Detecting Ultra-Violet Radiation by Using Titanium Dioxide Nanoparticles ()
1. Introduction
In recent years, UV photodetectors have got much consideration due to their civilian and military applications. These photodetectors are used in UV radiation measurement equipment, space research, detecting missiles at early stages of their launch, astronomy and biology [1,2]. In general, UV photodetectors are fabricated from large band gap inorganic semiconductor materials such as: GaN [3,4], CdS [5], diamond [6], or Si [7,8]. These materials are produced using complicated processing steps that are costly and are not appropriate for large scale applications. Recently semiconductor nanoparticles synthesized by chemical methods have been studied where in comparison to the semiconductor materials produced by previous techniques have advantages such as: low cost, simple preparation methods, capable of large scale applications, and vast options on substrate choice.
TiO2 is a wide band gap semiconductor material which can be used for UV photodetector fabrication [9]. This material crystalizes in three major phases: rutile, anatase, and brookite [10]. Rutile phase is thermodynamically more stable than the two other phases. Rutile TiO2 has direct and indirect band gaps. Its direct band gap makes TiO2 useful for photodetector fabrication. In this research TiO2 rutile phase nanoparticles were synthesized by a chemical method. Then by using these nanoparticles ultra-violet photodetectors were fabricated that are sensitive to ultra-violet radiation.
2. Synthesis of TiO2 Nanoparticles
We synthesized TiO2 nanoparticles by using 20 mL titanium (III) chloride solution (15%) in 10% hydrochloric acid from Merck Company. The solution was stirred vigorously in a reaction flask for 20 min at room temperature. Then 0.7 mL ammonia solution (25%) from Merck was diluted by 57.3 mL double distilled water and this 60 mL solution was added to the reaction flask drop wise very slowly (during 5 hours). Then the solution was left for 48 hours while it was stirred vigorously; during this time color of the solution gradually turns to white. White precipitation settle done properly. We centrifuged and washed the precipitation with water and ethanol a few times. This centrifuging and washing was repeated for three times.
We prepared three types of TiO2 samples, as-synthesized (sample A), annealed at 600˚C (sample B), and annealed at 800˚C. Duration of annealing time was one hour for both sample B and sample C.
3. Fabrication of Photodetectors
For fabricating UV photodetectors fluorine doped tin oxide (FTO) was coated on glass substrates by spray pyrolysis. For depositing FTO, 33.3 gr tin tetrachloride pentahydrate (SnCl4.5H2O, 98%) was dissolved in 50 mL double distilled water by using magnetic stirrer for a few minutes. Then 50 mL ethanol was added to the solution. Addition of alcohol turns the solution slightly blurred where by adding a few droplets of hydrochloric acid pH of the solution decreases. It leads to increasing of the solution solubility which causes the solution become clear again. Then to this solution we added 0.5 gr NH4F (99.99%). 10 mL of this solution was sprayed on glass slides where they were heated to 450˚C. The solution was sprayed at 2 atm air pressure with the speed of 10 mL/min where the spraying nozzle opening diameter was 0.3 mm and its distance from substrates was 35 cm. The deposited FTO layer had 200 nm thickness and its sheet resistance was 484 Ω.
After coating, FTO conducting oxide was disconnected by scribing a scribe line all along the width of glass slides. The scribe line was created by a diamond tip glass scriber and the width of the scribed line was 90 µm. Then TiO2 nanoparticles dispersed in diethylene glycol (50% by weight), and spin coated on the FTO film at 1000 rpm/min. TiO2 coating was done on the scribed line where the coating area was 5 mm by 5 mm. Then they are placed in an oven for 30 min and the oven temperature was 450˚C.
Three types of photodetectors were fabricated, one type with as synthesized TiO2 nanoparticles (type A), another one with TiO2 nanoparticles annealed at 600˚C (type B), and the other with TiO2 nanoparticles annealed at 800˚C (type C).
4. Characterization of TiO2 Nanoparticles
Some techniques were used for analyzing TiO2 nanoparticles such as: X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and UV-VIS spectroscopy. Figure 1 shows UV-VIS spectrum of the samples. As Figure 1 shows by increasing annealing temperature absorption at peak point is shifted to longer
Figure 1. UV-VIS spectrum of TiO2 nanoparticles samples: as-synthesized TiO2 nanoparticles (A); TiO2 nanoparticles annealed at 600˚C (B) and TiO2 nanoparticles annealed at 800˚C (C).
wavelengths (red shift) which indicates increasing nanoparticles size by increasing annealing temperature.
Figure 2 shows XRD spectrum of the samples. The Xray wavelength used for measurements was 0.154 nm. It