There's diodes, and there's diodes. Many are
not going to be useful as RF energy harvesting
devices at cell band frequencies - roughly
900MHz and 2.5GHz. A diode meant for sub-MHz
power supply applications could look like a
lousy capacitor at these frequencies. A RF
Schottky might do better, also letting you pull
current at a lower Vf (which is directly, RF
amplitude as captured by whatever is acting
as the antenna).
To which point, there's neither an antenna
nor a decent ground to be seen. It would be a
matter of luck if your "loop antenna" was a
fortunate multiple of the radio wavelength(s)
of interest. If it isn't, no antenna gain for you.