key: cord-0073552-1dszv7di
authors: Loka, Chadrasekhar; Lee, Kee-Sun
title: Dewetted Silver Nanoparticle-Dispersed WO(3) Heterojunction Nanostructures on Glass Fibers for Efficient Visible-Light-Active Photocatalysis by Magnetron Sputtering
date: 2021-12-28
journal: ACS Omega
DOI: 10.1021/acsomega.1c06239
sha: 91941848763dcf3b0f7ab6803a7e04c5b02f0fa2
doc_id: 73552
cord_uid: 1dszv7di

[Image: see text] Fabrication of hybrid-heterojunction nanostructures comprising the Z-scheme and localized surface plasmon resonance is essential for enhancing the photocatalytic degradation of organic compounds to enable environmental remediation. This study focuses on the dispersion of dewetted Ag nanoparticles over the 3D network-like silica glass fibers (SGFs) coated with a Cu-doped WO(3) heterojunction system by a high-throughput and cost-effective method using magnetron sputtering, followed by solid-state dewetting. The influence of Cu doping on the crystal structure, growth direction, and morphology of WO(3) and the effect of localized surface diffusion-driven dewetted Ag nanoparticles on the photocatalytic performance were investigated. The Cu doping changed the optical band gap, and the 2Cu–WO(3)/SGF exhibited excellent photocatalytic activity. The surface dispersion of dewetted Ag nanoparticles over Cu–WO(3)/SGFs exhibited lowest photoluminescence intensity, indicating the effective separation of photogenerated electrons–holes, which led to highest efficiency (∼98%) in photocatalytic degradation of methylene blue among all the fibers with a degradation rate constant (k = 0.0205 min(–1)) that was ∼18.6 times higher than that of pure WO(3) (k = 0.0011 min(–1)). The findings of this study can provide insights for designing low-cost and efficient visible-light-active photocatalysts for organic dye degradation, enabling environmental remediation.

The 21st century brought significant awareness about devastating environmental impacts and environmental consciousness in the society. Thus, the scientific community has developed substantial interest in the utilization of renewable energy, especially inexhaustible clean energy showered by the sun, for environmental remediation through the development of efficient visible-light-active photocatalysts to decontaminate air and water. 1,2 Tungsten oxide (WO 3 ) is a chemically stable non-toxic n-type semiconductor with a controllable optical band gap (2.4−2.9 eV) and good resistance against photocorrosion in aqueous media. 3 Therefore, WO 3 has potential applications in hydrogen production, virus deactivation, and harmful pollutant degradation, 4,5 which make it an appealing candidate for harvesting visible light to degrade organic compounds. However, the widespread application of WO 3 is inhibited due to the relatively low conduction band (CB) edge and the rapid recombination of photogenerated electrons and holes, which lead to poor photocatalytic reaction efficiency. 6 The photocatalytic efficiency can be significantly improved by improving the visible light absorption capacity, increasing the surface area, and reducing the recombination rate of photogenerated electron−hole pairs. To date, many scholars have invested a lot of energy to accomplish enhanced photocatalytic activity of WO 3 by implementing various strategies such as doping, metal loading, and heterojunction formation. 7−9 Among them, the Z-scheme heterojunction system has attracted increasing attention, which promotes the separation of charge carriers in addition to possessing high redox ability. 9−11 Nevertheless, most of the existing Z-scheme photocatalysts are powder-based materials that often tend to agglomerate because of their high surface energy and involve complexities in the separation process of photocatalytic nanoscale powders from aqueous solutions. Moreover, these residues can cause secondary contamination. As a result, exploring the immobilized functional materials with high photocatalytic activity, which does not require any additional separation process, is easily recyclable, and low cost is imperative for advanced and sustainable photocatalysis. In this context, it is expected that the exploitation of network-like fibers as a substrate material or a supporting skeleton for the photocatalyst could be a viable solution for environmental friendliness. The fibers possess potential advantages such as good adsorption ability, high surface area, good physicochemical and thermal stability, low cost, reusability, and recyclability. 12 Fukugaichi reported TiO 2 nanoparticle (NP) dispersions on glass fibers with enhanced photocatalytic degradation of organic dyes. 13 Heo et al. exploited filter fibers for antimicrobial applications using TiO 2 nanoparticle coverage through a simple aerosol deposition method, which is impressive in this COVID-19 pandemic situation. 14 Recently, the mere dispersion of plasmonic metal nanoparticles (such as Ag, Au, Cu, Pt, and Pd) onto a dielectric surface that can radically alter the localized surface plasmon resonance (LSPR) has been considered as the state of the art in the areas of photocatalysis, surface-enhanced Raman scattering, plasmon-enhanced fluorescence, photovoltaics, chemical, and biological sensing applications. 15, 16 However, fabrication of these nanoparticles with appropriate structure, size, density, and arrangement is crucial as they influence the LSPR phenomenon. To date, chemical synthesis and lithography were reported to fabricate plasmonic metal nanoparticles, 17, 18 which have limitations such as high cost, complexity in processing, high temperature, reproducibility, and time-consuming. In addition, most of these studies reported on the preparation of both patterned and randomly arranged nanoparticles on flat substrates rather than non-flat substrates. Silver has unique advantages over other plasmonic metals, such as low cost, bactericidal properties, high work function, and special behavior for oxygen adsorption. 19 Moreover, the dispersion of evenly distributed plasmonic Ag nanoparticles improves the visible light absorption range, in addition to substantially ameliorating the photocatalytic degradation efficiency of organic pollutants, by suppressing electron−hole recombination. 20 The loading of Ag on metal oxides was generally achieved by photoreduction, conventional impregnation, or chemical reduction methods for photocatalysis. 21−23 Solid-state dewetting, a spontaneous physical phenomenon that refers to the decomposition of a thin film into small particles or nanostructures, leaving voids on a substrate, driven by excess energy due to high fraction of grain boundaries, interfaces, surfaces, or residual stresses is another approach for the dispersion of Ag nanoparticles. 24−26 Although solid-state dewetting is the simplest method, the template or seed-mediated patterning is inevitable, 15, 17, 18 which again involves complex processing (such as lithography/shadowmask and so forth) for the 3D network-like substrates.

In this work, we report the fabrication of evenly dispersed Ag nanoparticles with a high-throughput and cost-effective method using magnetron sputtering, followed by solid-state dewetting. To the best of our knowledge, Ag dewetting-driven homogeneous Ag nanoparticle dispersion over 3D network-like fibers has rarely been reported. Utilization of low-cost, reusable, and recyclable 3D-network-like silica glass fibers (SGFs) as supporting materials and the preparation of hybrid nanostructures CuO−WO 3 /SGF and Ag nanoparticle dispersion based on the Z-scheme and LSPR phenomena for high-performance, practicable, visible-light-active photocatalysts are demonstrated. CuO is a direct band gap p-type semiconductor photocatalyst with a band gap of 1.3−1.7 eV, which is mostly coupled with other semiconductors to form a heterojunction that can potentially inhibit the recombination of photogenerated electron−hole pairs. 27, 28 In this work, the synergistic properties arising from the hybrid nanostructure composed of CuO−WO 3 composite coated on SGFs and the solid-state dewetting-driven surface dispersion of Ag nanoparticles are thoroughly investigated for an efficient visiblelight-active photocatalyst.

2.1. Crystal Structure and Microstructural Changes in Cu−WO 3 Fibers. Figure 1a shows the X-ray diffraction (XRD) results of the undoped and Cu-doped WO 3 -coated fibers annealed at 873 K. The undoped WO 3 fibers exhibited a monoclinic-WO 3 phase (JCPDS no. 01-083-0950) with characteristic peaks of (002), (020), and (200) facets at 2θ = 23.11, 23.55, and 24.24°, respectively. Among these three characteristic peaks, the (200) facet exhibited high intensity, indicating the preferred direction of growth. The 1Cu−WO 3 and 2Cu−WO 3 fibers also exhibited a monoclinic WO 3 phase (JCPDS no. 01-083-0950). However, with increasing Cu concentration, the crystal structure and growth direction of WO 3 significantly changed. The XRD intensity of the (002) peak increased with increasing Cu concentration, whereas the intensity of the (200) peak decreased. The change that occurred in the preferred orientation of WO 3 could be explained as follows: the as-coated fibers exhibited an amorphous phase (data not shown here), which recrystallized during the rapid thermal annealing (RTA) through surface reconfiguration, by evolving the crystal planes with low surface energy to lower the total surface energy. The literature suggested that the surface energies of monoclinic WO 3 followed the order (002) > (020) > (200) with corresponding surface energies of 1.56 > 1.54 > 1.43 J/m 2 , which implied that the (200) plane was a highly stable phase, while the (002) plane was unstable. 29 Among the three characteristic facets (002), (020), and (200) of WO 3 , the (002) facet was reported to exhibit significantly enhanced adsorption of reactive species during the photocatalytic reaction in comparison with the other facets. 30 Cu doping could be effective for the recrystallization of the Cu−WO 3 fibers in the (002) plane, which became predominant in 2Cu−WO 3 . A similar observation was reported by Kalanur and Seo where the WO 3 -preferred orientation was systematically tuned by controlling the Al doping; 30 a higher concentration of dopant changed the crystal structure, as the dopant took W positions in the lattice and was distributed uniformly throughout the crystal structure. Therefore, a further increase in the Cu content revealed the formation of the triclinic CuWO 4 (JCPDS no. 01-080-1918) phase due to the replacement of W by Cu in the lattice to form the CuWO 4 phase. All the Cu−WO 3 -coated fibers annealed at 873 K exhibited XRD peaks at 2θ of 35.5 and 38.6°attributed to the (−111) and (111) planes of the monoclinic CuO phase (JCPDS no. 45-0937), and a gradual increase in the CuO peak intensity was observed with an increase in Cu content. It has been reported that the thermodynamic equilibrium oxide phase (Cu−O) was wholly determined by the applied oxidation temperature, rather than growth kinetics, and as a result, CuO phase formation was ensured because of rapid O 2 diffusion above its critical temperature of 600 K. 31, 32 Thus, in this study, a higher oxidation temperature (873 K) could drastically enhance the O 2 diffusion process to form CuO. Consequently, the XRD results revealed that the 1Cu−WO 3 -and 2Cu−WO 3 -coated fibers annealed at 873 K comprised the WO 3 and CuO composite phases, and the 3Cu−WO 3 fibers consisted of CuWO 4 and CuO composite phases.

The surface morphologies of the undoped and Cu-doped WO 3 -coated fibers annealed at 873 K were observed by fieldemission scanning electron microscopy (FE-SEM), as shown in Figure 1b −e. The as-coated fibers are shown in Figure S1 , where all the fibers were aligned like the 3D network, with the existence of macropores; they exhibited a glossy and homogeneous surface morphology. As shown in Figure 1b , the pure WO 3 -coated fibers exhibited a flake-like surface microstructure and the formation of a few nanoplates with well-grown facets and sharp facet edges, which indicated the typical morphology of the crystalline WO 3 nanoplates. The 1Cu−WO 3 and 2Cu−WO 3 fibers exhibited densely covered large grains that could be composed of the CuO−WO 3 composite as shown in Figure 1c ,d, respectively. In addition, the 3Cu−WO 3 fibers exhibited a porous microstructure and agglomerated grains. It could be understood that in the case of high-concentration Cu-doped WO 3 fibers, the Cu atoms could rapidly diffuse into the adjacent WO 3 phase during the RTA process to occupy the W atom sites. 33 This would lead to the formation of localized pores, as shown in Figure 1e , forming the CuWO 4 phase, as confirmed by the XRD results.

2.2. Chemical Composition and Optical Properties of Cu−WO 3 Fibers. X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the surface chemical composition and state of the WO 3 and Cu−WO 3coated fibers annealed at 873 K. As shown in Figure 2 , the survey spectrum showed the peaks of W, Cu, and O in Cu− WO 3 -coated fibers. With an increase in Cu concentration, the Cu−WO 3 fibers exhibited a very intense signal of Cu along with the presence of strong satellite peaks, and the W signal was significantly decreased. The high-resolution W 4f spectra of WO 3 , 1Cu−WO 3 , and 2Cu−WO 3 were deconvoluted into three peaks with binding energies of 35.4, 37.5, and 41.0 eV, corresponding to W 4f 7/2 , W 4f 5/2 , and the satellite peak of WO 3 , respectively, which were consistent with the W 6+ state. 34 Moreover, the splitting energy of the W 4f doublet at 2.1 eV also confirmed the W 6+ state. The peak at 41.0 eV could be attributed to the binding energy of W 5p 3/2 . 35, 36 However, a slight shift in the W 4f spectra toward a higher energy of about 0.4 eV for 3Cu−WO 3 was observed, which is in good agreement with a similar observation reported by Thongpan et al. for the CuWO 4 phase. 37 The W 4f spectra of 3Cu−WO 3 were fitted into two peaks at 35.8 and 37.9 eV corresponding to W 4f 7/2 and W 4f 5/2 of W 6+ in the form of CuWO 4 , respectively. The deconvolution of high-resolution XPS spectra of Cu 2p for 1Cu−WO 3 and 2Cu−WO 3 showed that two major peaks centered at 933.65 and 953.45 eV are assigned to Cu 2p 3/2 and Cu 2p 1/2 of Cu 2+ , respectively, which confirmed the CuO phase. 38, 39 The binding energy difference between the two peaks Cu 2p 3/2 and Cu 2p 1/2 was 19.8 eV, which strongly indicated the presence of Cu 2+ . 40 In addition, the presence of strong satellite peaks around 940−944 and 962 eV in the Cu−WO 3 /SGFs corresponds to Cu 2p 3/2 and Cu 2p 1/2 , respectively, which indicate an open Cu 3d 9 shell corresponding to the bivalent (Cu 2+ ) state (CuO phase). 27, 39, 40 However, the peaks centered at 931.6 and 951 eV are attributed to Cu 2p 3/2 and Cu 2p 1/2 of Cu + , 41 which indicates the presence of a small amount of Cu 2 O phase in the 1Cu−WO 3 -coated fibers. The relative concentrations of Cu + and Cu 2+ species present on 1Cu−WO 3 was quantified to be about 18% (Cu + ) and 82% (Cu 2+ ) by considering the curve-fitting of the total Cu 2p emission peaks including the shake-up satellite peaks as reported by Biesinger et al. using the following equations 42,43

where B is the area of the shake-up peak and A is the total area of the Cu 2p main peak. The main emission line (A) contains both Cu 2+ (A1) and Cu + (A2) contributions, but the satellite intensity (B) is entirely from Cu 2+ . 42 In the case of 3Cu−WO 3 , the Cu 2p peaks were slightly shifted (about 0.7 eV) toward higher binding energies, and they were deconvoluted into two major peaks centered at 934.4 and 954.2 eV, attributed to Cu 2p 3/2 and Cu 2p 1/2 , respectively, due to Cu 2+ in the lattice sites of CuWO 4 . 44 The observed peak shifting in the Cu 2p spectra of the CuWO 4 phase is in good agreement with the previous reports. 45 The O 1s exhibited peaks at about 531 and 530 eV in both WO 3 and Cu−WO 3 fibers, which were attributed to the surface-adsorbed oxygen and the characteristic peak of metal ion-associated oxygen (W−O and Cu−O), respectively. 46 Raman spectroscopy is a well-known non-destructive method that is commonly used to investigate material properties including the structure and phase change owing to its high sensitivity to the electronic structure. Therefore, Raman spectroscopy was further utilized to evaluate the chemical bonds of Cu−WO 3 . The Raman spectra displayed in stretching modes of the monoclinic WO 3 . 47, 48 The peak at 978 cm −1 in the 2Cu−WO 3 /SGF is attributed to the symmetric stretching mode of WO bonds resulting from the adsorbing water molecules in the air. 37 Based on group theory calculations, the triclinic CuWO 4 exhibits 36 vibrational modes (Γ Raman+infrared = 18A g + 18A u ). 49 For the 3Cu−WO 3 / SGF, we have observed different Ag active Raman modes, among which the highest intense peak is observed at about 905 cm −1 , which is ascribed to the W−O stretching vibration ν 1 (A g ). The other peaks, 313 cm −1 attributed to the scheelite tungstate vibration ν 2 (A g ) in the triclinic lattice and 780 cm −1 associated with the scheelite structure vibration ν 3 (A g ), confirm the triclinic CuWO 4 phase. 50 The peaks at 357 and 398 cm −1 correspond to the deformation modes of r(B g ) and δ(A g ), respectively. The Raman peaks observed in 3Cu−WO 3coated fibers are in good coincidence with the previous reports of the CuWO 4 phase. 50 Consequently, the Raman spectroscopy studies are well corroborated with the XRD and XPS results, which confirm that the higher doping concentration of Cu leads to CuWO 4 phase formation.

Visible light absorption was an important precondition for efficient visible-light-active photocatalysts. Figure 4 shows the UV−visible absorption spectra of the WO 3 and Cu−WO 3coated fibers annealed at 873 K. The absorption spectra vary with Cu doping. It was observed that the red shift in the absorption spectra occurred with an increase in the Cu content, which was favorable for visible light photocatalysis. The optical band gaps of the WO 3 and Cu−WO 3 fibers were determined using Tauc's equation as follows

where α is the absorption coefficient, hϑ is the incident photon energy, A is an arbitrary coefficient, n = 1/2 for direct and n = 2 for indirect recombination, and E g is the optical band gap. WO 3 -coated fibers exhibited the lowest rate constant, which could be due to rapid electron−hole recombination. The 2Cu−WO 3 fibers exhibited approximately 10.7 times higher activity rate constant than pure WO 3 . In fact, the CB of WO 3 is more positive than the O 2 /O 2 •− redox potential (−0.33 eV), which indicates that the adsorbed O2 molecules on the surface of WO 3 cannot be reduced to superoxide radicals O 2

•− . The valence band (VB) of CuO (+1.43 eV) is less positive than OH − / • OH (+2.38 eV), which indicated that CuO could not directly produce the • OH radicals. Hence, the observed enhanced photocatalytic activity could not be explained by the conventional p−n heterojunction formation of CuO−WO 3 . A more credible mechanism could be the Z-scheme heterojunction formation; in the case of Cu−WO 3 samples, the photogenerated electrons from the CB of WO 3 could recombine with the holes in the VB of CuO. As the CB of CuO was more negative than the redox potential, the electrons could participate in the redox reactions to form the superoxide radical O 2

•− . The holes in WO 3 could contribute to the oxidation process to produce the • OH radicals because the VB of WO 3 was more positive than the oxidation potential to yield • OH. Thus, the electrons in the CB of CuO and holes in the VB of WO 3 could effectively perform redox/oxidation reactions. 2Cu−WO 3 possessed a higher concentration of Cu than 1Cu−WO 3 . This led to the formation of higher phase fraction of CuO while forming the CuO/WO 3 interfaces. This resulted in higher photocatalytic activity. Thus, the enhanced photocatalytic activity conformed to the CuO−WO 3 Z-scheme heterojunction mechanism, which greatly enhanced the separation of photogenerated electrons−holes. Moreover, the (002) crystal orientation of WO 3 in 2Cu−WO 3 predominantly accelerated the transfer of photoinduced charge carriers to ascertain high reactivity, compared to other orientations, as reported by several researchers. 55, 56 It is to be noted that the surface area of the photocatalyst greatly influenced the photocatalytic activity; herein, the 3D network-like fiber substrates obviously possess a high surface area compared to the 2D substrates. In the case of 3Cu−WO 3 fibers, the lower photocatalytic activity could be due to the CuWO 4 phase formation and porosity, which could be attributed to the smaller volume fraction of CuO. In general, the CuWO 4 phase has a CB edge of approximately 0.2−0.44 V versus NHE, which is more positive than the O 2 reduction potential; 57 hence, the lower photocatalytic activity can be obviously observed in 3Cu−WO 3 fibers. Overall, the 2Cu−WO 3 -coated fibers exhibited better photocatalytic activity among the Cu− WO 3 -coated SGFs.

2.4. Silver Dewetting on CuO−WO 3 Fibers. Ag nanoparticles were dispersed over CuO−WO 3 (2Cu−WO 3 ) fibers to further enhance the photocatalytic activity of the fibers, as shown in Figure 6a . A 10 nm-thick Ag film was sputtered on the CuO−WO 3 fibers and subsequently vacuumannealed by RTA at 873 K for 5 min to obtain the island formation and growth of Ag in the form of isolated nanoparticles. The crystal structure of the Ag-dispersed CuO−WO 3 fibers was studied by XRD, as shown in Figure  6b direction. The high-magnification (30 000×) surface microstructure of the Ag-dispersed CuO−WO 3 fibers clearly indicated the formation of evenly distributed Ag nanoparticles over the surface of CuO−WO 3 , as shown in Figure 6c . The corresponding low-magnification (5000×) image is shown in Figure S2 . The size distribution histogram of the Ag nanoparticles is shown in Figure 6d , where the solid line represents the Gaussian fitting to a normal distribution curve. Based on this fitting, the mean size (D) and standard deviation (σ) of the Ag nanoparticles were 78.8 ± 3 and 46.2 nm, respectively. The surface coverage of the Ag nanoparticles was determined to be approximately 24.9% using ImageJ software. The high-resolution XPS spectrum of Ag 3d is shown in Figure  6e , where two peaks with binding energies centered at 368.55 and 374.55 eV were observed with an energy gap of 6.0 eV, which corresponded to Ag 3d 5/2 and Ag 3d 3/2 of metallic Ag, respectively. 58 This result was in good accordance with the results from the XRD analysis, indicating the metallic nature of Ag.

Additional structural information of Ag-dispersed CuO− WO 3 fibers was examined using high-resolution transmission electron microscopy (HR-TEM), and TEM sampling was performed using the focused ion beam (FIB) H-bar technique, as shown in Figure 7a . Before milling, the surface of the target region of the Ag-dispersed CuO−WO 3 fiber was coated with a thin carbon layer, followed by a platinum protective coating to avoid milling of the target region. Milling was performed with a high-energy Ga ion beam. The milled specimen was picked and mounted on a TEM specimen grid. Subsequently, the specimen was milled again to obtain a very thin piece (electron transparent) with a sample thickness of less than 100 nm. The backscattered electron (BSE) images of the high- Figure 7b ) and final sample preparation (the rectangular region shown in the inset) are shown in Figure 7b . The cross-sectional TEM image ( Figure  7c ) shows the existence of Ag nanoparticles on the surface, and the thickness of the Ag-dispersed CuO−WO 3 coating was approximately 380 nm. From the HR-TEM image (Figure 7d ), a small near-spherical silver nanoparticle in the range of ∼50 nm was observed, which formed an interface with the monoclinic WO 3 phase. The crystal structure of silver was determined by the lattice fringes, and the corresponding fast Fourier transformation (FFT) and inverse FFT (IFFT) images of the marked region are depicted in Figure 7d . The interplanar distance of the lattice fringes was determined to be d = 0.235 nm, which corresponded to the d-spacing of the Ag(111) plane. Similarly, the FFT and IFFT images of WO 3 are shown in Figure 7d , which corresponded to monoclinic WO 3 with an interplanar distance d = 0.354 nm of the (002) plane. Based on the above results, Ag nanoparticle dispersion through Ag solid-state dewetting was successfully obtained. As shown in the schematic in Figure 8 , the as-deposited Ag layer on the CuO−WO 3 -coated fibers comprised numerous Ag adatoms. When subjected to RTA, the interfacial free energy between Ag and CuO−WO 3 spontaneously induced the separation and isolation of Ag (Ag solid-state dewetting). More specifically, from a thermodynamic point of view, dewetting occurred when the surface energy (E s ) was positive, and the driving force was defined as follows 3 3 where γ Ag , γ ( CuO−WO 3 ) , and γ Ag,(CuO−WO 3 ) are the surface energy densities of Ag, CuO−WO 3 , and Ag and CuO−WO 3 , respectively. The evolution of isolated Ag nanoparticles in an Ag thin film sputtered on a glass substrate is depicted in Figure  S3 for clearly understanding the formation of the Ag dewetting-induced Ag nanoparticle. The as-deposited Ag layer showed unstable or metastable Ag adatoms with a relatively smooth microstructure, which was ascribed to insufficient surface diffusion to obtain a low-energy configuration. The Ag adatoms started to diffuse on the surface with an increase in temperature and finally formed isolated island-like spherical or near-spherical nanoparticles owing to the lower surface energy. Surface diffusion was the major driving parameter for the Ag dewetting behavior, and the diffusion coefficient (D s ) of the atom could be related to the temperature (T) as follows 25

where D 0 is the pre-exponential factor of the surface diffusion coefficient, E a is the activation energy, k is the Boltzmann constant, and T is the applied temperature. From the above equation, it is obvious that D s is related to T. In the process of surface diffusion, voids were eventually formed, and the growth rate of the surface diffusion-induced voids accelerated with temperature. 59 Thus, the surface diffusion coefficient increased with an increase in temperature, which indicated that the Ag adatoms had a high probability of coalescence with the adjacent atoms to form a 3D island-like nanoparticle. In addition, the Ag nanoparticle density decreased, and the voids grew larger. The 873 K annealed films exhibited homogeneously distributed Ag island-like nanoparticles. As the CuO− WO 3 -coated fibers were composed of large grains and a seemingly rough surface, the Ag nanoparticle growth was physically limited to each grain of the substrate (CuO−WO 3 surface), and they could not physically migrate (hopping) from one grain to another, as illustrated in the schematic in Figure 8 . As a result, the formation of large nanoparticles, unlike the Ag nanoparticles which formed over a flat and smooth surface (for instance, glass substrate), was restrained. Therefore, in the case of CuO−WO 3 -coated fibers, small island-like Ag nanoparticles were formed owing to the localized surface diffusion-driven Ag dewetting, as observed in the FE-SEM image (Figure 6c ). 2.5. Influence of Dewetted Ag NPs on Photocatalytic Activity. The photocatalytic activity and MB degradation kinetics of the Ag-loaded CuO−WO 3 fibers are shown in Figure 9a . It was observed that 58% of MB was decomposed within the first 30 min and 98% degradation was observed after 180 min, which was much higher than that of the Cu−WO 3 fibers. The photocatalytic activity rate constant of WO 3 , Cu− WO 3 , and Ag-dispersed CuO−WO 3 fibers is shown in Figure  9b , where the Ag-dispersed CuO−WO 3 fibers exhibited remarkable photocatalytic activity among all the fibers with the highest MB degradation rate constant (k = 0.0205 min −1 ), which is approximately 1.73 times higher than that of 2Cu− WO 3 fibers and 18.6 times higher than that of pure WO 3 . The photocatalytic MB degradation stability of the Ag-dispersed CuO−WO 3 fibers was determined under the identical conditions and shown in Figure S4 . The efficiency was slightly changed with the degradation efficiency of about 90% during the third cycling, while the first cycling experiment showed about 98% efficiency. The XRD pattern and the FE-SEM image of the Ag@CuO−WO 3 sample after the photocatalytic MB degradation stability test for the third cycling are shown in Figure S5 .

The photocatalytic efficiency relies critically on the generation and recombination rates of electron−hole pairs upon light excitation. Photoluminescence (PL) spectroscopy is an important characterization tool for the investigation of surface processes involving photogenerated electron−hole pair recombination in semiconductors. The lower PL intensity implied more effective separation of electron−hole pairs; thus, higher photocatalytic activity could be achieved. PL was Figure 8 . Cross-sectional and plane-view schematic representation for before and after RTA of a thin Ag layer-coated CuO−WO 3 /SGF, illustrating the localized surface diffusion-driven Ag dewetting (3D island-like Ag nanoparticle formation). measured for WO 3 , Cu−WO 3 , and Ag-dispersed CuO−WO 3 fibers, as shown in Figure 9c , and the fibers showed emission peaks around 530 nm. The pure WO 3 fibers exhibited the strongest PL intensity due to the severe electron−hole recombination, while the Cu-doped WO 3 fibers exhibited lower PL intensity than WO 3 . 2Cu−WO 3 exhibited the lowest PL intensity among all the Cu-doped WO 3 fibers, indicating the effective retardation of electron−hole recombination. The PL intensity drastically decreased in Ag-dispersed CuO−WO 3 fibers, which indicated that Ag loading greatly inhibited the recombination rate of the electron−hole pairs, resulting in effective charge separation at the respective interfaces of Ag/ CuO/WO 3 . The mechanism for the enhanced photocatalytic activity of the Ag-dispersed CuO−WO 3 fibers could be understood as follows: the schematic is shown in Figure 9d on the synergetic effect of the Z-scheme and LSPR. The fast electron−hole pair recombination rate was greatly inhibited by the Z-scheme system of CuO−WO 3 , and the strong LSPR effect of dewetted Ag nanoparticles functions as visible light harvesting and electron-generating centers. The LSPR-excited electrons would be enriched on the surface of the dewetted Ag nanoparticles. As a result, the electrons transferred from Ag to the CB of the adjacent semiconductor (WO 3 or CuO at the respective interface with Ag) and increased the steady-state concentration of "chemically useful" (i.e., charge carriers at the surface of the semiconductor that participated in photocatalytic reactions) energetic charge carriers. 60 In this study, the dispersed Ag nanoparticles were approximately 79 nm, implying that the quadrupolar LSPR could be the most probable mode in which photon absorption was predominant rather than hot carrier generation. As a result, the excess electrons could also contribute to the production of more O 2

•− (as shown in the schematic in Figure 9d ). Bastuś et al. and Seemala et al. reported that the Ag nanoparticle size and distribution greatly influenced the LSPR effect. 61, 62 According to their reports, hot carrier generation was predominantly observed with relatively higher hot carrier density in relatively small Ag nanoparticles (∼67 nm), which exhibited dipolar plasmon resonance. Quadrupole moments could be observed above this critical size (∼67 nm), as the size of the Ag nanoparticle increased until ∼189 nm. This facilitated O 2 dissociation via direct interactions with the strong electric fields at the Ag surface due to the predominant photon absorption that effectively promoted photocatalysis. 62 Thus, the strong visible light absorption and charge separation ability of this Ag-dispersed CuO−WO 3 structure greatly enhanced the photocatalytic activity. Consequently, the Ag dispersion onto the fibers could remarkably improve the absorption of visible light due to the LSPR effect and significantly increased the probability of separation of charge carriers. Thus, the photogenerated electron−hole pairs were accompanied by the reduction/oxidation process for the degradation of the organic dye.

In summary, a novel and effective hybrid composite photocatalyst comprising a plasmonic metal (Ag)-dispersed CuO− WO 3 Z-scheme heterojunction was successfully coated on reusable and recyclable 3D network-like SGFs by an industrially well-adopted sputtering process. The XRD results revealed that the high concentration of Cu in 3Cu−WO 3 led to CuWO 4 phase formation at 873 K. Homogeneously dispersed, localized surface diffusion-driven Ag-dewetted nanoparticles of approximately 79 nm were successfully obtained. The MB photocatalytic degradation efficiency of Ag-dispersed CuO− WO 3 /SGF was about 18.6 times higher than that of pure WO 3coated fibers. The construction of the Z-scheme heterojunction system decreased the number of photogenerated electron−hole pairs. The dewetted Ag nanoparticle dispersion facilitated effective electron−hole transfer across the respective interfaces of Ag/CuO and Ag/WO 3 , and it was certainly beneficial to retard the rapid recombination of electron holes from WO 3 and CuO. This work provides insights into the practical applications of highly efficient fiber-supported visiblelight-active photocatalysts for environmental remediation.

4.1. Fabrication of Cu−WO 3 and Ag-Dispersed CuO− WO 3 Photocatalysts on SGFs. A 3D network-like commercially available SGF sheet was used as a substrate for the deposition of WO 3 and Cu-doped WO 3 by radio frequency magnetron sputtering at room temperature. A 2 in. planar tungsten oxide target and a Cu chip (99.999% purity, chip size: 1 × 1 × 0.5 mm)-embedded mosaic-type tungsten oxide target were used for the deposition of WO 3 and Cu−WO 3 over the SGF, respectively, under the same conditions. An Ar gas flow of 230 sccm, a target power of 150 W, a substrate rotation of 23 rpm, a target−substrate distance of 90 mm, and a deposition time of 30 min were maintained as deposition parameters. Before sputtering, the chamber was evacuated to a base pressure below 2 × 10 −6 Torr and pre-sputtered for 10 min to remove any adsorbed contaminants on the surface of the sputter target. Finally, the WO 3 and Cu−WO 3 -coated fibers were annealed at 873 K in ambient air for 30 min using the RTA system to obtain crystalline CuO−WO 3 composite nanostructures. The Cu concentration in the Cu−WO 3 -coated fibers was controlled by the mosaic structure of the WO 3 target by varying the number of Cu chips (1, 2, and 3) on the WO 3 target, which were named as 1Cu−WO 3 , 2Cu−WO 3 , and 3Cu−WO 3 , respectively. The Cu doping concentration was approximately 3−10 at. % based on the area fraction of Cu over the WO 3 target, respectively. Finally, an ultrathin Ag layer (∼10 nm) was sputtered over CuO−WO 3 (which was obtained by annealing 2Cu−WO 3 fibers annealed at 873 K) by the DC magnetron sputtering system using a 2 in. diameter Ag target of 99.999% purity with a target power supply of 30 W, and these fibers were subsequently annealed under vacuum for 5 min at 873 K to obtain Ag nanoparticle dispersion over the CuO−WO 3 /SGF. 4.2. Material Characterization and Organic Dye Degradation. The crystal structure was examined by XRD with a Rigaku SmartLab X-ray diffractometer using Cu Kα radiation at a scanning speed of 4°min −1 over the 2θ range of 15−70°. The surface morphology was investigated using FE-SEM (TESCAN-MIRA3, Korea). XPS measurements were carried out with a Thermo Scientific K-Alpha spectrometer using an Al Kα X-ray source with a constant analyzer mode to study the chemical composition of the constituent elements. Raman measurements were carried out by HORIBA LabRAM HR-800 with 514 nm laser excitation. TEM observations were performed using an FEI TECNAI G2 F20 (operating voltage: 200 kV) to determine the structure and cross-sectional microstructure of the dewetted Ag nanoparticles dispersed on CuO−WO 3 -coated fibers. The TEM sample was prepared by the H-bar technique using a TESCAN LYRA3 high-resolution FIB-SEM. The optical absorbance spectra were recorded using a UV−vis−NIR spectrophotometer (UV-3600, Shimadzu). The PL properties were studied using a HORIBA LabRAM HR-800, HORIBA France SAS. The photocatalytic activity was investigated by measuring the degree of decomposition of MB solution in a double-walled glass beaker (40 mL MB solution, 10 ppm) under visible light illumination using a 500 W Xe lamp with a 400 nm filter.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c06239.

High-magnification FE-SEM images of as-coated WO 3 and Cu−WO 3 on SGFs, low-magnification FE-SEM image of Ag-dispersed CuO−WO 3 /SGFs, evolution of the surface microstructure of the 10 nm-thick Ag film deposited (by magnetron sputtering) on glass substrates with respect to RTA temperature, photocatalytic stability test for three cycles, and the structure and morphology images of Ag-dispersed CuO−WO 3 /SGFs after three cycles (PDF)

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