Effects of Ag⁰-modification and Fe³⁺-doping on the structural, optical and photocatalytic properties of TiO₂

Abstract

Pure TiO₂, Ag⁰-modified TiO₂, Fe³⁺-doped TiO₂, and Ag⁰-modified/Fe³⁺-doped TiO₂ photocatalysts were synthesized via sol–gel technology. The crystal structure, element composition and surface morphology of the obtained photocatalysts were characterized via XRD, XPS, SEM and TEM, respectively. The results indicate that Ag–TiO₂ samples show higher photocatalytic activity than pure TiO₂. Unexpectedly, the photocatalytic activities of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are lower than pure TiO₂. To analyze the main factors affecting photocatalytic performance, the samples were further investigated by PL, DRS and BET. The results prove that the additions of Ag and Fe are advantageous for inhibiting the recombination of photoinduced pairs and improving the utilization of light. Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ exhibit smaller specific surface areas than pure TiO₂, which is the primary reason for their reduced photocatalytic performances.

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Introduction

In the last few years, titanium dioxide (TiO₂) has been widely used in the degradation of organic pollutants due to its strong photocatalytic ability, low cost, non-toxicity and chemical stability.^1–4 However, two main drawbacks limit its practical application. Firstly, with a wide energy gap of 3.2 eV, TiO₂ only absorbs ultraviolet light with a wavelength of less than 387 nm, therefore, the utilization rate of sunlight is limited.^5–9 Secondly, the high recombination rate of photogenerated electrons and holes results in low quantum yield.^10–12

Precious metal deposition is able to improve the utilization of sunlight and promote the separation of photogenerated pairs, which are in favor of photocatalytic activity.^13–16 Zhou et al.¹⁷ prepared Au-deposited TiO₂ films on indium–tin oxide glass by magnetron sputtering. Compared with pure TiO₂ film, Au/TiO₂ films show better photocatalytic activity because of their higher separation rate of photogenerated pairs and narrower band gaps. In the research of noble metal modifying, Ag-modified TiO₂ has attracted much attention owing to its cheapness and effectiveness.¹⁸ Wang et al.¹⁹ prepared Ag/TiO₂ nanotubes by electrospinning method and the photocatalytic performance is improved after Ag modification.

Besides, TiO₂ modification by metal ion doping has also been extensively studied.^20–24 Wu et al.²⁵ synthesized Cu-doped TiO₂ by hydrothermal synthesis and air heat treatment. 0.5 mol%-Cu/TiO₂ shows better photocatalytic activity than pure TiO₂ because it enlarges the visible light absorbance due to the presence of Cu 3d orbitals. However, the photocatalytic performance declines when the concentration of Cu is more than 1 mol%. They believe that the high content of Cu forms new recombination centers, reducing the separation of photoinduced pairs. Moradi et al.²⁶ reported that Fe-doped TiO₂ shows higher visible light photocatalytic activity than pure TiO₂ owing to its red-shift and lower recombination rate. However, there are also studies have shown that the photocatalytic activity of TiO₂ is reduced after metal ion doping.^23,27 For instance, Kundu et al.²⁷ synthesized Fe-doped and pure TiO₂ nanoparticles and it is found that pure TiO₂ shows the highest degradation rate of MB under sunlight. Cu-doped TiO₂ films were prepared by sol–gel dip-coating method, and their photodegradation rates are lower than that of pure TiO₂ film, which has been reported by Bensouici et al.²³

Compared with single element modification, multi-elements may produce a synergistic effect and further improve the photocatalytic activity of TiO₂.^1,4,7,22,28 Zhang et al.²⁸ synthesized Ln³⁺/Ag⁰–TiO₂, Ln³⁺–TiO₂, Ag⁰–TiO₂ and pure TiO_2, and photocatalytic tests show that Ln³⁺/Ag⁰–TiO₂ exhibits the best photocatalytic activity. Ln³⁺/Ag⁰–TiO₂ presents the lowest PL intensity because the synergistic effect of Ln³⁺ doping and Ag⁰ deposition provides more trap centers, which promotes the transfer of photoinduced electrons, suppressing the charge recombination effectively. Meanwhile, some studies have shown that single element displays better modification effect than multi-elements.^24,29 Khan et al.‘s research²⁹ indicates that Ga-doped TiO₂ exhibits better photocatalytic performance than N/Ga co-doped TiO₂. Malengreaux et al.²⁴ reported that Eu/Fe co-doped TiO₂ shows lower photocatalytic efficiency than Fe-doped TiO₂ and pure TiO₂.

Therefore, to explore the effects of single element modification and two elements co-modification on the photocatalytic performance of TiO₂, the pure, Ag-modified, Fe-doped and Ag/Fe-modified TiO₂ were prepared and their photocatalytic activities were investigated. The effects of Ag and Fe addition on the structure, morphology, optical and photocatalytic properties of TiO₂ were analyzed systematically.

Experimental

Preparation of materials

All the TiO₂ photocatalysts were prepared by sol–gel method. The specific process of preparation pure TiO₂ was as follows: solution A was prepared by adding butyl titanate in a volume ratio of 1 : 2 to anhydrous ethanol in a beaker. Solution B was prepared by adding anhydrous ethanol, glacial acetic acid and deionized water in a volume ratio of 4 : 5 : 5. Solution B was added to solution A dropwise. The resulting mixture was stirred until a sol formed. A gel formed after aging. The obtained gel was dried at 100 °C and then heat treated at 500 °C for 1 h to obtain pure TiO₂. For the preparation of Ag-modified TiO₂ and Fe-doped TiO₂, certain amounts of AgNO₃ or Fe(NO₃)₃·9H₂O were added into solution B. The atomic percentages of Ag/Ti or Fe/Ti were 0.5%, 1%, 2% and 4%, respectively. Keep other steps unchanged to obtain different concentrations of Ag modified TiO₂ and Fe doped TiO₂ which are recorded as 0.5% Ag–TiO₂, 1% Ag–TiO₂, 2% Ag–TiO₂, 4% Ag–TiO₂, 0.5% Fe–TiO₂, 1% Fe–TiO₂, 2% Fe–TiO₂ and 4% Fe–TiO₂. AgNO₃ and Fe(NO₃)₃·9H₂O were simultaneously added into solution B to prepare Ag/Fe co-modified TiO₂. Both the molar ratios of Ag/Ti and Fe/Ti are 1%, and the Ag/Fe co-modified TiO₂ is labeled as 1% Ag/1% Fe–TiO₂. The preparation process is shown in Fig. 1.

Fig. 1 The preparation process of TiO₂.

Characterization

The phase structure of photocatalysts was determined using an X-ray diffractometer (XRD). Element composition and valence state were analyzed by an X-ray photoelectron spectroscopy (XPS). Surface morphologies were observed by a field emission scanning electron microscopy (FESEM) and a transmission electron microscopy (TEM). Photoluminescence (PL) spectra were recorded through a luminescence spectrometer. UV-vis diffuse reflectance spectra (DRS) were recorded using a spectrophotometer. Specific surface areas were determined by the Brunauer Emmett Teller (BET) method.

Photocatalytic test

Add 100 mL rhodamine B (RhB) solution (10 mg L⁻¹) and 0 1 g of TiO₂ photocatalyst to the beaker. The mixture was ultrasonically dispersed for 10 minutes, and then stirred to establish the adsorption–desorption equilibrium in dark for 30 min. A 250 W xenon lamp was employed as a light source and a small amount of suspension was taken every 30 minutes. After centrifugation, the supernatant was extracted and the absorbance of RhB was measured with a spectrophotometer at the wavelength of 553 nm. The decolorization rate D (%) was calculated as follows:

D = (1−A_t/A₀)

In the formula, A₀ is the initial absorbance, and A_t is the absorbance at time t.

Results and discussion

XRD analysis

Fig. 2 shows the XRD patterns of pure TiO₂, Ag–TiO₂, Fe–TiO₂ and 1% Ag/1% Fe–TiO₂. In Fig. 2(a), the peaks of pure TiO₂ at 25.3°, 37.9°, 48.1°, 53.9°, 55.1°and 62.8° correspond to (101), (004), (200), (105), (211) and (204) crystal planes of anatase structure, respectively. It is observed that Ag–TiO₂, Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ also form anatase structure, suggesting that the crystal structure of TiO₂ is not significantly affected by Ag or Fe adding. However, the widths of diffraction peaks in Ag–TiO₂, Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are wider and the intensities are lower than pure TiO₂, indicating that the crystallinity of TiO₂ is reduced and the crystal grain is refined.^26,30 The grain sizes of TiO₂ are calculated by Scherrer formula²¹ and the results are shown in Table 1. Yu et al.³¹ believe that the addition of dopants hinders the contact between TiO₂ particles and inhibits the growth of grains during the heat treatment process, resulting in the decrease of grain size. The absence of diffraction peaks associated with Fe may be attributed to the fact that Fe³⁺ ions can replace Ti⁴⁺ ions into TiO₂ lattices due to their close ionic radius.^26,32,33 On the other hand, since the radius of Ag atom is much larger than that of Ti⁴⁺, it is difficult to enter TiO₂ lattices. Ag particles probably disperse on TiO₂ surface in the form of metallic Ag.^34,35

Fig. 2 XRD patterns of (a) Ag–TiO₂, (b) Fe–TiO₂ and (c) 1% Ag/1% Fe–TiO₂.

Table 1 The crystallite sizes of samples

Samples	Crystallite size (nm)	Samples	Crystallite size (nm)
Pure TiO2	16.0	0.5% Fe–TiO2	15.1
0.5% Ag–TiO2	11.2	1% Fe–TiO2	15.9
1% Ag–TiO2	11.6	2% Fe–TiO2	13.7
2% Ag–TiO2	11.9	4% Fe–TiO2	12.3
4% Ag–TiO2	12.0	1% Ag/1% Fe–TiO2	13.3

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XPS analysis

XPS is widely used to investigate the chemical valence of elements. To verify the valence states of Ag and Fe elements, XPS analysis of 1% Ag/1% Fe–TiO₂ has been carried out and the results are shown in Fig. 3. The C, Ti, O, Ag and Fe signal peaks appear in the full spectrum (Fig. 3(a)), meaning that there are C, Ti, O, Ag and Fe elements in the sample. The appearance of C 1s peak may be derived from C remaining in the organic calcination process. In Fig. 3(b), two peaks at 458.2 eV and 463.9 eV corresponding to Ti 2p_3/2 and Ti 2p_1/2 and the splitting energy between Ti 2p_3/2 and Ti 2p_1/2 (5.7 eV) indicate that the state of Ti ions is Ti⁴⁺.^36–38 Fig. 3(c) presents two peaks at 529.6 eV and 531.6 eV, ascribing to Ti–O bonds in TiO₂ lattices (O_L) and surface hydroxyl groups (O_H), respectively.^39,40 In Fig. 3(d), the peaks at 367.6 eV and 373.5 eV can be assigned to Ag 3d_5/2 and Ag 3d_3/2, respectively, which suggests that the Ag element exists as Ag⁰ in the sample.^34,41,42 Two peaks at 711.1 eV and 723.7 eV in Fig. 3(e) attribute to Fe 2p_3/2 and Fe 2p_1/2, which indicates Fe element exists as Fe³⁺.^27,32,43,44

Fig. 3 XPS spectra of 1% Ag/1% Fe–TiO₂: (a) total spectrum, (b) Ti 2p, (c) O 1s, (d) Ag 3d and (e) Fe 2p.

SEM and TEM images

Fig. 4 presents the SEM images of pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂, and 1% Ag/1% Fe–TiO₂. It is observed that the particles are nearly spherical. Among them, pure TiO₂ and 1% Ag–TiO₂ particles are fine and disperse relatively uniform, however, the particles in 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ present obvious agglomeration.

Fig. 4 SEM images of pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂.

Fig. 5(a) and (b) are TEM images of pure TiO₂ and 1% Ag/1% Fe–TiO₂, from which we can see that the particle size in pure TiO₂ is about 15–20 nm, while it is arranged from 10 to 15 nm in 1% Ag/1% Fe–TiO₂. Moreover, pure TiO₂ shows better particle dispersion than 1% Ag/1% Fe–TiO₂, which is consistent with SEM images. The HRTEM images of pure TiO₂ and 1% Ag/1% Fe–TiO₂ are presented in Fig. 5(c) and (d), respectively. Both the labeled interplanar spacing values in pure TiO₂ (0.352 nm) and 1% Ag/1% Fe–TiO₂ (0.364 nm) correspond to the anatase (101) crystal plane.⁴⁵ Since the radius of Fe³⁺ ions is larger than of Ti⁴⁺ ions, the substitution of Fe³⁺ for Ti⁴⁺ into crystal lattices causes lattice expansion, consequently, the anatase (101) plane spacing of 1% Ag/1% Fe–TiO₂ is slightly larger than that of pure TiO₂.²⁵ In Fig. 5(d), the marked interplanar spacing (0.236 nm) can be attributed to the (111) crystal plane of metallic Ag⁰,^34,39 which is in accord with XPS results.

Fig. 5 TEM images of (a) pure TiO₂, (b) 1% Ag/1% Fe–TiO₂, HRTEM images of (c) pure TiO₂, and (d) 1% Ag/1% Fe–TiO₂.

Photocatalytic performance

Photodegradation results. The photocatalytic activity of photocatalysts was evaluated via the decolorization rate of RhB and the results are presented in Fig. 6. The RhB decolorization rate of pure TiO₂ is 85.7% after 90 min of reaction. Ag–TiO₂ samples show higher photocatalytic activity than pure TiO₂. The decolorization rates of 0.5% Ag–TiO₂, 1% Ag–TiO₂, 2% Ag–TiO₂ and 4% Ag–TiO₂ are 88.8%, 95.4%, 92.8% and 92.5%, respectively. 1% Ag–TiO₂ exhibits the best photocatalytic activity and the decolorization rate of RhB decreases when Ag content exceeds 1%. Unexpectedly, the photocatalytic activity of TiO₂ decreases after Fe doping. The decolorization rates of 0.5% Fe–TiO₂, 1% Fe–TiO₂, 2% Fe–TiO₂ and 4% Fe–TiO₂ are 33.3%, 51.3%, 39.9% and 30.6%, respectively. It is clear that the addition of Ag improves the photocatalytic performance, while Fe is the opposite. The decolorization rate of 1% Ag/1% Fe–TiO₂ is 64.3%, suggesting that its photocatalytic activity is higher than Fe–TiO₂ but lower than pure TiO₂. The decolorization of RhB by TiO₂ can be considered as a first-order reaction,^17,19,46 and the calculation formula of the reaction rate constant k is as follows:

ln(C_t/C₀) = −kt

where t is the reaction time, C₀ and C_t are the initial concentration and the concentration at time t. The first-order reaction kinetics fit curves are shown in Fig. 7. It is calculated that the reaction rate constant of pure TiO₂ is 0.020 min⁻¹. Ag–TiO₂ samples show higher reaction rates than pure TiO₂, and the reaction rate constants of 0.5% Ag–TiO₂, 1% Ag–TiO₂, 2% Ag–TiO₂ and 4% Ag–TiO₂ are 0.024 min⁻¹, 0.034 min⁻¹, 0.030 min⁻¹ and 0.029 min⁻¹, respectively. The reaction rate constants of 0.5% Fe–TiO₂, 1% Fe–TiO₂, 2% Fe–TiO₂ and 4% Fe–TiO₂ are 0.0036 min⁻¹, 0.0075 min⁻¹, 0.0041 min⁻¹ and 0.0034 min⁻¹, respectively, implying that the reaction rates of Fe–TiO₂ are much lower than pure TiO₂. The reaction rate constant of 1% Ag/1% Fe–TiO₂ is 0.0099 min⁻¹, which is higher than Fe–TiO₂ but lower than pure TiO₂.

Fig. 6 Decolorization rate curves of RhB for (a) Ag–TiO₂, (b) Fe–TiO₂, and (c) 1% Ag/1% Fe–TiO₂.

Fig. 7 Kinetics linear simulation curves of (a) Ag–TiO₂, (b) Fe–TiO₂, and (c) 1% Ag/1% Fe–TiO₂.

PL spectra

In order to analyze the influence of addition with Ag and Fe on the photogenerated electron–hole recombination of TiO₂, the PL tests have been implemented and the results are shown in Fig. 8. The PL peaks originate from the recombination of photoinduced electrons and holes. Therefore, a lower PL peak intensity represents a lower recombination rate.^27,38 The PL peak intensity of Ag–TiO₂ is lower than that of pure TiO₂, which suggests that the recombination of photoinduced pairs is inhibited by Ag modification. When exposed to light source, electrons in the valence band are excited to the conduction band, forming photogenerated electrons, leaving photogenerated holes on valence band. Photoinduced electrons can be transferred to Ag⁰ particles which are deposited on TiO₂ surface, reducing the recombination.^6,18,19,34 Several researches have reported that there is an optimum concentration of Ag, above which new recombination centers will be formed thus increases recombination rate.^18,34 However, PL peak intensity decreases with the increasing Ag concentration in the present work, indicating that new recombination centers have not been formed when the Ag concentration reaches 4%. The PL peak intensity decreases with the increase of the amount of noble metal element, which also has been reported in previous work.⁴⁷ Photocatalytic experiments show that high Ag concentration is not conducive to photocatalytic activity and 1% Ag–TiO₂ exhibits the highest decolorization rate. The decrease in photocatalytic performance of 2% Ag–TiO₂ and 4% Ag–TiO₂ should be attributed to the fact that Ag⁰ particles are deposited on the surface of TiO₂ particles, and as the Ag concentration increases, excessive Ag particles will cover TiO₂ surface, reducing the utilization of light and reactive sites.^13,34

Fig. 8 PL spectra of (a) Ag–TiO₂, (b) pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂.

The photocatalytic activities of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are lower than pure TiO₂. Similar results have also been reported that the addition of metal ions reduces the photocatalytic performance of TiO₂.²³ There is a viewpoint that doping metal ions leads to a decrease in crystallinity and an increase in lattice defects. The formed lattice defects act as recombination centers, promoting the recombination of photoinduced pairs and suppressing the photocatalytic activity.^23,48,49 In contrast, from Fig. 8(b), it is clear that the PL intensities of 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are lower than that of pure TiO₂, proving that the addition of Fe is beneficial to decreasing the recombination. Fe³⁺ ions entering the crystal lattices to replace Ti⁴⁺ ions will cause lattice distortion and defects, capturing photogenerated electrons and reducing the recombination of photogenerated charges.^7,33 Moreover, 1% Ag/1% Fe–TiO₂ shows the lowest PL intensity, suggesting that there is a synergistic effect in suppressing photoinduced pairs recombination owing to adding Ag and Fe simultaneously. Therefore, we can conclude that the decrease in photocatalytic performance of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ should not be attributed to the promotion of recombination with Fe adding.

DRS analysis

The energy gap (E_g) of photocatalyst affects the absorption of light source, which is an important factor for photocatalytic performance. The influence of doping on the E_g of TiO₂ is controversial, and both redshift^{6,7,9,26,27,34} and blueshift²⁴ have been reported. Fig. 9 depicts the UV-visible absorption spectra of pure TiO₂ and 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂. It is observed that the addition of Ag and Fe causes a redshift in the absorption edge of TiO₂. The E_g was calculated based on the Kubelka–Munk equation and Tauc's plots.^3,32,35,50 The E_g of pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are estimated to be 3.20 eV, 3.09 eV, 2.69 eV and 2.87 eV, respectively. The results show that the addition of Ag and Fe is beneficial to increasing the absorption of visible light. It can be concluded from XPS and TEM results that Ag element exists in the form of Ag⁰. Due to the surface plasmon resonance (SPR) effect of Ag particles deposited on TiO₂ surface, the absorption of visible light can be enhanced, reducing the E_g.^{15,16,34,36,37} On the other hand, the substitutions of Ti⁴⁺ ions with Fe³⁺ ions form impurity energy levels between the conduction band and valence band in the forbidden band, thereby decreasing the E_g.^{5,7,10,21,33,43,51} Therefore, the change of E_g is not the main reason for the decreased photocatalytic performance of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂.

Fig. 9 (a) UV-visible absorption spectra and (b) plots of (αhν)^1/2 versus energy (hν) of pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂.

BET analysis

As is well known, photocatalyst with a larger specific surface area will provide more active reaction sites and increase light absorption, which is advantageous for photocatalytic performance.^19,22,27,33 It is observed from SEM images that the 1% Fe–TiO₂ particles have a larger agglomeration than pure TiO₂, which may lead to the decrease of specific surface area. To verify this assumption, we have performed BET specific surface area tests and the results are shown in Table 2. The specific surface area of 1% Fe–TiO₂ is 16.1 m² g⁻¹, which is much lower than that of pure TiO₂ (63.8 m² g⁻¹). Some researchers believe that reduction in grain size will lead to an increase in specific surface area.^48,51 In our work, the grain size of TiO₂ is reduced by Ag or Fe modification, which is proved by XRD results, however, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ possess smaller specific surface areas than pure TiO₂. Wang et al.⁵² and Adyani et al.²² are convinced that the specific surface area of TiO₂ is largely related to agglomeration degree. SEM and TEM images confirm an increase in particle agglomeration after Fe addition, which is the major reason for the decrease in specific surface area. It is the significant reduce in specific surface areas of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ that causes the decrease in their photocatalytic performance.

Table 2 Specific surface areas (S_BET) of pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂

Samples	SBET (m^2 g^−1)
Pure TiO2	63.8
1% Ag–TiO2	59.4
1% Fe–TiO2	16.1
1% Ag/1% Fe–TiO2	49.8

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From another perspective, as the surface areas of samples are quite different, the intrinsic photocatalytic activity (normalized by BET surface area) can be used as a reference. The intrinsic photocatalytic activity (I_PA) is calculated as follows:⁵³

I_PA (mg min⁻¹ m⁻²) = m_RhB/tS_BETm_s

where m_RhB is the weight of RhB which has been degraded, t is the reaction time (90 min), S_BET is the surface area, and m_s is the weight of sample used in each test (0.1 g).

The I_PA of pure TiO₂, 1% Ag–TiO₂, 1% Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are 0.00149 mg min⁻¹ m⁻², 0.00178 mg min⁻¹ m⁻², 0.00354 mg min⁻¹ m⁻² and 0.00143 mg min⁻¹ m⁻². The results show that 1% Fe–TiO₂ has relatively high intrinsic photocatalytic activity.

Conclusions

In summary, pure TiO₂, Ag–TiO₂, Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ were prepared by sol–gel method. The results of the decolorization rate of RhB indicate that the photocatalytic activities of Ag–TiO₂ are higher than pure TiO₂, while the photocatalytic activities of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are lower than pure TiO₂. The increased photocatalytic activity of Ag–TiO₂ can be attributed to the reduction of photoinduced pairs recombination rate and energy gap. The specific surface areas of Fe–TiO₂ and 1% Ag/1% Fe–TiO₂ are much lower than pure TiO₂, leading to the decreases in their photocatalytic properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Applied Basic Research Programs of Sichuan Province, China (Grant No. 2019JY0664, 2018JY0062), the Technology Innovation Research and Development Project of Chengdu City (Grant No. 2019-YFYF-00013-SN) and the Training Program for Innovation of Chengdu University, China (Grant No. 201911079002, S201911079042).

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