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. 2022 Feb 22;7(9):7595–7605. doi: 10.1021/acsomega.1c06109

NaNbO3 Nanorods: Photopiezocatalysts for Elevated Bacterial Disinfection and Wastewater Treatment

Aditi Sharma †, Upasana Bhardwaj †, Devendra Jain ‡, Himmat Singh Kushwaha †,*
PMCID: PMC8908499  PMID: 35284758

Abstract

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In the present work, ferroelectric sodium niobate (NaNbO3) nanorods are formulated to attain photopiezocatalysis for water pollutant degradation and bacterial disinfection. NaNbO3 nanorods, integrating the advantages of photocatalysis (generation of free charge carriers) and piezocatalysis (separation of these charge carriers), possess synergistic effects, which results in a higher catalytic activity than photocatalysis and piezocatalysis alone. Active species that are involved in the catalytic process are found to be •O2– < OH• < h+, indicating the significance of piezocatalysis and photocatalysis. The degradation efficiency of sodium niobate (NaNbO3) nanorods for Rhodamine B in the presence of both sunlight and ultrasonic vibration is 98.9% within 60 min (k = 7.6 × 10–2 min–1). The piezo potential generated by NaNbO3 nanorods was reported to be 16 V. The antibacterial activity of the produced sample was found to be effective against Escherichia coli. With inhibitory zones of 23 mm, sodium niobate has a greater antibacterial activity.

1. Introduction

Ferroelectric sodium niobate (NaNbO3) has exceptional characteristics such as nontoxicity, chemical stability, high crystallinity, d33 constant ∼12 m·V–1, and minimal ecological effect,1 making it suitable for piezoelectric and ferroelectric applications.2 Many innovative heterogeneous metal oxide semiconductor materials, such as TiO2, ZnS, ZnO, and Fe2O3 nanocatalysts, have been developed in recent years for environmental remediation. Among the investigated semiconductor photocatalysts, the NaNbO3 perovskite nanostructures have received a lot of interest recently for their advantageous qualities such as strong physicochemical stability, high crystallinity, low cost, abundance, and minimal environmental effect. NaNbO3 is a versatile oxide. Sodium niobate, NaNbO3, is an ecologically friendly photocatalyst that has received a lot of attention. Contrarily, sodium niobate (NaNbO3) can function as a photocatalyst in the photocatalytic hydrogen production reaction.3 Recent studies indicate that sodium niobate (NaNbO3) can perform a significant role in hydrogen production, CO2 removal, and pollutant deterioration because of its exceptional morphologies and properties.4−6 Unfortunately, the photoinduced charge carrier’s recombination rate and large band gap limit the photocatalytic activity of sodium niobate (NaNbO3) under ultraviolet (UV) light only. To solve this problem, different techniques such as coupling with narrow-energy-band-gap semiconductors, doping with some other elements, or self-doping have been used to reduce its energy band gap and accelerate the segregation of electron (e–)–hole (h+) pairs.7

Photocatalysis is an advanced oxidation process (AOP) that uses photoenergy to decompose dyes and pollutants. Under light illumination, electrons (e–) and holes (h+) are produced, which can then trigger certain active species like the superoxide ion (•O2–) and hydroxyl radical (•OH) with a great oxidation capacity to degrade harmful organic pollutants into harmless inorganic compounds without secondary contamination.8−10 However, although photocatalysis has been broadly reported, the practical uses of photocatalytic dye degradation are limited due to its high cost, lack of reactivity to dark conditions, low photoconversion efficiency, and charge recombination.11 Lately, there has been a surge in attention in piezoelectric polarization in wastewater treatment.12

Piezoelectric materials utilize mechanical energy to produce a substantial number of electrical charges that interact with water and result in dye degradation, a process known as piezocatalysis.13−17 Piezoelectric materials are extensively employed in transducers, sensing, actuators, etc. because they generate electricity when mechanically deformed.18 In the environment, there are many different types of vibration energies, including river flow, airflow, and human motion.19 Moreover, throughout the vibrational energy harvesting procedure of piezoelectric materials, a piezo potential is created, which helps enhance the charge carrier separation. Generally, piezoelectric polarization may be utilized in two manners to degrade organic contaminants. First, piezoelectric polarization is employed to boost photocatalytic performance by improving the segregation of photogenerated charge carriers. Second, it can operate as an independent tractive force to initiate a catalytic reaction.20 Wang et al. proposed a new mixed-oxide composite (MnOx-CeO2) photocatalyst that may generate synergistic effects between pyrocatalysis and photocatalysis, effectively increasing the organic dye degradation ratio.21

Any substance with antibacterial killing properties is desirable, and developing self-cleaning gadgets will be a bonus. Self-cleaning substances can help tackle a variety of environmental issues by disinfecting surfaces and removing dangerous bacteria. Researchers discovered that compounds having catalysts and antibacterial activity can help address several hospital-acquired disorders.22 In hospitals and medical laboratories, this will provide a sanitary environment.

In this work, the improved catalytic breakdown of Rhodamine B organic dye (pollutant) in the presence of hydrothermally produced ferroelectric NaNbO3 nanorods is accomplished and the antibacterial performance of the NaNbO3 nanorods was evaluated for the first time. This is due to the combined effect of photocatalysis and piezocatalysis, i.e., the synergistic effect. The photopiezocatalysis decomposition ratio is clearly larger than that of piezocatalysis and photocatalysis individually.

2. Catalyst Characterization

The crystal structures of the hydrothermally synthesized sodium niobate (NaNbO3) nanorods were identified by X-ray diffraction (XRD) with Cu Kα irradiation (2θ = 20–70°, λ = 1.5406 Å) at 27 °C. The morphology of the prepared samples was evaluated by EI Nova Nano FESEM 450 scanning electron microscopy (SEM) and transmission electron microscopy (The Tecnai G2 20 S-TWIN [FEI]). The elemental composition of sodium niobate (NaNbO3) was analyzed by X-ray photoelectron spectroscopy (XPS) (model ESCA+ Omicron Nano Technology), which consists of an ultrahigh-vacuum compartment linked to a monochromatic Al Kα-radiation outlet with an energy of 1486.7 eV and a 124 mm hemispherical electron detector. The optical characteristics of the prepared samples were investigated using a dual-beam U-3300 Hitachi spectrophotometer. The crystallographic orientation of NaNbO3 was assessed by an IRIX STR 500 Raman spectrometer. The samples were recorded at 27 °C with an Ar-ion laser excitation at 532 nm (∼1 mW, 50× objective). The production of the (OH•) hydroxyl radical was investigated using a Perkin Elmer LS 55 fluorescence spectrophotometer.

3. Results and Discussion

X-ray diffraction (XRD) patterns of the produced pristine NaNbO3 nanorods are depicted in Figure 1a. It was shown that all of the peaks (101), (121), (031), (220), (202), (141), (123), (242), (024), and (204) coincide well with the single orthorhombic phase of sodium niobate (NaNbO3) (JCPDS- 082-0606) with lattice parameters a = 5.56 Å, b = 7.79 Å, and c = 5.51 Å and having a space group P21ma, which corresponds to noncentrosymmetric and ferroelectric phases.23−25 There were no additional peaks, which indicates that the prepared sample was contamination-free. The crystallite size was determined using Scherrer’s equation from the highest intense peak, which is 17.69 nm.26

3. 1

Here, D is the crystallite size in nm, Scherrer’s constant k is 0.9, wavelength of the X-ray λ is 1.54 Å, β is FWHM (radians), and θ is the peak position.

Figure 1.

Figure 1

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Structural analysis of NaNbO3 nanorods: (a) X-ray diffraction pattern and (b) Raman spectra.

Figure 1b depicts the Raman spectra of the prepared pristine NaNbO3 nanorods. The spectra were taken at room temperature under 532 nm laser excitation. All of the peaks in the span of 150–1000 cm–1 are involved in the intrinsic modes of NbO6. The triply degenerate v5 (F2g) and v6 (F2u) are linked to the zone between 150 and 300 cm–123. The peak that appeared at 139 cm–1 is due to the NbO6 rotation. The peak corresponding to v5 (F1u) occurred at 433 cm–1. The peak at 602 cm–1 is associated with v1 (A1g). Lastly, the peak at 870 cm–1 corresponds to v1(A1g) + v5(F2g) modes,27 confirming the development of the ABO3 structure.28

Figure 2 shows the X-ray photoelectron spectroscopy (XPS) spectra of NaNbO3 across a wide energy range. Figure 2a is the survey spectra of NaNbO3 nanorods; the spectrum revealed no contamination. Long-term exposure to the atmosphere and/or remnants of the starting ingredients are likely to cause carbon contamination. High-resolution spectra of Na 1s, O 1s, and Nb 3d are depicted in Figure 2b–d. Figure 2b is the Na 1s orbital peak, which appeared at 1071.15 eV.29 The O 1s high-resolution spectra are shown in Figure 2c; the peaks are split into three contributions at 529.64, 531.14, and 534.53 eV. The most intense peak was observed at 529.64 eV, which is attributed to the lattice oxygen of sodium niobate (NaNbO3). The other peak that appeared at 532.26 eV has a higher binding energy, attributed to the surface-absorbed hydroxy group (−OH), and the peak at 534.53 eV appeared due to the surface-adsorbed water.30−32 The Nb 3d high-resolution spectrum is depicted in Figure 2c; it shows that the Nb 3d spectra further split into two peaks, one appears at 206.54 eV, which corresponds to Nb 3d5/2, and the other peak appears at 209.34 eV, which corresponds to Nb 3d3/2.33

Figure 2.

Figure 2

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XPS spectrum of NaNbO3 nanorods. (a) XPS survey, (b) binding energy spectra of Na 1s, (c) binding energy spectra of O 1s, and (d) binding energy spectra of Nb 3d.

The morphology of the prepared NaNbO3 sample was identified by a scanning electron microscope and a transmission electron microscope. Figure 3a displays the TEM image of NaNbO3, which clearly reveals the rodlike morphology. Figure 3b shows the SEM image of sodium niobate (NaNbO3), which has a rod morphology, with a length of 1–2 μm and a diameter of 100 nanometers.34 The elemental mapping scans (Figure 3c–f) demonstrate that NaNbO3 nanorods contain Na, Nb, and O elements with a homogeneous distribution.35

Figure 3.

Figure 3

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(a) TEM image, (b) SEM image, and (c–f) elemental mapping images of NaNbO3 nanorods.

The optical properties of prepared sodium niobate nanorods were evaluated by UV–vis spectroscopy. The optical spectra of NaNbO3 are depicted in Figure 4a, which has an absorbance below 350 nm. Figure 4b shows the direct band gap of the hydrothermally synthesized NaNbO3 nanorods, which can be obtained by Tauc’s equation

3. 2

Here, α is the absorption coefficient, hν is the energy of the incident photon, Eg is the band gap of the prepared sample, and n = 2 for the direct transition. The data acquired from the absorption spectrum may be plotted, (αhν)2vs (hν), as illustrated in Figure 4b. It shows that the plot achieved is the tangent to the linear component of the curves in a specific location.36 Extending this tangent to the (hν) axis, where (αhν)2 is zero, yields the energy band gap (Eg).37 The energy band gap of NaNbO3 nanorods has been obtained to be 3.6 eV.7,38 The segregation, emigration, and recombination mechanisms of electrons and holes are important variables in evaluating catalytic performances, which are revealed through photoluminescence (PL) spectroscopy. In general, a lower PL spectrum intensity equates to a quicker charge separation capability, as shown in Figure 4c.30Figure 4d shows the fluorescence spectra obtained by employing terephthalic acid as a photoluminescent capturing agent to trap the intermediate products hydroxyl radicals (•OH). The concentration of •OH produced in water determines the intensity of fluorescence under ultrasonic vibrations. The intensity of the •OH radical is directly proportional to the ultrasonic vibration time. As the vibration time increases, the formation of the •OH radical increases steadily. This indicates that the piezoelectric effect plays a substantial part in the synergistic photopiezocatalytic activity of NaNbO3 nanorods.39

Figure 4.

Figure 4

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Optical characterizations. (a) UV–vis (ultraviolet to visible) absorption spectra of NaNbO3 nanorods, (b) energy band gap of NaNbO3 nanorods, (c) photoluminescence spectra of NaNbO3 nanorods, and (d) photoluminescence spectrum for the detection of the OH• radical using terephthalic acid.

Accordingly, we assess the photocatalytic, piezocatalytic, and photopiezocatalytic performances of the sodium niobate (NaNbO3) nanorods by evaluating the decomposition ratio of Rhodamine B dye in an aqueous solution. To establish the adsorption–desorption equilibria between Rhodamine B molecules and NaNbO3 nanorods, 1 mg/mL catalyst was placed in a glass beaker, followed by steady stirring at 500 rpm for 30 min in the darkness. It was found that 12.5% Rhodamine B dye gets adsorbed on the catalyst surface in the dark. Figure 5a shows the photocatalytic activity of NaNbO3 nanorods in the Rhodamine B dye solution. The adsorption peak of Rhodamine B dye occurs at 554 nm and diminishes as the time of light exposure increases. The Rhodamine B solution is partially destroyed after 100 min of light exposure. Figure 5b shows the results of the piezocatalytic study using NaNbO3 nanorods. Rhodamine B dye has been shown to be substantially degraded after 80 min of reaction time. Figure 5c shows the deterioration of Rhodamine B organic dye in the presence of photo-piezo bicatalysis NaNbO3 nanorods. The figure illustrates that, in the presence of both ultrasonic vibrations and sunlight, the absorption peak of Rhodamine B rapidly decreases as the catalytic duration increases.40 A comparison of different catalytic environments is shown in Figure 5d. When sodium niobate (NaNbO3) nanorods were added to the Rhodamine B solution and exposed to ultrasonic vibrations, the Rhodamine B solution slowly deteriorated (red curve), i.e., the color of the Rhodamine B dye changes slightly. The deterioration rate of the Rhodamine B dye increases when the sodium niobate (NaNbO3) nanorods and the Rhodamine B dye are mixed and kept under sunlight (black curve). On the other hand, the color of the Rhodamine B solution fades from dark pink to a clear solution after being subjected to the combined effect of ultrasonic vibrations and sunlight (blue curve). In the absence of a catalyst, there is no apparent photocatalytic or piezocatalytic dye degradation. In addition, the Rhodamine B dye decomposition ratios rise approximately linearly with time in Figure 5e using the study data points from Figure 5a–c into a pseudo-first-order kinetic rate equation (kt = ln(C0/C)). In contrast, the slope of photopiezocatalysis is greater than that of photocatalysis and piezocatalysis, implying that the bicatalysis process has a faster decomposition rate.41Figure 5f depicts that the decomposition ratios for photocatalysis, piezocatalysis, and photo-piezo bicatalysis are 28.64, 75.59, and 98.9%, respectively. However, the synergistic catalytic activity of photocatalysis and piezocatalysis may be slightly lower than the total of individual catalysis. The combined impact of photoelectric effect and piezoelectric effect plays a key part in the synergistic effects of photopiezocatalysis. Simultaneously, the multiplier effect resulting from the pairing of photopiezocatalysis plays a significant role. Because of the modest connection between photoelectric and piezoelectric effects, the genuine degradation ratio of photopiezocatalysis is somewhat less than that of the total of the degradation ratios of piezocatalysis and photocatalysis.42−44 COD measurements were performed to analyze the mineralization of Rhodamine B during photocatalysis, piezocatalysis, and photopiezocatalysis, as depicted in Figure S3. The COD value drops from 25 to 6 mg/L before and after photopiezocatalysis. The degradation efficiency of the catalyst decreases as the dye concentration increases because at high dye concentrations the generation of •OH radicals on the surface of the catalyst is reduced since the active sites are covered by dye ions. Figure S3 shows that the degradation efficiencies of 10, 15, 30, and 50 mg/L reached 98.9, 75.4, 20, and 0% within 60 min of exposure time. The photopiezocatalytic activity of various catalysts with their removal efficiency is shown in Table 1.

Figure 5.

Figure 5

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(a) Photocatalytic activity, (b) piezocatalytic activity, and (c) photopiezocatalytic activity of NaNbO3 nanorods for the deterioration of Rhodamine B dye solution under the influence of sunlight, ultrasonic vibrations, and sunlight + ultrasonic vibrations. (d) Decomposition efficiency of NaNbO3 nanorods under various catalytic conditions. (e) Kinetic order curve with kinetic rate constants k1 = −0.006 46 min–1, k2 = −0.030 22 min–1, and k3 = −0.076 min–1. (f) Decomposition ratio curve.

Table 1. Different Photopiezocatalysts with Their Degradation Efficiencies toward Different Dye Pollutants.

photopiezocatalyst pollutant concentration (mg/L) degradation time (min) removal efficiency (%) refs.
BST-PDMS foam 25/Rhodamine B 90 97.8 (45)
ZnO nanorods 10/Acid orange 7 100 80.8 (40)
BiOIO3 10/Rhodamine B 60 98.6 (46)
KNbO3/ZnO 10/Methyl Orange 90 90 (47)
BiFeO3/TiO2 10/Methyl Violet 120 98 (48)
BiObr 10/Rhodamine B 80 100 (49)
NaNbO3 10/Rhodamine B 60 98.9 present work
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The surface area of the NaNbO3 catalyst was measured to be 7.12 m2/g. As depicted in Figure 6, a feasible approach for photopiezocatalysis of NaNbO3 nanorods has been offered. When the electrons in a sodium niobate (NaNbO3) nanorod capture photoenergy when exposed to sunlight, they can migrate to the conduction band (C.B.), leaving an equivalent number of h+ (holes) behind in the valence band (V.B.). Simultaneously, when sodium niobate (NaNbO3) nanorods are exposed to ultrasonic vibrations, it generates a piezoelectric potential.50 The spatial segregation of photoinduced electrons (e–) and holes (h+) is enhanced due to the generated piezoelectric potential. This improved segregation of (e–) and holes (h+) will accelerate the redox process,51 resulting in a synergistic relationship impact between photocatalysis and piezocatalysis. The overall reaction for the photopiezocatalysis can be expressed as

3. 3

The produced holes (h+) can immediately bind the OH– in the solution, resulting in the formation of the OH• radical, as depicted in eq (4)

3. 4

Conversely, the dissolved O2 in solution may absorb the produced electron (e–) and convert it into the superoxide ion (•O2–), as depicted in eq 5

3. 5

The superoxide ion (•O2–) can react to form the OH• radical, as depicted in eqs 6–8

3. 6
3. 7
3. 8

The OH• radical and the •O2– superoxide ion are highly active species, and they will react with Rhodamine B dye molecules, as shown in eq 9

3. 9

The catalytic activity of sodium niobate (NaNbO3) nanorods gets affected on addition of some active species, as shown in Figure 7. The photopiezocatalytic activity of NaNbO3 nanorods may be detected in the presence of different radical scavengers, although the decomposition ratios are substantially reduced when compared to the absence of scavengers. This is due to the fact that scavenging agents have caught a part of the active species, limiting the photopiezocatalysis activity. In Figure 7a, the catalytic process was substantially reduced when 1 mM IPA (isopropanol alcohol, OH• scavenger) was added, and only 17.42% Rhodamine B dye degraded. In Figure 7b, the catalytic process was also reduced when 1 mM EDTA (ethylenediaminetetraacetate, h+ scavenger) was added, and it was observed that only 33% Rhodamine B dye was degraded. In Figure 7c, the catalytic process was substantially inhibited when 1 mM BQ (benzoquinone, •O2– scavenger) was added, and only 44% Rhodamine B dye was degraded. In addition, Figure 7d represents the conclusions of the reactive species capturing study. It is apparent that the breakdown ratios of Rhodamine B in the presence and absence of various types of scavenging agents •O2– (superoxide radical), h+ (holes), and OH• (hydroxyl radical) are 98.9, 44, 33, and 17.42%, respectively, which signifies that all of these reactive species occur in the photopiezocatalytic process of NaNbO3 nanorods. Figure 7e illustrates the reusability of the sodium niobate nanorod (NaNbO3) catalyst in the presence of both sunlight and ultrasonic vibrations. It was observed that the degrading efficiency of sodium niobate nanorods (NaNbO3) was almost constant for up to four cycles, with the catalyst’s efficiency being marginally reduced in the fifth cycle. As the number of deterioration cycles increases with the reused sodium niobate nanorods (NaNbO3), the catalyst’s efficiency begins to decline after a few cycles.

Figure 6.

Figure 6

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Schematic demonstration of the synergistic effect of photopiezocatalysis for the deterioration of Rhodamine B dye.

Figure 7.

Figure 7

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Impact of various scavengers: (a) isopropanol alcohol (IPA), (b) ethylenediaminetetraacetate (EDTA), and (c) benzoquinone (B.Q) on NaNbO3 nanorods Rhodamine B decomposition. (d) Comparative decomposition ratios with and without scavengers. (e) Catalyst reusability.

In addition, Figure 8 illustrates an open-circuit voltage (OCV) recorded by a digital scanning oscilloscope (DSC). When NaNbO3 nanorods are subjected to ultrasonic vibrations, the potential generated in response to mechanical vibrations reaches the maximum value of 14.9 V, as shown in Figure 8a. When the mechanical vibrations are on, the produced piezo potential provides positive signals, indicating that stress was imposed on the NaNbO3 nanorods, and when the mechanical vibrations are off, it shows negative signals, indicating that stress was removed from the NaNbO3 nanorods. In the absence of mechanical vibrations, no potential was generated, indicating that there is no piezoelectric effect in the absence of mechanical stress.52 Furthermore, it is widely accepted that the efficiency of piezoelectric materials is regulated by the rate of stress applied and released; a quicker pace leads to an enhanced output owing to a greater quantity of stored charge over a given duration. Figure 8b shows an open-circuit voltage (OCV) when NaNbO3 nanorods are subjected to manual tapping, and it was observed that the voltage generated through tapping was 16.62 V. Although our thumb injected pressure in the second case (Figure 8b), the uncontrolled circumstances of imposed and withdrawn pressure can merely result in some abnormalities of negative and positive signals.53

Figure 8.

Figure 8

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Piezo-potential formation (a) under ultrasonic vibrations and (b) with hand tapping.

Antibacterial activities of the NaNbO3 nanorods against E. coli were studied by the well diffusion method using a Luria Bertani agar, and the 100 μL of 1 g L–1 NaNbO3 nanorods exhibited superior antibacterial activities by producing a 23 mm inhibition zone, as recorded in Figure 9a, which is higher than other catalysts. The photopiezocatalytic inactivation effect was also evaluated using NaNbO3 nanorods as a function of light and mechanical vibrations at different time intervals (as recorded in Figure 9b). The results depicted the increased antibacterial activities of NaNbO3 nanorods with increasing photopiezocatalytic time since the number of colonies produced on the LB agar medium reduced with increased reaction time. After 120 min of catalytic process, the NaNbO3 nanorods completely eliminated the E. coli. In the present study, NaNbO3 nanorods were found to have superior E. coli antibacterial activities, which could mostly be due to light absorption resulting in the generation of high reactive oxygen species, and the generated piezo potential results in the separation of these generated reactive species. The direct interaction between the catalyst (NaNbO3 nanorods) and E. coli is a vital factor in contact killing. According to reports, the hydroxyl radical (OH•) and anionic radical attack the cell wall of the bacteria from the outside, whereas H2O2 penetrates the cell directly.54 The concentration of E. coli with the treatment time is depicted in Table 2.

Figure 9.

Figure 9

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(a) Zone of inhibition in E.coli. (b) Photopiezocatalysis impact on E. coli bacterial colony formation in the presence of NaNbO3 nanorods.

Table 2. E. coli Concentration with Treatment Time.

time point (min) Log10 CFU mL–1
0 4.041393
30 2.60206
90 1.69897
120 0
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4. Conclusions

In conclusion, the improved catalytic performance of hydrothermally synthesized sodium niobate (NaNbO3) nanorods is achieved by harvesting sunlight and ultrasonic vibration energy jointly on the premise of the synergistic impact of photopiezocatalysis. The deterioration of Rhodamine B was studied under visible illumination, ultrasound vibrations, and combining visible illumination and ultrasonic vibration, yielding decomposition ratios of 28.64, 75.59, and 98.9%, respectively. The antibacterial performance of the sodium niobate (NaNbO3) nanorods against E. coli was examined, and it was revealed that the NaNbO3 has antibacterial properties. The increased catalytic performance of sodium niobate (NaNbO3) nanorods is because of the piezoelectric potential established during the piezocatalysis process, which can assist in the segregation of photogenerated charge carriers, culminating in a synergistic effect between piezocatalysis and photocatalysis. It depicts the proposed method of environmental purification by maximum utilization of photon energy and vibration energy. The investigated results show the potential of NaNbO3 for industrial wastewater treatment and biomedical applications.

5. Experimental Method

5.1. Synthesis of Sodium Niobate (NaNbO3) Nanorods

Sodium niobate (NaNbO3) nanorods were made in two processes via the hydrothermal method. In step one, 4 g of Nb2O5 was dissolved in 12 M sodium hydroxide (NaOH) solution and agitated overnight to produce Na2Nb2O6·H2O. Step two involved transferring the intermediate solution to a stainless-steel hydrothermal setup and keeping it at 150 °C for 4 h. After completing the reaction, the solid residue was collected by filtering or centrifugation after cooling (naturally), rinsed with water and ethanol, and evaporated in a vacuum oven for 6 h at 70 °C.55,25 Lastly, the prepared NaNbO3 nanorods were annealed at 450 °C for 3 h.56

5.2. Catalytic Activity Measurement

The degradation of the Rhodamine B dye (10 mg/L) was used to evaluate the synergetic photopiezocatalysis behavior of NaNbO3. The 50 mL Rhodamine B solution was divided into three glass beakers. To establish the adsorption–desorption equilibria between Rhodamine B molecules and NaNbO3 nanorods, 1 mg/mL catalyst was put in a glass beaker, followed by steady stirring at 500 rpm for 30 min in the darkness. One glass beaker was placed in the sunlight with continual stirring at 500 rpm to investigate the photocatalytic efficiency of the catalyst. During this experiment, cold water was pumped across the glass beaker to keep it at ambient temperature.57,58 The second glass beaker was subjected to an ultrasound bath (120 W, 40 kHz) to investigate the piezocatalyst effect of NaNbO3 nanorods, and the third beaker was subjected to the combined effect of ultrasonic vibration and sunlight to investigate the synergistic effect.

5.3. Catalyst Renewable Assessment

Following the photopiezocatalytic deterioration of Rhodamine B organic dye, the used sodium niobate (NaNbO3) nanorods were successfully retrieved by a centrifuge, rinsed three to four times using distilled water, and then dehydrated for 2 h in a vacuum heater at 60 °C. These regained nanorods were then utilized to degrade new Rhodamine B dye solution, and this procedure was repeated five times to investigate the repeatability of sodium niobate (NaNbO3) nanorods.

5.4. Scavenging Test

This experiment was performed to identify the active species responsible for the deterioration of Rhodamine B dye by photopiezocatalytic activity of the sodium niobate (NaNbO3) nanorods. The hydroxy radical (OH•) scavenger was isopropanol alcohol (IPA), the superoxide (•O2–) scavenger was para- benzoquinone (BQ), and the hole (h+) scavenger was ethylenediaminetetraacetate (EDTA). All of these scavengers were added to a separate beaker with the catalyst and Rhodamine B solution, and these mixtures were kept under the reaction setup of photopiezocatalysis.

5.5. Antibacterial Activities

5.5.1. Zone of Inhibition Assessment

The antibacterial activities of NaNbO3 nanorods were determined as the zone of inhibition using the agar well diffusion method against E. coli ATCC 25922 using the published protocol of Wong et al. (2019) with minor modifications.59 Initially, 200 μL of an overnight-grown E. coli suspension (>107 CFU mL–1) was spread onto the surface of LB agar Petri plates, and then, wells were punctured in LB agar Petri plates using a sterile cork-borer. Afterward, 100 μL of homogenized NaNbO3 nanorods (1 g L–1) was loaded into each well and incubated overnight at 37 °C for the development of the zone of inhibition. The measurement of zone of inhibition was performed by measuring the mean diameter around the NaNbO3 nanorods in millimeter.

5.5.2. Screening for Antibacterial Behavior

In the photopiezocatalytic inactivation experiment, 1 mL of overnight-grown culture was inoculated in 10 mL of fresh LB medium and incubated at 37 °C for 2 h to optimize the E. coli growth condition. Subsequently, 1 g L–1 NaNbO3 nanorod catalyst was added into the medium and then magnetically stirred for proper mixing in the dark for 10 min prior to mechanical vibrations using a vertex. At 0, 30, 90, and 120 min time intervals, the E. coli solutions (500 μL) were pipetted and spread onto the LB agar plates and incubated at 37 °C for 24 h for the formation of bacterial colonies.

Acknowledgments

The authors would like to thank the Department of Science and Technology (DST), India, and the Water Technology Initiative (WTI) for providing financial support for this scientific endeavor.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c06109.

  • XRD pattern of NaNbO3 before and after reaction (Figure S1); SEM images of NaNbO3 before and after reaction (Figure S2); COD analysis of Rhodamine B with different treatments: photocatalysis, piezocatalysis, and photopiezocatalysis (Figure S3); and degradation efficiency curve with different dye concentrations (Figure S4) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c06109_si_001.pdf (425.6KB, pdf)

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