Photochromism refers to the reversible change of a substance between two physical or chemical states, at least one of which is caused by light radiation [10]. The change process can be represented by Equation 17.1.
Its basic characteristics are: 1) T1 and T2 can exist stably under certain conditions; 2) The color difference between T1 and T2 is significant; 3) The change between T1 and T2 is reversible. Due to these excellent properties of the materials, they can be widely applied in display materials, sensors, and decorative materials. To truly achieve practical application, the following conditions must be met: a) T1 and T2 must have sufficient stability; b) T1 and T2 must have a sufficiently long cycle life; c) The absorption band is in the visible light region; d) The response speed of T1 and T2 should be fast; e) The sensitivity should be high; f) The preparation process should be simple and the cost should be low. However, so far, no practical materials that fully meet the above conditions have been developed.
Photochromic materials can be divided into two major categories: inorganic photochromic materials and organic photochromic materials. Inorganic photochromic materials mainly concentrate in the transition metal oxide system, such as MoO11~20], WO[21~32], V0,[33~36], NiO 37~39], RhpO, 401, Nb0,[4,42], etc., while organic photochromic materials mainly include phthalocyanine [143], spiropyran! etc. This section mainly introduces the photochromism of inorganic materials.
17.2.2 Photochromism of MoO3, WO
17.2.2 Photochromism of MoO, WO
WO and MoO are two metal oxides that are very competitive in the photochromic systems. Usually, MoO is a white powder, belonging to the hexagonal crystal system; WO is a yellow powder, belonging to the monoclinic crystal system. Both WO and MoO3 are n-type semiconductors, with band gaps E of 3.25 evi45] and 3.15 evi46] respectively. Under normal circumstances, only light with energy above ER can induce the photochromic reaction.
Color change property research
In a vacuum chamber, MoO and WO crystals were irradiated with 253.7nm ultraviolet light emitted by a mercury lamp. Both turned blue. XPSi47] research showed that there were low-valent tungsten and molybdenum in the color-changing substances. When MoO was irradiated in a hydrogen atmosphere, a blue substance containing +4, +5, +6 valence states was generated; if mercury vapor was added to the reaction atmosphere, more Mo* would be generated. For the WO system, there was no significant difference in photochromism in hydrogen and Hg/H atmospheres compared to pure vacuum. It should be noted that WO and MoO3 before irradiation were not compounds with a completely stoichiometric ratio, but had certain oxygen vacancies. It was found that when WO and MoO were made into amorphous films, the color change efficiency, color change depth, and response speed were all significantly improved.
There are many methods for preparing films, roughly divided into physical methods and chemical methods. Physical methods include vacuum evaporation method [12,24,28,48,49], electron beam evaporation method [50,51], etc.; chemical methods include chemical vapor deposition method [52531, sol-gel method (S4], reactive radio frequency sputtering method [5s], electrolytic deposition method [ 16,56,57], rotational coating method (58] and ion exchange method (9] etc. The conditions and raw materials of various preparation methods are different, and the microstructure and state of the films obtained are also different, and their color change performance is also different.
The microstructure of amorphous films prepared by different methods is different from that of powder crystals. The microstructure of the films was studied [60-62], with the aim of finding a universal structural expression that could represent all the films. However, up to now, the microstructure of the films has not been fully understood, let alone a universal structural expression has been found. Nevertheless, several models have been established to describe the structures of amorphous WO and MoO films. Shiojiri et al. analyzed the data from electron microscopy and believed that amorphous WO (a-WO) is composed of many microcrystals, and the arrangement of the microcrystals is the same as that in powder crystals. Arnoldssen [61] believed that the a-WO film is a kind of molecular-like structure, with trimer W,O as the basic unit, and it is combined with each other by weak water bridges, hydrogen bonds and van der Waals forces. The XRD (X-ray diffraction) [63] results suggest that the evaporated WO film is an unordered network structure composed of WO octahedra connected at the corners; by simulating the conditions of vacuum evaporation to prepare WO and MoO using ion mass spectrometry, (M,Oy)* form of positive ion clusters and (MO;) form of negative ion clusters (x = 1-4) were obtained. Among them, regardless of whether it is a positive ion or a negative ion, the abundance of clusters with x = 3, 4 is the largest. It can be considered that the evaporated films mainly exist in the form of MoO or WO trimer and tetramer. From another perspective, the degree of order of the films is around 3-4 [62]. Figure 17.1 is the mass spectrum of MoO3.
Since the evaporated films are in a high vacuum condition, the oxygen partial pressure is very low. When WO and MoO are heated at high temperatures, they are easily deoxygenated and form non-chemically balanced deficient oxidation compounds [11, 64, 65], such as u wO -, or MoO3-r. x is generally between 0 and 0.3. Moreover, it is impossible to ensure absolute dryness in the vacuum chamber, or when the film is taken out of the vacuum chamber and exposed to the atmosphere, some water is inevitably adsorbed on the surface or inside the film. Some people suggest that the structure of the film can be denoted as MoO5-, ·nHO or MoO3 – .(OH). The water content in the film varies with the humidity in the air. Other methods for preparing films also contain water molecules [30, 54, 55, 59]. Some studies [23] have shown that each unit of WO in the wO, film contains 1.74 water molecules. The water in the film can be divided into two types: one is chemically adsorbed water, which combines with MoO or WO through chemical bonding and is difficult to lose; the other is physically adsorbed water, which is generally weakly bound at the junction between particles. Zeller and Bey-zler [49] discovered two desorption peaks by differential thermal analysis between 150°C and 370°C, which belong to the desorption of chemically adsorbed water and the thermal desorption of physically adsorbed water. In the FTIR spectrum of the film, the change in the intensity of hydroxyl stretching vibration can monitor the change in water in the film [130]. After the film was annealed at 150°C for 1 hour, the intensity of OH decreased by 47%, and after annealing at 300°C for 1 hour, the intensity of OH decreased by 85%. This can roughly calculate that the ratio of chemically adsorbed water to physically adsorbed water is 1:1. If the WO film is immersed in a solution containing 1 mol/L LiCIO4 for 10 days, an ion exchange reaction as shown in Equation 17.2 will occur.
The photochromic performance of the ion-exchanged film is almost the same as that before the exchange. It can be considered that the chemical structure water has little effect on photochromism [66, 67], while the reduction of the physical structure water will reduce the photochromic performance. This indicates that the physical structure water participates in the color-changing process and plays an important role in photochromism. It can be summarized into the following two aspects of functions [68]: a) It is the source of photochromic ions; b) Acts as a bridge for the ion transmission within the membrane. There are OH and HO chains in the membrane, enabling hydrogen ions to be rapidly transmitted within the membrane, as shown in Figure 17.2.
· Nevertheless, the presence of water in the membrane also has its drawbacks: under the catalysis of light, it promotes the photolysis of the membrane.
As mentioned earlier, there are different viewpoints regarding the initial microscopic structure of the membrane, and the microscopic structure after photochromic transformation has not been completely clarified. That is, where the injected hydrogen ions are located and what kind of changes occur in the crystal lattice of the membrane. Regarding the injection position of hydrogen ions, there are different viewpoints. Some believe it is at the grain boundary [15], while others believe it is in the lattice gap [15]. A relatively reasonable explanation is that hydrogen ions enter the crystal lattice and combine with the M-O bond. The freshly deposited MoO3 membrane is amorphous, and in Raman spectroscopy, it presents three broad peaks at 951 cm-1, 863 cm-1, and 700 cm-1 corresponding to the stretching vibrations of Mo-O(3), Mo-Ozy, and Mo-O) [0]. As the light exposure time increases, the Raman vibration peaks also change accordingly. Mo-O) and Mo-Oz shift to the left, while Mo-Ozy shifts to the right. When ABS = 0.26, △vMo—O(1) = -14 cm-1, △Mo = -O(2) = -10 cm-1, Mo = O(3) = 6 cm-1. According to the relationship between the vibration peak position and bond strength, a left shift of the Raman peak indicates a decrease in vibration intensity, and a right shift indicates an increase in vibration intensity. The change in the Raman spectrum of MoO3 is due to the injection of hydrogen protons. Table 17.1 compares the bond strength and Raman peaks of MoO3 crystals, molybdenum bronze Ho.3MoO3, and H.6gMoO. [70-72], and it can be seen that during the photochromic transformation, the injected H* combines with Mo-Oz and occupies the lattice void position. Instead of destroying the Mo-O double bond as expected, it forms Mo—OH. As the coloration depth increases, the Raman peaks not only shift in position but also increase in peak width, indicating a decrease in the orderliness of the membrane and a reversible change during the coloring/decoloration process.
As mentioned above, MoO3 and WO3 are both n-type semiconductors, with band gaps of 3.25 eV and 3.15 eV respectively. Therefore, in general, only light with a wavelength greater than E can cause the coloration behavior of the membrane, that is, the excitation light wavelength cannot be greater than 380 nm, and for applications in the information display and recording materials field, it is necessary to match with current lasers (780 nm semiconductor laser, 632.8 nm He-Ne laser, or 514 nm/488 nm Ar* laser). This incompatibility undoubtedly limits its practical application. However, in 1992, this limitation was significantly improved. Yao Jianyan et al. were the first to report in the “Nature” journal that MoO has a coloration response to visible light radiation energy, thus shifting the coloration response wavelength of MoO to the visible light region, taking another important step on the practicalization path.
The MoO. film is prepared by vacuum evaporation, with a thickness of approximately 1000 nm. The substrate is a transparent conductive glass coated with ITO (InpO,-SnO2). After the membrane is colored by ultraviolet light exposure, it is irradiated with visible light, and no change occurs. However, if it is first subjected to weak cathode electrolysis coloring in a weak electrolyte solution of 0.1 mol/L LiCIO/propylene carbonate (PC, propylene carbonate) and then directly irradiated with visible light (wavelength ≥ 500 nm) in the air, the MoO film that was previously unresponsive to direct visible light irradiation becomes bluer. The changes in the absorption spectra before and after irradiation. Let the color change depth of the electrolytic pre-treatment be △ABS, = ABS – ABS., and the color change response caused by direct visible light radiation be △ABS2 = ABS. – ABS.. Then, the visible light color change efficiency α = AABSz / AABS, = 4. After the colored film is polarized at the anode in the electrolyte, its blue color will return to the initial state a. If the electrolytic pre-treatment is carried out again, the film still has the visible light color change effect. This cycle can be repeated many times, showing good reversibility. The application of visible light color change effect in information recording can be simply expressed.
In addition, after MoO, the visible light color change effect of WO was also discovered [73], enriching the family members of visible light color change, providing more evidence for revealing its color change mechanism, and providing selectivity for its practical application. Different from the visible light color change of MoO, the WO film deposited by vacuum evaporation undergoes electrolytic pre-treatment in a saturated KNO3/PC solution. The mechanism of visible light color change is still unclear and is currently in the exploration stage. During electrolytic pre-treatment, Li* ions or K* ions are injected into the film, generating molybdenum bronze Li, MoO or tungsten bronze K, WO, forming a metastable or stable trap energy level in the band gap. Thus, electrons in the valence band can be excited to the excited state under the irradiation of visible light, resulting in visible light color change. The WO does not show visible light color change when injected with Li* or Na* ions. When Li, Na ions are injected into MoO, there is a response to visible light irradiation. This may be caused by the difference in the lattice matching of the injected ions. The radii of Li, Na, and K* ions are quite different (0.039nm, 0.058nm, and 0.151nm respectively) [74], resulting in a large difference in trap energy levels, so it shows different responses to visible light irradiation. The deeper reason still needs further exploration.
In addition to the above visible light color change, there are also reports of another visible color change mode of WO [31]. This method is: first, deposit a layer of CdS nanocrystals on the substrate, and then deposit the WO film. Due to the absorption of 500nm light by CdS.