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Synthesis of Cu nanoparticles: Stability and conversion into Cu2S nanoparticles by decomposition of alkanethiolate Christian Rohner, Anna Pekkari, Hanna Härelind, and Kasper Moth-Poulsen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02117 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Synthesis of Cu nanoparticles: Stability and conversion into Cu2S nanoparticles by decomposition of alkanethiolate Christian Rohner, Anna Pekkari, Hanna Härelind and Kasper Moth-Poulsen* Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 41296 Gothenburg, Sweden Abstract

A lean synthesis of copper nanoparticles (Cu NP) from CuCl2 in dodecane via formation of Cu(I)-dodecanethiolate (Cu(I)-DDT) and their decomposition paths including spontaneous C-S bond cleavage of the alkanethiol on the surface of Cu NP is presented. The reduction of Cu(I)DDT by tert-butylamine borane complex (TBAB) in dodecane under N2 at elevated temperatures leads to the formation of thiol protected Cu NP with narrow size distribution in the size range of 3 to 10 nm depending on the reaction conditions. The Cu NP in the presence of excess dodecanethiol react further to Cu2S NP under decomposition of the ligand on the particle surface. The Cu2S formation occurs after short time at T > 175 °C or within ~12 h at RT. If excess thiol is removed immediately after the synthesis, the resulting colloid shows irreversible aggregation within days or hours. Our results suggest that alkanethiols are not long term stable on

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nanocopper surfaces and that the formation of copper(I) sulfide under the cleavage of the C-Sbond occurs even at room temperature.

Introduction Owing to the low cost of Cu, Cu NP are an interesting candidate for applications in catalysis,1,2 as well as plasmon sensing.3–5 Supported Cu catalysts have, for instance, been extensively studied for reactions like hydrogenolysis and steam reforming of alcohols,6,7 as well as for methanol synthesis.8 The major obstacle in preparing Cu NP is its sensitivity to oxidation. A number of methods are described in the literature trying to overcome this problem by using surfactants such as AOT,9 polymers,10,11 fatty acids and amines,12,13 with limited success regarding to the oxidation protection.14–16 Alkanethiols, widely used in noble metal nanoparticle synthesis, are considered a promising candidate for the oxidation protection of Cu NP, as several electrochemical studies on self-assembled monolayers on Cu electrodes show a stabilizing effect.17,18 Methods employing alkanethiols as stabilizers for Cu NP include variations of the Brust-Schiffrin or other synthetic methods in one-phase systems.15,19–24 At the same time, a partial decomposition of alkanethiols has been reported when the thiolate is bound to a Cu surface or cluster by scission of the C-S bond at room temperature.25,26 The thermal decomposition of Cu-thiolates at high temperatures (T > 180 °C) is employed in a multitude of methods in the fabrication of monodisperse copper(I) sulfide (Cu2S) NP.27–30 Here, we set out to develop a lean reaction system for the preparation of DDT (and hexadecanethiol) protected Cu nanoparticles (CuDDT NP, CuHDT NP) from readily available starting materials inspired by a method for monodisperse Au NP by Zheng et al.31 In brief, that method involves the reduction of AuPPh3Cl by tert-butylamine borane complex (TBAB) in benzene with dodecanethiol acting as the surfactant and yielding monodisperse Au NP with sizes


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in the 5-10 nm range. In our adaptation of the method, we substituted the Au(I) salt and benzene with CuCl2 and dodecane respectively, which initially lead to very encouraging results regarding the sizes, size distribution, shape and optical properties of alkanethiol stabilized Cu NPs. A number of reducing agents other than TBAB (1,2-Hexadecanediol, NaBH4, LiEt3BH) were tested with unsatisfactory results.32 Nevertheless, despite efforts to keep solutions of the Cu NP strictly oxygen free, it was found that storage of NP solutions for more than a few hours lead to irreversible aggregation of the Cu NP or the disappearance of the plasmon resonance band even under inert atmosphere. Interestingly, we observe that the thiol stabilized Cu particles are partially transformed to Cu2S even when stored at RT under N2 atmosphere and can be fully converted to Cu2S by heating the reaction mixture to > 145 °C for an extended period of time. Hence, our results suggest that alkanethiols are not long term stable on nanocopper surfaces and that the formation of copper(I) sulfide under the cleavage of the C-S-bond inevitably occurs even at room temperature. Experimental Section. Materials and Methods: Air-free Schlenk technique is used throughout with protection under N2 gas. Copper(II) chloride dihydrate (CuCl2.2H2O, 99.999%, Aldrich), dodecane (C12H25, ≥99%, Aldrich), dodecanethiol (DDT) (C12H24SH, 98%, Alfa Aesar ), hexadecanethiol (HDT) (C16H32SH, ≥95% Aldrich) and tert-butylamine borane complex (TBAB) ((CH3)3CNH2BH3, ≥97%, Aldrich) were purchased and used as received. Toluene was purified in a solvent purification system (MB SPS-800). Ethanol (99.5%, analytical reagent grade, Solveco) was degassed by using a freeze-pump-thaw method for 5 cycles. UV-Vis spectra were recorded using a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer. Transmission electron microscopy and electron diffraction were performed on a Tecnai T20


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TEM (200 kV, LaB6 cathode). Energy dispersive x-ray spectra (EDS) were recorded using an EDAX SiLi-detector. For TEM characterization, 5 µL of solution were dropcast on 200 mesh Ni or Cu grid with carbon coating (Ted Pella Inc.). Powder XRD were recorded using a Bruker D8 Advance with Cu Kα radiation. ICP-MS was conducted by ALS Scandinavia AB. Synthesis Cu NP and Cu2S NP: The detailed reaction conditions with final concentrations of reactants are listed in table S1-S3 in the supporting information. In the general procedure, a determined amount of CuCl2.2H2O is placed into a Schlenk flask equipped with a rubber septum. The flask is evacuated and heated to 60 °C in an oil bath until the blue crystallites turn into a fine brown powder (CuCl2). The flask is then flushed with N2 and an excess of the alkanethiol is added via syringe. After the initial reaction has receded, the flask is evacuated again until all CuCl2 is consumed and a white slurry is formed. Dodecane is added and the mixture is degassed thoroughly by pumping and flushing with N2 for 5 cycles at 60 °C. After heating to 145 °C, TBAB powder is added in one portion under N2 flow and the mixture is stirred at the same temperature for 5-90 min. Then the oil-bath is removed and the mixture is left to cool to room temperature. For Cu2S NP the mixture is reheated to 175 °C for 2 h or left overnight with N2 purging. To isolate the NP, a portion of the fresh reaction mixture is transferred to an N2 filled centrifuge tube, an excess of degassed ethanol is added (VEtOH/VRct.-mix > 2:1) and it is centrifuged at 5300 rcf for 20 min. The supernatant is discarded. For further purification, addition of degassed ethanol, centrifugation and removal of supernatant steps are repeated twice and followed by drying of the precipitate in N2 flow for 30 s. The material is then dissolved by addition of dry toluene with weak sonication to promote dissolution and once more centrifuged


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at 200 rcf. The supernatant colloidal solution and the precipitate are separately collected for characterization. Results and discussion The initial Cu NP colloid is synthesized from inexpensive starting materials (CuCl2.H2O) in a facile one-pot, one-phase method, where all reagents are dried and degassed directly in the reaction flask. The neat reaction between dodecanethiol (DDT) and CuCl2 gives Cu(I) dodecylthiolate (Cu(I)-DDT) and didodecyl disulfide under evolution of HCl as a white slurry in the excess DDT.33 Cu(I)-DDT dissolves in dodecane at ~135 °C which is evident by formation of a yellow solution. After addition of tert-butylamine borane (TBAB) powder, an immediate color change from yellow to colorless, brown and after continued heating blue indicates the formation of Cu NP. Concentrated solutions appear black. The particle solutions are generally not stable for longer than 12 hours and changes to the NP surface composition with time are indicated by color changes and the formation of precipitate. The formation and decomposition paths are summarized in Scheme 1

Scheme 1. (Top) Reaction scheme of the neat reaction of CuCl2 and DDT yielding Cu(I)-DDT and didodecyl disulfide. (Bottom) Formation of metastable thiolate protected Cu NP and their decomposition paths in the presence of excess thiol under ambient conditions or N2 atmosphere, respectively.


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If the mixture is opened to air, its color changes to brown within < 30 min. This is due to the oxidation of the NP surface and the disappearance of the plasmon resonance band.15,34 The copper oxide formed on the Cu NP is in turn dissolved by the excess thiol to give the Cu(I)thiolate precursor, which results in a white precipitate over the course of hours or days.35,36 If the isolation procedure is followed under ambient conditions, the product particles (Cu/Cu2O) in the colloid are in the size range of ~2-5 nm and mostly amorphous (see SI Figure S1). The UV-Vis spectrum of the yellow to brown solution is featureless with an absorption onset at 700 nm (SI Figure S1). The quality and quantity of the product NP varies with each batch as it strongly depends on the timing of the workup procedure due to the instability of the particles. This could be attributed to the oxidation effect, but even under strict N2 atmosphere the workup procedure yields varying results with respect to the amount of isolated NP soluble in toluene. Figure 1 shows the characterization of samples successfully isolated under N2-protection (for synthesis parameters and additional characterization see Table S2-S3 and Figure S2-S3, respectively). The average size and size distribution of the product particles proved relatively insensitive to the concentration of CuCl2 and DDT used in the synthesis when in the range of c(Cu2+) = 1-40 mM and c(DDT) = 500-1000 mM. Under these conditions the resulting NP are in the average size range of about 3-8 nm with size distributions in the range of 12-40% (see Table S2-S3). Below a DDT concentration of 400 mM, the resulting particles are polydisperse with particle sizes in the range of 2-100 nm (see samples A.5-A.7, B.6-8, C.7 in Figure S1-S3). The UV-Vis absorption spectrum of the blue solutions shows the plasmon resonance maximum of Cu NP at 597 nm (see Figure 1d). The electron diffractogram (ED) of the nanopowder shows a ring pattern that agrees


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with the reflex positions of Cu(0) and Cu2O (Figure 1e). The latter forms when the particles oxidize on the TEM grid (see Figure 1e, S7, S8).

Figure 1. Characterization of Cu NP synthesized in separate batches with c(DDT)=500 mM, c(TBAB)=40 mM at 145 °C. a) Sample C.1, TEM of 3.8 ± 0.8 nm Cu NP (t = 30 min, c(Cu+)=2 mM); b) Sample C.2, TEM of 5.5 ± 0.8 nm Cu NP (t = 90 min, c(Cu+) =2 mM); c) Sample C.4, TEM of 7.4 ± 1.0 nm Cu NP (t = 30 min, c(Cu+) =1.5 mM; d) UV-Vis spectra of 90 min sample with plasmon resonance maximum at 597 nm with photography of Cu NP


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solution (inset); e) SAED of sample shown in c with reference patterns of Cu(0), JCPDS-Nr. 40836 (white semicircles) and Cu2O, JCPDS-Nr. 5-0667 (red semicircles). The blue product solutions are in most cases not stable for longer than 12 h even under inert atmosphere where irreversible aggregation and precipitation of the particles occurs. When sonicated, the particles can be redispersed but precipitation or coating of the vial walls then occurs within 1 min. When the reaction mixture is left in the reaction flask with N2 protection overnight the blue mixture turns orange with only slight formation of precipitate (Figure 2c). Reheating of the aged mixture to 175 °C for 2 h produces monodisperse Cu2S NP (Figure 2a, 2b), where the orange mixture remains optically unchanged even after addition of another portion of TBAB, confirming that no Cu(I) DDT has formed. The UV-Vis spectrum of the purified orange colloid is featureless with an absorption onset at 600 nm (Figure 2d). In the SAED patterns of the NP, the reflexes belonging to the hexagonal Cu2S phase are clearly visible, the reflexes belonging to the oxide phase are notably absent (Figure 2e). Representative XRD and ICP-MS measurements confirm the evolution from Cu(0) to Cu2S via an amorphous phase (Figure S8). While the XRD of fully converted NP only shows reflexes belonging to the hexagonal Cu2S phase, the Cu:S ratio in this sample measured by ICP-MS is 1.6:1 compared to the theoretical 2:1. This might be due to presence of organosulfur or amorphous CuS compounds in the sample.


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Figure 2. a) TEM of 5.0 ± 0.3 nm Cu2S NP prepared from reaction mixture that was aged under N2 overnight and reheated to 175 °C for 2 h (same batch as C.3, compare Figure S3) with section of the FFT of the image (inset). b) TEM of 7.7 ± 0.6 nm Cu2S NP prepared by reheating a reaction mixture to 175 °C (same batch as C.4, compare Figure 1c). c) Photography of reaction mixture after aging overnight under N2 d) UV-Vis spectrum of the purified Cu2S NP. e) SAED of particles in b) shows the presence of crystalline Cu2S NP (reference pattern of hexagonal Cu2S, spacegroup P63/mmc, JCPDS-Nr. 46-1195, yellow semicircles) with Cu(0) still present (reference pattern of Cu(0), JCPDS-Nr. 4-0836, white semicircles),.


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Figure 3 shows the temporal evolution of the absorbance spectra of a reaction mixture where HDT was used in the synthesis of the Cu colloid. A sample solution (≈ 1 mL) was diluted by addition of dry Toluene (≈ 1 mL) and transferred to a UV-cuvette under N2 atmosphere. Measurements were taken periodically while the sample was at RT (Figure 3). In between measurements the cuvette was kept in an N2 filled cabinet. The plasmon resonance band disappears over the course of 4 h. The plasmon resonance maximum remains at 600 nm for 125 min and slightly shifts to longer wavelengths thereafter. The observed decrease in intensity is much slower and the shift of the resonance maximum to longer wavelengths much weaker than reported for an oxidation of copper.34 This leads us to the conclusion that the spectral changes are caused by a growth of Cu2S shell on the Cu NP.


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Figure 3. a) Temporal evolution of UV-Vis absorption spectra of freshly prepared Cu NP colloid/reaction mixture diluted with dry toluene in the presence of excess HDT. b) Photography of the solution at 0 min. c) Photography of the solution at 275 min. EDX measurements were performed on some of the samples (B.6, C.1, C.6, C.7), confirming a higher S content than would be expected for a monolayer of thiolate on the particles (< 5% for NP with D > 3.8 nm) in all of the examined samples. Nonetheless, in the ED or XRD measurements of samples aged at RT a copper sulfide phase could not be identified (Figures S4S7). This points to the formation of amorphous copper sulfides on the crystalline Cu(0) NP or


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very small (< 2 nm) copper sulfide clusters next to them at RT after the reduction of Cu(I)-DDT is complete. As the only source of sulfur in the system is DDT, a decomposition of the alkanethiolate on the Cu surface under cleavage of the C-S-bond can be concluded. This is in agreement with the findings of Vollmer et al.26 A probable mechanism of decomposition is α−β−elimination with the primary alkene leaving and H-S remaining bound on the surface of the NP, respectively. Albeit, we could not detect the presence of alkene in the solutions in NMR studies (not shown) or mass spectroscopy (not shown). Conclusion. We have presented a synthetic method for small alkanethiolate stabilized Cu NP (D = 3-10 nm), where we find that the alkanethiolate only provides a temporal stabilization of the nanoparticles while it undergoes decomposition under cleavage of the C-S-bond even under inert gas protection. We observe this decomposition indirectly in optical measurements. TEM characterization confirms the formation of Cu NP after reduction of Cu(I) thiolate and the product of the further reaction of the Cu NP with excess thiol as Cu2S NP . After reheating to 175 °C, the resulting Cu2S NP are crystalline, highly stable and monodisperse. We envision that the decomposition of the alkanethiolate shell could be utilized when depositing the Cu NP on mesoporous supports followed by removal of the alkyl moieties for applications where sulfur impurities are not an obstacle or wanted. Acknowledgements. The authors would like to thank the European Research Council and K & A Wallenberg foundation for financial support though an ERC starting grant (grant agreement 337221 SIMONE) and a KAW project grant (Single Particle Catalysis in Nanoreactors). Supporting information.


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