Metallic Ti3C2Tx MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio

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1 Supporting Information for Metallic Ti3C2Tx MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio Seon Joon Kim, 1, Hyeong-Jun Koh, 1, Chang E. Ren 2, Ohmin Kwon 3, Kathleen Maleski 2, Soo- Yeon Cho 1, Babak Anasori 2, Choong-Ki Kim 4, Yang-Kyu Choi 4, Jihan Kim 3, 5, Yury Gogotsi*, 2, Hee-Tae Jung*, 1, 5 1 National Research Laboratory for Organic Optoelectronic Materials, Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea 2 A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA 19104, USA 3 Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea 4 School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea 5 KAIST Institute for Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea These authors equally contributed to this work. S1

2 Supporting Figures Figure S1. (a) Photograph of Ti 3C 2T x aqueous solutions directly after centrifugation and after sequential dilution. (b) Optical images of a bare sensing electrode (left) and a Ti 3C 2T x deposited electrode (right). Figure S2. (a) Intensity distribution of individual flake size measured by dynamic light scattering (DLS), and transmission electron microscopy (TEM) (b) TEM image of multiple delaminated Ti 3C 2T x flakes on a carbon TEM grid. S2

3 Figure S3. Height profile of a typical Ti 3C 2T x film employed for channels in gas sensors, measured by atomic force microscopy (AFM). Figure S4. XPS spectra of as-filtered Ti3C2Tx films before transfer. (a-c) XPS measurements of films at three core levels: (a) Ti 2p, (b) C 1s, (c) O 1s, and their corresponding deconvoluted components. S3

4 Figure S5. Real-time gas response of Ti 3C 2T x MXene with 25 nm ~ 150 nm thickness to 100 ppm of acetone and ethanol. Figure S6. Signal-to-noise ratio (SNR) of Ti 3C 2T x sensors upon exposure to ppb concentrations (50~1000 ppb) of acetone, ethanol, and ammonia. S4

5 Figure S7. Log-log plot of signal-to-noise ratio of Ti 3C 2T x sensors upon exposure to ppb concentrations (50~1000 ppb) of acetone, ethanol, and ammonia. Dashed lines and equations at the bottom represent fitting curves derived from a power-law dependence. Figure S8. Morphological and chemical characterization of MoS2, black phosphorus (BP), and reduced graphene oxide (RGO) flakes and prepared films. (a-c) Intensity distribution, average size and polydispersity of (a) MoS 2, (b) BP, and (c) RGO flakes dissolved in solutions measured by dynamic light scattering (DLS). (df) Raman spectra of (d) MoS 2, (e) BP, and (f) RGO films acquired with a 514 nm laser. (g-i) Surface morphology of (g) MoS 2, (h) BP, and (i) RGO observed by SEM. S5

6 Figure S9. Gas response of ammonia (NH 3) and nitrogen dioxide (NO 2) for gas sensors based on two-dimensional materials. Only Ti 3C 2T x sensors showed a positive response for both NH 3 and NO 2. S6

7 Figure S10. Minimum energy (primary) adsorption configurations for acetone and ammonia. (a-e) Side and top views of the most favorable configurations for acetone on (a) Ti 3C 2O 2, (b) Ti 3C 2F 2, (c) BP, (d) MoS 2, and (e) graphene. (f-j) Side and top views of the most favorable configurations for ammonia on (f) Ti 3C 2O 2, (g) Ti 3C 2F 2, (h) BP, (i) MoS 2, and (j) graphene. The distance in the z-direction between the gas molecule and material surfaces are indicated in all configurations. S7

8 Figure S11. Secondary adsorption configurations for acetone. (a-f) Side and top view of the secondary favorable configurations for acetone on (a) Ti 3C 2(OH) 2, (b) Ti 3C 2O 2, (c) Ti 3C 2F 2, (d) BP, (e) graphene, and (f) MoS 2. The distance in the z-direction between the gas molecule and material surfaces are indicated in all configurations. S8

9 Figure S12. Secondary adsorption configurations for ammonia. (a-f) Side and top view of the secondary favorable configurations for ammonia on (a) Ti 3C 2(OH) 2, (b) Ti 3C 2O 2, (c) Ti 3C 2F 2, (d) BP, and (e, f) graphene. Secondary configurations for ammonia on MoS 2 were not calculated, as the most stable configuration among various combinations is reported in previous studies. The distance in the z-direction between the gas molecule and material surfaces are indicated in all configurations. S9

10 Supporting Tables Table S1. State-of-the-art performance of the empirical limit of detection (LOD) for room-temperature sensors based on two-dimensional materials to detect acetone and ammonia Gas Channel material Empirical limit of detection Operation temperature Reference Ti3C2Tx MXene 50 ppb RT This work Acetone Ag-doped tetralayer WS ppb 100 C S1 Thiol ligand-conjugated MoS 2 1 ppm RT S2 Monolayer MoS ppm RT S3 3D Graphene 300 ppm RT S4 Ti3C2Tx MXene 100 ppb RT This work MoS 2 film 300 ppb RT S5 WS 2 nanoflakes 1 ppm RT S6 Trilayer MoS 2 10 ppm RT S7 Graphene/MoS 2 heterostructure 20 ppm 100 C S8 Monolayer MoSe ppm RT S9 Ammonia Reduced graphene oxide 1 ppb RT S10 CVD graphene 200 ppt a RT S11 (with in-situ UV illumination during sensing) Monolayer graphene 1 ppm RT S12 Phosphorus-doped graphene 1 ppm RT S13 3D Graphene flowers 5 ppm RT S14 Black phosphorus 1 ppm RT S15 Black phosphorus (BP) flakes 10 ppm RT S16 * RT = Room temperature a The material s intrinsic limit of detection for the pristine CVD graphene was 83.7 ppb. In-situ UV illumination was performed during sensing measurements to remove contaminants and increase sensitivity, in which this type of UV treatment can boost the sensitivity of all other reported values including this work. Table S2. Signal-to-noise ratios of Ti 3C 2T x sensors under exposure to ppb concentrations of (50~1000 ppb) acetone, ethanol, and ammonia. Sensors were not tested for 50 ppb of ammonia and ethanol due to the unavailability of sufficiently diluted gas sources. 50 ppb 100 ppb 250 ppb 500 ppb 1000 ppb Acetone Ethanol Ammonia S10

11 Table S3. Gas response values (%) of Ti 3C 2T x, RGO, MoS 2, and BP sensors under exposure to 100 ppm of acetone, ethanol, ammonia, propanal, NO 2, SO 2, and ppm of CO 2. Ti3C2Tx RGO MoS2 BP Acetone Ethanol Ammonia Propanal NO SO CO Table S4. Average baseline resistance values of BP, MoS 2, RGO, and Ti 3C 2T x films measured in the gas sensing chamber at room temperature under N 2 flow. Channel material BP MoS 2 RGO Ti 3C 2T x Average baseline resistance 29.0 M Ohm 11.9 M Ohm 200 Ohm 108 Ohm Table S5. Signal-to-noise ratios of Ti 3C 2T x, RGO, MoS 2, and BP sensors under exposure to 100 ppm of acetone, ethanol, ammonia, propanal, NO 2, SO 2, and ppm of CO 2. Ti3C2Tx RGO MoS2 BP Acetone Ethanol Ammonia Propanal NO SO CO S11

12 Table S6. DFT calculation results for acetone and ammonia molecules with distinct molecular orientations on various sites of 2D materials. h and E a indicate the vertical distance and binding energy between gas molecules and 2D materials at equilibrium. Shaded areas indicate the material and configurations with the strongest binding for each gas. Gas Material Site a, b Orientation c, d h (Å) Ea (kj/mol) Ea (ev) Ti 3C 2(OH) 2 T Ti horizontal T C vertical Ti 3C 2O 2 T Ti horizontal T O vertical Ti 3C 2F 2 T Ti horizontal Acetone T F vertical BP H horizontal T P horizontal Graphene H horizontal H vertical MoS 2 T S horizontal T Mo vertical Ti 3C 2(OH) 2 T O down T C down Ti 3C 2O 2 T Ti down T O down Ti 3C 2F 2 T Ti down Ammonia T F down BP H down T P down Graphene H up H down T C down MoS 2 H down a Tx indicates that oxygen (for acetone) and nitrogen (for ammonia) atoms are placed above the indicated atom x (x = Ti, C, O, F, P, S, Mo) b H indicates that oxygen and nitrogen atoms are placed over the center of the hexagon (empty sites) c horizontal and vertical refer to the orientation of acetone with respect to the 2D materials basal plane d down and up refer to the oriented direction of nitrogen atoms in ammonia S12

13 Supporting Notes 1. Discussion of DFT calculations Molecule configuration To determine the most stable molecule configuration of the few initial configurations were adopted based on previous calculations for Ti3C2(OH)2/acetone S17, MoS2/acetone S18, graphene/acetone S19, Ti3C2(OH)2/ammonia S20, MoS2/ammonia S21, graphene/ammonia S22, and BP/ammonia S23. Supercell manipulation To avoid image cell interaction due to periodic boundary conditions, a large vacuum layer of 2 nm was adopted along z-axis. Also, there is possibility of the interaction between the cells along the xy-direction of plane. Therefore, supercell tests were conducted using 3 3 1, 4 4 1, cells to check whether the size of supercells have a significant influence in determining binding energy. Supercell tests were performed with MoS2, since Ti3C2Tx is computationally too expensive due to the large number of atoms and a larger supercell (63 atoms for 3 3 1, 112 atoms for 4 4 1, 175 atoms for 5 5 1) , 4 4 1, k-point grid was used for 3 3 1, 4 4 1, and supercell, respectively. Two different configurations for MoS2/acetone were tested. S13

14 Binding energy (kj/mol) Configuration 1 Configuration 2 Supercell size cell cell cell Table S7. Binding energy of acetone with MoS 2 with different supercell sizes. The results show that the energy difference between supercells with different size is within 1 kj/mol, while the energy difference between different configurations within the same supercell is about 5 kj/mol. Therefore, the effect of image cell interaction along the plane is relatively small, and we adopted the cell in all calculations to reduce computational cost. S14

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