The Myth of Visible Light Photocatalysis Using Lanthanide

Feb 6, 2018 - The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials ..... presumably provided no more than 100 mW/cm2 of to...
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The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials Sushant P. Sahu, Stephanie L Cates, Hyoung-il Kim, Jae-Hong Kim, and Ezra L. Cates Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05941 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion

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Materials

3 Sushant P. Sahu†, Stephanie L. Cates‡, Hyoung-Il Kim‡, Jae-Hong Kim‡, Ezra L. Cates†*

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Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, SC.

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Department of Chemical and Environmental Engineering, Yale University, New Haven, CT.

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ABSTRACT Upconversion luminescence is a nonlinear optical process achieved by certain engineered

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materials, which allows conversion of low energy photons into higher energy photons. Of particular

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relevance to environmental technology, lanthanide-based upconversion phosphors have appeared in

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dozens of publications as a tool for achieving visible light activation of wide-band gap semiconductor

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photocatalysts, such as TiO2, for degradation of water contaminants. Supposedly, the phosphor particles

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act to convert sub-band gap energy photons (e.g., solar visible light) into higher energy ultraviolet

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photons, thus driving catalytic aqueous contaminant degradation. Herein, however, we reexamined the

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photophysical properties of the popular visible-to-UV converters Y2SiO5:Pr3+ and Y3Al5O12:Er3+, and

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found that their efficiencies are not nearly high enough to induce catalytic degradations under the reported

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excitation conditions. Furthermore, our experiments indicate that the false narrative of visible-to-UV

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upconversion-sensitized photocatalysis likely arose due to coincidental enhancements of dye degradation

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via direct electron injection that occur in the presence of dielectric-semiconductor (phosphor-catalyst)

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interfaces. These effects were unrelated to upconversion and only occurred for dye solutions illuminated

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within the chromophore absorption bands. We conclude that upconversion using Pr3+ or Er3+-activated

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systems is not a technologically appealing mechanism for visible light photocatalysis, and provide

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experimental guidelines for avoiding future misinterpretation of these phenomena.

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TOC GRAPHIC

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INTRODUCTION Heterogeneous photocatalysis has maintained a strong foothold in water treatment technology

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research since the explosion of studies involving TiO2 began in the 1990s. Among the many uses of

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semiconductor photocatalysts proposed by academia in the environmental fields, advanced oxidation is

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the most common; therein, production of hydroxyl radicals by catalyst suspensions in photoreactors is

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seen as a “chemical free” alternative to H2O2 and O3-based unit processes for destruction of recalcitrant

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water contaminants.1, 2 While the most effective photocatalytic materials have band gap energies (Eg) that

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demand UV-range excitation wavelengths, many groups have pursued catalysts that are activated instead

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by lower-energy visible light.3-6 These efforts are motivated by the prospect of replacing energy-intensive

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UV lamp reactors with solar reactors, though the operational practicality of this concept has been

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debated.7 The primary route to visible light activation of semiconductors is through the use of materials

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with sufficiently low Eg or intra-band states to absorb such light and promote valence electrons into the

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conduction band. Inherently, however, a lower Eg generally implies weaker redox potential of the

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resulting conduction band electrons (e–cb), valence band holes (h+vb), or both.8

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To circumvent the weak activity of narrow-Eg materials, an alternative approach appearing with

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increasing frequency in the literature is the use of upconversion luminescent (UC) phosphors that convert

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the visible wavelengths into UV photons; these photons may then activate a more conventional wider-Eg

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material, such as TiO2 (Figure 1).8-12 Though stronger redox potential is preserved, the overall quantum

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yield of this approach is then intimately tied to the optical conversion efficiency of the UC system. The

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current authors have previously developed visible-to-UVC converting materials and investigated their

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application towards antimicrobial technologies.13-16 Despite the high sensitivity of microorganisms to

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germicidal UVC, we found that the conversion efficiency of even one of the most effective lanthanide

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(Ln3+) phosphor systems – X2-Y2SiO5:Pr3+,Li+ (YSO:Pr3+) – was prohibitively low under practical

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excitation conditions, yielding 10-3 % conversion of blue light to UVC under fluorescent lamp

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excitation.13, 14 (β-Y2Si2O7:Pr3+,Li+ was demonstrated to have 3.1× greater visible-to-UV conversion

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efficiency than YSO:Pr3+, the highest yet reported, though it has not been studied for environmental

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applications.17) We also experimented in the past with UC-sensitized photocatalysis using YSO:Pr3+

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coupled with TiO2 and found that even under 1.4 W/cm2 laser excitation, results were inconsistent and no

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obvious catalytic dye degradation resulting from upconverted light could be observed (unpublished). It

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has thus been unexpected to subsequently witness a steady stream of publications over the past ten years

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that report UC-photocatalyst composite systems that yield remarkable contaminant degradation rates

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under low intensity (non-laser) visible light excitation conditions.

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Figure 1. Mechanism of visible-to-UV upconversion sensitized photocatalysis disputed herein. Green

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circle – UC phosphor particle; white circles – semiconductor photocatalyst particles.

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The first studies reporting UC-sensitized visible light photocatalysis (UC-PC) were authored by

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Wang et al. in 2005 and 2006.18, 19 Though we count over 25 subsequent papers from these and other

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authors focusing on this topic, we have opted to cite only a limited number as representatives in order to

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minimize further validation of this body of work. Generally the studies share a common formula of

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coupling a catalyst (usually TiO2 or ZnO) with a UC phosphor (usually YAlO3:Er3+ or Y3Al5O12:Er3+,

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referred to herein as yttrium aluminum perovskite/YAP:Er3+ and yttrium aluminum garnet/YAG:Er3+,

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respectively) via mixing and heating to produce a composite powder. The catalytic activities under visible

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light exposure were then assessed by monitoring dye decolorization by catalyst suspensions via UV/vis

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spectroscopy.9, 20, 21 In all cases, significant enhancement of decolorization rates by the UC-PC composite

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were reported over the photocatalyst suspension alone, suggesting that such composites might be useful in

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solar-driven water treatment processes. More recently, Wu et al. reported UC-PC degradation of

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methylene blue (MB) by YSO:Pr3+ coupled with TiO2,22 with their findings contradicting the results of

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our unpublished experiments. Though most early papers on UC-PC were confined to specialty journals,

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the sheer number of them has resulted in substantial influence on the field. Now, visible-to-UV UC

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phosphors have begun to appear in studies concerning more complex catalytic systems, and in journals of

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broader scope, including ACS Applied Materials & Interfaces,23 and Environmental Science &

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Technology.24, 25 These reports appear to treat UC-PC as a well-established phenomenon by citing

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previous publications, and as a result the effect of UC in these experiments is largely assumed, rather than

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proven experimentally therein.

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In this work our objective was to disprove the role of UC in catalytic enhancements observed for

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Ln3+ phosphor-semiconductor composites under visible light irradiation, with more emphasis on control

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experiments than most previous studies. Though our past experience has involved YSO:Pr3+ and other

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Pr3+-activated converters,13, 16, 17 we also included YAP:Er3+ and YAG:Er3+ in the present study due to

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their prevalence in previous UC-PC literature.9, 10, 20, 21 Intended to result from this work is a rigorous

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understanding of the performance of UC materials under environmentally relevant excitation conditions,

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thus allowing the technological value of UC-PC to be objectively interpreted by the environmental

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engineering and applied materials chemistry communities.

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EXPERIMENTAL METHODS

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Stock Chemicals. Er(NO3)3•5H2O (> 99.99% purity) and LiNO3•xH2O (99.999%) were purchased from

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Alfa-Aesar. Y2O3 (99.999% purity) and MB were purchased from Acros Organics. Methyl Orange (MO,

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> 98% purity) and methyl red (MR) were supplied from TCI chemicals. Commercial TiO2 nanopowders

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(Aeroxide P25) was purchased from Acros Organics, and potassium hydrogen phthalate was obtained

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from EMD. Tetraethyl orthosilicate, TEOS (purity > 99.999%), Al (NO3)3•9H2O (99.997% purity),

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Pr(NO3)3•6H2O (> 99.99%), and HNO3 (trace metal grade) were purchased from Sigma-Aldrich. High

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purity phenol, ethanol, isopropanol, and anhydrous citric acid were obtained from Fisher Scientific and

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were used as received. Deionized water of resistivity >18 MΩ.cm was used in all experiments.

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Materials Synthesis. Powder X2-Y2SiO5 doped with 1.2 mol.% Pr3+ and 10 mol.% Li+ (i.e., prepared

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assuming a composition of Y1.776Pr0.024Li0.2SiO4.8, referred to herein as YSO:Pr3+) was prepared using a

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sol-gel decomposition synthesis, as described in our previous work.26 “Undoped” stoichiometric YSO: Li+

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samples without luminescent Pr3+ activation were prepared following the same procedure, without

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addition of Pr3+ ions. YAG:Er3+ phosphors of 1 mol.% Er3+ were prepared with molar Er:Y:Al ratio of

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0.01:0.99:1.67 by a sol-gel decomposition technique similar to methods described previously.27, 28 Briefly,

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stoichiometric amounts of Y(NO3)3 (4.373 mL, 1.77 M) and Er(NO3)3 (0.391 mL, 0.2 M) aqueous stock

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solutions were mixed in a beaker. Separately, 4.80 g Al (NO3)3•9H2O was dissolved in 5 mL deionized

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water, stirred for 1 h at room temperature, and then mixed dropwise with the Y3+/Er3+ solution. Citric acid

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(11.80 g) was added in a molar ratio of citric acid: metal ion of 3:1 under stirring. The solution was

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evaporated at ~85 ºC in a water bath until viscous gel was obtained and then dried at 130 ºC in an oven for

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24 h and ground to obtain a precursor powder. The material was then heated at 1200 ºC for 2 h in air

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using a tube furnace to obtain white YAG:Er3+(1%) sample. Undoped YAG containing no Er3+ ions was 5 ACS Paragon Plus Environment

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also prepared. A mixed-phase YAP/YAG phosphor material was also prepared according to Dong et al.,28

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and we further tested physically mixed phosphor and TiO2 powder samples prepared without the sintering

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step, in the same molar ratios as the composites.

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YSO:Pr3+/TiO2 and YAG:Er3+/TiO2 composite materials were prepared by mixing P25 TiO2 with

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UC phosphor powders at a molar ratio 1:0.16 (TiO2:phosphor). Powder mixtures were ground in a mortar

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and pestle followed by annealing at 550 ºC for 3 h in a tube furnace. The 16 mol% phosphor

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concentration was previously found to be optimum for dye removal by YAG:Er3+/TiO2 by Feng et al.10

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Characterization. The UC photoluminescence spectra of phosphor powders were acquired on a

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customized spectrometer by guiding a focused beam from an air-cooled argon ion laser with 488 nm line

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filter, and using phase-sensitive detection with components described previously.13 The laser was focused

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onto the center of each sample at a 45º angle from the sample face, and collection lenses aligned normal

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to the sample guided the emissions through a 450 nm short pass filter to the detector. The powder X-ray

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diffraction (XRD) patterns were collected on a Rigaku Miniflex using Cu Kα radiation (λ =1.54 Å). The

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morphology of the UC materials and composites were studied by scanning electron microscopy (SEM),

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using a SU6600 variable pressure SEM.

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Chemical Analyses. Concentrations of MB were determined by absorbance at 664 nm, based on a

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calibration curve, while 430 and 460 nm were used for MR and MO, respectively. A Varian Cary 50

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and/or Cary 300 UV/Vis spectrophotometer were employed. Phenol concentrations were monitored via

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HPLC using an Ultimate 3000 (Thermo Scientific) instrument coupled with variable wavelength UV

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detector and Hypersil Gold C10 column (3 µm, 150 mm × 2.1 mm; using acetonitrile/water mixture

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[32:68 v/v] as the mobile phase at a flow rate of 0.32 mL/min).

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Photocatalytic Experiments. Photocatalytic degradation of dyes and phenol by various materials were

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assessed under excitation by both a monochromatic laser and broad spectrum white light LED (W-LED)

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at room temperature. An argon ion laser (Modulaser Stellar-Pro-L-ML 1000) was used in conjunction

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with a 488 nm line filter, resulting in a 140 mW beam which was de-focused to a diameter matching the

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reactor cuvette yielding an intensity of approximately 180 mW/cm2. The W-LED employed was a 6 ACS Paragon Plus Environment

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mounted 800 mW cold white LED (Thorlabs; spectrum shown in Figure S1), coupled with 450 nm long

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pass filter to avoid any direct band gap excitation of TiO2 by violet or UV wavelengths, and collimated

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onto the suspension cuvette using an achromatic lens. Some tests experiments were also conducted using

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UVA radiation (λmax = 365 nm); suspensions were placed in quartz glass cuvette under stirring and

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photoirradiated by six 4 W black-light-blue lamps housed in an air-cooled irradiation chamber. Initial pH

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values of the suspensions were measured in all cases, but not adjusted, and ranged from 7.0-7.5.

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Photocatalysis experiments used aqueous suspensions of 2.8 g/L of catalyst or control materials

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and were magnetically stirred in the dark for 12 h (dyes) or 2 h (phenol) to establish an adsorption

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equilibrium. During photoirradiation, aliquots were withdrawn periodically, centrifuged to remove the

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solid catalysts and analyzed immediately by UV/vis or HPLC. Initial concentrations of 0.1 mM for dyes

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and 0.13 mM for phenol were used. Control experiments were performed using undoped YAG/TiO2 and

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YSO/TiO2, TiO2 alone, MB-only, and dark conditions. To study the effect of ·OH scavenging on MB

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degradation efficiency, isopropanol (18 mM) was added to suspensions prior to stirring and

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photoirradiation, and degradation kinetics were similarly monitored.

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RESULTS AND DISCUSSION

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Phosphor Characterization. According to XRD results (Figure S2), the YAG:Er3+ powder was single

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phase garnet structure (Ia3̄d).29 Also based on the present data, “YAP:Er3+” powders synthesized

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according to Dong et al.28 contained a mixture of perovskite and garnet phases. The perovskite phase is

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known to be thermodynamically unstable and single-phase powders are synthetically challenging.30, 31

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Diffraction patterns for YAP:Er3+ samples reported by Dong et al., as well as Wang et al., also showed

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impurity peaks that included YAG.27, 28 As expected, this mixed-phase material resulted in much weaker

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UC emission than single-phase YAG:Er3+, shown in Figure S3. Resultantly, YAG:Er3+ was deemed a

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more legitimate Er3+-activated UC phosphor and used herein, despite YAP:Er3+ appearing more

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frequently in the UC-PC literature. The diffraction patterns of YSO:Pr3+ and undoped YSO matched our

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previous reports (data not shown).26, 32 For UC emission spectroscopic analyses, 488 nm (“cyan”) was chosen for excitation, as this

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wavelength coincides with intermediate state absorption maxima for both Pr3+- and Er3+-activated

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systems.16, 33, 34 Figures 2A-B show the established mechanisms by which the two phosphors convert

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visible light into UV via energy transfer UC,16, 35 and the UC emission spectra recorded herein are shown

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in Figure 2C. Emission by YSO:Pr3+ was consistent with our earlier reports and results of other groups,

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showing broad 255-360 nm emission assigned to the 4f5d3HJ/3FJ interconfigurational transitions.16, 26, 34

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Emission by YAG:Er3+ showed sharper peaks characteristic of intra-4f transitions and qualitatively

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consistent with previous analyses of Er3+ UC in single crystals.33, 36, 37 Most notably, the detected UV-

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range UC peak of YAG:Er3+ at 319 nm (2P3/24I15/2)33 was much weaker than emission by the Pr3+-

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activated phosphor. In fact, with respect to integrated peak area, the cyan-to-UV conversion efficiency of

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YAG:Er3+ was 3 orders of magnitude lower than YSO:Pr3+.

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Based on the energy levels of the respective activator ions, this finding is not surprising.

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Upconversion by YSO:Pr3+ is known to be relatively efficient due to: (1) an energetically wide cluster of

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3

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the 3PJ and 1D2 states resulting in negligible nonradiative multiphonon relaxation from the intermediate

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state;39 (3) action of both [3PJ,3PJ][4f5d,3H4] and [3PJ,1D2][4f5d,3H4] energy transfer UC mechanisms,

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the latter of which involves the 1D2 singlet state which has an especially long lifetime of 27 µs (see ref. 16

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for additional details); and lastly (4), the vast majority of emission from the doubly excited 4f5d manifold

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is in the form of high-energy UV photons, rather than cascade-type visible light emission, owing to its

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parity-allowed nature. In contrast, the 4f electronic structure of Er3+ results in the crowded and evenly-

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spaced array of energy levels seen in Figure 2B, which are prone to multiphonon relaxation. Furthermore,

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even the Er3+ ions excited to the 2P3/2 emitting level which relax radiatively do so primarily in the form of

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>400 nm photons, with the UV emission peaks being much weaker by comparison.33, 36, 37 These data

PJ intermediate states resulting in relatively strong cyan/blue light absorption;34, 38 (2) a wide gap between

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show the first direct comparison of YSO:Pr3+ emission to Er3+-activated systems, and it is clear that the

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former has far greater UV conversion efficiency.

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Figure 2. (A) Mechanisms of visible-to-UVC conversion by YSO:Pr3+.16, 17 (B) Primary mechanism of

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visible-to-UV conversion by Er3+-doped phosphors.35 (C) Upconversion emission spectra of phosphors

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under 488 nm laser excitation (140 mW); inset shows YAG:Er3+ and undoped YAG spectra with adjusted

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axes. The peaks seen at >430 nm are due to leakage of the excitation light from the monochromator.

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To investigate UC-PC, composites were synthesized by sintering the phosphor powders together

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with the well-known polymorphic commercial P25 TiO2 nanoparticles. This method was used by Wang et

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al. with pure anatase TiO2 27 and we also believe it is theoretically optimal for UC-PC activity. As seen in

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in the SEM micrographs in Figure 3, it yields a porous TiO2 particulate layer, bound to the phosphor

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particles, that is likely conducive to contaminant and O2 diffusion to photoactivated catalyst sites. In

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Figure S4, the UC emission spectrum of YSO:Pr3+ is compared to that of the YSO:Pr3+/TiO2 composite

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material. No UV peak was observed for the composite, indicating that Pr3+ UC emission during focused

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laser excitation is entirely absorbed by the surrounding semiconductor nanoparticles. To confirm that the

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photocatalytic activity of the TiO2 was preserved upon sintering, MB degradation control experiments

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were performed using YSO:Pr3+/TiO2 under UVA irradiation (365 nm) to directly excite the TiO2. Shown

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in Figure S5, the photocatalyst component was confirmed to degrade MB effectively.

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Figure 3. SEM micrographs of powder phosphors materials and phosphor-TiO2 composites, including

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(A) YSO:Pr3+, (B) YSO:Pr3+/TiO2, (C) YAG:Er3+, and (D) YAG:Er3+/TiO2.

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Observation of UC-PC herein was first attempted using argon ion laser excitation of aqueous

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YSO:Pr3+/TiO2 and YAG:Er3+/TiO2 suspensions at 488 nm. Such monochromatic excitation is ideal for

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both Pr3+ and Er3+ visible-to-UV conversion, as portrayed in Figures 2A-B; however these conditions

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resulted in no observable MB degradation as seen in Figure 4A. For reference, past UC-PC studies by

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other authors typically used solar irradiation in their experiments,9, 20, 21 which presumably provided no

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more than 100 mW/cm2 of total power density (AM 1.5 solar irradiance), only a small fraction of which

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would resonate with the Ln3+ excitation transitions. The lack of any MB degradation under ~180 mW/cm2

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488 nm laser excitation herein shows that neither phosphor composite material resulted in substantial

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TiO2 activation via UC.

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Given that most prior UC-PC studies used broad spectrum excitation, we similarly investigated

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UC-PC activity of the suspensions irradiated by a W-LED equipped with a 450 nm long-pass filter. The

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filter was used to reject 400-410 nm wavelengths which could potentially result in direct excitation of the

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rutile portion of P25 TiO2. Degradation of MB is shown in Figure 4B, revealing that the composites do in

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fact induce a mild catalytic effect with the W-LED illumination, while the TiO2-only control showed no 10 ACS Paragon Plus Environment

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photocatalytic enhancement over the MB photolysis results. However, we further tested composite

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particles made using undoped YAG and YSO as controls (YAG/TiO2 and YSO/TiO2). Without the Ln3+

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activator ions, these materials were not capable of UC and yet showed the same degree of MB

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degradation enhancement as the doped phosphors (Figure 4B). It is clear then that catalytic action of the

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composites under W-LED excitation is related solely to a photochemical phenomenon and not related to

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UC emission. In addition to MB, we tested W-LED-induced degradation of two other commonly

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employed dyes – MO and MR. Catalytic enhancements were observed for neither UC-capable Ln3+-doped

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composites nor the undoped controls, as seen in Figure 4C. Catalytic degradation of phenol was also

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assessed, in order to contrast the composites’ photoactivity against dyes to their activity towards an

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organic species with no visible light absorption bands. The absorption spectra of phenol and MB are thus

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provided in Figure S6. As seen in Figure 4D, phenol degradation was not observed for any of the

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materials under W-LED irradiation, further refuting any generation of ·OH under these conditions.

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Figure 4. (A) Decoloration of MB by phosphor-TiO2 composites under 488 nm laser irradiation. (B) MB

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decoloration by composites and controls under white LED irradiation. (C) Degradation of various dyes

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by composite materials under white LED irradiation for 240 min; solid and striped bars depict Ln3+-

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doped and undoped phosphor materials, respectively. (D) Degradation of phenol under white LED

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irradiation. Error bars denote standard deviations of experiments performed in triplicate.

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We hypothesized that the reason for catalytic MB degradation occurring under W-LED excitation

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and not 488 nm excitation was a result of the former comprising wavelengths within the absorption range

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of MB. Semiconductors such as TiO2 are well-known to catalyze dye degradation through an alternative

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mechanism that does not involve band gap excitation to produce e–cb/h+vb pairs. Instead, upon absorption

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of light by the dye (which may be of lower photon energy than the TiO2 Eg), the excited dye molecule

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may transfer an electron into the TiO2 conduction band and become oxidized, in what is referred to as

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photosensitized oxidation or direct electron injection.40-43 The process relies on the resultant e–cb on TiO2

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becoming scavenged by O2 to produce O2·–, which may also contribute to dye decoloration.40-42 Since

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formation of h+vb and ·OH production are not significantly involved in this mechanism, addition of an

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·OH scavenger would not have any quenching effect on the degradation rates.

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We performed photocatalysis experiments with YSO:Pr3+/TiO2 and isopropanol as an ·OH

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scavenger 44 to probe the involvement of such a mechanism in the composite material suspensions. If UC

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were in fact contributing to degradation by exciting TiO2 with emitted UV, then isopropanol addition

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would scavenge ·OH and slow degradation kinetics. First we confirmed this quenching effect using direct

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UVA excitation of the composites; the suppressed MB degradation kinetics resulting from 18 mM

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isopropanol addition are seen in Figure 5. Under W-LED excitation, however, no change in kinetics was

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observed with isopropanol addition. The result confirms that UV-related mechanisms did not occur when

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visible light was used, excluding UC as a contributing factor.

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Figure 5. Effect of isopropanol ·OH scavenging on photocatalytic degradation of MB by YSO:Pr3+/TiO2

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composites particles under W-LED (visible) and UVA irradiation. Error bars indicate standard

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deviations of triplicate experiments.

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Above we have shown that UV UC is not responsible for enhanced dye degradation rates by

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phosphor/TiO2 composite materials; however, our experiments did indicate that the presence of the

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YAG:Er3+, YSO:Pr3+, or undoped host components may result in statistically significant MB degradation

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enhancements compared to TiO2 alone. This is enhancement is only observed when the system is excited

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within the MB absorption range, and it does not require absorption by the Ln3+/dielectric component.

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Control experiments using simple, unfused mixtures of phosphor and TiO2 particles resulted in no

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enhancement (Figure S7), thus clearly indicating that the presence of epitaxial YSO/YAG–TiO2 interface

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is required for the catalytic effect. Detailed explanation of this mechanism is beyond the scope of this

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work; however, numerous existing reports explain that certain surface defects on TiO2 can enhance direct

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electron injection. Improved degradation may arise due to increased adsorption of dye molecules onto the

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catalyst or by introducing electron-trapping defects into the TiO2 surface structure.45-49 Incorporation of

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Y3+ and the chemically similar La3+ into TiO2 as either dopants or as Y(La)2O3 nanoparticle

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heterojunctions has specifically been shown to enhance charge transfer from dyes, likely by inducing

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formation of oxygen vacancies.46-49 The catalytic enhancements by the composite materials studied here 13 ACS Paragon Plus Environment

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were most likely similarly related to the dielectric-semiconductor interface formed during the sintering

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step, with defects and/or heterojunctions playing the key role rather than the bulk YSO or YAG

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components. Degradation of MO or MR was not observed under the irradiation conditions tested,

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possibly a result of poor adsorption due to neutral charge of these dyes, though the data do not rule out the

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possibility of catalytic activity under more intense W-LED illumination.

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In our opinion, the overwhelming majoring of UC-PC literature exhibits three critical

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experimental flaws: first, the syntheses and quality of the UC materials are highly questionable – typically

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using low purity stock chemicals and showing XRD patterns that indicate phase impurities (e.g., see ref.

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21, Figure 1a and 1b therein). It is well known that optical properties –especially in UC materials– are

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highly affected by certain transition metal impurities, necessitating the use of 99.99% purity or greater

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stock chemicals.50 Secondly, in order to conclude that UC is responsible for the catalytic enhancements,

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the use of the non-activated phosphor host material (e.g., undoped YAP or YAG) in control experiments

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is required in order to confirm that optical – and not chemical – effects are responsible. Only the study by

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Feng et al. included such a control, finding that their YAG:Er3+ sample enhanced MB degradation to a

295

greater extent than YAG under white fluorescent lamp excitation (~12% vs. ~8% degradation after 2 h),

296

though both performed better than TiO2-only (~5% degradation).10

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Finally, UC-PC authors rarely include UC emission spectra of the materials in their publications,

298

which is certainly a minimum requirement for demonstrating UC capability prior to applying this

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phenomena to environmental technology. Studies unrelated to photocatalysis have shown visible-to-UV

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conversion by the same aforementioned Er3+-doped systems, though they used single crystals under pulse

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laser excitation with sophisticated detection systems.35-37, 51 Furthermore, the UC spectrum of YSO:Pr3+

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provided by Wu et al., as well as one in a more recent YSO:Pr3+ UC-PC paper, were measured with

303

fluorescence spectrometers equipped with a xenon lamps.22, 52 Based on our experience in characterizing

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YSO:Pr3+, cyan or blue laser excitation in excess of 50 mW and phase-sensitive detection are required to

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resolve the visible-to-UV anti-Stokes emission spectrum.13, 26 Their spectra instead appear to be normal 14 ACS Paragon Plus Environment

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Stokes emission that resulted from unintentional UV excitation of the sample by second order diffraction

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from the source monochromator in the absence of appropriate long-pass filters. This same instrumental

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blunder has been implicated in false reports of UC by carbon quantum dots,53 and illustrates the

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importance of avoiding accidental UV excitation when attempting to measure upconverted visible light.

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Overall, there is little convincing evidence that the powder UC materials used in past UC-PC experiments

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can produce enough UV emission under incoherent visible light excitation to drive aqueous contaminant

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degradation. As mentioned earlier, Cates et al. determined the blue-to-UV conversion of YSO:Pr3+ to be

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on the order of 0.001%;13 thus, even if the full solar irradiance of 100 mW/cm2 could be converted by

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such materials, the emitted UV intensity would amount to only 1 µW/cm2. Typical UV-driven advanced

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oxidation processes, on the other hand, require intensities in the mW/cm2 range.54, 55 We emphasize that our conclusions apply only to alleged photocatalytic enhancement by Ln3+

316 317

visible-to-UV upconversion materials. Reports on other types of UC-PC do not consistently contain the

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flaws described herein. Several studies, for example, have shown photocatalysis enhancement attributed

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to infrared-to-UV conversion by Yb3+/Tm3+ systems, such as YF3:Yb3+,Tm3+ and β-NaYF4:Yb3+,Tm3+ .12,

320

56-58

321

is generally orders of magnitude more efficient than the UC processes central to this work,8 making the

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approach much more feasible. Moreover, visible-to-visible UC by organic triplet-triplet annihilation

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systems has also been shown to enhance the action of visible light-active photocatalysts through

324

upconversion of sub-band gap photons 11, 59-61 – again, this process is enabled by highly efficient

325

conversion which is easily detected spectroscopically.

326

ASSOCIATED CONTENT

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Supporting Information. W-LED emission profile, XRD patterns, UC spectra of mixed-phase “YAP:Er3+”

328

and YSO:Pr3+/TiO2, UVA photocatalytic MB degradation data and degradation by physical mixtures,

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absorption spectra of MB and Phenol.

Conversion of 980 nm infrared radiation to UVA wavelengths by Yb3+-sensitized fluoride phosphors

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AUTHOR INFORMATION

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*Corresponding author. [email protected]. ph: 864-656-1540

332

The authors declare no competing financial interest.

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