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Remediation and Control Technologies 2

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†,* †

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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

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Women’s University, Seoul, 04310, Korea

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Submitted to

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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]

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Phone: +82-54-279-2283 1

ACS Paragon Plus Environment


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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

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{001} facet anatase. The photocatalytic degradation activity of toluene on 001-TNT was at

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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

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sign of catalyst deactivation under the same test conditions. Under visible light irradiation,

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the 001-TNT showed degradation activity for acetaldehyde and formaldehyde, while the TNT

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did not exhibit any degradation activity for them. The 001-TNT filter was successfully scaled

32

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.

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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

22

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

130

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

135

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+

139

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

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surface of TiO2 is positively charged. The surface fluorination reduces the surface energy of

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{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

178

system. The TNT film, a coiled Pt wire, and a Ag/AgCl electrode were employed as a

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working, a counter, and a reference electrode, respectively. The electrochemical impedance

180

spectroscopy (EIS) Nyquist plot was obtained in the frequency range of 10-2−106 Hz with an

181

alternating current (AC) voltage of 50 mV. The Mott-Schottky analysis was done with a

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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

186

investigated by the following spectroscopic analysis: high resolution transmission electron

187

microscopy (HR-TEM, JEOL, JEM-2200FS) with Cs correction, X-ray diffraction (XRD,

188

Max Science Co., M18XHF) using Cu-Kα radiation, X-ray photoelectron spectroscopy (XPS,

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ESCALAB 250), field emission scanning electron microscopy (FE-SEM, JEOL, JSM-7401F),

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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

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Jobin-Yvon LabRam Aramis) with 514.5 nm excitation laser and a 10 s exposure time.

194 195

Testing TNT Filter Installed on a Commercial Air Cleaner (Samsung, AX7000).

196

To prepare an enlarged TNT filter, a Ti plate (273(w) x 308(h) x 0.5(t) mm3) was punched

197

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

201

(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

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equipped with two 001-TNT filters and 30 UV-LEDs (Seoul Viosys, CUN66A1B) as the

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light source (total power 84 W) (Figure 7c).

205 206

VOCs Removal Test according to Air Cleaner Standards Protocol (SPS-

207

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

209

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

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sequence of HCHO, NH3, CH3CHO, CH3COOH, and C7H8. Each removal experiment was

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conducted with an initial concentration of 10 ppm for 30 min. After each test experiment, the

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chamber was cleaned by flushing with clean air and then refilled by the next target gas. The

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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

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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

221

and 001-TNT. Both have ordered channel structures with the tube diameter of 40 – 60 nm,

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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

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annealed above 450 ˚C, nanoscopic cracks were created on the nanotube walls and a rutile

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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

249

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

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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-

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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

266

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.

271

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

281

((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

13



303

. .

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= 0.07).51 Since the surface OH groups provide the hole trapping sites

304

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


Page 15 of 36


327

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.

15



Page 16 of 36

349

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


Page 17 of 36


373

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



Page 18 of 36

397

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


Page 19 of 36


420

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



Page 20 of 36

443

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

References

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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.

614

(60) Dozzi, M.; D’Andrea, C.; Ohtani, B.; Valentini, G.; Selli, E. Fluorine-doped TiO2

615

materials: photocatalytic activity vs time-resolved photoluminescence. J. Phys. Chem. C

616

2013, 117, 25586-25595.

617

(61) Dozzi, M.; Livraghi, s.; Giamello, E.; Selli, E. Photocatalytic activity of S- and F-doped

618

TiO2 in formic acid mineralization. Photochem. Photobiol. Sci. 2011, 10, 343-349.

619

(62) Li, D.; Haneda, H.; Labhsetwar, N.; Hishita, S.; Ohashi, N. Visible-light-driven

620

photocatalysis on fluorine-doped TiO2 powders by the creation of surface oxygen vacancies.

621

Chem. Phys. Lett. 2005, 401, 579−584.

622

(63) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N.; Labhsetwar, N. K. Fluorine-doped TiO2

623

powders prepared by spray pyrolysis and their improved photocatalytic activity for

624

decomposition of gas-phase acetaldehyde. J. Fluorine Chem. 2005, 126, 69−77.

625

(64) Khan, F. I.; Ghosal, A. K. Removal of volatile organic compounds from polluted air. J.

626

Loss Prev. Process Ind. 2000, 13 (6), 527-545. 27



Page 28 of 36

627

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.

630

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.

636 637 638 639 640 641 28


Page 29 of 36


642 643

Figure 1. Schematic illustration of the procedures used to fabricate TNT and 001-TNT.

644

29



Page 30 of 36

645 646

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.

650 651 652 653 30


Page 31 of 36


654 655

Figure 3. XRD patterns of TNTs and F-TNTs annealed at various temperatures.

31



Page 32 of 36

656 657

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.

32


Page 33 of 36


662 663

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



Page 34 of 36

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).

672 673 674 34


Page 35 of 36


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).

682 683 684 685 686 687 35



688

Page 36 of 36

Table of Content (TOC)

689

36


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