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Active {001} Facet Exposed TiO Nanotubes Photocatalyst Filter for Volatile Organic Compounds Removal: From Material Development to Commercial Indoor Air Cleaner Application Seunghyun Weon, Eunji Choi, Hyejin Kim, Jee Yeon Kim, Hee-Jin Park, Sae-mi Kim, Wooyul Kim, and Wonyong Choi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02282 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018
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Environmental Science & Technology
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Active {001} Facet Exposed TiO2 Nanotubes Photocatalyst Filter
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for Volatile Organic Compounds Removal: From Material
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Development to Commercial Indoor Air Cleaner Application
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Seunghyun Weon†, Eunji Choi†, Hyejin Kim†, Jee Yeon Kim‡, Hee-Jin Park‡, Sae-mi Kim‡,
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Wooyul Kim§, and Wonyong Choi†,* †
6 7 8 9
Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea
‡
Frontier Research Team, Samsung Research, Samsung Electronics Co., Seoul 06765, Korea
§
Department of Chemical and Biological Engineering, College of Engineering, Sookmyung
10
Women’s University, Seoul, 04310, Korea
11 12
Submitted to
13
Environmental Science & Technology
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2018
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* To whom correspondence should be addressed (W. Choi)
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E-mail:
[email protected] 17
Phone: +82-54-279-2283 1
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ABSTRACT
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TiO2 nanotubes (TNT) have a highly ordered open structure that promotes the diffusion of
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dioxygen and substrates onto active sites and exhibit high durability against deactivation
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during the photocatalytic air purification. Herein, we synthesized {001} facet-exposed TiO2
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nanotubes (001-TNT) using a new and simple method that can be easily scaled up, and tested
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them for the photocatalytic removal of volatile organic compounds (VOCs) in both a
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laboratory reactor and a commercial air cleaner. While the surface of TNT is mainly
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composed of {101} facet anatase, 001-TNT’s outer surface was preferentially aligned with
26
{001} facet anatase. The photocatalytic degradation activity of toluene on 001-TNT was at
27
least twice as high as that of TNT. While the TNT experienced a gradual deactivation during
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successive cycles of photocatalytic degradation of toluene, the 001-TNT did not exhibit any
29
sign of catalyst deactivation under the same test conditions. Under visible light irradiation,
30
the 001-TNT showed degradation activity for acetaldehyde and formaldehyde, while the TNT
31
did not exhibit any degradation activity for them. The 001-TNT filter was successfully scaled
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up and installed on a commercial air cleaner. The air cleaner equipped with the 001-TNT
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filters achieved an average VOCs removal efficiency of 72% (in 30 minutes of operation) in a
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8-m3 test chamber, which satisfied the air cleaner standards protocol (Korea) to be the first
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photocatalytic air cleaner that passed this protocol.
36 37
Keywords: TiO2 nanotubes, Photocatalytic air purification, Removal of VOCs, Facet
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engineering, Air cleaner.
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INTRODUCTION
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As modern people suffer from sick building syndrome (e.g., excessive fatigue, headache,
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skin trouble, etc.) due to exposure to poor indoor air quality (IAQ), various air purification
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technologies which are adequate for the indoor environments have been developed, such as
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filtration, adsorption, ionization, ultraviolet germicidal irradiation (UVGI), and photocatalytic
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oxidation.1-3 The leading technology for removing indoor air pollutants (e.g., volatile organic
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compounds (VOCs), odors (S, N-containing pollutants), and ozone) is the adsorption using
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filtration media such as activated carbon.4 However, it is doubtful whether the adsorption
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technology is efficient for indoor air purification because the equilibrium adsorption capacity
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is markedly reduced at sub-ppmv-level concentrations5 and the removal efficiency of
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adsorbents is adversely influenced by increasing relative humidity.6 Although various kinds
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of thermal catalysts for VOCs degradation have been developed and investigated, most
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thermal catalysts need expensive noble metals and elevated temperature well above room
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temperature for activation.7, 8 For indoor air cleaning, the need of noble metals and high
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temperature condition is inappropriate in view of cost, safety, and energy consumption.
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Among various air purification technologies, photocatalysis is a promising method for
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removing indoor VOCs because it can degrade VOCs completely to harmless CO2 and H2O
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without the need of noble metals under ambient conditions and maintain its removal
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efficiency even in the low concentration range (sub-ppmv levels).9-11 This technology is
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environmentally benign because the degradation of VOCs is driven by the reactive oxidants
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(e.g., ·OH, O2-) that are produced under ambient irradiated conditions (without the need of
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any chemical oxidants) of which mechanisms have been extensively investigated.12 However,
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the success cases of photocatalytic air cleaners in the market are few and limited largely due
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to low efficiency and durability. 3
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Although various photocatalysts have been widely tested, titanium dioxide (TiO2) has
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been most commonly employed as a practical photocatalyst for environmental remediation
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because of its abundance, low cost, non-toxicity, chemical stability, and strong oxidative
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power.13-15 Therefore, finding the most efficient form of TiO2 is one of the most important
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issues for photocatalytic air-purification technology development. The photocatalytic activity
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of TiO2 is determined by various physicochemical properties including morphology, crystal
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structure, defects, surface area, and exposed facets. The photocatalytic activity of TiO2 often
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exhibits rapid deactivation over time, which is caused by strong complexation and
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accumulation of carbonaceous intermediates that are in-situ generated from the degradation
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of VOCs on the photocatalyst surface. Since the formation of carbonaceous intermediates is
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often caused by an insufficient supply of O2, designing open structures (e.g., TiO2 nanotubes
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(TNT)) that allow facile O2 diffusion was proposed as a solution for preventing photocatalyst
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deactivation.16, 17 On the other hand, the intrinsic photocatalytic activity of TiO2 is closely
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related to its crystalline structure and surface facets.18, 19 Computational and experimental
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results indicate that the {101}, {010}, and {001} surface facets of anatase TiO2 exhibit
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different average surface energies of 0.44, 0.53, and 0.90 J/m2, respectively.20 Behavior of
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excess electrons at each surface facet is different, which can affect photocatalytic activity.
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Excess electrons in {101} facet surface tend to be trapped at a stable surface Ti3+-bridging
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OH complex, as a result, reductive conversion reactions are easily facilitated on the {101}
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facet surfaces. On the other hand, {001} facet surface attracts holes, which should facilitate
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oxidation reactions.21,
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predominance of five-coordinated Ti atoms, photocatalytic oxidation activity is much higher
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on {001} facets than on {101} facets.23,
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fabricated in various forms such as nanosheet, nanobox, and nanobelts and exhibited higher
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Owing to the high surface energy of {001} facet and surface
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The {001} facet-dominant TiO2 has been
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photocatalytic degradation activity for the degradation of aquatic pollutants than commercial
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TiO2.25-28 The {001} facet-dominant TiO2 showed superior photocatalytic activity for the
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degradation of gaseous pollutants.29, 30 In this respect, {001} facet-exposed TiO2 nanotubes
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(001-TNT), if successfully prepared, is expected to exhibit not only high photocatalytic
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activity but also resistance to the catalyst deactivation as an air-purifying photocatalyst,
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which has never been demonstrated.
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The general procedure to synthesize {001} facet-exposed TiO2 requires hydrofluoric acid
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as the capping agent under hydrothermal conditions.31 However, the general synthesis
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method (HF and hydrothermal method) of {001} facet-exposed TiO2 might easily collapse
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the morphological structure of TNT and is not desirable for scaling up the synthesis process
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because of the high toxicity of HF.32 Some studies have tried to synthesize 001-TNT by using
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surfactants33,
34
or adding Zn-assisted cathodic polarization procedures.35 Such previous
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methods are not very suitable for large-scale production. In addition, all of the reported usage
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of 001-TNT has been confined to photoelectrochemical energy devices such as dye sensitized
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solar cells (DSSCs) or supercapacitors that focus only on the electrochemical properties of
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TNT. Considering the unique open structure morphology and controlled reactive surface
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facets, 001-TNT can be proposed as an ideal photocatalyst for VOC degradation.
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In this study, we fabricated 001-TNT by developing a simpler and cost-effective facet-
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engineering method that is suitable for scale-up production and demonstrated its successful
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application as an efficient photocatalyst filter for air purification. The photocatalytic activity
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and durability of 001-TNT and TNT for the degradation of VOCs were compared to
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demonstrate the superior performance of 001-TNT as an air purifying photocatalyst. An
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enlarged 001-TNT filter (273 x 308 mm2) installed on a commercial air cleaner was prepared
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and successfully tested for VOCs removal performance that satisfied the air cleaner standards 5
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protocol (SPS-KACA002-132, Korea) for the first time as a photocatalyst-only air cleaner
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(without adsorbents and others).
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MATERIALS AND METHODS
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Materials. TNT films were synthesized by an electrochemical anodization method.
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The anodization was performed in an anodizer (Anodizer, Teraleader) with a two-electrode
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cell having Ti foil (Sigma-Aldrich, 0.127 mm thick, 99.7% purity) as a working electrode and
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a Pt-coiled wire as a counter electrode. Ti foil was cut into pieces measuring 3.5 cm x 3.5 cm
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in size. Before anodization, Ti foil was ultrasonically cleaned sequentially with acetone,
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ethanol, and water, then dried in air. The anodization was conducted at 50 V for 1 h in
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ethylene glycol electrolyte containing 0.3 wt% NH4F (Sigma-Aldrich, 98% purity) and 1 wt%
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H2O. The resulting TNT film was cleaned with ethanol and water, dried in air, and then
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annealed in air at 400 °C for 3 h with a heating rate of 2 °C min-1.
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To synthesize {001} facet-exposed TNT (denoted as “001-TNT”), NaF treatment
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was additionally applied to the amorphous TNT. Unlike the reported manufacturing processes
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of facet-exposed TNTs which employed hydrothermal method with toxic HF or surfactants,
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the present method greatly simplified the cumbersome manufacturing process by using NaF
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instead. The procedure for the fabrication of 001-TNT is illustrated in Figure 1. The as-
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prepared TNT (a-TNT) is amorphous before annealing. To fabricate 001-TNT, a-TNT was
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soaked in NaF (30 mM) at pH 3.5 for stabilizing {001} facet of TiO2 during 30 minutes. This
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method is commonly used to substitute the surface hydroxyl group of TiO2 with fluoride.36
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Simple ligand exchange between the surface hydroxyl groups on the TiO2 and the fluoride
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anions takes place on TiO2 surface (≡Ti−OH + F− → ≡Ti−F + OH−). After NaF treatment, the 6
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color of the amorphous TNT changed to black and showed clear absorption in visible region
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(Figure S1a). The {001} facet contains more oxygen vacancies than the {101} facet because
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each Ti atom in the {001} facet is coordinated with five oxygen atoms. The presence of Ti3+
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and oxygen vacancies induces the formation of mid-energy levels in the TiO2 bandgap,
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leading to a spectral absorption in the visible region.37 The presence of surface fluoride
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substituting the surface OH group is evidenced by FT-IR spectra, which show a significantly
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reduced O−H stretching band (3364 cm−2) on F-TNT in comparison with a-TNT (Figure S1b).
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The absorption of fluoride anions should be favored in acidic NaF solution in which the
144
surface of TiO2 is positively charged. The surface fluorination reduces the surface energy of
145
{001} facets, which makes their preferential exposure on the TNT surface easier. However, it
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is interesting to note that the XPS analysis of a-TNT and F-TNT shows that both samples
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have a similar surface F concentration (Figure S2a). This implies that the surface fluoride
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anions on a-TNT should exist as physisorbed anions while those on F-TNT should be
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chemically bonded (i.e., chemisorbed as ≡Ti−F). The black coloration of TNT took place
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only when the fluorination was done at acidic pH (Figure S3). After annealing, 001-TNT
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could be successfully obtained and the surface fluoride species were removed (Figure S2b).
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Reactor Setup and Experimental Procedure. The photocatalytic degradation of
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VOCs (toluene, acetaldehyde, and formaldehyde) was conducted in a closed-circulation
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reactor at ambient conditions.16 The TNT and 001-TNT films (size: 7 cm2) were compared
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for the photocatalytic degradation of VOCs under the same experimental conditions. The
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closed-circulation reactor contained a glass reactor (volume: 300 mL) with a quartz window
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(radius: 3 cm). A magnetic bar was placed at the bottom of the glass reactor to stir the air in
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it. A photoacoustic gas monitor (LumaSense, INNOVA 1412i), which can measure the 7
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concentrations of toluene, acetaldehyde, formaldehyde, carbon dioxide, and water vapor
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simultaneously, was connected to the glass reactor by a Teflon tube (2 mm radius). The
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reactor employed a 370 nm-emitting UV-LED (Luna Fiber Optic Korea, ICN15D-096) and a
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150-W halogen lamp with a long-pass cutoff filter (λ > 420 nm) as light sources of UV and
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visible light irradiation, respectively. The distance between the photocatalyst film surface and
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the light source was 4 cm. The intensity of UV and visible light flux was measured to be 8
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mW/cm2 and 30 mW/cm2, respectively, by a power meter (Newport, 1815-C). Before each
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experiment, the glass reactor was flushed by high-purity air and the photocatalyst film was
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pre-cleaned by illuminating under UV for 30 min to remove any adsorbed surface organic
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impurities. After the photocatalyst cleaning process, the concentration of toluene,
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acetaldehyde, or formaldehyde was adjusted by diluting the standard gas (300 ppmv toluene,
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1000 ppmv acetaldehyde, and 100 ppmv formaldehyde in Ar) with high-purity air. The
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humidity level was regularly checked by the photoacoustic gas monitor to maintain the
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relative humidity (RH) at ca. 65% by bubbling air through a stainless steel bottle containing
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deionized water.
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Photoelectrochemical Measurements. Photoelectrochemical (PEC) measurements
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were conducted using a potentiostat (Gamry, Reference 600) connected to a three-electrode
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system. The TNT film, a coiled Pt wire, and a Ag/AgCl electrode were employed as a
179
working, a counter, and a reference electrode, respectively. The electrochemical impedance
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spectroscopy (EIS) Nyquist plot was obtained in the frequency range of 10-2−106 Hz with an
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alternating current (AC) voltage of 50 mV. The Mott-Schottky analysis was done with a
182
potential range from -1.0 V to 1.0 V (vs Ag/AgCl) at a selected frequency of 1 kHz and AC
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voltage of 30 mV. 8
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Photocatalyst Characterization. The properties of TNT and 001-TNT were
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investigated by the following spectroscopic analysis: high resolution transmission electron
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microscopy (HR-TEM, JEOL, JEM-2200FS) with Cs correction, X-ray diffraction (XRD,
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Max Science Co., M18XHF) using Cu-Kα radiation, X-ray photoelectron spectroscopy (XPS,
189
ESCALAB 250), field emission scanning electron microscopy (FE-SEM, JEOL, JSM-7401F),
190
diffuse reflectance UV-Visible absorption spectroscopy using a spectrophotometer
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(Shimadzu UV-2401PC), attenuated total reflectance Fourier transform infrared spectroscopy
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(ATR-FTIR, Thermo Scientific iS50) using ZnSe crystal, and Raman spectroscopy (Horiba
193
Jobin-Yvon LabRam Aramis) with 514.5 nm excitation laser and a 10 s exposure time.
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Testing TNT Filter Installed on a Commercial Air Cleaner (Samsung, AX7000).
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To prepare an enlarged TNT filter, a Ti plate (273(w) x 308(h) x 0.5(t) mm3) was punched
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with multiple pores to minimize the pressure drop. The Ti plate with pores was anodized at
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150 V for 2 h in ethylene glycol electrolyte containing 0.3 wt% NH4F (Sigma-Aldrich, 98%
199
purity) and 1 wt% H2O to obtain TNT and 001-TNT filters (see Figure 7a). The resulting
200
001-TNT filter has channel pores with the diameter of 40 – 60 nm and a tube length of 12 µm
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(inset of Figure 7a). The 001-TNT filter was assembled with a reflector plate on the back side
202
to recover transmitted light (Figure 7b). A commercial air cleaner (Samsung, AX7000) was
203
equipped with two 001-TNT filters and 30 UV-LEDs (Seoul Viosys, CUN66A1B) as the
204
light source (total power 84 W) (Figure 7c).
205 206
VOCs Removal Test according to Air Cleaner Standards Protocol (SPS-
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KACA002-132). This standard, which was issued by Korea Conformity Laboratories (KCL), 9
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specifies the deodorization performance of air cleaners installed in indoor places such as
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houses and offices. The test was conducted in a chamber with a volume of 8.0(±0.2) m3
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(Figure 7d). Five VOC gases were tested and their removal efficiencies were measured in the
211
sequence of HCHO, NH3, CH3CHO, CH3COOH, and C7H8. Each removal experiment was
212
conducted with an initial concentration of 10 ppm for 30 min. After each test experiment, the
213
chamber was cleaned by flushing with clean air and then refilled by the next target gas. The
214
removal efficiency (E (%) = (1 – C30/C0) x 100) was calculated for each target gas. During
215
each test, the chamber was maintained at a temperature of 23°C and RH of 55%. An air
216
cleaner can be qualified to obtain a collective quality certification (CA) mark when the
217
average removal efficiency of five gases is above 70% after 30 minutes of operation.
218 219
RESULTS AND DISCUSSION
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Properties of Synthesized TNTs. Figure 2a and Figure 2c show the TEM images of TNT
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and 001-TNT. Both have ordered channel structures with the tube diameter of 40 – 60 nm,
222
which confirmed that NaF treatment did not change the morphological structure. However,
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the crystallographic structure was significantly different between TNT and 001-TNT (Figure
224
2b and 2d). The HR-TEM image of TNT shows the presence of many nanocrystallites in the
225
size range of 10 – 20 nm, which confirms that the wall of TNT is composed of many small
226
grains that should serve as the recombination centers of charge carriers.38 The lattice spacing
227
of 0.352 nm corresponds to the (101) planes, which implies that the surface of TNT is mainly
228
composed of {101} facets. The FFT (Fast Fourier Transform) pattern of TNT (inset of Figure
229
2b) shows a split dot-like pattern, which implies that the bare TNT is polycrystalline. On the
230
other hand, the HR-TEM image of 001-TNT shows a continuous lattice fringe in a wider area
231
(Figure 2d). The lattice spacing of 0.238 nm corresponds to the (004) planes, which implies 10
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that the outer wall of 001-TNT is mainly composed of {001} facets. The FFT pattern of 001-
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TNT (inset of Figure 2d) shows single-crystal-like spots attributed to the {001} facet anatase.
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Figure 3 shows the X-ray diffraction (XRD) patterns of TNTs and F-TNTs annealed
235
at various temperature from 300 ˚C to 400 ˚C. Annealing amorphous TNT induces the
236
formation of the crystal structures in TNT.39, 40 The XRD pattern spectra of TNT annealed at
237
300 ˚C for 3 h is still the same as that of the Ti foil, which implies that the TiO2 structure was
238
not crystallized (a-TNT). With increasing the annealing temperature above 350 ˚C,
239
amorphous TNT began to form anatase crystals. However, when amorphous TNT was
240
annealed above 450 ˚C, nanoscopic cracks were created on the nanotube walls and a rutile
241
phase started to emerge.41 Therefore, the TNTs used in this work were annealed at 400 ˚C.
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The XRD spectra of TNT and 001-TNT (both annealed at 400 ˚C) shows anatase patterns
243
(JCPDS NO. 00-021-1272). The XRD spectra of TNT exhibits main peaks of (101) relative
244
to (004) and (200), which implies that TNT has random crystallographic orientation like
245
typical anatase TiO2 powder. However, 001-TNT shows a higher (004) diffraction peak at
246
37.8° and a lower (101) diffraction peak at 25.3° compared to TNT. The relative intensity of
247
(004) diffraction to that of (101) (I004/I101) on 001-TNT was 6.15, while that of TNT was 0.33.
248
It indicates that 001-TNT is preferentially oriented along the [001] direction due to surface
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alignment with {001} exposed facets. Preferentially oriented TNTs shows excellent current
250
density and energy conversion efficiencies compared to bare TNT and TiO2 nanoparticles in
251
photoelectrochemical devices.42-44
252
crystallization of TNTs annealed at low temperatures. When TNT was annealed at 350 ˚C, it
253
crystallized incompletely. However, F-TNT showed higher XRD intensity of anatase crystals
254
after annealing at 350 ˚C, which indicates that NaF treatment facilitated the crystallization of
255
TNT.
In
addition, the NaF treatment facilitated the
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Electrochemical impedance spectroscopy (EIS) analysis was conducted to assess the
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charge transfer resistances of TNT and 001-TNT (Figure 4a). 001-TNT exhibited a lower
258
charge transfer resistance (showing a smaller semi-circle) than TNT, which indicates that the
259
photogenerated charge carriers are more efficiently transferred at the interface. Since 001-
260
TNT has more single-crystalline structure, the charge transfer in 001-TNT should be more
261
facilitated in polycrystalline TNT. To gain more information on 001-TNT characteristics,
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Mott-Schottky analysis was conducted with TNT and 001-TNT (Figure 4b). 001-TNT
263
exhibited a lower slope, which indicates a higher donor density in 001-TNT than TNT. This
264
should be caused by NaF treatment that might introduce fluoride ions into TiO2 crystal lattice
265
as dopants as well as on the TiO2 surface as complexing ligands. The spot-profile EDS
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analysis confirmed that F elements were doped in 001-TNT (Figure S4). However, the nature
267
of fluorine doping and the dopant concentration could not be fully characterized in this study.
268
Nevertheless, the notable properties of 001-TNT induced by NaF treatment seem to be related
269
with the enhanced the electrical conductivity and charge carrier density, which should
270
influence the photocatalytic activity.
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The absorption spectra of TNT and 001-TNT showed clear differences (Figure 4c).
272
TNT can absorb light only at λ < 400 nm because of the large bandgap of TiO2 (3.2 eV for
273
anatase). When TNT was synthesized in fluoride electrolytes (e.g., HF, NH4F), some fluoride
274
ions might be self-doped into the TiO2 lattice in TNT,45 although the majority of self-doped
275
F- ions can be driven out via annealing. The incorporation of F- ions and the concomitant Ti3+
276
introduces defect states in the bandgap, which should induce visible light absorption. As a
277
result, the 001-TNT exhibited an extended absorption spectrum to the visible region. This
278
implies that NaF treatment played a role in controlling not only the surface facet structure but
279
also the defect energy states in TNT. By assuming an indirect transition, the bandgap of TNT 12
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and 001-TNT was estimated to be ca. 3.1 and 2.95, respectively, according to Tauc’s plot
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((ahv)1/2 ∝ (hv - Eg)) (Figure 4d). The bandgap difference between TNT and 001-TNT is
282
consistent with the fact that TiO2 with {001} facet absorbs more photons than TiO2 with {101}
283
or {010} facets.46
284
The Raman spectra of both TNT and 001-TNT show the same peaks at 144 (Eg), 394
285
(B1g), 516 (A1g), and 637 cm-1 (Eg), which indicates the formation of TiO2 (Figure 4e).
286
Among these peaks, the intensity of Eg peaks at 144 and 637 cm-1 is mainly related to
287
symmetric stretching vibration of O-Ti-O in TiO2. B1g and A1g peak represents the symmetric
288
bending vibration and asymmetric bending vibration of O-Ti-O, respectively.47 There are four
289
kinds of bonding modes in anatase TiO2 including six-coordinated Ti atom (6c-Ti), three-
290
coordinated O atom (3c-O), five-coordinated Ti atoms (5c-Ti), and two-coordinated O atoms
291
(2c-O). However, only unsaturated 5c-Ti and 2c-O bonding mode are present on the surface
292
of {001} facet TiO2. Therefore, the symmetric stretching vibration modes (Eg) should be
293
reduced on {001} facets with 5c-Ti. As a result, 001-TNT indeed exhibited reduced Eg peak
294
intensity compared to TNT. The unsaturated 5c-Ti sites on {001} facets induce the
295
dissociative adsorption of water molecule with generating surface OH groups.48 A
296
computational analysis confirmed that H2O molecules are dissociatively adsorbed onto the
297
5c-Ti with the formation of surface hydroxyl groups, whereas 6c-Ti cannot adsorb H2O
298
dissociatively.49 Accordingly, 001-TNT showed higher intensity of the O−H band at 3364cm-
299
1
300
most of the adsorbed surface fluoride and formed OH-terminated surface,50 001-TNT
301
exhibited OH-terminated surface. On anatase TiO2, the surface number density of 5c-Ti
302
atoms is twice higher on {001} facets than {101} facets (N001-TNT = . = 0.14, NTNT =
compared to TNT in the FT-IR spectra (Figure 4f). Since the annealing process removed
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. .
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= 0.07).51 Since the surface OH groups provide the hole trapping sites
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with generating the surface-bound OH radicals, the high surface density of OH groups is
305
often related with the high photocatalytic activity for organic degradation.
306 307
Photocatalytic Degradation of VOCs on TNT vs 001-TNT.
308
tested for the degradation of model VOCs such as toluene, acetaldehyde and formaldehyde
309
(Figure 5, Figure 6). Each photocatalytic degradation test consisted of a dark circulation
310
period (10 min) for adsorption equilibrium and the following irradiation period. All VOCs
311
were not degraded at all under UV (370 nm) and visible light (> 420 nm) illumination in the
312
absence of photocatalysts. The 001-TNT showed a higher photocatalytic activity than TNT
313
for toluene degradation under the identical experimental condition (Figure 5a). In general,
314
{001} facet-exposed TiO2 have higher activities for the degradation of organic pollutants
315
compared to anatase TiO2.52,
316
higher photocatalytic activity for gaseous acetone degradation compared to P25.54 Though
317
{001} facet exposure is beneficial for the degradation reaction, there is an optimal exposure
318
(ca. 70%) of {001} facets on TiO2. When the ratio of {001} facet exceeds the optimal level,
319
photogenerated charge carriers on TiO2 is not efficiently separated due to low percentage of
320
{101} facet TiO2. It is also confirmed from this work that the photocatalytic activity of
321
toluene degradation (expressed in the pseudo first-order rate constant) of 001-TNT (k = 12.87
322
x 10-2 min-1) was doubled from that of TNT (k = 6.37 x 10-2 min-1). Considering that the
323
surface-exposed hydroxyl group (formed by 5c-Ti) density on 001-TNT is twice as high as
324
that of TNT, the photocatalytic degradation activity and the number of surface hydroxyl
325
groups seem to be closely correlated. Although the photocatalytic degradation efficiencies
326
(DE = ([C7H8]0 – [C7H8]30min)/[C7H8]0 × 100) of TNT (DE = 93%) and 001-TNT (DE =
53
The TNT and 001-TNT were
{001} facet-exposed TiO2 nanosheet exhibited nine times
14
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100%) were almost the same in 30 minutes, the mineralization efficiencies (ME = △[CO2]/(7
328
× [C7H8]0) × 100) of TNT and 001-TNT showed clear difference. 001-TNT completely
329
degraded toluene into CO2, while ME of TNT reached only 80%. This implies that toluene
330
degradation intermediates were significantly generated on TNT but not on 001-TNT.55 The
331
accumulation of degradation intermediates on the photocatalyst surface may induce the
332
catalyst deactivation.
333
To compare the durability of TNT and 001-TNT, five successive cycles of
334
photocatalytic degradation of toluene were conducted. The deactivation of TiO2 photocatalyst
335
during toluene degradation has been frequently reported in the literature.56 The color of TiO2
336
was changed from white to brown when the catalyst deactivation was observed as a result of
337
the surface accumulation of carbonaceous intermediates. These are more strongly adsorbed
338
on the TiO2 surface than the parent toluene molecules.9, 57 Our previous study observed that
339
TNT exhibited a markedly higher resistance against the photocatalyst deactivation compared
340
to TiO2 nanoparticulate film due to the presence of open channels which facilitates the mass
341
diffusion of VOCs and O2 molecules onto active sites of photocatalysts.16, 17 Nevertheless,
342
TNT exhibited a gradual deactivation with repeated cycles of photocatalytic degradation of
343
toluene. However, 001-TNT maintained its photocatalytic activity during five cycles without
344
showing any sign of deactivation (Figure 5b). Even if 001-TNT was deactivated after the
345
photocatalytic degradation of higher concentration of toluene, its photocatalytic activity could
346
be regenerated by UV irradiation in the clean air. With a combination of morphology control
347
(open channel structure) and facet engineering, 001-TNT could be developed as an efficient
348
and durable photocatalyst for air purification.
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Water vapor concentration is an important parameter that influences the
350
photocatalytic degradation of VOCs.9, 58 Since the surface OH groups that are depleted during
351
photocatalysis can be replenished by water adsorption, the presence of water vapor is
352
critically important for maintaining the photocatalytic activity. The effects of water vapor on
353
the photocatalytic activities are compared in Figure 5c. In the absence of water vapor, the
354
initial degradation rate of both TNT and 001-TNT were faster, but CO2 generation was
355
suppressed. Such behavior can be explained by two competing effects of water vapor on
356
photocatalytic reaction that (1) H2O molecules are adsorbed on TiO2 surface to block
357
adsorption site of toluene molecules and (2) H2O molecules are the precursor of OH radicals.
358
In the absence of water vapor, the surface OH groups are rapidly depleted, which limits the
359
sustained production of surface OH radicals. Therefore, the photocatalytic degradation
360
intermediates of VOCs tend to be more accumulated on the catalyst surface in water vapor-
361
deficient condition. With a lower humidity level, more benzaldehyde, generated from the
362
degradation of toluene, accumulated on the surface of TiO2.59 In the absence of humidity, the
363
ME of 001-TNT was 66% while that of TNT was 31%, which indicates that 001-TNT is
364
much more efficient in mineralizing VOCs probably because of the higher surface OH group
365
density on 001-TNT. In the presence of water vapor (RH 65%), the ME of 001-TNT and
366
TNT was 100% and 80% in 30 minutes of irradiation, respectively.
367
On the other hand, it is noted that 001-TNT exhibited visible light photocatalytic
368
activity for the degradation of acetaldehyde and formaldehyde, whereas TNT did not (only
369
moderate adsorption was observed) (Figure 6). CO2 was generated stoichiometrically from
370
the photocatalytic degradation of acetaldehyde and formaldehyde, which confirms that VOCs
371
were removed by degradation, not adsorption. The visible light activity of 001-TNT seems to
372
be related with the effect of F doping that induces the formation of Ti3+ surface states, 16
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localized below the bottom of the conduction band of TiO2.60 It has been reported that
374
fluorine-doped TiO2 exhibits visible light absorption and visible light photocatalytic
375
activity.61-63
376
Scale-up Demonstration on a Commercial Air Cleaner. TNT and 001-TNT
377
photocatalyst filters were scaled up to the size of 273 x 308 mm2 and installed on a
378
commercial air cleaner (Samsung, AX7000) to test their photocatalytic air purification
379
performance. The TNT and 001-TNT filters were fabricated on Ti plates by anodization
380
method. The pressure drop of the 001-TNT filter is 0.33 mmAq under 1 m/s air stream, which
381
rarely affects air cleaner operation. The structural morphology of the 001-TNT filter was
382
examined by FE-SEM (inset of Figure 7a). Both the TNT and 001-TNT filters have channel
383
pores with the diameter of 40 - 60 nm and a tube length of 12 µm. The XRD of the TNT and
384
001-TNT filters showed anatase patterns (JCPDS NO. 00-021-1272) (Figure S5). The XRD
385
of 001-TNT filter clearly showed the (004) diffraction peak at 37.8°, which indicates that the
386
001-TNT filter contains a higher amount of {001} facet TiO2 and preferentially oriented
387
compared to the TNT filter. The gas removal efficiencies of the TNT and 001-TNT filters
388
were measured according to the air cleaner standards protocol (SPS-KACA002-132) on a
389
commercial air cleaner (AX7000, Samsung) (Figure 7, Figure S6). The removal efficiency of
390
each target gas in the test chamber (8 m3) was measured in the sequence of HCHO, NH3,
391
CH3CHO, CH3COOH, and C7H8 at each concentration of 10 ppmv. The photocatalytic
392
degradation efficiency of VOCs highly varied depending on the kind of VOCs and decreased
393
in the order of: HCHO > CH3COOH > NH3 > CH3CHO > C7H8 (Figure S6). It is noted that
394
toluene is degraded at the slowest rate among the tested VOCs. Therefore, enhancing and
395
optimizing the photocatalytic air cleaner performance for the removal of toluene as we did in
396
this work is a proper strategy. The air cleaner standard protocol requires that an air cleaner 17
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should achieve an average removal efficiency of the five test gases above 70% within 30
398
minutes to be qualified for the CA mark. There has been no air cleaner equipped with solely
399
photocatalyst filter, which received the CA mark in Korean market. The 001-TNT filter
400
successfully achieved 72.1% of average removal efficiency, while TNT filter reached only
401
56.8% (see Table 1). The ordered open channel morphology and preferentially exposed {001}
402
facets on the 001-TNT filter seem to work synergically to maximize both the photocatalytic
403
activity and durability for VOCs degradation.
404 405
Environmental Importance.
406
Most air-purifying technologies utilize the adsorption process (e.g., activated carbon
407
filter). However, the adsorption technology has some intrinsic drawbacks that limit its
408
application to indoor air purification: 1) adsorption is not efficient in low-concentration
409
conditions (sub-ppmv),5 2) the removal efficiency is drastically reduced in humid condition,6
410
3) saturated adsorbents should be further treated or regenerated for reuse.64 Photocatalytic air
411
purification is attractive since it can avoid such problems. Although various photocatalytic
412
materials have been developed, low efficiency and poor durability are still main obstacles for
413
commercialization. The 001-TNT developed in this study combines two merits of
414
morphology control and facet engineering. The ordered open channel structure is highly
415
advantageous in hindering the accumulation of recalcitrant degradation intermediates on the
416
catalyst surface and {001} facet-aligned surface exhibits intrinsically higher photocatalytic
417
activity for VOC degradation. In addition, TNT filters are self-regenerative and are much
418
thinner (a few mm) compared with the conventional air filters with activated carbon beads (a
419
few cm), which enables the design of compact air cleaners. It should be also noted that 18
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activated carbon beads may release and spray activated carbon particles to cause a secondary
421
air pollution whereas TNT filters do not release any TiO2 particles under normal air stream.
422
The simple method developed in this study synthesized 001-TNT without hydrothermal
423
treatment and surfactant templates, which is highly advantageous for larger-scale fabrication.
424
The proposed anodizing method is an electrochemical process that has been already adopted
425
in large-scale manufacturing processes of various commercial products. In addition, the use
426
of NaF instead of toxic HF makes the manufacturing process more feasible and safer and the
427
rapidly declining cost of UV-LEDs will make the photocatalytic air cleaners more cost-
428
competitive compared with other kinds of air cleaners. Although the overall construction cost
429
of the TNT-based air cleaner might be higher than activated carbon filter-based air cleaners,
430
it should be counterbalanced by the lower operation cost owing to long lifetime of
431
regenerative TNT filters. All the considerations make the 001-TNT-based photocatalytic air
432
cleaner highly favorable for commercialization. The scaled-up 001-TNT filter installed on a
433
commercial air cleaner (AX7000, Samsung) demonstrated the air purification capability in a
434
8-m3 chamber and successfully satisfied the air cleaner standards protocol (SPS-KACA002-
435
132).
436
ACKNOWLEDGMENT.
437
This work was financially supported by the Global Research Laboratory (GRL) program
438
(2014K1A1A2041044) and X-project (2016R1E1A2A01953975), which were funded by the
439
Korea government (MSIP) through the National Research Foundation (NRF), and Samsung
440
Electronics.
441 442
Supporting Information Available. 19
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UV/visible absorption spectra and FT-IR spectra (Fig. S1); XPS spectra (Fig. S2); Color
444
change of a-TNT under various NaF treatment conditions (Fig. S3); EDS spectra (Fig. S4);
445
XRD pattern of TNT filters in the size of 273 x 308 mm2 (Fig. S5); Harmful gas removal
446
efficiencies (as a function of time) of TNT filters installed on a commercial air cleaner (Fig.
447
S6). This information is available free of charge via the internet at http://pubs.acs.org/.
448
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Catal., B: Environmental 2018, 220, 1−8.
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(56) Mo, J.; Zhang, Y.; Xu, Q.; Lamson, J. J.; Zhao, R. Photocatalytic purification of volatile
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organic compounds in indoor air: a literature review. Atmos. Environ. 2009, 43, 2229−2246.
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(57) Mo, J.; Zhang, Y. P.; Xu, Q. J.; Zhu, Y. F.; Lamson, J. J.; Zhao, R. Determination and
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risk assessment of by-products resulting from photocatalytic oxidation of toluene Appl.
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Catal., B: Environmental 2009, 89, 570-576.
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(58) Obee, T.; Brown, R. TiO2 photocatalysis for indoor air applications: effect of humidity
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and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-
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butadiene. Environ. Sci. Technol. 1995, 29, 1223−1231.
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(59) Guo, T.; Bai, Z.; Wu, C.; Zhu, T. Influence of relative humidity on the photocatalytic
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oxidation (PCO) of toluene by TiO2 loaded on activated carbon fibers: PCO rate and
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intermediates accumulation. Appl. Catal., B: Environmental 2008, 79, 171–178.
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(60) Dozzi, M.; D’Andrea, C.; Ohtani, B.; Valentini, G.; Selli, E. Fluorine-doped TiO2
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materials: photocatalytic activity vs time-resolved photoluminescence. J. Phys. Chem. C
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2013, 117, 25586-25595.
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(61) Dozzi, M.; Livraghi, s.; Giamello, E.; Selli, E. Photocatalytic activity of S- and F-doped
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TiO2 in formic acid mineralization. Photochem. Photobiol. Sci. 2011, 10, 343-349.
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(62) Li, D.; Haneda, H.; Labhsetwar, N.; Hishita, S.; Ohashi, N. Visible-light-driven
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photocatalysis on fluorine-doped TiO2 powders by the creation of surface oxygen vacancies.
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Chem. Phys. Lett. 2005, 401, 579−584.
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(63) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N.; Labhsetwar, N. K. Fluorine-doped TiO2
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powders prepared by spray pyrolysis and their improved photocatalytic activity for
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decomposition of gas-phase acetaldehyde. J. Fluorine Chem. 2005, 126, 69−77.
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(64) Khan, F. I.; Ghosal, A. K. Removal of volatile organic compounds from polluted air. J.
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Loss Prev. Process Ind. 2000, 13 (6), 527-545. 27
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Table 1. VOC gas removal test according to the air cleaner standards protocol (SPS-
628
KACA002-132) by employing an air cleaner (AX7000, Samsung) equipped with TNT and
629
001-TNT filters.
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VOC Removal Testa (SPS-KACA002-132) Target Gas
Removal Efficiency (%) TNT
001-TNT
HCHO
88.1
97.2
NH3
44.8
76.5
CH3CHO
40.8
58.7
CH3COOH
87.0
95.2
C7H8
23.2
32.7
Average
56.8
72.1
631 The volume of test chamber was 8.0±0.2 m3. The temperature and relative humidity in the
632
a
633
test chamber were controlled at 23°C and 55%, respectively. Each removal test was
634
conducted for 30 minutes in the following order of HCHO, NH3, CH3CHO, CH3COOH, and
635
C7H8. The concentration of each test gas was 10 ppmv.
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642 643
Figure 1. Schematic illustration of the procedures used to fabricate TNT and 001-TNT.
644
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Figure 2. TEM images obtained from (a) TNT and (c) 001-TNT. HR-TEM images obtained
647
from the outer channel surface of (b) TNT and (d) 001-TNT. The insets of (b) and (d) show
648
the FFT (Fast Fourier Transform) patterns of each TNT. The yellow dotted-line indicates the
649
grain boundaries.
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654 655
Figure 3. XRD patterns of TNTs and F-TNTs annealed at various temperatures.
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Figure 4. (a) Nyquist plots of TNT and 001-TNT in the range from 1 MHz to 0.01 Hz at
658
OCV under irradiation (λ>320 nm). (b) Mott-Schottky plots at a fixed frequency of 1 kHz in
659
aqueous KOH solution (0.1 M, pH 3.5). (c) DRS spectra and (d) Tauc plot for TNT and 001-
660
TNT. (e) Raman spectra and (f) FT-IR spectra of TNT and 001-TNT. The IR peak at 3364
661
cm-1 is assigned to O-H stretching. The reference FT-IR spectrum was obtained with a Ti foil.
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Figure 5. (a) Photocatalytic degradation of gaseous toluene on TNT and 001-TNT under UV
664
(370 nm) irradiation. (b) Repeated cycles of photocatalytic degradation of gaseous toluene on
665
TNT and 001-TNT. (c) Photocatalytic degradation of gaseous toluene on TNT and 001-TNT
666
in ambient air (RH 65%) (●, ■) and in the absence of H2O vapor (▼, ▲). All the closed
667
symbols represent [C7H8] and the open symbols represent [CO2]. 33
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668 669
Figure 6. Photocatalytic degradation of (a) gaseous acetaldehyde (C0 = 10 ppmv) and (b)
670
gaseous formaldehyde (C0 = 20 ppmv) on TNT and 001-TNT under visible light irradiation
671
(λ > 420 nm).
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675 676
Figure 7. The Photo image of (a) 001-TNT filter in the size of 273(w) x 308(h) x 0.5(t) mm3
677
(the inset shows a FE-SEM image of the cross sectional view of 001-TNT), (b) TNT filter
678
unit assembled with a 001-TNT filter and a reflector plate, (c) an inside view of an air cleaner
679
(AX7000, Samsung) equipped with two 001-TNT filter units and 30 UV-LEDs (Seoul
680
Viosys, CUN66A1B), and (d) a VOC removal test chamber (8.0±0.2 m3) in Korea
681
Conformity Laboratories (KCL).
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Table of Content (TOC)
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