Experimental Studies on TiO2 NT with Metal Dopants through Co-Precipitation, Sol–Gel, Hydrothermal Scheme and Corresponding Computational Molecular Evaluations

In the last decade, TiO2 nanotubes have attracted the attention of the scientific community and industry due to their exceptional photocatalytic properties, opening a wide range of additional applications in the fields of renewable energy, sensors, supercapacitors, and the pharmaceutical industry. However, their use is limited because their band gap is tied to the visible light spectrum. Therefore, it is essential to dope them with metals to extend their physicochemical advantages. In this review, we provide a brief overview of the preparation of metal-doped TiO2 nanotubes. We address hydrothermal and alteration methods that have been used to study the effects of different metal dopants on the structural, morphological, and optoelectrical properties of anatase and rutile nanotubes. The progress of DFT studies on the metal doping of TiO2 nanoparticles is discussed. In addition, the traditional models and their confirmation of the results of the experiment with TiO2 nanotubes are reviewed, as well as the use of TNT in various applications and the future prospects for its development in other fields. We focus on the comprehensive analysis and practical significance of the development of TiO2 hybrid materials and the need for a better understanding of the structural–chemical properties of anatase TiO2 nanotubes with metal doping for ion storage devices such as batteries.


Introduction
Titanium dioxide is a highly valued substance, used for its many beneficial properties in a variety of industrial products, including electronics, photovoltaics, paints, and foodgrade materials. The creation of nanotubes in TiO 2 phases, as well as their interactions with other metal dopants, biomolecule-polymer interactions, and interfacial processes, are leading to promising results in the fields of renewable energy and medicine. Nevertheless, much remains to be discovered about the properties of TiO 2 nanotubes, and considering that there are numerous TiO 2 phases, including rutile, anatase, brookite, hollandite, and metastable monoclinic phases. By stacking TiO 6 octahedra with warped edges, their crystal forms are created [1]. In addition, the anatase phase is one of the most effective photocatalysts due to its high photocatalytic activity, chemical stability, and negative carrier potential. The photocatalytic activity is affected by physicochemical parameters such as the grain size, crystallinity, surface area to volume ratio, surface texture, and geometry [1,2]. The fundamental drawback of TiO 2 for photocatalytic applications is that when exposed to visible light, only a very small fraction of the incident photons are utilized [2]. Rutile TiO 2 's large band gap accounts for this. Recent research on doped rutile TiO 2 has shown that it can be used in a wide variety of significant applications, such as photonic crystals, there is an urgent need to review the spectrum of molecular doping in terms of morphology, size, characterization, encapsulation, and nanocomposite behavior history. Therefore, the present review addresses various existing synthesis methods of TiO 2 NTs in the presence of metal dopants. We have observed the dimension and size of various TiO 2 NTs and the related physicochemical observations on the deposition of transition metals and their associated ions, such as Fe, Mo, Nb, Ru, Au, Ag, Pt, Cu, Co, Ni, V, Cr, and Mn. We have presented a DFT computational analysis for the doping of TiO 2 nanoparticles, followed by the experimental synthesis methods of TiO 2 NTs with metal dopants, the recent trends in the computational costs for the calculation of NT by classical and quantum molecular dynamic potentials, and, in the last section, we detail the future prospects of the synthesis methods for the metal doping of various TiO 2 nanostructures, such as nanobelts and nanohelices of NT, which are important for renewable energy and other potential biological applications. Although TiO 2 nanotubes present very valuable mechanical and optical properties, they have the drawback that the forbidden band is relatively large, so metal doping is an effective method to reduce this gap. In this regard, this review analyzes and evaluates the main findings of experimental methods for the metal doping of TiO 2 nanotubes; among the most prominent methods are the hydrothermal method, sol-gel method, co-precipitation method, and electrochemical anodization method. Similarly, this study discusses the importance of computational simulation as a very promising alternative method, because although DFT has been used as the only method for analysis, classical molecular dynamics is still an unexplored method for the study of doping in metal oxide nanostructures such as TiO 2 nanotubes.

Hydrothermal Method
Hydrothermal synthesis is primarily used to create metal oxide NTs [10]. By using it to create tiny-diameter TiO 2 NTs, Kasuga et al. [15] developed the hydrothermal approach. Currently, this method is widely used since it is a simple and versatile method; it requires a solvent (H 2 O or another) at a moderate temperature and high pressure. As observed in Figure 1a, the process begins with a precursor solution that can be aqueous or non-aqueous. The suspension is stirred vigorously for 2 h at 50°C. Then, the suspension is subjected to 130°C for 22 h in an autoclave. The product obtained must be centrifuged, filtered, and the solid sample must be subjected to an annealing wash at 400°C, with which the TiO 2 nanotubes will be obtained.

Self-Assembled Electrochemical Anodizing Method
Self-assembled electrochemical anodization (SOA) allows for the production of nanostructured arrays (such as aligned holes, nanochannels, and NTs) that are vertically oriented, size-controlled, and back-contacted (i.e., bonded to a metallic substrate) [16]. The metal of interest (M) can be used as the anode in a straightforward anode/cathode configuration to achieve anodization. Under the proper voltage, where the supply of oxygen ions is normally H2O in the electrolyte, it oxidizes to form a metal oxide as the basis for the oxide growth process; see Figure 1b. Template-assisted growth can produce TiO 2 NTs with a wide range of diameters and good homogeneity by simply changing the size and shape of the templates. For a procedure with a template, see [17]. Contrary to the electrochemical anodization method, it is compatible with a range of substrates, such as silicon and glass. The most common technique for creating NTs using positive template-assisted growth involves coating the templates' outer surfaces with TiO 2 and selectively etching them with wet chemicals. The first electrochemical deposition of TiO 2 NTs with template-assisted growth was described by Hoyer et al. [18] employing an ordered alumina template with a positive polymer form that was acetone-dissolvable.

Sol-Gel Method
In the sol-gel scheme, a precursor is dissolved in a solvent to produce a suspended emulsion that forms a gel after constant stirring [19]. Once the congruent suspension is attained, composite templates of anodized alumina or membranes made of polymers are soaked in the liquid medium. Further, the templates' pores are filled with suspended atoms and adopt their patterns; see Figure 1c. The required techniques of drying, calcining, and removing the gels produce the metal oxide NTs [20]. The template on which the particles are deposited, the content of the colloidal solution, the temperature, and the period of deposition all affect the particle size and the dimensions of the nanostructures [21][22][23][24][25]. The usability of the sol-gel nanostructures is constrained by the fact that they are frequently produced in bundled form [26]. The morphology of the final material is also impacted by the procedure needed to split the nanostructure derived from the template substance [26]. The studies on the synthesis of TNT by different methods are very limited, as noted by Shalini et al. [8], who discussed nearly thirty-five different methods for the preparation of TiO 2 NTs, all of which dealt with the anatase phase.

Experiments on Doping of Metals on TiO 2 NTs
TiO 2 NT has improved properties for photocatalytic applications compared to colloidal and nanoparticulate forms. TiO 2 NTs contain a higher concentration of OH. The strong photocatalytic activity of TiO 2 NTs was greatly attributed to both the increased capacity to absorb UV light and the large specific surface area of the NTs. Anatase TiO 2 NT has a band gap energy of 3.25 eV, which is somewhat higher than both rutile TiO 2 (3.2 eV) and anatase TiO 2 (3.2 eV) (3.0 eV). Rutile TiO 2 modification has a direct energy gap of (3.0 eV) (−0.1 eV) [27], brookite (−3.4 eV) (−0.1 eV), and anatase −4.2 eV. The indirect gap caused by anatase alteration is (3.2 eV) (−0.1 eV). In the context of chemical doping of TiO 2 NTs, the benefits of TiO 2 are its lack of toxicity, abundance of resources, and chemical stability. As a result, TiO 2 has received a lot of interest for a variety of applications, including lithium-ion batteries and biomedical ones [28][29][30][31][32].
TiO 2 can be used; however, its applications are constrained by its broad band gap and high rate of electron-hole recombination. The photo-response of TiO 2 must be expanded to the visible light area [33][34][35]; see Figure 2. In order to do this, a number of effective alterations have been made to improve the activity, including doping with metal ions-one of the most popular techniques-and decoration with metals, metal ions, and non-metals. Transition metals (TM) possess several valences and a d-electron structure that is not full in order to accept extra electrons, bring impurity levels into the TiO 2 band gap, and operate as a shallow trap for photo-generated electrons or holes in order to restrict the recombination of electron-hole pairs [36][37][38][39][40]. Physical methods such as laser ablation, magnetron or thermal sputtering, ion implantation, and so on, as well as chemical methods such as solvothermal, chemical, and electrodeposition (Figure 3a), sol-gel, hydrothermal (Figure 3b), direct oxidation, and so on, are available for the synthesis of doped TiO 2 . Numerous benefits come with the hydrothermal approach, including affordability, ease of use, adjustable nucleation, a suitable reaction time and temperature, and high productivity. The hydrothermal method is particularly attractive because it allows the production of uniform TiO 2 NTs with an outer diameter of around 10 nm [41][42][43]. Similar to NTs, TNTs have sparked a lot of interest in the biological fields due to their simplicity and the inexpensive cost of manufacture by electro-chemical anodization.  By altering the applied potential, pH, the amount of F ions in the electrolytes, and other variables, it is possible to accurately control their lengths, wall thicknesses, and diameters [44][45][46]. In the context of NT, Sn-doped TiO 2 (B) NTs had NT capacities of 241.6 mAh g −1 after 100 discharge/charge cycles at 0.1 • C, and 115.9 mAh g −1 after 10 cycles at 2 • C, according to Li et al. [47]. In order to reduce the Li+ transfer distance, Sn 2+ doping appears to improve the electrical conductivity and contact between the electrode and the electrolyte. According to their research, Sn-doped TiO 2 (B) NTs may have potential as a negative electrode component for creating reliable lithium-ion batteries. Although the results are promising, using a template-based liquid-phase deposition technique, Tu et al. [48] created Sn-TiO 2 NTs. They discovered that the NTs had a maximum methylene blue degradation rate of 88% in 6 h and 95% in 8 h when exposed to UV light. The light absorption only reaches the near-UV range due to the quick recombination of photo-generated charge carriers in Sn-TiO 2 and Sn doping in their research, which nevertheless has an impact on the photocatalytic efficiency. Li et al. [48] introduced an oxygen gap in TiO 2 as a solution to this issue and a practical method to enhance the functionality of Sn. Although the results are promising, using a template-based liquid-phase deposition technique, Tu et al. [48] created Sn-TiO 2 NTs. Because of the generation of an intergap containing electronic states as a result of the hybridization of the O 2p orbital with the Ti 3d orbital, the oxygen-coated TiO 2 's band gap would be reduced as a result of charge transfer from the O 2p orbital to the Ti 3d orbital (Vo-TiO 2 ). Figure 4 displays the photocatalytic performance and mineralization rates. Vo-TiO 2 demonstrated more photocatalytic activity in comparison to TiO 2 , Sn-TiO 2 , and Vo-TiO 2 due to more effective photoinduced charge separation and the use of visible light by introducing the oxygen gap and doping Sn into the TiO 2 NTs. (a) Rates of nitrobenzene photocatalytic mineralization by visible-light-exposed TiO 2 , Sn-347 TiO 2 , Vo-TiO 2 , and Vo-Sn-TiO 2 . (b) Rates of RhB photocatalytic mineralization when exposed to 348 TiO 2 , Sn-TiO 2 , Vo-TiO 2 , and Vo-Sn-TiO 2 . Ref. [48] is reprinted with permission. 2016 Elsevier B.V. 349 Copyright.
In the presence of an electric current, NTs serve as catalysts. As a result, they can provide molecules that come into contact with the reaction site's electrons. Anatase has more warped angles between the Ti-O bonds than a 90-degree angle. Due to these config-urational variations, anatase has a wider band gap (3.2 eV versus 3 eV) and less thermal stability than rutile [49,50]. However, anatase often has a greater specific surface area than rutile, which is very interesting for (photo)catalytic applications [29]. In fact, anatase and anatase-rutile combinations, such as P25, actually have stronger photocatalytic activity than rutile [46,51,52]. TiO 2 NTs doped with Zn 2+ had the highest catalytic activity, according to studies on doping with Zn metals, as a result of the weight fractions of the anatase phase, average crystallite sizes, SBET, energy band gap of the catalyst, and doped ions. These activities decreased as a result of doping with Mn 2+ or Ni 2+ . According to Figure 5 [53], the TiO 2 NTs exhibited a hollow, open structure with an average diameter of around 10 nm. The wall thickness of the tubes was also roughly 1 nm. Despite the fact that there is no known treatment for diabetes, diabetics should have their blood glucose levels closely monitored. Due to their versatility in terms of structural design and composition, ease of separation and storage, high stability, simplicity of fabrication, and tunable catalytic activity, these nanomaterials hold great promise for the development of colorimetric glucose biosensors. This activity is based on the peroxidase-like activity of these nanomaterials when used with glucose oxidase. The TiO 2 NT and CeO 2 combination provided by Zhao et al. [55] exhibits the largest concentration of Ce 3+ and the best peroxidase-like activity when compared to nanowires (NW) and nanorods (NR). The reaction offers a simple approach for the colorimetric detection of glucose and H 2 O 2 with detection thresholds of 3.2 and 6.1 M, respectively. This is due to the high activity of the CeO 2 /anatase phase of TiO 2 NT. The details of the three nanostructures are shown in Figure 6 with different doping concentrations of CeO 2 [55]. . XPS spectra were used to calculate the concentrations of Ce 3+ and Ce 4+ , which confirmed that the CeO 2 NT had the best peroxidase-like activity. The presence of TiO 2 promotes higher Ce 3+ concentrations in the following order: CeO 2 /NT-TiO 2 @0.1 > CeO 2 /NR-TiO 2 @0.1 > CeO 2 /NP-TiO 2 @0.1 > CeO 2 /NW-TiO 2 @0.1. In comparison to CeO 2 /NW-TiO 2 , CeO 2 /NR-TiO 2 , and CeO 2 /NP-TiO 2 . Reprinted with permission from Reference [56]. American Chemical Society 2015, all rights reserved.
Food safety and quality have shown a variety of uses for surface-enhanced Raman spectroscopy (SERS), including the analysis and detection of chemical and microbiological threats [57][58][59]. Recent advancements in SERS techniques for food safety and quality applications have concentrated on the development of enhanced SERS substrates and methods to (1) increase sensitivity and selectivity while reducing matrix interference, and (2) enable nondestructive sampling and in situ detection [57][58][59][60][61]. The activity and reproducibility of the plasmonic material used as the SERS substrate have a strong influence on the quality of the measured SERS spectra. Recently, according to Ambroziak et al. [62], the SERS activity of a layer of cubic silver nanoparticles on a tubular TiO 2 substrate was found to be eight times that of a typical electrochemically nanostructured silver electrode surface. The deactivation of the active surface was also slowed down by silver nanoparticles on TiO 2 NT substrates. The active surface had a milder deactivation process, which was also seen for silver nanoparticles placed on a TiO 2 NT substrate. The geometric dimensions and SEM details of bare TiO 2 NT and also cubic Ag nanoparticles/nanocubes (AgCNPs) @TiO 2 NT are shown in Figure 7a  To develop titanium-based implant materials, potentiodynamic polarization tests by Yu et al. [64] show that TNs modified with Au nanoparticles (AuNPs) are superior to unaltered TNs in terms of corrosion resistance. AuNPs are deposited on TiO 2 NT arrays (TN) by electrochemical deposition to improve the surface properties. The corrosion resistance increases with the amount of charged AuNPs. The adhesion and proliferation ability of osteoblast cells on the AuNP-modified surface of TNT is greater than on the unmodified surface of TNT; this type of AuNP-TNT array can produce titanium-based implant materials to improve bioactivity. Similarly, the electrochemical stability of TiO 2 NT deposited with Ag and Au nanoparticles was presented by Arkusz et al. [65], proving that it was possible to deposit Au nanoparticles with a size range of 20.3 ± 3 nm on the top, inner, and outer surfaces of TiO 2 NTs and that doing so did not cause structural damage to the TNT. According to TNT re-crystallization testing, the annealed anatase TNT was stable in aqueous environments and did not experience any significant structural changes, as shown in Figure 8A. According to Figure 8B, TNT has lower conductivity because its EIS displays a larger semicircle than those of the other modified electrodes. The excellent conductivity of AgNPs results in a total increase in conductivity when TNT is modified with them. This is mostly because of the quick electron transfer. Another indication that TNT-infused Au nanoparticles are more conductive than TNT alone is the charge transfer resistance of AuNPs/TNT. Both AgNPs and AuNPs reduce the electrical conductivity without acting as ion traps. The difference between TNT modified with AgNPs and AuNPs in charge transfer resistance (Rct) should be noted. The increase in Rct could have been caused by the steric restrictions that AgNPs and AuNPs on the electrode had induced. Gold nanoparticles are smaller than silver nanoparticles; therefore, the electrostatic barrier causes repulsive interactions between AuNPs and TNT, increasing Rct [65]. According to a study by Kim et al. [66], Cu-coated TiO 2 NTs are crucial for improving the performance of Li-ion batteries' electrochemical system. The differential capacitance (dQ/dV versus V) for the first cycle of TiO 2 NTs in the anatase phase and Cu-coated TiO 2 NTs at a current density of 50 mA g −1 is shown in Figure 9a for TiO 2 NTs in the anatase phase and Cu-coated TiO 2 NTs, respectively, via their cell potential versus differential capacitance curves. The pristine and Cu-coated TiO 2 NTs have a total discharge capacity of 253.9 and 259.4 mA h g −1 , respectively. The faster response with Li-ions was related to the improved charge transfer behavior, such that the Cu-coated TiO 2 nanotubes' irreversible capacity loss was lower than that of the uncoated TiO 2 NTs. These results support the copper coating of TiO 2 NTs as an electrochemical performance enhancer. As shown in Figure 9, electrochemical studies revealed that Cu-coated TiO 2 NTs exhibit better charge transfer behavior, better performance at high cycle rates, and higher discharge capacities than the original TiO 2 NTs. Additionally, the results of the uniform Cu coating can be seen in Figure 10 as SEM and TEM images. It is observed that anatase NTs have a wall thicknesses of approximately 15 nm and interior diameters of approximately 100 nm. Each TiO 2 NT in the array in Figure 10a is around 18 µm long and is equally spaced over the surface. The NTs have an inner diameter that varies between 40 and 50 nm, an inner wall thickness of around 30 nm, and an outside wall thickness of roughly 10 nm. Cu is consistently deposited on the surface of TiO2 NTs as shown in Figure 10b, in contrast to the original TiO 2 NTs in Figure 10a. The inner diameter of the Cu-coated TiO 2 NTs was smaller than that of the parent TiO 2 NTs as a result of the uniformly coated Cu on the surface of the TiO 2 NTs [66]. . The differential capacity plot (dQ/dV vs. V) corresponding to the first cycle of the parent and Cu-coated TiO 2 NTs at a current density of 50 mA g −1 is shown as (a) differential capacity vs. cell potential curves of parent and Cu-coated anatase-phase TiO 2 NTs at 50 mA g −1 . (b) Discharge-charge curves of the parent and Cu-coated TiO 2 NTs between 2.7 and 1.0 V at a constant current density of 50 mA g −1 show that the Cu-coated TiO 2 NTs' Coulombic efficiency was 13.7% higher than that of the parent TiO 2 NTs. The aforementioned findings point to a more pronounced charge transfer response in the Cu-coated TiO 2 NTs during the charge-discharge procedure than in the parent TiO 2 NTs. With permission from Ref. [66]. 2015 Elsevier B.V. All rights reserved. To achieve the efficient completion of the photoelectrochemical water-splitting process, it is possible to insert appropriately tailored nanostructures into the photoelectrode to improve the light-matter interactions for effective charge production, charge transport, and the activation of surface chemical processes [64,[67][68][69][70]. In plasmon-enhanced photoelectrochemical water splitting, the high electrochemically active surface area, optical absorption capability, and charge transfer rate play a key role in improving the photoelectrochemical activity of materials, as seen in the work by Cai et al. [71]. In particular, for ZnO-coated TiO 2 NT, as shown in the SEM image of Figure 9a,b, when light with 420 nm wavelength cutoffs is irradiated, the result is as depicted in Figure 11a,b. In comparison to the naked TiO 2 NTs, the ZnO-covered TiO 2 NTs have improved photoelectrochemical activities. In particular, for the 10-cycle ZnO-covered TiO 2 NTs, a considerable rise in the photoelectrochemical activity is seen. With the 10-cycle ZnO/TiO 2 NTs, the PC density rises from 1.4 µA/cm 2 for the naked TiO 2 NTs to 2.2 µA/cm 2 . In contrast to the bare TiO 2 NTs, the TiO 2 NTs coated by a 10-cycle ZnO deposit or by a 2.1 nm ZnO coating exhibit an increase in photoelectrochemical activity of over 60%. The 10-cycle ZnO/TiO 2 NT photoelectrode still displays quick and outstanding transient photocurrent responses in the PC density ( Figure 11c) [71] after around 1.5 h of intermittent visible illumination with a cutoff wavelength of 420 nm.
Due to the physicochemical importance of metal-doped TiO 2 NT, there are a number of variations and challenges associated with the various metal-doped TNTs. The following table (Table 1) explains the recent trends in the three main synthesis methods, size, phase, and dimension, as well as their characterization and physicochemical parameters, for different metal dopants of TNTs.  Si-TNTs have a uniform NT doped with 10% Si, whose photocatalytic methylene blue degradation efficiency triples compared to the undoped TiO 2 TNTs under ultraviolet light. Si-doped TiNTs are a promising substance for wastewater treatment in the industrial sector. FTIR confirms that the formation of the Si-O-Ti bonds contributes to the stabilization of the lattice and positively affects the photocatalytic activity of TiO 2 . [75] 5 Ultrasonic-assisted solhydrothermal method Anatase, 20 nm < length < 100 nm. Uniform values of diameter and outer diameter of ≈60 nm and ≈ 8 nm, respectively. Fe Higher photocatalytic activity than pure NTs, and R Fe = 1.75 was found to be the ideal doping concentration. By adding Fe, NTs' absorption edge was moved into the visible light spectrum, narrowing the band gap. When exposed to visible light, the absorption increased 2-to 4-fold above pure NTs. FeCu 3+ (upper) ions changed the composition of the phase and modified the catalyst's surface area, surface area distribution, and photocatalytic activity. [76] 6 Ultrasonic-assisted solhydrothermal method Anatase, rutile, average diameter 70-90 nm, wall thickness of 10-20 nm. SnO 2 -Sb SnO 2 NTs that have Sb doping. With a shelf life of 116 h, the TiO 2 NTs/SnO 2 -Sb electrode outlasted the Ti/SnO 2 -Sb electrode by a considerable amount (1.6 h). In comparison to electrodes made using the traditional dip-coating technique, those made utilizing hydrothermal synthesis have substantially better and larger SnO 2 crystals. [77] 7 Combination of sol-gel process with hydrothermal treatment Anatase, length > 100 nm, outer diameter of approximately 10 nm Fe With anatase Fe/TiO 2 NT, a greater photocatalytic effect was attained compared to Fe-doped NTs. The best photocatalytic activity was found in the 0.5% Fe/ TiO 2 NTs that were calcined at 300 • C; this activity was superior to that of the pure powder. Methyl orange degradation rate for pure powder and NT after two hours of calcination at 300 • C. 63.5% of the methyl orange decayed in the presence of pure NT after 3 h of irradiation, compared to 69.1% of the methyl orange in the presence of pure powder. [78] 8 Prior to using the hydrothermal procedure for NT, the solgel method was used to create their precursory nanopowders.
TNTs have a 20 nm pore size distribution, with the majority of them falling between 2.0 and 9.0 nm. The creation of the hysteresis loop, which diminishes marginally with increased Ce addition, is adversely affected by the greater pore size dispersion. The 2.5 mol% Ce-doped TNT had the greatest specific surface area (196.01 m 2 /g), lowest zeta potential (0.49 cm 3 /g), largest pore volume (0.49 cm 3 /g), and the best capacity to photocatalyze the dye MB quickly and remarkably effectively even when exposed to sun light. Ce is entirely dissolved in the titanium lattice during the hydrothermal synthesis as Ce 3+ and Ce 4+ , whose ratio rises with increasing Ce addition. [79] 9 Two-step hydrothermal method Anatase, length of > 100 nm, tubular with outer diameters of 8 to 12 nm. Ag The XRD patterns of the Ag-doped NTs show that the nanocrystalline anatase structure is preserved after doping. After Ag doping of the NTs, the FTIR spectrum hardly changes. In line with the findings of the XRD and FTIR spectra, the Raman spectra for both undoped and Ag-doped NTs confirm the anatase phase. The Ti-O vibrations of anatase TiO 2 are represented by the Raman bands at 144, 395, 515, and 635 cm −1 . The characteristic Raman peaks of the NTs become more potent with Ag doping. TiO 2 's particle size, crystal structure, specific surface area, and morphology can all be altered by metal doping. The increased crystallinity of the Agdoped NTs may be the reason that their Raman peaks are more intense than those of the undoped NTs. Ag-doped NTs are promising as effective antimicrobial reagents that are free of resistance. [80]  A superconducting quantum interference magnetometer was used to examine the magnetic characteristics of titanate nanotubes (NTs) doped with Fe 3+ , Ni 2+ , and Mn 2+ ions (SQUID). The as-prepared NTs changed as the calcination temperature exceeded beyond 350 K; they then changed again when the temperature rose, becoming a mixture of anatase and rutile titanate. The titanate NTs doped with Fe 3+ /Ni 2+ /Mn 2+ ions behaved para-magnetically. [84] 14 Hydrothermal treatment Anatase and rutile, 300 nm in length, Fe on-Pt-TiO 2 NT The rutile NTs collapsed, leaving rutile crystals that were around 50 nm in size, whereas the anatase NTs maintained their structural integrity. In comparison to the unaltered material, the hydrogen evolution rate for anatase NTs was six times higher. [85]

15
A straightforward sol-gel procedure, followed by an alkalithermal reaction, and finally an NH3 thermal treatment Anatase, diameter of 6 nm and a length of approximately 160 nm, Ni Due to its superior rate performance, higher initial Coulomb efficiency (65%), high reversible capacity (303 mA hg −1 after 500 cycles at 50 mA g −1 ), and superior long cycle life, the Ni-N/TNT electrode significantly outperforms the TNT and Ni/TNT electrodes in terms of sodium ion transport and storage (8000 cycles). Polarization can be lessened by co-doping with nickel and nitrogen. To improve the electrochemical performance of SIBs, co-doping with Ni and N can successfully tune the sodium ion diffusion rate and the electronic and phase structure of TiO 2 . [86] 16 One-step solvothermal method Anatase, diameter is around 0.1-1 µm, length is around 2-10 µm Si Si in TiO 2 exhibits more absorption than TiO 2 NTs, by 5%. Due to doping, Si-TiO 2 NTs require less energy than other materials to excite electrons from the valance band to the conduction band. TiO 2 NTs are inferior to Si-TiO 2 NTs in terms of photocatalytic activity. [87]  Before thermal treatment, the anodic TiO 2 NT arrays were amorphous, but they eventually crystallized to anatase at 400 • C and started to lose some of their abundance, although they lasted up to 1000 • C. Rutile started to form in modest amounts at 600 • C and its abundance quickly grew at 800 • C, which is consistent with anatase's rapid decomposition.
Anatase is known to be metastable, and by encouraging defects such as vacancies, the presence of Cr doping expedited the synthesis of anatase and rutile at a lower temperature. The conversion rate of anatase to rutile was significantly reduced by lowering the crystallization temperature of rutile from 600 to 500°C and of anatase from 400 to 400 • C. After annealing at 1000 • C, the rutile produced in TiO 2 was extremely stable, while it became unstable in Cr-doped TiO 2 , which broke down to TiO 2 at 900 • C. The presence of a Cr ion composition in doped TiO 2 was confirmed by ion beam analysis performed by RBS. [88] 18 The fact that Sn-doped TiO 2 NTs appear to have a bamboo joint structure is supported by Sn-doped TiO 2 NTs with varying Sn content (3, 5, and 7 at.%). After tin doping, the NTs have a better distribution. Doping greatly increases the electrical conductivity when the tin concentration is less than 5 atomic% by increasing the number of free carriers and Hall mobility. Both the carrier concentration and Hall mobility rapidly drop when the tin content rises from 5 to 7 atomic %. The 5 at.% Sn-doped TiO 2 NTs exhibit noticeably increased electrochemical performance and rate capability, according to XPS measurements. The continuous redox behavior of TiO 2 before and after Sn doping corroborates the results of XRD, Raman, and XPS and demonstrates that appropriate doping does not alter the anatase structure of pure TiO 2 . According to SEM analyses, the diameter is around 10 nm, and the average length is 150 nm. After tin doping, the tubes' diameter and length remain unaltered. In contrast, and in line with SEM, the NTs are more evenly distributed following tin doping. [92]  By inducing a red shift, moving the band gap to the visible light range, and successfully inhibiting the recombination of photoinduced electron-hole pairs, CeO 2 in TiO 2 NTs increased the photocatalytic activity. These substances have the potential to be employed as photocatalysts for the degradation of organic molecules in aqueous solutions because they are stable. Compared to P25, which had degradation efficiency of only 68% after 120 min of exposure, MB had photodegradation efficiency of 94.6% after 60 min and nearly 100% after 120. The capacity of TiO 2 NTs to deteriorate was successfully improved by the composite of TiO 2 and CeO 2 . When CeO 2 was used as a dopant for TiO 2 NTs, it was possible to see aggregated CeO 2 nanoparticles on the surface of the TiO 2 NTs that ranged in size from 5 to 10 nm. [95]
Anatase, ≈120 nm outside diameter, Cr, Co, Cu, Fe, Mn According to the findings of XRD, TEM, and SAED, doping with 3d TM ions has no effect on the crystal phase of The thin-film X-ray diffraction pattern (TF-XRD) confirms that the nanotubular structure of the sample annealed at 450 • C is preserved, whereas it is partially collapsed in the sample annealed at 600 • C.
Only the diffraction peaks of the matrix indicating the amorphous structure are seen in the TF-XRD patterns of the produced sample and the NT film annealed at 450 • C. In contrast, the typical peak of rutile TiO 2 emerged at 27.5 • C after the sample was annealed at • C. After annealing at 450 • C for 1 h, no anatase TiO 2 was found in the film NT, which may be easily obtained from the TiO 2 NT film formed on pure Ti under the same annealing circumstances. At 600 • C, the TiO 2 NT film grown on pure Ti undergoes a change from the anatase phase to the rutile phase, which might cause a partial collapse of the NT walls. The morphology, crystal structure, and surface chemistry of the Ni 2 O 3 -doped TiO 2 NT film are affected by changes in the preparation processes, such as the anodization temperature, time, and annealing temperature. These changes in turn affect the NiTi alloy's ability to resist corrosion, exhibit bioactivity, and be wettable. Rutile crystals were produced during annealing at temperatures of 600 • C, increasing the film's bioactivity. [97] 27 The ability of undoped TiO 2 to absorb light was enhanced by Ni doping. The separation of photo-generated electron-hole pairs was aided by moderate Ni doping. It was discovered that moderate Ni doping was a crucial design element for improved PEC properties.
At 550 • C, Ti1NiO NTs demonstrated improved PEC water-splitting ability. The anatase to rutile phase transition benefited from Ni doping. Rutile modes became stronger with an increase in annealing temperature from 600 • C to 700 • C, according to XRD patterns, suggesting that an additional rutile phase could occur. Ti1NiO, which mostly comprised the anatase phase, was annealed at 500 • C. When the material was annealed at 550 • C, rutile phase peaks became visible. [100]

30
TNTs were synthesized by a hydrothermal method. CeO 2 TNT hybrids were prepared by wet chemical deposition precipitation method.
Anatase, NTs with multilayered walls and the diameter is around 10 nm. CeO 2 CeO 2 and TNT hybrid materials were successfully synthesized, as demonstrated by XRD, XPS, and TEM, and CeO 2 particles were uniformly loaded on the surfaces of TNTs. Cone calorimeters were used to study samples' combustion qualities and thermogravimetric analysis (TGA). The thermal stability and flame retardancy of matrix materials can be significantly improved by adding loaded CeO 2 to TNTs, according to cone results. At 700 • C, the EP /0.1 CeO 2 TNTs have the largest residual carbon content (19.8%). It also has the slowest rate of degradation; the PHRR and THR are 680 kW/m 2 and 32.9 MJ/m 2 , respectively, reduced by 38.2% and 23.1% from the raw epoxy resin. TNT in EP creates a cross-linked network structure that can function as a physical barrier. The creation of the protected carbon layer and the efficient inhibition of the emission of both quality and quantity of heat from TNT loaded with CeO 2 can produce a flame-retardant effect. CeO 2 -loaded TNTs can significantly increase the structural stability and extensibility of the carbon layer.
[101] A maximum photocurrent density of 0.0016 A/cm 2 was displayed by CdSe/TiO 2 NTs after 30 min of immersion in a CdSe precursor solution, an improvement of around 10% over pristine NTs when exposed to solar light. When CdSe species were not loaded onto the TNTs in amounts greater than 0.5 at%, the characteristics of the potential photocurrent density could be seen. These might work well as electron scavengers to reduce recombination losses when exposed to solar radiation. A saturation level may be attained by overloading the CdSe-containing TNTs by 0.5%. The creation of CdSe spheres near the opening of the NT arrays could have adverse effects because of the significant number of charge carrier recombination locations. [102]

32
Alkaline hydrothermal process in a single step without additional calcination and reduction treatment Rutile and anatase, length 100-200 nm, pore diameter 9.5 to 11.5 nm. Ag The Brunauer-Emmett-Teller (BET) surface areas of the unmodified TNT and Ag 2 0 TNT samples were 392 and 330 m 2 g −1 , respectively. TEM images showed that the TNT sample had 100-200 nm long openings on both sides. The adsorption edge of the Ag-modified sample TNT clearly shifted to the blue region. OH groups, pre-adsorbed H2O, and oxygen vacancies were found on the surfaces of the resultant samples, according to FTIR, PL, and XPS investigations. By assessing the removal effectiveness for Hg0, MG, CV, or the combination of both dyes under UV irradiation, the photocatalytic activities of the resulting samples were examined.
The more AgCl and Ag species that were present on the surface TNT treated with Ag, the longer it took for the e − /h + couple to separate. For Hg0, CV, MG, and the mixture of dyes, the Ag 2 0-TNT sample had better removal efficiency than the unaltered TNT sample. Successfully used for photochemical degradation of carcinogenic dyes and Hg0 in the gas phase. With an initial capacity of 186.4 mA h g −1 and a capacity that remains constant after 100 cycles at a current density of 100 mA g −1 , the Li 4 Ti 5 O 12 NT/graphene composite specimen has the highest discharge specific capacities and cycle stability. [104] 34 Sol-gel process, hydrothermal process, subsequent annealing in N2 atmosphere Anatase, inner diameter of around 5 nm and an outer diameter of around 15 nm, pore diameter 9.37 nm. Sn As compared to TiO 2 , Sn-TiO 2 , and Vo-TiO 2 , Vo-Sn-TiO 2 has greater photocatalytic performance. With oxygen vacancies and Sn doping present in the TiO 2 NT, the photocatalytic performance was improved. This is attributed to increased light absorption, increased specific surface area, and decreased electron-hole pair recombination.

DFT for Materials
Among the conventional computational models, the robustness and generality of first-principles electronic structure methods come at enormous computational expense. The first-principle electronic structure theories or ab initio approaches also refer to these techniques; they rely on the Schrödinger equation being solved, which, from the ground up, defines how electrons and nuclei move in a chemical system [105][106][107][108][109]. These methods calculate the molecular energy and associated properties of a given molecular system based on fundamental physical concepts, such as the electrical and nuclear composition of atoms and molecules. First-principles calculations provide precise evaluations and are always aimed at improving the calculation of the atomic-and electronic-level properties of materials. They are based on quantum physics, electrodynamics, statistical thermodynamics, and classical mechanics. First-principles calculations can now efficiently and accurately solve a wide range of material-related issues-for instance, those relating to the design of materials for electrical power production, automotive applications, energy storage, microelectronics, and the chemical industry generally [107,[110][111][112][113]. Even though first-principles computations are time-consuming, the approximations can be improved in various situations, such as transition metal oxides, rare-earth compounds, van der Waals interactions, etc., by using so-called post density functional methods (DFT), which are computationally more difficult but still helpful [54,56,[114][115][116]. By solving these quantum physics puzzles, firstprinciples calculations are kept devoid of any empirical knowledge, and their predictive power is particularly useful when searching for new materials with as-yet-unidentified properties [117][118][119]. The discovery of the actual thermodynamic ground state is aided by the fact that fundamental thermodynamical properties, such as the heat of formation, can be calculated at least as accurately as the results of experiments, but much more quickly and for a much wider range of potential structures and compositions. DFT electronic structure calculations can easily determine the forces on atoms and stresses, which is necessary for improving the structural parameters and determining the vibrational properties. By incorporating phonons, the applicability of DFT research is substantially increased because the majority of the effects of temperature on the free energy can be taken into consideration. These matrix-diagnostic schemes in quantum and classical mechanics provide microscopic details for the system of interest. One such example is that the properties of a doped semiconductor material calculated at DFT have practical potential for increasing the efficiency of a photoelectrochemical cell [120][121][122][123]. Additionally, a 210,000 × 210,000 matrix must be created and diagonalized in order to perform a DFT calculation and obtain the lowest eigenvalues of very large matrices-for example, for the smallest systems of 1000 basis functions with around 300 electrons [124][125][126][127]. This is much too large. This matrix alone demands around 300 GB of storage space on disk. The helical phototranslation symmetry of these structures at NTs was used to compute NT with large unit cells (432 atoms in the case of the (36,36) NT) [128,129]. Automatic structure generation, one-and two-electron integral computation, generation of the complete Fock matrix by rotating only the irreducible fraction of the Fock matrix, and diagonalization of the Fock matrix are all made possible by this symmetry [128][129][130].

Quantum Computational Verification of Experimental Doping of Metals on TiO 2 Nanoparticles
A 40-nm-diameter cobalt-doped TiO 2 nanoparticle was created. Based on DFT, it was determined that cobalt doping changed the band structure of TiO 2 since the band gap of cobalt-doped TiO 2 was 72% smaller than that of pure TiO 2 . The structure simulation and mechanism analysis have demonstrated the co-doped TiO 2 gas sensor's improved characteristics and excellent gas-sensing capabilities at ambient temperature [131]. An ab initio study of the systems Ti 1−x R x O 2 (R = Mn, Fe, Co, Ni, Cu for various concentrations of substitutional impurities) in the rutile structure revealed that magnetic moments are present in Fe, Co, and Mn but not in Cu or Ni [132,133]. These calculations reveal an essential fact: doping reduces the energy required for vacancies to occur, resulting in doped systems having more vacancies than undoped systems. These findings are consistent with the idea that oxygen vacancies are crucial to the development of magnetism in doped TiO 2 and may help to explain the variety of magnetic moments that have been experimentally discovered in samples grown under various conditions [131]. Aspects of the electron paramagnetic resonance demonstrated that the co-precipitated TiO 2 with Fe ions in place of the Ti ions had increased photocatalytic activity. This was credited to the Fe ions integrated into the TiO 2 crystal lattice's synergistic effects of greater visible light absorption and low recombination of electron holes. DFT calculations using the CASTEP package based on the plane-wave pseudopotential approach confirmed the role of Fe in the electronic structure of TiO 2 [134]. In the DFT calculations, the hybrid B3LYP functional and the double zeta basis set LanL2DZ were used. In terms of photocatalytic activity for 4-nitrophenol degradation, Fe 3+ -doped TiO 2 outperformed undoped TiO 2 . DFT calculations show that the introduction of new electronic states within the band gap is what causes Fe 3+ -doped TiO 2 to become visible-light-active [135]. For a DFT computational examination of the formation energies of Si, Al, Fe, and F dopants of different charge states over different Fermi level energies in anatase and rutile, a local density approximation (LDA) study is a very well-liked and affordable technique [136]. The formation energy for F doping is shown to be the lowest at interstitial regions, whereas the substitutional sites of Ti have the highest stability for the cationic dopants. All dopants significantly stabilize anatase compared to the rutile phase, indicating that the conversion of anatase to rutile is prevented in such systems, with the dopants arranged in the order F > Si > Fe > Al according to the strength of anatase stabilization. The Al and Fe dopants act as shallow acceptors, with charge balance achieved by the formation of mobile charge carriers rather than anion vacancies [133,134,[137][138][139]. In Si-TiO 2 , the Si dopant acts as an inhibitor of the phase transformation from anatase to rutile; in Al-TiO 2 , the Al dopants act as weak inhibitors of the phase transformation from anatase to rutile, and the Fe dopants act as inhibitors of the phase transformation in TiO 2 [133]. TiO 2 (Li-TiO 2 ) have been studied by DFT [140]. Li itself is not magnetic; it creates holes in O-2p pi orbitals leading to magnetic moments in TiO 2 . The magnetic moments are delocalized but inhomogeneously distributed in the lattice, with long-range ferromagnetic interactions between them. The inhomogeneous distribution of the magnetic moments and the specific crystal symmetry lead to a distance dependence as well as a direction dependence of the stability of the ferromagnetism of the Li-TiO 2 system. Calculations suggest that Li-TiO 2 is a potential material for spintronics [140][141][142]. DFT calculations were carried out to examine the effects of Al or Cu doping on the electronic and geometrical structure as well as the photocatalytic properties of TiO 2 to support sol-gel-based Cu and Al doping studies, and the result was as follows. By optical measurements, the Cu and Al impurities were found to reduce the optical band gap values of the films produced. Finally, the doping process played an important role in improving the photocatalytic efficiency of the samples. Compared with Cu-doped TiO 2 , the Al-doped TiO 2 film showed the highest photocatalytic activity [143]. For Gram-negative E. coli and Gram-positive bacteria Aureus Staphylococcus, the antibacterial capacity of copper (Cu)-doped TiO 2 (Cu-TiO 2 ) was examined under visible light irradiation [144]. DFT studies showed that Cu + and Cu 2+ ions were introduced into the TiO 2 lattice instead of Ti 4+ ions, causing oxygen vacancies, and these improved the efficiency of the photocatalytic reaction [144][145][146]. Significantly high inactivation of bacteria (99.9999%) was achieved in 30 min of visible light irradiation by Cu-TiO 2 . First-principles calculations based on DFT [147] were carried out to examine the electrical and optical properties of pure rutile TiO 2 and TiO 2 doped with tin (Sn) and zinc (Zn). The SCAPS-1D simulator's DFT-extracted carrier mobility, band gap, and absorption spectra of TiO 2 were used to gauge how well solar cells performed in relation to the dopant concentration and TiO 2 thickness. Particularly when compared to the performance of PSCs with Sn-doped and Zn-doped TiO 2 , the functionality of perovskite solar cells (PSCs) with 3.125 mol% maximum power conversion efficiency (PCE) for Sndoped TiO 2 is 17.14, compared to 13.70% for undoped TiO 2 . Sn-doped TiO 2 has 0.63% better PCE than Zn-doped TiO 2 at the same doping concentration. The geometric structures and electronic properties of Ni-doped anatase and rutile TiO 2 have been successfully calculated and simulated using a pseudopotential method for plane waves based on DFT. Under O-rich growth conditions, the band gap for substitutional Ni in Ti-doped anatase TiO 2 shows a slight decrease of around 0.05 eV in comparison to pure anatase TiO 2 and contains a number of impurity energy levels that may be the cause of the experimental photocatalysis' red shift in the absorption edge and activity. The excitation energy of the photons decreases as a result of the impurity energy levels. When compared to pure TiO 2 , a roughly 0.05 eV smaller band gap is present [148]. Trends in the electronic structure and magnetic properties of TiO 2 -based strain gauges with impurities of transition metals (Sc, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) using DFT were assessed. There is a significant magnetic moment in TiO 2 -Fe and TiO 2 -Mn, with values of roughly 4.21 Bohr Mag/cell and 3.48 Bohr Mag/cell, respectively [149]. However, based on the density of states, TiO 2 -Sc and TiO 2 -Ni show no change in magnetic properties compared to pure TiO 2 [148,150]. TiO 2 showed that the more frequent oxygen vacancies and high surface basicity of Ni 2+ -doped TiO 2 contributed to higher nucleophilic attack activity for the hydrolysis of CWA [151]. To support the aforesaid conclusion, the surface structure and calculated charge distribution were obtained. The XRD spectra of NiO crystals, anatase-TiO 2 , with and without distinctive doping of Ni 2+ (TiO 2 -Ni), were simulated and the corresponding theoretical curves were considered in comparison with the experimental data. The outcomes display that the experimental curves are in ideal agreement with the curves calculated through the DFT method, indicating that Ni2+ doping does now no longer affect the crystal structure. The calculations performed with the Materials Studio program package in the CASTEP module show that metals used for doping, such as Co, Ni, Sb, Zn, Ag, and Mn, decreased the band gap of TiO 2 and shifted the position of DOS downward [149,[152][153][154][155]. These effects facilitated the migration of charge carriers and increased the photoactivity. Doping with cerium leads to greater stability of TiO 2 even at high temperatures, which offers greater advantages in industrial applications [156,157]. Ce and Zr doping of rutile TiO 2 was investigated using DFT +U and the HSE06 implementation of the hybrid exchange method DFT [156][157][158][159], revealing favorable dopant incorporation, lower oxygen hole formation energies for Ce doping, and higher oxygen hole formation energies for Zr doping [160][161][162]. Depending on the Ce content and the method employed to manufacture the coupled photocatalyst, the photocatalyst Ce-doped TiO 2 frequently has a lower band gap than TiO 2 [157,159,163,164], enabling visible light activation. The Ce content of Ce-doped TiO 2 determines the photocatalytic activity, which typically reaches a maximum when the Ce content is in the range of 0.025-0.6 mol% [157,159,[163][164][165]. The CeO 2 /TiO 2 interface is crucial for photocatalytic activity, because contact between CeO 2 /TiO 2 influences activity by encouraging efficient charge separation and opening up holes for chemical reaction. For photocatalytic applications, 10% or less ceria is suggested for TiO 2 -CeO 2 [164].

Current Demands to Understand the TiO 2 -NT-Metal Doping Through Classical and Quantum Molecular Dynamics (MD) to Support Experimental Synthesis and Characterization
Classical molecular dynamics methods make an important contribution to materials design. In reality, a true physical system is made up of numerous components. As a result, MD simulation continues to be a distinctive tool for investigation at the tiny scale. In particular, we must consider the impact of temperature, annealing time, and time between heating steps on crystallization, phase transition, and structure during the doping process in nano-substrates [131]. In the study by M. Predota et al. [166], which does not refer to NT but represents an important approach to understanding the effect of NT, classical MD simulations were performed. To describe the microstructure of the interface between aqueous solutions and the rutile (R-TiO 2 ) surface at the (110) temperature and to ascertain the impact of surface charge and hydroxylation on the adsorption of Rb + , Na + , Sr 2+ , Zn 2+ , and Ca 2+ ions at the interface between a metal oxide and water, M. Predota et al. [166] used classical MD simulations. Despite the fact that the study was not exclusively focused on NT, their findings from comparing the observations of ion adsorption sites obtained using MD and X-ray techniques show that the charged hydroxylated surface model is superior to the non-hydroxylated model as a good approximation of the actual rutile-water interface. Although there is overall agreement regarding the adsorption sites for all ions and the precise ion positions for Rb + and Sr 2+ , the height of the Zn 2+ ions above the Ti-O surface for these two sites is significantly different according to the X-ray and MD data. Additionally, in order to make accurate predictions of both the qualitative and the quantitative nature of interfacial phenomena, it has been claimed that the study of the MD results of ion adsorption and their interpretation is essential. Similarly, in the work of Sadaf Shirazi-Fard et al. [167], a study on the encapsulation and release of doxorubicin from TiO 2 NTs without doping with the anticancer drug doxorubicin (DOX) was presented. According to experimental studies and MD simulations with all atoms, the diffusion coefficients (Di) of DOX molecules are in the order of 10-10 m 2 /s. DOX molecules have numerous H-bonding interactions with TiO 2 NT walls and water, both short-and long-range. They calculated the strength of hydrogen bonds using radial distribution functions (RDFs) and combined radial/angular distribution functions (CDFs). In their study of Li-ion interaction with TiO 2 nanostructures, Kerisit et al. [168] used MD and a core-shell potential model to look at Li-ion diffusion in rutile and anatase TiO 2 . It was investigated how electron polarons affect Li diffusion. Li-ions and electron polarons will form strongly coupled pairs at low Li mole fractions. It is demonstrated that the Li diffusion in rutile is 4-5 orders of magnitude faster than that in anatase. In the research by Yildirim et al. [169], MD simulations using the DLPOLY-MD package and DFT are used to assess the energetics and dynamics of Li-ion transport in anatase and amorphous TiO 2 ( Figure 12). An investigation was performed on how diffusivity is affected by the Li-ion concentration. In comparison to anatase, the diffusion of Li is slower in amorphous TiO 2 . According to reports, in amorphous TiO 2 , the highest Li intercalation ratio happens at concentrations of 50% and higher. DFT and MD simulations have been used to examine the impact of the Li-ion concentration on diffusivity in rutile TiO 2 . At concentrations of roughly 50%, the energy barrier is at its lowest. This means that NTs, with their biocompatibility and chemical stability, are desirable materials for drug delivery systems. When employing TiO 2 NT as the anode for Li-ion batteries, larger storage capacities and higher charge/discharge rates can be attained. This was demonstrated by Li-ions on anatase NTs in a (MD) simulation [170], as shown in Figure 13a,b for the atomic configuration of a lithium-TiO 2 NT system; in their work, the simulation of adsorbed Li-ions reached saturation quickly depending on the temperature.
The number of adsorbed ions increases when the temperature reaches 1000 K, but the quantity of ions drops as the temperature rises more. Li-ion adsorption does not significantly alter the volume of the NTs. Tetrahedral or octahedral sites are where the Li-ions are adsorbed. The ions jump to the surrounding sites during the simulation because the thermodynamically unfavorable octahedral sites, in contrast to tetrahedral sites, have no effect on the number of adsorbed ions. When the surface coverage is modest, the Li-ion adsorption on anatase NTs follows a Langmuir adsorption curve, resulting in few interactions between the deposited ions. These simulations demonstrate that anatase NTs are good potential anodes for Li batteries, and their findings support this assertion. TiO 2 NTs were predicted by Zhenyu et al. [171] to have better electron transport than nanocrystal films. In the context of time-dependent density functional theory (TDDFT), their observations of the quantum-classical approach to non-adiabatic (MD) (NAMD) reveal that oxygen vacancies, which are frequent in TiO 2 , significantly increase non-radiative energy losses. Localized Ti3+ states are produced by oxygen vacancies hundreds of meV below the TiO 2 conduction band. These states encourage higher electron-phonon couplings, trap excited electrons, decrease the NT band gap, and make relaxation easier. These findings support the development of TiO 2 NTs' structure and charge transfer. Recently, Lei et al. [172] used MD simulations to examine the electrostatic characteristics of the interfaces between semiconductor TiO 2 and plasmonic nanoparticles of Ag or Cu. To explore the optical and photoelectrochemical properties, they were deposited on TiO 2 NT arrays (TiO 2 R/T) using a two-step pulse electrodeposition technique. The results ensured that the bimetallic system had a large potential drop from the Helmholtz layer simulated by MD. An ab initio study was utilized by Elham et al. [173] to better understand how ruthenium doping and hydrogen passivation impacted the structural alteration and, subsequently, the overall electronic band structure of an anatase TiO 2 NT (TNT) and its efficient water-splitting capacity. The electronic structure of Ru-doped a-TiO 2 (001) and H:a-TiO 2 (001) PscWf and post-processing programs from the Quantum Espresso package were used to calculate and optimize within DFT. A black Ru-doped TiO 2 -x sample with weak electronic coupling between its valance and conduction edge orbitals, which results in lower electron-hole recombination and rationalizes the experimental findings, is explained theoretically by their ab initio model of Ru-doped hydrogenated TiO 2 NTs, Ru-doped, and a pristine sample. Sergei et al. [174] used a modified B3LYP hybrid-exchange correlation functional in the context of density functional theory to perform all calculations on perfect, both single-and double-walled (SW vs. DW) TiO 2 NTs in the anatase and fluorite phases; their findings show that substitutional impurities can significantly change the electronic structures of both TiO 2 and SrTiOO 3 NTs. For N and S dopants, the difference between the highest occupied state and the lowest unoccupied state falls to 2.4 and 2.5 eV, respectively, and to 2.5 eV for C and Fe dopants in these flawed NTs. Because some impurity levels are located in the region between the two redox potentials, electron-hole recombination occurs. This makes the placement of the band gap edges less favorable for visible-light-driven photocatalysis. On the other hand, important studies on the mechanism of dynamic surface electron transfer have been carried out using Pt/rutile TiO 2 of 3 nm size and simulation cells at X, Y, and Z = 6.43 nm and 31.78 nm as a type of effective photocatalyst that allows one to demonstrate significant advantages [175], despite the fact that no classical MD has recently been performed for Pt-doped TiO 2 NT. The extremely small PT nanoparticle with a diameter of 3 nm that was deposited on the TiO 2 surface could be examined thanks to MD simulation in combination with a reactive force field (REAXFF). Their MD results confirm the experimentally observed method for the regulated protection of highly active sites (such as PT, AU, and PD) on supports, which is essential for the development of efficient and reliable photocatalysts. The force field of optimal atom potentials for liquid simulations was used to simulate lithium + ion doped with ionic liquid (IL) on the TiO 2 -B (1 0 0) surface, and fixing the TiO 2 substrate preserved the stable structure [176]. In the same way, simulations were carried out to investigate the mechanisms of wetting and regulation-specifically, how does the variation in the Li+ concentration affect the interfacial structure between ILS and TiO 2 ? What effect does this have on the contact length and angle of the electrolyte droplets on the TiO 2 -B (1 0 0 0) surface? Doping with Li+ was predicted to slow down the dynamic wetting process and alter the final shape of the ionic liquid droplet. For this purpose, optimized condensed phase molecular potentials were used for MD (compass) studies that allowed them to investigate the structures and dynamic properties of a 1 M KOH solution on anatase TiO 2 (001), (100), and (101) surfaces to understand the molecular details of the effect of KOH on the water splitting of the PEC phenomenon [177]. This study confirms that K+ ions prefer to bind to O2C on the TiO 2 surface. Furthermore, the layering of oxygen and water atoms on TiO 2 surfaces significantly regulates the kinetics of potassium ions. The first study of substantial amounts of water behavior on the surfaces of anatase (101) nanotubes with a diameter of 1 nm used traditional MD simulations [178], and the results show that water inside the tube diffuses more slowly than water in contact with the outer surface tint. The anatase phase of TiO 2 NTs would allow them to create nanoscale ion beams and serve in place of the huge magnets currently used for beam steering, according to studies that have recently focused on the relatively restricted channeling property of TNT [179]. Based on the Lindhard planar channeling potential, (MD) simulations were used to examine the channeling of HE++ mega-electron ions (MEV) in titania nanotubes. The ion channeling phenomenon made it possible for titania nanotubes to have a diameter of 100 nm and 2 m and a length of 1 m and 2 m, as shown in the simulated trajectories of ions projected onto them. To study the gas-sensing properties of TiO 2 , Natalia et al. [180] used all MD atoms to model a 150 × 150 surface area of atomic planes of 100 anatase TiO 2 and TiO 2 :MoO 3 compounds to study their interactions with water, hydrogen, methane, and ethanol molecules at temperatures of 300 K and 573 K. The synthesis of anatase-modified TiO 2 and TiO 2 : MoO 3 composite materials with variable MoO 3 content by the sol-gel method helped to verify the results of the simulations of molecular dynamics. On the basis of the synthesized oxide materials, single-electrode thermocatalytic chemical gas sensors were constructed, and experimental tests were carried out on their detection abilities for hydrogen, methane, and ethanol gases. According to IR spectroscopy data, the number of surface OH groups and adsorbed water molecules in the TiO 2 :TiO 2 composite is significantly higher than in bare TiO 2 , and decreases with increasing MoO 3 content. This experimental finding is reliable with the results of the (MD) simulation on the adsorption of the water molecule on the TiO 2 and TiO 2 surfaces. In recent years, the great differences in the properties of nanotubes and other TiO 2 nanostructures have captured the attention of many researchers, since geometry is decisive in the interaction with other elements, as well as the combination with other materials [181][182][183]. In the same direction, to explore the formation of non-abundant TiO 2 crystalline thin films (both rutile and anatase), Houska et al. [184] pulverized samples with a magnetron and performed atomic-level MD simulations with a Buckingham interaction potential, using a simulation technique that is iterative. They estimated the effect of the energy of the arriving atom, the structure of the substrate, and the lateral size (growing crystal) and the temperature of the substrate, and confirmed that the phenomena observed experimentally (in compared to anatase, the growth of rutile at higher energies per atom of the film and a higher temperature of the nucleation support temperature) were consistent with the MD results. The results shed light on the intricate relationships that exist between the process parameters and the structures of the TiO 2 films that are deposited. Using highresolution transmission electron microscopy (TEM), in situ ion irradiation TEM, and (MD) simulations, amorphous TiO 2 nanotubes were compared with their crystalline counterparts, anatase TiO 2 nanotubes. According to the calculations of MD [185], the internal stresses caused by the densification process during crystallization cause partially crystalline tubes to bend. TiO 2 nanotube-enhanced biomedical devices have been shown to significantly affect mesenchymal stem cell proliferation, differentiation, and adhesion. Cells react to the nanotubes via increasing adhesion, proliferation, and differentiation. In a biological environment, proteins act as an intermediate layer on the surface of the material to further promote cell adhesion and proliferation, and protein adhesion is the first event to occur when the implant surface makes contact with the biological environment (tissue, body fluids). Smaller-diameter TiO 2 NTs have a greater surface area for the binding of positively charged proteins than larger-diameter NTs [186]. Histone has the most adsorbed protein in 100-nm-diameter nanotubes (10 nm in length), while 15-nm-diameter nanotubes have higher values.

Conclusions
This review summarizes the common techniques used to synthesize and modify the properties of TNTs. TNTs have great potential for applications in the energy industry as efficient photocatalysts, for photoelectrochemical water splitting for hydrogen production, and as better photoanodes in photovoltaics of solar cells, while poor solubility factors enrich their applications in various biomedical fields. Nanostructured TNTs with factors such as a tunable length, wall density, and pore size result in a large surface area for highperformance solar energy utilization. Here, we have detailed some important observations on metal dopants for TNTs. In particular, thermogravimetric analysis has shown that the co-addition of Co 3 O 4 -TNTs increases the thermal stability of epoxy resin (EP). This implies that Co 3 O 4 -TNTs can effectively increase the flame retardancy of EP and have a good synergistic flame-retardant effect. According to cone calorimeter measurements, EP/Co 3 O 4 -TNTs exhibited the lowest peak heat release rate and had 35.4% lower overall heat release than pure EP. This implies that Co 3 O 4 -TNTs can effectively increase the flame retardancy of EP and have a good synergistic flame-retardant effect. According to calorimetric experiments, TNT-loaded CeO 2 can significantly increase the thermal stability and flame resistance of matrix materials. CdSe/TiO 2 loaded with CeO 2 may effectively slow the release quality and quantity of heat and encourage the creation of a protected carbon layer, which can achieve a flam-retardant effect. After 30 min of immersion in a CdSe precursor solution, CdSe/TiO 2 NTs displayed a maximum photocurrent density of 0.0016 A/cm 2 , an improvement of around 10% over the original NTs when exposed to sunlight. The adsorption edge of the Ag-modified sample TNT significantly shifted to the blue. The unmodified TNT and Ag 2 0-TNT samples had Brunauer-Emmett-Teller (BET) surface areas of 392 and 330 m 2 g −1 , respectively. The main challenge for doped TiO 2 photocatalysts is that the photocatalytic activity is lower in visible light than in ultraviolet light. As a result, more research into these photocatalysts is required. The development of optimized dopants and doping strategies is a critical area of future research. Self-assembled TiO 2 -based NTs are a particularly promising avenue, as alloy growth and the anodization process enable previously unexplored doping techniques. Doping titanium dioxide with metals/non-metals increases the electrical conductivity due to the formation of local energy states within the band structure and/or the formation of lattice defects (oxygen vacancies, Ti 3+ species). In addition, controlling the variation in the radii of host and doped ions and supporting oxygen depletion can lead to increased stability and activity of TiO 2 (B) during cycling in SIBs and LIBs. Combined with an extremely high degree of geometry control in reactive TiO 2 systems, experimental and quantum DFT and classical computational studies are currently in demand, and there is still room for understanding the electrochemical performance and various metal-doped TiO 2 NTs of anatase-based anodes for LIBs and SIBs using such effective nanostructures of NTs and in optimizing the comparative analysis of other anatase geometry states, such as nanorods, nanofibers, nanoribbons, nanowires, or nanoplates, with similar metal doping to produce hybrids and nanocomposites for use in metal-ion batteries and other related renewable energy applications including machine learning to improve classical (MD) simulations without increasing the computational complexity and providing a methodology that could be used to predict other electronic properties of a larger system of NTs with metal doping that cannot be calculated with classical simulations alone. Due to their high aspect ratio, exceptional flexibility, elasticity, and optical qualities, TiO 2 nanohelices (NHs) have garnered a lot of attention. These characteristics give rise to promising performance in a wide range of crucial domains, including optics, electronics, and micro/nanodevices. However, creating spatially anisotropic helical structures from stiff TiO 2 nanowires (TiO 2 NWs) is still difficult. To assemble individual TiO 2 Nws into a DNA-like helical structure, a pressure-induced hydrothermal method was developed [187]. Vertical TiO 2 nanohelix arrays were created by intertwining synthesized TiO 2 NHs (50 nm in diameter, 5-7 mm in length) with TiO 2 NHBs (20 nm in diameter) (NHAs). Thus, theoretical calculations also supported the finding that straight TiO 2 NWs preferentially change into helical conformations with the lowest possible entropy (S) and free energy (F) to continue growing in a small area. The excellent elastic characteristics have a lot of potential for use as buffer materials or in flexible devices. The creation of new hybrid materials will be greatly aided by additional computational and experimental verification of the NT nanohelices of TNTs, as with the doping of NT.
In the same way, another relevant aspect is that the classical (MD) simulations that are currently available and are experimentally challenging include the formation of nanotubes in the aforementioned TiO 2 phases and their interactions with other metal dopants, as well as the interactions between biomolecules and polymers and interfacial aspects. We believe that in experiments with various doping elements, such as DFT research, similar processes can be replicated using classical MD. Furthermore, a modern technique for simulating Ni-doped MoS 2 to understand the effect of doping was found to be the reactive force field (ReaxFF), connecting quantum mechanical and experiments [188]. This makes TNT metal doping using ReaxFF a particularly useful tool to understand the dynamics of doping research [189]. Funding: Financial support from the seed grant "Computational modeling of biomaterials and applications to bioengineering and classical and quantum machine learning for predicting social engineering (2022-2026, code: INV-0014-03-011)", Universidad Indoamérica, Ecuador, awarded to S.P.T.
Institutional Review Board Statement: Not applicable.