fun science experiment... fill a large beaker halfway with water and sprinkle a thin layer of powdered, activated charcoal (carbon/graphene) over the surface, cover the top of the beaker with saran wrap and place outside in the sun at midday. the temperature of the charcoal will get much warmer but not much else

Isn't that because it is fairly hydrophobic so it doesn't make good contact with the water?

yeah, there have been previous papers on creating membranes that wick some of the water up to be heated, and turned into vapor, but not so much that it cools down the carbon layer.