Growth, Structure, and Stability of KxWO3 Nanorods on Mica Substrate

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Growth, Structure, and Stability of KxWO3 Nanorods on Mica Substrate V. Potin,* S. Bruyere, M. Gillet, B. Domechini, and S. Bourgeois Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB), UMR 5209 CNRS-Universit
e de Bourgogne, 9 Av. A. Savary, BP 47 870, F-21078 Dijon Cedex, France ABSTRACT: KxWO3 nanorods, interesting as gas sensors, were elaborated on mica muscovite substrate and characterized by atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and mainly transmission electron microscopy. A combination of structural analyses allowed determining the morphology of these rods, and selected area electron diffraction experiments pointed out the simultaneous presence of the exotic hexagonal and stable monoclinic phases. Moreover, the presence of potassium inside the nanorods, coming from the mica substrate, was revealed. By combining all the observations, a growth model is proposed, consisting of the stacking of two different crystallographic structures. An interfacial hexagonal KxWO3 tungsten bronze grows in epitaxy on the mica substrate, followed by the growth of monoclinic trioxide tungsten. The stability of these nanometric objects was studied by annealing up to 870 K, indicating that the transformation of the presumed metastable hexagonal phase never occurred.

’ INTRODUCTION In the last decades, WOx tungsten oxide materials have attracted great interest due to their wide range of applications as electrochromic devices,1,2 dye-sensitized solar cells,3 optical recording devices, or photocatalysts.4,5 Great interest was also focused on this material as one of the most promising oxides for gas-sensing applications. In particular, WOx tungsten oxides were studied for the detection of NO2,6 H2,7 H2S,8 O3,9 H2O,10 alcohol, or organic gas.11 For more details about WO3, a general review of nanostructured WOx properties, methods of synthesis, and applications was recently published by Zheng et al.12 The crystallographic structure of WO3 is derived from the cubic structure of ReO3 in which the tungsten atoms are located at the cube corners, whereas the oxygen ones are located in the edge middles. Different crystallographic structures for WO3 are stable in a well-defined range of temperatures.13 It has been widely reported that phase transformation occurs in the following sequence for increasing temperatures: monoclinic II (ε-WO3), triclinic (δ-WO3), monoclinic I (γ-WO3), orthorhombic (β-WO3), and tetragonal P4/ncc and P4/nmm (α-WO3).14,15 Moreover, lower-temperature transitions are also reported in the case of nanoparticles compared to bulk materials.16 At room temperature and below 600 K, for bulk materials, the monoclinic γ-WO3 is the most stable phase; it has been extensively studied, and its parameters are well-known.17 Besides, the presence of hexagonal h-WO3 has also been reported.18 In contrast to the other structures, it cannot be obtained by crystal-phase transition after annealing or cooling from another WO3 structure but in specific conditions from dehydration of tungsten oxide hydrate19 or hydrogen bronze tungsten.20 It is mainly obtained after specific growth processes such as hydrothermal synthesis,21 vapor-phase deposition,22 or an indirect route.23 This hexagonal structure is characterized by the presence of small triangular and large hexagonal tunnels r 2011 American Chemical Society

along the c-axis, in which insertion of large cations like K+, Ca2+, Na+, or Pb2+ may occur.24,25 Due to this open-tunnel structure and intercalation chemistry, h-WO3 was extensively studied.26 Phase transition from hexagonal to monoclinic has been reported between 500 and 770 K in an exothermic reaction.27
30 Due to an increased surface to volume ratio, nanostructuration of oxide materials as nanowires, nanotubes, nanobelts, or nanorods enhances their properties compared to bulk materials.31 Especially, in the case of WO3, nanostructured objects were demonstrated to be more efficient gas sensors6,8 or photocatalysts32 than thin films. The influence of the different crystalline structures has also been investigated for various applications. For example, it appears that tetragonal and hexagonal WO3-based devices exhibit superior electrochromical and optical properties compared to the other structures.33 Similarly, h-WO3 has absolute selectivity to H2S in the presence of CH4, CO, H2, and NO, while m-WO3 has only relative selectivity.34 As WO3 exists in various crystal phases and as phase transitions between them are dependent not only on the temperature but also on the morphology, this system appears to be rather complex. Therefore, understanding the growth mechanism of the process is necessary to control these structures and therefore their properties. Our paper deals with the study of the structure and stability of WO3 nanorods grown on a mica substrate by a simple sublimation
condensation process in air at a temperature of about 640 K. Their morphology, crystallography, and composition have been characterized, confirming the simultaneous presence of the exotic hexagonal and stable monoclinic phases. The growth process of the nanorods and the thermal stability of these objects are discussed. Received: October 28, 2011 Revised: December 1, 2011 Published: December 06, 2011 1921

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’ EXPERIMENTAL METHODS The tungsten oxide nanorods were elaborated by a vapor
solid growth process, under air, at atmospheric pressure, on a (0001) freshly cleaved mica substrate.35,36 The nanorods were obtained after condensation of a sublimated oxide vapor. This oxide vapor was obtained by heating at 770 K a thick layer of WO3 previously deposited on silica. The mica substrate facing the vapor production is fixed above the heated silica plate by a series of wedges. By varying their number, the distance between them is comprised between 1 and 4 mm. The temperature of the mica substrate is then modulated between 630 and 650 K. Afterward, the whole system is cooled to room temperature. The obtained samples were first observed on the substrate by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). AFM experiments were performed in 00 tapping mode00 with a Nanoscope III Quadex and SEM ones with a TESCAN
MIRA I LMH. Afterward, they were characterized by Transmission Electron Microscopy (TEM). For TEM observations, a simple sample preparation was carried out by an extractive replica technique. After deposition of a thin carbon layer on the sample surface, the sample was put in diluted hydrofluoric acid during a few seconds. When put in water, the carbon replica with the extracted nanorods was separated from the mica substrate, floating on water. Portions of replica were then picked up from below with a forceps-held copper grid. Selective Area Electron Diffraction (SAED) and High Resolution Transmission Electron Microscopy (HRTEM) experiments were carried out with a JEOL 2100 LaB6 transmission electron microscope, whereas observations in scanning mode (STEM) were performed with a JEOL 2100 FEG one. The experimental diffraction patterns were saved with a GATAN Erlangshen digital camera. Their study was performed using the Gatan Digital Micrograph software, whereas the simulations of SAED patterns were performed with the Java Electron Microscopy software. The elemental chemical composition was determined by Energy Dispersive X-ray spectroscopy (EDX) in the electron microscopes by means of a JEOL 2300 EDT and a BRUKER Quantax XFlash 5030T SDD. To complete the morphological characterization, X-ray photoelectron spectroscopy (XPS) analyses were performed with variable emission angle using a VG Microtech CLAM 4 MCD analyzer system. Experiments were carried out by means of nonmonochromatized Al Kα radiation with emission angles varying from 0 to 85
with respect to the surface. The analyzer slit was set at 1 mm, giving an angular resolution of some degrees only. The analyzed sample size was then 1  1 cm2. The survey scan was recorded with a pass energy of 100 eV, while some lines (Si 2p and W 4f) were recorded specifically at a pass energy of 20 eV. Gaussian
Lorentzian (70
30) line shapes and Shirley background subtraction were then used for peak area measurements giving line intensities. ’ RESULTS 1. Structure and Morphology of the Nanorods. In a previous work,36 we have shown that nanorods grown on mica exhibit a particular organized pattern with an angle between them equal to 60
(Figure 1a). Their dimensions are comprised between several hundreds of nanometers and a few micrometers in length, several tens and hundreds of nanometers in width, and a few and several tens of nanometers in thickness, respectively.

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Figure 1. Typical (a) SEM and (b) AFM images showing the distribution of thin and thick nanorods.

Figure 2. (a) Dark-field STEM image of nanorods, (b) K map, (c) W map, (d) O map, (e) EDX spectra normalized on the Mα W line for black and red areas in (a), and (f) EDX linescan corresponding to the white arrow in (d).

When the totality of the substrate surface is observed by SEM, it appears that the nanorods do not cover the whole surface of the sample and that they are not homogeneously distributed. They are grouped together in several areas of many squared micrometers, whereas areas without any objects are also observed. Besides, if most of the nanorods grow alone, a few grow side by side or on top of each other, leading to increased width and/or thickness (Figure 1b). The thickest rods do not grow in a distinct pattern but appear to grow on the pattern previously formed by the thinnest nanorods. So, the rods can be divided in two categories in relation to their width and thickness. In the following, the aggregated and/or superposed nanorods will be referred to as thick and the single ones as thin. The elemental chemical composition was studied by EDX spectroscopy carried out in STEM mode. Thin and thick nanorods are all composed of tungsten and oxygen, with O/W ratio equal to 3.0 ((0.2). Therefore, the nanorods are mainly composed of tungsten trioxide. However, a small amount of potassium is systematically detected, varying from 0.5 to 4.5 at %, with the thickness of the nanorods. The dark-field STEM image of nanorods is presented in Figure 2a as well as maps obtained for K, W, and O elements (Figure 2b, c, and d, respectively) obtained exactly in the same area as the dark field image. These images clearly evidence that potassium is systematically detected in the whole area of the nanorods and only in it. It cannot come from pieces of the mica substrate scratched off during the replica 1922

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Figure 3. HRTEM images of (a) thin and (b) thick nanorods. The zones axes correspond to (a) [100] hexagonal structure with d020 = 0.640 nm and d002 = 0.380 nm and (b) [1
10] hexagonal and/or [010] monoclinic structures with d002 = 0.382 nm and d110h = d200m = 0.366 nm.

preparation. This is also confirmed by the fact that aluminum and silicon are never observed. These EDX maps were performed in 00 hypermap00 mode that allows extracting the EDX spectra obtained in each image pixel. Afterward, it is possible to sum up all of them, obtained from a given area. This was carried out in particular areas characterized by different contrasts and therefore by different thicknesses. Indeed, as the present species are exactly the same in the whole area, the difference of contrast is mainly due to a difference in thickness. The obtained spectra were normalized on the Mα ray of tungsten and on the background (Figure 2e). The black spectrum corresponding to the thickest area exhibits a lower potassium amount (0.9 at %) than the red one corresponding to the thinnest area (2.5 at %). This result is confirmed by EDX linescan performed along a nanorod with different thicknesses. When the probe scans the whole length of the rod from the thinnest part to the thickest one, the signal intensity emitted by potassium is constant, whereas the tungsten one increases by a factor of three (Figure 2d,f). By neglecting the absorption and secondary fluorescence, the intensities of detected signals correspond directly to amount of matter. The number of detected tungsten atoms decreases by an important quantity for the thinnest parts of the rods. In contrast, the number of potassium atoms appears not to vary along the rod. Therefore, the total amount of potassium is constant whatever the rod thickness is. As the rod density on the mica substrate is low, it is not possible to carry out X-ray diffraction experiments. Therefore, HRTEM and selected-area electron diffraction experiments in TEM were performed to study the crystallography. The main advantage of this technique is the possibility to obtain local information and therefore to discriminate between the different thin and thick areas of the rods. Previous work has pointed out the presence of hexagonal structure with [100] or [1
10] zone axes.37 As these results were obtained for the thinnest rods (Figure 3a), we put here emphasis on the results obtained for the thickest areas (Figure 3b). Typical HRTEM images and diffraction patterns may correspond to two different crystallographic structures exhibiting similar patterns: the hexagonal [1
10] pattern of KxWO3 is similar to the [010] monoclinic pattern of WO3. In both cases, as the exhibited distances are very close, it is difficult to distinguish between them without any ambiguity (Figure 3b). Indeed, the distances obtained for [1
10]h are equal to 0.380 nm for (002) and to 0.366 nm for (110), whereas the distances obtained for [010]m are equal to 0.384 nm for (002) and to 0.365 nm for (200). To discriminate between the

Figure 4. Distribution of the indexed hexagonal and/or monoclinic crystallographic structures in function of the tilt angle for each area labeled (a) and (b), respectively.

two possibilities, tilting of the sample in different directions was applied, and other zone axes were obtained. Series of diffraction patterns provided after tilting from the original zone axis to more than 45
were performed. The distances and the angles were determined from the patterns and compared to the values obtained from simulated patterns. This comparison allows establishing that the thick nanorods are composed of not only hexagonal but also monoclinic structures (Figure 3b). This is in contrast with the results obtained for thin nanorods in which only the hexagonal structure is pointed out (Figure 3a). To carry out a more detailed analysis, different areas were studied in the same nanorod (Figure 4): the area labeled (a) corresponds to the thinner end of the rod, whereas the (b) area corresponds to the middle of the rod where the width and therefore the thickness have increased. By referring to the AFM observations (Figure 1b), this increase indicates that it corresponds to superposed nanorods. For each area, different diffraction patterns were obtained after tilting of the sample holder. The results, displayed for both areas in Figure 4, show the distribution of the zone axes after indexation in function of the tilt angle. The tilt was applied around the direction indicated by the arrows in Figure 4, corresponding to d002 = 0.382 nm. For the zone labeled (a), both zone axes ([1
10]h before tilting and [100]h after tilting of 30
) correspond to the hexagonal structure. The absence of an important zone axis corresponding to the monoclinic structure after tilting of 45
confirms the unique presence of the hexagonal structure at the thinnest extremity of the rod. On the other hand, the comparison between experimental and simulated patterns indicates that both monoclinic and hexagonal zone axes are obtained from (b) area for different tilting angles. The following sequence for increasing tilt angles around d002 is pointed out: [1
10]h/[010]m before tilting, [
130]m, [100]h, and [
110]m after tilting of 18, 30, and 44
, respectively. A study of the stereographic projection was carried out. In theory, the hexagonal [1
10]h and [100]h zones axes and the monoclinic [010]m and [-110]m ones are linked to each other by angles equal to 30
and 45
, respectively. These values correspond exactly to the experimentally applied tilt angles to switch between the zone axes. So, the experimental zone axes correspond to only one initial orientation for each structure. These observations give important information on the growth mode of the rods. 1923

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The Journal of Physical Chemistry C The nanorod has first grown with a hexagonal structure, in orientation perpendicular to the substrate surface, in the whole area. This is shown by the observation of the same hexagonal zone axis in both thin and thick areas, indicating that the first layers of the rod have the same orientation. The monoclinic structure is only detected in the thicker areas, indicating that its growth takes place subsequently. Both structures seem to be superposed as the first zone axes obtained before tilting are the comparable hexagonal [1
10] and monoclinic [010]. The (110) and (001) planes of the hexagonal structure are parallel to the (010) and (001) planes of the monoclinic one, respectively. In this case, the mismatch is very low, less than 1% between (110)h and (010)m. Therefore, there is an epitaxy of the monoclinic structure on the hexagonal one. To complete the crystallographic description of these objects, the epitaxial relationships with the mica substrate were studied. This was carried out for a sample in which a part of the mica substrate was exceptionally torn away with the replica. By comparing the diffraction patterns obtained from areas corresponding to the WO3 thicker nanorods and the mica substrate, the following relationships are pointed out:
[010] monoclinic WO3 and [1
10] hexagonal KxWO3// [001] mica,
monoclinic WO3 and (001) hexagonal KxWO3//(010) mica. The (001) plane of the mica substrate is perpendicular to the Æ010æ and zone axes of monoclinic and hexagonal WO3, respectively. For the thinner rods, the epitaxial relationships correspond to:
[100] or [1
10] hexagonal KxWO3//[001] mica,
hexagonal KxWO3//(010) mica. In both cases, the growth direction of the nanorods (corresponding to the nanorod length) is parallel to the c-axis of the hexagonal and monoclinic structures. These relationships are in good agreement with Electron Channeling Pattern (ECP) experiments carried out on similar samples.38 In this study, Kikuchi lines of mica were observed and compared to the nanorod orientation. To complete the morphological study of tungsten oxide deposit, XPS experiments were carried out. In each case, aluminum (mainly from Al 2p line), potassium (mainly from K 2p line), silicon (mainly from Si 2p line), oxygen (O 1s), and tungsten (mainly from W 4f line) are detected. Using the respective evolutions of the intensities of two elements, the former being present in the substrate only while the latter is present in the deposit only, morphology of the deposit can be investigated using Fadley’s method.39 Fadley’s model considers a single overlayer A having a thickness t and surface coverage γ on a substrate B. The evolution of the experimental ratio IA/IB of the overlayer A line intensity to that of substrate B, as a function of the emission angle θ, can be fitted with the function R(θ) taking into account t and γ as well as fixed parameters such as the inelastic electron mean free path (IMFP) of detected photoelectrons (through the substrate and the deposit for the electrons coming from substrate B and through overlayer for electrons coming from the deposit A), the respective photoionization cross-section, and the concentration of each detected element in the corresponding bulk material.39 Theoretical concentrations of elements are known in substrate and deposit (considering WO3 deposit, whatever the crystallographic structure is); cross sections can be found in ref 40; and IMFP can be obtained from the QUASES-IMFP calculation software based on the Tanuma, Powell, and Penn formula (Table 1).41 In the present case, we chose the Si 2p line as a signal related to substrate,

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while the W 4f line was chosen for the deposit. The W 4f line area measurement was treated including the 4f7/2 and 4f5/2 components, whereas just one component was used for the Si 2p line. Considering WO3 nanorods having a thickness higher than ca. 10 nm (i.e., the thickness probed by XPS), Fadley’s model should give the surface area γ covered by WO3 rods. However, the experimental ratio IW/ISi plotted as a function of the emission angle sin θ cannot be well fitted with the function R(θ), as presented in Figure 5. Moreover, the best fit (the red curve on Figure 5) gives a rod thickness equal to 0.75 nm, while the surface coverage is 0.54. Such values are in complete disagreement with SEM and AFM observations (see Figure 1a and b). Considering this fact, we should admit that the real tungsten oxide deposit is not only composed of objects seen by SEM and AFM. Tungsten oxide should also be present on mica through a form other than nanorods. A simple way to confirm such a hypothesis is to use an evolution of Fadley’s model including two kinds of objects on the surface. Such an evolution was already developed in a previous paper in the case of a “double layer” deposit42 and can be easily modified to match with the present problem. Hence, in the “double layer” model, the equations integrate two different kinds of objects (A1 and A2), each having their own coverage (γ1 and γ2) and their own thickness (t1 and t2). In the expression of R(θ), the theoretical intensity of deposit A is then simply the sum of those of A1 and A2, while the signal of substrate B is now attenuated by two different deposits having each their own thickness and coverage. With this approach, a good fit is obtained (the black curve on Figure 5) with a first object A1 having a coverage γ1 = 0.11 and a thickness t1 larger than 10 nm and a second one A2 having a coverage γ2 = 1 and a thickness of t2 = 0.16 nm. Such a result is in good agreement with SEM and AFM observations with rods being the object A1. Due to the low thickness probe of the XPS technique, it is not possible to determine the thickness of rods by XPS, but the area occupied by rods (11% of the substrate surface) is anyway close to the one determined through SEM and images (see Figures 1a and b). However, XPS reveals also an ultrathin tungsten oxide film which completely covers the mica substrate (object A2). This latter object was not observed by SEM, AFM, or TEM. However, as such an ultrathin film does not exhibit any roughness, it cannot be seen by SEM and AFM. Moreover, concerning EDX analyses, the very small thickness of this film probably involves a tungsten amount below the EDX detection limit. Besides, this ultrathin film could not have been extracted from the mica substrate by the carbon replica. The sample, with a part of the mica substrate exceptionally torn away, was particularly studied to point out the presence of this ultrathin film. However, even in this case, due to the overlapping of the Si Kα and W Mα radiations, it was not possible to demonstrate without any ambiguity its presence. 2. Stability of the Nanorods. To study the thermal stability of these objects, annealing under air with different conditions was performed. As the elaboration temperature is just below 650 K and the mica substrate is damaged above 970 K, the annealing temperatures were between 670 and 920 K. First, the influence of the annealing time on the nanorod morphology was studied by annealing two parts of the same sample at 670 K during 3 and 6 h (Figure 6). The reference image of the sample before annealing shows a high density of nanorods with a pattern at 60
(Figure 6a), the atomic ratio of potassium determined by EDX being close to 2%. After 3 h of annealing at 670 K, the nanorods seem to be more dispersed on the mica surface (Figure 6b). Some of them are also shorter and broader than the reference ones. 1924

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Table 1. Fixed Parameters Used to Fit the Experimental IW/ISi Ratio of the W 4f XPS Line Intensity to That of Si 2p, as a Function of the Emission Angle θ analyzed material

line

overlayer: W 4f

cross

IMFP in

section40 concentration substrate41

IMFP in overlayer41

0.145

0.2

-

3.05 nm

0.107

0.143

3.1 nm

3.0 nm

WO3 substrate: Si 2p mica

Figure 6. TEM image of the reference sample (a) annealed at 670 K during three hours (b) and six hours (c). TEM image of supported nanorods on mica annealed during three hours at (d) 820 K, (e) 870 K, and (f) 920 K. Figure 5. Evolution of the IW/ISi ratio of the W 4f XPS line intensity to that of Si 2p, as a function of the emission angle θ. Two fits are given. The red one is obtained with one kind of object having single surface coverage γ and thickness t (here γ = 0.75 nm and t = 0.54), whereas the black one is obtained with two different deposits having each their own thickness and coverage (here γ1 = 0.11 and t1 > 10 nm, while γ2 = 1 and t2 = 0.16 nm).

The nanorod pattern with 60
between them is still present even if crystallized sheets of KxWO3 have grown around the nanorods linking them together. Apart from these sheets, the main difference is the variation of the atomic amount of potassium equal after annealing to 4% inside the nanorods and to 3% inside the new sheets. After six hours of annealing, the pattern of nanorods is destroyed (Figure 6c). Only a few intact rods with their initial length (more than a few micrometers) are still visible, the majority being damaged with a length less than 200 nm. In addition, small clusters of WO3 (less than 10 nm) are visible with no potassium. Second, the samples were annealed under air during 3 h at different annealing temperatures. For an annealing temperature below 830 K, the morphology of the annealed layers looks similar to the reference sample (Figure 6a and d). The pattern at 60
is still visible, and the growth of additional sheets between the nanorods increases with the annealing temperatures. Moreover, EDX spectra reveal that the atomic ratio of potassium increases from 2% in the nanorods before annealing to 4% in the nanorods after annealing at 670 K, reaching 6% after annealing at 820 K. As previously observed for the longest annealing times, for annealing temperatures above 870 K, the nanorods are completely destroyed, and only blocks of WO3 with less than 1% potassium are present on the substrate (Figure 6e and f). After the morphology, the crystallography of the annealed nanorods was studied by SAED in the same way as in part 1.

Two types of samples were selected to carry out this study, one with initially only thin rods of hexagonal structure and the second one with thick rods of hexagonal and monoclinic structure. After an annealing under air during three hours at 670 K, SAED experiments reveal that in the sample with the thinner rods the hexagonal structure is always the one and only structure. EDX experiments only point out a slight increase of the potassium amount after annealing. Moreover, even if the nanorod morphology is destroyed in the sample annealed during six hours, no transformation from the hexagonal to the monoclinic structure is observed. SAED patterns saved from the small pieces of nanorods after different tilting angles are all indexed as hexagonal. The same result is observed in all the samples previously annealed, with the observation of only the hexagonal structure. Annealing was carried out in the same conditions for thicker nanorods in which both the hexagonal and monoclinic structures were previously observed. After three hours of annealing at 670 K, both structures are always present (Figure 7). After tilting of 31 and 46
, around d1 = d110h = d200m = 0.365 nm of the initial superposed [1
10] hexagonal and [010] monoclinic zone axes (Figure 7a), the hexagonal [1
1
1] (Figure 7b) and monoclinic [0
11] (Figure 7c) zone axes are observed, respectively. This indicates that the crystalline structure of this nanorod is not modified by annealing at 670 K (Figure 7d). The same results are obtained after annealing at various temperatures from 670 to 820 K. There is no modification of the crystalline structure of the nanorods. Both structures are observed, the hexagonal one in the thinner areas of the nanorods and both hexagonal and monoclinic ones in the thicker ones. The only slight modification is the increase of the potassium amount reaching 6 at % for the higher temperatures. Besides, the crystallographic structure of the KxWO3 sheets growing around the nanorods was also studied by SAED experiments. For all annealing temperatures, these 1925

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Figure 7. SAED patterns and TEM image of a sample annealed at 670 K during three hours. (a) hexagonal [1
10] and/or monoclinic [010] zone axis with d1 = 0.365 nm and d2 = 0.382 nm. (b) Hexagonal [1
1
1] zone axis of WO3 with d1 = 0.365 nm = d110h and d3 = 0.333 nm = d
11
2h. (c) Monoclinic [0
11] WO3 zone axis with d1 = 0.365 nm = d200m and d4 = 0.269 nm = d022m. (d) TEM image of the corresponding nanorod (analyzed area indicated by the circle).

sheets are only formed by the hexagonal structure, the monoclinic structure never being observed. Therefore, annealing experiments up to 920 K only modified the morphology of the nanorods but not their crystallographic structure.

’ DISCUSSION The growth of WO3 on the (001) surface of the mica substrate leads to a very particular 2D pattern of rods at 60
with several micrometers in length, a few hundreds of nanometers in width, and less than 100 nm in thickness. This organization is in contrast with the results obtained on alumina, under vacuum, in which small grains with diameter under 3 nm were obtained.43 With exactly the same process under air, three-dimensional aggregates were obtained on Al2O3 or SiO2 substrate.35 In contrast, WO3 nanorods grow in epitaxy on the mica substrate. Another particularity of this system grown on mica muscovite is that the growth direction of the rods corresponds to only two of the three high-symmetry directions of the (001) surface of the substrate. In fact, these three directions are not equivalent as two types of (001) planes exist after cleavage at the (001) surface of the monoclinic muscovite mica. Due to a different stacking of the atoms at the surface, only two of the three directions are completely equivalent. This explains why the nanorods grow parallel to only two directions.38 This observation clearly points out the importance of the mica substrate, as the results are so different from the other substrates. Previous studies have pointed out the fact that the growth of WO3 objects is linked to the amount of water molecules in the atmosphere during the growth process.18 In particular, if the growth is carried out under primary vacuum, nanometric islands are obtained instead of nanorods. Nanometric islands are also obtained if the cleaved surface is exposed to air during several

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hours prior to deposition or if the mica substrate is annealed.38 This observation points out the importance of not only the type of substrate but also the substrate preparation and the quality of the surface. In this study, the surface of the mica substrate was always freshly cleaved just before the growth process, to be sure to obtain nanorods. The XPS study has revealed the simultaneous presence of an ultrathin film (coverage = 1, thickness of 0.16 nm) and of objects corresponding well to the nanorods (coverage = 0.11, thickness higher than 10 nm). This would correspond to the Stranski
Krastanov growth mode with the growth of an epitaxial wetting layer followed by the growth of oriented objects. This kind of growth was previously reported for the growth of silver44 and para-hexaphenyl45 on mica muscovite. This would explain why nanorods are not observed after annealing of the substrate or if the deposition is not carried out just after cleavage. In both cases, the mica surface is degraded and polluted, making impossible the epitaxial nanorod growth but leading to the growth of nanometric nonoriented islands. The ratio between W and O studied by EDX was determined to be constant and equal to 3.0 ((0.2). The systematic presence of potassium in the nanorods was also pointed out and is more surprising. This potassium comes from the substrate linked to the fact that the freshly cleaved surface of the mica substrate exhibits in theory a potassium atom plane. Moreover, a desorption experiment was carried out and has shown that potassium desorption occurs for temperatures between 540 and 670 K, with a maximum intensity between 620 and 650 K.37 These temperatures correspond exactly to the elaboration temperatures used for the growth of the nanorods. Furthermore, the reconstruction of the freshly cleaved mica surface was studied, and it was demonstrated that during the cleavage potassium atoms are not equally distributed between the two surfaces. Potassium atoms are present in more or less large areas in each face. This was clearly confirmed by observing the grown layers deposited on both faces of a cleaved substrate.38 Areas without objects on one face correspond to areas with objects on the other one. As potassium is systematically detected in each nanorod, the areas where nanorods are present would correspond to areas where potassium is still present at the surface after the cleavage. This would also explain the existence of areas without nanorods observed by SEM. Therefore, the presence of potassium atoms at the mica surface would be fundamental for nanorod growth. Moreover, the average potassium rate decreases with increasing nanorod thickness. For example, when a new nanorod has grown above another one, the thickness has without ambiguity increased. Then, it is observed that the relative proportion of potassium is systematically lower in this new thicker nanorod than in the initial thinner one. However, even if the relative potassium rate decreases with the thickness, the total amount of potassium is in fact constant along the whole nanorod whatever its thickness is, as confirmed by the linescan result (Figure 2f). The most credible explanation of this result is the localization of the potassium in the first grown layers at the interface with the mica substrate. Actually, with a constant potassium total amount, its relative proportion will be less detected as the nanorod is thicker. It is also important to report that neither silicon nor aluminum was detected by EDX, as well as no potassium around the nanorods. This confirms that the potassium presence results from the nanorods themselves and not from a piece of the mica substrate torn away with the carbon replica. Therefore, the potassium is homogenously distributed in the first layers along the whole nanorod. 1926

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The Journal of Physical Chemistry C Some rods and more particularly the thinnest ones correspond to the metastable single crystalline phase of hexagonal KxWO3. This metastable phase was previously reported in particular cases as the presence of water or alkali.26,29,46 In our case, the presence of this metastable hexagonal phase is linked to the presence of interfacial potassium. The first grown layers correspond to a KxWO3 tungsten bronze hexagonal phase. This tungsten bronze structure is obtained from the hexagonal structure in which potassium atoms are inserted in the large hexagonal tunnels along the c axis. Due to the very weak atomic weight of potassium compared to the tungsten one, no difference is observed in the simulated diffraction patterns for hexagonal KxWO3 and WO3. Therefore, it is impossible to distinguish them by electron diffraction experiments. However, we carried out calculations for a nanorod with a thickness of 30 nm and 4 at % of potassium. They have shown that there is a diffusion of the potassium along about ten atomic layers. So, it seems that after a first interfacial layer of KxWO3 the following layers of the rods during the growth are made of KxWO3, the x value decreasing with the nanorod thickness. The thicker rods present two different crystallographic structures of WO3, not in the length, but in the thickness. The hexagonal phase observed in thin rods is the basis of the nanorods growth model. For this hexagonal phase, two orientations (perpendicular to the mica substrate) are reported: the [100]h and [1
10]h zones axes. The monoclinic structure is only observed in the thicker part of the rods. In this case, the [010]m monoclinic zone axis (observed perpendicular to the substrate) has grown in epitaxy on [1
10]h. It should be noticed that the observed [1
10]h and [100]m zone axes have similar distances and angles, allowing the epitaxy between both structures. In contrast, we do not observe any monoclinic structure grown on [100]h hexagonal thin nanorods. By combining all these observations, a growth model can be proposed with two steps. First, a hexagonal KxWO3 layer (x decreasing with the thickness) grows in epitaxy on mica, thanks to potassium provided by the substrate. Second, when the presence of potassium and/or the effect of the epitaxy are not sufficient to stabilize the metastable hexagonal structure, the WO3 monoclinic structure occurs. The unique presence of the hexagonal phase is clearly pointed out in the thinnest nanorods, whereas the simultaneous presence of the exotic hexagonal and stable monoclinic phases is demonstrated in the thickest ones. By controlling the growth conditions, mainly the time deposition, it is possible to favor one type of nanorod and therefore one crystallographic structure. As the influence of the crystallographic structure on various applications was reported,33,34 the determining of the growth model is a key point to fully control the properties of WOx systems. To study the stability of this system, annealing experiments have been performed with different time and temperature annealing conditions. The evolution of the morphology and the crystallography of the objects were studied. For an annealing time less than three hours, the critical temperature to avoid any modification of the nanorod morphology on the mica substrate appears to be 870 K. Below this value, even if slight modifications of the morphology happen with the apparition of sheets between the nanorods, the characteristic pattern at 60
and the dimensions of the rods are preserved. In contrast, above this temperature, the morphology is completely destroyed. The annealing time is also an important parameter, pointing out a kinetic effect and the metastable character of the rods. When the annealing times increase,

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there is a gradual transformation of the morphology. The initial pattern of nanorods at 60
disappears for the benefit of KxWO3 sheets around the rods, followed by small clusters (less than 10 nm) of WO3 without potassium and pieces of nanorods. Furthermore, even if there is a modification of the nanorod morphology with destruction of the nanorod pattern after annealing, the hexagonal structure is always present. Therefore, the hexagonal structure and the initial morphology of the nanorods do not seem to be linked. The fact that the metastable hexagonal structure is always observed despite the annealing may be linked to the increase of the potassium amount. These additional potassium atoms have diffused from the mica substrate and have stabilized the interfacial hexagonal structure. So, the substrate and the potassium are key points for the stabilization of this metastable structure. The sheets of KxWO3 formed during the annealing process contain potassium atoms and correspond to the hexagonal structure. Moreover, the amount of potassium atoms in the nanorods has increased during the annealing process. The potassium coming from the substrate above 670 K is trapped inside the nanorods until these ones are destroyed above 870 K or by an extended annealing. The WO3 of the new sheets can only originate from the deposit, i.e., from the nanorods themselves or the ultrathin KxWO3 layer revealed by XPS. The TEM observations indicate that the rods are in average shorter after annealing than before. It means that due to the energy given by the annealing there is a new organization with clusters of KxWO3 coming from the nanorods (and maybe ultrathin film) forming sheets around nanorods with the help of potassium atoms diffusing from the mica substrate. This hypothesis implies that pure WO3 nanorods are less stable on the substrate than KxWO3 tungsten bronze. This is consistent with the fact that the tungsten bronze is the phase initially formed during the growth process. Its growth stops rapidly because the total amount of potassium is limited at the mica surface until the annealing process reaches a temperature allowing potassium diffusion from the volume. Furthermore, the blocks observed after extended annealing or for the highest temperatures are similar to the ones observed on disorganized surfaces. After such annealing conditions, it is possible that all potassium atoms close to the surface have desorbed from the mica substrate. Pure WO3 will then be formed with blocks morphology rather than the nanorods one observed only in the presence of potassium. Some studies on the stability of the hexagonal structure have been reported in the case of WO3 needles and sheets.27 Without any substrate, the structure is still hexagonal until 670 K under a mixed nitrogen and oxygen atmosphere. Above it, the transformation of the needles from the hexagonal to the monoclinic structures is achieved after five hours of annealing. In the case of the WO3 sheets, a temperature of 770 K and 17 h are necessary to obtain a complete transformation. After annealing of a thin film without substrate in air at 620 K, the coexistence of both hexagonal and monoclinic was reported, whereas after annealing at 670 K, only the monoclinic one is observed.28 A transition from hexagonal WO3 to monoclinic WO2.9 was also reported for polycrystalline thin films grown on Al2O3 after annealing at 620 K in air atmosphere with a mean humidity of 50%.47 From these examples, it seems that the initial morphology is an element to take into account in the study of the metastability of the hexagonal structure. Some annealing experiments of tungsten bronze have also been reported. Structural transformation from hexagonal to monoclinic was reported after annealing at 620 K until a complete 1927

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The Journal of Physical Chemistry C transformation at 670 K for LixWO3.46 The thermal behavior of differently prepared h-WO3 samples from Na2WO4 was studied, confirming the phase transition from hexagonal to monoclinic at 720 K. However, a better stabilization of the hexagonal phase was reported due to higher sodium content even if the transition occurred at 770 K.48 The same observations were made for samples containing NH4+ ions and NH3 molecules, linking the phase transition to their departure. The authors proposed that the impurities block the thermodynamically favored transition by making the network of corner sharing octahedral less flexible.49 Concerning the case studied in this work of supported KxWO3 nanostructures on mica substrate, different annealing experiments carried out with different temperature and time have not led to any transformation of the hexagonal structure. So, our results are in contrast with those previously reported. The oxidation/annealing behavior of initial WO3 amorphous structure grown on NaCl substrate was reported to be dominated by hexagonal WO3 phases, even if other tetragonal/monoclinic structure was present at high oxidation temperatures (773
873 K).50 The authors link the stability of the hexagonal phase to the initial amorphous state of the WO3 film before oxidation and annealing, but the possibility of a diffusion of sodium, comparable to the diffusion of potassium from the substrate, was not considered. Our system seems to be stabilized by the substrate and the potassium atoms. The nanorods are not only supported by the substrate but also in epitaxy. This epitaxy increases the stability. Moreover, potassium atoms originating from the substrate appear to be essential in the stabilization of the hexagonal structure. Therefore, the epitaxy on the substrate and the potassium present in the objects are key points for the stability of the hexagonal phase normally metastable.

’ CONCLUSIONS Supported tungsten oxide nanorods have been elaborated with a simple preparation method under air; these objects form a particular pattern on the muscovite surface of a mica substrate. After AFM, SEM, XPS, and TEM observations and analysis, a growth model is proposed. The thickest nanorods are composed by the stacking of two crystallographic structures. A hexagonal tungsten bronze layer KxWO3 (x decreasing with the thickness) grows in epitaxy on mica. Then, beyond a critical thickness, the monoclinic WO3 phase, stable at the elaboration temperature, grows in epitaxy. For the thinnest nanorods, which are only composed of the first layer, the stable monoclinic phase is not observed. A study of the thermal stability of these objects was carried out with different annealing conditions. The transformation of the hexagonal structure is never observed even if the monoclinic phase is initially present in the last layers of the nanorods. This study points out the importance of the epitaxy on the mica substrate and of the presence of potassium. They stabilize the hexagonal phase even though the nanorod morphology was completely destroyed. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (+33) (0)3.80.39.59.23. Fax: (+33) (0)3.80.39.38.19.

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