This study presents a novel approach to fabricating gold superlattice films through self-assembly of gold nanocubes for in situ ATR-SEIRAS study of CO reduction reactions (CORR). The research focuses on the mechanistic insights into C-C coupling in electrochemical CO reduction using gold superlattices. The study demonstrates the fabrication of highly ordered rhombic gold nanocube superlattices (GNSs) as substrates for surface-enhanced infrared absorption spectroscopy (SEIRAS) with significantly enhanced SEIRA effect. The enhanced SEIRA effect is controlled by manipulating the randomness of GNSs. Finite difference time domain simulations reveal that the electromagnetic effect accounts for the significantly improved spectroscopic vibrations on the GNSs. In situ SEIRAS results show that the vibrations of CO on the Cu₂O surfaces have been enhanced by 2.4 ± 0.5 and 18.0 ± 1.3 times using GNSs as substrates compared to those on traditional chemically deposited gold films in acidic and neutral electrolytes, respectively. Combined with isotopic labeling experiments, the reaction mechanisms for C-C coupling of CO electroreduction on Cu-based catalysts are revealed using the GNSs substrates. The study also shows that the GNSs have significantly improved reproducibility compared to traditional CDFs. The results provide a novel approach with high sensitivity and reproducibility to revealing the reaction mechanisms of surface-mediated electrochemical reactions for renewable energy applications. The study also demonstrates that the GNSs can be used as substrates for in situ spectroscopic study of C-C coupling of the CORR on Cu₂O catalysts in the presence of CH₃I. The results show that the vibrational signals of adsorbed CO are significantly enhanced by GNSs compared to traditional CDFs. The study also reveals that the coupling reactions between adsorbed CO and CH₃ group from CH₃I yield detectable amounts of ethanol via the CORR. The study provides an attractive strategy for revealing the reaction mechanisms of surface-mediated electrochemical reactions with high sensitivity and reproducibility.This study presents a novel approach to fabricating gold superlattice films through self-assembly of gold nanocubes for in situ ATR-SEIRAS study of CO reduction reactions (CORR). The research focuses on the mechanistic insights into C-C coupling in electrochemical CO reduction using gold superlattices. The study demonstrates the fabrication of highly ordered rhombic gold nanocube superlattices (GNSs) as substrates for surface-enhanced infrared absorption spectroscopy (SEIRAS) with significantly enhanced SEIRA effect. The enhanced SEIRA effect is controlled by manipulating the randomness of GNSs. Finite difference time domain simulations reveal that the electromagnetic effect accounts for the significantly improved spectroscopic vibrations on the GNSs. In situ SEIRAS results show that the vibrations of CO on the Cu₂O surfaces have been enhanced by 2.4 ± 0.5 and 18.0 ± 1.3 times using GNSs as substrates compared to those on traditional chemically deposited gold films in acidic and neutral electrolytes, respectively. Combined with isotopic labeling experiments, the reaction mechanisms for C-C coupling of CO electroreduction on Cu-based catalysts are revealed using the GNSs substrates. The study also shows that the GNSs have significantly improved reproducibility compared to traditional CDFs. The results provide a novel approach with high sensitivity and reproducibility to revealing the reaction mechanisms of surface-mediated electrochemical reactions for renewable energy applications. The study also demonstrates that the GNSs can be used as substrates for in situ spectroscopic study of C-C coupling of the CORR on Cu₂O catalysts in the presence of CH₃I. The results show that the vibrational signals of adsorbed CO are significantly enhanced by GNSs compared to traditional CDFs. The study also reveals that the coupling reactions between adsorbed CO and CH₃ group from CH₃I yield detectable amounts of ethanol via the CORR. The study provides an attractive strategy for revealing the reaction mechanisms of surface-mediated electrochemical reactions with high sensitivity and reproducibility.