Synergistic Effects of Ag Nanoparticles/BiV1-xMoxO4 with Enhanced Photocatalytic Activity

In recent years, BiVO4 has drawn much attention as a novel photocatalyst given its excellent ability to absorb visible light. This work reports the development of Ag-modified BiV1-xMoxO4 composites through a facile hydrothermal synthesis with the subsequent photoinduced reduction of Ag+ at almost neutral pH conditions. Metallic Ag nanoparticles were deposited on the (040) facet of Mo-doped BiVO4 powders. The crystal structure and morphology of the as-prepared samples were studied by XRD and SEM analyses. Moreover, the photocatalytic performance of BiVO4, Ag/BiVO4, and Ag-modified BiV1-xMoxO4 were evaluated by the degradation of rhodamine B (RhB). The Ag/BiV0.9925Mo0.0075O4 composite exhibited the most efficient photocatalytic performance. The present work provides greater insight into the application of BiVO4 in the field of photocatalysis.


Background
Given the increasing environmental pollution and energy crises, the development of efficient and promising solutions to reduce energy shortages and protect the environment is paramount [1,2]. Photocatalyst-based semiconductors, such as Bi 2 WO 6 [3,4], BiPO 4 [5,6], Ag 3 PO 4 [7,8], and BiVO 4 [9][10][11][12][13], have attracted much attention due to their applications in the degradation of organic pollutants or hydrogen production from water splitting. Nevertheless, most of the existing oxide photocatalysts have very low light-response efficiencies primarily because they only respond to ultraviolet light due to their narrow bandgaps [14][15][16]. Additionally, the photoinduced electrons can easily recombine with holes leading to a lower optical performance [17,18].
Due to its visible photocatalytic activity, wide bandgap of 2.42 eV, high stability, and non-toxicity, bismuth vanadate (BiVO 4 ) is a promising n-type semiconductor photocatalyst [19][20][21]. However, its resulting carrier transfer efficiency is relatively poor, leading to the recombination of photogenerated electrons and holes, which severely limits the photocatalytic performance of BiVO 4 . Various studies have assessed BiVO 4 modifications [20,[22][23][24], and substitution or metal doping on BiVO 4 has been shown as the most efficient method to change its carrier transport efficiency. Metal element doping introduces new defects or charges in the crystal lattice [25], influencing the motion of electrons and the creation of holes under light irradiation [26,27]. Adjustments to the distribution status or changes in the band structures can lead to changes in the activity of semiconductors [28]. For example, Thalluri et al. [29] introduced hexavalent molybdenum (Mo) at an almost neutral pH to substitute V while preserving the atomic ratio of fBiVO 4 , leading to the formation of a good crystal structure and considerable photocatalytic activity for water oxidation. Mo has a higher valence than V and therefore strengthens the n-type characteristics of the material [30]. Additionally, the photocatalytic activity of BiVO 4 is highly dependent on its various crystal facets. Recent studies on the deposition of noble metals, such as Ag, Cu, and Au, on the different facets of BiVO 4 have demonstrated good photocatalytic activity [31][32][33]. Li et al. [34] produced an Ag/BiVO 4 composite through the hydrothermal synthesis and photoreduction of Ag deposited on the (040) crystal facets of BiVO 4 , leading to an enhanced photoelectrochemical performance, as indicated by the fast separation of the electron-hole pairs.
In the present study, we build on the facile hydrothermal synthesis approach of Li et al. [29]

Synthesis of BiVO 4 and BiV 1-x Mo x O 4 Powders
Bismuth nitrate pentahydrate (Bi(NO 3 ) 3 ·5H 2 O, analytical grade), ammonium metavanadate (NH 4 VO 3 , analytical grade), ammonium carbonate, and ammonium molybdate ((NH 4 ) 2 MoO 4 ) were obtained from Sigma-Aldrich and used as received, without any further purification. All other chemicals used in the experiments were also of analytical grade, and deionized water was used for the preparation of the solutions. In a typical process, 3.7 mmol of Bi(NO 3 ) 3 ·5H 2 O, 3.7 mmol of NH 4 VO 3 , and 12 mmol of (NH 4 ) 2 CO 3 were dissolved in 75 mL of 1 M HNO 3 and stirred for approximately 30 min at room temperature until a clear solution was obtained. The pH of the mixture was adjusted to pH 8 with NaOH (2 M). The mixture was transferred into a 150-mL Teflon-lined stainless autoclave and heated for 12 h at 180°C under autogenous pressure in an oven. The precipitate was filtered and washed three times with distilled water followed by ethanol and dried for 12 h at 60°C in a drying oven.
The doped samples were prepared by replacing the equivalent weight of NH 4 VO 3 with different amounts of Mo. Mo precursors were introduced such that a nominal 0.5, 0.75, and 1% atomic substitution of V was achieved.

Preparation of Ag/BiVO 4 and Ag/BiV 1-x Mo x O 4 Samples
BiVO 4 (0.50 g) and AgNO 3 (0.05 g) were added to a (NH 4 ) 2 C 2 O 4 (0.8 g L −1 , 100 mL) aqueous solution in a 250-mL beaker in an ultrasonic bath until an evenly dispersed solution was formed. The resulting yellow mixture was then irradiated with a Xenon lamp for 30 min under magnetic stirring. The color of the system turned from a vivid yellow to grayish-green, indicating the generation of metallic Ag in the reaction system. The resulting samples were then filtered, washed with DI water, and dried at 60°C for 12 h to obtain the Ag/BiVO 4 and Ag/BiV 1-x Mo x O 4 composites.

Photocatalytic Activity
Assessment of the photocatalytic activity was performed using the degradation rate of RhB. The experimental system for photodegradation was calibrated at a UV cut-off wavelength below 420 nm, and the irradiation height of the Xenon lamp was close to the height of the 250-mL beaker. In a typical procedure, the as-prepared photocatalyst (0.1 g) was well dispersed in a RhB aqueous solution (150 mL, 10 mg L −1 ) under ultrasonication in a glass reactor equipped with a cooling water circulator to maintain a reaction system temperature of room temperature. The suspension was stirred for 30 min in the dark to reach the adsorption-desorption equilibrium and was then irradiated for 2 h with a Xenon lamp (300 W) under continuous stirring. A 5-mL aliquot of the suspension was taken every 30 min and centrifuged. The absorption spectrum of the obtained liquid

Characterization Techniques
The morphologies of the pure BiVO 4 and the decorated composites were investigated by field emission scanning electron microscopy (FESEM, S4800) and transmission electron microscopy (TEM; JEM-2100F, JEOL). Elemental analysis was performed by X-ray photoelectron spectroscopy (XPS; VGESCA-LAB MKII) with a nonmonochromatic Mg Kα X-ray source. The crystalline phase of the samples was determined by X-ray diffraction (XRD; Bruker D8) with Cu Kα radiation. Inductively coupled plasma (ICP) was employed to analyze the elemental composition of the samples. Additionally, UV-vis diffuse reflectance spectrum measurements were performed using a Shimadzu spectrophotometer (UV-2450) to evaluate the bandgap energy of BiVO 4 , Ag/BiVO 4 , BiV 1-x Mo x O 4 , and Ag/BiV 1-x Mo x O 4 over a wavelength range of 360-800 nm.

Results and Discussion
The crystallographic structure and phase of the prepared composites were characterized by XRD analysis (Fig. 1a).
The sharp diffraction peaks observed in the as-prepared BiVO4 were assigned to the conventional BiVO 4 phase since they were in good agreement with the standard (JCPDS) card no. 14-0688. According to the peak splitting observed at 18.7°and 30.5°, which indicate the (110) and (040) facets, the prepared BiVO 4 material possessed a single monoclinic scheelite structure. A diffraction peak at 38.1°was observed in the Ag-related photocatalysts (Fig. 1a) corresponding to the (111) crystal phase of metallic Ag (JCPDS file: 65-2871). This indicates that the photoreduction of Ag + ions indeed occurred, leading to the deposition of Ag nanoparticles on the BiVO 4 and BiV 1-x Mo x O 4 surfaces. Nevertheless, due to the low relative content of Ag, the XRD peaks were not intense.
As shown in Fig. 2a, EDS confirmed the presence of the Ag species, which agrees with the XRD results. The Bi (Fig. 2b), O (Fig. 2c), V (Fig. 2d), Mo (Fig. 2e), and Ag ( Fig. 2f ) elements are all distributed uniformly in the Ag/BiV 1-x Mo x O 4 composites, and the results verify the existence of Mo and Ag. The relative amounts of Mo did not appear to affect the crystal structure or phase. The Mo substitution ratio was assessed by ICP (Table 1); the practical Mo atomic content was calculated to be 0.16% in Ag/BiV 0.9925 Mo 0.0075 O 4 . It was observed that,  although the nominal dopant content introduced with the precursors was 0.75%, the final resulting amount of Mo in the doped materials was always lower than the expected. Similar results have also been found in previous research, and it is possible that intrinsic losses and the evaporation of the Mo dopant occur during the hydrothermal synthesis processes [35,36]. The morphology of the as-prepared pure BiVO 4 , Ag/ BiVO 4 , and Ag/BiV 1-x Mo x O 4 were investigated by SEM (Fig. 3). Pure BiVO 4 showed a slice-layer morphology with several clusters (Fig. 3a, b). For Ag/BiVO 4 , metallic Ag was observed to be well dispersed on the (040) crystal facet (Fig. 3c) (040) surface. This crystal facet has been shown to have a good charge carrier mobility [37]. Thus, the observed morphology should be beneficial to the photocatalytic performance of the synthesized doped BiVO 4 powders. The as-prepared BiVO 4 , Ag/BiVO 4 , and Ag/BiV 0.9925-Mo 0.0075 O 4 samples were further observed by TEM (Fig. 4a). Interplanar spacings of 0.475 nm were clearly observed in Fig. 4b, corresponding to the (110) corresponding to Bi 4f 7/2 and 4f 5/2 , respectively, confirming the Bi 3+ peaks in BiVO 4 (Fig. 5b). A typical O 1s spectrum was observed, as indicated by the main characteristic peak at 529.71 eV (Fig. 5c). The V 2p 3/2 and 2p 1/ 2 peaks observed at 516.5 and 524.1 eV, respectively, indicated the existence of V 5+ (Fig. 5d). The Ag 3d peaks at 367.98 and 374.0 eV, corresponding to Ag 3d 5/2 and 3d 3/2 (Fig. 5e), respectively, were observed in both Ag/ BiVO 4 and Ag/BiV 0.9925 Mo 0.0075 O 4 , confirming the existence of the metallic Ag species. Furthermore, the molar ratio of metallic Ag species accounted for 6.6% of all elements, as determined by XPS and in agreement with   (Table 1). Finally, the Mo 3d 5/2 and 3d 3/2 peaks located at 231.7 and 234.9 eV (Fig. 5f ), respectively, confirm the presence of Mo 6+ .
UV-vis diffuse reflectance spectrum measurements were taken to evaluate the optical bandgap and absorption properties of the photocatalysts, as shown in Fig. 5. The photocatalytic activity of a semiconductor is largely dependent on the size of the bandgap; the narrower the bandgap is, the greater the shift is of the absorption wavelength towards longer wavelengths. The bandgap of as-prepared BiVO 4 was approximately 2.3 eV (Fig. 6b), which agrees with the Kubelka-Munk bandgap estimation theory [38]. Compared with BiVO 4 , all the Modoped samples showed relatively narrow bandgaps (Fig. 6b). Furthermore, all Ag-deposited BiVO 4     The stability and reusability of photocatalysts are very important for their practical application. Therefore, we assessed the repeated cycles of Ag/BiV 0.9925 Mo 0.0075 O 4 in the photocatalytic degradation of RhB for 2 h under visible light irradiation. Overall, 99% of the RhB solution was degraded after five cycles (Fig. 9), indicating that the sample exhibited good photocatalytic stability.
To further assess the separation efficiency, the charge carrier lifetimes of pure BiVO 4 , Ag/BiVO 4 , and Ag/ BiV 0.9925 Mo 0.0075 O 4 were also analyzed (Fig. 10). The decay curves for the as-prepared photocatalysts fit well to a double-exponential function. The charge carrier decay lifetimes of BiVO 4 , Ag/BiVO 4 , and Ag/BiV 0.9925-Mo 0.0075 O 4 composites were 1.2304, 1.8220, and 2.0933 ns, respectively. Thus, the Ag-deposited samples, both with and without Mo doping, had much longer charge carrier lifetimes than pure BiVO 4 , achieving effective photocarrier separation and suggesting that a synergistic effect among Ag, Mo, and BiVO 4 led to enhancements of the photocatalytic activity.
To explore the underlying photocatalytic mechanism, RhB degradation was conducted under visible light irradiation [39], adding a hole (h + ) scavenger (ammonium oxalate ((NH 4 ) 2 C 2 O 4 )), a superoxide radical (•O 2− ) scavenger (1.4-benzoquinone, BQ) [40], or hydroxyl radical (•OH) scavengers (tert-Butanol, t-BuOH) [41]. Following the addition of BQ, no obvious decrease was observed, but an acceleration in the degradation rate was detected compared to that of Ag/BiV 0.9925-Mo 0.0075 O 4 (Fig. 11). The faster degradation rate may have resulted from the SPR-effect of metallic Ag in Ag/ BiV 0.9925 Mo 0.0075 O 4 , which would enhance the separation efficiency of electrons and holes. However, when t-BuOH was added, the catalytic efficiency decreased from 97.5 to 78.1%, indicating the presence of •OH as the active species. The photocatalytic activity was drastically reduced with the addition of (NH 4 ) 2 C 2 O 4 , suggesting that the holes acted as the main active species.
To further confirm the main active species generated in the photocatalytic process, electron spin resonance (ESR) was used. The principle of ESR is to react with free radicals using a spin-trapping agent to generate a relatively stable free radical adduct. A peak intensity was observed under visible light compared with dark conditions (Fig. 12a), demonstrating the existence of •O 2− . In  addition, obvious signals (Fig. 12b) suggested that •OH was produced in the photocatalytic process. In conclusion, the radical trap experiments and ESR analysis revealed that the photocatalytic process was governed by the combined effect of h + , •O 2− , and •OH active species.
According to the discussion above, a possible photocatalytic mechanism of Ag/BiV 0.9925 Mo 0.0075 O 4 was illustrated in Fig. 13

Conclusions
Herein, a simple hydrothermal synthesis procedure at almost neutral pH conditions and using ammonium carbonate as the structure-directing agent is reported for the preparation of Mo-doped BiVO 4 powders. Metallic Ag nanoparticles were then deposited on the (040) crystal facet of BiV 0.9925 Mo 0.0075 O 4 . Thus, a photocatalytic system has been successfully constructed by means of the reduction reaction. These synthesis conditions have  been shown to significantly influence the increase in the size of the (040) crystallographic facet, as confirmed by XRD and STEM analyses. The XRD indicated that the peak splitting observed at 30.5°is a result of the (040) facets. Ag nanoparticles deposited on the (040) facets can also be seen from the STEM. Furthermore, Ag/ BiV 0.9925 Mo 0.0075 O 4 showed a highly efficient photocatalytic performance for RhB degradation under visible light irradiation. This work could offer new inspiration for the rational utilization of BiVO 4 photocatalysts with high photocatalytic activity and their applications in the fields of energy production and environmental protection.