Elucidating the Surface Chemistry of Zinc Phosphide Nanoparticles

Jul 21, 2014 - On the basis of these data, a general mechanism for ligand exchange chemistry on the Zn3P2 nanoparticle surface was proposed to enable ...
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Elucidating the Surface Chemistry of Zinc Phosphide Nanoparticles Through Ligand Exchange Md Hosnay Mobarok†,‡ and Jillian M. Buriak*,†,‡ †

Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada National Institute for Nanotechnology (NINT), National Research Council (NRC), 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada

‡

S Supporting Information *

ABSTRACT: Zn3P2 nanoparticles, a potential earth abundant nanomaterial for photovoltaic applications, are prepared via a solution-based synthesis and end up capped with weakly bound phosphine ligands. These ligands are easily displaced from the nanoparticle surface, leading to an irreversible aggregation of particles. In this work, we elaborate the chemistry of Zn3P2 nanoparticles both to elucidate the surface functionalities present after synthesis, and to enable the production of stable solutions of Zn3P2 colloidal solutions. Three different types of anionic type ligands, formed from their neutral precursors of oleic acid, n-decylphosphonic acid, and 1octadecanethiol, were shown to be effective in yielding soluble functionalized nanoparticles. The functionalized Zn3P2 nanoparticles were thoroughly characterized by electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analyses, and FTIR spectroscopy. A combination of FTIR and multinuclear solution NMR spectroscopic studies on the starting agglomerated Zn3P2 nanoparticles and the functionalized particle solutions reveals that the particle surface is terminated by Zn−CH3 and −PHx(SiMe3)3−x groups. Using oleic acid as the workhorse ligand, it was shown that addition of oleic acid to agglomerated nanoparticles led to a homogeneous dispersion of Zn3P2 nanoparticles, in toluene, along with production of CH4 and C17H33COOSiMe3 as byproducts, as determined by 1H and 13C NMR spectroscopy. FTIR spectroscopy of the ligandexchanged particles indicated oleate coordination, along with the appearance of what has been assigned as a P−H stretch. Similar reaction chemistry was observed during ligand exchange with n-decylphosphonic acid and 1-octadecanethiol. On the basis of these data, a general mechanism for ligand exchange chemistry on the Zn3P2 nanoparticle surface was proposed to enable both the production of zinc phosphide nanoparticle solutions and the determination of various routes to surface functionalization of this material.

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INTRODUCTION Surface passivation is an important consideration in every nanoparticle synthesis as the ligands dictate solubility, much of the functionality, and long-term stability.1 For many applications, such as biomedical,2 electronic,3 sensing,4 and the construction of sophisticated hierarchical assemblies,5 the surface chemistry must be exquisitely tailored for the required function. Depending upon the intended use, the binding of the ligand to the nanoparticle surface may need to be strong and essentially irreversible,6 or for others, a metastable reversible binding motif may be preferable.7 Surface ligands also play an important role in the nucleation and growth of nanoparticles due to binding and other interactions with the synthetic precursors, which ultimately lead to control of nanoparticle size and shape.8−11 Much research effort has focused upon the surface chemistry of semiconductor nanoparticles, with the greatest amount on CdX (X = S, Se, Te), and PbX (X = S, Se).12−19 Simple molecules such as alkyl-substituted carboxylic acids, phosphonic acids, thiols, and amines serve as workhorse ligands, and yet, in spite of their chemical austerity, surprising new chemistry is still © 2014 American Chemical Society

being uncovered. It is typically assumed that Lewis basic ligands would interact with Lewis acid surface sites on a semiconductor nanoparticle surface as L-type (neutral donor, dative bond) or X-type (anionic ligand, covalent bond) donors.13−15 In the case of phosphonic acids on CdSe nanoparticles, it has been convincingly demonstrated that surface ligands are anionic (alkyl phosphonate, X-type) in nature rather than dative (Lewis basic L-type ligand), as shown by the corresponding reactivity with bis(trimethylsilyl)selenide or trimethylsilyl chloride.13 Recently, however, an unexpected Lewis acidic Z-type ligand displacement was reported.14 Z-type ligands act as Lewis acids that bind to Lewis-basic sites, and in this case, the metal center of a Cd−carboxylate complex acts as a Lewis acidic binding group. Even with a seemingly established semiconductor nanoparticle−ligand system, the actual chemistry through which the ligands bind may not be fully understood, and is worthy of further study. Received: June 20, 2014 Revised: July 12, 2014 Published: July 21, 2014 4653

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There has been growing interest in what are seen as unconventional semiconductor nanoparticles composed of nontoxic, earth-abundant elements.20 Phase-pure, nanoparticle forms of iron pyrite,21−27 copper sulfide,28 zinc phosphide,29,30 and others31−33 are of great interest as the basis for large-scale deployment of photovoltaics. The goal is the production of nanoparticle solutions that could be processed through spray coating, roll-to-roll printing, and other mass manufacturing approaches.31 Control of the surface chemistry and functionalization of the nanoparticles are critical to enable sufficient solubility to prepare films, and yet be removable, or short enough, to permit facile charge transfer from one nanoparticle to the next within the film.7a In addition, the surface chemistry needs to minimize or prevent high levels of defects that could hinder charge transport.34 The demands on the surface termination are, therefore, very strict, and if little is known about the surface chemistry, it would be challenging to enable the application of nanoparticle-based solutions for PV applications. Our group has recently described an approach for the synthesis of phase-pure, monodisperse zinc phosphide nanoparticles using dimethylzinc as the zinc source, and tris(trimethylsilyl)phosphine, P(SiMe3)3, as the phosphorus source (Figure 1a).30 Zn3P2 is an intriguing material for PV applications as it has a suitable band gap of 1.5 eV, a large absorption coefficient (>104 cm−1), a long minority-carrier diffusion length, and, in bulk form, was shown in the 1980’s to yield 6% efficiency solar cells.29,35−38 The zinc phosphide nanoparticles were fully characterized by XRD, high-resolution TEM and selected area electron diffraction, multinuclear solidstate and solution phase NMR, and XPS.29,30 In terms of the surface functionalization, NMR and FTIR suggested that the tri-n-octylphosphine (TOP) present in the synthesis was acting as the surface ligand (vide inf ra). It became clear, however, that TOP was a poor ligand for these nanoparticles since they were unstable in solution as a homogeneous dispersion and more than one washing step resulted in precipitation of the nanoparticles to an insoluble aggregate, as shown in Figure 1b, e. The instability of the zinc phosphide nanoparticle solution renders them unsuitable for solution-processed solar cell device fabrication. Understanding and harnessing the surface chemistry of these zinc phosphide nanoparticles is essential for successful PV applications. The small, but growing, body of literature regarding Zn3P2 nanoparticles has relied on the use of long-chain phosphines, amines, or phosphine oxides, all of which passivate the nanoparticles, presumably, by a dative interaction (L-type ligands).29,30,39−41 In this work, we investigated the surface chemistry of the as-prepared zinc phosphide nanoparticles and focused on their reactivity with an alkyl carboxylic acid, a phosphonic acid, and a thiol. The reactivity of these groups provides important insights into the surface chemistry of the starting zinc phosphide nanoparticles; understanding the starting point for the surface reactivity is important to develop the surface chemistry in a rational way for the development of colloidal nanoparticle solutions for photovoltaic applications.

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Figure 1. (a) Reaction scheme showing the chemistry to produce the Zn3P2 nanoparticles. (b) Agglomeration of the same particles upon washing due to ligand loss. (c, d) TEM image of tri-n-octylphosphine (TOP)-capped Zn3P2 nanoparticles, with size analysis. (e, f) TEM image and selected area electron diffraction (SAED) pattern of agglomerated Zn3P2 nanoparticles. (THF), hexane, and acetonitrile were dried using an Innovative Technologies solvent purification system. 2-Propanol (99.99%) was degassed via argon sparging and dried over 4 Å molecular sieves before use. All reactions were carried out under argon using Schlenk techniques with a dual manifold argon-vacuum system, or in an argonfilled glovebox. NMR Spectroscopy. Solution NMR spectra were recorded on a Agilent/Varian 400 or 500 MHz spectrometer operating at the resonance frequencies of 161.8 or 202.3 MHz, respectively, for 31P nuclei, 399.8 or 499.8 MHz, respectively, for 1H nuclei, and 100.6 or 125.7 MHz, respectively, for 13C nuclei. All NMR spectra were acquired using an adequate relaxation delay (≥5 T1). The 1H and 13C spectra were referenced internally to residual solvent proton signals relative to tetramethylsilane, whereas 31P{1H} spectra were referenced relative to external 85% H3PO4. All solution spectra were recorded at 27 °C unless otherwise noted. Solid-state 31P and 13C NMR spectra (Supporting Information) were obtained using Bruker Avance 300 and 500 NMR spectrometers, operating at 121.6 and 202.5 MHz, respectively, for 31P, and at 75.5 and 125.8 MHz for 13C. FTIR Spectroscopy. FTIR spectra were collected with a Nicolet Nexus 760 spectrometer with a DTGS detector and a nitrogen-purged sample chamber. All spectra were recorded on KBr salt plates inside a home-built inert atmosphere holder; air-sensitive samples were prepared in a nitrogen-filled glovebox. Electron Microscopy. High-resolution transmission electron microscope (HR-TEM), annular dark-field scanning transmission

EXPERIMENTAL SECTION

Chemicals. Dimethylzinc, ZnMe2 (96%), was obtained from AlfaAesar; tri-n-octylphosphine, TOP (97%), and tris(trimethylsilyl)phosphine, P(SiMe3)3 (98%), were obtained from STREM; oleic acid (99%), n-decylphosphonic acid (97%), and 1-octadecanethiol (98%) were obtained from Sigma-Aldrich. Toluene, tetrahydrofuran 4654

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described in the oleic acid synthesis) was suspended in 3 mL of toluene in a 20 mL vial with a magnetic stir bar; at this point, the nanoparticles form a cloudy suspension. A total of 24 mg of 1octadecanethiol (0.084 mmol), dissolved in 1 mL of toluene, was injected into the gently stirred cloudy suspension of nanoparticles. Immediately, the brownish cloudy suspension turned into a clear, dark red solution, and the solution was stirred for 12 h to ensure complete ligand exchange. During this time, the solution became cloudy. Centrifugation of the solution at 3000 rpm for 10 min led to the separation of the nanoparticles (24 mg). The supernatant was discarded, and the particle was redispersed in 2 mL of hexane for characterization. The nanoparticles can also be dispersed in toluene, but the solubility is lower compared to hexane. Ligand Exchange Reaction of Zn3P2 Nanoparticles with nDecylphosphonic Acid. A half batch of crude nanoparticles (∼45 mg of Zn3P2 nanoparticles suspended in 2 mL of TOP) was diluted with 3 mL of THF. At this point, the particles form a brown cloudy suspension in the solvent. In a separate vial, 10 mg of ndecylphosphonic acid (0.045 mmol) was dissolved in 1 mL of THF and added to the mildly stirred suspension of nanoparticles. The solution was stirred for 12 h, during which time the cloudy brown suspension become a dark red, homogeneous solution. Addition of 10 mL of 2-propanol led to flocculation of the nanoparticles, which were then collected by centrifugation at 4400 rpm for 30 min. The particles were redissolved in 2 mL of THF. An additional purification cycle was carried out, in which 8 mL of 2-propanol was added, followed by centrifugation (4400 rpm for 10 min) to remove excess ligands. After drying with a flow of argon, the obtained mass of nanoparticles was 37 mg. The particles were finally redispersed in THF for characterization. The use of acetonitrile to precipitate the Zn3P2 nanoparticles in toluene, as was the case for oleic acid treatment, did not lead to the flocculation of these nanoparticles. Synthesis of Oleic Acid-d1 (C17H33COOD). A 50 mL Schlenk flask was charged with 1 mL (3.15 mmol) of oleic acid and 10 mL of methanol-d4 (246.18 mmol) inside a nitrogen-filled glovebox. The flask was removed from the glovebox and connected to dual manifold argon-vacuum Schelnk line. The reaction mixture was stirred for 72 h under an argon atmosphere, followed by removal of solvent under vacuum. 2H NMR confirmed the synthesis of C17H33COOD. Integration of olefinic proton resonances (1H NMR spectrum) and the residual acidic proton of oleic acid suggested that the overall exchange yield of the oleic acid proton with deuterium was ∼85%.

electron microscope (STEM) imaging, and electron diffraction analysis were performed on using either an Hitachi 3300 TEM/STEM, operated with an accelerating voltage of 300 kV, or a JEOL 2200 FS S/ TEM, operated with an accelerating voltage of 200 kV. Selected area electron diffraction (SAED) patterns were analyzed using a Diffraction Ring Profiler.42 X-ray Diffraction (XRD) Analysis. XRD characterization was carried out using a Bruker AXS D8 Discover diffractometer, equipped with an area detector and a Cu Kα radiation source (λ = 1.54056 Å). All samples were prepared by drop-casting a concentrated solution of Zn3P2 nanoparticles onto native oxide-capped Si(100) wafers in an argon-filled glovebox. XRD scans were collected at an incident angle of ω = 15°, unless otherwise noted. X-ray Photoelectron Spectroscopy. XPS measurements were performed with a Kratos Axis Ultra, equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The spectra were referenced to the 84.0 eV binding energy of Au 4f7/2, referenced to the Fermi level, and calibrated to the C(1s) peak at 284.8 eV, under vacuum with a base pressure of less than 5 × 10−10 Torr before radiation. Peak fitting of the Zn(2p3/2) XPS spectra was done using 7:3 Gaussian− Lorentzian functions. The P(2p) region was fit using 3:7 Gaussian− Lorentzian functions, with the doublet area ratio constrained to 2:1 and the doublet separation fixed at 0.84 eV. The S(2p) region was fit using 3:7 Gaussian−Lorentzian functions, with the doublet area ratio constrained to 2:1 and the doublet separation fixed at 1.18 eV. Synthesis of Zn3P2 Nanoparticles. The synthesis of Zn3P2 was carried out via a slightly modified procedure as described in our previous report.30 Inside a nitrogen-filled glovebox, 290 μL (0.99 mmol) of P(SiMe3)3 and 100 μL of ZnMe2 (1.38 mmol) were mixed with 4 mL of tri-n-octylphosphine (TOP) in a 20 mL vial, which was placed on a hot plate. The reaction temperature was then increased to 160 °C within 15 min, and the reaction mixture was allowed to stir at this temperature for 2 h (in the previous report, the reaction was stirred for 1 h at 150 °C).30 The vial was then removed from the hot plate and allowed to cool to room temperature. At this point, the mixture appeared as a brown powder suspended in the solvent. Caution! On two occasions, we noted pyrophoric behavior of thoroughly dried, crude unf unctionalized samples (not yet subjected to ligand exchange) upon air exposure. Ligand Exchange Reaction of Zn3P2 Nanoparticles with Oleic Acid. In an argon-filled glovebox, a batch of crude Zn3P2 nanoparticles (∼100 mg), still suspended in the 4 mL of TOP in which they were synthesized, was diluted with 10 mL of dry toluene and centrifuged at 3000 rpm for 5 min. During this time, the particles settled to the bottom of the centrifuge tube to form a dark brown pellet. The supernatant was discarded, and the nanoparticle pellet was washed with 10 mL of dry toluene. The nanoparticles were then dried under vacuum, and 90 mg of Zn3P2 nanoparticles (∼0.35 mmol; calculated without considering the surface ligands) was obtained. At this point, the yield of the Zn3P2 nanoparticle synthesis (before ligand exchange) could be calculated, and the 90 mg of dried nanoparticles represents a yield of ∼75% relative to Zn (from ZnMe2). Complete drying of the nanoparticles was not necessary for any of the ligand exchange reactions; the masses were provided here to assist with reproducibility and yield calculations. The nanoparticles were then resuspended in 5 mL of toluene in a 20 mL vial with a magnetic stir bar. A total of 50 μL of oleic acid (0.08 mmol) was injected into the gently stirred cloudy suspension of nanoparticles in toluene, and immediately, the brownish cloudy suspension turned into a clear, dark red solution. The solution was stirred for 12 h to ensure complete ligand exchange. A total of 8 mL of dry acetonitrile was added to the Zn3P2 solution, which turned the solution into a cloudy brown suspension. The particles were subsequently separated by centrifugation at 3000 rpm for 10 min to form a brown pellet. The supernatant was discarded, and after drying under an argon flow, the total mass obtained was ∼78 mg. The pellet was dissolved in 3 mL of toluene for further characterization. Ligand Exchange Reactions with 1-Octadecanethiol. A 40 mg portion of isolated Zn3P2 particles (centrifuged and toluene washed as

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RESULTS AND DISCUSSION Zn3P2 Nanoparticle Functionalization. The synthesis of Zn3P2 nanoparticles using ZnMe2 and P(SiMe3)3 was slightly modified from our previous report.30 1H NMR analysis of the crude reaction mixture after 1 h of reaction time revealed the presence of unreacted starting materials, ZnMe2 and P(SiMe3)3 (Supporting Information, Figure S1), which could interfere with surface functionalization, the focus of this work. To ensure maximum conversion of the starting materials to the zinc phosphide product in order to reduce interference with surface functionalization, the reaction time was increased to 2 h, and the reaction temperature was raised slightly from 150 to 160 °C. These subtle changes in the reaction parameters resulted in complete consumption of ZnMe2, as shown by 1H NMR of the crude product (Supporting Information, Figure S1), with no significant change in the average particle size; the measured size of these nanoparticles is 16 ± 2 nm, as determined by TEM (Figure 1c, d), compared to the reported size of the previous report, 15 ± 2 nm.30 The as-prepared Zn3P2 nanoparticles, synthesized from dimethylzinc and tris(trimethyl)silylphosphine, P(SiMe3)3, required prolonged sonication (15−20 min) for complete dispersion in toluene.30 Loss of tri-n-octylphosphine (TOP) was observed, as suggested by the agglomeration of particles with more than one washing/ 4655

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red solution. Figure 3 shows the low- and high-resolution TEM images of the oleic acid-treated, soluble Zn3P2 nanoparticles; no changes in size, morphology, or crystallinity were noted after this treatment. FTIR spectroscopy was used to probe the surface chemistry of both the aggregated zinc phosphide nanoparticles (starting materials) and the oleic acid-treated nanoparticles. As shown in Figure 4, the FTIR spectrum of the aggregated Zn 3 P2 nanoparticles, acquired without air exposure, points to residual TOP bound to the surface, as exemplified by the ν(CHx) modes at 2950, 2925, and 2854 cm−1 (Figure 4a) arising from the n-octyl chains. The FTIR spectra of the neat P(SiMe3)3, ZnMe2 , and TOP starting materials are provided for comparison (Figure 4d−f, respectively). The features at 1242 and 837 cm−1 in the aggregated nanoparticles most likely arise from P(SiCH3)2 (vide inf ra) and/or P(SiCH3)3 vibrational modes of the ρ(Si−CH3) and the ν(Si−C), respectively (compare spectra (a) and (d) in Figure 4).43,44 The spectrum of the oleic acid-treated nanoparticles (Figure 4b) shows strong ν(CHx) modes, as would be expected for this long-chain ligand, with a visible ν(CH) mode at 3005 cm−1 arising from the C−H stretch of the internal cis-alkene. The carbonyl region of the oleic acid-treated zinc phosphide nanoparticles, 1700−1500 cm−1, is distinct from that of neat oleic acid (Figure 1b,c). The oleic acid is present as oleate, the deprotonated form, as evidenced by the appearance of strong asymmetric ν(COO) and symmetric ν(COO) vibration modes at 1549 and 1440 cm−1, respectively. The latter feature overlaps with the δ(CH2) vibration mode. Presumably, the oleates are coordinated to a Lewis acidic zinc-based surface site in a chelating or bridging bidentate fashion,14,45−47 similar to that observed in PbS nanoparticles.7b The disappearance of the feature at 1242 cm−1 and the significant reduction of the feature at 837 cm−1 suggest the loss of most of the bound P(SiMe3)3, whereas the appearance of a feature at 2305 cm−1 may correspond to ν(P−H), as described vide inf ra. X-ray photoelectron spectroscopy (XPS), shown in Figure 5a, of the oleic acid-treated Zn3P2 nanoparticles shows the expected doublet centered at 127.6 eV, attributed to the P(2p) of phosphide (P3−) in Zn3P2 (for survey XPS spectrum of the sample, see Figure S3, Supporting Information). No oxidized phosphorus was observed, as it would be expected to appear at 132−133 eV. A peak centered at 1021.5 eV in the Zn(2p) region is consistent with Zn in Zn3P2, with only a small feature

precipitation step (Figure 1b,e). When not subjected to sonication, centrifugation of a toluene suspension of these particles also led to irreversible aggregation, even when followed by prolonged sonication in toluene (up to 30 min). These ligand-stripped particles were extremely air- and moisture-sensitive, as determined by phosphorus XPS (Figure S2, Supporting Information); on two occasions, we have noted pyrophoric behavior of thoroughly dried samples upon air exposure. The TEM image (Figure 1e, no air exposure) shows the extent of aggregation upon ligand loss. A selected area electron diffraction (SAED) pattern confirmed the crystalline nature of these aggregated particles with a ring pattern matching that of tetragonal Zn3P2 (Figure 1f).30 As outlined in Figure 2, for the investigation of the functionalization of the Zn3P2 nanoparticles, three ligand

Figure 2. (a) Chemical approach to functionalization of agglomerated Zn3P2 nanoparticles with three different ligands to produce solubilized Zn3P2 nanoparticles. (b) Ligands chosen in this work.

types were chosen, oleic acid (a carboxylic acid), ndecylphosphonic acid, and 1-octadecanethiol, with oleic acid serving as the starting point. When 50 μL of neat oleic acid was added to 90 mg of the aggregated zinc phosphide nanoparticles in 5 mL of toluene, the particles almost instantly became solubilized upon stirring, producing an optically clear, dark red solution. These soluble nanoparticles could be precipitated with acetonitrile and centrifuged, and the pellet of zinc phosphide nanoparticles easily redissolved in toluene to reform the dark

Figure 3. (a) Low-resolution TEM image of oleic acid-treated Zn3P2 nanoparticles (dark-field STEM image inset) and (b) the corresponding highresolution TEM image. 4656

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features expected for the n-octadecyl chain, such as the intense ν(CHx) modes between 2960 and 2850 cm−1 and the δ(CH2) at 1466 cm−1, in addition to the new feature we tentatively assign to a P−H stretch, ν(P−H), at 2308 cm−1 (vide inf ra). The FTIR spectrum corresponding to the n-decylphosphonic acid-treated Zn3P2 nanoparticles (Figure 6c) also shows intense ν(CHx) features between 2900 and 2960 cm−1, as well as phosphorus−oxygen stretching modes, ν(PO), centered around 1000 cm−1, and a broad ν(OH) above 3000 cm−1. XPS of the 1-octadecanethiol- and n-decylphosphonic acidtreated Zn3P2 nanoparticles is shown in Figure 5b,c. In the case of 1-octadecanethiol, both Zn(2p) and P(2p) spectra show little oxidation, as substantiated by the lack of features at higher binding energies.29,35−38 The S(2p) feature was fitted with a doublet, arising from S(2p3/2) and S(2p1/2); the binding energy of this sulfur doublet, at 162.1 and 163.4 eV, corresponds to bonded thiolate on the surface; and the unbound (free) 1octadecanethiol feature would be expected at 164.5 and 165.7 eV, respectively.36 For n-decylphosphonic acid-treated Zn3P2 nanoparticles (Figure 5c), the XPS profile for the Zn(2p) is suggestive of a substantial amount of Zn bonded to oxygen, as shown by deconvolution; the feature centered around ∼1022.5 eV corresponds to ZnO or related oxygen-bound zinc atoms.37,48 The P(2p) spectrum is more complicated than those of the oleic acid- and 1-octadecanethiol-treated Zn3P2 nanoparticles because of the additional phosphorus in the ligand. The P(2p) feature corresponding to Zn3P2 is visible at ∼127.6 eV, but two additional higher energy doublets can be seen in the range of ∼129−134 eV. The features at 132.4 and 133.6 eV can be assigned to surface-bound phosphates/ phosphonates (POx/RPO(OH)2), while those at 129.7 and 130.5 eV might be attributed to phosphorus suboxides.48 Mechanistic Studies. In order to understand how the surface chemistry proceeded to produce these (presumably) oleate-, thiolate-, and phosphonate-terminated Zn3P2 nanoparticle surfaces, a series of multinuclear solution phase NMR experiments, to complement the FTIR results, were carried out. Starting with the aggregated Zn3P2 nanoparticles (those shown in Figure 1e), 44 mg of these nanoparticles was washed 10 times with toluene in an inert atmosphere glovebox, followed by centrifugation to ensure no residual ZnMe2 and P(SiMe3)3. The particles were then dried under vacuum and were placed in an NMR tube, followed by the addition of 0.7 mL of toluene-d8 and 10 μL (0.06 mmol) of diphenylmethane as an internal standard.. A 10 μL (0.016 mmol) portion of oleic acid was added, and the suspension was thoroughly mixed just before NMR analysis to produce a deep red solution. As a control experiment, a similar NMR sample was prepared but without addition of oleic acid, to detect any residual ZnMe2, TOP, P(SiMe3)3, or other side products in the toluene-d8; it was apparent that the 10 toluene washes had removed any residual nonbound starting materials or other impurities, to the detection limit of NMR in both 1H (see Figure 7a, and the Supporting Information, Figure S6) and 31P{1H} (Supporting Information, Figure S7). The 1H NMR spectrum, taken ∼3 min after the addition of oleic acid, displays two new peaks at δ 0.27 and 0.21 ppm (Figure 7a, and the Supporting Information, Figure S6), with the δ 0.27 feature showing a 29Si satellite with a 2JSiH value of 6.9 Hz. The 13 C{ 1 H} NMR spectrum revealed two corresponding singlet resonances (determined by gHSQC 2D experiment; Figure S8 in the Supporting Information) at δ 0.61 and −4.75 ppm. Upon acquiring proton-coupled 13C NMR, the

Figure 4. FTIR spectra of (a) agglomerated Zn3P2 nanoparticles, (b) oleic acid-treated Zn3P2 nanoparticles, (c) oleic acid, (d) tris(trimethylsilyl)phosphine, (e) dimethylzinc, and (f) tri-n-octylphosphine (TOP).

that could correspond to either ZnO or surface zinc bonded to the oleate ligand.29,35−38 The main carbon features in XPS correspond to the C(1s) of the aliphatic chain of oleate at 285.0 eV; a small feature centered at 288.3 eV in C(1s) was also observed owing to the carboxylate carbon of the bound oleate ligand (Supporting Information, Figure S4). A 1H NMR spectrum of these oleic acid-treated nanoparticles, in toluened8, shows broad features compared to those observed for free oleic acid, owing to the restricted rotational mobility upon attachment to the nanoparticle surface (Supporting Information, Figure S5).12,14 The signature olefinic protons of oleic acid that appear as a sharp multiplet at δ 5.4 ppm, are broader upon binding to the nanoparticle, and are shifted downfield to δ 5.5− 5.8 ppm. Other commonly used long-chain ligands for nanoparticle stabilization include alkylphosphonic acids and alkanethiols. Addition of either n-decylphosphonic acid or 1-octadecanethiol to aggregated Zn3P2 nanoparticles also resulted in optically transparent red solutions. 1-Octadecanethiol treatment was carried out in a manner similar to that for oleic acid treatment, in which the ligand was added to a toluene suspension of the aggregated Zn3P2 nanoparticles. Because of the poor solubility of phosphonic acids in toluene, THF was used instead of toluene for this procedure. The FTIR spectrum (Figure 6a) of the 1-octadecanethiol-treated Zn3P2 nanoparticles contains the 4657

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Figure 5. High-resolution XPS spectra of (a) Zn(2p) and P(2p) regions of oleic acid-treated nanoparticles, (b) Zn(2p), S(2p), and P(2p) regions of 1-octadecanethiol-treated nanoparticles, and (c) Zn(2p) and P(2p) regions of n-decylphosphonic acid-treated nanoparticles.

−P(SiMe3)2 phosphido groups, as well as Zn−CH3 functionalities. We cannot, however, rule out the existence of residual P(SiMe3)3 phosphine termination. It can be concluded that the ligand exchange process proceeds via simultaneous deprotonation of the acidic proton of the added ligand by the Zn−CH3 group, which leads to the release of CH4, and subsequent attachment of the oleate to surface zinc sites, as summarized in Figure 8. In the case of oleic acid, the release of C17H33COOSiMe3 is due to attack by oleic acid on the trimethylsilyl group in surface-bound phosphido −P(SiMe3)2 and/or phosphine P(SiMe3)3 terminations, which would result in −P(SiMe3)H and/or P(SiMe3)2H and, upon further reaction with oleic acid, would produce −PH2, P(SiMe3)H2, and/or PH3, depending upon the ratio of oleic acid to P(SiMe3) groups, as shown in Figure 8. All of these products were observed when P(SiMe3)3 and oleic acid were mixed in ratios of 1:1, 1:2, and 1:8 in toluene-d8 at room temperature (Supporting Information, Figures S9 and S10).50 The presence of the surface P−H bonds, as suggested by FTIR (Figure 4b),51 could be a product of this reaction. On the basis of the quantity of CH4 and C17H33COOSiMe3 produced with respect to an internal standard, the quantity of Zn−CH3 and

former peak split into a quartet of multiplets (Figure 7b) with a 1 JCH value of 119.5 Hz, whereas the latter peak split into a quintet with a 1JCH value of 125.6 Hz (Figure 7b). On the basis of the multiplicities, the resonance at −4.75 ppm in the 13C {1H} NMR spectrum, along with the corresponding peak at 0.21 ppm in the 1H NMR spectrum, was assigned to CH4. The peak at δ 0.61 in the 13C {1H} NMR spectrum, and the corresponding peak at 0.27 ppm in 1H NMR spectrum, was attributed to C17H33COOSiMe3.49 When partially deuterated oleic acid, C17H33COOD(H) (85% D), was used, elimination of both CH4 and CH3D was observed in the 1H NMR spectrum (Figure 7a, inset), which we attribute to the protonation and subsequent release of methane upon protonation of surface-bound methyl groups. The appearance of CH4 and C17H33COOSiMe3 upon addition of C17H33COOH implied the presence of unreacted Zn−CH3 and P−SiMe3 groups on the surface. The Zn3P2 nanoparticles were produced via the reaction of ZnMe2 and P(SiMe3)3, which, as described earlier, resulted in the formation of the precursor dimer [MeZn(μ-P(SiMe3)2]2 that then undergoes conversion to the Zn3P2 nanoparticle product.30 We, therefore, suggest that the surfaces of the Zn3P2 nanoparticles are capped with zinc-bound 4658

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Figure 6. FTIR spectra of (a) 1-octadecanethiol-treated Zn3P2 nanoparticles, (b) neat 1-octadecanethiol, (c) n-decylphosphonic acid-treated nanoparticles, and (d) neat n-decylphosphonic acid.

P−SiMe3 groups on the surface of the Zn3P2 nanoparticles was estimated using simple calculations (Supporting Information). Although these Zn3P2 nanoparticles are certainly not perfectly spherical, we can estimate that, on each ∼15 nm sized nanoparticle, there are conservatively ∼200−300 Zn−CH3 groups and ∼200 P−SiMe3 groups on the surface of each Zn3P2 nanoparticle. Since both 1-octadecanethiol and n-decylphosphonic acid contain acidic protons and nucleophilic conjugate bases (thiolate and phosphonate), similar reaction pathways can be postulated. Methane elimination was observed by 1H NMR spectroscopy upon addition of these molecules to an agglomerated Zn3P2 nanoparticle suspension (Supporting Information, Figures S11 and S12). Presumably, both the phosphonic acid and thiol passivation reactions with the Zn3P2 nanoparticles proceed via similar pathways with surface Zn− CH3 groups as compared to that observed in the case of oleic acid. In the case of n-decylphosphonic acid, the trimethylsilylsubstituted product, CH3(CH2)9P(O)(OSiMe3)2, was also observed, which corresponds to the chemistry shown in Figure 8b (Supporting Information, Figure 11).13 A subtle difference, however, was seen with 1-octadecanethiol; the FTIR spectrum of 1-octadecanethiol addition to agglomerated Zn3P2 nanoparticles shows residual SiMe3 groups, due to the observation of the ρ(Si−Me) at ∼1242 cm−1. When P(SiMe3)3 was reacted with 3 equiv of 1-octadecanethiol in toluene-d8 at room temperature for 24 h, most of the P(SiMe3)3 remained unreacted, with very little conversion to P(SiMe3)2H, P(SiMe3)H2, and PH3 (Supporting Information, Figure S13). The higher reactivity of oleic acid when compared to 1octadecanethiol is attributed to the oxophilicity of silicon. It is surmised, therefore, that the thiolate binding only results due to deprotonation by Zn−Me and formation of a Zn−SR bond and that the phosphido and/or phosphine groups remain intact.

Figure 7. (a) Selected area of the 1H NMR spectrum of the aggregated Zn3P2 nanoparticles in toluene-d8 (below), and the 1H NMR spectrum of the same area after (above) the addition of oleic acid; inset shows the elimination of CH3D upon addition of oleic acid-d1. (b) Selected area of 13C NMR spectrum of the nanoparticles in toluene-d8 after the addition of oleic acid, showing the appearance of CH4 and C17H33COOSi(CH3)3.

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CONCLUSIONS A combination of FTIR spectroscopy, XPS, chemical reactivity studies, and multinuclear NMR spectroscopy enabled the determination that the Zn3P2 nanoparticle surface, produced from the reaction of ZnMe2 and P(SiMe3)3, is most likely terminated by Zn−CH3 and phosphide −P(SiMe3)2 and/or P(SiMe3)3 groups. Reaction of these P−SiMe3/−CH3 terminated particles with acidic precursors to X-type ligands, such as carboxylic acids, phosphonic acid, and thiols, readily led to ligand passivation and a soluble Zn3P2 nanoparticle solution. On the basis of the byproduct released after ligand passivation, the ligand exchange process occurs through rapid deprotonation of acidic protons by surface-bound Zn−CH3 groups, accompanied by simultaneous release of CH4. In the case of oleic acid and n-decylphosphonic acid, an additional reaction of this nucleophilic ligand with P(SiMe3)x (x = 2, 3) occurs, leading to formation of XSiMe3 (X = oleate, n-decylphosphonate); the reaction of 1-octadecanethiol with P(SiMe3)3, however, appears to be negligible. All three approaches result in stable colloidal solutions of Zn3P2 nanoparticles, and the next 4659

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Chairs program. Li Peng is thanked for TEM imaging, Guy Bernard for acquiring the solid-state NMR spectra, and Jeffrey Murphy for assistance with particle size analysis. Brian Olsen is thanked for preparing the table of contents image. The staff of the NMR facilities at the University of Alberta and the Alberta Centre for Surface Engineering and Sciences (ACSES) are thanked for assistance with analyses.

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Figure 8. Reaction of P(SiMe3)3 with (a) oleic acid, (b) ndecylphosphonic acid, and 1-octadecanethiol. (c) Proposed surface termination of the starting aggregated Zn3P2 nanoparticles. (d) Substitution reactions on the surface of Zn3P2 nanoparticles, using oleic acid as the demonstrative ligand.

step is to use these nanoparticle solutions to prepare films for photovoltaic applications with short and/or conjugated ligands.

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AUTHOR INFORMATION

REFERENCES

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* Supporting Information S

Twenty-two figures composed of additional XRD, TEM, and NMR spectroscopic characterization data, additional experimental procedures, and calculations. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (J.M.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Alberta Innovates Energy and Environment Solutions (AIEES), NSERC, the National Institute for Nanotechnology (NRC-NINT), the Canadian Foundation for Innovation (CFI), and the Canada Research 4660

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