Length and Diameter Control of Ultrathin Nanowires of

Sep 26, 2012 - (3, 16) Thus, ultrathin NWs of substoichiometric tungsten oxide possess a combination of the best properties for EC devices, photocatal...
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Length and Diameter Control of Ultrathin Nanowires of Substoichiometric Tungsten Oxide with Insights into the Growth Mechanism Brian Moshofsky and Taleb Mokari* Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beersheva 84105, Israel S Supporting Information *

ABSTRACT: Tungsten oxide ultrathin nanowires have potential applications in electrochromic devices, dye sensitized solar cells, gas sensors, and as photocatalysts. Herein, we report a synthesis for solution phase ultrathin nanowires with independent control over the length and diameter. W(CO)6 is used as the tungsten source, and the solvents octadecanol and octadecene are used. Morphological control is obtained by varying the ratios between these three components as well as reaction conditions such as time and temperature. Such precise synthetic tuning will enable future investigations on the role of the aspect ratio and diameter for the above-mentioned applications. We can infer from experimental data a plausible nucleation pathway that involves the formation of tungsten alkoxide clusters. Raman spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, and X-ray and electron diffraction are used to characterize the nanowires, and the results indicate the phase to be a crystallographic sheer structure, such as W20O58. KEYWORDS: nanowires, ultrathin, tungsten oxide, growth mechanism, solution phase

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INTRODUCTION Tungsten oxide is perhaps most well-known for its electrochromic (EC) properties. Chromism in tungsten oxide was first demonstrated by Berzelius in 1815 by flowing hot H2 gas over WO3 to effect a color change.1 However, the ability to apply an electric field to induce either transparency or visible light absorbance was demonstrated by S. K. Deb in 1969.2 This technology is now the foundation for so-called smart windows. In spite of the large number of publications on tungsten oxide electrochromism, research is still being done to improve upon relevant properties such as coloration efficiency, stability, and coloration/bleaching time. Traditionally, the highest rates and efficiencies came from amorphous tungsten oxide films grown from vacuum deposition techniques. The drawbacks to amorphous films include their limited chemical stability and the fact that repeated ion insertion/removal degrades the films.3 However, nanocrystalline films have recently been shown to demonstrate similar coloration efficiency and equivalent coloration times while showing greater stability.3,4 These improvements for nanoparticle films were attributed to their large surface areas and low packing density. This rationale led to the use of vertically oriented nanowire (NW) arrays of tungsten oxide as an EC device layer and resulted in fast coloration/ bleaching times and enhanced contrast ratio between colored and bleached states.5,6 In addition to the crystallinity and morphology, stoichiometry of the crystalline phase is an © XXXX American Chemical Society

important parameter for tungsten oxide based EC device performance. It has been previously demonstrated that coloration efficiency of the substoichiometric W20O58 is higher than that for WO3 films.7 W18O49 has also been shown to exhibit good EC properties.8 Apart from EC applications, tungsten oxide has also been shown to be useful in a number of other applications including photocatalysis, gas sensing, dye sensitized solar cells, optical recording devices, and high Tc superconductors.9−14 Likewise, the properties of ultrathin NWs can improve performance in a number of these applications as well. For example, photocatalysis requires that a photogenerated charge carrier diffuses to the surface in order to react. Thus, the diffusion length should be longer than the radius of the wires for optimal activity. In the case of ultrathin NWs, no photogenerated carrier is more than a couple of nanometers from the surface. Also, smaller semiconductor nanocrystals show higher photocatalytic activity in general due to increased surface area, increased exciton energy (due to quantum confinement effects), and changes in the band bending.15 One specific application of tungsten oxide as a Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: June 28, 2012 Revised: September 9, 2012

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colored) of the washed nanowires in toluene onto a carbon/Formvar film on a copper grid (Ted Pella). X-ray Photoelectron Spectroscopy (XPS). The XPS was performed on an ESCALAB 250 from Thermo Fisher Scientific. Samples were prepared by drop casting the nanowires onto a Si(100) wafer. X-ray Diffraction (XRD). XRD measurements were performed on a Rigaku-Bede X-ray diffractometer with a 1.54 Å Cu X-ray source. The sample was prepared by dropcasting the nanowires onto a Si(100) wafer. Optical Measurements. UV−vis absorbance measurements were carried out on a Cary 5000 UV−vis−NIR spectrophotometer. Photoluminescence (PL) measurements were done using a Cary Eclipse fluorescence spectrophotometer. Washed nanowires were dissolved in toluene and placed in a quartz cuvette. Raman Spectroscopy. The Raman system that we used consists of a Jobin-Yvon LabRam HR 800 micro-Raman system, equipped with an LN2-cooled detector and a He−Ne laser that was supplied with the JY Raman spectrometer (514.5 nm). The sample was prepared by dropcasting the nanowires onto a Si(100) wafer.

semiconductor photocatalyst is for CO2 reduction. WO3 showed a higher rate of methanol production from CO2 and water versus TiO2 under illumination.9 A recent report also showed that ultrathin WOx NWs produced 0.86 mmol g−1 h−1 methane from CO2 and H2O without noble metal catalysts.10 For dye sensitized solar cell applications, ultrathin NWs have large surface areas, allowing for greater dye adsorption. Furthermore, NWs are an advantageous morphology because they provide a direct charge transfer pathway to the electrode. Lastly, sensors of tungsten oxide are based on the adsorption of molecules to the substrate surface resulting in a change in the conductivity. Small nanoparticle and ultrathin NW morphologies not only provide large surface area but also may exhibit easier carrier depletion resulting in enhanced sensitivity.3,16 Thus, ultrathin NWs of substoichiometric tungsten oxide possess a combination of the best properties for EC devices, photocatalysis, solar cells, and sensors, as well as other applications. In this report we present a tunable synthesis for ultrathin NWs and nanorods of tungsten suboxide. Our synthesis is performed using inexpensive reactants and simple laboratory equipment. The ultrathin NW morphology is achieved in a wide range of conditions and is extremely robust. By widely varying the reaction conditions we can alter the product to give different NW lengths (10−700 nm) and diameters (1 to 4 nm). Because the control over length and diameter is independent, very precise tailoring of the aspect ratio is obtainable in a range of 2.5−700. Furthermore, we discuss mechanisms for the growth of tungsten oxide ultrathin NWs based on past and present experimental evidence.

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RESULTS AND DISCUSSION Synthesis of WOx NWs. One dimensional NWs of tungsten oxide were among some of the first observed whisker-like nanostructures and were synthesized by heating of pure tungsten metal.17−19 While these syntheses are very straightforward, they lack precise control over the morphology. The first onedimensional solution phase synthesis of WOx was published by Park in 2003.20 Afterward, the development of tungsten oxide nanostructures was further advanced to numerous other morphologies by Niederberger et al. and Kim et al.11,21−23 Of particular relevance to our work from these publications were high aspect ratio W18O49 NWs. Kim and co-workers achieved NWs with ∼5 nm diameters and aspect ratios over 100 by using low tungsten precursor concentrations in ethanol, whereas higher concentrations gave nanorods. A similar product was obtained by Niederberger, only the NWs had ultrathin (1 ± 0.1 nm) diameters and were a micrometer long. One particularly interesting aspect of these syntheses was the large difference in synthetic conditions giving similar products. The first reaction utilized a pressurized vessel of WCl6 in ethanol at 200 °C, and the second used tungsten isopropoxide in benzyl alcohol in a sealed vial at 160 °C. In 2008, a synthesis using tungsten ethoxide in trioctylamine and oleic acid was reported that gave 5 nm diameter wires of ∼100 nm in length.24 They showed from FTIR studies that tungsten alkoxy acetate species form before the NWs are formed. Another solvothermal synthesis in water/ethanol with WCl6 found that a critical concentration of polyvinylpyrrolidone was needed for wire formation.25 This is not an exhaustive list of all ultrathin WOx NW syntheses, but it outlines the main synthetic developments. In each of the above reactions certain drawbacks could prohibit their usage such as requirement of an autoclave, long reaction times (>24 h), or little control over the final product. In this work we demonstrate a new and simple synthesis method for WOx ultrathin NWs with the ability to independently control the length and width. The synthesis consists of 1octadecanol (SA), 1-octadecene (ODE), and tungsten hexacarbonyl (W(CO)6). ODE is generally considered to be a noncoordinating solvent, and we assume that its only role is to dilute the SA and W(CO)6. The solvent mixture is first heated in a glass scintillation vial in a 200 °C oil bath in air, and then W(CO)6 powder is added. The solution becomes a faint yellowish color before starting to turn blue, indicating wire formation. The ultrathin nanowire product is 1.2 ± 0.2 nm in diameter and hundreds of nanometers in length (for size statistics

MATERIALS AND METHODS

Materials. 1-Octadecanol (SA) (99%), 1-dodecanol (98%), 1hexadecanol (99%), 1-octadecylamine (90%), 1-octadecanethiol (98%), 1-octadecanoic acid (95%), and tungsten hexacarbonyl (W(CO)6) (>97%) were purchased from Sigma-Aldrich and used as received. 1Octadecene (ODE) (90%) was purchased from Alfa Aesar and used as received. Synthesis of WOx NWs. For a standard ultrathin nanowire synthesis (diameter ≈ 1 nm, length ≈ 700 nm), 2.0 g of SA and 1.0 mL of ODE were added to a 20 mL glass scintillation vial with a Teflon coated stir bar. A scaled up reaction was also done using a crystallization dish covered by a watch glass. For other morphologies see details in the Supporting Information (Table S1). The uncapped vial was then placed into a hot oil bath at 200 °C with stirring in a fume hood. The solvent mixture was allowed to melt and the temperature to equilibrate. Next, 0.025 g of W(CO)6 was added directly to the scintillation vial as a powder. This addition marked the beginning of the reaction times discussed. The cap was then loosely placed on top of the vial. Caution, this reaction may evolve CO gas and must be done in a well-ventilated f ume hood for safe completion. After a few minutes the vial was swirled by hand to collect any material that had condensed on the side of the vial. A faint yellowish color was observed in the early stages of the reaction. After about five minutes the color of the solution became green/blue, and upon continued heating, a deep blue color was observed. After 20 min the reaction was stopped by removing the vial from the oil bath. For large wires the reaction product was washed with toluene only, and for smaller wires either toluene/ethanol/methanol or chloroform/methanol was used. The final product was dispersible in toluene or chloroform. UV irradiation experiments were done with a 365 nm high power LED from Thorlabs. Transmission Electron Microscopy (TEM). TEM analysis was performed using a Tecnai 12 G2 TWIN operated at 120 kV with a LaB6 filament. The dark field study was done with a JEOL JEM 2100-F operated at 200 kV with a FEG source. TEM samples were prepared by dropping approximately four μL of a dilute solution (faintly blue B

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concentration is increased. It is also possible to decrease the length of the NWs without altering the diameter. By keeping the amount of W(CO)6 constant at 0.030 g and diluting the SA with ODE we can decrease the wire length while maintaining the ultrathin morphology (Figure 2). We saw in Figure 1A that with

on all particles see the Supporting Information, Figures S1−S5 and Table S2). The wire diameter distribution is very narrow, and the wires are individually distinguishable in the TEM images, unlike the bundling observed in some previous reports.10,11 We find a strong dependence of the final wire morphology on many factors including the W(CO)6 concentration, alcohol concentration, temperature, and functional group. Using only SA as the solvent system without ODE and varying the amount of W(CO)6 added, we were able to simultaneously control the length and width (Figure 1). At low W(CO)6

Figure 2. TEM images of NWs obtained with increasing amount of SA relative to ODE (wt % SA in ODE). The amounts were (A) 20%, (B) 30%, (C) 40%, (D) 50%. All scale bars are 50 nm.

pure SA we produced wires with lengths of several hundreds of nanometers. However, in a very diluted solution (20 wt % SA in ODE) we obtained very short (∼10−50 nm) 1D structures with extensive branching (Figure 2A). Increasing to 30 wt % we see longer structures with the same diameter but with lengths on the order of 50 to 100 nm (Figure 2B). By 40 wt % SA in ODE longer NWs of hundreds of nanometers are already being formed (Figure 2C). Finally, at 50 wt % and above, wire lengths are quite long (>400 nm) and length differences between 50 wt % and pure SA are difficult to determine (Figure 2D). The reason for the NW formation in the weight percentage range greater than 40% may be due to complexation of W(CO)6 as will be discussed later along with the growth mechanism. Longer wire morphologies with increased diameters are also obtainable by increasing the reaction temperature (Figure 3). At the standard condition of 200 °C, we observe long wires of 1.2 nm diameter (Figure 3B). If we increase the temperature to 240 °C, a similar length structure but now with a diameter of up to 4 nm (Figure 3C) is obtained. The corresponding photoluminescence (PL) data can be seen in Figure 4 for the previously discussed wires. The band edge peak for thin wires is centered at 338 nm (Figure 4A), whereas the band edge peak for thicker wires is centered at 342 nm (Figure 4B). By comparison, 20 nm diameter nanowires of W18O49 have a band edge emission at 350, and 3 nm diameter nanorods were shown to have emission at 344 nm.20,29 Quantum confinement in oxides is usually very difficult to observe since their exciton Bohr radii are on the order of a nanometer.30 However, our results indicate weak quantum confinement effects due to the extremely small dimensions obtained here. We can also see in Figure 4 that at an

Figure 1. TEM images of NWs obtained with increasing amount of W(CO)6 added. The W(CO)6 amounts were (A) 0.02 g, (B) 0.04 g, (C) 0.07 g, (D) 0.10 g, (E) 0.14 g, and (F) 0.21 g. All scale bars are 50 nm.

concentration, long and thin NWs were formed with widths of 1.2 nm and lengths of hundreds of nanometers (Figure 1A). The average interwire spacing is 2.8 nm, while the octadecanol chain length is 2.38 nm when fully extended.26 The extensive interdigitation of ligands observed here has been studied previously and is prevalent in systems with longer alkyl chains.27 At higher W(CO)6 concentration the wires start to become shorter and thicker (Figure 1B−D). At a 10-fold increase in [W(CO)6] compared to the ultrathin wire formation we obtain nanorods 3.2 ± 0.9 nm in diameter with an average length of 14 ± 5 nm (Figure 1F). It is known from nucleation theory that a larger concentration of monomers leads to more nucleates.28 This would leave fewer monomers to grow onto the surface and explains why shorter structures are observed as the precursor C

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Figure 4. Photoluminescence data obtained for NWs prepared at (A) 200 °C and (B) 240 °C. TEM images of the NWs are inset. Gray lines are for an excitation wavelength of 300 nm and green lines are with excitation at 380 nm.

°C, and decomposition around 200 °C.32 Whether the W(CO)6 must melt before proper complexation or if precursor decomposition is required is still unclear. However, without the cap loosely placed on the vial during the reaction at 200 °C no product is formed. We think a tungsten complex (either W(CO)6 or an alcohol substituted product) is present in the headspace of the vial during the reaction and can escape without the vial being capped or sealed. This may allude to the importance of solvothermal methods for previous WOx syntheses. The use of a solvent with an alcohol functional group is critical to achieve NW morphology. The results using alternative solvents can be seen in Figure 5. Amine functional groups produce 1D structures, but they are embedded in what appears to be a matrix of organic molecules (Figure 5A). In this case, the oxide growth is limited due to the lack of readily accessible oxygen, in contrast to syntheses performed in alcohol solvents. Thiol and carboxylic acid functional groups resulted in non-1D structures (Figure 5B,C, respectively). Clearly, the functional group is critical for NW formation, which is not surprising as many nanoparticle syntheses are sensitive to the functional group. The main causes for this are that different functional groups have differing abilities to complex metal precursors and thus exhibit varying binding energies on growing nanoparticle surfaces. Shorter chain alcohols (C12, C16) give NWs (Figure 5D,E) and are bundled into larger superstructures. Hexadecanol gives urchin-like morphology while dodecanol gives spindle-like structures (Figure S6, Supporting Information). Eicosanol (C20) can give NWs similar to SA (Figure 5F) without apparent aggregation or assembly. Another important aspect of this synthesis is its simplicity and robustness. Nanoparticle syntheses are often fraught with very

Figure 3. TEM images of NWs obtained with increasing temperature. The reaction vial was placed in an oil bath at (A) 160 °C, (B) 200 °C, and (C) 240 °C. All scale bars are 50 nm.

excitation wavelength of 380 nm we can directly probe the defect emission of the nanostructures. It has been shown that increasing NW diameter with reaction temperature was observed for trigonal selenium.31 This increase was attributed to the larger seed diameter obtained prior to the wire formation. If we decrease the temperature to 160 °C (Figure 3A), only short, thin structures are found, and below 160 °C we did not recover any 1D nanomaterial. This indicates that the formation of the wires is thermally activated. We believe that around 160 °C particles start to be formed and function as the seeds for NW growth. At higher temperatures the initially formed seeds are larger due to increased growth rates prior to complete ligand capping. This mechanism also supports the trend of thicker wires with higher [W(CO)6] because both of these changes lead to faster growth kinetics at the early reaction stages. A possible reason for the onset of particle formation at the observed temperature could be due to the physical properties of W(CO)6. W(CO)6 has been reported to have a melting point of 166 °C, a boiling point at 184 D

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Figure 5. TEM images of NWs obtained with various solvents in place of SA. The solvents with 18 carbons and different function groups were (A) octadecylamine, (B) octadecanethiol, and (C) octadecanoic acid. Solvents with an alcohol function group but differing chain length were (D) dodecanol, (E) hexadecanol, and (F) eicosanol. All scale bars are 50 nm.

Figure 6. Characterization of the stoichiometry of WOx NWs was done with (A) Raman spectroscopy and (B) XPS. The gray plot in the Raman spectrum was obtained at a low laser power. The green plot was obtained using increased laser power and corresponds to tungsten trioxide. The black dotted line in the XPS spectrum is the total signal measured from the detector. The two blue plots show the peaks fitted to the W4f7/2 and W4f5/2 states corresponding to W6+. The two purple plots show the peaks fitted to the W4f7/2 and W4f5/2 states corresponding to W5+.

sensitive reaction parameters such as temperature ramp rate, chemical purity, and so forth.33−35 In this synthesis many factors have been tested and are accounted for. The standard synthesis is carried out in a 20 mL scintillation vial, but we have also successfully scaled up the reaction using 0.25 g of W(CO)6 in a crystallization dish covered by a watch glass. Testing the effect of the solvent by substitution of ODE with diphenyl ether did not affect the final product. This demonstrates that the important components to the synthesis are only the W(CO)6 and the alcohol. The addition time of the W(CO)6 does not make a difference for ultrathin NW formation. If the metal precursor starts in the vial or is added after 10 min of the solvent mixture heating in the oil bath we obtained the same product. The robust synthesis of ultrathin NWs is a positive attribute practically, but it also enables us to confidently exclude outside factors in our analysis of the growth mechanism. Characterization of the NWs. We sought to investigate the phase and stoichiometry of the NWs; however, definitive assignment is quite difficult for structures of such small diameter. Raman spectroscopy has been a useful tool in the analysis of tungsten oxide stoichiometry since it relies on phonon excitation instead of molecular vibrations in crystalline samples where the degree of crystalline order gives rise to different spectra.36,37 In the case of the tungsten suboxides, the packing of octahedra is less isotropic than in tungsten trioxide and thus the Raman spectrum shows broader peaks.38 In Figure 6A we show the results obtained using Raman spectroscopy on ultrathin WOx NWs. The initial spectrum was taken with a low laser power, and the result shows features at 230, 300, 395, 435, 730, 775, 870, and 945 cm−1 (Figure 6A). However, precise determination of the peak positions is complicated by their overlap due to extensive broadening. With increased laser power we observed new peaks

at 275, 330, 715, and 810 cm−1, indicating a transformation to WO3 by localized laser heating. This transformation behavior matches previous studies on both W18O49 and W20O58.36,37 The laser oxidation of tungsten suboxides is a well-known phenomenon and allows us to confirm that the initial spectrum contains information about the reduced form of the NWs. The peaks we observe at low laser power cannot be indexed to a reported phase of tungsten oxide. However, the diameter of the ultrathin NWs reported here is less than a unit cell, and, thus, their bonding is likely distorted from bulk structures. In general, there are three main regions in the Raman spectrum relevant to tungsten oxide structures. The highest frequency region around 950 cm−1 shows the WO stretch from hydrates or cluster boundaries; the next highest frequency region is from 600 to 800 cm−1 which relates to W−O−W stretching frequencies, and the low frequency region around 400 cm−1 correlates to W−O−W bending frequencies.38,39 In the low frequency region there are distinct features from our NWs. This supports the presence of a structure with crystallographic sheer planes (i.e., W20O58) and not one with pentagonal columns (i.e., W18O49), which have a relatively featureless spectrum due to a larger variation in bond lengths.38 In the high frequency region, we see two main peaks centered at 730 and 775 cm−1 and one less prominent peak at 870 cm−1. If we compare these peaks to the high frequency peaks of W20O58 (830 and 873 cm−1) we see a match at 870 cm−1; however, the lower energy peak at 830 cm−1 is not observed.37 The position of the Raman peak for the W−O stretch relates to the W−O bond length; thus, multiple peaks in the high E

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frequency region indicate multiple W−O bond lengths in the material.40 The peak values in our sample at 730 and 775 cm−1 are closer together than the two dominant peaks in WO3 (810 and 718 cm−1). This shows that the W−O bond lengths in our suboxide NWs have less variation than the bond lengths in WO3. This may be an effect of the surface ligands on the bonding within the entire NW or might reflect on the geometry of nucleates in early reaction stages. From the Raman spectrum alone it is difficult to precisely characterize the stoichiometry of the NWs. The evidence we obtained suggests that a crystallographic sheer structure, such as W20O58, is the most likely structure for our NWs. To further investigate the stoichiometry of the NWs, we used X-ray photoelectron spectroscopy (XPS). XPS can be used to measure the composition of tungsten oxides that have different ratios of W5+ to W6+. XPS is a surface sensitive technique; however, due to the small diameter of the NWs we can assume that we are collecting signal from the entire particle volume. Thus, XPS can be a useful tool for complete stoichiometric analysis of our NWs. XPS results in Figure 6B show the aggregate data and fitted peaks. The peaks at a binding energy of 35.5 and 37.6 eV match with reported values for the W6+ oxidation state whereas peaks at 34.2 and 36.3 eV match with values for the W5+ oxidation state.41 This analysis shows the W5+:W6+ ratio of about 1:6 and no indication of W4+. These factors together indicate that W18O49 is not formed in our synthesis but rather one of the phases closer to tungsten trioxide, such as W20O58 or W25O73.42 Whether this phase is indeed unique to our system or generally true for ultrathin NWs is not known since in-depth structural information on previous ultrathin NWs is limited. To further show that the NWs are WOx we used bright and dark field TEM and XRD. The dark field TEM in Figure 7B is collected from the diffraction ring corresponding to a d-spacing of 0.38 nm, which matches the spacing of many tungsten suboxides.43 By comparing the dark field image with the bright field image (Figure 7A,B), it can be clearly observed that the wires are responsible for the diffracted electrons at this spacing. The XRD data shown in Figure 7C is typical of ultrathin NWs. The sharp peak observed at 23.6° corresponds to a d-spacing of 0.38 nm and indicates the growth direction of the wires. Our results match the previous literature that shows the wire growth is along the [010] direction. The broad peaks in the plot are due to the small diameter of the wires and the associated Scherrer broadening in those lattice directions. Growth Mechanism. Despite the number of known syntheses for ultrathin WOx NWs, the nucleation and growth mechanisms are not yet fully understood. In a recent review of ultrathin NWs, Cademartiri and Ozin grouped the mechanistic pathways toward ultrathin NW growth into three modes: ligand assisted growth, templating/polymerization, and oriented attachment.44 We performed two experiments to further understand the reaction mechanism. First, we took aliquots of our reaction over time. In Figure 8, the evolution of the structures over the first 14 min can be seen. Prior to 6 min we recovered no product. At 6 min (Figure 8A) we see wires of 24 ± 11 nm in length, with a maximum measured length of 58 nm. By 10 min (Figure 8B) the wires have grown to 74 ± 40 nm, with a maximum measured length of 222 nm. At 14 min (Figure 8C) the wires are about 175 nm long on average, with a maximum length of 391 nm. By the end of the reaction the wires are too long to measure accurate averages, but the maximum length we measured unambiguously was 730 nm. We can see that the NW growth rate increases as the reaction proceeds. This might indicate that inter-NW coupling is

Figure 7. Structural characterization obtained with (A) bright field and (B) dark field TEM and also (C) XRD. The bright field image is of WOx NWs and the electron diffraction (inset) shows a strong diffraction peak at 0.38 nm, corresponding to one of the tungsten suboxides. The dark field image was obtained from diffraction at 0.38 nm. All scale bars are 50 nm.

taking place in addition to monomer growth. A similar mechanism was recently shown in an analysis of Bi2S3 ultrathin NW growth.45 The next experiment aimed to examine the nucleation stage of the reaction. We UV-irradiated a solution of SA with W(CO)6 (at 60 °C to melt the SA) with the standard solvent and precursor ratios (see details in experimental section). UV irradiation of W(CO)6 is known to remove one of the ligands which can then be replaced by a solvent molecule.46 We observed a yellow color develop upon irradiation, which is indicative of partially substituted W(CO)6.46,47 Repeating this same irradiation experiment with a 1:1 molar ratio of W(CO)6 and SA results in complete complexation of SA as seen by the disappearance of the OH stretch at ∼3300 cm−1 measured by FT-IR (Figure S7, Supporting Information). This likely indicates that the alcohol has complexed to the tungsten. Conducting the reaction under the standard conditions using this preformed tungsten alkoxide species, we get the same NW product as in the original conditions. Attempts to use FT-IR to analyze the reaction solution at early stages were unsuccessful as excess SA and low [W(CO)6] obstructed the results. Tungsten compounds have a very diverse chemistry, which makes the study of the mechanism of WOx NW growth both intriguing and challenging.48,49 For example, polyoxotungstates are well-known cluster anions consisting of tungsten, oxygen, and usually a cation.50 Additionally, tungsten alkoxides and oxoalkoxides are known cluster compounds of varying size.48 At our reaction temperature there is thermal decomposition of our tungsten precursor and an abundance of alcohol molecules. It is therefore reasonable that an alkoxide intermediate plays a role. Also, considering the literature in total, all the WOx ultrathin NW F

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Figure 9. Molecular and coordinating octahedron representations of a W4(OR)16 cluster.

There are also known complexes with two different ligands (heteroleptic complexes) that form when excess CO is introduced, that is, W4OR12 clusters with excess CO yielding W4OR12(CO)3.56 If a cluster of mixed alkoxide and CO ligand was present this could also control the condensation reaction to be limited to one direction. A heteroleptic compound involving CO would have different exchange kinetics and might also explain why our reaction proceeds much more rapidly than other reactions using WCl6. Lastly, we showed before that a minimum amount of SA is needed to form long wires, and below this amount short, branched structures occurred. If the amount of alcohol was not sufficient for fast or complete W-alkoxide formation this could prevent cluster formation or alter the cluster so that nonlinear structures can be formed. In summary, we believe that W(CO)6 reacts at elevated temperature with SA to form a multinuclear alkoxide cluster. The exact nature of this cluster can be altered with the reaction conditions. Due to anisotropy of the cluster and of the alkoxide/ carbon monoxide ligands, 1D growth is obtained.

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CONCLUSIONS We have described a new synthesis method for ultrathin nanowires of tungsten suboxide that allows for previously unachievable control over the nanowire morphology. Independent control over the length and width is readily achievable, which should lead to better performance of EC devices, photocatalysts, dye sensitized solar cells, and gas sensors. We have also shown XPS and Raman spectroscopy analysis of our NWs that support the W20O58 structure over the W18O49 phase. The synthesis is very robust and can be performed with simple, inexpensive equipment. Thus, this work enables more research groups the ability to test the properties and applications of NWs with carefully controlled aspect ratios. Lastly, we investigated the growth mechanism for WOx ultrathin nanowires in solution. We believe the process to take place through the reaction and coupling of small alkoxide clusters.

Figure 8. TEM images of reaction aliquots during a standard NW synthesis. The times that the aliquots were taken from the main solution were (A) 6 min, (B) 10 min, and (C) 14 min. All scale bars are 50 nm.

syntheses discussed previously involved an alcohol solvent or a tungsten alkoxide precursor. Thus, based on this information and the evidence we have collected, we propose that the growth mechanism for ultrathin NWs involves alkoxide species. In addition to the chemical evidence, alkoxide clusters have also shown behavioral trends that match what we observe in our synthesis. For example, Ti-alkoxide clusters can increase in size as the temperature increases and the size regime observed was similar to our NW diameters.51 Additionally, the well-studied Tialkoxide cluster, Ti4OR16, can also be chemically modified into many other cluster geometries.52,53 Some of the most common metal alkoxide clusters for tungsten are W 4 OR 12 and W4OR16.54,55 On each the two faces of the W4OR16 cluster only one alkoxide group is present that is triply bridged with the tungsten ions (Figure 9). From the side of the cluster the monoand bidentate alkoxides are packed more densely. Thus, there is a good reason to suspect that coupling of such clusters would take place from the top and bottom facets, primarily in one direction.

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ASSOCIATED CONTENT

S Supporting Information *

A table describing synthetic results with different reaction parameters is shown. TEM images of assemblies of NWs obtained with dodecanol and hexadecanol are presented. FT-IR data showing complexation of octadecanol to tungsten hexacarbonyl is also given. Nanowire length and diameter statistics are also tabulated. This material is available free of charge via the Internet at http://pubs.acs.org. G

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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 We thank I. Jen-La Plante for helpful discussions, M. Diab for XRD measurements, Dr. N. Froumin for XPS analysis, Dr. V. Ezersky for dark field TEM measurements, and Dr. L. Zeiri for Raman measurements. This work is supported by the Legacy Heritage Fund Program of the Israel Science Foundation (Grant No. 1728/09) and the Marie Curie Reintegration Grant (IRG)FP7 People-2009-RG. B.M. also thanks the Merage Foundation for financial support.

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dx.doi.org/10.1021/cm302015z | Chem. Mater. XXXX, XXX, XXX−XXX