HTA hybrids for multifunctional rewritable paper

applications. For example, Zhang et al. prepared hydrochromic dyes where water triggers the color switching.3 Yin et al. successfully prepared a serie...
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Novel PVP/HTA hybrids for multifunctional rewritable paper Dan Li, Jing Wei, Shun Dong, Huanan Li, Yuguo Xia, Xiuling Jiao, Ting Wang, and Dairong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15483 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Novel PVP/HTA hybrids for multifunctional rewritable paper Dan Li‡, Jing Wei‡, Shun Dong, Huanan Li, Yuguo Xia, Xiuling Jiao, Ting Wang* and Dairong Chen* †

School of Chemistry & Chemical Engineering, Shandong University; National

Engineering Research Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China.

‡ These authors contributed equally to this work.

Keywords: polyoxometalates · hybrids · photochromic · multifunctional rewritable paper · fast color switching

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ABSTRACT Owing to its benefits to reducing paper production and consumption, ink-free rewritable paper has attracted great attention and it is desirable to develop rewritable paper based on low-cost, robust and environmental benign color switching systems. Herein, we report the fabrication of a rewritable paper based on novel PVP/HTA hybrids with fast dual-mode color switching. As-prepared rewritable paper shows fast and reversible colorless-blue or blue-colorless color switching upon photoor hydro-printing, owing to the fast redox transformations of the unique HTA clusters. More interestingly, the rewritable paper can be used as a template for noble metal reduction and the noble metal can be deposited on the reduced area of the paper to form well-ordered patterns in high resolution. This rewritable paper can be produced in large scale and the composition can be facilely tuned with various POMs or polymers. It may not only be an attractive alternative to current paper prints, but also be potentially used for noble metal reduction, to prepare photolithographic circuits and opto-electronic devices.

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Introduction Paper is one of the most important products ever invented in human history which greatly promotes daily communication and information storage. Nowadays, although the rise of modern information technology has changed our ways of communication and data storage, the total consumption of paper has still been greatly increased and is expected to grow by 2.1 % per year until 2020.1 Most paper is disposed in a short period of time after being printed, which significantly increases the business operating cost and creates huge environmental problems.2 As an alternative approach, rewritable paper (RP), which can be used repeatedly, is beneficial for environmental conservation and cutting down the printing cost. To date, considerable progress has been made in this area. Various color switching materials based RPs have been proposed and shown potential in practical applications. For example, Zhang et al. prepared hydrochromic dyes where water triggers the color switching.3 Yin et al. successfully prepared a series of TiO2-dye systems for rewritable purpose.4 Klajn et al. proposed a novel light-controlled self-assembly of non-photoresponsive nanoparticles as rewritable media.5 Kim et al. designed full color light responsive diarylethenes as inks for RP.6 Meanwhile, a wide range of photochromic molecules, such as spiropyrans, fulgides, spironaphtooxazines, azobenzenes and traditional photochromic materials, such as WO3, have all been explored for RP.7-14 Polyoxometalates (POMs) are molecularly defined transition metal oxide clusters with sizes in nanometer scale (1-5 nm) which exhibit diverse topologies and show unique chemical and electronic properties.15 One interesting property of POMs is that a large number of POM clusters can be switched to blue color when reduced, and can return to their original color when oxidized. This reversible color switching is similar to common organic photochromic dyes and may potentially be used for RPs. Recently, Qu et al. demonstrated a POM-based rewritable paper by using the redox properties of phosphomolybdic acids (H3PMo12O40).16 Colorless patterns can be written by H2O2 3 ACS Paragon Plus Environment

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and erased by isopropyl alcohol under UV irradiation. However, the reported POM-based RP constructed by a four-layer structure, which is complex and expensive. Also, H2O2 is used for writing, which is inconvenient and the erasure of the pattern needs continuous UV irradiation for 6 hours, which is time-consuming and unpractical. In the 1980's and 90's, the studies on POMs have mainly focused on finding new POM structures and investigating their various properties.17-20 However, owing to the high crystal energies of the inorganic POM structure, the POMs are considered non-processable and it is hard to fabricate pure POM-based devices. In the last ten years, POMs conjugated with organic components have gained great interest owing to their high processabilities.21 Among various methods, one method is to integrate POMs into polymer matrices. The proper combination of POMs and polymer matrices can result in hybrid materials possessing both the unique properties of POMs and the processability and stability of polymers, which could find extensive applications. Polymer/POM hybrids have been a hot topic and there have been great progress made in the fabrication and properties of polymer/POM hybrid materials.22-24 Considering all the advantages of polymer/POM hybrid structures, we developed novel polymer/POM hybrids with fast dual-mode color switching for RP. Polyvinylpyrrolidone (PVP), a polymer well known for its good biocompatibility and high nucleophilicity, was used as the polymeric substrate and a lindqvist-type hexatungstic acid (HTA, H2W6O19) has been chosen as the model POM for its low reduction potential and small size. The PVP/HTA hybrids can be obtained with tunable HTA contents via a facile hydrothermal method. As-formed hybrids show fast dual-mode color switching which can be used for RP. The PVP/HTA-based RP was facilely fabricated by dip-coating a filter paper with the hybrids. As-obtained RP can be stable in aerobic conditions in either blue reduced state or white oxidized state. For the white RP, blue patterns can be photo-printed, for the blue RP, white patterns can be obtained through hydro-writing. Both the photo-printing and the hydro-printing can be completed within 2 minutes and can be reversed multiple times. Another 4 ACS Paragon Plus Environment

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interesting observation is that, owing to the low reduction potential of HTA, the RP can be used as a template for noble metal deposition to form well-ordered patterns in high resolution. Our research not only designs novel multifunctional rewritable paper but also provides new insights into the application of polymer/POM hybrids. Results PVP/HTA hybrids preparation. Homogenous dispersions of the HTA in the PVP matrix are difficult to obtain by using traditional methods, such as physical blending or layer by layer assembly, because of the large HTAs tendency to aggregate and precipitate out. Instead, we obtained PVP/HTA hybrids using a convenient hydrothermal method and the experimental details are provided in the SI. As is shown in Figure S1 in the SI, we noticed that the amounts of the PVP polymers are crucial in the formation of the PVP/HTA hybrids. Without PVP, the hydrothermal products are cubic WO3 nanoparticles which originate from condensation and crystallization of the HTA clusters.25 With small amounts of PVP added, the products are monoclinic WO3. In presence of

large amounts of PVP, the condensation of the HTA clusters is

prohibited, instead of forming WO3 nanocrystals, the HTA clusters conjugate with the PVP and form PVP/THA hybrids. Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) patterns in Figure S2 and S3 in the SI confirm the formation of the cubic, monoclinic-phase WO3 and the PVP/HTA hybrids. The hydrothermal preparation is facile and as shown in Figure S4 in the SI, with a 25 mL autoclave, we can obtain as much as 0.75 g products in a batch. The relative contents of the HTA in the hybrids can be tuned from 30 % to about 44 % by tuning the hydrothermal time, longer hydrothermal treatment results in more HTA clusters in the hybrids. The amounts of the HTA in the hybrids are confirmed by the thermal gravimetric analysis (TGA) measurements (Figure S5 in the SI). The first weight loss below 250 °C is due to the evaporation of residual ethanol and water. The second one in the range of 250 °C- 450 °C is attributed to the oxidation of PVP. Above 450 °C, the weight loss corresponds to the decomposition of POM skeleton. 5 ACS Paragon Plus Environment

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The lindqvist-type HTA structure is confirmed by the mass spectroscopy (MS), as shown in Figure S6 in the SI. One strong peak with a m/z value of 704 can be observed in the MS spectra, which can be assigned as the [W6O19]2- clusters. From the infrared spectroscopy (IR) of the PVP/HTA hybrids (Figure S7a in the SI), we noticed that below 1000 cm-1, there are three strong peaks at 971, 895, and 815 cm-1, which are the typical W-O vibrations being assigned to stretching vibrations of W-O, W-Ob-W (bridge oxygen), and W-Oc-W (corner oxygen), respectively, and the vibration bands between 1200 cm-1 and 1800 cm-1 can be attributed to the vibrations of PVP.26-27 Also, the peaks at 1655 cm-1 for the C=O vibrations are much broader and red shifted for the hybrid samples compared with that of the pure PVP. This observation indicates that the HTA clusters might interact with the oxygen atoms of the carbonyl group in PVP to form the PVP/HTA conjugates.28-29 The conjugation of the HTA clusters and the PVP polymers in the hybrids was also confirmed by the differential scanning calorimetry (DSC) results which show peak shifts towards lower temperature with increasing HTA contents (Figure S7b in the SI).

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Figure 1. (a) Illustrations of the color switching of the PVP/HTA hybrids with various HTA contents in solid state (left part) and in water (right part, 7 mg/mL, 10 mL). (b) UV-vis spectra of the PVP/HTA hybrids as a function of UV irradiation time, HTA concentration: 0.2 mM. (c) The absorption intensities of the LMCT band and the IVCT band evolutionally monitored for one reducing-oxidizing cycle. (d) Comparison of the ESR profiles between the reduced hybrids and the oxidized hybrids. (c-d 44 % HTA hybrids) Fast dual-mode color switching and the related mechanism. As-formed PVP/THA hybrids are blue color fluffy powders. In the solid state, the hybrids can stay blue for months without color fading, owing to the encapsulation and protection of the HTA by the polymer substrates. However, once spraying water into the hybrids, the polymer swells and allows the oxygen diffuses into the hybrids, and the blue THAs are oxidized to colorless. When the white hybrid powder is dried, upon UV illumination, the powders can be photo-reduced to blue color again. The phenomena of UV-reducing, hydro-reoxidizing of the hybrids can be reversibly processed multiple times in the solid state or in the aqueous solution (Figure 1a and Figure S8 in 7 ACS Paragon Plus Environment

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the SI). The absorption spectra obtained as a function of irradiation time for the PVP/HTA hybrids show two characteristic band changes in the UV and visible ranges (Figure 1b). Upon UV irradiation, the ligand-metal charge transfer (LMCT) band in the UV range decreases in intensity as the intervalence charge transfer (IVCT) band increases until the lowest energy LMCT band is totally absent in the spectra. By monitoring the absorption values at 632 nm and 320 nm, the absorption changes in a reducing-oxidizing cycle as a function of time are shown in Figure 1c. In the reducing and oxidizing processes, both the LMCT and the IVCT band changes show exponential shapes and both processes get saturated within 2 minutes. The electron spin resonance (ESR) spectra were measured at 108 K to verify the electronic states of the W6O192- clusters (Figure 1d). There are no detectable ESR signals for the colorless hybrids. Upon UV irradiation, the blue color hybrids exhibited ESR signals of W5+ at g=1.805, which is in agreement with previous reports and indicates the formation of one-electron-reduced W6O193- species.30-31 Also, another weak ESR signal at 3250 G with g=1.99 can be assigned as the organic radicals of PVP, which may be formed by hole transfer from the HTA clusters to the PVP. The fast dual-mode color switching of the hybrids related to a fast reducing-reoxidizing process, which is rarely reported. To explain the fast dual-mode color switching, the photochromic mechanism of the POM clusters need to be clarified. Yamase et al. have reviewed the photochromic properties of POMs in the 1990's and the conventional photochromic processes of POMs have been explained as follows.32 When POMs are photo-irradiated, excited electrons and holes are formed. The photogenerated holes react with surface hole scavengers (oxidizable molecules, such as primary and secondary alcohols, their ethers and aldehydes, denoted as S) while the excited electrons are trapped in electron traps such as heteroatoms, counterions, etc. (equation 1). These trapped carriers may be delocalized at room temperature and transfer among different metal nuclei. This kind of intervalence charge transfer (IVCT) processes induce the absorption bands in the near-IR region, and the reduced POMs are blue (equation 2). In the presence of air, the thermal 8 ACS Paragon Plus Environment

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oxidation of the POMs takes place and the reduced blue POMs are reoxidized and return to their original color (equation 3). hv H 2W6O19 + S → H 3W56 +W 5+ O19 + S ox

(1)

hv + W A5 + + W B6 + → W B5 + + W A6 +

(2)

2 H 3W56 +W 5+O19 + 1 O2 → H 2W6O19 + H 2O 2

(3)

To study the coloration kinetics, we prepared three different photochromic systems with same HTA contents: aqueous solution of the PVP/HTA hybrids, aqueous solution of the blended HTA-PVP mixtures and propan-2-ol as hole scavenger mixing with HTAs. As shown in Figure S9 in the SI, when propan-2-ol is used as hole scavenger, the system needs 5-6 hours to get photo-reduced. When using PVP instead of propan-2-ol, the reduction of the HTA-PVP mixtures is within only 5 minutes. Meanwhile, the PVP/HTA hybrids shows the fastest coloration kinetics and the blue color get saturated within 2 minutes. Our investigations reveal that the proper selection of the organic substrates and the homogeneous dispersion of the POM clusters in the polymeric substrates can efficiently promote the coloration kinetics, and polymers such as PVP are much better hole scavengers for photochromic responses compared with primary and secondary alcohols. The decoloration process the HTA clusters is compared with the decoloration of H3PW12O40 and H3PMo12O40 at 21 °C. Previous studies have indicated that, the reoxidation and decoloration rates of the POMs are related to the trap depth (∆E) of the localized electrons.20 As shown in Figure S10 in the SI, for Mo-based H3PMo12O40, the localized electrons are deeply trapped, ∆E is much larger than kT and the blue color of the H3PMo12O40 can stay for months. For H3PW12O40, a kind of hetero-polyblue, the ∆E is due to the presence of the P heteroatoms, and the decoloration can be achieved in 6-7 hours. For the HTA clusters, the trap is mainly induced by the counterions (protons), which is quite shallow and the HTA can be reoxidized in just a few minutes. Equation 3 indicates that the reoxidation of the 9 ACS Paragon Plus Environment

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POMs also involves the proton disassociation from POM clusters. We calculated and compared the activation energy for the disassociation of the protons from a HTA cluster and a H3PW12O40 cluster using DFT simulations (calculation details in the SI). Our calculation indicates that for an HTA cluster the activation energy for the cleavage of Obridge-H bond is -0.109 eV while for H3PW12O40 cluster it is 0.299 eV, indicating the HTAs are prone to proton disassociation. Previously, Papaconstantinou et al. have found that, for photocatalytic processes with POMs, the regeneration (reoxidation) of the POM clusters is the rate-determining step.33 The HTAs exhibiting fast reoxidation rate may potentially be used as room temperature photocatalysts for oxidation reactions, although the HTA-based photocatalysts have rarely been studied. PVP/HTA hybrids based multifunctional rewritable paper. The construction of the RP was simple. 0.14 g PVP/HTA hybrids (44 % HTA) were dissolved in 10 mL ethanol and the solution was dip-coated on a filter paper (diameter of 9 cm) and dried at 60 °C, and formed a PVP/HTA hybrid film with a photo- and hydro-active layer. More PVP polymers (~0.05 g) were further deposited on the rewritable paper to prevent the spontaneous oxidation of HTA by air. The photo-printing was achieved using a 5 W UV lamp as the light source through a photomask and aerated water was filled in a pen to do the hydro-handwriting. As shown in Figure 2a, patterns can be obtained within 2 minutes through either photo-printing under UV light or hydro-handwriting by aerated water on the RP, which is a big advantage over the existing

rewritable

systems.

Other

than

general

symbols

obtained

from

UV-illumination, English or Chinese characters can be clearly shown on the RP through hydro-handwriting. The printed pattern can be stored in dry air for 1-3 days without fading. The blue patterns can be hydro-erased and the white pattern can be photo-erased. We emphasize that the hydro-writing can not be achieved by damp air, since only a certain amount of water can swell the PVP layer and assist the penetration of oxygen into the hybrids. The repeatability was confirmed by recording the absorption at 650 nm after repetitive writing-erasing cycles. After 5 cycles, the absorption intensity slightly decreased, which may be induced by the consumption of 10 ACS Paragon Plus Environment

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the PVP polymers. Owing to the facile procedure and low fabrication cost, the RP can be highly competitive for commercial applications.

Figure 2. (a) Blue photo-printing, white hydro-handwriting and the photo-, hydro-erasing processes on the RP. The blue pattern can be erased thermally or hydro-treated, and the white characters can be erased by UV illumination. Scale bar is the same for all these pictures. (b) The absorption intensity at 650 nm of a RP recorded in 5 cycles of color switching between blue color and colorless states. The blue dots are for the photo-printing process, and red dots are hydro-erasing process. POMs for noble metal reduction has been a hot topic.34-36 However, the metal reductions have been mostly achieved in aqueous solutions and the POMs used are mainly keggin-type structures.37 The HTA clusters exhibit the lowest reduction potential among all POMs,38-39 indicating that the PVP/HTA hybrids may be used for noble-metal

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Figure 3. (a) UV-vis spectra and photographs showing the formation of Ag colloidal nanoparticles as a function of time. AgNO3 initial concentration: 0.1 M (5 mL), the hybrids added: 0.05 g, 44% HTA hybrids. (b) Multicolor display by Ag (first row), Pt (second row), Au (third row) reduction and deposition on the RP. First column: the RP immersed in a 0.001 M metal precursor solution; second column: 0.01 M; third column: 0.1 M. reduction. The blue hybrids were added into deaerated solutions of various noble-metal precursors. The hybrids can be totally dissolved within a few seconds to form transparent colorless solutions. Then the color of the solutions gradually turned black-brown (Ag), pink (Au), or brown (Cu) and the plasmon resonance absorbance peaks of the corresponding metal nanoparticles appeared in the UV-vis spectra (Figure 3a and Figure S11 in the SI). The Pd and Pt metal nanoparticles are also prepared using this method (Figure S12 in the SI). The TEM images of the metal nanoparticles verify the formation of the nanoparticles in the reduction process (Figure S13 in the SI). Interestingly, the hybrids-based rewritable paper also exhibits 12 ACS Paragon Plus Environment

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the metal reduction abilities. Immerse a pre-patterned RP into AgNO3 solutions with various concentration, the color of the reduced blue pattern on the RP immediately changed to green, yellow and brown, the color of the metal patterns depends on the amounts of reduced Ag. Other than Ag, the Pt, Au deposition can also show multiple colors depending on the amount of deposited nanoparticles (Figure 3b). UV-vis spectra and SEM images confirms the formation of Ag nanoparticles on the RP (Figure S14, S15 in the SI). Multicolor patterns can be obtained by metal deposition. The ability to reduce noble metals indicates the RP can potentially be used for metal-ion detection, reduction and recollection.

Figure 4. Optical microscope images of (a) colorless PVP/HTA (44% HTA) hybrid RP and (b) blue color reduced patterns on the RP. The patterns were made by 1 min UV illumination using a photomask. (c) One drop of 0.1 M AgNO3 solution dropped on the RP, Ag ions reduced and deposited on the RP and form a well-ordered metallic pattern. Other than filter paper, commercial paper, glass or silicon can also be used as substrates to fabricate the multifunctional RP. The resolution of the RP is examined using silicon as a flat substrate for potential optical applications. Copper grids with square pores were placed on the RP as photomasks to realize selected-area photoreduction. After UV irradiation for 1 min and remove the photomask, an image with blue color squares can be clearly observed by optical microscopy (Figure 4b). These patterns can be totally erased after hydro- or heat-treatment and reprinted upon UV irradiation. Impressively, silver ions can also be selectively reduced and deposited on the rewritable paper to form well-ordered patterns (Figure 4c and Figure S16). Other than optical data storage, construction of such a high-resolution well-ordered 13 ACS Paragon Plus Environment

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metallic structure may have potential applications in opto-electronics and plasmonic-related applications. Conclusions In summary, we have described a facile method for PVP/HTA hybrids preparation and the fabrication of the hybrids-based RPs, on which images with high resolution can be photo-printed or hydro-printed and used for multiple times. Compared with the existing organic color switching systems, such as the viologens, spiropyrans, leuco dyes which suffer from synthetic complexity and undergo photodegradation, the PVP/HTA based rewritable paper provides fast dual-mode color switching with superior stability. Advantages of the rewritable paper also include the easy, facile fabrication, simplicity, low cost and environmental benign. Additionally, the low reduction potential of HTA endows the RP a good template for noble-metal reduction and deposition on selected area of the RP. Applications of the multifunctional hybrid rewritable paper are far reaching, ranging from photo- or hydro-lithographic prints, noble metal reduction to rewritable optical memories. Our research also brings new insights into the application of POM-based hybrids and the investigation of lindqvist-type POM clusters. Our study proves that polymer/POM hybrids can be ideal for rewritable paper fabrication and we envision that the polymer/POM based rewritable paper can be extended to other POM clusters and polymers with various functionalities. ASSOCIATED CONTENT The supporting information is available. Experimental procedures, calculation details, schematic illustration and additional figures. AUTHOR INFORMATION Corresponding Author 14 ACS Paragon Plus Environment

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* (T.W.) E-mail: [email protected]. * (D.C.) E-mail: [email protected]. ACKNOWLEDGMENT This work was supported by the Young Scholars Program of Shandong University (YSPSDU, No. 2015WLJH27) and the Fundamental Research Funds (No. 2015TB004) of Shandong University, also funded by the National Natural Science Foundation of China (No. 21771118, 21701098), and the Taishan Scholars Climbing Program of Shandong Province (tspd20150201).

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