Influence of Solvent Reducing Ability on Copper Sulfide Crystal Phase

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Influence of Solvent Reducing Ability on Copper Sulfide Crystal Phase Nathaniel J. Freymeyer, Patrick D. Cunningham, Evan C. Jones, Brandon J. Golden, Alex M. Wiltrout, and Katherine E. Plass* Department of Chemistry, Franklin & Marshall College, Lancaster, Pennsylvania 17604, United States S Supporting Information *

ABSTRACT: Copper sulfide particles across a wide range of stoichiometries are obtained depending on the ratio of cosolvents from which they are grown. Copper sulfides are abundant, low-cost materials with phase-dependent properties relevant to solar energy conversion and (opto)electronic devices. For this reaction, the reducing ability of dodecanethiol versus oleic acid affects the speciation of the precursors, as determined using UV−vis absorption spectroscopy. The ratio of dodecanethiol to oleic acid in the synthetic medium affects the solid-state structure and stoichiometry, as determined by powder X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and detailed investigation of the band edge positions and plasmon behavior using visible−NIR optical absorption spectroscopy and cyclic voltammetry. A range of phases was obtained, including monoclinic chalcocite, tetragonal chalcocite, digenite, and covellite. The thermodynamic relationships between these phases were elucidated using equilibration experiments, revealing methods for postsynthetic property alteration. Particle size and morphology were also affected by solvent ratio, as shown using scanning or transmission electron microscopy. Oleic acid accelerated particle growth and resulted in particles with an unusual faceted shape.

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INTRODUCTION Copper sulfides constitute a class of materials with numerous phases comprised of different stoichiometries and lattice structures.1,2 Nanoparticles of these materials have received renewed attention due to their photovoltaic,3,4 plasmonic,5−9 sensing,4 photocatalytic,10,11 thermoelectric,12 ion-storage,13,14 superionic,15 and supercapacitor16 behavior. Particularly promising is their use in cost-effective solar energy conversion.7 All of these properties, however, are dependent upon the stoichiometry and crystal phase. One phase, covellite (CuS), has been utilized for solar hydrogen production,17 while chalcocite (Cu2S) catalyzes electron transfer to graphene anodes in QDSSCs.18 Synthetic control over the copper sulfide phase is therefore crucial for use of these materials. Composition control of nanoparticles of copper sulfides has been observed by alteration of the Cu/S ratio,19 starting materials,20 and stabilizing ligand,21 as well as by treatment with iodine.22 Demonstration of control over a wide range of compositions affords deeper understanding of the conditions under which the various phases are stable, a critical factor for employing these materials in any long-lived device. Copper chalcogenides were among the first plasmonic semiconductors reported.5−9 The intensity and position of plasmon absorption peaks varies with phase8 and postsynthetic treatment,23,24 both of which alter the number of holes in the valence band. Pure monoclinic chalcocite (Cu2S) shows no plasmon absorbance but spontaneously transforms to plasmonic djurleite (Cu1.97S).19,25 Exposure to oxidants or © 2013 American Chemical Society

reductants can alter the intensity of plasmon absorbance by altering the number of copper-related vacancies in the valence band.24 The wavelength of the plasmon band was found to depend on the copper-deficiency, getting smaller for the phases digenite (Cu1.8S) and covellite (CuS).8 In adapting a published procedure,26 utilizing 1:2 v/v dodecanethiol:oleic acid solvent to form mixed metal sulfides, plasmonic tetragonal chalcocite was generated. This hightemperature bulk phase has rarely been observed in nanoparticle form.27 Investigation of this phenomenon uncovered the fact that a variety of different copper sulfides with varying compositions, phases, and optical properties are obtained by alteration of the dodecanethiol:oleic acid solvent ratio (Figure 1). Solvent identity, including the cosolvent ratio, is known to affect the resultant crystal phase of copper chalcogenides.28−31 The reason for this, however, has not been investigated via detailed experiment. The solvent can induce a myriad of thermodynamic and kinetic changes32 to alter the resultant nanoparticle phase. In a significant advance on previous reports, we perform tests of the relative thermodynamic stability of these phases in different solvents, relate the observed solution species to experimental and literature discussion33−37 of the oxidation states of copper and sulfur species, and use the Received: June 13, 2013 Revised: July 9, 2013 Published: July 18, 2013 4059

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Visible/Near-Infrared Absorption Spectroscopy (vis-NIR). Spectra were collected using a Perkin-Elmer Lambda 950 spectrometer featuring an integrating sphere with PMT and InGaAs detectors. For solid samples, absorbance in the range from 2500 to 350 nm was measured at 525.55 nm/min. Detector response settings were 0.40 s. The InGaAs detector gain was 12 with a servo-controlled slit width. The PMT detector used autogain and a 1.5 nm slit width. Solutions were drop cast onto glass slides, and a cover slide was placed on top to contain the dried powder. Solid samples were sandwiched between NIR transparent quartz plates (Spectrocell) and analyzed at the reflectance port of the integrating sphere. Prior to sample measurement, a background was collected using one glass plate in front of a Spectralon reference. For liquid samples, absorbance in the range of 250 to 700 nm was measured at 405.07 nm/min. Detector response setting was 0.40 s and the slit width was 1.5 nm. For each sample, unheated solvent (oleic acid, dodecanethiol, or a mixture thereof) was used as a reference. Cu(II)(acac)2 spectra were collected using 3 mL of a 1.5 mM solution in CH2Cl2 injected with 200 μL of specified solvent. Transmission Electron Microscopy (TEM). TEM samples were prepared by solvent-casting samples onto Cu-supported TEM grids with lacey carbon coating. The microscope employed was a JEOL JEM-1005X electron microscope. Scanning electron microscopy (SEM). Powder samples were immobilized on conductive carbon tape and coated with gold. A JEOL 7500F scanning electron microscope and Evex MiniSEM (20 kV accelerating voltage) were used for imaging. Energy Dispersive X-ray Spectroscopy (EDS). EDS was carried out using a JEOL 7500F scanning electron microscope equipped with an Oxford Inca Thin Window 102 mm EDS housed in the Penn Regional Nanotechnology facility. Powder samples were immobilized on conductive carbon tape. X-ray Photoelectron Spectroscopy (XPS). XPS was carried out using a PHI VersaProbe 5000 XPS/Auger spectrometer housed at Drexel University. Particle samples cast on Si wafers were analyzed under high vacuum with monochromatic Al Kα X-rays. A flood gun was used for charge compensation. Wide energy scans from 0 to 1100 eV binding energy were employed to identify elements on the surface, and high resolution spectra were captured for Cu 3p, O 2s, and S 2p regions. Cyclic voltammetry (CV). CV was carried out using a CH Instruments CHI604C Electrochemical Analyzer. Pt wire mesh (Alfa Aesar, 99.9%) was used as the counter electrode and a silver wire quasireference electrode was used (Alfa Aesar, 99.9985%). Ferrocene (50 nM, Alfa Aesar, 99%)/ferrocenium (ferrocenium hexafluorophosphate, 50 nM, Sigma Aldrich) was used for calibration. The measured ferrocene/ferrocenium standard reduction potential (EFc/Fc+) was subtracted from the applied potentials for plots. Nanoparticle solutions were cast onto 2 mm diameter Pt working electrodes. A custom H-cell with two 50 mL compartments was used. Acetonitrile (Sigma-Aldrich, anhydrous and treated with 0.3 μm molecular sieves) was stored over molecular sieves (3 Å, Sigma Aldrich). Tetrabutylammoniumhexafluorophosphate (TBAPF6, 0.1 M, Sigma Aldrich, >99%) was used as a supporting electrolyte.

Figure 1. Representation of the influence of solvent (dodecanethiol/ oleic acid ratio) on copper sulfide particle phase. The copper ions are color-coded in the different phases. The sulfur ions are shown as yellow spheres.

uncovered solvent affects to devise postsynthetic alteration of the plasmonic properties of chalcocite nanoparticles. Presented here is a study of the influence of solvent on the obtained phase (Figure 1), size, and morphology of copper sulfide nanostructures. The composition has been shown to affect both the band structure and plasmonic behavior. Cu(II)(acac)2 and elemental sulfur are combined in dodecanethiol, oleic acid, or mixtures thereof, forming monoclinic chalcocite, tetragonal chalcocite, digenite, or covellite nano- or microparticles. Phase selectivity is attributed to differing precursor speciation, as supported by UV−visible spectroscopy. The electronic properties of both chalcocite forms are compared by visible/NIR spectroscopy and cyclic voltammetry.

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EXPERIMENTAL SECTION

The solvents used include 1-dodecanthiol (≥98%) and oleic acid (technical grade, 90%), which were all purchased from Sigma-Aldrich, as well as hexane (ACS grade, 95%) and acetone (ACS grade, 99.9%), which where purchased from Pharmco-Aaper. 1-Dodecanethiol and oleic acid were used as received. Reactants used include elemental sulfur (powder, 99.98%), copper(II) acetylacetonate [Cu(II)(acac)2, ≥99.99% metals basis], CuCl2 (anhydrous, 97%), and CuBr2 (99%) from Sigma-Aldrich. Copper(II) acetate [Cu(II)(OAc)2, anhydrous] was obtained from Alfa-Aesar. Synthesis of Cu2−xS Nanoparticles. In a typical synthetic procedure, a solution was prepared by dissolving Cu(II)(acac)2 (262 mg, 1.0 mmol) and elemental sulfur (64 mg, 2.0 mmol) in a mixture of 1-dodecanethiol and oleic acid with variable ratios in a 2-neck, roundbottom flask equipped with a reflux condenser. The reaction flask was purged with Ar(g) for a minimum of 20 min before it was heated to 180 °C for a period of two hours. After the solution was cooled to room temperature, the precipitate was centrifuged at 6000 rpm for 10 min in a 50 mL plastic centrifuge tube. The supernatant was then decanted, and the precipitate resuspended in a hexane/acetone solution (1:1 v/v). Centrifugation, decanting of the supernatant, followed by resuspension was repeated. Powder X-ray diffraction (PXRD). PXRD samples were prepared by casting an aliquot of nanoparticles suspended in hexane onto glass slides and then covering each slide in Parafilm. PXRD experiments were carried out using a PANalytical X’Pert Pro X-ray diffractometer using Cu Kα radiation (λ = 1.54 Å). The detector was a 13 channel X’celerator with a Ni filter. The accelerating voltage and current were 45 kV and 40 mA. The irradiated length was 10 mm and an automated diffraction slit was employed. Scans were collected from 6 to 70° 2θ. Ten repetitions were summed. The samples were then analyzed using the program PANalytical X’Pert HighScore Plus (version 2.2e), which allowed comparisons with the ICDD powder X-ray diffraction pattern database (PDF Release 2).

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RESULTS

Demonstration of Solvent Effect on Phase. As demonstrated below, variation of a binary solvent mixture results in particles of four different copper sulfide phases. PXRD (Figure 2), EDS, and XPS (Figure 3) show that the copper deficiency of the obtained product increases with the amount of oleic acid in the solvent. Four distinct copper sulfide phases were produced upon heating Cu(II)(acac)2 and elemental sulfur (1:2 molar ratio) in dodecanethiol, oleic acid, or mixtures thereof: monoclinic chalcocite, tetragonal chalcocite, digenite, and covellite. 1-Dodecanethiol as a solvent produces dark brown nanoparticles. PXRD of these particles (Figure 2d) shows very broad 4060

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2c). The pattern matches the ICDD pattern 00-029-0578. EDS gives an elemental composition of Cu2.0±0.2S. This encompasses that range over which tetragonal chalcocite has been reported in bulk: Cu2S and Cu1.96S.1 XPS (Figure 3) of the sulfur 2p, copper 2p, and oxygen 1s regions of these tetragonal chalcocite particles differ little from those of monoclinic chalcocite. While the solid-state packing is clearly different, tetragonal chalcocite differs only slightly in composition from monoclinic chalcocite. From EDS and XPS results (Figure 3), the compositions are not distinguishable. The electronic structure, measured by the band gap and plasmon absorbance, offers the primary evidence that tetragonal chalcocite, as synthesized here, is indeed more copper-deficient than monoclinic chalcocite. Using cyclic voltammetry, the band edge positions were measured, as were the electrochemical band gaps (Figure 4c).

Figure 2. PXRD patterns of copper sulfide particles obtained from the specified solvents, overlaid with the pattern from the ICDD for covellite (blue, 00-006-0464), digenite (orange, 00-023-0962), tetragonal γ-chalcocite (green, 00-029-0578), and monoclinic αchalcocite (red, 00-023-0961).

Figure 3. XPS of covellite (blue), digenite (orange), tetragonal chalcocite (green), and monoclinic chalcocite (red) particles, showing the S2p, O1s, and Cu 2p regions generated from solutions with varying dodecanethiol (ddt) to oleic acid (OA) ratios.

peaks that match monoclinic chalcocite (α-Cu2S) (ICDD pattern 00-023-0961). Monoclinic chalcocite is related to hexagonal chalcocite and djurleite through a nearly identical sulfide structure. The mobile copper cation positions are the structural difference, thus the PXRD patterns of hexagonal (βCu2S) and monoclinic chalcocite and djurleite (Cu1.94S) are very similar. The PXRD of the particles obtained here shows two overlapping peaks centered at 40.5° 2θ that distinguish the crystalline component as primarily monoclinic chalcocite.19 EDS shows an elemental composition of Cu2.0±0.1S. The copper 2p peak and the sulfur 2p positions are in agreement with published spectra (Figure 3).35,37 Monoclinic chalcocite is produced from solvent mixtures containing a majority of 1dodecanethiol (Figure S1, panels e−g, of the Supporting Information). A novel form of tetragonal chalcocite particles is produced from solvent mixtures of 1:2 to 1:3 dodecanethiol:oleic acid (Figure 2c and Figure S1, panels c and d, of the Supporting Information). Nanoparticles of this form have been reported once previously with different properties.27 In this typically high-temperature structure,38 copper ions occupy trigonal sites in a body-centered tetragonal sulfide matrix.39 The transformation from a monoclinic sulfide packing to body-centered packing in tetragonal chalcocite results in prominent new peaks in the PXRD pattern at 39.2 and 32.6° 2θ, as well as a change in relative intensities of the peaks around 46 and 48° 2θ (Figure

Figure 4. Vis/NIR absorbance spectra of (a) copper sulfide particles from dodecanethiol (red, bottom), 1:3 dodecanethiol (ddt):oleic acid (oa) (green), 1:19 dodecanethiol:oleic acid (orange), and oleic acid (blue, top). (b) Vis/NIR absorbance spectra of monoclinic (red, bottom) and tetragonal (green, top) chalcocite particles over time (0 days, 1/2 day, 1 day, 2 days). (c) Cyclic voltammograms of copper sulfide particles from dodecanethiol (monoclinic chalcocite, red, bottom) and 1:3 dodecanethiol:oleic acid (tetragonal chalcocite, green, top). (d) Vis/NIR spectra of monoclinic (red, bottom) and tetragonal (green, top) chalcocite particles after synthesis (solid line) and after overnight treatment with dodecanethiol (dashes) or oleic acid (dots), showing a systematic shift of the plasmon band to lower wavelength with oleic acid treatment.

Electrochemical band gaps of 1.37 eV for monoclinic chalcocite and 1.52 eV for tetragonal chalcocite were measured. Copper deficiency is known to increase the band gap due to a narrowing of the Cu 3d-derived conduction band.40 The CV further shows that the conduction band of the tetragonal phase is shifted to lower potential, indicating a higher conduction band edge.41 Evidence for the greater copper-deficiency of tetragonal chalcocite comes from the shift in the plasmon resonance from greater than 2000 nm for monoclinic to 1800 nm for tetragonal chalcocite. Monoclinic and tetragonal chalcocite showed significant plasmon absorption in the NIR region (Figure 4a). 4061

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DISCUSSION Origin of Phase-Selectivity. Reducing Abilities of the Solvents. Reduction of the precursors, Cu(II)(acac)2 and elemental sulfur, through interactions with the solvent is a necessary step in the formation of all phases of copper sulfide. Figure 5a and Figure S2 of the Supporting Information shows

Previous reports have shown that the plasmon frequency shifts to lower wavelength with a decrease in the copper:sulfur ratio because vacancies in the valence band increases the number of charge carriers.8 When left in air, Cu2−xS species are known to oxidize or lose copper via diffusion.8 Both processes increase the plasmon absorbance, though the plasmon peak intensity and energy remains greater for tetragonal than for monoclinic chalcocite (Figure 4b). The observation that monoclinic chalcocite made here exhibits this behavior despite having the PXRD pattern of Cu2S suggests that it must be slightly nonstoichiometric. Digenite particles were obtained from a 19:1 oleic acid:dodecanethiol solvent ratio (Figure 2b). Digenite forms a highly symmetric face-centered cubic anion structure (commonly referred to as the metastable form of digenite)42 that results in fewer PXRD peaks then either chalcocite form. The PXRD pattern is a match to ICDD pattern 00-023-0962. A small amount of tetragonal chalcocite (peaks 32.8 and 39.3° 2θ) is present. EDS gives a slightly copper-deficient stoichiometry (Cu1.9±0.2S) that is in agreement with the expected value of Cu1.8S.2 The peak in the sulfur 2p region of the XPS spectrum (Figure 3) is broader than that of the monoclinic or tetragonal forms of chalcocite, suggesting a greater variety of sulfur species. Thus, digenite likely contains sulfide and persulfide ions, as has been reported for covellite.35 Use of oleic acid as the solvent results in dark black particles with diffraction patterns that match all 10 peaks apparent in the reference pattern of covellite (ICDD pattern #00-006-0464) (Figure 2a); covellite adopts a hexagonal anion structure 37 containing both S2− and S2− 2 ions. EDS confirms an elemental composition of Cu0.9±0.2S, in agreement with the expected CuS formula. The XPS (Figure 3) shows that a copper 2p peak consisting of a sharp peak at 932.8 ± 0.2 eV with only a small tail at 935 eV in digenite and covellite are in agreement with literature reports.35 Furthermore, peak fitting suggests that while the sulfur 2p region of monoclinic and tetragonal chalcocite can be well fit by two peaks, covellite and digenite require more, broader peaks. Sulfur 2p regions showed that the fwhm increased from 2.0 eV for chalcocite to 2.6 eV for digenite and to 3.1 eV for covellite. Specifically, a shoulder doublet appears at lower binding energy due to the presence of persulfide ligands.35 XPS shows that oleic acid was present on the surface of those species grown in >90% oleic acid (Figure 3). Oleic acid adsorption is suggested by the broad oxygen 1s peak for covellite and digenite, indicating that oxygen is present in more than one environment, both as adventitious oxygen such as is found in tetragonal and monoclinic chalcocite and in carbonbound oxygen found in oleic acid. The near-doubling in peak width (fwhm of chalcocite is 1.8 eV, while that of digenite is 3.2 eV and covellite is 3.3 eV) and shift of the peak maximum from 530.4 eV for the chalcocites to 531.3 eV for digenite and covellite support this. The lack of a peak around 168 eV suggests that the oleic acid ligand is not binding to the sulfur in covellite.35 Oleic acid only binds to the surface when little dodecanethiol is present to displace it, such as occurs when digenite and covellite are obtained from 95 or 100% oleic acid solutions. Tetragonal and monoclinic chalcocite are capped by dodecanethiol, thus a broad O 1s feature is not observed in the XPS.

Figure 5. UV−visible spectra of (a) Cu(acac)2 (normalized to the Cu+ peak at 242 nm) and (b) elemental sulfur in the specified solvent (dodecanethiol in black and oleic acid in gray), demonstrating the change in speciation with solvent.

that the extent of reduction and dissociation of Cu(II)(acac)2 (λmax = 280 nm) to a copper +1 species (λmax = 235 nm) increases with the amount of dodecanethiol present. Upon heating to 200 °C, elemental sulfur (S8 rings) speciates differently in dodecanethiol versus oleic acid (Figure 5b). While no difference was observed in the 1H NMR spectra of the solvents when heated with sulfur, the UV−vis spectra exhibit a dependence on solvent. This suggests no alterations to the alkyl chains but can be explained by variation in the sulfur speciation. In dodecanethiol, sulfur dissolution narrows the UV−vis peak of the solvent (λmax = 278 nm), which may correspond to production of alkylated persulfide species. For comparison, spectroelectrochemical experiments show that S22− anions exhibit a λmax = 280 nm in DMF43 and addition of sulfur to methanethiol results in a series of CH3Sn2− species.44 After heating sulfur in oleic acid, a peak forms in the visible region that does not when oleic acid is heated alone. The peak observed in oleic acid is broader and at a larger wavelength, 2− overlapping the region where S2− 4 and S5 species absorb in 43 DMF. Further confirmation of the difference in reducing ability of oleic acid versus dodecanethiol comes from examination of the effect each solvent has on the plasmon absorbance of monoclinic chalcocite. It has recently been reported that the plasmon band in copper sulfides systematically shifts to longer wavelength when exposed to air due to oxidation of the valence band.24 Exposure to oleic acid or dodecanethiol influences this behavior. Oleic acid increases the intensity and extent of the plasmon band shift compared to dodecanethiol (Figure 4d), showing that the dodecanethiol better slows oxidation. Formation of the End-Members, Cu2S and CuS. The ability of the solvent to generate the end-members of the copper sulfide phase diagram, chalcocite (Cu2S) or covellite (CuS), can be explained straightforwardly based on the oxidation states of the S species alone. Covellite contains S2− and S2− 2 , while the only sulfur species in chalcocite is S2−.35,36 Any equilibria resulting in persulfides will be more favored in oleic acid than in dodecanethiol, leading to formation of covellite. This interpretation is supported by literature reports that covellite31 and chalcocite21 have been generated from oleic acid and dodecanethiol, respectively, using other synthetic parameters. It is further supported by the observation that other carboxylic 4062

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dodecanethiol-rich solutions. Covellite transformed completely to tetragonal chalcocite, while digenite and tetragonal chalcocite particles became a mixture of monoclinic and tetragonal chalcocite phases. While Cu2S can be generated from copper particles in dodecanethiol alone,46 the narrow PXRD peaks for the transformed monoclinic chalcocite compared to copper treated in dodecanethiol indicates that large tetragonal chalcocite or digenite particles transform. This transformation implies that monoclinic chalcocite is the most thermodynamically favorable phase in dodecanethiol, while tetragonal chalcocite is the next most stable phase, paralleling Ostwald’s rule of stages, describing the stepwise crystallization of polymorphs in order of increasing stability.47 Dodecanethiol further reduces or dissolves the existing persulfide anions, replacing them with sulfide anions in covellite. Heating synthesized particles in oleic acid induces transformation only of tetragonal chalcocite, which grows and becomes digenite (Figure S5 of the Supporting Information). Monoclinic chalcocite, digenite, and covellite remain the same phase. Solvent Effect on Size and Morphology. The size and morphology differences between the particles reflect the relative rates of nucleation and particle growth in dodecanethiol and oleic acid. TEM (Figure 6, panels b and c) shows that

acid solvents generate covellite, as well (Figure S3 of the Supporting Information). Why Obtain Tetragonal Chalcocite or Digenite? Obtaining the nonstoichiometric tetragonal chalcocite and digenite phases from moderately reducing media requires a more nuanced explanation, influenced by recent reports on the oxidation states of copper in copper sulfides, as well as experiments comparing the behavior with varying Cu(II)(acac)2:S ratios. Recent evidence shows that Cu2+ is present in Cu2S and CuS;33,36 previous XPS35,37 and EPR34 experiments suggest only Cu+ is present. These reports support the presence of Cu2+ and even Cu0 in both,33,36 with some speculation that Cu2+ transforms to Cu+ in solution or upon X-ray beam exposure, causing the disagreement between results from different experimental techniques. If nonstoichiometry is concurrent with transformation of Cu+ to Cu2+ to maintain charge neutrality, then it becomes clear why a progression in Cu deficiency with weaker solvent-reducing ability occurs. As the UV−vis spectra of Cu(II)(acac)2 in the various solvent ratios above shows, the concentration of Cu2+ will be greater in solvents with more oleic acid, leading to a larger x for Cu2−xS. Formation of tetragonal chalcocite from mixed dodecanethiol/oleic acid solvents was dependent not only on solvent ratio but also a Cu:S ratio. At 1:2 v/v ratio of dodecanethiol:oleic acid, tetragonal chalcocite was obtained using a 1:2 molar ratio of Cu(II)(acac)2:S, but monoclinic chalcocite was obtained using a 2:1 Cu(II)(acac)2:S ratio (Figure S4 of the Supporting Information). While it is known that monoclinic chalcocite is stabilized by excess copper,19 it is critical to our argument that this behavior is modulated by solvent identity. The requirement of excess sulfur to produce tetragonal chalcocite suggests that even for very slightly copperdeficient phases, a low total copper concentration is required to generate enough Cu2+ to stabilize the nonstoichiometric tetragonal species. Demonstration of Relative Thermodynamic Stability. The following sets of experiments demonstrate that the solvent environment affects the relative stabilities of the copper sulfide phases. This is unlike other systems where composition is determined by the rate of single-source precursor decomposition.21,45 The observed free energy ordering of the phases in dodecanethiol versus oleic acid reflects the presence of more highly reduced precursor species in dodecanethiol. To distinguish the free energy ordering of different phases in dodecanethiol and oleic acid, the as-synthesized particles were heated in pure solvent for four hours and examined by PXRD (Figure S5 of the Supporting Information). Transformation of phases occurred as depicted in Table 1. In dodecanethiol, particles transformed to phases originally obtained from more

Figure 6. (a) SEM and (b and c) TEM images of (a) covellite, (b) monoclinic α-chalcocite, and (c) tetragonal γ-chalcocite particles generated by heating Cu(II)(acac)2 and sulfur in oleic acid, dodecanethiol, and 2:1 oleic acid:dodecanethiol, respectively.

monoclinic and tetragonal chalcocite forms small, spherical particles with diameters of 5.2 ± 1.3 nm and 6.5 ± 2.4 nm, respectively. For particles grown in pure dodecanethiol or in 3:1 oleic acid:dodecanethiol, nucleation is very rapid, followed by slow growth. This is apparent from the small size and distribution of the monoclinic and tetragonal chalcocite particles, as well as the small size difference in tetragonal chalcocite particle size when the synthetic time was extended from 1 to 2 h (Figure S6 of the Supporting Information). Monoclinic chalcocite particles do not grow in either dodecanethiol or oleic acid solution after 4 h of heating. Tetragonal chalcocite particles do grow and transform phase, suggesting that dissolution is sufficiently facile to allow Ostwald ripening. The small, spherical monoclinic and tetragonal chalcocite particles are grown from the solvent mixtures with the most dodecanethiol. The presence of sulfur induces rapid

Table 1. Representation of the Phase Transformations That Take Place upon Equilibration of Pre-Formed Particles at 200 °C in Dodecanethiol and Oleic Acid initial phase monoclinic chalcocite tetragonal chalcocite digenite covellite

phase after heating in dodecanethiol monoclinic chalcocite monoclinic chalcocite + tetragonal chalcocite monoclinic chalcocite + tetragonal chalcocite + digenite tetragonal chalcocite

phase after heating in oleic acid monoclinic chalcocite digenite digenite covellite 4063

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instrument purchased under CBET-0959361, and SEM was carried out at the Penn Regional Nanotechnology Facility with assistance from Grant DMR-05-20020.

nucleation. Without sulfur, much copper-thiolate precursor remains unreacted (Figure S7 of the Supporting Information). Larger particles with preferred orientation are observed from more reactive copper precursors (Figure S8 of the Supporting Information), while nanowires are obtained from CuS2CNEt2.31 This confirms that the growth rate is tied to the rate of precursor decomposition.48 In comparison with chalcocite particles, particles grown from pure oleic acid or 19:1 oleic acid:dodecanethiol media exhibit rapid growth and unusual morphology. Digenite and covellite crystals are very large (>1 μm even after only 15 min of reaction). The “X-ball” morphology appears to form by growth of intermingled discs (Figure 6). This is reminiscent of the platelet growth of Cu2 S that occurs from solventless thermolysis of copper octanoate/thiolate precursor.49 Here, platelet growth was attributed to the relative stability of the (0001) face of hexagonal crystals in which the ratio of unit cell parameters c/a > 1.633,50 thereby inducing rapid growth along the {1000} planes. Ridged11,51 and platelet52 morphologies for covellite have previously been reported.

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CONCLUSIONS Copper sulfide particles are pursued for uses in renewable energy applications and advanced optical devices; this work reveals new ways of controlling the complex phase behavior that has limited use of copper sulfides in the past. A simple synthetic scheme offers pathways to a range of stoichiometries and crystal structures. The ability to obtain hexagonal, bodycentered, and face-centered cubic anion lattices suggests this could be a versatile platform for generating unique sulfide phases through cation exchange.53 Explorations of the relative thermodynamic stabilities of particles revealed a means of postsynthetic phase transformations and postsynthetic alteration of plasmon absorption bands. This work offers novel synthesis of tetragonal chalcocite particles for exploration and a route to accessing other cubic copper sulfide structures predicted to have increased stability relative to the hexagonal or monoclinic phases3 and decreased electrical conductivity.54

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ASSOCIATED CONTENT

S Supporting Information *

PXRD patterns and UV−vis spectra (Figures S1−S8). This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 717-291-4001. Fax: 717-2914343. Web site: http://www.fandm.edu/katherine-plass. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Franklin & Marshall College for faculty and student grants and for student support via the Hackman Summer Scholars Program. We are also grateful to the Henry and Camille Dreyfus Foundation for a New Faculty Startup Award and an NSF CAREER award (CHE-1149646). The National Science Foundation supported instrument purchase and use: the PXRD was purchased with EAR-0923224, XPS was carried out by Zhorro Nikolov or Siamak Nejati at the Centralized Research Facilities at Drexel University with an 4064

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