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Atmospheric-pressure cold plasma for synthesizing Ag modified Degussa P25 with visible light activity using dielectric barrier discharge
Catalysis Today 211 (2013) 143–146
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Atmospheric-pressure cold plasma for synthesizing Ag modified Degussa P25 with visible light activity using dielectric barrier discharge Lanbo Di ∗ , Zhijian Xu, Xiuling Zhang ∗ College of Physical Science and Technology, Dalian University, Dalian 116622, PR China
a r t i c l e
i n f o
Article history: Received 5 November 2012 Received in revised form 25 February 2013 Accepted 1 March 2013 Available online 22 April 2013 Keywords: Atmospheric-pressure cold plasma Dielectric barrier discharge TiO2 Silver Visible light activity
a b s t r a c t Atmospheric-pressure cold plasma was employed to fabricate Ag/TiO2 visible light photocatalyst. UV–vis, XPS and XRD spectra were used to investigate Ag valence in the Ag/TiO2 samples. UV–vis spectra indicated that metal Ag nanoparticles were present after plasma treatment, and the surface plasmon peak became narrower and shifted to shorter wavelengths when the discharge voltage was increased. XPS and XRD spectra proved that Ag ions were completely reduced to metallic Ag nanoparticles. The morphologies of the Ag/TiO2 samples were observed by transmission electron microscopy (TEM). TEM micrographs of the Ag/TiO2 sample fabricated at 36 kV showed that the average particle size of Ag nanoparticles was ca. 7.4 nm. Photocatalytic activity of the Ag/TiO2 samples was evaluated by photodegradation of methylene blue (MB) under visible light ( > 420 nm). Compared with Degussa P25, the Ag/TiO2 catalyst fabricated by DBD cold plasma exhibited a high activity for MB degradation. Atmospheric-pressure DBD cold plasma was highly efficient for preparation of supported metallic Ag catalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction TiO2 has received much attention due to its unique photocatalytic activity in the treatment of environmental contaminants [1,2]. However, TiO2 has a large band gap (3.2 eV), meaning that only ultraviolet light ( < 387.5 nm) can initiate the photocatalytic process, therefore limiting the practical use of TiO2 to <5% of the solar energy that reaches the earth. In order to solve this problem, noble metals such as Pt, Pd, Au and Ag were adopted to modify TiO2 surface to enhance the photocatalytic activity of TiO2 under visible light [3,4]. In particular, silver nanoparticles deposited on TiO2 surface (Ag/TiO2 ) have attracted significant attention because of nontoxicity of this metal with remarkable catalytic activity and antibacterial activity [5–7]. Moreover, silver is particularly suitable for industrial applications because of its relatively low price. Conventional methods for preparation of Ag/TiO2 composites are thermal reduction and photodeposition. Thermal reduction will result in the increase of the size of silver particles. Photodeposition can minimize the size of silver particles, but the process is complex and the amount of metal loaded is difficult to control. Compared with conventional methods, cold plasma, operated at low temperature, is a clean technology, and has been applied
∗ Corresponding authors. Tel.: +86 411 87402712; fax: +86 411 87402712. E-mail addresses: [email protected] (L. Di), [email protected] (X. Zhang). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.03.020
to reduce supported noble-metal ions [8–13]. However, most of the experiments were conducted at low pressure. Atmosphericpressure dielectric barrier discharge (DBD) is a simple and easily operated approach for generating cold plasma, and has been used to reduce noble-metal ions. Kim et al. [14] prepared supported metal catalysts by using atmospheric-pressure DBD cold plasma to reduce supported Pt and Co metal ions. In this study, by using Ar and H2 mixture as working gases, Ag modified Degussa P25 (Ag/TiO2 ) with visible light activity was successfully fabricated by atmospheric-pressure DBD cold plasma.
2. Experimental 2.1. Preparation of the Ag/TiO2 catalyst Commercially produced TiO2 powder was obtained from Degussa (P25 powder). Degussa P25 contains 75% anatase and 25% rutile with a specific BET-surface area of 50 m2 g−1 and primary particle size of 20 nm [15]. AgNO3 and commercially available Degussa P25 TiO2 powder were used as received. The catalyst was prepared by a conventional impregnation method. The required amount of an aqueous solution of AgNO3 was slowly added to Degussa P25 with thorough stirring at room temperature. The precursor was then dried at 373 K for 2 h for further investigation. The schematic diagram of dielectric barrier discharge cold plasma device for preparing Ag/TiO2 at atmospheric pressure was
L. Di et al. / Catalysis Today 211 (2013) 143–146
Fig. 1. Schematic diagram of the atmospheric-pressure DBD cold plasma device (1 – discharge electrode, 2 – quartz glass, 3 – ground electrode, 4 – cold plasma, 5 – sample).
shown in Fig. 1. Both of the high-voltage electrode and the ground electrode were stainless steel plates (50 mm in diameter). A reaction cell made of quartz was placed between the high-voltage electrode and ground electrode. The reaction cell consisted of two parts. The upper part was a quartz plate (90 mm in diameter, 2-mm thickness), while the lower part was a tank (75 mm in diameter, 6mm height). The reaction tank used to contain the samples was put in the center of the two electrodes. The discharge gap was 4 mm. The power source (CTP-2000 K, Nanjing Suman Electronic Co., Ltd) was capable of supplying a bipolar sine wave output with 0–40 kV peak-to-peak voltage (Up-p ) at a frequency of 7.5–30 kHz. All the individual gases used in this work were of high-purity grade (>99.99%). The gas flow rates were adjusted and controlled by a mass flow controller system (SevenStar Co., China). Our preliminary experiments showed that Ag ions could not be reduced by using Ar as working gas, indicating that electrons in our study were not the reducing agents. It has been reported that addition of Ar into H2 may promote the generation of active hydrogen species as a result of the collisions of metastable Ar atoms with H2 , thereby facilitating the reduction of metal ions [16]. So the gas mixture of Ar and H2 with a total flow rate of 100 mL min−1 was used as working gases. The influence of H2 content and the mechanism for reduction of Ag ions will be discussed in another paper. In this study, H2 content was 50%. Before the cold plasma treatment, 0.6 g Ag/TiO2 was uniformly put in the reaction tank. Unless otherwise specified, the Ag content was kept at 1 wt%. The modification process of Ag/TiO2 was conducted by applying a sine-wave high voltage at a frequency of 14.1 kHz without extra heating. In order to limit the bulk temperature at room temperature, the plasma treatment was not continuously operated. It was performed for three times with an interval of 10 min between two operations. Each treatment just took 2 min. 2.2. Characterization of the Ag/TiO2 catalyst UV–vis DRS spectra of the Ag/TiO2 samples were measured by a UV-vis spectrophotometer (Varian Cary 100, USA) with a BaSO4 plate as the reference. The chemical compositions of the Ag/TiO2 samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAN250 Thermo VG) using a monochromatized AlK␣ (1486.6 eV) X-ray source. The energy resolution was 0.5 eV for XPS. All binding energies were referenced to the XPS peak of carbon 1 s at 284.6 eV. X-ray diffraction was carried out on a China Dandong rotating anode X-ray diffractometer (DX-2700) with graphite˚ The Ag/TiO2 monochromatized Cu K␣1 radiation ( = 1.54178 A). samples were observed by transmission electron microscopy (TEM, G2 spirit, Hong Kong) at 200 kV accelerating voltage. 2.3. Photocatalytic activity test of the Ag/TiO2 catalyst The most often used methylene blue (MB) dye was used as the model dye contaminant. The photocatalytic reaction system consisted of a 300 W Xe lamp equipped with a cut-off filter ( = 420 nm) and a water filter placed between the Xe lamp and the reaction cell to prevent from thermal catalytic effect. The lamp was located 15 cm above the solution. All the experiments were
conducted at room temperature in air. The photocatalytic reaction was carried out with 0.1 g Ag/TiO2 suspended in 100 mL MB solution (the concentration of MB solution was about 10 mg L−1 ) in a Pyrex glass cell. The solution was stirred in the dark for 30 min to obtain a good dispersion and establish adsorption–desorption equilibrium between the organic molecules and the catalyst surface. At given time intervals, the slurry samples including the photocatalyst and MB were separated by a H1650 centrifuge (Xiangyi Centrifuge Instrument Co., Ltd., China), and then the solution was analyzed by a 721 UV-visible spectrometer (Shanghai Jinghua Group Co., Ltd., China). MB degradation was detected by measuring the absorption at the wavelength of 665 nm. The absorption was converted to the MB concentration referring to a standard curve showing a linear behavior between the concentration and the absorption at this wavelength. 3. Results and discussion 3.1. Ag valence in the Ag/TiO2 samples After the cold plasma treatment at atmospheric pressure, the color of Ag/TiO2 samples turned purple brown, which gave evidence of the change from silver ions to metallic silver particles. This was also observed by Liu et al. [17] by using glow discharge plasma at low pressure. The UV–vis DRS spectra for the Ag/TiO2 samples as prepared and treated by atmospheric-pressure cold plasma at different discharge voltages were shown in Fig. 2. Compared with the as-prepared Ag/TiO2 , all the four samples treated by cold plasma exhibited the obvious characteristic absorption around 510 nm, which can be attributed to the surface plasmon resonance peak of the spatially confined electrons in Ag nanoparticles [17]. These absorption spectra indicated that metal Ag nanoparticles were present after plasma treatment. However, these bands showed an unusually large red-shift from the typical plasmon peak of Ag nanoparticles at 400 nm. This redshift may be related to the close contact of adjacent silver particles [18]. Moreover, the surface plasmon peak became narrower and shifted to shorter wavelengths when the discharge voltage was increased, which indicated a decrease in size and size distribution of the Ag nanoparticles. This was inconsistent with the results previously reported by He et al. [8]. They thought that with the increase in plasma power, the interaction of the adjacent Ag nanoparticles was enforced and then redshift appeared. We recognize that the interaction of the adjacent Ag nanoparticles was enforced with the increase in plasma 1.0 0.9 28 kV
0.8
Absorbance (a.u.)
144
32 kV 34 kV
0.7
36 kV
0.6 0.5 0.4 As-prepared 0.3 0.2 0.1
400
500
600
700
800
Wavelength (nm) Fig. 2. UV–vis spectra of the Ag/TiO2 samples fabricated at different discharge voltages.
L. Di et al. / Catalysis Today 211 (2013) 143–146
Ag AgNO3
0
Intensity (a.u.)
Intensity (a.u.)
Ag
145
After plasma treatment
Before plasma treatment
380
375
370
365
360
Binding energy (eV) Fig. 3. Ag 3d XPS spectra in the Ag/TiO2 fabricated at 36 kV.
power. However, we suspected that more charged species were formed during the reduction of silver ions at atmospheric pressure, which may prevent the aggregation of silver nanoparticles, and then the decrease in particle size was observed with the increase in discharge voltage. Thus, discharge voltage of 36 kV was selected to fabricate Ag/TiO2 photocatalyst. In order to verify the chemical binding states and compositions, XPS spectra of Ag 3d in the Ag/TiO2 samples fabricated at a discharge voltage of 36 kV was collected and shown in Fig. 3. The Ag 3d spectra (Fig. 3) showed 3d5/2 at 367.6 eV and spin orbital splitting of 6.0 eV, and no other peaks could be deconvoluted. According to the reference data [17,19], they can be assigned to the electron transitions for metallic Ag. This result, consistent with the result of the UV–vis spectra, proved that the supported Ag ions had been completely reduced to their metallic states. In addition to UV–vis and XPS measurements, 6 wt% Ag/TiO2 samples were purposely fabricated at discharge voltage of 36 kV to further investigate the silver valence in Ag/TiO2 after DBD cold plasma treatment. The XRD patterns of the 6 wt% Ag/TiO2 samples as prepared and after cold plasma treatment were shown in Fig. 4, from which we can see that characteristic peaks corresponding to AgNO3 disappeared after cold plasma treatment. Moreover, new peaks at 38.0◦ , 44.1◦ , 64.3◦ and 77.3◦ appeared, which can be perfectly indexed to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections of the
20
30
40
50
60
70
80
2θ (degree) Fig. 4. XRD spectra of 6% Ag/TiO2 as prepared and after cold plasma treatment.
face-centered cubic (fcc) phase of Ag (JCPDS File 04-0783). The obvious and clear signals of the metallic Ag nanoparticles indicated that they are well crystallized, which was consistent with the results from the UV–vis and XPS observations. 3.2. Ag morphological structure in the Ag/TiO2 samples The TEM micrographs of Ag/TiO2 fabricated at 36 kV and their corresponding particle size distributions were depicted in Fig. 5, from which we can see that spherical Ag nanoparticles with narrow size distribution were highly dispersed on the surface of Degussa P25. The average particle size of Ag was determined to be 7.4 nm. It was a little larger than that prepared by cold plasma at low pressure (6.1 nm), but much smaller than that prepared by conventionally H2 -reduced (ca. 17.5 nm) [17]. This indicated that atmospheric-pressure cold plasma was more favorable than conventional reduction for enhancing the distribution of metal nanoparticles. 3.3. Photocatalytic activity of the Ag/TiO2 samples Fig. 6 represented the variation of C/C0 (C0 is the initial concentration of MB and C, the concentration at a given time) with irradiation time over Ag/TiO2 catalyst fabricated by DBD cold
Fig. 5. TEM image and histogram of Ag nanoparticles in Ag/TiO2 fabricated at 36 kV.
146
L. Di et al. / Catalysis Today 211 (2013) 143–146
Blank P25 Ag/TiO2
1.0
C/C 0
0.8
UV–vis DRS spectra indicated that metal Ag nanoparticles appeared after plasma treatment. XPS and XRD spectra proved that Ag ions were completely reduced to metallic Ag nanoparticles. TEM micrographs of the Ag/TiO2 sample fabricated at 36 kV showed that the average particle size of Ag nanoparticles was ca. 7.4 nm. The Ag/TiO2 catalyst fabricated by DBD cold plasma exhibited a high activity for MB degradation under visible light ( > 420 nm). Atmosphericpressure DBD cold plasma was highly efficient for preparation of supported metallic Ag catalyst.
0.6
Acknowledgements
0.4
This work is supported by National Natural Science Foundation of China (Grant No. 21173028) and Program for Liaoning Excellent Talents in University (LR2012042) 0
30
60
90
120
Time (min) Fig. 6. Photocatalytic MB degradation under visible light ( > 420 nm) at room temperature in air for 120 min over P25 and Ag/TiO2 , as well as MB photolysis. C0 is the initial concentration of MB and C, the concentration at a given time.
plasma at a discharge voltage of 36 kV. As a comparison, MB degradations over P25 and MB photolysis were also performed under the same conditions and were also shown in Fig. 6. Noncatalytic degradation of MB in solution also occurred and its trend was almost the same as P25. This was in line with results previously reported in the literature [20]. In contrast, the Ag/TiO2 catalyst fabricated by DBD cold plasma exhibited a high activity for MB degradation under visible light ( > 420 nm), and about 46% MB molecules were degraded after irradiation for 120 min. However, only 27% MB molecules were degraded for the Ag/TiO2 catalyst fabricated by thermal reduction. It was speculated that the plasma method produced an enhanced metal-support interaction [21–23], which may facilitate the electron transfer during photocatalytic reaction and be the major reason for the high activity of the plasma prepared photocatalysts. In addition, the smaller particle size of Ag (Fig. 5) compared with that obtained by thermal reduction may also enhance the activity of the photocatalysts. These were also confirmed by Zou et al. [21,23].
References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusion In this study, by using Ar and H2 mixture as working gases, Ag/TiO2 visible light photocatalyst was successfully fabricated by dielectric barrier discharge cold plasma at atmospheric pressure.
[20] [21] [22] [23]
R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. X. Chen, S.S. Mao, Chemical Reviews 107 (2007) 2891. F.B. Li, X.Z. Li, Chemosphere 48 (2002) 1103. S. Sakthivel, M.V. Shankar, M. Palanichamy, B. Arabindoo, D.W. Bahnemann, V. Murugesan, Water Research 38 (2004) 3001. V. Vamathevan, R. Amal, D. Beydoun, G. Low, S. McEvoy, Photochemistry and Photobiology Journal A 148 (2002) 233. P.K.J. Robertson, D.W. Bahnemann, J.M.C. Robertson, F. Wood, Handbook of Environmental Chemistry, vol. 2, Springer, Heidelberg, Germany, 2005, part M, pp. 367–423. Q. Li, S. Mahendra, D.Y. Lyon, L. Brunet, M.V. Liga, D. Li, P.J.J. Alvarez, Water Research 42 (2008) 4591. J. He, I. Ichinose, T. Kunitake, A. Nakao, Langmuir 18 (2002) 10005. O. Höfft, F. Endres, Physical Chemistry Chemical Physics 13 (2011) 13472. X. Liu, C.-Y. Mou, Szetsen Lee, Y. Li, J. Secrest, B.W.-L. Jang, Journal of Catalysis 285 (2012) 152. C. Shi, R. Hoisington, B.W.-L. Jang, Industrial and Engineering Chemistry Research 46 (2007) 4390. C. Zhou, X. Wang, X. Jia, H. Wang, C. Liu, Y. Yang, Electrochemistry Communications 18 (2012) 33. H. Wang, C. Liu, Applied Catalysis B 106 (2011) 672. S.-S. Kim, H. Lee, B.-K. Na, H.-K. Song, Catalysis Today 89 (2004) 193. R.I. Bickley, T.G. Carreno, L. Palmisano, R.J.D. Tilley, Journal of Solid State Chemistry 92 (1992) 178. H. Inui, K. Takeda, H. Kondo, K. Ishikawa, M. Sekine, H. Kano, N. Yoshida, M. Hori, Applied Physics Express 3 (2010) 126101. J.-J. Zou, Y.-P. Zhang, C.-J. Liu, Langmuir 22 (2006) 11388. H. Fujiwara, S. Yanagida, P.V. Kamat, Journal of Physical Chemistry B 103 (1999) 2589. J. Chastain (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation Physical Electronics Division, Eden Prairie, Minnesota, 1992. J. Wang, Z. Zou, J. Ye, Angewandte Chemie International Edition 43 (2004) 4463. J.-J. Zou, C. Chen, C.-J. Liu, Y.-P. Zhang, Y. Han, L. Cui, Materials Letters 59 (2005) 3437. J.-J. Zou, C.-J. Liu, Y.-P. Zhang, Langmuir 22 (2006) 2334. J.-J. Zou, H. He, L. Cui, H.-Y. Du, International Journal of Hydrogen Energy 32 (2007) 1762.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Atmospheric-pressure cold plasma for synthesizing Ag modified Degussa P25 with visible light activity using dielectric barrier discharge Lanbo Di ∗ , Zhijian Xu, Xiuling Zhang ∗ College of Physical Science and Technology, Dalian University, Dalian 116622, PR China
a r t i c l e
i n f o
Article history: Received 5 November 2012 Received in revised form 25 February 2013 Accepted 1 March 2013 Available online 22 April 2013 Keywords: Atmospheric-pressure cold plasma Dielectric barrier discharge TiO2 Silver Visible light activity
a b s t r a c t Atmospheric-pressure cold plasma was employed to fabricate Ag/TiO2 visible light photocatalyst. UV–vis, XPS and XRD spectra were used to investigate Ag valence in the Ag/TiO2 samples. UV–vis spectra indicated that metal Ag nanoparticles were present after plasma treatment, and the surface plasmon peak became narrower and shifted to shorter wavelengths when the discharge voltage was increased. XPS and XRD spectra proved that Ag ions were completely reduced to metallic Ag nanoparticles. The morphologies of the Ag/TiO2 samples were observed by transmission electron microscopy (TEM). TEM micrographs of the Ag/TiO2 sample fabricated at 36 kV showed that the average particle size of Ag nanoparticles was ca. 7.4 nm. Photocatalytic activity of the Ag/TiO2 samples was evaluated by photodegradation of methylene blue (MB) under visible light ( > 420 nm). Compared with Degussa P25, the Ag/TiO2 catalyst fabricated by DBD cold plasma exhibited a high activity for MB degradation. Atmospheric-pressure DBD cold plasma was highly efficient for preparation of supported metallic Ag catalyst. © 2013 Elsevier B.V. All rights reserved.
1. Introduction TiO2 has received much attention due to its unique photocatalytic activity in the treatment of environmental contaminants [1,2]. However, TiO2 has a large band gap (3.2 eV), meaning that only ultraviolet light ( < 387.5 nm) can initiate the photocatalytic process, therefore limiting the practical use of TiO2 to <5% of the solar energy that reaches the earth. In order to solve this problem, noble metals such as Pt, Pd, Au and Ag were adopted to modify TiO2 surface to enhance the photocatalytic activity of TiO2 under visible light [3,4]. In particular, silver nanoparticles deposited on TiO2 surface (Ag/TiO2 ) have attracted significant attention because of nontoxicity of this metal with remarkable catalytic activity and antibacterial activity [5–7]. Moreover, silver is particularly suitable for industrial applications because of its relatively low price. Conventional methods for preparation of Ag/TiO2 composites are thermal reduction and photodeposition. Thermal reduction will result in the increase of the size of silver particles. Photodeposition can minimize the size of silver particles, but the process is complex and the amount of metal loaded is difficult to control. Compared with conventional methods, cold plasma, operated at low temperature, is a clean technology, and has been applied
∗ Corresponding authors. Tel.: +86 411 87402712; fax: +86 411 87402712. E-mail addresses: [email protected] (L. Di), [email protected] (X. Zhang). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.03.020
to reduce supported noble-metal ions [8–13]. However, most of the experiments were conducted at low pressure. Atmosphericpressure dielectric barrier discharge (DBD) is a simple and easily operated approach for generating cold plasma, and has been used to reduce noble-metal ions. Kim et al. [14] prepared supported metal catalysts by using atmospheric-pressure DBD cold plasma to reduce supported Pt and Co metal ions. In this study, by using Ar and H2 mixture as working gases, Ag modified Degussa P25 (Ag/TiO2 ) with visible light activity was successfully fabricated by atmospheric-pressure DBD cold plasma.
2. Experimental 2.1. Preparation of the Ag/TiO2 catalyst Commercially produced TiO2 powder was obtained from Degussa (P25 powder). Degussa P25 contains 75% anatase and 25% rutile with a specific BET-surface area of 50 m2 g−1 and primary particle size of 20 nm [15]. AgNO3 and commercially available Degussa P25 TiO2 powder were used as received. The catalyst was prepared by a conventional impregnation method. The required amount of an aqueous solution of AgNO3 was slowly added to Degussa P25 with thorough stirring at room temperature. The precursor was then dried at 373 K for 2 h for further investigation. The schematic diagram of dielectric barrier discharge cold plasma device for preparing Ag/TiO2 at atmospheric pressure was
L. Di et al. / Catalysis Today 211 (2013) 143–146
Fig. 1. Schematic diagram of the atmospheric-pressure DBD cold plasma device (1 – discharge electrode, 2 – quartz glass, 3 – ground electrode, 4 – cold plasma, 5 – sample).
shown in Fig. 1. Both of the high-voltage electrode and the ground electrode were stainless steel plates (50 mm in diameter). A reaction cell made of quartz was placed between the high-voltage electrode and ground electrode. The reaction cell consisted of two parts. The upper part was a quartz plate (90 mm in diameter, 2-mm thickness), while the lower part was a tank (75 mm in diameter, 6mm height). The reaction tank used to contain the samples was put in the center of the two electrodes. The discharge gap was 4 mm. The power source (CTP-2000 K, Nanjing Suman Electronic Co., Ltd) was capable of supplying a bipolar sine wave output with 0–40 kV peak-to-peak voltage (Up-p ) at a frequency of 7.5–30 kHz. All the individual gases used in this work were of high-purity grade (>99.99%). The gas flow rates were adjusted and controlled by a mass flow controller system (SevenStar Co., China). Our preliminary experiments showed that Ag ions could not be reduced by using Ar as working gas, indicating that electrons in our study were not the reducing agents. It has been reported that addition of Ar into H2 may promote the generation of active hydrogen species as a result of the collisions of metastable Ar atoms with H2 , thereby facilitating the reduction of metal ions [16]. So the gas mixture of Ar and H2 with a total flow rate of 100 mL min−1 was used as working gases. The influence of H2 content and the mechanism for reduction of Ag ions will be discussed in another paper. In this study, H2 content was 50%. Before the cold plasma treatment, 0.6 g Ag/TiO2 was uniformly put in the reaction tank. Unless otherwise specified, the Ag content was kept at 1 wt%. The modification process of Ag/TiO2 was conducted by applying a sine-wave high voltage at a frequency of 14.1 kHz without extra heating. In order to limit the bulk temperature at room temperature, the plasma treatment was not continuously operated. It was performed for three times with an interval of 10 min between two operations. Each treatment just took 2 min. 2.2. Characterization of the Ag/TiO2 catalyst UV–vis DRS spectra of the Ag/TiO2 samples were measured by a UV-vis spectrophotometer (Varian Cary 100, USA) with a BaSO4 plate as the reference. The chemical compositions of the Ag/TiO2 samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAN250 Thermo VG) using a monochromatized AlK␣ (1486.6 eV) X-ray source. The energy resolution was 0.5 eV for XPS. All binding energies were referenced to the XPS peak of carbon 1 s at 284.6 eV. X-ray diffraction was carried out on a China Dandong rotating anode X-ray diffractometer (DX-2700) with graphite˚ The Ag/TiO2 monochromatized Cu K␣1 radiation ( = 1.54178 A). samples were observed by transmission electron microscopy (TEM, G2 spirit, Hong Kong) at 200 kV accelerating voltage. 2.3. Photocatalytic activity test of the Ag/TiO2 catalyst The most often used methylene blue (MB) dye was used as the model dye contaminant. The photocatalytic reaction system consisted of a 300 W Xe lamp equipped with a cut-off filter ( = 420 nm) and a water filter placed between the Xe lamp and the reaction cell to prevent from thermal catalytic effect. The lamp was located 15 cm above the solution. All the experiments were
conducted at room temperature in air. The photocatalytic reaction was carried out with 0.1 g Ag/TiO2 suspended in 100 mL MB solution (the concentration of MB solution was about 10 mg L−1 ) in a Pyrex glass cell. The solution was stirred in the dark for 30 min to obtain a good dispersion and establish adsorption–desorption equilibrium between the organic molecules and the catalyst surface. At given time intervals, the slurry samples including the photocatalyst and MB were separated by a H1650 centrifuge (Xiangyi Centrifuge Instrument Co., Ltd., China), and then the solution was analyzed by a 721 UV-visible spectrometer (Shanghai Jinghua Group Co., Ltd., China). MB degradation was detected by measuring the absorption at the wavelength of 665 nm. The absorption was converted to the MB concentration referring to a standard curve showing a linear behavior between the concentration and the absorption at this wavelength. 3. Results and discussion 3.1. Ag valence in the Ag/TiO2 samples After the cold plasma treatment at atmospheric pressure, the color of Ag/TiO2 samples turned purple brown, which gave evidence of the change from silver ions to metallic silver particles. This was also observed by Liu et al. [17] by using glow discharge plasma at low pressure. The UV–vis DRS spectra for the Ag/TiO2 samples as prepared and treated by atmospheric-pressure cold plasma at different discharge voltages were shown in Fig. 2. Compared with the as-prepared Ag/TiO2 , all the four samples treated by cold plasma exhibited the obvious characteristic absorption around 510 nm, which can be attributed to the surface plasmon resonance peak of the spatially confined electrons in Ag nanoparticles [17]. These absorption spectra indicated that metal Ag nanoparticles were present after plasma treatment. However, these bands showed an unusually large red-shift from the typical plasmon peak of Ag nanoparticles at 400 nm. This redshift may be related to the close contact of adjacent silver particles [18]. Moreover, the surface plasmon peak became narrower and shifted to shorter wavelengths when the discharge voltage was increased, which indicated a decrease in size and size distribution of the Ag nanoparticles. This was inconsistent with the results previously reported by He et al. [8]. They thought that with the increase in plasma power, the interaction of the adjacent Ag nanoparticles was enforced and then redshift appeared. We recognize that the interaction of the adjacent Ag nanoparticles was enforced with the increase in plasma 1.0 0.9 28 kV
0.8
Absorbance (a.u.)
144
32 kV 34 kV
0.7
36 kV
0.6 0.5 0.4 As-prepared 0.3 0.2 0.1
400
500
600
700
800
Wavelength (nm) Fig. 2. UV–vis spectra of the Ag/TiO2 samples fabricated at different discharge voltages.
L. Di et al. / Catalysis Today 211 (2013) 143–146
Ag AgNO3
0
Intensity (a.u.)
Intensity (a.u.)
Ag
145
After plasma treatment
Before plasma treatment
380
375
370
365
360
Binding energy (eV) Fig. 3. Ag 3d XPS spectra in the Ag/TiO2 fabricated at 36 kV.
power. However, we suspected that more charged species were formed during the reduction of silver ions at atmospheric pressure, which may prevent the aggregation of silver nanoparticles, and then the decrease in particle size was observed with the increase in discharge voltage. Thus, discharge voltage of 36 kV was selected to fabricate Ag/TiO2 photocatalyst. In order to verify the chemical binding states and compositions, XPS spectra of Ag 3d in the Ag/TiO2 samples fabricated at a discharge voltage of 36 kV was collected and shown in Fig. 3. The Ag 3d spectra (Fig. 3) showed 3d5/2 at 367.6 eV and spin orbital splitting of 6.0 eV, and no other peaks could be deconvoluted. According to the reference data [17,19], they can be assigned to the electron transitions for metallic Ag. This result, consistent with the result of the UV–vis spectra, proved that the supported Ag ions had been completely reduced to their metallic states. In addition to UV–vis and XPS measurements, 6 wt% Ag/TiO2 samples were purposely fabricated at discharge voltage of 36 kV to further investigate the silver valence in Ag/TiO2 after DBD cold plasma treatment. The XRD patterns of the 6 wt% Ag/TiO2 samples as prepared and after cold plasma treatment were shown in Fig. 4, from which we can see that characteristic peaks corresponding to AgNO3 disappeared after cold plasma treatment. Moreover, new peaks at 38.0◦ , 44.1◦ , 64.3◦ and 77.3◦ appeared, which can be perfectly indexed to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections of the
20
30
40
50
60
70
80
2θ (degree) Fig. 4. XRD spectra of 6% Ag/TiO2 as prepared and after cold plasma treatment.
face-centered cubic (fcc) phase of Ag (JCPDS File 04-0783). The obvious and clear signals of the metallic Ag nanoparticles indicated that they are well crystallized, which was consistent with the results from the UV–vis and XPS observations. 3.2. Ag morphological structure in the Ag/TiO2 samples The TEM micrographs of Ag/TiO2 fabricated at 36 kV and their corresponding particle size distributions were depicted in Fig. 5, from which we can see that spherical Ag nanoparticles with narrow size distribution were highly dispersed on the surface of Degussa P25. The average particle size of Ag was determined to be 7.4 nm. It was a little larger than that prepared by cold plasma at low pressure (6.1 nm), but much smaller than that prepared by conventionally H2 -reduced (ca. 17.5 nm) [17]. This indicated that atmospheric-pressure cold plasma was more favorable than conventional reduction for enhancing the distribution of metal nanoparticles. 3.3. Photocatalytic activity of the Ag/TiO2 samples Fig. 6 represented the variation of C/C0 (C0 is the initial concentration of MB and C, the concentration at a given time) with irradiation time over Ag/TiO2 catalyst fabricated by DBD cold
Fig. 5. TEM image and histogram of Ag nanoparticles in Ag/TiO2 fabricated at 36 kV.
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Blank P25 Ag/TiO2
1.0
C/C 0
0.8
UV–vis DRS spectra indicated that metal Ag nanoparticles appeared after plasma treatment. XPS and XRD spectra proved that Ag ions were completely reduced to metallic Ag nanoparticles. TEM micrographs of the Ag/TiO2 sample fabricated at 36 kV showed that the average particle size of Ag nanoparticles was ca. 7.4 nm. The Ag/TiO2 catalyst fabricated by DBD cold plasma exhibited a high activity for MB degradation under visible light ( > 420 nm). Atmosphericpressure DBD cold plasma was highly efficient for preparation of supported metallic Ag catalyst.
0.6
Acknowledgements
0.4
This work is supported by National Natural Science Foundation of China (Grant No. 21173028) and Program for Liaoning Excellent Talents in University (LR2012042) 0
30
60
90
120
Time (min) Fig. 6. Photocatalytic MB degradation under visible light ( > 420 nm) at room temperature in air for 120 min over P25 and Ag/TiO2 , as well as MB photolysis. C0 is the initial concentration of MB and C, the concentration at a given time.
plasma at a discharge voltage of 36 kV. As a comparison, MB degradations over P25 and MB photolysis were also performed under the same conditions and were also shown in Fig. 6. Noncatalytic degradation of MB in solution also occurred and its trend was almost the same as P25. This was in line with results previously reported in the literature [20]. In contrast, the Ag/TiO2 catalyst fabricated by DBD cold plasma exhibited a high activity for MB degradation under visible light ( > 420 nm), and about 46% MB molecules were degraded after irradiation for 120 min. However, only 27% MB molecules were degraded for the Ag/TiO2 catalyst fabricated by thermal reduction. It was speculated that the plasma method produced an enhanced metal-support interaction [21–23], which may facilitate the electron transfer during photocatalytic reaction and be the major reason for the high activity of the plasma prepared photocatalysts. In addition, the smaller particle size of Ag (Fig. 5) compared with that obtained by thermal reduction may also enhance the activity of the photocatalysts. These were also confirmed by Zou et al. [21,23].
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[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusion In this study, by using Ar and H2 mixture as working gases, Ag/TiO2 visible light photocatalyst was successfully fabricated by dielectric barrier discharge cold plasma at atmospheric pressure.
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