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J. Eur. Opt. Society-Rapid Publ. 21, 6( 2025)
changes in microstructure, crystallinity and interaction force. Moura et al. [ 16 ] fabricated V 2 O 5-TiO 2 thin films by sol-gel method and deposited on fluorine-doped tin oxide( FTO) substrates by using the dip coating technique. The films shown an orthorhombic crystal structure with the thickness of 617 nm, and had an optical modulation of 35 % at 633 nm after coloring and bleaching. Ivanova et al. [ 17 ] used sol-gel dip-coating technique to deposite TiO 2- V 2 O 5 composite thin films with V 2 O 5 as the dopant. Results showed that V 2 O 5-doping decreased the temperature of anatase-rutile transition of TiO 2, thefilm were visually uniform and smooth, and the color efficiency( CE) could reach 15 cm �2 C �1 at the wavelength of 525 nm. Rehman et al. [ 18 ] prepared TiO 2 / V 2 O 5 / TiO 2 multiple-layer-structure thin films by using electron-beam evaporation method. The films had a particle size of 20 – 40 nm with the optical bandgap of 3.51 eV, and showed an optical transmittance of ~ 78 % in the visible region. In addition, Wang et al. [ 19 ] have prepared the one-dimension( 1D) TiO 2 / V 2 O 5 heterostructures by RF reactive magnetron sputtering method. The photodecomposition rate of Rhodamine B( RhB) by the 1D TiO 2 / V 2 O 5 branched heterostructures under visible light was much faster than that of pure TiO 2 nanofibers, the visible-light-induced catalytic activity of the 1D TiO 2 / V 2 O 5 branched heterostructures was greatly improved. Sun et al. [ 20 ] synthesized a novel composite V 2 O 5 / BiVO 4 / TiO 2 photocatalyst by using sequentially hydrothermal and adhering method. Comparing to pure TiO 2 nanobelts and V 2 O 5 / BiVO 4 nanorods, the V 2 O 5 / BiVO 4 / TiO 2 composite film exhibited higher photocatalytic activity in decomposition of gaseous toluene under visible light irradiation( k > 400nm).
In addition, the metal-semiconductor-metal( MSM) structure under perpendicular voltage driven has been proved to enhance the optical and electrical properties of optoelectric devices [ 21 – 23 ]. Nowadays, most of studies focus on the electrochromic properties and the photocatalytic activities of V 2 O 5 films, and there are few studies on the optical and electrical properties of V 2 O 5 films before and after the phase transition happens. The reasons are probably that the devices based on V 2 O 5 films usually have a high resistance at room temperature, and the thermal and electrical performances of them need to be further improved. Moreover, TiO 2 is expected to refine the functions of V 2 O 5 films as mentioned above. So it is envisaged that introducing nanoscale TiO 2 powder to dope the V 2 O 5 film, improving their performance under certain conditions. In this investigation, the TiO 2-doped V 2 O 5 / FTO film is prepared by sol-gel method and post-annealing process with different proportions of TiO 2, andtheMSM structural device based on the prepared film is fabricated. Finally, the optical and electrical characteristics of the prepared films and the devices under thermal and electrical excitation are experimented.
2 Experiment
The preparation progress is shown in Figure 1. In this investigation, the 10 mm 20 mm 2.2 mm FTO conductive glass with the sheet resistance of 14 X was used as the substrate, while vanadium pentoxide powder( V 2 O 5, analytical purity of 99.9 %), nano titanium dioxide powder( TiO 2, 20 nm, analytical purity of 99.9 %) and hydrogen peroxide solution( H 2 O 2, 30 wt %) were selected as the raw materials. The TiO 2-doped V 2 O 5 / FTO film was prepared by sol-gel method. In priority, the rectangular FTO substrates were cleaned in acetone, absolute ethanol and deionized water in turn for 15 min each by using the ultrasonic cleaning machine( SK3300H). Drying them in the electric constant temperature drying box. Subsequently, V 2 O 5 powder was slowly added into a beaker with H 2 O 2 solution and stirring continuously for 10 – 15 min. In this process, the V 2 O 5 powder and the H 2 O 2 solution would undergo a violent exothermic reaction, releasing a large amount of oxygen. The reaction equations are listed as follow:
V 2 O 5 þ 2H 2 O 2 ¼ 2HVO 4 þ H 2 O 2HVO 4 þðn � 1ÞH 2 O ¼ V 2 O 5 nH 2 O þ O 2 "
After the reaction happened, added TiO 2 powder in moderation, and continuously stirring for about 3 h to fully diffuse the TiO 2 powder into the solution. While the stirring completed, the solution would become a gel with high viscosity. When the gel was made, placed the dried FTO substrate on the homogenizer, using the vacuum pump to inhale, absorbing the appropriate amount of gel with the rubber dropper and dropping it on the FTO substrate to make it be fully coated. Then, placed the sample in the annealing furnace after spin-coating at the speed of 4000 r / min, and the annealing temperature and time were set at 400 ° C and 2 h respectively [ 24 ]. The resistivity of the TiO 2-doped V 2 O 5 / FTO film is determined to be about 7.56 10 �4 X cm. Besides, the pure V 2 O 5 / FTO film was fabricated by the similar process above as a contrast.
Furthermore, sputtering an uppermost fluorine-doped SnO 2 film and a SiO 2 film with FTO and SiO 2 targets on the prepared film respectively, and then assembling it to form the MSM structure device after lithography and etching. Figure 2 displays the diagram of the device. The thicknesses of the FTO film and the TiO 2-doped V 2 O 5 film are about 350 nm and 420 nm respectively. 2 mm 2mm Au / Ni-Cr metallic contacts were used as the electrode. The device was formed to achieve transmittance modulation under perpendicular voltage driven.
3 Results and discussions
The 10 %, 12 %, 15 %, 20 % TiO 2-doped V 2 O 5 / FTO films had been fabricated by the above crafts and the transmittance curves were examined at the wavelength of 700 nm under heating and cooling process from 150 ° Cto330 ° C, as shown in Figure 3. It can be seen that all films show excellent thermal phase transition characteristics. During the heating process, the transmittance of the TiO 2-doped V 2 O 5 / FTO films under relative TiO 2 ratios decreased 16.381 %, 18.277 %, 16.264 %, 15.589 % respectively, in which the prepared film with the proportion of 12 % TiO 2 showed
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