The adsorption of small size Pd clusters on a g-C₃N₄ quantum dot: DFT and TD-DFT study

Abstract

In this study, the adsorption of Pd n cluster with different size ( n = 2–8) on graphitized carbon nitride (g-C 3 N 4 ) quantum dot has been studied using the density functional theory (DFT). The Pd 2 , Pd 4 and Pd 6 clusters showed tendency for landing at a corner of the six-fold cavity of g-C 3 N 4 while the Pd 8 cluster was landed on the top of the cavity of g-C 3 N 4 for efficient interaction with all pyridine like nitrogen atoms of the cavity, simultaneously. The charge of Pd n cluster in Pd n /g-C 3 N 4 ( n = 2–6) complexes was positive showing the charge transfer from the metal cluster to g-C 3 N 4 while the net charge of Pd 8 in Pd 8 /g-C 3 N 4 became negative showing an opposite direction of charge transfer in this complex. Moreover, it was found that by increasing the size of Pd n cluster, the energy band gap of Pd n /g-C 3 N 4 complex becomes smaller compared with the bare g-C 3 N 4 . The time-dependent density functional theory (TD-DFT) was employed to calculate the absorption spectra of the bare g-C 3 N 4 and Pd n /g-C 3 N 4 complexes. It was observed that the presence of Pd n clusters on the g-C 3 N 4 shifted the absorption peaks of g-C 3 N 4 to the lower wavelengths. Also, it increased the intensity of the absorption bands in the range of 400 to 800 nm depending on the Pd n cluster size which could lead to the increase in the photo-catalytic activity of Pd n /g-C 3 N 4 complexes compared to bare g-C 3 N 4 in visible region. Most of the UV–vis absorption bands of the Pd n /g-C 3 N 4 complexes in region of the 400–800 nm were related to the excitation of electron from the Pd n cluster to the g-C 3 N 4 which decreases the recombination of electron-hole in the complexes compared to the bare g-C 3 N 4 . Mixing of the states of g-C 3 N 4 with the states of Pd n clusters causes the decrease in the intensity of absorption bands of the g-C 3 N 4 in the region of 200–300 nm.