A FACILE NOVEL SYNTHESIS OF ThO2/Fe3O4 NANOCOMPOSITE WITH ENHANCED PHOTOCATALYTIC ACTIVITY FOR THE DEGRADATION OF MALACHITE GREEN UNDER VISIBLE LIGHT IRRADIATION

A FACILE NOVEL SYNTHESIS OF ThO₂/Fe₃O₄ NANOCOMPOSITE WITH ENHANCED PHOTOCATALYTIC ACTIVITY FOR THE DEGRADATION OF MALACHITE GREEN UNDER VISIBLE LIGHT IRRADIATION

Ramya Arumugam ¹, Prakash Periakaruppan ^{* 1} and Rayappan Selvanathan ²

Department of Chemistry ¹, Thiagarajar College, Madurai - 625009, Tamil Nadu, India.

Department of Chemistry ², Arul Anandar College, Karumathur - 625514, Tamil Nadu, India.

ABSTRACT: An innovative magnetically separable ThO₂/Fe₃O₄ nanocomposite was successfully synthesized by hydrothermal method. The synthesized ThO₂/Fe₃O₄ nanocomposite was characterized by UV-DRS, FT-IR, XRD, HR-TEM, BET, and TGA techniques. XRD and HR-TEM images displayed the overall spherical shape and confirmed the formation of well crystalline ThO₂/Fe₃O₄ nanocomposite. The catalytic efficiency of ThO₂/Fe₃O₄ was explored for the removal of malachite green dye (MG) from the aqueous phase. The degradation efficiency of about 99.53% was obtained, and the degradation reaction was maximal at pH 5. The results revealed that the synthesized ThO₂/Fe₃O₄ nanocomposite showed enhanced photocatalytic activity compared to the ThO₂ and Fe₃O₄ samples. Based on obtained results, the most plausible mechanism for removal MG has been proposed. The ThO₂/Fe₃O₄ photocatalyst remained a robust performer even after five cycles. This nanocomposite is an effective visible light responsive material which can be used for the removal of various organic pollutants from the environment.

ThO₂/Fe₃O₄ nanocomposite, Malachite green, Photocatalysis, Catalyst, the Degradation mechanism

INTRODUCTION: Contamination by organic pollutants has become a major environmental concern with industrial development and increases in the human population ^{1, 2}. In recent years, dyes are also an important class of pollutants that can be toxic and even carcinogenic to human beings ³. Among other pollutants, malachite green (MG) is a kind of triphenylmethane dye which has been widely used in the production of ceramics, leather, textile industry, food coloring, cell coloring, wool, jute, paper, acrylic industry, parasiticide, fungicide and bactericide in aquaculture industries globally ⁴.

However, malachite green has an adverse effect on the reproductive and immune system, and it shows potential genotoxic, carcinogenic effects, damaging effects on gill, intestine, kidney, liver, and it causes anemia, thyroid abnormalities ⁵. Therefore, it is necessary to find an effective method to remove MG from wastewater in an efficient and cost-effective way before its release into the environment ⁶.

In recent years, it has been proven that the photocatalytic degradation technology is an effective method for environmental remediation and efficient way for the removal of organic waste material from water system ⁷. For photocatalytic system, the nanosized metal oxides and their composites are developed because they possess high surface area and exhibit excellent physical and chemical properties due to its promising application in the energy production and environment-friendly and economical approach for the degradation of organic pollutants persisting in our environment ⁸. Recently, actinide - based nanomaterials have attracted more attention as they are considered very promising for the preparation of innovative nuclear fuels and highly efficient photocatalysts ^{9, 10, 11}. Therefore, the target synthesis of actinide oxide nanoparticles with specific sizes and shapes is of great importance for both applied and fundamental research. Among actinide oxides, ThO₂ has attracted increasing attention in recent years due to its usage as ceramics, sensors, solid electrolytes, optical materials, traditional nuclear industry, and catalysts ¹². However, a single photocatalyst always suffers from the drawback of deficient separation and transfer of charge carriers, which counteracts the high-activity performance of photocatalysts ¹³.

Among various photocatalysts, an iron oxide containing transition metal oxides are efficient in photocatalysis. Electron transport at the interface between two semiconductors is one of the important aspects for the design of this novel material. Thus, the photogenerated charge carriers, electrons, and holes are easily separated and are utilized for redox reaction ^{14, 15}. Recently, ThO₂ particles were doped with iron oxides particles because of their non-toxicity, low cost, compatible magnetic and electrochemical properties, which low cost, physical and chemical stability, high catalytic activity, environmental friendliness and ease of availability ^{16, 17}.

Herein, we have designed and developed a nano-structured ThO₂/Fe₃O₄ nanocomposite for the photocatalytic degradation of MG dye. The effects of the concentration of malachite green, pH value, and the concentration of photocatalyst on the degradation efficiency have been investigated. The fabricated ThO₂/Fe₃O₄ nanoparticles showed excellent photocatalytic activity for the degradation of MG dye under visible light irradiation.

EXPERIMENTAL:

Materials and Methods: All chemicals were analytical grade and used as purchased without further purification. Th(NO₃)₄.5H₂O and FeCl₃. 6H₂O, ethylene glycol, sodium acetate, trisodium citrate, toluene, and ethanol were purchased from Sigma Aldrich. Deionized (DI) water was used throughout the synthesis process.

Synthesis of ThO₂ Nanoparticles: In the hydro-thermal preparation, 1.5 g of Th(NO₃)₄.5H₂O powder was weighed into a 50 ml Teflon lined stainless steel autoclave, and subsequently 40 ml of toluene was added. The autoclave was sealed and maintained at 180 °C for 24 h in an electric oven, then cooled to room temperature naturally. The resulting precipitate was filtered, washed with ethanol and DI water for several times, and dried at 80 °C.

Synthesis of Fe₃O₄ Nanoparticles: The Fe₃O₄ magnetic nanoparticles were synthesized via a hydrothermal method according to previous report ¹⁸. Briefly, FeCl₃.6H₂O (3.3 g) and trisodium citrate (0.6 g) were dissolved in ethylene glycol (60 ml) to form a clear solution, followed by the addition of sodium acetate (6.0 g). The mixed solution was stirred vigorously for 30 min and then sealed in a Teflon lined stainless steel autoclave (100 ml capacity). The autoclave was heated and maintained at 200 °C for 10 h, and then allowed to cool to room temperature. The products were collected and washed several times with ethanol, DI water, and vacuum dried at 60 °C overnight.

Synthesis of ThO₂/Fe₃O₄ Nanocomposite: ThO₂ nanoparticles and Fe₃O₄ nanoparticles were added into absolute ethanol and ultrasonically dispersed for 10 min. After being stirred for 1 h at 75 ºC, the precipitate was separated, washed and dried at 80 ºC for 6 h. The obtained ThO₂/Fe₃O₄ product was calcined at 500 ºC for 2 h.

Characterization:

Material Characterization: The UV-Vis diffuse reflectance spectroscopy (UV-DRS) was recorded on a UV- 2400 spectrophotometer (Shimadzu Corporation, Japan) using BaSO₄ as the reference. Fourier transform infrared spectra (FT-IR) were recorded with a JASCO FT-IR 460 plus spectrometer. XRD analysis was carried out using Cu Ka radiation (k = 1.5418 Å) on a JEOL JDX 8030 X-ray diffractometer. The morphology of the material was ascertained High-resolution transmission electron microscopy and the corresponding selected- area electron diffraction (HR-TEM/ SAED) was carried out on a JEOL JEM 2100 analytical scanning transmission electron microscope. The samples for TEM were prepared by placing a drop of the prepared solution on carbon-coated copper grids, followed by drying. Photodegradation experiments were performed in a HEBER immersion type photoreactor (HIPR-MP125). TGA of samples was carried out with a TGA-50, SHIMADZU thermogravimetric analyzer at a heating rate of 10 ºC min^-1 up to 800 ºC under nitrogen gas.

Photodegradation Experiments: In the evaluation of photodegradation reaction, the experiment was conducted in a cylindrical immersion type photoreactor. The initial concentration of MG dye was taken 2 × 10^-5 M, and the 25 mg of the ThO₂/ Fe₃O₄ nanocomposite was added in the dye solutions to form a slurry. The reaction mixture was stirred in the dark for a certain time to establish the adsorption-desorption equilibrium of the dye molecules and dissolved water with the catalyst surface. Photolysis experiment of MG dye in the absence of photocatalyst under visible light was carried out by the aforementioned procedure. Further, the photocatalytic experiment was initiated by illuminating the system with visible light and after specific time intervals; certain aliquots of solution were withdrawn out with syringe at pre-determined time intervals, the dispersion was drawn and separated immediately by the adscititious magnet and collected. The residual amount of MG dye in the solution was analyzed using a UV-Vis spectrophotometer (JASCO V-530) based on its characteristic optical absorption at 619 nm. The decolorization efficiency (%) is calculated as:

Photodegradation (%) = C₀ – C / C₀ ....................1

Here, C is the absorption of MG solution at irradiation time of ‘t’ min and C0 is the initial absorption at t = 0 min.

RESULTS AND DISCUSSION:

UV-DRS: To understand the band structures of these materials, optical absorption study has been carried out in the wavelength (λ) range 200-900 nm, as shown in Fig. 1. The results reveal that the absorption edge shifted to longer wavelengths as the ThO₂ was doped into the surface of Fe₃O₄. The direct band gap was calculated from the UV-DRS spectra. We used the following formula for calculating band gaps from obtained diffuse reflectance spectra:

E = h*C / λ                .....................2

Where, h is Planck’s constant, C is the speed of light, and λ is cut off wavelength of recorded spectral data.

Fig. 1a-c shows the UV-DRS spectra of the ThO₂, Fe₃O₄ and ThO₂/Fe₃O₄ nanocomposite. The ThO₂, Fe₃O₄ nanoparticles and ThO₂/Fe₃O₄ nano-composite show the adsorption edge of 300, 450 and 475 nm, corresponding to a band gap of 1.8 eV, 2.2 eV, and 2.7 eV respectively, which signifies its photocatalytic activity under visible-light irradiation. The absorption of the ThO₂/Fe₃O₄ nanocomposites within the visible-light range significantly increased, and a red shift in comparison with the pure Fe₃O₄. These results are attributed to the interaction between the ThO₂ and Fe₃O₄ nanoparticles in the composite system. The enhanced light absorption of the ThO₂/Fe₃O₄ nanocomposite leads to the generation of more photoinduced electron-hole pairs under visible-light irradiation, which subsequently results in enhanced photocatalytic activity.

FIG. 1: UV-DRS SPECTRA OF ThO₂ (a), Fe₃O₄ (b), ThO₂ / Fe₃O₄ (c) NANOCOMPOSITE

FT - IR Spectroscopy: FT - IR measurements provided further evidence for the formation of ThO₂/Fe₃O₄ nanocomposite. Fig. 2 illustrates the FT-IR spectra of ThO₂ (a), Fe₃O₄ (b) and ThO₂/ Fe₃O₄ (c) nanocomposite. For all the samples, most typical band at 3420 cm^-1 was assigned to the stretching vibration of OH group and appearance of bands around 2927 cm^-1 and 2332 cm^-1 was attributed to stretching in C-H group respectively. The magnetite NPs can be seen by wide strong absorption band between 580 and 630 cm^-1, especially for Fe-O bond of bulk magnetite at 576 cm^{-1 19} as well as those at 1620 cm^-1 can be ascribed to carboxylate groups ²⁰. This indicates that magnetic Fe₃O₄ nanoparticles are capped with citrate groups that derived from the introduction of trisodium citrate during the solvothermal synthesis. These FT-IR spectra provided useful information that the Fe₃O₄ were successfully bound to the ThO₂.

FIG. 2: FT-IR SPECTRA OF ThO₂ (a), Fe₃O₄ (b), Fe₃O₄/ ThO₂ (c) NANOCOMPOSITE

XRD: The phase and crystalline composition of the as-prepared Fe₃O₄ and ThO₂ nanocomposite are examined by XRD measurements. The powder XRD pattern of the precursor is shown in Fig. 3 Fe₃O₄ (a), ThO₂ (b) and ThO₂/Fe₃O₄ (c). All the diffraction patterns obtained were indexed to the characteristic peaks of ThO₂ (JCPDS card no. 65-7222). Also a typical XRD pattern of the Fe3O4showed diffraction peaks at 2θ = 27.56°, 30.07°, 35.42°, 37.05°, 43.05°, 56.9°, 62.52°, 73.96° respectively corresponding to the (111), (220), (311), (222), (400), (333), (440) and (533) reflection face-centered cubic lattice of Fe₃O₄, which match well with the data from the JCPDS file (Card no. 82-1533) for  Fe₃O₄.  It was observed that Fe₃O₄/ ThO₂ shows the diffraction peaks at 2θ = 0.19 and 0.14° respectively.

The average crystalline structure was calculated from the most intense peak of 220 and it was found to be 8nm respectively. The XRD patterns that can be seen there are no additional peaks, implying the absence of impurity phases in the sample. This confirms the mesoporous and crystalline nature of the material. The average crystallite size of the synthesized ThO₂/Fe₃O₄ nanocomposites calculated by the XRD data according to Scherrer's equation.

D = k λ / β cos θ         ..................3

Where, d is the thickness of crystallite (nm), k is a constant dependent on the crystallite shape, λ is the X-ray wavelength (nm), B is the full width at half max in radians, and θ is the Bragg angle of the 2θ peak.

FIG. 3: (a-c) XRD PATTERN OF Fe₃O₄, ThO_2, AND ThO₂ /Fe₃O₄ NANOCOMPOSITE

TEM: The morphology, particle size, and structure of the products were characterized by TEM. The TEM images of different magnifications of ThO₂/ Fe₃O₄ nanocomposite are shown in Fig. 4a-d. To investigate the stability of Fe₃O₄ attached on the ThO₂ nanoparticles, a leaching test was conducted through treating the as-synthesized ThO₂/Fe₃O₄ sample fully dispersed with ethanol under sonication. Even after sonication for 30 min, the Fe₃O₄ nanoparticles are still firmly anchored on the ThO₂, suggesting the strong interaction between ThO₂ and Fe₃O₄. The morphology of pure ThO₂/ Fe₃O₄ nanocomposite shows spherical shape with uniform sizes and face-centered cubic structure. The average crystallite size of ThO₂/Fe₃O₄ nanocomposite is 8 nm, respectively. Fig. 4d illustrates Fe₃O₄ nanoparticles appearing black and the ThO₂ nanoparticles with a lighter color as observed for ThO₂/Fe₃O₄ nanocomposite. It is noticeable that ThO₂ nanoparticles are uniformly coated on the surface of the Fe₃O₄ nanoparticles.

The TEM images for the crystalline nature of ThO₂/Fe₃O₄ nanocomposite is displayed in Fig. 5a-b. The selected area electron diffraction (SAED) pattern of ThO₂/Fe₃O₄ nanocomposite is presented in Fig. 5c. The SAED pattern revealed several bright continuous concentric rings attributed to the diffraction from the (111), (220), (222), (400), (333) and (533) planes of ThO₂/Fe₃O₄, consistent well with the XRD data. The SAED pattern indicates the well crystalline nature of the prepared photocatalysts. EDX spectroscopy was employed to investigate the chemical composition and purity of the ThO₂/Fe₃O₄ nanocomposite. Fig. 5d shows that the corresponding results of the EDX pattern of the ThO₂/Fe₃O₄ nanoparticles contain four elements of Fe, Th, Cu, and O.

The energy-dispersive X-ray spectroscopy (EDS) demonstrates the coexistence of Fe, Th, O elements, which indicate the formation of ThO₂ and Fe₃O₄ nanocomposite.

FIG. 4: (a-d) HR-TEM IMAGES OF DIFFERENT MAGNIFICATIONS OF ThO₂/Fe₃O₄ NANOCOMPOSITE

FIG. 5: (a, b) HR-TEM IMAGES OF CRYSTALLINE NATURE OF ThO₂/Fe₃O₄ NANOCOMPOSITE, (c) SAED PATTERN OF ThO₂/Fe₃O₄ NANOCOMPOSITE, (d) EDX SPECTRUM OF ThO₂/Fe₃O₄ NANOCOMPOSITE

BET: The specific surface area, pore volume, and size analysis were calculated using the Brunauer-Emmet-Teller (BET) method. A typical nitrogen adsorption-desorption isotherm of all the photocatalysts are shown in Fig. 6. All the samples exhibit type IV nitrogen sorption isotherm with a capillary condensation step in the relative pressure (P/P₀) range 0-1.0, indicating a well-developed mesoporosity ²¹. The BET surface area of the nanocomposite was found to be 165.66 m²/g, calculated by the linear part of the BET plot. Fig. 6a (insert) shows that the total pore volume was observed by 0.55794 cm³g. The surface area, pore size, and pore volume of all the photocatalysts are presented in Table 1. Mesoporous structure and relatively high surface area could be responsible for its high photocatalytic activity.

TGA: The TGA plot in Fig. 7 shows the ThO₂/ Fe₃O₄ nanocomposite, which shows a steady weight loss in the temperature range of 25-700 ºC. The first weight loss (40.84%) was observed between 36-200 ºC, which can be attributed to the removal of physisorbed water molecules.

The second weight loss (8.77%) was occurring in the range of 270 to 400 ºC due to the decomposition of bioorganic compounds in ThO₂/ Fe₃O₄ nanocomposite.

TABLE 1: SURFACE AREA, PORE VOLUME, PORE SIZE OF SYNTHESIZED THO₂/FE₃O₄ NANOCOMPOSITE

Above the temperature of 400 ºC, the pure ThO₂ / Fe₃O₄ only exists, which is stable up to 600 ºC.

Photocatalytic Activity:

Effect of Time: Fig. 8 shows the changes in the MG absorption spectra during photocatalytic degradation with ThO₂/Fe₃O₄ at different irradiation times varying from 0 to 40 min. More than 75% of the MG is degraded within 40 min.

FIG. 8: PHOTOCATALYTIC DEGRADATION OF MG AT DIFFERENT TIME INTERVALS (0-40 min) USING ThO₂/Fe₃O₄ NANOCOMPOSITE

The results show that the absorption of the visible band at 618 nm decreased and a hypsochromic shift occurred simultaneously with increasing illumination time. The hypsochromic shift may be caused by an N-demethylation process. The absorbance peaks at 425 and 315 nm have declined, which indicates that the entire conjugated chromophore structure of MG has been destroyed ²².

Effect of pH on the Photodegradation of MG: The influence of different pH (3, 5, 7, and 9) value on the degradation efficiency of MG was investigated, which is shown in Fig. 9. The natural pH value of MG solution was found to be 5.2. The pH value was adjusted using dilute HCl or dilute NaOH solution for the experiment. The effect of pH on the degradation of MG was examined by changing the pH from 3, 5, 7 and 9 at a fixed amount of catalyst (0.25 g/L) and constant MG concentration (10 mg/L). Fig. 9a shows the effect of pH on the photodegradation of MG solution. At pH 5, it is observed that the absorption band of the MG is 619 nm. Since, the color from deep green changes to colorless, there is a rapid decrease with a slight hypochromic shift which is found at 619 nm, and there is no new absorption band in the UV range. It indicates that there would be a cleavage of the conjugated chromophore structure of the MG which would have occurred because of the formation of N-de-methylated intermediates. In the acidic pH 5, the absorption peak at 619 nm confirmed that there is adsorption of MG on ThO₂/Fe₃O₄ catalyst surface. It is found that degradation occurs (99.53%) within 40 min at pH 5. The electrons in the conduction band can be captured by the soluble O_2, and the holes can be trapped by the surface hydroxyl, these process resulting in the formation of hydroxyl radical species (^•OH), which are easily generated by oxidizing more hydroxide ions in acidic solution. Thus, the efficiency of the degradation process is enhanced in the acidic medium, especially at pH 5. Thus, the acidic medium favors the complete degradation of MG on the surface of the catalyst.

FIG. 9: (a) lN (C₀/C) vs. IRRADIATION TIME IN MG, (b) PHOTODEGRADATION EFFICIENCY OF MG

Similar kind of report is shown by Thu et al., ²³. The degradation yields of the cationic dyes are greater in the acidic medium than in neutral and alkaline media, which is correlated with the adsorption behavior of dyes on the catalyst surface. It shows that the degradation of malachite green followed the rate constant, 0.481×10^-2 min^-1 (R² = 0.9925). Fig. 9b illustrates the photodegradation efficiency of ThO₂/Fe₃O₄ nano-composite in which 99.53% of degradation is possible at pH 5.

Effect of Amount of Catalyst: The amount of photocatalyst is another factor affecting the performance of the photocatalyst. The photo-catalytic study was conducted under the following conditions: a source of irradiation, visible light, type of photocatalyst (ThO₂/Fe₃O₄ photocatalyst), the weight of photocatalyst (0.1 g to 0.5 g) and concentration of MG dye solution (2 × 10^-5 M). The effect of the loading of the ThO₂/Fe₃O₄ heterogeneous photocatalyst on the degradation of MG dye is shown in Fig. 10. The results reveal that the photocatalytic activity increased 80-95% after 40 min by increasing the weight of the ThO₂/Fe₃O₄ photocatalyst from 0.1 to 0.5 g/L due to increased. The degree of decomposition reaction increases with the increase in the reactive sites on the catalyst, beyond which, it shows that the degradation process is reduced, because of the scattering of light. There is a decrease in the penetration of light in the solution. When the amount of catalyst is higher in the solution, the activated molecules get deactivated by the collision of the lower energy molecules thereby increasing the turbidity of the medium and reduction in penetration of light leads to decrease in the rate of the reaction. Also, when the photocatalyst absorbs the highest light; hence, the surface area becomes high, as a result of agglomeration at higher concentration of catalyst.

FIG. 10: EFFECT OF CATALYST DOSE ON PHOTO-DEGRADATION OF MG IN AQUEOUS SOLUTION

Plausible Mechanism of MG Dye Degradation: The mechanism of MG dye degradation is initiated by the attack of OH radicals on the malachite green cation. First, the N-demethylation occurs on the attack of OH radicals, leading to the removal of all the four methyl groups present in MG dye.

As there is a continuous generation of OH radicals in the reaction system, the attack of OH radicals occurs spontaneously on the organic moiety. On further reaction, the by-products may undergo continuous degradation due to OH radical attack to bring about a benzene ring removal and ring opening to completely mineralize the material to yield carbon dioxide and water as the end by-products ^{24, 25, 26}. A schematic representation is shown in Scheme 1. Table 2 shows a comparison of previously reported MG degradation with different photocatalysts ^{27, 28, 29, 30}.

SCHEME 1: MECHANISM FOR THE DEGRADATION OF MG USING ThO₂/Fe₃O₄ NANOCOMPOSITE

TABLE 2: COMPARISON OF PHOTOCATALYTIC DEGRADATION OF MG USING VARIOUS CATALYSTS

Recyclability: The stability of a photocatalyst during the photocatalytic reaction is an important factor for a possible industrial application. To analyze the stability of the photocatalyst, prolonged recycling studies were performed. Fig. 11 shows the results of the repeated runs of MG degradation using the same photocatalyst. After each cycle, the photocatalyst was collected by centrifugation and then washed with distilled water until the complete removal of dye from the catalyst. Then, the catalyst was dried at 100 ºC overnight and used for another cycle. The photocatalytic activity was found to be nearly the same up to five cycles.

FIG. 11: REUSABILITY STUDY OF ThO₂/Fe₃O₄ CATALYST ON THE PHOTODEGRADATION OF MG

CONCLUSION: In this work, the ThO₂/Fe₃O₄ nanocomposite was prepared and investigated as effective material for removing MG from aqueous solution. Various characterization tools such as XRD, TEM strongly evidence the formation of ThO₂/Fe₃O₄ nanocomposite. Photocatalytic degradation of MG was 99.53% within 40 min. The photocatalyst was reused up to five cycles without significant change in the photocatalytic activity, which indicates the stability of the photo-catalyst. The ThO₂/Fe₃O₄ nanocomposite has a better specific surface area, more reactive sites, and higher stability. The facile preparation method and removal properties for MG imply the promising future of ThO₂/Fe₃O₄ nanocomposite in the practical application for wastewater treatment.

ACKNOWLEDGEMENT: The authors thank the management of Thiagarajar College for providing lab facilities.

CONFLICT OF INTEREST: There is no conflict of interest.

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    Arumugam R, Periakaruppan P and Selvanathan R: A facile novel synthesis of Tho₂/Fe₃O₄ nanocomposite with enhanced photocatalytic activity for the degradation of malachite green under visible light irradiation. Int J Pharm Sci & Res 2019; 10(6): 2902-10. doi: 10.13040/ IJPSR.0975-8232.10(6).2902-10.

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