Herein, Cu2O microcrystals are found to perform efficiently in hydrogen generation and formaldehyde degradation at room temperature, with MgO as a sacrificial agent, while other photocatalytic semiconductors such as TiO2, ZnO, Bi2O3, Fe2O3 and CuO exhibit poor capability under the same condition. This peculiarity of Cu2O microcrystals is comprehensively analyzed, focusing on exploring the formation of [Cu+]. paramagnetic during the reaction, which is first proposed as a principal factor for the hydrogen generation and formaldehyde degradation. Even more exciting, abundant hydroxyl radicals produced during this reaction process are detected via electron paramagnetic resonance spectrometry, which are particularly efficient in oxidizing formaldehyde and producing hydrogen. (C) 2015 Elsevier B.V. All rights reserved.