Methylene Blue Photocatalytic Degradation under Visible Irradiation on In2S3 Synthesized by Chemical Bath Deposition

In this work, we synthesized In2S3 powder through chemical bath deposition method (CBD) in acid medium; we used thioacetamide as sulphide source and InCl3 as indium ion source. X-ray diffraction, diffuse reflection, and Raman spectroscopy measurements were used for In2S3 powder physicochemical characterization. Optical analysis indicated that In2S3 was active in the visible region of electromagnetic spectrum; it had a band gap of 2.47 eV; the diffraction patterns and Raman spectroscopy suggested that powder had polycrystalline structure. Furthermore, we also studied the adsorption process of methylene blue (MB) on In2S3 powder; adsorption studies indicated that the Langmuir model describes experimental data. Finally, photocatalytic degradation of MB was studied under visible irradiation in aqueous solution; besides, pseudo-first-order model was used to obtain kinetic information about photocatalytic degradation; results indicated that the powder catalyst reduces 26% concentration of MB under visible irradiation.


Introduction
Nowadays, a subject of great research worldwide is associated with study of different ways to degrade pollutants (e.g., biological and chemical) present into water sources, because they have a negative impact on both quality ground (internal) and surface level of water reserves last reports indicate that more than 2 million tons of pollutants are generated daily in the planet; furthermore, about 1800 millions of people around the world are supplied by a source of water contaminated by biological or chemical pollutant [1].This is a serious environmental problem and currently is part of an objective study of different research institutes and government agencies.The World Health Organization (WHO) launched an international plan to evaluate water treatment technologies to establish work policy and to analyze new WHO criteria [2,3].
Conventional water decontamination processes contain (a) the possible self-purification (waterfalls and streams high speed), (b) desorption air, (c) ozone, (d) hydrogen peroxide, (e) potassium permanganate solutions (chemical oxidation), and (f) aerobiological methods; some of these processes achieve the mineralization of species [4,5].Precipitation of heavy metal ions is often performed by the addition of coagulants and flocculants; besides, activated carbon and other adsorbents can be used [6][7][8].However, these conventional methods do not degrade common recalcitrant organic pollutants in water, such as dyes, antibiotics, and pesticides.Effective removal from aquatic environments is necessary for water purification and conservation and maintaining human and ecological health [9].Heterogeneous photocatalysis (HP) is an option to degrade different recalcitrant pollutants, because HP is not a selective process; this methodology could act on almost any contaminant under suitable conditions [10].Conventionally, photocatalyst most studied in this field is titanium dioxide (TiO 2 ); however despite all the characteristics, TiO 2 has three main drawbacks: (a) fast recombination rate of photogenerated electron-hole pair, (b) low quantum yield in the photocatalytic reactions in aqueous solutions, and (c) high band gap value (3.2 eV, photocatalytic active only under UV-irradiation) [11].Today, it is common to modify both bulk and surface properties of TiO 2 to solve these problems; some options involve the following: (a) noble metal loading is used to reduce recombination rate of electronhole pair; (b) both ion implantation and nonmetal doping are applied to reduce band gap of the TiO 2 ; and (c) sensitization occurs by absorption of natural and/or synthetic organic dyes.These methodologies improve photoactivity of TiO 2 at visible region of electromagnetic spectrum [12].Another alternative includes change of TiO 2 for other semiconductor having lower band gap value and potential photocatalytic activity; In 2 S 3 is considered a promising material due to its chemical stability and its activity in visible range of the electromagnetic spectrum; it has three different crystalline structures and it has potential photocatalytic activity [13][14][15].It has been reported that this compound exhibits photocatalytic activity in the degradation of aqueous formic acid and methyl orange under visible light irradiation [16,17].
The aim of this study is to evaluate the photocatalytic activity of In 2 S 3 synthesized by CBD in the degradation of methylene blue as a potential alternative in photocatalysis.

Experimental
2.1.Compound Synthesis.Powder of In 2 S 3 was synthesized by CBD process from solution containing thioacetamide (Scharlau) (TA: 0.350 M) and indium chloride (InCl 3 : 0.020 M) as sources of S 2− and In 3+ , respectively; acetic acid (Es-Science) (HA) and sodium citrate (Riedel-de Haën) (Cit: 0.035 M) were used as complexing agents of In 3+ .The entire reaction can be written as follows: All reagents were mixed in pH at 2.5 and temperature at 70 ∘ C; after that substrate (soda lime glass) was immersed into reaction reactor and time reaction begins.After reaction finished, solution of reaction was filtered for using filter paper; powder-In 2 S 3 was cleaned with bidistilled water; after that, compound was dried on a hot plate at 40 ∘ C by 1 hour.After this process we obtained dry powder-In 2 S 3 .

Compound Characterization.
The optical properties of compound were studied through diffuse reflectance measurements; study was carried out in a Lambda 4 Perkin Elmer spectrophotometer equipped with an integrating sphere.Kubelka-Munk model and analysis based on differential reflection spectra were used to determine independently the energies of the fundamental optical transitions.X-ray diffraction patterns of the samples were recorded in Shimadzu 6000 diffractometer with a source of Cu-K  radiation ( = 0.15418 nm) in a range diffraction angle 2 between 20 ∘ and 80 ∘ .The Raman spectra were measured in a backscattering configuration at room temperature; the excitation radiation wavelength was 780 nm; the Raman peak analysis is based on Lorentzian-fitting performed in the range of 100-500 cm −1 .

Adsorption Study.
All reagents used in this work were of analytical grade.The MB was supplied by Sigma-Aldrich CAS Number 7220-79-3.We provided 4 different concentrations' MB on 0.025 g of In 2 S 3 .Powder was immersed in each solution and system was magnetically stirred in the dark until reaching the equilibrium of dye adsorption-desorption on In 2 S 3 surface; the concentration of MB dye was determined by spectrophotometry (HR2000CG UV-NIR ocean optics) at  = 665 nm.Content dye of the each solution was determined each five (5) minutes by spectrophotometry; prior to the concentration measurement, samples were filtered through 0.45 mm membrane; all adsorption processes were carried out at 298 K.

Photocatalytic Activity.
In photocatalytic process we used a batch photoreactor under visible irradiation 100 W OSRAM halogen immersion lamp.0.025 g of In 2 S 3 was immersed in the reactor with solution MB (10 ppm); during the photodegradation process, air was injected to reactor; the illumination time was beginning after 60 minutes of dark stirring to reach adsorption equilibrium; finally, the concentration of dye was determined by spectrophotometry at  = 665 nm.

Results and Discussions
3.1.Structural Study.Hydrodynamic synthesis conditions in CBD process (e.g., pH, ion concentration, and temperature) determine both the phase and preferential growth planes of crystalline structure of solid deposited [18].Figure 1 shows XRD diffraction pattern of powder synthesized by CBD process; preferential growth planes are shown inside figure .XRD pattern shows powder is polycrystalline; besides, main reflections can be assigned to cubic structure of In 2 S 3 (JCPDS number 65-0459); it has 4 the preferential growth planes (311), (222), (400), and (440); this result is according to others reposts; this crystalline phase has reported photocatalytic activity for water splitting under visible radiation [19].

Optical Study.
The optical properties of catalyst were determined from diffuse reflectance measurements in the range of 400-900 nm.UV-Vis diffuse reflectance spectra were analyzed with Kubelka-Munk remission function, given by the equation below [20]: Reflection (a.u.) 400 450 500 550 600 650 700 750 800 850 900 where   is the reflectance and (  ) is proportional to the absorption constant of the material, an indicative of the absorbance of the sample at a particular wavelength.For using Kubelka-Munk function (2) on the reflectance spectrum, we can obtain the graph of absorbance versus wavelength; Figure 2 shows the Kubelka-Munk function versus photon energy.The optical band gap of the films was determined by extrapolating the linear portion of ((  )ℎV) 2 versus ℎV plot on the -axis.
( (  ) ℎV) 2 =  (ℎV −   ) , where "  " is the band gap energy and "" is a constant depending on the transition probability.Figure 2 (insight) shows that the band gap of the compound was 2.47 eV.All signals of diffraction pattern correspond to In 2 S 3 ; however, this value is higher than the value reported to bulk In 2 S 3 value of 2.07 eV; in CBD process precipitation of other phases (e.g., oxides and hydroxides) had been reported; if they are present in the catalyst, they can increase the band gap value.Furthermore, band gap value indicates that catalyst can be active in the visible region of the electromagnetic spectrum; if semiconductor absorbs visible radiation it could be useful for degrading pollutant for using direct solar radiation [21,22].

Raman Spectroscopy Study.
Figure 3 shows modes obtained by Raman spectra for catalyst powder; definition and intensity of the signal in Raman spectra revealed high crystalline quality of catalyst; this result is according to XRD pattern diffraction; Raman spectra show five normal modes of vibration at 155 cm −1 , 186 cm −1 , 218 cm −1 , 243 cm −1 , and 305 cm −1 ; these signals can be assigned to modes of vibrations of In 2 S 3 phase; signals located at 186 cm −1 , 243 cm −1 , and 305 cm −1 can be assigned to A 1g vibration modes of In 2 S 3 phase; this result is according to other reports [23].to beginning of photodegradation step is a process poorly studied; it is well known that the adsorption of contaminant species on the surface of the photocatalyst is an important step prior to degradation process.In this study, Langmuir and Freundlich isotherm models were employed to describe the MB adsorption equilibrium.In Langmuir isotherm model, the adsorption process is taking place isothermally and adsorbent surface can be covered uniformly with a monolayer of adsorbate.The linear form of the Langmuir equation is where   is the equilibrium concentration of dye;  is the amount of dye adsorbed per gram of catalyst;   represents the maximum amount of dye that can be adsorbed; and  is related to the adsorption rate.The linear form of the Freundlich equation is where  is an indicator of adsorption capacity, as the higher the maximum capacity is, the higher the  value is; furthermore, ratio (1/) is a measure of intensity adsorption.The  and  values are empirical constants and they are specific to the system adsorbent/adsorbate [24].
Figure 4 shows the experimental results of adsorption equilibrium of different loads of MB on 0.25 g of In 2 S 3 powder.
Figure 4 shows adsorption kinetics of MB over In 2 S 3 ; results show that system reaches adsorption equilibrium point after 1 hour of dark agitation; from experimental results of Figure 4 and applying fitting of (4) and ( 5) we obtain adsorption isotherms for two theoretical models.Table 1 lists the fitting results and Figure 5 shows the experimental results of adsorption equilibrium of MB on In 2 S 3 and theoretical fitting to Langmuir model.The fitting results indicated that the Langmuir isotherm describes the experimental results MB adsorption equilibrium.photomineralization rate of methylene blue followed the Langmuir-Hinshelwood (L-H) kinetics model [25]: where V is the rate of dye mineralization,  is the rate constant, [MB] is the methylene blue concentration, and  is the adsorption coefficient.Equation ( 6) can be solved explicitly for () for using discrete changes in [MB] from the initial concentration to a zero reference point.In our case, an apparent first-order model can be assumed: Integration result in (( 6) and ( 7)) is as follows: where () is time in minutes and   is the apparent reaction rate constant (min −1 ); the assumption of a pseudo-first-order model was used in several studies to characterize the effect of different experimental conditions on the degradation rate [26].
Figure 6 shows change of methylene blue removal yield both in dark stirring and under visible light illumination, inside; we show pseudo-first-order model fitting.Figure 6 shows after 60 minutes a plateau zone is reached, under dark stirring; after that under visible irradiation, system reaches 86% of MB concentration reduction; this 26% could be assigned to photocatalytic process.These results indicate that In 2 S 3 has photocatalytic properties under visible irradiation.The   value for In 2 S 3 was established from the slope of the linear fitting of ln(MB/MB  ) versus time.We obtained a   value of 2.4 × 10 −2 min −1 .This is a significant result due to possibility of using visible radiation directly under catalysts without physical or chemical modification as TiO 2 required to be photocatalytically active under visible irradiation.

Conclusions
We synthesized In 2 S 3 by CBD process.Structural characterization indicated that catalyst powder was polycrystalline; besides, optical analysis indicated that compound was active in visible region of electromagnetic spectrum; it had a band gap of 2.47 eV.Furthermore, Langmuir adsorption model described equilibrium adsorption of MB on In 2 S 3 .Finally, In 2 S 3 had photocatalytic activity on MB degradation; results indicated that MB load reduced 26% after visible illumination.

3. 5 .
Photocatalytic Test.Figure6shows change of molar fraction () of MB under both dark and visible irradiation.

Figure 6 :
Figure 6: MB removal yield in dark stirring and under visible light irradiation.

Table 1 :
Linear fitting experimental results.