Summary SfES Project, Ivo Jak, 10767991
During February and March 2019, I have spent two months in Dr. Von Hauff’s group. Under daily supervision by Achidi Frick (former Chemistry SfES student, now doing his PhD in Dr. Von Hauff’s group), I have been busy helping assigning functional groups and chemical processes to peaks in Raman spectra. So the research question was the following: which peaks from the Raman spectra could be ascribed to which functional groups from our sample, i.e. LiMnBPO? The project I have been involved in, was a collaboration between the VU and the Technische Universität Berlin (TUB).
The material we have studied is LiMnBPO and is schematically shown in figure 1. The yellow balls are lithium, the red and white balls are H2O, the
green tetrahedra are PO4, the purple tetrahedra
are BO4 and the purple octahedra are MnO6. In
all “hedra”, the B, Mn or P atoms are in the centre, whereas the oxygen atoms are at the corners. All functional groups share one oxygen atom with their direct neighbours, which is schematically shown by the “hedra” touching each other at a certain point. For example, there could be Mn-O-P bonds. However, B-O-B and P-O-P bonds can never be present in this material. Around the anionic backbones (the blue, shadowlike vertical bands), parts of the material take the form of a double helix (Hausmann, 2019).
This material is not just studied for random reasons; there are good motives for the interest in LiMnBPO. After all, it is expected that this material can play a role, as catalyst, in the hydrogen evolution reaction and oxygen evolution reaction (personal communication). These reactions occur in the electrochemical water-splitting process, of which the goal is to form hydrogen and oxygen
Figure 1: Structure of LiMnBPO (Hausmann, 2019).
Figure 2: In the bottom right angle of the left picture, LiMnBPO is shown. This picture has been taken through the oculus of the Raman device. The magnification factor was twenty. In the right picture, it is shown that the Raman device is exactly focused on the angle of the material, which is shown in the left picture. The right picture makes clear what the sample looks like in reality, while put in the Raman device.
(Menezes et al., 2019). And if the process of creating hydrogen is successful, the hydrogen can be used as a fuel, which can replace fossil fuels. So the larger aim of this research is to help the world be a better place by studying ways to provide the world with clean, renewable energy sources. For more background on such catalysts and their uses, see Menezes et al. (2019).
Under normal circumstances, LiMnBPO is a white solid. The sample is shown in figure 2. The sample was prepared by the TUB group. However, it is not just this normal state we have investigated. Rather, the overall aim of this project was too see what happens if the material was covered with a solution (in our case KOH) and consequently exposed to a potential. When it is exposed to a potential, the material changes, which sounds rather strange when you realise that LiMnBPO should act as a catalyst. And after all, catalysts should not be influenced by a reaction. The changes are easily observed; the sample turn brown and the structure is not crystalline any more, as it used to be before having been exposed to a potential, but amorphous.
Nevertheless, during one of our measurements, on Friday 08-03-2019, the most remarkable thing occurred. When we were just measuring the sample before it had been exposed it to a potential, it turned black. Unfortunately, no pictures were taken of the blackened spots. The blackening was something we had not observed yet and did most certainly not expect to see. After all, the only thing the sample had been exposed to, was the monochromatic light of the laser. But when it happened even more than once, we saw a relation between changing the laser’s wavelength and the sample turning black. Sometimes, when we changed the laser from 532 nm to 785 nm, or the other way around, we observed this strange process. However, due to us not being able to explore this in further detail that day, we decided to look into it later. But here comes the strange part: when we did the exact same thing again (the only thing that differed was the fact that we were using one of the VU’s samples instead of one of the TUB’s samples), we did not see a colour shift. Hence, we have unfortunately not been able to find the causes of this colour shift.
The method we used to measure the spectra of LiMnBPO is Raman spectroscopy, which works as follows: when a laser’s monochromatic light interacts with a sample, the photons can be transmitted, absorbed or scattered. In the case of Raman scattering, as opposed to Rayleigh scattering, a very small part of the photons, approximately 10-5 %, undergoes an energy shift and thus a wavelength
shift. And the photons with a different wavelength are measured. The energy shifts occur because photons exchange energy with the vibrations of the atoms or molecules in the sample (Renishaw, 2019a). These particles can either go to a higher frequency (Stoke shift) or a lower frequency (anti-Stoke shift). The sample is slightly influenced by this method. Consequently, multiple Raman measurements will never give the exact same spectra. However, that does not mean that Raman spectroscopy is destructive method. Rather, one can always repeat the measurements. Raman spectroscopy is particularly useful for measuring the sample’s vibrational modes and structure (Joya & Sala, 2014; Yadav & Singh, 2015).
The VU possesses two different lasers in its Raman device (Renishaw, UK). One has a wavelength of 532 nm and the other one has a wavelength of 785 nm. Unfortunately, the former was broken when I arrived and it took until the second week of April to get it repaired. On the twelfth of April, in situ measurements were performed. But at the moment of writing, the results had not been analysed yet. For the in situ measurements, i.e. the measurements during exposing the sample to a potential, the 785 nm laser cannot be used, because this wavelength does not go well with the glass which covers the sample during in situ measurements (personal communication). This is due to the fact that glass can ‘mask’ the Raman signals when the 785 nm laser is used (Renishaw, 2019b). However, measuring the sample before and after the process is still possible.
The procedure for Raman measurements is not very complicated. What has to be done first, is controlling whether the device works well by measuring a silicon sample. After this, one can initiate
the real measurements. One puts the sample underneath an objective (there are objectives with factors 5, 20 and 63, normal or long range). By looking through the oculi, the sample has to be put in focus. When this is done, it has to be made sure that no light can interfere with the measurement. Then, the correct settings have to be installed and the measurement can be performed. Only when the measurement is started, the laser is put on. The Raman device is connected to a computer, on which the program WiRE (Renishaw, UK) is installed. Then, the data are exported to Origin, with which the final analyses are done.
The most important spectrum that has been investigated is the spectrum that is shown in figure 3. This spectrum has been obtained by Achidi and shows the results of measuring the sample before having been exposed to
a potential. According to the TUB group, we should have seen MnO6,
BO4 and PO4, which we
also did. By going through lots of scientific articles and comparing their results to ours, we have been able to assign the physical and chemical properties of the sample to each peak (e.g. El khalfaouy et al., 2016; Frost et al., 2013; Julien et al., 2003; Myint et al., 2006). See table 1 and the references for more details.
The main findings are the following. They are also shown in table 1. Most peaks below 300 cm-1 were
probably due to external lattice vibrations. Between 300 and 500 cm-1, the peaks are weak but broad,
making it hard to assign anything with certainty. Moreover, this region is not the main focus of the study (personal communication). So we have not really focussed on this region. Nevertheless, we have looked for possible assignments to these peaks. At the moment of writing, the TUB group is looking at the details. There is a small chance that these peaks could have been caused by lithium oxygen bonds, but otherwise they will just be assigned to manganese oxygen bonds or certain stretching processes. The peaks in the higher regions could have been caused by multiple functional groups, but it would make most sense if the peaks in the 500-700 cm-1 range had been caused by
MnO6 and the higher peaks by BO4 and PO4. Please note that in these assignments, the hypothesis to
find MnO6 within the 500-700 cm-1 range has been leading. One can find literature which says that
PO4 bending results in peaks in this region (e.g. El khalfaouy et al., 2013), but that is not very
plausible.
Peak values (cm-1) Assignment References
117.421, 179.024, 227.509 External lattice vibrations Frost et al, 2013 342.413, 385.609, 445.321 Lithium oxide events Julien et al., 2003
535.947 Stretching mode of the MnO6
octahedra
Julien et al., 2002 578.152 ν3(Mn-O) stretching vibration in Julien et al., 2003
Figure 3: Restults of Raman Spectography of the LiMnBPO sample before having applied a current.
the basal plane of [MnO6]
638.338 Symmetric stretching vibrations
ν2(Mn-O) of MnO6 groups
Julien et al., 2003; Ramana et al., 2005
No peak at 670-720 No B-O-B bonds Gautam & Yadav, 2013
No peak at 820-836 No B-O-B bonds Gautam & Yadav, 2013
948.671 Symmetrical stretching of B(3)-O
OR PO43- symmetric stretching
mode
Jun et al., 1995 OR Frost et al., 2013
1004.49, 1054.75 Asymmetrical stretching of B(4)
-O
Jun et al., 1995 Table 1:this table shows the most probable peak assignments. The peak values are the same as in figure 3.
In sum, we have contributed to a catalysis research by a TUB group, and eventually to a publication. However, since the writing is done by the other group, at the moment of writing this summary it was not yet known when the article would be published. However, it is my hope that the study will show some interesting results, but above all, I hope that new questions will be raised, studied and answered in the future. One could, for example, pose questions like if LiMnBPO is just a catalyst, why does it change colour and structure during the in situ measurement? Can this material be produced on a large enough scale to create enough hydrogen to act as fuel? Would the costs and environmental impacts of this method be lower than current methods? Are there similar, but more efficient materials? And finally, I would like to thank Achidi for his enthusiasm, help and wisdom, and Dr. Von Hauff for having let me join her group and research project for two months.
References:
El khalfaouy, R., El knidri, H., Belaabed, R., Addaou, A., Laajeb, A., & Lahsini, A. (2016). Synthesis and characterization of LiMnPO4 material as cathode for Li-ion batteries by a
precipitation method and solid-state blending. Journal of Materials and Environmental Science, 7(1), 40-49
Frost, R.L., Xi, Y., Scholz, R., Belotti, F.M., & Lopez, A. (2013). Infrared and Raman spectroscopic characterization of the phosphate mineral fairfieldite – Ca2(Mn2+,Fe2+)2(PO4)2·2(H2O). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 106, 216-223
Gautam, C.R., & Yadav, A.K. (2013). Synthesis and Optical Investigations on (Ba,Sr)TiO3
Borosilicate Glasses Doped with La2O3. Optics & Photonics Journal, 3(4A), 1-7
Hausmann, J.N (2019). LiMnBPO structure [PowerPoint slides]. TUB
Joya, K.S, & Sala, X. (2014). In situ Raman and surface-enhanced Raman spectroscopy on working electrodes: spectroelectrochemical characterization of water oxidation electrocatalysts. Physical Chemistry Chemical Physics, 17(33), 21094-21103
Julien, C., Massot, M., Baddour-Hadjean, R., Franger, S., Bach, S., & Pereire-Ramos, J.P. (2003). Raman spectra of birnessite manganese dioxides. Solid State Ionics, 159(3-4), 345-356
Julien, C., Massot, M., Rangan, S., Lemal, M., & Guyomard, D. (2002). Study of structural defects in γ-MnO2 by Raman spectroscopyo. Journal of Raman Spectroscopy, 33(4), 223-228 Jun, L., Shuping, X., & Shiyang, G. (1995). FT-IR and Raman spectroscopic study of hydrated
borates. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 51(4), 519-532
Menezes, P.W., Indra, A., Zaharieva, I. et al. (2019). Helical cobalt borophosphates to master durable overall water-splitting. Energy & Environmental Science, 12(3), 988-999
Myint, T., Thin, S.S. Kaung, P., & Htoon, S. (2006). Infrared Spectroscopy and Raman Scattering Studies on the Structure of Ag2O.B2O3.TeO2 Glass. Journal of the Myanmar Academy of Arts and Science, IV(2), 305-310
Ramana, C.V., Massot, M., & Julien, C.M., (2005). XPS and Raman spectroscopic characterization of LiMn2O4 spinels. Surface and Interface Analysis, 37(4), 412-416 Renishaw (2019a). Raman spectroscopy in more detail. Retrieved from
https://www.renishaw.com/en/raman-spectroscopy-in-more-detail--25806
Renishaw (2019b). Raman spectroscopy: important considerations. Retrieved from
https://www.renishaw.com/en/raman-spectroscopy-important-considerations--25804
Yadav, A.K., & Singh, P. (2015). A review of the structures of oxide glasses by Raman spectroscopy. RSC Advances, 5(85), 67583-67609
