Microstructure and Pseudocapacitive Properties of Electrodes Constructed of Oriented NiO-TiO2

Microstructure and Pseudocapacitive Properties of Electrodes Constructed ofOriented NiO-TiO2 Nanotube ArraysJae-Hun Kim, Kai Zhu1∗ Yanfa Yan, Craig L. Perkins and Arthur J. Frank2∗National Renewable Energy Laboratory, Golden, Colorado 80401-3393TABLE S1: XPS-derived composition of Ni-Ti foil, annealed Ni-Ti foil, as-grown NT array, and annealed NT array.Atomic Concentrations (%)SampleTi Ni O C F S N K Sputtered Ni-Ti foil 50.1 49.9 --- --- --- --- --- --- Annealed Ni-Ti foil a32.9 3.0 52.4 10.4 --- 0.3 0.4 0.6 As-grown NT array 26.8 8.2 45.6 11.4 7.3 --- 0.8 --- Annealed NT array a19.6 17.0 47.8 14.0 --- 0.7 0.9 ---a Annealed at a temperature of 600 o C.XPS was used to characterize the composition and chemical states of the as-grown and annealed films. Low energy resolution survey spectra were used to determine sample composition. Composition was determined using standard sensitivity factors that, when applied to the sputter-cleaned foil substrate, yielded values indicating that they were approximately correct. Table S1 lists the XPS-derived composition of the metallic foil standard, annealed Ni-Ti foil, the as-grown NT film, and the annealed NT film. Anodization of the Ni-Ti foil to form the as-grown NT array was found to increase markedly the Ti:Ni ratio relative to the starting foil material, consistent with chemically driven surface segregation of titanium observed1for this particular alloy. Interestingly, annealing this film in air brought substantially more nickel to the surface and restored the Ti:Ni ratio. Oxidation of Ni-Ti films generally results in the formation of an oxide that contains relatively little nickel;1,2 however, in certain oxidation regimes, the Ni:Ti ratio can increase.3 The as-grown NT film also shows the presence of fluorine, which is attributable to the fluoride-containing electrolyte.4,5Traces of the elements of S, N, and K are attributed to contamination.∗To whom correspondence should be addressed. E-mail: 1Kai.Zhu@; 2AFrank@.Figure S1. XPS spectra of the (a) Ti 2p and (b) Ni 2p regions of the as-grown NT film and the 600 o C annealed Ni-Ti foil and NT film.High resolution XPS spectra were used to probe the composition and chemical states of nickel and titanium in the NT films. As shown in Figure S1(a), the Ti 2p3/2 peak was found at 459.0 eV and to have a 1.29 eV full-width at half maximum (FWHM) in the annealed NT arrays. This binding energy is typical of that found for Ti4+ in rutile TiO2 (458.8 eV).6 Figure S1(b) is comprised of XPS spectra of the Ni 2p region of the as-grown and annealed NT films. As is typical of the 2p photoelectron spectra of first row transition metals, the Ni 2p data show energy loss features in addition to the 2p spin-orbit doublet.7 In the annealed film, the Ni 2p3/2 peak (3.0 eV FWHM) is found at a binding energy of 856.3 eV. With thermal annealing, a low binding energy shoulder appears at ca. 854.6 eV. Although some previous work8 has attributed these binding energy positions and shifts to the formation of NiTiO3, XRD data (Figure 2) and selected area electron diffraction measurement (Figure S2) did not show the presence of NiTiO3 in our case. It is generally agreed that the 2p3/2 peaks for Ni0, Ni2+, and Ni3+ appear at 852.6 eV, 854.6 eV, and 856.1 eV respectively.7 Therefore, the XPS results suggest that the nickel in the annealed NT films is in a mixture of 3+ and 2+ oxidation states.Figure S2. Selected area electron diffraction (SAED) pattern of a NiO-TiO2 NT array annealed at 600 o C in air for 1 h. The characteristic patterns of NiO and rutile TiOare indicated.2Figure S3. Cyclic voltammograms of the as-grown NT film and the 600 o C annealed Ni-Ti foil and NT film; CV plots of the NT films are the same as those shown in Figure 4. The potential scan rate was 20 mV/s.Figure S3 shows the cyclic voltammograms (CV) of the as-grown NT film and the Ni-Ti foil and NT film annealed at 600 o C. All samples have the same projected area. It is evident that the capacitance (current density) of the foil is inconsequential compared to that of the annealed NT film.Figure S4. SEM image of the surface view of the NT film annealed at 700 o C in air for 1 h.Figure S4 shows the SEM image of the NT film annealed at 700 o C. In comparison to the SEM image of the 600 o C annealed NT film (Figure 3a), it is clear that increasing the annealing temperaturefrom 600 to 700 o C causes the collapse of the NT structure.Figure S5. Dependence of the peak current density on the scan rates for an electrode with NT films annealed at 600 o C in air for 1 h.Figure S5 shows that the peak current density increases linearly with the scan rate.Figure S6. XRD patterns of NT films annealed from 400 to 700 o C. The peaks indicated by asterisks correspond to the NiTi and Ni3Ti phases present in the foil.Figure S6 shows the XPD patterns of the NT films annealed at temperatures between 400 and 700 o C. The NT films that were annealed at 400–500 o C exhibit one dominant diffraction peak at about 42.3o, which corresponds to the Ni-Ti foil. There are also very weak peaks at 34.0, 36.8, and 37.6o, which cannot be identified from JCPDS (Joint Committee on Powder Diffraction Standards). These peaks are likely associated with an intermediate phase(s) comprised of Ni, Ti, and O. For the film annealed at 500 o C, the characteristic peak of the rutile TiO2 phase (27.5o) is observed. When the film was annealed at 600 o C, the XRD measurements show peaks attributable to the rutile TiO2 and rock salt NiO (e.g., 2θ= 37.3o) phases. When the annealing temperature is raised to 700 o C, the intensities of rutile TiO2peaks increase substantially, and the characteristic NiO peaks are no longer evident. Recently, it was shown that thermal annealing of TiO2NT films grown on a Ti substrate in air at temperatures ≥ 500 o C leads to the formation of rutile crystallites in the Ti substrate, which initiate the growth and propagation of the rutile phase in the anatase NT walls. 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