Dual Photo‐ and Mechanochromisms of Graphitic Carbon Nitride/Polyvinyl Alcohol Film

Abstract

An adaptable stimuli responsive system is proposed based on defective graphitic carbon nitride (d‐GCN)/polyvinyl alcohol (PVA) composite attached to different substrates. Nitrogen vacancies, introduced during synthesis via potassium doping, act as internal electron donors to enhance the photoreduction of redox dyes for quick reversible photochromism. This system can be applied as visible light responsive rewritable paper that offers high resolution for printed text, rapid activation/recoloration, multicolor functionality, and high cyclability. The defects also inhibit the d‐GCN particle growth resulting in smaller particle sizes over bulk samples, which enable the formation of transparent mechanochromic films. By altering the PVA/d‐GCN ratio in the composites, two mechanical stimuli responses are introduced by taking advantage of wrinkle and crack formation on the film surface. With a low d‐GCN concentration, mechanical scratching of the film causes the formation of surface wrinkles that alter the film transparency without damaging the film, allowing for text to be written on the surface that can be erased by moisture. With a high d‐GCN concentration, cracks developed on the surface scatter light when stretched, enabling the development of a strain dependent smart window. Both these mechanochromisms can be combined with the photochromism to develop multifunctional smart materials with widespread application.

Smart Soft Materials with Multiscale Architecture and Dynamic Surface Topographies

“…By replacing the laponite in the PVA/laponite composite thin film with graphitic carbon nitride (GCN), we can introduce a photoactive material, thus photochromic responses, into the system. 3 Photocatalysts coupled with color switching redox dyes can show a photochromic effect under applied light as excited electrons from the photocatalyst chemically reduce the dye. The redox dye returns to the original color under ambient conditions because oxygen can oxidize the redox dye back to the original structure and color.…”

“…As shown in Figure 3b, if laponite is replaced by GCN, a dual rewritable surface can be developed on the basis of the wrinkle and light responsiveness of the PVA/GCN surface. 3 Under mechanical scratching, wrinkles developed on the surface can alter the transparency of the film, enabling information to be written on the film. These wrinkles can be readily erased by applying moisture on the film to permit subsequent writing cycles on the film.…”

“…This dynamic bilayer optics based on strain tunable morphologies can be further combined with other stimuli-responsive systems as shown in Figure d. By replacing the laponite in the PVA/laponite composite thin film with graphitic carbon nitride (GCN), we can introduce a photoactive material, thus photochromic responses, into the system . Photocatalysts coupled with color switching redox dyes can show a photochromic effect under applied light as excited electrons from the photocatalyst chemically reduce the dye.…”

“…The physical and/or chemical characteristics of smart soft materials can be dynamically converted when subjected to external stimuli, such as light, − solvent, ,, mechanical strain, − ,− temperature, ,− ,,, electric and/or magnetic fields, ,,, and so on. ,, These intriguing attributes permit the development of various smart soft photonics or electronics, which can lead to widespread application, including, smart windows, ,,, dynamic displays, ,,,,, rewritable media, ,,,, anticounterfeit tabs, ,,, encryption devices ,,,,, stretchable electronics, ,, wearable strain sensors, ,,,− ,, human–machine interface, , thermal camouflage, humidity sensors, ,,, and so on. ,, The design of such captivating materials has been largely insp...…”

“…For instance, Cephalopoda (octopus, squid, etc.) are capable of modulating their skin coloration for camouflage and communication by using their epidermal muscle to control the exposure of skin pigments, demonstrating a dynamic and convertible display strategy. ,, Under this inspiration, researchers have invented intriguing bilayered hybrid structures based on a rigid thin film tightly bonded to a soft substrate, which can exhibit analogous surface characteristics like mechanically tunable crack width, enabling researchers to dynamically and reversibly shield or expose the underlying color/luminescence/thermal radiation from the substrate. − ,, Various types of stimuli-responsive devices have been fabricated on the basis of this design strategy. ,, Another fascinating example is the wrinkling surface, which is vividly demonstrated in the petals of some flowers, , wings of insets, wet human fingers, and so on, exhibiting distinctive light scattering/diffusing characteristics and unique adaptive responses to external stimuli. Accordingly, researchers have fabricated similar wrinkling surfaces, which were achieved via applying compression to a bilayered structure containing a thin rigid film tightly bonded to a compliant soft substrate .…”