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Influence of the variation of the experimental conditions when obtaining the TiO2 antireflective thin film

The formation of thin films based on TiO2 plays a fundamental role in research involving solar energy (solar cell and solar thermal collectors) and from the part environmental (photocatalysis-artificial photosynthesis as a source of clean energy). In this sense, our work has focused on the experimental details directed towards the method of obtaining, for which as a precursor salt Titanium Oxyacetylacetonate (IV) (Sigma Aldrich with 90 % purity) it has been used dissolved in methyl alcohol (Fisher Chemical with 99.8% purity) in 0.05 M molar concentration. The effect on deposited films was studied by varying airflow (2 L/min - 6 L/min) and substrate temperature (150 °C - 450 °C). For the Optical and structural characterization was used UV-VIS spectroscopy and X-ray diffraction techniques. From the analysis of the results was determined the energy band gap with values around 3.5 eV for all films. The Anatase phase with tetragonal structure was find it, refined using the Rietveld method that determined network parameters of a = 3.807617 Å, b = 3.807617 Å, c = 9.547797 Å, the thickness of the films ranged in a range between 70 nm - 150 nm whose values depended on the deposit conditions. The previous evidence allows concluding that this material can be used as an anti-reflective layer to be implemented in solar collectors.


Introduction
To meet the challenges of the energy sector and in turn to the continuous deterioration of natural resources, both science and technology must converge into new systems to transform the way energy is produced and consumed, allowing a transition to sustainable energy systems. In particular, the integration of different types of materials (such as nanoparticles, thin films, etc.) to devices that use renewable energy can improve their efficiency and performance [1,2]. TiO2 thin films generally manufactured under a variety of techniques, have the potential to be designed according to the desired application, considering its various properties (optical, structural, morphological, etc.). The TiO2 films are often used in gas and humidity sensors [3], as gate electrodes for metal-oxide devices [4], as semiconductors in solar energy [5,6], photocatalysis [7] or as material for anti-reflective layers [8][9][10]. They can be implemented in solar cells as well as in solar thermal collectors. This work is focused on applications where it is necessary to capture as much light as possible, that is, it is necessary to add antireflective materials (for example TiO2). Since a significant combination of optical losses on the selective 9th SAC and 1st ICNNN -Colombia 2019 Journal of Physics: Conference Series 1541 (2020) 012008 IOP Publishing doi: 10.1088/1742-6596/1541/1/012008 2 surface of solar collectors is due to the high contrast index, which results in a high reflection of the absorber surface, whose value is around 35% over the entire electromagnetic spectrum [11].
One of the methods of obtaining TiO2 in the form of thin films is by means of the aerosol-assisted chemical vapor (AACVD) technique, which has advantages over other deposition techniques with higher cost (e.g. magnetron sputtering and other vacuum techniques), since it is a simple process and has a very low cost of implementation. In addition, the deposits can be made on substrates with complex geometries, acquire a uniform distribution of the precursor solution, forming high quality coatings [12]. The AACVD technique consists in atomizing a precursor solution in very fine drops that are then dragged by a flow of hot air through a mobile nozzle, towards a substrate with a certain temperature where the film is formed due to condensation of the atoms that move on the surface of the substrate [13]. The characteristics of the material deposited using this technique depend on variables such as the temperature of the substrate, the carrier air flow with the species to be deposited, the speed of the nozzle and the distance of the nozzle to the substrate (figure 1). In order to establish a deposit protocol and the optimization of the deposit process towards the manufacture of the anti-reflective layer (TiO2), using the AACVD technique, the influence of the deposit parameters and the consequences of their variations on the coating were systematically investigated. Therefore, this work describes the experimental method of the technique, varying two process parameters: the substrate temperature and the speed of the nozzle, in order to find their influence on the structural and optical properties of the deposited material. From the diffractograms, analytical methods were used to determine the structure of the deposits using Rietveld refinement, which approximates the intensities to certain positions of the X-ray measurement in order to obtain the specific diffractogram [14]. While using the Swanepoel method, the refractive index was obtained in a range of wavelengths and film thickness. [15,16]. The results obtained will help to optimize the manufacturing process of these coatings that will be the anti-reflective layers on the selective absorbent surfaces used in the manufacture of water heaters, air or solar energy installations of different types, like solar cells, photovoltaic panels, among others.

Deposit conditions
Thin films of TiO2 were obtained using the aerosol-assisted AACVD deposit technique, whose scheme with each of the electrical and mechanical components is shown in figure 1. Details about the configuration of the deposition system can be surveyed elsewhere [17,18]. All deposits were made on glass slides substrates with dimensions 2.5 cm x 7.5 cm. These were subjected to a cleaning treatment in an ultrasonic bath consisting of an immersion of acetone and subsequently methanol that lasted 40 minutes, in order to remove any grease from the surface. Titanium oxide (TiO2) was obtained from the precursor salt Titanium Oxyacetylacetonate (Sigma Aldrich, 99% purity) dissolved in methyl alcohol (Fisher Chemical 99.8%) with a molar concentration of 0.05 M. Filtered compressed air was employed as carrier gas it was fed at 5 L/min, the distance between the mobile nozzle and the substrate was 1 mm, the mobile nozzle scrolled across the substrates, applying 4 layers. Coatings were deposited in two stages: (a) Variation of substrate temperature from 150 °C to 450 °C in steps of 50 °C; (b) Variation of the speed of the mobile nozzle from 0.6 cm/min to 0.8 cm / min and substrate temperature of 400 °C. Table 1 summarizes the deposit parameters used for both stages.

Characterization methods
The thermal stability of the precursor salt was studied by Thermogravimetric analysis (TGA). Simultaneously, a TGA-DSC device (TA Instruments SDT-650) was used, with an alumina tray of 90 ul. The curves were measured up to 800 °C at a heating rate of 10 °C/min under a constant nitrogen flow. The crystalline structure was analyzed by grazing incidence x-ray diffraction. For this purpose, a Diffractometer XPERT-PRO system was used, operating at 45 keV and 20 mA that works with Cu Kα radiation (0.1540 nm and 0.1544 nm). The parameters used to make the measurements were: scanning range 2Θ from 20° to 100°, a scanning time of 14 s, a step size of 0.04° and a grazing incidence angle set at 0.5°. Phase identification and Rietveld refinement were done using X-pert High Score Plus and GSAS software with a Pseudo-Voigt function, respectively [19]. The transmittance and reflectance spectra were measured in the UV-Vis interval using a Thermo Scientific -Evolution 220 spectrophotometer equipped with integrating sphere accessory. The Swanepoel method was used by adjusting the envelopes of the interference bands of the transmission spectrum in order to obtain the refractive index and the film thickness [15,16].

Thermal stability: Influence of substrate temperature
In order to determine the influence of the temperature on the formation of thin films of TiO2, the thermal behavior (TGA-DSC) of the precursor salt (TiO(acac)2), Titanium(IV) Oxyacetylacetonate) was analyzed up to a value of 800 °C. The purpose was to obtain information about the temperatures of reaction and products during the thermal decomposition. However, it is important to take into account that this result is only an indicator of the possible precursor transformations, due to the different thermal conditions during the synthesis of the films in the AACVD [20]. For this reason, deposits were made in the temperature range between 150 °C and 450 °C and additionally the transmittance spectra in the UV-Visible were analyzed. Figure 2 shows the TGA curve (continuous line) and the DTG curve (dotted curve) of the precursor salt of Titanium Oxyacetylacetonate. From the TGA curve, the reagent decomposition can be observed, while the DTG curve can accurately identify the two steps in which mass loss occurs. There is an initial slow endothermic mass loss of approximately 1% up to 152 °C, related to the loss of water present in the reagent. The first decomposition step for Oxyacetylacetonate corresponds to the temperature range between 162 °C -284 °C (endothermic reaction), that results in a mass loss of 54%, which is equivalent to the maximum mass loss rate. This first decomposition can be tentatively explained with the loss of mass of the chelated ligands and the burning point corresponding to acetylacetone [21,22]. The second step of mass loss occurs in the range of 439 °C -502 °C that begins as a rapid endothermic reaction with a constant mass change of (3%). The decomposition of the reagent is completed practically from 480 °C. Figure 3 shows the DSC curve of the same salt, where only one endothermic peak is observed between 200 °C -250 °C, which can be attributed to the onset of Oxyacetylacetonate decomposition. According to the results obtained, it is concluded that the temperature range to make the deposits is above 350 °C since at lower values of this temperature there is both an irreversible transition and a greater loss of mass (TGA), which is observable in the DSC thermogram and the greater loss of mass (TGA) that the reagent has.   One way to corroborate that the films correspond to TiO2 and that they also comply with the properties to be implemented as antireflective layers is through the study of their optical characteristics. Figure 4 shows the layers transmittance spectra in the Uv-Visible range, of layers for different substrate temperatures. It is observed that as the temperature increases, the transmittance decreases depending on the absorption zone, that is, either in the zone of strong absorption (UV-Visible) or the zone that goes from the mean to the weak absorption (400 -Near-InfraRed), this fact is given by the interference bands that is a well-known optical phenomenon [23]. The transmittance measurements allowed to determinate of the energy of the band gap Eg using the square of the product of the absorption coefficient (α 2 ) and the energy of the incident photons ((hν) 2 ), as a function of the energy (hν), such as it shown in Figure 5. From the high absorption region, the graph of (αhν) 2 as a function of photon energy (hν) and extrapolating the linear region of this curve (αhν) 2 = 0 get it the energy value of the band gap Eg is calculated for each thin film. In all cases, it can be seen that a direct transition takes place and, in addition, the presence of different Eg values according to the substrate temperature.
In the case of deposit temperatures in the range of 150 °C -350 °C, values between 1.5 eV and 2.7 eV are obtained (figure 5a), which are below of the values reported in the literature for TiO2 (3.4 eV [24]) as a potential antireflective layer. However, this behavior could be explained based on the results reported from the thermal study, where it was observed that at temperatures below 350 °C the TiO2 formation is not stable producing changes in mass loss, even induced other phases. In the case of deposit temperatures between 400 °C and 450 °C (Figure 5b), the Eg values found are 3.5 eV, which are around the values reported in the literature [24]. The Eg value obtained, allows to establishing that for temperatures between 400 °C and 450 °C, the layers of TiO2 with antireflective characteristics can be achievable and deposited using a versatile and low-cost technique such as AACVD.

Influence of speed mobile nozzle
The study of the influence of the parameters of the deposition process, showed that the temperature to obtain the films should be above 400 °C on glass substrates. In fact, the structural characteristics and the validation of the optical properties that have been studied when varying the speed of the nozzle confirmed it. In this sense, two TiO2 films were deposited by varying the speed of the mobile nozzle at a substrate temperature of 400 °C with speeds of 0.6 cm/min and 0.8 cm/min and whose other parameters are shown in table 1. Figure 6 shows the standard diffraction patterns for films deposited at speeds of 0.6 cm / min and 0.8 cm / min. In both cases it is observed that they have a polycrystalline structure and to which 10 planes can be associated, two of them of high intensity and eight planes of low intensity. The planes with higher intensity in the diffractogram are oriented in 2θ around the values 25.31 ° -plane (101) and 48.05 ° -plane (200). The other eight planes can be seen in figure 6. It should be noted that the planes around 2θ in 36.85 ° -plane (103), 37.86 ° -plane (004) and 38.68 ° -plane (112), only appear in the deposited film with a nozzle speed (VN) equivalent to 0.6 cm /min. All above crystalline planes correspond to TiO2 with tetragonal structure and anatase phase, according to the ICSD database card No. 9852 [26]. It is important to note that at the higher nozzle speed of (0.8 cm/min), the crystallinity of the film decreases, showing planes with very low intensity and even some planes do not appear at all. This behavior could be taken as a technology indicator for the optimization of the deposit parameters, in order to improve antireflective layers. After exposing the latter information, it could be said that the most appropriate speed with the conditions used in table 1 is likely 0.6 cm/min. The Rietveld refinement was performed for both samples in order to corroborate the atomic positions and find evidence of possible structural changes with respect to the reference pattern and also calculate the crystallite size using the parameters obtained from the refinement. The phase and structure formed for each velocity were determined using the unit cell (tetragonal -referenced from the database) and the known spatial groups (I41/amd-141) for TiO2, whose values are shown in table 2. Figure 7 shows the experimental diffraction pattern and its respective refinement for the sample with speed 0.6 cm/min, with values of (Chi) 2 = 1.5 and R (F 2 ) = 0.77.   [15,16] the thickness of the films and the refractive index were calculated according to the wavelength, whose results will confirm that TiO2 is obtained by the AACVD technique successfully.   [15].

X-Ray diffraction.
In the spectra of figure 8, it can be observed that the films are highly transparent in the range of Visible -NIR (Near InfraRed) with transmittance values around 70 % and that they also have an absorption edge of around 350 nm, which is equivalent to the edge obtained for the deposited films when varying the temperature (figure 4). The highly transparency in that range, is a good indicator of the characteristics of an antireflective material, considering that the interference fringes along the electromagnetic spectrum, may contribute as optical traps when the antireflective layer is on top of the absorbent layer, causing the light to be absorbed again, this due to top surface to be out of phase with the wave reflected from the another surface [5]. Another optical property that confirms the obtaining of TiO2 is the refractive index (n), which was determined with the envelope method (Swanepoel) and is shown in figure 9.
The variation of n for the film obtained at a nozzle speed equal to 0.6 cm/min has a nearly constant value between 400 nm and 800 nm with n = 2.2, such value is in accordance with the values reported in the literature [27,28]. For the sample obtained at a speed of 0.8 cm/min a value is even below n = 2.0, which can be attributed to lower crystallinity and the presence of possible defects such as dislocations possibly caused by the higher speed, showing that the speed of the nozzle is a decisive parameter to obtain TiO2 layers with optimal values. The calculated thickness value was approximately 470 nm for the speed of 0.6 cm/min and 505 nm for the speed of 0.8 cm/min. Finally, Figure 10 shows the graph of (αhν) 2 as a function of the energy of the photons (hν) from which values of the energy band gap were calculated, Eg = 3.5 eV for both speeds 0.6 cm/min and 0.8 cm/min. These values are equivalent to those obtained when varying the substrate temperature. Therefore, it can be said that the technological parameters of the process have been varied successfully. Both the temperature and the speed of the mobile nozzle, provide reproducible results and ideal characteristics that are conditions necessary to meet, so that the TiO2 can be employed as an antireflective layer and incorporated into the absorber surface of a flat solar collector.  Figure 10. Plot of (αhν) 2 vs hν for the samples deposited at 0.6 cm/min -0.8 cm/min.

Conclusions
The variation of the speed of the mobile nozzles during the deposit via AACVD, has allowed to determine that there is a speed limit equivalent to 0.6 cm/min, in order to obtain polycrystalline TiO2 with a well-defined structure, with a crystallite size around 20 nm, optimal refractive index, a value of the energy band gap Eg = 3.5 eV that confirms that the proposed material was obtained, and fundamentally, that allows the crossing of up to 70% of the light in the Visible-NIR range, working as optical traps with well-defined interference fringes, which make it have the ideal deposit conditions to be used as antireflective material to be integrated into a solar collector.