Article pubs.acs.org/cm
Ultrathin Sheets of Metal or Metal Sulfide from Molecularly Thin Sheets of Metal Thiolates in Solution Balanagulu Busupalli,† Sreenivas Kummara,‡ Guruswamy Kumaraswamy,*,‡ and Bhagavatula L. V. Prasad*,† †
Physical and Materials Chemistry Division and ‡Complex Fluids and Polymer Engineering, Polymer Science and Engineering Division, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune, 411008, India S Supporting Information *
ABSTRACT: Materials that exist as single molecule thick twodimensional sheets are in great demand because they hold promise as precursors for synthesis of layered functional materials. We demonstrate that metal thiolates, that exist as lamellar assemblies in the neat state, can be disassembled into individual molecular sheets simply by dilution in apolar organic solvents and that these can form ultrathin metallic layers on substrates upon heat treatment. We establish the pathway to the disassembly of metal thiolates in solution using a combination of techniques, including X-ray diffraction, light scattering, FTIR, and TEM. Our results indicate that the lamellar structure of Pd-thiolates is preserved in toluene up to a concentration of 300% w/v and the average intersheet distance is unchanged. Interestingly, the dynamics of the Pd-thiolate sheets remain correlated even on diluting them up to 30% w/v, though the disorder within the lamellar stacks increases with a decrease in their coherence length. Finally, at dilutions less than about 5% w/v, individual sheets of these structures can be accessed that are isolated and directly observed using TEM. Heat treatment of the ultrathin films of metal thiolates deposited on appropriate substrates resulted in the formation of metal or metal sulfides with retention of sheetlike morphologies.
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covalently linked to sulfurs of alkanethiols (Figure 1b inset).14 This raises an important questionwill the covalent bonding presumed in the proposed Pd-thiolate structure stay intact in organic solvents that solubilize it? Or will the thiolates disintegrate into individual monomer units? While the use of metal thiolates (especially, palladium thiolates) for synthesis of functional nanomaterials15−19 has been reported, details about their solution state structure remain unexplored. We conjectured that if metal thiolates indeed retain a two-dimensional sheetlike structure when diluted in organic solvents and if these sheets can be readily harvested, they could represent important building blocks for the synthesis of layered structures. To explore this possibility, unraveling the solution state structure of metal thiolates, in various solvents and at various levels of dilution, is of paramount importance. Here, we report the solution state structure of these materials in detail. We demonstrate that Pd-octanethiolate and several other metal thiolates are indeed sheetlike covalently bonded structures that are amenable to facile disassembly into molecularly thin sheets, simply by diluting in nonpolar organic solvents such as toluene, chloroform, or CCl4. We use WAXD (wide angle X-ray diffraction), DLS (dynamic light scattering), and FTIR (Fourier-transform infrared) spectroscopy to investigate the
INTRODUCTION Even as graphene1 and alternate two-dimensional atomic crystals2−4 continue to inspire intense research efforts, there is currently tremendous interest in novel “designer” twodimensional structures, driven by their promise for unprecedented plasmonic and catalytic properties.5 There is an urgent need to devise suitable precursors and establish synthetic routes to such materials. Specifically, sheetlike metal−organic hybrids that can be readily exfoliated and that are amenable to solution deposition might find utility in preparation of layered heterostructures for devices in the postgraphene era. In this context, layered materials that are compatible with existing solution processing technologies and that allow deposition of a variety of metals might have significant value. However, the de novo synthesis of free floating sheets of metal−organic hybrids in solution is not reported. Therefore, we chose to investigate metal thiolates, since it appeared that these materials might have several advantages for such applications. Several metal thiolates exist as lamellar structures,6−13 which opens up the exciting possibility of exploring such materials as building blocks for the preparation of novel two-dimensional structures. Further, metal thiolates can be easily synthesized in large quantities, and they have been found to be readily soluble in nonpolar solvents such as chloroform and toluene. A case in point is the palladium thiolate system. The first reported synthesis of Pd-thiolates dates back to 1935 and suggested that the structure of Pd-thiolates involved a network of Pd2+ ions © 2014 American Chemical Society
Received: February 26, 2014 Revised: May 4, 2014 Published: May 6, 2014 3436
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Solution State FTIR. Solution state FTIR spectra for the sample at various concentrations starting from 500% w/v (500% w/v, 100% w/v, 90% w/v, 80% w/v, 70% w/v, 60% w/v, 50% w/v, 40% w/v, 30% w/v, 20% w/v, 10% w/v, 5% w/v, 1.667% w/v, and 0.56% w/v) were recorded on a Bruker Optics ALPHA-E spectrometer operated at a resolution of 4 cm−1 with a universal Zn−Se ATR accessory in the 600−4000 cm−1 region. Samples at different concentrations were prepared separately, and each sample was drop-cast onto the base in the solution state FTIR instrument for the measurement. The base was thoroughly cleaned with the solvent after each measurement. Dynamic Light Scattering (DLS). Dynamic light scattering measurements were performed on the sample at different concentrations using a 3D DLS instrument (LS instruments, with a λ = 632.8 nm He−Ne laser and an inbuilt autocorrelator). 1000 mg of the sample dissolved in 1 mL of toluene (analytical reagent grade) to get a 100% w/v sample that was carefully filtered through a 0.2 μm hydrophobic PTFE filter into a precleaned cylindrical glass sample cell. The sample cell was immersed into a vat prefilled with dust free toluene maintained at 25 °C for the scattering measurements. DLS was recorded on the 100% w/v sample. The 100% w/v sample was then diluted to obtain concentrations of 60% w/v, 50% w/v, 40% w/v, 30% w/v, 20% w/v, 10% w/v, 5% w/v, 1.667% w/v, and 0.56% w/v by filtering toluene (analytical reagent grade) directly into the sample cell to dilute the concentrated samples. DLS measurements were then made on each sample at a constant angle of 90° for 30 s. Multiangle DLS was performed on a Pd-octanethiolate sample at 50% w/v, 40% w/v, 30% w/v, 20% w/v, and 10% w/v. The data were collected at angles of 45, 60, 75, 90, 105, and 120°. Each measurement was repeated for at least three times to ensure that the results were consistent. Static Light Scattering. Static light scattering was performed on palladium octanethiolate starting from 50% w/v and then subsequently diluted to 40% w/v, 30% w/v, 20% w/v, 10% w/v, 5% w/v, and 1.667% w/v. Intensity versus q was plotted on a log−linear scale. Wide Angle X-ray Diffraction (WAXD). Wide angle X-ray diffraction (WAXD) data for samples at different solution concentrations were collected on an R-axis IV image plate with a Rigaku diffractometer equipped with a rotating anode source (Cu Kα, λ = 1.54 Å). Samples were packed into hollow cylindrical borosilicate glass capillaries of average diameter = 2 mm, and WAXD was performed on such capillaries. The scanning time was set at 1 min for all the samples. The distance between the sample and detector was kept constant at 80 mm for all samples. Rotational oscillations of the rotating anode were set at ±0.5° per minute with the measurements being taken at the constant angle 0°. A high density polyethylene film (HDPE film) was used as a reference. WAXD experiments were performed on 500% w/v, 300% w/v, 100% w/v and 90% w/v samples of palladium octanethiolate in toluene. Images were analyzed using ImageJ software from NIH and azimuthally averaged 1D data was obtained. Scattering from the empty cylindrical borosilicate glass capillary and for the capillary filled with toluene were measured. WAXD measurements were performed on the metal thiolate samples at concentrations of 500%, 300%, and 100% w/v and are presented after background correction. TEM/HRTEM. Samples dissolved in chloroform at various concentrations (1.667% w/v, 10% w/v) were drop cast onto separate 200 mesh carbon coated copper grids (Ted Pella, Inc.) and studied using a transmission electron microscope (TEM, FEI model TECNAI G2 F20) operating at an accelerating voltage of 200 kV. A high resolution transmission electron microscope (HRTEM, FEI model TECNAI G2 F30) operating at an accelerating voltage of 300 kV was employed to visualize the ultrathin metal/metal sulfide sheets. Electron Dispersive X-ray Analysis. Energy dispersive X-ray analysis (EDX) measurements on the palladium octanethiolate sample were obtained from a scanning electron microscope (SEM, FEI model Quanta 200 3D) equipped with EDX attachment at an operating voltage of 30 kV. Energy dispersive X-ray analysis (EDX) measurement on the ultrathin palladium metallic layers was performed using the HRTEM operating at an accelerating voltage of 300 kV.
Figure 1. (a) Powder XRD pattern of Pd-octanethiolate showing lamellar (00l) reflections (inset shows photographs of Pd-octanethiolate in a solvent-free state (1 g material) and after solvent addition (50 mg in 5 mL of toluene)). (b) Plausible structure of the palladium thiolate lamellae. (c, d, e, and f) 2D WAXD patterns for the samples at 500% w/v, 300% w/v, 100% w/v, and 90% w/v. (g) The corresponding 1D plots of the WAXD for the same samples.
route to solution disassembly of lamellar Pd-thiolates in detail, and show that solubilization in nonpolar organic solvents allows us to access ultrathin sheets of these layered metal thiolates, including palladium, nickel, mercury, and lead thiolates. Our work represents the first definitive proof for the solution structure of metal thiolates. After conclusively establishing the solution structure of Pd-thiolate, we show that these can be deposited as thin films from dilute solutions on desired substrates. Finally, we demonstrate that heat treatment of these thin films under appropriate conditions affords a route to layers of palladium metal or palladium sulfide, depending on the choice of processing conditions. Thus, this work provides a complete link between detailed structural studies of metal thiolates in solution and the use of these sheetlike dilute solution constructs for controllable fabrication of metal or metal sulfide thin films on solid subtrates.
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EXPERIMENTAL SECTION
Materials. Palladium acetate, sodium tetrachloropalladate(II), mercuric nitrate, octanethiol, and dodecanethiol were purchased from Sigma-Aldrich and used as received. Nickel acetylacetonate and lead acetate (II) trihydrate were purchased from Merck chemicals. Toluene (analytical reagent grade) purchased from Merck chemicals was distilled and used for the synthesis as well as for the spectroscopic and microscopic studies, and 4-tert-butyl-toluene was used as received from Sigma-Aldrich. Distilled CCl4 was used for the solution state FTIR analysis. UV−Vis Spectroscopy. UV−vis spectra were recorded on a Jasco V-570 UV/vis/NIR spectrophotometer operated at a resolution of 2 nm. Powder XRD. A few drops of the sample dissolved in toluene were drop-cast onto a glass plate, and the sample was dried at room temperature to yield a thin film on the glass plate. Powder XRD profiles of these samples were recorded on an X’pert Pro model PANalytical diffractometer from Philips PANalytical instruments operated at a voltage of 40 kV and a current of 30 mA with Cu Kα (1.5418 Å) radiation. The samples were scanned in a 2θ range from 3° to 15° with a scan rate of 0.4° per minute. 3437
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Thermogravimetric Analyses (TGA). Thermogravimetric analyses of the palladium octanethiolate sample were performed on a TG50 analyzer (Mettler-Toledo) or a SDT Q600 TG-DTA analyzer under nitrogen atmosphere at 10 °C min−1 heating rate within a temperature range of 20−900 °C. Synthesis of Materials. a. Synthesis of Palladium Octanethiolate. Palladium octanethiolate was synthesized by a modification of a reported protocol6 to allow synthesis in scaled-up quantities. In this procedure 500 mg palladium of acetate was taken in an eppendorf tube of 2 mL capacity. To this, 900 μL of octanethiol was added and the tube was shaken vigorously. The reaction mixture turned orange-red instantaneously. The mixture was washed thoroughly (5−6 times) using ethanol, and the orange-red product was air-dried at room temperature. The obtained orange-red powder is readily soluble in organic solvents such as chloroform, toluene, CCl4, etc. We note that we add a slight excess of thiol over the stoichiometric requirement in this modified procedure. This modified procedure worked equally well with other metal−thiolates, as described below, and furnished gram quantities of materials. b. Synthesis of Nickel Octanethiolate. To 500 mg of nickel acetylacetonate taken in an eppendorf tube of 2 mL capacity, 900 μL of octanethiol was added and the tube was shaken vigorously. The reaction mixture turned black instantaneously. The mixture was washed thoroughly with ethanol, and the black product was air-dried at room temperature. The obtained black powder goes readily into organic solvents such as chloroform, toluene, CCl4, etc. The powder was characterized using PXRD, TEM, and UV−visible spectroscopy. c. Synthesis of Mercury Octanethiolate. To 500 mg of mercuric nitrate taken in an eppendorf tube of 2 mL capacity, 900 μL of octanethiol was added, and the tube was shaken vigorously. Caution should be maintained while shaking the reaction mixture, as the reaction is highly exothermic. The reaction mixture turned colorless instantaneously. The mixture was washed thoroughly with ethanol, and the white product thus obtained was air-dried at room temperature. The white powder goes readily into organic solvents such as chloroform, toluene, CCl4, etc. The powder was characterized using PXRD and TEM. d. Synthesis of Lead Octanethiolate. To 500 mg of lead acetate (II) trihydrate taken in an eppendorf tube of 2 mL capacity was added 900 μL of octanethiol, and the tube was shaken vigorously. The reaction mixture turned yellow in color instantaneously. The mixture was washed thoroughly with ethanol, and the yellow product was airdried at room temperature. The obtained yellow powder goes readily into organic solvents such as chloroform, toluene, CCl4, etc. The powder was characterized using PXRD and TEM. e. Synthesis of Palladium Dodecanethiolate Hexamer. Synthesis of palladium dodecanethiolatehexamer was carried out according to the reported literature.16 To 0.588 g of Na2PdCl4 taken in 10 mL of 4tert-butyltoluene in a round-bottom flask was added 0.98 mL of dodecanethiol, and the reaction mixture was refluxed in an oil bath under argon atmosphere at 192 °C for 1 h. The flask was then removed and cooled, and the mixture was poured into 100 mL of ethanol taken in another round-bottom flask. This mixture was stirred vigorously. Overnight stirring resulted in orange precipitates. The product was centrifuged and washed with ethanol and dried. f. Synthesis of Palladium Dodecanethiolate. To 500 mg of palladium acetate taken in an eppendorf tube of 2 mL capacity was added 900 μL of dodecanethiol, and the tube was shaken vigorously. The reaction mixture turned yellowish-red instantaneously. The mixture was washed thoroughly with ethanol, and the yellowish-red product was air-dried at room temperature. The obtained yellowishred powder goes readily into organic solvents such as chloroform, toluene, CCl4, etc. The powder was characterized using PXRD and TEM. g. Preparation of Ultrathin Palladium or Palladium Sulfide Layers. Palladium octanethiolate (10% w/v in chloroform) was spin coated onto a 2 cm × 2 cm quartz plate and then heated to 950 °C in an argon atmosphere for 6 h. The pale gray layer formed was separated from the quartz substrate through mechanical vibration into ethanol using a bath sonicator. This ethanolic dispersion was drop-cast onto a
carbon coated copper TEM grid and observed under TEM and HRTEM. PXRD was used to characterize the as prepared sample. The same procedure was used to prepare ultrathin palladium sulfide layers except that the sample was heated to 350 °C.
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RESULTS AND DISCUSSION Palladium-octanethiolate was synthesized following a modification of a previously reported protocol6 (details in the Experimental Section). In the neat state (viz. undiluted by solvent), the product is a waxy solid (Figure 1a inset, neat Pdoctanethiolate) that readily forms an orange colored solution upon addition of nonpolar solvents such as toluene, CHCl3, CCl4, etc. (Figure 1a inset, diluted Pd-octanethiolate). Neat Pdoctanethiolate was characterized using energy dispersive X-ray analysis (see Supporting Information (SI) Figure S1) to confirm the stoichiometry. NMR (see SI Figure S2) and TGA (see SI Figure S3) were performed to characterize palladium octanethiolate. Powder X-ray diffraction (using Cu Kα radiation) shows peaks at 2θ values of 3.9, 7.8, and 11.7°, corresponding to q values of 0.28, 0.56, and 0.83 Å−1, respectively (Figure 1a). These peak positions are in the ratio of ≈1:2:3, indicating that the Pd-octanethiolate is characterized by lamellar order, with an interlayer spacing of 22.6 ± 0.5 Å. This spacing corresponds to a structure with partially interdigitated alkyl chains and accords well with literature reports (Figure 1b).20 2D WAXD patterns (corrected for glass and solvent scattering) were recorded as a function of dilution at concentrations of 500%, 300%, 100%, and 90% (w/v) in toluene (Figure 1c−f). At 500% w/v, the WAXD pattern displays multiple concentric rings, indicating orientational isotropy over the diffracting sample volume. The rings correspond to diffraction peaks at the same 2θ values as for the “powder” XRD from the neat Pd-octanethiolate (Figure 1g). However, there is a decrease in the diffracted intensity and an increase in peak width relative to the neat sample (see SI Figure S4, peak width or fwhm increased from from 0.54 degrees for 500% w/v sample to 0.63 degrees for 300% w/v sample). The regular spacing of the diffraction rings indicates that lamellar order is preserved in the solution state, at this concentration (Figure 1g). Interestingly, there is no swelling of the lamellar phase, viz. no change in the 2θ values for the lamellar diffraction peaks on dilution with toluene to 500% w/v. Thus, the X-ray data indicates that, on dilution of the Pdoctanethiolate to 500% w/v in toluene, the average spacing between the thiolate lamellae is unchangedhowever, there is a decrease in the coherence length of the diffracting lamellar stacks and increased disorder within the stacks. Samples diluted further (up to 300% w/v) showed similar WAXD patterns as that of the 500% w/v sample (as is evident from the corresponding 1D plots, Figure 1g). Increasing the dilution to 100% w/v and 90% w/v of the Pd-octanethiolate in toluene resulted in complete disappearance of the diffraction rings (Figure 1e and f), indicating a loss of lamellar order (Figure 1g). Such “unbinding” phase transitions, leading to loss of lamellar order, are commonly observed for lamellar surfactant bilayer systems, and result from temperature or dilutioninduced changes in the balance of attraction and repulsion between the sheetlike structures.21 Pd-octanethiolate is a yellow colored materialhowever, it does not absorb in the visible above a wavelength of 520 nm (see SI Figure S5), thus making it possible to perform light scattering experiments on the sample using red He−Ne laser light (λ = 632.8 nm). The intensity−intensity time correlation 3438
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Figure 2. (a) DLS for samples at different concentrations viz. from 100% w/v to 1.67% w/v in toluene. The solid lines represent fitting curves obtained using eq 2. (b) Plot showing the relation between relaxation times and the concentration. Concentrations from 30% w/v to 100% w/v show two distinct decay times (τ1 and τ2) whereas samples at 20% w/v and below show single relaxation (τ1). (c) Fast relaxation time scales from multiangle DLS of a palladium octanethiolate sample at different concentrations from 50% w/v to 10% w/v through 40% w/v, 30% w/v, and 20% w/ v plotted as a function of 1/q2 show linear dependence.
with q−2 (Figure 2c). Literature reports23 indicate that hydrodynamic undulation modes observed in DLS from dilute lamellar systems show relaxation times that scale with q−2. It is plausible that the fast relaxation time observed in our DLS experiments arises from such hydrodynamic undulation modes of sheetlike Pd-octanethiolate structures. We observe that τ1 is strongly concentration dependent below 30% w/v, suggesting that, even at these low concentrations, weak sheet−sheet interactions influence the single sheet undulation modes. τ1 is approximately constant for higher concentrations between 50 and 100% w/v, suggesting that the undulation modes persist at higher concentrations. The contribution of these undulation modes, x, is in the range 0.01−0.08 at Pd-thiolate concentrations above 30% w/v (see SI Figure S8). We interpret the slow time scale, τ2, as resulting from cooperative relaxation of interacting Pd-octanethiolate sheets. The q−2 scaling does not hold for τ2, the slow relaxation time. To ensure that the decrease in τ1 at lower concentrations indeed corresponds to its constituent sheets rather than complete disintegration into monomeric units, we performed DLS on toluene solutions of two model monomeric compounds: octanethiol and a well-defined hexameric complex of Pd-dodecanethiolate (Pd-dodecanethiolate hexamer was synthesized following literature reports).16 For the details of the synthesis and characterization of Pd-dodecanethiolate hexamer, see Experimental Section and SI Figures S9a and S9b and S10). The neat hexameric Pd-dodecanethiolate is a solid, and we were unable to filter toluene solutions of this sample at concentrations higher than 30% w/v. Therefore, DLS measurements were performed on samples from 5% to 30% w/ v. For the hexameric Pd-dodecanethiolate at concentrations between 5 and 30% w/v, and for solutions of octanethiol in toluene up to a concentration of 30% w/v, the time correlation of the scattered intensity decay very rapidly, and we are unable to measure the decay of g2(t) (see SI Figures S9c and S9d, respectively). Thus, in comparison to the hexameric Pddodecanethiolate and octanethiol, τ1 relaxation processes in Pdoctanethiolate, even at concentrations as low as 1.667% w/v, are characterized by longer time scales and, therefore, correspond to slower relaxing, larger structures. This indicates that diluting with toluene does not result in disintegration of the Pd-octanethiolate to monomeric (or oligomeric) units. Static light scattering on toluene solutions of Pd-octanethiolate (see SI Figure S11) is in qualitative accord with DLS data, indicating a structural transition at a concentration around 30%
function, g(2)(q,t), was obtained at 90° for Pd-octanethiolate samples at concentrations from 100% to 1.667% (w/v) in toluene: g(2)(q , t ) = I(q , t )I(q , 0) / I(q , 0)2
(1)
Samples at higher concentrations (above 100% w/v) could not be filtered using a 0.2 μm filter, precluding light scattering measurements. DLS was repeated at least twice for independently prepared samples at each concentration and was observed to be quantitatively reproducible. For concentrations ≤20% w/v, the correlation function shows a single stage decay. At concentrations between 20% and 30% w/v, there is an abrupt, qualitative change in the relaxation behavior. For concentrations ≥30% w/v, the correlation function shows a two-stage decay (Figure 2a). We fit the correlation function with an empirical expression of the following form: g(2)(q , t ) − 1 = A[x e−t / τ1 + (1 − x)e−t / τ2]2
(2)
where τ1 and τ2 are time scales that characterize a fast and slow relaxation process, respectively; x is the contribution of the fast relaxation process to the correlation decay; and A is an instrumental constant. We note that attempts to fit the correlation function for sample concentrations ≥30% w/v with a single relaxation time or with a stretched exponential yielded poor fits (see SI Figure S6). A linear combination of two simple exponentials was observed to fit our data well, and employing stretched exponentials did not significantly improve the goodness of fit. This is in contrast to dynamic light scattering for caged systems where the slow relaxation process associated with cage reorganization is typically fitted with a stretched exponential.22 In the present case, for concentrations ≤20% w/v, we set x = 1; that is, the correlation function is fitted with only one relaxation time, τ1. We note that τ1 increases with sample concentration from the lowest concentration measured, up to about 50% w/v (Figure 2b). For concentrations above 30% w/v, the time scales for the slow and fast processes are separated by one to 2 orders of magnitude. Above a concentration of 50% w/v, τ1 (≈2 × 10−4 s) and τ2 (≈10−2 s) are not strongly concentration dependent. Multiangle DLS data were recorded for Pd-octanethiolate samples at concentrations of 50%, 40%, 30%, 20%, and 10% w/ v (see SI Figure S7), and the correlation functions, g(2)(q,t), were fitted using eq 2. We observe that for all concentrations, above and below 30% w/v, the fast relaxation time, τ1, scales 3439
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30% w/v is also observed for the rocking band at 725 cm−1 (see SI Figure S12d). Moreover, other prominent IR bands corresponding to CH2 scissoring at 1438 cm−1 and umbrella motions of CH3 at 1365 cm−1 follow the same trend as the C− H stretching progressions of the end CH3 group (see SI Figure S13). Based on our data we suggest the following sequence of structural changes on diluting Pd-octanethiolate with organic solvents such as toluene or CCl4. Neat Pd-octanethiolate is organized as a lamellar structure, with an interlayer spacing of ≈2.26 nm. On diluting to 500% w/v, there is increased disorder within the lamellar Pd-octanethiolate stacks, and a decrease in the coherence length that characterizes stacking, as reflected by the decrease in the WAXD intensity and increased peak width (inset in Figure 1d). We attribute this to decreased inter alkyl chain interactions as evidenced by the FTIR data (Figure 3a). Increased disorder on further dilution to 100% w/v, consistent with FTIR evidence for decreased chain−chain interactions, eliminates WAXD reflections that signify lamellar ordering. However, even at concentrations ≥30% w/v, Pd-octanethiolate structures still exhibit a two stage microstructural relaxation in DLS measurements, and they show evidence for large, O(100 nm) scale structures from static light scattering. Thus, while Pdoctanethiolate sheets at concentrations between 100% and 30% w/v in toluene exhibit cooperative relaxation, they are not sufficiently strongly correlated to exhibit lamellar stacking in WAXD. For concentrations ≥30% w/v in toluene, Pdoctanethiolate is characterized by two widely separated relaxation times, τ1 and τ2. We interpret the fast time scale obtained from the DLS data as arising from undulation modes of individual lamellae, while the slow time scales arise from larger scale cooperative relaxations of correlated Pd-thiolate sheets. DLS and SLS data show a qualitative change between 30% w/v and 20% w/v, consistent with a change in slope of the I(2852 cm−1)/I(2920 cm−1) from FTIR. At even greater dilution (≤20% w/v), the correlations between the Pdoctanethiolate sheets that give rise to cooperative relaxation disappear, and only one relaxation time, τ1, is required to fit the DLS data. Even in dilute solution, τ1 is concentration dependent and increases strongly with concentration, suggesting that weak intersheet interactions persist to concentrations as low as 1.667% w/v. However, Pd-octanethiolate does not disassemble into monomeric units at the lowest concentrations that we investigated, as τ1 is significantly larger than that for model noncovalently linked Pd-thiolate oligomeric aggregates. Thus, we anticipate that extreme dilutions should afford us access to single sheets of the Pd-octanethiolate. Therefore, we prepared samples by evaporating Pd-octanethiolate onto TEM grids. At extremely low concentrations, we were indeed able to reproducibly image single sheets of the Pd-octanethiolate (Figure 3c−e). Thus, TEM evidence strongly supports our contention that the sheetlike structure of Pd-octanethiolate is preserved at extreme dilutions in organic solvents and that individual sheets can be easily harvested. We also note that this sequence of structural transitions is not specific to Pd-octanethiolate but can be observed for other thiolates of palladium, such as, for example palladium dodecanethiolate (see SI Figure S14). Solvent-induced exfoliation to individual molecularly thin sheets is also evidenced for several other soluble metal thiolates that are known to exhibit a layered architecture, such as nickel thiolate, mercury thiolate, and lead thiolate (Figure 4) (more
w/v. At concentrations ≤20% (w/v), the scattered intensity of the Pd-octanethiolate is approximately q-independent over a qrange of 0.01 to 0.07 nm−1. At concentrations ≥30% w/v, there is a qualitative change in the q-dependence of the scattered intensity, with enhanced scattering at low q, indicating the formation of large length scale structures. To probe the molecular implications of the changes in τ1 and τ2 on dilution, and to understand the emergence of lamellar order in Pd-octanethiolate, we used solution state FTIR to interrogate changes in the C−H stretching progressions of the CH3 end group of the octanethiol moiety. Analysis of data obtained for dilution with toluene is complicated due to interference from toluene vibrations. Therefore, we present data for Pd-octanethiolate serially diluted using CCl4, after normalization with respect to intensity at 3027.3 cm−1 (Figure 3a). We note that qualitatively similar data was obtained for
Figure 3. (a) Solution state FTIR plot highlighting the C−H stretching progressions of the end CH3 for different concentrations of the sample in CCl4. (b) The corresponding graph for ratio of the C−H symmetric to antisymmetric stretching bands of the end CH3. Parts c, d, and e are the TEM images of palladium octanethiolate prepared from a sample at 1.67% w/v.
dilution with toluene (see SI Figure S12a, S12b, and S12c). We observe a gradual decrease in the relative intensity of the stretching bands on serial dilution of the sample with CCl4. The ratio of the intensities of the C−H symmetric stretching band (at 2852 cm−1) to the C−H antisymmetric stretching band (at 2922 cm−1) of the C−H stretching progressions of the end CH3 group systematically increases with Pd-octanethiolate concentration (Figure 3b). In accordance with literature reports,24 our data suggest that, on increasing sample dilution, there is a decrease in the intermolecular interactions between the alkyl chains and enhanced conformational disorder due to an increased number of gauche conformers (g+, g−) with respect to the trans (t+, t−) conformers. Further, the head band of rocking progressions at 725 cm−1 increases in intensity as the sample becomes diluted (see SI Figure S13b), in contrast to the decrease in intensity observed for the C−H stretching bands. This is consistent with the reduced interchain attractive interactions on diluting with solvent. Our data also reveal that the increase in I(2852 cm−1)/I(2920 cm−1) with concentration is steeper as the concentration increases to 30% w/v, and it shows a more gradual increase from 30% w/v to 500% w/v. This change in slope around the concentration of 3440
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such heat treatment revealed the formation of ultrathin Pd(0) or PdS sheets, depending on the heat treatment temperature. More specifically, when the spin coated palladium octanethiolate was heat treated at 950 °C, the formation of Pd-sheets could be seen, which is indicated by the PXRD profile (Figure 5a), which matched with that of Pd (0) sheets (JCPDS card no. 05-0681).5 Furthermore, the HRTEM image revealed lattice fringes that correspond to the (111) planes (Figure 5b) of Pd(0). The low magnification TEM image revealed a sheetlike morphology with lateral dimensions of several micrometers. Clearly, these Pd(0) films are sufficiently thin to allow transmission of electrons through them (Figure 5b, inset). The selected area electron diffraction also matched with that of Pd(0) (Figure 5c). EDS analysis of the heat treated sample on a quartz substrate revealed that the sample contains only Pd(0) and that the sulfur content is negligible (see SI Figure S16). On the other hand, when a similar palladium octanethiolate sample on a quartz substrate was heat treated to 350 °C, the formation of palladium sulfide (PdS) sheets was observed (see SI Figure S17). Thus, dilution in organic solvents represents a general route to access molecularly thin sheetlike structures of a variety of metal thiolates. Apart from their potential for novel materials synthesis, our work demonstrates an easy way to prepare molecular precursors that can be readily exfoliated into single sheets that might inspire realization of novel two-dimensonal structures (ultrathin metal layers). Exfoliation of lamellar systems into individual sheets,25−29 and the crumpling or rolling up30−32 of molecular paper33,34 in solution remain fascinating open problems in fundamental materials science. Given the enormous synthetic challenges associated with denovo synthesis of free-standing two-dimensional sheets in solution,35−37 this facile dilution based route to molecularly thin sheets of metal thiolates and their use as precursor materials in the preparation of thin metallic sheets is especially attractive.
representative TEM images of these metal thiolates are shown in SI Figure S15).
Figure 4. (a) PXRD plots of (i) nickel octanethiolate, (ii) mercury octanethiolate, and (iii) lead octanethiolate; and TEM images of (b) nickel octanethiolate, (c) mercury octanethiolate, and (d) lead octanethioalte. Insets of parts b, c, and d, are photographs of the clear solutions of nickel octanethiolate, mercury octanethiolate, and lead octanethiolate, respectively, in chloroform.
We further envisaged that, as the thiolates readily go into nonpolar organic solvents, heat treatment of the single sheets of metal thiolates coated on appropriate substrates could lead to the formation of thin metallic/metal sulfide layers with retention of sheetlike morphologies. Accordingly, different concentrations of palladium octanethiolate, viz. 10% w/v and 100% w/v samples, were spin-coated onto quartz substrates of 2 cm × 2 cm size and were heated to 350 or 950 °C under argon gas atmosphere. Analysis of the resulting product after
Figure 5. (a) PXRD pattern of Pd(0) sheets indexed as per JCPDS card no. 05-0681. (b) Enlarged HRTEM image of the Pd(0) sheets showing lattice fringes with separation of 0.23 nm between the fringes (inset shows HRTEM image of Pd(0) crystals showing sheetlike morphology). (c) SAED pattern obtained from the image in part b indexed for the Pd(0) sheets. 3441
dx.doi.org/10.1021/cm5006949 | Chem. Mater. 2014, 26, 3436−3442
Chemistry of Materials
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ASSOCIATED CONTENT
S Supporting Information *
Detailed fitting of DLS data, solution state FTIR of the materials in toluene, TEM images of different metal thiolates, TGA of palladium octanethiolate, and characterization details of Pd-dodecanethiolate hexamer. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Mr. Manas Sule for his help with the WAXD experiments. B.B. acknowledges CSIRNew Delhi for a fellowship. B.L.V.P. acknowledges DST-UNANST for financial support. We gratefully thank Prof. G. U. Kulkarni, JNCASR, Bangalore, for many useful discussions. We thank Dr. Nandini Devi, NCL, Pune, for helping us with drawing the structures.
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REFERENCES
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