ABSTRACT

In this paper sensitivity of chemically synthesized titanium dioxide (TiO₂) nanoparticles to ultra-violet (UV) radiation is investigated. For this purpose three types of ultra-violet photodetectors were fabricated, one with as-synthesized TiO₂ nanoparticles, another one with TiO₂ nanoparticles annealed at 600˚C, and the third one with TiO₂ nanoparticles annealed at 800˚C. In each case the annealing time duration was one hour. The photodetectors were fabricated on glass slides where on them a thin layer of fluorine doped tin oxide (FTO) was deposited by spray pyrolysis. The results show that all photodetectors are sensitive to UV radiation where the photodetectors fabricated from annealed TiO₂ nanoparticles are more sensitive than the photodetectors fabricated from as-synthesized TiO₂ 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 (SnCl_4.5H₂O, 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

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