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Sat Feb 24, 2018, 06:41 AM

Highly sensitive, uranium based UV detectors.

I am fascinated by the remarkable chemistry of the actinide elements because of the interesting chemistry of the 5f orbitals.

(One of the "actinides," thorium, strictly has limited or no 5f chemistry, although its considered an actinide nonetheless, for convenience.)

One of the interesting things about the actinides, all of which are radioactive, is that they are excellent shielding materials for high energy radiation, owing to the fact that they have so many electrons - uranium, for example has 92 - making it possible for them to have many electronic transitions, and because they are massive, their inner electrons can absorb very high energy radiation to emit "Auger electrons."

Thus I was fascinated by a paper published recently in the wonderful - if overly dense - journal ACS Appl. Mater. Interfaces, specifically, this one: Highly Sensitive Detection of UV Radiation Using a Uranium Coordination Polymer, published by scientists at the Key State Laboratory of Radiation Medicine and Protection and the School for Radiological and Interdisciplinary Sciences and a few other institutions in Sozhou, China. (Wang et al, ACS Appl. Mater. Interfaces, 2018, 10 (5), pp 4844–4850)

(The Chinese government doesn't hate science quite as much as our government hates it, which is why they are going to eat us alive in the 21st century.)

Here's the introductory text from the paper:

Ultraviolet radiation is widely used in chemical industries, such as curing and photolithography, sterilization, surface modification technique, and so forth,1−3 but can exhibit either positive or negative impacts on human health. For instance, UV radiation is crucial for assisting human skin to produce vitamin D that is necessary in physiological processes.4 Excessive doses of UV radiation, however, impose great damage on the human body and may result in the development of cutaneous malignant melanoma (CMM) and non-melanoma skin cancer (NMSC),5 leading to premature skin-aging and eye disorders. 6,7 Besides these physical impacts, developing efficient UV photodetectors is also highly desirable in automotive, aerospace, environmental, and biological researches.7 Currently, various techniques have been developed to detect UV radiation both qualitatively and quantitatively. The most developed semiconductor photodetectors, including metal−semiconductor− metal (MSM) detectors,8 PIN photodiodes detectors,9 p− n junction diodes,10 and Schottky barrier detectors,11 often suffer from several disadvantages, such as sophisticated synthesis and manufacturing procedure, not being able to measure the accumulated UV dosage as well as high defect density in the material. The latter greatly lowers the detection sensitivity and efficiency.12


The authors propose a uranium based detector for the following reasons:

Uranium, the most critical 5f element in the nuclear fuel cycle, is chosen in this work as the metal center based on the following considerations. First, depleted uranium is an abundant long-half-life radioactive byproduct of the nuclear power industry that receives limited studies in luminescent coordination polymer systems compared with other metals. Second, uranyl luminescence originating from the HOMO− LUMO transition of hybridized molecular orbitals often exhibits brighter emission and more efficient absorption of UV light than trivalent lanthanides owing to the non Laporte forbidden nature which greatly extends the detection limit.34 Third, given the 5f/6d orbitals of uranyl are deeply involved in coordination, the luminescence is highly sensitive to the coordination environment, which affords more opportunities for developing detection ability (i.e., more efficient energy transfer).35


LUMO here refers to the "lowest unoccupied molecular orbital" and HOMO to the "lowest occupied molecular orbital." Transitions between molecular orbitals (or in some cases atomic orbitals), defined by quantum mechanics, determine the properties of radiation absorption and emission, not only at high energy levels such as those observed for UV, X-rays, and gamma radiation, but also in the visible range: Color is a function of these effects.

The authors synthesize a "MOF" - a "metal organic framework" - a class of materials that has been the subject of vast amounts of research in recent years. This particular framework is built from uranium atoms, nitroisophtalic acid and dimethylformamide.

Here's a graphic describing the structure of this framework:



The caption:

Figure 1. Crystal structure depictions of 1, where hydrogen atoms are omitted for clarity: (a) coordination environment of uranium(VI); (b) asymmetric unit of [UO2(L)(DMF)]; (c) 1D metal–organic chain of 1 composed of 5-nitroisophthalic acid linked asymmetric units; (d) pseudolayered structure comprising 1D chains coalescing due to π···π interactions. Atom colors: U = green, O = red, C = black, N = blue


Although the molecules luminescence nicely, after long term irradiation, the intensity of the luminescence fades:



The caption:

Figure 2. (a) UV dosage dependent luminescence spectra of 1 performed on a single crystal to show the quenching effect under 365 nm UV light. (b) Correlation between the quenching ratio and radiation dosage. Inset is the correlation between D/[(I0 – I)/I0 %] and the UV dosage. (c) is the corresponding luminescence photographs of a single crystal after receiving continuous UV radiation.


Surprisingly however, this effect seems not to relate to structural degradation of the molecular organic framework, which demonstrates remarkable structural integrity even upon irradiation with higher energy wavelengths, to wit, x-rays and gamma rays, as is shown in the XRD (X-ray diffraction pattern) graphics shown:



The caption:

Figure 3. (a) PXRD patterns for samples irradiated with UV, 100 Gy X-ray and 100 kGy γ-ray radiation. (b) EPR spectra of 1 before and after UV, 100 Gy X-ray, 100 kGy γ-ray radiations.


The EPR (Electron Paramagnetic Resonance) spectra clearly shows the persistence of free radicals, thought to reside on the dimethylformamide ligand:



The caption:

Figure 4. (a) Optimized geometry structure and bond parameters of ground state DMF molecule. (b) Optimized geometry structure, bond parameters (left), and net spin density (right) of triplet DMF· radical. (c, d) Simulated radical-free and radical-bearing coordination structures of the fragment, named as uranyl-5-NIPA-DMF and uranyl-5-NIPA-DMF·, respectively. Bond parameters are labeled below each structure.


I love this last graphic, because one doesn't get to look at electron density diagrams of molecular orbitals resulting from the mixing of f orbitals all that much:



The caption:

Figure 5. Density of states (DOS) of (a) the isolated uranyl molecule, (b) the uranyl-5-NIPA-DMF complex, and (c) the uranyl-5-NIPA-DMF· complex. The gray-filled and empty areas below DOS curves indicate the occupied and unoccupied states, respectively. For each DOS, the lowest unoccupied U(5f) orbital is normalized at 0 eV for convenience.


The authors thus conclude:

In summary, a highly stable uranium coordination polymer was successfully synthesized through solvothermal method that exhibits superior sensing property. The intrinsic luminescence of 1 could be quenched by UV which makes it suitable for monitoring UV radiation. The radical-induced quenching mechanism confirmed by EPR, X-ray crystallography, and DFT calculations studies corroborates this property of 1...

... This work provides us new opportunity for searching powerful UV responsive materials by taking advantage of efficient UV light asbsorber (uranyl) as metal center. We further noticed that many other uranyl hybrid materials constructed from different types of ligands and solvents may exhibit similar properties, which can be therefore fine-tuned by varying uranyl coordiation enviorments, crystal structures, and chemical constituents (e.g., light sensitizer), and the systematic investigations are in progress. We also believe this work offers new insight into methods in which depleted uranium may be reused for beneficial purposes.


It's a fine paper, but I will note that my preferred use for depleted uranium is as a precursor to plutonium as a nuclear fuel.

The interesting thing for me about this paper is the stability of this framework in a high radiation field. This suggests it's use as a "breathable" nuclear fuel, albeit one that would operate in a thermal spectrum, thus of use in thorium derived U-233 systems as opposed to plutonium breeding systems.

An interesting paper I think.

Have a nice weekend.

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Reply Highly sensitive, uranium based UV detectors. (Original post)
NNadir Feb 2018 OP
eppur_se_muova Feb 2018 #1
NNadir Feb 2018 #2
eppur_se_muova Feb 2018 #3
NNadir Feb 2018 #4
eppur_se_muova Feb 2018 #5
NNadir Mar 2018 #6

Response to NNadir (Original post)

Sat Feb 24, 2018, 10:28 AM

1. 1 Gy (1 Gray) = 100 rad

Last edited Sat Feb 24, 2018, 12:33 PM - Edit history (2)

https://en.wikipedia.org/wiki/Gray_(unit)
Had to look that up.

Reminds me: need to download the latest version of DIRAC.

http://diracprogram.org/doku.php

Wow, I haven't visited their site in some time. Many updates since, and lots more info now. Used to have a little beginner's introduction to relativistic effects -- transcript from a short presentation given in Dutch.

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Response to eppur_se_muova (Reply #1)

Sat Feb 24, 2018, 04:42 PM

2. Reminds me of the famous story about Oppenheimer told in "American Prometheus."

Oppenheimer was a professor at UC Berkeley and decided to go to Europe for a semester and asked one of his colleagues to teach one of his classes.

His colleague asked Oppenheimer what book he should use for the course, and Oppenheimer told him.

The colleague looked into the book and went back to Oppenheimer and said, "But Oppenheimer, the book is in Dutch!"

"Yes," Oppenheimer replied, "but it's easy Dutch..."

I once took my boys and a friend of theirs to meet Freeman Dyson, one of the most amazing afternoons ever, a strange story in itself. We talked about so many things, and I never got around to asking him about Oppenheimer...

Damn...

Anyway.

I wasn't familiar with DIRAC at all; never heard of it in fact. Thanks! Looks like fun! I was aware of relativistic effects in the core electrons of heavy and superheavy elements, but not of this program.

I just recalled that I once wrote elsewhere about "Feynmanium."

Oh. Oh. Plutonium Contamination Suspected.

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Response to NNadir (Reply #2)

Sat Feb 24, 2018, 06:51 PM

3. Hey, you jumped ahead to the punch line ! :D

I've always wanted to find a real-life situation into which I could drop that quote. But there's that risk that anyone who is likely to get the joke has probably already heard it ...

(To split hairs: the speed of a core electron is predicted to exceed c only in a non-relativistic Bohr model; in the Dirac equation it approaches infinitesimally closer as Z increases. There's a famous quote that the Schrodinger equation tells us everything there is to know about chemistry, if only we knew how to solve it. In fact that would only be true in a non-relativistic universe ! In our real universe we need to solve the Dirac equation.)
(I read once -- don't have the reference at hand, sad to say -- that above a certain Z the electric field gradient at the surface of the nucleus becomes so great that electron-positron pairs can be created from the vaccuum without further energy input. The positron gets ejected, while the electron combines with a proton in the nucleus. So this is the ultimate upper limit to nuclear charge, apparently. Maybe around 173(?) but as I say I'm working from an old memory.)

That Dutch article was probably around the same level as this review by Pyykkö: https://www.annualreviews.org/doi/abs/10.1146/annurev-physchem-032511-143755 , so maybe there's nothing new there for you. (Pyykkö was the one who predicted that Oganesson (eka-Radon) would form a stable anion).

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Response to eppur_se_muova (Reply #3)

Sat Feb 24, 2018, 07:59 PM

4. Well, to be honest, I never actually considered the situation on that level...

...since my interest sort of wanes after einsteinium. I have this bad habit of being interested only in elements I can see, and one can, in fact, see photographs of Einsteinium.

I was actually at the 1998 ACS meeting in San Diego where the Seaborgium name was announced, and I attended a lecture on Einsteinium - I actually forgot who was speaking - but Seaborg himself showed up and sat not too far from where I was sitting. They acknowledged him, and he stood and waved to everyone. The title was, if I recall correctly, "Einsteinium, the last visible element.

It was a small room.

I was kind of in awe, sort of like a kid encountering his favorite rock star.

I do have a number of pretty good books on actinide chemistry, including the Seaborg/Loveland classic, but they're old now, as I am old.

I did recently download the 4th edition of The Chemistry of the Actinide and Transactinide Elements (3rd ed., Volumes 1-5), which Princeton recently put on line in their library. (The link here is to the third edition.)

I won't tell you I've read much of it, but I probably will wander through it eventually.

I wasn't aware of the Dirac approach to approaching high Z. There had to be a solution and thank you for providing it. I always thought of it as a kind of joke.

I will download the paper you linked though; it looks interesting.

I am aware of course of relativistic effects in electron configurations; Seaborg covered this in his book years and years and years ago, but I'm sure your link is far more up to date.

More and more I'm interested in my favorite actinide, plutonium, and I'm also getting very interested in neptunium because of its remarkable non-proliferation value.

These two elements I think offer the last best hope for humanity and what's really, really, really, really cool about them is that they form a very low melting eutectic in the metallic state.

Here's the connected phase diagram of the actinides:



( Seigfried Hecker, “Plutonium and Its Alloys, From Atoms to Microstructure” Los Alamos Science No. 26, 290-335 (2000) pg. 301)

Very cool those um, visible actinides...

My youngest son is locked and loaded with that Oppenheimer joke by the way. He actually is fluent in French, decent in Chinese and Spanish, and has a working knowledge of Japanese, Russian and Italian. His ABC friends, (American Born Chinese) are all impressed that he can read and write Chinese characters and they, though they speak the language fluently having learned it from their parents, apparently can't.

He doesn't know Dutch though, as far as I know, also doesn't know German.

He's way smarter than his old man on every level though, way smarter.

Thanks.

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Response to NNadir (Reply #4)

Sat Feb 24, 2018, 11:06 PM

5. It's been said that we run out of stable nuclei "just in time" ...

... because the electronic configurations threaten to get really hairy further on, with the gap between spin α and β e-s getting larger than the gaps between sublevels, so that the next d-subshell might fill halfway before e-s begin to occupy f orbitals -- or likewise f half-filled then starting on g. Yikes, a Periodic Table with variable-length rows !


I read the "baby" version of Seaborg back in high school -- some of it I really didn't understand at the time, and tracked down a copy of it this year to re-read. Really amazed me how **once you had an intense neutron flux available**, the synthesis of several new elements proceeded according to fairly straightforward logic. Seaborg also made it pretty clear that there really wasn't much chance of making macroscopic quantities of further elements, and why not. And those cool pics of microscopic amounts of the new elements ! But mostly, what a unique period for science that was !



That and Jolly really piqued my interest for something beyond the chemistry I knew (I actually read these before high school chemistry). Ireland just didn't have the same effect on me (mostly because I didn't understand all the terms and abbreviations in what was really a college-level book), though the synthesis of cubane managed to catch my attention. Kind of odd that I ended up in organic, given that.



It only lately occurred to me to wonder why one particular library in a small Florida town had these particular books on its shelves, given how sparse their collection was otherwise. Two paths in a wood, and all that.

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Response to eppur_se_muova (Reply #3)

Wed Mar 7, 2018, 10:42 AM

6. I picked that Annual Reviews paper up.

I was familiar, in a general sort of way, with relativistic effects being responsible for the color of gold, but not with its effect on the color of ammonium hexachloroplumbate or Pb2(NO2)2 or the violet color of pentaphenyl bismuth.

An interesting read.

Thanks.

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