Unveiling the Critical Role of Surface Oxidation of Electroresponsive

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Unveiling the Critical Role of Surface Oxidation of ElectroResponsive Behaviors in Two-Dimensional TiCTx MXenes 3

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Wen Ling Zhang, Li Deng, Jingquan Liu, Yang Liu, Jianbo Yin, Hongbo Zeng, Wen Zheng, and Aitang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11525 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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The Journal of Physical Chemistry

Unveiling the Critical Role of Surface Oxidation of Electro-responsive Behaviors in Two-Dimensional Ti3C2Tx MXenes Wen Ling Zhang,*,†,‡ Li Deng,† Jingquan Liu,† Yang Liu,§ Jianbo Yin,§ Hongbo Zeng,‡ Wen Zheng,† and Aitang Zhang† †College

of Materials Science and Engineering, Institute for Graphene Applied

Technology Innovation, Qingdao University, Qingdao 266071, China ‡Department

of Chemical and Materials Engineering, University of Alberta,

Edmonton, Alberta T6G 1H9, Canada §Smart

Materials Laboratory, Department of Applied Physics, Northwestern

Polytechnical University, Xi’an 710129, China

ABSTRACT Here, two-dimensional (2D) C/TiO2 hybrids synthesized via oxidation of Ti3C2Tx MXene are reported as an ideal ER candidate in pursuit of high ER efficiency. The lamellar structure of conductive carbon layers provides abundant electron transport channels, aiming to shorten the response time to an electric field while the TiO2 nanoparticles deliver high surface polarization leading to the high ER effects. The oxidation degrees of Ti3C2Tx MXene are systematically controlled through the hydrothermal method (h-C/TiO2) or CO2 calcination (c-C/TiO2). We comprehensively studied the relationship between the surface oxidation and the ER behaviours of these C/TiO2 hybrids. It was found that the h-C/TiO2 hybrid exhibited superior ER behaviors compared with c-C/TiO2 hybrid. The holey h-C/TiO2 architecture creates 1

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large electron and oil transport pathways, which is responsible for the superior ER performances. Overall, this research provides original insights into the exciting electro-responsive of this quickly growing 2D MXene family as well as provides a guideline for processing oxidation of MXene for diverse applications.

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INTRODUCTION Recently,

an

emerging

class

of

two-dimensional

(2D)

transition-metal

carbides/nitrides labeled as MXene, is attracting particular interests in vast fields, such as catalysis, sensors, biomedicine and energy storage.1,2 These monolayers of MXenes are generally produced by selectively etching Al element from their corresponding bulk MAX phase using fluoride containing solutions.3 Hence, the surfaces of MXenes are spontaneously terminated with functional groups, giving a formula Mn+1XnTx, where Tx denotes the surface terminations (e.g., -OH, -O, or -F groups).4 The surface groups terminated at the MXenes sheets play an important role in modulating their electrochemical, thermoelectric or dielectric properties.5 Taking advantage of unusual lamellar structure, superior density to some traditional dielectric particles, (e.g., SiO2, TiO2) and rich surface chemistry, 2D nanomaterials have been recognized as promising electro-responsive materials in pursuit of high activity and dispersion stability.6-8 In our previous study, graphene oxide (GO) based hybrid systems

with

abundant

polar

functional

groups

have

exhibited

dramatic

electro-responsive performances.9,10 Smart materials are becoming a predominant frontier owing to their fast response to specific stimuli, such as temperature, pressure, humidity, pH, salt concentration, light, CO2, or electric/magnetic fields.11,12 Electrorheological (ER) suspensions are one of the fascinating smart materials, consisting of dielectric micro/nano particles in an insulating carrier medium, such as silicone oil, corn oil, or mineral oils.13 The ER fluids behave as Newtonian or slightly pseudoplastic fluids in normal conditions. By application of an external electric field, the well-dispersed particles get polarized and form fibril/column-like structures along the electric field direction, resulting in a remarkable liquid-like to solid-like phase transition.14,15 Due to their advantages of 3



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effectively controllable rheological properties (such as viscosity, shear stress, and storage/loss modulus), fast response and low power consumption, ER materials have been designed for a broad promising applications, such as shock absorbers, vibration dampers, supercapacitors, artificial skins, microfluidic pumps, and robotics,16-21 etc. However, two main challenges remain which hinder its further commercial applications: 1) the lack of material systems with high ER activity and 2) the incomplete understanding of the critical factors affecting the ER effect.22,23 In terms of the previous study, ER performances can be greatly enhanced using highly polarizable and large surface area materials dispersing in their best mediums. Particularly, the intrinsic characteristics of organic/inorganic nanoparticles influence their ER performances directly. The most studied ER materials are TiO2, SiO2, natural clays, conducting polymers, such as polyaniline, polypyrrole, and so on.24-28 However, the most inorganic particles based ER fluids need a large applied voltage resulting from their insulating property, while the conducting polymers trend to aggregate and exhibit poor dispersion stability in medium oils. Therefore, it is highly necessary and challenging to explore promising material systems with excellent ER activities. Besides, many efforts have been devoted to modulate the ER behaviors or reveal the ER mechanisms for achieving suitable ER effects as well as offering a guideline to design optimal ER material systems.29-32 Hao et al. have studied the influence of particle volume fraction and electric field on the rheological properties of an ER fluid.33 Tian et al. have proposed a structure parameter considering the frictional interaction among the polarizable particles to reveal the origin in ER effect.34,35 Choi and Seo et al. have proposed a normalization equation to study the correlation between the yield stress and the applied electric/magnetic field.36,37 Jang et al. have pointed out that the particle geometry plays an important role on the ER effects.38 To 4


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date, the fundamental and comprehensive understanding of the correlation between ER behaviors and the critical features for a complete material system remains lacking. In this work, Ti3C2Tx, the first discovered and the most studied MXenes was chosen as the primary focus of ER behaviors. The out Ti layer of Ti3C2Tx can be easily oxidized to TiO2 nanoparticles.39 Hence, the oxidized Ti3C2Tx (C/TiO2) with conductive carbon layers and high polarizable TiO2 nanoparticles, is expected to have a dramatic increase in ER activities. The oxidation of Ti3C2Tx was obtained via using the hydrothermal method (h-C/TiO2) or calcination in the CO2 atmosphere (c-C/TiO2). It is worth noting that the proportions of the terminated functional groups (or called as the oxidation degree) of the Ti3C2Tx are dependent on the oxidation methods and conditions. We comprehensively studied the relationship between oxidation degree and the ER behaviors of these C/TiO2 hybrids.

EXPERIMENTAL SECTION Materials. The Ti3AlC2 powder (Shanghai Yuehuanxin Material Technology Co., Ltd), hydrogen fluoride, (HF, 49 wt%, Shanghai Macklin Biochemical Co., Ltd), and hydrogen peroxide (H2O2, 30 wt%, Sinopharm Chemical Reagent Co., Ltd) were used as received. Ultrapure water was used in the experiments. Synthesis of Ti3C2Tx MXene. The Ti3C2Tx MXene was synthesized according to a previous report.40,41 Briefly, the Ti3AlC2 powder was slowly added into 49 wt% HF acid and kept magnetic stirring for 70 h at room temperature. After the HF etching, the mixture was centrifuged and washed in ultrapure water until the pH ~ 7. The Ti3C2Tx MXene was obtained after freeze-dried. Synthesis of 2D-layered C/TiO2 hybrids. The 2D-layered C/TiO2 hybrids were 5



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fabricated by hydrothermal method or CO2 oxidization, respectively. In a hydrothermal method,42,

43

0.7 g Ti3C2Tx powder was added into 70 mL ultrapure

water under stirring and a controlled amount of H2O2 (1.5 mL) was added. The mixture was transferred to a 200 mL Teflon-lined autoclave and heated at 200 ºC for 20 h. The resulting sediment named as h-C/TiO2-1.5-20 hybrid was collected by centrifugation, washed with ultrapure water and freeze-dried. The h-C/TiO2 hybrids with different amount of H2O2 (m) and reaction time (n) were also prepared, which named as h-C/TiO2-m-n hybrid. As for the CO2 calcination,44-45 the as-prepared Ti3C2Tx powders were put into a quartz tube furnace at 800 °C for 1 h with a heating rate of 10 °C/min in flowing CO2 gas. This sample was named as c-C/TiO2-800 hybrid. The Ti3C2Tx powders were also calcined at 650, 700 and 750 °C with the same heating rate in the CO2 atmosphere, which marked as c-C/TiO2-650, c-C/TiO2-700 and c-C/TiO2-750, respectively. Preparation of h-C/TiO2-1.5-20 based ER fluids. The h-C/TiO2-1.5-20 hybrid (ρ=3.57 g/cm3) were dried in a vacuum oven at 80 ºC for 24 h, The dried powder was dispersed in silicone oil (viscosity=100 cSt, ρ=0.965 g/cm3) at different volume fraction (4 vol%, 8 vol%, and 12 vol%), followed by sonication. Characterization. The microstructural morphology and element distribution of the samples were conducted with field-emission scanning electron microscopy (FE-SEM, JEOL JSM-2010), high-resolution TEM (HRTEM) and scanning transmission electron microscopy (STEM, Tecnai G2 F20, 200 kV) equipped with an energy dispersive X-ray spectrometer. The crystallization of the samples was recorded on a 6


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powder X-ray diffractometer (XRD, DX-2700, λ=0.15406 nm). Raman spectra were obtained

from

a

Raman

spectrometer

(Thermo

Fisher

DXR2).

The

Brunauer-Emmett-Teller (BET) surface area and the distribution of pore size were conducted using N2 as adsorption gas on a physisorption analyzer (V-sorb 2800S). The elemental composition and chemical bonds were obtained via X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi). The density was characterized by an automatic density analyzer (AccuPyc II 1340). The ER performances were evaluated on a rotational rheometer (MCR302, Anton Paar) using a Couette-type cylinder geometry (CC 17, the gap distance is 0.71 mm). Optical microscope (OM) images were obtained from an optical microscope (Nikon Alphaphot-2) attached with a voltage-generating apparatus.

RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of the fabrication of C/TiO2 hybrids by hydrothermal process or CO2 calcination.

The C/TiO2 hybrids were prepared as illustrated in Scheme 1. Briefly, the 2D Ti3C2Tx sheets were prepared via etching the aluminum layer from Ti3AlC2 using hydrofluoric acid (HF) solution. During the hydrothermal process, the H2O2 molecules can oxidize the Ti atoms into TiO2 nanoparticles and etch the more active sites of carbon layers, 7



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which gradually evolving into porous carbon nanosheets. As for the C/TiO2 achieved by CO2 calcination, the lamellar architecture is well preserved owing to the supporting of TiO2 nanoparticles decorated and intercalated the carbon layers.

Figure 1. SEM and TEM images of Ti3C2Tx (a, b) and h-C/TiO2-1.5-20 hybrids (c, d), respectively. Elemental mapping images of h-C/TiO2-1.5-20 hybrid (e). SEM images of c-C/TiO2-650 (f), c-C/TiO2-700 (g), c-C/TiO2-750 (h) and c-C/TiO2-800 (i). Inset is the schematic structure of the samples.

The morphology of the samples was characterized with SEM and TEM technologies. Compared with the tightly stacking structure of Ti3AlC2 bulk (Figure S1a), a laminar structure with the loosely accordion-like structure of Ti3C2Tx layers could be observed after the removal of Al layers (Figure 1a, b). After oxidation, the TiO2 nanoparticles are anchored homogeneously on and between the ultrathin carbon layers (Figure 1c-i). In the hydrothermal treatment, various hetero-structures were observed by adjusting the ratio of H2O2 and Ti3C2Tx or the reaction conditions (e.g. hydrothermal time) shown in Figure S2, 3. The different ratio of H2O2 and MXene is based on the starting amount of H2O2 (0.5, 1.0, 1.5, 2.0 ml), the hydrothermal time is 20 h. At relatively low amounts of H2O2 (e.g. 0.5, 1.0 ml), the TiO2 nanostructures are only 8


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sparsely located on the Ti3C2Tx nanosheets (Figures S2a-d). When the amount of H2O2 is appropriate (~1.5 ml), the TiO2 nanoparticles cover the whole surfaces of the Ti3C2Tx uniformly and densely (Figure 1c, d). As shown in Figure S2e, f, the TiO2 nanoparticles are tightly aggregated on the surface accompanied by the collapse of carbon layers, confirming that the excessive H2O2 (e.g. 2.0 ml) would lead aggressive oxidation and etching. Unlike the black colors of other h-C/TiO2 hybrids, the color of h-C/TiO2-1.5-20 hybrids changed to milky white. The effect of hydrothermal time was also discussed shown in Figure S3. With the increase of hydrothermal time (e. g. 25, 30 h), the carbon layers gradually collapsed and densely covered by the TiO2 nanoparticles. However, the bare carbon layers were observed obviously at shorter reaction time (e. g. 5, 10 h). The layered architecture is well-preserved in the case of CO2 calculation, and the 2D Ti3C2Tx sheets were transformed to carbon layers decorated by TiO2 nanosheets/nanoparticles (Figure 1f–i). After increasing the calcination temperature, more TiO2 nanoparticles have been generated, both confirmed from not only the TEM images but also the sharply increased contents of oxygen (Table S1). Compared with other C/TiO2 hybrids, the relatively high content of Fluorine of c-C/TiO2-650 may be due to the broad bare Ti3C2Tx nanosheets as shown in Table S1. The appropriate electric conductivity of the active ER particles is in the range of 10-6~10-10 S/cm to keep the electrical sensitivity as well as avoid electric breakdown. Unfortunately, the electric breakdown of the c-C/TiO2 based ER fluid occurs at the electric field strength of 0.3 kV/mm (Figure S4). We speculate that this phenomenon 9



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is due to the unusual structure of c-C/TiO2 hybrids. During the hydrothermal process, the H2O2 molecules can not only oxidize the Ti atoms into TiO2 nanoparticles but also aggressively etch the carbon layers, which gradually transform into porous carbon pieces. Therefore, the TiO2 nanoparticles can wrapped densely on the ultrathin carbon pieces in the h-C/TiO2 hybrids. However, the c-C/TiO2 hybrids with well-reserved lamellar structure possess numerous bare conductive carbon layer, which will generate short-circuit during the bulk rheological test. As noted above, we select the h-C/TiO2-1.5-20 as an optimal ER candidate owing to the high loading of functional groups, simple synthesis procedure as well as the high yield for the following bulk rheological measurements.

Figure 2. XRD patterns (a, b) and Raman spectra (c) of the samples. Ti 2p (d), O 1s (e) and C 1s (f) XPS for h-C/TiO2-1.5-2.0 hybrid. N2 adsorption-desorption isotherms of Ti3C2Tx (g) and h-C/TiO2-1.5-2.0 hybrid (h), respectively. The height square of oil capillary rise vs. time for the 10


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samples (i).

The crystal structures of the Ti3AlC2, Ti3C2Tx, and h-C/TiO2-1.5-20 hybrid were characterized using an X-ray diffractometer (XRD) shown in Figure 2a. The bulk Ti3AlC2 shows a prominent peak at 39.04°, which is the characteristic peak of the (104) plane (JCPDS card no. 52-0875). It decreases sharply and even disappears in Ti3C2Tx and h-C/TiO2-1.5-20 hybrid. A sharp peak at 9.49° assigned to the (002) plane of Ti3AlC2 shifts to a lower value (8.81°) of Ti3C2Tx and becomes broader, indicating the enlarged d-spacing from 0.93 nm to 1.00 nm based on Bragg’s equation.45 In the XRD pattern of h-C/TiO2-1.5-20 hybrid, the obvious (002) peak disappears meanwhile new diffraction peaks of anatase-TiO2 (JCPDS card no. 73-1764) along with the rutile-TiO2 (JCPDS card no. 65-0192) emerge.45 As for the c-C/TiO2 hybrids, the peaks become sharp and strong with the increase of calcination temperature (Figure 2b). Moreover, when the temperature increased to 800 °C, only thermally stable rutile TiO2 phase was observed (JCPDS Card No. 21-1276).44 Raman spectra were then carried out to identify the structure of the samples shown in Figure 2c. As for the h-C/TiO2-1.5-20 hybrid, the apparent peaks at 398 and 636 cm-1 are indexed to the rutile-TiO2 while the characteristic peaks at 147 and 517 cm-1 confirm the formation of anatase-TiO2, consistent with the XRD patterns.44, 46,47 All of the three samples show two peaks at 1374 cm-1 (D-band) and 1592 cm-1 (G-band), which can be assigned to the disordered carbon and ordered sp2 carbons, respectively.48 Like the other exfoliated 2D materials, the Ti3C2Tx spectrum shows weak and broad D and G peaks after removal of Al layers from the Ti3AlC2 phase. Compared with Ti3C2Tx, the intensities of D and G bands of h-C/TiO2-1.5-20 hybrid 11



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decrease obviously, which mean that part of carbon has been covered or oxidized during the hydrothermal process. However, the decreased value of ID/IG from 1.02 for Ti3AlC2 to 0.86 for h-C/TiO2-1.5-20 hybrid may imply that the oxidation process is favoured to arrange the carbon atoms regularly, which is beneficial to shorten the response time to the external electric field. X-ray photoelectron spectroscopy (XPS) spectra provide valuable details concerning their surface chemistry. As shown in the survey curves of Ti3AlC2 and Ti3C2Tx (Figure S1b): the Al element disappeared after etching with HF, and the F element arises. The presences of Ti 2p, O 1s and C 1s in c-C/TiO2 confirm the successful CO2 oxidation of Ti3C2Tx (Figure S1c). Figure S1g-i and Figure 2d-f show the deconvolution of Ti 2p, O 1s and C 1s of XPS spectra for Ti3C2Tx and h-C/TiO2-1.5-20, respectively. In the Ti 2p region (Figure S1g and Figure 2d), the Ti 2p3/2 (455.2 eV) and Ti 2p1/2 (461.3 eV) of Ti-C bands for Ti3C2Tx were replaced with Ti 2p3/2 (458.7 eV) and Ti 2p1/2 (464.5 eV) of Ti-O bands after oxidation of h-C/TiO2-1.5-20. From the O 1s peaks of Ti3C2Tx (Figure S1h), it can be observed that the Ti3C2Tx possesses many hydroxyl functional groups which endow it with a hydrophilic nature. There are no obvious hydroxyl groups for the h-C/TiO2-1.5-20 (Figure 2e), demonstrating that h-C/TiO2-1.5-20 holds great promise to enhance dispersion stability in oils for ER applications. The peak intensity of Ti-O band increased significantly due to the generation of TiO2.45 Remarkably, in the C 1s spectra of the Ti3C2Tx, the distinct peak at 282.7 eV (Figure S1i), which is ascribed to C−Ti bonds, disappeared completely in h-C/TiO2-1.5-20 (Figure 2f), suggesting the 12


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Ti-C band has been destroyed.44 Besides, the high heterocarbon components of Ti3C2Tx, such as C-O (~286.1 eV) and O-C=O (~288.4 eV) bands, weakened after the hydrothermal process. A peak at 286.2 eV (C-O-Ti band) of h-C/TiO2-1.5-20 hybrids appeared,45 suggesting that hydrothermal process can break the Ti-C band followed by the re-arrangement of carbon and generation of Ti-O bonds. Compared with the Ti 2p, O 1s and C 1s of bulk Ti3AlC2 (Figure S1d-f) and Ti3C2Tx, the O-C=O/C-F band at 288.4 eV appears after the HF etching. N2 adsorption-desorption isotherms were carried out to investigate the porosity of

Ti3C2Tx

and

h-C/TiO2-1.5-20.

As

shown

in

Figure

2g,

h,

the

Brunauer-Emmett-Teller (BET) surface area of h-C/TiO2-1.5-20 hybrid was significantly improved from 3.81 (bare Ti3C2Tx) to 43.83 m2/g, owing to the formation of TiO2 nanoparticles and irregular slits during the hydrothermal process. Notably, the accordion-like Ti3C2Tx with more visible wrinkle and slits can adsorb more N2 gas compared to that of Ti3AlC2 bulk (0.05 m2/g, Figure S1a). As shown in the inset of Figure 2h, the pore size distribution of h-C/TiO2-1.5-20 hybrid mainly focused on 17.42 nm. The large specific surface area of h-C/TiO2-1.5-20 hybrid generally leads to fast polarization and good infiltration of medium oil in ER measurements. The wettability refers to the interactions between particles and dispersing medium oil, is one of the crucial factors that determine the ER activity. In general, the dielectric particles with high dispersion stability exhibit improved ER activities owing to the enhanced particle mobility in the insulating medium. However, the particle with 13



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inferior wettability will tend to aggregate into clusters, resulting in poor dispersion stability. The permeabilities of Ti3C2Tx and h-C/TiO2-1.5-20 hybrid were measured and compared by the Washburn method.49 It can be observed that the wettability of h-C/TiO2-1.5-20 hybrid was enhanced compared to that of Ti3C2Tx (Figure 2i), which may generate good ER performances.

Figure 3. Shear stress vs. shear rate ramped from 1 to 1000 1/s of h-C/TiO2-1.5-20 based ER fluids with particle concentration at 4 vol.% (a) and 8 vol.% (b), respectively. The flow curves in (a) were fitted with a CCJ model (solid line) and the Bingham model (dashed line). Amplitude sweep of the h-C/TiO2-1.5-20 based ER fluids at a fixed angular frequency of 6.28 rad/s (c). Frequency sweep of the ER fluid with a particle fraction of 4 vol.% (d) and 8 vol.% (e) with a fixed strain amplitude of 0.003% in the LVE range. G′ (closed symbols) and G" (open symbols). Cyclic on-off testing at a fixed shear rate (1/s) response to an electric field with a square voltage pulse (t=30 s) (f). Dynamic yield stress vs. electric field strength (g) and universal fitting of the h-C/TiO2-1.5-20 based ER fluids (h, i), b=0.66.

The bulk rheological behaviors of the ER fluids based on h-C/TiO2-1.5-20 particles at different particle fractions (4, 8 and 12 vol.%) were assessed with a 14


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rotational rheometer in both rotational and oscillation modes. As shown in Figure 3a and 3b, similar flow curves trends were obtained even at different particle concentrations (4 and 8 vol.%), except for the enhanced yield stress according to the increasing particle volume ratio. In the absence of an electric field, ER fluids behave as a Newtonian fluid, in which the shear stress significantly increases in proportion to the shear rate without yielding. While under an electric field, rigid pre-yield stress was observed, and the yield stresses increase step by step with the increased electric field strength. In addition, the h-C/TiO2-1.5-20 particles based ER fluids exhibited a concave shape, in which in shear stress decreased slightly at the low shear rate region and after passing the critical shear stress, Newtonian fluid behaviors re-appeared. The phenomenon can be explained as follows: The formation of fibril-like chains of polarizable particles in shear flow is a balancing process originating from the cooperation of electrostatic force among the polarized particles and hydrodynamic forces from the mechanical shearing.50 In a low shear rate region, the shear stress exhibited a plateau value, confirming that the fibril-like structures sustained well owing to the dominant electrostatic interactions among the particles. When the shear rate is further increased (~above 130 1/s), the shear flow induced hydrodynamic interactions become dominate compared to the electrostatic interactions. The aligned particular structures are totally destroyed, and Newtonian fluid phenomenon re-appeared. Two different models are used to evaluate the flow behaviours of h-C/TiO2-1.5-20 particles based ER fluid. Bingham model is a simplest and general 15



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model for an ER suspension, which expressed as follows:51 τ = τ0 + η0γ, τ ≥ τ0 γ=0

(1)

τ < τ0

Where 𝛾 is the shear rate, τ is the shear stress, η is the shear viscosity, and τ0 is the yield stress. However, this simple model cannot fit the complex flow behaviors in some practical situations well over the whole shear rate, such as the h-C/TiO2-1.5-20 particle-based ER fluid. Dashed lines deriving from the Bingham model in Figure 3a shows an apparent deviation from the experimental data, especially for lower shear rate region. Hence, a constitutive rheological equation with 6 parameters for this type of ER fluid, known as the Cho-Choi-Jhon (CCJ) model, was suggested for this situation. The proposed equation can be written as follows: τ=

τ0 1 + (t1γ)α

[

+ η∞ 1 + (

1 t2γ)β

]γ

(2)

Where t1 and t2 denote the time constants, η∞ indicates the shear viscosity at infinite shear rate, β is in the range of 0 and 1 because dτ/dγ ≥ 0. The two terms in the right-hand side of the CCJ model can describe the shear stress in a wide range of shear rate regions. The first one supports the decrease in the shear stress at the low shear rate region. Moreover, the second one controls the fitting for the increasing shear stress at a high shear rate region. From the fitting result in Figure 3a, we confirm that the CCJ model provides a better explanation for the unique flow curves in both the low and high shear rate regions. Optimal values of the parameters derived from fitting these two equations are listed in Table 1.

Table 1. Fitting parameters of Bingham and CCJ model equations to the flow curve of 16


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h-C/TiO2-1.5-20 based ER fluids (4 vol.%, particle concentration).

An oscillation test was carried out to determine the viscoelastic behaviors of the h-C/TiO2-1.5-20 based ER fluid. Prior to a frequency sweep test, an amplitude sweep test was examined to find the linear viscoelastic range (γLVE) at an angular frequency of 6.28 rad/s. At the small amplitude region (Figure 3c), the storage modulus (G′) and loss modulus (G") show nearly linearly relation up to a critical strain, which defined as the γLVE, where the deformation in the structure is reversible. The frequency sweep was examined in the angular range of 1 to 100 rad/s at a strain of 0.003%, within the bound of γLVE. As shown in Figure 3d, e, the values of G′ is much higher than G" at a constant electric field strength, indicating the elastic properties of the ER fluids.52 To explore the sensitivity and the stability of at a relevant electric field, the steady shear flow was evaluated under applied square voltage pulse (t=30 s) with a shear rate of 1 s-1. As shown in Figure 3f, the shear viscosity of the ER fluids jumps to a high value from the zero-field viscosity when the electric field applied, and drops to zero-field level again straightforward at the moment of the electric field removed. The h-C/TiO2-1.5-20 particle-based ER fluid exhibits higher shear viscosity with a higher particle volume fraction at the same electric field strength, which consistent with the 17



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shear stress data in Figure 3a and b. The fantastic switching performance responding to the electric field signal will trigger new applications. To explore the mechanism of the ER effects of the novel h-C/TiO2-1.5-20 particle based ER fluid, the dynamic yield stress (τy) corrected from the original flow curves in Figure 3a, b was re-plotted vs. electric field strength (E) in a log-log scale. The correlation between τy and E was fitted by a power-law equation, which is expressed as follows: τ=Em. In general, the exponent m of 1.5 is corresponding to the conduction model while the dielectric polarization model is responsible for the quadratic relation. As plotted in Figure 3g, both types of ER fluids show an alternative yield stress model, in which the τy is directly proportional to E2 at a low electric field region, and approaches to 1.5 at a high electric field region. The crosspoint of the electric field, which called as a critical electric field (Ec), was introduced to describe the derivation of the hybrid yield stress equation as follows:53 τy(E0) =

(

αE20

tanh

E0

E0

)

Ec

Ec

(3)

As shown in Figure 3g, the points of the h-C/TiO2-1.5-20 based ER fluid at different volume fractions of particles are fitted by two lines with the slopes alternated from 2 to 1.5, by which Ec is 2 kV/mm. To better understand and reveal the ER mechanism of this novel ER fluid, the experimental points of the h-C/TiO2-1.5-20 based ER fluids at different particle concentration were fitted into a single line by normalizing Eqn. (3) using E and τ: τ = 1.313E

3/2

tanh E

E ≡ E0 Ec, τ ≡ τy(E0) τy(Ec) (4) 18


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The experimental points (τy, E0) in Figure 3g are then fitted using the normalized Eqn. (4), the results are shown in Figure 3h. It can be noticed that an obvious bias of the experimental points and universal line was observed. Hence, a corrected parameter b was introduced in Eqn. (4): 54 3/2

τ = 1.313H 1 + 2𝑏

E≡𝐸

tanh H 4𝑏

and τ ≡ τ𝐸

(5)

Figure 3i demonstrates that the experimental points of our ER fluids can be fitted well using a single curve deriving from Eqn. (5) independent of particle volume concentration. The fitting parameter b for the h-C/TiO2-1.5-20 based ER fluids is 0.66. Considering that ER behavior is closely related to the polarization of particles in suspensions, we conducted a dielectric study, including the permittivity (ε′) and dielectric loss (ε″). In general, a proper ER fluid should have a large achievable polarizability (Δε), and fast relaxation time (λ). The former Δε denotes the dielectric relaxation strength, and Δε=ε0-ε∞, where ε0 and ε∞ are the limit values when frequency approaches to 0 and at the high limit, respectively. The latter λ is the relation time, where λ=1/2πfmax, (fmax is the frequency at a maximum dielectric loss). To gain more insight into the correlation of ER performances on dielectric constants, the dielectric constants of h-C/TiO2 based ER fluid (4 vol%, particle concentration) were investigated (Figure S5). The h-C/TiO2 based ER fluid exhibits a large Δε of 3.02. According to the polarization model, the strong interfacial polarizability should be the origin of the high ER response. However, the relaxation time of h-C/TiO2 based ER 19



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fluid is 0.016 s, which is still not fast enough as an ideal ER fluid (~10-3-10-6 s). The relaxation time may influence by the hierarchical structure, in which the conductive carbon layer was densely wrapped by TiO2 nanoparticle. Therefore, a deliberate design of h-C/TiO2 combining fast polarization rate and high achievable polarizability will be pursued in future.

Figure 4. OM images of the h-C/TiO2-1.5-20 (a, f), c-C/TiO2-650 (b, g) and c-C/TiO2-700 (c, h), c-C/TiO2-750 (d, i) and c-C/TiO2-800 (e, j) based ER fluids (2 vol.%, particle concentration) without (the top row) and with (the middle row) the electric field, respectively. Scale bar is 180 μm. The proposed mechanisms of the corresponding OM phenomena (c, f, i, l, o). The green dots denote the dipole.

The structural change of C/TiO2 based ER fluids (2 vol.%, particle concentration) was observed through using an optical microscope (OM) (Figure 4). Without the electric field, all of the C/TiO2 particles dispersed randomly in the insulating oil as shown in Figure 4 a, d, g, j, m. At the moment of an electric field applied, the C/TiO2 particles aligned along the electric field direction (Figure 4b, e, h, k, and n). Furthermore, there are some differences among these C/TiO2 particles based ER suspensions. Without the electric field, the h-C/TiO2-1.5-20 particles dispersed more 20


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uniformly in the silicone oils than that of corresponding c-C/TiO2 particles. When exposed to an external electric field, the h-C/TiO2-1.5-20 particles formed denser and more closely linked fibril-structures than those of c-C/TiO2 particles based ones. These phenomena indicate that the ER responses of the h-C/TiO2-1.5-20 based ER fluid is superior to those of c-C/TiO2 particles based ones, which may due to the synergistic effect of unique porous 2D architecture and abundant functional groups.

Figure 5. Schematic mechanism of h-C/TiO2 based ER fluid at different particle concentrations.

As shown in Figure 5, the mechanism for the unique ER performances of the h-C/TiO2 hybrids based ER fluids is proposed: during the hydrothermal process, the H2O2 molecules can etch the active carbon layers to porous structure, which enable the carrier oil to access through the h-C/TiO2 layers, resulting in good dispersion stability. Thus, more TiO2 nanoparticles anchored on the porous carbon layers, affording superior polarization and rational electric conductivity during the ER tests. Meanwhile, the large amount of TiO2 nanoparticles can prevent the crimp of the lamellar h-C/TiO2 hybrid based ER fluid when subjected to the violent shearing process. Hence, superior ER behaviors were obtained. To study the advantage and persistence of the lamellar architecture of the 21



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h-C/TiO2 hybrids on their ER behaviors, their TEM image was carried out after the robust ER measurement. The h-C/TiO2-1.5-20 based ER fluid was first washed with ethanol to remove the silicone oil. After thorough washing, TEM image of the particle was characterized as shown in Figure S6. The h-C/TiO2-1.5-20 exhibited well-reserved lamellar structure decorated by TiO2 nanoparticles, which is very close to the previous TEM result (Figure 1d). The slightly fuzzy stain on the h-C/TiO2-1.5-20 surface might due to the residual silicone oil. The result further complicates the influence of the specific lamellar architecture of our designed oxidized MXene hybrids on their excellent ER performance.

CONCLUSIONS In summary, lamellar C/TiO2 hybrids were synthesized via the oxidation of Ti3C2Tx MXene using a hydrothermal method (h-C/TiO2) or CO2 calcination (c-C/TiO2), respectively. As for the c-C/TiO2 hybrids, the size and amount of TiO2 granules increased with the increasing calcination temperature, leading to an interesting electro-responsive

behavior.

The

h-C/TiO2-1.5-20

with

advanced

lamellar

architectures delivered an optimal ER performance. In details, the conductive carbon layers with oxygen functional groups significantly reduced the response time to an electric field meanwhile the polarizable TiO2 nanoparticles enhanced the ER effects. This research broadens the application of MXene and provides a possible pathway to predict and optimize other 2D materials with specific architectures.

ASSOCIATED CONTENT Supporting Information SEM image and N2 adsorption-desorption isotherms of the Ti3AlC2. The XPS survey 22


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of the samples and Ti 2p, O 1s and C 1s for Ti3AlC2 and Ti3C2Tx. SEM images of h-C/TiO2 obtained at different conditions. Flow curves of c-C/TiO2-650 based ER fluid (~2.91-3.40 vol.%, particle concentration). Dielectric spectra of h-C/TiO2 based ER fluid (4 vol.%, particle concentration). TEM image of the h-C/TiO2-1.5-20 after bulk rheological test. Summary of the elemental ratio from EDS results (Atomic %) for the samples and their densities.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID

Wen Ling Zhang: 0000-0003-2445-320X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21603115), Shandong Provincial Natural Science Foundation, China (Grant No. ZR2016BB04, ZR2017JL025), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chairs Program (H. Zeng).

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Unveiling the Critical Role of Surface Oxidation of Electroresponsive

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