1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 2D Titanium Carbide/Reduced Graphene Oxide Heterostructures for Supercapacitor Applications Adriana M. Navarro-Sua ´rez + , [a, b] Kathleen Maleski, [a] Taron Makaryan, [a] Jun Yan, [a, c] Babak Anasori, [a] and Yury Gogotsi* [a] Solution-processable two-dimensional (2D) materials offer the possibility of manufacturing heterostructures with various properties, creating a way to tune materials towards a specific application. Two different 2D materials, titanium carbide MXene (Ti 3 C 2 T x ) and reduced graphene oxide (rGO), have shown promising results for supercapacitor applications due to their flake-like morphology, high conductivity; and ability to interca- late molecules or ions for charge storage. Here, we demonstrate the self-assembly of a heterostructure between negatively charged Ti 3 C 2 T x and positively charged modified rGO after shear mixing. Changes in zeta (z) potential, X-ray diffraction (XRD) patterns; and Raman spectra confirm the assembly of this heterostructure. The produced rGO : Ti 3 C 2 T x heterostructures were used as electrodes for supercapacitors. The addition of rGO to Ti 3 C 2 T x allowed some widening of the voltage window. Moreover, due to the synergistic effect of these materials, an increase of the capacitance value was observed. An electrode film composed of rGO (1 wt.%) and Ti 3 C 2 T x (99 wt.%) achieved capacitance values up to 254 F · g 1 at 2 mV·s 1 and 193 F · g 1 at 100 mV · s 1 . 1. Introduction Current energy storage devices are based on the capture and release of metal ions and electric charges, i. e. batteries and supercapacitors. [1] These can be found in several configurations, dimensions, and can contain many types of materials, but the current technology relies on particulate-like nanomaterials, which may lose their mechanical integrity upon bending. [2] In contrast, two-dimensional (2D) materials are inherently flexible and have a high surface reactivity, high electrical conductivity, and large surface area, which are fundamental characteristics of energy storage devices. The exfoliation of graphite into single layers of graphene, in 2004, resulted in a new explosion of interest in 2D materials. [3] A large family of 2D transition metal carbides and nitrides, called MXenes, are gaining interest in a variety of applications due to their promising properties. [4,5] MXenes have a general formula of M nþ1 X n T x , where M is an early transition metal (e.g., Ti, Nb, V, Mo), X can be carbon or nitrogen, n is equal to 1, 2 or 3 and T x represents surface functional terminations such as  OH,  O, or  F. The 2D structures are made of sheets of transition metals that are interleaved with sheets of carbon or nitrogen in a [MX] n M arrangement. [5] Stacking two-dimensional materials, such as graphene and MXene, can be made into layered heterostructures. [6–8] This architecture leads to the possibility of creating and designing layered artificial structures with “on-demand” properties, mak- ing them of notable interest for a range of applications, including energy storage. [1,9] A 3D heterostructure constructed from MXene and rGO could have the potential to stop aggregation and restacking of MXene or rGO nanosheets, which has been demonstrated to deteriorate the intrinsic electro- chemical performance of the 2D materials. [10] Titanium carbide MXene (Ti 3 C 2 T x ) and reduced graphene oxide (rGO) are 2D materials which have shown promising results in supercapacitors devices. [5,11–16] So far, there are two reports on the assembly of stacked structures of titanium carbide/rGO heterostructures. [17,18] However, designing a homo- geneous and uniform heterostructure comprising these two materials is a challenge given that Ti 3 C 2 T x and rGO are both negatively charged. To achieve a self-assembled layered structure with two inherently negatively charged materials, surface functionalization can be used to alter the surface charges of the 2D sheets. Recently, we reported a strategy to prepare Ti 3 C 2 T x /rGO films by using electrostatic self-assembly between positively charged rGO with poly(diallyldimethylam- monium chloride) (PDDA) and negatively charged Ti 3 C 2 T x MXene nanosheets. After electrostatic assembly, rGO nano- sheets were inserted in between Ti 3 C 2 T x layers. [19] Here, we show that functionalizing rGO with amine groups can effectively change the surface charge on rGO from negative to positive. Thus, when the positively charged rGO nanosheets [a] Dr. A. M. Navarro-Suµrez, + K. Maleski, Dr. T. Makaryan, Dr. J. Yan, Dr. B. Anasori, Prof. Y. Gogotsi Department of Materials Science & Engineering and A.J. Drexel Nanomaterials Institute, Drexel University Philadelphia, PA 19104, USA E-mail: gogotsi@drexel.edu [b] Dr. A. M. Navarro-Suµrez + CIC energiGUNE Albert Einstein 48, 01510 MiÇano, Alava, Spain [c] Dr. J. Yan Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering Harbin Engineering University Harbin 150001, China [ + ] Current address: Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/batt.201800014 33 Batteries & Supercaps 2018, 1, 33 – 38  2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Articles DOI: 10.1002/batt.201800014