Photocatalytic activity of Al/Ni-doped TiO 2 films synthesized by sol-gel method: Effect of sunlight photocatalysis on the catalysts properties

Photocatalytic activity of semiconductors is affected by the nature of metal dopant. To study the effect of non-transition and transition metal on the physical and optical properties of TiO 2 based photocatalysts; Al and Ni-doped TiO 2 thin films respectively were prepared via a sol-gel dip-coating method. The effect of the photocatalysis process on the properties of TiO 2 based thin films was investigated. The photocatalytic activity was calculated from methylene blue dye degradation under sunlight irradiation. XRD results show that un-doped TiO2 films were grown with anatase phase, whereas, the Ni and Ni/Al-doped TiO 2 films show Ti 4 O 7 single phase. The presence of Al preferred the rutile phase. No phases related to NiO or Al 2 O 3 were detected. Ni-TiO2 photocatalyst shows high photocatalytic activity (~93%) thanks to the high content of O and Ti, wide bandgap (3.35 eV), low crystal size (6.87 nm), high film thickness (288 nm), and high surface roughness (44.5 nm). After photocatalysis, all the films show a decrease in O content and thickness, whereas the indirect bandgap values were increased which suggesting the reuse with low photocatalytic activity.

As well known, there is two technologic ways for the illumination of organic substances from wastewater by the heterogeneous photocatalytic processes, i.e. suspended and fixed type. The suspended photocatalyst has more active surface sites than the fixed one due to its movement and nature of the material used (powder); however, its separation from treated solution is an issue [15]. The fixed photocatalyst has an advantage to mostly employ as thin films that are easy to use on an industrial scale at low cost.
In recent years, variety of researches have involved studying the photocatalytic properties of TiO 2 thin films exposed under sunlight [16][17][18] and UV light radiations [19][20][21]. Wen et al. [22] studied the effect of doping TiO 2 catalyst by I and F on the degradation rate of methylene blue and disclosed that TiO 2 photocatalyst exhibited much higher activity under UV light than that under simulated sunlight. Yu et al. [23] reported that the photocatalytic activity of TiO 2 thin film was remarkably enhanced by Ni doping at 0.5 at. % dopant concentration. The general mechanism of TiO 2 semiconductor photocatalysis is described step by step in the literature [24,25].
In this study, we purposed a cost-effective solution within the fields of wastewater treatment before putting it up.
Aluminum and Nickel doped TiO 2 photocatalyst thin films were synthesized on glass substrates by sol-gel dip-coating technique and then characterized. Al and Ni are chosen as non-transition and transition metal respectively, to study the effect of metal doped TiO 2 thin films on the photodegradation rate of methylene blue dye under sunlight irradiation.

Materials
All chemicals were used as received from Sigma Aldrich without any purification. Glacial acetic acid was used as a chelating agent.

Titanium dioxide-Ni-Al thin films synthesis
The sol has been formed by dissolving TTIP in water and isopropanol (iPrOH) then stirred for 30 min. Later, Glacial acetic acid was added dropwise into the sol with vigorous stirring for 2 h at room temperature. The molar ratio of the components was optimized at H 2 O: iPrOH: TTIP: Glacial acetic acid = 2:25:1:1. The final solution is a transparent yellowish. To synthesis Al and Ni doped TiO 2 thin films, Nickel (II) nitrate hexahydrate (Ni 3 wt. %) and Aluminum nitrate nonahydrate (Al 3 wt. %) were dissolved in 10 ml ethanol for 60 min and then added to the previous solution respectively.
Glass substrates were cleaned in acetone for 5 min to avoid any organic contamination and then washed with ethanol and water. The substrates were dipped in the prepared solutions for 30 seconds, and then withdrawn at a speed of 0.8 mm.s -1 and dried at 100 °C for 5 min to evaporate organic solvents. The dip-coating procedure was repeated five times. All the TiO 2 based thin films were calcined at 450 °C for 3 hours.

Al/Ni/TiO 2 thin films caracterization
The crystallographic structure of the films was evaluated by X-ray diffraction using a Bruker diffractometer (D8 Advance model) with CuKa radiation (λ=1.5406 Å), scanning from 20-80° at a rate of 0.03° s -1 . The 3D surface topography and the surface roughness of the films were analyzed by mechanical profilometer (Tencor P-7). The surface morphology of Al/Ni/TiO 2 films was studied using (JEOL JSM 5800) scanning electron microscope (SEM) coupled with an energy dispersive X-ray analyzer (EDX) to study the chemical composition of the photocatalysts. The optical transmission spectra of the films were conducted with a UV-VIS JASCO V-770 spectrophotometer at a wavelength from 300-1500 nm.

Photocatalytic caracterization
The photocatalytic experiments by exploiting sunlight irradiation were conducted on the 15 th of December 2020, under the conditions of the weather for that day (Biskra-Algeria). We intended to mention the date, as both the mean air temperature and relative humidity affect the solar radiation and therefore the photocatalytic efficiency of the films [26][27][28][29][30]. In addition, we aim for an in-depth study to compare the photocatalytic performance of TiO 2 based thin films under sunlight irradiation every 15 th of each month.
In this study, Methylene blue (MB) dye has chosen as a model of wastewater contaminants. Photocatalytic degradation of MB solutions by TiO 2 based thin films was investigated under sunlight irradiation. The photocatalyst samples (20 cm 2 ) were dipped in 100 ml of MB solution (2ppm), in a Pyrex beaker at an angle of 36 ° ± 1 with tracking the direction of the sun manually during the experiments as an imitation of the solar panel system. Before the reaction, the photocatalyst was kept in the MB solution in the dark for 30 min to reach adsorption equilibrium, and then the irradiation tests were carried out from 8:30 am to 3:30 pm. During these experiments, the MB solution has stirred with a speed of 250 rpm. Figure 1 shows the XRD patterns of Al/Ni/TiO 2 thin films. The XRD pattern of pure TiO 2 exhibits a tetragonal crystal structure with a single anatase phase (A) at the diffraction line 25.4° (JCPDS: 00-002-0387); this result is similar to a previous study [31].

TiO 2 based thin films properties
The Ni-TiO 2 film reveals that Ni doping TiO 2 produced a change in the crystalline structure from anatase to anorthic structure of respectively. The Al-Ni-TiO 2 thin film reveals a similar XRD results to that of Ni-TiO 2 film. Rutile crystal phase has tighter band gap (3.0 eV) than that of anatase phase (3.4 eV) [33], therefore, it suggest low photocatalytic activity due to the recombinition of electrons from the conduction to the valence band (hole). Crystal size, porosity, and roughness of TiO 2 material are ones of important factors influencing its catalytic activity. Choi et al. [34] disclosed that small crystal size is of importance for high photocatalytic activity of TiO 2 material. The primary crystal size was valued based on the effective crystal dimension D computed using Scherrer equation which is given in the form Where 0.9 is the Scherrer constant, λ is the wavelength of X-ray (1.5406 Å), B is the full wave half-maximum, and is the Bragg angle [35]. D values were calculated for the most intense diffraction line ( Table 1). The crystal size of TiO2 based thin films was around 6-9 nm. Based on the literature [36,37], Ni-TiO2 catalyst promises high photocatalytic efficiency compared to other catalysts as it has small particle size (6.87 nm). Moreover, Ni and Al doping decrease the crystal size due to the incorporation of Ni 2+ and Al 3+ ions into cation sites of TiO2 lattice.  The 3D surface topography of un-doped TiO 2 and Al/Ni doped TiO 2 thin films is shown in Fig. 2. The root mean square (R q ) surface roughness was calculated with the use of Apex analysis software (    Table 1). The thickness (t) of TiO 2 based thin films was estimated by the gravimetric method using the relation Where t is the films thickness (cm), A is the surface area of the films (cm 2 ), M is the mass of the films (g), and g is the density of the film material (g.cm -3 ) [39]. Table 1 shows that films thickness values confirm the 3D film growth as shown in Fig. 2. TiO 2 films with the best high (1980 nm) and much high crystal size (8.38 nm) is the thickest film. The doping of TiO 2 films by Ni and Al reduce the thickness of the films to 288 nm and 284 nm, respectively.
Where α is the absorption coefficient (m -1 ) (eq. 4), which should be identified by the equation 4, h is Planck's constant (4.136*10 -15 eV.s), ν is Frequency of light (s -1 ), C is a Constant, Eg is Optical indirect band gap (eV), t is the film thickness (m) and T is the optical transmittance (%) [40]. The band gaps of the films slightly decreased with doping TiO 2 thin films by Ni and Al ions owing to the change in crystallographic phase from anatase to Ti 4 O 7 and rutile, respectively. In other hand, both degree of crystallinity [19], crystal size [17] and film thickness [39] are the of importance parameters controlling the film band gap value. Table 1 shows that the indirect band gap widens with increasing thickness [41]. The wide band gap (3.41 eV) for TiO 2 thin film agrees with high crystal size (8.38 nm) and thickness (316 nm).

Photocatalytic performance
The photocatalytic performance of TiO 2 based thin films was evaluated through the photodegradation of MB in an aqueous solution under sun irradiation. Fig. 5(a) shows that MB decomposes continuously throughout the entire irradiation time for Nidoped TiO 2 thin films. The rate of degradation varies according to the hour of irradiation, since solar radiation is affected by temperature, humidity, wind speed, etc. The photocatalytic efficiency (γ) is determined from the following equation [42]: Where C 0 and C t are the concentrations of MB dye at times t = 0 and t, respectively. The photodegradation of MB can be fitted by a pseudo-first-order reaction kinetic model [43,44]: Where k is the pseudo first-order rate constant (h -1 ) and t is the irradiation time (hours). The degree of MB degradation was in the order Ni/TiO 2 > Al/Ni/TiO 2 > TiO 2 > Al/TiO 2 as shown in Fig. 5. Al/TiO 2 (rutile phase) film shows lower photocatalytic efficiency (~64%), than that of anatase (~84%) TiO 2 film (Table 1); it is well known that rutile phase has lower photocatalytic efficiency than anatase phase due to the low indirect band gap. Wang et al [45] studied the sonocatalytic degradation of methyl orange in the presence of nanometer anatase and rutile TiO 2 powders. The compared sonocatalytic activities showed that the catalytic activity of nanometer anatase TiO 2 powder is higher than that of nanometer rutile TiO 2 powder. Ni/TiO 2 (~93%), and Al/Ni/TiO 2 (~86%) as sun catalysts show an effective degradation rate compared to that of anatase TiO 2 catalyst due to the low crystal size, high roughness, and crystallographic phase (Ti 4 O 7 ) ( Table 1). Roughness enhancement increases surface area of the films [46], and consequently increases the quantity of adsorbed contaminant. Hence photodegradation predominates. In addition, the modification of TiO 2 films surface by Ni 2+ improves photocatalytic activity under sun light which can be attributed to the decreased recombination rate electron-hole pairs. As a transition metal, nickel metal is well known as it most often has remarkable catalytic properties and can form a wide variety of ionic species in a wide range of oxidation states which gives rise to variously colored complexes due to the different electronic transitions within the incomplete 3d 8 sub-shell. The doped metal plays an effective role in charge separation by capturing electrons. Begum et al. [47] discussed that Ni ions capture the electron that left a freer hole at the valence band which increases the holes concentration and leads to migrates to surface layer by withdrawing the electrons from the surface.
K values were determined by regression analysis of lines slopes in Fig. 5(c). Table 1 shows K values which increase with irradiation time for all TiO 2 based photocatalysts. Figure 5(d) shows that the discoloration rate constant is largest with Ni doped TiO 2 due to the low crystal size and high surface roughness. Al-TiO 2 (rutile) photocatalysts shows the smallest (k) value maybe owing to the high electron recombinition rate. Eva et al. [33] reported that as rutile adsorbs OH • more effectively than anatase, the amount of OH • generated on TiO 2 and diffusing to the bulk solution would be lower in rutile crystallites. This result leads to conclude that the doping metal type on TiO 2 photocatalysts is of most important effective parameter on the photodegradation of MB. As discussed in above, Ni doped TiO 2 as transition metal increased the degree of photocatalysis under sunlight irradiation. In other hand, Al was chosen as non-transition metal (sub-shell: 3p 1 ), which decreased the photocatalytic activity of TiO 2 thin films. photolysis process. Hence, it is suggesting that the reuse of these samples for second cycle of photolysis gives low photodegradation rate than the first one. EDX results show that the content of Ti and O decreased by the photocatalysis application of these films as shown in Fig. 6(b). Hoffmann et al. [24] reported that unless oxygen is supplied on a continuous basis to a photocatalytic reactor, the rate of photocatalytic oxidation will decrease dramatically after depletion of the primary electron acceptor due to charge-carrier recombination. Overall, Ti and O contents are decreased owing to the photocatalytic oxidation of MB on bulk-phase TiO 2 .    (Table 2). Kim et al. [48] found that the optical transmission is not exponentially related to the film thickness. This result suggests that the transmission of the TiO2 based thin films is affected by other physical properties of the films. In this study, the transmission decreases is attributed to the decreased O content in the films by the sun photocatalysis process.   Table 2 shows an increase in band gap with varying values according to the photocatalytic activities of the samples. This increase may lead to the low electron transfer kinetics from the valence band to the conduction band driving photo-oxidation of contaminants when using these samples for a second cycle. Trapping electron-hole of wide band gap semiconductors require high energy for photocatalysis than that of tighter one. This result suggests that using the samples for a second photocatalytic cycle may give less photodegradation rate than the first cycle. Bansode et al. [49] reused the Fe doped TiO2 thin films for three cycles as MB bleaching photocatalysts under UV light, and found that the nature of degradation remains unaltered.

Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.