TiO2 Nanotubes with Open Channels as Deactivation-Resistant

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TiO Nanotubes with Open Channels as Deactivation-Resistant Photocatalyst for the Degradation of Volatile Organic Compounds Seunghyun Weon, and Wonyong Choi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05418 • Publication Date (Web): 08 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016

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Environmental Science & Technology

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TiO2 Nanotubes with Open Channels as Deactivation-Resistant

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Photocatalyst for the Degradation of Volatile Organic

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Compounds

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Seunghyun Weon and Wonyong Choi*

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School of Environmental Science and Engineering, Pohang University of Science and

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Technology (POSTECH), Pohang 790-784, Korea

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Submitted to

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(Revised)

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2016

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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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We synthesized ordered TiO2 nanotubes (TNT) and compared their photocatalytic activity

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with that of TiO2 nanoparticles (TNP) film during the repeated cycles of photocatalytic

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degradation of gaseous toluene and acetaldehyde to test the durability of TNT as an air-

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purifying photocatalyst. The photocatalytic activity of TNT showed only moderate reduction

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after the five cycles of toluene degradation, whereas TNP underwent rapid deactivation as the

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photocatalysis cycles were repeated. Dynamic SIMS analysis showed that carbonaceous

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deposits were formed on the surface of TNP during the photocatalytic degradation of toluene,

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which implies that the photocatalyst deactivation should be ascribed to the accumulation of

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recalcitrant degradation intermediates (carbonaceous residues). In more oxidizing atmosphere

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(100% O2 under which less carbonaceous residues should form), the photocatalytic activity of

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TNP still decreased with repeating cycles of toluene degradation, whereas TNT showed no

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sign of deactivation. Because TNT has a highly ordered open channel structure, O2 molecules

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can be more easily supplied to the active sites with less mass transfer limitation, which

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subsequently hinders the accumulation of carbonaceous residues on TNT surface. Contrary to

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the case of toluene degradation, both TNT and TNP did not exhibit any significant

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deactivation during the photocatalytic degradation of acetaldehyde, because the generation of

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recalcitrant intermediates from acetaldehyde degradation is insignificant. The structural

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characteristics of TNT is highly advantageous in preventing the catalyst deactivation during

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the photocatalytic degradation of aromatic compounds.

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Keywords: Titanium dioxide, TiO2 nanotubes, Photocatalyst deactivation, air purification, Removal of VOCs

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INTRODUCTION

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Volatile organic compounds (VOCs) are major components of indoor air pollution. VOCs are

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composed of various alcohols, aromatics (benzene, toluene), aldehydes (acetaldehyde,

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formaldehyde), and halocarbons.1-3 These chemicals are emitted from household products

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such as construction materials, paints, furniture, and other consumer products. VOCs cause

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adverse effects on human health and are a main culprit for sick building syndrome.4-6

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Photocatalysis is an ideal method for removing VOCs present at low concentrations in indoor

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environment because it operates at ambient temperature and pressure to degrade them to CO2

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and H2O.7-9

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Titanium dioxide (TiO2) has been commonly used as a photocatalyst for environmental

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remediation because of its abundance, low cost, non-toxicity, chemical stability, and strong

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oxidative power.10-12 Application of TiO2 for air purification has been widely investigated

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and successfully demonstrated.13-17 However, TiO2 suffers from the catalyst deactivation that

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the activity gradually decreases during the continued use for VOC degradation.18-21 To be

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used for VOC degradation, TiO2 nanoparticles (TNP) should be immobilized on a substrate

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surface. Since the immobilized TNPs are closely packed on the substrate, VOC and O2

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molecules slowly diffuse to the active sites of TiO2 nanoparticles through the interstitial

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space. In addition, in-situ generated recalcitrant intermediates of VOC degradation, which are

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believed to be responsible for the photocatalyst deactivation, may not be easily removed from

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the surface of TNPs unless O2 molecules are sufficiently provided through the closely packed

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TNPs. The photocatalyst deactivation might be reduced by developing more active catalysts

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via heterojunction with other semiconductor,22 metal loading,23-24 and synthesis of new

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photocatalytic material.25 However, these are not intrinsic solutions for reducing catalyst

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deactivation that is commonly observed during the photocatalytic degradation of VOCs. 3



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TiO2 nanotube (TNT) has attracted much attention due to its unique structure.26 It can be

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easily and cost-effectively fabricated on Ti-foil by electrochemical anodization. It has well-

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ordered open channel structure with length of 100 nm – 100 µm and pore size of 10 nm – 500

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nm, which can be controlled by varying the anodization time and applied bias.27 Because of

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the TNT’s structural characteristics of openness, TNT has an advantage of facile mass

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diffusion of VOC and O2 molecules. Unlike TNPs, most of which surface is unexposed

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because of the closely packed structure, the surface of TNTs is mostly exposed due to its

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open channel structure and easily accessible to diffusing gas molecules. In addition, light can

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penetrate deeper into the TNT film than TNP film because of the open structure of TNT.

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With such properties, TNT might inhibit the accumulation of recalcitrant carbonaceous

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intermediates on its surface during the photocatalytic degradation process because any

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intermediates (as soon as they are formed) can be immediately degraded under high flux of

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O2 supply. Although TNT has demonstrated successful performances in photocatalytic

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purification of contaminated water and air,28-32 it has not been recognized that the unique

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structural characteristics of TNT may have an intrinsic resistance to the catalyst deactivation.

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This study investigated the durability of TNT as an air-purifying photocatalyst during the

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degradation of gaseous toluene and acetaldehyde that were employed as model VOCs. The

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photocatalytic activities of TNT and TNP were compared under the identical experimental

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conditions and with repeating the photocatalysis cycles to test if there is any significant

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difference in the catalyst durability between them. It was clearly demonstrated that TNT is

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more resistant to catalyst deactivation compared with TNP. The phenomena of TiO2

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photocatalyst deactivation

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characterized with employing TNP and TNT films. How the catalyst deactivation is affected

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by the structure of TiO2 and other factors is discussed in detail.

and re-activation were systematically investigated and

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MATERIALS AND METHODS

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Materials. For TNP preparation, commercial TiO2 (P25) with an average surface area of 50

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m2 g-1 and primary particle size of 20 – 30 nm was used. P25 was coated on a glass substrate

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(2 cm x 2 cm) using a doctor-blade method.33 P25 powder was vigorously mixed with ethanol

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in a concentration of 0.15 g/mL. The mixed slurry was cast on a glass substrate, dried under

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air and then heated at 200 °C for 1 h to remove residual ethanol in the resulting TNP film of

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10 µm thickness.

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TNT was synthesized by two-step electrochemical anodization to obtain more uniform

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and stable structure.34-35 Anodization was performed in a two-electrode cell with a Ti foil

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(Aldrich, 0.127 mm thick, 99.7% purity) as a working electrode and a Pt-coiled wire as a

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counter electrode. Ti foil was cut into 3 cm x 2 cm pieces. Before anodization, Ti foil was

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ultrasonically cleaned sequentially with acetone, ethanol and water, then dried in air. The first

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anodization was conducted at 60 V for 10 min in ethylene glycol electrolyte containing 0.5

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wt% NH4F (Sigma-Aldrich, 98% purity) and 3 wt% H2O. The resulting TNT layer was

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removed from the Ti foil substrate by ultrasonication in a concentrated H2O2 solution (Junsei,

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30% purity), then washed with water. The first anodized Ti foil was reused as a template for

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the second anodization and anodized again at 50 V for 1 h in ethylene glycol electrolyte

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containing 0.3 wt% NH4F and 1 wt% H2O. After the second anodization, the resulting TNT

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film was cleaned with ethanol and water, dried in air, then annealed in air at 450 °C for 3 h

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with a heating rate of 2 °C min-1.

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Reactor Setup and Experimental Procedure. The photocatalytic degradation of toluene

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and acetaldehyde was conducted in a closed-circulation reactor (Figure 1) at ambient 5



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conditions. The photocatalyst films of TNT and TNP were prepared in the size of 2 cm x 2

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cm and compared for the photocatalytic degradation of VOCs under the same experimental

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conditions. The glass reactor had a volume of 300 mL, and a quartz window with 3 cm

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radius. A magnetic bar was placed at the bottom of the glass reactor to circulate the air in it.

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The glass reactor and a photoacoustic gas monitor (LumaSense, INNOVA 1412i) were

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connected by a Teflon tube (2 mm radius). The photoacoustic gas monitor can simultaneously

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measure the concentrations of toluene, acetaldehyde, carbon dioxide and water vapor. The

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reactor employed a 370 nm-emitting UV-LED (Luna Fiber Optic Korea, ICN14D-096) as a

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light source. The distance between the photocatalyst surface and the UV-LED was 4 cm. The

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intensity of UV light flux was measured to be 12 mW/cm2 by a power meter (Newport, 1815-

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C). Before each experiment, the glass reactor was purged with high-purity air. All

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photocatalysts were pre-cleaned by illuminating under UV for 1 h before each experiment to

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remove any adsorbed organic impurities. After the cleaning process, the concentration of

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toluene or acetaldehyde was adjusted by diluting the standard gas (300 ppmv toluene, 1000

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ppmv acetaldehyde in Ar) with high-purity air. The typical inlet concentration of toluene and

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acetaldehyde was 50 ppmv and 180 ppmv, respectively. The initial concentration of

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acetaldehyde was adjusted to 180 ppmv, which has almost the same carbon content as 50

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ppmv of toluene. The relative humidity (RH) was adjusted at ca. 65% by bubbling air through

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a stainless steel bottle that contained deionized water of which temperature was controlled

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during the experiment. The humidity level was regularly checked by the photoacoustic gas

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monitor to maintain RH constant.

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Photocatalyst Characterization.

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The properties of TNP and TNT photocatalysts were investigated by X-ray diffraction (XRD,

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Max Science Co., M18XHF) using Cu-Kα radiation, field emission scanning electron

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microscopy (FE-SEM, JEOL, JSM-7401F), diffuse reflectance UV-Visible absorption

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spectroscopy using a spectrophotometer (Shimadzu UV-2401PC), high resolution

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transmission electron microscopy (HR-TEM, JEOL, JEM-2200FS) with Cs correction, and

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electron energy loss spectrum (EELS). The carbonaceous residues deposited on TiO2 were

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analyzed by secondary ion mass spectrometry (SIMS, CAMECA, IMS 6F, O2+ Gun) and FT-

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IR spectroscopy (Thermo Scientific Is50)

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RESULTS AND DISCUSSION

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Properties of Photocatalysts. The XRD analysis of photocatalysts (Figure S1) showed that

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the immobilization of P25 TiO2 to form TNP did not change any crystalline phase property.

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The synthesized TNT showed mainly anatase diffraction patterns. The structural

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morphologies of TNP and TNT were examined by FE-SEM analysis (Figure 2). The

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thicknesses of the TNP and TNT films were controlled to be similar. The thickness of the

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TNP film was about 9.8(±1.2) µm while TNT has channel pores with the diameter of 40-60

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nm and a tube length of 9.5(±0.9) µm. The light absorption and reflection by TNT film is

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clearly different from that by TNP film. The diffuse reflectance spectra of TNT film exhibits

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a broad band in the entire visible region (see Figure S2), which implies that the light incident

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onto the TNT film penetrates deeper into the film and is more attenuated within the open

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channel with yielding lower intensity of diffuse reflecting light.

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Photocatalytic Degradation of Toluene on TNP vs TNT.

Five successive cycles of

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photocatalytic degradation of toluene with an initial concentration of 50 ppmv were 7



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conducted using TNP and TNT films (Figure 3a, b). The concentration of 50 ppmv toluene

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was optimal for comparing the deactivation effect between TNP and TNT films. Each

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experiment cycle consisted of 10 min of circulation period for adsorption equilibrium and 30

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min of photocatalytic degradation period after which a clean air purging followed to remove

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remaining toluene. The subsequent photocatalysis cycle resumed after the reactor was filled

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with a fresh gas containing 50 ppmv toluene. The photocatalytic activity of TNT was similar

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to (or slightly higher than) that of TNP in the first degradation cycle, which indicates that the

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photocatalytic activities of TNP and TNT were similar at the beginning. Incidentally,

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considering that the total mass of TiO2 in TNT should be lower than that of TNP film of the

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similar thickness (because of the large open channel volume in TNT), the intrinsic

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photocatalytic activity of TNT normalized by the mass of TiO2 should be higher than that of

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TNP. When the number of photocatalytic degradation cycles increased, the activities of TNP

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and TNT changed in very different ways. The pseudo-first order degradation rate constants

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(for both TNP and TNT) are summarized for successive photocatalysis cycles of toluene

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degradation (Table 1). TNP exhibited a rapid deactivation (k decreased to < 5% of the initial

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value after 5th cycle).

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The degree of photocatalyst deactivation depends on various experimental conditions

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(e.g., mass of photocatalyst, light intensity, type of reactor) and TiO2 photocatalyst suffers

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severe deactivation particularly during toluene degradation. Einaga et al. reported that the

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activity of TNP decreased 86% within 1 h toluene degradation.36 Cao et al. also observed

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severe TNP deactivation (74% decrease of activity within 1 h toluene degradation).37 Unlike

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TNP, the activity of TNT was only moderately reduced under the same condition (k

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decreased to ~50% of the initial value after 5th cycle). This indicates that TNT is more

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durable than TNP as an air-purifying photocatalyst. However, it should be noted that we are 8


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comparing the pure anatase (TNT) with a mixed crystalline TiO2 (P25 with anatase-rutile

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mixture). Since this different crystalline characteristics may be responsible for the different

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deactivation behavior between TNT and TNP, we tested anatase-only (rutile-free) TNP film

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as well. Pure anatase power was obtained by a selective etching of rutile phase in P25 and

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used for preparing anatase-P25 film according to the literature procedure.38-39 As shown in

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Figure S3a, both P25 and anatase-P25 films exhibited a similar deactivation behavior, which

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rules out the possible role of rutile in deactivation mechanism. We also carried out additional

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deactivation experiments with employing TNP films of various thickness (7.1–13.6 µm). As

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shown in Figure S3b, the deactivation behavior was similarly observed with all TNP films.

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This confirms that the photocatalyst deactivation is an intrinsic phenomenon which is not

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dependent on the TiO2 mass.

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Carbonaceous Deposits Formed on TiO2.

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light brown after five cycles of photocatalytic degradation, which is also evident in the

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diffuse reflectance spectrum of deactivated TNP film showing an elevated background

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(Figure 4a). This indicates that carbonaceous deposits were formed on the TNP surface due to

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the incomplete degradation of toluene.36 During the photocatalytic oxidation of toluene,

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hydroxyl radicals attack the methyl group of toluene, which subsequently leads to the

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formation of byproducts such as benzyl alcohol, benzaldehyde, and benzoic acid. FT-IR

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analysis of the deactivated TiO2 film clearly shows the presence of carbon-containing

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functional groups originating from the carbonaceous intermediates formation (see Figure S4).

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These intermediates are more strongly adsorbed on TiO2 surface than toluene and some of

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them can be further transformed into condensed products (with higher molecular weight)

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unless they are rapidly decomposed to CO2.37 In particular, benzoic acid is known as a main

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deactivation-causing intermediate which is a solid at room temperature and strongly adsorbed

The color of TNP film changed from white to

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on the surface of the catalyst.40 The strong interaction between recalcitrant carbonaceous

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intermediates and the TiO2 surface should decrease the number of active sites, which is the

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main reason for the photocatalyst deactivation.

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To estimate the amount of carbonaceous materials deposited on the photocatalyst surface,

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the TNT and TNP films that had been used in 5 successive photocatalysis cycles of toluene

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degradation were illuminated under UV irradiation in the clean air (without any toluene). The

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deactivated photocatalyst can be often regenerated by photooxidizing carbonaceous deposits

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in a post-photocatalytic process.41 During the post-photocatalytic process, CO2 was evolved

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from the photocatalytic decomposition of in-situ formed carbonaceous deposits as shown in

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Figure 4b. The concentration of CO2 rapidly increased in the initial irradiation stage and was

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saturated after 1 h UV irradiation, which means all deposited carbonaceous materials were

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oxidized to CO2. Fresh TNT and TNP samples also generated non-negligible amount of CO2,

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which should be ascribed to the ambient organic impurities adsorbed on TiO2 surface. The

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net CO2 generated from the carbonaceous deposit degradation was estimated from (∆[CO2]net

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= [CO2]deac-TiO2 − [CO2]fresh

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deactivated TNP (after 5 cycles) was almost three times higher than that of deactivated TNT.

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This clearly indicates that more carbonaceous deposits were formed on TNP than TNT,

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which is consistent with the observation that TNP film was more severely deactivated than

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TNT film after 5 photocatalysis cycles (see Figure 3). In addition, it is interesting to note that

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the initial decomposition rate of the carbonaceous deposit (i.e., CO2 generation rate) on TNT

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(k = 1.21 min-1) was much higher than that of TNP (k = 0.78 min-1) although TNT has less

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carbon deposits than TNP. As a result, the photogeneration of CO2 from the deactivated TNT

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reached the saturation level much faster compared with TNP (Figure 3b). This indicates that

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the photocatalytic removal rate of in-situ generated carbonaceous deposits is much faster on

TiO2)

after 1 h UV irradiation and ∆[CO2]net measured with

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TNT. This is probably ascribed to the open channel structure of TNT that allows facile O2 (an

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essential oxidant needed for mineralization) transfer to the inner tube surface on which the

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carbonaceous deposits are oxidized and decomposed into CO2. After the carbon deposits are

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photocatalytically removed under clean air and UV irradiation, the deactivated TNP and TNT

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films were regenerated with exhibiting the activity similar to the fresh samples (Figure 4c,

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d).41

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The carbonaceous intermediates that were deposited on the surface of TNP and TNT

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(after the 5th cycle of toluene degradation) were analyzed by dynamic SIMS for depth

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profiling (Figure 5a, 5b).42 The intensity of carbon signal was clearly higher with the

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deactivated TNP compared with fresh TNP and TNT samples, which is consistent with the

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above evidences that TNP suffered from more rapid deactivation. The carbon signal

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intensities of TNP were higher in the surface region (< 0.5 µm) and rapidly decreased with

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the depth (Figure 5a). It should be noted that the carbon signal of the deactivated TNP was

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consistently higher than that of the fresh TNP up to the depth of 2 µm, which implies that the

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deposition of carbon residues is not limited to the surface region but takes place throughout

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the TNP film (leaving carbon residues in the interstitial pores), because light penetrates into

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the TNP film up to the depth of a few µm.43 On the other hand, TNT showed little difference

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in the carbon signal distribution between the fresh and deactivated TNTs, which indicates the

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presence of much less carbon residues on the deactivated TNT (Figure 5b). TEM images and

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EELS elemental maps of C also showed that carbonaceous deposits were formed on the

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surface of TNP after the 5th cycle of toluene degradation (Figure 5c-f), whereas they were not

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clearly observed on TNT (Figure 5g-j). This is consistent with the SIMS data as well.

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Factors that Affect Catalyst Deactivation.

If the main cause of the photocatalyst

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deactivation is the accumulation of recalcitrant carbonaceous residues on TiO2 and the

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formation of carbonaceous deposits is ascribed to the incomplete photocatalytic oxidation of

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toluene, the deactivation process might be retarded under higher O2 concentration under

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which the photocatalytic oxidation is less limited by the mass transfer of O2 molecules.

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Therefore, the photocatalytic degradation of toluene was carried out in pure O2 atmosphere

262

(instead of air) and the repeated photocatalysis cycles were compared between TNP and TNT

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films as shown in Figure 6. In comparison with Figure 3, it is clear that the photocatalyst

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deactivation is markedly reduced under pure oxygen than air atmosphere. The initial

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degradation rates were much enhanced under pure oxygen as well. The toluene degradation

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rate constants (for successive cycles with TNP and TNT) are compared between the air and

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pure oxygen atmosphere in Table 1. The deactivation of TNP was much retarded (Figure 6a)

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and TNT showed almost no sign of deactivation (Figure 6b) under pure O2 atmosphere. The

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O2-rich condition generally accelerates the photocatalytic oxidation and mineralization rate of

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organic pollutants.44 This is because dioxygen molecules serve not only as a main acceptor of

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conduction band (CB) electrons (eq. 2) that are generated upon UV excitation (eq 1) but also

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as an essential reagent required for the mineralization of organic substances (i.e., conversion

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to CO2) (eq 3). TiO + hν → TiO e + h

(1)

O + e → O

(2)

R ∙ + O → ROO ∙ →→ CO

(3)

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The effects of pure oxygen shown in Figure 6 and Table 1 clearly suggest that a facile O2

275

supply is a critical factor for preventing catalyst deactivation and a main reason for the higher

276

durability of TNT against deactivation during VOC degradation. In the TNP film where

277

nanoparticles are closely packed with restricted interstitial pores, dioxygen molecules cannot

278

diffuse freely into the inner surface whereas the TNT film that has an open channel structure

279

allows free diffusion of O2 into its inner tube surface without any geometrical barrier.45 This

280

is schematically illustrated in Scheme 1. Because of the openness of TNT structure, the

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carbonaceous deposits (whose formation is favored under O2-deficient condition) are less

282

formed on TNT and any carbonaceous intermediates formed on TNT can be more rapidly

283

removed via photocatalytic oxidation. On the other hand, the photocatalyst deactivation can

284

be also influenced by the substrate concentration because the higher substrate concentration

285

should produce more intermediates (precursors of carbonaceous deposits). The concentration-

286

dependent (toluene: 20 ppmv vs. 40 ppmv) deactivation was compared between TNP and

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TNT films (see Figure S5a-d). TNT completely degraded toluene during all five cycles at

288

both 20 ppmv and 40 ppmv without showing any deactivation. In contrast, the deactivation of

289

TNP was observed at 40 ppmv. This also confirms that TNT has an inherently higher

290

resistance to catalyst deactivation than TNP.

291

Although TiO2 photocatalyst deactivation has been observed with various VOCs,36-37,40-

292

41,46

293

particular, aromatic compounds such as toluene induce severe deactivation as demonstrated

294

in this study. To compare the effect of VOC type (toluene vs. acetaldehyde) on the

295

photocatalyst deactivation, the photocatalytic degradation of acetaldehyde was also

296

conducted with repeating the five degradation cycles on TNP and TNT (see Figure S6a, b).

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Contrary to the case of toluene degradation, both TNT and TNP exhibited complete

the degree of deactivation can be very different depending on the kind of VOCs. In

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degradation of acetaldehyde during all five cycles without showing any sign of deactivation.

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This implies that the degradation of acetaldehyde leaves little intermediates that may

300

accumulate on TiO2 surface, contrary to the case of toluene. Although the photocatalytic

301

degradation of acetaldehyde may generate intermediates like acetic acid and formic acid,47

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they should be immediately decomposed as soon as they are produced without leaving any

303

carbonaceous residues.48-50

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The deactivation of photocatalyst has been the most serious problem in commercializing

305

this technology. In particular, the deactivation is more serious when the photocatalysts are

306

applied to air purification than water treatment.36, 51 The degradation of VOCs often leave

307

recalcitrant carbonaceous residues on the photocatalyst surface as a result of incomplete

308

degradation, which causes the catalyst deactivation. This study demonstrated that the TNT

309

film as a photocatalyst for VOCs degradation is markedly more resistant to the deactivation

310

than the conventional TNP film. The open straight channel structure of TNT facilitates the

311

mass transfer of dioxygen molecules onto the active surface sites, accelerates the

312

photocatalytic degradation of recalcitrant intermediates, and consequently retards the

313

accumulation of carbonaceous residues. Such structural characteristics of TNT is highly

314

advantageous in preventing the catalyst deactivation during the photocatalytic degradation of

315

aromatic compounds.

316 317

ACKNOWLEDGMENT.

318

This work was supported by the Global Research Laboratory program (2014K1A1A2041044)

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funded by the Korea government (MSIP) through NRF.

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Supporting Information Available.

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XRD of TiO2 samples (Fig. S1); Absorption spectra of TNP and TNT films (Fig. S2);

323

Toluene degradation rate constants obtained using various TNP films (Fig. S3); FT-IR

324

spectra of fresh and deactivated TiO2 (Fig. S4); Repeated cycles of photocatalytic

325

degradation of toluene (Fig. S5); Repeated cycles of photocatalytic degradation of

326

acetaldehyde (Fig. S6). This information is available free of charge via the Internet at

327

http://pubs.acs.org/.

328 329

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(2) Ayoko, G. A. Volatile organic compounds in indoor environments. The Handbook

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Environ. Chem. 2004, 4 (Part F), 1-35.

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(3) Chaudhary, A.; Hellweg, S. Including indoor offgassed emissions in the life cycle

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inventories of wood products. Environ. Sci. Technol. 2014, 48, 14607-14614.

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(4) Salthammer, T.; Mentese, S.; Marutzky, R. Formaldehyde in the indoor envrionment.

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Chem. Rev. 2010, 110, 2536-2572.

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456

viability of photocatalysis for air purification. Molecules 2015, 20, 1319-1356.

457 20


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458

Table 1. Change of the toluene degradation rate constants during the successive

459

photocatalysis cycles on TNP and TNT films.

460 Pseudo first-order rate constant (10-2 min-1)

Cycle TNP (Air)

TNT (Air)

TNP (O2)

TNT (O2)

1

3.83

4.08

7.39

7.77

2

1.26

3.14

5.2

8.34

3

0.53

2.91

4.33

8.62

4

0.42

2.53

3.34

7.79

5

0.1

2.26

2.69

7.66

461 462 463

21



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464 465

Figure 1. Schematic diagram of the experimental setup for the photocatalytic degradation.

466

22


Page 23 of 29


467 468

Figure 2. FE-SEM images of cross-sectional and top view of (a, c) TNP and (b, d) TNT film.

23



Page 24 of 29

469

470 471

Figure 3. Repeated photocatalytic degradation cycles of gaseous toluene on (a) TNP and (b)

472

TNT in the air (●: [Toluene], ○: [CO2]).

24


Page 25 of 29


473 474

Figure 4. (a) Diffuse reflectance UV-Visible absorption spectra of fresh TNP and deactivated

475

TNP after 5th cycle. The Inset shows photo image of fresh TNP and deactivated TNP after 5th

476

cycle. (b) Profile of CO2 generation from fresh and deactivated TNP and TNT under UV

477

irradiation in the fresh air (without toluene). (c) Profile of photocatalytic degradation of

478

toluene using fresh TNP, deactivated TNP after 5th cycle, and reactivated TNP after 1 h UV

479

treatment. (d) Profile of photocatalytic degradation of toluene using fresh TNT, deactivated

480

TNT after 5th cycle, and reactivated TNT after 1 h UV treatment.

481 482 483 25



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484 485

Figure 5. (a) Dynamic SIMS depth profiles of fresh TNP and deactivated TNP after 5th cycle.

486

(b) Dynamic SIMS depth profiles of fresh TNT and deactivated TNT after 5th cycle. TEM

487

images and EELS elemental maps of C (represented by a white color) in (c, d) fresh TNP, (e,

488

f) deactivated TNP after 5th cycle, (g, h) fresh TNT, and (i, j) deactivated TNT after 5th cycle.

489 490 491

26


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492

493 494

Figure 6. Repeated photocatalytic degradation cycles of gaseous toluene on (a) TNP and (b)

495

TNT in pure oxygen (100% O2) gas (●: [Toluene], ○: [CO2]).

27



Page 28 of 29

496 497

Scheme 1. Schematic illustration of TNP and TNT films. Red colored parts indicate

498

carbonaceous deposits that are in-situ formed during the photocatalytic degradation of

499

toluene.

500 501

28


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502


Table of Content (TOC)

503

29


TiO2 Nanotubes with Open Channels as Deactivation-Resistant

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