WO3 Nanorods Created by Self-Assembly of Highly Crystalline

Article pubs.acs.org/Langmuir

WO3 Nanorods Created by Self-Assembly of Highly Crystalline Nanowires under Hydrothermal Conditions Julien R. G. Navarro,*,†,‡,§ Arnaud Mayence,‡ Juliana Andrade,§ Frédéric Lerouge,† Frédéric Chaput,† Peter Oleynikov,‡ Lennart Bergström,‡ Stephane Parola,† and Agnieszka Pawlicka§ †

Ecole Normale Supérieure de Lyon, CNRS, Université Lyon 1, Laboratoire de Chimie UMR 5182, 46, Allée d’Italie, F-69364 Lyon cedex 07, France ‡ Arrhenius Laboratory, Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden § Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador Sãocarlense 400, 13566-590 São Carlos, SP, Brazil

ABSTRACT: WO3 nanorods and wires were obtained via hydrothermal synthesis using sodium tungstate as a precursor and either oxalic acid, citric acid, or poly(methacrylic acid) as a stabilizing agent. Transmission electron microscopy images showed that the organic acids with different numbers of carboxylic groups per molecule influence the final sizes and stacking nanostructures of WO3 wires. Threedimensional electron diffraction tomography of a single nanocrystal revealed a hexagonal WO3 structure with preferential growth along the c-axis, which was confirmed by high-resolution transmission electron microscopy. WO3 nanowires were also spin-coated onto an indium tin oxide/glass conducting substrate, resulting in the formation of a film that was characterized by scanning electron microscopy. Finally, cyclic voltammetry measurements performed on the WO3 thin film showed voltammograms typical for the WO3 redox process.

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INTRODUCTION

In this work, hydrothermal synthesis is used to obtain anisotropic WO3 nanostructures (nanorods and nanowires). The main advantages of this method are its low cost compared with that of expensive vacuum technologies, its ease of handling, and the further possibility of controlling the deposition of nanostructures on a substrate.19 We report on the tungsten trioxide nanowires and nanorods obtained through hydrothermal synthesis in the presence of three different organic acids, i.e., oxalic acid, citric acid, and poly(methacrylic acid) (poly acid), as capping agents (Scheme 1). Their three-dimensional structure and morphology have been investigated by transmission electron microscopy (TEM). We show that the number of acids per stabilizing molecule plays an important role in the size and stacking of wires. Three-dimensional electron diffraction tomography (3D EDT) was utilized to determine the hexagonal structure of a single WO3 nanorod, and the preferential direction of growth along the c-axis was confirmed by high-resolution transmission electron microscopy (HRTEM). The electrochemical

Tungsten oxide thin films are interesting candidates for application in electrochromic devices.1−3 Other properties have already been reported such as negative capacitance for the development of new signal amplifier devices,4 high sensitivity for gas sensor development,5 or field emission properties.6 WO3 thin films can be synthesized through several approaches such as the sol−gel process,2,7,8 chemical vapor deposition,9−11 anodic oxidation,12,13 or laser ablation.14,15 WO3 can crystallize into several structures (monoclinic and hexagonal phase) depending on the synthetic conditions, e.g., temperature.16 Govender et al.17 synthesized WO3 nanostructures by a CO2-laser pyrolysis technique and observed morphological changes (nanowires and nanostars) by adjusting the precursor concentration. The 300−1000 nm long and 1 ± 0.1 nm thick nanowires that were promising for gas sensor applications were obtained by Polleux et al.5 Li et al.18 obtained quasi-aligned single-crystalline W18O49 nanowires in a high-vacuum system under different atmospheric pressures. These field-emitting nanomaterials with a diameter of 20−100 nm are promising candidates for photochromic or electrochromic applications. © XXXX American Chemical Society

Received: July 1, 2014 Revised: August 8, 2014

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or poly acid) to stabilize the rods during the crystal growth process. The synthesized WO3 nanostructures were characterized by TEM (Figure 1). As shown in Figure 1a, the thin nanowires exhibited a tendency to self-assemble into micrometer-sized rods when oxalic acid was used as the stabilizing agent. A similar tendency was previously reported in the case of micrometer-sized tubes of tungsten oxide.30 The obtained nanorods composed of several stacked wires were approximately 1.0 ± 0.5 μm long and 100 ± 60 nm wide. TEM images revealed a nonhomogeneous rod thickness distribution, suggesting that the width of the nanorods strongly depended on the number of packed wire units. Interestingly, these self-assembled micrometer-sized rods presented the same length; i.e., no heterogeneous wire aggregates were observed. Figure 1b shows the nanorods obtained using citric acid as a stabilizing agent. The WO3 nanostructures display a width smaller than that of the nanorods presented in Figure 1a. The use of citric acid allowed the decrease in the width of the nanorods (number of packed wires). As shown in Scheme 1, citric acid contains three carboxylic acid groups while oxalic acid has only two. Therefore, the additional -CO2H group improves the stability of the WO3 structures, preventing wires from stacking into larger rods. The width and length of the nanowires were estimated to 20 ± 5 nm and 1.0 ± 0.5 μm, respectively. To avoid the stacking of wires, the number of carboxylic groups per stabilizing molecule was increased. Thus, aiming to obtain isolated wires, we chose poly(methacrylic acid) (PMAA). As a result, single wires with a mean width of 5 ± 3 nm and a length of 0.7 ± 0.3 μm were obtained (Figure 1c). Moreover, the wires tended to form circles probably because of their high length:width aspect ratio. The bending of some nanowires has been observed by TEM when they are undergoing high-flux electron beam exposition (not shown here). Consequently, the acid function number per molecule (stabilizing agent) strongly influenced the final WO3 morphology, resulting in a drastic change in the rod width and a stacking effect. Additionally, the influence of stirring during the WO3 synthesis with PMAA was also investigated. It was found that constant stirring contributed to the yield of isolated nanowires (Figure 1c), and its absence promoted nanorods with a needlelike shape (Figure 1d). The hydrothermal synthesis of the WO3 nanowires may be related to a nucleation and subsequent Ostwald ripening process.31 Previous work showed that tungstate anions in an acidic solution at a high temperature polymerize through a condensation reaction yielding WO3 nuclei.29,32 All synthesized WO3 nanostructure shapes, lengths, and widths are summarized in Table 1. The purified powder of WO3 nanowires, obtained using PMAA, was investigated by the powder X-ray diffraction (PXRD) technique. The synthesized nanostructures resulted in a hexagonal structure in space group P6/mmm with lattice parameters of a = 7.3339(5) Å and c = 3.8721(3) Å, corroborating the simulated PXRD pattern shown in Figure 2. Three-Dimensional Electron Diffraction Tomography (3D EDT). 3D electron diffraction tomography22 (3D EDT) has been utilized to collect 3D data from an individual nanorod. Figure 3a shows the TEM picture of a WO3 single-crystal nanorod used for 3D EDT data collection. The approximate hexagonal cell from the reconstructed data set was uniquely determined as follows: a = 7.487 Å, b = 7.524 Å, c = 3.826 Å, α = 90.34°, β = 89.23°, and γ = 119.96°. The calculated unit cell, taking into account the distortions introduced by the lenses of the microscope and imperfections of the goniometer tilt angle read-out, matches well with the unit cell [a = 7.3339(5) Å,

Scheme 1. Chemical Structures of the Acids Used for the Hydrothermal Synthesis

properties were investigated using cyclic voltammetry (CV) after deposition on an indium tin oxide (ITO) conducting substrate.

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EXPERIMENTAL SECTION

Materials. Sodium tungstate dihydrate (Na2WO4·2H2O), oxalic acid (C2H2O4), citric acid (C6H8O7), a sodium salt of poly(methacrylic acid) solution (PMAA, MW = 4000−6000), and hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich and used as received. All suspensions were synthesized in Milli-Q water. General Procedure for WO3 (rods and wires) Synthesis. The WO3 nanostructures were synthesized using a procedure adapted from the literature.20,21 One hundred milligrams of the tungsten precursor (Na2WO4·2H2O) and the organic acid (either 200 mM oxalic acid, 200 mM citric acid, or 2 mM poly acid) were dissolved in 14 mL of Milli-Q water. The pH was fixed to 2.2 by using a 2 M HCl solution. The final volume was adjusted to 16 mL. The resulting mixture was then heated at 180 °C for 3 h in an autoclave while being constantly stirred. The final pale blue solution was then purified by several centrifugation− dispersion steps using Milli-Q water. Deposition of WO3 (rods and wires) on ITO Glasses. First, the conductive ITO-coated glasses (Delta Technologies, 5−15 Ω cm−2, 2.5 cm × 2.5 cm × 0.5 mm) were cleaned before WO3 deposition. In parallel, dispersion of WO3 was achieved by mixing 10 mg of WO3 powder in 2 mL of H2O. Finally, this solution was spin-coated on the ITO substrates at a rotation speed of 1000 rpm and dried at 100 °C overnight. Characterization Techniques. Transmission electron microscopy images were acquired using a TOPCON Em-002b instrument (120 kV). The sample phase composition was determined by X-ray diffraction analysis. Powder X-ray diffraction (PXRD) patterns were collected on a PANalytical X’Pert Pro instrument equipped with Cu Kα1 radiation (45 kV, 40 mA), an irradiation length of 10 mm, a mask fixed length of 10 mm, and a step size of 0.013° in the Bragg−Bretano configuration. The scanning electron microscopy (SEM) images were obtained with a LEO model 440 scanning microscope. CV analysis using Autolab PGSTAT302N was performed in a classical three-electrode electrochemical cell within applied potentials of ±1.0 V. In this setup, agglomeration of WO3 thin nanostructures was achieved with a working electrode, a Ag wire as a reference electrode, and a Pt plate as an auxiliary electrode. The electrolyte was 0.1 M LiClO4 in propylene carbonate (PC). The electron diffraction patterns and high-resolution images were acquired on a JEOL JEM-2100 LaB6 electron microscope operated at 200 kV. The electron diffraction data set (3D EDT) was collected using Analitex EDT-COLLECT.22 The data collection beam tilt step was 0.2°; the total range of stage tilt angles covered during the experiment was 84.9°, resulting in 450 individual electron diffraction frames. The frame exposure time was 0.5 s, and the total data collection time was ∼32 min. Data processing was conducted using Analitex EDT-PROCESS.22 The intensities extracted from reconstructed reciprocal space were used to determine the structure by direct methods using Sir2011.23

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RESULTS AND DISCUSSION Synthesis and Characterization of Nanowires. WO3 rod and wire nanostructures were prepared using hydrothermal synthesis.20,24−29 The initial acidic solution (pH 2.2) containing the tungsten precursor and the molecular ligands was heated in a Teflon-lined stainless steel autoclave at 180 °C for 3 h while being constantly stirred. Three different organic acids (Scheme 1) were used as the protecting ligand (either oxalic acid, citric acid, B

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Figure 1. TEM pictures of the WO3 nanostructures obtained, under constant stirring, with (a) oxalic acid, (b) citric acid, and (c) poly(methacrylic acid), and (d) poly(methacrylic acid) synthesized without agitation.

On the basis of experimental data shown in Figure 3, the mean WO3 structure geometry is then presented in Figure 4. This structure can be described as a three-dimensional network, in which tungsten atoms are inside the octahedron and represented in Figure 4a by small blue spheres. The tungsten atoms are bound to six oxygen atoms represented by red spheres in an octahedral coordination. As shown in panel a of Figure 4 and more clearly in panel b of Figure 4, each octahedron is connected to six others by corner sharing of its oxygen atoms. Thus, this structure can be visualized as a two-dimensional network of interconnected octahedra arranged such that channels are created along the c-direction. Those two-dimensional layers are connected to each other perpendicular to the basal plane of the hexagonal lattice. HRTEM images of a single-crystal nanorod confirmed the preferential growth orientation along the c-axis (Figure 5). The lattice fringes were identified as (001) and (110) lattice planes.

Table 1. Shapes and Dimensions of Synthesized WO3 Nanorods and Nanowires stabilizing reagent

shape

length (μm)

width (nm)

oxalic acid citric acid poly acid

rods wires and rods wires

1.0 ± 0.5 1.0 ± 0.5 0.7 ± 0.3

100 ± 60 20 ± 5 5±3

and c = 3.8721(3) Å] refined from the powder X-ray profile. As shown in panels b and c of Figure 3, the analysis of the 3D EDT data revealed that the c-axis of the unit cell coincides with the principal direction of growth of the rods. This preferential growth has been confirmed by comparing the orientation matrix, from the collected 3D EDT data sets, acquired on several WO3 wires. The analysis of intensities in reciprocal space shows that there are defects in the structure caused by the presence of diffuse scattering in the a*−b* plane around some spots. C

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Figure 2. Experimental (top) and simulated (bottom) PXRD pattern of WO3 nanowires obtained with poly(methacrylic acid).

Figure 3. TEM image and projections of the reconstructed reciprocal space volume of an individual WO3 nanorod. (a) TEM picture of a WO3 singlecrystal nanorod used for 3D electron diffraction tomography data collection. The circle shows the area from where the data set was acquired. (b and c) 3D reconstructed reciprocal space shown along the c*-axis and [−120] direction, respectively.

using spin-coating, to study their electrochemical properties. The physical thickness of the spin-coated film was estimated by atomic force microscopy (data not shown). WO3 on the ITO substrate displayed a rough topology with a root-mean-square (rms) roughness of 16.7 nm. The initial ITO surface was also analyzed, highlighting a flat and smooth morphology with a rms roughness of 4.9 nm. The coverage percentage and topology of deposited WO3 nanowires were investigated using SEM (Figure 6). The SEM pictures revealed a relatively homogeneous coverage of the substrate surface with bundled WO3 wires. Large agglomerates (0.7 ± 0.3 μm) are sometimes observed in some region of the substrate. Electrochemical Properties of the Deposited WO3 Wires. The cyclic voltammograms of WO3 annealed at 100 °C were recorded at different scan speeds (50−500 mV/s) by sweeping the potential from −1.0 to 1.0 V versus Ag in a 0.1 M LiClO4/PC (propylene carbonate) solution (Figure 7). The cyclic voltammograms shown in Figure 7 displayed shapes that are characteristic of WO3 thin films,34,35 in which the WO3 coating reduction process to LixWO3 occurs through an increase in the cathodic current density to −110 mA/cm2 at −1.0 V for a rate of 500 mV/s. After a reversal potential is applied, an anodic

Figure 4. Model of the WO3 hexagonal structure illustrated by a 3D network of WO6 octahedra. (a) Each octahedron is connected to six others by corner sharing. (b) Projection along the preferential growth direction of the nanorod, [001].

The mean lattice fringe spacings were measured to be 3.9 Å for (001) and 3.6 Å (110). The HRTEM picture in Figure 5b shows disorder between two crystalline regions, yet both regions have the same crystallographic orientation. This indicates that the stacking of WO3 nanowires occurs through oriented attachment of preformed crystalline nanowires along their common c-axis, while nonaligned nanowires within the aggregate may dissolve.33 Deposition of WO3 Wires on ITO and SEM Characterization. WO3 nanowires were deposited on ITO substrates, D

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Figure 5. Bright field TEM image of a nanorod synthesized with PMAA without agitation. (a) Low-magnification image (inset, SAED pattern) and (b and c) HRTEM pictures of the tagged area in panel a along [1−10] (insets, FFT of an individual picture).

Figure 6. SEM images of spin-coated WO3 nanowires stabilized with poly(methacrylic acid) at different magnifications.

the wire width and stacking effect was observed when the number of -CO2H groups per capping agent was increased. PMAA allowed the stabilization of the structure into a single nanowire that tends to form coils. Additionally, it was observed that constant stirring contributed to the generation of isolated nanowires, while an absence of stirring promoted the formation of nanorods with a needlelike shape. 3D EDT, conducted on a single nanorod, revealed a hexagonal WO3 structure with a preferential direction of growth along the c-axis, which was confirmed by HRTEM. WO3 nanowires obtained in the presence of PMAA were also spin-coated onto the ITO/glass substrate. SEM of the coatings revealed an almost uniform deposition of WO3 nanowire bundles. The CV measurements on WO3 thin films revealed a redox process that is associated with the formation of tungsten oxide bronze LixWO3. These nanomaterials can be further used as photocatalysts or in optoelectronic devices.

Figure 7. Cyclic voltammograms of the poly(methacrylic acid)stabilized WO3 nanowires spin-coated on ITO and recorded at scan speeds of 50, 100, 200, 300, and 500 mV/s.

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peak at −0.75 V is observed and attributed to the oxidation process. To verify if the redox process was due to WO3 nanowires, a CV analysis of the ITO/glass substrate was also performed. An increase in the cathodic current density to −0.43 mA/cm2 at −2.0 V and an anodic peak of 0.16 mA/cm2 at −1.0 V were observed (data not shown here). Thus, the cyclic voltammograms in Figure 7 are associated with electrochemical responses of the deposited WO3 nanostructures.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The financial support of FP7-PEOPLE-2009-IRSES (Proposal N° 247544) and the financial support of the Brazilian agencies Capes, CNPq, and FAPESP are gratefully acknowledged. This work was partially supported by ANR P3N Project nanoPDT ANR-09-NANO-027. This work was also partially supported by the Swedish Research Council (VR). The Knut and Alice Wallenberg (KAW) Foundation is acknowledged for providing the electron microscopy facilities and financial support (3DEMNATUR project, A.M. and L.B.).

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CONCLUSIONS WO3 nanostructures (rods and wires) were obtained by hydrothermal synthesis using different stabilizing agents. The resulting nanocrystal morphologies depend strongly on the number of acid groups per capping agent molecule. A decrease in E

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