3.1. Morphology and microstructure
The synthesis process of creating highly crystalline BiOCl nanosheets using a one-step hydrothermal method is described in
Figure 1. As described in
Figure 1, Bi(NO
3)
3·5H
2O is used as the Bi source, while the Cl
− is provided by KCl. The BiOCl nanosheet structure can be obtained by continuous heating at 160 °C in a pure water solvent for 12 h.
Figure 2a and b show scanning electron microscopy (SEM) images of BiOCl, showing that the BiOCl structure is a two-dimensional (2D) nanosheet with uniform morphology. The width of the nanosheet ranges from 1 to 2 μm. The formed nanosheets are clustered together by electrostatic interactions [
28,
29,
30,
31]. In the magnified image, it can be observed more clearly that the surface of the material is smooth and free from particles, thus increasing the wear resistance of the material. The SEM image of BiOCl 24 h was shown in
Figure 2c. Although it has a similar nanosheet structure, its morphology is irregular and has tiny impurities.
The transmission electron microscopy (TEM) images of
Figure 3a,b present the nanosheet structure of single-phase BiOCl and the appearance of regular and clear arrays of square diffraction spots in selected areas of electron diffraction (SAED) images of the corresponding nanosheets (
Figure 3c), indicating that BiOCl is a single-crystal material [
32]. The high-resolution transmission electron microscopy (HRTEM) in
Figure 3d demonstrates clear lattice stripes. The exposed (102) crystal plane of BiOCl has a lattice spacing of 0.268 nm.
The crystal structures of BiOCl and BiOCl 24 h were analyzed by the XRD pattern in
Figure 4a. All the diffraction peaks correspond well to the BiOCl tetragonal structure crystalline phase, which has very high crystallinity and no other spurious peaks, indicating that there is only one substance of BiOCl in the material. Its corresponding standard card is JCPDS no. 82-0485 and the space group is P4/mmm (NO. 129). The cell parameters of BiOCl are a= b = 3.878 Å and c = 7.403 Å. The highly crystalline BiOCl and BiOCl 24 h have eleven major diffraction peaks in the XRD patterns, 2
θ = 12.0, 24.2, 25.9, 32.5, 33.5, 36.6, 41.0, 46.7, 49.8, 55.2 and 58.7°, and the diffraction peaks are located on the crystal plane of (001) (002) (101) (110) (102) ( 003) (112) (200) (113) (104) (212), respectively. High crystallinity also indicates excellent dimensional stability and tight molecular alignment within the structure that is less prone to fracture. To further investigate the surface state of the samples, Raman tests were performed on the BiOCl catalysts to determine the stretching or vibrational modes of the molecules. Three main diffraction peaks appear in the Raman (Raman) spectrum of
Figure 4b, among which 144 and 199 cm
-1 bands belong to the Bi-X bond stretching vibrations within
A1 g and
Eg of BiOCl, respectively. The peak at 395 cm
-1 can be ascribed to the
Eg and
B1 g energy bands generated by the motion of O atoms [
33,
34]. To get a clear understanding of the structural atomic bonding within BiOCl, Fourier transform infrared spectroscopy (FTIR) was utilized to detect the chemical bonds in BiOCl molecules. As shown in
Figure 4c, the vibrational peak at 1621 cm
-1 can be attributed to the stretching and bending motion of the hydroxyl (−OH) group, which is caused by the surface hydration phenomenon. The strong characteristic peak at 526 cm
-1 is generated by the stretching of the Bi-O bond within BiOCl [
35]. The formation of highly crystalline BiOCl was further verified. The optical properties were measured by ultraviolet-visible diffuse reflectance (UV-vis), as shown in
Figure 4d, using UV-visible diffuse reflectance spectroscopy. BiOCl has a continuous absorption band in the range of 360-800 nm. It can be determined that the maximum absorption wave edge occurs at about 378 nm. Thus the bandgap position can be obtained by Equation 1:
The estimated bandgap value of BiOCl is about 3.37 eV (
Figure 4e), which is suitable for light absorption in the UV region. Although UV light accounts for a relatively small fraction of solar energy, such a forbidden bandwidth can absorb UV light sufficiently to achieve the catalytic application of BiOCl in the UV region. As shown in
Figure 4f, the nitrogen adsorption-desorption isotherm was obtained for the analysis of the porous structure. In the relative high-pressure region, BiOCl exhibits type IV isotherm and H hysteresis loop. Type IV isotherm is similar to type II isotherm in the relative low-pressure region, but is prone to capillary coalescence in the region of higher relative pressure, making the latter half of the curve climb sharply [
36]. It also indicates that BiOCl nanosheets are mesoporous structures. The specific surface area, pore volume, and pore diameter of BiOCl catalysts are further derived from the Brunauer-Emmett-Teller (BET) theory to give a total surface area of 1.0 m
2g
-1. The pore volume and pore diameter of BiOCl is 0.002 cm
3g
-1 and 16.6 nm, respectively.
In addition to the internal structural analysis of BiOCl, we further investigated the surface elements of BiOCl and its chemical state. The X-ray photoelectron spectra (XPS) in
Figure 5 perfectly demonstrate the elemental states in BiOCl. Among them, the fitted peaks of Bi
3+ fall in the binding energy of 164.45 and 159.15 eV (
Figure 5a), and the two peaks belong to Bi 4f
7/2 and Bi 4f
5/2 orbitals, respectively [
37]. The fitted peaks of O elements are at 532.28 and 529.88 eV binding energy (
Figure 5b), which can be ascribed to oxygen-containing components adsorbed to the surface of BiOCl and the hydroxyl and lattice oxygen in BiOCl. [
38]. The two fitted peaks for elemental Cl at 201.95 and 200.31 eV (
Figure 5c) correspond to the Cl 2p
3/2 and Cl 2p
1/2 orbitals, respectively, confirming the presence of Cl
− [
39]. The elemental composition was also analyzed in the XPS full spectrum (
Figure 5d), containing characteristic peaks of the orbitals of three elements, Bi, O, and Cl. Bi 4p, O 1s, Bi 4d, C 1s, Cl 2p, Bi 4f, and Bi 5d orbitals can be observed in the full spectrum. Among them, C 1s is from the carbon in air adsorbed on the surface of the sample at the time of testing. For Bi elements, Bi 4f has the highest intensity, the smallest peak width, and the best symmetry, and is the mainline spectrum of Bi elements. There are also weak line spectra of Bi 4p, Bi 4d, and Bi 5d. Since Bi elements have multiple internal electrons, multiple XPS signals of Bi elements were produced. In addition, there is no interference from any other elements in BiOCl. It indicates that the sample synthesized by BiOCl is clean and free of impurities. The XPS valence band spectra of BiOCl also provided the valence band structure information of BiOCl. The energy corresponding to the valence band position is around 2.21 eV obtained in
Figure 5e. As shown in
Figure 5f, the Mott-Schottky curves tested at three frequencies of 1.0, 1.5, and 2.0 kHz to determine the flat-band valence. The intercept of the curve in the
x-axis can be derived from the flat-band potential is -0.99 V. It can be seen that BiOCl's is a positive slope curve, which proves that this material is an n-type semiconductor, and the participation in the conductivity is dominated by free electrons. The carrier concentration can be obtained from Equation 2:
Where
e0 is the charge per unit charge,
ε is the relative permittivity,
ε0 is the vacuum permittivity,
C is the interfacial capacitance, and
V is potential. We obtain the vacuum dielectric constant of BiOCl
ε0 = 55 [
40]. The carrier concentration of BiOCl is calculated as
Nd = 1.22×10
19. Furthermore, the flat-band potential can be deduced from the position of the conduction band (CB) by using Equation 3:
The calculated CB value of BiOCl falls at -0.38 V. As a classical photocatalyst suitable for degradation, the oxidation capacity of BiOCl itself deserves to be studied in depth.
3.2. Photocatalytic activity and mechanism
Although the content of antibiotics in water resources is not high, it is still harmful to human beings and cannot be ignored. In this work, the degradation of norfloxacin in an aqueous solution was carried out under simulated sunlight irradiation using BiOCl as the photocatalyst. As can be seen in
Figure 6a, after one hour of dark treatment to reach an adsorption balance of norfloxacin on BiOCl, the degradation rate of norfloxacin reached 84 % within 180 min under light irradiation, while the degradation rate of norfloxacin over BiOCl 24 h was only 72 %, indicating that BiOCl under 12 h reaction conditions possessed a better degradation ability. And the rate constants of 0.010 min
-1 and 0.007 min
-1 for BiOCl and BiOCl 24 h were obtained from the rate curves, respectively (
Figure 6b). The rate constant of BiOCl was about 1.43 times higher than that of BiOCl 24 h, which once again highlighted that the BiOCl degradation process was efficient. To examine the recyclability of the BiOCl material, five cycling experiments were performed (
Figure 6c). After five consecutive degradations, the catalytic activity remained almost unchanged. Such stability of BiOCl material indicates that it can be recycled after several times of wastewater treatment. In the radical capture experiments in
Figure 6d, Tert-butanol (
t-BuOH) was used to capture hydroxyl groups, potassium iodide (KI) to capture holes, silver nitrate (AgNO
3) to capture electrons, and benzoquinone (BQ) to capture superoxide anions, respectively [
41,
42]. Compared with the blank experiment, we know that the main active substances in the BiOCl system are electrons and superoxide anions. In conclusion, BiOCl photocatalysts have both great photocatalytic activity and excellent recyclability, which further confirms the feasibility of BiOCl materials in practical applications.
As shown in the photocatalyst mechanism in
Figure 7, the separation of electrons and holes occurs in highly crystalline BiOCl nanosheets under light irradiation. The excited electrons are transferred from the valence band to the conduction band, and the holes are left in the valence band. The CB value of BiOCl obtained from equation 3 is -0.38 V and the bandgap value obtained from equation 1 is 3.37 eV. Therefore, the VB position can be obtained to be 2.99 V. The hydroxide (OH
−) at the VB position loses an electron and is converted into a hydroxyl radical (
•OH), while the oxygen (O
2) molecule at the CB position accepts an electron and produces the reduction product superoxide anion (
•O
2−). Both of them play an important role in the photocatalytic reaction. Norfloxacin was photocatalytically converted to carbon dioxide, water, and other small molecule impurities over BiOCl materials. The photocatalytic carrier complex is prevented under the conditions of photoexcitation, which promotes the oxidation ability of BiOCl, increases the active site, captures the norfloxacin molecule, and then degrades it into a non-toxic and harmless substance. Such BiOCl with high crystallinity not only has excellent photocatalytic activity but also presents great wear and chemical resistance [
43].