Nanostructured TiO2 Arrays for Energy Storage

Because of their extensive specific surface area, excellent charge transfer rate, superior chemical stability, low cost, and Earth abundance, nanostructured titanium dioxide (TiO2) arrays have been thoroughly explored during the past few decades. The synthesis methods for TiO2 nanoarrays, which mainly include hydrothermal/solvothermal processes, vapor-based approaches, templated growth, and top-down fabrication techniques, are summarized, and the mechanisms are also discussed. In order to improve their electrochemical performance, several attempts have been conducted to produce TiO2 nanoarrays with morphologies and sizes that show tremendous promise for energy storage. This paper provides an overview of current developments in the research of TiO2 nanostructured arrays. Initially, the morphological engineering of TiO2 materials is discussed, with an emphasis on the various synthetic techniques and associated chemical and physical characteristics. We then give a brief overview of the most recent uses of TiO2 nanoarrays in the manufacture of batteries and supercapacitors. This paper also highlights the emerging tendencies and difficulties of TiO2 nanoarrays in different applications.


Introduction
Due to its numerous benefits, including affordability, earthiness, and superior chemical stability, titanium dioxide is a crucial multifunctional substance with several uses in batteries, photocatalysis, and sensors [1][2][3][4][5]. Due to their increased specific surface area and decreased diffusion length, nanostructured TiO 2 materials perform better electrochemically when compared to bulk TiO 2 materials [6]. Compared to bulk materials, nanostructured TiO 2 obtains a larger specific contact area and a decreased diffusion length and therefore performs better electrochemically. In this regard, nanostructured TiO 2 is substantially studied as the electrode for Li-ion batteries, Na-ion batteries, supercapacitors, and emerging aqueous batteries [7]. In contrast to thin-film electrodes, which could boost conductivity at the interface between active materials with current collectors despite having low specific surface areas, traditional powder-based electrodes have high specific surface areas but inevitably require binders and conductive agents [8]. Nanostructured arrays have the ability to be free of binders and conductive materials while possessing a significant area of specific surface. Several previous reviews have been reported on the regulation and preparation of TiO 2 hierarchical morphologies [9] and their applications [10,11]. Yet, it remains challenging to controllably create nanostructured TiO 2 arrays nowadays. Although there are several evaluations concerning TiO 2 materials [12], there are very few assessments of the energy storage capabilities of nanostructured TiO 2 arrays.
In this study, we emphasize the significant developments in the creation of nanostructured TiO 2 arrays in various sizes [13][14][15][16][17]. The uses of nanostructured TiO 2 arrays for energy storage are then discussed, with a focus on methods for enhancing electrochemical performance [6,[18][19][20][21][22]. It is possible to summarize and predict the optimization of energy there are very few assessments of the energy storage capabilities of nanostructured TiO arrays.
In this study, we emphasize the significant developments in the creation o nanostructured TiO2 arrays in various sizes [13][14][15][16][17]. The uses of nanostructured TiO arrays for energy storage are then discussed, with a focus on methods for enhancin electrochemical performance [6,[18][19][20][21][22]. It is possible to summarize and predict th optimization of energy storage capabilities by contrasting the electrochemical an morphological characteristics of various TiO2 nanostructured arrays.
(1) Nanowire arrays Alkaline hydrothermal techniques are frequently used to prepare nanowire array [31,32]. The alkaline hydrothermal method uses diluted alkaline solution to heat Ti foil moderately high temperatures (usually 150-220 °C) in a Teflon-lined autoclave to produc a vertical alignment of protonated sodium titanate nanowire arrays on Ti foil. With furthe proton exchange and annealing, the phase changes from sodium titanate to anatase o rutile TiO2 [32]. The alkaline hydrothermal preparation usually results in nanotube
(1) Nanowire arrays Alkaline hydrothermal techniques are frequently used to prepare nanowire arrays [31,32]. The alkaline hydrothermal method uses diluted alkaline solution to heat Ti foil at moderately high temperatures (usually 150-220 • C) in a Teflon-lined autoclave to produce a vertical alignment of protonated sodium titanate nanowire arrays on Ti foil. With further proton exchange and annealing, the phase changes from sodium titanate to anatase or rutile TiO 2 [32]. The alkaline hydrothermal preparation usually results in nanotubes, nanowires or nanorods, and nanoribbons [33]. Nanosheets are usually observed at an early stage of the preparation or as a small impurity in the final product. Nanobelts are also usually produced at relatively high temperatures (for example, 180 • C) during hydrothermal treatment. Nanowires are often obtained by a calcination of nanotubes at temperatures above 400 • C or Materials 2023, 16, 3864 3 of 31 a hydrothermal reaction at high temperatures (typical above 200 • C). The size and shape of the arrays can be controlled by varying the hydrothermal parameters and solution concentration (e.g., reaction time can affect the length of the nanowires) [34,35]. Liu et al. produced single crystalline nanowires orientated in the (100) direction, as shown in Figure 2a,b [31]. Similarly, Boercker et al. prepared polycrystalline TiO 2 nanowires grown on titanium foil [29]. Moreover, by adjusting the calcination temperatures, TiO 2 -B nanowires that ranged in diameter from 20 to 40 nm were created by Armstrong's group [36]. The solvent ethanol was also used to create TiO 2 -B nanowires with similar size as the former [37].
A variety of methods for creating TiO 2 nanowire arrays under mild circumstances have been disclosed. By employing the electrospinning technique on an FTO substrate, Krishnamoorthy's group created a simple and efficient to create a vertical alignment of anatase TiO 2 nanowires [38]. Aligned TiO 2 nanoribbons with a length of 25 µm were manufactured by applying an improved electrospinning method on a TCO substrate. The vertical TiO 2 nanowires were approximately 27 µm in length and had an average wire width of 90 nm (Figure 2c,d) [39]. In addition, Wu et al. reported an efficient and inexpensive technique to grow TiO 2 nanowire arrays on arbitrary substrates (e.g., stainless steel and carbon cloth) by using H 2 O 2 solutions at a low temperature (Figure 2e-i) [23]. early stage of the preparation or as a small impurity in the final product. Nanobelt also usually produced at relatively high temperatures (for example, 180 °C) du hydrothermal treatment. Nanowires are often obtained by a calcination of nanotub temperatures above 400 °C or a hydrothermal reaction at high temperatures (typical a 200 °C). The size and shape of the arrays can be controlled by varying the hydrothe parameters and solution concentration (e.g., reaction time can affect the length o nanowires) [34,35]. Liu et al. produced single crystalline nanowires orientated in the direction, as shown in Figure 2a,b [31]. Similarly, Boercker et al. prepared polycrysta TiO2 nanowires grown on titanium foil [29]. Moreover, by adjusting the calcin temperatures, TiO2-B nanowires that ranged in diameter from 20 to 40 nm were cre by Armstrong's group [36]. The solvent ethanol was also used to create TiO2-B nanow with similar size as the former [37].
A variety of methods for creating TiO2 nanowire arrays under mild circumsta have been disclosed. By employing the electrospinning technique on an FTO subs Krishnamoorthy's group created a simple and efficient to create a vertical alignme anatase TiO2 nanowires [38]. Aligned TiO2 nanoribbons with a length of 25 μm manufactured by applying an improved electrospinning method on a TCO substrate vertical TiO2 nanowires were approximately 27 μm in length and had an average width of 90 nm (Figure 2c,d) [39]. In addition, Wu et al. reported an efficient inexpensive technique to grow TiO2 nanowire arrays on arbitrary substrates (e.g., stai steel and carbon cloth) by using H2O2 solutions at a low temperature ( (2) Nanorod arrays Rutile TiO2 nanorod arrays can be produced by employing concentrated HCl sol [40][41][42], while SO4 2− , CH3CO 2− , and C2O4 2− cloud create anatase titania [43]. The qua anatase TiO2 nanorod array made of orientated nanocrystals on a FTO glass was cre by a solvothermal technique with tetrabutyl titanate and H2SO4 [43]. Orientated m crystal rutile TiO2 nanorod films were manufactured via a simple hydrothermal techn (concentrated HCl) (Figure 3a,b) [27]. The morphology of the nanorods can be mod (2) Nanorod arrays Rutile TiO 2 nanorod arrays can be produced by employing concentrated HCl solution [40][41][42], while SO 4 2− , CH 3 CO 2− , and C 2 O 4 2− cloud create anatase titania [43]. The quasi-1D anatase TiO 2 nanorod array made of orientated nanocrystals on a FTO glass was created by a solvothermal technique with tetrabutyl titanate and H 2 SO 4 [43]. Orientated mono-crystal rutile TiO 2 nanorod films were manufactured via a simple hydrothermal technique (concentrated HCl) (Figure 3a,b) [27]. The morphology of the nanorods can be modified by adjusting experimental factors such as reaction circumstances and solution concentration and additives. By adjusting the growth parameters, such as growth time, reaction temperature, initial reactant concentration, acidity, and additives, the diameter, length, and density of the nanorods may be altered [44]. Additionally, the epitaxial relationship (lattice mismatch) between active materials and substrate significantly affects the nucleation and development of nanorods [45]. Titanium metal substrates can also be used to grow vertical rutile TiO 2 nanorods grown along the [001]-axis, as illustrated in Figure 3c-f [46]. Different reaction conditions can lead to the formation of different crystal structures of 1D TiO 2 . In general, the nanorods obtained by the acid hydrothermal methods are rutile phases [47]. Those synthesized by alkaline hydrothermal methods [48] and hydrogen peroxide methods [49] are sodium titanate or titanic acid; subsequently, titanic acid can be transformed to titanium dioxide after heat treatment, and the crystal phases of the obtained TiO 2 depends on the heat treatment temperature (TiO 2 (B) at low temperatures, anatase at moderate temperatures, and rutile at high temperatures) [50]. Mixed phases can be obtained by controlling the heat treatment temperature. reaction temperature, initial reactant concentration, acidity, and additives, the diameter length, and density of the nanorods may be altered [44]. Additionally, the epitaxia relationship (lattice mismatch) between active materials and substrate significantly affects the nucleation and development of nanorods [45]. Titanium metal substrates can also be used to grow vertical rutile TiO2 nanorods grown along the [001]-axis, as illustrated in Figure 3c-f [46]. Different reaction conditions can lead to the formation of different crysta structures of 1D TiO2. In general, the nanorods obtained by the acid hydrotherma methods are rutile phases [47]. Those synthesized by alkaline hydrothermal methods [48 and hydrogen peroxide methods [49] are sodium titanate or titanic acid; subsequently titanic acid can be transformed to titanium dioxide after heat treatment, and the crysta phases of the obtained TiO2 depends on the heat treatment temperature (TiO2(B) at low temperatures, anatase at moderate temperatures, and rutile at high temperatures) [50] Mixed phases can be obtained by controlling the heat treatment temperature.
Wu et al. produced ordered titania nanorod arrays on titanium surfaces using a method similar to that described above for creating TiO2 nanowire arrays [14]. A coating of condensed anatase nanoparticles (2 μm thick) was laid on the basis of the aligned nanorod arrays (1 μm thick). The majority of the as-deposited nanorods were a combination of rutile and anatase. Rutile nanorods cultivated alongside the path of the [001]-axis.  Wu et al. produced ordered titania nanorod arrays on titanium surfaces using a method similar to that described above for creating TiO 2 nanowire arrays [14]. A coating of condensed anatase nanoparticles (2 µm thick) was laid on the basis of the aligned nanorod arrays (1 µm thick). The majority of the as-deposited nanorods were a combination of rutile and anatase. Rutile nanorods cultivated alongside the path of the [001]-axis. (3) Nanotube arrays Titanium dioxide nanotube arrays can be made by using numerous methods. The main routes can be divided into anodic self-organization, templating, and electrospinning methods [51][52][53].
Anodization techniques can successfully create TiO 2 nanotube arrays. In contrast to the uncontrollable hydrothermal method, the anodic oxidation method can modulate the size of nanotubes. The nanotubular structures are affected by several factors, including the application of potential, the electrolyte's concentration, and the fluoride content [54][55][56]. For instance, the applied voltage can be used to regulate the tube diameter [55], and the anodization period affects the tube length (larger tubular diameters and layer thickness are observed for larger voltage and longer anodization times) [54,57]. The regulation of electrolytes can also affect the tube width and length [54]. The tube length is limited to 500-600 nm in electrolytes at lower pH because of the faster etching rate in acidic electrolytes [58]. In aqueous electrolytes, tubes with widths between 10 and 100 nm generally develop when a voltage of 1 to 25 V is applied in an electrolyte containing 0.1 to 0.5 wt % F − [59,60].
Larger diameters can be obtained in organic solutions (e.g., F − with a concentration of <1 M in C 6 H 10 O 4 [61][62][63][64]); the diameter of the tubes can reach 800 nm in the most ideal organic electrolyte [64]. However, aqueous solutions containing F have severe electrochemical etching rates for electrodes, resulting in the length of nanotubes in aqueous electrolytes being capped at a few micrometers. The electrolyte temperature is another important factor that greatly influences the thickness of tubes [65,66]; in fact, by performing anodizing at a low temperature, it can even be possible to practically seal the interior of the tubes and create rod-like structures [66]. Complexing agents such as EDTA (41 µm h −1 ) [67] or lactic acid (1200 µm h −1 ) [68] can also be employed to speed up the formation of nanotubes (Figure 4a,b). Under certain anodizing conditions, this can lead to peculiar morphologies, such as the tube-in-tube structure [69]. Titanium dioxide nanotube arrays can be also achieved by alkaline hydrothermal methods. The formation process of the nanotubes can be divided into several stages: the dissolution of TiO 2 raw material in alkaline solution occurs simultaneously with the epitaxial growth of sodium trititanate layered nanosheets, followed by the crystallization of dissolved titanate on the exfoliated nanosheets and the generation of physical tension, which induces bending of the nanosheets and the formation of nanotubes [70].
In template methods for the synthesis of nanotubes, modulating the hydrolysis rate of the titanium-containing compound solution (containing the template agent) enables the polymerization of TiO 2 in or on the surface of self-assembled template molecules, followed by the selective removal of the template agent. High-aspect-ratio cellulose, micelles, or hard templates can be used as templates to synthesize tubular structures by a range of deposition methods, such as atomic layer deposition [71][72][73][74][75]. Hoyer et al. prepared titanium dioxide nanotubes by electrodeposition on ordered alumina templates [76]. The sol-gel technique can also be used to manufacture TiO 2 [77][78][79][80]. An alumina template, for instance, can be selectively dissolved before TiO 2 sol has been absorbed into its pores [80]. By reproducing different ZnO nanomorphologies, complex hollow TiO 2 nanostructures are successfully created (Figure 4c-i). For example, NH 3 − saturated deposition can produce TiO 2 nanotube arrays with 70 nm diameters [81].
By creating arrays of randomly oriented nanotube electrodes Figure 5a-d), both sealed (Figure 5e-h) and unsealed (Figure 5i-l), Han's team systematically studied variables that influence the performance of TiO 2 electrodes [82]. Their theoretical study and experimental findings revealed that the highly ordered titania nanotube electrode outperforms conventionally random electrodes and cap-sealed electrodes in terms of rate capability. Additionally, electrospinning is another efficient technique for producing TiO 2 nanotube arrays. By adding the precursor solution through a capillary spinneret, Li's group obtained an array of TiO 2 hollow nanotubes [83].

2D Nanostructured Arrays
Two-dimensional materials include nanobelts and nanosheets featuring a significant aspect ratio [84,85]. Additionally, 2D materials, such as graphene and MoS 2 , have a variety of unusual physical, chemical, optical, electrical, and magnetic properties [84]. Larger specified areas, more exposed active sites, and certain specifically exposed crystal surfaces can all be provided by 2D TiO 2 nanostructured arrays.
(1) Nanosheets arrays TiO 2 nanosheets with special exposed surfaces such as (010), (101), (001), and (105) facets are attractive for renewable energy due to their different reaction activities [86][87][88][89]. For instance, by etching with hydrofluoric acid, Yang et al. prepared a highly exposed (001) surface of single-crystalline anatase titanium dioxide, and reactive (001) facets have promising applications in photocatalysis [89]. Liu et al. [90] hydrothermally processed Ti foil in a 5 M NaOH solution to create titanate nanosheet arrays on Ti foil. These titanate nanosheets underwent the proper post-processing to produce anatase TiO 2 /brookite TiO 2 heterostructures. Yang et al. [91] used tetrabutyl titanate and ammonium hexafluorotitanate as Ti precursors, where F − functions acted as termination agents, producing anatase TiO 2 nanosheet arrays with optimal exposure (001) facets on FTO. Lu et al. [92] reported a two-step method to fabricate anatase TiO 2 nanosheet arrays with the preferred exposure of [93] facets, where tetrabutyl titanate in hydrofluoric acid/toluene was hydrothermally treated to yield hexagonal TiOF 2 nanosheets at first, and then the TiO 2 nanosheets could be obtained by calcinating the TiOF 2 nanosheets at 500 • C. Nevertheless, each of the aforementioned techniques used a strongly acidic or basic solution, which is corrosive and dangerous. Therefore, Zhong et al. exploited a novel approach to grow anatase TiO 2 nanosheet arrays on FTO substrate via a relative green approach by using Na 2 EDTA and TEOA as co-coordination agents under weak basic conditions (Figure 6a-c) [94]. Gan [95]. The developed process involved a TiO nanorod-derived synthesis of upstanding hexagonal kassite nanosheet arrays and a transformation of the kassite to TiO 2 . The single-crystalline hexagonal kassite was hydrothermally treated with diluted HNO 3 aqueous solution and transformed into nanoporous rutile TiO 2 nanosheet arrays with shape preservation (Figure 6d). The Ti-O bond of TiO 2 nanorods was broken under the hydrothermal treatment of concentrated NaOH, and the binding of Ca 2+ ions released from the substrate into the solution with Ti 2 O 4 (OH) 2− caused the heterogeneous nucleation and growth of CaTi 2 O 4 (OH) 2 . After the nucleation, CaTi 2 O 4 (OH) 2 nanosheets grew along the (100) direction to reduce the lattice mismatch with the FTO substrate. Subsequently, the density of TiO 2 nanorods decreased, the yield and size of nanosheets increased, and finally, TiO 2 nanorods were completely dissolved and converted into nanosheets ( Figure 6e).
(2) Nanobelt arrays TiO 2 nanobelt arrays are typically fabricated through an alkaline hydrothermal treatment, followed by a subsequent proton exchange and heat treatment. For instance, Zhuo et al. investigated the route of Ti foil with the alkaline solution (5 M) [16]. Titanate was then heated to transform titanate into TiO 2 nanobelt arrays without changing the morphology. Unlike conventional alkali-hydrothermal approaches to titanates, we demonstrated the synthesis of titanate ultrathin nanobelt arrays using a novel and robust H 2 O 2 -assisted wetchemistry route at ambient conditions (Figure 7a-f) [96]. In the absence of any seed layers, the synthesis technique is effective for synthesizing thin films of one-or three-dimensional arrays on a variety of substrates at low temperatures. (2) Nanobelt arrays TiO2 nanobelt arrays are typically fabricated through an alkaline hydrothermal treatment, followed by a subsequent proton exchange and heat treatment. For instance, Zhuo et al. investigated the route of Ti foil with the alkaline solution (5 M) [16]. Titanate was then heated to transform titanate into TiO2 nanobelt arrays without changing the morphology. Unlike conventional alkali-hydrothermal approaches to titanates, we demonstrated the synthesis of titanate ultrathin nanobelt arrays using a novel and robust H2O2-assisted wet-chemistry route at ambient conditions (Figure 7a-f) [96]. In the absence of any seed layers, the synthesis technique is effective for synthesizing thin films of oneor three-dimensional arrays on a variety of substrates at low temperatures. TiO2 nanobelt arrays can also serve as a substrate for supporting other active materials. For instance, Luo's group synthesized the core-shell structure of TiO2 nanobelts@MnO2 nanosheets by the hydrothermal method (Figure 8a,b) [97]. Benefiting from this unique three-dimensional structure, the ultrathin nanosheets are uniformly TiO 2 nanobelt arrays can also serve as a substrate for supporting other active materials. For instance, Luo's group synthesized the core-shell structure of TiO 2 nanobelts@MnO 2 nanosheets by the hydrothermal method (Figure 8a,b) [97]. Benefiting from this unique three-dimensional structure, the ultrathin nanosheets are uniformly dispersed on the nanobelts, resulting in a larger contact area. Meanwhile, the vertically grown nanobelt arrays not only act as a strong supporter but also facilitate the charge transport. Similar structures include graphene-wrapped TiO 2 @Co 3 O 4 coaxial nanobelt arrays, as illustrated in Figure 8c,d [98].

3D Nanostructured Arrays
Three-dimensional nanoarrays have a larger specific surface area than that of 1D and 2D arrays. Additionally, they are usually prepared by one-step or two-step methods, namely one-step growth [99] and the multi-step surface branching decoration of preformed 1D morphology [100]. The preparation is simpler and more convenient in the former, but the controllability is not as good as that of the latter.
Decorating trunks with various branches, such as nanowires, nanosheets, nanorods, or nanoparticles, is useful to increase the contact surface area. By modifying the experimental parameters (e.g., precursor solution [101] and reaction rate [102]), the shape of branches can be controlled, for example, in the case of rutile TiO2 nanowire branches on a rutile TiO2 nanowire [88] and a hyperbranched hierarchical anatase TiO2 nanowire on pre-performed anatase TiO2 nanowire trunks [103]. Yang's group developed rutile TiO2 nanoarrays by the acidic evaporation method, and the rutile (101) twinning structure promoted the form of nanotrees (Figure 9a-f) [104]. The reaction mechanism of this process was the oxidation of rutile TiO2 by HCl vapor and the induction of its nucleation and growth along the c-direction. Then, the acidic vapor was able to induce (101) twinning of the sample, thus inducing the nucleation and growth of branches on the (100) side of the surface with higher surface energy. With the growth of the second branch and chemical erosion on both sides of the branch, the third branches could be generated on the initially grown nanorods at the previously eroded sites. In addition, we produced 3D anatase TiO2 nanostructured arrays with TiO2 nanoparticle branches (Figure 9g,h) [105]. Specifically, TiO2 nanowire grown by H2O2-assisted method was subsequently deposited

3D Nanostructured Arrays
Three-dimensional nanoarrays have a larger specific surface area than that of 1D and 2D arrays. Additionally, they are usually prepared by one-step or two-step methods, namely one-step growth [99] and the multi-step surface branching decoration of pre-formed 1D morphology [100]. The preparation is simpler and more convenient in the former, but the controllability is not as good as that of the latter.
Decorating trunks with various branches, such as nanowires, nanosheets, nanorods, or nanoparticles, is useful to increase the contact surface area. By modifying the experimental parameters (e.g., precursor solution [101] and reaction rate [102]), the shape of branches can be controlled, for example, in the case of rutile TiO 2 nanowire branches on a rutile TiO 2 nanowire [88] and a hyperbranched hierarchical anatase TiO 2 nanowire on pre-performed anatase TiO 2 nanowire trunks [103]. Yang's group developed rutile TiO 2 nanoarrays by the acidic evaporation method, and the rutile (101) twinning structure promoted the form of nanotrees (Figure 9a-f) [104]. The reaction mechanism of this process was the oxidation of rutile TiO 2 by HCl vapor and the induction of its nucleation and growth along the c-direction. Then, the acidic vapor was able to induce (101) twinning of the sample, thus inducing the nucleation and growth of branches on the (100) side of the surface with higher surface energy. With the growth of the second branch and chemical erosion on both sides of the branch, the third branches could be generated on the initially grown nanorods at the previously eroded sites. In addition, we produced 3D anatase TiO 2 nanostructured arrays with TiO 2 nanoparticle branches (Figure 9g,h) [105]. Specifically, TiO 2 nanowire grown by H 2 O 2 -assisted method was subsequently deposited in a liquid deposition technology to obtain a three-dimensional structure with abundant anatase TiO 2 nanoparticles. Typical cases include rutile TiO2 nanorods on pre-formed rutile TiO2 nanorod tru [106] and anatase TiO2 nanorod/nanosheet branches on pre-performed anatase nanowire [107,108]. Using the multi-step hydrothermal preparation method, Wu e constructed directional layered heterogeneous TiO2 hyperbranched array materials. T three-dimensional structures were composed of basic units such as nanow nanosheets, and nanorods. (Figure 10a-f) [109], and the 3D monoblocks led substantially enlarged contact area. Typical cases include rutile TiO 2 nanorods on pre-formed rutile TiO 2 nanorod trunks [106] and anatase TiO 2 nanorod/nanosheet branches on pre-performed anatase TiO 2 nanowire [107,108]. Using the multi-step hydrothermal preparation method, Wu et al. constructed directional layered heterogeneous TiO 2 hyperbranched array materials. These three-dimensional structures were composed of basic units such as nanowires, nanosheets, and nanorods. (Figure 10a-f) [109], and the 3D monoblocks led to a substantially enlarged contact area.
Liu et al. built nanotube networks, namely hierarchical TiO 2 nanorod arrays composed of rutile trunks and anatase nanotube branches, by using ZnO nanorods served as the template (Figure 10g-i) [25]. The backbones of three-dimensional TiO 2 arrays mainly include nanowires, nanotubes, and nanorods. A pre-formed nanowire trunk is often created using the alkaline hydrothermal process as the aforementioned method for 1D nanowire arrays [103]. Additionally, using an H 2 O 2 -assisted approach [105], we can obtain bramblelike anatase TiO 2 nanoarrays [110]. We also obtained three-dimensional TiO 2 nanorod arrays with a biphasic mixture (rutile/anatase) of balsam-pear-like shapes through a series of simple and green preparation methods [111]. Liu et al. built nanotube networks, namely hierarchical TiO2 nanorod composed of rutile trunks and anatase nanotube branches, by using ZnO nanorods as the template (Figure 10g-i) [25]. The backbones of three-dimensional TiO2 mainly include nanowires, nanotubes, and nanorods. A pre-formed nanowire t often created using the alkaline hydrothermal process as the aforementioned met 1D nanowire arrays [103]. Additionally, using an H2O2-assisted approach [105], obtain bramble-like anatase TiO2 nanoarrays [110]. We also obtained three-dime TiO2 nanorod arrays with a biphasic mixture (rutile/anatase) of balsam-pear-like through a series of simple and green preparation methods [111].
Using nanotube arrays as structural support allows for an increased surfa (more inner surface exposure) and decreased electrolyte/electrode interfacial res Roh's team disclosed a simple method for creating hierarchical TiO2 nanotube (Figure 11a-c) [112]. The hierarchical TiO2 nanotube arrays are made up of short n branches and long nanotube trunk with a vertical orientation. Various morpholo the hierarchical TiO2 nanotube arrays can be obtained by adjusting the prep parameters. We prepared a 3D electrode that consists of anatase TiO2 mes branches and single-crystal-like TiN nanowire trunks (Figure 11d,e) [113]. These provide a stable structure during charging and enhance electron migration. Anatas TiO2 nanoflower arrays were also synthesized by proton exchange of titanate w support of K2S2O8 (Figure 12a-c) [114]. Using nanotube arrays as structural support allows for an increased surface area (more inner surface exposure) and decreased electrolyte/electrode interfacial resistance. Roh's team disclosed a simple method for creating hierarchical TiO 2 nanotube arrays (Figure 11a-c) [112]. The hierarchical TiO 2 nanotube arrays are made up of short nanorod branches and long nanotube trunk with a vertical orientation. Various morphologies for the hierarchical TiO 2 nanotube arrays can be obtained by adjusting the preparation parameters. We prepared a 3D electrode that consists of anatase TiO 2 mesocrystal branches and single-crystal-like TiN nanowire trunks (Figure 11d,e) [113]. These trunks provide a stable structure during charging and enhance electron migration. Anatase/rutile TiO 2 nanoflower arrays were also synthesized by proton exchange of titanate with the support of K 2 S 2 O 8 (Figure 12a-c) [114].     Single-crystal nanoarrays are highly attractive because they can simultaneously enlarge the contact area and enhance the reaction kinetics of ions. Sheng's team developed single-crystal rutile TiO 2 nanowire arrays [88]. The branches grew through the backbone along the (100) and (010) crystallographic planes in four symmetric directions of epitaxy, with a typical angle of 65 • between the c-axes of the backbone and branches. These branched nanowires extending from the backbone not only expanded the contact area between the electrode material and the electrolyte but also accelerated the charge transport. In addition, strain engineering can optimize the physical and chemical properties of TiO 2 [116]. We prepared quasi-single-crystal anatase TiO 2 branched nanowire arrays and adjusted the lattice constant by the "convertible precursor induced growth" method, extending the lattice constant α by 0.37% (Figure 12d-f) [115]. During the growth of anatase TiO 2 mesocrystals on the surface of single-crystal sodium titanate using the liquid-phase deposition method, an extended lattice parameter a (3.804 Å) was introduced because the lattice parameter b of sodium titanate was slightly higher than the lattice parameter a of anatase TiO 2 (Figure 12g,h). In addition, TiO 2 3D nanoarrays can also be used for the synthesis of TiN nanoarrays with unique structures. We uncovered a novel "surface-induced" Kirkendall effect that causes titanate-decorated TiO 2 3D nanoarrays to be nitridated to form distinctive hollow TiN nanotrees (Figure 13a-f) [117]. The stacking of titanate nanosheets can accelerate chemical reactions and transform into a nitride layer. The hollow structures are caused by the Kirkendall effect. Table 1 provides brief summary of various synthesis methods of TiO 2 nanoarrays.
Single-crystal nanoarrays are highly attractive because they can simulta enlarge the contact area and enhance the reaction kinetics of ions. Sheng's team de single-crystal rutile TiO2 nanowire arrays [88]. The branches grew through the b along the (100) and (010) crystallographic planes in four symmetric directions of with a typical angle of 65° between the c-axes of the backbone and branche branched nanowires extending from the backbone not only expanded the cont between the electrode material and the electrolyte but also accelerated the transport. In addition, strain engineering can optimize the physical and c properties of TiO2 [116]. We prepared quasi-single-crystal anatase TiO2 b nanowire arrays and adjusted the lattice constant by the "convertible precursor growth" method, extending the lattice constant α by 0.37% (Figure 12d-f) [115] the growth of anatase TiO2 mesocrystals on the surface of single-crystal sodium using the liquid-phase deposition method, an extended lattice parameter a (3.804 introduced because the lattice parameter b of sodium titanate was slightly higher lattice parameter a of anatase TiO2 (Figure 12g,h). In addition, TiO2 3D nanoarrays be used for the synthesis of TiN nanoarrays with unique structures. We uncovered "surface-induced" Kirkendall effect that causes titanate-decorated TiO2 3D nanoa be nitridated to form distinctive hollow TiN nanotrees (Figure 13a-f) [117]. The of titanate nanosheets can accelerate chemical reactions and transform into a nitri The hollow structures are caused by the Kirkendall effect. Table 1 provides brief su of various synthesis methods of TiO2 nanoarrays.

Energy Storage Applications of Nanostructured TiO 2 Arrays
Using aligned TiO 2 nanoarray materials as electrodes for energy storage has many benefits [118][119][120][121]. Firstly, nano-array structures reduce the impedance at the interfaces of electrodes and electrolytes. Secondly, aligned nanostructures can provide direct electrical transfer pathways. Finally, the direct connection of electrodes to the current collectors eliminates the usage of binders and conducting additives [122][123][124].

Lithium-Ion Batteries
Battery performance is highly influenced by the characteristics of the electrodes. There is a need for fundamental advancements in terms of high energy and low costs. Since TiO 2 has an appropriate discharge plateau (approximately 1.5-1.75 V vs. lithium), which prevents organic electrolytes from decomposition, it has been regarded as a good alternative to graphite [125][126][127]. However, it has been found that the achievable capacities of bulk TiO 2 are quite low [128,129] due to the repulsive force among Li ions. Highly ordered TiO 2 nanoarrays facilitate Li-ion transportation [130][131][132][133]. The relevant works are outlined in this section based on the TiO 2 polymorphs.
Since lithium ions are more easily transported along the c-direction [134,135], rutile TiO 2 can improve the electrochemical performance by adjusting the growth direction of nanoarrays [136][137][138]. The c-direction TiO 2 nanorod arrays displayed outstanding cycling performance, with 93% capacity containing after 600 cycles [139]. Similarly, Dong et al. also prepared rutile TiO 2 nanorod arrays growing along the direction of the c-axis. The capacity of the active materials for Li-ions insertion/extraction is 10 times higher than that of the compact layer. And it remained to 133 µAh cm −2 (15 µA cm −2 ) after 50th cycle [46]. Several studies have reported other nanoarray configurations, including nanotubes, with outstanding electrochemical performances. For instance, Guan et al. developed rutile TiO 2 nanotube arrays by anodizing Ti foils. These arrays exhibited enhanced performances, maintaining 77% capacity after 100th cycle [140].
When charged and discharged with low currents, an anatase phase can reversibly absorb 0.5 Li per formula unit of TiO 2 through a biphasic process [141]. Tang et al. produced TiO 2 nanowire arrays with dense nanocavities with 305.8 mAh g −1 (0.2 C) [142].
Compared to the above two phases, TiO 2 -B is more suitable for storing lithium ions [143,144], and many works showed the excellent electrochemical performances of TiO 2 -B nanoarrays. Considerable efforts have been devoted to the synthesis of TiO 2 -B nanomaterials with various morphologies through different methods such as hydrothermal, sol-gel, and solvothermal methods [104,126,[145][146][147]. Liu et al. grew vertically oriented single-crystalline TiO 2 -B nanowire arrays by placing Ti foil in a hydrothermal solution during the hydrothermal process. Similar single-crystal TiO 2 -B nanowire arrays by Liu's group had an outstanding cycling calendar with 120 mAh g −1 even at 1.8 C [148]. Other approaches, such as Tang's team's, synthesized TiO 2 (B) nanowire arrays with 124.9 mAh/g at 2 C [149].
Phase mixing further enhances the lithium ions' storage [150][151][152][153]. We constructed 3D TiO 2 nanotrees by depositing ultrathin nanoribbons of the anatase/TiO 2 -B hybrid phase on single-crystal anatase nanowire arrays. The anatase is responsible for electron reception, while the TiO 2 (B) performs the intercalation of ions, leading to the effective separation of ions and electrons, thus enhancing the battery's performance. In addition, the ultra-long branching structure of the nanoribbons results in the excellent electrochemical performance of the electrode (Figure 14a-e) [154]. The morphology of TiO2 nanoarrays also has a huge impact on the performances of batteries [121]. For example, layered structures can ensure high capacities and good structural stability during Li ion insertion/extraction, which is attributed to shorter transport lengths for Li + . In addition, a larger specific surface area could increase the electrode-electrolyte contact area and facilitate the lithium ions' insertion and extraction [157][158][159][160][161][162][163].
Element doping and compositing TiO2 nanostructured arrays with other materials can also be good strategies to enhance the performance of electrode materials [164,165]. For instance, Kyeremateng's group developed Sn doping TiO2 nanotube arrays. The phase transformation of TiO2 was affected by Sn (From anatase to rutile) [166][167][168][169]. With a current density of 70 μA cm −2 (1 C), Sn-doped TiO2 nanotubes delivered much higher capacity values compared to TiO2 nanotubes. An electrochemical test showed that at 70 μA cm −2 , TiO2 nanotubes with Sn doping possessed a significantly larger capacity than conventional nanotubes. High-capacity Fe2O3 was deposited onto the surface of 1D TiO2 nanoarrays to improve the capacity of the electrodes [170]. To boost the electrode's capacity, composition with high-capacity Fe2O3 is an additional option. Furthermore, we prepared core-shell nanowire arrays to enhance the electrical conductivity and ion diffusion rate [113]. The single-crystal TiN nanowires acted as conductive collectors, while the branching shells were composed of nanoporous anatase TiO2. This specially structured 3D array exhibited pseudocapacitive-dominated charge storage with excellent electrochemical properties (Figure 15a-g).  [46,153,154,[154][155][156]. Reprinted with permission from [154], Copyright 2016 The Royal Society of Chemistry.
The morphology of TiO 2 nanoarrays also has a huge impact on the performances of batteries [121]. For example, layered structures can ensure high capacities and good structural stability during Li ion insertion/extraction, which is attributed to shorter transport lengths for Li + . In addition, a larger specific surface area could increase the electrode-electrolyte contact area and facilitate the lithium ions' insertion and extraction [157][158][159][160][161][162][163].
Element doping and compositing TiO 2 nanostructured arrays with other materials can also be good strategies to enhance the performance of electrode materials [164,165]. For instance, Kyeremateng's group developed Sn doping TiO 2 nanotube arrays. The phase transformation of TiO 2 was affected by Sn (From anatase to rutile) [166][167][168][169]. With a current density of 70 µA cm −2 (1 C), Sn-doped TiO 2 nanotubes delivered much higher capacity values compared to TiO 2 nanotubes. An electrochemical test showed that at 70 µA cm −2 , TiO 2 nanotubes with Sn doping possessed a significantly larger capacity than conventional nanotubes. High-capacity Fe 2 O 3 was deposited onto the surface of 1D TiO 2 nanoarrays to improve the capacity of the electrodes [170]. To boost the electrode's capacity, composition with high-capacity Fe 2 O 3 is an additional option. Furthermore, we prepared core-shell nanowire arrays to enhance the electrical conductivity and ion diffusion rate [113]. The single-crystal TiN nanowires acted as conductive collectors, while the branching shells were composed of nanoporous anatase TiO 2 . This specially structured 3D array exhibited pseudocapacitive-dominated charge storage with excellent electrochemical properties (Figure 15a-g). Post-annealing TiO2 nanoarrays in a reducing atmosphere can also substantially improved the capacity [172,173]. Lu et al. demonstrated that hydrogenation treatment processed an abundance of oxygen vacancies, which improved the electronic conductivity and thus resulted in excellent rate performances [21]. Table 2 provides an overview of the performance parameters of nanoarrays of TiO2 in lithium-ion batteries.  Post-annealing TiO 2 nanoarrays in a reducing atmosphere can also substantially improved the capacity [172,173]. Lu et al. demonstrated that hydrogenation treatment processed an abundance of oxygen vacancies, which improved the electronic conductivity and thus resulted in excellent rate performances [21]. Table 2 provides an overview of the performance parameters of nanoarrays of TiO 2 in lithium-ion batteries.

Sodium-Ion Batteries
Due to their availability, affordability, and environmental friendliness, sodium-ion batteries have drawn a lot of interest [194,195]. Nonetheless, there is still a critical demand for the discovery of superior anode materials [196,197].
Highly ordered TiO 2 nanoarrays feature the intrinsic merits for a larger contact area and the ability to release inner stress [197]. Ruffo et al. synthesized nanostructured TiO 2 with different morphologies. Thus, the exposition of different crystalline surfaces brought essentially different performances [198]. Bella et al. produced and compared the amorphous and anatase TiO 2 nanotubular arrays obtained by an anodic oxidation [199]. After the first cycling, anatase TiO 2 had superior electrochemical properties compared to its amorphous counterpart. In addition, they also demonstrated that anatase had optimal cyclic stability, and the channels along the [001] direction facilitated the transport of Na ions [200,201].
It was reported that TiO 2 @C nanotube arrays exhibited an excellent capacity with 232 mAh g −1 after 500th [202]. And three-dimensional Ni@TiO 2 core/shell nanoarrays showed ∼312 mAh g −1 after 100th at 50 mA g −1 [203]. The co-doping technique (such as Ni and N doping) can further improve the Na-ion storage [19]. It was also reported that compositing with the high capacity and conductivity of 2D MoS 2 nanosheets could improve the electronical performance [204]. The capacity can also be enhanced by the treatment of functionalized salts. Ni et al. produced phosphorylated TiO 2 nanotube arrays with a large capacitance of 147 mAh g −1 at 3350 mA g −1 and an excellent cycling calendar [205]. Table 3 provides an overview of the performance parameters of nanoarrays of TiO 2 in sodium-ion batteries.

Supercapacitors
TiO 2 nanoarrays can also be used for aqueous supercapacitors. Aqueous supercapacitors are safer and can be charged and discharged fast, despite having a low energy density. TiO 2 nanoarrays are favorable for their faster charge transport and interfacial ion mobility, apart from a higher surface area for ion adsorption [208,209].
For example, Kim's group applied TiO 2 nanotube array electrode materials to an EDLC-type device, and the experimental results were 2.4 mF cm −2 at 50 mV s −1 [210]. The TiO 2 nanoarray structure used directly as an electrode material had an area capacitance (1 mV s −1 /911 µF cm −2 ) several orders magnitudes larger when compared to the capacitance of nanoparticles (1 mV s −1 /180 µF cm −2 ) [211]. Additionally, the performance of the supercapacitor was proportional to the aspect ratio of the nanotubes [212].
Despite the inherent low area capacitance properties of TiO 2 , relatively high performance can be obtained by a range of methods [208,[213][214][215]. Compositing with C materials [216][217][218], H 2 treatment [219], plasma treatment [220], electrochemical doping [221][222][223], and synthesizing black or blue TiO 2 [224,225], have also been performed to improve the conductivity and capacitances of TiO 2 nanoarrays. For example, Kim et al. reported that NiO-TiO 2 nanotube arrays were obtained by the anodic oxidation of a Ni-Ti alloy, which showed an excellent performance for supercapacitors [226]. Additionally, Patil et al. fabricated 3D-1D TiO 2 hierarchical nanostructures by using straightforward chemical methods, which had the benefits of improved electron transportation properties (contributed by 1D TiO 2 nanotubes) and high surface area (as a result of the 3D TiO 2 flower) for improved area capacitance [227].
In recent times, we can also enhance the performance of supercapacitors by surface fluorine-modified anatase TiO 2 nanoarrays [105]. With an improved capacity of more than 6.4-fold, the calculation results showed that the fluorine modification altered the storing sites and transport routes, which caused a substantial reduction in the diffusion potential (Figure 16a-g). Table 4 provides an overview of the performance parameters of nanoarrays of TiO 2 in supercapacitors.

Other Batteries
Due to their enormous theoretical energy density, multivalent ion batteries, such as those based on magnesium [246], aluminum [247], and zinc [248], have attracted a lot of research. The Al 3+ with a small radius (53.5 pm) can perform the intercalation reaction [248]. For instance, anatase TiO 2 nanotube arrays possess reversible capacity of 75 mAh g −1 in 1 M AlCl 3 aqueous solution [249]. In addition, the experimental results show that ions can reversibly insert/extract into/from TiO 2 .
Battery voltage is constrained by the water's electrochemical window [250], thus limiting the energy density. By increasing the salt concentration, the activity of water can be drastically reduced, and the potential window can be extended [251]. Zhou's group developed TiO 2 -B nanotube arrays as an anode in the electrolyte of 50 M LiTFSI and 25 M TMBTFSI, and 194.5 mAh/g and 150 Wh/kg could be obtained when the anode mated with LiMn 2 O 4 [252].
Hydrogen ions, which possess the smallest ionic radius, are ideal charge carriers [253]. However, H + are normally present as a form of H 3 O + with high dehydration energy [254]. Recently we have demonstrated that anatase TiO 2 (001) enables the decomposition of H 3 O + to H + [110], and the anatase TiO 2 nanowire array functions superbly as the anode of the proton battery ( Figure 17).

Summary and Outlook
This review summarizes and compares the design principles, s applications of TiO2 nanoarrays. Emerging opportunities for TiO2 nanostru in a variety of domains are also discussed in detail. The goal of this review the current applications and challenges associated with TiO2 nanostru proving an in-depth look at TiO2 nanostructured arrays. TiO2 nanostru represent an important category of nanomaterials in energy applications, su ion batteries, sodium-ion batteries, supercapacitors, and other emerg However, the preparation scale is still at the laboratory level, and the contro material growth is still insufficient. For industrial applications, simple an scale production methods need to be developed in the future. The current m  [110,253,[255][256][257][258]. Reprinted with permission from [110], Copyright 2021 American Chemical Society.

Summary and Outlook
This review summarizes and compares the design principles, synthesis, and applications of TiO 2 nanoarrays. Emerging opportunities for TiO 2 nanostructured arrays in a variety of domains are also discussed in detail. The goal of this review is to figure out the current applications and challenges associated with TiO 2 nanostructured arrays, proving an in-depth look at TiO 2 nanostructured arrays. TiO 2 nanostructured arrays represent an important category of nanomaterials in energy applications, such as lithium-ion batteries, sodium-ion batteries, supercapacitors, and other emerging batteries. However, the preparation scale is still at the laboratory level, and the controllability of the material growth is still insufficient. For industrial applications, simple and controllable scale production methods need to be developed in the future. The current material growth mechanism is also not very clear, which impedes the accurate design of the material. It needs in situ spherical differential TEM, in situ synchrotron radiation, and other advanced characterizations to reveal the relevant mechanism, which is conducive to the accurate design and controllable preparation of titanium dioxide arrays. Moreover, there has not been much research showing that these materials have a solid theoretical design. Scientists will have the capability to forecast the characteristics of TiO 2 nanostructured arrays based on their material components, size, and morphology when the theoretical foundations and computational power are improved. Therefore, TiO 2 nanostructured arrays' components and structure may be tailored to meet the desired characteristics for various applications. The possibility for more multidisciplinary research on this subject will increase, resulting in further TiO 2 nanostructured array applications in several new fields.