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Anodic growth of ultra-long Ni-Ti-O nanopores
Electrochemistry Communications 71 (2016) 28–32
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Anodic growth of ultra-long Ni-Ti-O nanopores Ruiqiang Hang a,⁎, Mingxiang Zong a, Long Bai a, Ang Gao b, Yanlian Liu a, Xiangyu Zhang a, Xiaobo Huang a, Bin Tang a,⁎, Paul K. Chu b a b
Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 12 June 2016 Received in revised form 19 July 2016 Accepted 3 August 2016 Available online 05 August 2016 Keywords: Anodization Nickel-titanium alloy Nanopores
a b s t r a c t Although long nanotubes with a large specific surface area are desirable in many applications, it is difficult to produce long Ni-Ti-O nanotubes on nearly equiatomic NiTi alloy in fluoride (F)-containing ethylene glycol (EG) by anodization. In this work, a new electrolyte composed of EG, H2O, and HCl is designed and adopted to fabricate long nanopores rather than nanotubes on the NiTi alloy. By applying an anodization voltage of 10 V to the EG solutions containing 5.0–11.0 vol% H2O and 0.125–0.75 M HCl, Ni-Ti-O nanopores with a diameter of about 70 nm are produced. The nanopore length increases almost linearly with anodization time and by optimizing the experimental conditions, nanopores with a length of 160 μm can be prepared. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Self-assembly electrochemistry has been known for N60 years [1] and is still commonly practiced nowadays [2–5]. In particular, anodic growth of TiO2 nanotubes (NTs) on titanium (Ti) has attracted much interest due to potential applications in energy, environmental, biomedical, and other fields [6–8]. It has been indicated that improved and/or new properties can be obtained from TiO2 NTs by doping with the proper elements [9–12] and one of the reliable and effective doping approach is anodization [8,13]. An example is anodic growth of Ni-Ti-O NTs on nearly equiatomic NiTi alloy [14]. In an electrolyte composed of ethylene glycol (EG), H2O, and NH4F, well-defined NTs can be fabricated on the NiTi alloy. The NTs annealed at 600 °C exhibit rapid charge/discharge kinetics, excellent cycling stability, as well as high rate capability and are promising as electrodes in supercapacitors. Other potential applications of Ni-Ti-O NTs include electrodes in electrocatalysis, sensors, and biomedical coatings [15–22]. In all the previous studies, owing to the limitation of the electrolyte system proposed by Kim and co-workers, the tube length is limited (b 800 nm) because of rapid etching of F− to the NTs. However, in many applications, longer NTs are more desirable [23–25] but our previous study indicates it may not be possible to prepare long Ni-Ti-O NTs in this particular electrolyte [26]. Herein, synthesis of ultra-long Ni-Ti-O nanopores (NPs) on the NiTi alloy in a new electrolyte system composed of EG, H2O, and HCl is described. Our results show that under optimal conditions, NPs with a
⁎ Corresponding authors. E-mail addresses: [email protected] (R. Hang), [email protected] (B. Tang).
http://dx.doi.org/10.1016/j.elecom.2016.08.002 1388-2481/© 2016 Elsevier B.V. All rights reserved.
length of 160 μm can be produced and the longer NTs are more suitable for many applications. 2. Material and methods Mirror-polished NiTi alloy (50.8 at.% Ni) sheets (Φ9.0 mm × 2 mm) were used as the substrates. Anodization was conducted in a 250 ml two-electrode cell with a NiTi sheet as the anode and platinum sheet as the counterpart at 35 °C. 100 ml of the electrolyte composed of EG, H2O, and concentrated HCl were used in the anodization experiments on a power supply (IT6123, ITECH, China). The H2O content mentioned hereafter represented that H2O in the electrolyte together with that in concentrated HCl. The anodization time was 20 min unless otherwise stated. The anodization voltage and H2O and HCl concentrations in the electrolyte were varied to investigate their respective influence on the anodic behavior of the NiTi alloy. Under the optimized experimental conditions, the anodization time was extended to observe the influence on the NP length. A field-emission scanning electron microscope (FESEM, JSM-7001F, JEOL) equipped with an energy-dispersive X-ray spectrometry (EDS, QX200, Bruker) was used to examine the morphology and determine the elemental composition. 3. Results To investigate the influence of HCl on the anodic behavior of the NiTi alloy, EG electrolytes with different HCl concentrations are used. Fig. 1(a) shows the surface, sub-surface, and cross-sectional FE-SEM images of the samples anodized in electrolytes with different HCl concentrations. The surface morphology of the sample anodized without HCl is similar to that of the mechanically polished one. After adding 0.016 M
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Fig. 1. (a) Surface, sub-surface, and cross-sectional FE-SEM images of the samples anodized at 10.0 V in the EG containing 5.0 vol% H2O and different HCl concentrations. (b) and (c) Crosssectional FE-SEM images of the NPs anodized at 10 V in the EG containing 5.0 vol% H2O and 0.25 M HCl. (d) EDS spectrum of the NPs fabricated in the EG electrolyte containing 0.5 M HCl. (e) Corresponding j-t curves during anodization.
HCl into the electrolyte, cracks can be observed and doubling the concentration produces an irregular nanoporous structure. Irregular NPs with a limited length are observed when the HCl concentration is increased to 0.063 M and further increasing the HCl concentration produces irregular NPs that cover the sample surface uniformly. After taking away the irregular layers, ordered NPs can be observed. The pore diameter is about 70 nm irrespective of HCl concentrations. However, the length increases from 5.0 μm to 29.8 μm when the HCl concentration is increased from 0.125 M to 0.75 M. Fig. 1(b) shows higher magnification cross-sectional image of the NPs anodized in the EG containing 0.25 M HCl compared with that in Fig. 1(a). It seems that the NPs span the entire oxide layer. Local enlarged image of Fig. 1(b) (Fig. 1(c)) clearly shows the NPs are relatively straight, which indicates they grow perpendicular to the surface of the substrate. Fig. 1(d) shows the EDS spectrum of the anodized NPs revealing the presence of Ni, Ti, O, and
Cl. Since Cl comes from the Cl-containing electrolyte, the NPs are composed of Ni, Ti, and O and designated as Ni-Ti-O NPs. Fig. 1(e) presents the current density-time curves of the samples during anodization. The current density maintains at zero during the experiment when no HCl is added into the electrolyte. In the presence of HCl, the current density drops rapidly initially and then decreases gradually until reaching a relatively stable value. The steady-state current density (SSCD) increases with HCl concentrations. The H2O content in the electrolyte is another key parameter that influences the anodic behavior of the NiTi alloy. As shown in Figs. 2(a), 3.0 vol% H2O results in an irregular surface morphology. However, when the H2O content is between 5.0 vol% and 11.0 vol%, irregular NPs emerge, but further increase decreases the nanoporous structure. The cross-sectional images show that the NP length increases from 18.3 μm for 5.0 vol% H2O to 41.0 μm for 11.0 vol% H2O, but no ordered
30
R. Hang et al. / Electrochemistry Communications 71 (2016) 28–32
Fig. 2. (a) Surface and cross-sectional FE-SEM images of the samples anodized at 10.0 V in EG containing 0.5 M HCl and different H2O contents. (b) Corresponding j-t curves during anodization.
cross-section can be observed when the H2O content is 19.0 vol%. The corresponding j-t curves in Fig. 2(b) indicate that the current density depends largely on the H2O content in the electrolyte. Increasing the H2O content elevates the initial and steady-state current densities. However, the SSCD determined from the electrolyte without H2O is
larger than that from the electrolytes containing 5.0 vol% and 7.0 vol% H2O. Another important factor that influences the anodic behavior of the NiTi alloy is the applied voltage. As shown in Fig. 3(a), the surface morphology of the sample anodized at 2.5 V is similar to that of the
Fig. 3. (a) Surface FE-SEM images of the samples anodized at different voltages in EG containing 0.5 M HCl and 5.0 vol% H2O. The voltage for each sample is shown in the upper left corner in each image. (b) Corresponding j-t curves during anodization.
R. Hang et al. / Electrochemistry Communications 71 (2016) 28–32
mechanically polished one, even though some scattered white dots can be observed. Anodization at 5.0 V produces irregular large pores (100– 300 nm in diameter) in comparison with that anodized at 10.0 V. Although a larger voltage (20 V) generates the nanoporous structure, the NP length is very limited. The j-t curves reveal a voltage dependence and increase in the H2O content significantly elevates the SSCD. Finally, the influence of the anodization time on the NP length is investigated. As shown in Fig. 4(a), the NP length increases with anodization time and Fig. 4(b) reveals a nearly linear relationship between the NP length and anodization time. The NP length increases from 4.0 μm at 5 min to 160 μm at 320 min. Further increase anodization results in spontaneous detachment of the NPs from the substrate, which may be ascribed to their relatively high internal stress. 4. Discussion Fabrication of Ni-Ti-O NTs on nearly equiatomic NiTi alloy by anodization has many applications [14,16,18,22,26]. However, previous use of Fcontaining electrolytes in the fabrication limits the NT length compromising the properties. In this work, a Cl-containing electrolyte is designed to prepare ultra-long Ni-Ti-O NPs instead of NTs. By adopting the optimal experimental conditions, the NP length can reach 160 μm and the materials which have a larger specific surface area are more suitable for applications including drug delivery, electrocatalysis, photocatalysis, and sensing. Formation of self-assembled nanostructures by anodization is mainly ascribed to the balance between growth of the oxide film and its dissolution under the attack of aggressive ions. In this system, an initial
31
compact layer composed of TiO2 and NiO may be formed by hydrolyzing the constituents of the NiTi alloy under an electric field according to the following reactions: Ti þ 2H2 O → TiO2 þ 4Hþ þ 4e−
ð1Þ
Ni þ H2 O → NiO þ 2Hþ þ 2e−
ð2Þ
Formation of the oxide layer decreases the current density because of the poor electrical conductivity. After the compact layer is formed, TiO2 and NiO start to dissolve under the attack of Cl− by the following reactions [27]: −
TiO2 þ 4Hþ þ 6Cl → TiCl6 2 −
NiO þ 2Hþ þ 4Cl → NiCl4 2
−
−
þ 2H2 O
ð3Þ
þ H2 O
ð4Þ
Thereafter, a balance between formation and dissolution of the oxide layer is gradually established as manifested by the relatively stable current density. The concentrations of Cl− and H2O in the electrolyte and anodization voltage affect the dynamic balance and consequently formation and characteristics of the NPs. Our results show that Cl− is essential to the anodic growth of Ni-Ti-O NPs. A larger Cl− concentration increases the NP length possibly due to fast etching of the newly formed oxide layer at the bottom. H2O is also fundamental to anodic growth of the NPs because it offers the source of O for electro-oxidation of Ti and Ni. Although there is some H2O in concentrated HCl, the amount is too
Fig. 4. (a) Cross-sectional FE-SEM images of the samples anodized for different time at 10.0 V in EG containing 0.5 M HCl and 5.0 vol% H2O. Anodization time for each sample is shown in the upper left corner of each image. (b) NP length as a function of anodization time.
32
R. Hang et al. / Electrochemistry Communications 71 (2016) 28–32
small for fast growth of the oxide layer in order to balance dissolution under attack by Cl−. When the H2O content is in the range between 5.0 vol% and 11.0 vol%, NPs are formed and the length increases with H2O contents. One possible explanation is that a larger H2O content enables fast oxidation of Ti and Ni and lowers the viscosity of the electrolyte thus accelerating Cl− etching of the oxide layer and the overall electrochemical reaction. However, too much H2O may disrupt the balance thus resulting in the formation of the irregular structure. The voltage window pertaining to anodization of NiTi in Cl-containing electrolytes is relatively small compared to that for F-containing electrolytes [26]. Only a voltage of 10.0 V can generate ordered NPs. A smaller voltage may not be sufficient to drive oxidation and migration of Cl− and a larger voltage may produce excessive etching of the newly formed oxide layer by Cl− resulting in a limited NP length. It is recognized that long NPs with high specific surface area can contribute to the improvement of their desired properties [23]. Accordingly, the NPs prepared by the present work may substantially improve a variety of properties that Ni-Ti-O NTs possessed, because the length of Ni-Ti-O NPs (160 μm) after anodization for 320 min is N120 times of the longest Ni-Ti-O NTs reported previously [26]. 5. Conclusion Ordered Ni-Ti-O NPs are fabricated on nearly equiatomic NiTi alloy by anodization in an electrolyte composed of EG, H2O, and HCl. The optimal H2O content in the electrolyte is 5.0–11.0 vol%, HCl concentration is 0.125–0.5 M, and anodization voltage is 10 V. The diameter of NPs is about 70 nm. The NP length increases almost linearly with anodization time and reaches 160 μm after anodization for 320 min under the optimal experimental conditions. Acknowledgements This work was jointly supported by the National Natural Science Foundation of China (31400815), Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (201626), Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 11301215, and City University of Hong Kong Applied Research Grant (ARG) No. 9667122. References [1] F. Keller, M. Hunter, D. Robinson, Structural features of oxide coatings on aluminum, J. Electrochem. Soc. 100 (1953) 411–419. [2] T. Yoshida, K. Terada, D. Schlettwein, T. Oekermann, T. Sugiura, H. Minoura, Electrochemical self-assembly of nanoporous ZnO/Eosin Y thin films and their sensitized photoelectrochemical performance, Adv. Mater. 12 (2000) 1214–1217. [3] T. Yoshida, H. Minoura, Electrochemical self-assembly of dye-modified zinc oxide thin films, Adv. Mater. 12 (2000) 1219–1222. [4] H. Masuda, K. Yada, A. Osaka, Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution, Jpn. J. Appl. Phys. 37 (1998) L1340.
[5] V. Sadasivan, C. Richter, L. Menon, P. Williams, Electrochemical self-assembly of porous alumina templates, AICHE J. 51 (2005) 649–655. [6] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, D. David, M.-Y. Perrin, M. Aucouturier, Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy, Surf. Interface Anal. 27 (1999) 629–637. [7] K. Lee, A. Mazare, P. Schmuki, One-dimensional titanium dioxide nanomaterials: nanotubes, Chem. Rev. 114 (2014) 9385–9454. [8] P. Roy, S. Berger, P. Schmuki, TiO2 nanotubes: synthesis and applications, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. [9] A. Ghicov, M. Yamamoto, P. Schmuki, Lattice widening in niobium-doped TiO2 nanotubes: efficient ion intercalation and swift electrochromic contrast, Angew. Chem. Int. Ed. 47 (2008) 7934–7937. [10] L. Deng, S. Wang, D. Liu, B. Zhu, W. Huang, S. Wu, S. Zhang, Synthesis, characterization of Fe-doped TiO2 nanotubes with high photocatalytic activity, Catal. Lett. 129 (2009) 513–518. [11] F. Fabregat-Santiago, E.M. Barea, J. Bisquert, G.K. Mor, K. Shankar, C.A. Grimes, High carrier density and capacitance in TiO2 nanotube arrays induced by electrochemical doping, J. Am. Chem. Soc. 130 (2008) 11312–11316. [12] A. Gao, R. Hang, X. Huang, L. Zhao, X. Zhang, L. Wang, B. Tang, S. Ma, P.K. Chu, The effects of titania nanotubes with embedded silver oxide nanoparticles on bacteria and osteoblasts, Biomaterials 35 (2014) 4223–4235. [13] Y.C. Nah, I. Paramasivam, P. Schmuki, Doped TiO2 and TiO2 nanotubes: synthesis and applications, ChemPhysChem 11 (2010) 2698–2713. [14] J.-H. Kim, K. Zhu, Y. Yan, C.L. Perkins, A.J. Frank, Microstructure and pseudocapacitive properties of electrodes constructed of oriented NiO-TiO2 nanotube arrays, Nano Lett. 10 (2010) 4099–4104. [15] H. He, G. Ke, P. He, J. Liang, F. Dong, Ordered NiO-TiO2 nanotube arrays as an efficient catalyst support for methanol oxidation, Phys. Status Solidi A 212 (2015) 2085–2090. [16] G.-Y. Hou, Y.-Y. Xie, L.-K. Wu, H.-Z. Cao, Y.-P. Tang, G.-Q. Zheng, Electrocatalytic performance of Ni-Ti-O nanotube arrays/NiTi alloy electrode annealed under H2 atmosphere for electro-oxidation of methanol, Int. J. Hydrog. Energy 41 (2016) 9295–9302. [17] Z. Li, D. Ding, Q. Liu, C. Ning, Hydrogen sensing with Ni-doped TiO2 nanotubes, Sensors 13 (2013) 8393–8402. [18] R. Hang, Y. Liu, A. Gao, L. Bai, X. Huang, X. Zhang, N. Lin, B. Tang, P.K. Chu, Highly ordered Ni-Ti-O nanotubes for non-enzymatic glucose detection, Mater. Sci. Eng. C 51 (2015) 37–42. [19] R. Hang, Y. Liu, S. Liu, L. Bai, A. Gao, X. Zhang, X. Huang, B. Tang, P.K. Chu, Sizedependent corrosion behavior and cytocompatibility of Ni-Ti-O nanotubes prepared by anodization of biomedical NiTi alloy, Corros. Sci. 103 (2016) 173–180. [20] R. Hang, X. Huang, L. Tian, Z. He, B. Tang, Preparation, characterization, corrosion behavior and bioactivity of Ni2O3-doped TiO2 nanotubes on NiTi alloy, Electrochim. Acta 70 (2012) 382–393. [21] P.P. Lee, A. Cerchiari, T.A. Desai, Nitinol-based nanotubular coatings for the modulation of human vascular cell function, Nano Lett. 14 (2014) 5021–5028. [22] P.P. Lee, T.A. Desai, Nitinol-based nanotubular arrays with controlled diameters upregulate human vascular cell ecm production, ACS Biomater. Sci. Eng. 2 (2016) 409–414. [23] C.-Y. Lee, Z. Su, K. Lee, H. Tsuchiya, P. Schmuki, Self-organized cobalt fluoride nanochannel layers used as a pseudocapacitor material, Chem. Commun. 50 (2014) 7067–7070. [24] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting, Nano Lett. 6 (2006) 24–28. [25] J.M. Macak, M. Zlamal, J. Krysa, P. Schmuki, Self-organized TiO2 nanotube layers as highly efficient photocatalysts, Small 3 (2007) 300–304. [26] R. Hang, Y. Liu, L. Zhao, A. Gao, L. Bai, X. Huang, X. Zhang, B. Tang, P.K. Chu, Fabrication of Ni-Ti-O nanotube arrays by anodization of NiTi alloy and their potential applications, Sci. Rep. 4 (2014) 7547. [27] N.K. Allam, C.A. Grimes, Formation of vertically oriented TiO2 nanotube arrays using a fluoride free HCl aqueous electrolyte, J. Phys. Chem. C 111 (2007) 13028–13032.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Anodic growth of ultra-long Ni-Ti-O nanopores Ruiqiang Hang a,⁎, Mingxiang Zong a, Long Bai a, Ang Gao b, Yanlian Liu a, Xiangyu Zhang a, Xiaobo Huang a, Bin Tang a,⁎, Paul K. Chu b a b
Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, China Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 12 June 2016 Received in revised form 19 July 2016 Accepted 3 August 2016 Available online 05 August 2016 Keywords: Anodization Nickel-titanium alloy Nanopores
a b s t r a c t Although long nanotubes with a large specific surface area are desirable in many applications, it is difficult to produce long Ni-Ti-O nanotubes on nearly equiatomic NiTi alloy in fluoride (F)-containing ethylene glycol (EG) by anodization. In this work, a new electrolyte composed of EG, H2O, and HCl is designed and adopted to fabricate long nanopores rather than nanotubes on the NiTi alloy. By applying an anodization voltage of 10 V to the EG solutions containing 5.0–11.0 vol% H2O and 0.125–0.75 M HCl, Ni-Ti-O nanopores with a diameter of about 70 nm are produced. The nanopore length increases almost linearly with anodization time and by optimizing the experimental conditions, nanopores with a length of 160 μm can be prepared. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Self-assembly electrochemistry has been known for N60 years [1] and is still commonly practiced nowadays [2–5]. In particular, anodic growth of TiO2 nanotubes (NTs) on titanium (Ti) has attracted much interest due to potential applications in energy, environmental, biomedical, and other fields [6–8]. It has been indicated that improved and/or new properties can be obtained from TiO2 NTs by doping with the proper elements [9–12] and one of the reliable and effective doping approach is anodization [8,13]. An example is anodic growth of Ni-Ti-O NTs on nearly equiatomic NiTi alloy [14]. In an electrolyte composed of ethylene glycol (EG), H2O, and NH4F, well-defined NTs can be fabricated on the NiTi alloy. The NTs annealed at 600 °C exhibit rapid charge/discharge kinetics, excellent cycling stability, as well as high rate capability and are promising as electrodes in supercapacitors. Other potential applications of Ni-Ti-O NTs include electrodes in electrocatalysis, sensors, and biomedical coatings [15–22]. In all the previous studies, owing to the limitation of the electrolyte system proposed by Kim and co-workers, the tube length is limited (b 800 nm) because of rapid etching of F− to the NTs. However, in many applications, longer NTs are more desirable [23–25] but our previous study indicates it may not be possible to prepare long Ni-Ti-O NTs in this particular electrolyte [26]. Herein, synthesis of ultra-long Ni-Ti-O nanopores (NPs) on the NiTi alloy in a new electrolyte system composed of EG, H2O, and HCl is described. Our results show that under optimal conditions, NPs with a
⁎ Corresponding authors. E-mail addresses: [email protected] (R. Hang), [email protected] (B. Tang).
http://dx.doi.org/10.1016/j.elecom.2016.08.002 1388-2481/© 2016 Elsevier B.V. All rights reserved.
length of 160 μm can be produced and the longer NTs are more suitable for many applications. 2. Material and methods Mirror-polished NiTi alloy (50.8 at.% Ni) sheets (Φ9.0 mm × 2 mm) were used as the substrates. Anodization was conducted in a 250 ml two-electrode cell with a NiTi sheet as the anode and platinum sheet as the counterpart at 35 °C. 100 ml of the electrolyte composed of EG, H2O, and concentrated HCl were used in the anodization experiments on a power supply (IT6123, ITECH, China). The H2O content mentioned hereafter represented that H2O in the electrolyte together with that in concentrated HCl. The anodization time was 20 min unless otherwise stated. The anodization voltage and H2O and HCl concentrations in the electrolyte were varied to investigate their respective influence on the anodic behavior of the NiTi alloy. Under the optimized experimental conditions, the anodization time was extended to observe the influence on the NP length. A field-emission scanning electron microscope (FESEM, JSM-7001F, JEOL) equipped with an energy-dispersive X-ray spectrometry (EDS, QX200, Bruker) was used to examine the morphology and determine the elemental composition. 3. Results To investigate the influence of HCl on the anodic behavior of the NiTi alloy, EG electrolytes with different HCl concentrations are used. Fig. 1(a) shows the surface, sub-surface, and cross-sectional FE-SEM images of the samples anodized in electrolytes with different HCl concentrations. The surface morphology of the sample anodized without HCl is similar to that of the mechanically polished one. After adding 0.016 M
R. Hang et al. / Electrochemistry Communications 71 (2016) 28–32
29
Fig. 1. (a) Surface, sub-surface, and cross-sectional FE-SEM images of the samples anodized at 10.0 V in the EG containing 5.0 vol% H2O and different HCl concentrations. (b) and (c) Crosssectional FE-SEM images of the NPs anodized at 10 V in the EG containing 5.0 vol% H2O and 0.25 M HCl. (d) EDS spectrum of the NPs fabricated in the EG electrolyte containing 0.5 M HCl. (e) Corresponding j-t curves during anodization.
HCl into the electrolyte, cracks can be observed and doubling the concentration produces an irregular nanoporous structure. Irregular NPs with a limited length are observed when the HCl concentration is increased to 0.063 M and further increasing the HCl concentration produces irregular NPs that cover the sample surface uniformly. After taking away the irregular layers, ordered NPs can be observed. The pore diameter is about 70 nm irrespective of HCl concentrations. However, the length increases from 5.0 μm to 29.8 μm when the HCl concentration is increased from 0.125 M to 0.75 M. Fig. 1(b) shows higher magnification cross-sectional image of the NPs anodized in the EG containing 0.25 M HCl compared with that in Fig. 1(a). It seems that the NPs span the entire oxide layer. Local enlarged image of Fig. 1(b) (Fig. 1(c)) clearly shows the NPs are relatively straight, which indicates they grow perpendicular to the surface of the substrate. Fig. 1(d) shows the EDS spectrum of the anodized NPs revealing the presence of Ni, Ti, O, and
Cl. Since Cl comes from the Cl-containing electrolyte, the NPs are composed of Ni, Ti, and O and designated as Ni-Ti-O NPs. Fig. 1(e) presents the current density-time curves of the samples during anodization. The current density maintains at zero during the experiment when no HCl is added into the electrolyte. In the presence of HCl, the current density drops rapidly initially and then decreases gradually until reaching a relatively stable value. The steady-state current density (SSCD) increases with HCl concentrations. The H2O content in the electrolyte is another key parameter that influences the anodic behavior of the NiTi alloy. As shown in Figs. 2(a), 3.0 vol% H2O results in an irregular surface morphology. However, when the H2O content is between 5.0 vol% and 11.0 vol%, irregular NPs emerge, but further increase decreases the nanoporous structure. The cross-sectional images show that the NP length increases from 18.3 μm for 5.0 vol% H2O to 41.0 μm for 11.0 vol% H2O, but no ordered
30
R. Hang et al. / Electrochemistry Communications 71 (2016) 28–32
Fig. 2. (a) Surface and cross-sectional FE-SEM images of the samples anodized at 10.0 V in EG containing 0.5 M HCl and different H2O contents. (b) Corresponding j-t curves during anodization.
cross-section can be observed when the H2O content is 19.0 vol%. The corresponding j-t curves in Fig. 2(b) indicate that the current density depends largely on the H2O content in the electrolyte. Increasing the H2O content elevates the initial and steady-state current densities. However, the SSCD determined from the electrolyte without H2O is
larger than that from the electrolytes containing 5.0 vol% and 7.0 vol% H2O. Another important factor that influences the anodic behavior of the NiTi alloy is the applied voltage. As shown in Fig. 3(a), the surface morphology of the sample anodized at 2.5 V is similar to that of the
Fig. 3. (a) Surface FE-SEM images of the samples anodized at different voltages in EG containing 0.5 M HCl and 5.0 vol% H2O. The voltage for each sample is shown in the upper left corner in each image. (b) Corresponding j-t curves during anodization.
R. Hang et al. / Electrochemistry Communications 71 (2016) 28–32
mechanically polished one, even though some scattered white dots can be observed. Anodization at 5.0 V produces irregular large pores (100– 300 nm in diameter) in comparison with that anodized at 10.0 V. Although a larger voltage (20 V) generates the nanoporous structure, the NP length is very limited. The j-t curves reveal a voltage dependence and increase in the H2O content significantly elevates the SSCD. Finally, the influence of the anodization time on the NP length is investigated. As shown in Fig. 4(a), the NP length increases with anodization time and Fig. 4(b) reveals a nearly linear relationship between the NP length and anodization time. The NP length increases from 4.0 μm at 5 min to 160 μm at 320 min. Further increase anodization results in spontaneous detachment of the NPs from the substrate, which may be ascribed to their relatively high internal stress. 4. Discussion Fabrication of Ni-Ti-O NTs on nearly equiatomic NiTi alloy by anodization has many applications [14,16,18,22,26]. However, previous use of Fcontaining electrolytes in the fabrication limits the NT length compromising the properties. In this work, a Cl-containing electrolyte is designed to prepare ultra-long Ni-Ti-O NPs instead of NTs. By adopting the optimal experimental conditions, the NP length can reach 160 μm and the materials which have a larger specific surface area are more suitable for applications including drug delivery, electrocatalysis, photocatalysis, and sensing. Formation of self-assembled nanostructures by anodization is mainly ascribed to the balance between growth of the oxide film and its dissolution under the attack of aggressive ions. In this system, an initial
31
compact layer composed of TiO2 and NiO may be formed by hydrolyzing the constituents of the NiTi alloy under an electric field according to the following reactions: Ti þ 2H2 O → TiO2 þ 4Hþ þ 4e−
ð1Þ
Ni þ H2 O → NiO þ 2Hþ þ 2e−
ð2Þ
Formation of the oxide layer decreases the current density because of the poor electrical conductivity. After the compact layer is formed, TiO2 and NiO start to dissolve under the attack of Cl− by the following reactions [27]: −
TiO2 þ 4Hþ þ 6Cl → TiCl6 2 −
NiO þ 2Hþ þ 4Cl → NiCl4 2
−
−
þ 2H2 O
ð3Þ
þ H2 O
ð4Þ
Thereafter, a balance between formation and dissolution of the oxide layer is gradually established as manifested by the relatively stable current density. The concentrations of Cl− and H2O in the electrolyte and anodization voltage affect the dynamic balance and consequently formation and characteristics of the NPs. Our results show that Cl− is essential to the anodic growth of Ni-Ti-O NPs. A larger Cl− concentration increases the NP length possibly due to fast etching of the newly formed oxide layer at the bottom. H2O is also fundamental to anodic growth of the NPs because it offers the source of O for electro-oxidation of Ti and Ni. Although there is some H2O in concentrated HCl, the amount is too
Fig. 4. (a) Cross-sectional FE-SEM images of the samples anodized for different time at 10.0 V in EG containing 0.5 M HCl and 5.0 vol% H2O. Anodization time for each sample is shown in the upper left corner of each image. (b) NP length as a function of anodization time.
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small for fast growth of the oxide layer in order to balance dissolution under attack by Cl−. When the H2O content is in the range between 5.0 vol% and 11.0 vol%, NPs are formed and the length increases with H2O contents. One possible explanation is that a larger H2O content enables fast oxidation of Ti and Ni and lowers the viscosity of the electrolyte thus accelerating Cl− etching of the oxide layer and the overall electrochemical reaction. However, too much H2O may disrupt the balance thus resulting in the formation of the irregular structure. The voltage window pertaining to anodization of NiTi in Cl-containing electrolytes is relatively small compared to that for F-containing electrolytes [26]. Only a voltage of 10.0 V can generate ordered NPs. A smaller voltage may not be sufficient to drive oxidation and migration of Cl− and a larger voltage may produce excessive etching of the newly formed oxide layer by Cl− resulting in a limited NP length. It is recognized that long NPs with high specific surface area can contribute to the improvement of their desired properties [23]. Accordingly, the NPs prepared by the present work may substantially improve a variety of properties that Ni-Ti-O NTs possessed, because the length of Ni-Ti-O NPs (160 μm) after anodization for 320 min is N120 times of the longest Ni-Ti-O NTs reported previously [26]. 5. Conclusion Ordered Ni-Ti-O NPs are fabricated on nearly equiatomic NiTi alloy by anodization in an electrolyte composed of EG, H2O, and HCl. The optimal H2O content in the electrolyte is 5.0–11.0 vol%, HCl concentration is 0.125–0.5 M, and anodization voltage is 10 V. The diameter of NPs is about 70 nm. The NP length increases almost linearly with anodization time and reaches 160 μm after anodization for 320 min under the optimal experimental conditions. Acknowledgements This work was jointly supported by the National Natural Science Foundation of China (31400815), Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (201626), Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 11301215, and City University of Hong Kong Applied Research Grant (ARG) No. 9667122. References [1] F. Keller, M. Hunter, D. Robinson, Structural features of oxide coatings on aluminum, J. Electrochem. Soc. 100 (1953) 411–419. [2] T. Yoshida, K. Terada, D. Schlettwein, T. Oekermann, T. Sugiura, H. Minoura, Electrochemical self-assembly of nanoporous ZnO/Eosin Y thin films and their sensitized photoelectrochemical performance, Adv. Mater. 12 (2000) 1214–1217. [3] T. Yoshida, H. Minoura, Electrochemical self-assembly of dye-modified zinc oxide thin films, Adv. Mater. 12 (2000) 1219–1222. [4] H. Masuda, K. Yada, A. Osaka, Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution, Jpn. J. Appl. Phys. 37 (1998) L1340.
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