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Enhancing the photocatalytic efficiency of TiO2 nanotube arrays for H2 production by using non-noble metal cobalt as co-catalyst
Materials Letters 165 (2016) 37–40
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhancing the photocatalytic efficiency of TiO2 nanotube arrays for H2 production by using non-noble metal cobalt as co-catalyst Xiujie Wang a, Shengsen Zhang b,n, Biyu Peng a, Hongjuan Wang a, Hao Yu a, Feng Peng a,n a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 September 2015 Received in revised form 14 November 2015 Accepted 22 November 2015 Available online 23 November 2015
Non-noble metals are attractive alternatives to Pt co-catalysts for hydrogen production. Herein, Co nanoparticles were designed to uniformly deposit on the surface of TiO2 nanotube arrays (Co/TNAs) via photo-deposition and reduction method. The structure and optical property of the Co/TNAs were characterized. Compared with pure TNAs, Co/TNAs showed a highly photocatalytic activity for hydrogen production without the Pt co-catalyst under UV–vis light irradiation. The study presents a promising practical way using non-noble metal as co-catalyst for photocatalytic splitting of water into hydrogen. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Solar energy materials Photocatalyst TiO2 nanotube arrays Co-catalyst
1. Introduction Hydrogen as a clean, environmentally-benign and a sustainable energy carrier has been investigated widely in the world [1–3]. The TiO2 nanotube arrays (TNAs) prepared by anodization method as photocatalyst for hydrogen production have been studied extensively due to their vertically aligned structures and excellent photo-elctrochemical properties [2,4,5]. However, the hydrogen production efficiency of photocatalytic water-splitting over the pure TNAs film remains quite limited, mainly because of the recombination rate of photogenerated electrons and holes is still very high. To resolve this problem, many methods have been proposed to enhance the photocatalytic activity of TiO2 nanotube arrays, such as non-metal doping, metal loading, and semiconductor coupling [6]. General speaking, noble metals, such as Au [7], Pt [8], and Pd [5], were incorporated onto the surface of TNAs to enhance the electrons transfer and act as active sites, resulting in the enhanced efficiency of hydrogen production [9]. But the noble metals are both expensive and rare. It is of particular attraction to undertake non-noble metals as co-catalysts for photocatalytic splitting of water into hydrogen. Some important advances have been achieved in mimicking the catalytic active sites of natural hydrogenases to synthesize n
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (F. Peng). http://dx.doi.org/10.1016/j.matlet.2015.11.103 0167-577X/& 2015 Elsevier B.V. All rights reserved.
catalyst for hydrogen evolution based on earth-abundant elements such as Fe, Ni and Co [10–12]. For example, Tran et al. [13] demonstrated that Co nanoclusters as an efficient co-catalyst loaded onto the TiO2 nanopowder surface are attractive alternatives to Pt co-catalyst for photocatalytic splitting water. It is interesting to design Co as co-catalyst loaded on TNAs to prepare a novel immobilized and easy reusable photocatalyst for H2 production. In this work, Co nanoparticles (NPs) with different contents were deposited on TNAs (Co/TNAs) through photo-deposition-reduction method. The hydrogen production efficiency of photocatalytic splitting of water on the Co/TNAs has been studied.
2. Experimental section The TNAs were prepared by an electrochemical anodization method according to the previous report [14]. CoOx was deposited on TNAs by photo-deposition. Firstly, the TNAs film was immersed into 0.1 M Co(NO3)2 for 30 s under ultrasonic. Secondly, the film was irradiated under UV light for 120 s, and then cleared with deionized water. The above step was repeated for several cycles. Finally, the obtained samples were reduced in a tubular furnace at 250 °C for 1 h under Ar-H2 (volume ratio of 80/20) atmosphere. The as-prepared samples were denoted by Co/TNAs-M (M denotes the repeated photo-deposition cycles). In addition, the Pt/TNAs catalyst was also prepared by the photo-reduction method as a reference [15].
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X-ray diffraction (XRD) patterns were obtained on a D/max-IIIA (Japan) using Cu Kα radiation source. The morphology and structure of samples were examined by scanning electron microscope (SEM, LEO 1530VP) and transmission electron microscope (TEM, JEOL JEM-2010F, Japan). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Krato Axis Ultra DLD with Al Kα X-ray (hv ¼1486.6 eV) at 15 kV and 150 W. The UV–vis diffused reflectance spectra (UV–vis DRS) were recorded by a U3010 spectrometer (Hitachi). Photoluminescence (PL) spectra were measured at room temperature on a F-7000 Fluorescence spectrohpotometer (Hitachi). Photocatalytic hydrogen evolution was performed in a Lab Solar II photocatalytic hydrogen evolution system (Perfect light, Beijing) as our previous work [16]. A 4.0 cm2 of Co/TNAs sheet was immersed in 80 mL aqueous solution of 5% ethylene glycol (pH¼7.40) and was placed under the light source (PLS-SXE300UV Xe lamp, 150 mW cm 2). Then the system was sealed and vacuumized to keep the pressure as 0.09 MPa. The reaction temperature was maintained at 16 °C by circulating cooling water and the reactor. A gas chromatograph (GC-14C, Shimadzu) with TCD detector connected to the reaction system was used to determine the gas evolution online.
3. Results and discussion Fig. 1A shows the FESEM image of ordered TNAs with the uniform diameter of about 100 nm and the length of approx. 900 nm. Fig. 1B and C clearly display the dispersed Co NPs with the diameter of ca. 10 nm. Compared with the cross-sections of TNAs (insets in Fig. 1A) and Co/TNAs-15 (Fig. 1C), it can be found that the Co NPs were embedded into the surface of the external and internal walls of the TNAs. The amount of Co NPs was more on the top than on the bottom of TNAs. As shown in Fig. 1D and E, TEM image confirms that both the external and internal of the tubes
were loaded with nanoparticles with average size of 10 nm for the Co/TNAs-15. Fig. 1F displays the interplanar spacing is 0.216 nm, which can be indexed to the (100) plane of element Co. This result reveals that the uniformly dispersed Co NPs could be embedded into TNAs by photo-deposition-reduction method. XRD patterns of the as-prepared samples are shown in Fig. 2A. For TNAs sample (curve a), except for the peaks at 2θ ¼40.3°, 53.1° and 70.7° coming from the Ti substrate (JCPDS no. 89-2762 ), the rest peaks at 2θ ¼25.4°, 48.2° and 63.0° are assigned to the (101), (200) and (204) faces of anatase TiO2 (JCPDS no. 21-1272) [17]. Fig. 2A–e shows a new peak at 2θ value of 43.6°, which is attributed to the metal Co (JCPDS no. 65-9722). In addition, the intensity of the peak of Co increases with the increase of deposition cycles, indicating the content of Co NPs on TNAs increases gradually. Fig. 2Ae displays that the peaks of TiO2 decrease rapidly and the peak of Co increases obviously due to the congregation of Co NPs on the surface of TNAs. XPS was carried out for the Co/TNAs-15 sample as shown in Fig. 2B and C. Main doublet peaks located at 777.9 eV and 793.0 eV are corresponded to the Co 2P3/2 and Co 2P1/2 states of metallic Co atoms. Another doublet peaks located at 781.1 eV and 796.8 eV are assigned to the Co 2P3/2 and Co 2P1/2 states of Co–O bond [18], indicating a little fraction of CoOx on the surface of Co/TNAs-15. The CoOx could be formed due to the interaction between the surface Co atoms and O2 in the atmosphere. However, no CoOx peaks are shown in the XRD pattern probably because the content of CoOx in bulk phase is very low. Fig. 2D shows the pure TNAs sample has the typical absorption in the visible region, which is attributed to the light scattering of nanotube [19]. For Co/TNAs, the curves b–e in Fig. 2D shows that the absorption intensity in 420–700 nm increases with the increase of Co deposition cycles, which mainly due to the light gray color of Co NPs on the surface of TNAs. The inset of Fig. 2D shows the PL spectra of TNAs and Co/TNAs-15, illustrating that the recombination ability of photogenerated electron–hole of Co/TNAs-
Fig. 1. SEM images of (A) TNAs and (B, C) Co/TNAs-15; TEM (D,E) and HRTEM (F) of Co/TNAs-15.
X. Wang et al. / Materials Letters 165 (2016) 37–40
39
Fig. 2. (A) XRD images of different samples. (B) The whole XPS spectrum of Co/TNAs-15. (C) The high-resolution XPS spectra of Co 2p of Co/TNAs-15. (D) UV–vis absorption spectra of different samples and photoluminescence spectra for the TNAs and Co/TNAs-15 (inset). (a) TNAs, (b) Co/TNAs-5, (c) Co/TNAs-10, (d) Co/TNAs-15 and (e) Co/TNAs20.
15 is lower than that of TNAs. This result predicts the Co/TNAs-15 has a higher photocatalytic activity than pure TNAs. The catalytic activity of all samples was evaluated by photocatalytic splitting of water into hydrogen. Yu et al. discovered that ethylene glycol was a favorable sacrificial agent in photocatalytic H2 evolution reaction [10], so ethylene glycol aqueous solution was
chosen in this work. Fig. 3A shows that TNAs have a very low photocatalytic activity because of the relatively rapid recombination of electrons and holes in pure TNAs. However, Co deposited TNAs obviously facilitated the photocatalytic hydrogen evolution. The photocatalytic activity of Co/TNAs increased with deposition cycles increasing from 5 to 15, but then decreased with further
Fig. 3. (A) The performance of hydrogen production on different samples: (a) TNAs, (b) Co/TNAs-5, (c) Co/TNAs-10, (d) Co/TNAs-15, (e) Co/TNAs-20 and (f) Pt/TNAs. (B) The scheme of possible mechanism for the photocatalytic hydrogen evolution on Co/TNAs under the vis–UV light irradiation. The inset in (A) shows a relationship between cycles and average rates of hydrogen production.
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increasing deposition cycles to 20. The inset in Fig. 3A shows a relationship between cycles and average rates of hydrogen production. The Co/TNAs-15 possesses the highest photocatalytic activity, which hydrogen production rate (11.34 μmol cm 2 h 1) is almost 10 times as high as that of pure TNAs. It can be explained that the moderate Co NPs uniformly distributed on the surface of the TNAs favorably enhanced the transfer of photogenerated electrons from TiO2. But the excessive Co NPs fully covered the surface of the TNAs and decreased the adsorption of incident light, leading to the significantly decreased photocatalytic activity (Co/ TNAs-20). Compared with the Pt/TNAs, the Co/TNAs-15 shows a comparable activity. Although the hydrogen evolution rate of Co/ TNAs-15 is only 62.5% of Pt/TNAs, the unit-mass price of Pt is approximately 3000 times that of Co and the reserves of Co in the earth’s crust is about 10000 times that of Pt. Therefore, Co as a cocatalyst instead of Pt is of great significance for photocatalytic splitting of water into hydrogen. Fig. 3B presents a possible mechanism for cobalt nanoparticledecorated TiO2 nanotube arrays with enhanced photocatalytic hydrogen generation. Under UV–vis light, the TiO2 absorbs photons and the electrons in the valance band (VB) are excited and transfer into the conductive band (CB), resulting in the formation of photogenerated electron–hole pairs. Owing to the well electro conductibility of Co NPs which have lower Fermi energy than TiO2, the photogenerated electrons favorably transfer to the surface of Co NPs where the H þ can be reduced to H2. This process enhances the electrons transfer and separation. In addition, Co NPs like noble metal Pt act as active sites to decrease the over-potential of hydrogen evolution, leading to the great improvement of photocatalytic activity.
4. Conclusions Co/TNAs composite photocatalyst was prepared via photo-deposition-reduction method. In comparison of pure TNAs, the asprepared Co/TNAs composite exhibited enhanced photocatalytic activity due to the stronger ability on separation of electron–hole pairs than pure TNAs. The hydrogen evolution rate of Co/TNAs-15 was near 10 times as high as that of pure TNAs. Co NPs as an
efficient co-catalyst loaded onto the TiO2 nanotube arrays are attractive alternatives to Pt co-catalyst for photocatalytic splitting of water into hydrogen.
Acknowledgments We acknowledge the financial support from the NSFC (No. 21328301), the Guangdong Provincial NSFC (No. 2014A030312007) and the Fundamental Research Funds for the Central Universities.
References [1] F.K. Butt, C. Cao, F. Idrees, M. Tahir, R. Hussain, R. Ahmed, W.S. Khan, Int. J. Hydrog. Energy 40 (2015) 9359–9364. [2] N.T. Nguyen, M. Altomare, J.E. Yoo, P. Schmuki, Adv. Mater. 27 (2015) 3208–3215. [3] F.K. Butt, F. Idrees, M. Tahir, C. Cao, R. Hussain, R. Ahmed, B.U. Haq, Mater. Lett. 155 (2015) 15–17. [4] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215–218. [5] Q. Wang, J. Qiao, S. Gao, Mater. Lett. 131 (2014) 354–357. [6] K. Lee, A. Mazare, P. Schmuki, Chem. Rev. 114 (2014) 9385–9454. [7] Z. Zhang, L. Zhang, M.N. Hedhili, H. Zhang, P. Wang, Nano Lett. 13 (2013) 14–20. [8] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, J. Phys. Chem. C 111 (2007) 8677–8685. [9] J. Yang, D. Wang, H. Han, C. Li, Acc. Chem. Res. 46 (2013) 1900–1909. [10] D. Merki, H. Vrubel, L. Rovelli, S. Fierro, X. Hu, Chem. Sci. 3 (2012) 2515–2525. [11] M.M. Momeni, Y. Ghayeb, J. Electroanal. Chem. 751 (2015) 43–48. [12] C. Wu, Y. Bai, F. Wu, Mater. Lett. 62 (2008) 4242–4244. [13] P.D. Tran, L. Xi, S.K. Batabyal, L.H. Wong, J. Barber, J.S. Chye Loo, Phys. Chem. Chem. Phys. 14 (2012) 11596–11599. [14] S. Zhang, S. Zhang, F. Peng, H. Zhang, H. Liu, H. Zhao, Electrochem. Commun. 13 (2011) 861–864. [15] A. Kar, Y.R. Smith, V. Subramanian, Environ. Sci. Technol. 43 (2009) 3260–3265. [16] S. Zhang, B. Peng, S. Yang, Y. Fang, F. Peng, Int. J. Hydrog. Energy 38 (2013) 13866–13871. [17] M. Akram, F.K. Butt, A.Z. Alshemary, Y.F. Goh, W.A.W. Ibrahim, R. Hussain, Mater. Lett. 158 (2015) 95–98. [18] F. Zhang, A. Yamakata, K. Maeda, Y. Moriya, T. Takata, J. Kubota, K. Teshima, S. Oishi, K. Domen, J. Am. Chem. Soc. 134 (2012) 8348–8351. [19] H. Zhu, B. Yang, J. Xu, Z. Fu, M. Wen, T. Guo, S. Fu, J. Zuo, S. Zhang, Appl. Catal. B: Environ. 90 (2009) 463–469.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhancing the photocatalytic efficiency of TiO2 nanotube arrays for H2 production by using non-noble metal cobalt as co-catalyst Xiujie Wang a, Shengsen Zhang b,n, Biyu Peng a, Hongjuan Wang a, Hao Yu a, Feng Peng a,n a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 September 2015 Received in revised form 14 November 2015 Accepted 22 November 2015 Available online 23 November 2015
Non-noble metals are attractive alternatives to Pt co-catalysts for hydrogen production. Herein, Co nanoparticles were designed to uniformly deposit on the surface of TiO2 nanotube arrays (Co/TNAs) via photo-deposition and reduction method. The structure and optical property of the Co/TNAs were characterized. Compared with pure TNAs, Co/TNAs showed a highly photocatalytic activity for hydrogen production without the Pt co-catalyst under UV–vis light irradiation. The study presents a promising practical way using non-noble metal as co-catalyst for photocatalytic splitting of water into hydrogen. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Solar energy materials Photocatalyst TiO2 nanotube arrays Co-catalyst
1. Introduction Hydrogen as a clean, environmentally-benign and a sustainable energy carrier has been investigated widely in the world [1–3]. The TiO2 nanotube arrays (TNAs) prepared by anodization method as photocatalyst for hydrogen production have been studied extensively due to their vertically aligned structures and excellent photo-elctrochemical properties [2,4,5]. However, the hydrogen production efficiency of photocatalytic water-splitting over the pure TNAs film remains quite limited, mainly because of the recombination rate of photogenerated electrons and holes is still very high. To resolve this problem, many methods have been proposed to enhance the photocatalytic activity of TiO2 nanotube arrays, such as non-metal doping, metal loading, and semiconductor coupling [6]. General speaking, noble metals, such as Au [7], Pt [8], and Pd [5], were incorporated onto the surface of TNAs to enhance the electrons transfer and act as active sites, resulting in the enhanced efficiency of hydrogen production [9]. But the noble metals are both expensive and rare. It is of particular attraction to undertake non-noble metals as co-catalysts for photocatalytic splitting of water into hydrogen. Some important advances have been achieved in mimicking the catalytic active sites of natural hydrogenases to synthesize n
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (F. Peng). http://dx.doi.org/10.1016/j.matlet.2015.11.103 0167-577X/& 2015 Elsevier B.V. All rights reserved.
catalyst for hydrogen evolution based on earth-abundant elements such as Fe, Ni and Co [10–12]. For example, Tran et al. [13] demonstrated that Co nanoclusters as an efficient co-catalyst loaded onto the TiO2 nanopowder surface are attractive alternatives to Pt co-catalyst for photocatalytic splitting water. It is interesting to design Co as co-catalyst loaded on TNAs to prepare a novel immobilized and easy reusable photocatalyst for H2 production. In this work, Co nanoparticles (NPs) with different contents were deposited on TNAs (Co/TNAs) through photo-deposition-reduction method. The hydrogen production efficiency of photocatalytic splitting of water on the Co/TNAs has been studied.
2. Experimental section The TNAs were prepared by an electrochemical anodization method according to the previous report [14]. CoOx was deposited on TNAs by photo-deposition. Firstly, the TNAs film was immersed into 0.1 M Co(NO3)2 for 30 s under ultrasonic. Secondly, the film was irradiated under UV light for 120 s, and then cleared with deionized water. The above step was repeated for several cycles. Finally, the obtained samples were reduced in a tubular furnace at 250 °C for 1 h under Ar-H2 (volume ratio of 80/20) atmosphere. The as-prepared samples were denoted by Co/TNAs-M (M denotes the repeated photo-deposition cycles). In addition, the Pt/TNAs catalyst was also prepared by the photo-reduction method as a reference [15].
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X-ray diffraction (XRD) patterns were obtained on a D/max-IIIA (Japan) using Cu Kα radiation source. The morphology and structure of samples were examined by scanning electron microscope (SEM, LEO 1530VP) and transmission electron microscope (TEM, JEOL JEM-2010F, Japan). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Krato Axis Ultra DLD with Al Kα X-ray (hv ¼1486.6 eV) at 15 kV and 150 W. The UV–vis diffused reflectance spectra (UV–vis DRS) were recorded by a U3010 spectrometer (Hitachi). Photoluminescence (PL) spectra were measured at room temperature on a F-7000 Fluorescence spectrohpotometer (Hitachi). Photocatalytic hydrogen evolution was performed in a Lab Solar II photocatalytic hydrogen evolution system (Perfect light, Beijing) as our previous work [16]. A 4.0 cm2 of Co/TNAs sheet was immersed in 80 mL aqueous solution of 5% ethylene glycol (pH¼7.40) and was placed under the light source (PLS-SXE300UV Xe lamp, 150 mW cm 2). Then the system was sealed and vacuumized to keep the pressure as 0.09 MPa. The reaction temperature was maintained at 16 °C by circulating cooling water and the reactor. A gas chromatograph (GC-14C, Shimadzu) with TCD detector connected to the reaction system was used to determine the gas evolution online.
3. Results and discussion Fig. 1A shows the FESEM image of ordered TNAs with the uniform diameter of about 100 nm and the length of approx. 900 nm. Fig. 1B and C clearly display the dispersed Co NPs with the diameter of ca. 10 nm. Compared with the cross-sections of TNAs (insets in Fig. 1A) and Co/TNAs-15 (Fig. 1C), it can be found that the Co NPs were embedded into the surface of the external and internal walls of the TNAs. The amount of Co NPs was more on the top than on the bottom of TNAs. As shown in Fig. 1D and E, TEM image confirms that both the external and internal of the tubes
were loaded with nanoparticles with average size of 10 nm for the Co/TNAs-15. Fig. 1F displays the interplanar spacing is 0.216 nm, which can be indexed to the (100) plane of element Co. This result reveals that the uniformly dispersed Co NPs could be embedded into TNAs by photo-deposition-reduction method. XRD patterns of the as-prepared samples are shown in Fig. 2A. For TNAs sample (curve a), except for the peaks at 2θ ¼40.3°, 53.1° and 70.7° coming from the Ti substrate (JCPDS no. 89-2762 ), the rest peaks at 2θ ¼25.4°, 48.2° and 63.0° are assigned to the (101), (200) and (204) faces of anatase TiO2 (JCPDS no. 21-1272) [17]. Fig. 2A–e shows a new peak at 2θ value of 43.6°, which is attributed to the metal Co (JCPDS no. 65-9722). In addition, the intensity of the peak of Co increases with the increase of deposition cycles, indicating the content of Co NPs on TNAs increases gradually. Fig. 2Ae displays that the peaks of TiO2 decrease rapidly and the peak of Co increases obviously due to the congregation of Co NPs on the surface of TNAs. XPS was carried out for the Co/TNAs-15 sample as shown in Fig. 2B and C. Main doublet peaks located at 777.9 eV and 793.0 eV are corresponded to the Co 2P3/2 and Co 2P1/2 states of metallic Co atoms. Another doublet peaks located at 781.1 eV and 796.8 eV are assigned to the Co 2P3/2 and Co 2P1/2 states of Co–O bond [18], indicating a little fraction of CoOx on the surface of Co/TNAs-15. The CoOx could be formed due to the interaction between the surface Co atoms and O2 in the atmosphere. However, no CoOx peaks are shown in the XRD pattern probably because the content of CoOx in bulk phase is very low. Fig. 2D shows the pure TNAs sample has the typical absorption in the visible region, which is attributed to the light scattering of nanotube [19]. For Co/TNAs, the curves b–e in Fig. 2D shows that the absorption intensity in 420–700 nm increases with the increase of Co deposition cycles, which mainly due to the light gray color of Co NPs on the surface of TNAs. The inset of Fig. 2D shows the PL spectra of TNAs and Co/TNAs-15, illustrating that the recombination ability of photogenerated electron–hole of Co/TNAs-
Fig. 1. SEM images of (A) TNAs and (B, C) Co/TNAs-15; TEM (D,E) and HRTEM (F) of Co/TNAs-15.
X. Wang et al. / Materials Letters 165 (2016) 37–40
39
Fig. 2. (A) XRD images of different samples. (B) The whole XPS spectrum of Co/TNAs-15. (C) The high-resolution XPS spectra of Co 2p of Co/TNAs-15. (D) UV–vis absorption spectra of different samples and photoluminescence spectra for the TNAs and Co/TNAs-15 (inset). (a) TNAs, (b) Co/TNAs-5, (c) Co/TNAs-10, (d) Co/TNAs-15 and (e) Co/TNAs20.
15 is lower than that of TNAs. This result predicts the Co/TNAs-15 has a higher photocatalytic activity than pure TNAs. The catalytic activity of all samples was evaluated by photocatalytic splitting of water into hydrogen. Yu et al. discovered that ethylene glycol was a favorable sacrificial agent in photocatalytic H2 evolution reaction [10], so ethylene glycol aqueous solution was
chosen in this work. Fig. 3A shows that TNAs have a very low photocatalytic activity because of the relatively rapid recombination of electrons and holes in pure TNAs. However, Co deposited TNAs obviously facilitated the photocatalytic hydrogen evolution. The photocatalytic activity of Co/TNAs increased with deposition cycles increasing from 5 to 15, but then decreased with further
Fig. 3. (A) The performance of hydrogen production on different samples: (a) TNAs, (b) Co/TNAs-5, (c) Co/TNAs-10, (d) Co/TNAs-15, (e) Co/TNAs-20 and (f) Pt/TNAs. (B) The scheme of possible mechanism for the photocatalytic hydrogen evolution on Co/TNAs under the vis–UV light irradiation. The inset in (A) shows a relationship between cycles and average rates of hydrogen production.
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increasing deposition cycles to 20. The inset in Fig. 3A shows a relationship between cycles and average rates of hydrogen production. The Co/TNAs-15 possesses the highest photocatalytic activity, which hydrogen production rate (11.34 μmol cm 2 h 1) is almost 10 times as high as that of pure TNAs. It can be explained that the moderate Co NPs uniformly distributed on the surface of the TNAs favorably enhanced the transfer of photogenerated electrons from TiO2. But the excessive Co NPs fully covered the surface of the TNAs and decreased the adsorption of incident light, leading to the significantly decreased photocatalytic activity (Co/ TNAs-20). Compared with the Pt/TNAs, the Co/TNAs-15 shows a comparable activity. Although the hydrogen evolution rate of Co/ TNAs-15 is only 62.5% of Pt/TNAs, the unit-mass price of Pt is approximately 3000 times that of Co and the reserves of Co in the earth’s crust is about 10000 times that of Pt. Therefore, Co as a cocatalyst instead of Pt is of great significance for photocatalytic splitting of water into hydrogen. Fig. 3B presents a possible mechanism for cobalt nanoparticledecorated TiO2 nanotube arrays with enhanced photocatalytic hydrogen generation. Under UV–vis light, the TiO2 absorbs photons and the electrons in the valance band (VB) are excited and transfer into the conductive band (CB), resulting in the formation of photogenerated electron–hole pairs. Owing to the well electro conductibility of Co NPs which have lower Fermi energy than TiO2, the photogenerated electrons favorably transfer to the surface of Co NPs where the H þ can be reduced to H2. This process enhances the electrons transfer and separation. In addition, Co NPs like noble metal Pt act as active sites to decrease the over-potential of hydrogen evolution, leading to the great improvement of photocatalytic activity.
4. Conclusions Co/TNAs composite photocatalyst was prepared via photo-deposition-reduction method. In comparison of pure TNAs, the asprepared Co/TNAs composite exhibited enhanced photocatalytic activity due to the stronger ability on separation of electron–hole pairs than pure TNAs. The hydrogen evolution rate of Co/TNAs-15 was near 10 times as high as that of pure TNAs. Co NPs as an
efficient co-catalyst loaded onto the TiO2 nanotube arrays are attractive alternatives to Pt co-catalyst for photocatalytic splitting of water into hydrogen.
Acknowledgments We acknowledge the financial support from the NSFC (No. 21328301), the Guangdong Provincial NSFC (No. 2014A030312007) and the Fundamental Research Funds for the Central Universities.
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