Fabrication of a Plasmonic Heterojunction for Degradation of Oxytetracycline Hydrochloride and Removal of Cr(VI) from Water

Abstract

A novel Ag/Ag₂CO₃/BiVO₄ plasmonic photocatalyst was successfully prepared by depositing Ag nanoparticles on the surface of Ag₂CO₃/BiVO₄ through the photoreduction reaction. Due to the existence of this novel heterojunction photocatalyst structure, not only can it prevent the photogenerated charge recombination, but the unique properties of Ag also have a great advantage in the absorption of light. The Ag/Ag₂CO₃/BiVO₄ photocatalyst showed good catalytic performance in the degradation of oxytetracycline hydrochloride (OTH) and removal of Cr⁶⁺, and the degradation rate of OTH reached 98.0% after 150 min of illumination. The successful preparation of Ag/Ag₂CO₃/BiVO₄ was confirmed by a series of characterization methods, and the importance of •OH and h⁺ radicals in the degradation of OTH was demonstrated. In addition, the photocatalytic mechanism of Ag/Ag₂CO₃/BiVO₄ photocatalyst was systematically studied in terms of degradation of OTH and reduction of Cr⁶⁺. This study is of great importance for the development of novel plasmonic heterojunction photocatalysts and to meet future environmental requirements.

1. Introduction

Photocatalysis is an environmentally friendly technology and has become a promising method for the elimination of organic pollutants, heavy metals and medical residues [1,2,3,4,5,6,7,8,9]. As an efficient, environmentally friendly, and chemically stable photocatalytic material, titanium dioxide (TiO₂) has inestimable application potential in photocatalysis [10,11,12,13,14,15]. However, due to its wide band gap and narrow UV response range, its application is limited. Therefore, it is urgent to find a photocatalyst driven by visible light with an appropriate light response range [16,17,18,19,20,21,22,23,24,25].

Bismuth vanadate (BiVO₄) has a fine effect on the elimination of organic pollutants due to its unique visible light response, excellent chemical stability, and photocatalytic activity [26,27,28,29]. However, the practical application of BiVO₄ is greatly limited due to its poor separation ability of photogenerated carriers and weak light utilization force. Therefore, further research is needed to remedy these deficiencies [30,31,32,33,34,35].

For improving the catalytic performance of BiVO₄ samples, researchers synthesized heterojunction photocatalysts containing BiVO₄, such as Fe₂O₃/BiVO₄, BiVO₄/TiO₂, Ag@AgBr/BiVO₄, etc. [36,37,38]. For instance, Li group prepared g-C₃N₄/BiVO₄ photocatalyst, which showed excellent performance in methylene blue degradation [39]. Wang et al. constructed BiVO₄/Co₃O₄ heterojunction photocatalyst, which showed high photocatalytic performance [40]. Gao reported BiVO₄/Bi₂S₃ photocatalyst synthesized by anion exchange method, which showed superior photocatalytic activity in the reduction of Cr⁶⁺ [41]. Based on the above results, it can be concluded that the heterojunction photocatalyst has better photocatalytic activity.

Moreover, the photocatalytic performance of the photocatalyst may be improved when it is modified with precious metals [42,43]. Due to unique properties of noble metals, namely, the local surface plasmon resonance (LSPR). This feature greatly improves the utilization of light in the visible region and the recombination of charge carriers is prevented to optimize the performance of the photocatalyst. Zhao reported that Ag/AgVO₃/RGO photocatalyst’s performance in BPA degradation is unmatched [44]. Based on the research and learning of predecessors, we were inspired to use Ag nanoparticles to modify BiVO₄-based heterojunction photocatalyst to generate LSPR. In order to verify this hypothesis, flower spherical BiVO₄ was synthesized by one-step hydrothermal method. Then Ag₂CO₃ was deposited on the surface of BiVO₄ to synthesize Ag₂CO₃/BiVO₄ photocatalyst. The Ag/Ag₂CO₃/BiVO₄ plasma photocatalyst was further synthesized by photoreduction reaction.

The photocatalytic properties of Ag/Ag₂CO₃/BiVO₄ were studied by photocatalytic removal of Cr⁶⁺ and photodegradation of OTH. The photocatalytic mechanism of Ag/Ag₂CO₃/BiVO₄ photocatalyst in the degradation of OTH was systematically studied. Figure 1 shows the synthesis route of Ag/Ag₂CO₃/BiVO₄ photocatalyst. The plasma heterojunction photocatalyst was synthesized by photoreduction reaction, which provides ideas for the development of new semiconductor heterojunction photocatalysts.

2. Results and Discussion

2.1. Morphology of Sample

The structure, morphology, and phase purities of BiVO₄, Ag/Ag₂CO₃/BiVO₄ and Ag₂CO₃/BiVO₄ samples were detected by XRD (Figure 2a). In the XRD pattern of the standard phase of BiVO₄ (JCPDS: 14-0688), almost all diffraction peaks are consistent with it. In addition, no Ag diffraction peak was found in the Ag₂CO₃/BiVO₄ sample, possibly because Ag₂CO₃ is highly dispersed on the BiVO₄ surface, or because the Ag₂CO₃ content is low. However, for Ag/Ag₂CO₃/BiVO₄, the corresponding diffraction peak is not shown in the figure because of the poor crystallinity or too little content of Ag. To further verify the presence of Ag and Ag₂CO₃, XPS analysis was performed on the prepared samples.

In order to prove the successful synthesis of Ag/Ag₂CO₃/BiVO₄ and the existence of Ag₂CO₃, the surface chemical composition and electronic state of the samples were detected by X-ray photoelectron spectroscopy (XPS). In Figure 2b, the peak at 284.8 eV may be derived from hydrocarbon contaminants [45]. However, the main peak at 288.4 eV originates from Ag₂CO₃, indicating the existence of Ag₂CO₃ [46]. In Figure 2c, Ag 3d consists of independent peaks centered at 367.7 and 373.7 eV, corresponding to Ag 3d_5/2 and Ag 3d_3/2, respectively. These two independent peaks can deconvolve four bands, Ag⁺ 3d_5/2 (367.5 eV) and 3d_3/2 (373.5 eV), and Ag⁰ 3d_5/2 (368.6 eV) and 3d_3/2 (374.7 eV), thus confirming the existence of Ag [47]. As can be seen in Figure 2d, the peak located at 529.5 eV, indicating the presence of the Bi-O band [48]. In addition, the peaks of O 1 s at 531.2 and 532.1 eV, correspond to CO₃²⁻ and -OH, respectively [49,50]. In this case, oxygen atoms undergo chemical changes on the surface of the photocatalyst, and then capture a large number of photoexcited electron-hole pairs, thus promoting the separation rate of photogenerated charges and finally improving the catalytic activity of the photocatalyst. For Figure 2e,f, the peaks of Bi 4f and V 2p located at 159.4, 164.7 eV, and 517.0, 524.2 eV are basically corresponding to Bi³⁺ and V⁵⁺, respectively.

The structure morphologies of the samples were studied by FESEM. In Figure 3a, the size of flower globular BiVO₄ is about 2–4 μm. The surface of Ag/Ag₂CO₃/BiVO₄ is rougher than that of BiVO₄, indicating the presence of Ag ions. Therefore, the existence of Ag or Ag₂CO₃ is confirmed, which is consistent with the XRD results.

In order to understand the microstructure morphology of Ag/Ag₂CO₃/BiVO₄, TEM and HRTEM were performed to characterize the Ag/Ag₂CO₃/BiVO₄. In Figure 4a,b, the morphological characteristics of Ag/Ag₂CO₃/BiVO₄ can be clearly observed. From Figure 4c, the diameters of Ag and BiVO₄ are 0.238 and 0.312 nm, respectively, corresponding to the (111) and (121) crystal planes [51]. This unique structure will help prevent photogenerated carrier recombination.

2.2. Optical Properties

The optical properties of the samples were measured by UV–vis diffuse reflectance spectra and photoluminescence spectra. The UV–vis absorption spectra of BiVO₄, Ag₂CO₃/BiVO₄ and Ag/Ag₂CO₃/BiVO₄ are shown in Figure 5a. BiVO₄ has a strong absorption at about 500 nm, which may be band gap absorption. However, the use of light by Ag/Ag₂CO₃/BiVO₄ is obviously stronger than that of BiVO₄ and Ag₂CO₃/BiVO₄. This shows that Ag/Ag₂CO₃/BiVO₄ will have excellent photocatalytic performance under visible light irradiation. There are two main reasons for this phenomenon. First, Ag nanoparticles enhance the absorbance in the process of darkening. Second, under local surface plasmon resonance (LSPR), strong absorption occurs. Moreover, the optical absorption near band edge can be calculated according to the equation αhυ = A (hυ-Eg)ⁿ, where α, h, υ, A and Eg are the absorption coefficient, Plank’s constant, frequency of the incident photon, a constant and band gap, respectively. The index n equals 2 for direct-gap semiconductor and it equals 1/2 for indirect-gap semiconductor. As to all the samples, n equals 1/2. The band gap values of BiVO₄, Ag₂CO₃/BiVO₄ and Ag/Ag₂CO₃/BiVO₄ are calculated by the equation, which are 2.50, 2.43 and 2.37 eV, respectively.

The recombination rate of photogenerated carriers is an important factor affecting the photocatalytic activity of photocatalysts. Therefore, the efficiency of photoinduced electron and hole migration was studied by PL spectrum. Simply put, a higher PL intensity means a lower number of photoelectrons and a lower hole separation efficiency. The PL spectral intensity of the prepared sample was emitted at a wavelength of 425 nm. In Figure 5b, compared with the BiVO₄, the PL intensity of Ag₂CO₃/BiVO₄ is lower, indicating that the fast carrier movement rate between Ag₂CO₃ and BiVO₄ interface is related. However, when Ag nanoparticles are deposited on the surface of Ag₂CO₃/BiVO₄, the PL intensity of Ag/Ag₂CO₃/BiVO₄ is lower, which indicates that the LSPR of Ag can enhance the absorbance and accelerate the separation rate of electron-hole pairs.

In order to study the photoresponse of photocatalyst, the separation efficiency of photogenerated carriers was analyzed by photocurrent spectroscopy. Ag/Ag₂CO₃/BiVO₄ has the highest photocurrent intensity, which means that the electron–hole pairs recombination rate is extremely low.

2.3. Photocatalytic Performance

By degrading OTH and removing Cr⁶⁺, the photocatalytic activity of the samples was systematically studied in the range of lighting. At the beginning of the experiment, it had experienced dark adsorption for 1 h and the adsorption and desorption on the surfaces of the catalysts were balanced. Furthermore, the photodegradation experiment of OTH without photocatalyst was also carried out as blank control.

Figure 6a shows the degradation of OTH by photocatalyst. Among all the samples, only Ag/Ag₂CO₃/BiVO₄ showed excellent photocatalytic performance, and the degradation rate of OTH reached 98.0% after 150 min of illumination. Nevertheless, the oxidation degradation of OTH over the pure BiVO₄ and Ag₂CO₃/BiVO₄ are only 57.0% and 69.3%, respectively. According to the equation: ln(C₀/C_t) = k_appt, C₀ is the initial concentration and C_t is the concentration at time t, k_app is the apparent reaction rate constant. For Figure 6b, it can be seen that the k_app value of Ag/Ag₂CO₃/BiVO₄ is the highest (0.023 min⁻¹). Furthermore, stability is also a very important factor in photocatalytic degradation. Therefore, we studied the stability of the sample (Figure 6c). The Ag/Ag₂CO₃/BiVO₄ photocatalyst has the best stability and excellent performance in degrading OTH.

As observed in Figure 7a, the photocatalytic reduction activity of the sample was systematically investigated by photocatalytic removal of Cr⁶⁺ under illumination. The results also show that Ag/Ag₂CO₃/BiVO₄ photocatalyst has the highest photocatalytic performance. In addition, we also verified the stability of the sample in the process of removing Cr⁶⁺, as expected, showing excellent stability in Figure 7b.

In addition, the total organic carbon (TOC) in degradation of OTH was also determined for evaluation of mineralization rate of OTH. From Figure 8, the mineralization rate of OTH in the reaction system were 34.2%, 46.8% and 68.9% corresponding to BiVO₄, Ag₂CO₃/BiVO₄ and Ag/Ag₂CO₃/BiVO₄, respectively, suggesting that OTH is effectively mineralized during the photocatalytic degradation process and that Ag/Ag₂CO₃/BiVO₄ has a higher mineralization rate.

2.4. Photocatalytic Mechanism

Generally speaking, the role of photogenerated carriers in photocatalytic processes cannot be denied, and there are many active species in photocatalytic reactions. The trapping agents used in the experiment are ammonium oxalate (AO), isopropanol (IPA) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL), which are used to scavenge free radicals. These scavengers can capture h⁺, •OH and •O₂⁻, and the effects of these active species on OTH degradation were examined [52,53,54]. In Figure 6d, the low effect of TEMPOL on photocatalytic degradation of OTH suggests that •O₂⁻ is not the predominant active species in photocatalytic reactions. However, when AO and IPA are added, the photocatalytic degradation rate of OTH is significantly reduced, which is inseparable from the role of h⁺ and •OH. Thus, h⁺ and •OH are the most active species in the photocatalytic process.

As shown in Figure 9, the photocatalytic degradation mechanism of Ag/Ag₂CO₃/BiVO₄ photocatalyst was proposed. Under visible light irradiation, photogenerated electrons and photogenerated holes are generated on the conduction band and valence band of Ag₂CO₃ and BiVO₄, respectively. As shown in Equations (1) and (2):

BiVO₄ + hυ → BiVO₄ (e⁻ + h⁺)(1)

Ag₂CO₃ + hυ → Ag₂CO₃ (e⁻ + h⁺)(2)

Since the conduction band (CB) and valence band (VB) position of BiVO₄ are lower than Ag₂CO₃, the conduction band of photogenerated electrons transferred from Ag₂CO₃ to BiVO₄, nevertheless the valence band of photogenerated holes are transferred from BiVO₄ to Ag₂CO₃. As shown in Equations (3) and (4):

Ag₂CO₃ (e⁻) + BiVO₄ → Ag₂CO₃ + BiVO₄ (e⁻)(3)

BiVO₄ (h⁺) + Ag₂CO₃ → Ag₂CO₃ (h⁺) + BiVO₄(4)

In addition, the photogenerated electrons on Ag surface will migrate to the conduction band of Ag₂CO₃, and these photogenerated electrons will further transfer from Ag₂CO₃ to the conduction band of BiVO₄. As shown in Equations (5) and (6):

Ag + hυ → Ag*(5)

Ag* + Ag₂CO₃ → Ag₂CO₃ (e⁻) + Ag⁺*(6)

In this case, due to the conduction band of BiVO₄ is surrounded by many photogenerated electrons, which possesses strong reducibility. Therefore, the photogenerated electrons on the conduction band of BiVO₄ can participate in the reduction of hexavalent chromium ions to trivalent chromium ions. As shown in Equations (7) and (8):

Ag₂CO₃ (h⁺) + OH−/H₂O → •OH(7)

OTH + •OH/h⁺ → products → CO₂ + H₂O(8)

Besides, many photogenerated holes are enriched in the valence band of Ag₂CO₃ and are captured by H₂O and OH− on the surface of the photocatalyst to form •OH. Therefore, they are directly involved in the degradation of OTH.

3. Materials and Discussion

3.1. Materials

Bi(NO₃)₃·5H₂O (purity 99%), Na₃VO₄·12H₂O (purity 99%), AgNO₃ (purity 99%), NaHCO₃ (purity 99%) and all chemicals were analytically pure (purity 99%). The chemical reagents used were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Preparation of Flower Globular BiVO₄

We studied the synthesized sample with reference to previous experiments [55]. First, Bi (NO₃)₃·5H₂O (60 mg) was added to ultrapure water (40 mL), followed by 5 min ultrasound treatment. Then Na₃VO₄·12H₂O (100 mg) solution was added, and the mixture turns yellow. It was finally transferred to a Telfon-lined stainless steel autoclave and heated for 8 h at 160 ℃. After cooling the solid powder to room temperature, the BiVO₄ sample was successfully prepared by washing with ultrapure water and ethanol several times.

3.3. Synthesized of Ag₂CO₃/BiVO₄ Heterojunction Photocatalyst

Firstly, 1 mmol BiVO₄ was dissolved in 50 mL ultrapure water and treated with ultrasound treatment for 5 min. In addition, AgNO₃ (0.20 mmol) solution was put in the mixture and stirred at a constant speed for about 30 min, then 0.10 mmol NaHCO₃ solution was added to make the pH neutral. Finally, the mixed solution was placed in the dark for 7 h and centrifuged to obtain the Ag₂CO₃/BiVO₄ samples.

3.4. Preparation of Ag/Ag₂CO₃/BiVO₄ Photocatalyst

First, Ag₂CO₃/BiVO₄ (0.5 g) was put in ethanol (50 mL), then 5 min ultrasonic treatment, and then exposure to ultraviolet light for 3 h. Finally, the final samples were obtained by centrifugation.

3.5. Photocatalytic Procedures

The catalytic performance of the samples for degradation of OTH was investigated using a 500 W Xe lamp (100 mW/cm²) with a cut-off wavelength of ≤420 nm. The weight of catalyst was 0.4 g/L, the original concentration of OTH and Cr⁶⁺ is 20 and 15 mg/L, respectively. Before illumination, the mixture was stirred in the dark for 60 min to reach an adsorption-desorption equilibrium. After exposure to visible light for different times, take some mixed solutions and measure the pollutant concentration after centrifuging to remove the powder samples.

3.6. Characterization

In this paper, X-ray diffraction (XRD), Field emission scanning electron microscope (FESEM), X-ray photoelectron spectra (XPS), photoluminescence spectroscopy (PL) and transmission electron microscope (TEM) etc. The specific details of these characterized instruments, SEM-EDS elemental mapping of Ag/Ag₂CO₃/BiVO₄ (Figure S1) and N₂-adsorption isotherms of Ag/Ag₂CO₃/BiVO₄ (Figure S2) will be supplemented in the supplementary materials.

4. Conclusions

A novel Ag/Ag₂CO₃/BiVO₄ plasmonic photocatalyst was achieved by depositing Ag nanoparticles on the surface of Ag₂CO₃/BiVO₄ through the photoreduction reaction. The morphology, structure and optical properties of the prepared samples were characterized by XRD and XPS, FESEM and TEM, UV–vis and PL. The catalytic properties of the sample were investigated by removing Cr⁶⁺ and degradation of OTH, and the degradation rate of OTH reached 98.0% after 150 min of illumination. Comparing BiVO₄, Ag₂CO₃/BiVO₄ and Ag/Ag₂CO₃/BiVO₄, the latter possess excellent photocatalytic performance by enhancing light utilization and inhibiting the recombination by charge transfer and electron-hole pairs in Ag/Ag₂CO₃/BiVO₄. In addition, the results showed that h⁺ and •OH were the main active species in the photocatalytic degradation of OTH, and the enhanced photocatalytic activity mechanism of Ag/Ag₂CO₃/BiVO₄ photocatalyst was systematically studied in terms of degradation of OTH and reduction of Cr⁶⁺.

Author Contributions

J.C. designed this work and prepared the manuscript. M.Z., X.Y., X.Z., Y.Y. and Y.L. carried out the experiments and data analysis. H.Z. and W.Z. participated in the discussion of the results. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The study did not report any data.

Acknowledgments

The authors greatly acknowledge the National Natural Science Foundation of China (No. 41573061, 51578279 and 51278242), the China Postdoctoral Science Foundation funded the 61th batches surface (2017M610336), and the Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF).

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Figure 1. Schematic illustration of the synthesis process of Ag/Ag2CO3/BiVO4 photocatalyst.

Figure 2. (a) XRD patterns of the as-prepared samples, (b–f) XPS spectra of each element.

Figure 3. (a) The FESEM image of BiVO4, (b,c) The SEM image of Ag/Ag2CO3/BiVO4.

Figure 4. (a,b) The TEM image of Ag/Ag2CO3/BiVO4, and (c) HRTEM image of Ag/Ag2CO3/BiVO4.

Figure 5. (a) The samples of UV—vis absorption spectra, (b) PL spectra, (c) photocurrent.

Figure 6. (a) Photocatalytic activities of the samples and (b) pseudo-first-order kinetics of OTH degradations, (c) five recycling runs of OTH degradations, (d) the effect of different quenchers on the degradation of OTH.

Figure 7. (a) photocatalytic reduction activities of Cr6+ reduction, (b) five recycling runs of Cr6+ reduction.

Figure 8. TOC removal of OTH for the different samples.

Figure 9. The photocatalysis mechanism of plasmonic heterojunction photocatalyst.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121498/s1, Characterization; Analytical methods; Figure S1: SEM-EDS elemental mapping of Ag/Ag₂CO₃/BiVO₄; Figure S2: N₂-adsorption isotherms of Ag/Ag₂CO₃/BiVO₄.

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