[Ifeffit] [Ext] Re: Any coorelation about 'white line" in K-edge XAS with oxidation states?

[Qingying Jia]

Interpretation of the K-edge XANES spectrum of 3d metal is not easy. The
forbidden s-d transition contributes only a small part of the XANES spectra
(normally the "pre-edge" peak). A rather comprehensive explanation of
several features of the Fe K-edge XANES spectra can be found in a paper ( *ACS
Nano* 2015, 9, 12, 12496–12505, Figure 2 and discussions). Ni should be
similar. The "white line intensity" of 3d metals is affected by many
factors including oxidation state but actually not that strongly. O
coordination number and especially source (O, OH, H2O) also matter.
Regarding the in-temperature XAS, you may refer to our recent publication (*J.
Am. Chem. Soc.* 2020, 142, 3, 1417–1423) that gives a good example showing
how FeOx transforms to metallic Fe or Fe-Nx with increasing temperature,
providing temperature-dependent both XANES and FT-EXAFS spectra.

  Jia

On Wed, Dec 15, 2021 at 8:59 AM 王茂林 <maolinwang at pku.edu.cn> wrote:

> Dear Robert:
>
>     Thanks for your such a detailed and quick reply!
>
>     Here I get some points: Because s→d transition is forbidden, we cannot
> assure that the higher "white line" intensity, the higher the metals'
> oxidation states are. But we can also get metal's oxidation information via
> comparation the edge positions and features in spectrum with standard
> sample (in this case is Ni foil and NiO, the "white line" intensity of NiO
> is higher, so when the "white line" of spectrum in samples is getting
> higher, the formal oxidation number is higher). (First we assume that
> ligands are all oxygen atoms.)
>
>     I'm a beginner of EXAFS, is that right? Thanks again for your
> patience!
>
>
>
> Marco Wang
>
>
>
>
>
> -----原始郵件-----
> *發件人:*"Robert Gordon" <ragordon at alumni.sfu.ca>
> *發送時間:*2021-12-12 12:04:35 (星期日)
> *收件人:* ifeffit at millenia.cars.aps.anl.gov
> *抄送:*
> *主題:* Re: [Ifeffit] Any coorelation about 'white line" in K-edge XAS with
> oxidation states?
>
> Hi Marco,
>
> Yes, a s - d transition is dipole forbidden, but the quadrupole transition
> can occur, as can a transition
> s - mixed p/d. The edge position in XAFS for metals like Ni and Cu is
> taken at the inflection point leading to
> the first main absorption feature, but is considered to have s - d
> character. For NiO, features near this edge
> indicator in the figure would be considered pre-edge. For NiO, the edge
> position would be considered
> shifted from that of Ni foil.
>
> The white line for a K-edge generally corresponds to a noticeable number
> of unoccupied states with p character
> on the absorbing atom in the presence of the core hole (i.e. intermediate
> states) with "small" energy dispersion.
> Look at the energy difference between the edge indicator and "white line"
> indicator. That is about 18 eV.
> For most materials, the work function is 4 - 6 eV between the highest
> occupied states and the continuum
> in the ground state. Edge position is sensitive to charge transfer between
> target atom and ligands, not specifically
> a formal oxidation state. What you see is not a measurement of the ground
> state Density of States, but an influence
> of the core hole on intermediate states, and of the emitted photoelectron
> during the lifetime of the corehole.
>
> Much of this terminology also arises from consideration of bulk materials,
> not nanoscale.
> Features in the XANES can arise from long range interactions, which are
> lessened as the scale
> gets considerably reduced.
>
> The figure shows a clear transition from something with a pronounced white
> line feature
> to something with a less-pronounced feature and increased intensity near
> the foil edge energy, which I would
> interpret as a loss of coordinating oxygen yielding something more
> metallic, or at least more covalent,
> in character on the substrate. I am curious as to what Ni2Mo3N Ni K-edge
> would reveal in comparison
> to their results. It is not unreasonable for the authors to conclude a
> reduction of the Ni is occurring by loss
> of coordinating oxygen - "fully reduced to Ni(0)"...well, that is a bit of
> an overstatement - "extent of electron transfer
> from the Ni reduced to that of a more metallic/covalent environment" would
> be more exact, but I understand
> what is meant and would not argue with another author's writing style if
> it is not an egregious overstatement.
>
> So, to answer your question:
> "how should we correctly interpret the “white line” and edge energy from
> XAS spectrum of metals’ K-edge and L-edge?"
>
> We do this by a careful consideration of known standards and knowledge of
> the influence of bonding
> environment and extent of electron transfer between absorber and ligands
>
> The concept of formal oxidation state (oxidation/reduction) is a
> convenient means of discussing electron
> transfer in more ionic systems but less useful as the degree of covalency
> increases.
>
>
> -R.
>
>
>
>
>
> On 2021-12-11 6:11 a.m., Marco Wang wrote:
>
> Dear everyone:
>
>
>
> Ihave some questions about the interpretation of "white line", from
> anarticle (https://doi.org/10.1038/s41467-021-27116-8) and the figure was
> attached. In the article, it is said that *“the sample was fully reduced
> to Ni (0) state at ~480 °C judging from the edge position and ‘white-line’
> profiles”*.
>
>
>
> First, we assumed that the metal was coordinated with the same anion.
>
>
>
> We all know that *K*-edge transition in 3d and 4d metalis' spin
> forbidden, so it may be inappropriate to conclude that the intensity of
> the "white line" is correlated with metals' oxidation states. But in other
>  words, the probability for s→d transition may be proportional to
> thenumber of empty d orbitals, so it seems like if the the intensity of "white
> line" is higher, the oxidation state is higher.
>
>
>
> So, how should we correctly interpret the “white line” and edge energy
> from XAS spectrum of metals’ *K*-edge and *L*-edge?
>
>
>
> Thank you in advance for your time.
>
>
>
> Marco Wang
>
>
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