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Piezotronics enhanced photocatalytic activities of Ag-BaTiO3 plasmonic photocatalysts
Journal of Alloys and Compounds 801 (2019) 483e488
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Piezotronics enhanced photocatalytic activities of Ag-BaTiO3 plasmonic photocatalysts Shuya Xu a, b, Zhihong Liu a, Maolin Zhang a, Limin Guo b, c, * a
School of Advanced Materials and Nanotechnology, Xidian University, Xi'an, 710126, China Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, PR China c State Key Laboratory of Information Photonics and Optical Communications, School of Science, Beijing University of Posts and Telecommunications, Beijing, 100876, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2019 Received in revised form 2 June 2019 Accepted 9 June 2019 Available online 11 June 2019
Ag-BaTiO3 piezo-photocatalysts were fabricated by precipitating Ag nanoparticles on BaTiO3 nanopiezoelectric through a photochemical reducing approach. The mechanism of piezo-photocatalysis and the effect of the size and distribution of Ag nanoparticles on the properties of the photocatalyst were investigated. The surface plasmon resonance (SPR) of Ag nanoparticles endows the catalyst an absorption in the visible light region. The piezoelectric field originated from the deformation of BaTiO3 can further enhance the separation of the photogenerated carriers of SPR and promote the formation of oxidizing radicals that could accelerate the degradation of organic dyes. The 1mAg-BaTiO3 showed an excellent photocatalytic performance of degrading 83% Rh B in 75 min under full-spectrum light irradiation with ultrasonic excitation. The piezoelectric charges on the surfaces of the BaTiO3 and formed piezoelectric potential in the nanocrystal have been confirmed to express an increment of the catalytic activity more than 20% (compared with the sole photocatalysis). This work provides an effective technology for environment purification and could be extended to other piezoelectric materials. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ag/BaTiO3 Piezoelectric effect Photocatalysis Localized surface plasmon resonance
1. Introduction The piezoelectric effect induced by deformation of nano-scale piezoelectric materials has been discovered and researched for decades [1e4]. This discovery arouses the interest in harvesting low-frequency vibrational energies in surroundings and converted them into electricity as well as applied for micro-devices [5,6]. The related research findings have been widely applied in the designs of piezoelectric nanogenerators and self-powered nanosystems [7]. Recently, piezoelectric catalysis for environmental purification and sewage treatment has attracted much attention [8e16]. Moreover, this valuable process is always involved with photocatalysis [17,18] (e.g., photocatalytic organic degradation [19e21], H2 generation [22,23]). Hong et al. investigated the piezoelectric-chemical effect for water splitting and dye degradation under ultrasonic vibration of ZnO microfibers and BaTiO3 microdendrites [19,24]. Subsequently, ultrasonic was proposed as an effective excitation source to
* Corresponding author. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, PR China. E-mail address: [email protected] (L. Guo). https://doi.org/10.1016/j.jallcom.2019.06.115 0925-8388/© 2019 Elsevier B.V. All rights reserved.
induce the piezoelectric field into the catalytic process [15,25]. Where, piezoelectric potential drives a stronger separation of photogenerated electron-hole pairs and suppresses the carrier recombination. Various materials (e.g., ZnO [26,27], ZnSnO3 [28,29], SrBi4Ti4O15 [9,30], Bi4Ti3O12 [31,32] and BaTiO3 [33]) have been studied as bifunctional agents for both piezoelectronic effect and photocatalytic activity [34,35]. Among them, tetragonal-phase BaTiO3 is a typical dielectric material with excellent chemical and structure stability, and widely used in Multi-layer Ceramic Capacitors or piezoelectric resonators [36,37]. An indirect approach of combining piezoelectric BaTiO3 with semiconducting Ag2O is demonstrated by Li et al. [38]. Where, an enhanced charge transporting property of semiconductor is obtained by the force of local electric field into BaTiO3. However, the work by integrating piezoelectric materials with photoactive catalysts (denoted as piezophotocatalysis) is still inadequate. Plasmonic photocatalysis with characteristic localized surface plasmon resonance (LSPR) effect has recently attracted much attention as a promising technology for high-performance photocatalysis [39e42]. The LSPR derived from the optical response of the noble metal nanoparticles have facilitated the photocatalytic progress under solar light illumination [43e45]. For the LSPR media
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(mostly for noble metals), Ag is the most popular materials used for plasmonic photocatalysis because its low cost and wide optical response range of visible light [46e50]. Therefore, we propose to grow Ag nanoparticles on BaTiO3 nanocrystals to combine piezoelectricity and plasmonic photocatalysis in a bifunctional agent. Thus, it is highly hoped that the Ag-BaTiO3 pizeo-plasmonic photocatalyst could deliver a high piezo-photocatalytic activity. In this article, we fabricate the Ag-BaTiO3 heterostructure materials for degrading of Rh B. The effects of microtopography, concentration of Ag, optical properties and degradation mechanism were discussed. The optical sample (1mAg-BaTiO3) showed an excellent photocatalytic performance for degrading 82% Rhodamine B (Rh B) in 75 min under a full-spectrum light irradiation with ultrasonic excitation. When taking the BaTiO3 as nano-piezoelectric force and the Ag nanoparticles as photo-harvestor and reactive sites, a ‘fast lane’ for the electrons-hole pairs separation and transmission was achieved in this synergistic reaction. This work offers a feasible technology of hybridizing piezoelectric materials with photocatalyst for the environment improvement and other electrochemical applications, especially for the large scaled production. 2. Experimental section 2.1. Preparation of Ag-BaTiO3 heterostructures BaTiO3 nanocubes are synthesized by a hydrothermal method at 200 C for 48 h. Typically, 0.5 mmol Ba(OH)2, 0.5 mmol C16H36O4Ti, and 2 ml NaOH are mixed together dissolved in 25 ml distilled water. 2 mL 1 M NaOH solution was added to adjustment the pH value to 13. Then, the solution was intensely mixed for 30 min and transferred to a 30 mL Teflon-lined stainless-steel autoclave. The autoclave was put into an oven with a temperature of 200 C and kept for 24 hours. After the reaction, the samples were washed with deionized water and ethanol for three times, and BaTiO3 powders were obtained after drying. Ag nanoparticles were deposited on the surface of BaTiO3 powder using a photo-reduction reaction. 0.5 g BaTiO3 was put into a quartz beaker which loading with 25 mL AgNO3 solution (0.01 M). The beaker was placed under a UV-light illumination and irradiated for different time intervals under a constant stirring. The powder was then washed and separated by a centrifuge and dried at room temperature. 2.2. Material characterization The crystal structures were characterized by an X-Ray (Panalytical Xpert3 Powder) using Cu Ka radiation source (l ¼ 0.15406 nm). Scanning electron microscopy (SEM) studies were carried out on Nova NanoSEM 450 for the morphological characterization. The Transmission electron microscopy morphology of the samples was analyzed using FEI TECNAI F20, meanwhile, the EDS spectra were taken on the chosen area for an elemental analysis. X-Ray photoelectron spectroscopy (XPS) data were obtained using a PHI-5400 electron spectrometer. Raman spectra were recorded using a Renishaw In Via Microscope set-up with excitation wavelength of 633 nm. The photocurrent was measured using an electrochemical system (CHI 760E) with a conventional three-electrode cell. UVevisible absorption spectra were performed on an UVevis spectrophotometer (UV-3600, Shimadzu, Japan). 2.3. Photoelectrocatalytic experiments The photocatalytic activity of the catalysts was evaluated by the decolorization of Rh B in aqueous solutions. The excitation source is a 300 W Xe lamp. 50 mg of the as-prepared photocatalyst was
mixed with Rh B (50 mL, 0.01 mM) and then stirred in the dark for 30 min, to reach a complete adsorptionedesorption equilibrium. Afterwards, the suspension was exposed to full-spectrum light irradiation with maximum illumination time up to 75 min. During the irradiation, the suspension was magnetically stirred or driven by ultrasonic waves. The reaction temperature was kept at room temperature. At fixed intervals of 15 min, about 4 mL aliquots were sampled, centrifuged and filtered. The optical absorption intensity of Rh B at 554 nm was measured using an UV-vis spectrophotometer to reflect the dye concentration. 3. Results and discussion 3.1. Morphological and structural characterization A typical synthetic route for xAg-BaTiO3 photocatalysts was shown in Fig. S1. BaTiO3 were prepared by hydrothermal method, and Ag was grown on BaTiO3 by photochemical reduction method under different reaction time. xAg-BaTiO3(x ¼ 1 m, 5 m, 10 m, 30 m, where x is the photo-reduction time (abbreviation of min)) with different size and distribution was successfully prepared. The microscopic morphology of Ag-BaTiO3 is shown in Fig. 1 and Fig. S2. The BaTiO3 nano-crystals show a cuboidal shape with the size of ~100 nm for all samples. Ag nanoparticles are loaded uniformly on the surface of BaTiO3. These two counterparts contact closely with each other that may provide a favorable channel for the transfer of hot electrons from the excited Ag to the BaTiO3. In order to study the effect of Ag size on the photocatalytic performance, four groups of xAg-BaTiO3 heterostructures were obtained by controlling the reaction time during preparation. It is obvious that the size of Ag nanoparticles increases gradually with the photo-reduction time. Meanwhile, the amount of Ag nanoparticles is decreased with the reaction time which may be caused by the falling off of the bigger Ag attachments (Fig. S2). More proofs can be seen in TEM images. Ag nanoparticles with a size of 6 nm are obtained when the reduction time was 1 min (1mAg-BaTiO3) and homogeneously loaded onto BaTiO3 nanocrystals (Fig. 1b). The high resolution TEM image (Fig. 1c) shows the detailed structure of the 1mAg-BaTiO3. Both BaTiO3 and Ag are highly crystallized. The lattice fringes are 0.238 nm and 0.405 nm, respectively, corresponding to (111) plane of Ag and (100) plane of tetragonal BaTiO3. Moreover, the EDS mapping spectrum analysis (Fig. 1dei) of 1mAg-BaTiO3 shows the uniform distribution of elements, such as Ag, O, Ti, Ba, etc, in these Ag-BaTiO3 heterostructures. The content of Ag is about 10 wt% from EDS analysis in the sample 1mAg-BaTiO3 (Fig. S3). The surface chemistry and valence states of the samples are studied by high resolution XPS. As it can be seen in Fig. 2a, AgBaTiO3 consists of Ag, Ba, Ti, O and C, of which C is due to a small amount of carbonaceous compounds in the system of the instrument. The high-resolution XPS spectrum of Ag (Fig. 2b) in the 1mAg-BaTiO3 shows that the centroids of the Ag 3d3/2 and Ag 3d5/2 peaks are located at 374.2 eV and 367.9 eV, respectively, indicating that Ag exists in a metallic state in the heterostructures [28]. Metallic Ag is potentially significant for the collective oscillation of surface electrons with incident light and enhanced electromagnetic field. The binding energies of Ba 3d3/2 and Ba 3d5/2 are located at 796.82 eV and 778.43 eV, respectively (Fig. S4) [51]. Ti 2p3/2 and Ti 2p1/2 are derived from the quadrivalent Ti in the BaTiO3 (Fig. S4) [52]. Fig. 2c shows the XRD patterns of the tetragonal phase BaTiO3 and the Ag-BaTiO3 sample. All samples are consistent with the PDF standard card of BaTiO3, and no peaks assigned to Ag are detected even after the photo chemical precipitation. It is due to the tiny amount of Ag loading in the samples. Two divided peaks at 44.8 and 45.4 (Fig. S5), corresponding to the (002) and (200) facets of BaTiO3, implied that the tetragonal structure of BaTiO3 was
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Fig. 1. (a) SEM image of 1mAg-BaTiO3, (b) TEM image of 1mAg-BaTiO3, (c) HRTEM image of 1mAg-BaTiO3 (d-i) STEM elemental mapping of 1mAg-BaTiO3.
Fig. 2. (a) XPS survey for Ag-BaTiO3; (b) XPS spectra of Ag 3d in Ag-BaTiO3; (c) XRD patterns of the xAg-BaTiO3; (d) UV-vis of xAg-BaTiO3; (e) the PL spectra of xAg-BaTiO3; (f) Photocurrent response of the xAg-BaTiO3 hybrids under full-spectrum light irradiation.
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synthesized. The bigger c/a ratio of tetragonal BaTiO3 and excellent piezoelectricity are more suitable as piezoelectric substrate and piezo-photocatalysis [18]. 3.2. Optical properties The optical properties of Ag-BaTiO3 with different Ag particle size were studied. The UV-vis absorption spectra of the samples are shown in Fig. 2d. Pure BaTiO3 has no absorbance in the visible light region, while the absorption edge is located around 355 nm, which corresponds to the band gap of about 3.31 eV. Compared with pure BaTiO3, the light absorption intensity of the Ag-BaTiO3 heterostructures is significantly enhanced in the visible light ranged from 400 to 700 nm. This phenomenon is attributed to the plasmonic absorption of Ag metals by surface plasmon resonance (SPR), thus arouse the utilization of visible light in the Ag-BaTiO3 nanopiezoelectric system. In addition, the light absorption cross section was significantly reduced along with photo-reduction time and disordered distribution of Ag particles. The falling off of Ag decoration and decreased active sites should be the principal causes. The photoluminescence (PL) spectra of Ag-BaTiO3 at 325 nm excitation wavelength are measured to clarify the improved charge separation of Ag-BaTiO3 heterostructures during the photocatalytic activity (Fig. 2e). Usually, stronger PL peaks represent a higher possibility of recombination between electrons and holes. Compared with other samples, the intensity of PL peak for 1mAg-BaTiO3 heterostructure is much lower, which reveals a lower recombination rate of photogenerated electron-hole pairs. Due to the long lifetime of photogenerated carriers, more photogenerated carriers will be captured to form active free radicals (OH or O2-), which will enhance the ability to oxidize Rh B and significantly improve photocatalytic efficiency. In addition, the conduction band of the BaTiO3 can act as an electron trap, thereby which improves the separation of the photogenerated carrier, thus enhancing the photocatalytic activity. The time-resolved photocurrents of the Ag-BaTiO3 are measured under intermittent irradiation of full spectrum light (Fig. 2e). Generally, the higher photocurrent response always means the higher carrier density and the higher charge transfer efficiency which is beneficial to the enhancement of photocatalytic performance. The photocurrent density of the 1mAg-BaTiO3 is higher than that of other samples, resulting from the superior electron separation and transfer properties in Ag-BaTiO3 heterostructure. This is consistent with the PL spectra and photocatalytic performance. 3.3. Piezo-photocatalytic performance To demonstrate the piezotronic effect on photocatalytic activity of Ag-BaTiO3 heterostructures, we studied the degradation of Rh B under the full-spectrum light irradiation, piezoelectric excitation and both of these two excitation. In the experiment, ultrasound was used as the excitation source to induce piezoelectric filed. When ultrasonic stimulation was applied alone (Fig. 3a) under dark condition, the degradation efficiency of all samples was very slow. The degradation rate of all samples were less than 15%. It indicates that the polarization charges generated by the built-in electric field in the BaTiO3 crystal is limited in the crystals and could not be involved into the catalytic reaction. Milewhile, the comparison of the piezo-catalysis activity of pure BaTiO3 and Ag/ BaTiO3 is determined in Fig. 3a, where it shows a limit enhancement after the decoration of Ag nanoparticles on BaTiO3 (from 10% to 15%). The photodegradation of all the samples was evaluated under full spectrum irradiation (Fig. 3b), BaTiO3 still showed the slowest activity, which means that the photocatalytic effect of BaTiO3 is limited due to the wide band gap. After modified by Ag, the photocatalytic ability of Ag-BaTiO3 is significantly improved on
account of the SPR effect. The 1mAg-BaTiO3 acts out the best photocatalytic activity that induces a degradation rate of approximately 61% in 75min. So it can proposed that the plamonic effect and SPR ceased hole-electron pairs are the crucial factors in formation of OH and oxidation of Rh B. Moreover, the appropriate size and distribution of Ag modifiers play a positive role in the photocatalysis, for example, the 5mAg-BaTiO3, 10mAg-BaTiO3 and 30mAg-BaTiO3 show a slight deterioration with respect to the 1mAg-BaTiO3. By integrating the results of absorption spectra and SEM characterization, it is concluded that the optimized photocatalysis are obtained on a suitable Ag containing due to the balance of plasmonic excitation and hole-electron recombination (isolated dispersion of Ag particles onto BaTiO3 are proven to be advanced in suppressing hole-electron recombination) [53]. When the piezoelectric excitation and photo-driving are utilized together in the catalytic process, Rh B degradation is enhanced distinctly. Similarly, the 1mAg-BaTiO3 heterostructure also has the best performance and reaches 83% degradation of Rh B after 75 min (Fig. 3c). The degradation rate under both excitation increased 22% compared with that of stimulated by full spectrum light only. All above results illuminated that the piezotronic effect can enhance the photocatalytic performance by the piezo-potential of AgBaTiO3 heterostructures. Fig. 3d summarized the Rh B degradation rate for all samples, compared with the case of single ultrasonic stimulation, catalytic enhancement is obvious among the light excitation. So, it comes to the fact that photo-excitation and photogenerated carriers are essential to the Rh B oxidation. Surface holes from photocatalytic process can transfer to the adjacent solution and form free hydroxyl groups (OH) or superoxide radicals (O2-) and thus oxidize the organic compounds, resulting in photocatalytic efficiency [54,55]. Meanwhile, a further degradation elevation is observed for each sample by the participation of ultrasonic vibration, meaning that the piezoelectric field plays an auxiliary role to greatly improve separation of electron hole pairs and promote free radical generation. It further confirms that the greatly improved photocatalytic performance of Ag-BaTiO3 is attributed to the synergistic effect of the piezoelectric fields in the BaTiO3 nano-piezoelectric crystals and the surface plasmon resonance of Ag nanoparticles.
3.4. Photocatalytic mechanism analysis The mechanism of photogenerated charge carrier separation and transfer in the Ag-BaTiO3 heterostructures are illuminated in Fig. 4. For the natural contacted Ag and BaTiO3, the charge carriers are redistributed in the interface between Ag and BaTiO3 due to the different Fermi level (electrons transfer from BaTiO3 to Ag). Therefore, a band bending and Schottky barrier in formed at the Ag-BaTiO3 interface, it constructs an internal field E pointing from BaTiO3 to Ag (Fig. 4a). When Ag-BaTiO3 is illuminated by full-spectrum light, BaTiO3 absorbs UV light and generates photo-charge carriers (conduction band and valence band of BaTiO3 as well as the Fermi level of Ag are labeled in Fig. 4). The electrons and holes are then captured by the acceptor and donor in solution to initiate the redox reactions. On the other hand, the Ag nanoparticles response to the incident visible light by LSPR and create more exciting electrons and holes. The excited electrons in Ag have sufficient energy to go across the natural Schottky barrier and fed into the conduction band of BaTiO3 (Fig. 4b). Consequently, more oxiditive source on surface of Ag can get involved into redox reactions. Finally, when the BaTiO3 nanocrystals are placed in a compressive stress induced by ultrasonic waves, the polarization potential on the surface of BaTiO3 will move more photoinduced charge carriers toward Ag to BaTiO3, that can continuously strengthen the degradation process.
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Fig. 3. Degradation of Rh B under (a) ultrasonic excitation, (b) full spectrum light irradiation, (c) both piezoelectric and full spectrum light irradiation, (d) piezo-catalytic, piezophotocatalytic, and photocatalytic degradation of Rh B in the presence of xAg-BaTiO3 and BaTiO3 for 75 min.
Fig. 4. Schematic illustration of charge transfer mechanism in photocatalytic and piezo-photocatalysis process of Ag-BaTiO3.
4. Conclusion
Appendix A. Supplementary data
In this paper, a novel piezo-photocatalyst Ag-BaTiO3 was successfully prepared. The coupling of piezoelectric effect with the plasmonic photocatalytic process can not only broaden the photo absorption range of the catalyst, but also enhance the separation efficiency of photogenerated charge carriers. A significant piezoelectric photocatalysis is achieved. Among them, the degradation rate of 83% is obtained on the 1mAg-BaTiO3 heterostructures (within 75 minutes). The degradation efficiency increased 22% by the addition of piezoelectric effect. The results show that piezoelectric photocatalysis is a novel and effective technology for sewage treatment.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.115.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51802021), The basic research Fund of BUPT (2019RC22). The fund of Education and Teaching Reform Project of BUPT (2019Y015).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Piezotronics enhanced photocatalytic activities of Ag-BaTiO3 plasmonic photocatalysts Shuya Xu a, b, Zhihong Liu a, Maolin Zhang a, Limin Guo b, c, * a
School of Advanced Materials and Nanotechnology, Xidian University, Xi'an, 710126, China Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, PR China c State Key Laboratory of Information Photonics and Optical Communications, School of Science, Beijing University of Posts and Telecommunications, Beijing, 100876, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2019 Received in revised form 2 June 2019 Accepted 9 June 2019 Available online 11 June 2019
Ag-BaTiO3 piezo-photocatalysts were fabricated by precipitating Ag nanoparticles on BaTiO3 nanopiezoelectric through a photochemical reducing approach. The mechanism of piezo-photocatalysis and the effect of the size and distribution of Ag nanoparticles on the properties of the photocatalyst were investigated. The surface plasmon resonance (SPR) of Ag nanoparticles endows the catalyst an absorption in the visible light region. The piezoelectric field originated from the deformation of BaTiO3 can further enhance the separation of the photogenerated carriers of SPR and promote the formation of oxidizing radicals that could accelerate the degradation of organic dyes. The 1mAg-BaTiO3 showed an excellent photocatalytic performance of degrading 83% Rh B in 75 min under full-spectrum light irradiation with ultrasonic excitation. The piezoelectric charges on the surfaces of the BaTiO3 and formed piezoelectric potential in the nanocrystal have been confirmed to express an increment of the catalytic activity more than 20% (compared with the sole photocatalysis). This work provides an effective technology for environment purification and could be extended to other piezoelectric materials. © 2019 Elsevier B.V. All rights reserved.
Keywords: Ag/BaTiO3 Piezoelectric effect Photocatalysis Localized surface plasmon resonance
1. Introduction The piezoelectric effect induced by deformation of nano-scale piezoelectric materials has been discovered and researched for decades [1e4]. This discovery arouses the interest in harvesting low-frequency vibrational energies in surroundings and converted them into electricity as well as applied for micro-devices [5,6]. The related research findings have been widely applied in the designs of piezoelectric nanogenerators and self-powered nanosystems [7]. Recently, piezoelectric catalysis for environmental purification and sewage treatment has attracted much attention [8e16]. Moreover, this valuable process is always involved with photocatalysis [17,18] (e.g., photocatalytic organic degradation [19e21], H2 generation [22,23]). Hong et al. investigated the piezoelectric-chemical effect for water splitting and dye degradation under ultrasonic vibration of ZnO microfibers and BaTiO3 microdendrites [19,24]. Subsequently, ultrasonic was proposed as an effective excitation source to
* Corresponding author. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, PR China. E-mail address: [email protected] (L. Guo). https://doi.org/10.1016/j.jallcom.2019.06.115 0925-8388/© 2019 Elsevier B.V. All rights reserved.
induce the piezoelectric field into the catalytic process [15,25]. Where, piezoelectric potential drives a stronger separation of photogenerated electron-hole pairs and suppresses the carrier recombination. Various materials (e.g., ZnO [26,27], ZnSnO3 [28,29], SrBi4Ti4O15 [9,30], Bi4Ti3O12 [31,32] and BaTiO3 [33]) have been studied as bifunctional agents for both piezoelectronic effect and photocatalytic activity [34,35]. Among them, tetragonal-phase BaTiO3 is a typical dielectric material with excellent chemical and structure stability, and widely used in Multi-layer Ceramic Capacitors or piezoelectric resonators [36,37]. An indirect approach of combining piezoelectric BaTiO3 with semiconducting Ag2O is demonstrated by Li et al. [38]. Where, an enhanced charge transporting property of semiconductor is obtained by the force of local electric field into BaTiO3. However, the work by integrating piezoelectric materials with photoactive catalysts (denoted as piezophotocatalysis) is still inadequate. Plasmonic photocatalysis with characteristic localized surface plasmon resonance (LSPR) effect has recently attracted much attention as a promising technology for high-performance photocatalysis [39e42]. The LSPR derived from the optical response of the noble metal nanoparticles have facilitated the photocatalytic progress under solar light illumination [43e45]. For the LSPR media
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(mostly for noble metals), Ag is the most popular materials used for plasmonic photocatalysis because its low cost and wide optical response range of visible light [46e50]. Therefore, we propose to grow Ag nanoparticles on BaTiO3 nanocrystals to combine piezoelectricity and plasmonic photocatalysis in a bifunctional agent. Thus, it is highly hoped that the Ag-BaTiO3 pizeo-plasmonic photocatalyst could deliver a high piezo-photocatalytic activity. In this article, we fabricate the Ag-BaTiO3 heterostructure materials for degrading of Rh B. The effects of microtopography, concentration of Ag, optical properties and degradation mechanism were discussed. The optical sample (1mAg-BaTiO3) showed an excellent photocatalytic performance for degrading 82% Rhodamine B (Rh B) in 75 min under a full-spectrum light irradiation with ultrasonic excitation. When taking the BaTiO3 as nano-piezoelectric force and the Ag nanoparticles as photo-harvestor and reactive sites, a ‘fast lane’ for the electrons-hole pairs separation and transmission was achieved in this synergistic reaction. This work offers a feasible technology of hybridizing piezoelectric materials with photocatalyst for the environment improvement and other electrochemical applications, especially for the large scaled production. 2. Experimental section 2.1. Preparation of Ag-BaTiO3 heterostructures BaTiO3 nanocubes are synthesized by a hydrothermal method at 200 C for 48 h. Typically, 0.5 mmol Ba(OH)2, 0.5 mmol C16H36O4Ti, and 2 ml NaOH are mixed together dissolved in 25 ml distilled water. 2 mL 1 M NaOH solution was added to adjustment the pH value to 13. Then, the solution was intensely mixed for 30 min and transferred to a 30 mL Teflon-lined stainless-steel autoclave. The autoclave was put into an oven with a temperature of 200 C and kept for 24 hours. After the reaction, the samples were washed with deionized water and ethanol for three times, and BaTiO3 powders were obtained after drying. Ag nanoparticles were deposited on the surface of BaTiO3 powder using a photo-reduction reaction. 0.5 g BaTiO3 was put into a quartz beaker which loading with 25 mL AgNO3 solution (0.01 M). The beaker was placed under a UV-light illumination and irradiated for different time intervals under a constant stirring. The powder was then washed and separated by a centrifuge and dried at room temperature. 2.2. Material characterization The crystal structures were characterized by an X-Ray (Panalytical Xpert3 Powder) using Cu Ka radiation source (l ¼ 0.15406 nm). Scanning electron microscopy (SEM) studies were carried out on Nova NanoSEM 450 for the morphological characterization. The Transmission electron microscopy morphology of the samples was analyzed using FEI TECNAI F20, meanwhile, the EDS spectra were taken on the chosen area for an elemental analysis. X-Ray photoelectron spectroscopy (XPS) data were obtained using a PHI-5400 electron spectrometer. Raman spectra were recorded using a Renishaw In Via Microscope set-up with excitation wavelength of 633 nm. The photocurrent was measured using an electrochemical system (CHI 760E) with a conventional three-electrode cell. UVevisible absorption spectra were performed on an UVevis spectrophotometer (UV-3600, Shimadzu, Japan). 2.3. Photoelectrocatalytic experiments The photocatalytic activity of the catalysts was evaluated by the decolorization of Rh B in aqueous solutions. The excitation source is a 300 W Xe lamp. 50 mg of the as-prepared photocatalyst was
mixed with Rh B (50 mL, 0.01 mM) and then stirred in the dark for 30 min, to reach a complete adsorptionedesorption equilibrium. Afterwards, the suspension was exposed to full-spectrum light irradiation with maximum illumination time up to 75 min. During the irradiation, the suspension was magnetically stirred or driven by ultrasonic waves. The reaction temperature was kept at room temperature. At fixed intervals of 15 min, about 4 mL aliquots were sampled, centrifuged and filtered. The optical absorption intensity of Rh B at 554 nm was measured using an UV-vis spectrophotometer to reflect the dye concentration. 3. Results and discussion 3.1. Morphological and structural characterization A typical synthetic route for xAg-BaTiO3 photocatalysts was shown in Fig. S1. BaTiO3 were prepared by hydrothermal method, and Ag was grown on BaTiO3 by photochemical reduction method under different reaction time. xAg-BaTiO3(x ¼ 1 m, 5 m, 10 m, 30 m, where x is the photo-reduction time (abbreviation of min)) with different size and distribution was successfully prepared. The microscopic morphology of Ag-BaTiO3 is shown in Fig. 1 and Fig. S2. The BaTiO3 nano-crystals show a cuboidal shape with the size of ~100 nm for all samples. Ag nanoparticles are loaded uniformly on the surface of BaTiO3. These two counterparts contact closely with each other that may provide a favorable channel for the transfer of hot electrons from the excited Ag to the BaTiO3. In order to study the effect of Ag size on the photocatalytic performance, four groups of xAg-BaTiO3 heterostructures were obtained by controlling the reaction time during preparation. It is obvious that the size of Ag nanoparticles increases gradually with the photo-reduction time. Meanwhile, the amount of Ag nanoparticles is decreased with the reaction time which may be caused by the falling off of the bigger Ag attachments (Fig. S2). More proofs can be seen in TEM images. Ag nanoparticles with a size of 6 nm are obtained when the reduction time was 1 min (1mAg-BaTiO3) and homogeneously loaded onto BaTiO3 nanocrystals (Fig. 1b). The high resolution TEM image (Fig. 1c) shows the detailed structure of the 1mAg-BaTiO3. Both BaTiO3 and Ag are highly crystallized. The lattice fringes are 0.238 nm and 0.405 nm, respectively, corresponding to (111) plane of Ag and (100) plane of tetragonal BaTiO3. Moreover, the EDS mapping spectrum analysis (Fig. 1dei) of 1mAg-BaTiO3 shows the uniform distribution of elements, such as Ag, O, Ti, Ba, etc, in these Ag-BaTiO3 heterostructures. The content of Ag is about 10 wt% from EDS analysis in the sample 1mAg-BaTiO3 (Fig. S3). The surface chemistry and valence states of the samples are studied by high resolution XPS. As it can be seen in Fig. 2a, AgBaTiO3 consists of Ag, Ba, Ti, O and C, of which C is due to a small amount of carbonaceous compounds in the system of the instrument. The high-resolution XPS spectrum of Ag (Fig. 2b) in the 1mAg-BaTiO3 shows that the centroids of the Ag 3d3/2 and Ag 3d5/2 peaks are located at 374.2 eV and 367.9 eV, respectively, indicating that Ag exists in a metallic state in the heterostructures [28]. Metallic Ag is potentially significant for the collective oscillation of surface electrons with incident light and enhanced electromagnetic field. The binding energies of Ba 3d3/2 and Ba 3d5/2 are located at 796.82 eV and 778.43 eV, respectively (Fig. S4) [51]. Ti 2p3/2 and Ti 2p1/2 are derived from the quadrivalent Ti in the BaTiO3 (Fig. S4) [52]. Fig. 2c shows the XRD patterns of the tetragonal phase BaTiO3 and the Ag-BaTiO3 sample. All samples are consistent with the PDF standard card of BaTiO3, and no peaks assigned to Ag are detected even after the photo chemical precipitation. It is due to the tiny amount of Ag loading in the samples. Two divided peaks at 44.8 and 45.4 (Fig. S5), corresponding to the (002) and (200) facets of BaTiO3, implied that the tetragonal structure of BaTiO3 was
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Fig. 1. (a) SEM image of 1mAg-BaTiO3, (b) TEM image of 1mAg-BaTiO3, (c) HRTEM image of 1mAg-BaTiO3 (d-i) STEM elemental mapping of 1mAg-BaTiO3.
Fig. 2. (a) XPS survey for Ag-BaTiO3; (b) XPS spectra of Ag 3d in Ag-BaTiO3; (c) XRD patterns of the xAg-BaTiO3; (d) UV-vis of xAg-BaTiO3; (e) the PL spectra of xAg-BaTiO3; (f) Photocurrent response of the xAg-BaTiO3 hybrids under full-spectrum light irradiation.
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synthesized. The bigger c/a ratio of tetragonal BaTiO3 and excellent piezoelectricity are more suitable as piezoelectric substrate and piezo-photocatalysis [18]. 3.2. Optical properties The optical properties of Ag-BaTiO3 with different Ag particle size were studied. The UV-vis absorption spectra of the samples are shown in Fig. 2d. Pure BaTiO3 has no absorbance in the visible light region, while the absorption edge is located around 355 nm, which corresponds to the band gap of about 3.31 eV. Compared with pure BaTiO3, the light absorption intensity of the Ag-BaTiO3 heterostructures is significantly enhanced in the visible light ranged from 400 to 700 nm. This phenomenon is attributed to the plasmonic absorption of Ag metals by surface plasmon resonance (SPR), thus arouse the utilization of visible light in the Ag-BaTiO3 nanopiezoelectric system. In addition, the light absorption cross section was significantly reduced along with photo-reduction time and disordered distribution of Ag particles. The falling off of Ag decoration and decreased active sites should be the principal causes. The photoluminescence (PL) spectra of Ag-BaTiO3 at 325 nm excitation wavelength are measured to clarify the improved charge separation of Ag-BaTiO3 heterostructures during the photocatalytic activity (Fig. 2e). Usually, stronger PL peaks represent a higher possibility of recombination between electrons and holes. Compared with other samples, the intensity of PL peak for 1mAg-BaTiO3 heterostructure is much lower, which reveals a lower recombination rate of photogenerated electron-hole pairs. Due to the long lifetime of photogenerated carriers, more photogenerated carriers will be captured to form active free radicals (OH or O2-), which will enhance the ability to oxidize Rh B and significantly improve photocatalytic efficiency. In addition, the conduction band of the BaTiO3 can act as an electron trap, thereby which improves the separation of the photogenerated carrier, thus enhancing the photocatalytic activity. The time-resolved photocurrents of the Ag-BaTiO3 are measured under intermittent irradiation of full spectrum light (Fig. 2e). Generally, the higher photocurrent response always means the higher carrier density and the higher charge transfer efficiency which is beneficial to the enhancement of photocatalytic performance. The photocurrent density of the 1mAg-BaTiO3 is higher than that of other samples, resulting from the superior electron separation and transfer properties in Ag-BaTiO3 heterostructure. This is consistent with the PL spectra and photocatalytic performance. 3.3. Piezo-photocatalytic performance To demonstrate the piezotronic effect on photocatalytic activity of Ag-BaTiO3 heterostructures, we studied the degradation of Rh B under the full-spectrum light irradiation, piezoelectric excitation and both of these two excitation. In the experiment, ultrasound was used as the excitation source to induce piezoelectric filed. When ultrasonic stimulation was applied alone (Fig. 3a) under dark condition, the degradation efficiency of all samples was very slow. The degradation rate of all samples were less than 15%. It indicates that the polarization charges generated by the built-in electric field in the BaTiO3 crystal is limited in the crystals and could not be involved into the catalytic reaction. Milewhile, the comparison of the piezo-catalysis activity of pure BaTiO3 and Ag/ BaTiO3 is determined in Fig. 3a, where it shows a limit enhancement after the decoration of Ag nanoparticles on BaTiO3 (from 10% to 15%). The photodegradation of all the samples was evaluated under full spectrum irradiation (Fig. 3b), BaTiO3 still showed the slowest activity, which means that the photocatalytic effect of BaTiO3 is limited due to the wide band gap. After modified by Ag, the photocatalytic ability of Ag-BaTiO3 is significantly improved on
account of the SPR effect. The 1mAg-BaTiO3 acts out the best photocatalytic activity that induces a degradation rate of approximately 61% in 75min. So it can proposed that the plamonic effect and SPR ceased hole-electron pairs are the crucial factors in formation of OH and oxidation of Rh B. Moreover, the appropriate size and distribution of Ag modifiers play a positive role in the photocatalysis, for example, the 5mAg-BaTiO3, 10mAg-BaTiO3 and 30mAg-BaTiO3 show a slight deterioration with respect to the 1mAg-BaTiO3. By integrating the results of absorption spectra and SEM characterization, it is concluded that the optimized photocatalysis are obtained on a suitable Ag containing due to the balance of plasmonic excitation and hole-electron recombination (isolated dispersion of Ag particles onto BaTiO3 are proven to be advanced in suppressing hole-electron recombination) [53]. When the piezoelectric excitation and photo-driving are utilized together in the catalytic process, Rh B degradation is enhanced distinctly. Similarly, the 1mAg-BaTiO3 heterostructure also has the best performance and reaches 83% degradation of Rh B after 75 min (Fig. 3c). The degradation rate under both excitation increased 22% compared with that of stimulated by full spectrum light only. All above results illuminated that the piezotronic effect can enhance the photocatalytic performance by the piezo-potential of AgBaTiO3 heterostructures. Fig. 3d summarized the Rh B degradation rate for all samples, compared with the case of single ultrasonic stimulation, catalytic enhancement is obvious among the light excitation. So, it comes to the fact that photo-excitation and photogenerated carriers are essential to the Rh B oxidation. Surface holes from photocatalytic process can transfer to the adjacent solution and form free hydroxyl groups (OH) or superoxide radicals (O2-) and thus oxidize the organic compounds, resulting in photocatalytic efficiency [54,55]. Meanwhile, a further degradation elevation is observed for each sample by the participation of ultrasonic vibration, meaning that the piezoelectric field plays an auxiliary role to greatly improve separation of electron hole pairs and promote free radical generation. It further confirms that the greatly improved photocatalytic performance of Ag-BaTiO3 is attributed to the synergistic effect of the piezoelectric fields in the BaTiO3 nano-piezoelectric crystals and the surface plasmon resonance of Ag nanoparticles.
3.4. Photocatalytic mechanism analysis The mechanism of photogenerated charge carrier separation and transfer in the Ag-BaTiO3 heterostructures are illuminated in Fig. 4. For the natural contacted Ag and BaTiO3, the charge carriers are redistributed in the interface between Ag and BaTiO3 due to the different Fermi level (electrons transfer from BaTiO3 to Ag). Therefore, a band bending and Schottky barrier in formed at the Ag-BaTiO3 interface, it constructs an internal field E pointing from BaTiO3 to Ag (Fig. 4a). When Ag-BaTiO3 is illuminated by full-spectrum light, BaTiO3 absorbs UV light and generates photo-charge carriers (conduction band and valence band of BaTiO3 as well as the Fermi level of Ag are labeled in Fig. 4). The electrons and holes are then captured by the acceptor and donor in solution to initiate the redox reactions. On the other hand, the Ag nanoparticles response to the incident visible light by LSPR and create more exciting electrons and holes. The excited electrons in Ag have sufficient energy to go across the natural Schottky barrier and fed into the conduction band of BaTiO3 (Fig. 4b). Consequently, more oxiditive source on surface of Ag can get involved into redox reactions. Finally, when the BaTiO3 nanocrystals are placed in a compressive stress induced by ultrasonic waves, the polarization potential on the surface of BaTiO3 will move more photoinduced charge carriers toward Ag to BaTiO3, that can continuously strengthen the degradation process.
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Fig. 3. Degradation of Rh B under (a) ultrasonic excitation, (b) full spectrum light irradiation, (c) both piezoelectric and full spectrum light irradiation, (d) piezo-catalytic, piezophotocatalytic, and photocatalytic degradation of Rh B in the presence of xAg-BaTiO3 and BaTiO3 for 75 min.
Fig. 4. Schematic illustration of charge transfer mechanism in photocatalytic and piezo-photocatalysis process of Ag-BaTiO3.
4. Conclusion
Appendix A. Supplementary data
In this paper, a novel piezo-photocatalyst Ag-BaTiO3 was successfully prepared. The coupling of piezoelectric effect with the plasmonic photocatalytic process can not only broaden the photo absorption range of the catalyst, but also enhance the separation efficiency of photogenerated charge carriers. A significant piezoelectric photocatalysis is achieved. Among them, the degradation rate of 83% is obtained on the 1mAg-BaTiO3 heterostructures (within 75 minutes). The degradation efficiency increased 22% by the addition of piezoelectric effect. The results show that piezoelectric photocatalysis is a novel and effective technology for sewage treatment.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.115.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51802021), The basic research Fund of BUPT (2019RC22). The fund of Education and Teaching Reform Project of BUPT (2019Y015).
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