Using Carbon Laser Patterning to Produce Flexible, Metal-Free Humidity Sensors

A relative humidity sensor was produced by carbon laser patterning of a carbon precursor ink on a flexible substrate. Citric acid and urea, both inexpensive and naturally abundant molecules, are used as initial precursors to obtain a porous carbon foam after CO₂ laser irradiation. The laser-patterned material is characterized by electron microscopy, Raman spectroscopy, and vertical scanning interferometry. An intrinsic p-type semiconducting behavior was confirmed by thermoelectric and Hall measurements. The resistance of this porous, metal-free material is sensitive to atmospheric variations, namely, temperature and relative humidity (≈5 Ω·%). Under dry atmosphere, the sensor acts as a thermometer with a linear relationship between temperature and relative variation of resistance (0.07%·K⁻¹). The evolution of the sensor resistance at different relative humidities and temperatures is studied by electrical impedance measurements. The kinetic transitory regime of water desorption from the carbonaceous surface of the sensor is analyzed using Langmuir's model. The equilibrium constant of adsorption K_ads has been determined, and the standard enthalpy of adsorption of water on the sensor surface is estimated at Δ_adsH° = −42.6 kJ·mol⁻¹. The simple and inexpensive production and its high, stable sensitivity make laser-patterned carbon interesting for humidity sensing applications, and the method allows for the large-scale production of printed sensor arrays.