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Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light
G Model JIEC 3853 No. of Pages 6
Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light Seung Hwa Yooa,1, Sung-In Leea,2 , Han-Ik Johb , Sungho Leea,c,* a Carbon Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324, Republic of Korea b Department of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea c Department of Nano Material Engineering, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
A R T I C L E I N F O
Article history: Received 20 November 2017 Received in revised form 13 January 2018 Accepted 27 January 2018 Available online xxx Keywords: Titanium dioxide Cadmium selenide Carbon nanofibers Photocatalyst Electrospinning
A B S T R A C T
A highly effective photocatalyst based on CdSe quantum dots (QDs) and TiO2 nanoparticles welldispersed on carbon nanofibers (CNFs), named as CdSe/TiO2/CNF was synthesized by simple successive ionic layer adsorption and reaction (SILAR) method on TiO2/CNF. It was verified by spectroscopic analysis that CdSe QDs were successfully synthesized and some aggregations and oxide phases existed in CdSe/ TiO2/CNF. The photocatalytic activity of CdSe/TiO2/CNF was demonstrated for decomposition of methylene (MB) aqueous solutions. The tests were performed for different MB concentrations and light irradiation. It was found that CdSe/TiO2/CNF showed high photocatalytic activity under visible light irradiation and even high MB concentrations. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction Photocatalysts work under lights, which are absorbed to provide electron-hole pairs with free radicals by reactions [1–3]. One of the mostly used photo-catalysts is titanium dioxide (TiO2) because of its high catalytic performance to create hydroxyl radicals in water, leading to decomposition of organics by photooxidation [4,5]. It is well known that TiO2 absorbs only ultraviolet (UV) light due to a large band gap energy (3.2 eV), which restrict more various and efficient usage under sunlight because sunlight consists of infrared of 49%, visible of 46% and UV light of 5% [6,7]. Therefore, maximizing photocatalytic activity by visible light has been studied with semiconducting inorganics, metal oxides or non-metal element doping which generate electron-hole pairs when visible light is absorbed [8–16]. Cadmium selenide (CdSe) has been used as semiconductor, having a band gap of 1.75 eV, which is much narrower with a higher conduction band-edge compared to TiO2 [8,17]. These render CdSe
to be an excellent candidate for transferring photo-generated electrons from CdSe to TiO2 under visible light in CdSe decorated TiO2, which enhance photo-catalytic activities. Wang et al. reported that CdSe quantum dots (QD) have higher energy level difference between each CdSe QD and TiO2 conduction bandedges, compared to bulk CdSe [18,19]. Therefore, CdSe QD is more efficient in charge carrier separation by much faster electron transferring. There are several representative routes to synthesize CdSe-QD on TiO2 such as ligand exchange [20], linker-assisted hybridization process [17,21], chemical bath deposition [22], and electrophoresis [23]. However, these methods require chemical synthesis to obtain CdSe QD using cadmium precursor and selenium powder and further steps to decorate QD on TiO2. In this study, CdSe was simply deposited on TiO2 supported on carbon nanofibers (CNFs), named as CdSe/TiO2/CNF, and was utilized as a photocatalyst to decompose aqueous methylene blue as a model pollutant.
* Corresponding author at: Carbon Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324, Republic of Korea. E-mail address: [email protected] (S. Lee). 1 Current address: Biomedical Manufacturing Technology Center, Korea Institute of Industrial Technology, 59 Yangho-gil, Yeongcheon-si, Gyeongsangbuk-do 38822, Republic of Korea. 2 Current address: LG Chem R&D Campus Daejeon, 188 Munji-ro, Yuseong-gu, Daejeon 34122, Republic of Korea. https://doi.org/10.1016/j.jiec.2018.01.034 1226-086X/© 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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Experimental Materials Polyacrylonitrile (PAN), titanium isopropoxide, cadmium nitrate tetrahydrate, selenium dioxide, sodium borohydride, and dimethylformamide (DMF) were purchased from the Sigma Aldrich Co. All chemicals were analytical grade and used without purification. Preparation of TiO2/CNF Solution preparation and electrospinning TiO2 precursor/PAN nanofiber mat 8 w% PAN solution was prepared by dissolving 4 g of PAN in 46 g of DMF for 2 h at 60 C. Separately, 4 g of titanium isopropoxide was mixed with 0.3 g of acetic acid dropwise, and the mixture was slowly added to as-prepared PAN solution. The final solution was rigorously stirred until it was completely dissolved. Heat treatment to convert TiO2 precursor/PAN to TiO2/CNF nanofiber mat The as-prepared polymeric nanofiber mat was stabilized followed by carbonization. For stabilization, the polymeric nanofiber mat was heated in air at 250 C for 120 min in a forced convection oven. Then, the stabilized mats were carbonized at 1600 C under N2 atmosphere. The temperature was raised from 25 to 1600 C at 5 C/min rate with no holding time at the final temperature. Subsequently, the carbonized mats were activated in oxygen atmosphere with 60 min holding time at 500 C. Preparation of CdSe/TiO2/CNF CdSe/TiO2/CNF was prepared through a successive ionic layer adsorption and reaction (SILAR) method. The as-prepared TiO2/ CNF mat was dipped into an ethanol solution containing 0.03 M cadmium nitrate tetrahydrate for 30 s to allow Cd2+ to be absorbed; then, it was rinsed with ethanol. Subsequently, the sample was
dipped into an ethanol solution containing 0.03 M selenium dioxide and sodium borohydride for 30 s, where the pre-adsorbed Cd2+ reacted with Se2 to form CdSe. After finishing the SILAR method, the treated samples were grounded with an agate mortar to finally give a powder form of CdSe/TiO2/CNF. Photocatalytic decomposition test 0.1 g of CdSe/TiO2/CNF powder was added in 100 mL methylene blue (MB) solution of 50, 100, and 150 ppm initial concentration. The mixture of MB solution and CdSe/TiO2/CNF powder was stirred for 2 h to achieve adsorption-desorption equilibrium. A Xe lamp (US 66983 Arc Lamp Source 200–500 W, Newport.) with a 420 nm cut-off filter (GG-420, 200 Sq. Longpass Filter, Edmund optics.) was used for the photocatalytic decomposition tests by utilizing visible light. In case to utilize visible + UV light, the cut-off filter was eliminated to use the full spectrum of Xe lamp. At given time intervals (2 min), 3 mL of sample solutions were collected and CdSe/TiO2/CNF powder was removed by filtration. The residual concentration of MB for each time intervals were measured with an UV–vis spectrometer (V670, JASCO Inc.). Analysis of material properties The morphology of CdSe/TiO2/CNF was observed using a field emission scanning electron microscope (FE-SEM, NOVA Nano SEM 450) and a transmission electron microscope (TEM, Tecnai G2 F20, FEI). X-ray diffraction was carried out using a D/MAX2500 V PC Xray diffractometer (Rigaku, Japan) with monochromated Cu Ka as a source between 2u of 10 and 80 at a scan rate of 3 /min. X-ray photoelectron spectra were obtained (Thermo Scientific, K-alpha) using monochromated Al Ka (1486.6 eV) X-rays at a pressure less than 3 107 Torr. Results and discussion In our previous study, well-dispersed TiO2 nanoparticles on carbon nanofibers (CNFs) were prepared, and a significantly
Fig. 1. (a, c) SEM and TEM images of CdSe/TiO2/CNF powder. (b, d) Higher magnification of (a) and (c), respectively.
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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effective photocatalytic activity was observed under UV light [24]. To extend their performance under visible light, TiO2/CNF composites were manipulated with CdSe QD using a simple dip coating into solutions of Cd and Se precursors in sequence. To investigate the photocatalytic activity of resulting samples, degradation of methylene blue (MB) was carried out. We demonstrated highly effective degradation of MB in 2 min under visible light, which is the fastest performance reported in the open literature to the best of our knowledge (Table S1). TiO2/CNF composites were prepared by electrospinning of polyacrylonitrile and titanium isopropoxide mixture, stabilization, carbonization, and activation. Details are in the previous work [24]. Using successive ionic layer adsorption and reaction (SILAR) method, CdSe QDs were loaded on the surface of TiO2/CNF composites observed by scanning electron microscope (SEM) images in Fig. 1a and b. Even though TiO2 nanoparticles were welldispersed on the CNFs, CdSe QDs and their aggregation were found on TiO2 nanoparticles and the surface of CNFs, indicating that SILAR method could not render selective decoration of CdSe on TiO2. Transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDX) (Fig. 2) were conducted to evaluate morphology and composition of the prepared samples. In consistent with the SEM micrographs, 20–50 nm sized nanoparticles were well dispersed on carbon nanofibers, which were observed by TEM (Fig. 1c). Further higher magnification TEM observation revealed that these nanoparticles were consisted of several nanocrystals as depicted in Fig. 1d. Based on the lattice spacing of these crystals, it was verified that nanoparticles were consisted of TiO2 and CdSe (TiO2 (101), CdSe (311), and CdSe (220) planes were observable). EDX point mapping of the nanoparticle (the point 1 in Fig. 2a) showed existence of Ti, Cd, and Se along with C. The point mapping of the fiber (the point 3 in Fig. 2a) showed that Ti was absent compared to the nanoparticle, however, Cd, and Se elements were still detected. Therefore, it was conclusive that CdSe QDs were deposited on both TiO2 and CNF. The XRD diffractograms of TiO2/CNF and CdSe/TiO2/CNF are shown in Fig. 3. The phase of TiO2 was revealed as anatase (JCPDF No. 00-004-047). After depositing CdSe QDs on TiO2/CNF, the
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Fig. 3. XRD diffractograms of TiO2/CNF and CdSe/TiO2/CNF.
crystallographic planes of CdSe (111) and (220) (JCPDF No. 00-019019) were observed in case of CdSe/TiO2/CNF [25]. Additionally, diffraction peaks of cadmium hydroxide (Cd(OH)2, JCPDF No. 00020-0179) were also observed. This phase was assumed to be formed during QD deposition, since ethanol was used as a solvent of the SILAR method. Further composition analysis was performed with XPS. As shown in Fig. 4, the spectra of Cd 3d5/2 and 3d3/2 were centered at 405.6 and 412.4 eV, respectively (Fig. 4e) [26]. The two peaks at 54.2 and 55.3 eV corresponded to Se 3d5/2 and 3d3/2, respectively, which was originated from the Cd-Se bond (Fig. 4f) [26,27]. The binding energies of 459.1, 464.8, and 530.6 eV were Ti 2p3/2, Ti 2p1/2, and TiO bonds, respectively, of anatase TiO2 (Fig. 4d) [28,29]. The activation of CNF resulted in formation of various carbon-oxygen groups exhibited in the C 1s and O 1s spectra (Fig. 4b and c) [30–32]. The photocatalytic activity of TiO2/CNF and CdSe/TiO2/CNF was tested to decompose MB under visible and visible + UV light. For
Fig. 2. EDX point spectra of CdSe/TiO2/CNF. (a) TEM microgram of CdSe/TiO2/CNF for EDX point measurement. Point spectra of red circle 1 and 3 are displayed in (b) and (c), respectively (the box 1 and the point 2 showed similar EDX spectra). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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Fig. 4. XPS (a) survey scan of CdSe/TiO2/CNF. The deconvoluted peaks of (b) C 1s, (c) O 1s, (d) Ti 2p, (e) Cd 3d, and (f) Se 3d.
Fig. 5. MB photocatalytic decomposition curve tested under (a, b) visible and (c, d) visible + UV light. The photocatalysts used for the tests were (a, c) TiO2/CNF and (b, e) CdSe/ TiO2/CNF.
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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visible light irradiation, a UV cut-off filter (lcut-off < 420 nm) was used to illuminate the photons in the UV range. As shown in Fig. 5a, the decomposition of MB with TiO2/CNF was negligible under visible light irradiation (approximately 5, 3, and 2% was reduced for 50, 100, and 150 ppm MB, respectively during 10 min). We presume the small amount of MB reduction due to the absorption of lights in the range of 390–420 nm (3.2–3.0 eV) which might contribute to MB photodecomposition. However, in case of CdSe/TiO2/CNF, the photodecomposition was significantly enhanced under visible light irradiation (Fig. 5b). The MB concentration was reduced to 9, 11, and 27% within 2 min for 50, 100, and 150 initial MB concentrations, respectively. The deterioration of the photocatalytic activity at higher initial MB concentrations might be due to the light absorption of MB (lmax = 633 nm) that could reduce the light absorption by the photocatalyst. After 2 min, the MB concentration gradually and slightly reduced until 10 min. On the other hand, TiO2/CNF showed high photocatalytic activity even for high MB concentrations under visible + UV light irradiation (Fig. 5c). Note that effective photodecomposition of high concentration MB (150 ppm) was achieved by TiO2/CNF. The photocatalytic activity was further enhanced with CdSe/TiO2/CNF under visible + UV light irradiation (Fig. 5d). MB concentration was reduced as 98, 92, and 88% for initial concentration of 50, 100, and 150 ppm, respectively, indicating that the photocatalytic activity of CdSe/TiO2/CNF was highly efficient compared to previous literatures [9,18,19,33–35]. Along with the high performance, the
Fig. 6. MB (50 ppm) photocatalytic decomposition cyclic test under visible light of CdSe/TiO2/CNF.
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reusability of CdSe/TiO2/CNF was evaluated by five cyclic tests in 50 ppm MB aqueous solution. As shown in Fig. 6, CdSe/TiO2/CNF exhibited no deterioration of photocatalytic activities for five cycles. Therefore, it was verified that CdSe/TiO2/CNF showed high stability under repetitive use which is important for real world application. The amount of Cd leached from CdSe/TiO2/CNF was evaluated to prove the stability of our samples once more and reveal the potential threat of Cd poisoning of the treated water. For typical experiment, CdSe/TiO2/CNF 1 g/L was immersed in water for 6.5 days and the residual amount of Cd was measured by inductively coupled plasma mass spectroscopy (ICP-MS) after filtering out CdSe/TiO2/CNF powders. It was found that the residual amount of Cd was 0.1 mg/L (0.1 ppm), which is a negligible amount compared with the deionized water (0.28 mg/L, 0.28 ppm) used as blank. In addition, photocataylsts for the decomposition of MB similar to CdSe/TiO2/CNF and their performance are listed in Table 1. As shown, the performance of the CdSe/TiO2/CNF is remarkable compared to previously reported photocatalysts. These results suggest that CdSe/TiO2/CNF is the highly efficient photocatalyst to degrade organic substances without leaching in solution. Conclusions In conclusion, CdSe QDs were easily decorated on as-prepared TiO2/CNF (CdSe/TiO2/CNF) by SILAR method. The SEM and TEM observations showed that CdSe QDs were deposited on TiO2 nanoparticles and CNF surfaces with some aggregations. The composition of CdSe/TiO2/CNF was verified by EDX, XRD, and XPS, which also indicated that CdSe was well synthesized along with some amount of Ce hydroxide on TiO2/CNF. The photocatalytic activity of TiO2/CNF and CdSe/TiO2/CNF was tested to decompose MB aqueous solution under visible and visible + UV light irradiation. TiO2/CNF showed negligible amount of MB decomposition under visible light irradiation due to the high band gap of TiO2 (3.2 eV). However, CdSe/TiO2/CNF showed effective decomposition of MB under visible light due to the light absorption of CdSe and efficient charge transfer to TiO2. Additionally, a highly effective decomposition of MB was achieved by CdSe/TiO2/CNF under visible + UV light irradiation for high concentration MB aqueous solutions. The CdSe/TiO2/CNF was stable enough that the catalytic performance was maintained for five repetitive uses with no deterioration. Based on our results, CdSe/TiO2/CNF exhibited very
Table 1 List of photocatalysts and their activity measurement with methylene blue. Photocatalyst
Catalyst amount (g)/1 L water
Dye
Dye concentration (ppm)
Irradiation
Decomposition (%), time (min)
Reference
CdS/rGO/TiO2 CdSe/TiO2 CdSe/TiO2 CdS/TiO2 CdS/TiO2 CdS/CNT-TiO2 CdS/TiO2 CdS-graphene/TiO2 CdS/TiO2 CdS-Au-TiO2
? (13 10 mm size) 3.3 1 0.1 1.3 0.2 0.75 ? (75 50 mm size) 0.5 ? (20 20 mm size)
MB MB MB MB MB MB MB MB MB MB
10 17 3.2 3.2 10 3.2 7 50 20 10
Visible Visible Visible Visible Visible Visible Visible Visible Visible Visible
70, 120 60, 120 60, 240 100, 175 100, 180 35, 150 80, 60 30, 150 90, 180 70, 120
[9] [33] [34] [36] [37] [38] [39] [40] [41] [42]
CdSe/TiO2/CNF
1
MB
50, 100, 150
Visible
91, 2 89, 2 73, 2
This work
CdSe/TiO2/CNF
1
MB
50, 100, 150
Visible + UV
98, 2 92, 2 88, 2
This work
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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high performance and high stability compared with the reported works to the best of our knowledge. Acknowledgments This work was supported by a grant from the Korea Institute of Science and Technology (KIST) Institutional program and the Industrial Core Technology Development Program (10052760) and the High Performance Carbon Fiber Development Program (17CM-MA-24) funded by the Ministry of Trade, Industry and Energy and Development Program (NRF-2017M2A2A6A02071124) funded by the National Research Foundation of Korea, Republic of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.01.034. References [1] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24 (2012) 229. [2] J.M. Herrmann, Catal. Today 53 (1999) 115. [3] M. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997. [4] A. Fujishima, X. Zhang, D. Tryk, Surf. Sci. Rep. 63 (2008) 515. [5] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, ACS Nano 4 (1) (2009) 380. [6] S. Huang, N. Zhu, Z. Lou, L. Gu, C. Miao, H. Yuan, et al., Nanoscale 6 (2014) 1362. [7] N. Prakash, D. Thangaraju, R. Karthikeyan, M. Arivanandhan, Y. Shimura, Y. Hayakawa, RSC Adv. 6 (2016) 80655. [8] P.V. Kamat, J. Phys. Chem. C 112 (2008) 18737. [9] H. Li, Z. Xia, J. Chen, L. Lei, J. Xing, Appl. Catal. B: Environ. 168–169 (2015) 105. [10] Y. Peng, L. Shang, Y. Cao, G.I. Waterhouse, C. Zhou, T. Bian, et al., Chem. Commun. 51 (2015) 12556. [11] P.S. Saud, Z.K. Ghouru, B. Pant, T. An, J.H. Lee, M. Park, H.Y. Kim, Carbon Lett. 18 (2016) 30. [12] H.U. Lee, S.C. Lee, Y.C. Lee, B. Son, S.Y. Park, J.W. Lee, et al., Sci. Rep. 4 (2014) 6740.
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Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light Seung Hwa Yooa,1, Sung-In Leea,2 , Han-Ik Johb , Sungho Leea,c,* a Carbon Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324, Republic of Korea b Department of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea c Department of Nano Material Engineering, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
A R T I C L E I N F O
Article history: Received 20 November 2017 Received in revised form 13 January 2018 Accepted 27 January 2018 Available online xxx Keywords: Titanium dioxide Cadmium selenide Carbon nanofibers Photocatalyst Electrospinning
A B S T R A C T
A highly effective photocatalyst based on CdSe quantum dots (QDs) and TiO2 nanoparticles welldispersed on carbon nanofibers (CNFs), named as CdSe/TiO2/CNF was synthesized by simple successive ionic layer adsorption and reaction (SILAR) method on TiO2/CNF. It was verified by spectroscopic analysis that CdSe QDs were successfully synthesized and some aggregations and oxide phases existed in CdSe/ TiO2/CNF. The photocatalytic activity of CdSe/TiO2/CNF was demonstrated for decomposition of methylene (MB) aqueous solutions. The tests were performed for different MB concentrations and light irradiation. It was found that CdSe/TiO2/CNF showed high photocatalytic activity under visible light irradiation and even high MB concentrations. © 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction Photocatalysts work under lights, which are absorbed to provide electron-hole pairs with free radicals by reactions [1–3]. One of the mostly used photo-catalysts is titanium dioxide (TiO2) because of its high catalytic performance to create hydroxyl radicals in water, leading to decomposition of organics by photooxidation [4,5]. It is well known that TiO2 absorbs only ultraviolet (UV) light due to a large band gap energy (3.2 eV), which restrict more various and efficient usage under sunlight because sunlight consists of infrared of 49%, visible of 46% and UV light of 5% [6,7]. Therefore, maximizing photocatalytic activity by visible light has been studied with semiconducting inorganics, metal oxides or non-metal element doping which generate electron-hole pairs when visible light is absorbed [8–16]. Cadmium selenide (CdSe) has been used as semiconductor, having a band gap of 1.75 eV, which is much narrower with a higher conduction band-edge compared to TiO2 [8,17]. These render CdSe
to be an excellent candidate for transferring photo-generated electrons from CdSe to TiO2 under visible light in CdSe decorated TiO2, which enhance photo-catalytic activities. Wang et al. reported that CdSe quantum dots (QD) have higher energy level difference between each CdSe QD and TiO2 conduction bandedges, compared to bulk CdSe [18,19]. Therefore, CdSe QD is more efficient in charge carrier separation by much faster electron transferring. There are several representative routes to synthesize CdSe-QD on TiO2 such as ligand exchange [20], linker-assisted hybridization process [17,21], chemical bath deposition [22], and electrophoresis [23]. However, these methods require chemical synthesis to obtain CdSe QD using cadmium precursor and selenium powder and further steps to decorate QD on TiO2. In this study, CdSe was simply deposited on TiO2 supported on carbon nanofibers (CNFs), named as CdSe/TiO2/CNF, and was utilized as a photocatalyst to decompose aqueous methylene blue as a model pollutant.
* Corresponding author at: Carbon Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeollabuk-do 55324, Republic of Korea. E-mail address: [email protected] (S. Lee). 1 Current address: Biomedical Manufacturing Technology Center, Korea Institute of Industrial Technology, 59 Yangho-gil, Yeongcheon-si, Gyeongsangbuk-do 38822, Republic of Korea. 2 Current address: LG Chem R&D Campus Daejeon, 188 Munji-ro, Yuseong-gu, Daejeon 34122, Republic of Korea. https://doi.org/10.1016/j.jiec.2018.01.034 1226-086X/© 2018 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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Experimental Materials Polyacrylonitrile (PAN), titanium isopropoxide, cadmium nitrate tetrahydrate, selenium dioxide, sodium borohydride, and dimethylformamide (DMF) were purchased from the Sigma Aldrich Co. All chemicals were analytical grade and used without purification. Preparation of TiO2/CNF Solution preparation and electrospinning TiO2 precursor/PAN nanofiber mat 8 w% PAN solution was prepared by dissolving 4 g of PAN in 46 g of DMF for 2 h at 60 C. Separately, 4 g of titanium isopropoxide was mixed with 0.3 g of acetic acid dropwise, and the mixture was slowly added to as-prepared PAN solution. The final solution was rigorously stirred until it was completely dissolved. Heat treatment to convert TiO2 precursor/PAN to TiO2/CNF nanofiber mat The as-prepared polymeric nanofiber mat was stabilized followed by carbonization. For stabilization, the polymeric nanofiber mat was heated in air at 250 C for 120 min in a forced convection oven. Then, the stabilized mats were carbonized at 1600 C under N2 atmosphere. The temperature was raised from 25 to 1600 C at 5 C/min rate with no holding time at the final temperature. Subsequently, the carbonized mats were activated in oxygen atmosphere with 60 min holding time at 500 C. Preparation of CdSe/TiO2/CNF CdSe/TiO2/CNF was prepared through a successive ionic layer adsorption and reaction (SILAR) method. The as-prepared TiO2/ CNF mat was dipped into an ethanol solution containing 0.03 M cadmium nitrate tetrahydrate for 30 s to allow Cd2+ to be absorbed; then, it was rinsed with ethanol. Subsequently, the sample was
dipped into an ethanol solution containing 0.03 M selenium dioxide and sodium borohydride for 30 s, where the pre-adsorbed Cd2+ reacted with Se2 to form CdSe. After finishing the SILAR method, the treated samples were grounded with an agate mortar to finally give a powder form of CdSe/TiO2/CNF. Photocatalytic decomposition test 0.1 g of CdSe/TiO2/CNF powder was added in 100 mL methylene blue (MB) solution of 50, 100, and 150 ppm initial concentration. The mixture of MB solution and CdSe/TiO2/CNF powder was stirred for 2 h to achieve adsorption-desorption equilibrium. A Xe lamp (US 66983 Arc Lamp Source 200–500 W, Newport.) with a 420 nm cut-off filter (GG-420, 200 Sq. Longpass Filter, Edmund optics.) was used for the photocatalytic decomposition tests by utilizing visible light. In case to utilize visible + UV light, the cut-off filter was eliminated to use the full spectrum of Xe lamp. At given time intervals (2 min), 3 mL of sample solutions were collected and CdSe/TiO2/CNF powder was removed by filtration. The residual concentration of MB for each time intervals were measured with an UV–vis spectrometer (V670, JASCO Inc.). Analysis of material properties The morphology of CdSe/TiO2/CNF was observed using a field emission scanning electron microscope (FE-SEM, NOVA Nano SEM 450) and a transmission electron microscope (TEM, Tecnai G2 F20, FEI). X-ray diffraction was carried out using a D/MAX2500 V PC Xray diffractometer (Rigaku, Japan) with monochromated Cu Ka as a source between 2u of 10 and 80 at a scan rate of 3 /min. X-ray photoelectron spectra were obtained (Thermo Scientific, K-alpha) using monochromated Al Ka (1486.6 eV) X-rays at a pressure less than 3 107 Torr. Results and discussion In our previous study, well-dispersed TiO2 nanoparticles on carbon nanofibers (CNFs) were prepared, and a significantly
Fig. 1. (a, c) SEM and TEM images of CdSe/TiO2/CNF powder. (b, d) Higher magnification of (a) and (c), respectively.
Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034
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effective photocatalytic activity was observed under UV light [24]. To extend their performance under visible light, TiO2/CNF composites were manipulated with CdSe QD using a simple dip coating into solutions of Cd and Se precursors in sequence. To investigate the photocatalytic activity of resulting samples, degradation of methylene blue (MB) was carried out. We demonstrated highly effective degradation of MB in 2 min under visible light, which is the fastest performance reported in the open literature to the best of our knowledge (Table S1). TiO2/CNF composites were prepared by electrospinning of polyacrylonitrile and titanium isopropoxide mixture, stabilization, carbonization, and activation. Details are in the previous work [24]. Using successive ionic layer adsorption and reaction (SILAR) method, CdSe QDs were loaded on the surface of TiO2/CNF composites observed by scanning electron microscope (SEM) images in Fig. 1a and b. Even though TiO2 nanoparticles were welldispersed on the CNFs, CdSe QDs and their aggregation were found on TiO2 nanoparticles and the surface of CNFs, indicating that SILAR method could not render selective decoration of CdSe on TiO2. Transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDX) (Fig. 2) were conducted to evaluate morphology and composition of the prepared samples. In consistent with the SEM micrographs, 20–50 nm sized nanoparticles were well dispersed on carbon nanofibers, which were observed by TEM (Fig. 1c). Further higher magnification TEM observation revealed that these nanoparticles were consisted of several nanocrystals as depicted in Fig. 1d. Based on the lattice spacing of these crystals, it was verified that nanoparticles were consisted of TiO2 and CdSe (TiO2 (101), CdSe (311), and CdSe (220) planes were observable). EDX point mapping of the nanoparticle (the point 1 in Fig. 2a) showed existence of Ti, Cd, and Se along with C. The point mapping of the fiber (the point 3 in Fig. 2a) showed that Ti was absent compared to the nanoparticle, however, Cd, and Se elements were still detected. Therefore, it was conclusive that CdSe QDs were deposited on both TiO2 and CNF. The XRD diffractograms of TiO2/CNF and CdSe/TiO2/CNF are shown in Fig. 3. The phase of TiO2 was revealed as anatase (JCPDF No. 00-004-047). After depositing CdSe QDs on TiO2/CNF, the
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Fig. 3. XRD diffractograms of TiO2/CNF and CdSe/TiO2/CNF.
crystallographic planes of CdSe (111) and (220) (JCPDF No. 00-019019) were observed in case of CdSe/TiO2/CNF [25]. Additionally, diffraction peaks of cadmium hydroxide (Cd(OH)2, JCPDF No. 00020-0179) were also observed. This phase was assumed to be formed during QD deposition, since ethanol was used as a solvent of the SILAR method. Further composition analysis was performed with XPS. As shown in Fig. 4, the spectra of Cd 3d5/2 and 3d3/2 were centered at 405.6 and 412.4 eV, respectively (Fig. 4e) [26]. The two peaks at 54.2 and 55.3 eV corresponded to Se 3d5/2 and 3d3/2, respectively, which was originated from the Cd-Se bond (Fig. 4f) [26,27]. The binding energies of 459.1, 464.8, and 530.6 eV were Ti 2p3/2, Ti 2p1/2, and TiO bonds, respectively, of anatase TiO2 (Fig. 4d) [28,29]. The activation of CNF resulted in formation of various carbon-oxygen groups exhibited in the C 1s and O 1s spectra (Fig. 4b and c) [30–32]. The photocatalytic activity of TiO2/CNF and CdSe/TiO2/CNF was tested to decompose MB under visible and visible + UV light. For
Fig. 2. EDX point spectra of CdSe/TiO2/CNF. (a) TEM microgram of CdSe/TiO2/CNF for EDX point measurement. Point spectra of red circle 1 and 3 are displayed in (b) and (c), respectively (the box 1 and the point 2 showed similar EDX spectra). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. XPS (a) survey scan of CdSe/TiO2/CNF. The deconvoluted peaks of (b) C 1s, (c) O 1s, (d) Ti 2p, (e) Cd 3d, and (f) Se 3d.
Fig. 5. MB photocatalytic decomposition curve tested under (a, b) visible and (c, d) visible + UV light. The photocatalysts used for the tests were (a, c) TiO2/CNF and (b, e) CdSe/ TiO2/CNF.
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visible light irradiation, a UV cut-off filter (lcut-off < 420 nm) was used to illuminate the photons in the UV range. As shown in Fig. 5a, the decomposition of MB with TiO2/CNF was negligible under visible light irradiation (approximately 5, 3, and 2% was reduced for 50, 100, and 150 ppm MB, respectively during 10 min). We presume the small amount of MB reduction due to the absorption of lights in the range of 390–420 nm (3.2–3.0 eV) which might contribute to MB photodecomposition. However, in case of CdSe/TiO2/CNF, the photodecomposition was significantly enhanced under visible light irradiation (Fig. 5b). The MB concentration was reduced to 9, 11, and 27% within 2 min for 50, 100, and 150 initial MB concentrations, respectively. The deterioration of the photocatalytic activity at higher initial MB concentrations might be due to the light absorption of MB (lmax = 633 nm) that could reduce the light absorption by the photocatalyst. After 2 min, the MB concentration gradually and slightly reduced until 10 min. On the other hand, TiO2/CNF showed high photocatalytic activity even for high MB concentrations under visible + UV light irradiation (Fig. 5c). Note that effective photodecomposition of high concentration MB (150 ppm) was achieved by TiO2/CNF. The photocatalytic activity was further enhanced with CdSe/TiO2/CNF under visible + UV light irradiation (Fig. 5d). MB concentration was reduced as 98, 92, and 88% for initial concentration of 50, 100, and 150 ppm, respectively, indicating that the photocatalytic activity of CdSe/TiO2/CNF was highly efficient compared to previous literatures [9,18,19,33–35]. Along with the high performance, the
Fig. 6. MB (50 ppm) photocatalytic decomposition cyclic test under visible light of CdSe/TiO2/CNF.
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reusability of CdSe/TiO2/CNF was evaluated by five cyclic tests in 50 ppm MB aqueous solution. As shown in Fig. 6, CdSe/TiO2/CNF exhibited no deterioration of photocatalytic activities for five cycles. Therefore, it was verified that CdSe/TiO2/CNF showed high stability under repetitive use which is important for real world application. The amount of Cd leached from CdSe/TiO2/CNF was evaluated to prove the stability of our samples once more and reveal the potential threat of Cd poisoning of the treated water. For typical experiment, CdSe/TiO2/CNF 1 g/L was immersed in water for 6.5 days and the residual amount of Cd was measured by inductively coupled plasma mass spectroscopy (ICP-MS) after filtering out CdSe/TiO2/CNF powders. It was found that the residual amount of Cd was 0.1 mg/L (0.1 ppm), which is a negligible amount compared with the deionized water (0.28 mg/L, 0.28 ppm) used as blank. In addition, photocataylsts for the decomposition of MB similar to CdSe/TiO2/CNF and their performance are listed in Table 1. As shown, the performance of the CdSe/TiO2/CNF is remarkable compared to previously reported photocatalysts. These results suggest that CdSe/TiO2/CNF is the highly efficient photocatalyst to degrade organic substances without leaching in solution. Conclusions In conclusion, CdSe QDs were easily decorated on as-prepared TiO2/CNF (CdSe/TiO2/CNF) by SILAR method. The SEM and TEM observations showed that CdSe QDs were deposited on TiO2 nanoparticles and CNF surfaces with some aggregations. The composition of CdSe/TiO2/CNF was verified by EDX, XRD, and XPS, which also indicated that CdSe was well synthesized along with some amount of Ce hydroxide on TiO2/CNF. The photocatalytic activity of TiO2/CNF and CdSe/TiO2/CNF was tested to decompose MB aqueous solution under visible and visible + UV light irradiation. TiO2/CNF showed negligible amount of MB decomposition under visible light irradiation due to the high band gap of TiO2 (3.2 eV). However, CdSe/TiO2/CNF showed effective decomposition of MB under visible light due to the light absorption of CdSe and efficient charge transfer to TiO2. Additionally, a highly effective decomposition of MB was achieved by CdSe/TiO2/CNF under visible + UV light irradiation for high concentration MB aqueous solutions. The CdSe/TiO2/CNF was stable enough that the catalytic performance was maintained for five repetitive uses with no deterioration. Based on our results, CdSe/TiO2/CNF exhibited very
Table 1 List of photocatalysts and their activity measurement with methylene blue. Photocatalyst
Catalyst amount (g)/1 L water
Dye
Dye concentration (ppm)
Irradiation
Decomposition (%), time (min)
Reference
CdS/rGO/TiO2 CdSe/TiO2 CdSe/TiO2 CdS/TiO2 CdS/TiO2 CdS/CNT-TiO2 CdS/TiO2 CdS-graphene/TiO2 CdS/TiO2 CdS-Au-TiO2
? (13 10 mm size) 3.3 1 0.1 1.3 0.2 0.75 ? (75 50 mm size) 0.5 ? (20 20 mm size)
MB MB MB MB MB MB MB MB MB MB
10 17 3.2 3.2 10 3.2 7 50 20 10
Visible Visible Visible Visible Visible Visible Visible Visible Visible Visible
70, 120 60, 120 60, 240 100, 175 100, 180 35, 150 80, 60 30, 150 90, 180 70, 120
[9] [33] [34] [36] [37] [38] [39] [40] [41] [42]
CdSe/TiO2/CNF
1
MB
50, 100, 150
Visible
91, 2 89, 2 73, 2
This work
CdSe/TiO2/CNF
1
MB
50, 100, 150
Visible + UV
98, 2 92, 2 88, 2
This work
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high performance and high stability compared with the reported works to the best of our knowledge. Acknowledgments This work was supported by a grant from the Korea Institute of Science and Technology (KIST) Institutional program and the Industrial Core Technology Development Program (10052760) and the High Performance Carbon Fiber Development Program (17CM-MA-24) funded by the Ministry of Trade, Industry and Energy and Development Program (NRF-2017M2A2A6A02071124) funded by the National Research Foundation of Korea, Republic of Korea. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.01.034. References [1] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Adv. Mater. 24 (2012) 229. [2] J.M. Herrmann, Catal. Today 53 (1999) 115. [3] M. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997. [4] A. Fujishima, X. Zhang, D. Tryk, Surf. Sci. Rep. 63 (2008) 515. [5] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, ACS Nano 4 (1) (2009) 380. [6] S. Huang, N. Zhu, Z. Lou, L. Gu, C. Miao, H. Yuan, et al., Nanoscale 6 (2014) 1362. [7] N. Prakash, D. Thangaraju, R. Karthikeyan, M. Arivanandhan, Y. Shimura, Y. Hayakawa, RSC Adv. 6 (2016) 80655. [8] P.V. Kamat, J. Phys. Chem. C 112 (2008) 18737. [9] H. Li, Z. Xia, J. Chen, L. Lei, J. Xing, Appl. Catal. B: Environ. 168–169 (2015) 105. [10] Y. Peng, L. Shang, Y. Cao, G.I. Waterhouse, C. Zhou, T. Bian, et al., Chem. Commun. 51 (2015) 12556. [11] P.S. Saud, Z.K. Ghouru, B. Pant, T. An, J.H. Lee, M. Park, H.Y. Kim, Carbon Lett. 18 (2016) 30. [12] H.U. Lee, S.C. Lee, Y.C. Lee, B. Son, S.Y. Park, J.W. Lee, et al., Sci. Rep. 4 (2014) 6740.
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Please cite this article in press as: S.H. Yoo, et al., Highly effective photocatalysts based on carbon nanofibers decorated with TiO2 and CdSe under visible light, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.01.034