Excellent photocatalytic activity of ternary Ag@WO3@rGO nanocomposites under solar simulation irradiation

ry ir Le mda m ersit The Faculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang, 550000, Viet Nam a r t i c l e i n f o For instance, Cong et al. intercalated the cations of H , Li , N , K , Mgþ, and Al3þ into WO3 for surface-enhanced Raman scattering applications [4]. Khan et al. enhanced the visible-light-induced ce of WO3 nano- eets [7]. Moreover, antibacterial, and oasted rice beads hu et al. prepared hance the photo- ght irradiation [9]. characteristics by he WO3 film [10]. Rh2O3 clusters and tallites [11]. However, there is still plenty of room for studying the catalytic properties of the WO3-based nanocomposite materials. Among the various materials used for compositing with WO3, graphene and its derivatives are one of the most utilized materials due to their large surface area and high carrier mobility [12,13]. The large surface area and the highmobility are favorable for the charge transfer process between WO3 and graphene or its derivatives. Besides, the formation of composites with other materials could * Corresponding author. ** Corresponding author. E-mail addresses: lswha@gachon.ac.kr (S.-W. Lee), tu.nguyencong@hust.edu.vn (C.T. Nguyen). Contents lists availab Journal of Science: Advanc .e l Journal of Science: Advanced Materials and Devices 6 (2021) 108e117Peer review under responsibility of Vietnam National University, Hanoi.WO3 and the surrounding environment or other materials [4e6]. þ þ þ þ WO3 crystallites to control the sensing characteristic of WO3 crys-1. Introduction Tungsten oxide (WO3) is a widely studied metal oxide semi- conductor for many cutting-edge applications such as photo/elec- trocatalysis, energy storage, smart window, antibacterial, anticancer agents, and pathogens control [1e3]. Recently, a brand- new application of tungsten oxide as the non-noble metal surface- enhanced Raman scattering substrate makes the interest in WO3 one more time a blooming/rising issue [4,5]. Most of the afore- mentioned applications are based on the charge transfer between photocapacitive and photocatalytic performan rods by decorating them with graphene nanosh Selvamani et al. studied the photocatalytic, anticancer activity of the Ag@Ag8W4O16 nanor prepared by the facile microwave synthesis [8]. Z WO3 composited with carbon nanotubes to en degradation of sulfamethoxazole under visible li Su et al. enhanced the NO2 gas-sensing combining the reduced graphene oxide with t Staerz et al. created the heterojunction betweenArticle history: Received 25 July 2020 Received in revised form 30 November 2020 Accepted 7 December 2020 Available online 11 December 2020 Keywords: Ternary nanocomposite Hydrothermal Ag@WO3@rGO Adsorption Photocatalytic activity Solar simulation irradiationhttps://doi.org/10.1016/j.jsamd.2020.12.001 2468-2179/© 2020 The Authors. Publishing services b license ( b s t r a c t The ternary nanocomposite of Ag@WO3@rGO was synthesized via a two-step hydrothermal method in which the binary composite of the Ag-doped WO3 (Ag@WO3) prepared by the 1st hydrothermal step was combined with the reduced graphene oxide (rGO) via the ultrasonic-assisted 2nd hydrothermal step. The composition with Ag and rGO helped to reduce the optical bandgap energy of WO3, and thereby to enhance the photocatalytic activity of the WO3ebased nanocomposites. When compared with the pristine WO3 sample under the solar simulation irradiation, the binary (Ag@WO3) nanocomposite showed the increased photocatalytic activity towards the degradation of Rhodamine B (RhB) from 23% (pristine WO3) to 80% (Ag@WO3). Notably, the ternary nanocomposite exhibited an excellent photo- catalytic efficiency of RhB degradation (99.5%). The significantly enhanced photocatalytic activity was assigned to the combined synergy effects of the electron-reservoir of Ag and the highly absorptive r-GO with the visible-band gap WO3 semiconductors, which suggests a promising procedure to construct ternary nanocomposite-based high-performance photocatalysts under the solar light. © 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY-NC-ND license ( nc-nd/4.0/).e School of Engineering Physics, Hanoi University of Science and Technology, No 1, Dai Co Viet Street, Ha Noi, Viet NamOriginal Article Excellent photocatalytic activity of terna nanocomposites under solar simulation Vy Anh Tran a, b, Thang Phan Nguyen a, Van Thuan Cong Tu Nguyen e, * a Department of Chemical and Biological Engineering, Gachon University, 1342 Seongna b Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Na c Center for Advanced Chemistry, Institute of Research and Development, Duy Tan Univ d journal homepage: wwwy Elsevier B.V. on behalf of Vietnam d/4.0/).Ag@WO3@rGO radiation c, d, Il Tae Kim a, Sang-Wha Lee a, **, ero, Sujeong-gu, Seongnam-si, 13120, Republic of Korea y, Da Nang, 550000, Viet Nam le at ScienceDirect ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY-NC-ND bring new properties to WO3, such as surface plasmonic resonance (SPR) under visible light irradiation [14]. This SPR effect originates from the noble metals like Ag and Au, which could help to increase the antibacterial, anticancer activity of WO3 e the synergistic effect between WO3 and noble metals [14]. There are many reports of Ag-doped WO3 or WO3 composited with the reduced graphene oxide (rGO). However, as to our knowledge, the photodecomposition of the azo dye (Rhodamine B) over the Ag@WO3 nanostructure decorated with rGO has not been studied until now. In this contribution, we report a facile method for the synthesis of the ternary Ag@WO3 nanostructure decorated with rGO as a catalyst for highly efficient photode- composition of Rhodamine B. Firstly, the binary Ag@WO3 nano- structures were directly synthesized by the one-step hydrothermal synthetic method. Then the obtained binary com- posite Ag@WO3 was wrapped with the rGO using the ultrasonic- assisted hydrothermal method to form the ternary nano- composite Ag@WO3@rGO. The chemical composition, the atomic binding energy, and the interaction characteristics of the samples Ltd. (China). Silver nitrate (AgNO3, 99.8%) was a product of 2.2. Material preparation 2.2.1. Synthesis of Ag@WO3 nanocomposite Ag@WO3 nanocomposites were directly synthesized using the hydrothermal method via the following steps: (i) preparing 25 mL Na2WO4 solution (1M) by dissolving 0.025 mol Na2WO4.2H2O in 25 mL double-distilled (DD) water at room temperature (rt) under vigorous stirring for 20 min; (ii) pouring gradually 5 mL nitric acid (HNO3) into the Na2WO4 solution under continuous stirring to form a yellow homogenous suspension; (iii) putting 50 mL solution of AgNO3 with different concentrations into the yellow suspension to get the precursor suspension; (iv) stirring the precursor suspension for 4 h, then putting it into a 100 mL Teflon-lined stainless steel autoclave. The hydrothermal process was carried out at 180 C for 24 h. After cooling naturally to rt, the obtained precipitates were thoroughly rinsed and filtered four times by DDwater using the 15- mm pore filter paper. The cleaned slurry was dried at 80 C for 24 h, then ground to obtain the desired powder. In this study, the amount of AgNO3 was varied from 0.09 to 0.48 g that corresponds to the V.A. Tran, T.P. Nguyen, V.T. Le et al. Journal of Science: Advanced Materials and Devices 6 (2021) 108e117Changzhou Guoyu Environmental S&T Co., Ltd (China). Graphene oxide (GO, 4 mg/mL) was purchased from Bioneer (South Korea). Ethanol and destilled water (HPLC grade) were used as received without further purification. All other chemicals were of the highest commercially available quality and were used as received. Glasswares were cleaned using an acidic solution of HNO3: HCl (1:3 volume ratio) and then rinsed several times with deionized (DI) water.were examined by the energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectros- copy, and diffuse reflectance spectroscopy (DRS). The photo- catalytic activities of WO3, Ag@WO3, and Ag@WO3@rGO were evaluated towards the degradation of Rhodamine B (RhB) under simulated solar-irradiation. Based on the characterization and catalytic performance, the enhanced photocatalytic mechanism of the WO3-based composites was discussed. 2. Experimental 2.1. Chemical materials Sodium tungstate dihydrate (Na2WO4.2H2O, 99.5%) and nitric acid (HNO3, 37% wt.) were purchased from Xilong Scientific Co.,Scheme 1. Synthetic process of Ag@WO3@rGO sample using t 109mass ratio of Ag/WO3 in the range from 1.0 to 5.0 wt.%. The samples were named as WO3, Ag-1@WO3, Ag-3@WO3, and Ag-5@WO3 corresponding to 0, 1, 3, and 5 wt.% of Ag in Ag/WO3, respectively. The scheme for fabricating the binary Ag@WO3 nanocomposite is illustrated in Fig. S1. 2.2.2. Preparation of Ag@WO3@rGO nanocomposite The hydrothermal method was used to synthesize the ternary nanocomposites of Ag-3@WO3@rGO. At first, 8 mL of graphene oxide suspension (4 mg/mL) was dissolved in 100 mL of DD water and ultrasonicated for 10 min. Then, 200 mg of Ag-3@WO3 were added to the GO solution and sonicated for 5 min to make the homogeneous suspension. This mixture was transferred into the Teflon sealed autoclave, and then the hydrothermal process at 180 C for 24 h was performed. The obtained product was washed several times with distilled water and ethanol, and finally dried in the oven at 60 C for 12 h. The synthetic process of the Ag- 3@WO3@rGO ternary nanocomposite is presented in Scheme 1. 2.3. Adsorption and photocatalytic activity testing The photocatalytic activity of the sample was evaluated by measuring the photo-degradation efficiency of RhB in an aqueous medium under solar simulation irradiation. The solar simulatorhe combined ultrasonication and hydrothermal methods. (Atlas Material Testing Technology LLC) was equipped with an air- cooled 1700 W xenon lamp. A daylight filter with a cut-off wave- length <290 nm was used to simulate the sunlight, and the tem- perature of the reactor glass was maintained at 25 C using a water reflux system. The irradiation intensity was set to 700 ± 5 Wm2 [15]. The degradation solution was prepared by diluting the stock solution of RhB to its desired concentration. Each photocatalyst (50 mg) was added to a photoreactor containing 100 mL of 5 ppm RhB. The temperature of the photoreactor was maintained at 25 C by circulating the cooling water around the reactor. Before the light irradiation, the dye solution was stirred for 30 min in the darkness to reach an adsorptionedesorption equilibrium. The mixture was then exposed to a solar simulation system. After each interval of 10 min, 3-mL aliquots were collected from the photo-reactor and centrifuged to remove the photocatalysts. The supernatant was used to monitor the photo-degradation efficiency (PDE) of the RhB dye. The degradation efficiency of RhB was determined by the equation (1) below. The PDE was determined from the concentra- tion of RhB in the sampled solution which is in proportion with the intensity of the absorption peak at 551 nm in the UV-Vis spectrum of RhB. So, the PDE was estimated by the following formula: PDEð%Þ ¼ Co  Ct Co  100 ¼ Io  It Io  100; (1) where Co is the initial concentration of RhB (mg/L) and Ct is the concentration of RhB (mg/L) in an aliquot at a definite interval of time at the end of each experiment; Io is the intensity of the 551 nm peak in UV-Vis spectrum of initial RhB solution; It is the intensity of the 551 nm peak in UV-Vis spectrum of aliquots at a definite in- terval of time at the end of each experiment. 2.4. Instrumental analytical methods The low-magnification SEM images and mapping energy- dispersive X-ray spectroscopy (EDS) images of the samples ob- tained using Tabletop Microscope HITACHI TM4000Plus were used to analyze the distribution ofW, O, and Ag elements in the Ag@WO3 nanocomposite. The field emission scanning electron microscope (HITACHI S4800) images were used for the morphological analysis of Ag@WO3 nanocomposite. Transmission electron microscopy (TEM) specimens were pre- pared by placing one drop (6 mL) of the aqueous dispersion of the sample onto a carbon-coated copper grid and drying at rt. As with other techniques, all sample solutions were treated in an ultrasonic bath (30 s) and vortex mixer (5 s) before the deposition of samples to reduce aggregation. TEM and selected area diffraction pattern (SADP) analysis were performed on a HITACHI H-7600 (Japan) microscope operated at 80 kV, and a Tecnai G2 F30 (Germany) microscope operated at 300 kV. X-ray diffraction (XRD) patterns of the samples were obtained by an automatic X-ray diffractometer (Rigaku Rint 2200 Series, Rigaku, Tokyo, Japan) using monochromatized CuK-a1 radiation of wavelength l ¼ 1.5406 Å, at a 40-kV voltage and 30-mA current with the continuous-scanning 2q mode in the range of 10e70. Raman spectra were obtained with a Renishaw Invia Raman Microscope (Renishaw PLC, Great Britain) using 633 nm laser and Leica N PLAN L50x/0.50 BD Microscopy objective. Absorption spectra of the samples were measured by UV-Vis spectrophotometer JASCO V- 750 (Japan) with 60 mm integrating sphere ISV-922. XPS measurements were operated by the K-alpha XPS system (Thermo Fisher Scientific, Korea Basic Science Institute) with monochromated Al Ka (hw¼ 1486:6 eVÞ to measure the chemical V.A. Tran, T.P. Nguyen, V.T. Le et al.composition and atomic binding energy. 110The isotherms were measured at 196 C on a volumetric in- strument. Samples were outgassed in a vacuum at room tempera- ture for at least 24 h before the sorptionmeasurements. The surface areas were estimated by applying the BrunauereEmmetteTeller (BET) equation. The BarretteJoynereHalenda (BJH) method was used to determine the mesopore size distribution at the Smart Materials Research Center for IoT at Gachon University. 3. Results and discussion 3.1. SEM and TEM analysis To confirm the existence of the Ag element in the binary nanocomposite, EDSmeasurement was performed over a large area (26 20 mm2). Fig.1a-d show the lowmagnification SEM image and the distribution ofW, O, and Ag elements. The EDSmapping images indicate that Ag, O, and W elements are uniformly distributed. Fig. 2a presents the TEM image of the Ag-3@WO3 sample. The morphology of the sample exhibits a mixture of nanoflake and nanopolygon shapes. Both types of nanoflakes and nanopolygons have the dimension of less than 50 nm. The distribution of the nanoflakes and nanopolygons are the favorable morphologies for photocatalyst materials because of the high surface area. The field emission scanning electron microscopy (FE-SEM) analyses show that the WO3, Ag-1@WO3, and Ag-5@WO3 samples have similar morphologies with the Ag-3@WO3 sample (Fig. S2), indicating that the Ag doping (1e5 wt. %) did not affect the morphology of the binary Ag@WO3 samples. Fig. 2b shows the TEM image of the Ag- 3@WO3 sample after the incorporation with the GO nanosheets which is referred to as Ag-3@WO3@rGO. The nanoflake and nano- polygon like granular structures of WO3 are also observed in the TEM image of the ternary Ag-3@WO3@rGO, which is similar to that of the binary Ag-3@WO3 (Fig. 2a). This result indicates that the hybridization process with GO does not affect the morphology of the binary nanocomposites. The Ag and GO components were confirmed in the TEM image as seen from Fig. 2b. Fig. 2c shows the selected area diffraction pattern (SAED) of the Ag-3@WO3@rGO sample that shows the presence of theWO3 hexagonal and Ag face- centered cubic structures in Ag-3@WO3@rGO - indicated with the white and red colors, respectively. Moreover, the diffraction planes of WO3 are well matched with the XRD pattern as will be discussed later. Thus, the ternary Ag@WO3@rGO was successfully prepared. 3.2. XRD and Raman scattering analysis Fig. 3a presents the XRD patterns of theWO3, Ag-3@WO3, and Ag- 3@WO3@rGO samples. In the XRD spectra of the ternary nano- composite, no diffraction peak is observed in the range of 10e15, indicating that GO is completely reduced to rGO [6]. The XRD pat- terns of all the samples are similar to each other, confirming that the direct composition of Ag and rGO nanosheets does not affect the crystal structure ofWO3. The XRD patterns of the Ag-1@WO3 and Ag- 5@WO3 samples are similar to the those of the Ag-3@WO3 sample, irrespective of the Ag amounts (Fig. S3). In particular, the XRD pat- terns of the samples are similar to that of a new hexagonal WeO- based structure, which has been reported by Besnardiere et al. in 2019 [16]. This hexagonal structure is assigned to the space group P6/ mmm with the corresponding structural parameters of a ¼ b ¼ 9.997 Å, c ¼ 3.9199 Å, a ¼ b ¼ 90
, and g ¼ 120
[16]. The Raman analysis also confirms the hexagonal structure of the WO3 and Ag-3@WO3 composite. Fig. 3b represents the Raman spectra of the WO3, Ag-3@WO3, and the Ag-3@WO3@rGO samples. The inset of Fig. 3b shows the Raman spectrum of rGO in the range from 1000 to 1800 cm1, which clearly shows the presence of the D Journal of Science: Advanced Materials and Devices 6 (2021) 108e117and G bands. All the samples show the same pattern of the Fig. 1. (a) Low magnification SEM image e x5000 and EDS mapping images of Ag-3@WO3 sample, (b) the distribution of W, (c) the distribution of O element, and (d) the dis- tribution of Ag elements. Fig. 2. TEM images of (a) Ag-3@WO3 and (b) Ag-3@WO3@rGO; (c) the selected area diffraction pattern (SADP) of Ag-3@WO3@rGO. V.A. Tran, T.P. Nguyen, V.T. Le et al. Journal of Science: Advanced Materials and Devices 6 (2021) 108e117 111 3, A V.A. Tran, T.P. Nguyen, V.T. Le et al.hexagonal WO3 [17]. In the Raman spectrum of the ternary nano- composite, the characteristic peaks of both hexagonalWO3 and rGO are observed. In the Raman spectrum of the pristine WO3, the distinct peaks at 233, 319, and 431 cm1 originate from the vibra- tion of WeW,W5þeO, andW5þ]O bonds, respectively [18,19]. The vibrations at 638, 816, and 950 cm1 are attributed to the WeO, OeWeO, and W6þ]O bonds, respectively [19]. The bands at 1318, 1559, 2602, and 2877 cm1 are the characteristic D, G, 2D, and DþG bands of graphene [20]. The Raman spectrum of Ag-3@WO3@rGO in the range of 2300e3100 cm1 is presented in Fig. S4. The position of the characteristic Raman peaks of WO3 and rGO in pristine and nanocomposite samples are listed in Table 1. Ac- cording to Table 1, some characteristic peaks of WO3 are shifted to a lower wavenumber when theWO3 is composited with Ag. In detail, the characteristic peaks of the OeWeO and W6þ]O bonds are shifted from 817 and 950 cm1 to 814 and 940 cm1, respectively. The blue shift is attributed to the shortening of the WeO bonds caused by the appearance of Ag, in which Ag takes oxygen from WO3, and causing the lack of oxygen on the surface ofWO3. The lack of oxygen results in a stronger and shorter bond between W and O [21,22]. When rGO is introduced to the Ag-3@WO3 nanocomposite, the pattern of Raman spectra of WO3 is changed dramatically, as illustrated by the disappearance of theW5þ]O andW6þ]O peaks, the strong shift of W5þeO and the appearance of a peak at 881 cm1. These changes are assigned to the interaction between Ag-3@WO3 and rGO, i.e., rGO neutralizes the dangling bonds of W5þ]O and W6þ]O. This interaction also causes the redshift of Fig. 3. (a) XRD patterns of WO3, Ag@WO3, and Ag@WO3@rGO; (b) Raman spectra of WO the range of 1000e1800 cm1. The dotted line is d