Synthesis and characterization of Bi1.56Sb1.48Co0.96O7 pyrochlore sun-light-responsive photocatalyst

Materials Research Bulletin 74 (2016) 491–501

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Synthesis and characterization of Bi1.56Sb1.48Co0.96O7 pyrochlore sun-light-responsive photocatalyst Benhadria Naceura , Elaziouti Abdelkadera,b,* , Laouedj Nadjiaa,b , Mayouf Sellamia , Bettahar Noureddinea a Laboratory of Inorganic Materials Chemistry and Application, Department of Materials Engineering, University of Science and Technology of Oran (USTO M. B), BP 1505, El M’naouar, 31000 Oran, Algeria b Dr Moulay Tahar University, Saida, Algeria

A R T I C L E I N F O

A B S T R A C T

Article history: Received 14 August 2015 Received in revised form 3 November 2015 Accepted 8 November 2015 Available online 12 November 2015

Novel nanostructure pyrochlore Bi1.56Sb1.48Co0.96O7 was successfully synthesized via solid state reaction method in air. The as-synthesized photocatalyst was characterized by X-ray diffraction, Scanning electron microscopy and UV–vis diffuse reflectance spectroscopy techniques. The results showed that the BSCO was crystallized with the pyrochlore-type structure, cubic crystal system and space group Fd3m. The average particle size and band gap for BSCO were D = 76.29 nm and Eg = 1.50 eV respectively. Under the optimum conditions for discoloration of the dye: initial concentration of 20 mg L
1 RhB, pH 7, 25  C, 0.5 mL H2O2 and BSCO/dye mass ration of 1 g L
1, 97.77 and 90.16% of RhB were removed with BSCO/H2O2 photocatalytic system within 60 min of irradiation time under UVA- and SL irradiations respectively. Pseudo-second-order kinetic model gave the best fit, with highest correlation coefficients (R2  0.99). On the base of these results, the mechanism of the enhancement of the discoloration efficiency was discussed. . ã 2015 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures B. Crystal growth C. X-ray diffraction D. Catalytic properties D. Crystal structure

1. Introduction The increase in the use of organic pollutants in various industries nowadays causing several harmful effects to humans as well as ecological systems. The waste water from the industries is directly released into the water bodies, thus contaminating the entire fresh water resources. Wastewater treatment and recycling is an important concern and the researchers are looking forward for inexpensive and suitable technologies. Hence, pollution treatment should be a major concern. One of the best and green environmentally friendly processes is advanced oxidation processes (AOPs) [1]. AOPs are important technologies in environmental restoration applications [1,2]. Glaze et al. have established the concept of AOPs and they defined them as processes involving the generation of highly reactive oxidizing agents able to degrade organic molecules [3–6]. Nowadays, AOPs are considered high efficiency physical–chemical processes due to their thermodynamic viability and capable to produce important changes in the chemical structure of the contaminants via the contribution of free

* Corresponding author. E-mail addresses: [email protected] (B. Naceur), [email protected] (E. Abdelkader), [email protected] (L. Nadjia), [email protected] (M. Sellami), [email protected] (B. Noureddine). http://dx.doi.org/10.1016/j.materresbull.2015.11.012 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

radicals [7]. These reactive species, mainly hydroxyl radicals (OH), are of particular interest due to their high reactivity together with low selectivity [8,9]. However, other studies have suggested that, besides hydroxyl radicals, AOPs can also generate other oxidizing agents [10,11]. Free radicals in AOPs, may be produced by processes using irradiation and those without irradiation; Heterogeneous photocatalysis (TiO2/UV), irradiation/H2O2/O3, homogeneous photocatalysis (Fenton and Fenton-like processes), electrochemical oxidation and wet hydrogen peroxide catalytic oxidation (WHPCO), as well as their combinations, are considered as the most efficient ways for contaminants degradation [12–18]. In general, these methods combine an oxidizing agent with a catalyst and/or a radiation to generate the hydroxyl radical [19].The different AOPs may be thus viewed as different ways in which reactive radicals can be generated from the different reaction sources. The combination of irradiation with catalysts and hydrogen peroxide is one of such methods which have described significant attention in recent years, due to its effectiveness in mineralization of organic compounds, including removal of dye pollutants from residual textile treatment waters [20]. In this context, several catalysts have been developed in the recent years for dyes degradation. The pyrochlore-type compounds A2B2O7 structure (where A is usually the larger trivalent cation and B is the smaller tetravalent cation) have received much attention due to

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their interesting properties such as electrical, magnetic, thermophysical, catalytic and optical properties [21–25], and have been used in different promising applications such as solid electrolytes, anodes and cathodes for fuel cells and sensors, catalysts, dielectrics, and thermal barrier coatings [26–31]. Recently, many pyrochlore-type compounds (A2B2O7) have been studied to evaluate their semiconductor properties for photocatalytic applications such as Bi2Ti2O7 [32,33], Pb2Nb2O7 [34], Fe2BiSbO7, [35], Bi2Zr2O7 [36], other pyrochlore compounds that have been evaluated are rare earths, such as Sm2FeTaO7 [37], Tm2Ti2O7 [38], Bi2Ce2O7 [39], Ln2Ti2O7 (Ln = Nd, Gd, Er) [40]. As far as we have been able to ascertain, the catalytic efficiency of pyrochlore as a catalyst in combined process using hydrogen peroxide as oxidizing agent has not been reported. In this study, novel pyrochlore-type compound Bi1.56Sb1.48Co0.96O7 (BSCO) was synthesized via conventional solid state method and then characterized by XRD, SEM and UV–vis DRS techniques to be assessed as potential photocatalyst for the degradation of Rhodamine B (RhB) dye, as a probe reaction, under UVA-light irradiation. Various operating parameters such as temperature of the dye solution and volume of hydrogen peroxide (H2O2) were investigated. Moreover, the discoloration of RhB was undertaken under different oxidations systems that include (i) RhB dye solution in the presence of UVA-light and sunlight (RhB self-photolysis process), (ii) mixture of RhB dye solution and H2O2 in the absence of UVA-light and sunlight (dark oxidation process), (iii) Mixture of RhB dye solution and H2O2 in the presence of UVA-light and sunlight (homogeneous photolysis process), (iv) reaction mixture of RhB dye solution and BSCO/H2O2 system catalytic in the absence of UVA-light and sunlight (dark adsorption/oxidation process) and (v) RhB dye solution in the presence of the BSCO/H2O2 catalytic system under irradiation of UVA-light and sunlight (BSCO/photo-oxidation process), were monitored by absorption spectrophotometry. The discoloration of RhB in the presence of H2O2/BSCO photocatalytic system was correlated by a pseudo-first order kinetic model. On the basis of their structural, morphological, optical features of BSCO photocatalyst and maxima absorption of the dye, the discoloration performance of BSCO catalyst was discussed. 2. Experimental 2.1. Chemical reagents The starting materials introduced in the synthesis of pyrochlore compound: Bi2O3 (99.98%), Sb2O3 (99%) and CoO (99%) were all provided from Aldrich Chemical Company Ltd. Rhodamine B (RhB, C.I. Basic violet, C.I. number 45170, chemical class: xanthenes, molecular formula C28H31N2O3Cl, molecular weight: 479.01 g/

mole, IUPAC name: N-[9-(ortho-carboxyphenyl)-6-(diethylamino)-3H-xanthen-3-ylidine]diethylammonium chloride) were from Sigma–Aldrich. The molecular structure, chemical properties and UV–vis absorption spectrum of aqueous solution of RhB are shown in Table 1. Other chemicals used in the experiments (NH4OH and H2SO4) were purchased from Sigma–Aldrich. Distilled water was used for preparation of various solutions. 2.2. Synthesis of Bi56Sb48Co96O7 pyrochlore compound The sample with pyrochlore structure was prepared by solidstate reaction method in air with a reduced time of temperature treatment in comparison with that previously published [41]. Stoichiometric proportions of the reactants were thoroughly ground for 1 h in an agate mortar and heated in alumina crucibles at 1073 K for 24 h.Then the resulting powders were calcined at different temperatures 1073, 1173 and 1273 K for 24 h for each temperature with intermediate grindings. Finally, the powder was re-milled and pressed into disk with diameter of 13 mm and thickness of 3 mm. The disk was sintered in air at 1273 K for 24 h. 2.3. Characterization The formation of different phases was confirmed by X-ray diffraction (XRD) pattern recorded on a pan Analytical X’pert Pro Xray diffraction system using CuKa radiation. The XRD pattern was collected in the 10–80 intervals with a step size of 0.01313 . The crystallite size was determined from the broadening of corresponding XRD (2 2 2) plane by the Scherrer formula Eq. (1): D¼

0:9l FWHM cosu

ð1Þ

where l is the X-ray wavelength (1.5406 Å) and u is the Bragg angle and FWHM is the full-width at half maximum of (h k l) diffraction peak. FWHM was calculated from the peak having highest intensity in the sample. The lattice constant and the cell volume of the sample calculated from their corresponding XRD pattern data were determined using the Dicvol program implemented in Fullprof program [42].Scanning electron microscopy (SEM) image was performed with a JEOL JSM-6610LA analytical scanning electron microscope. UV–vis diffuse reflectance spectroscopy measurements were carried out using a Perckin Elmer Lambda 650 spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 200 to 800 nm, and polytetrafluoroethylene (PTFE, Teflon) was used as a reflectance standard. The Kubelka–Munk formalism and Tauc plots were used to determine the band gap energy.

Table 1 The molecular structure, chemical properties and UV–vis absorption spectrum of aqueous solution of Rhodamine B (Rh B). Molecular structure

Chemicals properties Chemical formula lmax (nm) Chemical class Mw (g mol
1) C.I. number

UV–visible absorption spectrum C28H31ClN2O3 554 Triphenylmethane 4696.665 22120

B. Naceur et al. / Materials Research Bulletin 74 (2016) 491–501

The residual pollutant concentrations during degradation were monitored with Shimadzu UV mini-1240 spectrophotometer in the range 200–800 nm, using 1 cm optical pathway cells.

493

where half-life time, t1/2, is defined as the amount of time required for the catalytic degradation of 50% of RhB dye in aqueous solution. 3. Results and discussion

2.4. Photocatalytic study 3.1. Characterization The discoloration potential of BSCO catalyst was assessed through the degradation of Rhodamine B (RhB) dye, as a probe reaction, under UVA (365 nm), sunlight (SL) irradiation and dark conditions. In a typical process, 200 mg of photocatalyst was dispersed in RhB solution (200 mL, 20 mg L
1) in quartz cell tube. The suspension pH value was previously adjusted at 7 using NaOH/ H2SO4 solutions and (Hanna HI 210) pH meter. Prior to degradation experiments, the suspension was stirred for 30 min in the dark at 298 K using magnetic stirrer (SpeedsafeTM Hanna) to ensure the establishment of adsorption/desorption equilibrium between the photocatalyst and RhB substrate. The sample was then irradiated at 298 K using 6 W ultraviolet (l = 365 nm, BLX-E365) photoreactor with long wave UV light (UV-A) extending from 320 to 380 nm with an energy peak at 365 nm. Long wave is also called “black light”. Black light (BL): lamp producing 365 nm ultraviolet light with visible light. Spacious UVA exposure chamber in stainless steel: 14.5  36  35 cm, and external dimensions: H = 30.5 cm, D = 36 cm and W = 35 cm) or solar light radiation (SL). At regular time intervals, the samples were collected and centrifuged using centrifuge (EBA-Hetlich) at 3500 rpm for 15 min to completely remove catalyst particles. The residual RhB concentrations during degradation were determined based on absorption at 554 nm as measured with UV mini-1240 spectrophotometer. The effect of some parameters such as the volume of hydrogen peroxide ((v (H2O2) = 0–1 mL)), the temperature of the dye solution (T = 298– 333 K), the various oxidations processes, and the nature of irradiation upon catalytic performance was investigated. The discoloration efficiency of the catalyst was calculated by the following equation Eq. (2):   C
Cf h0 ð%Þ ¼ i ð2Þ 100 Ci where Ci: dye initial concentration (mg L
1) and Cf: dye residual concentration during the reaction (mg L
1). According to the Planck’s law and some further calculation, the relationship between the absorption wavelength of the photoreactor and the band gap value can be determined by Eq. (3): Eg ¼

hC

l

¼

1239

l

ð3Þ

3.1.1. XRD analysis The X-ray diffraction patterns of Bi1.56Sb1.48Co0.96O7 (BSCO) compound are shown in Fig. 1. It could be seen from Fig. 1 that the full-profile structure refinements of the collected data were also obtained by the the Dicvol program implemented in Fullprof program [42]. The cell parameters of synthesized BSCO sample are showed in Table 2. The X-ray diffraction patterns revealed that BSCO compound was single phase. BSCO was found to be pyrochlore-type structure and a cubic crystal system with a space group Fd3m. The cell parameter a was found to be 10.4238 Å. Crystal size calculation from the broadening of the main peak (2 2 2) using the Scherrer formula revealed that BSCO exhibited smaller crystallite size (76.29 nm vs 93.14 nm) than that reported in the previous work [41]. This highlight is in accordance with the fact that, by solid state route, BSCO compound is undergoing sintering due to higher thermal treatment (1273 K) and/or longer calcinations time, casing a high crystallization degree and severe loss in the effective surface area. From the above results, it is clear that two requirements, namely, a large surface area and reasonable crystallinity degree are wholly satisfied by heat treatment at 1273 K for 5 days only. However, these requirements are in general conflict with each other, because the crystallinity increases with the heat treatment temperature and/or extended heat treatment time, while the surface area decreases. Several studies have shown that the crystallinity is improved with the calcination of the particles leading to increased photocatalytic degradation efficiency. Otherwise, numbers of papers have shown that most of pyrochlore-type oxides synthesized via solid-state reaction method, have given smaller specific area than that prepared by sol–gel method [43–47]. 3.1.2. SEM analysis As shown in Fig. 2, the morphological observations by SEM reveal that the external area of the BSCO pyrohclore pellet sintered at 1273 K for 24 h is irregular shaped morphology, and both large and small grains were observed, indicating an un-homogeneous grain growth taken place in this sample. The grain size distributions are in the range of 0.8–10 mm.

where h is Planck’s constant (4.13566733  10
15 eV s); C is the speed of light (2.99792458  1017 nm s
1) and l is the UVA-light wavelength (320–380 nm). From the calculation, in order to absorb a UVA-light wavelength, the band gap value of the photoreactor has to be below 3.87 eV and above 3.26 eV. The discoloration efficiency of BSCO for RhB dye was quantified by measurement of dye apparent first order rate constants under operating parameters Eq. (4): log

C0 ¼ K app t C

ð4Þ

where Kapp is the apparent pseudo-first order rate constant, C and C0 are the concentrations at time ‘t’ and ‘t = 0’, respectively. The plot of ln C0/C against irradiation time t should give straight lines, whose slope is equal to Kapp. The half-life of dye degradation at various process parameters was raised from Eq. (5): t1=2 ¼

0:693 K app

ð5Þ

Fig. 1. XRD pattern of BSCO pyrochlore compound prepared at 1273 K for 5 days. Inset; XRD pattern for (2 2 2) diffraction peak.

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B. Naceur et al. / Materials Research Bulletin 74 (2016) 491–501

Table 2 The crystal structure parameters and crystallite size of the BSCO pyrochlore compound. Photocatalysts

2u ( )

FWHM (radian)

Crystallite size D (nm)

Cell parameter (Å)

Cell volume (Å)3

Reaction time (day)

Reference

Bi1.56Sb1.48Co0.96O (BSCO)

29.695 29.672

0.00154 0.00188

93.14 76.29

10.4489 10.4238

1140.825 1132.610

11 5

[32] This work

calculated using the Tauc approach Eq. (6) [49,50]:

ahv ’ Aðhv
Eg Þn=2

Fig. 2. SEM image of BSCO pyrochlore compound prepared at 1273 K for 5 days and sintered at 1273 K for 24 h.

where a, h, n, Eg and A are the linear absorption coefficient, Planck’s constant, light frequency, band gap energy of the material and a constant involving properties of the bands, respectively. The exponent n depends on the type of transition between the semiconductor bands. The values of n for directly allowed, directly forbidden, indirectly allowed, or indirectly forbidden transition are n = 1, 2, 3 and 4, respectively. By applying n = 4, the indirect band gap (Eg) of the as-prepared photocatalyst is determined from the plot of (ahn)1/2 versus hn, as indicated in Fig. 4. By extrapolating the straight line to the x-axis in this plot, the Eg value of BSCO was found to be 1.50 eV. The estimated value indicates that BSCO possessed narrower band gap and the optical transition for BSCO was indirectly allowed [51]. This value can be attributed to the ligand-metal charge transfer transition O(II) ! Co(II), which is in good agreement with the reported literature [52,53]. Furthermore, we can calculate the valence band (VB) and conduction band (CB) positions of BSCO sample through the following empirical formulas Eq. (7) and Eq. (8) 1 EVB ¼ x
Ee þ Eg 2

ð7Þ

ECB ¼ EVB
Eg

ð8Þ

where xis the absolute electronegativity of the atom semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. Herein, the electronegativity of an atom is the arithmetic mean of the atomic electron affinity and the first ionization energy; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap of the semiconductor; ECB is the conduction band potential and EVB is

10

1.75

8

1.55

6

(αhν)1/2

Absorbance (a. u.)

3.1.3. UV–vis-DRS analysis Fig. 3 shows UV–visible diffuse reflectance spectrum for BSCO sample recorded in the wavelength range of 200–800 nm. As can be seen, the spectrum is characterized by four broad bands at 250, 380, 540, 720 nm and a shoulder at 420 nm. The first band from 250 to 350 nm involves the charge transfer transitions O2– ! Co2+ and O2– ! Co3+ and the Co(III) is in an octahedral site. According to the literature [48] the absorption bands at 380 and 540 nm are assigned to Co(II) ions in tetrahedral coordination. The appearance of a band at 720 nm can be associated to Co(III) in octahedral environment. The shoulders at 420 nm and at longer wavelength region (>551 nm) indicate the presence of Co (II) species in octahedral coordination. The observed absorption bands (540 and 720 nm) give the first notation that the BSCO photocatalyst can be employed as a visible light responsive photocatalyst. The UV–visible absorption spectrum of the BSCO compound was taken and subsequently its optical band gap was

1.35

ð6Þ

4

Eg=1.50 eV

1.15

2 0.95

0 0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.75 200

300

400

500

600

700

800

Wavelength ʎ (nm) Fig. 3. UV–visible diffuse reflectance spectrum for BSCO pyrochlore compound.

hν (eV) Fig. 4. Plot of (ahn)1/2 against (hn) from the UV–vis diffuse reflectance for BSCO pyrochlore compound. In the inset, the band gap value is described.

B. Naceur et al. / Materials Research Bulletin 74 (2016) 491–501 Table 3 Band gap (Eg), conduction band (ECB), and valence band (EVB) potentials potential of BSCO pyrochlore compound. Photocatalyst Bi1.56Sb1.48Co0.96O7 (BSCO)

Table 4 Kinetics parameters for the (BSCO/H2O2/UVA) system at various temperature of dye solution ([BSCO] = 1 g L
1, [RhB] = 20 mg, v(H2O2) = 1 mL, lUVA = 365 nm and pH 7).

Absorption edge l (nm)

Eg (eV)

ECB (eV)

EVB (eV)

Temperature medium T (K)

h0

K1 (min
1)103

t1/2 (min)

R2

(%)

827

1.50

+1.06

+2.56

298 313 333

76.97 91.45 91.7

24 45 41

28.76 15.57 16.91

0.99 0.97 0.93

the valence band potential. The xvalue for BSCO is ca.6.31 eV. According to the above equations (Eqs. (7) and (8)), the top of the VB is found to be +2.56 eV, as well as the bottoms of the CB is calculated to be +1.06 eV. The calculated CB and VB edge potentials for BSCO compound are shown in Table 3. 4. Photocatalytic processes 4.1. Photocatalytic processes under UVA light 4.1.1. Effect of temperature of the dye solution The influence of temperatures of the dye solution on the discoloration of RhB was investigated under UVA-light irradiation between 298 and 333 K. As shown in Fig. 5 and Table 4, an increase in temperature up to 313 K induces the collision frequency of molecule in solution [54] and promotes the mass transfer of the organic contaminants to the bulk solution as well as helps the reaction to compete more efficiently with e
/h+ recombination [55]. Further increase in temperature beyond 313 K, no significant change was found for the discoloration efficiency after 60 min of irradiation time. According to Eq. (4), the linear plot of Ln C0/C against irradiation time t (Fig. 6) is a straight line with a slope equal to Kapp. The firstorder rate constants are shown in the Table 4, where regression coefficients values (R2 = 0.93–0.99), confirm that the dye discoloration efficiency of BSCO/H2O2 catalytic system under UVA-light irradiation at various temperatures of the dye solution, follows apparently a pseudo first-order kinetics model.

The rate constants were calculated to be 24  10
3, 45  10
3 and 41 10
3 min
1 for temperatures of 298, 313, and 333 K, respectively. 4.1.2. Effect of H2O2 volume In order to find out the optimal H2O2 volume for the discoloration of RhB, the catalytic oxidation runs was carried out for different H2O2 concentrations ranging from 0 to 1 mL. As shown in Fig. 7, the photocatalytic efficiency of BSCO without any addition of H2O2, nearly 1.07 % RhB was degraded under UVA-light illumination within 60 min. This may be attributed to the saturation effect due to the high doping concentration of cobalt in the BSCO catalyst matrix, might work as recombination centers for the photo-induced electrons and holes, thus significantly reducing the discoloration efficiency [56]. However, after addition of 0.25 and 0.5 mL of H2O2, the discoloration efficiency achieved to 72. 98 and 97.77% under UVA-light irradiation within 60 min, respectively. Based on the above results, the H2O2 is considered to have a double function in the photocatalytic oxidation process. It could act as an alternative electron acceptor to oxygen, thereby enhancing the rate of photocatalytic oxidation Eq. (9). H2O2 + e
! OH + OH


(9)

Furthermore, peroxide oxygen also produces hydroxyl radicals (OH) via photodecomposition, and it inhibits the e
and h+ recombination according to the following equations Eqs. (10) and (11). In aqueous solution, ultraviolet radiation above 290 nm splits hydrogen peroxide into OH and suggested that this reaction could be an important source of hydroxyl radical.

1

RhB/BSC/H2O2 (1 ml)/UVA T=298 K RhB/BSC/ H2O2 (1ml)/UVA

T=313 K

RhB/BSC/H2O2(1ml) /UVA

T=333 K

0.8

Time (min) 0 0

0.6

10

20

30

40

50

60

-0.5

0.4

Ln (C/C0)

Redueced concentration (C/C0)

495

0.2

0 0

10

20

30

40

50

-1

-1.5

60

Time (min) RhB/BSC/H2O2 (1 ml)/UVA T=298 K RhB/BSC/ H2O2 (1ml)/UVA T=313 K RhB/BSC/H2O2(1ml) /UVA T=333 K Fig. 5. The effect of temperature of dye solution on the discoloration kinetics of RhB by using (BSCO/H2O2/UVA) system ([BSCO] = 1 g L
1, [RhB] = 20 mg L
1, v(H2O2) = 1 mL, lUVA = 365 nm and pH 7).

-2

-2.5 Fig. 6. The plot of ln(C/C0) versus irradiation time (t) for the (BSCO/H2O2/UVA) system at various temperature of dye solution ([BSCO] = 1 g L
1, [RhB] = 20 mg L
1, v (H2O2) = 1 mL, lUVA = 365 nm and pH 7).

496

B. Naceur et al. / Materials Research Bulletin 74 (2016) 491–501 Table 5 Kinetics parameters at various volume of H2O2 ([BSCO] = 1 gL-1, [RhB] = 20 mgL-1, T = 298 K, lUVA = 365 nm and pH 7).

0.8

0.6

h0

K1 (min
1)103

t1/2 (min)

R2

(%)

0 0.25 0.5 1

1.07 72.98 97.77 76.97

0.2 20.8 65.2 24.1

3465.74 33.32 10.63 28.76

– 0.99 0.99 0.99

0.4

0.2

0 0

10

20

30

40

50

60

Irradiation time (min) RhB/BSCO/ H2O2 (0 ml)/UVA RhB/BSCO/H2O2 (0,25 ml)/UVA RhB/BSCO/H2O2 (0,5 ml)/UVA RhB/BSCO/H2O2 (1 ml)/UVA

H2O2 + hv ! 2OH

(10)

H2O2 + O2
! OH + OH
+ O2

(11)

In the other hand, when the volume of H2O2 was further increased to 1 mL, approximately 76.97% RhB discoloration was achieved within 60 min. This fact may be explained by the scavenging effect of hydroxyl radicals upon increasing the volume of H2O2, generating perhydroxyl radicals (HO2) (Eq. (12)). Actually HO2 is less strong oxidant as compared to hydroxyl radical (OH). Therefore the rate of discoloration of dye decreases when the volume of H2O2 is increased beyond 0.5 mL. (12) H2O2 + OH ! HO + HO2 For this reason, the volume of H2O2 of 0.5 mL was selected for subsequent experiments. Plotting ln (C/C0) against irradiation time t (Fig. 8) gives the apparent rate constant for discoloration of RhB from the slope of curve fitting line, and the intercept is equal to zero. Table 5

RhB/BSCO/ H2O2 (0 ml)/UVA RhB/BSCO/H2O2 (0,25 ml)/UVA RhB/BSCO/H2O2 (0,5 ml)/UVA RhB/BSCO/H2O2 (1 ml)/UVA

Time (min) 0 0

10

20

30

40

displayed the first-order rate constants. The straight line relationship between ln (C/C0) and t indicates that the photocatalytic discoloration of RhB by BSCO/H2O2 system at various volume of H2O2 also follows the pseudo first-order model with regression coefficients R2 higher than 0.99. The apparent rate constants were calculated to be 0.2  10
3, 20.8  10
3, 65.2  10
3 and 24.1 10
3 min
1 for 0, 0.25, 0.5 and 1 mL of H2O2, respectively. 5. Effect of the various oxidations systems

Fig. 7. Discoloration kinetics of RhB by using the (BSCO/H2O2/UVA) system at various volume of H2O2 ([BSCO] = 1 g L
1, [RhB] = 20 mg L
1, T = 298 K, lUVA = 365 nm and pH 7).

50

60

-0.5 -1

Ln (C/C0)

v (H2O2) (mL)

-1.5

5.1. Discoloration of RhB in different oxidations systems under UVA light The discoloration of RhB dye was undertaken under different oxidations systems that include (i) RhB dye solution in the presence of UVA-light and sunlight (RhB self-photolysis process), (ii) mixture of RhB dye solution and H2O2 in the absence of UVAlight and sunlight (dark oxidation process), (iii) mixture of RhB dye solution and H2O2 in the presence of UVA-light and sunlight (homogeneous photolysis process), (iv) reaction mixture of RhB dye solution and BSCO/H2O2 system catalytic in the absence of UVA-light and sunlight (dark adsorption/oxidation process) and (v) RhB dye solution in the presence of the BSCO/H2O2 catalytic system under irradiation of UVA-light and sunlight (BSCO/photo-oxidation process) and the results are depicted in Fig. 9 and Table 6. The discoloration of RhB was not observable for both direct photolysis and photo catalysis processes (1.07%). However, a maximum discoloration efficiency of 92.86 and 97.77% was obtained with homogeneous photolysis process (H2O2/UVA) and BSCO/photooxidation process (BSCO/H2O2/UVA system) within 60 min of irradiation time, respectively, which was approximately 4 times 1

Reduced concentration (C/C0)

Reduced concentration (C/C0)

1

0.8

0.6

0.4

0.2

0 0

-2 -2.5 -3 -3.5

10

20

30

40

50

60

Time (min) RhB/H2O2 (0.5 mL)/25°C RhB/H2O2 (0,5 mL)/UVA/25°C RhB/BSCO/H2O2 (0.5 mL)/25°C RhB/UVA /25 °C RhB/BSCO/25°C RhB/BSCO/UVA/25°C RhB/BSCO/H2O2 (0,5 mL)/UVA

-4 Fig. 8. The plot of ln(C/C0) versus irradiation time (t) at various volume of H2O2 ([BSCO] = 1 g L
1, [RhB] = 20 mg L
1, T = 298 K, lUVA = 365 nm and pH 7).

Fig. 9. Discoloration kinetics of RhB by various oxidations systems with or without UVA light ([BSCO] = 1 g L
, [RhB] = 20 mg L
1, v(H2O2) = 0.5 mL, T = 298 K, lUVA = 365 nm and pH 7).

B. Naceur et al. / Materials Research Bulletin 74 (2016) 491–501 Table 6 Kinetics parameters of the discoloration of RhB at various oxidation systems under or without UVA light ([BSCO] = 1 gL-1, [RhB] = 20 mgL-1, v(H2O2) = 0.5 mL, T = 298 K and pH 7).

RhB/UVA/25  C RhB/H2O2 (0.5 mL)/UVA/25  C RhB/BSCO/UVA/25  C RhB/BSCO/H2O2 (0.5 mL)/UVA/25  C RhB/BSCO/H2O2 (0.5 mL)/25  C RhB/H2O2 (0.5 mL)/25  C RhB/BSCO/25  C

h0

K1 (min
1)

t1/2 (min)

R2

(%) 0 92.86 1.07 97.77 54.2 26.54 9.96

– 0.042 0.0002 0.065 0.015 0.0069 –

– 16.503 3465.74 45.15 46.209 100.456 –

– 0.99 0.58 0.98 0.34 0.47 –

RhB/H2O2 (0,5 mL)/UVA/25°C RhB/BSCO/UVA/25°C RhB/BSCO/H2O2 (0,5 mL)/UVA RhB/H2O2/25°C RhB/BSCO/H2O2 (0.5 mL)/25°C

Time (min)

higher than that observed for dark oxidation process (H2O2) (26.54% system), and 2 times higher than RhB adsorption catalyzed by BSCO/H2O2 system (54.2%). Such an enhancement in the discoloration efficiency in both H2O2/UVA and BSCO/H2O2/UVA systems was primarily attributed to the synergetic effect between active radical species from catalytic decomposition of H2O2 by BSCO powder (dark conditions) Eq. (13) and direct oxidation of RhB by OH radicals Eq. (15), generated from direct photolysis of H2O2 Eq. (14). The efficiency of 54.2% observed in the BSCO/H2O2 system can be explained by the ability of the BSCO catalyst to decompose H2O2 into active radicals (HO, HOO, or O2
). The similar trend was observed when H2O2 was added (as source of free radicals) with MnO2 oxide embedded polymer composite catalyst in the oxidative degradation of dyes [57]. Metal oxides like CeO3 doped Fe2O3/gAl2O3 and Fe2O3/carbon are known as efficient catalysts in the catalytic wet peroxide oxidation method for dyes removal [58,59].

0 0

10

20

30

40

50

60

-0.5 -1

Ln (C/C0)

Photocatalytic systems

497

-1.5 -2 -2.5 -3 -3.5 -4

Fig. 10. ln(C/C0) against irradiation time during the discoloration of RhB by various oxidations systems with or without UVA light ([BSCO] = 1 g L
1, [RhB] = 20 mg, v (H2O2) = 0.5 mL, lUVA = 365 nm, T = 298 K and pH 7).

Eq. (20). The possible reactions involved in the photooxidation are illustrated below (Fig. 10): BSCO + hv(UVA) ! BSCO(e
BC + h+BV)

(16)

H2O + h+!OH

(17)

BSCO + H2O2 ! Active radicals

(13)

R + h+ ! R+

(18)

H2O2 + hv(UVA)!2OH (photolysis)

(14)

RhB + (OH, R+) ! degraded or mineralized products

(19)

RhB + (OH, H2O2, active radical species) ! degraded or mineralized products (15) All these combined processes are able effectively to produce active radicals which can participating in the degradation of RhB Eq. (15). Hence the sequential discoloration efficiency was conducted as follows;

hBSCO/H2O2/UVA > hH2O2/UVA > hBSCO/H2O2 > hH2O2 > hBSCO/UVA > hselfphotolysis

According to the band edge position (Table 3), although, under UVA (lUVA = 320–380 nm) light irradiation, the energy of the excitation light is sufficient to directly excite the BSCO (l = 827 nm ! Eg = 1.5 eV), electrons (e
) in the VB can be excited to the CB with simultaneous generation of the same amount of holes (h+) in the VB Eq. (16). The CB edge potential of BSCO (+1.06 eV/NHE) is more positive than that for superoxide radical (
0.28 V/NHE) and than that required for H2O2 splitting reaction (+0.06 eV). Hence photoinduced electron in the CB cannot reduced O2 into O2
as well as split H2O2 into OH. However, the VB edge potential of BSCO (+2.56 eV/NHE) is more active than those of hydroxyl radical (+1.9 V/NHE) and organic specie (+1 V/NHE); thus photoproduced holes in the VB are able to directly oxidize them into hydroxyl radicals (OH) Eq. (17) and cationic radical R+ Eq. (18). Generated radicals have effectively the ability to degrade organic molecules Eq. (19). The impact of recombination of photo-induced electron– hole (e
–h+) pairs is the possible main reason for the less catalytic efficiency of low band gap BSCO sample with large grain size

BSCO (e
BC + h+BV) ! BSCO (Recombination)

(20) 2

The plot of ln (C/C0) versus t gives a straight line with R values (higher than 0.98) suggesting that the RhB photo-discoloration follows a pseudo-first-order kinetic mechanism. The first-order rate constants are displayed in the Table 6. 5.2. Discoloration of RhB in different oxidations systems under sunlight (SL) 5.2.1. Quantitative study Fig. 11 presents the discoloration of RhB dye in various oxidations systems under sunlight (SL). The order of discoloration efficiency showed similar trend as observed in Table 6 (under UVAlight condition), and both homogeneous photolysis (H2O2/SL system) and BSCO/photo-oxidation (BSCO/H2O2/SL system) processes exhibited higher efficiency, as a result of 76.00 and 90.16% of RhB discoloration within 60 min of irradiation time. However, in BSCO/SL catalytic system, insignificant discoloration efficiency of RhB was observed. According to the UV–vis-DRS results, the BSCO with narrow band gap of 1.50 eV, corresponding to irradiation of the visible range (827 nm) and RhB molecule, which is only effective under visible irradiation, absorbs at 554 nm. Upon irradiation by sunlight (SL), BSCO particles will form a paired electron (e
) and hole (h+) in the CB (+1.06 eV/NHE) and VB (+2.56 eV/NHE), respectively Eq. (21). Simultaneously, electrons in adsorbed RhB-homo (+0.95 V/NHE) are excited to RhB*-lumo (
1.42 V/NHE) (Eq. (22)). According to the band edge position, the

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of any direct contact between the photons and immobilized BSCO [60]. The dye sensitized with electron–hole recombination mechanism was illustrated below:

Reduced concentration (C/C0)

1

0.8

0.6

0.4

0.2

BSCO + hn (sunlight) ! BSCO (e
BC + h+BV)

(21)

RhB + hn (sunlight) ! RhB

(22)

RhBads + BSCO (e
BC) ! RhB + + BSCO (e
BC)

(23)

0 0

10

20

30

40

50

60

Time (min) RhB/SL/25°C RhB/H2O2 (0,5mL)/SL/25°C RhB/BSCO/SL/25°C RhB/BSCO/H2O2 (0,5 mL)/25°C RhB/BSCO/H2O2 (0,5 mL)/SL/25°C RhB/H2O2 (0.5 mL)/25°C RhB/BSCO/25°C Fig. 11. Discoloration kinetics of RhB by various oxidations systems under or without SL irradiation ([BSCO] = 1 g L
1, [RhB] = 20 mg L
1, v(H2O2) = 0.5 mL, T = 298 K and pH 7).

electrons are injected from adsorbed RhB*-lumo into CB of an adjacent BSCO particles, thus generating dye cationic radical RhB+ (Eq. (23)). All generated active radical species are able to decompose RhB molecules into degraded or mineralized products Eq. (24). However, the discoloration efficiency of BSCO alone under SL irradiation is strongly affected by the high recombination rate Eq. (25). This fact in accordance with the high cobalt content in the BSCO photocatalyst matrix, as pointed out in Section 4.1.2.The chemical structure of the dye indicates that RhB has more complex structure, making it less photodegradable. The absorption of light photon by dye itself leading to a less availability of photons for hydroxyl radical generation. The strong absorption of light by the dye molecules is thought to have an inhibitive effect on the photogeneration of holes or hydroxyl radicals, because of the lack

RhB + (OHads and RhB+ads) ! degraded or mineralized products (24)

BSCO (e
BC + h+BV) ! BSCO recombination

(25)

This enhancement in discoloration performance is primary ascribed to the synergetic effect of direct oxidation of RhB by H2O2, free active species from H2O2 catalytic decomposition by BSCO powder (dark conditions) Eq. (27), hydroxyl radicals generated from direct photolysis of H2O2 Eq. (26) and by dye sensitized with electron–hole recombination mechanism Eqs. (21)–(25). All generated active species are able to degrade RhB molecules into less toxic molecules or mineralized products Eq. (28). H2O2 + hn ! 2OH (photolysis)

(26)

BSCO + H2O2 ! Active radicals

(27)

RhB + (OHads, H2O2, active radical species, RhB+ads) ! degraded or mineralized products (28) Hence the sequential discoloration efficiency was conducted as follows;

Scheme 1. Heterogeneous photo Fenton process with dye sensitized mechanisms of RhB by Bi1.56Sb1.48Co0.96O7 photocatalyst.

B. Naceur et al. / Materials Research Bulletin 74 (2016) 491–501

554

RhB/H2O2 (0,5mL)/SL/25°C RhB/BSCO/SL/25°C RhB/BSCO/H2O2 (0,5 mL)/SL/25°C RhB/H2O2 (0.5 mL)/25°C RhB/BSCO/H2O2 (0.5 mL)/25°C

553

10

20

30

40

50

60

Ln (C/C0)

-0.4

-0.9

Wavelength ʎ (nm)

552

Time (min) 0

499

551 550 549 548 547 546 545 544

-1.4

0

10

20

30

40

50

60

Time (min) RhB/BSCO/H2O2 (0,5 mL)/SL/25°C RhB/SL/25°C RhB/H2O2 (0,5mL)/SL/25°C RhB/BSCO/SL/25°C RhB/BSCO/H2O2 (0,5 mL)/25 °C

-1.9

-2.4 Fig. 12. ln(C/C0) against irradiation time during the discoloration of RhB by various oxidations systems under or without SL irradiation ([BSCO] = 1 g L
1, [RhB] = 20 mg L
1, v(H2O2) = 0.5 mL, T = 298 K and pH 7).

Fig. 13. Variation of the wavelengths of RhB solution at different experimental conditions ([RhB] = 20 mg L
1, [catalyst] = 1 g L
1, pH 7, T = 298 K, lmax = 365 nm and irradiation time 60 min).

hBSCO/H2O2/SL > hH2O2/SL > hBSCO/H2O2 > hH2O2 > hBSCO/SL > hself-photolysis Based on what has been discussed above in our photodegradability, heterogeneous photo Fenton process with dye sensitized mechanisms of RhB by Bi1.56Sb1.48Co0.96O7 compound is proposed and illustrated in Scheme 1. The linear relationship between ln (C/C0) vs irradiation time (t) (Fig. 12) indicates that data were well fitted to a pseudo-first-order kinetic model with high regression coefficient (R2 > 0.99) for H2O2/ SL and BSCO/H2O2/SL catalytic systems only. Table 7 displays the first-order rate constants of RhB discoloration at various oxidations systems under sunlight (SL). 5.2.2. Qualitative study Rhodamine B (N,N,N',N0 -tetraethylrodamine) belongs to the oxygen-containing heterocyclic xanthene dyes family. It could be seen from Table 1, that there is an absorption band at (554) nm, which originates from the p ! p* transitions from the binding HOMO (highest occupied molecular orbital) to the anti-binding LUMO (lowest unoccupied molecular orbital) along the longest dimension of the conjugated system. However, the shoulder around (521 nm) nm is usually ascribed to the dimmer. The bands below 450 nm represent the transitions to the mesomeric limit structures with shorter conjugation units and originate from n ! p* transitions from the NHOMO (next highest occupied molecular orbital) to the LUMO [61]. Under visible light, and in the presence of an appropriate semiconductor, rhodamine B degrades either via an efficient Ndeethylation sensitization mechanism or by a photocatalytic mechanism [62]. Characteristic differences in the mechanism of Table 7 Kinetics parameters of the discoloration of RhB at various oxidation systems under or without SL irradiation ([BSCO] = 1 gL-1, [RhB] = 20 mg, v(H2O2) = 0.5 mL, T = 298 K and pH 7). Photocatalytic systems

h0 (%)

K1 (min
1)

t1/2 (min)

R2

RhB/SL/25  C RhB/H2O2 (0.5 mL)/SL/25  C RhB/BSCO/SL/25  C RhB/BSCO/H2O2 (0.5 mL)/SL/25  C RhB/BSCO/H2O2 (0.5 mL)/25  C RhB/H2O2 (0.5 mL)/25  C RhB/BSCO/25  C

0 76 0.21 90.16 54.2 26.54 9.96

– 0.023 0.0002 0.04 0.015 0.0069 –

– 30.1368339 3465.74 17.3286795 46.20981204 100.4561131 –

– 0.99 0.58 0.99 0.34 0.47 –

RhB discoloration during SL irradiation under different experimental conditions were monitored by absorption spectrophotometry and the results are shown in Fig. 13. As shown in Fig 13, the irradiated RhB solution in the BSCO/H2O2 system depicted an absorption maximum wavelength shift slightly from 554 to 545 nm with light irradiation time. On the other hand, for both catalytic processes (self-RhB photolysis and BSCO/H2O2 systems), the absorption maxima blue shifted from 554 to 547 nm as the light irradiation time was increased up to 30 min, whereas, no change was observed in the bands maxima beyond 30 min. It is known that the N-deethylation derivative (s) of RhB from the aromatic rings causes a significant blue shift in the maximum absorption of the dye. The N-deethylation products of RhB such as triethylrhodamine, diethylrhodamine, ethylrhodamine and rhodamine correspond to an optical absorption maximum, lmax, at 555, 539, 522, 510 and 498 nm, respectively [62]. The slight blue shift in the maximum absorption of the RhB dye under various oxidation systems did not exceed 10 nm. Such observed blue shifted absorption maximum could be attributed possibly to the change in the environmental polarity/acidity of BSCO surface with respect to water surrounding, suggesting that the phtocatalytic process is spontaneous. The BSCO photocatalyst used by us in the present work, which has an appropriate band gap (Eg = 1.50 eV), tend to degrade RhB under sunlight (SL) irradiation, not by a Ndeethylation sensitization mechanism but via a photocatalytic mechanism which is manifested by a direct attack of OH radicals in the bulk solution mainly at the aromatic chromophore ring, leading to the destruction of RhB structure. This behavior was explained by the lack of interaction affinity between this photocatalyst and the RhB ions, which prevents charge transport from the dye to the photocatalyst. Similar result has been noticed for the photodecomposition of RhB dye in the presence of Nb2O5 (Eg = 3.4) nanoparticles [63], Bi2O3 (Eg = 2.85 eV) and BiVO4 (Eg = 2.44 eV) [64] under visible light irradiation. The depletion of the absorbance happened without a shift of the absorption maximum to blue region, indicating that the main mechanism for RhB photodegradation occurs by OH radicals attack on the RhB chromospheres. This statement was clearly demonstrated in another study of visible-light photocatalytic degradation of methyl orange (MO) on carbon-doped TiO2 and on Pt/WO3 [65]. Here, degradation of MO dye on the Pt/WO3 photocatalyst did not yield any

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Table 8 Comparison of photocatalytic activity of some photocatalysts with the present photocatalyst for the degradation of Rhodamine B (RhB). Typical photocatalytic systems

Photocatalytic performance h0 (%)

Reference

Nb2O5/photo-Fenton hybrid process (Fe2+/H2O2/Nb2O5/UV) TiO2 nanoparticles Nb2O5 nanoparticles Nb2O5 nanoparticles TiO2 coated activated carbon H3PW12O40/SiO2 sensitized by H2O2 UV/S2O8 2
system Cr-codoped TiO2 Bi2Sn2O7 Ti/TiO2–NiO photoanode Gd2InSbO7 and Gd2FeSbO7 Nanosized pyrochlore Y2Ti2O7
xNy Graphene/Bi2WO6 RGO/Bi2WO6 QDs Graphene/Bi2O2CO3 Bi2MoO6/carbon nanofibers (CNFs) Bi2O2CO3/Bi2MoO6 Bi2O2CO3/BiOI BiOI/ZnTiO3 Bi2MoO6/Bi3.64 Mo0.36O6.55 Ag/AgCl/BiOCl Ag/AgBr/BiOBr Er3+-doped-Bi2O3 Ag2CO3 nanorods K0.51Sb2.67O6.26 LaFeO3 nanosheets Fe2O3/ BiVO4 porous nanoplates g-C3N4/MIL-125(Ti) heterostructures Sb-doped (BiO)2CO3 nanoplates g-C3N4 (graphitic carbon nitride)/rGO (reduced graphene oxide)-800 Ag3PO4 Ag3PO4@AgCl Biosynthesized SnO2 NPs Heterogeneous photo Fenton process (H2O2/BSCO/UVA) (H2O2/BSCO/Sunlight)

78 99.7 >99.7 43 82.21 97.7 85 98 97.8 96.3 86.7 66 100 100 74 87 100 100 38 100 95 100 68 94 53 90.66 100 95.2 96.7 90 80 100 95 97.77 90.16

[60] [67

intermediate products that absorbed light in the visible part of the spectrum. An analogous degradation pathway has been reported in the efficient oxidation of Congo red azo-dye over 20 wt% SrO– CuBi2O4 composite photocatalyst under UVA-light irradiation [66]. The photocatalytic performance of the BSCO nanoparticles is better or comparable with other typical photocatalysts for the degradation of RhB (Table 8). 6. Conclusion In Summary, novel nanostructure pyrochlore Bi1.56Sb1.48Co0.96O7 has been successfully synthesized via improved solid state reaction and characterized by XRD, SEM and UV–vis DRS techniques to be assessed as potential photocatalyst for the degradation of Rhodamine B (RhB) dye, as a probe reaction, under UVA- and SL-light irradiations. The results showed that the BSCO was single phase and crystallized in a cubic pyrochlore-type structure with space group Fd3m. The cell parameter a was found to be 10.4238 Å. The average particle size and band gap for BSCO were D = 76.29 nm and Eg = 1.50 eV respectively. Under the optimum conditions for discoloration of the dye: initial concentration of 20 mg L
1, RhB, 0.5 mL H2O2, BSCO/dye mass ratio of 1 g L
1, pH 7 and 25  C, 97.77 and 90.16% of RhB were degraded with BSCO/H2O2 photocatalyst system within 60 min of irradiation under UVA- and SL-light respectively. Pseudo-second-order kinetic model gave the best fit, with highest correlation coefficients (R2  0.99). The catalytic efficiency observed in the dark conditions is due to the synergetic effect between direct oxidation of RhB by H2O2 and active radicals from BSCO catalyzed H2O2 decomposition. The low photocatalytic efficiency of BSCO alone under UVA and SL-light

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