Nano Res.

Electronic Supplementary Material

Aqueous and mechanical exfoliation, unique properties,and theoretical understanding of MoO3 nanosheets madefrom free-standing α-MoO3 crystals: Raman mode softening and absorption edge blue shift

Hongfei Liu1 (), Yongqing Cai2, Mingyong Han1, Shifeng Guo1, Ming Lin1, Meng Zhao1, Yongwei Zhang2,

and Dongzhi Chi1

1 Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way,

Singapore 138634, Singapore 2 Institute of High Performance Computing (IHPC), A*STAR (Agency for Science, Technology and Research), 1 Fusionopolis Way, Singapore

138632, Singapore

Supporting information to DOI 10.1007/s12274-017-1733-x

Figure S1 Schematic illustration of the thermal vapor transport (TVT) set up employed for producing α-MoO3 submicron thick belts, the crystals nucleated on the inner wall of the quartz reactor (1-inch in diameter), hierarchically grew along their length directions, and eventually loosely fill the chamber at the highlighted outlet location.

Address correspondence to liuhf@imre.a-star.edu.sg

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Figure S2 (a) Transmittance spectrum collected under micro-spectrophotometer from an individual α-MoO3 belt, the solid curve is a fragment spectral fitting using the optical index of α-MoO3 presented in (b). With the obtained layer thickness of the free-standing α-MoO3 belt, the optical absorption coefficients can be derived from the absorbance spectrum (see Fig. 2 and the main text).

Figure S3 Comparison of the vibrational patterns of Raman active phonons for monolayer (a) and bulk (b) α-MoO3. The calculations are based on the density functional perturbation theory (DFPT), using Quantum Espresso package with local density functional approximation. Norm-conserving Troullier-Martins pseudo potentials with energy cutoff up to 70 Ry are used. The first Brillouin zone is sampled with an 8 × 3 × 8 and 8 × 1 × 8 Monkhorst-Pack grids for bulk and monolayer α-MoO3, respectively. For monolayer MoO3, a vacuum region with thickness of 1.6 nm is adopted. All the structures are fully relaxed until the forces exerted on the atoms are < 0.1 eV/nm.

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Figure S4 Plots of square of Abs. h as a function of photon energy for (a) the aqueous exfoliated MoO3-BSA samples A and B and (b) the submicron-thick α-MoO3 single belt. Those of the difference between the MoO3-BSA sample A and the BSA solution are also plotted in (b) for easier comparisons.

Detailed Procedures for Structural Analysis

Mechanical Exfoliation: The scotch-tape method is used in this work for the mechanical exfoliations, which is

well understood as a common technique that peels flakes off from van der Waals crystals in their layer-stacking

directions.

BSA-Assisted Aqueous Exfoliation: This method has recently been developed in our group (see ref. 20, i.e., J.

Am. Chem. Soc. 2015, 137, 6152–6155). In this process, the BSA molecules interact with, and partly attached on,

the surface of the bulk crystals while the other parts of the molecule body are floating in the solution. Upon

sonication, the surface layer of the van der Waals crystals can be peeled off the attached BSA molecules. The

new exposed surfaces will be again attached by BSA molecules. The interactions between BSA and crystal surfaces

in aqueous solutions as well as their influence on crystal exfoliations under various aqueous conditions can be

found in ref. 20 and the Electronic Supplementary Information therein.

A General Comparison between the Mechanical and BSA-Assisted Aqueous Exfoliations: To compare the

mechanically and the aqueous exfoliated α-MoO3 nanosheets, we realized that (i) the surface of the mechanically

exfoliated nanosheets is much cleaner and free of contaminations, which is more suitable for fabricating nanoscale

electrical devices while the aqueous exfoliated nanosheets suffered from residual BSA contaminations. The

aqueous exfoliated nanosheets have to be deposited/coated on desired substrates for further characterizations

and/or applications and, during this process, surface-related challenges (e.g., hydrophobicity) could be faced in

promoting the uniformity; (ii) an additional step, e.g., plasma treatment, to get rid of the residual BSA from the

aqueous exfoliated MoO3 nanosheets is generally needed for their further applications. This step may also

cause nanosheets damaging. In fact, we have already observed plasma treatment-induced pores in an aqueous

exfoliated MoO3 nanosheet under TEM. The formation of such pores is probably related to the intrinsic oxygen

vacancies that were incorporated in their mother bulks during the crystal growth. Upon plasma treatment, the

vacancies may diffuse and coalesce to form voids within the 2D structure. In comparison, such an addition step

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Nano Res.

is not needed for the mechanically exfoliated nanosheets; (iii) the aqueous exfoliated nanostructures are

generally smaller than those produced by mechanical exfoliations, they are not even big enough for optical

characterizations such as photoluminescence and Raman scattering; and (iv) aqueous exfoliated MoO3, feasibly

produced in large scale, may have better catalytic applications, e.g., in chemical sensing, energy storage, etc.,

when compared with those of mechanically exfoliated ones.

XRD: GADDS (general-area-detector diffraction system) equipped with a Cu-Kα1 X-ray source was used for

measuring the XRD patterns of the belt crystals. The X-ray beam, ~500 m in beam size, is aligned by using

a visible laser (650-nm) to the sample area on the holder. The beam alignment is monitored by a CCD

(charge-coupled device) camera. This system has an advance of being sensitive to crystal phase structures. For the

measurement in this work, the belt crystals were gently pressed on a double side tape that was already pasted

on the standard XRD sample holder. The XRD curve was collected in the standard 2/ mode.

TEM: Structural analysis by TEM was conducted on an FEI Titan 80-300 S/TEM (scanning/transmission

electron microscope) operated at 200 keV. The TEM was carefully calibrated using standard Si single crystals.

The orientation of the nanoplate was determined by Fast Fourier Transform (FFT) image of selected areas in

high resolution TEM images. The specimen were prepared by fishing the nanopalte from the supernatant using

TEM grid followed by Ar-ion plasma cleaning to remove the residual BSA.

EDX: EDX spectra were collected in an SEM system (JSM 7600) for the belt crystals which were generally

pressed a double side tape that was already pasted on the standard SEM holder. For the EDX analysis, the

working distance was set at 15 mm and the electron beam energy was set at 15 keV while the probe current was

set at 12 A for an optimized count rate. The elemental analysis was done in a squared area of about 20 m

20 m on the belt surface.

Raman Scattering: Witec alpha-300 system was used for the confocal micro-Raman spectroscopy at room

temperature. The 532-nm laser was guided through an optical fiber into the microscope system and the phonon

scatted light was collected with the same microscope in a backscattering configuration. For individual belt

samples, they can be picked up by tweezers and put on silicon or sapphire holder, the holder is also used as a

Raman-shift calibration reference.

UV-VIS Spectroscopy: UV-VIS-NIR micro-spectrophotometer (QDI 2010, CRAIC Technologies) was used for

measuring the absorbance and transmittance of the solid-state samples. For these measurements, the samples

were put on sapphire substrates which are also used as reference for the optical spectra measurements because

of their larger optical bandgap (~6.2 eV). For the liquid samples, quartz cuvette was used, and during

measurement empty cuvette was used a reference.

AFM: The Nanowizard III instrument (JPK Instruments AG, Berlin, Germany) equipped with the NanoWizard

head and controller was employed for the AFM measurements. The nanostructures were imaged by using standard

silicon tips (k ~ 40 N/m, Tap 300AL-G, Budget sensors) working in a tapping mode under ambient conditions.

All the images were processed using the JPK data processing software (Version 4.2).

Detailed Procedures for Theoretical Calculations

The bulk α-MoO3 consists of atomically thin layered structures with each layer bonded by van der Waals forces.

For the theoretical calculations in this study, the relaxed lattice constants of bulk α-MoO3 are taken a = 0.3808, b =

1.3156, and c = 0.3676 nm; they are slightly underestimated when compared with experimental values. For the

monolayer MoO3, its in-plane lattice constants are (a = 0.3771 and c = 0.3744 nm) slightly larger than those of bulk

α-MoO3. There are three types of oxygen sites in α-MoO3 and they are the terminal oxygen at the surface (O1c),

the twofold-coordinated oxygen (O2c) bridging two Mo atoms, and the threefold-coordinated oxygen atom (O3c).

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The calculation for Raman intensity of phonons was via Porezag’s approach by averaging orientations of

incident and scattered photons [S1].

Although the Raman mode frequencies of α-MoO3 are well recorded previously, the vivid picture of atomic

vibrating patterns is less reported. In particular, to our best knowledge, the mode frequencies and intensities of

monolayer MoO3 are still lacking in the literature. In this light, in Figure-S3 shown above we summarized the

atomic vibrating patterns and their vibration frequencies for all the Raman active modes of monolayer and bulk

MoO3, respectively. Since the unit cell of bulk α-MoO3 contains two MoO3 sheets, its phonon numbers are twice

that of monolayer. Most of the phonon modes of monolayer MoO3 correspond to two modes of bulk MoO3

which differing from each other with respect to the relatively vibrating phase of the neighboring sheets in the

unit cell.

Reference

[S1] Porezag, D.; Pederson, M. R. Infrared intensities and Raman-scattering activities within density-functional theory. Phys. Rev. B

1996, 54, 7830–7836.