Cairo Pentagonal C–N–P Monolayers with Tunable Auxeticity and Photocatalytic Activity: Computational Insights

Designing two-dimensional (2D) materials that integrate auxetic behavior with functional properties remains desirable. Here, we propose two Cairo pentagonal monolayers, C2N3P and C2NP3, and demonstrate that N-rich and P-rich compositions provide an effective route to tuning their bonding configurations and physical properties. The variation in composition drives a transformation in dimer configurations from N–N to P–P, accompanied by distinct local bonding environments. Stronger and more directional C–N bonds enhance structural rigidity, whereas larger P atoms and P–P dimers increase structural flexibility. As a result, N-rich C2N3P exhibits higher stiffness and a pronounced negative Poisson’s ratio (−0.124), while P-rich C2NP3 shows lower stiffness and a reduced auxetic response (−0.063). Both monolayers are direct-band gap semiconductors with tunable electronic properties. Increasing N content enhances charge localization and widens the band gap, thereby improving photocatalytic activity for water splitting. During water splitting, N and P atoms act as preferential active sites for the hydrogen and oxygen evolution reactions, respectively. Accordingly, C2N3P exhibits more favorable overall photocatalytic water-splitting characteristics than C2NP3. These results suggest a correlation between composition-driven bonding modulation and the coupling of auxetic behavior with photocatalytic functionality.