Sun Yat-sen University Reports in Nature Communications: Silicon-Based Catalyst with Isolated Cobalt Atoms Enables Highly Efficient Photocatalytic CO₂ Reduction

Release Date:

2023-04-06 11:48

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01 Quick Overview of the Entire Text

Currently, many reported heterogeneous single-atom photocatalysts are limited by low photon utilization and low metal loading. Crystalline silicon, with its ideal bandgap and strong light-absorption capability, is an excellent substrate material for single-atom photocatalysts. However, because crystalline silicon is an atomic crystal characterized by high structural rigidity and high formation energy, effective synthetic methods for preparing silicon-based single-atom catalysts remain lacking. This work presents a solid-phase epitaxial growth approach to synthesize a class of silicon-based catalysts with a relatively high single-atom cobalt loading (3.4 wt%), which, in model experiments for photocatalytic CO₂ reduction to syngas, achieves high atom utilization, high total catalytic turnover number, and high quantum yield.

02 Background Introduction

Single-atom catalysts (SACs) are of great significance in energy conversion, environmental remediation, chemical synthesis, and other fields due to their ability to maximize atomic utilization. In recent years, a widely recognized strategy for fabricating photocatalysts involves loading metal sites onto semiconductor supports to construct SACs, as this unique host–guest configuration facilitates enhanced photon utilization and promotes the efficient separation of photogenerated electrons and holes between the bulk phase and the surface of the catalyst. Representative SAC-support systems include metal oxides, chalcogenides, and metal–organic frameworks. However, the intrinsic light-absorption capabilities of these support materials are often suboptimal, resulting in insufficient generation of photogenerated electrons and thereby severely limiting the photocatalytic activity of the material.

Crystalline silicon possesses an ideal bandgap structure and exhibits excellent light absorption across the entire visible and near-infrared regions; at the same time, it is inexpensive, readily available, and highly stable. If metal catalytic sites can be effectively supported on crystalline silicon, outstanding catalytic performance can be expected. However, since crystalline silicon is an atomic crystal with a highly rigid structure and high formation energy, its synthesis typically requires high-temperature conditions. Moreover, silicon’s dense crystalline structure tends to repel dopant atoms during high-temperature growth. Consequently, the loading of metallic active sites in currently reported silicon-based heterogeneous catalysts remains suboptimal. Therefore, the synthesis of high-performance silicon-based single-atom photocatalysts is of great significance for breakthroughs in heterogeneous photocatalysis.

03 Highlights of This Article

1. This work reports a solid-state chemical synthesis method that, through precise control of solid-phase reaction conditions, yields silicon-based catalysts with atomically dispersed cobalt. This synthetic approach first facilitates the formation of cobalt silicide intermediates, thereby chemically confining the cobalt atoms and preventing their aggregation, followed by the promotion of epitaxial growth of crystalline silicon to obtain CoSi. ₂ The Si structure, upon complete completion of the solid-phase reaction, yields an atomically dispersed silicon-based catalyst, namely Co@Si SAC.

2. By tuning the solid-phase reaction conditions, the cobalt loading in the catalyst can be varied from 0.4 wt% to 3.4 wt%. In all cases, effective atomic-level dispersion of cobalt is maintained.

3. The catalyst prepared using this method achieved 2×10 during a six-hour continuous experiment on visible-light-driven photocatalytic reduction of carbon dioxide to synthesize syngas. ⁴ the total catalytic turnover number and an external quantum yield as high as 10%, with the product H ₂ The CO ratio can be adjusted between 0.8 and 2.

04 Illustrated Explanation

1. Material Synthesis Strategy

Epitaxial growth is a synthetic method that leverages the similarity in crystal structure and close lattice orientation to grow heterostructures on pre-existing crystalline substrates. Moreover, because epitaxial growth allows precise control over the aggregation state of guest atoms within the host material, it can favor the uniform distribution of metals in crystalline silicon. In this work, we observed that cobalt silicide and silicon exhibit remarkably similar crystal structures and a low lattice mismatch (Fig. 1a). Based on this characteristic, we proposed a strategy for synthesizing silicon nanocrystals with atomically dispersed cobalt via epitaxial growth (Fig. 1b). Specifically, we first in situ generate a cobalt silicide intermediate in a system where silicon and cobalt are uniformly dispersed, and then, under solid-state reaction conditions, utilize the cobalt silicide to promote the epitaxial growth of silicon, thereby obtaining CoSi. ₂ On the Si interface, as the solid-phase reaction proceeds, cobalt silicide undergoes thermal decomposition while crystalline silicon continues to grow. During the overall structural transformation from a disordered to an ordered state, cobalt atoms become dispersed within the crystalline silicon, resulting in atomically dispersed cobalt in silicon nanocrystals.

▲Figure 1: Synthetic design strategy for the catalyst; (a) CoSi ₂ and the crystal structure characteristics of Si and CoSi ₂ /Si epitaxial interface; (b) Schematic diagram of the synthesis design of Co@Si SAC

2. Material Property Characterization

1. Powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed on Co@Si SAC samples synthesized under various conditions. The results indicate that, during the synthesis process, cobalt atoms are captured and bonded by silicon atoms to form CoSi. ₂ intermediate, thereby promoting the formation of silicon nanocrystals and significantly lowering the final crystallization temperature (Fig. 2a–c). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) clearly reveals the distribution of Co atoms within the silicon nanocrystals, and electron energy-loss spectroscopy (EELS) is used to confirm the signal corresponding to cobalt atoms (Fig. 2d–g), demonstrating the effective isolated dispersion of cobalt atoms in the catalyst. Finally, based on the above characterization results, this study has deduced the overall process by which Co atoms are dispersed in the catalyst via this synthesis method (Fig. 2h). First, a siloxane copolymer precursor with uniform dispersion of cobalt and silicon is obtained through co-hydrolysis. Under annealing conditions in a reducing atmosphere, a cobalt silicide intermediate is formed in situ, which then drives the epitaxial growth of silicon crystals to yield CoSi. ₂ /The structure of Si, finally CoSi ₂ Upon thermal decomposition, silicon atoms re-enter the silicon crystal growth process, while cobalt atoms are dispersed throughout the resulting silicon crystals and encapsulated by a silica matrix formed via the silicon disproportionation reaction, thereby establishing a stable catalytic composite structure.

The authors further obtained more detailed local chemical environment information for catalyst samples with different cobalt loadings through synchrotron radiation characterization (Fig. 3a–b). The results show that, in all catalyst samples with varying cobalt loadings, cobalt atoms are distributed in a manner distinct from either metallic aggregation or oxidation states. By fitting and modeling the data, parameters such as the chemical coordination number (Fig. 3d) and bond lengths and distances (Fig. 3e) provide a more accurate understanding of the distribution of cobalt atoms across catalysts with different loadings. The findings indicate that when the cobalt loading is below 3.4 wt%, cobalt atoms remain effectively isolated and dispersed within the catalyst; however, once the loading reaches 3.4 wt%, the fraction of cobalt existing in aggregated and oxidized forms begins to increase.

▲Figure 2 Synthesis and characterization of the Co@Si SAC catalyst ; Co@Si SAC at different synthesis temperatures (a) XRD results; (b) If 2 p XPS spectrum; (c) Carbon dioxide p XPS spectrum; (d) HAADF-STEM image of silicon nanocrystals with atomically dispersed Co; (e) High-resolution STEM image of a single silicon nanocrystal; (f) Fourier transform results for the red-boxed portion in Figure e; (g) EELS results for the STEM image of silicon nanocrystals in the selected region; (h) Schematic illustration of the formation of the Co@Si SAC catalyst

▲Figure 3 Synchrotron radiation characterization results of the Co@Si SAC catalyst ; Catalyst samples with different Co loadings and the blank (a) XANES spectra at the Co-K edge; (b) Fitted Co K-edge EXAFS spectrum; (c) Distribution of Co atoms and theoretical models; (d) The simulated results of the chemical coordination number and the corresponding (e) Results for Chemical Bond Distances and Bond Lengths

3. Catalytic Performance Analysis

The synthesis and characterization of the above materials demonstrate that this catalyst system exhibits a high dispersion of active metal sites. The authors further investigated the photocatalytic CO2 reduction performance of this series of catalysts. A series of experiments indicate that, under visible-light irradiation, the silicon-based catalyst facilitates the catalytic reduction of CO2 to produce syngas (CO + H2) with high efficiency. ₂ ) Yield. In a performance comparison with cobalt-containing blank control samples, this series of catalysts demonstrated a significant advantage in atomic utilization (Figure 4a), while also exhibiting a high total catalytic turnover number (Figure 4b). Comparison with reported heterogeneous catalysts in terms of metal loading and quantum yield further confirms the superior performance of this catalyst (Figure 4d). During a continuous six-hour experiment, a yield of 2 × 10 was achieved. ⁴ the total number of catalytic conversions and an external quantum yield as high as 10%, and in syngas, H ₂ The CO ratio can be adjusted between 0.8 and 2.

▲Figure 4 Photocatalytic CO reduction over the Co@Si SAC catalyst ₂ Performance of Reconstructed Synthesis Gas ; Catalysts under different Co loading levels (a) Catalytic atom utilization; (b) Catalytic total turnover; (c) CO reduction yield as a function of time; (d) Comparison with reported heterogeneous catalysts in terms of metal loading and quantum yield.

05 Summary and Outlook

This work reports a synthetic strategy that employs solid-phase reactions to in situ generate cobalt silicide intermediates, followed by epitaxial growth to successfully fabricate single-atom silicon-based catalysts. This approach achieves effective dispersion of isolated atoms on silicon nanocrystals and thereby constructs a highly efficient photocatalytic system for CO₂ reduction. By overcoming the traditional bottlenecks in the preparation of silicon catalysts, this study provides a novel synthetic route for integrating silicon substrates with metal active sites and elucidates the structural evolution mechanisms of single-atom silicon catalysts during synthesis, offering a new and viable design paradigm for developing high-performance silicon-based photocatalytic materials.

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