01 C to reduce Ce3+ into elemental Ce deposition on TNTs. This modified sample was named as TNTs-Ce. Secondly, several TNTs-Ce samples were oxidized by potentiostat powered by an anodic potential E = 1.0 V to the sample in supporting electrolyte SB203580 (0.01 M Ce(NO3)3) for total electricity Q = 0.00001, 0.00025, 0.005, and 0.01 C, respectively. The oxidized samples were denoted as TNTs-0.00001 C, TNTs-0.00025 C, TNTs-0.005 C, and TNTs-0.01 C, correspondingly. The morphologies were observed using

field emission scanning electron microscope (FE-SEM, JSM-7500 F) with energy dispersive X-ray spectroscopy (EDX). The crystal phases and composition were characterized by X-ray diffraction (XRD, Y-2000) and X-ray photoelectron spectroscopy (XPS, MT-500, with Al monochromator with C1s at 284.8 eV). The photocurrent response measurements were carried out in an improved three-electrode electrochemical cell with a quartz window and 0.1 M Na2SO4 as supporting electrolyte. A 450-W Xeon lamp, a CT110 monochromator LDE225 chemical structure (1/8, Crowntech), and a potentiostat (PARSTAT2273, Princeton Applied Research, Oak Ridge, TN, USA) were also applied

for electrochemistry measurements. The Mott-Schottky plots were performed with frequency 1,000 Hz and applied potential from -1.0 to 0.5 V by 0.1 V steps. Results and discussion Figure 1 shows the SEM images of the (A) TNTs, (B) TNTs-Ce, (C) TNTs-0.00025 C, and (D) TNTs-0.01 C. Figure 1A indicates an average diameter of 50 Bay 11-7085 nm and tube length of 2 μm of TNTs. After deposition, the morphology of the TNTs was changed by reductive Ce or oxidative Ce. Cross section SEM and EDX are also employed

to confirm the decoration of Ce in the tubes from Figure 1C,D,E,F. From the EDX spectra, the nanotubes near the top contained more Ce (Ti/Ce = 3.17) than the nanotubes near the bottom (Ti/Ce = 10.98). Figure 1 SEM images. Of (A) TNTs with inset cross section image, (B) TNTs-Ce, (C) TNTs-0.00025 C with inset cross section image, (D) TNTs-0.01 C, (E) and (F) corresponding EDX spectra of e and f in (C). According to XRD patterns in Figure 2A, TNTs indicate anatase crystal phase. The simple substance Ce can be identified on TNTs-Ce. After anodic oxidation, the elemental Ce and CeO2 are detected in the deposited materials. They agree well with the reported values from JPCDS card (TiO2 73-1764), (Ti 44-1294), (Ce 38-0765), and (CeO2 44-1001). Figure 2 XRD patterns and XPS spectrum survey. (A) XRD patterns for (a) TNTs, (b) TNTs-Ce, and (c) TNTs-0.00025 C. (B) XPS spectrum survey of various samples. XPS spectrum of (C) Ce3d, (D) O1s, and (E) Ti2p of TNTs-0.00025 C. Figure 2B shows the survey of various samples, and Figure 2C,D,E shows the XPS spectra of TNTs-0.00025 C. The characteristic peaks of Ce3d are splitted to multipeak structure and fitted according to reference [14], besides O1s and Ti2p. The oxidative Ce is a mixture of Ce, Ce2O3, and CeO2. The relative proportions are calculated from the fitting data as Table 1.