of anodic tantalum oxides as well as their morphologies and physical properties.The pioneering work of D. A. Vermilyea in 1953 70 marks the earliest exploration of anodic oxidation of Ta and growth of tantalum oxide at electrolyte/oxide.In this study, both theoretical and experimental attempts were made to understand the rate of growth of tantalum oxide films under various anodizing conditions.The relations among the rate of oxide growth, electric field, and temperature were established.Notably, a growth rate of 5.24 nm s −1 and 0.05 nm s −1 was observed at the anodizing current density of 10 mA cm −2 and 0.1 mA cm −2 , respectively, in 0.1 wt % Na 2 SO 4 at 0 °C.Furthermore, this investigation demonstrated variations in the oxide film thickness as a function of voltage and time at different anodizing currents.The study also successfully correlated the rate of oxide formation with heat generated at the oxide layer through the passage of current.X-ray and electron diffraction studies revealed that the oxide films formed in this study were amorphous Ta 2 O 5 with the capability of crystallization through annealing at temperatures at 500−800 °C in a vacuum.
In the following study, D. A. Vermilyea studied the growth of ATO films on both rough and polished tantalum surfaces. 71he effect of polishing the tantalum sheet on the surface morphology of ATO films was investigated by optical microscopy.In this study, specimens with deep scratches exhibited no evidence of oxide formation for an anodizing current density up to 2 mA cm −2 .However, at higher anodizing current densities, tantalum oxide was observed to grow on both scratched and polished tantalum substrates.This study, for the first time, demonstrated the migration of metal ions through the oxide layer during its growth.Subsequent efforts were made to understand the rate controlling step and the movement/migration of metal ions during the anodic oxide growth on Ta. 90,103 For example, B. Verkerk et al. 103 investigated the mechanism of tantalum oxide growth utilizing     perchlorate, sodium borofluoride, sodium orthosilicate, sodium hydroxide, sodium sulfate, ammonium sulfate, nickel sulfate, and sodium sulfite.Utilizing the secondary ion mass spectrometry (SIMS) technique, they analyzed the incorporation of electrolyte species into the oxide films during anodization.This study revealed no incorporation of Li + , Na + , K + , B, and N in the oxide layer.In addition, the researcher thoroughly investigated the amount of incorporated elements, such as sulfur, chlorine, and fluorine, as well as the depth of incorporation with variations in the concentration of the corresponding anodizing electrolytes.The study further demonstrated that the amount of incorporated sulfur and chlorine along with the relative depth of incorporation increased with an increase in the concentration of H 2 SO 4 and HCl.On the other hand, V. E. Borisenko et al. 66 studied the oxidation of tantalum by anodizing in 0.5 M H 3 PO 4 followed by thermal annealing.Using Rutherford back scattering (RBS) and SEM techniques, they explored the composition, thickness, and morphology of the oxidized tantalum.This study also disclosed the incorporation of phosphorus (1.6 to 1.8 atom %) into the formed ATO films.Further, J. M. Albella et al. 100 conducted Ta anodization in an H 3 PO 4 solution and demonstrated that phosphorus species incorporated into the oxide layer are in the form of PO 3− and BO − throughout the outer layer.However, for tungstate, molybdate, chromate, and vanadate electrolytes, the incorporated impurities (W, Mo, Cr, and V) or anion species were suggested to be present only in the outermost layer, with the thickness too small to be determined by SIMS.Utilizing the XPS technique, the authors revealed that the oxy-anions of various metals, e.g., B, W, Mo, V, and Cr, may dissociate to release corresponding cation species (B 3+ , W 6+ , Mo 6+ , V 5+ , and Cr 6+ ) which migrate outward at different rates.For the electrolytes containing halide ions, it was observed that Cl − , Br − , and I − ions may not be incorporated into the oxide structure, whereas F − ions can be incorporated into the growing oxide layer and migrate inward at a faster rate in comparison to that of the O 2− ions.As a result, they accumulate at the metal/oxide interface, forming a distinct layer of TaF 5 approximately 5 nm thick (Figure 3). 75e existence of this layer at the metal-oxide interface, reported for the first time, has significant implications, particularly in terms of diminished adhesion between the oxide film and the metal substrate.It was assumed that a higher ionic radius of Cl − , Br − , and I − ions when compared to oxygen ion is the main reason behind their nonincorporating behavior.Subsequently, advanced analytical techniques, such as highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS mapping, were employed to investigate this oxide doping effect.Figure 4 shows the HAADF-STEM and EDS mapping images of an ATO nanotube wall formed in a mixture H 2 SO 4 :HF (8:0.5 vol %) at 15 V, which clearly indicates a random distribution of F and S in the surface layer of the ATO nanotube. 49Recent studies by M. M. Momeni et al. 36,37 have corroborated the presence of F and S in ATO nanotubes prepared in an H 2 SO 4 + HF + H 2 O electrolyte.Additionally, similar findings regarding F incorporation were also observed in anodizing electrolytes of an aqueous−organic mixed type.L. Fialho et al., 38 through EDX and XPS studies, demonstrated the presence of F − ions in the ATO layer synthesized in the electrolyte based on EG + H 2 O + NH 4 F.
From a practical standpoint, it is anticipated that the incorporation of electrolyte species and/or impurities in the anodic layer can significantly modify its physical properties.For example, the incorporation of phosphate in titanium or tantalum oxide layer may alter the crystallization temperature of the amorphous anodic oxide layer, enhance the breakdown strength of the material, or can reduce the dielectric constant of the material. 81,85,99,104J. R. Sloppy et al. 85 showcased the superior properties of phosphate incorporated ATO layers for use as a dielectric material in electrolytic capacitors.Therefore, the impact of anodizing electrolyte composition on the impurity level in the ATOs and, consequently, their properties is a topic of discussion that warrants further attention.

Effect of Electrolyte Composition.
−73,75,76,87,88,93−95 The barrier ATO films are formed at both tantalum/oxide and oxide/electrolyte interfaces as a result of migration of O 2− /OH − and Ta 5+ ions across the oxide layer, assisted by a       nm thick forms at 10 V. 102 The incorporation of citrate anions in the ATO films was also evidenced from XPS analysis.This study further revealed that physical properties such as dielectric constant and interfacial capacitance values increase with the concentration of anodizing electrolyte (citric acid).M. V. Diamanti et al. showed a linear increase in thickness (45−180 nm) of ATO films with respect to preparation potentials (20− 100 V) in either 0.5 M H 2 SO 4 or 0.5 M H 3 PO 4 electrolyte. 67n this study, it was demonstrated that the memristive behavior of the ATO films changes with film thickness.It is thoroughly investigated that barrier-type oxide films on tantalum are formed in almost all electrolytes, except for those containing hydrofluoric/fluoride ions.In the early 21 st century, substantial efforts were directed toward producing nanostructures of anodic Ta 2 O 5 in electrolytes containing HF or NH 4 F. 43,53,92,96−98 As previously mentioned, the evidence of the structural and morphological evolution of ATO nanostructures dates back to 2005 when H 2 SO 4 containing a small amount of HF was employed as the anodizing electrolyte, with the anodizing potential intentionally kept well below the dielectric breakdown potential. 47A noticeable change in polarization behavior was observed in the presence of HF, which was characterized by higher current densities and the emergence of multiple broad peaks (Figure 5a).Furthermore, an increase in the concentration of HF up to a certain value (i.e., 1 and 2 wt %) resulted in a decrease in current densities, attributed to the formation of a new phase (TaF 5 ) or changes in the morphology (i.e., pore formation) or effective surface area of the electrode.The formation of porous oxide layers and variations in the pore diameter with changes in the anodization time were observed for the first time (Figure 5b and c).
Additionally, an increase in the thickness of the porous tantalum oxide layer with an increase in anodization time was observed.This study clearly indicated the role of HF in the formation of porous morphology and demonstrated that higher concentration (3 wt % or more) of HF can lead to partial or Table 4. continued    complete etching of porous oxide.This phenomenon may be attributed to an accelerated rate of oxide layer dissolution compared to the rate of oxide layer formation at the metal/ electrolyte interface.Later on, the same research group undertook further optimization of the HF content in the electrolyte, aiming to produce porous tantalum oxide films with highly regular pore arrays featuring about 20 nm pore diameters. 43,105−109 SEM images of the materials prepared in this study are presented in Figure 6, which once again indicates a pivotal role of HF concentration in the generation of the nanoporous structure. 105This study also sheds light on the influence of other anodizing parameters on the oxide morphology, namely extending the duration of anodization (from 1 to 4 h) increases both the pore diameter (from 5 to 35 nm) and thickness (from 160 to 800 nm) of the porous ATO layer.Subsequent studies illustrated well-defined control over the morphology and dimensions of tantalum oxides by optimizing the ratio of electrolyte components, specifically HF:H 2 O:H 2 SO 4 .J. E. Barton et al. 42 conducted an intensive study on the impact of the HF:H 2 O ratio and applied potential on the morphology of anodic Ta 2 O 5 nanotubes encompassing such parameters as the inner diameter, outer diameter, and wall thickness (Figure 7). 42−109 As mentioned earlier, when anodization starts, a compact oxide layer of TaO x forms on the surface of tantalum. 29,53,91As the anodization progresses, the formation of pores occurs due to both phenomena, dissolution of oxide at the oxide/electrolyte interface and generation of stress at the metal/oxide interface.However, the first of these factors is more important in the initial formation of pores, 110,111 and resulting Ta ions (Ta 5+ ) appearing at the oxide/electrolyte interface, are soluble by forming [TaF 7 ] 2− . 29,109This process creates pores on the surface of the metal oxide films.It is hypothesized that the complex [TaF 7 ] 2− , having a lower charge-to-size ratio compared to the F − ions, can easily be replaced by the electro-migrated F − ions from the bulk solution. 91,105−109 Thus, a new interface between oxide pores and electrolyte (specifically F − ions) forms, and the above process repeats in a continuous manner during the entire length of anodization, potentially deepening the pores.The growth rate of nanoporous/nanotube structures may be determined by the diffusion rate of the [TaF 7 ] 2− complex and F − ions. 53,91It is predicted that the nanoporosity could be related to the molecular dimension of [TaF 7 ] 2− and the formation kinetics of Ta 2 O 5 .A detailed analysis of various hypotheses for the formation of nanoporous/nanotube ATO structures and the proposed reaction mechanisms is discussed in Section 5 of this review.
Further investigations into the variation of the composition of inorganic anodizing electrolytes include the addition of small quantities of organic solvents/electrolytes to the parent electrolyte or the creation of mixed inorganic−organic type electrolytes. 24,26,31,37,48,57  Optimizing the anodization conditions, they demonstrated that the addition of 5 vol % ethylene glycol and the application of anodizing potentials of 10, 12, and 15 V are suitable for production of highly ordered ATO nanotubes (Figure 8a−c).They produced nanotube membranes up to ∼19 μm in length and demonstrated that the addition of DMSO resulted in the formation of nanotubes of larger length compared to those formed in EG containing solutions (Figure 8d).
Similarly, nonaqueous anodization electrolytes, such as ethylene glycol (EG)/glycerol containing a small amount of NH 4 F, have been successfully formulated for the fabrication of ATO nanostructures. 37,40Moreover, numerous studies have further incorporated a small amount of H 2 O into the anodizing electrolyte containing EG/diethylene glycol (DEG)/glycerol + NH 4 F and examined the effect of water content on the morphology of anodic Ta 2 O 5 layers. 21,26,44,62W. Wei et al. 62 demonstrated the formation of nanoporous Ta 2 O 5 layers by anodization of Ta in electrolytes composed of glycerol and NH 4 F (0.1−0.5 M).In this study, they highlighted the crucial role of the NH 4 F concentration and applied potential on the pore diameter and thickness of the anodic layers (Figure 9b, c).The study also revealed an increasing anodization current density with an increase in the concentration of NH 4 F in the electrolyte (Figure 9a).These results may suggest a higher growth rate of anodic structures in electrolytes containing higher NH 4 F concentrations.However, the volcano-shaped relationship between the concentration of NH 4 F and layer thickness suggests the possibility of dissolution of the anodic layer at high concentrations of NH 4 F (above 0.2 M).Similarly, for 0.2 M NH 4 F in EG as an electrolyte, S. Xia et al. 40 observed a nanoporous ATO layer at anodizing potential of 15 V (Figure 9d).L. Fialho et al. 38   revealed a gradual decrease in the diameter of NTs from 143 to 90 nm with an increase in the electrolyte temperature.K. Lee  et al. showed the relationship between anodizing potential and electrolyte temperature for the formation of highly ordered nanoporous ATO films. 48For 1 h of anodization at 20 V (10 wt % K 2 HPO 4 containing anhydrous glycerol), it is demonstrated that an electrolyte temperature of 180 °C is suitable for the production of ordered nanoporous ATO films with a pore diameter of ∼25 nm.It is further illustrated that higher (200 °C) or lower (150 °C) electrolyte temperatures may promote irregularity in the pores.
4.4.Effect of F − and Water Content.−98 However, the influence of H 2 O addition to nonaqueous electrolytes or the variation of the F − ions to H 2 O ratio in both aqueous and nonaqueous electrolytes can prompt a new growth model for ATO structures and therefore needs separate discussion. 49,58,62n the case of concentrated mixtures of H 2 SO 4 (8 vol %) + HF (0.125−0.5 vol %), it has been observed that the anodization current density decreases as the HF concentration decreases to 0.125 vol % (Figure 10a). 49It has been previously noted that F − ions can attack and dissolve the anodic oxide layer through pitting corrosion.Therefore, the decrease in the HF concentration can be associated with a decrease in the dissolution rate of the ATO layer.Consequently, a decrease in the F − concentration and an increase in the H 2 O content can help stabilize the anodic oxide by decreasing the dissolution rate.Therefore, nanostructures (nanoporous/nanotubular) of ATO cannot form on the initial compact oxide layers unless the H 2 O/F − ion ratio is adequately balanced in the electrolyte.M. A. A. Talip et al. and W. Wei et al. 58,62 systematically explored the influence of the H 2 O content on the morphology of ATO layers.It is observed that the addition of H 2 O (1−5 mL) to 200 mL of glycerol +0.2 M NH 4 F resulted in a rapid decay of the anodizing current density from the initial value (Figure 10b).SEM image studies demonstrated that the addition of water to glycerol + NH 4 F-based electrolytes promotes the formation of a compact or inhomogeneous porous oxide layer (Figure 10c).M. A. A. Talip et al. 58 varied the H 2 O content (0, 2, 4, and 6 vol %) in the anodizing electrolyte (EG containing 1.35 wt % NH 4 F and 0.25 vol % of H 2 SO 4 ) and demonstrated that no nanostructures could be produced without H 2 O.In contrast, nanotubular structures could be formed in the electrolyte containing 2 and 4 vol % of H 2 O. Hence, these studies confirmed that an optimum amount of H 2 O in both aqueous and nonaqueous electrolytes may help promote the nanostructural ATO morphology by manipulating the dissolution rate.

Effect of Anodization Potential, Current Density, and Time.
3][14][15][16][17][18]28 M. Fernańdez demonstrated a linear increase in voltage during a constant current (0.04 A) anodization of the Ta electrode in 0.01−10% H 3 PO 4 or 0.01 H 3 PO 4 + 0.1−0.15% citri acid solutions.94 They observed a gradual increment in voltage over time until an oscillation appeared in the voltage range of 100−200 V.The oscillations in the V vs t curve indicated breakdown events, and the voltage at which the oscillations start is known as the scintillation voltage.These oscillations suggest poor electrical conductivity or high resistivity of the oxide layer.Many earlier studies on Ta anodization demonstrated minimal effects of anodization potential/current and time on the morphology of anodic tantalum oxide layer when anodized in one component electrolytes, such as Na 2 SO 4 , H 2 SO 4 , H 3 PO 4 , chromic acid, citric acid etc. 41,52,61,63,66,70−73,75,76,79,87−89,94−96,101 However, studies in the early 21 st century revealed a profound effect of anodization potential/current and time on the morphology of anodic tantalum oxide layer when formed in fluoridecontaining electrolytes.33 −40 ,43 ,53 ,92 ,96 −98 In this section, we summarize some notable investigations in these directions.
T. Wen et al. 74 conducted a study on the anodic growth of Ta 2 O 5 nanotubes under the constant anodization current (75− 175 mA cm −2 ) in a H 2 SO 4 (90−95 vol %) + HF (5−10 vol %) electrolyte.This research revealed an increase in the length and diameter of nanotubes with an increase in the anodizing current up to 150 mA cm −2 .It was specifically observed that only the length of nanotubes is directly proportional to the anodizing time and the growth rate of nanotubes decreases with an increase in time or HF concentration.A growth rate of 9.0 μm min −1 was observed for the first 2 min at 125 mA cm −2 in an electrolyte containing 5 vol % HF (Figure 11a Similar to the anodizing current, the anodizing voltage also plays an essential role in the evolution of the nanostructural morphology of anodic ATO (Figure 12a).W. Wei et al. 62 observed a volcano-shaped relationship between the thickness of the porous ATO layers and the applied potential (10−50 V) in glycerol containing 0.2 M NH 4 F (anodization time of 3 h).Initially, the thickness of the porous oxide film increases steeply from about 5.9 μm at 10 V to 15.7 μm at 20 V and then decreases with an increase in the anodizing potential beyond 20 V (Figure 12b).In this study, the pore diameter of the ATOs was demonstrated to depend linearly on the applied potential.Hence, it was predicted that high anodizing voltage (above 20 V) can speed up the dissolution or etching process, which can directly affect the thickness of the layer and pore diameter of ATO.Moreover, anodization voltage can also regulate the interpore distances in the nanopore/nanotube ATO films. 42,49,109In H 2 SO 4 containing 0.25 vol % HF and 4.3 vol % H 2 O, J. E. Barton et al. 42 demonstrated that the spacing between ATO nanotubes is directly proportional to the anodization voltage, with a proportionality constant of nearly 0.0025 μm/V for the applied voltage range of 10 to 90 V.This study further demonstrated that the NTs produced at 10 V and at higher voltages (40−90 V) were disordered with variations in pore diameter along the nanotubes. 42For example, the outer diameters varied from ∼30 nm at the ends to ∼20 nm in the middle for the NTs produced at 10 V. W. Chen et al. 45 fabricated three types of ATO films (free-standing ATO nanotube membranes, highly ordered amorphous adhered ATO nanotube arrays, and spark discharge films of ATO crystal phase) in the same electrolyte composed of concentrated H 2 SO 4 and HF.This study further confirmed an increase in the diameter of ATO nanotubes with an increase in anodization voltage (Figure 12c).L. Fialho et al. 38 synthesized nanoporous ATO layers at 40 and 60 V in ethylene glycol containing 3 vol % H 2 O and 0.6−2.4wt % NH 4 F.They observed randomly ordered porous layers of ATOs at both anodizing potentials.It is assumed that an imbalance between the rate of oxide formation and dissolution is the reason behind the formation of disordered porous structures.
However, the debate on conditions for the separation of nanopores and its transformation to nanotubular ATO films can be clearly understood from the work of C. F. A. Alves et al. 49 As demonstrated in Section 4.2, the EDX elemental mapping of the single free-standing ATO NT showed the presence of S and F with a high F/S atomic ratio (1.8) on the NT wall (Figure 4).From this, it is hypothesized that F − ions, having a higher migration velocity compared to that of O 2− , can migrate from the outer part of the oxide layer to the inner part, creating a wall of separation between pores and thus being responsible for the separation of pores.Thereafter, the degree of solubility of the [TaF 7 ] 2− complexes present on these walls, and their diffusion in an electrolyte containing optimized amount of H 2 O, may influence the nanotube morphology.This study demonstrated the formation of free-standing ATO NTs with a diameter of ∼45 nm at 15 V in a H 2 SO 4 electrolyte containing 0.5:1.5 vol % of HF:H 2 O.Moreover, efforts have also been made to understand the effect of anodizing voltage and time on ATO structures in fluoride free electrolytes.K. Lee et al. 48synthesized porous ATOs in a F − ion free nonaqueous electrolyte containing glycerol and 10 wt % K 2 HPO 4 at 180 °C.This study showed the formation of highly ordered self-organized porous structures (pore size of ∼25 nm) at the anodization potential of 20 V and irregular porous structures at 2 and 50 V.However, the reason behind the observed anomaly in the morphology with the change in applied potential was not properly described in this study.From the above discussions, it can be concluded that the anodizing voltage/current and time are equally important parameters as the composition of the electrolyte.We anticipate plenty of opportunities to generate new kind nano/micro structures of ATOs simply by controlling these parameters.More details on the effect of anodizing potential/current density and time on the morphology can be found in Table 4.

Problems with Adhesion of Anodic Layers to Substrates.
The synthesized ATO layers can undergo further processing depending on their utility or the mode of application.28,53,66,72,93 This poses a significant challenge for their direct application in many sectors.It is evident that very long NTs of few μm in length carrying high mass pose a risk of detachment from the metal substrate.
Many studies have indicated that the incorporation of fluoride ions in the anodic films is the major reason behind the detachment of the film. 27,28,53,92,97As discussed earlier, the inward mobility of fluoride ions is almost twice to that of O 2− ions, leading to the formation of a thin layer of fluoride and/or oxyfluoride species (TaF 5 , TaO 2 F and TaOF 3 ) between the anodic oxide film and tantalum substrate. 97Alternatively, it can be said that a gradual build-up process of fluoride matrix at the Ta/Ta 2 O 5 interface occurs throughout the anodization process. 53,57,92These species are predicted to be fairly soluble in water and can therefore be easily detached by washing or during ultrasonication treatments (Figure 13).Some studies have even reported that ATO films can be detached completely with minimal handling. 53,66,72,92It is noteworthy that in the case of other anodic structures, such as TiO 2 and WO 3 , the films are difficult to detach due to differences in the solubility of the oxyfluoride−metal complexes for these samples. 97owever, in the previous decade, many efforts were made to synthesize ATO NTs that are well adhered to the Ta substrate. 27,51M. A. Baluk et al. 51 studied the crucial role of anodization potential, time and post annealing effects on the adhesion properties of ATO NTs prepared in F − ioncontaining H 2 SO 4 solutions.It was demonstrated that ATO NTs (length of 1.68−2.19μm) formed at 15 V for 5 min were uniform and strongly adhered to the Ta substrate.Moreover, the morphology and adhesion of the NTs seemed to be unaffected at an annealing temperature up to 450 °C in a nitrogen atmosphere.On the other hand, NTs produced at 20 V for 10 min possessed weak adhesion properties and can be easily detached from the surface.Most importantly, they demonstrated that an annealing temperature of 600−700 °C can defragment the NT arrays and the base Ta foil.R. V. Goncalves et al. 27 explored the crucial role of electrolyte temperature on the adhesion of Ta 2 O 5 NTs.They systemati-cally investigated the effect of electrolyte (H 2 SO 4 + 1 vol % HF + 4 vol % H 2 O) temperature on the adherence properties of ATO NTs and demonstrated that ATO NTs produced at 0, 10, and 20 °C were well adhered to the substrate.Meanwhile, the ATO NTs produced at above 30 °C started to detach from the Ta substrate or could be easily removed by an adhesive tape.

VARIOUS HYPOTHESES LINKED TO ANODIZATION AND ANODIZATION OF TANTALUM
−105 As discussed earlier, anodizing conditions like appropriate F − ions containing electrolytes, voltage range, and anodizing time are necessary to shape the metal oxide layer into a nanotube/nanopore geometry, and this can be correlated to an equilibrium condition between the oxide formation and its dissolution.In this section, we will discuss all possible growth mechanisms of ATO in electrolytes containing F − ions or free of F − ions.

Field-Assisted Model for the Porous and Tubular ATO Growth.
The most important information regarding the growth mechanism of tantalum oxide can be understood from the current−time (i−t) plot for the constant potential anodization, and from the potential-time plot for galvanostatic anodization (Figure 14a,b). 38,49,74Typically, three different stages of the ATO growth can be identified in the i−t curve recorded during anodization in an electrolyte containing fluoride ions (Figure 14a). 38An exponential decay in current/voltage at stage (i) is proposed to be associated with the formation of a resistive compact oxide layer through Ta hydrolysis at the metal/electrolyte interface (Figure 14a, c,  stage I).This phenomenon is expressed in eq 1.Thereafter, the anodic current reaches a minimum value, believed to be the onset of electric-field-enhanced dissolution of the oxide film through fluoride ion etching.These processes may lead to the initiation of small pores on the oxide layer.From this point onward, ions present in the electrolyte (OH − and F − ) and in the oxide layer (Ta 5+ and O 2− ) can interact and move through the barrier-type oxide layer.This results in the continuation of the oxidation process with an increase in the anodic current.At this stage, porosity is thought to be induced by the acidic environment containing F − ions (Figure 14a and c, stage II).As the pores extend, the surface area consecutively increases, potentially enhancing the rate of ATO dissolution (eq 2).Finally, a steady state of the anodization/oxidation current can be maintained thereafter (Figure 14a, stage III).This may indicate an equilibrium between the oxide formation and dissolution reactions: According to L. Fialho et al., 38 there are two possibilities in the third stage.For the first one, a decrease in the anodization current density over time is observed.This can result in the formation of stable nanotube/pore structures (Figure 14a, stage III-ii).Alternatively, a steady-state anodic current or a very slow increase in the current may occur.hese researchers put forth a plausible explanation for the growth of ATOs in the H 2 SO 4 + HF electrolyte by analyzing HAADF-STEM images and STEM-EDS compositional maps of the dimple and pore structures.Their proposal involves a chemical dissolution of Ta 2 O 5 at the oxides/electrolyte interface, which is enriched with F − ions, by fluoride/sulfate ions.It is considered that a small ionic radius of F − allows to penetrate the growing Ta 2 O 5 lattice and thus be transported through the oxide layer by the applied field.In accordance with eq 1, the concentration of H + ions may increase in the vicinity of the oxide layer with a nanodimple morphology (Figure 14c, (ii).To maintain electroneutrality, F − ions are hypothesized to migrate to the H + sites, potentially competing for the position of the O 2− in the oxide lattice.Subsequently, the dissolution of Ta 2 O 5 is initiated through the formation of the [TaF 7 ] 2− complex, as described by eq 2, leading to the creation of the porous layer structure (Figure 14c, (iii).The dissolution of Ta generates negatively charged cation vacancies at the oxide surface, potentially enhancing the reaction described in eq 1.Consequently, Ta 5+ ions can easily migrate to nearby vacancy sites, leading to deeper pore formation.The transition from a porous to a tubular structure can be visually represented in Figure 14c, (iii), and Figure 15.The fluoride-rich layer that separates the pores is susceptible to dissolution in water, giving rise to a nanotubular morphology. 15,49,59Therefore, the interplay between the rate of electric field-assisted metal oxide formation in a fluoride containing electrolyte and its chemical etching favors the development of the nanoporous/ nanotubular structure.Moreover, it is proposed that the bottom of the pores/tubes are more acidified compared to their top. 49,15As a result, the rate of oxide dissolution at the bottom of pores/tubes is higher compared to that of pores/ tubes at the top, which contributes to an increase in the length of the pore/tubular structure.Consequently, it can be inferred that the length and diameter of nanopores/nanotubes are dependent on parameters such as the concentration of fluoride ions, applied potential/current density, and the duration of the process.

POST-TREATMENT OF ANODIC FILMS AND THEIR CHARACTERIZATION
The freshly prepared anodic tantalum oxide films on Ta are often amorphous.Therefore, when examining the freshly prepared ATOs, the XRD pattern of the Ta substrate (JCPDS,  No. 89−5158) is observed. 112,113Additionally, as discussed in earlier sections, the anodic structures may contain a significant amount of impurities from the electrolytes, e.g., F, S, P, etc. [36][37][38][39]69,75 Numerous studies have demonstrated that thermal treatment plays an important role in the elimination of the impurity content, and enhances the crystallinity of the nanostructures, which can ultimately alter their physiochemical properties.27,28 R. V. Goncalves et al. 28 systematically investigated the crystallization process of free-standing ATO NTs by annealing them at 750, 800, and 900 °C in air for 0.5 or 1 h. They obsrved the orthorhombic β-Ta 2 O 5 phase for all of the annealed NT samples with minimal impact on the nanotubular morphology (Figure 16a,b).However, slight distortion in the nanotube structure was observed for samples treated at 800 °C for 1 h.It was further indicated that annealing above 800 °C may cause significant deformation in NT walls.XPS studies revealed the complete removal of S and F in the heat treated Ta 2 O 5 NTs compared to the freshly prepared NT samples (6 and 5 at.% of S and F) (Figure 16c). An inrease in the O/Ta ratio was also observed in all of the annealed NTs compared to the untreated ones.However, another study of Ta 2 O 5 nanotubes prepared in NH 4 F containing electrolytes showed a significant amount of fluorine (4.15−6.36at. %)in the nanostructures even after thermal treatment at 450 °C for 1 h in air.44 Through XPS studies, S. Singh et al. 97 demonstrated that the fluoride content in freestanding anodic Ta 2 O 5 layers can be reduced simply by washing them with water for a few seconds, as the tantalum oxyfluoride layer present in the outer layer of the pores/tubes is soluble in water.Furthermore, various studies have highlighted that the crystallization temperature and thermal stability of anodic tantalum oxide strongly depend on the preparation conditions.It is anticipated that the introduction of dopants or impurities from the electrolyte can induce changes in the crystallization temperature and thermal stability of the anodic nanostructures.50 For example, anodic tantalum oxide nanostructures prepared in H 2 SO 4 + HF electrolytes with additives, such as H 3 PO 4 , ethylene glycol, or dimethyl sulfoxide exhibited crystallization temperatures in the range of 300−550 °C.57 In contrast, free-standing NTs prepared in H 2 SO 4 + HF electrolytes with additional additives showed a crystallization temperature above 750 °C.27,42 Similarly, several studies have investigated the impact of annealing temperature, and time on the crystallinity of anodic structures.36,37,40,59 M. M. Momeni et al. 36,37 demonstrated that anodic tantalum oxide nanotubes obtained after 2 h of annealing in air at 400 °C are partially crystalline, displaying some XRD diffraction patterns of the Ta 2 O 5 phase.M.A.A. Talip et al. 59 observed XRD diffraction patterns of the orthorhombic Ta 2 O 5 (JCPDS, No. 25−0922) for the nanotube samples annealed at 500 °C for 2 h in air.H. Yu et al. 26 investigated the effect of annealing on the crystallinity of coral-like anodic tantalum oxide in air (550 °C, 3 h), observing diffraction peaks of β-Ta 2 O 5 with an orthorhombic phase (JCPDS, No. 89−2843) in this case.Using Scherrer's equation, they estimated the average crystallite size to be around 27.6 nm.It is noteworthy that high-temperature treatment above 1000 °C can induce a transition from β-Ta 2 O 5 to α-Ta 2 O 5 phase.86,114 In addition, post-treatment methods such as annealing ATOs in H 2 S and NH 3 gas were performed to widen their applicability.21,22,40 P. Li et al. demonstrated the partial and complete conversion of ATO nanotube films to TaS 2 nanotubes by annealing in H 2 S atmosphere at 625 °C.22 The XRD and electron diffraction studies revealed complete conversion to the TaS 2 phase when treated under these conditions for 24 h.The TaS 2 nanotubes produced in this study exhibited a higher superconductivity compared to bulk TaS 2 .S. Banerjee et al. synthesized TaON nanotubes by treating ATO nanotubes at 700 °C for 6 h in an NH 3 atmosphere.The formation of the TaON phase was revealed through XRD and electron diffraction studies.21 In this case, the UV−vis absorption properties and band gap energy (2.07 eV) of TaON nanotubes were found to be more favorable for solar water splitting compared to untreated ATO nanotubes.Under AM 1.5 conditions in a 1 M KOH solution, it is demonstrated that TaON nanotubes generated a photocurrent density of 2.6 mA cm −2 at 0.5 V (vs Ag/AgCl), which was higher compared to ATOs (0.25 mA cm −2 ).Moreover, the photocatalytic activity of the TaON nanotubes was found to be stable for up to 2 h.Similarly, S. Xia et al. annealed nanoporous ATO films in an argon or air atmosphere for 2 h at 350, 450, and 550 °C.40 This study demonstrated that thermal annealing of anodic Ta 2 O 5 in an inert atmosphere can lead to an oxygen deficiency in the metal oxide layer, which may be beneficial for Li-ion storage.The Ta 2 O 5 films produced under post-treatment conditions in an argon atmosphere at 450 °C demonstrated a high lithium capacity of ∼480 mAh g −1 with excellent cycling stability (up to 8000 cycles) compared to Ta 2 O 5 films treated under similar conditions in an air atmosphere.They hypothesized that a high level of oxygen defects with an amorphous nature is beneficial in this case.These results underscore the significant role of thermal treatment conditions in shaping the crystalline phases of anodic Ta 2 O 5 , with additional effects on the morphology and elemental composition.Details of the annealing conditions and  their effects on the crystalline phases of anodic tantalum oxide can be found in Table 5.
Apart from this, post-treatment also includes decoration of secondary nanoparticles of metals, metal oxides, metal hydroxides, and metal sulfides (e.g., Fe 2 O 3 , Mn 3 O 4 , NiO, Co(OH) x , Bi 2 S 3 , and Au) on ATO or its derived materials by various multistep methods. 24,34,35,44,51,114The chronology of the fabrication of ATO structures and their post-treatment conditions are presented in Scheme 1 for a better understanding of these methods.Furthermore, some notable examples of these strategies to enhance their performance are discussed in the next section.

APPLICATIONS OF ANODIC TANTALUM OXIDES
−115 In this section, we summarize some notable physiochemical properties and applications of ATOs or their derived materials.
7.1.Capacitors and Memristive Devices.Capacitors based on the Ta or Ta 2 O 5 have been extensively investigated and find applications in many electronic devices including mobile phones, computers, and space equipment, etc. 87,93,94,115 M. A. Mohammed et al. 94 studied the capacitance−voltage characteristics of Ta 2 O 5 films synthesized by multiple methods, such as anodization, thermal annealing, and ion-implantationbased enhanced thermal oxidation.It was demonstrated that the room temperature capacitance of anodic Ta 2 O 5 films increased with applied bias after annealing the sample at 500 °C.The enhanced capacitance behavior was attributed to a structural transformation of Ta 2 O 5 from an amorphous to a partially polycrystalline state.More recently, a Ta 2 O 5 /Fe 2 O 3 composite, based on 3D nanotube arrays, has been explored as an electrode material for supercapacitor application. 115In this case, Fe 2 O 3 nanoparticles were anchored on the walls of Ta 2 O 5 nanotubes using a wet chemical deposition method, and the structural arrangement was believed to efficiently relieve strain during the cycles.The Ta 2 O 5 /Fe 2 O 3 nanocomposite arrays exhibited excellent capacitive characteristics with remarkable stability over 10000 cycles (Figure 17).
In addition to this, efforts have been dedicated to explore the memristive behavior of anodic Ta 2 O 5 films. 11,67,80M. Diamanti et al. 67 conducted a systematic investigation, comparing the memristive properties of anodic Ti, Ta, and Nb oxide films formed in phosphoric acid at different potentials (10, 20, 25, and 30 V).The oxide film thickness was controlled in the range 30−200 nm.It was observed that the thickness of the oxide films plays a major role in the memristive switching properties.The synthesized Nb 2 O 5 , TiO 2 , and Ta 2 O 5 oxide films with thicknesses in the range of a few tens of nanometers, demonstrated superior memristive performance compared to thicker films.Similarly, I. Zrinski et al. 80 studied memristive properties of the Hf−Ta system, wherein a 15 nm thick anodic oxide layer was obtained by anodization of the sputter deposited Ta−Hf layer in a citrate buffer solution.

Catalytic Activity and Degradation of Chemical Substances.
The n-type semiconductor properties of nanostructured ATOs, with a band gap in the range of 3.8−4.5 eV, allow it to generate electron−hole pairs upon exposure to photons with energies equals to or higher than its band gap. 21,26,27,36Consequently, this material can serve as a catalyst for various photochemical and photoelectrochemical reactions.M. M. Momeni et al. 36 demonstrated a photocatalytic degradation of methylene blue (MB) on anodic tantalum oxide nanotube films formed under varying anodizing conditions.The high photocatalytic activity observed for the nanotube films (2 μm thick) is attributed to the large surface area and semiconducting properties of the material, believed to enhance the adsorption efficiency of light and facilitate high diffusion rates of reactants and products.In another study, A. Merenda et al. 84 designed sub-10 nm surface pore size titanium−tantalum oxide substrates by electrochemical oxidation of a Ta−Ti alloy matrix.They further adjusted the band gap of the material by altering the Ta/Ti surface ratio, demonstrating that a composite with a minimum band gap exhibits higher activity in the degradation of MB under visible light compared to pure TiO 2 (Figure 18a).
H. Yu et al. 26 developed nanoporous tantalum oxide films with a coral-like morphology for the photocatalytic decomposition of phenol.In this study, the material with a band gap of 3.8 eV and pore diameter of 30−50 nm showed high efficiency in the photocatalytic decomposition of phenol compared to a compact oxide layer (Figure 18b).In another work, ATO nanotubes decorated with Bi 2 S 3 quantum dots (QDs), with external and internal nanotube diameters of 41 and 19 nm, o and an oxide thickness of 1.27 μm, demonstrated a high photocatalytic activity toward toluene degradation.About 99% of toluene (200 ppm) was degraded after 5 min of UV−Vis irradiation. 51oreover, ATOs have also exhibited catalytic activity for gaseous phase reactions.R.V. Goncalves et al. 28 compared the catalytic CO oxidation activity of commercial Ta 2 O 5 with freestanding ATO nanotubes treated under various annealing conditions (Figure 18c).In this study, thermally treated ATO nanotubes showed higher activity compared to amorphous and commercial Ta 2 O 5 .NTs.The study demonstrated that high crystallinity, high surface area, and low surface contamination in the ATO NTs can enhance their photocatalytic activity for hydrogen production.Specifically, ATO NTs treated at 800 °C for 1 h in air exhibited 1.6 times higher activity compared to ATO NTs treated at 800 °C for 0.5 h.The same research group further developed a heterogeneous structure of anodic ATO NTs decorated with p-type NiO nanoparticles as a photocatalyst for hydrogen evolution from a water−ethanol solution. 34Ni nanoparticles were deposited onto the ATO NTs by direct-current (dc) magnetron sputtering in a vacuum.The duration of sputtering was varied from 5 to 180 s to increase the Ni loading from 0.01 to 0.47 wt %.After deposition, the heterostructures were calcined in air for 2 h at 500 °C to convert the deposited Ni to its oxide form (NiO).In this case, the ATO NTs loaded with 0.16 wt % of NiO showed higher hydrogen evolution activity (up to 7.7 ± 0.3 mmol h −1 g −1 ) compared to pure ATO NTs (4.9 ± 0.3 mmol h −1 g −1 ) (Figure 19a).It is suggested that the electrons from the conduction band of NiO can migrate to Ta 2 O 5 during water reduction, and photogenerated holes from Ta 2 O 5 can move to the valence band of NiO to assist the ethanol oxidation reaction.−25 For example, S. Grigorescu et al. 24 converted ATO NTs into Ta  RHE compared to Ta 3 N 5 /Co−Pi and Ta 3 N 5 /Co(OH) x photoelectrodes (Figure 19b).Z. Su at al. 25 also investigated PEC water splitting activity of Ta 3 N 5 /Co−Pi and Ta 3 N 5 / Co(OH) x photoelectrodes.They observed remarkable photocurrents of 5.9 mA cm −2 at 1.23 V RHE and 12.9 mA cm −2 at 1.59 V RHE for Co−Pi/Ta 3 N 5 .Additionally, they synthesized Ba-doped Ta 3 N 5 and studied the PEC water splitting activity of Co−Pi/Ba−Ta 3 N 5 , which showed an enhanced photocurrent density of 7.5 mA cm −2 at 1.23 V RHE (Figure 19c).Beyond nanotube structures, Y. Li et al. 86 successfully synthesized Co− Pi modified nanorod structures of Ta 3 N 5 and investigated their PEC water splitting activity in a 0.5 M K 2 HPO 4 electrolyte.This material achieved a maximum solar conversion efficiency of 0.36% and a photocurrent density of 1.5 mA cm −2 at 1 V versus RHE under an AM of 1.5 G.

Anticorrosion Coatings.
Tantalum or tantalumbased materials are recognized for their stability under acidic conditions, making them excellent candidates for anticorrosion coatings. 31,44,83J. Hou et al. 83 investigated corrosion-resistance behavior of a 150 nm thick Ta 2 O 5 coating produced by anodic oxidation of a Ta/Mo matrix.In a 3.5% NaCl solution, the anodic Ta 2 O 5 /Ta/Mo matrix displayed a higher corrosion potential (E corr = 0.140 V) and a lower corrosion current density (j corr = 0.009 A cm −2 ) compared to that of the pure Ta/Mo sample (Figure 20a).SEM studies indicated an improvement in the compactness of the Ta 2 O 5 coating on the Ta/Mo matrix, which is believed to be responsible for enhanced resistance against Cl − ions.More recently, A. Bordbar-Khiabani et al. 44 designed Mn 3 O 4 NPs coated ATO films for the corrosion resistance in a biochemical environment (i.e., simulated inflammatory conditions around metallic implants in the human body). 44In this case, the Mn 3 O 4 NPs were coated through the electrophoretic deposition (PED) method, utilizing anodized-tantalum as the cathode.The TEM studies confirmed the deposition of spherically shaped Mn 3 O 4 NPs on the ATO substrate.They observed improved corrosion resistance activity of Mn 3 O 4 NPs coated films with a positive shift in corrosion potential (E corr ) compared to that of anodized and polished Ta (Figure 20b).7.5.Medical Applications.As mentioned earlier, tantalum has the ability to form a spontaneous passive oxide film on the surface, providing protection against harsh acidic and biological environments. 31,46,83Therefore, tantalum, tantalum oxide, or its alloys with hierarchical micro/nanostructured surfaces hold great potential for orthopedic and dental implant applications.For example, J. W. Ma et al. 46 designed an anodic tantalum surface with a nanodimpled morphology.In this study, the dimple diameter ranged from 40 to 180 nm, and the dimples exhibited a hydrophilic nature.The researchers observed an increase in the dimple diameter with an increase in the anodization potential in H 2 SO 4 + HF electrolytes.In the in vitro studies, they demonstrated a significant promotion in the adhesion, proliferation, and differentiation of MC-3T3-E1 cells on nanodimpled structures compared to a pure Ta surface.It was further shown that the Ta surface with a small dimple size (40 nm) is more beneficial for cell proliferation and osteogenic differentiation compared to the surface with a larger dimple size (Figure 21a).Recently, L. Fialho et al. 33 implemented strategies for incorporating osteoconductive elements, such as Ca, P, and Mg into the surface of anodic tantalum oxide layers, aiming to enhance the bioactivity and bone implant applications.They designed a bone-like structure of porous tantalum oxide using an anodic oxidation process.The surface oxide layer was doped with Ca, P, and Mg through a cathodic polarization or anodization method in an electrolyte containing Ca, P, and Mg precursors.Contact angle studies revealed improved hydrophilicity for both doped and undoped anodic tantalum oxide structures compared to the pristine or polished Ta substrate (Figure 21b).Additionally, the doped anodic tantalum oxide structures showed higher surface roughness compared to that of undoped anodic tantalum oxide.The researchers speculated that the modified surface properties of these materials could promote osseointegration processes and remineralization of dental implants.

CONCLUSIONS AND OUTLOOKS
In this comprehensive review, we delve into the recent advancements in the synthesis and applications of anodic tantalum oxide-based materials.The anodization process has emerged as a versatile method for fabricating tantalum oxides with diverse morphologies, including a compact oxide layer and nanoporous, nanotubular, nanocoral, and nanodimple structures.We meticulously present and discuss the anodization conditions governing the fabrication of these oxide forms, shedding light on the intricacies of the process.Notably, we emphasize various anodization parameters crucial for controlling the morphology of nanoporous/nanotubular structures, such as pore size and tube length, and we explore the physical characterization methods used for detailed analyses.
Our review extends to the elucidation of the growth mechanism underlying anodic nanostructures, providing insights into the chemical processes that dictate their formation.By summarizing the latest research findings, we contribute to advancing the understanding of the chemistry governing the synthesis of anodic tantalum oxide nanostructures.Additionally, we delve into the significance of the annealing method as a means to enhance the physiochemical properties of anodic tantalum oxides.This section provides a comprehensive overview of the strategies employed for annealing, offering valuable insights into optimizing the properties of these materials.
Furthermore, our review encompasses the modifications applied to anodic tantalum oxides, including surface modifications and/or doping with various metals, nonmetals, and metal oxides.Notably, surface modifications have emerged as a promising strategy to enhance photochemical and photoelectrochemical activities of anodic tantalum oxides.We thoroughly discuss the methods and outcomes of such modifications, highlighting their impact on the performance of tantalum oxide-based materials.As we navigate through these discussions, it becomes evident that surface modification holds tremendous potential for elevating the functionalities of anodic tantalum oxides.The nuanced exploration of the interplay between surface modifications and the resulting improvements in photochemical and photoelectrochemical activities underscores the significance of this approach.
In our closing remarks, we posit that future investigations in two key directions will be pivotal.First, we advocate for a concerted focus on further exploration and refinement of surface modifications for anodic tantalum oxides.This avenue presents a promising trajectory for enhancing the overall performance and applicability of these materials in various technological domains.Second, we underscore the importance of manipulating anodizing parameters to engineer novel nanostructures, such as nanowires, nanorods, nanospheres, and more.This strategic manipulation of anodization conditions offers an exciting platform for designing tantalum oxide-based materials with vastly improved properties.The exploration of novel nanostructures not only expands the material landscape but also opens new avenues for tailoring tantalum oxide materials to meet specific application requirements.
In conclusion, this review encapsulates a holistic overview of recent developments in the synthesis and applications of anodic tantalum oxide-based materials.From elucidating anodization conditions and growth mechanisms to exploring surface modifications and doping strategies, we provide a comprehensive resource for researchers and practitioners in the field.We anticipate that this review will inspire further innovations and discoveries in the pursuit of advanced materials with enhanced properties and diverse applications.

■ AUTHOR INFORMATION
95 examined the anodization of Ta in electrolytes, such as chromic acid, phosphoric acid, sulfuric acid, and citric acid with variations in concentrations.This study suggested, for the first time, the possibility of incorporation of sulfur and phosphorus from electrolytes into the anodic oxide layer.Moreover, it also indicated the incorporation of organic electrolytes during anodization in citric acid and suggested that these foreign species may influence the oxygen distribution in the oxide layer.Similarly, earlier studies on understanding of ATO formation and the incorporation of electrolyte species into the oxide matrix can be traced back to 1978 with the work of F. Arifuku et al. and later in 1996 by K. Shimizu et al.69,75 F. Arifuku et al.69 examined the anodization of Ta in various electrolytes, such as phosphoric acid, nitric acid, hydrochloric acid, potassium fluoride, sodium chloride, lithium sodium

Figure 1 .
Figure 1.Pictorial representation showing the fabrication of ATOs with various morphologies.

Figure 2 .
Figure 2. (a) SEM image of Ta anodized by sweeping the potential from E OCP to 20 V in 1 M H 2 SO 4 + 2 wt % HF (scan rate of 10 mV s −1 ).Reproduced with permission from ref 43.Copyright 2006 Springer.(b) SEM images after anodization of polycrystalline Ta surface for 5 s and (c) 10 s in 16.4 M H 2 SO 4 + 2.9 M HF at 15 V. Reproduced with permission from ref 53.Copyright 2009 American Chemical Society.
Photocatalytic water splitting field.93−95This compact structure is attributed to a very low solubility of anodic oxide layers in theses electrolytes.For example, Y. Kim et al. grew ATO films on Ta foil in citric acid solutions (0.01−0.2 M) at anodizing potentials up to 10 V and demonstrated that, for all the concentrations of citric acid, a compact ATO layer about 22 photoelectrochemical, MB − methylene blue.

Figure 3 .
Figure 3. Secondary ion mass spectroscopy (SIMS) depth profile of the film formed on tantalum at 1 mA cm −1 (to the voltage of 100 V) in a 1 M ammonium fluoride electrolyte at 25 °C.The decreasing TaO − yield associated with the region of enrichment of fluoride species is indicated by the arrow.Reproduced with permission from ref 75 Copyright 1996 Taylor and Francis.

Figure 4 .
Figure 4. (a) HAADF-STEM and (b−h) EDS spectrum images showing the elemental distribution for the anodic nanotube wall formed in H 2 SO 4 :HF electrolyte (8:0.5 vol %) at room temperature under 15 V for 120 s.The area underlined by the black and white boxes in (g) was used to estimate the F/S ratio using the EDS spectrum of each specific region.Reproduced with permission from ref 49.Copyright 2020 Elsevier.

Figure 5 .
Figure 5. (a) Anodic polarization curves of tantalum in 1 M H 2 SO 4 containing different concentrations of HF recorded at the scan rate of 10 mV s −1 , (b) SEM image of the anodic porous tantalum oxide layer formed in 1 M H 2 SO 4 + 2 wt % HF after 2 h of polarization at 20 V, (c) pore diameter of porous ATO layers formed in 1 M H 2 SO 4 + 2 wt % HF for different anodization times.Reproduced with permission from ref 47.Copyright 2005 The Electrochemical Society.

Figure 6 .
Figure 6.SEM images of the Ta surface anodized at 20 V (after a potential ramp from the E OCP to 20 V with a scan rate 10 mV s −1 in H 2 SO 4 (1 M) with different concentrations of HF (a) 0.5 wt %, (b) 1 wt %, (c) 2 wt %, and (d) 3 wt %.Reproduced with permission from ref 105.Copyright 2005 The Electrochemical Society.

Figure 7 .
Figure 7. TEM images of (a) an open end of a Ta 2 O 5 nanotube with thinner walls compared to (b) the other end of the same tube.(c) Graph showing a structural dependence of the inner diameter, outer diameter, and wall thickness on the HF concentration (4% H 2 O and 30 V). Reproduced with permission from ref 42.Copyright 2009 Royal Society of Chemistry.
N. K. Allam et al.57demonstrated the fabrication of highly ordered nanoporous/nanotube ATOs in an H 2 SO 4 + HF + H 2 O electrolyte containing low concentrations of other additives, such as ethylene glycol (EG), dimethyl sulfoxide (DMSO), or glycerol.They proposed that organic additives can alter various physiochemical parameters (e.g., viscosity, ion mobility, and current density) of the parent anodizing electrolyte and may play a crucial role in the development of the nanostructural morphology.

Figure 8 .
Figure 8. FESEM image of tantalum oxide surfaces formed in an electrolyte containing HF + H 2 SO 4 (1:9 vol %) + 5 vol % ethylene glycol at (a) 10, (b) 12, and (c) 15 V.(d) Variation of the nanotube length as a function of anodization voltage for ATO samples fabricated in the presence of ethylene glycol (EG) and dimethyl sulfoxide (DMSO).Reproduced with permission from ref 57.Copyright 2008 American Chemical Society.
achieved a nanoporous ATO layer by anodizing Ta foil in EG containing 3 vol % H 2 O and NH 4 F (0.3−2.4 wt %) at 40 and 60 V.They demonstrated that under different concentrations of NH 4 F (1.2 and 2.4 wt %) in the electrolyte, an increase in the applied electric field (60 V) could generate uniform pores on the surface layer.However, the precise role of NH 4 F in the formation of ATO layers was not clearly indicated in this study.Additionally, some studies demonstrated the effect of electrolyte temperature on the nanoporous/nanotube morphology of ATO films. 27,48R. V. Goncalves et al. systematically investigated the change in ATO NT length and pore diameter with variation in electrolyte temperature.27In H 2 SO 4 + 1 vol % HF + 4 vol % H 2 O electrolytes at an anodizing potential of 50 V for 20 min, it was demonstrated that the length of NTs increased gradually from 1.3 to 4.6 μm with an increase in electrolyte temperature from 0−50 °C.27This study further

Figure 9 .
Figure 9. (a) Current density−time curves recorded during the anodization of Ta at 20 V for 3 h in glycerol with different amounts of NH 4 F. (b) SEM cross-sectional images of porous Ta 2 O 5 layers grown as in (a) with 0.1, 0.3, and 0.5 M NH 4 F, the insets show corresponding top views of the anodic layers.(c) The average pore diameter (triangles) and the thickness (squares) of porous Ta 2 O 5 layers prepared as in (a).Reproduced with permission from ref 62.Copyright 2008 Elsevier.(d) SEM image of top view of anodic Ta 2 O 5 produced in EG with 0.2 M NH 4 F at 15 V for 2 h.Reproduced with permission from ref 40.Copyright 2018 Elsevier.

Figure 10 .
Figure 10.Current density vs time curves for Ta anodization in (a) H 2 SO 4 (8 vol %) with different concentrations HF (from 0.5 to 0.125 vol %) at room temperature under 15 V for 120 s.Reproduced with permission from ref 49.Copyright 2020 Elsevier.(b) in glycerol +0.2 M NH 4 F electrolyte containing different amounts of water at 20 V for 3 h, and (c) SEM cross-sectional images of Ta 2 O 5 layers grown at the conditions indicated in (b).Reproduced with permission from ref 62.Copyright 2008 Elsevier.
−c).In another work, Z. Su et al. 25 demonstrated constant current anodization at 1.2 A cm −2 with an unprecedently high growth rate of Ta 2 O 5 nanotubes, reaching the length of ∼7 μm in just 2 s and ∼10 μm in just 4 s.In this case, H 2 SO 4 with 13.6 wt % H 2 O and 0.8 wt % NH 4 F was chosen as the anodizing electrolyte (Figure 11d−f).

Figure 11 .
Figure 11.SEM images of the cross-sectional views of Ta 2 O 5 nanotubes obtained in the electrolyte containing 95 vol % H 2 SO 4 + 5 vol % HF at 125 mA cm −2 for different anodizing times: (a) 1 min and (b) 2 min.(c) The length of Ta 2 O 5 nanotubes as a function of anodizing time in the electrolyte containing 90−95 vol % H 2 SO 4 + 5−10 vol % HF at different current densities.Reproduced with permission from ref 74.Copyright 2021 Elsevier.(d) Top and (e) cross section SEM images of Ta 2 O 5 nanotube arrays grown in sulfuric acid−based electrolyte containing 13.6 wt % H 2 O and 0.8 wt % NH 4 F under galvanostatic anodization at 1.2 A cm −2 for 2 s.(f) corresponding the layer thickness vs anodization time curve.Reproduced with permission from ref 25.Copyright 2015 Elsevier.

Figure 12 .
Figure 12.(a) SEM images of the anodic nanoporous Ta 2 O 5 film formed in glycerol +0.2 M NH 4 F electrolyte during 3 h-anodization at 20 V showing top-, bottom-, cross-sectional, and a magnified cross-sectional view.(b) The average pore diameter (triangles) and the thickness (squares) of porous Ta 2 O 5 layers prepared in glycerol +0.2 M NH 4 F electrolyte for 3 h at different potentials ranging from 10 to 50 V, insets show SEM images of the nanoporous Ta 2 O 5 layer grown at 50 V.Reproduced with permission from ref 62 Copyright 2008 Elsevier.(c) Variation of the nanotube diameter (top diameter and bottom diameter) and nanotube length as a function of anodization voltage (10−170 V) for samples fabricated after 1 h anodization.Reproduced with permission from ref 45 Copyright 2017 Elsevier.

Figure 13 .
Figure 13.(a) Pictorial demonstration of the film peeling process.The architecture of the oxide film composition is shown as separated layers.The film detaches due to the dissolution of the fluoride-rich layer in water and floats off due to the skin effect.(b) Micrograph of a partially detached and floating tantalum oxide film over the water surface.Reproduced with permission from ref 97.Copyright 2008 American Chemical Society.

Figure 14 .
Figure 14.(a) A visual representation of current density-time response during Ta anodization with the typical three stages: (I) a compact oxide layer formation, (II) pore formation and extension, and III) steady-state growth with (i) dimples (DPs) formation or (ii) nanotubes (NTs) formation.Reproduced with permission from ref 38.Copyright 2020 Elsevier.(b) Voltage−time curves recorded during constant current anodization in the electrolyte containing 90−95 vol % H 2 SO 4 + 5−10 vol % HF at different anodizing current densities.Reproduced with permission from ref 74.Copyright 2021 Elsevier.(c) Schematic representation of the nanopore-nanotube assembly mechanism: (i) a surface with pits before anodization; (ii) dissolution of the Ta 2 O 5 anodic layer and formation of dimples; (iii) growth of nanopores (length grown by electric field); (iv) formation of nanotubes; (v) dimple-shaped morphology.Reproduced with permission from ref 49.Copyright 2020 Elsevier.

Figure 15 .
Figure 15.Schematic representation of nanotube formation from nanopores by a selective dissolution of the fluoride-rich layer by H 2 O molecules from the electrolyte.(A) Nanoporous anodic oxide with the fluoride-rich layer at cell boundaries and pore bottoms.(B) Anodic oxide nanotubes formed by selective etching of the fluoriderich layer.Reproduced with permission from ref 15.Copyright 2020 Elsevier.

Figure 16 .
Figure 16.(a) XRD diffractograms of as-anodized and annealed freestanding Ta 2 O 5 NTs.NTs were prepared at 50 V in an electrolyte (H 2 SO 4 + 1 vol % HF + 4 vol %) at 50 °C and then annealed at 550, 750, 800 °C for 0.5 h and at 800 °C for 1 h.The top diffractogram was recorded for the commercial Ta 2 O 5 powder sample (SP).(b) SEM and TEM images of the freestanding Ta 2 O 5 NTs prepared at a potential of 50 V under the same anodization conditions as in (a).All samples were annealed at different temperatures for 0.5 h, except the bottom image that shows Ta 2 O 5 annealed for 1 h at 800 °C.(c) Survey XPS data of the freestanding Ta 2 O 5 NTs prepared at a potential of 50 V with the anodization electrolyte at 50 °C.All samples were annealed at different temperatures for 0.5 h, except the top spectrum that shows Ta 2 O 5 annealed for 1 h at 800 °C.Reproduced with permission from ref 27.Copyright 2012 American Chemical Society.

Scheme 1 .
Scheme 1. Chronology of the Fabrication of ATO Structures and Their Post-treatment Methods

Figure 17 .
Figure 17.(a) Electrochemical capacitance vs frequency for Ta 2 O 5 /Fe 2 O 3 NTs layers.(b) Capacitance retention and Coulombic efficiency during cycling tests for the material measured at 1 mA cm −2 for 10000 cycles.Reproduced with permission from ref 115.Copyright 2014 American Chemical Society.

Figure 18 .
Figure 18.(a) Degradation of MB for different anodized Ti−Ta substrates with various Ta at % loadings.A 320−480 nm filter was applied to the light source.Reproduced with permission from ref 84.Copyright 2019 American Chemical Society.(b) Variation of phenol concentration with time during degradation tests at compact surface tantalum oxide (CSTO) and coral like tantalum oxide (CLTO) photocatalysts.A 450 W highpressure mercury lamp served as the UV light source.Reproduced with permission from ref 26.Copyright 2013 PLOS.(c) CO conversion as a function of temperature over commercial Ta 2 O 5 , freestanding amorphous Ta 2 O 5 nanotubes, and Ta 2 O 5 nanotubes heat-treated at various temperatures.Reproduced with permission from ref 28.Copyright 2014 Royal Society of Chemistry.

Figure 19 .
Figure 19.(a) Hydrogen evolution for pure Ta 2 O 5 NTs and NTs with different concentrations of NiO NPs on their surface in an H 2 O/ethanol (4:1 vol.) solution.A xenon lamp of 300 W and a light intensity of 400 mW cm −2 was utilized for this study.Reproduced with permission from ref 34.Copyright 2017 American Chemical Society.(b) Photocurrent−potential curves of open Ta 3 N 5 nanotubes loaded with different cocatalysts (Co(OH) x , Co-Pi, Co(OH) x + Co-Pi, and Co-Pi + Co(OH) x ) under chopped AM 1.5 G simulated sunlight in a 1 M KOH solution (pH = 13.7) at a scan rate of 2 mV s −1 .Reproduced with permission from ref 24.Copyright 2015 Elsevier.(c) Current−potential curves for Co-Pi/Ba−Ta 3 N 5 photoelectrode under chopped AM 1.5 G simulated sunlight in a 1 M KOH solution (pH = 13.7) at a scan rate of 10 mV s −1 .Reproduced with permission from ref 25.Copyright 2015 Elsevier.
In the previous decade, Ta 2 O 5 or modified Ta 2 O 5 , has been extensively investigated as a photocatalyst or photoelectrochemical catalyst for water splitting under UV irradiation/solar light conditions. 21,23−25,27,34 R.V. Goncalves et al. 27 elucidate the crucial role of annealing conditions on the photocatalytic hydrogen generation activity of freestanding anodic Ta 2 O 5

3 N 5
NTs by nitridation in an ammonia atmosphere and modified the surface of Ta 3 N 5 NTs with cobalt phosphate (Co−Pi) or Co(OH) x or combination of Co−Pi + Co(OH) x .In this case, Ta 3 N 5 −Co(OH) x was prepared by depositing Co(OH) x on Ta 3 N 5 NTs by a simple wet chemical method, while Ta 3 N 5 −Co−Pi was prepared by the electrodeposition of Co−Pi from a Co(NO) 3 + potassium phosphate electrolyte bath.The Ta 3 N 5 photoanode modified with both Co−Pi and Co(OH) x (i.e., Co−Pi + Co(OH) x /Ta 3 N 5 ) exhibited an enhanced photocurrent density up to 3.1 mA cm −2 at 1.23 V vs

Figure 21 .
Figure 21.(a) Alkaline phosphatase (ALP) activity tests for culturing MC-3T3-E1 cells for 7, 14, and 21 days on nanodimpled surfaces with different diameters formed on tantalum metal (activity values of all groups in 21 days are significantly different from those of the corresponding groups in 7 and 14 days); * indicates a significant difference between the 40 nm group and all other groups with the same cell culture time; # indicates a significant difference between the 70 nm group and all other groups with the same cell culture time.Reproduced with permission from ref 46.Copyright 2019 Springer.(b) Relationship between roughness (column plot) and wettability (scatter plot) for each surface modification along with photographs of wetting behavior of surface with a Milli-Q ultrapure water drop where Ta-CaP3 is synthesized by 2 step anodization in 0.18 M calcium acetate (CaA) + 0.12 M β-glycerolphosphate (β-GP) at 100 V for 30 min and Ta-CaPMg4 is synthesized by 2 step anodization in 0.18 M CaA + 0.12 M β-GP + 0.08 M magnesium acetate (MgA) at 80 V for 30 min.Reproduced with permission from ref 33.Copyright 2019 Elsevier.

Table 1 .
Characteristic of Ta Substrates Used for Anodization radioactive labeling techniques.In 1972, E. Gunzel

Table 2 .
Conditions for the Chemical Polishing of Ta Substrates

Table 3 .
Conditions for Electrochemical Polishing of Ta Substrates

Table 4 .
Selected Procedures of Ta Anodization along with the Observed Morphology of the Anodic Film, Post-treatment Conditions, and Applications a 4 75 V, up to 1 mA cm −2