Photo-controllable thermoelectric properties with reversibility and photo-thermoelectric effects of tungsten trioxide accompanied by its photochromic phenomenon

Abstract

The addition of photo-controllable properties to tungsten trioxide (WO3) is of interest for developing practical applications of WO3 as well as for interpreting such phenomena from scientific viewpoints. Here, a sputtered crystalline WO3 thin film generated thermoelectric power due to ultraviolet (UV) light-induced band-gap excitation and was accompanied by a photochromic reaction resulting from generating W5+ ions. The thermoelectric properties (electrical conductivity (σ) and Seebeck coefficient (S)) and coloration of WO3 could be reversibly switched by alternating the external stimulus between UV light irradiation and dark storage. After irradiating the film with UV light, σ increased, whereas the absolute value of S decreased, and the photochromic (coloration) reaction was detected. Notably, the opposite behavior was exhibited by WO3 after dark storage, and this reversible cycle could be repeated at least three times. Moreover, photo-thermoelectric effects (photo-conductive effect (photo-conductivity, σphoto) and photo-Seebeck effect (photo-Seebeck coefficient, Sphoto)) were also detected in response to visible-light irradiation of the colored WO3 thin films. Under visible-light irradiation, σphoto and the absolute value of Sphoto increased and decreased, respectively. These effects are likely attributable to the excitation of electrons from the mid-gap visible light absorption band (W5+ state) to the conduction band of WO3. Our findings demonstrate that the simultaneous, reversible switching of multiple properties of WO3 thin film is achieved by the application of an external stimulus and that this material exhibits photo-thermoelectric effects when irradiated with visible-light.

The fast and reversible intrinsic photochromic response of hydrated tungsten oxide nanosheets

“…Although a variety of methods have been applied in material preparation, such as chemical or physical deposition, the sol-gel method, the photochromism of WO 3 has been severely prohibited and only exhibited slight color change, slow and irreversible photochromic response, as compared to the electrochromic process. 16,17 First, the photo coloration rate is too slow. For hours or days of UV exposure, the optical absorption only showed slight changes.…”

Transparent and photochromic poly(hydroxyethyl acrylate–acrylamide)/WO3 hydrogel with antibacterial properties against bacterial keratitis in contact lens

“…Tungsten oxide (WO 3 ) is a naturally abundant, low cost and non‐toxic transition metal oxide with broad and robust antibacterial properties that has been extensively investigated by many researchers 28–31 . It is also characterize by interesting reversible photochromic response in different matrices 32–35 …”

Controllable Anomalous n- and p-Type Photothermoelectric Effects of Platinum Oxide and Tungsten Trioxide Layers with and without Chromic Reaction

“…Such changes involve two separate phenomena. One is induced by the temperature gradient caused by heating with light (thermal contribution of light), − and the other is the contribution of the photon energy of light, which is termed as a photothermoelectric (photo-TE) effect. ,− …”