Photocatalytic degradation of some organic dyes under solar light irradiation using TiO2 and ZnO nanoparticles

INTRODUCTION

       With the development of the textile industry, intense concerns were focused on the contamination of the environment caused by dye pollutants, which caused severe environmental pollution and health problems due to the variety, toxicity and persistence characteristics [1-6]. Most of dyes are characterized by complicated constitution and high chemical stability, hence persist for long distances in flowing water, retards photosynthetic activity, inhibit the growth of aquatic biota by blocking out sunlight and utilizing dissolved oxygen and also decrease the recreation value of stream [7]. Therefore degradation of the dyes in industrial wastewaters has generated considerable attention due to their huge volume of production, slow biodegradation, low decoloration and high toxicity [8-11].

      Metal oxides can adopt a vast number of structural geometries with an electronic structure that can exhibit metallic, semiconductor or insulator character and therefore play  a   very  important  role  in  many  areas  of  chemistry,physics and materials science [12-16]. In recent years, an enormous interest has been devoted to heterogeneous photocatalysis using metal oxides owing to its potential applications to both environmental applications and organic synthesis [17-21]. Many attempts have been made to study photocatalytic activity of different metal oxides such as SnO₂, ZrO₂, CdS [22,23].

      Titanium dioxide (TiO₂) and zinc oxide (ZnO) that are characterized by chemical stability, environmentally friendly, no toxicity and cheap production cost, are used in a large variety of different fields of photochemistry ranging from large-scale products to more advanced applications, for example, in environmental remediation, photo-electrolysis of water and dye-sensitized solar cells [24-27].

      Sunlight is abundantly available natural source of energy which can be conveniently exploited for the irradiation of semiconductors in the photodegradation of pollutants and can make the process economically more viable [28,29]. Dyes can be degraded in the presence of photocatalyst upon irradiation with visible light because of their absorption in the visible region.

      In the present study  the  degradation  of  the  three  dyes Rhodamine B (RB), Methylene blue (MB) and Acridine orange (AO) were investigated using solar light irradiation and the efficiencies of various metal oxide catalysts were compared (Scheme 1). With using TiO₂ and ZnO nanoparticles as a photocatalyst under solar irradiation, the effects of different operational parameters on the degradation of dyes were studied.

EXPERIMENTAL

 Materials

      Chemicals and solvents were purchased from Merck and Sigma-Aldrich and were used without further purification.

Characterization

    Transmission electron microscopy (TEM) was conducted on carbon-coated copper grids using a FEI Technai G2 F20 Field Emission scanning transmission electron microscope (STEM) at 200 kV (point-to-point resolution

Synthesis of ZnO Nanoparticles

      Nanoparticles of ZnO were prepared using a polyethylene glycol (PEG) sol-gel method. Zn(NO₃)₂.6H₂O, ethanol, and PEG (Mw = 1000) were used as starting raw materials. 1.0 g of Zn(NO₃)₂.6H₂O was dissolved in the transparent solution of PEG (1.5 g) and ethanol (20 ml) with constant stirring at 150 °C until a viscous gel was formed. Viscous gel was dried at 350 °C for 30 min. To obtain ZnO nanoparticles, the resulting dried porous precursors were ground into powder and calcined in air at 600 °C for 6 h.

Scheme 1. Structure of Rhodamine B (RB), Methylene blue (MB) and Acridine orange (AO)

Synthesis of TiO₂ Nanoparticles

      The controlled hydrolysis of titanium isopropoxide under ice-cold (0 °C) condition with vigorous stirring gives white precipitate of TiO(OH)₂. The precipitate was washed several times in distilled water and then dissolved in nitric acid to get a clear, transparent titanyl nitrate [TiO(NO₃)₂] solution [30]. Aqueous solution of 1:1 molar ratio of titanyl nitrate and urea were taken in a 250 ml beaker and introduced into a muffle furnace maintained at 673 K. Solid products were obtained within 2 h of total preparation time.

Photocatalytic Degradation of the Dye under Solar Irradiation

      Experiments were carried out under solar light. To 50 ml of dye solution (10 mg l^-1), 10 mg of catalyst was added and suspension was subjected to solar light irradiation. The aqueous suspension was magnetically stirred throughout the experiment. For a given time interval, a small quantity of the mixture solution was pipetted into a quartz cell, and its absorption spectrum was measured using an UV-Vis spectrophotometer.

RESULTS AND DISCUSSION

Catalyst Characterization

      XRD patterns were recorded for nanoparticles to determine structure and phase formation of ZnO and TiO₂ photocatalysts (Fig. 1). XRD patterns of the ZnO show peaks at the positions of 31.75°, 34.38°, 36.21°, 47.44°, 56.35°,  62.73°,  66.27°,  67.79°  and  69.10°,  which  are  in good agreement with the standard JCPDS file for ZnO (JCPDS 36-1451, a = b = 3.249 Å, c = 5.206 Å, α = β = 90° and γ = 120°) and can be indexed as the hexagonal wurtzite structure of ZnO having space group P63mc [31]. The structure of ZnO is characterized by two interconnecting sublattices of Zn²⁺ and O^2-, and each Zn ion is surrounded by a tetrahedral of O ions, and vice versa. Also XRD pattern of TiO₂ nanoparticles could be indexed to rutile phase of titania. The samples consisted of a pure ZnO and TiO₂ phases with no possible impurities.

Fig. 1. XRD pattern of a) TiO₂; b) ZnO nanoparticles.

                                                                                                               Fig. 2. IR spectra of TiO₂ and ZnO nanoparticles.

      Figure 2  shows  the  FT-IR  spectra  of  the  synthesized ZnO and TiO₂ nanoparticles. The peaks at ν = 617 cm^-1 and 435 cm^-1 are related to the stretching vibrations of Zn-O bonds [32]. Also, TiO₂ nanoparticles show broad band in the region of 400-900 cm^-1 due to unresolved stretching vibrations of Ti-O-Ti. The weak peak at 1620 cm^-1corresponds to the bending vibrations of O-H, and the broad band at around 3427 cm^-1 are attributed to the surface adsorbed water and hydroxyl groups [33].

      Particle morphology and textural properties of ZnO and TiO₂ catalysts were also studied carefully by SEM and TEM. Representative SEM and  TEM  images  recorded  for ZnO and TiO₂ materials are shown, respectively, in Figs. 3 and 4. Aggregated nanoparticles of ZnO and TiO₂ with spherical-like shapes are observed with particle diameter in the range of 80-100 nm.

Fig. 3. SEM images of a) TiO₂; b) ZnO nanoparticles.

Fig. 4. TEM images of (a) TiO₂; b) ZnO nanoparticles.

Catalytic Performance

      Photodegradation  experiments  were  carried  out  under irradiation of the dye solution under solar light (~400          W m^-2). The extent of dye degradation was monitored using UV-Vis spectroscopic techniques. The first photo-degradation of Rhodamine B (RB) by various metal oxides as a photocatalyst was investigated. In the absence of a photocatalyst, the photodegradation of RhB under solar light irradiation was very slow, and  almost  no  degradation of RhB was observed after 120 min. But When the TiO₂ catalyst was added to the RB solution under solar irradiation, the degradation of RB was complete after 35 min, showing that TiO₂ facilitated the degradation of RB by solar light (Fig. 5a). The catalytic activity of the system decreased when TiO₂ was replaced by ZnO and loss of RB took more than 120 min (Fig. 5b). TiO₂ is the most promising photocatalyst as it is chemically inert and stable with respect to photocorrosion and chemical corrosion, but ZnO is unstable due to incongruous dissolution to yield Zn(OH)₂ on the ZnO particle surfaces and thus leading to catalyst inactivation [34]. Other catalysts, MoO₃, WO₃, RuO₂, Co₃O₄, α-Fe₂O₃, Mn₂O₃ and Fe₃O₄ have negligible activity.

Fig. 5. Changes of UV-Vis spectra during the degradation of RB on a) TiO₂; b) ZnO under solar light irradiation. 50 mg  catalyst; 50 ml RB (10 mg l^-1).

 Table 1. Effect of the Amount of Catalyst on the Time for Complete RB Degradation

      As shown in Table 1, the amount of catalysts TiO₂ and ZnO could also significantly affect the rate of degradation of RB. When the amount of catalysts TiO₂ and ZnO were increased from 0.01-0.05 g, the time for complete RB degradation was reduced from 70 min to 35 min and 360 min to 120 min, respectively.

      In final, photocatalytic activity of ZnO and TiO₂ nanoparticles was evaluated by degrading two other dyes, MB and AO in aqueous suspensions under solar light irradiation at room temperature. In Fig. 6, UV-Vis spectra after different illumination times of a MB solution with TiO₂ and ZnO catalysts are shown. As can be seen from the figure, the maximum absorption peak at 663 nm gradually decreases during the  illumination  and  the  removal of  MB for two catalysts are complete after 50 min. The reaction in the absence of solar light irradiation, confirmed that adding only TiO₂ or ZnO catalyst into the solution within 60 min, little removal of MB (about 5%) occurred, indicating that the effect of adsorption on MB removal is not obvious (Figs. 6c, d).

Fig. 6. Changes of UV-Vis spectra during the degradation of  MB  on a) TiO₂;  b) ZnO under solar light irradiation; c) TiO₂; d) ZnO in the absence of solar light irradiation. 50 mg catalyst; 50 ml MB (10 mgl^-1).

      It is known that the adsorption capacity on catalyst of dye is a key factor, which the more target molecules adsorbed on catalyst, the faster or more complete these molecules decomposed. So, the adsorption experiments were carried out to study the adsorption of MB dye on TiO₂ and ZnO nanoparticles at various pH.  The  effect  of  pH on  the degradation efficiency of MB was examined in the range of 5-9. The initial pH was adjusted by addition of 1.0 M NaOH or HCl. As shown in Fig. 7, TiO₂ and ZnO are efficient catalysts over a wide pH range from 5-9, which is the range of pH found in most natural and polluted waters. The activity follows the order pH 5 > 7 > 9 and 9 > 7 > 5, respectively, for TiO₂ and ZnO nanoparticles.

      Also when TiO₂ and ZnO were used as a catalyst for degradation of AO under solar light irradiation, the degradation percentage of AO increased almost linearly with time and the AO degradation was complete after 35 min  and  50 min,  respectively  (Fig. 8).  The  percentage of removal efficiency was calculated by using (I₀ - I) × 100/I₀.

Fig. 7. Effects of pH on the degradation of MB in the presence of TiO₂ and ZnO.

Fig. 8. Percentage degradation of AO with a) TiO₂; b) ZnO under solar light irradiation. 50 mg  catalyst; 50 ml AO(10 mg l^-1).

 CONCLUSIONS

      Nanoparticles of ZnO and TiO₂ were successfully synthesized and characterized by XRD, IR, SEM, EDX and TEM. These new materials were found to be an effective catalyst for the destruction of industrial dyes Rhodamine B (RB), Methylene blue (MB) and Acridine orange (AO) under solar irradiation at room temperature.

ACKNOWLEDGEMENTS

       M. Amini thanks the Research Council of the University of Maragheh for financial support of this work.