Strontium-induced phase, energy band and microstructure regulation in Ba_1−xSr_xTiO₃ photocatalysts for boosting visible-light photocatalytic activity

Abstract

A polyacrylamide gel method has been developed for fabrication of Ba_1−xSr_xTiO₃ (BST, x = 0.1–0.5) photocatalysts. When x = 0.1–0.2, the BST photocatalyst is in a tetragonal phase, and when x is other value, the BST photocatalyst is in a cubic phase. The microstructure of BST photocatalysts changes from an irregular polyhedron in the tetragonal phase to cuboid in the cubic phase. The change of the phase structure makes the color parameter b* value of BST photocatalysts change from negative to positive, and the E_g value and photocatalytic activity increase with the increase of the x value. The optimum catalyst content, drug concentration, pH value and illumination time for the degradation of tetracycline hydrochloride (TC-HCl) by BST photocatalysts were 1 g L⁻¹, 20 mg L⁻¹, 6.77 and 120 min, respectively. The degradation pathway of TC-HCl and the toxicity of the intermediate products were also analyzed. This technique of phase structure, microstructure, energy band engineering and photocatalytic activity regulation is helpful for the development of other similar photocatalysts.

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1. Introduction

Photocatalytic technology driven by light energy is a common and efficient green technology used to degrade dyes, persistent organic pollutants (POPs), and drugs, and reduce heavy metal ions.^1–6 At present, titanium dioxide (TiO₂) and strontium titanate (SrTiO₃) have been used in industrial applications as photocatalysts, but they can only respond to UV light because of their large band gap (E_g), and the UV content of sunlight is only about 7%, which greatly limits their application in the field of photocatalysis.^7–10 Therefore, the development of novel semiconductor photocatalysts that can respond to visible light has become a key problem to solve the degradation of pollutants.^11–15

Barium titanate (BaTiO₃) is also an ABO₃ perovskite photocatalyst with industrial potential for photocatalytic degradation of dyes, drugs and POPs due to its high photocatalytic activity.^16–18 However, BaTiO₃ also faces the disadvantages of high E_g value, difficulty in responding to visible light and low efficiency of charge carrier separation and transfer, which limit its application in the field of photocatalysis.¹⁹ To improve the visible light utilization rate of BaTiO₃ photocatalysts, researchers made the following efforts: (1) BaTiO₃ with a special defect structure was constructed under extreme conditions to make it respond to visible light and degrade pollutants.²⁰ (2) Ion doping BaTiO₃ improves its E_g value to make it respond to visible light and degrade pollutants.^21,22 (3) BaTiO₃-based heterojunction photocatalysts were constructed by combining with other excellent semiconductors to enhance interfacial charge carrier transfer and transport capacity and degrade pollutants.^23,24

Among these methods, ion doping BaTiO₃ can effectively adjust the E_g value of BaTiO₃ and make it respond to visible light. Recently, the use of strontium ions to occupy the A-site ions of BaTiO₃ to form Ba_1−xSr_xTiO₃ (BST) photocatalysts can be widely used in the photocatalytic decomposition of water to produce hydrogen and dye degradation.^25–32 However, no researchers have yet regulated the phase structure, morphology and photocatalytic activity of BST photocatalysts for photocatalytic degradation of tetracycline hydrochloride (TC-HCl) under simulated sunlight irradiation by adjusting the content of strontium ions. The polyacrylamide gel method is an effective method to synthesize ion-doped semiconductor photocatalysts, but it has not been used to synthesize BST photocatalysts.^33,34 Therefore, it is of great significance to synthesize BST photocatalysts by the polyacrylamide gel method and study the evolution of the phase structure, microstructure, energy band structure and photocatalytic activity of BST photocatalysts induced by strontium ions.

In this paper, we propose to synthesize strontium-induced BST photocatalysts by the polyacrylamide gel method, and then enhance their photocatalytic activity by regulating the phase structure, microstructure and energy band structure of the BST photocatalysts by adjusting the content of strontium ions. The effects of the x value, catalyst content, drug concentration, pH value and irradiation time on the photocatalytic activity of the BST photocatalysts were investigated with tetracycline hydrochloride (TC-HCl) as the degradation target for drugs. The main active species that play a key role in the photocatalysis process were detected through capture experiments and free radical detection experiments. The degradation pathways and the toxicity of intermediates for the photocatalytic degradation of TC-HCl by BST photocatalysts were also estimated by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS) and the Toxicity Estimation Software Tool (T.E.S.T.). A possible photocatalytic mechanism of BST photocatalysts has been proposed based on photocatalytic experiments, verification experiments of active species and energy band theory.

2. Experimental section

2.1 Synthesis of Ba_xSr_1−xTiO₃ photocatalysts

Anhydrous barium nitrate (Ba(NO₃)₂), anhydrous strontium nitrate (Sr(NO₃)₂) and tetrabutyl titanate (C₁₆H₃₆O₄Ti) were purchased from Chengdu Aike Reagent Co., LTD. Citric acid (C₆H₈O₇), glucose (C₆H₁₂O₆), acrylamide (C₃H₅NO), N,N′- methylene bisacrylamide (C₇H₁₀N₂O₂) and ammonia (NH₃·H₂O) were purchased from Tianjin Kemeiou Reagent Co., LTD. The reagents mentioned above were not further purified and used directly in the process.

To obtain the Ba_1−xSr_xTiO₃ (BST) photocatalysts with molar ratios of n_Ba : n_Sr = 9 : 1, 8 : 2, 7 : 3, 6 : 4 and 5 : 5, appropriate amounts of Ba(NO₃)₂, Sr(NO₃)₂ and C₁₆H₃₆O₄Ti were weighed, and the corresponding samples were named BST1, BST2, BST3, BST4 and BST5, respectively. The corresponding volume of tetrabutyl titanate was measured with a measuring cylinder and added into Ba(NO₃)₂ and Sr(NO₃)₂ solutions dissolved in 20 mL deionized water. Then, 4.7282 g citric acid, 20 g glucose, 9.5958 g acrylamide and 1.9192 g methylene diacrylamide were added to the above solution, and deionized water was added in time to make it reach 100 mL. Finally, a precursor solution with 0.015 mol L⁻¹ metal ion concentration was formed. After the precursor solution was stirred at room temperature for 2 hours until clear, a small amount of ammonia was added to adjust the pH value of the solution to 11, and the temperature was increased to about 100 °C until a jelly-like gel was formed. The gel was dried at 120 °C for 24 hours and then cooled. After grinding, the target product was obtained by sintering at 950 °C in a tube furnace for 5 hours. The related synthesis process of the BST photocatalysts is shown in Fig. 1.

Fig. 1 The synthesis process of BST photocatalysts.

2.2 Material characterization

The phase evolution process of the BST photocatalysts was characterized by means of a DX-2007BH X-ray powder diffractometer (XRD) with Cu Kα radiation. The functional groups of the BST photocatalysts were studied using a Fourier transform infrared (FTIR) spectrophotometer (FTIR 650) in a wavenumber range of 400–4000 cm⁻¹. The charge states, electronic energy levels and compositions of the BST photocatalysts were measured using a KRATOS X SAM 800 X-ray photoelectron spectrometer (XPS). The surface morphologies of the BST photocatalysts were observed using an SU8010 field-emission scanning electron microscope (FE-SEM) and JEM 2100F transmission electron microscope (TEM). Ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) of the BST photocatalysts were measured using a UV1901 UV-visible spectrophotometer with BaSO₄ used as a reference.

2.3 Photocatalytic experiments

To study the effects of phase transition, band structure change and morphology evolution on the photocatalytic activity of the BST photocatalysts, tetracycline hydrochloride (TC-HCl) was selected as the target degradation drug to perform the photocatalytic experiments. The initial drug concentration, catalyst content and pH value were 10–50 mg L⁻¹, 1 g L⁻¹ and 3–13, respectively. At the beginning of the experiment, a half hour adsorption experiment was performed to eliminate the effect of adsorption on the photocatalytic activity of the BST photocatalysts. After the photocatalytic experiment began, samples were taken every half an hour until the experiment was completed 2 hours later. The above extracted samples were marked with time numbers and the absorbance curves of the samples at time t were measured using a UV-1800 UV-visible spectrophotometer. Disodium ethylenediamine tetraacetic acid (EDTA-2Na), 2-propanol (IPA) and 1,4-benzoquinone (BQ) were used to perform capture experiments to determine the contribution of holes, hydroxyl radicals, and superoxide radicals to photocatalysis. The holes (room temperature, X-band (9.34 GHz)), hydroxyl radicals (5,5-dimethyl-1-pyrroline N-oxide (DMPO)), and superoxide radicals (5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO)) are also confirmed by the electron spin/paramagnetic resonance (ESR/EPR) experiments on a Bruker A300 spectrometer. High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS) was performed to explore the degradation ways and products of the BST photocatalysts for the degradation of TC-HCl by a positive ionization mode with a full scan range from m/z 50 to 1000. Detailed experimental parameters are described in the literature.³⁵

3. Results and discussion

3.1 Phase structure and purity

The introduction of strontium ions into BaTiO₃ results in the transformation of the phase structure, which can be observed by XRD. Fig. 2(a) displays the XRD patterns of the BST photocatalysts. With the increase of the x value, the position of the diffraction peaks of the BST photocatalysts does not change obviously. However, after magnifying the diffraction peaks in the range of 40–50° (Fig. 2(b)), it is found that the BST photocatalysts with x = 0.1 and 0.2 has two peaks belonging to the tetragonal phase with the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 44-0093, while the BST photocatalysts with x = 0.3–0.5 have only one peak and the peak position shifts to a higher angle with the increase of the x value, belonging to the cubic phase, which can be referred to JCPDS card no. 39-1395.^36,37 The results show that the BST photocatalysts change from the tetragonal phase to the cubic phase with the increase of x values. Fig. 2(c) shows the lattice parameters of the BST photocatalysts. The lattice parameters of the BST photocatalysts can be estimated by eqn (1) and (2):where, d is the interplanar spacing, (h, k, l) are the crystal indices, and (a, c) are the lattice parameters. The tetragonality (c/a ratio) of the BST1 and BST2 photocatalysts was 0.9963 and 0.9992, respectively. The radius of the strontium ions is smaller than that of barium ions, and the incorporation of BaTiO₃ significantly affects the lattice parameters of the BST photocatalysts. With the increase of the x value, the lattice parameter (a) of the BST photocatalysts decreases. The crystallite size (D) of the BST photocatalysts can be obtained by eqn (3):where k is the shape factor, and it takes the value of 2.08. β is the full-width at half maximum (FWHM). The FWHM of the BST photocatalysts is the sum of the Gaussian (G) (FWHM)_G and Lorentzian (L) (FWHM)_L components.λ and θ are the X-ray wavelength and the Bragg diffraction angle, respectively. The crystallite sizes of the BST1, BST2, BST3, BST4 and BST5 photocatalysts are 78.56, 102.51, 92.49, 89.42 and 72.08 nm, respectively. With the increase of the x value, the crystallite size of the BST photocatalyst increases first and then decreases. The change of the crystallite size is mainly attributed to the phase transformation of the BST photocatalyst caused by different Sr ion contents.

Fig. 2 (a) XRD patterns, (b) XRD enlarged drawing, (c) lattice parameters and (d) FTIR spectra of the BST photocatalysts.

The chemical environment of the BST photocatalyst was characterized by FTIR. Fig. 2(d) displays the FTIR spectra of the BST photocatalysts. Two special vibration modes at 3460 and 1639 cm⁻¹ exist in all samples, which are attributed to the stretching vibration of O–H and the bending vibration of H–O–H, respectively.^38–41 The main reason for the above two peaks is the use of KBr during the test, which tends to absorb moisture from the air. A weak characteristic peak at 1400 cm⁻¹ indicates that the sample contains a small amount of C–O functional groups.^42–44 The four characteristic peaks at 887, 671, 538 and 414 cm⁻¹ in the range of 400–1000 cm⁻¹ were attributed to the Ba–O stretching vibration, the Sr–Ti–O bonds, the Ti–O vibrations, and metal–oxygen bonds, respectively.^45–47

3.2 XPS analysis

The composition, binding energies and charge states of the BST photocatalysts were further characterized by XPS. Fig. 3(a) displays the XPS survey scan spectra of the BST1, BST3 and BST5 photocatalysts. The survey scan spectra of the BST1, BST3 and BST5 photocatalysts display the characteristic peaks of the elements required by the target except for C, which can be attributed to the C–O functional group and the calibration peak of the XPS instrument.⁴⁸ The details of the high resolution spectra of C 1s for the BST photocatalysts will be discussed later. Fig. 3(b) demonstrates the Ba 3d spectra of the BST1, BST3 and BST5 photocatalysts. The Ba 3d photoelectron peaks of the BST1, BST3 and BST5 photocatalysts have two components in the BST photocatalysts designated as the Ba 3d_5/2 peak at around 778.13 eV and the Ba 3d_3/3 peak at around 793.49 eV.⁴⁹ The Sr 3d spectra of the BST1, BST3 and BST5 photocatalysts are presented in Fig. 3(c)–(e). The photoelectron peaks of Sr 3d for the BST1, BST3 and BST5 photocatalysts were well split off from the other photoelectron peaks, and the photoelectron peaks of Sr 3d_3/2 and Sr 3d_5/2 had asymmetric profiles.^50,51Fig. 3(f)–(h) reveal the Ti 2p spectra of the BST1, BST3 and BST5 photocatalysts. In Fig. 3(f)–(h), the deconvolution of the Ti 2p signal obtained from the BST1, BST3 and BST5 photocatalysts presents the peaks at 457.74, 458.45 and 463.49 eV. The photoelectron peaks at 457.74 and 463.49 eV are assigned to Ti 2p_3/2 and Ti 2p_1/2, respectively. The photoelectron peak at 458.45 eV can be attributed to the satellite peak.⁵²

Fig. 3 (a) XPS survey scan spectra and (b) Ba 3d spectra of the BST1, BST3 and BST5 photocatalysts. Sr 3d spectra of the (c) BST1, (d) BST3 and (e) BST5 photocatalysts. Ti 2p spectra of the (f) BST1, (g) BST3 and (h) BST5 photocatalysts.

Fig. 4(a)–(c) display the C 1s spectra of the BST1, BST3 and BST5 photocatalysts. The C1s spectra can be fitted into three photoelectron peaks at 284.82, 286.44 and 289.09 eV by the Avantage software. The first spike is mainly due to the calibration peak used by the XPS instrument.⁴⁸ The second peak is mainly due to the presence of a small amount of C–OH functional groups in the BST photocatalyst.⁵³ The last peak is the presence of a small number of O=C–OH functional groups in the BST photocatalyst.³⁵ This result was consistent with the FTIR results, which confirmed the presence of C–O functional groups in the BST photocatalysts. The O 1s spectra of the BST1, BST3 and BST5 photocatalysts are displayed in Fig. 4(d)–(f). The O 1s spectra can also be fitted into three photoelectron peaks, which are located at 528.94, 531.44 and 534.18 eV, respectively. The peaks at 528.94, 531.44 and 534.18 eV are assigned to the lattice oxygen of SrTiO₃ (O_LSTO), the lattice oxygen of BaTiO₃ (O_LBTO) and the adsorbed oxygen, respectively.^54,55 Simultaneously, the adsorbed oxygen may contain a small amount of oxygen in the C–O functional group.⁵⁶

Fig. 4 C 1s spectra of the (a) BST1, (b) BST3 and (c) BST5 photocatalysts. O 1s spectra of the (d) BST1, (e) BST3 and (f) BST5 photocatalysts.

3.3 Microstructural analysis

Fig. 5 displays the SEM photographs of the BST1, BST3 and BST5 photocatalysts. When x = 0.1, some particles in the BST photocatalysts are irregularly triangular, some are approximately spherical, and some are tetragonal as presented in Fig. 5(a). When x = 0.3, the regular tetragonal becomes more and more numerous and the growth of particles becomes obvious as displayed in Fig. 5(b). When x reaches 0.5, the large particles become larger and have been transformed into regular cuboid or cubic particles. Combined with the XRD and SEM results, it is found that the crystal structure of the BST photocatalysts changes from the tetragonal phase to the cubic phase, and the morphology changes from irregular spherical particles to regular cubic particles.

Fig. 5 SEM photographs of the (a) BST1, (b) BST3 and (c) BST5 photocatalysts.

Fig. 6(a) shows the TEM photograph of the BST5 photocatalyst. The BST5 photocatalyst is mainly composed of some regular cuboids or cubes of different sizes, which is consistent with the SEM observation results. The HRTRM characterization (Fig. 6(b)) of large particles shows that the diffraction pattern at the spacing of 0.28 nm corresponds to the (110) crystal plane of Ba_0.5Sr_0.5TiO₃. Fig. 6(c)–(h) displays the BF-TEM photograph and the corresponding elemental mapping photographs of the BST5 photocatalyst. In Fig. 6(c)–(i), each element is evenly distributed in the BST photocatalysts, indicating that the Sr ions occupy the positions of Ba ions to form Ba_0.5Sr_0.5TiO₃, rather than the solid solution of BaTiO₃ and SrTiO₃.

Fig. 6 (a) TEM, (b) HRTEM, and (c) BF-TEM photographs and (d)–(i) the corresponding elemental mapping photographs of the BST5 photocatalyst.

3.4 Optical properties

Fig. 7(a) shows the DRS of the BST photocatalysts. With an increasing wavelength, the reflectance drops sharply in the wavelength range of 190–250 nm, while remaining almost constant in the wavelength range of 250–380 nm. In the wavelength range of 380–420 nm, the reflectance increases sharply with the increasing wavelength, and then slowly increases to 900 nm with the increase of wavelength. With the increase of x, the reflectance increases first and then decreases in the wavelength range of 200–380 nm. Based on the DRS and formula (6), the color parameters (L*, a*, b*) of the BST photocatalysts can be calculated and are given in Table 1:where, (X_n, Y_n, Z_n) = (94.81, 100, 107.32). Based on formula (7), (X₁₀, Y₁₀, Z₁₀) can be estimated:where, λ, (x₁₀(λ), y₁₀(λ), z₁₀(λ)), S(λ), R(λ) and Δλ are the wavelength, the color matching function, the relative spectral power distribution, the reflectance, and 1 nm, respectively. It can be seen from Table 1 that the lowest value of L* is 94.322, indicating that the BST photocatalyst is white. When x = 0.4, L* is the largest. When x = 0.1–0.2, the a* value is negative, and when x = 0.3–0.5, the a* value is positive. This result further confirms that the phase transition of the BST photocatalysts has a great influence on color parameters.

Fig. 7 (a) DRS, (b) ABS and (c) E_g values of the BST photocatalysts.

Table 1 Color parameters (L*, a*, b*) and E_g values of the BST photocatalysts

Sample	Color parameters			E g value (eV)
	L*	a*	b*
BST1	94.322	−0.528	8.837	2.70
BST2	96.349	−0.174	6.259	2.79
BST3	96.738	0.584	3.521	2.83
BST4	96.893	0.037	4.178	2.88
BST5	96.408	0.203	6.185	2.91

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According to the DRS and the Kubelka–Munk (K–M) formula (8), the absorption spectra (ABS) of the BST photocatalysts are displayed in Fig. 7(b).

R is the reflectance of the BST photocatalysts, α is the absorption coefficient of the BST photocatalysts, and S is the scattering coefficient of the BST photocatalysts. The results reveal that the BST photocatalysts have a high UV optical absorption coefficient at 190–400 nm and a weak visible optical absorption coefficient at 400–800 nm. In the ultraviolet range, the BST5 photocatalysts have the highest optical absorption coefficient, while the BST1 photocatalysts have the highest optical absorption coefficient in the visible range. This finding further demonstrates that the BST photocatalysts have visible light response capability.

Based on the Tauc relation (9), the relationship between (αhv)^1/2 and hv of the BST photocatalysts is displayed in Fig. 7(c).

(F(R)E)ⁿ = A(E − E_g) (9)

where E = hv is the photon energy, A is a constant, and E_g is the optical band gap value of the BST photocatalysts, that is, the focus value of the slope at the steepest point of the curve and the horizontal coordinate. For the indirect band gap semiconductor, n = 1/2. The E_g values of the BST1, BST2, BST3, BST4 and BST5 photocatalysts are 2.70, 2.79, 2.83, 2.88 and 2.91 eV, respectively. Compared with BaTiO₃ (Fig. S1†), the E_g value of the BST photocatalysts has been greatly reduced. With the increase of the x value, the E_g value of the BST photocatalysts increases gradually. The results also confirm that the E_g value of BaTiO₃ can be regulated by the partial substitution of A-site ions for Sr ions in BaTiO₃.

3.5 Enhanced photocatalytic performances

3.5.1 Effect of the x value on photocatalytic activity. TC-HCl is an antibiotic that is difficult to be degraded naturally, and was selected as a target degradation drug to gain insight into the photocatalytic activity of the BST photocatalysts. Fig. 8(a) displays the degradation efficiency for TC-HCl on the BST photocatalysts. After half an hour of adsorption, the adsorption percentage of the BST5 photocatalyst reached almost 20%, but TC-HCl was no longer adsorbed by increasing the adsorption time, indicating that the adsorption equilibrium was reached within half an hour. With the increase of illumination time, the photocatalytic activity of the BST photocatalysts was enhanced. Meanwhile, a comparison experiment was also performed, and the degradation percentage was about 30% after 120 minutes of light exposure. With the increase of the x value, the degradation percentage of the BST photocatalysts also increased.

Fig. 8 (a) The degradation efficiency for TC-HCl on the BST photocatalysts. (b) Plots of ln (A_t/A₀) vs. irradiation time of the BST photocatalysts for the degradation of TC-HCl. (c) The apparent degradation rate constant (k) value of the BST photocatalysts for the degradation of TC-HCl. Effects of (d) drug concentration and (e) pH value on the degradation of TC-HCl on the BST5 photocatalyst. (f) PZC of the BST5 photocatalyst.

The first order kinetic curve can better reflect the photocatalytic activity of the BST photocatalysts:

ln(A_t/A₀) = −kt (10)

where A_t is the absorbance of TC-HCl at time t, A₀ is the initial absorbance of TC-HCl, and k is the kinetic rate constant. Fig. 8(b) displays the plots of ln (A_t/A₀) vs. irradiation time of the BST photocatalysts for the degradation of TC-HCl. There is a linear dependence between ln (A_t/A₀) and irradiation time. The k values of the BST photocatalysts for the degradation of TC-HCl are presented in Fig. 8(c). The k values of the BaTiO₃, BST1, BST2, BST3, BST4 and BST5 photocatalysts are 0.0030, 0.0053, 0.0083, 0.0102, 0.0118 and 0.0136 min⁻¹, respectively. The photocatalytic degradation rate of the BST5 photocatalyst is 4.53 times that of BaTiO₃.

3.5.2 Effect of drug concentration on photocatalytic activity. Fig. 8(d) reveals the effect of drug concentration on the degradation of TC-HCl on the BST5 photocatalyst. The catalyst content and drug concentration were 1 g L⁻¹ and 10–50 mg L⁻¹, respectively. With the increase of drug concentration, the photocatalytic degradation percentage of the BST5 photocatalyst first increased and then decreased. The optimal drug concentration was 20 mg L⁻¹, and the degradation percentage was 87.1%. The main reason why the degradation percentage of the BST5 photocatalyst decreased with the increase of drug concentration was that the large number of drugs attached to the surface of the BST5 photocatalyst increased the path length of the photon entering the reaction solution.^57–59 Similarly, the optimal content of the catalyst (1 g L⁻¹) can be determined as shown in Fig. S2.†

3.5.3 Effect of the pH value on photocatalytic activity. Fig. 8(e) displays the effect of the pH value on the degradation of TC-HCl on the BST5 photocatalyst. When the pH of the reaction solution is not adjusted, the initial pH of the reaction solution is 6.77. When the pH value of the reaction solution was adjusted to acidic conditions, the degradation percentage of TC-HCl by the BST5 photocatalyst was not more than 40%. However, after adjusting the pH value of the reaction solution to alkaline conditions, the BST5 photocatalyst has a high photocatalytic activity, and the photocatalytic percentage increases slowly with the increase of the pH value. To explain this phenomenon, the point of zero charge (PZC) experiment was performed and is displayed in Fig. 8(f). It can be seen from the figure that the PZC value of the BST5 photocatalyst is 6.69. Sharifian⁶⁰ and Gao⁶¹et al. reported that the PZC value of SrTiO₃ synthesized by different methods was close to 8.5. Klusáčková⁶²et al. believed that the PZC value of BaTiO₃ was between 6 and 7. When an Sr ion partially occupies the A-site ion in BaTiO₃, the PZC value is close to the PZC value of BaTiO₃. When the pH of the reaction solution is greater than the PZC value, the surface charge of the BST5 photocatalyst is negative, which will be easy to react with TC-HCl and improve the photocatalytic activity of the BST5 photocatalyst.⁶³

3.6 Stability and free radical detection

3.6.1 Stability. As an efficient photocatalyst, whether it can be recycled is an important index to evaluate its practicability. To perform the cyclic stability experiment of the photocatalyst, the photocatalyst will be filtered, dried and sintered after each photocatalysis, and then the next photocatalytic experiment will be performed. Fig. 9(a) shows the cycle index on TC-HCl degradation onto the BST5 photocatalyst. After five cycles, the photocatalytic degradation percentage of the BST5 photocatalyst decreased by 8.6%. This decrease is mainly caused by the loss of the photocatalyst during the cycle, the decrease of surface active sites and the decrease of light permeability.^64,65

Fig. 9 (a) Cycle index on TC-HCl degradation onto the BST5 photocatalyst. (b) XRD patterns of the BST5 photocatalyst before and after degradation of TC-HCl. (c) Trapping experiment of the BST5 photocatalyst for the degradation of TC-HCl. ESR/EPR spectra of the BST5 photocatalyst for the (d) oxygen vacancy, (e) hydroxyl radical and (f) superoxide radical. (g) The emission spectra with 2-hydroxyterephthalic acid for the degradation of TC-HCl in the presence of the 1 g L⁻¹ BST5 photocatalyst.

To further determine the stability of the BST5 photocatalyst, XRD characterization of the BST5 photocatalyst before and after the photocatalysis was performed. Fig. 9(b) displays the XRD patterns of the BST5 photocatalyst before and after the degradation of TC-HCl. The position of the diffraction peak of the BST5 photocatalyst did not change before and after the photocatalysis. Although the intensity of the diffraction peak changed, the phase purity of the BST5 photocatalyst did not change, indicating that the BST5 photocatalyst was stable. Combined with the above experiments, it is proved that the BST5 photocatalyst can be recycled and is stable. In addition, the XPS spectra (Fig. S3†) of the BST5 photocatalyst after photocatalysis are measured, which also show that the BST5 photocatalyst is stable.

3.6.2 Capture experiments. The photocatalytic experiments revealed that the BST5 photocatalyst had high photocatalytic activity in the degradation of TC-HCl. To confirm the role of the free radical in the photocatalytic process, the capture experiments were performed. During the experiment, 1 mmol EDTA-2Na, IPA or BQ was added to the reaction solution to confirm the role of holes, hydroxyl radicals and superoxide radicals in the photocatalytic process, and the other processes were consistent with the photocatalytic experiment. Fig. 9(c) presents the trapping experiment of the BST5 photocatalyst for the degradation of TC-HCl. When EDTA-2Na, IPA and BQ were added to the reaction solution, the photocatalytic degradation percentage of the BST5 photocatalyst dropped sharply, and the maximum value was no more than 40%, indicating that the holes, hydroxyl radicals and superoxide radicals played key roles in the whole photocatalytic process.

3.6.3 ESR/EPR characterization. ESR and EPR experiments were performed to further confirm the dominant role of oxygen vacancies, hydroxyl radicals and superoxide radicals in the degradation of TC-HCl by the BST5 photocatalyst. Fig. 9(d)–(f) displays the ESR/EPR spectra of the BST5 photocatalyst for the oxygen vacancies, hydroxyl radicals and superoxide radicals. The EPR spectra of the BST5 photocatalyst are presented in Fig. 9(d). A symmetric and sharp peak at g = 2.003 was observed, indicating the presence of a large number of oxygen vacancies in the BST5 photocatalyst.^66,67 This result is consistent with that observed in the O1s spectra by XPS. The EPR spectra of the BST5 photocatalyst for the hydroxyl radicals and superoxide radicals are displayed in Fig. 9(e) and (f). Almost no hydroxyl radical and superoxide radical signals were detected when there was no exposure to simulated sunlight. The signals of the hydroxyl radicals and superoxide radicals were enhanced significantly when the solution was irradiated by simulated sunlight, and their intensities increased with the increase of illumination time. ESR/EPR experiments confirmed that the oxygen vacancy, hydroxyl radical and superoxide radical were the main active species during the degradation of TC-HCl by the BST5 photocatalyst.

3.6.4 Hydroxyl free radical detection. Hydroxyl radicals can also be detected by adding 2-hydroxyterephthalic acid to the reaction solution.^68–70Fig. 9(g) reveals the emission spectra with 2-hydroxyterephthalic acid for the degradation of TC-HCl in the presence of the 1 g L⁻¹ BST5 photocatalyst with the excitation wavelength of 320 nm. A distinct emission peak at 425 nm can be observed. The intensity of the emission peak increases with the increasing of the illumination time of simulated sunlight. The results further confirmed that the hydroxyl radical is the main active species in the degradation of TC-HCl by the BST5 photocatalyst.

3.6.5 HPLC-MS analysis and toxicity estimation. HPLC-MS and the Toxicity Estimation Software Tool (T.E.S.T.) were used to detect the intermediate products of TC-HCl degradation and evaluate the toxicity of TC-HCl and the intermediate products, respectively. Fig. S4–S6† display the relevant results and the degradation pathway of TC-HCl in the presence of the BST5 photocatalyst. The results confirm that TC-HCl can be degraded into CO₂, H₂O and NH⁴⁺ under the action of the BST5 photocatalyst and simulated sunlight. By toxicity estimation, the toxicity of some intermediates decreased and some products remained. Therefore, during the photocatalytic degradation of TC-HCl by the BST5 photocatalyst, the illumination time should be extended to reduce the toxicity of the reaction products.

3.7 Photocatalytic mechanism

With the increase of the x value, the E_g value of the BST photocatalyst increases, the morphology changes from an irregular polyhedron to a regular cuboid, and the phase structure changes from a tetragonal phase to a cubic phase, and the percentage of photocatalytic degradation also increases. Optical property analysis and free radical detection experiments confirmed that the BST photocatalysts have visible light response ability, and the oxygen vacancies, holes, hydroxyl radicals and superoxide radicals are the main active species in the degradation process of TC-HCl. Based on the above conclusions and the band theory, the conduction and valence band potentials of the BST photocatalysts were obtained on the basis of eqn (11) and (12):

E_CB = X − E^e − 0.5E_g (11)

E_VB = E_CB + E_g (12)

where, the E_g values of the BST1, BST2, BST3, BST4 and BST5 photocatalysts are 2.70, 2.79, 2.83, 2.88 and 2.91 eV, respectively (E^e = 4.5 eV). The absolute electronegativity (X) values of the BST1, BST2, BST3, BST4 and BST5 photocatalysts are 5.1103, 5.0917, 5.0731, 5.0547 and 5.0363, respectively, which can be calculated by eqn (13):

The E_CB values of the BST1, BST2, BST3, BST4 and BST5 photocatalysts are −0.7397, −0.8033, −0.8419, −0.8853 and −0.9187 eV, respectively, while the E_VB values of the BST1, BST2, BST3, BST4 and BST5 photocatalysts are 1.9603, 1.9867, 1.9881, 1.9947 and 1.9913 eV, respectively. Therefore, a photocatalysis mechanism of the BST photocatalysts is presented in Fig. 10. The E_g values of the BST photocatalysts are relatively large, which make it difficult for the BST photocatalysts to respond to visible light. However, there are a few C–O functional groups and oxygen vacancies in the BST photocatalysts, which makes the transfer and separation of electrons and holes in the BST photocatalysts possible. When the simulated sunlight shines on the surface of the BST photocatalysts, the electrons in their valence band are excited to transition to the impurity level formed by the C–O functional group or vacancy, and then transition to the conduction band of the BST photocatalysts under the action of the impurity level, leaving holes in the valence band.

Ba_1−xSr_xTiO₃ + hv → e⁻ + h⁺ (14)

Fig. 10 The photocatalysis mechanism of the BST photocatalysts.

In the valence band of the BST photocatalysts, the following reactions can occur due to the redox potentials of OH⁻/˙OH being +1.89 V versus NHE.^71,72

h_VB⁺ + OH⁻ → ˙OH (15)

In addition, the holes can directly react with TC-HCl to produce non-toxic and harmless small molecular organic compounds.

h_VB⁺ + TC-HCl → CO₂ + H₂O + NH₄⁺ (16)

In the conduction band of the BST photocatalysts, electrons can react with oxygen in the reaction solution to form superoxide free radicals due to the redox potential of O₂/˙O₂⁻ being −0.13 V.^73–75

e_CB⁻ + O₂ → ˙O₂⁻ (17)

It is worth noting that the free electrons of the conduction band can also react with O₂/H₂O₂ to form hydroxyl radicals.

˙O₂⁻ + 2H⁺ + e_CB⁻ → H₂O₂ (18)

2e_CB⁻ + O₂ + 2H⁺ → H₂O₂ (19)

e_CB⁻ + H₂O₂ → ˙OH + OH⁻ (20)

˙O₂⁻ + H₂O₂ → ˙OH + OH⁻ + O₂ (21)

Finally, both the hydroxyl radicals generated by the conduction band and the valence band can react with TC-HCl to produce non-toxic and harmless products.

˙OH + TC-HCl → CO₂ + H₂O + NH₄⁺ (22)

Due to the difference of the phase structure, surface morphology, E_g value, C–O functional group and oxygen vacancy concentration of the BST photocatalysts, the percentage of the photocatalytic degradation of the BST photocatalysts increases with the increase of the x value. Therefore, it can be concluded that the BST photocatalysts with a cubic phase have a similar cuboid morphology, large E_g values and high photocatalytic activity.

4. Conclusion

The BST photocatalysts with different Sr contents were produced by the polyacrylamide gel method and adjusting the pH of the reaction solution to 11. When x = 0.3–0.5, the BST photocatalyst changes from a tetragonal phase at x = 0.1–02 to a cubic phase. The phase transition changes the value of b* in the color parameter of the BST photocatalysts from the negative of the tetragonal phase to the positive of the cubic phase. The E_g value or photocatalytic degradation percentage of the BST photocatalyst is linearly dependent on the x value. The optimal drug concentration, catalyst content, pH value and reaction time were 20 mg L⁻¹, 1 g L⁻¹, 6.77 and 120 min, respectively. The capture experiment and free radical detection experiment revealed that the hole, vacancy, hydroxyl radical and superoxide radical were the main active species in the degradation of TC-HCl by the BST photocatalysts under simulated sunlight irradiation. Their synergistic effect endow the BST photocatalysts with high charge carrier transfer and separation efficiency and photocatalytic activity. HPLC-MS and toxicity analysis revealed the photocatalytic degradation pathway of TC-HCl and the toxicity of the intermediate products. Based on the results of various experimental characterization and energy band theory, a reasonable explanation of the mechanism of photocatalysis has been proposed. This phase structure, energy band and photocatalytic regulation technique provides technical support and theoretical basis for the synthesis of other similar photocatalysts.

Author contributions

Yan Han, Shifa Wang: experiment, data analysis and paper writing. Shifa Wang, Leiming Fang, and Dengfeng Li: supervision and paper revision. Maoyuan Li, Huajing Gao, Mengjun Han, and Hua Yang: experimental testing and analysis. Jagadeesha Angadi V., A. F. Abd El-Rehim, and Atif Mossad Ali: writing – review and editing.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

This work was supported by the NSAF joint Foundation of China (U2030116), the National Natural Science Foundation of China (12075215), the Science and Technology Research Program of Chongqing Education Commission of China (KJQN202201204 and KJZD-K202001202), the Chongqing Key Laboratory of Geological Environment Monitoring and Disaster Early-warning in Three Gorges Reservoir Area (No. ZD2020A0401), the Talent Introduction Project (09924601) of Chongqing Three Gorges University and the Deanship of Scientific Research at King Khalid University for funding this work through large group Research Project under grant number RGP2/325/44.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy00278k

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