Nanostructured thin films of TiO2 tailored by anodization

Although nanostructured TiO2 layers have been widely prepared by anodization, thin films with thicknesses under 1 μm, over substrate other than Ti foils, with structures beyond the nanopores, had remained a challenge. In this work, such nanostructured TiO2 thin films were synthesized by anodization of Ti films deposited by sputtering on FTO/glass substrates. Anodization was performed in an electrolyte based on 0.6 wt% of NH4F, a graphite cathode and the application of 30 V during lapses ranging from 3 to 14 min. The amorphous TiO2 structures acquired the crystal anatase phase after a post-annealing treatment at 450 °C/4 h. Porous morphologies were observed for anodizing times of 3 and 4 min, sponges were formed with 5 and 6 min and vertical tubular structures were achieved by using 7 up to 9 min; dissolution was observed for longer times. Pore diameters of the structures were in the range of 27 to 47 nm, lengths were within the 330 and 1000 nm interval, transmittance was in the visible range of 70 ± 10%, the energy gap was 3.37 ± 0.02 eV and the wet contact angle was between 20 to 27°. One major contribution of the findings herein developed, is that they can be extended to TiO2 thin films, with a specific nanostructure, grown on a wide gamma of substrates, relevant for particular applications.

Typically, an ETL is a compact layer that can be made from semiconductors such as ZnO, Zn 2 SO 4 , In 2 S 3 , SnO 2 , Zn 2 SnO 4 and Nb 2 O 5 [17], but TiO 2 is the most widely used because of its excellent optoelectronic properties, photoelectrochemical stability [18] and compatible band gap upper level (∼4.05 eV) with the perovskite (3.97 eV) [19].When on top of this compact TiO 2 layer, a mesoporous layer is placed, the conversion energy efficiency of the HPSC is increased due to the generation and conduction of charge carriers is improved as well [20].This efficiency can be further increased if the mesoporous film is nanostructured, as in the case of TNS rods [21] or cones [22] synthesized using the hydrothermal procedure, and columns obtained through thermal oxidation [23].However, anodization, a low-cost, simple and reproducible technique [24], has been barely used, and only tubular structures have been reported for ETLs [25][26][27].In the anodization technique, the concentration of the NH 4 F electrolyte controls the oxidation speed [28], the applied voltage influences the diameter of pores and tubes [29] and the synthesis time determines the length of the TNS [30,31].
TNS for hybrid solar cells are challenging to obtain due to the particular requirements for such application.Among these requirements are the use of a glass substrate, annealing temperatures below 500 °C, structure lengths in the range of 300 to 1000 nm [27,28] and a desirable (004) preferred orientation for higher efficiencies [32].These characteristics are fulfilled under the conditions we report in this work, in which TNS with different morphologies were attained by anodization, a valuable technique due to its simplicity and low cost.

Materials and methods
Titanium films were deposited by DC magnetron sputtering, using a 99.99% Ti target (Kurt J Lesker), and FTO/ glass substrates (Ossila TEC 15 S304).The deposition conditions were an applied power of 100 W, 38 sccm of Ar (2 mTorr), a target-substrate distance of 5 cm, growth rate of 15 nm min −1 and deposition times of 20 and 40 min for film thicknesses of 300 and 600 nm, respectively.The base pressure was 6 × 10 −6 Torr.Prior deposition the target was sputter-cleaned for 5 min applying 100 W. Morphology and roughness of the as grown Ti films were characterized by AFM (SPM Park Systems XE-70) and the thickness was verified in a SEM JEOL JIB-4500.
The TNS were synthesized by anodization of the Ti/FTO/glass films, using a graphite rod cathode (6 mm diameter, 4 cm length) placed 1.5 cm apart.The electrolyte solution was 0.6 wt% NH 4 F, 2 wt% of deionized water and the balance of ethylene glycol.A DC voltage of 30 V was applied between the film and rod during periods ranging from 3 to 14 min, while the solution was magnetically stirred.For the shorter times (3 to 6 min), Ti films 300 nm thick were used to keep the final TiO 2 structure in the length range of 300 to 1000 nm; for times between 7 and 14 min, 600 nm Ti films were needed for the same purpose.The current versus time response was monitored in an automatized homemade system.
The amorphous TiO 2 samples, previously rinsed in ethanol and deionized water, were annealed at 450 °C for 4 h.The TNS morphologies and cross section images were acquired in a SEM JEOL JSM-7600F.XRD analysis were conducted in a Philips X'pert MPD diffractometer, with a λ Cu Kα = 1.5405Å, in a Bragg-Brentano configuration, using steps of 0.02°and 0.5 s.Raman measurements were carried out in a Thermo Scientific DXR Raman Microscope (532 nm wavelength, 10 mW).The water contact angle (WCA) was determined in a homemade device with a high precision digital camera (108 mega pixels) and drops of 9 μl; measurements, corroborated in different areas of the samples, were performed at room temperature and a relative humidity of 40%.A UV-vis spectrometer (Agilent Carry 60) was used to acquire the transmittance spectra in the 200 to 1100 nm range.Band gap energies were obtained from Tauc plots.Pore diameters were determined by the ImageJ v1.53k software, average values were calculated with at least thirty data in each sample.The same software was used for the contact angle determination.All measurements were confirmed in a second set of samples prepared in the same conditions.

Results and discussion
The Ti films deposited by sputtering showed a granular homogeneous morphology (appreciated in the AFM image of figure 1(a)) with a RMS roughness of 20 nm and a uniform thickness (as the observed in the SEM micrograph in figure 1(b) for the sample deposited during 20 min).
Current density versus anodization time curves were taken during the anodization process to provide a guide of the TNS formation and dissolution.Figure 2 shows a representative curve in which four stages (s1-4) are appreciated.The chemical reactions at the Ti-electrolyte interface [33,34] give rise to a formation of a superficial compact oxide layer, known as the barrier layer due to the conductivity reduction of the once pure metallic surface, being the origin of the current density drop at s1 [33,34].The appearance of pits (s2) gives rise to a peak [33,34].In our study, three of them (s2a-c) that are associate with a 1st, 2nd and even a 3rd family of pits, which act as nucleation sites for pores.With the increment of the synthesis time, the material at pore edges gradually get dissolve under the influence of F -ions, removing the thin oxide layer that covers the structure formed underneath.The electric field generated by the applied voltage also leads to a gradual increase in pore diameter over time.The nanotubes are formed at a constant current (starting at s3), then dissolution overcomes the formation rate at s4 [33,34].Beyond this point, there is an increment in the current, which indicates the approximation to the conductive FTO film (needed in HPSC), different from non-conductive substrates for which no current should be observed.
SEM images of the samples anodized at different times during the constant current stage (s3 to s4), were taken before and after a post-annealing treatment at 450 °C/4 h, verifying that the annealing did not change the morphology.Porous structures were observed in the 3 min and 4 min samples, as is depicted from figures 3(a)-(b).For 5 and 6 min, dissolution of material between pores was appreciated (figures 3(c)-(d)), indicative of the formation of a transition structure between the just porous and tubular, known as nanosponges [33].Films with larger pore sizes show lower contact angles (see table 1), which are preferred for a higher surface wettability [36,37].The contact angles obtained from images as the ones in figure 6(bottom), were between 20 to 27°, values low enough to promote a more homogeneous growth of a hybrid perovskite film.
Raman spectra of all samples exhibit only the characteristic vibrational modes associated to TiO 2 in the anatase phase, as seen in figure 7 (top) for the 7 min sample.The anatase phase was confirmed by XRD using de JCPDS No. 21-1272 card.
Figure 7 (bottom) shows the diffractograms for porous and tubular structures (4 and 8 min).A crystallite size of 25 nm was calculated by the Debye-Scherrer equation [38][39][40].A preferential (004) orientation was observed in both structures, also reported for other TNS [32], but the preferential orientation was stronger in the tubular structures in which the intensity ratio of the (101) to the (004) peaks was 1:3 in comparison to the 1:2 for the porous.
The UV-vis transmittance curves of anodized TiO 2 samples obtained from 3-9 min are shown in figure 8 (top).Transmittance was in the 70 ± 10% range for wavelengths over 500 nm, typical and usable values for HPSC [41].In general, the difference in transmittance can be associated to the differences in morphology and thickness of samples [38] (except for the lower transmittance of the 6 min sample which should be related to an incomplete oxidation of the Ti layer [31,38]).The observed oscillations are attributed to multiple reflections at the interfaces between the film and the substrate as well as the film and air [42].The small difference in the intersections may be associated to the morphologic differences of samples [43].The average indirect band gap of   1 summarizes the pore and/or tube inner diameters as well as the lengths of the structures.The length of the sample anodized for 3 min (516 nm) is longer than the one in the initial Ti film (300 nm), which is due to the formation of the oxide.When the anodizing time increases to 4 min, the pore size increases and the length decreases (to 339 nm) because of the dissolution of the structure.The same trend in the length is observed in the TNS obtained by anodization of the 600 nm Ti films for times between 5 and 10 min, but the pore size remains similar, in agreement for the anodization process at a fixed applied voltage [35,51,52].All the obtained TNS lengths are in the interval of 300 to 1000 nm required for applications in perovskite solar cells [41].

Conclusions
Nanostructured TiO 2 films with porous, sponges and tubular morphologies were attained by anodization of Ti/ FTO/glass films, using 30 V and anodizing times ranging from 3 to 9 min.After a heat treatment at 450 °C/4 h, all the structures exhibited the anatase phase with a (004) preferential orientation.Pore diameters were in the 27 ± 4 to 47 ± 6 nm range.Lengths were kept within the 300 to 1000 nm interval.Transmittance in the visible range was 70 ± 10%, the energy gap was 3.37 ± 0.02 eV and the wet contact angle was between 20 to 27°.These characteristics fulfill the technical requirements, such as a glass substrate, nanometric pore size, length range, preferred orientation, transmittance, energy gap and wet contact angle, that make the achieved nanostructured TiO 2 thin films, potential candidates for the electron transport layers in hybrid perovskite solar cells.In addition, the results of this work open the possibility to prepare, at a low-cost, TiO 2 in thin film form, with a specific nanostructure, on diverse substrates relevant for particular applications (as the mentioned in references [6,7] and [12][13][14][15][16]).

Figure 1 .
Figure 1.(a) AFM and (b) SEM of the transversal section of a Ti film deposited by sputtering during 20 min.

Figure 2 .
Figure 2. Current density versus Anodization time curve taken during the anodization of a 300 nm Ti film.

For
longer anodizing times, 7 to 9 min, morphologies were as shown in figure 4. The well define structures attained with 8 and 9 min (figures 4(b)-(c)), different from the fuzzy obtained with 7 min (figure 4(a)), are associated to the total formation of nanotubes, which is in accordance to literature [31].Formation of tubes was corroborated by SEM images of the cross section, as the one in figure 4(d).The tubular structures begin to collapse at 10 min because of their dissolution (figure 5(a)), which is increased for longer times as is observed in figure 5(b) for the sample anodized for 14 min.

Figure 3 .
Figure 3. SEM images of TiO 2 structures obtained by anodization of Ti films 300 nm thick, during (a) 3 min and (b) 4 min; and Ti films 600 nm thick anodized for (c) 5 min and (d) 6 min; all of them post-annealed at 450 °C/4 h.

Figure 4 .
Figure 4. SEM images of the TiO 2 structures obtained by anodization of 600 nm Ti films during (a) 7 min, (b) 8 min and (c) 9 min, post-annealed at 450 °C/4 h.(d) SEM of the cross section of the sample in (b).

Figure 6 .
Figure 6.Wet contact angles of the FTO substrate and compact TiO 2 (top), and nanostructured TiO 2 samples obtained by anodization for 3 and 7 min (bottom) and a post-annealing at 450 °C/4 h.

Figure 7 .
Figure 7. Top: Raman spectra of the sample with 7 min of anodization.Bottom: (a) Ti/FTO/glass substrate and the TiO 2 structures obtained by anodization during (b) 4 min and (c) 8 min and a post-annealing at 450 °C/4 h.

Table 1 .
Pore and/or inner diameters, lengths, band gaps and contact angles of the TNS samples prepared by anodization of Ti films, using different times and post-annealed at 450 °C/4 h.