Photoinduced water splitting is a feasible way to mitigate the energy crisis and the associated environmental issues (Lewis, 2007). Given the high atom utilization efficiency, unique electronic structure, precisely identified active site, and excellent catalytic activity and selectivity, single-atom catalysts (SACs) have emerged as a new frontier in heterogeneous catalysis including photocatalytic water splitting in recent years (Qiao et al., 2011; Lang et al., 2020; Zhuo et al., 2020). As reported by Yang et al., compared to atomically dispersed Pd and Rh, single Pt sites anchored on TiO₂ exhibit excellent efficiency, high stability, and photo-corrosion resistance for solar-driven water splitting (Xing et al., 2014). Afterward, a series of single-atom catalysts on different substrates had sparked tremendous attention in photoinduced water splitting (Sui et al., 2017; Wu et al., 2018; Hejazi et al., 2020). For instance, Schmuki et al. found that the rate of water splitting on the Pt site of the thin TiO₂ layer was enhanced 150 times higher than that on Pt nanoparticles (Hejazi et al., 2020). However, these processes face great challenges in suppressing the recombination of photogenerated carriers and further enhancing the efficiency of charge transfer and chemical reactions. One way to address this weakness is to combine the advantages of SACs and plasmonic excitations in metal clusters to strengthen the light-matter interactions, thanks to the superior optical absorption and extended lifetime of excited carriers of the latter (Nie and Emory, 1997; Xu et al., 1999; Prodan et al., 2003).

Plasmonic metal clusters such as Au, Ag, Cu, and Al can concentrate and channel the energy of solar light into the absorbates after plasmon excitation, which have been prevalently utilized in chemical and solar energy conversion, especially in plasmon-driven photocatalysis including O₂ dissociation (Christopher et al., 2011; Christopher et al., 2012; Seemala et al., 2019) and H₂O splitting (Liu et al., 2011; Primo et al., 2011; Thimsen et al., 2011; Warren and Thimsen, 2012; Qian et al., 2014; Sigle et al., 2015; Yan et al., 2016; Yan et al., 2018). However, plasmon-induced photocatalysis using SACs has seldom been investigated, especially its underlying carrier dynamics and reaction mechanism. Mark et al. reported a theoretical study about the plasmon-mediated N₂ dissociation on a single-atom Fe-functionalized Au cluster (Martirez and Carter, 2017). It was revealed that the strong localized surface plasmon of Au and the active Fe site worked together to lower the dissociation barrier after the consecutive resonance energy transfer. However, the carrier dynamics and underlying charge transfer mechanism was absent. Zhou et al. quantitatively explored the hot carriers and thermal contributions in a plasmon-assisted ammonia photolysis using atomically dispersed Ru on Cu nanoparticles under light irradiation (Zhou et al., 2018). However, the detailed role of the Ru site in the reaction process remains elusive and needs to be further investigated.

Here, we investigate plasmon-induced water splitting on the Ag cluster doped by a single Pt atom at the single-molecule level using real-time time-dependent density functional theory (rt-TDDFT). Through the carrier and structural dynamics analysis, we find that the introduction of single Pt atom improves light absorption and electronic level alignment as given by the strong light-matter interactions. Specially, it opens up different charge transfer channels to magnify the charge transfer rates to water molecules, enabling high-efficiency water splitting. The aforementioned findings offer new prospects for solar water splitting and for the design of optimal photocatalysts with high efficiency.

Atomic Configuration and Absorption Spectrum. Here, the tetrahedral Ag₂₀ cluster is used as a model system, while larger clusters such as Ag₅₅ are also tested. The reasons for choosing Ag₂₀ are as follows: first, the tetrahedral Ag₂₀ nanoparticle is structurally stable among many other clusters (Wang et al., 2003). Second, it has been widely used as a good model system for investigating the interactions between small molecules and metal clusters under illumination (Zhao et al., 2006a; Zhao et al., 2006b). Third, the tetrahedral Au₂₀ clusters have already been obtained experimentally on ultrathin NaCl films (Li et al., 2020), which gives guidance for the synthesis of tetrahedral Ag₂₀ on the supported substrates. After the relaxation of the adsorption geometry of the representative high-symmetry configurations, we selected the most stable atomic structures for Ag₁₉Pt–H₂O and Ag₂₀–H₂O, as shown in Figures 1A,B. Here, compared to the initial distance of 3 Å, the distance of Pt–O is 2.11 Å, less than that for Ag–O (2.24 Å), implying that there exists a stronger interaction between the Ag₁₉Pt and H₂O molecule than Ag₂₀. In addition, the ∠H–O–H bond angle in water is 107.6° and 108.3° for Ag₁₉Pt-H₂O, Ag₂₀-H₂O, respectively, compared to the value of 104.5° in intact water molecules, suggesting that the water molecule is activated after adsorption.

The absorption spectra of Ag₁₉Pt–H₂O, Ag₂₀–H₂O, Ag₂₀, and a freestanding H₂O molecule have been calculated and are shown in Figure 1C. The absorption peak of freestanding H₂O is located at >8 eV, corresponding to the transition from the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of water. Compared to the highest absorption peak located at 4.62 eV for Ag₂₀, Ag₁₉Pt–H₂O and Ag₂₀–H₂O complexes both display a blue shift where the major absorption peak moves to 5.07 and 4.83 eV, respectively. This means that the Ag₁₉Pt cluster has a stronger electronic coupling between the metal cluster and H₂O molecule than the Ag₂₀ cluster, which can also be confirmed by the photo absorption spectra calculated for Ag₅₅Pt–H₂O and Ag₅₅–H₂O, as shown in Supplementary Figure S1.

Ultrafast Molecular Dynamics After Photoexcitation. To investigate the photoinduced response, we calculate the time-dependent changes in the bond length of O–H for the two complexes mentioned earlier upon laser illumination under different maximum field strength E_max, as shown in Figures 2A,B. The couplings between atomic and electronic motions are governed by the Ehrenfest approximation (Alonso et al., 2008). Given that the equilibrium O–H bond length of gaseous H₂O is 0.98 Å, we assume that the interaction between the OH and H atom can be considered negligible when the H–OH distance is >2.0 Å. So, for simplicity, we take the O–H bond length of 2.0 Å as the criterion for determining the breaking of the O–H bond. Obviously, one finds that without laser illumination, the O–H bond length exhibits a stable oscillation around 0.98 Å and the bond does not break, indicating that water splitting does not occur directly after adsorption on the metal clusters. Upon illumination, the O–H bond length increases with E_max for the two systems. In particular, the O–H bond breaks at 40 fs with E_max = 0.5 V/Å for Ag₁₉Pt–H₂O, while it does not break but only shows an elongation of 0.2 Å for Ag₂₀–H₂O under the same field strength. These results suggest that photocatalytic water splitting can be attributed to the cooperation of the photoexcitation and the introduction of the Pt atom.

In order to uncover the underlying mechanism of water splitting, we first compare the time-evolved total charge Q located on the H₂O molecule under different field strengths for the two complexes, as shown in Figures 2C,D. For the two systems under irradiation, Q rises quickly when the field strength increases and then gradually decays with an oscillation to a value around 8 e. Here, 8 e corresponds to the initial total charge of a freestanding H₂O molecule, that is, there exists a charge transfer ΔQ (ΔQ = Q − 8) from the metal cluster to the H₂O molecule if Q > 8. In particular, compared to the charge transfer ΔQ = 0.75 e to the H₂O molecule for Ag₂₀–H₂O at E_max = 0.5 V/Å, a charge transfer ΔQ = 0.92 e takes place for the Ag₁₉Pt–H₂O cluster under the same field strength, implying that the introduction of the Pt atom enhances the amount of charges transferred to water.

Time-Evolved Kohn–Sham States. To further explore the difference in charge transfer for the two systems, the time-evolved occupation of Kohn–Sham (KS) states and the corresponding projected local density of states (LDOS) under a field strength of 0.1 V/Å are calculated, as shown in Figure 3. Here, the occupation of KS states is calculated by projecting the time-dependent KS state onto KS orbitals at time t = 0. At first, there exists an obvious oscillation in the electronic occupation for the KS states near the Fermi level for the two systems, as shown in Figures 3A,C, indicating charge density oscillations around the metal cluster surface (Townsend and Bryant, 2011). Then, the electrons at the deep energy region can be photoexcited to high-energy levels, implying the rapid plasmon decay into hot electron–hole pairs following photoexcitation (Townsend and Bryant, 2014; Ma et al., 2015). It can be seen that the energy region of excitation almost extends all over the range of 0–5 eV for the Ag₁₉Pt–H₂O cluster (Figure 3A), while it mainly locates at 3.5–5 eV for Ag₂₀–H₂O (Figure 3C), further confirming that more charge transfer to the H₂O molecule is favored in the Ag₁₉Pt–H₂O system. In order to explain the difference in charge density oscillations for the two systems, we calculated the charge density at t = 40 fs under different field strengths, as shown in Supplementary Figure S2, indicating that the introduction of a single Pt atom alters the distribution of charge oscillations within and around the metal surface. For the sake of analysis, a weak laser pulse with a field strength of 0.1 V/Å is used. Similar results are observed under stronger laser pulses with the relative contribution of different excitation channels varying, such as the case of E_max = 0.5 V/Å, as shown in Supplementary Figure S3.

From the LDOS shown in Figures 3B,D, we can find that the introduction of the Pt atom displays a metallic character for the Ag₁₉Pt–H₂O cluster, while a semiconducting behavior is identified for Ag₂₀–H₂O, suggesting a qualitative change in the electronic structure. Furthermore, the LDOS located on H₂O species is very diffusive, suggesting strong electronic couplings between water and the nanoparticle in both cases.

Time-Dependent Occupation of Kohn–Sham States. To offer a direct description of ultrafast carrier dynamics, the time evolution of the occupation of Kohn–Sham states are shown in Figure 4. In Figure 4A, the change of occupation for the HOMO-14 and HOMO-15 (corresponding to the HOMO level of the H₂O species as shown in Figure 4B) is 15% and 7.5%, respectively, which means that the intramolecular charge transfer exists for Ag₁₉Pt–H₂O. In contrast, the change of occupation for HOMO-9 and HOMO-11 (corresponding to the HOMO level of water as shown in Figure 4D) is 0, ruling out the intramolecular charge transfer channels for the Ag₂₀–H₂O case. In other words, the introduction of the Pt atom opens up additional charge transfer channels to facilitate the plasmon-induced chemical reaction, that is, an intramolecular charge transfer in the case of Ag₁₉Pt–H₂O. Moreover, there is a contrary variation trend between LUMO, LUMO+1, LUMO+11 and HOMO, HOMO-1, and HOMO-8 for Ag₁₉Pt–H₂O (Figure 4A), implying a charge transfer among these orbitals, so does the orbitals between LUMO+11, LUMO+16 and HOMO-2, HOMO-5 for the Ag₂₀–H₂O case. These channels stand for the hot electron generation and intermolecular charge transfer pathways between the metal nanoparticle and water.

Charge Transfer Mechanisms. To confirm the underlying charge transfer mechanisms for the two systems, we calculated the time-evolved transition coefficients from all occupied states to the orbitals mentioned earlier, as shown in Figure 5. The charge transfer mechanism for the two systems is analyzed as follows.

The Ag₁₉Pt–H₂O Case. Given the fact that the total number of electrons in Ag₁₉Pt–H₂O is an odd number, the HOMO of the system is half-occupied in spin-unpolarized calculations. Figure 5A reveals that the main contribution to the HOMO level stems from the HOMO-1 level. In addition, the main wave function component of the two orbitals is dominantly originated from the metal nanoparticle, which can be deduced from Figure 4B. Similar results can be obtained by the same analysis, as shown in Figures 4B, 5B,C, that is, the HOMO-1 and HOMO levels make a dominant contribution to the excitation to the LUMO and LUMO+1 state, where the corresponding wave functions are mainly distributed on the metal nanoparticle. Therefore, the channels of indirect charge transfer can be open via inelastic electron tunneling (Christopher et al., 2011; Brongersma et al., 2015; Linic et al., 2015; Thrall et al., 2013; Mukherjee et al., 2013; Mukherjee et al., 2014; Zhang et al., 2018; Yan et al., 2015). Second, there is a significant charge transfer from the HOMO-8 to LUMO+11 states, as shown in Figure 5D. Through the analysis given in Figure 4B, the orbital component of HOMO-8 is mainly contributed by the metal cluster, while the LUMO+11 orbital is mainly distributed on the water molecule. Meanwhile, the energy gap of (LUMO+11)-(HOMO-8) is equal to the photoenergy corresponding to the highest absorption peak for Ag₁₉Pt–H₂O, implying that this is a direct charge transfer via localized surface plasmon resonances (Yan et al., 2011; Kale et al., 2013; Kale et al., 2014; Yan et al., 2016; Kumar et al., 2019). In summary, the introduction of the Pt atom opens up more channels of charge transfer including intramolecular, indirect, and direct charge transfer pathways, resulting in efficient charge transfer and subsequent water splitting.

The Ag₂₀–H₂O Case. Figures 5E,F show that there exists obvious charge transfer from the HOMO-5 and HOMO-2 states to the LUMO+11 and LUMO+16 states, respectively. In addition, the analysis of the wave function component, as shown in Figure 4D, shows that the HOMO-5 and HOMO-2 states are mainly located on the metal nanoparticle, while the LUMO+11 and LUMO+16 states are mainly located on the H₂O molecule. Meanwhile, the energy gap between the HOMO-5, HOMO-2 and LUMO+11, LUMO+16 states equals to the photoenergy of the major peak in the photoabsorption spectra for the Ag₂₀–H₂O system. These facts suggest that the direct charge transfer is the main charge transfer mechanism for the Ag₂₀–H₂O case.

In summary, we probe the difference in the underlying charge transfer mechanism for plasmon-driven water splitting for two representative systems: Ag₁₉Pt–H₂O and Ag₂₀–H₂O. Through the analysis of non-adiabatic molecular dynamic trajectories, we find that water may split within 40 fs after photoexcitation in the Ag₁₉Pt–H₂O case, where the O–H bond is only slightly elongated and does not break in the simulations for the Ag₂₀–H₂O system. Our ultrafast carrier dynamic analysis finds that there is more charge transferred to the H₂O molecule in the Ag₁₉Pt–H₂O system than in the Ag₂₀–H₂O system. More importantly, it can be inferred that the introduction of single Pt atom in Ag₁₉Pt–H₂O strengthens the optical absorption as well as the interactions between the water and metal nanoparticle, opening up more charge transfer channels than that in Ag₂₀–H₂O, resulting in successful water splitting events. These results provide a new microscopic picture for solar water splitting and may facilitate the design of high-efficiency single-atom photocatalysts.