Piezophotodegradation and piezophotoelectrochemical water splitting of hydrothermally grown BiFeO3 films with various morphologies

https://doi.org/10.1016/j.jece.2022.107213Get rights and content

Highlights

  • Hydrothermal fabrication of pure p-BiFeO3 films with various morphologies [microparticle (BFOMP), microplate (BFOMPL), and microsheet (BFOMS)] on FTO substrates.

  • Piezotronic and piezophototronic measurement: BFOMPL outperforming BFOMP and BFOMS.

  • Electrochemical and PL analyses: BFOMPL possessing the highest electrochemical surface area per unit mass (22.6 mF cm−2 mg−1) and the weakest PL emission.

  • BFOMPL exhibiting excellent piezophotodegradation performance (19.8 × 10–3 min−1), a piezophotoelectrochemical current density [− 0.83 mA cm−2 at −0.6 V (vs. Ag/AgCl)], and a maximum piezo-induced applied bias photon-to-current conversion efficiency (0.54% at −0.51 V)

  • Our samples: promising for application in piezoelectricity-related fields

Abstract

This paper reports the use of the hydrothermal method in the fabrication of pure BiFeO3 (BFO) films with various morphologies [microparticle (BFOMP), microplate (BFOMPL), and microsheet (BFOMS)] on fluorine-doped tin oxide substrates and their applications in piezophotodegradation and piezophotoelectrochemical water splitting. These samples exhibited p-type characteristics, band gaps of approximately 2.1 eV, and favorable crystal. Piezoresponse force microscopy revealed that the piezoelectric coefficients of BFOMP, BFOMPL, and BFOMS were 13.0, 18.6, and 15.3 pm V−1, respectively. The associated piezotronic and piezophototronic characteristics of the samples were verified through facile current–voltage measurement, which revealed that BFOMPL outperformed BFOMP and BFOMS. Electrochemical and photoluminescence (PL) analyses demonstrated that BFOMPL possessed the highest electrochemical surface area per-unit mass (22.6 mF cm−2 mg−1) and the weakest PL emission. The combination of these characteristics and the favorable energy band positions under stress of BFOMPL contributed to its excellent piezophotocatalytic performance, with a piezophotodegradation rate constant of approximately 19.8 × 10–3 min−1 for methylene blue solutions, a piezophotoelectrochemical current density of approximately − 0.83 mA·cm−2 at −0.6 V (vs. Ag/AgCl) and a maximum piezo-induced applied bias photon-to-current conversion efficiency of 0.54% at −0.51 V. Based on the enhanced piezoelectricity, which can effectively enhance the separation of photogenerated electron-hole pairs, thus improving photocatalyst performance. BFOMPL also exhibited good reusability and versatility toward degrading different organic pollutants. The results indicate that our samples are promising for application in environmental sustainability.

Introduction

Water pollution, especially caused by toxic, persistent organic dyes, is a serious environmental problem. Various efficient techniques for pollution remediation have been developed accordingly, including absorption methods, electrochemical treatments, precipitation approaches, and photocatalysis [1], [2], [3], among which solar irradiation-driven photocatalysis is particularly attractive owing to its cost-effectiveness, water splitting ability, and potentially high photodegradation efficiency. However, the rapid recombination of photoinduced electron−hole (e−h+) pairs in the bulk and on the surface of photocatalysts is one of crucial factors that deteriorate the photocatalytic performance [4], [5], [6]. Thus, increasing the lifetime of photogenerated carriers and creating high photocatalytic surface area are essential [7], [8], [9]. Studies on piezoelectric materials (e.g., ZnO [10], ZnSnO3 [11], NaNbO3 [12], MoS2 [13] and BaTiO3 [14], [15], [16] have investigated piezoelectricity-enhanced photocatalysis, also known as piezophotocatalysis. Specifically, the enhancement is attributable to the buildup of external stress-induced piezopotentials, leading to the excellent separation (low recombination) of photogenerated charges. For example, ZnO nanowires exhibited enhanced photodegradation of methylene blue (MB) under ultraviolet (UV) illumination and external periodic force [10]. Lin et al. [14] examined the piezophotocatalytic water splitting performance of BaTiO3 nanowires through ultrasonication. Liu et al. [15] reported the piezophotodegradation activity of BaTiO3 nanowires as being approximately 1.3 and 2.2 times that of piezocatalysis and photocatalysis, respectively. In another study, ZnSnO3 nanowire arrays exhibited excellent charge carrier separation through an ultrasonication-induced electric field [11]. A decrease in the Schottky barrier of Au/MoS2 interface promotes the separation of photogenerated carriers induced by the piezoelectric potential under ultrasonic treatment [13].

Although satisfactory photocatalytic results were achieved in these studies, the studied materials had a large band gap Eg, limiting their effective absorption in the visible light spectrum. Environmentally friendly and Pb-free BiFeO3 (BFO) is a promising material for multifunctional applications because of its unique characteristics, including high ferroelectric Currie (Tc ≈ 1103 K) and antiferromagnetism Néel (TN ≈ 643 K) temperatures, large remnant polarization (Pr ≈ 100 μC cm−2) along a [111] direction, and excellent dielectric properties [17], [18], [19]. Moreover, BFO possesses a direct and narrow Eg (≈2.1 eV) [20] and excellent chemical stability [21], which are favorable for photoexcitation-related applications [22], and photocatalysis [20]. Furthermore, another study observed a strong piezoelectric response in BFO films (d33 ≈ 22.2 pm·V−1) [23], which can be coupled with their optical properties to achieve piezophotocatalysis. BFO-nanosheet powders fabricated by You et al. [24] effectively piezodecomposed Rhodamine B (RhB) dyes over 50 min with an efficiency of up to approximately 94%. The powders also exhibited a strong piezocatalytic H2 harvesting rate of approximately 124 μmol g−1 under 1 h of irradiation and 100-W vibration at a resonant frequency. In another study, BFO nanosheets and nanowires degraded RhB in 1 h with an efficiency of approximately 71% and 97%, respectively, under UV−visible (UV–Vis) illumination and external stress [20].

Various solution-based methods have been developed to characterize BFO with various nanostructures, including sol−gel methods [22], [25], [26], chemical solution deposition (CSD) [27], coprecipitation [28], hydrothermal processes [23], and microwave-assisted hydrothermal synthesis [29]. For example, Xu et al. synthesized BFO nanoparticles on Pt/Ti/SiO2/Si, indium tin oxide (ITO), and fluorine-doped tin oxide (FTO) substrates by using a simple sol−gel spin coating method [25], with the Eg being approximately 2.0, 2.6, and 2.7 eV, respectively. By using the same approach, Wang et al. fabricated multilayered BFO films on FTO substrates, with Bi(NO3)3 and Fe(NO3)3 precursors dissolved in ethylene glycol solvents. The enhanced photovoltaic effect was also observed with an open-circuit voltage and short-circuit current of approximately 0.15 V and 4.58 mA·cm−2, respectively [26]. Gupta et al. employed CSD to fabricate BFO films codoped with Ce and Mn on ITO substrates [27]. The induced short-circuit photocurrent density Jph increased linearly from 33 to 241 µA·cm−2 ascribable to the ferroelectric photovoltaic effect with the incident light intensity increasing from 15 to 160 mW·cm−2. Through coprecipitation, Malathi et al. [28] synthesized BFO nanoparticles, which were integrated with hydrothermally grown BiOI nanoparticles. The photodegradation ability was then examined.

Among these manufacturing processes, hydrothermal synthesis possesses a strong advantage in that the phases, purity, crystallinity, composition, and morphology of resulting materials can be modified by adjusting the precursor type, temperatures, reaction time, pressure, complex agents, mineralizers, and pH values. Thus, this technique is ideal for synthesizing BFO [30], [31], [32], [33]. In one study, for example, BFO films were hydrothermally grown on (100) SrTiO3 substrates, and distinct mosaic-like islands measuring approximately 0.38 µm were observed. The leakage current density at an applied field of 50 kV·cm−1 was approximately 3.3 × 10−2 A cm−2, and the Pr and coercive field Ec were approximately 11.7 μC cm−2 and 27.1 kV cm−1, respectively [30]. Other studies have also reported the ferroelectric switching and high piezoelectricity of BFO films as well as their diode-like rectifying characteristics [17], [31], [32], [33]. For example, Guo et al. indicated the coexistence of 71° and 109° domain walls in hydrothermally grown microcrystalline BFO films on Nb-doped SrTiO3 substrates [17]. In our previous study, we observed the growth of BFO nanorods (Eg ≈ 2.3 eV) on ITO substrates as well as the piezophotodegradation of MB solutions under visible light irradiation [33].

Although researchers have noted the feasibility of the hydrothermal synthesis of BFO, high-density BFO films with tunable morphologies and excellent adhesion to transparent conducting oxide substrates and their advanced piezoelectricity-related applications have not been investigated. In this study, pure p-type BFO films were hydrothermally fabricated on FTO substrates through the manipulation of the following: (1) the seed layers of BFO coatings to enhance film–substrate adhesion and to adjust the crystallite sizes, densities, and surface areas of the resulting films; (2) Bi(OH)3 and Fe(OH)3 coprecipitates to prevent the formation of impurities; and (3) NaOH and KOH mineralizers and polyethylene glycol [PEG, molecular weight (MW): 200 K] to tune the film morphology. The piezoelectric response of the BFO films was theoretically and experimentally evaluated. The resulting piezotronic and piezophototronic characteristics were determined and extended to photocatalytic applications. The synergistic effect of the induced piezopotentials, energy band bending, large electrochemical surface areas (ECSAs), low recombination of photogenerated charges, and efficient mass transfer in solutions was confirmed to contribute to the excellent piezophotocatalytic performance of the samples.

Section snippets

Experimental

Prior to experimentation, FTO substrates (2.4 cm × 2.4 cm) were ultrasonically cleaned using ethanol, isopropyl alcohol, and deionized (DI) water in sequence for 10 min each to remove organic and inorganic contaminants. The cleaned substrates were then dried using N2 gas. To fabricate the seed layers of the BFO coatings on the FTO substrates, the sol solution was first prepared by dissolving 2.67 g of Bi(NO3)3.5 H2O and 2.02 g of Fe(NO3)3.9 H2O in the following solvents under stirring: 0.7 mL

Results and discussion

The primary function of the seed layer of the BFO coatings was to enhance the adhesion of the BFO films to FTO substrates, in addition to tailoring the crystallite size, densities, and surface areas of the BFO films. Furthermore, to minimize the formation of impurities (e.g., Bi25FeO40, Bi2O3, or Fe2O3) after the hydrothermal reaction, Bi(OH)3 and Fe(OH)3 coprecipitates were centrifugally collected and employed as hydrothermal precursors. The film morphology was modulated by NaOH and KOH

Conclusions

Pure BFO films with various morphologies [microparticles (BFOMP), microplates (BFOMPL), and microsheets (BFOMS)] were hydrothermally grown on FTO substrates by using Bi(OH)3 and Fe(OH)3 coprecipitates. Favorable sample crystallinity and the Eg of approximately 2.1 eV were observed. The energy band diagram and Mott–Schottky measurement respectively indicated and confirmed that the fabricated BFO films had p-type characteristics. Theoretical simulation revealed that BFOMPL had the favorable

CRediT authorship contribution statement

Thi Nghi Nhan Nguyen: Experiment, Characterizations, Data analysis, Writing – original draft. Kao-Shuo Chang: Supervision, Project administration, Resources, and Manuscript revision and editing. Both authors contributed substantially to this study. Both have read and approved the final version of the manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was partially supported by the Ministry of Science and Technology (MOST), Taiwan, under grant number MOST 106-2221-E-006-053-MY3.

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