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Keywords:

  • electrochromics;
  • photonic crystals;
  • nanoparticle thin films;
  • nanoporous bragg mirrors;
  • tuneable color

We describe herein the first example of an electrochromic Bragg mirror (ECBM), combining nanoporous multilayers made of NiO and WO3 nanoparticles. Because NiO and WO3 are complementary in their coloration effects (e.g. cathodic coloration for WO3 and anodic coloration for NiO)1 and their corresponding change in refractive index, tunability can be achieved by combining these electrochromic components in a 1D Bragg mirror tandem arrangement. The high nanoporosity of this ECBM allows protons and electrons to be quickly shuttled into and out of the multilayers, altering the mix of intervalence charge transfer optical effects within the layers and Bragg diffraction effects between the layers. A proper choice of electrolyte guarantees cycling of the optical properties with negligible degradation.

Electrochromic2, 3 devices change their electronic structure and color via electrically-induced storage of ions and electrons in the material, which can be reversed by applying an opposing electrical bias. In comparison, photonic crystals change their color by alterations in the dimension and/or refractive index of the photonic lattice, which can also be changed electrically and reversibly. A prominent example for reversible color changes emanating from alterations of the geometrical structure are voltage-driven, swellable and shrinkable 3D inverse opals built from cross-linked polyferrocenylsilane.3–5 This effect is known as the electrophotonic effect.4, 5 Electrochromic materials present themselves as a good alternative, as the coloration usually comes with a change in the refractive index. In the field of electrochromic devices, tungsten trioxide W(VI)O3 and nickel oxide Ni(II)O are the inorganic material archetypes, and function according to Equations 1–2.6–8

  • equation image((1))
  • equation image((2))

Especially WO3 has widely been used for tuning the optical properties of three-dimensional inverted nanocrystalline photonic crystals opals (i-ncWO3-o), in which the change of color is due to increasing absorption as well as tuning of the photonic stop-band arising from the corresponding index change.9, 10

Here, we combine for the first time two electrochromic materials in a photonic crystal architecture in a proof-of-concept nanoporous 1D electrochromic Bragg mirror (ECBM) made of tungsten and nickel oxide nanoparticle multilayers. An applied potential allows switching of both materials simultaneously between W(V,VI) and Ni(II,III) oxidation states. In contrast to electrochromic photonic crystals based on a single material,9, 10 this does not lead to a strong spectral shift of the photonic stop-band but rather to a broadband modulation of its reflectance, potentially useful for tunable mirror devices and grayscale control in reflective displays.

In our ECBM WO3 represents the high-refractive-index component (n = 2.1) and NiO the low-refractive-index one (n = 1.6). In Figure 1(a) spectroscopic-ellipsometry (SE) measurements of the different refractive indices for the colored and bleached states of thin films of each of the two materials with electrolyte present are shown. For details of the measurement and the fitting see the Experimental Section. The solid lines correspond to the colored state, the dashed ones to the bleached state. Applying a negative bias voltage, the WO3 changes to the colored state (decreasing its refractive index) while the NiO bleaches (increasing its refractive index), leading to an overall decrease of the refractive index contrast between the two materials and, hence, to a reduction of the reflectance of the stop-band if combined in an ECBM. Due to the different dispersion of the refractive indices, spectral positions can be found in which the increase in refractive index of one material just balances the decrease of the other material, hence, keeping the overall effective index the same. However, for the case of a center wavelength of 650 nm the overall effective index is slightly reduced, as the refractive index of WO3 decreases more than the refractive index of NiO increases. Hence, a slight blue-shift of the stop-band will accompany the reduction of the reflectance in this spectral range. Applying a positive bias voltage increases the index difference again, leading to a recovery of the reflectance of the photonic stop-band. Note that the overall change in refractive index of both materials is huge compared to values which can be reached, for example, by non-linear optical effects like the optical-Kerr effect or by electro-optical effects (Δn = 10−3).

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Figure 1. Spectroscopic ellipsometry measurements of the coloration state-dependent refractive index of WO3 and NiO.

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From the modeling of the spectroscopic ellipsometry (SE) data we retrieve that only for the NiO films a very slight absorbance in the region between 400 nm and 1000 nm has to be accounted for, while for the WO3 absorbance it is negligible, hence, tuning will most probably be due to the change in refractive index alone.

ECBMs are fabricated from these materials by spin-coating alternate layers of NiO and WO3 nanoparticles from 3–8 nm colloidal dispersions11 onto glass substrates with a transparent fluorine-doped tin oxide (FTO) electrode (2500–5000 rpm for 20–40 s followed by thermal post-treatment in air at 450 °C for 15–20 min), see also Figure S1(a,b) in the Supporting Information (SI). This process is repeated until an ECBM with the desired number of NiO/WO3 double layers is obtained. This procedure provides ECBM with a high structural quality and optical transparency. The nanoparticles also endow the NiO/WO3 BM with porosity (e.g., NiO ≈ 25.2% and WO3 ≈ 10%) allowing the rapid and reversible insertion of ions such as H+ or Li+ into and out of the NiO/WO3 ECBM. Porosity-based applications in nanoparticle multilayers of 1D photonic crystals have already been reported on electrolyte infiltration12, 13 and diffusion14 studies as well as with regard to sensing.15 The structural quality of the NiO/WO3 ECBM sample is exemplary, as shown in Figure 2 in a scanning electron micrograph of a cross section on a 4 double-layer (DL) stack. The layer thicknesses have been chosen such that the ECBM satisfies the quarter-wave requirement in the bleached state.

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Figure 2. Scanning electron cross-sectional image of a 4 DL nanoparticle NiO/WO3 nanoporous ECBM with NiO = 87 nm and WO3 = 62 nm.

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Nanoporosity, directly connected to the nanocrystal size, and layer thickness are crucial for controlling switching times (1–5 s), as they determine the ion mobility and insertion within the NiO and WO3 layers.16–21

To electrically tune the optical properties of the ECBM, a cell has been constructed containing the electrolyte and FTO as a transparent counter-electrode. Applying a bias voltage of +2.5 V switches the WO3 from the bleached to the colored state and simultaneously the NiO from the colored to the bleached state. To drive the systems into their respective other states, a bias voltage of -2.5 V is applied. Starting from an ECBM in its totally bleached initial state (orange appearance, see Figure 3(a) center), the color changes to dark green (Figure 3(a) left) and then cycles between dark green and yellow (Figure 3(a) right).

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Figure 3. (a) Color effects recorded at a 60° angle for cathodic coloration = yellow stack and anodic coloration = green stack of an orange-red 7 DL nanoparticle NiO/WO3 ECBM stack. (b–d) Time-dependent reflectance measurements with a recording frequency of 350 ms; (b) initial coloration of a 7 DL nanoparticle NiO/WO3 ECBM infiltrated in 0.5 M LiClO4 aqueous electrolyte under a negative bias (–2.5 V) (WO3 coloration), (c) positive bias (+2.5 V) right (NiO coloration), and (d) negative bias (–2.5 V).

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The resulting changes in the reflectance spectra are shown in Figure 3(b) for the initial bias voltage of –2.5 V and the accompanying reduction in refractive-index contrast of the WO3 layers. As expected, the overall reflectance is reduced and the position of the stop-band strongly blue-shifts, as only one of the materials changes its refractive index during this initial coloration. Spectra are taken with a time separation of 350 ms, indicating a relatively fast switching of the materials. Reversing the bias voltage to +2.5 V leads to recovery of the stop-band reflectance (Figure 3(c)). As now both materials simultaneously change their refractive index, a small shift (≈20–25 nm) in the spectral position can be observed. The initial state in Figure 3(b) will not be reached as, during cycling, one of the materials will always be in the colored state. In Figure 3(d) the subsequent WO3 coloration is plotted, where it can be seen that it essentially reverses the behavior shown in Figure 3(c), demonstrating that both materials simultaneously change their properties. The cycling between dark green and yellow colors can also be directly observed spectroscopically. While the strong peak around 650 nm (yellow) corresponds to the photonic stop-band, the remaining peaks are Fabry-Pérot fringes and change their intensity much less than the photonic stop-band during the refractive index changes. As the prominent peak can be found roughly at 510 nm, it dominates the visual impression in the WO3 colored state as the photonic stop-band is decreased to merely ≈30% reflectance, comparable to the strength of the Fabry-Pérot fringes.

That these dominant colors are due to photonic crystal effects is demonstrated in Figure 4. Here, just one double layer is shown. In Figure 4 left-hand-side WO3 is colored and NiO is bleached, leading to a transparent state, supporting the negligible absorption found from the SE analysis. In Figure 4 right-hand-side coloration (bleaching) of NiO (WO3) has taken place. The obvious darkening is due to slightly increased absorption of the NiO film under coloration. Supporting powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS) measurements for the colored and bleached nanoparticle NiO and WO3 films are to be found in the SI, Figures S2–S4.

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Figure 4. Coloration of a nanoparticle NiO 80 nm/WO3 100 nm DL (double-layer) immersed in 0.5 M LiClO4 electrolyte under -2.5 V negative (left) and +2.5 V positive (right) bias.

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An important property of the ECBM is their cycling capability, for which the right choice of electrolyte is crucial. We determined 0.5 M LiClO4 to be the ideal electrolyte-system. The single, double and multilayer thin films could be reproducibly cycled at ±2.5 V. This single electrolyte was consistently used throughout this study. Switching and cycling is also possible with other electrolytes such as 0.5 M AcOH, but they limit the stability and number of possible cycles due to corrosion of the WO3 film components.

Cycling of the ECBM is demonstrated in Figure 5. Here, we show switching and cycling between the two possible electrochromic states and the changes in reflectance over several cycles. Plotted is the peak reflectance at its corresponding wavelength. The reflectance fully recovers after each cycle. However, a small decrease of the reflectance can be observed, most probably due to a kinetic surface aspect and a higher resistance of injected ions (e.g., H+ or Li+) with the time, resulting in slower switching times and less reflectance over the number of possible cycles.

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Figure 5. Reflectance and spectral cycling between the colored NiO and WO3 states.

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In conclusion, we have synthesized and demonstrated the operation of the first example of a nanoporous ECBM that combines two nanoparticulate electrochromic materials, NiO–WO3, with complementary behavior, leading to a broadband change of the reflectance under varying bias voltage. This proof-of-concept study demonstrates that the electrically tunable refractive index of the electrochromic materials strongly influences the optical properties of the photonic crystal. The ECBM operates by shuttling ions and electrons into and out of nanoporous [NiOOH/WO3] [LEFT RIGHT ARROW] [Ni(OH)2/HxWO3] multilayers to alter the mix of intervalence charge transfer within the layers and Bragg diffraction between the layers. These ECBMs are a distinctive means of creating and manipulating color through the synergistic control of structural and electronic color effects. Potentially useful applications for ECBMs include tunable reflectors and control of grayscale in full color reflective displays. Constructing graded and tandem structures and combining several electrochromic materials will open new avenues for electrically tunable photonic materials.

Experimental Section

  1. Top of page
  2. Experimental Section
  3. Supporting Information
  4. Acknowledgements
  5. Supporting Information

Materials and Methods: The NiO films were prepared using a nickel precursor solution. Nickel hydroxide Ni(OH)2 (0.5 g) was added to glacial acetic acid (5 mL) and allowed to stir at room temperature for 8 days. The solution was then filtered using a Buchner funnel, and the filtrate collected and stored at room temperature on the bench top.

Two different synthetic methods were used to prepare thick and thin WO3 films. For thinner WO3 layers a nanoparticle WO3 dispersion was used (method A), while for thicker WO3 layers (>60 nm) a tungsten precursor sol (method B) was employed.

Method A) WO3 nanoparticle dispersions were synthesized by dissolution of elemental tungsten powder (ASP 1–5 μm or mesh 325) 5.53 g (30.1 mol) in 50 mL H2O2 (30% mean per analysis (p.a.)) and the addition of a small amount of 1–3 mL of AcOH (Glacial Acid) at 0 °C by cooling the reaction mixture in a 500 mL reaction vessel with an ice-bath. An exothermic oxidation/dissolution process leads to a light-yellow WO3 dispersion, which was stored in a plastic bottle at 4 °C and used for spin-coating thin films.11

Method B) WO3 thick films were produced using a tungsten sol precursor that has been reported previously.22 Briefly, 6.5 g of tungsten powder (ASP 1–5 μm or mesh 325) was dissolved in 80 mL of H2O2 solution (30 wt%) at 0 °C. The mixture was slowly allowed to turn into a clear solution, at which time 100 mL of glacial AcOH was added to the solution. The solution was then refluxed at 55 °C for 16 h. The solvent was then removed in vacuo at 65 °C via distillation, and the resulting flaky yellow powder was re-dissolved in EtOH at 55 °C.

CAUTION: Very Exothermic Reaction, Immediate Ice-Bath Cooling is Necessary in a Well-Ventilated Hood. Precautions should be Taken During the Whole Synthetic Process!!

Instrumentation: Scanning electron microscopy images were obtained using a Hitachi S-5200 operating at 1–5 kV for films on silicon substrates. The crystal phases of the dried dispersions were analyzed by powder X-ray diffraction (PXRD) using a Siemens D5000 diffractometer and Cu-Kα line as the X-ray source. An Autosorb-1 instrument from Quantachrome Instruments (Boynton Beach, Florida, USA) was used for measuring adsorption/desorption isotherms, surface areas, pore volumes, and pore size distributions. Adsorption/desorption isotherms for nanoparticle NiO and WO3 were measured with N2 gas (99.9995% purity), multipoint and single point BET methods were used to determine the surface area (m2/g), and average pore size and pore size distribution were determined through BJH methods. The autosorb software (Quantachrome AS1Win) for data analysis was provided by the manufacturer. Before measuring, nanoparticle samples were degassed under vacuum at 200–220 °C for at least 4–6 h for drying purposes and for removal of solvent impurities and humidity. BET surface area of dried and calcined powder samples of NiO results in 110 m2/g, with a specific porosity of 0.27 cc/g and 41 m2/g for WO3, with a specific porosity of 0.07 cc/g nanoparticles respectively.

Spectroscopic Ellipsometry (SE) & Ellipsometric Porosimetry (EP): SE analyses were performed in a J. A. Woollam instrument at incident angles of 65.0°, 70.0°, and 75.0°. The measurements were performed on films deposited on both silicon and glass substrates in the spectral range between 1–3 eV. The modeling and regression of the ellipsometric spectra were performed using the software provided by the manufacturer. The experimental data were fitted by representing each sample with a four-layer model (i.e., glass substrate, FTO layer, electrochromic film and ambient/air). By using known dielectric functions for both glass and FTO, the indices of refraction and the extinction coefficient of nanoparticle WO3 and NiO layers were recovered. The index of refraction was represented by a Cauchy formula, while the extinction coefficient was represented by a decaying exponential.

Ellipsometric Porosimetry (EP) measurements were performed with a Sopra GES-5E ellipsometer at a fixed incidence angle of 70.15° on Si-Substrates in the range 1.2–4 eV under humid water atmosphere. Modeling and regression of the ellipsometric measurements were performed using the software Winelli provided by the manufacturer.

Optical Measurements: Optical spectra and micrographs were acquired with an Ocean Optics SD2000 fiber optic spectrophotometer coupled to an optical microscope. Time-resolved reflectance measurements were done in a frequency of 350 ms or 500 ms, which was used to record the time dependence change in photonic-stop-band shifting during a recording time of 45 s. All spectra were normalized to the reflectance of a silver mirror assumed to reflect 100% of the incoming light. Transmission measurements are performed on a Lamba 900 UV–vis–NIR spectrometer in the range 300 nm to 900 nm and 300 nm to 2500 nm.

Preparation of Nanoparticle NiO/WO3 Electrochromic Bragg Mirrors: Prepared metal oxide dispersions were filtered through a 0.45 μm Titan 2 HPLC Filter Amber (GMF Membrane), to remove any agglomerates and subsequently diluted to the desired concentration, used for making ECBMs. The WO3 colloidal dispersion was diluted with DI water to specific concentrations and polyethylene glycol (PEG, [(C2H4O)n·H2O], MW: 20000 g/mol) was added to the prepared WO3 nanoparticle dispersion (≈7 wt%) and ≈10 wt% for NiO dispersions. Before spin-coating the NiO films were prepared using the nickel precursor solution (see above), to which 0.1 g of PEG was added per 1 mL of solution (10 wt% PEG) The solution was allowed to stir for 20 min, and subsequently spin-coated onto FTO glass. The films were then annealed at 450 °C for 20 min, yielding transparent nanoporous NiO thin films (the thickness of the films can be controlled by dilution of the original precursor solution using glacial acetic acid).

Bleaching and Coloring of Electrochromic Nanoporous NiO/WO3 Bragg Mirrors: Coloration and bleaching studies were performed in four different electrolytes to find an ideal medium: e.g. a strong acid (0.01–1 M HClO4), a weak acid (0.01–1 M AcOH), a strong base (0.01–1 M NaOH), and a salt solution (0.01–1 M LiClO4). Nanoporous electrochromic WO3 and NiO films were prepared on 1-inch × 1-inch FTO-coated glass, with a strip of FTO exposed for electrical contact. These samples were submerged separately in 50 mL beakers containing ∼20 mL of electrolyte, and suspended using copper tape. Voltages of ±2.0 V were applied and the coloration and bleaching times were recorded. The ideal system for switching NiO/WO3 ECBMs was found to be a 0.5 M LiClO4 electrolyte at ±2.5 V and which was used throughout this study. This system maximized the number of reproducible coloration/bleaching cycles as both WO3 and NiO layers were stable in this electrolyte at the applied voltage.

Chronoamperometric Cycling Studies: These measurements were performed on a BASi Epsilon EC instrument under the same conditions as described above for bleaching and coloring in the time range of 20 s.

Supporting Information

  1. Top of page
  2. Experimental Section
  3. Supporting Information
  4. Acknowledgements
  5. Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

  1. Top of page
  2. Experimental Section
  3. Supporting Information
  4. Acknowledgements
  5. Supporting Information

G.A.O. is Government of Canada Research Chair in Materials Chemistry and Nanochemistry. He is deeply indebted to the Natural Sciences and Engineering Research Council for the strong and sustained support of his work. E.R. thanks the Alexander von Humboldt (AvH) Foundation for a Feodor Lynen Postdoctoral Fellowship.

Supporting Information

  1. Top of page
  2. Experimental Section
  3. Supporting Information
  4. Acknowledgements
  5. Supporting Information

Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.

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