Hierarchical porous TiO2 photocatalytic nanomaterials were fabricated by impregnation and calcination using a peanut shell biotemplate, and TiO2/BiFeO3 composite nanomaterials with different doping amounts were fabricated using hydrothermal synthesis. The micromorphology, structure, element composition and valence state of the photocatalyst were analyzed using a series of characterization methods, including X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), BET surface area (BET), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance (UV-vis), fluorescence spectroscopy (PL) and other technological means. Finally, the degradation mechanism and efficiency of BiFeO3 composite photocatalyst on the target pollutant triclosan were analyzed using a xenon lamp to simulate sunlight. The results showed that TiO2/BiFeO3 catalyst fabricated using a peanut shell biotemplate has a specific surface area of 153.64 m2/g, a band gap of 1.92 eV, and forms heterostructures. The optimum doping amount of TiO2/BiFeO3 catalyst was 1 mol/mol, and the degradation rate was 81.2%. The main active substances degraded were ·O2and ·OH. The degradation process measured is consistent with the pseudo-first-order kinetic model.

  • TiO2 loading can increase the specific surface area and empty volume of BiFeO3 and provide more active sites.

  • Through total organic carbon analysis, the mineralization rate of total organic carbon of TiO2/BiFeO3 photocatalyst is 58.9%.

  • h+ and ·OH are the active substances for degradation of triclosan. Relatively speaking, · OH has more obvious inhibition on triclosan and is the main active oxidizing substance.

The anti-fungal and antibiotic agent triclosan (TCS) can be commonly found in hygiene products such as soaps and toothpaste. However, the ubiquity of TCS has led to concerns about its increasing presence in the water environment and its potential to cause harm. TCS is continuously accumulated and enriched by domestic sewage systems, and ultimately discharged into the environment. The sewage and sludge containing TCS seriously affect the water environment, not only endangering aquatic animals and plants but also threatening human health through food chain disruption (Chen et al. 2019a; Zheng et al. 2020). As a result of human activities and the large consumption of chemicals, many pollutants are released into the environment. Pharmaceuticals and personal care products (PPCPs) are frequently detected worldwide, which has become an emerging risk contaminant and attracted increasing international attention. PPCPs mainly include pharmaceuticals and other chemical consumer goods, veterinary medicine, nursing products and additives and inert ingredients used in the production and processing process. Triclosan (TCS) is a typical PPCP chemical, which is often added to consumers' daily care products as a broad-spectrum fungicide. It can be found in soap, toothpaste, cosmetics, medical disinfectants and other sanitary products. According to statistics (Zhao et al. 2013a), as early as 2006, the annual consumption of TCS in Europe exceeded 450 tons, and the annual consumption in the United States exceeded 600 tons. Many nursing products after daily use enter the sewage treatment system along with sewage and eventually enter the environment. At present, as one of the ten most common organic pollutants in organic wastewater, TCS has been listed as one of the most concerning emerging pollutants. With the continuous detection of TCS in the environment, the demand for efficient removal of TCS technology is particularly urgent. Research on the toxic effects of TCS can be traced back to the 1970s when researchers discovered that people with sensitive skin would develop allergic reactions when using soap, and the higher the content of TCS, the more obvious the degree of allergy (Ouyang et al. 2020; Farhadi et al. 2021; Saravanakumar & Park 2021). Now, researchers have found that the long-term presence of TCS in the environment may contaminate soil and harm aquatic organisms, mammals and human beings (Castro et al. 2017; Sharipova et al. 2017; Chen et al. 2019b; Subramanian et al. 2019).

Photocatalytic technology is a newly emerging technology, involving multiple fields, such as nanometer materials, catalytic chemistry and materials science (Schneider et al. 2014; Zhao et al. 2014). Since photocatalytic technology uses renewable solar energy to degrade pollutants under natural light (Dona-Rodriguez & Pulido Melian 2021) and is environmental-friendly, it has currently become an important means to treat micropollutants in sewage. In recent years, the development of new photocatalysts has become a much discussed subject, having far-reaching significance for the degradation of environmental organic matter. Photocatalysis refers to the process where semiconductor photocatalytic materials convert the energy gained from ultraviolet or visible light exposure into chemical energy. Through a series of reactions, this chemical energy is then used to decompose harmful organic matter into non-polluting organic matter (Zhang et al. 2007; Mao et al. 2020). The energy band theory can also explain the principles of semiconductor photocatalysis. The electronic energy levels at the interior of semiconductors are relatively independent, mainly including conduction band and valence band (Soltani & Lee 2016a; Singh & Rajput 2019). When the light shines on the surface of the semiconductor catalyst with a photon energy greater than the band gap width (Eg), the electrons on the semiconductor valence band are excited to transition from the valence band (VB) to the conduction band (CB), forming photogenerated electrons (e) and holes (h+). The strong oxidation potential of h+ can generate hydroxyl radicals (•OH) from H20 adsorbed on the catalyst surface and generate superoxide radicals (•O2) through the interaction of photogenerated electrons (e) and O2. These free radicals also with strong oxidation potential can degrade organic pollutants such as dyes into non-toxic or less toxic organic small molecules, even CO2 and H2O, to achieve the purpose of advanced treatment of pollutants.

The forbidden bandwidth of semiconductor materials determines the intrinsic electrical and optical properties of the materials. Specifically, under illuminated conditions, if the energy absorbed by valence electrons is greater than or equal to the forbidden band width energy gap, then electrons in the valence band of the material can be excited, and the electrons can migrate to the conduction band of the material. When the conduction band receives electrons to form photogenerated electrons, the valence band will lose electrons and form photogenerated holes (Kim et al. 2013; Soltani & Lee 2016b; Jiao et al. 2017; Chang et al. 2019). Photogenerated holes and electrons (photogenerated carriers) have oxidation and reduction potential and can oxidize organic pollutants into small inorganic molecules, and also generate strong oxidizing •OH through H2O on the surface of semiconductor materials to oxidize organic pollutants (Abraham & Mathew 2021).

Bismuth ferrite (BiFeO3) is widely studied as a multiferroic material (Rong et al. 2016; Sazali et al. 2019). Since the forbidden band width of bismuth ferrite is around 2.2 eV to 2.8 eV (Maleki et al. 2017; Wu et al. 2018; Xu et al. 2018; Yan & Liu 2019), it can respond to visible light to some extent, and belongs to a new type of photocatalytic materials (Maleki 2018; Greculeasa et al. 2019; Lu & Qi 2019). However, like most semiconductor materials, its low quantum efficiency prohibits its application in wastewater treatment (Soltani & Lee 2017; Li et al. 2018, 2019a). Therefore, the key to enhancing the activity of BiFeO3 materials is to control the migration and separation of photogenerated charges. At present, the common methods to improve the photocatalytic activity of BiFeO3 materials mainly include controlling the morphology and particle size, doping (Zhang et al. 2014; Yu et al. 2015; Guan et al. 2018; Abdul Satar et al. 2019; Li et al. 2019b) and forming composites with other semiconductor materials. The modified BiFeO3 can significantly inhibit the recombination of photogenerated carriers and enhance its response to visible light (Wang et al. 2019). While the research is currently in the early stages, the performance of BiFeO3 in current experiments shows great potential for future developments, such as the design and preparation of multi-dimensional nanostructured photocatalytic materials with more practical properties such as nanotubes, nanorods and nanofibers (Zhu et al. 2012; Pi et al. 2017; Zatsepin et al. 2017). In addition, the photocatalytic mechanism of the BiFeO3 complex can be further explored to comprehensively analyze its energy band structure and interface characteristics, and more intuitively understand the photon conversion and photocatalytic process. It can also be used to develop economical and environmentally friendly modified materials with strong light stability, and further explore the applications of BiFeO3 in the photocatalytic reduction of CO2 and the decomposition of water for hydrogen production (Pang & Lei 2016; Chandel et al. 2017; Wei et al. 2017).

By combining wide band-gap semiconductor TiO2 with narrow band-gap semiconductor BiFeO3, the TiO2/BiFeO3, heterostructure exhibits higher photocatalytic activity than either pure TiO2 or BiFeO3. The formation of the TiO2/BiFeO3 heterostructure improves the photocatalytic performance, enhances the photoinduced charge transfer and promotes the separation of photogenerated electrons and holes. As far as we know, there were no reports on the preparation of TiO2/BiFeO3 photocatalytic materials using peanut shell biotemplate. The biotemplate is used to prepare photocatalysts based on predetermined structures and functions, that have unique advantages such as variable structure and superior functions. The original morphology and microstructure of the template can be completely replicated with peanut shell biomimetic materials. In the following research, the preparation of hierarchical porous TiO2 catalyst from bio-templated peanut shells is presented in detail (Zhao et al. 2013b; Chandel et al. 2017; Wang et al. 2018). On this basis, a new type of TiO2/BiFeO3/biochar composite is explored using a hydrothermal synthesis method doped with different amounts of BiFeO3. Discussions focus on the synthesis and characterization of TiO2/BiFeO3 hierarchical porous material and the influencing factors on the photocatalytic degradation of TCS. Thus, the possible degradation mechanism can be deduced, which may contribute to the provision of a new green and efficient method for photocatalytic TCS degradation.

Chemicals and reagents

Titanium dioxide (analytically pure, MacLean Biochemical Technology Co., Ltd), hydrochloric acid, ammonia, anhydrous ethanol (analytically pure, Beijing Chemical Plant), bismuth nitrate, iron nitrate (Tianjin Guangfu Fine Chemical Research Institute), triclosan (Shanghai Aladdin Bio-Chem Technology Co., Ltd), sodium oxalate, isopropanol, sodium thiosulfate, p-Benzoquinone (analytically pure, Tianjin Guangfu Fine Chemical Research Institute), and peanut shells (harvested cropland).

Glass instruments used in the experiment (Tianjin Tianbo Glass Instrument Co., Ltd), PL-X300D simulated fluorescent xenon lamp source (Beijing Prinsess Technology Co., Ltd), muffle furnace (KSI-60-16 Longkou Xianke Co., Ltd), high-performance liquid chromatography (1200LC Agilent), total organic carbon analyzer (TOC-VCPH, Shimadzu, Japan), X-ray photoelectron spectrometer (SmartLab (3KW), Japan Science of Physics), X-ray diffractometer (XD-3 Shimadzu, Japan), scanning electron microscope (XL-30ESEM, USA FEI), transmission electron microscope (FEI G2 F20, USA FEI), specific surface area and pore size analyzer (ASAP2020, USA Micrometrics), ultraviolet-visible spectrophotometer (UV-2550 Shimadzu, Japan).

Preparation of biomorphic TiO2/BiFeO3

Pretreatment of peanut shells

A number of peanut shells were selected and washed in water 3-5 times to remove impurities from the surface. The washed peanut shells were then soaked in 5% hydrochloric acid for 3 h to remove excess ions, to provide more growth sites, and then washed with distilled water repeatedly until pH neutral. Extracted peanut shells were soaked in hydrochloric acid with 5% dilute ammonia water for 3 h to increase the absorbability. The peanut shells after ammonia extraction and acid soaking were then washed with distilled water repeatedly, and dried in a 60 °C oven for standby.

Preparation of hierarchical porous TiO2

  • (1)

    Preparation of tetrabutyl titanate solution: 200 mL tetrabutyl titanate was added to 600 mL ethanol, and then 2 mL glacial acetic acid was added to prevent hydrolysis of the tetrabutyl titanate.

  • (2)

    The pretreated peanut shells were immersed in the prepared tetrabutyl titanate solution. This was sealed and placed aside for 48 h, and then the solution was poured out, leaving behind the peanut shells. The peanut shells were then washed with anhydrous ethanol three times to remove the tetrabutyl titanate adhering to the surface. The peanut shells were then washed three times with distilled water and dried.

  • (3)

    The dried peanut shells were calcined at around 100 °C in a muffle furnace, the temperature was then increased to 550 °C at a rate of 2 °C/min, this was then left to anneal for 4 h to remove the peanut shell template completely. Finally, pure TiO2 with a template was obtained.

In this experiment, tetrabutyl titanate (Ti(OC4H9)4) abbreviated as Ti-OR was used as the precursor to TiO2, absolute ethanol (C2H5OH) was used as the solvent, and glacial acetic acid (CH3COOH) was used as the inhibitor to prepare TiO2 sol. Ti(OBu)4 contains active butoxy reaction groups, which will generate a strong hydrolysis reaction when meeting with water. Once the hydrolysis reaction occurs, the dehydration polycondensation and alcohol loss polycondensation will be carried out successively. The main reactions that occurred are as follows:
formula
(1)
formula
(2)
formula
(3)
formula
(4)

Preparation of BiFeO3 and TiO2/BiFeO3 with different doping ratios by hydrothermal synthesis

10 mmol (4.85 g) Bi (NO3)3•5H2O and 10 mmol (4.04 g) Fe (NO3)3•9H2O were dissolved in 200 mL 0.1M dilute nitric acid. White templated TiO2 powder was added to the mixed solution. 3M preprepared NaOH was then added to the solution under strong stirring, leaving a reddish-brown precipitate to form. The precipitate was evenly mixed using ultrasound for 30 min. The precursor was then transferred to a 100 mL stainless steel PTFE-lined kettle with a filling degree of 70–80%, and the kettle was heated in an oven at 180 °C for 24 h. After the reaction, the solution was allowed to cool to room temperature undisturbed. The mixture was then centrifuged at 6,000 r/min for 30 min and the supernatant removed. The precipitate was then washed with anhydrous ethanol three times to obtain TiO2-BiFeO3 powder and dried in an oven at 60 °C. TiO2 of different mass percentages was prepared (0wt% (0.00 g), 5wt% (0.469 g), 8wt% (0.774 g), 10wt% (0.989 g) and 15wt% (1.571 g) with the above method, and named as BiFeO3, 0.5TiO2-BiFeO3, 0.8TiO2-BiFeO3, 1.0TiO2-BiFeO3 and 1.5TiO2-BiFeO3 composites, respectively. The main reactions are as follows:
formula
(5)
formula
(6)
formula
(7)

Photocatalytic performance evaluation

In this experiment, TiO2, BiFeO3 and TiO2/BiFeO3 with different doping ratios (0.5 mol/mol, 0.8 mol/mol, 1 mol/mol, and 1.5 mol/mol, respectively) were selected as photocatalysts to degrade TCS. The optimal doping ratio and photocatalyst were determined by comparing the concentration variation of TCS degraded. To study the effect of photocatalysts on the degradation of TCS, a xenon lamp was used to simulate sunlight to carry out a degradation experiment. In this experiment, a pseudo-first-order dynamics model was then used to fit the photocatalytic degradation process of TCS. The quasi-first-order kinetic model was used to analyze the effect of the photocatalyst on the degradation rate of TCS. The peak area was then detected by HPLC to allow the calculation of the concentration of TCS. The degradation rate was calculated as shown in Formula (8). The catalyst was added to a final concentration of 0.5 g/L in a 10 mg/L TCS solution. The solution was dark adsorbed for 30 min before illumination to achieve the adsorption-desorption equilibrium state of the catalyst and TCS solution. The xenon lamp was used as the light source in the experiment, and the power of the xenon lamp was set to 300 W. During the whole light irradiation experiment, a magnetic stirrer was used to keep the catalyst and TCS solution uniformly mixed. The whole experimental device is shown in Figure 1. The solution was sampled every 30 min, passed through 0.22 μm organic filter membrane filtration, and then the peak area was measured by high-performance liquid chromatography to calculate the concentration value of TCS. The calculation formula of TCS is shown in (9):
formula
(8)

TCS detection method:

X-bridge@C18 (4.6 mm*250 mm, 5 μm) reverse chromatographic column and Waters e2695 high-performance liquid chromatograph with Waters 2998 diode array tube detector detect TCS.

Liquid phase detection conditions: the mobile phase is acetonitrile ultrapure water (75:25 v/v), the total flow rate was 1.0 mL/min, the sample volume was 10 μL, the detection wavelength was 230 nm, the column temperature was 35 °C, the peak time of the target substance was 7.8 min, and the total detection time of the sample was 10 min.
formula
(9)
wherein: A0 was the initial peak area of the sample; At was the peak area of the sample at time t; Ct was the initial peak concentration of the sample; C0 was the concentration of the sample at time C0.

Characterization results of catalyst structure

To analyze the phase composition and crystal structure of the material, the TiO2/BiFeO3 photocatalyst was analyzed using XRD. Figure 2 shows the TiO2/BiFeO3 photocatalysts with different doping amounts (respectively 0.2 mol/mol, 0.5 mol/mol, 0.8 mol/mol and 1 mol/mol). It can be seen from the figure that TiO2/BiFeO3 photocatalyst was composed of TiO2 photocatalyst and BiFeO3 photocatalyst, and there were no characteristic peaks of other substances. Moreover, the crystal structure and main phase composition can be clearly seen, and the characteristic diffraction peaks at 2θ = 25.34°, 37.91°, 48.12°, and 55.07° directly correspond to the crystal planes (101), (111), (200) and (211) (Huo et al. 2019), which are consistent with the TiO2 photocatalytic material of JCPDS No.74-0534; the characteristic diffraction peaks at 2θ = 23.15°, 32.46°, 45.91°, and 57.32° directly correspond to the crystal planes (100) (Dumitru et al. 2019), (110), (200), and (211), which are consistent with the BiFeO3 photocatalytic materials of JCPDS No. 26-1044. The results show that the peanut shell biotemplate may be removed after calcination at high temperatures, and the doping of these two photocatalysts does not affect their crystal structures, wherein, the average particle size of 1TiO2/BiFeO3 photocatalyst is 14.98 nm. With the increase in doping amount of BiFeO3 photocatalyst, the diffraction peak of TiO2 photocatalyst increased, so the diffraction peak of BiFeO3 photocatalyst first increased and then decreased. This is mainly because BiFeO3 photocatalyst can cover the surface of TiO2 photocatalyst, while TiO2 inhibits the growth direction of BiFeO3. With increasing doping ratio, the photocatalysis first increased and then decreased, this may have been caused by the different mass percentages of TiO2. We can infer from Table 1 that the doping of TiO2 will reduce the band gap width and increase edge absorption, resulting in different diffraction peak intensities. However, too much TiO2 occupies the material surface, which reduces the active sites of the photocatalyst, thus reducing the contact between the photocatalyst and TCS, and so the degradation rate is reduced.
Table 1

Band gap widths of different catalysts

Name of catalystTiO2 photocatalystBiFeO3 photocatalyst0.5TiO2/BiFeO3 photocatalyst0.8TiO2/BiFeO3 photocatalystTiO2/BiFeO3 photocatalyst1.5TiO2/BiFeO3 photocatalyst
Absorption edge (nm) 378 664 665 681 684 700 
Band gap (eV) 3.26 2.23 2.57 2.37 1.92 2.49 
Name of catalystTiO2 photocatalystBiFeO3 photocatalyst0.5TiO2/BiFeO3 photocatalyst0.8TiO2/BiFeO3 photocatalystTiO2/BiFeO3 photocatalyst1.5TiO2/BiFeO3 photocatalyst
Absorption edge (nm) 378 664 665 681 684 700 
Band gap (eV) 3.26 2.23 2.57 2.37 1.92 2.49 
Figure 1

Schematic diagram of the experimental device.

Figure 1

Schematic diagram of the experimental device.

Close modal
Figure 2

XRD patterns of TiO2, BiFeO3 (a) and TiO2/BiFeO3 (b) composite photocatalysts.

Figure 2

XRD patterns of TiO2, BiFeO3 (a) and TiO2/BiFeO3 (b) composite photocatalysts.

Close modal
To describe the morphology and particle size of nanomaterials more intuitively and clearly, scanning electron microscopy (SEM) was used to test and analyze the nanomaterials in this chapter. Figure 3(a) and 3(b) shows the SEM images of peanut shells, and Figure 3(c) and 3(d) shows the SEM images of peanut shells loaded with TiO2. Therefore, peanut shells can composite with photocatalytic materials to improve the morphology and structural characteristics of the photocatalyst.
Figure 3

The SEM results of peanut shell (a) and (b) and peanut shell loaded with TiO2 (c) and (d).

Figure 3

The SEM results of peanut shell (a) and (b) and peanut shell loaded with TiO2 (c) and (d).

Close modal

The change in crystalline form of silica has a significant impact on the collapse of the residual structure of photocatalytic materials, indicating that the silica carbon network structure can act as a specific template (Gong et al. 2013). This template can provide natural microchannels for the photocatalytic materials, increasing the specific surface area of the TiO2 catalytic materials. After the peanut shells were soaked in tetrabutyl titanate, the peanut shells were completely immersed in the titanium solution, and the porous structure on the inner surface of the peanut shells was opened by high-temperature calcination, which allowed tetrabutyl titanate solution to enter its internal structure and react with carbon to form TiO2-CX structures (Khan et al. 2020). It can be seen from Figure 3(c) and 3(d) that the TiO2 photocatalyst was prepared using a peanut shell biotemplate had a spherical structure and smooth surface. Its structure was closely connected and closely arranged, which can present the characteristics of the hierarchical porous structure of peanut shells, increase the number of active sites and hence effectively improve the photocatalytic activity. However, it can be seen from the figure that there were some incomplete fragments. The reason for this may be that the peanut shell itself was damaged or the internal structure of the peanut shell template had defects.

Figure 4 shows the SEM diagrams of 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate under different magnification conditions. After comparing Figures 3 and 4, we can see that the TiO2 photocatalytic material was of spherical structure, closely arranged, BiFeO3 photocatalyst adhered to its surface, showing a cotton-like structure, and BiFeO3 photocatalyst inhibited the growth direction of TiO2 photocatalyst. It can be seen from Figure 4(c) and 4(d) that the BiFeO3 photocatalyst was almost completely covered with TiO2 photocatalyst, forming a heterostructure. This kind of structure can inhibit the growth of BiFeO3 nanoparticles and reduce the size of the composite to provide more reaction sites for the photocatalyst. By comparison, it can be seen that bismuth ferrite composite material had a better degradation effect on TCS than TiO2 photocatalytic material alone. The reason is that the size of the composite is reduced and the number of active sites is increased, thus the photocatalytic efficiency is improved.
Figure 4

The SEM results of TiO2/BiFeO3 photocatalyst 5k (a),10k (b, c), 20k (d) prepared by biological template in peanut shell.

Figure 4

The SEM results of TiO2/BiFeO3 photocatalyst 5k (a),10k (b, c), 20k (d) prepared by biological template in peanut shell.

Close modal
To analyze the interior structure and lattice fringe size of the material, the 1TiO2/BiFeO3 photocatalyst was analyzed using transmission electron microscopy (TEM). Figure 5 shows the TEM diagrams of 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate under different magnification conditions. From Figure 5(a) and 5(b), it can be seen that the growth composition of different crystal faces of 1TiO2/BiFeO3 particles was relatively rough and compact. It can be seen from Figure 5(c) and 5(d) that the BiFeO3 photocatalyst was not only embedded in the surface of the TiO2 photocatalyst, but also in the interior. The results show that the lattice fringes of BiFeO3 photocatalyst embedded in the interior were well distributed and clear. Measuring the lattice spacing with digital micrograph and comparing with the data of JCPDS standard card, the spacing of one group of lattice fringes was 0.19 nm (Mohanty et al. 2022), corresponding to the lattice plane of TiO2 photocatalyst (101), and the spacing of the other group of lattice fringes was 0.22 nm, corresponding to the lattice surface of BiFeO3 photocatalyst (110) (Nazir et al. 2021). The results confirm the existence of a heterostructure which expands the visible light response range and improves photocatalytic activity.
Figure 5

The TEM results of TiO2/BiFeO3 photocatalyst 100 nm (a), 50 nm (b), 5 nm (c), 2 nm (d) prepared by biological template in peanut shell.

Figure 5

The TEM results of TiO2/BiFeO3 photocatalyst 100 nm (a), 50 nm (b), 5 nm (c), 2 nm (d) prepared by biological template in peanut shell.

Close modal
To understand the nitrogen adsorption-specific surface area and pore size of 1TiO2/BiFeO3 photocatalyst, nitrogen isothermal adsorption-desorption was used for analysis. Figure 6(a) shows the nitrogen adsorption-desorption isotherm of 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate, 1TiO2/BiFeO3 has a specific surface area of 59.59 m2/g, it can be seen that the adsorption curve of 1TiO2/BiFeO3 photocatalyst is V type and there is H3 type hysteresis loop. Figure 6(b) shows the BJH pore size distribution curve of 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate, indicating that 1TiO2/BiFeO3 photocatalyst had microporous and mesoporous structure, and aperture distribution is mainly 1–20 nm.
Figure 6

Nitrogen adsorption isotherm (a) and pore size distribution curve (b) of 1TiO2/BiFeO3 photocatalyst.

Figure 6

Nitrogen adsorption isotherm (a) and pore size distribution curve (b) of 1TiO2/BiFeO3 photocatalyst.

Close modal
X-ray photoelectron spectroscopy (XPS) was used to analyze the valence state of the composite photocatalytic materials to clearly understand the main element types, chemical composition, element valence state, chemical bond stability of the samples, and whether the valence state of each element in the composite has changed. Figure 7(a) shows the full XPS spectrum of 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate. The main elements are Ti, Fe, Bi, O and C. Wherein, C element may mainly have come from two sources: first, the presence of C pollution in the air, and second, the carbon source pollution calibrated with 248.8 eV peak as the reference peak. Figure 7(b) is the XPS spectrum of Ti2p of 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate. Wherein, the binding energy peak values of Ti2p3/2 and Ti2p1/2 are about 458.4 eV and 464.8 eV, respectively, indicating that there is a positive tetravalent state of Ti (Dai et al. 2013; Masingboon & Maensiri 2013; Zhu et al. 2014a; Cai et al. 2017). Figure 7(c) shows the XPS spectrum of Fe2p in 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate. Wherein, the binding energy peak values of Fe2p3/2 and Fe2p1/2 are around 710.9 eV and 724.5 eV, respectively, indicating that the Fe valence in 1TiO2/BiFeO3 photocatalyst is positively trivalent (Jiang et al. 2011; Katiyar et al. 2014; Liu et al. 2017). Figure 7(d) shows the XPS spectrum of Bi4f in 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate. Wherein, the binding energy peak values of Bi4f7/2 are Bi4f5/2 are around 159.0 eV and 164.3 eV, respectively, indicating that the Bi valence state in the photocatalyst is positively trivalent (Zhu et al. 2011). Figure 7(e) shows the O1s spectrum in 1TiO2/BiFeO3 photocatalyst prepared with peanut shell biotemplate. By fitting the O1s peaks with background removed using Lorentzian Gaussian in Xpseak 4.1 program, the O1s spectrum in 1TiO2/BiFeO3 photocatalyst can be divided into two peaks of 529.8 eV and 531.5 eV, and a Ti-O bond may be formed at 529.8 eV (Nazir et al. 2021). The peak at the binding energy of 531.5 eV may be related to the oxide adsorbed by the catalytic material. The above analysis shows that TiO2 is successfully loaded into BiFeO3, and 1TiO2/BiFeO3 composite photocatalyst is obtained without altering the electronic structure of BiFeO3. Figure 7(f) shows 284.93 eV, which demonstrates that there is inorganic carbon residue in peanut shell after calcination.
Figure 7

XPS pattern of 1TiO2/BiFeO3 (a) full pattern; (b) Ti 2p; (c) Fe 2P; (d) Bi 4f; (e) O 1s; and (f) C 1s.

Figure 7

XPS pattern of 1TiO2/BiFeO3 (a) full pattern; (b) Ti 2p; (c) Fe 2P; (d) Bi 4f; (e) O 1s; and (f) C 1s.

Close modal
In order to test the light response ability of photocatalyst materials, we used UV-Vis DRS to analyze the composite photocatalyst. Figure 8 shows the UV-Vis DRS spectra of TiO2, BiFeO3 and TiO2/BiFeO3 (Liao et al. 2021) with different doping ratios. Table 1 shows the absorption edge and band gap width of different catalysts. It can be seen from the table that TiO2 photocatalyst materials had light absorption characteristics only in the ultraviolet light region, with the absorption edge of about 378 nm. The absorption edge of BiFeO3 photocatalyst materials expanded by about 286 nm, to an absorption edge of 664 nm, indicating that there would also be good light absorption performance in the visible light region, the doping of BiFeO3 with TiO2 further broadened the edge absorption. The edge absorption of different doping ratios widened by 1 nm 17, 20 and 36 nm, respectively. This means that the composite material has a stronger response to light than the pure phase material, which may be caused by the doping of C element of peanut shell biological template or the formation of heterostructures. The calculated band gap width of the pure phase TiO2 photocatalyst, BiFeO3 photocatalyst and TiO2/BiFeO3 photocatalyst with different doping ratio are 3.26 eV, 2.23 eV, 2.57 eV, 2.37 eV, 1.92 eV and 2.49 eV, respectively (Umar et al. 2019; Cao et al. 2021). The minimum band gap width of the photocatalyst is 1TiO2/BiFeO3 photocatalyst.
Figure 8

UV-Vis DRS spectra of TiO2, BiFeO3 and TiO2/BiFeO3 with different parameter ratios.

Figure 8

UV-Vis DRS spectra of TiO2, BiFeO3 and TiO2/BiFeO3 with different parameter ratios.

Close modal
The band gap is calculated according to the following formula (Akhtar et al. 2022):
formula
(10)
where A is a constant; α is the light absorption index; h is Planck constant; ν is the incident photon frequency; is the band gap width of the semiconductor.
Photoluminescence spectra (PL) was mainly used to analyze and compare the recombination rate of photo-generated carriers in photocatalytic materials. Figure 9 shows the photoluminescence spectra of BiFeO3 and 1TiO2/BiFeO3 photocatalysts. Compared with the pure phase BiFeO3 photocatalyst, the PL emission intensity of 1TiO2/BiFeO3 photocatalyst was lower, indicating that the recombination rate of photo-generated carriers in 1TiO2/BiFeO3 photocatalyst is lower. It is possible that the tight binding between BiFeO3 and TiO2 makes the photo-generated carriers migrate at the interface, and the electrons generated in the conduction band of BiFeO3 transfer rapidly through TiO2, which reduces the recombination of photo-generated electrons and holes and improves the photocatalytic degradation activity.
Figure 9

Photoluminescence spectra of BiFeO3 and 1TiO2/BiFeO3 photocatalysts.

Figure 9

Photoluminescence spectra of BiFeO3 and 1TiO2/BiFeO3 photocatalysts.

Close modal

Study on the activity of photocatalytic degradation of TCS

In this experiment, TiO2, BiFeO3 and TiO2/BiFeO3 with different doping ratios (respectively 0.5 mol/mol, 0.8 mol/mol, 1 mol/mol, and 1.5 mol/mol) were selected as photocatalysts to degrade TCS. The optimal doping ratio and photocatalyst were determined by comparing the concentration variation of TCS degraded. To study the effect of photocatalysts on the degradation of TCS, TCS was selected as the target pollutant, and a xenon lamp was used to simulate the sunlight to carry out degradation experiment. The quasi first-order kinetic model was used to analyze the effect of photocatalyst on the degradation rate of TCS. The fitting formula of quasi first-order kinetics is as follows:
formula
(11)
  • wherein: C0 - the initial reaction concentration of TCS after dark reaction;

  • Ct – the concentration of TCS at time t after the beginning of the photoreaction process;

  • t – reaction time.

To understand the photocatalytic activity of the material, we carried out a photocatalytic experiment on TCS. Figure 10 shows the effects (A) and kinetics (B) of TCS degradation by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts. It can be seen from Figure 10(a) that the degradation rates of TCS by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts are 38.7, 49.3 and 77.8%, respectively. The degradation rate of TiO2 photocatalyst may be attributed to the biotemplate peanut shells silicon-carbon structure and C element doping, which not only increases the specific surface area, but also enriches the pore structure. The degradation rate of TCS by BiFeO3 photocatalyst prepared by hydrothermal synthesis was 49.3%, which was lower than that of composite photocatalyst 1TiO2/BiFeO3, indicating that the catalytic activity of composite BiFeO3 photocatalyst was better than that of a single photocatalyst. Table 2 shows the pseudo-first-order kinetic equations and R values of three photocatalysts for degradation of TCS. The photocatalyst with the best R value was 1TiO2/BiFeO3 photocatalyst (Sood et al. 2016).
Table 2

Kinetic equations of different catalysts

Catalyst nameQuasi first-order kinetic equationR2
TiO2 photocatalyst y = 0.10511t + 0.25393 0.9852 
BiFeO3 photocatalyst y = 0.14214t + 0.23107 0.9887 
1TiO2/BiFeO3 photocatalyst y = 0.17143t + 0.31112 0.9952 
Catalyst nameQuasi first-order kinetic equationR2
TiO2 photocatalyst y = 0.10511t + 0.25393 0.9852 
BiFeO3 photocatalyst y = 0.14214t + 0.23107 0.9887 
1TiO2/BiFeO3 photocatalyst y = 0.17143t + 0.31112 0.9952 
Figure 10

Effects (a) and kinetics (b) of TCS degradation by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts.

Figure 10

Effects (a) and kinetics (b) of TCS degradation by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts.

Close modal
The results of degradation of TCS by TiO2/BiFeO3 photocatalysts with different doping ratios are shown in Figure 11(a), and the degradation kinetics are shown in Figure 11(b). With the increase of doping ratio, the photocatalytic activity first increased and then decreased, and the degradation rates were 67.8, 68.5, 81.2, and 74.9%, respectively. This may be related to their specific surface area, and we can see the doping ratio with the best degradation was 1 mol/mol. The hierarchical porous TiO2 photocatalyst can effectively inhibit the size of BiFeO3, limit its growth, increase the effective sites, excite more electrons and holes, and inhibit the size enhancement. There was no agglomeration phenomenon, and the performance is enhanced. Table 3 shows the pseudo first-order kinetic equations and R values of TiO2/BiFeO3 with different doping ratios for degradation of TCS. The photocatalyst with the best R value is 1TiO2/BiFeO3 (Si et al. 2018).
Table 3

Kinetic equation of TiO2/BiFeO3 with different doping ratios

Catalyst nameQuasi first-order kinetic equationR2
0.5 TiO2/BiFeO3 photocatalyst y = 0.20357t + 0.6275 0.9532 
0.8 TiO2/BiFeO3 photocatalyst y = 0.23929t + 0.6937 0.9687 
1 TiO2/BiFeO3 photocatalyst y = 0.29786t + 0.9961 0.9737 
1.5 TiO2/BiFeO3 photocatalyst y = 0.25511t + 0.8075 0.9701 
Catalyst nameQuasi first-order kinetic equationR2
0.5 TiO2/BiFeO3 photocatalyst y = 0.20357t + 0.6275 0.9532 
0.8 TiO2/BiFeO3 photocatalyst y = 0.23929t + 0.6937 0.9687 
1 TiO2/BiFeO3 photocatalyst y = 0.29786t + 0.9961 0.9737 
1.5 TiO2/BiFeO3 photocatalyst y = 0.25511t + 0.8075 0.9701 
Figure 11

Effects (a) and kinetics (b) of TCS degradation by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts of different doping ratios.

Figure 11

Effects (a) and kinetics (b) of TCS degradation by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts of different doping ratios.

Close modal

In order to explore the degradation process and mechanism of TCS degradation by TiO2/BiFeO3 composite photocatalyst, the 1TiO2/BiFeO3 photocatalyst with the best degradation effect was selected to for degradation process and mechanism analysis through the analysis of total organic carbon and trapping experiments.

Figure 12(a) shows the variation trend of total organic carbon concentration of TCS solution degraded by 1TiO2/BiFeO3 photocatalyst with reaction time, and Figure 12(b) shows the mineralization kinetics. It can be seen from Figure 12(a) that the mineralization rate of TCS with initial concentration of 10 mg/L was 58.9% after being degraded by 0.5 g/L 1TiO2/BiFeO3 photocatalyst under simulated sunlight for 3 h. As shown in Figure 12(b), the photocatalytic time and mineralization rate had a linear relation. The degradation kinetics of TCS by 1TiO2/BiFeO3 was analyzed with the pseudo-first-order dynamic model. The pseudo-first-order kinetic equation was ln(TOC0/TOCt) = 0.20143t + 0.15786, and the fitting correlation coefficient R2 was 0.9921, which was consistent with the pseudo-first-order kinetics.
Figure 12

Variation of TOC concentration (a) and kinetics (b) of photocatalytic degradation of TCS by TiO2/BiFeO3.

Figure 12

Variation of TOC concentration (a) and kinetics (b) of photocatalytic degradation of TCS by TiO2/BiFeO3.

Close modal
Figure 13 shows the results of active species trapping for degradation of TCS solution by 1TiO2/BiFeO3 (Huang et al. 2020) photocatalyst. In the active species trapping experiment, sodium oxalate was used as h +(hole) inhibiting trapping agent, isopropanol as •OH inhibiting trapping agent, sodium thiosulfate as e-inhibiting trapping agent, and p-Benzoquinone as •O2 inhibiting trapping agent (Li et al. 2011; Zhu et al. 2014b). The active substances for degrading TCS were determined by adding 0.1 mmol sodium oxalate, isopropanol, sodium thiosulfate and p-Benzoquinone to the solution individually (Yin et al. 2022).
Figure 13

Results of active species trapping for degradation of TCS solution by TiO2/BiFeO3 photocatalyst.

Figure 13

Results of active species trapping for degradation of TCS solution by TiO2/BiFeO3 photocatalyst.

Close modal
The photodegradation mechanism of 1TiO2/BiFeO3 was studied by using different scavengers to trap free radicals. Isopropanol was used as hydroxyl radical (•OH) scavenger, p-Benzoquinone as superoxide radical (•O2) scavenger, sodium thiosulfate as an electron (e) scavenger and sodium oxalate as hole (h+) (Jayababu et al. 2021) scavenger. As shown in the results of Figure 13, the degradation rate decreases after adding sodium oxalate and isopropanol, indicating these two have inhibitory effect; after adding sodium thiosulfate and p-Benzoquinone, the degradation rate was almost the same as that without trapper, indicating that these two have no inhibition effect to the photocatalytic experiment; therefore, h+ and •OH are active substances for degradation of TCS (Abdul Satar et al. 2019). Relatively speaking, •OH has a more obvious inhibitory effect on TCS and is the main reactive oxidation substances.
Figure 14

Schematic diagram of a mechanism for the TiO2/BiFeO3 under visible light irradiation.

Figure 14

Schematic diagram of a mechanism for the TiO2/BiFeO3 under visible light irradiation.

Close modal

Through the analysis of the results of the trapping experiment, we can infer the possible mechanism of degradation of TCS by the 1TiO2/BiFeO3 photocatalyst. When the light energy is greater than the forbidden band width of BiFeO3, electrons will be excited from the valence band and transferred to the conduction band, h+ (Zhang et al. 2022) will be generated around the valence band, electrons will be transferred to TiO2 photocatalyst, and oxygen molecules will be captured to produce •O2–. These h + oxidized water molecules in the valence band will produce •OH, •OH and •O2 (Wei et al. 2020) to oxidize the TCS molecules adsorbed onto the surface of BiFeO3. The peanut shell biotemplate has mesoporous structure, so it increases the specific surface area of TiO2 photocatalyst, the number of active sites and the photocatalytic activity (Jaffari et al. 2021). In addition, TiO2 and BiFeO3 form heterostructures, which can be fully confirmed by TEM, XPS and other characterization techniques. The heterostructure can effectively reduce the recombination rate of electron-hole and increase the photocatalytic activity. The forbidden band width of BiFeO3 catalyst is 2.23 eV, easily excited to generate electron hole pairs. The forbidden band width of TiO2 catalyst is 3.26 eV, not easy to be excited. When these two catalysts are doped with each other, as shown in Figure 14, the forbidden band width is 1.92 eV, forming a uniform lattice fringe. When the electrons are irradiated by xenon lamp, the conduction band of TiO2 photocatalyst will be transferred from BiFeO3 photocatalyst, thus inhibiting the electron-hole recombination, and improving the performance of photocatalysis.

It can be seen from the comparison in Table 4 that in terms of TCS removal, 1TiO2/BiTiO3 can achieve the same removal rate and specific surface area, and has good development prospects.

Table 4

Comparison of TCS removal by different materials

SampleTarget pollutantBET(m2/g)Active speciesRemoval rate(%)Reference
1TiO2/BiTiO3 TCS 59.59 h+ /•OH 81.6 This research 
Bi7O9I3/Bi TCS •OH/•O2 85 Chang et al. (2021)  
SnO2@ZnS TCS 123.1 h+ /•OH 40 Hojamberdiev et al. (2020)  
Bi2O3/BiOI TCS •OH/•O2 73.2 Xu et al. (2022)  
rGO–TiO2 TCS 50 •OH 87 Kaur et al. (2020)  
SampleTarget pollutantBET(m2/g)Active speciesRemoval rate(%)Reference
1TiO2/BiTiO3 TCS 59.59 h+ /•OH 81.6 This research 
Bi7O9I3/Bi TCS •OH/•O2 85 Chang et al. (2021)  
SnO2@ZnS TCS 123.1 h+ /•OH 40 Hojamberdiev et al. (2020)  
Bi2O3/BiOI TCS •OH/•O2 73.2 Xu et al. (2022)  
rGO–TiO2 TCS 50 •OH 87 Kaur et al. (2020)  

The possible mechanism of light degradation of TCS by 1TiO2/BiFeO3 photocatalyst is as follows (Liu et al. 2018; Hu et al. 2019; Li et al. 2019c):
formula
(12)
formula
(13)
formula
(14)
formula
(15)
formula
(16)
formula
(17)

In this experiment, peanut shells were used as a biotemplate to prepare a hierarchical porous TiO2 catalyst, which was then used to synthesize TiO2/BiFeO3 photocatalysts with different doping amounts using the hydrothermal method. Through a series of studies on their morphology, structure and photocatalytic activity, combined with the total organic carbon and capture experimental results, the possible degradation mechanism was determined, and the main conclusions are as follows:

  • (1)

    Through XRD analysis and comparison, it can be seen that the characteristic diffraction peak of 1TiO2/BiFeO3 photocatalyst is consistent with the standard card, showing a better phase crystalline form. Through scanning electron microscopy (SEM), we can see that 1TiO2/BiFeO3 photocatalyst shows a cotton-like structure, and nanometer-grade particle size, which was closely arranged and formed a heterogeneous structure. The lattice fringes of 1TiO2/BiFeO3 photocatalyst can be seen from the TEM image, wherein, the spacing of one group of lattice fringe is 0.19 nm, corresponding to the lattice plane of TiO2 photocatalyst (101), and the spacing of the other group of lattice fringes is 0.22 nm, corresponding to the lattice surface of BiFeO3 photocatalyst (110). The results confirm the existence of heterostructures, which expand the light frequency and wavelength response range. The average diameter of 1TiO2/BiFeO3 photocatalyst was 14.98 nm according to BET analysis, and the specific surface area was 153.64 m2/g. The XPS spectra show that the valence of Ti is tetravalent, and that of Fe and Bi is trivalent. It shows that the crystal structures of TiO2 and BiFeO3 do not affect each other, and each element keeps its original valence. The position of the composite diffraction peak of the two XRD patterns is consistent with that of the single diffraction peak. UV-Vis diffuse reflection analysis shows that the absorption edge of 1TiO2/BiFeO3 photocatalyst is about 550 nm, and its forbidden bandwidth is 1.92 eV. PL analysis shows that the recombination rate of photogenerated carriers in 1TiO2/BiFeO3 photocatalyst is low, which may be because the close bond between BiFeO3 and TiO2 makes the photo-generated carriers migrate at the interface so that the electrons generated in the BiFeO3 conduction band quickly transfer through TiO2, thus reducing the recombination probability of photo-generated electrons and holes and enhancing photocatalytic degradation activity.

  • (2)

    The degradation process and kinetics of TCS by TiO2, BiFeO3 and 1TiO2/BiFeO3 photocatalysts were compared and analyzed. The results showed that the degradation process conformed to quasi-first-order kinetics. The degradation rates of TiO2 photocatalyst, BiFeO3 photocatalyst and 1TiO2/BiFeO3 photocatalyst were 38.7%, 49.6% and 81.2%, respectively. By comparing the photodegradation effect of different doping amounts of TiO2/BiFeO3 photocatalyst, it is concluded that the best doping amount of photocatalyst is 1 mol/mol, that is, 1TiO2/BiFeO3 photocatalyst, and the degradation process conforms to the quasi first-order kinetics, indicating that the doping amount of BiFeO3 affects the photocatalytic performance of TiO2/BiFeO3 photocatalyst.

  • (3)

    The mineralization rate of total organic carbon in 1TiO2/BiFeO3 photocatalyst is 58.9%, which accords with the quasi-first-order kinetics. Through the analysis of the trapping experiment, it is concluded that OH plays the main role of strong oxidation in the degradation process, and O2 plays the secondary role of strong oxidation. We can generally infer the possible mechanism of degradation, that is when the light energy is greater than the forbidden band width of BiFeO3, electrons will be excited from the valence band and transferred to the conduction band, h+ will be generated around the valence band, electrons will be transferred to TiO2 photocatalyst, and oxygen molecules will be trapped to produce •O2. The h+ oxidized water molecules in the valence band will produce •OH, •OH and •O2 - to oxidize the TCS molecules adsorbed on the surface of BiFeO3.

Our work was supported by the National Natural Science Foundation of China (Grant No. 51778267), the National Water Pollution Control and Treatment Science and Technology Major Project (No. 2012ZX07408001), the Jilin Province Science and Technology Department Project (No. 20220203047SF).

Gen Liu: Conceptualization, methodology, analysis, data curation, writing – original draft preparation, visualization, and investigation. Yingzi Lin: Reviewing. Siwen Li: Supervision. Chunyan Shi: Validation. Daihua Zhang: Methodology.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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