Effects of morphology, size and crystallinity on the electrochromic properties of nanostructured WO₃ films

Abstract

WO 3 nanostructures with various morphologies and crystallinity (such as cylinder-like WO 3 nanorod arrays (WNRs-1), porous WO 3 nanofibers, WO 3 nanoflake arrays, sharp cone-like nanorod arrays (WNRs-2) and annealed cylinder-like WNRs-1) were prepared on FTO substrates by using a hydrothermal method without organic additives. The effects of morphology, size and crystallinity on the electrochromic properties of WO 3 nanostructures were systematically investigated by scanning electron microscopy, X-ray diffraction, cyclic voltammetry and chronoamperometry. The WNRs-1 exhibited excellent cyclic stability, wide optical modulation (64%), and relatively high coloration efficiency (61 cm 2 C -1 ). A fast switching speed of 5 s and 6 s for bleaching and coloration are achieved for the WNRs-1 after annealing. Moreover, the relationship between the micro-morphologies/structures and electrochromic performance of WO 3 nanostructures was also discussed. the deintercalation) compared with the unannealed WNRs-1, which may be closely related to the different crystalline forms. The h-WO 3 could accommodate abundant Li-ions due to its three possible locations (trigonal cavity, hexagonal window and four-coordinated square window).However, when the h-WO 3 is translated to m-W 18 O 49 , a large amount of hexagonal windows disappear which result in low capacity for Li-ions. Moreover, the cyclic reversible value is only 71% calculated from the charge/discharge densities, which is mainly ascribed to the oxygen deficiency.Greater oxygen deficiency in W 18 O 49 could capture a certain amount of electrons in the deintercalation. Furthermore, this experiment has settled the disputes of electrochromic mechanism between the double injection model 33 and the small polaron model 34 . A major focus of the debate is the valence transformation of tungsten element in the color-changed process. The double injection model suggested that the valence transformation of tungsten element changed between plus five and plus six. While the small polaron model believed that it should be between plus five and plus four.From Fig. 8, we know that there indeed is a mixture of two kinds of valence changes in the annealed WNRs-1 during the charging and discharging process.

Novel Spark Method for Deposition of Metal Oxide Thin Films: Deposition of Hexagonal Tungsten Oxide

“…Chronoamperometry measurements were also conducted switching the voltage between −1000V and +1000 mV at an interval of 300 s and the respective results are shown in Figure (down). The intercalation and de‐intercalation charge densities (obtained by the integration of the excess current density) were found equal to 0.7 and 0.25 mC cm −2 , respectively. The observed difference between the intercalation and the de‐intercalation charge densities can be attributed to the remaining Li + ions within the WO 3 lattice and as a result, the layer remained slightly colored after several cycles which favors a degradation in its performance .…”

“…The observed difference between the intercalation and the de‐intercalation charge densities can be attributed to the remaining Li + ions within the WO 3 lattice and as a result, the layer remained slightly colored after several cycles which favors a degradation in its performance . The intercalation and de‐intercalation time response (defined as the time needed for the excess current density to reduce by 10% of the absolute maximum value) were found 22 and 8 s, respectively. These values are slightly lower than those reported in previous works for electrochromic WO 3 .…”

Fabrication of Mo-Doped WO3 Nanorod Arrays on FTO Substrate with Enhanced Electrochromic Properties

“…The results suggested that under optimum conditions for the preparation of WNRAs, with good morphologies and excellent properties at hydrothermal temperature of 180 °C and a growth time of 5 h, Mo-doped WNRAs with an optical modulation of 61.7% at a wavelength of 660 nm are produced with a coloration/bleaching time of 3 s/9 s and a coloration efficiency of 73.1 cm 2 /C. Especially, the coloration/bleaching time is faster than that in other literature [ 21 , 22 ].…”

“…Crystalline WO 3 nanorod arrays (WNRAs) synthesized on transparent substrates by the hydrothermal method offer an excellent quality, well-ordered, and uniform structure; multiple oxidation states; as well as a high capability to accommodate intercalated positive ions [ 20 ]. However, in practice, a poor coloration efficiency (~68 cm 2 /C) and long switching time (~27 s) are normally yielded by pure crystalline WO 3 and these difficulties have not been resolved thoroughly [ 21 ]. Hence, the use of EC devices in commercially viable applications requires further improvement on their electrochromic characteristics (reversibility, stability, optical modulation, etc.).…”

Nanofilament array embedded tungsten oxide for highly efficient electrochromic supercapacitor electrodes

“…During the cathodic potential scan, the intercalation of Li + ions induces the reduction of the W 6+ state to the W 5+ state, resulting in the electrochromic effect (blue coloration of the film). [69,70] By contrast, during the anodic potential scan, an oxidation reaction at the electrode-electrolyte interface causes deintercalation of Li + ions from the W 5+ state to the W 6+ state, making the electrode transparent (bleached). To evaluate the electrochromic properties of both the pristine WO 3 and NFA-embedded WO 3 electrodes, we conducted optical transmittance measurements by applying potential steps of ±0.75 V (vs. SCE) for a fixed time of 30 s. The transmittance spectra for the electrodes were recorded in the colored and bleached states, as shown in Fig.…”