Size-Dependent Physical and Electrochemical Properties of Two

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Size-Dependent Physical and Electrochemical Properties of Two-Dimensional MXene Flakes Kathleen Maleski, Chang E. Ren, Meng-Qiang Zhao, Babak Anasori, and Yury Gogotsi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04662 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

Size-Dependent Physical and Electrochemical Properties of Two-Dimensional MXene Flakes Kathleen Maleski1˟, Chang E. Ren1˟, Meng-Qiang Zhao1,2, Babak Anasori1, Yury Gogotsi1* 1

2

Department of Materials Science and Engineering, and A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA 19104, United States of America

Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, United States of America ˟

Authors contributed equally to this work

ABSTRACT: Two-dimensional (2D) particles, including transition metal carbides (MXenes), often exhibit large lateral size polydispersity in delaminated colloidal solutions. This heterogeneity results in challenges when conducting fundamental studies, such as investigating correlations between properties and the 2D flake size. To resolve this challenge, we have developed solutionprocessable techniques to control and sort 2D titanium carbide (Ti3C2Tx) MXene flakes after synthesis based on sonication and density gradient centrifugation, respectively. By tuning sonication conditions, Ti3C2Tx flakes with varied lateral sizes, ranging from 0.1 to ~5 µm, can be obtained. Furthermore, density gradient centrifugation was used to sort Ti3C2Tx flakes with different lateral sizes into more monodisperse fractions. These processing techniques allow for characterization of size-dependent optical and electronic properties by measuring absorption spectra and film conductivity, respectively. Additionally, by testing the material as electrochemical capacitor electrodes, we show the Ti3C2Tx flake size dependence on electrochemical performance. Ti3C2Tx films made of flakes with lateral sizes of ~1 µm showed the best capacitance of 290 F/g at 2 mV/s and rate performance with 200 F/g at 1000 mV/s. The work provides a general methodology which can be followed to control the size of MXene flakes and other 2D materials for a variety of applications and fundamental size-dependent studies. KEYWORDS: two-dimensional material, MXene, flake size, sonication, density gradient centrifugation

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1. Introduction Two-dimensional (2D) materials, such as graphene, transition metal oxides, transitional metal dichalcogenides, etc., have recently received increased consideration for many applications due to their diverse mechanical, optical, and electronic properties.1 Transition metal carbides and/or nitrides, named as MXenes, have further expanded the family of 2D materials by adding close to 30 new materials (and many predicted), each with unique properties.2 MXenes are synthesized by selectively etching layered materials, such as MAX phases, which have a formula Mn+1AXn, where M commonly stands for a transition metal (Ti, Mo, V, etc.), A is group 13 to 16 elements of periodic table (Al, Si, Ga, etc.), X is either carbon and/or nitrogen, and n is an integer from 1 to 3.2-5 Etched in fluorine-containing aqueous acids, the resulting MXene flakes contain surface terminations (Tx), such as =O, –OH, and –F, rendering them hydrophilic and capable of solution processing.6 In addition, MXene flakes are negatively charged,7 show high electrical conductivity and mechanical strength, as well as promising intercalation properties of ions and large organic molecules.2 These advantageous properties have allowed the family of materials to show early promise in applications such as energy storage,8-9 electromagnetic interference shielding,10 and optoelectronics.11-12 The properties and the corresponding performance of MXenes in various applications are largely dependent on their stoichiometric compositions, surface chemistry, flake thickness, and lateral size.2 Numerous efforts have been devoted to investigate the correlations between the former three parameters and the physiochemical properties of MXenes; however, difficulties in controlling the lateral size of MXene flakes hinder the studies of their size-dependent optical, mechanical, electrochemical, and electronic properties.2 Additionally, synthesis of MXene flakes can be conducted in a variety of routes, for example, different etching solutions can be used,


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ranging from aqueous hydrofluoric acid (HF) to a mixture of lithium fluoride (LiF) and hydrochloric acid (HCl).4, 13-14 Changing the synthesis route can affect the delamination into single flakes, producing Ti3C2Tx flakes with lateral sizes ranging from a few hundred nanometers to a few microns.14-16 Regardless of synthesis route, precise control over the size of MXene flakes in solution remains as a challenge. By varying the input of mechanical energy, sonication can be used as a simple method to decrease the size of 2D materials, such as graphene.17 Density gradient ultracentrifugation (DGU) has been utilized to separate nanomaterials both by size and thickness, taking advantage of differences in particle sedimentation rates.18-19 In particular, density gradients have also been utilized to separate 2D materials such as transition metal dichalcogenides (TMDs, such as MoS2),20 and graphene by number of layers,21 sheet size and surface chemistry.19, 22 In this regard, both sonication and density gradient methods are promising to control or sort the lateral size of MXenes to allow studies on their size-dependent optical, electronic, and electrochemical properties. In this study, these two methods are explored to control the flake size of Ti3C2Tx MXene. First, we discuss the sonication conditions to achieve control over the lateral size of Ti3C2Tx MXene flakes. Additionally, we use inexpensive and environmentally friendly sucrose as the main component in a density gradient separation22-24 to isolate Ti3C2Tx flakes with different sizes by centrifugation. Using this principle to create controlled dispersions, we probed the relationships between lateral size of the nanosheet and the resulting optical, electrical, and electrochemical properties.



2. Experimental Methods Preparation of Ti3C2Tx MXene Ti3C2Tx powder was obtained from etching of Ti3AlC2 MAX powder (10 mg/mL) on top of a 9-mL density gradient (66 w/v%, 1 mL step volume and 60-20 w/v% 2 mL step volume). After separation (3500 rpm for 30 minutes), 3-mL fractions were vacuum filtered to create films composed of flakes with different lateral sizes. The samples are named based upon the line of the centrifuge tube (in milliliters) from which they were fractioned (e.g., 12-9 mL or 6-3 mL).



Characterization X-ray diffraction (XRD) measurements were performed using a powder diffractometer (Rigaku SmartLab) with Cu Kα radiation at a step of 0.02º and a collection time of 0.5 s per step. Transmission electron microscopy (TEM, JEOL JEM-2100F) was used to gather information about the size of the flakes as well as the flake size distribution. Dynamic Light Scattering (DLS) was performed by pipetting 1 mL of solution into a polystyrene cuvette (Zetasizer Nano ZS, Malvern Instruments, USA). DLS average was taken over a total of 5 measurements from each sample. Characterization of optical properties was measured by UV-vis absorbance spectroscopy (10 mm optical path length cuvette, Evolution 201, Thermo Scientific), scanning from 200-1000 nm and normalized at 264 nm. Electrical properties were probed by measuring conductivity by 4point probe technique (Jandel Engineering Limited). The distance between probes was 1.0 mm. Cross-section scanning electron microscopy (SEM) was conducted on a Zeiss Supra 50VP to measure the thickness of the free-standing electrodes. Correlation of DLS and SEM size distributions Scanning electron microscopy (SEM) samples were prepared by vacuum filtering lowconcentration (< 0.1 mg/mL) Ti3C2Tx colloidal solutions over a porous alumina membrane (Anodisc, 0.1 µm pore size, Whatman™). SEM analysis was conducted at various magnifications. Microscopy-derived distributions were determined by measuring the length (longest distance) and width (shortest distance) of 100 flakes using ImageJ image analysis software25, followed by using a histogram (30 bins from 0 to 6 µm) to create log-normal distributions. Distributions were also calculated for SEM length and SEM width (Table 1). The largest DLS intensity peak by percentage (%) was named aDLS, similar to previous reports.26 Z-average and aDLS were used to compare to SEM distributions. Results for dynamic light scattering are shown in Table 1. 6 ACS Paragon Plus Environment

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Electrochemical Characterization Various flake sizes were measured electrochemically by recording their performance in an asymmetric supercapacitor. Ti3C2Tx electrodes were fabricated by vacuum filtering to create a film. The films were cut to size and used in a three-electrode Swagelok. The cell was structured as Ti3C2Tx (working), over capacitive activated carbon (counter), silver/silver chloride (Ag/AgCl) in 1M KCl as reference electrode, and glassy carbon electrodes as the current collectors, to avoid hydrogen evolution reactions.27 Electrochemical testing was performed in 3M sulfuric acid (H2SO4) electrolyte due to the higher ionic conductivity compared to 1M H2SO4.28 The performance of Ti3C2Tx was evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge and discharge (GCPL) using a BioLogic VMP3 potentiostat (BioLogic, France). After initially pre-cycling the electrodes at 20 mV/s for 500 cycles, cyclic voltammetry was performed at scan rates from 2 mV/s to 10,000 mV/s. The BS-0.25 material was used to measure long-term cyclability at 10 A/g. The cell was cycled for 10,000 cycles with a 0.9 V potential window (-0.6 V to 0.3V vs. reference). 3. Results and Discussion Detailed illustration of how the sonication and sucrose density gradient centrifugation techniques are utilized to control the size of MXene flakes after synthesis is schematically shown in Figure 1. Exfoliation or delamination of 2D materials produces heterogeneous dispersions with large degrees of polydispersity (nm to µm) and varying thicknesses (that is, single- to multi-layer flakes) (Figure 1a, b).1, 19 When conducting Ti3C2Tx synthesis, centrifugation is a key part of the procedure to isolate delaminated Ti3C2Tx flakes from multi-layered Ti3C2Tx or the unreacted MAX phase particles that were used as the starting precursor.14 The as-synthesized Ti3C2Tx solution is usually subjected to centrifugation at 3500 rpm for 1 hour, removing multi-layer Ti3C2Tx flakes 7 ACS Paragon Plus Environment


and leaving a Ti3C2Tx colloidal solution largely composed of single-layer flakes.18 On one hand, the size of Ti3C2Tx flakes can be reduced by sonication, with control achieved by tuning the sonication time and power (Figure 1c). Another approach is sucrose density gradient centrifugation, which renders the isolation of Ti3C2Tx flakes with different lateral sizes (Figure 1d). In the most simplistic manner, sonication is used to directly break flakes in solution due to the input mechanical energy, producing Ti3C2Tx lateral sizes which are smaller compared to the as-synthesized MXene (Figure 2). Depending on the sonication time and power, the Ti3C2Tx solutions obtained by bath sonication at 100 W for 0.25, 1.5, and 3 h are denoted as BS-0.25, BS1.5, and BS-3.0, respectively. The Ti3C2Tx solution without sonication is denoted as BS-0, and the sample obtained by probe-sonication at 250 W for 1 h is denoted as PS-1.0. Figure 2 shows the resulting transmission electron microscopy (TEM) images and distribution of flake size by percentage obtained by statistical analysis of over 300 flakes. The lateral size of Ti3C2Tx flakes in the BS-0 ranges from 1.0 to 10.0 µm, with an average size of 4.4±1.5 µm (Figure 2a). Just after 0.25 hours of sonication (BS-0.25), polydisperse flakes are reduced to smaller, more monodisperse flakes ranging from less than 0.5 to 2.5 µm, with an average size of 1.0±0.56 µm (Figure 2b). As expected, when bath sonication time is increased, Ti3C2Tx continues to decrease in lateral size below 1 µm. The average flake sizes for BS-1.5 and BS-3.0 are 0.57±0.30 and 0.35±0.15 µm, respectively (Figure 2c and d). By introducing more power, probe sonication method produces the smallest and most monodisperse flakes (

Size-Dependent Physical and Electrochemical Properties of Two

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