Capacitive and Oxidant Generating Properties of Black Colored TiO 2 Nanotube Array Fabricated by Electrochemical Self-Doping

Transcription

1 Supporting Information Capacitive and Oxidant Generating Properties of Black Colored TiO 2 Nanotube Array Fabricated by Electrochemical Self-Doping Choonsoo Kim a, Seonghwan Kim a, Jaehan Lee, a Jiye Kim a and Jeyong Yoon a a School of Chemical and Biological Engineering, College of Engineering, Institute of Chemical Process, Seoul National University (SNU), Gwanak-gu, Daehak-dong, Seoul , Republic of Korea Corresponding author. jeyong@snu.ac.kr (Phone: ; Fax:

2 Figure S1. (a) Schematic process for fabricating the Black TiO 2 NTA and the Blue TiO 2 NTA: For the preparation of Black TiO 2 NTA, as-prepared TiO 2 NTA was thermally treated at 200 o C under atmospheric condition (A 1 ) leading to the Red TiO 2 NTA (amorphous). Then, the Red TiO 2 NTA was electrochemically doped by electrochemical self-doping and annealed at 450 o C under nitrogen condition (A 2 ); The Blue TiO 2 NTA was fabricated with the annealing of as-prepared TiO 2 NTA at 450 o C under atmospheric condition (B 1 ) and electrochemical self-doping (B 2 ): The electrochemical self-doping was conducted under constant current (0.017 A/cm 2 ) in phosphate buffer solution (([KH 2 PO 4 ] 0 = 0.1 M, ph = 7.2); (b) Mott-Schottky plot ([KH 2 PO 4 ] 0 = 0.1 M (ph = 7.2), AC potential = 10 mv, DC potential = open circuit potential, 25 o C) of the Black and Blue TiO 2 NTA Figure S1a and b show the fabricating protocols and level of dopants of two TiO 2 NTA (the Black TiO 2 NTA and the Blue TiO 2 NTA), respectively. As shown in Figure S1a, the Black TiO 2 NTA was prepared with electrochemical self-doping of the Red TiO 2 NTA (amorphous) and N 2 annealing, whereas the Blue TiO 2 NTA was fabricated by electrochemical self-doping of pristine TiO 2 NTA (anatase). Note that the Black TiO 2 NTA can be only fabricated by the 2

3 electrochemical self-doping of amorphous TiO 2 NTA, while the Blue TiO 2 NTA is fabricated by the electrochemical self-doping of anatase TiO 2 NTA. Moreover, the Black TiO 2 NTA exhibited the higher level of dopant than the Blue TiO 2 NTA as supported by the lower slope of the Black TiO 2 NTA in Mott-Schottky plot (Figure S1b) in comparison with that of the Blue TiO 2 NTA. The high doping level of Black TiO 2 NTA can be attributed the larger spatial channel with disordered structure and defects of amorphous TiO 2 NTA for proton intercalation by the electrochemical self-doping than those of anatase TiO 2 NTA 1-2. Figure S2. XRD patterns of the TiO 2 nanotube array (NTA) samples (as-prepared, Red and pristine TiO 2 NTA) in reference to the JCPDS (PDF ): The A and T represent anatase and titanium, respectively. 3

4 Figure S2 presents the crystal structure of as-prepared, Red and pristine TiO 2 NTAs investigated by High-Resolution X-ray Diffraction (XRD, Bruker D8 DISCOVER, Germany) in reference to the anatase phase TiO 2 NTA obtained from the JCPDS (PDF ). Note that the XRD patterns of the pristine TiO 2 NTA were from the result of Figure 2b. As shown in Figure S2, typical peaks related with the anatase and rutile phases were not observed for the as-prepared and Red TiO 2 NTAs except for the titanium peaks, and it indicates they are the amorphous phase 3. On the other hand, the peaks in the pristine TiO 2 NTA were identified as the anatase phase through reference to the JCPDS (PDF ). In summary, the asprepared and Red TiO 2 NTA were, as a result, the amorphous phases, while the pristine TiO 2 NTA was an anatase phase. (b) Figure S3. Comparison of cyclic voltammograms for the Black TiO 2 NTA and amorphous 4

5 TiO 2 NTA at a scan rate of 100 mv/s ([KH 2 PO 4 ] 0 = 0.1 M with NaOH (ph = 7.2). Figure S3 shows the cyclic voltammograms of the Black TiO 2 NTA compared with the Red TiO 2 NTA. Note that as-prepared TiO 2 NTA did not reveal any remarkable difference for the shape of the cyclic voltammograms from that of the Red TiO 2 NTA (data not shown). As shown in Figure S3, a significant difference with respect to the charging current and oxygen evolution reaction (OER) is seen between the Black and Red TiO 2 NTAs. The charging current of the Black TiO 2 NTA within V was detectably higher than that of the Red TiO 2 NTA. In terms of OER, the Black TiO 2 NTA showed an evident OER under a potential of 1.2 V. However, no OER was observed for the Red TiO 2 NTA. These results suggest the Red TiO 2 NTA does not possess good capacitive and oxidant generating properties, and it implies that electrochemical self-doping has a critical role in providing the capacitive and oxidant generating properties of the Black TiO 2 NTA. Figure S4. Comparison of cyclic voltammograms (a) and Mott-Schottky plot (b) of the Black 5

6 TiO 2 NTA, the Annealed TiO 2 NTA, and the pristine TiO 2 NTA: The cyclic voltammogram and Mott-Schottky plot were obtained at a scan rate of 100 mv/s in 1 M of Na 2 SO 4 and at an AC potential of 10 mv and frequency of 100 Hz (b) ([KH 2 PO 4 ] 0 = 1 M with NaOH (ph = 7.2)), respectively: The Annealed TiO 2 NTA indicates the fabricated TiO 2 NTA without electrochemical self-doping with annealing of the Red TiO 2 NTA at 450 o C under N 2 gas condition without electrochemical self-doping. In order to prove the effect on the electrochemical self-doping of the Black TiO 2 NTA, N 2 annealed TiO 2 NTA without electrochemical self-doping (denoted as the Annealed TiO 2 NTA) was prepared by annealing of Red TiO 2 NTA at 450 o C under N 2 gas condition without electrochemical self-doping. Figure S4a and b show the comparison of cyclic voltammograms, Mott-Schottky plot of the Black TiO 2 NTA, Annealed TiO 2 NTA and pristine TiO 2 NTA, respectively. As can be seen in Figure S4a, the Annealed TiO 2 NTA exhibited the nearly rectangular shape of the cyclic voltammograms, and its plateau current was higher than that of the pristine TiO 2 NTA in the potential range of V. This result indicates a good capacitive property of the Annealed TiO 2 NTA based on the doping effect caused by annealing under N 2 condition. This doping effect which is called carbon doping effect was presumed to be attributed to the residual ethylene glycol on surface of TiO 2 NTA which was used as electrolyte in the anodization. The Hu and coworkers 4 proposed that the residual ethylene glycol plays a critical role in doping process as the carbonaceous precursors during the annealing under an inert gas condition. Nevertheless, the plateau current of the Black TiO 2 NTA exhibited even higher than that of the Annealed TiO 2 NTA, and its areal capacitance (ca.15.6 mf/cm 2 ) was approximately 40% enhanced in comparison with that of the Annealed TiO 2 NTA (ca mf/cm 2 ), indicating the electrochemical self-doping can 6

7 provide the additional doping effect along with carbon doping. These results can be further supported by the level of dopant which can be evaluated from the inverse slope in Mott-Schottky plot (Figure S4b). As shown in Figure S4b, the linear slope of the Annealed TiO 2 NTA showed much lower than that of the pristine TiO 2 NTA, but higher than that of Black TiO 2 NTA (the inset of Figure S4b). It suggests that the electrochemical self-doping leads to the additional level of dopant in the Annealed TiO 2 NTA. Additionally, fluorine also can be one of the candidates triggering the doping effect of Black TiO 2 NTA since electrolyte containing fluoride ion was employed in anodization step to fabricate the as-prepared TiO 2 NTA (amorphous) in order to fabricate Black TiO 2 NTA. However, it was generally known that the doping effect of fluorine is expected to be dramatically reduced with annealing under high temperature 3. Therefore, it can be explained by that the fluorine doping effect would be minimal as compared to the doping effect based on carbon and electrochemical self-doping. Figure S5. XRD patterns (a) and XPS spectra (b) of the Black TiO 2 NTA and Annealed TiO 2 NTA 7

8 Despite of these distinct electrochemical properties between Black and Annealed TiO 2 NTA (Figure S4), it was observed that no remarkable difference of the surface characteristics examined by XRD patterns (Figure S5a) and XPS spectra (Figure S5b) of the Black TiO 2 NTA and Annealed TiO 2 NTA. Figure S6. Cycling stability of the Black TiO 2 NTA as supercapacitor; the cycle performance of the Black TiO 2 NTA was examined with cyclic voltammetry measurements a scan rate of 100 mv/s in 1 M of Na 2 SO 4 : The inset displays the representative cyclic voltammograms of the Black TiO 2 NTA for 5,000 th cycles. Figure S6 presents the cycling stability of the Black TiO 2 NTA as supercapacitor. The cycling stability was examined with cyclic voltammetry measurements at a scan rate of 100 mv/s for 5,000 cycles. As can be seen in Figure S6, the capacitance retention reduces only ca. 4% for 8

9 5,000 th cycles, indicating the superior stability of the Black TiO 2 NTA as supercapacitor. Cited literatures (1) Fang, H. T.; Liu, M.; Wang, D. W.; Sun, T., Guan, D. S.; Li, F.; Zhou, J.; Sham, T. K.; Cheng, H. M. Comparison of the Rate Capability of Nanostructured Amorphous and Anatase TiO 2 for Lithium Insertion Using Anodic TiO 2 Nanotube Arrays. Nanotechnology 2009, 20, (2) Xiong, H; Slater, M. D.; Balasubramanian, M.; Johnson, C. S.; Rajh, T. Amorphous TiO 2 Nanotube Anode for Rechargeable Sodium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, (3) Grimes, C. A.; Mor, G. K. TiO 2 Nanotube Arrays: Synthesis, Properties, and Applications Springer; New York, 2009 (4) Hu, L.; Huo, K.; Chen, R.; Gao, B.; Fu, J.; Chu, P. K. Recyclable and High-sensitivity Electrochemical Biosensing Platform Composed of Carbon-doped TiO 2 Nanotube Arrays. Anal. Chem.2011, 83,