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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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Environmental Science & Technology

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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†*

4 5 6

†

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

7

‡

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

8 9 10

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

27 28 29 30

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

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

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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,

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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

301

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

302

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

304

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

305

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

307

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

308

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

309

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

310

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

311

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

312

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

313

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

314

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

315

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

318

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

319

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

323

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.

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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,

329

absorption spectra of MB and Phenol.

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



330

AUTHOR INFORMATION

331

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

332

The authors declare no competing financial interest.

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