Sm2FeTaO7 Photocatalyst for Degradation of Indigo Carmine Dye under Solar Light Irradiation

This paper is focused to study Sm2FeTaO7 pyrochlore-type compound as solar photocatalyst for the degradation of indigo carmine dye in aqueous solution. Sm2FeTaO7 was synthesized by using conventional solid state reaction and sol-gel method. X-ray diffraction results indicated that Sm2FeTaO7 exhibit a monoclinic crystal structure. By scanning electron microscopy analysis, it was observed that sol-gel material presents particle size of around 150 nm. The specific surface area and energy bandgap values were 12 m2 g−1 and 2.0 eV, respectively. The photocatalytic results showed that indigo carmine molecule can be degraded under solar light irradiation using the synthesized materials, sol-gel photocatalyst was 8 times more active than solid state. On the other hand, when Sm2FeTaO7 was impregnated with CuO as cocatalyst the photocatalytic activity was increased because CuO acts as electron trap decreasing electron-hole pair recombination rates.


Introduction
Nowadays, most of the investigations on photocatalysts are conducted in order to modify the nanostructure to promote their use under visible-light, especially when solar light is used because sun light is an available free energy source [1][2][3][4][5].
Actually, most of the common visible-light-sensitive photocatalysts are binary compounds such as CdS, CdSe, WO 3 , TiO 2 , ZnO, and Fe 2 O 3 that are unstable or have low activity during the photocatalytic process [6][7][8][9]. While ternary oxides are a promising family of interesting compounds that can offer the properties desired for an ideal visible-light photocatalyst [10].
Photodegradation using mixed oxides such as A 2 BB'O 7 pyrochlore-type compounds has attracted considerable attention because those compounds could be acting as photocatalyst under visible-light irradiation. The slight modification into their crystal structure will cause a variation in their electronic properties provoking an enhancement in the photocatalytic activity. Those compounds have the advantage that A and B sites can be substituted by several metal ions in order to develop visible responsive photocatalysts.
Previous works reported that pyrochlore containing metal ions like Bi 3+ , Gd 3+ , Sm 3+ , In 3+ , Fe 3+ , Ta 5+ , and Sb 5+ improve the photocatalytic activity for dye degradation in aqueous solution under visible-light irradiation. This is because those metals favor the mobility of photoinduced electrons and holes to reach easily the reactive sites on catalyst surface [11][12][13][14][15][16]. However, to our knowledge, there are no reported studies concerning the photocatalytic activity of pyrochlore compounds under solar light.
Recently in our group, a novel pyrochlore-type compound Sm 2 FeTaO 7 has been synthesized with attractive characteristics to be evaluated as potential photocatalyst [17]. For that reason in this paper, the attention is focused to study Sm 2 FeTaO 7 as photocatalyst for the degradation of indigo carmine dye in aqueous solution under real and varying solar light illumination. Samples were taken at different times to monitor the progress of the reaction by UV-vis spectroscopy and total organic carbon analysis. Additionally it is proposed the relation between preparative methods and the reaction mechanism of the photocatalytic degradation of indigo carmine dye.

2.4.
Characterization. Sm 2 FeTaO 7 materials were characterized by X-ray powder diffraction (XRD) using a Bruker D8 Advance diffractometer and CuKα radiation (λ = 1.5406Å) as the incident X-ray source. XRD data were collected at room temperature from 10 to 100 • with a step interval of 0.01 • and a counting time of 1s/step. Morphology of the samples was observed using a JEOL 6490 LV Scanning Electron Microscope (SEM). All samples were stuck to graphite tape and then placed on an aluminum sample holder and located in the SEM chamber. The content of CuO of the impregnated catalysts was determined by energy dispersive X-ray spectroscopy (EDS) analyzing five random zones.
The optical absorption properties of the samples were analyzed in the range of 200-900 nm at room temperature with a UV-vis spectrophotometer (Lambda 35 Perkin Elmer Corporation) equipped with an integrating sphere attachment. The energy bandgap of Sm 2 FeTaO 7 was determined by reflection spectra following the equation [13,18] Here, α, v, A, and Eg are absorption coefficient, light frequency, proportional constant, and bandgap, respectively.
The specific surface area (S BET ) was determined by nitrogen adsorption isotherms from the BET method using the Quantachrome NOVA 2000e equipment. The samples were degassed for 3 hours at 300 • C prior to the analysis.

Photocatalytic
Activity. The degradation of indigo carmine dye (Indigo-5,5 -disulfonic acid disodium salt) was carried out using a glass container as reactor under solar light irradiation. Four photodegradation experiments were carried out with an indigo carmine solution (10 mg/L). In each of the experiments, 400 mL of the solution were mixed with 400 mg of catalyst (SS, SG, CuO/SS, and CuO/SG) in the glass reactor. The system was allowed to reach adsorption equilibrium under dark conditions. Then, the reactors were placed under solar illumination. In addition, a similar experiment without using any catalyst was also carried out to monitor pure photochemical degradation reactions.
During the experiment, the reaction mixture was kept under stirring. The temperature was controlled at 30 • C running cold water on the external surface of the glass reactor. Samples for analysis were taken at different times to monitor the reaction. Each sample was analyzed by UV-vis spectrophotometer (Lambda 35 Perkin Elmer Corporation) and TOC apparatus (TOC-VCSH, Shimadzu Corporation). All tests were carried out simultaneously to ensure identical experimental conditions. Solar irradiation data were obtained from the Environmental Integrated Monitoring System (SIMA) at Monterrey, Nuevo León, México.

Characterization of Pyrochlore-Type
Compound Sm 2 FeTaO 7 3.1.1. X-Ray Diffraction Analysis. Sm 2 FeTaO 7 was obtained as a single phase at 1400 • C by solid state reaction and at 800 • C by sol-gel method; see Figure 1. This compound crystallized in a monoclinic system with the space group C2/c [17]. According to XRD patterns both materials showed the same crystal structure but with different crystallinity degree. This result is related to the differences in the synthesis conditions, 1400 • C/36 h and 800 • C/6 h for solid state reaction and sol-gel method, respectively. Crystal size calculation from the broadening of the main peak (2θ = 29.1 • ) using the Scherrer formula revealed values of 77 nm for solid state and 43 nm for sol-gel samples. This situation is in agreement with the fact that, by solid state method, material is suffering sintering due to higher thermal treatment than sol-gel, reaching a high crystallization degree.
When CuO/SS and CuO/SG samples were analyzed any XRD peaks attributed to CuO phase were detected since the CuO content in the impregnated catalysts is very low (1 wt.%).
The cell parameters of synthesized compounds are showed in Table 1. Figure 2 shows the morphology of Sm 2 FeTaO 7 synthesized by solid state and sol-gel. It can be seen that morphology and dimensions of particles were strongly dependent on the synthesis route. The particles prepared by solid state reaction have a smooth surface and size of 1 to 4 μm due to the high temperature  of synthesis. While the particles prepared by sol gel have spherical shape and size of around 100 to 150 nm, the micrographs also show these particles are forming aggregates of large size. Also in Figure 2 are showed the EDS analysis of a specific zone and its respective spectra corresponding to CuO/SG material. The results of this analysis are reported in Table 2.

Morphology and Specific Surface Area.
The results of the BET analysis are also reported in Table 2. Sm 2 FeTaO 7 catalysts prepared by sol-gel had a specific surface area of one order of magnitude higher than the corresponding area of materials prepared by solid state reaction. This result corroborates XRD and SEM analysis, and it was observed that sol-gel material has small particle size. Therefore the highest specific surface area of sol-gel material is due to the soft chemical route used.  In the case of CuO/SS and CuO/SG surface analysis, it is observed that surface area variation after wet impregnation is not significant. Although it is well known that the oxide support nature and metal oxide cocatalyst play an important role in surface area modification [19,20], it seems that our results are depending on the amount of cocatalyst used. Figure 3 shows the UV-vis spectra for all synthesized samples. It is observed a strong absorption in the visible light region from 400 nm to 900 nm. CuO/SS and CuO/SG showed an additional absorption band at 600-800 nm, which is indicative of the presence of CuO dispersed at the surface [21]. It could be noted that absorption of this band is more evident in CuO/SS sample due to its low surface area, that is, CuO particles are covering much of the available support surface in comparison with CuO/SG.

UV-Vis Analysis.
The bandgap (Eg) of Sm 2 FeTaO 7 synthesized by solid state and sol-gel was determined by the Tauc plot [18] of (αhv) 1/n versus hv showed in Figure 4. The Eg value obtained was 1.99 eV and 2.01 eV for material synthesized by solid  state and sol-gel, respectively. According to Eg results, we assumed that the ability of Sm 2 FeTaO 7 to absorb in the visible light region is directly associated to the presence of Fe. Iron generally acts as electron donor and its 3d electrons are easily excited by the visible light allowing the shift of bandgap to the visible region [10,16,22].

Photocatalytic Evaluation. The photodegradation tests
were realized under the solar light irradiation conditions presented in Figure 5(a). Samples were exposed to irradiation from 9 : 00 AM to 7 : 00 PM, where the maximum energy was received. Figure 5(b) shows transmittance spectra of glass container used as photocatalytic reactor. It can be noted that the glass container allows the incidence of radiation with wavelength larger 300 nm. During the photocatalytic experiment, the intensity of the blue color of indigo carmine solution decreased with time. The analysis of the reaction samples by UV-vis spectroscopy shows a slight decrement of all absorption bands (610, 286, 252, and 206 nm). Figure 6 shows the absorption spectra for indigo carmine photodegradation using CuO/SG. It is clear that indigo carmine molecule is suffering degradation under solar light irradiation.
Degradation of indigo carmine solution as a function of irradiation time using the synthesized materials is showed in Figure 7. It can be seen that photolysis reached around 20% due to the small portion of UV light of the solar spectrum, which participate directly in the photochemical reaction, whereas in the presence of the materials synthesized by solid state reaction only 5-10% of degradation is achieved. We can assume that a part of photons are absorbed by materials for the subsequent formation of the electron (e − ) and hole (h + ) pairs limiting the photolysis effect. Besides the photogenerated charges are not participating efficiently for the indigo carmine degradation according to the low activity observed. This fact could be due to several factors such as low interaction between material and dye solution, the low surface area as well as the high rate of the recombination process.
On the other hand, materials synthesized by sol-gel route showed the higher activity for degradation of the aqueous solution of indigo carmine. In concordance with our results, sol-gel material exhibited an activity 8 times higher than solid state catalyst. It is known that photoactivity is strongly influenced by the nature of material. In our case, sol-gel materials present high surface area and small particle size allowing a better interaction between solution and catalyst material. The presence of small particles provides more active sites and decreases the migration distance of photogenerated electrons and holes promoting the reaction on the catalyst surface [23].
With respect to the use of CuO as cocatalyst promoter, it increases the photocatalytic activity of Sm 2 FeTaO 7 . In particular CuO/SG material reached 38% of photodegradation; see Figure 7. In this case CuO acts as an electron trap [24] decreasing the rate of recombination of the photogenerated electron-hole pairs and more holes will be available to oxidize the organic molecules [25]. Additionally, it is known that the recombination competes strongly with the photocatalytic process being it the major limitation in photocatalysis as it reduces the overall quantum efficiency [26].
Additionally, photocatalytic tests were followed by TOC analysis. Results of all samples showed that mineralization is occurring during the reaction. Materials SG and CuO/SG showed 7% and 11% of TOC reduction, respectively, while mineralization for solid state materials is less than 5%.
On basis of the above results, the relation between preparative methods and the reaction mechanism of the photocatalytic degradation of indigo carmine dye is schematically illustrated in Figure 8. It is an attempt to explain why sol gel samples presented higher activity than solid state samples during the photocatalytic process.
In the first step, both CuO/SS and CuO/SG absorb photons with energy greater than their bandgap to form electron-hole pairs. Then, the charge carriers migrate to the surface where redox reactions will occur; simultaneously recombination process is occurring and competes with the photocatalytic process. In our case, both processes are affected by the preparation method. When samples were prepared by sol gel, particle size is smaller than solid state samples; consequently the distance that electrons and holes 6 International Journal of Photoenergy have to migrate to the surface becomes short decreasing the recombination rate and providing more active sites for the surface redox reactions. Finally, CuO is acting as electron trap reducing the electron-hole recombination rate, whereas holes are forming oxidative species like hydroxyl radicals that react with indigo carmine dye molecule until forming CO 2 and H 2 O [23][24][25].
According to our results Sm 2 FeTaO 7 can be considered as photocatalyst for organic compounds degradation under real and varying solar light illumination as an alternative material for the treatment of wastewater from textile and dying industries.

Conclusions
The photophysical characteristics of Sm 2 FeTaO 7 allowed its use as a visible-light photocatalyst for degradation of indigo carmine dye under solar light irradiation. Its activity was enhanced 8 times when this material was synthesized by solgel route due to the high surface area and small particle size compared with solid state material. In addition, the presence of CuO as cocatalyst increases the photocatalytic activity because CuO acts as electron trap avoiding electron-hole pair recombination rates, which favors the oxidation of the indigo carmine molecule.