Reaction engineering studies of the continuous synthesis of CuInS2 and CuInS2/ZnS nanocrystals
Graphical abstract
Introduction
Nanocrystalline semiconductors exhibit unique size-tunable electronic and optical properties distinct from their bulk counterparts due to the spatial confinement of the electronic wave function within the narrow boundaries of the very small particles [1], [2], [3], [4], [5]. Until now most popular is their application as efficient fluorescent agents since their stability and color purity is superior to organic dyes. Varying chemical composition and particle size allows flexible tuning of the semiconducting properties of the nanocrystals. This enables many applications which make use e.g. of the strong fluorescence employed for many applications like LEDs for display and lighting, light converters, labels for biomolecules etc. Furthermore the special light harvesting properties are utilized for solar cells or even of special charge transport properties proposed for various transistor types. Some of these applications already reached commercial status even for high volume production [6], [7], [8], [9]. The physics and chemistry as well as potential applications are well described in many reviews listing historical highlights and major milestones in the field, e.g. [10]. Typically, II–VI-type semiconductor nanocrystals corresponding to visible or near-infrared fluorescence contain Cd, Hg and Pb. Due to their toxicity, those heavy metals are becoming difficult to use for commercial applications. Possible alternative materials with a tunable emission in the visible range include III–V-type semiconductors like InP being less toxic but more challenging to produce, or ternary or quaternary systems like IB–IIIA–VIA (IB = Cu, Ag; IIIA = In, Ga, Al; VIA = S, Se, Te) [11], [12], [13], [14], [15], [16], [17].
Among them, CuInS2 gained the largest interest recently due to its tenability of its efficient fluorescence and other applications. With a bulk direct bandgap of 1.5 eV (827 nm) [18], CuInS2 nanocrystals could realize the emission from the visible to near-infrared (NIR) [14]. Different from binary nanocrystals like CdSe, no clearly distinguishable excitonic peak is visible in the UV–vis spectra and their photoluminescence spectra shows broad emission peak (FWHM: 100–150 nm), long life-time (hundreds of ns) and a significant Stokes shift (0.5–0.6 eV) [4]. The PL properties can be explained by the donor–acceptor pair (DAP) recombination [19]. Since donor–acceptor defects like vacancies or anti-site defects of the cations are generally more likely due to deviation from the ideal stoichiometry, the real stoichiometry and crystal structure strongly influence the PL mechanism [20], [21], [22], [23], [24], [25]. ZnS, an appropriate wide band gap material, can be used for an inorganic shell that is necessary for almost all types of quantum dots in order to achieve and maintain a high quantum yield (QY). ZnS can efficiently passivate the surface traps, preventing largely a leakage of the created charge carriers and therefore their non-radiative recombination. Moreover, other interpretations like Zn incorporation, gradient shell and original CuInS2 core size reduction are also suggested for improved QY by ZnS shell coating [21]. After two decades’ studies, the QY of CuInS2 in the recent reports has been as high as 80% [13], [20], [23], [26], [27]. Until now, different synthesis routes have been conducted, such as most commonly used one-pot thermolysis [12], [20], [22], [26], solvothermal method [9], [28], [29], single source precursor routes [30], [31], [32] and hot injection [33]. However, most of these synthesis routes are based on the batch method, which inevitably meets the difficulties of heat and mass transfer so that to face challenges of large scale production and quality control of different batches. Recently, Lee and Han developed a hybrid flow reactor combining a batch-type mixer and a flow type furnace to synthesize CuInS2/ZnS. With ZnS shell growth, the PL QY strongly increases from 20% to 60% [34]. However, also their approach is hard to scale-up since the reactions are performed in flasks.
In recent years, microreaction technology (MRT) intrinsically demonstrates advantages on super-efficient mass and heat transfer due to the small size of the channels and the sophisticated channel structure [42]. This is essential in the nanocrystal synthesis since good mixing and efficient heat transfer allow fast temperature change, resulting in homogenous nanocrystals and narrow size distribution. MRT also offers highly efficient quality control and small amount of online volume which is much safer compared to batch processes. Moreover, it gives the flexibility to adapt processes to dynamic market demands by modular plant configurations and quickly realizes large-scale production by numbering-up strategy instead of geometrical scaling-up. The development of the MRT supported synthesis of QDs was set on Cd based systems [53], [56]. With this technology scaling-up of nanoparticle production can be done economically to enable high performance quantum dot based applications at reasonable costs. Lately, the technology has been also transferred to the QD-free systems. Kwon et al. [55] developed in situ synthesis of ZnSe/ZnS core–shell in a microfluidic system with a through-flow in the range of μl/min. Xie et al. [57] synthesized InP nanocrystals in two types of microfluidic reactors, i.e. in a silicon based chip reactor and in tube reactor. Kim et al. introduced a continuous synthetic method in a micro-tubular reactor for synthesizing CuInSe2 nanocrystals inks with potential scalability [35].
In the present study a facile continuous route based on MRT is developed to synthesize CuInS2 and CuInS2/ZnS nanocrystals for large scale production of highly fluorescing semiconducting nanocrystals which might be applied to other ternary or quaternary systems like IB–IIIA–VIA as well. The broad research project included studies aiming at development of recipe yielding QDs with high photoluminescence and stability. Various aspects of the optimization of recipes and mode of operation are described elsewhere [43], [44], [45], [46], [47], [48], [49]. For example procedure for green synthesis of CuInS2/ZnS led to nanocrystals with 80% quantum yield [50], [61]. Our technology utilized experience from the process that we developed for the Cd-based QDs [41]. The project reported in this paper aimed at development of the continuous synthesis process technology and rules for transferring the recipes from batch to continuous manufacturing of QDs. The approach applied in this study differs from the usually applied and reported in publications. This refers to the type of the equipment, i.e. scalable, commercially available, microstructured modules that have been used. Their capacity is significantly higher compared to the usually applied microfluidic devices. Furthermore, the state of the art for continuous syntheses is based on the segmented flow technique. In our approach the active radial mixing and, in turn, the plug flow characteristic has been achieved by applying microreactors equipped with the static mixing elements. Our studies aimed at verification if with this type of equipment highly luminescent nanocrystals can be produced.
Section snippets
Precipitation unit
For the continuous synthesis of QDs an experimental unit that bases the commercially available micro-modules of the Ehrfeld microreaction (EMB) system has been constructed (Fig. 1). The unit consists of the supply of precursors, a two stage synthesis sequence that is necessary for making core–shell nanocrystals and post-reaction treatment. Precursors were fed by means of high performance liquid chromatography (HPLC) pump with the variable flowrate in the range of the flowrate range is 1–100
Precursor selection
Continuous synthesis by means of microreaction technology implies that the precursors have to be fed into the microreactor as liquid at ambient temperature. Therefore, OA, OLA, TOP were selected as solvents. A mixture of Zn-precursor in OLA and TOP yielded clear solution that was stable at room temperature even overnight. Initially, CuAc was used as precursor. However, it was very active and easy to be oxidized into Cu(Ac)2. Accordingly, it should be stored in the glove box. The sensitivity of
Conclusions
In the reported studies, recipies for synthesis of CuInS2/ZnS core/shell nanocrystals have been successfully translated into the continuous flow synthesis applying scalable, commercially available MRT equipment. The capacity of this equipment is significantly larger than the throughputs reported in the open literature. Stable operation and very good reproducibility of the results have been achieved. QYs that amounted to ca. 50% for the CuInS2/ZnS core/shell nanocrystals were on the same level
Acknowledgement
We gratefully acknowledge Susan Tian and Shaoqiang Tang for their contribution to the experimental work.
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