Ag2WO4 nanorods decorated with AgI nanoparticles: Novel and efficient visible-light-driven photocatalysts for the degradation of water pollutants

To develop efficient and stable visible-light-driven (VLD) photocatalysts for pollutant degradation, we synthesized novel heterojunction photocatalysts comprised of AgI nanoparticle-decorated Ag2WO4 nanorods via a facile method. Various characterization techniques, including XRD, SEM, TEM, EDX, and UV–vis DRS were used to investigate the morphology and optical properties of the as-prepared AgI/Ag2WO4 catalyst. With AgI acting as the cocatalyst, the resulting AgI/Ag2WO4 heterostructure shows excellent performance in degrading toxic, stable pollutants such as rhodamine B (RhB), methyl orange (MO) and para-chlorophenol (4-CP). The high performance is attributed to the enhanced visible-light absorption properties and the promoted separation efficiency of charge carriers through the formation of the heterojunction between AgI and Ag2WO4. Additionally, AgI/Ag2WO4 exhibits durable stability. The active species trapping experiment reveals that active species (O2•− and h+) dominantly contribute to RhB degradation. The AgI/Ag2WO4 heterojunction photocatalyst characterized in this work holds great potential for remedying environmental issues due to its simple preparation method and excellent photocatalytic performance.

The integration of VLD components with wide bandgap semiconductors having well-matched energy bands has provided a new opportunity for the development of VLD photocatalysts [12]. As a consequence, some Ag 2 WO 4 -based composites containing VLD components such as Ag 2 S/Ag 2 WO 4 [40], C 3 N 4 /Ag 2 WO 4 [39], Bi 2 MoO 6 /Ag 2 WO 4 /Ag [42] etc., have been reported to show improved VLD performance in the degradation of pollutants. To the best of our knowledge, application of AgI/Ag 2 WO 4 as a VLD photocatalyst for the degradation of toxic pollutants remains unreported.
In this study, to enhance the photocatalytic performance of Ag 2 WO 4 , AgI (possessing matched energy band levels) was chosen as a suitable component to combine with Ag 2 WO 4 , AgI/Ag 2 WO 4 heterojunctions at different mole ratios. These heterojunctions were prepared via an in situ ion-exchange approach, utilizing Ag 2 WO 4 nanorods as the Ag source. The as-prepared AgI/Ag 2 WO 4 heterojunctions exhibited remarkably higher photocatalytic activity than pure Ag 2 WO 4 toward the degradation of rhodamine B (RhB), methyl orange (MO) and para-chlorophenol (4-CP) under visible light. Based on a systematic characterization and study, a possible photocatalytic mechanism over AgI/Ag 2 WO 4 was also elucidated in this work.

Results and Discussion
Preparation and characterization of catalysts Ag 2 WO 4 nanorods decorated with AgI nanoparticles were prepared via an in situ anion-exchange method. Ag 2 WO 4 nanorods were first synthesized by mixing AgNO 3 and Na 2 WO 4 aqueous solutions at room temperature [37]. Subsequently, AgI nanopar-ticles were readily anchored onto Ag 2 WO 4 nanorods via an in situ anion-exchange between I − in the solution and the lattice W 2  Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the morphology and microstructure of AgI/Ag 2 WO 4 heterojunctions ( Figure 2). are uniformly coated on the surface of Ag 2 WO 4 nanorods, signifying the formation of the AgI/Ag 2 WO 4 core-shell heterostructure. To more clearly observe the microstructure of the AgI/Ag 2 WO 4 composite, the TEM and high-resolution TEM In addition, the composition of 0.3AgI/Ag 2 WO 4 was further identified by energy-dispersive X-ray spectroscopy (EDX). As shown in Figure 3, 0.3AgI/Ag 2 WO 4 is composed of Ag, I, W and O elements. The results further confirm the formation of the AgI/Ag 2 WO 4 heterojunctions, thereby facilitating the charge transfer between them [12].
Subsequently, the optical properties of as-prepared photocatalysts were investigated by UV-vis diffuse reflectance spectroscopy (DRS) analysis ( Figure 4). As indicated from Figure 4,  pure Ag 2 WO 4 has a clear absorption edge at about 400 nm [31], while AgI has a broader absorption band with the absorption edge located at around 460 nm [22]. For AgI/Ag 2 WO 4 heterojunctions, a pronounced enhancement in visible-light absorption range is achieved when AgI content was progressively increased. This suggests that the introduction of AgI nanoparticles can optimize the light absorption capacity owing to the formation of a nanojunction between AgI and Ag 2 WO 4 . These facts indicate that AgI/Ag 2 WO 4 heterojunctions can harvest more light and thus can be expected to be efficient VLD photocatalysts.

Photocatalytic performance
RhB, MO, and 4-CP, three types of toxic pollutants with stable chemical structures, were used to evaluate the activity of AgI/Ag 2 WO 4 heterojunctions. Figure 5a shows the visible-light degradation results of RhB (10 mg L −1 ) over as-prepared samples. Bare Ag 2 WO 4 exhibits poor visible-light photocatalytic activity with an RhB degradation rate of 16.2% after 60 min irradiation due to the unsatisfactory visible-light absorption and fast recombination of photoinduced charge carriers [31,38,44,45]. Apparently, the introduction of a proper amount of AgI can markedly facilitate the separation of electron-hole pairs, leading to the marked photocatalytic activity, which is in accordance with previous reports. As the amount of AgI is further increased, the activity of 0.4AgI/Ag 2 WO 4 decreases, indicating that excessive AgI is unfavorable for photocatalytic reaction. The possible reason is that a large amount of AgI particles having a larger diameter could interfere with the light absorption of reactive sites. It has also been found that the degradation process of RhB can be fitted well with the apparent firstorder model (Figure 5b). As we can see, the k value for RhB decomposition over 0.3AgI/Ag 2 WO 4 is about 0.0386 min −1 , which is much higher than those over other samples.
Besides RhB, MO ( Figure 6a) and 4-CP ( Figure S1, Supporting Information File 1) also could be efficiently degraded by 0.3AgI/Ag 2 WO 4 under visible light, indicating the outstanding photocatalytic activity of 0.3AgI/Ag 2 WO 4 . In addition, the degradation rate constant of MO ( Figure 6b) and 4-CP ( Figure S1b, Supporting Information File 1) over catalysts were also calculated by the pseudo-first-order model. It is found that 0.3AgI/Ag 2 WO 4 still achieves the highest apparent rate constant (0.0292 min −1 for MO degradation and 0.0129 min −1 for 4-CP degradation) among all these samples.  The mineralization of organic pollutants is crucial for pollutant treatment [46]. Thus, the total organic carbon (TOC) removal efficiency of RhB over 0.3AgI/Ag 2 WO 4 was examined ( Figure 7). After 360 min of reaction, the TOC removal effi-ciency reached 67.2%, signifying that 0.3AgI/Ag 2 WO 4 can effectively mineralize RhB.
The operational lifetime of the photocatalysts is crucial for practical application [47]. To reveal the durability of 0.3AgI/Ag 2 WO 4 , the cycling photocatalytic degradation of RhB was performed. As shown in Figure 8a, no apparent activity decrease was observed after five successive runs, demonstrating the good stability of the catalyst. Furthermore, the XRD pattern of the used 0.3AgI/Ag 2 WO 4 is similar to that of the fresh one ( Figure 8b). These facts suggest that 0.3AgI/Ag 2 WO 4 possesses long-term stability for photocatalytic reaction.

Photocatalytic mechanism
To elucidate the degradation mechanism, active-species trapping tests were performed during RhB degradation over 0.3AgI/Ag 2 WO 4 ( Figure 9) [13,48]. Figure 9 shows the effects of various trapping agents on the RhB degradation efficiency under visible-light irradiation. When IPA was introduced, the RhB degradation efficiency slightly reduced from 91.3 % to 70.7%, suggesting that very little •OH was involved in the reac-   Electrochemical impedance spectroscopy (EIS) measurement was applied to study the charge transport and separation [49]. A smaller arc radius commonly signifies a higher charge transport rate. As displayed in Figure 10, the arc radius of 0.3AgI/Ag 2 WO 4 is smaller than that of AgI, suggesting that 0.3AgI/Ag 2 WO 4 holds a higher charge transfer rate and a more effective separation of charge carriers. On the basis of the above discussion, the excellent photocatalytic activity of AgI/Ag 2 WO 4 is concluded to be due to the broadening of the photo-absorption range from the ultraviolet to the visible light range (Figure 4) and the formation of a heterojunction between AgI and Ag 2 WO 4 . The matched band alignments lead to a fascinating separation and transfer of the photo-generated electrons and holes ( Figure 11) [50][51][52]. A heterostructure is well constructed after the in situ growth of AgI on Ag 2 WO 4 . Under visible light irradiation, AgI is excited to produce electrons and holes. Given the negative potential of conduction band (CB) of AgI to that of Ag 2 WO 4 , electrons tend to migrate from the CB of AgI to that of Ag 2 WO 4 , whereby the separation rate of electron-hole pairs is boosted. Consequently, the accu-mulated electrons in the CB of AgI (and more holes left behind in the valence band (VB)) could readily attack the pollutant molecules, resulting in the remarkable photocatalytic performance of AgI/Ag 2 WO 4 .

Conclusion
In summary, a novel heterojunction photocatalyst comprised of AgI nanoparticle-decorated Ag 2 WO 4 nanorods exhibiting remarkable photocatalytic performance has been prepared via a facile method. This resulting AgI/Ag 2 WO 4 catalyst exhibits exceptionally high and stable photocatalytic activity for the degradation of RhB, MO and 4-CP due to its extended light absorption range and the formation of a heterojunction between AgI and Ag 2 WO 4 . This work not only offers a high-efficiency AgI/Ag 2 WO 4 heterojunction photocatalyst, but also provides new inspiration for the development of visible-light-driven Ag-based heterojunction photocatalysts.

Experimental Photocatalyst synthesis
All reagents were purchased from Shanghai Sinopharm Chemical Reagent Ltd. and used as received.
Ag 2 WO 4 nanorods were prepared according to a previous report [41]. Briefly, AgNO 3 (0.01 mol L −1 , 100 mL) and Na 2 WO 4 (0.005 mol L −1 , 100 mL) aqueous solutions were first prepared. Then, the AgNO 3 aqueous solution was slowly poured into Na 2 WO 4 aqueous solution and incubated at room temperature for 12 h in the dark. Finally, the white precipitate was collected, washed successively with distilled water, and dried in vacuum at 60 °C for 12 h.

Characterization
The crystalline structure of the samples was studied by using a Bruker D8 Advance X-ray diffractometer (XRD). The images of the morphological structure were observed by a field emis-sion scanning electron microscope (FE-SEM, Hitachi S-4800) and a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010F). Energy-dispersive X-ray (EDX) spectroscopy coupled with SEM was employed to identify the chemical composition of the sample. UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded on an UV-vis spectrophotometer (Shimadzu, UV-2600).

Photocatalytic tests
The photocatalytic activity of the as-prepared samples was evaluated by the removal of RhB, MO or 4-CP under visible-light irradiation. A 300 W Xe lamp with a cut-off filter (λ > 400 nm) was used as the light source during the reaction. In each experiment, 10 mg of catalyst was dispersed in RhB (100 mL, 10 mg L -1 ), MO (100 mL, 5 mg L -1 ) or 4-CP (100 mL, 5 mg L -1 ) aqueous solution. Before light illumination, the suspensions were first magnetically stirred in the dark for 1 hour. Then 2 mL of the suspension was collected and the light was switched on. With the light on and under magnetic stirring, 2 mL of the suspension was sampled at given time intervals. All suspensions were centrifuged to remove the catalyst particles. The RhB and MO concentrations were monitored by a UV-2600 spectrometer. The 4-CP concentrations were monitored by high-performance liquid chromatography (HPLC, Agilent 110 series).
The total organic carbon (TOC) experiment was carried out by dispersing 100 mg of 0.3AgI/Ag 2 WO 4 in RhB (50 mg L -1 , 100 mL) solution. During the reaction, a 10 mL suspension was sampled every hour and monitored by a TOC analyzer (Shimadzu TOC-VCPH).

Supporting Information
Supporting Information File 1