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Titanium dioxide modification with cobalt oxide nanoparticles for photocatalysis
G Model
JIEC 2630 1–5 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7 8
Titanium dioxide modification with cobalt oxide nanoparticles for photocatalysis Q1 Heon
Lee a, Young-Kwon Park b, Sun-Jae Kim c, Byung-Hoon Kim d, Sang-Chul Jung a,*
a
Department of Environmental Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeonnam 540-950, Republic of Korea School of Environmental Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 130-743, Republic of Korea c Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 143-747, Republic of Korea d Department of Dental Materials, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 501-759, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 July 2015 Received in revised form 20 August 2015 Accepted 31 August 2015 Available online xxx
Liquid phase plasma (LPP) method was used to synthesize a photocatalyst that can respond to visible light by precipitating cobalt oxide nanoparticles on the surface of TiO2 powders. Uniform precipitation of cobalt oxide nanoparticles on the surface of TiO2 powders was observed, resulting in successful synthesis of Co oxide nanoparticles on TiO2 photocatalysts (CTP). The CTP synthesized in this study showed a photocatalytic activity under visible blue light. However, its photocatalytic activity was lower than that of bare TiO2 under UV light because the cobalt oxide precipitate acted as crystal defects deteriorating the catalytic activity. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Titanium dioxide Cobalt oxide nanoparticle Energy band gap Liquid phase plasma Photocatalytic activity
9 10
Introduction
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Titanium dioxide (TiO2) is a cheap metal oxide with superior photocatalytic properties and chemical durability. Since its photocatalytic properties were discovered in 1972 by FujishimaHonda research team, TiO2, which had been used exclusively as white pigment before, has been widely used as basis materials for manufacturing sunscreen, photocatalyst, and solar cell [1]. Because of relatively large band gap of this compound (e.g. 3.2 eV for anatase), it has to absorb ultraviolet (UV) rays to exert photocatalytic activity or to act as solar cell. However, the solar energy that TiO2 can utilize is very limited for two reasons. Firstly, the portion of UV energy in the total solar energy is less than 5%. Secondly, all the UV energy is not used because the quantum efficiency of TiO2 is low. This hinders wider use of TiO2 in solar energy applications. Efforts have been made to develop new photocatalysts that can use visible and near-infrared lights, which account for more than 60% of total solar energy, with high quantum efficiency. The simplest way to realize this is to alter TiO2 so that it can utilize visible and near-infrared with high quantum efficiency [2–10].
* Corresponding author. Tel.: +82 61 750 3814; fax: +82 61 750 3810. E-mail address: [email protected] (S.-C. Jung).
A liquid-phase plasma (LPP) method using glow discharge in liquid was recently suggested as a promising method to synthesize metal nanoparticles [11,12]. The strength of the LPP method is very high reaction rate due to activated species and radicals produced under high pressure. Because of the reaction field with an excited energy state formed by LPP, the addition of reducing agents is not required in an LPP process. In this study, we evaluated a new material, Co oxide nanoparticles on TiO2 powder, to reduce the large band gap of pure TiO2 powder (P25). To investigate the structural effect of the catalyst on photocatalysis, Co oxide nanoparticles on TiO2 photocatalysts (hereafter CTP) with anatase and rutile structures were prepared using an LPP method. This method offers cobalt nanoparticles impregnation in a single-step process. The photocatalytic activity of CTP was investigated with decomposition of dye in its aqueous solution.
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Experimental
46
Materials
47
The titanium dioxide used in this study was Degussa P25 (ca. 80% anatase, 20% rutile, with a BET surface area of 50 m2/g and an average particle size of 25 nm). All the chemicals used in this work were of reagent grade and used without any further purification.
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http://dx.doi.org/10.1016/j.jiec.2015.08.025 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: H. Lee, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.08.025
G Model
JIEC 2630 1–5 H. Lee et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
2
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Cobalt chloride hexahydrate (CoCl26H2O, Daejung Chemicals & metals Co., Ltd) was used as the precursor to produce cobalt nanoparticle using the LPP method. Cetyltrimethylammonium bromide (CTAB, CH3(CH2)15N(CH3)3Br, Daejung Chemicals & metals Co., Ltd) was used as the dispersant to enhance the dispersion of and to avoid the coagulation between cobalt particles produced in aqueous solution during the LPP reaction. Ultrapure water (Daejung Chemical & metals Co. Ltd.) was used as the solvent for the LPP reaction and the decomposition reaction. Acid orange 7 (AO7, C16H11N2NaO4S, Aldrich Co. Ltd.) was used as the model pollutant to decompose in the evaluation of photocatalytic activity of the synthesized CTP.
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Apparatus
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CTP was prepared using the LPP apparatus. Similar apparatus was used in our previous study to generate nanoparticles dispersed in an aqueous solution using the LPP process [13]. Detailed information of the experimental setup can be found in that paper [13]. Fig. 1 shows the schematic of the experimental apparatus used in this study to evaluate the photocatalytic activity of CTP. An LED light module was prepared by installing 100 UV-LEDs (Nichia corp., NSPU510CS) and 100 Blue-LEDs (Nichia corp., NSPB510BS) with regular intervals inside a cylindrical plastic pipe with an inner diameter of 45 mm and a height of 120 mm. A light control box was used to control the intensity of radiation supplied to the decomposition reactor by switching onoff the LED lamps of the LED light module. The decomposition reaction was carried out by circulating the AO7 solution with dispersed CTP between a reactant tank and a quartz reactor (K = 22 mm, height = 220 mm) with a constant flow rate using a metering pump.
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Preparation of CTP
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CTP was generated by the LPP process. The LPP is defined as a discharge in aqueous solution, stabilized by the exchange of electrons and ions between gas and liquid phases. This liquid plasma generates a number of highly active species in the aqueous solution. The reactants of the desired reaction are added into the solution and they react with the active species, e.g. hydrogen, oxygen, and hydroxyl radicals, at the interface of gas and liquid phases. The Co nanoparticle production using the LPP method via the reduction of metal ions and hydrogen radicals was found to be an effective process [14]. An LPP system with a high frequency bipolar pulse-type power supply (Nano technology lnc., NTI-1000 W) was used for the reduction of cobalt ions dissociated from cobalt chloride. The
Fig. 1. Schematic diagram of photocatalytic degradation experimental apparatus with LED light.
power supply was operated under a pressure of 250 V, a frequency of 30 kHz, and a pulse width of 5 mm. Electric power was supplied into the double tube type LPP reactor through tungsten electrodes installed in both sides of the reactor with a distance between them of 1.0 mm. In order to synthesize CTP using the LPP method, 5 mM of CoCl26H2O and 2.5 mM of CTAB were dissolved in 250 mL of ultrapure water and then 0.5 g of TiO2 powders (P25) were added. The mixture was stirred for 10 min for uniform dispersion and introduced into the LPP reactor. CTP was produced by operating the LPP process for 90 min to precipitate cobalt nanoparticles on the surface of TiO2 powders. After the completion of the LPP reaction, the reaction solution underwent centrifugation (4000 rpm) and washing repeated five times each to remove remaining chemicals and CTAB. The precipitate was dried for 48 h in a vaccum oven maintained at 353 K. The elemental composition of the synthesized CTP was analyzed using a Field Emission Scanning Electron Microscope (FESEM, JSM-7100F, and JEOL). A High Resolution Field Emission Transmission Electron Microscope (HR-FETEM, JEM2100F, JEOL) was used to examine the elemental distribution of the particle surface. X-photoelectron spectroscopy (XPS, Multilab 2000 system, SSK) was used to analyze the chemical composition of CTP. UVvis spectroscopy (Avantes AvaLight-DHS) was used to measure the band gap energy of CTP.
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Evaluation of photocatalytic activity
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Experiments were performed to evaluate the photocatalytic degradation of AO7 by using a re-circulation-type reaction system with UV or visible light LED lamps. CTP was mixed with 600 mL of AO7 solution with an AO7 concentration of 1.38 102 mg/L. This reactant solution was circulated between the reactant tank and the quartz reactor installed inside the LED light module using a metering pump to allow photocatalytic decomposition of AO7. Samples were collected from the reactant solution with a uniform time interval. After centrifugation, the samples were passed through a 0.2 mm syringe filter to remove CTP. The AO7 concentration of the samples was determined by measuring their absorptivity at l max = 485 nm using a UVvis spectrometer (UV1601, Shimadzu). The experimental conditions are as follows: reaction temperature of 293 K, pH of 6.17, and reaction time of 350 min.
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Results and discussion
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Characteristics of the CTP
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Fig. 2 shows the energy-dispersive X-ray spectroscopy (EDS) spectrum for the CTP synthesized using the LPP method. The two elements comprising TiO2 powder (P25), titanium, and oxygen
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Fig. 2. EDS spectrum of CTP prepared by LPP method.
Please cite this article in press as: H. Lee, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.08.025
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JIEC 2630 1–5 H. Lee et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Fig. 3. HR-TEM image of (a) CTP powder and mapped element of CTP powder prepared by LPP method: (b) titanium, (c) oxygen, and (d) cobalt.
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were observed at 4.50, 4.93 and at 0.52 keV, respectively. The cobalt peak representing cobalt nanoparticles was observed at 6.92 keV. The mass fractions of the elements determined from the EDS spectrum were Ti:O:Co = 59.20:40.16:0.64 (%), while the atomic fractions were Ti:O:Co = 32.90:66.81:0.29 (%). The tungsten peak appearing in the EDS spectrum is due to the tungsten electrodes used in the LPP process and the bromine peak is due to the dispersant CTAB. The EDS analysis result shows that cobalt nanoparticles were precipitated on the surface of TiO2 powders during the LPP process resulting in successful synthesis of CTP. The surface of CTP powders was observed using HR-TEM. Fig. 3 shows the mapping images of each element (titanium, oxygen, and cobalt). Fig. 3a is the HR-TEM image of CTP powders. The particle size was 2550 nm, which is in good agreement with the size of TiO2 particles (P25) used in this study. Fig. 3b and c show the mapping images of titanium and oxygen, appearing as red and white dots, respectively. Fig. 3d shows the mapping image of cobalt (green). The locations of the green dots (d) agree well with those of
3
red (b) and white (c) dots, indicating that cobalt nanoparticles were dispersed well onto the TiO2 powders. This result shows that cobalt nanoparticles were precipitated uniformly on the surface of TiO2 powders, resulting in successful synthesis of CTP. X-photoelectron spectroscopy was used to examine the chemical bond structure of CTP. Fig. 4 shows the high-resolution XPS spectra for titanium (a) and cobalt (b). In Fig. 4a, the titanium absorption peaks were observed at the binding energy levels of 458.6 and 464.2 eV, representing Ti 2p3/2 and Ti 2p1/2 peaks, respectively, exhibited by Ti4+ that comprises TiO2 [15]. In Fig. 4b, the cobalt absorption peaks were observed at 780.4 and 796.1 eV, representing Co 2p3/2 and Co 2p1/2 peaks, respectively. Metallic cobalt (Co0) peak is generally observed at 778 eV but it was not observed for the synthesized CTP in this study. Cobalt oxide peak is known to be observed at 780.0780.2 eV [16]. The energy splitting DE (difference between 2p3/2 and 2p1/2) of CTP was determined to be 15.7 eV, indicating these were peaks due to Co2+ [17]. In addition, a shoulder peak observed at high energy side and the peak shift to a binding energy slightly higher than CoO imply the existence of CoTiO3 [18]. These XPS analysis results indicate that the nanoparticles precipitated on TiO2 powder surface by the LPP process are mostly cobalt oxide, whereas a part of them are CoTiO3 produced by combination of TiO2 and cobalt in a powerful plasma field. The optical absorbance of TiO2 (P25) and CTP measured using UVvis spectroscopy and calculated using the Kubelka-Munk function is shown in Fig. 5. Kubelka-Munk function is known to be able to determine the optical energy band gap easily using the h slope estimated from (F(R1)hv)(1/ ) calculated from the optical absorbance [19]. Bare TiO2 is commercial P25, whose energy band gap is known to be 3.20 eV. The energy band gap of P25 measured in this study was 3.19 eV, being in good agreement. The energy band gap of the CTP synthesized in this study was 2.97 eV, which is smaller than that of P25. It is believed that the cobalt oxide nanoparticles precipitated on the surface of TiO2 reduced the band gap [20]. Therefore, the CTP synthesized in this study with a small band gap can act as a photocatalyst at visible light range.
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Photocatalytic activity of CTP
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The results of the AO7 decomposition experiment carried out to evaluate the photocatalytic activity of CTP synthesized by the LPP process are shown in Fig. 6. Because an objective of this study was to synthesize a photocatalyst that can response to visible light, the light supply was produced using visible LED to examine whether the CTP could work with visible light. The LED light module was prepared using 100 Nichia UV LED lamps (peak wavelength 375 nm, power dissipation 80 mW) and 100 blue LED
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Fig. 4. Narrow-range XPS spectra of (a) Ti element and (b) Co element of CTP prepared by LPP method.
Please cite this article in press as: H. Lee, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.08.025
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H. Lee et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Fig. 7. Photocatalytic degradation of AO7 at various blue light intensities using CTP. Fig. 5. Optical absorbance for the bare TiO2 and CTP.
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lamps (peak wavelength 465 nm, power dissipation 120 mW). In a preliminary experiment green LED, amber LED, and red LED were used but CTP was not activated by these LEDs. A reactant solution was prepared by dispersing 1 mg/mL CTP in aqueous AO7 solution. For comparison purpose, another reactant solution was preparing using bare TiO2 (P-25) instead of CTP. When UVLED lamp was used as the light source, both CTP and bare-TiO2 showed photocatalytic activity for the decomposition of AO7 but the activity of bare-TiO2 was higher. When blue-LED lamp was used, on the other hand, bare-TiO2 showed little photocatalytic activity, whereas CTP showed a certain activity, though not high, which was attributed to the reduced band gap by precipitated cobalt oxide. Cobalt oxide precipitated on TiO2 crystal acts as defects, promoting electron-hole recombination and hence reducing active species participating the decomposition reaction of AO7 [21,22]. This resulted in a lower catalytic activity of CTP than that of bare-TiO2 when UVLED lamp was used. In this study, the light control box was used to control the intensity of the radiation onto the decomposition reactor by switching on/off the LED lamps of the LED light module. The AO7 decomposition results obtained using CTP with different blue light radiation levels are compared in Fig. 7. When the blue light radiation levels were 4800, 7200, 9600, and 12,000 mW, the AO7 decomposition rate coefficient was determined to be 1.9125 104, 3.2762 104, 5.2034 104, and 7.8653 104 min1, respectively. The proportional increase of the decomposition rate with
Fig. 6. Decolorization of acid orange 7 using various photocatalyst with different LED lamps.
Fig. 8. Photocatalytic degradation of AO7 at various initial dosage of CTP on blueLED lamp.
increasing radiation level testifies the excitement of photocatalyst by light source. Therefore, this result demonstrates the excitement of CTP synthesized using the LPP method by blue light. Fig. 8 shows the AO7 decomposition results obtained with four different CTP dose levels (0.15, 0.30, 0.45, and 0.60 g in 600 mL of aqueous AO7 solution) with the blue light radiation of 12,000 mW. The decomposition rate was very low when 0.15 g of CTP was used. As the CTP dose level was increased, however, the AO7 decomposition rate increased proportionally. The decomposition rate coefficient k was determined to be 7.3100 105, 2.1299 104, 4.1721 104, and 7.8653 104 min1 at the above-mentioned CTP dose levels.
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Conclusions
240
The conclusions obtained in this study are summarized as the following:
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The EDS and XPS analyses of the CTP synthesized using the LPP method showed that the cobalt nanoparticles precipitated on the surface of TiO2 powders are mostly cobalt oxide with some CoTiO3, which was produced by the combination of TiO2 and cobalt during the LPP process. HR-TEM mapping images showed uniform precipitation of cobalt oxide nanoparticles produced using the LPP method on the surface of TiO2 powders resulting in successful synthesis of CTP.
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Please cite this article in press as: H. Lee, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.08.025
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JIEC 2630 1–5 H. Lee et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
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The energy band gap of the CTP synthesized using the LPP method measured using the Kubelka-Munk function method was 3.03 eV, which was smaller than that of P25, 3.13 eV. The smaller band gap of CTP was attributed to the cobalt oxide nanoparticles precipitated on the surface of TiO2 powders. Although the CTP synthesized using the LPP method exhibited photocatalytic activity with visible blue light, its photocatalytic activity was lower than that of bare TiO2 under UV radiation because precipitated cobalt oxide acted as crystal defects deteriorating catalytic activity.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Acknowledgment This research was supported by Basic Science Research Program Q2 through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education (2013R1A1A2A10004797). References
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W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan, Z. Zou, Appl. Catal. 69 (2007) 138. Y. Taga, Thin Solid Films 517 (2009) 3167. S. Sato, R. Nakamura, S. Abe, Appl. Catal. A 284 (2005) 131. T. Umebayashi, T. Yamaki, H. Itoh, K. Asai, Appl. Phys. Lett. 81 (2002) 454. D. Chen, D. Yang, Q. Wang, Z. Jiang, Ind. Eng. Chem. Res. 45 (2006) 4110. D. Li, N. Ohashi, S. Hishita, T. Kolodiazhnyi, H. Haneda, J. Solid State Chem. 178 (2005) 3293. N. Vinothkumar, M. De, Int. J. Hydrogen Energ. 39 (2014) 11494. H. Lee, S.H. Park, S.C. Jung, J.J. Yun, S.J. Kim, D.H. Kim, J. Mater. Res. 28 (2013) 1105. H. Lee, S.H. Park, S.G. Seo, S.J. Kim, S.C. Kim, Y.K. Park, S.C. Jung, Curr. Nanosci. 10 (2014) 7. H. Lee, S.H. Park, S.J. Kim, Y.K. Park, B.J. Kim, K.H. An, S.J. Ki, S.C. Jung, Int. J. Hydrogen Energ. 40 (2015) 754. J. Hieda, N. Saito, O. Takai, J. Vac. Sci. Technol. A 26 (2008) 854. S. Yang, Y. Guo, N. Yan, D. Wu, H. He, Z. Qu, C. Yang, Q. Zhou, J. Jia, ACS Appl. Mater. Interfaces 3 (2011) 209. M.A. Barakat, G. Hayes, S.I. Shah, J. Nanosci. Nanotechnol. 5 (2005) 759. S. Thota, N. Narang, S. Nayak, S. Sambasivam, B.C. Choi, T. Sarkar, M.S. Andersson, R. Mathieu, M.S. Seehra, J. Phys. Condens. Matter 27 (2015) 166001, 18. M. Voß, D. Borgmann, G. Wedler, J. Catal. 212 (2002) 10. H.K. Ku, H.J. Oh, K.J. Noh, S.C. Jung, K.S. Park, W.J. Lee, S.J. Kim, J. Nanosci. Nanotechnol. 11 (2011) 7269. K. Esquivel, G. Ma, J. Garcia, F.J. Rodriguez, M.V. Gonzalez, L. Escobar-Alarcon, L. Oriz-Frade, L.A. Godinez, J. Nanopart. Res. 13 (2011) 3313. E.B. Gracien, J. Shen, X.R. Sun, D. Liu, M. Li, S.D. Yao, J. Sun, Thin Solid Films 515 (2007) 5287. A.O. Roberto, J.B. Jaime, J. Mex. Chem. Soc. 2010 (2010) 164.
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JIEC 2630 1–5 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7 8
Titanium dioxide modification with cobalt oxide nanoparticles for photocatalysis Q1 Heon
Lee a, Young-Kwon Park b, Sun-Jae Kim c, Byung-Hoon Kim d, Sang-Chul Jung a,*
a
Department of Environmental Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeonnam 540-950, Republic of Korea School of Environmental Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 130-743, Republic of Korea c Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 143-747, Republic of Korea d Department of Dental Materials, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 501-759, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 July 2015 Received in revised form 20 August 2015 Accepted 31 August 2015 Available online xxx
Liquid phase plasma (LPP) method was used to synthesize a photocatalyst that can respond to visible light by precipitating cobalt oxide nanoparticles on the surface of TiO2 powders. Uniform precipitation of cobalt oxide nanoparticles on the surface of TiO2 powders was observed, resulting in successful synthesis of Co oxide nanoparticles on TiO2 photocatalysts (CTP). The CTP synthesized in this study showed a photocatalytic activity under visible blue light. However, its photocatalytic activity was lower than that of bare TiO2 under UV light because the cobalt oxide precipitate acted as crystal defects deteriorating the catalytic activity. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Titanium dioxide Cobalt oxide nanoparticle Energy band gap Liquid phase plasma Photocatalytic activity
9 10
Introduction
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Titanium dioxide (TiO2) is a cheap metal oxide with superior photocatalytic properties and chemical durability. Since its photocatalytic properties were discovered in 1972 by FujishimaHonda research team, TiO2, which had been used exclusively as white pigment before, has been widely used as basis materials for manufacturing sunscreen, photocatalyst, and solar cell [1]. Because of relatively large band gap of this compound (e.g. 3.2 eV for anatase), it has to absorb ultraviolet (UV) rays to exert photocatalytic activity or to act as solar cell. However, the solar energy that TiO2 can utilize is very limited for two reasons. Firstly, the portion of UV energy in the total solar energy is less than 5%. Secondly, all the UV energy is not used because the quantum efficiency of TiO2 is low. This hinders wider use of TiO2 in solar energy applications. Efforts have been made to develop new photocatalysts that can use visible and near-infrared lights, which account for more than 60% of total solar energy, with high quantum efficiency. The simplest way to realize this is to alter TiO2 so that it can utilize visible and near-infrared with high quantum efficiency [2–10].
* Corresponding author. Tel.: +82 61 750 3814; fax: +82 61 750 3810. E-mail address: [email protected] (S.-C. Jung).
A liquid-phase plasma (LPP) method using glow discharge in liquid was recently suggested as a promising method to synthesize metal nanoparticles [11,12]. The strength of the LPP method is very high reaction rate due to activated species and radicals produced under high pressure. Because of the reaction field with an excited energy state formed by LPP, the addition of reducing agents is not required in an LPP process. In this study, we evaluated a new material, Co oxide nanoparticles on TiO2 powder, to reduce the large band gap of pure TiO2 powder (P25). To investigate the structural effect of the catalyst on photocatalysis, Co oxide nanoparticles on TiO2 photocatalysts (hereafter CTP) with anatase and rutile structures were prepared using an LPP method. This method offers cobalt nanoparticles impregnation in a single-step process. The photocatalytic activity of CTP was investigated with decomposition of dye in its aqueous solution.
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
Experimental
46
Materials
47
The titanium dioxide used in this study was Degussa P25 (ca. 80% anatase, 20% rutile, with a BET surface area of 50 m2/g and an average particle size of 25 nm). All the chemicals used in this work were of reagent grade and used without any further purification.
48 49 50 51
http://dx.doi.org/10.1016/j.jiec.2015.08.025 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: H. Lee, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.08.025
G Model
JIEC 2630 1–5 H. Lee et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
2
52 53 54 55 56 57 58 59 60 61 62 63
Cobalt chloride hexahydrate (CoCl26H2O, Daejung Chemicals & metals Co., Ltd) was used as the precursor to produce cobalt nanoparticle using the LPP method. Cetyltrimethylammonium bromide (CTAB, CH3(CH2)15N(CH3)3Br, Daejung Chemicals & metals Co., Ltd) was used as the dispersant to enhance the dispersion of and to avoid the coagulation between cobalt particles produced in aqueous solution during the LPP reaction. Ultrapure water (Daejung Chemical & metals Co. Ltd.) was used as the solvent for the LPP reaction and the decomposition reaction. Acid orange 7 (AO7, C16H11N2NaO4S, Aldrich Co. Ltd.) was used as the model pollutant to decompose in the evaluation of photocatalytic activity of the synthesized CTP.
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Apparatus
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CTP was prepared using the LPP apparatus. Similar apparatus was used in our previous study to generate nanoparticles dispersed in an aqueous solution using the LPP process [13]. Detailed information of the experimental setup can be found in that paper [13]. Fig. 1 shows the schematic of the experimental apparatus used in this study to evaluate the photocatalytic activity of CTP. An LED light module was prepared by installing 100 UV-LEDs (Nichia corp., NSPU510CS) and 100 Blue-LEDs (Nichia corp., NSPB510BS) with regular intervals inside a cylindrical plastic pipe with an inner diameter of 45 mm and a height of 120 mm. A light control box was used to control the intensity of radiation supplied to the decomposition reactor by switching onoff the LED lamps of the LED light module. The decomposition reaction was carried out by circulating the AO7 solution with dispersed CTP between a reactant tank and a quartz reactor (K = 22 mm, height = 220 mm) with a constant flow rate using a metering pump.
81
Preparation of CTP
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CTP was generated by the LPP process. The LPP is defined as a discharge in aqueous solution, stabilized by the exchange of electrons and ions between gas and liquid phases. This liquid plasma generates a number of highly active species in the aqueous solution. The reactants of the desired reaction are added into the solution and they react with the active species, e.g. hydrogen, oxygen, and hydroxyl radicals, at the interface of gas and liquid phases. The Co nanoparticle production using the LPP method via the reduction of metal ions and hydrogen radicals was found to be an effective process [14]. An LPP system with a high frequency bipolar pulse-type power supply (Nano technology lnc., NTI-1000 W) was used for the reduction of cobalt ions dissociated from cobalt chloride. The
Fig. 1. Schematic diagram of photocatalytic degradation experimental apparatus with LED light.
power supply was operated under a pressure of 250 V, a frequency of 30 kHz, and a pulse width of 5 mm. Electric power was supplied into the double tube type LPP reactor through tungsten electrodes installed in both sides of the reactor with a distance between them of 1.0 mm. In order to synthesize CTP using the LPP method, 5 mM of CoCl26H2O and 2.5 mM of CTAB were dissolved in 250 mL of ultrapure water and then 0.5 g of TiO2 powders (P25) were added. The mixture was stirred for 10 min for uniform dispersion and introduced into the LPP reactor. CTP was produced by operating the LPP process for 90 min to precipitate cobalt nanoparticles on the surface of TiO2 powders. After the completion of the LPP reaction, the reaction solution underwent centrifugation (4000 rpm) and washing repeated five times each to remove remaining chemicals and CTAB. The precipitate was dried for 48 h in a vaccum oven maintained at 353 K. The elemental composition of the synthesized CTP was analyzed using a Field Emission Scanning Electron Microscope (FESEM, JSM-7100F, and JEOL). A High Resolution Field Emission Transmission Electron Microscope (HR-FETEM, JEM2100F, JEOL) was used to examine the elemental distribution of the particle surface. X-photoelectron spectroscopy (XPS, Multilab 2000 system, SSK) was used to analyze the chemical composition of CTP. UVvis spectroscopy (Avantes AvaLight-DHS) was used to measure the band gap energy of CTP.
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Evaluation of photocatalytic activity
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Experiments were performed to evaluate the photocatalytic degradation of AO7 by using a re-circulation-type reaction system with UV or visible light LED lamps. CTP was mixed with 600 mL of AO7 solution with an AO7 concentration of 1.38 102 mg/L. This reactant solution was circulated between the reactant tank and the quartz reactor installed inside the LED light module using a metering pump to allow photocatalytic decomposition of AO7. Samples were collected from the reactant solution with a uniform time interval. After centrifugation, the samples were passed through a 0.2 mm syringe filter to remove CTP. The AO7 concentration of the samples was determined by measuring their absorptivity at l max = 485 nm using a UVvis spectrometer (UV1601, Shimadzu). The experimental conditions are as follows: reaction temperature of 293 K, pH of 6.17, and reaction time of 350 min.
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Results and discussion
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Characteristics of the CTP
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Fig. 2 shows the energy-dispersive X-ray spectroscopy (EDS) spectrum for the CTP synthesized using the LPP method. The two elements comprising TiO2 powder (P25), titanium, and oxygen
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Fig. 2. EDS spectrum of CTP prepared by LPP method.
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Fig. 3. HR-TEM image of (a) CTP powder and mapped element of CTP powder prepared by LPP method: (b) titanium, (c) oxygen, and (d) cobalt.
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were observed at 4.50, 4.93 and at 0.52 keV, respectively. The cobalt peak representing cobalt nanoparticles was observed at 6.92 keV. The mass fractions of the elements determined from the EDS spectrum were Ti:O:Co = 59.20:40.16:0.64 (%), while the atomic fractions were Ti:O:Co = 32.90:66.81:0.29 (%). The tungsten peak appearing in the EDS spectrum is due to the tungsten electrodes used in the LPP process and the bromine peak is due to the dispersant CTAB. The EDS analysis result shows that cobalt nanoparticles were precipitated on the surface of TiO2 powders during the LPP process resulting in successful synthesis of CTP. The surface of CTP powders was observed using HR-TEM. Fig. 3 shows the mapping images of each element (titanium, oxygen, and cobalt). Fig. 3a is the HR-TEM image of CTP powders. The particle size was 2550 nm, which is in good agreement with the size of TiO2 particles (P25) used in this study. Fig. 3b and c show the mapping images of titanium and oxygen, appearing as red and white dots, respectively. Fig. 3d shows the mapping image of cobalt (green). The locations of the green dots (d) agree well with those of
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red (b) and white (c) dots, indicating that cobalt nanoparticles were dispersed well onto the TiO2 powders. This result shows that cobalt nanoparticles were precipitated uniformly on the surface of TiO2 powders, resulting in successful synthesis of CTP. X-photoelectron spectroscopy was used to examine the chemical bond structure of CTP. Fig. 4 shows the high-resolution XPS spectra for titanium (a) and cobalt (b). In Fig. 4a, the titanium absorption peaks were observed at the binding energy levels of 458.6 and 464.2 eV, representing Ti 2p3/2 and Ti 2p1/2 peaks, respectively, exhibited by Ti4+ that comprises TiO2 [15]. In Fig. 4b, the cobalt absorption peaks were observed at 780.4 and 796.1 eV, representing Co 2p3/2 and Co 2p1/2 peaks, respectively. Metallic cobalt (Co0) peak is generally observed at 778 eV but it was not observed for the synthesized CTP in this study. Cobalt oxide peak is known to be observed at 780.0780.2 eV [16]. The energy splitting DE (difference between 2p3/2 and 2p1/2) of CTP was determined to be 15.7 eV, indicating these were peaks due to Co2+ [17]. In addition, a shoulder peak observed at high energy side and the peak shift to a binding energy slightly higher than CoO imply the existence of CoTiO3 [18]. These XPS analysis results indicate that the nanoparticles precipitated on TiO2 powder surface by the LPP process are mostly cobalt oxide, whereas a part of them are CoTiO3 produced by combination of TiO2 and cobalt in a powerful plasma field. The optical absorbance of TiO2 (P25) and CTP measured using UVvis spectroscopy and calculated using the Kubelka-Munk function is shown in Fig. 5. Kubelka-Munk function is known to be able to determine the optical energy band gap easily using the h slope estimated from (F(R1)hv)(1/ ) calculated from the optical absorbance [19]. Bare TiO2 is commercial P25, whose energy band gap is known to be 3.20 eV. The energy band gap of P25 measured in this study was 3.19 eV, being in good agreement. The energy band gap of the CTP synthesized in this study was 2.97 eV, which is smaller than that of P25. It is believed that the cobalt oxide nanoparticles precipitated on the surface of TiO2 reduced the band gap [20]. Therefore, the CTP synthesized in this study with a small band gap can act as a photocatalyst at visible light range.
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Photocatalytic activity of CTP
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The results of the AO7 decomposition experiment carried out to evaluate the photocatalytic activity of CTP synthesized by the LPP process are shown in Fig. 6. Because an objective of this study was to synthesize a photocatalyst that can response to visible light, the light supply was produced using visible LED to examine whether the CTP could work with visible light. The LED light module was prepared using 100 Nichia UV LED lamps (peak wavelength 375 nm, power dissipation 80 mW) and 100 blue LED
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Fig. 4. Narrow-range XPS spectra of (a) Ti element and (b) Co element of CTP prepared by LPP method.
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Fig. 7. Photocatalytic degradation of AO7 at various blue light intensities using CTP. Fig. 5. Optical absorbance for the bare TiO2 and CTP.
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lamps (peak wavelength 465 nm, power dissipation 120 mW). In a preliminary experiment green LED, amber LED, and red LED were used but CTP was not activated by these LEDs. A reactant solution was prepared by dispersing 1 mg/mL CTP in aqueous AO7 solution. For comparison purpose, another reactant solution was preparing using bare TiO2 (P-25) instead of CTP. When UVLED lamp was used as the light source, both CTP and bare-TiO2 showed photocatalytic activity for the decomposition of AO7 but the activity of bare-TiO2 was higher. When blue-LED lamp was used, on the other hand, bare-TiO2 showed little photocatalytic activity, whereas CTP showed a certain activity, though not high, which was attributed to the reduced band gap by precipitated cobalt oxide. Cobalt oxide precipitated on TiO2 crystal acts as defects, promoting electron-hole recombination and hence reducing active species participating the decomposition reaction of AO7 [21,22]. This resulted in a lower catalytic activity of CTP than that of bare-TiO2 when UVLED lamp was used. In this study, the light control box was used to control the intensity of the radiation onto the decomposition reactor by switching on/off the LED lamps of the LED light module. The AO7 decomposition results obtained using CTP with different blue light radiation levels are compared in Fig. 7. When the blue light radiation levels were 4800, 7200, 9600, and 12,000 mW, the AO7 decomposition rate coefficient was determined to be 1.9125 104, 3.2762 104, 5.2034 104, and 7.8653 104 min1, respectively. The proportional increase of the decomposition rate with
Fig. 6. Decolorization of acid orange 7 using various photocatalyst with different LED lamps.
Fig. 8. Photocatalytic degradation of AO7 at various initial dosage of CTP on blueLED lamp.
increasing radiation level testifies the excitement of photocatalyst by light source. Therefore, this result demonstrates the excitement of CTP synthesized using the LPP method by blue light. Fig. 8 shows the AO7 decomposition results obtained with four different CTP dose levels (0.15, 0.30, 0.45, and 0.60 g in 600 mL of aqueous AO7 solution) with the blue light radiation of 12,000 mW. The decomposition rate was very low when 0.15 g of CTP was used. As the CTP dose level was increased, however, the AO7 decomposition rate increased proportionally. The decomposition rate coefficient k was determined to be 7.3100 105, 2.1299 104, 4.1721 104, and 7.8653 104 min1 at the above-mentioned CTP dose levels.
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Conclusions
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The conclusions obtained in this study are summarized as the following:
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The EDS and XPS analyses of the CTP synthesized using the LPP method showed that the cobalt nanoparticles precipitated on the surface of TiO2 powders are mostly cobalt oxide with some CoTiO3, which was produced by the combination of TiO2 and cobalt during the LPP process. HR-TEM mapping images showed uniform precipitation of cobalt oxide nanoparticles produced using the LPP method on the surface of TiO2 powders resulting in successful synthesis of CTP.
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The energy band gap of the CTP synthesized using the LPP method measured using the Kubelka-Munk function method was 3.03 eV, which was smaller than that of P25, 3.13 eV. The smaller band gap of CTP was attributed to the cobalt oxide nanoparticles precipitated on the surface of TiO2 powders. Although the CTP synthesized using the LPP method exhibited photocatalytic activity with visible blue light, its photocatalytic activity was lower than that of bare TiO2 under UV radiation because precipitated cobalt oxide acted as crystal defects deteriorating catalytic activity.
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Acknowledgment This research was supported by Basic Science Research Program Q2 through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education (2013R1A1A2A10004797). References
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