Sol-gel deposited Ag/TiO2/Pt tri-layers exhibit unipolar resistive switching (URS) with a negative turn-on voltage when fired in air. Variation in both the turn-on voltage and the on-state current at turn-off threshold is observed in successive current-voltage (I-V) measurements. Based on the same composition and processing flow, bipolar resistive switching (BRS) showing positive turn-on voltage by contrast is obtained by applying two alternative procedures: one procedure under oxygen ambience and another one in which indium-tin oxide is used as the bottom electrode. Conduction mechanisms induced by reduction/oxidation reactions explain how firing ambient and bottom electrode affect the switching mode, as well as the distinct turn-on voltage polarity. URS and BRS are related to the amount of ambient oxygen and the arrangement of internal oxygen vacancy, thus determining whether the direction-independent thermochemical heating or polarity-dependent electrochemical oxidation near the anode interface is responsible for the filament rupture. Additionally, the space-charge-limited transport is analyzed to examine how URS and BRS-activated samples significantly differ from each other. Importantly, the temperature-dependent I-V data helps to elucidate the dominant carrier behavior in the regime of low and high electric field. (C) 2012 Elsevier B.V. All rights reserved.