A study on the preparation and performance of a graphene-supported,Ce ion-doped,high-efficiency,magnetic TiO 2 photocatalyst

Abstract

The preparation of a magnetic core-shell Z-scheme heterojunction Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material by sol–gel and hydrothermal methods is described. The catalytic activity of the composite photocatalyst doped with metal Ce and loaded with graphene is significantly improved. Under standard experimental conditions (pH = 7, [methylene blue] = 10 mg L⁻¹, magnetic composite photocatalyst concentration = 0.1 g 50 mL⁻¹), the Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite photocatalyst lead to a maximum degradation of methylene blue of 98.2% in 120 min.

Introduction

With the continuous advancement of the industrial revolution, the problem of water pollution has become increasingly serious. Polluted water sources seriously endanger human health, and it is extremely important to solve the problem of water pollution. ¹ There are many types of pollutants in water sources, such as organic pollutants, inorganic pollutants, toxic metal ions, and harmful nitrogen oxide. Traditional sewage treatment methods are low in efficiency, high in cost, and unselective towards different pollutants. In particular, there are secondary pollution problems. Therefore, sewage treatment has not been well solved. ² As early as 1972, Fujishima and Honda discovered that TiO₂ in photocells could participate in a redox process with water to release clean energy (H₂/O₂) when exposed to light. ³ In 1976, Carey and coworkers ⁴ used TiO₂ semiconductors to degrade organic pollutants. Since then, photocatalytic oxidation has entered a stage of rapid development as a new water treatment technology.

TiO₂ as a photocatalyst has the characteristics of good stability, low cost, strong catalytic activity, and is not harmful to the environment. The photocatalytic efficiency of nano titanium dioxide is related to the crystal phase, particle size, and specific surface area. It has been demonstrated that anatase is the most stable and effective polymorph on a nanoscale due to its relatively low surface energy.^5–7 However, due to the large band gap of TiO₂, the high recombination rate of photogenerated carriers, and difficult to recycle characteristics, the application of TiO₂ in water treatment is limited. At present, a small amount of doping of TiO₂ with transition-metal lanthanide series and actinide series metals can reduce the band gap width of TiO₂, and also make it photo-responsive in the visible light region, thereby improving the utilization of sunlight. ⁸ Studies on doping of transition metals, including Ce, Co, Ni, and so on, have been reported, which show the modification effect of Ce is better compared to those of other transition metals.^9–13 In addition, the main disadvantage of TiO₂ as a photocatalyst is that it is difficult to completely recover from wastewater. At present, several research groups have developed TiO₂-based magnetic catalysts that can be quickly separated from wastewater.^14,15 However, the magnetic core can reduce the photocatalytic efficiency. ¹⁶ In fact, SiO₂, as a magnetic isolation material, can be used as a protective film to avoid interactions between the magnetic core and the TiO₂ coating.^17–19 Meanwhile, graphene is a two-dimensional nanomaterial with high specific surface area and high conductivity.^20,21 It can improve the migration efficiency of photogenerated carriers, and therefore, the recombination of photogenerated carriers can be effectively suppressed.

In this research, a Z-scheme heterojunction Fe₃O₄@SiO₂@TiO₂-Ce/rGO core-shell material has been prepared by sol–gel and hydrothermal methods. TiO₂ doped with the rare earth metal Ce is loaded on graphene oxide to improve the photocatalytic activity. CeO₂ acts as an electron receiver to reduce electron-hole recombination, while graphene has good electrical conductivity and a large specific surface area. It can increase the contact area between pollutants and the catalyst and improve the catalytic activity of the catalyst. The principle is shown in Figure 1.

Figure 1.

Fe₃O₄@SiO₂@TiO₂-Ce/rGO the morphology.

Results and discussion

X-ray diffraction analysis

As shown in Figure 2(a), characteristic diffraction peaks appear at 30.0, 35.3, 43.0, 53.5, 56.9 and 62.8, corresponding to the (220), (311), (400), (422), (511) and (440) crystal planes (JCPDS No. 26 -1136), which are the characteristic diffraction peaks of Fe₃O₄. Comparing Figure 2(a) and (b), it is not difficult to see that there is no obvious change between Fe₃O₄ and Fe₃O₄@SiO₂. This may be because the SiO₂ coating is amorphous; hence, no new diffraction peak occurs. Figure 2(c) and (d) shows that Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO have similar characteristic peaks. In addition to the established diffraction peaks of Fe₃O₄, new diffraction peaks appeared at 25.8, 37.8, 48.0, 53.8, 55.0, 62.1, 68.8, 70.3 and 75.0, corresponding to the (101), (004), (200), (105), (211), (213), (116), (220) and (215) crystal planes (JCPDS No. 21-1272). This shows that the introduced TiO₂ is mainly in the anatase phase. As shown in Figure 2(c) and (d), characteristic diffraction peaks of rGO and Ce were not observed. This may be because the amount of Ce and GO is too small and detection is not obvious.

Figure 2.

XRD spectrum of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂@TiO₂-Ce and (d) Fe₃O₄@SiO₂@TiO₂-Ce/rGO.

Scanning electron microscope image analysis

The typical lamellar structure of GO can be clearly seen in Figure 3(a), and Figure 3(b) shows that the Fe₃O₄ particles have a tetragonal structure. The particle size is about 400 nm and the particles are evenly distributed. Figure 3(c) clearly shows that the Fe₃O₄ surface becomes smooth, while the edges and corners are no longer clear; at the same time the particle size has increased, indicating that SiO₂ has successfully coated the surface of Fe₃O₄. Figure 3(d) shows that on addition of Ce, the surface becomes rough and granular. The EDS spectrum shows that the Ce content is 3%, indicating that Ce metal has been successfully doped on the surface of the TiO₂. Figure 3(e) shows that Fe₃O₄@SiO₂@TiO₂-Ce has been successfully loaded onto rGO.

Figure 3.

SEM images of (a) rGO, (b) Fe₃O₄, (c) Fe₃O₄@SiO₂, (d) Fe₃O₄@SiO₂@TiO₂-Ce, (e) Fe₃O₄@SiO₂@TiO₂-Ce/rGO and (f) the energy-dispersive spectrometer of Fe₃O₄@SiO₂@TiO₂-Ce.

X-ray photoelectron spectroscopy analysis

In order to determine the chemical composition, the sample was analysed by X-ray photoelectron spectroscopy (XPS). It can be clearly seen from Figure 4 that the sample is composed of Fe, Si, Ti, Ce, C and O. This proves that Ce has been successfully doped on the photocatalyst. Compared with the Ti_2p peak, the Fe_2p peak is very weak, indicating that the Fe₃O₄ core is almost completely covered by SiO₂, forming a core-shell structure. The Ce3d spectrum is shown in Figure 4(b) and (c). The V′, V″ and V′″ peaks correspond to Ce³⁺3d_5/2 and Ce³⁺3d; u, u′, u″ and u′″ correspond to Ce⁴⁺3d_3/2, Ce⁴⁺3d_5/2, Ce⁴⁺3d_3/2 and Ce⁴⁺3d_3/2^22–26 (V and u are used to refer to the different positions of the peaks of Ce). The CeO₂ peak shifts, which may be caused by the energy transfer after CeO₂ and TiO₂, form a heterojunction. Figure 4(f) shows the VB-XPS plots of CeO₂ and TiO₂. It can be estimated that the VB positions of CeO₂ and TiO₂ are 2.22 eV and 2.64 eV, respectively ²⁷ (see Figure S1 in the Supplementary Material). Fourier transform infrared (FTIR) spectroscopy analysis was used to study the bonding interactions in the fabricated Fe₃O₄@SiO₂@TiO₂–Ce and Fe₃O₄@SiO₂@TiO₂–Ce/rGO photocatalyst. The wide band at 3419 cm⁻¹ is attributed to the tensile vibration of surface water, the hydroxy (OH) group. The characteristic peak of Si-O-Si corresponds to the antisymmetric stretching vibration at 1080 cm⁻¹, while the Fe–O characteristic peak is at 573 cm⁻¹ (an asymmetric stretching vibration). ²¹ Absorptions due to Ti–O–Ti, Ti–O–Si ¹⁶ and C=C occur at 478 cm⁻¹ (a stretching vibration), 807 cm⁻¹ (a symmetrical stretch) and 1623 cm⁻¹ (double bond stretching vibration). ²⁸ The two plots do not change significantly and there is an obvious C–O–C impurity at 1226 cm⁻¹ in Fe₃O₄@SiO₂@TiO₂-Ce. On the addition of graphene, the peak due to C=C in Fe₃O₄@SiO₂@TiO₂-Ce/rGO is more obvious.

Figure 4.

(a) X-ray diffraction pattern of Fe₃O₄@SiO₂@TiO₂-Ce/rGO; (b–e) Ce and Ti peak patterns; and (f) the VB-XPS patterns of CeO₂ and TiO₂.

Transmission electron microscopy image analysis

In order to further clarify the morphology of the sample, we performed transmission electron microscopy (TEM) image analysis on Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO. Figure 5(a) shows the tetragonal configuration of Fe₃O₄ more clearly, while Figure 5(b) shows that a translucent region is formed on the surface of Fe₃O₄, indicating that Fe₃O₄ and SiO₂ form a core-shell structure. Figure 5(c) shows that there are many obvious particles on the outer surface of the material. According to the scanning electron microscope (SEM) and EDS results, the obvious particulate matter may be Ce, which proves that it has been successfully doped on the surface of Fe₃O₄@SiO₂@TiO₂. Figure 5(d) shows that Fe₃O₄@SiO₂@TiO₂-Ce is tightly supported on the surface of rGO, confirming the successful preparation of the Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material.

Figure 5.

TEM images of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂@TiO₂-Ce and (d) Fe₃O₄@SiO₂@TiO₂-Ce/rGO.

UV–Vis spectroscopy analysis

It can be seen from the UV–Vis diffuse reflectance spectrum of the catalyst (Figure 6) that pure TiO₂ samples have strong absorption in the ultraviolet region, but there is almost no absorption in the visible light region; CeO₂ also absorbs in the ultraviolet region but, again, almost no absorption occurs in the visible region. The absorption intensity of the photocatalyst doped with Ce is significantly enhanced under visible light and exhibits a red shift of the absorbing boundary. This shows that Ce doping can expand the response range of TiO₂ in the visible light region. The photocatalyst loaded with rGO further improves the absorption performance of the catalyst under visible light. Figure 6(b) is the Tauc graph driven by the UV-DRS spectrum. ²² It can be estimated that the energy band gaps of Fe₃O₄@SiO₂@TiO₂-Ce/rGO, Fe₃O₄@SiO₂@TiO₂-Ce, TiO₂ and CeO₂ are 2.07 eV, 2.05 eV, 3.20 eV and 2.71 eV, respectively. The band gap of Fe₃O₄@SiO₂@TiO₂-Ce/rGO is narrowed, which further proves that the addition of CeO₂ can significantly improve the photocatalytic efficiency of TiO₂. The adsorption performance of the photocatalyst is characterized by an N₂ physical adsorption experiment, and the corresponding N₂ adsorption-desorption isotherm is shown in Figure S2 (see the Supplementary Material). It can be seen that the Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO samples show the shape of a type IV isothermal curve and that they are mesoporous structures. Electron microscopy showed that the particle size was about 40 nm. This was confirmed by previous electron microscopy. ²⁸ These studies showed that the specific surface areas of Fe₃O₄, Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO were 3.7660, 40.6405 and 46.9017 m² g⁻¹, respectively. The increase in specific surface area is conducive to the adsorption of pollutants and the promotion of photocatalytic degradation of pollutants.

Figure 6.

(a) and (b) UV–Vis DRS spectra and Tauc plots of Fe₃O₄@SiO₂@TiO₂–Ce/rGO, Fe₃O₄@SiO₂@TiO₂–Ce, CeO₂ and TiO₂.

Electrochemical impedance spectroscopy analysis

Electrochemical impedance spectroscopy is a method used to characterize charge transfer. The smaller the curve radius, the smaller the resistance. The Nyquist curve in Figure 7 exhibits the largest radius for TiO₂, the second is Fe₃O₄@SiO₂@TiO₂-Ce, and the smallest is for the Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material. This shows that the Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material has the highest charge transfer efficiency. This is due to the large π-bond structure of the loaded graphene, which promotes photogenerated electron transfer. Graphene is also an excellent electron acceptor, thereby reducing recombination with holes. This is consistent with the UV test.

Figure 7.

EIS of TiO₂, Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO.

Photoluminescence analysis

The recombination of photogenerated carriers reduces the activity of the TiO₂ photocatalyst. Therefore, photoluminescence (PL) analysis is used to measure the luminescence intensity of the fluorescence generated by the recombination of photogenerated carriers. Figure 8 clearly shows that the fluorescence intensities of pure TiO₂, Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO gradually decrease. It also shows that doping Ce and loading rGO reduces the recombination efficiency of photogenerated carriers. This is consistent with the EIS results. Therefore, the photocatalytic activity is improved.

Figure 8.

PL spectra of the nano-photocatalyst samples.

Photocatalytic activity and magnetic test

As shown in Figure 9(a), the photocatalytic activity of the sample was studied via the photodegradation rate of methylene blue (MB) under visible light irradiation. The absorbance of MB after degradation was measured with an UV–Vis near-infrared spectrophotometer. The degradation efficiency of the photocatalyst increases with time. The photocatalytic activity of Fe₃O₄@SiO₂ is 0, while the photocatalytic activity of TiO₂ is very low, and the degradation rate is 8.7%. The photocatalytic activity of the Fe₃O₄@SiO₂@TiO₂-Ce composite material obtained after metal Ce doping improves greatly and the degradation rate is 75.4%. That of the Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material is 98.2%. This shows that the supported graphene plays a significant role in improving the photocatalytic activity of the photocatalyst. This is because the excitation TiO₂ conduction band electrons can be transferred to the graphene sheet which prevents photogenerated electrons from recombining with holes. This is consistent with the PL test results. The UV–Vis DRS also proved that the band gap width of the modified TiO₂ photocatalyst was reduced and the visible light absorption region was greatly improved. The EIS test results are consistent with the UV–Vis DRS and PL tests. Therefore, the photocatalytic activity is improved. As shown Figure 9(b), the Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material can be easily aggregated by magnetic substances. The figure shows the magnetic hysteresis loop of Fe₃O₄@SiO₂@TiO₂-Ce/rGO, indicating that it is magnetic. The saturation magnetization of Fe₃O₄@SiO₂@TiO₂-Ce/rGO is 8.37 emu g⁻¹ and so achieved the purpose of convenient recycling. The degradation efficiency of Fe₃O₄@SiO₂@TiO₂-Ce/rGO remained about 90% after recycling six times.

Figure 9.

(a) Catalytic activity of Fe₃O₄@SiO₂, TiO₂, Fe₃O₄@SiO₂@TiO₂-Ce and Fe₃O₄@SiO₂@TiO₂-Ce/rGO; (b) the magnetic test of Fe₃O₄@SiO₂@TiO₂-Ce/rGO and (c) the degradation of the photocatalyst over six cycles.

Comparison of the degradation performances

Table 1 shows a comparison of the degradation efficiency of the photocatalytic materials and other similar materials in this experiment (the degradation rates are those under the optimal degradation conditions). The samples prepared in this experiment have high degradation efficiency over relatively short times.

Table 1.

Comparison of the photocatalytic degradation rates of similar materials.

Sample	Magnetic	Targeted contaminant	Degradation time	Rate	Ref.
Fe3O4@TiO2-Ce/rGO	Yes	Methylene blue	120 min	98.2%	This work
Ag@Fe3O4@C-Au@TiO2	Yes	Methylene blue	360 min	97.0%	Shen et al. 13
Fe3O4@SiO2@TiO2	Yes	Rhodamine B	180 min	95%	Ma et al. 15
TiO2/MMT/Fe3O4	Yes	Methylene blue	80 min	94%	Zhang et al. 29
γ-Fe2O3@SiO2@TiO2–Ag	Yes	Basic blue 41	3 h	94%	Dabirvaziri et al. 18
Ag2O/TiO2 NTA	No	Methylene blue	120 min	93.89%	Hou et al. 10

Mechanistic analysis

According to the above analysis, the catalytic degradation principle of the Fe₃O₄@SiO₂@TiO₂@Ce/rGO nanocomposite can be deduced (Figure 10). Under visible light irradiation, the electrons on the valence band of TiO₂ and CeO₂ are excited by the light and transition to the conduction band. Due to the band structure of the two materials, the photogenerated electrons excited to the conduction band of TiO₂ recombine with the photogenerated holes on CeO₂, and as a result, a large number of holes are generated in the TiO₂ valence band, along with a large number of electrons in the CeO₂ conduction band; the two materials form a Z-type heterostructure.

Figure 10.

Fe₃O₄@SiO₂@TiO₂-Ce/rGO the principle.

Conclusion

In this experiment, a Z-scheme heterojunction magnetic composite material, Fe₃O₄@SiO₂@TiO₂-Ce/rGO, was prepared by the sol–gel and hydrothermal methods. The morphology and structure are characterized by TEM, SEM, X-ray diffraction (XRD) and FTIR, and performance tests via UV–Vis DRS, PL, N₂ gas adsorption, EIS and so on. All proved that the prepared Fe₃O₄@SiO₂@TiO₂-Ce/rGO composite material has high catalytic activity. SiO₂ plays a protective role, isolates Fe₃O₄ and TiO₂, and prevents the performance of TiO₂ from being diminished. The cerium and Ti metals undergo charge transfer and form a Z-type heterojunction structure. The supported reduced graphene oxide increases the specific surface area of the composite material and enhances the electrical conductivity. The prepared high-efficiency magnetic recyclable composite material has broad application prospects in the treatment of factory wastewater.

Experimental

Materials and equipment

Fe₃O₄ was obtained from Luoyang Haorun Information Technology Co. Ltd. Graphene Oxide was purchased from Shenzhen Tuling Evolution Technology Ce. Ltd. Ethyl orthosilicate (TEOS), tetrabutyl titanate (TBOT), MB, sodium dodecylbenzene sulfonate (SDBS), HNO₃, glacial acetic acid, Ce(NO₃)₃·6H₂O and C₂H₅OH were obtained from Tianjin Komiou Chemical Reagent Ce, Ltd. All chemical reagents are of analytical grade and were used without further purification.

A D8 Advance X-ray diffractometer (Bruker-AXE, Germany), S-3400 scanning electron microscope (Hitachi, Japan), Lambda750 UV–Vis-NIR spectrophotometer (PE, USA), a CEL-LAB500 series of multi-position photochemical reactions instrument (photolysis instrument) (Beijing Zhongjiao Jinyuan Technology Co., Ltd.), a UV-5100B UV–Vis spectrophotometer (Shanghai Analytical Instrument Co., Ltd.) and an FLS920 transient steady-state fluorescence spectrophotometer (Edinburgh, UK) were employed.

Synthesis of Fe₃O₄@SiO₂ nanoparticles

Fe₃O₄ (0.3 g) was weighed in a 100 mL beaker and diluted HCl (50 mL, 0.1 mol L⁻¹) was added. The mixture was subjected to sonication for 15 min. Next, the Fe₃O₄ solid was magnetically separated and washed three times with deionized water. The magnetically separated Fe₃O₄ solid was placed in a 250 mL three-necked flask, and deionized water (18 mL) and absolute ethanol (80 mL) were added. Then, after stirring for 30 min at room temperature, NH₃H₂O was added. After stirring for 30 min, TEOS was added slowly (0.6 mL) and the stirring was continued for 12 h. The product, Fe₃O₄@SiO₂, was separated magnetically and was washed and dried. The product was then dried at 60 °C for 12 h.

Synthesis of Fe₃O₄@SiO₂@TiO₂-Ce nanoparticles

Ce(NO₃)₃ is calcined at high temperature to generate CeO₂. CeO₂(NO₃)₂ (0.1 g) was weighed in a 100 mL beaker and H₂O (1.8 mL), HNO₃ (0.2 mL) and absolute ethanol (20 mL) were added to make solution A. Fe₃O₄@SiO₂ (0.2 g) was weighed in a 100 mL beaker and absolute ethanol (20 mL) and acetic acid (0.25 mL) were added. The mixture was subjected to sonication for 40 min, tetrabutyl titanate (5 mL) was added, and the mixture was then mechanically stirred in a 35 °C water bath for 30 min to prepare liquid B. Solution A was slowly added to liquid B through a peristaltic pump, the mixture was stirred evenly until a sol was formed, and aged at 30 °C for 18 h. The obtained gel was dried in an oven at 80 °C for 24 h, and the dried gel was ground and calcined under a nitrogen atmosphere at 450 °C for 2 h to obtain Fe₃O₄@SiO₂@TiO₂@Ce powder.

Synthesis of Fe₃O₄@SiO₂@TiO₂-Ce/rGO nanoparticles

Graphene oxide (0.08 g) was weighed in a 100 mL beaker and absolute ethanol (40 mL) and deionized water (20 mL) were added successively. The mixture was subjected to sonication for 1 h, and SDBS (0.15 g) and Fe₃O₄@SiO₂@TiO₂@Ce (0.2 g) powder were added 10 min later followed by further sonication for 1 h. The reaction solution was transferred to a 100 mL high-pressure reactor and placed in a 120 °C oven to continue the reaction for 3 h. After the high-pressure reactor had cooled, the product was magnetically separated, and washed three times with absolute ethanol and deionized water. The product was dried at 60 °C for 24 h (Oxidation of graphene to graphene oxide).

Performance test on Fe₃O₄@SiO₂@TiO₂-Ce/rGO

Using visible light as the light source, the photocatalytic performance of the photocatalyst was evaluated by assessing the photodegradation rate of MB. The magnetic photocatalyst (0.1 g) was dispersed in a MB solution (50 mL, 10 mg L⁻¹), and testing was performed in a multi-position photochemical reactor (the light source was a 300 W xenon lamp). The suspension was stirred for 20 min in the light before changing to dark to establish the MB adsorption equilibrium. A 5 mL aliquot of the supernatant was removed after exposure to light every 10 min and analysed with detection with a spectrophotometer after centrifugation. The degradation rate was used to measure the degree of degradation of MB.

Supplemental Material

sj-docx-1-chl-10.1177_17475198211072812 – Supplemental material for A study on the preparation and performance of a graphene-supported, Ce ion-doped, high-efficiency, magnetic TiO2 photocatalyst

Supplemental material, sj-docx-1-chl-10.1177_17475198211072812 for A study on the preparation and performance of a graphene-supported, Ce ion-doped, high-efficiency, magnetic TiO2 photocatalyst by Weiyan Cao, Xijun Liu, Yuwei Wang, Yong Zhang and Congzhi Fu in Journal of Chemical Research