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Article

Piezo-Enhanced Photocatalytic Performance of Bismuth Ferrite-Based Thin Film for Organic Pollutants Degradation

by
Vasile Tiron
1,
Roxana Jijie
1,
Teodora Matei
2,
Ioana-Laura Velicu
2,
Silviu Gurlui
2 and
Georgiana Bulai
3,*
1
Research Center on Advanced Materials and Technologies, Department of Exact and Natural Science, Institute of Interdisciplinary Research, Alexandru Ioan Cuza University of Iasi, 700506 Iasi, Romania
2
Faculty of Physics, Alexandru Ioan Cuza University of Iasi, 700506 Iasi, Romania
3
Integrated Center of Environmental Science Studies in the North-Eastern Development Region (CERNESIM), Department of Exact and Natural Science, Institute of Interdisciplinary Research, Alexandru Ioan Cuza University of Iasi, 700506 Iasi, Romania
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(8), 1416; https://doi.org/10.3390/coatings13081416
Submission received: 8 June 2023 / Revised: 7 August 2023 / Accepted: 10 August 2023 / Published: 12 August 2023
(This article belongs to the Section Thin Films)
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Versions Notes

Abstract

:
This work addresses the global sustainable development concerns by investigating the enhancement of piezo-photocatalytic efficiency in bismuth ferrite-based thin films synthesized using reactive high-power impulse magnetron sputtering. The influence of substrate type and Cr addition on structural, optical and ferroelectric properties of bismuth ferrite (BFO) based thin films was investigated. X-ray diffraction measurements showed the formation of different phases depending on the substrate used for sample growth. Compared to the BFO film deposited on FTO (F-SnO2), the Cr-doped BFO (BFCO) sample on SrTiO3 (STO) exhibits higher photodegradation efficiency (52.3% vs. 27.8%). The enhanced photocatalytic activity of BFCO is associated with a lower energy band gap (1.62 eV vs. 1.77 eV). The application of ultrasonic-wave vibrations simultaneously with visible light improved 2.85 times and 1.86 times the photocatalytic degradation efficiencies of BFO/FTO and BFCO/STO catalysts, respectively. The piezoresponse force microscopy (PFM) measurements showed that both catalysts exhibit ferroelectric behavior, but a higher piezoelectric potential was evidenced in the case of the BFO/FTO thin film. The enhancement of piezo-photodegradation efficiency was mainly attributed to the piezoelectric-driven separation and transport of photo-generated carriers toward the surface of the photocatalyst.

1. Introduction

In contrast to conventional p-n junction solar cells, which operate based on the well-known photovoltaic (PV) effect, PV cells based on the ferroelectric effect function differently. In these cells, the separation and transport of photo-generated carriers occur through a polarization-induced internal electric field, often referred to as a depolarization electric field [1,2,3]. The strength of the polarization-induced field in a ferroelectric material is significantly higher, up to two orders of magnitude, compared to the electric field existing in a p-n junction. This strong electric field extends throughout the entire ferroelectric material, enabling efficient separation and transport of electron-hole pairs. The fundamental research on oxide ferroelectrics was extended to a technological concept from the photovoltaic standpoint [4] after several reported results showed that the low conversion efficiencies could be overcome by large, above-bandgap, photovoltages [5] or by engineering the ferroelectric domain architectures and interfaces and reducing the film thickness [5,6,7]. However, it is worth noting that the wide bandgap of ferroelectric materials exceeds the ideal value of 1.4 eV, which makes them less compatible with traditional PV devices due to their relatively poor sunlight absorption capabilities. Narrowing the band gap of ferroelectric materials while preserving their ferroelectric properties holds significant promise for designing solar energy conversion devices with high PCE. A commonly employed strategy to address this challenge is to introduce different types of ions in the crystal lattice of the material. The flexibility of cation mixing in oxide ferroelectrics provides an excellent opportunity to finely tune their physical and chemical properties [8,9,10,11,12,13,14]. Another way to increase the PCE of a device is by reducing the recombination of photogenerated electron-hole pairs caused by defects (including those on the surface), impurities, grain boundaries, and clusters [15].
Compared to chemical methods used for the synthesis of photocatalytic materials, plasma processing material techniques provide the advantage of enabling control over the physicochemical properties of a material. This control is achieved through access to a wide range of process parameters, allowing precise manipulation and customization of the material’s characteristics. In this context, the films reported in the present study were deposited by High Power Impulse Magnetron Sputtering (HiPIMS). The use of this method enables the deposition of smooth thin films with high microstructural density, mechanical hardness, and strong adhesion to the substrate [16,17]. These characteristics can improve the photovoltaic and photocatalytic response of ferroelectric structures by reducing the recombination rate of the photogenerated carriers and by enhancing chemical and mechanical stability. Furthermore, HiPIMS offers advantages for fabricating multi-layered devices over large surfaces and temperature-sensitive substrates. These features are particularly relevant when considering solar energy conversion systems, especially on a commercial scale. To date, there are no reported results specifically on Cr-doped bismuth ferrite-based films deposited by HiPIMS. However, a study conducted by Benetti et al. focused on ferroelectric films deposited using a combination of magnetron sputtering and pulsed laser deposition (PLD) techniques [18] showed encouraging results. The crystallinity of the thin film has a very important impact on the ferroelectric properties and photovoltaic capability of the tested sample. Compared to Cr-doped bismuth ferrite thin films obtained by chemical routes where additional thermal treatments are necessary to crystallize the otherwise amorphous materials or to the ones grown by pulsed laser deposition technique where a uniform layer can be deposited on a limited area, HiPIMS allows the growth of a highly crystalline sample with limited structural defects over large surfaces [19,20,21,22,23]. The choice of substrate type can have a significant impact on the crystallographic growth of the deposited samples. A study by Qi et al. demonstrated the influence of substrate selection on the epitaxial growth of BiFeO3 thin films. Specifically, they achieved epitaxial growth of BiFeO3 on a SrTiO3 substrate and observed a dominant Fe3O4 phase on a LaAlO3 substrate using the same experimental conditions [24]. Kartavtseva et al. discovered that the substrate-induced strain has a significant impact on the crystalline structure, ferroelectric, and magnetic properties of the material [25]. The compressive coherent strain resulting from the deposition of bismuth ferrite thin films on SrTiO3 substrates using PLD is believed to promote Fe/Cr cation ordering and enhance the saturation magnetization of the films [26].
In photocatalytic applications, the photogeneration of charge carriers and their subsequent separation are key processes. Unlike in PV devices, where the goal is to generate an electric current, photocatalysis aims to initiate redox reactions. Nowadays, photocatalysis has drawn increasing attention due to its potential to address current environmental and energy issues by converting solar light into chemical energy [27,28]. BFO nanoparticles were employed as an efficient catalyst for the degradation of methyl orange [29] and Congo red [30] dyes under visible light irradiation. Several studies have shown that the doping of Gd3+ [31] or co-doping of Gd3+ and Sn4+ [27] into A and B-sites of BiFeO3 can contribute towards the enhancement of photocatalytic properties of BFO. Irfan et al. reported that about 95% of Congo red dye removal takes place in the presence of Gd3+ and Sn4+ co-doped BFO nanoparticles under visible irradiation after 180 min, while the photocatalytic degradation efficiency attained by the BFO nanoparticles was 44% [27]. The remarkable enhancement of the photocatalytic activity of the composite material was attributed to the reduced particle size, increased surface area and low recombination rate of charge carriers. Fatima et al. [30] observed that the lanthanum/manganese co-doping and incorporation of graphene nanoplatelets significantly enhanced the Congo red removal compared to pure BFO.
The review by Sportelli et al. highlights the exceptional performance of PV materials as photocatalysts. It emphasizes the significance of integrating PV structures into research related to the degradation of environmental pollutants [28]. Mabuti et al. conducted a study on bismuth oxyiodide nanocomposites and reported an increase in photocurrent density, along with an improved degradation rate of methylene blue (MB). This enhancement was attributed to the addition of graphene, which resulted in an increased generation of photogenerated carriers [32]. In recent years, ferroelectric materials have emerged as promising candidates for piezocatalytic applications. This is primarily due to their ability to separate photogenerated carriers through the piezoelectrically induced built-in electric field. This phenomenon opens up new possibilities for utilizing these materials in piezocatalytic systems, where mechanical energy can be harnessed to drive catalytic reactions and improve overall efficiency [33,34,35]. Xu et al. obtained an increase in the degradation rate of RhB by Ba0.7Sr0.3TiO3 based composites from 26.1% to 88.0% and further to 98.8% through photocatalysis, piezo-catalysis and combined photo-piezo-catalysis, respectively [36]. The utilization of BFO-based material in piezocatalytic degradation of dyes has attracted increasing attention. For instance, a 59% and 92% degradation of Rhodamine B (RhB) was reported for BFO nanosheets and nanowires catalyst under ultrasonic vibration within 1 h [37]. Wang et al. demonstrated that about 82% of RhB molecules were removed within 90 min of ultrasonic vibration (40 kHz, 120 W) in the presence of CoOx/BiFeO3, with a rate of 1.29 h−1, which is 2.38 times higher than that of BiFeO3 [38].
Both the photovoltaic effect and photo/piezo-catalytic efficiency can be correlated to the ferroelectric properties studied by the piezoresponse force microscopy (PFM) [39,40,41,42]. Nd and V co-doped BiFeO3 thin films obtained by PLD presented a more evident domain back switching from PFM results and higher open circuit voltage and short circuit current than the un-doped sample [43]. This method can be used to obtain a preliminary insight into the photovoltaic effect even for films that are not deposited on a conductive layer and for which a current-voltage characteristic cannot be recorded.
The aim of this study was to investigate how the choice of substrate type and the addition of Cr influence various properties of BiFeO3 films. Specifically, we focused on examining the structural, optical, piezoelectric, and ferroelectric characteristics of these thin films deposited using the HiPIMS technique. To our knowledge, these are the first reported results on Cr-doped bismuth ferrite thin films obtained by HiPIMS. Furthermore, we aimed to assess the potential of these thin films as piezo-photocatalysts. To explore the impact of substrate type, three different substrates were utilized in this study. The first substrate was SrTiO3 (STO), chosen for its similar crystallographic structure to BiFeO3 and minimal lattice mismatch. The second and third substrates were fluorine-tin oxide (FTO) and indium-tin oxide (ITO) coated glass, respectively. These two types of substrates are commonly used in multi-layered photovoltaic systems as transparent electrodes. The potential of ferrite bismuth-based thin films in piezo-photocatalysis was demonstrated through experiments involving the degradation of MB organic dye.

2. Materials and Methods

Thin films based on bismuth ferrite (BFO) and chromium-doped bismuth ferrite (BFCO) were fabricated using the reactive HiPIMS technique. The deposition process was carried out on different types of substrates, maintaining similar experimental conditions for both cases. By employing reactive HiPIMS, the films were synthesized with controlled composition and desired properties. For the deposition of the bismuth ferrite-based thin films, commercially available targets of BiFe and Bi2FeCr alloys were utilized as source materials. The deposition process involved setting the Ar:O2 gas ratio at 20:1 while maintaining a mixed gas pressure of 0.75 Pa. These specific parameters were chosen to achieve the desired composition and characteristics of the thin films during the deposition process. To promote the growth of crystalline thin films, an in situ substrate temperature of 700 °C was maintained for all samples during the deposition process. The substrates used included (100) single-side polished SrTiO3 (STO), fluorine-tin oxide (FTO), and indium-tin oxide (ITO) coated glass substrates. These substrates were positioned at a distance of 8 cm in front of the cathode, ensuring optimal deposition conditions for the growth of high-quality crystalline films. The pulse duration was set at 5 µs, the frequency at 1.5 kHz, and the amplitude at 700 V. These specific parameters were chosen based on previous studies that investigated the impact of various experimental processing parameters on the phase evolution and crystalline structure of thin films. Factors such as gas pressure, the ratio of argon to oxygen flow, deposition temperature, pulse duration, and target voltage were considered in order to optimize the deposition conditions and achieve the desired properties of the samples.
To analyze the crystalline structure of the deposited films, X-ray diffraction (XRD) measurements were performed using a Shimadzu LabX XRD-6000 Diffractometer. The instrument, manufactured by Shimadzu in Kyoto, Japan, utilized CuKα radiation with a wavelength of 1.54 Å. This technique provided valuable information about the crystallographic properties of the films, allowing for the characterization and identification of specific phases present in the material.
To investigate the optical properties of the films, UV-VIS spectroscopy was employed. Transmission spectra were recorded in the range of 180 nm to 1000 nm, allowing for the analysis of the film’s optical response across a wide wavelength range. The derived Tauc plots were utilized to estimate the optical band gap values, providing information about the energy required for electronic transitions in the material.
The piezoelectric and ferroelectric properties of the bismuth ferrite-based thin films were examined using atomic force microscopy (AFM) with a piezoresponse force microscopy (PFM) module. The specific AFM system used was the Solver-Pro from NT-MDT. The experimental setup, probe calibration tests, and measurement procedures for studying the local piezoresponse behavior, ferroelectricity, and polarization switching were extensively described in our previous papers [44,45]. This methodology allowed for the characterization and understanding of the piezoelectric and ferroelectric properties at the nanoscale level in the obtained films.
The samples that exhibited the best formation of the BiFeO3 phase were selected for further investigation of their dye degradation capability. Specifically, the photocatalytic and piezo-photocatalytic behaviors of two types of samples were examined: BFO thin films deposited onto FTO substrates with a size of 1 × 1 cm2 and BFCO thin films deposited onto STO substrates with the same dimensions. The evaluation of their photocatalytic and piezo-photocatalytic activities was conducted by studying the degradation of MB dye in an aqueous solution under visible light irradiation (wavelength > 400 nm), for 4 h, with an incident light flux of 120 mW/cm2 uniformly distributed over an area of 1.6 cm2. The distance between the light source and the samples was maintained at 6 cm. Additionally, the experiments were carried out with and without the application of ultrasonic vibration generated by an ultrasonic cleaner (Elmasonic Easy 30 H, 37 kHz, 80 W).
For the piezo-photocatalytic experiments, the samples were placed in a glass Petri dish with dimensions of 40 mm × 12 mm. The Petri dish contained 3 mL of an aqueous solution with a concentration of 10 mg/L of MB dye. To achieve adsorption–desorption equilibrium, the samples were kept in the dark, at room temperature, for 30 min. Subsequently, the piezo-photocatalytic performance of the samples was evaluated by measuring the maximum absorbance value of the treated solutions at a wavelength of 664 nm using a Specord 210 Plus UV-Vis spectrophotometer from Analytik Jena, Germany. The remanent MB concentration was determined according to a previously reported calibration curve [46] with the equation y = −0.011 + 0.039x, where y represents the concentration of the dye and x represents the absorbance value. Initially, the spectra of several MB solutions with concentrations ranging from 0 to 60 uM (24 mg/L) were recorded between 200 and 900 nm. Then the calibration curve was created by plotting the absorbance at λmax = 664 nm as a function of the corresponding concentrations. The degradation efficiency was calculated using the following equation: %Degradation = (C0 − C)/C0 × 100%, where C0 is the initial dye concentration after 30 min in the dark, and C is the dye concentration at a given time (t).

3. Results and Discussions

3.1. Thin Films Characterization

3.1.1. Thin Films Microstructure

Figure 1 shows the XRD patterns obtained for all samples in the 10–70° 2θ range. The BFO thin film deposited on the FTO substrate exhibited diffraction lines corresponding to both the substrate and the BiFeO3 hexagonal structure (card no. ICSD 98-016-2264). No residual phases were detected in this sample. However, for the bismuth ferrite-based thin films deposited on ITO and STO substrates, secondary phases were observed. The BFO/ITO sample displayed additional diffraction lines corresponding to the Bi2Fe4O9 mullite structure (card no. ICSD 98-002-0067). In the case of the BFO/STO sample, the XRD pattern exhibited extra peaks attributed to Bi25FeO40 (selenite—card no. ICSD 98-006-2719) and Bi2O3 (monoclinic—card no. ICSD 98-061-6890). These secondary phases indicate differences in the composition and crystal structure of the films deposited on different substrates. Similar observations were reported in [24], where bismuth ferrite films were deposited on SrTiO3 and LaAlO3 substrates using RF magnetron sputtering under similar experimental conditions.
In the case of BFCO samples, the XRD patterns exhibited different behaviors depending on the substrate used. For the BFCO deposited on ITO substrates, the film displayed two main peaks that were associated with the mullite phase. On the other hand, for the BFCO grown on an STO substrate, most of the diffraction lines matched the BiFeO3 orthorhombic pattern (card no. ICSD 98-016-2895). The XRD signal of the BFCO/FTO sample was considerably lower compared to the other samples, and the observed peaks were attributed to the Bi2Fe4O9 mullite phase and CrO3 structure (card no. ICSD 98-003-8125). The structural analysis results indicated that both the substrate type and the presence of Cr influenced the crystallographic structure of the material. While for BFO, the sample deposited on FTO showed the best formation of BiFeO3, for BFCO, the use of STO substrate was found to be more suitable.
The crystallite size (d) and strain due to the broadening of the observed structures were determined by considering the FWHM of the main peaks. For crystallite size estimations, we used the Debye–Scherrer equation while the strain (ε) was given by: ε = βhkl/4tanθ, where βhkl is the FWHM of the peak and θ is the Bragg angle [47]. The estimated values are listed in Table 1. A small variation in crystallite size was observed for the BiFeO3 structure in BFO films and for Bi2Fe4O9 in BFCO samples as the substrate was modified. The smallest d value and the highest strain were estimated for the BiFeO3 structure in BFCO/STO sample, which also presented the highest photocatalytic activity, as described in the last section.

3.1.2. Optical Properties

The theoretical study by density functional theory revealed that the electronic structures of BiFeO3 present an indirect band gap [48]. This was confirmed by both the local spin-density approximation [49] and the screened-exchange density functional theory approximation [50]. The optical properties of the deposited samples were studied by UV-VIS spectroscopy measurements in the 200–1000 nm spectral range. The indirect band gap (Eg) values were determined from the derived Tauc plots presented in Figure 2. The lowest Eg values of 1.77 eV and 1.62 eV were estimated for the BFO/FTO and BFCO/STO samples, respectively, for which the XRD results showed BiFeO3 phase formation with no or limited secondary phases. Comparable values were reported in the theoretic study of McDonnell et al. [51] and in experimental investigations [51,52], and higher ones in [53]. The BFO thin films deposited on STO and ITO substrates presented higher band gap values, probably due to the presence of secondary phases of mullite, selenite and bismuth oxide. A similar observation can be made for the BFCO samples. The BFCO/FTO and BFCO/ITO thin films presented higher Eg values of 1.84 and 1.96 eV, respectively, together with additional Bi2Fe4O9 and CrO3 structure formation. The presence of internal stress can also affect the optical properties of the bismuth ferrite films. The study of Himcinschi et al. on the influence of BiFeO3 thin films deposited by PLD on different types of substrates showed a blue shift of the optical band gap as the compressive strain increased [54].

3.1.3. Piezo-Ferroelectric Properties

Piezoresponse force microscopy was utilized to examine the piezoelectric and ferroelectric properties of BFO and BFCO films. In addition to surface topography, PFM provided valuable insights into local piezoelectric response and ferroelectric behavior. This was achieved by capturing out-of-plane piezoresponse amplitude and phase maps, as well as recording piezoresponse amplitude butterfly loops and phase hysteresis curves within a specific grid (10 × 10 points matrix). Figure 3a,d presents the morphology of the BFO/FTO and BFCO/STO thin films, respectively, observed within a 3 × 3 μm2 area. Both topographic images indicate the presence of evenly distributed small grains on the surface of the films. In contrast to the BFO sample, which exhibits a compact grain structure, the BFCO film displays distinct and well-dispersed grains on its surface. Usually, the existence of well-dispersed particles can ensure a higher surface area and can lead to an increased photocatalytic response as more active sites participate in the degradation process. In addition, the built-in electric field formed at the BiFeO3 and Bi2Fe4O9 heterojunction enhances, even more, the photocatalytic performance, as described in the next section. Figure 3b,e illustrates the out-of-plane piezoresponse amplitude maps of the BFO/FTO and BFCO/STO thin films, respectively. The BFO thin film (Figure 3b) exhibits a noticeable amplitude variation in different regions, indicating the presence of random and distinct piezoelectric domains with a remarkable piezoelectric response. The corresponding out-of-plane phase images are depicted in Figure 3c,f. The phase angle, which represents the relationship between the applied tip bias voltage and the piezoresponse signal, reveals the presence of domains with different polarization and orientation within the films. Figure 3f highlights the presence of well-defined and regularly shaped domain walls.
The PFM polarization switching curves depicted in Figure 4 demonstrate noticeable differences in the butterfly curves of piezoresponse amplitude and changes in phase angle within the local hysteresis loops. These variations confirm the presence of piezoelectricity and ferroelectricity in both BFO and BFCO thin films. The observed changes in the PFM amplitude butterfly curves are attributed to the alteration in strain under an external electric field applied by the AFM tip. The piezoresponse curves of BFO and BFCO exhibit the characteristic butterfly shape, with BFO reaching a maximum amplitude of approximately 250 pm under ±5 V DC tip bias voltage (Figure 4a) and BFCO achieving a maximum amplitude of around 130 pm under ±10 V DC tip bias voltage (Figure 4b).
Under the same external field of ±5 V DC tip bias voltage, the strain induced in the BFO film is five times higher than that induced in the BFCO tin film, indicating a remarkable piezoelectric potential. Moreover, the BFO film exhibits a well-defined 180° phase angle inversion and a wide hysteresis loop of 5.0 V, indicating strong ferroelectric spontaneous polarization. In contrast, the BFCO film shows a 150° phase angle inversion and a phase hysteresis loop of only 2.5 V width, suggesting a weaker ferroelectric behavior. Notably, the widths of the hysteresis loops closely align with the gaps between the piezoresponse amplitude butterfly curves. The presence of ferroelectric spontaneous polarization creates a built-in electric field that facilitates the separation and transport of photogenerated electrons and holes, which are crucial for redox reactions. The higher piezoresponse and ferroelectric polarization in the BFO thin film control the migration rate of photoinduced charge carriers, leading to reduced recombination and enhanced piezo-photocatalytic efficiency. The difference in piezoelectric potential is not given by the grain size variation but rather by the crystalline quality of the film and crystal orientation. The larger crystallite of BFO thin film could improve the piezoelectric potential and piezocatalytic performances. The piezoresponse amplitude butterfly loops and phase hysteresis curves are not affected by the surface topography since they provide information about the local polarization switching and ferroelectric phase. The thicknesses of the two samples are almost the same. The profilometry measurements revealed a thickness of 95 nm for BFO/FTO and 102 nm for the BFCO/STO. Thus, the piezoelectric properties are not influenced by the sample thickness. The PFM measurements consistently demonstrate that the BFO thin film exhibits superior ferroelectric behavior and outstanding piezoelectric potential compared to the BFCO thin film.

3.2. Piezo-Photodegradation Efficiency Assessment

The development of new catalysts that can effectively harness multiple energy sources holds great promise for addressing current environmental and energy challenges. One such versatile catalyst is based on BFO nanostructured material, which has demonstrated its efficacy in efficiently degrading organic contaminants using both light and vibrational energy. In a study conducted by Mushtaq et al. [37], the degradation of RhB dye was investigated using BFO nanosheets and nanowires as catalysts under solar light and mechanical vibrations. The results revealed that BFO nanowires exhibited enhanced photo-piezocatalytic performance compared to BFO nanosheets. Remarkably, within just 1 h of treatment, BFO nanowires achieved an impressive removal rate of 97% for the dye, with a constant degradation rate of 0.058 min−1. These findings underscore the potential of BFO nanowires as highly efficient catalysts for the degradation of organic dyes through the synergistic utilization of light and mechanical vibrations.
Moreover, recent research has explored the application of Ag/BFO nanocomposites as efficient piezo-photocatalysts for the degradation of methyl orange [55]. The combination of silver nanoparticles with BFO further enhances the catalytic performance by facilitating charge separation and promoting redox reactions. The Ag/BFO nanocomposites have exhibited promising results in degrading methyl orange, highlighting their potential for the removal of organic pollutants using the combined effects of piezoelectric and photocatalytic processes.
These studies collectively emphasize the potential of BFO-based materials, such as BFO nanowires and Ag/BFO nanocomposites, as highly efficient and versatile catalysts for the degradation of organic contaminants. By harnessing the capabilities of both light and mechanical vibrations, these catalysts offer new opportunities for addressing environmental pollution and advancing sustainable energy solutions. Further research and exploration in this field hold promise for the development of innovative catalysts that can effectively utilize multiple energy sources, contributing to a cleaner and more sustainable future.
In order to explore the degradation potential of BFO and BFCO films under visible light and ultrasonic wave vibrations, their ability to degrade MB dye was studied. Figure 5 illustrates the results of the experiments. The control experiment without any catalyst (referred to as the “blank”) exhibited a negligible self-degradation response, with only approximately 3.2% degradation observed. However, upon 240 min of visible light irradiation, the BFO and BFCO catalysts demonstrated significant degradation of MB, with approximately 27.8% and 52.3% degradation achieved, respectively.
The superior photocatalytic activity of the Cr-doped BFO can be attributed to its reduced energy band gap (1.62 eV compared to 1.77 eV for undoped BFO), which allows for better utilization of visible light, resulting in increased electron-hole (e−/h+) pairs available for catalytic reactions. While the photo-absorption property of the photocatalyst plays a crucial role in light-harvesting efficiency, it is not the sole determinant of overall photocatalytic performance. Other factors, such as the efficient separation and transfer of photogenerated charge carriers to the surface of the photocatalyst, also significantly influence the efficiency of the light-harvesting [56]. The BFCO film exhibited superior photocatalytic activity compared to [57], where a 25% degradation of MB was reported for BFO thin films synthesized via the spray pyrolysis method using a 4 mM precursor concentration after 10 h of visible light exposure. Additionally, pure BFO nanoparticles achieved a decomposition rate of 40% for Congo red molecules within 2 h [30] and 19% degradation of methyl violet within 3 h [27]. These findings highlight the enhanced photocatalytic performance of BFCO thin films and suggest their potential for effective degradation of organic dyes. Overall, the results demonstrate that both BFO and BFCO thin films exhibit significant photocatalytic activity for MB degradation under visible light irradiation. The Cr-doped BFO shows enhanced performance due to its reduced energy band gap, allowing for efficient utilization of visible light. The high photocatalytic activity of BFCO thin films indicates the potential of these materials for advanced photocatalytic applications. Further research in this field is essential to uncover the underlying mechanisms and optimize the photocatalytic performance of BFO and BFCO thin films for broader environmental remediation and energy-related applications.
When mechanical vibrations are applied in conjunction with visible light, the degradation efficiency of MB can be further enhanced. The BFO/FTO catalyst exhibited a degradation efficiency of up to 63%, while the BFCO/STO catalyst achieved a degradation efficiency of 72.7%. These results highlight the synergistic effect of combining mechanical vibrations with visible light in enhancing photocatalytic activity.
Moreover, the piezo-photodecomposition ratio of MB for the BFO/FTO catalyst was found to be up to 3 times higher (0.004 min−1) compared to the photocatalytic decomposition rate (0.0014 min−1). This significant increase in the degradation rate can be attributed to the efficient separation of photo-excited charges facilitated by the in situ generated electric field induced by the mechanical stress assistance.
The intrinsic electric field generated by the mechanical stress, in this case, the ultrasonic wave vibrations, acts as a driving force for the efficient separation and transport of the photo-induced charge carriers. This enhanced charge separation and transport further amplify the catalytic process, leading to a more efficient decomposition of the organic dye under visible-light irradiation.
Overall, the combination of mechanical vibrations and visible light demonstrates a synergistic effect that enhances the degradation efficiency of MB. The in situ generated electric field by the mechanical stress aids in the efficient separation and transport of photo-induced charge carriers, thereby promoting the catalytic decomposition process. These findings highlight the potential of piezo-photocatalysis as an effective approach for advanced organic dye degradation and offer promising prospects for the development of multifunctional catalysts that can harness multiple energy sources.
In addition to the efficient separation of photo-electrons and holes facilitated by the piezoelectric potential, the enhanced photocatalytic activity of the BFCO/STO thin film in MB dye decomposition can be attributed to the extra-separation effect. The heterojunction formed between BiFeO3 and Bi2Fe4O9 in the film enables the further separation of photo-generated charge carriers, leading to a reduction in the recombination rate and a strengthening of the photodegradation efficiency. This effect has been observed in one of our previous study [45], where the heterojunction between Bi2Fe4O9 and ε-Fe2O3 was found to significantly improve the photocatalytic and piezocatalytic performance of Bi2Fe4O9 thin films.
Another contributing factor to the enhanced photocatalytic activity is the high crystalline order of the BFO/FTO and BFCO/STO samples. The well-defined crystal structure allows an efficient transfer of photo-excited charge carriers toward the surface of the photocatalyst, where they participate in redox reactions. This efficient charge transfer process promotes the overall photocatalytic performance.
Similar findings have been reported for other composite materials. For example, Ag/BiFeO3 composites showed significantly higher piezo-photocatalytic degradation rates compared to pure photocatalysis. The degradation efficiency was found to be 200% higher in the piezo-photocatalytic process compared to photocatalysis alone [55]. Additionally, in the case of BFO nanowires and nanosheets, the simultaneous application of light and mechanical energy resulted in much higher RhB degradation rates compared to photocatalysis alone [37]. The degradation efficiency was observed to increase from 66% to 71% for BFO nanosheets and from 60% to 97% for BFO nanowires.
The degradation kinetics of MB by both BFO and BFCO catalysts were found to follow a first-order kinetic reaction, as shown in Figure 5a,b. The plots of ln(C/C0) against exposure time, for both light irradiation and in combination with ultrasonic vibrations, exhibited a linear relationship. This indicates that the degradation of MB follows a rate equation of the form ln(C/C0) = −kt, where C is the concentration of MB at a given time, C0 is the initial concentration of MB, k is the degradation rate constant, and t is the exposure time.
The calculated degradation rate constant values for MB degradation over the BFCO catalyst in the presence of visible light were determined to be 3 × 10−3 min−1, which is two times higher than that of BFO. This suggests that the BFCO catalyst has a higher efficiency in degrading MB under visible light irradiation alone.
Furthermore, an enhancement in the degradation performance was observed for the Cr-doped BFO thin film when both visible light and mechanical energy (ultrasonic vibrations) were used simultaneously. The calculated degradation rate constant for this case was found to be 5.6 × 10−3 min−1, indicating a further improvement in the degradation efficiency compared to the BFCO catalyst under light irradiation alone. The higher k value for the BFCO sample demonstrates its superior performance in the simultaneous removal of MB dye using visible light and mechanical vibrations.
Both films present an enhancement of the piezo-photodegradation efficiency, and this enhancement is justified by the piezoelectric-driven separation and transport of photo-generated carriers. The photocatalytic efficiency of the BFCO/STO thin film is higher than the one of the BFO/FTO sample, mainly due to the lower band gap values (that allows more visible light to be used for charge carrier generation) and probably to the built-in electric field formed at the BiFeO3 and Bi2Fe4O9 heterojunction (that aids the separation process of the charge carriers). Furthermore, due to the more elevated ferroelectric response of BFO/FTO, this sample showed a higher piezo-enhanced degradation efficiency. However, the overall piezo-photocatalytic activity of the BFCO/STO sample is still higher than the one of the BFO/FTO.
These results highlight the significance of factors such as band gap energy, heterojunction formation, crystalline order, and simultaneous application of light and mechanical energy in enhancing the photocatalytic performance of BFO and BFCO thin films. By exploiting these factors, the efficiency of organic dye degradation can be significantly improved, offering promising possibilities for the development of advanced catalytic systems for environmental remediation and energy applications.
It is important to emphasize that while there have been numerous studies on the piezo-photocatalytic degradation of organic contaminants using nanostructures and nanoparticles, research focused on thin films is relatively limited. Nanostructures and nanoparticles often offer high surface area, providing more active sites for catalytic reactions. In contrast, thin films have a lower specific surface area, which can limit the number of active sites available for the catalysis [45].
However, working with thin films has its advantages. Thin films are more practical in terms of application and scalability. Unlike nanostructures and nanoparticles, thin films do not require a separation process after the completion of the reaction, making them more convenient for use in practical applications. Additionally, thin films are well-suited for PV applications, where they can be integrated directly into devices and systems for energy conversion.
While the use of thin films for piezo-photocatalytic degradation may present some challenges related to surface area and active sites, their practicality, ease of integration, and suitability for PV applications make them a promising option for the catalytic degradation of organic contaminants in water. Further research and optimization of thin film catalysts can lead to improved performance and expand their applications in water treatment and environmental remediation.

4. Conclusions

The present study is focused on investigating the influence of substrate type and chromium addition on the piezo-photocatalytic activity of thin films based on BFO material. The experimental results provided valuable insights into the dual properties of these thin films, namely photocatalysis and piezo-catalysis. The findings demonstrated the feasibility of simultaneously harnessing visible light and vibration energy, leading to improved degradation rates for organic contaminants.
Notably, the results indicated that BFCO/STO films exhibited superior piezo-photocatalytic activity compared to BFO/FTO films. The BFCO/STO configuration achieved a remarkable 72.7% degradation of the harmful MB dye within a period of 4 h, with a constant degradation rate of 5.6 × 10−3 min−1. This enhanced performance can be attributed to the synergistic effects resulting from the coupling of photocatalytic and piezoelectric properties in the bismuth ferrite thin films. Although the BFO/FTO sample showed a higher piezo-enhanced degradation efficiency due to the more elevated ferroelectric response, the overall piezo-photocatalytic activity of the BFCO/STO thin film was still higher.
By combining the capabilities of photocatalysis and piezoelectricity, the efficiency of organic pollutant degradation was significantly enhanced. The simultaneous utilization of both energy sources allowed for a higher degradation rate compared to using either method individually. This demonstrates the potential of bismuth ferrite-based thin films as effective catalysts for the degradation of organic pollutants.
These findings contribute to the growing body of knowledge on piezo-photocatalysis and highlight the potential of bismuth ferrite thin films for practical applications in environmental remediation and water treatment. The results emphasize the importance of exploring synergistic effects between different energy sources for the development of efficient and sustainable catalysts. Further research in this field could lead to the design and optimization of advanced materials with enhanced piezo-photocatalytic properties for effective pollutant degradation.

Author Contributions

Conceptualization, V.T. and G.B.; methodology, V.T., R.J. and G.B.; validation, V.T., R.J. and G.B.; formal analysis, V.T., R.J., T.M. and G.B.; investigation, V.T., R.J. and G.B.; resources, V.T., S.G. and G.B.; data curation, V.T., R.J. and G.B.; writing—original draft preparation, V.T., R.J. and G.B.; writing—review and editing, V.T., R.J., I.-L.V. and G.B; visualization, I.-L.V.; supervision, G.B.; project administration, G.B.; funding acquisition, G.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Ministry of Research, Innovation and Digitalization, CNCS-UEFISCDI, project number PN-III-P1-1.1-TE-2021-0265, within the PNCDI III program.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

It is not the case here.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 01416 g001
Figure 1. The XRD results of the BFO (a) and BFCO (b) thin films deposited on STO, FTO, and ITO substrates.
Figure 1. The XRD results of the BFO (a) and BFCO (b) thin films deposited on STO, FTO, and ITO substrates.
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Figure 2. Tauc plots derived from UV-VIS spectroscopy measurements for the BFO (a) and BFCO (b) thin films.
Figure 2. Tauc plots derived from UV-VIS spectroscopy measurements for the BFO (a) and BFCO (b) thin films.
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Figure 3. Topography images (a,d), out-of-plane piezoresponse amplitude images (b,e), out-of-plane phase images (c,f) of BFO/FTO (first row) and BFCO/STO (second row) thin films.
Figure 3. Topography images (a,d), out-of-plane piezoresponse amplitude images (b,e), out-of-plane phase images (c,f) of BFO/FTO (first row) and BFCO/STO (second row) thin films.
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Figure 4. Piezoresponse amplitude butterfly loops and phase hysteresis of BFO/FTO (a) and BFCO/STO (b) thin films.
Figure 4. Piezoresponse amplitude butterfly loops and phase hysteresis of BFO/FTO (a) and BFCO/STO (b) thin films.
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Figure 5. Photocatalytic and piezo-photocatalytic performance of (a) BFO (FTO) and (b) BFCO (STO) thin films.
Figure 5. Photocatalytic and piezo-photocatalytic performance of (a) BFO (FTO) and (b) BFCO (STO) thin films.
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Table 1. Crystallite size and microstrain values of the main structures found in the deposited thin films.
Table 1. Crystallite size and microstrain values of the main structures found in the deposited thin films.
SamplePhaseCrystallite Size (±1 nm)Strain (±0.02%)
BFO/STOBiFeO3141.3
Bi25FeO40360.24
Bi2O3500.34
BFO/FTOBiFeO3220.78
BFO/ITOBiFeO3170.93
Bi2Fe4O9220.61
BFCO/STOBiFeO361.5
Bi2Fe4O9121.2
BFCO/FTOBiFeO3240.54
Bi2Fe4O9210.7
CrO3310.43
BFCO/ITOBi2Fe4O9101.07
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Tiron, V.; Jijie, R.; Matei, T.; Velicu, I.-L.; Gurlui, S.; Bulai, G. Piezo-Enhanced Photocatalytic Performance of Bismuth Ferrite-Based Thin Film for Organic Pollutants Degradation. Coatings 2023, 13, 1416. https://doi.org/10.3390/coatings13081416

AMA Style

Tiron V, Jijie R, Matei T, Velicu I-L, Gurlui S, Bulai G. Piezo-Enhanced Photocatalytic Performance of Bismuth Ferrite-Based Thin Film for Organic Pollutants Degradation. Coatings. 2023; 13(8):1416. https://doi.org/10.3390/coatings13081416

Chicago/Turabian Style

Tiron, Vasile, Roxana Jijie, Teodora Matei, Ioana-Laura Velicu, Silviu Gurlui, and Georgiana Bulai. 2023. "Piezo-Enhanced Photocatalytic Performance of Bismuth Ferrite-Based Thin Film for Organic Pollutants Degradation" Coatings 13, no. 8: 1416. https://doi.org/10.3390/coatings13081416

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