1. Introduction
Rapid and widespread industrialization worldwide is the primary cause of liquid waste generation by various industries. The textile industries contribute significantly to the global wastewater volume due to their enormous water consumption, thereby posing significant risks of water and soil pollution [
1]. Textile industries consume a wide range of manufactured colorants and discharge a significant quantity of colored organic dyes, which have a negative impact on the photosynthetic function of plants and aquatic life [
2,
3,
4]. The dyes utilized in the textile industry are stable under various environmental conditions, including exposure to light and fluctuations in temperature [
3,
5]. The persistence and resistance to degradation of dyes present significant challenges for wastewater treatment plants. Therefore, the photocatalytic approach for water treatment has recently gained extensive attention in academic communities [
6].
The rationale behind the photocatalytic approach is based on the in situ generation of highly reactive transient species for the mineralization of refractory organic compounds, ultimately transforming them into harmless carbon dioxide and water. A variety of semiconductor catalysts have demonstrated its efficiency in degrading a wide range of refractory organics, such as TiO
2 [
7], ZnO [
8], CdS [
9], BaTiO
3 [
10], etc., and the degradation process exhibits excellent performance, operating at ambient pressure and temperature, low cost, and with a lack of secondary waste formation [
11]. BaTiO
3 is considered an efficient semiconductor photocatalyst among various photocatalytic materials due to its perovskite structure, appropriate valance band (VB)/conduction band (CB) positions, availability in a wide variety of sizes and morphologies, environmental friendliness, and good stability [
12]. In a typical study, Alammar et al. [
13] synthesized BaTiO
3 nanoparticles by one-step room-temperature ultrasound synthesis in an ionic liquid. The photocatalytic activity of the obtained sample was evaluated by the degradation of methylene blue (MB) under UV irradiation. The removal rate for the MB solution (10 mg/L) was approximately 40% within 180 min, which highlighted the promising application potential of BaTiO
3 nanoparticles in the field of photocatalytic degradation of organic dyes. Nevertheless, the photocatalytic activity of a general BaTiO
3 nanoparticle photocatalyst is inherently limited by its low specific surface area [
14] and the high recombination rates of photoinduced charged species (electrons and holes) [
15]. Surface morphology and size control is a crucial approach for enhancing the photocatalytic efficiency of materials, as it facilitates an increased number of reactive sites. Numerous studies have been dedicated to synthesizing BaTiO
3-based materials with various morphologies, such as one-dimensional (1D) nanotubes [
16], two-dimensional (2D) nanoflowers [
17], and three-dimensional (3D) aerogels [
18], aiming to enhance their photocatalytic efficiency. Currently, the literature on BaTiO
3-based materials predominantly focuses on 1D and 2D materials rather than 3D aerogel materials. Despite the exceptional potential of aerogel materials in adsorption and photocatalysis due to their unique surface area and abundant pore structure [
19,
20,
21], the synthesis of BaTiO
3 aerogel remains a highly intricate challenge that has yet to be fully resolved [
18,
22]. Chau et al. [
18] employed chitin nanocrystals as a liquid crystal template for the synthesis of BaTiO
3 aerogel. The obtained BaTiO
3 aerogel exhibited an attenuated specific surface area of only 50 m
2/g due to the removal of the chitin template at a high temperature (900 °C). The photocatalytic degradation efficiency towards MB exhibited a slight improvement compared to that of P25. Rechberger et al. [
22] present a strategy for assembling surface-functionalized nanocrystalline BaTiO
3 particles into a highly porous macroscopic framework. The gelation mechanism is based on a rapidly induced destabilization of the BaTiO
3 nanoparticles in dispersion. The bulk aerogel remained stable after supercritical drying and exhibited an unprecedentedly high surface area of over 300 m
2/g, but the photocatalytic property has not been discussed. The aforementioned strategy, while capable of yielding aerogel samples with a substantial specific surface area, entails a complex synthesis process that is unsuitable for large-scale industrial production. The deposition of noble metals is frequently employed to effectively tackle the critical issue of high recombination rates of photoinduced charged species (electrons and holes). The nanostructured metals can act as electron traps, leading to improvement in the separation of electrons and holes, and consequently enhancing photocatalytic activity [
23,
24]. Liu et al. [
15] successfully prepared the flower-like BaTiO
3 nanotube arrays (NTAs) by a hydrothermal method using TiO
2 NTAs as precursors. Ag-loaded BaTiO
3 NTAs were formed by a photochemical reduction method. The photocatalytic activity of Ag-BaTiO
3 NTAs was evaluated from the analysis of the photodegradation of methyl orange (MO). It can be seen that silver-modified BaTiO
3 NTAs have a much higher photocatalytic degradation rate than that of pure BaTiO
3 NTAs. Loading noble metal nanoparticles onto BaTiO
3 can establish a Schottky barrier, wherein the Fermi level of the metal nanoparticles is positioned lower than that of the CB of BaTiO
3. Consequently, photogenerated electrons migrate from the CB of BaTiO
3 to the metal nanoparticles until their Fermi levels align, resulting in photogenerated holes’ formation within the VB. Thus, spatial separation of photogenerated electron–hole pairs occurs. The noble metal nanoparticles serve as electron traps capable of capturing excited electrons, thereby facilitating efficient separation and reduced recombination of electron–hole pairs [
25]. However, the dispersibility of noble metal nanoparticles on the catalyst surface may be constrained due to the small specific surface area of the parent material. Previous studies frequently employed graphene as a support material for loading photocatalysts, aiming to augment the specific surface area and improve the distribution of metal particles on the catalyst surface. Passi et al. [
26] designed the Ag-BaTiO
3/GO nanocomposite, and the results showed that GO nanosheets served as the ground for both photodeposited Ag and BaTiO
3. To date, research on using a bare catalyst material with a large specific surface area for surface modification by noble metal nanoparticles has been less frequently reported.
Herein, a liquid-phase and template-free synthetic route was employed to obtain a novel BaTiO3 aerogel through the co-gelling of metallic alkoxides and supercritical drying without any heat treatment. Subsequently, noble metal Ag nanoparticles were deposited onto the aerogel surface via the photochemical deposition method. The exceptional surface area and abundant pore structure of the as-prepared aerogel photocatalyst provide a great number of active sites, while the surface modification with silver nanoparticles effectively enhances the capture of photogenerated electrons, thereby ensuring an elevated level of photocatalytic efficiency. The preparation process of aerogel solely entails facile agitation and supercritical drying, rendering it highly amenable to industrialization. Furthermore, this method exhibits the potential for broad application in the synthesis of various perovskite aerogels. Consequently, the experimental methodologies exhibit universality, ease of operation, and suitability for large-scale industrial production.
2. Results and Discussion
The conventional process for synthesizing aerogels involves five steps: sol–gel, solvent exchange, aging, supercritical drying, and heat treatment [
27,
28]. The BaTiO
3 nanoparticles (BTO NPs) were synthesized utilizing the aforementioned procedure. The aged gel underwent supercritical drying and calcination, resulting in the formation of crystalline BaTiO
3. As shown in
Figure 1a, the obtained diffraction peaks of BTO NPs positioned at 21.9° (100), 31.3° (110), 38.7° (111), 45.0° (200), 50.7° (210), 55.9° (211), and 65.5° (220) can be well assigned to a cubic phase of BaTiO
3 (JCPDS 31-0174) [
29]. Nitrogen adsorption/desorption studies (
Figure 1b and
Table 1) show that BTO NPs have a mesoporous structure with a pore size distribution at ~20 nm and a low specific surface area of ~22 m
2/g. The absence of an aerogel structure in the prepared BTO NPs is evident due to their limited specific surface area and lack of a well-developed pore structure. We assume that the primary impediment to the successful synthesis of BaTiO
3 aerogels through conventional methods lies in the limited hydrolytic capacity of inorganic salts (Ba
2+), thereby hindering their involvement in gel network formation (tends to form TiO
2). Only when the calcination of the gel precursor can lead to the incorporation of Ba
2+ into TiO
2 can perovskite BaTiO
3 crystals be generated. The material undergoes significant shrinkage due to high-temperature calcination, resulting in the deterioration of its pore structure and subsequent failure in aerogel formation. Given the above, we have employed metal alkoxides with hydrolysis rates comparable to that of titanium (IV) isopropoxide to optimize the preparation process. The synthesis of BaTiO
3 aerogels was accomplished through the co-gelation of metal alkoxides and supercritical drying, eliminating the need for additional heat treatment. The specific preparation process can be found in
Section 4 (Materials and Methods). The results are presented in
Figure 1 and
Table 1, where the as-synthesized BTO-1, BTO-2, and BTO-3 aerogels also exhibit a crystalline cubic phase of BaTiO
3 (JCPDS 31-0174) and reveal a significantly enhanced specific surface area in comparison to that of BTO NPs. As summarized in
Table 1, the specific surface areas of the three aerogels are ~120 m
2/g (BTO-1), ~220 m
2/g (BTO-2), and ~90 m
2/g (BTO-3), respectively. These values are several to even ten times larger than those of BTO NPs. The adsorption/desorption isotherms of all the aerogels are identified as type IV with an H-1 type hysteresis loop, which is a typical characteristic of mesoporous materials [
30]. The presence of a three-dimensional continuous mesoporous structure is further supported by the exceptional total pore volume and average pore sizes (
Table 1). BTO-2 exhibits a more remarkable porous structure than the other two samples, and its specific surface area is more than four times higher than that of the reported BaTiO
3 aerogel prepared via the template methods in the literature [
18]. Therefore, the modified aerogel procedure offers a more user-friendly and efficient approach to synthesizing BaTiO
3 aerogels. Since the metal alkoxide is based on the zero-valent metal reacting directly with the alcohol, the modified aerogel procedure demonstrates significant potential for wide-ranging applications in synthesizing various perovskite aerogels. In order to explore the optical properties of BaTiO
3 aerogel samples, the UV-Vis absorption spectra were recorded. The UV-Vis absorption spectra of BaTiO
3 aerogel samples were baseline-corrected. In
Figure 1c, the absorption spectra of the three aerogel samples exhibit a high degree of similarity, indicating a clear and evident trend. The bandgaps of BTO-1, BTO-2, and BTO-3 samples were further determined by analyzing the (Ahν)
1/2-hν relationship curve, which was derived using the formula proposed by Tauc, Davis, and Mott et al. The (Ahν)
1/2-hν curve for bandgap calculation was conducted according to the baseline-corrected absorption spectra. Here, hν was set as the
x-axis, (Ahν)
1/2 was set as the
y-axis, and the reverse extension of the tangent line intersected with the
x-axis. The intersection on the
x-axis represents the value of the optical bandgap. As shown in
Figure 1d, the calculated bandgaps for all the samples are ~3.2 eV. The results are consistent with those reported in the literature [
10,
12].
In the preparation of aerogels, alcohols are frequently selected as solvents to effectively reduce the shrinkage of the materials during gelation and supercritical drying processes. Through extensive and ongoing investigation into the synthesis of aerogels, it has been found that that a combination of alcohol and a hydrophobic solvent promotes the formation of high-quality aerogels. According to the literature [
31], the inclusion of toluene in the binary solvent appears to act as a surfactant, which reduces surface tension at the gas–liquid–pore interfaces. As a result, wet gels are formed that have lower density and higher porosity, ultimately resulting in products with larger surface areas. Different alcohols in mixture with toluene lead to aerogels with different properties: micromorphology, specific surface area, pore volume, and pore size distribution. The impact of this binary solvent on the aerogel process necessitates further investigation.
Figure 2 shows SEM images of BTO NPs and BaTiO
3 aerogels. Obviously, BTO NPs present a disordered structure with an irregular arrangement of nanoparticles, as shown in
Figure 2a. Among all the three aerogel samples, only the BTO-2 sample exhibits a continuous three-dimensional network skeleton, as shown in
Figure 2c. The distinct three-dimensional network skeleton structure is not observed in BTO-1 and BTO-3 samples, while the micromorphology appears to exhibit particle agglomeration, as shown in
Figure 2b,d. The aforementioned findings are in line with the data presented in
Table 1. In conclusion, the ethanol and toluene mixture emerges as the optimal solvent for synthesizing BaTiO
3 aerogel, with superior characteristics in terms of specific surface area and pore structure. In contrast to the conventional sol–gel approach used for synthesizing BaTiO
3 NPs, barium ethylate can participate in gel network formation through hydrolysis. During the initial stages of gelation in this system, a Ti-rich oxide network is formed, accompanied by a significant presence of unreacted Ba ions within the liquid filling the pores due to the high solubility of Ba(OH)
2 (formed by hydrolysis) in the ethanol and toluene mixture solvent. The Ba(OH)
2 in the ethanol and toluene mixture solvent subsequently undergoes co-gelation, leading to the formation of chemical bonds within the gel network and resulting in the crystalline BaTiO
3 particulate gel. The bonding between the Ti and Ba species is speculated to involve a repeating unit of (-Ti-O-)
nBa
2+, according to the literature [
32]. Therefore, the corresponding aerogel can be obtained by supercritical drying of the crystalline BaTiO
3 particulate gel without additional heat treatment. Although further investigation has been conducted on the impact of binary solvents on the aerogel process, regrettably, due to the insufficient crucial data and experimental evidence, a clear mechanism elucidating why ethanol–toluene in the binary solvents is more suitable for BaTiO
3 aerogel preparation than methanol–toluene and isopropanol–toluene remains elusive.
The micromorphology was further studied by transmission electron microscopy (TEM).
Figure 3 shows TEM micrographs of BTO-1, BTO-2, and BTO-3 aerogel samples. The BTO-1 aerogel sample exhibits a morphology characterized by short rod-shaped particles (
Figure 3a) while BTO-2 and BTO-3 aerogel samples exhibit a morphology characterized by irregular spherical particles (
Figure 3b,c). Evidently, the directional growth of BaTiO
3 grains is constrained by the binary solvent system comprising methanol and toluene. Surface morphology and size control is a crucial approach for enhancing the photocatalytic efficiency of materials. The impact of micromorphology on photocatalytic efficiency is further investigated in the subsequent photodegradation experiments. The high-resolution TEM (HRTEM) micrographs further prove that the lattice stripe spacing is 0.404 nm (inset in
Figure 3a), 0.285 nm (inset in
Figure 3b), and 0.134 nm (inset in
Figure 3c), corresponding to the (100), (110), and (300) crystal planes of the cubic phase BaTiO
3, respectively. The results are consistent with the XRD spectrum (
Figure 1a).
The BTO-2 aerogel sample synthesized by using an ethanol–toluene binary solvent exhibits superior microstructure and pore structure, which is a very suitable method for Ag nanoparticles’ surface modification in the next work.
Figure 4 shows TEM and HRTEM micrographs of BTO-2 aerogel samples with varying amounts of Ag deposition. It is evident in
Figure 4a–c that Ag nanoparticles are uniformly distributed on BTO-2 aerogel in all modified samples. Upon increasing the Ag deposition amount from 1% to 5%, a noticeable augmentation in the size of Ag nanoparticles is observed, from several nanometers to dozens of nanometers (
Figure 4d–f). Furthermore, the size differences of Ag nanoparticles grown on different exposed crystal planes can also be observed. The dimension of Ag nanoparticles on the (100) crystal plane of BaTiO
3 in the 3% Ag/BTO-2 sample shown in
Figure 4e is approximately 10 nm, while that on the (210) crystal plane measures around 5 nm. The observed phenomenon suggests that the prepared BaTiO
3 aerogel exhibits a polycrystalline nature, with distinct crystal planes exposed. Some of these crystal planes demonstrate enhanced suitability for Ag nanoparticle growth, while others exhibit slower rates of Ag nanoparticle formation. Works of literature [
33,
34] have revealed that the phenomenon is primarily associated with the surface energy of the exposed crystal plane. High-energy surface atoms exhibit high activity and are easy to combine with foreign atoms such as the Ag or Au atoms to form a stable structure and significantly increase the number of loading noble metal nanoparticles on the material surface. The impact of different Ag deposition amounts on photocatalytic efficiency is further investigated in the subsequent photodegradation experiments.
The degradation of MO by BTO NPs, BTO-1, BTO-2, and BTO-3 aerogel under ultraviolet light irradiation was studied. In a typical adsorption experiment, we investigated the adsorption of aerogel of MO over a longer period of time (60 min) and observed consistent equilibrium adsorption amounts at both 30 min and 60 min. The C/C
0 values at 30 min and after 1 h of stirring in the dark are shown in
Table 2. The results indicate that the adsorption–desorption equilibrium of the MO and the aerogel can be established in 30 min. To strike a balance between experimental efficiency and achieving the complete establishment of the adsorption–desorption equilibrium, we determined the optimal adsorption time to be 30 min. As shown in
Figure 5a, after the adsorption equilibrium is reached in the dark (although
Figure 5a–c only provide the C/C
0 value at the adsorption endpoint, it can be determined that the adsorption–desorption equilibrium has been reached), the removal efficiency (adsorption) of all BaTiO
3 aerogel samples of MO surpasses that of bare BaTiO
3 NPs, owing to their porous structure. Specifically, the adsorption efficiency of BaTiO3 NPs was found to be less than 1%, whereas the three aerogel samples exhibited adsorption efficiencies of 5% (BTO-1), 14% (BTO-2), and 2% (BTO-3), respectively. There is no doubt that the three-dimensional continuous porous structure of BaTiO
3 aerogels provides a convenient transfer channel for the adsorption of MO. The BTO-2 aerogel sample exhibited superior adsorption efficiency, attributed to the exceptional specific surface area and total pore volume, whereas the adsorption efficiency of BTO-1 and BTO-3 samples was only marginally higher than that of BTO NPs due to their inadequate specific surface area and total pore volume. When it comes to photocatalysis after ultraviolet light irradiation, the removal efficiency of all BaTiO
3 aerogel samples was also higher than that of bare BaTiO
3 NPs. It is noteworthy that despite BTO-1 exhibiting a larger specific surface area and total pore volume compared to BTO NPs, the removal efficiencies of both samples were remarkably similar, with 43% for the former and 42% for the latter. Possible reasons include the potential impact of micromorphology, as nanorod-shaped particles (BTO-1) may exhibit lower photocatalytic efficiency compared to spherical particles (BTO-2, BTO-3, and even BTO NPs). Similar results were reported in the literature [
17], demonstrating that the photocatalytic activity of the BaTiO
3 material with square particles was higher compared to spherical nanoflowers and short rods. The removal efficiency of the BTO-2 aerogel sample for MO was almost 80% after 60 min of illumination, which is nearly twice as high as that of the BTO NPs sample. On the one hand, the three-dimensional continuous porous structure of aerogel provides a convenient transfer channel for the in situ degradation of MO [
35]. On the other hand, it facilitates an increased number of reactive sites [
36,
37]. Therefore, the BTO-2 sample exhibits enhanced photocatalytic efficiency. In order to further improve the photocatalytic efficiency of the sample and increase its commercial value, we also studied the effect of the introduction of noble metal Ag nanoparticles on the performance of the samples. As shown in
Figure 5b, the surface modification with Ag nanoparticles effectively enhanced the MO removal efficiency. MO was almost completely degraded by the Ag nanoparticles deposited on the BaTiO
3 aerogel samples after 60 min of illumination, while the optimum removal efficiency of the BaTiO
3 aerogel samples was 80% compared with the unmodified sample. The introduction of noble metal Ag nanoparticles onto BaTiO
3 leads to the spatial separation of photogenerated electron–hole pairs through a Schottky barrier [
25]. As a result, it contributes to the enhanced photocatalytic activity of Ag nanoparticles deposited onto BaTiO
3 aerogel. A similar trend of enhancement was observed in the adsorption efficiency of MO. The three Ag nanoparticles deposited on the BaTiO
3 aerogel samples exhibited adsorption efficiencies of 26% (5% Ag/BTO-1), 54% (5% Ag/BTO-2), and 27% (5% Ag/BTO-3), respectively. The enhanced adsorption of dye molecules by noble metals can be attributed to the potential alteration in the electronic state of the substrate resulting from the deposition of noble metal nanoparticles on its surface [
23]. In acidic media of pH below 6.5, BaTiO
3 surfaces are positively charged, while MO is negatively charged [
38,
39]. Due to the strong electrostatic attraction between these charged species, the facile adsorption of MO on the surface of BaTiO
3 can be observed. The interaction between Ag and BaTiO
3 may potentially alter the electronic state of BaTiO
3 by the possible transferred charge carriers, thereby resulting in an enhanced adsorption phenomenon. Relevant occurrences have previously been documented [
40,
41]. However, the previously mentioned photocatalyst was constrained by its limited specific surface area, and it exhibited a slight increase in adsorptive capacity. The adsorption capacity was significantly enhanced when using a bare catalyst material with a large specific surface area for surface modification by metal nanoparticles. This may be attributed to the fact that the substrate material, which has a larger specific surface area, provides the necessary conditions for achieving uniform dispersion of Ag on its surface. As the sample with the highest adsorption and photocatalytic efficiency, BTO-2 was further studied for its photocatalytic performance with different Ag deposition amounts. The results are shown in
Figure 5c. Evidently, the augmentation of Ag deposition quantity leads to varying degrees of enhancement in both the adsorption efficiency and photocatalytic efficiency of the modified BTO-2 aerogel samples. The Ag nanoparticles deposited on the BTO-2 aerogel samples exhibited adsorption efficiencies of 15% (1% Ag/BTO-2), 19% (3% Ag/BTO-2), 24% (4% Ag/BTO-2), and 54% (5% Ag/BTO-2), respectively. The size of Ag nanoparticles exhibited a positive correlation with the deposition amount (
Figure 4). In general, larger Ag nanoparticles exhibited an enhanced adsorption capacity for MO due to the potential alteration in the electronic state of the substrate resulting from the deposition of noble metal nanoparticles on its surface, and they exhibited higher photocatalytic efficiency due to the spatial separation of photogenerated electron–hole pairs through a Schottky barrier.
To further investigate the photocatalytic activities of BTO NPs, BaTiO
3 aerogel samples, and Ag nanoparticles deposited onto BaTiO
3 aerogel samples, pseudo-first-order kinetics were employed to analyze the photocatalytic degradation kinetics. The kinetics of photocatalytic degradation of MO solution were calculated from the simplified form of the Langmuir–Hinshelwood model, which can be described as Equation (1):
where
C0 is the initial concentration (mg/L) of the MO solution and
C is the concentration (mg/L) of the MO solution at any time
t (min), and
k is the pseudo-first-order rate constant (min
−1). Obviously, the degradation rate
k can be obtained from the slope of
ln(
C0/
C) vs.
t and the time
t1/2 required to degrade half of the MO solution can be calculated by
ln2/
k. The calculation results of various dynamics parameters have been summarized and presented in
Table 3. The squared correlation coefficient (
R2) falls within the range of 91.44–99.67%, indicating the reliability of the fitting curve for the pseudo-first-order kinetic equation. The photocatalytic degradation of MO catalyzed by the 5% Ag/BTO-2 aerogel sample exhibited the highest reaction rate, with
k = 0.1181 min
−1, which was almost 4.9-fold higher than that of the initial BTO-2 sample (
k = 0.0243 min
−1) and 13.7-fold higher than that of the BTO NPs (
k = 0.0086 min
−1).
Table 4 lists the photocatalytic performance of various catalysts for the degradation of MO. The BaTiO
3 aerogel in this work (BTO-2) exhibits enhanced photocatalytic degradation performance compared to the traditional BaTiO
3 nanomaterials and commercialized P25. The use of a bare BaTiO
3 aerogel with a large specific surface area as the ground for photodeposited Ag nanoparticles has also achieved impressive results. The 5% Ag/BTO-2 aerogel exhibits superior or comparable photocatalytic degradation performance to other BaTiO
3 composite photocatalysts. Thus, the utilization of material modification techniques based on high-performance photocatalysts with exceptional specific surface areas presents unparalleled advantages in augmenting the capabilities and broadening the scope of photocatalysts for future applications. The recycling and long-term stability of catalysts are pivotal attributes for practical applications. The stability and reusability of the Ag nanoparticles deposited onto the BaTiO
3 aerogel photocatalysts were further evaluated through additional experiments, as depicted in
Figure 5d. After five cycles of photocatalytic degradation of MO under identical experimental conditions, the 5% Ag/BTO-2 aerogel exhibited a remarkable preservation rate of 96.4% for the removal efficiency. Furthermore, the aerogel can be effectively recycled for subsequent cycles through a straightforward filtration and washing process prior to each cycle. The error in each experimental group was exceptionally small, indicating the robustness of the synthesis process and the consistent performance of the material.
In order to comprehend the photocatalytic mechanism of BaTiO
3 aerogels, it is imperative to determine their energy-band potentials, as the redox ability of photogenerated electrons and holes is closely associated with these potentials. The VB and CB potentials of BaTiO
3 can be calculated by Equations (2) and (3) [
42,
43]:
where
EVB and
ECB are the VB edge and CB edge potentials (eV vs. NHE),
χ is the absolute electronegativity of the BaTiO
3 aerogel. The term is defined as the arithmetic mean of the electron affinity and the first ionization energy of the constituent atoms and can be calculated to be 5.25 eV, according to the literature [
44].
Ee is the energy of free electrons on the hydrogen scale (~4.5 eV). The bandgap energy
Eg of BaTiO
3 aerogel is 3.15 eV, as shown in
Figure 1d. As a result, the VB edge and CB edge potentials are estimated to be 2.32 and −0.83 eV vs. the normal hydrogen electrode (NHE), respectively.
Figure 5e shows an energy diagram with the valence and the conduction bands of BaTiO
3 and the energies of O
2/
and H
2O/·OH couples. It is clearly visible that the
EVB of BaTiO
3 aerogel is more positive than the redox potential of H
2O/·OH (2.27 eV vs. NHE) and the
ECB of BaTiO
3 aerogel is more negative than the standard redox potential of O
2/
(−0.046 eV vs. NHE) [
45], resulting in the formation of a large amount of
and ·OH radicals. The schematic diagram depicting the photocatalytic mechanisms of Ag nanoparticles deposited onto BaTiO
3 aerogel is presented in
Figure 5f. Under ultraviolet light, photogenerated electrons and holes are produced in the 5% Ag/BTO-2 sample. The photogenerated electrons transfer from the O 2p orbital (VB) to the Ti 3d orbital (CB), subsequently migrating towards the metallic Ag and accumulating on its surface. These electrons can be rapidly transferred to the adsorbed oxygen on the Ag surface, resulting in the generation of reactive oxygen species (
). Concurrently, the photogenerated holes can efficiently transfer to the aerogel surface, leading to the generation of highly reactive ·OH radicals. Both ·OH radicals and
radicals possess potent oxidizing capabilities, facilitating the complete oxidation of MO into H
2O and CO
2. Acting as photogenerated electron traps, Ag nanoparticles enhance the rate of electron transfer to molecular oxygen and suppresses the recombination of photogenerated electrons and holes. Meanwhile, the three-dimensional porous structure of the BaTiO
3 aerogel provides suitable transport channels for adsorption and photocatalysis and generates more active sites. As a result, these structure and components contribute to the enhanced photocatalytic activity of Ag nanoparticles deposited onto BaTiO
3 aerogel.
Table 2.
The C/C0 values at 30 min and after 1 h of stirring in the dark.
Table 2.
The C/C0 values at 30 min and after 1 h of stirring in the dark.
Adsorption Time (min) | C/C0 (%) |
---|
BTO NPs | BTO-1 | BTO-2 | BTO-3 | 5% Ag/ BTO-1 | 5% Ag/ BTO-2 | 5% Ag/ BTO-3 | 1% Ag/ BTO-2 | 3% Ag/ BTO-2 | 4% Ag/ BTO-2 |
---|
30 | 99.37 | 95.06 | 85.75 | 98.47 | 73.88 | 45.51 | 72.76 | 84.87 | 80.94 | 76.01 |
60 | 99.35 | 95.10 | 85.83 | 98.44 | 73.96 | 46.02 | 72.68 | 84.64 | 81.06 | 75.97 |
Table 3.
The various calculated dynamics parameters for photodegradation.
Table 3.
The various calculated dynamics parameters for photodegradation.
Sample | k (min−1) | t1/2 (min) | R2 (%) |
---|
BTO NPs | 0.0086 | 80.6 | 96.08 |
BTO-1 | 0.0099 | 70.0 | 99.67 |
BTO-2 | 0.0243 | 28.5 | 91.44 |
BTO-3 | 0.0182 | 38.1 | 98.09 |
5% Ag/BTO-1 | 0.0518 | 13.4 | 92.30 |
5% Ag/BTO-2 | 0.1181 | 5.9 | 98.96 |
5% Ag/BTO-3 | 0.0521 | 13.3 | 95.32 |
Table 4.
Photocatalytic performance of various catalysts for the degradation of MO.
Table 4.
Photocatalytic performance of various catalysts for the degradation of MO.
Photocatalyst | Morphology | Photocatalytic Performance | Photodegradation Kinetics | Ref |
---|
BTO-2 (1g/L) | Aerogels | 80% for MO, 60 min (10 mg/L, 100 mL) | 0.0243 min−1 | This work |
BTO-NPs (1g/L) | Irregular NPs | 42% for MO, 60 min (10 mg/L, 100 mL) | 0.0086 min−1 | This work |
BaTiO3 (—) | Coral cluster | 65% for MO, 150 min (Not mentioned) | 0.0074 min−1 | [46] |
BaTiO3 (—) | Nanocube | 76% for MO, 45 min (20 mg/L, 60 mL) | Not mentioned | [47] |
P25 (0.04 g) | Irregular NPs | ~52% for MO, 90 min (20 mg/L, 150 mL) | Not mentioned | [48] |
5% Ag/BTO-2 (1g/L) | Aerogels | 99% for MO, 40 min (10 mg/L, 100 mL) | 0.1181 min−1 | This work |
Ag/BaTiO3 (6 cm2) | Nanotube and flower | 98% for MO, 60 min (20 mg/L, 75 mL) | 0.0707 min−1 | [15] |
BaTiO3@g-C3N4 (0.5 g/L) | Irregular NPs | 76% for MO, 360 min (5 mg/L) | Not mentioned | [43] |
BaTiO3/In2S3 (0.5 g/L) | Core–shell | 93% for MO, 90 min (10 mg/L, 100 mL) | 0.0334 min−1 | [49] |
Bi2O3/BaTiO3 (2 g/L) | Irregular NPs | 99% for MO, 50 min (10 mg/L, 300 mL) | 0.1100 min−1 | [14] |
BaTiO3/rGO (0.05 g) | Nanosheet and NPs | 70% for MO, 20 min (0.05 mM, 50 mL) | 0.0556 min−1 | [50] |