The researchers used a commercial pencil to fabricate electrodes, and oxidized multiwalled carbon nanotubes (o-MWCNTs) were utilized as the sensitive material for the ink marker.
The marker was fabricated by injecting the ink (aqueous dispersion of o-MWCNTs) in the refill of a blank maker and drawing on the sensitive region of devices.
The team reports that the o-MWCNTs exhibit excellent dispersibility in water with good stability, which is beneficial for the solution process.
Compared with solid-state process, solution processes could introduce the sensitive material into the top layer of paper and form a structure of o-MWCNTs loaded porous cellulose layer by a permeating process.
Such structure could increase the contact area between the sensitive material and water molecules; meanwhile, the top layer of hydrophilic cellulose paper could participate in the water adsorption process and enhance the response of the humidity sensor.
Furthermore, the formation of sensitive layer does not rely on the abrasion between the substrate and bulk made from the sensitive material, which is beneficial for maintaining the completeness of electrodes.
According to the authors, the structure of the obtained paper-based humidity sensor mainly exhibits the following advantages:
1) o-MWCNTs could permeate into the top layer of paper substrates, and such structure could increase the contacting area between o-MWCNTs and gaseous water molecules; meanwhile, the top layer of hydrophilic cellulose paper could participate in the water adsorption process and further enhance the response of humidity sensors;
2) the dispersion of sensitive materials alleviates the inner stress of sensitive layer, leading to good mechanical stability;
3) the paper substrates, electrodes, and sensitive material are all carbon-based, demonstrating the environmental friendliness and recyclable superiority;
4) drawing on paper technique with the ubiquitous materials, paper, pencil, and marker provides a facile method to fabricate flexible sensitive device on-site and gives users great freedom to design sensors on-demand.
The resultant devices are flexible, disposable, and potentially wearable, and meanwhile, the solution-based process is feasible for inkjet printing, which is possible for batch production.