Structurally and Compositionally Tunable Absorption Properties of AgCl@AgAu Nanocatalysts for Plasmonic Photocatalytic Degradation of Environmental Pollutants

Composite nanomaterials having Ag nanoparticles (NPs) that decorate nanostructured AgCl (Ag/AgCl) are promising as plasmonic photocatalysts because of the visible-light absorption of Ag NPs. However, the narrow absorption bands of Ag NPs near 400 nm cause inefficient absorption in the visible range and, consequently, unsatisfactory photocatalytic activity of Ag/AgCl nanomaterials. In this study, we introduce a new class of AgCl-based photocatalysts that are decorated with bimetallic Ag and Au NPs (AgCl@AgAu NPs) for visible-light-driven photocatalytic degradation of organic pollutants. Polyvinylpyrrolidone induces selective reduction of noble metal precursors on AgCl while leaving AgCl intact. The extended composition of the decorating NPs red-shifts the absorption band to 550–650 nm, which allows the catalysts to take advantage of more energy in the visible range for improved efficiency. Furthermore, we control the structures of the AgCl@AgAu NPs, and investigate their correlation with photocatalytic properties. The versatility, chemical stability, and practical application of the AgCl@AgAu NPs are demonstrated using various organic pollutants, recycling experiments, and natural aqueous media, respectively. Our fundamental investigation on the synthesis and applications of AgCl-based nano-photocatalysts is highly valuable for designing plasmonic photocatalysts and expanding their utilization.


Introduction
Silver chloride (AgCl) is a crystalline material that is readily prepared by aqueous precipitation at room temperature because of its low water solubility (K sp = 1.77 × 10 −10 at 25 • C) [1]. Based on its facile synthesis and semiconductive electronic properties (band gap = 3.19 eV), AgCl has been used in various applications such as photosensitizers, reference electrodes, and photocatalysts [2][3][4]. Recently, the scientific interest in AgCl has been extended to its nanoscale synthesis, leading to significant progress in chemical control over the shape and size of AgCl. As a result, AgCl nanomaterials with various shapes such as plate, cubic, spherical, and octahedral could be synthesized [5][6][7][8][9][10]. In addition to pure AgCl, composite nanostructures composed of AgCl and noble metals have recently been synthesized [5, 6,[11][12][13]. However, most of the composite nanostructures were still compositionally limited to Ag/AgCl, whose sizes and shapes, and corresponding chemical and physical properties, were hardly controlled [14][15][16][17]. These shortcomings mainly originated from the limited synthetic conditions of noble metal/AgCl composite nanostructures. The bottom-up synthesis of noble two chemical mechanisms that can explain our observations. First, the large MW of PVP can inhibit the initial crystallization of AgCl, which is attributed to the decreased seed concentration and increased size of the NPs in the context of seed-mediated growth [35][36][37]. Second, the increase in PVP MW can inhibit the ripening of silver halide NPs, resulting in smaller silver halide NPs [38]. In our work, the first mechanism was dominant as the MW of PVP increased from 10k to 55k, while the second mechanism was dominant from 55k to 360k, resulting in the largest AgCl@AgAu NPs with 55k PVP. The MW of PVP also influenced the LSPR properties of the AgCl@AgAu NPs (Figure 1f). The metallic bumps on the AgCl@AgAu NPs with 10k and 360k PVP were more pronounced, resulting in strong plasmonic absorption near 550 nm, while the large two the AgCl@AgAu NPs with 29k and 55k PVP exhibited weaker plasmonic absorption. The monomeric vinyl pyrrolidone led to broad scattering owing to aggregated structures, as observed in Figure 1a. The photocatalytic activities of the AgCl@AgAu NPs for degradation of an organic dye were evaluated using amaranth as a model dye, which is a common organic pollutant in the dye industry, but is hardly degradable (Figure 1g) [39]. Interestingly, 55k PVP-AgCl@AgAu NPs exhibited the largest photocatalytic activity, which could be attributed to their reasonably strong LSPR and chemical stability based on their large diameter (220 nm in diameter).
Catalysts 2020, 9, x FOR PEER REVIEW 3 of 14 explain our observations. First, the large MW of PVP can inhibit the initial crystallization of AgCl, which is attributed to the decreased seed concentration and increased size of the NPs in the context of seed-mediated growth [35][36][37]. Second, the increase in PVP MW can inhibit the ripening of silver halide NPs, resulting in smaller silver halide NPs [38]. In our work, the first mechanism was dominant as the MW of PVP increased from 10k to 55k, while the second mechanism was dominant from 55k to 360k, resulting in the largest AgCl@AgAu NPs with 55k PVP. The MW of PVP also influenced the LSPR properties of the AgCl@AgAu NPs (Figure 1f). The metallic bumps on the AgCl@AgAu NPs with 10k and 360k PVP were more pronounced, resulting in strong plasmonic absorption near 550 nm, while the large two the AgCl@AgAu NPs with 29k and 55k PVP exhibited weaker plasmonic absorption. The monomeric vinyl pyrrolidone led to broad scattering owing to aggregated structures, as observed in Figure 1a. The photocatalytic activities of the AgCl@AgAu NPs for degradation of an organic dye were evaluated using amaranth as a model dye, which is a common organic pollutant in the dye industry, but is hardly degradable (Figure 1g) [39]. Interestingly, 55k PVP-AgCl@AgAu NPs exhibited the largest photocatalytic activity, which could be attributed to their reasonably strong LSPR and chemical stability based on their large diameter (220 nm in diameter).  Catalysts 2020, 9, x FOR PEER REVIEW 3 of 14 explain our observations. First, the large MW of PVP can inhibit the initial crystallization of AgCl, which is attributed to the decreased seed concentration and increased size of the NPs in the context of seed-mediated growth [35][36][37]. Second, the increase in PVP MW can inhibit the ripening of silver halide NPs, resulting in smaller silver halide NPs [38]. In our work, the first mechanism was dominant as the MW of PVP increased from 10k to 55k, while the second mechanism was dominant from 55k to 360k, resulting in the largest AgCl@AgAu NPs with 55k PVP. The MW of PVP also influenced the LSPR properties of the AgCl@AgAu NPs (Figure 1f). The metallic bumps on the AgCl@AgAu NPs with 10k and 360k PVP were more pronounced, resulting in strong plasmonic absorption near 550 nm, while the large two the AgCl@AgAu NPs with 29k and 55k PVP exhibited weaker plasmonic absorption. The monomeric vinyl pyrrolidone led to broad scattering owing to aggregated structures, as observed in Figure 1a. The photocatalytic activities of the AgCl@AgAu NPs for degradation of an organic dye were evaluated using amaranth as a model dye, which is a common organic pollutant in the dye industry, but is hardly degradable (Figure 1g) [39]. Interestingly, 55k PVP-AgCl@AgAu NPs exhibited the largest photocatalytic activity, which could be attributed to their reasonably strong LSPR and chemical stability based on their large diameter (220 nm in diameter). Scheme 1. Synthetic scheme of cubic and spherical AgCl@AgAu nanoparticles (NPs). The shape of the AgCl@AgAu NPs is controlled by the initial concentration of polyvinylpyrrolidone (PVP). The final concentration of PVP is adjusted to be the same for the same reductive conditions. The resultant AgCl@AgAu NPs are used for the photocatalytic degradation of organic pollutants under visible light irradiation.  Plasmonic absorption significantly affects the photocatalytic activity because it facilitates the separation of absorbed visible light into holes and electrons. Therefore, the enhanced absorption by larger metallic bumps improves the photocatalytic properties. On the other hand, the presence of such larger bumps decreases the exposed surfaces of AgCl NPs where the catalytic reactions actually occur, resulting in lower photocatalytic properties. In order to investigate which effect is dominant, we chose the AgCl@AgAu NPs synthesized with 55k PVP as a control and grew their bumps in a growth solution, which was analyzed using SEM (Figure 2a,b). The growth of the bumps was further analyzed using UV-vis spectroscopy, where the extinction dramatically increased owing to the increased plasmonic absorption and scattering of the enlarged metallic bumps (Figure 2c). The reduction of Ag + on the initial metallic bumps also induced a significant spectral blue shift from 550 to 420 nm. After confirming the successful growth of the metallic bumps, we compared the photocatalytic properties of the two types of the AgCl@AgAu NPs with different sizes of metallic bumps (Figure 2d). Interestingly, despite the increased plasmonic absorption, the photocatalytic activity decreased substantially. The control AgCl@AgAu NPs induced the completion of the reaction after 90 min (95% completion), while the reaction in the presence of the AgCl@AgAu NPs with larger bumps was still in process even at 180 min (70% completion). This limited photocatalytic activity caused by the larger metallic bumps clearly indicates that the effect of surface masking of AgCl by the metallic bumps is strong enough to suppress the effect of increased plasmonic absorption. Based on our observations, it is necessary that the metallic bumps are designed to be small enough to expose sufficient AgCl surfaces for the photocatalytic reaction. However, the bumps should at least be large enough to exhibit plasmonic properties. We also evaluated the effect of the size of AgCl nanomaterials on the photocatalytic activity of the AgCl@AgAu NPs which, however, turned out to be negligible ( Figure S1) [5].
Catalysts 2020, 9,  Plasmonic absorption significantly affects the photocatalytic activity because it facilitates the separation of absorbed visible light into holes and electrons. Therefore, the enhanced absorption by larger metallic bumps improves the photocatalytic properties. On the other hand, the presence of such larger bumps decreases the exposed surfaces of AgCl NPs where the catalytic reactions actually occur, resulting in lower photocatalytic properties. In order to investigate which effect is dominant, we chose the AgCl@AgAu NPs synthesized with 55k PVP as a control and grew their bumps in a growth solution, which was analyzed using SEM (Figures 2a and 2b). The growth of the bumps was further analyzed using UV-vis spectroscopy, where the extinction dramatically increased owing to the increased plasmonic absorption and scattering of the enlarged metallic bumps (Figure 2c). The reduction of Ag + on the initial metallic bumps also induced a significant spectral blue shift from 550 to 420 nm. After confirming the successful growth of the metallic bumps, we compared the photocatalytic properties of the two types of the AgCl@AgAu NPs with different sizes of metallic bumps ( Figure 2d). Interestingly, despite the increased plasmonic absorption, the photocatalytic activity decreased substantially. The control AgCl@AgAu NPs induced the completion of the reaction after 90 min (95% completion), while the reaction in the presence of the AgCl@AgAu NPs with larger bumps was still in process even at 180 min (70% completion). This limited photocatalytic activity caused by the larger metallic bumps clearly indicates that the effect of surface masking of AgCl by the metallic bumps is strong enough to suppress the effect of increased plasmonic absorption. Based on our observations, it is necessary that the metallic bumps are designed to be small enough to expose sufficient AgCl surfaces for the photocatalytic reaction. However, the bumps should at least be large enough to exhibit plasmonic properties. We also evaluated the effect of the size of AgCl nanomaterials on the photocatalytic activity of the AgCl@AgAu NPs which, however, turned out to be negligible ( Figure S1) [5]. The shape of NPs is crucial to determine their usage as catalysts. The exposed particle surface exhibits different surface potentials and consequently different catalytic properties depending on the crystalline facets. Therefore, cubic crystalline AgCl particles with (200) facets are expected to exhibit different photocatalytic activity from that of spherical AgCl particles with mixed facets. As the PVP The shape of NPs is crucial to determine their usage as catalysts. The exposed particle surface exhibits different surface potentials and consequently different catalytic properties depending on the crystalline facets. Therefore, cubic crystalline AgCl particles with (200) facets are expected to exhibit different photocatalytic activity from that of spherical AgCl particles with mixed facets. As the PVP concentrations required for the synthesis of cubic AgCl particles and for the reduction of metallic precursors are different (0.0357 g/L (1X PVP) and 5.356 g/L (150X PVP), respectively) [5], we first synthesized cubic AgCl NPs at 0.0345 g/L of PVP, and then formed metallic bumps on the cubic AgCl NPs by increasing the PVP concentration to 5.356 g/L using concentrated PVP solution. To evaluate the effect of the MW of PVP on the shape and size of the resultant NPs, we conducted the synthesis using PVP with various MWs (monomeric vinyl pyrrolidone, 10k, 29k, 55k, and 360k). While monomeric vinyl pyrrolidone resulted in highly aggregated NPs with a poorly controlled shape ( Figure 3a) as we observed with the spherical AgCl particles (Figure 1a), the other polymeric vinyl pyrrolidone successfully resulted in the cubic AgCl@AgAu NPs with tiny metallic bumps on the surfaces, and with similar sizes and structures regardless of the MWs (Figure 3b-e). Interestingly, the bumps were much smaller than those on the spherical AgCl@AgAu NPs (Figure 1b-e). This observation indicates that Au and Ag were more actively nucleated on the spherical AgCl@AgAu NPs than the cubic ones, probably because of their high surface potentials based on the high-index facets [40]. Although the size difference of the tiny metallic bumps was hardly observed using SEM, their plasmonic properties were still observed by the shoulders of the UV-vis spectra around 600 nm (Figure 3f). Based on the absorption of the cubic AgCl@AgAuNPs in the visible range, the photocatalytic activities of the cubic AgCl@AgAuNPs depending on the MW of PVP were investigated using amaranth ( Figure 3g). As the visible-range absorption was relatively weak, the photocatalytic activity was mainly determined by the size-dependent stability of the cubic AgCl@AgAuNPs. The largest AgCl@AgAuNPs (edge length =~340 nm), synthesized with 55k PVP (Figure 3d), exhibited the highest photocatalytic activity, which was followed by that of the AgCl@AgAuNPs synthesized with 29k PVP ( Figure 3c). As observed with the spherical AgCl@AgAu NPs (Figure 1), the cubic AgCl@AgAu NPs synthesized with 55k PVP were determined to be the most stable and the fastest photocatalysts. The importance of the MW of PVP was also demonstrated by the almost consistent photocatalytic activity of the AgCl@AgAu NPs synthesized with 55k PVP regardless of their shapes ( Figure S2). In fact, however, the shape of the AgCl@AgAu NPs played an important role in determining their chemical stability under elongated reaction conditions (24 h), proposing the one synthesized with 10X PVP during the AgCl formation and 150X PVP during the reduction exhibited the greatest stability (10X AgCl@AgAu NPs; Figure S3).
Catalysts 2020, 9, x FOR PEER REVIEW 5 of 14 concentrations required for the synthesis of cubic AgCl particles and for the reduction of metallic precursors are different (0.0357 g/L (1X PVP) and 5.356 g/L (150X PVP), respectively) [5], we first synthesized cubic AgCl NPs at 0.0345 g/L of PVP, and then formed metallic bumps on the cubic AgCl NPs by increasing the PVP concentration to 5.356 g/L using concentrated PVP solution. To evaluate the effect of the MW of PVP on the shape and size of the resultant NPs, we conducted the synthesis using PVP with various MWs (monomeric vinyl pyrrolidone, 10k, 29k, 55k, and 360k). While monomeric vinyl pyrrolidone resulted in highly aggregated NPs with a poorly controlled shape ( Figure 3a) as we observed with the spherical AgCl particles (Figure 1a), the other polymeric vinyl pyrrolidone successfully resulted in the cubic AgCl@AgAu NPs with tiny metallic bumps on the surfaces, and with similar sizes and structures regardless of the MWs (Figure 3b-e). Interestingly, the bumps were much smaller than those on the spherical AgCl@AgAu NPs (Figures 1b-e). This observation indicates that Au and Ag were more actively nucleated on the spherical AgCl@AgAu NPs than the cubic ones, probably because of their high surface potentials based on the high-index facets [40]. Although the size difference of the tiny metallic bumps was hardly observed using SEM, their plasmonic properties were still observed by the shoulders of the UV-vis spectra around 600 nm ( Figure 3f). Based on the absorption of the cubic AgCl@AgAuNPs in the visible range, the photocatalytic activities of the cubic AgCl@AgAuNPs depending on the MW of PVP were investigated using amaranth ( Figure 3g). As the visible-range absorption was relatively weak, the photocatalytic activity was mainly determined by the size-dependent stability of the cubic AgCl@AgAuNPs. The largest AgCl@AgAuNPs (edge length = ~340 nm), synthesized with 55k PVP (Figure 3d), exhibited the highest photocatalytic activity, which was followed by that of the AgCl@AgAuNPs synthesized with 29k PVP (Figure 3c). As observed with the spherical AgCl@AgAu NPs (Figure 1), the cubic AgCl@AgAu NPs synthesized with 55k PVP were determined to be the most stable and the fastest photocatalysts. The importance of the MW of PVP was also demonstrated by the almost consistent photocatalytic activity of the AgCl@AgAu NPs synthesized with 55k PVP regardless of their shapes ( Figure S2). In fact, however, the shape of the AgCl@AgAu NPs played an important role in determining their chemical stability under elongated reaction conditions (24 h), proposing the one synthesized with 10X PVP during the AgCl formation and 150X PVP during the reduction exhibited the greatest stability (10X AgCl@AgAu NPs; Figure S3).  The crystalline structures of the AgCl@AgAu NPs were analyzed at each step of the synthesis using X-ray diffraction (XRD) (Figure 4). Based on the excellent stability and photocatalytic performance, 10X AgCl@AgAu NPs were chosen as a representative photocatalyst for the analysis. The quantum yield of the degradation of amaranth using 10X AgCl@AgAu NPs as photocatalysts is determined to be~0.35%, which is as good as those reported with regard to the conventional TiO 2 photocatalysts (~0.2%) [41]. Before the reduction of the metallic precursors, the 10X AgCl NPs (AgCl NPs that is synthesized with 10X PVP) exhibited a strong (200) peak at 32 • owing to stacking of the truncated cubic AgCl particles, along with a few weaker peaks at 28, 46, and 67 • in the XRD pattern. This pattern exactly matched that of the standard AgCl reference (JCPDS No. 01-1013), elucidating the crystalline nature of the AgCl NPs. After the formation of metallic bumps, we observed a few more new peaks at 38, 44, 64, 78, and 82 • in the XRD pattern, which showed the formation of metallic Au and Ag bumps on the AgCl NPs. Because the metallic bumps were too small to be clearly observed using SEM, their peaks were broader than those of the AgCl NPs [42]. However, their overall XRD pattern was distinctive enough to identify the presence of metallic Au and Ag, indicating that the number of metallic bumps was still considerable.
Catalysts 2020, 9, x FOR PEER REVIEW 6 of 14 The crystalline structures of the AgCl@AgAu NPs were analyzed at each step of the synthesis using X-ray diffraction (XRD) (Figure 4). Based on the excellent stability and photocatalytic performance, 10X AgCl@AgAu NPs were chosen as a representative photocatalyst for the analysis. The quantum yield of the degradation of amaranth using 10X AgCl@AgAu NPs as photocatalysts is determined to be ~0.35%, which is as good as those reported with regard to the conventional TiO2 photocatalysts (~0.2%) [41]. Before the reduction of the metallic precursors, the 10X AgCl NPs (AgCl NPs that is synthesized with 10X PVP) exhibited a strong (200) peak at 32° owing to stacking of the truncated cubic AgCl particles, along with a few weaker peaks at 28, 46, and 67° in the XRD pattern. This pattern exactly matched that of the standard AgCl reference (JCPDS No. 01-1013), elucidating the crystalline nature of the AgCl NPs. After the formation of metallic bumps, we observed a few more new peaks at 38, 44, 64, 78, and 82° in the XRD pattern, which showed the formation of metallic Au and Ag bumps on the AgCl NPs. Because the metallic bumps were too small to be clearly observed using SEM, their peaks were broader than those of the AgCl NPs [42]. However, their overall XRD pattern was distinctive enough to identify the presence of metallic Au and Ag, indicating that the number of metallic bumps was still considerable. The metallic bumps were further investigated with respect to their structural and plasmonic properties using transmission electron microscopy (TEM) and UV-vis spectroscopy. As the observation of the metallic bumps on the surface of the AgCl@AgAu NPs was limited due to the presence of AgCl, we first eliminated the AgCl NPs of the 10X PVP AgCl@AgAu NPs using NH3 and collected the remaining bimetallic AgAu NPs for TEM investigation. The metallic bumps were in general smaller than 10 nm (Figure 5a), which explains why they were not clearly observed using SEM in Figure S2c. The distribution of Au and Ag was determined by energy dispersive X-ray spectroscopy and shown in elemental maps (Figures 5b, 5c, and 5d). Most of the bumps contained both Au and Ag at an atomic ratio of 25 and 75, which is similar to the mole ratio of remaining AuCl4and Ag + (approximately 1:3 or 1:4) after the synthesis of the AgCl NPs. [43,44] The interplanar spacing of the AgAu NP bumps was determined to be 0.23 nm using high-resolution TEM (Figure 5e), which matches the literature values of (111) planes of pure Au and Ag [45]. These dispersed metallic bumps exhibited a strong absorption band in the visible range (~550-650 nm), which is due to their LSPR as a result of their composition (Au and Ag) and size (approximately 10 nm or smaller) (Figure 5f). Interestingly, this absorption band is hardly observed in the UV-vis spectrum of the initial AgCl@AgAu NPs (Figure S2f), probably because of the short interparticle distances between the bumps on the AgCl NPs and consequent interference of electromagnetic waves [46]. The metallic bumps were further investigated with respect to their structural and plasmonic properties using transmission electron microscopy (TEM) and UV-vis spectroscopy. As the observation of the metallic bumps on the surface of the AgCl@AgAu NPs was limited due to the presence of AgCl, we first eliminated the AgCl NPs of the 10X PVP AgCl@AgAu NPs using NH 3 and collected the remaining bimetallic AgAu NPs for TEM investigation. The metallic bumps were in general smaller than 10 nm (Figure 5a), which explains why they were not clearly observed using SEM in Figure S2c. The distribution of Au and Ag was determined by energy dispersive X-ray spectroscopy and shown in elemental maps (Figure 5b-d). Most of the bumps contained both Au and Ag at an atomic ratio of 25 and 75, which is similar to the mole ratio of remaining AuCl 4 − and Ag + (approximately 1:3 or 1:4) after the synthesis of the AgCl NPs. [43,44] The interplanar spacing of the AgAu NP bumps was determined to be 0.23 nm using high-resolution TEM (Figure 5e), which matches the literature values of (111) planes of pure Au and Ag [45]. These dispersed metallic bumps exhibited a strong absorption band in the visible range (~550-650 nm), which is due to their LSPR as a result of their composition (Au and Ag) and size (approximately 10 nm or smaller) (Figure 5f). Interestingly, this absorption band is hardly observed in the UV-vis spectrum of the initial AgCl@AgAu NPs (Figure S2f), probably because of the short interparticle distances between the bumps on the AgCl NPs and consequent interference of electromagnetic waves [46]. In addition to the correlation between the structures and photocatalytic properties of the AgCl@AgAu NPs, we examined the effect of visible light irradiation on the photocatalytic reactions. In fact, it is possible that the catalytic reactions rely on metal-based catalysis owing to the presence of metallic nanostructures of the AgCl@AgAu NPs. To evaluate this hypothesis, we conducted the oxidation of amaranth in the presence of 10X PVP AgCl@AgAu NPs and 10X AgCl NPs with or without visible light irradiation. In the absence of visible light, the difference between the photocatalytic activities of the AgCl NPs and the AgCl@AgAu NPs was negligible. This observation indicates that metal-based catalysis based on the metallic bumps is insignificant. In the presence of visible light, however, more than 80% of amaranth was oxidized with the AgCl@AgAu NPs after 180 min, while the reaction proceeded to less than 50% with the AgCl NPs at the same time. The enhanced reaction rate by visible light irradiation clearly proves the visible light-driven photocatalytic properties of the AgCl@AgAu NPs based on the increased visible light absorption of the metallic bumps, compared to that of the AgCl NPs (Figure 6a, inset). To further evaluate the role of the visible light irradiation, it was stopped at 60 min. Without the irradiation, only negligible degradation of amaranth was observed. Therefore, the photocatalytic degradation can be easily quenched by simply eliminating the light irradiation. Moreover, even if the metallic bumps were not intentionally grown, the AgCl NPs can still absorb a certain amount of visible light, potentially owing to the presence of tiny metal clusters naturally occurring on the AgCl surface [7]. Taken together, the plasmonic absorption of the bimetallic AgAu NPs assists the generation of electron-hole pairs in AgCl, which would have been hardly possible with pure AgCl NPs. The generated electrons and holes in AgCl induce the formation of hydroxyl radical (•OH) from H2O2, and hydroxyl/Cl radical (•OH/•Cl) from OH − /Cl − , respectively. Eventually, the radical species (•OH and •Cl) degrade the organic pollutants [47][48][49]. The versatility of the AgCl@AgAu NP photocatalysts for degradation of toxic organic pollutants was evaluated using other harmful organic dyes (Figure 6b). Rhodamine B (RB), Reactive Red 120 (RR), and Methylene Blue (MB) were chosen as representative harmful organic pollutants, because they are generally used in the dye industry but are highly hazardous for the environment [50][51][52][53]. Without the photocatalysts, all of the organic dyes were degraded about 20% after 180 min despite visible light irradiation. However, the batches containing the AgCl@AgAu NP photocatalysts could remove over 80% of all the harmful organic pollutants at the same time, which demonstrated In addition to the correlation between the structures and photocatalytic properties of the AgCl@AgAu NPs, we examined the effect of visible light irradiation on the photocatalytic reactions. In fact, it is possible that the catalytic reactions rely on metal-based catalysis owing to the presence of metallic nanostructures of the AgCl@AgAu NPs. To evaluate this hypothesis, we conducted the oxidation of amaranth in the presence of 10X PVP AgCl@AgAu NPs and 10X AgCl NPs with or without visible light irradiation. In the absence of visible light, the difference between the photocatalytic activities of the AgCl NPs and the AgCl@AgAu NPs was negligible. This observation indicates that metal-based catalysis based on the metallic bumps is insignificant. In the presence of visible light, however, more than 80% of amaranth was oxidized with the AgCl@AgAu NPs after 180 min, while the reaction proceeded to less than 50% with the AgCl NPs at the same time. The enhanced reaction rate by visible light irradiation clearly proves the visible light-driven photocatalytic properties of the AgCl@AgAu NPs based on the increased visible light absorption of the metallic bumps, compared to that of the AgCl NPs (Figure 6a, inset). To further evaluate the role of the visible light irradiation, it was stopped at 60 min. Without the irradiation, only negligible degradation of amaranth was observed. Therefore, the photocatalytic degradation can be easily quenched by simply eliminating the light irradiation. Moreover, even if the metallic bumps were not intentionally grown, the AgCl NPs can still absorb a certain amount of visible light, potentially owing to the presence of tiny metal clusters naturally occurring on the AgCl surface [7]. Taken together, the plasmonic absorption of the bimetallic AgAu NPs assists the generation of electron-hole pairs in AgCl, which would have been hardly possible with pure AgCl NPs. The generated electrons and holes in AgCl induce the formation of hydroxyl radical (•OH) from H 2 O 2 , and hydroxyl/Cl radical (•OH/•Cl) from OH − /Cl − , respectively. Eventually, the radical species (•OH and •Cl) degrade the organic pollutants [47][48][49]. The versatility of the AgCl@AgAu NP photocatalysts for degradation of toxic organic pollutants was evaluated using other harmful organic dyes (Figure 6b). Rhodamine B (RB), Reactive Red 120 (RR), and Methylene Blue (MB) were chosen as representative harmful organic pollutants, because they are generally used in the dye industry but are highly hazardous for the environment [50][51][52][53]. Without the photocatalysts, all of the organic dyes were degraded about 20% after 180 min despite visible light irradiation. However, the batches containing the AgCl@AgAu NP photocatalysts could remove over 80% of all the harmful organic pollutants at the same time, which demonstrated the superior and versatile photocatalytic properties of the AgCl@AgAu NPs. To demonstrate the capability of AgCl@AgAu NPs for practical applications, we evaluated the photocatalytic properties of AgCl@AgAu NPs in natural water obtained from the Han River (Seoul, Republic of Korea) using the same four dyes (amaranth (AM), RB, RR, MB) (Figure 6c). After 3 h, approximately 60% of the dyes was degraded by the AgCl@AgAu NPs in a photocatalytic manner. Without the catalyst, however, less than 40% of the dyes were degraded. In particular, RR degraded almost negligibly (<5%) without the aid of the AgCl@AgAu NPs. Even though the photocatalytic efficiency of the AgCl@AgAu NPs was decreased in natural media, the versatile and practical capability of AgCl@AgAu NPs for the removal of environmental pollutants was clearly demonstrated.
Catalysts 2020, 9, x FOR PEER REVIEW 8 of 14 the superior and versatile photocatalytic properties of the AgCl@AgAu NPs. To demonstrate the capability of AgCl@AgAu NPs for practical applications, we evaluated the photocatalytic properties of AgCl@AgAu NPs in natural water obtained from the Han River (Seoul, Republic of Korea) using the same four dyes (amaranth (AM), RB, RR, MB) (Figure 6c). After 3 h, approximately 60% of the dyes was degraded by the AgCl@AgAu NPs in a photocatalytic manner. Without the catalyst, however, less than 40% of the dyes were degraded. In particular, RR degraded almost negligibly (<5%) without the aid of the AgCl@AgAu NPs. Even though the photocatalytic efficiency of the AgCl@AgAu NPs was decreased in natural media, the versatile and practical capability of AgCl@AgAu NPs for the removal of environmental pollutants was clearly demonstrated. The stability of heterogeneous catalysts is an important issue for reaction efficiency [54,55]. Under harsh oxidative or reductive conditions, NP catalysts could easily become non-catalytic due to their irreversible aggregation in the reaction mixture, where different phases of reactants and catalysts coexist. If NP catalysts exhibit sufficient stability to be collected after use and redispersed in a new reaction mixture, they could be repeatedly reused in multiple catalytic reactions. This recyclability would be highly advantageous in terms of cost and efficiency. Although the AgCl@AgAu NPs exhibit excellent photocatalytic properties, their stability and consequent recyclability need to be investigated, considering that they had to suffer under oxidative conditions. Therefore, the stability of the AgCl@AgAu NPs was evaluated by recycling them for the degradation of MB. The 10X PVP AgCl@AgAu NPs were used as a model photocatalyst for the recyclability evaluation owing to their excellent stability ( Figure S3). After each photocatalytic reaction, the AgCl@AgAu NPs were collected by centrifugation and redispersed for the repeated reactions. Interestingly, MB was fully oxidized even at the 7th photocatalytic reaction (Figure 7). Moreover, the The stability of heterogeneous catalysts is an important issue for reaction efficiency [54,55]. Under harsh oxidative or reductive conditions, NP catalysts could easily become non-catalytic due to their irreversible aggregation in the reaction mixture, where different phases of reactants and catalysts coexist. If NP catalysts exhibit sufficient stability to be collected after use and redispersed in a new reaction mixture, they could be repeatedly reused in multiple catalytic reactions. This recyclability would be highly advantageous in terms of cost and efficiency. Although the AgCl@AgAu NPs exhibit excellent photocatalytic properties, their stability and consequent recyclability need to be investigated, considering that they had to suffer under oxidative conditions. Therefore, the stability of the AgCl@AgAu NPs was evaluated by recycling them for the degradation of MB. The 10X PVP AgCl@AgAu NPs were used as a model photocatalyst for the recyclability evaluation owing to their excellent stability ( Figure S3). After each photocatalytic reaction, the AgCl@AgAu NPs were collected by centrifugation and redispersed for the repeated reactions. Interestingly, MB was fully oxidized even at the 7th photocatalytic reaction (Figure 7). Moreover, the photocatalytic capability of the AgCl@AgAu NPs was maintained nearly 70% even at the 11th reaction. This result demonstrates that the AgCl@AgAu NPs could be remarkably stable enough to be catalytically functional after repeated reactions, which is somewhat unexpected in view of the chemical instability of AgCl [6]. The superior recyclability and stability of the AgCl@AgAu NPs would be highly attractive for their versatile and practical photocatalytic applications.
Catalysts 2020, 9, x FOR PEER REVIEW 9 of 14 photocatalytic capability of the AgCl@AgAu NPs was maintained nearly 70% even at the 11th reaction. This result demonstrates that the AgCl@AgAu NPs could be remarkably stable enough to be catalytically functional after repeated reactions, which is somewhat unexpected in view of the chemical instability of AgCl [6]. The superior recyclability and stability of the AgCl@AgAu NPs would be highly attractive for their versatile and practical photocatalytic applications.

Synthesis of AgCl@AgAu NPs
An aqueous PVP solution (MW of PVP: monomer, 10k, 29k, 55k, and 360k; 7.045 mL, 5.356 g/L) was combined with an aqueous solution of HAuCl4 (5 mM, 40 μL), followed by injection of an aqueous AgNO3 solution (5 mM, 200 μL). For the growth of AgAu NP bumps, the mixture was incubated with vigorous stirring at 25 °C for 24 h. The resultant AgCl@AgAu NPs were washed three times by centrifugation at 5000 rpm for 5 min, removal of the supernatant, and redispersion in a 0.01% Tween 20 solution.

Synthesis of AgCl@AgAu NPs
An aqueous PVP solution (MW of PVP: monomer, 10k, 29k, 55k, and 360k; 7.045 mL, 5.356 g/L) was combined with an aqueous solution of HAuCl 4 (5 mM, 40 µL), followed by injection of an aqueous AgNO 3 solution (5 mM, 200 µL). For the growth of AgAu NP bumps, the mixture was incubated with vigorous stirring at 25 • C for 24 h. The resultant AgCl@AgAu NPs were washed three times by centrifugation at 5000 rpm for 5 min, removal of the supernatant, and redispersion in a 0.01% Tween 20 solution.

Synthesis of AgCl@AgAu NPs with Larger Metallic Bumps
An aqueous PVP solution (7.045 mL, 5.356 g/L) was combined with an aqueous solution of HAuCl 4 (5 mM, 40 µL) and AgNO 3 (5 mM, 200 µL), and vigorously stirred at 25 • C for 24 h. Subsequently, the mixture was injected into an ascorbic acid solution (180 µM, 8 mL) using a syringe in a drop-by-drop manner, followed by the injection of an AgNO 3 solution (5 mM, 360 µL). The mixture was incubated with vigorous stirring for 3 h. The synthesized AgCl@AgAu NPs with large metallic bumps were washed three times by centrifugation at 5000 rpm for 5 min, removal of the supernatant, and redispersion in a 0.01% Tween 20 solution.

Synthesis of Cubic AgCl@AgAu NPs
An aqueous PVP solution (MW of PVP: monomer, 10k, 29k, 55k, and 360k; 7.045 mL, 0.0357 g/L) was combined with an aqueous solution of HAuCl 4 (5 mM, 40 µL) and AgNO 3 (5 mM, 200 µL), followed by vigorous stirring at 25 • C for 30 min. After the formation of the AgCl NPs, a concentrated aqueous PVP solution (111.8 g/L, 352 µL) was quickly injected to adjust the final concentration of PVP to 5.356 g/L. For the growth of the AgAu NP bumps, the mixture was incubated with vigorous stirring at 25 • C for 24 h. The resultant cubic AgCl@AgAu NPs were washed three times by centrifugation at 5000 rpm for 5 min, removal of the supernatant, and redispersion in a 0.01% Tween 20 solution.

Synthesis of AgCl@AgAu NPs with Various Sizes
An aqueous 55k PVP solution (7.045 mL, 5.356 g/L) was combined with an aqueous solution of HAuCl 4 (5 mM), to which an aqueous solution of AgNO 3 (5 mM) was added. The volume ratio of the AgNO 3 and HAuCl 4 solutions was determined to be 7:1, 5:1, 3:1, 2:1, or 1:1, depending on the desired size of the resultant AgCl nanostructures. The sum of the volumes of the two precursor solutions was 240 µL. For the growth of AgAu NP bumps, the mixture was incubated with vigorous stirring at 25 • C for 24 h. The resultant AgCl@AgAu NPs were washed three times by centrifugation at 5000 rpm for 5 min, removal of the supernatant, and redispersion in a 0.01% Tween 20 solution.

Synthesis of Various Shapes of AgCl@AgAu NPs
An aqueous PVP solution with various initial concentrations (7.045 mL, 0.0357 g/L; 1X, 0.178 g/L; 5X, 0.357 g/L; 10X, 3.571 g/L; 100X, 5.356 g/L; 150X) was combined with an aqueous solution of HAuCl 4 (5 mM, 40 µL) and AgNO 3 (5 mM, 200 µL), followed by vigorous stirring at 25 • C for 30 min. After 30 min, a concentrated aqueous PVP solution (111.8 g/L) was quickly injected to adjust the final concentration of PVP to 5.356 g/L. For the growth of AgAu NP bumps, the mixture was incubated with vigorous stirring at 25 • C for 24 h. The resultant AgCl@AgAu NPs were washed three times by centrifugation at 5000 rpm for 5 min, removal of the supernatant, and redispersion in a 0.01% Tween 20 solution.

Photocatalytic Degradation of Organic dyes
The mass of the synthesized AgCl@AgAu NPs was calculated based on the solubility product constant (Ksp) of AgCl and the precursor mole ratio of AgNO 3 and HAuCl 4 . Subsequently, the AgCl@AgAu NP solutions were diluted to have equal mass for comparison of the photocatalytic activities. The ultrapure water was added until the diluted AgCl@AgAu NPs to be dispersed in 438 µL of water and combined with an organic dye solution (2 mM, 50 µL; amaranth, Reactive Red 120, and rhodamine B) and H 2 O 2 solution (30%, 12 µL). In the case of methylene blue, a methylene blue solution (1 mM, 50 µL) and an H 2 O 2 solution (1%, 5 µL) were mixed with 445 µL of the redispersed AgCl@AgAu NPs solution. The photocatalytic degradation reactions were conducted under visible light irradiation for 3 h using a metal-halide lamp equipped with UV cut-off sheets (λ > 400 nm). The degradation of organic dyes was analyzed by observing the changes in the characteristic absorption of each organic dye every 30 min using UV-vis spectroscopy.

Conclusions
The controlled synthesis of AgCl-based nanomaterials has been almost unexplored, or resulted in only limited success. The largest concern in this field has been the inherent chemical instability of AgCl. Therefore, although particulate plasmonic structures are the most common in previous reports, their composition was mainly limited to Ag. Unlike previous studies, this work demonstrates a universal and straightforward route to synthesize bimetallic Ag and Au nanobumps on the surface of AgCl nanomaterials, which significantly expanded the window for visible light absorption. Importantly, the results shown in this work provide a facile approach to control the reduction of metal ions into nanostructured bumps with controlled and desired plasmonic properties at the interface of reducible solid AgCl and liquid water. We also demonstrate the correlation between the structural, compositional, and photocatalytic properties of AgCl-based nanomaterials. The extended visible light absorption range of the AgCl@AgAu NPs is highly advantageous for their enhanced photocatalytic properties because of the following two reasons. First, the significant red-shift of the absorption band (∆λ MAX = 200 nm) implies the utilization of more energy, which would otherwise not have been used. Therefore, the AgCl@AgAu NPs, when used with conventional Ag/AgCl nanomaterials, would be an ideal combination for building up photocatalytic systems with efficient energy absorption. Second, the spectral irradiance of sunlight is higher in the ranges where the AgCl@AgAu NPs (550-650 nm) exhibit the absorption than in those of the conventional Ag/AgCl nanomaterials (~400 nm) [56]. Since sunlight is the ultimate source of visible light irradiation, the AgCl@AgAu NPs would be more appropriate for the decontamination of natural water than the conventional Ag/AgCl nanomaterials. For practical applications in the future, large-scale synthesis and long-term stability of the AgCl@AgAu NPs need to be further investigated.