Lecture 8: Assignment – PB fading

What was requested Analysis of research strategy through paper structure What, Why, Where, How From data interpretation to redox mechanism Putting the study into perspective

Reminder about What, Why, Where, How What? Setting up the foundations of the study Why? Confirming worth to do it Where? Specificities of the material/object of study How? Techniques implemented to perform the study The art of materials characterization lies in choosing technique and measurement scale to the property of interest while ensuring enough statistics.

1. WHAT? Situate the study within its context Prussian blue is a pigment with formula Fe(III)hexacyanoferrate(II) used in many domains, from art to materials sciences. Prussian blue discolors in cultural heritage artefacts exposed to light. The fading is due to a photoreduction involving many factors but the exact redox mechanism remains unknown. Prussian blue is sensitive to radiation damage, thus difficult to analyze with high photon flux techniques such as XAS. Aim of the study To characterize precisely X-ray radiation damage To study the influence of the substrate, cations and protons on Prussian blue reduction behavior.

2. WHY? Explain the importance of this study Prussian blue is widely used in many domains, with promising applications in materials sciences and a large presence in diverse cultural heritage obejcts. The fading of Prussian blue depends a lot on the object, so that is depicted as a problematic and mysterious pigment. Motivation of the study (to characterize potential X-ray radiation damage) and thus assess the validity of XAS to study Prussian blue. (to decipher mechanism behind the fading) and thus better apprehend its capricious fading behavior in art objects.

3. WHERE? From concept study to concrete material Prussian blue model samples: PB in cellulose, cellulose loaded with KCl, and cellulose acidified. No constraints on sample amount or care about destruction of precious sample Heterogeneity of Prussian blue in fibers (crusts, micro and nanoparticles) Need to take it into account for measurement (200x200microns) Prussian blue known to be sensitive to radiation damage. Need to take it into account for measurement (fast measurement)

4. HOW? Practical choice of technique and modalities Theory, concept Practical aspects HOW

2. From data interpretation to redox mechanism 1 4 3 5

3. Putting it into perspective “We cannot solve our problems with the same thinking we used when we created them” Albert Einstein

Example in “real presentation” Presentation given by Claire Gervais at the international conference on X-ray absorption spectroscopy, XAFS16, Karlsruhe, August 2015

Prussian blue conservation issue Discovered accidentally in 18th century and widely used until 20th Fades under strong UV-visible light or anoxic treatment Fading depends a lot on the object investigated, « mysterious » pigment watercolor painting cyanotype trichromie dye

Current knowledge on Prussian blue Structure of Prussian blue is complex Properties and redox process depend on many structural features Complex interplay structure - properties Absorption ~ 640nm e- Fe(III) C N Fe(II) Vacancies Water network Cations Geometry of building blocks Samain et. al., JSR, 20:460-473, 2013

Current knowledge on Prussian blue Fading is due to photoreduction of Prussian blue Influence of the substrate and atmosphere evidenced Exact redox mechanism remains unknown K+, e- Source of cations? Transport phenomena? Corresponding oxidation? Reversibility? Gervais et. al., JAAS, 28:1600-1609, 2013

Photoreduction mechanism in PB papers K+, e- X-ray absorption and emission spectroscopies Oxydation state upon fading Local environment of iron Monitoring of fading kinetics UV-vis, Raman, FTIR spectroscopies Photochemistry and color Substrate degradation Monitoring of fading kinetics Controled history Plenty of material Controled manufacturing Model samples X-ray anomalous diffraction location of FeII/FeIII Disorder in vacancies 14

Outline Prussian blue in heritage materials Kinetics of photoreduction by XANES Radiation damage of Prussian blue X-ray photochemistry of Prussian blue Conclusion

Work on Prussian blue paper model samples ? K+, e- cellulose +KCl 20μm +HCl Model samples 16

Control of environmental conditions Gas RH% X-ray source UV-vis light source Gervais et. al., Applied Physics A, 111:15-22, 2013

Kinetics of photoreduction followed by XANES Beam Size 25 x 30 microns Sampling area: 200 x 200 microns Energy range Fe K-edge (7050 – 7300eV, ~ 1eV) Optics Double crystal Si(111) monochromator Detection transmission mode Data robustness Several kinetics on several samples Image: SPring8, Japan synchrotron website

Kinetics of photoreduction followed by XANES 0 minutes

Kinetics of photoreduction followed by XANES 40 minutes

Kinetics of photoreduction followed by XANES Less intense and broader absorption peak 70 minutes in agreement with iron(III) reduction Energy shift Pre-peak region increases

Kinetics of photoreduction followed by XANES Absorption edge

Kinetics of photoreduction followed by XANES

Kinetics of photoreduction followed by XANES ~2eV PB PB reduced

Kinetics of photoreduction followed by XANES Shift of absorption edge with time ≈ measure of fading kinetics -1eV after 25min -2eV after 70min

Kinetics of photoreduction followed by XANES We can follow the kinetics …

Kinetics of photoreduction followed by XANES We can follow the kinetics of radiation damage! Kinetics obtained without UV-visible light

Outline Prussian blue in heritage materials Kinetics of photoreduction by XANES Radiation damage of Prussian blue X-ray photochemistry of Prussian blue Conclusion

Gervais et. al., Langmuir, 31, 8168-8175, 2015 X-ray radiation damage of Prussian blue goethite Gervais et. al., Langmuir, 31, 8168-8175, 2015

X-ray radiation damage of Prussian blue Two-step process: PB reduction Degradation Gervais et. al., Langmuir, 31, 8168-8175, 2015

So what to do with this beam damage? X-ray radiation damage of Prussian blue So what to do with this beam damage?

X-ray photochemistry X-ray radiation damage of Prussian blue X-ray photoreduction decoupled from other structural degradations X-ray photochemistry UV-visible photoreduction

X-ray photochemistry of Prussian blue in paper K+, e- cellulose 50%RH Gas RH% X-ray source +KCl 70%RH +HCl 50%RH Anoxia 33

Photoreduction varies in rate and extent with the substrate cellulose+HCl cellulose cellulose+KCl

Refinement of the redox mechanism in Prussian blue Solvated cations migrate within lattice during Prussian blue reduction cellulose +KCl (70%RH)

Acidity promotes re-oxidation of Prussian blue via oxygen reduction Refinement of the redox mechanism in Prussian blue Acidity promotes re-oxidation of Prussian blue via oxygen reduction +HCl

In anoxia, the influence of acidity disappears Refinement of the redox mechanism in Prussian blue In anoxia, the influence of acidity disappears N2 50%RH +HCl

Conclusion X-ray radiation damage for micro/nano XAS study of Prussian blue characterized Radiation damage  X-ray photochemistry Role of substrate and atmosphere on Prussian blue redox evidenced, can tune PB chemistry Conservators: to mitigate PB fading, focus on substrate Material scientists: focus on composite PB materials O2 H2O

Perspectives UV-visible photochemistry with quick-EXAFS and lower photon flux (Time-resolved) RIXS to understand spin and charge transitions in PB Cryo-nanoscopectroscopy imaging to visualize PB-fiber interface Acknowledgements: Francois Baudelet, Lucie Nataf, Swiss National Science Foundation for Professorship grant (138986) O2 H2O