Supporting Information

Supporting Information Laminated and Two-Dimensional Carbon-Supported Microwave Absorbers Derived from MXenes Meikang Han, Xiaowei Yin,*, Xinliang Li, Babak Anasori, Litong Zhang, Laifei Cheng, and Yury Gogotsi Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi an, Shaanxi 710072, China A. J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States *Corresponding author: yinxw@nwpu.edu.cn S-1

Figure S1. Photograph of the as-prepared samples of C/TiO 2 hybrids in the paraffin matrix for dielectric measurement. The reflection coefficient (RC) was calculated using the measured complex permittivity at a given layer thickness and frequency by the following equations: RC db = 20log10 Zin -1 Z in +1 (1) Z in = tanh j2 fd c where Z in is the normalized input impendence of the microwave absorption layer, ε and μ are the relative permittivity and permeability of the samples, f is the EM wave frequency, d is the matching thickness, c is the light velocity in vacuum. RC value is < -10 db (denoted effective absorption), more than 90% of the EM wave is absorbed. The corresponding frequency range (RC < -10 db) is defined as the effective absorption bandwidth. (2) Figure S2. (a) Nitrogen adsorption desorption isotherms for Ti 3 C 2 T x, TiO 2 and C/TiO 2 hybrids (S-800); (b) The TG-DSC curves of the as-prepared C/TiO 2 hybrids in an air atmosphere at the temperature ranging from 30 to 800 C. S-2

The N 2 adsorption desorption isotherms (Figure S2a) show an increase in the specific surface area (SSA) from Ti 3 C 2 T x to C/TiO 2 hybrids. The SSA of C/TiO 2 hybrids estimated using the BET equations was found to be ~31.4 m 2 /g, a factor of 4 greater than that of Ti 3 C 2 T x, which had a SSA of 8.1 m 2 /g. In particular, to evaluate the SSA of carbon layers derived from MXene, pure TiO 2 was obtained by oxidation of the as-synthesized C/TiO 2 hybrids at 800 C in air, and its SSA was ~8.5 m 2 /g. The weight ratio of carbon to TiO 2 is about 1:24, as shown in the TG-DSC curves of C/TiO 2 hybrids in air atmosphere (Figure S2b). The SSA of carbon derived from MXene which was estimated from the above data was ~584 m 2 /g. In addition, based on the ratio of carbon in C/TiO 2 hybrids (~3.98 wt.%), it can be calculated that the mass ratio of carbon in the sample with 45 wt.% filler loading is ~1.79 wt.% (0.45 3.98 wt.%). Figure S3. Schematic of the structural evolution from Ti 3 C 2 T x to C/TiO 2 hybrids with the increasing temperature, and the corresponding SEM images. Figure S3 shows a schematic of the structural evolution from Ti 3 C 2 T x to C/TiO 2 hybrids, coupled with the corresponding SEM images. After annealing at 600 C, small crystalline TiO 2 nanoparticles and disorder carbon form on the surface of Ti 3 C 2 T x layers. At this stage, no visible morphological changes were observed, which is ascribed to localized oxidation of the external Ti layers terminated with hydroxyl groups, oxygen and fluorine. As the temperature increases, the smooth flakes become rough, and the interlayer spacing broadens obviously, which is related to the increasing density and size of anatase and rutile TiO 2 particles during the intensified oxidation process. A significant transformation occurs when Ti layers undergo the complete oxidation of Ti at the annealing temperature of 800 C. It leads to the exfoliation of the intermediated C layers. 2D disordered carbon layers support TiO 2 nanocrystals, which separate carbon sheets and prevent them from restacking. S-3

Figure S4. 3D plots of reflection coefficient (RC) versus frequency and thickness for the samples with mass ratios of (a) 35, (b) 40 and (c) 50 wt.% C/TiO 2 hybrids (S-800) in the paraffin matrix. Figure S5. (a) ε' and (b) ε" versus frequency for the composites of Ti 3 C 2 T x, S-600, S-700 and S-800 in the paraffin matrix with 45 wt.% filler mass ratio. As shown in Figure S5, both ε' and ε" of S-800 are higher than those of Ti 3 C 2 T x, S-600 and S-700, especially in the low frequency (lower than 10 GHz). The complete exfoliation of carbon layers from multi-layer Ti 3 C 2 T x makes a major contribution to the increased permittivity, because the pure TiO 2 has a relatively low ε". 1 The typical relaxation peaks appear after annealing treatment, while no obvious relaxation can be S-4

observed in Ti 3 C 2 T x. It is attributed to the enhanced orientation polarization, which arises from defects and the oxygen functional groups. It should be specifically noted that the response frequency of S-800 is different from that of S-600 and S-700. This is related to the increasing crystallization of carbon layers and the crystal transition of TiO 2 from anatase to rutile. Figure S6. 3D plots of reflection coefficient values versus frequency and thickness for the composites of (a) Ti 3 C 2 T x, (b) S-600, (c) S-700 and (d) S-800 in the paraffin matrix with 45 wt.% mass ratio. The calculated RC values of all the samples versus thickness and frequency are shown in Figure S6. Based on our calculations, there is no effective absorption by Ti 3 C 2 T x with a thickness from 1 to 5 mm, which can be ascribed to its extremely low dielectric loss (Figure S6a). As for S-600 and S-700, both of them present a narrow absorption bandwidth and thick absorption layer (Figure S6b and c). For example, the effective absorption bandwidth of S-700 is only 2.4 GHz (6.6-9 GHz), and the sample thickness is 3.2 mm, although the RC min value can reach -36 db (Figure S6c). S-5

RGO/magnetic materials RGO/nonma gnetic materials RGO/carb on Table S1. EM wave absorption properties of graphene-based materials in recent years The G Optimum RC Type Filler Matrix content min effective thickness Refs (db) bandwidth (wt.%) (mm) (GHz) This Laminated C/TiO 2 wax 1.79-36 1.7 5.6 work RGO NBR 10-57 3 4.5 2 Magnetic RGO epoxy 40-25.2 4 ~1 3 RGO foam wax - -30.5 10 52.2 4 RGO foam/cnts PDMS <5-55 2.75 3.5 5 RGO/C spheres wax 8.8 ~-25 2 ~4 6 RGO/ZnO spheres wax 6-45 2.2 3.3 7 RGO/ZnO wax 6.84-54.2 2.4 6.7 8 RGO/MoS 2 wax 4.3 ~-33 2 5.7 9 RGO/silica textile PF 4.1-37 3.5 ~3.9 10 RGO/SiC nanowires SiOC 3-69.3 2.35 3.4 11 GN/PANI wax - -52.5 2 ~4 12 RGO/γ-Fe 2 O 3 wax 5-59.6 2.5 3 13 RGO/α-Fe 2 O 3 wax 3.75 ~-32 3 9.3 14 RGO/Fe 3 O 4 wax ~6-23 3 5.8 15 RGO/ Fe 3 O 4 wax - -36.4 1.5 4.5 16 RGO/ Fe 3 O 4 wax 15.7-40 4.5 ~3 17 RGO/ Fe 3 O 4 PANI 8.97 ~-25 1.5 4.5 18 RGO/ Fe 3 O 4 epoxy - ~-20 2 2.7 19 RGO/ Fe 3 O 4 wax ~9.4-24 2 4.9 20 RGO/Fe 3 O 4 /SiO 2 /NiO wax - -51.5 1.8 5.1 21 RGO/Fe 3 O 4 /ZnO epoxy - -40 5 2 22 RGO/Fe 3 O 4 /CNTs wax 2-36 2 3.6 23 RGO/Fe 3 O 4 /SiO 2 wax - -31.9 2.5 ~5 24 RGO/ Fe 3 O 4 /Fe/ ZnO wax - -32.5 2.5 ~5.5 25 RGO/Fe 3 O 4 /C/PANI wax - -44.2 3 5.8 26 RGO/CuS wax - -54.5 2.5 4.5 27 RGO/MnFe 2 O 4 PVDF 1.35-29 3 4.8 28 RGO/NiFe 2 O 4 wax 14.7-29.2 2 4.4 29 RGO/NiFe 2 O 4 wax ~20-42 7 ~2 30 RGO/ CoFe 2 O 4 wax ~9.8-18.5 2 3.7 31 RGO/ ZnFe 2 O 4 wax 6-29.3 1.6 2.6 32 RGO/BaFe 12 O 19 PANI - -58 3 4 33 G/Fe wax ~8-31.5 2.5 4.7 34 RGO/Fe wax 1.4 ~-27 2 4.4 35 RGO/Fe/MnO 2 wax 2.4 ~-18 1.5 4 36 RGO/Co wax ~15-47.5 2 ~5 37 RGO/Ni wax ~19 ~-20 2.5 ~5.5 38 RGO/NiCoP wax - -17.8 1.5 ~3 39 References (1) Xia, T.; Zhang, C.; Oyler, N. A.; Chen, X. Hydrogenated TiO 2 Nanocrystals: A Novel Microwave Absorbing Material. Adv. Mater. 2013, 25, 6905-6910. (2) Singh, V. K.; Shukla, A.; Patra, M. K.; Saini, L.; Jani, R. K.; Vadera, S. R.; Kumar, N. Microwave Absorbing Properties of a Thermally Reduced Graphene Oxide/Nitrile Butadiene Rubber Composite. Carbon 2012, 50, 2202-2208. (3) Kowsari, E.; Mohammadi, M. Synthesis of Reduced and Functional Graphene Oxide with Magnetic Ionic Liquid and Its Application as an Electromagnetic-absorbing Coating. Compos. Sci. Technol. 2016, 126, 106-114. S-6

(4) Zhang, Y.; Huang, Y.; Zhang, T.; Chang, H.; Xiao, P.; Chen, H.; Huang, Z.; Chen, Y. Broadband and Tunable High-performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam. Adv. Mater. 2015, 27, 2049-2053. (5) Kong, L.; Yin, X.; Yuan, X.; Zhang, Y.; Liu, X.; Cheng, L.; Zhang, L. Electromagnetic Wave Absorption Properties of Graphene Modified with Carbon Nanotube/Poly (dimethyl siloxane) Composites. Carbon 2014, 73, 185-193. (6) Wang, Y.; Du, Y.; Qiang, R.; Tian, C.; Xu, P.; Han, X. Interfacially Engineered Sandwich-like rgo/carbon Microspheres/rGO Composite as an Efficient and Durable Microwave Absorber. Adv. Mater. Interfaces 2016, 3, 1500684. (7) Han, M.; Yin, X.; Kong, L.; Li, M.; Duan, W.; Zhang, L.; Cheng, L. Graphene-wrapped ZnO Hollow Spheres with Enhanced Electromagnetic Wave Absorption Properties. J. Mater. Chem. A 2014, 2, 16403-16409. (8) Feng, W.; Wang, Y.; Chen, J.; Wang, L.; Guo, L.; Ouyang, J.; Jia, D.; Zhou, Y. Reduced Graphene Oxide Decorated with In-situ Growing ZnO Nanocrystals: Facile Synthesis and Enhanced Microwave Absorption Properties. Carbon 2016, 108, 52-60. (9) Wang, Y.; Chen, D.; Yin, X.; Xu, P.; Wu, F.; He, M. Hybrid of MoS 2 and Reduced Graphene Oxide: A Lightweight and Broadband Electromagnetic Wave Absorber. ACS Appl. Mater. Interfaces 2015, 7, 26226-26234. (10) Song, W. L.; Guan, X. T.; Fan, L. Z.; Zhao, Y. B.; Cao, W. Q.; Wang, C. Y.; Cao, M. S. Strong and Thermostable Polymeric Graphene/Silica Textile for Lightweight Practical Microwave Absorption Composites. Carbon 2016, 100, 109-117. (11) Han, M.; Yin, X.; Duan, W.; Ren, S.; Zhang, L.; Cheng, L. Hierarchical Graphene/SiC Nanowire Networks in Polymer-derived Ceramics with Enhanced Electromagnetic Wave Absorbing Capability. J. Eur. Ceram. Soc. 2016, 36, 2695-2703. (12) Wang, Y.; Wu, X.; Zhang, W. Synthesis and High-performance Microwave Absorption of Graphene Foam/Polyaniline Nanorods. Mater. Lett. 2016, 165, 71-74. (13) Kong, L.; Yin, X.; Zhang, Y.; Yuan, X.; Li, Q.; Ye, F.; Cheng, L.; Zhang, L. Electromagnetic Wave Absorption Properties of Reduced Graphene Oxide Modified by Maghemite Colloidal Nanoparticle Clusters. J. Phys. Chem. C 2013, 117, 19701-19711. (14) Chen, D.; Wang, G. S.; He, S.; Liu, J.; Guo, L.; Cao, M. S. Controllable Fabrication of Mono-dispersed RGO-hematite Nanocomposites and Their Enhanced Wave Absorption Properties. J. Mater. Chem. A 2013, 1, 5996-6003. (15) Hu, C.; Mou, Z.; Lu, G.; Chen, N.; Dong, Z.; Hu, M.; Qu, L. 3D Graphene-Fe 3 O 4 Nanocomposites with High-performance Microwave Absorption. Phys. Chem. Chem. Phys. 2013, 15, 13038-13043. (16) Wang, G.; Gao, Z.; Wan, G.; Lin, S.; Yang, P.; Qin, Y. High Densities of Magnetic Nanoparticles Supported on Graphene Fabricated by Atomic Layer Deposition and Their Use as Efficient Synergistic Microwave Absorbers. Nano. Res. 2014, 7, 704-716. (17) Zheng, X.; Feng, J.; Zong, Y.; Miao, H.; Hu, X.; Bai, J.; Li, X. Hydrophobic Graphene Nanosheets Decorated by Monodispersed Superparamagnetic Fe 3 O 4 Nanocrystals as Synergistic Electromagnetic Wave Absorbers. J. Mater. Chem. C S-7

2015, 3, 4452-4463. (18) Luo, J.; Xu, Y.; Yao, W.; Jiang, C.; Xu, J. Synthesis and Microwave Absorption Properties of Reduced Graphene Oxide-magnetic Porous Nanospheres-polyaniline Composites. Compos. Sci. Technol. 2015, 117, 315-321. (19) Sun, D.; Zou, Q.; Qian, G.; Sun, C.; Jiang, W.; Li, F. Controlled Synthesis of Porous Fe 3 O 4 -decorated Graphene with Extraordinary Electromagnetic Wave Absorption Properties. Acta. Mater. 2013, 61, 5829-5834. (20) Xu, H. L.; Bi, H.; Yang, R. B. Enhanced Microwave Absorption Property of Bowl-like Fe 3 O 4 Hollow Spheres/Reduced Graphene Oxide Composites. J. Appl. Phys. 2012, 111, 07A522. (21) Wang, L.; Huang, Y.; Sun, X.; Huang, H.; Liu, P.; Zong, M.; Wang, Y. Synthesis and Microwave Absorption Enhancement of Graphene@Fe 3 O 4 @SiO 2 @NiO Nanosheet Hierarchical Structures. Nanoscale 2014, 6, 3157-3164. (22) Sun, D.; Zou, Q.; Wang, Y.; Wang, Y.; Jiang, W.; Li, F. Controllable Synthesis of Porous Fe 3 O 4 @ZnO Sphere Decorated Graphene for Extraordinary Electromagnetic Wave Absorption. Nanoscale 2014, 6, 6557-6562. (23) Zhang, H.; Hong, M.; Chen, P.; Xie, A.; Shen, Y. 3D and Ternary rgo/mcnts/fe 3 O 4 Composite Hydrogels: Synthesis, Characterization and Their Electromagnetic Wave Absorption Properties. J. Alloy. Comp. 2016, 665, 381-387. (24) Ren, Y.; Zhu, C.; Zhang, S.; Li, C.; Chen, Y.; Gao, P.; Yang, P.; Ouyang, Q. Three-dimensional SiO 2 @Fe 3 O 4 Core/Shell Nanorod Array/Graphene Architecture: Synthesis and Electromagnetic Absorption Properties. Nanoscale 2013, 5, 12296-12303. (25) Ren, Y. L.; Wu, H. Y.; Lu, M. M.; Chen, Y. J.; Zhu, C. L.; Gao, P.; Cao, M. S.; Li, C. Y.; Ouyang, Q. Y. Quaternary Nanocomposites Consisting of Graphene, Fe 3 O 4 @Fe Core@Shell, and ZnO Nanoparticles: Synthesis and Excellent Electromagnetic Absorption Properties. ACS Appl. Mater. Interfaces 2012, 4, 6436-6442. (26) Wang, L.; Huang, Y.; Li, C.; Chen, J.; Sun, X. Hierarchical Composites of Polyaniline Nanorod Arrays Covalently-grafted on the Surfaces of Graphene@Fe 3 O 4 @C with High Microwave Absorption Performance. Compos. Sci. Technol 2015, 108, 1-8. (27) Liu, P.; Huang, Y.; Yan, J.; Yang, Y.; Zhao, Y. Construction of CuS Nanoflakes Vertically Aligned on Magnetically Decorated Graphene and Their Enhanced Microwave Absorption Properties. ACS Appl. Mater. Interfaces 2016, 8, 5536-5546. (28) Zhang, X. J.; Wang, G. S.; Cao, W. Q.; Wei, Y. Z.; Liang, J. F.; Guo, L.; Cao, M. S. Enhanced Microwave Absorption Property of Reduced Graphene Oxide (RGO)-MnFe 2 O 4 Nanocomposites and Polyvinylidene Fluoride. ACS Appl. Mater. Interfaces 2014, 6, 7471-7478. (29) Fu, M.; Jiao, Q.; Zhao, Y. Preparation of NiFe 2 O 4 Nanorod-graphene Composites via an Ionic Liquid Assisted One-step Hydrothermal Approach and Their Microwave Absorbing Properties. J. Mater. Chem. A 2013, 1, 5577-5586. (30) He, J. Z.; Wang, X. X.; Zhang, Y. L.; Cao, M. S. Small Magnetic Nanoparticles Decorating Reduced Graphene Oxides to Tune the Electromagnetic Attenuation Capacity. J. Mater. Chem. C 2016, 4, 7130-7140. S-8

(31) Fu, M.; Jiao, Q.; Zhao, Y.; Li, H. Vapor Diffusion Synthesis of CoFe 2 O 4 Hollow Sphere/Graphene Composites as Absorbing Materials. J. Mater. Chem. A 2014, 2, 735-744. (32) Yang, Z.; Wan, Y.; Xiong, G.; Li, D.; Li, Q.; Ma, C.; Guo, R.; Luo, H. Facile Synthesis of ZnFe 2 O 4 /Reduced Graphene Oxide Nanohybrids for Enhanced Microwave Absorption Properties. Mater. Res. Bull. 2015, 61, 292-297. (33) Durmus, Z.; Durmus, A.; Kavas, H. Synthesis and Characterization of Structural and Magnetic Properties of Graphene/Hard Ferrite Nanocomposites as Microwave-absorbing Material. J. Mater. Sci. 2015, 50, 1201-1213. (34) Chen, Y.; Lei, Z.; Wu, H.; Zhu, C.; Gao, P.; Ouyang, Q.; Qi, L. H.; Qin, W. Electromagnetic Absorption Properties of Graphene/Fe Nanocomposites. Mater. Res. Bull. 2013, 48, 3362-3366. (35) Zhao, X.; Zhang, Z.; Wang, L.; Xi, K.; Cao, Q.; Wang, D.; Yang, Y.; Du, Y. Excellent Microwave Absorption Property of Graphene-coated Fe Nanocomposites. Sci. Rep. 2013, 3, 3421. (36) Lv, H.; Ji, G.; Liang, X.; Zhang, H.; Du, Y. A Novel Rod-like MnO 2 @Fe Loading on Graphene Giving Excellent Electromagnetic Absorption Properties. J. Mater. Chem. C 2015, 3, 5056-5064. (37) Pan, G.; Zhu, J.; Ma, S.; Sun, G.; Yang, X. Enhancing the Electromagnetic Performance of Co through the Phase-controlled Synthesis of Hexagonal and Cubic Co Nanocrystals Grown on Graphene. ACS Appl. Mater. Interfaces 2013, 5, 12716-12724. (38) Zhu, Z.; Sun, X.; Li, G.; Xue, H.; Guo, H.; Fan, X.; Pan, X.; He, J. Microwave-assisted Synthesis of Graphene-Ni Composites with Enhanced Microwave Absorption Properties in Ku-band. J. Magn. Magn. Mater. 2015, 377, 95-103. (39) Ye, W.; Fu, J.; Wang, Q.; Wang, C.; Xue, D. Electromagnetic Wave Absorption Properties of NiCoP Alloy Nanoparticles Decorated on Reduced Graphene Oxide Nanosheets. J. Magn. Magn. Mater. 2015, 395, 147-151. S-9