Structural-Plasmonic Relationship of Crystalline Copper Oxide Microcubes Decorated with Plasmonic Gold Nanoparticles

Copper oxide semiconductors are the most prominent catalyst for CO₂ electrochemical reduction. In recent years, a myriad of properties has been arising from new combinations between distinct nanomaterials. The superficial decoration of copper oxide with noble metal nanoparticles presents plasmonic synergistic effects resulting from local crystalline structure perturbation. In this way, different characterization methods of materials can be used to elucidate the structural, electronic, and optic properties of nanomaterials. In this work, we combined these three approaches to understand the role of the addition of plasmonic gold nanostructures to semiconductive copper oxide microcubes. The green synthesis of nanomaterials is an important factor. This way, we used a CuCl₂ salt in basic medium and later a soft reductant addition. To add the Au nanoparticles, we added a Au³⁺ solution and due to the reduction potential of Cu₂O being lower than gold, nanoparticles are naturally formed. On our SEM images of synthesized nanoparticles, we observed that Cu₂O nanoparticles have a well-defined cubic shape and flat surface and mean size of 536.40 nm ± 179.41 nm. Also, the gold addition to the Cu₂O nanoparticles does not change its shape but creates a rugous surface with size distribution of rough structures of about 35.47 nm ± 9.38 nm. EDS analysis of Au-Cu₂O confirms that this roughness can be attributed to gold nanostructures. A XPS analysis shows that on Cu₂O the surface is composed of Cu⁰ (931.1 eV), CuO (934.2 eV) and Cu₂O (932.5 eV) while Au-Cu₂O surface is composed by CuO (932.0 eV) and Cu₂O (932.4 eV) due to copper oxidation on gold presence. To understand about the crystalline structure of our materials, we used a XRD analysis which shows very sharp peaks for both materials, indicating a monocrystal behavior. The peaks present in Cu₂O sample can be attributed to Cu₂O cubic (ICSD: 98-062-8621) with the main facet (111). The addition of gold to the Cu₂O does not change the crystallinity but we observed the arising of peaks attribute to gold structures (ICSD: 98-016-3723). These results are consistent with the SEM images and show the success of the proposal synthesis. Cyclic voltammetry was performed to understand the electrochemical behavior of materials and shows a reduction event occurring at -0.40 V on Cu₂O sample, which can be associated with the reduction of Cu⁺ to Cu⁰ (-0.36 V). On Au-Cu₂O, the voltammogram does not show the -0.40 V event, but a new event is observed at - 0.90 V. The presence of gold on the surface could protect the Cu⁺ on the surface by the spillover effect. When oxygen atoms are adsorbed on the catalyst surface, the gold structure act as an electron source reducing O₂ to active species. Electronic properties were analyzed by UV-vis-NIR showing two bands of Cu₂O in 506 and 651 nm and 624 nm for Au-Cu₂O, there were no observed gold plasmonic bands for Au-Cu₂O but a broad shoulder on the infrared region. Dark field microscopy data shows the optical properties. Two bands with a maximum at 488 and 583 nm are attributed to Mie resonance of copper oxide. When gold is added to the Cu₂O bands positions change to 476 and 623 nm, respectively. According to the literature, gold nanoparticles around 40 nm have a λ_scattering at 500 nm. Applying potential from 0.00 V to -0.80 V (vs. Ag/AgCl), we observed that Cu₂O sample does not present changes in the spectral pattern, however on Au-Cu₂O, as more cathodic the applied potential, the higher the intensity of 476 nm band. To apply a negative potential to a metallic structure means to inject electrons on its Fermi level, dislocating it to a different energy. Combining these results with Raman spectroscopy in different potentials, we observed changes in position and intensity for Cu₂O bands. Thus, our hypothesis is that gold nanoparticles cause distortion on Cu₂O crystalline lattice, favoring electronic density on gold nanostructures rather than a distribution over the surface.