Structural Modification of Sol-Gel Synthesized V2O5 and TiO2 Thin Films with/without Erbium Doping

Comparative work of with/without erbium(Er-) doped vanadium pentoxide (V 2 O 5 ) and titanium dioxide (TiO 2 ) thin films were carried out via sol-gel technique by dissolving erbium (III) nitrate pentahydrate (Er(NO 3 ) 3 ⋅5H 2 O) in vanadium (V) oxoisopropoxide (OV[OCH(CH 3 ) 2 ] 3 ) and titanium (IV) isopropoxide (Ti[OCH(CH 3 ) 2 ] 4 ). Effect of Er doping was traced by Fourier transform IR (FTIR), thermogravimetric/differential thermal (TG/DTA), and photoluminescence measurements. UV-Vis transmission/absorption measurement indicated a blue shift upon Er doping in V 2 O 5 film due to the softening of V=O bond while appearance of typical absorption peaks in Er-doped TiO 2 film. Granule size of the films increased (reduced) upon Er substitution on host material compared to undoped V 2 O 5 and TiO 2 films, respectively.


Introduction
Titanium dioxide (TiO 2 ) and vanadium pentoxide (V 2 O 5 ) thin films have drawn considerable attention with their outstanding properties that make them key elements for optical coatings [1], gas sensors [2], electrode materials for Li ion batteries [3], and electrochromic devices [4].These oxides have also potentials as host materials for rare earth ion implantation due to the suitability of oxygen inclusion and their wide band gap nature that enhance the photoluminescence (PL) emission of dopant ion [5].Among them, erbium (Er) has a potential interest for telecommunication due to sharp photoluminescence emission at 1540 nm, which corresponds to a minimum loss window for silica optical fibers and ascertains its importance [6].Additionally, Er doping, especially in TiO 2 film, leads to enhancing photocatalytic activity due to the absorption peaks located at 490, 523, and 654 nm, being attributed to the transitions of 4f electrons from 4 I 15/2 → 4 F 7/2 , 4 I 15/2 → 2 H 11/2 , and 4 I 15/2 → 4 F 9/2 , respectively.Furthermore, red shift in absorption edge of TiO 2 might be observed due to Er doping.Consequently, it is important to investigate how Er ion acts and locates in the network of host material.In this work, we monitored structural changes by Er doping on V 2 O 5 and TiO 2 films through UV-Vis transmittance spectroscopy, photoluminescence (PL) measurement, and Fourier transform infrared spectroscopy (FTIR).Additionally, we correlated the charge capacity and structural alteration by Er doping on V 2 O 5 and TiO 2 films through cyclic-voltammetry and AFM measurements as well as the surveyed techniques.

Experimental
Ti[OCH(CH 3 ) 2 ] 4 and OV[OCH(CH 3 ) 2 ] 3 were used as sol precursors.For TiO 2 sol, 2.4 mL of Ti[OCH(CH 3 ) 2 ] 4 was added to 30 mL ethanol and mixed in a magnetic stirrer for 1 h. 10 mL glacial acetic acid (CH 3 CO 2 H) and 20 mL ethanol was introduced into the mixture and stirred.Finally, 3 mL triethylamine ((C 2 H 5 ) 3 N) was added and solution was mixed for 4 h.For V 2 O 5 sol, 10.2 mL OV[OCH(CH 3 ) 2 ] 3 was added into 40 mL isopropyl alcohol ((CH 3 ) 2 CHOH) and solution was mixed for 2 h. 1 mL glacial acetic acid was added to the mixture and stirred.Er doping was carried out using Er(NO 3 ) 3 ⋅5H 2 O powder which was dissolved in TiO constant speed of 100 mm/min.Prior to deposition, substrates were cleaned in an ultrasonic bath using acetone, isopropyl alcohol, and deionized (DI) water, respectively.Deposited films were dried at room temperature and then heat-treated at 150 ∘ C for 20 min.The processes were resumed for double layer, resulting in uniform films.

Results and Discussion
TG/DTA experiment was carried out with the temperature range from 25 ∘ C to 1000 ∘ C for V 2 O 5 and TiO 2 powders, respectively.Figure 1(a) showed the first weight loss up to 320 ∘ C with a large endotherm, associated with the volatilization and combustion of organic species for undoped V 2 O 5 film.Second change occurred around the 343 ∘ C, corresponding to the phase transition while other mass losses started at 610 ∘ C, reflecting the melting point of V 2 O 5 .TG/DTA curves of the TiO 2 powder, on the other hand, showed two mass losses that were associated with endothermic and exothermic events and depicted in Figure 1(b).The first endothermic event took place around 90 ∘ C, denoting elimination of water while exothermic events were due to the volatilization and combustion of CH Figure 2 showed FTIR spectra of the films in which deconvolution process was applied to identify the IR modes.V 2 O 5 films exhibited two large bands at ∼1600 and ∼3400 cm −1 .The peaks between 1400 and 1650 cm −1 were OH bending and OH-H stretching from water [8,9].Moreover, H 2 O and H 3 O + bonds appeared at 3362 and 3200 cm −1 .In the 400-1100 cm −1 , the V 2 O 5 film exhibited three characteristic vibration modes: V=O vibrations at 1017 cm −1 [10], the V-O-V symmetric stretch around 516 cm −1 [11], and the V-O-V asymmetric stretch at 756 cm −1 [12].The group of bands presented below 600 cm −1 corresponded to the edge sharing 3V-O C stretching [13] and the bridging V-O B -V deformations [14].Peaks at 932 cm −1 and 1000 cm −1 corresponded to V 4+ =O and V 5+ =O bands by indicating nonstoichiometric V 2 O 5 film while the band at 830-840 cm −1 showed disorder (or amorphous phase) of V 2 O 5 film [15].For TiO 2 film, the presence of Ti-O-Ti and Ti-O polymeric chains was clearly evident from the bands at 471 and 789 cm −1 .Also vibration of the Ti-O-O was identified from the band at 693 cm −1 [16].Moreover, the bands at 1009, 1122, and 1138 cm −1 were ascribed to stretching of Ti-O-C [17].LO mode of amorphous TiO 2 [18] appeared at 874 cm −1 and the broadband from 3000 to 3600 cm −1 associated with the stretching vibration modes of hydroxyl groups [19].The bands at 1288 and 1368 cm −1 were vibration mode of the C-O-O group and the doublet in 1441 and 1538 cm −1 designated the symmetric and asymmetric stretching vibration of the carboxylic group coordinated to Ti as a bidentate ligand [17].Upon Er doping, the bands around 400-450 cm −1 corresponded to Er-O bond.Also the huge band at 3000-3500 cm −1 was attributed to water related bonds.The bands between 1300 and 3000 cm −1 represented the carbon related bonds.Moreover, as deposited, V 2 O 5 film showed small peaks at 440 and 600 cm −1 that could be assigned to phonon bands of crystallized Er 2 O 3 cubic phase [20].Optical transmittance spectra were given in Figure 3.For V 2 O 5 films, transmittance curve is strongly affected by Er doping, causing a blue shift (see the inset of Figure 3(a)) in the optical band gap.Contrary to V 2 O 5 films, though no absorption peaks arose in undoped TiO 2 film within visible region [21,22], absorption peaks related to Er doping in TiO 2 film were observed, located at 490, 523, and 654 nm in absorption measurement (given as inset of Figure 3(a)), and responsible for improvement in photocatalytic activity of TiO 2 .  's of the films were calculated as to Tauc's law as follows: where  was constant, ℎ] = photon energy, and  was the fingerprint of the transition.Best fit for all the films was given by a direct allowed transition where  = 1/2.remained almost the same.To verify the formation of Erdoped vanadia/titania films, PL measurements were carried out and depicted in Figure 4.The peaks in the spectra, located at 840 and 980 nm, owing to the transition of 4 I 15/2 → 4 I 9/2 and 4 I 15/2 → 4 I 11/2 , directly related to Er substitution [23].Moreover, PL peaks, which appeared at the range of 300-400 nm, was attributed to oxygen vacancy in V 2 O 5 film since, due to the weakness of V=O bond, its oxygen was easily removed [24].In titania films, apart from the peaks emerging in absorption measurement, a slightly shifted and new emerged peaks were present in PL measurement, confirming the successful of Er doping in TiO 2 film [22][23][24][25].Figure 5 displays AFM results of the films.As to the analysis, crack-free and homogeneous films are synthesized and, upon Er doping, the size of the grains increases (decreases) in V 2 O 5 (TiO 2 ) films, similarly to the ZnO:Er films [26,27].
To ascertain the proposition, X-Ray diffractograms (XRD) were obtained using X-ray diffractometer using CuK radiation and illustrated in Figure 6.The weak and broad peak around 25 ∘ in V 2 O 5 film indicated (003) growth direction while, in TiO 2 film, it gave (101) direction with verifying anatase phase [28,29].Noteworthily, weaker and broader XRD peaks implied reduced grains size and low extent of crystallinity.Indeed, this was exactly observed in Er-doped TiO 2 film.In V 2 O 5 film, relatively strength and narrow XRD diffraction peaks were observed after Er doping.Such results were consistent with the one obtained in absorption measurement on Er-doped TiO 2 and V 2 O 5 films.V 2 O 5 can exhibit multielectrochromism regarding its layered structure and thickness with the following reaction [30] V while TiO 2 shows cathodic electrochromism upon Li + and electron insertion into the films with the following reaction [26]: V 2 O 5 films demonstrated orange-yellow to green to greyishblue color with a high contrast while TiO 2 films changed from transparent to greyish-blue color.CVs of the films were illustrated in Figure 7. Anodic: (cathodic) charge capacities upon doping enhanced the charge capacities such that 34.

Conclusion
The erbium-undoped/erbium-doped vanadium pentoxide and titanium dioxide thin films were produced via dip coating technique.FTIR and TG/DTA measurements were performed to find out Er substitution.Upon Er doping, UV-Vis spectroscopy indicated a blue shift on the band gap values of V 2 O 5 due to the softening of V=O bond.Due to the impact of Er on host material structure, granule size of the V 2 O 5 film increased (UV-Vis and AFM measurements) yielding more space for intercalation of ion in host materials.In TiO 2 , reduced granule size by Er doping caused increase in surface area and hence dramatic increase in ion storage capacity that were deduced by CV.

Figure 2 :
Figure 2: FTIR (a) transmittance and (b) deconvolution spectra of the films deposited on ITO coated glass substrates.

Figure 3 :
Figure 3: (a) Optical transmittance spectra, (b) Tauc plots of the films deposited on ITO coated glass substrates.The inset in (a) demonstrated the absorption measurement whereas in (b) resumed Tauc plot for Er-doped films.
Er doping resulted in the softening of V=O bond in V 2 O 5 film and O deficit in anatase TiO 2 film.However, keep in mind that Er local structure was determined by the Ti-O arrangement in anatase TiO 2 whereas, in rutile TiO 2 , by Er-O chemical property rather than the Ti-O arrangement [7].
3OH, (CH 3 ) 2 CHOH, and CH 3 COOH species.The two peaks in the DTA curve located at 367 ∘ C and 510 ∘ C, respectively, corresponded to the crystallization of the amorphous into anatase phase.Above 600 ∘ C, the anataserutile phase transition occurred since there was no mass loss in TG curve.Upon Er doping, similar features appeared except Er 2 O 3 cubic phase crystallization in TiO 2 film due to requirement of higher ambient temperature.As partial conclusion, [14]films, which might be related to increase in the oxygen vacancies.It was reported that increase of the interlayer distances due to softening of V=O leads to decrease of interlayer interactions and made Li diffusion easier[14].In Er-doped TiO 2 film, presence of absorption peaks and