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WO3 photocatalyst and its photocatalytic activity under simulated solar light
Journal of Solid State Chemistry 197 (2013) 560–565
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Microwave assisted hydrothermal synthesis of Ag/AgCl/WO3 photocatalyst and its photocatalytic activity under simulated solar light Rajesh Adhikari a, Gobinda Gyawali a, Tohru Sekino b, Soo Wohn Lee c,n a
Research Center for Eco-Multifunctional Nano Materials, Sun Moon University, Korea Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan c Department of Environmental Engineering, Sun Moon University, Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 April 2012 Received in revised form 22 July 2012 Accepted 5 August 2012 Available online 15 August 2012
Simulated solar light responsive Ag/AgCl/WO3 composite photocatalyst was synthesized by microwave assisted hydrothermal process. The synthesized powders were characterized by X-Ray Diffraction (XRD) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Diffuse Reflectance Spectroscopy (UV–Vis DRS), and BET surface area analyzer to investigate the crystal structure, morphology, chemical composition, optical properties and surface area of the composite photocatalyst. This photocatalyst exhibited higher photocatalytic activity for the degradation of rhodamine B under simulated solar light irradiation. Dye degradation efficiency of composite photocatalyst was found to be increased significantly as compared to that of the commercial WO3 nanopowder. Increase in photocatalytic activity of the photocatalyst was explained on the basis of surface plasmon resonance (SPR) effect caused by the silver nanoparticles present in the composite photocatalyst. & 2012 Elsevier Inc. All rights reserved.
Keywords: Solar simulator Microwave Hydrothermal Nanocomposite Surface plasmon resonance
1. Introduction Photocatalytic degradation of organic compounds on semiconductor surface under UV light irradiation offers a viable approach to the solution of a variety of environmental problems. TiO2 photocatalyst has been the most widely used material in photocatalysis because of its higher catalytic activity, stability, and low cost [1,2]. However, its limited UV driven activity largely inhibits its overall efficiency under natural sunlight, which consists of 5% UV, 43% visible and 52% infrared [3]. One of the potential solutions for effective utilization is to shift the absorption of the semiconductor from the UV region into the visible region by allowing for more photons to be absorbed and utilized in decomposing the pollutants. Many researches have been carried out in the visible light driven photocatalysts by using several approaches such as doping of metal and non-metal elements, dye sensitization, deposition of noble metals, and making composite photocatalyst by forming heterojunctions [4–8]. Among the various techniques for the enhancement of visible light effective photocatalysis, composite photocatalysts have drawn more attention owing to their significant increase in the photocatalytic activity. Formation of heterojunction between two semiconductors allows the interaction of the band structure
n
Corresponding author. E-mail address: [email protected] (S. Wohn Lee).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.012
which effectively prevents the electron–hole recombination thereby enhance the photocatalytic activity [9–11]. Among many visible light active photocatalysts, tungsten oxide (WO3) is an ideal candidate due to its smaller band gap (ca. 2.4–2.8 eV), high oxidation power of valance band (VB) holes (þ 3.1 3.2 VNHE), nontoxicity, and stability. However, pure WO3 is not an efficient photocatalyst because of the lower conduction band (CB) edge (þ 0.3 0.5 VNHE) that does not provide a sufficient potential to reduce O2 E0(O2/O2 0)¼ 0.33 VNHE and E0(O2/HO02)¼ 0.05 VNHE and the inability of O2 to scavenge CB electron in WO3 results in the fast recombination and the lower photocatalytic activity [12]. Noble metal nanoparticles (NPs) show strong visible light absorption because of size and shape dependent plasmon resonance which has wide variety of applications such as colorimetric centers, photovoltaic devices, photochromic devices and photocatalysts. In particular, silver NPs show efficient plasmon resonance in the visible light region which has been utilized to develop a plasmonic photocatalyst. Recently, Hayato et al. [13] fabricated Au nanoparticles on TiO2 for the photocatalytic hydrogen evolution under visible light. Wang et al. [14] synthesized Ag@AgCl plasmonic photocatalysts through ion exchange and the photoreduction method, which showed excellent photocatalytic activities under visible light irradiation. It is reported that silver halides are generally considered as photosensitive materials, can coexist stably with Ag NPs during the whole photocatalytic process. Kakuta et al. [15] also observed that Ag NPs could contribute to the smooth separation of electron–hole pairs on
R. Adhikari et al. / Journal of Solid State Chemistry 197 (2013) 560–565
the AgBr/SiO2 NPs and enable it to catalyze H2 production from alcohol radicals. Youngping Liu et al. [16] did one pot pyridineassisted synthesis of Ag/Ag3PO4 as a visible light driven photocatalyst with enhanced photocatalytic activity. Yuanguo Xu and coworkers [17] reported the enhanced photocatalytic activity of Ag/AgCl/ZnO photocatalyst. Peng Wang et al. synthesized composite semiconductor H2WO4 H2O/AgCl as an efficient and stable photocatalyst under visible light and they focused on charge transfer process between small band gap semiconductors (SBG) coupled with AgCl [18,19]. Jiaguo Yu et al. [20] reported the fabrication and characterization of Ag/AgCl/TiO2 nanotube arrays. Guo et al. [21] synthesized Ag/AgCl@TiO2 for the treatment of aqueous hazardous pollutants. All these literatures mentioned above have proved that the Ag/AgCl supported with metal oxide semiconductor have enhanced photocatalytic activity under visible or simulated solar light. However, very little work has been done on the fabrication of WO3 and Ag/AgCl nanoparticles based nanocomposite and their photocatalytic activities. In this work, a new simulated solar light driven photocatalyst Ag/AgCl/WO3 has been synthesized by the microwave assisted hydrothermal method. The synthesized Ag/AgCl/WO3 samples showed higher photocatalytic activity for the degradation of aqueous rhodamine B solution and the photocatalyst exhibited higher stability.
2. Materials and methods 2.1. Materials Sodium Tungstate Dihydrate (Na2WO4 2H2O), Silver Nitrate (AgNO3) and Hydrochloric Acid (HCl) were received from Sigma Aldrich. All the chemicals were of analytical grade and were used for the synthesis of Ag/AgCl/WO3 without any further purification. Double distilled water was used for all synthesis and treatment processes. 2.2. Preparation of photocatalyst 5 g of Na2WO4 2H2O was dissolved in 30 ml of 1 M HCl solution and was stirred continuously for 1 h. Similarly, 5 ml of the aqueous solution containing 0.1 mole percent of AgNO3 was prepared in distilled water and added to the above solution. The pH of each solution was maintained to be 2. After the pH adjustment, the mixture was treated with UV light (300–420 nm, homemade design) for 1 h to reduce silver ions to silver nanoparticles. UV treated solutions were finally transferred to Teflon flask for microwave hydrothermal treatment. The mixture was heated at 2001 C for 2 h. The resulting products were washed with distilled water several times, dried at 70 1C, and calcined in air at 600 1C for 5 h. In a similar manner, two more samples of Ag/AgCl/WO3 photocatalysts were synthesized with different Ag/AgCl contents (0.3 mole percent and 0.5 mole percent of AgNO3) and have been assigned as S-0.1, S-0.3, and S-0.5, respectively for convenience.
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by PHI 5000C ESCA system with MgKa source operating at 14.0 kV and 25 mA. All the binding energies were referenced to the C 1s peak at 284.6 eV. Photocatalytic experiments were performed by UV–Vis spectrophotometer (optizen view, Mecasys) and portable solar simulator. 2.4. Photocatalytic experiment Rhodamine B was selected as a model dye in order to check the photocatalytic activity under simulated solar light conditions. It is due to the fact that the most of the dyes are sensitized in presence of light even in absence of photocatalyst. Xiaoli et al. [22] tested the decomposition of Methylene Blue under visible light and studied the suitability of probe molecule for photocatalytic activity test. In comparison to Methylene Blue, rhodamine B is more stable under visible light. Therefore, rhodamine B was selected as a model dye in this work. In the photocatalytic experiment, 0.1 g of the catalyst powder was placed in a Pyrex glass cell (50 mm 70 mm) and then dispersed in absolute ethanol by using ultra sound for 10 minutes. The solution with dispersed catalyst particles was then dried for 24 h in an oven at 70 1C to make a thin layer of catalyst coating. 50 ml of 10 ppm rhodamine B solution was slowly poured in the Pyrex glass and kept in the dark for half an hour to achieve adsorption–desorption equilibrium. After the equilibrium was reached, photocatalytic experiments were conducted by using solar simulator under simulated solar light and the absorbance of the solution was measured by UV–Vis spectrophotometer. The photodegradation of rhodamine B by the different samples was observed until 240 minutes at every 30 minutes time interval.
3. Results and discussion 3.1. Structural and morphological analysis XRD, TEM and XPS analysis were performed to study the crystal structure and morphology of the as synthesized samples. Fig. 1 shows the XRD patterns of as prepared samples. XRD patterns of composite photocatalyst are compared with that of commercial WO3 powder. From Fig. 1, it is evident that all the diffraction peaks are well indexed to monoclinic WO3 (JCPDS card
2.3. Characterization The crystalline structure of the catalysts was characterized by powder X-Ray diffraction (XRD) employing a scanning rate of 41 per minute in a range from 51 to 701 by Rigaku X-Ray Diffractometer using monochromatized CuKa (l ¼1.54 A1 ) radiation. The morphologies and sizes of the samples were observed by TEM. UV– Vis Diffuse Reflectance Spectra (DRS) were recorded by UV–Vis spectrophotometer (NIRJASCO 570, Micrometrics instrument Corp. Edinburgh Instrument—F900). XPS measurement was performed
Fig. 1. XRD patterns of as synthesized samples and commercial WO3 nanopowder.
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number 72-0677), cubic Ag (JCPDS cards no. 65-2871) and cubic phase AgCl (31-1238). The peaks appeared at 27.821 and 32.241 are assigned to the (111) and (200) plane of cubic phase of AgCl and a peak appeared at 44.31 is assigned to the (200) plane of metallic silver. Hence from the XRD patterns, it is clear that as synthesized samples are composed of Ag, AgCl and WO3 particles and no additional peaks were appeared for any other impurities. Fig. 2 Show the typical TEM images of Ag/AgCl/WO3 (S-0.3) sample, insets show the HRTEM images of AgCl and Ag nanoparticles and the corresponding EDX spectrum. Fig. 2a reveals that the composite photocatalyst has mixed morphology composed of rectangular plate like and round shaped WO3 particles. AgCl nanoparticles have average diameter of 30 nm and possess two different morphologies in which some particles are spherical and some are of oval shaped. Moreover, AgCl nanoparticles are attached to the WO3 nanoparticles and heterojunction has been formed in between them. Similarly, Fig. 2b shows the TEM image of same sample in which Ag nanoparticles having diameter ranging from 10 to 20 nm are present in the composite. It can also be revealed from the figure that the distribution of Ag and AgCl nanoparticles in the composite photocatalyst is found to be heterogeneous which might be due to the hydrothermal treatment which was carried out after the photoreduction of Ag nanoparticles. Furthermore, EDX spectrum also confirms the presence of Ag and AgCl nanoparticles present in the composite photocatalyst. Compositions and the chemical states of the components present in the composite photocatalyst were investigated by XPS analysis. Fig. 3a shows the general XPS survey of the composite photocatalyst (S-0.3). It is clear from the spectrum that the surface morphology of the Ag/AgCl/WO3 composite photocatalyst consists of W, O, Ag, and Cl element and no any
other impurity peaks were detected. Peak at 34.1 eV is ascribed to W which is originated from W6 þ ion of the WO3. Two prominent peaks (Fig. 3b) corresponding to ca. 368.9 eV(Ag3d5/2) and 374.5 eV(Ag3d3/2) confirmed the presence of metallic Ag nanoparticles [23]. Fig. 3c and 3d present the high resolution XPS spectra for O and Cl present in the sample. The peak at 199 eV can be assigned to be Cl (2p) peak which is originated from the AgCl nanoparticles whereas peak at 534 eV corresponds to the Oxygen. The peak for C 1s at 284.8 eV is due to the hydrocarbon from the XPS instrument itself. Hence, from the TEM and XPS analysis, it can be proved that the composite photocatalyst consists of Ag/ AgCl and WO3. 3.2. BET surface area analysis BET surface area of as synthesized samples compared with that of commercial WO3 nanopowder is shown in Table 1. It is revealed from the table that the composite photocatalysts have smaller BET surface area as compared to commercial WO3 nanopowder. Moreover, BET surface area decreased with increase in dopant concentrations. Decrease in surface area might be due to the occupation of pores and interstitial lattice positions of WO3 by silver nanoparticles and it is the common characteristic of heterogeneous phase of the composite materials. Similar results have been reported for composite photocatalysts [24]. 3.3. UV–Vis DRS analysis Fig. 4 displays the UV–Vis diffuse reflectance spectra of the as prepared catalyst samples and commercial WO3 powder as a reference. All the samples show a strong absorption in the visible region which confirms that the composite catalyst is active under
Fig. 2. TEM images of sample, S-0.3. a. showing AgCl particles, b. showing Ag nanoparticles, c. EDX spectrum showing the components of composite photocatalyst.
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Fig. 3. XPS Spectra of a. XPS survey spectrum of sample S-0.3, b. XPS spectrum for Ag 3d band, c. for O 1s band, d. for Cl 2p band.
Table 1 BET surface area of different samples. Sample
BET surface area (m2/g)
Com. WO3 S-0.1 S-0.3 S-0.5
3.642 2.504 3.455 0.211
visible light. From DRS analysis, Ag/AgCl/WO3 photocatalyst exhibited the absorption in the full range of sunlight including visible range as well as UV region. The enhanced light-absorption for visible light was ascribed to silver nanoparticles with localized surface plasmonic resonance, indicating that silver nanoparticles were successfully introduced into the composite photocatalyst [25]. These results verify that silver nanoparticles were reduced from silver ions
on the outer surface of WO3 and Ag/AgCl/WO3 photocatalyst exhibited the enhanced absorption for the visible light. 3.4. Photocatalytic activity The kinetic plot of the photocatalytic degradation of aqueous rhodamine B solution under simulated solar light is shown in Fig. 5. Photocatalytic activities of composite samples were compared with commercial WO3 powder under identical conditions. Photodegradation of rhodamine B under simulated solar light without using catalyst was also carried out. Among the three types of Ag/AgCl/WO3 samples, S-0.3 sample exhibited the best photocatalytic performance. The photodegradation of rhodamine B by this composite photocatalyst was almost 10 times higher than that of commercial WO3 powder. The rate of photocatalysis was found to be in the order: S-0.34S-0.54S-0.1 4commercial WO3 powder 4without photocatalyst. Kinetic plot reveals that the photodegradation of rhodamine B follows a pseudo-first order
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Fig. 6. Schematic diagram of mechanism of photocatalysis.
Fig. 4. UV–Vis diffuse reflectance spectra of different samples.
Fig. 7. Recycle tests of the sample S-0.3.
Fig. 5. Kinetic plot for the degradation of rhodamine B by various photocatalysts under simulated solar light.
kinetics which is almost similar for all dyes degradation experiments at lower concentrations [20]. 3.5. Proposed mechanism of the photocatalysis Although WO3 photocatalyst has appropriate band gap energy, it cannot show the better photocatalytic activity for practical purpose due to the lower conduction band as compared to Normal Hydrogen Electrode [12]. When we compare the UV–Vis DRS absorbance of composite photocatalyst with the WO3 powder, it was observed that the composite photocatalyst is more efficient than the WO3 photocatalyst. This result can be explained on the basis of surface plasmon resonance effect. The main characteristic of plasmon exciton coupling interaction is to induce electromagnetic field in the vicinity of the semiconductor and the exciton energy is transferred from the semiconductor to the metal nanoparticles. Conjugation of Ag/AgCl with WO3 induces local surface plasmon resonance (LSPR) which is produced by the collective oscillation of surface electrons on the Ag nanoparticles
that enhances the local inner electromagnetic field [14,20,26]. Therefore, the photogenerated electrons on the WO3 photocatalyst are transferred to the Ag nanoparticles which prevents the recombination of electron–hole pairs and then reduces the adsorbed oxygen to produce super oxygen anionic free radicals (O2 d) [27]. Similarly, holes formed on the valence band of WO3 are responsible for the oxidation of dyes molecules leading to the formation of various degraded products. Hence, we believe that the increase in photocatalytic activity of the composite photocatalyst is mainly due to the plasmon resonance caused by Ag nanoparticles. The photocatalytic mechanism of the Ag/AgCl/WO3 is schematically illustrated in Fig. 6. According to the literatures [22–25], the appropriate ratio of Ag0/Ag þ is essential for effective charge separation, that plays a key role for the effective photocatalysis. If the ratio is high, there is the formation of Ag cluster, which then turns out to be the combination center for photoinduced carriers and ultimately counteracts the activity improvement [28–31]. Hence, we believe that the optimum ratio is achieved in S-0.3 and exhibited the best photocatalytic activity as compared to other samples. The schematic of the photocatalytic mechanism of the Ag/AgCl/WO3 photocatalyst is illustrated in Fig. 6. 3.6. Stability of the photocatalyst Stability of the photocatalyst is a key factor that determines the long term efficiency. Therefore, the stability of the as synthesized photocatalyst was studied by several recycle test experiment. The recycle test was performed four times on S-0.3 sample and the
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results were compared in Fig. 7. It is revealed from the figure that the activity of the photocatalyst remained almost constant which shows the Ag/AgCl/WO3 composite is stable photocatalyst; however, small decrease in its stability might be due to the sparingly soluble nature of Ag þ ion in aqueous medium.
4. Conclusions Simulated solar light responsive Ag/AgCl/WO3 photocatalyst was successfully synthesized by the microwave assisted hydrothermal process. The composite photocatalyst exhibited much higher photocatalytic activity as compared to commercial WO3 powder that can be ascribed to the surface plasmon resonance effect induced by Ag nanoparticles. Moreover, this work provides an insight for the further fabrication of Ag/AgCl/WO3 composite system in order to obtain highly active visible light responsive photocatalyst.
Acknowledgment This research work has been supported by Global Research program of the National Research Foundation of Korea (NRF) funded by Ministry of Education, Science and Technology (MEST), Korea grant number (2010-00339). References [1] A. Fujishima, T.N. Rao, D.N. Tryk, J. Photochem. Photobiol. C 1 (2000) 1–21. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [3] J. Zhoua, Y. Cheng, J. Yu, J. Photochem. Photobiol. A 223 (2011) 82–87. [4] D. Dana, B. Vlasta, M. Mazur, M.A. Malati, Appl. Catal. B 37 (2002) 91–105.
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Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Microwave assisted hydrothermal synthesis of Ag/AgCl/WO3 photocatalyst and its photocatalytic activity under simulated solar light Rajesh Adhikari a, Gobinda Gyawali a, Tohru Sekino b, Soo Wohn Lee c,n a
Research Center for Eco-Multifunctional Nano Materials, Sun Moon University, Korea Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan c Department of Environmental Engineering, Sun Moon University, Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 April 2012 Received in revised form 22 July 2012 Accepted 5 August 2012 Available online 15 August 2012
Simulated solar light responsive Ag/AgCl/WO3 composite photocatalyst was synthesized by microwave assisted hydrothermal process. The synthesized powders were characterized by X-Ray Diffraction (XRD) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Diffuse Reflectance Spectroscopy (UV–Vis DRS), and BET surface area analyzer to investigate the crystal structure, morphology, chemical composition, optical properties and surface area of the composite photocatalyst. This photocatalyst exhibited higher photocatalytic activity for the degradation of rhodamine B under simulated solar light irradiation. Dye degradation efficiency of composite photocatalyst was found to be increased significantly as compared to that of the commercial WO3 nanopowder. Increase in photocatalytic activity of the photocatalyst was explained on the basis of surface plasmon resonance (SPR) effect caused by the silver nanoparticles present in the composite photocatalyst. & 2012 Elsevier Inc. All rights reserved.
Keywords: Solar simulator Microwave Hydrothermal Nanocomposite Surface plasmon resonance
1. Introduction Photocatalytic degradation of organic compounds on semiconductor surface under UV light irradiation offers a viable approach to the solution of a variety of environmental problems. TiO2 photocatalyst has been the most widely used material in photocatalysis because of its higher catalytic activity, stability, and low cost [1,2]. However, its limited UV driven activity largely inhibits its overall efficiency under natural sunlight, which consists of 5% UV, 43% visible and 52% infrared [3]. One of the potential solutions for effective utilization is to shift the absorption of the semiconductor from the UV region into the visible region by allowing for more photons to be absorbed and utilized in decomposing the pollutants. Many researches have been carried out in the visible light driven photocatalysts by using several approaches such as doping of metal and non-metal elements, dye sensitization, deposition of noble metals, and making composite photocatalyst by forming heterojunctions [4–8]. Among the various techniques for the enhancement of visible light effective photocatalysis, composite photocatalysts have drawn more attention owing to their significant increase in the photocatalytic activity. Formation of heterojunction between two semiconductors allows the interaction of the band structure
n
Corresponding author. E-mail address: [email protected] (S. Wohn Lee).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.012
which effectively prevents the electron–hole recombination thereby enhance the photocatalytic activity [9–11]. Among many visible light active photocatalysts, tungsten oxide (WO3) is an ideal candidate due to its smaller band gap (ca. 2.4–2.8 eV), high oxidation power of valance band (VB) holes (þ 3.1 3.2 VNHE), nontoxicity, and stability. However, pure WO3 is not an efficient photocatalyst because of the lower conduction band (CB) edge (þ 0.3 0.5 VNHE) that does not provide a sufficient potential to reduce O2 E0(O2/O2 0)¼ 0.33 VNHE and E0(O2/HO02)¼ 0.05 VNHE and the inability of O2 to scavenge CB electron in WO3 results in the fast recombination and the lower photocatalytic activity [12]. Noble metal nanoparticles (NPs) show strong visible light absorption because of size and shape dependent plasmon resonance which has wide variety of applications such as colorimetric centers, photovoltaic devices, photochromic devices and photocatalysts. In particular, silver NPs show efficient plasmon resonance in the visible light region which has been utilized to develop a plasmonic photocatalyst. Recently, Hayato et al. [13] fabricated Au nanoparticles on TiO2 for the photocatalytic hydrogen evolution under visible light. Wang et al. [14] synthesized Ag@AgCl plasmonic photocatalysts through ion exchange and the photoreduction method, which showed excellent photocatalytic activities under visible light irradiation. It is reported that silver halides are generally considered as photosensitive materials, can coexist stably with Ag NPs during the whole photocatalytic process. Kakuta et al. [15] also observed that Ag NPs could contribute to the smooth separation of electron–hole pairs on
R. Adhikari et al. / Journal of Solid State Chemistry 197 (2013) 560–565
the AgBr/SiO2 NPs and enable it to catalyze H2 production from alcohol radicals. Youngping Liu et al. [16] did one pot pyridineassisted synthesis of Ag/Ag3PO4 as a visible light driven photocatalyst with enhanced photocatalytic activity. Yuanguo Xu and coworkers [17] reported the enhanced photocatalytic activity of Ag/AgCl/ZnO photocatalyst. Peng Wang et al. synthesized composite semiconductor H2WO4 H2O/AgCl as an efficient and stable photocatalyst under visible light and they focused on charge transfer process between small band gap semiconductors (SBG) coupled with AgCl [18,19]. Jiaguo Yu et al. [20] reported the fabrication and characterization of Ag/AgCl/TiO2 nanotube arrays. Guo et al. [21] synthesized Ag/AgCl@TiO2 for the treatment of aqueous hazardous pollutants. All these literatures mentioned above have proved that the Ag/AgCl supported with metal oxide semiconductor have enhanced photocatalytic activity under visible or simulated solar light. However, very little work has been done on the fabrication of WO3 and Ag/AgCl nanoparticles based nanocomposite and their photocatalytic activities. In this work, a new simulated solar light driven photocatalyst Ag/AgCl/WO3 has been synthesized by the microwave assisted hydrothermal method. The synthesized Ag/AgCl/WO3 samples showed higher photocatalytic activity for the degradation of aqueous rhodamine B solution and the photocatalyst exhibited higher stability.
2. Materials and methods 2.1. Materials Sodium Tungstate Dihydrate (Na2WO4 2H2O), Silver Nitrate (AgNO3) and Hydrochloric Acid (HCl) were received from Sigma Aldrich. All the chemicals were of analytical grade and were used for the synthesis of Ag/AgCl/WO3 without any further purification. Double distilled water was used for all synthesis and treatment processes. 2.2. Preparation of photocatalyst 5 g of Na2WO4 2H2O was dissolved in 30 ml of 1 M HCl solution and was stirred continuously for 1 h. Similarly, 5 ml of the aqueous solution containing 0.1 mole percent of AgNO3 was prepared in distilled water and added to the above solution. The pH of each solution was maintained to be 2. After the pH adjustment, the mixture was treated with UV light (300–420 nm, homemade design) for 1 h to reduce silver ions to silver nanoparticles. UV treated solutions were finally transferred to Teflon flask for microwave hydrothermal treatment. The mixture was heated at 2001 C for 2 h. The resulting products were washed with distilled water several times, dried at 70 1C, and calcined in air at 600 1C for 5 h. In a similar manner, two more samples of Ag/AgCl/WO3 photocatalysts were synthesized with different Ag/AgCl contents (0.3 mole percent and 0.5 mole percent of AgNO3) and have been assigned as S-0.1, S-0.3, and S-0.5, respectively for convenience.
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by PHI 5000C ESCA system with MgKa source operating at 14.0 kV and 25 mA. All the binding energies were referenced to the C 1s peak at 284.6 eV. Photocatalytic experiments were performed by UV–Vis spectrophotometer (optizen view, Mecasys) and portable solar simulator. 2.4. Photocatalytic experiment Rhodamine B was selected as a model dye in order to check the photocatalytic activity under simulated solar light conditions. It is due to the fact that the most of the dyes are sensitized in presence of light even in absence of photocatalyst. Xiaoli et al. [22] tested the decomposition of Methylene Blue under visible light and studied the suitability of probe molecule for photocatalytic activity test. In comparison to Methylene Blue, rhodamine B is more stable under visible light. Therefore, rhodamine B was selected as a model dye in this work. In the photocatalytic experiment, 0.1 g of the catalyst powder was placed in a Pyrex glass cell (50 mm 70 mm) and then dispersed in absolute ethanol by using ultra sound for 10 minutes. The solution with dispersed catalyst particles was then dried for 24 h in an oven at 70 1C to make a thin layer of catalyst coating. 50 ml of 10 ppm rhodamine B solution was slowly poured in the Pyrex glass and kept in the dark for half an hour to achieve adsorption–desorption equilibrium. After the equilibrium was reached, photocatalytic experiments were conducted by using solar simulator under simulated solar light and the absorbance of the solution was measured by UV–Vis spectrophotometer. The photodegradation of rhodamine B by the different samples was observed until 240 minutes at every 30 minutes time interval.
3. Results and discussion 3.1. Structural and morphological analysis XRD, TEM and XPS analysis were performed to study the crystal structure and morphology of the as synthesized samples. Fig. 1 shows the XRD patterns of as prepared samples. XRD patterns of composite photocatalyst are compared with that of commercial WO3 powder. From Fig. 1, it is evident that all the diffraction peaks are well indexed to monoclinic WO3 (JCPDS card
2.3. Characterization The crystalline structure of the catalysts was characterized by powder X-Ray diffraction (XRD) employing a scanning rate of 41 per minute in a range from 51 to 701 by Rigaku X-Ray Diffractometer using monochromatized CuKa (l ¼1.54 A1 ) radiation. The morphologies and sizes of the samples were observed by TEM. UV– Vis Diffuse Reflectance Spectra (DRS) were recorded by UV–Vis spectrophotometer (NIRJASCO 570, Micrometrics instrument Corp. Edinburgh Instrument—F900). XPS measurement was performed
Fig. 1. XRD patterns of as synthesized samples and commercial WO3 nanopowder.
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number 72-0677), cubic Ag (JCPDS cards no. 65-2871) and cubic phase AgCl (31-1238). The peaks appeared at 27.821 and 32.241 are assigned to the (111) and (200) plane of cubic phase of AgCl and a peak appeared at 44.31 is assigned to the (200) plane of metallic silver. Hence from the XRD patterns, it is clear that as synthesized samples are composed of Ag, AgCl and WO3 particles and no additional peaks were appeared for any other impurities. Fig. 2 Show the typical TEM images of Ag/AgCl/WO3 (S-0.3) sample, insets show the HRTEM images of AgCl and Ag nanoparticles and the corresponding EDX spectrum. Fig. 2a reveals that the composite photocatalyst has mixed morphology composed of rectangular plate like and round shaped WO3 particles. AgCl nanoparticles have average diameter of 30 nm and possess two different morphologies in which some particles are spherical and some are of oval shaped. Moreover, AgCl nanoparticles are attached to the WO3 nanoparticles and heterojunction has been formed in between them. Similarly, Fig. 2b shows the TEM image of same sample in which Ag nanoparticles having diameter ranging from 10 to 20 nm are present in the composite. It can also be revealed from the figure that the distribution of Ag and AgCl nanoparticles in the composite photocatalyst is found to be heterogeneous which might be due to the hydrothermal treatment which was carried out after the photoreduction of Ag nanoparticles. Furthermore, EDX spectrum also confirms the presence of Ag and AgCl nanoparticles present in the composite photocatalyst. Compositions and the chemical states of the components present in the composite photocatalyst were investigated by XPS analysis. Fig. 3a shows the general XPS survey of the composite photocatalyst (S-0.3). It is clear from the spectrum that the surface morphology of the Ag/AgCl/WO3 composite photocatalyst consists of W, O, Ag, and Cl element and no any
other impurity peaks were detected. Peak at 34.1 eV is ascribed to W which is originated from W6 þ ion of the WO3. Two prominent peaks (Fig. 3b) corresponding to ca. 368.9 eV(Ag3d5/2) and 374.5 eV(Ag3d3/2) confirmed the presence of metallic Ag nanoparticles [23]. Fig. 3c and 3d present the high resolution XPS spectra for O and Cl present in the sample. The peak at 199 eV can be assigned to be Cl (2p) peak which is originated from the AgCl nanoparticles whereas peak at 534 eV corresponds to the Oxygen. The peak for C 1s at 284.8 eV is due to the hydrocarbon from the XPS instrument itself. Hence, from the TEM and XPS analysis, it can be proved that the composite photocatalyst consists of Ag/ AgCl and WO3. 3.2. BET surface area analysis BET surface area of as synthesized samples compared with that of commercial WO3 nanopowder is shown in Table 1. It is revealed from the table that the composite photocatalysts have smaller BET surface area as compared to commercial WO3 nanopowder. Moreover, BET surface area decreased with increase in dopant concentrations. Decrease in surface area might be due to the occupation of pores and interstitial lattice positions of WO3 by silver nanoparticles and it is the common characteristic of heterogeneous phase of the composite materials. Similar results have been reported for composite photocatalysts [24]. 3.3. UV–Vis DRS analysis Fig. 4 displays the UV–Vis diffuse reflectance spectra of the as prepared catalyst samples and commercial WO3 powder as a reference. All the samples show a strong absorption in the visible region which confirms that the composite catalyst is active under
Fig. 2. TEM images of sample, S-0.3. a. showing AgCl particles, b. showing Ag nanoparticles, c. EDX spectrum showing the components of composite photocatalyst.
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Fig. 3. XPS Spectra of a. XPS survey spectrum of sample S-0.3, b. XPS spectrum for Ag 3d band, c. for O 1s band, d. for Cl 2p band.
Table 1 BET surface area of different samples. Sample
BET surface area (m2/g)
Com. WO3 S-0.1 S-0.3 S-0.5
3.642 2.504 3.455 0.211
visible light. From DRS analysis, Ag/AgCl/WO3 photocatalyst exhibited the absorption in the full range of sunlight including visible range as well as UV region. The enhanced light-absorption for visible light was ascribed to silver nanoparticles with localized surface plasmonic resonance, indicating that silver nanoparticles were successfully introduced into the composite photocatalyst [25]. These results verify that silver nanoparticles were reduced from silver ions
on the outer surface of WO3 and Ag/AgCl/WO3 photocatalyst exhibited the enhanced absorption for the visible light. 3.4. Photocatalytic activity The kinetic plot of the photocatalytic degradation of aqueous rhodamine B solution under simulated solar light is shown in Fig. 5. Photocatalytic activities of composite samples were compared with commercial WO3 powder under identical conditions. Photodegradation of rhodamine B under simulated solar light without using catalyst was also carried out. Among the three types of Ag/AgCl/WO3 samples, S-0.3 sample exhibited the best photocatalytic performance. The photodegradation of rhodamine B by this composite photocatalyst was almost 10 times higher than that of commercial WO3 powder. The rate of photocatalysis was found to be in the order: S-0.34S-0.54S-0.1 4commercial WO3 powder 4without photocatalyst. Kinetic plot reveals that the photodegradation of rhodamine B follows a pseudo-first order
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Fig. 6. Schematic diagram of mechanism of photocatalysis.
Fig. 4. UV–Vis diffuse reflectance spectra of different samples.
Fig. 7. Recycle tests of the sample S-0.3.
Fig. 5. Kinetic plot for the degradation of rhodamine B by various photocatalysts under simulated solar light.
kinetics which is almost similar for all dyes degradation experiments at lower concentrations [20]. 3.5. Proposed mechanism of the photocatalysis Although WO3 photocatalyst has appropriate band gap energy, it cannot show the better photocatalytic activity for practical purpose due to the lower conduction band as compared to Normal Hydrogen Electrode [12]. When we compare the UV–Vis DRS absorbance of composite photocatalyst with the WO3 powder, it was observed that the composite photocatalyst is more efficient than the WO3 photocatalyst. This result can be explained on the basis of surface plasmon resonance effect. The main characteristic of plasmon exciton coupling interaction is to induce electromagnetic field in the vicinity of the semiconductor and the exciton energy is transferred from the semiconductor to the metal nanoparticles. Conjugation of Ag/AgCl with WO3 induces local surface plasmon resonance (LSPR) which is produced by the collective oscillation of surface electrons on the Ag nanoparticles
that enhances the local inner electromagnetic field [14,20,26]. Therefore, the photogenerated electrons on the WO3 photocatalyst are transferred to the Ag nanoparticles which prevents the recombination of electron–hole pairs and then reduces the adsorbed oxygen to produce super oxygen anionic free radicals (O2 d) [27]. Similarly, holes formed on the valence band of WO3 are responsible for the oxidation of dyes molecules leading to the formation of various degraded products. Hence, we believe that the increase in photocatalytic activity of the composite photocatalyst is mainly due to the plasmon resonance caused by Ag nanoparticles. The photocatalytic mechanism of the Ag/AgCl/WO3 is schematically illustrated in Fig. 6. According to the literatures [22–25], the appropriate ratio of Ag0/Ag þ is essential for effective charge separation, that plays a key role for the effective photocatalysis. If the ratio is high, there is the formation of Ag cluster, which then turns out to be the combination center for photoinduced carriers and ultimately counteracts the activity improvement [28–31]. Hence, we believe that the optimum ratio is achieved in S-0.3 and exhibited the best photocatalytic activity as compared to other samples. The schematic of the photocatalytic mechanism of the Ag/AgCl/WO3 photocatalyst is illustrated in Fig. 6. 3.6. Stability of the photocatalyst Stability of the photocatalyst is a key factor that determines the long term efficiency. Therefore, the stability of the as synthesized photocatalyst was studied by several recycle test experiment. The recycle test was performed four times on S-0.3 sample and the
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results were compared in Fig. 7. It is revealed from the figure that the activity of the photocatalyst remained almost constant which shows the Ag/AgCl/WO3 composite is stable photocatalyst; however, small decrease in its stability might be due to the sparingly soluble nature of Ag þ ion in aqueous medium.
4. Conclusions Simulated solar light responsive Ag/AgCl/WO3 photocatalyst was successfully synthesized by the microwave assisted hydrothermal process. The composite photocatalyst exhibited much higher photocatalytic activity as compared to commercial WO3 powder that can be ascribed to the surface plasmon resonance effect induced by Ag nanoparticles. Moreover, this work provides an insight for the further fabrication of Ag/AgCl/WO3 composite system in order to obtain highly active visible light responsive photocatalyst.
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