Tailored Remote Photochromic Coloration of in situ Synthesized CdS Quantum Dot Loaded WO₃ Films

Abstract

WO 3 has a lower conduction band level ( + 0.5 eV), thus it is reasonable to expect that charge transfers can occur between the QDs and WO 3 . The QDs can absorb light in the visible region to generate electron-hole pairs, and the excited electrons are effi ciently transferred from QDs to WO 3 . But the synthesis conditions of most QDs are not conducive to the in situ loading of WO 3 fi lms with QDs, because WO 3 is thermodynamically unstable in alkaline environments and QDs are mostly prepared by colloidal synthesis in an alkaline medium. Thus in situ loading of WO 3 fi lms with QDs using suitable methods is still a challenging issue.On the other hand, WO 3 exhibits pronounced photochromism upon bandgap photoexcitation, which makes it attractive for applications in many areas, ranging from smart windows and nonemissive displays to information storage media. [ 8 , 9 ] However, the development of photochromic devices based on WO 3 is impeded to a considerable degree because the sensitivity of the photochromic effect of WO 3 is limited to the near-ultraviolet range. To overcome this limitation, Bechinger et al. demonstrated that the sensitivity to light exposure of WO 3 , that is, photochromism, is shifted from the near-ultraviolet into the visible range if a thin CdS fi lm is deposited underneath. [ 10 ] In the study reported here, we focused on the remote photochromic characteristics in CdS loaded WO 3 . The bandgap of CdS is reported to be 2.25 eV, [ 11 ] which is narrower than that of WO 3 (2.7 eV). [ 12 , 13 ] When visible light irradiation with wavelengths greater than 500 nm excites only CdS, the electrons photogenerated in CdS are injected into WO 3 to cause the photochromism of WO 3 ; CdS does not show any photochromism. Still, some issues remain to be uncovered in the system, such as how to improve the effi ciency of energy transfer between CdS and WO 3 , and how to deliberately manipulate the photochromism of this system on a microscopic level. [ 14 ] A potential strategy to achieve these goals is to design a photochromic system with tunable coloration behavior by coupling CdS QDs of different sizes to WO 3 fi lms, which relies on the tunable optical and electronic properties of QDs. The shorttime chemical bath deposition (CBD) technique was employed for in situ introduction of CdS QDs of different sizes and in different amounts into WO 3 fi lms in this study. [ 2 , 15 ] The visiblelight-driven photochromic coloration in the CdS QD-WO 3 coupling system associated with a broad absorption peak ranging Tailored Remote Photochromic Coloration of in situ Synthesized CdS Quantum Dot Loaded WO 3 FilmsCdS quantum dot (QD) loaded WO 3 fi lms, fabricated by screen printing and short-time chemical bath deposition (CBD) techniques, have been proven to have an effi cient visible-light-driven photochromic response. One of the striking features of such a photochromic system is its remote photochromic characteristic. The photogenerated electrons in CdS are injected into WO 3 to cause the color change of WO 3 , while...

Optical Properties of Hybrid Organic‐Inorganic Materials and their Applications

“…Only one paper [232] reports tungsten oxide included in a fluoro-polymer matrix but yield a modest photochromic response. The photochromic properties of these oxides can be modified by interaction with species such as CdS NPs, [233] Ag [234] /Cu [227] species, TiO 2 , [235,236] silica, [237] or cellulose. [238] These nanocomposite materials are not strictly hybrid organic-inorganic materials, so these issues are out of the scope of this manuscript.…”

Vertically aligned hexagonal WO3 nanotree electrode for photoelectrochemical water oxidation

“…In photochromic or electrochromic devices, electrons localize at tungsten ions to form W 5+ species, resulting in coloration from the visible to near-infrared region. [39] The fabricated WO 3 nanotree film appeared blue just after hydrothermal treatment, but turned yellowish-green in color after the washing and annealing treatment in nanotree is more positive than the redox potential required for hydrogen generation; thus, this material cannot generate hydrogen without the application of bias potential. The UV-Vis spectrum results suggest that the bandgap of the WO 3 nanotree structure is approximately 2.5 eV (Figure 3) and indicate that the valence band of the WO 3 nanotree is sufficiently deep to oxidize water.…”

Proton exchange growth to mesoporous WO3·0.33H2O structure with highly photochromic sensitivity

“…Nanostructured WO 3 /WO 3 Á xH 2 O exhibit pronounced photochromism, which makes them attractive for applications in many areas, such as smart windows, nonemissive displays, photochromic switches and information storage media [5]. However, the development of photochromic devices based on WO 3 /WO 3 Á xH 2 O is impeded to a considerable degree because of the sensitivity of photochromism [6]. Enhancing surface area is one of the most efficient ways to improve their sensitivity of photochromic coloration.…”

“…Soft-template and hard-template routes are often used for the preparation of mesoporous WO 3 / WO 3 Á xH 2 O nanostructures [9]. In soft template route, W(CO) 6 , WCl 6 and W(C 2 H 5 O) 6 are used as tungsten sources. However, they are too expensive to produce in large scale.…”

“…Herein, we report hierarchical WO 3 Á 0.33H 2 O mesoporous nanorod assemblies which were synthesized by proton exchange using Na 2 WO 4 , much cheaper than W(CO) 6 , WCl 6 and W(C 2 H 5 O) 6 , as the tungsten source. To the best of our knowledge, there have been no reports on template-free synthesis of WO 3 /WO 3 Á xH 2 O nanorods with mesoporous structure.…”