Semiconductor photocatalysts, used for solar-energy conversion with application in environment and energy, have attracted extensive interest ^{[1, 2, 3]}. The search for highly efficient photocatalysts is the key to practical applications. The photocatalysis efficiency is mainly associated with the optical absorption, charge separation, and interfacial transfer ^{[4, 5, 6, 7]}. Generally, a single component photocatalyst suffers from poor visible-light absorption or/and a high charge recombination rate. To address these major problems, much effort has been devoted to investigating strategies such as doping, deposition, grafting, surface sensitization, and heterostructure construction ^{[8, 9, 10, 11, 12]}.

Constructing heterostructures is not only appealing in enhancing the performance of the individual photocatalysts but also for introducing some novel and unique properties ^{[13, 14, 15, 16]}. Recently, Ag/AgX (X = Cl, I, Br) have been suggested as efficient cocatalysts to construct ternary photocatalysts and improve the photocatalytic performance ^{[17, 18, 19]}. Especially, Ag/AgCl has been successfully applied to boost the photocatalytic activity of semiconductors with narrow or wide bandgaps. For instance, Yu et al. ^[20] prepared Ag/AgCl/TiO₂ nanotube array and found that it exhibited a high visible-light photocatalytic activity for the degradation of methyl orange. Zhang et al. ^[21] reported Ag/AgCl/Bi₂MoO₆ composites with superior visible-light photocatalytic activities in the decomposition of rhodamine B solution. Xu et al. ^[22] found that Ag/AgCl/WO₃ hollow spheres displayed excellent visible-light-response photocatalytic activity and recycling ability for the degradation of 4-chlorophenol.

Various types of semiconductor photocatalysts have been developed, including monometal oxides, metal sulfides, and composite oxides ^{[23, 24, 25, 26]}. Much attention has been given to Bi-based photocatalysts, such as Bi₂O₃, Bi₂WO₆, BiOCl, Bi₂O₂CO₃, BiVO₄, and NaBiO₃, for hydrogen production and environmental remediation ^{[27, 28, 29, 30, 31]}. Recently, nanostructured BiOIO₃ possessing two lone-pair cations and displaying an Aurivillius-type (BiO)₂²⁺ layer has been reported to exhibit high photocatalytic activity ^{[32, 33, 34]}. Unfortunately, owing to its large bandgap, it can be only activated by ultraviolet light, which impedes its application. Some efforts have been made for the enhancement of the photocatalytic ability of BiOIO₃ through heterojunction construction, including the synthesis of BiOIO₃/Bi₂WO₆,BiOIO₃/ g-C₃N₄, BiOIO₃/RGO, and BiOIO₃/BiOI ^{[35, 36, 37, 38]}. Nonetheless, the photocatalytic activity of BiOIO₃ is far from efficient for practical applications, and it is highly desirable to improve the photocatalysis efficiency. To our knowledge, using Ag/AgCl as cocatalyst to create ternary BiOIO₃-based composites with enhanced photocatalytic performance is not reported yet.

Herein, we prepared ternary Ag/AgCl/BiOIO₃ composites by a two-step method. The as-prepared composites showed enhanced visible-light absorption owing to the surface plasmon resonance of metallic Ag. Strong interfacial interactions existed in these components, and the significantly improved charge separation and migration in Ag/AgCl/BiOIO₃ composites were demonstrated by the photocurrent response and electrochemical impedance spectroscopy. The Ag/AgCl/BiOIO₃ composites showed higher visible-light photocatalytic performance than the Ag/AgCl and BiOIO₃ for the removal of NO. The possible photocatalytic mechanism responsible for the improved photocatalytic performance was also proposed. The outstanding properties associated with the novel Ag/AgCl/BiOIO₃ composite materials suggest that they can be used as visible-light harvesting photocatalysts for air purification applications.

All the chemicals were of analytical reagent grade and were used without further purification. BiOIO₃ was obtained by a simple hydrothermal method ^[37]. In a typical synthesis, 0.485 g of Bi(NO₃)₃·5H₂O was added to 70 mL distilled water and stirred vigorously for 30 min. Then, 0.214 g of KIO₃ was added into the above aqueous solution and stirred continuously for 10 min. The resulting suspension was then hydrothermally treated at 150 °C for 6 h. Finally, the BiOIO₃ product was collected and dried at 60 °C for 12 h.

Ag/AgCl/BiOIO₃ nanocomposites were synthesized by a chemical precipitation method at room temperature. A certain amount of NaCl was dissolved in 80 mL H₂O, and 0.48 g of BiOIO₃ (1.2 mmol) was added into the above aqueous solution and stirred for 30 min. Then, 30 mL aqueous solution containing AgNO₃ (equal molar amount to NaCl) was added dropwise into the above solution and stirred for 2 h. After the stirring was completed, the resulting suspension was aged for 1 h. Finally, the resulting products were collected by filtration, washed with water and ethanol four times and dried at 60 °C to obtain the final products. Depending on the molar ratio of BiOIO₃ to NaCl (6:1, 3:1, 1:1, 1:3, and 1:6), different composites can be synthesized. Ag/AgCl was synthesized by a chemical precipitation method at room temperature without adding BiOIO₃, and Ag metal was produced by photoreduction from the surrounding light.

The crystal phases of the samples were analyzed by X-ray diffraction (XRD) with Cu K_α radiation (model D/max RA, Rigaku Co., Japan). Scanning electron microscopy (SEM; model JSM-6490, JEOL, Japan) was used to characterize the morphology of the obtained products. The morphology and structure of the samples were examined by transmission electron microscopy (TEM; JEM-2010, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) with Al K_α X-rays (hν = 1486.6 eV) radiation operated at 150 W (Thermo ESCALAB 250, USA) was used to investigate the surface properties. The UV-vis diffuse- reflectance spectrometry (DRS) spectra were obtained for the dry-pressed disk samples using a scanning UV-vis spectrophotometer (TU-1901, China) equipped with an integrating sphere assembly, using 100% BaSO₄ as the reflectance sample. N₂ adsorption-desorption isotherms were obtained on N₂ adsorption apparatus (ASAP 2020, Micromeritics, USA). All the samples were degassed at 100 °C prior to measurements. The photocurrent response and electrochemical impedance spectra measurements were performed in three-electrode quartz cells with a 0.1 mol/L Na₂SO₄ electrolyte solution. Platinum wire was used as the counter electrode, and saturated calomel electrodes were used as the reference electrodes. The as-prepared samples film electrodes on ITO served as the working electrode. The photoelectrochemical experiment results were recorded using an electrochemical system (CHI-660B, China). All the photoelectrochemical measurements were performed under visible light of a 500 W Xe lamp coupled with 420 nm cutoff filters with average light power of 45 mW/cm².

The photocatalytic activity was investigated by removal of NO at ppb levels in a continuous flow reactor at ambient temperature. The volume of the rectangular reactor, made of polymeric glass and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). For the visible-light photocatalytic activity test, a 150-W commercial tungsten halogen lamp was vertically placed outside the reactor, and a UV cut-off filter (420 nm) was adopted. The as-prepared sample (0.20 g) was dispersed in distilled water (50 mL) in a beaker by ultrasonic treatment for 10 min, coated onto two glass dishes (12.0 cm in diameter) and then pre-treated at 70 °C to remove the water. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N₂ balance). The initial concentration of NO was diluted to about 550 ppb by the air stream supplied by a zero air generator. The desired relative humidity (RH) level of the NO flow was controlled at 50% by passing the zero air stream through a humidification chamber. The flow rates of the air stream and NO were controlled at 2.4 L/min and 15 mL/min, respectively. After the adsorption-desorption equilibrium was achieved in the dark, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc., model 42c-TL), which monitors NO, NO₂, and NO_x (NO_x represents NO + NO₂) with a sampling rate of 1.0 L/min. The removal ratio (η) of NO was calculated as η = (1 - c/c₀) ×100%, where c and c₀ are the concentrations of NO in the outlet steam and the feeding stream, respectively.

The crystallographic structures of BiOIO₃, Ag/AgCl, and the Ag/AgCl/BiOIO₃ ternary composites were characterized by XRD, as shown in Fig. 1(a). All the peaks for the as-prepared BiOIO₃ and Ag/AgCl are indexed to the orthorhombic phase of BiOIO₃ (ICSD ^#262019) and cubic phase of AgCl (JCPDS 31- 1238), respectively. In the composites, the peaks at 2θ = 27.82°, 32.24°, and 46.25°, which are assigned to the (111), (200), and (220) planes of AgCl, are detected, and the intensity of these peaks becomes stronger with the increased molar ratio of NaCl to BiOIO₃. In contrast, the diffraction peaks attributed to BiOIO₃ gradually decrease as the amount of BiOIO₃ is reduced in the composites. Notably, there are no XRD peaks about Ag⁰ observed in the as-prepared Ag/AgCl and ternary composites mainly because of the low content and high dispersity of Ag⁰. Moreover, from the enlarged view (Fig. 1(b)), the diffraction peak positions of BiOIO₃ in the composites experience shifts, demonstrating that strong interactions exist between the introduced Ag/AgCl and BiOIO₃.

Fig. 1. XRD patterns (a) and enlarged view of 2θ = 26°-34° diffraction region (b) of BiOIO3, Ag/AgCl, and the Ag/AgCl/BiOIO3 ternary composites prepared using different BOIO3 to NaCl molar ratios.

XPS was further used to investigate the composition of Ag/AgCl and the ternary composites photocatalyst (1:3). As shown in Fig. 2(a), all peaks can be indexed to Ag, Cl, O, and C elements, and no peaks of other elements are observed for Ag/AgCl. The C peak is mainly derived from the adventitious carbon from the XPS instrument, and the O peak is attributed to the surface adsorbed H₂O. For the ternary composite (1:3), besides the C peak, peaks arising from Bi, O, I, Cl, and Ag elements are observed. Fig. 2(b) shows the high-resolution XPS spectra of Bi 4f, and the peaks located at 159.0 and 164.4 eV are ascribed to Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺, respectively ^[33]. The O 1s peaks at 530.1 and 532.2 eV in Fig. 2(c) are attributed to the Bi-O bonds and the O-H bonds of the surface adsorbed water, respectively. The binding energies of I 3d_5/2 and I 3d_3/2 of I⁵⁺ are 623.8 and 635.3 eV (Fig. 2(d)) ^[33]. In the Ag/AgCl sample (Fig. 2(e)), the Ag 3d spectrum consists of two peaks at about 367.80 and 373.8 eV, which can be assigned to Ag 3d_5/2 and Ag 3d_3/2 of Ag⁺, respectively. Moreover, the two peaks at 368.5 and 374.7 eV, corresponding to metallic Ag, are observed, suggesting that partial Ag⁺ ions are reduced to Ag^{0 [39]}. Thus, the as-prepared Ag/AgCl does contain some metallic Ag, which mainly results from the photoreduction by the surrounding light. For the ternary composite (1:3), peaks of both Ag⁺ and Ag⁰ can be detected. In terms of Cl 2p (Fig. 2f), the binding energies of Cl 2p_3/2 and Cl 2p_1/2 are approximately 198.3 and 199.9 eV, respectively, which are detected in Ag/AgCl and the composite (1:3). It can be seen that the peaks of Ag⁰, Ag⁺, and Cl⁻ undergo chemical shifts in the composite (1:3) relative to those of Ag/AgCl, indicating the presence of strong interactions among the three components, which is consistent with the XRD result. The XPS results suggest the successful coupling of Ag/AgCl with BiOIO₃.

Fig. 2. XPS spectra of Ag/AgCl and the Ag/AgCl/BiOIO3 ternary composite (1:3). (a) Survey spectra; (b) Bi 4f; (c) O 1s; (d) I 3d; (e) Ag 3d; (f) Cl 2p.

The morphology of Ag/AgCl, BiOIO₃, and the composite (1:3) were investigated by SEM and TEM. As shown in Fig. 3(a) and (b), particle-like AgCl can be observed, while pure BiOIO₃ is mainly composed of nanosheets, as revealed by Fig. 3(c) and (d). In the composite (1:3), particle-like AgCl and nanosheet- shaped BiOIO₃ can be observed (Fig. 3(e) and (f)), implying that the shapes of BiOIO₃ and Ag/AgCl do not show significant changes after compositing. The TEM image further confirms that the composites have two morphologies, as particles and nanosheets (Fig. 3(g)). From the HRTEM image (Fig. 3(h)), the distance between the two adjacent planes is 0.37 nm, corresponding to the (111) plane of BiOIO₃, and the distance of 0.32 nm belongs to the (111) plane of AgCl. As can be seen, the intimate interfacial contact exists in these components, which is beneficial for charge separation and transfer.

Fig. 3. SEM images of Ag/AgCl (a, b), BiOIO3 (c, d), and the ternary composite (1:3) (e, f); TEM and HRTEM images of the ternary composite (1:3) (g, h).

The optical properties of the as-obtained samples were characterized by UV-vis DRS spectra and the results are shown in Fig. 4. BiOIO₃ has a clear edge around 390 nm, and no significant absorption in the visible-light region is observed. The bandgap of BiOIO₃ is 3.1 eV ^[37]. The Ag/AgCl sample exhibits a wide and strong light absorption centered at 545 nm, and this strong visible-light absorption can be attributed to the strong plasmon resonance absorption of Ag particles ^{[40, 41, 42]}. After construction of the ternary composites, none of the composites show an obvious enhanced absorption in the ultraviolet-light region in contrast to the pure BiOIO₃, but display a boost in the visible-light absorption. The absorption intensity in the visible-light region continually increases as the molar ratio of BiOIO₃ to NaCl is increased. The enhanced visible-light absorption can be ascribed to the plasmon resonance absorption of Ag particles. It can be concluded that the introduction of Ag/AgCl into BiOIO₃ benefits the optical absorption property and the usage efficiency of visible light. Similar phenomena are also observed in the cases of Ag/AgCl/TiO₂ and Ag/AgCl/WO₃ ^{[20, 22]}. The remarkable absorption enhancement in the visible region is beneficial to improve the photocatalytic performance.

Fig. 4. UV-vis DRS spectra of the as-prepared samples.

Photoelectrochemical measurements were performed to investigate the excitation, separation, transfer, and recombination of photoinduced charge carriers ^[43]. Fig. 5(a) shows the photocurrent of the samples irradiated under visible light. Obviously, the enhanced photocurrent is observed in the ternary composites, implying an enhanced separation efficiency of photoinduced electrons and holes ^[44]. The typical electrochemical impedance spectra (EIS) were adopted to investigate the photogenerated charge separation process, as shown in Fig. 5(b). It is observed that with the addition of Ag/AgCl, although in a small amount, the diameters of the arc radius on the EIS Nyquist plot of the ternary composites are smaller than those of pure BiOIO₃, which reveals a decrease in the interface layer resistance and the charge transfer resistance on the surface. Among these composites, the composite (1:6) shows the strongest photocurrent and the most efficient separation ability of charge carriers. Overall, charge transfer is accelerated and charge recombination is suppressed owing to the interfacial interaction between the Ag/AgCl and BiOIO₃. Thereby, a high efficiency in photocatalysis over the ternary composites would be achieved.

Fig. 5. Transient photocurrent response (a) and electrochemical impedance spectroscopy Nyquist plots (b) of BiOIO3 and the ternary composites.

The N₂ adsorption-desorption isotherms of all the prepared photocatalysts are shown in Fig. 6(a). From the isotherms, the Ag/AgCl sample displays the type III (Brunauer- Deming- Deming-Teller classification) isotherm, which characteristically indicates a weak interaction existing between N₂ and the sample. However, it is observed that the other samples show type IV isotherms with H3 hysteresis loops ^[45]. In view of the morphological structure, the H3 hysteresis loops might arise from the aggregation of nanosheet-like particles possessing slit-shaped pores. Fig. 6(b) displays the corresponding pore-size distributions. No pores exist in Ag/AgCl. The composites, as well as pure BiOIO₃, contain macropores and large mesopores over a broad range with the peaks centered at approximately 50 nm. These results demonstrate the formation of hierarchically nanoporous structures. The BET surface areas of Ag/AgCl, BiOIO₃, and the composites (6:1), (3:1), (1:1), (1:3), and (1:6) are calculated to be 1, 14, 10, 10, 8.5, 4.8, and 5 m²/g, respectively. It is clear that the specific surface areas of these photocatalysts are relatively low.

Fig. 6. N2 adsorption-desorption isotherms curves (a) and pore-size distribution (b) of the as-prepared samples.

The as-synthesized ternary samples show high visible absorption and efficient charge transfer, so they are expected to show high visible-light photocatalytic activity. NO, which is chemically stable and hard to remove, was chosen as the representative air pollutant to evaluate the photocatalytic performance of the as-synthesized photocatalysts. Fig. 7(a) shows the photocatalytic activity of the seven photocatalysts. BiOIO₃ exhibits poor photocatalytic activity with a removal ratio of 13.0% owing to its large bandgap. The as-prepared Ag/AgCl sample shows a certain photocatalytic performance (removal ratio of ca. 30%) because of the existence of Ag⁰. Metal Ag with plasmon resonance effects can be excited by visible light to generate carriers, and electrons can react with the species at the interfaces while holes inject into the nearby AgCl to oxidize Cl^- ions to yield Cl atoms, which are reactive for oxidizing species in the surrounding environment. Meanwhile, Cl atoms would be reduced back to Cl^- after redox reactions ^{[40, 41]}. With the assistance of composite photocatalysts, the high visible-light photocatalytic activity is obviously superior to those observed from the BiOIO₃ and Ag/AgCl. Adding a small amount of Ag/AgCl can result in significantly increased photocatalytic activity. The composite (6:1) displays a high removal ratio of 56.0%. With an increased Ag/AgCl content in the composites, the NO removal ratio gradually increases and finally reaches a stable value. The composites (1:3) and (1:6) show a comparable and high removal ratio of approximately 64.0% for the removal of NO. To evaluate the stability of the photocatalytic performance of the composite (1:3), circulating runs were carried out. As shown in Fig. 7(b), the composite (1:3) shows a slight decrease in activity after four recycles, which can be attributed to the products or intermediates blocking the active sites. Furthermore, XRD, combined with the UV-vis DRS spectra, reflects the good stability in the phase structure and light absorption properties of the composite (1:3) (Fig. 7(c) and (d)).

Fig. 7. Photocatalytic oxidation of NO over the as-prepared samples (a), cycling runs of the composite (1:3) in air under visible-light irradiation (b), and XRD pattern (c) and UV-vis DRS spectrum (d) of the composite (1:3) after cycling testing.

Based on the previously reported photocatalyst systems, such as Ag/AgCl/TiO₂, Ag@AgCl, and the above experimental results ^{[20, 40]}, the photocatalytic mechanism of the Ag/AgCl/BiOIO₃ system is proposed, as depicted in Fig. 8. Under visible-light irradiation, photogenerated electron-hole pairs are created in Ag particles owing to surface plasmon resonance. A Schottky barrier is formed at the Ag/BiOIO₃ interface owing to the different work functions. As the plasmon-induced electrons of Ag particles can transfer to the conduction band (−0.2 V vs NHE) of TiO₂ under visible-light irradiation ^[46], it is believed that the photo-induced electrons of Ag particles can easily transfer to the conduction band (+0.99 V vs NHE) of BiOIO₃ ^[37]. The potential of the electrons on the conduction band of BiOIO₃ are not negative enough to induce the generation of ^•O₂⁻. Alternatively, electrons can be consumed by multi-electrons reactions to yield H₂O ^[47]. Meanwhile, the holes left on the surface of Ag particles can transfer to the surface of AgCl to oxidize Cl^- to Cl⁰. The light irradiation induces reduction of partial Ag⁺ ions and leaves Cl^- ions, so that the surface of AgCl particles is negatively charged and terminated by Cl^- ions. Cl⁰ atoms are reactive radical species, which are able to oxidize NO and then be reduced to Cl^- again. I n this case, BiOIO₃ contributes to the rapid transfer of photogenerated electrons, leading to a lower recombination rate and enhanced photocatalytic activity.

Fig. 8. Proposed photocatalytic mechanism of the as-prepared ternary composites.

Ag/AgCl/BiOIO₃ was constructed through a two-step synthesis route. The generation of AgCl was accompanied with the in situ formation of metallic Ag by photoreduction of Ag⁺ ions by the surrounding light. Remarkable improvement of NO removal capability for Ag/AgCl/BiOIO₃ was a result of the enhanced visible-light harvesting and decreased recombination of photogenerated electron-hole pairs owing to the strong interaction of Ag/AgCl and BiOIO₃. This originated from the surface plasmon resonance absorption of Ag particles under visible-light irradiation and the charge separation at Ag/BiOIO₃ surface, which included electrons transferring from the photo-excited Ag to BiOIO₃ and holes transferred to AgCl to oxidize Cl^-. The present work provides fundamental insights into the role of BiOIO₃ in tuning interfacial charge transfer processes. The highly efficient Ag/AgCl/BiOIO₃ composites are widely applicable in various fields, such as air purification and waste water treatment.