X-ray Photochemistry of Prussian Blue Cellulosic ... - ACS Publications

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X-ray photochemistry of Prussian blue cellulosic materials: Evidence for a substrate-mediated redox process Claire Gervais, Marie-Angélique Languille, Giulia Moretti, and Solenn Reguer Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00770 • Publication Date (Web): 30 Jun 2015 Downloaded from http://pubs.acs.org on July 4, 2015

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X-ray photochemistry of Prussian blue cellulosic materials: Evidence for a substrate-mediated redox process Claire Gervais,∗,† Marie-Angélique Languille,‡,§ Giulia Moretti,† and Solenn Reguer¶ Bern University of Applied Sciences, Fellerstrasse 11, Bern, Switzerland, Centre de recherche sur la conservation (CRC, USR 3224), Sorbonne Universités, Muséum national d’Histoire naturelle, Ministère de la Culture et de la Communication, CNRS ; CP21, 36 rue Geoffroy-Saint-Hilaire, 75005 Paris, France, and DiffAbs beamline at Synchrotron SOLEIL, Saint-Aubin BP 48, 91192 Gif-sur-Yvette, France

Abstract Beside its promising applications in the design of multifunctional materials, batteries and biosensors, the pigment Prussian blue is still studied in heritage science because of its capricious fading behavior due to a complex light-induced redox mechanism. We studied model heritage materials composed of Prussian blue embedded into a cellulosic fiber substrate by means of X-ray absorption near-edge spectroscopy. Significant X-ray radiation damages were observed and characterized. X-ray radiation induced first a ∗

To whom correspondence should be addressed Bern University of Applied Sciences, Fellerstrasse 11, Bern, Switzerland ‡ Centre de recherche sur la conservation (CRC, USR 3224), Sorbonne Universités, Muséum national d’Histoire naturelle, Ministère de la Culture et de la Communication, CNRS ; CP21, 36 rue Geoffroy-SaintHilaire, 75005 Paris, France ¶ DiffAbs beamline at Synchrotron SOLEIL, Saint-Aubin BP 48, 91192 Gif-sur-Yvette, France § IPANEMA (CNRS, MCC, USR 3461), Saint-Aubin BP 48, 91192 Gif-sur-Yvette, France †

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reduction of Prussian blue, in a similar way as visible light does, followed by a complete degradation of the pigment and formation of iron(III) oxyhydroxide. We took advantage of this X-ray photochemistry to investigate in depth the redox behavior of Prussian blue. We could particularly demonstrate that the rate, extent and quality of Prussian blue photoreduction can be tuned by modifying pH and alkali cation content of the cellulosic substrate. The present study represents a step further in the understanding of Prussian blue heritage materials from an electrochemical viewpoint, and provides evidence for a substrate-mediated photochemistry applicable to a wider class of Prussian blue composite materials.

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Introduction

Nowadays, Prussian blue (PB, iron(III) hexacyanoferrate(II)) is better known as electrochemically synthesized thin films, or as the original compound of a series of analogues (PBA) where iron is substituted by other transitional metal atoms. 1,2 The magnetic, electrochemical and optical properties of Prussian blue and its analogues can be tuned by external stimuli, 3–7 leading to very promising applications for memory devices, batteries and biosensors. Before becoming a star compound in materials sciences, Prussian blue got an equal success as a pigment and dye. Thanks to its high tinting strength and its cheap and straightforward synthesis, it was used in various heritage artefacts, such as oil paintings, textiles, early-photographic processes or watercolors. 8–13 Despite this wide use, Prussian blue is still depicted as a problematic and mysterious pigment because of its capricious fading behavior under light exposure, which depends a lot on the object investigated. 14 The fading is due to the photoreduction of Prussian blue, a process where the substrate, the environment, impurities, as well as the structure of Prussian blue are thought to play a role. 14–17 Much remains to be done to understand the redox mechanism behind Prussian blue fading. For instance, the structural and electronic reorganization of Prussian blue upon fading is unknown, as well as the possible migration pathways of cations and electrons within the structure. 2 ACS Paragon Plus Environment

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X-ray Absorption Near Edge Structure (XANES) spectroscopy is an appropriate technique to probe the two iron sites, FeII (CN)6 and FeIII , present in the cubic lattice of Prussian blue and to follow possible structural modifications in response to environmental or endogenous stimuli. 14–16,18 However, the synchrotron radiation used for XANES may cause damages to sensitive Prussian blue heritage materials, in particular by inducing a reduction of the pigment. 15 Those radiation damages are problematic because they interfere with the process of Prussian blue light fading and may remain overlooked in XANES studies when not consciously considered and detected. Besides, they prevent precious heritage samples to be analyzed or worse, may lead to their unperceived damage. Finally, they raise concerns about possible X-ray radiation damages in other PB or PBA composite materials. We hereby characterize in detail X-ray radiation damages of Prussian blue embedded in a cellulosic fiber substrate, a model used to study the mechanism of Prussian blue fading in cyanotypes and watercolors. We demonstrate that X-ray induced reduction can be fruitfully used to get insights into the redox chemistry of those materials and more particularly analyze the roles of alkali cations and protons on the rate, extent and quality of Prussian blue photoreduction. We finally discuss the major role of the substrate in the sensitivity of Prussian blue towards both radiation damage and light-induced fading.

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Experimental Section

Prussian blue paper materials. Prussian blue (PB) was synthesized as follows: 14 50 mL of an aqueous solution of K4 FeII CN6 .3H2 O (0.2 M) were added dropwise to an equal volume of an aqueous solution of FeIII 2 (SO4 )3 .5H2 O maintained under vigorous magnetic stirring. The precipitates were filtered under vacuum using a 2 µm filter. The filtrates were washed several times with deionized water and dried at 60◦ C. The powder was ground in an agate mortar before characterization. Elemental analysis by SEM-EDX gave an average composition of PB close to that of soluble Prussian blue, that is KFeIII [FeII CN6 ]. The PB colored paper


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samples (PB:WHA) were prepared with Whatman paper no. 1, composed of 100% cotton cellulose fibers. A colloidal suspension of the synthesized Prussian blue was prepared (25 mg in 1 mL of deionised water) and small strips of the Whatman papers were immersed into the sol for 5 seconds under moderate magnetic stirring. The samples were dried at room temperature and 50% relative humidity. The immersion–drying process was repeated nine times to achieve a PB concentration of 6 mg/cm2 . In order to analyze the roles of alkali cations and acidity, PB:WHAKCl and PB:WHAHCl samples were prepared similarly, with Whatman paper soaked in KCl(1M) and HCl(1M) and dried before PB immersion.

Raman spectroscopy. Spectra were recorded using a Renishaw Raman microscope equipped with a 785 nm diode laser and a grating of 1200l/m. The spectral resolution was in the 1 cm−1 range. A 50× objective lens was used, leading to an estimated laser spot of approximatively 900 nm in diameter. Each spectrum was collected during 10 s and 20 acquisitions, with a power on the sample estimated to 0.6 mW. No observable laser-induced alteration of the samples was found.

X-ray absorption spectroscopy. Energy-dispersive XANES at the Fe K-edge was performed at the ODE beamline of synchrotron SOLEIL. The beam size was about 30µm FWHM in horizontal and 35µm FWHM in vertical and the photon flux hitting the samples was 1.1 × 1011 photons/s. The samples were placed in an environmental chamber described previously, 15 to keep relative humidity within ±2 of the targeted value (50%RH or 70%RH). Reproducibility of the results was ensured by measuring at least two spots on each sample. Spectra were recorded in transmission geometry at room temperature using a CCD camera that was regularly exposed to the direct beam to record the I0 spectrum. The CCD exposure time was chosen from 5 to 25 ms depending on the pigment concentration into the samples. Up to 150 successive exposures (frames) were saved and averaged to get the final XANES spectrum. Spectra were recorded every 30 s. Due to sample intrinsic heterogeneities, an oscillating movement of the sample over 200µm in horizontal and vertical was performed 4 ACS Paragon Plus Environment

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during acquisition of the 150 frames. This allowed the final spectrum to be averaged over an area larger than the initial beam spot size. The energy range of the XANES spectra acquired on the samples was 440 eV from 7034 eV to 7475 eV. The spectrum of a metallic iron foil was also recorded and used for energy calibration based on the inflection point of the absorption edge at 7112 eV. Data treatment was performed using the Iffefit 3.0.3 package with the Athena software. 19 The pre-edge range background was removed using an empirical polynomial function, and the absorption background was removed using a cubic spline with a normalization range between 40 and 220 eV after the edge. In order to assess the composition ratio between Prussian blue, reduced Prussian blue and iron(III) oxyhydroxide (goethite), a linear combination of the XANES normalized spectra were realized with the Athena software between -20 eV and +60 eV of the absorption edge. All linear combinations were allowed between the reference spectra and the final ratio was forced to sum to 1. Quality of the fit was assessed by the R-factor, which was always found below 5 × 10−3 .

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Results

Optical microscopy and SEM of PB:WHA shows that Prussian blue is heterogeneously distributed within the cellulosic matrix and presents different sizes (Figure 1). It is found either in the form of crusts between the fibers of the paper substrate, as micro-particles or as nanoparticles. In order to investigate further the structure of Prussian blue in this type of heterogeneous materials, XANES studies were performed with a small beam size (30µm × 35µm) scanned over a larger area (200µm × 200µm). This allowed to keep a high photon flux and get an average spectrum representative of the material.

Characterization of X-ray Radiation damages. The samples were found to drastically change during XANES, even without external stimuli such as light. Figure 2 shows two spots exposed continuously to X-rays respectively during 110 min (spot 1) and 420 min (spot 2) at 5 ACS Paragon Plus Environment

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b)

a)

3 µm Figure 1: Optical and Scanning Electron Microscopy (back-scattered electron) of PB:WHA showing the heterogeneous distribution of Prussian blue particles around and within the fibers. 50%RH. After X-ray exposure, spot 1 shows a discoloration of the pigment. In addition, spot 2 shows a red coloration of the pigment at the edges of the exposed area.

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These visual

color changes are associated with a photoluminescence of the paper visible in UV-light, indicating that X-ray damaged also the cellulosic substrate. Alteration of the pigment Prussian blue was characterized by Raman spectroscopy before and after X-ray exposure, as well as XANES recorded ”live” during X-ray exposure. Raman spectra indicate that the discoloration of the pigment is associated with a shift of the ν(CN) band from 2157 to 2152 cm−1 and an increase of the shoulder at 2152 cm−1 (see supplementary materials). In red areas, no Raman signal could be measured, indicating that the pigment was probably destroyed. The evolution of the XANES spectra upon exposure to the X-ray beam is shown for spot 2 (Figure 2, left). Two spectral modifications were observed: a shift of the absorption edge towards lower energies happening in the first 40 minutes, followed by a flattening of the oscillations after the edge in later acquisitions. Comparison with reference spectra of different iron oxides indicates that the final spectrum is very close to that of iron(III) oxyhydroxide, either goethite or the disordered phase ferrihydrite. 20 The 1

This heterogeneity comes from the response latency of the motor used to move the sample over an area larger than the beam. It results in a longer exposure at the edges than in the middle of the scanned area.


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X-ray exposure time

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Figure 2: X-ray induced radiation damages observed on two spots of PB:WHA. Left: Evolution of XANES spectra with X-ray exposure time for spot 2 (constant sample environment fixed at 50%RH, air, no UV-visible light). For clarity, spectra are displayed every minutes for the first 40 minutes, and every 10 minutes for the rest of the exposure (40 min–420 min). Dashed XANES spectrum: goethite reference. The XANES derivatives of all spectra acquired the first 40 minutes are shown in the inset. Right: Optical micrographs in visible and fluorescence light (emitting light: 420-495 nm) of spots 1 and 2 after X-ray exposure. two iron phases feature both a large proportion of FeIII octahedra, arranged differently in the lattice. Hence they cannot be easily distinguished by XANES, except by the intensity of the pre-edge that requires high-resolution data. 21

The evolution with X-ray exposure time of the absorption edge, defined as the energy at the maximum of the XANES derivative is shown Figure 3. The absorption edge shifts of 2 eV from 7128 eV to 7126 eV at t = 16min and may be inferred to a lowering of the average oxidation state of iron due to the reduction of FeIII into FeII . After this shift, the energy of the absorption edge does not change until the very end at t = 420min, where it increases again. This can be explained by the degradation of Prussian blue into iron(III) oxyhydroxide, which has a higher absorption edge (7127.7 eV for goethite). The ratio between the different iron phases was quantified by a linear combination fit


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Composition ratio Prussian blue

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Figure 3: Evolution of absorption edge and composition ratio with X-ray exposure time (log scale). The dashed line at t = 40 min is used as a guideline to indicate the separation between the reduction of Prussian blue and the degradation of the reduced Prussian blue into iron(III) oxyhydroxide. In the cubic lattice of the iron phases, FeII and FeIII sites are colored in blue and red respectively. of the XANES spectra. Three reference spectra were selected for the original Prussian blue (spectrum at t = 0 min), the reduced Prussian blue (spectrum at t = 40min), selected as those presenting the maximum shift towards lower energies and a minimum change in the postedge region, and goethite, selected to represent the iron(III) oxyhydroxide phase. The general evolution of the composition ratio upon X-ray exposure time is shown Figure 3 (an example of LCF is given in supplementary materials). Prussian blue undergoes two transformation steps that are successive in time. The first phase lasts about 40 minutes and consists in a reduction of Prussian blue. The proportion of reduced Prussian blue reaches 50% at t ≈ 10min, in good agreement with the drop of the absorption edge occuring at t = 16min. Once the entire Prussian blue is reduced at t = 40min, a second, much slower transformation step occurs, where the reduced Prussian blue degrades into iron(III) oxyhydroxide.


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Analogy between X-ray-induced and light-induced reduction of Prussian blue. Similarly to X-ray radiation, UV-visible light induces a reduction of Prussian blue, leading to the loss of color (fading) observed in PB paper heritage artefacts. 14 Both reduction processes were compared by XANES of PB:WHA samples exposed to X-rays at 7 keV (this study) and exposed to UV-visible light during 5 days (data taken from a previous study 14 ). In both cases, the spectra are evolving similarly, with a slight shift of the absorption edge

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Figure 4: XANES spectra of PB:WHA samples before (dark blue curves) and after exposition to X-rays at 7 keV (this study, light blue curve) and UV-visible light (taken from reference, 14 orange curve). Inset: fluorescence of uncolored paper exposed to 5 minutes X-rays in the same conditions (emitting light: 510-560 nm). towards lower energies (Figure 4). However, it required only 5 minutes of exposure to X-rays to match the same amount of Prussian blue reduction obtained after 5-days light exposure. This difference of reduction kinetics might be straightforward (X-rays are more energetic than UV-visible radiation) but two points are requiring a deeper analysis of the results: First the total energy required to achieve a similar amount of Prussian blue reduction is ≈500 µJ/µm2 with UV-visible light (HMI lamp, irradiance 1200 W/m2 ) and ≈1 µJ/µm2 with X-rays (7100eV, with a photon flux of 1.11011 ), indicating that the rate of Prussian blue reduction is not only a question of total radiation dose 22 received by the sample but depends also on the type of radiation. Second, Prussian blue powder, without substrate,


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remains stable during hours under the X-ray beam or in UV-visible light. Thus, it seems that the Prussian blue structure alone is not readily affected, at least directly, by radiation damage. To the contrary, uncolored paper let 5 minutes under the X-ray beam showed significant fluorescence (Figure 4, inset) while none was detected after 5 days of light exposure. Therefore, the alteration of the cellulosic substrate is much more significant with X-rays than with UV-visible light. This in turn initiates the reduction of Prussian blue, in agreement with previous studies that highlighted the role of the substrate on Prussian blue reduction. 14

X-ray photochemistry of Prussian blue paper materials. For electroneutrality reasons, the reduction of Prussian blue must be accompanied by either a reorganization or a migration of ionic species within the structure. 23,24 We thus studied cation and proton fluxes susceptible to accompany the photoreduction of FeIII by performing XANES time series on two modified systems: PB:WHAHCl where the paper matrix was pre-acidified by immersion in a solution of hydrochloric acid (HCl, 1M), and PB:WHAKCl where the paper matrix was artificially loaded in potassium by immersion in a solution of potassium chloride (KCl, 1M). This latter sample was additionally kept in a high relative humidity environment (70% RH instead of 50%RH for the other systems). Like for PB:WHA, X-ray radiation induces a reduction of Prussian blue in those two systems, visible by a shift of the absorption edge towards lower energies (Figure 5a). However, the kinetics and extent of the reduction greatly differ: While the reduction of Prussian blue in PB:WHA induces a shift of the edge of 2 eV after ≈ 16 minutes, it is less important and happens later in PB:WHAHCl (shift of 1 eV only, after ≈ 50 − 60 minutes). To the contrary, PB reduction is more important and starts earlier in PB:WHAKCl kept at 70%RH (shift of the edge of 3 eV after ≈ 10 minutes). These results shows that it is possible to modify the reduction kinetics of Prussian blue by modifying the substrate composition. To get further insight into the redox mechanism, we looked at the shape of the XANES derivatives and considered the maximum height of the XANES derivative, combined with the


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Figure 5: Use of X-ray induced reduction of Prussian blue to study the redox chemistry of PB composite materials. a) Evolution of the absorption edges with X-ray exposure time. Evolution of the XANES derivatives for b) PB:WHAHCl and c) PB:WHAKCl at 70% RH. Spectra are colored according to a rainbow color scheme, from t = 0 (blue) to t = 120 min (red). d) Evolution of the height of the derivatives at 7128.5 eV (dark blue curve) and at 7125.5 eV (light blue curve) in the system PB:WHAKCl (70% RH). Center: General redox equation for the Prussian blue – paper system. half-full width of the peak, as an indicator for the number of iron environments within the system. A large, low-intense XANES derivative indicates that the egde is ill-defined due to the presence of several iron sites that absorb X-rays at slightly different energies. This is the case for the reduced Prussian blue structure in PB:WHAHCl which presents a less intense


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and broader XANES derivative than the initial, non-reduced Prussian blue (Figure 5b). This is a hint that iron reduction led to structural disorders. To the contrary, a high, narrow absorption edge indicates that one or few well-defined iron sites are predominant. This is the case for the reduced Prussian blue in PB:WHAKCl , which features a XANES derivative similar to the initial Prussian blue, only shifted to a lower energy (Figure 5c). We come up with a redox mechanism that explains those results and highlights well the interactions between Prussian blue and the cellulosic substrate. According to the redox equation proposed in Figure 5, the reduction reaction (PB→PBreduced ) requires that cations enter within the Prussian blue structure for electroneutrality purposes. Those cations need to be solvated to migrate through the zeolite-like channels of the lattice, where the water molecular network acts as a cation carrier. For the system PB:WHAKCl at 70%RH, the substrate loaded in potassium and the high relative humidity provide solvated cations that act together with the X-ray irradiation in favor of Prussian blue reduction. Hence a welldefined redox reactivity, with two clear oxidation states at 7128.5 eV for PB and 7125.5 eV for PBreduced . The reduction does not induce any other type of disorder within the structure and the absorption edge remains well-defined. In that case, it is even possible to estimate the kinetic rate of the reaction PB→PBreduced by considering the height of the XANES derivative at 7128.5 eV as being the amount of non-reduced Prussian blue remaining in the system (Figure 5d). Its evolution with time follows an exponential which corresponds to a first-order process with a reaction rate k = 0.018. Similarly, the height of the XANES derivative at 7125.5 eV indicates the amount of reduced Prussian blue appearing into the system. It follows an exponential slightly lower, due to the partial degradation of PBreduced into iron(III) oxyhydroxide after 40–50 minutes. The redox behavior of the system PB:WHAHCl can be explained by considering the counter-reaction (PBreduced →PB), which spontaneously occurs in presence of oxygen and requires protons for oxygen reduction to occur. 25 Protons brought into the paper matrix of PB:WHAHCl , together with ambient oxygen, promote the re-oxidation of the reduced


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Prussian blue. As such, they act in opposition to X-ray radiation. The opposite effect of Xray radiation and acidity enhances disorder within the structure, with regions more reduced, and others more re-oxidized, depending on the ability of protons to be transported from the cellulosic substrate to the Prussian blue structure and to react with oxygen. The end result is a heterogeneous distribution of different oxidation states within the Prussian blue lattice, resulting in ill-defined absorption edges (Figure 5c), and a slower reduction kinetics on average.

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Discussion

Already well established for biological materials, 26,27 synchrotron radiation damages is now becoming a major issue for the study of ancient materials, being cultural heritage, archaeological or paleontological. 28 This awareness stems not only from the significance of those materials –they are unique testimonies of our past–, but also from their very nature: because they are mostly heterogeneous in chemical composition, in phase distribution and in topology, their spectroscopic investigation requires the use of micro-probes usually coupled to high photon fluxes, leading to high photon doses absorbed by the sample. The pigment Prussian blue is no exception to the rule and its sensitivity to X-ray radiation damage when embedded into a cellulosic paper was hereby highlighted and characterized. We could particularly show that the pigment undergoes two distinct transformation steps, first a reduction of FeIII followed by a complete degradation of the reduced pigment into iron(III) oxyhydroxide, either goethite or ferrihydroxide. Ferrihydroxide is also found as an impurity in Prussian blue pigments synthesized with eighteenth century historical recipes. 29 Its presence has been attributed to an excess of iron salt (e.g. iron sulfate) not consumed during the synthesis of Prussian blue, which leaves FeII ions in the system. Those free FeII ions subsequently oxidize and hydrolyze to give ferrihydrite, by a mechanism well studied in environmental chemistry. 30 The formation of iron(III) oxyhydroxide that we observe here might be explained


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by a similar process. Indeed, the solubility of Prussian blue, almost null at low pH and potential, strongly increases in slightly basic or reducing conditions. 31 At high pH (above 9 to 10), Prussian blue is not stable and hydrolyzes. During X-ray radiation, basic conditions are unlikely because the photodegradation of cellulose releases acidic species. 32 However, X-rays also induce a dehydroxylation of the pyranose units associated with a significant release of water. 33 This water, combined with the reduction conditions brought by the X-ray beam, 27 are factors that may be sufficient to induce the dissolution of the reduced Prussian blue, with a release of free FeIII ions and the successive formation of iron(III) oxyhydroxide. Clearly, a further understanding of the mechanism leading to the degradation of Prussian blue requires to better characterize the structural and chemical changes happening in cellulose during long X-ray exposures, a study which would also be beneficial in the context of digitalization of paper archives by X-ray microtomography. 34 Before the complete degradation of Prussian blue, the first effect of X-ray radiation is the reduction of Prussian blue, via a redox mechanism that we could characterize. With that respect, Prussian blue paper materials are very similar to Prussian blue modified electrodes, where Prussian blue thin films are electrochemically synthesized on electrodes, and reduced or oxidized ad libitum by tuning the voltage. 35,36 In this type of system, voltammetric studies confirmed that the redox process depends on the overall ability of cations to enter and exit the structure upon reduction and oxidation, respectively. 23–25,37–39 In Prussian blue paper materials, the electrolytic solution is replaced by the cellulosic matrix, and we showed here that the redox process was indeed accelerated, amplified and of better quality when the substrate was loaded with solvated K+ alkali cations. As such, the equation that we propose in Figure 5 emphasizes the importance of ion transport, but many other factors are susceptible to influence the redox process. Indeed, studies performed on electrochemically synthesized thin films showed that the ionic compensation following the change in ratio FeIII :FeII may be given by electrolytes within the solution (K+ ) but might also be obtained by changes in the composition of the water molecules (H+ /H3 O+ ) present within the Prussian


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blue lattice. 24,40 Other factors influencing the redox behavior are the degree of crystallinity and disorder of the Prussian blue structure, or the presence of alkali metals and hydrogen ions within FeII (CN)6 vacancies. 39 Studies on Prussian blue electrodes might be of help to better refine the redox process in Prussian blue heritage materials. However, a straightforward shift of knowledge from one to the other would be hasty because the two systems differ in two ways. First, a shift from wet chemistry in electrodes to solid state chemistry for Prussian blue paper materials, implies that the ion transport is slower, more heterogeneous, and depends on the distribution of the pigment within the cellulosic fibers. As such, Prussian blue paper materials have a slower redox kinetics than Prussian blue electrodes and may feature structural disorders induced by a ill-defined redox process. This was particularly pregnant here for PB:WHAHCl samples. A second major difference lies in the activation of the redox process, that is the source of electrons in the equation in Figure 5. Controlled by voltage in Prussian blue modified electrodes, it is triggered by light or X-ray exposure of the substrate in Prussian blue paper materials: Light-induced photo-oxidation of cellulose releases free radicals 32 and X-ray induced damages to cellulosic paper are thought to follow a similar process. 41 However, transition metals influence the degradation of cellulose via a complex catalytic process 42 that remains to be investigated for Prussian blue. What is clear however, is that the photochemistry of Prussian blue heritage materials is intimately linked to the interaction of the pigment with its substrate. With that respect, they are related to the wider class of Prussian blue multifunctional materials. Recent studies have shown that the combination of PB or PBA with organic or inorganic materials could emulate new magnetic, electrochromic or optical properties. 43–47 Our findings that Prussian blue redox properties can be modified by acting on the substrate go fully along this line.


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5

Conclusion

This study highlights the fundamental role played by the cellulosic substrate on the photochemistry of Prussian blue. The X-ray induced degradation of cellulose generates environmental conditions (radicals, water release) that triggers the reduction of Prussian blue and the concomitant migration of alkali cations. Like the two sides of the same medal, evidence for a substrate-mediated photochemistry of Prussian blue has two consequences. In terms of radiation damage, the present findings support the fact that the substrate, and not the pigment Prussian blue itself, is determining the stability of Prussian blue composite materials under the beam. Strategies to mitigate radiation damages should thus be targeted on the substrate, especially if this latter is organic and has a tendency to oxidize. In cultural heritage artefacts, this is the case for paper, but also textiles and gelatin, all substrates particularly sensitive to radiation effects. 14,48 This warning about X-ray radiation damage is in line with recent studies 22,28 and applies equally to composite materials such as assembled Prussian blue-polymer nanocomposites for rechargeable batteries and biosensors 49 or the recent growing class of Prussian blue multifunctional materials. 43,44 On the positive side, we could demonstrate that the kinetics and extent of Prussian blue photoreduction can be tuned by modifying adequately the composition of the cellulosic matrix. This not only represents a step further in the understanding of Prussian blue heritage materials from an electrochemical viewpoint, but it opens an interesting perspective in the design of electrochromic printing systems 36 and more generally multifunctional materials based on Prussian blue. Indeed, having a glimpse on the substrate instead of the pigment could be an unconventional but potentially effective way to tune properties... out of the blue.

Acknowledgement We would like to warmly thank Lucie Nataf, Francois Baudelet, Frédéric Picca, the detector group of synchrotron SOLEIL and Mathieu Jacot-Guillarmod for their invaluable help during


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the synchrotron sessions, as well as Emeline Pouyet. Claire Gervais acknowledges the Swiss National Science Foundation for the professorship grant no. 138986.

Supporting Information Available. Raman spectroscopy of Prussian blue and linear combination fitting of XANES spectra are supplied as supplementary materials. This information is available free of charge via the Internet at http://pubs.acs.org.

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TOC Graphic. PBreduced PB

H+

7115 7120 7125 7130 7135

Energy (eV)

hν

K

u cell

+


fibe c i los

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X-ray Photochemistry of Prussian Blue Cellulosic ... - ACS Publications

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