Self-organized and self-assembled TiO2 nanosheets and nanobowls on TiO2 nanocavities by electrochemical anodization and their properties

In this research work, we prepared for the first time TiO2 nanosheets and nanobowls assembled on an arrangement of TiO2 nanocavities, and studied their morphological, optical, and structural properties. The assembled nanostructures were synthesized by a fast two-step electrochemical anodization using fluorides and ethylene glycol. By Field Emission Scanning Electron Microscopy, we showed that these nanostructures have a morphology well organized and ordered with a homogeneous distribution. Also, other characteristics such as photoluminescence, reflectance spectra, band gap energy, and Raman spectra were studied and compared with the optical and structural properties of TiO2 nanotubes. We found that the time of anodization is a key parameter to control the final shape of the individual elements in the nanostructure. Our results show that when nanobowls or nanosheets are self-assembled on nanocavities the morphological, optical, and structural properties change significantly in comparison to TiO2 nanotubes. Furthermore, the emission was improved considerably and the band gap energy was modified to higher energy values. Likewise, the interference fringes are generated in the reflectance spectra by the length of the nanocavities and by the thickness of the nanobowls and the nanosheets. Finally, a reduction on the displaced the Eg(1) Raman mode was observed with decreasing of the length of the nanocavities.


Introduction
Due to their unusual physical, electrical and chemical properties and attractive applications in the field of the electrochemistry and electronics, nanosheets are being widely studied [1,2]. Nanosheets are nanostructured materials that have a shape of sheet whose thickness is lesser than 100 nm and their length is by the order of a few micrometers [2,3]. Commonly, nanosheets are synthesized either by exfoliation or hydrothermal and solvothermal processes [1][2][3][4][5][6][7] and they have been applied to Li-ion batteries and supercapacitors for improving the charge and discharge time, and in solar cells [1][2][3][4][5][6][7][8]. In 2019, Zhao et al investigated GeSe nanosheets, and demonstrated that these can be used in surface plasmon resonance sensor by depositing it by spin coating, improving the sensitivity of the sensor in the detection of heavy metals and chemical molecular identification [9]. However, all these fabrication processes are highly expensive and do not allow a precise control of the periodicity of the nanostructures [3][4][5][6][7][8][9].
In the same way, nanobowls are nanostructured materials that have a shape of bowl or cup whose outer diameter is about or smaller than 100 nanometers [10]. Because of their excellent physical, optical and chemical properties, nanobowls are used in the field of the nanophotonics and photoelectrochemistry, in applications such as sieves for selecting particles, as nanocontainers or as light trappings [11,12]. Typically, nanobowls are synthesized by a colloidal crystal template, a facile solid state dewetting, microwave heating, atomic layer deposition or template-sol-gel processes [10][11][12][13][14][15][16][17]. Nonetheless, nanobowls are usually fabricated in larger dimensions, closed to 500 nm of outer diameter, and having a high manufacturing cost [12][13][14][15][16][17]. Recently, Umh et al showed that TiO 2 nanobowls can be used as photonic crystals. They reported that TiO 2 nanostructures had two bands of reflection and can be modified with increasing of the cavity diameter of nanobowls by modifying the voltage of anodization [18].
Another technique widely employed in the synthesis of nanostructures is electrochemical anodization, since it improves the morphology of the material, creates highly defined geometrical structures and decreases the cost of synthesis [19]. This process commonly uses fluorides and ethylene glycol, and results in a periodic array for the creation and growth of self-ordered and self-organized TiO 2 nanotubes (TiO 2 -nt) like a honeycomb [20]. The self-organization process, morphology, properties, and chemical composition of the nanomaterial can be controlled using the correct anodization parameters such as voltage, current and chemical reagents (water, fluorides, and ethylene glycol) [21][22][23][24][25][26]. For example, Bae et al fabricated a nanotemplate of Al 2 O 3 nanotubes with Au nanosurfaces for refractometric sensing applications. The sensor was fabricated by electrochemical anodization and by sputtering. The authors demonstrated that the sensor changes its reflectance spectrum with the different refractive index of the target due to the nanotubes morphology and Au nanosurfaces [27].
For the above, in this work, we show for the first time a study about TiO 2 nanosheets (TiO 2 -ns) and nanobowls (TiO 2 -nb) assembled on the top part of an ordered array of TiO 2 nanocavities (TiO 2 -nc) by electrochemical anodization, which in turn were grown on a template of Ti nanobowls (Ti-nb). In order to demonstrate the quasi-periodicity of our materials, we show and analyze images of the morphology of the selfassembled structures by Field Emission Scanning Electron Microscopy (FESEM) of each of them and we study the effect of the morphology on their optical properties, such as photoluminescence (PL) and reflectance, and on their structural properties by Raman spectroscopy.

Experimental details
In order to prepare the self-assembled materials, a titanium rectangular foil (1.5 cm 2 ) with a thickness of 100 μm was used as anode while a platinum electrode as counter electrode with an Inter-Electrode Spacing of 1 cm between them. Ti foils were firstly degreased by sonication process with trichloroethylene, acetone and deionized H 2 O, followed by a dry nitrogen blowing. For the fabrication of nanosheets and nanobowls assembled on nanocavities (TiO 2 -nb/TiO 2 -nc and TiO 2 -ns/TiO 2 -nc), a two-step electrochemical anodization was carried out using an organic electrolyte solution containing 0.255 wt% NH 4 F and 1 v/v% water in ethylene glycol at a constant voltage of 30 V, in both cases. In the first-step anodization (1 h), we grew a preliminary arrangement of TiO 2 -nt on the Ti foil, as shown in figure 1(a), right away we made a detachment process with a solution of phosphoric acid and hydrochloric acid to remove the first package of nanotubes and create a template of Ti-nb, that is, a network of bowls-shaped sites to promote a better nucleation ( figure 1(b)). Finally, both self-ordered and self-assembled TiO 2 -ns and TiO 2 -nb on the top of TiO 2 -nc were gotten after the second-step anodization during 20 and 50 min, respectively, all of them at room temperature (figure 1(c)).
To understand the effect of the morphology of nanosheets and nanobowls on their morphological, optical, and structural properties, we fabricated: (1) TiO 2 nanocavities without self-assembled nanostructures, for this, we grew nanocavities with the same procedure as the nanosheets (figure 1), but with a time of just 12 min in the second-step anodization, and (2) standard TiO 2 nanotubes with same electrolyte solution, voltage, and anodecathode spacing as in the synthesis of our nanosheets but with a three-step anodization process with a anodization times of 4 h, 20 h and 1 h for the first, second and third step, respectively, and with a detachment process after the first and second step of anodization to create and improve the template of Ti-nb [28][29][30][31][32]. Likewise, we also studied the effect of the morphology of self-assembled nanostructures on the crystalline structure of the anatase phase, thus, through thermal treatment at 400°C for 2 h by a hot plate process, we converted the amorphous structure of our materials to the anatase crystalline phase. After the synthesis, the superficial and transversal morphologies of the resulting nanomaterials were analyzed by FESEM (FEI SCIOS). PL spectra at room temperature using a fluorescence spectrometer (Fluoromax-3, Jobin Ybon) were calculated and studied. A specular reflectance spectrum was determined by a Semiconsoft MProbe UVVisSR thin film measurement System and Raman spectra were analyzed by Raman microscope model alpha 300 R, utilizing the green laser excitation (532 nm).

Results and discussion
The time of the second-step anodization is a key factor for the control of the final shape of the nanostructure.
A time of 20 min gave as a result the formation of self-organized nanostructures in the form of sheets with a quasi-homogeneous distribution density. The upper part of figure 2 shows the superficial morphology of TiO 2 -ns. The diameter and thickness of TiO 2 -ns were about 80-90 nm and 37 nm, respectively, and a distance between one nanosheet and another of approximately 29 nm, however, these nanosheets were not completely flat because their edges were slightly clearer (figure 2(b)), in other words, they had a slightly concave shape. For this, we assume that a treatment time of less than 20 min in the second-step anodization could result in flatter nanosheets.
In the other hand, the time of 50 min in the second-step anodization turned out in self-organized nanostructures with a shape of bowls and a quasi-homogeneous distribution density. This nanostructure can be seen in figures 2(c) and (d). Apparently, the surface size of the nanobowls is smaller than that of the nanosheets (figure 2(b)) because we cannot see the depth in these 2D images. The diameter and thickness were about 76 nm and 74 nm, respectively; and the separation distance between nanobowls of around 30 nm. Likewise, our TiO 2 -nb had shorter dimensions (thickness and diameter) than another published in literature [9][10][11][12][13][14][15][16][17]. Additionally, in figures 2(b) and (d), it can be seen the top of the nanocavities (TiO 2 -nc) as some little black circles, like pores, between the spaces of the nanosheets and nanobowls. Both TiO 2 -ns and TiO 2 -nb were placed preferably among the nanocavities to achieve the most stable structure. In figure 2(b), the inner diameter of the TiO 2 -nc was about 25 nm, while in figure 2(d) the TiO 2 -nc are ellipse shaped with an inner major axis of 52 nm and an inner minor axis of 37 nm, approximately. Figure 3(a) shows the successive arrangement of the synthesis of nanosheets assembled on nanocavities: firstly, the bottom of the image shows the template of Ti-nb on which, secondly, we made TiO 2 -nc grow, whose length is about 330 nm and, finally, on the top these short nanotubes (nanocavities), we synthesized TiO 2 -ns. The Ti-nb template showed a good homogeneous organization which helped to have a preferential growth and get a better organization of the short TiO 2 -nt [22]. Hence, if short nanotubes get a good organization, nanosheets will be organized too. Additionally, an oxide layer between nanocavities and nanosheets is observed (see figure 3(a)) and this layer is similar to 'the entire surface layer' reported by another study [29]. Figure 3(b) shows a FESEM image of TiO 2 -nb assembled on TiO 2 -nc. Due to the longer anodization time, 50 min, a dissolution process occurred which caused the nanosheets to get a concave shape in the form of bowls. We assume that the [TiF 6 ] 2− ions migrated to the top part of 'the entire surface layer', etching the inner part of the nanotubes and the nanobowls. According to Chong et al, 'the entire surface layer' decreases and ruptures as the anodization time and amount of electrolyte inside the short nanotubes increase [29]. The reduction in the length of the short nanotubes can also be caused by the increase in the pressure of the oxygen bubbles generated in the compact oxide layer (first stage of anodization) [29]. However, several variables must be considered to understand the effect of the anodization time on the length of short nanotubes such as the titanium foil area and inter-electrode distance. Consequently, the length of the short nanotubes was about 128 nm, considerably smaller than that of the TiO 2 -ns (330 nm) and, conversely the inner diameter of nanocavities was longer with respect to the diameter of the short nanotubes in figure 3(a). The ratio between the superficial area of a nanosheet and the internal cross section area of a nanotube was around 11, while the same relationship for the TiO 2 -nb was barely about three. From FESEM images, we can observe that new complex nanomaterials were synthesized. As a result of a fast twostep anodization process, a hybrid material was developed [33]. Namely, two types of nanostructures were assembled: nanosheets or nanobowls (two-dimension) on short nanotubes (one-dimensional). According to Povolotskaya et al, hybrid material can create new properties, so we expect that the properties of the selfassembled nanostructures improve.
To analyze the morphology over a short range, a comparison between the diameter size distribution of nanobowls and nanosheets assembled on short nanotubes is shown in figure 4. The images were taken at 45°w ith respect to the vertical of the nanobowls and nanosheets. Figure 4(a) shows the superficial morphology of nanobowls on nanocavities, the nanostructures are shaped like small bowls or cups organized in hexagonal controlled-packed, where each nanobowl is surrounded by several TiO 2 -nb, i.e., nanobowls grew among the space between the nanocavities. Also, figure 4(b) shows a histogram where we did an analysis about the diameter of TiO 2 -nb. The diameter with the highest frequency was 75 nm with a standard deviation of 19 nm, an arithmetic mean of 76 nm and coefficient variation of 24% respect to average. Figure 4(c) shows the superficial morphology of nanosheets on nanocavities, we can observe that the nanosheets have a greater variation in diameter than the nanobowls. Figure 4(d) shows a frequency histogram of the diameter of TiO 2 -ns. The highest frequency was 88 nm with a standard deviation of 72 nm, an arithmetic mean of 88 nm and a coefficient variation of 82% respect to average. This means that we have more control over the diameter size in the nanobowls because their standard deviation was lower than that of the nanosheets. Figure 5(a) shows a micrograph of short TiO 2 -nt before the growth of nanosheets. These nanocavities, also named 'U-tubes' are connected by 'the entire surface layer' on the top part of the nanostructures [29,34]. The length of the nanocavities was about 135 nm, and the inner diameter of about 21 nm. The image of the crosssection is shown in figure 5(b), here, we can see the roughness in the top part of nanocavities due to the organic electrolyte. According to the chemical dissolution model, the first stage is the creation of the compact oxide layer, secondly, the dissolution process that creates initial pores on the surface of the oxide layer, however, because we formed the Ti-nb template by the detachment process of the nanotubes, the new nanotubes can be grown more efficiently and quickly in the third stage of anodization [35]. In the red box, we can see the internal shape of the nanocavities; this cross-section image was obtained after the Focus Ion Beam process (FIB).
Additionally, figure 5(c) shows the superficial morphology of standard nanotubes whose the inner diameter and length were about 93 nm and 2.5 μm, respectively. As we can see, the nanostructures are organized in a closed-packed of nanotubes due to three steps of anodization and two detachments, since a better template of Ti-nb improve the periodicity of the nanostructures [20]. Because the nanosheets and nanobowls were assembled on the nanocavities, a new morphology was obtained that changed significantly the reflectance spectra. Figure 6(a) shows the reflectance spectra for TiO 2 -nb/TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, TiO 2 -nc and TiO 2 -nt. The reflectivity of Ti foil was 34% in the violet band and it increases as the wavelength increases. Also, the larger nanotubes have a similar phenomenon, however, the reflectivity decreases to 18% in the violet band. Furthermore, a considerable reduction in reflectance is observed because the absorption edge for TiO 2 -nt had a band gap energy of 3.0 eV and these results agree with other reports [36,37].  For nanocavities, the reflectance spectrum changes considerably because of interference fringes (see figure 6(a)). In other words, two bands with a high reflection, the main peak at 565 nm and secondary at 395 nm. In this way, when the nanosheets were assembled on the top part of the nanocavities, their spectrum had two dips of reflectance at 430 nm and 680 nm. As the nanocavities, the spectra of nanosheets presented interference fringes with two maximum reflection peaks displaced to the near ultraviolet (522 nm and 380 nm). For the nanobowls assembled on nanocavities, three bands of high reflection are observed at 400 nm, 510 nm and 760 nm ( figure 6(a)). This manipulating of the propagation of light in the different reflectance spectra is attributed to two variables: length of the nanocavities and self-assembled nanostructures. A similar phenomenon of the interference fringes in the reflectance spectrum was observed in multilayer films, Sta et al reported the optical properties of the TiO 2 thin films, when they increased the account layers of thin films, the number of the interference fringes increased and the band gap energy decreased [38]. In fact, TiO 2 -nb/TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, and TiO 2 -nc can be used as a partial photonic crystal because they have a reflectance spectrum similar to other reports where structural color has been studied in birds, butterflies, turkeys, and mosquitos [39][40][41]. For instance, TiO 2 -nc shows a reflectance spectrum similar to the study reported by Eliason et al where they demonstrated that the microstructures of birds wings were organized in a hexagonal packed of melanosomes and this created a partial photonic band gap on the visible spectrum [41].
On the other side, to determine the consequence of the assembly of TiO 2 nanostructures on the optical properties, the reflectance spectra were utilized to calculate the optical band gap energy of the indirect transitions by Kubelka-Munk function [42]. whereas, an increasing of band gap energy was shown for TiO 2 -nb/TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, and TiO 2 -nc. The band gap for the TiO 2 -ns/TiO 2 -nc was higher (3.18 eV) than the nanotubes. In the case of nanocavities, the band gap was 3.25 eV, while that TiO 2 -nb/TiO 2 -nc had the highest band gap energy (3.28 eV). It is clear the band gap energy increases as an effect of the length of the nanotubes and the thickness of the nanobowls and nanosheets. Defect states such as oxygen vacancies can displace the band gap energy to high energy [42][43][44]. For this reason, we carried out a PL study to validate how defect states can be modified the optical properties. Figure 7(a) shows PL spectra of TiO 2 -nb/TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, TiO 2 -nc and TiO 2 -nt. In order to understand the effect of the self-assembled nanostructures (TiO 2 -nb and TiO 2 -ns) on the emission, we compared their PL intensity with the nanocavities and nanotubes. We can see that the nanobowls assembled on nanocavities had a greater intensity than the TiO 2 -nc, TiO 2 -ns/TiO 2 -nc and TiO 2 -nt. In addition, TiO 2 -nb/TiO 2 -nc had approximately the same PL intensity than the nanosheets because the self-assembled nanostructures created more defect states than the nanotubes and nanocavities.
To identify the main mechanisms that generate the emission, we deconvolved the PL spectra for TiO 2 -nb/TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, TiO 2 -nc and TiO 2 -nt and they are illustrated in figures 7(b)-(e), respectively, and shown in table 1. The first mechanism of emission is generated by self-trapped excitons (STE) at 415 nm band (peak 1 for all structures) [45]. Other mechanisms are single-ionized oxygen vacancies (Vo * ) at   [46][47][48][49][50][51]. These nanomaterials had another emission band of 750 nm to 850 nm (peak 4) that is associated with excited states of Ti 3+ [52]. Usually, the excited states are related to the rutile phase. In fact, TiO 2 -nb/TiO 2 -nc showed the highest emission in violet, blue, and green bands of the electromagnetic spectrum at 418 nm, 448 nm, and 498 nm, respectively, caused by STE and oxygen vacancies. Particularly, the PL intensity of the TiO 2 -nb/TiO 2 -nc film by STE was 1.2 times larger than the TiO 2 -ns/TiO 2 -nc film (peak 1 in figures 7(b) and (c)), 2.3 times larger than the TiO 2 -nc film and 8.7 times larger than the TiO 2 -nt film. Similarly, the area by Vo * of the TiO 2 -nb/TiO 2 -nc film was 1.14 times larger than the TiO 2 -ns/TiO 2 -nc film, 3.8 times larger than the TiO 2 -nc film and 5.58 times larger than the TiO 2 -nt film (peak 2 and 3 in figures 7(b)-(e)). Besides, the PL intensity in the infra-red band of the TiO 2 -nb/TiO 2 -nc film was 1.4 times higher than TiO 2 -ns/TiO 2 -nc film, 2.6 times higher than the TiO 2 -nc film and 5.6 times higher than the TiO 2 -nt film (peak 4 in figures 7(b)-(e)). Figure 7(f) shows the area of the defect states of TiO 2 -nb/TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, TiO 2 -nc and TiO 2 -nt. Here, TiO 2 -nb/TiO 2 -nc had the greatest area of Vo * than other structures. We can clearly see that a relationship is found between Vo * and the band gap energy (E g ), the band gap energy increases with increasing of Vo * . We assume that the increase in emissions is caused by the assembly of nanostructures and quasi-periodic arrangement of nanobowls and nanosheets, because we developed a hybrid nanostructure with a unique and complex morphology.
In summary, the optical properties of the nanotubes are modified by the change of morphology. These nanomaterials can be used to gas sensors, multiplicators, and LEDs. For example, the high amount of oxygen vacancies is related to the increasing of the reactivity in a gas sensor and this, in turn, can enhance the sensor response of the device [53,54].
The physical origin of the luminescence of our TiO 2 nanostructures can be explained by the mechanism of emission that is shown in figure 8. For nanotubes of larger length ( figure 8(a)), the emission in violet, cyan, and infra-red bands are created by a PL process.
The mechanism starts with the excitation of electrons, when the excitation energy is greater than the band gap energy (3.00 eV). So, the electrons are transferred from the valence band to the conduction band, and after by a relaxation process, an energy decay of the electrons is realized. Consequently, the electrons are trapped and recombined by the STE and this recombination generates the emission of a photon. Then, the electrons are trapped and recombined by Vo * and Vo ** , and finally, by the states of Ti 3+. The energy levels of STE, both types of oxygen vacancies, and Ti 3+ defects are 0.18 eV, 0.35 eV, 0.36 eV, and 1.69 eV, below the conduction band minimum. Figures 8(b)-(d) show the physical mechanism of emission for TiO 2 -nc, TiO 2 -ns/TiO 2 -nc, TiO 2 -nb/TiO 2 -nc, respectively. Similar color emissions of nanotubes and nanobowls assembled on nanocavities are observed, however, an increase in the energy levels below the conduction band minimum is detected because the STE, Vo * , Vo ** , and Ti 3+ defects are localized at 0.42 eV, 0.62 eV, 0.9 eV, and 1.86 eV, correspondingly. In the case of nanosheets assembled on nanocavities, violet, blue and infrared bands are observed whose energy levels are 0.27 eV, 0.46 eV, 0.67 eV, and 1.73 eV, correspondingly. For nanocavities, a red band of emission is observed at 1.33 eV below the conduction band minimum.
In order to study the effect of the morphology on the vibration modes and phases of nanomaterial, Raman spectra were calculated for all TiO 2 nanomaterials after the synthesis and thermal treatment, see figures 9(a) and (b), respectively, in the region of −20 cm −1 to 1800 cm −1 . In figure 9(a) we can observe that the nanotubes with length in the order of micrometers had the highest intensity at 161 cm −1 , this band is related to the Raman active mode of symmetries of E g (1) which corresponds to the anatase phase [55]. Usually, the amorphous nanotubes do not present the E g(1) mode. However, the anatase phase is observed in these spectra because the nanotubes are organized in a closed-packed array. In another study, the crystallinity of the amorphous nanotubes can be increased only by a water vapor treatment [56]. It is clear that the intensity of the anatase phase increases with increasing of the thickness of the TiO 2 film. Other Raman scattering bands were located at 391 cm −1 (B 1g mode), 509 cm −1 (A 1g +B 1g mode), and 638 cm −1 (E g mode) [56]. Besides, the Raman modes of ethylene glycol from 1000 cm −1 to 1400 cm −1 were observed [56]. At 1593 cm −1 , the graphite mode that corresponds to a monolayer of graphene was noticed and it is caused by ethylene glycol [56,57]. Furthermore, the self-assembled nanostructures films had a lower E g(1) mode intensity, and higher intensity in the amorphous phase that corresponding to Ti-O phase (215 cm −1 , 425 cm −1 ), and Ti-O stretching (610-635 cm −1 ) than the nanotubes [56]. Another important vibration mode is 7 cm −1 (see inner graph in figure 9(a)), these bands are produced only in nanocavities and self-assembled nanostructures. The highest intensity at 7 cm −1 was TiO 2 -nb/TiO 2 -nc, secondly, TiO 2 -ns/TiO 2 -nc, and thirdly, TiO 2 -nc. From these spectra, we can observe that vibrations at low frequency decrease with increasing of the length of the nanocavities and thickness of the two-dimension structures. The origin of the vibrations at 7 cm −1 is associated with the confinement of acoustic phonons, this can give information about size, form, and concentration of the nanomaterials [58][59][60].
On the other hand, significant changes in the Raman spectra after thermal treatment were observed (see figure 9(b)). The main band of these spectra corresponds to symmetries of E g(1) at 154 cm −1 . The intensity of the E g(1) mode for the self-assembled nanostructures and nanocavities after annealing was more defined than the Raman spectra after synthesis. The bands at 391 cm −1 , 509 cm −1 and 638 cm −1 were higher and more defined than the bands in the samples without annealing. Also, as an effect of the thermal treatment, the vibrations in the range of 1000 cm −1 to 1600 cm −1 were not observed in these spectra [61]. Besides, the peak related to graphene disappeared. In summary, the intensities of the anatase modes increase for all nanostructures with the length of the nanotubes.
For low frequencies, the peaks at 7 cm −1 increase with decreasing of the intensity of the E g(1) mode in the self-assembled nanostructures and nanocavities. In addition, the peaks of the films after the thermal treatment were lower in intensity than the peaks of TiO 2 nanomaterials after the synthesis. However, the intensity of nanocavities was higher than the same material without annealed because it had lower amount of defect states and did not have nanobowls and nanosheets. Also, in this spectrum the nanotubes did not show vibration modes.
Figures 9(c) and (d) show the correlation between the defect states and the crystallinity of the TiO 2 films. A notable displacement of the E g(1) peak corresponding to the anatase phase is noticed because typically the E g(1) mode has been observed at 146 cm −1 (see figure 9(c)). This displacement is related to the existence of oxygen vacancies in the amorphous structure [62]. From PL characterization, it can be noted that the nanotubes had the least amount of self-trapped excitons and oxygen vacancies than the other nanostructures. Therefore, the oxygen vacancies are not the only cause of distortion. Other origins could be the confinement of acoustic phonons and the different grain size according to the study carried out by Amoresi et al [62]. Additionally, a relationship between the final thickness of the TiO 2 film and wavenumber of the E g(1) mode is observed. In other words, a greater thickness of the TiO 2 film increases the displacement of E g(1) , on the contrary, a thinner thickness approaches the value of E g (1) . After the thermal treatment, the displacement is lower than that of the TiO 2 films after anodization because the thermal treatment passivates the surface of the material and reduces the defects that distort the lattice of the anatase phase ( figure 9(d)).

Conclusions
We investigated for the first time TiO 2 nanosheets and nanobowls assembled on a TiO 2 nanocavities array. To grow and assemble the nanostructures, a rapid two-step electrochemical anodization process was developed. We demonstrated by FESEM that these nanostructures have a morphology well organized and were self-ordered among several short TiO 2 -nt to achieve the most stable nanostructure. The time of the second anodization is a key parameter in the shape of the nanostructure: a shorter time, resulted in nanosheets while a longer time in nanobowls. Interestingly, the PL study showed that self-assembled nanostructures enhance emission eight times more than nanotubes. Also, the reflectance characterization showed that nanotubes with length in the order of nanometers increase the number of interference fringes. In addition, nanocavities, self-assembled nanobowls and nanosheets notably changed the reflectance spectra because they create a color interference fringe and partial optical band gap, being similar to other reports that utilized multilayer films or photonic crystals in nature. Moreover, as a consequence of the length of the nanotubes and thickness of the self-assembled nanostructures, the band gap is displaced to high energy values. Likewise, the Raman spectra showed an improvement in the crystallinity of the anatase phase with the increase in the length of the nanotubes, and the self-assembled nanostructures and nanocavities had a vibration mode at low frequencies that can be associated with the phonon acoustic confinement. To conclude, we developed several morphologies of the TiO 2 films by modifying parameters in the synthesis. As a consequence of this, the emission was enhanced, the reflectance spectrum was changed, band gap energy was increased and the intensity of the Raman spectra was modified. These nanostructures could apply to gas sensors, optical sensors, LEDs, and materials for plasmon effects. In the immediate future, we will study the mechanism of the formation of these nanobowls and nanosheets assembled on nanocavities.