1. Introduction
Titanium dioxide which is known as Titania occurs in four forms: Rutile (Eg = 3.0 eV), Anatase (Eg = 3.2 eV), Brookite (Eg = 2.96 eV) and TiO2-β. Brookite and Anatase phases are rare and have more photocatalytic activity than Rutile phase [1]. TiO2 is desirable due to its inertness, stability, and low cost [2]. It is also self regenerating and recyclable. Its redox potential of the H2O/*OH couple (−2.8 eV) lies within the band gap. However, its large band gap (Eg = 3.2 eV) only allows the UV absorption of the solar spectrum. Titania nano particles are highly considered for the technological and industrial applications because of its photo catalytic activities [3], such as self-cleaning [4], anti bacterial [5], fog proofing [6]; and also, as a new source of energy saving in dye sensitized solar cells [7-9]. Titania nano particles are prepared by many techniques such as sol-gel [10], hydrothermal [11] and co-precipitation processes [12]. But, there are still many ongoing researches [13] to develop new features and simplification of nanoparticles synthesis within a desired size distribution. In this study, we used a simple Pechini sol-gel process [14,15] with two different complex agents in order to study the role of polymeric and non-polymeric agents in phase purity and size formation.
2. Experimental Details
The titanium oxide nano particles have been prepared by Sol-gel process in two steps. Titanium isopropoxide (TTIP) was used as precursor. Acetyl acetone (AcAc: CH3COCH2COCH3) and acetic acid (HOC(COOH)(CH2COOH)2) were used as both complex and polymer agent and finally absolute ethanol as solvent. In first step, starting solution was prepared with diluting 0.1 M TTIP by 40 cc absolute ethanol with the same volume ratio and then mixed with 0.1 M AcAc (Citric Acid) and stirred with magnet for 1 hr in 40˚C to establish initial sol. This sol was red (yellow)-color and pH = 6.5 (3.0). For gaining transparent sol witch has nano particles of Titania as complex compounds, the sol was refluxed in oil bath at 130˚C for 4 h. After this step, sol was orange(yellow-green)-color and pH = 8.4(6.0). In the next step, we used oil bath to heat the sol, indirectly at T = 80˚C for 36 hrs until forming gel. The gel firstly was dried up by direct heating on hot plate at 120˚C for 2hrs and then for final drying, the gel was heated at 150˚C for 30 minutes. The titanium oxide nano particles cannot be obtained unless the dried gel calcinated at 400, 500, 650˚C for 1 hr. the weight of powders measured before and after calcinations that the results have come in Tables 1 and 2.
The X-ray diffractometer (D8 Advance Bruker), with
Table 1. Powder prepared by sol-gel process using AcAc as complex and polymer agent.
Table 2. Powder prepared by sol-gel process using citric acid as complex and polymer agent.
Cu-Kα radiation of wavelength 1.5405 Å (operated at 25 kV, 20 mA) was employed for structural studies. TEM was employed for nano particles size analysis. The UvVis spectra were prepared by spectrophotometer (Agilent 8453) for determining band gap of nano particles.
3. Results and Discussion
Structural characterization: the XRD patterns of titanium oxide nano particles (Figures 1 and 2) show Rutile and Anatase phases formation. The (101)-plane for Anatase and (110)-plane for Rutile are the main diffraction planes seen in the figures. the mean crystalline size of nanoparticles is estimated using scherrer formula, and Anatase/Rutile phase ratio also is determined using Equation (1) [11] in which, xA is the Anatase phase contribution in the powder, IR and IA are the intensity of the highest peaks of Rutile and Anatase in XRD patterns, respectively.
(1)
Figure 1(a) show phase formation of Anatase and small crystalline size (Table 3) in lowest sintering temperature, which may due to using organo-metalic precursor and organic complex agent, it allows Ti-O bonds formation and postpones the condensation process rather than hydrolysis [12]. Comparison of Tables 3 and 4 shows that in the sintering temperature at which both Anatase and Rutile phases coexist, AcAc case results in a uniform crystalline growth process other than Citric Acid one. The observation of mixed Anatase and Rutile phases at 500˚C (Figure 1(b)) and Rutile phase at 650˚C (Figure 1(c)) indicates phase transition of Anatase to Rutile. Moreover, application of AcAc as complex agent results in obtaining single phase TiO2 nanoparticles in 400˚C and 600˚C sintering temperatures. Actually powders did not show any crystalline phase below 400˚C sintering tem-
Table 3. Nano crystalline size of calcined powders prepared by AcAc as complex agent.
Table 4. Nano crystalline size of calcined powders prepared by citric acid as complex agent.
perature. thus, comparing citric acid and AcAc chemical formula one can be said that the lower the amount of carbon existing in the dried powders, the lower the sintering temperature at which phase formation occurs.
TEM images (Figures 3 and 4) show that with increasing of sintering temperature, agglomeration of nano particles has been occurred. Acetyl Acetone causes a more crystalline structure and more uniformity of size distribution in 400˚C sintering temperatures. On the other hand, in highest sintering temperature, the particles obtained from polymeric agent tends to agglomerate larger in size than the acidic one.
Optical characterization: Using Uv-Vis spectra and Tauc formula, The optical absorption edge was analyzed by the following relationship [16], αhν = A (hν − Eg)m where A is a constant, m value is respectively 1/2 and 2
(a)
(b)
(c)
Figure 1. XRD pattern of nano powder prepared by AcAc as complex agent, calcined at (a) T = 400˚C, (b) T = 500˚C, (101) red-peak and (110) blue-peak implies to Anatase and Rutile phases, respectively, and (c) T = 650˚C.
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(b)
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Figure 2. XRD pattern of nano powder prepared by citric acid as complex agent, calcined at (a) T = 400˚C, (b) T = 500˚C, (101) red-peak and (110) blue-peak implies to Anatase and Rutile phases, respectively, and (c) T = 650˚C.
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(b)
(c)
Figure 3. TEM images of nano powder prepared by AcAc as complex agent, calcined at (a) T = 400˚C, (b) 500˚C, (c) 650˚C.
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(b)
Figure 4. TEM images of nano powder prepared by citric acid as complex agent, calcined at (a) T = 500˚C, (b) 650˚C.
for direct and indirect transitions and Eg is the optical band gap. We estimated the direct energy band gap of nano particles. The optical results show that nano particles with smaller size which sintered at 500˚C have smaller band gap rather than nano particles which sintered at 650˚C (Figure 5). This may be due to existing of traps that lies within the band gap. Nano particles that have been sintered at 500˚C which contain two phases (Anatase and Rutile) have more traps rather than those who sintered at 650˚C.
Figure 5. energy gap of nano particles prepared with solgel process and using AcAc complex agent (T: sintering temperature).
4. Conclusion
TiO2 nanoparticles have been synthesized simply via complexing-polymerizing sol-gel method. The effect of polymer agent on size distribution of particles shows that AcAc is a proper candidate to synthesize particles in pure Anatase phase below 10 nm. Also, sintering at 650˚C shows pure Rutile phase with narrow size distribution about 100 nm. Moreover, application of Citric acid gives rise to a less agglomeration of particles after annealing at 650˚C which does not have pure phase.