Keywords

Photo catalytic oxidation; Tio₂; Rodamin B; Ionic species

Introduction

Rhodamine dyes (RB) are important class of xanthene dyes. They have high quantum yield of fluorescence and high absorption factors. They have been widely studied due to their properties and practical applications in dye lasers [1]. Furthermore, RB dyes have been used for some specific applications, such as biological stains, electrochemical luminescence sensitizer, water- tracing agents, molecular probes, chromo-ionophores in optical chemical sensors and solar collectors and many others devices so treatment extra dye in wastewater is very important [2]. Clearly, wastewater is full of different material with different range of the pH values. Photo-degradation efficiency of dyes is affected by pH of the solution in the presence of Tio₂.

Heterogeneous photo-catalysis is a processing which the irradiation of an oxide semi-conductor produces photo-excited electrons (e⁻) and positive charged holes (h⁺) to the form hydroxyl radical and superoxide radical anion, that both of them play as the oxidizing species in the photo-catalytic oxidation method. The efficiency of photo-excitation of the semi-conductor is dependent on the energy state of the electrons valence band and conduction band. The extent of dye adsorption depends on several factors such as, dye nature of the dye, surface area of the used photo-catalyst, concentration and pH of the solution. The pH controls the surface charge of the photo-catalyst and dye. Adsorption of the dye is minimum when the pH of the solution is at the isoelectric point (point of zero charge). The surface of the photo-catalyst is positively charged below isoelectric point and carries a negative charge above that. As a result, the structure of dye is important to be found in due course, i.e., the efficiency of adsorption on the surface of a photo-catalyst can be low or high in acidic and basic media [3].

In Tio₂/UV system positive charged holes can be scavenged by the negatively charged anions and therefore depends on the positive charged or negatively charged adsorption of dyes on the surface of Tio₂ is considerably inhibited or promoted [4]. Previous studies indicated that inorganic anions can scavenge •OH to form the corresponding anion radicals [5-7]. The anionradicals may themselves oxidize organic and inorganic compounds [5,8,9], which can also influence on the total rates of the photo- catalytic oxidation. It is also reported that competitive adsorptionof the inorganic anions for active sites on the Tio₂ surface may contribute to the photo-catalytic degradation of organic compounds [10-13]. For instance, Cl− decreases the degradationrate of 2-chlorophenol and 2-nitrophenol at pH values lower thanthe Tio₂ point of zero charge (pHpzc), while Cl− had no inhibitory role at pH values greater than the pHpzc due to slight adsorptionto the negatively charged Tio₂ surface [12]. Inorganic anions canalso change the rates of dye photo-catalytic oxidation in one of the following ways: (i)•OH scavenging by inorganic anions, (ii)direct oxidation of dye by anion radicals, or (iii) adsorption ofinorganic anions to the Tio₂ surface [14,15].

The photo-catalytic activity of Tio₂ for the oxidative degradation of (RB) may be enhanced by first, surface modification using metals, second, transition metal doping and third, coupled photo-catalysts. In this research Ag metals were used for increasing in photo-degradation of (RB) dyes. Improving the photocatalytic activity by Ag deposition may be associated to different mechanisms:

(1) Ag nanoparticles deposited on Tio₂ act as electron traps, enhancing the electron-hole separation and the later transfer of the trapped electron to the adsorbed O₂ acting as an electron acceptor [1-3].

(2)self-photosensitization pathway, that means,extendthe lightabsorption into the visible range and enhancesurface electronexcitation by plasmon resonances excitedby visible light, injectingan electron into the conduction band of the Tio₂ semiconductor,so, the injected electron on the Tio₂ particle reacts with adsorbedoxidants to produce reactive oxygen radicals and dye is degradedby these oxygen radicals. Under visible light irradiation, thesemiconductor Tio₂ acts only as an electron-transfer mediatorand.

(3)modify the surface properties of photocatalyst [4-8].Zakerhamidi and co-author studied solvent effects on thedipolemoment and photophysical properties of rhodamine dyes.This study describes the effective parameters such as pH of thesolution, type of solvent and cationic and anionic species on thephoto-bleaching of (RB) in the presence of Tio₂ nanoparticles[16,17]. The amount of degradation of RB were discussed indifferent circumstances.In this experience it was investigated theeffects of pH and common salts and solvent on photo degradationof dye Rhodamine (RB).

Materials and Methods

Titanium dioxide nanoparticles (purity>99.5 %,) with average particle size 30 nm and specific surface of 50 m²/g was purchased from Degussa Corporation (USA).

Compounds of CuCl₂. 2H₂O, Cu(NO₃)₂. 3H₂O, Al(NO₃)₃, Co(NO₃)₂, Fe(NO₃)₃ .9H₂O, CoCl₂ .6H₂O and FeCl₂ were obtained from Fluka (Buchs, Switzerland) and used without further purification. The pH (2,6 and 12) values of the solutions were adjusted using HCl (0.01 M) and KOH (0.01 M) solutions. RB dye (10^-5 M) was prepared from Fluka (Buchs, Switzerland). The photo-catalyst was added to dye solution and the suspension was irradiated under UV-Vis light. Then fluorescence spectra of the dye solutions were recorded and de-colorization process was monitored in terms of change in intensity at λmax excitation (550 nm) and λmax emission (580 nm) of the dye after 0, 2, 5, 8, 10 minute irradiation.

Ag-Tio₂ was synthesized by photo-reduction of AgNO₃. In this way, Ag⁺ ions are deposited as Ag metal on the Tio₂ surface by photo-reduction of AgNO₃ under UV irradiation [8]. One gram of Tio₂ was added into 250 mL of double-distilled water and then irradiated with UV-Vis light for 10 min to remove any impurities present on Tio₂ surface. Then approperiate amount of AgNO₃ was added into the suspension of Tio₂ solution and the suspension adjusted to pH 3.5, finially it was irradiated using UV light for 40 min with continuous stirring under N₂ atmosphere. The suspension was then filtered,washed,dried and then ground into a fine powder [9,10].

General procedure

The photo-catalytic reaction was carried out in an inner irradiation type reactor. The photo-catalyst powder was dispersed by magnetic stirrer. A 300 W high-pressure mercury lamp from Oriel GMBH, Model 8500 (Cheltenham, England) was used as the light source and spectro-fluorimeter of Perkin-Elmer LS-3B Luminescence (USA) was used. The cell was made with Quartz (3 mL content) that it was put it in the close space in front of the light in reactor. Analysis of the crystalline structures was performed by XRD Diffracto-meter (GBC MMA made in Australia) with wavelength of Cu Ka radiation in 2Ө range from 15° to 80° , SEM analysis was performed by a VEGA TESCAN (USA) instrument.

Results

The effect of pH

To study the effect of pH on photo-degradation, several experiments were done using 1.0 × 10^-5 M dye concentration and it was exposed under UV-Vis light in the presence of nanoparticles of Tio₂. Figure 1 shows increasing of % degradation of RB in acidic (pH=2), normal (ca. pH=6) and basic medium (ca. pH=12) with increasing of anionic species, after ten minute UV-Vis irradiation in every experiments and in present of 0.1 g/L titanium dioxide. As can be seen acidic, normal and basic medium is significantly depended on the concentration of anions either NO3- or Cl-. The behavior of dye degradation in acidic, normal and basic medium are nearly the same so that in higher concentration of anions the percentage of dye degradation as function of anion concentration is smooth (Figure 1).

Figure 1: Effect of anions on the %degradation of RB under UV-Vis irradiation for 5 min at the present of 0.1 g/L titanium dioxide, in acidic (■), normal (⧫) and basic (5) medium (a) Cl⁻ and (b) NO⁻.

Effect of cations on the titanium dioxide-based photo-catalytic oxidation of aqueous R

Wastewater is full of material, effect anion and cation for increase adsorption dye on catalyst was investigated in different pH, however the kind of cation is important. Salts of NO3^- and Cl^- are very common. In a quartz cell, 3 mL of the solution containing 0.01 M of (a) NO3^- and (b) Cl^- are exposed by UV-V irradiation for five minutes in the presence of 0.1 g/L titanium dioxide. Experiments were performed using 50 mL of solution with constant initial concentration of dye RB (10^-5 M) and in the present of salts nitrate and chloride. Figure 2 shows the effect of cations on % degradation of RB in normal medium with increasing of anionic species. The counter ions are the same, i.e., (a) NO₃^- and (b) Cl^- for the cations of Al³⁺, Cu²⁺ and Co²⁺ and Fe³⁺. Figure 2 illustrates the influence of the cations on the % degradation of RB. Results reveal that % degradation of RB is highly enhance in the presence of Fe²⁺ and in contrast the degradation is very low in the presence of Cu²⁺. The other cations (Al³⁺, Co²⁺ ) are also have differences effects, in this case both cations of Al³⁺, Co²⁺ have almost similar effect on the % degradation of RB so that % degradation almost have the nearly the same values. The order of % degradation for the cations are as follows Fe³⁺ > Co²⁺ > Al³⁺ >Cu²⁺ (Figure 2).

Figure 2: Effect of caions on the % degradation of RB under UV-Vis irradiation for 5 min at the present of 0.1 g/L titanium dioxide. (a) NO3⁻ and (b) Cl⁻.

Effect of solvent

The percentage of dye degradation was also examined using different of solvents i.e., water, methanol and ethanol. Figure 3 illustrate the effect of dye degradation in aqueous (water) and nonaqueous (alcoholic medium). Results indicated that rate of % degradation in water is the highest amount (Figure 3).

Figure 3: Effect of solvents on the %degradation of RB under UV-Vis irradiation for 5 min at the present of 0.1 g/L titanium dioxide, in ethanol (■), water (⧫) and methanol (5) medium.

Characterization of Tio₂ and Ag-Tio₂ nanoparticles

In order to investigate the changes in the crystal structure due to silver deposit, XRD measurements were taken in the range of 2Ө 15-80 for pure Tio₂ and Ag-Tio₂ nanoparticles (Figure 1). It is seen that the XRD patterns of Ag-Tio₂ sample are almost the same as for pure Tio₂, except for the intensities of the peaks. Also, theXRD patterns show no diffraction peaks due to silver deposition.This may be due to the fact that deposition alters the crystallinitybut not the crystal structure of Ag-Tio₂ and thus suggest that thesilver deposits are merely placed on the surface of the crystals.Diffractions that are attributable to anatase phase of Tio₂ crystals Tio₂samples. The signal intensity of unloaded sample is very strong and shows evident characteristics of the radiative recombinations of electrons and holes. The PL peak at 367 nm arises from the band-to-band recombination which evidently decreases with the loaded Ag particles. Two factors affect the PL intensity after Ag deposition. One is the migration of the electrons from Tio₂ to Ag particles; the other is the Ag plasmon absorption. The PL band from 390 to 440 nm is from the recombination transition related to the surface structure, which also decreases with the loaded Ag, indicating the photo- deposited Ag has affected the surface structure of Tio₂, leading to the decrease of surface recombination but if silver amount became above the optimum loading, silver particles can also act as recombination centers so The signal intensity of loaded sample is very strong [11,12] (Figure 5).

Figure 4: XRD patterns for TiO2 nanotubes and AgTiO₂.

Figure 5: The PL spectra of unloaded and loaded TiO₂ samples.

Figure 6: SEM images of AgTiO₂ composites.

Figure 7: The results of elemental analysis from the EDX spectra of AgTiO₂ with (a) 23, and (b) 45 at% weight of Ag composites.

Figure 2 shows SEM images of the AgTio₂ composites. The general morphology of the AgTio₂ can be clearly observed in these micrographs. Tio₂ particles were uniformly distributed. It can be considered that the better dispersion provides a larger number of active catalytic centers for the photocatalytic reaction (Figure 6).

Figure 4 shows the results of elemental analysis from the EDX spectra of AgTio₂ with 23, 45 at % weight of Ag composites. These spectra show the presence of peaks from Ag and Ti. The elemental composition analysis of the composite series is listed in Tables. Ti was the major elements in the composite series. Figure 7a is 23 at % weight of Ag composites and Figure 7b is 45 at % weight of Ag composites. As expected, it was observed that the Ag content of the AgTio₂ composites showed an increase with increasing the at % weight of Ag (Figure 7).

Photo-catalytic degradation of RB

Figure 5 shows the change in the intensity of fluoresans spectra of RB (1.0 × 10⁵ M) in the presence of the Tio₂ and Ag-Tio₂nanoparticles (100 mg/L) as photo-catalyst. After irradiation with UV-Vis light, the intensity of spectra decreased due to decolouration of the dye. Figure 5 indicated that speed of de- colouration, after 5 min irradiation in present of AgTio₂ was faster than in present of Tio₂ (Figure 8).

Figure 8: Shows the change in the intensity of fluoresans spectra of RB (1.0 × 10⁵ M) in the presence of the TiO₂ and Ag-TiO₂ nanoparticles (100 mg/L) as photo-catalyst.

Effect of catalyst amount

The effect of Ag-Tio₂ amount on the photo-catalytic degradation of RB was studied at fixed concentration of RB M in aqueous solutions. Figure 6 shows the photo-degradation efficiency of RB (% E) as a function of the amount of at % weight of Ag composites. An optimum silver ion loading of 23 at % weight of Ag was found. For all silver ion loadings used, Ag/Tio₂ performed significantly better than bare Tio₂ .Above the optimum silver ion loading, the activity of Ag/Tio₂ particles decreased. There are several reasons for the existence of an optimum metal loading. It is also possible that above the optimum loading, silver particles can also act as recombination centres [5,6,13] (Figure 9).

Figure 9: Shows the photo-degradation efficiency of RB (% E) as a function of the amount of at % weight of Ag composites.

Effect of electron acceptors in dye photo- degradation

The use of inorganic oxidants, such as S2O8^-2 in Tio₂ system increased the quantum efficiencies either by inhibiting electron- hole pair recombination through scavenging conduction band electron at the surface of Tio₂ or by offering additional oxygen atom as an electron acceptor to from the superoxide radical ion and along reaction electron acceptor with AgTio₂ due production of active radicals. Figure 7 gives the effect of electron acceptors in photo-catalytic activity of AgTio₂. Dye exposed under UV-Vis for 2 min, (a) in present only AgTio₂=0.12 g/lit, (b) In present Tio₂=0.1 g/lit and in present of S2O8^-2=1 × 10-3 M [10] (Figure 10).

Figure 10: The effect of electron acceptors in photo-catalytic activity of AgTiO₂. Dye exposed under UV - Vis for 2 min, (a) in present only AgTiO₂ = 0.12 g/lit, (b) In present TiO₂ = 0.1 g/lit and in present of S2O8^-2 = 1 × 10^-3 M.

Discussion

The effect of pH

The pH of the solution is an important parameter which strongly influences the surface charge properties in aqueous dispersion. Generally, surface of the net Tio₂ is positively charged in acid media and negatively charged in alkaline media with an isoelectric point of around pH 6~7 [18]. Therefore, adsorbates in the aqueous solution tend to adsorb on the surface of Tio₂by the negatively charged or electron abundant group in acidic solution and by positive charged in alkaline solution because of the electrostatic interaction [19]. The ζ-potential of Tio₂ with different pH amounts was investigated before. Photo-degradation efficiency of dyes is affected by pH of the solution in the presence of Tio₂. The amount of pH solution changes the surface charge of Tio₂ particle [3]. As a result, the adsorption of dye on the surface is altered thus causing a change in the reaction rate. Under acidic or alkaline condition the surface of Tio₂ can be protonated or deprotonated (Eq. 1 and 2) respectively according to the following reactions, therefore, the pH effect is often dependent on the nature of the dye. To study the effect of pH on photo- degradation, several experiments were done using 1.0 × 10^-5 M dye concentration and it was exposed under UV-Vis light in the presence of nanoparticles of Tio₂ [20]. Figure 1 shows increasing of % degradation of RB in acidic (pH=2), normal (ca. pH=6) and basic medium(ca. pH=12) with increasing of anionic species, after ten minute UV-Vis irradiation in every experiments and in present of 0.1 g/lit Titanium dioxide. Figure 1 shows the effect of anions (i.e., NO3^- and Cl^-) on the % degradation of RB under UV-Vis irradiation (for 5 min) at the present of 0.1 g/L titanium dioxide in acidic, normal and basic medium. As can be seen acidic, normal and basic medium is significantly depended on the concentration of anions either NO3^- or Cl^-). The behavior of dye degradation in acidic, normal and basic medium are nearly the same so that in higher concentration of anions the percentage of dye degradation as function of anion concentration is smooth. It is pertinent to point out that in both normal and basic medium the concentration of anion at concentration of about 25 mg/L has lowest value of degradation either in the presence of NO3- or Cl-. Results also revealed that the % degradation of RB is hardly depends on the concentration of anion concentration in acidic media. As a result the % degradation of RB is depended on medium, i.e., acid, neutral and basic. This evidence can be attributed to the surface of nanoparticles as well as the chemical structure of the dye in question.

TiOH + H+→ TiOH₂+ (1)

TiOH +OH-→ TiO- + H₂O (2)

Acidic medium

In acidic medium, the rationale behind this is that at low pH values, more H⁺ are available for adsorption to mask the surface of the catalyst thus inhibiting the photo-excitation of semiconductor particles, [3]. In addition, adsorption of dye on the surface positively charged of Tio₂ is not favorite, results indicated that adsorption of RB on Tio₂ is significantly increased in the presence of the anionic species in acidic medium and hence increasing RB % degradation. Figure 11 shows theoretically suggestions for increasing adsorption of RB on Tio₂ in the presence of the anion species (Cl^- or NO3^-) in acidic solution with 10^-2 molar of HCl with different concentrations of salt (Figure 11).

Figure 11: Theoretically suggestions for increasing adsorption of RB on TiO₂in the presence of the anion species (Cl^- or NO3^-) in acidic solution, a no salt, b has moderate concentration and c with higher concentration than b.

Neutral medium

In neutral solution, the pH value of the reaction solutions under the experiment conditions was controlled at about 6, in which the surface of Tio₂ was positively charged. The acid dissociation constant of the carboxyl group of RB is about 4.1 ± 0.1. Thus, under the experimental conditions, most of the carboxyl group of RB was dissociated and negatively charged at −COO− state and RB tends to adsorb on the surface positively charged of Tio₂ by the negatively charged carboxyl group of RB [19]. So rate of photo degradation of dye (RB) without salt in neutral medium is the highest. Therefore, RB Dyeis zwitter-ionic form when the solution pH is higher than the acid dissociation constant of RB [21]. Adsorption of RB molecules through the positively charged significantly increased in the presence of the anionic species in normal medium. Figure 12 shows theoretically suggest ions for increasing adsorption of RB on Tio₂ in the presence of the anionic species (Cl^- or NO3-) in normal medium (Figure 12).

Figure 12: Theoretically suggestions for increasing adsorption of RB on TiO₂ in the presence of the anionic species (Cl⁻ or NO3⁻) in normal medium, a no salt, b has moderate concentration and c with higher concentration than b.

Basic medium

In basic solution, RB is zwitter-ionic and the surface of Tio₂ is negatively charged. As a result, adsorption of RB molecules through the positively charged is chosen on the surface negatively charged of Tio₂. Besides H₂O₂ molecules that are formed during photo-degradation process could easily self-decomposed in basic medium. The species of •OOH could also react with both •OH radical as well as H₂O₂ in the basic medium. Thus, it woulddecrease the concentration of •OH radicals and affect degradationefficiency of RB. The degradation rate was found to increasewith oxidation potential of ·OH radical in the acidic medium asreported by Wang additionally in the alkaline conditions there isa competitive adsorption between hydroxyl groups and the dyemolecule [3,22,23].

It is pertinent to point out that the adsorption of RB molecules through the negatively charged carboxyl group on the surface negatively charged of Tio₂ was also significantly increased in the presence of the cationic species in basic medium. Results indicated that positively copper complex, leads to increase adsorption of RB on Tio₂ and hence increasing color removal from the solution. Figure 13 shows theoretically suggest ions for increasing adsorption of RB on Tio₂ in the presence of the cation species Cu⁺² inbasic medium using 10^-2 M of KOH with different concentrations of salt (Figure 13).

Figure 13: Theoretically suggestions for increasing adsorption of RB on TiO₂ in the presence of the cation species Cu⁺² in basic medium, a no salt, b has moderate concentration and c with higher concentration than b.

Effect of cations on the titanium dioxide-based photo-catalytic oxidation of aqueous RB

It can be justified that suppression of OH• radicals takes place due to trapping of the conduction band electrons by the adsorbed metal ions and decreasing color removal might be expected [24]. Results indicated that significant decreasing in the initial degradation rate of photo-catalysis, is occurred in the presence of cations, among them Cu⁺² has the largest effect (Figure 2). It can deduce that the retardation is due to Cu⁺² ions, which it scavenges the electrons required for the formation of hydroxyl radicals. Results specified the Fe ions have an important role (increasing of % degradation) in the photo-degradation process which also has been reported by many researchers [25-27]. Fe+3 ions can accelerate the decomposition of many organic compounds in water using hydroxyl radicals formed in the following process [28].

Fe⁺³+OH-+h+→Fe⁺²+OH (7)

This effect could be attributed to the scavenging of OH radicals by solvents or solubility of electrons, which are vital for generating OH radicals. If latter is the reason, one can expect higher degradation in more polar solvents than in less polar solvents. The dielectric constants of methanol and ethanol are 32.6 and 24.3, respectively. Therefore the possibility of generation OH radical in protic solvent like methanol is much higher compared to that of ethanol and consequently increases the rate of the degradation [29].

This is not surprising since excited electrons are less solvated in organic solvents than in aqueous solution. It may be that as the organic portion of the reaction mixture is increased, free electrons becomes less easily solvated, and the possibility of recombination with the positive hole (h⁺) increases. Another possibility is that the acetonitrile (and the other solvents examined) scavenge surface holes, which might be expected to retard photo-oxidation [30,31].

As a result, solvent also can play a significant role in photo- degradation process. These effects are closely related to the nature and degree of the solute dipole moment changes in the process of solute solvent interactions [32]. In organic solvents, free electrons are less solvated and hence recombination of electron with hole is increased. Dye (RB) is zwitterionic form in neutral medium. Results specified that the degradation of dye in methanol is less than the others, this probably due to more solvation of dye in methanol. Since solvation can inhibit adsorption of dye on the surface of nano particles since solvation of dye in ethanol is less and hence degradation of dye in ethanol is more than methanol. Figure 3 presents the effect of solvents (Water, Ethanol and methanol) on the % degradation of RB by use Tio₂ and in present of 0.1 g/lit Titanium dioxide.

In previous studies, the effect of parameters such as pH, adsorbent amount and initial dye concentration and the adsorption of (RB) dye were carried out using sodium montmorillonite [16]. Results obtained from the present work specified that using Tio₂nanoparticles, the photodegridation of RB is significantly.

Conclusion

Photo-degradation of (RB) using nanoparticle Tio₂ under UV light was investigated. The basis of reaction is photoredox process. Different parameters have important role in this process. Results showed that pH of the solution, type of solvent and the type of ions species can influence on dye degradation. The amount of pH solution changed the surface charge of Tio₂ particle. As a result, the adsorption of dye on the surface is altered thus causing a change in the reaction rate. In neutral medium, the adsorption of dye on the surface positively charged of Tio₂ is favourite, therefore % degradation of (RB) in this medium was the most amount. Adsorption of (RB) molecules on the surface of Tio₂ significantly changed in the presence of more amounts of the ionics in the normal, acidic and basic solutions so was influenced on the photo-bleaching of (RB) dye. Plus, in the normal solutions and in presence of the same amount of different salts, kind of salt was very important. Results indicated that significant decreasing in the initial degradation rate of photo-catalysis, is occurred in the presence of various cations, among them Cu⁺² has largest effect but Fe⁺² increased photo degradation of (RB). Results indicated that, solvent also can play a significant role in variation of photo- physical properties in solutions, and results indicated that rate of % degradation in water is the highest amount between ethanol and methanol and water.

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