Supporting Information A non-van der Waals 2D Material from natural titanium mineral ore Ilmenite

Transcription

1 Supporting Information A non-van der Waals 2D Material from natural titanium mineral ore Ilmenite Aravind Puthirath Balan 1,2, Sruthi Radhakrishnan 1, Ritesh Kumar 3, Ram Neupane 4, Shyam Kanta Sinha 5, Liangzi Deng 6, Carlos A. de los Reyes 7, Amey Apte 1, B. Manmadha Rao 4, Maggie Paulose 4, Robert Vajtai 1, Ching Wu Chu 6,8, Gelu Costin 9, Angel A. Martí 7, Oomman K Varghese 4 *, Abhishek K. Singh 3 *, Chandra Sekhar Tiwary 1,10 *, Maliemadom R. Anantharaman 1,2,11 *, and Pulickel M Ajayan 1 * 1 Department of Materials Science and NanoEngineering, Rice University, Houston, Texas-77005, USA. 2 Department of Physics, Cochin University of Science and Technology, Kochi , India. 3 Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka , India. 4 Department of Physics, University of Houston, Houston, Texas-77204, USA. 5 Department of Materials Engineering, Indian Institute of Science, Bangalore , India. 6 Texas Center for Superconductivity, University of Houston, Houston, Texas-77004, USA. 7 Department of Chemistry, Rice University, Houston, Texas-77005, USA. 8 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California-94720, USA. 9 Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, Texas-77005, USA. 10 Materials Science and Engineering, Indian Institute of Technology, Gandhinagar, Gujarat , India. 11 Inter University Center for Nanomaterials and Devices, Cochin University of Science and Technology, Kochi , India. correspondence to: okvarghe@central.uh.edu, abhishek@iisc.ac.in, cst.iisc@gmail.com, mraiyer@yahoo.com, ajayan@rice.edu authors contributed equally 1

2 Illmenite ore The ilmenite ore used for exfoliation is currently used as a secondary EPMA standard at the EPMA laboratory, Earth, Environmental and Planetary Sciences, Rice University. It represents picro-ilmenite, a Mg-rich ilmenite from De Beers mine, Kimberley, South Africa (Bultfontein mine). The sample represents a 0.5 mm size fragment from a mega crystal of Mg-rich ilmenite (initial crystal size was 1.1 cm). The composition details are summarized in Table S1 X-ray diffraction Figure. S1 shows the XRD of pristine ilmenite powder. The XRD pattern confirms the ilmenite phase with the rhombohedral crystal structure (JCPDS file ) with space group R. Scanning Electron Microscopy (SEM) SEM images of pristine powder and exfoliated ilmenite are shown in Figure S2. SEM micrographs of ilmenene (Figure. S2b& S2c) confirm the exfoliation of pristine sample into ultrathin sheets. Wavelength Dispersive Spectroscopy (WDS) elemental analysis confirm the purity of exfoliated ilmenite, having MgO impurities in small quantities, which is usual in ilmenite ore materials. Transmission Electron Microscopy The color maps (Figure. S3a) indicates the constituent elements. TEM (Figure. S3b&c), HRTEM (Figure. S3d) images and associated Fourier transform (Figure. S3d inset) further confirms the formation of monolayers and their crystalline nature. Atomic force microscopy (AFM) 2

3 Atomic force microscopy was carried out to find the thickness of exfoliated sheets. Figure. S4 summarizes the AFM analysis. Most of the flakes are of thickness were of around 2-3 nm which consists of roughly 5-6 layers thick sheets of ilmenene. Mechanism of exfoliation Schematic illustration showing the bulk FeTiO 3 structure projected along [110] is shown in the figure S5. Layers of O atoms represented by large red spheres, Ti atoms by small white spheres, and Fe atoms by small brown spheres. In this projection, the metal atoms are aligned normal to the plane shown. Within the cation layers, there are two possible termination planes due to the low and high position of cations, one through Fe cation layers and another one through Ti cation layers. Among the two available planes, the stable termination planes of the (001) bulk surface, through which the cleavage occurs during ultrasonic cavitation is shown by yellow arrow mark and is through the Fe cation layers which is substantiated by theoretical calculations. Optical and Magnetic Characterizations Figure. S6a is the absorption analysis of the bulk powder and the Tauc plot gives a direct band gap of 2.75 ev (DFT calculations confirm the band gap, and it is ~2.72 ev; refer Fig. S7a(iii)). We already saw (Figure. 3a in the main text) that the bandgap enhanced to 3.15 ev on exfoliation due to the quantum confinement in two dimension. The Photoluminescence spectrum of ilmenene is shown in the Figure. S6b. There is an intense blue emission at 410 nm which could be attributed to the F- centers and weak absorption at 383 could be due to the excitons around the F-centers. 1 Further, a very weak emission at 465 nm can be due to the defects like anion vacancies. Figure. S6c&d shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of pristine and exfoliated FeTiO 3 measured with applied fields 10 Oe. As the temperature 3

4 increases the magnetic moment in the FC and ZFC curves increases, reaches a maximum position around 20 K and thereafter at 26.8 K FC and ZFC merge together and steadily decrease up to 300 K. The point at which the differential plot of the ZFC curve (inset) takes an abrupt dip is assigned to be the Neel temperature of FeTiO 3 which is around 55K. Above the Neel temperature, Ilmenite is paramagnetic, while below T N the Fe 2+ layers couple antiferromagnetically and are separated by magnetically inert Ti 4+ layers. Figure. S6e&f highlights the full-range room temperature hysteresis along with the zoom in view in the inset, showing the coercive field and remanent magnetization. At room temperature (300 K), the bulk sample exhibits a coercivity of 26 Oe which increases to 31 Oe in the case of ilmenene and the remanent magnetization measured for pristine ilmenite is emu/g while it is emu/g for ilmenene. Photoelectrochemical studies Figure S7a&b shows the SEM images of bare and ilmenene loaded TiO 2 nanotube arrays which were employed for the photoelectrochemical and hydrogen evolution studies. Figure S7c represents a comparison of absorbance spectrum of ilmenene and photocurrent spectrum of ilmenene-titania nanotube array electrode in the visible light region. Theoretical Calculations Theoretical calculations were performed for the bulk ilmenite system as well for comparison. The optimized lattice parameters of bulk ilmenite are found to be: a = b = 5.02 Å and c = Å (Fig. S8 (i), (ii) & (iii)). The antiferromagnetic phase of bulk ilmenite has been found to be more stable than that of ferromagnetic in previous theoretical reports. 2-3 The results for magnetic ordering in the antiferromagnetic phase is summarized in Table S3. We could that µ Fe 3.5 µb and µ Ti 0 µb, corresponding to Fe 2+ and Ti 4+ ions, respectively. Experimental observations also 4

5 yielded major proportions of these ions in the bulk. The electronic bandstructure for the antiferromagnetic phase is shown in Fig. S8a(iv), where its band gap was obtained to be 2.72 ev. Fig. S8b(i)-(iv) summarizes the top and side view of Fe-terminated and Ti-terminated ilmenene and Fig. S8 (v) shows the phonon stability of Ti-terminated ilmenene. Table. S2 summarizes the energy and bandgap values of antiferromagnetic and ferromagnetic configurations of Titerminated ilmenene as a function of U. Supplementary Figures Figure S1. XRD of pristine Ilmenite powder 5

6 Figure S2. SEM micrographs of (a) pristine ilmenite (scale bar 400µm). (b) & (c) ilmenene (scale bar 1µm & 3µm respectively), and (d) WDS elemental analysis of ilmenene. 6

7 Figure. S3. (a) Color maps of constituent elements. (b)& (c) TEM (0.5µ, 5nm respectively), and (d) HRTEM (scalebar 2nm) and Fourier transform of ilmenene. 7

8 Figure. S4. AFM of an ilmenene sheets. (a) Ilmenene sheets drop casted on a silicon substrate. (b) An enlarged view of one single sheet. (c) Line profile of section AB gives the thickness of the ilmenene sheet (~1.7 nm). 8

9 Figure S5. Mechanism of exfoliation of bulk ilmenite in [001] direction. Exfoliation direction is indicated by yellow arrow. Atom color codes; Fe (brown), Ti (white) and O (red). The structure is adapted from AFLOW.ORG web entry generator V and could be accessed from the following link - Please note that the data included within the aflow.org repository is free for scientific, academic and noncommercial purposes. 9

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11 Figure. S6. (a) Absorption of pristine ilmenite; (b) Photoluminescence spectra of ilmenene; FC-ZFC magnetization of (c) pristine and (d) ilmenene; Room temperature hysteresis of (e) bulk ilmenite and (f) ilmenene, with the respective zoom in views in the inset. (c) Figure. S7. SEM images showing the (a) lateral and (b) top views of the titania nanotubes loaded with ilmenene. While FeTiO3 aggregates can be seen on the top surface of the nanotubes, 11

12 the 2D sheets on the walls of the nanotubes cannot be discerned. (c) A comparison of absorbance spectrum of ilmenene and photocurrent spectrum of ilmenene-titania nanotube array electrode in the visible light region. Figure S8(a) (i), (ii) & (iii) Crystal structure of bulk ilmenite as seen from a, c and b axes, respectively. Brown, blue and red colors denote Fe, Ti and O atoms, respectively. (iv) Electronic bandstructure and projected density of states (PDOS) of bulk ilmenite, as calculated from DFT+U method. (b) (i) & (ii) Crystal structure of Fe-terminated (001) ilmenene as seen from top and side views, respectively. (iii) & (iv) Crystal structure of Ti-terminated (001) ilmenene as 12

13 seen from top and side views, respectively. (v) Phonon bandstucture of Ti-terminated (001) ilmenene. Table S1. The constituent compounds of ilmenite and cation numbers. Constituent Compounds Cation numbers normalized to 3 Oxygen atoms Chemical Formula Percentage Name Cation Number TiO Ti FeO Fe MgO Mg Fe 2 O Fe Cr 2 O Cr MnO 0.55 Mn Al 2 O Al NiO 0.17 Ni CaO 0.13 Ca SiO Si Total Total

14 Table S2. Energy and bandgap values of antiferromagnetic and ferromagnetic configurations of Ti-terminated ilmenene as a function of U U [ev] E AFM [ev/unit cell] E FM [ev/unit cell] E FM-AFM [mev/unit cell] E g (AFM) [ev] E g (FM) [ev] Table S3. Local magnetic moments of transition metal atoms of bulk ilmenite for antiferromagnetic phase (U=7, J=1) Species Local magnetic moment ( µ ) Species Local magnetic moment ( µ ) Fe Ti Fe Ti Fe Ti Fe Ti Fe Ti Fe Ti µ avg 3.49 µ avg

15 References 1. Surdo, A.; Kortov, V.; Pustovarov, V., Luminescence of F and F+ centers in corundum upon excitation in the interval from 4 to 40eV. Radiation measurements 2001, 33, Wilson, N. C.; Muscat, J.; Mkhonto, D.; Ngoepe, P. E.; Harrison, N. M., Structure and properties of ilmenite from first principles. Physical Review B 2005, 71, Ribeiro, R. A. P.; de Lazaro, S. R., Structural, electronic and elastic properties of FeBO3 (B = Ti, Sn, Si, Zr) ilmenite: a density functional theory study. RSC Advances 2014, 4,