Experimental evidence on RH-dependent crossover from an electronic to protonic conduction with an oscillatory behaviour

Abstract

An oxide semiconductor changes its resistance with exposure of water molecules and is accepted to be governed by electronic and protonic conduction in low and high humid atmosphere, respectively, without any experimental evidences. Here, we report on the experimental evidence of a relative humidity (RH) dependent crossover, from an electronic to protonic conduction and its oscillatory behaviour in mesoporous SnO2. Interestingly, oscillatory conduction observed in the intermediate humidity range (70%–90% RH) lies in between two monotonic variations that substantiate the competitive adsorption and desorption processes of oxygen species and water molecules. In addition, we have shown that the conduction process can be tuned predominantly electronic or protonic by pre- and post-UV treatment. The conductance increases by 2–3 orders as the conduction changes from pure electronic to protonic, suggesting an insulator-to-metal like transition.

Harvesting energy via stimuli‐free water/moisture dissociation by mesoporous SnO 2 –based hydroelectric cell and CuO as a pump for atmospheric moisture

“…The absorption spectrum shown in Figure 1D reveals a sharp absorption edge at~340 nm related to the direct band edge transition of SnO 2 . 18,25,26 Here, the strong electrostatic field, generated due to presence of oxygen vacancies 27,28 (existence of oxygen vacancies were confirmed with photoluminescence spectra shown in Figure S1), dissociates the water molecules (as seen in Figure 2B) according to Tauc plot, as shown in the inset of Figure 1D, confirms a direct bandgap of~3.62 eV and is in accordance with earlier report.…”

“…34,35 When the CuO is kept at 25°C with 76% RH, the color of the sample changes from dark black (0 minutes) to light green in 60 to 70 minutes ( Figure 4A(iii)). 26,27 Figure 4B(ii) shows that the mesoporous SnO 2 -based hydroelectric cell can generate the potential in the presence of the moisture absorbing layer of CuO. The structural analysis before (red curve in Figure 4A(iii)) and after (blue curve in Figure 4A(iii)) moisture absorption confirms the Cu (OH) 2 complex formation.…”

“…[22][23][24] The water surrounding the pallet generates pressure, which also helps the water molecules to penetrate inside the pores. 18,25,26 Here, the strong electrostatic field, generated due to presence of oxygen vacancies 27,28 (existence of oxygen vacancies were confirmed with photoluminescence spectra shown in Figure S1), dissociates the water molecules (as seen in Figure 2B) according to…”

“…41-0254. [26][27][28] As discussed above, OHoxidized the Al electrode to release the 3e in the system to develop the potential via subsequent H 2 evolution. The presence of small amount of Cl helps the CuO to slowly react with the atmospheric moisture, which latter getting pumped to porous SnO 2, which leads to the formation of Cu (OH) 2 complex.…”

“…During moisture pumping, the water molecules create the various chemisorb and physisorb layers on the SnO 2 25,26 and then subsequently penetrate into the pores via capillary condensation 22,23 where they split, into OHand H + /H 3 O + , due to an electrostatic field generated by defects and trapped hydronium ions (Equations (1), (2)). [26][27][28] As discussed above, OHoxidized the Al electrode to release the 3e in the system to develop the potential via subsequent H 2 evolution. 26,27 Figure 4B(ii) shows that the mesoporous SnO 2 -based hydroelectric cell can generate the potential in the presence of the moisture absorbing layer of CuO.…”