Synthesis, characterization and synergistic photocatalytic properties of yeast-assisted composite La_0.7Sr_0.3MnO₃/TiO₂

Abstract

In order to identify a high-efficiency solar-light-driven magnetic photocatalyst, a series of powders such as perovskite La_0.7Sr_0.3MnO₃, yeast-assisted La_0.7Sr_0.3MnO₃ (Y-La_0.7Sr_0.3MnO₃), Y-La_0.7Sr_0.3MnO₃/TiO₂ and TiO₂ were prepared and characterized by photoluminescence spectroscopy (PL), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption (N₂-BET). The magnetic and photocatalytic properties of the powders were also investigated. The experimental results show that, when the content of TiO₂ in the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ is 3.51 wt%, the Y-La_0.7Sr_0.3MnO₃/TiO₂ exhibits stronger absorption in the UV-Vis light region. A composite p–n heterojunction was formed and had the lowest photoluminescence (PL) intensity, indicating a higher separation efficiency of photogenerated charge carriers and an enhancement of the photocatalytic activity of the Y-La_0.7Sr_0.3MnO₃/TiO₂ composite. Under solar irradiation, Y-La_0.7Sr_0.3MnO₃/TiO₂ shows an obvious synergistic photocatalytic effect on orange (MO) wastewater, and the synergetic index is calculated to be 1.79. Furthermore, it still exhibits high photocatalytic activity after being reused 8 times. Moreover, Y-La_0.7Sr_0.3MnO₃/TiO₂ displays soft ferromagnetic behavior, has much higher magnetization and lower coercivity and remanence. Therefore, Y-La_0.7Sr_0.3MnO₃/TiO₂ can be recycled by an external magnetic field more easily and can be used repeatedly. Radical trapping experiments confirmed that the main reactive oxygen species (ROS) involved in the process of the photocatalytic reaction are the superoxide radical anion (˙O₂⁻), hole (h⁺), and hydroxyl radical (˙OH), and the sequence of the contributions of the ROS to the photocatalytic activity of the composite is ˙O₂⁻ > h⁺ > ˙OH. Furthermore, the photocatalytic mechanism of Y-La_0.7Sr_0.3MnO₃/TiO₂ is proposed in this work.

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Introduction

It is well known that multifunctional homogeneous photocatalysts with intrinsic magnetic properties can be recycled via a magnetic field. Therefore a lot of homogeneous catalysts with magnetization and photocatalytic properties have been prepared and the corresponding properties investigated recently, for example, rare-earth mixed chalcogenide Dy₄S₄Te₃,¹ multiporous ZnFe₂O₄ nanotubes,² Ni₂FeVO₆ nanoparticles,³ (La_0.5Bi_0.2Ba_0.2Mn_0.1)FeO_(3−δ),⁴ porous tetragonal-CuFe₂O₄ nanotubes,⁵ Ba-doped BiFeO₃ nanoparticles,⁶ ferromagnetic ZrO₂ nanostructures,⁷ CdS nanostructures,⁸ Bi₅Fe_0.95Co_0.05Ti₃O₁₅ via europium doping,⁹ yttrium ferrite ceramic¹⁰ etc. Because of the excellent magnetism and photocatalytic activity of perovskite oxides under solar irradiation, it has become the focus of our research group and some perovskite oxides have been investigated and reported, such as LaFe_0.5Mn_0.5−xO_3−δ, LaFe_0.9Mn_0.1−xO_3−δ, La_0.8Ba_0.2Fe_0.9Mn_0.1O_3−δ and La_0.8Sr_0.2Mn_1−xTi_xO_3−δ (x = 0–0.15).^11–14

It is reported that La_1−xSr_xMnO₃ (x = 0–0.5) materials show superparamagnetic behaviors and the highest saturation magnetization could be obtained when x = 0.3.¹⁵ Furthermore, La_1−xSr_xMnO₃ materials also show good electrical and catalytic properties, so they are always used as multifunctional materials. For example, they have been applied as cathode materials in solid oxide fuel cells,^16–18 and used as catalysts in the degradation of NO¹⁹ and MO.¹⁴ Due to the narrow band gap, La_1−xSr_xMnO_3−δ could exhibit good photocatalytic activity under solar light irradiation,²⁰ and TiO₂ displays excellent photocatalytic activities under UV irradiation. So, a composite La_0.7Sr_0.3MnO₃/TiO₂ could improve the utilization of solar energy. Besides, the construction of a heterojunction structure is also a means used in recent years to improve the catalytic activities of pure catalysts. This is because the heterojunction photocatalyst is favorable for the separation of photogenerated electron–hole pairs, so the photocatalytic properties are improved. Therefore, a composite La_0.7Sr_0.3MnO₃/TiO₂ was investigated in this study.

Generally speaking, perovskite oxides are usually synthesized at a relatively high calcination temperature, which makes perovskite oxides with very low specific surface areas. To overcome the obstacle of low specific surface areas of perovskite oxides, a lot of approaches have been studied. One of the most effective approaches is to synthesize porous nanostructures, prepared via a template method,^21,22 emulsion method,^23,24 spray reaction method,²⁵ etc. Among the template methods, bio-templating has drawn more attention in the field of oxide material synthesis.^26,27 Among many bio-templating agents, yeast can easily be obtained in a large amount in a short time without environmental pollution. Simultaneously, it has a short generation time and can be easily incubated. In addition, yeast as a template may significantly simplify the processing routes because both the synthesis of the template and modification of the template surface can be omitted.²⁸ So yeast was chosen as a template to increase the specific surface area of La_0.7Sr_0.3MnO₃ and improve its photocatalytic activity.

In a word, in order to identify a high-efficiency solar-light-driven magnetic photocatalyst, a series of powders such as perovskite La_0.7Sr_0.3MnO₃, yeast-assisted La_0.7Sr_0.3MnO₃ (Y-La_0.7Sr_0.3MnO₃), Y-La_0.7Sr_0.3MnO₃/TiO₂ and TiO₂ were prepared and characterized. Simultaneously, the magnetism, the synergistic photocatalytic properties and reusability were also investigated. In addition, we used a range of chemical probes and scavengers for reactive oxygen species (ROS) to interrogate the mechanism of photocatalytic degradation of MO.

Experimental

Materials

The starting chemicals for the target compounds were of analytical grade without further purification. La(CH₃COO)₃ (99.9%), Sr(CH₃COO)₂ (99%) and Mn(CH₃COO)₂·4H₂O (99.9%) etc. were purchased from Aladdin Industrial Corporation. Butyl titanate was purchased from Fuchen Chemical Company (Tianjin, China). The powdered yeast was purchased from Angel Yeast Company. The solvents used in this work were purchased from Damao Chemical Company (Tianjin, China). MO was purchased from Tianjin Chemical Reagent Factory.

Preparation of photocatalysts La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃/TiO₂ and TiO₂

Preparation of La_0.7Sr_0.3MnO₃. La_0.7Sr_0.3MnO₃ was synthesized according to previous literature.¹⁵ In the typical experiment, the mole ratio of La : Sr : Mn was 0.7 : 0.3 : 1, and metal acetate salts were dissolved in deionized water; the mixed solution was named A. A was thermally decomposed in an oven under normal atmosphere at 800 °C for 1.5 h and left to cool down to room temperature before being ground to obtain La_0.7Sr_0.3MnO₃.

Preparation of Y-La_0.7Sr_0.3MnO₃. Y-La_0.7Sr_0.3MnO₃ was synthesized using yeast-assisted thermal-hydro decomposition. In this experiment, the solution A was prepared and poured into the activated yeast solution (0.5000 g powdered yeast cultivated in 20.0 ml 37 °C water for 0.5 h). The mixed solution was stirred with a magnetic stirrer at 37 °C for an hour, and then decomposed in an oven at 800 °C for 1.5 h. The obtained powder was ground to obtain Y-La_0.7Sr_0.3MnO₃.

Preparation of TiO₂. TiO₂ was synthesized using a sol–gel method.²⁹ 10.0 ml butyl titanate was added to a mixed solvent of 70.0 ml absolute ethanol, 10.0 ml H₂O and 4.0 ml acetic acid, which was on a magnetic stirrer to obtain a uniform sol. Then, the mixture was dried in an oven at 80 °C and calcined in a muffle furnace at 550 °C for 1.5 h and left to cool down to room temperature before being ground to obtain a TiO₂ powder.

Preparation of Y-La_0.7Sr_0.3MnO₃/TiO₂. Y-La_0.7Sr_0.3MnO₃/TiO₂ was prepared using a sol–gel method. Firstly, two mixed solutions were prepared. 0.2000 g Y-La_0.7Sr_0.3MnO₃ powder was dispersed in a mixed solvent of 35.0 ml absolute ethanol, 10.0 ml H₂O and 4.0 ml acetic acid, which was named B. Different amounts of butyl titanate were added to 35.0 ml absolute ethanol, which was on a magnetic stirrer to obtain a uniform sol (named C). Then, C was slowly dropwise added to B under vigorous mechanical stirring in a 40 °C water bath for 2 h. Then, the mixture was dried in an oven at 80 °C and calcined in a muffle furnace at 550 °C for 1.5 h and left to cool down to room temperature before being ground to obtain Y-La_0.7Sr_0.3MnO₃/TiO₂ powders. The mass fraction of TiO₂ on Y-La_0.7Sr_0.3MnO₃ was determined from the titanium dioxide converted from the butyl titanate and the composite substance was recorded as Y-La_0.7Sr_0.3MnO₃/TiO₂ (χ wt%) (χ = 0.47, 1.17, 3.51 and 5.86).

Characterization

X-ray diffraction (XRD) was used to identify the phase and crystalline nature of the samples, and was performed on a D/max-RB X-ray diffraction apparatus (Rigaku) with a Ni-filtered CuKα radiation source (λ = 1.5418 Å). UV-Vis DRS was conducted using a UV-Vis spectrophotometer (Hitachi, Japan), using BaSO₄ as a reflectance sample, and the mass of all of the as-prepared samples was equal to 0.0020 g. The measurement of PL spectra was performed on a fluorescence spectrophotometer (F-2700, Hitachi) with a Xe lamp as the excitation source (a perovskite catalyst with a wavelength of 220 nm was used; under this wavelength the emission band of TiO₂ was covered by a frequency-doubled peak, so the excitation wavelength of TiO₂ was 265 nm). N₂-physisorption analysis of the prepared catalysts was performed at 200 °C on a NOVA touch-LX⁴ surface area & pore size analyzer to obtain the textural properties of the catalysts (specific surface area and pore volume). The particle size and morphology of the samples were characterized by a JEM-2100 (200 kV) transmission electron microscope (TEM). The selected area electron diffraction (SAED) patterns from the TEM and high resolution TEM (HRTEM) images were analyzed. XPS analysis was performed by a PHI Quantera SXM apparatus, equipped with a standard Al Kα excitation source. The binding energy (BE) scale has been calibrated by measuring the C 1s peak (BE = 284.6 eV) from the ubiquitous surface layer of adventitious carbon. The magnetic measurements of the samples were carried out in a vibrating sample magnetometer (VSM) (VersaLad, Quantum Design Co., America) at room temperature in an applied magnetic field sweeping between ±30 000 Oe. An image of the yeast cells was recorded with a polarizing microscope (TL-1530, Shanghai Di Lun Optical Instrument Co., Ltd.)

Photocatalytic experiments. Photocatalytic experiments were conducted out of the chemistry department building between 9:30 a.m. and 3:30 p.m. on a sunny day, July 2016. In that period, the solar intensity fluctuation was minimal and the intensity was about 97 100 lux in the city of Taiyuan (112°33′00′′-E, 37°51′9′′-N). The photocatalytic activity of the as-prepared catalysts was evaluated using MO as a probe subject. In this experiment, 0.0050 g Y-La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃/TiO₂ and TiO₂ were dispersed into 50.0 ml MO solution (10 mg l⁻¹, pH = 3.0) in a glass reactor, which was put in the dark for 10 min to achieve adsorption–desorption equilibrium between MO and the photocatalyst. After that, the reactor was irradiated with sunlight under mechanical stirring. After the photocatalysis experiment, the powder was recovered from the solution with a magnet (50 mm × 50 mm × 10 mm, surface field of ∼3000 G), and the concentration of the MO solution was determined using the UV-Vis spectrophotometer according to its absorbance wavelength at 505 nm. 4.0 ml MO solutions were taken at regular 5 min intervals during the irradiation, and were analyzed using the UV-Vis spectrometer. The MO degradation percentage was calculated as:

Degradation rate (%) = 100(C₀ − C_t)/C₀ (1)

where C₀ and C_t (mg L⁻¹) are the concentrations of the MO solution before and after the photocatalytic degradation, respectively.

Scavenger studies. Sacrificial substrates that have high affinities for particular ROS have been reported as a method to differentiate the contribution of each ROS to the photocatalytic activity of novel materials. In this study, isopropanol (IPA, 0.1 mM), p-benzoquinone (BQ, 0.1 mM), and potassium iodide (KI, 0.1 mM) were adopted to be scavengers of the hydroxyl radicals (˙OH), superoxide radicals (˙O₂⁻) and holes (h⁺), respectively,^30,31 and the experiment was carried out according to the steps of “Photocatalytic experiments” described above.

Results and discussion

XRD analysis

The XRD patterns of the as-prepared La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) are shown in Fig. 1. The samples exhibit a clear rhombohedral structure with characteristic 2θ values at 32.7°, 40.1°, 46.8°, 58.1° and 68.5° (JCPDS no. 89-0649). No other peak is found, indicating that the perovskite oxides La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ were formed. In addition, it is noted that all of the recorded diffraction spectra reveal only the diffraction peaks of perovskite oxide La_0.7Sr_0.3MnO₃, without any additional diffraction peaks relating to Ti, which could be attributed to the low content of TiO₂ (ref. 32) that did not result in any significant change in the crystalline phase of Y-La_0.7Sr_0.3MnO₃/TiO₂. As shown in Fig. 1, the diffraction peak positions of La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ do not change, but the intensities of the characteristic diffraction peaks of Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ are weaker than those of La_0.7Sr_0.3MnO₃. Additionally, the peaks of La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) become wider and wider in sequence, indicating that the crystallite sizes decrease in sequence according to Debye–Scherrer’s formula, which is consistent with the results of the TEM. Those results indicate that the crystallite volumes indeed decrease with the addition of yeast, and that may result in bigger surface areas and higher photocatalytic activity of the as-prepared powders.

Fig. 1 XRD patterns of (a) La_0.7Sr_0.3MnO₃, (b) Y-La_0.7Sr_0.3MnO₃, and (c) Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%).

PL analysis

Photoluminescence (PL) is a powerful technique to investigate the separation efficiency of photogenerated electrons and holes of photocatalysts. PL emission results from the recombination of photogenerated electrons and holes and the PL intensity indicates the recombination rate of electron–hole pairs under light irradiation.^33,34 In Fig. 2(a), the peaks at 405–500 nm are frequency-doubled peaks of the prepared powders (a–f). The PL spectra of Y-La_0.7Sr_0.3MnO₃/TiO₂, Y-La_0.7Sr_0.3MnO₃ and La_0.7Sr_0.3MnO₃ exhibit a wide and strong emission band which is observed between 250 and 405 nm. Here, it is worth noting that the PL spectrum of TiO₂ is shown in Fig. 2(b) (where the excitation wavelength was 265 nm), and the emission band is between 360 and 500 nm. However, with an excitation wavelength of 220 nm, the emission band of TiO₂ was covered by a frequency-doubled peak, so the PL intensity of TiO₂ cannot be compared with the perovskite oxide Y-La_0.7Sr_0.3MnO₃/TiO₂. Therefore, the PL intensity of Y-La_0.7Sr_0.3MnO₃/TiO₂ was compared with that of Y-La_0.7Sr_0.3MnO₃ to investigate the recombination rate of electron–hole pairs of the composites. As can be seen from Fig. 2(a), although the PL curve of Y-La_0.7Sr_0.3MnO₃ is similar to that of La_0.7Sr_0.3MnO₃, the PL intensity of Y-La_0.7Sr_0.3MnO₃ is obviously weaker than that of La_0.7Sr_0.3MnO₃, indicating that the recombination rate of electron–hole pairs is decreased and the catalytic activity of Y-La_0.7Sr_0.3MnO₃ could be higher than that of La_0.7Sr_0.3MnO₃. So Y-La_0.7Sr_0.3MnO₃ was chosen to synthesize the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ in this study. As shown in Fig. 2(a), the PL intensity decreases with increasing content of TiO₂ from 0 to 3.51 wt%, and the PL intensity increases when the TiO₂ content is up to 5.86 wt%. This result means that when the content of the TiO₂ in Y-La_0.7Sr_0.3MnO₃/TiO₂ is 3.51 wt%, the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) has the lowest PL intensity, indicating that a Y-La_0.7Sr_0.3MnO₃/TiO₂ heterojunction can effectively suppress the recombination of electron–hole pairs and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) could be the best photocatalyst among the prepared powders.

Fig. 2 (a) Photoluminescence spectra of the catalysts (with an excitation wavelength of 220 nm): (a) La_0.7Sr_0.3MnO₃, (b) Y-La_0.7Sr_0.3MnO₃, (c) Y-La_0.7Sr_0.3MnO₃/TiO₂ (0.47 wt%), (d) Y-La_0.7Sr_0.3MnO₃/TiO₂ (1.17 wt%), (e) Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) and (f) Y-La_0.7Sr_0.3MnO₃/TiO₂ (5.86 wt%). (b) Photoluminescence spectra of TiO₂ (with an excitation wavelength of 265 nm).

UV-Vis Diffuse Reflection Spectroscopy (DRS) analysis

The photoabsorption properties of the as-prepared TiO₂, La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) (the four samples had the same mass in the UV-Vis DRS test) were detected by UV-Vis DRS, as shown in Fig. 3. It can be observed that only UV light is absorbed by the TiO₂, which is ascribed to electron excitations of TiO₂ from the valence band to the conduction band. While La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) all exhibit broad absorption from UV to visible light, and the absorbance increases in sequence, which indicates that Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%), with higher absorbance, could have higher photocatalytic activities, and would be a high-efficiency solar-light-driven photocatalyst.

Fig. 3 UV-Vis diffuse reflectance spectra of TiO₂, La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃, and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%).

XPS analysis

To verify if titanium exists in Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%), an XPS experiment was performed and the results are shown in Fig. 4. Fig. 4(a) shows that La, Sr, Mn, Ti, O and C are detected on the surface of the sample. Fig. 4(b) shows the XPS spectrum of the Ti 2p region. It demonstrates that two peaks for Ti 2p_3/2 and Ti 2p_1/2 are located at 457.8 eV and 463.6 eV, respectively.³⁵ The spin orbit splitting energy between Ti 2p_3/2 and Ti 2p_1/2 is 5.8 eV and in agreement with the result of Ti 2p corresponding to Ti⁴⁺ and only consisting of the Ti⁴⁺ chemical state.³⁶ Thus the existence of titanium in Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) is verified, and is in the form of TiO₂.

Fig. 4 XPS spectra of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) sample, (a) survey scan, (b) Ti 2p, (c) Mn 2p, (d) O 1s.

The XPS spectra of Mn 2p_1/2 and Mn 2p_3/2 of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) are shown in Fig. 4(c). The full width at half maximum (FWHM) of the Mn 2p_3/2 peak for the sample is larger than 6 eV, which explicitly indicates the coexistence of Mn²⁺, Mn³⁺ and Mn⁴⁺ ions.³⁷ It is reported that the binding energy (BE) values of Mn 2p_3/2 and Mn 2p_1/2 for the manganese cations in Mn²⁺, Mn³⁺ and Mn⁴⁺ are 641.0, 641.9, 642.1, 652.7, 653.1, and 653.8 eV, respectively.^37–39 While in this study, the peaks of Mn 2p_3/2 and Mn 2p_1/2 for the manganese cations in Mn²⁺, Mn³⁺ and Mn⁴⁺ are 641.3, 642.3, 644.55, 652.55, 653.7, and 655.5 eV, respectively. It is obviously shown that the Mn 2p_3/2 and Mn 2p_1/2 shift to higher BEs, which have been found in previous literature.³⁷ Meanwhile, the multivalent state of Mn can result in a higher catalytic oxidation activity. So Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) could be used as a high efficiency photocatalyst.

In Fig. 4(d), O 1s is decomposed into two peaks at binding energies of 529.3 eV and 531.2 eV,⁴⁰ where the two peaks stand for the lattice oxygen (O_latt) and surface-adsorbed oxygen (O_ads), respectively. O_ads is the important component for improving photocatalytic properties.

TEM analysis

TEM images of La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) are displayed in Fig. 5(a–c), and a high resolution TEM (HRTEM) image of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) with the corresponding SAED pattern is shown in Fig. 5(d). It is evident from the TEM images of the samples in Fig. 5(a–c) that the particle sizes of the catalysts are about 40–150 nm, with the average particle sizes of La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) being about 132 nm, 63 nm and 43 nm (measured by nano measurer software), respectively. The results indicate that the addition of yeast reduces the particle volume of the samples (corresponding to the results of the XRD) and could enhance their photocatalytic activity. From Fig. 5(d), by measuring the lattice fringes, the resolved interplanar distances are determined to be around 2.82 and 3.89 Å, corresponding to the (102) plane of TiO₂ (JCPDS no. 65-6429) and the (012) plane of Y-La_0.7Sr_0.3MnO₃, respectively. As Y-La_0.7Sr_0.3MnO₃ is a p-type semiconductor⁴¹ and TiO₂ is an n-type semiconductor,⁴² the results above further demonstrate that composite Y-La_0.7Sr_0.3MnO₃/TiO₂ is successfully synthesized and a p–n heterojunction is formed. Here, the heterojunction interface formed in the Y-La_0.7Sr_0.3MnO₃/TiO₂ may help to effectively separate electron–hole pairs by shortening the charge transfer distance.⁴³ This is in agreement with the PL results, indicating that the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ could enhance the photocatalytic activity.

Fig. 5 TEM images of (a) La_0.7Sr_0.3MnO₃, (b) Y-La_0.7Sr_0.3MnO₃, (c) Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) and (d) lattice fringes from HRTEM image with corresponding SAED pattern of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%).

BET analysis

The results of the adsorption–desorption isotherms indicate that TiO₂, La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) have type III isotherm shapes, which exhibit type H3 hysteresis loops as can be seen in Fig. 6(a).⁴⁴ The adsorption and desorption lines overlaps completely in the low relative pressure range, which is mainly due to a presence of non-pores. Even though the samples in this study belong to the type III-class, there are hysteresis loops in the high relative pressure region (P/P₀ > 0.8), which are proposed to be a result of bulk pores due to agglomeration of particles.

Fig. 6 (a) Adsorption–desorption isotherms of N₂ for TiO₂, La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%). (b) Optical image of yeast cells.

As shown in Table 1, the addition of yeast in the synthesis process of Y-La_0.7Sr_0.3MnO₃ leads to an increase in the BET surface area. This can be attributed to the effect of the yeast template. An optical image of yeast cells shows that the shape of the yeast cells is spherical Fig. 6(b). During the preparation of Y-La_0.7Sr_0.3MnO₃, hybrid precursors formed on the yeast cell walls were scattered evenly by the yeast template. When the hybrid precursors were calcined, the aggregation of the precursor was inhibited by the scattered spherical yeast cells. Hence, compared with La_0.7Sr_0.3MnO₃ which was not added to yeast in the synthetic process, the BET surface area of Y-La_0.7Sr_0.3MnO₃ increases.

Table 1 Overview of the surface properties of as-prepared samples

Sample	SBET (m^2 g^−1)	Pore radius Dv(r) (nm)
TiO2	1.85	8.67
La0.7Sr0.3MnO3	11.86	16.64
Y-La0.7Sr0.3MnO3	14.02	15.46
Y-La0.7Sr0.3MnO3/TiO2	12.77	16.41

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Magnetic properties

The room temperature magnetic hysteresis curves obtained from VSM measurements are shown in Fig. 7(a), and the magnetic parameters such as the saturation magnetization M_s, (M_s measured at 28.6 kOe), coercivity H_c and remanent magnetization M_r are given in Table 2. It is seen that the powders show no obvious magnetic hysteresis loops and low coercivities H_c and remanences M_r, and exhibit soft ferromagnetic behavior. The M_s value for Y-La_0.7Sr_0.3MnO₃ is similar to the value for La_0.7Sr_0.3MnO₃, but the corresponding value for Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) is lower than those of both of them. The decrease in the value for Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) is attributed to magnetization shielding provided by the non-magnetic TiO₂. It is noted that in spite of the decrease in the M_s of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%), the value is still large enough to achieve simple separation with an external magnetic field (50 mm × 50 mm × 10 mm, surface field of ∼3000 G) (Fig. 7(b)). As shown in the above experimental data, the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) exhibits stronger absorption in the UV-Vis light region, higher separation efficiency of photogenerated charge carriers and higher saturation magnetization, therefore, this study continuously investigates its photocatalytic activities under sunlight.

Fig. 7 (a) M–H hysteresis loops of La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ at room temperature. (b) Magnetic composite Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) was absolutely recovered from the solution by a magnet.

Table 2 Magnetic data for La_0.7Sr_0.3MnO₃, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%)

Sample	Hc (Oe)	Mr (emu g^−1)	Ms (emu g^−1)
La0.7Sr0.3MnO3	41.4	2.0	40.1
Y-La0.7Sr0.3MnO3	38.3	1.9	39.4
Y-La0.7Sr0.3MnO3/TiO2	24.8	0.7	32.6

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Synergetic photocatalytic effect of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%)

Before the photocatalysis, the adsorption performance of the photocatalysts was investigated. As shown in Fig. 8(a), in the dark the adsorption capacities of TiO₂, Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) become stronger and stronger in sequence. Here, the adsorption capacities could be attributed to the specific surface areas and pore radii which could promote the adsorption capacity of the organic pollutants. In this study, the BET surface area and pore radius of TiO₂ are smaller than those of Y-La_0.7Sr_0.3MnO₃ and Y-La_0.7Sr_0.3MnO₃/TiO₂ (see Table 1), and hence it has the lowest adsorption capacity among the examined powders. The pore radius of Y-La_0.7Sr_0.3MnO₃/TiO₂ is bigger than that of Y-La_0.7Sr_0.3MnO₃ while the specific surface area of the composite is smaller. The higher adsorption capacity of the composite shows that the pore radius is the deciding factor for the adsorption of large MO molecules.

Fig. 8 (a) Evolution of MO concentration according to sunlight radiation exposure time in the presence of different catalysts. (b) The UV-Vis spectral changes of MO degraded by Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%). (c) First order kinetics curves of the degradation of MO with different catalysts.

After adsorption, the photocatalytic activities of the as-prepared samples were evaluated by exposing them to the irradiation of solar light. For a blank experiment without catalysts, MO degradation under sunlight illumination is negligible (data not shown). As shown in Fig. 8(a), one can observe that Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) possesses remarkably improved photocatalytic activity over Y-La_0.7Sr_0.3MnO₃ and TiO₂, and exhibits the best degradation efficiency with about 90% MO removal after 40 min of sunlight irradiation. In Fig. 8(b), it shows the change of absorbance with time. In addition, the degradation of MO approximately followed first order kinetics, as evidenced by the linear plot of −ln(C/C₀) vs. photocatalytic time t (Fig. 8(c)). We obtained the rate constants of k₁ (Y-La_0.7Sr_0.3MnO₃/TiO₂), k₂ (Y-La_0.7Sr_0.3MnO₃) and k₃ (TiO₂) to be 3.67 × 10⁻² min⁻¹, 2.103 × 10⁻² min⁻¹ and 0.546 × 10⁻² min⁻¹, respectively. Furthermore, the MO degradation rate constant of Y-La_0.7Sr_0.3MnO₃/TiO₂ was found to be even higher than the sum of Y-La_0.7Sr_0.3MnO₃ and TiO₂, which suggests that the synergetic effect of Y-La_0.7Sr_0.3MnO₃/TiO₂ could contribute to the whole catalytic activity. Here, we quantitatively evaluate the synergetic effect in the MO degradation system by using the SI:⁴⁵

SI = k₁/[(1 − x)k₂ + xk₃] (2)

where x is the content of TiO₂ in Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%); here x = 3.51%. k₁, k₂ and k₃ are the first-order rate constants of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%), Y-La_0.7Sr_0.3MnO₃ and TiO_2, respectively. The value of SI was obtained to be 1.79, implying that this synergistic effect can enhance the catalytic activity of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) by 79.0%.

The photocatalytic recyclability and repeated performance of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) were studied using the photocatalytic degradation of MO under sunlight irradiation for 50 min. After each cycling photocatalytic experiment, the photocatalyst was collected using a magnet for the next recycling. From Fig. 9, the photocatalytic degradation efficiency slightly dropped over 8 cycles, however, the photocatalyst Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) still maintained high photocatalytic activity, indicating that Y-La_0.7Sr_0.3MnO₃/TiO₂ is very stable under the photocatalytic reaction conditions, and it can be recycled by magnetic field and possesses high reusability.

Fig. 9 The recycled photocatalytic experiments of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) for the degradation of MO under sunlight.

Photocatalytic mechanism

Active species trapping experiments. In the photocatalytic degradation process, a series of photogenerated main active species of h⁺, ˙OH and ˙O₂⁻ are suspected to be involved in the photocatalytic reaction. To further evaluate the roles of these active species during the photodegradation of MO, different scavengers were introduced to quench the relevant active species. As shown in Fig. 10, the photocatalytic degradation of MO over the Y-La_0.7Sr_0.3MnO₃/TiO₂ composite was affected by the addition of different scavengers, and the sequence of the photocatalytic degradation efficiency of MO in descending order is isopropanol (IPA: the scavenger of ˙OH) > potassium iodide (KI: the scavenger of h⁺) > p-benzoquinone (BQ: the scavenger of ˙O₂⁻), indicating that superoxide radicals (˙O₂⁻), holes (h⁺) and hydroxyl radicals (˙OH) are all active species and the sequence of the contributions of the ROS to the photocatalytic activity of the composite catalyst is ˙O₂⁻ > h⁺ > ˙OH.

Fig. 10 Plots of photogenerated carrier trapping for the photocatalytic degradation of MO.

Photocatalytic mechanism. For explaining the photocatalytic degradation mechanism, the valence band edge positions of Y-La_0.7Sr_0.3MnO₃ and TiO₂ should be confirmed. The band gap energy (E_g) can be estimated from the extrapolation to the energy axis of a plot of (Ahν)² against hν. Fig. 11 illustrates the plots of (Ahν)₂ versus hν for TiO₂ and Y-La_0.7Sr_0.3MnO₃ derived from the data of the UV-Vis diffuse reflectance spectra (Fig. 3),⁴¹ using the equation:⁴⁶

(Ahν)ⁿ = hν − E_g (3)

where E_g, A, h, and ν are the band gap, absorbance, Plank constant and light frequency, respectively. n is an index which depends on the electronic transition of the semiconductor. n = 2 for a direct-gap semiconductor and n = 1/2 for an indirect-gap one. Y-La_0.7Sr_0.3MnO₃ is a direct semiconductor,⁴⁷ thus, the value of n equals 2. According to Fig. 11, the band gaps energies are estimated as 1.3 and 3.2 eV for Y-La_0.7Sr_0.3MnO₃ and TiO₂, respectively.

Fig. 11 The estimated E_g values of TiO₂ and Y-La_0.7Sr_0.3MnO₃ using UV-Vis DRS data.

The migration direction of the photogenerated charge carriers in the composite is related to the band edge positions of the semiconductors. The valence band (VB) and conduction band (CB) edge position of a semiconductor is estimated according to the following equation:³¹

E_CB = X − E_C − 0.5E_g (4)

E_VB = E_CB + E_g (5)

where X is the electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms, E_g, E_C, E_CB and E_VB are the band gap energy, the energy of free electrons on the hydrogen scale (about 4.5 eV), and the conduction band and valence band edge potentials, respectively. The X values for Y-La_0.7Sr_0.3MnO₃ and TiO₂ were estimated to be 5.34 and 5.81 eV, respectively.

Based on the band gap structure calculations from the data of the UV-Vis diffuse reflectance spectra (Fig. 3), the calculated E_CB and E_VB edge positions of TiO₂ are −0.29 and 2.91 eV, respectively. As for Y-La_0.7Sr_0.3MnO₃, the E_CB and E_VB edge positions are 0.19 and 1.49 eV, respectively. The energy band structures of Y-La_0.7Sr_0.3MnO₃ and TiO₂ are schematically illustrated in Fig. 12, and it seems that the nested band structure is unfavorable for the separation of photogenerated carriers. However, it is known that Y-La_0.7Sr_0.3MnO₃ is a p-type semiconductor, whose Fermi energy level (E_F) is close to the valence band, and TiO₂ is an n-type semiconductor, whose Fermi energy level is close to the conduction band. Moreover, Fermi energy levels of semiconductors can be substantially shifted and/or pinned by surface states in a composite.⁴⁸ When the Y-La_0.7Sr_0.3MnO₃ semiconductor is hybridized by TiO₂ to form a p–n heterojunction structure, the electrons will diffuse in the opposite direction, resulting in an accumulation of negative charges in the Y-La_0.7Sr_0.3MnO₃ region near the junction. Also, holes diffuse from the Y-La_0.7Sr_0.3MnO₃ to the n-TiO₂ region, forming a positive section in the vicinity of the heterojunction. When the Fermi levels of Y-La_0.7Sr_0.3MnO₃ and TiO₂ align under thermodynamic equilibrium conditions, an internal electric field directed from the n-TiO₂ to the p-Y-La_0.7Sr_0.3MnO₃ is simultaneously built to stop the charge diffusion. Meanwhile, the all energy band of Y-La_0.7Sr_0.3MnO₃ was shifted upwards in this process whereas that of TiO₂ was shifted downwards until an equilibrium state of Fermi levels of p-Y-La_0.7Sr_0.3MnO₃ and n-TiO₂ was obtained after hybridization. As a result, the CB bottom of Y-La_0.7Sr_0.3MnO₃ became higher than that of the n-TiO₂ semiconductor. Under solar irradiation, Y-La_0.7Sr_0.3MnO₃ is excited and there is the generation of electron–hole pairs. According to the schema of Fig. 12, the photogenerated electrons move to the positive field (n-type TiO₂) and then react with dissolved O₂ in solution to produce reactive ˙O₂⁻. At the same time, the photogenerated holes (h⁺) move into the negative field (p-type Y-La_0.7Sr_0.3MnO₃) and oxidize OH⁻ or H₂O to ˙OH as well as directly oxidizing MO. Thus an efficient separation of photoinduced electron–hole pairs can be achieved in the formed heterojunction between the semiconductors, which leads to better photocatalytic activity than pure Y-La_0.7Sr_0.3MnO₃ and pure TiO₂. This is in agreement with the synergetic photocatalytic effect of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) described above.

Fig. 12 The proposed photocatalytic mechanism for the photodegradation of MO over Y-La_0.7Sr_0.3MnO₃/TiO₂.

On the basis of the above results and previous studies, a possible enhancement mechanism for the photocatalytic activity of the Y-La_0.7Sr_0.3MnO₃/TiO₂ composite can also be proposed. Firstly, the hybridization of Y-La_0.7Sr_0.3MnO₃ with TiO₂ is beneficial to the improvement of the solar light absorbing capacity in this study, which has been demonstrated using UV-Vis DRS analysis. Secondly, the La_0.7Sr_0.3MnO₃/TiO₂ heterojunctions can enhance charge transfer and inhibit the recombination of photogenerated electron–hole pairs, which is very important for the improvement of photocatalytic activity. Hence, the Y-La_0.7Sr_0.3MnO₃/TiO₂ composite is a high-efficiency solar-light-driven magnetic photocatalyst.

Conclusions

In summary, the solar-light-driven Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) with intrinsic magnetic properties can be synthesized and exhibits an obvious synergistic photocatalytic effect on the degradation of MO wastewater under sunlight. The enhanced photocatalytic performance could be mainly attributed to a decrease in the recombination of photogenerated carriers on the surface of the p–n heterojunction structure in Y-La_0.7Sr_0.3MnO₃/TiO₂. The photocatalytic activity of Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) does not obviously decrease after 8 times recycling, indicating that the Y-La_0.7Sr_0.3MnO₃/TiO₂ can be recycled using an external magnetic field more easily and can be used repeatedly. It is believed that the composite Y-La_0.7Sr_0.3MnO₃/TiO₂ (3.51 wt%) photocatalyst would have potential applications in wastewater treatment in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project was supported by the Natural Science Fund Council of China (No. 21371159) and the Science and Technology Item in Taiyuan Shanxi (No. 120247-13).

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