English

• 0.   Introduction

  Photocatalytic technology can be used to simulate natural photosynthesis, which can change solar energy into chemical energy, and degrade organic pollutants in sewage into harmless substances such as CO₂ and H₂O under normal temperature and pressure^[1-3], thus avoiding the secondary pollution problem with traditional methods. TiO₂ is an n-type semiconductor catalyst that is non-toxic, highly active, chemically stable, cheap, environmentally friendly, and it has been widely studied as an ideal photocatalyst^[4-7]. However, in the process of photocatalysis, TiO₂ has some defects, such as low quantum efficiency, easy recombination of electron - hole pairs, and low utilization of sunlight, which greatly restricts its extensive industrial application. The solution to these problems depends on in - depth and systematic basic research.

  To improve the photocatalytic activity of TiO₂, the researchers used a variety of methods, such as controlling the morphology^[8-11], doping transition metal ions and non - metallic ions^[12-16], surface sensitization^[17-18], semiconductor composite^[19-20]. Recent studies show that the selection of semiconductors with appropriate energy bands to couple with TiO₂, such as Bi₂WO₆^[21-22], g-C₃N₄^[23-25], CdS^[26-27], CeO₂^[28-29], is conducive to separating electrons and holes, and improving the visible light catalysis of TiO₂. CeO₂ has high conductivity, thermal stability, oxygen storage capacity, and has a narrow energy gap (2.92 eV). Moreover, Ce⁴⁺ and Ce³⁺ ions are easy to reciprocal transformation, which makes CeO₂ have good electron transfer ability and light absorption ability. The bandgap difference between TiO₂ and CeO₂ can promote the separation of photogenerated electronhole pairs and improve catalysis activity^[30]. Although TiO₂ and CeO₂ composite materials have received extensive attention, the research of CeO₂/TiO₂ as promising photocatalytic materials is not deep enough. In particular, the photocatalytic efficiency of CeO₂/TiO₂ is far from practical application. Therefore, it is necessary to further improve the photocatalytic performance of CeO₂/TiO₂ by optimizing the experiment. In this work, we prepared CeO₂/TiO₂ photocatalyst materials with a three-dimensional flower structure by solvothermal method. Under xenon lamp irradiation, flower-like CeO₂/TiO₂ photocatalyst had high activity for methyl orange degradation.

  1.   Experimental

  1.1   Preparation of the samples

  Preparation of CeO₂: All the chemical reagents were chemically pure and were used directly without further processing. The water used was distilled water. Under strong stirring, 0.26 g cerium nitrate was dissolved in 100 mL water. After stirring frequently for 30 min, NaOH was added to the solution to control the pH to 9 - 10, followed by hydrothermal treatment at 180 ℃ in a Teflon-lined autoclave for 24 h. The product was centrifugally separated, washed with ethanol and distilled water, then dried. The sample was collected and then put into the annealing furnace at 500 ℃ for 2 h to obtain CeO₂.

  Preparation of CeO₂/TiO₂: polyethylene glycol, cetyltrimethyl ammonium bromide, and carboxamide were immersed into 70 mL acetic acid solution, and after vigorous stirring to dissolve them, CeO₂ was added into the above-mixed solution, finally added 2 mL butyl titanate by dropping and stirring for 20 min, and then moved the solution to 100 mL stainless steel autoclave lined with polytetrafluoroethylene. The reaction time was different at 150 ℃, and cooling with the furnace to room temperature. The precipitates were washed with ethanol and water thoroughly three times, drying at 80 ℃ and calcining at 450 ℃ for 1 h. According to the above preparation method, the samples prepared with Ce/Ti molar ratios n_Ce/n_Ti of 0.05, 0.1 and 0.2 in the reaction system were marked as 0.05CeO₂/TiO₂, 0.1CeO₂/TiO₂, 0.2CeO₂/TiO₂ respectively.

  1.2   Characterization

  Under the conditions of Cu target, 40 kV and 40 mA with Cu Kα X-ray radiation source (λ=0.154 nm) and 2θ range of 20°-80°, the samples were recorded by X - ray diffractometer of Dandong Haoyuan instrument company; the morphologies of the synthetic samples were used by scanning electron microscope (SEM, Zeiss Merlin field emission) at the acceleration voltage of 5 kV; the specific surface area was measured using the measurement instrument (ASAP2460). The U-3900 ultraviolet - visible spectrophotometer with integrating sphere in Japan was used to measure the absorbance of powder. X -ray photoelectron spectroscopy (XPS) measurements were measured on an Escalab 250 Xi spectrometer. Photoluminescence (PL) spectra were measured using FLS 980 fluorescence spectrophotometer. The photocurrent response and electrochemical impedance spectroscopy (EIS) were carried by an electrochemical workstation (CHI660E).

  1.3   Photocatalytic activity measurement

  CeO₂/TiO₂ was added to methyl orange (MO) solution, then the MO solution was illuminated. The photocatalytic performance of the sample was tested by measuring the degradation rate of MO. The specific processes were listed as follows: 0.02 g of catalyst sample was added to 80 mL MO solution (10 mg·L^-1), and ultrasonic agitation was performed for 30 min to achieve adsorption-desorption equilibrium in the dark. A 300 W xenon lamp was used to simulate and irradiate from the top of the MO solution. The xenon lamp was 10 cm away from the liquid surface. A small portion of the solution was taken every 10 min to be centrifuged and separated. The absorbance of the residual MO was analyzed by an ultraviolet-visible spectrophotometer.

  2.   Results and discussion

  2.1   Characterization of the samples

  Fig. 1 shows the XRD patterns of CeO₂/TiO₂ heterojunction prepared by adding different amounts of CeO₂. There were several different diffraction peaks of CeO₂/TiO₂ heterojunction nanoflowers at 2θ =25.3°, 37.9°, 48.1°, 54.1°, 55.2°, 62.6°, and 70.3° respectively, corresponding to anatase TiO₂ (PDF No. 21 - 1272). The diffraction peaks with 2θ=28.6°, 33.2°, 56.6°, and 59.5° belong to the characteristic diffraction peaks of CeO₂ (PDF No. 34 - 0394), indicating that the heterostructure nanocomposite composed of TiO₂ and CeO₂. It can be seen from the figure that the intensity of the diffraction peak of CeO₂ increased gradually with the increase of CeO₂ content.

  Figure 1

  

  Figure 1.  XRD patterns of CeO₂/TiO₂

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  Fig. 2 showed that the prepared CeO₂/TiO₂ heterojunction had a three-dimensional flower-like structure, and nano-CeO₂ particles adhered to the petals of TiO₂. With the increase of CeO₂ content, the number of CeO₂ nanoparticles on the petals of TiO₂ increased gradually.

  Figure 2

  

  Figure 2.  SEM and TEM images of (a, b) 0.05CeO₂/TiO₂, (c, d) 0.1CeO₂/TiO₂, and (e, f) 0.2CeO₂/TiO₂

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  Solvothermal time can affect the morphology and properties of the samples. When the molar ratio of Ce and Ti was 0.1, and the samples were labeled as CeO₂/TiO₂-t, where t min was the reaction time. Fig. 3 shows that the diffraction peaks correspond to the characteristic diffraction peaks of TiO₂ and CeO₂ respectively.

  Figure 3

  

  Figure 3.  XRD patterns of (a) CeO₂/TiO₂-4, (b) CeO₂/TiO₂-6 and (c) CeO₂/TiO₂-12

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  Fig. 4 shows the SEM images of CeO₂/TiO₂. It can be seen that under solvothermal conditions for 4 h, the CeO₂/TiO₂ heterojunction was a three-dimensional flower-like microsphere structure. The diameter of the microspheres was between 0.61 and 0.96 μm. The average diameter was 0.77 μm. The flower structure was formed by the directional aggregation of nanoparticles. When the reaction time increased up to 6 h, the diameter of the flower-like microspheres ranged from 0.58 to 1.29 μm, with an average diameter of 0.59 μm. When the solvothermal time was 12 h, the diameter of the three-dimensional flower-like structure was 0.88-1.89 μm, with an average diameter of 1.36 μm. CeO₂ particles were oriented and integrated into a shuttle shape embedded between thin plates. With the increase of solvothermal time, the diameter of flower - like TiO₂ became smaller at the beginning and larger at the next stage, and CeO₂ gradually aggregated from nanoparticles to shuttle shape.

  Figure 4

  

  Figure 4.  SEM images of (a, b) CeO₂/TiO₂-4, (c, d) CeO₂/TiO₂-6 and (e, f) CeO₂/TiO₂-12

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  Fig. 5 shows the N₂ adsorption - desorption isotherms and BJH (Barrette - Joyner - Halenda) pore size distribution curves of samples. The Brunauer-EmmettTeller specific surface area (S_BET), pore volume (V_P), and average pore size of the samples are shown in Table 1. The results showed the prepared samples had high S_BET and large V_P, providing more active sites and light-harvesting capacity, and improving the utilization efficiency of light, thereby contributing to the degradation of organic pollutants.

  Figure 5

  

  Figure 5.  (a) N₂ adsorption-desorption isotherms and (b) pore size distribution curves for CeO₂/TiO₂-t

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  Table 1

  

  Table 1.  S_BET, V_P, and average pore diameter of CeO₂/TiO₂-t

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  Sample	SBET / (m2·g-1)	VP/ (cm3·g-1)	Average pore diameter / nm
  CeO2/TiO2-4	159	0.41	8.68
  CeO2/TiO2-6	143	0.48	11.44
  CeO2/TiO2-12	110	0.36	11.48

  Fig. 6 shows the full spectrum of CeO₂/TiO₂-6 and the high-resolution XPS spectra of Ti2p, O1s, and Ce3d. It can be seen from Fig. 6a that the sample only contained C, O, Ti, and Ce elements. C was mainly derived from the residual carbon of some organic precursors during heat treatment and the oily carbon from the XPS instrument itself. The binding energies of 458.78 and 464.48 eV in Fig. 6b correspond to the characteristic peaks of Ti2p_2/3 and Ti2p_1/2 orbits respectively, which are the standard bond energies of Ti2p in pure TiO₂, indicating that Ti exists in form of Ti^{4+ [31]}. In the O1s spectrum of Fig. 6c, one peak at around 530.10 eV corresponds to the oxygen in the TiO₂ lattice, and the other peak at around 531.58 eV corresponds to the hydroxyl (—OH) on the surface of TiO₂^[32-33]. In Fig. 6d, V (881.52), V″ (888.13), and V‴ (898.41) correspond to Ce3d_5/2 spin-orbital bands; U (900.11), U″ (906.83), and U ‴ (915.81) correspond to Ce3d_3/2 spin - orbital bands. The peaks labeled as V, V″, V‴, U, U″, and U‴ are attributed to the existence of Ce⁴⁺. The peaks at V' (885.13) and U' (903.12) are attributed to the presence of Ce³⁺ in the composite^[34]. Ce³⁺ is mainly due to the strong interaction between TiO₂ and CeO₂, which makes Ce⁴⁺ reduced to Ce^{3+ [35]}.

  Figure 6

  

  Figure 6.  XPS spectra of CeO₂/TiO₂-6: (a) survey, (b) Ti2p, (c) O1s, (d) Ce3d

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  Because the intensity of light emission depends on the recombination ability of excited electrons and holes, we can analyze the ability of semiconductor materials to capture and migrate photogenerated holes and electrons. The low intensity of the PL spectrum indicates that the recombination rate of electron - hole pairs is low and the separation efficiency of electron hole pairs represents reverse. Fig. 7 shows the PL spectra excited at 350 nm. The PL intensity of CeO₂/TiO₂-12 was lower than that of CeO₂/TiO₂-6, indicating that CeO₂/TiO₂-12 presented high separation efficiency.

  Figure 7

  

  Figure 7.  PL spectra of CeO₂/TiO₂-6 and CeO₂/TiO₂-12

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  2.2   Photocatalytic activity

  To investigate the photocatalytic activity of the sample, the photocatalytic degradation of MO (xenon lamp simulated sunlight) was carried out. The degradation rate of MO was calculated as follows: D=(1-A/A₀)× 100%, where D is the degradation rate of MO solution; A₀ is the absorbance of MO solution before irradiation; A is the absorbance of MO solution at the wavelength of 464 nm. The experimental results of photocatalysis under light were shown in Fig. 8.

  Figure 8

  

  Figure 8.  Photocatalytic degradation rate of MO for the samples

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  Fig. 8a shows the curve of the photocatalytic degradation rate of MO under simulated sunlight for the samples prepared with various molar ratios of CeO₂ and TiO₂. Fig. 8b shows the photocatalytic degradation rate curves of MO under simulated sunlight irradiation for the samples prepared under different solvothermal times when the molar ratio of CeO₂ to TiO₂ was 0.1 (The material prepared without polyethylene glycol, cetyltrimethylammonium bromide, and carboxamide was recorded as CeO₂/TiO₂-B). It can be seen that the degradation rate of MO with catalyst increased with the extension of illumination time. The degradation rate of CeO₂/TiO₂ was better than that of TiO₂ after 50 min illumination. The photocatalytic performance of flower like CeO₂/TiO₂ was higher than that of CeO₂/TiO₂ - B. 0.1CeO₂/TiO₂ had the best photocatalytic performance under 50 min illumination and the photocatalytic activity of CeO₂/TiO₂ - 6 was the best, and the degradation rate reached 95% after 50 min illumination (Fig. 8b). The degradation rate of MO solution added with pure TiO₂ or CeO₂ was only 78% or 70% respectively after 50 min illumination, which indicated that the composite of CeO₂ and TiO₂ enhances the photocatalytic activity of TiO₂.

  Fig. 9 is the UV-Vis diffuse reflectance spectra of the samples. It can be seen that the absorption band edges of TiO₂, CeO₂, CeO₂/TiO₂ - 4, CeO₂/TiO₂ - 6, and CeO₂/TiO₂ - 12 were 393, 432, 463, 481, and 469 nm respectively. According to the formula E_g=1 240/λ_g (λ_g is absorption edge), the bandgaps (E_g) of TiO₂, CeO₂, CeO₂/TiO₂ - 4, CeO₂/TiO₂ - 6, and CeO₂/TiO₂ - 12 were about 3.16, 2.87, 2.68, 2.58, and 2.64 eV respectively, which indicates that CeO₂/TiO₂ broadens the absorption range compared with TiO₂ and CeO₂.

  Figure 9

  

  Figure 9.  UV-Vis diffuse reflectance spectra of the samples

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  Fig. 10 shows the effects of reuse times of CeO₂/TiO₂ - 6 catalyst on photocatalytic activity. It can be seen that the degradation rates of MO by CeO₂/TiO₂-6 were 95%, 94%, and 92% respectively when the catalyst was reused for the first time, the second time, and the third time. The catalytic activity was not significantly reduced, indicating that the photocatalyst has certain stability and can be recycled many times.

  Figure 10

  

  Figure 10.  Effect of reuse degradation times of CeO₂/TiO₂-6 on the degradation rate of MO

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  In the process of photocatalysis, water molecules or hydroxyl radicals can be oxidized by holes to generate hydroxyl radicals, and superoxide anion radicals may be generated when dissolved oxygen in water receives photogenerated electrons. Electron spin resonance (ESR) is generally used to detect hydroxyl radical (·OH) radical and superoxide radical (·O₂^-). Fig. 11 presents the ESR spectra of DMPO-·OH and DMPO·O₂^- obtained with 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as the radical scavenger. Under xenon lamp irradiation, the ESR spectra of ·OH showed four characteristic peaks, and that of ·O₂^- showed six characteristic peaks. However, there was no signal in the dark. It indicates that ·OH and ·O₂^- exist in the reaction system with CeO₂/TiO₂.

  Figure 11

  

  Figure 11.  ESR spectra of (a) DMPO-·OH and (b) DMPO-·O₂^- for CeO₂/TiO₂-6 in the dark and under xenon lamp irradiation

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  The interface charge transfer and photogenerated charge recombination of the catalyst were investigated by electrochemical characterization. Fig. 12a shows the photocurrent response of the catalyst under xenon lamp irradiation. It suggests that CeO₂, TiO₂, and CeO₂/TiO₂-6 all had obvious photocurrent responses. When the light source was turned off, the current signal returned to the original level, and the response current of CeO₂/TiO₂-6 was higher than that of pure CeO₂ or pure TiO₂ under the light. Generally, the stronger the separation ability of photo-generated carriers, the stronger the photocurrent of the material. That shows the separation ability of CeO₂/TiO₂ - 6 photo - generated carriers was better than pure CeO₂ and TiO₂, which is mainly due to the formation of heterojunction between CeO₂ and TiO₂. EIS can further confirm the effective separation of photogenerated electrons and holes. The arc radius in EIS (Fig. 12b) is related to the charge transfer resistance of the material. In general, the smaller the arc radius, the faster the separation or transfer speed of photogenerated carriers, and the photocurrent intensity is also increased. It can be seen that CeO₂/TiO₂ - 6 had the smallest arc radius, which indicates that CeO₂/TiO₂ - 6 has the smallest electron transfer resistance and the best charge separation efficiency, which is consistent with the photocurrent response.

  Figure 12

  

  Figure 12.  (a) Transient photocurrent responses of CeO₂, TiO₂, and CeO₂/TiO₂-6; (b) EIS spectra of CeO₂, TiO₂, and CeO₂/TiO₂-6

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  Fig. 13 shows the photocatalysis mechanism of CeO₂/TiO₂. Under simulated sunlight, CeO₂/TiO₂ can absorb not only ultraviolet light but also visible light. Both CeO₂ and TiO₂ can be excited by ultraviolet light, then the electrons jump to the conduction band to form the conduction band electron (e^-) while leaving holes (H⁺) in the valence band. Because the conduction band (CB) of CeO₂ is higher than that of TiO₂, the electrons in CB of CeO₂ transfer to CB of TiO₂ through the interface. On the other hand, the valence gap (VB) of CeO₂ is lower than that of TiO₂, and the holes of VB of TiO₂ are transferred to VB of CeO₂, which is prone to the separation of photogenerated electron-hole pairs^[30]. Under visible light irradiation, electrons from VB of CeO₂ are transferred to CB of TiO₂, and photogenerated electrons in CB of CeO₂ can be transferred to CB of TiO₂, thus inhibiting the recombination of photogenerated electrons and hole^[36]. The results were consistent with the photocurrent response and EIS. Subsequently, the e^- was reacted with the O₂ to form ·O₂^-. The H₂O could be oxidized by h⁺ to produce ·OH. The pollutant was oxidized by ·O₂^- and ·OH to produce CO₂ and H₂O. Simultaneously, the h⁺ in VB of CeO₂ was directly involved in the oxidation of pollutants.

  Figure 13

  

  Figure 13.  Photocatalysis mechanism of CeO₂/TiO₂

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  3.   Conclusions

  The three-dimensional flower-like CeO₂/TiO₂ heterojunction was prepared by the solvothermal method. Compared with TiO₂, flower-like CeO₂/TiO₂ heterojunction showed better photocatalytic performance under simulated sunlight. Among them, the degradation rate of MO reached 95% when CeO₂/TiO₂-6 was illuminated for 50 min, and the photocatalytic performance reached the best. The flower-like CeO₂/TiO₂ heterojunction had excellent catalytic performance, which is mainly due to the following factors. First of all, the three - dimensional hierarchical structure, with a large specific surface area and a different size of pore structure, greatly improves the utilization of light. Secondly, the heterojunction effect can enhance the efficiency of charge separation and interface charge transfer greatly.

  
  
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  Figure 1  XRD patterns of CeO₂/TiO₂

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  Figure 2  SEM and TEM images of (a, b) 0.05CeO₂/TiO₂, (c, d) 0.1CeO₂/TiO₂, and (e, f) 0.2CeO₂/TiO₂

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  Figure 3  XRD patterns of (a) CeO₂/TiO₂-4, (b) CeO₂/TiO₂-6 and (c) CeO₂/TiO₂-12

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  Figure 4  SEM images of (a, b) CeO₂/TiO₂-4, (c, d) CeO₂/TiO₂-6 and (e, f) CeO₂/TiO₂-12

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  Figure 5  (a) N₂ adsorption-desorption isotherms and (b) pore size distribution curves for CeO₂/TiO₂-t

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  Figure 6  XPS spectra of CeO₂/TiO₂-6: (a) survey, (b) Ti2p, (c) O1s, (d) Ce3d

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  Figure 7  PL spectra of CeO₂/TiO₂-6 and CeO₂/TiO₂-12

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  Figure 8  Photocatalytic degradation rate of MO for the samples

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  Figure 9  UV-Vis diffuse reflectance spectra of the samples

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  Figure 10  Effect of reuse degradation times of CeO₂/TiO₂-6 on the degradation rate of MO

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  Figure 11  ESR spectra of (a) DMPO-·OH and (b) DMPO-·O₂^- for CeO₂/TiO₂-6 in the dark and under xenon lamp irradiation

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  Figure 12  (a) Transient photocurrent responses of CeO₂, TiO₂, and CeO₂/TiO₂-6; (b) EIS spectra of CeO₂, TiO₂, and CeO₂/TiO₂-6

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  Figure 13  Photocatalysis mechanism of CeO₂/TiO₂

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  Table 1.  S_BET, V_P, and average pore diameter of CeO₂/TiO₂-t

  Sample	SBET / (m2·g-1)	VP/ (cm3·g-1)	Average pore diameter / nm
  CeO2/TiO2-4	159	0.41	8.68
  CeO2/TiO2-6	143	0.48	11.44
  CeO2/TiO2-12	110	0.36	11.48

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