Phase-Pure Crystalline Zinc Phosphide Nanoparticles: Synthetic

Feb 20, 2014 - Use of tri-n-octylphosphine (TOP) with ZnMe2 takes place at high temperatures (∼350 °C) and appears to proceed via rapid in situ red...
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Phase-Pure Crystalline Zinc Phosphide Nanoparticles: Synthetic Approaches and Characterization Md Hosnay Mobarok,†,‡ Erik J. Luber,†,‡ Guy M. Bernard,‡ Li Peng,† Roderick E. Wasylishen,‡ and Jillian M. Buriak*,†,‡ ‡

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

†

S Supporting Information *

ABSTRACT: Zinc phosphide may have potential for photovoltaic applications due to its high absorptivity of visible light and the earth abundance of its constituent elements. Two different solution-phase synthetic strategies for phase-pure and crystalline Zn3P2 nanoparticles (∼3−15 nm) are described here using dimethylzinc and vary with phosphorus source. Use of tri-n-octylphosphine (TOP) with ZnMe2 takes place at high temperatures (∼350 °C) and appears to proceed via rapid in situ reduction to Zn(0), followed by subsequent reaction with TOP over a period of several hours to produce Zn3P2 nanoparticles. Some degree of control over size was obtained through variance of the TOP concentration in solution; the average size of the particles decreases with increasing TOP concentration. With the more reactive phosphine, P(SiMe3)3, lower temperatures, ∼150 °C, and shorter reaction times (1 h) are required. When P(SiMe3)3 is used, the reaction mechanism most likely proceeds via phosphido-bridged dimeric Zn(II) intermediates, and not metallic zinc species, as is the case with TOP. In all cases, the nanoparticles were characterized by a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and solution and solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) analyses. Surface investigation through a combination of MAS 31P NMR and XPS analyses suggests that the particles synthesized with TOP at 350 °C possess a core−shell structure consisting of a crystalline Zn3P2 core and an amorphous P(0)-rich shell. Conversely, the ligand and phosphorus sources are decoupled in the P(SiMe3)3 synthesis, resulting in significantly reduced P(0) formation.

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since a 0.7 cm2 bulk sample of Zn3P2 was integrated into a Schottky PV cell and was reported to have a power conversion efficiency of 6%.9 Although solution syntheses of nanocrystalline morphologies of most of the many earth abundant inorganic materials with potential for photovoltaics (FeS2,10−16 Cu2S,17 CZTS,18 and Cu2O19) have already been realized, the successful synthesis of crystalline Zn3P2 nanoparticles remained elusive until very recently. A number of solid-phase syntheses of nanostructured Zn3P2 morphologies have been described, including nanotubes,20,21 nanobelts,22,23 nanowires,24−27 and hyperbranched structures,28 but these structures are not ideal for the production of smooth thin films in typical PV device architectures. In terms of solution-phase syntheses of Zn3P2 nanoparticles, the earliest report dates back to 1985 when Weller and co-workers briefly described very small Zn3P2 nanoparticles, prepared through injection of phosphine gas into an aqueous solution of zinc salts; the material was,

INTRODUCTION The solution-phase synthesis of inorganic semiconductor nanomaterials composed of earth abundant elements is drawing increasing interest because of the potential for large-scale deployment in nanoparticle-based photovoltaics (PV).1 Materials such as FeS2, Cu2S, Cu2O, copper zinc tin sulfide (CZTS), and Zn3P2 are considered to be promising for PV applications due their ideal combination of a predicted reasonable-to-good theoretical performance, low material requirements of these materials, and low cost due to their abundance and accessibility in the earth’s crust.2 A colloidal nanoparticle form of these materials is attractive because such dispersions could be used for the production of thin films through wet manufacturing processes such as spin-coating, doctor-blading, roll-to-roll printing, and inkjet printing.3−5 Among the earth abundant candidate materials, zinc phosphide has an ideal band gap (1.5 eV), a large absorption coefficient (>104 cm−1), and long minority diffusion length (5−10 μm) for photovoltaic applications.6−8 Moreover, the precursor materials (Zn and P) are abundant, inexpensive, and nontoxic. Early work from the 1980s underlines the potential of Zn3P2 for PV applications, © 2014 American Chemical Society

Received: February 15, 2014 Published: February 20, 2014 1925

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however, only characterized by ultraviolet−visible (UV−vis) spectroscopy as the paper was focused on Cd3P2, an isoelectronic congener of Zn3P2.29 In 1994, Buhro produced Zn3P2 nanoparticles through the reaction of Zn(OMe)2 with P(SiMe3)3 and revealed the formation of small particles (∼10 nm diameter) via transmission electron microscopy (TEM); determination of the underlying atomic structure of the resulting materials was not carried out [e.g., using X-ray diffraction (XRD) or selected area electron diffraction (SAED) characterization methods], as this was not the focus of the work.30 Later, Green and O’Brien looked at the reaction of ZnMe2 and PtBu3 at 150−250 °C, and while TEM clearly showed spherical nanoparticles, the crystalline structure of these particles was not determined.31 Mézailles and co-workers demonstrated the synthesis of extremely small Zn3P2 nanoparticles (1−5 nm, as determined by TEM) through the reaction of Zn(0) nanoparticles and white phosphorus, but no other data were provided. The related material, InP, was the primary target of the work.32 More recently, Miao and coworkers reported a Zn3P2@ZnO nanohybrid material by preparing preformed Zn3P2 nanoparticles from either PH3 (g) or P(SiMe3)3, and diethylzinc, zinc stearate, or zinc bis[bis(trimethylsilyl)amide] as the zinc source, at room temperature, followed by partial hydrolysis to form ZnO in situ; the resulting product was a hybrid Zn3P2:ZnO material.33 Our group recently reported synthesis of phase-pure and highly crystalline Zn3P2 nanoparticles using tri-n-octylphosphine (TOP) as a phosphorus source at ∼350 °C and integrated them into ZnO/ Zn3P2 heterojunction devices.34 The zinc phosphide nanoparticles were found to possess the tetragonal α-Zn3P2 structure, as confirmed by XRD and TEM, and the surface composition was analyzed by X-ray photoelectron spectroscopy (XPS). While the devices fabricated from these particles showed high photosensitivity and excellent rectification behavior, they displayed no photocurrent. It was determined that the nanostructure of these zinc phosphide nanoparticles was the most likely cause of the nonexistent photocurrent, as detailed XPS studies suggested that the crystalline Zn3P2 cores were surrounded by an amorphous shell of phosphorus. Bulk zinc phosphide is also known to form an amorphous shell of either P(0)35−37 or phosphorus suboxide,38 and this surface layer would need to be removed to enable good electrical contact. Attempts to remove this surface layer with dilute bromine and HF (aq) resulted in complete destruction of the nanoparticle films. A detailed investigation of the synthetic approach toward the production of Zn3P2 nanoparticles may therefore lead to insights that will enable a cleaner synthesis, avoiding the formation of the insulating phosphorus or phosphorus suboxide shell. In this paper, we expand upon the solution phase synthetic approach described for Zn3P2 nanoparticles and examine several variations, accompanied by detailed XRD, XPS, solid-state, and solution nuclear magnetic resonance spectroscopic (NMR) studies, as summarized in Scheme 1. The result of these studies is a new synthetic protocol that requires lower temperatures (∼150 °C versus ∼350 °C) and results in monodisperse and crystalline Zn3P2 nanoparticles, with minimal quantities of P(0) on the surface.

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Scheme 1. Synthetic Approaches for Zn3P2 NPs [Reactions 1.1 and 1.2]

Sigma-Aldrich. Toluene was degassed and distilled before use. 2Propanol (99.99%) was degassed under bubbling argon and dried over 4 Å molecular sieves before use. All reactions were carried out under argon using Schlenk techniques or in an argon-filled glovebox. NMR Spectroscopy. Solid-state 31P and 13C NMR spectra of the Zn3P2 nanoparticles 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, respectively, for 13C. Direct polarization 31P{1H} NMR spectra of magic angle spinning (MAS) samples were acquired with a rotor-synchronized Hahn echo pulse sequence (90° − τ − 180° − τ − ACQ); the 31P 90° pulse was 4.0 μs, the 180° pulse was 8.0 μs, and the recycle delay was 120 s. 31P and 13C NMR spectra were also obtained with ramped cross-polarization (CP): the 1H 90° pulses were 4.0 μs, contact times of 2.0 to 5.0 ms were used, and the recycle delays were 6.0 s. The two-pulse phase modulated (TPPM) 1H decoupling scheme of Bennett and co-workers was used in the acquisition of all 31P and 13C NMR spectra.39 The former were referenced to 85% phosphoric acid (δ = 0) by setting the isotropic peak of an external ammonium dihydrogen phosphate sample to 0.81 ppm, and 13C NMR spectra were referenced to TMS (δ = 0) by setting the high frequency peak of an external adamantane sample to 28.56 ppm.40 13C NMR spectra were also acquired using the dipolar dephasing (often referred to as nonquaternary suppression) technique.41 A 40 μs dephasing period with an 8.0 μs refocusing pulse was used to acquire these spectra. Solution NMR spectra were recorded on Varian Inova-400 spectrometer operating at the resonance frequencies of 161.8 MHz for 31P nuclei and 399.8 MHz for 1H nuclei. The 1H 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. Peak fitting of solid-state 31 P MAS NMR spectra was performed with Gauss-Lorentzian lineshapes using a Levenberg−Marquardt nonlinear least-squares optimization algorithm. Electron Microscopy. High-resolution TEM imaging and electron diffraction analysis were performed on a Hitachi 3300 TEM/STEM, operated with an accelerating voltage of 300 kV. Electron tomography and elemental mapping were acquired on a JEOL 2200 FS S/TEM, operated with an accelerating voltage of 200 kV and equipped with an in-column omega energy filter. SAED patterns were analyzed using Diffraction Ring Profiler.42 Two-dimensional elemental mappings were acquired by using energy-filtered TEM and scanning transmission electron microscopy−electron energy loss spectroscopy (STEM− EELS) spectral imaging. High-angle annular dark field (HAADF) STEM was utilized for electron tomography, as it provided the most suitable contrast mechanism for tomographic reconstruction of a crystalline sample, where the intensity is approximately proportional to Z2 and has little or no diffraction effects.43 HAADF−STEM images were collected from ±60° with 3° steps. The TEMography software package from System in Frontier Inc.44 was used for reconstruction and visualization. The effect of gold particles, used as fiducial markers, was minimized by the simultaneous iterative reconstruction techniques algorithm.45,46

EXPERIMENTAL SECTION

Chemicals. Dimethylzinc (96%) was obtained from Alfa-Aesar. Tri-n-octylphosphine (TOP, 97%) and tris(trimethylsilyl)phosphine [P(SiMe3)3, 98%] were obtained from STREM. Tri-n-octylamine (98%) and 1-octadecene (90%, technical grade) were obtained from 1926

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26.7 μL of P(SiMe3)3 (0.092 mmol), and 100 μL of TOP were placed in a J Young NMR tube and sealed with a Teflon cap in an inert atmosphere glovebox. The tube was then removed from the glovebox and immersed in an oil bath set at ∼110 °C (just under the boiling of toluene). The tube was left sitting at this temperature for ∼48 h. Carrying out the reaction in o-xylene at 144 °C significantly reduced the reaction time (∼18 h).

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 from a concentrated solution (∼40 mg/mL) of Zn3P2 NPs onto (100) native oxide-capped Si wafers in a nitrogen filled glovebox. XRD scans were collected at an incident angle of ω = 15°, unless otherwise noted. Peak fitting (Voigt functions) and background subtraction (spline) were performed using Fityk.47 Peak parameters used for Scherrer analysis (Gaussian FWHM and Lorentzian FWHM) were adjusted for instrumental broadening using a calibrated LaB6 NIST standard (SRM-660b). X-Ray Photoelectron Spectroscopy. XPS measurements were performed using 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 corrected to the C(1s) peak at 284.8 eV, and under a vacuum of 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 doublet separation fixed at 0.83 eV. Thermogravimetric Analysis (TGA). TGA measurements were carried out in a nitrogen environment in a Mettler Toledo TGA/ DSC1. The nanoparticles were dried overnight under vacuum prior to the analysis. A total 3.67 mg of sample was heated at a rate of 10 °C/ min. UV−Vis Analyses. UV−vis spectra were acquired on an Agilent 8453 spectrometer. Synthetic Methods. Reaction 1.1, Scheme 1. To investigate the effects of TOP concentration, several mixtures of 1-octadecene and TOP were prepared, consisting of 8 − x mL of 1-octadecene + x mL of TOP (x = 3, 4, 5, 6, and 8) in four separate three-neck reaction flasks connected to reflux condensers. The mixtures were heated to 320 °C followed by rapid injection of a mixture of 100 μL of dimethylzinc and 2 mL of TOP, in each reaction flask. The temperature of the mixtures was increased to 348 °C over a period of 2 h, followed by stirring of the mixture for 3 h at this temperature. The crude mixture was then diluted with 10 mL of dry toluene. Any large aggregates formed during the reaction were separated by centrifuging the crude solution, followed by filtration through a 0.45 μm filter. The addition of 20 mL of 2-propanol led to the flocculation of nanoparticles, which were separated by centrifuging at 4400 rpm. This washing cycle was repeated, at least three times, to remove residual ligand and solvent (ODE). Reaction 1.2, Scheme 1. In a typical experiment in an argon-filled glovebox, 4 mL of TOP in a 20 mL vial was stirred at 100 °C for 1 h. After the TOP temperature was cooled to ambient temperature, a total of 105 μL (1.41 mmol) of dimethylzinc and 267 μL (0.92 mmol) of P(SiMe3)3 was added to the vial. The vial was then sealed with a septum, and a temperature probe was inserted through the septum. The temperature of the mixture was gradually increased to 150 °C within a period of ∼10 min. The solution was then stirred at this temperature for 1 h. During this time, a brown to dark red precipitate appeared. A total of 5 mL of toluene was added to 1 mL of the crude suspension of nanoparticles. The toluene suspension needed to be sonicated for 10 to 15 min for full dispersion of particles (sonication was carried out outside the glovebox with a septum-capped tube). The addition of 15 mL of 2-propanol led to the flocculation of nanoparticles with an average diameter of ∼20 nm, which were separated by centrifugation. The particles were redispersed in toluene for characterization. More than one washing cycle was found to be detrimental to the redispersivity of the particles, suggestive of ligand loss during washing. Caution: We have had two instances of dried samples prepared via reaction (1.2) spontaneously igniting in air ambient. Possible causes include residual ZnMe2 and/or P(SiMe3)3, ZnMe groups remaining on the nanoparticle surface, or the reaction of unprotected Zn3P2 nanoparticles with water and/or oxygen. Further studies are underway to understand this phenomenon. Nanoparticle Synthesis in an NMR Tube. In an argon-filled glovebox, 600 μL of toluene-d8, 10 μL of dimethylzinc (0.138 mmol),

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RESULTS AND DISCUSSION For solution-phase syntheses of metal phosphides, reaction conditions are dictated by the choice of source reagents. Sources such as tri-n-octylphosphine (TOP) or tri-n-octylphosphine oxide (TOPO) require high reaction temperatures (∼300 °C) to cleave the strong P−C bonds but are more convenient and safer to work with than P4 or PH3 gas, for example.48−58 The use of phosphine reagents as a phosphorus source is, however, tempered by the fact that the phosphorus itself must undergo a multielectron reduction process in the overall reaction [eq 2.1; Scheme 2], since the phosphorus is in a Scheme 2. Synthetic Outline for Metal Phosphide Syntheses Using TOP and P(SiMe3)3

+3 or +5 formal oxidation state in TOP or TOPO, respectively, and in the −3 oxidation state in the product metal phosphide. Owing to the weaker P−Si bond (363.6 kJ/mol) in P(SiMe3)3, compared to the P−C bonds (508 ± 9 kJ/mol)59 in TOP, and a −3 formal oxidation state of phosphorus in P(SiMe3)3, this reagent may provide a redox neutral path for the production of metal phosphides (eq 2.2; Scheme 2). P(SiMe3)3 has been referred to as a “direct PH3 replacement” and has been shown to have wide applicability for the clean and direct synthesis of phosphide materials such as bulk InP.60 Earlier work in our group suggested that a thin phosphorus-rich shell forms around the Zn3P2 core when TOP is used,34 and so P(SiMe3)3, with its formal phosphorus oxidation state of P(−3), may avoid unwanted P(0) byproduct formed en route between the P(+3) in TOP, and P(−3). Reaction 1.1. In our previously reported high-temperature synthetic approach [reaction 1.1, Scheme 1], dimethylzinc and TOP, in a 1:7 molar ratio, were heated together in 1octadecene as a solvent at 350 °C for 6 h.34 In this reaction, it is assumed that TOP is both a phosphorus source and a surface passivating ligand. The details of X-ray diffraction and electron microscopy characterization for particles obtained by this method were discussed in our previous report, but no attempts to control the size of the resulting nanoparticles were made.34 In this present study, a more systematic approach for controlling the size of the nanoparticles has been undertaken. Among various reaction parameters, the concentration of ligand can have a significant influence on the size of the particles;61,62 it has also been demonstrated that the use of an “innocent” and 1927

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presumably unreactive high-boiling solvent can provide a means to control the concentration of the ligand.61 In a typical experiment, dimethylzinc (diluted in 2 mL of TOP) was injected into a hot mixture of TOP and 1-octadecene at 320 °C, followed by a slow increase in temperature to 348 °C over 2 h from the time of injection, at which point the reaction was maintained at this temperature for another 3 h. The color of the solution changed to a gray suspension immediately after the injection of dimethylzinc. The gray material was characterized to be metallic zinc as confirmed by X-ray diffraction analysis (XRD; see Supporting Information, Figure S1). Upon raising the temperature to 348 °C, the solution gradually turned to a dark red over a period of 3 h, indicating the formation of Zn3P2 nanoparticles. The resulting mass of nanoparticles, following centrifugation, filtration, and precipitation, as described in the Experimental Section, is ∼80 mg per batch. As reported earlier, TGA analysis suggests ∼10% organic content present in the sample,34 and therefore the approximate yield of Zn3P2 is 72 mg per batch (61% overall yield; calculations in the SI). As can be observed from the XRD and TEM analysis, by varying the ratio of TOP/1-octadecene, the size of the particles can be tuned (detailed reaction conditions described in the Experimental Section). Shown in Figure 1a−d are TEM images of different sized nanoparticles with histograms outlining the distribution of particle sizes. With an increase of the TOP/1octadecene ratio (v/v) 3:7 to 3:2, it was found that the size of the nanoparticles decreased from 6.3 to 4.4 nm. This result is expected since the rate of growth of the nanoparticles is expected to decrease with increasing competition from ligand binding on the nanoparticle surface.63 As confirmed by XRD, these nanoparticles are crystalline α-Zn3P2 (Figure 2a). Congruent with TEM observations, the features observed by XRD broaden with increasing TOP concentration (Figure 2b), pointing to a decrease in average particle size, according to the Scherrer equation (Figure 2c). The average size of the particles obtained by XRD and TEM are summarized in Table 1. Analysis of these histograms reveals that the particle sizes are roughly normally distributed (see fits in Figure 1). As such, we can estimate the polydispersity (p) of these particles, which is simply given p = σ/μ, where σ is the standard deviation and μ is the mean of the size distribution.64 Inspection of Table 1 reveals that the polydispersity is essentially unchanged with TOP concentration, with values fluctuating between 23% and 26% polydispersity. Given that nanoparticle size decreases monotonically with increasing TOP concentration, the smallest particle size achievable using this synthetic method was determined by carrying out the reaction in neat TOP without 1-octadecene. From TEM, the average size of the particles was found to be 3.7 ± 1.0 nm (p = 26%; Figure 3a and b) while Scherrer analysis of the strongest peak in the XRD spectrum suggests that the average particle size is 3.6 nm. In a typical experiment, TOP was brought to reflux (∼340 °C) under an inert atmosphere, followed by rapid injection of a mixture of dimethylzinc (100 μL, diluted in 2 mL of TOP). The color of the reaction mixture changes rapidly in about 5 min from colorless to dark red. The mixture was then allowed to stir at this temperature for another ∼5 h. A rapid change of color suggests that the formation of nanoparticles in pure TOP is faster than the reaction in which 1-octadecene is used as a diluent. XRD analysis confirms the formation of Zn3P2 with the expected tetragonal crystal structure (PDF no. 01-073-4212), in which the three strongest peaks (for the 202, 004, and 400/224 planes) are clearly

Figure 1. (Left) TEM images of Zn3P2 nanoparticles synthesized via reaction 1.1 using different TOP/1-octadecene ratios: (a) 3 mL of TOP and 7 mL of 1-octadecene, (b) 4 mL of TOP and 6 mL of 1octadecene, (c) 5 mL of TOP and 5 mL of 1-octadecene, (d) 6 mL of TOP and 4 mL of 1-octadecene. (Right) Corresponding histograms of the particle size distributions and best-fit Gaussian distribution (solid line).

distinguishable (Figure 3c). Consistent with the smaller size of the particles, the peaks are broadened significantly due to incoherent scattering from the edges of the small crystallites, as compared to the larger particles synthesized in the diluted 1octadecene/TOP mixture.34 To obtain a better sense of the surface and core chemical composition, multinuclear NMR analyses of these nanoparticles in both solution65 and solid state66 were carried out. Solutionphase 31P{1H} and 1H NMR spectra of the ∼6.5 nm Zn3P2 nanoparticles were acquired in toluene-d8 (see Supporting Information, Figure S2). As expected, no peaks for the Zn3P2 core were observed via solution phase 31P NMR,31 in contrast to 31P MAS NMR spectrum (vide inf ra). However, four broad resonances, centered at δ 54, 30, 6 and −30 ppm are observed, 1928

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Table 1. Average Sizes and Polydispersity of Zn3P2 Nanoparticles Produced via Reaction 1.1 TOP/ODE 3 4 5 6

mL/7 mL/6 mL/5 mL/4

mL mL mL mL

XRD particle size (nm) 6.3 5.4 4.7 4.4

TEM particle size (nm)

polydispersity (%)

± ± ± ±

25 23 26 26

6.2 5.3 4.6 4.4

1.5 1.2 1.2 1.2

Shown in Figure 4 is a solid-state 31P{1H} NMR spectrum of the ∼6.5 nm particles, obtained with magic angle spinning (MAS) and acquired with a Hahn echo. Several distinct features comprise this spectrum: a cluster of low intensity narrow peaks ranging from δ 55 to 0 ppm; four broad features centered at δ 70, −10, −100, and −260 ppm; and two sharp high intensity resonances at δ −197 and −228 ppm. The resonances at δ −197 and −228 ppm, are assigned to the P(−3) in the core of the Zn3P2 nanoparticle, as similar peaks have been reported for the solid-state 31P{1H} NMR spectrum of bulk Zn3P2, where they appear at δ −195 and −228 ppm.67 By utilizing 1H → 31P cross-polarization (CP), it is possible to distinguish which resonances correspond to the surface bound ligands on the nanoparticle, since CP enables the transfer of magnetization from abundant spins (in this case 1H) to less receptive NMR nuclei (31P).68−70 When magnetization is transferred from the 1H nuclei of the surface-bound ligands to proximal 31P nuclei (i.e., those at or close to the surface of the nanoparticles), the intensity of 31P resonances corresponding to the ligand and phosphorus at the nanoparticle surface is significantly enhanced compared to the core resonances. The CP spectrum of the particles, shown in Figure S3a, shows significant enhancement of peaks in the high-frequency region, where five peaks can be clearly identified at δ 54, 30, 5, −6, and −18 ppm. In contrast, the relative intensities of the core Zn3P2 peaks at δ −197 and −228 ppm are drastically reduced. As such, we assign the cluster of peaks from δ 55 to 0 ppm in Figure 4 to surface bound ligands on the nanoparticles. These surface bound ligands could correspond to a range of species, including various phosphonic acids and phosphines that result from TOP decomposition at the high reaction temperatures or are present as trace impurities in the TOP starting material.71−73 To test this possibility, neat TOP was heated to the reflux temperature (∼350−370 °C) for 1 h under an inert atmosphere; a 31P{1H} solution phase NMR spectrum of this refluxed sample confirms the formation of a number of new peaks, some of which can be assigned on the basis of literature values (Supporting Information, Figure S4 and Table S1).71−73 This assignment is further substantiated by the corresponding solid-state 13C{1H} NMR spectrum of these ∼6.5 nm particles, acquired using 1H → 13C cross-polarization, displaying four peaks at δ 32.7, 30.4, 23.5, and 14.6 ppm that correspond to the 13 C nuclei of the octyl group of the ligands (Supporting Information, Figure S5). The presence of zinc and phosphorus oxidation products on the surface would favor binding of species such as phosphonic acid derivatives, and thus the selective bonding of even low concentrations of such molecules could be competitive over TOP surface coordination.74,75 The last unassigned feature in the solid-state 31P{1H} NMR spectrum is the set of broad resonances at δ 70, −10, −100, and −260 ppm. These features display moderate enhancement with CP (less than the ligands, but more than core peaks, see Figure S3a). As such, these phosphorus containing species are

Figure 2. (a) XRD spectrum of Zn3P2 nanoparticles synthesized via reaction 1.1 and reflections of bulk Zn3P2 (PDF no. 01-073-4212). (b) XRD spectra of the strongest reflection of the Zn3P2 nanoparticles (45°), synthesized using different volumes of TOP (inset). (c) Correlation of particle size versus the TOP/1-octadecene ratio, showing the size variation of the resulting nanoparticles.

which can be assigned to the surface bound ligands (see, Supporting Information, Figure S2).31 The 1H NMR spectrum of the ∼6.5 nm particles show significant broadening of the octyl resonances (δ 0.88 for −CH3 and δ 1.25 for −CH2− groups) at ambient temperature, which is consistent with the restricted rotational mobility of the ligands bound to the nanoparticle surfaces (Supporting Information, Figure S2).65 1929

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Figure 4. 31P MAS NMR spectrum of Zn3P2 nanoparticles synthesized via reaction 1.1 with a mixture of 3 mL of TOP and 7 mL of 1octadecene. Spinning sideband peaks labeled with an asterisk. Spectrum acquired with a Hahn echo at 121.6 MHz, spinning at 10 kHz.

shell coated the nanoparticles.34 From XPS analysis, it was determined that the phosphorus-rich shell consisted primarily of P(0) species. However, after extended atmospheric exposure, a significant increase in Zn−O and P−O species was found on the surface. As such, we tentatively assign this series of broad resonances to the phosphorus-rich shell surrounding the Zn3P2 core. Last, it is noted that the set of broad peaks is consistent with the spectra typically observed in solid-state MAS NMR characterization of zinc phosphate compounds.76−78 Shown in Figure 5a is a solid-state 31P{1H} NMR spectrum of the nanoparticles prepared in neat TOP. We see that the relative intensity of the peaks corresponding to the ligands and the phosphorus rich shell is significantly increased relative to the Zn3P2 core peaks. From the P(2p) XPS spectrum (Figure 5b), we see that only 20% of the phosphorus atoms belong to the Zn3P2 core. This further supports the core−shell structure of these nanoparticles, since we would expect the shell to occupy a larger volume fraction of the nanoparticle as the size decreases. Moving beyond simply varying TOP concentration, the use of different ligands could also have an influence on the size and shape of the nanoparticles.79 In addition to phosphines, amines have been used as ligands for metal phosphide nanoparticles.80,81 This class of ligands seems to have weaker surface-ligand interactions, which may accelerate the growth kinetics of the particles, leading to larger particles.63 To test whether amines influence the size of the particles, tri-noctylamine was chosen as the obvious amino analogue of TOP. Detailed experimental procedures and characterization of the particles are provided in the Supporting Information (Figures S7−S10). Use of tri-n-octylamine as a solvent/ligand with TOP resulted in the synthesis of Zn3P2 nanoparticles, as determined by XRD and XPS, but with a broader size distribution (TEM). High resolution TEM showed that the particles were polycrystalline, and thus this approach was not pursued further. Reaction 1.2. Using the synthetic method described for reaction 1.1, Scheme 1, we have shown that it is possible to produce crystalline Zn3P2 nanoparticles with tunable sizes ranging from ∼3−7 nm. However, it has also been demonstrated that all of these particles possess a core−shell

Figure 3. (a) TEM micrograph, (b) corresponding histogram analysis, and (c) XRD spectrum of Zn3P2 nanoparticles synthesized via reaction 1.1 in neat TOP.

expected to be located between the ligands and the Zn3P2 core, suggesting a core−shell structure. A core−shell structure is further substantiated by increasing the CP contact time, where a significant increase in the relative intensity of these broad resonances is observed (Supporting Information, Figure S6). In a previous study, we have characterized the surface of identically prepared Zn3P2 nanoparticles using XPS and UPS, where it was proposed that an amorphous phosphorus-rich 1930

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resulting nanoparticles during the toluene workup. To contrast, an excess of P(SiMe3)3, 1.1 equiv relative to dimethylzinc, led to nanoparticles that aggregated and could not be fully redispersed in toluene during workup. After the reaction was complete, the crude mixture was suspended in toluene, followed by sonication for ∼15 min for complete dispersion; following sonication, the addition of 2-propanol caused the nanoparticles to precipitate. More than one precipitation step led to considerable aggregation of the nanoparticles (see Supporting Information). The resulting mass of the purified nanoparticles is approximately 108 mg after the first washing cycle. TGA analysis suggests ∼12% organic content present in the sample, as shown in the Supporting Information. As a result, the approximate yield of Zn3P2 is 95 mg per batch (81% overall yield). An additional wash led to drastic ligand loss as evident from negligible mass loss (see Supporting Information for yield calculation and TGA analyses). To determine if TOP could be a source of phosphorus, a control experiment was carried out: ZnMe2 (100 μL, 1.38 mmol) was added to 4 mL of neat TOP and the solution stirred in the absence of P(SiMe3)3 at 150 °C for 3 h. The solution remained colorless, and no Zn3P2 formation was noted. This result suggests that at these lower temperatures, TOP is sufficiently stable to not result in the formation of zinc phosphide. Reaction 1.2 can also be carried out in unreactive solvents such as 1-octadecene, dodecane, and toluene (NMR tube reaction, Figure S16). In a typical reaction, ZnMe2 (100 μL, 1.38 mmol), P(SiMe3)3 (267 μL, 0.92 mmol), and TOP (1 mL) were added to 4 mL of 1-octadecene and stirred for ∼1 h at 170 °C. In all cases, the solution turned dark, and formation of zinc phosphide nanoparticles was observed, but particle aggregation was a far more serious problem, even before any washing was carried out. All further reactions were thus carried out in TOP as the solvent. Shown in Figure 6a is a TEM micrograph of nanoparticles produced through reaction 1.2, Scheme 1. The average size of these nanoparticles was found to be ∼14.7 ± 2.1 nm, with a polydispersity of 14%. Relative to reaction 1.1, we see that the monodispersity of the nanoparticles produced via reaction 1.2 is significantly improved, as it is approximately doubled. Moreover, the polydispersity is approaching a value of 10%, which is generally considered the benchmark of high monodispersity for nanoparticle solutions.64 The HRTEM micrographs in Figure 6c,d clearly demonstrate that the nanoparticles consist of single crystals, and lattice fringe spacing of 0.33 nm in Figure 6c matches that of the (202) planes in α-Zn3P2. From the SAED pattern in Figure 6e and XRD spectrum (PDF no. 01-073-4212, which corresponds to the crystal structure of tetragonal Zn3P2) in Figure 6f, we clearly see that these nanoparticles adopt the crystalline structure of tetragonal α-Zn3P2. The larger size of the nanoparticles produced by this method permits a facile study of the shape of these particles by tomographic analysis. STEM tomography suggests that the shapes of the particles are irregular bipyramids (see Supporting Information, Figures S12 and S13). The 31P{1H} NMR spectrum of these particles, acquired with Hahn echo, shows two core peaks centered at δ −227 and −195 ppm, which is consistent with the formation of crystalline Zn3P2 (Figure 7a). The intensity of the ligand peaks in the 31 1 P{ H} NMR spectrum was found to be very low compared to the particles synthesized at a higher temperature. However, a 31 1 P{ H} NMR spectrum, acquired in CP mode, shows the

Figure 5. (a) 31P MAS NMR spectrum of Zn3P2 nanoparticles synthesized in neat TOP via reaction 1.1 and (b) high-resolution XPS spectra of the P(2p) region. The 31P MAS NMR spectrum was acquired with a Hahn echo technique at 121.6 MHz spinning at 10 kHz.

structure, where an amorphous phosphorus-rich shell surrounds the crystalline Zn3P2 core, which forms during the reaction. This phosphorus-rich shell is undesirable, as it is electrically insulating in nature and will greatly impede charge transport in devices made from these nanoparticles. It is believed that the phosphorus-rich shell forms as a result of using TOP as a phosphorus source, where excess P(0) is a byproduct formed en route between the P(+3) in TO, and P(−3) in Zn3P2. As such, we have devised a new synthesis inspired by recent reports describing lower temperatures for nanoparticle syntheses using P(SiMe3)3.80,82 We further investigated the use of this reagent for Zn3P2 nanoparticle growth, as it has seen usage for the growth of nanostructured Zn3P2.33 For instance, Ojo and co-workers demonstrated a low-temperature (RT to 90 °C) synthetic protocol for the synthesis of Cd 3 P 2 nanoparticles in common organic solvents using a soluble cadmium salt, Cd(OAc)2(OAm)2, where OAc = acetate and OAm = octylamine, and P(SiMe3)3 as the phosphorus source.80 Following a similar strategy, the reaction of dimethylzinc and P(SiMe3)3 in neat TOP (here viewed as both a solvent and a ligand) resulted in the rapid formation of Zn3P2 nanoparticles in ∼1 h of reaction time at 150 °C [eq 1.2, Scheme 1]. During the 1 h, the color of the reaction changed from colorless to dark red. A slight stoichiometric excess of dimethyl zinc [1.05 equiv to P(SiMe3)3] was found to be necessary for dispersing the 1931

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Figure 6. (a) TEM micrograph of Zn3P2 nanoparticles produced via reaction 1.2 and (b) corresponding histogram of particle size distribution. (c,d) High resolution TEM micrographs of single crystal particles. (e) SAED pattern (left) and simulation of α-Zn3P2 ring pattern (right). (f) Corresponding XRD pattern and reflections of bulk Zn3P2 (PDF no. 01-073-4212).

Figure 7. (a) 31P MAS NMR spectrum of Zn3P2 nanoparticles synthesized as described in eq 1.2, (b) corresponding P(2p) XPS spectrum. The 31P MAS NMR spectrum was acquired with a Hahn echo technique at 202.5 MHz, spinning at 12 kHz. Spinning side bands are labeled with an asterisk.

ligand peaks at δ 62, 36, 32, and 5 ppm more clearly (Supporting Information, Figure S3c). The lower intensity of ligand resonances might be attributed to the combination of two factors: (i) a lower ratio of surface/bulk for larger particles83 and (ii) significant ligand loss from the surface of the particles during the removal of excess ligand by precipitation. Contrary to the particles synthesized via reaction 1.1, no broad peaks are visible at δ −110, −10, and 70 ppm; however, there is still a very weak shoulder peak at δ −260 ppm. Looking to the P(2p) XPS spectrum of these particles (Figure 7b), we see that it is dominated by P−Zn bonding, with 72% of the P atoms in the Zn3P2 core. However, P−P bonding (28%) is still apparent in the XPS spectrum, evidenced by the doublet centered at ∼129 eV. Given the minimal amounts of phosphorus oxidation observed in the P(2p) XPS spectrum, and the lack of NMR resonances at δ −110, −10, and 70 ppm, we hypothesize that the broad NMR resonance at δ −260 ppm is a consequence of the P−P bonding environment surrounding the Zn3P2 core. A 13C{1H} NMR of this sample shows the usual octyl resonances at δ 32.0, 30.0, 23.0, and 14.0 ppm, along with some unidentified peaks (see Supporting Information, Figure S14). Among these peaks, the ones at δ 69.0 and 8.0 ppm were assigned to −CH and −CH3 of 2-propanol, which might

coordinate to the particle surface during washing. These assignments were confirmed by nonquaternary carbon suppression experiments.41 The origin of these other peaks is not understood at this time. Although these data still show the presence of a phosphorusrich shell in reaction 1.2, it is greatly reduced from that of reaction 1.1. More importantly, the phosphorus source, zinc source, and ligand are decoupled in reaction 1.2. In this way, it should be possible to independently fine-tune the reaction parameters such that the desired bulk and surface stoichiometry are obtained, making this a promising future route for the production of Zn3P2 nanoparticles. One potential challenge related to future use of these nanoparticles relates to their poorly understood surface chemistry and hence their stability in ambient atmosphere. The fact that washing steps for the Zn3P2 nanoparticle formed via reaction 1.2 remove most of the ligand suggests that phosphines bind weakly. Current investigation is underway to find suitable ligands to increase the oxidative and hydrolytic stabilities of the particles under ambient conditions. Mechanistic Insights. Reaction 1.1 is based upon the injection of a diluted solution of ZnMe2 in TOP, into a hot (348 °C) solution of TOP and 1-octadecene. As mentioned vide supra, the dimethylzinc, which is colorless, turns gray 1932

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stable at temperatures approaching 300 °C, and so at 150 °C, it is expected to be stable.85 It therefore appears that the mechanism is not reliant on independent decomposition pathways for ZnMe2 and P(SiMe3)3, and that the in situ formation of the phosphide-bridged intermediate is the starting point for the ultimate chemistry that leads to Zn3P2.

within 5 min of injection into the hot solution, and this gray material was determined to be metallic zinc. As outlined with reaction 3.1 in Scheme 3, the Zn3P2 most likely proceeds via the Scheme 3. Proposed Mechanisms for Formation of Zn3P2 for Reactions 1.1 and 1.2

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CONCLUSIONS Two approaches toward the solution phase synthesis of phasepure Zn3P2 nanoparticles were explored. The first synthetic route, reaction 1.1, involves the rapid in situ decomposition of ZnMe2 to Zn(0) species in TOP at 348 °C, followed by conversion of the metallic zinc to the Zn3P2 product. Control over the average size of the resulting Zn3P2 nanoparticles was enabled through dilution of the TOP capping ligand with the high boiling point solvent, 1-octadecene. The second reaction investigated, reaction 1.2, uses the phosphine, P(SiMe3)3, which has been previously considered as a liquid-phase source of PH3. In this case, the reaction appears to proceed via a phosphidobridging dinuclear zinc compound that decomposes at 150 °C to Zn3P2 nanoparticles; the large excess of TOP (the solvent) serves to cap the resulting particles and limit growth to result in a relatively monodisperse colloidal solution. Solid-state NMR, XRD, and high-resolution electron microscopy clearly demonstrate that the nanoparticle cores, synthesized by both reaction approaches, are indeed phase pure, monocrystalline Zn3P2 nanoparticles, but the surface chemistries vary. The ZnMe2/ TOP reaction 1.1 appears to lead to residual P(0) on the surface, accompanied by some oxidation, whereas reaction 1.2, which uses ZnMe2 and P(SiMe3)3, has substantially less P(0), presumably because the phosphorus source in tris(trimethylsilyl)phosphine is in the same −3 formal oxidation state as Zn3P2. Solid-state 31P{1H} NMR suggests that the surface makeup of the nanoparticles, prepared via both routes, is complex and may result from binding of phosphonic acid or other impurities in TOP (or TOP decomposition products) to a partially oxidized Zn3P2 surface. While the cores of these nanoparticles are well-defined, the surface functionalization of the zinc phosphide remains poorly understood and will be the source of future work.

chemistry described by Schaak and Henkes, in which various metals, including zinc, were shown to form the corresponding phosphide in the presence of TOP at 370 °C for 2 h.48,49 In that work, metallic zinc foil, powders, and other forms were converted into bulk Zn3P2, as determined by XRD.48 Reaction 1.2, the chemistry based upon ZnMe2 and P(SiMe3)3, carried out at 150 °C in TOP as the solvent, is similar to the known reactivity of a variety of alkyl main group compounds and P(SiMe3)3. Steigerwald and co-workers showed in 1991 that Me3In and P(SiMe3)3 lead to formation of the dimer [Me2In(μ-P(SiMe3)2]2, a compound with two bridging phosphido-P(SiMe 3) 2 groups, along with two equivalents of SiMe4.60 A similar reaction can be proposed for ZnMe2 and P(SiMe3)3, in which a dimetallic zinc complex with bridging phosphide groups is formed, [MeZn(μ-P(SiMe3)2]2, as outlined in eq 3.2 in Scheme 3. Formation of this dinuclear zinc complex under these conditions was confirmed with solution phase multinuclear magnetic resonance experiments in toluene-d8 in which 10 μL (0.13 mM) of dimethylzinc and 27 μL (0.09 mM) of P(SiMe3)3 were heated at 110 °C. After 12 h, the signature singlet in the 31P{1H} NMR spectrum at δ −247.5 ppm was observed, as well as the expected features in the 1H, 13C{1H}, and 29Si{1H} NMR spectra (Supporting Information, Figures S21−S27).84 The formation of SiMe4 was also observed. In the case of [Me2In(μ-P(SiMe3)2]2, high temperatures induced the decomposition of this dimeric complex to form bulk InP. [MeZn(μ-P(SiMe3)2]2 is also presumably an intermediate along the reaction pathway connecting ZnMe2 and P(SiMe3)3, as suggested by reaction 3.3 in Scheme 3. Direct decomposition of P(SiMe3)3 to form other phosphorus species was not observed upon heating this phosphine in deuterated toluene at ∼110 °C for 14 h [reaction 3.4, data in the Supporting Information]. Dimethylzinc is also found to be stable at this temperature (∼110 °C) with no sign of decomposition after 14 h of heating [reaction 3.5, data in the Supporting Information]. Earlier work has shown that ZnMe2 is

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ASSOCIATED CONTENT

S Supporting Information *

Twenty-nine figures composed of additional XRD, TEM, SEM, NMR, and UV−vis spectroscopic characterization data and synthetic methods for producing Zn3P2 nanoparticles using trin-octylamine. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSERC, the National Institute for Nanotechnology (NRC-NINT), the Canadian Foundation for Innovation (CFI), Alberta Innovates Energy and Environment Solutions (AIEES), and the Canada Research Chairs (CRC) program. The staff of the NMR facilities at the University of Alberta and the Alberta Centre for Surface Engineering and 1933

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Sciences (ACSES) are thanked for assistance with analyses. Brian Olsen is thanked for the Table of Contents (TOC) figure.

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