Ezra L. Cates

Engineering Light: Advances in Wavelength Conversion Materials for Energy and Environmental Technologies

Upconversion photoluminescence (UC) occurs in optical materials that are capable of absorbing low energy photons and emitting photons of higher energy and shorter wavelength, while downconversion (DC) materials may absorb one high energy photon and emit two of lower energy for quantum yields exceeding unity. These wavelength conversion processes allow us to transform electromagnetic radiation so it may be more effectively utilized by light-capturing devices and materials. Progress in designing more efficient organic and inorganic photochemical conversion systems has initiated a recent surge in attempts to apply these processes for practical uses, including enhancement of many energy and environmental technologies. In this review, we introduce important concepts in UC and DC materials and discuss the current status and challenges toward the application of wavelength conversion to solar cells, photocatalysis, and antimicrobial surfaces.

Converting Visible Light into UVC: Microbial Inactivation by Pr3+-Activated Upconversion Materials

Herein we report the synthesis and properties of light-activated antimicrobial surfaces composed of lanthanide-doped upconversion luminescent nano- and microcrystalline Y(2)SiO(5). Unlike photocatalytic surfaces, which convert light energy into reactive chemical species, this work describes surfaces that inactivate microorganisms through purely optical mechanisms, wherein incident visible light is partially converted into germicidal UVC radiation. Upconversion phosphors utilizing a Pr(3+) activator ion were synthesized and their visible-to-ultraviolet conversion capabilities were confirmed via photoluminescence spectroscopy. Polycrystalline films were prepared on glass substrates, and the extent of surface microbial inactivation and biofilm inhibition under visible light excitation were investigated. Results show that, under normal visible fluorescent lamp exposure, a sufficient amount of UVC radiation was emitted to inhibit Pseudomonas aeruginosa biofilm formation and to inactivate Bacillus subtilis spores on the dry surfaces. This new application of upconversion luminescence shows for the first time its ability to deter microbial contamination and could potentially lead to new material strategies for disinfection of surfaces and water.

Photocatalytic Water Treatment: So Where Are We Going with This?

D the Cold War era race to the moon between the United States and the USSR, the Soviets are now believed to have possessed a working lunar orbiter, lunar lander, and functional moon suits at the time of the Apollo 11 success. They lacked, however, a reliable rocket capable of getting this payload to the moon. While the quest for photocatalytic water treatment (PWT) is a bit less awe-inspiring, parallels can be drawn between the moon race story and the failures of this enticing form of water treatment. In a 1996 interview with Chemical & Engineering News, chemist James R. Bolton commented on the surge of recent studies within this new field: “This may be a strong statement, but I think the interest in TiO2 [in aqueous systems] is a good example of scientific hype.” Twenty years later, it is surprisingly difficult to argue that he was wrong. The field has ballooned within academia and spawned a growing number of subfields pursuing new applications of PWT, improved catalysts, and reaction mechanisms. And yet, the fundamental technology has scarcely demonstrated a capability to survive outside the lab. While I am a hopeful believer in PWT and active participant in the field, “hype” is perhaps still the best descriptor of its present driving forces and academic allure. In fact, among the water treatment fields, photocatalytic processes arguably show the widest disconnect between research directions and the actual needs of the water industry. Several recognizable motifs lead off the introduction sections of typical PWT research articles. The more practical studies are motivated by the prospect of enabling large-scale “chemicalfree” ultraviolet PWT reactors for municipal water and wastewater treatment plants. With the growth of the direct potable reuse sector and associated needs for advanced oxidation processes (AOPs), progress on this front needs only to exceed the cost-effectiveness of existing H2O2 and ozone technologies. I believe this is the most promising direction; however, academics often label photocatalytic AOPs as energy-saving, due to their catalytic nature, which is misleading. The Purifics Photo-Cat, for example, is the sole PWT packaged unit that has seen legitimate commercialization for small-scale applications. While it is an impressive design, Benotti et al. found that treatment efficiencies of the system for 1-log reduction of many representative contaminants were in the approximate range of 1−3 kWh/m when considering the energy consumption by the UV lamps. Even this energy requirement places the process on par with seawater reverse osmosis desalination. Moreover, when pumping and catalyst recovery via crossflow ultrafiltration were factored into the Photo-Cat study, the values for total energy consumption rates increased 4-fold. In fact, efficiency improved greatly if the TiO2 in the unit was simply replaced with H2O2 dosing. 2 This result illustrates the breakdown of cost-effectiveness of PWT when translating it from a bench experiment to a useable process. Other ongoing PWT research examines the degradation of specific contaminants for which no “go-to” conventional approaches exist. Examples include photocatalytic reduction of oxyanion contaminants (e.g., NO3 −, ClO4 −), toxic metals oxidation/reduction, and destruction of per/polyfluoroalkyl substances. Even if effective material systems are developed, however, the same implementation hurdles that plague more generic PWT (see ref 3) will be every bit as prohibitive. Until reactor design and catalyst recovery aspects are solved, any new embodiments are unfortunately nothing more than laboratory demonstrations. The majority of remaining studies emphasize the need for visible light catalysts for sustainable solar PWT technology. In an unofficial Google Scholar survey of articles relating to PWT published in the last two years (which excluded air or surface applications), I found that approximately 45% targeted visible light activity or sunlight-driven treatment. I assert, however, that the seemingly urgent need for visible light catalysts is only invoked by those who study them, and the broader water treatment community has little interest in solar processes. This is primarily due to the exorbitant area footprints that would result in replacing UV lamp reactors with solar irradiation. How much solar collection area is required to replace one 1000 W low pressure high-output mercury lamp? Assuming a 40% lamp efficiency, and that the hypothetical visible light catalyst is activated by wavelengths of 450 nm or shorter, 2.6 m of solar exposure (AM 1.5) would be needed to achieve the treatment power of one lamp. For a full-scale process, this area would

Bench-scale evaluation of water disinfection by visible-to-UVC upconversion under high-intensity irradiation

The feasibility of applying visible-to-UVC upconversion (UC) luminescence to enhance the kinetics of solar water disinfection was evaluated using Lu7O5F9:Pr(3+) ceramics incorporated into a solar reactor containing E. coli suspensions. Inactivation was assessed in batch conditions using both laser and lens-concentrated sunlight excitation conditions. Under 840-mW argon laser excitation, the UC efficiency was estimated to be 1 order of magnitude greater than previously reported under lamp excitation and UVC emitted by the reactors resulted in 3.6-log inactivation in 20 min. However, experiments using ~1500 mW of concentrated natural sunlight showed no additional inactivation that could be attributed to UC within the timescale studied. Due to the fundamental and practical limitations of solar focusing, the optical concentration ratio employed herein prevented the excitation beam from achieving the power densities required to attain UC efficiencies comparable to the laser experiments. We also observed that the high intensity of both the laser and sunlight induced rapid photoreactivation by the bacteria, which detracted from net disinfection performance. The results suggest that current UC materials perform inadequately for environmental application; nonetheless, valuable qualitative and quantitative insight was gained that more explicitly defines materials development goals and considerations for application of UC to environmental technology.

The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials

Upconversion luminescence is a nonlinear optical process achieved by certain engineered materials, which allows conversion of low energy photons into higher energy photons. Of particular relevance to environmental technology, lanthanide-based upconversion phosphors have appeared in dozens of publications as a tool for achieving visible light activation of wide-band gap semiconductor photocatalysts, such as TiO2, for degradation of water contaminants. Supposedly, the phosphor particles act to convert sub-band gap energy photons (e.g., solar visible light) into higher energy ultraviolet photons, thus driving catalytic aqueous contaminant degradation. Herein, however, we reexamined the photophysical properties of the popular visible-to-UV converters Y2SiO5:Pr3+ and Y3Al5O12:Er3+, and found that their efficiencies are not nearly high enough to induce catalytic degradations under the reported excitation conditions. Furthermore, our experiments indicate that the false narrative of visible-to-UV upconversion-sensitized photocatalysis likely arose due to coincidental enhancements of dye degradation via direct electron injection that occur in the presence of dielectric-semiconductor (phosphor-catalyst) interfaces. These effects were unrelated to upconversion and only occurred for dye solutions illuminated within the chromophore absorption bands. We conclude that upconversion using Pr3+ or Er3+-activated systems is not a technologically appealing mechanism for visible light photocatalysis, and provide experimental guidelines for avoiding future misinterpretation of these phenomena.

Balancing intermediate state decay rates for efficient Pr3+ visible-to-UVC upconversion: the case of β-Y2Si2O7:Pr3+

Materials that convert visible light into ultraviolet radiation have recently been studied for potential application to new catalytic and antimicrobial technologies. Conversion efficiencies of reported phosphors, however, have remained much lower than the more well-known IR-to-visible upconversion systems, due to a lack of lanthanide activator ions well suited for the required energy transfer mechanisms. Herein, the ceramic pyrosilicate β-Y2Si2O7:Pr3+ was prepared for the first time and its visible-to-UVC upconversion efficiency was optimized. The resulting material showed the most efficient upconversion of this type to date, with a 3.9-fold improvement over the compositionally similar X2-Y2SiO5:Pr3+ benchmark material. Analysis of both the steady-state and time-dependent luminescence behavior revealed that a relatively short-lived 3PJ intermediate state – normally considered detrimental to Pr3+ upconversion – in fact contributed to its efficacy. The pyrosilicate host also resulted in a much longer 1D2 luminescence lifetime, the origins of which are speculated to relate to the precise spacing of the 4f states. Based on the results, we discuss revised criteria for the “ideal” Pr3+ photophysical behavior and host requirements for maximizing UVC conversion efficiency.

Comment on “Intimate Coupling of Photocatalysis and Biodegradation for Degrading Phenol Using Different Light Types: Visible Light vs UV Light”

Biodegradation for Degrading Phenol Using Different Light Types: Visible Light vs UV Light” R published experiments by Zhou et al. describe a hybrid photocatalysis−biodegradation reactor (“ICPB”) for treating water contaminated with recalcitrant organic compounds. One particular aspect of their work is scrutinized herein, within the context of what I believe to be a flawed body of work, with contributions from several groups. The paper perpetuates poorly supported claims of upconversion-sensitized photocatalysis involving erbium-doped phosphors, the implications of which likely misguided the interpretation of their results. The novel aspect of the Zhou et al. paper was the use of visible light irradiation and a visible light-active photocatalytic system in the ICPB. A TiO2/YAlO3:Er 3+ composite material was used, which allegedly converted visible wavelengths to UV radiation via upconversion; these UV photons were then implicated in photocatalytic degradation of phenol by the TiO2 component. Based on existing literature, however, I believe that YAlO3:Er 3+ is not capable of emitting significant amounts of UV via upconversion under their experimental conditions. The appearance of this claim in ES&T gives undue validation to similar misguided claims regarding the efficiency of visible-toUV upconversion materials, which have appeared in lower impact journals over the past 9 years. Herein, the past literature on YAlO3:Er 3+ upconversion is summarized and an alternative explanation for the behavior of the ICPB is offered. Upconversion luminescence (UC) is the process whereby two or more photons are sequentially absorbed by an ion or molecular system to reach a doubly excited state and emit one higher energy photon upon relaxation. Compared to other nonlinear optical processes, UC by lanthanide-doped materials is relatively efficient, particularly under laser excitation. Still, using UC in technologies that involve less intense pump sourcessuch as sunlight or lampshas been a major challenge. Measurable spectral response enhancements of solar cells and photocatalytic systems have been achieved by several groups using benchmark IR-to-visible or IR-to-UVA converters (e.g., NaYF4:Er ,Yb and YF3:Tm ,Yb) under solar irradiation, though they were marginal. It should be noted that Yb-sensitized UC systems−a class to which YAlO3:Er 3+ does not belong−are the most intensively studied materials in the field and have exceptional conversion efficiencies on the order of 1%. By comparison, the efficiencies of the most well-known visible-to-UV materials, which employ Pr as an activator, have been estimated at ∼0.001% under fluorescent lamp excitation. As stated above, Zhou and coauthors claimed to have utilized UC by YAlO3:Er 3+ to indirectly impart visible light activity on TiO2. Separate works by Xu and Jiang, and Hai-Gui et al. first studied the visible light upconverting characteristics of this material, using pulse laser excitation and sophisticated detection systems. Both groups observed only one very weak emission peak in the 300−400 nm range, located at 320 nm and assigned to the P3/2 → I15/2 transition. Wang and coworkers were the first to publish TiO2 sensitization by YAlO3:Er 3+ in 2009; however, they were unable to provide a UC emission spectrum to support this phenomenon. Six publications by the same group describing degradation of various dyes by YAlO3:Er 3+ composites followed within two years. Emission spectra were not provided in these papers either, and one work even claimed to have achieved photocatalysis by upconverting light emitted by cavitation bubbles during sonication. An ability to produce significant UC emission by exciting a material with visible light undetectable to the naked eye is highly questionable. Several of these works were cited in the recent Zhou et al. paper. Zhou et al. refer the reader to one of their previous publications for their synthesis methods and characterization of YAlO3:Er 3+. Upconversion emission spectra are shown therein, however they do not match those measured by Xu and Jiang or Hai-Gui et al. and show poor signal-to-noise ratio. The legitimacy of these signals as anything other than artifacts from monochromator interference effects or normal Stokes emission may be doubtful. The data were obtained using a standard fluorescence spectrometer, whereas laser excitation is typically required to measure UC spectra of lanthanide materials. Furthermore, if a long-pass filter is not used to reject second-order diffraction from the excitation beam, the sample will be simultaneously excited by both the selected wavelength and by photons of wavelength λ/2 (276 or 227 nm in their case). Thus, normal photoluminescence may be mistaken for UC emission. This misleading effect has already been implicated in several reports of UC by carbon quantum dots, which turned out to be artifactual. One of the two UC emission spectra in the less recent Zhou et al. paper was, according to the authors, obtained by exciting YAlO3:Er 3+ at 553 nm. The absorption spectrum of YAlO3:Er 3+ has been reported previously, showing poor overlap of the I15/2 → S3/2 transition with this wavelength. The other UC spectrum, with 455 nm excitation, shows an entirely different set of peaks. Such behavior is uncharacteristic of lanthanide UC, wherein varying the excitation wavelength may affect peak intensity ratios, but not the spectral distribution of the peaks. One of the main conclusions of Zhou et al.’s recent ES&T paper was that irradiating the ICPB system with visible light was less detrimental to bacteria than UVC radiation. The authors used an unfiltered LED panel stated to emit at 420− 700 nm, but did not provide the output spectrum of this source. The panel may have included “420 nm” InGaN LEDs, which can have emissions tailing into the UV range. Particularly since the irradiation intensity for the visible-light experiments was 2 orders of magnitude greater than with UVC irradiation, any minor UV component to the lamp output during the former could have easily resulted in comparable photocatalysis Correspondence/Rebuttal

Challenges and prospects of advanced oxidation water treatment processes using catalytic nanomaterials

Centralized water treatment has dominated in developed urban areas over the past century, although increasing challenges with this model demand a shift to a more decentralized approach wherein advanced oxidation processes (AOPs) can be appealing treatment options. Efforts to overcome the fundamental obstacles that have thus far limited the practical use of traditional AOPs, such as reducing their chemical and energy input demands, target the utilization of heterogeneous catalysts. Specifically, recent advances in nanotechnology have stimulated extensive research investigating engineered nanomaterial (ENM) applications to AOPs. In this Perspective, we critically evaluate previously studied ENM catalysts and the next-generation treatment technologies they seek to enable. Opportunities for improvement exist at the intersection of materials science and treatment process engineering, as future research should aim to enhance catalyst properties while considering the unique roadblocks to practical ENM implementation in water treatment.This Perspective evaluates catalysts based on engineered nanomaterials and the next-generation water treatment technologies they seek to enable.

Porous Silicon’s Photoactivity in Water: Insights into Environmental Fate

Interest in porous silicon (pSi) (and, more broadly, silicon nanoparticles (NPs)) has increased along with their concomitant use in various commercial and consumer products, yet little is known about their behavior in the natural environment. In this study, we have investigated the photosensitization, optical, and surface properties of pSi as a function of time in aqueous systems. Samples were prepared via an anodic electrochemical etching procedure, resulting in pSi particles with diameters of ca. 500 nm, composed of a porous network of Si nanocrystallites of 2-4 nm. Initially, pSi particles generated significant amounts of (1)O2, yet they rapidly lost much of this ability due to the formation of an oxide layer on the surface, as determined by X-ray photoelectron spectroscopy, which likely prevented further photosensitization events. Addition of natural organic matter (NOM) did not significantly impact pSis photosensitization abilities. The pSi lacked any intrinsic bactericidal properties on Escherichia coli and did not produce enough (1)O2 to considerably affect populations of a model virus, PR772, highlighting its relatively benign nature toward microbial communities. Results from this study suggest that the photoactivity of pSi is unlikely to persist in aqueous systems and that it may instead behave more similarly to silica particles from an environmental perspective.