Articleshttps://doi.org/10.1038/s41565-018-0134-y

Exfoliation of a non-van der Waals material from iron ore hematiteAravind Puthirath Balan   1,2,11, Sruthi Radhakrishnan1,11, Cristiano F. Woellner   3, Shyam K. Sinha4, Liangzi Deng5, Carlos de los Reyes6, Banki Manmadha Rao7, Maggie Paulose7, Ram Neupane7, Amey Apte1, Vidya Kochat1, Robert Vajtai   1, Avetik R. Harutyunyan8, Ching-Wu Chu5,9, Gelu Costin10, Douglas S. Galvao3, Angel A. Martí6, Peter A. van Aken4, Oomman K. Varghese7, Chandra Sekhar Tiwary1*, Anantharaman Malie Madom Ramaswamy Iyer1,2* and Pulickel M. Ajayan1*

1Department of Materials Science and NanoEngineering, Rice University, Houston, TX, USA. 2Department of Physics, Cochin University of Science and Technology, Kochi, Kerala, India. 3Applied Physics Department and Center for Computational Engineering and Sciences, State University of Campinas – UNICAMP, Campinas, Brazil. 4Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research, Stuttgart, Germany. 5Texas Center for Superconductivity, University of Houston, Houston, TX, USA. 6Department of Chemistry, Rice University, Houston, TX, USA. 7Department of Physics, University of Houston, Houston, TX, USA. 8Honda Research Institute USA Inc., Columbus, OH, USA. 9Lawrence Berkeley National Lab, Berkeley, CA, USA. 10Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX, USA. 11These authors contributed equally: Aravind Puthirath Balan, Sruthi Radhakrishnan. *e-mail: cst.iisc@gmail.com; mraiyer@yahoo.com; ajayan@rice.edu

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Supplementary Information

Hematene: A new non van-der Waals 2D material Puthirath Balan Aravind 1,2,¥, Sruthi Radhakrishnan1, ¥, Cristiano F. Woellner3,

Shyam K. Sinha4, Liangzi Deng5, Carlos de los Reyes6, Banki Manmadha Rao7, Maggie Paulose7, Ram Neupane7, Amey Apte1, Vidya Kochat1, Robert Vajtai1, Avetik R. Harutyunyan8,

Ching-Wu Chu5,9, Gelu Costin10, Douglas S. Galvao3, Angel A. Martí6, Peter A. van Aken4, Oomman K Varghese7, Chandra Sekhar Tiwary1*, Maliemadom R. Anantharaman1,2* and

Pulickel M. Ajayan1,*

1 Department of Materials Science and NanoEngineering, Rice University, Houston, Texas, USA-77005.

2 Department of Physics, Cochin University of Science and Technology, Kochi, India-682022. 3Applied Physics Department and Center for Computational Engineering & Sciences, State

University of Campinas – UNICAMP, 13083–859–Campinas, Sao Paulo, Brazil 4 Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research,

Heisenbergstraße 1, 70569 Stuttgart 5 Texas Center for Superconductivity, University of Houston, Houston, Texas, USA-77004.

6 Department of Chemistry, Rice University, Houston, Texas, USA-77005. 7 Department of Physics, University of Houston, Houston, Texas, USA-77204.

8 Honda Research Institute USA Inc., Columbus, Ohio, USA, 43212. 9Lawrence Berkeley National Lab, Berkeley, California, USA, 94720

10Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, Texas, USA-77005

* Address correspondence to: ajayan@rice.edu (P.M.A.), mraiyer@yahoo.com (MRA), cst.iisc@gmail.com (C.S.T)

¥ These authors contributed equally

1. Hematite ore – var. specularite

The lamellar variety (called specularite or oligist) of hematite was obtained from a mine in

Northern Cape Province, Kuruman Iron Formation. They crystallize to form a lamellar structure

rather than granular mass. Specularite variety is commonly formed in shear zones, with a few

millimetric dimensions. The composition as determined by EPMA laboratory, Rhodes University

and confirmed by EPMA laboratory Rice University shows 99.82% Fe2O3 and 0.14% Al2O3. Table

S1 shows the composition of the crystal.

Table S1. The composition of the hematite crystal as determined by EPMA

Composition Percentage Std. Dev.

Al2O3 0.14 0.03

Fe2O3 99.82 0.80

Crystals commonly contain planes of atoms along which the bonding of the atoms is weaker

compared to other planes. In hematite with the trigonal crystal structure, {001} has the highest

broken bond density1. Alternating layers of Fe3+ and O2- form a dipole moment perpendicular to

{001} and tend to be less stable. On application of ultrasonic energy, the hydrodynamic forces

cleave the hematite crystal along the [001] and [010] directions. A pictorial representation is shown

in Figure S1.

Figure S1. A representative figure showing the exfoliation of hematite into layered hematene in [001] direction.

2. Scanning Electron Microscopy

Scanning electron microscopy (SEM) was performed to examine the morphology of hematene.

Figure S2a & S2b shows a typical low-magnification SEM image of the pristine and exfoliated

samples, which indicates the formation of 2D sheets with a few micrometres of lateral dimensions

on exfoliation from solid lumps of pristine hematite powder. The elemental composition of the

hematene is confirmed by wavelength dispersive spectroscopy (WDS), which shows that the

primary components are Fe and O (Figure S2c)

Figure S2. a) Scanning electron microscopy (SEM) image of pristine hematite (the scale bar is 20

µm) b) SEM micrograph of hematene (the scale bar is 5 µm) c) EPMA-WDS analysis of hematene

3. Transmission Electron Microscopy (TEM)

Figure S3. a) The STEM images of hematene, with the line profiles. The step-wise nature of the

line profile shows the 2D nature of hematene; b) STEM image of monolayer and intensity profile;

c) (i) Thickness map and (ii) bright field (iii) Intensity profile of a monolayer and bilayer hematene;

d) Low magnification bright field (i-iv) images of a monolayer, bilayer and few (three to four)

layers of hematene sheets. (vii-viii) Dark-field images of observed hematene sheets.

The exfoliated suspension was transferred to a holey carbon grid for observing through the

TEM. The line-intensity profile of high angle annular dark field STEM (HAADF-STEM) image

(mass-thickness image) of several randomly stacked 2D hematene is given in Figure S3 (a). The

intensity plot shows a step but not continuous intensity variation. Generally, HAADF intensity

gradually increases for the bulk materials as a function of thickness (from edge to core, when Z

remain constant), but here the intensity step is a clear indication for the formation of well-separated

2D materials which are stacked over each other. We have used such intensity variation to quantify

the single layer as shown in Figure S3b. Furthermore, we performed energy filtered TEM

(EFTEM) (above image), which shows thin 2D sheets of hematene Figure S3c. In the EFTEM

technique, a relative specimen thickness map can also be computed (given in the graph shown on

the right) by acquiring an unfiltered and a zero-loss image (same region and identical conditions)

using Poisson statistic of inelastic scattering. It gives the relative measure of the specimen

thickness in units of the local inelastic mean free path. Absolute thickness can be calculated from

cross-section TEM of the specific layer, which is complicated for the present study. The color map

clearly distinguishes monolayer and bi-layer. Also, the thickness is found to be uniform across the

sheet. The intensity profile across the bi-layer further confirm two sheets stacked randomly on top

of each other. Few more low magnification bright field images are shown in Figure S3d.

4. Atomic Force Microscopy

Atomic force microscopy (AFM) was employed to determine the thickness of exfoliated layers.

The AFM tapping mode was used in the appropriate regime of hematene transferred onto a silicon

substrate. The acquired topographic images (Figure S4) shows ultrathin 2D sheets of hematite

with a thickness of ~ 1 nm. Height profiling of the AFM image is indicated on the right-hand side

implies the apparent height of the sheets about the substrate is around 1 nm and would be a bi/tri-

layer of hematene sheets.

Figure S4. a) - f) AFM images of the 2D hematene sheets with corresponding 3D view on the

right and the representative height profile in between. g) The thickness and lateral size distribution

as observed from the AFM images.

In the exfoliation of 2D hematene from hematite, we have observed layers in the thickness range

from 2 to 10 layers. We have seen from experiments that the properties like magnetism and

photocatalysis that we report in the manuscript are observed in these layers. Minor variations in

the exfoliation and isolation procedure result in hematite bulk crystals and cause a change in the

properties we observe.

5. Raman Spectroscopy

Raman analysis is carried out to confirm retention of hematite phase in hematene and also to extract

evidence for the formation of ultrathin layers. Raman spectra confirm the hematite phase along

with some striking changes in intensities of the dominant modes and shift in frequencies of all the

observed Raman modes. The peaks are fitted to Gaussian functions to get the parameters, and the

results are summarized in Table S2. The Ag modes undergo a redshift while the Eg modes undergo

a blue shift on exfoliation and the degree of shift increases with the wavenumber. The lower modes

associated with bending movements of the molecule (223 and 289 cm-1) show pronounced

intensity variations when compared to the higher modes associated with stretching vibrations.

Table S2. The variation of Raman frequency and intensity of various observed Raman Modes of

hematite due to exfoliation is summarized.

Mode Bulk Exfoliated

Raman Shift(cm-1) Intensity (norm) Raman Shift(cm-1) Intensity (norm)

Ag 222.8 1 222.6 0.920 Eg 288.3 0.84 289.3 1 Eg 401.7 0.24 405.2 0.35 Ag 493.7 0.10 490.1 0.10 Eg 603.2 0.17 607.7 0.24 Eu - - 662.6 0.18 Magnon 1300.9 0.68 1304.1 0.64

We also observe the appearance of defects peaks in the Raman spectra on exfoliation of hematite

into hematene. The commonly observed LO and T modes ascribed to the loss of symmetry and

formation of tetragonal defects respectively in hematite are observed in hematene2,3. The ratio of

LO mode to the Eg mode in hematite increases from 0.18 in hematite to 0.77 in hematene. Besides

the loss of symmetry, the compensation of the surface by oxygen atoms leads to the appearance of

T mode in hematene which is found to be absent in hematite (see Figure S5). This has been

previously observed in nanoparticles on decreasing their size which increases the surface area2.

Figure S5. The defect peak in hematite and hematene

6. X-ray photoelectron spectroscopy (XPS)

Figure S6 shows the XPS spectrum of pristine hematite. The Fe2p spectrum is characteristic of

Fe3+ oxidation state. No other oxidation states corresponding to impurity phases are found and it

shows the purity of the bulk powder. From the O1s spectra, the lattice oxygen at lower binding

energy is predominant when compared to defective oxygen at higher binding energy.

Figure S6. XPS spectrum of pristine hematite

7. X-ray Diffraction

Figure S7. a) X-ray diffraction of pristine hematite and hematene with proper indexing; b)

An enlarged view of major peaks (104) and (110) to show the broadening and upshift of

peaks on exfoliation.

Figure S7a shows XRD pattern of pristine hematite and hematene. All the peaks of hematene can

be perfectly indexed to hematite corundum crystal structure (JCPDS card 33-664, a = 5.035Å and

c =13.74 Å). No peaks corresponding to β-FeOOH, Fe3O4, γ-Fe2O3, and other inorganic iron-

bearing phases detected in the diffraction spectra. In the case of hematene, the corresponding peaks

appear broadened and shifted w.r.t pristine hematite (Figure S7b). The uniform broadening of

diffraction peaks is due to the confinement since strain effects would increase with the order of

diffraction4,5.

8. Infrared Spectroscopy

The Fourier transform infrared spectra (FTIR) of pristine and exfoliated samples were

recorded yielding two modes near 520 and 433 cm-1(Figure S8), which are the A2u/Eu and Eu

bands, respectively. The former is due to the overlapping of A2u and Eu vibrations having dipolar

moments along and perpendicular to the c axis, respectively. All the observed vibrations were due

to stretching of Fe–O bonds6.

Figure S8. FTIR spectrum of pristine and exfoliated samples

9. Absorption and Photoluminescence

Figure S9a is the UV-Visible absorption spectrum of pristine hematite. The absorption

edge is around 660 nm. The optical band gap 𝐸𝐸𝑔𝑔 is estimated from the UV-Visible absorption

spectra Tauc-Plot (inset) using the formula for direct inter-band transitions given below

(𝛼𝛼ℎ𝜗𝜗)2 = 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐. (ℎ𝜗𝜗 − 𝐸𝐸𝑔𝑔) (1)

where ℎ𝜗𝜗 is the incident photon energy, 𝐸𝐸𝑔𝑔 is the optical band gap, and 𝛼𝛼 is the linear

absorption coefficient. The band gap energy of pristine sample is found to be 1.83 eV. Figure S9b

is the photoluminescence (PL) emission spectra of pristine hematite powder (the inset shows the

PL of hematene). It has a broad blue and a couple of ultraviolet emission bands which are due to

the ligand - to - metal charge transfer (LMCT) transitions (direct transitions)7,8. It is reported earlier

that an emission above the band-gap energy of hematite has been rarely observed. These emission

bands measured at higher energy than the bandgap are due to the intrinsic properties of the

electronic transitions in iron oxides. The Fe3+ ligand field and the Fe3+-Fe3+ pair transitions

(indirect transitions) extending into the visible region are not appreciable9,10. The same behavior

is observed for hematene too. However, the broad emission band slightly shifted to higher energy

(blue shift) which is acceptable since the band gap of hematene is higher compared to bulk due to

confinement effects. These similar kind of emissions in both pristine and exfoliated samples

implies that quantum confinement does not have great influence in photoluminescence.

Figure S9. a) UV-Visible absorption spectra of pristine hematite; the Tauc-plot for band gap

determination is shown in the inset; b) Photoluminescence emission spectra of pristine hematite

powder at an excitation wavelength of 280 nm.

10. Magnetic Properties

Figure S10 showcases the additional magnetic measurements to support our findings. Figure S10a

is the comparison of magnetization of pristine hematite and hematene at 10K. The pristine powder

behaves antiferromagnetic (AF) with very low coercivity and without saturation.

Figure S10. a) Low temperature (10K) hysteresis loop of pristine hematite and hematene; b) FC-

ZFC of hematene at 10 Oe and derivative of ZFC curve (dM/dT) shown in the inset; c) Comparison

of low temperature (10K) and room temperature (300K) hysteresis of pristine hematite.

However, hematene clearly shows a large coercivity (>200 Oe) and a higher degree of saturation

compared to pristine hematite, which confirms the presence of a WF phase at a lower temperature,

which in turn confirms the suppression of the Morin transition in hematene. Figure S10b is the

FC-ZFC measurements of hematene at a low field (10 Oe) to verify the consistency of the

behaviour (the suppression of the Morin transition is clear from dM/dT plot in the inset). Figure

S10c is the comparison of the magnetization of the pristine powder at room temperature (300K)

and low temperature (10 K). The behaviour is on expected lines. At 10 K, no hysteresis loop is

observed (very low coercivity and remanence), showing an antiferromagnetic (AF) behaviour. At

300 K, a hysteresis is observed (coercivity ~ 150 Oe), due to the weak ferromagnetic (WF)

behaviour of the material. The values of loop parameters for the pristine hematite and hematene

are shown in Table S3.

Figure S11. The hysteresis of hematene and hematite, the zoomed inset shows the magnetization

at the zero applied fields and the coercivity.

Table S3. Comparison of magnetic properties of pristine hematite and hematene

The influence of 2D morphology on the Morin transition could be understood in terms of

magnetic anisotropy energy as discussed by Wheeler et. al 11. The Morin transition; spin

reorientation at TM, arises from a change of sign of the total magnetic anisotropy energy Emag. This

energy term is the sum of magnetocrystalline anisotropy energy Emca, the shape anisotropy energy

Es and the surface anisotropy energy Esurface (lower symmetry and reduced co-ordination),

𝐸𝐸𝑚𝑚𝑚𝑚𝑔𝑔 = 𝐸𝐸𝑚𝑚𝑚𝑚𝑚𝑚 + 𝐸𝐸𝑠𝑠 + 𝐸𝐸𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑚𝑚𝑚𝑚𝑠𝑠 (2)

In the case of pristine hematite, Emca and Es contributions are dominant, which are almost

the same in magnitude but with opposite signs. At T>TM, negative Es becomes predominant to

Emca thus forcing the spins to orient perpendicular to c-axis. At T<TM, the positive Emca term

dominates Es such that spins align parallel to c-axis. In the case of hematene, the spin orientation

at room temperature remains intact, down to very low temperatures, implying that Emag remains

negative. Since the values of Emca and Es are almost the same, Esurface has a greater influence on the

orientation of spins. The surface contribution is likely to arise from anisotropy and exchange field

effects that occur around the surface iron atoms, where the coordination of iron atoms deviates

from that found in bulk material.

11. Photocatalysis

Titanium foils (0.25 mm thickness, 99.7% pure, Sigma-Aldrich) were ultrasonically cleaned with

Hysteresis Parameters Bulk Hematene Temperature

Hc (Oe) 160 200

300 K Mr (emu/g) 0.65 -

Ms (emu/g) 0.35 1.60

Hc (Oe) 40 230

10 K Mr (emu/g) 0.20 0.40

Ms (emu/g) - 2

a soap solution followed by acetone and isopropyl alcohol for 5 min each and dried in a nitrogen

jet. The anodization was performed in a two-electrode electrochemical cell with the titanium foil

as the working electrode (anode) and platinum (Pt) foil as the counter electrode (cathode) under a

constant voltage of 55 V at room temperature, in ethylene glycol electrolyte which contains 3 vol%

of deionized water and 0.3 wt% of NH4F. After completing the anodization process, the TiO2

nanotubes film was ultrasonically cleaned in isopropyl alcohol for 3 to 5 minutes to remove the

debris on the top of the nanotubes. The samples were annealed at 530 °C for 3 h in an oxygen

atmosphere. The 2D Fe2O3 nanosheets (hematene) were coated on nanotube films by drop casting

until its colour turned into brick-red. The Fe2O3 loaded nanotube samples were heated at 200 °C

for 20 minutes to remove the surface adsorbed organic compounds.

The sample is characterized by means of SEM and Raman spectroscopy (shown in Figure S12a-

d). Figures S8a and S8b respectively show the photographs and SEM images of the bare titania

nanotube array film and the hematene loaded titania nanotubes. As evident from Figure S12c

(inset), the greyish titania film turned into brick red upon loading hematene. The SEM images (see

Fig. S12 (a,b)) showed no significant difference between the surfaces of bare and hematene loaded

nanotubes except that some chunks of aggregated 2D sheets were seen on the surface (Figure.

S12c). The Raman spectrum of the hematene loaded TiO2 nanotube film is given in Figure S12d.

All peaks could be assigned to either the anatase phase of titania or Fe2O312,13. The low intensities

of the two peaks, 225.5 cm-1 (A1g) and 292.3 cm-1 (Eg), arising from hematite are presumably due

to the ultralow thickness of the sheets. It could also be due to low coverage on the nanotube surface.

The Raman study indicates that the Fe2O3 sheets stayed intact on TiO2 and no intermixing,

whatsoever, took place.

The photocurrent measurements were done using photoelectrochemical (PEC) cells in three

electrode mode using a CH instrument (model 660C) electrochemical analyzer. 0.5 M Na2SO4 was

used as the electrolyte. The hematene/TNT film, Ag/AgCl and Pt foil were used as working,

reference and counter electrodes respectively. A Newport (Model No: 67005) xenon arc lamp

source fitted with an AM 1.5 G filter was used for illumination. The output intensity was kept at

100 mW/cm2 using a National Renewable Energy Laboratory (NREL) calibrated silicon solar cell.

The Mott-Schottky measurements were also performed in a three-electrode configuration under

dark conditions at a frequency of 1000 Hz in 0.5 M Na2SO4.The incident photon to current

conversion efficiency (IPCE) was measured using a setup consisting of Newport monochromator

(Newport, Model No: 74125) and power meter. A two-electrode PEC cell configuration consisting

of a bare or hematene loaded titania nanotube photoanode and a platinum.

IPCE was calculated using the relation IPCE (%) = 100 h c j(λ)/e λ P(λ), where h is the Planck

constant, c the velocity of light, e the electron charge, λ the wavelength, j(λ) the photocurrent

density at λ and P(λ) the power density of light at λ. The solar photocurrent (AM 1.5G irradiance)

density j was calculated using the relation

𝑗𝑗 = ∫ 𝐼𝐼𝐼𝐼𝐼𝐼𝐸𝐸∞𝜆𝜆𝑚𝑚𝑚𝑚𝑚𝑚

𝐼𝐼(𝜆𝜆) 𝜆𝜆 � 𝑠𝑠ℎ𝑚𝑚� 𝑑𝑑𝜆𝜆 (3)

Here I(λ) is the power density of AM 1.5G solar radiation at the wavelength λ.

Figure S12. SEM images of a) bare titania nanotube array; b) Hematene loaded nanotubes and; c)

the nanotube surface having aggregated 2D sheets, and; d) Raman spectrum of hematene loaded

titania nanotubes.

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