SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION SUPPLEMENTARY DISCUSSION AND FIGURES 1. Chemical and Structural Characterization (a) Grazing-incidence small-angle X-ray scattering (GISAXS) The structural evolution of the mesoporous MoO 3 thin films upon thermal treatment was followed by GISAXS. The data shown in figure S1 were collected at an angle of incidence, β, of 0.2. By using 30 micron thick silicon substrates, it was also possible to carry out experiments in transmission mode, i.e. at β = 90 (inset in figure S1). For small angles of incidence, the films produce patterns with distinct scattering maxima which can be indexed in terms of a face centered cubic (fcc) system with (111) orientation relative to the plane of the substrate. Both fundamental and second order diffraction peaks can be observed, indicating the highly ordered nature of the nanoscale porosity. We note that in order to see the higher ordered peaks such as the (2 0 2 ), the images must be color mapped to saturate the fundamental diffraction peaks. Some of these peaks are already slightly elongated due to stacking faults as there is a statistical occurrence of ABA stacking in the regular ABCABC sequence. 1 These two effects combine to make the (200) and (1 1 1 ) diffraction peaks appear as lines, rather than distinct spots. From the relative position of the out-of-plane reflections, a unidirectional lattice contraction of % normal to the substrate is determined for films heated above 400 C. Patterns taken in transmission mode show isotropic diffraction rings, characteristic of randomly oriented domains parallel to the plane of the substrate. GISAXS also demonstrates that the cubic pore-solid architecture can be retained up to almost 500 C. nature materials 1

2 supplementary information Figure S1: Structural evolution as a function of temperature. Grazing-incidence SAXS on mesoporous MoO 3 films with amorphous (200 C) and crystalline walls (> 400 C). Data were collected at angles of incidence of 0.2 as well as 90 (inset, obtained on a sample heated to 450 C) and show the evolution of the fcc mesophase upon thermal treatment. Scattering vector s components are given in units of 1/nm; s = 2/λ sinθ. 1 Ruland, W. & Smarsly, B. M. Two-dimensional small-angle X-ray scattering of selfassembled nanocomposite films with oriented arrays of spheres: determination of lattice type, preferred orientation, deformation and imperfection. J. Appl. Cryst. 40, (2007). (b) X-ray photoelectron spectroscopy (XPS) XPS was used to determine the elemental makeup and the oxidation state of molybdenum. Figure 2d shows a typical survey scan, which confirms the absence of any contaminants. Aside from a weak C1s peak that we associate with adventitious carbon, only molybdenum and oxygen core levels can be observed. XPS further reveals symmetric peaks for the Mo3d region (single doublet with binding energies of / ev and / ev for the d 3/2 and d 5/2 2 nature MATERIALS

3 supplementary information lines, respectively, inset in figure 2d) indicating only the Mo 6+ oxidation state. For films calcined at 450 C in oxygen, the atomic oxygen-to-molybdenum ratio is found to be / The slight deviation from stoichiometric MoO 3 can be attributed to surface oxygen species, such as OH-groups. (c) Raman spectroscopy We have used Raman spectroscopy to determine the presence of any β-moo 3 phase in our calcined mesoporous films. The spectra were acquired using a confocal Raman microscope (CRM200, WITEC, Germany) equipped with an objective from Nikon (x20, NA = 0.17) and a linear polarized laser (diode pumped green laser, λ = 532 nm, CrystaLaser). Mesoporous crystalline samples heated to various temperatures between 400 and 450 ºC were examined. 1D-WAXD patterns were obtained as an initial screen of crystalline phase. Figure S2a shows patterns for two samples, one calcined at 420 C and the other at 450 C. The line patterns correspond to reference card # for orthorhombic α-moo 3 (black lines) and # for monoclinic β-moo 3 (red lines) according to JCPDS. α-moo 3 (molybdite) is the thermodynamically stable phase and consists of a layered arrangement of edge and corner-linked MoO 6 octahedra. β-moo 3, by contrast, is metastable and crystallizes in a monoclinic ReO 3 structure in which the MoO 6 octahedra share only the corners with each other. Neither WAXD pattern shows the presence of β-moo 3 as all reflections can be attributed to α-moo 3. Because X-ray diffraction can be insensitive to poorly crystallized materials, Raman measurements were carried out to determine whether the β-moo 3 phase was present (Figure S2b). In the figure, (A) is the bare silicon wafer used as the substrate for the films. (B) and (C) refer to mesoporous films that had been calcined at 400 and 450 C, respectively. The Raman nature materials 3

4 supplementary information measurements indicate the presence of a small amount of β-moo 3 for samples that had been calcined at 400 C. Spectra obtained on samples that had been heated above 400 C are consistent with pure-phase α-moo 3. The films calcined at 450 C show the characteristic vibrations for α-moo 3 : 825 cm -1, 672 cm -1, 342 cm -1 and 294 cm The sample calcined at 400 C exhibits these modes along with a low intensity vibration at 902 cm -1 which is characteristic of the β-moo 3 phase. 2 This result implies that β-moo 3 is completely transformed into α-moo 3 at temperatures above 400 C, which is in good agreement with WAXD. All electrochemical measurements made on mesoporous crystalline films were carried out on samples which had been calcined at 450 C in oxygen for 2 min. 4 nature MATERIALS

5 supplementary information Figure S2: Atomic structure of 180 nm thick mesoporous α-moo 3 films with layered nanocrystalline walls. (a) WAXD data obtained on samples heated to 420 and 450 ºC shows the presence of only α-moo 3. (b) Raman spectra for bare Si substrates (A), mesoporous MoO 3 heated to 400 ºC (B), and mesoporous MoO 3 heated to 450 ºC (C). Raman peaks in the sample heated to 450 ºC are consistent with pure-phase α-moo 3. In contrast, the sample heated to 400ºC shows the presence of a small amount of β-moo 3. 2 McEvoy, T. M. & Stevenson, K. J. Spatially resolved imaging of inhomogeneous charge transfer behavior in polymorphous molybdenum oxide. I. Correlation of localized structural, electronic, and chemical properties using conductive probe atomic force microscopy and raman microprobe spectroscopy. Langmuir 21, (2295). 2. Charge Storage (a) Distinguishing between capacitive and intercalation processes Capacitive effects were characterized by analyzing the voltammetric response at various sweep rates according to: i = av b (1) in which the measured current i obeys a power law relationship with the sweep rate v. 3 Both a and b are adjustable parameters. The kinetic limitations associated with traditional cation insertion are quite different from those associated with surface redox processes. Insertion is a nature materials 5

6 supplementary information diffusion controlled process and, thus, the current flow at any given voltage is expected to vary with the square root of the sweep rate (b = 0.5) according to: i = nfac*d 1/2 v 1/2 (αnf/rt) 1/2 π 1/2 χ(bt) (2) where C* is the surface concentration of the electrode material, α the transfer coefficient, D the chemical diffusion coefficient, n the number of electrons involved in the electrode reaction, A the surface area of the electrode material, F the Faraday constant, R the molar gas constant, T the temperature, and the function χ(bt) represents the normalized current. 4 In contrast, surface redox processes are not diffusion controlled and, thus, the current should vary linearly with the sweep rate (b = 1) according to: i = vc d A (3) in which C d represents the capacitance. 4 The calculated b-values for mesoporous α-moo 3 films are shown in figure S3. With the exception of the insertion peak potentials, the b-values are close to 1, indicating that the current is predominantly capacitive in nature. At 2.4 V and 2.1 V vs. Li/Li +, respectively, the b-values are smaller, which shows that the current arises from capacitive effects mixed with contributions from the diffusion-controlled Li + insertion reaction. The analysis, however, indicates that the major part of the total stored charge is capacitive. 6 nature MATERIALS

7 supplementary information Figure S3: Power-law dependence of charge storage kinetics for mesoporous α-moo 3 as a function of potential (cathodic sweep). 3 Lindstrom, H. et al. Li + ion insertion in TiO 2 (anatase). 2. Voltammetry on nanoporous films. J. Phys. Chem. B 101, (1979). 4 Bard, A. J. & Faulkner L R. Electrochemical methods: fundamentals and applications. John Wiley and Sons, New York, (b) Double-layer capacitance and cycling behavior The double layer capacitance for mesoporous MoO 3 films was determined by using 1.0 M tetrabutylammonium (TBA + ) perchlorate in propylene carbonate as the electrolyte in 3- electrode cells with lithium serving as both the reference and counter electrodes. These results were compared to those obtained using identical 3-electrode cells but where 1.0 M LiClO 4 served as the salt. The sweep rate in figure S4 was 10 mv/s. The TBA + result gives the doublelayer capacitance while the Li + experiment is comprised of contributions from pseudocapacitance and Li + intercalation (i.e., faradaic processes) in addition to the double-layer capacitance. From nature materials 7

8 supplementary information the data, it is evident that the double layer capacitance contributes only a small amount to the total charge storage. We find that the double layer capacitance, (~ 10 to 20 C/g) contributes < 5 % of the total capacitive charge storage for mesoporous α-moo 3 and ~ 10 % for mesoporous amorphous MoO 3. Thus, most of the capacitive charge storage for mesoporous MoO 3 comes from pseudocapacitive processes. In addition, we see that the mesoporous films exhibit good reversibility and that the total charge storage decreases only slightly between the second and tenth cycles. Figure S4. Voltammetry sweeps (for mesoporous amorphous MoO 3 ) comparing lithium and tetrabutylammonium electrolytes. Much lower charge storage is observed with TBA + than with Li +, indicating that only a small fraction of the charge storage stems from double layer capacitance. The materials also show good retention of capacity upon cycling. (c) Further discussion on intercalation vs. redox pseudocapacitance Some final insight on insertion processes versus capacitance can be gained from examining charge storage as a function of time. The data in figure S5 were generated by measuring the sweep rate dependence of the current, enabling capacitive and diffusion-controlled 8 nature MATERIALS

9 supplementary information contributions to be separated as a function of charging time. For very short charging times (here 20 s), one expects to have only capacitive contributions to charge storage. This is indeed observed, however, the magnitude of charge storage for α-moo 3 is approximately three times larger than that of the mesoporous amorphous films (280 C/g vs. 90 C/g). These amounts are both much higher than one would obtain for double-layer effects, which are calculated to be about 20 C/g based on the data shown in figure S4. However, the fact that so much more charge is being stored with α-moo 3 in this short time period suggests that there must be a pseudocapacitive mechanism contributing to charge storage in the crystalline material which is not contributing to charge storage in the amorphous one. As discussed in the text, the most plausible explanation is a contribution from intercalation pseudocapacitance. The charging time of 200 s provides an interesting period because it is at this time when diffusion-controlled intercalation begins to take place. At 200 s of charging, the films have achieved their maximum capacitive contributions of 450 C/g and 150 C/g, respectively. The factor of three difference in capacity between the two samples is maintained as shown in figure 3c. nature materials 9

10 supplementary information Figure S5: Capacitive and diffusion-controlled contributions to charge storage. Comparison of the total charge stored in mesoporous α-moo 3 films (indicated by C) with amorphous (indicated by A) samples after various charging times. 10 nature MATERIALS