In recent years, lead-free perovskite materials represented by Cs2NaInCl6 have achieved rapid breakthroughs from zero to hundred in luminous efficiency by relying on the alloying scheme of Ag/Na and In/Bi. Thanks to the self-trapped exciton (STE), it exhibits the luminescence characteristics of full coverage of the visible spectrum, which pleases people’s eyes, and the warm white light emission of a single component avoids the multi-component light reabsorption problems that are easy to occur in the previous “blue + yellow” or “blue + green + red” schemes. This makes it show high commercial value in scenarios such as LED lighting and LCD white light backplanes.
On the basis of not affecting the luminous efficiency, it is a prerequisite to achieve commercial application by seeking a synthesis strategy that can meet multiple ideal conditions such as mass production, rapid response, eco-friendliness, low cost, and preparation at room temperature and pressure. Traditional high-temperature solid-phase and hydrothermal methods generally exhibit high reaction temperatures (and pressures) and reaction times of several hours. In contrast, the recrystallization method based on liquid-phase reaction has the characteristics of rapid nucleation growth, but it is limited by the solubility of raw materials (such as Ag source, etc.) and often leads to a large amount of solvent consumption, which undoubtedly increases the preparation cost. Therefore, exploring a synthesis strategy that can meet the above needs at the same time has become one of the difficulties for the commercialization of lead-free perovskite materials.
Recently, Li Xiyan’s research group of Nankai University proposed a universal preparation strategy for synthesizing lead-free perovskites, which is based on the hydrochloric acid environment to realize the rapid mass production and preparation of raw material powder to product powder, and the whole preparation process is carried out at room temperature and pressure, and the synthesis time is only a few minutes or even a few seconds, which creates the possibility for lead-free perovskites to achieve commercial mass production.
Figure 1 Preparation of schematic diagram and photos
2. Research background
The preparation methods of materials are diversified, and for microcrystals, high-temperature solid phase method, hydrothermal method and recrystallization method are more common.
2.1 High temperature solid phase method
The high-temperature solid-phase method is often applied to the synthesis of oxide materials, which is more friendly to materials with poor solubility, high melting point, more stable chemical bonds or special atmosphere requirements, and this method has also been used to prepare lead-free perovskite materials, for example: in 2019, Zhao et al. used the high-temperature solid-phase method to prepare Cs2AgInCl6:Cr3+ to achieve near-infrared luminescence of Cr3+. However, the high temperature environment used in this scheme is easy to cause partial reduction of Ag and may produce chlorine, which is difficult to achieve industrial mass production.
2.2 Hydrothermal method
Compared with solid-phase reactions, hydrothermal methods provide a solvent environment while greatly reducing the synthesis temperature, and are often used as nanocrystalline, microcrystalline or single crystal preparation. Chlorine-based lead-free perovskites can be prepared by using concentrated hydrochloric acid as a solvent environment. Since 2018, Luo et al. first prepared Cs2Ag0.6Na0.4InCl6 by hydrothermal method to achieve bright STE emission, and hydrothermal method has become one of the most popular strategies for luminescence control of lead-free perovskite materials.
However, the hydrothermal method itself relies on extremely high pressure to provide reaction energy, and the reactor volume is limited, which is difficult to meet the industrial mass production. In addition, this strategy is still difficult to avoid the problem of solubility of raw materials (such as AgCl), which leads to obvious differences between the ratio of Na/Ag and the feeding ratio in the product, showing obvious accidents.
2.3 Recrystallization method
The liquid-phase reaction-based recrystallization method (or precipitation method) usually mixes the configured precursor solution in the anti-solvent, so that the ion explosive nucleation growth is the target product, and the crystal growth rate of this protocol is relatively fast and can be completed in seconds. For example, in 2020, Zhang et al. prepared Cs2Ag0.6Na0.4InCl6:Bi, Yb by recrystallization method, and obtained materials with dual-band luminescence properties. However, the solubility of AgCl in concentrated hydrochloric acid is extremely poor, which leads to a large amount of hydrochloric acid consumption (for example, only 1 mmol product is obtained from 30 mL of hydrochloric acid) and it is difficult to control industrial production costs. In the same year, Wang et al. increased the solubility of AgCl in hydrochloric acid by increasing the temperature, however, the heating device and the condensing device introduced to inhibit the volatilization of HCl gas limited its possibility of industrial mass production.
3. Innovative research
Just when scholars were considering how to improve the solubility of raw materials, Li Xiyan’s team at Nankai University took a different approach, ignoring the solubility of raw materials, and proposed a universal “powder-to-powder” synthesis strategy. Taking Cs2Ag1-xNaxIn1-yBiyCl6 as an example, the authors only used 1-2 mL of concentrated hydrochloric acid solution to add drops to the mixed raw material powder, stirred and sonicated appropriately, and obtained a 1 mmol pure phase product, and the whole reaction time lasted only a few minutes or even seconds.
Fig. 2 Synthesis of lead-free perovskite Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 (left) and Cs2ZrCl6 (right) with hydrochloric acid
The authors found that in the Na/Ag enrichment region where a small amount of NaCl/AgCl was dissolved, the raw material underwent “free ions-target product” from the initial powder state. In the Na/Ag barren area, the raw material needs to undergo “free ions-intermediate products-further growth” to obtain the target product, and the further growth process needs to continuously consume Na+ and Ag+ ions, thereby promoting the chemical equilibrium of NaCl and AgCl to move to the ionic state, and then forming positive feedback until the raw materials and intermediate products are consumed. Pure phase XRD proves the reliability of the synthesis strategy, with the alloying of Na/Ag and In/Bi, the emission spectrum, excitation spectrum and fluorescence lifetime show regular changes, and the luminescence properties can be adjusted after doping such as Ln3+ or TM3+, which proves the excellent response of the strategy to the preparation of a variety of doped materials. In this experiment, complete dissolution of raw materials does not seem to be necessary, which will provide a new idea for the understanding of the mechanism of traditional recrystallization methods.
Concentrated hydrochloric acid, as the only solvent used, exhibits multiple functions in this strategy, which are summarized as follows:
1) Provide a reliable solvent environment for rapid reaction;
2) Provide a reverse dissolution environment for the product to improve the chemical yield;
3) Halogen enrichment environment to achieve anion passivation to improve luminous efficiency;
4) Guide the growth direction of the product to ensure the pure phase.
In addition, the same policy applies to Cs2InCl5· H2O, CsMnCl3, Cs2ZrCl6, Cs4MnBi2Cl12 and other lead-free perovskite derivatives. Interestingly, when concentrated hydrochloric acid is replaced with the corresponding hydrobromic acid or hydroiodic acid, perovskite materials corresponding to halogen groups, such as Cs2AgBiBr6 or Cs3Bi2I9, can also be prepared, and this series of extended experiments proves that the strategy has wide universality.
Fig. 3 Growth mechanism diagram and LaMer model
4. Application and prospects
The strategy proposed in this paper can meet the ideal preparation conditions for mass production, rapid response, eco-friendliness, low cost and normal temperature and pressure preparation at the same time, and provides an industrialization strategy for the application of lead-free perovskites in the future fields of white LED lighting and LCD backlight panels. The products with a variety of luminescent properties obtained by doping in this paper also indicate that this strategy is expected to become another new preparation scheme after high-temperature solid-phase method, hydrothermal method and recrystallization method, and the induction of reaction mechanism and summary of hydrochloric acid function will also provide a strong reference for the further theoretical development of hydrothermal method and recrystallization method.
The results of the study were published online in Light: Science & Applications under the title “A universal hydrochloric acid-assistant powder-to-powder strategy for quick and mass preparation of lead-free perovskite microcrystals.”
The first author of this paper is Dr. Yang Huanxin, School of Electronic Information and Optical Engineering, Nankai University, and the corresponding author is researcher Li Xiyan. Collaborators include Professor Yuan Mingjian and Professor Long Guankui of Nankai University, Professor Zhang Libing of Tianjin University and Professor Zhang Yuhai of Jinan University. (Source: LightScience Applications WeChat public account)
Related paper information:https://www.nature.com/articles/s41377-023-01117-2
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