We developed a novel solar steam generator (SSG) with high solar conversion efficiency and excellent salt resistance. A CO2 laser was used to convert the surface of basswood to graphitic carbon layers (GCL), and various grid patterns of GCL were created on wood. The low thermal conductivity of wood suppressed heat loss to bulk water, and the presence of the grooves in the grid increased the evaporation rate by increasing the surface area to absorb more sunlight. In addition, the supply of bulk water through the grooves endowed the SSG with salt resistance and self-regeneration properties. The salt resistance was maintained in a 20-wt % NaCl solution for the duration of the experiment (2 weeks), which indicates that the developed SSG can be used in saline water for long-term operation.

A CO2 laser was employed to create a rectangle (4 × 2 mm2) of a conductive graphitic carbon layer (GCL) directly on a cellulose substrate. By tilting the substrate while keeping the laser power constant, the laser power density was gradually changed while scanning in the direction of the long side of the rectangle, due to deviation of the laser focus. As the laser beam defocus distance increased, the laser intensity at the substrate decreased, and the oxygen-to-carbon ratio (O/C) of the GCL increased. Upon exposing the GCL substrate to water vapor, the hydrogen-containing groups (carboxyl and hydroxyl groups) in the GCL were hydrolyzed, and a density gradient of hydrogen ions was induced due to the preformed O/C gradient. The resulting voltage and current outputs reached 0.23 V and 0.4 μA/cm2, respectively, at 70% relative humidity. Additionally, it was demonstrated that the electricity obtained during breathing could turn on a green light-emitting diode operating at an onset potential of 2 V when an array of the GCLs was attached to a filter mask.

Ultrafast and reversible colorimetric responses to humidity changes are observed with disordered, nanoporous titania microspheres, optimized in speed and intensity with a monolayer coverage. The response times, defined by intracrystalline diffusion of water molecules, represent the fastest values for colorimetric humidity sensors.