Iced photochemical reduction to synthesize atomically dispersed metals by suppressing nanocrystal growth

Photochemical solution-phase reactions have been widely applied for the syntheses of nanocrystals. In particular, tuning of the nucleation and growth of solids has been a major area of focus. Here we demonstrate a facile approach to generate atomically dispersed platinum via photochemical reduction of frozen chloroplatinic acid solution using ultraviolet light. Using this iced-photochemical reduction, the aggregation of atoms is prevented, and single atoms are successfully stabilized. The platinum atoms are deposited on various substrates, including mesoporous carbon, graphene, carbon nanotubes, titanium dioxide nanoparticles, and zinc oxide nanowires. The atomically dispersed platinum on mesoporous carbon exhibits efficient catalytic activity for the electrochemical hydrogen evolution reaction, with an overpotential of only 65 mV at a current density of 100 mA cm−2 and long-time durability (>10 h), superior to state-of-the-art platinum/carbon. This iced-photochemical reduction may be extended to other single atoms, for example gold and silver, as demonstrated in this study.

with a large capacity relative to the volume of solution. For example, the thickness of frozen solution was just ~1.3 mm when a 3 mL of H2PtCl6 solution was put into the container (55 mm in diameter). 3. Preparation of the Pt1/MWCNT and Pt1/G. A 10 ml Pt single atoms solution and a 10 ml mg ml -1 multi-walled nanotubes (or graphene) solution were mixed, filtered, cleaned and dried at room temperature. Then, the Ag1/MC powders were collected. 4. Preparation of Pt1/TiO2 and Pt1/ZnO. A 10 mL Pt single atoms solution and 1 mg TiO2 (or ZnO) were mixed together. The mixed solution was filtered, cleaned and dried at room temperature.

Preparation of Ag single atoms solution and Ag1/MC.
A 0.3 mg ml -1 AgNO3 solution was frozen quickly and entirely using liquid nitrogen, followed by UV irradiation (0.89 mW cm -2 ) for one hour at -25°C in a lyophilizer. Then the Ag single atoms solution was obtained after the ice melting process at room temperature in a dark place. A 10 ml mesoporous carbon solution (5 mg ml -1 ) and a 25 ml the obtained Ag single atoms solution were mixed. Then, the Ag1/MC samples were cleaned and dried at room temperature. 6. Preparation of Au single atoms solution and Au1/MC. A 0.1 mg ml -1 HAuCl4 solution was frozen quickly and entirely using liquid nitrogen. The subsequent processes of Au1/MC were similar to these of Ag1/MC.  11. XPS characterization.
X-ray photoelectron spectroscopy measurements were performed using an X-ray photoelectron spectrometer (Escalab 250Xi) equipped with an Al Kα radiation source (1487.6 eV) and hemispherical analyzer with a pass energy of 30.0 eV and an energy step size of 0.05 eV. The collected data were corrected for charging effect-induced peak shifts using the binding energy (BE) of C 1s peak of substrate (284.8 eV). Spectral deconvolution was performed by Shirley background subtraction using a Voigt function convoluting the Gaussian and Lorentzian functions.

ICP-MS characterization.
The Pt loading contents of Pt single atoms materials were measured and confirmed using ICP-MS measurements (ELAN DRC-e). Confirmed by ICP-MS test, the Pt loading of Pt1/MC was 2.6%, which can be regarded as a relatively high level compared with other single Pt atom catalysts. As shown in Table S8, Pt metal remained atomically dispersed with a low Pt loading (< 2 wt.%) while Pt clusters or nanoparticles from the aggregation of single atoms emerged with the increase of Pt loading.  The DFT calculations were carried out using a graphene (10 × 10 × 1) supercell containing 200 C atoms to identify the adsorption strengths of single Pt atom on the common defects, such as SV, DV, and edges. Considering the existence of all kinds of defects of mesoporous carbon in experiment, the Pt single atoms adsorbed on the defects and edges were carefully considered. All inequivalent Pt-adsorption configurations at these defects or edges have been investigated, and the most stable configurations are shown in Supplementary Fig. 13. The adsorption energies (Ea) for each configuration were calculated using the following equation:

Supplementary
Where Ept is the energy of an isolated Pt atom, EM is the total energy of the defects/edges systems, and EM+nPt is the total energy of the Pt adsorption on substrates with defects or edges, respectively. A positive Ea indicates the stable Pt adsorption. As shown in Supplementary Table 4, all the adsorption energies are positive for the Pt adsorption on perfect carbon, defects and edge sites, indicating the stability of the Pt single atoms on these systems. As for the Pt single atoms adsorbed on perfect graphene, almost no charge transfer occurs between Pt and graphene. Different from the perfect graphene, the large charge transfer appears between Pt and C in the SV/DV and the edge, which confirms the relatively large Pt adsorption energy at these defect/edge sites.

Supplementary Note 3. Stability of Pt single atoms on mesoporous carbon and graphene.
To further examine the distribution of the Pt atoms on the mesoporous carbon, the energy differences between the isolated Pt atom and the Pt cluster, ΔEc, at both defects and edge sites were calculated. Here the total energy of the isolated configuration was used as a reference energy. In this scheme, a relatively large positive value of ΔEc indicates that Pt atoms energetically prefer to adsorbed as the single-atom instead of clustering. For comparison of substrates, various possible Pt cluster configurations on pristine graphene were also examined.

Supplementary Note 4.
Oxidation state of Pt on the MC substrate.
We further characterized the oxidation state of Pt on the MC substrate before and after HER catalytic step using first-principle calculations. As shown in Supplementary Fig. 30 and Supplementary where ΔSH is the difference in entropy. ΔEZPE is the difference in zero-point energy between the adsorbed and the gas phase. ΔEZPE -ΔSHT is about 0.24 eV 5 . ΔEH is the hydrogen chemisorption energy defined as: where n is the number of H atoms in the calculation. The most desirable value for |ΔGH*| should be zero 6 . A (2 × 2) Pt (111) were used to calculate the catalytic activity of the typical Pt (111) catalyst. The calculated free energy diagram for hydrogen adsorption on Pt (111) was consistent with the previous report 7 (Supplementary Fig. 27).

Supplementary Figures
Supplementary Figure 1