Correspondence/Rebuttal pubs.acs.org/est
Comment on “Intimate Coupling of Photocatalysis and Biodegradation for Degrading Phenol Using Different Light Types: Visible Light vs UV Light” workers were the first to publish TiO2 sensitization by YAlO3:Er3+ in 2009; however, they were unable to provide a UC emission spectrum to support this phenomenon.18 Six publications by the same group describing degradation of various dyes by YAlO3:Er3+ composites followed within two years.19−24 Emission spectra were not provided in these papers either, and one work even claimed to have achieved photocatalysis by upconverting light emitted by cavitation bubbles during sonication.23 An ability to produce significant UC emission by exciting a material with visible light undetectable to the naked eye is highly questionable. Several of these works were cited in the recent Zhou et al. paper.1 Zhou et al. refer the reader to one of their previous publications for their synthesis methods and characterization of YAlO3:Er3+.25 Upconversion emission spectra are shown therein, however they do not match those measured by Xu and Jiang or Hai-Gui et al.16,17 and show poor signal-to-noise ratio. The legitimacy of these signals as anything other than artifacts from monochromator interference effects or normal Stokes emission may be doubtful. The data were obtained using a standard fluorescence spectrometer, whereas laser excitation is typically required to measure UC spectra of lanthanide materials. Furthermore, if a long-pass filter is not used to reject second-order diffraction from the excitation beam, the sample will be simultaneously excited by both the selected wavelength and by photons of wavelength λ/2 (276 or 227 nm in their case). Thus, normal photoluminescence may be mistaken for UC emission. This misleading effect has already been implicated in several reports of UC by carbon quantum dots, which turned out to be artifactual.26,27 One of the two UC emission spectra in the less recent Zhou et al. paper was, according to the authors, obtained by exciting YAlO3:Er3+ at 553 nm.25 The absorption spectrum of YAlO3:Er3+ has been reported previously, showing poor overlap of the 4I15/2 → 4S3/2 transition with this wavelength.28 The other UC spectrum, with 455 nm excitation, shows an entirely different set of peaks. Such behavior is uncharacteristic of lanthanide UC, wherein varying the excitation wavelength may affect peak intensity ratios, but not the spectral distribution of the peaks.16,29,30 One of the main conclusions of Zhou et al.’s recent ES&T paper was that irradiating the ICPB system with visible light was less detrimental to bacteria than UVC radiation.1 The authors used an unfiltered LED panel stated to emit at 420− 700 nm, but did not provide the output spectrum of this source. The panel may have included “420 nm” InGaN LEDs, which can have emissions tailing into the UV range.31 Particularly since the irradiation intensity for the visible-light experiments was 2 orders of magnitude greater than with UVC irradiation, any minor UV component to the lamp output during the former could have easily resulted in comparable photocatalysis
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ecently published experiments by Zhou et al. describe a hybrid photocatalysis−biodegradation reactor (“ICPB”) for treating water contaminated with recalcitrant organic compounds.1 One particular aspect of their work is scrutinized herein, within the context of what I believe to be a flawed body of work, with contributions from several groups. The paper perpetuates poorly supported claims of upconversion-sensitized photocatalysis involving erbium-doped phosphors, the implications of which likely misguided the interpretation of their results. The novel aspect of the Zhou et al. paper was the use of visible light irradiation and a visible light-active photocatalytic system in the ICPB. A TiO2/YAlO3:Er3+ composite material was used, which allegedly converted visible wavelengths to UV radiation via upconversion; these UV photons were then implicated in photocatalytic degradation of phenol by the TiO2 component. Based on existing literature, however, I believe that YAlO3:Er3+ is not capable of emitting significant amounts of UV via upconversion under their experimental conditions. The appearance of this claim in ES&T gives undue validation to similar misguided claims regarding the efficiency of visible-toUV upconversion materials, which have appeared in lower impact journals over the past 9 years. Herein, the past literature on YAlO3:Er3+ upconversion is summarized and an alternative explanation for the behavior of the ICPB is offered. Upconversion luminescence (UC) is the process whereby two or more photons are sequentially absorbed by an ion or molecular system to reach a doubly excited state and emit one higher energy photon upon relaxation.2,3 Compared to other nonlinear optical processes, UC by lanthanide-doped materials is relatively efficient, particularly under laser excitation.2 Still, using UC in technologies that involve less intense pump sourcessuch as sunlight or lampshas been a major challenge. Measurable spectral response enhancements of solar cells and photocatalytic systems have been achieved by several groups using benchmark IR-to-visible or IR-to-UVA converters (e.g., NaYF4:Er3+,Yb3+ and YF3:Tm3+,Yb3+) under solar irradiation, though they were marginal.4−9 It should be noted that Yb3+-sensitized UC systems−a class to which YAlO3:Er3+ does not belong−are the most intensively studied materials in the field and have exceptional conversion efficiencies on the order of 1%.3,10−12 By comparison, the efficiencies of the most well-known visible-to-UV materials, which employ Pr3+ as an activator, have been estimated at ∼0.001% under fluorescent lamp excitation.13−15 As stated above, Zhou and coauthors claimed to have utilized UC by YAlO3:Er3+ to indirectly impart visible light activity on TiO2. Separate works by Xu and Jiang, and Hai-Gui et al. first studied the visible light upconverting characteristics of this material, using pulse laser excitation and sophisticated detection systems.16,17 Both groups observed only one very weak emission peak in the 300−400 nm range, located at 320 nm and assigned to the 2P3/2 → 4I15/2 transition. Wang and co© XXXX American Chemical Society
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DOI: 10.1021/acs.est.5b03105 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
(13) Cates, E. L.; Cho, M.; Kim, J.-H. Converting visible light into UVC: Microbial inactivation by Pr3+-activated upconversion materials. Environ. Sci. Technol. 2011, 45 (8), 3680−3686. (14) Cates, E. L.; Wilkinson, A. P.; Kim, J.-H. Visible-to-UVC upconversion efficiency and mechanisms of Lu7O6F9:Pr3+ and Y2SiO5:Pr3+ ceramics. J. Lumin. 2015, 160 (0), 202−209. (15) Cates, S. L.; Cates, E. L.; Cho, M.; Kim, J.-H. Synthesis and characterization of Visible-to-UVC upconversion antimicrobial ceramics. Environ. Sci. Technol. 2014, 48 (4), 2290−2297. (16) Xu, H.; Jiang, Z. Dynamics of visible-to-ultraviolet upconversion in YAlO3: (1%)Er3+. Chem. Phys. 2003, 287 (1−2), 155−159. (17) Yang, H.-G.; Dai, Z.-W.; Sun, Z.-W. Ultraviolet and visible upconversion dynamics in Er3+:YAlO3 under 2H11/2 excitation. Chin. Phys. 2006, 15 (6), 1273. (18) Wang, J.; Li, J.; Zhang, L.; Li, C.; Xie, Y.; Liu, B.; Xu, R.; Zhang, X. Photocatalytic degradation of organic dyes by a high efficient TiO2based catalysts under solar light irradiation. Catal. Lett. 2009, 130 (3), 551−557. (19) Wang, J.; Xie, Y.; Zhang, Z.; Li, J.; Chen, X.; Zhang, L.; Xu, R.; Zhang, X. Photocatalytic degradation of organic dyes with Er3+:YAlO3/ ZnO composite under solar light. Sol. Energy Mater. Sol. Cells 2009, 93 (3), 355−361. (20) Wang, J.; Li, J.; Xie, Y.; Zhang, L.; Han, G.; Li, Y.; Xu, R.; Zhang, X. Preparation of Er3+:YAlO3/ZnO coating compound by sol-gel method and photocatalytic degradation of organic dyes under sun light irradiation. Inorg. Mater. 2010, 46 (4), 399−404. (21) Wang, J.; Xie, Y.; Zhang, Z.; Li, J.; Li, C.; Zhang, L.; Xing, Z.; Xu, R.; Zhang, X. Photocatalytic degradation of organic dyes by Er3+:YAlO3/TiO2 composite under solar light. Environ. Chem. Lett. 2010, 8 (1), 87−93. (22) Xu, R.; Li, J.; Wang, J.; Wang, X.; Liu, B.; Wang, B.; Luan, X.; Zhang, X. Photocatalytic degradation of organic dyes under solar light irradiation combined with Er3+:YAlO3/Fe- and Co-doped TiO2 coated composites. Sol. Energy Mater. Sol. Cells 2010, 94 (6), 1157−1165. (23) Gao, J.; Jiang, R.; Wang, J.; Kang, P.; Wang, B.; Li, Y.; Li, K.; Zhang, X. The investigation of sonocatalytic activity of Er3+:YAlO3/ TiO2-ZnO composite in azo dyes degradation. Ultrason. Sonochem. 2011, 18 (2), 541−548. (24) Gao, J. Q.; Luan, X. Y.; Wang, J.; Wang, B. X.; Li, K.; Li, Y.; Kang, P. L.; Han, G. X. Preparation of Er3+:YAlO3/Fe-doped TiO2ZnO and its application in photocatalytic degradation of dyes under solar light irradiation. Desalination 2011, 268 (1−3), 68−75. (25) Dong, S.; Zhang, X.; He, F.; Dong, S.; Zhou, D.; Wang, B. Visible-light photocatalytic degradation of methyl orange over spherical activated carbon-supported and Er3+:YAlO3-doped TiO2 in a fluidized bed. J. Chem. Technol. Biotechnol. 2015, 90 (5), 880−887. (26) Wen, X.; Yu, P.; Toh, Y.-R.; Ma, X.; Tang, J. On the upconversion fluorescence in carbon nanodots and graphene quantum dots. Chem. Commun. 2014, 50 (36), 4703−4706. (27) Tan, D.; Zhou, S.; Qiu, J. Comment on “Upconversion and downconversion fluorescent graphene quantum dots: Ultrasonic preparation and photocatalysis. ACS Nano 2012, 6 (8), 6530−6531. (28) Pollnau, M.; Heumann, E.; Huber, G. Time-resolved spectra of excited-state absorption in Er3+ doped YAlO3. Appl. Phys. A: Solids Surf. 1992, 54 (5), 404−410. (29) Takahashi, M.; Shojiya, M.; Kanno, R.; Kawamoto, Y.; Kadono, K.; Ohtsuki, T.; Peyghambarian, N. Nonradiative decay processes and mechanisms of frequency upconversion of Er3+ in ZrF4−BaF2−LaF3 glass. J. Appl. Phys. 1997, 81 (7), 2940−2945. (30) Cates, E. L.; Kim, J.-H. Upconversion under polychromatic excitation: Y2SiO5:Pr3+,Li+ converts violet, cyan, green, and yellow light into UVC. Opt. Mater. 2013, 35 (12), 2347−2351. (31) Hikosaka, T.; Tanikawa, T.; Honda, Y.; Yamaguchi, M.; Sawaki, N. Fabrication and properties of semi-polar (1−101) and (11−22) InGaN/GaN light emitting diodes on patterned Si substrates. Phys. Status Solidi C 2008, 5 (6), 2234−2237.
rates. Their results may thus simply suggest that UVA irradiation is a better alternative to UVC, as it is less germicidal. To conclusively demonstrate that TiO2/YAlO3:Er3+ is capable of UC-sensitized photocatalysis using blue/green wavelengths from lamp sources, the catalytic performance of this composite must be compared to that of TiO2/YAlO3(undoped) while employing a < 420 nm cutoff filter. It is my opinion that such materials should not be included in applied technology research until the UC-sensitized photocatalysis ability is rigorously confirmed.
Ezra L. Cates*
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Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29631, United States
AUTHOR INFORMATION
Corresponding Author
*Phone: 864-656-1540; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.est.5b03105 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
