DOI: 10.1021/cg9010295
Synthesis of Nanostructured Tungsten Oxide Thin Films: A Simple, Controllable, Inexpensive, Aqueous Sol-Gel Method
2010, Vol. 10 430–439
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Michael Breedon,*,† Paul Spizzirri,‡ Matthew Taylor,§ Johan du Plessis,§ Dougal McCulloch,§ Jianmin Zhu, Leshu Yu,z Zheng Hu,z Colin Rix,^ Wojtek Wlodarski,† and Kourosh Kalantar-zadeh† School of Electrical and Computer Engineering, RMIT University, Australia, ‡School of Physics, University of Melbourne, Australia, §School of Applied Sciences, Applied Physics, RMIT University, Australia, National Laboratory of Solid State Microstructures, Physics Department, Nanjing University, China, zKey Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, China, and ^School of Applied Sciences, Applied Chemistry, RMIT University, Australia )
†
Received August 26, 2009; Revised Manuscript Received September 26, 2009
ABSTRACT: Among the available metal oxide nanostructures, tungsten oxide has remained, at times, troublesome to fabricate, with many synthetic methods often requiring exotic equipment and or reagents. In this work, we present a systematic investigation demonstrating a new method for the deposition of anhydrous and hydrated nanostructured tungsten oxide thin films via spin coating. The attributes of these materials include the following: high surface area, controllable deposition, and compatibility with existing semiconductor fabrication infrastructure making this method a suitable candidate for application in the manufacture of gas sensors and dye sensitized solar cells. We will show that it is possible to form micrometer thick highly crystalline nanostructured thin films and, using Raman, SEM, XRD, XPS, and TEM analysis, will prove that these nanostructures can be rendered into anhydrous or partially or fully hydrated tungsten oxides. We further demonstrate the application of these materials in the fabrication of an inexpensive NO2 gas sensor, capable of sensing sub-ppm levels of NO2 concentrations at a modest operating temperature of 175 C.
1. Introduction Tungsten Oxides. Tungsten oxides are intrinsically semiconducting metal oxides, capable of withstanding low pH environments and high temperatures, making them excellent choices for robust, nanostructured, inorganic frameworks. Tungsten trioxide (WO3) and tungsten oxide hydrates (WO3 3 nH2O) have been fabricated via a number of different experimental routes, which include but are not limited to: acid precipitation,1,2 chemical etching,3 sol-gel synthesis,4 thermal evaporation,5 RF-sputtering,6 chemical vapor deposition (CVD),7 anodization,8-10 electrodeposition,11 electrospinning,12 pulsed laser deposition,10,11 and hotwire CVD.13 However, each of these methods has one or more characteristic drawbacks, including: being highly energetic, being vacuum dependent, or requiring exotic and often dangerous reagents. Some of these methods are also unable to generate nanocrystalline tungsten oxide morphologies. This paper describes a controlled, safe, room temperature synthesis that uses readily available laboratory reagents, effectively negating the need for expensive processing equipment and hazardous reagents. The precipitation of WO3 3 nH2O from a tungstate ion solution using concentrated acid is a well-known synthetic route described as the following:1,14 H2 O
H2 O
WO4 2 - þ 2Hþ sf H2 WO4 sf WO3 3 nH2 O
ð1Þ
however, this reaction occurs rapidly upon mixing the two reagents, often yielding micrometer (or larger) sized WO3 3 nH2O particles. *Corresponding author e-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 10/29/2009
Unlike many other methods which generate powdered nanodimensional tungsten oxides, we demonstrate how spin coating and aging of the deposited film in a humid environment makes it possible to generate contiguous, nanostructured thin films. This method is compatible with existing semiconductor fabrication infrastructure and could easily be scaled to suit larger substrates. The methodology is described in detail in the experimental section. Briefly, two separate solutions (nitric acid at pH -0.1, and sodium tungstate at pH ∼12) are mixed, generating a sol-gel, which is then spun onto a quartz substrate. Coated substrates are then dried in either a low or high humidity environment. This method, especially when combined with an aging step in a humid environment, produces highly crystalline hydrated tungsten oxide nanostructures which can be directly deposited onto technologically important surfaces, such as those on transducers. Deposited films can be used directly as hydrated tungsten oxides or annealed to induce the desired pure or mixed nanostructured WO3 polymorphs (i.e., monoclinic, triclinic, hexagonal, or orthorhombic). This permits some degree of freedom in tailoring the physical and chemical characteristics of the processed material. Experimental Procedure. In this process, two 20 mL syringes were placed into a programmable syringe pump (Harvard Instruments PhD 2000). The first syringe was filled with a nitric acid (1.25 mol dm-3) solution, while the second contained a Na2WO4 (0.25 mol dm-3) solution which had been adjusted to pH 12.1. The syringe pump was programmed to deliver 5 mL of each solution onto a spinning quartz substrate rotating at 8000 rpm with a constant flow rate of either 0.25 mL/min, 0.5 mL/min, 1.0 mL/min, 2.5 mL/ min, or 5.0 mL/min. The outlets of each syringe were r 2009 American Chemical Society
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extended using tygon laboratory tubing and fed separately into a microfluidic Y-connector. This Y-connector was then suspended over the spinning substrate in a manner which ensured that the mixed liquid was dropped directly onto the center of the rotating substrate. Quartz substrates were diced into 11 11 mm2 and cleaned with acetone, isopropyl alcohol, and DI water prior to deposition. Conductometric substrates were patterned with four sets of interdigitated transducer pairs and were cleaned similarly. In search of improved uniformity and surface adhesion, a variety of different hydrophilic and superhydrophilic susbstrates were also appraised. However, while it was possible to deposit onto these substrates, there were no visual improvements in uniformity nor mechanical adhesion of the film; as such, their use was discontinued in favor of less expensive substrates. Samples were dried for 24-72 h either (i) under ambient conditions (typically 35% RH), identified as “dried in air”, or (ii) in a high humidity environment (around 55% RH or greater), identified as “as prepared”. It should be noted that all spectroscopic and diffraction data presented in this paper has been offset for purposes of comparison and clarity. The resulting thin films were analyzed using the following equipment: FEI Nova Nano FE-SEM; JEM-40001X and JEOL-2010 HR-TEM; Olympus SZ-CTV optical microscope with Pixel link CCD camera; Bruker D8 XRD; Micro-Raman measurements were performed on a Renishaw RM1000 spectrometer in a backscattering geometry. The 514.5 nm line from an argon ion laser was used as the excitation source at low excitation powers to prevent localized heating during the measurement. The notch filter spectral response profile of the instrument prevented measurement below ∼100 cm-1. XPS measurements were performed on a VG-310F instrument using Al nonmonochromated X-rays (20 kV, 15 mA) with the hemispherical energy analyzer set at a pass energy of 100 eV for the survey spectrum and 20 eV for the peak scans. Photoelectrons were taken off normally to the surface. All spectra presented were charge shift corrected, taking the C 1s peak at 285.0 eV. Experimental Considerations. Tungstate ions exhibit a number of different species in an aqueous environment (WO42-, HWO4-, H2WO4, HW6O215-, and W6O216-),15 with the major species present at pH 12.1 being WO42. As such, the precursor tungstate reagent was adjusted to pH ∼12. Deviation from the optimal solution concentrations and pH’s may result in unfavorable growth. Nitric acid with a concentration of 1.25 mol dm-3 (-0.1 pH) forces the (WO42-) to precipitate from solution via an intermediate H2WO4 complex, yielding WO3 3 nH2O and water, as outlined in reaction 1. It should be noted that once the two solutions are mixed, the viscosity increases as the resultant solution gels. Control of the gelation process is achieved by feeding the two separate reagents into a Y-connector, making it possible to tune the viscosity of the droplets exiting the connector prior to deposition on a suitable rotating substrate (i.e., a quartz substrate on a spin coater). Optical properties such as opacity, which is a function of film thickness, can also be controlled via the flow rate. Flow rates as low as 0.25 mL/ min result in a comparatively viscous gel. Conversely, larger flow rates (5 mL/min) have a viscosity approximating that of water, leading to significantly thinner films. Flow rate was also found to have an effect on the type and distribution of the WO3 3 nH2O nanostructures. High flow rates often reduced porosity and resulted in microtextured WO3 3 nH2O structures.
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The condensation polymerization reaction that forms the basis of the sol-gel method is schematically represented in Figure 1, which was adapted from refs 16 and 17. Here, H2WO4 is formed when the two reagents are mixed in the Y connector. As H2WO4 comes into contact with other H2WO4 molecules, they undergo partial condensation polymerization where two hydroxyl ligands combine to eliminate water and form a shared oxygen bond between the two tungsten centers. Further addition of acid to the partially polymerized system causes tungsten oxo ligands (WdO) to become progressively hydrated, resulting in the formation of dihydroxy ligands W;(OH)2. These ligands then further condense to increase the degree of cross-linking, eventually leading to a 3-dimensional structure in which tungsten centers are completely cross-linked to adjacent tungsten centers via four or six shared oxygen bonds, resulting in structures similar to those found in polyoxo-tungstates.17 These polyoxo-tungstates (tungstic clusters) later coalesce, forming extended oxide sheets16 which cause the formation nanoplatelets, schematically represented in Figure 1. 2. Results and Discussion Humidity Treatment and Annealing. Samples were observed to intensify in color, turning pale yellow during syneresis. During drying in a fume cupboard with a humidity of ∼35% (“dried in air” samples), the thin films often developed cracks, presumably due to the interfacial tension caused by the hydrophobic quartz substrate and the gel, or due to crystallite expansion compensating for the residual strain in the deposited film. High humidity environments have been previously shown to be an effective means of crystallizing stable WO3 3 2H2O.16,18 In light of this, samples were placed in a humid environment of approximately 55% RH for 24-72 h with samples identified as “as prepared” samples, since these samples were utilized as the basis for the realization of functional devices. This process had a dramatic impact on the morphology, uniformity and adhesion of the dried tungsten oxide films. Comparing the electron microscope images in parts a and b of Figure 2, samples left to dry in water vapor produced platelets with an increased diameter when compared with samples dried under ambient conditions. In this investigation, electron microscopy has been used to confirm that the as prepared product forms rectangular WO3 3 nH2O platelets with dimensions ranging from several hundred nanometers to several micrometers. The platelet size depends upon the deposition parameters and subsequent processing as depicted in the SEM and TEM images presented in images of Figure 2. The nanoplatelets shown have thicknesses of the order of 10-30 nm and had a high affinity for each other, readily agglomerating during the deposition process to form thick films (several micrometers). Parts a and b of Figure 2 show that many of the nanoplatelets attempt to align orthogonally to their nearest neighbors, generating a small degree of long-range order in the thin film. Humid environments may also retard the rate at which water can be eliminated (in its capacity as a leaving group) from the cross-linking sol-gel, thus lowering the speed at which adjacent clusters cross-link. This appears to assist in minimizing the interfacial stress between the substrate and the aging sol-gel that would otherwise cause characteristic cracks to emerge in the drying film. Furthermore, this atmosphere dampens samples, providing a physical means for
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Figure 1. Sol-gel condensation polymerization reaction and cluster condensation.
aqueous polyoxo-tungstate species to fortify the emerging crystalline tungstite platelets, normally extending the high surface area facet of the crystallite. As seen in Figure 2c, the sample dried in a fume cupboard is polycrystalline with a concentric diffraction pattern. In contrast, the sample dried in a humid atmosphere was highly crystalline with a set of bright diffraction spots in the SAED pattern indicating the single crystal feature (inset in Figure 1d). Fortification of platelets during the aging process is evidenced by an increase in the dimensionality and improvement of crystallinity of samples prepared via humid atmosphere. Samples presented in Figure 2c had been dried under ambient conditions and are polycrystalline with a small number of secondary nanocrystalline precipitates. In
contrast, samples dried in a high humidity environment appear more crystalline, with only minor secondary nanocrystallites observed forming on the surface of the larger nanoplatelet (see Figure 2d). For the fabrication of many functional devices, anhydrous tungsten oxide may be required. Considering this, samples were annealed for 2 h at either 300 or 550 C in an oxygen rich atmosphere (90% O2(g) - 10% Ar(g)) to drive water from the crystal lattice and to prevent the formation of substoichometric tungsten oxides. Electron microscopy of the annealed samples revealed that they were well ordered and crystalline, as shown in Figures 3 and 4. High resolution transmission electron microscopy (HRTEM) observations indicate that the tungsten oxide
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Figure 2. Electron microscopy (SEM and HRTEM) of materials (a) left to dry at low humidity, “dried in air” (inset is a low magnification optical image), (b) dried in a humid environment, “as prepared” (inset is a low magnification optical image), (c) HRTEM of tungsten oxide nanoplatelets dried at low humidity, and (d) HRTEM of tungsten oxide nanoplatelets dried in a humid environment. Insets for parts c and d are the corresponding SAED patterns. The HRTEM scale bar is 5 nm, and all samples were processed at 8000 rpm with a dispensing rate of 1 mL/min.
Figure 3. HRTEM of WO3 nanoplatelets annealed at (a) 300 C or (b) 550 C. Figure insets are the corresponding SAED patterns.
nanoplatelets were formed from the coalescence of smaller tungstic acid precipitates, visible as raised areas in the HRTEM images. The corresponding SAED of the annealed samples consisted of many bright diffraction spots, indicating that the nanoplatelets had become highly crystalline. In all samples, lattice fringes were present and have been used to identify the respective phases of the annealed tungsten oxide samples. At an annealing temperature of 300 C, tungsten oxide was calculated to have a lattice fringe d-spacing of 2.64 A˚, which is in excellent agreement with the (201) plane of monoclinic WO3 (ICDD card file 75-2072). It was elucidated
that the sample annealed at 550 C exhibited a characteristic d-spacing of 3.79 A˚, corresponding to the (020) plane of orthorhombic WO3 (ICDD 71-0131). X-ray Diffraction. X-ray diffraction (XRD) investigations, presented in Figure 5, indicate that the as prepared sample contained tungsten oxide hydrates, with all three being tungsten oxides with different degrees of hydration, i.e. WO3 3 nH2O, where n is typically 0, 1/3, 1/2, 1, or 2. The nanostructured thin film had a high degree of crystallinity, as indicated by the sharp reflections, which corresponded to the ICDD card files: 84-0886 (tungsten oxide
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Figure 4. SEM image representing a tungsten oxide film at different stages of processing (10 μm scale bar).
Figure 5. XRD patterns of tungsten oxide at the different stages of fabrication.
hydrate), 18-1420 (tungsten hydrogen oxide hydrate), and 89-0758 (tungsten oxide hydrate). These films were polymorphic, matching primitive crystal systems being monoclinic, with the addition of some minor orthorhombic characteristics. It should be noted that, in these XRD
measurements, a large area of the sample was measured, effectively collecting an average of the different crystal structures found throughout the material. The observed polymorphism in the XRD pattern is ascribed to the aqueous nature of the sol-gel preparative method. Smaller secondary
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Figure 6. Representative Raman spectra of the WO3 samples prepared this work. The spectral range shown depicts various WO3 vibrations between 200 and 1000 cm-1. The samples are identified on each plot and represent dried in air and as prepared materials prior to annealing.
nanoprecipitates localized on some of the larger platelet structures can be seen in Figure 2c and d. Annealing as deposited samples for 2 h in an oxygen rich atmosphere at 300 C rendered the tungsten oxide hydrates to monoclinic WO3, which was in good agreement with the ICCD cardfile 75-2072. Annealing at temperatures of 550 C generated multiple crystalline phases of WO3, with the dominant crystallographic structure being orthorhombic (ICDD 710131). In addition to this there are also some reflections which are in good agreement with hexagonal WO3 (ICDD 75-2187) and WO3 3 1/3H2O (ICDD 87-1203). It should be noted that in both annealed samples, the presence of crystalline sodium nitrate (ICDD 89-0311) was observed as this material was formed as a byproduct of the reaction between sodium tungstate and nitric acid. This species was also identified using both Raman and XPS analyses. XRD patterns of samples dried in air are not reported, as there was insufficient material coverage to obtain meaningful results. Raman Spectroscopic Analysis. Raman spectroscopy was employed to study the differences between the preparation methods, including post-deposition annealing. Tungsten trioxide exists as one of several allotropes in addition to a number of hydrates (WO3 3 nH2O), and Raman spectroscopy is well suited to their analysis.19,20 The bending modes of WO3 are located between 600 and 960 cm-1 while the stretching modes can be found between 200 and 500 cm-1. Lattice modes of crystalline material have been reported below 200 cm-1.19,21 Due to the transmission properties of the notch filter used for these measurements, lattice modes could not be studied in this work. Results for WO3 3 nH2O prepared in humid (as prepared) and dry environments (dried in air), as well as several as prepared samples annealed at 300 and 550 C, are presented in Figures 6 and 7, respectively, where the vibrational modes associated with WO3 and WO3 3 nH2O are evident. Measurements from dried in air and as prepared materials exhibit broad features with bands indicative of hydrated, so-called amorphous or nanocrystalline WO3 3 nH2O.19 The peak
observed at 378 cm-1, which can be attributed to stretching modes arising from W-OH2, is also consistent with the presence of WO3 3 nH2O phases. Peaks arising from OH stretches were also observed (not shown) around 3200 cm-1, which further support this assignment. The band situated at 960 cm-1 has been attributed to the symmetric stretching mode of terminal (W6þdO) groups and is often associated with WO3 3 nH2O.19,21 For the as prepared sample, the band is resolved as a doublet at around 960 cm-1 and may be attributed to multiple and distinct (W6þdO) groups which may also be vibrationally coupled.22 These peaks represent lattice discontinuities which lead to short-range (lattice) order. Bridging (O-W-O) vibrations, which occur around 700 cm-1, are influenced significantly by hydration, and as a result, the 660 cm-1 band can be used as a spectral marker for the hydration level of WO3.19 For the sample as prepared in Figure 6, the broad transitions at 660 and 690 cm-1 resemble that of the dihydrate (WO3 3 2H2O) whereas the spectrum for the sample dried in air, which corresponds to a sample aged in a low humidity environment, may be attributed to the monohydrate (WO3 3 H2O).19 Care should be exercised using this approach, since the crystalline hexagonal phase (h-WO3) also exhibits bands at these frequencies but is likely to be absent in materials prepared without a thermal annealing step. Several other bands also appear in the spectra of these materials. For the sample dried in air, the band observed at 1052 cm-1 is not characteristic of WO3 3 nH2O and is thought to originate from NaNO3, which is known to crystallize at room temperature into an ordered calcite structure.23 This salt likely forms during the reaction between nitric acid and sodium tungstate. Other weak transitions can also be seen at ∼510 cm-1 and ∼430 cm-1 and may be associated with tungsten peroxo groups24 and water librations, respectively.19 For the as prepared sample, the small peak at 466 cm-1 may be indicative of a reduced tungsten (W5þdO) mode.25 Following thermal treatment, the spectral features of these materials changed dramatically; as observed in Figure 7,
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Figure 7. Representative room temperature Raman spectra of annealed WO3 samples prepared in this work. The annealing temperature for each of the samples is identified in the figure.
Raman bands become sharper, suggestive of an increase in both crystallinity and order. For the 300 C annealed sample (Figure 7), the most intense peaks are located at 797 cm-1 and 711 cm-1. Based on this, the dominant phase in this material is assigned as monoclinic WO3.19 This is consistent with the known temperature dependent sequence for bulk WO3 phase transitions.26 The terminal (W6þdO) stretching mode at ∼950 cm-1 has been significantly reduced and replaced with bands at 797 cm-1 and 711 cm-1 with a shoulder at 687 cm-1. These arise from (O-W-O) stretching modes in the WO6 octahedral units.27 The 711 cm-1 mode, which is sensitive to lattice distortion, has also shifted from ∼689 cm-1 in the hydrated material (Figure 6), indicating that there has been a change in crystal symmetry following the annealing.28 Residual dihydrate phase likely remains, as indicated by the 687 cm-1 band. Peaks at 267 cm-1 and 328 cm-1 can be ascribed to the corresponding δ(W-O-W) bending modes of the bridging oxygens.29 The position of the ∼270 cm-1 band can be used to estimate the crystalline grain size,30 which in this work corresponds to ∼35 nm. Residual intensity in the ν(W6þdO) mode also supports the assignment of mixed phases (e.g., hydrated and crystalline) in this material. Peaks located at 191 cm-1 and 1068 cm-1 likely result from lattice vibrations of WO3 and residual NaNO3, respectively. Similarly, following annealing at 550 C, there is a further reduction in the spectral line width consistent with greater crystalline phase formation. Well-defined bands arising from the hydrous WO3 3 1/3H2O phase (948 cm-1, 805 cm-1, 679 cm-1, 337 cm-1, and 268 cm-1) are present in this material.19 Other bands at 790 cm-1, 775 cm-1, and 679 cm-1 may be indicative of the hexagonal WO3 phase.28 The weaker band at 918 cm-1 can be assigned to either a hydrous phase or, more likely, a sodium tungstate phase,28 which is consistent with the observed reduction in the 1068 cm-1 NaNO3 band. There is also evidence for tungsten-peroxo bonds with a band at 540 cm-1.24 Grain size analysis for this material30 using the δ(W-O-W) mode was complicated by the presence of overlapping peaks from 268 cm-1 to 274 cm-1, indicating that there is a range of grain sizes in this material.
X-ray Photoelectron Spectroscopy (XPS). XPS survey studies of as deposited and annealed samples presented in Figure 8 reveal the presence of tungsten, oxygen, sodium, and (adventitious) carbon. Spectra of O 1s and W 4f7 peaks are presented in Figure 9a and b, respectively. As discussed previously, the Na 1s peak at 1072 eV is due to byproducts of the sodium tungstate precursor. The O 1s spectra of the as prepared sample show a smaller peak occurring at 533.2 eV labeled (I) in Figure 9a at a higher binding energy compared to the main peak at 531 eV. The peak position is in good agreement with binding energies previously identified with oxygen atoms in H2O molecules bound with or in WO3.31 This weak oxygen peak (I) located at 533.1 eV disappears as samples were annealed and water was liberated from the sample. The position of peak (II) at 530.6 eV, which corresponds to WdO bonding modes, remains static and intensifies after annealing, suggesting that the tungsten in the tungsten oxide hydrate sample exists as W6þ. The W 4f orbital is clearly resolved into W 4f5/2 and W 4f7/2 contributions, centered upon 37.5 eV (I) and 35.4 eV (II), respectively. There was no evidence of the formation of substoichiometric WO3-x, owing to the absence of W5þ peaks, typically manifesting at 34.8 eV.31 The results presented in Figure 9b are in good agreement with previously identified stoichiometric WO3 (W6þ).32,33 Application of Nanostructured WO3 for NO2 Sensing. The WO3 nanoplatelets prepared in this work have been investigated as potential candidates for conductometric NO2 gas sensing. After spin coating and exposure to a high humidity environment, coated conductometric transducers were covered in a bright yellow film, which after annealing at 550 C was rendered white/beige. These processing steps and associated SEM images are presented in Figure 4. The fabricated sensor was appraised as a candidate for nitrogen dioxide (NO2) sensing; which is a toxic, irritating gas, posing serious repercussions for those afflicted with respiratory ailments and disorders. Upon repeated exposure to high concentrations of NO2 (1-5 ppm), there is some evidence of disordered mechanics of breathing and ventilatory function in healthy adults.34 These effects include increased breathing
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Figure 8. XPS survey spectra of tungsten oxide samples.
Figure 9. XPS of tungsten oxide hydrate and annealed samples (a) O 1s and (b) W 4f7.
frequency, while subchronic exposures of
