Sodium (Na)- and potassium (K)-doped δ-MnO2, which presented different band gaps, were synthesized by a hydrothermal method. Then, uniform Au nanoparticles (NPs) were deposited on MnO2 to form metal-semiconductor nano-heterojunctions (MnO2-Au). By comparing their temperature-dependent thermal catalytic performances, p-aminothiophenol to p, p'-dimercaptoazobenzene conversion was used as proof-of-concept transformations. MnO2-Au hybrid materials demonstrated better thermal catalytic performances relative to individual Au NPs. Meanwhile, K-doped MnO2-Au, with a MnO2 support displaying a narrower bandgap, displayed superior catalytic activities relative to Na-doped MnO2-Au. To get the same catalytic performance by individual Au NPs, it can be ∼50 K less by Na-doped MnO2-Au and ∼100 K less by K-doped MnO2-Au. The enhancement is mainly attributed to the thermally excited electrons in MnO2, which were transferred to Au NPs. The additional electrons in Au NPs increase the electron density and thus contribute to the improvement of thermal catalysis. Our findings show that the establishment of a nano-heterojunction formed by metal NPs on a semiconductor support has a significant impact on thermal catalysis, where a narrower band gap can facilitate thermally excited carriers and thus bring about better catalytic performances. Thus, the results presented here shed light on the design of a nano-heterojunction catalyst to approach reactions with superior performance under moderate conditions.
Keywords: Au nanoparticles; MnO2; electron transfer; heterojunction; thermal catalysis.