Organic-Intercalant-Free Liquid-Exfoliation Route to Layered Metal

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Organic-Intercalant-Free Liquid-Exfoliation Route to Layered Metal Oxide Nanosheets via the Control of Electrostatic Interlayer Interaction Jang Mee Lee, Bohyun Kang, Yun Kyung Jo, and Seong-Ju Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00566 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Materials & Interfaces

Organic-Intercalant-Free Liquid-Exfoliation Route to Layered Metal Oxide Nanosheets via the Control of Electrostatic Interlayer Interaction

Jang Mee Lee,† Bohyun Kang,† Yun Kyung Jo, and Seong-Ju Hwang*

Center for Hybrid Interfacial Chemical Structure (CICS), Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans University, Seoul 03760, Republic of Korea

†

Both the authors equally contribute to the present work.

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* To whom all correspondences should be addressed.

Tel) +82-2-3277-4370

Fax) +82-2-3277-3419

E-mail) [email protected]


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Abstract

A scalable organic-intercalant-free liquid-exfoliation route to 2D nanosheets (NSs) of layered transition metal oxides (TMOs) is developed by employing hydronium-intercalated derivatives as precursors. The replacement of interlayer alkali metal ions with larger hydronium ions via acid-treatment makes possible the efficient liquid-exfoliation of TMOs without any assistance of organic intercalant cations. Not only a weakening of interlayer electrostatic interaction upon hydronium-intercalation but also an efficient solvation of deintercalated hydronium ions via hydrogen bonding with polar solvents is mainly responsible for the high efficacy of hydroniumintercalated TMOs as precursors for liquid-exfoliation. The nature of solvent employed also has profound effect on the exfoliation yield of these TMO NSs; viscosity, surface tension, density, and Hansen solubility parameter as well as a capability to solvate the exfoliated NSs and hydronium ions are crucial factors for determining the exfoliation efficiency of hydroniumintercalated precursor. All the obtained Ti1xO2, MnO2, and RuO2 NSs show highly anisotropic 2D morphologies and distinct negative surface charges with zeta potential of 30~50 mV. Such distinct surface charges of these NSs render them versatile hybridization matrices for the synthesis of novel nanohybrids with enhanced functionalities. The hybridization with the liquidexfoliated TMO NSs is quite effective in improving the photocatalytic activity of CdS and the ACS Paragon Plus Environment

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electrode functionalities of graphene and graphenelayered double hydroxide nanohybrid. The present study underscores the usefulness of the present liquid-exfoliation method in synthesizing organic-free TMO NSs and their nanohybrids as well as in widening the application field of exfoliated TMO NSs.

Keywords: Organic-intercalant-free liquid-exfoliation, layered metal oxide nanosheets, hydronium-intercalation, electrostatic interlayer interaction, hydrogen bonding

Introduction

Recently 2D nanosheets (NSs) of inorganic solids have received prime attention because of their unique physicochemical properties originating from highly anisotropic 2D morphology with nanometer-level thickness and wide lateral dimension.1

In comparison with graphene,

inorganic 2D NSs like transition metal oxides (TMOs), transition metal dichalcogenides (TMDs), transition metal carbide, and layered double hydroxides (LDHs) show much greater diversity in chemical

compositions

and

physicochemical

properties,

which

provide

promising

functionalities in a wide spectrum of application areas ranging from renewable energy ACS Paragon Plus Environment

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technologies to nanobio technologies.28

To dates, many synthesis methods have been

developed for 2D inorganic NSs, which can be categorized into top-down and bottom-up approaches.912 While most of bottom-up processes rely on the controlled crystal growth of inorganic solid into highly anisotropic 2D nanostructure,1315 top-down process is typically achieved by the exfoliation of the pristine layered compounds into thin 2D NSs.1618 The topdown exfoliation route boasts many advantages such as the great tunability of the composition of inorganic NS and the high yield synthesis of monolayered NS.17 Depending on the type of host materials, several exfoliation routes, i.e. liquid-exfoliation, intercalation of bulky organic species, and lithiationhydroxylation, should be carefully selected to obtain 2D inorganic NSs.2023 Among them, the liquid-exfoliation process is most suitable for economically-feasible large-scale synthesis of few-layered 2D inorganic NSs. Several swellable inorganic solids like LDH and aluminosilicate clay are readily exfoliated into individual NS via the liquid-exfoliation process.24,25 In case of TMDs with weak van der Waals-type interlayer interaction, the loading of ultrasonic shear stress leads to the liquid-exfoliation of these materials into few-layered TMD NSs.2628 Conversely, except for few van der Waals-type oxides like MoO3 and V2O5, an efficient liquid-exfoliation method is not developed yet for most of layered TMOs with strong electrostatic interlayer interaction,29 although diverse valuable functionalities of these materials evoke great deal of research efforts for the exfoliation of TMO NSs.30 Such a difficulty in the ACS Paragon Plus Environment

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liquid-exfoliation of TMO originates from a strong electrostatic interaction between negativelycharged host layers and interlayer cations. Instead, the exfoliation of these TMO materials can be achieved by the intercalation of bulky organic cations like tetrabutylammonium (TBA) ions with significant weakening of interlayer interaction, i.e., chemical exfoliation.3133 Although this chemical exfoliation process with an assistance of organic intercalant is highly efficient in preparing monolayered TMO NSs, the use of organic cations offers several demerits such as increase of production costs, prolonged reaction time, environmental pollution by organic cations, and surface contamination of NS by adsorbed organic cations. Taking into account diverse excellent functionalities and superior thermal/chemical stabilities of TMO NSs over TMD/LDH NSs,34 it is highly demanded to develop an organic-intercalant-free scalable liquidexfoliation route to TMO NSs for circumventing the drawbacks of currently-utilized chemical exfoliation route. For the successful liquid-exfoliation of TMO, a strong electrostatic interlayer interaction of this material should be weakened before the loading of ultrasonic shear stress. Considering that the interlayer alkali metal ions in the TMO material can be easily exchanged with larger hydronium (H3O+) ions via acid-treatment,3133 the resulting hydronium-intercalated derivatives with markedly expanded basal spacing are supposed to be useful precursors for the liquid-exfoliation of these TMO materials. Despite intense research activities devoted for the exfoliation of TMO NSs,35 at the time of this submission, we are unaware of any other ACS Paragon Plus Environment

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reports on organic-intercalant-free liquid-exfoliation for layered TMO with strong electrostatic interlayer interaction. Here we report novel organic-free scalable synthetic route to the exfoliated 2D NSs of diverse TMOs by employing the corresponding hydronium-intercalated derivatives as precursors for liquid-exfoliation.

Several polar solvents with different densities, dielectric

constants, and molecular structures are tested as exfoliation media to probe crucial factors governing the exfoliation efficiency of TMO NSs. The obtained liquid-exfoliated TMO NSs are applied as hybridization matrices for synthesizing novel functional nanohybrids to verify the usefulness of the present liquid-exfoliation method as an economically feasible route to functional NS-based materials.

Results and Discussion Employing acid-treated TMOs as precursors for liquid-exfoliation. The liquid-exfoliation of layered solids can occur when the interlayer interaction of the pristine material is compensated by solvation energy with solvent molecule.36,37

However, since the electrostatic interlayer

interaction of layered TMO is too strong to be compensated by solvation energy, it is difficult to synthesize exfoliated TMO NS via liquid-exfoliation process. Such a strong interlayer bonding of layered TMO material can be significantly weakened by the intercalation of hydronium ion ACS Paragon Plus Environment

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via acid-treatment, because the exchange of interlayer alkali metal ions with larger hydronium (H3O+) ions leads to the notable expansion of basal spacing and thus the weakening of interlayer interaction.

Simultaneously, the presence of hydronium ions in the acid-treated

TMO precursor can provide an additional stabilization effect for its exfoliated state via an efficient hydrogen bonding between released hydronium ions and polar solvent molecules. Since both effects of hydronium-intercalation can facilitate the liquid-exfoliation process of TMO, the hydronium-intercalated derivatives are supposed to be useful precursors for synthesizing the liquid-exfoliated TMO NSs with monolayer and few-stacked layers, as illustrated in Figure 1A. A loading of ultrasound shear force on the hydronium-intercalated derivatives is helpful in increasing the exfoliation yield of layered TMO NSs.

Figure 1. (A) Schematic diagram to the liquid-exfoliation of ionic TMO material using hydronium-intercalated derivatives (hydrogen atoms are in white, oxygen atoms are in red, ACS Paragon Plus Environment

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carbon atoms are in black, nitrogen atoms are in clear blue, transition metal atoms within nanosheets are in faint blue, and alkali metal atoms between nanosheets are in dark yellow). (B) Crystal structures of (a) Ax/4Ti1xO2, (b) AxMnO2, and (c) AxRuO2 (A = alkali metal).

In this study, three kinds of layered TMOs with dissimilar physicochemical natures are employed as host materials for liquid-exfoliation, i.e. wide bandgap semiconducting Ax/4Ti1xO2, narrow bandgap semiconducting AxMnO2, and metallic AxRuO2 (A = alkali metal). As depicted in Figure 1B, the lepidocrocite (-FeOOH)-structured Ax/4Ti1xO2 lattice is composed of corrugated host layers with edge sharing and interlayer alkali metal (A) ions.38 The presence of Ti vacancies in this Ti1xO2 lattice provides negative layer charge for this TMO material, which is compensated by the presence of interlayer alkali metal ions. The other layered AxMnO2 and AxRuO2 materials are crystallized with rocksalt-type structure, in which alkali metal (A) and transition metal layers are interstratified, see Figure 1B. Since both the Mn and Ru ions possess mixed trivalent and tetravalent oxidation states, the corresponding MnO2 and RuO2 layers show negative layer charge. The Ti1xO2 layer has a thicker thickness of 0.7 nm than those of MnO2 and RuO2 layers (0.35 nm).3133 Also, several types of polar organic solvents with different molecular structures are employed as exfoliation media to probe the


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effect

of

chemical

interaction

between

solvent

molecules

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and

exfoliated

TMO

NSs/deintercalated hydronium ions on the efficiency of the liquid-exfoliation of TMO NSs. The structural evolution of the pristine Ax/4Ti1xO2, AxMnO2, and AxRuO2 materials upon acidtreatment with 1 M HCl aqueous solution is examined with powder X-ray diffraction (XRD) analysis. As presented in Figure S1 of Supporting Information, all the acid-treated derivatives commonly show the displacement of (0k0)/(00l) reflections toward low angle region, indicating the marked increase of basal spacing caused by the exchange of interlayer alkali metal ions with larger hydronium ions. The interlayer distances of the pristine TMOs and their acidtreated derivatives are calculated as 0.85 and 0.95 nm for Ti1xO2, 0.69 and 0.74 nm for MnO2, and 0.71 and 0.75 nm for RuO2, respectively.

Taking into consideration of the significant

effect of the charge density of layered TMO on its intercalation chemistry, the interlayer interaction is weaker for Ti1xO2 than MnO2 and RuO2 since the former has a lower charge density.39 The weaker charge density and interlayer interaction lead to more efficient cointercalation of water molecules, which is responsible for the greater basal spacing increment for Ti1xO2.

These increments in interlayer distance suggests the weakening of the

electrostatic interlayer interaction of layered TMO material by acid-treatment. The loading of ultrasonic shear stress on the hydronium-intercalated TMO materials dispersed in polar solvents yields opaque colloidal suspension of liquid-exfoliated TMO NSs, ACS Paragon Plus Environment

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as presented in Figure 2A. After the ultrasonic treatment, a small amount of precipitate is isolated by keeping the dispersions during overnight or centrifuging the solutions at 1000 rpm, resulting in the colloidal suspension of exfoliated TMO NSs.

The obtained colloidal

suspensions of TMO NSs remain unprecipitated for longer than 1 week, reflecting their good colloidal stability without any organic surfactant ion. As illustrated in Figure 2B, all the colloidal suspensions of Ti1xO2, MnO2, and RuO2 exhibit distinct Tyndall phenomena, confirming the formation of exfoliated 2D TMO NSs. The zeta potential measurement clearly demonstrates the negative surface charges of the obtained TMO NSs, which is in good agreement with those of chemically-exfoliated TMO NSs,3133 see Figure 2C.


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Figure 2. (A) Photoimages of the colloidal suspensions, (B) Tyndall phenomena, and (C) zeta potentials for the colloidal suspensions of liquid-exfoliated (a) Ti1xO2, (b) MnO2, and (c) RuO2 NSs.

Effect of solvent on the efficiency of the liquid-exfoliation of TMO. To determine the optimal solvents for liquid-exfoliation of TMO NSs as well as understand the crucial factors determining the exfoliation efficiency of TMO, several polar solvents with various physicochemical properties such as formamide, dimethyl sulfoxide (DMSO), methanol, ethanol, isopropyl alcohol (IPA), 1-butanol, and acetone are employed as exfoliation media. Diverse physical parameters including Hansen solubility parameter (HSP) of all the solvents employed here are summarized in Table 1, together with the exfoliation yields of TMO NSs. The exfoliation yield calculated from the ratio of exfoliated TMO NS over unexfoliated TMO material and the solution concentration of exfoliated TMO NSs are listed in Tables S1 and S2 of Supporting Information.

Table 1. Diverse physical parameters and the exfoliation yields of TMO NSs for several polar solvents.

T, D, P, and H represent the total, dispersive, polar, and hydrogen bonding

solubility parameters, respectively. The exfoliation yield is calculated by dividing the weight of


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floating NSs by the original weight of precursor material. The quantity of floating NSs is assessed by subtracting the weight of sedimented powder from the original weight of precursor after keeping it for overnight. Solvent

Dielectric

Viscosity

Surfac

Densit

Hansen solubility

Exfoliation yield

constant

(cP)

e

y (g

parameter (MPa1/2)

(wt%)

tensio

mL1)

(ε)

T

D

P

H

n (mJ

Ti1xO

MnO2

RuO2

2

m2) Formamid

111

3.34

57.0

1.13

36.6

17.2

26.2

19.0

77%

43%

11%

DMSO

46.7

1.99

42.9

1.092

26.7

18.4

16.4

10.2

61%



11%

Methanol

32.7

0.54

22.1

0.791

29.6

15.1

12.3

22.3

34%

20%

11%

Ethanol

24.5

1.07

22.0

0.789

26.5

15.8

8.8

19.4



31%

16%

IPA

18.2

2.04

23.3

0.803

23.6

15.8

6.1

16.4



32%



1-butanol

17.8

2.54

25.0

0.8095

23.2

16.0

5.7

15.8

70%

9%

14%

Acetone

20.7

0.31

23.0

0.7845

19.9

15.5

10.4

7.0







e

In principle, the colloidal stability of exfoliated TMO NS is determined by a compromise between gravity and buoyancy of NS. Since generally the liquid exfoliation yields much thicker NS than the chemical exfoliation,2628 a fraction of exfoliated TMO NS with large thickness suffers from huge contribution of gravity overwhelming that of buoyancy. Thus, the observed settlement of a fraction of liquid-exfoliated TMO NSs can be ascribed to their large thickness ACS Paragon Plus Environment

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with the remarkable contribution of gravity. The contribution of buoyancy can be enhanced by an efficient chemical interaction between TMO NS and solvent to overcome the counter-effect of gravity.

Among many solvents employed here, formamide can act as very efficient

exfoliation media for many TMO NSs. This observation can be rationalized by the largest values of formamide for dielectric constant, viscosity, surface tension, density, and HSP,40 reflecting its best capability to interact with polar inorganic species. Of prime interest is that the alcoholic solvents are quite useful as an exfoliation media for thinner MnO2 and RuO2 NSs rather than for thicker Ti1xO2 NS. As shown in Tables 1 and S1 of Supporting Information, like formamide, alcoholic solvents have relatively high hydrogen bonding parameters (H) of HSP than do the other solvents, indicating their high ability to form strong attractive interaction with polar inorganic species. This finding strongly suggests a crucial role of surface hydrogen bonding in the liquid-exfoliation of thin TMO NS. Additionally, the Mn K-edge X-ray absorption near edge structure (XANES) analysis demonstrates the partial reduction of redoxable MnO2 NS via the interaction with alcoholic solvent (see Figure S2 of Supporting Information), leading to the increase of the negative surface charge of MnO2 NS. The enhanced surface charge of liquid-exfoliated MnO2 NS is helpful for stabilizing its colloidal state via the enhanced interaction with solvent. Since such a change of surface charge is negligible for non-redoxable TMO NS like Ti1xO2, the Ti1xO2 NS shows only a negligible exfoliation yield in ethanol and ACS Paragon Plus Environment

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IPA solvent.


Among the alcoholic solvent employed here, 1-butanol shows the highest

exfoliation yields, which can be ascribed to its high viscosity, surface tension, and density. Conversely, acetone is the poorest solvent for the exfoliation of TMOs, because it has quite low values for all the above-mentioned parameters. Furthermore, the absence of hydroxyl and amide groups in acetone gives a significantly low H value of HSP, making it impossible to form hydrogen bonding with TMO NSs and hydronium ions. Such a molecular structure of acetone is also responsible for its poor efficiency as an exfoliation media for TMO NS. Characterization of the physicochemical properties of liquid-exfoliated TMO NSs. The optical properties of the liquid-exfoliated Ti1xO2, MnO2, and RuO2 NSs are investigated with UVvis absorbance spectroscopy to determine their molar absorption coefficients. Figure 3 presents the UVvis spectra and absorbances at specific wavelength (i.e. 300 nm for Ti1xO2, 430 nm for MnO2, and 350 nm for RuO2) for the colloidal suspensions of liquid-exfoliated TMO NSs with different concentrations.

The absorbances of all the present TMO NSs are linearly

proportional to the concentration of colloidal suspensions in a given concentration range, strongly suggesting no agglomeration of liquid-exfoliated NS. From these data, the molar absorption coefficients of Ti1xO2, MnO2, and RuO2 are determined to be 1.47103, 1.46103, and 1.19103 L mol1 cm1, respectively. The obtained molar absorption coefficients of liquidexfoliated TMO NSs are slightly smaller than the previously reported values of chemicallyACS Paragon Plus Environment

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exfoliated TMO NSs,3133,41 reflecting the slightly lower concentration of liquid-exfoliated TMO NS due to the formation of few-layered NSs. Furthermore, the formation of exfoliated TMO NSs in various organic solvents is evidenced by the observation of distinct absorbance peak at specific wavelength, see Figure S3 of Supporting Information. As compared to the chemicallyexfoliated TMO NSs, the present liquid-exfoliated homologues display distinct red shifts of the absorption edge. Considering the quantum confinement effect induced by exfoliation,42 the observed lower edge energies for the liquid-exfoliated NSs than the chemically-exfoliated NSs can be ascribed to the formation of few-layer stacked NS rather than monolayered NS. It is worthwhile to note that the type of solvents has notable influence on the intensity and energy of absorption peak, which is attributable to their dissimilar efficacy in stabilizing liquidexfoliated NSs. Due to the quantum confinement effect,42 the thinner TMO NS formed by efficient liquid-exfoliation process shows a higher energy for absorption peak. Simultaneously, the efficient exfoliation of TMO into thinner NS yields a higher concentration of colloidal suspension, leading to the enhancement of absorption peak. In one instance, the formamidebased suspensions of MnO2 and Ti1xO2 NSs show more intense absorption peaks at higher energies than the other suspensions (Figure S3), which is in good agreement of the high efficiency of formamide solvent for the liquid-exfoliation of TMO into thinner NSs.


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Figure 3. UVvis spectra (top) and absorbances at specific wavelength (bottom) for the colloidal suspensions of liquid-exfoliated (A) Ti1xO2, (B) MnO2, and (C) RuO2 with various concentrations: (a) 1/2, (b) 1/4, (c) 1/8, (d) 1/16, and (e) 1/32 of initial concentration.

The lateral crystal sizes, crystal structures, and layer thicknesses of the liquid-exfoliated TMO NSs are examined with transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and atomic force microscopy (AFM), respectively. As illustrated in Figure 4A, all the present liquid-exfoliated TMO NSs prepared in various solvents display submicrometer-sized 2D crystallites with faint contrast, underscoring successful exfoliation of bulk layered crystallite into very thin 2D NS. Some thick particles on the surface of exfoliated nanosheets are inevitably caused by shattering of the inorganic crystallite boundary under the


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ultrasonic shear stress. Facile controls in ultrasonic conditions such as sonication power and time as well as centrifugal force can allow to effectively remove the unnecessary thick particles. In addition, the maintenance of their original crystal structures upon the liquid-exfoliation is confirmed by the SAED analysis showing the clear (271) and (031) planes for Ti1xO2 NS, (1010) and (011) planes for MnO2 NS, and (102) plane for RuO2 NS, see Figure S4 of

Supporting Information. As shown in the AFM images of Figure 4B, the thicknesses of the present TMO NSs are determined as ~6.8, ~6.1, and ~11.1 nm for Ti1xO2, MnO2, and RuO2, respectively.

The present AFM results clearly demonstrate the successful exfoliation of

layered TMO materials into few-layered NS by employing the hydronium-intercalated derivatives as precursors.


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Figure 4. (A) TEM and (B) AFM images of liquid-exfoliated (a) Ti1xO2, (b) MnO2, and (c) RuO2 NSs with diverse polar solvents. The present colloids are spread on the surface of O2 plasmatreated silica substrate for AFM measurement.

The distribution plots of the lateral dimensions and layer thickness of TMO NSs are presented in Figure 5. The lateral dimensions of the present TMO NSs are estimated from the TEM images of 200 NSs. For this analysis, the polar solvents of formamide, DMSO, and ethanol are employed as media for the liquid-exfoliation of Ti1xO2, MnO2, and RuO2 NSs, respectively. The average lateral dimensions of these NSs are determined to be 217.5±113.3, 288.5±128.2, and 154.5±74.5 nm for Ti1xO2, MnO2, and RuO2, respectively. These lateral dimensions of liquid-exfoliated TMO NSs are somewhat smaller than those of conventional chemically-exfoliated NSs.41 This result is attributable to the application of strong ultrasonic shear stress, resulting in the cracking and shattering of layered crystallites. The present TEM results are cross-confirmed by dynamic light scattering (DLS) measurements, see Figure S5 of

Supporting Information. With the assumption that the shape of TMO NS is rectangular and the diagonal lines correspond to the lengths of NSs, the lateral dimensions determined from DLS measurement are similar to those estimated from TEM analysis. Also, the thicknesses of the obtained TMO NSs are evaluated from the AFM data of 200 NSs. The average thicknesses of


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the TMO NSs are determined as 9.0±3.2 nm for Ti1xO2 NS, 12.3±4.4 nm for MnO2 NS, and 10.4±4.3 nm for RuO2 NS, respectively, reflecting the successful formation of few-layer stacked NS with nanometer-level thickness. The percentages of the monolayers and a few layers ( 5 nm) are determined as 4.5% for Ti1xO2 NS, 1.5% for MnO2 NS, and 6.5% for RuO2 NS, respectively. The relatively larger thickness of these TMO NSs than the previously reported value of liquid-exfoliated TMD NSs can be mainly ascribed to the strong electrostatic interlayer interaction of TMO materials.2628

Figure 5. Distribution plots of (top) lateral dimensions and (bottom) layer thicknesses of liquidexfoliated (A) Ti1xO2, (B) MnO2, and (C) RuO2 NSs.

Statistical analyses of nanosheet

dimension and thickness are carried out with many TEM and AFM images.


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To verify the universal applicability of the present synthetic strategy, the liquid-exfoliated NSs of other complex TMOs (CaLaNb2TiO10 and [Mn1/3Co1/3Ni1/3]O2) are also synthesized by employing their hydronium-intercalated derivatives as precursors. As illustrated in Figure S6A of Supporting Information, the CaLaNb2TiO10 lattice is composed of 2D perovskite slabs (An1BnO3n1) with Ca/La ions in A site and Nb/Ti ions in B site, whereas the [Mn1/3Co1/3Ni1/3]O2 lattice is crystallized with 2D rock salt-type structure consisting of the ternary Mn/Co/Ni ions in octahedral sites. As can be seen clearly from the Tyndall phenomena and TEM images in Figures S6B and S6C of Supporting Information, both the TMOs can be well-delaminated into highly anisotropic colloidal 2D NSs with nanometer-level thickness. The liquid-exfoliation of both the materials can occur in various polar solvents, confirming that the present liquidexfoliation method can be employed for diverse TMOs with strong electrostatic interlayer interaction. Thermodynamic and kinetic aspects of the liquid-exfoliation of TMO NSs with hydroniumintercalated precursors. The thermodynamic and kinetic aspects of liquid-exfoliation process with hydronium-intercalated TMO precursors are investigated to understand the effect of the acid-treatment for the precursors on the efficiency of liquid-exfoliation.

In terms of

thermodynamics, the liquid-exfoliation of TMO NS corresponds to the replacement of the lattice energy of TMO precursor with the solvation energies for exfoliated TMO NS and ACS Paragon Plus Environment

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deintercalated interlayer cations. As illustrated in Figure 6A, the initial state of liquid-exfoliation process consists of host TMO crystals and solvent molecules which interact negligibly each other.

Thus, the energy of the initial state is determined by the lattice energy of TMO

precursor that depends on the electrostatic interaction between negatively-charged TMO layers and interlayer cations, and also by the dipoledipole interaction between solvent molecules. Since the lattice energy of the precursors is inversely proportional to the distance between interlayer cations and anionic TMO layers,43 the expansion of basal spacing upon the acid-treatment leads to the depression of lattice energy and the subsequent destabilization of the initial state, see Figure 6B. Based on the interlayer distances determined by XRD analysis, the weakening of the lattice energy of TMO precursor upon acid-treatment is determined to be ~11% for layered Ti1xO2, ~7% for MnO2, and ~5% for RuO2, respectively.


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Figure 6. Evolution of energetics for the liquid-exfoliation process (A) before and (B) after the acid-treatment for the TMO precursor (hydrogen atoms are in white, oxygen atoms are in red, carbon atoms are in black, nitrogen atoms are in clear blue, transition metal atoms within nanosheets are in faint blue, and alkali metal atoms between nanosheets are in dark yellow). Ea indicates the activation energy.

As depicted in Figure 6, the final state of the liquid-exfoliation process contains deintercalated interlayer cations and exfoliated TMO NSs, which are significantly solvated by solvent molecules.

While the acid-treatment for TMO precursors does not cause any significant

change in the interaction between exfoliated TMO NS and solvent molecules in the final state, the solvation energy of the deintercalated interlayer cations is remarkably altered by the replacement of alkali metal ions with hydronium ions. In the case of the acid-treated TMO precursor, the hydronium ions are released from the host lattice during the exfoliation process. The deintercalated hydronium ions can be efficiently stabilized by a stronger hydrogen bonding with polar solvent molecules. The strong hydrogen bonding of hydronium ions with solvent molecules helps to avoid the restacking of the exfoliated TMO nanosheets by depression of the interaction between hydronium and TMO nanosheets. Instead of hydronium ions, the liquid-exfoliation process of the as-prepared host material (Ax/4Ti1xO2, AxMnO2, and


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AxRuO2) releases alkali metal ions, which experience weaker iondipole interaction with solvent molecule. Thus, the final state of liquid-exfoliation process is more stable for the hydronium-intercalated TMO than for the as-prepared TMO, see Figure 6. Taking into account the destabilization of the initial state and the stabilization of the final state caused by the acidtreatment for the TMO precursor, employing the acid-treated TMOs as precursors is effective in improving the thermodynamic spontaneity of liquid-exfoliation process, which is proportional to G = Gfinal state  Ginitial state. This is responsible for the observed high efficiency of liquidexfoliation with the acid-treated precursors. For the estimation of reaction kinetics of liquid-exfoliation, the energy of transition state is quite crucial, since the activation energy (Ea) for this process corresponds to the energy difference between the initial state and the transition state, as illustrated in Figure 6. Since the liquid-exfoliation process is proceeded by the diffusion of solvent molecules into the interlayer space of TMO precursors, it causes a gradual increase of basal spacing and the weakening of the lattice energy of TMO precursor. Considering the fact that the transition state of liquidexfoliation process has the highest position in energy, the depressed lattice energy of TMO in the transition state would not be completely compensated by the coordinative stabilization of interlayer cations by limited number of diffused solvent molecules in the interlayer space of host TMO lattice. Since the lattice energy of TMO precursor in the transition state is also ACS Paragon Plus Environment

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proportional to basal spacing rather than the type of interlayer cation, the TMO precursor in the transition state is supposed to have similar basal spacing and similar lattice energy before and after acid-treatment.

Under this supposition, the highly efficient stabilization of interlayer

hydronium ions by strong hydrogen bonding with diffused solvent molecules strongly suggests the lower energy of transition state for the acid-treated precursor than for the as-prepared precursor.

In summary, the acid-treatment for the TMO precursor gives rise to both the

destabilization of initial state and the stabilization of transition state, indicating the depression of activation energy, as shown in Figure 6B. The resulting improvement of reaction kinetics also contributes to the increase of liquid-exfoliation yield achieved with the acid-treated TMO precursor. Hybridization effect of liquid-exfoliated TMO NS on the photocatalytic activity of CdS. To verify the usefulness of the present liquid-exfoliated TMO NSs as hybridization matrices, the obtained RuO2 NS is hybridized with photocatalytically-active CdS quantum dot (QD).44 The electrostatic interaction between negatively-charged RuO2 NS and positively-charged CdS QD yields intimately-coupled CdSRuO2 nanohybrid.44 As depicted in the powder XRD patterns of Figure 7A, the CdSRuO2 nanohybrid shows nearly identical diffraction features to the precursor CdS QD, indicating the negligible effect of RuO2 hybridization on the crystal structure of hexagonal CdS. No observation of any RuO2-related reflections strongly suggests ACS Paragon Plus Environment

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the homogeneous dispersion of RuO2 NSs in the present nanohybrid without phase segregation of layered RuO2 phase. The maintenance of the original crystal and electronic structures of CdS QD upon the hybridization with liquid-exfoliated RuO2 NS is further evidenced by the Cd K-edge XANES spectroscopy showing CdS-typical spectral feature for the CdSRuO2 nanohybrid, see Figure S7 of Supporting Information. Although no RuO2related XRD peak is discernible for the present CdSRuO2 nanohybrid, the observation of typical Ru K-edge XANES features A and B of layered RuO2 phase provides strong evidence for the incorporation of layered RuO2 NS in the CdSRuO2 nanohybrid, see Figure 7B. As plotted in diffuse reflectance UVvis spectra of Figure 7C, a remarkable absorption enhancement in the range of visible light occurs by the hybridization with liquid-exfoliated RuO2 NS, indicating the significant visible light harvesting ability of CdSRuO2 nanohybrid. Additionally, the hybridization with RuO2 NS gives rise to a significant decrease of the photoluminescence (PL) intensity of CdS (Figure 7D), clearly demonstrating the effective depression of charge recombination and the increase of the life-times of photogenerated electrons and holes. This observation underscores a highly efficient role of liquid-exfoliated RuO2 NS as an electron reservoir, as found from density functional theory (DFT) calculation.44 Of prime importance is that the PL depression is much more prominent for the CdSRuO2


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nanohybrid than for the physical mixture of CdS and H0.2RuO2, confirming the high efficiency of liquid-exfoliated RuO2 NS as an electron reservoir.

Figure 7. (A) Powder XRD patterns, (B) Ru K-edge XANES spectra, (C) diffuse reflectance UVvis spectra, (D) PL spectra, (E) photocatalytic H2 generation data, and (F) comparison plots of visible light-induced H2 evolution for the precursor CdS QD, the CdSRuO2 nanohybrid, and the physical mixture of CdS and H0.2RuO2.

The obtained CdSRuO2 nanohybrid is employed as photocatalyst for visible light-induced H2 generation, as compared with the precursor CdS QD and the physical mixture of CdS and H0.2RuO2.

In Figures 7E and 7F, the CdSRuO2 nanohybrid shows much higher

photocatalytic activity for H2 production under visible light irradiation ( > 420 nm) than do the precursor CdS QD and the physical mixture of CdS and H0.2RuO2, indicating the beneficial ACS Paragon Plus Environment

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effect of hybridization with liquid-exfoliated RuO2 NS. The present result underscores the usefulness of liquid-exfoliated RuO2 NS as an effective hybridization matrix for improving the photocatalytic performance of semiconductor nanocrystal as well as for depressing the charge recombination rate. The physicochemical properties and photocatalytic activity of the present CdSRuO2 nanohybrid are quite similar to those of the CdSRuO2 nanohybrid with chemicallyexfoliated NS,44 highlighting the usefulness of the liquid-exfoliated TMO NS in synthesizing efficient hybrid-type photocatalyst.

Of noteworthy is that the photocatalytic activity of the

physical mixture is even lower than that of the precursor CdS, which is attributable to the lowering of CdS content and the negligible coupling effect with bulk H0.2RuO2. Hybridization effect of liquid-exfoliated TMO NS on the electrode functionalities of reduced graphene oxide (rGO)-based nanohybrids. The liquid-exfoliated MnO2 NS is incorporated into the CoAl-LDHrGO nanohybrid to study the hybridization effect of liquid-exfoliated TMO NS on the supercapacitor electrode functionality of nanostructured material.45

As depicted in

Figure 8A, the Co2 and Al3 cations are adsorbed on the surface of negatively-charged mixed nanosheets of rGO and MnO2, which is followed by the crystal growth of CoAl-LDH crystallites in the hybrid matrix of two kinds of nanosheets.

Such a strong electrostatic

interaction between precursor metal ions and rGO/MnO2 nanosheet leads to the synthesis of intimately-coupled CoAl-LDHrGOMnO2 nanohybrid.45 For comparison, the control sample ACS Paragon Plus Environment

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of MnO2-free CoAl-LDHrGO nanohybrid is also synthesized by the identical synthetic process in the absence of MnO2 NS. According to powder XRD analysis (Figure 8B), both the MnO2-incorporated CoAl-LDHrGOMnO2 and MnO2-free CoAl-LDHrGO nanohybrids commonly display typical Bragg reflections of CoAl-LDH phase, indicating the negligible influence of MnO2/rGO hybridization on the crystal structure of CoAl-LDH. The absence of MnO2/rGO-related Bragg reflections in the present XRD patterns indicates the homogeneous dispersion of MnO2/rGO NSs in the present nanohybrids without phase segregation of MnO2 and rGO phases. According to the combined analysis of inductively coupled plasma (ICP) spectroscopy and energy dispersive spectrometry (EDS), the obtained nanohybrids show the CoAl-LDH:rGO:MnO2 weight ratios of 77.8:20.5:0.564 for CoAl-LDHrGOMnO2 and 80.8:19.1:0 for CoAl-LDHrGO.

As shown in Figure S8 of Supporting Information, the

incorporation of MnO2 and rGO NSs in the present CoAl-LDHrGOMnO2 nanohybrid is confirmed by Mn K-edge XANES and micro-Raman spectroscopic analyses exhibiting the typical spectral features of layered MnO2 and rGO, respectively. In Figure 8C, the FE-SEM images of both the nanohybrids clearly demonstrate the formation of the mesoporous stacking structure of component NSs. According to the N2 adsorptiondesorption isotherm analysis (Figure S9 of Supporting Information), the CoAl-LDHrGOMnO2 nanohybrid has a larger surface area of 34 m2 g1 than that of CoAl-LDHrGO nanohybrid (22 m2 g1), indicating the ACS Paragon Plus Environment

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beneficial hybridization effect of liquid-exfoliated MnO2 NSs in expanding surface area of nanohybrid.

Figure 8. (A) Schematic illustration, (B) powder XRD patterns, (C) SEM images, (D, E) CV curves of the 5000th cycles, and (F) specific capacitance plots of (a) CoAl-LDHrGO and (b) CoAl-LDHrGOMnO2 nanohybrids.

The hybridization effect of liquid-exfoliated MnO2 NS on the specific capacitance of CoAlLDHrGO nanohybrid is examined with cyclic voltammetry (CV). As plotted in Figures 8D and 8E, both the CoAl-LDHrGOMnO2 and CoAl-LDHrGO nanohybrids commonly exhibit typical CV curve of CoAl-LDH phase,4649 indicating the main contribution of CoAl-LDH component for the overall electrode activities of these nanohybrids.

The incorporation of


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liquid-exfoliated MnO2 NS makes larger the integral area of CV curve, underscoring the beneficial role of MnO2 NS in improving the electrode performance of CoAl-LDHrGO nanohybrid. The CoAl-LDHrGOMnO2 nanohybrid delivers a large specific capacitance of ~410 C g1 at scan rate of 5 mV s1, which is much greater than those of the MnO2-free CoAlLDHrGO nanohybrid (~260 C g1). Of noteworthy is that the specific capacitance of CoAlLDHrGOMnO2 nanohybrid is notably larger than that of the physical mixture of CoAl-LDH, rGO, and MnO2 NSs (~230 C g1), confirming the usefulness of liquid-exfoliated MnO2 NS as a hybridization matrix in improving supercapacitor electrode performance (Figure S10 of

Supporting Information). As illustrated in Figure 8F, the CoAl-LDHrGOMnO2 nanohybrid shows better cyclability upto the 5,000th cycle than do the MnO2-free CoAl-LDHrGO nanohybrid and the physical mixture of CoAl-LDH, rGO, and MnO2 NSs, highlighting the improvement of electrochemical stability upon the incorporation of liquid-exfoliated MnO2 NS. The beneficial role of liquid-exfoliated MnO2 NS in improving the electrode performance of LDHrGO nanohybrid can be ascribed to the depression of  interaction between rGO NSs by the intervention of TMO NS, leading to the enhancement of the porosity and charge transfer kinetics of graphene-based nanohybrid.50

This interpretation is further supported by the

results of electrochemical impedance spectroscopy (EIS) showing lower solution/charge transfer resistances and higher ion diffusivity for the CoAl-LDHrGOMnO2 nanohybrid than ACS Paragon Plus Environment

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for the CoAl-LDHrGO nanohybrid, as plotted in Figure S11 and Table S1 of Supporting

Information. Additionally, the liquid-exfoliated MnO2 NS is utilized as a hybridization matrix for the restacked rGO NSs to examine the hybridization effect of liquid-exfoliated TMO NS on the electrode performance of rGO for secondary batteries.

As depicted in Figure S12A of

Supporting Information, the HrGOMnO2 nanohybrid is synthesized by the restacking of the colloidal mixture of rGO and liquid-exfoliated MnO2 NSs with protons. Also, the control sample of HrGO nanohybrid is prepared by the same synthetic method except for no addition of MnO2 NS. As shown in Figure S12B, both the present nanohybrids display broad XRD feature corresponding to typical Bragg reflections of rGO phase in the 2 region of ~2040, indicating negligible influence of MnO2 incorporation on the crystal structure of rGO.

The FE-SEM

results in Figure S12C clearly demonstrate porous stacking morphologies of both the HrGOMnO2 and HrGO nanohybrids. To probe the hybridization effect of MnO2 NS on the electrode functionality of rGO, both the nanohybrids are employed as anode materials for lithium ion batteries (LIBs).

As plotted in Figures S12DF, the MnO2-incorporated

HrGOMnO2 nanohybrid delivers much larger discharge capacity than does the HrGO nanohybrid, highlighting the beneficial role of liquid-exfoliated MnO2 NS in improving the anode performance of rGO for LIBs. According to EIS results (Figure S13 and Table S2 of ACS Paragon Plus Environment

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Supporting Information), the incorporation of liquid-exfoliated MnO2 NS results in a significant lowering of solution resistance and charge transfer resistance, which is responsible for the enhanced LIB electrode performance of restacked HrGOMnO2 nanohybrid.

All the

experimental findings presented here provide strong evidence for the beneficial role of liquidexfoliated TMO NSs as efficient hybridization matrices for optimizing many functionalities of inorganic solids.

Conclusion In this study, we develop a facile and scalable organic-intercalant-free liquid-exfoliation route to highly anisotropic 2D TMO NSs by employing the acid-treated derivatives as precursors. The acid-treatment for layered TMOs is quite effective in enhancing the efficiency of liquidexfoliation via the exchange of interlayer alkali metal ions with larger hydronium ions. The beneficial role of hydronium-intercalated TMO derivative as a precursor for liquid-exfoliation is attributable both to the weakening of interlayer interaction upon hydronium-intercalation and to the efficient solvation of hydronium ion released by liquid-exfoliation process, highlighting the importance of the fine-control of thermodynamic and kinetic factors for efficient liquidexfoliation. The present results underscore that the acid-treatment for layered TMO enables to develop an economic and scalable liquid-exfoliation route to 2D TMO NSs.

Of prime


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importance is that the resulting liquid-exfoliated TMO NSs can be used as effective hybridization matrices for diverse nanostructured materials to improve their electrode and photocatalyst performances. Taking into account the high economic feasibility and scalability of the present liquid-exfoliation synthesis, the hybridization with liquid-exfoliated TMO NSs can provide a practical and facile methodology to explore high-performance TMO-based functional nanohybrids appropriate for industrial uses. Additionally, it is worthwhile to note that the absence of organic intercalant in the colloidal suspension of liquid-exfoliated TMO NS would allow to study their intrinsic physical and surface properties and also to exploit new application areas such as electronic devices. In contrast to the present liquid-exfoliation route, the conventional chemical-exfoliation process using ammonium intercalant ions leads to the unavoidable surface contamination of negatively-charged TMO NS with organic intercalant cations, which prevents from the reliable measurements of the intrinsic transport and surface properties of TMO NSs.

Also, the

surface-adsorption of insulating organic cations is quite detrimental for the application of chemically-exfoliated TMO NSs for electronic devices like field emission transistors and diodes, since the functionalities of these devices are strongly dependent on interfacial electrical contact.

Conversely, the liquid-exfoliation process developed here is free from the

aforementioned drawbacks of conventional chemical-exfoliation process originating from the ACS Paragon Plus Environment

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use of organic intercalant cations. Such merits of liquid-exfoliation would provide valuable opportunity to widen the application and research fields of exfoliated 2D TMO NSs. Judging from the fact that there are many types of layered inorganic solids with exchangeable interlayer ions, our current project is to apply the present liquid-exfoliation strategy for the scalable synthesis of various inorganic NSs and their nanohybrids.

Methods Preparation of the colloidal suspensions of liquid-exfoliated Ti1xO2, MnO2 and RuO2 NSs. The pristine layered TMOs of Ax/4Ti1xO2, AxMnO2, and AxRuO2 (A = alkali metal) were synthesized by conventional solid-state reaction with stoichiometric mixtures of alkali metal carbonates and metal oxides, as reported previously.3133 The hydronium-intercalation into layered TMO materials was carried out by the reaction of the pristine TMO powder with 1 M HCl solution at 10 g L1 concentration.3133 For liquid-exfoliation, the obtained hydroniumintercalated derivatives were immersed in various polar solvents such as formamide, DMSO, ethanol, methanol, IPA, 1-butanol, and acetone in the initial exfoliation concentration of 1 g L1, which was followed by the loading of ultrasonic stress (Branson 5510 Model 40 kHz) at room temperature for 5 h. The dispersion was obtained by keeping it at room temperature for overnight to remove out the unexfoliated layered materials. The colloidal suspension of liquidACS Paragon Plus Environment

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exfoliated TMO NS was restored from supernatant. The concentration and exfoliation yield of the colloidal suspension were calculated by dividing the weight of floating NSs by the original weight of precursor. The weight of floating NS was evaluated by subtracting the weight of sediment from the original weight of precursor after restoring the supernatant. Preparation of CdSRuO2 nanohybrid. For the synthesis of CdSRuO2 nanohybrid with liquid-exfoliated RuO2 NS, the precursor of cationic CdS QD was prepared by the reaction with cadmium acetate dehydrate, 2-aminoethanethiol hydrochloride, and thioacetamide in deionized water with reflux condition at 40 C for 5 h, which was followed by the extended reaction at 60 C for 5 h.51 The colloidal suspension of liquid-exfoliated RuO2 NS (0.5wt%) was added drop-wise into the aqueous solution of CdS QD. After the reaction at 60 C for 3 h, the resulting product was isolated by centrifugation, washed by deionized water and ethanol, and then dried at 50 C. Preparation of CoAl-LDHrGOMnO2 and CoAl-LDHrGO nanohybrids. For the synthesis of CoAl-LDHrGOMnO2 nanohybrid with liquid-exfoliated RuO2 NS,41 the colloidal suspension of graphene oxide (GO) NS was prepared by the modified Hummers' method.52 The colloidal mixture of GO and liquid-exfoliated MnO2 NSs was obtained by mixing each precursor suspension.

For the crystal growth of CoAl-LDH material, CoCl26H2O and

AlCl36H2O were dissolved in the deionized water with molar ratio of 2:1 and then urea was ACS Paragon Plus Environment

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added to adjust pH. After stirring for 30 min, homogeneous colloidal mixtures of GO and MnO2 NSs were added drop-wise and the reaction was proceeded at 97 C for 24 h. After the reaction, the resulting product was isolated by centrifugation, washed by deionized water and ethanol, and then dried at 50 C. During the reaction, the urea acted as reductant to transform GO to rGO. The reference CoAl-LDHrGO nanohybrid was also synthesized by the identical synthetic process except for no addition of MnO2 NS.

The weight ratio of CoAl-

LDH:rGO:MnO2 was adjusted to be 92.3:7.7:0 for CoAl-LDHrGO and 92.3:7.0:0.7 for CoAlLDHrGOMnO2. Preparation of HrGOMnO2 and HrGO nanohybrids. To synthesize rGO NS, ammonia solution (1400 L, 28wt%) and hydrazine solution (200 L, 35wt%) were added to the suspension of GO and then reacted at 90 C under reflux condition. For the synthesis of HrGOMnO2 nanohybrid, both the suspensions of rGO NS and liquid-exfoliated MnO2 NS (2.5wt%) were mixed under stirring for 30 min and then restacked by adding 0.1 M HCl solution. For comparison, the HrGO material was synthesized as a control under the same procedure in the absence of MnO2 NSs.

After the reaction, the resulting products were

isolated by centrifugation, washed by deionized water and ethanol, and then freeze-dried. Characterization. The crystal structures of the present materials were studied by powder XRD (Rigaku D/Max-2000/PC, Ni-filtered Cu K radiation,  = 1.5418 Å, 25 C). The zeta ACS Paragon Plus Environment

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potential and DLS data of the colloidal suspensions of liquid-exfoliated TMO NSs were measured by the Zetasizer Nano ZS (Malvern Instruments). To probe the optical properties and to calculate the molar absorption coefficients of the exfoliated TMO NSs, their UVvis spectra were collected with spectrophotometer (V-760, JASCO). The crystallite shapes and crystal structures of the hydronium-intercalated TMO materials and their liquid-exfoliated NSs were probed by performing HR-TEM/SAED measurements with a Jeol JEM-2100F microscope at an accelerating voltage of 200 kV. The thickness and lateral dimension distributions of the liquid-exfoliated TMO NSs were investigated by using AFM (XE-100, Park Systems) and HRTEM. The crystal morphologies of the present nanohybrids were examined with FE-SEM analysis (Jeol JSM-6700F). The oxidation states and local atomic arrangements of Mn, Ru, and Cd ions in the present materials were investigated using XANES analysis at Mn K-edge, Ru K-edge, and Cd K-edge at beam line 10C in the Pohang Accelerator Laboratory (PAL, Pohang, Korea). All the present XANES spectra were calibrated by measuring the spectrum of Mn, Ru, and Cd elements.

The electronic structures of the present materials were

investigated with diffuse reflectance UVvis and PL spectroscopies.

The chemical

compositions of the CoAl-LDHrGOMnO2 and CoAl-LDHrGO nanohybrids were examined with ICP spectroscopy (PerkinElmer NexION 300) and EDS measurement (Jeol JSM-6700F). ACS Paragon Plus Environment

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Photocatalyst and electrode activity tests of nanohybrids. For the photocatalytic activity tests, the CdSRuO2 nanohybrid, the precursor CdS, and the physical mixture of CdS and H0.2RuO2 were employed as photocatalysts for H2 evolution using Newport Xe lamp (300 W). The optical cut-off filter (  420 nm) was used to block out the UV light and 50 mg of the photocatalyst was suspended in 100 mL of a mixed solution of 0.1 M sodium sulfide and 0.02 M sodium sulfite (hole scavenger). The amount of evolved H2 gas was determined with gas chromatography (Shimadzu GC-2014). The supercapacitor electrode performances of the CoAl-LDHrGOMnO2 and CoAlLDHrGO nanohybrids were investigated with CV measurement.

All electrochemical

measurements were carried out using a standard three-electrode electrochemical cell at room temperature with a potentiostat/galvanostat (WonA Tech.). An active material coated Ni-foam, a Pt mesh, and a saturated calomel electrode (SCE) were employed as working, counter, and reference electrodes, respectively. The working electrodes were manufactured by mixing the active material, acetylene black, and polyvinylidenefluoride (PVDF) with a mass ratio of 80:15:5 in N-methylpyrrolidone and coating on a Ni foam. The supercapacitor performance was evaluated using the active material with mass loading of 23 mg cm2 and 30 mL of 1 M KOH solution as an electrolyte. The actual area of the active material was adjusted as 1 cm2. The fabricated electrodes were dried under vacuum at 80 C for 2 h. The CV data were ACS Paragon Plus Environment

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collected in the potential region of 0.00.55 V at a scan rate of 20 mV s1. The EIS data were measured with an IVIUM impedance analyzer in the frequency range of 0.01105 Hz. The LIB anode performances of the HrGOMnO2 and HrGO nanohybrids were measured by galvanostatic chargedischarge cycling tests. The resulting materials were assembled into a 2016 coin-type cell in the Ar-filled glove box. The working electrodes were manufactured by mixing the active material and polyvinylidenefluoride (PVDF) with a mass ratio of 90:10 in Nmethylpyrrolidone and the subsequent coating of the slurry on Cu metal foil. These obtained electrodes were vacuum-dried at 120 C for 12 h. The electrolyte was composed of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC:DEC = 1:1 by volume).

For the battery

performance evaluation, the active material with mass loading of 12 mg cm2 were used together with approximately 300 L of organic electrolyte. The actual area of active material for battery test was 2 cm2.

The electrochemical cycling tests were performed in a

galvanostatic mode using Maccor multichannel galvanostat/potentiostat.

The constant

potential range of 0.01–3.0 V (vs. Li/Li+) with current density of 100 mA g1 was employed. The EIS measurement was performed with an IVIUM impedance analyzer in the frequency range of 0.1105 Hz.

Acknowledgment


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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2017R1A2A1A17069463) and by the Korea government (MSIT) (No. NRF-2017R1A5A1015365). The experiments at PAL were supported in part by MOST and POSTECH.

Supporting Information Available

Powder XRD patterns of the pristine TMOs and their hydronium-intercalated derivatives. Mn K-edge XANES spectra of layered MnO2 before and after liquid-exfoliation.

UVvis

absorbance spectra, SAED patterns, DLS data for liquid-exfoliated Ti1xO2, MnO2, and RuO2 NSs.

Crystal structures, Tyndall phenomena, and TEM images for liquid-exfoliated

CaLaNb2TiO10 and [Mn1/3Co1/3Ni1/3]O2 NSs.

Cd K-edge XANES spectra of CdSRuO2

nanohybrid and CdS. Mn K-edge XANES, micro-Raman, N2 adsorptiondesorption isotherm, and EIS data of CoAl-LDHrGOMnO2 nanohybrid and references. Specific capacitance plot of the physical mixture of CoAl-LDH, rGO, and MnO2 NSs. Structural, morphological, and electrochemical characterization data for the HrGOMnO2 and HrGO nanohybrids.

References and notes


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A graphic for the table of contents (TOC):


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Organic-Intercalant-Free Liquid-Exfoliation Route to Layered Metal

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