Chromogenic applications

Chromogenic applications

4.1

4

Background

There exist several forms of chromism, but this chapter only describes the phenomena that involve metal oxide materials. Chromism refers to changes in visible optical properties in response to an external stimulus such as electrical, optical, thermal, or mechanical [1]. It includes changes in color or intensity. It can also refer to a transition from transparent to a scattering reflective state. The chromogenic family of materials has been ever expanding over the past few years, with the development of new materials and nanomaterials. A schematic of the evolution of chromogenic effect and materials is shown in Fig. 4.1. The first reference to chromogenic behavior of materials is about photochromic materials and dates back to ancient times, to the era of Alexander the Great (356
323 BC). At that time, the Macedonian head warriors were equipped with photochromic bracelets that would change color when exposed to sunlight [2]. More recently, photochromism was first observed and reported by Fritsch in 1867 [3]. He noticed that tetracene (C18H12) with air and light produced a colorless material (orange), which regenerated the color in the dark. Later, in 1876, ter Meer observed a color change in potassium salt of dinitroethane that changed from yellow in the dark to red in the daylight [4]. The term “photochromism,” from the Greek words phos (light) and chroma (color), was introduced by Hirshberg only in 1950, to describe the observed phenomenon of color changing with exposure to light [4].

Figure 4.1 Historic events in the development of chromogenic materials. Metal Oxide Nanostructures. DOI: https://doi.org/10.1016/B978-0-12-811512-1.00004-7 © 2019 Elsevier Inc. All rights reserved.

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The first report of electrochromism dates from the 1930s when Kobosew and Nekrassow first noted electrochemical coloration in bulk tungsten oxide [5]. But it was with Deb’s demonstrations of electrically induced and reversible coloration in transition metal oxides, between the 1960s and 1970s, that an increase in studying electrochromic materials for devices applications (like windows, mirrors, and displays) was observed [6]. Thermochromics materials started to emerge from the laboratory in the 1960s and were based on liquid crystal technologies [7]. In the 1960s
70s growing interest in using metal oxides in chromogenic applications was observed. David Adler studied the effect on crystallographic structure when this type of material is heated, a phenomenon that explains the thermochromic effect on metal oxide materials [8]. These metal oxides are one of the most interesting classes of materials that can exhibit a variety of structures and properties. The nature of metal
oxygen bonding can vary between highly covalent to nearly ionic [9]. Transition metallic elements possess a d orbital partially filled, which is the basis for the vast range of oxides that they can form, presenting a high metastability in multiple oxidation states. Transition metal oxides are formed by the charge exchange between the highly electronegative oxygen atoms and the less electronegative transition metal atoms. By losing different numbers of electrons in these orbitals to the oxygen atom, the transition metals can form different oxides. This exchange leads to a wide range of bonding and also to different structures and phases. All these properties are of great importance when switching between these states and structures, leading to reversible changes in their optical and electrical properties, offering a vast number of applications for this type of material [10,11]. These switches can provide small activation energies in the form of heat, light, electric field, and pressure, which makes the use of metal oxide materials in chromogenic applications advantageous. The chromism effect can be observed in organic or inorganic materials, but these ones have some advantages: they possess higher thermal stability, strength, and chemical resistance, and can easily be shaped into thin film or other suitable forms [12]. Around 1980
90 a growing interest was observed in the use of transition metal oxides in chromogenic devices due to these unique properties. Since the beginning of the 21st century, the use of nanostructures in different fields of application has grown exponentially. The use of nanostructured metal oxide is more advantageous when compared with thin films due to the surface area increasing and the chromogenic effect being enhanced, by enhancement of the range and control of the optical properties, improving the switching performance [10,13]. This observed enhancement is due to the high internal surface, high grain scattering boundary scattering, and high porosity. It was discovered by S. Corr that when going to nanoscale, new magnetic, electronic, or optical characteristics on materials may appear, which are usually not exhibited in their bulk state [11]. The use of nanomaterials in chromogenic applications will improve coloration efficiencies, and provide faster switching times and longer cycles lives. It also may provide a cost reduction in device production, due to being compatible with solution processing [6,14
17].

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Various types of materials and structures can be used to construct chromogenic devices, depending on the specific applications, like electrochromic, thermochromic, photochromic, chemichromic, and magnetochromic properties.

4.2

Electrochromic nanomaterials

Electrochromic coloration of materials is the most widely used and studied form of chromogenic effect, due to the ease of control and reversibility of optical properties just by applying an electric field to the material or device. These electrochromic materials are characterized by presenting a reversible change in their optical properties (like reflectance, transmittance, or absorbance) with the application of a small electric field. Their working behavior is basically characterized by the amount of light that they allow to pass through them when an external voltage is applied [1,18,19]. This phenomenon is observed due to electrochemical redox and oxidation reactions of the material nanostructure caused by an applied electrical current or potential [20,21]. When coloration occurs, it is due to a double charge injection into the material—protons are extracted from the electrolyte (that can be solid or liquid) and electrons are injected from an opposing electrode [22]. Electrochromic metal oxide materials can be divided into two main groups: cathodic and anodic electrochromic materials. In cathodic oxides, the color transition happens upon ion insertion, while in anodic oxides the optical transition occurs upon ion extraction [23]. The most studied electrochromic materials are tungsten oxide (WO3) [24
27] and nickel oxide (NiO) [28
32], as cationic and anodic electrochromic materials, respectively, but many other electrochromic metal oxides exist, such as V2O5, TiO2 MoO3, Nb2O5, and Ta2O5 (cathodic electrochromic materials) [21,33
37], and IrO2, CoO2, MnO2, FeO2, Cr2O3, and RhO2 (anodic electrochromic materials) [21,38
41]. These last materials have the particularity that they do not bleach completely. In Fig. 4.2 is shown the periodic system of the elements, where it is possible to observe which transition metals form cathodic or anodic electrochromic metal oxides. For characterization of an electrochromic material, the switching speed, durability/stability, and coloration efficiency (CE) must be analyzed. Gillaspie et al. [21] defined the coloration efficiency as the “change in optical density, OD, per unit of inserted charge, Q,” expressed by the following equation [40,42]: CE 5 DðODÞ=ΔQ

(4.1)

Therefore, when using cathodic materials, the coloration reaction of the material occurs upon simultaneous electrochemical insertion of small cations, such as hydrogen (H1), lithium (Li1), potassium (K1), or sodium (Na1) (present in the electrolyte) and of charge-balancing electrons [21]. This phenomenon can be

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Figure 4.2 Periodic system of the elements, showing transition metals with oxides that can act as cathodic (in blue) (dark gray in print version) or anodic (in orange) (light gray in print version) electrochromic materials.

Figure 4.3 Schematic of electrochemical redox and oxidation reactions observed in electrochemical materials.

explained by the schematic shown in Fig. 4.3. When a voltage is applied, electrons are injected into the electrolyte, and cations from the electrolyte will compensate the charge injection, originating a change in the color of the active electrochemical layer. When a reversible voltage is applied, the inverse process occurs and the active electrochromic layer becomes bleached, the former chromic state [43,44]. The basic redox reactions of a cathodic and anodic electrochromic transition can be described by the following simplified reactions, referring to WO3 and NiO materials [23]:

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107

ðWO3 1xM1 1xe2 Þbleached 2ðMx WO3 Þcolored

(4.2)

½Mx NiObleached 2ðNiOx 1xM1 1xe2 Þcolored

(4.3)

with (M 5 Li1, H1, Na1) The coloration mechanism in WO3 materials is related to the redox process between W41, W51, and W61 ions (different oxidation states) [21]. In NiO nanostructures are related to the charge transfer process between Ni21/Ni31 and Ni31/ Ni41 [1,28]. Table 4.1 describes the redox/oxidation reactions that occur by applying a voltage to metal oxide nanostructures.

4.2.1 Electrochromic applications One of the most interesting and most studied applications for electrochromic metal oxide nanostructures is their use in smart windows [6,18,24,51,52]. However, in the past few years other applications have shown growing interest, such as optical displays [53], rear-view mirrors [54], and biosensors [55]. These electrochromic devices can be built either on rigid (glass) or flexible (polymeric or paper) substrates [55
57], or even on textile substrates [58]. The use of flexible substrates can bring some advantages, allowing “retrofit” installation on existing structures and in this way reducing production and installation costs [59].

4.2.1.1 Smart electrochromic windows/displays on glass substrate This is one of the most studied applications and can be applied in automobiles, buildings, or airplanes [6,24,51,52]. The majority of electrochromic smart windows use tungsten oxide, WO3, as an active electrochromic layer [25,26,60
62]. These electrochromic windows are constructed on a rigid or flexible substrate, using different processing techniques. As an electrode, the glass substrates can be covered with a conductive transparent metal oxide thin film, such as ITO (indium thin oxide), FTO (fluorine thin oxide), or even GZO (zinc oxide doped with gallium). To produce metal oxide nanostructures of WO3, different techniques can be used, such as a hydrothermal method [48,55,63
65], chemical bath deposition [66,67], electrodeposition [68], spray pyrolysis [69,70], sol
gel [71
73], electrospinning [74], thermal evaporation [75], or chemical vapor-phase deposition [76,77]. The hydrothermal method is one of the most commonly used to produce metal oxide nanostructures. Kadam et al. developed a seed layer-free hydrothermal method for prepare nanostructured WO3 films on ITO-coated glass substrate, with improved electrochromic performance [48,63]. By using ethylene glycol on WO3 synthesis, Kadam et al. were able to improve the nanostructures’ adhesion to the substrate and by this was obtained an enhancement in coloration efficiency. They obtained a CE of 135.8 cm2/C and a response time of 2.5 s, when applying 61.5 V.

Table 4.1 Transition electrochromic metal oxides—redox reaction Metal

Oxidized form

Reduced form

Redox reaction

References

Ti V Nb Ta Mo W Mn Fe

TiO2-Colorless V2O5-Brown/yellow Nb2O5-Colorless Ta2O5-Colorless MoO3-Pale blue WO3-Very pale yellow MnO2-Brown Fe2O3-Brown Fe3O4-Black CoO-Pale yellow Ni(OH)2-Colorless RhO2.2H2O-Green Ir2O3-Colorless

MxTiO2-Blue-gray MxV2O2-Very pale blue MxNb2O2-Multi color MxTa2O2
Deep blue MxMoO3-Very intense blue MxWO3-Intense blue MnxO2-Yellow Fe3O4-Black FeO-Colorless Co3O4-Dark brown NiOOH-gray/brownish Rh2O3.5H2O-Yellow Ir2O4.H2O-brown

TiO2(bleach) 1 xM1 1 xe22MxTiO2(blue-gray) V2O5(bleach) 1 xM1 1 xe22 MxV2O5(pale blue) Nb2O5(bleach) 1 xM1 1 xe22 MxNb2O5(multicolor) Ta2O5(bleach) 1 xM1 1 xe22MxTa2O5(deep blue) MoO3(bleach) 1 xM1 1 xe22MxMoO3(intense blue) WO3(bleach) 1 xM1 1 xe22MxWO3(blue) MxMnxO2 (Yellow) 1 xM1 1 e22MnO2 (Brown) Fe2O3 1 xM1 1 xe22 MxFe2O3

[45] [46] [36] [47] [17] [48] [40] [49]

Co3O4(Dark brown) 1 xM1 1 e223CoO(Pale yellow) 1 2OH2 NiOx(Gray/brownish) 1 yM1 1 e22 MyNiOx(Bleached) Rh2O3.5H2O(Yellow) 1 yM1 1 e22RhO2.2H2O(Green) Ir2O4.H2O (Brown) 1 yM1 1 e22 My Ir2O3 (Bleached)

[39] [30] [41] [50]

Co Ni Rh Ir

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Haizeng Li et al. and Zhihui Jiao et al. also used a hydrothermal method to synthesize WO3 directly onto an FTO glass substrate [63,64]. They produced 2D WO3 nanostructures (nest-like and plate-like nanostructures) that present a large surface area and permeable channels, which are responsible for the significant enhancement observed in electrochromic CE. It is thought that porous structures that bleaching and coloration response and efficiency are improved, by reducing the ion diffusion path length and resistance [64]. Li was able to achieve a CE of 126.34 cm2/C, with a switching speed of 26 s, while Jiao obtain a CE of 112.7 cm2/C, with a switching speed of 4.3 s, when applying 61 V. Fig. 4.4 show the 2D WO3 nanostructures produced by a seedless hydrothermal method. Zhang et al. produced WO3 nanowire arrays, also by a hydrothermal method, by adding a capping agent (ammonium sulfate) [65]. This nanowire arrays form a well-aligned array with a highly porous surface morphology, allowing a fast switching coloration speed of 7.6 s and a high CE of 102.8 cm2/C. As described earlier, hydrothermal synthesis is the most common method used to produce WO3 electrochromic devices. But other techniques are also used, as reported by Poongodi et al., who produced WO3 by electrodeposition (presenting a nanoflake morphology) with a CE of 154.93 cm2/C and a switching time of 2.87 s [68] or by Denayer et al. [69], who used the spray-pyrolysis technique and obtained a CE of 50.6 cm2/C with a switching time of 6.4 s. Moreover, Li et al. [72] reported the production of WO3 by sol
gel method in electrochromic applications. They reported a coloration efficiency of 50 cm2/C and a switching time of 6 s. The sol
gel method produced a mesoporous film comprised of nanocrystalline domains that can provide an increase in performance and on stability and durability.

Figure 4.4 Scanning electron image (SEM) of (A) 2D WO3 nest-like nanostructure (Source: Adapted from H. Li, G. Shi, H. Wang, Q. Zhang, and Y. Li, Self-seeded growth of nest-like hydrated tungsten trioxide film directly on FTO substrate for highly enhanced electrochromic performance J. Mater. Chem. A 2(2014) 11305
11310), with permission of The Royal Society of Chemistry (2017); (B) 2D WO3 plate-like nanostructure. Source: Adapted from Z. Jiao, X. Wang, J. Wang, L. Ke, H.V. Demir, T.W. Koh. et al., Efficient synthesis of plate-like crystalline hydrated tungsten trioxide thin films with highly improved electrochromic performance Chem. Commun. 48(2012) 365
367, with permission of The Royal Society of Chemistry (2017).

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Metal Oxide Nanostructures

On the other hand, electrospinning is a powerful process that allows the formation of solid fibers with a diameter of just some nanometers. These 1D nanostructures offer direct electrical pathways for electrons, increasing their transport rate, as well as increasing the surface area and device porosity, allowing the electrolyte to penetrate more easily and shorten the proton diffusion within the bulk material [74]. Dulgerbaki et al. produced WO3 nanofibers by electrodeposition method. They achieved a CE of 64.58 cm2/C for electrospun WO3 nanofibers [74]. Thermal evaporation and chemical vapor deposition were used by Patel et al. [78] and Deshpande et al. [79] to obtain nanostructured WO3 to be used in electrochromic devices. They achieved ECs of up to 57.7 and 42 cm2/C, respectively. In Fig. 4.5 it is possible to observe some WO3 nanostructures, produced with different synthesis techniques, to be used in electrochromic devices. Some examples of smart windows on glass substrate, using WO3 as electrochromic material, are shown in Fig. 4.6 [80,81]. Xie et al. [80] developed an electrochromic “window” (Fig. 4.6A) with nanostructured WO3 film produced by a simple and seedless method at room temperature. They dipped an activated FTO conductive glass (by applying 6 V for 10 s) into a precursor solution for 15 min, followed by annealing at 300 C for 1 h in air. It was estimated that a CE of 107.8 cm2/C and a coloration and bleach time of 3.2 and 1.2 s were achieved, respectively. Another example is shown in Fig. 4.6B, where Kondalkar et al. display photographs of a “smart window” coloration by applying different voltage intensities [81]. They produced an electrochromic layer with honeycomb nanostructured single-crystalline hexagonal WO3, and were able to achieve a switching time of 4.29 s and a CE of 87.23 cm2/C. A very interesting work is shown on Fig. 4.7. Pawel Wojcik et al. [82] produced an inkjet printed electrochromic 8 3 8 passive matrix, with a good optical contrast. This matrix consists of two 100 mm by 100 mm ITO-coated glass substrates with 64 pixels, 1 cm2 each, and with a layer configuration ITO/α-WO3/TiO2/WOX/ITO. When a voltage is applied to a selected pair of electrodes, the ions will be driven from the electrolyte into the α -WO3/TiO2/WOX film, where they are intercalated, causing a change in color (Fig. 4.7A). By reversing the voltage the ions will return to the electrolyte, resulting in the pixels bleaching (Fig. 4.7B). More recently, a grow interest in using silver (Ag) nanoparticles incorporated in WO3 nanostructured electrochromic devices for the enhancement of coloration efficiency and switching response has been observed [66]. Hoseinzadeh et al. [66] reported the production of WO3 nanopowder by a chemical bath deposition method that was deposited on FTO-coated glass. Silver nanoparticles were then deposited on WO3 by PVD (pulse vapor deposition) method. By applying 61 V, they were able to achieve a decrease in response time from 6.9 to 5.3 s and an increase in coloration efficiency from 63.5 to 74.2 cm2/C (a difference of 10.7 cm2/C), for devices produced without and with Ag nanoparticles. Titanium dioxide (TiO2) is another cathodic metal oxide that can be used in electrochromic devices. Some authors have reported the synthesis of TiO2 aiming at this type of application. Dinh et al. [45] prepared TiO2 anatase nanostructure films by a doctor blade technique. They were able to produce electrochromic devices

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Figure 4.5 Scanning electron image of WO3 nanostructures produced by (A) electrodeposition (Source: adapted from S. Poongodi, P.S. Kumar, D. Mangalaraj, N. Ponpandian, P. Meena, Y. Masuda et al., Electrodeposition of WO3 nanostructured thin films for electrochromic and H2S gas sensor applications, J. Alloys Compd. 719(2017) 71
81), with permission of Elsevier (2017); (B) electrospinning (Source: adapted from C. Dulgerbaki, N. N. Maslakci, A.I. Komur, and A.U. Oksuz, Electrochromic device based on electrospun WO3 nanofibers, Mater. Res. Bull. 72(2015) 70
76), with permission of Elsevier (2017); (C) thermal evaporation (Source: adapted from I.T.S. Garcia, D.S. Corrˆea, D.S. de Moura, J.C.O. Pazinato, M.B. Pereira and N.B.D. da Costa, Multifaceted tungsten oxide films grown by thermal evaporation, Surf. Coating. Technol. 283(2015) 177
183), with permission of Elsevier (2017); and (D) chemical vapor deposition. (Source: Adapted from Z.S. Houweling, P.-P.R.M.L. Harks, Y. Kuang, C.H.M. van der Werf, J.W. Geus, and R.E.I. Schropp, Hetero- and homogeneous three-dimensional hierarchical tungsten oxide nanostructures by hot-wire chemical vapor deposition, Thin Solid Films 575 (2015) 76
83), with permission of Elsevier (2017).

with a coloration efficiency of 33.7 cm2/C and a switching time of 2 s. The big advantage of using TiO2 is that it easily forms nanostructured films with a porosity superior to 50% and the internal surface area exceeds the geometric area. These properties lead to a rapid accumulation of electrons and the charge may be readily compensated by adsorbed ions at the electrode
electrolyte interface, which may lead to a rapid switching time of the device [83]. The big disadvantage of this

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Metal Oxide Nanostructures

Figure 4.6 (A) Schematic of a typical sandwich-like solid-state electrochromic device (top) and photographs of the electrochromic device in the bleached and colored states, respectively (bottom) (Source: adapted from Z. Xie, L. Gao, B. Liang, X. Wang, G. Chen, Z. Liu, et al., Fast fabrication of a WO3  2H2O thin film with improved electrochromic properties, J. Mater. Chem. 22(2012) 19904), with permission of the Royal Society of Chemistry; (B) photographs of the electrochromic device produced with honeycomb nanostructured WO3 layer (Source: adapted from V.V. Kondalkar, S.S. Mali, R.R. Kharade, K.V. Khot, P.B. Patil, R.M. Mane, et al., High performing smart electrochromic device based on honeycomb nanostructured h-WO3 thin films: hydrothermal assisted synthesis, Dalt. Trans. 44(2015) 2788
2800), with permission of the Royal Society of Chemistry.

Figure 4.7 Inkjet printed electrochromic 8 3 8 passive matrix on ITO-coated glass substrate in (A) colored and (B) bleached states. Source: Adapted from P.J. Wojcik, L. Pereira, R. Martins, and E. Fortunato, Statistical mixture design and multivariate analysis of inkjet printed a—WO3/TiO2/WOX electrochromic films, ACS Comb. Sci. 16(2014) 5
16, with permission of the American Chemistry Society.

material is the lower coloration efficiency observed. Chen et al. [84] produced a TiO2 electrode with antireflective and electrochromic properties, with a CE up to 13.87 cm2/C and a switching speed of 11.3 s.

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Another good candidate to be used in smart electrochromic windows, and one of the most studied, is NiO (an anodic electrochromic material), a low-cost material, with a good cyclic color reversibility, high coloration efficiency, and durability. Many authors have reported the production of NiO nanostructures for use in electrochromic devices [28,30
32,85
87]. Xia et al. [32] and Dalavi et al. [87] both produced a highly nanoporous network of NiO by chemical bath deposition at room temperature. The common CE value obtained for this type of nanostructures is 42 cm2/C. Furthermore, Dalavi et al. investigated the influence of film thickness on optical properties of the electrochromic device. They were able to obtain a CE of 95 cm2/C for the thicker film, with a response time of only 3.5 s. This nanoporous network provides a high specific surface area with open space between individual pores that will facilitate the contact between the electrolyte and the oxide surface, allowing easier diffusion of ions between them. Large thickness in an open structure will offer a larger amount of active mass deposited and allows for more easy diffusion of ions along its length. Pereira et al. [28] were able to demonstrate that by combinig NiO and WO3 on the same electrochromic device it is possible to improve the optical modulation, achieving a value of 77%. The obtained NiO films were nanostructured with a needle-like grain shape (Fig. 4.8). These nanostructured films show a response time for coloration of 6.3 s and bleaching of 4.4 s, calculated as the necessary time to achieve 10% and 90% of the initial and final transmittance values, respectively.

Figure 4.8 (A) Spectral transmittance at 630 nm as a function of thickness, of the NiO films for colored and bleached states (the insets show photos of NiO films on the colored and bleached states); (B) SEM image of NiO nanostructured film; (C) proof-of-concept of an ITO/WO3/LiClO4-PC/NiO/ITO electrochromic window. The transmittance modulation was achieved by applying 62 V. Source: Adapted from S. Pereira, A. Gonc¸alves, N. Correia, J. Pinto, L. Pereira, R. Martins, et al., Electrochromic behavior of NiO thin films deposited by e-beam evaporation at room temperature Sol. Energy Mater. Sol. Cells 120(2014) 109
115, with permission of Elsevier.

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Bo et al. produced a composite of NiO/TiO2 nanorods array by hydrothermal/solvothermal method on FTO-coated glass [85]. When comparing with a NiO nanostructured film, NiO/TiO2 composite achieved a CE of 147.6 cm2/C, as against the 55.1 cm2/C achieved only with NiO, with response times of 3.8 and 0.7 s for NiO/ TiO2 and NiO, respectively. The purpose of using this type of composite is to increase the color contrast (and by this way the CE), decrease the response time, and decrease the degradation on cycling. Bo et al. state that NiO deposited on TiO2 nanorod arrays will present a higher porous surface area, which will promote the penetration capacity of electrolytes and promote electrochromic reactions. Besides the above-mentioned metal oxides (WO3 and NiO, that are the most commonly used metal oxides in electrochromic applications), some authors have reported the use of other nanostructured metal oxides. Lu et al. [46] and Tong et al. [88] studied the coloration of electrochromic V2O5 nanostructured films. These materials can be deposited in the form of a mesoporous film, with a high surface area, that will provide porous channels, which will facilitate ion diffusion and effective strain relaxation upon cycling of Li ion intercalation/deintercalation [46]. A CE of 32 cm2/C and a switching time of 8.9 s were obtained. By doping V2O5 with Ti by a spin coating method, Wei et al. [89] were able to improve the electrochromic stability. By adding Ti, the V2O5 layers are distorted, providing more free space for Li ion movement. The device produced by Wei et al. remained stable through 200,000 cycles, which, according to the authors, is the electrochromic device based on V2O5 with the longest lifetime [89]. Nb2O5 is a very interesting metal oxide for use in electrochromic applications. Although presenting a very low CE with values ranging from less than 10 to 26 cm2/C [90,91], it possesses a big advantage when compared with WO3. Nb2O5 presents a very complex crystalline structure with more than 10 identified polymorphs. For these reasons, it possesses a multicolor capability from brown (observed for amorphous layers) to blue (observed for more crystalline layers). Moreover, it also presents a long-term cyclic stability [36,91]. More recently, Yao et al. [36] were able to produce a nanoporous Nb2O5 film by an anodization process, that presented a CE of 47 cm2/C and a switching time of 13 s. Kirchgeorg et al. developed an alloy with tantalum pentoxide [92] to increase the switching speed and coloration efficiency of a nanoporous tungsten
tantalum oxide film. Kirchgeorg et al. illustrated that the use of Ta2O5 would protect the oxide layer against dissolution in acidic media and at higher cathodic potentials. The increase in the switching speed is assumed to result from crystallographic defects in the structures of W
Ta. Zheng et al. synthesized MoO3 nanobelts [93] by a hydrothermal method and applied these nanostructures into an ITO-coated glass. The use of MoO3 shows a stronger and more homogeneous chromogenic response in this colored state and presents a better open-circuit memory when compared with the majority of metal oxides. Only a few authors have reported the use anodic coloration metal oxides [39
41,50]. Iridium oxide (IrOx) is a good material to be used as an anodic counter electrode, with strong coloration, when compared with tungsten oxide. However,

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presently it has the big disadvantage of being a very high-cost material. Being a hydrate crystal, it changes from transparent to brown, by extraction of a proton from H2O, changing the electric charge of iridium. Manganese oxides present various crystal phases and morphologies [50]. The reduction of Mn41 to the Mn31 state changes the color from brown to yellow. There have been reports of CE ranging from 6 to 130 cm2/C for this type of material [40,94]. Rhodium oxide nanocrystals present a good electrochromic response, with reversibility in green/yellow states (oxidized and reduced states, respectively) and a long lifetime. It was achieved a CE of 29 cm2/C for this type of anodic metal oxides [41].

4.2.1.2 Electrochromic displays on flexible substrate In the last decade there has been a growing interest in the development of flexible devices. These devices bring the advantages of being less expensive with their production methods being easily accessible, such as sol
gel or inkjet printing. Several authors have reported the production of electrochromic devices, based on metal oxide nanostructures, on flexible substrates like polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) [26,56,57,59,95
98]. Liang et al. [59] reported a very interesting work where they use 2D ultrathin WO3 nanosheets (obtained by exfoliation of tungstate-based inorganic
organic nanohybrids) in flexible devices. They used PET as substrate, covered with ITO thin film as a working electrode and counter electrode. As an electrolyte they used a solution containing lithium perchlorate in propylene carbonate. In Fig. 4.9 is represented the flexible electrochromic device developed by Liang et al. This

Figure 4.9 (A) Schematic of a flexible electrochromic smart window; (B) optical transmittance as a function of time, after 400 cycles under extending (red) (black in print version) and bending (green) (gray in print version) configuration. Source: Adapted from L. Liang, J. Zhang, Y. Zhou, J. Xie, X. Zhang, M. Guan, et al., Highperformance flexible electrochromic device based on facile semiconductor-to-metal transition realized by WO3-2H2O ultrathin nanosheets, Sci. Rep. 3(2013) 1936, with permission of Springer Nature.

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device presents coloration/bleached transmittance of 20/65%, by applying
3.0 and 13.0 V, in each state for 30 s, and after being bent 400 times. Their device shows a CE of 120.9 cm2/C and a switching speed of 9.7 s. In Fig. 4.10 it is possible to observe the work developed by Costa et al. [95,96]. An inkjet printing technique was used to deposit WO3 and V2O5 nanoparticles on ITO-coated flexible substrate. WO3 and V2O5 nanoparticles were produced by a hydrothermal method with the aim of achieving very small nanoparticles, so that they can be printable (with sizes below 200 nm). With WO3 a CE of 133 cm2/C was achieved, with a switching time of .6 s (these values were obtained with a minimum of 0.9 V), while for V2O5 a CE of 42 cm2/C was observed (for applied voltages as low as 0.1 V), with a switching time of approximately 200 s. Both WO3 and V2O5 display excellent cycling stability after 30,000
50,000 cycles, having a degradation of only 18%
25%. Electrochromic flexible devices, based on 1D WO3 nanobundle structures, were produced by Chen et al. [98]. These nanostructures were produced by a hydrothermal method associated with electrophoretic deposition. With this type of nanostructure they obtained a CE of 89.8 cm2/C with a fast response time of 9 s in coloration. This high EC value was obtained due to the increased surface area using 1D nanostructures. Lin et al. compared the electrochromic performance of tungsten flexible devices with and without the addition of tantalum [99]. The purpose of doping with tantalum is for enhancement of the proton intercalation reversibility. They were able to reduce the coloration and bleaching time to 4
5 s and 8 s, respectively, and increase the difference between coloration and bleaching transmittance. Values of 25% and 91% of transmittance for colored and bleach films, respectively, were obtained. Also reported by Lin et al. [100], were NiO nanostructured films deposited on flexible substrates with an EC of 67 cm2/C, after 1100 cycles.

Figure 4.10 Photograph of flexible electrochromic display with (A) WO3 and (B) V2O5 nanoparticles, deposited by an inkjet printing technique. Source: Adapted from C. Costa, C. Pinheiro, I. Henriques, and C.A.T. Laia, Inkjet printing of sol
gel synthesized hydrated tungsten oxide nanoparticles for flexible electrochromic devices, ACS Appl. Mater. Interfaces 4(2012) 1330
1340; C. Costa, C. Pinheiro, I. Henriques, and C.A.T. Laia, Electrochromic properties of inkjet printed vanadium oxide gel on flexible polyethylene terephthalate/indium tin oxide electrodes, ACS Appl. Mater. Interfaces 4(2012) 5266
5275, with permission of the American Chemical Society.

Chromogenic applications

117

4.2.1.3 Cellulosic-based devices—colorimetric biosensors As it is the major biopolymer on Earth, and is biocompatible and biodegradable, cellulose is most suitable to be used in low-cost and disposable applications [101,102]. Paper-based colorimetric sensors are novel and of great interest, with just a few reports existing that apply metal oxides to this type of sensor. One of the most interesting applications of chromogenic materials, and taking advantage of electrochemically active bacteria (EAB), is in biosensor applications for the detection of specific bacteria. Marques et al. have developed a platform in common office paper for bioelectrochromic detection of an electrochemically active bacteria, using WO3 nanoparticles [55]. Electrochemically active bacteria have the ability to transfer electrons outside their cells. By using an electrochromic material, it is possible to use this electron transfer for their detection. When present, the cells in the sample induce the formation of tungsten bronze which displays a deep blue color, contrasting with the white background of the paper platform. In Fig. 4.11, the device production is illustrated. The sensor was patterned using a wax printer (a method that was previously optimized by Costa et al. [103]). The patterned paper was then impregnated with WO3 nanoparticles by a drop casting method, creating an electrochromic layer that in the presence of bacteria will change color. Another interesting colorimetric sensor based on metal oxides deposited on cellulosic substrates was reported by Sharp et al. [104]. They developed a metal oxidebased array for field application in the characterization of phenolic antioxidantcontaining samples. These sensors are based on colored charge transfer complexes between polyphenols and metal oxide nanoparticles. The nanoparticles used were TiO2, ZnO, Fe2O3, and ZrO2, which possess different polyphenol-binding properties that create distinct and detectable optically signal. Polyphenols, like epigallocatechin gallete (EGCG), gallic acid (GA), resveratrol (RV) and trolox, were able to be detected with the produced sensor. In Fig. 4.12 is shown the colorimetric result on the detection of the phenol antioxidant GA, using different metal oxide nanoparticles. The detection of low concentrations of proteins is also of great importance in allergy testing, clinical treatment, or in the early diagnosis of certain diseases. Xiaoning Li et al. used Fe3O4 nanoparticles for colorimetric detection of certain proteins [105]. The nanoparticles were functionalized with dopamine (Dop) and with trimethylammonium (TMA) allowing differential interactions with analyte proteins and causing simultaneous alteration in catalytic efficiency. Distinct patterns are generated by Fe3O4 nanoparticle catalyzed oxidation, changing color from a colorless state to green in the presence of hydrogen peroxide (H2O2). In Table 4.2 is presented a summary of the state-of-the-art of the application of metal oxide nanostructures to electrochromic devices, showing the coloration efficiency obtained with each material.

4.3

Thermochromic nanomaterials

Thermochromism is a reversible change observed in spectral properties of a material as a result of heating and/or cooling [106,107]. Most often, this change will

118

Metal Oxide Nanostructures

Figure 4.11 (A) Colorimetric biosensor fabrication—hydrophobic barriers formation; (B) photograph of a positive result when using a paper-based sensor with WO3 nanoprobes in detection of EAB (The Geobacter sulfurreducens cells appears yellow in online version). Source: Adapted from A.C. Marques, L. Santos, M.N. Costa, J.M. Dantas, P. Duarte, A. Gonc¸alves et al., Office paper platform for bioelectrochromic detection of electrochemically active bacteria using tungsten trioxide nanoprobes, Sci. Rep. 5(2015) 9910, with permission of Springer Nature.

affect the amount of ultraviolet and/or infrared radiation allowed to pass through the material. As the amount of infrared radiation increases, the amount of diffracted radiation also increases due to the increase in temperature. At a critical temperature, TC, the thermochromic material undergoes a structural transformation below which the material is semiconducting and relatively nonabsorbing in the infrared region,

Chromogenic applications

119

Figure 4.12 Colorimetric metal oxide sensors on paper substrate, made from TiO2, ZnO, ZrO2, and F2O3 nanoparticles, when detecting the phenolic antioxidant GA. Source: Adapted from E. Sharpe, R. Bradley, T. Frasco, D. Jayathilaka, A. Marsh, and S. Andreescu, Metal oxide based multisensor array and portable database for field analysis of antioxidants, Sensors Actuators B Chem. 193(2014) 552
562, with permission of Elsevier.

and above TC the material is metallic, reflecting the infrared radiation [106,108]. In Fig. 4.13 is a schematic of the thermochromic mechanism of a coated surface. For metal oxide materials this color change is observed with an increase/ decrease in temperature, and is most often due to changes in crystalline phase or in the ligand geometry and that undergo phase transition or exhibit charge-transfer bands near the visible or infrared region [109]. The most commonly used and studied metal oxide materials with thermochromic properties are vanadium oxides (VO2, V2O3, or V2O5) and titanium oxide, Ti2O3 [14,107,110
113]. Vanadium pentoxide, V2O5, undergoes a semiconductor/metal phase transition at 257 C (this particular oxide also exhibits electrochromic properties). Below TC it presents an orthorhombic structure, composed of corner and edge sharing VO6 octahedra. On the other hand, V2O3 at room temperature presents a corundum structure with rhombohedral symmetry. The phase transition occurs at a very low temperature of 2123 C, going from monoclinic phase to the rhombohedral metallic phase. Vanadium dioxide (VO2) exhibits a thermochromic phase transition at a temperature that is close to room temperature, with a TC value around 68 C [10,108]. The phase transition occurs from a most stable monoclinic phase (characteristic of temperatures below TC) to a tetragonal rutile phase (characteristic of temperatures above TC). This phase transition is accompanied by a modification of electrical parameters, changing from a semiconductor behavior (low conductivity) to a metallic behavior (high conductivity) [14,107,114]. In Fig. 4.14 it is possible to observe the crystallographic structures of tetragonal rutile and monoclinic phases of VO2, characteristic of high- and low-temperature phases, respectively. The tetragonal rutile phase belongs to the P42/mm space group while the monoclinic phase belongs to the P21/c space group [115]. Nevertheless, the value of TC 5 68 C is considered to be too high for applications in buildings or cars, restricting its energy-saving efficiency. Some techniques can be employed to overcome this disadvantage: it is possible to replace some vanadium atoms by higher valence ions like tungsten, W61, molybdenum, Mo51,

Table 4.2 State-of-the-art of chromogenic materials applied to electrochromic devices and systems Author

Material

Morphology

Substrate

Production method

Colouration efficiency (cm2/C)

Kadam et al.

WO3

Glass

Hydrothermal

135.18

2.5

[48]

Li et al. Jiao et al. Zhang et al. Poongodi et al. Denayer et al. Hoseinzadeh et al. Li et al. Dulgerbaki et al. Patel et al.

WO3 WO3 WO3 WO3

Nanostructured film Nest-like Plate-like Nanowires Nanoflake

Glass Glass Glass Glass

Hydrothermal Hydrothermal Hydrothermal Electrodeposition

126.34 112.7 102.8 154.93

26 4.3 7.6 2.87

[63] [64] [65] [68]

WO3

Mesoporous

Glass

Spray-pyrolysis

50.6

6.4

[69]

WO3/Ag

Nanostructured film Mesoporous Nanofibers

Glass

Chemical bath

74.2

5.3

[66]

Glass Glass

Sol-gel Electrospinning

50 64.58

6


[72] [74]

Glas

57.7




[78]

Deshpande et al. Dinh et al.

WO3

42




[79]

TiO2

Glass

Thermal evaporation Chemical Vapour Deposition Doctor blade

33.7

Tong et al. Yao et al. Dalavi et al.

V2O5 Nb2O5 NiO

Glass Glass Glass

Anodic deposition Anodization Chemical bath

32 47 95

WO3 WO3 WO3

Nanostructured film Nanostructured film Nanostructured film nanofibre Nanoporouse Nanoporouse network

Glass

Response time (s)

References

2

[45]

8.9 13 3.5

[88] [36] [87]

Pereira et al.

NiO NiO/ TiO2 MnO2

Nanostructured film Nanostructured film Nanosheets

Bo et al.

Chen et al.

RhO2 WO3 WO3 V2O5 WO3

Nanoflowers Nanosheets Nanoparticle Nanoparticles Nanobundles

Glass PET PET PET PET

Lin et al.

NiO

Nanostructured film

PET

Sakai et al. Wang et al. Liang et al. Costa et al.

Glass

e-beam evaporation

55

3.6

[28]

Glass

Hydrothermal

147.6

3.8

[85]

Glass

Layer-by-layer assembly Sol-gel Exfoliation Inkjet printing Inkjet printing Electrophoretic deposition Sputtering

36.7




[40]

29.1 120.9 133 42 89.8


9.7 6 200 22

[41] [59] [95] [96] [98]




[100]

67

122

Metal Oxide Nanostructures

Figure 4.13 Schematic of the mechanism of a thermochromic material when coating a surface for heat protection.

Figure 4.14 Crystallographic structures of the (A) high-temperature (T . TC) tetragonal rutile phase and (B) the low-temperature (T , TC) monoclinic phase of VO2. Source: Adapted from Y. Wu, L. Fan, W. Huang, S. Chen, S. Chen, F. Chen, et al., Depressed transition temperature of WxV1 2 xO2 : mechanistic insights from the X-ray absorption fine structure (XAFS) spectroscopy, Phys. Chem. Chem. Phys. 16(2014) 17705, with permission of the Royal Society of Chemistry.

niobium, Nb51, or rhenium, Re51 [108,115]. Dopants incorporated into VO2 lattice allow decreasing or increasing of thermochromic switching temperature (depending on the desired application). Tungsten doping has been proven to be the one with the greatest temperature percentage decrease. The effect on TC of vanadium oxide may depend on different factors, such as dopant ion size, charge, and electron carrier density. Metal ions

Chromogenic applications

123

with atomic radii larger than V41 ions or that create V51 defects, like tungsten, niobium, and titanium can decrease the TC to around 25 C [107]. Gonc¸alves et al. were able to reduce the TC from 68 C to 56 C by doping vanadium with tungsten oxide (VaWO) [14]. By co-doping VO2 with manganese (Mg) and tungsten (W), Wang et al. [116] were also able to reduce the TC to 30 C. On the other hand, the introduction of trivalent cations (lower valence ions) like chromium, Cr31, aluminum, Al31, and gallium, Ga31, will increase the TC [115,117]. Some authors have noted that the change of TC values due to donor/acceptor doping is related to changes in the electron density in the conduction band of VO2, which will affect the band structure and the activation energy, and thus change the TC. Also, other possible reason is that the doping may induce disorder on the VO2 host lattice [118
120]. Nevertheless, other metal oxide nanoparticles are also intensely studied to be used in applications that require a thermochromic transition, such as Ti2O3, F3O4, and Mo9O26 [117]. As has been said before, the thermochromic transition is usually due to changes in crystalline phase, but unlike vanadium oxides, Ti2O3 does not undergo a structural phase transition. In this case, the Ti31
Ti31 bonds become shorter as the temperature decreases [107].

4.3.1 Thermochromic applications The thermochromic effect offers a vast number of applications, such as thermometers (with fever indicator or design applications) and temperature sensors (for safety and warning) [107]. However, when applied as a smart coating they offer the most interesting application as they can be used to increase the energetic efficiency of a building, when applied to the roof or windows [114].

4.3.1.1 Smart thermochromic coatings Materials with thermochromic applications have in recent years been applied in architectural structures. Coating materials with a thermochromic thin or nanostructured film will change the amount of ultraviolet and/or infrared radiation allowed to pass through the surface [106]. Vanadium oxide is by far the most studied thermochromic metal oxide [14,16,110,112,116,121
125], presenting a phase transition at about 68 C. The use of nanoparticles, instead of metal oxide thin films, may bring the advantage of reducing the TC and widening hysteresis, which are two factors of interest for their applications in smart windows. Several authors have reported the synthesis of different VO2 nanostructures for applications in thermochromic systems. Whittaker et al. [122] have synthesized VO2 nanosheets that exhibit a well-defined metal insulator phase transition. These nanosheet structures were produced by a hydrothermal approach using bulk V2O5 as the starting material. Whittaker et al. were able to observe a phase transition at 67 C during the heating cycle, and at 60 C during the cooling cycle. Baik et al. have synthesized VO2 nanowires using

124

Metal Oxide Nanostructures

atmospheric-pressure and physical vapor deposition [123]. They were able to obtain TC ranging from 62 C to 70 C, depending on the width of the nanowire. It was possible to conclude that TC decreases with a decrease in VO2 nanowire width, probably due to interactions of the nanoparticles with the substrate. Qi et al. developed a new process for the synthesis of vanadium oxide and were able to obtain nanoparticles with a mixture of morphologies [124]. By pyrolyzing NH4VO3 from room temperature to 1300 C, they were able to obtain VO2 nanoparticles with two types of morphologies: spherical and pentagonal prisms. With these two types of morphologies a TC of 64 C and 68 C was achieved. With this studies, Qi et al. were able to conclude that the grain morphology highly affected the phase transition parameters. Tiles with thermochromic properties are being used for two purposes: for esthetic reasons, changing color to suit the environmental mood of the room; and also for energy and environmental safety. By absorbing infrared radiation, it will be a control in the buildings’ internal temperature, improving the energy efficiency of buildings (avoiding excessive energy consumption with cooling in summer). But, as said before, the use of VO2 in these cases is limited, due to the fact that the transition phase occurs usually at temperatures close to 68 C. A way to solve this problem is using dopants in VO2 crystallographic structures. Gonc¸alves et al. were able to synthesize VO2 nanoparticles assisted by microwave radiation [14]. These nanoparticles were sprayed on ceramic glassy tiles, forming a continuous and protecting nanostructured film. By incorporating WO3 in VO2, it was possible to reduce the transition temperature by 20 C, leading to a final transition temperature of 49 C, achieved with 3% WO3. In Fig. 4.15 the effect of this smart coating on tiles when heating in a hot plate can be observed. Gao et al. were able to dope vanadium dioxide with antimony (Sb). They produced quasi-spherical VO2 nanoparticles with uniform size doped with antimony (Sb31), with hexagonal structure by a hydrothermal method. Sb31 ions are larger in radius and lower in valence than V41 ions, which results in the introduction of extra oxygen vacancies during the nucleation and growth of VO2 nanoparticles [126]. Gao et al. were able to reduce the TC to 61 C. When doping with fluorine, F, it is possible to reduce the VO2 nanoparticle size. Dai et al. [127] produced F-doped VO2 by hydrothermal synthesis and were able to reduce the phase transition temperature to 35 C at 2.93% of F in VO2. The purpose of using fluorine as a dopant was not only to decrease the TC, but also to increase the VO2 film transmittance. According to Dai et al. “Widening the bandgap between V3d and O2p states could provide an effective strategy to achieve a colorless VO2 film.” Fluorine presents an electronegativity value of 3.90 eV, a higher value when compared with the electronegativity of oxygen, 3.44 eV. So, substituting oxygen by a fluorine atom would decrease the O2p state and increase the bandgap between the V3d and O2p states. Reports of doping VO2 with Mg and Mo have been presented by Shu-Yi Li et al. and Dengbing Li et al. [128,129]. The use of Mg as a dopant is of great potential as it can simultaneously decrease the TC and increase the luminous transmittance, while Mo doping allows the reduction of TC to values very close to room

Chromogenic applications

125

Figure 4.15 (A, B) Photographs of a pristine regular ceramic tile without and with a VO2 coating, respectively; (C) apparent temperature as function of the hot plate temperature, detected by an IR camera, for both tiles. The inset shows the optical and infrared photographs of both tiles, respectively. Source: Adapted from A. Gonc¸alves, J. Resende, A.C. Marques, J.V. Pinto, D. Nunes, A. Marie, et al., Smart optically active VO2 nanostructured layers applied in roof-type ceramic tiles for energy efficiency, Sol. Energy Mater. Sol. Cells 150(2016) 1
9, with permission of Elsevier.

temperature. Dengbing Li et al. studied the influence of Mo doping of VO2. The nanostructures were produced by hydrothermal method using 5.62 atom% of dopant. Doping with Mo may induce distortion of the VO6 octahedra when Mo atoms substitute the V atoms, making it easier to break the octahedral interconnections, thus reducing the activation energy of the formation of monoclinic VO2. With this process a TC of 24 C for 5.62 atom% of dopant was achieved [129]. Another material that can be used in coatings is V2O5, with a higher critical temperature when compared with VO2, above 250 C. Kumar et al. [130,131] applied V2O5 nanoparticles to coat silicon substrates. They were able to show that depending on the oxidation temperature (varying from 350 C to 550 C); the color change upon phase transition would be different. The coatings change from green to honey yellow, which is attributed to a decrease in oxygen efficiency. In Fig. 4.16A this effect can be observed. The thermochromic color changes upon heating from room

126

Metal Oxide Nanostructures

Figure 4.16 (A) Photographs of V2O5 coatings on silicon, obtained by oxidation at different temperatures; (B) surface scanning electron micrographs of the V2O5 oxidized films at different temperatures (350, 450, 550 C). Source: Adapted from S. Kumar, F. Maury, and N. Bahlawane, Tunable thermochromic properties of V2O5 coatings, Mater. Today Phys. 2(2017) 1
5, with permission of Elsevier.

temperature to 300 C. In Fig. 4.16B the nanoparticle morphology can be seen. At higher oxidation temperatures, a coalescence to form larger crystallites is observed. In Table 4.3 is presented a summary of the state-of-the-art relative to the use of metal oxide nanostructures in thermochromic applications, showing the critical temperature, TC, obtained for each material.

4.4

Photochromic nanomaterials

Photochromism is a chemical process in which a compound undergoes a reversible change between two states having separate absorption spectra, which mean different colors. Usually, the photochromic properties of a material are observed when it is under the influence of an electromagnetic radiation, usually UV or visible light, but it can be extended to all electromagnetic radiation spectra [3]. Fires et al. gave the following general definition of a photochromic behavior “. . .a reversible transformation of a single chemical species being induced in one or both directions by electromagnetic radiation between two states having distinguishable absorption spectra.” This reversible change in color, that may appear upon exposure to ultraviolet (UV) illumination [29], infrared light [3] or visible light, can be described by the following photochromic reaction: Aðλ1 Þ

!hν 1 Bðλ2 Þ ’Δ;hν 2

(4.4)

The starting material A will absorb electromagnetic radiation, with a certain wavelength λ1, and undergoes a transformation into material B, absorbing at a longer wavelength λ2. This reversible change in color may be due to alteration in physical properties and changes in refractive index, dielectric constant, oxidation and reduction potentials, and geometrical structure [20]. The back reaction can be thermally or photochemically activated [3].

Table 4.3 State-of-the-art of chromogenic materials applied to thermochromic systems Author Whittaker et al. Baik et al. Qi et al. Gonc¸alves et. al Gao et al. Dai et al. Li et al. Kumar et al.

Material

Morphology

Production method

Critical temperature, TC ( C)

References

VO2

Nanosheets

Hydrothermal from bulk V2O5

67

[122]

VO2 VO2

Nanowires Nanospheres/pentagonal prisms Round/facet nanoparticles Nanospheres Small nanoparticles Small nanoparticles nanostructures

Physical vapor deposition Pyrolysis of NH4VO3

62 , TC , 71 64
68

[123] [124]

Hydrothermal assisted by microwave radiation Hydrothermal Hydrothermal Hydrothermal Thermal oxidation

49

[14]

61 35 24 250

[126] [127] [129] [131]

W
VO2 Sb
VO2 F
VO2 Mo
VO2 V2O5

128

Metal Oxide Nanostructures

When a photochromic effect occurs, the change in color is not the only alteration observed. There are also changes in the material refractive index, dielectric constant, oxidation/reduction potentials, and geometrical structure [20]. Many metal oxide materials exhibit photochromic properties upon band gap excitation by electromagnetic radiation. Some examples are molybdenum oxide, MoO3, titanium dioxide, TiO2, vanadium pentoxide, V2O5, niobium pentoxide, Nb2O5, tungsten oxide, WO3, and zinc oxide, ZnO [12,17,20,132
135]. The use of nanoparticles in photochromic applications, with a high surface area and enhanced band gap, will cause an increase in the generation of more free or trapped charge carriers, that will result in a slow rate of electron-hole recombination, improving the photochromic efficiency [135].

4.4.1 Photochromic applications Photochromics are the most attractive and promising materials for a vast number of applications in many different fields. Some of the best known applications for photochromic materials are: in ophthalmics, especially in lenses for sunglasses, cosmetics, actinometrical, and heat measurements, optical memories for data storage, photo-optical switches, filters, self-developing photography, and many others. The use of this type of material in ophthalmic glasses, for example, relies on the fact that they change their opacity depending on the ultraviolet radiation conditions. Lenses will darken when exposed to strong sunlight and reverse back to being colorless in low light. Molybdenum oxide is one of the most studied metal oxides regarding application as a photochromic material, when absorbing UV radiation. The photochromism mechanism from MoO3 can be explained by the following equations [136,137]: MoO3 1 hλ ! MoO3  1 e2 1 h1

(4.5)

2h1 1 H2 O ! 2H1 1 O

(4.6)

V MoO3 1 xH1 1 xe2 ! Hx MoVI 12x Mox O3

(4.7)

When the film is irradiated with UV light (with excitation energy above the band gap, hν $ Eg), electrons and holes are formed (Eq. 4.5). The photogenerated electrons are then injected into the conduction band of the MoO3 and the holes react with the adsorbed H2O, originating protons (Eq. 4.6). These protons will then diffuse into the MoO3 lattice forming the hydrogen molybdenum bronze (Eq. 4.7). The result will be a change in the optical absorption of MoO3 that will change from colorless to blue [106]. The photochromic properties of MoO3 make it suitable for applications such as displays, erasable optical storage devices, and smart windows/glasses [134,138]. Tungsten oxide is another good candidate for photochromic applications [133,137]. Exposure of amorphous WO3 particles to ultraviolet radiation will

Chromogenic applications

129

induce a color change from colorless to an intense blue or brown. This color change is caused by the reduction of W61 atoms into W51 and/or W41, followed by the formation of tungsten bronzes (HxWO3) [106]. Fig. 4.17 is a schematic representation of the photochromic mechanism that occurs on WO3 nanoparticles, when exposed to UV radiation. Electrons (e2) and holes (h1) are formed and the protons (H1) necessary for coloration will be produced from the reaction of water absorbed on the surface or interior of the holes. Then the photogenerated electrons that were injected into the conductive band will react with H1 and WO3, producing hydrogen VI tungsten bronze (Hx WV x W12x O3 ) [138,139]. Tungsten oxide photochromism sensitivity is usually limited by its bandgap energy of 3.17 eV, corresponding to the near-UV range. For an efficient use of solar radiation or laser, which covers the entire wavelength range, it is important to extend the coloration response to the visible light region. This is possible by the insertion of a small amount of cations in the crystallographic structure (like K1 for WO3 and Li1 for MoO3) through cathodic polarization that will induce the formation of KxWO3 and this way a slight deformation in the oxide microstructure. Intermediate metastable trap states with energy levels lying within the bandgap region will be formed, which are accessible to visible light coloration [133]. A very interesting work has been reported by Bin Hui et al. where the photo responsivity of 2D sheet-like nanostructures of WO3, grown on wood surfaces, was studied [139]. Wood is widely used in buildings and in decoration, with the advantages of being lightweight, esthetic, and with renewable properties. The growth of these nanostructures in wood was performed by a low-temperature hydrothermal method, using ethanol as an inducer. It was observed that the prepared nanostructures react very intensely when exposed to UV radiation, with color change. Hocevar et al. reported studies on how to increase the coloration in photochromic devices [140]. They developed a system of WO3 layers that shown an increase in symmetry by titanium, Ti, incorporation into the WO3 crystalline matrix. The Ti ions induce ordering and increase the symmetry of WO6 octahedral in the WO3 crystalline structure. It was observed that there was a transformation of the cell symmetry from monoclinic through tetragonal and then to cubic, when the atomic

Figure 4.17 Schematic representation of the photochromic mechanism that occurs on WO3 nanoparticles when exposed to UV radiation.

130

Metal Oxide Nanostructures

ratio between W and Ti reached 100:10. Further increase in Ti results in a collapse of symmetry. The photochromic device was evaluated under solar illumination with a power of 1000 W/cm2, showing a high coloration contrast and fast kinetics, for the cubic structure layers. Tungsten oxide/methylcellulose composite films were produced by Yamazaki et al. [138]. The use of cellulose brings the advantage of being a biodegradable natural polymer and allows forming films into a substrate. Therefore, Yamazaki et al. were able to produce cellulosic composite films with tungsten oxide nanoparticles incorporated that presented a reversible color: colorless in the dark, to dark blue under UV irradiation. These composites are expected to be applied in recyclable display media and in detachable films (or labels) for glass windows (where the light transmission changes with sunlight—exhibiting a dark blue when exposed to sunlight in the summer and turning to transparent during the night—with it not being necessary to apply an external voltage). By combining two metal oxide nanostructures, WO3 and MoO3, Yuehong Song et al. produced hollow microspheres with enhanced photochromic properties, when compared with MoO3 or WO3 crystals with single composition [137]. In Fig. 4.18 the photochromic response of the different nanoparticles produced can be observed. Sample S1 has a greater concentration of MoO3 and less of WO3 and sample S3 has a greater concentration of WO3 and less of MoO3. Yuehong Song et al. observed that the sample with a greater concentration of MoO3 presented a better coloration enhancement. This good photochromic performance can be attributed to the intervalence
charge transfer among Mo, W, and O atoms [137,141]. Titanium dioxide is a prominent photochromic material, with bandgap energy of 3.2 eV, long-term stability against photo and chemical corrosion, and with biological and chemical inerts. Songara et al. reported on the photochromic properties of vanadium-doped TiO2 nanoparticles [135]. The doped films showed a reversible color change from beige (not exposed to UV light) to brownish violet, when exposed to UV light, while for undoped samples no change was observed. The metal ion of V will influence the photo-reactivity of TiO2 by acting as an electron/ or hole trap center within the band gap, altering the electron/hole pair recombination rate process. Therefore, V doping will create energy levels below the conduction band edge of TiO2 nanoparticles and will act as trap centers for photogenerated electrons. When V-TiO2 nanostructures are exposed to UV light, pairs of electrons/ holes are photogenerated and the excited electrons will be trapped at V51 trap centers, and V51 is reduced to the V41 state. Color change is then observed from beige to brownish-violet [135,142]. Naoi et al. combined silver nanoparticles with TiO2 nonporous films and produced a material that exhibited multicolor photochromism [143]. The nanoporous films were prepared by photoelectrochemical reduction of Ag1 to Ag under UV light. The color of Ag-TiO2 nanoporous film presents a gray coloration that changes to the color of the incident light, returning to the original state under UV light. As described before, the development of photochromic glasses is an area of major importance. The shorter wavelengths emitted by the Sun are more energetic and have great potential for harm to the human eye. Although small amounts of UV

Chromogenic applications

131

Figure 4.18 Photographs of Mo/W oxide samples (S1 to S3) and pure MoO3 and WO3 samples, before and after coloration. Source: Adapted from Y. Song, J. Zhao, Y. Zhao, and Z. Huang, Aqueous synthesis and photochromic study of Mo/W oxide hollow microspheres, RSC Adv. 6(2016) 99898
99904, with permission of Royal Society of Chemistry.

radiation are essential for the production of vitamin D, overexposure to this radiation may cause eye problems. Photochromic glasses have the ability to adjust their transmittance according to the intensity of the solar irradiation, depending on the wavelength. El-Zaiat et al. studied the effect of UV radiation on photochromic glasses doped with metal oxide nanoparticles, like TiO2, CoO, and Cr2O3, that were prepared using the melt quench technique [144]. They were able to increase the glass reflectance after exposure to the UV source in 44% when using CoO nanoparticles. Also, Mirhadi et al. [145] showed how doping sodium silicate glasses with chromium oxide changes the UV light absorption. Transmission of light though the glass is reduced due to ligand field and charge transfer mechanisms, leading also to the formation of new crystallographic phases. The UV-Vis spectrum showed several absorption peaks, which are responsible for the development of color in glass.

132

4.5

Metal Oxide Nanostructures

Gasochromic nanomaterials

Those materials that change color as a response to chemical changes or reactions are called chemochromic materials. Within this group of chromogenic materials exists the gasochromics [10] that involve a color change under exposure to a certain gas or atmosphere, possessing the capacity to return to the initial state once the cause which led to the color change is removed [146]. Examples of this type of metal oxide materials are SnO2, WO3, MoO3, and V2O5, that present strong colorations when exposure to hydrogen gas [132,147].

4.5.1 Gasochromic applications 4.5.1.1 Sensors and biosensors The group of gasochromic materials is the mostly studied type of chemochromism and is mainly used in sensors and biosensors. An important application of MoO3and WO3-based gasochromic devices is in the detection of hydrogen leakage, changing color from transparent to a dark blue. Regarding molybdenum oxide, the hydrogen atoms will intercalate into MoO3, forming HxMoO3, which decomposes into MoO2 1 1/2 H2O with a fast reduction of Mo61 into Mo51 states and slow and simultaneous formation of Mo41 states [147]. Oxygen has a larger electronegativity when compared to hydrogen. Therefore, most of the binary d-metal oxides are more stable than the corresponding hybrids. When exposed to gas mixtures with hydrogen, most transition metals form binary oxides. Hydrogen can intercalate into metal oxides acting as a donor or acceptor of electrons, inducing a variation in optical and electrical properties of the material, by 61 forming molybdenum bronze, Hx Mo51 x Mo12x O3 . Hydrogen atoms occupy sites in the van der Waals gaps between double layers of MoO6 octahedral as well as interlayer sites. This results in a small increase in the cell volume and distortion of the lattice changing the overall crystal symmetry from orthorhombic to monoclinic [141,147] (Fig. 4.19). As described earlier, WO3 is also a metal oxide with gasochromic properties. Reducing gases, such as H2, H2S, and NH3, will increase the surface conductivity of WO3 by creating oxygen vacancies and will result in color changes. Oxidizing gases, such as O3, NO2, and also CO2, will reduce the number of vacancies, resulting in a more transparent material, with a reduction in conductivity [10]. Ranjbar et al. produced WO3 by laser ablation activated by Pd and/or a layer that is optically switchable from a bleach state (colorless) to a dark blue state, when exposed to H2 gas [148,149]. This process is completely reversible by replacing the H2 by O2. The color change is attributed to H2 intercalation with a WO3 lattice that promotes the formation of small polarons as quasiparticle states [150]. Hydrogen can be used as a clean fuel in transports and in household applications, being a colorless, odorless, and tasteless gas. It burns cleanly without releasing

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Figure 4.19 (A) Simplified crystal structure of MoO3 and hydrogen bronze; (B) energy bands diagram. Source: Adapted from A. Borgschulte, O. Sambalova, R. Delmelle, S. Jenatsch, R. Hany, and F. Nu¨esch, Hydrogen reduction of molybdenum oxide at room temperature, Sci. Rep. 7 (2017) 40761, with permission of Springer Nature.

hazardous gases or greenhouse gases. A chemochromic WO3/H2 sensor can be used for selection of specific mutant green algae that produces H2 gas while tolerating high levels of O2 presence [151]. One example is the Chlamydomonas that are adapted to anaerobiosis in the dark and when illuminated they produce H2 which is detected by a Pd/WO3 system which turns blue [151]. The selection of this algae will allow the development of biohydrogen production under atmospheric levels of O2 [152].

4.6

Magnetochromic nanomaterials

Magnetochromic nanomaterials possess the property of changing color when an external field is applied. The spin-induced electric polarization can be magnetically controlled, creating a reorganization of the magnetic particles, and being a reversible process [10]. The observed color will depend on the particle size and the intensity of the applied magnetic field [153]. Under an external magnetic field, the magnetic nanoparticles will form a longrange ordered structure. Due to the chain-like distribution along the direction of the field, the nanoparticles can be viewed as a one-dimensional structure. By increasing the magnetic field intensity, the magnetic nanoparticles would become closer to one another, and at the same time, the magnetic attraction between the nanoparticles, the repulsion between solvent shells, and the electrostatic repulsion of the Stern layer will balance each other. Only when all these forces are in equilibrium state, can the magnetic nanoparticles become an ordered structure, then being visible due to the phenomenon of magnetochromism [154]. The nanoparticle spacing dictates the wavelength of the light reflected by the material, according to Braggs law: mλ 5 2ndsinθ

(4.8)

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Metal Oxide Nanostructures

where m is the order of scattering, λ is the diffraction wavelength, n is the refraction index of water, d in the lattice plane spacing, and θ is the Bragg angle (the angle between the incident light and the normal diffraction plane). When a stronger field is applied, reflections with a shorter wavelength will be observed, and by using Braggs law it is possible to make an estimation of the distance between adjacent nanoparticles along the direction of the magnetic field [154]. Using a magnetic field as an external stimulus may improve the spectral tenability and the response rate of materials in chromogenic applications, and presents a facile integration into the existing photonic systems [155].

4.6.1 Magnetochromic applications The most studied metal oxide nanoparticle with magnetochromic application is iron oxide, Fe3O4 [153,156
158]. This material can be used in applications, such as magnetic recording media, ferrofluid, and for biomedical purposes [159
161], and is considered to be a new platform of novel optoelectronic devices, sensors, and color displays [155,162,163]. Zhuang et al. were able to synthesize Fe3O4 nanosheets that can form onedimensional photonic crystals under an external magnetic field [164,165]. These magnetic nanoparticles are able to diffract visible light and display a variety of colors with changing intensity of the magnetic field (Fig. 4.20). Wang et al. also observed the same effect [166] when using Fe3O4 monodisperse nanoclusters in water. As can be seen in Fig. 4.20D, by increasing the magnetic field intensity, the diffraction color changes from red to blue. As explained by Wang, the shift to blue is due to a decrease in the interparticle distance as the magnetic field increases, and as can be explained by Braggs law (see Eq. 4.8), the shorter the distance between the interparticles, the shorter the observed wavelengths. In Fig. 4.20 several examples of magnetochromic effect when using suspensions with Fe3O4 nanoparticles and by applying an external magnetic field can be observed. The behavior observed when using Fe3O4 monodisperse nanoclusters in water is very useful for applying this material in displays. By applying an external magnetic field to these suspensions, assembly of 1D periodic photonic chains is induced, being a direct result of dipole
dipole interactions between magnetic particles. If the nanoparticles have uniform size, the interparticle space becomes equivalent for all neighboring pairs and visible light will be diffracted with a certain color, depending on the interparticle space [167]. The optical response of these magnetochromic materials to an external magnetic stimulus is fast, being a critical and important feature for potential applications in magnetic responsive color displays. Zhuan et al. developed a chip where they integrated an Fe3O4 suspension into microwells, sealed with a PDMS (polydimethylsiloxane) microchannel to form a microfluidic system [165]. By applying an external magnetic field along the microchannel, a change in the microwells is

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135

Figure 4.20 Photographs of Fe3O4 nanosheet colloidal solution formed in response to external magnetic field: (A) side and (B) top view (Source: adapted from L. Zhuang, W. Zhang, Y. Zhao, H. Shen, H. Lin, and J. Liang, Preparation and characterization of Fe3O4 particles with novel nanosheets morphology and magnetochromatic property by a modified solvothermal method, Sci. Rep. 5(2015) 9320), with permission of Springer Nature. (C, D) Color changes as a response to an increase in the magnetic field intensity. Source: Adapted from L. Zhuang, Y. Zhao, H. Zhong, J. Liang, J. Zhou, and H. Shen, Hydrophilic magnetochromatic nanoparticles with controllable sizes and super-high magnetization for visualization of magnetic field intensity, Sci. Rep. 5(2015) 17063; W. Wang, B. Tang, B. Ju, S. Zhang, Y. Liu, Q.Y. Zhang, et al., Size-controlled synthesis of water-dispersible superparamagnetic Fe3O4 nanoclusters and their magnetic responsiveness, RSC Adv. 5(2015) 75292
75299, with permission of the Royal Society of Chemistry and Springer Nature.

observed (Fig. 4.21A, B). Also, Ge et al. were able to produce a small display using a suspension of Fe3O4 encapsulated in PDMS film (Fig. 4.21C, D). By applying an external magnetic field, color change is induced and some letters appear [153]. Another possible and important application of Fe3O4 nanoparticles acting as a magnetochromic material is described by Hu et al. [157]. Hu et al. describe how a magnetochromic effect can be applied in anticounterfeiting. By using Fe3O4 nanoparticles of different sizes and by applying an external magnetic field, these nanoparticles will present different colors depending on their size. Also, Ge et al. [153] describe how it is possible to tune the diffraction wavelength by controlling the overall size of the nanoparticles: diffractions in red can be obtained with relatively large particles (.150 nm), in the blue region they can be obtained with smaller particles (,100 nm), while the medium-sized nanoparticles (100
150 nm) may present a diffraction wavelength tunable from blue to green, yellow, and red. In Fig. 4.22 the color change when the magnetic field of constant intensity is applied to Fe3O4 nanoclusters with different particle sizes can be observed [157].

Figure 4.21 (A) Photographs of a chip with an Fe3O4 suspension integrated into microwells before and after applying an external magnetic field (the microchannel is highlighted with red ink); (B) colors of single sealed microsquares under different intensities of magnetic field (the colorintensity bar is shown on the right) (Source: adapted from L. Zhuang, Y. Zhao, H. Zhong, J. Liang, J. Zhou, and H. Shen, Hydrophilic magnetochromatic nanoparticles with controllable sizes and super-high magnetization for visualization of magnetic field intensity, Sci. Rep. 5 (2015) 17063), with permission of Springer Nature. (C, D) Schematic of fabrication procedure of a field-responsive PDMS film embedded with Fe3O4 suspension and (E) photograph of a patterned PDMS film embedded with Fe3O4 suspension displaying letters after applications of an external magnetic. Source: Adapted from J. Ge, S. Kwon, and Y. Yin, Niche applications of magnetically responsive photonic structures, J. Mater. Chem. 20(2010) 5777
5784, with permission of the Royal Society of Chemistry.

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Figure 4.22 Photograph of suspensions with Fe3O4 nanoparticles, under the same magnetic field of 0.1 T, and with different particles size of: (A) 200 nm, (B) 140 nm, (C) 125 nm, (D) 200 nm 1 125 nm, (E) 200 nm 1 140 nm, and (F) 140 nm 1 125 nm. (G) Photograph of an anticounterfeiting label without and with an external magnetic field applied, showing the change in color. Source: Adapted from H. Hu, Q.-W. Chen, J. Tang, X.-Y. Hu, and X.-H. Zhou, Photonic anti-counterfeiting using structural colors derived from magnetic-responsive photonic crystals with double photonic bandgap heterostructures, J. Mater. Chem. 22(2012) 11048, with permission of the Royal Society of Chemistry.

4.7

Conclusions

In this chapter, a review has been presented on nanostructured metal oxides applied to different chromogenic systems. The use of nanostructured metal oxide nanoparticles in different chromogenic applications brings numerous advantages, such as an enhancement in the chromogenic effect, chromogenic transitions, and stability. The use of “smart materials” in a variety of applications allows improving the efficiency of some devices. By using electrochromic glass in a car, it is possible to keep the interior cooler, increasing the fuel economy as well as the comfort of passengers, while protecting the vehicle components from exposure to heat and solar radiation [52]. The most studied electrochromic material is WO3 (with cathodic coloration) and NiO (with anodic coloration). When compared to other metal oxide nanostructures, WO3 presents a higher coloration efficiency (refer to Table 4.2). On the other hand, the use of electrochromic materials in biosensing of bacteria in cellulosic-based substrates allows the production of biocompatible and biodegradable devices, suitable for low-cost and disposable applications. Thermochromic metal oxide nanoparticles are now being applied in architectural structures with coating materials that change the amount of ultraviolet and/or infrared radiation allowed to pass through the surface. Vanadium oxide is by far the most studied thermochromic metal oxide, presenting a phase transition at about 68 C or close to 50 C when doped with other materials, like W. By doping with F

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and Mo, it is possible to achieve phase transition temperatures close to room temperature values (24 C). The photochromic materials also have enormous potential to be applied in smart windows, for energy consumption reduction, with the advantage of it not being necessary to apply an external voltage for color change. Most of the photochromic metal oxides change their color with UV irradiation, making them suitable to be integrated in ophthalmic glasses or sunglasses. Tungsten oxide is also largely used in gasochromic colorimetric sensors. These nanostructures are able to detect small levels of hydrogen concentrations, which allow the detection of certain types of algae that produce hydrogen. The collected hydrogen can be used as a clean fuel in transport and in household applications, being a colorless, odorless, and tasteless gas. Finally, the magnetochromic metal oxide materials were also addressed in this chapter, being presented as a new area of development that takes advantage of using the variation of applied external field intensity, where this field is responsible for color changing of magnetic nanoparticles, like Fe2O3. This type of nanostructure is most useful to be used in displays.

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