Enhancing Photocatalysis of Ag Nanoparticles Decorated BaTiO3 Nanofibers through Plasmon-Induced Resonance Energy Transfer Turned by Piezoelectric Field

Abstract

Revealing the charge transfer path is very important for studying the photocatalytic mechanism and improving photocatalytic performance. In this work, the charge transfer path turned by the piezoelectricity in Ag-BaTiO₃ nanofibers is discussed through degrading methyl orange. The piezo-photocatalytic degradation rate of Ag-BaTiO₃ is much higher than the photocatalysis of Ag-BaTiO₃ and piezo-photocatalysis of BaTiO₃, implying the coupling effect between Ag nanoparticle-induced localized surface plasmon resonance (LSPR), photoexcited electron-hole pairs, and deformation-induced piezoelectric field. With the distribution density of Ag nanoparticles doubling, the LSPR field increases by one order of magnitude. Combined with charge separation driven by the piezoelectric field, more electrons in BaTiO₃ nanofibers are excited by plasmon-induced resonance energy transfer to improve the photocatalytic property.

1. Introduction

The increasing worsening of global water pollution seriously threatens human health and social development. Solar-induced photocatalytic degradation of pollutants based on semiconducting photocatalysts is an effective method for wastewater treatment [1,2,3,4]. Under light irradiation with applicable energy, the semiconductor photocatalysts are excited to generate free electron-hole pairs, which migrate to the solid/liquid interface and generate reactive oxygen species (ROSs) for participating in the degradation of various pollutants [5,6,7,8]. However, the electron-hole pairs are easy to recombine due to the conduction of the Coulomb force; only a few carriers can successfully migrate to the surface of semiconductor particles to participate in the photocatalytic reaction [9,10,11,12,13]. It is a key problem to effectively improve the separation of electron-hole pairs for photocatalysis.

Recently, a novel method, named “piezo-photocatalysis” has been developed based on piezoelectric photocatalysts by coupling the semiconductor, photoexcitation, and piezoelectric effect to achieve simple and efficient separation of electron-hole pairs [14,15,16,17]. BaTiO₃ (BTO) is one of the most promising piezoelectric materials that can generate a strong internal field through crystal deformation under mechanical strain [18,19,20], which is considered a potential piezoelectric photocatalyst [21,22,23,24]. However, pure BTO can only absorb UV light due to its wide band gap (~3.6 eV). On the other hand, piezo-photocatalysis can be further improved by coupling with other photocatalytic enhancement methods, including heterojunctions, plasmons, and defect projects. In particular, the plasmons raised from metal nanoparticles loading on semiconductors with an appropriate amount are efficient to enhance the catalytic performance, which has been widely studied in photocatalytic degradation, nitrogen fixation, CO₂ reduction, and electrocatalysis [25,26,27,28].

In this work, BTO nanofibers are synthesized to study the photocatalytic and piezo-photocatalytic performances through degrading methyl orange (MO). Ag nanoparticles with different densities are coated on the BTO surface to further improve the catalytic performance. The BTO nanofibers with high Ag loading mass exhibit better photocatalytic performance, which is significantly improved by ultrasonication-induced piezoelectricity. To study the charge transfer paths, the BTO and Ag-BTO are excited by UV and visible light, respectively. Combining the simulation results, the energy transfer mechanism of piezo-photocatalysis of Ag-BTO is proposed.

2. Results and Discussion

2.1. Characterization of Ag-BTO Nanofibers

Figure 1a shows the SEM image of pure BTO nanofibers, illustrating the ultralong un-directional nanofibers with few fractures. Figure 1b is the SEM image of Ag-BTO-2 nanofibers, displaying a similar morphology with pure BTO. The low-resolution TEM image of Ag-BTO-2 nanofibers is shown in Figure 1c, depicting the uniform distribution of Ag nanoparticles on the whole surface of BTO nanofibers. The corresponding SAED pattern is shown in Figure 1d, where the diffraction rings depending on (110), (111), (200), (211), and (202) crystal planes of BTO are observed [29]. The HRTEM images of Ag-BTO-1 and Ag-BTO-2 are shown in Figure 1e,f, respectively. The distances of 0.235 and 0.422 nm belong to the (111) and (001) crystal planes of Ag and BTO, respectively. For Ag-BTO-2, the size of the Ag nanoparticle is equal to Ag-BTO-1, while the distribution density is double.

The XRD and Raman curves of pure BTO, Ag-BTO-1, and Ag-BTO-2 are compared in Figure 2. As shown in Figure 2a, all the major diffraction peaks of the three samples are assigned to BTO (PDF No. 79-2265) [30]. Particularly, the peak splitting around 45° indicates the high purity of tetragonal BTO. The trace peak at 38.62° assigned to (111) plane of Ag crystal can only be observed in Ag-BTO-2, which is attributed to the low proportion of Ag nanoparticles. In Figure 2b, the three normalized Raman curves are basically the same, where the peaks at 265, 309, 517, and 716 cm⁻¹ correspond to BTO [31]. A sharp peak appearing around 309 cm⁻¹ further confirms the presence of tetragonal BTO. The DRS spectra of the three samples are shown in Figure 2c. Both the samples display two similar adsorption peaks in UV range around 200 and 300 nm. For Ag-BTO-1, a gentle peak appears around 500–700 nm due to the LSPR of Ag nanoparticles. For Ag-BTO-2, the intensity of the gentle peak triples, implying the LSPR of Ag nanoparticles drastically increases. To characterize the crystal structure of the synthesized BTO nanofibers in detail, the refined XRD pattern of pure BTO nanofibers is tested and analyzed by Rietveld refinement, as shown in Figure 3. The refined XRD pattern is indexed by tetragonal BaTiO₃ (piezoelectric, space group P4mm) and cubic BaTiO₃ (non-piezoelectric, space group Pm–3m) with a ratio of 2:1, as well as a small portion of BaCO₃. The refinement results are shown in Table 1, where the R_wp = 6.06%, R_p = 3.98%, and GOF = 3.32. The lattice parameters for tetragonal and cubic BTO are calculated to be a = 4.0150, c = 3.9984, and a = 4.0081 Å, respectively, which are very close to the standard data. These results suggest that the piezoelectric BaTiO₃ are successfully synthesized under a relatively low temperature.

The XPS spectra of pure BTO and Ag-BTO-2 are almost the same, as shown in Figure 4. The survey spectra depict the presence of C, O, Ti, and Ba in pure BTO and C, O, Ti, Ba, and Ag in Ag-BTO, respectively (Figure 4a). The C 1s spectra are divided into three peaks around 284.80, 286.48, and 288.78 eV corresponding to the environmental C–C (or C–H) groups, CO₃²⁻ ions, and C–O groups, respectively (Figure 4b) [32]. The O 1s spectra are also divided into three peaks assigning BaTiO₃, CO₃²⁻ ions, and C–O groups, respectively (Figure 4c). The Ba 3d spectra shows two peaks around 793.65 and 778.48 eV, which are assigned to Ba 3d_3/2 and Ba 3d_5/2, respectively. Furthermore, the peaks are well divided into two couple of peaks assigned to BaTiO₃ and BaCO₃, respectively (Figure 4d). Ti 2p photoelectron peaks reveal the purity of BaTiO₃ without TiO₂ or sodium titanates (Figure 4e). For Ag-BTO-2, the energy difference between Ag 3d_3/2 (373.68) and 3d_5/2 (367.67) is 6.01 eV, indicating the zero-valent state of Ag element (Figure 4f). These results imply that Ag nanoparticles are successfully loaded on the surface of BTO nanofibers without affecting the morphology, microstructure, and chemical state of BTO nanofibers.

2.2. Piezo-Photocatalytic Property of Ag-BTO Nanofibers

The piezo-photocatalytic properties of the nanofibers are evaluated by degrading MO solution under ultrasonication and solar irradiation, as shown in Figure 5. Figure 5a–c show the MB degradation rates catalyzed by pure BTO, Ag-BTO-1, and Ag-BTO-2 under different conditions for 120 min, respectively. Under only ultrasonication, the degradation rates of the three samples are relatively low, indicating the ultrasound-induced piezoelectric field failed to efficiently degrade MO. Under light irradiation, the loading of Ag nanoparticles greatly improves the degradation rate, which further increases with the increasing Ag loading mass. The maximum degradation rate (82.7%) is achieved by Ag-BTO-2 under the cooperation of ultrasonication and light irradiation, suggesting the synergistic effect between piezoelectricity and photocatalysis. Figure 5d shows the degradation kinetic curves, indicating all the degradation processes follow the pseudo-first-order reaction kinetics.

To further study the function of the piezoelectricity on photocatalysis, the catalytic performances of BTO and Ag-BTO-2 samples are measured under UV and visible light, respectively, as shown in Figure 6. All the catalytic degradation rates of Ag-BTO-2 catalysts are higher than pure BTO. For pure BTO nanofibers (Figure 6a), the photocatalysis and piezo-catalysis under visible light are very weak (2.4% and 13.7%), and those under UV light are also limited (19.4% and 26.6%). As shown in Figure 6b, the UV-driven photocatalysis of the Ag-BTO-2 sample is moderately improved from 26.0% to 38.5% by piezoelectricity. In comparison, the visible-driven photocatalysis is developed from 32.4% to 51.7% by piezoelectricity, displaying a giant enhancement.

The stability of the catalysts is shown in Figure 7. Figure 7a is a TEM image of Ag-BTO-2 after piezo-photocatalytic process, keeping the original morphology and nanoparticle-nanofiber structure. The corresponding XRD pattern is shown in Figure 7b, also showing similar data with the sample before piezo-photocatalysis. Particularly, the minor peak related to Ag nanoparticles is still obtained, and no Ag₂O peak is observed, indicating that the Ag nanoparticles maintain metallic state.

2.3. Mechanism Analysis

Based on the above experimental results, the mechanisms of photocatalysis and piezo-photocatalysis of Ag-BTO nanofibers are developed. The possible photocatalytic mechanism of charge transfer in Ag-BTO is simply discussed. The generation and separation of photoexcited electron-hole pairs are the most important for photocatalysis. BTO nanofibers and Ag nanoparticles form Schottky contact with band offset (Figure 8a) [33]. BTO can absorb UV light to generate electron-hole pairs but cannot absorb visible light due to the wide bandgap (Figure 8b). Ag nanoparticles can absorb visible light through located surface plasmon resonance (LSPR) [34,35,36], but the UV absorption is very low due to the large detuning from resonance frequency (Figure 8c). As the Ag nanoparticles are synthesized on the BTO surface, there are three possible paths to enhance the solar-driven photocatalysis: the first is that the photogenerated electrons in BTO migrate to Ag nanoparticles due to the lower work function, leading to the separation of electron-hole pairs (Figure 8d) [37,38,39]; the second is that the LSPR-induced hot electrons in Ag nanoparticles migrate to BTO, increasing the number of free electrons in BTO (Figure 8e) [40,41,42,43]; the last is that the plasmons in Ag can transfer energy to BTO to generate electron-hole pairs, named “plasmon-induced resonance energy transfer” (PIRET) through dipole-dipole resonance (Figure 8f) [44,45,46,47]. In this case, the UV photocatalytic property of Ag-BTO is a little higher than that of BTO, ruling out the existence of the first path. On the other hand, the Ag-BTO samples exhibit good visible photocatalytic properties, indicating that the visible light is absorbed by Ag nanoparticles through LSPR and transfers hot electrons or energy to BTO, corresponding to the second or third path.

The LSPR electromagnetic fields of Ag-BTO-1 and Ag-BTO-2 are simulated by FEM, as shown in Figure 9a,b, respectively. As the distance of Ag nanoparticle arrays decreases from 10 to 5 nm, the average strength of the LSPR electromagnetic field at 550 nm increases by one order of magnitude. Thus, the photocatalysis and piezo-photocatalysis of Ag-BTO-2 are much higher than those of Ag-BTO-1. Under ultrasonication, BTO is deformed by ultrasound-induced cavitation blasting and generates a built-in piezoelectric field, which promotes the electron-hole separation in BTO and increases the Ag-BTO interface barrier (Figure 9c). As reported, the hot-electron injection path is significantly affected by the interface barrier, while the PERET path is a non-contact route rising superior to the interface barrier [48]. Under solar irradiation, the electron density of state around Ag nanoparticles is much higher than that in BTO, and the migrating of free electrons from BTO to Ag is restrained. The piezoelectric-enhanced interface barrier prevents the hot electrons from injecting into BTO. With a large number of Ag nanoparticles loading on the BTO surface, the first path and second path are both excluded during the piezo-photocatalysis process. Thus, the piezo-photocatalytic mechanism of Ag-BTO nanofibers is described below: under solar irradiation, BTO is excited by UV light to generate electron-hole pairs. The piezoelectric field contributes to the separation of electron-hole pairs and increases the Ag-BTO interface barrier. On the other hand, visible light is captured by high-density Ag nanoparticles through LSPR, transferring energy to further excite more electron-hole pairs for catalysis.

3. Materials and Methods

3.1. Synthesis of BaTiO₃ Nanofibers

All reagents were analytical reagents and purchased from Sinophram Chemical Reagent, Shanghai, China. A piece of Ti foil (Haiyuan aluminum, Xining, China) was ultrasound-cleaned in deionized water/acetone for 10 min, heated to 650 °C with a heating rate of 10 °C min⁻¹, and maintained for 5 h in air. Then, the heat-treated Ti foil was immersed in 30 mL NaOH aqueous solution (12 M), sealed, and heated at 160 °C for 6 h, followed by immersion in Ba(OH)₂ aqueous solution (0.02 M) at 210 °C for 7 h. After the reaction, the white film was carefully peeled from the surface of Ti foil and ground in an agate mortar for 30 min. Finally, the powders were ultrasound-dispersed in deionized water for 10 min and centrifuged at 4000 rpm for 3 min. To synthesize Ag-BTO nanofibers, the sample was dispersed in 50 mL AgNO₃ ethanol solution (0.02 and 0.05 M) under 100 rpm stirring (JoanLab MS5s, Huzhou, China) and 300 W Hg lamp (PerfectLight CHF-XM 300, Beijing, China) irradiation for 2 h, labeled as Ag-BTO-1 and Ag-BTO-2, respectively.

3.2. Characterization and Measurements

The morphology and microstructure were obtained by scanning electron microscope (SEM, Hitachi S-8100, Tokyo. Japan), transmission electron microscope (TEM, FEI Tecnai F20, Portland, OR, USA), X-ray powder diffractometer (XRD, Rigaku SmartLab, Tokyo, Japan), Raman spectrometer (HORIBA LabRAM Nano, Montpellier, France), and X-ray photoelectron spectroscope (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA). Conventional XRD patterns were measured with a sweeping rate of 10° min⁻¹. The refined XRD pattern of pure BTO nanofibers was tested in a slow rate of 0.5° min⁻¹ with a wide sweeping range from 5° to 120°, and Rietveld refinement is employed by TOPAS academic. The photocatalysis and piezo-photocatalysis were evaluated by degrading MO in a mechano-photo reaction apparatus (homemade). A 300 W Xe lamp (PerfectLight, Microsolar 300, Beijing, China) was used to directly provide the light source. UV and visible light were obtained by short-pass and long-pass filters, respectively. The ultrasound was generated by a 120 W ultrasonic vibrator (DongSen DS-120STS, Shenzhen, China) with a frequency of 24 kHz. During the catalytic process, 10 mg catalyst was used to degrade 50 mL MO solution (10 mg L⁻¹). The MO concentration was determined by absorption spectroscopy at 464 nm.

4. Conclusions

In summary, the piezo-photocatalytic performance of BaTiO₃ nanofibers is enhanced by loading with Ag nanoparticles. FEM simulation indicates that the visible-light absorbance of Ag nanoparticles exponentially increases with the distribution density. With the loading of Ag nanoparticles, the piezo-photocatalytic MO degradation rate increases from 30.8% to 50.8%, and further increases to 82.7% with doubling Ag density. The UV-driven and visible-driven catalytic performances were individually measured to study the contribution of Ag nanoparticles and BTO nanofibers, displaying that the visible-driven catalysis dominates the enhancement during the piezo-photocatalytic process. Finally, the piezo-photocatalytic mechanism of Ag-BTO was discussed. Under solar irradiation, electron-hole pairs are generated in BTO by UV light, separated by the piezoelectric field, and enhanced by the high-density Ag nanoparticles through PIRET. This work develops the investigation of plasmon-improved piezo-photocatalysis.

Author Contributions

Conceptualization, Y.F. and J.M.; methodology, Z.R. and P.L.; investigation, P.C. and X.L.; writing—original draft preparation, P.C. and Y.L.; writing—review and editing, Y.F. and J.W.; visualization, P.C. and W.L.; supervision, Y.F. and J.M.; project administration, Y.F. and J.M.; funding acquisition, J.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank Zhenyu Duan from Shiyanjia Lab (www.shiyanjia.com) for the helps on material characterizations.

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 and Table

Figure 1. The characterizations of the samples. (a,b) SEM images of BTO and Ag-BTO nanofibers. (c) TEM image of Ag-BTO nanofibers. (d) SAED pattern of Ag-BTO nanofibers. (e,f) HRTEM images of Ag-BTO-1 and Ag-BTO-2.

Figure 2. (a) XRD patterns, (b) Raman spectra, and (c) DRS spectra of the three samples.

Figure 3. (a) Rietveld refined XRD pattern of the BTO nanofibers. (b–d) The refined crystal lattice structures of (b) BaTiO3-Pm-3m, (c) BaTiO3-P4mm, and (d) BaCO3-Pmcn.

Figure 4. The XPS spectra of BTO and Ag-BTO. (a) Survey spectra. (b) C1s. (c) O 1s. (d) Ba 3d. (e) Ti 2p. (f) Ag 3d.

Figure 5. Catalytic performance comparison. (a–c) The piezocatalysis, photocatalysis, and piezo-photocatalysis of (a) BTO, (b) Ag-BTO-1, and (c) Ag-BTO-2. (d) The reaction kinetics.

Figure 6. The photocatalysis and piezo-photocatalysis under UV and visible light of (a) BTO and (b) Ag-BTO-2.

Figure 7. (a) XRD pattern and (b) TEM image of Ag-BTO-2 after piezo-photocatalysis.

Figure 8. The schematic diagrams of charge transfer paths of Ag–BTO. (a) Schottky contact of Ag–BTO. (b) UV excitation. (c) Visible light excitation. (d–f) The transfer paths of (d) charge separation, (e) hot-electron injection, and (f) PIRET.

Figure 9. The proposed mechanism of piezo-photocatalysis. (a,b) The FEM simulation of the LSPR electromagnetic field of (a) Ag-BTO-1 and (b) Ag-BTO-2. (c) The hot-electron injection process in photocatalysis. (d) The PIRET process in piezo-photocatalysis.

Table 1

Rietveld refined cell parameters of the synthesized BTO nanofibers.

Cell Parameters	Tetragonal BaTiO3	Cubic BaTiO3	BaCO3
Proportion	64.32%	32.20%	3.48%
Space Group	P4mm	Pm-3m	Pmcn
a (Å)	4.0150	4.0081	5.3130
b (Å)	4.0150	4.0081	8.9038
c (Å)	3.9984	4.0081	6.4361
α (°)	90	90	90
β (°)	90	90	90
γ (°)	90	90	90
Volume (Å3)	64.45	64.392	304.47

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