Sol–gel synthesis of Sm2InTaO7 and its photocatalytic activity on degradation of crystal violet...

Sol–gel synthesis of Sm2InTaO7 and its photocatalyticactivity on degradation of crystal violet dyeand reduction of Cr(VI) ions

Leticia M. Torres-Martınez • Edgar Moctezuma •

Miguel A. Ruiz-Gomez • Isaıas Juarez-Ramırez •

Mayra Z. Figueroa-Torres

Received: 10 February 2011 / Accepted: 4 August 2011 / Published online: 14 June 2012

� Springer Science+Business Media B.V. 2012

Abstract This paper reports the synthesis of Sm2InTaO7 and its photocatalytic

activity on the degradation of crystal violet dye and reduction of Cr(VI) ions in aqueous

solution. Sm2InTaO7 was prepared by sol–gel method at 1,200 �C. For comparison

purposes, Sm2InTaO7 was prepared also by solid-state reaction at 1,400 �C. The evo-

lution of the crystalline phases of the sol–gel sample with the annealing temperature was

followed by X-ray diffraction. The cubic crystalline structure of Sm2InTaO7 was

determined by Rietveld refinement method. Specific surface area was 5 m2g-1 for sol–

gel sample and 1 m2g-1 for solid-state sample, whereas energy band gap was around

3.5 eV. Sm2InTaO7 prepared by sol–gel method showed better photocatalytic activity

than Sm2InTaO7 prepared by solid-state reaction for the mineralization of crystal violet

dye and reduction of Cr(VI) solution under UV-light irradiation.

Keywords Sm2InTaO7 � Sol–gel � Photocatalysis � Crystal violet dye � Cr(VI) ions

Introduction

Oxide materials exhibiting pyrochlore structure have received much attention due to

their interesting properties and potential applications. These compounds are used as

photocatalytic materials [1, 2], magnetic materials [3, 4], and in the immobilization

and treatment of nuclear waste [5, 6], among others. In particular, pyrochlore oxides

with general formula A23?B3?B05?O7 have been widely investigated for

L. M. Torres-Martınez (&) � M. A. Ruiz-Gomez � I. Juarez-Ramırez � M. Z. Figueroa-Torres

Departamento de Ecomateriales y Energıa, Facultad de Ingenierıa Civil, Universidad Autonoma

de Nuevo Leon, Cd. Universitaria, 66450 San Nicolas de los Garza, N.L., Mexico

e-mail: [email protected]

E. Moctezuma � M. A. Ruiz-Gomez

Facultad de Ciencias Quımicas, Universidad Autonoma de San Luis Potosı, Av. Manuel Nava #6,

78290 San Luis Potosı, S.L.P., Mexico

123

Res Chem Intermed (2013) 39:1533–1544

DOI 10.1007/s11164-012-0618-6

photocatalytic water-splitting reaction [7–9] and photodegradation of different kinds

of dyes such as methylene blue, rhodamine B, and alizarin red S [10–12].

Most of the pyrochlore oxides are synthesized by solid-state reaction by mixing

and heating the precursors at high temperature for long periods of time. This method

yield solids with low surface area and high particle size [9, 13]. Sol–gel synthesis

method offers new approaches to prepare novel catalysts with specific chemical

composition and well defined. Crystallite size and surface area of catalysts particles

can also be well controlled [2, 14, 15].

In recent years, photocatalysis processes have been widely investigated in the field

of environmental pollution control. Organic compounds that may contaminate

industrial and domestic wastewaters can be mineralized to CO2 and water. For

example, toxic and mostly non-biodegradable organic dyes can be degraded to

uncolored and innocuous chemical compounds [16–19]. Since any photocatalytic

process involves oxidation and reduction reactions, it can be used to transform metallic

cations to their reduced form. In this way, Cr(VI) that is a toxic, carcinogenic and

mobile contaminant which is a byproduct from various industrial processes can be

reduced to a lesser toxic Cr(III) cation [17]. Furthermore, pure hydrogen can be

produced by the water-splitting reaction that it is also a photocatalytic process [7].

Commercial titanium dioxide is the most efficient photocatalyst for the complete

mineralization of organic pollutants or reduction of metallic cations, but TiO2 has a

large band gap around 3.2 eV and low quantum yield [17, 18]. Therefore, several

research groups around the world have been working in the development of more

active novel catalysts with a narrow band gap that can be activated with visible light.

Recently, the synthesis of Sm2InTaO7, which is a new pyrochlore oxide, by solid-

state reaction has been reported. This oxide was tested for the photocatalytic

production of pure hydrogen by the water- splitting reaction [7], but the use of this

material for degradation of organic dyes or heavy metals reduction has not been

reported. In fact, Sm2InTaO7 has not been synthesized by a route different than

solid-state reaction method, which could enhance its structure and electronic and

photophysical properties in order to improve its photocatalytic activity.

In this paper, we present the synthesis, characterization, and photocatalytic activity of

Sm2InTaO7. This catalyst was prepared by the sol–gel method using indium

acetylacetonate, samarium acetate, and tantalum ethoxide as starting materials.

Catalysts were characterized by X-ray diffraction (XRD) and Rietveld refinement,

UV–Vis spectroscopy, specific surface area measurement, and scanning electron

microscopy (SEM). The photocatalytic activity of Sm2InTaO7 prepared by sol–gel

method was evaluated for the degradation of crystal violet dye and the reduction of

Cr(VI) ions in aqueous solution. In addition, for comparison purposes, Sm2InTaO7 was

prepared by solid state and tested as photocatalyst on the reactions mentioned above.

Experimental

Synthesis of Sm2InTaO7 by sol–gel method and solid-state reaction

In order to prepare Sm2InTaO7 by sol–gel method, a stoichiometric amount of

indium(III) acetylacetonate was dissolved in acetylacetone. The reaction mixture

1534 L. M. Torres-Martınez et al.

123

was kept in magnetic stirring for 3 h and refluxed at 70 �C. Samarium(III) acetate

was then dissolved in ethylene glycol and water and refluxed for 1 h at 70 �C.

Glacial acetic acid was added to obtain a colorless samarium solution. At the same

time, the tantalum ethoxide was mixed with ethanol. Both the samarium and

tantalum solutions were slowly added to the indium solution and it was refluxed at

70 �C for 48 h. After this time, pH was adjusted to 10 using ammonium hydroxide.

The solution was kept under the same conditions for 48 h. The final product was

dried for 24 h at 100 �C to obtain the fresh sample. Portions of this material were

thermal treated at 1,000, 1,100, and 1,200 �C for 12 h under air atmosphere using a

heating rate of 1 �C/min.

Sm2InTaO7 was also prepared by solid-state reaction method. The chemical

reagents Sm2O3, In2O3, and Ta2O5 (Aldrich purity [99.9 %) were utilized as

starting materials. The powders were dried at 200 �C for 4 h before the synthesis

and stored in a desiccator. Then, stoichiometric amounts of each reactant were

perfectly mixed with acetone in an agate mortar. This solid was placed into a

platinum crucible and calcined at 1,400 �C for 3 days under air atmosphere with

intermediate regrinding to complete reaction.

Characterization

All the Sm2InTaO7 samples were well characterized by powder XRD using a Bruker

D8 Advance diffractometer equipped with a Vantec high-speed detector and a CuKaradiation (k = 1.5406 A) incident X-ray source. XRD data were collected at room

temperature from 10 to 1008 with a step interval of 0.018 and a counting time of

1 s/step. A detailed analysis of the crystal structure was performed by the Rietveld

refinement method using TOPAS R3 software [20].

Morphology of samples was observed using a JEOL 6490 LV SEM. All samples

were placed on an aluminum sample holder using carbon tape and located in the

SEM chamber.

The optical absorption properties of the samples were analyzed in the range of

200–600 nm at room temperature with a UV–Vis spectrophotometer (Lambda 35

Perkin Elmer Corporation) equipped with an integrating sphere attachment. The

energy band gap (Eg) value was obtained by the Kubelka–Munk function based on

the diffuse reflection (DR UV–Vis absorption spectra).

The specific surface area was determined by nitrogen adsorption isotherms from

the BET method using the Quantachrome NOVA 2000e equipment. The samples

were degassed for 3 h at 300 �C prior to the analysis.

Crystal violet photodegradation

The photocatalytic tests were performed in a home-made reactor equipped with an

immersion UV Pen Ray light lamp (UV Products, 254 nm and 4,400 lW/cm2). The

lamp is covered with a quartz tube. For each experiment, 1 L of aqueous solution of

crystal violet was prepared by dissolving 10 mg of solid in deionized water. The pH

of the solution was adjusted to pH = 3 by addition of concentrated H2SO4 solution.

Then, 150 mL of this solution was placed in the glass reactor and mixed with

Sol–gel synthesis of Sm2InTaO7 and its photocatalytic activity 1535

123

100 mg of Sm2InTaO7 catalyst. This slurry was kept under stirring for 30 min in the

dark to establish the adsorption–desorption equilibrium. The UV lamp was turned

on to activate the catalyst. During the photocatalytic reaction, samples were

collected every 10 min and the catalyst particles were removed by centrifugation.

Each sample was analyzed with a UV–Vis spectrophotometer (Lambda 35 Perkin

Elmer Corporation) calibrated at 590 nm and with a TOC analyzer (TOC-VCSH,

Shimadzu Corporation).

Cr(VI) ion photoreduction

The photocatalytic reduction experiments were also carried out in the home-made

reactor described above. In this case, Cr(VI) aqueous solution (20 mg/L) was prepared

using K2Cr2O7 and the pH was adjusted to 2 by the addition of H2SO4 solution. Then

100 mg of photocatalyst was dispersed in 150 mL of Cr(VI) solution under vigorous

stirring for 30 min in dark conditions in order to establish the adsorption–desorption

equilibrium. Solution samples were collected every 20 min and solid catalyst was

removed by centrifugation. Cr(VI) concentration was monitored by measuring the

absorbance at 348 nm as a function of the irradiation time.

Results and discussion

Synthesis and characterization

XRD patterns of Sm2InTaO7 synthesized by sol–gel method and calcined at 1,000,

1,100 and 1,200 �C are shown in Fig. 1. It is noted that samples prepared by sol–

gel and annealed at 1,000 and 1,100 �C showed reflections associated with

Sm2InTaO7 cubic pyrochlore, in addition some small peaks attributed to unreacted

Fig. 1 X-ray powder diffraction patterns of Sm2InTaO7 synthesized by sol–gel


123

In2O3 were also found. Finally, the sample heated at 1,200 �C showed the presence

of the single-phase Sm2InTaO7.

Rietveld refinement of Sm2InTaO7

X-ray Rietveld method was used in order to elucidate the crystal structure of the

oxide Sm2InTaO7 prepared by sol–gel. The analysis was done considering previous

information based in the space group Fd-3m (No. 227) for cubic pyrochlore

Sm2InTaO7 [7].

The X-ray refinement diffractogram of Sm2InTaO7 synthesized by sol–gel and

annealed at 1,200 �C during 12 h it can be seen in Fig. 2. Good agreement was

observed between the simulated and experimental XRD patterns, which means that

all the reflections could be indexed on the basis of a cubic crystal structure.

A similar result was obtained for the refinement of the solid-state reaction.

Cell parameters and atomic positions of Sm2InTaO7 synthesized by both sol–gel

and solid state are reported in Tables 1 and 2, respectively. Table 1 also shows a

comparison with the reported cell parameters data for this phase.

Fig. 2 Experimental and simulated diffractograms of Sm2InTaO7 prepared by sol–gel method

Table 1 Crystal data and

Rietveld refinement results for

Sm2InTaO7

Parameter Compound

Sm2InTaO7

sol–gel

Sm2InTaO7

solid state

Sm2InTaO7

reported [7]

a (A) 10.5521 10.5676 10.5448

Volume (A3) 1174.963 1180.144 No data

Space group Fd-3m Fd-3m Fd-3m

Z 8 8 8

Rwp0 9.8 8.6 10.1


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It is important to mention that in cubic pyrochlore, all of the atoms occupy special

positions, except for the oxygen atom at the 48f position with the unconstrained

x parameter. According to refinement results, Sm2InTaO7 synthesized by sol–gel and

solid state showed a closed value: x48f = 0.3352(4) and x48f = 0.3334(6), respectively.

SEM morphological analysis of the catalysts prepared by the sol–gel method

revealed the presence of small particles with an average size below 1 lm (Fig. 3a).

It is also observed that some particles has been started to sinter increasing their size

may be due to the neck formation among these particles. On the other hand, the

average size of particles of catalysts prepared by solid-state reaction is higher than

1 lm. Furthermore, Fig. 3b shows the presence of particles with neck growths due

to the sintering process caused by the high temperature of synthesis and long

reaction time.

Table 2 Structural parameters of Sm2InTaO7 prepared by sol–gel and solid state

Compound Atom Site x y z Occupancy

Sm2InTaO7 sol–gel Sm 16(d) 0.5000 0.5000 0.5000 1

In/Ta 16(c) 0.0000 0.0000 0.0000 0.5

O1 48(f) 0.3352(4) 0.1250 0.1250 1

O2 8(b) 0.3750 0.3750 0.3750 1

Sm2InTaO7 solid state Sm 16(d) 0.5000 0.5000 0.5000 1

In/Ta 16(c) 0.0000 0.0000 0.0000 0.5

O1 48(f) 0.3334(6) 0.1250 0.1250 1

O2 8(b) 0.3750 0.3750 0.3750 1

Sm2InTaO7 reported [7] Sm 16(d) 0.5000 0.5000 0.5000 1

In/Ta 16(c) 0.0000 0.0000 0.0000 0.5

O1 48(f) 0.3302(3) 0.1250 0.1250 1

O2 8(b) 0.3750 0.3750 0.3750 1

Fig. 3 SEM images for Sm2InTaO7 synthesized by: a sol–gel and b solid state


123

Photophysical properties

The DR UV–Vis absorption spectra of Sm2InTaO7 samples synthesized in this work

are presented in Fig. 4. Both catalysts, the one prepared by sol–gel method and the

prepared by solid-state reaction present several absorption peaks in the visible light

region (k = 400 nm). According to the literature, this could be attributed to internal

transitions of the partly filled samarium 4f shell [7]. It could also be noted that these

materials showed major absorption at k \350 nm, indicating that Sm2InTaO7

synthesized in this work have a large Eg.

The results of the UV–Vis analysis and the surface area measurements are

presented in Table 3. This table indicates that Sm2InTaO7 prepared in our

laboratories has an Eg in the order of 3.5 eV corresponding to a k = 355 nm. The

difference with the Eg value reported in the literature [7] may be due to the fact that

the Kubelka–Munk analysis was performed without taking into account the

samarium 4f transition. Table 3 also shows that all the Sm2InTaO7 catalysts have

very low specific surface (SBET) areas.

According to the photophysical properties of Sm2InTaO7 catalysts prepared in this

work, they will show their best photocatalytic activity under UV light irradiation.

Photodegradation of crystal violet dye

Due to the large Eg values shown by Sm2InTaO7 samples, a high-energy UV light lamp

(k = 254 nm, intensity = 4,400 lW/cm2) was selected as irradiation source for the

photocatalytic degradation experiments. Figure 5 shows the results of the

Fig. 4 UV-Vis absorbance spectra for Sm2InTaO7 synthesized by sol–gel and solid-state reaction

Table 3 Energy band gap and

specific surface area values of

Sm2InTaO7 prepared by sol–gel

and solid-state reaction

* Synthesized in this work

** Reported in the literature [7]

Compound Synthesis method Eg (eV) SBET (m2g-1)

Sm2InTaO7 Sol–gel* 3.5 5

Sm2InTaO7 Solid state* 3.6 1

Sm2InTaO7 Solid state** 2.8 0.47


123

photocatalytic degradation of an aqueous solution of crystal violet (150 mL, 10 mg/L)

with Sm2InTaO7 synthesized by sol–gel. It can be seen that the maximum absorption

band of crystal violet solution at k = 590 nm disappeared completely after 60 min of

irradiation. Also noticeable is the decrease of absorption band at 208 nm, which

indicates the decomposition of the aromatic ring during the photocatalytic process.

In order to compare the photocatalytic degradation of crystal violet using

Sm2InTaO7 catalysts prepared by sol–gel and solid-state reaction with pure

photochemical reaction, the experimental data of three experiments are shown in

Fig. 6. The upper line indicates that crystal violet can be degraded by pure

photochemical reaction. It also shows that the presence of Sm2InTaO7 catalyst

accelerate the photodegradation reactions.

According to the photocatalytic results shown by Sm2InTaO7 on the photodeg-

radation of crystal violet, it could be assumed that the crystalline structure of

photocatalysts play an important role in photoactivity. It is well documented that the

Fig. 5 UV-Vis absorption spectra of the photodegradation of crystal violet using Sm2InTaO7 synthesizedby sol–gel. (V = 150 mL, Co = 10 mg/L, catalyst = 100 mg, one UV pen ray lamp, k = 254 nm)

Fig. 6 Photodegradation of crystal violet under UV-light irradiation and using Sm2InTaO7 synthesizedby sol–gel and solid state (V = 150 mL, Co = 10 mg/L, catalyst = 100 mg, one UV pen ray lamp,k = 254 nm)


123

distortion of BO6 polyhedra in pyrochlore structures results in lattice distortion,

which is one of the important parameters for charge separation and contributes to

improve the photoactivity [1, 7].

On the other hand, although a photolysis effect was observed during these

reactions, which indicates a high decoloration of crystal violet solution, from an

environmental point of view one important objective is to reach the complete

mineralization of organic compounds. For this reason, the samples of the

photocatalytic reaction were also analyzed with TOC apparatus. Figure 7 shows

the mineralization degree obtained after 60 min of UV irradiation. The results

revealed that mineralization of crystal violet increases due to the presence of the

photocatalysts. According to this result, it is confirmed that Sm2InTaO7 presents

photocatalytic activity to mineralize crystal violet dye to CO2 and water.

Photoreduction of Cr(VI) ions

Sm2InTaO7 materials were used for the photocatalytic reduction of Cr(VI) cations.

The experiments were carried out with an aqueous Cr(VI) solution (150 mL,

20 mg/L). Since the literature indicates [19] that photoreduction reaction is

favorable in acidic conditions, the pH of the solution was adjusted to a value of 2.

Figure 8 shows the absorption spectra of Cr(VI) solution in the interval of

220–500 nm (Fig. 8a) and in the interval of 400–700 nm (Fig. 8b) as a function of

the time during the photocatalytic process using Sm2InTaO7 synthesized by the sol–

gel method. It can be seen in Fig. 8a that a drastic reduction in the intensity of the

absorption band at k = 348 nm after 180 min of UV-light irradiation is indicative

that photoreduction of Cr(VI) to Cr(III) is occurring, whereas in Fig. 8b one

absorption band near to 585 nm appears in the spectra and it is increased as function

of time. The appearance of this absorption band is clear evidence of the presence of

Cr(III) in the solution. This result was corroborated by the analysis of a

chromium(III) nitrate solution used as reference, where the UV–Vis absorption

Fig. 7 TOC results of the photodegradation of crystal violet under UV-light irradiation and usingSm2InTaO7 synthesized by sol–gel and solid state. (V = 150 mL, Co = 10 mg/L, catalyst = 100 mg,one UV pen ray lamp, k = 254 nm)


123

spectra shows two characteristic absorption bands at 425 and 585 nm. Additionally,

in order to determine if Cr(III) was absorbed by the catalysts, Sm2InTaO7 powders

used as photocatalysts were filtered and dried at 50 �C and then analyzed by DR

UV–Vis absorption spectra. However, only the absorption bands corresponding to

Sm2InTaO7 were detected. These results suggest that Cr(III) is not adsorbed on the

surface of the Sm2InTaO7 catalyst and that Cr(III) remains in the solution.

The results of the photoreduction of Cr(VI) in the presence and in the absence of

Sm2InTaO7 are presented in Fig. 9. The upper line indicates that by 28 % of the

Cr(VI) is reduced by photochemical reactions. This decrease in Cr(VI) concentra-

tion has been previously reported [21] and it was attributed to the irradiation of

UV-light provoking H2O photolysis and subsequent Cr(VI) reduction (see

Reactions 1 and 2). When the catalyst (Sm2InTaO7) was added to the reaction

mixture, the overall reduction percentage of Cr(VI) was increased up to 52 %.

2 H2O �!UVlightO2 þ 4 Hþ þ 4 e� ð1Þ

Cr2O2�7 þ 14 Hþ þ 6 e� ! 2 Cr3þ þ 7 H2O ð2Þ

These results indicate that Sm2InTaO7 is able to cause the photoreduction of

Cr(VI) ions in aqueous solution. Although there is a slight variation using sol–gel or

Fig. 8 UV-Vis absorption spectra changes of Cr(VI) solution during the photocatalytic reaction bySm2InTaO7 sol–gel under UV-light irradiation. V = 150 mL, Co = 20 mg/L, catalyst = 100 mg, oneUV pen ray lamp, k = 254 nm. a from k = 220–500 nm, b from k = 400–700 nm


123

solid-state catalysts, the dissimilarity in photoreduction activities must be due to the

differences in the surface area of each catalyst. Another possibility is that the

presence of smaller particles obtained by the sol–gel method modify the diffusion

length and accelerate the migration rate of the excited electron and hole to the active

site of the particle and then participate in subsequences oxidation–reduction reaction

[22].

Conclusions

According to our results, the sol–gel method allows the synthesis of the Sm2InTaO7

pyrochlore type structure compound at lower temperature and less thermal treatment

time than by conventional solid-state method and it crystallizes in the typical cubic

pyrochlore structure. In addition, sol–gel Sm2InTaO7 showed better photocatalytic

activity for crystal violet degradation and also for the photoreduction of Cr(VI) ions

in aqueous solution. This high activity of sol–gel Sm2InTaO7 is attributed to the

crystalline structure and the specific surface area, respectively.

Acknowledgments The authors would like to thank CONACYT for financial support through the CB-

98740-2008, CB-84809-2007, CB-103532-2008 and CB-83923-2007, and PAICYT-UANL2009 for the

projects IT176-09 and IT171-09. Also Edgar Moctezuma would like to thank the projects UASLP C10-

FRC-07-03. M.Z. Figueroa-Torres thanks CONACYT for the financial support through the program

‘‘Apoyos complementarios para la consolidacion institucional de grupos de investigacion-retencion’’ No.

144226. Miguel A. Ruiz-Gomez thanks CONACYT for PhD scholarship No. 239336.

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Fig. 9 Photoreduction of Cr(VI) solution in the absence and the presence of Sm2InTaO7 photocatalysts.V = 150 mL, Co = 20 mg/L, catalyst = 100 mg, one UV pen ray lamp, k = 254 nm


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