Effect of Light and Heavy Rare Earth Doping on the Physical Structure of Bi₂O₂CO₃ and Their Performance in Photocatalytic Degradation of Dimethyl Phthalate

Abstract

In order to solve the problem of environmental health hazards caused by phthalate esters, a series of pure Bi₂O₂CO₃ and light (La, Ce, Pr, Nd, Sm and Eu) and heavy (Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) rare earth-doped Bi₂O₂CO₃ samples were prepared by hydrothermal method. The crystalline phase composition and physical structure of the samples calcined at 300 °C were studied, and we found that the rare earth ion doping promoted the transformation of Bi₂O₂CO₃ to β-Bi₂O₃ crystalline phase, thus obtaining a mixed crystal phase photocatalyst constituted by rare earth-ion-doped Bi₂O₂CO₃/β-Bi₂O₃. The Bi₂O₃/Bi₂O₂CO₃ heterostructure had a lower band gap and more efficient charge transfer. The fabricated samples were applied to the photocatalytic degradation of dimethyl phthalate (DMP) under a 300 W tungsten lamp, and it was found that the rare earth ion doping enhanced the photocatalytic degradation activity of DMP, in which the heavy rare earth of Er-doped sample reached 78% degradation for DMP at 150 min of light illumination. In addition, the doping of rare earths resulted in a larger specific surface area and a stronger absorption of visible light. At the same time, the formation of Bi₂O₂CO₃/β-Bi₂O₃ heterogeneous junction enhanced the separation efficiency of photogenerated electrons and holes.

1. Introduction

Phthalate esters (PAEs) are widely used as plasticizers which can change polyvinyl chloride from a hard plastic to an elastic plastic. In recent years, the environmental health hazards caused by such compounds as phthalates have attracted widespread attention from the environmental and health communities. Numerous studies have shown that phthalates are human endocrine disruptors with reproductive and developmental toxicity and carcinogenicity. In 2017, the World Health Organization’s International Agency for Research on Cancer classified di (2-ethylhexyl) phthalate as a Group 2B carcinogen. PAEs, also known as environmental hormone pollutants, are among the most significant trace organic pollutants in the water environment. Therefore, it is important to develop effective methods to remove PAEs from water bodies.

Currently, the main methods for the removal of PAEs-like compounds in the aqueous environment are adsorption separation, biodegradation and advanced oxidation [1]. The adsorption method is highly adaptable, but because of the limited capacity of the adsorbent, it requires frequent replacement of the adsorbent and therefore consumes a lot of time and human resources. The adsorption method has strong adaptability, but due to the limited capacity of the adsorbent, the adsorbent needs to be replaced frequently, which consumes a lot of manpower and material resources. Biodegradation has the advantages of low treatment cost and stable metabolites, but its disadvantage is that the removal rate is not high, is time-consuming, and sometimes there is incomplete degradation. Photocatalytic oxidation is a promising method for the treatment of organic pollutants in water bodies that has been rapidly developed in recent years, with the advantages of using natural sunlight, fast degradation rate and mild reaction conditions [2]. Currently, the studied photocatalysts for PAEs removal mainly focus on TiO₂ photocatalysts with the diethyl phthalate degradation efficiency of 44.5–67.4% [3,4,5], Fe, Ag-ZnO with the dibutyl phthalate reduction efficiency of 95% [6], V₂O₅/MoO₃ with the methylene blue degradation efficiency of 89.23% [7], ZrO_x/ZnO with the dimethyl phthalate (DMP) degradation half-life time of shortening 45% [8], α-Fe₂O₃ with the dibutyl phthalate degradation efficiency 93% [9], g-C₃N₄/Bi₂O₃ with the dibutyl phthalate degradation efficiency of 60% [10], BiVO₄ with bisphenol A degradation efficiency 99% [11] and other photocatalytic materials.

Bismuth-based semiconductors are characterized by environmental friendliness, easy availability of raw materials and easily tunable optical properties, but are less used in photocatalytic removal of PAEs. In addition, the photocatalytic oxidative removal performance of bismuth-based semiconductors can be enhanced by physical structure modulation such as crystal morphology, crystal phase, and dimensionality. On the other hand, rare earth elements have a unique 4f sublayer electronic structure, large atomic magnetic moments, strong spin–orbit coupling and variable coordination numbers. Doping of rare earth atoms into semiconductors can modulate the semiconductor photocatalyst lattice, energy level structure and surface adsorption properties, thus improving the optical absorption capacity, the migration and separation rate of photogenerated carriers and the adsorption capacity of the reactants [12]. In addition, lanthanides have well-defined 4f→4f leap electron orbitals as well as high photochemical stability with distinct luminescence spectra in the ultraviolet(UV)-visible near-infrared region [13]. Therefore, rare earth elements are often used in the synthesis of fluorescent/phosphorescent materials and up-conversion photocatalytic materials [14,15,16,17,18,19].

Our investigations show that rare earth doping has a better modulating effect on the phase transition, crystal and crystal plane growth of bismuth-based semiconductors, which can create unique microstructures such as heterogeneous junctions. For example, Gd³⁺-doped Bi₂MoO₆ photocatalysts were prepared by hydrothermal-roasting method, in which Gd³⁺ was doped into Bi₂MoO₆ lattice to replace part of Bi³⁺, inducing distortion of Bi₂MoO₆ lattice, adjusting the energy band structure of Bi₂MoO₆, reducing the forbidden band width of Bi₂MoO₆, improving the separation of photogenerated electrons and holes and thus significantly boosting the photocatalytic activity [20]. The incorporation of Sm³⁺ into Bi₂WO₆ can cause lattice contraction, increase specific surface groups and surface hydroxyl groups, and generate more oxygen vacancies, resulting in the increased activity and stability of Bi₂WO₆ [21]. Eu³⁺ can induce BiVO₄ crystal morphology from nanoparticles to microspheres, and make its crystalline transformation from monoclinic to orthorhombic crystal system to form heterogeneous phase junction [22]. Yb³⁺ and Er³⁺ co-doping can modulate the WO₃-0.33H₂O and W₁₈O₄₉ crystalline phase composition and form Z-type heterogeneous phase junctions to enhance the separation and up-conversion efficiency of photogenerated electrons and holes [23]. Highly dispersed Er atoms doped with n-type oxygen vacancy-containing NiO (Er/NiO_1−x) crystals can change the symmetry of the crystal, enhance the polarization and internal electric field and greatly strengthen the effective separation of photogenerated carriers, thus improving the CO₂ adsorption and activation and excellent photocatalytic reduction performance [24].

In this paper, a series of rare earth-doped Bi₂O₂CO₃ samples were prepared by hydrothermal method, and under the calcination condition at 300 °C, the effects of light and heavy rare earth doping on the physical structures of Bi₂O₂CO₃ such as crystalline phase transformation, specific surface area, grain morphology and light absorption were investigated. The effect of different rare earth doping on the degradation activity of the catalyst was also investigated by using visible light photocatalytic degradation of DMP, and the reasons for the enhancement of photocatalytic performance caused by rare earth doping were analyzed.

2. Results and Discussion

2.1. X-ray Powder Diffraction (XRD) Analysis

XRD was used to study the effect of doping with different rare earths on the crystallinity and crystalline phase of the samples. Figure 1 show the XRD patterns of the pure and doped samples after calcination at 300 °C. The main characteristic diffraction peaks of the pure samples appear at 2θ = 12.9°, 23.9°, 30.3° and 32.7°, indicating that the synthesized samples belong to the tetragonal system Bi₂O₂CO₃ (JCPDS [00-025-1464]). After doping with different rare earth ions, a clear change in the diffraction peaks of the doped samples can be clearly observed when compared with the pure samples heat-treated at the same temperature. New diffraction peaks appear at 2θ = 27.9°, 31.7° and 46.2°, corresponding to the characteristic diffraction peaks on the (201), (002) and (222) crystal planes of the tetragonal crystal system β-Bi₂O₃ (JCPDS [00-027-0050]), which indicates that the rare earth-doped samples are mostly transformed into β-Bi₂O₃. According to the relevant literature, it is known that β-Bi₂O₃ is generated before α-Bi₂O₃ when Bi₂O₂CO₃ is heat-treated at lower temperatures, while β-Bi₂O₃ is also slowly converted to α-Bi₂O₃ during the cooling down process [25]. However, the presence of doped samples mainly in the form of β-Bi₂O₃ indicates that proper rare earth doping inhibits the phase transition from β-Bi₂O₃ to α-Bi₂O₃, thus improving the stability of β-Bi₂O₃ at low temperatures. It may also be due to the fact that the ion of Bi³⁺ is 103 pm, which is similar to the radius of rare earth ions, and rare earth ions can easily enter the Bi₂O₃ lattice to replace part of Bi³⁺ and form a rare earth bismuth complex oxide, which further forms a heterogeneous structure with the rare earth ions uniformly dispersed in the β-Bi₂O₃ crystal [26]. Furthermore, the aggregation of Bi₂O₃ grains was hindered, thus reducing the energy of the system to a certain extent, and the resulting β-Bi₂O₃ has not enough energy to cross the phase transition potential barrier, and thus eventually β-Bi₂O₃ can be stable at room temperature. In addition, the β-Bi₂O₃ that can be stabilized with the small amount of Bi₂O₂CO₃ present in the system can also form Bi₂O₂CO₃/β-Bi₂O₃ heterojunctions. Additionally, in the absence of rare earths, β-Bi₂O₃ can easily phase into α-Bi₂O₃ with strong adsorption capacity during the cooling process, which can reabsorb CO₂ in the air into Bi₂O₂CO₃.

2.2. Thermogravimetric (TG) Analysis

The undoped samples and Nd-doped samples were analyzed under air atmosphere using thermogravimetry. Figure 2 shows the TG−DTG curves of the undoped sample and Nd-doped samples, respectively. In the whole process, the total weight loss of the pure sample and Nd-doped samples were 11.1% and 10.18%, respectively. Additionally, the theoretical weight loss of Bi₂O₂CO₃→Bi₂O₃ + CO₂ in Bi₂O₂CO₃ decomposition reaction is 8.6%. So there is also a part of weight loss caused by water adsorbed on the catalyst surface [27]. All DTG curves have two peaks, indicating that the samples decompose in two stages throughout the heating process. In the first stage, the weight loss is mainly caused by the loss of water molecules adsorbed on the catalyst surface, and in the second stage, a chemical inversion occurs: Bi₂O₂CO₃→Bi₂O₃ + CO₂. Combined with XRD analysis, the pure sample at 300 ℃ is still Bi₂O₂CO₃ crystal phase; the reason may be that although the phase has changed to β-Bi₂O₃ during calcination at 300 °C, the pure sample β-Bi₂O₃ will be transformed to α-Bi₂O₃ during cooling. α-Bi₂O₃ has good adsorption and adsorbs CO₂ in the air to further form Bi₂O₂CO₃.

2.3. Scanning Electron Microscope (SEM) Analysis

In order to investigate whether the doped rare earth has an effect on the morphology of the samples, we carried out SEM characterization analysis of the undoped and doped samples after heat treatment at 300 °C. Figure 3 and Figure 4 show the SEM images of pure samples, light rare earth- and heavy rare earth-doped samples at the same magnification. As shown in Figure 3a, the pure sample heat-treated at 300 °C is a smooth surface lamellar structure. However, as shown in Figure 3b–d and Figure 4, after heat treatment at the same temperature, the integrity of the nanosheet structure of the sample is destroyed to a certain extent by rare earth doping. It can be seen from the photos in Figure 3b–d that although the nanosheet structure is slightly damaged due to the doping of a small amount of light rare earth, it is not difficult to find that the particles doped with light rare earth are refined and the particle size distribution of the sample is narrowed. As can be seen from the photographs of Figure 4, the heavy rare earth doping also has an effect on the phase morphology compared to the nanosheet structure of the pure sample. And these morphological changes caused by rare earth doping could facilitate the adsorption of the catalyst to the substrate molecules, thus enhancing its photocatalytic activity.

2.4. TEM and EDX Analysis

The TEM photographs of the light rare earth Nd- and heavy rare earth Gd-doped samples under heat treatment at 300 °C are given in Figure 5, and it can be found that the catalysts are composed of irregular nanosheets. The (2 1 0) crystal plane lattice spacing of Bi₂O₃ after Nd and Gd doping are indicated in Figure 5b,d, respectively, where the (2 1 0) lattice spacing is 0.336 nm for Nd doping and 0.338 nm for Gd doping. Compared to the (2 1 0) lattice spacing (0.346 nm) in JCPDS Standard Card 00-027-0050 for bare Bi₂O₃, the lattice shrinks in doped samples is due to the substitution of large Bi³⁺ ion by small Nd³⁺ and Gd³⁺ ion [28]. To explore the elemental composition of the irregular nanosheets, EDX selected area elemental analysis was performed. Figure 6 shows the EDX spectra of Nd- and Gd-doped catalysts, respectively. Figure 6a contains three elements of Nd, Bi and O and Figure 6b contains three elements of Gd, Bi and O, indicating that the rare earth-ion-doped samples synthesized by hydrothermal method do have the presence of rare earth ions.

2.5. Light Absorption and Specific Surface Area Analysis

The UV-Vis diffuse reflectance spectroscopy was used to analyze the light absorption ability of the samples. It can be seen from Figure 7 that the light absorption of the Bi₂O₂CO₃ sample after calcination at 300 °C is mainly concentrated in the UV region with an absorption edge of 435 nm. After doping with different rare earth ions, the light absorption of the samples shows a significant red shift in the visible region, which is mainly due to the presence of earth ions. In addition, the introduced rare earth ions have strong electron capture ability, which can adjust the energy band structure of β-Bi₂O₃ and reduce its forbidden bandwidth. From Figure 7a, it can be found that the light response range of the light rare earth-doped samples is mainly concentrated in 539~546 nm. From Figure 7b, it can be seen that the light response range of the heavy rare earth-doped samples is mainly concentrated in 536~548 nm.

The band gap energy of the sample was calculated by drawing the Tauc plots as shown in Figure 8. Here, this linear region to the horizontal axis yields the energy of the optical band gap of the material.

The calculated band gap energy is shown in

Table 1. Band gap energy and specific surface area of Bi₂O₂CO₃ and rare earth-doped Bi₂O₂CO₃/β-Bi₂O₃.

Sample	Eg (eV)	SBET (m2/g)	Sample	Eg (eV)	SBET (m2/g)
Bi2O2CO3	3.18	5.4	Gd-Bi2O2CO3/β-Bi2O3	2.49	16.1
La-Bi2O2CO3/β-Bi2O3	2.53	11.2	Tb-Bi2O2CO3/β-Bi2O3	2.52	15.2
Ce-Bi2O2CO3/β-Bi2O3	2.42	10.9	Dy-Bi2O2CO3/β-Bi2O3	2.50	16.8
Pr-Bi2O2CO3/β-Bi2O3	2.48	11.1	Ho-Bi2O2CO3/β-Bi2O3	2.56	17.3
Nd-Bi2O2CO3/β-Bi2O3	2.57	14.2	Er-Bi2O2CO3/β-Bi2O3	2.54	14.6
Sm-Bi2O2CO3/β-Bi2O3	2.42	13.6	Tm-Bi2O2CO3/β-Bi2O3	2.48	15.3
Eu-Bi2O2CO3/β-Bi2O3	2.57	14.5	Yb-Bi2O2CO3/β-Bi2O3	2.47	14.9
			Lu-Bi2O2CO3/β-Bi2O3	2.44	13.9

. The band gap energy of Bi₂O₂CO₃ is 3.18 eV, and the effect of light rare earth and heavy rare earth doping on band gap energy is not much different. The band gap energy of doped Bi₂O₂CO₃/β-Bi₂O₃ is 2.42~2.57 eV. The specific surface area of the sample was measured by N₂ physical adsorption–desorption, and the results are shown in Table 1. From the table, it can be seen that the specific surface area of the Bi₂O₂CO₃ sample is very small, only 5.4 m²/g, which is consistent with the previous SEM image analysis that the sample has a flake structure with a smooth surface. The specific surface area of the light rare earth- and heavy rare earth-doped samples has a significant increase, where the specific surface area of the light rare earth-doped Bi₂O₃/β-Bi₂O₃ increases to 10.9~14.5 m²/g, and the heavy rare earth-doped increases the specific surface area more significantly, with the specific surface area increasing to 13.9~17.3 m²/g. With the conclusion of the previous SEM analysis, it is concluded that rare earth doping makes the distribution of the sample particles narrower, with particle refinement and increased dispersion, thus causing an increase in specific surface area.

2.6. Fourier Transform Infrared Spectroscopy (FT-IR) Analysis

The FI−IR spectra of the pure sample and the light and heavy rare earth-doped samples are shown in Figure 9. The broad absorption peak near 3440 cm⁻¹ is attributed to the OH-stretching vibration peak of the hydroxyl functional group on the sample surface, while the absorption peak at 1600 cm⁻¹ corresponds to the bending vibration peak of H-O-H of physically adsorbed water on the catalyst surface. The absorption peak near 1390 cm⁻¹ for the pure sample corresponds to the C-O stretching vibration peak in CO₃²⁻. The composite rare earth samples also have C-O absorption peaks near 1390 cm⁻¹ due to the small amount of Bi₂O₂CO₃ contained in the samples, but a significant decrease in the intensity of C-O absorption peaks can be observed before and after doping. The absorption peaks in the range of 400–700 cm⁻¹ are attributed to Bi-O bonds and the absorption peaks near 835 cm⁻¹ are Bi-O-Bi bonds. Comparing with the IR spectra of each sample, it can be found that the rare earth doping causes the stretching vibration peak at 835 cm⁻¹ to be significantly weakened. Combined with the XRD characterization results, it is clear that the introduction of different rare earths promotes the stable existence of β-Bi₂O₃ at room temperature, and RE³⁺ may replace part of Bi³⁺ in the Bi₂O₃ lattice and form Bi-O-RE bond, thus making the Bi-O-Bi bond vibration peak at 835 cm⁻¹ weakened. In addition, the formation of Bi-O-RE caused a change in the peak shape in the range of 400–700 cm⁻¹.

2.7. Photocatalytic Activity

The photocatalytic activity of the samples was carried out by degrading 15 mg•L⁻¹ DMP under light irradiation. As can be seen in Figure 10a, DMP is hardly degraded under light irradiation in the absence of a catalyst, which indicates that photolysis can be neglected. Figure 10a,b show the comparison of the degradation rate of DMP over catalysts doped with light rare earth and heavy rare earth, where C is the concentration of DMP after irradiation for t time and C₀ is the initial concentration before light illumination. As seen from Figure 10, the degradation activity of Bi₂O₂CO₃ for DMP was poor, with a degradation rate of only 17% at 150 min, mainly due to the large forbidden bandwidth of Bi₂O₂CO₃ (3.18 eV) with poor absorption of visible light. The performance of the catalyst is greatly improved by doping rare earth, but the effect of each rare earth on the improvement of catalytic activity is quite different. Some of the rare earths showed distinct enhancement of photocatalytic activity, among which the light rare earths were Nd, Pr and Sm, and the heavy rare earths were Gd, Er and Yb. Among the light rare earths, Nd has the most obvious improvement in photocatalytic performance, and the degradation rate of DMP reaches 72% for 150 min of photoreaction, followed by Pr, with a degradation rate of 66%. Among the heavy rare earths, Er showed the most significant increase in photocatalytic activity, with 78% degradation rate, followed by Gd with 69% degradation rate.

In order to study the final mineralization efficiency of DMP in photocatalytic degradation, the total organic carbon (TOC) content during the reaction was tested and analyzed. The results of TOC removal rate are shown in

Table 2. Degradation (D%) and TOC removal (R%) of DMP by Bi₂O₂CO₃ and different rare earth-doped Bi₂O₂CO₃/β-Bi₂O₃.

Sample	D (%)	R (%)	Sample	D (%)	R (%)
Bi2O2CO3	17	14	Gd-Bi2O2CO3/β-Bi2O3	69	62
La-Bi2O2CO3/β-Bi2O3	35	30	Tb-Bi2O2CO3/β-Bi2O3	40	34
Ce-Bi2O2CO3/β-Bi2O3	38	34	Dy-Bi2O2CO3/β-Bi2O3	52	47
Pr-Bi2O2CO3/β-Bi2O3	66	59	Ho-Bi2O2CO3/β-Bi2O3	35	32
Nd-Bi2O2CO3/β-Bi2O3	72	67	Er-Bi2O2CO3/β-Bi2O3	78	69
Sm-Bi2O2CO3/β-Bi2O3	53	48	Tm-Bi2O2CO3/β-Bi2O3	46	41
Eu-Bi2O2CO3/β-Bi2O3	40	31	Yb-Bi2O2CO3/β-Bi2O3	57	49
			Lu-Bi2O2CO3/β-Bi2O3	30	26

, which shows that DMP can be basically degraded and mineralized into inorganic small molecules such as CO₃²⁻, CO₂ and H₂O in the photocatalytic reaction, resulting in a high removal rate of TOC. The enhancement effect of light rare earth and heavy rare earth doping on the TOC removal rate of DMP is basically the same.

Usually, the active species involved in the photocatalytic degradation of organic pollutants are O₂^•−, h, and •OH radicals. Here, we selected p-benzoquinone (BZQ), disodium EDTA (Na₂-EDTA) and isopropanol as the quenchers for O₂^•−, hole and •OH, respectively. Figure 11a shows that with the addition of BZQ or Na₂-EDTA, a fast deactivation of Nd-Bi₂O₂CO₃/β-Bi₂O₃ was observed, indicating that both O₂^•− and holes are the main active species for DMP degradation. However, the role of •OH radicals in DMP degradation could be ignored. The stability of the as-prepared Nd-Bi₂O₂CO₃/β-Bi₂O₃ was evaluated by cyclic utilization under the same reaction conditions. As shown in Figure 11b, the degradation rate of DMP after 150 min visible light irradiation was 74% in the first run, and the degradation rate value was ~64% in the fourth run, indicating its good stability.

Photogenerated charge transfer kinetics is studied using electrochemical impedance spectroscopy (EIS). As shown in Figure 12a, the sample doped with light and heavy rare earth ions has a much smaller arc than that of pure Bi₂O₂CO₃, which indicates that the doping ions accelerate the photogenerated electron–hole pair separation and carrier migration at the electrode/electrolyte interface. The separation efficiency of the photogenerated charge carriers was detected using the transient photocurrent response. It is also shown in Figure 12b that the composites have higher photocurrent density after doping with ions and exhibit better separation efficiency of photogenerated charge carriers.

We used Mott–Schottky (MS) curves to reveal the energy band positions of β-Bi₂O₃ and Bi₂O₂CO₃. Since the slopes of both curves are positive, we assume that β-Bi₂O₃ and Bi₂O₂CO₃ are n-type semiconductors. From Figure 13a,b, we know that the flat-band potentials of β-Bi₂O₃ and Bi₂O₂CO₃ are −0.026 V and 0.15 V, respectively (vs. Ag/AgCl, pH = 7). In addition, the conduction band (CB) of general n-type semiconductor is more negative than the flat potential (about 0.1 V). Therefore, the CB potentials of β-Bi₂O₃ and Bi₂O₂CO₃ are about −0.126 V and 0.05 V, respectively, which are equivalent to 0.114 V and 0.29 V compared to the normal hydrogen electrode (NHE, pH = 7). We have known the band gap values of β-Bi₂O₃ and Bi₂O₂CO₃ by Formula:

E_CB = E_VB − E_g

We used Mott–Schottky (MS) curves to reveal the energy band positions of β-Bi₂O₃ and Bi₂O₂CO₃. Since the slopes of both curves are positive, we assume that β-Bi₂O₃ and Bi₂O₂CO₃ are n-type semiconductors. From Figure 13a,b, we know that the flat-band potentials of β-Bi₂O₃ and Bi₂O₂CO₃ are −0.026 V and 0.15 V, respectively (vs. Ag/AgCl, pH = 7). In addition, the conduction band (CB) of general n-type semiconductor is more negative than the flat potential (about 0.1 V). Therefore, the CB potentials of β-Bi₂O₃ and Bi₂O₂CO₃ are about −0.126 V and 0.05 V, respectively, which are equivalent to 0.114 V and 0.29 V compared to the normal hydrogen electrode (NHE, pH = 7). We have known the band gap values of β-Bi₂O₃ and Bi₂O₂CO₃ by Formula:

E_CB = E_VB − E_g

We can calculate the β-Bi₂O₃ and Bi₂O₂CO₃ valence band (VB) potentials, which are 2.33 V and 3.28 V (vs. NHE, pH = 7), respectively.

The VB edge positions of Bi₂O₂CO₃ and α-Bi₂O₃ were estimated according to the electronegativity. The CB and VB potentials of the two semiconductors at the point of zero charge can be calculated by the following equation:

E_VB = X − E^e + 0.5 E_g

where X is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms; E^e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); E_VB is the VB edge potential; and E_g is the band gap of the semiconductor obtained from the UV-visible diffuse reflectance absorption. Therefore, the band gaps of Bi₂O₂CO₃ and α-Bi₂O₃ are 3.28 and 2.22 eV, respectively. The CB position can be deduced from the equation E_CB = E_VB − E_g. The X values for Bi₂O₂CO₃ and α-Bi₂O₃ are 6.47 and 5.77 eV respectively. On the basis of the above equations, the top of the VB and the bottom of the CB of Bi₂O₂CO₃ are calculated to be 3.56 and 0.28 eV, respectively. Accordingly, the VB and CB of α-Bi₂O₃ are estimated to be 2.33 and 0.11 eV, respectively.

The main reason for the enhanced photocatalytic degradation efficiency of DMP by doped light and heavy rare earth samples is that the rare earth-doped samples calcined at 300 °C produced a mixed crystalline phase of Bi₂O₂CO₃ and β-Bi₂O₃. The forbidden bandwidth of β-Bi₂O₃ is smaller (2.22 eV), so the absorption of visible light by the samples is enhanced. The Bi₂O₂CO₃/β-Bi₂O₃ heterogeneous phase junction formed by the simultaneous mixing of crystalline phases can enhance the separation efficiency of photogenerated electrons and holes [28]. Secondly, the doping of rare earths changes the grain size and morphology of the samples, and the rare earth-doped samples have finer grains and a larger specific surface area, which improves the adsorption capacity of the molecules of the degradation target DMP. Finally, the doped rare earth ions (RE³⁺) can trap photogenerated electrons and further promote the separation of photogenerated holes [29,30], which generates more holes to participate in the degradation reaction of DMP and enhances the photocatalytic degradation ability. The reaction mechanism of DMP degradation by Bi₂O₂CO₃/β-Bi₂O₃ doped with rare earth elements is shown in Figure 14. The difference in the photocatalytic performance of Bi₂O₃ caused by different rare earth ion doping may be related to the filled state of the electronic structure of the 4f layer of rare earth elements [31].

3. Experimental

3.1. Materials

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, 99.99%), acetate (CH₃CO₂H, ≥99.8%), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O, 99.99%), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.99%), praseodymium nitrate hexahydrate (Pr(NO₃)₃·6H₂O, 99.99%), neodymium nitrate hexahydrate (Nd(NO₃)₃·6H₂O, 99.99%), samarium (III) nitrate hexahydrate (Sm(NO₃)₃·6H₂O, 99.99%), europium nitrate hexahydrate (Eu(NO₃)₃·6H₂O, 99.99%), gadolinium (III) nitrate hexahydrate (Gd(NO₃)₃·6H₂O, 99.99%), terbium (III) nitrate hexahydrate (Te(NO₃)₃·6H₂O, 99.9%), dysprosium nitrate hexahydrate (Dy(NO₃)₃·6H₂O, 99.99%), holmium (III) nitrate hexahydrate (Ho(NO₃)₃·6H₂O, 99.99%), erbium nitrate hexahydrate (Er(NO₃)₃·6H₂O, 99.99%), thulium (III) nitrate hexahydrate (Tm(NO₃)₃·6H₂O, 99.99%), ytterbium nitrate hexahydrate (Yb(NO₃)₃·6H₂O, 99.99%), lutetium nitrate hexahydrate (Lu(NO₃)₃·6H₂O, 99.99%), sodium sulfate (Na₂SO₄, 99.99%), potassium ferrocyanide trihydrate (K₄FeC₆N₆·3H₂O, 99.99%), p-Benzoquinone (C₆H₄O₂, 99%), ethylenediaminetetraacetic acid disodium salt dihydrate (C₁₀H₁₄N₂Na₂O₈·2H₂O, 99.99%) and isopropyl alcohol (C₃H₈O, 99%) were purchased from Aladdin. All chemicals were used directly without further purifications.

3.2. Sample Synthesis

First, 2.3 g Bi(NO₃)₃·5H₂O was weighed and dissolved in a mixed solution of 3 mL of glacial acetic acid and 20 mL of deionized water. Under magnetic stirring, after the Bi(NO₃)₃·5H₂O was completely dissolved, an appropriate amount of RE(NO₃)₃·6H₂O (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy Ho, Er, Tm, Yb, Lu) was added, and the molar ratio of RE³⁺:Bi³⁺ ions was controlled to be 4%. Then, 1 mol·L⁻¹ NaOH solution was added drop by drop to adjust the pH of the solution to 6–7. Vigorously stirring for 1 h, it was put into a 100 mL polytetrafluoroethylene liner, heated at 140 °C for 14 h, and cooled to room temperature to obtain a precipitate which was washed several times with deionized water, dried in a constant temperature drying oven at 60 °C for 6 h. Finally, the prepared precursor was calcined in a muffle furnace at 300 °C for 4 h (the heating rate was controlled at 3 °C/min) to obtain a yellow powder. The preparation of pure Bi₂O₂CO₃ is prepared as shown above, except that no additional RE(NO₃)₃-6H₂O needs to be added.

3.3. Characterization

The sample crystal phase structure was analyzed by Bruker D8 Advance type XRD instrument. The copper target was Cu K_α with λ = 0.15 nm, the operating voltage was 40 kV and the current was 40 mA. Scanning electron microscope (SEM) is an XL30 SEM produced by Philips in the Netherlands, which can be used to observe the apparent morphology and particle size distribution of the photocatalyst samples. The shape, size, dispersion and particle size distribution of the nanoparticles were observed with a Philips CM-120 TEM from the Netherlands. The FT−IR spectra were measured on a Nicolet-470 infrared spectrometer. The UV-visible diffuse reflectance spectra were measured on a Shimadzu 2550 UV-visible spectrophotometer with a large integrating sphere solid powder diffuse reflectance attachment, using BaSO₄ as the standard reference material, and the scanning range of 200–600 nm. Thermogravimetric and differential thermal analyses of the samples were performed on a Perkin Elmer TGA7 thermogravimetric-differential thermal analyzer (air atmosphere and reference alumina). The temperature rise range was 30 ℃~600 ℃, and the temperature rise rate was 10 ℃/min.

3.4. Photocatalytic Activity Evaluation

The photocatalytic activity of the prepared samples was tested by the degradation of DMP in aqueous solution under tungsten lamp (300 W) irradiation. The experimental setup is shown in Figure 15. The tungsten lamp was placed in a cylindrical Pyrex vessel surrounded by a circulating cooling water jacket. A 0.05 g photocatalyst was suspended in 80 mL aqueous solution of DMP (C₀ = 0.015 g/L). Before the lamp was turned on, the suspension was stirred in the dark for 40 min to reach the physical adsorption–desorption equilibrium of DMP over photocatalyst. The pH value of the suspension is around 7. The suspension was vigorously stirred during reaction, and its temperature was maintained at 22 ± 2 °C by circulation of water through an external cooling coil. The role of the cooling jacket is to cool the suspension liquid. The trapping test was performed by adding BZQ as an O²⁻ quencher, Na₂-EDTA as a cavity quencher, and isopropanol as an -OH quencher to the above containers.

At given intervals, 4 mL solution was sampled and centrifuged by high speed to remove catalyst particles. The concentration of DMP was determined by high-performance liquid chromatography (HPLC, Agilent 1200, Palo Alto, CA, USA) using a system with a UV detector. The TOC content in the solution was monitored by a TOC analyzer (Aurora 1030C, OI Analytical, College Station, TX, USA).

3.5. Photoelectrochemical Measurements

The photocurrent (IT) response of the catalyst, EIS analysis and MS were measured in a three-electrode system on an electrochemical analyzer (CHI660E) with platinum wire as the counter electrode, Ag/AgCl as the reference electrode and fluorine-doped tin oxide (FTO) glass coated with photocatalyst as the working electrode. A 1.5 mL centrifuge tube containing 5 mg of photocatalyst was added to 0.5 mL of ethanol and sonicated for 30 min to disperse the catalyst in the ethanol, then 10 uL of Nafion 117 perfluorinated resin was added to the solution and shaken well. A 1 cm × 1 cm FTO conductive glass sheet was then painted with the solution prepared above and the FTO conductive glass was allowed to dry with the solution and nail polish was brushed around it. Finally, the FTO conductive glass was dried in an oven at 60 °C for 2 h. IT, EIS analysis and MS were performed in electrolyte solutions of 0.1 M Na₂SO₄, 0.1 M K₄[Fe(CN)₆] and 0.5 M Na₂SO₄, respectively.

4. Conclusions

A series of pure Bi₂O₂CO₃ and light and heavy rare earth-doped Bi₂O₂CO₃ samples were prepared by hydrothermal method. The changes of crystalline phase composition and physical structure of the samples after calcination at 300 °C were investigated, and it was found that the samples without rare earth doping were basically Bi₂O₂CO₃ crystalline phase after calcination. Under the calcination condition, the rare earth doping promoted the transformation of Bi₂O₂CO₃ to β-Bi₂O₃ crystalline phase to obtain the rare earth-doped Bi₂O₂CO₃/β-Bi₂O₃ mixed crystalline phase photocatalyst. The photocatalytic degradation of phthalates showed that rare earth doping exhibited a good enhancement of the photocatalytic degradation activity of DMP. Nd, Pr, and Sm in light rare earths and Gd, Er and Yb in heavy rare earths display the most obvious effect. The rare earth doping made the obtained samples have a strong absorption of visible light. Moreover, the formation of Bi₂O₂CO₃/β-Bi₂O₃ heterogeneous phase junction enhanced the separation of photogenerated electrons and holes. In addition, the rare earth-doped samples had larger specific surface area and better adsorption ability to the molecules of the degradation target DMP, which could promote the photocatalytic degradation ability.