Efficient Photo-Driven Electron Transfer from Amino Group-Decorated Adamantane to Water

Abstract

Nanodiamonds in water can generate solvated electrons under ultraviolet (UV) excitation, but UV light constitutes only a small portion of solar energy. To harvest solar energy in the visible range, we investigate band gap reduction via surface amino functionalization and examine its impact on photo-excited charge transfer to water. Adamantane, the smallest nanodiamond, is used as a model due to its electron emission properties. Liquid water is first represented using water dimers and then complete solvation shell structures surrounding the adamantane. By systematically analyzing different functionalized adamantane structures, we find that nitrogen serves as the primary electron donor to nearby water molecules. Furthermore, the negative electron affinity of adamantane, which determines its emission capability, is preserved with half of the amino group functionalization on the surface. Our findings motivate further experimental verification using nanodiamonds with amino-functionalized surfaces.

Keywords: adamantane; amino group; electron transfer; excited states; linear response time-dependent density functional theory (LR-TDDFT).

Figure 1

The structures as well as the HOMOs of adamantane with varying amino surface functionalization are shown. The upper row displays only the structures, while the lower row presents the HOMO of the corresponding structure. The naming convention for these structures includes “Ad” for adamantane followed by the number of amino groups. The final segment in each name distinguishes different isomers. The isovalue for all orbital plots was set to $2\times {10}^{-2}{a}_0^{-3/2}$. The bottom of the figure shows the color bar corresponding to the isovalue contour plot. The yellow and cyan colors represent the positive and negative values of the wave function, respectively.

Figure 2

Dependence of HOMO and LUMO energies on the number of surface amino groups in functionalized adamantane. (a) The HOMO energy (in eV) as a function of the number of amino groups. The dashed line shows a linear fit. (b) The LUMO energy versus an increasing number of amino groups. Legend and units are the same as in (a).

Figure 3

Schematics showing the structures used to calculate electron transfer from amino-functionalized adamantane to water molecules. ${\mathrm{C}}_{\mathrm{S}}\mathrm{C}$ water dimers are depicted in various positions and orientations relative to adamantane: above (a), in front (f), to the left (l), and to the right (r). Additionally, two types of amino-functionalized adamantane were considered, namely Ad_10NH2, with nitrogen atoms uniformly distributed, and Ad_7NH2_2, with nitrogen atoms concentrated on one side.

Figure 4

The total ground state energy as a function of the center-of-mass (COM) distance between the water dimer and the Ad_7NH2_2 cluster in the Ad_7NH2_2_CsC_l structure is shown. The energy is given in eV units, and the COM distance is in Å units.

Figure 5

Photo-excited charge transfer from amino-functionalized adamantane to water dimers at various distances. The structure corresponds to the Ad_7NH2_2_CsC_lr configuration, as shown in Figure 3 in this study. (a) The left and right water dimers at their respective equilibrium distance $d_0$. (b) The structures after shifting both the left and right water dimers 1 Å further away from their respective equilibrium distances. The left column displays the molecular orbitals, including the HOMO, LUMO, and LUMO+1. The middle and right columns show the NTO densities for the left and right charge transfer states. Qualitative properties, including the order of the excited state, lowest charge transfer excitation energies ($E_{ct}$), and charge transferred to the water molecules ($q_w$), are also provided alongside the corresponding NTO plots. The isovalue for all orbital plots was set to $2\times {10}^{-2}{a}_0^{-3/2}$. The isovalue for the NTO plots was set to $4\times {10}^{-4}{a}_0^{-3}$.

Figure 6

Solvation shell structures and charge transfer excited-state properties of one exemplary MD snapshot. (a) The water solvation structure around the amino group-functionalized adamantane (Ad_7NH2_2). (b) Zoomed-in view of the hydrogen bonds formed between the amino groups and water molecules. (c) Hole NTO density distribution of the excited states; the isovalue was $1\times {10}^{-3}{a}_0^{-3}$. (d) Electron NTO density distribution; the isovalue was $2\times {10}^{-4}{a}_0^{-3}$. The isovalues were selected based on the maximum density distribution. The excited-state energy was $5.258\mathrm{eV}$. The charge transfer to the water molecules ($q_{\mathrm{w}}$) was 0.943.