Highly efficient photocatalytic reduction of Cr(VI) over hierarchical–like In2S3 hollow microspheres

*Rengaraj Selvaraj1), Salma M. Z. Al-Kindy1), Tharaya Al Fahdi1),Kholood Al

Nofli1),Iman Al Maadi1), Younghun Kim2)

1)Department of Chemistry, College of Science, Sultan Qaboos University, P.C. 123, Al-Khoudh, Muscat, Sultanate of Oman

2)Department of Chemical, Engineering, Kwangwoon University, Seoul 139-701, Korea

1)E-mail: srengaraj1971@yahoo.com; rengaraj@squ.edu.om

ABSTRACT

Hexavalent chromium (Cr(VI)) is a common heavy metal pollutant in the wastewaters, which mainly comes from electroplating, pigments, mining and chromate manufacturing industries. Cr(VI) is mobile and highly toxic, whereas Cr(III) is less toxic and can be readily precipitated with alkaline or neutral solutions. Cr(VI) has attracted considerable attention from society and regulation authorities around the world because of its high acute toxicity and strong carcinogenic activity to humans. The world health organization (WHO) has regulated that the concentration of Cr(VI) should be below 0.05 mg/L in drinking water. Hence, it is of great importance to explore how to remove the Cr(VI) in water effectively. Recently, the photocatalytic reduction of Cr(VI) using the semiconductor photocatalysis technology has received considerable attention. The development of high performance photocatalysts is indispensable for the application of the photocatalytic process to large scale Cr(VI) wastewater treatment. Unfortunately, most of the semiconductors have a major drawback stemming from its large band gap, which means it can absorb mainly UV light. For overcoming this problem, intensive research efforts have been recently focused on the development of visible light active photocatalysts. In this study photocatalytic reduction of Cr(VI) was investigated by using a indium sulphide (In2S3) hollow microsphere under visible light in aqueous solution. In this work we demonstrate that the one step solvothermal synthesis of three-dimensional (3D) hierarchical like In2S3 hollow microspheres, which are composed of two-dimensional (2D) nanosheets. The synthesized products have been characterized by a variety of methods, including X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) analysis, and ultraviolet visible diffused reflectance spectroscopy (UV-vis DRS). The optical properties of In2S3 were also investigated by UV-vis DRS, which indicated that our In2S3 microsphere samples possess a band gap of 2.0 eV. Furthermore, the photocatalytic activity studies revealed that the synthesized In2S3 hollow microspheres exhibit an excellent photocatalytic performance in rapidly reducing aqueous Cr(VI) to Cr(III) under visible light irradiation. These results suggest that In2S3 hollow microspheres will be an interesting candidate for photocatalytic detoxification studies under visible light radiation.

1. INTRODUCTION Chromium is one of the most abundant heavy metals, causing pollutionof

groundwaters and soil due to its frequent industrial application. Chromium occurs naturally mainly in the trivalent Cr(III) and hexavalent Cr(VI) forms. The majority of its adverse effects is caused by Cr(VI) because of its solubility, mobility and high oxidizing potential leading to generally higher toxicity causing health problems such as liver damage, pulmonary congestion, vomiting and severe diarrhea(Nriagu and Nieboer, 1988). On the other hand, Cr(III) is less reactive and toxic and can be readily precipitated out of solution. Therefore, the majority of in situ treatment methods employed at the present time utilize geofixation of Cr(VI) by its reduction to Cr(III) and formation of insoluble Cr(III) compounds (Jardineet al., 1999). A number of articles have been published to date describing various applications of individual biological or chemical approaches to precipitate chromium into its insoluble Cr(III) form.

The methods employed for the removal of Cr(VI) includechemical precipitation,

reverse osmosis, ion exchange, foam flotation, electrolysis, photocatalytic reduction, adsorption, etc. However, most of these methods require either high energy or large quantities of chemicals, and in this respect the photocatalytic process is found to be superior to other conventional treatment techniques.

An alternative clean route of treating Cr(VI) that has receivedconsiderable

attention is photocatalytic reduction in the presence of a semiconductor material such as ZnO, TiO2, CdS or WO3 under visible or UV radiation (Rengarajet al., 2007; Sun et al., 2005; Wang et al., 2004; Schranket al., 2002; Kabraet al., 2004). The photocatalytic method is based on the reactive properties of an electron–hole pair generated in the semiconductor when irradiated by UV/vis light having energy greater than the band-gap energy of the semiconductor. This method is widely used for treatment of the drinking water and industrial wastewater by oxidizing the organic pollutants. The reducing capacity of the semiconductor photocatalyst which uses the electrons generated on the semiconductor surface is, however, less explored. It is more attractive compared to the conventional processes since it works at near-neutral pH thereby requiring less alkali during coagulation–precipitation. This method, if conducted with solar radiation as the source of activation energy, can be more economical and eco-friendly (Mohapatraet al., 2005). Among the photocatalysts, WO3 is less available, cadmium itself is a toxic heavy metal and CdS is prone to easy deactivation and photocorrosion (Reuterglrdh and Langphasuk., 1997).

Application of the photocatalytic technique for remediation of inorganic and

organic pollutants in wastewater has been reviewed by Kabraet al. (2004). Cieslaet al. (2004)have made an exhaustive survey of the work on environmental photocatalysis by transition metal complexes. Photo-reduction of a metal ion is accelerated if it is accompanied by simultaneous oxidation of an organic molecule that plays the role of a ligand or a ‘sacrificial electron donor’. Concurrent generation of one or more of the species O2•, HO2•, OH•, H2O2 and HO2−during the photo reduction process has been reported. This occurs because of the fact that the medium (water) acts as the electron

donor simultaneously with the sacrificial organic species. The phenomenon has been called ‘ligand to metal charge transfer’(LMCT). Reduction of Cr(VI) to Cr(III) leads to drastic decrease in bioavailability and toxicity of this element. The photocatalytic reduction of Cr(VI) with semiconductor such as CdS, ZnS, WO3, ZnOand TiO2 in UV light has been reported in literature (Wang et al., 1992; Domenech and Munoz., 1987; Yoneyama., 1979).

So in the present work, we have studied the activity of In2S3nanoflowers towards

the photocatalytic reduction of hexavalent chromium in aqueous suspension using HCOOH as a hole scavenger in the presence of visible light. 2. EXPERIMENTAL 2.1 Materials Analytical grade indium nitrate (In(NO3)3) (Sigma-Aldrich, 99%) and thiosemicarbazide (TSC, NH2NHCSNH2) (Sigma-Aldrich 99%) were used as Indium and sufur source for the preparation of In2S3 hollow microsphere photocatalyst. Potassium dichromate was obtained from the Aldrich Chemical Company, USA, and Used without further purification as the Cr(VI) source. Deionized , doubly distilled water was used for the preparation of all solutions. 2.2 Preparation of In2S3 hollow microsphere

The hierarchical-like In2S3 hollow microsphere were synthesized using analytical

grade indium nitrate In(NO3)3 (Sigma-Aldrich 99%) and thiosemicarbazide (CSN3H5) (Sigma-Aldrich 99%) without further purification. In a typical synthesis, 3.324 mmol of In(NO3)3 and 6.648 – 13.297 mmol (1: 2 - 4 molar ratio) of thiosemicarbazide (TSC) were dissolved in 70 mL of ethanol/water (v/v = 1:1) and continuouly stirred for 30 minutes to form a clear solution. Here TSC serve both as a sulfur source and as capping ligand. The solution was then transferred into an autoclave and maintained at 180oC for 10 h to 24h and then cooled to room temperature naturally. The orange colour precipitate was harvested by centrifugation and washed several times using deionized water and ethanol to remove the possible remaining cations and anions, and then dried in an airoven at 70oC for 24 h for further characterisation purpose. It is noted that the post treatment of the products after the reaction were carried out in fume hood to keep from excess H2S (generated in the solvothermal process). 2.3 Characterization of In2S3 hollow microsphere

The structural analysis of the samples were performed using a Bruker (D5005)

X-ray diffractometer equipped with graphite monochromatizedCuK radiation ( = 1.54056 Å). An accelerating voltage of 40kV and emission current of 30 mA were adopted for the measurements. The morphology and microstructure were characterized by field-emission scanning electron microscopy (Hitachi S-4800). The absorption spectrum of the samples in the diffused reflectance spectrum (DRS) mode was

recorded in the wavelength range between 200 and 1000 nm using a spectrophotometer (Jasco – V670), with BaSO4 as reference. From the absorption edge the band gap values were calculated by extrapolation method. 2.4 Experimental Procedure

In order to investigate the photocatalytic activity of In2S3 hollow microsphere, the

photocatalytic reduction of chromium(VI) was carried out in the In2S3 suspension under visible light irradiation. The reaction suspensions were prepared by adding 0.100 g of the catalyst to 250 ml of aqueous Cr(VI) solution with an initial Cr(VI) concentration of 10 mg L-1. Prior to the photoreaction, the suspension was magnetically stirred in the dark for 30 min to establish an adsorption/desorption equilibrium condition. The aqueous suspension containing Cr(VI) and the photocatalyst was then irradiated by visible light with constant stirring. At given time intervals, analytical samples were taken from the suspension and immediately centrifuged at 5000 rpm for 15 min, followed by filtering through a 0.45 mm millipore filter to remove any particles. The filtrate was analyzed by means of a spectrophotometer. A blank experiment was carried out using dichromate solution without a photocatalyst in order to determine the extent of reduction of the concentration of hexavalent chromium due to the visible radiation. After irradiation and adsorption, the suspension was filtered and the Cr(VI) content was analyzed quantitatively by measuring the absorption band at 350 nm using the spectrophotometer (Colon et al., 2002). 2.5 Analytical methods

The chromium(VI) content was analysed quantitatively by measuring the

absorption band at 350 nm using a SHIMADZU (UV2450) UV-Vis spectrophotometer. In each test, the total chromium content [Cr(III) +Cr(IV)] was measured by Atomic Absorption Spectrometer (Varian – Spectra 220 FS) and it was observed that the total chromium content remained constant as the reaction proceeded. This indicates that there is no precipitation of Cr(III) and no irreversible adsorption on the catalyst. 3. RESULTS AND DISCUSSION 3.1 XRD analysis of In2S3nano flowers.

The crystal structures of the samples were characterized by XRD. Figure 1

shows the typical XRD pattern of the samples prepared by simple template free solvothermal method. All the reflection peaks in Figure 8 were carefully compared with the JCPDS data base and identified as the formation of the tetragonal phase. The observed peak positions, 2 = 11.8, 27.4, 33.5, 43.8 and 47.9 were respectively indexed as (1 0 1), (1 0 9), (0 0 1 2), (1 0 15) and (2 2 1 2) reflections of the tetragonal -In2S3 phase (a = 7.623Å, c = 32.36 Å, JCPDS: 73-1366) (Liu et al., 2011; Lee et al., 2009). The peaks are considerably strong and narrow, which indicate that the product is well crystallized. No characteristic peaks arising from the possible impurities are visible, such as InS, In2O3 and other phases of In2S3. This clearly indicates that pure

crystalline -In2S3 tetragonal phase was formed via solvothermal process. From Figure 8, it can also be seen that (1 0 9), (0 0 12), and (2 2 12) planes show the three strongest diffraction peaks, and the relative diffraction intensity of either (0 0 12)/(1 0 9) or (2 2 12)/(1 0 9) is unusually higher than the corresponding conventional values (JCPDS: 73-1366). This observation revealed that the resultant hollow In2S3 microspheres are grown predominantely along the (0 0 12) and (2 2 12) directions. From these XRD measurements it is interesting to note that similar XRD patterns were observed for all In2S3 hollow microspheres (In2S3-1:2, In2S3-1:3 and In2S3-1:4) prepared with different indium nitrate to thiosemicarbazide ratio, indicating that the samples possess similar crystalline structure and the structure does not change due to change in precursors ratios.

Fig. 1.XRD patterns of hierarchical like In2S3 hollow microspeheres prepared with

different indium nitrate and thiosemicarbazide ratio, (a) In2S3 - 1:2, (b) In2S3 - 1:3, (c) In2S3 - 1:4.

3.2 EDX analysis

In order to identify the components of the synthesized In2S3 hollow microsphere the EDX microanalysis and elemental mappings have been carried out in a SEM. The EDX spectrum and the corresponding elemental mappings recorded for Sample-In2S3 1: 4 are shown in Figure 2, which illustrate the actual distribution of In and S separately in the sample, with a In/S molar ratio of approximately 2:3. The EDX analysis confirmed that there are no elements other than In and S present in the sample. From the Figure 9 it is clear that the EDX spectrum displayed three intense peaks between 2.8 and 3.5

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Fig.2.EDX analysis of In2S3 hollow microsphere.

3.3 SEM analysis

The morphology of the samples were examined by SEM. Fig. 3 shows the typical SEM images of the In2S3 prepared at 180oC for 24 h with a molar ratio of 1:4. Low magnification SEM observations show that the as-prepared In2S3 samples have spherical morphology with size from 3 to 7 m in diameters (Figure 3). High magnification SEM images reveal that In2S3 hollow micro-spheres are built from small nanosheets with a thickness of 10 to 20 nm and nanofibers with a length of 80 to 100 nm. The wall thickness of the hollow microsphere is around 300- 500nm. These nanosheets and nonorods construct the surface of the hollow In2S3 microspheres. The surfaces of the hollow microspheres are analogous to hierarchical structure.

The relationship between the observed morphology and the synthesis time has

been investigated by SEM. The SEM image of the samples prepared at 24 h is shown in Figure 3. Initially, after 10 hrs of synthesis time, it has been observed that the spherical morphology is not completely achieved. But after 15 h of synthesis clear regular sphere morphology has been observed. These spheres were constructed by

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theory(Rengarajet al., 2006). The Kubelaka-Munk function F(R) = (1-R)2/2R, is used as the equivalent of absorbance (and is plotted in Figure 4). The absorption edges of the In2S3 hollow microspheres were found between 610 and 620 nm, corresponding to the absorption edge of a semiconductor material. For bulk like In2S3, the band gap (Eg) is reported to be vary between 2.0 and 2.2 eV which correspond to 620 to 550 nm. For our samples, the absorption edges were found vary between 610 and 620 nm, that correspond to the absorption edge of a semiconductor material. It is also interesting to note that the absorption edges for all samples were very close to each other indicating similar optical properties. Furthermore, the steep absorption edge is an indication of a narrow size distribution and uniform crystallites of In2S3 microspheres (Revathiet al., 2008); the particle size and uniform distribution is confirmed by SEM (Figure 3). The band gap values of these samples were calculated by extrapolating the absorption edge by a linear fit method. For all samples the band gap values were found very close to each other around 1.9 eV, indicating that there is no change in band gap energy values while changing the molar ratio of the precursors. The steep shape of the visible region reveals that the absorption band of In2S3 is due to the transition from the valence band to the conduction band (Liu et al., 2006), and is not due to the transition from the metal impurity level to the conduction band, as observed for the metal ion-doped semiconductors(Liu et al., 2009). The band structure indicates that charge transfer upon photoexcitation occurs from the S 3p orbital to the In 5p empty orbital.Thus, the steep absorption edge implies single-phase In2S3, which is in good agreement with our XRD studies. Since the In2S3 absorb significant amount of visible light, it can be used as a visible-light active photocatalyst.Hence the photocatalytic properties of these In2S3 microspheres have been systematically studied and are discussed in the following sections.

Fig. 4. UV-Vis DRS spectrum of In2S3 microsphere.

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3.5 Photocatalytic reduction of Cr(VI) in aqueous solution 3.5.1 Photocatalytic activity for Cr(VI) reduction with In2S3 hollow microsphere.

To determine the photocatalytic activity of the prepared catalysts, two experiments were first carried out with HCOOH as a hole scavenger to reduce Cr(VI) in aqueous solution with an initial concentration of 10 mg/l at pH 3.00 using In2S3 catalyst. The concentration of Cr(VI) with reaction time are shown in Fig. 5. The concentration of formic acid used in this study is based on a molar ratio of Cr(VI) : HCOOH = 1:12, which is slightly more than theoretical demand of 1:6 as shown in latter part of discussion. The experimental results shown in Fig. 5demonstrated that after 2 minutes reaction time, Cr(VI) in all the suspensions were reduced by more than 90%. In2S3 catalysts achieved the highest efficiency of Cr(VI) reduction by more than 99%. In this study, the In2S3 hollow microsphere sample under visible light irradiation demonstrated a considerable reduction of Cr(VI) in aqueous solution.

Fig. 5.pH Change and variation of Cr(VI) and Cr(III) conentrations during reaction. [Conditions: Cr(VI) coentration = 10mg/L; pH = 3.00; amount of catalyst = 1 g/L]

3.5.2 Effect of reductants

In addition to the effect of catalysts dosoage, another way to promote photocatalytic performance is to add sacrificial electron donors (hole scavenger) to the reaction system. Distinct kinds of sacrificial reagents were commonly found to have different effects in various systems. Accordingly, choosing a suitable and effective sacrificial reagent becomes especially important for the improvement of catalytic performance. In this study, formic acid with a simple one-carbon molecular structure

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was chosen as a hole scavenger to investigate its effect on Cr(VI) reduction. Hence, its oxidation to carbon dioxide is straightforward and involves minimal intermediate products(Aguado and Anderson., 1993). Also, formic acid is capable of forming reducing radicals, which could help in the reduction reaction(Kaiseet al., 1994). In the presence of formic acid, Cr(VI) reduction can be expressed as follows:

Cr2O72- + 14H+ +6e- 2Cr3+ + 7H2O (1)

One mol of Cr(VI) requires 6 mol of electrons to be reduced to Cr(III), which needs equivalent 6 mol of HCOOH to scavenge the holes. Which is having strong agreement with the previous literature(Papadamet al., 2007). The organic species such as HCOOH accept holes from the valence band either directly or indirectly and subsequently oxidized and thereby suppressing the electron – hole recombination process, increasing the reduction efficiency. Thus it can reduce Cr(VI) to Cr(III). Our experiments confirmed that using a hole scavenger such as formic acid is an essential condition to proceed this photocatalytic reduction reaction, since no catalytic activity of In2S3 was observed in the Cr(VI)solution without a hole scavenger. In the same time, it was found that the pH increased from 1.69 to 1.85 due to the consumption of formic acid as shown in Fig. 5. 3.5.3 Time dependence of photocatalytic reduction of Cr(VI)

The time dependence of the photocatalyticCr(VI) reduction reaction over In2S3

catalyst is shown in Fig. 5. At the beginning of reaction, the concentration of Cr(VI) decreased rapidly, but only trace of Cr(VI) appeared in the solution. With the proceeding of the reaction, it had been found that the concentration of the Cr(III) increased quickly, and the pH value of reaction solution also increased accordingly. Fig. 5 describes that after certain reaction period, the concentration of Cr(VI) was slowly reduced. Actually photocatalysis is suitable for both oxidation and reduction reactions, since e- and h+ are generated simultaneously on In2S3 under visible light illumination. In our experiments, reduction reaction was dominated in the initial stage due to the presence of formic acid as a hole scavenger. The pH rising might result from the reduction reaction from loss of formic acid. These results suggested that such a photocatalytic reduction reaction could affect pH in the reaction solution significantly. Furthermore, such a heterogeneous reaction in aqueous In2S3 suspension is also dependent on its adsorption rate on the catalyst surface.

Therefore, the adsorption properties of catalysts can be greatly changed at different pH values. In an acidic environment, H+ ions are adsorbed onto the surface of In2S3, which was reported to have a large surface proton exchange capacity. The photogenerated electrons can be captured by the adsorbed H+ to form Hoads, which is able to reduce Cr(VI)(Zhang et al., 2005). At high pH, the surface of catalyst has a net negative charge. Photocatalytic effectiveness was known to depend on how long the photo-generated electrons are trapped on the conduction band and how efficiently they are used in sub-sequent reduction reactions(Kominamiet al., 2001). Moreover, the morphology of the catalyst are another crucial factors in determining catalytic activity(Ranjit and Viswanathan., 1997).

In this study, In2S3 hollow microspheres are not only should prolong the life time of the photo-generated electrons very well, but should also act as efficient reduction sites. Since equal numbers of e- and h+ must be consumed in the photocatalytic systems, the acceleration of reduction reaction with e- at the homogeneously dispersed catalyst surface enhances the overall reaction as observed. 3.5.4 Discussion about photocatalyticCr(VI) reduction From the above studies it is observed that photocatalytic reduction of Cr(VI) to Cr(III) takes place when Cr(VI) solution containing In2S3 microsphere is illuminated in light having photon energy greater than the band gap energy of the semiconductor. During photocatalysis, adsorption occurred first when In2S3 was dispersed in the aqueous solution containing metal ions. This process is reversible and even takes place in absence of light illumination. The reduction of Cr(VI) to Cr(III) occurs because under illumination electron – hole pairs are created inside the semiconductor particles. After migration of these species to the surface of the particle, the photogenerated electrons reduce Cr(VI) to Cr(III) and holes oxidize water and sacrificial electron donor.

The possible mechanism of the photocatalytic reduction of hexavalent chromium is as follows:

In2S3In2S3 (h+ + e-) (2) Cr2O72- + H+ + e- Cr3+ + H2O (3) H2O + h+ O2 + H+ (4)

As the mentioned above, In2S3 hollow microsphere showed higher photocatalytic

activity in Cr(VI) reduction. It has been generally believed that the conversion of Cr(VI) into Cr(III) is structure-sensitive and geometric effect of catalyst plays an important role in catalytic reduction of Cr(VI). From the experimental results, it is clear that In2S3 hollow microsphere shows very good candidate for the reduction of Cr(VI). 3.6 Mechanism of the enhanced photocatalytic activity

The photocatalytic activity of In2S3 for the reduction of Cr(VI) may be enhanced by the electron-hole separation and the subsequent transfer of the trapped electron to the adsorbed O2 or Cr(VI) acting as an electron acceptor. After the photogeneration of electrons and holes by photons of appropriate energy (h EG, EG = ~2.0 ev) as shown in Equation 5. Because of the shorter band gap in the In2S3 nanostructure the electron-hole separation takes place. This reaction enables the positive photoholes p+ to react with OH- adsorbed species to create oOH as shown in Equation 8, which are generally assumed to be the degradating oxidative agents. In the same way photogenerated electron reduces Cr(VI) to Cr(III) as shown in equation 8.

In2S3 + h e- + p+ (5)

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Cr2O72- + 14H+ +6e- 2Cr3+ + 7H2O (8)

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In2S3 as the semiconductor photocatalyst under visible light radiation in the presence of sacrificial electron donor. The enhancement of the photocatalytic activity of the In2S3 was confirmed in the reduction of Cr(VI) in the presence of formic acid. The improved separation of electrons and holes on the In2S3 surface allows more efficient channelling of the charge carriers into useful reduction and oxidation reactions rather than recombination reactions. The photocatalyticreduction of hexavalent chromium is enhanced in the presence of sacrificial electron donors such as HCOOH. The experiments demonstrated that Cr(VI) was effectively degraded in aqueous In2S3 suspension by more than 99% within 180 min, while the pH of the solution increased from 1.65 to 1.85 due to the reduction of Cr(VI) and consumption of formic acid. ACKNOWLEDGEMENTS

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