Photocatalysis technology is considered a promising method for air and sewage purification because it is efficient, economical, and environmentally viable ^{[1, 2, 3, 4, 5, 6]}. Titanium dioxide (TiO₂) has also received increased attention because of its nontoxicity, high chemical stability, and low cost ^{[7, 8, 9]}. However, TiO₂ can only exhibit photocatalytic activity under UV light. Furthermore, its poor adsorption capacity and high recombination rate of photogenerated electron-hole pairs hinder its practical application ^{[10, 11, 12]}. To resolve this problem, various methods, such as chemical modification ^[13], metallization ^[14], and the development of novel heterostructure semiconductors ^{[15, 16]}, have been employed to enhance the charge-separation efficiency. For example, Tang et al. ^[17] reported that the Ag/AlO₂/TiO₂ heterojunction presented higher photocatalytic activity for decomposing formaldehyde under sunlight irradiation than did pure TiO₂ and AgAlO₂. This was attributed to the heterojunction providing a matching energy band structure. Lee et al. ^[18] found that TiO₂/CuO nanofiber exhibited high photocatalytic properties for degrading and cleaning the organics produced from dye wastewater, which can be ascribed to the high quantum efficiency.

In addition, researchers have found that silver oxide (Ag₂O) can act as a novel visible-light-driven semiconductor material because of its unique band gap, good sensitivity to light, facile preparation, and inexpensiveness, meaning that this could be an excellent charge-separation promoter and built-in acceptor ^{[19, 20, 21]}. Ag particles are also used as a cleaning agent to remove bacteria from contaminated water ^{[22, 23, 24]}. Theoretically, the conduction band (CB) of Ag₂O is more negative than the corresponding band of TiO₂, and the valence band (VB) of TiO₂ is more positive than that of Ag₂O ^[25]. Their matching band gaps and positions might favor the formation of Ag₂O-TiO₂ heterostructures, which expands the photoabsorption range and facilitates the separation of photogenerated electrons and hole pairs ^{[26, 27, 28, 29, 30, 31]}. However, their application is also limited by weak adsorption and the difficulty in separating them from wastewater.

Sepiolite is a microfibrous clay mineral consisting of a 2:1 layered structure built by two tetrahedral silica sheets with a central magnesia sheet ^[32]. Its peculiar structural tunnels lead to interesting surface properties and high absorption capacity ^[33]. Zhang et al. ^[34] studied bicrystalline TiO₂ by supporting it on porous sepiolite clay for the photocatalytic degradation of gaseous formaldehyde, wherein the TiO₂/sepiolite showed a higher photocatalytic activity than commercial Degussa P-25 or bare TiO₂. Moreover, the layered structure of sepiolite exhibited higher physicochemical stability. Thus, sepiolite may also be a good support for the Ag₂O-TiO₂ heterojunction catalyst, which can improve the catalyst adsorption capacity and catalyst separation, and its recycling from wastewater.

In the present work, heterostructure Ag₂O-TiO₂/sepiolite composites were synthesized by a simple two-step method. Their photocatalytic activities were investigated by the photocatalytic degradation of acid red G (ARG) aqueous solution and gaseous formaldehyde under visible-light irradiation (λ > 420 nm). The structural features of the composites were analyzed through systematic characterization, and the visible-light photocatalytic mechanism of the Ag₂O-TiO₂/sepiolite composites is discussed in detail.

The sepiolite sample used in this study was from Hunan Province, China. All chemical reagents were of analytical grade and used without further purification. The synthesis of Ag₂O-TiO₂/sepiolite composite comprises two main processes.

TiO₂ sol was prepared by an acid-catalyzed sol-gel method from a TiCl₄ precursor (Sinopharm Chemical Reagent Co., Ltd., China). TiCl₄ (8 mL) was added gradually to a HCl solution (22 wt%) under continuous stirring for 0.5 h at 25 °C, and then aged for 6 h to obtain a transparent TiO₂ sol. Then, the TiO₂ sol was added dropwise into sepiolite powder saturated with deionized water (1 wt%) under vigorous stirring for 0.5 h at 70 °C, before being aged for 12 h to obtain individual TiO₂ loadings of 30 wt% on the sepiolite support (30%TiO₂/sepiolite). The resulting mixed suspensions were centrifuged at 5000 r/min several times and then washed with deionized water to neutralize the supernatants until no Cl⁻ was detected by AgNO₃ solution. After removing all excessive chloride, the samples were dried at 70 °C for 5 h.

Ag₂O-TiO₂/sepiolite composites were synthesized via an impregnation method. TiO₂/sepiolite powder (1.00 g) was dispersed in 18.5 mL of AgNO₃ solution (0.05 mol/L) to produce a suspension (Ag/powders = 10 wt%). After stirring in the dark for 2 h, the resulting suspension was dried at 60 °C and then calcined at 100, 200, 350, and 450 °C for 30 min under ambient conditions. The resulting samples were stored in the dark in the form of fine white powders (< 400 °C), in which the Ag₂O phase may exist on the surface of the TiO₂/sepiolite composites. The color of the Ag₂O-TiO₂/sepiolite composites changed to gray by increasing the calcination temperature to 450 °C, owing to the decomposition of Ag₂O to metallic Ag ^[25]. For comparison and to investigate the effect of Ag₂O particles on the photocatalytic performance of sepiolite clay, pure Ag₂O-TiO₂ and Ag₂O/sepiolite composites were also prepared following the same procedure to provide 10%Ag₂O-30%TiO₂/ sepiolite (200 °C).

X-ray diffraction (XRD, Rigaku D/MAX-RB diffractometer, operating at 40 kV and 50 mA with Cu K_α radiation, λ = 0.15406 nm) was used to determine the structure and crystallinity of the as-prepared samples. X-ray photoelectron spectroscopy (XPS) data were collected using an ESCALAB II XPS system operating in hybrid mode, with a monochromatic Mg K_α source and a charge neutralizer. All binding energies obtained by the XPS spectral analysis used the C 1s peak at 284.5 eV as a reference. The morphologies of the samples were observed by scanning electronic microscopy (SEM, JSM5610LV). The absorption edge of the samples was measured by an ultraviolet-visible (UV-vis) spectrophotometer (UV-2550, Shimadzu), for which the raw sepiolite and TiO₂-supported catalyst samples were used as the reflectance standard. A N₂ adsorption-desorption isotherm was obtained at liquid nitrogen temperature (-196 °C) using a Quantachrome AUTOSORB-1 nitrogen adsorption apparatus. The specific surface area was determined by the multi-point BET method and the pore sizes were measured by the BJH method of adsorption. The Fourier transform infrared (FT-IR) spectra of the chemical bonds on the surface of the samples were obtained using a Thermo Nicolet Nexus spectrometer. Photoluminescence (PL) spectra were recorded via a fluorescence spectrophotometer (RF-5301PC, Shimadzu) with an excitation wavelength of 312 nm, which is capable of detecting the hydroxyl radical (^•OH) during the photocatalytic degradation process.

The photocatalytic degradation experiments of ARG dye and gaseous formaldehyde were performed under visible-light irradiation at room temperature. A desired amount (0.15 g) of the as-prepared catalyst was homogeneously suspended in the ARG dye aqueous solution (100 mg/L) under continuous stirring for 30 min in a dark environment to establish the adsorption-desorption equilibrium. The reaction was started by direct exposure to visible-light irradiation (300 W Dy lamp) with a 420-nm cutoff filter. At given time intervals of illumination, small aliquots of the stirred suspension were drawn and centrifuged to remove the photocatalyst. The supernatant liquor of the ARG dye was analyzed by UV-vis spectrophotometry (UV751GD, China) at its maximum absorption wavelength (λ_max) of 505 nm.

Gaseous formaldehyde degradation was conducted in a closed stainless steel photoreactor with an inner volume of 0.8 L. The as-prepared catalyst, with a dosage of 0.15 g, was dispersed in a dish of diameter 70 mm. Then the sample was set into the gas photoreactor, and formaldehyde solution (2 μL) was injected into the photoreactor by microsyringe. The reactor was blown by a fan for the formaldehyde to slowly volatilize until the initial concentration reached equilibrium. After several minutes, a 300 W Dy lamp was turned on to irradiate the photoreactor with a 420-nm cutoff filter. A 1412 photoacoustic field gas-monitor (Innova AirTech Instruments, Denmark) was applied to detect the concentration of gaseous formaldehyde, and the mineralization products (CO₂ and H₂O) were also monitored. The tests were performed at room temperature (25 °C).

XRD was used to investigate the crystal structure of the catalyst particles. Fig. 1(a) shows the XRD patterns over a scan range of 5° to 70° for the sepiolite, TiO₂/sepiolite, and Ag₂O-TiO₂/sepiolite composites obtained at different temperatures. It is found that all samples show the main phase of sepiolite, for which the diffraction peaks at 2θ = 8.78°, 12.2°, 24.8°, and 26.6° are indexed to the (110), (130), (231), and (080) planes, respectively, of the pristine sepiolite (JCPDS 26-1226).

Fig. 1. (a) XRD patterns of sepiolite (1), TiO2/sepiolite (2), Ag2O-TiO2/sepiolite (100 °C) (3), Ag2O-TiO2/sepiolite (200 °C) (4), Ag2O-TiO2/sepiolite (350 °C) (5), and Ag2O-TiO2/sepiolite (450 °C) (6); The corresponding enlarged diffraction areas from 30° to 34° (b) and 43.5° to 45° (c) for some typical samples.

Meanwhile, the intensities of the (130), (231), and (080) peaks of the sepiolite gradually decreased by increasing the calcination temperature, which indicates that the layered structure of sepiolite was, to some extent, destroyed ^{[35, 36]}. In addition, the diffraction peaks at 2θ = 25.3°, 48.1°, and 55.1° are indexed to the (101), (200), and (211) planes, respectively, of the anatase phase of TiO₂ (JCPDS 83-2243), whereas the peaks at 2θ = 27.4° and 36.0° are indexed to the (110) and (101) planes, respectively, of the rutile phase of TiO₂ (JCPDS 78-2485) ^[37]. There are commonly two naturally occurring phases of titania (rutile and anatase) when the calcination temperature is above 400 °C ^[33], and the thermal stability of the anatase crystalline phase for Ag₂O-TiO₂/sepiolite composites can likely be attributed to the stabilizing effect of the silica that exists in the sepiolite framework ^[32]. In Fig. 1(a), the peaks corresponding to the Ag₂O phase are not clearly detectable. To further demonstrate the phase structures of the Ag on the surface of the sepiolite, the weak diffraction peaks of Ag₂O at 2θ = 32.7° (JCPDS 12-0793) and metallic Ag at 2θ = 44.3° (JCPDS 04-0783) were further detected, as shown in Fig. 1(b) and (c). The samples obtained at 200 and 350 °C were found to contain the Ag₂O phase (Fig. 1(b)). Metallic Ag was observed as the calcination temperature was increased to 450 °C (Fig. 1(c)), which can be attributed to the decomposition of Ag₂O ^[38]. In this study, TiO₂ could exist as a mixture of nanosized anatase and rutile in the composites, and the formation of the Ag₂O phase could be inhibited by controlling the calcination temperature below 450 °C.

XPS was employed to analyze the chemical state of the elements on the surface of the Ag₂O-TiO₂/sepiolite composites, and the results are shown in Fig. 2(a). In Fig. 2(b), the Ag 3d region shows XPS peaks with two individual peaks at 368.3 and 374.3 eV, which are assigned to the Ag 3d_5/2 and Ag 3d_3/2 peaks in Ag₂O phase ^{[39, 40]}, respectively. In Fig. 2(c), the Ti 2p region exhibits two individual peaks at 458.7 and 464.3 eV, which indicate that Ti⁴⁺ of TiO₂ is the major Ti species in the Ag₂O-TiO₂/sepiolite composites ^{[41, 42, 43]}. In addition, it can be seen that the binding energy values of the Ti 2p region are slightly increased, which is attributed to the interaction of Ag ions with the TiO₂ surface ^{[44, 45]}. In Fig. 2(d), the main peak of the O 1s region with a binding energy of 530.1 eV is assigned to the oxygen in the TiO₂ crystal lattice ^{[38, 39, 43]}, while the peak at 532.4 eV is assigned to the metal-OH bonds ^[46]. The formation of silver oxide is attributed to the Ag⁺ ions absorbing oxygen ions originating from the TiO₂ metal crystal lattice or from dispersed -OH in the clay ^[47].

Fig. 2. XPS spectrum of Ag2O-TiO2/sepiolite composites (a) and high-resolution XPS spectra of Ag 3d region (b), Ti 2p region (c), and O 1s region (d).

The morphologies of the sepiolite, TiO₂/sepiolite, and Ag₂O-TiO₂/sepiolite composites were characterized by SEM. In Fig. 3(a), the sepiolite exhibits a smooth and flat surface, while the TiO₂/sepiolite composite presents a relatively rough and loose-knit section (Fig. 3(b)). From Fig. 3(c), it can be seen that a large number of the TiO₂ and Ag₂O particles assembled on the surface of the sepiolite, and their shapes are quite irregular. The morphology of the sepiolite clay shows no obvious difference, while the layered structures of the mineral congeries were partially destroyed when the TiO₂ nanoparticles were introduced into the sepiolite by the sol-gel process.

Fig. 3. SEM images of raw sepiolite (a), TiO2/sepiolite (b), and Ag2O-TiO2/sepiolite composites (c).

HRTEM was used to further investigate the phase structure of the Ag₂O-TiO₂/sepiolite composite. Fig. 4(a) and (b) show typical TEM images of the Ag₂O-TiO₂/sepiolite composite with a layered structure, which is consistent with the SEM observations. Fig. 4(c) and (d) show HRTEM images recorded from the white framed areas indicated in Fig. 4(b), in which it can be seen that two crystals of Ag₂O and TiO₂ are tightly interconnected, and the lattice fringe spacing of 0.272 nm (Fig. 4(c)) corresponds to the (111) crystallographic plane of Ag₂O. The observed lattice spacings of 0.351 and 0.324 nm (Fig. 4(d)) correspond to the (101) plane of the anatase and the (110) plane of the rutile phase, respectively. The HRTEM images further confirm that heterostructures of Ag₂O and TiO₂ have been formed in the composites ^[48].

Fig. 4. TEM (a,b) and HRTEM (c,d) images of the Ag2O-TiO2/sepiolite composite prepared at 200 °C.

The N₂ adsorption-desorption isotherm and pore size distribution curve of the Ag₂O-TiO₂/sepiolite composite (200 °C) are shown in Fig. 5. The BET surface area, pore volume, and average pore size of the different samples are summarized in Table 1. The surface area (60.39 m²/g) of the Ag₂O-TiO₂/sepiolite composite prepared at 200 °C is five times higher than that of the raw sepiolite (12.21 m²/g). Taking into account the calcination temperature, Ag₂O-TiO₂/sepiolite composites show a higher thermal stability and a loss of specific surface area of between 15% and 33% ^[49]. The N₂ adsorption-desorption isotherm of the composite seems to be type IV, according to the IUPAC classification, which demonstrates the presence of mesopores (2-50 nm) ^[26]. The shape of the hysteresis loops is of type H3, which is the main feature of a multiporous structure ^{[32, 50]}. The continuously increased adsorption branch at relative pressure (p/p₀ = 0.01-1.0) indicates capillary condensation of N₂ molecules inside the different pore-sized structure (micropores and mesoporous) ^{[50, 51, 52]}.

Fig. 5. N2 adsorption-desorption isotherm of the Ag2O-TiO2/sepiolite composite prepared at 200 °C. The inset is the corresponding pore size distribution.

Table 1 Specific surface areas and pore parameters of sepiolite, TiO2/sepiolite, Ag2O-TiO2, and the Ag2O-TiO2/sepiolite composites prepared at different temperatures. Sample BET surface area (m2/g) Pore volume (cm3/g) Average pore size (nm) Sepiolite 12.21 0.06 69.46 TiO2/sepiolite 63.21 0.10 37.60 Ag2O-TiO2 3.13 0.01 42.19 Ag2O-TiO2/sepiolite (100 °C) 51.21 0.09 71.29 Ag2O-TiO2/sepiolite (200 °C) 60.39 0.10 43.66 Ag2O-TiO2/sepiolite (350 °C) 40.44 0.07 47.74 Ag2O-TiO2/sepiolite (450 °C) 44.99 0.07 90.60

Table 1 Specific surface areas and pore parameters of sepiolite, TiO2/sepiolite, Ag2O-TiO2, and the Ag2O-TiO2/sepiolite composites prepared at different temperatures.

Typical FT-IR spectra of sepiolite and the Ag₂O-TiO₂/ sepiolite composite before and after photocatalytic degradation of ARG dye are illustrated in Fig. 6. The bands observed at 3668 cm⁻¹ can be attributed to the stretching vibrations of hydroxyl groups (Mg₃OH) and water molecules bound to the octahedral sheets of Mg ions in the sepiolite clay ^{[53, 54, 55]}. The bands between 3435 and 1637 cm⁻¹ can be assigned to the O-H stretching mode and the H-O-H bending mode from the interbedded water molecule of the clay ^[37]. The bands at 1114 and 1033 cm⁻¹ are due to the stretching of Si-O in the Si-O-Si groups of the tetrahedral sheet, and the peak at 637 cm⁻¹ is assigned to OH bending vibrations ^[56]. As reported, the Si-O-Al (octahedral) bending vibrations are present at 469 cm^{−1 [57]}. As shown in Fig. 6(b), the disappearance of the band at 430 cm⁻¹ is assigned to Si-O-Si bending vibrations ^[54], and the shift of the bands at 3424 and 1631 cm⁻¹ might be caused by changes to the chemical bond vibration for TiO₂ particles inserted into the layer of sepiolite or the bonding of Ti-OH ^[58], indicating that the layered structure of sepiolite has been destroyed during the preparation process. Moreover, the characteristic bands of Si-O in the Si-O-Si groups of the tetrahedral sheets (around 1114, 1033, and 469 cm⁻¹) still exist, and the new peak at 908 cm⁻¹ is caused by a Ti-O-H stretching vibration. In addition, the intensities of the vibration modes at 637-1033 cm⁻¹ are decreased, which indicates that the organic matter on the clay was removed in the calcination process. The FT-IR spectrum of the recovered composite does not show the characteristic peak of ARG dye, which confirms that other than that which was adsorbed by the composite, the ARG was degraded completely by the photocatalysis. Meanwhile, the result also indicates that the Ag₂O-TiO₂/sepiolite composite has good stability.

Fig. 6. FT-IR spectra of sepiolite (1) and the Ag2O-TiO2/sepiolite composite before (2) and after (3) photocatalytic reaction.

The UV-vis diffuse reflectance spectra of sepiolite, TiO₂, Ag₂O, TiO₂/sepiolite, and Ag₂O-TiO₂/sepiolite composite are shown in Fig. 7. The absorption edge of the Ag₂O-TiO₂/sepiolite composite tends to red-shift compared with that of the TiO₂/sepiolite sample. The band gaps of pure TiO₂ and pure Ag₂O are calculated to be 2.9 and 1.0 eV, respectively. The Ag₂O-TiO₂/sepiolite composite exhibited enhanced photoabsorption from the UV light region to the visible-light region in the range 370-430 nm. The results indicate that the Ag₂O nanoparticles loaded onto the TiO₂/sepiolite have a suitable band gap for photocatalytic decomposition of organic pollutants in the visible-light region.

Fig. 7. UV-vis diffuse reflectance spectra of sepiolite, TiO2, Ag2O, TiO2/sepiolite, and Ag2O-TiO2/sepiolite composite.

The photocatalytic activities of the Ag₂O-TiO₂/sepiolite composites were evaluated by photocatalytic degradation of ARG dye solution and gaseous formaldehyde under visible-light irradiation. As shown in Fig. 8, it is clear that the TiO₂/sepiolite composites showed little photocatalytic performance. However, after coating Ag₂O nanoparticles, the Ag₂O-TiO₂/sepiolite composites showed an obviously enhanced photocatalytic activity and ARG was completely decomposed with an increase in irradiation time. Specifically, when the calcination temperature is 200 °C, the obtained Ag₂O-TiO₂/sepiolite composite shows superior photocatalytic activity and the concentration of ARG was decreased by 98% after 40 min, whereas only about 24% of ARG could be removed over the TiO₂/sepiolite after 40 min irradiation. Furthermore, the photocatalytic degradation of the ARG solution followed a pseudo-first-order reaction rate ^[59]. Thus, the rate constant (k, min⁻¹) of the ARG decomposition over Ag₂O-TiO₂/sepiolite composites can be estimated by ln(C₀/C) = kt. Fig. 9 shows the comparison of the apparent rate constants of the Ag₂O-TiO₂/sepiolite composites at different calcined temperatures for the degradation of ARG solution. The k values for the ARG decomposition by the catalysts calcined at 100, 200, 350, and 450 °C are 0.064, 0.104, 0.048, and 0.020 min⁻¹, respectively. Fig. 10 shows the photocatalytic activities of the samples with different amounts of Ag₂O under visible-light irradiation. It is clearly shown that the samples with a content of Ag₂O from 2% to 10% exhibited enhanced photocatalytic activity. However, when the content of Ag₂O was further increased to 15%, no further enhancement of ARG degradation can be observed. Hence, when the content of Ag₂O was increased to 10%, a heterostructure of Ag₂O and TiO₂ was formed on the surface of sepiolite, which made the photocatalytic process more efficient.

Fig. 8. Photocatalytic degradation of ARG by TiO2/sepiolite and the Ag2O-TiO2/sepiolite catalysts obtained at different temperatures under visible-light irradiation.

Fig. 9. Comparison of apparent rate constants for the Ag2O-TiO2/ sepiolite catalysts calcined at different temperatures for the degradation of ARG solution.

Fig. 10. Visible-light photocatalytic degradation of ARG by Ag2O-TiO2/ sepiolite catalysts with different Ag2O contents.

Fig. 11 displays the photocatalytic degradation of the ARG solution by the different photocatalysts. It is clear that the Ag₂O-TiO₂/sepiolite composite exhibited the highest photocatalytic activity of the systems studied. ARG is almost completely degraded after 40 min irradiation over Ag₂O-TiO₂/ sepiolite under visible light, whereas only about 7% of ARG could be removed over P-25, 11% over pure sepiolite, 24% over TiO₂/sepiolite, 62% over TiO₂-Ag₂O, and 87% over Ag₂O/ sepiolite after 40 min irradiation. Therefore, it is possible that a steady system is formed between Ag₂O and TiO₂ nanoparticles based on multiphase heterostructures, which exist in this coordination to improve the photocatalytic activities of the composites. Formaldehyde, which is a typical volatile organic compound, was selected to further evaluate the photocatalytic properties of the Ag₂O-TiO₂/sepiolite composites.

Fig. 11. Visible-light photocatalytic degradation of ARG solution by different samples.

Fig. 12 shows the photocatalytic degradation of gaseous formaldehyde under visible-light irradiation. The concentration of formaldehyde in air was reduced and the decomposition product of CO₂ gas was gradually increased with an increase in the irradiation time. Meanwhile, Fig. 12 also shows the results of a blank contrast experiment for gas formaldehyde exposed to visible light without catalyst, implying that the catalyst was critical for the degradation of gaseous formaldehyde to occur. These results suggest that the Ag₂O-TiO₂/sepiolite composite also exhibits relatively high photocatalytic activity for the degradation of gaseous formaldehyde under visible-light illumination.

Fig. 12. Visible-light photocatalytic degradation of formaldehyde with and without Ag2O-TiO2/sepiolite catalyst.

Photoinduced active species (^•OH, h⁺, and O₂^•−) are very important for the mineralization of organic contaminants. Therefore, the terephthalic acid photoluminescence (TAPL) technique was conducted to detect the formation of ^•OH. Scavengers were also employed to determine the oxidative species during the photocatalytic degradation process. As shown in Fig. 13(a), the intensity of the peaks increased slightly with an increase in the illumination time, indicating that ^•OH was not the dominant species for the degradation of ARG solution. From Fig. 13(b), ^•OH radicals trapping experiments (IPA) further determine that the ^•OH was not the main active species in the photocatalytic process. KI was used as the scavenger of h⁺ and ^•OH, sodium oxalate (Na₂C₂O₄) as the scavenger of h⁺, and benzoquinone (BQ) as the scavenger of O₂^{•− [60]}. It can be seen that the KI and Na₂C₂O₄ had significant effects on the photocatalytic activity of the Ag₂O-TiO₂/sepiolite composite, which indicates that the h⁺ is one of the main active species in the photocatalytic system. In the presence of BQ, the degradation efficiency of Ag₂O-TiO₂/ sepiolite largely decreased, which suggests that O₂^•− is also one of the major active species in the photocatalytic system. O₂^•− could be generated by the adsorbed oxygen molecule on the surface of the catalyst capturing active electrons ^[26]. Thus, oxidative h⁺/O₂^•− radicals could play an important role in the photocatalytic solution under visible-light irradiation and in the photocatalytic ARG degradation ability of the composite capturing the reactive species h⁺, ^•OH, and ^•O₂^-.

Fig. 13. (a) •OH trapping PL spectra of terephthalic acid (TA) solution in the presence of the Ag2O-TiO2/sepiolite composite; (b) Photocatalytic degradation of ARG over the Ag2O-TiO2/sepiolite with and without the quenchers.

The enhanced photocatalytic mechanism of Ag₂O-TiO₂/ sepiolite heterostructure is presented in Fig. 14. The high photocatalytic activity of the Ag₂O-TiO₂/sepiolite composite can be attributed to Ag₂O-TiO₂ heterostructure being in the composite and the porous structure of sepiolite. Under visible-light irradiation, Ag₂O nanoparticles can be excited to h⁺ and e⁻ because of their narrow band gap. Meanwhile, Ag₂O has excessive negative charges that can easily adsorb onto positively charged TiO₂ nanoparticles ^{[48, 61, 62]}. In this case, the photogenerated electrons on the CB of Ag₂O can transfer into the CB of TiO₂ and the photogenerated holes gather in the VB of Ag₂O. Finally, the migration of photogenerated carriers is promoted because of the difference in band positions between those in the Ag₂O-TiO₂ heterojunction. Thereafter, the photogenerated electrons on the surface of Ag₂O can transfer into the CB of the TiO₂ that has reacted with molecular oxygen to produce O₂^•−, and the holes generated in Ag₂O directly oxidize the adsorbed dyes or participate in the reaction to generate other radical species that oxidize the organic compounds. Thus, the series of oxidation-reduction reactions caused by the heterostructure of Ag₂O-TiO₂ can restrain the fast recombination of photoinduced electron-hole pairs effectively. Meanwhile, the porous structure of the sepiolite clay provides numerous nucleation sites for the formation of Ag₂O-TiO₂ heterostructure. This synergistic effect leads to the enhancement in photocatalytic activity ^[63].

Fig. 14. Proposed photocatalytic mechanism of Ag2O-TiO2/sepiolite catalysts for the degradation of ARG solution under irradiation of visible light.

Novel Ag₂O-TiO₂/sepiolite composites with heterostructure were synthesized by a simple two-step method. The as-prepared Ag₂O-TiO₂/sepiolite composites exhibited enhanced visible-light photocatalytic activity for the degradation of ARG. This enhancement may be ascribed to the porous structure of sepiolite and the high quantum efficiency of Ag₂O-TiO₂ heterostructure. Under visible-light irradiation, Ag₂O nanoparticles as a visible-light active component enhanced the Ag₂O-TiO₂ heterostructure photocatalytic activity via synergetic effects on the electron-hole separation and efficient electron transmission at the Ag₂O-TiO₂ interface. Sepiolite clay as the substrate for Ag₂O-TiO₂ heterostructure could provide more active sites and enhance the adsorption properties. In addition, the h⁺ and O₂^•− radicals could be the main active species during the photo-oxidation process. This result may provide a new strategy for the design and development of high-performance visible-light photocatalysts.