Combining BiOCl with TiO2 nanomaterials is beneficial to enhance the photocatalytic activity and optoelectronic activity. In this paper, BiOCl nanosheet–TiO2 nanotube array composites were synthesized to enhance the photocatalytic degradation performance for methyl orange (MO) of TiO2 under ultraviolet light irradiation. BiOCl nanosheets were deposited on TiO2 nanotube arrays by the straightforward impregnation method. X-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, and photocurrent (i–t) were used to evaluate the composites of BiOCl nanosheets–TiO2 nanotube arrays. The results showed that the tetragonal BiOCl nanosheets clustered together on the surface of the TiO2 nanotubes and grew along the (110) crystal plane. The composites outperformed pure TiO2 regarding outstanding structure and overall photocatalytic performance, and the MO photocatalytic degradation rate was 98.5%. For the 30-BiOCl–TiO2, its photocurrent intensity (58 µA) was 4.5 higher than TiO2 (13 µA). The degradation rate of 87% can still be reached after three cycles.

In the past few decades, a significant amount of research has been carried out to address both energy deficiency and the issue of environmental pollution. This has attracted a great attention within the research community.1–3 Organic pollutants are known to infiltrate via a variety of sources, including wastewater treatment plants. These plants are a major source of organic contaminants due to the highly heterogeneous discharges from households, hospitals, and industries.4 However, certain traditional physical/biological wastewater treatment techniques fall short in their ability to eliminate numerous organic contaminants, which causes pollutants to be released into the environment.5 Even trace amounts of exposure to them can have negative impacts on aquatic life and human health.6,7 Photocatalysis is thought to be a promising technique for the treatment of organic pollutants with the ability to convert toxic organic matter in industrial waste streams into non-toxic carbon dioxide and water through the formation of oxygen-containing free radicals.8,9

TiO2 is an important inorganic photosensitive semiconductor material, which stands out among many photocatalysts because of its high photocatalytic activity, excellent hydrophilicity, and stable physicochemical properties.10,11 It is widely used in organic waste degradation, photolysis of water to produce hydrogen, gas sensors, solar cells, and other fields.12–16 Meanwhile, the application of TiO2 as a photocatalyst is also affected by a wide spectral response range and a low photogenerated electron–hole separation rate.17 How to overcome this effect and make the application of photocatalysts wider has become a research hotspot.

Pure TiO2 nanoparticles have severe restrictions. The bandgap of anatase TiO2 is 3.2 eV, which prevents it from acting as a semiconductor under any other light than ultraviolet (UV). However, just 5% of natural sunshine is UV light.18 What is worse, the catalytic activity of pure TiO2 is significantly reduced after UV radiation excites electron–hole pairs, since their rate of recombination is significantly higher than that of their separation.19 It is imperative to study the modification of TiO2. Introducing new metal ions (Fe3+,20 Cr3+,21 Sn2+22) into the TiO2 lattice can lead to the formation or change of lattice type or defects in TiO2. This can alter the energy band structure and the motion trajectories of photogenerated electron–hole pairs, resulting in a reduction of electron–hole pairs’ recombination rate. The bandgap of TiO2 can be significantly lowered, allowing the spectral response range to be expanded from a specific ultraviolet region to the visible region by replacing oxygen vacancies in the TiO2 lattice with new impurity levels.23,24 In addition, the deposition of a noble metal such as Pt25 and Ag,26 as well as the recombination of semiconductors such as Fe2O327 and CdS,28 can also effectively improve the separation efficiency of photogenerated carriers and broaden the corresponding range of TiO2 to light. So far, multitudinous photocatalysts have been synthesized for photodegradation of organic dyes in an aqueous solution, such as Cr–TiO2/C,29 TiO2@CNTs/AgNPs,30 and TiO2/SiO2,31 which have been reported to have the effect of degrading organic dyes in recent years, which enhanced the photocatalytic performance of TiO2.

Among a series of semiconductor materials, BiOCl has attracted great attention from researchers due to its suitable bandgap and highly anisotropic layered nanostructures.32,33 The hybrid orbital of BiOCl can stimulate the formation of a self-induced internal electric field to a certain extent.34 This enables the effective separation of photogenerated electron–hole pairs, which helps us improve the photocatalytic activity and optoelectronic performance. Combining the economical, high photocatalytic activity of non-toxic and harmless BiOCl with TiO2 nanomaterials and optimizing the best preparation process can have significant research potential.35,36

In this work, we successfully fabricated highly ordered, precisely aligned TiO2 nanotube arrays on Ti foil substrates. BiOCl nanosheets were then applied to the surface of the nanotube arrays using a simple dipping method. Furthermore, we looked at the effects of precursor concentration on the material’s microscopic morphology, crystal structure, elemental composition, photoelectric response, and photocatalytic activity.

Pure titanium foil came from Jilitai Metal Material Co., Ltd. in Shanxi, China. Bismuth chloride (BiCl3) was purchased from Yuanye Biotechnology Co., Ltd. in Shanghai, China. Huifengda Chemical Co., Ltd. in Shantou, China, provided the ammonium fluoride (NH4F), acetone (C3H6O), and ethylene glycol (C2H6O2) used in this experiment. Anhydrous ethanol (C2H5OH), hydrofluoric acid (HF), and nitric acid were bought from Dongke Chemical Products Sales Co., Ltd. in Henan, China. Methyl orange (MO) powder was bought from Ruifeng Chemical Co., Ltd. in Shanghai, China.

In acetone, ethanol, and deionized water, the titanium foil was subjected to sonicate for 5 min, respectively. After 30 s in a polishing liquid with a volume ratio of 1:2:5 (HF:HNO3:H2O), the titanium foil was sonicated and then rinsed with deionized water. Ethylene glycol solutions containing 0.1M NH4F and 5 vol. % water served as the electrolyte. At 60 V for 1 h, titanium foil was electrochemically anodized. The titanium foil was then sonicated in deionized water until the oxide film and impurities were completely removed from the obtained titanium dioxide nanotube arrays. The second step reaction was carried out under the same reaction. To obtain anatase phase TiO2, the anodized TiO2 nanotube arrays were annealed at 500 °C at a 5 °C/min pace for 2 h.

BiCl3 was added to ethanol at various quantities (10, 30, 60, and 100 mM). Using an impregnation technique, the TiO2 nanotube arrays were submerged in the solution for 30 min. After being dried at 80 °C for 30 min, the sample was submerged in deionized water for the following 30 min. The sample was eventually annealed at 400 °C for 2 h after drying naturally. The schematic flow chart is shown in Scheme 1.

SCHEME 1.

TiO2 nanotube arrays and BiOCl–TiO2 composite preparation procedure.

SCHEME 1.

TiO2 nanotube arrays and BiOCl–TiO2 composite preparation procedure.

Close modal

The phase structure of the composites of nanotube arrays was examined using x-ray diffractometry (XRD, Bruker, Germany). Monochromatic Cu K1 radiation was used in an x-ray diffractometer that ran at 40 mA and 45 kV (incident angle = 0.5°, = 1.541°, step size = 0.5°, integration time = 20 s/step). The morphology of the nanotube array composites was determined using a scanning electron microscope (SEM, JSM-6701F, 20 kV, JEOL, Tokyo, Japan); experimental testing needs a high vacuum. The device included a high-performance x-ray energy spectrometer capable of scanning small sections of the sample surface along a point line to get the findings of elemental analysis. The composites of nanotube arrays were examined using a high-resolution transmission electron microscope (TEM, JEM-2100, 200 kV, JEOL, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) studies were carried out on an ESCALAB 25OXi (Thermo Fisher Scientific, China). The best energy resolution is ≤0.45 eV, the best spatial resolution is ≤3 µm, the best sensitivity is 1000 kcps for the monochromatic light source, and the best vacuum of the analysis chamber is up to 1.7–10–10 mbar. UV–Vis (M16070024, Agilent Corporation) diffuse reflectance absorption spectroscopy was used to analyze the absorption of light by a sample and was measured in the wavelength range of 200–800 nm. Photocurrent tests (i–t) impedance spectra were performed using a Chenhua CHI660E electrochemical workstation as a way to record and reflect the intensity of the photoresponse current.

A 20 mg/L concentration of methyl orange (MO) solution was created as a mock wastewater for photocatalytic degradation. The sample was prepared for photocatalytic degradation by cutting it into a 10 mm square and placing it in a quartz test tube. A high-pressure mercury (Hg) lamp was used as the photocatalytic system with a primary wavelength of 365 nm and a power of 500 W. The light intensity in the reaction was 160 mW/cm2. The absorbance of the solution was measured every 20 min. The degradation rate (η) was used to assess the catalyst’s photocatalytic performance. The detailed calculation formula is as follows:
η=C0Ct/C0×100%=A0At/A0×100.
(1)
Among them, C0 is the initial concentration of MO and Ct is the concentration of MO at time t. A0 is the initial absorbance of MO at 466 nm, and At is the absorbance of MO at time t in ultraviolet (UV) light irradiation of 600 nm.
  1. The synthesis of TiO2 barrier layer: as shown in Scheme 2, an oxide film is formed between the titanium substrate and the electrolyte, because titanium loses electrons, an oxide coating develops, which can be expressed by Eqs. (2) and (3).

  2. The synthesis of nano-pores: The oxide film is corroded by F, and some pores are etched on the surface of the oxide film, and the processes can be expressed by Eq. (4).

  3. The synthesis of nanotubes: the pores expand to the titanium substrate to form a tubular structure,

Ti4eTi4+,
(2)
Ti4++2H2OTiO2+4H+,
(3)
TiO2+6F+4H+TiF62+2H2O.
(4)
SCHEME 2.

Synthesis process of TiO2 nanotube arrays.

SCHEME 2.

Synthesis process of TiO2 nanotube arrays.

Close modal

Figure 1 depicts the crystal phase of TiO2 nanotubes and BiOCl–TiO2 composites as determined by XRD. The diffraction peaks of TiO2 nanotubes at 25.56°, 38.09°, 48.19°, 54.27°, and 55.24° point to the (101), (004), (200), (105), and (211) crystal facets of the anatase phase TiO2 (JCPDF 21-1272). The samples included no additional contaminant peaks, demonstrating the exceptional purity of the TiO2 nanotube. The distinctive phase diffraction peaks of BiOCl can be seen at 2θ of 32.5° and 46.6° point to the (110) and (200) planes corresponding to the tetragonal BiOCl crystal planes (JCPDF 06-0249). With the increase in BiOCl content, the corresponding BiOCl characteristic peaks became more and more sharp, which proved that BiOCl–TiO2 composites were, indeed, formed.

FIG. 1.

XRD of all samples.

FIG. 1.

XRD of all samples.

Close modal

Figure 2 shows the SEM images of TiO2 nanotube arrays and BiOCl–TiO2 composites. As shown in Fig. 2(a), the TiO2 nanotube arrays were neatly and densely arranged, and the surface was smooth and had no covering. Figure 2(b) shows a side view of the nanotube arrays, where TiO2 can be seen growing vertically to the titanium substrate. After measuring, the diameter, wall thickness, and tube length of TiO2 nanotubes were 130 nm, 30 nm, and 10 µm, respectively. When Bi3+ was present at a quantity of 0.01 mol/L, it can be seen from Fig. 2(c) that there was only a small amount of BiOCl on the TiO2. The BiOCl nanosheets were stacked together in a circular shape, with a diameter of about 500 nm. The nanosheets were stacked into clusters in direct contact with the nanotube arrays, which provided the basic conditions for the formation of p–n heterojunctions. With the increase in Bi3+ concentration, the number of BiOCl increased gradually. Under the condition that Bi3+ was 0.03 mol/L, the ideal distribution of BiOCl and TiO2 can be obtained from Fig. 2(d). At this time, both dense and neat nanotube arrays and densely distributed nanosheets can be seen, and the two were in close contact with each other and left space, which provided favorable conditions for the formation of p–n heterojunctions and for receiving sufficient light sources. Figures 2(e) and 2(f) show that under too high Bi3+ concentration, massive BiOCl was generated and completely covered the surface of the nanotube array. This made the TiO2 nanotube arrays unable to function, resulting in a significant decrease in the function of the composites.

FIG. 2.

SEM images of the composites with different Bi3+ concentrations. (a) and (b) pure TiO2, (c) 10 mM, (d) 30 mM, (e) 60 mM, and (f) 100 mM.

FIG. 2.

SEM images of the composites with different Bi3+ concentrations. (a) and (b) pure TiO2, (c) 10 mM, (d) 30 mM, (e) 60 mM, and (f) 100 mM.

Close modal

Figures 3(a) and 3(b) show the energy dispersive x-ray spectroscopy (EDX) spectrum of TiO2 and composites. The appearance of O and Ti was consistent with the experimental expectation, which proved that the sample was a compound of titanium oxide. In addition, the existence of Bi proved that BiOCl was successfully synthesized in Fig. 3(b). Figure 3(c) shows the TEM image of the composites, the composite structure of nanotubes and nanosheets can be clearly seen, and the nanosheets surrounded the nanotubes. Figure 3(d) shows a high-resolution TEM of a single TiO2 nanotube, demonstrating the TiO2 nanotube’s remarkable crystallinity. The (110) planes of atomic were represented by the clear lattice fringes with an interplanar lattice spacing of 0.33 nm. It showed that TiO2 was anatase phase, which agreed with the findings of the XRD patterns.

FIG. 3.

TEM images of samples: (a) EDX spectrum of TiO2, (b) EDX spectrum of the BiOCl–TiO2 sample, (c) TEM image of the BiOCl–TiO2 sample, and (d) HRTEM image of TiO2.

FIG. 3.

TEM images of samples: (a) EDX spectrum of TiO2, (b) EDX spectrum of the BiOCl–TiO2 sample, (c) TEM image of the BiOCl–TiO2 sample, and (d) HRTEM image of TiO2.

Close modal

XPS was used to investigate the surface makeup and valence condition of TiO2 samples and BiOCl–TiO2 composites, and the results are shown in Fig. 4. From Fig. 4(a), it can be observed that the TiO2 sample contained only two elements, Ti and O, except for the C 1s extraneous peak, which may be caused by carbon pollution during XPS testing. In the BiOCl–TiO2 composites, there were only four elements, Ti, Bi, O, and Cl, which proved the high purity of the sample. At the same time, it showed that the BiOCl–TiO2 composites were successfully prepared. The result was very consistent with the EDX pattern described above. The Ti 2p peak positions in the TiO2 sample, as indicated in Fig. 4(b), emerged at 458.8 and 464.5 eV corresponding to Ti 2p3/2 and Ti 2p1/2, suggesting that Ti in TiO2 mostly resided as Ti4+. In addition, the XPS peak appeared at 458.6 and 464.3 eV can be related to Ti 2p3/2 and Ti 2p1/2. The addition of BiOCl did not affect the lattice change of TiO2, as evidenced by the unchanged binding energy position of Ti in the composites. Figure 4(c) shows the O 1s peak position in the TiO2 sample appeared at 530.0 eV, which corresponded to the Ti–O band. In addition to the Bi–O characteristic peak at 529.7 eV, the composites also had a small O 1s peak at 531.7 eV. It may be derived from oxygen-containing groups, such as hydroxyl radicals (·OH) adsorbed on the surface. According to Fig. 4(d), the characteristic peaks at the binding energy positions of 158.9 and 164.2 eV were associated with Bi 4f7/2 and Bi 4f5/2, respectively. It showed that Bi existed in the trivalent oxidation state of Bi3+ at this time. The strong characteristic peak of Cl 2p in Fig. 4(e) was found to be present at binding energies between 197.5 and 198.9 eV, demonstrating the presence of Cl in BiOCl. These findings provided more evidence for the coexistence of TiO2 and BiOCl in the composites.

FIG. 4.

XPS spectrum of TiO2 and BiOCl–TiO2 composites: (a) survey scan, (b) Ti 2p spectra, (c) O 1s spectra, (d) Bi 4f spectra, and (e) Cl 2p spectra.

FIG. 4.

XPS spectrum of TiO2 and BiOCl–TiO2 composites: (a) survey scan, (b) Ti 2p spectra, (c) O 1s spectra, (d) Bi 4f spectra, and (e) Cl 2p spectra.

Close modal

The photogenerated electron–hole separation ability of the samples was measured by the transient photocurrent response, which can prove the photocatalytic ability. As can be seen in Fig. 5, under exposure to visible light, the photocurrent rapidly rose to a peak value. When the lamp was turned off, the photocurrent quickly returned to zero. After three cycles, the photocurrent intensity barely changed, demonstrating the samples’ excellent durability. Further observation revealed that, in comparison with TiO2, the photocurrent intensity of composites was much higher. When compared to TiO2, 30-BiOCl–TiO2’s photocurrent intensity (58 A) was 4.5 times higher. Excessive loadings of bismuth can reduce light injection and thus affect photogenerated electron–hole separation ability. Therefore, the maximum photocurrent density may be seen in a 30-BiOCl–TiO2 sample with a moderate quantity of nanosheets.

FIG. 5.

Transient photocurrent response diagram of the TiO2 and BiOCl–TiO2 composites.

FIG. 5.

Transient photocurrent response diagram of the TiO2 and BiOCl–TiO2 composites.

Close modal

UV–Vis diffuse reflectance spectroscopy was used to analyze the response wavelength of samples, and the results are shown in Fig. 6. The absorption edge of BiOCl–TiO2 occurred before 470 nm. Compared with the UV–Vis–DRS spectra of TiO2 and BiOCl, we can find that BiOCl–TiO2 is shifted toward the longer wave direction.

FIG. 6.

UV–visible DRS spectra and plots of (Ahν)2 vs photon energy (hν) of TiO2, BiOCl, and 30-BiOCl–TiO2.

FIG. 6.

UV–visible DRS spectra and plots of (Ahν)2 vs photon energy (hν) of TiO2, BiOCl, and 30-BiOCl–TiO2.

Close modal
The relationship of absorbance and incident photon energy hν can be described by the following equation:
Ahν=ChνEg1/2,
(5)

where A, Eg, h, and ν represent the absorption coefficient, the bandgap energy, the Planck constant, and the incident light frequency, respectively, and C denotes a constant. Therefore, the bandgap energy (Eg) of samples can be estimated from a plot of (Ahν)2 vs hν (Fig. 6). The bandgap energy of prepared BiOCl–TiO2 is 2.90 eV, which is narrower than that of BiOCl (3.25 eV) and TiO2 (3.20 eV).

Photocatalytic (PC) experiments on MO degradation revealed the photocatalytic activity of the material. As shown in Figs. 7(a) and 7(b), the optimal composites 30-BiOCl–TiO2 exhibited a great PC performance. The photocatalytic efficiencies can reach 98.5% within 120 min, which was 1.18 times higher than pure TiO2. This can be explained by the composites’ increased light absorption and quicker carrier separation.

FIG. 7.

(a) PC activities of samples, (b) degradation rate of samples, (c) catalytic pseudo-first-order kinetic, and (d) cycling photocatalytic tests of 30-BiOCl–TiO2, and (e) XRD spectra pattern after 3rd run.

FIG. 7.

(a) PC activities of samples, (b) degradation rate of samples, (c) catalytic pseudo-first-order kinetic, and (d) cycling photocatalytic tests of 30-BiOCl–TiO2, and (e) XRD spectra pattern after 3rd run.

Close modal
A pseudo-first-order model is utilized to assess the catalytic reaction kinetics and predict the kinetic parameters of the reactions,
lnCC0=kt,
(6)
where C is the concentration of MO, C0 is the initial concentration of MO, t is the reaction time, and k is the constant of the pseudo-first-order reaction. BiOCl content had an impact on the reaction rate to some extent, and 30-BiOCl–TiO2’s kinetic constant reached 0.031—two times greater than that of pure TiO2. Insufficient concentration of Bi3+ will not produce enough BiOCl, and excessive Bi3+ concentration will generate a large amount of BiOCl and completely cover the surface of the nanotube array, which will make the TiO2 nanotube arrays unable to function. To learn the photocatalytic stability of 30-BiOCl–TiO2, the recycling degradation experiments of MO are executed, as shown in Fig. 7(d), and the XRD analysis of the catalyst before and after the reaction is shown in Fig. 7(e). Notably, 30-BiOCl–TiO2 showed excellent stability in the photocatalytic degradation of MO solution over three cycles. In addition, no noteworthy difference was found between the fresh and the used 30-BiOCl–TiO2 catalyst samples, confirming the good retention of crystal structure. The results indicate that the samples have high reusability and stability. Table I shows a comparison of the photocatalytic activity of BiOCl–TiO2 and previously reported ones for photocatalysts.
TABLE I.

Comparison of reported photocatalysts for MO degradation efficiency.

ReferenceMaterialReaction systemPhotodegradation efficiency (%)
37  NiFe2O4@TiO2 1 mg photocatalyst; UV light 90.06 
MO (8 mg/L 125 ml); 90 min 
38  Ag/TiO2/biochar 10 mg photocatalyst; UV light 97.48 
MO (20 mg/L 40 ml); 60 min 
39  MoS2/TiO2-NTA UV–visible light 96 
MO (10 mg/L); 180 min 
40  Fe/TiNTs 10 mg photocatalyst; UV light 93 
MO (10 mg/L 10 ml); 480 min 
This work BiOCl–TiO2 1 cm2 photocatalyst; UV light 98.5 
MO (20 mg/L 35 ml); 120 min 
ReferenceMaterialReaction systemPhotodegradation efficiency (%)
37  NiFe2O4@TiO2 1 mg photocatalyst; UV light 90.06 
MO (8 mg/L 125 ml); 90 min 
38  Ag/TiO2/biochar 10 mg photocatalyst; UV light 97.48 
MO (20 mg/L 40 ml); 60 min 
39  MoS2/TiO2-NTA UV–visible light 96 
MO (10 mg/L); 180 min 
40  Fe/TiNTs 10 mg photocatalyst; UV light 93 
MO (10 mg/L 10 ml); 480 min 
This work BiOCl–TiO2 1 cm2 photocatalyst; UV light 98.5 
MO (20 mg/L 35 ml); 120 min 

As shown in Fig. 8(a), the photocatalysis mechanism of the composite material shows that after the p-type semiconductor BiOCl and the n-type semiconductor TiO2 form a p–n heterojunction, their Fermi levels will be aligned in the same position, which can effectively reduce the energy demand for external ultraviolet light and thus broaden the spectral response range. When the composite semiconductor was exposed to external ultraviolet light, both BiOCl and TiO2 were stimulated to generate photogenerated electrons and holes. An intrinsic electric field is produced at the interface between BiOCl and TiO2 as a result of the creation of a depletion area due to the formation of a p–n heterojunction. The separation of photogenerated electrons and photogenerated holes is accelerated by the electric field, which also modifies the energy band at the interface. When light is absorbed by the BiOCl–TiO2 heterojunction photocatalyst, negatively charged electrons are produced. These electrons transfer from the negatively charged BiOCl conduction band to the TiO2 conduction band. At the same time, positively charged holes are produced and move the TiO2 valence band to the BiOCl valence band, as Fig. 8(b) illustrates. This separation of electrons and holes prevents their recombination, leading to better utilization of light and improved photocatalytic activity.

FIG. 8.

(a) Possible photocatalytic mechanism diagram of BiOCl–TiO2 composites and (b) transfer process of photogenerated carriers under UV illumination.

FIG. 8.

(a) Possible photocatalytic mechanism diagram of BiOCl–TiO2 composites and (b) transfer process of photogenerated carriers under UV illumination.

Close modal

In summary, a secondary anodization process was used to create TiO2 nanotube arrays. BiOCl nanosheets were effectively loaded on TiO2 nanotube arrays by the solution impregnation approach to increase the photoresponse range of TiO2, decrease the recombination rate of photogenerated electrons and photogenerated holes, and further enhance the photocatalytic activity. At 30 mM/L of Bi3+, the composites showed optimal photocatalytic efficacy, decomposing MO by 98.5%. In addition, cyclic tests demonstrated the composites’ strong stability.

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 32060577 and 32360619) and the Natural Science Foundation of Jiangxi Province (Grant Nos. 20212BAB203034 and 20224ACB203016).

The authors have no conflicts to disclose.

Yude Liu: Data curation (lead); Formal analysis (equal); Investigation (equal); Writing –original draft (lead). Mengqin You: Investigation (equal); Resources (equal). Rui Li: Investigation (equal); Resources (equal). Jun Du: Conceptualization (lead); Funding acquisition (lead); Investigation (equal); Supervision (lead); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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