Fig. 4.

Extrusion printing of MXene viscous aqueous inks. a Photo of MXene aqueous ink, showing its viscous nature. b Optical images of all-MXene printed patterns, including MSCs with various configurations, on paper. The yield is high using the extrusion printing method, producing > 70 MSCs (<N> = 1) on paper based on 1 mL of the MXene ink. c Low-magnification SEM image of printed MXene MSC. Scale bar = 200 µm. d High-magnification SEM image of MXene MSC of the framed area in (c), showing stacked, interconnected MXene nanosheets forming a continuous film. Scale bar = 500 nm. e The width distribution and the resultant width spatial uniformity of the extrusion-printed MXene MSCs. f Raman spectrum of the extrusion-printed lines along line A, inset shows the sum intensity of peak C along line B. The MXene characteristic peaks are well retained, indicating no oxidation occurred during the extrusion printing. g SEM images of printed MSCs with <N> = 1 (top) and <N> = 5 (middle). Cross-sectional SEM image of MSC with <N> = 5 is shown on the bottom, demonstrating a well-stacked, continuous nanostructure. Scale bar = 100 µm (top and middle) and = 1 µm (bottom). h The sheet resistance, R_s, plotted as a function of <N>, showing an exponential decay. i Extrusion-printed tandem devices (two MSCs in serial and two in parallel) on a paper substrate (left panel), showing a great flexibility (right panel). j Comparison of the conductivity of the as-printed lines plotted as a function of ink concentration, showing the advantage of our work in printing highly concentrated inks for highly conductive networks. The data in (j) come from ref. ¹³ and references within