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Article

A C-Doped TiO2/Fe3O4 Nanocomposite for Photocatalytic Dye Degradation under Natural Sunlight Irradiation

by
Mamo Gebrezgiabher
1,
Gebrehiwot Gebreslassie
1,
Tesfay Gebretsadik
1,
Gebretinsae Yeabyo
2,
Fikre Elemo
1,
Yosef Bayeh
1,
Madhu Thomas
1,* and
Wolfgang Linert
3
1
Department of Industrial Chemistry, College of Applied Sciences, Addis Ababa Science and Technology University, P.O. Box 16417 Addis Ababa, Ethiopia
2
Department Chemistry, College of Natural and Computational Sciences, Mekelle University, P.O. Box 231 Mekelle, Ethiopia
3
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060 Vienna, Austria
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2019, 3(3), 75; https://0-doi-org.brum.beds.ac.uk/10.3390/jcs3030075
Submission received: 11 June 2019 / Revised: 4 July 2019 / Accepted: 16 July 2019 / Published: 22 July 2019
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Abstract

:
Magnetically recyclable C-doped TiO2/Fe3O4 (C-TiO2/Fe3O4) nanocomposite was successfully synthesized via a sol–gel method. The synthesized samples were characterized using SEM, energy-dispersive X-ray spectroscopy (EDS), FTIR, and UV-VIS diffuse reflectance spectroscopy (DRS) techniques. The results clearly showed that a C-TiO2/Fe3O4 nanocomposite was produced. The photocatalytic activities of the prepared pristine (TiO2), C-doped TiO2 (C-TiO2) and C-TiO2/Fe3O4 were evaluated by the photodegradation of methyl orange (MO) under natural sunlight. The effect of catalyst loading and MO concentration were studied and optimized. The C-TiO2/Fe3O4 nanocomposite exhibited an excellent photocatalytic activity (99.68%) that was higher than the TiO2 (55.41%) and C-TiO2 (70%) photocatalysts within 150 min. The magnetic nanocomposite could be easily recovered from the treated solution by applying external magnetic field. The C-TiO2/Fe3O4 composite showed excellent photocatalytic performance for four consecutive photocatalytic reactions. Thus, this work could provide a simple method for the mass production of highly photoactive and stable C-TiO2/Fe3O4 photocatalyst for environmental remediation.

1. Introduction

Recently, TiO2-based photocatalysts have been used in several applications, such as antimicrobial activity [1], water splitting [2], hydrogen production [3], carbon dioxide reduction [4], organic pollutant degradation [5,6,7,8], solar cells [9,10,11], batteries [12], and super capacitors [13,14]. Currently, the search has intensified for an abundant, inexpensive, efficient, safe, and recyclable photocatalyst that can be used for degradation of organic contaminants, for instance, methyl orange (MO) in wastewater treatment and/or water purification. Over the last decade, scientific research interest for developing such a photocatalyst has been growing exponentially [6,15,16,17,18,19]. Among the reported photocatalysts, TiO2-based photocatalysts seem to be more popular because of their chemical stability, strong oxidizing power, nontoxicity, and low cost [20,21,22]. However, there are still some drawbacks that need to be addressed for this type of photocatalyst. For example, its band gap (i.e., 3.2 eV) only absorbs UV light at less than 388 nm. Therefore, several strategies have been developed to enhance the optical absorption of TiO2, such as colorant sensitization, metal- or nonmetal-doped TiO2-based nanoparticles (NPs), and modification of TiO2 by combining it with another metal oxide-based semiconductor [23]. In addition to this, TiO2 NPs are difficult to recover from treated water, which results in the discharge of TiO2 NPs into the environment. One alternative solution to this problem is to apply an NP photocatalyst that is immobilized on a permanent magnetic carrier, which permits it to be recollected from the treated water using an external magnetic field and then reused again. Immobilization of photocatalytic TiO2 NPs on magnetic transporter can be done by sol–gel method [24], plasma spraying [25], and precipitation [26].
Magnetic photocatalysts are a more attractive and economical method of water treatment for the future because they are capable of treating a large volume of wastewater with a small amount of magnetic photocatalyst over its useable lifespan, which is impossible with conventional purification methods. Iron oxides are the most common magnetic materials, which are cheap, easy to prepare, strongly magnetic, and environmentally benign [27,28]. Extensive efforts have been made to combine TiO2 photocatalyst with magnetic iron oxide NPs as a feasible strategy to build an effectively stable and easily recoverable composite photocatalyst [29,30,31]. Over the past few years, a number of TiO2-based hybrid photocatalysts with iron oxide-based magnetic materials have been reported, such as TiO2/Fe3O4 [32,33], Ag3PO4/TiO2/Fe3O4 [34], Ni–Zn–ferrite/TiO2 [35], and N-doped TiO2/ZnFe2O4 [36], to improve photocatalytic activity, cycling stability, and long-term durability in the photodegradation of dye pollutants. However, the magnetic properties and photodissolution of magnetic iron oxides happen upon direct deposition with the active photocatalysts, which reduces the photocatalytic activity of the photocatalyst [37].
Here, we synthesized a C-TiO2/Fe3O4 nanocomposite via a sol–gel method and characterized it using several characterization techniques, such as XRD, SEM, energy-dispersive spectroscopy (EDS), FTIR, and UV-VIS diffuse reflectance spectroscopy (DRS). The photocatalytic activity of the as-prepared pristine and nanocomposites were evaluated for MO (Scheme 1) degradation under natural sunlight. The C-TiO2/Fe3O4 nanocomposite photocatalyst showed better photocatalytic activity than the TiO2 and C-TiO2 photocatalysts. Therefore, the prepared nanocomposite photocatalyst would exhibit enhanced photocatalytic activity under natural sunlight with excellent stability and recyclability.

2. Experimental Part

2.1. Chemicals

Hydrochloric acid, methyl orange, ethanol, titanium chloride, ammonia, glucose, iron(II)chloridetetrahydrate, and iron(III)chloridehexahydrate were purchased from Merck, (Mumbai, India). All the chemicals were of analytical grade and used without any purification. Distilled water was used in all the experiments.

2.2. Synthesis of Photocatalysts

TiO2 NPs were prepared using a sol–gel method. Twenty milliliters of the precursor TiCl3 was added to 180 mL of distilled water. The mixed solution was stirred vigorously for 30 min. Then, 2 M ammonia aqueous solution was added drop-wise to the above solution with simultaneous stirring until a white precipitate was observed. Thereafter, the obtained precipitate was washed repeatedly until no chloride was detected in the filtrate. The precipitate was dried in an oven at 200 °C for 3 h, calcined at 450 °C for 1 h, cooled to room temperature, and stored in a moisture-free atmosphere for the next step. C-TiO2 photocatalyst was synthesized by mixing a stoichiometric amount of precursor and the desired amount of powdered glucose (99%) (i.e., C:Ti mole ratio of 1:6) [38]. These mixtures were then transported to muffle furnace and calcinated at 300 °C for 5 h. After natural cooling, C-TiO2 photocatalysts were collected and kept for the next steps.
Fe3O4 was prepared using a titration coprecipitation approach [39]. One hundred milliliters of 0.15 mol/L FeCl3·6H2O aqueous solution and 200 mL of 0.15 mol/L FeCl2·4H2O aqueous solution were mixed in a three-necked flask. The mixed solution was stirred at 40 °C. Then, ammonia solution was dropped into the mixed solution until the pH value reached 9. After that, a large amount of black precipitate was produced. The obtained Fe3O4 precipitate was washed with distilled water and ethanol numerous times until the pH value reached 7. Lastly, the Fe3O4 was dried in an oven at 40 °C and collected.
C-TiO2/Fe3O4 composite was synthesized using facile thermal sol–gel method. Typically, Fe3O4 (0.01 mol) was dispersed in the mixture solution of water–ethanol with a volume ratio of 1:20. After stirring the suspension for about 15 min using a magnetic stirrer, a diluted HCl aqueous solution was added into the suspension until the pH value of the mixture reached 5. Then, 6 mg of C-TiO2 dissolved in 50 mL of distilled water was slowly dropped into the above suspension. The suspension was vigorously stirred for 30 min to ensure a uniform composition. The precipitates were recovered by magnetic separation and washed with ethanol and deionized water until the pH value reached 7. The nanocomposite was aged for 5 h and then dried in open air oven at 60 °C. Then, the prepared C-TiO2/Fe3O4 composite was calcined at 450 °C for 3 h.

2.3. Characterization Techniques

The following were used as characterization techniques: XRD (Shimadzu Corp., Kyoto, Japan, operating at 40 kV and 30 mA, using Cu Kα radiation (1.5406 Å), step scan mode with step time and degree (2θ) of 0.4 s and 0.02°, respectively, for the range of 10° to 80°); UV-VIS DRS (Perkin Elmer Lamda 35 spectrometer (PerkinElmer, Inc., Waltham, MA, USA), at a wavelength range of 200–800 nm); FTIR spectrophotometer (IR prestige 21 (Shimadzu Corp.), in the range of 400–4000 cm−1 using KBr pellets); SEM (INSPECTTM F50, FEI Company, Hillsboro, OR, USA); and EDS equipped with SEM (JEOL JSM-5610 SEM, JEOL, Ltd., Tokyo, Japan).

2.4. Photocatalytic Activity

The photocatalytic activity of the prepared pristine and nanocomposite photocatalysts were studied for the degradation of MO under natural sunlight irradiation. Typically, the photocatalytic degradation experiments were carried out by adding 20 mg photocatalyst to MO dye aqueous solution (5 mg/L, 100 mL sample volume) in a 500 mL beaker (Scheme 2). Then, the mixture was stirred for 1 h in a dark place to obtain absorption–desorption equilibrium. The mixed solution was then exposed to natural sunlight. To significantly minimize the fluctuation of sunlight intensity, all photodegradation experiments were carried out around midday (between 11 a.m. to 2 p.m.) in the same month (December, 2018) and in the same place (Addis Ababa, Ethiopia). Thereafter, a 3 mL sample was taken at every 30 min interval of sunlight irradiation. The magnetic photocatalysts were separated using a magnet, and nonmagnetic photocatalysts were separated using a centrifuge. The residual concentration of MO was analyzed by the change in absorbance using a UV-VIS spectrophotometer (Optizen POP, Mecasys Co., Ltd, Daejeon, Korea) at 464 nm (λmax of MO). The effect of the catalyst dose was studied by varying the amount of photocatalyst loading from 10 to 50 mg by keeping the MO concentration (5 mg/L, 100 mL) and other parameters constant. The effect of the initial dye concentration was examined by varying the concentration of MO from 5 to 25 mg/L, keeping the catalyst dose at 20 mg in 100 mL volume of MO solution, with other parameters remaining constant.

3. Results and Discussion

3.1. Characterization Results

The morphology of pristine and nanocomposite samples was investigated by XRD. Figure 1a shows the XRD patterns of TiO2, C-TiO2, Fe3O4, and C-TiO2/Fe3O4 nanocomposites. The XRD peak at 35.6° is a characteristic peak of magnetic phase of Fe3O4 [40]. The XRD peaks corresponding to Fe3O4 clearly resembled the standard diffraction pattern of magnetite (JCPDS file No. 03-0863). The average crystallite size of the as-prepared Fe3O4 powder, as estimated by the Scherrer formula, was around 14 nm from the (311) peak, which showed the as-prepared particles were in the nanosize range [41]. There were no diffraction peaks of carbon in the C-TiO2/Fe3O4 nanocomposites due to their amorphous nature and low content compared to Fe3O4 and TiO2. In addition to all diffraction peaks of Fe3O4, four other diffraction peaks appeared at 2θ = 25.3°, 37.8°, 48.1°, 55.1°, and 63.5°, which corresponded to (101), (004), (200), and (211) planes of anatase TiO2 (JCPDS file No. 21-1272), respectively, in the C-TiO2/Fe3O4 nanocomposites. XRD results demonstrated that the crystalline anatase TiO2 and Fe3O4 coexisted in the C-TiO2/Fe3O4 sample. It could be observed that the intensity of the Fe3O4 peak decreased after being combined with TiO2. This reduction might be due to the effect of the TiO2 layer coating on Fe3O4 NPs.
The FTIR spectra of the synthesized samples (Figure 1b) showed broad bands in the range 3200–3460 cm−1, associated with the stretching vibration of the –OH species [32,42]. In addition, peaks at about 1650 cm−1 were related to the H–OH bending of the adsorbed water molecules [42]. The FTIR spectrum of TiO2 showed a broad characteristic peak of Ti–O–Ti vibration at 500–900 cm−1 [32,42], while a strong absorption band of Fe–O stretching vibration at 600 cm−1 [42,43,44] was clearly observed. Strong OH stretching frequency suggested high intensity of surface hydroxide, which enabled better dispersion of Fe3O4 nanoparticles and also enhanced the affinity between Fe3O4 nanoparticles and the precursor TiO2 [32]. The characteristic peaks of both C-TiO2 and Fe3O4 were noticed in the spectrum (Figure 1b), suggesting that the C-TiO2/Fe3O4 nanocomposite was successfully prepared.
The morphologies of TiO2, Fe3O4, C-TiO2, and C-TiO2/Fe3O4 samples were examined using SEM and are presented in Figure 2. Figure 2a displays the SEM image of the TiO2 nanoparticles in which many nearly monodispersed spheres could be seen. The surfaces of the TiO2 spheres were rough. Figure 2b shows the SEM image of the Fe3O4 spherically shaped colloidal particles. As shown in Figure 2c, agglomerated C-TiO2 could be observed. This might be due to the carbon matrix doped on the TiO2 spheres. Figure 2d is the SEM image of the C-TiO2/Fe3O4 nanocomposite, where many tiny C-TiO2 nanoparticles were adhered to the Fe3O4. The surfaces of the C-TiO2/Fe3O4 nanocomposites were smooth, which was distinctly different from the initial C-TiO2 spheres that are shown in Figure 2c. Such a difference can be attributed to the loading of C-TiO2 on Fe3O4. Furthermore, the elemental analysis of TiO2, C-TiO2, and C-TiO2/Fe3O4 samples were investigated using EDS, and the images are displayed in Figure 3. The EDS peaks of TiO2 samples were ascribed to Ti and O elements, as shown in Figure 3a. For the C-TiO2 sample, the peaks were associated with C, Ti, and O elements (Figure 3b). As shown in Figure 3c, C, Ti, Fe, and O elements were present in the EDS spectra of C-TiO2/Fe3O4 nanocomposites. Hence, we can conclude that TiO2, C-TiO2, and C-TiO2/Fe3O4 nanocomposite samples were prepared successfully.
The optical properties of the photocatalysts were studied using UV-VIS DRS, and the results for TiO2, C-TiO2, and C-TiO2/Fe3O4 nanocomposites are given in Figure 4a. It shows that the absorption edge of TiO2 was 375 nm and that it could only be absorbed in the UV region. After doping of TiO2 by carbon, its optical response shifted into the visible region. The UV-VIS DRS spectra demonstrated that the C-TiO2/Fe3O4 material could adsorb significantly more light in the 200–800 nm regions compared to TiO2 and C-TiO2 materials. As a result, it was found that the C-TiO2/Fe3O4 nanocomposite showed better photocatalytic activity in the visible light region than TiO2 and C-TiO2. Moreover, the band gap energy (Eg) of the as-prepared samples were estimated using Tauc’s equation [45] by extrapolation of the linear part of the curves obtained by plotting of (α hv)2 vs. hv. As shown in Figure 4b, the Eg values of pure TiO2, C-TiO2, and C-TiO2/Fe3O4 samples were 3.0, 2.5, and 2.4 eV, respectively. As a result, the C-TiO2/Fe3O4 nanocomposite exhibited an enhanced absorption in the visible region compared to the pure TiO2 and C-TiO2 photocatalyst.

3.2. Photocatalytic Activity

The photocatalytic degradation rate of the MO pollutant depends on the MO concentration and dose of the photocatalyst. As shown in Figure 5a, the degradation efficiency decreased as concentration of MO increased from 5 to 25 mg/L at constant catalyst dose (20 mg/100 mL). Therefore, the initial concentration of MO was taken to be 5 mg/L. Furthermore, Figure 5b shows the effect of photocatalyst loading on the degradation efficiency at constant MO concentration (100 mL, 5 mg/L). When the amount of photocatalyst loading increased from 10 to 20 mg, the degradation efficiency was enhanced. However, as the catalyst dose increased from 20 to 50 mg, the degradation efficiency decreased. Therefore, the optimum photocatalyst dose was found to be 20 mg/L.
After optimizing the MO concentration (5 mg/L) and catalyst dose (20 mg/L), the photocatalytic activity of TiO2, C-TiO2, and C-TiO2/Fe3O4 materials were investigated for MO degradation under solar light irradiation. The C-TiO2/Fe3O4 nanocomposite showed higher photocatalytic activity than TiO2 and C-TiO2 samples (Figure 6a). The C-TiO2/Fe3O4 nanocomposite degraded 99.68% of MO pollutant within 150 min, while the TiO2 and C-TiO2 degraded about 55.41% and 70%, respectively. This could be due to the fast electron-hole separation and low-rate photogenerated electron-hole recombination in the nanocomposite than in the TiO2 and C-TiO2 samples. Photocatalyst recyclability is very important in practical application. For this reason, the C-TiO2/Fe3O4 nanocomposite was recovered using external magnetic field and reused for photodegradation reactions after being washed and dried. As shown in Figure 6b, the C-TiO2/Fe3O4 nanocomposite showed almost the same photocatalytic performance for four consecutive photocatalytic reactions, indicating the nanocomposite photocatalyst has excellent photocatalytic stability.
We also compared the photocatalyst efficiency of the synthesized C-TiO2/Fe3O4 nanocomposite with previous TiO2-based studies (Table 1). As shown, the C-TiO2/Fe3O4 nanocomposite prepared here showed excellent photocatalytic activity compared to the previous works.

4. Conclusions

A visible-light photoactive and magnetically separable C-TiO2/Fe3O4 nanocomposite was successfully prepared by facile sol–gel method and fully characterized in its crystal structure, optical properties, and morphologies. The photocatalytic activity of the prepared pristine and nanocomposite catalysts were investigated for MO degradation under natural sunlight irradiation. The C-TiO2/Fe3O4 nanocomposite showed higher catalytic activity than TiO2 and C-TiO2. The enhancement in the photodegradation efficiency could be attributed to the synergistic effect of C-TiO2 and Fe3O4 components. In addition, it could be easily separated using external magnetic field and showed excellent photocatalyst stability for four consecutive cycles.

Author Contributions

All the authors participated in conceptualizing, executing, analyzing and writing the original draft except the last two authors (M.T. and W.L.) who were responsible for supervision, editing, and reviewing of this article.

Funding

This research project was supported by the Internal Research grant of Addis Ababa Science and Technology University (Project/Reference Number: IRG 02/09).

Acknowledgments

The authors are grateful to Addis Ababa Science and Technology University for providing necessary facilities for this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Jcs 03 00075 sch001 550
Scheme 1. Structure of methyl orange.
Scheme 1. Structure of methyl orange.
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Scheme 2. Schematic illustration for the photocatalytic degradation of methyl orange (MO).
Scheme 2. Schematic illustration for the photocatalytic degradation of methyl orange (MO).
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Figure 1. (a) XRD pattern of TiO2, Fe3O4, C-TiO2, and C-TiO2/Fe3O4 nanocomposites and (b) FTIR spectra of TiO2, Fe3O4, C-TiO2, and C-TiO2/Fe3O4 composites.
Figure 1. (a) XRD pattern of TiO2, Fe3O4, C-TiO2, and C-TiO2/Fe3O4 nanocomposites and (b) FTIR spectra of TiO2, Fe3O4, C-TiO2, and C-TiO2/Fe3O4 composites.
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Figure 2. SEM images of (a) TiO2, (b) Fe3O4, (c) C-doped TiO2, and (d) C-doped TiO2/Fe3O4 nanocomposites.
Figure 2. SEM images of (a) TiO2, (b) Fe3O4, (c) C-doped TiO2, and (d) C-doped TiO2/Fe3O4 nanocomposites.
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Figure 3. Energy-dispersive spectroscopy (EDS) images of (a) TiO2 (b) C-TiO2, and (c) C-TiO2/Fe3O4 samples.
Figure 3. Energy-dispersive spectroscopy (EDS) images of (a) TiO2 (b) C-TiO2, and (c) C-TiO2/Fe3O4 samples.
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Figure 4. (a) UV-VIS diffuse reflectance spectra (DRS) of TiO2, C-TiO2, and C-TiO2/Fe3O4 composite and (b) (α hv)2 vs. hv curve of TiO2, C-TiO2, and C-TiO2/Fe3O4 photocatalysts.
Figure 4. (a) UV-VIS diffuse reflectance spectra (DRS) of TiO2, C-TiO2, and C-TiO2/Fe3O4 composite and (b) (α hv)2 vs. hv curve of TiO2, C-TiO2, and C-TiO2/Fe3O4 photocatalysts.
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Figure 5. Effect of (a) initial dye concentration and (b) catalyst dosage on the photodegradation efficiency.
Figure 5. Effect of (a) initial dye concentration and (b) catalyst dosage on the photodegradation efficiency.
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Figure 6. (a) Photolysis of MO and photocatalytic degradation of MO over pure TiO2, C-TiO2, and C-TiO2/Fe3O4 photocatalysts and (b) recyclability study of C-TiO2/Fe3O4 nanocomposite photocatalyst.
Figure 6. (a) Photolysis of MO and photocatalytic degradation of MO over pure TiO2, C-TiO2, and C-TiO2/Fe3O4 photocatalysts and (b) recyclability study of C-TiO2/Fe3O4 nanocomposite photocatalyst.
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Table
Table 1. Comparison of photocatalytic activities of some selected nanocomposites with the present work.
Table 1. Comparison of photocatalytic activities of some selected nanocomposites with the present work.
CatalystPreparation MethodCatalyst Load (mg/mL)Pollutant Load (mg/L)Type of PollutantLight Source% DegradationRef.
Fe3O4/TiO2Solvothermal0.310Natural red30 W Xenon100% in 120 min[32]
Fe3O4 /TiO2Emulsion copolymerization0.45Rhodamine B250 W Hg100% in 120 min[33]
Fe3O4/TiO2Chemical coprecipitation0.120Methyl blue10 W LED90% in 300 min[40]
N-TiO2/ZnFe2O4Vapor–thermal0.45Rhodamine B500 W Hg100% in 240 min[36]
C-TiO2/Fe3O4Sol–gel0.25Methyl orangeNatural sunlight99.68% in 150 minThis work

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Gebrezgiabher, M.; Gebreslassie, G.; Gebretsadik, T.; Yeabyo, G.; Elemo, F.; Bayeh, Y.; Thomas, M.; Linert, W. A C-Doped TiO2/Fe3O4 Nanocomposite for Photocatalytic Dye Degradation under Natural Sunlight Irradiation. J. Compos. Sci. 2019, 3, 75. https://0-doi-org.brum.beds.ac.uk/10.3390/jcs3030075

AMA Style

Gebrezgiabher M, Gebreslassie G, Gebretsadik T, Yeabyo G, Elemo F, Bayeh Y, Thomas M, Linert W. A C-Doped TiO2/Fe3O4 Nanocomposite for Photocatalytic Dye Degradation under Natural Sunlight Irradiation. Journal of Composites Science. 2019; 3(3):75. https://0-doi-org.brum.beds.ac.uk/10.3390/jcs3030075

Chicago/Turabian Style

Gebrezgiabher, Mamo, Gebrehiwot Gebreslassie, Tesfay Gebretsadik, Gebretinsae Yeabyo, Fikre Elemo, Yosef Bayeh, Madhu Thomas, and Wolfgang Linert. 2019. "A C-Doped TiO2/Fe3O4 Nanocomposite for Photocatalytic Dye Degradation under Natural Sunlight Irradiation" Journal of Composites Science 3, no. 3: 75. https://0-doi-org.brum.beds.ac.uk/10.3390/jcs3030075

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