Flexible electronic devices are gaining momentum in various applications including touch screen panels, solar cells, wearable devices, etc. Susceptibility to the environmental conditions limits their performance. Researchers at Shanghai Institute of Ceramics, Chinese Academy of Sciences, China have suggested a novel strategy for the design of all-weather flexible superhydrophobic, electrically conductive paper with flame-resistant property.
Hydroxyapatite nanowires (HNs), Ketjen black (KB), and polydimethylsiloxane (PDMS) were used to fabricate the all-weather flexible electrically conductive paper. The HNs were prepared by mixing an aqueous solution (500 ml) containing 2.2 g of CaCl2, 10 g of NaOH and 2.8 g of NaH2PO4·2H2O with an ethanolic solution (140 g) containing 100 g of oleic acid under stirring followed by hydrothermal treatment at 180 °C for 24 h in a Teflon-lined stainless steel autoclave. The HNs were dispersed in ethanol to form a colloidal suspension. KB was dispersed in ethanol under ultrasonication to obtain a KB colloidal suspension. The colloidal suspension of KB was mixed with the colloidal suspension of HNs under stirring for 10 min. Vacuum-assisted filtration was adopted to fabricate the KB + HNs paper and it was peeled off after drying at 90 °C for 5 min. The KB + HNs paper was immersed in dilute PDMS solution (ratio of PDMS:curing agent:ethylacetate is 10:1:100) at room temperature for 30 min and subsequently cured at 100 °C for 1 h (Fig. 1(a)). The as-prepared KB+HNs+PDMS paper (KHP paper) exhibits a rough morphology (Fig. 1(b)), higher water contact angle (>150°) and low sliding angle (<10°) (Fig. 1(c)) high flexibility (twisted and bent without breaking for 500 cycles) (Fig. 1(d)) and electrically conductive (Fig. 1(e)).
Fig. 1 (a) Schematic illustration of various stages involved in the preparation; (b) morphology; (c) water contact angle; (d) flexibility; and (e) electric conductivity of flexible electrically conductive KB + HNs + PDMS paper.
The high water contact angle (>150°) and a low sliding angle (<10°) enables water droplets to bounce off from the surface and automatically rolled away even at a small tilting angle, thus keeping the KHP paper to keep dry (Fig. 2(a)). The water repellent ability of KHP paper is retained under highly corrosive conditions (pH: 2-13), when heating up to 300 °C for 12 h and when exposed to humid conditions (50 -90% RH) for 24 h. The KHP paper also exhibits self-cleaning ability, which is evidenced by the easy removal of soil by water droplets (Figs. 2(b-e)). Real-time electrical performance of the KHP paper upon wetting, monitored by measuring the resultant current upon applying a potential of 3 V indicates that the water droplets remains stable on the surface without wetting (Figs. 2(f) and 2(g)) and the electrical current was steady from 0 to 10 s (Fig. 2 h and 2(i)). Real-time electrical performance underwater (Fig. 2(j)) also indicates the ability of the KHP paper to exhibit a good stability, as evidenced by the brightness of the LED lamp from 0 to 120 s (Figs. 2(k) and 2(l)).
Fig. 2 (a) Bouncing-off of water droplets; (b-e) self-cleaning ability; (f-l) Real-time monitoring of electrical conductivity: (f-i) with a few water droplets; (j-l) after total immersion in water
The electrothermal effect of the KHP paper is ascertained by applying a direct voltage to the paper covered by copper foils at the edges and measuring change in surface temperature using an infrared thermal imaging camera (Fig. 3(a)). The surface temperature is increased quickly within 10 s and then leveled off (Fig. 3(b)); the higher the applied voltage, the higher the surface temperature of KHP paper. The ability of the KHP paper to retain the rapid thermoresponsive behavior for five cycles indicated its recyclability (Fig. 3(c)). Due to its electrothermal effect, the KHP paper in capable of quickly evaporating tiny water droplet within 128 s (Fig. 3(d)) and deicing of ice within 23 s (Fig. 3(e)).
Fig. 3 (a) Schematic illustration of the surface temperature measurement of the KHP paper; (b) Change in surface temperature with time; (c) stability upon repeated heating/cooling cycles; (d) evaporation of tiny water droplet (3 µL); and (e) deicing of ice.
The KHP paper also exhibits flame retarding characteristics and its electrical conductivity is increased from 11.92 mA to 13.39 mA after exposure to flame for 60 s and stabilized to 13.34 A after 7 min (Fig. 4(a)). Real-time monitoring of the electrical current and the brightness of an LED lamp up to 7 min confirm the ability of the KHP paper to retain electrical conductivity even under extreme condition of combustion (Fig. 4(b)).
The KHP paper exhibits superhydrophobicity, better flexibility, enhanced mechanical properties, good electrical conductivity, high thermal stability, suitable electrothermal effect and good flame retardancy. Due to its ability to perform well under extreme conditions (underwater as well as in flame), the KHP electrically conductive paper seems to be promising for applications in flexible electronic devices.
T.S.N. Sankara Narayanan
Fig. 4 Real-time monitoring of electrical conductivity of the KHP paper in flame up to 7 min : (a) change in current; and (b) brightness of LED lamps.
For more information, the reader may kindly refer: Fei-Fei Chen et al., Hydroxyapatite Nanowire-Based All-Weather Flexible Electrically Conductive Paper with Superhydrophobic and Flame-Retardant Properties, ACS Appl. Mater. Interfaces, DOI: 10.1021/acsami.7b09484
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