| Summary: | The interactions between plasma and liquids cause complex physical and chemical reactions at the gas–liquid contact surface, producing numerous chemically active particles that can rapidly reduce noble metal ions. This study uses atmospheric-pressure surface dielectric barrier discharge (DBD) plasma to treat ethanol aqueous solutions containing noble metal precursors, and stable gold, platinum, and palladium colloids are obtained within a few minutes. To evaluate the mechanism of the reduction of noble metal precursors by atmospheric-pressure surface DBD plasma, the corresponding metal colloids are prepared first by activating an ethanol aqueous solution with plasma and then adding noble metal precursors. It is found that the long-lived active species hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) plays a dominant role in the synthesis process, which has distinct effects on different metal ions. When HAuCl<sub>4</sub> and H<sub>2</sub>PdCl<sub>4</sub> are used as precursors, H<sub>2</sub>O<sub>2</sub> acts as a reducing agent, and AuCl<sub>4</sub><sup>−</sup> and PdCl<sub>4</sub><sup>2−</sup> ions can be reduced to metallic Au and Pd. However, when AgNO<sub>3</sub> is the precursor, H<sub>2</sub>O<sub>2</sub> acts as an oxidising agent, and Ag<sup>+</sup> ions cannot be reduced to obtain metal colloids because metallic Ag can be dissolved in H<sub>2</sub>O<sub>2</sub> under acidic conditions. A similar phenomenon was also observed for the preparation of Pd colloid-PA with a plasma-activated ethanol aqueous solution using Pd(NO<sub>3</sub>)<sub>2</sub> as a Pd precursor.
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