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Visible light range of Nb-doped titanate nanostructures synthesized with Nb oxide
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Photocatalytic activity under UV/Visible light range of Nb-doped titanate nanostructures synthesized with Nb oxide Jong Min Byun a,b , Hye Rim Choi b , Young Do Kim a,b , Tohru Sekino c , Se Hoon Kim d,∗ a
The Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea Department of Materials Science & Engineering, Hanyang University, Seoul 04763, Republic of Korea The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan d Materials Convergence & Design R&D Center, Korea Automotive Technology Institute, Cheonan 31162, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 11 July 2016 Received in revised form 25 August 2016 Accepted 26 August 2016 Available online xxx Keywords: Titanate nanostructures Hydrothermal process Niobium Doping Photocatalytic activity
a b s t r a c t In this work, using economical and stable niobium oxide (Nb2 O5 ) powder as niobium source, visible light responsive Nb-doped titanate nanostructures were synthesized by hydrothermal process. The synthesized Nb-doped titanate nanostructures were composed of two types of titanate nanostructures (nanotubes and nanosheets) and TiO2 nanoparticles. They have a smaller band gap energy of 3.24 eV compared to pure TNTs that were synthesized under the same experimental conditions. The photocatalytic activity of the synthesized Nb-doped titanate nanostructures was evaluated under visible light irradiation through the degradation of methylene blue (MB) and rhodamine B (RhB). Consequently, the synthesized Nb-doped titanate nanostructures exhibited much higher photocatalytic activity under visible light irradiation than pure TNTs. The photocatalytic activity of the synthesized Nb-doped titanate nanostructures was 1.4 times (MB) and 3.1 times (RhB) higher than of pure TNTs because the Nb-doping narrowed the band gap and it accelerated the separation of photo-induced electron-hole pairs. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Photocatalytic reaction with solar energy and semiconductors is of great importance in solving global energy and environmental issues [1–3]. Various semiconductors, such as TiO2 , ZnO, WO3 , CdS, and NiO, have been attempted to apply the photocatalysts, and it is generally accepted that TiO2 is the most reliable material due to its relatively low cost, nontoxicity, high reactivity and chemical stability under ultraviolet light (< 387 nm), whose energy exceeds the band gap of 3.0 ∼ 3.3 eV in the anatase or rutile crystalline phase [4,5]. Despite such advantages, however, the practical application of TiO2 for photovoltaics and photocatalysts is extremely limited because of its poor absorption capability under visible light (> 400 nm) due to its intrinsically large energy band gap and low quantum yield caused by the rapid recombination of photogenerated electrons and holes [6,7]. Therefore, there have been several attempts to develop modified TiO2 that exhibits high reactivity under visible light irradiation. As a result, several approaches for TiO2 modification have been proposed, including metal-ion doped TiO2 using transition metals [8], reduced TiOx photocatalysts [9,10],
∗ Corresponding author. E-mail address: [email protected] (S.H. Kim).
non-metal doped TiO2 [11–13], composites with TiO2 and other semiconductors having lower band gap energy [14,15], and sensitizing of TiO2 with dyes [16]. Among these approaches, it is well-known that the metal-ion doping is a widely used and effective way to improve the photocatalytic activity and stability of a semiconductor. In particular, niobium (Nb) doping is thought to be a promising approach to improve the photocatalytic activity of TiO2 under visible light irradiation. Nb can easily form a solid solution in the lattice of TiO2 because the ionic radius of Nb5+ (0.64 Å) is similar to that of Ti4+ (0.605 Å) [17]. The effect of Nb on the photocatalytic activity of TiO2 has been demonstrated in many existing studies. Michalow et al. fabricated Nb-doped TiO2 powder by a flame spray synthesis using titanium tetraisopropoxide (Ti(C3 H7 O)4 ) and niobium chloride (NbCl5 ). To form the Nb-doped TiO2 powder, Nb and Ti precursors were dissolved in a solvent and sprayed into a high temperature acetylene-oxygen flame using a reactive atomizing gas (oxygen). The photocatalytic activity of the Nb-doped TiO2 powder was assessed by the photo-degradation of a methylene blue solution under a UV irradiation. TiO2 with Nb concentration up to 1 at.% showed higher first-order photodecomposition kinetic rate constant towards MB decomposition under irradiation, than commercial P25 [18]. Also, Kaleji et al. prepared an Nb-doped TiO2 thin film by a sol-gel method using titanium butoxide (Ti(OC4 H9 )4 ) and
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niobium chloride (NbCl5 ). To fabricate the Nb-doped TiO2 thin film, the solution of two starting materials was aged for 24 h in order to complete all reactions, and calcined at 400 ◦ C for 1 h. The photocatalytic activity of the film was tested using the degradation of methylene blue. The 1 mol% Nb-doped TiO2 sample indicated higher photocatalytic activity than the on-doped TiO2 in the UV light region [19]. However, these methods, which is used for synthesizing various metal oxide nanostructures, expensive and unstable niobium alkoxide or niobium chloride (NbCl5 ) is required as a precursor of Nb. Also, complex processing such as flame experimental setup and calcination at higher than 400 ◦ C is necessary. Therefore, economical and easy processes should be examined for the synthesis of Nb-doped TiO2 . In this study, Nb-doped titanate nanostructures with improved reactivity under visible light irradiation were synthesized by a hydrothermal process using economical TiO2 and Nb2 O5 powders as an Nb precursor. Photocatalytic activity under UV/visible light range of synthesized Nb-doped titanate nanostructures was also investigated. 2. Experimental 2.1. Synthesis of Nb-doped titanate nanostructures In this study, Nb-doped titanate nanostructures were synthesized by a hydrothermal process. Commercial anatase TiO2 (99.9%, Kojundo Chemical Lab. Co., Japan) and Nb2 O5 (99.9%, Wako Pure Chemical Industries Ltd., Japan) powders were used as starting materials. Nb2 O5 powder at a concentration of 3 mol% was mixed with TiO2 powder, and then the mixed powders were homogeneously dispersed in a 10 M NaOH aqueous solution by ultrasonification for 30 min to form an amorphous Ti-Na-O. The mixture was refluxed at 135 ◦ C for 24 h. The resulting product was washed with deionized water until it reached pH 7 ∼ 8. Next, the product was treated with 0.1 M HCl to exchange sodium ions. The treated product was washed repeatedly with deionized water and ethyl alcohol until the solution conductivity reached below 10 S/cm. Finally, the synthesized powder was dried in an oven at 70 ◦ C for 48 h. 2.2. Characterization The phase analysis of synthesized Nb-doped titanate nanostructures was conducted by X-ray diffractometer (XRD, D8 Advance, Bruker AXS GmbH) using Cu K␣ radiation. Microstructure observation and composition analysis of synthesized Nb-doped titanate nanostructures were performed by a field emission scanning electron microscopy (FESEM, NanoSEM 450, FEI) and transmission electron microscopy (TEM, JEM-2100F, JEOL Co.) with an accelerating voltage of 200 kV equipped with an energy dispersive spectroscopy (EDS). The optical properties were analyzed with UV–vis spectrophotometer (V-650 spectrophotometer, Jasco Co.) equipped with the integrating sphere accessory for diffuse reflectance spectra. BaSO4 was used as a reference. 2.3. Photocatalytic activity The adsorptive and degradative properties of synthesized Nbdoped titanate nanostructures were analyzed using methylene blue (MB, Wako Pure Chemical Industries Ltd.) and rhodamine B (RhB, Wako Pure Chemical Industries Ltd.). The photocatalytic activity of the synthesized Nb-doped titanate nanostructures was compared with that of commercial Degussa P-25 and TNTs synthesized under the same experimental conditions. MB and RhB solutions were prepared at a concentration of 20 mg/L. Then, 20 mg of synthesized
Fig. 1. XRD pattern of synthesized Nb-doped titanate nanostructures.
Nb-doped titanate nanostructures was suspended in 100 mL of this solution. The degradation of MB and RhB under UV irradiation was evaluated after exposure to UV light (mercury–xenon lamp with a center wavelength of 365 nm, UVF-204S, SAN-EI Electric Co.) and degradations under visible light was performed by exposure to visible light (solar simulator with cutoff glass filter under 400 nm) at room temperature. Also, the adsorption capacity of TNTs and Nbdoped titanate nanostructures for MB and RhB dye was compared with Degussa P-25 under dark condition over 6 h. The concentrations of MB and RhB solutions were analyzed with a UV–vis spectrophotometer (UV mini-1240, Shimadzu Co.) by measuring the absorption band at 663 and 552 nm, respectively. 3. Results and discussion Fig. 1 shows the XRD pattern of the synthesized Nb-doped titanate nanostructures. It clearly showed the peaks of anatase TiO2 and titanate (H2 Ti4 O9 ·H2 O), which indicates that the nanotubes were synthesized through a solution chemical route, such as pure TNTs. In particular, Nb2 O5 peaks could not be found in the XRD pattern of synthesized Nb-doped titanate nanostructures. It was considered that the Nb2 O5 powder was completely dissolved in the NaOH aqueous solution and Nb ion (Nb5+ ) might have been substituted for the Ti ion (Ti4+ ) site in titanate [20]. Fig. 2 shows the high-resolution XPS spectra of pure TNTs and synthesized Nb-doped titanate nanostructures, confirming the presence of Ti 2p, O 1s, and Nb 3d in the samples. For both samples, two peaks at about 458–464 eV correspond to the Ti 2p3/2 and Ti 2p1/2 states, indicating that Ti is present in a valence state of 4+ , and the at 529 eV, which is attributed to bulk oxygen bonded to titanium, were observed as shown in Fig. 2(a) and (b). In particular, for synthesized Nb-doped titanate nanostructures, two peaks at about 206–209 eV correspond to the Nb 3d5/2 and Nb 3d3/2 states, confirming that Nb is present in the 5+ valence state [21]. The shape and morphology of synthesized Nb-doped titanate nanostructures observed using FE-SEM are shown in Fig. 3. In general, non-doped titanate nanostructures are known to exist as nanotubes with large aspect ratio [22]. However, Nb-doped titanate nanostructures were found to exist as a mixture of agglomerated nanotubes and nanosheets. The reason for the poor yield of nanotubes is that the additives, including Nb ions, interfere with the rolling of nanosheets into nanotubes during the hydrothermal process [23]. Also, it is thought that the separation of layered nanosheets was limited because of complicated interconnection. Therefore, some platelet TiO2 nanoparticles (NPs) with diameters of about 100–200 nm were present.
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Fig. 3. FE-SEM images of synthesized Nb-doped titanate nanostructures; (a) low magnification (b) high magnification.
Fig. 2. High-resolution XPS spectra of pure TNTs and synthesized Nb-doped titanate nanostructures, which confirms the presence of (a) Ti 2p, (b) O 1s, and (c) Nb 3d.
Fig. 4 shows the TEM images of the synthesized Nb-doped titanate nanostructures. Synthesized Nb-doped titanate nanostructures were shown to be composed of two types of titanate nanostructures (nanotubes and nanosheets) and TiO2 NPs, which are shown in the high-resolution image (red arrow) of Fig. 4(b). The TiO2 NPs and titanate nanostructures formed a porous network structure, with TiO2 NPs at the core site, and titanate nanostructures tangled with adjacent titanate nanostructures surrounding the TiO2 NPs. Fig. 5 shows the UV–vis diffuse reflectance spectra and Tauc plot of the band gap energy of pure TNTs and synthesized Nb-doped titanate nanostructures. As shown in Fig. 5(a), the reflectance of the synthesized Nb-doped titanate nanostructures presented a relatively red-shifted reflectance edge compared to that of the pure
TNTs and a low reflectance in the visible light region around 560 nm. Also, as shown in the tuac plot of Fig. 5(b), the band gap energy of pure TNTs was 3.34 eV, in agreements with other reports [24,25] and synthesized Nb-doped titanate nanostructures have a smaller band gap energy of 3.24 eV. To evaluate the adsorption ability of the synthesized TNTs and Nb-doped titanate nanostructures, MB and RhB removal tests were performed under dark conditions. Fig. 6 depicts the behavior of various MB and RhB concentrations under dark conditions. Dark conditions result in an observed decrease in MB concentration in TNTs and Nb-TNT over 2 h, after which no further changes were observed for up to 4 h, indicating an adsorption equilibrium for MB. In terms of RhB, the TNTs and Nb-doped titanate nanostructures exhibited a negligible degree of absorption. It is well-known that the rhodamine B dye cannot be easily absorbed by catalysts. In the case of P-25, the concentrations of MB and RhB changed little. Removal test for MB and RhB were performed under UV and visible light irradiation to evaluate the photocatalytic activity of synthesized Nb-doped titanate nanostructures. Fig. 7(a) and (b)
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Fig. 5. UV–vis diffuse reflectance spectra (a) and Tauc plot (b) for band gap energy of pure TNTs and synthesized Nb-doped titanate nanostructures.
Fig. 6. Dye adsorption ability of synthesized TNTs and Nb-doped titanate nanostructures under dark condition over 6 h.
Fig. 4. TEM images and schematic diagram of synthesized Nb-doped titanate nanostructures: (a) low magnification, (b) high magnification, and (c) schematic diagram of TiO2 /titanate network structure of Nb-doped titanate nanostructures. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
depict the degradation properties of MB by Degussa P-25, pure TNTs, and synthesized Nb-doped titanate nanostructures under UV and visible light irradiation, while Fig. 7(c) and (d) show the degradation property of RhB. The degradation rates of MB and RhB by Nb-doped titanate nanostructures under UV and visible light irradiation were significantly higher than those resulting from pure TNTs. In the MB solution, considering the adsorption ability, P25 has better photocatalytic properties than Nb-doped titanate nanostructures under UV light. Table 1 presents a summary of photocatalytic activity under the UV/visible light range for P-25,
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Fig. 7. Photocatalytic activity of synthesized Nb-doped titanate nanostructures under UV–vis light range: (a) Methylene blue degradation under UV light irradiation, (b) Methylene blue degradation under visible light irradiation, (c) Rhodamine B degradation under UV light irradiation, and (d) Rhodamine B degradation under visible light irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Summarized photocatalytic activity under UV/Visible light range of P-25, TNT, and Nb-doped titanate nanostructures. Sample
P-25 TNT Nb-TNT
Methylene Blue (MB)
Rhodamine B (RhB)
Structure
Degradation under UV
Degradation under Vis.
Degradation under UV
Degradation under Vis.
Excellent Fair Very good
Poor Excellent Excellent
Excellent Fair Very good
Good Good Excellent
TNT, and Nb-doped titanate nanostructures. It is thought that the improved photocatalytic activity of the Nb-doped titanate nanostructures was caused by the TiO2 /titanate network structure of Nb-doped titanate nanostructures. It is known that the photocatalytic activity is affected by the electron-hole pair recombination rate within a semiconductor [6–8]. Furthermore, the recombination rate of titanate is known to be faster than that of TiO2 in the UV range [26]. Therefore, the photocatalytic activity of TiO2 is higher than that of titanate. The reason for the increase in photocatalytic activity compared with that of pure TNTs, is thought to be a combination of titanate nanosheets/nanotubes and TiO2 nanoparticles on Nb-doped titanate nanostructures. This indicates that the transitions of photo-generated electrons from the conduction band to the valance band in the titanate and TiO2 were blocked, and that the recombination of photo-generated electron-hole pairs was suppressed due to the synergetic effect between the titanate nanosheets/nanotubes and the TiO2 nano-particles. As shown in Fig. 7(b), the MB degradation rate of Nb-doped titanate nanostruc-
NPs NTs NTs@NPs
tures was similar to that of other synthesized TNTs because the TiO2 nearly failed to serve as a photocatalyst in the visible light range. However, the photocatalytic activity of the synthesized TNTs could not be confirmed in the degradation of MB because the degradation rate generally decreased as the dye concentration decreased. The photocatalytic activity under visible light range was confirmed by the degradation of RhB as shown in Fig. 7(d). The degradation rate of the Nb-doped titanate nanostructures increased significantly, compared with that of Degussa P-25 and pure TNTs. This is speculated to be a result of the effect of doped metal ions, such as Nb5+ . These doping ions produce vacancies, excess ions, and/or aliovalent cations, which result in enhancement of the photocatalytic activity by restraining electron-hole pair recombination. 4. Conclusions In this study, Nb-doped titanate nanostructures with improved photocatalytic activity under visible light irradiation were suc-
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cessfully synthesized by a hydrothermal process using TiO2 and Nb2 O5 powders. Synthesized Nb-doped titanate nanostructures were composed of nanotubes, nanosheets, and TiO2 NPs. They had network a structure, with TiO2 NPs at the core site, and titanate nanostructures tangled with adjacent titanate nanostructure. The synthesized Nb-doped titanate nanostructures had a smaller band gap energy of 3.24 eV compared to that of pure TNTs (3.34 eV). The photocatalytic activity, which was evaluated by MB and RhB removal tests under UV/visible light range of synthesized Nb-doped titanate nanostructures was 1.4 ∼ 3.1 times as higher than that of pure TNTs because Nb-doping narrowed the band gap and it accelerated the separation of photo-induced electron-hole pairs. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03013422). Also, one of the authors (S. H. Kim) acknowledges the support of Japan Society for the Promotion of Science (JSPS) Fellowship. This research was supported in part by the JSPS under the Grant-in-Aid for JSPS Fellows(No.23-01332) and under the Grants-in-Aid for Scientific Research (A) (No. 22241017). References [1] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett. 6 (2006) 215–218. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor, Chem. Rev. 95 (1995) 69–96. [3] S.H.S. Chan, T.Y. Wu, J.C. Juan, C.Y. Teh, Recent developments of metal oxide semiconductors as photocatalysts in advanced oxidation processes (AOPs) for treatment of dye waste-water, J. Chem. Technol. Biotechnol. 86 (2011) 1130–1158. [4] C.Y. Wang, C. Bottcher, D.W. Bahnemannc, J.K. Dohrmann, A comparative study of nanometer sized Fe(III)-doped TiO2 photocatalysts: synthesis, characterization and activity, J. Mater. Chem. 13 (2003) 2322–2329. [5] Y. Kim, S. Yang, E.H. Jeon, J. Baik, N. Kim, H.S. Kim, H. Lee, Enhancement of photo-Oxidation activities depending on structural distortion of Fe-Doped TiO2 nanoparticles, Nanoscale Res. Lett. 11 (2016) 1–8. [6] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C-Photochem. Rev. 1 (2000) 1–21. [7] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Roja, Design of a highly efficient photoelectrolytic cell for hydrogen generation by water splitting: application of TiO2-x Cx nanotubes as a photoanode and Pt/TiO2 nanotubes as a cathode, J. Phys. Chem. C 111 (2007) 8677–8685.
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Perspective Article
Photocatalytic activity under UV/Visible light range of Nb-doped titanate nanostructures synthesized with Nb oxide Jong Min Byun a,b , Hye Rim Choi b , Young Do Kim a,b , Tohru Sekino c , Se Hoon Kim d,∗ a
The Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea Department of Materials Science & Engineering, Hanyang University, Seoul 04763, Republic of Korea The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan d Materials Convergence & Design R&D Center, Korea Automotive Technology Institute, Cheonan 31162, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 11 July 2016 Received in revised form 25 August 2016 Accepted 26 August 2016 Available online xxx Keywords: Titanate nanostructures Hydrothermal process Niobium Doping Photocatalytic activity
a b s t r a c t In this work, using economical and stable niobium oxide (Nb2 O5 ) powder as niobium source, visible light responsive Nb-doped titanate nanostructures were synthesized by hydrothermal process. The synthesized Nb-doped titanate nanostructures were composed of two types of titanate nanostructures (nanotubes and nanosheets) and TiO2 nanoparticles. They have a smaller band gap energy of 3.24 eV compared to pure TNTs that were synthesized under the same experimental conditions. The photocatalytic activity of the synthesized Nb-doped titanate nanostructures was evaluated under visible light irradiation through the degradation of methylene blue (MB) and rhodamine B (RhB). Consequently, the synthesized Nb-doped titanate nanostructures exhibited much higher photocatalytic activity under visible light irradiation than pure TNTs. The photocatalytic activity of the synthesized Nb-doped titanate nanostructures was 1.4 times (MB) and 3.1 times (RhB) higher than of pure TNTs because the Nb-doping narrowed the band gap and it accelerated the separation of photo-induced electron-hole pairs. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Photocatalytic reaction with solar energy and semiconductors is of great importance in solving global energy and environmental issues [1–3]. Various semiconductors, such as TiO2 , ZnO, WO3 , CdS, and NiO, have been attempted to apply the photocatalysts, and it is generally accepted that TiO2 is the most reliable material due to its relatively low cost, nontoxicity, high reactivity and chemical stability under ultraviolet light (< 387 nm), whose energy exceeds the band gap of 3.0 ∼ 3.3 eV in the anatase or rutile crystalline phase [4,5]. Despite such advantages, however, the practical application of TiO2 for photovoltaics and photocatalysts is extremely limited because of its poor absorption capability under visible light (> 400 nm) due to its intrinsically large energy band gap and low quantum yield caused by the rapid recombination of photogenerated electrons and holes [6,7]. Therefore, there have been several attempts to develop modified TiO2 that exhibits high reactivity under visible light irradiation. As a result, several approaches for TiO2 modification have been proposed, including metal-ion doped TiO2 using transition metals [8], reduced TiOx photocatalysts [9,10],
∗ Corresponding author. E-mail address: [email protected] (S.H. Kim).
non-metal doped TiO2 [11–13], composites with TiO2 and other semiconductors having lower band gap energy [14,15], and sensitizing of TiO2 with dyes [16]. Among these approaches, it is well-known that the metal-ion doping is a widely used and effective way to improve the photocatalytic activity and stability of a semiconductor. In particular, niobium (Nb) doping is thought to be a promising approach to improve the photocatalytic activity of TiO2 under visible light irradiation. Nb can easily form a solid solution in the lattice of TiO2 because the ionic radius of Nb5+ (0.64 Å) is similar to that of Ti4+ (0.605 Å) [17]. The effect of Nb on the photocatalytic activity of TiO2 has been demonstrated in many existing studies. Michalow et al. fabricated Nb-doped TiO2 powder by a flame spray synthesis using titanium tetraisopropoxide (Ti(C3 H7 O)4 ) and niobium chloride (NbCl5 ). To form the Nb-doped TiO2 powder, Nb and Ti precursors were dissolved in a solvent and sprayed into a high temperature acetylene-oxygen flame using a reactive atomizing gas (oxygen). The photocatalytic activity of the Nb-doped TiO2 powder was assessed by the photo-degradation of a methylene blue solution under a UV irradiation. TiO2 with Nb concentration up to 1 at.% showed higher first-order photodecomposition kinetic rate constant towards MB decomposition under irradiation, than commercial P25 [18]. Also, Kaleji et al. prepared an Nb-doped TiO2 thin film by a sol-gel method using titanium butoxide (Ti(OC4 H9 )4 ) and
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niobium chloride (NbCl5 ). To fabricate the Nb-doped TiO2 thin film, the solution of two starting materials was aged for 24 h in order to complete all reactions, and calcined at 400 ◦ C for 1 h. The photocatalytic activity of the film was tested using the degradation of methylene blue. The 1 mol% Nb-doped TiO2 sample indicated higher photocatalytic activity than the on-doped TiO2 in the UV light region [19]. However, these methods, which is used for synthesizing various metal oxide nanostructures, expensive and unstable niobium alkoxide or niobium chloride (NbCl5 ) is required as a precursor of Nb. Also, complex processing such as flame experimental setup and calcination at higher than 400 ◦ C is necessary. Therefore, economical and easy processes should be examined for the synthesis of Nb-doped TiO2 . In this study, Nb-doped titanate nanostructures with improved reactivity under visible light irradiation were synthesized by a hydrothermal process using economical TiO2 and Nb2 O5 powders as an Nb precursor. Photocatalytic activity under UV/visible light range of synthesized Nb-doped titanate nanostructures was also investigated. 2. Experimental 2.1. Synthesis of Nb-doped titanate nanostructures In this study, Nb-doped titanate nanostructures were synthesized by a hydrothermal process. Commercial anatase TiO2 (99.9%, Kojundo Chemical Lab. Co., Japan) and Nb2 O5 (99.9%, Wako Pure Chemical Industries Ltd., Japan) powders were used as starting materials. Nb2 O5 powder at a concentration of 3 mol% was mixed with TiO2 powder, and then the mixed powders were homogeneously dispersed in a 10 M NaOH aqueous solution by ultrasonification for 30 min to form an amorphous Ti-Na-O. The mixture was refluxed at 135 ◦ C for 24 h. The resulting product was washed with deionized water until it reached pH 7 ∼ 8. Next, the product was treated with 0.1 M HCl to exchange sodium ions. The treated product was washed repeatedly with deionized water and ethyl alcohol until the solution conductivity reached below 10 S/cm. Finally, the synthesized powder was dried in an oven at 70 ◦ C for 48 h. 2.2. Characterization The phase analysis of synthesized Nb-doped titanate nanostructures was conducted by X-ray diffractometer (XRD, D8 Advance, Bruker AXS GmbH) using Cu K␣ radiation. Microstructure observation and composition analysis of synthesized Nb-doped titanate nanostructures were performed by a field emission scanning electron microscopy (FESEM, NanoSEM 450, FEI) and transmission electron microscopy (TEM, JEM-2100F, JEOL Co.) with an accelerating voltage of 200 kV equipped with an energy dispersive spectroscopy (EDS). The optical properties were analyzed with UV–vis spectrophotometer (V-650 spectrophotometer, Jasco Co.) equipped with the integrating sphere accessory for diffuse reflectance spectra. BaSO4 was used as a reference. 2.3. Photocatalytic activity The adsorptive and degradative properties of synthesized Nbdoped titanate nanostructures were analyzed using methylene blue (MB, Wako Pure Chemical Industries Ltd.) and rhodamine B (RhB, Wako Pure Chemical Industries Ltd.). The photocatalytic activity of the synthesized Nb-doped titanate nanostructures was compared with that of commercial Degussa P-25 and TNTs synthesized under the same experimental conditions. MB and RhB solutions were prepared at a concentration of 20 mg/L. Then, 20 mg of synthesized
Fig. 1. XRD pattern of synthesized Nb-doped titanate nanostructures.
Nb-doped titanate nanostructures was suspended in 100 mL of this solution. The degradation of MB and RhB under UV irradiation was evaluated after exposure to UV light (mercury–xenon lamp with a center wavelength of 365 nm, UVF-204S, SAN-EI Electric Co.) and degradations under visible light was performed by exposure to visible light (solar simulator with cutoff glass filter under 400 nm) at room temperature. Also, the adsorption capacity of TNTs and Nbdoped titanate nanostructures for MB and RhB dye was compared with Degussa P-25 under dark condition over 6 h. The concentrations of MB and RhB solutions were analyzed with a UV–vis spectrophotometer (UV mini-1240, Shimadzu Co.) by measuring the absorption band at 663 and 552 nm, respectively. 3. Results and discussion Fig. 1 shows the XRD pattern of the synthesized Nb-doped titanate nanostructures. It clearly showed the peaks of anatase TiO2 and titanate (H2 Ti4 O9 ·H2 O), which indicates that the nanotubes were synthesized through a solution chemical route, such as pure TNTs. In particular, Nb2 O5 peaks could not be found in the XRD pattern of synthesized Nb-doped titanate nanostructures. It was considered that the Nb2 O5 powder was completely dissolved in the NaOH aqueous solution and Nb ion (Nb5+ ) might have been substituted for the Ti ion (Ti4+ ) site in titanate [20]. Fig. 2 shows the high-resolution XPS spectra of pure TNTs and synthesized Nb-doped titanate nanostructures, confirming the presence of Ti 2p, O 1s, and Nb 3d in the samples. For both samples, two peaks at about 458–464 eV correspond to the Ti 2p3/2 and Ti 2p1/2 states, indicating that Ti is present in a valence state of 4+ , and the at 529 eV, which is attributed to bulk oxygen bonded to titanium, were observed as shown in Fig. 2(a) and (b). In particular, for synthesized Nb-doped titanate nanostructures, two peaks at about 206–209 eV correspond to the Nb 3d5/2 and Nb 3d3/2 states, confirming that Nb is present in the 5+ valence state [21]. The shape and morphology of synthesized Nb-doped titanate nanostructures observed using FE-SEM are shown in Fig. 3. In general, non-doped titanate nanostructures are known to exist as nanotubes with large aspect ratio [22]. However, Nb-doped titanate nanostructures were found to exist as a mixture of agglomerated nanotubes and nanosheets. The reason for the poor yield of nanotubes is that the additives, including Nb ions, interfere with the rolling of nanosheets into nanotubes during the hydrothermal process [23]. Also, it is thought that the separation of layered nanosheets was limited because of complicated interconnection. Therefore, some platelet TiO2 nanoparticles (NPs) with diameters of about 100–200 nm were present.
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Fig. 3. FE-SEM images of synthesized Nb-doped titanate nanostructures; (a) low magnification (b) high magnification.
Fig. 2. High-resolution XPS spectra of pure TNTs and synthesized Nb-doped titanate nanostructures, which confirms the presence of (a) Ti 2p, (b) O 1s, and (c) Nb 3d.
Fig. 4 shows the TEM images of the synthesized Nb-doped titanate nanostructures. Synthesized Nb-doped titanate nanostructures were shown to be composed of two types of titanate nanostructures (nanotubes and nanosheets) and TiO2 NPs, which are shown in the high-resolution image (red arrow) of Fig. 4(b). The TiO2 NPs and titanate nanostructures formed a porous network structure, with TiO2 NPs at the core site, and titanate nanostructures tangled with adjacent titanate nanostructures surrounding the TiO2 NPs. Fig. 5 shows the UV–vis diffuse reflectance spectra and Tauc plot of the band gap energy of pure TNTs and synthesized Nb-doped titanate nanostructures. As shown in Fig. 5(a), the reflectance of the synthesized Nb-doped titanate nanostructures presented a relatively red-shifted reflectance edge compared to that of the pure
TNTs and a low reflectance in the visible light region around 560 nm. Also, as shown in the tuac plot of Fig. 5(b), the band gap energy of pure TNTs was 3.34 eV, in agreements with other reports [24,25] and synthesized Nb-doped titanate nanostructures have a smaller band gap energy of 3.24 eV. To evaluate the adsorption ability of the synthesized TNTs and Nb-doped titanate nanostructures, MB and RhB removal tests were performed under dark conditions. Fig. 6 depicts the behavior of various MB and RhB concentrations under dark conditions. Dark conditions result in an observed decrease in MB concentration in TNTs and Nb-TNT over 2 h, after which no further changes were observed for up to 4 h, indicating an adsorption equilibrium for MB. In terms of RhB, the TNTs and Nb-doped titanate nanostructures exhibited a negligible degree of absorption. It is well-known that the rhodamine B dye cannot be easily absorbed by catalysts. In the case of P-25, the concentrations of MB and RhB changed little. Removal test for MB and RhB were performed under UV and visible light irradiation to evaluate the photocatalytic activity of synthesized Nb-doped titanate nanostructures. Fig. 7(a) and (b)
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Fig. 5. UV–vis diffuse reflectance spectra (a) and Tauc plot (b) for band gap energy of pure TNTs and synthesized Nb-doped titanate nanostructures.
Fig. 6. Dye adsorption ability of synthesized TNTs and Nb-doped titanate nanostructures under dark condition over 6 h.
Fig. 4. TEM images and schematic diagram of synthesized Nb-doped titanate nanostructures: (a) low magnification, (b) high magnification, and (c) schematic diagram of TiO2 /titanate network structure of Nb-doped titanate nanostructures. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
depict the degradation properties of MB by Degussa P-25, pure TNTs, and synthesized Nb-doped titanate nanostructures under UV and visible light irradiation, while Fig. 7(c) and (d) show the degradation property of RhB. The degradation rates of MB and RhB by Nb-doped titanate nanostructures under UV and visible light irradiation were significantly higher than those resulting from pure TNTs. In the MB solution, considering the adsorption ability, P25 has better photocatalytic properties than Nb-doped titanate nanostructures under UV light. Table 1 presents a summary of photocatalytic activity under the UV/visible light range for P-25,
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Fig. 7. Photocatalytic activity of synthesized Nb-doped titanate nanostructures under UV–vis light range: (a) Methylene blue degradation under UV light irradiation, (b) Methylene blue degradation under visible light irradiation, (c) Rhodamine B degradation under UV light irradiation, and (d) Rhodamine B degradation under visible light irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Summarized photocatalytic activity under UV/Visible light range of P-25, TNT, and Nb-doped titanate nanostructures. Sample
P-25 TNT Nb-TNT
Methylene Blue (MB)
Rhodamine B (RhB)
Structure
Degradation under UV
Degradation under Vis.
Degradation under UV
Degradation under Vis.
Excellent Fair Very good
Poor Excellent Excellent
Excellent Fair Very good
Good Good Excellent
TNT, and Nb-doped titanate nanostructures. It is thought that the improved photocatalytic activity of the Nb-doped titanate nanostructures was caused by the TiO2 /titanate network structure of Nb-doped titanate nanostructures. It is known that the photocatalytic activity is affected by the electron-hole pair recombination rate within a semiconductor [6–8]. Furthermore, the recombination rate of titanate is known to be faster than that of TiO2 in the UV range [26]. Therefore, the photocatalytic activity of TiO2 is higher than that of titanate. The reason for the increase in photocatalytic activity compared with that of pure TNTs, is thought to be a combination of titanate nanosheets/nanotubes and TiO2 nanoparticles on Nb-doped titanate nanostructures. This indicates that the transitions of photo-generated electrons from the conduction band to the valance band in the titanate and TiO2 were blocked, and that the recombination of photo-generated electron-hole pairs was suppressed due to the synergetic effect between the titanate nanosheets/nanotubes and the TiO2 nano-particles. As shown in Fig. 7(b), the MB degradation rate of Nb-doped titanate nanostruc-
NPs NTs NTs@NPs
tures was similar to that of other synthesized TNTs because the TiO2 nearly failed to serve as a photocatalyst in the visible light range. However, the photocatalytic activity of the synthesized TNTs could not be confirmed in the degradation of MB because the degradation rate generally decreased as the dye concentration decreased. The photocatalytic activity under visible light range was confirmed by the degradation of RhB as shown in Fig. 7(d). The degradation rate of the Nb-doped titanate nanostructures increased significantly, compared with that of Degussa P-25 and pure TNTs. This is speculated to be a result of the effect of doped metal ions, such as Nb5+ . These doping ions produce vacancies, excess ions, and/or aliovalent cations, which result in enhancement of the photocatalytic activity by restraining electron-hole pair recombination. 4. Conclusions In this study, Nb-doped titanate nanostructures with improved photocatalytic activity under visible light irradiation were suc-
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cessfully synthesized by a hydrothermal process using TiO2 and Nb2 O5 powders. Synthesized Nb-doped titanate nanostructures were composed of nanotubes, nanosheets, and TiO2 NPs. They had network a structure, with TiO2 NPs at the core site, and titanate nanostructures tangled with adjacent titanate nanostructure. The synthesized Nb-doped titanate nanostructures had a smaller band gap energy of 3.24 eV compared to that of pure TNTs (3.34 eV). The photocatalytic activity, which was evaluated by MB and RhB removal tests under UV/visible light range of synthesized Nb-doped titanate nanostructures was 1.4 ∼ 3.1 times as higher than that of pure TNTs because Nb-doping narrowed the band gap and it accelerated the separation of photo-induced electron-hole pairs. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03013422). Also, one of the authors (S. H. Kim) acknowledges the support of Japan Society for the Promotion of Science (JSPS) Fellowship. This research was supported in part by the JSPS under the Grant-in-Aid for JSPS Fellows(No.23-01332) and under the Grants-in-Aid for Scientific Research (A) (No. 22241017). References [1] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett. 6 (2006) 215–218. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor, Chem. Rev. 95 (1995) 69–96. [3] S.H.S. Chan, T.Y. Wu, J.C. Juan, C.Y. Teh, Recent developments of metal oxide semiconductors as photocatalysts in advanced oxidation processes (AOPs) for treatment of dye waste-water, J. Chem. Technol. Biotechnol. 86 (2011) 1130–1158. [4] C.Y. Wang, C. Bottcher, D.W. Bahnemannc, J.K. Dohrmann, A comparative study of nanometer sized Fe(III)-doped TiO2 photocatalysts: synthesis, characterization and activity, J. Mater. Chem. 13 (2003) 2322–2329. [5] Y. Kim, S. Yang, E.H. Jeon, J. Baik, N. Kim, H.S. Kim, H. Lee, Enhancement of photo-Oxidation activities depending on structural distortion of Fe-Doped TiO2 nanoparticles, Nanoscale Res. Lett. 11 (2016) 1–8. [6] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C-Photochem. Rev. 1 (2000) 1–21. [7] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Roja, Design of a highly efficient photoelectrolytic cell for hydrogen generation by water splitting: application of TiO2-x Cx nanotubes as a photoanode and Pt/TiO2 nanotubes as a cathode, J. Phys. Chem. C 111 (2007) 8677–8685.
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