Effect of Dispersants on Photochromic Behavior of Tungsten Oxide Nanoparticles in Methylcellulose

Kinetic Studies of WO3-Based Photochromism in Polyvinyl Alcohol Film

Tungsten oxide (WO3) has been extensively studied for various photochromic applications. Blue coloration of WO3 is explained in terms of the intervalence charge transfer (IVCT) transition of electrons between W6+ and W5+. However, various absorption spectra with different shapes have been reported. Herein, a transparent film was prepared by drying aqueous solutions containing polyvinyl alcohol, WO3 nanoparticles and ethylene glycol (EG). For comparison, the photochromic behavior of an aqueous WO3 colloidal solution containing EG was also investigated. Under UV irradiation, a single intense peak was always observed at ca. 777 nm in the colloidal solution, but the absorption spectra of the film changed from a peak at 770 nm to two distinct peaks at 654 and 1003 nm. All absorption spectra observed with the film and the colloidal solution were deconvoluted into five peaks at 540, 640, 775, 984, and 1265 nm. Kinetic studies using the colloidal solution indicated that the coloration rates (r0) estimated at the deconvoluted peaks of 640, 775, and 984 nm followed the same rate law. On the other hand, in the case of the film, r0 evaluated at 640 or 984 nm was independent of the water amounts but increased proportionally to the EG amounts and the light intensity, although r0 at 775 nm significantly increased with the increasing amounts of water and EG. Raman and electron spin resonance spectroscopic observations of the film revealed that the photogenerated electrons migrated toward the terminal W═O moiety to accumulate and then a small anisotropic electron spin resonance signal appeared. Our study demonstrates that the absorption at 775 nm is due to IVCT between W6+ and W5+, which is stabilized with water in the bulk and the absorption peaks at 640 and 984 nm are attributable to IVCT on the WO3 surface.

Coupling Ti doping with oxygen vacancies in tungsten oxide for high-performance photochromism applications

A series of Ti-doped W₁₈O₄₉ samples were prepared using a convenient solvothermal route. Due to the synergistic effect of doped Ti and oxygen vacancies, the samples showed excellent visible-light photochromic properties. Their performances as light-printable rewritable paper and smart windows showed great application value and promotion value.

Functional Aerogels Composed of Regenerated Cellulose and Tungsten Oxide for UV Detection and Seawater Desalination

Functional aerogels composed of regenerated cellulose and tungsten oxide were fabricated by implanting tungsten-oxide nanodots into regenerated cellulose fiber. This superfast photochromic property benefitted from the small size and even distribution of tungsten oxide, which was caused by the confinement effect of the regenerated cellulose fiber. The composite was characterized using XRD and TEM to illustrate the successful loading of tungsten oxide. The composite turned from pale white to bright blue under ambient solar irradiation in five seconds. The evidence of solar absorption and electron paramagnetic resonance (EPR) demonstrated the fast photochromic nature of the composite and its mechanism. Furthermore, carbon fiber filled with preferential growth tungsten-oxide nanorods was obtained by annealing the photochromic composite in a N₂ atmosphere. This annealed product exhibited good absorption across the whole solar spectrum and revealed an excellent photothermal conversion performance. The water evaporation rate reached 1.75 kg m^-2 h^-1 under one sun illumination, which is 4.4 times higher than that of pure water. The photothermal conversion efficiency was 85%, which shows its potential application prospects in seawater desalination.

Photochromism of TiO2 Nanoparticles Doped with Molybdenum Ions: Neutral Color Change from Colorless to Black via Gray

Transition metal oxide nanoparticles have been extensively studied for the development of smart windows which are expected to be a promising technology to save energy in buildings. However, most of them turn blue under UV irradiation. Since the blue coloration affects the color of objects through the windows, the development of materials with a neutral color which hardly disturbs the view is more beneficial. In this work, we prepared a colorless-transparent TiO₂ colloidal solution containing Mo⁶⁺ ions (Mo-TiO₂), which turns black via gray in a nitrogen atmosphere under UV irradiation. An absorption peak was observed at 535 nm, which increased with an increase in the UV irradiation time and reached a constant value (Abs_max). The Abs_max value increased linearly with an increase in the Mo doping amount. We demonstrated that the photochromic behavior of the Mo-TiO₂ nanoparticles is completely different from that of pure MoO₃ or the mixture of MoO₃ and TiO₂. In addition, we performed a kinetic study to elucidate the mechanism and found that the coloration rate at the initial stage decreased with an increase in Mo doping amount. Based on the kinetic analysis, the following results are obtained: a color center is formed at a deeper energy level than the Mo dopant level; the number of the color center depends on the Mo doping amount, and the color center traps the photogenerated electrons more rapidly when it is isolated. The black color was bleached by purging the air in the solution. In particular, the gray state which is observed as a transient color is promising for the development of smart windows to shield the sunlight while allowing a clear and undistorted view.

Determination of W(V) in WO3 Photochromism Using Localized Surface Plasmon Resonance of Ag Nanoparticles

A reversible color change of WO₃ has been widely studied to develop new energy-saving technologies such as smart windows, rewritable paper, and information displays. A blue coloration arises from the intervalence charge transfer between W(VI) and W(V), which is partially formed by the reduction of WO₃ under UV light or an applied voltage. This means that WO₃ has a mixed-valence state of W(V) and W(VI) upon the reduction. However, despite many studies for various applications, how many W(V) atoms are formed and contribute to the intervalence charge transfer (IVCT) transition remains unclear because W(V) formed in WO₃ cannot be determined quantitatively. We determined the amount of the photogenerated W(V) in an aqueous WO₃ colloidal solution containing ethylene glycol (EG) by observing the localized surface plasmon resonance (LSPR) peaks of Ag nanoparticles which were produced by a redox reaction between W(V) and Ag⁺. EG acted as a hole scavenger to suppress the recombination between the photogenerated holes and electrons. First, we explored the reaction condition where only the IVCT transition was observed under UV irradiation, and then it decreased in response to the increase in the LSPR peak in the dark. Under such a condition, the absorbance at 775 nm (A₇₇₅) due to the IVCT transition was observed after the UV irradiation for 30 s, and the absorbance at 410 nm (A₄₁₀) due to the LSPR absorption was obtained when A₇₇₅ completely disappeared in the dark. Experiments were performed at various UV intensities to confirm a proportional relationship between A₇₇₅ and A₄₁₀. Electron spin resonance measurements revealed that A₇₇₅ was proportional to the amount of W(V). Furthermore, Ag nanoparticles were synthesized by a polyol reduction method to obtain the relationship between the LSPR peak intensity and the Ag⁺ concentration, which was consumed for the formation of Ag. On the basis of all of these relationships, A₇₇₅ of 1.669 corresponded to 2.53 × 10^-4 mol dm^-3 W(V), which was estimated to be only 0.21% of 0.12 mol dm^-3 WO₃ used in this study, and the molar absorption coefficient for the IVCT transition between W(V) and W(VI) was evaluated to be 6.85 × 10³ dm³ mol^-1 cm^-1.

UV Photochromism in Transition Metal Oxides and Hybrid Materials

Limited levels of UV exposure can be beneficial to the human body. However, the UV radiation present in the atmosphere can be damaging if levels of exposure exceed safe limits which depend on the individual the skin color. Hence, UV photochromic materials that respond to UV light by changing their color are powerful tools to sense radiation safety limits. Photochromic materials comprise either organic materials, inorganic transition metal oxides, or a hybrid combination of both. The photochromic behavior largely relies on charge transfer mechanisms and electronic band structures. These factors can be influenced by the structure and morphology, fabrication, composition, hybridization, and preparation of the photochromic materials, among others. Significant challenges are involved in realizing rapid photochromic change, which is repeatable, reversible with low fatigue, and behaving according to the desired application requirements. These challenges also relate to finding the right synergy between the photochromic materials used, the environment it is being used for, and the objectives that need to be achieved. In this review, the principles and applications of photochromic processes for transition metal oxides and hybrid materials, photocatalytic applications, and the outlook in the context of commercialized sensors in this field are presented.

Highly Efficient Photochromic Tungsten Oxide@PNIPAM Composite Spheres with a Fast Response

Coloration efficiency and a fast response are important in developing materials for optical switching. A novel, highly efficient photochromic tungsten oxide@poly(N-isopropylacrylamide) (PNIPAM) hybrid sphere is reported, whose colors can be rapidly converted between yellow and blue under different lights. The color change can be seen clearly even if the tungsten oxide content in the hybrid sphere is very low, exhibiting outstanding coloration efficiency of tungsten oxide. A photochromic mechanism is proposed in which the amide group in PNIPAM spheres participates in electron injection and the transition of valence states between W⁵⁺ and W⁶⁺ in the photochromic process. The interaction between tungsten oxide and PNIPAM plays a key role in enhancing the coloration efficiency of tungsten oxide and accelerating the switchable speed of color transformation, which is very useful in developing new photochromic materials. These hybrid spheres can be used in rewritable record displays and have wide potential applications in controlling energy transmittance in smart windows or in detecting UV light in optical sensors.

Facile synthesis of fluorescent tungsten oxide quantum dots for telomerase detection based on the inner filter effect

The traditional detection of telomerase activity is mainly based on the polymerase chain reaction (PCR), which has the disadvantages of being time-consuming and susceptible to interferences; thus, here, we propose a facile method for the fabrication of fluorescent tungsten oxide quantum dots (WO_x QDs) and employ them for telomerase activity sensing. It is found that the fluorescence of WO_x QDs can be significantly quenched by hemin based on the inner filter effect (IFE). However, in the presence of telomerase, the primer-DNA can be extended to generate repeating units of TTAGGG to form G-quadruplex and thus, hemin can be encapsulated to reduce its absorbance, resulting in decreased IFE and efficient fluorescence recovery of WO_x QDs. Based on the fluorescence changes of IFE between hemin and WO_x QDs, the telomerase activity within the range of 50-30 000 HeLa cells can be detected and the lowest detection amount can reach 17 cells. The method exhibits good versatility that can also be applied to telomerase detection in A549 and L929 cells. In addition, because of the good biocompatibility of the sensor, it can be used for the real-time monitoring of telomerase activity in living cells, thus showing great potential in tumor diagnosis and inhibitor drug screening.

Photoreductive BiOCl Ultrathin Nanosheets for Highly Efficient Photocatalytic Color Switching

The reversible photocatalytic color switching systems (PCSSs) driven by semiconductor nanoparticles have attracted considerable attention because of their wide applications. However, the developed semiconductor nanoparticles with photoreductive activity are mainly limited to TiO₂-based photocatalysts, which greatly hinder their broad applications. Here we report a cocapping ligand-assisted strategy for the development of photoreductive BiOCl ultrathin nanosheets with abundant oxygen vacancies. Both the cocapping ligands and oxygen vacancies in BiOCl ultrathin nanosheets act as sacrificial electron donors to efficiently scavenge the photogenerated holes, endowing the BiOCl ultrathin nanosheets high photoreductive activity and thus enabling the photocatalytic color switching of redox dyes, such as methylene blue (MB) and neutral red. By successfully integrating the BiOCl ultrathin nanosheet/MB/H₂O color switching system with poly(vinyl alcohol) hydrogel to fabricate a twistable gel film and simultaneously solving the dye-leaching issue of the gel film in a water environment, we further demonstrate its application in a colorimetric oxygen indicator for food packaging, exhibiting high sensitivity to monitor oxygen leakage by the naked eye. We believe the work opens a new avenue for designing photoreductive semiconductor nanomaterials to enrich the PCSSs and their applications.

Selective gas detection using Mn3O4/WO3 composites as a sensing layer

Pure WO₃ sensors and Mn₃O₄/WO₃ composite sensors with different Mn concentrations (1 atom %, 3 atom % and 5 atom %) were successfully prepared through a facile hydrothermal method. As gas sensing materials, their sensing performance at different temperatures was systematically investigated for gas detection. The devices displayed different sensing responses toward different gases at specific temperatures. The gas sensing performance of Mn₃O₄/WO₃ composites (especially at 3 atom % Mn) were far improved compared to sensors based on pure WO₃, where the improvement is related to the heterojunction formed between the two metal oxides. The sensor based on the Mn₃O₄/WO₃ composite with 3 atom % Mn showed a high selective response to hydrogen sulfide (H₂S), ammonia (NH₃) and carbon monoxide (CO) at working temperatures of 90 °C, 150 °C and 210 °C, respectively. The demonstrated superior selectivity opens the door for potential applications in gas recognition and detection.