Inorganic compounds with different crystalline and amorphous states may show distinct properties in catalytic applications. In this work, we control the crystallization level by fine thermal treatment and synthesize a semicrystalline IrOx material with the formation of abundant boundaries. Theoretical calculation reveals that the interfacial iridium with a high degree of unsaturation is highly active for the hydrogen evolution reaction compared to individual counterparts based on the optimal binding energy with hydrogen (H*). At the heat treatment temperature of 500 °C, the obtained IrOx-500 catalyst has dramatically promoted hydrogen evolution kinetics, endowing the iridium catalyst with a bifunctional activity for acidic overall water splitting with a total voltage of only 1.554 V at a current density of 10 mA cm-2. In light of the remarkable boundary-enhanced catalysis effects, the semicrystalline material should be further developed for other applications.