Magnetic Properties of Cu<sub><i>m</i></sub>O<sub><i>n</i></sub> Clusters: A First Principles Study

Experimental evidence shows that small Cu<sub>2</sub>O nanoparticles exhibit ferromagnetic or paramagnetic properties, allowing for the promising possibility to recycle the catalyst Cu<sub>2</sub>O easily in wastewater treatment. In this paper, theoretical calculation studying the magnetic property of copper/oxide clusters is reported. A series of Cu<sub><i>m</i></sub>O<sub><i>n</i></sub> ((<i>m</i>, <i>n</i>) = (4, 1); (4, 2); (4, 5); (16, 15); (28, 15); (44, 15); (28, 27)) clusters were investigated using generalized gradient approximation (GGA) and the Hubbard U (GGA+U) method within density functional theory (DFT). It is found that the electronic structures of bulk Cu<sub>2</sub>O calculated by the GGA and GGA+U are similar. The structures of Cu<sub><i>m</i></sub>O<sub><i>n</i></sub> ((<i>m</i>, <i>n</i>) = (4, 1); (4, 2); (4, 5)) are all planar. For the bulk-product Cu<sub><i>m</i></sub>O<sub><i>n</i></sub> ((<i>m</i>, <i>n</i>) = (16, 15); (28, 15); (44, 15); (28, 27)), O atoms prefer to be the outermost atoms. We classified two types of clusters on the basis of their O to Cu atomic ratios. One is O-rich clusters, i.e., Cu<sub>4</sub>O<sub>5</sub>, Cu<sub>16</sub>O<sub>15</sub>, and Cu<sub>28</sub>O<sub>27</sub>. The other is O-poor clusters, i.e., Cu<sub>4</sub>O, Cu<sub>4</sub>O<sub>2</sub>, Cu<sub>28</sub>O<sub>15</sub>, and Cu<sub>44</sub>O<sub>15</sub>. The calculation results show that the O-rich clusters have longer average Cu−Cu bonds and larger binding energy than those of the O-poor ones. More interestingly, the former are magnetic and give ferromagnetic ordering while the latter are nonmagnetic. The hydrogenation of O-terminated clusters can improve its stability but suppress its magnetism. The study may be extremely useful for the potential applications of Cu<sub>2</sub>O nanopaticles in the catalysis and semiconductor fields.