Layered Phosphonates in Colloidal Synthesis of Anisotropic ZnO

Article pubs.acs.org/cm

Layered Phosphonates in Colloidal Synthesis of Anisotropic ZnO Nanocrystals Bryan M. Tienes, Russell J. Perkins, Richard K. Shoemaker, and Gordana Dukovic* Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80304, United States S Supporting Information *

ABSTRACT: We describe the role of phosphonic acids in the synthesis of anisotropic colloidal ZnO nanocrystals (nanorods) and, specifically, the discovery of an insoluble layered Zn−phosphonate intermediate. This compound is formed by the reaction of the molecular Zn precursor Zn(OAc)2 with phosphonic acids, and it acts as a heterogeneous Zn source during nanocrystal formation. Layered metal phosphonates have been studied extensively but have not been described in the context of nanocrystal synthesis. Layered Zn−octadecylphosphonate can be used as a sole precursor to obtain isotropic, soluble ZnO nanocrystals. However, for anisotropic rod-like shapes both the heterogeneous and the homogeneous (Zn(OAc)2) sources of Zn are necessary. The existence of a heterogeneous metal source described here is in contrast to the mechanisms of particle nucleation and growth from homogeneous molecular precursors often used to describe nanocrystal formation. Since many metals form layered phosphonates, our findings have implications for synthesis of nanocrystals of other semiconductors. KEYWORDS: zinc oxide, colloidal nanocrystals, nanocrystal synthesis, anisotropic growth, metal phosphonates

■

the PA, bulk ZnO was obtained).28 TBPA was also found to act as a surface-capping ligand.28 In the synthesis of ZnO nanocrystals by reaction of zinc stearate with 1-octadecanol, addition of octadecylphosphonic acid (ODPA) after ZnO nanocrystal formation resulted in marked sharpening of size distribution of inhomogeneously sized quantum dots and conversion of nanopyramids into quantum dots.29 Finally, in a reaction of Zn(OAc)2 with 1,12-dodecanediol, addition of tetradecylphosphonic acid (TDPA) in the synthetic mixture resulted in a drastic change of the product morphology, from hierarchically ordered spheres consisting of aggregated ZnO nanocones to highly soluble nanorods with a relatively uniform size distribution.21,30 These reports suggest that, when employed in synthesis of ZnO nanostructures, PAs may provide the degree of synthetic control currently available with CdSe nanocrystals. In this manuscript, we focus on the role of aliphatic PAs in the synthesis of anisotropic ZnO nanocrystals. We discovered that, in the synthesis of ZnO nanorods, the reaction of Zn(OAc)2 with aliphatic PAs (e.g., octadecylphosphonic acid, ODPA) leads to the formation of an insoluble, highly ordered reaction intermediate. Characterization by TEM, STEM-EELS, low-angle XRD, FTIR, and solid-state 31P NMR revealed that Zn(OAc)2 reacts with PAs to form insoluble layered Zn− phosphonates. Such materials are known in the literature but have not been previously reported in the context of nanocrystal synthesis. Layered Zn−phosphonates can react with 1-

INTRODUCTION Alkyl phosphonic acids (PAs) have played a crucial role in the development of synthetic methods for colloidal semiconductor nanocrystals, most notably CdSe-based materials. Their use has enabled a remarkable degree of control over CdSe nanocrystal shape through fine-tuning of experimental parameters.1−10 When used in CdSe nanocrystal synthesis, aliphatic PAs react with Cd sources such as CdO and Cd(CH3)2 to form cadmium phosphonates.4,11 These precursor molecules react with Se sources, such as trialkylphosphine selenides, to nucleate and grow the nanocrystals.12,13 It has been proposed that the strong binding of phosphonate ligands to Cd2+ results in relatively stable Cd precursors, allowing for the accumulation of monomers in high concentrations, which is necessary for growth of anisotropic shapes (e.g., nanorods).3,4 Anisotropic nanocrystal growth has also been attributed to selective passivation of crystalline facets.14−16 In addition to controlling the reaction kinetics and the resulting nanocrystal shapes, PAs serve as strong surface-capping ligands that stabilize nanocrystals against precipitation in organic solvents.7,13,17 ZnO is another ubiquitous II−VI wurtzite semiconductor of fundamental and technological importance.18 There is considerable interest in synthetic control of the shape of ZnO nanostructures for optoelectronic and other applications.18−21 While several high-temperature colloidal synthetic procedures with control of particle size and shape have been reported,20,22−27 relatively few utilize PAs.28−30 In thermal decomposition of zinc acetate (Zn(OAc)2) in a long-chain alkylamine solvent, tert-butylphosphonic acid (TBPA) enabled the synthesis of quantum confined ZnO nanocrystals (without © 2013 American Chemical Society

Received: July 22, 2013 Published: October 8, 2013 4321

dx.doi.org/10.1021/cm402465w | Chem. Mater. 2013, 25, 4321−4329

Chemistry of Materials

Article

While stirring, NaOH was added until the pH was greater than 6. Upon NaOH addition, a white precipitate formed and was collected by centrifugation. The precipitate was then washed with methanol. Characterization. Electron Microsopy. All TEM samples were prepared by dropping dilute solutions on carbon coated copper 300 mesh grids (Electron Microscopy Sciences PN: CF300-Cu). Low resolution transmission electron microscope (TEM) images were collected on a Phillips CM100 microscope operating at 80−100 kV. High resolution TEM (HR-TEM) images were collected on a JEOL JEM 2000-FX TEM operating at 200 kV. STEM-EELS imaging and elemental mapping were performed by FEI Company (Hillsboro, OR) on a FEI Tecnai Osiris TEM. SEM samples were prepared by dropping sample solutions on a piece of silicon wafer attached to an aluminum stub with conductive carbon tape and evaporating the solvent. SEM images were collected on a JEOL JSM-7401F scanning electron microscope at 1.0 kV acceleration voltage. ZnO Nanocrystal Size Measurements. Nanocrystal sizes were measured using the software package ImageJ available as freeware. For the nanorods prepared with C10, C14, C18, and phenyl PAs, the longest axis was measured as the length, and the width was measured perpendicular to that axis at 50% of the length. Nanocones prepared with C6 PA were measured from the tip of the cone to the base for the length and the length of the base of the cone was defined as the width. 189, 151, 155, 338, and 113 particles were measured for the nanocrystals made from the C6, C10, C14, C18, and phenyl PAs, respectively. Low Angle Powder X-ray Diffraction (XRD). XRD data was collected on a Scintag Pad V diffractometer operating at 25 mA and −40 kV using the Cu Kα X-ray line (0.1540562 nm). XRD samples were prepared by depositing sample solutions on glass slides and drying at 60 °C in air. Spectroscopy. UV−visible absorption spectra were collected on an Agilent 8453 UV−visible spectrophotometer. Fourier transform infrared (FTIR) samples were prepared by either drying samples on International Crystal Laboratories KBr Real Crystal IR Cards or using an International Crystal Laboratories SL-3 Liquid cell with KBr windows and a 0.1 mm path length. Neat 0.5 mL reaction aliquots were injected directly into the liquid cell for the quantitative FTIR experiments. For the high-temperature data shown in Supporting Information Figure S11, neat aliquots were first injected into liquid N2 to quench the reaction. Spectra were collected on a Thermo Nicolet Avatar 360 FTIR using an average of 64 scans at a 1 cm−1 resolution. 31 P Solid State NMR. NMR spectra were acquired on a narrow-bore Varian INOVA 400 NMR spectrometer system, operating at 161.988 MHz for 31P observation. Magic angle spinning (MAS) experiments were performed using a 5 mm MAS spinning assembly and zirconia rotors manufactured by Revolution NMR, Inc. (Fort Collins, CO), which provides excellent spinning stability of ±10 Hz for the spinning sideband analysis. Single-pulsed excitation MAS NMR experiments were performed using a 45° excitation pulse of 2.8 μs, with broadband 1 H TPPM decoupling, using 72 kHz cw decoupling field strength. Spectra shown are the result of 128−256 scans with a 10 s relaxation delay between scans. Chemical shifts were referenced indirectly to zero ppm 1H NMR, using the absolute configuration frequency of the spectrometer. This referencing was checked against a sample of pure triphenylphosphine (TPP), yielding an observed absolute chemical shift of −8.4 ppm for TPP.32 Spectral simulations were performed to determine principal components of the chemical shift tensors via spinning sideband analysis under slow MAS using the STARS (Spectrum Analysis of Rotating Solids) simulation package developed by the Jakobsen group at the University of Aarhus, as implemented in the VNMRJ 3.2 software (Agilent Technologies).33

undecanol to produce isotropic, soluble ZnO nanocrystals. However, to obtain anisotropic rod-shaped nanocrystals, both the molecular Zn(OAc)2 and the heterogeneous Zn− phosphonate precursors are required. The dimensions of the anisotropic ZnO nanocrystals are relatively insensitive to the length of the aliphatic chain of the PA in the C10−C18 length range. The presence of an insoluble source of precursor in the reaction mixture presents a contrast to the homogeneous nucleation and growth mechanisms commonly invoked to describe formation of nanocrystals (e.g., CdSe). Since many metals are known to form layered phosphonates, these materials may be important intermediates in syntheses of other nanocrystalline materials when PAs are used. Interaction of heterogeneous and homogeneous precursors of the same element, such as the example described here, may open new opportunities for synthetic control of colloidal nanocrystals.

■

EXPERIMENTAL SECTION

Synthesis. Chemicals. All chemicals were purchased from a commercial source and used without further purification: Zn(OAc)2 (99.99%), ZnCl2 (98+%), trioctylphosphine oxide (TOPO, 99%), dioctyl ether (99%), 1-undecanol (99%), propyl (C3) phosphonic acid (95%), and phenyl phosphonic acid (98%) were purchased from Sigma Aldrich. Octadecyl (C18) phosphonic acid (ODPA, 99+%), hexyl (C6) phosphonic acid (99+%), and tetradecyl (C14) phosphonic acid (99+%) were purchased from PCI Synthesis (9 Opportunity Way, Newburyport, MA 01950, 978-463-4853). Decyl (C10) phosphonic acid (98%) was purchased from Alfa Aesar. ZnO Nanorod Synthesis. This synthesis is a modified version of a previously reported procedure.30 Zn(OAc)2 (1 mmol), R-PA (0.45 mmol, R = C3, C6, C10, C14, C18, phenyl), TOPO (3 mmol), and octyl ether (7.5 mL) were combined in a three-neck round-bottom flask fitted with a condenser, septum, and glass thermocouple adaptor. In the drying step, the mixture was magnetically stirred under vacuum at 100 °C for 30 min. The flask was then placed under argon and heated to 200 °C for 1 h (precursor formation step). To isolate the insoluble intermediate, the reaction was cooled to room temperature at this point, and the solid was precipitated using a 1:1 ethanol/ methanol mixture. This solid was then washed three times by suspending in chloroform and precipitating with a 1:1 ethanol/ methanol mixture. To synthesize anisotropic ZnO nanocrystals, the precursor mixture at 200 °C was cooled to 60 °C and argon-purged 1undecanol (6 mmol) was added via syringe using air-free techniques. The flask was heated to 250 °C for 2 h under argon. After cooling to room temperature, the nanorods were isolated by precipitating with methanol. The resulting solid was then dissolved in hexanes and centrifuged, leaving a supernatant containing the nanorod product. Control Reactions To Isolate the Role of the Zn Precursor. Control A: The ZnO nanorod synthesis starting with Zn(OAc)2 (0.55 mmol), TOPO (3 mmol), and octyl ether (7.5 mL) without the presence of PA was carried out as described above, including the drying, precursor formation, and nanocrystal formation steps. Control B: Zn-ODP (∼0.45 mmol) (isolated and purified as described above) was reintroduced into a solvent of octyl ether (7.5 mL) and TOPO (3 mmol) and dried under vacuum at 100 °C for 30 min. After cooling to 60 °C, argon-purged 1-undecanol (6 mmol) was added via syringe using air-free techniques. The flask was heated to 250 °C for 2 h under argon and then cooled to room temperature. The product was isolated as described above. Control C: The procedure was identical to that for Control B except that Zn(OAc)2 (0.55 mmol) was added to the octyl ether and TOPO solvents along with the isolated Zn-ODP. Aqueous Zinc Phosphonate Hydrate Synthesis. This procedure is a modified version of a previously reported synthesis.31 For Zn-ODP, ZnCl2 (1 mmol) was dissolved in 30 mL of 18.2 MΩ water and ODPA (1 mmol) was dissolved in 100 mL of methanol. For Zn− phenylphosphonate, 7.3 mmol of ZnCl2 and an equimolar amount of phenylphosphonic acid were each dissolved in 20 mL of 18.2 MΩ water. In both cases, the PA solution was added to the ZnCl2 solution.

■

RESULTS AND DISCUSSION To explore the role of PAs in ZnO nanorod synthesis, we adapted a previously reported procedure in which addition of a long-chain aliphatic PA to the reaction mixture drastically transformed the product from ZnO nanocone aggregates to highly soluble ZnO nanorods.30 Briefly, our air-free synthetic 4322



Article

before the addition of 1-undecanol. The solid was isolated by precipitation and purified for structural and chemical characterization (see Experimental Section). Figure 2A shows an SEM image of the insoluble intermediate formed during ZnO nanorod synthesis. The material consists of

procedure consisted of the following steps: (i) drying of Zn(OAc)2 (1.0 mmol) dissolved in a mixture of octyl ether and trioctylphosphine oxide (TOPO) by heating under vacuum at 100 °C for 30 min; (ii) precursor formation, in which the reaction mixture was heated to 200 °C under an argon atmosphere, incubated at that temperature for 60 min, and then cooled to 60 °C; and (iii) nanocrystal formation, in which 1undecanol (6 mmol) was added, and the reaction was heated to 250 °C, and then allowed to proceed for 2 h. A TEM image of the ZnO nanocrystals resulting from the synthesis described above is shown in Figure 1A. The particles are aggregated,

Figure 1. (A) TEM image of ZnO nanocrystals synthesized from a reaction of Zn(OAc)2 with 1-undecanol in a solvent of octyl ether and TOPO. The crystals are aggregated, relatively large, and polydisperse. (B) TEM image of ZnO nanocrystals synthesized by reaction of Zn(OAc)2 and ODPA with 1-undecanol in the same solvent mixture. The particles are highly anisotropic with rod-like shapes. Figure 2. Images of the insoluble intermediate collected after the precursor formation step in the ZnO nanorod synthesis. (A) An SEM image showing that the intermediate consists of three-dimensional quasi-cylindrical structures with diameters around 100 nm and lengths on the order of 1 μm. (B) TEM image demonstrating the periodic structure within the quasi-cylinders. (C) HR-TEM image showing thin layers of dark contrast material and thicker layers of a light contrast material. (D) STEM-HAADF image of the periodic layered structure correlated with STEM-EELS images for Zn, O, and C, showing that the layers consist of carbon-rich regions alternating with zinc- and oxygen-rich regions.

relatively large (smallest dimension > 10 nm), and polydisperse in size and shape. We refer to this synthetic procedure as the PA-f ree synthesis. In contrast, when ODPA was included in the reaction mixture in a 0.45:1 ODPA:Zn molar ratio, the resulting ZnO nanocrystals were anisotropic irregularly shaped nanorods, with diameters of 7 ± 2 nm, lengths of 18 ± 5 nm, and the average aspect ratio of 2.5:1 (Figure 1B). We refer to the procedure that includes ODPA as the ZnO nanorod synthesis. In this work, we explore how the addition of ODPA impacts the ZnO nanocrystal synthesis to generate a drastically different product. Isolation and Structural Characterization of a Layered Intermediate. The PA-f ree and ZnO nanorod syntheses described above exhibited very different appearances of the reaction mixture at the various stages of the reaction. In the PAfree synthesis, the mixture of Zn(OAc)2, octyl ether, and TOPO turned optically clear at 90 °C, during the drying step, indicating dissolution of the starting materials. The solution remained clear during the precursor formation step. Only upon the addition of 1-undecanol did the reaction turn cloudy, as the insoluble ZnO nanocrystals (Figure 1A) were formed. In the ZnO nanorod synthesis, the ODPA-containing reaction mixture also turned clear upon reaching approximately 90 °C during the drying step. However, the mixture quickly became cloudy before reaching 100 °C and at the same time the pressure increased in the Schlenk line, indicating the release of a volatile product. The cloudiness persisted throughout the remainder of the reaction (both the precursor formation and nanocrystal formation steps), suggesting the existence of an insoluble intermediate. The insoluble material could be separated from the ZnO nanocrystals, which were soluble in hexane. To investigate the structure and the chemical identity of the insoluble intermediate, the ZnO nanorod synthesis was halted

quasi-cylindrical three-dimensional structures with average diameters of approximately 100 nm and lengths on the order of 1 μm. A closer look via TEM (Figure 2B) reveals that each quasi-cylinder consists of alternating layers of dark and light contrast. HR-TEM (Figure 2C) shows the dark-contrast layers to be significantly thinner than the light-contrast layers. Notably, there is no evidence of appreciable crystalline domains in the HR-TEM images. Information about the elemental composition of the alternating layers is provided by electron energy loss spectroscopy (EELS) mapping via scanning TEM (STEM) (Figure 2D). In the high angle annular dark field (HAADF) image (Figure 2D, top), the contrast is reversed such that the bright regions in HAADF correspond to the dark regions in Figure 2B,C. EELS maps reveal that the bright regions in the HAADF image are rich in Zn and O, while the dark HAADF regions contain high amounts of C. The images shown in Figure 2 suggest that the solid formed in ZnO nanorod synthesis consists of periodic structures that are inorganic−organic hybrids with relatively thin Zn- and Ocontaining layers and relatively thick organic layers. In order to characterize the periodic structure of the layered intermediate in ZnO nanorod synthesis, we employed low angle powder X-ray diffraction (XRD). By examining diffraction 4323



Article

PAs in the reaction with Zn(OAc)2. All these reactions resulted in the formation of periodic structures (Supporting Information Figure S1), which could also be used as intermediates to form ZnO nanocrystals, as we discuss later in the text. Low-angle XRD patterns for the layered structures formed using C3, C6, C10, C14, and C18 PAs (Figure 3A) demonstrate an increase in d-spacing with increasing alkyl chain length. The d-spacing as a function of PA length, as measured from an O atom of the OH group to the terminal H on the alkyl chain, is shown in Figure 3B. The relationship is linear, with a remarkably good fit (R2 = 0.999) and a slope of 1.5, which is discussed in a later section. This linear dependence strongly suggests that the organic layers originate from the PAs. Extrapolating to zero PA length, we find the thickness of the Zn-containing layer to be 0.35 nm. This length is on the order of the lattice parameter a of wurtzite ZnO,18 suggesting that the Zn-containing layers are a single Zn atom thick. Chemical Identification of the Layered Intermediate. On the basis of the periodic layered structure, STEM-EELS elemental analysis, and the thinness of the Zn-containing layer, we hypothesized that the solid intermediates formed by reaction of Zn(OAc)2 and PAs are layered Zn phosphonates. Layered divalent metal phosphonates are a well-known class of materials.31,34−40 Interest in these materials stems from their porous structure and their potential applications as ion exchangers, sorbents, proton conductors, catalysts, and catalyst supports.35,36,41 Zn phosphonates are typically made through aqueous routes by reacting a zinc salt, usually ZnCl2, with the PA and excess base.31,38,39,42−44 The base deprotonates the PA, which allows it to dissolve and form a complex with Zn2+. The Zn phosphonate precipitates from solution and is isolated by filtration. Nonaqueous routes to Zn phosphonates, such as the one described here, are not common,45 and such structures have not been previously reported in the context of nanocrystal synthesis. Examples of characterization of layered metal phosphonates by FT-IR 38,40,43−46 and 31 P solid-state NMR37,39,46 provide a means for comparing the structures generated in the ZnO nanorod synthesis with these known materials. An FTIR spectrum of the layered intermediate in the ZnO nanorod synthesis is shown in Figure 4A. The FTIR spectra of the pure starting materials (Zn(OAc)2 and ODPA) are shown in Supporting Information Figure S2 for comparison. The peaks at 2800−3000 cm−1 correspond to the C−H stretching modes of the alkyl chains, and the peak at 1463 cm−1 corresponds to alkyl chain C−H scissor mode.47−49 The region between 1250 cm−1 and 900 cm−1 is associated with the phosphonate functional group, and the peaks correspond to PO, P−O, and P−O−H stretches.45,47,50,51 Very similar FTIR spectra were obtained when TOPO was excluded from the solvent (Supporting Information Figure S3). For comparison, an FTIR spectrum of Zn-octadecyl phosphonate (Zn-ODP) synthesized using the previously reported aqueous methods31,35,36 is shown in Figure 4B. This spectrum is nearly identical to the one in Figure 4A with two exceptions. A peak in the O−H region (3440 cm−1) is present in Figure 4B and is characteristic of a hydrated product.43,45 The two sharp P−O peaks centered at 1150 cm−1 have a larger splitting in Figure 4B, which we attribute to the presence of water molecules incorporated into the crystal lattice of the hydrated product. Very similar differences were previously observed between the FTIR spectra of hydrated and dehydrated Zn-phenyl phosphonates.43 To provide further evidence that the small

in the 2Θ range of 2−20°, we can examine features that correspond to intermolecular periodicity. The solid intermediate shown in Figure 2 exhibits several regularly spaced peaks at 2Θ values below 15° (Figure 3A, green trace). We assign them

Figure 3. (A) Low angle XRD patterns of Zn-(C3, C6, C10, C14, C18) phosphonates prepared by reaction of Zn(OAc)2 with C3, C6, C10, C14, C18 PAs. The position of the (001) reflection was converted to d-spacing using Bragg’s law. (B) d-spacing measured by XRD as a function of the PA length. The linear fit is extrapolated to zero (infinitely small PA) to give a Zn layer thickness of 0.35 nm. The slope of the line is 1.5.

to multiple orders of diffraction (e.g., (001), (002), (003), etc.) corresponding to the repeat unit length (i.e., d-spacing) in the direction along the length of the quasi-cylinders shown in Figure 2. The well-defined peak positions indicate that the dspacing is uniform throughout the sample. The repeat unit length, calculated using Bragg’s law, is 3.9 nm, which is consistent with the TEM observations (Figure 2). Notably, we do not observe diffraction features at higher angles (30−80°), which would correspond to the lattice spacings of a crystalline solid such as ZnO. Next, we consider the origin of the organic layers. To determine whether the length of the repeat unit depends on the alkyl chain length of the PA, we replaced ODPA (C18) with propyl (C3), hexyl (C6), decyl (C10), and tetradecyl (C14) 4324



Article

Figure 4. FTIR spectra of samples dried on a KBr crystal. (A) Insoluble intermediate in the ZnO nanorod synthesis. (B) Zn-ODP prepared by an aqueous method using ZnCl2, ODPA, and NaOH. The spectra match well with the exceptions of an O−H stretch at 3440 cm−1 and splitting of the phosphonate peaks centered at 1150 cm−1. These differences are attributed to the presence of water incorporated into the structure made under aqueous conditions.

differences in the spectra in Figure 4 are due to presence of water in the aqueous product, we synthesized Zn-phenyl phosphonate both by our method and by the previously reported aqueous method.43 Their FTIR spectra (Supporting Information Figure S4) match the previously reported spectra of hydrated and dehydrated Zn-phenyl phosphonates.43 The FTIR data support the hypothesis that the product of reaction of Zn(OAc)2 with ODPA in the synthesis of ZnO nanorods is layered Zn-ODP. This compound can form by the displacement of acetate with phosphonate, with acetic acid as a product: nZn(OAc)2 + nODPA → [Zn(ODP)]n + 2nHOAc

Figure 5. (A) 31P MAS (10 kHz) solid state NMR spectra of ODPA (red) and layered Zn-ODP (black). ODPA spectrum contains a single peak while the layered Zn-ODP spectrum contains four peaks with chemical shifts similar to those of ODPA. (B) Schematic of the atomic arrangement in (112) Zn-ODP as determined by comparison of the 31 P tensor parameters with literature values.37,39 The alkyl chains point in and out of the plane of the page and have been removed for clarity.

(1)

This reaction is consistent with the observation of a volatile product (HOAc) during the drying step at 100 °C under vacuum. Previously described layered Zn phosphonates can have varying bonding motifs between the zinc atoms and the oxygen atoms of the phosphonate. The possibilities are described as (011), (111), (112), and (122).37,39,46 In this notation, each of the three numbers represents an oxygen atom in the phosphonate and the value corresponds to the number of metal ions bound to that oxygen. For example, a value of 2 indicates that the oxygen atom is bridging two metal ions. The different bonding arrangements can be distinguished by 31P solid state magic angle spinning (MAS) NMR, specifically by determining the associated chemical shift tensor parameters.37,39 These parameters have been determined for several bonding motifs in Zn phosphonates synthesized by aqueous methods, and the NMR data was correlated with the chemical structure determined by single crystal X-ray crystallography.37 Here, we utilize 31P solid state NMR to determine the bonding arrangement in layered Zn-ODP formed during ZnO nanorod synthesis. Figure 5A shows isotropic 31P solid state NMR spectra of ODPA (red trace) and the insoluble intermediate in the ZnO nanorod synthesis (black trace), obtained using MAS at 10 kHz. ODPA exhibits one peak at 30 ppm. In the Zn-ODP spectrum, four separate isotropic peaks are observed in the 27− 34 ppm region. Peaks corresponding to TOPO, which has a

main peak at 47 ppm (Supporting Information Figure S5), were not observed in the spectra of the product. Chemical shift tensor parameters of the four Zn-ODP peaks, determined by spinning the sample slowly (2.0 to 2.2 kHz) and simulating the powder spectra of each the four isotropic peaks (see Experimental Section), are shown in Supporting Information Table S1. The 2 kHz MAS spectra, their deconvolution, and the simulated powder spectra are shown in Supporting Information Figures S6−S8. The values of the asymmetry parameter (η) and the chemical shift anisotropy (Δ) for each of the four phosphorus peaks are a close match to those previously reported for the (112) bonding motif observed in Zn− phosphonates.37,46 Indeed, the (112) configuration has been reported for Zn-ODP.39 In this configuration, one of the phosphonate O atoms bridges two Zn atoms, while the remaining two O atoms are singly bound to one Zn atom each (Figure 5B). Each Zn is tetrahedrally coordinated by O atoms such that sites are not available for coordination of additional species in the material. Packing Geometry of the Alkyl Interlayers. With the identification of the layered periodic structures that form upon the reaction of Zn(OAc)2 and ODPA as Zn-ODP, we can determine the packing geometry of the alkyl chains that separate the single-Zn atom planes. The slope of the line 4325



Article

correlating the length of the periodic repeat unit (i.e., the dspacing) with the PA length (Figure 3B) is 1.5, corresponding to a 1.5-fold increase in d-spacing per increase in PA length. A slope of 1 would correspond to completely interdigitated aliphatic chains, while a value of 2 would suggest aliphatic chains that are stacked in fully extended configurations perpendicular to the Zn planes. The slope of 1.5 suggests that the aliphatic layers are oriented at an angle τ from the plane of the Zn atoms such that their projection along the axis perpendicular to the planes is 1.5 times the length of one chain (Figure 6).31,35 The parallel and herringbone configurations of

Figure 7. Quantitative FTIR spectra in the acetate carbonyl region of the crude reaction mixture collected after the precursor formation step and before the addition 1-undecanol for (A) PA-f ree reaction and (B) ZnO nanorod synthesis. The area under the peak of B is 47% less than the area under A, indicating that all of the ODPA in the reaction is converted to Zn-ODP, and the remaining Zn2+ is in the form of Zn(OAc)2. Figure 6. Schematic representation of the packing of the alkyl chains in the layered Zn-ODP structure. The diagram on the right depicts the geometry used to calculate the angle τ using the d-spacings measured by XRD and the PA length measured from the O atom of the OH group to the terminal H on the alkyl chain.

that that the reaction described by eq 1 proceeds to completion, i.e., until no more ODPA is available. Consequently, prior to the nanocrystal formation step, the reaction mixture of the ZnO nanorod synthesis contains Zn(OAc)2 and layered Zn-ODP in approximately equal amounts. Relationship between Zn Precursor and Shape of ZnO Nanocrystal Products. Given the presence of two distinct Zn precursors in ZnO nanorod synthesis, it is critical to understand how each contributes to the final synthesis outcome. In the nanocrystal formation step, following the addition of 1undecanol to the precursor mixture at 60 °C, the reaction is heated to 250 °C and allowed to proceed for 120 min. Throughout the course of the reaction, the mixture remains markedly cloudy, suggesting that the layered Zn-ODP does not dissolve into molecular components. This is in contrast to the case of CdSe synthesis via Cd−phosphonate, which is optically clear in solution at elevated temperatures, indicating that the Cd precursor is a molecular Cd−phosphonate species.13 The difference could be attributed to the fact that both the Cd-ODP formation and CdSe nucleation and growth are carried out at higher temperatures (>300 °C). (Such high temperatures are not feasible in this ZnO synthesis because of boiling point considerations.) We note that similar ZnO nanorods were obtained when TOPO was removed from the reaction mixture (Supporting Information Figure S10), which suggests that TOPO mainly serves the role of a solvent. We performed three control reactions that aim to differentiate the effects of the two Zn precursors on the appearance of the final ZnO product (see Experimental Section). Figure 8A shows the product of a reaction similar to ZnO nanorod synthesis, with the exception that ODPA was removed and Zn(OAc)2 was present in the amount of 0.55 mmol. This experiment removes the Zn-ODP precursor from the reaction mixture while the Zn(OAc)2 concentration matches that of the ZnO nanorod synthesis. We refer to this reaction as Control A. The resulting nanocrystals are relatively large (tens of nm), highly polydisperse, isotropic in shape, and aggregated. This result suggests that Zn-ODP precursor is needed for the

aliphatic chains (Pattern 1 and Pattern 2 in Figure 6) cannot be distinguished in this model. The value of τ determined from the slope (cos(90° − τ) = slope/2) is 49°, which has been predicted to be the ideal value for alkyl phosphonates of divalent metals based on the local bonding geometry at the phosphonate−metal interface.31,35,36 Stoichiometry of Zn Precursors in ZnO Nanorod Synthesis. Having identified and characterized layered ZnODP as the insoluble intermediate in the ZnO nanorod synthesis, we now turn our attention to the role of this material in the synthesis. First, we establish the proportion of Zn2+ present as Zn-ODP relative to Zn2+ in Zn(OAc)2. The molar ratio of Zn(OAc)2 and ODPA in the starting material mixture for ZnO nanorod synthesis is 1:0.45. Equation 1 suggests that, if the reaction proceeded to completion, after the precursor formation step, 45% of the Zn precursor would be present in the form of Zn-ODP and the other 55% as Zn(OAc)2. We evaluated the extent of Zn(OAc)2 → Zn-ODP conversion by quantitative FTIR (Figure 7). In this experiment, we acquired FTIR spectra of the neat reaction mixtures using a 0.10 mm path-length liquid IR cell. The absorption from solvents overwhelms most parts of the spectrum (Supporting Information Figure S9) but leaves a window (2500 cm−1 to 1500 cm−1) where a minimal solvent absorption can be subtracted. This window is particularly useful for quantification in the carbonyl region. In Figure 7, the absorbance of the acetate carbonyl peak around 1600 cm−1 after the precursor formation step and before the addition of 1-undecanol is shown for the PA-f ree reaction and the ZnO nanorod synthesis. Both reactions started with 1.0 mmol of Zn(OAc)2 in the reaction mixture. Figure 7 shows that, in the ZnO nanorod synthesis, the acetate intensity decreased by 47% (based on the area under the peak) during the precursor formation step. This indicates 4326



Article

(Supporting Information Figure S1). ZnO nanocrystals can be obtained by replacing ODPA with any of these PAs. Supporting Information Figure S1 shows TEM images of both the layered Zn−phosphonate structures and the resulting ZnO nanocrystals for a range of PAs. In the case of propyl (C3) PA, the resulting nanocrystals form aggregates. This suggests that this short PA on the nanocrystal surface does not enable sufficient solubility in organic solvents. When hexyl (C6) PA is used, the resulting nanocrystals are primarily conical in shape, with bases of 11 ± 3 nm and heights of 15 ± 5 nm. With decyl (C10), tetradecyl (C14), and phenyl PAs, the nanocrystals have rodlike anisotropic shapes and are similar in size and aspect ratio, within one standard deviation, to those obtained with C18 PA (ODPA) (Supporting Information Table S2). We note that this relatively weak dependence of the final particle dimensions on the aliphatic portion of the PA, provided that it is sufficiently long to enable solubilization, is in contrast to a much stronger impact in the case of CdSe nanorods. One report demonstrates an increase in aspect ratio of CdSe nanorods from 2.5 to 6.7 when the PA aliphatic chain changes from C18 to C10 and an aspect ratio of 18 for C6.52 Chemical Reactions in ZnO Nanorod Synthesis. In this final section, we discuss possible chemical pathways for formation of ZnO in the presence of PAs. It has been shown that in the PA-f ree ZnO synthesis, Zn(OAc)2 reacts with alcohols via ester elimination to produce Zn(OH)2 (Figure 9A), which in turn leads to ZnO nanocrystals, such as the ones

Figure 8. Results of ZnO synthesis with varying Zn precursors. (A) Aggregated polydisperse isotropic nanocrystals resulting from the Zn(OAc)2-only reaction (Control A). (B) The small soluble isotropic nanocrystals synthesized in the Zn-ODP-only reaction (Control B). (C) Anisotropic nanocrystals resulting from the reaction that includes Zn-ODP and Zn(OAc)2 (Control C).

formation of smaller, anisotropic, relatively monodisperse, and soluble ZnO nanocrystals. The product of the Control A reaction is similar to the one obtained in the PA-f ree reaction described earlier in the text, where 1.0 mmol of Zn(OAc)2 was used (Figure 1A). This similarity indicates that Zn(OAc)2 concentration is not the deciding factor in the particle shape. In the experiment we refer to as Control B, the isolated and purified Zn-ODP (approximately 0.45 mmol) was reintroduced into the octyl ether/TOPO solvent mixture and subjected to the reaction with 1-undecanol under conditions identical to those of the ZnO nanorod synthesis. The product consists of relatively small, polydisperse, soluble ZnO nanocrystals that are isotropic in shape (Figure 8B). Zn-ODP also serves as the source of octadecylphosphonate, the surface-capping agent that enables solubility in organic solvents.28 Finally, when the isolated and purified Zn-ODP (0.45 mmol) was combined with 0.55 mmol of Zn(OAc)2 in the solvent mixture (Control C), the resulting product was small, anisotropic, relatively monodisperse ZnO nanocrystals (Figure 8C) very similar in appearance to those shown in Figure 1B. This set of control experiments demonstrates that, while Zn-ODP alone can produce small, soluble, isotropic ZnO nanocrystals, both Zn(OAc)2 and Zn-ODP are required for the formation of ZnO nanorods. The relationship between Zn precursors and the appearance of the resulting ZnO nanocrystals is summarized in Scheme 1. Role of Aliphatic Chain Length in ZnO Nanocrystal Shape. Formation of Zn phosphonates is a general phenomenon that occurs for a range of aliphatic chain lengths (C3, C6, C10, C14), as well as for phenylphosphonic acid Scheme 1. Summary of the Reaction Pathways for Zn(OAc)2 to ZnO in the Presence and Absence of ODPAa

Figure 9. (A) Reaction scheme for an ester elimination producing Zn(OH)2 from Zn(OAc)2 and 1-undecanol. Zn(OH)2 condensation leads to formation of ZnO and H2O. Quantitative FTIR spectra of the crude reaction at room temperature after the addition of the 1undecanol (black trace) and after heating for 2 h at 250 °C (red trace) for (B) Control A and (C) ZnO nanorod synthesis.

shown in Figure 1A, via a condensation reaction that eliminates H2O:21,30 nZn(OH)2 → (Zn‐O)n + nH 2O

a

(2)

The Zn(OAc)2 reactant and the ester product are readily identifiable by FTIR.30 In the presence of PAs, the possibilities for chemical reactions become more numerous. Figure 9B shows the quantitative FTIR spectra in the carbonyl region (with solvent signal subtracted) for ZnO nanorod synthesis and

NC = nanocrystal. 4327



Article

nanocrystals, inclusion of the molecular precursor Zn(OAc)2 is needed to form relatively monodisperse anisotropic rod-like nanocrystals. In contrast to previous observations with CdSe nanorods, the length of the alkyl PA chain within the C10−C18 range does not significantly alter the size and shape of the resulting ZnO nanocrystals. This example of how two chemically distinct metal precursors, one heterogeneous and one homogeneous, together enable the formation of anisotropic ZnO nanocrystals points to new opportunities for chemical control in nanocrystal synthesis. Considering that many metals are known to form layered phosphonates, such structures may play an important role in the synthesis of nanocrystals of other semiconductors.

for Control A reaction at room temperature (immediately after injection of 1-undecanol) and after heating at 250 °C for 2 h. In both cases, upon reaction with 1-undecanol, the peak around 1600 cm−1 assigned to Zn(OAc)2 disappears and another peak grows in near 1750 cm−1, corresponding to the carbonyl group of the ester formed when Zn(OAc)2 reacts with alcohol to form Zn(OH)221,30 (Figure 9B,C). In fact, the acetate → ester conversion follows very similar kinetics in the presence and absence of ODPA (Supporting Information Figure S11), suggesting that the Zn(OAc)2 → Zn(OH)2 conversion is not strongly perturbed by the presence of Zn-ODP. Thus we consider cross-reactions between Zn(OAc)2 and Zn-ODP less likely than reactions of Zn-ODP with Zn(OH)2, 1-undecanol, and H2O. A reaction of Zn(OH)2 with Zn-ODP can be considered as an analogy to a reaction of −OH with a phosphoester, resulting in a condensation that liberates ODPA: nZn(OH)2 + nZn‐ODP → (Zn−O)2n + nH 2ODP

■

S Supporting Information *

TEM images of the layered phosphonates and the corresponding ZnO nanocrystals made with C3, C6, C10, C14, C18, and phenyl PAs; FTIR spectra of Zn(OAc)2 and ODPA; FTIR spectra of Zn-ODP made with and without TOPO; FTIR spectra comparing Zn−phenylphosphonates made through aqueous and nonaqueous methods; MAS (2 kHz) 31P NMR spectrum of Zn-ODP; overlaid simulated MAS (2 kHz) 31P NMR spectra for the individual peaks in the Zn-ODP spectrum; simulated static powder 31P NMR spectra for the four peaks in the Zn-ODP spectrum; MAS (10 kHz) 31P NMR spectrum of TOPO; experimentally derived 31P NMR tensor parameters for ODPA and Zn-ODP; liquid FTIR of the octyl ether/TOPO solvent mixture; TEM image of ZnO nanorods made without TOPO; liquid FTIR spectra showing the kinetics of Zn(OH)2 formation in ZnO nanorod synthesis and Control A; size measurements for the ZnO nanocrystals made with the C6, C10, C14, C18, and phenyl PAs; and powder XRD pattern of ZnO nanorods. This material is available free of charge via the Internet at http://pubs.acs.org.

(3)

where ODPA is written as H2ODP to emphasize the two acidic protons. The ODPA released in this reaction can become the surface phosphonate ligand makes ZnO nanorods soluble in organic solvents. The reaction in eq 3 may be in equilibrium with the reverse reaction.29 Another possible reaction of ZnODP involves 1-undecanol, in analogy to the Zn(OAc)2 reaction with 1-undecanol: Zn‐ODP + 2C11H 23‐OH → Zn(OH)2 + (C11H 24)2 ODP (4)

followed by the condensation of Zn(OH)2 to make ZnO (eq 2). This reaction is the likely pathway leading to ZnO nanocrystals in Control B above (Zn-ODP-only reaction). A reaction of Zn stearate and 1-octadecanol has been shown to lead to polydisperse ZnO particles due to irreversible ester formation.29 A similar effect here may account for the polydispersity of particle sizes in Control B (Figure 8B). Finally, the H2O released in condensation of Zn(OH)2 could react with Zn-ODP before evaporation: Zn‐ODP + 2H 2O → Zn(OH)2 + H 2ODP

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: (G.D.) [email protected].

( 5)

This would produce additional Zn(OH)2, which in turn could undergo condensation to ZnO (eq 2). The reverse reaction would replace OH termination of ZnO nanocrystal surface with phosphonate groups. Clearly, the presence of Zn-ODP in the reaction mixture that leads to ZnO nanorods introduces a multitude of possible chemical pathways. Further efforts are needed to unravel this chemical complexity and understand the detailed mechanism of ZnO nanorod formation.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research. B.M.T. acknowledges fellowship support from Conoco Phillips. R.J.P. thanks the Undergraduate Research Opportunities Program at University of Colorado, Boulder, for partial support. We also thank Roy Geiss for assistance with HR-TEM imaging, FEI Company (Hillsboro, OR) for HAADF and STEM-EELS images in Figure 2D, and Tarek Sammakia for helpful discussions.

■

CONCLUSIONS In this work, we examined how inclusion of PAs, such as ODPA, impacts the synthesis of ZnO nanocrystals. We found a behavior that is drastically different than the nucleation and growth of nanocrystals from homogeneous molecular precursors usually observed in prototypical cases (e.g., CdSe quantum dots and rods). Instead, we found that a molecular precursor, Zn(OAc)2, can react with PAs to form insoluble, layered, highly periodic Zn−phosphonate (e.g., Zn-ODP) structures that act as heterogeneous intermediates in the synthesis of ZnO nanorods. Such structures have been studied previously but have not been described in the context of nanocrystal synthesis. While layered Zn-ODP can be used as a precursor to form relatively small, polydisperse, soluble

■

REFERENCES

(1) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122 (51), 12700−12706. (2) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404 (6773), 59−61. (3) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123 (7), 1389− 1395. (4) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124 (13), 3343− 3353. 4328



Article

(39) Gao, W.; Dickinson, L.; Morin, F. G.; Reven, L. Chem. Mater. 1997, 9 (12), 3113−3120. (40) Clearfield, A. Chapter 1 The Early History and Growth of Metal Phosphonate Chemistry. In Metal Phosphonate Chemistry: From Synthesis to Applications; The Royal Society of Chemistry: 2012; Vol. 0, pp 1−44. (41) Clearfield, A. Curr. Opin. Solid State Mater. Sci. 1996, 1 (2), 268−278. (42) Fredoueil, F.; Penicaud, V.; Bujoli-Doeuff, M.; Bujoli, B. Inorg. Chem. 1997, 36 (21), 4702−4706. (43) Frink, K. J.; Wang, R. C.; Colon, J. L.; Clearfield, A. Inorg. Chem. 1991, 30 (7), 1438−1441. (44) Poojary, D. M.; Zhang, B. L.; Bellinghausen, P.; Clearfield, A. Inorg. Chem. 1996, 35 (18), 5254−5263. (45) Deemie, R. W.; Knight, A.; Rao, M. J. Organomet. Chem. 1999, 585 (1), 162−166. (46) Gerbier, P.; Guerin, C.; Henner, B.; Unal, J. R. J. Mater. Chem. 1999, 9 (10), 2559−2565. (47) Luschtinetz, R.; Seifert, G.; Jaehne, E.; Adler, H.-J. P. Macromol. Symp. 2007, 254 (1), 248−253. (48) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86 (26), 5145−5150. (49) Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R. J. Am. Chem. Soc. 1997, 119 (30), 7084−7094. (50) Thomas, L. C.; Chittenden, R. A. Spectrochim. Acta, Part A 1970, 26 (4), 781−800. (51) Thomas, L. C.; Chittenden, R. A. Spectrochim. Acta 1964, 20 (3), 467−487. (52) Wang, W.; Banerjee, S.; Jia, S.; Steigerwald, M. L.; Herman, I. P. Chem. Mater. 2007, 19 (10), 2573−2580.

(5) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J. B.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430 (6996), 190−195. (6) Yin, Y.; Alivisatos, A. P. Nature 2005, 437 (7059), 664−670. (7) Huang, J.; Kovalenko, M. V.; Talapin, D. V. J. Am. Chem. Soc. 2010, 132 (45), 15866−15868. (8) Wang, F.; Tang, R.; Kao, J. L. F.; Dingman, S. D.; Buhro, W. E. J. Am. Chem. Soc. 2009, 131 (13), 4983−4994. (9) Wang, F.; Tang, R.; Buhro, W. E. Nano Lett. 2008, 8 (10), 3521− 3524. (10) Wang, F.; Dong, A.; Sun, J.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45 (19), 7511−7521. (11) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123 (1), 183− 184. (12) Liu, H. T.; Owen, J. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2007, 129 (2), 305−312. (13) Owen, J. S.; Chan, E. M.; Liu, H.; Alivisatos, A. P. J. Am. Chem. Soc. 2010, 132 (51), 18206−18213. (14) Puzder, A.; Williamson, A. J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P. Nano Lett. 2004, 4 (12), 2361−2365. (15) Rempel, J. Y.; Trout, B. L.; Bawendi, M. G.; Jensen, K. F. J. Phys. Chem. B 2006, 110 (36), 18007−18016. (16) Manna, L.; Wang; Cingolani, R.; Alivisatos, A. P. J. Phys. Chem. B 2005, 109 (13), 6183−6192. (17) Gomes, R.; Hassinen, A.; Szczygiel, A.; Zhao, Q.; Vantomme, A.; Martins, J. C.; Hens, Z. J. Phys. Chem. Lett. 2011, 2 (3), 145−152. (18) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98 (4), 041301. (19) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J. Adv. Funct. Mater. 2002, 12 (5), 323−331. (20) Andelman, T.; Gong, Y.; Polking, M.; Yin, M.; Kuskovsky, I.; Neumark, G.; O’Brien, S. J. Phys. Chem. B 2005, 109 (30), 14314− 14318. (21) Kwon, S. G.; Hyeon, T. Acc. Chem. Res. 2008, 41 (12), 1696− 1709. (22) Niederberger, M. Acc. Chem. Res. 2007, 40 (9), 793−800. (23) Shim, M.; Guyot-Sionnest, P. J. Am. Chem. Soc. 2001, 123 (47), 11651−11654. (24) Demir, M. M.; Munoz-Espi, R.; Lieberwirth, I.; Wegner, G. J. Mater. Chem. 2006, 16 (28), 2940−2947. (25) Choi, S. H.; Kim, E. G.; Park, J.; An, K.; Lee, N.; Kim, S. C.; Hyeon, T. J. Phys. Chem. B 2005, 109 (31), 14792−14794. (26) Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y.; Neumark, G. F.; O’Brien, S. J. Am. Chem. Soc. 2004, 126 (20), 6206− 6207. (27) Zhang, Z.; Lu, M.; Xu, H.; Chin, W.-S. Chem.−Eur. J. 2007, 13 (2), 632−638. (28) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107 (20), 4756−4762. (29) Chen, Y. F.; Kim, M.; Lian, G.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2005, 127 (38), 13331−13337. (30) Joo, J.; Kwon, S. G.; Yu, J. H.; Hyeon, T. Adv. Mater. (Weinheim, Ger.) 2005, 17 (15), 1873−1877. (31) Cao, G.; Lee, H.; Lynch, V. M.; Mallouk, T. E. Inorg. Chem. 1988, 27 (16), 2781−2785. (32) Harris, R. K.; Becker, E. D.; De Menezes, S. M. C.; Granger, P.; Hoffman, R. E.; Zilm, K. W. Pure Appl. Chem. 2008, 80 (1), 59−84. (33) Skibsted, J.; Nielsen, N. C.; Bildsoe, H.; Jakobsen, H. J. J. Magn. Reson. 1991, 95 (1), 88−117. (34) Cunningham, D.; Hennelly, P. J. D. Inorg. Chim. Acta 1979, 37 (1), 95−102. (35) Cao, G.; Mallouk, T. E. Inorg. Chem. 1991, 30 (7), 1434−1438. (36) Guang, C.; Hong, H. G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25 (9), 420−427. (37) Massiot, D.; Drumel, S.; Janvier, P.; Bujoli-Doeuff, M.; Bujoli, B. Chem. Mater. 1997, 9 (1), 6−7. (38) Martin, K. J.; Squattrito, P. J.; Clearfield, A. Inorg. Chim. Acta 1989, 155 (1), 7−9. 4329


Layered Phosphonates in Colloidal Synthesis of Anisotropic ZnO

Recommend Documents