Bi2O3-BiFeO3 Glass-Ceramic: Controllable β-/γ-Bi2O3 Transformation and Application as Magnetic Solar-Driven Photocatalyst for Water Decontamination.

Glass and glass-ceramic materials containing photoactive and magnetic crystalline phases were prepared from Fe2O3 and Bi2O3 using the conventional melt method. All samples were characterized in terms of formed phases, morphological analyses, optical properties, and magnetic properties. Formation of the photoactive tetragonal β- and body-centered cubic γ-Bi2O3 phases along with the magnetic BiFeO3 and Fe3O4 phases was revealed. However, the crystalline structure relied on the composition and the applied heat-treatment time. β-/γ-Bi2O3 transformation could be controlled by the heat-treatment time. The samples exhibited variable magnetic properties depending on their composition. All of the samples showed excellent absorbance in visible light with an optical band gap of 1.90-2.22 eV, making them ideal for solar-light-driven photocatalysis. The best performance was recorded for the sample containing equal amounts of Fe2O3 and Bi2O3 due to the formation of γ-Bi2O3/BiFeO3 heterojunction in this sample.

Introduction

Water quality has been deteriorating and became the main issue in many countries. In an effort to combat the problem of water pollution, rapid and significant progress in water/wastewater treatment has been made, including photocatalytic oxidation.[1] However, the practical application of photocatalytic oxidation is usually restricted by two main factors, namely, photocatalyst separation/recovery and the utilization of costly commercial lamps. A plausible solution to overcome these obstacles is to use magnetically separable solar-driven photocatalyst, such as bismuth ferrite (BiFeO3).

BiFeO3 belongs to multiferroic materials possessing electric, magnetic, and structural order parameters that yield simultaneous effects of ferroelectricity, ferromagnetism, and ferroelasticity in the same material.[2] BiFeO3 has magnetic properties and has the highest ferroelectric polarization.[3] Usually, BiFeO3 present in a rhombohedrally distorted perovskite (ABO3) structure with the R3c space group. It was demonstrated as a fairly attractive visible-light photocatalyst for the photo-organic pollutant decomposition because of its narrow band gap energy (2.2 eV), superior magnetic properties, and excellent chemical stability.[4] BiFeO3 nanoparticles were used in the photocatalytic degradation of different dyes like methyl orange, rhodamine B, methylene blue, congo red, and reactive black-5 as summarized by Ponraj et al. (2017).[5] However, BiFeO3 usually exhibits limited photocatalytic activity.[6,7] Different strategies were employed to enhance its photocatalytic activity including doping with rare-earth metals, nonmetal, or metal dopants.[4,5,8] Additionally, better charge carrier separation and higher photocatalytic activity could be achieved via the formation of heterojunction between BiFeO3 and other semiconductors such as TiO2, BiVO4, CuO, and g-C3N4.[7,9−13]

Bismuth oxide (Bi2O3) is a good candidate for the formation of such heterojunction. This semiconductor has attracted great interest as a visible-light photocatalyst with a direct band gap ranging from 2 to 3.9 eV.[14,15] Superior photocatalytic activity of Bi2O3 is principally attributed to the Bi–O layered crystal or special Bi–O polyhedron structures providing high surface area and more active sites. Additionally, the strong hybridization between Bi 6s 6p and O 2p electrons favors the mobility of photoinduced holes.[16] Therefore, the photocatalytic activity of Bi2O3 relies on its crystalline structure and morphology.[17] Usually, Bi2O3 exists in one of the polymorphs α-, β-, γ-, δ-, ε-, ω-, or high-pressure hexagonal phases. However, α-, β-, γ-, and δ-Bi2O3 are the most predominant polymorphic forms. α-Bi2O3 (monoclinic) and δ- Bi2O3 (face-centered cubic) phases are stable at room and high temperature, respectively. Meanwhile, β-Bi2O3 (tetragonal) and γ-Bi2O3 (body-centered cubic, BCC) phases are metastable, formed at high temperature, and usually transformed into α-Bi2O3 when the temperature is reduced.[18,19] Among the different polymorphs of Bi2O3, the α-, β-, and γ-phases were demonstrated to possess high photocatalytic activity in the order of α- <β- < γ-Bi2O3.[17,19−21] The superior photocatalytic activity of γ-Bi2O3 has been attributed to its disordered truncated octahedral BiO5 units providing more active sites and to its unique valence band structure.[17,19] However, β- and γ-Bi2O3 phases are metastable and cannot be obtained in pure form.[18,19]

To the best of our knowledge, the photocatalytic behavior of the Bi2O3/BiFeO3 heterostructure has been studied a few times in literature. In these very recent publications,[22,23] authors prepared this heterostructure using the costly nitrates precursors and the conventional sol–gel or hydrothermal methods. In this study, we provide, for the first time, the traditional melt technique for combining the highly photoactive metastable β- and γ-Bi2O3 phases with BiFeO3 in one glass-ceramic material. Different preparation conditions were examined such as parent glass composition and the heat-treatment regime. The photocatalytic efficiency of the different as-prepared samples and glass-ceramic samples to remove organic contaminants from water/wastewater was studied. Reactive Yellow 160 dye was used as a model substrate for organic contaminants. The effects of initial pH of solution and etching on the photocatalytic activity were investigated as well.

Results and Discussion

Characterization of the Prepared Materials

To investigate the thermal behavior of the prepared samples, differential scanning calorimetry (DSC) was conducted for the BF as-prepared sample containing equal starting amounts of Bi2O3 and Fe2O3. The recorded DSC graph is presented in Figure . A clear exothermic peak was observed at 640 °C, which indicates the crystallization of the parent glass at this temperature. Therefore, the heat-treatment scheme was designed to heat the samples at 640 °C for 2, 4, and 10 h to monitor the change in characteristics of the developed glass-ceramic.

Figure 1

DSC trace of BF as-prepared sample.

X-ray diffraction (XRD) analysis was performed for both the parent glass and the corresponding glass-ceramic samples of different compositions that were prepared at different heat-treatment times. The collected XRD patterns are illustrated in Figure . Different crystalline structures were observed as summarized in Table . When equal amounts of Bi2O3 and Fe2O3 were used (i.e., BF samples), two major phases, tetragonal β-Bi2O3 and BiFeO3, were observed in the as-prepared sample (Figure a). Heating this sample at 640 °C for 2 h resulted in the complete phase transformation from tetragonal β-Bi2O3 to the body-centered cubic (BCC) γ-Bi2O3 along with the formation of a new phase, Fe3O4. These phases coexisted with the BiFeO3 phase that was already observed in BF as-prepared sample. The phase transformation from β- to γ-Bi2O3 phase might be the result of the grain growth at the grain boundary or within the grains of β-Bi2O3. Consequently, larger grains of γ-crystallites are formed after heat-treatment for 2 h due to the formation of γ-nuclei within the β-crystallites.[24] This transformation could be additionally confirmed by the appearance of new peaks at 2θ values of 24.95, 30.7, 33.22, 42, 52.92, and 62°, which recognized as the phase γ-Bi2O3 (ICCD No. 1–74–1375). Increasing the heating time to 4 h caused, surprisingly, the partial reverse transformation from the γ-Bi2O3 back to the β-Bi2O3 phase (ICCD No. 1-75-0993), while the BiFeO3 and Fe3O4 phases continue to exist. This backward phase transformation could be supported by the following two observations. First, the intensity of the peak at 2θ = 31.92° slightly increased after heating for 4 h and further increased after 10 h of heat-treatment. The second observation is that the intensity of the main peak of γ-Bi2O3 at 2θ = 27.96° decreased gradually from the sample heat-treated for 4 h to that heat-treated for 10 h. This backward phase transition from γ- to β-Bi2O3 might be due to the reformation of the disordered truncated octahedral units BiO5 and tetrahedral BiO4, which are the building blocks of cubic γ-Bi2O3[17] after heating for a prolonged time. These results clearly demonstrate that the crystal structure of Bi2O3 depends critically on the heat-treatment time.

Figure 2

XRD patterns of the as-prepared samples and the corresponding glass-ceramic samples developed at 640 °C and different heating times: (a) BF, (b) GF, and (c) GB.

Table 1

Crystallization Characteristics of the Studied Glasses

sample code	heat-treatment time	crystallized phases (ICCD No.)
BF	as-prepared	T β-Bi2O3 (01-78-1793); BiFeO3 (01-74-2016)
	2 h	BCC γ-Bi2O3 (01-74-1375); BiFeO3 (01-74-2016); Fe3O4 (01-88-0866)
	4 h	BCC γ-Bi2O3 (01-74-1375); T β-Bi2O3 (01-78-1793); BiFeO3 (01-74-2016); Fe3O4 (01-89-0951)
	10 h
GF	as-prepared	γ-Fe2O3 (00-039-1346)
	2 h	Fe3O4 (01-89-0951); T β-Bi2O3 (01-78-1793); BiFeO3 (01-74-2016); γ-Fe2O3 (00-039-1346)
	4 h
	10 h
GB	as-prepared	amorphous
	2 h	Bi2O3 (03-065-3319); BiFeO3 (01-74-2016); α-Fe2O3 (01-85-0987, 01-88-2359)
	4 h
	10 h

On contrary, mainly amorphous structure with low-intensity peaks belonging to the γ-Fe2O3 phase was observed in the as-prepared sample with a high content of Fe2O3 (coded GF (as-prepared) in Figure b) likely due to iron acting as a glass former, which led to increased glass stability and retardation of the Bi2O3 crystallization. Thereby, only low-intensity peaks relevant to the γ-Fe2O3 phase was observed in this sample. However, heat-treatment enhanced, to some extent, the crystallinity of the GF glass-ceramic samples. The heat-treated GF samples possessed Fe3O4, γ-Fe2O3, tetragonal β-Bi2O3, and BiFeO3. However, Bi2O3, BiFeO3, and γ-Fe2O3 phases were observed in the heat-treated GB sample (Figure c) due to the high content of Bi2O3. Additionally, peaks at 12 and 23° were detected in both GB and GF samples. These diffraction peaks do not match with any known crystal structure and may be due to the formation of new or mixed phases.

The prepared materials were intended to be used as magnetic photocatalysts. Therefore, the magnetic behavior of the prepared materials was assessed from the recorded vibrating sample magnetometer (VSM) hysteresis loops depicted in Figure . It is interesting to note that the magnetic properties were found to be significantly dependent on the starting ratio between Fe2O3 and Bi2O3 of the materials rather than the heat-treatment. For instance, all BF samples prepared at different heat-treatment times depict nearly the same hard ferromagnetic behavior[25] with coercivity (Hc) values of about 750 G and remanent magnetization (Mr) of 0.16 emu/g (Figure a). A plausible explanation is that BF samples (either the as-prepared or the corresponding glass-ceramics) contain the same crystalline magnetic phases. The heat-treatment changed the crystallinity of Bi2O3, which is a nonmagnetic component. As a result, all BF samples exhibited the same magnetic behavior. In Figure b, the GF as-prepared sample exhibited interesting superparamagnetic behavior of 1.4 emu/g saturation magnetization (Ms) with Hc and Mr values of 146 G and 0.17 emu/g, respectively. It is well-established that the decrease in particle size is usually accompanied with a surface spin disorder, and the coercivity may approach zero if the crystal size is small enough.[26,27] Consequently, this may give an indication of the formation of magnetic nanocrystallites in this sample;[28] in this case, γ-Fe2O3 phase. The heat-treatment caused the growth of these nanocrystallites and the formation of the magnetic Fe3O4 and BiFeO3 phases along with the nonmagnetic β-Bi2O3 phase as presented in the relevant XRD patterns (Figure b). Accordingly, heat-treated GF sample showed ferromagnetic properties with Ms, Hc, and Mr values of 0.45 emu/g, 529 G, and 0.12 emu/g, respectively.

Figure 3

VSM hysteresis loops of the as-prepared samples and the corresponding glass-ceramic samples: (a) BF, (b) GF, and (c) GB.

The GB as-prepared sample presents a linear magnetization-field strength (M–H) relationship with nearly neither Mr nor Hc, which are typical characteristics of paramagnetic materials (Figure c).[28] This would be expected, as this sample is mostly amorphous with a random arrangement of atoms. After heat-treatment for 10 h, the Bi2O3 and BiFeO3 phases were developed in the GB glass-ceramic sample. The presence of these crystalline phases converted the GB glass-ceramic sample into a weak ferromagnetic material of very small Hc and Mr.

To identify the morphology and the surface structure of the samples, imaging by field emission scanning electron microscopy (FE-SEM) was performed on the selected sample, BF. As Figure demonstrates, the SEM image of the BF sample heat-treated at 640 °C for 2 h contains spherical particles embedded in a glassy matrix. However, after etching this sample for 150 s, the glassy matrix corroded, allowing a clearer appearance of the formed crystals as spherical particles in a homogeneous and uniform texture. In addition, some particles were diffused with each other and precipitated on the surface in the form of rod-shaped particles (inset of Figure b). Evidently, increasing the soaking time to 10 h caused the growth of the granules (Figure c).

Figure 4

SEM images of (a) BF (2 h) sample before etching, (b) BF (2 h) after etching, and (c) BF (10 h).

The optical properties of the as-prepared samples and the corresponding glass-ceramic samples were evaluated from the collected diffuse reflectance spectra. The absorption coefficient was calculated using the Kubelka–Munk equation (eq )[29]where R denotes the reflectance of an infinite film and the function F(R∞) is equivalent to the absorption coefficient, α. The obtained absorbance spectra are presented in Figure .

Figure 5

Kubelka–Munk transformation of the diffuse reflectance UV–vis spectra of the different as-prepared samples and the corresponding glass-ceramic photocatalysts: (a) BF, (b) GF, and (c) GB.

The optical band gap (Eg) of the prepared samples could be determined by eq where α, hv, Eg, and K are optical absorption coefficient, photon energy, band gap, and a constant, respectively. n is a parameter related to the type of optical transition process in a semiconductor. That is, n = 1/2 for direct transition, whereas n = 2 for an indirect transition.[15] Plotting (αhv)2 versus photon energy (hv) (Tauc plot) gives the optical band gap (Eg) by extrapolation of the linear part of this curve,[29] as depicted in Figure . According to the presented data, the band gap energy of the prepared materials is in the range of 1.90–2.22 eV, confirming their absorption in the visible-light range (λ = 542–634 nm). Additionally, it is noticeable that the heat-treatment time does not affect the optical properties in the case of the GB and GF samples. It only has a minor effect in the case of the BF sample where the band gap was red-shifted from 2.1 to 2.0 eV by applying the heat-treatment and conversion from glass to glass-ceramic.

Figure 6

Tauc’s plot of the prepared samples: (a) BF, (b) GF, and (c) GB samples.

Photodegradation Efficiency of the Prepared Samples

To assess and optimize the photocatalytic efficiency of the prepared materials, four main factors have been studied, namely, the composition, heat-treatment time, initial pH of the dye solution, and the etching of the glass-ceramic surface. Figure illustrates the time variation of the dye concentration with simulated solar-light irradiation in the presence of the prepared materials. It is worth mentioning that the studied dye was not removed either by photolysis or adsorption in dark in these experiments. According to Figure , it is noticeable that the degradation rate was influenced significantly by the composition and time of heat-treatment; in other words, by the crystalline structure. For a better comparison, the achieved dye removal after 6 h of irradiation was compared in Figure d. The highest removal percentage was observed mainly in the BF sample that was heat-treated for 2 h. This BF sample is composed mainly of the photoactive γ-Bi2O3 and BiFeO3 phases according to the XRD analysis (see Figure a). This metastable γ-Bi2O3 was reported to be the most active polymorph among the Bi2O3 polymorphs because of its unique valence band structure, surface defects, strains, and reconstructions.[16,17] Additionally, BiFeO3 is the second crystalline phase of this sample, which also has photocatalytic activity, as illustrated by other researchers.[5,22,30] It is worth noting that the combination between these two phases in one material should reinforce the photocatalytic activity due to the formation of heterojunction that enhances the accumulation of charge carriers in BiFeO3[31] as depicted in Figure . Other BF samples contain the β-Bi2O3 phase of lower photocatalytic activity, leading to a lower performance for the degradation of the RY160 dye.

Figure 7

(a–c) Change in the dye concentration with irradiation time using BF, GF, and GB samples, respectively, (d) dye removal after 6 h irradiation as a function of composition and the heat-treatment time [50 mL of the 10 mg/L RY160 dye solution, 2 g/L of photocatalyst, and natural pH = 5.9].

Figure 8

Band positions of the formed phases versus normal hydrogen electrode (band positions were extracted from refs (7, 32)).

Surprisingly, the amorphous GB as-prepared sample showed a slightly lower photodegradation efficiency than that of the BF sample heat-treated for 2 h but higher than those of other samples. Consequently, it can be deduced that this sample might contain very tiny photoactive crystals, which could not be detected by the XRD analysis. These crystals provide a sufficient number of active sites for the photodegradation of the RY160 dye. Other samples showed lower activity, which can be attributed to the formation of the less photoactive crystalline phases such as β-Bi2O3, Fe2O3, and Fe3O4. Therefore, this study supports the fact that stabilization of the metastable γ-Bi2O3 can provide excellent photoactive material.

To further understand and correlate the phase composition to the photocatalytic activity of the different prepared materials, BiFeO3 and Bi2O3 phases were quantified and the results are displayed in Figure .

Figure 9

BiFeO3:Bi2O3 ratio in the prepared materials.

Among the different BF samples, the ratio of BiFeO3 was the lowest in the as-prepared sample. Nevertheless, after heat-treatment for 2 h, an appreciable amount of BiFeO3 phase was formed. Further heat-treatment for 4 and 10 h caused a reduction in the amount of BiFeO3 phase. Contrary, the heat-treatment time had an insignificant effect on the BiFeO3:Bi2O3 ratio in the GB, and GF glass-ceramic samples. For the GF group, all samples had comparable amounts of Bi2O3 and BiFeO3 phases. Keeping in mind the low photocatalytic activity of pure BiFeO3 illustrates the low photocatalytic activity of GF glass-ceramic samples. On the other hand, all GB glass-ceramic samples showed the BiFeO3 ratio of about 24%. This ratio is smaller than that of the corresponding BF samples, explaining the lower photocatalytic activity of the GB group relative to that of the BF group.

Correlating the results of the phases quantification and the photocatalytic activity indicates that the enhancement of the photocatalytic activity is ruled not only by the formation of the Bi2O3–BiFeO3 heterojunction but also by the ratio of BiFeO3:Bi2O3 phases. Changing the BiFeO3:Bi2O3 phases ratio has a considerable effect on the photocatalytic activity. The BF 2 h sample achieved the highest photocatalytic activity, indicating that the optimum ratio of BiFeO3:Bi2O3 phases in this composition is 34/66.

It is well known that the pH of the medium is a key factor in photocatalytic efficiency. Photodegradation of the RY160 dye was performed at different initial pH values to define the optimum pH of the photocatalytic reaction. The obtained results are depicted in Figure . The point at zero time corresponds to the normalized dye absorbance after the dark adsorption period. The photocatalytic degradation of the RY160 dye was possible only under acidic pH conditions because of the preferred adsorption of the dye on the photocatalyst surface under this condition. Therefore, more dye molecules can be attacked by the reactive oxidizing species that are photogenerated at the photocatalyst surface. The highest photodegradation rate was obtained at pH 3, with the pseudo-first-order rate constant of 0.90 ± 0.03 min–1 (R2 = 0.99). However, at pH 5, the rate constant dropped significantly to the value of 0.19 ± 0.02 min–1 (R2 = 0.90). Therefore, pH 3 can be considered as the optimum value for the degradation of the RY160 dye using the BF sample heat-treated at 640 °C for 2 h as a photocatalyst.

Figure 10

Photocatalytic degradation of the RY160 dye using the BF sample heat-treated at 640 °C for 2 h at different initial pH values [50 mL of the 10 mg/L RY160 dye solution and 2 g/L of photocatalyst].

To improve the photocatalytic activity, the glass component was etched from the surface of the BF sample heat-treated at 640 °C for 2 h using an aqueous solution containing 0.2% HF and 0.2% HNO3 for 150 s. Figure compares the adsorption and photocatalytic performance of the nonetched and etched samples. The point at zero time corresponds to the normalized dye absorbance after the dark adsorption period. Etching had an insignificant effect on the amount of RY160 adsorbed during the dark period. In contrast, the enrichment of the surface photoactive crystalline phases on the expense of glassy matrix could accelerate the photodegradation rate of the RY160 dye by ∼1.5 times, specifically from k = 0.90 ± 0.03 min–1 (R2 = 0.99) to 1.42 ± 0.16 min–1 (R2 = 0.93) for nonetched and etched samples, respectively.

Figure 11

photocatalytic performances of the BF sample heat-treated at 640 °C for 2 h before and after etching [50 mL of the 10 mg/L RY160 dye solution, 2 g/L of photocatalyst, and pH = 3].

Conclusions

Glass-ceramic materials containing the highly photoactive metastable β- and γ-Bi2O3 phases with BiFeO3 were successfully prepared using the traditional melt technique. The XRD analysis of these materials revealed the formation of different crystalline phases of Bi2O3 and iron oxide depending on the chemical composition of the parent glass and the heat-treatment schedule applied to develop the corresponding glass-ceramic. Remarkably, the transformation between β- and γ-Bi2O3 phases was highly dependent on the heat-treatment time. The magnetic behavior was demonstrated to be significantly dependent on the starting ratio between Fe2O3 and Bi2O3 of the materials rather than the applied heat-treatment. The calculated optical band gap energy of all prepared materials confirmed their high absorptivity to visible light. Photocatalytic activity tests for the degradation of the RY160 dye illustrated that the sample containing equal starting amounts of Fe2O3 and Bi2O3 and heat-treated at 640 °C for 2 h exhibits the best performance due to the formation of the γ-Bi2O3/BiFeO3 heterojunction in this sample. The performance could be enhanced by carrying out the photodegradation experiment at pH 3 and after etching the surface of this sample.

Experimental Section

Materials

Reagent grades bismuth oxide (Bi2O3) and iron(III) oxide (Fe2O3) were purchased from Loba Chemie Company (India). The reactive yellow 160 (RY160) dye was supplied by a local dyeing factory. The chemical structure of this dye is presented in Figure .

Figure 12

Chemical structure of the RY160 dye.

Preparation of Glass-Ceramic Samples

Glass samples of different compositions were prepared using the conventional melting method. The samples’ codes, chemical compositions, and temperature of preparation are illustrated in Table . Briefly, a 100 g batch was melted in an electrical furnace (Nabertherm, Germany). The batch was swirled repeatedly to ensure the homogenization of the molten. Then, the molten was poured on a preheated mold. The cast samples were annealed in another preheated furnace to remove any stress in the cast sample. Finally, the samples were crushed to fine powders before assessing their photocatalytic activity.

Table 2

Sample Codes, Chemical Compositions, and Preparation Temperatures of the Prepared Materials

Characterization of the Prepared Samples

The thermal behavior of the as-prepared samples was investigated using DSC analysis. The DSC traces were collected using a STD Q600 instrument (TA Instruments Company) at a 10 °C/min heating rate under nitrogen atmosphere (30 mL/min). α-Al2O3 was used as the reference material in this analysis. Crystalline phases developed in the glass-ceramic samples were identified from the XRD patterns obtained by an Empyrean XRD diffractometer (Malvern Panalytical Company). The XRD analysis was performed using Cu Kα (1.54060 Å) radiation in the 2θ range of 10–70° with 0.0260 step size and 18.87 s scan step time. The microstructure was examined for the intact specimen and for samples etched for 150 s in aqueous solution containing 0.2% HF and 0.2% HNO3. A field emission scanning electron microscope FE-SEM (Philips XL30 model) was used for this purpose. Magnetic properties were assessed by vibrating sample magnetometer (VSM, Riken Denshi BH-55) at room temperature. UV–vis diffuse reflectance spectroscopy (DRS) was used to study the optical properties. DRS spectra, within the range of 200–1000 nm, were recorded by a JASCO spectrophotometer (model V570, Japan) using BaSO4 as the reference material.

Photocatalytic Activity Tests

Photocatalytic activity of the samples prepared at different conditions was evaluated by the degradation of the RY160 dye under simulated sunlight. Irradiation system (model UVACube 400) was purchased from Honle UV Technology Company (Germany). This system is equipped with a halogenide high-pressure lamp (model SOL 500) emitting light of a spectrum comparable to that of the natural sunlight. The emitted light passes through a filter (model H2) with a cutoff at 295 nm. In typical photocatalytic experiments, 2 g/L of the material under investigation was suspended in 50 mL of the 10 mg/L RY160 dye aqueous solution. The suspension was first agitated in dark before starting the irradiation. The reaction temperature was kept constant at 20 °C by means of a chiller (Julabo FC400). During the reaction course, 3 mL of aliquots was withdrawn from the reaction medium at different time intervals and filtered through a 0.22 μm filter (Agilent) before the analysis. Variation in the dye absorbance was monitored spectrophotometrically using Jasco V630 spectrophotometer (Japan).