Formation of Kirkendall void of lead-sulfide cubes

Synthesis of hollow micro-/nanostructures with sizes ranging from 10?nm to 10?μm have been intensively studied in the last few decades due to their promising applications. The synthesis of these micro-/nanomaterials having different nanostructures such as core–shell, and hollow structures are mainly based on Ostwald ripening [1], layer-by-layer (LBL) assembly [2] and the Kirkendall effect [3], [4], [5]. However, the Kirkendall effect for the formation of shape engineered hollow nano-micro structures of metal oxides and sulfides has attracted great interest due to its promising applications in modern physics [6], [7], [8], [9]. In particular, the binary and ternary semiconducting metal sulfides such as CdS, Ag2S, ZnS, PbS, AgInS2 and their nanostructures have attracted attention due to application in various fields including light emitting diodes (LED), biological labeling, near-infrared (NIR) emission and photovoltaics [10], [11], [12], [13]. Due to the multi-functionality of these metal-sulfides based on compositional diversity and shape engineering, these materials have attracted great interest.

Smigelkas and Kirkendall introduced the Kirkendall effect using copper and zinc in alpha brass and investigated the different diffusion rates at the diffusion couple interface. As a result, formation of hollow voids in a bulk diffusion couple [14]. This phenomenon is called the Kirkendall effect. This is the first experimental proof demonstrating the atomic diffusion occuring at the two metal interfaces through vacancy exchange. Later, Yin et al. applied this concept firstly in cobalt oxide and chalcogenides for the formation of hollow compound nanocrystals [15]. So far, metal oxides with compositional diversity have been synthesized and proved by the Kirkendall effect. However, most of these metal oxide hollow structures are spherical in shape. In contrast, synthesis of shaped-controlled, especially non-spherical hollow nano-micro structures, is a tedious job due to the limited availability of non-spherical templates and uncontrolled growth [16]. From a hollow material applications point of view; the hollow cubic shapes provide excellent active sites results in impressive performance [9], [17].

Among the metal sulfides, reports on the synthesis of PbS binary metal sulfide material having micro-cube structures is significantly limited. Here, the PbS microcube structure has been synthesized by a simple single-step hydrothermal method. In the hydrothermal method, we optimized the reaction temperature, precursor concentration and time in such a way that the Pb-cations from lead nitrate (Pb(NO3)2) and S-anions from thiourea (CH4N2S) reacts with each other to form PbS microcubes. Briefly, the precursor molar concentration was 3.4?mmol Pb(NO3)2, 6.8?mmol CH4N2S and added 0.08?g of sodium dodecyl sulfate CH3(CH2)11SO4Na (SDS) as a surfactant in 20?ml water solvent. A clear solution was loaded in a 30?ml Teflon-lined stainless-steel autoclave reactor at 160?°C for 72?h for hydrothermal reaction maintained. The black colored product was washed several times and dried in an electric oven for 24?h. Noteworthy that, these hollow microcubes were obtained only for 160?°C.

In this process, the reaction between Pb and S requires diffusion of S or Pb through the compound shell. Due to their small sizes and the relatively low melting point, it is expected that, the Pb particles exist as liquid droplets in the reaction bath. Initially, the Pb nuclei centers are generated and start the growth of cubic shapes due to the weak coordination between NO3− and Pb2+. On the other hand, at elevated temperature CH4N2S starts to produce H2S in aqueous medium will initiate the formation of PbS through reaction between H2S and Pb2+. However, the SDS medium, partially terminates the H2S and Pb2+ growth kinetics due to slow release of S2− anions. Due to the high reactivity of Pb2+, the outward dominant diffusion happens, accompanied by the Kirkendall effect to create hollow architecture.

This issue’s cover image shows a typical Field Emission Scanning Electron Microscope (FE-SEM, S-4700, Hitachi, Japan, operated at 15?kV) image of PbS hollow microcubes synthesized by the hydrothermal method. The synthesized ∼5?μm microcube is composed of stacking of eight tiny microcubes with ∼2.4?μm in size. The main microcube comprises stacking of eight tiny microcubes containing cracks and voids revealing the formation of the hollow interior with nanosized voids. In this study, we demonstrated the Kirkendall effect in PbS microcube formation through controlled reactions between Pb2+ and S2- atoms. This effect has been explained with the help of the Kirkendall effect. These hollow PbS microcubes formed because Pb atoms diffuse faster than S which leads to the formation of a hollow or cracked interior. Currently, these PbS microcubes have been used for toxic nitrogen oxide (NOx) sensors and our results show excellent sensitivity and selectivity.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016H1D3A1909289) for an outstanding overseas young researcher. This research is also supported by the National Research Foundation of Korea (NRF-2020R1A2C2004880). This work was also supported by Priority Research Centers Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (NRF-2018R1A6A1A03024334). This work was also supported by the National Research Foundation of Korea grant funded by the Korea Government (NRF-2018R1C1B6008218).

Formation of Kirkendall void of lead-sulfide cubes

Declaration of Competing Interest

Acknowledgements

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