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Abstract

In this study, a 0.2CoFe₂O₄/0.8TiO₂-5%La nanocomposite was synthesized by a co-precipitation and a hydrothermal method and then was applied as a high activity photocatalyst for degradation of methylene blue under visible light irradiation. The 0.2CoFe₂O₄/0.8TiO₂-5%La material was characterized by X-ray diffraction, Raman spectra, scanning electron microscope/transmission electron microscope and UV-Vis techniques. The photocatalytic activity of the material was initially investigated through the decomposition of methylene blue. The synthetic 0.2 CoFe₂O₄/0.8 TiO₂-5% La showed excellent photocatalytic activity for degradation of methylene blue under visible light irradiation. Methylene blue could be degraded by more than 99.14% after only 50 min.

Keywords

Fenton reaction methylene blue (MB)nanomaterials photocatalysts photodegradation visible light irradiation

Introduction

Every year, large amounts of organic pigments are found in wastewater from industries such as textiles, tanning, paper, and cosmetics. These organic substances are very stable but they cause pollution of surface water as well as groundwater if they are not treated thoroughly and thereby seriously affect human health and ecosystems in water.^1–10 Therefore, looking for effective methods of treating persistent organic pollutants such as dyes is a concern of many scientists and research groups. In particular, the semiconductor photocatalytic process, an advanced oxidation process, has shown efficiency in the field of wastewater treatment.^11–20 Titanium dioxide (TiO₂) is known as a ‘green’ photocatalyst, with non-toxic properties and excellent optical endurance. However, because of the rapid recombination of electron-hole pairs, and no visible light absorption, the optical and electrical efficiencies of pure TiO₂ are quite limited.^21–29 The photocatalytic potential of TiO₂ can be improved by doping with metals or non-metals, or by combining TiO₂ with other narrow-bandgap semiconductors to enhance photon absorption of TiO₂ in the visible light region.^{2–4,12,22–24,26,27,29} Cobalt ferrite is a magnetic semiconductor material that has some unique properties such as a narrow optical bandgap, nontoxicity and a low cost.^30,31 Because of the magnetic properties of cobalt ferrite, it is also convenient to perform magnetic separation of cobalt ferrite from the aqueous solution when it is combined with TiO₂ as a photocatalyst.

In this study, for the purpose of producing photocatalysts with high dye-decomposition activities in ultraviolet (UV) and visible light, we have studied the fabrication of the 0.2CoFe₂O₄/0.8TiO₂-5%La nanocomposite material (CFTLa) by a co-precipitation method combined with a thermal method. In our synthetic process, a low temperature (<200 °C) was used, leading to the formation of the nanocomposite material with a small particle size. The small particle size increases the surface contact area and the number of active centres, therefore, facilitating the application of the heterogeneous catalyst to photocatalysis. The nanocomposite material exhibits very good photocatalytic activity for the decomposition of methylene blue (MB). The performance of MB decomposition reaches from 97.31% to 99.14% after 50 min under visible light.

Results and discussion

Characterization of the synthetic CFTLa nanocomposite

X-ray diffraction (XRD) patterns of powdered samples of TiO₂-5%La, CoFe₂O₄ and CFTLa nanocomposite are shown in Figure 1(a). On the XRD pattern of TiO₂-5% La (curve a), the appearance of the characteristic peaks of a TiO₂ anatase phase at 2θ = 25.40°, 38°, 48°, 54° and 62.73° corresponding to the (101), (004), (200), (105) and (204) reflection planes, respectively, (JCPDS card Number 21-1272) can be observed. The XRD pattern of CoFe₂O₄ (curve b) has the characteristic peaks of an CoFe₂O₄ phase at 2θ = 30.1°, 35.4°, 43.13°, 57.4° and 63.1° corresponding to the (220), (311), (400), (511) and (440) reflection planes, respectively (JCPDS card Number 22-1086). Moreover, in both cases, no other crystalline phase was detected, indicating that all the synthetic samples are a single phase. Finally, the XRD pattern of the CFTLa powder (curve c) reveals the characteristic peaks of both of the phases of TiO₂ anatase and CoFe₂O₄ at 2θ = 25.40°, 30.1°, 48°, 54° and 63°. The above results prove that in this study, the CFTLa composite material has been successfully synthesized using our new synthetic protocol. In order to study more about the structure of the material, we conducted a survey of the Raman Spectra of TiO₂-5%La, CoFe₂O₄ and CFTLa (Figure 1(b)).

Figure 1.

(a) XRD patterns of: (a) TiO₂-5%La, (b) CoFe₂O₄ and (c) 0.2CoFe₂O₄/0.8TiO₂-5%La nanocomposite (CFTLa). (b) Raman spectra of TiO₂-5%La, CoFe₂O₄ and CFTLa. (c) Nitrogen adsorption/desorption isotherms. (d) Pore-size distribution of CFTLa, respectively. (e) Ultraviolet-visible spectra of (1) TiO₂-5%La and (2) 0.2CoFe₂O₄/0.8TiO₂-5%La nanocomposite. (f, g) SEM. (h, i) TEM images of the synthesized CFTLa, respectively.

TiO₂ anatase phase has a tetragonal structure, with space group I41/amd. The sharp peak at ~156 cm⁻¹ is consistent with Ti–Ti bonding present in the octahedral chains. Cobalt ferrite has a cubic spinel structure with the Fd3m space group. It is noticed from Figure 1(b) that the cobalt ferrite sample shows four major bands at ~290, ~463, ~625 and ~673 cm⁻¹, respectively. The Raman spectrum of CFTLa shows several characteristic bands at 156, 290, 463, and 625 cm⁻¹, which can be attributed to the vibrations of TiO₂-5%La and CoFe₂O₄. The magnetic results for CFTLa showed that the saturation magnetization (M_s), remanence magnetization (M_r), coercivity (H_c) and squareness (S_r=M_r/M_s) are 50.0 emu.g⁻¹, 12.7 emu.g⁻¹, 348.7 Oe and 0.257, respectively. These results indicated that the CFTLa sample exhibited typical paramagnetic behaviour with high saturation magnetization (M_s), therefore, the CFTLa can be easily removed from solutions and recycled by applying an external magnetic field for reuse.

Figure 1(c) shows the nitrogen adsorption/desorption isotherms of the CFTLa nanocomposite material. Compared with the Brunauer-Emmett-Teller (BET) surface of TiO₂³² or CoFe₂O₄ nanoparticles^33,34 or multi-walled carbon nanotubes (MWCNTs) or MWCNTs/TiO₂ nanocomposites,³⁵ the synthetic CFWTLa nanocomposites have a much bigger BET surface (i.e. approximately 177 m² g⁻¹). With this large BET surface area, the nanocomposite contains more active centres, which can improve the catalytic ability of the material. UV-Vis measurements show that, compared to TiO₂-5%La material, the absorption edge of the CFTLa composite shifts and expands to the visible light region (λ > 400 nm) (Figure 1(e)). The presence of Co^2⁺ and Fe^3⁺ ions in the CFTLa composite helps to reduce the energy of electron transferences and the maximum absorption of the composite material is shifted towards the longer wavelength compared to TiO₂. Therefore, the band gap energy of TiO₂ is reduced by combining CoFe₂O₄ and TiO₂-5% La in the CFTLa composite material. These results reveal that the CFTLa composite material has more potential for photocatalytic applications. Figure 1(e) and (f) present scanning electron microscope (SEM) images of the CFTLa nanocomposite. SEM images clearly show a highly homogeneous surface of the CFTLa composite. It can be seen that the CFTLa particles have small sizes and are quite uniform with spherical-like shapes. The small size of these particles facilitates the application of the heterogeneous catalysts for photocatalysis, leading to increased surface area and the number of active centres. Transmission electron microscope (TEM) images of the CFTLa composite are shown in Figure 1(h) and 1(i).

Photocatalytic activity of the CFTLa nanocomposite

When the material absorbs UV/visible radiation, electrons from the valence band move to the conduction band, creating electron-hole pairs:

Semiconductor materials + h υ \to {e_{CB}}^{-} + {h_{VB}}^{+}

(1)

{e_{CB}}^{-} + H_{2} O_{2} \to {OH}^{.} + {OH}^{-}

(2)

Fe^3⁺ ions act as electron trapping centres that can prevent the recombination of electron holes. In addition, the combination of Fe^3⁺ and H₂O₂ in the Fenton reaction increases the number of OH radicals, thereby increasing the catalytic activity of the material. According to previous studies,^23,30,31,35 the rate of OH radical formation in the Fenton process increases significantly with the participation of UV or visible irradiation (photo-Fenton), so that the organic pollutants will be degraded easily. The results of the photocatalytic activity of the materials also show that the CFTLa nanocomposite exhibits excellent photocatalytic activity in the visible light region, especially, when combining with the photo-Fenton reaction. The MB degradation results of CFTLa are shown in Table 1. The degradation efficiency is in the range of 97.31%–99.14% depending on the H₂O₂ volume after irradiation for only 50 min using visible radiation.

Table 1.

MB degradation using CFTLa as photocatalyst combining with photo-Fenton reaction under visible radiation.

Time (min)	Dosage: 0.8 g CFTLa per 1 L of MB
	0.5 mL H₂O₂^a/L MB		1 mL H₂O₂^a/L MB		2 mL H₂O₂^a/L MB
	C/C₀	H%	C/C₀	H%	C/C₀	H%
10	0.538	69.24	0.308	82.39	0.176	89.94
20	0.224	87.19	0.128	92.68	0.073	95.83
30	0.184	89.48	0.105	93.10	0.060	96.57
40	0.081	95.39	0.046	97.37	0.026	98.51
50	0.047	97.31	0.027	98.46	0.015	99.14

MB: methylene blue; CFTLa: 0.2CoFe₂O₄/0.8TiO₂-5%La.

H₂O₂ concentration was 20 v/v.%.

In comparison, MB degradation happens very slowly in the non-catalytic system on irradiation by a compact lamp where only H₂O₂ is present (Table 2).

Table 2.

MB degradation using only H₂O₂ without any catalysts.

Time (min)	0.5 mL H₂O₂ 20%/L MB		1 mL H₂O₂ 20%/L MB		2 mL H₂O₂ 20%/L MB
Time (min)	C/C₀	H%	C/C₀	H%	C/C₀	H%
10	0.974	2.63	1.00	0.00	1.00	0.00
20	0.958	4.21	0.981	1.93	0.953	4.69
30	0.905	9.47	0.976	2.42	0.917	8.33
40	0.895	10.53	0.918	6.28	0.854	14.58
50	0.895	10.53	0.937	8.21	0.844	15.63

MB: methylene blue.

After 50 min of illumination, the MB degradation efficiency is 10.53% with the sample containing 0.5 mL H₂O₂ 20%/L MB, 8.21% corresponding to 1 mL H₂O₂ 20%/L MB, and the MB degradation efficiency is highest, 15.63%, with the sample containing 2 mL H₂O₂ 20%/L MB. These above results indicate that, in this work, MB is degraded mainly due to the role of the CFTLa photocatalyst. The results of MB degradation are presented graphically in Figure 2.

Figure 2.

(a) MB degradation of CFTLa and H₂O₂. (b) MB degradation of TiO₂-5%La, CoFe₂O₄ and CFTLa. (c) ln(A₀/A)-t diagram with the CFTLa catalyst.

On the other hand, in both cases using only TiO₂-5%La or CoFe₂O₄ as photocatalysts, after 50 min irradiation under visible radiation, the MB degradation efficiency is very low, 14.86% or 7.849%, respectively, Table 3 and Figure 2. When using the CFTLa nanocomposite as the photocatalyst, under same experimental conditions, the MB degradation efficiency reaches the highest value, 97.31%. This result can be explained by the excellent characteristics of the CFTLa nanocomposite for photocatalytic applications, such as the small particle size and the shifting and expanding of the absorption edge to the visible light region as mentioned above, which are not present in the individual materials (TiO₂-5%La or CoFe₂O₄). Supposing that the decomposition reaction of MB by catalyst CFTLa nanocomposite is a first-order reaction, the kinetic equation for the reaction is

Table 3.

MB degradation of TiO₂-5%La, CoFe₂O₄ and 0.2CoFe₂O₄/0.8TiO₂-5%La (CFTLa).

Time (min)	TiO₂-5%La		CoFe₂O₄		CFTLa
Time (min)	C/C_o	H%	C/C_o	H%	C/C_o	H%
10	0.999	0.06	1.00	0.00	0.538	46.20
20	0.951	4.91	0.992	0.753	0.224	77.64
30	0.919	8.09	0.963	3.656	0.184	81.59
40	0.883	11.62	0.939	6.075	0.081	91.88
50	0.851	14.86	0.921	7.849	0.047	97.31

MB: methylene blue; CFTLa: 0.2CoFe₂O₄/0.8TiO₂-5%La.

\ln \frac{C_{0}}{C} = \ln \frac{A_{0}}{A} = k t

(3)

where $C_{0}$ : the initial concentration of MB, C: the concentration at time t, A₀: the optical absorbance of the solution at the initial time, A: the optical absorbance of the solution at time t, k: the reaction rate constant, t: the reaction time. It can be seen that the graphs are linear with a high correlation coefficient R², showing that the MB decomposition reaction of the CFTLa nanocomposite follows the first-order kinetic equation. The k and R² values are, respectively, 0.060, 0.9820 and 0.069, 0.9560 for decomposition processes using 0.5 and 1.0 mL of H₂O₂/L of MB solution. The MB degradation efficiency of CFTLa nanocomposite can be compared with those of other previous studies (Table 4).

Table 4.

Comparison of the MB degradation efficiency of CFTLa nanocomposite with other photocatalyst materials.

Catalyst materials	MB degradation efficiency (%)	Treatment time (min)	Irradiation	Refs
Fe₃O₄/rGO/ZnSnO₃ nanoparticles	64	60	Visible light	Gnanamoorthy et al.³⁶
ZnSnO₃@Er, ZnSnO₃@Eu, and ZnSnO₃@Pr	65–70	120–150	Visible light	Gnanamoorthy et al.³⁷
Bi₂Sn₂O₇/rGO	80	70	Visible light	Gnanamoorthy et al.³⁸
CFTLa	99.14	50	UV light	This work

MB: methylene blue; CFTLa: 0.2CoFe₂O₄/0.8TiO₂-5%La; UV: ultraviolet.

Conclusion

The CFTLa nanocomposite was successfully synthesized using our new synthetic protocol, a two-stage combined coprecipitation and hydrothermal method. Structural measurements and analysis confirmed the presence of the two-phase CoFe₂O₄ and TiO₂-5%La in this composite material. With our new synthetic process, a low temperature (<200 °C) was used, leading to the formation of the nanocomposite material with small particle sizes. The small particle size increases the surface contact area and the number of active centres, therefore, facilitating the application of this heterogeneous catalyst for photocatalysis. Compared to TiO₂-5%La, the absorption edge in the UV-Vis measurements of the CFTLa composite shifted and expanded significantly to the visible light region (λ > 400 nm). The photocatalytic activity of CFTLa was shown to be higher in comparison with TiO₂-5%La or CoFe₂O₄. The synthetic nanocomposite material expressed excellent photocatalytic activity especially under visible light. The MB degradation of CFTLa reached 99.14% after irradiation with visible radiation for only 50 min. The MB decomposition using CFTLa followed first-order kinetics with a high correlation coefficient R².

Experimental

Chemicals

Tetra-isopropyl titanate (Ti(i-OC₃H₇)₄), lanthanum(III) nitrate hexahydrate (La(NO₃)₃.6H₂O), cobalt(II) nitrate hexahydrate (Co(NO₃)₂.6H₂O), iron(III) nitrate nonahydrate (Fe(NO₃)₃.9H₂O), acetylacetone (C₅H₈O₂) and ethanol (C₂H₅OH) were purchased from Merck Millipore. Distilled water was produced in our laboratory. MB (C₁₆H₁₈N₃SCl, 99 wt.%) was purchased from Xilong Chemical (China).

Preparation of CFTLa nanocomposite

Synthesis of CoFe₂O₄

A mixture of 10 mL of Co(NO₃)₂ 0.05M and 20 mL of Fe(NO₃)₃ 0.05M in aqueous solution was stirred, then aqueous of 19 mL of NaOH 0.5 M was added twice as much as needed was added slowly and the mixture was stirred for 2 h, at a temperature of 80 °C–90 °C. The precipitate was isolated by filtration and washed with distilled water.

Synthesis of TiO₂-5%La

A mixture of 10 mL ethanol (C₂H₅OH), 1 mL acetylacetone (C₅H₈O₂) and 0.7 mL tetra-sopropyl orthotitanate Ti(i-OC₃H₇)₄ and a mixture of 10 mL ethanol, 1 mL acetylacetone and 0.0433 g La(NO₃)₃.6H₂O and distilled water were stirred separately. The second solution was added dropwise into the first solution and the combined solution was stirred for 30 min. It was then treated hydrothermally at 180 °C for 12 h. The precipitate so obtained was filtered, washed many times with distilled water, dried at 120 °C in 6 h.

Synthesis of CFTLa nanocomposite

The mixture of CoFe₂O₄ and ethanol was added dropwise into the solution of TiO₂-5%La and the mixture stirred for 30 min. It was then subjected to hydrothermal treatment at 180 °C for 12 h. The precipitate was then isolated by filtration, washed many times with distilled water, dried at 120 °C in 6 h, milled and the CFTLa obtained as a powder of 1.0 g of CFTLa.

Characterization

The phase purity of the synthetic samples was characterized by a X-ray D5005-Siemens (Germany) diffractometer equipped with Cu-Kα radiation (k = 0.15405 Å, 40 kV, 30 mA) over the range from 10 °C to 70 °C with a scanning rate of 0.02 deg s⁻¹. Fourier transform infrared (FTIR) spectrometry was carried out on a Nicolet 6700 FTIR spectrometer, over the range from 400 to 4000 cm⁻¹. The absorption spectra were measured on an Agilent 8453 UV-Vis spectrophotometer system with wavelengths in the range of 200–1200 nm. Morphologies and crystal structures of the nanoparticles were characterized using a Field Emission Scanning Electron Microscope JEOL JSM-9300F. TEM images were taken using a TECNAI F20-G2 high-resolution transmission electron microscope operated at an accelerating voltage of 300 kV. The magnetic behaviours of the samples were measured at room temperature using a vibrating sample magnetometer (VSM 880 DMS/ADE Technologies, USA) at fields ranging from −10 to 10 kOe at 25 °C, with an accuracy of 10⁻⁵ emu.

Evaluation of the photocatalytic activity of CFTLa nanocomposite for degradation of MB under visible light irradiation

80 mg of the powdered CFTLa nanocomposite was added into 100 mL of 32 µmol/L MB solution. Different volumes of 20% H₂O₂ (2: 1; and 0.5 mL/L MB solution) were added. The mixture was stirred gently in a dark box for 5 min. Then the solution was irradiated with a High-Pressure Mercury Lamp simulating visible light (Osram, 250 W). After each experimental period, 5 mL of the dye solution was extracted and analysed to determine the residual concentration of MB. The rate of disappearance (H%) of MB was calculated by the following formula:

\begin{array}{l} H (%) = (C_{0} - C) * 100 % / C_{0} \\ = (A_{0} - A) * 100 % / A_{0} \end{array}

(4)

where, H (%) is per cent of removal MB by the photocatalytic reaction; C₀ is the initial concentration of MB and C is the remaining concentration at the time of the measurement; A₀ is the initial absorbency of the MB in solution and A is the absorbency of the reaction solution.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Vietnam Ministry of Education and Training under the Program Code CT2022.04 (via the Grant number CT2022.04.BKA.03).

ORCID iDs

Luyen T Tran

Hoang V Tran

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Synthesis and photocatalytic activity of a lanthanum doped CoFe 2 O 4 /TiO 2 nanocomposite for photodegradation of methylene blue organic dye

Abstract

Keywords

Introduction

Results and discussion

Characterization of the synthetic CFTLa nanocomposite

Photocatalytic activity of the CFTLa nanocomposite

Conclusion

Experimental

Chemicals

Preparation of CFTLa nanocomposite

Synthesis of CoFe2O4

Synthesis of TiO2-5%La

Synthesis of CFTLa nanocomposite

Characterization

Evaluation of the photocatalytic activity of CFTLa nanocomposite for degradation of MB under visible light irradiation

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References

Synthesis of CoFe₂O₄

Synthesis of TiO₂-5%La