Article pubs.acs.org/est
Plasmon-Enhanced Sub-Bandgap Photocatalysis via Triplet−Triplet Annihilation Upconversion for Volatile Organic Compound Degradation Hyoung-il Kim,† Seunghyun Weon,‡ Homan Kang,§,⊥ Anna L. Hagstrom,† Oh Seok Kwon,†,¶ Yoon-Sik Lee,§,∥ Wonyong Choi,*,‡ and Jae-Hong Kim*,† †
Department of Chemical and Environmental Engineering, School of Engineering and Applied Science, Yale University, New Haven, Connecticut 06511, United States ‡ School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea § Interdisciplinary Program in Nano-Science and Technology, Seoul National University, Seoul 08826, Republic of Korea ∥ School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea S Supporting Information *
ABSTRACT: This study demonstrates the first reported photocatalytic decomposition of an indoor air pollutant, acetaldehyde, using low-energy, sub-bandgap photons harnessed through sensitized triplet−triplet annihilation (TTA) upconversion (UC). To utilize low-intensity noncoherent indoor light and maximize photocatalytic activity, we designed a plasmon-enhanced sub-bandgap photocatalyst device consisting of two main components: (1) TTA-UC rubbery polymer films containing broad-band plasmonic particles (Ag-SiO2) to upconvert sub-bandgap photons, and (2) nanodiamond (ND)-loaded WO3 as a visible-light photocatalyst composite. Effective decomposition of acetaldehyde was achieved using ND/WO3 (Eg = 2.8 eV) coupled with TTA-UC polymer films that emit blue photons (λEm = 425 nm, 2.92 eV) upconverted from green photons (λEx = 532 nm, 2.33 eV), which are wasted in most environmental photocatalysis. The overall photocatalytic efficiency was amplified by the broad-band surface plasmon resonance of AgNP-SiO2 particles incorporated into the TTA-UC films.
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INTRODUCTION
can utilize only a fraction (blue portion) of the whole visible spectrum (approximately ≤480 nm).7−9 Amplifying the frequency of incident light through upconversion (UC) of more than one sub-bandgap photon to a single high-energy photon corrects this spectral mismatch by enabling a photocatalyst to utilize photons that would otherwise be wasted, effectively extending its absorption range.10 This class of photoluminescence processes has been investigated for a myriad of applications, including photovoltaics,11 photoelectrochemical water splitting,12 bioimaging,13 and photodynamic therapy.14 Of the two primary UC mechanisms, only lanthanidebased UC has been shown to improve the photocatalytic degradation performance of semiconductors.15,16 However, because lanthanide cations have weak, narrow absorption bands, this UC mechanism typically requires high-power laser excitation to
The potential of semiconductor photocatalysis as a sustainable method for environmental remediation has long been recognized and investigated but has yet to be fully realized, partly due to a mismatch between the wavelengths prevalent in sunlight/ artificial light sources and the absorption range of effective photocatalysts.1−3 Semiconductor photocatalysts absorb light and generate electron−hole pairs that produce reactive oxygen species (ROS) such as hydroxyl radicals (•OH), which can effectively degrade air and water pollutants. Unfortunately, the photocatalysts most suitable for this purpose are wide-bandgap semiconductors with poor visible light absorption abilities. TiO2, the most popular environmental photocatalyst because of its low toxicity, high stability, and superior photooxidation power,1,4 has a bandgap energy (Eg) of 3.2 eV (anatase) and can therefore absorb UV photons only.5,6 UV photons make up less than 5% of solar radiation and a far smaller fraction of common indoor light sources such as halogen or fluorescent lamps. Even common visible-light photocatalysts such as WO3, C3N4, and Bi2WO6 © XXXX American Chemical Society
Received: May 31, 2016 Revised: August 15, 2016 Accepted: September 7, 2016
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DOI: 10.1021/acs.est.6b02729 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology reach efficiencies above 1%.17 On the other hand, triplet−triplet annihilation UC (TTA-UC) employing two organic chromophores (a sensitizer and an acceptor) in tandem to achieve UC18 achieves much higher quantum efficiencies, up to 40%,19,20 even under low-intensity noncoherent radiation such as sunlight or indoor light. However, two critical limitations have thus far stood in the way of its use in environmental remediation applications.19−21 Because TTA-UC relies on a series of Dexter energy transfers between the long-lived triplet excited states of adjacent chromophores that are insoluble in water, TTA-UC requires a nonpolar host medium that (1) allows chromophores to diffuse to collide with each other and (2) protects chromophores from exposure to oxygen (3O2), which readily quenches triplet excited states. The deoxygenated organic solvents that are typically employed in fundamental studies of TTA-UC are, unfortunately, impractical for long-term applications under ambient conditions. As a result, alternative host media for practical TTA-UC systems have been actively sought after. TTA-UC materials suitable for environmental remediation should have sufficient efficiency and stability in oxygen-rich environments. Researchers seeking to translate TTA-UC to oxygen-rich environments while maintaining the high diffusivity and efficiency of solution-based systems have investigated a variety of host media, including microemulsions,22 oxygenscavenging solvents,23 dendrimers,24 microscopically fluidic gel matrices,25 and liquid-core capsules.26,27 To integrate TTA-UC materials with photocatalysts, we previously fabricated micro/nanocapsules and employed them in aqueous suspensions to sensitize photocatalysts such as WO3 and CdS for ROS production using sub-bandgap photons.28,29 However, the critical limitations of their shell materials kept us from proceeding onward: the microcapsules’ rigid crystalline polymer shell effectively blocked oxygen, but its surface did not allow anchoring of photocatalyst particles,28 whereas the nanocapsules’ silica shell was easily functionalized with photocatalyst particles but allowed oxygen infiltration that caused rapid chromophore degradation.29 In this study, we shift our focus to solid-state TTA-UC materials slightly lower in efficiency than their liquid-based counterparts but far better suited to longterm operation in oxygen-rich conditions. We embed TTA-UC chromophores into thin films of a rubbery polyurethane whose high chain mobility allows efficient energy transfers between chromophores, making it one of the most efficient solid-state TTA-UC materials to date.21 These films can be easily fabricated in different sizes and shapes for integration with a wide variety of solar devices without any need for deoxygenation. Employing these solid-state systems, we herein report the first successful use of TTA-UC to harness sub-bandgap photons for the photocatalytic degradation of acetaldehyde, a volatile organic carbon (VOC) that is a common indoor air pollutant. We fabricated a photocatalyst device (Scheme 1) in which a polyurethane TTA-UC film absorbs incident green photons (532 nm/2.33 eV) and emits blue photons (425 nm/2.92 eV) that sensitize an underlying film of visible-light photocatalyst WO3 (Eg = 2.8 eV) loaded with cocatalyst nanodiamond (ND). Under low-intensity LED irradiation in a closed-circulation gasphase reactor, this device effectively oxidized acetaldehyde to carbon dioxide. To further enhance the absorption of incident light and thereby enhance the efficiency of sub-bandgap photocatalysis, we incorporated plasmonic particles consisting of silica (SiO2) spheres coated with closely assembled silver nanoparticles (AgNPs) into the TTA-UC films. This unprecedented
Scheme 1. Illustration of the Plasmon-Enhanced SubBandgap Photocatalyst Device for Acetaldehyde Degradation by Triplet−Triplet Annihilation-Based Upconversion
utilization of sub-bandgap photons for the photocatalytic degradation of a VOC provides the crucial first step toward the practical application of TTA-UC for photocatalytic air purification.
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EXPERIMENTAL SECTION Preparation of TTA-UC Films. Stock solutions of TTA-UC chromophores were prepared by dissolving a sensitizer, palladium(II) octaethylporphyrin (PdOEP; 1.6 mM) or platinum(II) octaethylporphyrin (PtOEP; 1.6 mM), and an acceptor, 9,10-diphenylanthracene (DPA; 15.1 mM), in tetrahydrofuran (THF). Precursor mixtures for the TTA-UC films were prepared by mixing Clear Flex 50 precursors (polyester polyol and methylene bis(4-cyclohexyl isocyanate); Smooth-On, Inc.) in a 1:2 ratio by volume and adding appropriate amounts of the chromophore stock solutions to bring the films’ final chromophore concentrations to 0.5 mM for PdOEP/PtOEP and 1.3−16.1 mM for DPA. Films were prepared by casting these precursor mixtures onto either round (12 mm diameter) or square (22 mm width) microscopy cover glasses and mounting a second cover glass on top to yield flat films. Films were cured at room temperature in the dark for 16 h and then heated in a convection oven at 55 °C for 48 h to remove residual THF. We further incorporated AgNP-SiO2 plasmonic particles into select TTA-UC films (PtOEP/DPA). SiO2 spheres were synthesized using the Stöber method30 and functionalized with thiol groups. A dense layer of silver nanoparticles (AgNPs) was grown on their surface in a solution of AgNO3 and octylamine, followed by a thin coating of silica formed through further treatment with tetraethylorthosilicate (TEOS). Detailed synthetic procedures, adopted from a previous report,31 are provided in the Supporting Information. AgNP-SiO2 particles were added directly to the precursor mixture in concentrations of 5−80 wt % with respect to PtOEP and otherwise following the same film fabrication procedure detailed above. This had no significant effect on the thickness of the resulting films: the average thickness of the unmodified PtOEP/DPA films was 150 ± 40 μm, whereas that of films containing 20 wt % AgNPSiO2 (with respect to PtOEP) was 160 ± 30 μm (Figure S1). Preparation of ND-Loaded WO3 (ND/WO3) Films. WO3 is known to be ineffective as a standalone environmental photocatalyst due to its low-lying conduction band (CB) potential (0.4 VNHE), which is not thermochemically favorable for utilizing O2 as an acceptor of CB electrons.32,33 It is therefore a B
DOI: 10.1021/acs.est.6b02729 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
detonation-synthesized ND agglomerates (uDiamond Allegro, ca. 5 wt %, ca. 4−6 nm diameter) followed by several rounds of acid treatment (1 M H2SO4) and rinsing with deionized water (DI). ND was loaded onto WO3 particles (Aldrich,
