Article pubs.acs.org/crystal
In Situ Formation of Reactive Sulfide Precursors in the One-Pot, Multigram Synthesis of Cu2ZnSnS4 Nanocrystals Anthony S. R. Chesman,*,† Joel van Embden,† Noel W. Duffy,† Nathan A. S. Webster,‡,# and Jacek J. Jasieniak*,† †
CSIRO Materials Science and Engineering, Bayview Avenue, Clayton, Victoria 3168, Australia CSIRO Process Science and Engineering, Bayview Avenue, Clayton, Victoria 3168, Australia # Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, NSW 2234, Australia ‡
S Supporting Information *
ABSTRACT: Herein we outline a general one-pot method to produce large quantities of compositionally tunable, kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) through the decomposition of in situ generated metal sulfide precursors. This method uses air stable precursors and should be applicable to the synthesis of a range of metal sulfides. We examine the formation of the ligands, precursors, and particles in turn. Direct reaction of CS2 with the aliphatic primary amines and thiols that already constitute the reaction mixture is used to produce ligands in situ. Through the use of 1H and 13C nuclear magnetic resonance, Fourier transform infrared spectroscopy, and optical absorption spectroscopy, we elucidate the formation of the resulting oleyldithiocarbamate and dodecyltrithiocarbonate ligands. The decomposition of their corresponding metal complexes at temperatures of ∼100 °C yields nuclei with a size of 1−2 nm, with further growth facilitated by the decomposition of dodecanethiol. In this way the nucleation and growth stages of the reaction are decoupled, allowing for the generation of NCs at high concentrations. Using in situ X-ray diffraction, we monitor the evolution of our reactions, thus enabling a real-time glimpse into the formation of Cu2ZnSnS4 NCs. For completeness, the surface chemistry and the electronic structure of the resulting CZTS NCs are studied.
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INTRODUCTION
Although these disadvantages are problematic, they are not inherent to these systems and may be surmounted through the development of more convenient in situ synthetic methods to generate the metal complexes, which are designed to thermally decompose without generating detrimental coproducts. The addition of carbon disulfide to a solution of either a neat primary amine or alkyldiamine at room temperature results in the rapid formation of an alkylammonium alkyldithiocarbamate salt14 or zwitterionic alkyldithiocarbamatoammonium species,15,16 respectively. Over 70 years ago, this reaction pathway was utilized for the synthesis of cetyldithiocarbamate,17 demonstrating its applicability for generating long chain aliphatic dithiocarbamates. This method has been since been employed for the in situ formation of alkyldithiocarbamate ligands for passivating both gold films and gold nanoparticles.14a,18 Despite the success achieved using this facile route for alkyldithiocarbamate formation, to the best of our knowledge it has yet to be used for the in situ formation of metal complexes that can act as metal sulfide precursors for nanocrystal synthesis. The applicability of this approach to the synthesis of a wide variety of metal sulfide NCs is evident; however, in this work
Metal complexes incorporating ligands with dithiocarbamate (NCS2−) or xanthate (OCS2−) functional groups are becoming widely adopted as precursors for the synthesis of metal chalcogenide nanocrystals (NCs).1 This has arisen because unlike other highly reactive sulfide sources, such as hydrogen sulfide (H2S) and bis(trimethylsilyl)sulfide [(TMS)2S], dithiocarbamate and xanthate ligands are nonvolatile, as well as moisture, air, and light stable under ambient conditions.2 The facile thermal decomposition of metal dithiocarbamate and xanthate complexes has been exploited for the synthesis of a multitude of metal sulfides, including ZnS,3 CdS,1b,2d Sb2S3,4 NiS,5 Bi2S3,6 PbS,7 PdS,8 EuS,9 HgS,10 CuInS2,11 and CuInxGa(1−x)S2.12 However, there are drawbacks to their use that may preclude their employment in commercial scale, solution based, syntheses. First, their use introduces additional steps into the synthetic procedure. The alkali metal salt of the ligand must be synthesized and subsequently complexed with a transition or lanthanoid metal. The resultant product must then be isolated and purified prior to use in metal sulfide syntheses. Second, the thermal decomposition of short alkyl chain functionalized dithiocarbamate and xanthate ligands during metal sulfide formation generates a range of low boiling point coproducts,3b,11,13 which cause problems such as “bumping” of large scale reaction solutions at elevated temperatures. Published XXXX by the American Chemical Society
Received: January 6, 2013 Revised: February 4, 2013
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dx.doi.org/10.1021/cg4000268 | Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Importantly, in addition to being highly reactive, the thermal decomposition of the dithiocarbamate ligand proceeds without the generation of any unwanted volatile coproducts. In this report we present a general single step route to the synthesis of large quantities of metal sulfide NCs, with a specific focus on CZTS. The reaction media used are all comprised of cheap and readily available chemicals. It is important to note here that the precursors themselves and their chemistries are both air and moisture insensitive. We explore the formation of the sulfide precursors as well as the growth and nucleation stages of the reaction in turn. Finally, we investigate the effect of using various dithiocarbamate ligand concentrations on the physical properties of the synthesized CZTS NCs.
we selectively demonstrate its viability by focusing on the quaternary chalcogenide semiconductor Cu2ZnSnS4 (CZTS). Our motivation to study this system stems from its promise to be the material of choice for next generation thin film solar cells.19 CZTS contains only earth abundant and nontoxic elements, has a band gap of 1.4−1.5 eV, and a high optical absorption coefficient of ∼104 to 105 cm−1, which makes CZTS ideal for inclusion in thin-film solar cells as a photon absorbing layer.20 The synthesis of CZTS, and the fabrication of CZTS thin-films, have been achieved using a variety of techniques, including spray pyrolysis;21 thermal evaporation;22 sulfurization of stacked Cu/Zn/Sn metallic layers; 23 chemical bath deposition;24 spin-coating, dip-casting and doctor blading of preprepared nanocrystals;25 and decomposition of solution processed molecular precursors.26 After deposition, CZTS thinfilms are typically selenized at elevated temperatures to give Cu2ZnSnSxSe(4−x) (CZTSSe), which results in higher efficiency PV devices. An approach reported by Mitzi et al., in which metal chalcogenides are deposited from hydrazine as a slurry,27 has given record efficiencies for a CZTSSe device of 11.1%,28 although the use of highly toxic and flammable hydrazine is a deterrent for the adoption of this technique by industry. A similar route, in which a slurry of Cu2S, Zn, Sn, and S in ethanol were ball milled to give a composite ink, was used to fabricate PV devices with an efficiency of 5.14%.29 Guo et al. used the solution deposition of CZTS NCs, followed by annealing and selenization, to give a 7.2% efficiency solar cell,25a while doctor bladed CZTGeS NCs (Ge/(Sn + Ge) = 0.25) were used in the fabrication of PV devices with efficiencies of up to 8.4%.30 To date, the majority of solution based methods used to synthesize CZTS NCs employ a “hot-injection” technique that uses elemental sulfur, thiolate, dithiocarbamate, or xanthate ligands, as sulfide sources.31 The uncontrolled nucleation rates associated with prolonged injection times prohibit the scale-up of “hot-injection” based techniques to commercial levels. This limitation can be avoided by the use of a “heat-up” method.32 Such “heat-up” methods are an excellent means to generate large quantities of nanocrystals. However, they do require specific control over both the ligand and precursor chemistries in order to obtain the necessary balance between the nucleation and growth rates needed to afford high quality NCs. Furthermore, the decomposition products of the precursors themselves, which are often overlooked, must be taken into account to ensure that they are not detrimental to the desired properties of the NCs or the scalability of the reaction. These criteria must be met before a NC synthesis may be considered truly scalable. In our unique approach to the one-pot, multigram synthesis of CZTS NCs, two sulfide sources of different reactivities were employed to decouple the nucleation and growth stages of the reaction.33 The thermal decomposition of an O-ethyl xanthate metal complex at temperatures below 100 °C initiated nucleation, with growth of the CZTS NCs facilitated by the decomposition of an alkylthiol at elevated temperatures. However, thermal decomposition of the ligand resulted in the formation of ethanol as the primary coproduct, necessitating either the use of an extended degassing period or the appending of a distillation apparatus to the reaction vessel to avoid “bumping” of the reaction solution, which can cause irreversible aggregation of the NCs. In order to overcome this and simplify the reaction method, we have adopted an alternative approach that involves the in situ formation of a long chained alkyldithiocarbamate anion, which acts as a sulfide source.
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EXPERIMENTAL SECTION
Materials. CuI (99%), dodecanethiol (98%) (DDT), technical grade oleylamine (70%) (OLA), SnCl4·5H2O (98%), and ZnCl2 (99%) were purchased from Aldrich. Acetone (99.8%), chloroform (99.8%), CS2 (99.9%), ethanol (99.5%), and methanol (99.8%) were purchased from Merck. All reagents and solvents were used as received without further purification. CZTS Nanocrystal Synthesis. CS2 (1.2 mL, 1.52 g, 20.0 mmol) was added dropwise to a OLA/DDT (3:1 v/v) (64 mL) solution with stirring, resulting in an immediate color change from cloudy white to orange. CAUTION: As the formation of the oleyldithiocarbamate (ODC) and dodecyltrithiocarbonate (DTC) ligands is an exothermic reaction, caution must be taken to add the CS2 at a slow rate to ensure the temperature of the reaction solution does not exceed the boiling point of CS2. The reaction solution was stirred under nitrogen for 10 min, after which CuI (2.66 g, 14.0 mmol), SnCl4·5H2O (2.80 g, 8.0 mmol), and ZnCl2 (1.36 g, 10.0 mmol) were added as powders, resulting in a mustard colored suspension. The reaction solution was placed under a vacuum (ca. 2.0 × 10−1 mbar) to degas and remove residual water, and then heated over a 10 min period to 100 °C. During this heating phase at approximately 70 °C, the yellow/orange suspension dissolved to give a deep red solution. After degassing at 100 °C for 30 min, the reaction solution was placed under an atmosphere of nitrogen and heated to 250 °C over a 10 min period, with the temperature then maintained for a further 30 min. After cooling to 60 °C over a 30 min period, CHCl3 (30 mL), acetone (50 mL), and MeOH (50 mL) were added to the reaction solution, which was centrifuged to separate out CZTS NCs. The colorless supernatant was discarded, and the precipitate was redissolved in CHCl3 (20 mL) to give a deep brown solution, which was centrifuged to precipitate any NC aggregates (
