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
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).
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.
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).
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.
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.
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] ↔ [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.