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Sun Apr 22, 2018, 08:58 AM

A Minor Problem With Visible Light Up-conversion to UV for Pollutant Destruction: It's a Myth.

Since the mid-20th century, a large amounts of seriously recalcitrant organic pollutants - most often organohalides - have been accumulating, sometimes at dangerous levels, particularly in the tissue of animals high in the food chain, including but not limited to human beings. One can read lots of papers about these molecules in breast milk (and the milk of other species) for example.

The long term persistence of these molecules is related to the strength of the chemical bonds that define them, and these in turn are defined, in quantum mechanical terms by a set of molecular orbitals, most often characterized mathematically as three dimensional wave functions. In quantum mechanics, the transition from one state (represented by its wave function) to another state (represented by a different wave function) requires a certain minimal energy, to wit: It is not possible for a large number of energy packets (quanta of light) to break a bond whose energy transition exceeds the energy associated with its wavelength/frequency.

The energy of light is of course, described by the famous Planck equation, which most people who take science courses encounter in high school, or in the worst case, early in their college careers:

An unfortunate feature of our times, wherever environmental issues are discussed, is sun worship - an enthusiasm for all things "solar" - that may hark back to some ancient religious fervor, I don't know, as I'm not a participant.

Scientists are not magical beings separate from humanity; they are pretty much like everyone else, and as such they can get caught up in popular fads, and run with ideas that prove to be nonsensical. Some years back, stuck in some airport somewhere and bored out of my mind with nothing to read, I came across a delightful little book all about these kinds of adventures in scientists kidding themselves, this one: Yes, We Have No Neutrons: An Eye-Opening Tour through the Twists and Turns of Bad Science. It's all about "Cold Fusion," "IQ testing" "N-rays" etc, etc, etc...

A fun read, take it as you will...

By the way, bad science does not always involve bad scientists. Highly intelligent and highly competent people can often mislead themselves terribly. Martin Fleischmann of "Cold Fusion" fame was throughout his career highly regarded as an electrochemist, the author of more than 250 scientific papers, which is why when he erroneously reported "cold fusion" he was taken - initially at least - seriously.

To return to the point of recalcitrant pollutants, it should be pretty clear that since they often appear in surface waters - the Hudson River has long been polluted with polychlorobiphenyls (PCBs) - solar energy doesn't do a very good job at decomposing them. If it did, they would tend to disappear, at least on sunny days in summer.

They, um, don't.

This is because visible light, although it is sufficiently energetic to break some chemical bonds - notably the hydrogen-oxygen bond in water, which is, of course, how photosynthesis works - is insufficiently energetic to break the (aromatic) carbon chlorine bond in a PCB.

At a minimum, higher frequency radiation is required to break these bonds, at a minimum short wavelength ultraviolet radiation, although x-rays and gamma rays are even better. Now even though humanity is working hard at destroying the ozone layer which absorbs and degrades UV radiation, it's a good thing that we can't have "solar remediation" of PCB's, since energy at the wavelength that destroys them would also destroy many of the molecules in our flesh; a fact that is connected with the ever rising rates of melanoma, a largely still incurable cancer that is involved in primitive or more sophisticated sun worship.

However the culture of enthusiasm of all things solar so much as it appears in science has lead many people to speculate that it is possible to "upconvert" visible wavelengths of light into UV light which is known to work, particularly in the presence of titanium dioxide catalysts to degrade recalcitrant pollutants.

I am hardly an expert in the theory of this stuff, by the way. In fact the first time I heard of it is in the paper from the primary scientific literature written by scientists at Yale and Clemson, that I will now discuss, which claims, that the whole affair is, in fact, a myth: The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials. (Cates et al, Environ. Sci. Technol., 2018, 52 (5), pp 2973–2980)

From the introduction, which covers the very real basics about titanium dioxide catalysts and the mechanism of their actions, and a brief summary of the controversy:

Heterogeneous photocatalysis has maintained a strong foothold in water treatment technology research since the explosion of studies involving TiO2 began in the 1990s. Among the many uses of semiconductor photocatalysts proposed by academia in the environmental fields, advanced oxidation is the most common; therein, production of hydroxyl radicals by catalyst suspensions in photoreactors is seen as a “chemical free” alternative to H2O2 and O3-based unit processes for destruction of recalcitrant water contaminants.1,2 While the most effective photocatalytic materials have band gap energies (Eg) that demand UV-range excitation wavelengths, many groups have pursued catalysts that are activated instead by lower-energy visible light.3−6 These efforts are motivated by the prospect of replacing energy-intensive UV lamp reactors with solar reactors, though the operational practicality of this concept has been debated.7 The primary route to visible light activation of semiconductors is through the use of materials with sufficiently low Eg or intraband states to absorb such light and promote valence electrons into the conduction band. Inherently, however, a lower Eg generally implies weaker redox potential of the resulting conduction band electrons (e− cb), valence band holes (h+ vb), or both.8

Here's a picture of the proposed mechanism that people reporting "upconversion" claim:

The caption:

Figure 1. Mechanism of visible-to-UV upconversion sensitized photocatalysis disputed herein. Green circle, UC phosphor particle; white circles, semiconductor photocatalyst particles.

In the paper the authors describe the synthesis of some of these putative upconversion catalysts and their characterization and the proposed mechanism under which they work.

The authors discuss the mechanism of upconversion, which by the way is according to them, as I understand it, a known phenomenon, but one which has very limited intensity:

The caption:

Figure 2. (A) Mechanisms of visible-to-UVC conversion by YSO: Pr3+.(16,17) (B) Primary mechanism of visible-to-UV conversion by Er3+-doped phosphors.(35) (C) Upconversion emission spectra of phosphors under 488 nm laser excitation (140 mW); inset shows YAG:Er3+ and undoped YAG spectra with adjusted axes. The peaks seen at >430 nm are due to leakage of the excitation light from the monochromator.

Anyway, they make the catalysts and then test them against a series of well known persistent organic pollutants, dyes used in the preparation of clothing.

Figure 4. (A) Decoloration of MB by phosphor-TiO2 composites under 488 nm laser irradiation. (B) MB decoloration by composites and controls under white LED irradiation. (C) Degradation of various dyes by composite materials under white LED irradiation for 240 min; solid and striped bars depict Ln3+-doped and undoped phosphor materials, respectively. (D) Degradation of phenol under white LED irradiation. Error bars denote standard deviations of experiments performed in triplicate.

They compare these unimpressive results with actual direct irradiation of "MB" methylene blue with UV radiation not resulting from "upconversion:"

Figure 5. Effect of isopropanol •OH scavenging on photocatalytic degradation of MB by YSO: Pr3+/TiO2 composite particles under W-LED (visible) and UVA irradiation. Error bars indicate standard deviations of triplicate experiments.

In their discussion the authors write:

Above we have shown that UV UC is not responsible for enhanced dye degradation rates by phosphor/TiO2 composite materials; however, our experiments did indicate that the presence of the YAG:Er3+, YSO : Pr3+, or undoped host enhanced dye degradation rates by phosphor/TiO2 composite materials; however, our experiments did indicate that the presence of the YAG : Er3+, YSO : Pr3+, or undoped host components may result in statistically significant MB degradation enhancements compared to TiO2 alone. This is enhancement is only observed when the system is excited within the MB absorption range, and it does not require absorption by the Ln3+/dielectric component. Control experiments using simple, unfused mixtures of phosphor and TiO2 particles resulted in no enhancement (SI Figure S7), thus clearly indicating that the presence of epitaxial YSO/YAG− TiO2 interface is required for the catalytic effect.

They offer some reasons for why they think that many of the papers on this subject are not reproducible or valid:

In our opinion, the overwhelming majoring of UC-PC literature exhibits three critical experimental flaws: first, the syntheses and quality of the UC materials are highly questionable, typically using low purity stock chemicals and showing XRD patterns that indicate phase impurities (e.g., see ref 21, Figure 1a and b therein). It is well-known that optical propertiesespecially in UC materialsare highly affected by certain transition metal impurities, necessitating the use of 99.99% purity or greater stock chemicals.50 Second, in order to conclude that UC is responsible for the catalytic enhancements, the use of the nonactivated phosphor host material (e.g., undoped YAP or YAG) in control experiments is required in order to confirm that opticaland not chemicaleffects are responsible. Only the study by Feng et al. included such a control, finding that their YAG:Er3+ sample enhanced MB degradation to a greater extent than YAG under white fluorescent lamp excitation (∼12% vs ∼8% degradation after 2 h), though both performed better than TiO2-only (∼5% degradation).10

Finally, UC-PC authors rarely include UC emission spectra of the materials in their publications, which is certainly a minimum requirement for demonstrating UC capability prior to applying this phenomena to environmental technology. Studies unrelated to photocatalysis have shown visible-to-UV conversion by the same aforementioned Er3+-doped systems, though they used single crystals under pulse laser excitation with sophisticated detection systems.35−37,51 Furthermore, the UC spectrum of YSO: Pr3+ provided by Wu et al., as well as one in a more recent YSO: Pr3+ UC-PC paper, were measured with fluorescence spectrometers equipped with a xenon lamps.22,52 Based on our experience in characterizing YSO: Pr3+, cyan or blue laser excitation in excess of 50 mW and phase-sensitive detection are required to resolve the visible-to-UV anti-Stokes emission spectrum.13,26 Their spectra instead appear to be normal Stokes emission that resulted from unintentional UV excitation of the sample by second order diffraction from the source monochromator in the absence of appropriate long-pass filters. This same instrumental blunder has been implicated in false reports of UC by carbon quantum dots,53 and illustrates the importance of avoiding accidental UV excitation when attempting to measure upconverted visible light.

Too bad. One kind of wishes it worked. We could just dump titanium oxide doped with lanthanides in the Hudson River and be done with it, but we won't done with it.

Right now the PCB contaminated riverbed of the Hudson River is being dredged, but the real problem is that this is just moving a pollutant from one place to another: There is no safe place to dispose of the dredged soils really.

Speaking of another "solar will save us" degradation, the degradation of the planetary atmosphere, the vast "solar revolution" has been notoriously ineffective on that score as well. The Mauna Loa observatory seemed to be off line for a few days - I thought Trump and Pruitt had defunded it in my less than inappropriate paranoia, but it came back on line. The reported concentration on April 21, 2018 was 411.14 ppm which is, to my knowledge, the highest value ever measured there.

Be that as it may, we seem to be falling asleep worshiping the sun.

Metaphors aside, don't do it yourself. Melanoma is a tough disease: I lost two good friends to it.

Have a pleasant Sunday.

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