Wavelength- and Humidity-Controlled Reversible Photoconductivity Switching in a Novel Hybrid Polyoxoniobate

Reversing the sign of photoconductivity in a single material has long been a formidable challenge, yet achieving this goal could open the door to a new generation of adaptive optoelectronics. Here, we report the first case of an inorganic-organic hybrid polyoxometalate (POM), [Ni(BTP)]₄[Nb₁₀O₂₆]·35H₂O (1, BTP = Bis-tris propane), capable of fully reversible switching between positive photoconductivity (PPC) and negative photoconductivity (NPC). This switching is dually controlled by excitation wavelength and relative humidity. Short-wavelength excitation and low humidity give conventional PPC, whereas long-wavelength excitation and high humidity induce NPC. This anomalous polarity inversion stems from the competition between two coexisting mechanisms: one is the photoelectric effect, which generates charge carriers within the POM; the other is the light-induced water desorption process, which suppresses the electron donation from adsorbed H₂O molecules, thereby reducing the carrier density. Resonant microcantilever measurements directly confirm the light-induced water desorption process, and systematic wavelength/humidity-dependent photoconductivity mapping reveals a well-defined phase boundary that governs reversible PPC-NPC interconversion. This work represents a rare polarity-switchable photoconductive material, expanding POM applications in optoelectronics.