Key Words

TiO2 nanoparticles, Photo catalytic degradation, Methyl Orange, XRD, TEM, FTIR

Introduction

Paper, dyeing, plastic and textile industries use color for dyeing their products and thus use a huge amount of water which results in the production of a dye-containing wastewater with hazardous effects on the environment [1][2][3][4]. At present, 100000 different types of dyes with annual production rate of 7×105 are produced. Among them textile industries consume about 36000 ton/year dye, 10 to 20 percent of which remains in wastewater [5]. The presence of these dye pollutants in water streams causes numerous problems related to their carcinogenicity, toxicity to aquatic life and easily detected and undesirable esthetic aspect [6]. Dyeing effluents are very difficult to treat, due to their resistance to biodegradability, stability to light, heat and oxidizing agents [7]. MO causes eye and skin irritation and may cause respiratory and digestive tract irritations and it is also responsible for toxic effects. In addition to standard technologies for the degradation and/or removal of dyes, several new specific technologies, the socalled advanced oxidation processes (AOPs), have been developed. Heterogeneous photo catalysis, as one of the AOPs, could be effective in the oxidation /degradation of organic dyes. The initial interest in the heterogeneous photo catalysis was aroused in 1972 when Fujishima and Honda discovered the photochemical splitting of water into hydrogen and oxygen by the UV irradiated TiO2 [8]. After that, research on the heterogeneous photo catalysis started growing rapidly [9] in many area covering water and air treatment technologies.

Among the metal oxide semiconductors, TiO2 is widely used catalyst because of its fascinating physicochemical properties, high photo activity, photo-corrosion stability, nontoxic and low cost [10][11]. It founds its application as solar cells, gas sensors, photo catalysts, capacitors, and catalyst supports [12][13][14][15][16]. TiO2 self-cleaning property can be bestowed on many different types of surface, and some TiO2 based self-cleaning products such as tiles, glass, and plastics have been commercially available.TiO2 self-cleaning coatings are finding increasing applications in buildings, public furniture and auto industry. The self-cleaning mechanism is mainly based on TiO2 photo catalysis, where photo-induced electron-holes catalyze reaction on the surface [17][18]. The electrons and holes form hydroxyl radicals which are assumed to be the main reactants in the degradation of many pollutants like herbicides [19], fungicide [20], aliphatics and aromatics [21], dyes [22] and bacteria [23].TiO2 has many polymorphs, among which anatase TiO2 shows the highest photo catalytic activity toward photo degradation of most organic pollutants in waste water.

In this case, photosensitization of the dye may occur upon excitation by UV-light in addition to the hydroxyl radical attacking the dye molecules. The photo generated electrons might also transfer from the excited dye to the semiconductor particle. It can also reduce the adsorbed oxygen in the suspensions to form superoxide radicals, than the reaction between the dye radical and the other active oxygen-containing species might also occur, which can accelerate the photo degradation process of the dye [24]. In the present investigation, TiO2 nanoparticles were synthesized via Sol-Gel route using Titanium Tetraisopropoxide as a metal precursor and was characterized by X-ray diffraction (XRD), Fouriertransform infrared absorption spectrophotometry (FT-IR), Transmission Electron Microscopy (TEM), moreover, water pollutant substance from industry, one of the dyes, methyl orange was studied in aqueous medium for photo catalytic degradation under irradiation of ultraviolet light.

Methodology

Materials

Titanium Tetraisopropoxide (97%) was provided by Sigma Aldrich Co, 3050, St.Louis, MO, USA, Glacial Acetic Acid (99%) was purchased from Hi-Media. Methyl Orange was of analytical reagent grade and used without further purification. All solutions used in the experiments were prepared by using double distilled demineralised water. The chemical structure, IUPAC name, molecular weight and Colour Index (CI) number of Methyl Orange is represented in Table 1.

Table 1: Structure and characteristics of malachite green

Catalyst Preparation

The Sample was prepared by novel and simple Sol-Gel route. 12 mL titanium isopropoxide was added to 23 mL acetic acid with continuous stirring. Hydrolysis of titanium tetraisopropoxide solution was carried out by adding distilled water (72 ml) slowly at the rate of 0.5 ml/min with continuous stirring. The solution was kept stirring for 6 h until achieving a clear transparent sol. Dried at 100◦C, after that it was calcined at 600◦C for 2 h at a ramp rate of 5◦C/min. [25].

Catalytic characterization

Powder XRD patterns were recorded with a Phillips X’pert MPD system, Holland using CuKa radiation (λ=1.5405 Aº) in a 2y range of 5–601 at a scan speed of 0.11 s-1 and patterns were compared with the standard anatase diffractograms [26]. Transmission Electron Microscope (TEM) studies were carried out on the sample using a model Philips Tecnai 20, Holland with an accelerating voltage of 100kV for the details of size, and morphological structure. An FT-IR spectrophotometer (SPECTRUM GX, Perkin- Elmer) was used to determine the specific functional groups in TiO2 samples. The spectrum is recorded in the range of wave-number 400–4,000 cm-1 .

Photo catalytic experiment

The photo catalytic activities of the materials were studied by examining the degradation reactions [27]. Methyl Orange stock containing four different concentration of dye i.e. 10ppm,20ppm,30ppm,40ppm were prepared in borosilicate glass tubes containing various doses of synthesized catalytic TiO2 nanoparticles . Catalysts containing tubes were placed on UV- radiation lamp. Two 15 W low pressure mercury UV tubes (Spectronics) emitting near UV radiation with a peak at 365 nm were used at a light intensity of 3.48 mW/cm2 measured near the film surface. The photo catalytic oxidation process started when UV radiation reached the TiO2 photo catalyst. Afterwards at one hour time interval sample were collected, centrifuged and analyzed with a UV-Vis spectrophotometer. A blank study was also carried out only in the presence of UV light without any catalyst. This shows that though during UV irradiation, direct photolysis of dyes could occur, mineralization of dyes only takes place in the presence of a photo catalyst [28]. In the photo degradation experiments the extent of removal of the dye, in terms of the values of percentage removal has been calculated using the following relationship:

Equation 1

Percentage Removal (%R) = 100*(Ci-Cf)/Cf (1)

Where, Ci= initial concentration of dye (ppm); Cf = final concentration of dye (ppm) at given time.

Results and Discussion

Crystallinity and crystallite size

Figure 1 XRD powder pattern of synthesized TiO2 by Sol-Gel

Functional Group analysis of TiO2 nanoparticles

Fourier transform infrared (FTIR) spectrum of as-synthesized anatase TiO2 nanoparticles is shown in Figure 2. It was observed that the strong band in the range of 700–500 cm-1 is associated with the characteristic vibration modes of TiO2. This confirms that the TiO2 phase has been formed. The absorption in the range from 3,500 to 2,500 cm-1 may be related to the presence of O–H stretching vibration (Monomer, intermolecular, intra molecular and polymeric) [34]. The absorption band at 1,637 cm-1 due to the presence of O–H bending vibration which is probably because of the reabsorption of water from the atmosphere has occurred [35].

Figure 2: FTIR spectra of synthesized TiO2

Structural characteristics of TiO2 nanoparticles

The above results indicated that the precursor titanate obtained was in nanostructure, which is further confirmed by TEM observation. The homogeneity, uniformity and the size of the resulting TiO2 crystals were studied by TEM, as shown in Figure 2. TEM study indicated that all the crystals were completely separated from each other and uniformed with a particle analytical grade with size of 20 -30 nm. These particles do not grow together to form bigger particles, even after an extensive period of time. It is worth noting that only a small percentage of the total particles exhibit a diameter size bigger than 30 nm. The crystallites had sets of clearly resolved lattice fringes giving evidence that the TiO2 material was highly crystalline [36].

Figure 3: TEM image of synthesized TiO2 nanoparticles

However, there was a slight discrepancy between the particles sizes determined by XRD analysis and TEM. This could be due to the fact that the effective mass approximation is relatively less correct for small nanoparticles and statistical effects of spatial confinement also influence the optical properties of Nano crystalline semiconductors [37].

De colorization efficiency of UV/TiO2 photo catalysis process

Photo catalytic properties of the as-prepared samples were examined by degradation of MO dye solution under UV light irradiation at room temperature. In order to identify possible losses of methyl orange in the system, control experiments without catalyst added were preformed. The course of methyl orange photo catalytic degradation used pure catalyst TiO2 at different amount added to different dye concentration. It was found that no obvious methyl orange loss was observed in control experiment which confirmed that the methyl orange was stable in our experiment. However, four dye concentrations- 10ppm, 20ppm, 30ppm, 40ppm were irradiated with different catalytic doses of 5mg, 10mg, 15mg and 20mg. At 20mg catalytic dye was found to be almost completely decolourised on irradiation for 120 minutes. Catalyst loading of 20mg showed better result than other 3 doses (Figure 4). For 30 and 40 ppm, about 60% degradation was obtained within 1 hour for 20mg concentrations of catalyst. However, 99% degradation was obtained for catalyst loading of 20mg at 3 hours (Figure 5). The degradation increases with increase in catalytic dose. There is no doubt that electron injection from the dye to the positive holes of TiO2 yields the dye cationic radical. After this stage, the cationic radical, Dye•+, can undergo hydrolysis and/or de protonation pathway of the dye cationic radicals, which in turn are determined by the different adsorption modes of MO on the TiO2 particles surface[38]. Total mineralization of the organic dye pollutants usually follows proposed mechanism described below [39][40].

Photo catalysis occurs by following proposed mechanism given in step wise manner.

Absorption of efficient photons (hν≥EG=3.2 ev) by titania

(TiO2) + hν e−CB+ h+VB (1)

Oxygen ionosorption (first step of oxygen reduction; oxygen’s oxidation degree passes from 0 to −1/2)

(O2) ads + e−CB O2• (2)

Neutralization of OH− groups by photo holes which produces OH◦ radicals

(H2O ⇔ H + + OH )ads + h+VB H + + OH• (3)

Neutralization of O2◦− by protons

O2◦− + H+ HO2◦ (4)

Transient hydrogen peroxide formation and dis mutation of oxygen

2HO2◦− → H2O2 + O2 (5)

Decomposition of H2O2 and second reduction of oxygen

H2O2 + e OH◦ + OH (6)

Oxidation of the organic reactant via successive attacks by OH• radicals

R + OH◦ → R• + H2O (7)

Direct oxidation by reaction with holes

R + h + R + degradation products (8)

As an example of the last process, holes can react directly with carboxylic acids generating CO2

RCOO + h + → R• + CO2 (9)

Increase in the concentration of catalyst shows an increase in dye degradation for the first few minutes. This is due to the fact that number of dye molecules adsorbed and photon absorbed increases with increase in catalyst loading. The increase in degradation was probably due to an increase in availability of catalytic sites and adsorption sites. It could be due to decolorization of MO which undergone de methylation, methylation and hydroxylation processes.

Conclusion

In this research, A novel, easy and reproductive method was followed for the synthesis of TiO2 and photo catalytic activity of TiO2 nanoparticles on Methyl Orange was studied. It’s physical and chemical characterization was done by TEM, XRD and FT-IR. The obtained results comply with that of standard. The ultraviolet (UV) light irradiation of the dye by using nanoanatase TiO2 as a catalyst has yielded percentage decolouration of greater than 90% for a catalyst loading of 20mg and initial concentration of the dye solution of 10-40ppm.