S-1

Supporting Information

Phase-Controlled Electrochemical Activity of Epitaxial Mg-Spinel Thin Films Zhenxing Feng

1,2, *, Xiao Chen

3, Liang Qiao

6, Albert L. Lipson

1,2, Timothy T. Fister1,2, Li Zeng

3,

Chunjoong Kim2,7

, Tanghong Yi2,7

, Niya Sa1,2, Danielle L. Proffit

1,2, Anthony K. Burrell1,2, Jordi

Cabana2,5

, Brian J. Ingram1,2, Michael D. Biegalski

4, Michael J. Bedzyk

3,4 5, Paul Fenter1,2,*

1Chemical Science and Engineering Division, 2Joint Center for Energy Storage Research

(JCESR), Argonne National Laboratory, Lemont, Illinois, 60439, United States

3Applied Physics Program, 4Department of Materials Science and Engineering, 5Department of

Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA

6Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge,

Tennessee, 37831, United States

7Department of Chemistry, University of Illinois at Chicago, Chicago, IL, 60607, United States

*Corresponding author email addresses: fengz@anl.gov (Z. Feng), fenter@anl.gov (P. Fenter)

Index Page

Supplementary Methods S-2 – S-3

Table S1 S-4

Figures S1 – S8 S-5 – S-10

S-2

Supplementary Methods

Pulsed Laser Deposition (PLD) Target Synthesis and Growth.

The MgMn2O4 (MMO) and La0.7Sr0.3FeO3 (LSFO) targets with 2 inch diameter were

synthesized using solid-state methods from stoichiometric mixtures of MgO and Mn2O3 (Alfa

Aesar, USA) powders for MMO, as well as La2O3, SrCO3, and Fe2O3 (Alfa Aesar, USA)

powders for LSFO, respectively. Both targets were calcinated at 1100 °C in air for 72 hours. The

TiC was a commercial target (PVD products, Inc). The MMO target was mixed phase, with the

majority as tetragonal phase, as confirmed by X-ray diffraction (XRD) in Figure S1.

Epitaxial TiC thin films on MgO(001) were prepared at 700 °C under vacuum condition

for 10000 pulses (~50 nm). The LSFO thin films on MgO(001) were prepared at 750 °C under

200 mTorr O2 for 7500 pulses (~50 nm). The MMO thin films on both TiC and LSFO films were

prepared at 500 °C under 10 mTorr O2 for 10000 pulses (~70 nm, estimated from the MMO

growth rate). PLD was done under the following conditions: KrF excimer laser (λ = 248 nm), 10

Hz pulse rate, ~1.5J/cm2 energy density. Reflection high-energy electron diffraction (RHEED)

was utilized for diagnostic in situ monitoring of the film growth.

The successful growth of different pure phases of MMO on either LSFO or TiC can be

seen from the specular θ-2θ scan in Figure S3.

X-ray Characterization.

Based on the specular XRD (Figure 1) and off-specular XRD (Figure S4) measurements

on tetragonal phase MMO (MMOT), the following epitaxial relationship of plane directions has

been obtained: MMOT[001]//LSFO[001]//MgO[001] and MMOT[100]//LSFO[110]//MgO[110].

The MMOT film has a 45° in-plane rotation with respect to LSFO film and MgO substrate. In

contrast, the cubic phase MMO (MMOC) has different epitaxial alignment with respect to the

substrates. As shown in Figure S5, the green arrows are the X-ray scan directions. The observed

peaks for corresponding thin films and MgO substrate in Figure 1c and 1d clearly indicate the

reciprocal diffraction patterns in Figure S5, and thus the following epitaxial relationship:

MMOC[001]//TiC[001]//MgO[001] and MMOC[100]//TiC[100]//MgO[100].

X-ray Photoelectron Spectroscopy Analysis.

The XPS spectra for Mn 2p3/2 can be decomposed and fitted quantitatively for MMOC at

charged (Figure 4) and uncharged (Figure S9) conditions according to multiplet theory.[2] The

S-3

data are fitted to three chemical states, namely Mn(II), Mn(III) and Mn(IV). The spectrum from

the standard MnO2 powder sample is best fit with Mn(IV) only.

S-4

Table S1: Epitaxial relationship as well as bulk and thin film lattice constants of materials in

this work. The in-plane strains are calculated using , where afilm_bulk is the

in-plane bulk lattice constant of the film material, and aunder is the in-plane lattice constant of the

underlying film (TiC and LSFO). For comparison, the last row lists the calculated in-plane

strains if MMOC were grown on LSFO and MMOT were grown on TiC. R45° denotes the in-

plane 45° rotation.

Materials MgO TiC MMOC La0.7Sr0.3FeO3 MMOT

aBulk/cBulk (Å) 4.212 4.328 8.600 3.929 5.727/9.284

Epitaxy − Cube-on-cube Cube-on-cube Cube-on-cube R45° in-plane

cFilm (Å) − 4.224 8.339 3.892 9.244

aFilm (Å) − 4.218 8.440 3.931 5.722

− −

1.94%

MMOC/TiC

− 3.02%

MMOT/LSFO

εxx − − -3.99%

MMOC/LSFO

− 9.39%

MMOT/TiC

εxx =a film _ bulk − aunder( )

aunder

εxx =

a film _ bulk − aunder( )aunder

S-5

Figure S1. X-ray diffraction pattern of MgMn2O4 (MMO) target matches the tetragonal phase of

MMO from PDF reference card 00-023-0392. The X-ray is from Cu Kα1 (λ = 1.5406 Å)

radiation.

S-6

Figure S2. 5 × 5 μm2 AFM images of (a) blank MgO substrate with clear terraces separated by

~ 4 Å. (b) Tetragonal MgMn2O4 grown on LSFO/MgO(001). (c) 2 × 2 μm2 AFM image of

cubic MgMn2O4 grown on TiC/MgO(001). Both MMOT and MMOC have same root-mean-

square roughness of 0.2 nm.

Figure S3. (a) θ-2θ specular X-ray diffraction on MMOT/LSFO/MgO(001) and

MMOC/TiC/MgO(001). No other phase of MMO and TiC were found, suggesting the phase pure

thin films have been synthesized. (b) The zoom-in of XRD around MgO(002) show the

successful stabilziation of different MMO phases on either TiC and LSFO layers.

S-7

Figure S4. (a) Off-specular X-ray diffraction on MMOT/LSFO/MgO(001) around MgO(202).

(b) φ scan of off-specular thin film peaks and MgO substrate peak reveals the 45o in-plane

rotation for MMOT.

S-8

Figure S5. The crystallographic epitaxial relationships of MMOC, TiC and MgO as seen in

reciprocal space. Circles indicate Bragg peak locations as a function of momentum transfer

along the specular (i.e., surface normal) and in-plane directions, which are shown in the vertical

and lateral directions, respectively. The color codes are: red for MgO, blue for TiC and orange

for MMOC. The green arrows are the X-ray scan directions. This clearly shows the following

epitaxial relationship of plane directions: MMOC[001]//TiC[001]//MgO[001] and

MMOC[100]//TiC[100]//MgO[100].

S-9

Figure S6. Radial X-ray diffraction spectra of cubic MMOC near the MgO(004). The MMOC

(008) shift is better observed.

S-10

Figure S7. Cyclic voltammetry of (a) epoxy on stainless steel and (b) TiC/MgO(001) with

epoxy on stainless steel in coin cells at 1 mV/s scan rate with BP2000 carbon on stainless steel

as the anode and 0.2 M Mg(TFSI)2 in PC as the electrolyte. The epoxy and TiC/MgO(001) on

stainless steel represent the background for MMO thin films. The figure is plotted using the

same scale in Figure 2.

Figure S8. Mn 2p3/2 X-ray photoelectron spectra (XPS) of (a) cubic MMOC thin film cathode at

uncharged condition and (b) MnO2 powder. Data is deconvoluted and fitted to the mixture of

Mn(III) and Mn(II) at the MMOC charged condition, and only Mn(IV) for MnO2 powder.