Growth Mechanism and Surface State of CuInS2 Nanocrystals

Article Cite This: J. Am. Chem. Soc. 2017, 139, 15748-15759

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Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol Marina Gromova,† Aurélie Lefrançois,‡ Louis Vaure,‡ Fabio Agnese,† Dmitry Aldakov,‡ Axel Maurice,‡ David Djurado,‡ Colette Lebrun,‡ Arnaud de Geyer,† Tobias U. Schülli,§ Stéphanie Pouget,† and Peter Reiss*,‡ †

Université Grenoble Alpes, CEA, INAC, MEM, Grenoble 38000, France Université Grenoble Alpes, CEA, CNRS, INAC, UMR5819 SyMMES, Grenoble 38000, France § The European Synchrotron ESRF, BP 220, Grenoble 38043 Cedex 9, France ‡

S Supporting Information *

ABSTRACT: Ternary metal chalcogenide nanocrystals (NCs) offer exciting opportunities as novel materials to be explored on the nanoscale showing optoelectronic properties tunable with size and composition. CuInS2 (CIS) NCs are the most widely studied representatives of this family as they can be easily prepared with good size control and in high yield by reacting the metal precursors (copper iodide and indium acetate) in dodecanethiol (DDT). Despite the widespread use of this synthesis method, both the reaction mechanism and the surface state of the obtained NCs remain elusive. Here, we perform in situ X-ray diffraction using synchrotron radiation to monitor the pre- and postnucleation stages of the formation of CIS NCs. SAXS measurements show that the reaction intermediate formed at 100 °C presents a periodic lamellar structure with a characteristic spacing of 34.9 Å. WAXS measurements performed after nucleation of the CIS NCs at 230 °C demonstrate that their growth kinetics depend on the degree of precursor conversion achieved in the initial stage at 100 °C. NC formation requires the cleavage of S−C bonds. We reveal by means of combined 1D and 2D proton and carbon NMR analyses that the generated dodecyl radicals lead to the formation of a new thioether species R−S−R. The latter is part of a ligand double layer, which consists of dynamically bound dodecanethiolate ligands as well as of head-to-tail bound R−S−R molecules. This ligand double layer and a high ligand density (3.6 DDT molecules per nm2) are at the origin of the apparent difficulty to functionalize the surface of CIS NCs obtained with the DDT method.

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INTRODUCTION Ternary metal chalcogenide nanocrystals M+M3+E2 (M+ = Cu, Ag; M3+ = In, Fe; E = S, Se) have drawn tremendous research interest in the past decade, triggered by the discovery of their appealing optical properties.1−8 Going from binary to ternary (or multinary) systems opened many novel perspectives enabled by the large scope of materials to be explored in the form of colloidal NCs for the first time. In these compounds, the optical and electronic properties can be tuned not only with the size and surface chemistry of the NCs but also by changing their composition.9 Of considerable importance is also the possibility to develop materials showing visible or near-infrared absorption/emission without relying on the use of toxic heavy metals such as Cd, Pb, or Hg.10,11 Because of these features, ternary and multinary metal chalcogenide NCs are extensively investigated as active materials in many types of applications ranging from display and lighting over biological imaging to photovoltaics, photocatalysis, and thermoelectrics, to name just the most important ones.9,12−14 Copper indium disulfide (CIS) NCs are the most studied type of this class of materials and have been used either directly or in the form of their alloys with Zn and/or Se in these applications.15 On the fundamental © 2017 American Chemical Society

research side, many efforts have been made to understand the origin of the photoluminescence in these systems, characterized by a broad emission peak and large Stokes shift.16 Only very recently could it be shown by means of single-particle spectroscopy that the line width of individual NCs can be much narrower than that of ensembles (∼130 meV vs ∼350 meV).17 Importantly, the particle-to-particle variation of the PL energy, giving rise to the broad ensemble line width, has been attributed to the random positioning of the emitting center (copper-related defect) within the NCs. This indicates that there is in principle no physical limit inhibiting the formation of CIS-based NCs of similar optical quality as established II−VI QDs, provided that chemical synthesis methods are capable of delivering precisely controlled particles. On the other hand, the understanding of the chemical reactions underlying the formation of CIS NCs and the investigation of their surface state is much less advanced than the optical studies. Improving this knowledge is the goal of this Article. Received: July 19, 2017 Published: October 10, 2017 15748

DOI: 10.1021/jacs.7b07401 J. Am. Chem. Soc. 2017, 139, 15748−15759

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Journal of the American Chemical Society

Figure 1. In situ synchrotron WAXS measurements as a function of the heating time at 230 °C. For comparison, the diffraction pattern of chalcopyrite (JCPDS: 04-002-6388) CIS is indicated as red diamonds. Duration of the preheating phase of the precursor solution at 100 °C: 30 min (A), 60 min (B).

dodecanethiolate ligands, which themselves interact strongly with a second layer of thioether (didodecyl sulfide) molecules, formed in the course of the reaction. Although this leads to a dense organic coverage of the surface of the NCs, the microscopic binding mode of the dodecanethiolate ligands is dynamic; their desorption/adsorption processes are accompanied by their protonation/deprotonation.

The most widely used synthesis method for CIS NCs consists of the reaction of the metal precursors (e.g., copper iodide, indium acetate) with 1-dodecanethiol (DDT).4,6,18 In this reaction, DDT is used either alone or in a mixture with 1octadecene (ODE) and has the triple role of sulfur source, surface ligand, and solvent. The synthesis is generally conducted in two phases, first at around 100 °C (“complexation”) and then around 230 °C (“nucleation and growth”). This method has many advantageous features due to its simple implementation (heat-up approach),19,20 high reproducibility and reaction yield, as well as facile scalability. Moreover, the obtained CIS NCs show size-dependent optical properties, simply adjustable with reaction time. Nonetheless, many aspects related to the reaction mechanism and surface state of the NCs remain elusive to date. Among the open questions, the nature of the intermediate product formed during the complexation stage at lower temperature is of crucial importance. This reaction intermediate plays a central role in controlling the reaction kinetics as well as the stoichiometry, crystalline phase, and shape of the subsequently formed NCs.13 Concerning the surface state, many studies showed that it is very challenging to replace quantitatively the surface ligands of the as-prepared, supposedly DDT-capped CIS NCs.5,21−24 This is a potential drawback in comparison with DDT-free synthesis methods,23 as ligand exchange is mandatory for many applications. Examples are the aqueous phase transfer of the NCs for biological studies4,25 and the replacement of bulky DDT ligands with shorter ones to improve the electronic coupling between particles or between particles and electrodes in solar cells.26,27 In this Article, we are attempting to give a full picture of the reaction mechanism and surface state of CIS NCs synthesized with the DDT method. In the first part, we are using synchrotron X-ray diffraction for following in situ the pre- and postnucleation stages of the synthesis. In complementary laboratory studies, we identify the reaction intermediate, which shows a 2D polymeric structure forming a lamellar phase. This intermediate contains a significant amount of iodine for charge compensation. Interestingly, also the obtained CIS NCs, regardless of their size, contain 20−30% iodine with respect to indium. In the second part, we are focusing on the NC surface state using 1D and 2D proton and carbon NMR. We demonstrate that the surface is capped by a first layer of

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RESULTS AND DISCUSSION Synchrotron X-ray In Situ Study of the Nucleation and Growth Mechanism. The present system is particularly well suited for in situ X-ray studies using synchrotron radiation. First, the synthesis is of heat-up type; that is, all precursors can be mixed and then heated to the growth temperature (230 °C) within a thin glass capillary (Lindemann tube of 1 mm inner diameter) placed into the X-ray beam (cf., Figure S1). To have a homogeneous mixture, the metal salts (copper(I) iodide and indium(III) acetate) are mixed with DDT and heated to 100 °C for 30−60 min prior to their transfer into the capillary. Second, the studied reaction uses a comparably high concentration of precursors, which results in a strong signal despite the small sample volume probed. During the in situ measurements, a large q range was accessible by varying the distance of the 2D detector and working at high energy (λ = 0.774 Å). Wide-angle X-ray scattering (WAXS) measurements did not show any exploitable signals for heating times 99.9%), indium(III) acetate (>99.9%), dodecanethiol (99%), and anhydrous solvents were purchased from Sigma-Aldrich and used as received. The synthesis is based on the procedure reported in ref 18. CuInS2 NCs were synthesized by reacting copper(I) iodide (190 mg, 1 mmol) with indium(III) acetate (292 mg, 1 mmol) in dodecanethiol (5 mL) using a 50 mL three-neck flask equipped with a condenser. The reaction mixture was first degassed for 15 min under primary vacuum using a Schlenk line. Under argon atmosphere, the temperature was then raised to 100 °C. The mixture was stirred for 30 min (“sample A”) or 1 h (“sample B”) before degassing once again for 1 min and backfilling with argon. In both cases, a yellow, transparent solution is obtained. Stirring was continued, and the temperature was raised to 230 °C at 30 °C/min, resulting in a color change from yellow over orange and red to dark brown. The heating time can be varied (typically in a range of 10 min to 1 h) to reach the desired size. After cooling to room temperature, the NCs were purified by adding 5 mL of methanol and centrifugation at 5000 rpm for 5 min. A second cycle or purification was carried out by adding 10 mL of chloroform to the precipitate, followed by 5 mL of methanol and centrifugation as before. The precipitate was finally solubilized in 10 mL of chloroform or toluene. Samples used for NMR analyses were precipitated once again with 5 mL of methanol and separated from the supernatant by centrifugation. The precipitate was thoroughly dried under Ar flow and redispersed in deuterated solvent (CDCl3 or toluene-d8). For the in situ synthesis, the first steps including the heating at 100 °C were carried out identically in the lab using a Schlenk line. The obtained precursor solutions were transferred to a glovebox and filled into Lindemann tubes. Gentle heating was required to decrease the solution viscosity, and the tubes were flame-sealed after taking them out of the glovebox. EDX. EDX spectra were recorded on a ZEISS Ultra 55+ electron microscope equipped with an EDX probe (acceleration tension, 20 keV; distance sample/electron source, 7 mm). For sample preparation, a concentrated colloidal solution of CIS NCs in chloroform or hexane is drop-cast on the surface of a silicon substrate, which has been cleaned and sonicated in acetone and ethanol prior to the deposition. After evaporation of the solvent, the substrate is rinsed with methanol. XPS. X-ray photoelectron spectroscopy (XPS) analyses were carried out with a Versa Probe II spectrometer (ULVAC-PHI) equipped with a monochromated Al Kα source (hν = 1486.6 eV). The core level peaks were recorded with a constant pass energy of 23.3 eV. The XPS spectra were fitted with CasaXPS 2.3.15 software using Shirley background and a combination of Gaussian (70%) and Lorentzian (30%) distributions. Binding energies are referenced with respect to adventitious carbon (C1s BE = 284.6 eV). Electron Microscopy. An aberration corrected 200 kV FEI Themis (S)TEM microscope was used for fast EDX spectrum imaging (SI) and simultaneous HAADF STEM imaging. This system offers a high brightness electron gun, four symmetrically arranged windowless silicon drift detectors (SDD) with large solid angle (Ω = 0.9 sr), and improved tilt response. SI is carried out by progressive scanning of individual frames to reduce beam damage, followed by summing multiple frames; each frame is obtained with a dwell time of 100 μs. The 256 × 256 pixel SI was acquired for 15 min using Bruker Esprit software. Elemental maps were obtained using a beam current of 1 nA and an accelerating voltage of 200 kV. The semiconvergence angle (α) of the probe was 21 mrad.

I = exp( −(γHgδ)2 D(Δ − δ /3 − τ /2)) I0 where I is the echo intensity at g and I0 is the echo intensity extrapolated to zero gradient, γH is the hydrogen gyromagnetic ratio, D is the self-diffusion coefficient of the considered species, δ and Δ are the gradient pulse duration and delay during which the diffusion is observed, and finally τ is the time interval between the bipolar gradient pulses. The experiments were done with δ/2 ranging between 0.3 and 1.5 ms, Δ varying from 20 to 1000 ms, and τ was equal to 0.2 ms. The recycle delay was adjusted to 4 s. The diffusion measurements were performed without temperature regulation to minimize convection phenomena into the sample. For precise measurements of D, the sample was analyzed in a capillary tube (OD: 2 mm, ID: 1.2 mm). All spectra were processed with the Bruker Topspin software package. To estimate the hydrodynamic diameter dH from the diffusion coefficient D, we used the Stokes−Einstein formula: dH =

kBT 3πηD

where kB is the Boltzmann constant, and taking η = 0.57 × 10−3 kg s−1 m−1 for the value of the dynamic viscosity of chloroform at 293 K.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07401. 15757

DOI: 10.1021/jacs.7b07401 J. Am. Chem. Soc. 2017, 139, 15748−15759

Article

Journal of the American Chemical Society

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Experimental setup for the synchrotron measurements, mass spectrometry data, TGA data, XPS data, and additional SAXS and NMR data (PDF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Colette Lebrun: 0000-0001-7908-2202 Peter Reiss: 0000-0002-9563-238X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the French National Research Agency ANR for financial support (Grants NIRA ANR-13-BS08-011-03, QuePhelec ANR-13-BS10-0011-01, and NEUTRINOS ANR16-CE09-0015-03). We further gratefully acknowledge the Labex ARCANE (Grant QDPhotocat) as well as CAPES/ COFECUB (Grant no. 858/15). We thank Dr. Tugce Akdas for help with TGA analysis. This Article is dedicated to Prof. Dieter Fenske on the occasion of his 75th birthday.

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DOI: 10.1021/jacs.7b07401 J. Am. Chem. Soc. 2017, 139, 15748−15759

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Journal of the American Chemical Society (51) Garcia-Cardona, E.; Wong, E. H.; Barkey, D. P. J. Electrochem. Soc. 2011, 158, D143−D148.

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DOI: 10.1021/jacs.7b07401 J. Am. Chem. Soc. 2017, 139, 15748−15759