The removal of 2 , 4-dichlorophenol under visible light irradiation by silver indium sulfide nanoparticles synthesized by microwave

Article history: Received October 28, 2012 Received in Revised form December 6, 2012 Accepted 27 February 2013 Available online 1 March 2013 Silver indium sulfide (AgInS2) nanoparticles were synthesized by microwave method. These nanopartricles were characterized by FT-IR, XRD, DRS, SEM and TEM techniques. The band gap energy of 1.96 eV was determined by UV-Vis diffuse reflection spectrum (DRS). The photocatalytic activity was studied by photodegradation reaction of 2,4-dichlorophenol (2,4DCP) under visible light irradiation. The influence of initial concentration, initial solution pH on the degradation percentage of 2,4-DCP and also, the kinetics of photodegradation were investigated. The removal efficiency up to 95% proved the superior capability of AgInS2 (AIS) nanoparticles for water purification. © 2013 Growing Science Ltd. All rights reserved.


Introduction
The chlorophenols are extensively used as fungicide, herbicide, wood preservative, dyes and drugs [1][2] .These materials have been listed as toxic pollutants by the various environmental protection agencies such as the United States Environmental Protection Agency 3 .The increase of discharging these materials from the industrial and domestic activities into the environment leads to enhance the waters pollution.Being toxic and harmful for environment and living organisms and the shortage of water sources, these pollutants should be removed.Nowadays, the variety of photocatalysts is employed for solving the serious environmental and pollution challenges [3][4][5] .In fact, the use of photocatalysis technology for destruction of the organic pollutants in water is an interesting method, which can be replaced to the traditional methods [6][7][8][9][10] .In this technique, the active species such as OH • , O 2 •-and hole (h + ) are generated by light energy and performed the degradation of organic pollutants [10][11] .Although, the semiconductors such as TiO 2 and ZnO with a wide band gap of 3.23-3.37eV are the best operational available, these are only activated under UV light irradiation, which accounts for around 5% of the total solar energy.Therefore, a decreased band gap can extend the photocatalytic application from UV light region into the visible light range [12][13] .
Many studies have been carried out to exploit the new semiconductor materials with a narrow band gap adapted to the visible light domain.AgInS 2 is a ternary chalchogenide with direct band gap about 1.86-2.03eV and high extinction coefficient in visible light region [14][15][16][17][18][19] .Thus, this compound can be introduced as a favorable material for photovoltaic and photocatalytic applications.There are several routes for preparation of AgInS 2 nanoparticles 10, 14-16, 18, 20-26 .One of these methods is microwave irradiation process.
In the present study, AgInS 2 nanoparticles with the pure orthorhombic phase were synthesized by the microwave method.The photocatalytic activity of AgInS 2 nanoparticles was investigated for degradation of 2,4-dichlorophenol (2,4-DCP) under visible light irradiation.

Structural and morphological study
Fig. 1 shows the XRD pattern of the synthesized AIS sample.All of the diffraction peaks confirm the crystalline phase of orthorhombic AgInS 2 (JCPDS No. 25-1328).In addition, no characteristic peaks of the impurities are observed in this pattern, implying that a pure orthorhombic phase of AgInS 2 has been synthesized.FT-IR spectrum shown in Fig. 2 confirmed the purity of the obtained product.There are no peaks in this spectrum, which represents the removal of organic sections via washing with ethanol and distilled water.The morphology of as-synthesized product was studied by SEM and TEM techniques.The SEM images revealed the uniform particulate morphology with the average particle size of 70 nm (Figs.3a and b).
The TEM image shown in Fig. 3c further confirmed the results of SEM technique.The average particle size was estimated using a microstructure measurement program and Minitab statistical software 27,28 .The histogram of the particles size distribution of the obtained sample has been illustrated in Fig. 3d.Where, α is the absorption coefficient, hν is the photon energy, B is a constant value and Eg is the band gap energy.In equation ( 1), the absorption coefficient (α) is estimated by the following equation: where, T is the transmittance value and t is the thickness of the substance that is exposed to the UV-Vis irradiation.Based on the UV-Vis DRS, the plot of (αhν) n vs. hν in eV was drawn.The band gap value was obtained by extrapolating the linear region of this curve.Fig. 5 shows the band gap energy plot with the UV-Vis absorption spectrum in the inset.The measured band gap was found to be 1.96 eV.This value indicates that this nanomaterial can be liable for the phtocatalytic activity in visible light domain.

Photodegradation performance of AIS nanoparticles
In order to study the phtodegradation performance of AIS nanoparticles, a series of the comparative experiments carried out in purification of polluted water with 2,4-DCP under visible light irradiation.After 6 h of light irradiation time, the remained concentration of pollutant in aqueous solution was determined.
The removal efficiency of phtocatalytic activation was concluded from Eq. 3: where, C 0 is the initial 2,4-DCP concentration and C t is the pollutant concentration at time, t.All C 0 and C t values were determined by the maximum absorption at the wavelength of 248 nm.The progress of the photocatalytic degradation of 2,4-DCP under visible light illumination was monitored by UV-Vis spectrophotometer.It was found that no degradation of 2,4-DCP was occurred in the absence of the photocatalyst under visible light irradiation.When the photocatalytic reaction was carried out in the dark, the removal efficiency of 3% is resulted.In fact, it is originated from the partial adsorption of the organic molecules on the surface of photocatalyst particles.Degradation performance was significantly improved up to 95%, when the 2,4-DCP solution containing AIS nanoparticles was exposed to the visible light illumination (Fig. 6).

Initial solution pH
The effect of solution pH on the degradation of 2,4-DCP was investigated via adjusting solution pH to three values of 3, 6 and 10 by 0.01 mol L −1 HCl and NaOH, respectively.Each of these runs was performed in the aqueous solutions of 2,4-DCP with 15 mg L -1 concentration (50 mL) 0.02 g AIS photocatalyst.Fig. 7 shows the degradation operation at different pH values.An enhanced removal percentage (95%) was observed at pH=3.This value declines to 81 and 43% at adjusted solution pH to 6 and 10, respectively.According to the mentioned results, the initial solution pH of 3 was selected for further studies.

Initial 2,4-DCP concentration
In order to study the influence of initial 2,4-DCP concentration on the removal efficiency, three initial pollutant concentrations i.e., 15, 25 and 50 mg L -1 were examined.The results have been exhibited in Fig. 8. Due to increase the 2,4-DCP concentration from 15 to 25 and 50 mg L -1 , the removal percentage of 2,4-DCP decreased from 95 to 80 and 70%, respectively.This result can be ascribed to reduce the active sites on the catalyst surface.Meanwhile, the increase of pollutant concentration can decrease the light transmittance within suspension.Both of these reasons lead to decline the amount of hydroxyl radicals (OH • ) in medium and subsequently, the reduction of removal efficiency.

Kinetic evaluation
The scanning of photocatalytic reaction indicated that this operation can be described by the simplified kinetic model of pseudo first-order equation and its formula is given as: where, C 0 is the initial concentration (mg L -1 ) and C t is the concentration at time, t, the slope, k, is the apparent rate constant.By plotting Ln (C t /C 0 ) vs. time, a linear relationship was observed for each of three experiments with different concentrations (Fig. 9).The results represented that the photocatalytic activity of AIS nanoparticles for removing the 2,4-DCP from aqueous solution follows the pseudo-first-order kinetic model.As seen in Fig. 9, the correlation coefficients (R 2 ) of pseudofirst-order model are 0.955, 0.978 and 0.919 for applied 2,4-DCP concentrations (15, 25 and 50 mg L -1 ), respectively.The all of three experiments show good fit to pseudo-first-order model, but the photocatalytic experiment with the 2,4-DCP concentration of 25 mg L -1 has a better fit than the other initial 2,4-DCP concentrations applied in this work.

Photodegradation mechanism
Generally, in a photocatalytic process, the excitation of electrons from valence band (VB) to conduction band (CB) and generation of electron-hole pairs take place on the surface of the photocatalyst by exposing to light irradiation.The exited electrons in CB and hole (h + ) in VB are captured by oxygen molecules and OH‾ or H 2 O species dissolved in the suspension, respectively.The transfer of electron to oxygen molecule generates the superoxide anion and also, the presence of h + on VB band leads to produce hydroxyl radical (OH • ).These species depredate the organic pollutant adsorbed by the catalyst surface.The attack of produced radicals to aromatic carbon with its unpaired electron of 2,4-DCP molecule leads to break the π-bond of the aromatic ring, dechlorination and the C-O bond formation.As a result, some by-products such as hydroquinone and quinoid structures maybe produced.Finally, degradation of 2,4-DCP is completed through forming of CO 2 , H 2 O and Cl - as the final products [1][2][3] , as shown in Scheme 1.

Conclusions
In summary, AgInS 2 nanoparticles were successfully prepared by a rapid and facile microwave heating technique using AgNO 3 , InCl 3 .4H 2 O and thioacetamide as the starting materials.The structural and morphological studies confirmed the formation of a pure orthorhombic phase of AgInS 2 nanoparticles with the average size of 70 nm.The conformity of the band gap energy of this product (1.96eV) with the visible region of solar energy suggested that the obtained nanoparticles can be employed as an appropriate phtocatalyst in this domain.A series of phtocatalytic reactions for degradation of 2,4-DCP as a pollutant model under visible light illumination were carried out.Meanwhile, a comparative study of the effective parameters on the photocatalytic performance was studied.As a result, the degradation process of 2,4-DCP was dependent on pH, and maximum removal efficiency (95%) was obtained at pH of 3. It was found that the phtodegradation of 2,4-DCP on the AIS nanoparticles at different concentrations follows a kinetics model of pseudo-first-order rate.The results suggested the high capability of AIS nanoparticles for water and wastewater treatment under visible light irradiation.

Characterization
The X-ray diffraction (XRD) pattern was recorded by a JEOL diffractometer using Cu Kα radiation (wavelength = 1.5418Å).Scanning electron microscopy (SEM) images were taken on a Philips XL-30E SEM with gold coating.Transmission electron microscopy (TEM) images were measured on a ZEISS EM900 microscope working at 50 kV.Fourier transform infrared (FT-IR) spectrum was recorded on a Shimadzu-8400S spectrometer in the range of 400-4000 cm -1 using KBr pellets.The surface area of the product was obtained by using Brunauer-Emmett-Teller (BET) technique with Micromeritics (Gemini) in the range of relative pressures from 0.0 to 1.0.Before employing, the sample was degassed at 200 ºC for 2 h.In addition, the pore size distribution was determined from the desorption branch of the isotherm curve using the Barrett-Joyner-Halenda (BJH) model.The diffuse reflectance spectroscopy (DRS) was obtained using Shimadzu-UV-2550-8030 spectrophotometer in the range of 190-800 nm with slit width of 5.0 nm and light source with wavelength of 360.0 nm at room temperature.

General procedure
AgInS 2 nanoparticles were prepared by microwave method 20 .In a summary, 58 mg silver nitrate (AgNO 3 ), 100 mg indium (ІII) chloride tetrahydrate (InCl 3 .4H 2 O) and 51 mg thioacetamide (C 2 H 5 NS) were dissolved in ethylene glycol, separately.After mixing the solutions, sodium dodecyl sulfate (SDS) as a surfactant was added to this solution.The resulting mixture was stirred and placed into a domestic microwave oven with the power of 900 W for 5 min.The obtained precipitation was filtered and dried at 70 ˚C for 4 h in a vacuum oven.

Photocatalytic activity measurements
In this study, the photocatalytic behavior of the prepared AIS nanoparticles was evaluated via a probe reaction on the degradation of 2,4-dichlorophenol (2,4-DCP) in aqueous medium under visible light irradiation.A series of experiments were conducted to evaluate the influence of initial solution pH (with pH values of 3, 6 and 10) and initial 2,4-DCP concentration (15, 25 and 50 mg L -1 ) on the photodegradation percentage.The fixed amount of photocatalyst (0.02 g) was charged into the reaction vessels.The solution was stirred for 1 h under dark conditions to establish the adsorptiondesorption equilibrium.At given intervals of irradiation (60 min), 3 mL of suspension was taken away from the reaction vessels, centrifuged for 20 min to remove the remained catalyst from the solution.The visible light irradiation was carried out by a High-pressure mercury-vapor lamp (500W and λ=546.8nm).The progress of photocatalytic reactions was monitored using the UV-Vis spectrophotometer at the wavelength of 315 nm.

Fig. 1 . 2 .
Fig. 1.XRD pattern of AgInS 2 nanoparticles Fig. 2. FT-IR spectrum of AgInS 2 nanoparticles Fig. 3. SEM imThe nitro area of these category of calculated fo of the nitro which was nanoparticle sites on the

Fig. 6 .
Fig. 6.Dependence of degradation of 2,4-DCP to the presence of light and catalyst

Fig. 7 .
Fig. 7.The effect of initial solution pH on the degradation percentage of 2,4-DCP

Fig. 8 .Fig. 9 .
Fig. 8. Dependence of the removal efficiency to initial 2,4-DCP concentration in various irradiation times Fig. 9.The kinetics model of photodegradation performance of the AgInS 2 nanoparticles, initial 2,4-DCP concentration of 15, 25 and 50 mg L -1 , initial solution pH of 3 with the charge of fixed amount of photocatalyst (0.02 g)

2  1 .
Scheme 1.The probable mechanism of phtocatalytic degradation of 2,4-DCP on the surface of AgInS2 nanoparticles under visible light irradiation