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Multi-color Reversible Photochromisms via Tunable Light-Dependent Responses
Article
Multi-color Reversible Photochromisms via Tunable Light-Dependent Responses Andrew T. Smith, Hao Ding, Alicia Gorski, ..., Guqiao Ding, Huidan Zeng, Luyi Sun [email protected]
HIGHLIGHTS A highly active photochromic system was achieved by TiO2 x/ rGO nanocomposites Four multi-colored chromic designs were realized within a single system Wavelength, intensity, time, and dual-response (photo-/hydro-) chromism are displayed
A highly active oxygen vacant titanium dioxide/reduced graphene oxide photocatalyst that exhibits significantly enhanced photochromic performance and high cyclability is reported. The catalyst is used in the design of a single photochromic system that can be tuned to exhibit response to changes in the properties of the applied light. Four devices are achieved that can respond to changes in wavelength, intensity, time of illumination, or multiple stimuli, and all exhibit vivid multi-colored functionality.
Smith et al., Matter 2, 680–696 March 4, 2020 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.matt.2019.12.006
Article
Multi-color Reversible Photochromisms via Tunable Light-Dependent Responses Andrew T. Smith,1,2 Hao Ding,1,2 Alicia Gorski,1 Monica Zhang,1 Philip A. Gitman,1 Chanhyun Park,1 Zirui Hao,1 Yejia Jiang,3 Brandon L. Williams,1,2 Songshan Zeng,1,2 Akhil Kokkula,1 Qingkai Yu,4 Guqiao Ding,5 Huidan Zeng,3 and Luyi Sun1,2,6,7,*
SUMMARY
Progress and Potential
Reversible multi-color chromic systems are critical for the development of advanced sensors, encryption devices, and rewritable media. While many systems show chromic responses to applied stimuli, few show distinct multicolored states within the same system and exhibit limited flexibility in manipulation of the stimuli source. Light-responsive materials have shown promise because of high precision and controllable parameters of the incident light but suffer from slow kinetics limiting the versatility of the photochromic systems. Here, we report a highly active oxygen vacant titanium oxide/reduced graphene oxide photocatalyst that significantly accelerates the color switching of commercial redox dyes. With the improved catalyst, four photochromic devices are realized: (1) multi-wavelength photochromism, (2) intensitydependent photochromism, (3) time-dependent photochromism, and (4) dualresponse hydro-/photochromism. These devices are based on manipulating the interactions between the light and semiconductor and exhibit a broad range of photochromic behaviors with multi-color states and high reversibility.
Designing systems that change color under stimuli is not a trivial problem and neither is meeting the long list of required properties for rewritable chromic systems (e.g., easy/quick printing and erasing, high contrast, cyclability, multi-coloring). Current efforts have focused on exploring a variety of stimuli to induce single color changes, but most rely on complex synthetic means to impart a material with colorchanging properties, which often rely on sacrificial components and complex machinery and exhibit limited control. Here, we demonstrate that through alteration of the stimuli, i.e., light, and the kinetics of the colorchanging mechanism, i.e., the semiconductor/dye, different interactions within a single system can be achieved. Enabling the design of a diverse set of reversible multi-colored devices that show excellent contrast, high cyclability, and multi-color functionality allows the system to meet the challenges facing chromic materials and may lead to the design of many functional devices.
INTRODUCTION Smart materials that show reversible optical responses to different chemical and physical stimuli have led to the development of numerous optical devices finding wide applications as sensors,1–3 security devices,4,5 smart windows,6,7 and information displays.8–11 The stimuli sources including pH,12 electricity,13 temperature,14 solvent,15 mechanical deformation,16 and light17 show promise as external stimuli to control the response of the materials in both liquid and solid phases. Among the various stimuli applicable to induce reversible color-changing responses, light has become of particular interest because of the absence of direct contact with the sample, the possibility for long-distance control, and versatile control over the various properties of light such as wavelength, intensity, and time of irradiation. In addition, the ability to apply mass illumination over a wide area or precise photoprinting allows for light-responsive materials to find widespread applications. As such, photoreversible chromic systems have developed into an important group of smart materials, with semiconductor/redox,18–20 photoacid,21 and transition metals19,22 showing excellent reversibility and color contrast. Conventionally, photochromic systems rely on a single chromophore to transition from one colored state to another upon the direct illumination of the sample with light. This reliance on a single chromophore has led to a great limitation in the versatility of photochromic systems and hindered the development of potential applications.23 Although there have been some efforts made to design multi-color photochromic systems, so far there has been limited success.24–30 It is therefore important to revolutionize the
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design to achieve fast color-switching capabilities and incorporate multiple colorchanging responses to significantly broaden the applications of these materials in different environments. Here, we propose to develop an oxygen vacant titanium dioxide (TiO2 x) semiconductor through a polyol hydrothermal synthesis. The synthesized semiconductor will act as photocatalyst for the photoreduction of color-changing redox dyes, including methylene blue (MB), neutral red (NR), and indigo carmine (IC), with the vacancies acting as internal sacrificial electron donors (ISED). The ISED scavenge holes left behind by the photoexcitation of valence band electrons in order to prevent irreversible oxidation of the organic dyes.31–33 To improve the photocatalytic activity of the color-changing reaction, we couple reduced graphene oxide (rGO) with the TiO2 x nanocrystals by an in situ reduction of graphene oxide (GO) powders during the hydrothermal TiO2 crystallization, which has been shown to improve the charge separation in TiO2 semiconductors.34–38 This resulting semiconductor nanocomposite shows fast switching rates and excellent cyclability, and is used in the design of four different reversible chromic systems that each show distinct multicolor responses, namely (1) multi-wavelength photochromism (MWP), (2) intensitydependent photochromism (IDP), (3) time-dependent photochromism (TDP), and (4) dual-response hydro-/photochromism (DRHP). All of these devices can rapidly and reversibly change their color through the precise control of the applied stimuli and show distinct multi-color states. For example, the MWP device can reversibly switch among four color states by illuminating the device with different wavelengths of light. The IDP can reversibly turn from purple to either red or white depending upon the intensity of the light applied. The TDP will reversibly transition to different colors during continuous illumination of the sample by a single wavelength of light, showing five distinct colors within 1 min. The DRHP will reversibly change color to both applied humidity and light independently. The success of these designs lies in the development of a highly reductive catalyst and through manipulation of the film structure and the dye reduction kinetics to elicit unique responses to the applied stimuli.
RESULTS Synthesis and Design of Photochromic Films To induce oxygen vacant TiO2, we carried out a solvothermal reaction using titanium(IV) butoxide as the titania precursor and ethylene glycol as the polyol solvent at 220 C for 3 h. The resultant TiO2 x is composed of small nanocrystals of ca. 6 nm (Figure S1) and is confirmed to be anatase-phase TiO2 by X-ray diffraction (XRD) (Figure 1C). To verify the existence of oxygen vacancies and the presence of Ti3+, we used both electron paramagnetic resonance (EPR; Figure 1D) and X-ray photoelectron spectroscopy (XPS; Figures S2 and S3) to probe the as-synthesized particles. Pure anatase TiO2 shows no signal in EPR (Figure 1D) as there are no unpaired electrons. When TiO2 is synthesized in the presence of ethylene glycol, a peak centered at g = 2.003 appears, which is assignable to oxygen vacancies/Ti3+ in the lattice, indicating the existence of oxygen vacancies in the as-synthesized catalyst.39,40 The XPS survey spectrum of the polyol synthesized TiO2 x (Figure S2) shows no detected impurities, with only Ti, O, and C signals detected. In the high-resolution Ti 2p XPS spectrum (Figure S3A), peaks located at binding energies of 458.5 and 464.2 eV can be assigned to Ti 2p3/2 and 2p1/2, respectively. The two peaks can be further deconvoluted into Gaussian curves of Ti3+ and Ti4+, which confirms the presence of oxygen vacancies in the catalyst. The O 1s spectrum (Figure S3B) shows peaks at 529.7 and 531.4 eV, which are attributed to Ti–O bonds and adsorbed O2, respectively.39
1Department
of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, USA
2Polymer
Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA
3Key
Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
4Ingram
School of Engineering, Texas State University, San Marcos, TX 78666, USA
5State
Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
6Department
of Biomedical Engineering, University of Connecticut, Storrs, CT 06269, USA
7Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.12.006
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Figure 1. Characterization of TiO2 x and TiO2 x/rGO Catalyst (A) TEM image of TiO2 x /rGO 15 . Scale bar, 50 nm. (B) Size distribution of TiO2 x /rGO 15. (C) XRD patterns of the catalysts with different amounts of GO introduced to synthesis: (I) TiO2 x/rGO0, (II) TiO2 x/rGO5, (III) TiO2 x/rGO10, (IV) TiO2 x/rGO15, and (V) GO. (D) EPR spectrum of pure anatase TiO 2 , TiO2 x , and TiO 2 x /rGO15 . (E and F) High-resolution Ti 2p XPS spectrum of TiO 2 x /rGO 15 (E) and high-resolution C 1s XPS spectrum of TiO 2 x /rGO 15 (F).
TiO2 x/rGOy (y = 5, 10, or 15 mg GO) was synthesized by adding different amounts of GO to the above reaction system. The resulting nanocomposites are composed of small nanocrystals of 7 nm as verified by transmission electron microscopy (TEM) (Figures 1A and 1B). XRD patterns of the nanocomposites (Figure 1C, I–V) confirm that all of the TiO2 x/rGOy form anatase TiO2 and that the reaction temperature is effective in reducing the GO to rGO with the disappearance of the peak at 2q = 12.85 in all of the samples.41,42 The EPR spectrum (Figure 1D) provides strong evidence of oxygen vacancies in the TiO2 x/rGO15 nanocomposite with the peak centered at g = 2.003. XPS was further used to verify the presence of vacancies in the nanocomposite (Figure 1E). For Ti 2p, the XPS spectrum exhibits two peaks located at 458.7 and 464.4 eV for Ti 2p3/2 and 2p1/2, respectively. A slight shift of 0.2 eV to higher binding energy is observed from that of the TiO2 x catalyst without rGO. As the deconvoluted Gaussian curves indicate, there is an increased portion of Ti4+ that coincides with the increase of the rGO in the system, as seen in Figures 1E and S4, but Ti3+ is still present on the surface of the catalyst, indicating that the addition of GO to the synthesis does not completely inhibit the formation of oxygen vacancies. The C 1s spectrum of the TiO2 x/rGO15 nanocomposite (Figure 1F) can
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be deconvoluted into five peaks at 288.5, 286.3, 285.2, 284.6, and 283.7 eV, which correspond to O–C=O, C–O, C–C, C=C, and Ti–C bonds in the catalyst, respectively.43,44 The Ti–C interaction demonstrates the successful growth of the titania catalyst on the rGO surface and is the key to the interfacial charge transfer between TiO2 and rGO resulting in the improved performance.34 The O 1s spectrum (Figure S6) shows peaks at 530.0, 531.4, and 532.7 eV, which correspond to Ti–O, absorbed O2, and Ti–OH bond formation, respectively.39 To form a photochromic film, the TiO2 x/rGO15 nanocomposite can be coupled with the redox dye MB and encapsulated in a hydrophilic polymer, hydroxypropyl cellulose (HPC), which upon spray coating onto a plastic substrate results in a uniform blue film. The film under direct illumination of UV light (365 nm) will rapidly change from the blue MB to the reduced colorless form of the leuco MB as the photogenerated electrons, aided by the rGO, transfer from the catalyst to the MB, returning to the initial blue state if left under ambient conditions or in the presence of oxygen (Figures 2A and S7). As shown in Figure 2B, the absorption spectrum of the TiO2 x/rGO15/MB/HPC film exhibits two main peaks at 610 and 664 nm, corresponding to the dimers and monomers of MB, respectively.45 Under UV light, both absorption peaks disappear within 15–20 s as the film transitions from a light-blue to colorless film. Upon mild heating of 65 C, to accelerate the diffusion of oxygen, the film reverts back to the colored oxidized state in 10 min with a decrease in the peak at 610 nm as some dimers are converted to monomers during the heating process (Figure 2C).20 To demonstrate the writability of the system, a mask can be placed over the film to selectively block the light, and highly precise and detailed images can be photoprinted onto the film (Figure 2D), showing the potential to generate high-contrast and intricately detailed images that can be printed by simple illumination of the film. As a result of the ISED, the system shows remarkable cyclability with reversible color changing able to reach at least 50 cycles (Figure 2E). After the 50th cycle the film still shows excellent color-changing performance, with the 51st cycle of the sample showing high contrast even after being left for 2 weeks under ambient conditions before illumination (Figure S8). To evaluate the effect of coupling rGO to the catalyst, we tested the color-changing kinetics for the four TiO2 x/rGO(0,5,10,15) catalysts by monitoring the change in absorbance over time for the different catalysts coupled with MB in an HPC film (Figure S9). For the sample with no rGO, the color-changing reaction took 60 s to completely change the film from blue to colorless (Figure 2F). With the addition of only 5 mg of rGO, the sample showed greatly improved kinetics going from 60 to 40 s. As the rGO content increased the catalyst showed improved kinetic performance, with the best results coming with the TiO2 x/rGO15 achieving a 3-fold reduction in color-changing time over the bare TiO2 x, changing in 15–20 s. These results show the benefit of coupling the titania catalyst with conductive supports to improve the photochromic performance of semiconductor/redox systems and opens the avenue to explore other supports to potentially further improve the color-changing performance of semiconductor/redox systems. Wavelength-Dependent Optical Properties of MWP Based on the above experiments, it is clear that coupling TiO2 with rGO can lead to excellent photoreversible chromic systems. As mentioned, however, a critical problem facing the wider adoption of photochromic systems is the inability of films to show multiple colors within a single system. A unique property of the semiconductor light interaction is that the excitation of an electron from the valence band into the conduction band requires energy exceeding the band gap of the semiconductor. As
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Figure 2. Photoreversible Color Switching of the TiO2 x/rGO15/MB/HPC Film (A) Color switching of the film upon photoreduction and oxidation under ambient conditions. (B) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation. (C) Recoloration of the film by mild heating in air to accelerate the diffusion of oxygen. (D) Digital photograph of the film covered with a photomask to print a high-resolution image with 365-nm light. (E) Continuous cycling of the film recorded at an absorption intensity of 610 nm for 50 cycles. (F) Color-switching kinetics of different TiO 2 x /rGO y /MB/HPC films.
a result, this phenomenon can be used to control the color-changing interaction of the semiconductor/redox system by coupling differently colored dyes with semiconductors of sufficiently different band gaps. For the titania system, 365-nm light is sufficient to activate the system, so by combining with a semiconductor with a larger band gap there should be a sufficient window to apply light selectively to control the color-changing reactions. A promising semiconductor to use in conjunction with the TiO2 x/rGO system is SnO2, which has been shown to be a good candidate for reversible color-changing reactions and has a larger band gap then TiO2.46,47 To provide efficient color-changing performance for the SnO2 catalyst, we adopted a similar synthetic method to design an efficient SnO2/rGO catalyst. According to XRD (Figure S10), the as-synthesized SnO2/rGO shows rutile crystal structure and a reduction of the GO to rGO. Under 254-nm light, the SnO2/rGO catalyst when coupled with NR in a polyvinyl alcohol (PVA) matrix with glycerol (Gly) and phosphoric acid (H3PO4) shows a quick color change from a light-red to colorless upon reduction of the dye (Figure S11A). The addition of Gly and H3PO4 are necessary to stabilize the NR dye because the
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redox reaction is pH sensitive, requiring low pH and higher water content for improved performance.48 As a result of the larger band gap, the UV-visible (UV-vis) absorption spectrum of the SnO2/rGO/NR/PVA/Gly/H3PO4 film exhibits no shift in the NR peak at 520 nm under 365-nm light, even after 60 s of continuous illumination (Figure S11B). Under 254-nm light (Figure S11C), the film exhibits a reduction and blue shift in 10–20 s, corresponding to a color change in the film. Under heating for 10 min at 65 C the absorption peak returns to the original value, showing the reversibility of the SnO2/rGO/NR system (Figure S11D). To test the reversibility of this system, we cycled the SnO2/rGO/NR system up to 12 times, whereby it exhibited good reversible photochromism under 254-nm light (Figure S11E). Based on the unique interactions of semiconductors with light, the MWP can be designed as schematically shown in Figure 3A. To achieve wavelength selectivity, the key is to structure the film into multiple layers with the larger-band-gap semiconductor on top. The film will exhibit a color based on the mixture of the dyes and can be manipulated to obtain a desired color. In this study, MB and NR are used as model redox dyes. The structure of the MWP should consist of (1) substrate, (2) TiO2/dye, (3) passivation layer (which is necessary to avoid electron transfer between the semiconductors),49,50 and (4) SnO2/dye. Since the transmission of 365-nm light is high through the SnO2/rGO/dye film (40%, Figure S12), when the layered film is illuminated with 365-nm light, the light is transmitted through the top SnO2/rGO layer reaching the TiO2 x/rGO15, thus activating that layer and changing the dye coupled to the TiO2 x/rGO15 only. Consequently if 254-nm light is then applied to the film, the SnO2/rGO layer will change color and go to a colorless state. Interestingly, based on this unique design of the film, the MWP can exhibit an orthogonal design to the applied light, in that if 254-nm light is applied first then only the SnO2/rGO layer will change color because the light is sufficiently absorbed so as not to reach the TiO2 x/rGO layer. The sample can then be exposed to 365-nm light in order to reach the same colorless state. The absorption spectrum of the initial MWP is shown in Figure 3B. The layered film exhibits three broad peaks, two for MB at 610 and 664 nm and one for NR at 520 nm. Upon 365-nm light irradiation, the peaks corresponding to MB are reduced and the film becomes red after 20 s. When the MWP is heated, the color reverts to the initial mixed dye color and the peaks for MB return, with the transition of some dimers to monomers upon heating (Figure 3C). When the recovered sample is then exposed to 254-nm light, the NR peak disappears as the film turns to the lightblue of MB in 20 s. Again, upon heating the peaks return as the leuco NR is oxidized back to a red state (Figure 3D). If both 365- and 254-nm UV radiations are applied simultaneously, both MB and NR will be reduced and the film will turn white. To recover from this white state, simply heating at 65 C for 10 min is sufficient to return the film to the original purple/deep-blue color (Figure 3E). As it appears that under 254-nm light both NR and MB peaks change (Figure 3D), it may be that the MB would eventually change to become colorless under further 254nm light exposure. To test whether the MWP is truly orthogonal, we further exposed the MWP to extended 254-nm light exposure, well beyond what is required to change the color of the film from the mixed dye purple to blue. As seen in Figure S13, after 5 min of exposure to 254-nm light the film will remain blue, as the MB is not fully reduced and the change in the absorption spectrum ceases to change under further light exposure. The small change is most likely due to an overlap of the absorption bands of NR and MB and limited exposure to the 254-nm light in the top region of
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Figure 3. Design Strategy and Wavelength Selective Color Switching of MWP (A) Schematic illustration of orthogonal color-switching process whereby each layer can be selectively switched to change the overall color of the layered film. (B) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation. (C) UV-vis spectrum showing the recoloration of the sample in (B) and the decoloration of the film upon 254-nm UV irradiation. (D) UV-vis spectrum showing the recoloration of the sample in (C) and the decoloration of the film upon UV irradiation with both 365-nm and 254-nm light simultaneously. (E) Recoloration of the film in (D) by mild heating in air to accelerate the diffusion of oxygen. Insets show the corresponding color of the film after UV irradiation at the specified conditions (B–D) and heating (E).
the TiO2 x/rGO/MB film. Only upon 365-nm light exposure will the MB in the MWP be reduced completely to a colorless state. The versatility of this color-changing performance allows for the design of different functional devices. By covering the sample with a light blocking mask, complex images can be easily created in different colors (Figure S14). The orthogonal design allows for the MWP to take the mixture of two redox dyes and generate four distinct
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Figure 4. Design Strategy and Intensity-Selective Color Switching of IDP (A) Schematic illustration of color-switching process whereby the film can be selectively switched by controlling the intensity of light to change the overall color of the single-layer film. (B) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation at an intensity of 88 mW/cm 2 . (C) UV-vis spectrum showing the recoloration of the sample in (B) and the decoloration of the film upon 365-nm UV irradiation at an intensity of 5 mW/cm 2 . (D) Recoloration of the film in (C) by mild heating in air to accelerate the diffusion of oxygen. Insets show the corresponding color of the film after UV irradiation (B and C) and heating (D).
colors, offering the potential for this device to find wide applications as light sensors or for use in rewritable media.51,52 Intensity-Dependent Optical Properties of IDP Since the reliance on multi-layer films can in many circumstances be difficult to deal with, it would be beneficial to design a system that can show multi-color images within a single film. By taking advantage of the different redox potentials of differently colored dyes, multiple colors can be imparted into a film by controlling the parameters of a single wavelength of light. Inherent to the rate kinetics of any photocatalyst is a dependence on the light intensity. By controlling the intensity of the light, it is possible to manipulate the kinetics of the color-switching reaction and develop an IDP film. As schematically illustrated in Figure 4A, under sufficiently low-intensity light, a film with a mixture of redox dyes of different redox potentials coupled with the TiO2 x/rGO catalyst will only allow for the dye with a lower redox potential to be selectively changed. Conversely, if a light source of higher intensity is illuminated over the IDP, both dyes will change and give a third color. Of course, it is also possible to shine lights sequentially as well, which will allow one to distinguish between the different light intensities. The redox potentials of MB ( 0.229 V [versus standard calomel electrode]) and NR ( 0.325 V [versus standard calomel electrode]) are sufficiently different as to allow for a distinction to be made in the color-changing kinetics under different light intensities.47
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Under 365-nm light and a measured intensity at the film surface of 88 mW/cm2, the IDP will transition from the purple of the mixed dye film to the red of the NR only. The absorption spectrum of the IDP (Figure 4B) shows the corresponding peaks of both dyes in the initial film. As the light is applied, the peaks for MB disappear as the film turns red. Upon heating the film recovers, showing the MB and NR peaks once more. When 365-nm light of higher intensity (5 mW/cm2) is applied to the film, the peaks corresponding to both dyes reduce at roughly the same rate and the film turns white after 30 s of illumination (Figure 4C). Again, upon heating at 65 C (Figure 4D) the IDP will recover to the original purple color as both dyes are oxidized back to a colored state. To verify whether this phenomenon is not just a result of mixing the dyes into a single film, we coupled each of the dyes with the TiO2 x/rGO15 catalyst independently. For MB under the higher-intensity light, it has already been shown that the film will change quickly upon illumination (Figure 2). For the NR dye coupled with the TiO2 x/rGO15 catalyst, the absorption spectrum (Figure S15) shows only the peak corresponding to NR. When the 365-nm light at 5 mW/cm2 is applied to the film, the NR is quickly reduced to a colorless state within 20 s. Under a low-intensity light source (Figure S16), the MB absorption spectrum (Figure S16A) shows the reduction of MB under the applied light in the same 60-s illumination. Meanwhile, the NR absorption spectrum (Figure S16B) shows no change in the NR absorption under the applied light, indicating that the reduction kinetics in the IDP can be controlled using dyes of different redox potentials and sufficiently low-intensity light. To apply this phenomenon as a rewritable system that exhibits multi-colored states, masks can be used to cover the film to selectively apply the different lights to print images in different colors on the film, making this system useful as an intensity sensor or rewritable media (Figure S17). Time-Dependent Optical Properties of TDP The observation that the kinetic parameters could be manipulated to distinguish between dyes in a single film encourages us to explore other methods to control the multi-coloration of the system. As the intensity and time of illumination are inherently linked, it would be beneficial to manipulate the system to show different colored states by controlling the optical color change over time. However, for the IDP under the 5-mW/cm2 light source, the MB and NR dyes change at roughly the same rate under the applied light, giving no clear color distinction over time. As mentioned above, however, the kinetics of the system can be manipulated by adding acid to the system. By changing the amount of acid added, the single film with two dyes can be manipulated to exhibit different color-changing phenomena. With a lower amount of acid added than in the IDP system, it turns out that the MB and NR reduction kinetics can be separated so that the system exhibits timecontrolled color changes under the 5-mW/cm2 light source (Figure S18). As the film is exposed to 365-nm light for varying amounts of time, the film will transition from an initial purple of the mixed dyes to a color more dominated by the NR dye as the MB dye is reduced faster. Upon sufficient exposure, however, the NR will continue to be reduced until reaching the final white state, giving a transition from purple to red to white. Interestingly, when more acid is added than in the IDP case, the kinetics of the individual dyes swap with the NR, seeming to change faster than MB (Figure S19). In this case of increased acid concentration the film, when exposed to 365-nm light, will transition from the same initial purple state as the low acid and IDP case to an
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Figure 5. Design Strategy and Time-Dependent Color Switching of TDP (A) Schematic illustration of color-switching process whereby the film changes color as the duration under UV irradiation is increased. (B) The initial as-formed TDP and the corresponding color change of the film with continuous 365nm UV irradiation. (C) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation. (D) Recoloration of the film in (C) by mild heating in air to accelerate the diffusion of oxygen.
intermediate blue/purple state to finally a white state as both dyes are reduced, but in a shorter amount of time, as it only takes 20 s to fully reduce both dyes. With this idea of manipulating the dyes in the system, we further explored whether this effect could be coupled with different combinations of dyes. IC is another commercially available redox dye that shows reversible chromism upon reduction (Figure S7) but exhibits a different color transition compared with MB. When a film is formed with TiO2 x/rGO15 and the IC dye instead of MB, a blue film is still formed (Figure S20) but upon exposure to 365-nm light the film transitions not to a colorless state but instead to yellow, with an intermediate green state as some of the IC is in the oxidized and reduced forms. This makes IC an interesting choice to design a photochromic film to exhibit extended colors over the NR/MB films. For the TDP, the design is illustrated in Figure 5A, with the idea being that a film of TiO2 x/rGO15 with NR and IC can show reversible color-changing kinetics by simply applying a single wavelength of light at a constant intensity for different durations of time, with the reversible oxidation occurring under ambient oxygen or accelerated with heating to improve the diffusion. As Figures 5B and S21 show, the film, when formed with the assistance of a PVA matrix (and the Gly and H3PO4), has an initial color of red. When the TDP is exposed to UV light through a photomask, printed images can be generated and after only 4 s the TDP turns to blue/purple, which is
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a rapid change in color. After 8 s most of the red disappears, so the TDP becomes predominately blue/green. Then after 30 s of light exposure the IC starts to turn green/yellow, and finally at 60 s becomes yellow. This is a significant improvement over many reversible photochromic systems that show a single color change in the same time span that the TDP shows five distinct colors that span the visible spectrum.11,20 From the UV-vis spectrum (Figures 5C and S22), one can observe that the TDP exhibits two main peaks, one at 610 nm and the other at 550 nm, with the 610-nm peak corresponding to IC and the 550-nm peak to NR. The slight red shift from 520 to 550 nm of the NR peak should be due to overlap of the absorption bands of the two dyes.53 Under illumination with 365-nm light, the NR peaks in the TDP begin to disappear almost immediately upon the introduction of UV light while the IC shows slower reduction kinetics. After 60 s of illumination both dyes are fully reduced, reaching the final reduced color of the TDP. Upon heating, the film reverts back to the original red color as both dyes are oxidized back to their original state (Figure 5D), showing the potential for the TDP to find use as a rewritable multi-color system or timer under UV sensor. Humidity- and Light-Responsive DRHP As is evident, the color-changing kinetics of NR can be easily controlled by the addition of an acid into the polymer matrix. However, with the color of NR being sensitive to pH with a color transition around a pH of 8, we wondered whether a film could be designed to respond to other stimuli. Interestingly when a little NaOH (50 mL of 0.06 M NaOH) was added, instead of acid, to a solution of TiO2 x/rGO15/NR/ HPC/LiCl and spray coated onto a plastic substrate, the film stayed red (Figure S22A). When humidity is applied to the film, either by simple human breath or from a humidifier, the red film quickly and reversibly transitioned to yellow while under humidity and back to red when dried, indicating an increase in local pH around the NR in the humidified film. When UV light is applied to the film, there is little change in the absorption spectrum of the NR film, indicating that the catalyst is highly reductive, as no oxidation occurs, and that NR requires low pH to show any photoinduced color change (Figure S23B). However, when humidity is applied the peak at 520 nm is reduced and shifts to 460 nm (Figure S23C), indicating sensitivity to humidity response. The above observations led us to design the DRHP, which takes advantage of the humidity response of the NR to combine with the photochromic response of MB. As shown in Figure 6A, the idea of the DRHP is that the initial film will show a color of the mixed dyes, but when humidity is applied to the film it will change to a different color. Conversely, when UV light is applied to the film, it will then change to a third color. An additional fourth color can then be generated by applying both stimuli consecutively, giving the DRHP four colored states under dry/humid and dark/light with the ability to print text/picture onto the film (Figure 6B). The absorption spectrum of the film under the different conditions can be seen in Figure 6C, in which under the different stimuli the three peaks for MB and NR shift depending upon the conditions under which the film is placed. The DRHP is highly cyclable, as both the humidity response and the UV response can be cycled at least ten times (Figure 6D), indicating good viability as a reversible dual-response sensor or rewritable surface. The response of the film to humidity can be removed by adding more NaOH (100 mL of 0.06 M NaOH), which results in an increase in the local pH, turning the film green when dried. The film is thus beyond the critical transition point to change with humidity, limiting the response to only light (Figure S24). This film with more base is effective in creating a green color that transitions reversibly to yellow under light, offering a way to generate new color transitions and showing the ability of the system
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Figure 6. Design Strategy and Stimuli-Dependent Color Switching of DRHP (A) Schematic illustration of color-switching process whereby the film can reversibly change color as different stimuli are applied. (B) The initial as-formed DRHP in the dry/dark state and the corresponding color change of the film with different dark/light dry/humid conditions. The film was covered with a light/humidity blocking mask to show printed text. (C) UV-vis spectrum showing the DRHP under different environmental conditions: no stimuli, 365-nm UV irradiation, and humidity applied. (D) Continuous cycling of the film recorded at an absorption intensity of 520 nm for ten cycles under humidity cycling and absorption intensity of 610 nm for ten cycles under 365-nm UV irradiation.
to use inert chromophores with the highly reductive catalyst to create new colors that are difficult to produce with current commercial redox dyes.
DISCUSSION In summary, we have demonstrated a method to develop a highly reductive and active catalyst through the introduction of oxygen vacancies to a TiO2 catalyst coupled with a rGO support. Through the introduction of oxygen vacancies, the reliance on external sacrificial electron donors can be avoided through the ISED. With the absence of highly oxidative holes in the semiconductor, the problems of degradation of organic dyes can be avoided to allow for highly reversible and consistent color-changing performance. With the addition of rGO, the activity and color-changing rate can be significantly improved to make semiconductor/ redox systems more viable for many potential applications. The highly reductive nature of the TiO2 x/rGO catalyst allows for high cyclability (>50 cycles), as the organic dyes experience no degradation due to surface holes in the semiconductor. The ability of the catalyst to avoid degradation of organic dyes allows for the potential to introduce inert chromophores to potentially extend the possible colors to any desired, greatly expanding the potential color palate for photochromic films.
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Furthermore, we have extended the scope of photochromic systems from a single colored dye transition to a breadth of self-contained multi-color rewritable films and have demonstrated how, through precise manipulation of the properties of light and redox dyes, highly controllable and distinct multi-color systems can be fabricated. The problems of designing different complex color-changing dyes can be avoided, as commercially available redox dyes are sufficient to show multiple colors within a single system. This development of multi-color rewritable photochromic films is expected to inspire further development of better rewritable media, sensors, and light-responsive devices.
EXPERIMENTAL PROCEDURES Materials Titanium(IV) butoxide (Ti(OBu)4), HPC, MB, NR, and IC were purchased from Alpha Aesar and used as received without further purification. Tin(IV) chloride pentahydrate, >98% (SnCl4$5H2O), IC, and ethylene glycol (EG) were purchased from Acros Organics. PVA (Mowiol 8-88; Kuraray), polyvinyl butyral (PVB; Mowital B 60 HH; Kuraray), HCl (37%; Sigma-Aldrich), H3PO4 (85%; Fisher Scientific), LiCl (Fisher Scientific), and ammonium hydroxide solution (NH4OH, 28% in H2O; Sigma-Aldrich) were used as received. An improved Hummers’ method54 was used to synthesize GO from natural graphite flakes with a size of ca. 325 mesh. The procedures involved mixing 3.0 g of graphite powders with 18.0 g of KMnO4 and 3.0 g NaNO3 followed by slow addition of 360 mL of 18 M H2SO4. It was then kept under stirring for 12 h. Thereafter, the solution was charged to 500 mL of deionized (DI) water at a temperature close to 0 C, and 14 mL of H2O2 (30%) was added into the mixture carefully. After stirring for 1 h, the solution was allowed to stand for 1 day. The yellowish-brown material that settled down was collected and repeatedly washed using DI water, 30% HCl, and ethanol to remove impurities. After drying in an oven at 60 C, the GO powder was ground. Synthesis of TiO2 x/rGO Nanocomposites In a typical experiment, a predetermined amount of GO powders (0, 5, 10, and 15 mg) was added to 15.0 mL of EG and ultrasonicated for 1 h, and then 1.0 mL of Ti(OBu)4 was added under vigorous stirring. After ca. 1 min, 1.0 mL of H2O was added to the mixing solution. Finally, after mixing the solution for 5 min, the pH was adjusted to 2 with HCl and mixed for another 10 min, forming a gray solution with GO and milky-white solution without GO. The solution was sealed inside a 45-mL Teflon-lined stainless-steel autoclave in an oven for 3 h at 220 C and then cooled to room temperature. The product was washed with water three times to remove residual and centrifuged at 8,000 3 g for 10 min to collect the particles. The as-synthesized particles were dried at 65 C in an oven before being dispersed in water at a concentration of 20 mg/mL. Synthesis of SnO2/rGO Nanocomposite A sample of 10 mg of GO powders was added to 7.5 mL of EG and 7.5 mL of H2O as mixed solvents and ultrasonicated for 1 h, and then 0.5 g of SnCl4$5H2O was added under vigorous stirring. After ca. 1 min, 1.0 mL of NH4OH was added to the mixing solution and mixed for another 5 min. The solution was sealed inside a 45-mL Teflonlined stainless-steel autoclave in an oven for 18 h at 180 C and then cooled to room temperature. The product was washed with water three times to remove residual and centrifuged at 8,000 3 g for 10 min to collect the particles. The as-synthesized particles were dried at 65 C in an oven before being dispersed in water at a concentration of 20 mg/mL.
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Characterization TEM images were collected by an FEI Talos F200X scanning transmission electron microscope. The catalyst particles were dispersed in ethanol with the assistance of ultrasonication and then deposited dropwise on 400-mesh copper grids. Powder XRD patterns were recorded on a Bruker D2 Phaser with Cu radiation. EPR spectra were acquired on a Bruker EMX-8/2.7 at X-band at room temperature. XPS characterization was carried out on a Thermo Fisher ESCALAB 250 Xi X-ray photoelectron spectrophotometer. UV-vis spectroscopy was performed on a PerkinElmer UV/visible/near-infrared Lambda 900 spectrophotometer in the 400to 800-nm range under transmission mode at room temperature. Preparation of Devices Stock solutions of TiO2 x/rGO15 (20 mg/mL), SnO2/rGO10 (20 mg/mL), HPC/H2O (50 mg/mL), PVA/H2O (50 mg/mL), Gly/H2O (50 mg/mL), MB/H2O (1 mg/mL), NR/H2O (1 mg/mL), IC/H2O (1 mg/mL), LiCl/H2O (10 mg/mL), PVB/ethanol (20 mg/mL), 0.1 M H3PO4, and 0.06 M NaOH were first prepared. All samples were spray coated using a Master Airbrush G444-SET equipped with a 0.2-mm needle nozzle and a Royal Mini Air Compressor at 25 psi onto a polystyrene substrate. A typical film sample with a thickness of ca. 1.5 mm was prepared by spray coating a mixture of stock solutions of 0.1 mL of HPC, 0.25 mL of TiO2 x/rGO15, and 0.15 mL of MB that was stirred for 30 min and ultrasonicated for 30 min to form a homogeneous solution. Kinetic Study Samples were made by spray coating a mixture of stock solutions of 0.1 mL of HPC, 0.25 mL of TiO2 x/rGOy (y = 0, 5, 10, 15 mg, all catalysts were dispersed as 20 mg/mL solutions), 0.15 mL of MB, and 100 mL of 0.1 M H3PO4 that was stirred for 30 min and ultrasonicated for 30 min to form a homogeneous solution, and sprayed onto a precleaned foundation with a thickness of ca. 1.5 mm. The ratio of C/C0 was calculated by monitoring the change in absorbance at 610 nm. Preparation of MWP First, a mixture of stock solutions of 0.1 mL of HPC, 0.25 mL of TiO2 x/rGO15, and 0.15 mL of MB was stirred for 30 min and ultrasonicated for 30 min to form a homogeneous solution, then sprayed onto a precleaned foundation with a thickness of ca. 1.5 mm to form the first layer. Next, 1.0 mL of PVB solution was sprayed on top of the MB layer to form a passivation layer (ca. 2.5 mm). The SnO2/NR layer was formed by first stirring and sonicating for 30 min a solution of 0.2 mL of PVA, 0.2 mL of Gly, 0.25 mL of SnO2/rGO10, 0.15 mL of NR, and 50 mL of 0.1 M H3PO4 stock solutions, then sprayed on top of the PVB layer to form the top layer of the MWP with a thickness of ca. 2 mm. Preparation of IDP A mixture of stock solutions of 0.4 mL of PVA, 0.4 mL of Gly, 0.3 mL of TiO2 x/rGO15, 0.15 mL of MB, 0.15 mL of NR, and 35 mL of 0.1 M H3PO4 was stirred for 30 min and ultrasonicated for 30 min, followed by spray coating onto the polystyrene substrate with a thickness of ca. 2.5 mm. Preparation of TDP A mixture of stock solutions of 0.4 mL of PVA, 0.4 mL of Gly, 0.3 mL of TiO2 x/rGO15, 0.15 mL of IC, 0.15 mL of NR, and 50 mL of 0.1 M H3PO4 was mixed for 30 min and
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ultrasonicated for 30 min, followed by spray coating onto the polystyrene substrate with a thickness of ca. 2.5 mm. Preparation of DRHP A mixture of stock solutions of 0.1 mL of HPC, 0.3 mL of TiO2 x/rGO15, 0.2 mL of MB, 0.2 mL of NR, 50 mL of LiCl, and 60 mL of 0.06 M NaOH was stirred for 30 min and ultrasonicated for 30 min, followed by spray coating onto the polystyrene substrate with a thickness of ca. 1.5 mm. Photoirradiation Photoirradiation was performed using a typical laboratory UV lamp (4-W Spectoline ENF-240 C equipped with 365- and 254-nm light sources) and UV LED flashlight (5-W JacobsParts FLT-D at 365 nm). Intensity was measured with Santacary iU2 UV A/B Light Meter for low irradiation intensity and a General UVAP Digital Light Meter for high irradiation intensity.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2019.12.006.
ACKNOWLEDGMENTS The authors acknowledge the free PVA samples provided by Kuraray. A.T.S. acknowledges the GAANN Fellowship (no. P200A150330).
AUTHOR CONTRIBUTIONS A.T.S. and L.S. conceived the idea. L.S. supervised the research. A.T.S. designed the chromic films and optimized preparation steps for all the photochromisms. A.T.S., A.G., M.Z., P.A.G., C.P., Z.H., B.L.W., S.Z., and A.K. prepared all the photochromic samples. A.T.S. synthesized all the catalysts, and Q.Y. and G.D. synthesized the GO. A.T.S., H.D., Y.J., and H.Z. ran the characterizations. A.T.S. and L.S. wrote the first draft of the manuscript, and all authors contributed to revising the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: August 20, 2019 Revised: November 5, 2019 Accepted: December 6, 2019 Published: January 22, 2020
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Multi-color Reversible Photochromisms via Tunable Light-Dependent Responses Andrew T. Smith, Hao Ding, Alicia Gorski, ..., Guqiao Ding, Huidan Zeng, Luyi Sun [email protected]
HIGHLIGHTS A highly active photochromic system was achieved by TiO2 x/ rGO nanocomposites Four multi-colored chromic designs were realized within a single system Wavelength, intensity, time, and dual-response (photo-/hydro-) chromism are displayed
A highly active oxygen vacant titanium dioxide/reduced graphene oxide photocatalyst that exhibits significantly enhanced photochromic performance and high cyclability is reported. The catalyst is used in the design of a single photochromic system that can be tuned to exhibit response to changes in the properties of the applied light. Four devices are achieved that can respond to changes in wavelength, intensity, time of illumination, or multiple stimuli, and all exhibit vivid multi-colored functionality.
Smith et al., Matter 2, 680–696 March 4, 2020 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.matt.2019.12.006
Article
Multi-color Reversible Photochromisms via Tunable Light-Dependent Responses Andrew T. Smith,1,2 Hao Ding,1,2 Alicia Gorski,1 Monica Zhang,1 Philip A. Gitman,1 Chanhyun Park,1 Zirui Hao,1 Yejia Jiang,3 Brandon L. Williams,1,2 Songshan Zeng,1,2 Akhil Kokkula,1 Qingkai Yu,4 Guqiao Ding,5 Huidan Zeng,3 and Luyi Sun1,2,6,7,*
SUMMARY
Progress and Potential
Reversible multi-color chromic systems are critical for the development of advanced sensors, encryption devices, and rewritable media. While many systems show chromic responses to applied stimuli, few show distinct multicolored states within the same system and exhibit limited flexibility in manipulation of the stimuli source. Light-responsive materials have shown promise because of high precision and controllable parameters of the incident light but suffer from slow kinetics limiting the versatility of the photochromic systems. Here, we report a highly active oxygen vacant titanium oxide/reduced graphene oxide photocatalyst that significantly accelerates the color switching of commercial redox dyes. With the improved catalyst, four photochromic devices are realized: (1) multi-wavelength photochromism, (2) intensitydependent photochromism, (3) time-dependent photochromism, and (4) dualresponse hydro-/photochromism. These devices are based on manipulating the interactions between the light and semiconductor and exhibit a broad range of photochromic behaviors with multi-color states and high reversibility.
Designing systems that change color under stimuli is not a trivial problem and neither is meeting the long list of required properties for rewritable chromic systems (e.g., easy/quick printing and erasing, high contrast, cyclability, multi-coloring). Current efforts have focused on exploring a variety of stimuli to induce single color changes, but most rely on complex synthetic means to impart a material with colorchanging properties, which often rely on sacrificial components and complex machinery and exhibit limited control. Here, we demonstrate that through alteration of the stimuli, i.e., light, and the kinetics of the colorchanging mechanism, i.e., the semiconductor/dye, different interactions within a single system can be achieved. Enabling the design of a diverse set of reversible multi-colored devices that show excellent contrast, high cyclability, and multi-color functionality allows the system to meet the challenges facing chromic materials and may lead to the design of many functional devices.
INTRODUCTION Smart materials that show reversible optical responses to different chemical and physical stimuli have led to the development of numerous optical devices finding wide applications as sensors,1–3 security devices,4,5 smart windows,6,7 and information displays.8–11 The stimuli sources including pH,12 electricity,13 temperature,14 solvent,15 mechanical deformation,16 and light17 show promise as external stimuli to control the response of the materials in both liquid and solid phases. Among the various stimuli applicable to induce reversible color-changing responses, light has become of particular interest because of the absence of direct contact with the sample, the possibility for long-distance control, and versatile control over the various properties of light such as wavelength, intensity, and time of irradiation. In addition, the ability to apply mass illumination over a wide area or precise photoprinting allows for light-responsive materials to find widespread applications. As such, photoreversible chromic systems have developed into an important group of smart materials, with semiconductor/redox,18–20 photoacid,21 and transition metals19,22 showing excellent reversibility and color contrast. Conventionally, photochromic systems rely on a single chromophore to transition from one colored state to another upon the direct illumination of the sample with light. This reliance on a single chromophore has led to a great limitation in the versatility of photochromic systems and hindered the development of potential applications.23 Although there have been some efforts made to design multi-color photochromic systems, so far there has been limited success.24–30 It is therefore important to revolutionize the
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design to achieve fast color-switching capabilities and incorporate multiple colorchanging responses to significantly broaden the applications of these materials in different environments. Here, we propose to develop an oxygen vacant titanium dioxide (TiO2 x) semiconductor through a polyol hydrothermal synthesis. The synthesized semiconductor will act as photocatalyst for the photoreduction of color-changing redox dyes, including methylene blue (MB), neutral red (NR), and indigo carmine (IC), with the vacancies acting as internal sacrificial electron donors (ISED). The ISED scavenge holes left behind by the photoexcitation of valence band electrons in order to prevent irreversible oxidation of the organic dyes.31–33 To improve the photocatalytic activity of the color-changing reaction, we couple reduced graphene oxide (rGO) with the TiO2 x nanocrystals by an in situ reduction of graphene oxide (GO) powders during the hydrothermal TiO2 crystallization, which has been shown to improve the charge separation in TiO2 semiconductors.34–38 This resulting semiconductor nanocomposite shows fast switching rates and excellent cyclability, and is used in the design of four different reversible chromic systems that each show distinct multicolor responses, namely (1) multi-wavelength photochromism (MWP), (2) intensitydependent photochromism (IDP), (3) time-dependent photochromism (TDP), and (4) dual-response hydro-/photochromism (DRHP). All of these devices can rapidly and reversibly change their color through the precise control of the applied stimuli and show distinct multi-color states. For example, the MWP device can reversibly switch among four color states by illuminating the device with different wavelengths of light. The IDP can reversibly turn from purple to either red or white depending upon the intensity of the light applied. The TDP will reversibly transition to different colors during continuous illumination of the sample by a single wavelength of light, showing five distinct colors within 1 min. The DRHP will reversibly change color to both applied humidity and light independently. The success of these designs lies in the development of a highly reductive catalyst and through manipulation of the film structure and the dye reduction kinetics to elicit unique responses to the applied stimuli.
RESULTS Synthesis and Design of Photochromic Films To induce oxygen vacant TiO2, we carried out a solvothermal reaction using titanium(IV) butoxide as the titania precursor and ethylene glycol as the polyol solvent at 220 C for 3 h. The resultant TiO2 x is composed of small nanocrystals of ca. 6 nm (Figure S1) and is confirmed to be anatase-phase TiO2 by X-ray diffraction (XRD) (Figure 1C). To verify the existence of oxygen vacancies and the presence of Ti3+, we used both electron paramagnetic resonance (EPR; Figure 1D) and X-ray photoelectron spectroscopy (XPS; Figures S2 and S3) to probe the as-synthesized particles. Pure anatase TiO2 shows no signal in EPR (Figure 1D) as there are no unpaired electrons. When TiO2 is synthesized in the presence of ethylene glycol, a peak centered at g = 2.003 appears, which is assignable to oxygen vacancies/Ti3+ in the lattice, indicating the existence of oxygen vacancies in the as-synthesized catalyst.39,40 The XPS survey spectrum of the polyol synthesized TiO2 x (Figure S2) shows no detected impurities, with only Ti, O, and C signals detected. In the high-resolution Ti 2p XPS spectrum (Figure S3A), peaks located at binding energies of 458.5 and 464.2 eV can be assigned to Ti 2p3/2 and 2p1/2, respectively. The two peaks can be further deconvoluted into Gaussian curves of Ti3+ and Ti4+, which confirms the presence of oxygen vacancies in the catalyst. The O 1s spectrum (Figure S3B) shows peaks at 529.7 and 531.4 eV, which are attributed to Ti–O bonds and adsorbed O2, respectively.39
1Department
of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, USA
2Polymer
Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA
3Key
Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
4Ingram
School of Engineering, Texas State University, San Marcos, TX 78666, USA
5State
Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
6Department
of Biomedical Engineering, University of Connecticut, Storrs, CT 06269, USA
7Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.12.006
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Figure 1. Characterization of TiO2 x and TiO2 x/rGO Catalyst (A) TEM image of TiO2 x /rGO 15 . Scale bar, 50 nm. (B) Size distribution of TiO2 x /rGO 15. (C) XRD patterns of the catalysts with different amounts of GO introduced to synthesis: (I) TiO2 x/rGO0, (II) TiO2 x/rGO5, (III) TiO2 x/rGO10, (IV) TiO2 x/rGO15, and (V) GO. (D) EPR spectrum of pure anatase TiO 2 , TiO2 x , and TiO 2 x /rGO15 . (E and F) High-resolution Ti 2p XPS spectrum of TiO 2 x /rGO 15 (E) and high-resolution C 1s XPS spectrum of TiO 2 x /rGO 15 (F).
TiO2 x/rGOy (y = 5, 10, or 15 mg GO) was synthesized by adding different amounts of GO to the above reaction system. The resulting nanocomposites are composed of small nanocrystals of 7 nm as verified by transmission electron microscopy (TEM) (Figures 1A and 1B). XRD patterns of the nanocomposites (Figure 1C, I–V) confirm that all of the TiO2 x/rGOy form anatase TiO2 and that the reaction temperature is effective in reducing the GO to rGO with the disappearance of the peak at 2q = 12.85 in all of the samples.41,42 The EPR spectrum (Figure 1D) provides strong evidence of oxygen vacancies in the TiO2 x/rGO15 nanocomposite with the peak centered at g = 2.003. XPS was further used to verify the presence of vacancies in the nanocomposite (Figure 1E). For Ti 2p, the XPS spectrum exhibits two peaks located at 458.7 and 464.4 eV for Ti 2p3/2 and 2p1/2, respectively. A slight shift of 0.2 eV to higher binding energy is observed from that of the TiO2 x catalyst without rGO. As the deconvoluted Gaussian curves indicate, there is an increased portion of Ti4+ that coincides with the increase of the rGO in the system, as seen in Figures 1E and S4, but Ti3+ is still present on the surface of the catalyst, indicating that the addition of GO to the synthesis does not completely inhibit the formation of oxygen vacancies. The C 1s spectrum of the TiO2 x/rGO15 nanocomposite (Figure 1F) can
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be deconvoluted into five peaks at 288.5, 286.3, 285.2, 284.6, and 283.7 eV, which correspond to O–C=O, C–O, C–C, C=C, and Ti–C bonds in the catalyst, respectively.43,44 The Ti–C interaction demonstrates the successful growth of the titania catalyst on the rGO surface and is the key to the interfacial charge transfer between TiO2 and rGO resulting in the improved performance.34 The O 1s spectrum (Figure S6) shows peaks at 530.0, 531.4, and 532.7 eV, which correspond to Ti–O, absorbed O2, and Ti–OH bond formation, respectively.39 To form a photochromic film, the TiO2 x/rGO15 nanocomposite can be coupled with the redox dye MB and encapsulated in a hydrophilic polymer, hydroxypropyl cellulose (HPC), which upon spray coating onto a plastic substrate results in a uniform blue film. The film under direct illumination of UV light (365 nm) will rapidly change from the blue MB to the reduced colorless form of the leuco MB as the photogenerated electrons, aided by the rGO, transfer from the catalyst to the MB, returning to the initial blue state if left under ambient conditions or in the presence of oxygen (Figures 2A and S7). As shown in Figure 2B, the absorption spectrum of the TiO2 x/rGO15/MB/HPC film exhibits two main peaks at 610 and 664 nm, corresponding to the dimers and monomers of MB, respectively.45 Under UV light, both absorption peaks disappear within 15–20 s as the film transitions from a light-blue to colorless film. Upon mild heating of 65 C, to accelerate the diffusion of oxygen, the film reverts back to the colored oxidized state in 10 min with a decrease in the peak at 610 nm as some dimers are converted to monomers during the heating process (Figure 2C).20 To demonstrate the writability of the system, a mask can be placed over the film to selectively block the light, and highly precise and detailed images can be photoprinted onto the film (Figure 2D), showing the potential to generate high-contrast and intricately detailed images that can be printed by simple illumination of the film. As a result of the ISED, the system shows remarkable cyclability with reversible color changing able to reach at least 50 cycles (Figure 2E). After the 50th cycle the film still shows excellent color-changing performance, with the 51st cycle of the sample showing high contrast even after being left for 2 weeks under ambient conditions before illumination (Figure S8). To evaluate the effect of coupling rGO to the catalyst, we tested the color-changing kinetics for the four TiO2 x/rGO(0,5,10,15) catalysts by monitoring the change in absorbance over time for the different catalysts coupled with MB in an HPC film (Figure S9). For the sample with no rGO, the color-changing reaction took 60 s to completely change the film from blue to colorless (Figure 2F). With the addition of only 5 mg of rGO, the sample showed greatly improved kinetics going from 60 to 40 s. As the rGO content increased the catalyst showed improved kinetic performance, with the best results coming with the TiO2 x/rGO15 achieving a 3-fold reduction in color-changing time over the bare TiO2 x, changing in 15–20 s. These results show the benefit of coupling the titania catalyst with conductive supports to improve the photochromic performance of semiconductor/redox systems and opens the avenue to explore other supports to potentially further improve the color-changing performance of semiconductor/redox systems. Wavelength-Dependent Optical Properties of MWP Based on the above experiments, it is clear that coupling TiO2 with rGO can lead to excellent photoreversible chromic systems. As mentioned, however, a critical problem facing the wider adoption of photochromic systems is the inability of films to show multiple colors within a single system. A unique property of the semiconductor light interaction is that the excitation of an electron from the valence band into the conduction band requires energy exceeding the band gap of the semiconductor. As
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Figure 2. Photoreversible Color Switching of the TiO2 x/rGO15/MB/HPC Film (A) Color switching of the film upon photoreduction and oxidation under ambient conditions. (B) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation. (C) Recoloration of the film by mild heating in air to accelerate the diffusion of oxygen. (D) Digital photograph of the film covered with a photomask to print a high-resolution image with 365-nm light. (E) Continuous cycling of the film recorded at an absorption intensity of 610 nm for 50 cycles. (F) Color-switching kinetics of different TiO 2 x /rGO y /MB/HPC films.
a result, this phenomenon can be used to control the color-changing interaction of the semiconductor/redox system by coupling differently colored dyes with semiconductors of sufficiently different band gaps. For the titania system, 365-nm light is sufficient to activate the system, so by combining with a semiconductor with a larger band gap there should be a sufficient window to apply light selectively to control the color-changing reactions. A promising semiconductor to use in conjunction with the TiO2 x/rGO system is SnO2, which has been shown to be a good candidate for reversible color-changing reactions and has a larger band gap then TiO2.46,47 To provide efficient color-changing performance for the SnO2 catalyst, we adopted a similar synthetic method to design an efficient SnO2/rGO catalyst. According to XRD (Figure S10), the as-synthesized SnO2/rGO shows rutile crystal structure and a reduction of the GO to rGO. Under 254-nm light, the SnO2/rGO catalyst when coupled with NR in a polyvinyl alcohol (PVA) matrix with glycerol (Gly) and phosphoric acid (H3PO4) shows a quick color change from a light-red to colorless upon reduction of the dye (Figure S11A). The addition of Gly and H3PO4 are necessary to stabilize the NR dye because the
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redox reaction is pH sensitive, requiring low pH and higher water content for improved performance.48 As a result of the larger band gap, the UV-visible (UV-vis) absorption spectrum of the SnO2/rGO/NR/PVA/Gly/H3PO4 film exhibits no shift in the NR peak at 520 nm under 365-nm light, even after 60 s of continuous illumination (Figure S11B). Under 254-nm light (Figure S11C), the film exhibits a reduction and blue shift in 10–20 s, corresponding to a color change in the film. Under heating for 10 min at 65 C the absorption peak returns to the original value, showing the reversibility of the SnO2/rGO/NR system (Figure S11D). To test the reversibility of this system, we cycled the SnO2/rGO/NR system up to 12 times, whereby it exhibited good reversible photochromism under 254-nm light (Figure S11E). Based on the unique interactions of semiconductors with light, the MWP can be designed as schematically shown in Figure 3A. To achieve wavelength selectivity, the key is to structure the film into multiple layers with the larger-band-gap semiconductor on top. The film will exhibit a color based on the mixture of the dyes and can be manipulated to obtain a desired color. In this study, MB and NR are used as model redox dyes. The structure of the MWP should consist of (1) substrate, (2) TiO2/dye, (3) passivation layer (which is necessary to avoid electron transfer between the semiconductors),49,50 and (4) SnO2/dye. Since the transmission of 365-nm light is high through the SnO2/rGO/dye film (40%, Figure S12), when the layered film is illuminated with 365-nm light, the light is transmitted through the top SnO2/rGO layer reaching the TiO2 x/rGO15, thus activating that layer and changing the dye coupled to the TiO2 x/rGO15 only. Consequently if 254-nm light is then applied to the film, the SnO2/rGO layer will change color and go to a colorless state. Interestingly, based on this unique design of the film, the MWP can exhibit an orthogonal design to the applied light, in that if 254-nm light is applied first then only the SnO2/rGO layer will change color because the light is sufficiently absorbed so as not to reach the TiO2 x/rGO layer. The sample can then be exposed to 365-nm light in order to reach the same colorless state. The absorption spectrum of the initial MWP is shown in Figure 3B. The layered film exhibits three broad peaks, two for MB at 610 and 664 nm and one for NR at 520 nm. Upon 365-nm light irradiation, the peaks corresponding to MB are reduced and the film becomes red after 20 s. When the MWP is heated, the color reverts to the initial mixed dye color and the peaks for MB return, with the transition of some dimers to monomers upon heating (Figure 3C). When the recovered sample is then exposed to 254-nm light, the NR peak disappears as the film turns to the lightblue of MB in 20 s. Again, upon heating the peaks return as the leuco NR is oxidized back to a red state (Figure 3D). If both 365- and 254-nm UV radiations are applied simultaneously, both MB and NR will be reduced and the film will turn white. To recover from this white state, simply heating at 65 C for 10 min is sufficient to return the film to the original purple/deep-blue color (Figure 3E). As it appears that under 254-nm light both NR and MB peaks change (Figure 3D), it may be that the MB would eventually change to become colorless under further 254nm light exposure. To test whether the MWP is truly orthogonal, we further exposed the MWP to extended 254-nm light exposure, well beyond what is required to change the color of the film from the mixed dye purple to blue. As seen in Figure S13, after 5 min of exposure to 254-nm light the film will remain blue, as the MB is not fully reduced and the change in the absorption spectrum ceases to change under further light exposure. The small change is most likely due to an overlap of the absorption bands of NR and MB and limited exposure to the 254-nm light in the top region of
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Figure 3. Design Strategy and Wavelength Selective Color Switching of MWP (A) Schematic illustration of orthogonal color-switching process whereby each layer can be selectively switched to change the overall color of the layered film. (B) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation. (C) UV-vis spectrum showing the recoloration of the sample in (B) and the decoloration of the film upon 254-nm UV irradiation. (D) UV-vis spectrum showing the recoloration of the sample in (C) and the decoloration of the film upon UV irradiation with both 365-nm and 254-nm light simultaneously. (E) Recoloration of the film in (D) by mild heating in air to accelerate the diffusion of oxygen. Insets show the corresponding color of the film after UV irradiation at the specified conditions (B–D) and heating (E).
the TiO2 x/rGO/MB film. Only upon 365-nm light exposure will the MB in the MWP be reduced completely to a colorless state. The versatility of this color-changing performance allows for the design of different functional devices. By covering the sample with a light blocking mask, complex images can be easily created in different colors (Figure S14). The orthogonal design allows for the MWP to take the mixture of two redox dyes and generate four distinct
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Figure 4. Design Strategy and Intensity-Selective Color Switching of IDP (A) Schematic illustration of color-switching process whereby the film can be selectively switched by controlling the intensity of light to change the overall color of the single-layer film. (B) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation at an intensity of 88 mW/cm 2 . (C) UV-vis spectrum showing the recoloration of the sample in (B) and the decoloration of the film upon 365-nm UV irradiation at an intensity of 5 mW/cm 2 . (D) Recoloration of the film in (C) by mild heating in air to accelerate the diffusion of oxygen. Insets show the corresponding color of the film after UV irradiation (B and C) and heating (D).
colors, offering the potential for this device to find wide applications as light sensors or for use in rewritable media.51,52 Intensity-Dependent Optical Properties of IDP Since the reliance on multi-layer films can in many circumstances be difficult to deal with, it would be beneficial to design a system that can show multi-color images within a single film. By taking advantage of the different redox potentials of differently colored dyes, multiple colors can be imparted into a film by controlling the parameters of a single wavelength of light. Inherent to the rate kinetics of any photocatalyst is a dependence on the light intensity. By controlling the intensity of the light, it is possible to manipulate the kinetics of the color-switching reaction and develop an IDP film. As schematically illustrated in Figure 4A, under sufficiently low-intensity light, a film with a mixture of redox dyes of different redox potentials coupled with the TiO2 x/rGO catalyst will only allow for the dye with a lower redox potential to be selectively changed. Conversely, if a light source of higher intensity is illuminated over the IDP, both dyes will change and give a third color. Of course, it is also possible to shine lights sequentially as well, which will allow one to distinguish between the different light intensities. The redox potentials of MB ( 0.229 V [versus standard calomel electrode]) and NR ( 0.325 V [versus standard calomel electrode]) are sufficiently different as to allow for a distinction to be made in the color-changing kinetics under different light intensities.47
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Under 365-nm light and a measured intensity at the film surface of 88 mW/cm2, the IDP will transition from the purple of the mixed dye film to the red of the NR only. The absorption spectrum of the IDP (Figure 4B) shows the corresponding peaks of both dyes in the initial film. As the light is applied, the peaks for MB disappear as the film turns red. Upon heating the film recovers, showing the MB and NR peaks once more. When 365-nm light of higher intensity (5 mW/cm2) is applied to the film, the peaks corresponding to both dyes reduce at roughly the same rate and the film turns white after 30 s of illumination (Figure 4C). Again, upon heating at 65 C (Figure 4D) the IDP will recover to the original purple color as both dyes are oxidized back to a colored state. To verify whether this phenomenon is not just a result of mixing the dyes into a single film, we coupled each of the dyes with the TiO2 x/rGO15 catalyst independently. For MB under the higher-intensity light, it has already been shown that the film will change quickly upon illumination (Figure 2). For the NR dye coupled with the TiO2 x/rGO15 catalyst, the absorption spectrum (Figure S15) shows only the peak corresponding to NR. When the 365-nm light at 5 mW/cm2 is applied to the film, the NR is quickly reduced to a colorless state within 20 s. Under a low-intensity light source (Figure S16), the MB absorption spectrum (Figure S16A) shows the reduction of MB under the applied light in the same 60-s illumination. Meanwhile, the NR absorption spectrum (Figure S16B) shows no change in the NR absorption under the applied light, indicating that the reduction kinetics in the IDP can be controlled using dyes of different redox potentials and sufficiently low-intensity light. To apply this phenomenon as a rewritable system that exhibits multi-colored states, masks can be used to cover the film to selectively apply the different lights to print images in different colors on the film, making this system useful as an intensity sensor or rewritable media (Figure S17). Time-Dependent Optical Properties of TDP The observation that the kinetic parameters could be manipulated to distinguish between dyes in a single film encourages us to explore other methods to control the multi-coloration of the system. As the intensity and time of illumination are inherently linked, it would be beneficial to manipulate the system to show different colored states by controlling the optical color change over time. However, for the IDP under the 5-mW/cm2 light source, the MB and NR dyes change at roughly the same rate under the applied light, giving no clear color distinction over time. As mentioned above, however, the kinetics of the system can be manipulated by adding acid to the system. By changing the amount of acid added, the single film with two dyes can be manipulated to exhibit different color-changing phenomena. With a lower amount of acid added than in the IDP system, it turns out that the MB and NR reduction kinetics can be separated so that the system exhibits timecontrolled color changes under the 5-mW/cm2 light source (Figure S18). As the film is exposed to 365-nm light for varying amounts of time, the film will transition from an initial purple of the mixed dyes to a color more dominated by the NR dye as the MB dye is reduced faster. Upon sufficient exposure, however, the NR will continue to be reduced until reaching the final white state, giving a transition from purple to red to white. Interestingly, when more acid is added than in the IDP case, the kinetics of the individual dyes swap with the NR, seeming to change faster than MB (Figure S19). In this case of increased acid concentration the film, when exposed to 365-nm light, will transition from the same initial purple state as the low acid and IDP case to an
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Figure 5. Design Strategy and Time-Dependent Color Switching of TDP (A) Schematic illustration of color-switching process whereby the film changes color as the duration under UV irradiation is increased. (B) The initial as-formed TDP and the corresponding color change of the film with continuous 365nm UV irradiation. (C) UV-vis spectrum showing the decoloration of the film upon 365-nm UV irradiation. (D) Recoloration of the film in (C) by mild heating in air to accelerate the diffusion of oxygen.
intermediate blue/purple state to finally a white state as both dyes are reduced, but in a shorter amount of time, as it only takes 20 s to fully reduce both dyes. With this idea of manipulating the dyes in the system, we further explored whether this effect could be coupled with different combinations of dyes. IC is another commercially available redox dye that shows reversible chromism upon reduction (Figure S7) but exhibits a different color transition compared with MB. When a film is formed with TiO2 x/rGO15 and the IC dye instead of MB, a blue film is still formed (Figure S20) but upon exposure to 365-nm light the film transitions not to a colorless state but instead to yellow, with an intermediate green state as some of the IC is in the oxidized and reduced forms. This makes IC an interesting choice to design a photochromic film to exhibit extended colors over the NR/MB films. For the TDP, the design is illustrated in Figure 5A, with the idea being that a film of TiO2 x/rGO15 with NR and IC can show reversible color-changing kinetics by simply applying a single wavelength of light at a constant intensity for different durations of time, with the reversible oxidation occurring under ambient oxygen or accelerated with heating to improve the diffusion. As Figures 5B and S21 show, the film, when formed with the assistance of a PVA matrix (and the Gly and H3PO4), has an initial color of red. When the TDP is exposed to UV light through a photomask, printed images can be generated and after only 4 s the TDP turns to blue/purple, which is
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a rapid change in color. After 8 s most of the red disappears, so the TDP becomes predominately blue/green. Then after 30 s of light exposure the IC starts to turn green/yellow, and finally at 60 s becomes yellow. This is a significant improvement over many reversible photochromic systems that show a single color change in the same time span that the TDP shows five distinct colors that span the visible spectrum.11,20 From the UV-vis spectrum (Figures 5C and S22), one can observe that the TDP exhibits two main peaks, one at 610 nm and the other at 550 nm, with the 610-nm peak corresponding to IC and the 550-nm peak to NR. The slight red shift from 520 to 550 nm of the NR peak should be due to overlap of the absorption bands of the two dyes.53 Under illumination with 365-nm light, the NR peaks in the TDP begin to disappear almost immediately upon the introduction of UV light while the IC shows slower reduction kinetics. After 60 s of illumination both dyes are fully reduced, reaching the final reduced color of the TDP. Upon heating, the film reverts back to the original red color as both dyes are oxidized back to their original state (Figure 5D), showing the potential for the TDP to find use as a rewritable multi-color system or timer under UV sensor. Humidity- and Light-Responsive DRHP As is evident, the color-changing kinetics of NR can be easily controlled by the addition of an acid into the polymer matrix. However, with the color of NR being sensitive to pH with a color transition around a pH of 8, we wondered whether a film could be designed to respond to other stimuli. Interestingly when a little NaOH (50 mL of 0.06 M NaOH) was added, instead of acid, to a solution of TiO2 x/rGO15/NR/ HPC/LiCl and spray coated onto a plastic substrate, the film stayed red (Figure S22A). When humidity is applied to the film, either by simple human breath or from a humidifier, the red film quickly and reversibly transitioned to yellow while under humidity and back to red when dried, indicating an increase in local pH around the NR in the humidified film. When UV light is applied to the film, there is little change in the absorption spectrum of the NR film, indicating that the catalyst is highly reductive, as no oxidation occurs, and that NR requires low pH to show any photoinduced color change (Figure S23B). However, when humidity is applied the peak at 520 nm is reduced and shifts to 460 nm (Figure S23C), indicating sensitivity to humidity response. The above observations led us to design the DRHP, which takes advantage of the humidity response of the NR to combine with the photochromic response of MB. As shown in Figure 6A, the idea of the DRHP is that the initial film will show a color of the mixed dyes, but when humidity is applied to the film it will change to a different color. Conversely, when UV light is applied to the film, it will then change to a third color. An additional fourth color can then be generated by applying both stimuli consecutively, giving the DRHP four colored states under dry/humid and dark/light with the ability to print text/picture onto the film (Figure 6B). The absorption spectrum of the film under the different conditions can be seen in Figure 6C, in which under the different stimuli the three peaks for MB and NR shift depending upon the conditions under which the film is placed. The DRHP is highly cyclable, as both the humidity response and the UV response can be cycled at least ten times (Figure 6D), indicating good viability as a reversible dual-response sensor or rewritable surface. The response of the film to humidity can be removed by adding more NaOH (100 mL of 0.06 M NaOH), which results in an increase in the local pH, turning the film green when dried. The film is thus beyond the critical transition point to change with humidity, limiting the response to only light (Figure S24). This film with more base is effective in creating a green color that transitions reversibly to yellow under light, offering a way to generate new color transitions and showing the ability of the system
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Figure 6. Design Strategy and Stimuli-Dependent Color Switching of DRHP (A) Schematic illustration of color-switching process whereby the film can reversibly change color as different stimuli are applied. (B) The initial as-formed DRHP in the dry/dark state and the corresponding color change of the film with different dark/light dry/humid conditions. The film was covered with a light/humidity blocking mask to show printed text. (C) UV-vis spectrum showing the DRHP under different environmental conditions: no stimuli, 365-nm UV irradiation, and humidity applied. (D) Continuous cycling of the film recorded at an absorption intensity of 520 nm for ten cycles under humidity cycling and absorption intensity of 610 nm for ten cycles under 365-nm UV irradiation.
to use inert chromophores with the highly reductive catalyst to create new colors that are difficult to produce with current commercial redox dyes.
DISCUSSION In summary, we have demonstrated a method to develop a highly reductive and active catalyst through the introduction of oxygen vacancies to a TiO2 catalyst coupled with a rGO support. Through the introduction of oxygen vacancies, the reliance on external sacrificial electron donors can be avoided through the ISED. With the absence of highly oxidative holes in the semiconductor, the problems of degradation of organic dyes can be avoided to allow for highly reversible and consistent color-changing performance. With the addition of rGO, the activity and color-changing rate can be significantly improved to make semiconductor/ redox systems more viable for many potential applications. The highly reductive nature of the TiO2 x/rGO catalyst allows for high cyclability (>50 cycles), as the organic dyes experience no degradation due to surface holes in the semiconductor. The ability of the catalyst to avoid degradation of organic dyes allows for the potential to introduce inert chromophores to potentially extend the possible colors to any desired, greatly expanding the potential color palate for photochromic films.
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Furthermore, we have extended the scope of photochromic systems from a single colored dye transition to a breadth of self-contained multi-color rewritable films and have demonstrated how, through precise manipulation of the properties of light and redox dyes, highly controllable and distinct multi-color systems can be fabricated. The problems of designing different complex color-changing dyes can be avoided, as commercially available redox dyes are sufficient to show multiple colors within a single system. This development of multi-color rewritable photochromic films is expected to inspire further development of better rewritable media, sensors, and light-responsive devices.
EXPERIMENTAL PROCEDURES Materials Titanium(IV) butoxide (Ti(OBu)4), HPC, MB, NR, and IC were purchased from Alpha Aesar and used as received without further purification. Tin(IV) chloride pentahydrate, >98% (SnCl4$5H2O), IC, and ethylene glycol (EG) were purchased from Acros Organics. PVA (Mowiol 8-88; Kuraray), polyvinyl butyral (PVB; Mowital B 60 HH; Kuraray), HCl (37%; Sigma-Aldrich), H3PO4 (85%; Fisher Scientific), LiCl (Fisher Scientific), and ammonium hydroxide solution (NH4OH, 28% in H2O; Sigma-Aldrich) were used as received. An improved Hummers’ method54 was used to synthesize GO from natural graphite flakes with a size of ca. 325 mesh. The procedures involved mixing 3.0 g of graphite powders with 18.0 g of KMnO4 and 3.0 g NaNO3 followed by slow addition of 360 mL of 18 M H2SO4. It was then kept under stirring for 12 h. Thereafter, the solution was charged to 500 mL of deionized (DI) water at a temperature close to 0 C, and 14 mL of H2O2 (30%) was added into the mixture carefully. After stirring for 1 h, the solution was allowed to stand for 1 day. The yellowish-brown material that settled down was collected and repeatedly washed using DI water, 30% HCl, and ethanol to remove impurities. After drying in an oven at 60 C, the GO powder was ground. Synthesis of TiO2 x/rGO Nanocomposites In a typical experiment, a predetermined amount of GO powders (0, 5, 10, and 15 mg) was added to 15.0 mL of EG and ultrasonicated for 1 h, and then 1.0 mL of Ti(OBu)4 was added under vigorous stirring. After ca. 1 min, 1.0 mL of H2O was added to the mixing solution. Finally, after mixing the solution for 5 min, the pH was adjusted to 2 with HCl and mixed for another 10 min, forming a gray solution with GO and milky-white solution without GO. The solution was sealed inside a 45-mL Teflon-lined stainless-steel autoclave in an oven for 3 h at 220 C and then cooled to room temperature. The product was washed with water three times to remove residual and centrifuged at 8,000 3 g for 10 min to collect the particles. The as-synthesized particles were dried at 65 C in an oven before being dispersed in water at a concentration of 20 mg/mL. Synthesis of SnO2/rGO Nanocomposite A sample of 10 mg of GO powders was added to 7.5 mL of EG and 7.5 mL of H2O as mixed solvents and ultrasonicated for 1 h, and then 0.5 g of SnCl4$5H2O was added under vigorous stirring. After ca. 1 min, 1.0 mL of NH4OH was added to the mixing solution and mixed for another 5 min. The solution was sealed inside a 45-mL Teflonlined stainless-steel autoclave in an oven for 18 h at 180 C and then cooled to room temperature. The product was washed with water three times to remove residual and centrifuged at 8,000 3 g for 10 min to collect the particles. The as-synthesized particles were dried at 65 C in an oven before being dispersed in water at a concentration of 20 mg/mL.
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Characterization TEM images were collected by an FEI Talos F200X scanning transmission electron microscope. The catalyst particles were dispersed in ethanol with the assistance of ultrasonication and then deposited dropwise on 400-mesh copper grids. Powder XRD patterns were recorded on a Bruker D2 Phaser with Cu radiation. EPR spectra were acquired on a Bruker EMX-8/2.7 at X-band at room temperature. XPS characterization was carried out on a Thermo Fisher ESCALAB 250 Xi X-ray photoelectron spectrophotometer. UV-vis spectroscopy was performed on a PerkinElmer UV/visible/near-infrared Lambda 900 spectrophotometer in the 400to 800-nm range under transmission mode at room temperature. Preparation of Devices Stock solutions of TiO2 x/rGO15 (20 mg/mL), SnO2/rGO10 (20 mg/mL), HPC/H2O (50 mg/mL), PVA/H2O (50 mg/mL), Gly/H2O (50 mg/mL), MB/H2O (1 mg/mL), NR/H2O (1 mg/mL), IC/H2O (1 mg/mL), LiCl/H2O (10 mg/mL), PVB/ethanol (20 mg/mL), 0.1 M H3PO4, and 0.06 M NaOH were first prepared. All samples were spray coated using a Master Airbrush G444-SET equipped with a 0.2-mm needle nozzle and a Royal Mini Air Compressor at 25 psi onto a polystyrene substrate. A typical film sample with a thickness of ca. 1.5 mm was prepared by spray coating a mixture of stock solutions of 0.1 mL of HPC, 0.25 mL of TiO2 x/rGO15, and 0.15 mL of MB that was stirred for 30 min and ultrasonicated for 30 min to form a homogeneous solution. Kinetic Study Samples were made by spray coating a mixture of stock solutions of 0.1 mL of HPC, 0.25 mL of TiO2 x/rGOy (y = 0, 5, 10, 15 mg, all catalysts were dispersed as 20 mg/mL solutions), 0.15 mL of MB, and 100 mL of 0.1 M H3PO4 that was stirred for 30 min and ultrasonicated for 30 min to form a homogeneous solution, and sprayed onto a precleaned foundation with a thickness of ca. 1.5 mm. The ratio of C/C0 was calculated by monitoring the change in absorbance at 610 nm. Preparation of MWP First, a mixture of stock solutions of 0.1 mL of HPC, 0.25 mL of TiO2 x/rGO15, and 0.15 mL of MB was stirred for 30 min and ultrasonicated for 30 min to form a homogeneous solution, then sprayed onto a precleaned foundation with a thickness of ca. 1.5 mm to form the first layer. Next, 1.0 mL of PVB solution was sprayed on top of the MB layer to form a passivation layer (ca. 2.5 mm). The SnO2/NR layer was formed by first stirring and sonicating for 30 min a solution of 0.2 mL of PVA, 0.2 mL of Gly, 0.25 mL of SnO2/rGO10, 0.15 mL of NR, and 50 mL of 0.1 M H3PO4 stock solutions, then sprayed on top of the PVB layer to form the top layer of the MWP with a thickness of ca. 2 mm. Preparation of IDP A mixture of stock solutions of 0.4 mL of PVA, 0.4 mL of Gly, 0.3 mL of TiO2 x/rGO15, 0.15 mL of MB, 0.15 mL of NR, and 35 mL of 0.1 M H3PO4 was stirred for 30 min and ultrasonicated for 30 min, followed by spray coating onto the polystyrene substrate with a thickness of ca. 2.5 mm. Preparation of TDP A mixture of stock solutions of 0.4 mL of PVA, 0.4 mL of Gly, 0.3 mL of TiO2 x/rGO15, 0.15 mL of IC, 0.15 mL of NR, and 50 mL of 0.1 M H3PO4 was mixed for 30 min and
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ultrasonicated for 30 min, followed by spray coating onto the polystyrene substrate with a thickness of ca. 2.5 mm. Preparation of DRHP A mixture of stock solutions of 0.1 mL of HPC, 0.3 mL of TiO2 x/rGO15, 0.2 mL of MB, 0.2 mL of NR, 50 mL of LiCl, and 60 mL of 0.06 M NaOH was stirred for 30 min and ultrasonicated for 30 min, followed by spray coating onto the polystyrene substrate with a thickness of ca. 1.5 mm. Photoirradiation Photoirradiation was performed using a typical laboratory UV lamp (4-W Spectoline ENF-240 C equipped with 365- and 254-nm light sources) and UV LED flashlight (5-W JacobsParts FLT-D at 365 nm). Intensity was measured with Santacary iU2 UV A/B Light Meter for low irradiation intensity and a General UVAP Digital Light Meter for high irradiation intensity.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2019.12.006.
ACKNOWLEDGMENTS The authors acknowledge the free PVA samples provided by Kuraray. A.T.S. acknowledges the GAANN Fellowship (no. P200A150330).
AUTHOR CONTRIBUTIONS A.T.S. and L.S. conceived the idea. L.S. supervised the research. A.T.S. designed the chromic films and optimized preparation steps for all the photochromisms. A.T.S., A.G., M.Z., P.A.G., C.P., Z.H., B.L.W., S.Z., and A.K. prepared all the photochromic samples. A.T.S. synthesized all the catalysts, and Q.Y. and G.D. synthesized the GO. A.T.S., H.D., Y.J., and H.Z. ran the characterizations. A.T.S. and L.S. wrote the first draft of the manuscript, and all authors contributed to revising the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: August 20, 2019 Revised: November 5, 2019 Accepted: December 6, 2019 Published: January 22, 2020
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