Article pubs.acs.org/JPCC
Visible-Light-Driven Photochromism of Hexagonal Sodium Tungsten Bronze Nanorods Tao Gao*,† and Bjørn Petter Jelle‡,§ †
Department of Architectural Design, History and Technology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway ‡ Department of Materials and Structures, SINTEF Building and Infrastructure, 7465 Trondheim, Norway § Department of Civil and Transport Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway ABSTRACT: Single-crystalline sodium tungsten bronze (Na-WO3) nanorods with typical diameters of 10−200 nm and lengths of several micrometers were prepared via hydrothermal synthesis. The as-prepared Na-WO3 nanorods crystallized in a hexagonal structure (space group P6/mmm) with unit cell parameters a = 7.3166(8) Å and c = 3.8990(8) Å and elongated along the ⟨001⟩ direction. Chemical analyses indicated a stoichiometry of Na0.18WO3.09·0.5H2O, revealing the existence of tunnel Na+ ions and water molecules in the structure, as confirmed also by the vibrational spectroscopic study. The as-prepared Na-WO3 nanorods exhibited a direct-allowed electronic transition with band-gap energy of about 2.5 eV, which allows a visible-light-driven photochromism related to photogenerated carriers and a proton−electron double injection process. The proposed photochromism was discussed in detail by means of Fourier transform infrared spectroscopy. The involved local structural evolutions such as water decomposition and ion intercalation during the photochromic process were identified.
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INTRODUCTION Tungsten bronzes represent an important class of nonstoichiometric compounds of general formula MxWO3,1−3 where the solute ions M (usually alkali metals such as Li, Na, K, and Rb) occupy voids or tunnels within a network of WO6 octahedra. Depending on the size of the interstitial M ion and its concentration x (0 < x < 1), various lattice configurations, such as cubic, tetragonal, and hexagonal phases, can be formed,1−3 resulting in the structural chemistry of tungsten bronzes being very rich and complex. In general, light alkali metals such as Li and Na usually form cubic or tetragonal structures, whereas tungsten bronzes with heavy alkali metals such as K and Rb usually exhibit hexagonal symmetry. The hexagonal tungsten bronzes have an interesting tunnel structure,2−5 where one-dimensional (1D) chains of alkali ions M are embedded in the open hexagonal channels of corner-linked WO6 octahedra (Figure 1). The tunnel ions M are loosely bounded to the cross-linked WO6 octahedral framework and are exchangeable with other cations.6−9 The presence of tunnel cations in hexagonal tungsten bronzes brings about interesting temperature- and/or M-concentration-dependent lattice dynamics.10−13 Moreover, the tunnel alkali ions can donate their s electrons to the conduction band of WO3, resulting in metallicity and eventually a superconducting transition, which has been the subject of a large number of investigations for more than 50 years.14−17 The recent research interest in tungsten bronzes MxWO3 has been devoted to their prominent photoelectrochemical proper© XXXX American Chemical Society
Figure 1. Polyhedral representation of hexagonal tungsten bronzes MxWO3. The structure is viewed down the c-axis of a hexagonal unit cell. Tunnel species M are represented by gray balls.
ties and potential applications in, e.g., catalysts, batteries, and smart windows.18−22 Among these, smart windows allow the transmittance/reflectance of visible light and solar energy to be modulated in architectural windows to adapt to ambient conditions or user preferences, therefore offering a great energy-saving potential for highly glazed buildings.23 So far, the application of electrochromic materials in smart windows is still of priority to some extent;24,25 however, it is worth pointing out that materials with other chromogenic properties such as Received: May 8, 2013 Revised: June 6, 2013
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dx.doi.org/10.1021/jp404597c | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
was prepared first, which acts as the precursor for the subsequent hydrothermal growth of tungsten bronzes with controlled sizes and compositions. In a typical synthesis, 11.4 g of Na2WO4·2H2O was dissolved in 150 mL of water, and to this solution 15 mL of concentrated H2SO4 was added dropwise under constant stirring. White precipitation was formed immediately upon the mixing and gradually changed color to light-yellow. The precipitation was separated from the reaction solution by centrifugation, washed four times with water, and finally dissolved in 300 mL of oxalic acid aqueous solution (0.4 M). Thirty milliliters of the obtained TOC solution was transferred into a Teflon-lined autoclave (capacity 40 mL). After adding a certain amount of Na2SO4 powder (1−3 g), the autoclave was sealed and heated at 180 °C for 24 h. After the reaction, the autoclave was cooled down to room temperature by tap water. The obtained white participate was filtered, washed with water to remove the residual ions/chemicals, and then dried at 60 °C for 5 h to give the as-prepared sodium tungsten bronze materials. By changing the templating cations in the TOC solution (e.g., adding Li2SO4 or K2SO4), different tungsten bronzes were also prepared. Structural Refinement. Phase analysis was based on powder X-ray diffraction (XRD) data collected on a Siemens D5000 powder X-ray diffractometer (with Cu Kα1 radiation) equipped with an automatic antiscatter slit and a scintillation detector. The XRD data were measured between 5 and 100° in 2θ in reflection geometry. The obtained XRD patterns were analyzed by the Rietveld method. Starting atomic coordinates for Na, W, and O were taken from those reported for sodium tungsten bronzes, Na0.17WO3.085·0.23D2O and Na0.17WO3.085·0.19H2O.33 Difference Fourier calculations revealed two significant maxima in the electron density map at (0 0 0) and (0 0 0.5); these positions were believed to be occupied by Na+ ions and/or water molecules, e.g., H3O+.34 However, it was difficult to refine the thermal displacement parameters and populations of tunnel species Na+ and H3O+; therefore, these values were fixed during the refinement. Preparation of Photochromic Samples. Samples for reflectance measurement were prepared by hydrostatically pressing the as-prepared powders into small disks with diameters of 15 mm and thickness of about 2 mm. During the sample preparation, a slightly color change from white to blue was also noticed for the as-prepared materials. Thereafter, visible light irradiation was performed with an Olympus Europe Highlight 3100 cold-light illumination system (halogen lighting: 400−800 nm) under an irradiation powder of ∼500 W/m2 for 30 min. The bleaching process was performed in an oven preheated at 60 °C for 30 min; one small drop of water was also applied on the surface of the colored samples to promote a complete bleaching. Characterization. Crystal structure of the as-synthesized materials was determined by XRD (Bruker AXS D8 Advance diffractometer with Cu Kα1 radiation). The morphology and chemical composition of the as-prepared materials were investigated by field-emission scanning electron microscopy (SEM, Zeiss Supra 55VP) and transmission electron microscopy (TEM, JEOL JEM-2010), both equipped with energydispersive X-ray spectrometers (EDS). Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449C thermo-microbalance in nitrogen atmosphere at a heating rate of 10 °C min−1. Attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra were recorded on a Nicolet
photochromism and gasochromism are also of interest in smart window applications, where a simple device structure and a good compatibility with architectural windows can be expected.26 In this regard, an improved understanding on the intrinsic chromogenic properties of tungsten bronzes MxWO3 will have an impact on their practical applications. It is also worthwhile to note that, compared to tungsten oxide WO3, tungsten bronzes MxWO3 may involve different chromogenic mechanisms due to the presence of tunnel or interstitial cation M, therefore representing an interesting material system to understand the structure−property relationship for their practical applications. In this work, we discuss a visible-light-driven photochromism observed in sodium tungsten bronze (denoted hereafter as NaWO3) nanorods with a hexagonal phase structure. The present work has two aims: to revisit the structural property of recently synthesized Na-WO3 nanorods with small sizes (typical diameters of 10−200 nm and lengths of several micrometers) and to reveal their intrinsic chromogenic properties. Previously, Wang et al. reported a hydrothermal synthesis of WO3 nanorods with a hexagonal phase structure by reacting Na2WO4, NaCl, and HCl at 180 °C.21,22 It must be pointed out that, since the cation intercalation has a great impact on the formation of hexagonal WO3 nanorods,27 the possible presence of tunnel Na+ ions in the hexagonal structure (see Figure 1) may indicate that the previously reported tungsten trioxide WO3 nanorods21,22 are actually tungsten bronzes Na-WO3. Consequently, size- and/or composition-dependent properties can be expected for Na-WO3 when compared to WO3, which has indeed been observed in the present work. For example, the as-prepared Na-WO3 nanorods exhibit an interesting photochromism induced by visible light irradiation, which, to the best of our knowledge, has not been reported previously. Usually, the sensitivity of the photochromic effect in WO3 and related materials is limited to the near-ultraviolet (UV) range;28−30 therefore, the development of new photochromic materials with coloration response expanded to the visible-light region represents a critical challenge to their practical applications. For such purpose, WO3−CdS composites,30 WO3-based organic− inorganic hybrid materials,28 and more recently magadiite− polytungstate composite31 have been reported in the literature. The visible-light-driven photochromism is usually attributed to the efficient charge separation and intermolecular charge transfer between different components.30,32 Obviously, it is of great interest to study the intrinsic photochromic properties of the single phased Na-WO3 nanorods. Apart from a more detailed understanding of the visible-light-driven photochromism, it may also have relevance for their technical applications.
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EXPERIMENTAL PROCEDURES Chemicals and Materials. Reagent-grade sulfuric acid (H2SO4, 96%), sodium tungstate dihydrate (Na2WO4·2H2O), oxalic acid (H2C2O4), potassium sulfate (K2SO4), sodium sulfate (Na2SO4), lithium sulfate monohydrate (Li2SO4·H2O), and tungsten trioxide (WO3, monoclinic, particle size
