Regulated ion exchange method to achieve a variety of precious metal sulfur compound custom synthesis

On July 25, 2022, Beijing time, Professor Huang Xiaoqing of Xiamen University and Professor Xu Yong of Guangdong University of Technology published a research paper entitled “Synthesis of noble metal chalcogenides via cation exchange reactions” in Nature Synthesis.

This achievement realizes the custom synthesis of a variety of precious metal sulfur compounds by regulated ion exchange method. By regulating the kinetic and thermodynamic factors in the ion exchange method, the noble metal chalcogenetic compounds with adjustable morphology and controllable structure were synthesized, including zero-dimensional, one-dimensional, two-dimensional and three-dimensional platinum, palladium, rhodium, ruthenium, gold and silver-based precious metal chalcogenous chalcogenous compounds, and combined with experimental and theoretical calculations, the importance of kinetic and thermodynamic regulation to the structure of the compound was confirmed, providing a rational and simple solution for the custom synthesis of such compounds.

The corresponding authors of the paper are Professor Xu Yong and Professor Huang Xiaoqing; The first authors are Dr. Feng Yonggang and Dr. Ji Yujin.

Precious metal chalcogenides have attracted much attention because of their unique properties and are widely used in the fields of electronics, optoelectronics, catalysis and sensing. For a long time, researchers have poured a lot of effort into the controllable synthesis of such compounds, and have developed mechanical stripping, chemical vapor deposition, molecular beam epitaxial growth, wet chemical synthesis and other methods for the synthesis of precious metal chalcogenous compounds. However, the controlled synthesis of such compounds still faces huge challenges, which greatly limits their practical application.

Since the 1990s, the ion exchange method has been widely used in nanostructure synthesis. Compared with the traditional bottom-up method, the synthesis of nanostructures by ion exchange method is mainly achieved by exchanging with the target ions and templates, so the morphology and structure of the target product depends largely on the template. Although the ion exchange method is widely used in the synthesis of some non-precious metal sulfur compounds, it is rarely reported on the synthesis of precious metal sulfur compounds, mainly for the following two reasons: 1) The synthesis of precious metal sulfur compounds undergoes complex kinetic and thermodynamic processes; 2) Compared with non-precious metal elements, the reduction potential of precious metal ions is larger, so it is very easy to be reduced by some solvents or surface ligands during ion exchange to form metal nanoparticles during ion exchange. In view of the above research difficulties, this work focuses on the kinetic and thermodynamic factors in the ion exchange method, and develops a universal controllable synthesis scheme of precious metal chalcogenous compounds, which lays the foundation for expanding the application of such materials in many fields such as energy and catalysis.

In this work, the authors first synthesized CuTe as a template and characterized its two-dimensional flake structure using a series of means (Figure 1a). Using CuTe as a template, the two-dimensional Pd20Te7, PtTe2, Rh1.398Te2, RuTe2, AuTe2, and Ag2Te nanosheet structures were obtained after adding different precious metal ion precursor solutions (Figure 1b-g). From the transmission electron microscopy photos, the prepared precious metal sulfur compounds retain the morphology of the CuTe template, which fully confirms the universality of the method.

Figure 1: Pd20Te7, PtTe2, Rh1.398Te2, RuTe2, AuTe2, Ag2Te and other nanosheet structures were synthesized by ion exchange method using two-dimensional CuTe nanosheets as templates.

In addition, the authors also found that the structure of the target product could be regulated by changing the solvent used during the ion exchange process. Taking palladium telluride as an example, when ion exchange occurs in dimethyl sulfoxide (DMSO), the product is PdTe; When using acetone, methanol, ethanol, ethylene glycol these solvents, the product is Pd20Te7 (Figure 2a). In order to further reveal the structural transformation of the products in different solvents, the authors studied the evolution of the product structures in different solvents with the ion exchange time. Studies have shown that CuTe gradually evolves into CuTe/Te/PdTe2/PdTe after 10 min and eventually into Pd20 Te7 after 1 h (Figure 2b); In glycol (EG), CuTe gradually evolves into CuTe/Te/PdTe2/PdTe (5 min) as the exchange time lengthens, then PdTe/PdTe2 (30 min) and finally forms PdTe after 1 h (Figure 2c).

Figure 2: Effect of solvation effect on the structure of ion exchange products.

To study the kinetics of the ion exchange process, the authors further studied the concentration of individual ions in the exchange product and solution. Studies have shown that Cu2+ can be rapidly precipitated from the CuTe template, whether in EG or DMSO solvents, but the concentration of Pd2+ in solution is very different. Specifically, the concentration of Pd2+ in EG is much higher than in DMSO (Figures 2c, 2d), indicating that the solvation effect of Pd2+ in the two solvents is different. The above experimental results show that the ion exchange process is affected by both dynamic and thermodynamic factors. First, Cu2+ precipitates rapidly from the CuTe template and undergoes ion exchange with Pd2+ in solution (kinetic control), where in different solvents, the free Pd2+ in solution is affected by the solvation effect and ultimately determines the structure of the product (thermodynamic control) (Figure 3a). The above conclusions are further supported by first-principles computations. The kinetic energy barrier precipitated by Cu2+ from the CuTe template was close to that of cu2+ in EG and DMSO solvents, respectively, which were 3.83 eV and 3.02 eV, indicating that the solvent had little influence on the precipitation rate of Cu2+ (Figure 3b); The interaction between the S=O functional group and Pd2+ in DMSO is much stronger than the interaction between EG and, which in turn affects the concentration of free Pd2+ in solution (Figure 3c-e).

Figure 3: Study of the mechanism of dynamics and thermodynamic regulation during ion exchange.

In addition to two-dimensional nanosheets, the authors also used this ion exchange method for the synthesis of other morphological noble metal chalcogenous compounds, such as zero-dimensional nanospheres, one-dimensional nanowires, and three-dimensional nanoflower-like structures (Figure 4). Similarly, the modulation of the structure of the noble metal chalcogenetic compounds of different dimensions can be achieved by regulating the type of solvent. Finally, the authors further extended the method to the synthesis of precious metals selenides and sulfides, and successfully prepared the synthesis of Pd17Se15 and Pd16S7 nanosheets (Figure 5). The above experimental results fully confirm that the ion exchange method described herein can realize the customized synthesis of precious metal tulphide compounds with different morphologies and structures.

Figure 4: Ion exchange is used to synthesize other morphological palladium-based precious metal tulphide compounds.

Figure 5: The ion exchange method is used to synthesize two-dimensional nanosheets of palladium selenide and palladium sulfide.

Finally, this work elaborates the influence of kinetic and thermodynamic factors on the product structure in the ion exchange method, and realizes the customized synthesis of precious metal tulphides with different morphologies and structures through the dual regulation of kinetic and thermodynamic factors. This work not only develops a general formulation strategy for precious metal chalcogenous compounds, but also lays the foundation for expanding the application of these materials in many fields such as catalysis and energy. This research was supported by the Ministry of Science and Technology, the National Foundation of Guangdong Province, the Guangdong Provincial Natural Science Foundation Outstanding Youth Fund, the Jiangsu Provincial Natural Science Foundation, Xiamen University and Guangdong University of Technology. (Source: Science Network)

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