1. Lecture 8: Assignment – PB fading

2. 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

3. 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.

4. 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.

5. 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.

6. 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)

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

8. 2. From data interpretation to redox mechanism1 4 3 5

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

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

11. 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

12. 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: , 2013

13. 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: , 2013

14. 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

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

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

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

18. 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

19. Kinetics of photoreduction followed by XANES
0 minutes

20. Kinetics of photoreduction followed by XANES
40 minutes

21. 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

22. Kinetics of photoreduction followed by XANES
Absorption edge

23. Kinetics of photoreduction followed by XANES

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

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

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

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

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

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

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

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

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

33. 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

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

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

36. 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

37. 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

38. 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

39. 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