Synthesis of Wurtzite Cu 2 ZnGeSe 4 Nanocrystals and their Thermoelectric Properties

Anion exchange induced formation of kesterite copper zinc tin sulphide-copper zinc tin selenide nanoheterostructures

We report the colloidal synthesis of quaternary kesterite CZTS-CZTSe heterostructures via anion exchange reactions on a kesterite CZTS template. The crystal phase selectivity during the synthesis (kesterite vs. wurtzite) is due to the initial nucleation of cubic Cu9S5 seeds, followed by incorporation of Zn and Sn. Upon injection of Se-precursor, which triggered simultaneous anion exchange and overgrowth of the pristine CZTS template, sandwich CZTS-CZTSe (core-tip) nanoheterostructures were obtained. X-ray photoelectron spectroscopy (XPS) and optical band gap measurement results suggest a change of intrinsic electronic structure of CZTS by Se-treatment. Our study not only provides insight into mechanisms of formation of kesterite CZTS nanocrystals (NCs) and subsequent anion exchange reactions, but also opens doors to access novel CZTSSe nanostructures for potential applications.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Graphene boosted Cu2GeS3 for advanced lithium-ion batteries

Ternary Cu₂GeS₃ (CGS) serves as lithium ion battery anode materials for the first time, whose electrochemical performance is significantly improved by the introduction of reduced graphene oxide.

Multinary I-III-VI2 and I2-II-IV-VI4 Semiconductor Nanostructures for Photocatalytic Applications

Semiconductor nanostructures that can effectively serve as light-responsive photocatalysts have been of considerable interest over the past decade. This is because their use in light-induced photocatalysis can potentially address some of the most serious environmental and energy-related concerns facing the world today. One important application is photocatalytic hydrogen production from water under solar radiation. It is regarded as a clean and sustainable approach to hydrogen fuel generation because it makes use of renewable resources (i.e., sunlight and water), does not involve fossil fuel consumption, and does not result in environmental pollution or greenhouse gas emission. Another notable application is the photocatalytic degradation of nonbiodegradable dyes, which offers an effective way of ridding industrial wastewater of toxic organic pollutants prior to its release into the environment. Metal oxide semiconductors (e.g., TiO2) are the most widely studied class of semiconductor photocatalysts. Their nanostructured forms have been reported to efficiently generate hydrogen from water and effectively degrade organic dyes under ultraviolet-light irradiation. However, the wide band gap characteristic of most metal oxides precludes absorption of light in the visible region, which makes up a considerable portion of the solar radiation spectrum. Meanwhile, nanostructures of cadmium chalcogenide semiconductors (e.g., CdS), with their relatively narrow band gap that can be easily adjusted through size control and alloying, have displayed immense potential as visible-light-responsive photocatalysts, but the intrinsic toxicity of cadmium poses potential risks to human health and the environment. In developing new nanostructured semiconductors for light-driven photocatalysis, it is important to choose a semiconducting material that has a high absorption coefficient over a wide spectral range and is safe for use in real-world settings. Among the most promising candidates are the multinary chalcogenide semiconductors (MCSs), which include the ternary I-III-VI2 semiconductors (e.g., AgGaS2, CuInS2, and CuInSe2) and the quaternary I2-II-IV-VI4 semiconductors (e.g., Cu2ZnGeS4, Cu2ZnSnS4, and Ag2ZnSnS4). These inorganic compounds consist of environmentally benign elemental components, exhibit excellent light-harvesting properties, and possess band gap energies that are well-suited for solar photon absorption. Moreover, the band structures of these materials can be conveniently modified through alloying to boost their ability to harvest visible photons. In this Account, we provide a summary of recent research on the use of ternary I-III-VI2 and quaternary I2-II-IV-VI4 semiconductor nanostructures for light-induced photocatalytic applications, with focus on hydrogen production and organic dye degradation. We include a review of the solution-based methods that have been employed to prepare multinary chalcogenide semiconductor nanostructures of varying compositions, sizes, shapes, and crystal structures, which are factors that are known to have significant influence on the photocatalytic activity of semiconductor photocatalysts. The enhancement of photocatalytic performance through creation of hybrid nanoscale architectures is also presented. Lastly, views on the current challenges and future directions are discussed in the concluding section.

Colloidal synthesis of wurtz-stannite Cu2CdGeS4nanocrystals with high catalytic activity toward iodine redox couples in dye-sensitized solar cells

Wurtz-stannite Cu₂CdGeS₄nanocrystals were synthesizedviaa facile hot-injection method at a low temperature.

Thermoelectric Properties of Solution Synthesized Nanostructured Materials

Thermoelectric nanocomposites made by solution synthesis and compression of nanostructured chalcogenides could potentially be low-cost, scalable alternatives to traditional solid-state synthesized materials. We review the progress in this field by comparing the power factor and/or the thermoelectric figure of merit, ZT, of four classes of materials: (Bi,Sb)2(Te,Se)3, PbTe, ternary and quaternary copper chalcogenides, and silver chalcogenides. We also discuss the thermal conductivity reduction associated with multiphased nanocomposites. The ZT of the best solution synthesized materials are, in several cases, shown to be equal to or greater than the corresponding bulk materials despite the generally reduced mobility associated with solution synthesized nanocomposites. For the solution synthesized materials with the highest performance, the synthesis and processing conditions are summarized to provide guidance for future work.

Heterovalent substitution to Enrich electrical conductivity in Cu2CdSn(1-x)GaxSe4 series for high thermoelectric performances

Serials of Ga doping on Sn sites as heterovalent substitution in Cu2CdSnSe4 are prepared by the melting method and the spark plasma sintering (SPS) technique to form Cu2CdSn(1-x)GaxSe4 (x = 0, 0.025, 0.05, 0.075, 0.01, and 0.125). Massive atomic vacancies are found at x = 0.10 by the heterovalent substitution, which contributes significantly to the increase of electrical conductivity and the decrease of lattice thermal conductivity. The electrical conductivity is increased by about ten times at 300 K after Ga doping. Moreover, the seebeck coefficient only decreases slightly from 310 to 226 μV/K at 723 K, and a significant increase of the power factor is obtained. As a result, a maximum value of 0.27 for the figure of merit (ZT) is obtained at x = 0.10 and at 723 K. Through an ab initio study of the Ga doping effect, we find that the Fermi level of Cu2CdSnSe4 is shifted downward to the valence band, thus improving the hole concentration and enhancing the electrical conductivity at low doping levels. Our experimental and theoretical studies show that a moderate Ga doping on Sn sites is an effective method to improve the thermoelectric performance of Cu2CdSnSe4.

Colloidal nanocrystals of orthorhombic Cu2ZnGeS4: phase-controlled synthesis, formation mechanism and photocatalytic behavior

The orthorhombic polymorph of Cu2ZnGeS4 (CZGS) is a metastable wurtzite-derived phase that can only be prepared in the bulk form by extensive heating at high temperatures (≥790 °C) when using the conventional solid-state reaction route. By employing a facile solution-based synthetic strategy, we were able to obtain phase-pure orthorhombic CZGS in nanocrystalline form at a much lower reaction temperature. Prior to this work, the colloidal synthesis of single-phase orthorhombic CZGS on the nanoscale has never been reported. We find that the use of an appropriate combination of coordinating solvents and precursors is crucial to the sole formation of this metastable phase in solution. A possible formation mechanism is proposed on the basis of our experimental results. Because CZGS consists of environmentally benign metal components, it is regarded as a promising alternative material to the technologically useful yet toxic cadmium-containing semiconductors. The orthorhombic CZGS nanocrystals display strong photon absorption in the visible spectrum and are photocatalytically active in dye degradation under visible-light illumination.

Synthesis of wurtzite-zincblende Cu2ZnSnS4 and Cu2ZnSnSe4 nanocrystals: insight into the structural selection of quaternary and ternary compounds influenced by binary nuclei

Nearly monodispersed wurtzite-dominant Cu2ZnSnS4 and zincblende-dominant Cu2ZnSnSe4 nanocrystals were successfully synthesized by mixing metal salts with heated thiourea or selenourea in oleylamine. A perspective of the structural relationship between quaternary and ternary semiconductors was investigated through the application of different anion sources to prepare Cu2SnS3 and Cu2SnSe3 nanocrystals. Investigations on copper-based binary compounds found that CuSe (or CuS) and Cu2Se (or Cu1.96S, Cu9S5) nuclei were primarily responsible for the formation of zincblende or wurtzite structures, respectively. Further management over these binary intermediates corresponded to slight structural transformations of the quaternary nanocrystals which could be observed not only in XRD patterns, but from optical and electrical properties as well. According to these results, Cu2ZnGeS4 nanocrystals with wurtzite-dominant structures were first reported using SC(NH2)2, which also verified that the binary semiconductors are the determinative factors.